THE MEDICAL FACULTY OF Cazenave's Manual of Diseases of the Skin, trans- lated by Dr. Burgess. Fcap. Price 7s. Ward's Outlines of Human Osteology. 32mo. cloth. Price 5s. G-uthrie's Commentaries on the Surgery of the War in Portugal, Spain, France, and the Netherlands. Fifth Edi- tion, revised to 1853. cr. 8vo. Price 14s. " We can thoroughly and safely recommend this work." Medical Times. Valentin's Text-Book of Physiology, translated and edited from the Third German Edition by William Brinton, M.D. 500 Figures on wood, copper, and stone. 8vo. cloth. Price 25s. " The best Text-book of Physiology ever published " Dublin Med. Quarterly. "An excellent translation of this admirable work." Lancet. MR. RENSHAFS PUBLICATIONS. THE MOST COMPLETE SYSTEM EVEB BTJBLISHED. t SURGERY CHELIUS' accom John I of Surj " This Sys be patronizec "The mosj descriptions department, I "This woJ Medical Tim\ " We earm system of mo> GENERAL P princip John Si pital. Cont< Produci Evacua Infecth " Mr. Simo advantage." " We strong London Jourt THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID THE CMNIQUE MEDICARE; or Reports of Medical Cases by G. Andral : translated, with Observations from the Writings of the most distinguished Medical Authors, by D. Spillan, M.D., Fellow of the College of Physicians in Ireland. " We strongly recommend all our young friends who wish to know what is most excellent in the modern pathology of France to lose no time in adding this translation to their stock of Medical authorities ' British and Foreign Medical Review. " VVe recommend Dr. Spillan's translation to our readers." Lancet. TEXT-BOOK OF PHYSIOLOGY. TEXT BOO OF PHYSIOLOGY. DR. G. VALENTIN, PROFESSOR OP PHYSIOLOGY IN THE UNIVERSITY OP BERN. TRANSLATED AND EDITED FROM THE THIRD GERMAN EDITION, BY WILLIAM BRINTON, M.D., LICENTIATE OF THE ROYAL COLLEGE OF PHYSICIANS ; PHYSICIAN TO THE ROYAL FREE HOSPITAL ; MEDICAL TUTOR IN KING'S COLLEGE.* WITH UPWARDS OF FIVE HUNDRED ILLUSTRATIONS ON WOOD, COPPER, AND STOKE. y " M LONDON: HENRY RENSHAW, 356, STRAND. . . , LONDON : Printed by SAMUKL BENTLKT and Co. Bangor House, Shoe Lane. PREFACE. THE work now introduced to the English public is best described as an abridgment, by Professor Valentin, of the last edition of his larger systematic treatise, the u Lehrbuch der Physiologic." The preface to a translation is no place to enlarge on the merits of the original, or the repute of its Author. The reader probably thinks it enough to be told the main features of the book that follows. The Author's chief objects have been brevity and completeness. In pursuance of the latter, he has laid great stress on the numerous physico- chemical researches which have done so much for modern Physiology. This has involved a short account of those parts of the physical sciences on which such researches are based. Hence the advanced student of Physiology may use this book as a convenient summary of many experiments hitherto imperfectly known in this country. While the beginner will certainly find that, in addition to a full though condensed treatment of the first principles of this science, it comprehends so much of various kindred subjects as may either obviate, or what is better, fructify, a reference to the ordinary text-books of each. And in this respect it seems peculiarly adapted to that increasing number of the educated public, who, although unable to devote themselves to an extended course of study, still desire some insight into the natural laws which regulate their own life and welfare. In order to secure, if possible, a faithful, as well as readable, transla- tion, the original was first rendered into a literal English; the words and phrases of which, where necessary, were afterwards exchanged for simpler or smoother equivalents. Measurements are everywhere reduced to English feet and inches ; and iy PREFACE. weights to the Avoirdupois standard. The calculations necessary for these changes have been made with much care. The number of decimals in the several estimates generally corresponds with those given in the original. In some instances the reader may perhaps think too many have been set down. But by neglecting one or more of the right hand figures, he can always obtain a more simple, though less exact, state- ment. Finally, notes have been added by the Editor, in the few instances where he has thought it advisable to modify or explain the meaning of the text. WILLIAM BRINTON. KING'S COLLEGE, April, 1853. CONTENTS. CHAP. PAGE I. ORGANIZATION AND LIFE .... 1 II. ORGANIC AND ANIMAL FUNCTIONS ... 5 III. INDEPENDENCE OP THE VITAL MANIFESTATIONS IN ANIMALS 8 IV. PHYSICAL PROPERTIES OF THE HUMAN BODY . . 13 V. CHEMICAL COMPOSITION OF ORGANIZED BEINGS . 93 VI. DIGESTION . . . . . .114 VII. ABSORPTION . . . . . .156 VIII. CIRCULATION . . . . . .173 IX. RESPIRATION ...... 222 X. EVAPORATION . . . . . 254 XI. SECRETION . . . . . .262 XII. THE VASCULAR GLANDS . . . .297 XIII. NUTRITION ...... 303 XIV. ANIMAL HEAT ...... 345 XV. LOCOMOTION ...... 354 XVI. VOICE . . . . . . 416 XVII. FUNCTIONS OF THE SENSES .... 427 Sight ...... 428 Hearing . . . . . .476 Smell ...... 484 Taste ...... 488 Touch ...... 492 XVIII. INNERVATION ...... 500 XIX. GENERATION AND DEVELOPMENT 608 LIST OF THE WOODCUTS. 1 . DIAGRAM of the curve of elasticity . 21 2. Diagram of the extension of a cylinder .... 3. Diagram representing the action of friction . . . .27 4. Diagram to illustrate hydrostatic pressure . . . .29 5. Diagram to illustrate hydrostatic equilibrium . . . ,30 6. Barometer . . . * . . 31 7. Another variety of barometer . . . . .31 8. Manometer . 31 9. Vertical section of the trunk, showing the situation of the thoracic and abdo- minal viscera . . . . . .34 10. Instrument to illustrate the atmospheric pressure . . .34 11. Pelvis and hip-joints in vertical section f . . .35 12. Diagram to illustrate the flow of liquids . 36 1 3. Diagram showing the pressure of current liquids ... 37 1 4. Diagram showing the effect of widening a channel 15. Diagram to illustrate capillary attraction . . .40 16. Diagram to illustrate capillary repulsion . . . .41 17. Diagram to illustrate the interstices of animal tissues . .41 18. Diagram to illustrate the mechanical results of evaporation 43 1 9. Apparatus showing endosmose and exosmose . . .44 20. Diagram illustrating their mechanism . . . .44 21. Dutrochet's endosmometer . . . 46 22. Apparatus to illustrate the diffusion of two liquids through the coats of a vessel 50 23. Diagram of a circular undulation . . . . .53 24. Diagram of the ray of undulation . . . , .53 25. Diagram to illustrate polarization . . . . .53 26. Diagram to illustrate the axes of elasticity . . . .54 27. Diagram to illustrate refraction of coloured light . . .55 28. Diagram to illustrate vibrations of the luminous aether . . .55 29. Diagram to illustrate partial interference of two rays . . .56 30. Diagram to illustrate their complete interference . . .56 31. Diagram to illustrate the bands produced by inflection of light . . 57 32. Diagram to illustrate refraction of light . . . .57 33. A Nicol's prism in different positions . 58 34. The polarizing microscope , . . .59 35. Tube for examining the polarization of fluids . . . .60 36. Diagram to illustrate the polarization of doubly-refracting objects . . 61 37. Diagram to illustrate the polarization of a starch-granule . . .64 38. The same in the opposite position of the upper prism . . .64 39. Apparatus to illustrate the tension of various vapours . . .66 40. Apparatus to illustrate the conduction of heat . . .70 41. Insulated coil of wire . 76 viii LIST OF THE WOODCUTS. FIG. p AOE 42. Astatic magnetic needle . . 76 43. Galvanometer . .... 76 44. Tangential compass .... .77 45. Diagram representing the muscular current . . . .78 46. Diagram representing the muscular current in a cylindrical muscle . . 78 47. Diagram to illustrate the peripolar arrangement of the molecules of muscle . 79 48. Galvanic circuit in which two conductors may be compared with each other . 79 49. Apparatus to illustrate the peripheric or centrifugal current 80 50. Apparatus showing the central or centripetal current . 80 51. The galvanic column or pile 80 52. Bunsen's constant circuit . . . 81 53. Diagram representing the changes of electrical tension by curves . 82 54. Apparatus for the electrolysis of fluids . .84 55. Apparatus illustrating electrical polarization . . . .85 56. Compound (inductive and inducteous) coil . . . .85 57. Diagram representing the induction produced by interrupting a galvanic circuit 86 58. Electro-magnetic apparatus . . . . .87 59. Magneto-electric apparatus . . . . .89 60. Diagram representing magnetism and diamagnetism . . .89 61. Apparatus exhibiting the diamagnetic properties of the living frog . . 90 62. Apparatus for determining the rotation of the plane of polarization producible by electricity . . . . . . .91 63. Incisor or cutting tooth ...... 121 64. Canine or eye tooth ...... 121 65. Molar or grinding tooth . . . . . .121 66. Skull showing the mechanism of mastication . . . .122 67. Muscles of the tongue . . . . . .124 68. Vertical section of the head, to illustrate the mechanism of deglutition . 125 69. Soft palate ... . .126 70. The same during the act of deglutition . . . 126 71. Peristalsis of the oesophagus during deglutition . . 127 72. The same more advanced . . . . .127 73. Diagram of the undulations of the same . . . .127 74. Human stomach ...... 129 75. Stomach, duodenum, and gall-ducts of the human subject . .132 76. Small intestine of a newly killed rabbit, during vermicular contraction . 134 77. Human caecum, with the ilio-csecal and ilio-colic valves . . . 134 78. Stomach- tubes in a vertical section of the gastric mucous membrane . 142 79. Separation of muscular fibre into discs when undergoing digestion . . 145 80. Apparatus for illustrating filtration . . . . .157 81. Diagram illustrating the structure of an intestinal villus . . .161 82. Course of the larger lymphatics of the trunks . . . .164 83. Trunk of an absorbent injected with mercury . . 1 68 84. 85. Diagrams illustrating the mechanism of the valves of the absorbents. (In 84 these are open; in 85, shut) . . . .168 86. Apparatus showing the force of suction exerted by a current of liquid in a tube 170 87. Anterior view of the adult human heart . . . .174 88. Posterior view of the same . . . . .174 89. Diagram of the pulmonary and systemic circulations . . .175 90. Right heart, slit up along its right border . . . .178 91. Left heart, laid open along its left border . . . .178 92. Auriculo- ventricular valve during the systole of the ventricle . .180 LIST OF THE WOODCUTS. ix FIG. PAGE 93. Auriculo- ventricular valve during the diastole of the same . .180 94. Upper surface of the ventricles during their systole . . .181 95. Upper surface of the ventricles during their diastole . . 1 82 96. Apparatus for imitating the action of the heart and its valves . . 183 97. The heart of a rabbit ; showing the contrast of its outline during diastole and systole ....... 183 98. Heart of an etherized frog during ventricular diastole . . . 185 99. The same during ventricular systole . . . 1 85 1 00. Thoracic cavity of an eight months' child which only lived two days after its premature birth: showing the situation of its thoracic viscera . .186 101. The same after artificial inflation of the lungs . . . 187 102. Vertical section of a stethoscope . . . . .187 103. Diagram to illustrate the blood- waves in the aorta , . .188 1 04. Diagram showing the effect of the elongation of an artery on its form and situation 191 1 05. Diagram to illustrate the mode of applying the haemadynamometer . 191 106. Diagram of the blood- and breathing-pressures . . .193 107. Diagram illustrating the equal tension of the blood in different animals . 194 108. Diagram explaining the increase of pulsatory movement at the end of a limb 197 1 09. Diagram illustrating the relations borne by the areas of arterial bifurcations to their trunk . . . . . .199 110. Capillaries of the tactile papillae . . . . .199 111. Capillaries of an intestinal villus . . . . .199 112. Capillaries of the mesentery ..... 200 113. Capillaries of the dried lung ..... 200 114. Capillary of the web of the frog's foot .... 200 115. Diagram to explain the apparent velocity of the blood-corpuscles under the microscope . . . . .201 116. 117. Diagrams illustrating the mechanism of the valves of the veins. (In 116 they are open; in 117, shut,) ..... 204 118. Diagram representing the systemic, pulmonic, and portal circulation . * 207 119. Viscera and vessels of the trunk of an eight months' foetus which only lived two days after birth : showing its portal circulation . . . 208 120. Heart of the same . ..... 223 121. Excised auricle of the same, laid open to show its interior . . 224 122. Diagram to illustrate the rarefaction produced by the sudden mechanical dila- tation of a space containing air ..... 225 123. Diagram illustrating the arrangements of the thorax . . . 226 124. Vertical section of the trunk, showing the mechanism of respiration . 227 125. Profile of the respiratory movements of the adult male . . . 228 126. The same in the adult female ..... 228 127. Adult skeleton . . . . . .230 128. Vertical median section of the head, to show the channels of breathing . 232 129. Pneumatometer ...... 234 130. Apparatus for determining the quantity of watery vapour contained in the air expired within a given time ..... 237 131. Apparatus for determining the same with respect to the total air of expiration 238 132. Hutchinson's spirometer ..... 243 133. The same filled with air ..... 243 134. Eudiometer-tube for explosion by the electrical spark . . . 245 135. Eudiometric apparatus for analysis by weight . . .246 1 36. Apparatus for experimenting on the asphyxia of small animals . . 253 137. Apparatus for the eudiometric analysis of the total transpiration of small animals 255 X LIST OP THE WOODCUTS. FIG. PAGE 1 38. Apparatus which effects the same purpose without danger to the animal, and with larger quantitative results . . . 256 139. Longitudinal section of a tendon and its burs,a . 262 140. Diagram of three different arrangements of a secreting surface . . 263 141. Diagram of the papillary and cavitary increase of a surface . . 263 1 42. Villi and follicles as seen in a vertical section of the intestine . . 263 143. Spiral tubular gland of the skin . . . . .264 144. Ramified gland ending in vesicular dilatations : being a portion of the parotid 264 145. Vertical section of the stomach-tubes of the pig . . , 266 146. Hepatic cells of the human liver ..... 266 147. Outline of the villi and follicles of Lieberkuehn . . .271 148. Outline of the villi and Brunn's glands . . . .271 1 49. Knee-joint the synovial membrane of which has been filled with tallow . 272 150. Anterior view of the eye and its appendages .... 273 151. Arrangement of the liver and its ducts .... 280 1 52. Median longitudinal section of the human kidney . . .283 153. Diagram showing the minute structure of the kidney . . . 284 154. The urinary organs . . . . . . 285 155. Diagram to illustrate the opening of the ureter into the bladder . . 286 1 56. Apparatus for applying the fermentation-test to sugary urine . . 292 157. Tube for the quantitative determination of sugar by Trommer's test . 292 158. Fibrous network of the spleen ..... 296 159. Fibre-cells of the spleen . . . 296 1 60. Malpighian corpuscle of the spleen . . 298 161. Another such corpuscle . . . 298 162. Portion of the thyroid gland . . 300 163. Cells of the thyroid gland . / . . .300 164. Crystalline plates of cholestearine from an enlarged thyroid . .301 165. Calcified vessels from the thyroid. After Ecker . . .301 166. Portion of the thymus gland. After Ecker . . . .301 167. Fibrinous coagula from the heart . . . . .301 168. Margin of a lobule of healthy thymus .... 302 169. Similar margin in the diseased thymus .... 302 170. Flakes of coagulated fibrine ..... 305 171. Diagram of the structure of a villus . . . .307 172. Diagram to illustrate the nutrition of the various epithelial strata . . 308 173. Nucleated epithelial cells of the cuticle . . .308 174. Ciliated epithelia from the bronchi .... 309 175. Bundle of areolar tissue which has been exposed to the action of acetic acid. After Henle . . 312 176. Capillaries of a diseased ovary. After Har ting . . .316 177. Cells formed during the development of exsudations .318 178. Cicatrix of a divided muscle ...... 320 179. Nerve after transverse section . . . . .321 180. The same after reunion of its divided segments . , . 321 181. Fractured tibia after reunion ..... 322 182. Vertical section of the same, showing its callus . . . 322 183. Diagram to illustrate the result of deligation on an artery and its collateral branches ....... 323 184. Stump of an arm three years after amputation . . . 324 185. Thermometer for determining the animal heat . . . 346 186. Thermo-electric apparatus for the same purpose ... 347 LIST OF THE WOODCUTS. XI FIG. PAGE 187. Diagram of the Brownian movement as exhibited by molecules of pigment . 354 188. Portion of ciliated mucous membrane from the palate of the frog . . 356 189. Separate ciliated epithelia from the same part . . . 358 190. Mass of such epithelia . . . . . 358 191. Diagram illustrating the flexion of the cilia .... 358 1 92. Diagram illustrating their circular movement .... 358 1 93. Wavy ciliary border and current ..... 358 1 95. Irregular ciliated surface producing rotary currents . . . 358 196. Altered ciliated epithelia from the mucous membrane of the nose during catarrh 361 197. Ciliated spores of the Hcematococcus pluvialis . . . 362 198. Ciliated spore of the Ectosperma davata .... 362 199. Ciliated episporium of the Pelvetia caniculata . . . 362 200. Spermatozoids of the vertebrate semen : viz. the forms seen in (a). Most osseous fishes. (6). Cartilaginous fishes and birds. (c). Mammals . . . . . .363 201. Cells containing seminal filaments. From the mature antheridia of a foliaceous moss . . . . . .364 202. Similar cells having two filaments ..... 364 203. Body of the Loxopliyllum meleagris; containing globules of fluid which are produced by the contraction of its sarcode .... 365 204. Periodic contractions seen in the cells of the ova of the Planarice . . 365 205. Interlacement of the striped fibres in the auricle of the frog's heart . 367 206. Prepared leg of the frog . ... 368 207. Prepared leg of the frog, insulated ..... 369 208. Diagram to illustrate the contractions produced by the muscle itself . 370 209. Matteucci's column .. . . . . .370 210. Prepared frog, as adapted to experiments on this column . . 371 211. Muscular fibres showing parallel zig-zags .... 376 212. Other fibres with irregular zig-zags .... -376 213. Muscular fibre exhibiting constrictions of its surface . . . 377 214. Peculiar forms assumed by the ends of muscular fibres . . . 378 216. Muscular fibre which has coiled up under water . . 378 217. Apparatus for determining the bulk of muscles during their contraction . 379 218. E. Weber's apparatus for determining the elasticity of contracting muscle . 380 219. Diagram to illustrate the electrical properties of the molecules of a muscular fibre 381 220. Mode of exhibiting the induced contraction . . . .. 382 221. Galvanic forceps . . . . .383 222. Diagram to illustrate the state of equipoise .... 388 223. Diagram to illustrate the state of counterpoise . . . 389 224. Diagram to illustrate the action of the one-armed lever . .389 225. Diagram to illustrate the effect of the direction of a force applied to a horizontal lever . . . . . . .390 226. Diagram to illustrate the centre of gravity of a sphere . . 39 1 227. Diagram to illustrate the centre of gravity of an ellipse . . .391 228. 229. Diagrams illustrating the centre of gravity of the laden and unladen man in the erect attitude ...... 392 230. Lumbar vertebra ...... 393 231. Adult skeleton . . . . . .394 232. Diagram illustrating the mechanical use of the processes of bones . . 39fr 233. Knee-joint, illustrating the same advantages .... 396 234. Vertical section of a portion of the spine .... 397 Xii LIST OF THE WOODCUTS. FIG. PAGE 235. Shoulder-joint 39 ? 236. Muscles of the upper extremity, as seen anteriorly . . 398 237. The eye in situ, as seen from above . . 399 238. Diagram illustrating the effective action of muscles . . . 400 239. Diagram illustrating the effect of the oblique attachment of muscular fibres to their tendon . . . . 402 240. Diagram illustrating the leverage of the fore-arm . . .406 241. Diagram illustrating the advantages secured by the same . 407 242. Diagram illustrating the elasticity of the limbs . 410 243. Regnier's dynamometer . . .412 244. The same as used to determine tractile force . . . .413 245. Diagram of a sonorous undulation . . .416 246.) j 416 04 H C Diagrams illustrating the waves of alternate rarefaction and condensation . (417 248. Metallic tongued instrument . . .417 249. Membranous tongued instrument . 417 250. Anterior view of the human larynx . . . .418 251. Lateral view of the same . . . - .418 252. Superior view of the same . , . . .418 253. Anterior muscles of the same . . . . .419 254. Posterior muscles of the same . . . . .419 255. An organ-pipe ...... 420 256. The same in situ . . . . . .420 257. Apparatus for verifying the action of the larynx in the dead subject . 420 258. Ordinary registers of voice . . . . .423 259. Muscles of the eyes as seen from above .... 429 260. Side view of the eye in situ ..... 430 261. Diagram illustrating the action of the two oblique muscles . . 430 262. Diagram illustrating the associated movement of the eyes . . 431 263. Diagram illustrating the act of squinting .... 334 264. Diagram illustrating the law of reflection of light . . . 435 265. Diagram illustrating the real focus of the concave mirror . . 435 266. Diagram illustrating the virtual focus of the concave mirror . . 435 267. Diagram illustrating the laws of refraction . . . .436 268. Diagram illustrating the index of refraction . . . .436 269. Diagram illustrating refraction by a convex lens . . .437 270. Various forms of lens : viz. a. The double convex, ft. The plano-convex. , [ The meniscus. a. ) e. The double concave. f. The plano-concave . . . . .438 271. Diagram illustrating spherical aberration .... 439 272. Diagram illustrating the use of a septum in diminishing spherical aberration 439 273.) 274 ) Dia rams illustrating the images of convex lenses . . . 440 275. Section of the camera obscura ..... 440 276. Vertical section of the eye . . . . . .441 277. The same arranged so as to illustrate the mirrored image . . 442 278. Diagram illustrating the stratified structure of the lens . . . 444 279. Diagram illustrating the formation of dispersive circles within the eye . 445 LIST OP THE WOODCUTS. Xlll FIG. PAGE 280. Diagram illustrating the visual angle .447 281. Diagram illustrating the optometer . 447 282. Another diagram illustrating the principle of the same . . 448 283. Diagram illustrating the chromatic aberration . 452 285 ' Diagrams illustrating the formation of the prismatic spectrum . . 452 286. Diagram illustrating the dark lines of the prismatic spectrum . . 453 287. Vertical section of an achromatic combination of lenses . . . 453 288. Diagram of an apparatus for determining the maximum visual angle . 454 289. Diagram illustrating the minimum visual angle . 456 290. Diagram illustrating the magnifying action of a convex lens . . 457 291. Diagram illustrating the construction of the compound microscope . . 457 292. Diagram illustrating one of the theories of erect vision . . . 458 293. Diagram illustrating the deceptive estimation of surface . . 459 O q/ ( Diagram illustrating the irradiation caused by white surfaces . . 460 296. Diagram illustrating Plateau's method of determining the duration of visual impressions . . . . . .461 297. The thaumatrope ...... 462 298. Diagram for determining the complementary colours . . . 463 299. Diagram illustrating the subjective complementary colours of vision . 464 300. Diagram illustrating the subjective complementary shadows of vision . 464 301. Diagram illustrating the relations of the width and distance of the visual field 465 302. Diagram illustrating vision with both eyes, as in various animals . . 466 303. Diagram illustrating vision with both eyes, as in man . . 466 - " [ Diagrams illustrating the circuit of vision . . .467 305. ; ' * [ Diagrams illustrating the perception of solidity by the eye . . 469 307. i 308. The stereoscope .... .469 309. Diagram illustrating the action of the stereoscope . . 470 310. Diagram illustrating the effects of shadow .... 472 311. Diagram illustrating these effects in the eye .... 473 312. Entoptic figures ...... 474 313. Entoptic view of the vessels of the retina .... 474 314. Diagram to illustrate the experiment of Mariotte . . . 475 315. Anterior view of the organ of hearing .... 476 316. Auditory ossicles ...... 477 317. Magnified view of the auditory ossicles in situ . . . 477 318. Auditory ossicles and their muscles, in situ, as seen from above . . 478 319. Labyrinth partly laid open so as to show its nerves . . .448 320. Branching of the auditory nerve ..... 448 321. Diagram illustrating the pulsation produced by notes having nearly the same number of vibrations ...... 483 322. Segment of the head showing various structures of the nose, mouth, and pharynx ....... 484 323. The tongue and its nerves, as seen from above . . . 489 324. Mode of illustrating the double and single tactile perception . . 499 325. Human cerebro- spinal centre ..... 500 326. Nervous system of a beetle ..... 501 327. Nerve-fibre and its axis-cylinder . . . . .501 328. Diagram illustrating the branching of a nerve . . . 502 XIV LIST OF THE WOODCUTS. PIG. PAGE 329. Diagram illustrating the anastomosis of nerves , . . 502 330. Diagram illustrating the division of nerve-fibres . . .504 331. Diagram illustrating the looping of nerve-fibres . .. . 504 332. Vertical section of a Pacinian corpuscle .... 504 333. Diagram illustrating the specific conduction of the nerve-fibres . . 506 334. Sciatic plexus of a frog, in situ ..... 507 335. Portion of the human spinal cord . . . . .510 336. Nervous centre of the frog, as seen from behind . . .510 337. Vertical median section of the human head .... 512 338. Viscera of the frog, in situ showing the distribution of the vagus nerve . 519 339. Ganglion on the posterior root of a cat's cervical nerve . . . 519 340. Isolated ganglion-corpuscles of the same . . . .521 341. Ganglion-corpuscle covered by a nuclear sheath . . .521 342. Similar corpuscles with interlacing fibres . . . . 521 343. Fine section of a ganglion and nerve at their place of union; showing the vaginal process of a ganglion- corpuscle . . . .521 344. Another such section, exhibiting medullary fibres with nucleated vaginal processes . . . . . 522 345. Thoracic ganglion of the sympathetic. From a cat . . 523 346. Fine section from a sympathetic ganglion of a mammal showing corpuscles which appear to be unconnected with true nerve-fibres . . 524 347. Ganglion-corpuscle giving off two peripheric processes. From the eel-pout (Gaduslota) . . . . .526 348. Anterior lymph-heart of the frog, in situ .... 531 349. Posterior lymph-heart of the frog, in situ .... 531 350. Excised heart of the frog, at the commencement of the contraction produced by mechanical irritation ...... 532 351. Prepared leg of the frog . . . . ,541 352. Diagram illustrating the electro-tonic state of the nerves . . 542 353. Diagram illustrating the polarity of the nervous molecules during the electro- tonic state ....... 543 354. Diagram illustrating the paradoxical contraction . . . 545 355. Apparatus for testing the contractions of the prepared frog . . 548 356. Prepared leg of the frog . . . K . .555 357. Gymnotus or electrical eel ..... 556 358. Posterior view of the torpedo ; showing the surface of its left electrical organ 557 359. Columns of the electrical organ of the torpedo, cut across transversely . 557 360. Laminae seen by a vertical section of the same . . . 567 361. Columns and laminae of the gymnotus .... 557 362. Portion of the nervous centre of the torpedo . . . 559 363. Ganglion-corpuscles of the electric lobe of the torpedo . . . 559 364. Reunion of a divided nerve ...... 563 365. Median vertical section of the human brain . . . 566 366. Diagram of a portion of the spinal cord, with its motor and sensitive trunks . 572 367. Skull of the new-born infant ..... 586 368. Nervous centre of the frog ..... 589 369. Ovum of the pike ...... 609 370. Ovum of the frog ...... 609 37 1. Diagram representing the action of the pollen-tubes upon the ovary of the plant 611 372. Vertical section of the ovule of the plant . . . 612 373. Diagram representing the action of the pollen-tube on the ovule . . 612 374. Ovum of the Medusa in different stages of development . . .614 LIST OF THE WOODCUTS. XV PIS. PAGE 375. Tetrarhynclms, being probably a young Taenia . . .615 376. Young tapeworm (Bothriocephalus latus) . . . .615 377. Mature links of the same tapeworm .... 615 378. Nursing embryo of the Distoma paciftca. After Steenstrup . . 616 379. Mould-filaments from the scalled head of a child . . . 617 380. View of a thread- worm (the Trigla), showing its internal organs . 620 381. Bundle of spermatozoids ..... 625 382. Urinary and generative organs of the male .... 626 383. Bladder and adjoining generative structures of the male . . 626 384. Part of the male organs of generation. The bladder is laid open, and the penis divided by a horizontal and longitudinal section . . . 627 385. Transverse section of the penis showing its spongy textures . . 628 386. Internal female organs of generation .... 630 387. Imaginary section of the Graafian follicle .... 631 388. Change undergone by the unimpregnated ovum of the rabbit. After Bischoff 638 389. Ovum of the rabbit, surrounded by spermatozoids. After Bischoff . 642 390. Ovum at the latter end of pregnancy, in situ . . ; 645 391. Cleaving of the yolk in the ovum of the rabbit. After Bischoff . . 648 392. Partial cleaving of the yolk, as seen in the ovum of the pike . . 649 393. Tubular glands of the uterus, as seen in a vertical section of its mucous mem- brane ....... 650 394. Ovum during the second month . . . . .651 395. Diagram illustrating the development of the amnion . . . 652 396. Diagram showing the same ovum at a later date . . . 652 397. Uterus and placenta at the end of pregnancy . . . 654 398. Right auricle of an eight months' foetus . . 658 399. Heart of the same ...... 658 400. Abdominal viscera and vessels of the same .... 659 401. Position of the foetus in the ninth month of utero-gestation . . 664 402. Skull of the new-born infant . . . . .664 403. Pelvis of the female 665 EXPLANATION OF THE PLATES. (THE FRACTIONS BENEATH THE FIGURES STATE THE NATURAL SIZE OF THE CORRESPONDING OBJECTS). PLATE I. FIG. 1. MINUTE crystals of common salt. Deposited from a solution by evaporation on a glass plate. 2. Minute crystals of uric acid. Thrown down by hydrochloric acid from partially eva- porated human urine which had been diluted with water. 3. Minute crystals of oxalate of lime. Deposited from the residuum of human urine by standing. 4. Crystals of carbonate of lime. From the otolithe of the green frog. 5. Some of the same crystals, under the polarizing microscope, with parallel Nicol's prisms. 6. The same crystals, with the upper Nicol turned round 60. 7. The same crystals, with the upper Nicol turned round 90. 8. Crystalline globules from the urine of the horse under the polarizing microscope, with the Nicol's prisms at right angles. 9. A large starch-granule under the polarizing microscope, with the two Nicol's prisms in parallel planes. 10. The same granule, with the prisms at right angles. 1 1. The same granule, with the prisms at right angles, and the object plate turned round 45. 12. The crystalline lens of an adult, as seen under the polarizing apparatus, with Nicol's prisms crossing at right angles. Natural size. 13. Tufts of Haidinger, as seen when looked at through a Nicol's prism against the grey sky. The yellow tuft corresponds to the greater diameter of the prism ; and the violet, to the smaller diameter. a b. The longer transverse axis of the prism. c d. The shorter transverse axis. 14. Posterior half of the retina of an adult. It lies within the sclerotic, and presents the medullary elevation that corresponds to the entry of the optic nerve ; together with the vessels which radiate from this point, the central fold, the yellow spot, and the central fossa. Natural size. 15. The central fossa of the retina of another adult. Magnified. a. The granular layer of the retina. b. The shallow terminal bifurcation of the central fossa. c. The radiating rods or papillae of the rnembrana Jacobi which are seen at the bottom of the fossa. d. The deepest part or apex of the central fossa. 16. Gall-stone from the gall-bladder of a girl. It consists of crystals of cholestearine. Magnified from two to three times. 17. Collection of the various substances which are found admixed in the healthy human faeces. All the bodies here represented were found in the same excrement. Here the relics of the various animal and vegetable tissues are only brought close to each other for the sake of economizing space. EXPLANATION OF THE PLATES. XV11 FIG. . A starch-granule, exactly in focus, and hence exhibiting its concentric layers. b c. Two other starch-granules. Their surfaces, being out of focus, have margins which are as dark as those of the oil-drops. d. Part of the skin of a walnut. e. A fragment of vegetable epidermis. f. A reticular vessel from the vegetable food. g. A striped muscular fibre from the animal food. This has only become more transparent. h. Other striped fibres, which are beginning to separate into transverse fragments. i k I. Crystals of ammoniaco-phosphate of magnesia. m. A cell of the pavement epithelium from the neighbourhood of the rectum, saturated with biliary colouring matter. n. Large brown particles. o. Small granules which vary from a greyish-white to a brownish colour, and are met with in large numbers in the faeces. PLATE II. 18. Sarcina ventriculi from the matters vomited by a sick man. 19. Torula cerevisia from yeast. 20. Crystalline globules from the urine of the horse. 21. Crystalline globules from the sandy matter of the middle choroid plexus of a woman. a. Globule with a side process. b. Semi-calcified and transparent globule. 22. Lymph from a wounded lymphatic of the upper part of the leg of an adult man. It was taken from the living body; but having been kept for some time in a closed vessel, offered various after-deposits. a a. Altered lymph-corpuscles. b. Larger granules. c. Fine molecular corpuscles in still larger quantity than the preceding. d. A crystal of cholestearine. e. A crystal of some other substance. 23. Blood- and lymph-corpuscles of the green frog. a. The wall of the blood-corpuscle. 6. Its nucleus. c. Lymph-corpuscle of the blood. 24. Blood- and lymph-corpuscles of the human blood. a. Blood-corpuscles of blood which had been just taken from the living subject, and was unmixed with any foreign fluid. Seen in surface : and to the right, in edge. b. Ordinary lymph-corpuscles of the blood; clear, bright, and colourless. c. Somewhat darker (and unusual) lymph-corpuscle. d d. Blood-corpuscles aggregated in piles or rouleaux : from the serum of venous blood taken from a person suffering under inflammation of the lungs. 25. Constituents of the sanguineous plug found in the cervix uteri of a woman aged 47 years, who died of pneumonia during menstruation. a. Coagulated fibrine. b. Swollen blood-corpuscles below the surface. c. The same close to the surface. d. Small granular globules. e. Large dark granular globules. f. Large clear granular globules. 26. Substance covering the inner surface of the mucous membrane of the uterus in the same person. This is reddish or red in some points, but is elsewhere transparent and greyish-white. EXPLANATION OF THE PLATES. FIG. a. Colourless globules apposed to each other like a pavement, and united by a mucous fluid to form the above transparent substance. b. A few heaps of partially changed blood-corpuscles, giving rise to the red spots. No coagulated fibrine can here be observed. 27. Fat-cells from the subcutaneous adipose tissue of a woman's thigh. 28. Various forms of pigment-molecules from the human choroid. 29. Hexagonal pigment-cells with transparent nuclei, from the choroid of the frog. 30. Branched pigment- cells from the tissues surrounding an abdominal artery of the same animal. 31. Solid matters of the human saliva. a b. The oldest shed cells of the pavement-epithelium of the mouth, as seen by shadowed light, c d. Salivary corpuscles seen under the light of a lamp. 32. Epithelial cells from the middle epidermal layers of the under surface of the great toe of an adult girl. a. Cells with structures resembling nuclei. b c. Separated cells without nuclei. d. Two epidermal cells which are connected to each other and excavated laterally. 33. Pavement-epithelium of the conjunctiva covering the cornea of a man ; seen by shadowed light. a. Polygonal cells. b. Their nuclei. 34. Cylindrical cells of epithelium, from the cystic duct of a man. a. The free surface, which resembles that of a drum. b. The cylindrical cells, which have granular horny walls. c. Oval cells. 35. Several cylinders from the same situation. These are arranged like a palisade; being placed obliquely, and somewhat curved below. 36. Ciliated cylinders from the trachea of another man. a. Larger cylinders, with nuclei above. b c. Smaller cylinders, with nuclei below. 37. Fresh horny substance from the free extremity of the nail of an adult man's middle finger. 38. A fragment of the same horny substance after repeated boiling in a solution of potash. Here the bright transparent horny cells are seen in their natural position , and many of them retain their nuclei. 39. Fragment of a red hair from the beard of a man. a. Lines produced by the margin of the cells of the epidermis which clothes the hair. b. The fibrous streaks of the cortical substance. c. Continuous medullary substance, which is richer in pigment, and therefore darker. d. The clearer medullary substance which succeeds its partial interruption. e. Matters which have collected on the surface of the hair; consisting of shed epithelial cells, fatty substances, and foreign impurities. PLATE III. 40. Bundles of areolar tissue from between the striped fibres of the trapezius muscle. 41. A portion of the above, treated with acetic acid. a. Gelatinous basis. b. Investing filaments. 42. Elastic fibres from the outermost elastic layer of the aorta of the ox. 43. Fenestrated membrane of the same vessel. a. The membrane. b. Its apertures. EXPLANATION OF THE PLATES. xix FIG. 44. Layer of small reticular fibres from the same arter} T . 45. Thin transverse section of cartilage from a tracheal ring of an adult. a. Granular basis. b. Large cartilage-corpuscles. c. Small and simple cartilage- corpuscles. d. Primary enclosing structures. e. Secondary enclosing structures. 46. Thin transverse section of the human femur, slightly magnified. a. The basic substance. b. Medullary canals cut across. c. Lacunae. 47. Part of the same section more strongly magnified. a. The basic substance. b. Medullary canal cut across. c. Medullary canal descending obliquely. d. Osseous lamina arranged concentrically around the medullary canal, c. Lacunae, with their radiating canaliculi . 48. Thin longitudinal section of the horse's femur. a. Medullary canal running longitudinally. b. Long lacunae and their radiating canaliculi. c. Round ones which lie more deeply. d. Network of the latter. 49. Part of a thin section of a horse's molar tooth. a b. True dentine. b c. Enamel (which occupies) c. The free surface of the tooth. d. Tooth-tubes or fibres, taking an arched course here and there. These form enlargements such as are not seen in the healthy human tooth. Above b the ends of the tooth-fibres branch in the neighbourhood of the enamel. Above b c are the fibres of the latter substance. The dark streaks are cracks produced in making the section. 50. A portion of the cement of the same molar tooth. a. Its basic substance. b. Lacunae lying deeply. c. Others in focus. d. Medullary canals, which do not exist in the human teeth. PLATE IV. 5 1 . Transition of ossifying cartilage into bone. From the upper epiphysis of the tibia of the human embryo at the eighth month. a I. Cartilage. b c. Adjacent bone. At a b are seen the cartilage corpuscles partially arranged in rows; and at be the partitions of the young and spongy bone, separated by medullary spaces. 52. Two Meibomian glands from the lower eyelid of a child ; seen on a black ground. a. Substance of the eyelid. b. Vesicular ends of the glands. 53. Gastric glands from a vertical section of the mucous membrane of a healthy man's stomach. a. Simple gastric glands. b c. The two tubes of a ramified gastric gland. d. Openings of the gastric glands on the surface of the mucous membrane. XX EXPLANATION OF THE PLATES. FIG. 54. a. A striped muscular fibre from the trapezius of a human corpse some time after death. Here no sarcolemma can be seen. b. A fibre having a sarcolemma with some nuclei on one side. 55. Outermost layer of the lens. From an adult human eye. a. Globules of liquor Morgagni. . b. A globular mass which appears to have a caudate prolongation. c. Deeper fibrous substance. 56. Lenticular fibres from the same lens. 57. Lenticular fibres from the same lens, having fine striae at right angles to the long axis of the fibres themselves. 58. Bundles of primitive fibres from the radiation of the optic nerve. From the retina represented in Plate I. Fig. 14. 59. Bundle of unstriped muscular fibres from the muscular coat of a man's stomach. 60. Fibre-cells into which these muscular fibres are split up by tearing with fine needles. a a. Their nuclei seen indistinctly. 61. A piece of unstriped muscle treated with acetic acid. a. The muscular substance which has become transparent. b. The now distinct nuclei. 62. Vertical section of the skin of the sole of an adult man's foot, from the surface to the subcutaneous adipose tissue. a b. Thickness of the epidermis, the undulating layers of which are indicated by transverse lines. b. Region of the Malpighian mucus. c. Some of the oldest epidermal scales, partially shed. d. Tactile papillae of the corium. e. The same with loops of blood-vessels appearing indistinctly through them. f. The remainder of the corium. g. Part of the subcutaneous adipose tissue. h. Spiral gland taking a tortuous course through the epidermis to open at i. k. Its straighter transit through the corium. I. A second of these, taking wider curves through the corium. m. Continuation of the latter. n. The place where it divides. o p. Its terminal tubes, the coils of which occupy the adipose tissue. Their epi- thelial structures are partially dissolved by the action of the dilute solution of ammonia by which the whole has been rendered more transparent. g r. Proper capsular investment of these terminal tubes. 63. The base of a small hair from the fore-arm of the same man. a b. Border of the epidermis. c. Horny shaft of the hair. d. Horny bulb of the hair. ef. Outer and inner root-sheaths. g. Canal of the hair-follicle. Ji i. Two sebaceous glands, the ducts of which open into the canal of the hair-follicle. k. A third sebaceous gland injured by the section. PLATE V. 64. A portion of the free extremity of the frog's pancreas, slightly magnified. a. Basement membrane of the gland. b. Groups of distended tenninal vesicles. 65. Portion of an urinary tubule from the kidney of an adult. a. Epithelial cells. b. Basement membrane from which the cells have been partially removed in the act of preparing the specimen for examination. EXPLANATION OF THE PLATES. XXI FIG. 66. A Malpighian corpuscle, with its appended urinary tubule. From the kidney of a green frog. a. Coil of blood-vessel, in the interior of which we may recognise some blood- corpuscles. b c. Its limitary membrane. d. The capsule itself. e. The adjoining urinary tubule, with its epithelial cells. 67. Proper corpuscles of the cortical substance of the supra-renal capsule, showing their radiate arrangement. From an adult man. a. The side towards the surface. b. That towards the interior. c. Clear intervening substance. 68. Various primitive nerve-fibres from the sciatic nerve of a newly-killed frog. a. A fibre, the contents of which have not yet coagulated, while its margins have become varicose in being prepared for examination. b. A large fibre which has almost retained its original cylindrical form. c. A small fibre, with notched margins and a distinctly oleaginous content. d. A varicose fibre of medium size. 69. A number of primitive nerve fibres from a human corpse somewhat longer after death. a. A fibre in the contents of which coagulation is commencing. Its margins are beginning to become varicose. b. A fibre (partially concealed), the contents of which have completely coagulated. c d. Partial coagulation of the same. e. Small fibres, lying deeper, the contents of which have become cloudy here and there. f. A fibre in which coagulation is beginning at a part that has been compressed and constricted by the act of preparation. g. A fine fibre, which has become varicose at h; while its altered contents have protruded at i. 70. Large nerve-fibre at its division. From an abdominal nerve of the eel. a. The trunk. b c. The branches of division. 71. Ganglion-corpuscles with vaginal processes. From the superior thoracic ganglion of the human sympathetic trunk. a. Substance of the corpuscle. b. Its bright nucleus. c. Nucleolus. d. Nucleated investment. e. Vaginal processes. /. Apposed nuclei. g. A true nerve-fibre of average diameter running in the neighbourhood. 72. Ganglion-corpuscle with processes of nerve-fibre. From the Gasserian ganglion of a newly-killed eel-pout. (Gadus lota). a. Corpuscle with its nucleus and nucleolus. b c. Its upper and its lower nerve-fibres. d. Transparent outline of the membrane which surrounds the ganglion-corpuscle. 73. A similar preparation from the same ganglion; after the animal had been dead many days, and was considerably advanced in putrefaction. a. The ganglion-corpuscle, which has become paler and redder. b c. The two processes of nerve-fibre; the contents of which have coagulated, and assumed a paler greyish-red aspect. d. The upper and constricted point of union, where nervous contents can no longer be recognized. 74. Ganglion-corpuscles from the Gasserian ganglion of a new-born infant. XX11 EXPLANATION OF THE PLATES. PIG. a. Angular ganglion-corpuscle. b. Single process of the same. c. A process which divides into two subordinate branches, d and e. ftm&g. Isolated corpuscles devoid of processes. 7 5. Drops of the liquid contents of primitive nerve-fibres. From the putrid medulla oblon- gata of the ox. 76. Very large and pale ganglion-corpuscle from the grey substance of the anterior columns- From the inferior cervical region of the spinal cord of a newly-killed ox. a. The pale substance of the corpuscle. b. The heaps of granules in its interior. c. The nucleus. d. The nucleolus. e. A simple pale process. /. A larger process. Here it is impossible to decide whether the primitive fibre g really proceeds from it, or only lies on it. 77. Cerebral substance of a new-born male infant. From the upper part of the middle lobe of the brain, at about ths of an inch below the surface. a. Granular substance. b. Clear cells. c. Granular nuclei. d. Similar but less distinct nuclei, apparently free. 78. Elements of human semen. a. Spermatozoids, exactly in the focus of the microscope. b. Others outside this point. c. Smaller seminal globules. d. Parent-cells of the spermatozoids. 79. Milk of a woman, one day after parturition. a. Large * b. Medium Mnilk- corpuscles. c. Small ) d. Colostrum-corpuscles. e. Scales of epithelium admixed with them. 80. Milk of a woman, ten weeks after parturition. a. Milk-corpuscles. b. Colostrum-corpuscles of very small size and number. la&I Litk Jinsl. v. J. fi. Ser^A, Taf. ff. Fig.xxn. '!'(,; A' A III. " Fig. A'.YV. Tiy.xxvm. Tig. XXXIV. Fia. XXXVII. liy. XXIV . Fig. XXXTL. Fig. XXXV. Fig.xxxvm. Fig.XX. Fig. /. 7.JH7/7. \ III \ \ Fn,.I.XXH. Fig LXXIV. d o Fiy. LXXI. d. Fiy.LJfXVI. a... . Fig. LXJfV. /'///. LXXVR. Fiff. Fig.IJXK. A TEXT-BOOK OF HUMAN PHYSIOLOGY. CHAPTER I. ORGANIZATION AND LIFE. 1. THE different constituents of every organized being together form a nicely calculated whole, the particular parts of which only vary, within certain limits, with immediate circumstances. The mixture, the form, the arrangement, arid the alteration, of the few or many substances which we meet with in every plant and animal correspond to a chief plan, which pervades all their details, and places the results in dependence on the attainable means. The vital functions are the visible expression of this arrangement; and health, disease, or death, are functions of the ratio of those conditions which the several molecules exhibit. 2. The capacities of self-preservation and propagation recur in every kind of living being. As it was necessary that the order of the organic world should maintain itself without external and supplementary sup- port as it was necessary that the individual should be able to accom- modate itself to internal and external change, and preserve the species in spite of the destruction of the individual, both of these capacities were indispensably called for. At the same time they constitute the characteristic means of distinguishing the organic creation from those contrivances which are the result of human handywork. 3. Every such apparatus requires a physical or chemical stimulus a food as we may call it to maintain the activity of its machineiy, and thus bring about the intended effect. In this way the clock-weight con- ditionates the movement of the clock, the steam that of the steam-engine, and the combustion of its constituents, the light of the candle-wick. The like phenomenon recurs in living creatures. Their manifestations of force are always connected with a change of molecular proportion, or with a chemical interchange of substance. In this way particular combinations are produced, which leave the body, and which must therefore be replaced by others, in order that it may subsist. But the food thus needed not merely serves to compensate those unavoidable losses ; its surplus is frequently applied to the formation of new organs, to the perfection B 2 GENERAL ARRANGEMENT OF ORGANISMS. [CHAP. I. of old ones, and to the restoration of lost parts. And while, in the case of our artificial contrivances, all these changes can only be induced through the instrumentality of the mind and hand of a human being foreign to the machine, organic bodies accomplish them by their own in- herent forces, so that the living being fulfils, at one and the same time, the different functions of machine, attendant, and architect. 4. Since we are unable to furnish a trace of inherent independence to any apparatus we can construct, the instant the necessary food fails, it comes to a stand still. The hungry animal, on the other hand, at first withdraws the compensative matters out of the mass of its own body. Organs, on which the play of the whole depends, gradually diminish in bulk, until the too great loss of substance seriously interferes with the action of the machinery, and finally results in death. The arrangement on which this peculiarity depends has this additional advantage, that the matters once used are capable of being again employed to a much wider extent than in our machines, and are therefore only set at liberty after repeated exchanges. The conditional causes of independence are thus associated with a rule of much greater economy. 5. Reproduction is but another expression of the circumstances just enunciated. The organic being, which possesses the capacity of ap- plying the food it receives, not only to the nutrition of existing parts, but also to the construction of new organs, and which can defray un- avoidable expenses or necessary restorations from the already existing structures of the body, presents an embryo as an additional product of its nutrition. This embryo includes a certain sum of parts, which only require a particular food, in order that limb should arrange itself on lirnb, after a definite plan, until a new and independent being is created. But since the parent organisms only attain the capacities necessary for generation after a certain duration of life, the parents and their progeny are separated by an interval of time, the continual repetition of which secures the continuance of genera and species. 6. The independence of organized creatures has frequently led to the notion that the arrangement of the organism is based upon a peculiar vital force, which imparts to it properties differing from those of inor- ganic nature. It was thought that the vital functions could only thus be possible. Either this force was represented as an attendant upon a machine, who arranged at will inert substances with given properties ; or it was presumed that combinations otherwise inanimate received a higher grade of activity by the communication of vital force. When this was again withdrawn, they became subject to the laws which hold good for the" inorganic world ; and thus after death, underwent putre- faction. 7. But the assumption of such a vital force is neither useful as afford- ing a clue to a series of phenomena otherwise unknown, nor even harmless CHAP. I.] GENERAL ARRANGEMENT OF ORGANISMS. 3 in its influence upon our ideas. It impedes a correct recognition of the fundamental principles on which the existence of living creatures is based ; and leads to results which are decisively opposed by more exact physiological investigations. It separates the physical and chemical phe- nomena of dead and living nature by a line of demarcation which does not really exist. And although it captivates us at the first glance, by claiming a higher influence for these vital appearances, yet a more care- ful examination soon teaches us, that this supposition, so flattering to our vanity, prevents all insight into that much more remarkable manner in which nature accomplishes the most peculiar as well as transitory opera- tions, by the bare use of forces everywhere present. 8. We have but to imagine that the vital functions are the result of an infinitely wise plan of organization, to comprehend all this from a simpler, more accurate, and even higher point of view. We can first of all suppose, that the embryo includes a number of conditionating causes, by means of which structures corresponding to the general object are extracted from fitting nutritive materials. In this way, for instance, vesicles or cells are produced, the properties of which react on the elements already present, and assist to determine the mode in which the subsequent food is consumed. This process is continually repeated by the physico- chemical conditions of the several parts once formed ; and their fluc- tuating influences operate in such a way that an organism conformable to its object is continually present. The sum of the particles existing at any particular moment excites the vital phenomena then present, and at the same time conditionates those which appear in the time imme- diately following. And if limb be properly arranged on limb the embryo grows on, conformably to rule, and results in a vigorous being which corre- sponds to the perfect plan of organization. While on the other hand, if imperfections appear at an early date, the young being is crippled by a deficiency in the number and development of its organs : that only is effected which the general mass of existing structures can accomplish by means of their physico-chemical powers. So that we get imperfect, mis- shapen, or sickly creatures, whose capacity of life depends on the amount of opposition between what is required and what can be effected. The same obtains subsequently. The favourable or unfavourable influences only determine whether the already existing parts persist, increase, or diminish, whether the qualities necessary to the varied play of thousands of organs remain, or whether the machinery of life is arrested by their annihilation. At every instant, and under all circumstances, nature offers the sum of those operations which to the advantage or disadvantage of the creature are effected by the great series of microscopic elementary constituents. So that all irregular activities, degenerations, disturbances, and pains, which immediately follow them are just as necessary, and are based upon the same fundamental circum- B 2 4 GENERAL ARRANGEMENT OF ORGANISMS. [CHAP. 1. stances, as those results of more favourable and healthy immediate causes, which are distinguished by their conformability to the general purpose. 9. If we regard the individual phenomena of life from this point of view if we recollect that the smallest creature possesses a greater number of parts, and these more conformable to a specific purpose, and more inti- mately united to each other, than those of the most artful machine which the hand of man has produced we shall not be surprised to find that matters apparently dead experience a transition into parts of the body, and become possessed of vital functions ; and that, conversely, portions se- vered from the organism become subject to what are called chemical laws. And such a view at once frees us from many teleological theories, which but too often betray the childhood of knowledge, and which seek to limit all nature to the short-sighted horizon of anthropomorphous ideas. 10. The great majority of parts formed by the organism subserve more or less important vital functions, which are brought about in such a manner as is most conformable with the end in view, and are maintained with the least possible waste of matter. Still we have no right to presume that every structure must subserve a determinate and independent phy- siological use. On the contrary, the general arrangement of the organism sometimes causes the formation of many parts, as mere supplementary productions, in order either to their being subsequently applied suit- ably to a given object, or, by their origin or presence, to allow of the intermission or proper action of other organs. But the function is never the exciting cause, but only the possible result, of the production of the several constituents. It may further happen that a part is less protected, or even less adapted to its purpose than it might be, because a better arrangement of the particular organ could only have resulted from another kind, or another sum, of conditions. Just as diseases, with their lamentable results, are, under certain circumstances, the neces- sary consequences of the constitution of the organism, so we may often see the conflict of instincts (which are equally the mere expression of given organizations) leading to the most refined cruelties of one animal towards another leading to actions which we, with our short-sighted view, are only too ready to condemn as wrong. Our eye remarks the subordinate masses of colouring, but is incapable of including the whole picture. We ought, therefore, never to forget, that we can but perceive particular points of the great natural whole that we have not the capa- city to trace the connection of all its mutually entangled threads. Hence we easily discover final causes which are not present, while that harmony of the thousand-fold machinery which really exists, is subject to such manifold fluctuations that it only too easily confuses our limited human capacity. CHAPTER II. ORGANIC AND ANIMAL FUNCTIONS. 11. THE constant physical and chemical changes which accompany life? depend upon various exchanges which are produced by the work of the dif- ferent parts of the body : the extrusion of what is useless, the assimilation of what is received, and the restoration of the organs by which all these operations are effected. The whole of the vegetable or general organic functions, on which nutrition and generation depend, are repeated in every living body. It has often been supposed that all their particulars correspond in the two organic kingdoms: that there is a digestion, a respiration, a perspiration, and an excretion, in plants as well as animals. But a more accurate examination teaches that this is not the case. Vegetables possess no tissues which allow of the same kind of nutritive absorption, of distribution of juices, or of secretion, that we meet with in at least the higher animals. They have no large cavities in which considerable quantities of food can be collected, and dissolved by special fluid secretions. They possess no point midway in the movement of their juices, and no mechanism other than that of a casual and secondary apparatus for the inhaustion or expulsion of the respiratory gases. They are devoid of the changeable epithelial coverings which play an important part in many of the animal excretory organs. In one word, the general organic functions are introduced into the two living kingdoms of nature, and probably even into their subordinate divisions, by two different ways. This difference leads at once to the conclusion, that the structure of the animal is not a simple repetition of that of the plant, with the addition of a series of new apparatus. The nature of the tissues, the mode of their action and change, the form, division, and destiny of the organs, all these rather teach us that animals of any development are constructed upon an altogether different plan. 12. Vegetables always remain passively subject to the outer world. They mutually act upon it, and are acted upon by it, only through meteo- rological influences, and through the ingesta and egesta of their bodily substance. Many of the phenomena of their growth depend upon the heat, the degree of moisture, and perhaps the electrical condition, of the atmosphere. From it they withdraw carbonic acid, and, in exceptional cases, oxygen. Water and the constituents dissolved in it are absorbed by them from the soil or the fluid in which they live. They give off to 6 ANIMAL FUNCTIONS. [CHAP. II. the atmosphere oxygen, and, under certain circumstances, carbonic acid, and sometimes other gases, watery vapour, and volatile organic or inor- ganic combinations. They allow different kinds of fluid mixtures to transude and appear on their surface. But all independent perception of the things around them, and all voluntary change of place, are completely absent. The phenomena of movement which are here and there mani- fested in the vegetable kingdom do not depend on a voluntary principle. Light and heat, and, in many cases, a particular time of the day, consti- tute the general conditions under which they occur. 13. It is the distinguishing characteristic of the animal that it recog- nizes the objects which surround it, that it enters into manifold and independent relations of exchange with them, and that it makes use of them to a great extent at will. An active personality, and a free will, dictate and pervade its most frequent and important relations to the external world. So that we find here special animal functions which allow of the reception of impressions, of the change in place of particular parts or of the whole organism, aud finally, of mental emotions. The senses, together with the organs of locomotion, which can be subjected to a self-calculated guidance, and the nervous structures, complete the circle of those organs which subserve the highest vital manifestations of the animal, and whose actions have no parallel in the vegetable king- dom. 14. These apparatus are also of essential service to those parts which are the seats of the general organic functions. The contractile tissues which belong to the general plan are frequently made use of to intro- duce or expel food, respiratory gases, juices of the body, secretions, and even the developed embryo; to alter the phenomena of nutrition by changing the porosity of particular parts, or to accelerate many processes of metamorphosis. 15. In the series of functions met with in man and the more highly organized animals, that of digestion elaborates the food, while the useless remainder, mixed with excretory matters, is rejected in the fseces. That of absorption provides for the transmission of whatever is to be added to the blood the mother-fluid of nutrition. The circulation sends this in closed canals throughout the body, in order both to the mainte- nance of the particular parts, and to the renovation of the fluid itself. The respiration effects the greater part of the exchange of its gases, while the cutaneous transpiration repeats the same occurrence on a smaller scale. Besides this, watery vapour and other matters are thrown off both by the lungs and the skin. The organs of excretion offer certain secretions, which are either destined to leave the body immediately, or may serve other purposes, and then be discharged, or may, when wanted, be returned into the mass of the blood. Finally, nutrition maintains, increases, or diminishes the mass of the constituents of which the entire CHAP. II.] ANIMAL FUNCTIONS. 7 organism is composed : and forms in this manner the result of the general organic functions of the animal being. While the senses receive the impressions of the external world, the phenomena of motion lead to the change in space of particular parts, or of the entire mass of the creature. The organ of voice results from a suitable connection of the organs of respiration and movement. The nervous system, which receives and elaborates excitements, coerces the muscular fibres to contraction, and constitutes the immediate instrument of the mental functions, while it at the same time exercises a mediate control over most other organs of the body, since it can alter their movable pieces within certain limits. Generation and Development certainly belong to the general organic functions. But since they do not subserve to the maintenance of the individual, but to the preservation of the species, they have been very properly separated from the remaining phenomena of nutrition. The description of the evolution of the embryo belongs, however, to an especial branch of science, to the history of development. CHAPTER III. INDEPENDENCE OF THE VITAL MANIFESTATIONS IN ANIMALS. 16. THE physical and chemical contrivances of our body often closely correspond with those of an artificial apparatus. The heart may be com- pared to a pump provided with suitable valves, the arteries to elastic conducting tubes, the eye to an optical camera obscura, and the organ of voice to a tongued musical pipe. The bones and muscles work like levers and tractile forces, while at least many organs of secretion act in part like a filtering apparatus. And just as we seek to promote evapora- tion by increasing its surface, and removing the air already saturated with watery vapour, so something very similar to this recurs in the lungs of man and animals. 1 7. But if we compare any such apparatus with those which nature exhibits in the animal body, we find that the latter are distinguished not only by a far greater adaptation to their ends, and by the advantage of self- maintenance already mentioned, but also by the independent and manifold character of their operations. The muscles with which they are provided, and which are regulated by nerves, render them capable of shifting and adjusting themselves, while the machines which we contrive exhibit a more or less permanent helplessness. For instance, the rays of light experience such a diversion in the eye that the focus of the images of objects at suitable distances lies on some part of the retina ; and we can easily construct an artificial eye, the semi- transparent back-ground of which repeats this arrangement ; but the hand of the observer must be brought to its assistance, just as in the telescope and microscope. It must shift the instrument so that the recti- linear rays of light enter the dioptric apparatus. It must insert screens, provided with greater or smaller openings, so as to regulate the mass of the entering light. It must alter the mutual distance of the lenses, or change certain of the refractile media, so as to obtain distinct images of objects at different distances. While our organ of vision can itself pro- vide for all this series of improvements on the instrument, the muscles of the eye move the eyeball as it is required for the sight, the iris, which plays the part of a screen, enlarges or contracts its aperture according to the quantity of the entering light. And certain other changes allow of our distinctly recognizing objects both far and near. In one word, our organ of vision forms a much more independent con- CHAP. III.] FLUCTUATIONS OF THE SEVERAL FUNCTIONS. 9 trivance than any dioptric instrument which human industry has ever been able to construct. 18. And frequently, Nature requires but one organ to accomplish that for which we find many necessary. One and the same muscle can pro- duce the most varied attitudes of the parts which correspond to it, according as it varies its point of action. Since the phenomena of filtra- tion depend upon the size of the spaces which the filter offers, and since the capacity for diffusion additionally varies with the nature of the porous body, the different membranes of our body can altogether change their operations when their contractile tissues are drawn together, or when nutritive phenomena lead to the deposit of different matters in their substance. The contraction of the muscular fibres is intimately connected with certain changes of their molecular properties, and thus with a complete alteration of their physical condition. 19. This automatic change in the condition of the several organs per- tains to that series of phenomena to which animals owe their greater independence. But since for the most part it only completes and perfects functions otherwise possible, or diminishes the number of contrivances required, it is not so essential as self-maintenance and reproduction. The comparison of plants and animals plainly shows how Nature makes use to this end of substances which, even without this requirement, would necessarily be present, and thus seeks to derive from them the greatest possible amount of advantage. The rigid tissues of vegetables are much less fit to construct any movable apparatus than the soft contractile elements of the animal creation. And hence this self-improvement is much less frequent in the former than in the latter. 20. In contemplating the phenomena of the nutrition and growth of living beings, we are struck by a certain visible independence of particular tissues. The mixture which transudes the walls of the blood-vessels in order to distribute itself amongst the other tissues the nutritional fluid, as we may call it reaches the most various elements of the corporeal organs, where they lie close to each other. Yet in spite of this, the mus- cular fibre selects from it materials altogether different to those chosen by the neighbouring nerve-fibre, and these again differ from those taken up by the mass of areolar tissue which encloses them both. This phenome- non has been named the organic attraction. But it is not necessary to adduce special vital forces to assist in explaining these circumstances. The mere arrangement of the parts, and the properties which they already possess, make it quite conceivable why the muscular fibre, rich in fibrine, exhibits different elective affinities from the nerve-fibre, which is filled with a mixture of oil and albumen. Indeed, one sometimes sees how Nature proceeds deliberately, step by step, to allot to every tissue its proper combination. The epidermis consists of a number of cells depo- sited in a stratiform manner. The outermost layers contain the most 10 FLUCTUATIONS OF THE SEVERAL FUNCTIONS. [CHAP. III. horny matter, the middle, less, and the most interior, least of all. The most internal substratum is formed of nuclei and cells, rich in albumen. The blood-vessels are distributed solely beneath the epidermis in the mass of the corium. The nutritional fluid furnished with dissolved albumi- nous substances, in passing out of the vessels, first reaches the nuclei and albuminous cells which lie next to the corium. When it has here given off the proper materials, and has undergone a corresponding change, it passes to the least horny layers, so that these increase their mass. Finally, the oldest horny cells only get that which the younger and needier ones have already rejected as superfluous. 21. Many of the functions are complementary to others, so that a single task, as it were, divides itself, to be discharged by a number of different organs. For example, the matters which are to be removed from the blood, and then from the body, divide themselves according to their cohesion. The gases and vapours pass out by the lungs and the skin, and the liquid portion chiefly by the urine. Just as the heavier scale of a balance goes down, and the lighter flies up, so the quantity of the urine is increased according as less water is evaporated by the skin and lungs; while, on the other hand, when the sweat drives off more fluid, other circumstances being equal, less urine is secreted. We have here a quantity composed of two varying members, which always endeavour to give about the same sum. 22. These fluctuations of equilibrium are sometimes connected with certain relations of adaptation. The woman capable of childbearing discharges every month a certain quantity of blood from* the organs of generation. When she becomes pregnant this evacuation ceases. In this way, a part of the matters formerly excreted is rendered applicable to the embryo. When the child is born, that surplus of the fluids which is no longer given to the embryo, is determined to the breasts. These organs furnish the milk, provided that the act of suckling excites and maintains in them an increased activity. During this time, the menstruation remains absent. But on the contrary, if the milk is not suckled from the breasts, it soon disappears; and, on the inactivity of these glands, menstruation again appears. 23. If we limit our attention to shorter intervals of time, we shall find the same striving after equipoise in all the phenomena of nutrition. Following the weight of the body of an adult man from day to day, we shall find that the differences are usually not greater than one feecal and one urinary evacuation. And in most instances, something of the same kind may even be observed when a man or an animal increases or emaciates. The small daily increments and decrements which here obtain only form more considerable and visible values by being added together in large numbers. The organs of reception and extrusion are so arranged that, as a general rule, the corporeal mass is never suddenly altered, but CHAP. III.] FLUCTUATIONS OF THE SEVERAL FUNCTIONS. 11 income and expenditure are very nearly equal, if we limit observation to a small space of time. We have a regular clock-work, which is always correct within a certain limit, and which strives to maintain its ordinary rate in spite of many disturbances. 24. This latter circumstance has frequently led the physician to assume a natural healing power* The organism itself was supposed to possess the capacity of getting rid of certain morbid matters by means of crises, so as finally to recover the freedom of its ordinary healthy play. But closer observation teaches that the entire opinion is based upon a mistake. The course of every disease which is left to itself, depends immediately on the condition of the parts of the body. If these elements are so degenerate as to afford none but abnormal or even disturbing results, their destruction follows as a matter of course. No counter -plan exists which could restore them. In such a case there is just as little of a natural healing power, as there is of a vital force for the ordinary circumstances of life. But, on the other hand, it may certainly happen, that the variable organs of excretion may take on an activity, which is greater than that of their immediately preceding operations, and that this disturbance of equilibrium may unload other parts, and so allow of a return to a more regular activity. In this way the sweat or the urine may take up and discharge the fluid of dropsical effusions, or a diarrhoea may counteract an effusion which loads the brain, or means which promote absorption may discuss solid deposits. But all these phenomena are the results of the conditions naturally or artificially present, and are not the consequences of a special remedial plan, or of a special force, opposed to disease, and previously present in the organism. 25. Every animal has a time for development and growth, a period of middle life, in which the body strives to maintain an unaltered mass, and an epoch of decrease, which is concluded by natural death. These periodical changes are explained by the mode of arrangement which prevails in the organic creation. The younger elements can always so elaborate their surplus as to produce from it new and suitable tissues. The older are only capable of maintaining themselves. And gradually their energy sinks, so that finally their mass diminishes. And if there are many changes connected with definite years of life, the dates of which only fluctuate within certain limits, such as the milk and permanent teething, the appearance of puberty, the cessation of menstruation in old women ; this may be explained as due to the fact, that such phenomena constitute the necessary results of certain stages of growth, and follow inevitably from the previous and accom- panying conditions. A truly periodic repetition cannot be distinctly made out. * Commonly termed the vis medicatrix naturae. 12 PERIODICITY OF SOME FUNCTIONS. [CHAP. III. 26. In many functions, however, and especially in those of the sexual organs, periodicity does appear. The rutting season of many animals recurs only at regular times of the year. Meteorological and other circumstances, especially temperature, often exert a visible influence in this respect. But since many creatures prepare during the cold of winter for the approaching rut of spring ; and since woman menstruates every four weeks during all seasons indifferently, it follows that these fluctuating phenomena must be based upon other causes than the sur- rounding temperature. But it does not follow that the female organism derives this periodicity from itself, or that it would obtain if we could remove all other influences without destroying the living being. We shall rather find that these phenomena of regular recurrence are probably connected with causes which do not exclusively lie in the organism. CHAPTER IV. PHYSICAL PROPERTIES OF THE HUMAN BODY. 27. THE small size and extraordinary number of the pieces with which Nature works in the animal creation, ensure a number of advantages, which we are unable to attain to the same extent in our artificial contrivances. The various tissues which enjoy the functions of separate organs require the aid of the microscope to be seen, and even its highest magnifying powers are often insufficient to expose all their more influential consti- tuents. What we call an organ is only the determinate aggregate of a large number of microscopic tissues of various kinds, every one of which possesses its own special function. The limits of our senses render it im- possible to form the remotest conception of this vast arrangement of microcosms. 28. Any exact estimate of the number of histological elements which go to a given part is met by insuperable difficulties, since their form corresponds to no simple mathematical figure, and their sizes and distances, and the minuteness with which they are divided, vary remarkably, even within small extents of space. The investigation has a yet more uncer- tain basis when foreign tissues intervene between those which constitute the essential characteristics of the organ. With the exception of a few organs, as, for instance, the muscles, there is scarcely any means of accu- rately estimating the number of these tissues. But even the vaguest estimates suffice to show that the higher animals may possess, in each of their larger organs, millions of separate physical and chemical labo- ratories. Harting has calculated that in the adult on an average 700,645 to 789,677 epidermoid cells (Tab. n. Fig. 32) are collected in the space of a square inch. Now since the surface of the author's body amounts to about 2325 square inches, there are 1750 millions of horny cells in a single layer of epidermis. But even the parts where the skin is thinnest have more than one layer. They possess at least more than a dozen of such strata, each of which exhibits more than a billion of small horny scales, complete in themselves. The air-tubes of the rabbit sustain about twenty millions of cilia, and those of man about one hundred and fifty millions, each of which is in perpetual movement. The fat of the adult appears to be some- what less minutely divided than that of the new-born infant. Yet, in 14 NUMBER OP THE SMALLEST ORGANIC CONSTITUENTS. [CHAP. IV. spite of this, there would be about * sixty-five millions and a half of fat- cells (Tab. n. Fig. 27) in the space of a single cubic inch. According to Harting the most superficial layer of the crystalline lens in a woman, whom he examined, contained two thousand lenticular fibres (Tab. i. Fig. 56), and the entire choroid of the eye about eleven millions of pigmentary cells. (Tab. n. Fig. 29.) According to Rosenthal the roots of the human cerebral nerves con- tain more than a hundred thousand primitive fibres, and it is probable that even this estimate is too small. If we add to these the spinal nerves, and consider that many of these fibres divide in their further course, we again get a number of several hundred thousands for these conductors, which, after all, do but work the biddings of the mental emotions on the peripheric part of the nervous system. According to Harting the median nerve of the adult includes from twelve thousand to twenty-one thousand primitive fibres, and the femoral nerve twenty- one thousand to fifty thousand. 29. But it is only the person unacquainted with other natural pheno- mena who will be astonished at these numbers. Wherever we go, we come upon quantities, which our limited faculty of apprehension generally regards as infinitely large. Although light courses through one thousand millions of feet in a second, and the length of each separate undulation amounts on an average to about 1-5 0,000th of an inch, yet a ray of light which comes from a star of the twelfth magnitude, or from a star on an average 3261 times removed, requires about four thousand years in order to reach the earth. On the other hand, the above-mentioned animal tissues contain a number of different subordinate constituents, the smallest of which cannot be recognized even under the most powerful microscope. The ultimate molecules of all bodies have so small a size, and are so similarly composed, that the eye of man will never be able to see them. 30. But it is not so much the number, as the systematic arrangement and union of its different pieces, which makes the organic creation so wonderful. Nature has so arranged every organism that it everywhere operates in limited spaces, which are not only the smallest possible, but are, to a great degree, independent of each other. This arrangement leads of itself to many important advantages to which we shall return again. But it is so intimately connected with the very nature of things that it recurs in even the most unimportant circumstances. For instance, when the carbonate of lime contained in the lime-saccule of the frog is precipi- tated we obtain minute crystals, the greatest of which has a maximum diameter of less than 1-1 000th of an inch, while on the other hand the smallest are less than 1-1 5,000th. (Tab. i. Fig. 6, 7.) 31. The enlargement of the free and active surface constitutes one of the most striking advantages of this subdivision into smaller masses. A * 65,541,537. CHAP. IV.] ADVANTAGES OF MINUTENESS OF ORGANIC CONSTITUENTS. 15 pulverised body is dissolved much more easily than an entire piece of much larger size, because more molecules of the solid and the fluid substance are brought into mutual contact. The smallness of the blood- corpuscles (Tab. n. Fig. 24) floating in the blood thus enables them to exercise the most extensive reactions on the mother-fluid which surrounds them, and on the gases which they have taken up. In man their average diameter amounts to l-3600th of an inch, and their medium thickness to 1-1 5,000th of an inch. In this way nature greatly gains in active surface, since at the same time that the entire mass of blood-corpuscles is divided into rolls of 1-3 6 00th transverse diameter, these are again separated into discs of 1-1 5,000th in height. But since the upper and under surface of the blood-corpuscles are either somewhat hollowed out, as in man, mammalia, and some fishes, or elevated, as in birds, reptiles, and certain other fishes, it is evident that this circumstance still further increases the surface than if every blood-corpuscle had exhibited the level surfaces which bound the ends of a mathematical cylinder. And hence we may conjecture that those animals which possess smaller blood-corpuscles, with proportionally more extensive surfaces, and which have also a much * greater number of them, are endowed with a greater activity. The subdivision of the mass of blood leads to similar phenomena. The circulating blood fulfils two objects. It gives off matters to the various tissues of the body, and, in the lungs and skin, exchanges certain elastic fluid compounds with the atmosphere. Both of these functions are increased when larger surfaces of the blood come into contact with more extensive walls of vessels. The finer canals of the capillary vessels are therefore interposed between the larger conduit pipes of the arteries and veins. The smallest capillaries of the retina and brain, when filled with blood, measure, according to Henle, l-5000th of an inch, and are 2700 times finer than the aorta at its commencement. It results from hence that the surfaces of contact would be multiplied in exact proportion with the walls of the vessels, if we regard as invariable their length and the mass of the blood. But since capillary vessels, which are almost half as fine again as those above-named, occur in some preparations which have been artificially injected and thus over-distended, and since the arteries and veins only experience a transition into capillaries by gradual and successive divisions, it follows that the enlargement of surface which Nature obtains by means of the capillary system must be even more considerable. The secreting glands offer a third example of the great advantages thus obtained by Nature for particular organs. They constitute a number of * The original has " equal or even greater number," and hence the Editor feels bound to explain why he has altered these words to "much greater." Dismissing the less measurable influence of concavity or convexity, one may broadly state that to give an equal total surface and total activity, the number of blood-discs should be inversely as some power between the square and the cube of their diameter. The influence of minuteness in increasing activity would therefore presuppose an increase of number even exceeding this. 16 WATERY CONTENTS OF THE ORGANS. [CHAP. IV. coiled, or arborescent and branching, tubules. We shall hereafter see how greatly the cavitary surfaces of these secreting canals may thus be increased. At present we will only adduce in illustration that a square inch of mucous membrane from the stomach of the rabbit contains about 451,600 gastric glands, and that the average secreting surface of both kidneys, which only amount to from 1-1 48th to 1 -246th of the corporeal mass, is about six times as large as the whole outer surface of the skin. 32. A consideration of the physical and chemical properties of the par- ticular tissues will show us many other advantages dependent upon the division of the organic implements into microscopic and independent pieces. And both this and the study of their several functions will prove at every step, that the small size and large number of these agents of the vital actions essentially contribute to the exquisite perfection of their operations. 33. The pliability of most of the organs depends in great part upon the considerable quantity of water which they contain. If muscles, ten- dons, and other soft tissues are thoroughly dried, they form brittle masses, which may not unfrequently be broken like glass. But if they are again softened by immersion in water, they recover a great part of their original flexibility. About three-fourths of the entire weight of an animal is composed of combinations, which volatilize at a temperature of 212 Fahrenheit. A frog was killed under olive oil, and carefully cleaned. In the fresh state it had weighed 461 grs., but after undergoing desiccation it left a solid residuum of only 84 grs., or 18 per cent. ; so that the process had deprived it of nearly 5-6ths of its weight. 34. The fluids of the human body obviously contain more water than the solid structures. But the quantity of this substance varies in a very high degree with the circumstances of excretion and nutrition. Taking the ordinary estimate, the blood, which usually contains about 70 to 80 per cent, of fluid matters, constitutes as it "were the neutral ground between the fluid and solid constituents of the human body. Evaporation to diyness withdraws from the sweat and mixed saliva about 99 to 99f per cent, of fluid ingredients. The liquor amnii, the gastric juice, and the aqueous humour of the eye have 98 to 99 parts : the lymph, the semen, the pancreatic fluid, and the mucus of the nose, 90 to 97 : and, finally, the bile, 87 to 90, and the milk, 83 to 92 per cent. The watery ingredient of the urine varies according to circum- stances, but is in general 93 to 98 per cent. The loose areolar tissue, which is saturated with the general nutri- tional fluid of the body, gives 80 per cent. ; the brain, 75 to 78 ; the glands and the muscles, 72 to 79 ; and the cartilages, ligaments, tendons, and crystalline lens, 57 to 70 parts. Even clean fresh bones lose more than 14 per cent, when placed in the water-bath. CHAP. IV.] SPECIFIC GRAVITY OF THE ANIMAL TISSUES. 17 35. External appearance frequently deceives us with regard to the watery content of bodies. Substances which appear to be quite dry often lose a considerable quantity of their weight when exposed for some time to a heat of from 212 to 258. Powdered crystallized cane-sugar only gives off a very inconsiderable quantity, for instance, - 6 per cent., and even this is probably due to watery vapour, or some other volatile substances contained in the interstices of the fine powder : while harts- horn contains 1 4 per cent. ; and bread, in its ordinary state of dryness, 43 to 46 -parts. The specific gravity of the different constituents of the animal body i.e., the proportion of their weight to their cubic contents varies within rather narrow limits. A given quantity of water (1 cubic inch) has, at its temperature of greatest density (39^-), a certain weight (253*5 grains). Taking the specific weight of this water as a starting point, we need only divide the quantity of grains which any other mass weighs, by the number of cubic inches which it contains, in order to get its precise specific gravity. 36. There is but one large ingredient of the human body which is lighter than water. The specific gravity of the human fat amounts only to '932. There is no fluid of our organism which has a specific gravity of exactly 1 , or that of distilled water, since every one of them contains dissolved solid ingredients. The bones, which have the highest specific gravity, scarcely reach double that of pure water. 37. Here again we find the blood, with a specific gravity of 1-06, form- ing a line of partition between the fluid and solid constituents of the organism. Those mixed fluids which contain a large quantity of water, as the liquor amnii, the saliva, the gastric juice, and the urine, range from 1-004 to 1-02. The bile, the lymph, and the milk have a gravity of from 1 -02 to 1 -04. The brain, the specific gravity of which is dimi- nished by the amount of its fatty constituents, also amounts but to from 1-009 to 1-03. Animal structures which are soaked in a large quantity of nutritional fluid not unfrequently exhibit specific gravities less than the average of the whole mass of blood. For instance, the gravity of certain muscles amounts to 1-020, while that of many nerves is 1-040. But with these exceptions, we shall find that the estimates for the solid tissues of the body hitherto examined, either surpass the average specific gravity of the blood, or are at least equal to it. Thus, that of the arteries amount to 1-06 1-10; the veins, 1-08 1-11; the nerves, 1-05 1-13; the tendons I'll 1-13; the cartilages, 1-1; the fresh bones covered by their peri- osteum, 1-2 1 -5 ; while cleaned bones, and fragments of compact substance, reach a gravity of 1-9 2-0. 38. The medium specific gravity of the whole body is of course deter- mined by that of its particular constituents, compared with their absolute 18 SPECIFIC GRAVITY OF MAN. [CHAP. IV. quantities. Thus a disproportionate amount of bone, or of other solid tissues, may raise the general specific gravity, while, conversely, it is lowered by large fatty deposits. 39. Frogs examined in spring, during their rutting period, gave an average specific gravity of 1-03 to 1-04: three mice, -96 to 1-09. An eight months' child, which had lived two days, showed a gravity of 1-008. But the adult, which is provided with a stronger and heavier skeleton, has a somewhat greater average gravity: and 1*06 to 1 -07 would probably be the estimate nearest to the truth. This does not much differ from the specific gravity of the blood. Disregarding differences of sex, we may estimate the weight of a human being of thirty years old at 130 Ibs. : according to which valuation the cubic capacity would amount in round numbers to 20 cubic feet. 40. Every man is able to alter his specific gravity in an instant, by drawing air into his lungs. Regarding water as unity, the specific gravity of the air amounts to -001299 : that is, it is nearly 770 times as light. So that air has a much greater power in lightening the body than any arrangement of cork, which has a specific gravity of '24. But since the quantity of air which we are able to take into the lungs is but a small one, it follows that we cannot produce any very great difference in this way. 41. Sea-water has a specific gravity of 1'03. River-water is something less. Hence man sinks in both of these, so soon as he is completely immersed, unless he sustains himself from time to time by appropriate movements of the body. And the inhalation of the greatest possible quantity of air into the lungs enables him to keep on the surface much more easily. But the capacity of the lungs is not sufficient to allow of the reception of such quantities of air as would lower his specific gravity from 1-065 even to 1 -03. The appearance of the bodies of drowned persons on the surface of the water is probably the result of three contri- buting causes : the increase of their volume in the water, the dissolution of their substance, and the access of putrefaction ; the latter chiefly acting by the gases developed during the process. 42. Large deposits of fat (Tab. n. Fig. 27) lower the specific gravity in proportion as they predominate over the other parts, and especially over the bones and the muscles. In rare and exceptional instances, the extra- ordinary fatness of a man enables him to float like a cork on the surface of sea- water. 43. It is usual to express the cohesion or absolute solidity of a solid body by the weight which suffices to rend asunder a mass of definite thickness. Thus we will suppose that a cylindrical wire, whose transverse section amounts to 3-1000ths of a square inch, hangs perpendicularly from a fixed upper end, and requires to be loaded with a weight of 330 Ibs. before it is torn asunder at any point. We may therefore state its CHAP. IV.] SOLIDITY OF THE TISSUES. 19 cohesion, as the amount of weight required for 1-1 000th of a square inch, or as 110 Ibs. 44. But the body made use of in these experiments is subject to a double burden. It is not only laden with the weight which we have voluntarily added, but every upper part has to suspend all those which lie beneath it. Thus a metallic wire must tear asunder without any foreign burden, when the weight necessary to its disruption is supplied by its own mass. For instance, supposing it to be of iron, with a specific gravity of 7 '5, and a cohesion of 110, this event would necessarily occur as soon as there were more than four miles of length, with only 1-1 000th of a square inch of transverse surface. 45. Thin iron wires have a greater cohesive value than thicker ones of the same composition. If the solidity of the moist animal tissues similarly increases with the enlargement of their surfaces; the division into microscopic fibres or scales (Tab. HI. Fig. 40) will have the effect of vastly increasing the resistance. 46. One hundred threads, which are properly united into a cord, will bear more than one hundred times the weight which each separate one could sustain. And where nature has united many thousand of the finest elements of the tissues into a whole, the solidity of its organs will thus gain an additional increase. 47. Flat ropes are stronger than round ones of the same nature. Many of the hairs of the head, and a large number of the tendons, which have an oblong instead of a circular section, are probably thus endowed with more strength. 48. It often happens that two parts which consist of precisely the same tissues yet essentially differ in cohesive force. The one may be nine times as strong as the other. These differences are partly inherent, partly, however, depend on the way in which the experiments are made. The original differences of the molecules, the mode in which they are united to each other, the compressed or diffused subdivision of their bundles, the mixture of more or less solid constituents, and the more or less favourable form of the entire mass all these circumstances cause many of the differences which we meet with on comparing a series of dead bodies, or a number of corresponding parts in the same body. Inequalities in length, deviations from a simple mathematical form, the access of putrefaction after death, and the way in which tractile weight acts upon the particular microscopic elements, these circumstances equally increase the fluctuations which, in spite of the greatest care, appear in such experiments. 49. Thin leaden wires have on an average 27, copper wires 27'5, and iron wires, with a transverse surface of 1-7 7 Oth of a square inch, 76-2, of comparative cohesive force. Very thin threads from the cocoon of the silk-worm, consisting of 8 to 10 microscopic filaments, gave a force of 20 SOLIDITY OF THE TISSUES. [CHAP, iv- 28, and a well twisted silken cord 42-5. If we compare these with the results which Wertheim 2 ) and myself obtained with different portions of human corpses some days after death. Part. Cohesion. Part. Cohesion. Mean. Extremes. Mean. Extremes. Muscles 06 02 to '13 Tendons 53 2-3 to 10-4 Arteries Veins 14 23 1 to -2 1 to '3 | Bone (after Wertheim) 8' 4'3 to 15- Nerves Hair 98 9-9 G to 3-5 f Bone (after Be van) 37-9 25-11 to 75-81 The sequence in which the particular tissues are here placed corresponds to a general increase of cohesion. But we may remark that the absolute solidity of no part of the animal body equals that of the thin iron wire : even that of the silken cord is considerably below this substance. All the supplementary advantages which the organic tissues enjoy cannot compensate for the original difference in the nature of their molecules. 50. But we must not think it an imperfection that no part of our body is made as strong as iron. We shall hereafter point out what important properties are in this way acquired. And, in spite of this deficiency, those parts of the body which have to support great weights are con- structed of such a strength that their cohesion is far beyond their most extraordinary requirements. Even taking the lowest valuations given above, we find that it would require more than seven-fold the weight of the whole body, or more than eight hundred and eighty pounds, to tear the extensor tendons of the foot. And if, in exceptional instances, this rupture of continuity is effected by convulsive contractions of the corre- sponding muscles, this fact only shows what enormous force the shortening of these organs is able to exert. 51. If a suspended body be laden with a continually increasing weight, it is as continually elongated, until finally, it ruptures at the weakest place. But if the weight be removed before this happens, it endeavours to return to its original length. If it succeeds in doing this, it is per- fectly elastic ; but if, on the other hand, it remains permanently elon- gated to a certain extent, it possesses only an imperfect elasticity. 52. One and the same body may exhibit both of these properties with different weights. A strap, for instance, which is originally 4-5ths of an inch long, can return to its previous length after a temporary weight of 28^ drachms. But if laden with 34 drachms, it will only return to a length of 24-25ths. So that 28| drachms indicate the limit of its com- plete elasticity. That of the incomplete elasticity obviously coincides with the limit of cohesive force. 53. Simple as these circumstances may appear, there are many dim- CHAP. IV.] ELASTICITY OF THE TISSUES. 21 culties which prevent their correct determination. Molecular composi- tion, temperature, the mode of applying the weight, and the duration of its application, greatly influence the results. There are many substances which, immediately after the removal of the weight, only shorten to a certain extent, although their length subsequently undergoes a consider- able diminution. Silk and most parts of the human body behave in this way. 54. As regards the elongations produced by different weights, they are of two kinds. They either increase regularly or irregularly with the weight. The soft structures of the human body offer in this respect a peculiar phenomenon (Fig. 1). In order to exhibit the fact mathemati- cally, we may consider the increasing weights as the abscisses, (ab, ac, ad, Fig. 1), and the corresponding FIG. 1. elongations as the ordinates, of a curve. Now if both increase in the same proportion, so that the proportion of b e to cf is equal to that of a b to ac, the line of elasticity will be a straight one ; since the correspond- ing sides of similar triangles vary in proportion with each other. But if this is not the case, if, for instance, the elongations b k, c i, d k, increase disproportionately to the weights ab, ac, ad, the line of elasticity will be a curve, the course of which will depend on its abscisses and ordinates. Many inorganic bodies offer us the simplest of these two cases. Their elongations increase in exact proportion with the weights ; they, there- fore, possess a straight line of elasticity a efg. According to Wertheim and Chevandier, this holds good for wood, and also for bone, especially for dried pieces of its compact tissue ; while, on the contrary, the soft tissues of the human body invariably exhibit a curved line of elasticity, which, according to Wertheim, corresponds with an hyperbola ahik, so long as no immoderate weight is applied. The quantity of their watery constituent forms a very essential cause of this phenomenon. If ten- dons or nerves are dried in the air, their line of elasticity approximates to a straight one. 55. If a cylinder (A BCD, Fig. 2) possesses a straight line of elasticity, we may get the simplest idea of its exten- sibility by taking the weight (P), reduced to a certain unit of transverse section, and required in order to extend it to twice its length. If its extensibility and compressibility are proportionate to each other, the weight P, laid upon AB, will bring the cylinder A B F G again to half its length, or will restore it to A B C D. The idea of a coefficient, an index, or a scale of elasticity, is based upon these assumptions. We designate by these words the weight compared with an unit ^^ of transverse section, the tractile forces of which would double the 22 INDEX OF ELASTICITY. [CHAP. IV. original length of a body, while its compressing force would reduce it to the half. 56. This index of elasticity has frequently only an ideal value, since there are many substances which tear rather than elongate to twice their length. And in many instances, even where highly elastic bodies are concerned, its estimate rests upon an inaccurate basis, since the capacities for extension and compression do not go hand in hand. This is especially the case with bodies which are mixtures of different kinds of constituents. Finally, when the line of elasticity does not form a straight line, or a curve which may be mathematically estimated, it does not safely exhibit the extension for additional weights. 57. In applying this method to the constituents of the animal textures we meet with as many varieties as were previously found in the amounts of cohesion (48). Wertheim obtained 1-15 Ibs. for the membrane of the human femoral artery when reduced to 39 -4 inches in length, and -0016 square inch of transverse surface. The femoral vein had 1-85 to 1-94 Ibs. ; the sartorius muscle '574 to 2-8 ; various nerves of the thigh 22-17 to 71-53 Ibs. ; the tendons 283-27 to 442-36 ; and strips of the compact tissue of the thigh and fibula 4013 to 5979. Ludwig found 1-99 to 3-2 for transverse strips of the arch of the aorta in the horse, and 2-87 was found by myself for the same part in the cow. According to Wertheim the result in the dog was only -8. . Pieces of arteries at a greater distance from the heart invariably gave a smaller value than the arch of the aorta. 58. Very much depends on the fact whether the parts have or have not previously been displaced. If they have once exceeded the limit of their complete elasticity, the weight first laid upon them must also have thinned them, either generally or in some particular places. They also undergo a smaller extension on the application of further weights. Each of these causes contributes to raise the index of elasticity very considerably. For instance, a square strip from the aorta of the cow, presented at first only 2-7; but after passing the limit of complete elasticity, it amounted to 6-9. 59. A body which is to yield but little to a tractile force, must neces- sarily possess a great index of elasticity. And conversely, the opposite condition necessitates a smaller magnitude. The bones and soft struc- tures of the animal body may explain this antithesis. The former have an index which averages fourteen times that of the tendons, although these belong to the most solid of the soft textures. This difference principally depends on the constitution of the cartilage which forms the basis of bone, and on the numerous salts of lime which it contains. 60. The watery contents greatly contribute to the smallness of the index found for the softer animal tissues. For instance, the fresh tendon of the long flexor of the great toe, had an index of 283-27. But when dried in the open air, it amounted to 412-2. CHAP. IV.] ARRANGEMENT OF THE BONES. 23 61. Few if any parts of our body ever require to extend to twice their length. And on this account, even in very elastic tissues, the limit of perfect elasticity is less than this index. But the latter is inferior to the amount of cohesion; at least in the pieces of aorta above mentioned it was found to be so. The coats of the arteries are, on account of their small index, easily extended, and, after the removal of the weight, quickly return to their previous condition. We shall hereafter see how much more necessary this property is to their function, than an unusual exten- sion of the limit of perfect elasticity. While the function of the blood- vessels demands a high degree of elastic extensibility, the tendons, on the other hand, require to be solid cords, which shall yield as little as possible to the powerful force to which they are exposed. Their average index of elasticity is therefore raised to 360-5, while that of the nerves, which rank next to them, among the tissues formerly mentioned, amounts only to 4545. 62. The bones may suffice to show what great mechanical advantages result from a proper subdivision of a mass. We have already seen ( 37) that they possess a greater specific gravity than any of the soft tissues. Tf their substance had completely filled all the space which they take up, the weight of the skeleton would have been uselessly increased, and the movement of a large part of the animal machinery would have been ren- dered proportionally more difficult. And hence Nature protects its free surfaces with dense compact tissue, and stows away, in the interior of the skeleton, the marrow, the cavities of which enclose lighter compounds. In this way, substance, weight, and muscular force are all economized; while the other mechanical relations are at the same time improved. 63. Let us suppose the same mass of matter arranged in two different ways, in one as a dense solid cylinder, and in the other as a hollow one, it is evident that the latter will offer a larger extent of surface. So that the presence of the medullary cavities of bones produces a greater extent of free surface, which latter may either enclose other tissues, or may afford a multitude of points for the attachments of muscles, tendons, and ligaments. And with this it affords a better provision for many circumstances of solidity, which either generally or exceptionally occur. 64. Reactive solidity obtains when a given body is loaded above ; relative solidity is shown when a weight strives to bend a horizontal and fixed mass. A cylinder which is solid throughout is more unfavourably cir- cumstanced, with respect to both of these, than a hollow one, the mode- rately thick walls of which contain the same mass of solid matter. Hence it is obvious that the subdivision of the osseous cavity may afford important facilities in this respect. The proper mixture of cartilage with salts of lime, of compact with cancellated tissue, of rounded with angular forms, of uniformly continu- ous segments with numerous elevations, depressions, enlargements and 24 ACTIVE FORCE OF MECHANICAL IMPRESSIONS. [CHAP. IV, processes all this results in making the pieces of the skeleton hard levers, bases of support, and protective textures, such as the artifice of man could never imitate. But even Nature herself is unable always to retain these advantages, or to protect the bones from unusual attacks, as it does many soft parts. A deficient development, such as happens in scrofula, produces curvatures of the bones, and especially of those which have to sustain a heavy weight, such as the vertebral column, the pelvis, and the long and hollow bones of the lower extremities. Hence many of the forms which correspond to the other vital objects, will be either very imperfectly developed, or not at all. Softening of the bones leads to the most extraordinary curves, and, to fractures, on the slightest disturbance. In more advanced life the salts of lime are sometimes too abundantly deposited, and the bones become more brittle than they should be. Their great vascularity, and their chemical constitution, cause them to be more liable to suppuration than many other tissues, such as horn, carti- lage, and tendon. Many of them, such as the clavicle, the hollow bones of the trunk, and the extremities, are very liable to be broken. Others, such as the cancellated bodies of the vertebrae, and the bones of the tarsus and carpus, are frequently visited by caries; and others, such as the bones of the nose and palate, are attacked by syphilitic degenerations. The epiphyses and diaphyses of the healthy long bones are so loosely united, that the tibia, for instance, which is suspended from its upper end, and transmits a weight by its lower one, is often torn asunder at one of these points of union by a force which is less than the cohesive energy of its compact substance. 65. The points where many soft tissues are attached exhibit a similar phenomenon. Very strong tendons, such as the tendo Achillis, or thick ligaments, like the great ligament which strengthens the hip-joint, may often be torn away from their attachment to the bones in the dead body, rather than ruptured in their own substance. 66. The active force of a body in movement is the product of its mass multiplied by the square of its velocity. It is therefore increased an hundredfold when the velocity is only ten times multiplied. If the same object be struck by two cannon-balls, one of which has four times the velocity of the other, the latter will penetrate sixteen times deeper. And since the spaces which a body falls through are as the squares of the final velocity, so a body must inflict a greater injury in proportion to the height from which it falls. The construction of many technical varieties of the hammer and the pile-driving machine depend upon the fact, that active force is gained by multiplying the final velocity. For instance, if a mass, the weight of which is unity, falls with a velocity of 5 feet in a second, its force equals 15-145 times that weight. A hammer which weighs only 9 oz. exercises a force of 12024 oz., when a man, in using it, imparts a final velocity of 638 feet. CHAP. IV.] COMPRESSIVE ELASTICITY OF THE TISSUES. 25 It is thus highly important to estimate the velocity or acquired force of bodies which inflict an iiijury. But the resistance offered by the different tissues varies greatly. It not unfreqiiently happens that a bullet which has penetrated the softer tissues goes completely round the ribs, either in. consequence of originally wanting the force necessary to break these bones, or from having lost it in a previous part of its course. If the bone has been broken, the velocity with which this has been done exer- cises an important influence on the nature and extent of the fracture. Since relative solidity diminishes in proportion to increase of length and decrease of thickness, it becomes evident why the long and small cylin- drical bones, and the broad and thin flat ones, are most exposed to the danger of fracture, especially where they possess a more brittle layer of compact tissue on their exterior. In this way it often happens that the bones of the extremities are fractured, while the soft parts which encase them preserve their continuity. 67. Gases and vapours have the property of unlimited expansion, so that the space they occupy is only limited by the amount of pressure to which they are exposed. This ability of altering volume in corre- spondence with pressure, is usually called elasticity of compression; although elasticity, in the strict sense of the word, belongs only to solid bodies. 68. The law of Mario tte teaches us, that the volumes of gases and vapours is inversely as the pressure to which they are exposed. A quantity of gas, which, under the pressure of one atmosphere, or with the barometer at 30 inches of mercury, occupies the space of 1000 cubic inches, takes a bulk of 2000 cubic inches when the barometer is reduced to 15. Regnault has indeed shown that many degrees of heat, and particular amounts of pressure, constitute exceptions to this sup- posed universal law. But these deviations scarcely affect those physio- logical phenomena which are connected with the gaseous state. Hence it follows, that the atmospheric gases which are enclosed within our bodies, change their volume or their weight, according to the barometric condition of the atmosphere. 69. Liquids possess a very inconsiderable elasticity of compression. Water at the temperature of its greatest density, or at 38-75, loses only th of its bulk on being subjected to the additional pressure of 47000000 another atmosphere. And in the cases of those parts of our bodies which are saturated with fluids, it is probable that this compressibility is still further diminished. So that we are justified in assuming, that no external or internal forces can modify the condition of these fluids, except by pressing them out, or driving them into other places. 70. We must be careful to distinguish decrease of volume from mere alteration of outward form. However efficiently our bodies resist the first of these, very slight alterations of pressure suffice to produce alter- 26 COMPRESSIVE ELASTICITY OF THE TISSUES. [CHAP. IV. ations of shape, especially in those structures which possess a small index of elasticity. Nature makes use of this to produce many important phenomena. The blood-corpuscles of the frog (Tab. IL, Fig. 23, a, b), possess large long diameters, and small transverse ones. But even these exceed the width of some of the finer blood-vessels. And just as an elastic ball flattens on striking against an object, and recovers its original rounded form as it rebounds, so these corpuscles become elongated when the stream of the blood drives them into the capillaries, while their subse- quent passage into vessels of a larger diameter allows them to return to their original form. We shall subsequently see, that fluids which circu- late in small tubes, cling with great tenacity to their surface. But the requirements of nutrition necessitate a change in the contents of the capillaries. The blood-corpuscles, which are in this way forced hither and thither through the finest capillaries, are thus enabled to dislodge, and as it were, to strip off this tenacious peripheric layer of fluid. 71. An essential peculiarity of liquids, is the great mobility of their particles. A pressure which is exerted upon their mass in one point, pro- pagates itself equally in all directions. If we completely fill an elastic bladder with a fluid, it yields to all external mechanical force, in a degree varying with its capacity of resistance ; and if its elasticity remains un- impaired, it subsequently returns to its original form. Nature makes use of such an arrangement to constitute the fat of our bodies a kind of self- regulating cushion. Thousands of small vesicles, bounded by elastic walls, enclose masses of oil, which the warmth of the human body prevents from congealing (Tab. n. Fig. 27). The pressure of a body which possesses an extensive surface of contact thus divides itself amongst a great num- ber of small elastic cells, the interstices of which being filled by areolar tissue and nutritional fluid, easily permit these evasive movements. 72. Yielding solid textures moderate the pressure which is transmitted to them from without. In this way the horny cells of the epidermis save us many pains to which we should otherwise be exposed at every step. The sole of the foot, which has to bear the weight of the whole body, is for this reason endued with the greatest number of epidermal cells, especially at those parts which are pressed against the ground in the acts of standing and walking. 73. If a compressing force acts slowly and continuously, the parts so acted upon are extended to the utmost limits of their cohesion. The changes to which most parts are liable in the course of disease, offer striking examples of this. The skin, the nerves, and even the bones, are frequently thus extended to a considerable degree. Physical properties, and the capacity of growth, generally assist in producing these results. The skin covers the most enormous tumours. Particular portions of bone, such as the epiphyses and diaphyses of the cylindrical bones, may CHAP. IV.] INDEX OF FRICTION. 27 be thus converted into large cavitary shells. And if the cause of such an extension be overcome, the parts often return in a short time to their original form. 74. Here and there Nature allows this distension to go so far that the obstacle offered by cohesion is completely overcome, and the parts tear at the most yielding place. In this way the ovarian follicle of the mammal gradually absorbs extraneous matter, until it finally bursts at the thinnest part, and the ovule which it contains is set free. And although it is as yet undecided whether this phenomenon is merely physical, or whether it depends on a real growth, yet it cannot be doubted that the former cause produces the same results in the progress of disease. Arterial tumours frequently burst solely by reason of a constant increase in the contents of the sac having at last extended it beyond the limits of its cohesion. 75. We know of no body which possesses perfectly uniform surfaces. A highly magnified view of the smoothest solid mass discloses uneven- nesses, projections and elevations, which are usually irregularly disposed. If we look at this on a larger scale, a solid body (Fig. 3, A) grasps, with its projections, the depressions of any other mass, against which it is pressed, either by its own weight, or by external force. If it be moved in the horizontal direction, it must either be raised and depressed in cor- respondence with these irregularities, or be injured at its surface of con- tact. Either of these alternatives requires the application of a certain force, which is indicated by the name of the "index or coefficient of friction." The absolute value of this increases directly with the weight of A. But the size of the surfaces in contact cannot affect it. So that when one states that the friction of iron upon copper amounts to -17, this means that an iron cube, moved on a plate of copper, requires the application of 17 Ibs. of force for every -100 Ibs. of iron. It is, how- ever, perfectly indifferent whether the surface of friction amounts to 1 or 10 square inches. When a sphere is rolled upon a surface, the index of friction determines how much force is lost upon that surface. Other things being equal, this is less than the index of friction first mentioned. 76. As friction forms an obstacle to movement, it furthers the stability of a body. If it were altogether absent, the slightest touch would suffice to produce a displacement ; so that it forms a kind of counterpoise, which requires to be overcome in order that movement should be produced. And in point of fact, we find that Nature makes use of both of these relations in various parts of the organism. 77. The deepest layers of epidermis (Tab. n. Fig. 32) are intimately 28 INDEX OF FRICTION. [CHAP. IV. and uniformly united; while the superficial horny cells are more loosely connected, and are continually scaling off. Hence the sole of the foot has a very uneven surface, which is of essential use in standing with naked feet. A man in new or ironshod boots easily slips and falls. Felt shoes allow their wearers to walk over ice without falling, where smoother and less yielding coverings would be dangerous. 78. Many internal parts require to be moved to and fro upon others. The movements of the brain and spinal cord, of the abdominal and pelvic viscera, and of the joints and tendons, lead to varieties of gliding and rolling friction. But Nature has here adopted a combination of nume- rous means, so as greatly to diminish the loss of power which would otherwise result. 79. In mechanical arts we make use of various lubricating substances to overcome the resistance of friction. We cover the surfaces of contact with viscid substances, because these are more efficient than limpid water. They more or less fill up the opposed depressions, and in this way pro- duce a greater smoothness and mobility. And they thus not only diminish friction, but prevent the wearing of the gliding solids. If one piece of oak wood rubs against another the index of friction varies from 48 to -34, according as the movement takes the direction of the fibres, or is transverse to them. But if there be water between the two pieces of wood, it sinks from -34 to - 25. Soap similarly interposed reduces it from 48 to '16. The dry friction of cast-iron on oak wood amounts to -49 ; but when the opposed surfaces are smeared with hog's lard or oil, it sinks to -078. 80. If we compare the various organs of our body with the arrange- ments just mentioned, we find that their surfaces are generally much smoother than those of many polished machines. In addition to this precaution, Nature makes use of albuminous and mucous fluids as means of lubrication. These easily adhere to the surfaces of the organs, and offer very little obstacle to movement. The contents of the various serous sacs and bursae mucosa3, and the synovial fluid of the joints, thus exert a very important influence in diminishing the impediment which friction affords. 81. The liquid state probably results from the circumstance that cohe- sion and pressure, i.e. the two forces which keep the molecules of matter closely connected together, attain nearly a counterpoise with those of warmth or repulsion, so that the particles become easily moveable upon each other. And if a sufficient obstacle is present to prevent the evasion of a liquid, a pressure in any one direction is transmitted in all others. This fact forms the basis of the laws which regulate the movement and rest of liquids. Suppose abed (Fig. 4) to be a receiver with unyielding walls, and for the present closed at c e. Let it be open above at a b, and filled with water CHAP. IV.] PRESSURE OF LIQUIDS. 29 up to this point. The perpendicular and gravitating line of liquid from / to g presses downwards, with a force corresponding to its specific gravity and its length. But since the pressure exerted at g diffuses itself equally in all directions, all of the molecules in the transverse line between i and k have to sustain the same pressure. And if the linefg be prolonged to h, the transverse pressure d c is of course proportionally increased. It is obvious that the size of this transverse section makes no difference in the result. The hydrostatic pressure of a liquid which remains in equi- librium and at rest, is, therefore, wholly and solely dependent upon the height of the pressure. That is to say, it is measurable by the perpen- dicular line which may be drawn from the upper to the under surface of the liquid, and which exactly corresponds to the direction of gravitation. Let us now suppose the vessel abed con- verted into abon : it is evident that this can no way affect the hydrostatic pressure, since the height of the column fh remains unaltered. Or altering its form to aim b, it would only have the pressure of fg, and this, though less than anob and a deb, is equal to a i k b. In one word, the hydrostatic pres- sure which is exerted at the bottom of any vessel is quite independent of all alterations of its form. Let us now suppose abed connected with the long and open tube qwxr, by means of e p q c. Let it be filled to the same level s t, as the water in the larger vessel : it will be in a state of perfect equili- brium and rest. Since the columns a d and fr have the same height, the same hydrostatic pressure is exercised on the entire transverse section d r. And this holds good for all parallel sections up to a b and s t. But if a b be now shut in by a solid wall, and the fluid in qw XT so increased by new additions that its surface rises to u v, the additional column v t will not only weigh upon qstr, but also upon abed. And a b will have to sustain the increase of pressure, just as much as s t. And since the influence is no way dependent on the size of the transverse section, we can in this way produce the most important changes in the large shut receiver by the instrumentality of this small column connected with it. 82. We have already seen that the amount of the hydrostatic pressure depends, not merely on the length of the column, but also on the specific gravity of the liquid. A body, the specific gravity of which is twice as high, will exert twice the hydrostatic pressure of another fluid. The arrangement of the barometer and manometer is based upon this pheno- menon ; and it has also considerable influence in the movements of the animal fluids. 30 BAROMETER. [CHAP. IV. 83. If we fill the two perpendicular ends of a bent cylindrical tube with the same liquid it will remain in a state of hydrostatic equilibrium ; i.e. the two surfaces, one of which is at ef (Fig. 5), must lie at the same horizontal level. If we now pour in a quantity of the same fluid corresponding to the cylinder ikfe, the equilibrium will in a short time again attain the two surfaces a b and c d. The fluid on each side is in- creased to the extent of the column a bfe, or to half the height of the superadded quantity. But if, on the other hand, we add the same quantity of a liquid which is twice as light, the height of the column in the other limb of the tube would only be raised to half the amount. 84. Atmospheric air at 32, and under the pressure which obtains at the level of the sea, has a specific gravity which is 1046745 times lighter than mercury. So that a vertical column of air so many inches in length would correspond to 1 inch of mercury. And taking this fluid as our index, we may construct the tube, whose contents shall represent the atmospheric pressure, 10467 times shorter than such a column of air. 85. If we made use of a tube open on both sides, like that just spoken of, the air would press on a b and c d with equal force, and the quick- silver would have the same height in both limbs of the tube, so that the influence of the air could not be estimated. In order to do this there must be a vacuum over the indicating fluid. The atmospheric pressure will then impel it to a height which will be the exact counter- poise of itself. This may be better seen by examining the diagram of a barometer given in Fig. 6. The atmosphere presses on the surface of the mercury at a b. But since there is no air at s to exert a counter-pressure, the quicksilver is maintained in the tube, from a b to s, as a surplus column which exactly equals the external atmospheric pressure. The same ob- tains in the ordinary barometer, Fig. 7. The horizontal surface of quick- silver in the shorter limb corresponds to the under surface of the column of quicksilver, which exhibits the pressure of the air. It is thus equal to a b in Fig. 6. Since observation shows that at the sea level the barometer has a height of 29-9 inches, this corresponds to a pressure of 313200 inches of air at a temperature of 32, and in the same place. Under different temperature and pressure, however, the quantity would be different. Thirty inches of mercury are in this respect equal to about 33 feet of water. 86. A manometer (Fig. 8), which measures the pressure of a liquid, or the force of its stream, consists of a suitably curved tube, fixed perpendicu- larly, and containing an indicating fluid in a state of equilibrium ; i.e. re- CHAP. IV.] MANOMETER. 31 acting to the level of in b and c. If we now allow the entry of another fluid into b to compress the indicating liquid, this will experience a de- FIG. 6. FIG. 7. FIG. 8. pression suppose of 30 and a similar elevation will take place in c. But in order that the proof liquid thus raised in c should maintain its equilibrium without this pressure, b must contain so much of this proof liquid as to be not 30 below, but 30 above, 0. The pressure which we are examining, therefore, amounts to twice 30, or to 60. That is to say, the amount to which the proof-liquid falls in one or rises in the other limb of a completely uniform manometer tube, forms the half of the pressure sought for. The hsemadynamometer is an instrument which in this way estimates the strength of the current of the blood. The pneumamometer is a similar apparatus, which measures the altered tension of the air re- spired in the act of breathing. For instance, if the hsemadynamometer shows that the blood in the carotid of a dog depresses one limb of the mercurial column 3-15 inches, it follows that the stream in this vessel has a force of 6-3 inches of quicksilver. Water would give nearly four- teen times'* as great an estimate, being so much lighter than mercury. 13-598. PRESSURE OF AIR ON THE BODY. [CHAP. IV. Hence when small amounts of pressure are to be estimated this fluid is preferable : thus, for example, it is made use of in examining the current blood of the large venous trunks. 87. In order to reduce the results obtained by one proof liquid to those of a second fluid, we need only multiply tfre numbers by the quotient of the specific gravities of the first and second fluid. A mer- curial pressure of 6*3 inches amounts to 85-68 of water. If we assume the average specific gravity of the blood to be 1 -06, a quarter of a line of quicksilver corresponds to -6547 inches of blood-pressure, and the same amount of water to -05342 inches. 88. Since every column of fluid acts, with a force corresponding to its height, on every point of the lower surface of the containing vessel, we might find the absolute or total pressure which this has to sustain by multiplying its surface by the height of the liquid, and reducing this cubic capacity to the weight of the fluid, the pressure of which had been previously ascertained. Every square inch of surface which receives the atmospheric pressure at the sea-level and freezing temperature, has 29*9 inches to sustain. Thus we have a total of 29-9 cubic inches of mercury. And since a cubic inch of water weighs 252-6 grains, a cubic inch of mercury will be 252-6 x 13-598 = 3460 grains, so that every square inch exposed to the surface of the atmosphere at the level of the sea, has to sustain a pressure of 14*78 pounds. 89. The outer surface of the author's body, which has a length of 63 inches, and a weight of 119-14 pounds, amounts to about 2325 square inches. So that the atmosphere exerts upon it a total pressure of 34,366 pounds, or 287 times the weight of the body. Quetelet supposes that the outer surface of a very large man measuring 68-11 inches in height, and weighing 167-677 pounds, amounts to 2549-75 square inches. According to this estimate, the total weight would be 37682-7 pounds, or 224 times the weight of the body. 90. This considerable amount of pressure need not surprise us, when we reflect, that it is not only borne by ourselves, but by all the masses which surround us, and that their several degrees of cohesion are only thus preserved. A closer examination of these phenomena will show how the several parts of our organism behave under alterations of these conditions. 91. Since the crust of the earth is separated from the airless realms of space by a girdle of atmosphere, this latter must have its deepest layers more pressed upon, and its higher, less. Omitting all considera- tion of the exceptions to Mariotte's law, the atmosphere must become denser and heavier, in proportion to the amount of its own mass which it has to sustain. And the condition of the barometer which serves to indicate the several amounts of pressure, will also take cognizance of these states. So that we may make use of this instrument in the measurement of CHAP. IV.] ACTION OF THE ATMOSPHERIC PRESSURE. 33 heights ; i. e. may apply it to determine the elevation of any place above the level of the sea. If the barometer stands at 29'9 inches, the height of the quicksilver column will diminish l-25th of an inch on making a vertical ascent of 37-73 feet. And if I find that the barometer at Bern has a height of 28-16 inches, I am enabled apart from all other corrections to calcu- late that this place is 17454 feet above the level of the sea. The total weight of the atmosphere is thus diminished by more than 1-1 7th. As the barometer sinks 16-93 inches on the summit of Mont Blanc, this would give a weight of atmosphere corresponding to little more than 2-5ths of that at the level of the sea. 92. If we place a bell-glass on the plate of an air-pump, and com- pletely exhaust it, every square inch of its surface will have to sustain a pressure of nearly 15 Ibs. And if its cohesion be not sufficient to resist this weight, it breaks. But if we allow the air to re-enter, while the exterior atmosphere exerts the same pressure, the air contained within its cavity presses from within outwards with equal force. We have, in fact, just the same counterpoise as in the manometer, when both limbs of the tube are allowed to remain open. A similar condition is present in the human body. While on the one hand the atmospheric column is pressing on its outer surface, it possesses, on the other hand, numerous internal cavities, which are filled with air, and the contents of which exert an opposite pressure with equal force. If this were not the case (apart from the considerations of heat and vapour), our organism would resemble a barometer, or an exhausted bell-glass, and would be perpetually exposed to the pressure of the atmosphere. But as it is constituted, we may rather compare it to a receiver with air playing freely upon both its surfaces. 93. There are certain closed cavities of our body which are filled with liquid, and are so arranged, that the pressure of the air upon them is used as a mechanical force. In this way Nature obtains facilities for the movement of the blood and lymph, which will hereafter be again referred to, as well as for the gliding of tendons and other moveable parts. And an inspection of the serous membranes and joints will teach us what various results may thus be obtained. 94. In Fig. 9, e rj are exhibited the air-tight walls of the belly. A fluid, the peritoneal fluid, occupies the peritoneal cavity, and the inter- stices of the abdominal viscera. The external atmospheric pressure on the walls of the belly fits all its contents exactly to one another. So that any substance remaining under the influence of the pressure of the air can only enter the stomach q r, and the remainder of the alimentary canal s t u v, when driven forward by some additional force. If it passes further, the atmosphere which presses on the whole of the receiving organ, and keeps all its parts in contact, brings together the walls of the 34 EFFECT ON THE SEROUS SACS. [CHAP. iv. emptied portion. If particular loops of intestine move upon each other, the peritoneal fluid is compressed into the interstices thus formed. If a portion of the contents of the canal is expelled in the shape of faeces, or urine, the external atmospheric pressure again brings the cavity into coaptation with its diminished con- tents. In short, it enables every- thing to lie in the smallest possible space, at the same time that it allows the slightest preponderance of pres- sure to effect changes of space ne- cessary for entering or emerging sub- stances. The same general arrange- ment is required in the other serous sacs. They contain a liquid, which immediately occupies all the spaces caused by movement or by alteration in the size and situation of their walls. A serous vapour is not pre- sent. 95. If two hollow hemispheres (a and b, Fig. 10) be pressed together, and the air which they contain pumped out of them by means of the side tube c, and if the entrance of fresh air be prevented by a sud- den closure of the stop- cock at c, we shall find that in all cases where a and b possess any con- siderable extent of sur- face, it will be quite impossible to separate them. Since there is no counter-pressure by air from within, the atmosphere holds the two hemi- spheres together with a force of 1748 pounds for every square inch of surface which they possess. 96. In the joints a similar arrangement has been brought to the assistance of the muscles. The capsules of the joints form air-tight cavi- ties, which are filled with a certain quantity of liquid synovia, and unite the opposed articular extremities of the bones. Since there is no air in their interior to exert a counterpressure, the head of the femur (Fig. 11, g) is retained in the cotyloid cavity, like the hemispheres a and b when the ajf is pumped out of them. The accuracy of this conclusion FIG. 10. CHAP. IV.] EFFECT ON THE JOINTS. 35 has been experimentally shown by W. and Ed. Weber. They found that on bringing a suitably prepared hip-joint under the receiver of an air- pump, and exhausting the air, the weight of the piece of femur (b) caused it to drop out of its socket, while, conversely, the readmission of the air again raised it to its place. FIG. 11. 97. It is easy to see that this circumstance must cause the articulat- ing surfaces to fit into each other much more accurately than could the capsules and the ligaments alone. Without it the muscles must have borne the additional burden of the weight of the thighbone every time the leg was raised. The air-tight capsule, and the absence of gases and vapours from the interior of the joint, rid it of this useless load ; and hence there is less danger of their being fatigued. 98. Many travellers have remarked that, at great elevations, a man suddenly becomes fatigued, moves his legs with difficulty, and is finally either unable to proceed, or is obliged to rest after proceeding a very short distance. Since the atmospheric pressure diminishes considerably in the higher regions, it has been supposed that in such circumstances he is no longer relieved of the entire weight of the leg, but that the muscles are compelled to sustain a part. This affords a very simple explanation of the fatigue just mentioned. But practised mountaineers have felt none of this embarrassment on the summits of the highest mountains in Europe, such as Mont Blanc, the Jungfrau, and Monte Rosa. And the exertions necessary in climbing a mountain, especially to those unused to them, might readily deceive any one. But, even apart from this, it may be shown, that the greatest height hitherto reached by man allows of sufficient atmospheric pressure to relieve us of the entire weight of the extremities. The hip-joint is that which has proportionably the smallest surface, and the thigh-bone 36 THEORETICAL VELOCITY OF THE FLOW OF LIQUIDS. [CHAP. IV. FlG. 12. is the heaviest weight : so that if we can verify the above statement for this articulation, it will hold good, a fortiori, for the several segments of the arm, and the remaining joints of the leg. Gay Lussac ascended in a balloon to a height of 7632-2 yards. Omit- ting all collateral circumstances, this would correspond to a barometric height of 1346 inches. The severed thigh of a labourer, aged sixty- seven years, who had died accidentally, weighed, with its muscles and vessels, 18-53 pounds. The surface of pressure in the hip-joint amounts to 2-8 square inches. So that the diminished atmospheric pressure would still suffice to remove the weight of the thigh : it would indeed exactly do so ( 1 89 4 -98 x 14 ' 78 x 2 ' 8 55 = 18 ' 53 lbs -)- The right thigh of a new-born male child, when similarly isolated, had a weight of -5231 Ibs., and the hip-joint a surface of -274 square inches. With the barometer at the height above mentioned, this would sustain a weight of 1-0 12 Ibs. 99. If a liquid flows out of an opening, i Jc, Fig. 12, in the under surface, b c, of a vessel, a, b, c, d, the amount discharged will depend partly on the velocity with which every mole- cule of the column g h moves, and partly on the number of such columns passing through i k. The velocity of the flow and the size of the aperture are therefore the chief conditions to be regarded. 100. In hydraulics we distinguish two kinds of velocities, theoretical and real. The first may be immediately deduced from the laws of attraction, or from the general law of gravita- tion. But the second can only be empirically determined .for each particular instance. All that the theory can do is to give certain approximative values. 101. Let us suppose that just so much is continually added above as flows off below. The surface of the liquid, ef t Fig. 12, or the height of the column g h, will remain unchanged in spite of the exit of fluid below. Now if we confine ourselves to the theoretical velocity of the flow, the theory of Toricelli states, that the molecule h comes to the aperture of exit with the self-same velocity which it has attained during its free passage from g to h. The distance between g and h forms, as it were, the space through which the molecule falls. But since the final velocities of masses are to each other as the square roots of the different heights through which they fall, it follows that the velocity of exit must have the same proportion. For instance, if the experiment be so managed that the level of the liquid is at first that of ef, and then that of I n, where g h is to nil as 4 to 1, it will follow that h will have twice the velocity in the former as in the latter case. CHAP. IV.] REAL VELOCITY OF FLOW. 37 FIG. 13. 102. It is thus obvious that it is the degree of pressure, gh or nh, which essentially determines the velocity of exit. It is therefore called the index of velocity for a flowing liquid ; and every other force which acts upon a fluid mass, may be reduced to a previously denned degree of velocity. It is only necessary to convert it into a column of fluid of cor- responding height, which is undergoing a similar exit. Let us suppose that a surface of water, amounting to one square inch, is pressed upon with a force of 1000 grains (996-31 grains, -14233 Ibs.); we may estimate the degree of velocity at 4 (3-9371) inches, since one cubic inch of water weighs 250 grains (252-6 grains, -03615 Ibs.). 103. The form of the aperture of exit, the shape of the stream com- mencing at this place, the nature, diameter, and course of the tubes through which the fluid mass is driven, and the resistance of the bodies which it meets with on its way ; these are the principal circumstances which cause the real to differ from the theoretical velocity. Their effect is almost always to consume a considerable part of the original force. But they also are reducible to a definite estimate, which is called the degree of resistance. So that the real velocity is that residue of the theoretical which is left after the subtraction of the entire resist- ance. Hence, in order to get its value we must multiply the theoretical value of the velocities and the quantities discharged by cer- tain fractions, the coefficients or indices of resistance. We may represent the force that propels a fluid through a conducting tube (ac Fig. 13) under the form of a neighbour- ing vessel which contains a column of definite height. The velocity with which the stream rushes out at c must be less __ than the pressure of the column ^^ would alone produce, from two ^ causes. Firstly, the molecules of the water have to overcome the resistance of the atmosphere at c ; and in addition to this they are liable to be thrown into various curves. Adhesion to the walls of the tube a b c, and friction against them, consti- tute a second element of the index of resistance. All these obstacles obtain when a stream of urine is expelled from the urethra. Its stream is therefore discharged with less force than that which the contraction of the bladder and the assisting abdominal muscles together impress upon it. But on the other hand, that part of the resist- ance which depends on the passage into a different medium vanishes when 38 INFLUENCE OF TUBES ON VELOCITY. [CHAP. IV. the heart drives the arterial blood into the capillaries. So that here it is only the influence possessed by the walls of the tubes which remains. 104. If we place two side tubes, a d and be, perpendicularly in a and b, the water will ascend in them to a height corresponding with their situation. They are " Piezometers " i. e. they measure the pressure which the fluid exerts at these two points, by the height of the columns they respectively contain. And they thus show how much of the original pressure has been hitherto consumed by the resistance. But the column b e must be shorter than a d, since the portion of tube between a and b has added a certain total of obstacles. Applying this to the vessels of our body, we shall find that the different heights simultaneously ob- served in two heemadynamometers, whereof one is fixed into a main artery near the heart, and the other into the same vessel further off, will acquaint us with that amount of resistance which is offered by the intervening portion of tube. 105. The detrimental influence thus exerted by the inner surface of the tube depends upon two causes. The peripheric particles of the fluid must strike against the irregularities of the wall. And adhesion tends to retain them in contact. The amount of resistance offered by the first of these causes is as the square of the velocity, while that of the second is simply proportionate to it.* We thus see what important advantages Nature obtains by the extraordinary smoothness of the internal surface of the blood-vessels and absorbents. 106. Leaving for the present the changes produced by the walls of a tube, we may notice that the same quantity of liquid would pass in the same unit of time through any transverse section of an uniform cylin- drical tube, a be, Fig. 13. But if we suppose that in its course from AB, Fig. 14, it experiences a dilata- tion at D c G E, the mass of flowing water will have to diffuse itself over a larger space. If we regard the fluid transmitted in an unit of time as a cylinder which has the transverse sec- tion of the tube for its base, and the degree of velocity for its side, the latter will lose just as much as the former gains in extent. So that, other things being equal, the velocity of a fluid is inversely as the size of its channel. * This will be evident if we consider that the shock of these fluid particles against the -regularities will depend upon, Istly. their number ; and, 2ndly, their force. Now, since both number and force vary with velocity, the entire resistance of the shock (s) will be s = v* bo ot the next constituent -the quicker the stream, the oftener is the adhesion of each particle overcome, hence the total adhesion (a) will be a = v. Or, putting both these elements of resistance together, we get r = ^ + v . And if we suppose that the amounts of force and of adhesion specific to the matters made use of are known from experiment, and are indicated by b and c respectively, we may arrange the whole resistance as an equation ; r = Itf + cv. Editor. CHAP. IV.] TRANSIT OF LIQUIDS THROUGH CAPILLARY TUBES. 39 The blood-vessels, the absorbents, the ramifications of the bronchi, and the ducts of many glands, agree in this one circumstance; viz., that the sum of the transverse sections of a number of their subordinate branches is greater than that of their chief trunks. So that the channels widen in the same direction as that in which this subdivision occurs. Therefore the velocity of a fluid which is expelled in the same direction must decrease, while that of one which is coming in the contrary one must increase. In this way, the nearer the blood approaches to the capillaries, the slower is its flow. While in the return of this fluid towards the heart, or in the passage of the lymph onwards, or in the progress of an excreted fluid in ramified ducts of the glands, the velocity of movement is continually increasing. Under similar circumstances, the stream of air in inspira- tion has a decreasing, and on expiration an increasing, velocity. 107. Those peripheric or most external molecules which suffer from the irregularities and adhesiveness of the internal surface, form a layer which is proportionally thinner, the greater the diameter of the tube.* The resistances offered by very narrow canals to the transit of fluid are thus rendered extremely great. 108. The fine canals with which Nature operates in our bodies, are much more minute than any capillary tubes with which hydraulic expe- riments have been made. The fluid which is driven through them generally passes into the same medium. But sometimes liquid masses are expelled into the air : the excretory ducts of the sweat glands, and the sebaceous follicles of the skin and external ear, are instances of this. 109. The velocity with which a liquid runs through a capillary tube to pass into another uniform fluid, varies with the nature of the moving fluid. According to Poiseuille, 4 ) solutions of saltpetre and acetate of ammonia move more quickly than pure water, while alcohol and blood- serum have a slower rate of movement. When the length of capillary tubes does not exceed a certain proportion to their transverse section, the quantities passing through in a given unit of time are directly as the fourth power of the diameter, and inversely as the length of the tube. It hence results that the resistances are greatly increased by narrowing the tube. The finest capillaries of the body are only 1-1 1000th of an inch in diameter. Their transverse section is thus 1,2 10,000 times smaller than that of a capillary tube 1-1 Oth of an inch in diameter. Hence, * We may, perhaps, explain this statement by rendering it more exact. The areas of circles of diiferent size have the proportions of the squares of their diameters; while the circles themselves are but as these diameters. And it is evident that we may regard the number of molecules contained in a circle as proportionate to its area ; while the number of these in contact with the circle will vary with the length of the line forming it. So that an addition to the diameter, which is only multiplied by 3^- to increase the latter number, is involved to a higher power to represent the former one; and thus gives the whole contents a continually increased proportion, or vice versa, as continually decreases the proportionate number of the limitary molecules. Editor. 40 CAPILLARY ATTRACTION. [CHAP. iv. under similar circumstances, the quantity of liquid flowing out of it would - th of that delivered by the larger tube. diminish to --- g - 110. Warmth quickens the passage of fluid through minute tubes in a very great degree. The other conditions remaining the same, a capillary tube, which at 39 -2 allows one cubic inch to pass through in a given unit of time, at 99-5 permits 2-314 cubic inches to pass. It results from hence, that the higher temperature which is offered by the warm-blooded animals may, in this respect, be of great advantage. And it also explains why the flow of blood in the cutaneous capillaries is retarded, or even altogether checked, when the temperature of the skin is considerably lowered by the application of cold. 111. When a fluid passes through a capillary tube, a peripheric layer is formed, in which the particles have a much slower stream than those of the middle. This is often called the immovable layer. Its power necessarily increases with the magnitude of the resistances offered by the inner sur- face of the wall. We shall hereafter see that this obtains even in the finest capillaries of our body : although to a much less extent than in glass tubes with a larger transverse section a fact which is a fresh in- stance of the way in which Nature avoids all unnecessary loss of power. 112. The powerful influence possessed by the phenomena of adhesion in minute intervals of space leads to numerous peculiarities which receive the collective name of capillary attraction. Since all organized parts are porous, we meet with these phenomena in every organ of the body. And most of the conditions which determine the metamorphosis of matter are intimately connected with them. 113. If we dip a capillary tube (a, Fig. 15), the inner surface of which is moistened with the same fluid, into water, a watery solution, alcohol, ether, or oil, we find that it fills, not only up to the level of the sur- rounding fluid, I c, but even higher, to d e. This surplus of elevation, df, is called the capillary height. 114. If a liquid is contained in a large re- ceiver, its upper surface forms a plane, which is at right angles to all the columns of fluid de- scending in the direction of gravity. Hence it appears to us in the shape of a horizontal sur- face, while, contrary to this, the level d e, (Fig. 15) is excavated. Its greatest elevation is at the margin, and its least is in the middle. 115. If a solid and a fluid body are in a state of mutual adhesion or " prosaphy," the latter seeks to moisten the former to an indefinite extent. The cohesion or "synaphy" of the fluid, i.e. the force with which, its molecules mutually attract each other, opposes itself to this attempt. CHAP. IV.] CAPILLARY REPULSION. 41 Both these operations counteract each other in the capillary tube. And the excavated surface constitutes a visible expression of this antagonism. 116. If we select a fluid which has no adhe- sion for the wall of the tube, or which will not moisten it, we shall obtain phenomena exactly the reverse of the preceding. If we plunge the same capillary tube (a, Fig. 16) into quicksilver, or any other melted metal, the level, d e, of the fluid contained in it does not rise to the height of b c. It remains at df, below this. So that we have a capillary repulsion, instead of a capillary at- traction. At the same time the form of d e is convex; so that the highest point of its curve occupies the middle of the tube. 117. If the walls of the capillary tube consist of inorganic matter, their nature exercises no influence on the capillary height, df. Thus water rises equally in glass or metallic tubes. 118. But if the inner surface of the tube has been covered with an uninterrupted layer of fat, the water is repelled. While if it can penetrate the fatty mass, and so moisten the tube, the increase of height re- appears. The elevations of adhesive, and the depressions of non-adhesive, fluids are inversely as the size of the tubes made use of. But they also vary considerably with the nature of the fluid applied. Thus with a tube of l-2oth of an inch in diameter, and a temperature of 39*2, water rises 608, olive oil -296, and ether -208 inches. These quantities diminish at an increased temperature. Adopting the empirical formula proposed by Brunner, 5 ) the fluids just named rise -5708, -283, -1613 inches at a temperature of 98-6. 119. We may regard every part of an organ as a mass which is tra- versed by interstices in all possible directions, as shown in the diagram, Fig. 17. This structure gives the aggregate tissues the power of absorbing fluids when FlG - 17 - dry. When a liquid body presses on c, while an elastic one is present at d, it also renders them capable of serving as a filter. And finally, when two fluids with proper action on each other are subject to its influence at c and d, it gives rise to diffusion. 120. When a portion of dry animal matter is placed in a fluid which is capable of moistening it, the latter gradually penetrates its interstices ; and the whole increases in size and weight. The amount of this absorp- tion or imbibition depends upon the nature of the organized body, and of the fluid; together with the temperature, the pressure, and the duration of the operation. But hitherto there have been no experiments which 42 FILTRATION THROUGH ANIMAL MEMBRANES. [CHAP. IV. would enable us to reduce the influence of these numerous elements to definite laws. For even assuming the volumes hitherto determined to be accurate, they offer but an approximative value ; since adhesion allows layers of fluid which do not belong to the question to attach themselves to the moistened side. Nevertheless they plainly show how greatly the result depends on the nature of the fluid. 121. According to Chevreul 3j- ounces of tendon took up, in the course of twenty-four hours, 10 '8 6 cubic inches of water, 6-96 of salt-water, and 5*25 of oil. Similar experiments by Liebig have shown that alcohol is about midway between a solution of salt and oil. The nature of these fluid compounds renders these differences sufficiently explicable. The small amount of oil depends on the low degree of attraction which this fluid possesses for the animal tissues. Alcohol exhibits the same cir- cumstance, and would in addition rather shrink up than expand the mass. Watery solutions are inferior to pure water, since a certain amount of the attractive force is expended in the process of solution. Besides this, we ought not in these cases to forget that many fluids such as pure water, all incompletely saturated solutions, and alcohol not only penetrate the interstices of the organized mass, but can chemically take up some of its constituents, and essentially change others. 122. Under ordinary circumstances our skin is dry. But when brought into contact with a liquid by bathing, it gradually becomes saturated. In particular places, the fatty particles of the cutaneous secretion oppose themselves to the imbibition of the water. And the air is often obsti- nately retained in the small interstices between the skin and the larger or smaller hairs. But if these obstructing substances are dislodged, the integument becomes more and more saturated. It loses that capacity of resistance which it ordinarily possesses, and which is so necessary to the sense of touch ; and the fluid is not only imbibed by the interstices of the horny cells, but also softens their substance. 123. A filter is a porous partition, which first absorbs the liquid con- stituents of the fluid submitted to it. The pressure exercised by the numerous strata of the mixture then drives the fluid through the pores, as through a system of fine tubes. The difficulty of the transit increases with its length, and is also especially augmented by the fineness of the pores. The better kinds of filtering paper are, therefore, very thin. They thus form shorter and more simply subdivided canals. The particles of fluid, which have been pressed through, aggregate into drops on its free side of the filter ; and when gravity preponderates over cohesion, these drops finally fall. 124. Delicate animal membranes, such as the pleura, the peritoneum, or the other serous coverings, make excellent filters. The corpuscles of milk, which will pass through even good filtering-paper, are retained on these membranes. Very considerable amounts of pressure may not only CHAP. IV.] EVAPORATION THROUGH ANIMAL MEMBRANES. 43 increase the rapidity of transudation, but can also extend the skin itself ; and by thus enlarging its interstices may allow thicker fluids to pass through, which would otherwise be incapable of doing so. The albumen of an albuminous solution, or of the serum of the blood, is unable to pass with the pressure of a column of small height. But if the pressure be increased it soon transudes. 125. The evaporation which takes place through porous partitions constitutes a kind of inverted filtration. If a bent tube filled with a watery fluid be shut by an organized membrane, (Fig. 18) the mercury which limits it below will gradually ascend in the tube. The membrane becomes first soaked through FlG 18 * with fluid. The most external particles, which are in contact with the atmosphere, gradually evaporate according to the pressure, temperature, and hygro- metric condition, of the air. Other fluid neces- sarily takes its place. The uninterrupted continu- ance of this process diminishes the mass of fluid enclosed in the tube, and therefore the space which it fills. The pressure of the external air operating unchecked on the surface of the mercury, drives it up the tube as far as the tension of its contents will allow. It is obvious that such an apparatus may be used to measure the results of evaporation. The free surfaces of plants and animals, which give off water and other combinations in the form of vapours, must permit of similar indirect results. Unimpeded counter-pressure will force up compensating fluids. 126. It is quite unnecessary that the liquid mass should be in imme- diate contact with the organized partition. If it can evaporate at the existing pressure and temperature, it saturates with its vapour the space of air above. And this vapour again transudes the organized partition to become free as soon as the exterior atmosphere has also become saturated in less degree. Hence the fluid undergoes a continual diminution. If we partially fill a glass with water, and exactly close its aperture by an animal membrane, we shall find that, in spite of this, the height of the fluid gradually decreases. 127. The results of this experiment are essentially determined by the affinity which the vapour has for the isolating texture. If we use a pig's bladder and a mixture of alcohol and water, the alcohol gradually becomes more concentrated, because the pores of the membrane attract and trans- mit more watery than alcoholic vapour. And from a similar reason, if we exchange the bladder for a thin sheet of india-rubber, the reverse obtains. 128. If the interstices of a porous body imbibe a fluid of any kind, it will probably be to a certain extent condensed when it is in immediate 44 DIFFUSION OF LIQUIDS. [CHAP. iv. contact with solid walls. And if it contains solid matters in solution these may be in great proportion kept back during the nitration. A solution of salt which Matteucci conducted through 26 feet of sand was found to have lost 1-1 Oth of its previous specific gravity. 129. If we fill a vessel with water (a, Fig. 19) and plunge into it a tube, which is shut below by a porous partition b c, and is filled to a certain height, d, with a saline solution, a diffusion of the two fluids will FIG. 19. FIG. 20. be produced. The pores of b c allow the molecules of the dissolved solid to pass from d to a, and those of the water to pass from a to d. The current from without inwards (indicated by the clear arrow) is called endosmose, and that in the opposite direction, exosmose. 130. With the aid of the accompanying diagram (Fig. 20), the causes and chief conditions of the phenomena may be very clearly and simply shown. The two cavities a I c d and efg h are filled with the fluids W and S, which are separated from each other by means of the porous partition b c h e. The aperture of com- munication, iklm, corresponds with one of the interstitial openings which produce the diffusion. If W were water and S oil, no change would occur without the aid of un- usual forces ; whether iTclm were filled with the former or the latter of these fluids. The atoms of the fluids on either side would have no mutual attraction. The fundamental con- dition of endosmose consists in the affinity of z Tc f the fluids which are separated by the porous partition. Hence we say that it is only miscible fluids which are susceptible of diffusion. And CHAP. IV.] DIFFUSION OF LIQUIDS. 45 since the membranes of the living animal are soaked with water, unless under special adjuvant circumstances, oils will be rejected. 131. If we exchange the water, W, for a solution of soda or potash, diffusion is rendered possible by saponification. And since the blood- vessels and absorbents of the human body inclose the alkaline lymph and blood, they exhibit conditions which are more favourable to the taking up of fat than if they contained pure water. 132. When fluids are driven through by a mechanical force which we can express as a determinate amount of pressure, the resistance is vastly increased by a diminution in the diameter of the conducting tubes ( 109). If i k, Fig. 20, is but small, the influence which differences of hydrostatic pressure would exert on the two fluids may be disregarded as inapprecia- ble. Under such circumstances the phenomena of diffusion are therefore independent of hydrostatic influences. But if, on the other hand, one fluid can exert a much stronger pressure than the other, this causes the organized membrane to yield, to acquire larger pores, and thus to offer fewer obstacles to transudation. So that not only diffusion but filtration occurs. 133. The behaviour of the partition in this respect, the form, size, and subdivision of its interstices, and the attraction which the walls i m and k I exert on the fluid which they contain all these circumstances are liable to great variation. And they not only differ in different animal mem- branes, but also in different portions of the same bladder or other part. It is often not indifferent which side of a membrane is in contact with a particular fluid. And since these circumstances influence the strength and rapidity of diffusion, it follows that in separating the same fluids by unequal membranes we are instituting different kinds of experiments. The manifold character of the partitions which are used to transmit the fluid compounds of our bodies allows of an infinite variety of diffusive results. And since porosity is capable of being affected by the influence of the nerves and other circumstances, one and the same membrane may, at different times, produce very different results. 134. Let us suppose the partition, be he, to have been originally moistened with water, so that iklm enclosed a column of that liquid. Let W be also water, and S a solution of salt. In this case a transverse section made at i k would not meet with any foreign fluid ; while at I m it would intersect the saline solution ; a body of greater density, in which mutual attraction holds together a certain number of atoms of salt and water. Limiting our attention to this intermediate column of water, we shall find that it seeks to equalize the difference of density between i k and I m, so that its mass may contain an uniformly divided quantity of saline molecules. Molecules of salt must therefore undergo an endosmose in the direction I k, from S towards W ; and vice versa, particles of water pass outwards by exosmose in k I from W towards S. But when iklm 46 ENDOSMOMETER. [CHAP. iv. has begun to take up saline particles, the struggle for equality is repeated at both its surfaces, ik and Im. W takes up salt, and somewhat increases its density, while S is diluted with water. If no obstacle intervenes, the diffusion only ceases when the atoms of water and of salt are equally divided amongst all parts of both fluids; i.e. when both of these exhibit the same density or specific gravity. 135. The number of saline particles which pass over from S through iTclm towards W, and the number of watery particles which take the reverse path during the same period of time, will vary with the nature of the substances dissolved in S. Other circumstances remaining unaltered, there are about four units of water to one of salt, and twelve of water to one of sulphate of soda. Jolly, 6 ) who first drew attention to these pro- portions, named them the endosmotic equivalents of the particular bodies. 136. Since these results are so variously affected by the density of the solution, by the nature of the membrane, and by temperature and pressure, it is not difficult to explain why different endosmotic equivalents have been obtained from a series of different experiments made with two similar fluids. If it were possible to reduce all but one of these conditions to an exact equality, the differences in the endosmotic equivalents would per- mit a more exact investigation of this remaining one. 137. The phenomena of diffusion have been investigated in two ways by volume and by weight. Here, as in most other cases, the use of the balance is greatly to be preferred, since the deter- mination of space is open to many errors which are scarcely to be avoided, and often not to be remedied. 138. Fig. 21 exhibits the endosmometer first used by Dutrochet, which gives at least an approximation to the changes in the volume of the diffused fluids. A vessel (6, Fig. 21) which contains one of the fluids, and is shut below by the porous membrane c d, is connected above with a tube, a. This is fixed verti- cally, and is plunged to a certain depth into the second fluid, n. If b is originally a solution of salt, and n pure water, the former is gradually diluted and increased in volume. If the saline solution at first stood on the same level with n, it rises in course of time towards n' ; while the surface of n is depressed. And by means of a scale the rise of the fluid in the ascending tube a may be followed numerically. If the capacity of b is also known, the increased bulk of the saline solution may be easily estimated. This arrangement has the disadvantage, that the hydrostatic pressure upon the membrane b c becomes greater, the more n rises in the tube. On FIG. 21. CHAP. IV.] ENDOSMOTIC EQUIVALENTS. 47 this account, Vierordt ?) constructed an endosmometer in which this is somewhat provided against. If we determine the interchange of fluids by the alteration of weight instead of size, we may constantly depress the inner tube in the course of the experiment, so that no considerable excess of pressure can ever occur. The error which evaporation allows of is best avoided by making air-tight the vessel whose contents diminish in volume, and closing the other one with two layers of asbestos and sulphuric acid. The first, which receives the vapours of the endosmotic fluid, may be weighed with it ; but the second, which is to guard against the evapora- tion of the air, must be removed before every weighing. 8 139. Supposing d, Fig. 22, to be a saturated solution of salt, and a dis- tilled water, the diffusion will continue until the densities of a and d are as nearly identical as collateral circumstances will permit. If the quan- tity of d be only a small fraction of a, still a and d will contain more or less considerable per-centages of salt ; while, on the other hand, if d is originally very small, or a very large, we get at last so weak a solution of salt in both vessels that we may almost disregard the per-centage of solid matter, and consider the fluid contained in d as distilled water. And if a be repeatedly filled up with pure water, the like will happen, even when its quantity is much less considerable. Jolly made use of this method to determine the endosmotic equivalents. If the tube shut by b c contains a certain quantity of a soluble solid, or a solution of the same, while the distilled water a is frequently changed, the resulting fluid finally becomes so dilute that it may be fairly regarded as pure water. The proportion of the weight of solid matter originally present to that of. the fluid which has entered gives the endosmotic equivalent. Thus, for instance, when the inner tube originally contained 37 grains of dry salt, it showed at the end of the experiment 142-95 grains of water. Hence the endosmotic equivalent here amounted to 3-99. If a solution of salt had been used containing 15-4 grains of salt and 46 -33 grains of water, under similar circumstances 107-646 grains of water would have been finally present. 140. If we arrange in a series the endosmotic equivalents, as obtained by Jolly by means of a pig's bladder prepared with alcohol, they will be as follows : Substance. Endosmotic Equivalent. Substance. Endosmotic Equivalent. Hydrated Sulphuric ) Acid . . . $ Bisulphate of Potash 308 to -391 2-345 Sulphate of the Oxide } of Copper . . $ 9-564 1179? Salt 820 to 4-58 Sulphate of Soda . . 11 -033 to 12-44 Alcohol 4-140 to 4-336 7-064 to 7'25 Sulphate of Magnesia Sulphate of Potash . 11-503 to 11-802 Jl-42 to 1276 Hydrate of Potash . 200-09 to 231-4 48 CIRCUMSTANCES AFFECTING THE DIFFUSION OF LIQUIDS. [CHAP. IV. Consequently the equivalent of hydrate of sulphuric acid is on an average 617 times as small as that of hydrate of potash. 141. We have already seen how the ascent of water, ether, and oil diminishes under the influence of a higher temperature ; while on the other hand, it increases the amounts passing through a capillary tube. The latter phenomenon is more connected with adhesion, the former with cohesion. It had been previously remarked, that under higher tempera- tures endosmose was increased. Jolly found that the endosmotic equi- valent increased with increased temperature. Glaubersalt had an equiva- lent of 11-07 at 42-08, and of 19-53 at 80-6. Common salt showed the reverse of this : its equivalent was 4-43 at 32-45 and 4-12 at 53-15. 142. We may easily assume that, other things being equal, the quanti- ties of substances transferred within a definite time correspond to the difference in the densities of the two fluids. But since the longer the diffusion lasts, the closer becomes their equality, it follows that the same unit of time effects a less change of weight at a later than at an earlier period. Hence it follows, that the rapidity of endosmose increases with the difference in density, and is therefore greatest at the commencement of the process. In a series of experiments, Vierordt9) allowed 6-103 cubic inches of a saline solution of different degrees of density to act upon 6-103 cubic inches of water, through 2 square inches of bladder : -and examined the bulk five hours after the commencement of diffusion. Where the 6-103 cubic inches had contained half an ounce of salt, the water had lost -214 cubic inches. When the original quantity of salt was 1-066 ounces, it had lost -32894 cubic inches. Although the temperature in both cases amounted to 50, the more concentrated saline solution exhibited a lesser decrease of the water opposed to it, and consequently a smaller increase of its own volume. Since there are here only 6-103 cubic inches of water opposed to the same quantity of saline solution, they must take up more salt from the dense than from the dilute fluid in this long space of time. So that the process of equalization is at first to a certain extent accelerated for the concentrated mixture. But since the changes continually diminish as the period of equalization approaches, the more concentrated solution is then placed at a disadvantage compared with the more dilute one. 143. Tenacious fluids may, under circumstances otherwise identical, form a temporary or permanent hindrance to diffusion. A solution of albumen, when mixed with intestinal mucus, takes up less water. Ac- cording to Vierordt, a saline solution containing 5*242 ounces of gum, and having a bulk of 6-1 cubic inches, increases one-fourth less than when the viscous mixture is not present. Similar differences affect the initial rapidity of endosmose. Bruecke separated water and a solution of albumen, or serum of the blood, by the shell membrane of the egg. He CHAP. IV.] COLLATERAL CIRCUMSTANCES AFFECTING DIFFUSION. 49 found that the salt first passed through, with a small quantity of organic matter; while the albumen came later. Similar phenomena are seen in the secretions of the living body. 144. When an animal membrane is extended by a considerable hydro- static pressure, diffusion is facilitated in three ways. The active surface is increased, as well as the diameter of the interstices, while new pores are here and there produced. Hence under such circumstances, albumen transudes in much greater quantity to gain the water on the other side. And tenacious mixtures, which from the smallness of the interstices were hitherto altogether retained, can now take part in the diffusive currents. 145. When two different solutions are separated by a porous partition, a chemical attraction is added to the other causes of this mutual opera- tion. If a precipitate be formed, it may fill the interstices, so as first to diminish diffusion, and then altogether to prevent it. If the pores are filled with a mass which can only be expelled with difficulty or not at all, similar effects are produced. 146. Even if thicker membranes be made use of, such as the coats of the small intestine, the aorta, or the inferior cava, still the first endos- motic fluid transudes in a very short time. A solution of prussiate of potash, which diffuses itself with a solution of chloride of iron, requires less than a second to pass through the mucous membrane of the small intestine and its ordinary fluid (amounting to -059 inch), under a pressure of -063 inch of mercury. And if the pressure be increased to from 1*18 to 1*57 inch of mercury, the time occupied is so short as to be quite inappreciable. So that in the living tissues diffusion is almost instantaneous when the necessary conditions are present. It may be approximatively estimated, that the first interchange of matter effected by the walls of the capillaries of our body occurs in l-300th to l-800th of a second, at a pressure of only -05532 inch of mercury. 147. If we connect a blood-vessel a (Fig. 22), with a funnel b, and allow a fluid continually to flow towards e, while the surrounding mix- ture remains at rest, the phenomena of diffusion must be greatly facili- tated. For at every instant, fresh particles of the fluid in a are brought into mutual action with d. The difference of density will be maintained at a higher point than if the fluid in a were at rest. The diffusion is thus increased, especially when the duration of time necessary to the mutual action of the fluids is less than the velocity with which the mole- cules of one of them pass. It is hence evident, that the movement of the blood is of great use in the phenomena of the interchange of matter now under consideration. 148. When the molecules of a fluid transude a porous partition, it is by no means a matter of indifference whether they pass in the vaporous E 50 DIFFUSION OF GASES. [CHAP. IV. or liquid form. The same membrane may behave very differently in the two cases. Thus our integuments, in their ordinary state of dryness, allow an easy transit to watery vapour in large quantity ; while liquid water is altogether prevented from passing. FIG. 22. 149. Under equal pressure, two dry gases, which are chemically indif- ferent to each other, and separated by a porous substance, are exchanged in inverse proportion to the square roots of their densities. This law of the diffusion of gases, which may be theoretically deduced from the movement of fluids, is named Graham's law, he having originally proved it by experiment. Taking the density of the atmosphere at 2 9 '92 inches barometer, and 32, as unity, the specific gravity of oxygen is 1-10563, and that of carbonic acid 1-5291. When these two gases diffuse themselves under the circumstances mentioned above, '85 volumes of carbonic acid are exchanged with 1 of oxygen. Thus the vessel containing carbonic acid receives 3-20ths more oxygen than it gives off carbonic acid. 150. Porous solid bodies and liquid masses can take up part of the air in contact with them quite independently of any chemical affinity. Other circumstances being equal, the quantities thus taken up vary with the nature of the absorbing and absorbed substances. Every liquid possesses a specific capacity of absorption for any given gas. For instance, one volume of water takes up -05 volumes or a twentieth of its bulk of atmospheric air. And when deprived of this air, it absorbs 042 volumes of nitrogen, -065 of oxygen, and 1-06 of carbonic acid, or 43-78 of sulphurous acid gas. One volume of alcohol absorbs 2-60 volumes of air, one volume of ether, 2-17. A volume of a nearly saturated solution of salt only takes up -329, and a volume of solution of chloride of calcium only -261, volumes. CHAP. IV.] ABSORPTION OF GASES. 51 151. If a number of gases be presented to one fluid, each is absorbed in a different proportion to that in which it would be taken up if alone. One volume of water at 64-4 was exposed to 3-90 volumes of a mixture of equal bulks of oxygen and carbonic acid: it took up -471 volumes of the latter, and only -05 volumes of the former gas. 152. So long as other circumstances, and especially those of tem- perature, remain the same, pressure exerts no influence on the volumes of the different gases taken up by any particular fluid. One volume of water always absorbs -065 volumes of oxygen, whether the external pressure amounts to 29 -92 or 14-96 inches. Omitting the exceptions to Mariotte's law already mentioned ( 68), the weight of a definite volume of a gas increases in direct proportion to the pressure exerted upon it. So that the weight of the quantity of gas absorbed will increase or decrease with its tension. If 61 cubic inches of water take up 3-9668 cubic inches of oxygen, this bulk, which at 29*92 in. barometer, and 32, weighs 1-4363 grains, at 14-96 in. barometer, or half the pressure, weighs but -718 grains, or is only half the quantity. 153. The Daltonian theory rests upon this law laid down by Henry, and upon another proposition which is derived from the laws of vapour, and will be considered hereafter. Let us suppose a space of 6-103 cubic inches to be filled with atmospheric air, which contains 21 volumes per cent, of oxygen, and 79 of nitrogen. Each of these gases is exposed to a pressure of 29-92 in. barometer, or a single atmosphere; therefore each has the tension which it would possess as the sole occupant of this space. If the 1-28 cubic inches occupied 6-103, their tension would sink from 1- to '21 atmospheres, and similarly that of the nitrogen would be -79. If the temperature remain unchanged, every fluid will absorb the same volume under all pressures, and Dalton supposes that this equally obtains with each gas of a mixture. But we must calculate its tension as though it occupied the entire space; and make this estimate at the end of the process of absorption. These conditions are best obtained by exposing to the atmosphere a vessel filled with water which has been deprived of all air; since the quantities of nitrogen and oxygen absorbed remain utterly inconsider- able in comparison with those contained in the whole atmosphere. Hence neither the bulk nor the pressure of the gases experience the slightest change during the course of absorption. If we had exposed one volume of water free from air to pure oxygen, it would have absorbed -065 volumes under the pressure of one atmo- sphere. If it be exposed to air it absorbs just as much oxygen, with the difference, that this has a tension, not of 1-, but of -21. If we reduce this to a pressure of one atmosphere we get -065 x -21 = -01 365. Since one volume of water would take up -042 nitrogen, if this alone were present, 52 DALTONIAN THEORY. [CHAP. IV. we get *042 x '79 = -0331 8. Hence one volume of water absorbs a total of -04683 volumes of air, composed of -01365 volumes of oxygen, and 03318 volumes of nitrogen ( 150). 154. If this experiment be repeated in a closed vessel, the circum- stances vary at every instant. Since the water takes up unequal quan- tities of the two gases, either their volumes become altered during the time of absorption, or supposing that this is artificially prevented, their tension is affected. Hence a better method of estimation must be in- troduced. 155. Keeping to the Daltonian hypothesis, that one gas has no influence on the pressure of another, and may therefore be regarded as a vacuum in this respect, we may easily determine the exchange which takes place between a liquid containing a certain gas and a space of air. We will suppose a non-oxidizable liquid, saturated with carbonic acid, to be exposed to the atmosphere, which we will assume to be devoid of this gas : the carbonic acid contained in the liquid will behave just as if the space above it were a vacuum. It will continue to 'diffuse itself into the air, until the tension of the quantity there present equals that portion of it which is retained by the liquid. And vice versa, the oxygen of the air is absorbed just as if it alone had been present, and had divided itself over the whole of this limited space. The behaviour of the nitrogen depends on exactly the same causes. And if a given volume of air be used in the operation, a more careful theoretical consideration will show, that its proportion to the volume of the liquid must also be considered. The Daltonian theory has not yet been fully corroborated by experi- ment. Many phenomena, such as for instance the aeriform contents of snow-water, are rather against than for it. The experiments made upon the proportionate absorption of gases depend almost solely upon older and more insecure methods of Eudiometry, the results of which do not offer the delicacy requisite for a satisfactory proof of this theory. On the whole it is probable that the main laws enunciated by Dalton approxi- mate to the truth. But it is not unlikely that their elementary condi- tions differ from those which his theory would suppose. Since the blood which circulates in the blood-vessels of the lungs gives off carbonic acid and very small quantities of nitrogen, it is evident that porous membranes, which separate a liquid from a space filled with air, must allow of an interchange of gases. But more accurate physical research fails us. It is therefore impossible even to theorize on the details of the result. 156. The vibrations of elastic bodies play an important part in the vital functions of the organism. The perception of such a change consti- tutes the function of the two highest organs of sense. The eye perceives the undulations of the luminous sether; and the ear those of ponderable matter, whether fluid or solid. Since we shall hereafter examine in CHAP. IV.] UNDULATOEY MOVEMENTS. 53 detail into the functions of these organs, we need here only acquaint our- selves with the general relations of the tissues of our bodies to LIGHT. 157. When a molecule, a, Fig. 24, of a substance which is uniformly elastic in all directions, falls into a state of vibration, the disturbance is propagated equally on all sides. Circular waves are produced, as shown at Fig. 24. This is best seen by looking at a surface of stagnant water, into which a stone has been cast. Any radius whatever for instance, FIG. 23. FIG. 24. a c, Fig. 24, which is drawn from the centre, a, to any point of the circle or sphere around it, forms a ray of undulation. The time necessary to communicate the disturbance from one molecule to another along a ray of given length, from a to & or c, constitutes the velocity of propagation. Taking an unit of a second of time, this velocity amounts to 928 millions of feet for light, and to 13071 feet for atmospheric air at 32. 158. In considering the kinds of vibrations recognized by the eye and the ear, we find an essential difference. If a molecule, 6, Fig. 25, be disturbed, it may possibly move to and fro in all imaginable planes. But, according to Fresnel, the impression of light depends solely on those vibrations which occur at right angles to the direction of swift propaga- tion, a c, or in the transverse plane, f g. Sounds depend on exactly the reverse circumstances : they proceed from the molecular movements which correspond to the vertical longitudinal plane. Hence we see the trans- verse vibrations of the particles of the luminous ether, and hear the longi- tudinal ones of ponderable elastic substances. 159. Supposing b c d e, Fig. 25, to represent a part of the transverse plane, f g, Fig. 24, the luminous molecule, a, may vibrate in all possible directions, f g, Ji i, k I. In ordinary light, this uncertainty, and the change of path thus per- mitted, really obtains. But on the other hand, polarized light has a predetermined, fixed, and one-sided course. If it be rectilineally polarized, a vibrates in only one definite path ; for instance, in f g. This f g is therefore the projection of that transverse plane of vibration possessed FIG. 25. 54 ORDINARY AND POLARIZED RAYS OF LIGHT. [CHAP. IV. by those particles of the sether which occupy the plane of polarization, k I. But besides a rectilineal, there is a circular and an elliptical polarization : the names of which sufficiently indicate the difference in their modes of vibration. 160. We have hitherto assumed that the vibrating luminous sether is equally elastic in all directions. And this is certainly true of those bodies which refract light simply. But on the other hand, there are bodies which possess a power of double refraction, so that the elasticity of the luminous sether which they contain varies with the three different dimensions of space. We may best depict this fact by representing the degree of elasticity in any given dimension as a line, which may be named the axis of elasti- city. We get, in all, three such axes; one (Fig. 26), a b, from before backwards ; one, c d, transverse to this from right to left; and a third, ef, which is perpendicular to both, and passes from above downwards. These lines, a b, c d, ef, have equal lengths in all simply refractive substances, such as air, water, or common glass. But if this is not the case, we get a doubly refractive body. In such circumstances, one of two things may occur. Two of the axes of elasticity for instance, a b and c d are alike, while the third is different : or all three may exhibit different lengths. The first case obtains in bodies which have one axis of double refraction : and the second in those which have two such axes. Calcareous spar and phosphate of lime are optically uniaxial ; while mica, sulphate of lime, potassio-tartrate of soda, and sugar, are binaxial. 161. If a ray of light passes out of one medium into another of dif- ferent density, it is bent aside out of its rectilineal course. This change of its course produces the refraction of light. Its amount depends directly on the nature of the two media traversed by the light. So that when a ray of light coming out of the air passes through glass, the result may be estimated by that index of refraction which is offered by glass in connection with the atmosphere. The different colours red, orange, yellow, green, light blue, dark blue, and violet form a second element of the change. Thus the red rays experience the least, and the violet the greatest refraction. If the former pass from air into water they have an index of refraction amounting to but 1-3309 ; while the latter have one of 1-3442. If we suppose that a colourless ray, I i, which contains a mixture of all possible coloured undulations, has to pass through the body, a b, Fig. 27, whose limitary surfaces are not parallel, the red rays will appear as i r, and the violet as i u. And if the several tracts of colorific undulations, r e and u e, emerge in such a way as to allow their differences of colour to be verified by the eye, it is evident CHAP. IV.] CHROMATIC ABERRATION. 55 that we shall get a coloured image instead of a colourless one. On this fact depends the chromasia or coloration of a lens, and of the human eye. And it FlG - 27 - hence becomes an object to make the microscope and telescope achromatic : i.e. to arrange them so that as little colour as possible may be produced by the refraction of light. 162. Let us suppose that A , Fig. 28, is the path of a ray of light, and that the movement of the molecules of luminous aether begins at b j it will require a certain time to propagate the disturbance from hence to B. Hence by the time that its neighbouring molecule begins to move, b must have already travelled over a part of its path of semivibration, bb'. And since this movement proceeds from atom to atom, there is a mole- cule, c, which only begins to vibrate when b has completed a perfect cycle of vibration, from b to &', back again from b' to b, and thence from b to b", and back from b" to b. The distance from b to c is called the length of a wave. It is the interval between two molecules, which vibrate similarly at all periods of time, and one of which precedes the other by a single complete and perfect vibration. Under these circumstances bf corresponds to half an undulation. 1C 3. The colours differ in the length of their waves. Each undula- tion of the most external red of the prismatic spectrum has a length of 25J millionths of an inch ; while that of the other rays, or the interval b c, Fig. 28, is for orange 23, yellow 21 f, green 20, light blue 18f, dark blue 17f, and extreme violet 16, millionths. 164. If the velocity of propagation be unchanged, the number of vibrations occurring in an unit of time must obviously be inversely as the length of the undulations, so that the extreme red will have the smallest, and the extreme violet the greatest number. The former has 439, and the latter 697, billions of vibrations in the second. But this is only true of the atmosphere. If a ray of light passes into a more refrac- tive medium, the velocity of its propagation is diminished. 165. The rest or movement of a molecule of the aether, b, Fig. 28, will depend on the amount and direction of the forces by which it is acted upon. Supposing that two excitements of equal amount impel it in the same path, the intensity of its vibration will be doubled. But as it is this INTERFERENCE OF LIGHT. [CHAP. IV. which determines the luminous intensity of each elementary ray, this latter is also increased. On the other hand, if we imagine that one force seeks to impel b. Fig. 28, towards b b", and a second of equal amount at the same time urges b towards bb', the two disturbances must mutually destroy each other. And the molecule b remains at rest, i.e. in spite of the double impulse, darkness is produced. This phenomena is called the complete interference of light. 166. Let us imagine A B and C D, Fig. 29, to be two rays of light which meet each other at an inconceivably small angle, and whose cor- responding undulations take a similar course, the molecules b c d d' will vibrate with a velocity twice as great as if only one of the two rays FIG. 29. were present. But this can only happen when the path of the rays differs by twice, thrice, or generally by some multiple of the length of an undulation a d'. Here the light is increased. While, on the other hand, where the paths differ by half the length of an undulation, we get a complete interference, such as is exhibited at Fig. 30. The molecules of FIG. 30. the Eether, which lies midway between a and c, would vibrate till b, if only the ray A B were present. But it is evident that the dotted track of undulation which corresponds to C D, simultaneously moves upward with precisely the same force. Therefore it remains at rest. But it is obvious, that any difference in the paths of the rays A B and C D which amounts to half an undulation, or an inexact multiple of the same, must produce this mutual annihilation of the forces. It is therefore this difference which determines whether light, added to light, produces light, or darkness. 167. When the vibrations of the luminous sether are compelled to propagate themselves through a narrow fissure, their undulations suffer certain changes, the results of which are named inflection of light. The phenomenon may be illustrated by comparing it with the meeting and subsequent diffusion of the waves in water streaming through a small sluice. Such an alteration in the waves of light leads to the most mani- fold phenomena of interference. CHAP. IV.] INFLECTION OF LIGHT. 57 FIG. 31. 168. If rays of light of only one colour, which have passed through a very small round opening, are received upon a screen, or looked at with a magnifying-glass, we see in the middle a bright and coloured circular area, which is surrounded by a series of dark and light rings, as shown at Fig. 31. The complete interference of the luminous waves which diffuse themselves behind the narrow orifice pro- duces the black circular bands, the breadth of which depends on the length of the undulations. Hence the appearance differs with different colours. For instance, if we interpose a red glass between the solar ray and the aperture through which it passes, we get the broadest stripes, because the luminous undulations which interfere are in such a case the longest. On the other hand, violet exhibits the smallest bands. So that we have here a means of measuring the length of the waves in the different colours. 169. If we repeat this experiment with colourless sunlight, we find that the bright median area is enclosed by a series of circular bands possessing all the colours of the rainbow. Colourless light consists of luminous waves of all possible colours in a state of mixture, which renders it impossible for the eye to detect the particular ingredients. But since the breadths of the dark and light bands vary with the difference in length of the waves of the several colours, many of these are almost or quite extinguished in particular places, while others gain in luminous intensity. Hence what is called colourless light produces interferential or eritoptic colours. These are found when light has to pass through very small apertures, or through thin layers of fluid, or unequal strata of solid, bodies. The iridescence of tendons, of the tapetum in the eye of some mammalia, and of the peritoneum of some fishes, together with the reddish -colour exhibited by the fibrous-like elements of the areolar tissue of some animals when under high magnifying powers, are instances of such physical and accidental developments of colours which do not originally exist. 170. When a ray of light, a b (Fig. 32), has to pass through a simply refractive body, it experiences a deviation which is exactly proportionate to the refrangibility of this substance. For instance, it passes towards b c or b d, according as this exerts a more or less powerful influence. In all cases it re- mains single, as at first. But if it passes through calcareous spar, or any other doubly refracting body, it exhibits for the most part two paths, or two rays, b c and b d ; so that on looking through it we are able to see two images of one and the same object. The one, called the ordinary ray, follows the laws of FIG. 32. 58 DOUBLE REFRACTION. [CHAP. IV. ordinary refraction, and has a definite index of refraction. But the second, or extraordinary ray, presents an index of refraction which, within certain limits, increases or decreases in proportion to the direc- tion of the original ray at its entrance. Both rays contain polarized light, and their planes of polarization are at right angles to each other. If we suppose b c d e, (Fig. 25, p. 53) to be the transverse plane which is perpendicular to the ray of light, and fg to be the plane of polariza- tion of the ordinary ray, then if / a k be a right angle, Ic I will be that of the extraordinary ray. 171. A Nicol's prism consists of two prisms of calcareous spar, ab d, and d b c, (Fig. 33,) which are cut in definite directions, and are united by means of Canada balsam at b d. If a ray of light passes upwards through it from b c, the ordinary image is reflected by the Canada balsam, b d, while, on the other hand, the extraordinary ray passes out at a d ; and since the main sections a b d and a b c are parallel, it is single and not double. So that such a prism has the advantage of offering only one polarized luminous image. 172. Many optical properties of the animal tissues are examined by means of a microscope provided with a polarizing apparatus such as is delineated at Fig. 34. A Nicol's prism, b, is fixed under the stage, between the illuminating mirror, /, and the object disc, e. The latter is divided into parts of a circle, and is capable of being rotated horizon- tally. The uppermost part of the microscope tube, a, bears a circular plate, d, divided into 360 degrees, on which the index of the frame around a second such prism, c, plays horizontally. This latter is fixed upon the eye-piece of the microscope, and is called the analyzing prism, while that below is the polarizing one. If we imagine that c is absent, and that all side-light is cut off by a screen, the object present at e can only be perceived in polarized light by means of the lower prism b. And, on adding the upper Nicol's prism, we obtain results which are modified by its position. Supposing that fg (Fig. 25, p. 53) is the plane of polarization of those rays of light which emerge from the under prism, they will pass unimpeded when the upper one presents a plane of polarization having a similar posi- tion. This, for instance, is the case when the graduated disc, d (Fig. 34), rests at and 180 ; we then get the greatest possible strength of illumina- tion. On the other hand, if we turn the upper prism, c (Fig. 34), so that the index points to 90 or 270, its plane of polarization corresponds to k I (Fig. 25, p. 53); while the pencils of light ascending from the lower Nicol are polarized in the plane fg. So that those rays of light, the POLARIZING MICROSCOPE. 59 CHAP. IV.] molecules of which vibrate at right angles to the plane of polarization, cannot pass through the upper prism, and we get the greatest possible obscurity. And since all intermediate directions allow one part of the light to pass, and repulse another, we might argue, ct, priori, what phe- nomena must result from our changing the position of the upper prism. If we turn it round a whole circle, we shall have two places of greatest illumination, viz. and 180, and two of greatest obscurity, 90 and 270; the illumination will decrease from to 90, and from 180 to 270; and will increase from 90 to 180, or from 270 to 0. FIG. 34. 173. This fact sufficiently explains the way in which we may deter- mine the saccharine or albuminous constituents of an animal fluid for instance, of the urine by means of the polarizing apparatus. Many fluids and gases and amongst solids, the rock-crystal possess the property of causing the plane of polarization of those rays of light which 60 CIRCULAR POLARIZATION. [CHAP. IV. pass through them to deviate in the horizontal direction to a definite angle. Suppose that this plane occupied f g, Fig. 25, p. 53, it would be in h i when moved towards the right, and in m n when towards the left. In this way what is called circular polarization is produced : and the body which causes it, is said to possess a certain rotative capacity. Pure powdered starch as for instance, inulin turns polarized light towards the left ( -^ ), while the dextrine into which it is trans- formed, as it undergoes fermentation to become grape-sugar, turns it towards the right ( >> ). And if grape-sugar has not been solid, but has been from the beginning in a state of solution, it turns the plane of polarization towards the left side. Pure cane-sugar, sugar-candy, and sugar of milk, exhibit a deviation towards the left. Other circumstances being equal, the magnitude of the angle of rotation depends on the density of the solution. For instance, in a solution containing only I per cent, of sugar-candy, the rotation forms an arc of 53' : but with II per cent, it amounts to 10 10'. Let us imagine we had interposed a tube (Fig. 35) closed below by a transparent glass plate, between the polarizing and analysing prism ; the relations of brightness and darkness, just mentioned, FIG. 35. would be no way altered by the fact of this tube j containing water or any other indifferent fluid. If the upper prism corresponds to/ where b ^H^^ it mixes with the bile and pancreatic fluid. The former secretion may flow downwards either by the hepatic (n) duct, or by the cystic (m)from the gall- bladder (). Both of these canals are united to form the ductus com- munis choledochus, or common biliary duct, which is visible at r, and which penetrates the descending duodenum close to the excretory duct CHAP. VI.] MOVEMENT OF THE SMALL INTESTINES. 133 of the pancreas. These efferent canals of the liver and pancreas pass for a certain distance between the muscular fibres of the duodenum, before they open on an elevation of mucous membrane in the cavity of the intestine. They are thus closed at the instant in which their neigh- bouring muscular fibres contract; and are protected in this way from any disturbing reflux of fluid chyme. Their own secretions only pass into the duodenum at certain suitable times. 398. The small intestines (s, Fig. 9, p. 34) gradually drive their con- tents onward. But this peristaltic movement is slower and more inter- rupted than might be expected from many experiments instituted on dead animals. If we open the abdominal cavity of a suffocated rabbit, we generally find a vigorous and tempestuous movement going on in many parts of the small intestines. But it would be a great mistake to conclude that a similar storm of peristalsis obtains in the living human subject. If we examine the intestines of a living rabbit we find them much more tran- quil. The small intestine of mice which have been killed by the vapour of ether often exhibits not a trace of peristalsis, even under the influence of the electro-magnetic machine. In surgical and obstetric operations as, for instance, in hernia or the caesarean section it occasionally hap- pens that some loops of small intestine are exposed ; but they are either altogether still, or at any rate their contractions are less vigorous than those sometimes seen in criminals after execution. And persons who have a fistula of the small intestine after an operation for hernia (i.e. who have in the intestine an opening which passes through the wall of the belly to the outside) only pass fluid faecal matters at intervals. This fact affords a fresh corroboration to the statement that undigested food generally passes but slowly through the small intestine. 399. An examination of the dead rabbit proves that the peristaltic movements exhibit certain alternations of activity and repose. If dab c Fig. 76, be a segment of small intestine, the vermicular movement will pass gradually from d to c, the contraction and relaxation moving on- wards as in the oesophagus ( 380), except that the undulations are slower and shorter. The movement is often arrested at c. But it not unfre- quently begins anew at d, to pass through an equal, d c, & shorter, a b, or a longer, distance. Every undulatory contraction can drive the contents forwards either completely, or imperfectly, or not at all, according as the maximum force of contraction can produce a contact of opposite points in a transverse section of the tube, or can only approximate them to each other. And if we add to this that the vermicular contraction is not always peristaltic, but sometimes antiperistaltic,* it will follow, that * It appears highly improbable that peristalsis is ever reversed in any part of the living intestinal canal, while the contractions here alluded to are so irregular, that the rule might almost be extended to the dead subject. Editor. 134 VALVES OF THE INTESTINE. [CHAP. vi. the contents of the small intestines of a recently killed rabbit are urged onwards more slowly than appears at first sight to be the case. And we have already seen that the velocity is still less in the living animal. Whether it is greater in the jejunum than in the ileum is at present unknown. FIG. 76. FIG. 77. 400. The commencement of the small intestine has its pyloric valve (h, Fig. 74, p. 129), which tends to prevent an antiperistaltic reflux of the chyme into the stomach. The end of the ileum (a, Fig. 77), pos- sesses the ilio-ccecal valve, which executes a similar service. Under ordi- nary circumstances this renders it impossible that there should be any antiperistalsis of the contents of the caecum (b) or the ascending colon (c and /) towards the small intestine (a) ; while on the other hand, it easily allows all substances present in a to pass through its fissure k towards b and c. 401. The pyloric valve forms a double fold of mucous membrane, which is almost levelled by the removal of the corresponding muscular bands. The ilio-coecal valve (h c, Fig. 77) is formed by the peculiar mode in which the ileum (a) sinks into the common commence- ment of the csecum (b) and the ascending colon (/). And if in the dead body we in- ject fluid in the direction from / towards a, he is often so completely shut, that the cor- responding portion of the large intestine bursts rather than allow the fluid to pass into the ileum. Something similar to this is generally repeated in the living subject. It is only in the most CHAP. VI.] MOVEMENT OF THE LARGE INTESTINES. 135 urgent cases that excrements pass into the small intestine and thence upwards. Hence the genuine faecal vomiting as distinguished from that apparently stercoraceous vomiting, by means of which the con- tents of the small intestine are discharged from the mouth only occurs in those obstinate cases of strangulated hernia of the large intestine in which the exit per anum is immutably shut. 402. When the residue of the food contained in the ileum (a, Fig. 77) is impelled through the fissure (k) of the ilio-colic valve (h c), two ways are open, that toward the caecum (6), and that towards the ascending colon (/). But it is probable that the muscular fibres here present have a definite mode of action. A large part of the food is frequently found in the caecum ; indeed, in well nourished herbivorous animals it is usually more or less thus distended. The vermiform appendix d, also receives alimentary matters. Its narrowness may be the cause of danger to life. For instance, a cherry-stone which has been swallowed now and then sticks fast in the vermiform appendix, and gives rise to inflammation and suppuration. And if this produces perforation, the faecal matters pass into the abdominal cavity, and lead to an inflammation of the peritoneum which is generally fatal. 403. The remains of the food, after spending a certain time in the caecum, subsequently enter the commencement of the colon : so that we have here a definite alternation of movement in two opposite directions. 404. The caecum (6), the ascending (/, Fig. 77), the transverse (I, Fig. 9, p. 34), and the descending colon u, Fig. 9, possess greatly dilated pouches or sacculi (e, Fig. 77), constituting a kind of supple- mentary cavities, which increase the surface of contact, and retard the movement of the substances occupying their interior. The falciform folds (g, Fig. 77) which occupy the transverse constrictions of their inte- rior have the same direct use as the valvulae conniventes of the small intestine. They retard the advance of the substances which glide along their cavities, so that certain portions are confined between them. And since they are occupied by transverse or circular muscular fibres which take the same direction, they may occlude corresponding portions of the large intestine. It probably depends on the mechanism of the falciform fold g that the remains of the food a descending from the ileum pass first into the caecum b, and not into the ascending colon, /. The longi- tudinal bands which are indicated at /, Fig. 77, and which proceed from an aggregation of the longitudinal muscular fibres, shorten the large intestine in a direction from the ileum towards the rectum : they can also to a certain extent depress it, and under many circumstances are capable of widening it. 405. On observing the movement of the large intestine in a newly killed rabbit, we find a recurrence of the same phenomena which we have already seen in the small intestine. We not unfrequently remark 136 MOVEMENTS OF THE RECTUM. [CHAP. VI. undulations, which pass more or less completely around it, and the effect of which on the contents seems to be quite superficial, so as not to drive them on to any visible extent. To this it may be added, that the move- ments during life are probably slower, and only return after certain intervals of rest : while the more solid faecal masses present greater resist- ance, and the mechanical obstacles of the intestine itself are also greater. Hence it follows, that the forward movement of the large intestines is even slower than that of the small. The phenomena of constipation corroborate this view. 406. The uniformly cylindrical rectum (y, Fig. 9, p. 34) possesses a muscular coat which is of great proportional strength, and which, as far as the anus, is composed of unstriped fibres similar to those of the stomach and the small and large intestines. The movements which it exhibits in the newly killed animal, as a result of irritation of its nerves, are, however, essentially different from those of the remainder of the alimentary canal. It moves up and down by fits and starts, and some- times the fsecal pellets are visibly expelled by the aid of a vigorous peri- stalsis. Finally, sections which have just emptied themselves may be seen to collapse, so as to prevent the reflux of the contents upwards. 407. The sigmoid flexure of the colon (v, Fig. 9, p. 34) which depends from a loop of mesentery, and through which the descending colon t passes into the rectum y, probably often serves as a receptacle of faeces. Its free attachment allows it to change its position according to its weight, while its curvature prevents too much being impelled at any one time from the colon into the rectum. 408. Since the lowest part of the rectum lies outside the cavity of the peritoneum y, Fig. 9, the less hindrance is offered by the pelvic struc- tures, the more easily will the abdominal pressure ( 393) urge the faeces towards the anus. And if these parts assist, the evacuation will be so much the more easily effected. 409. The peristaltic or impulsive movements of the rectum probably give rise to the feeling of necessity for an evacuation. Under certain abnormal conditions for instance, in violent diarrhoeas it sometimes happens that a person feels compelled to go to stool every instant, but in spite of every exertion, is unable to evacuate anything, or, at most, only small quantities of faeces, mucus, or blood. It is the irritation of the mucous membrane of the rectum, and the movements of the terminal portion of the alimentary canal thus produced, that give rise to these deceptive sensations, which often continue in spite of all evidence to the contrary. 410. The movements proper to the rectum, in conjunction with the assistance given by the abdominal pressure, expel the faeces from the anus. The contractile structures here concerned do not act uninter- ruptedly: but contraction, relaxation, and rest, each have their sys- CHAP. VI.] CHEMISTRY OF DIGESTION. 137 tematic influence. And the muscles of the pelvic outlet also take a determinate share, by means of which they assist the process. 411. It is probable that the external sphincter of the anus, which is provided with transversely striated fibres, is in a state of moderate con- traction when no faeces are passing. But since a foreign body can penetrate the anus without meeting any great amount of resistance, it may be conjectured that the ordinary contraction of this circular muscle is inconsiderable. If large masses of fseces are to pass through, the sphincter yields : but if they are to be retained, it remains more ener- getically contracted. 412. The internal sphincter of the anus, which is composed of un- striped muscular fibres, and is only a greater aggregation of the circular fibres of the rectum, must obviously be relaxed at the instant the fseces pass. And by subsequently contracting it may either divide what has already passed from what remains, or may only generally prevent all exit. 413. The levator ani probably widens and shortens the rectum, and partially prevents the mucous membrane being prolapsed from its loose and yielding attachment. The influence which is exerted by the coccy- geus and the transverse perineal muscles has yet to be established. 414. The chief object of the chemical phenomena of digestion consists in the solution, as far as possible, of those solid compounds which we receive in the food. Since it is only gaseous fluids or liquids which can enter the lymph or blood by way of diffusion ( 129), many alimentary substances require to be liquefied in order to attain their object. It also follows that most drinks do not require any special process of diges- tion ; so that the alimentary canal only affords them the means of transit. But it is by no means impossible that the chemical influences exerted by the different parts of the intestine are made use of to assist in the digestion of many fluid aliments. 415. Many drinks, as, for instance, beer or coffee, are mixtures of solid and fluid substances. Water containing much lime becomes turbid when mixed with the food. And milk, as we shall see, precipitates the greater part of its casein in the stomach. These solid substances will, therefore, require an additional process of solution, in order to their entering the blood. Broths, and similar drinks, contain many oil-drops, which only enter the chyle and the blood under the influence of a chemical process of digestion. 416. In order to dissolve the largest possible quantity of solid alimen- tary substances in the course of the intestinal canal, nature makes use of two methods. The first consists in a series of watery solutions, which are faintly acid or alkaline, contain certain salts and suitable organic compounds, and which have the ordinary temperature of the interior of the body, viz. 98-6, or rather more. All substances which are, unable to resist these influences are at once liquefied. But as this method alone 138 RELATION OP FERMENTATION TO DIGESTION. [CHAP. VI. would not suffice, definite processes of metamorphosis are also induced, so as to form new and more soluble compounds. But they are not always carried to their utmost extent, since bodies less serviceable to the organism would be ultimately produced : on the contrary, their soluble first products are removed at once from the control of the alimentary canal. 417. As all the substances really alimentary belong to the vegetable or animal kingdom, fermentation and putrefaction readily suggest themselves as possibly applicable to such an object. The most essential processes of digestion are founded upon definite metamorphoses of this kind, which are confined to a suitable period of time. They present us with various circumstances of the several kinds of fermentation, of the saccharine, the mucous, the lactic, and (less frequently) of the vinous and the acetous fermentations, and with many stages of the putrefaction of azotized substances, for which chemistry has made out no sufficiently definite subdivisions. 418. The contactive operations ( 299) which play so important a part in every series of the spontaneous decompositions of organic matter, obtain at almost every step of the chemical phenomena of digestion. Many of the substances admixed with the food such as the mixture of saliva and mucus of the mouth, the gastric juice, the pancreatic fluid, and the several kinds of mucus furnished by the numerous segments of the intestine contain ferments which, either alone or with the help of definite and accompanying substances, induce spontaneous decomposition of the solid alimentary matters. . 419. In addition to this, there are other operations which exercise a manifold and important influence upon the whole process. Mastication reduces the food to a state of minute division, and mixes it with the fluids of the mouth. The peristaltic movement of the different sub- sequent sections of the digestive tube intimately mixes it with the solvent fluids at its disposal. But as these either possess mucous characters, or are directly mixed with quantities of mucus, nature in this way obtains a threefold advantage. Since the mucus diminishes the obstacle which friction offers, the solid substances glide more easily over the surface ( 79). Besides this, it contains mucous corpuscles (Tab. II. Fig. xxxi, c d), which are variable microscopic constituents that plainly show it to be itself undergoing a metamorphosis, and hence to be peculiarly adapted to excite a contactive operation. Finally, its tenacity adapts it to retain fatty substances in the form of an emulsion, and to hold solid bodies suspended in a similar state of minute division. But since the surface of contact is thus increased ( 31), the solvent juices subsequently poured out, or originally contained in the mucus, are enabled to act with greater efficacy. Finally, it is obvious that all these processes will be greatly furthered by the warmth of the internal organs. CHAP. VI.] SALIVARY FLUIDS. 139 420. The different varieties of mucus which are furnished by the several segments of the alimentary canal, the saliva, the pancreatic fluid, the gastric juice, and the bile, all contain a large proportion of water. In addition to this, the gastric juice is slightly acid, and the saliva faintly alkaline. Hence compounds which do not resist dilute alkaline or acid solutions are at once taken up without further preparation. If, in spite of this, the assistance of drinks is required, the fact depends, not upon chemical, but solely on quantitative, relations. The absolute quantities of the digestive fluids are only too small, when large amounts of soluble alimentary compounds have to be overcome. But the study of absorp- tion will teach us, that nature knows how to remedy this. Failing the necessary fluid appliances, large quantities of these soluble matters are not always rejected, but are mostly only more slowly taken up. 421. Sugar, salt, saltpetre, sulphate of soda, and sulphate of mag- nesia, are thus dissolved in the mouth. The acid gastric juice can drive off the carbonic acid of alkaline and earthy salts; and salt, or the alkaline phosphates which occur in most animal juices, will con- tribute, although only to a small extent, to the solution of the earthy phosphates. 422. The fluid of the mouth (Tab. II. Fig. xxxi), which we sometimes eject in the form of what is called spittle, consists in reality of a mixture of different secretions. It is probable that the mucous membrane of the mouth itself sets free certain solutions on its surface. The numerous excretory organs which it includes such as the labial and buccal glands, or those of the gums, tongue, and palate, yield a series of products which empty themselves into the cavity, and together form the mucus, of the mouth. The larger salivary glands situated in its neighbourhood, viz. the parotid, submaxillary, and sublingual glands, and probably also those of the apex of the tongue prepare the proper saliva, which is afterwards mixed with the mucus of the mouth. Under these circum- stances it depends upon secondary causes, which of these constituents predominates in the mixture formed by the spittle. 423. The recollection or the sight of pleasant food, the acts of masti- cation, tobacco-smoking, titillation of the soft palate, speaking, and singing, all increase the quantity of the salivary fluids. And irritation of the gastric mucous membrane may lead to the same result. Frerichs n ) found that, on introducing food into the stomach of a dog provided with a gastric fistula, more saliva was instantly secreted into the mouth. And if salt was substituted for food, the quantity was still greater. This explains how a morbid irritation of the gastric membrane may produce an energetic conflux of the salivary fluids. 424. When large quantities of saliva are required for chemical or physiological observations, various artificial means are made use of. Tobacco-smoking, and, especially, irritation of the soft palate, are very 140 ACTION OF THE SALIVA. [CHAP. VI. useful in this respect. But such an experiment furnishes a mixture which is in all probability more diluted than usual. 425. Such observations teach us, that the salivary fluid, after the removal by filtration of the mucus and epithelium which is mixed with it, is one of the most aqueous fluids of the body. For instance, my saliva leaves only -77 per cent of solid residue. And in eighteen special analyses Frerichs obtained extremes of -51 to 1-05, with an average of 72 per cent. 426. The act of mastication, which minutely divides the food so as to produce a number of interstices, also causes it to be moistened with con- siderable quantities of the salivary fluid. We might hence expect, d, priori, that the amount of fluid taken up would generally vary with the dryness and the capacity for minute division possessed by the food. A series of experiments undertaken by Lassaigne confirms this presump- tion. The increase of weight after complete mastication amounted to only 4 per cent for apples, 8 per cent for nuts, 28 for biscuits, 43 to 45 for beef, and from 30 to 127 for the different kinds of bread. 427. Since the healthy saliva which is poured forth in large quantity during mastication has an alkaline reaction, all those constituents of the food which are soluble in a weak alkaline solution are instantly dissolved, as far as their quantitative proportions will allow. The mucous quality of the fluids of the mouth may contribute to smoothen the alimentary bolus. And as this is afterwards pressed through the isthmus of the fauces, it scrapes off mucus from the root of the tongue, the soft palate, and the tonsils ( 377), so as to envelope itself with a slippery covering, which allows it to glide onwards so much the more easily. 428. The salivary fluids are incapable of dissolving fats or coagulated albumen. Hence portions of meat which remain in the interstices of the teeth are only softened and decolorized by even a long sojourn. But under favourable circumstances of temperature, the saliva possesses the property of converting paste into dextrin and grape sugar, and in this way renders it soluble. Higher degrees of temperature accelerate this metamorphosis in an extraordinary manner. But the temperature of 96-8 to 9 8 -6, which is possessed by the completely masticated food, is quite incapable of at once inducing this change. Hence the saccharine fermentation of boiled starch is never completed at the time the bolus descends into the pharynx and oesophagus. In spite of this, however, the admixture of saliva is by no means useless, but forms, to a certain extent, a provision for the future. We shall see that the presence of the gastric juice does not arrest this action of the saliva. Hence it proceeds in the stomach, where the food remains for a considerable time. 429. Many ruminants which secrete much saliva introduce large quantities of it into their first two stomachs, and especially into the CHAP. VI.] ACTION OP THE SALIVA. 141 paunch. This phenomenon has an obvious relation to the influence exerted by saliva upon starch which has been boiled or broken up by the action of warm water. 430. Raw starch also may certainly succumb to the power of the saliva. But it opposes a much greater obstacle. If slices of raw pota- toes are mixed with the salivary fluids, and exposed to a temperature of 104 during twenty-four hours, it will often be found that most, if not all of them, have retained their form, and become blue with tincture of iodine. And an examination of the contents found in the first stomach of ruminants under high magnifying powers not unfrequently affords similar results. According to J. Vogel, some of the matters vomited by the human subject teach the same fact. And we shall also find, that on the other side of the stomach nature has applied digestive juices which are also intended for the metamorphosis of starch. The swal- lowed saliva only forms a kind of substitute for the stomach, whose gastric juice is exclusively occupied with the solution of other consti- tuents of the food. 431. The fluids of the mouth, as they are generally evacuated, forming a mixture of the secretions of its mucous membrane, the glands contained in it, and the pure saliva, are at all events capable of inducing a most energetic saccharine fermentation in boiled starch. Many observers, such as Bidder, Jacubowitsch, and Schmidt, altogether deny this capacity to the pure saliva and the pure salivary fluid of the mouth. While others, as Lassaigne, Magendie, Kayer, Bernard and Barreswill, ascribe it to the latter of these two fluids only. Finally, it was found by Frerichs that the watery extract of the proper salivary glands, or that of the mucous mem- brane of the mouth, only furnished traces of sugar; while a mixture of the two gave much more considerable quantities. Mialhe named the substance precipitable by alcohol diastase, because it had the power of inducing the saccharine fermentation of starch, just like the vegetable diastase ( 299). But hitherto it has been found impossible to exhibit this agent of the metamorphosis in a pure state. Besides, the power of inducing saccharine fermentation also belongs to many kinds of mucus, to blood in which putrefaction has commenced, to the hepatic or renal substance, and probably to numerous other structures of the body when not quite fresh. 432. The short time during which the mucous and slippery alimentary bolus remains in the oesophagus allows of no very important chemical changes. But at present we are ignorant whether the glands of this part of the alimentary canal furnish a mixture which is capable of continuing its operation in the stomach. 433. The greater part of the internal coat of the stomach consists of the vertical gastric glands (Fig. 78 and Tab. IV. Fig. LIIL); clusters of which may easily be recognized by a low magnifying power in proper 142 GASTRIC JUICE. [CHAP. VI. sections of the mucous membrane. They furnish the gastric juice, which is a mixture of a tenacious fluid with granules, nuclei, and pavement- cells. These denser corpuscles of different kinds pro- bably form the visible expression of that continual meta- morphosis, that perpetual change, which the whole is undergoing, and on which the solvent powers of the gastric juice, and the qualities ascribed to the pepsin or theoretical digestive substance, may be supposed to de- pend. But since the secretions of many other small glands which exhibit similar microscopic constituents are devoid of similar properties, it fol- lows that the characteristics recognizable in this mixture by the eye will not suffice to explain its intimate constitution. 434. The empty stomach contains a mucus which has a neutral or faintly acid reaction. If this be scraped off, we come upon a fluid below it which strongly reddens vegetable colours. If the exposed gastric mucous membrane be titillated, or if stones and other insoluble substances be introduced into the uninjured stomach of a living dog, there exsudes a more considerable quantity of a decidedly acid secretion. The food acts in a similar way. Its mere consistence forms, as it were, a stimulus, which educes the acid gastric juice necessary to its elaboration. 435. We have seen ( 420) that the fluids of the mouth which descend with the food have an alkaline reaction; and we shall find that the blood, the lymph, and many mixtures which are added to the chyme in the sub- sequent parts of the alimentary tube, usually exhibit the same quality. Since, notwithstanding this, the gastric juice always exhibits a free acid, there must be something in the walls of the gastric glands, or in the me- tamorphosis excited in their interior, to which this strong acid reaction is due. According to Bernard, if warm blood be injected into the arteries of the stomach of a newly killed animal, an acid gastric juice imme- diately exsudes. 436. The cause on which the free acid depends has been much con- tested. At present we know that hydrochloric acid is not, as was sup- posed, the basis of the phenomenon; and that the gastric juice of granivorous birds which swallow flint-stones, does not contain any free hydrofluoric acid. The acid is most probably'the lactic. But it may not only be furnished by the stomach, but also from the spontaneous decom- position of the hydrates of carbon, from altered milk, or from other substances. Butyric and other volatile fatty acids, or acetic acid, are also exceptionally met with in the replete stomach. But chemistry has hitherto been unable to decide whether the acid reaction depends on free acids, acid salts, or acid organic compounds. 437. We shall soon see that the most important operations of stomach digestion may be imitated by mixing the watery extract of the gastric mucous membrane with small quantities of acid, so as to form an arti- CHAP. VI.] AETIFICIAL GASTRIC DIGESTION. 143 ficial digestive fluid. Various acids may be used with effect, so long as they are present in corresponding minimal quantities. But the necessary quantities of acid, and the rapidity of the operation, vary with the diffe- rent fluid which is applied. Hence the question, which is the active acid of the gastric juice, is so far of importance, as that it is an inquiry into the acid compound which nature originally makes use of, and which will probably correspond to the most suitable collateral conditions. 438. The gastric juice is not intended for the transformation of the hydrates of carbon or the fats. These continue to be attacked by the admixed saliva, since its influence is not annihilated by the acid gastric juice. It may also happen that fatty acids are produced. But the chief object of gastric digestion is the solution of coagulated albuminous sub- stances. 439. If the stomach of a newly killed animal be filled with water and allowed to stand twenty-four hours, and if the fluid be then filtered and concentrated at a moderate heat, we obtain a mixture which, by a proper addition of acid, forms a tolerable artificial digestive fluid. Small portions of gastric mucous membrane may also be directly mixed with faintly acidulated water. And a stomach which has been rapidly dried and kept for years serves almost as well as a fresh one. Many experimenters extract the gastric mucous membrane with water, evaporate the filtered fluid, and either treat it at once with alcohol, or only after precipitation with acetate of lead and separation of the precipitate from the metal with the aid of sulphuretted hydrogen. 440. It is obvious that, supposing nature to require a free acid, this will only be made use of in a very dilute state, since otherwise the tissues themselves would be attacked. Experiments on artificial digestion also teach us that it is only very small quantities which lead to the desired end ; such " microlytic " quantities of hydrochloric acid, for instance, as either cause but a faint precipitate in a watery solution of albumen, or do not affect it at all. The more energetic action of larger quantities injures or disturbs the results. 441. The favourable influence of moderate degrees of heat, small quantities of acid, and proper fermentative substances, may be easily verified in every kind of artificial digestive fluid. If we imitate the gastric temperature of the warm-blooded animals by a constant heat of 96-8 to 105, the solution occurs much more rapidly than at from 59 to 68. A boiling heat annihilates the digestive power of the fluid. And all temperatures above 122 remarkably injure it. 442. If the non-acidulated gastric juice be allowed to remain at a tem- perature of 96-8 to 105, it soon developes an intense putrefactive smell. And albuminous bodies added to it are not immediately dissolved, but are at most only conducted to a partial and ineffective spontaneous decom- position. But if a small quantity of acid be present, this putrefaction 144 ARTIFICIAL GASTRIC DIGESTION. [CHAP. VI. does not appear. The whole takes the acid smell of matters vomited, retains it for a very long time, and effects a genuine solution of coagulated albuminous substances. 443. Very dilute mineral acids are certainly capable of slowly dissolv- ing small pieces of meat or albumen. But even an imperfect effect of this kind demands many days and a high temperature. We see from this what advantages nature obtains by combining moderate heat, slight acidulation, and a suitable ferment. 444. Although artificial digestion repeats the most essential opera- tions of the gastric juice of the living stomach, yet the two exhibit many important differences. If the mucous membrane of the stomach be ex- tracted with water, foreign compounds, such as soluble albumen, may be taken up from the tissues themselves. Hence this digestive fluid offers reactions, which are not only different from those of the gastric juice itself, but are in some respects more variable. If small pieces of stomach be digested in acidulated water, they as it were consume themselves. Their fibrous substances are gradually liquefied, and we finally obtain a thick mixture, from which the large portions of gastric mucous membrane formerly present are generally absent. Something similar may occur to a partial extent in the dead body. Many cases of apparent gelatinous softening met with in the dead stomach depend upon the fact, that the strongly acid gastric juice has attacked the neighbouring tissues after death. But it is obvious that in the living animal, this casual effect is either wanting, or is rendered harmless by a compensative development. 445. Setting aside these differences, the changes which we meet with are tolerably uniform, whether we follow them in an artificial digestive fluid, or in a living stomach. In the latter, we have two ways of accom- plishing our object, the examination of matters vomited, or the institu- tion of gastric fistulse. 446. Beginning with the albuminous substances, we shall again find that much depends upon their state of aggregation and their other qualities. The fibrine of blood offers less resistance than the albumen of hard-boiled eggs. The muscular fibres are more easily overcome than the denser fibrous masses of tendons or ligaments ; and hard cheese succumbs so slowly, that a part of it often passes on to the duodenum. 447. Sharply-cut dice of albumen first become more transparent at their corners, while a less transparent nucleus remains in their middle. The effect thus proceeds from without inwards, just as in every other solution. The corners are rounded off, softened, and finally altogether dissolved. The nucleus undergoes similar changes. And finally, we have a greyish-white and cloudy mass, in which a greater part, if not all, of the albumen is not merely mechanically subdivided, but chemically taken up. 448. Flesh at first behaves just as it would in any other faintly acidu- CHAP. VI.] DIGESTION OP FLESH AND MILK. 145 lated fluid. The areolar tissue which is interposed between the muscular fibres, and the substance of these structures themselves, become more gelatinous and transparent. The sarcolemma (Tab. IV. Fig. LIV. b) and its nuclei (c) are more easily distinguished. Later, these structures disappear. But the transverse strise of the mus- cular fibres may be long recognized, especially FlG - 79 - under the influence of shadow. The mass of fibres is then separated into fragments, such as are represented in Fig. 79 j and we finally get a thick and imperfect solution of the whole. According to Frerichs, the thicker muscles of the adult resist longer than the smaller ones of the young animal. Other circumstances being equal, boiling or moderate roasting furthers the rapidity of the solution. 449. Casein may demand the digestive action of the stomach under two different forms. We often introduce it as a solid in cheese. And under the influence of the gastric mucous membrane, milk soon coagu- lates, so that the precipitated casein requires a new dissolution. 450. Cheese belongs to that class of albuminous compounds which are overcome by the digestive fluid, although able to offer it a con- siderable resistance. Hence relics of it not infrequently pass into the duodenum. 451. Very small portions of gastric mucous membrane, or of the acidulated artificial digestive fluid, precipitate considerable quantities of casein from milk. This effect probably depends, not so much upon the immediate neutralization of the alkali of the milk, as upon a special contactive agency. According to Mitscherlich, the quantities of lactic acid produced from the sugar of milk at first bear no proportion to the amount of precipitated casein. The albuminous wall which surrounds every milk-corpuscle (Fig. LXXX. a) is not unfrequently dissolved, so that the globules of oil or butter become free, and subsequently run together into large drops by their accidental contact. The other fluids are for the most part absorbed in the stomach. The casein which remains, and the fatty deposit, diminish under the influence of the digestive action, until finally the residue passes on into the duodenum. 452. Vegetable albumen, legumin, and gluten are dissolved in the stomach like the animal albuminous substances. Gelatin is also overcome. But just as the albumen dissolved in an acidulated artificial digestive fluid is not precipitated by a boiling heat, so the solution of gelatin has alto- gether lost the power of coagulating on cooling. Softer tissues which yield gelatin, such as the different kinds of cellular tissue, easily succumb to the influence of gastric digestion. Denser ones, on the contrary, such as tendons, ligaments, or elastic fibres, often resist the most continuous action. In thin laminse of cartilage the intercellular substance (Tab. III. L 146 CHYME. [CHAP. vi. Fig. XLV. a) is first attacked. Their dissolution is on the whole slow ; but they finally disappear to the extent of the greater part of their nuclear structure. The bones only lose a part of the large quantity of calcareous salts which they contain. But their cartilage yields more quickly to the gastric juice. It appears to behave like the substance of the permanent cartilages. 453. We know that other circumstances being equal, a powder is more quickly dissolved than a larger solid body, since the subdivision into small masses increases the surface of mutual contact ( 31). The like is found to obtain in gastric digestion. The same quantity of coagulated albumen is liquefied far sooner when cut up into a great number of thin slices than when exposed in the form of a single large cube. Hence the thorough mastication of the food not only subserves the use of mixing it more intimately and copiously with the fluids of the mouth, but also assists to accelerate the subsequent gastric digestion. 454. The movements of the stomach also afford important advantages. They knead up the most superficial layers with the gastric juice, and finally move them onwards so as to allow of a repetition of the process with deeper ones. And since the more dilute and completely fluid sub- stances are immediately absorbed by the stomach, those denser ones alone remain which yet require the action of the gastric juice, just as a precipitate which we seek to separate by filtration is left more accessible to subsequent extraction. The withdrawal of the fluids has also the advantage of preventing that disturbing influence which would result from too great a dilution. 455. The quality of the chyme which passes immediately into the duodenum must evidently vary with the different nature of the food. It generally constitutes a mechanical mixture of a grey gelatinous semi-trans- parent mass with all those relics that are capable of resisting the gastric digestion. The former constituent contains those dissolved substances which circumstances will no longer permit to be received into the lymph or the blood. Hence we may recognise in it relics of gum, sugar, lactic acid, and pectin, after the use of potatoes, bread, or other vegetable food ; or of albuminous and gelatinous bodies after a corresponding animal diet. The mechanical admixture may consist of raw starch granules; decolor- ized simple, dense, or woody cells, and bundles of vessels ; fat ; pulverulent relics of hard albumen or casein ; fragments of cartilages ; muscular fibres ; tendons ; ligaments ; splinters of bone ; or salts of the osseous tissue. The fluid substances, which are chiefly absorbed, but partially conveyed into the duodenum, seem always to include considerable quantities of ashes. Herbivora furnish, on an average, more than carnivora. 456. We have already seen that a great influence is exercised by the state of aggregation of the food. Hence the digestibility of particular alimentary substances must not be decided from their chemical characters CHAP. VI.] CONDITION OF THE GASTRIC MUCOUS MEMBRANE. 147 only. An additional reason for this caution is found in the possible state of the gastric mucous membrane itself. The intensity with which it is aroused from its state of rest immediately depends upon the moderate degree of friction exerted by the matters that are introduced into the stomach. Small quantities of cold fluids favour the secretion, while it visibly suffers under the influence of larger masses of ice. Blondlot and Bernard 12 ) believe themselves to have found that the application of a weak solution of carbonate of potash furthers the solution of meat more energetically than that of a little wine- vinegar ; and hence that the faintly alkaline character of the saliva, and of many kinds of food, such as albumen, assists the gastric function. But on the other hand, larger quantities of alkali are injurious. Large quantities of salt limit the artificial digestion of coagulated albuminous substances. It is hence self- evident that not only the nature, but also the mixture, of the aliments has a decided influence. The mucous membrane of the stomach appears to be itself one of the most sensitive parts of the body. Violent febrile excite- ments, or the direct application of strong mechanical or chemical irrita- tions, easily alter the secretion which it furnishes, and the movements which it takes on. We thus get a gastric juice which either digests badly or not at all : and unusual phenomena of fermentation appear. Bile regurgitates into the stomach - } and nausea or vomiting frequently accompany these irregularities. 457. That residue of the food which passes into the small intestines next encounters the intestinal juices ; that is, the different secretions which are furnished by the glands of Brunner, Lieberkuehn, and Peyer, and probably also by the mucous membrane itself. In the descending portion of the duodenum they also meet with the pancreatic fluid and the bile. 458. Since we cannot completely separate the secretions of these small intestinal glands from the other admixtures, it is impossible to decide what functional differences correspond to the structural peculiarities of these secretory organs. According to Middeldorpf 13 ) the fluid of the glands of Brunner, which are more developed in herbivora, is incapable of dissolving pieces of flesh or albumen, while it is able to convert starch into grape sugar. 459. Many collateral circumstances may essentially alter the reactions of the mucous substances present in the intestines. In testing the upper segments it is possible that the acid chyme which has entered conceals the alkaline character of the secretions. The -production of lactic acid from the hydrates of carbon taken in the food may induce similar errors. If we add to this, that small quantities of fluids having a weak reaction may be very faintly or even incorrectly indicated by the vegetable colours usually employed, we shall not be surprised at the contradictions which have resulted from such examinations. L 2 148 PANCREATIC FLUID. [CHAP. VI. For instance, Middeldorpf found that the secretion from the Brunne- rian glands of the pig had an acid reaction ; while on the other hand Frerichs found both it and that of the follicles of Lieberkuehii to be alka- line. It must however be admitted, that the originally alkaline quality of the intestinal juice becomes more decided the further we descend from the duodenum. 460. The mucous juice of the intestines assists the residuum of the food in gliding onwards. Its tenacity causes the more fluid fats to be minutely subdivided in the form of an emulsion, and allows them to retain this condition. It can also convert starch into sugar, although not very energetically. And since the mucus includes substances which are undergoing metamorphosis, it is possible that under favourable circumstances it may furnish an organic ferment. It must however be remarked, that the intestinal mucous membrane, when mixed with faintly acidulated water, is generally unable to dissolve coagulated protein-compounds. 461. The pancreatic fluid of healthy animals appears to be poured out in considerable quantity during the time of digestion only. Its characters seem to be very easily altered by abnormal circumstances. Leuret and Lassaigne, Tiedemann and Gmelin, Bernard, and Frerichs, inserted a tube into the pancreatic duct of living animals and birds, either from the duo- denal orifice of this canal, or in some part of its course. But the fluids thus obtained differed essentially from each other : for certain of these observers found considerable quantities of albumen, while others did not. On the other hand, almost all remarked an alkaline character of the pan- creatic fluid so obtained. But the watery extract of the fresh pancreas of the cow has sometimes an acid reaction. 462. The pancreatic fluid must contain certain substances which have a tendency to decomposition. If the pancreas be triturated with water, or extracted as completely as possible, the whole putrefies within a short time at the blood-heat. This phenomenon furnishes an indication of the reason why the pancreatic fluid so powerfully excites fermentation. 463. If the pancreatic mass be triturated with water and mixed with paste, the latter is quickly dissolved, and is converted into grape-sugar. If the whole be allowed to remain at a proper temperature, fermentation soon goes further. A strongly acid reaction may be remarked, and may be conjectured to depend upon lactic acid. While an energetic develop- ment of gas seems to indicate a copious production of carbonic acid. 464. An acid character of the fluid, or the presence of bile, intestinal mucus, or pieces of small intestine, does not suspend the powerful fer- mentative influence of the artificial pancreatic fluid. We may thence con- clude, that this influence obtains during life, and greatly contributes to the production of lactic, and even of carbonic, acid from the suitable hydrates of carbon. But granules of raw starch are much more slowly overcome CHAP. VI.] BILE. 149 than a mass of paste. Thus if a large quantity of the former be mixed with pancreatic fluid, and allowed to remain for three days at blood-heat, the greater part of the starch granules are found unchanged. The whole may have an offensive acid smell, although only small quantities have been dissolved. This explains why many starch granules are not elabo- rated in the upper part of the small intestine, but are carried on further intact. 465. The pancreatic fluid certainly contributes to the minute division of fluid fat in the form of an emulsion. But this effect, which also belongs to other digestive juices, probably constitutes no essential element of its function. 466. Nor is the necessity of this secretion sufficiently explained by the energetic fermentation which it excites in paste. And since it does not dissolve pieces of albumen, it remains for further investigations to show what finer changes may be produced in the azotized alimentary substances under the influence of the pancreatic juice. It appears to facilitate the deposit of resinous substances from the bile. 467. The numerous attempts of chemists to investigate the consti- tuents of the bile have not hitherto led to any satisfactory physiological results. The ease with which this mixture is decomposed causes the simplest chemical processes to furnish products of metamorphosis which are not unfrequently viewed as its essential constituents. And although it is probable that the bile undergoes some alteration in the intestine, and even in the gall-bladder itself, still it has hitherto been impossible to exhibit with sufficient clearness the course of the changes which obtain in these situations. 468. We shall hereafter see that the bile separates from the blood cer- tain substances previously useless. Apart of these substances is gradually rendered insoluble and discharged in the excrements. But those com- pounds which retain their liquid form, and perhaps many which are gradually redissolved, are returned anew into the blood. That constant arrangement in the animal kingdom, by means of which the bile is poured out into the commencement of the small intestine, admits of a double interpretation. Since this secretion must accompany the relics of the food for a great part of their course, it may be intended to afford some essential assistance to digestion. But it is also possible that the co-operation of the small and large intestines is required in order to separate the bile into constituents which are soluble and susceptible of absorption, and into others which are denser and destined to expulsion. The changes which it provokes in the residue of the food would thus be mere secondary phenomena, having a more or less subordinate import. 469. The experiments hitherto made are incapable of deciding between these two views. Since digestion does not completely cease in cases of jaundice or of artificial biliary fistula, it follows that the bile is not 150 ACTION OP THE BILE. [CHAP. VI. indispensable to the general process. But the physical properties of the excrements suffice to show that this fluid is to a certain extent a regulator, and perhaps a definite and not unimportant condition of that peculiar spontaneous decomposition which the relics of the food undergo. But much of this is very indefinite, since chemistry has scarcely penetrated these peculiar phenomena of putrefaction. 470. Since fresh bile is by no means strongly alkaline, but is usually either neutral, or at most, but faintly alkalescent, the view which was formerly so frequently propounded, that it was intended to neutralize the acid chyme, is at once negatived. Even when the contents of the upper part of the small intestine have taken up a large quantity of bile," they generally preserve their acid character. On the other hand, however, the free acid reacts on the bile. The colours of the substances present in the intestine are tolerably explained by this fact. 471. If the intestinal contents be followed along the small and large intestines they will be generally found to become first yellowish or yellow- ish-green, then green, and finally brown. Separate brown microscopic masses appear before this colour is present to the naked eye. If bile be treated with small quantities of acid, or with an acidulated digestive fluid, yellowish-green or green precipitates may be artificially produced. The unacidulated gastric mucus, salt, or muriate of ammonia, do not give rise to this change. One may thence conclude, that it is caused by the biliary precipitate which is produced by the acid contents of the upper part of the small intestine. The precipitate consists chiefly of cystic mucus, of cholepyrrhin or biliary colouring matter, and of fatty bodies : and it probably contains other constituents of the bile. 472. As the bile gradually descends along the course of the intestinal canal, it slowly undergoes important changes. It is probable that the fluid which remains after the separation of the solid precipitate is gradu- ally absorbed. And on an average, the lower the residuum of the food has descended, the less the quantity of bile which can be extracted from it by water. We here meet with biliary compounds which are only soluble with difficulty ; such as, for instance, the supposed modifications of cholepyrrhin, taurin, dyslysin, and the like. But it is at present impossible to specify the details of this metamorphosis with sufficient accuracy. We only know that these combinations usually leave the body with the faeces. 473. By drying the precipitate which is thrown down from putrefying human bile we get a brown substance which diffuses the strongest smell of human ordure, especially after the application of a small quantity of water. The same results may be obtained with the semifluid contents of the csGcum. If the experiment be repeated with ox-bile, we obtain a yellowish-green substance which smells like cowdung. 474. Many combinations which are produced by the artificial treat- CHAP. VI.] ACTION OF THE BILE. 151 ment or the putrefactive decomposition of albuminous substances have also a more or less distinct odour of feeces. But we should be wrong if we therefore attributed the smell of the excrements to. the food. For when these putrefy alone, this smell is generally absent. And if the entry of the bile is prevented in jaundice, the greyish-white and clayey faeces have a smell which is intensely putrefactive, but quite different from that of healthy excrements. And of many animals supplied with the same food, each will furnish its peculiar faecal odour, an odour which some- times recurs in a weaker degree in the blood, the urine, and the cuta- neous evaporation. It therefore follows that the two most prominent physical qualities of the excrements the colour and the smell chiefly depend upon the bile. 475. Hitherto the bile has not been shown to possess any peculiar solvent powers. Morsels of albumen or cheese, and more or less dense masses of fibrine, resist its influence with great obstinacy. Just as little does it possess the power of converting starch into grape-sugar with any considerable force ; or of inducing the lactic or acetic fermentation. In one word, in the present state of our knowledge it may be regarded as a mixture which is incapable of affording any direct support to the influence of either the gastric juice, the intestinal mucus, or the pancreatic fluid. 476. Comparative researches rather lead to the conviction, that the bile leaves intact many of the phenomena of metamorphosis which appear in the intestinal canal, while there are others which it has the power of limiting. Hence it seems to be serviceable by its negative, rather than by its positive, action. 477. The pancreatic fluid operates as a powerful excitant of decompo- sition, whether bile be present or not. If large quantities of bile be mixed with the artificial acidulated digestive fluid, the latter loses the capacity of dissolving morsels of albumen. Having mixed acidulated water with pieces of the mucous membrane of the human caecum, and with bile, I found that its action upon beef was weaker than usual. The addition of the biliary precipitate obtained by acetic acid seemed to delay solution more than an admixture of pure unfiltered human bile. The penetrating putrefactive smell offered by the faeces of jaundiced sub- jects had long led to the conclusion that the bile was opposed to putre- faction or had an antiseptic agency. And Frerichs found that after deliga- tion of the biliary duct, the filtered albuminous contents of the intestine were made rose-coloured by nitric, and violet by hydrochloric acid. This reaction, which has also been observed in the alvine evacuations of per- sons suffering from cholera and nervous fevers, indicates a substance evolved by the putrefaction of albuminous matters. 478. We may hence conceive that the bile sets definite limits to the decomposition of many albuminous substances ; while it does not hinder the metamorphoses which are induced by the pancreatic fluid, and pro- 152 DIGESTION IN THE (IffiCUM. [CHAP. VI. bably by the intestinal mucus. It certainly possesses antiseptic properties, but only for certain stages and kinds of spontaneous decomposition. It is probable that an extension of our knowledge of the putrefactive pro- cess would allow a more accurate estimate of the agency of the bile. 479. The colours of the excrements show that the biliary constituents which are expelled from the rectum only attain their perfect and normal metamorphosis after the alimentary residuum has remained a certain time in the intestinal canal. The yellow evacuations of diarrhoea con- tain a certain quantity of pure bile. And since the excrements of the sucking child are green or yellowish-green, and fluid, while those of the older infant are yellow, more solid, and finally brown, we may con- jecture that the character of the bile, or the mode of its decomposition, or both of these circumstances, vary with the age. 480. We shall subsequently see that the greater part of the fluid fat is converted into chyle in the course of the small intestine. The movement of the intestine intimately kneads it up with the mixture of intestinal juices, pancreatic fluid, and bile. It then forms very minute and finely- divided drops. Possibly a very small quantity is converted into fatty acids. But, as above mentioned, these may proceed from the hydrates of carbon : and the greater part of it certainly remains unchanged. 481. Hitherto we know extremely little of that series of changes which accompanies digestion, and the formation of fseces, in the large intestine. The cause of this lies not so much in the physiological as in the chemical circumstances. The chemistry of the present day furnishes no sufficiently clear insight into those limited phenomena of putrefaction which appear in the course of the large intestine. 482. Confining our attention to the outward appearance, the semifluid contents of the caecum have a consistence which tolerably corresponds to that of the contents of the lower part of the small intestine. While in the course of the colon we find denser excrementitious substances ; which, however, always contain 3-4ths of their weight of water and other volatile matters. The combinations destined for expulsion are condensed in the large intestine of man and certain of the mammalia, as, for instance, the rabbit, the sheep, and the horse. 483. Although the contents of the caecum not unfrequently offer a dis- tinctly fsecal smell, yet in the remaining course of the large intestine, this odour is remarkably increased. The human excrements only acquire a brown colour by degrees. If we also consider, that compounds of hydro- gen, such as carburetted and sulphuretted hydrogen, constantly appear in the large intestine, and that ammonia is more abundantly present (in the form of its double salt, the ammoniaco-phosphate of magnesia), we can scarcely doubt that the residue of the food and of the biliary precipitates is subjected to a limited process of putrefaction, such as that which occurs spontaneously under water. CHAP. VI.] DIGESTION IN THE CAECUM. 153 484. While the purely carnivorous animals possess a very small csecum, in the herbivora it attains a considerable size. In the rabbit and hare the great sac it forms is generally distended by an alimentary residuum. Those raw vegetable substances which require a longer elaboration, and therefore a longer delay, in the alimentary canal, probably find in the caecum their most favourable digestive receptacle. 485. The chief agents of the metamorphosis of the hydrates of carbon, are the saliva, and the secretions poured into the small intestine; while that of the less soluble albuminous and other azotized substances, is effected by the gastric juice. The fluid fats are mostly absorbed in the small intestine. And the addition of a special digestive system in the large intestine can only have the object of making useful that which has escaped the previous portions of the canal. So that the csecum and colon form, as it were, the second frontier guard, by means of which the alimentary residuum is subjected to an additional examination, in order that as little useful matter as possible may be lost. 486. A collateral circumstance probably favours this repeated extrac- tion. The azotized substances which the gastric juice has not overcome immediately pass through the small intestine. They are here mixed with fermenting matters, which have the power of unloosing them, or unlocking their molecules to a certain extent. They are thus more easily dissolved in the large intestine. And if we remember that many nutritious vege- table substances are enclosed in coverings of cellulose, we may readily imagine that the metamorphoses which occur in the small intestine will considerably facilitate their being subsequently overcome. And even where the walls of the cells are not dissolved, they may acquire a con- siderable increase in permeability and diffusive capacity. 487. At present we do not know of any definite and special use which is subserved by the probably essentially alkaline secretion of the csecum. If this intestine contains hydrates of carbon, they frequently undergo the lactic fermentation. The lactic acid thus produced might offer two collateral advantages. It would independently dissolve many compounds, especially those of vegetable food, and salts, such as the carbonate of lime and magnesia. And in conjunction with the organic matters of the csecal secretion, it would furnish a mixture capable of overcoming coagu- lated albumen. I have found that a mixture of acidulated water with pieces of the mucous membrane of the caecum has a weaker and slower action than a digestive fluid prepared from the gastric membrane; but that the final results prove the solvent power to exist. 488. It is probable that the alkaline secretion of the large intestine can also assist in taking up albuminous substances. The filtered contents of the colon frequently contain albumen. The residuum of vegetable food here continues its fermentation, so that not only lactic, but butyric, acid may appear. 154 CONSTITUENTS OF THE PIECES. [CHAP. VI. 489. The excrements contain constituents of three kinds, the unela- borated solid parts of the food, the insoluble or difficultly-soluble cellular deposits, and rnucus or other organic combinations which are evacuated per anum. The hybernating animals show that the absence of food does not suspend the formation of excrement. For instance, we shall here- after see that a hedgehog which has taken no food during some months of its winter sleep, from time to time evacuates considerable quantities of solid or semi-solid fleeces. 490. The densely ligneous vegetable tissues which are introduced in large quantity with the food, are either expelled unchanged from the anus, or are decolorized and partially extracted. In this way the horse gets rid of a large quantity of the vegetable stalks which it has eaten. The human faeces sometimes contain hard envelopes of seeds, and cherry or plum-stones. And even when a more common and apparently more wholesome food is made use of, the faeces still generally contain micro- scopic relics which are exactly similar to others that have been overcome by the digestive fluids. For example, Tab. I. Fig. xvn. exhibits the constituents of healthy human excrements diluted with water. Here a is a starch granule, the surface of which being in focus allows its concentric laminae to be visible \ b c are other starch granules, which lie more deeply, and hence look almost like drops of oil. At d e f are lignified epidermoid cells, and reticular vessels of the vegetable food. At g is seen a muscular fibre from the meat which has been eaten, and which has only become clear and colourless : h is another fibre which has broken down into transverse fragments. Crystals of ammoniaco-phosphate of magnesia are shown at ik I, m represents epithelial scales from the neighbourhood of the anus, n biliary masses, and o numerous small molecules. 491. Hitherto we have no large and complete series of analyses of the excrements. Berzelius, who examined the faeces of a labourer fed upon bread and mixed food, found 75 per cent of water, -9 of bile, - 9 of albu- men, 2 -7 of extractive matters, 1*2 of salts, 7 of insoluble remains of the food, and 14' of mucus, biliary resin, fat, and other animal compounds. The elementary composition of the excrements will again occupy our attention in the chemical phenomena of nutrition. 492. The spontaneous decomposition undergone by the food in the alimentary canal evidences itself by two supplemental occurrences ; by a change in the nature of the gases contained in the intestine, and by the occasional formation of mould. 493. The frothy saliva contains a certain quantity of air in mechanical union; and not unfrequently peculiar movements of deglutition intro- duce larger quantities into the stomach. But the gases contained in this organ do not possess the composition of pure atmospheric air. They contain more carbonic acid, less oxygen, somewhat less nitrogen, and in CHAP. VI.] INTESTINAL GASES AND MOULD. 155 rare instances, small quantities of hydrogen. The cause of this pheno- menon is twofold. We shall hereafter see that the mediate contact of the atmosphere with the blood causes oxygen to be given off, and carbonic acid to be taken up. Carbonic acid and hydrogen may be set free by the fluids which are drunk, by that fermentation of the hydrates of car- bon which is induced in the stomach itself, and especially, by the produc- tion of butyric acid. That solution of the dense albuminous substances which is effected by the gastric juice furnishes no gaseous substances. Nor does it require the presence of atmospheric air, although this is indispensable to many forms of fermentation. 494. The small intestine contains considerably more carbonic acid, less nitrogen, little or no oxygen, and considerable quantities of hydrogen. The progress of fermentation in the hydrates of carbon sufficiently ex- plains this mixture. 495. A more or less considerable content of carburetted hydrogen distinguishes the air contained in the large from that in the small intes- tine. Sulphuretted hydrogen, and the odorous matter of the excrements, are usually present in flatus. The presence of carburetted hydrogen points to the fact that the quantity of free oxygen is insufficient to con- vert all the carbon into carbonic acid, and that a certain quantity of water or some other hydrogenous substance must be decomposed, in order to the conversion of part of the carbon into a gaseous compound. The ammonia which is set free from the decomposition of azotized bodies, and which may also be developed from the changed bile, appears to be chiefly combined with other substances, and especially with sulphate of mag- nesia. 496. Fermenting mixtures which have a free acid reaction greatly favour the multiplication of the various kinds of mould. Hence we frequently meet with them in the course of the alimentary canal. The acid gastric juice can first allow of their development. The acid fermen- tation of the hydrates of carbon, or a morbid production of acid, also frequently form favouring circumstances. Infusoria appear on the whole with less frequency. The tartar or mucous earthy mass which frequently covers the under part of the crown of the teeth contains peculiar articulated threads which probably belong to the vegetable kingdom, and moveable creatures resembling vibriones. Vomiting not unfrequently furnishes those pecu- liar vegetable parasites which have been named the sarcina ventriculi (Tab. II. Fig. XVIIL). The yeast plant (Tab. II. Fig. xix.), some species of hygrocrocis, and other thready fungi, may be met with in all parts of the alimentary canal, especially in herbivorous animals. CHAPTER VII. ABSORPTION. 497. THE porous structure of the animal tissues leads to numerous and mutual actions between the juices of the body and the fluids which come into contact with its organs. Special means for favouring these actions allow of foreign solutions being taken up in large quantity, or, as it is usually expressed, absorbed. That solution of solid bodies, which is necessary to diffusion, may be effected by the organic mixtures them- selves. The transit these induce is sometimes distinguished by the term resorption. 498. All phenomena of this kind conduct into the blood the com- binations which are exposed to them. Solutions first gain the nutritive fluid which soaks the tissues. They can thence pass immediately into the mass of the blood. But certain collateral objects often require that they should previously enter the lymph, and flow onward with this liquid in the absorbents or lymphatics, so as to be only mixed with the blood at a later period. Hence we must distinguish as much as possible between the immediate and the mediate transit into the blood j or between the entry into the blood and into the lymph. 499. The alimentary substances liquefied by digestion form an im- portant source of the phenomena of absorption. The lymphatics of the alimentary canal often specially take up the fatty matters contained in the food. Since the oil-drops in the lymph are minutely divided as in an emulsion, so as to give this fluid a milky appearance, the lymph thus altered during the digestive act is named the chyle ; and the lymphatics which pass from the intestinal canal, and especially from the small in- testine, are called the lacteal or chyliferous vessels. But this time- honoured distinction is based upon outward and collateral circumstances only. The lymphatics of the small intestine only enjoy more frequent opportunities than others for taking up fatty matters. Hence during fasting, their contents correspond with ordinary lymph. And if other lymphatics absorb fat, they also contain chyle. This may be seen, for instance, in the lymphatics of the large intestine, if a meat-broth which is rich in fatty matter be introduced per anum. 500. Since absorption depends upon the porosity of the walls of the vessels which limit the blood and the lymph, it would seem to be possible that not only chemical solutions, but solid bodies of very small size, CHAP. VII.] MECHANISM OF ABSORPTION. 157 FIG. 80. might enter or emerge. All that is necessary is, that these should be smaller than the interstices* of the limitary walls. It has certainly been supposed that blood-corpuscles, and particles of indigo or finely powdered charcoal, can pass directly through. But more exact observations mili- tate against this notion. We can easily imagine that very small and angular pieces of charcoal may be accidentally driven into the blood- vessels under the influence of the pressure furnished by the intestine, and may then be washed off by the circulating blood. But the process is essen- tially exceptional, and affords no support to ordinary absorption. 501. We shall hereafter see that the porosity of animal membranes is capable of being altered by numerous collateral circumstances. The relaxation of the limitary walls probably leads to the enlargement of their interstices. These may also be dilated under the influence of very powerful pressure. One might thence imagine, that such extraordinary circumstances would allow of the exit of the blood or of other minute structures. But these unusual conditions, the first step towards actual rapture, constitute rare exceptions, which for the most part only appear in cases of disease. 502. Experiments on filtration may easily convince us how close is the normal texture of the animal mem- branes. If I close a tube a (Fig. 80), with the wall, b c, of the thoracic duct of a horse, covered by a stratum of red human serum (a) 26 inches high, and enclose the whole in the vapour apparatus, e f g, no blood-corpuscle (Tab. II. Figs. xxiv. to xxvi.), but only fluid, will pass through. Filtering paper which retains freshly precipitated oxalate of lime allows milk corpuscles (Tab. IV. Fig. LXXX. a) to pass through with a columnar pressure of 5| inches; while the mucous membrane of the washed human small intestine does not afford the same result with a column of milk of ten times this length. * It seems necessary to remind the reader that most of the organized membranes through which these fluids transude offer no visible interstices whatever, even to the highest powers of the microscope. In assuming such apertures we ought therefore always to remember, that it is only requisite that they should be larger than the atoms of the transuding fluids. And without pausing to inquire whether this physical condition might not sometimes be satisfied by the mere atomic grouping of the septum and fluid, we need only notice, that the ordinary colorific tests of dilute salts show the size of such atoms to be more minute than the mind can conceive of e.g. something far smaller than the 100000000th of a cubic inch, in the case of the salts of iron. Editor. 158 ABSORPTION OF DRINKS. [CHAP. VII. 503. We have seen ( 130) that membranes moistened with water reject fluids, such as quicksilver and oil, which are not attracted by watery solutions. Hence we here meet with obstacles which may in many respects be compared with those offered by large solid bodies. It has certainly been believed, that the globules of quicksilver contained in mercurial ointment may directly enter the blood. But later researches are opposed to this view. The way in which fatty matters behave will shortly engage our express attention. 504. We will first consider the absorption of the alimentary sub- stances, and will subsequently enumerate the peculiarities offered by the lymph. If we introduce into the stomach a large quantity of spring water, containing a twentieth per cent of solid residue ( 338), it will dissolve many of those organic matters which it meets with, and which the neigh- bouring juices of the body retain in a less dilute form. Thus the first current of diffusion is concerned with the nutrient and secreted fluids which moisten the coats of the stomach. But since these again are in contact with the walls of the blood-vessels and lymphatics, the blood and lymph must themselves be influenced within a short space of time. The average watery content of human blood amounts to 79 parts per cent, and that of the lymph to about 93 or 94. Hence both of these fluids are more concentrated than the considerable quantities of water which we drink. They must therefore tend to dilute themselves at its expense. The blood, which contains more solid residuum, will hence act with greater energy than the lymph, which has a more watery con- stitution. 505. Setting aside the indeterminate influence exerted by the nature of the walls of the vessels and the other interposed tissues, it is evident that the process of diffusion must continue, until the densities of the two fluids have become equal to each other ( 134). If all remained at rest, the blood and lymph of the walls of the stomach would thus be consi- derably diluted, and the remainder of water it contained would be cor- respondingly concentrated. But since the blood and lymph continue in movement, new portions of these denser mixtures are every instant exposed to the operation. Hence this arrangement, which also holds good for absorption generally, supports the interchange of the fluids in a very important manner. 506. We shall hereafter see that the lymph moves more slowly than the blood. Hence the favourable effect accomplished by this change of sub- stance is less valid for it. And all this explains why the greater part of the water drunk enters the blood. The percentage of solid residue in the blood may thus be visibly decreased. 507. Drinks, such as coffee, tea, lemonade, or wine, give rise to similar phenomena. Alcohol and sether also easily enter the blood. A CHAP. VII.] ABSORPTION OF SOLUTIONS. 159 part of them is subsequently vaporized in the lungs, and at other free surfaces. 508. If a man takes a large quantity of an easily soluble substance, such as salt, the process of the phenomena may be followed with toler- able completeness. The fluids of the mouth, and the gastric juice ( 434) which is poured forth in large quantity, will first dissolve as much as possible. Hence we first get a saturated solution of salt, which is therefore at any rate denser than the lymph and the nutritional fluid. This solution will next give off salt, and take up water. And if a certain quantity of the salt has remained undissolved, this also tends to form a concentrated brine. This process is repeated until the concen- trated solution of salt is alone present. And the constant withdrawal of water at every instant may result in the sensation of thirst. Putting out of consideration the unknown influence of the partitions which effect the diffusive process, it is evident that a time finally arrives, in which the solution of salt can act little or not at all on the blood, while it can still operate most energetically on the lymph and the nutri- tional fluid. And when its dilution has been thus effected, it is neces- sarily followed by a more vigorous interchange of action with the blood. So that we finally get phenomena similar to those that resulted from the introduction of the water which was so poor in solid constituents. 509. But this purely physical method of regarding the process is insuffi- cient to a satisfactory explanation of all the phenomena which occur in the living body. Small quantities of salt, which are incapable of draining the entire mass of the blood in any considerable degree, nevertheless lead to remarkable feelings of thirst. This fact is very likely due to the circumstance, that this sensation may possibly depend upon local influences, exerted upon the nerves of the stomach by the blood. Larger quantities of a solution of salt which contains more of this substance than the blood itself not unfrequently produce diarrhoea, probably in conse- quence of the irritating effect of the brine stimulating the organs of secre- tion, and exciting the vermicular movements of the alimentary tube. But although the purgative properties of large quantities of sulphate of soda or magnesia may be partially explained in the same way, still we are far from any sufficient explanation of the causes of the action of the different cathartics. 510. The fluids of the mouth contain about 99 per cent of water; and the gastric juice, which is poured out in larger quantity, about 98 per cent. Hence even supposing them to have extracted from the food, salts, dextrin, sugar, albuminous substances, or other compounds, still they will generally contain more water than the blood, and often even than the lymph. This explains why more dilute solutions generally disappear in the stomach itself ( 454). It is possible that the acid character of the gastric juice may have some influence in this respect. Still there are at 160 ABSORPTION OF FAT. [CHAP. VII. present no experiments on diffusion which are sufficiently trustworthy to allow even a theoretical decision of this question. 511. When the stomach has thus given up to the blood and the lymph all that it can communicate, there remains a more tenacious and resisting residuum. We have already ( 455) seen that a part of this mass passes into the duodenum with the chyme. 512. In the duodenum a series of new watery solutions is added. The intestinal mucus originally contains only from 4 to 5 per cent of solid residuum; probably often less than this. The pancreatic fluid has 98 to 99, and the bile 87 parts of water. Hence we here meet with new sources of dilution. The watery solutions transmitted through this part take possession of these fluids, and absorption again begins. While the intes- tinal villi, which are richly provided with blood-vessels and lymphatics, increase the active surface. The vermicular movement maintains a continual change of the fluids which are undergoing absorption. The amount of influence exerted will evidently depend upon the relative values of the masses and times concerned in the operation. 513. Since the gastric juice leaves fat untouched, and, as it were, liberates it from the albuminous membranes in which it is sometimes contained, its absorption may possibly commence in the stomach. In point of fact, in the suckling rabbit we often find that the lymphatics of the stomach include a white fluid. But if the adult animal has eaten considerable quantities of fat, it is generally in the small intestine that the absorbents begin to contain any considerable quantity of white chyle. The lymphatics of the stomach are usually filled with a yellowish lymph; and at most, some of them exhibit whitish streaks. 514. The absorption of fat (Tab. II. Fig. xxvu.) is not yet sufficiently explained. Since the coats of the intestine are moistened by watery solu- tions, the present state of our knowledge forbids us to suppose that the fluid fats enter the blood or lymph by way of simple diffusion. For if this were the case, we might expect them to be already absorbed in the stomach ; and large quantities of oil taken in the food could scarcely pass unchanged per anum. But however correct this reflection may otherwise appear, still we must remember that many glands secrete fluid fats, although their walls are saturated with watery solutions. 515. If all the fatty matters taken in at the mouth were converted into fatty acids, and into glycerin which is soluble in water, the process might be more easily explained. We have already seen in the study of digestion, that fatty acids are certainly capable of saponifying with the alkalies present in neighbouring fluids. The alkalinity of the blood and lymph would therefore favour its reception. But the special conver- sion of fat into chyle, and the microscopic constituents of this fluid which we shall presently refer to, speak rather against than for this opinion. 516. We have seen ( 460) that the fatty ingesta undergo an ex- CHAP. VII.] FORMATION OF THE CHYLE, 161 tremely minute division in the small intestine. If small oil-drops be brought into an albuminous solution, each of them becomes surrounded by a layer of albumen which has been called the haptogenous membrane. If the mucous or albuminous substances in which the smallest oil-globules are finally diffused act in the same way, we obtain a protective mem- brane consisting of watery combinations. This would facilitate the further passage of the molecules of fat by diffusion. 517. Under favourable circumstances the absorption of fatty matter may be immediately followed by the microscope. Let us imagine Fig. 81 to be a diagram of an intestinal villus, of which a is the epithelium, con- FIG. 81. sisting of columnar cells (6) that clothe the whole. At c is the homo- geneous limitary membrane, beneath which is the remaining structure that forms the basis of the villus. At d are the blood-vessels, and at e the lymphatics, which ascend in its centre. We frequently find a large quantity of finely divided oil- globules closely adhering to the outer surface of the columnar epithelium. This latter structure is not thrown off during diges- tion, but is only removed as a consequence of maceration or of violent diarrhoea. The point of the villus is on this account seen dark by trans- mitted, and greyish-white by direct, light. According to some observers, the individual oil-globules enter into the interior of the columnar epithelia (6), and subsequently find their way to the vascular tubes which exist in the villus. Fig. 81 renders it evident that, in such a course, they meet with the blood-vessel d, before coming upon the trunks of the absorbents which occupy the middle of the villus. But since the greater part of the fat passes into the chyle, it must either be to a great extent rejected by the blood, or be at once given off by it to the chyle. 518. The arrangement represented by Fig. 81 reminds us to a certain extent of some circumstances which will shortly be mentioned in speak- ing of the secreting glands. These contain a number of ducts, in the interior of which the secretion appears, while the blood-vessels which surround the exterior of the canal supply the necessary mother-fluid. We may conceive something similar to be repeated in the formation of the chyle. Hence this does not consist of a simple fluid of transmission, but of a mixture which is only produced under the influence of the blood. According to Fenwick, ligature of the blood-vessels renders the formation of a normal chyle impossible. 519. Although the absorption of the liquefied food and of the mixtures M 162 CUTANEOUS ABSORPTION. [CHAP. VII. which are added to it proceeds along the whole course of the large intestine, still the processes here met with are similar to those exhi- bited in the stomach and small intestine. But though the absorbents can take up watery solutions or fats, yet under ordinary circumstances, white chyle is not formed here. 520. The introduction of food renders the intestine the most fre- quent recipient of foreign fluids. But absorption may also obtain in every other part of the body, to a greater or less extent. 521. Since the skin is dry, and, until moistened, only allows liquids to pass through it with great difficulty, these only begin to be rapidly taken up after the complete moistening of the epidermis. Hence it depends on the time that a man has remained in a bath whether any of the compounds dissolved in it have entered the juices of his body, and how much has thus been absorbed. 522. Parts which are provided with very delicate integuments, such as the conjunctiva of the eye, allow absorption to go on with greater ease and rapidity. But we are altogether devoid of sufficient comparative observations on the behaviour of the different mucous membranes in this respect. The imperfect action sometimes exhibited by solutions intro- duced into the ramifications of the air-tubes, indicates that differences do obtain, which could not have been safely predicted. 523. In endermic experiments we seek to obviate the difficulties which the dry skin opposes, by its removal. For instance, we first pro- duce a blister by cantharides, and after removing the epidermis raised by the effused fluid, we strew upon the moistened surface the remedies, the action of which is desired such as morphia or veratria. 524. Solid deposits, such as inflammatory exsudations, or deposits of pus which have penetrated between the internal parts of the tissues, are not ^infrequently liquefied and absorbed. But nature frequently fails in this attempt, even although aided by the alkaline or saline character of the neighbouring juices. In many cases a solid residuum which has been thus corroded for years still obstinately remains. Foreign bodies which have been introduced by accident, such as needles, knife- points, or bullets, resist with equal frequency and success. Not unfrequently, new exsudations surround them with a capsule. It may also happen that they are gradually urged forwards through the softer tissues. 525. From what has been previously brought forward it follows, that the matters absorbed may either pass at once into the blood, or may first enter the chyle or the lymph, or, finally, may pass into both these fluids simultaneously. But a great deal of this uncertainty depends, not so much on the nature of the proffered compounds, as upon the existing density of their solutions, and even upon the places at which they are absorbed. Many salts, especially those of the metallic combinations, as well as sugar, lactic acid, and many albuminous bodies, are thus liable CHAP. VII.] TRANSIT INTO THE LYMPH OB BLOOD. 163 to great variety of action. The few experiments hitherto made seem to indicate, that the transit of some substances essentially depends upon their chemical constitution. Alcohol, and colouring matters for instance, turmeric or madder enter more easily into the blood than into the chyle. The latter also seem to be more frequently found in the lymph than in the chyle. We have already seen ( 499) that fatty substances are taken up in large quantity by the absorbents, and that the milky appearance of the chyle principally depends upon these. Hence it is absent in the fasting state, as well as after the use of food which contains no fat, and out of which no considerable quantity can be produced in the course of the digestive metamorphosis. 526. Narcotic poisons, such as strychnine, kill much more quickly when the circulation is so free, that their transit into the blood is un- impeded. On the other hand, if a ligature be applied to the aorta of a rabbit, immediately below the origin of the two renal arteries, and so that the circulation in the hind legs is in great part obstructed, still, in spite of this, the introduction of strychnine into a wound of the thigh induces convulsions and even death. The only difference is, that the result is less rapid and complete. 527. Since both chyle and lymph are sooner or later mixed with the blood, the question intrudes itself Why has nature provided absorbents as well as blood-vessels 1 A consideration of the course of the lymphatics may best enable us to answer this question. 528. The absorbents of the small intestine principally take their rise in the intestinal villi. If we examine them during the period of diges- tion, we frequently see central vessels, which are filled with white chyle, and appear finally to dilate into club-shaped extremities. But more suc- cessful observation shows bifurcating branches, and mutual commu- nications between the primary tubes. If the valves hereafter to be mentioned offer no important obstacle to their backward or peripheral injection, we may sometimes succeed in exhibiting the absorbents, as beginning by numerous reticulations such as may, for instance, be seen on the surface of the horse's liver. These appearances are at any rate more trustworthy than those blind extremities of the absorbents which are described by some ancient and modern authors as visible with the naked eye. Still it must be admitted that we are as yet absolutely ignorant of the mode in which the absorbents begin in most parts of the body. 529. Be that as it may, we next meet with trunks which, passing onward, either join with each other to form a dense network, or are at least united together by large transverse branches. They subse- quently enter the various lymphatic glands, which are coils of lymphatics with numerous blood-vessels distributed between them. 164 COURSE OF THE LYMPHATICS. [CHAP. vii. FIG. 82. 530. Occasionally some of the lacteals of the mesentery or of other parts open immediately into subordinate venous branches. But the thoracic and cephalic ducts form the chief conduits which transmit the chyle and lymph into the blood. 531. The lymphatics which ascend from the legs, the pelvis, and the hypogastrium, first unite to form the receptaculum chyli (a, Fig. 82). This then passes into the thoracic duct (6), which also receives the remaining lymphatics of the belly and the greater part of the chest, together with some of the branches coming from the arm. It forms the largest lymphatic trunk, and finally empties itself into the union (e) of the left jugular (c) with the left subclavian vein (d). The cephalic trunk (/, Fig. 82), into which open the lymphatics of the head and neck, and of part of the arms (. CO CO CO CO 05 05 CO 05 <~3, &> 05 t> to Q '5*2 H> (N 05 ^ I 1 *O CO fe o , 2 - - p & GNAULT IK 1 CO cb CO 05 (M "^ CO r-l CO O5 ^J* CO ^4 l^- (N CO x g S o fa Q 1 fa fa V rt suoijBAiasqo jo aaqurn^ CO " to ^ r-l CO CO CO CO CO to o ^ o . (N tO CO CO CO (M >. CO O *Jj t-% 9 v ^ j< CO CO "^ r-l CO CO CO 111" 31 CO 2 p 05 O 05 05 O5 CO -^ 1 1 CO ^H CO 05 CO CO 05 CO cp p c to 05 ^ ^ O5 * r-< t- O5 to t> 3 CO d CO O O CO t>. CO to CO +3 1 " O ^ JjS o CO *o J^ t- O CO r ) ^ t^ r-H S '! *-p Q P.| CO (M O5 CO 11 H =r "o >> t.5, a l o J g - CO r-H 05 CO ^* r-H 1O g S oc 1 *^ CO to CO C: r.: O !>. CO o Q 2 CO CO o s CO to r-l 05 r-H CO CO CO co co pfl o 8 II CO CO 3 8 CO 05 *S S o 3 IF II 1 frl 'EL CHAP. X.] CARBONIC ACID AND OXYGEN GIVEN OFF. 261 846. Hitherto no attempt has been made directly to determine the total quantity of oxygen consumed, and carbonic acid given off, in the pulmonary and cutaneous transpiration of the human subject. But in- directly, they have been determined by Barral, from statistical researches which will be hereafter referred to in treating of nutrition. Still these are necessarily an unsafe basis ( 286). And if, from the absolute values given by this inquirer, we calculate the elementary composition per cent of the solid residuum of the urine and faeces, we shall find that it is the same for all the persons referred to. Hence it is pro- bable that these percentages are not the results of special examinations, but have been reduced from one chief analysis. But since the various kinds of food produce differences in the quality of the urine and faeces, a new and important source of error is thus opened up. We may collect the estimates given by Barral as follows : Quantity in Twenty-four hours expressed in ounces. Weight of Person, Age, and Bodily Weight. Carbonic Acid to that of Oxygen consumed. Carbonic Acid given off. Oxygen as 1. Barral himself, 29 years, 104-8 Ibs. 37-47 43-45 1-16 The same .... 27-3 31-36 1-15 Male, 59 years, 129'5 Ibs. 31-39 38-42 1-23 Female, 32 135- 31-3 35-54 1-14 Boy, 6 33-1 14-95 18-07 1-21 The average was 1-17, which almost exactly corresponds with the diffu- sive proportion ( 815). But the reasons above mentioned prevent us from regarding these numbers as a confirmation of any theory. 847. Supposing that the somewhat forced respiration in my experi- ments about made up for the small quantity of carbonic acid in the cutaneous exhalation, it would follow that, on an average, I take up 28-52 oz. of oxygen, and give off 33-16 oz. of carbonic acid, in the 24 hours. Here the relative weight is 1-16. 848. Recent eudiometric researches have not proved that any consi- derable quantities of other gases escape through the skin. Volatile organic matters, and even traces of inorganic compounds, frequently appear. The acid smell generally diffused by a perspiring portion of skin appears chiefly due to volatile fatty acids ; as, for instance, caprylic acid ( 310). The sebaceous secretion of the skin, and the sweat, can furnish abundance of the requisite materials. 849. The function of the skin is capable of being, to a certain extent, inverted by a water-bath. The surrounding liquid not merely hinders or restricts the interchange of air : it gradually soaks through the cuticle ( 120) and thus allows of endosmose ( 129). In this way the materials of the bath reach the nutritive fluid and the blood. CHAPTER XL FIG. 139. SECRETION. 850. THE porous walls of the vessels permit a reciprocal diffusive current between the blood, which is propelled under a certain pressure, and the nutritional fluid, which soaks the tissues. If the vessels occupy structures which abut on a cavity shut off from the outer air, they will give off fluids, until the tension of these forms a counterpoise to that of their blood, and the chemical attractions are satisfied. This simplest form of secretion obtains in the membranes of the brain and spinal cord, the pericardium, the pleura, the peritoneum, the vaginal tunic of the testicle, the synovial membranes of the joints, the sheaths of the tendons, and the bursse mucosse : in short, in all the serous membranes, and their congeners. The relations of a tendinous sheath are exhibited in Fig. 1 39. Let a be a tendon, the surface of which is covered by the membranous layer b c, reflected into the free lamella, d e. The blood circulating in d e will fill the cavity, /, with fluids, until all active difference of pressure or che- mical composition ceases. And if any collateral cause alters the fluid contained in /, the portions of blood circulating in the neighbouring vessels are always ready to equalize the difference. 851. If the cavity that bounds the secreting membrane is open towards one side, the exsudation may be carried off or excreted. This absence of the counter-pressure which finally checks secretion in the closed cavity, ensures its unobstructed continuance in the open one. The skin, the mucous membranes, and the numerous secreting glands, all possess this important advantage. For instance, the bile furnished by the liver (a, Fig. 75, p. 132) flows through the hepatic and biliary duct (n and r) into the duodenum (i). In this way the mixture already produced is no obstacle to subsequent secretion. 852. Other circumstances being equal, the quantity of fluid which passes off in a given unit of time must vary with the size of the free surface. Hence nature increases this as much as possible. The structure of the secreting glands may in great part be thus explained. CHAP. XI.] INCREASE OF SECRETING SURFACE. 263 853. Let us suppose a b, Fig. 140, to be the transverse section of a plane limitary surface ; if we roll up a b into c d e, or fold it into f g h i k, its superficies will remain the same. But in the two latter cases the membrane possessing the surface occupies a smaller space. So that if this suffice to the object of the whole, it will be possible to compress an active surface of great extent into a very small organ. FIG. 140. FIG. 141. 854. The secreting membrane, g b } Fig. 141, may thus gain surface in two ways. If it expands into folds, villi, or other prominences (a b c), these will form a new mass, the interior of which may be occupied by blood-vessels. But the free surface, a b c, exceeds the rectilineal one, a c, by a certain amount, varying with its form. And its secretion passes immediately into the neighbouring space at i. On the other hand, if g k sinks into a depression, d e f, the mere increase of surface is repeated j but we obtain a follicle, which secretes its exsudation, first into &, and thence into the neighbouring space, i. The first case represents a simple secretion ; the second a secretion, followed by an excretion. 855. And just as gills increase the respiratory surface by external rami- fications, and lungs by internal cavities ( 725), so something like this is repeated in the organs of secretion. The mucous membrane of the small FIG. 142. intestine is studded by numerous villi, such as are represented in Fig. 1 42. These increase the secretion of intestinal mucus, as well as the capacity 264 DIFFERENT FORMS OF GLANDS. [CHAP. XI. for absorption. A similar purpose is fulfilled by folds j which occupy the mucous membrane of the alimentary canal, as well as the papillae (Tab. IV. Fig. 62, d e) of the corium, the conjunctiva, the mucous mem- brane of the nose, the cavity of the mouth, the urinary bladder, the vagina, and many other parts. 856. The structure of the secreting glands is based upon the formation of depressions, as shown in Fig. 141. The glands of the stomach exhibited n Fig. 78 offer one of the simplest examples. The tube (a) in the walls of which the blood-vessels run encloses a space, which opens by one extre- mity into the cavity of the stomach. 857. There are two methods by which surface can be still more densely packed. The secreting cavity is produced into a long and small tube, which is rolled up as closely as possible (Tab. IV. Fig. 62, o p) ; or it branches continually like a tree, into an increasing number of subordi- nate excretory ducts, which are compressed together in the same manner (Tab. IV. Fig. 53, b c). The tubular glands are an example of the first case ; and the ramified, of the second. Fig. 143 is a diagram of a cutaneous gland, such as is called a spiral or sweat gland. The common duct first divides into two subordinate tubes, which are then variously coiled upon themselves. Fig. 144 re- presents a portion of the parotid. Here the branching continues, until FIG. 143. FIG. 144. there are none but fine secreting tubules, which are visible under a low magnifying power. These terminate by rounded blind dilatations, or vesicles. (Tab. IV. Fig. 52, 6.) 858. It is obvious that nature is not exclusively bound to any of the types just described. We find, in fact, that the same organ of secre- tion sometimes enlarges its surface in different ways. The tubes of the cutaneous glands (Tab. IV. Fig. 52, n and Fig. 143), and those of the kidney, (Tab. V. Fig. 55 and Fig. 153) bifurcate a few times before they attain a great length, and become coiled up. The terminal vesicles of the ramified glands generally exhibit a smooth internal surface. But CHAP. XI.] DEPENDENCE OP SECRETION ON THE BLOOD. 265 we have already seen ( 725) that the pulmonary vesicles possess folds, which additionally increase the respiratory surface. 859. We have no means of accurately determining the extent of secreting surface contained in the glands. But estimates which are rather too small than too large teach us, that amounts are often attained in this way, such as seem incredible at first sight. The secreting surface of one parotid gland may be set down as about two-thirds that of the skin. A testicle presents about 2 square feet, a kidney about 48^, and a lung at least 64J. 860. The quantity, velocity, and quality, of the blood that circulates in a secreting organ, must exercise an important influence upon the exsudation itself. The kidneys, which are intended for the rapid re- moval of superfluous water, are therefore supplied by the wide renal arteries. The large circumference of the pulmonary artery (a, Fig. 87, p. 174) has also the object of allowing as much blood as possible to be carried to the lungs in a short time. Other circumstances being equal, a gland provided with more numerous capillary vessels may furnish a very different secretion, since the dilatation of the channel ( 689) diminishes velocity ( 106) and permits a different process for equalizing diffusion. If the channels of the blood are so arranged that pressure is raised or depressed, we get a second important influence. We shall hereafter find that phenomena of this kind are best seen in the kidneys and the liver. The instance of the bile plainly shows what an influence is exerted by the admixture of the blood upon the quality of the secretion : since the liver which prepares it offers a marked exception to the other glands, in being supplied with blood from the portal vein, instead of from the arterial system. 861. The coats of the vessels, the limitary membrane, and the endo- thelium, of the glandular ducts, form a partition, the molecular consti- tution of which probably plays an important part in all excretory pro- cesses. Hitherto this subject has not received a very careful physical investigation. But one may foresee that the truth will only be brought to light by very round-about ways. For the tunics of the finest excretory- canals are too minute to be subjected to special experiments on their diffusive capacity. Their physical characters are often essentially altered shortly after death. It is for the most part impossible to imitate in frag- ments of the dead body, those collateral conditions of pressure, current, and proper egress, which exist during life. And finally, it is evident that the operations producible by independent contraction cannot possibly be repeated in the dead parts. 862. Recent microscopic researches strongly indicate that many se- cretions are the result of still more complicated phenomena. The interior of the stomach-tube of the pig (which is represented highly magnified by Fig. 145), is covered by cylindrical epithelia apposed to 266 INFLUENCE OF THE GLAND-CELLS ON SECRETION. [CHAP. XI. FIG. 145. each other like a palisade. And the gastric juice which they secrete is not a mere chemical solution, but a mixture of a mucous fluid with granules, nuclei, and cells. We shall hereafter see that something similar to this is the case with all kinds of mucus. A homogeneous fluid which exsudes from the blood-vessels dissolves cer- tain constituents of the epithelia that are simultaneously produced ; while others are mechanically mixed with it, to form a solid residuum. Hence the resulting secretion contains three constituents, a fluid which is rapidly secreted, the dissolved products of an endothelium which is more slowly produced, and insoluble relics of these or other structures. 863. A microscopic examination of the liver, kidneys, and testicles, also leads to the conviction that many of the compounds found in the secretions formerly occupied the interior of gland-cells. The human Fi 1 46 li yer contains a great number of peculiar cells, some forms of which are shown in Fig. 146, as they appear under a mag- nifying power of 255 diame- ters. These often contain yellow globules, or yellowish masses, of indefinite outline, possessing the chemical qualities of certain constituents of the bile. The cells in the kidney-tubes of many in- vertebrata (compare Tab. V. Fig. 45, a) contain granules of uric acid. The moving seminal elements wrongly called spermatic animalcules (Tab. V. Fig. 76) are begun in parent-cells, which gradually break up and disappear. Here Nature works by a continuous slow growth during the period of rest. So that materials, which require a long time for their development, can be thrown off at any time with great rapidity. From these and similar facts many observers conclude, that the formation of parent-cells is an important sign of secretory activity. 864. The question has frequently been raised, whether the action of the glands is based upon mere phenomena of filtration and diffusion, or upon more recondite organic processes. This inquiry may to a great extent be answered by what we have already seen. The original fluid mixture comes from the blood and the nutritional fluid. The result will therefore be greatly determined by the surplus pressure of these juices, and by the character of the partitions (861). These are simple physical actions, CHAP. XI.] VERMICULAR MOVEMENT OF THE GLAND-DUCTS. 267 the effects of which must vary with the structure and circumstances of the secretory organs. This may perhaps explain why many serous secre- tions contain large quantities of salts ( 143) ; why the urea previously formed in the blood passes off almost exclusively by the urine without any decomposition ; and why the yellow colour of this fluid, and of the bile, is wanting in the saliva and the different kinds of mucus. On the other hand, the facts mentioned in 863 show that many secretions require the organic development of cells. If there is not time for this process, the secreted fluid has a watery and more or less abnormal constitution. 865. We shall hereafter find that some serous or mucous fluids are given off at various parts of the body. But others can only be prepared by special glands. The sperm is only produced in the testicles. And since the human spermatic artery is distinguished from all others by its great length and small width, one might imagine that the peculiarities of circulation thus produced essentially aided the development of the secretion. But, in most animals, this arrangement is absent. And Berthold's experiments show that, in birds, the proper vascular and nervous connection is not essential. For, after allowing the excised testicle of a cock to become attached to the intestine, he found that it still contained spermatic elements. Hence the structure of the spermatic tubes, and their previous contents, together suffice to this continuance of development. 866. It is obvious that the fluid contained in the tubes of a gland offers a certain obstacle to subsequent secretion. But, in most instances, the open ducts prevent the accession of a counter-pressure sufficient to check all further exsudation. Now, since towards the blind extremities of the ramified glands their surface increases, and the sum of the cali- bres of the subordinate tubes is greater than that of their trunk, the con- tinuous process of secretion furnishes a vis cb tergo ( 545), which propels their fluid contents with an increasing velocity ( 106). But as this alone would rarely offer a sufficiently certain or rapid means of pro- pulsion, nature aids it by others. 867. The excretory ducts of the larger glands, such as the ureters (u, Fig. 119, p. 208) or seminal ducts, are capable of vermicular move- ments as rapid and vigorous as those of the intestine ( 399). Under the microscope, unstriped muscular fibres (Tab. IV. Fig. 59) may be seen in their middle coat. It is true that the smaller ducts of glands generally exhibit an uniform middle tunic (Tab. V. Figs. 64, a, 65, I). Still we shall find that this does not disprove their contractile power. 868. The contraction of neighbouring muscles sometimes extrudes the contents of excretory organs. It is probable that the muscles of masti- cation act thus upon the parotid gland. But nature does not trust 268 SWEAT. [CHAP. xi. exclusively to such assistance. All these glands can give out large quantities of fluid, even when the neighbouring muscles are at rest. 869. The Secretion of the Skin. The comparative extent of the cuta- neous surface varies greatly at different periods of life. We have already seen that ( 89) the skin of an adult man forms a surface of about 2325 to 2550 square inches. Hence one pound of the body corresponds to about 20 square inches, and a cubic inch of its volume to from -58 to '74 square inches. While a new-born female infant, with a weight of 31bs. 14| ounces, and a bulk of 105-76 cubic inches, presented a cuta- neous surface of 185 square inches. So that a pound of its weight, and a cubic inch of its volume, had a surface of 47*44 and 1-75 square inches respectively. This difference does not exclusively depend upon the greater development of the adult skeleton; but appears rather connected with the fact, that small bodies have proportionably larger surfaces.* 870. We have already ( 839) learnt the chief condition under which sweat is poured out. Since the skin warms the surrounding atmosphere, watery vapour is constantly passing off, even when the air is saturated with vapour at its own temperature (191). In sweating, the organism sets free larger quantities of fluid. In general, the act is due, either to a greater distension of the skin with blood, or to an alteration in its porosity, or to an union of both these causes.t 871. Many have supposed the sweat to be formed by the glands represented in Fig. 143, p. 264, (Tab. IV. Fig. 42, i to p), and which, from the twisting of their excretory ducts, have been named spiral glands. Indeed they are often expressly called the sweat-glands. It is certainly probable that a liquid secretion would exsude from the free openings of these gland-tubes (Tab. IV. Fig. 42, i) more easily than it would pene- trate the dense cutaneous tissues themselves. But since sweat is often poured out in places where few or none of these tubes are present, the skin itself must form its means of egress. 872. The sweat is one of the most dilute fluids of the body ( 34). Its solid residue forms but J to 1 per cent. It contains chloride of sodium and ammonium, and phosphate of lime, with other salts, and organic matters. It also contains epithelial scales (Tab. II. Fig. 32) and fatty substances; which are due to the admixture of the sebaceous secre- tion of the skin. The remarkably sour smell of many kinds of sweat appears to depend, partly on acetic acid, partly on volatile fatty acids ( 848). * It may be useful to illustrate this well known fact by an example. A sphere having a radius of 1 inch would possess a surface and capacity of 12- 6 square, and 4'2 cubic, inches respectively; which would be increased to 50'4 and 33*6 in one having a radius of 2 inches. So that each inch of volume would correspond to 1 \ of surface in the larger, and 3 in the smaller, body. EDITOR. f These vaporous and liquid secretions are often distinguished from each other as the insensible and sensible perspiration. EDITOR. CHAP. XI.] SEBACEOUS SECRETION. 269 873. When sweat ceases to be produced, the water and other volatile compounds which have collected on the surface of the body gradually evaporate. Other substances are precipitated in the solid form ; so that microscopic crystals of chloride of sodium (Tab. I. Fig. 1) or other salts are sometimes found on and between loosened scales of the cuticle (Tab. IV. Fig. 62, i, c, a). 874. Violent perspiration gets rid of so much water that the necessity of compensation is evinced by thirst ; and the quantity of urine is visibly diminished. Many morbid phenomena indicate that the conditions under which sweat is poured out are not yet accurately known. Persons suffering from subcutaneous dropsy usually exhibit a dry skin. Sweat concludes many febrile attacks. Copious perspiration often accompanies fatal dege- nerations of the infantile brain. The yascularity and temperature of the skin do not always vary in proportion to these alterations. The cold sweat which accompanies vomiting or syncope is an instance of this fact. In these cases we are justified in supposing that the porosity of the organic partition, which depends upon the nervous system, plays an important part. 875. Many portions of the skin possess special glands which, from the fatty matter they secrete, are called sebaceous. Those which secrete the wax of the ear are constructed after the type of the tubular glands (figured in Fig. 143, p. 264). The Meibomian glands of the eyelids form tubes, the whole length of which is studded with lateral vesicles (Tab. IV. Fig. 52). On the other hand, the numerous small glands which accompany the hairs (Tab. IV. Fig. 63, k i k) ramify like a tree, and ter- minate by globular heads. They pour their fatty contents into the cavity out of which the hair passes (Tab. IV. Fig. 63, g) : while other larger glands, such as those which occur in the neighbourhood of the alee nasi, in other parts of the face, and in the labia pudendi, open immedi- ately upon the surface of the skin. 876. The sebaceous glands of the hair furnish a fatty mixture which anoints its shaft, (Tab. II. Fig. 39, Tab. IV. Fig. 63, cd) making it smoother and more pliant. It thus forms a kind of pomatum, provided by nature itself. Its superfluous part, together with the fatty secretion of other sebaceous glands, becomes mixed with the desquamated scales of the cuticle (Tab. II. Fig. 32). This mixture, the fat of which is easily altered by the atmosphere, forms the sebaceous secretion of the skin, and the wax of the auditory duct. The fatty substance on the corona of the glans penis is produced by the simple sebaceous glands of this part. 877. Serous secretions. To this class belong the fluids contained in the shut serous sacs of the cerebral and spinal membranes, the pericardium, the pleura, and the vaginal tunic of the testicle. We have already ( 850) learnt the cause which normally limits their quantity : in many morbid states they are greatly increased ; so as to give rise to a dropsy of the particular sac affected. 270 MUCUS. [CHAP. XL 878. The fluids under consideration generally form limpid, colourless, or at most faintly yellowish, solutions. It is questionable whether the epithelial scales they sometimes contain are not the results of a desquama- tion which occurs after death. Most of them have 1 to 2 per cent of solid residuum, which generally contains albumen and other organic matters, with what is often a very large proportion of ashy constituents. Their mechanical use has ( 94) already been explained. 879. The aqueous humour of the eye closely resembles the serous secretions. It also contains less than 2 per cent of solid matters; in which the chloride of sodium amounts to nearly half as much again as the organic compounds. In the fluid which moistens the vitreous sub- stance the above proportion of this salt is increased. And Millon found that both these fluids contain urea. 880. Mucus. It has already been remarked ( 862) that mucus is not a simple secretion. A dilute and richly saline exsudation dissolves those epithelial cells which are not too horny; and thus acquires a mucous character. Hence we find it mechanically mixed with fine granules; with the nuclear structures called mucous corpuscles (Tab. II. Fig. 31, d); and often, with perfect or half-corroded cells. The contactive effects of many kinds of mucus probably depend upon these variable constituents ( 418). These facts also explain why, when the secretion of the nose is hurried by the contact of cold air or by catarrh, a thin saline mixture is poured out. 881. For the same reasons, the quality of mucus is liable to great variation. The ordinary mucus of the nose contains from 88 to 94 per cent of water, and that of the pulmonary membrane about 95 ; while the gastric juice, which is secreted more copiously, has 98 to 99 per cent. The compounds afterwards added by solution generally cause the organic to predominate over the ashy constituents. 882. Mucus is produced in two ways : in special glands, or on free sur- faces. The former may either simply increase the secreting surface, or may assist to prepare special juices, which are subsequently mixed with the rest of the mucus. To the latter class probably belong the cylin- drical follicles of Lieberkuehn ; which crowd the mucous membrane of the small intestines, and which are represented by the accompanying diagram (Fig. 147). They form a mass below, while the intestinal villi project above : so that both the types of increase of surface exhibited in our diagram (Fig. 141, p. 263) are here present together. The glands of Brunn, which occupy the duodenum, and terminate by dilatations like grapes upon a stalk, probably furnish a mixture different from the neighbouring intestinal mucus. Their outline is represented by Fig. 1 48. 883. As yet we know very little about the import of those shut cap- sules, which are irregularly met with in many mucous membranes. They are scattered over various parts of the stomach and intestine; where they CHAP. XI.] LENTICULAR GLANDS. 271 are called lenticular, solitary, and aggregated glands. To these also belong the glandulce Nabothi of the mucous membrane of the uterus. The Peyerian patches are aggregations of vesicles, such as are represented by the globular enlargements in Fig. 142. Cylindrical follicles of Lie- berkuehn are also found around them. And, according to Bruecke, the absorbents of the intestine may be injected from their interior. FIG. 147. FIG. 148. 884. Each of these closed vesicles contains a mucous and usually alkaline fluid ; in which we again find a mechanical admixture of cells, nuclei, and granules. Since in many dead bodies they are very numerous, while in' others they are very few, it may be conjectured that their number depends upon secondary causes, of which we are as yet almost ignorant. But it is probable that their disappearance is due to their contents being either absorbed during life, or poured out from a rupture which occurs after death. 885. The different kinds of mucus are useful either by their mechanical or chemical properties. We have already ( 80) seen that they greatly reduce the obstacle which friction offers. Many of them have the power of uniting bubbles of air into a kind of emulsion ( 493), so as to present a frothy appearance. Others, such as the intestinal mucus, contribute to the minute division of the fats and the residue of the food ( 365). All of them protect the surfaces they clothe from the injurious effect of the fluids with which they are in contact ; such as the atmospheric air, the bile, the urine, or the menstrual blood. Finally, the chemical qualities of some particular kinds of mucus endow them with contactive powers. 886. The characters of the synovia, which is furnisned by the synovia! 272 SYNOVIA. [CHAP. xi. membranes, and collects in the articular cavities of the joints place it, as it were, midway between the serous and mucous secretions. It forms an alkaline, colourless, or faintly yellowish mixture; the tenacity 'of which, according to Frerichs, 30 ) depends upon mucus. Here again the mucous character is produced by the solution of epithelia : and, according to this observer, it increases with the frequency of movement. In the latter case the density of the synovia therefore increases. That of the knee-joint of a stall-fed ox contained 97 per cent of water; while that of an animal which had spent the whole summer at pasture had but 95 per cent. Mucus, albumen, and other organic matters as yet but imperfectly known together with fat, chloride of sodium, sulphates and phosphates of the alkalis, carbonate of lime, and earthy phosphates are found in the fresh fluid, and in the ashes of its solid residuum. Microscopic research teaches that the mechanical admixture consists of fragments of epithe- lium. At any rate this is the case with synovia obtained after death. The fat globules sometimes remarked seem to be only aggregated by violence. 887. The fluid is prepared by the synovial membranes themselves, and not by any special glandular structures imbedded in them. The Haversian glands of the knee joint are mere cushions of fat. But, in this joint, certain free portions of the synovial membrane are clothed with villi, which increase the secreting surface. 888. The synovial membranes of many joints, (such as those of the shoulder, the knee, and often the hip), are prolonged into supplementary sacs called bursce mucosce. According to Weber, when FIG. 149. the knee-joint is filled by a congealed mass, it has the shape represented in Fig. 149. A mucous bursa, a, passes under the common tendon, 6, of the extensor femoris; and a second and third (c and d) run with those of the hamstring muscles. Thus, according to the difference of position, the viscid synovia which diminishes friction ( 80) can either pass into these supplementary sacs, or can retreat into the cavity of the joint. 889. The closed bursce mucosce, and the sheaths ( 850) frequently found under the muscles and tendons, contain a fluid similar to synovia. Hitherto, however, its properties have not been further examined. But it is probable that it also is a serous fluid, which only becomes viscid subse- quently. 890. The lachrymal secretion. The opposite wood-cut (Fig. 150) repre- sents the left human eye. The lachrymal gland (a), consisting of an upper and lower division, occupies the upper, outer, and anterior part of the orbit. It belongs to that class of secreting organs which ramify, CHAP. XI.] THE TEARS. 273 and end by vesicular dilatations (Fig. 144, p. 264). Delicate excretory ducts convey its fluid to the outer angle of the eye. Here it reaches the sac of the conjunctiva, the small space which lies between the surface of the eye (b, c, d) and the eyelids (e and/). From hence it is con- ducted onwards by a twofold process. It advances in the fissure of the conjunctiva, just as water rapidly spreads between two moistened glass plates in close apposition to each other. Besides this, it mixes with the fluid previously present. 891. Several other secretory organs in this situation furnish mixtures of different kinds, which are united with the product of the lachrymal glands. The conjunctiva itself which clothes the surface of the cornea (6, Fig. 150), the sclerotic (d), and the interior of the eyelids, adds a fluid; which is chiefly secreted by its more vascular parts, and by the few mucous glands it contains. The Meibomian glands imbedded in the eyelids (Tab. IV. Fig. 152) prepare a fatty secretion, which is poured out at their margins. The glandular caruncula lachrymalis at the inner angle of the eye (g, Fig. 150), probably adds another mucous secretion. 892. Since the fluid covering the eye is exposed to the air, a variable quantity will evaporate ( 186). A special apparatus carries the remainder into the cavity of the nose. 893. On everting the margin of either eyelid before a mirror, we observe a dark point, called the punctum lachrymale (h i, Fig. 150). It o r I f c is the aperture of a small excretory duct, the lachrymal canal. The two canals (Jc and Z, Fig. 150), open into the lachrymal sac (m), which passes off into the lachrymal duct (n). This finally opens into the nasal fossa, below the anterior part of the inferior turbinated bone. 894. The propulsion of the tears is aided by a frequent repetition of the act of winking. They thus pass into the lachrymal canals (k I, Fig. 150), the lachrymal sac (m), and the lachrymal- duct (n), to discharge themselves into the cavity of the nose and mingle wi^h the mucus there. The ciliated cylindrical epithelium (Tab. II. Fig. 36), which occupies the surface of the mucous membrane of the nose, is continued into the T 274 THE TEARS. [CHAP. XT. lachrymal duct and sac. If the current it produce be directed down- wards, it will aid in the discharge of the tears. 895. Many observers have supposed that the respiratory movements also assist this propulsion. The lower outlet (o) of the lachrymal duct (n) often possesses a more or less complete fold, which is capable of acting as a valve. It permits fluids to pass unhindered from the lachrymal duct into the nasal cavity. While if air or any other substance attempts to take the reverse direction, it shuts. It is therefore imagined, that the draught which accompanies inspiration ( 735, et seq.) assists the passage of the tears into the nose, while the valve prevents the entrance of air or mucus during strong expiration. 896. From what has just been stated, it is evident that a more or less continuous stream of tears runs from the outer towards the inner angle of the eye. Apart from evaporation ( 892), the quantities received and discharged are equal. Thus the eye is only maintained moist ; neither swimming in tears, nor allowing any surplus to pass out between the eyelids. And since the Meibomian glands open upon the margin of the eyelid, their fatty secretion enables the surface in this situation in some extent to resist the passage of watery fluid. 897. Sometimes the tears are secreted so copiously, that the current towards the nasal cavity becomes insufficient to carry off all the fluid. A large quantity therefore collects in the conjunctival sac, and finally emerges from the fissure of the eyelids. This unusual mode of dis- charge constitutes weeping, an act which has some analogy to that of sweating ( 839). 898. Another abnormal form of discharge is seen in lachrymal fistula. When the lachrymal duct (n o, Fig. 150^ p. 273) is obstructed, the sac (m) is distended by a constantly increasing quantity of fluid, a fact which proves that the act of inspiration is not the exclusive condition of its passage hither. An external fistulous opening is thus finally produced, the closure of which is prevented by the chemical irritation of the tears that constantly pass through it. If the natural outlet is restored, the supplementary opening gradually becomes less disturbed by the passage of the injurious secretion, and so heals up. 899. The quantity of tears which moistens the conjunctival sac in the ordinary state of repose, is too small to be subjected to analysis. Hence the only fluids which have been used in physico-chemical researches are those poured out during weeping ; i. e., during a flow of tears excited by unusual collateral causes. It might be expected that this mixture, which is prepared and poured out with unusual rapidity, would contain more water and salts, and less mucus, than under ordinary circumstances ( 864). According to Frerichs this dilute secretion consists of 98-7 to 99 per cent of water. It contains epithelia, and sometimes fatty substances, CHAP. XI.] THE SALIVA. 275 together with mechanical admixtures. Its organic matters consist partly of albuminous and mucous substances. In its ashes are found chloride of sodium, and phosphates of the alkalis and earths. 900. In morbid states, the foreign admixtures may exceed the original solid residuum. On collecting the turbid fluid which constantly flowed from a blind human eye, I found that it left 5-9 per cent of residue. It contained numerous more or less perfect epithelial scales, very small fatty molecules, crystalline structures (in all probability chiefly inorganic salts which had only been precipitated on standing), and minute mole- cular corpuscles, the exact nature of which could not be determined. 901. The crust not unfrequently found in the inner angle of the eye, especially in the morning, contains large quantities of more or less perfect epithelial scales, with small granules and oil-globules. Hence in many respects it resembles the wax of the ear, and the sebaceous secretion of the skin ( 875). The globe of the eye is so arranged, that the eyelids exert a pressure, which tends to propel the substances contained in the conjunctival sac towards the inner angle of the eye. The flow of the tears is thus facilitated from its very commencement ( 894). In this way a particle of dust which has fallen into the eye, after some time passes to its inner angle. The same fact explains why the crusts just mentioned are oftener met with at the inner angle of the eye than at the margins of the eyelids. 902. From the above statements it is easy to see why the eye remains moist, even after the destruction of the lachrymal gland, or the loss of a great part of the conjunctiva. The idea that the aqueous humour transudes the cornea to gain the conjunctiva has little support from theory; and receives none from the experiments instituted by Frerichs. 903. The saliva. A series of large glands furnish the secretions called the saliva and the pancreatic fluid. Each of the two parotids forms a tolerably flat mass, which is thicker posteriorly, and lies before and beneath the ear. The duct of each opens on the inner surface of the cheek, opposite the first or second molar tooth of the upper jaw. The duct of each of the two submaxillary glands behind and beneath the lower jaw opens by an orifice near the frcenulum linguae. Each of the two sublingual glands has a number of efferent canals, which also open beneath the tongue. Occasionally some of these are united with others from the submaxillary glands, to form a single duct. The lingual glands are situated at the apex of the tongue. The pancreas (p q, Fig. 551, p. 280) empties itself into a duct which opens into the descending portion of the duodenum (i). 904. Taking the volume of the sublingual gland as 1, the submaxillary has a bulk of 4, the parotid from 8 to 9, and the pancreas 27 to 28. Thus the salivary glands of both sides form a total of 27 : or something less than the single pancreas. T 2 276 QUANTITY OF THE SALIVA. [CHAP. XI. 905. We have already seen, ( 422) that the spittle is a mixture of pure saliva with the fluid secreted by the mucous membrane of the mouth, and by the various glands imbedded in it. 906. The activity of these organs varies greatly according to circum- stances. In a state of rest, the salivary glands scarcely secrete any fluid; while, under other circumstances ( 423), they pour out a large quantity. In such cases the amount of mucus is also increased, though to a smaller extent. Salivation, or ptyalism, is a morbid increase of the spittle. 907. Such variations render it difficult to determine how much of these secretions generally escapes in the 24 hours. And when we add that some of the fluid of the mouth is involuntarily swallowed during waking, and still more during sleeping, it will be evident that all the estimates hitherto made are necessarily inaccurate. 908. It is generally supposed that an adult man secretes about 10 to 14 ounces of spittle daily. Mitscherlich, who had the opportunity of examining a man with a fistula of the parotid duct, obtained an average of about 2-8 ounces fluid from this opening in the 24 hours. And sup- posing that all the salivary glands together make up a secreting surface 3J times as large as that of one parotid, and act simultaneously, we should get a total of 9-4 ounces. The remainder would be due to the lingual glands and the mouth. Jacubowitsch 31 ) sought for an approximative solution of this pro- blem by tying the efferent ducts of the several salivary glands in dogs, and then determining the amount of fluids poured into the cavity of the mouth. But the numbers thus obtained are invalid. For not only were the animals placed in a morbid state by the artificial inter- ference, but the cavity of the mouth was stimulated with vinegar, and a part of its fluids was undoubtedly lost by evaporation and deglutition. According to his researches, the quantities per hour amounted to 1'737 ounce for the two parotids, 1'37 for the two submaxillaries, and '875 for the orbital and sublingual glands and the mucous membrane of the mouth. But since this would give 95*3 ounces for 24 hours, it is evident that the disturbances above mentioned although to some extent correc- tive of each other caused a great increase in the total quantity of fluid secreted. 909. We have already become acquainted ( 425) with the watery and other constituents of the oral spittle. Epithelium (Tab. II. Fig. 31) and mucus usually form more than J of the solid residuum. Besides this it contains ptyaline, and other still more recondite nitrogenous com- pounds, with traces of fat. And in addition to the ordinary salts such as chloride of potassium and sodium, with phosphates of the alkalis and earths, and some oxide of iron it contains sulphocyanide of potassium. This salt, which betrays itself by the red colour it produces with the CHAP. XI.] PANCREATIC FLUID. 277 salts of iron, amounts at most to r^^th of the fresh fluid. So that if we suppose an adult man to secrete 9 '4 ounces of mixed spittle in 24 hours, this quantity would correspond to *42 grains of the sulphocyanide : a quantity so small, that it could not possibly produce any injurious effects. 910. In salivation ( 906), which sometimes arises spontaneously, but is generally due to the use of metals, and especially of quicksilver, the secretion is not necessarily more watery than the ordinary fluid of the mouth. It frequently contains mucus in large quantities; and albumen, or even urea, as stated by Wright. According to Gmelin, we may some- times establish the presence of mercury in the saliva of persons under- going a mercurial course. The salivary calculi, which scarcely ever occur save in the ducts of the sublingual glands, are composed of carbonate and phosphate of lime, to the extent of more than f ths of their substance. The remainder consists of an organic basis, with small quantities of alka- line salts. 911. Just as a large quantity of saliva is poured into the mouth during the act of eating, so the attempt to collect the pancreatic fluid of living animals only succeeds at the period of intestinal digestion. We have already ( 461) seen, that the composition of this secretion varies greatly according to the circumstances under which it is obtained; and that it is also very much inclined to decomposition, a peculiarity to which its digestive action is probably chiefly due. 912. Dogs undergo extirpation of the pancreas without injury. And since men may live for years with complete degeneration of this secreting organ (p q, Fig. 151, p. 280), we are justified in supposing that its func- tion is not essential to the maintenance of life. Such persons sometimes vomit large quantities of watery fluid ; and this has been regarded as pan- creatic juice which has undergone regurgitation into the stomach. Since irritation of the salivary glands of the mouth often causes the secretion of large quantities of fluid, a similar process may perhaps occur with the pancreas. But it is more probable that these fluids do not come from this gland, but consist of saliva, which has been secreted in answer to gastric irritation ( 423), and afterwards swallowed. 913. The Bile. The liver is distinguished, not merely by volume and density, but also by its peculiar relations to the vascular system. We have seen ( 681) that the veins of the stomach, intestine, pancreas, and spleen, unite in the portal vein. This ramifies in the interior of the liver, receives the veins of the gall-bladder and hepatic ducts, unites with the finest branches of the small hepatic artery, and finally ends in the capillaries which are continuous with the roots of the hepatic vein. 914. If the hepatic artery, the portal vein, and the hepatic veins, be injected to their finest branches with substances of different colours, an 278 THE BILE. [CHAP. xi. examination of suitable sections under the microscope will show that the liver consists of a number of roundish grains or lobules, more or less distinct from each other. In the intervals of these run the branches of the hepatic artery and portal vein. A number of capillaries, united with each other so as to form a network, pass into the substance of the lobule, and converge towards its centre. A common trunk which de- scends in the middle of the lobule, forms one of the numerous rootlets of the hepatic vein, the terminal trunk of which opens into the inferior vena cava (I, Fig. 119, p. 208). Hence the network of capillaries, from which the secretion chiefly proceeds, contains a dark-red portal blood. While the capillaries of other glands receive a bright-red blood from their arteries. 915. The precise mode in which the hepatic ducts terminate is as yet undecided. If the hepatic canals be injected from the duct with some congealing substance, they may be seen to bifurcate ; and subsequently they unite with each other by occasional cross-branches. In some of the lobules we may remark a network of the coloured substance, having meshes which appear to be either empty, or are penetrated by extremely fine capillaries. But, on the other hand, the fresh liver chiefly exhibits regular aggregations of the liver cells (Fig. 146, p. 266), some- times separated by clear radiating intervals. Many observers suppose these cells to occupy the interior of a structureless membrane which is continued from the middle tunic of the biliary canals. But from in- jected preparations, Gerlach 32 ) concludes that these canals, after surround- ing the lobules of the liver with a network, and sending nets into their interior, are continued into mere dilated spaces, which are devoid of walls and occupy the intervals of the hepatic cells. A third view makes them terminate in flattened blind sacculi lying close to each other. 916. The obscurity which attaches to the minute anatomy of the liver is again met with in most of its physiological relations. The liver of the adult weighs from 2-2 to 3-75 pounds. Assuming its average to be 46 '3 ounces, and that of the parotid -7 ounce, the former would be 66 times as great. And if the quantities secreted by these glands were proportionate to their weight, the liver would furnish about 11 pounds of bile daily. But it is very improbable that so much is really secreted. And since the tissue of the parotid is looser, and its ducts larger, than those of the liver, a cubic inch of the latter would contain more secreting surface. Hence it is probable that the bile is secreted much more slowly than the saliva. 917. It has often been attempted to solve this question by means of experiments on animals. Nasse and Platner found that a dog weighing from 19 to 21 pounds gave off from 5 to 6 ounces of bile daily through a biliary fistula. Stackmann 33 ) tied the common hepatic duct (n r, Fig. 151) in living cats, and placing a canula in the evacuated gall- CHAP. XI.] QUANTITY OP THE BILE. 279 bladder (I), collected the bile which flowed out in definite periods of time. Reducing these safer results to a pound of bodily weight gave as follows Accompanying Circumstances. Quantity of Bile, in grains, for each pound of bodily weight. The hours referred to are those which followed the commencement of the experiment. First Hour. Second Hour. Third Hour. From 2 to 3 hours after feeding From 12 to 15 hours after feeding Twenty-four hours after feeding Fasting from 48 to 240 hours 3-92 to 6-51 574 . 9-1 2-52 4-34 1-54 4-48 2-59 to 5-11 5-39 6-23 1-82 3-92 63 2-52 1-54 to 5-04 4-55 5-88 1-12 3-99 35 2-24 From thig we see that cats pour out the largest quantities of bile from 12 to 15 hours after feeding. Tf they are made to fast for some days, the secretion gradually diminishes. Hence it is not merely due to that meta- morphosis of the blood which is produced by the various functions, but is evidently in part the result of the elaboration of the food. 918. Assuming an average of 4*2 grains per hour and pound, for cats fed as usual, we shall get 100-8 grains for the twenty-four hours. Ac- cording to this, the daily quantity of bile would be about ^th the weight of the whole body. 919. In the cat, the proportionate weight of the liver is liable to great variations. For instance, it may form -^th or ^th of the entire body. Usually, however, it ranges from ^th to j^nd. In the adult man, the healthy liver generally constitutes from -^rd to ^nd of the weight; a fraction which is rather a small one. Disregarding this, as well as the scarcely comparable circumstances of secreting surface and vascularity, and taking ^th as a basis for calculation, a man weighing 132-38 pounds would secrete an average of 30^ ounces of bile in the twenty-four hours. And supposing the average weight of the human liver to be 46 J ounces, its specific gravity 1 -07, and hence its volume 74-7 cubic inches, each cubic inch of its substance would furnish only 177- grains of bile daily. While, since a cubic inch of parotid probably furnishes 1013 grains of saliva, the liver must work from 5 to 6 times as slowly. 920. The trunk and branches of the portal vein have a comparatively large diameter. If we add to these the ramifications of the hepatic artery, the extremely numerous and dense capillaries, together with the large efferent hepatic veins, it will be obvious that the gland receives a 'large quantity of blood. But, in spite of this, the amount of its secretion is but small. Hence we might expect that the difference between the afferent portal, and the efferent hepatic, venous blood would be so very slight, as to elude our present means of analysis, and be, to a great extent, concealed by collateral circumstances. 280 SOURCES OF THE BILE. [CHAP. XI. 921. According to Schultz, Simon, and Schmid, the water of the portal blood varies greatly in quantity : so that it is sometimes more, sometimes less, dense than the blood of the carotid artery, the jugular, or the hepatic veins. For instance, its solid residue ranges from 18 to 28 per cent. Fasting horses and dogs have a watery portal blood, while that of the well- fed animals contains more solid matters. Sometimes it contains more fat, colouring matter, and salts, but less fibrine, than other kinds of blood. A comparison of the numbers on which these statements are based shows that their differences are much too great to depend only on a slow secretion of the bile. On the contrary, they may rather be ascribed to unavoidable errors of analysis, together with the influence exerted by the absorption of foods and drinks, and other circumstances. 922. Such an explanation may also illustrate the great ^difficulty of deciding, whether the most essential Constituents of the bile are pre- viously formed in the blood. We shall see that the kidneys secrete much more rapidly than the liver. But, in spite of this, the most essential element of the urine viz. its urea exists in the blood in such very small quantity, that its presence can only be established with ex- treme care. Now, since the daily quantity of bile is probably not greater than the quantity of urine secreted in the 24 hours, while more blood seems to pass through the liver than through both kidneys, we can only expect such traces as may long elude chemical analysis. This remark especially holds good for the chief biliary substances which will shortly be mentioned. It is true that certain of its collateral ingredients, such as its cholesterine and colouring matter may be verified in the blood. But they are also met with in other fluids. 923. The bile which descends from the hepatic canals may either immediately enter the common biliary duct (r, Fig. 151) from the he- patic duct (ri); or may be collected FlG - 15h in the gall-bladder (I) after passing through the cystic duct (m). The un striped muscular fibres of the gall- fa ; bladder, which are less marked in man than in many mammalia such as the ox afterwards impel the collect- ' * ed bile into the cystic duct (m), the common biliary duct (r), and the duodenum (i k). The numerous folds which occupy the cystic duct probably render this movement a slow one. 924. The bile contained in the hepatic duct (n, Fig. 172) already possesses a certain amount of viscidity. As yet, however, we are ignorant whether this quality is present in the finer binary canals, or whether it depends upon the secretion of those small tubes which are contained in CHAP. XI.] COMPOSITION OF THE BILE. 281 the walls of the larger ducts, and to which Theile has drawn attention. In the gall-bladder, its density and viscidity increase. It is possible that chemical metamorphoses are at the same time induced. 925. We have already ( 467) learnt the difficulties which oppose a chemical analysis of the bile. Berzelius and Mulder suppose that a neutral organic substance, bilin, forms the most important constituent : while, on the other hand, Liebig and his school think that certain organic acids are united with soda to form fatty salts. According to the latter view, the bile resembles a soap. The latrochemists of the seventeenth century sought to establish a similar theory. 926. The question whether this soap is a compound of one or many biliary acids, has likewise been differently answered. According to the recent researches of Strecker, the bile contains a mixture of two soda- compounds; one of which is formed by the cholic, and the other by the choleic, acid. The cholic acid contains 67*1 per cent of carbon, 9 '3 of hydrogen, 3-0 of nitrogen, and 20-6 of oxygen. Its equivalent ( 279) is CsjjH^NiO^. By boiling with alkalis, it is converted into cholalic acid (C 4 H 3 N t 2 ) ; which, united with two atoms of water, gives glycin or sugar of glue ( 319). The choleic acid the hypothetical formula of which is C 52 H 45 Nj O u S 2 is decomposed by putrefaction into ammonia, taurin ( 320), and a resinous mass. Its sulphur is probably due to the taurin, which Redtenbacher regards as an acid sulphite of aldehyd- ammonia. The dyslysin, which is precipitated in the course of the alimentary canal, ( 472) may be obtained from cholic acid by continued boiling in dilute hydrochloric acid. 927. Solid deposits of gall-stones frequently occur in the biliary ducts, and especially in the gall-bladder, which sometimes contains hundreds or thousands of such concretions. In very rare cases they consist of carbonate and phosphate of lime. Usually they are composed of cho- lesterine, colouring matter, margarine, and margarates. When these softer gall-stones lie closely compressed in the gall-bladder, they often acquire smooth surfaces, being polished by mutual friction. But, in many instances these cholesterine gall-stones form complete aggrega- tions of crystals. A concretion of this kind is shown in Tab. I. Fig. 16, slightly magnified. Many of the bezoar-stones, which are regarded as the gall-stones of the antelope, consist of lithofellic acid or of bezoaric acid ; while others are composed of phosphate of lime, and ammoniaco- phosphate of magnesia. 928. We have already ( 467, et seq.) been made acquainted with what is at present known concerning the relations of the bile to the digestive function. Since certain insoluble products of its decomposition are given off with the faeces, it is, at least, in part an excretion. It has long been supposed that the formation of bile was necessary to the purification of the blood. Some have thought that the older blood-corpuscles are dis- 282 THE URINE. [CHAP. xr. solved in the liver of the adult, where they are either wholly or partially applied to the formation of bile. But this view is unsupported by any trustworthy proofs. Dogs in whom biliary fistulse have been established may live for months, but they finally perish with appearances of inani- tion ( 361). And though the loss of hepatic function destroys life, still this injurious effect only comes on gradually. So that the real import of the secretion of bile, or of its natural flow into the intestine, can only be deduced from the gradual synthesis of many different operations. 929. According to Bernard, the blood and liver of mammals, birds, and reptiles, contain a considerable quantity of grape-sugar. This is produced in the gland itself, independently of all amylaceous food ( 304) ; but its formation ceases on cutting through the vagus nerve. 930. The Urine. The secretion of urine by the kidneys fulfils two chief objects. It withdraws from the blood superfluous water, and soluble ashy constituents. At the same time, it carries off certain azotized sub- stances which cannot otherwise be got rid of. The water of the urine, as it were, rinses the body on leaving it; so that it frees the organism from a series of compounds which have either been introduced with the food, or have been rendered unfit for use by the action of the various organs. 931. Although many of the substances given off in the urine are pro- bably products of muscular movement, still this does not immediately necessitate its evacuation. The same remark applies to the use of nitro- genized food, which gives rise to similar results. But if, on the other hand, we introduce into the stomach considerable quantities of water, food, or drink, the urinary bladder soon becomes distended. Hence water is rapidly carried off by the kidneys. 932. From this fact it is obvious, that the function of the urinary organs must vary greatly with the accompanying circumstances. If the blood have taken up much water, an unusual quantity will exsude. This circumstance and the varying activity of the skin ( 835) even affect the total quantity per day. Thus the same person may give off less than two pounds of urine in one day, and more than four the next. 933. Disregarding such variations, an adult of average size may be stated to secrete from 2 to 3 pounds of urine in the 24 hours. The quantity of fluid thus set free is somewhat less than that originally secreted by the kidneys, since the urine becomes slightly condensed by a long continuance in the bladder. But this circumstance does not essentially affect the limits above mentioned. Supposing the average volume of both kidneys to be 17 cubic inches, every cubic inch would daily furnish about 1000 grains, or about 2-3 ounces: an activity which would be somewhat greater that that of the parotid ( 919), and much greater than that of the liver. 934. On making a longitudinal median section of the human kidney, CHAP. XT.] STRUCTURE OP THE KIDNEYS. 283 we find that the cortical substance (/, Fig. 152) exhibits a dark brownish- red colour, while the medullary substance (e) is of a clearer and whiter hue. Both of these contain urinary tubules, which are the ducts of the glands. In the medullary substance these take a comparatively straight course j while in the cortex they are very tortuous. The straight tubules open on the free surface of some peculiar projections, the mamillse (d, Fig. 152). The cavities between these prominences are partially covered FIG. 152. with a white membrane, and are called the calyces (c, Fig. 152); they open into a common receiver, the pelvis of the kidney (6), which is pro- longed into the ureter (a). Finally, the latter tube opens into the bladder, in which the urine collects before leaving the body. 935. A large artery, the renal, which leaves the aorta at nearly a right angle, ramifies in the interior of the kidney. Its branches, after attaining a certain fineness, form peculiar coils the Malpighian tufts (Tab. V. Fig. 66, a). The arterial twigs which emerge from these are continued into a network of capillaries that surrounds the tortuous urinary tubule. The veins of the kidney unite to form a large trunk, which empties itself into the inferior cava. A section of the renal substance under a moderate magnifying power is shown by Fig. 153. At a are the Malpighian coils just mentioned, b shows the fine arterial twigs from which they arise, c the tortuous, and d the straight, urinary tubules ; the latter are seen to bifurcate here and there. 936. A special capsule (Tab. V. Fig. 66, led) surrounds each tuft to form the Malpighian body. This capsule is the dilated end of an urinary tubule (Tab. V. Fig. 66, e). Fine cilia (Tab. II. Fig. 36) frequently maintain a vigorous current over a greater or less extent of the internal surface of the capsule. Sometimes, however, they cannot be recognized at all ; and in many cases they are only seen where the capsule becomes continuous with the neighbouring urinary tubule. 937. The use of this peculiar arrangement cannot be exactly deter- 284 EXPULSION OF THE URINE. [CHAP. XL mined. Since curves increase resistance ( 103), the blood will pass more slowly through these tufts, and will exert a stronger pressure on the walls of the vessels. They FlG 153 would thus allow the ex- sudation of a dense solution containing peculiar sub- stances ; and would trans- mit a comparatively watery blood to the plexus of ca- pillaries around the urinary tubules. But this hypothe- sis does not clearly explain why the water of the blood together with its soluble matters, such as the pecu- liar compounds of urea, uric, and hippuric acid ( 321) chiefly pass off by the urine. 938. The urine, which is prepared in the tortuous urinary tubules (c, Fig. 153) passes from these into the straight ones, d. It es- capes from the latter through their orifices on the mamillary processes (d, Fig. 152, p. 283). The fluid then gains the calyces (c) and pelvis (b) of the kidney, and finally enters the blad- der (m, Fig. 154) through the ureter, (k I). All these parts possess unstriped mus- cular fibres (Tab. IV. Fig. 59), the contraction of which propels the urine onwards. Artificial irritation of the ureter (Ic I, Fig. 154) or its nerves in recently killed ani- mals, often produces vermi- cular movements, which are directed from the kidneys (c d), towards the urinary bladder (m). This collects the urine drop by drop, and gradually becomes distended in proportion as its contents increase. CHAP. XI.] EXPULSION OF THE URINE. 285 939. Occasionally a rare malformation, the prolapsus of the bladder, affords an opportunity for verifying this exit of the urine by drops from the ureter. In this malformation, the symphysis of the pubis (below k Fig. 27, p. 230) and the anterior wall of the bladder are wanting : while the posterior part of the cavity is exposed as a reddish irregular mass, covered with mucus. In such cases, the orifices of the ureters are seen opening from time to time to allow the passage of a drop. of urine; the right ureter often acting at different times from the left. 940. A hsBmadynamometer, which was introduced by Ludwig and Loe- bell 34 ) into the ureter of a dog, exhi- bited two chief variations of pressure. One of these depended upon the ver- micular movement of the ureter j the momentary elevation thus produced rarely exceeded 4 inches of mer- cury. But the second, which was more constantly visible, amounted to a positive pressure of from -28 to '39 inches. We cannot from hence conclude, that the latter ex- tremes really correspond to the vary- ing tension under which the urine passes into the commencement of the urinary tubules, since they may be assisted by slight contractions of the ureter, and perhaps also of the calyces and pelvis of the kidney. But it may be conjec- tured that the original pressure is at least as great as these numbers would indicate. 941. The act of opening the lower orifice of the urinary bladder (above 2, Fig. 175), forms the next step in the evacuation of the urine. The first impulse to this is probably given by the compressor urethrce of the mem- branous urethra (z, Fig. 154), which is provided with striped muscular fibres (Tab. IV. Fig. 54). This is followed by the relaxation of the sphincter vesicce (above z). The several bundles of unstriped fibres in the ftmdus and body of the bladder (m n) compress this receptacle, so that the urine is driven forth through the urethra (z a b), as through the u. 286 EXPULSION OF THE URINE. [CHAP. XI. canula of a syringe. The mode in which the ureters (n) penetrate the bladder prevents all hurtful retrogression of the fluid at this period. For instance, supposing g h i Tc, Fig. 155, to be the lowest part of the ureter, it passes for a certain distance between the muscular bundles, n, o, of the bladder, before opening into its cavity. When these bundles contract, they instantly close the terminal segment of the ureter, g h I m. 942. Since the male has a longer and nar- rower urethra than the female, the width, form, and velocity of the stream of urine (z to /', Fig. 154), differ in the two sexes. But in both, the abdominal pressure assists, (393) when any difficulty occurs. When the orifice leading to the urethra is obstructed, urine collects in the bladder in constantly increasing quantities. The bladder thus becomes so distended, that its fundus (m, Fig. 154) ascends into the umbilical region (from v beyond x, Fig. 9, p. 34). In such cases it finally bursts in some part of its extent, generally below, where it is not covered by peritoneum (comp. Fig. 9). The urine is thus effused in the neigh- bouring areolar tissue ; the scrotum and penis swell considerably ; and inflammation, gangrene, and death, shortly follow. In rare instances, nature seeks another and a less dangerous outlet. In the embryo a hollow duct, the urachus (d, Fig. 119, p. 208), runs from the fundus of the bladder towards the navel, and from thence passes to the membranes of the ovum. It is subsequently converted into a solid cord. In some cases of retention of urine, it has happened that this remnant of the former urachus, or the umbilical ligament of the bladder, has opened afresh, and the urine has gushed out at the navel (, Fig. 9, p. 34). 943. In the bladder, the urine probably becomes denser and more mucous. But since all the quantitative analyses of this fluid have been made upon urine which was evacuated from the body, this circumstance must be borne in mind in considering the numerical estimates. 944. The specific gravity of the urine may vary from 1004 to 1050 : the usual average seems to be from 1015 to 1019. The quantity of water which it contains is from 92 to 99 per cent. Since, in the course of the day, many watery substances are introduced into the alimentary canal, the urine, and especially that which is passed after drinking (urina potus), is more watery than that which comes away on arising in the morn- ing (urina sanguinis). That which is evacuated during the digestion of solid food (urina cibi) is generally between the two former in this respect. But neither the specific gravity, nor the amount of water, allows us to judge of the nature of the solid residuum ; since its several constituents vary remarkably in quantity. CHAP. XIr] CONSTITUENTS OP THE URINE. 287 945. Under ordinary circumstances, the urine may give off more water than the pulmonary or cutaneous evaporation. We have seen ( 836), that these removed somewhat less than 21bs. 8| oz. from the author's body daily. The average quantity of urine in the twenty-four hours was 31bs. 3-^ oz. Taking its watery constituent at 94*6 per cent, we obtain 31bs. and ^ oz. of water. It is obvious, that even a mode- rate increase in the transpiration would suffice to reverse this rela- tion. Thus Barral gives 40-4 to 45-5 oz. of water as the quantity lost from his body by evaporation in twenty-four hours, and only 34-5 to 41-4 for the urine. 946. The fresh urine of mammalia gives off a certain quantity of car- bonic acid on coming into contact with the air. The details of this gaseous interchange require a further examination. From causes with which we have already ( 327) become acquainted, the putrefying urine frequently contains free ammonia, or its carbonate. 947. The urine contains a mechanical admixture of vesical mucus- ( 880), and fragments of epithelium (Tab. II. Fig. 31). In addition to this, its concentrated residue and ashes exhibit the following substances : urea; uric acid; hippuric acid (521); kreatin (321); oxalates, car- bonates, phosphates, and sulphates; sulphurets and chlorides of the metals; silicic acid; iron; and manganese. To these may be added a colouring matter; some peculiar (and probably variable) mixed substances which are as yet little known, and are included under the name of extractive matters; and sometimes, compounds of fluorine. Blood, pus, semen, and fragments of various tissues, constitute its morbid mechanical admixtures : and sugar, butyric acid, albumen, and unusual red and blue colouring matters, its abnormal chemical constituents. Besides these, a great variety of substances may appear in the urine as a result of their being taken in the food. 948. The percentage composition of the urine varies so considerably from all these circumstances, that it is scarcely possible to talk of averages of its several constituents. We have no complete analyses of urine which correspond to the recent progress of science. Hence we will only quote an older example for the sake of its completeness. Lehmann found in human urine 93-2 per cent, of water, 3 '29 of urea, -11 of uric acid, -15 of free lactic acid (kreatin with other substances), 1*15 of extractive matters, 01 of vesical mucus, -73 of sulphate of potash and soda, -4 of phosphate of soda and acid phosphate of ammonia, *37 of chloride of sodium and ammonium, -11 of phosphate of lime and magnesia, and -17 of lactic salts. 949. A part of the urea is due to the fact, that the elements of this compound are formed by the ordinary vital functions, and especially by the locomotive organs ; and are then dismissed as effete from the blood into the urine. A second portion arises from the metamorphosis of 288 UREA AND URIC ACID. [CHAP. xi. the digested azotized food, and especially of the albuminous substances. Hence the amount of urea is diminished in fasting animals, and rises considerably after the use of highly azotized food, such as eggs or meat. With vegetable diet its quantity is less, while muscular activity remark- ably augments it. 950. Since the quantity of uric acid is much smaller than that of urea ( 948), it is far more influenced by any errors of analysis. This fact, together with the considerable variations which seem to occur under similar collateral circumstances, explains the obscurity which attaches to the relations of uric acid to the changes of the organism. The quantity of uric acid in the urine is sometimes visibly increased by the continuous use of a meat diet. 951. The observations instituted by Lehmann upon himself will ex- plain much of what has just been said. This observer found as follows : Average daily Quantities. M Accompanying Per cent. Absolute Weight in Grains. II Solid Resi- duum. Urea. Uric Acid. Total Quantity of Urine. Solid Residue. Urea. Uric Acid. H During 14 days only \ the necessary food and drink was 1 taken. Twohours' f 6-412 3-072 112 16336-7 1047-4 501-9 18-27 1:2-09 movement daily in the open air .J Animal food only i during 12 days . I 7-272 4-424 123 18571-4 1350-4 821-6 22-83 1:1-64 Exclusively vege-1 table food for 12 I 6-517 2-473 112 14038-6 914-8 347-2 15-77 1:2-64 days . . . J Food absolutely free*! from nitrogen for > 643-7 232-4 11-35 1:2-71 two days . . . J 952. The urine of well-fed carnivora contains considerable quantities of urea. While, according to Frerichs, dogs fed upon substances devoid of nitrogen give off about the same quantities of urea as in the fasting state. The quantity thus secreted is that produced by the functions indispensable to life. 953. According to Lecanu, men give off, on an average, 432-4 grains of urea : according to Becquerel only 295 grains. The latter observer esti- mates the quantity for women to be 270-3 and 240-9 grains. Lecanu found 125- for old age ; and 69-5 to 208-5 for the latter part of child- hood. 954. We have already seen ( 322) that urea contains more nitrogen CHAP. XI.] UREA AND URIC ACID. 289 than any other organic compound (46-73 per cent). It therefore con- veys large quantities of this element out of the organism. The 432 grains of urea contain 201 of nitrogen; and the 270 grains, 108. Barral estimates the nitrogenous constituent of the proper mixed food of adult men as amounting to from 324 to 432 grains daily. 955. Small quantities of urea are often present in other fluids besides the urine. In spite of the presence of albuminous substances ( 297), it may be recognized in the blood. It is also frequently found in morbid exsudations (such as dropsical effusions) ; in the aqueous humour, and vitreous substance, of the eye ; and according to some, may rarely be met with in the saliva and other secretions. 956. These facts justify the conclusion, that the urea of the urine is not produced in the kidneys, but transudes from the blood. These glands do but permit it to pass unchanged into their secreted product ; while the others either reject it, or immediately decompose it, so that it can no longer be recognized. 957. The fact that the blood contains but very small quantities of urea speaks rather for, than against, this theory. For when we con- sider the large quantity of blood which circulates daily through the kidneys, it becomes evident that if the blood contained much, and was only partially unloaded in the urinary organs, the urine would contain more urea than it does. A simple calculation will corroborate this statement. If the urine of a person exclusively fed on meat gives off an average of 821 grains of urea in the 24 hours, this gives -57 grains as the mean quantity per minute. Assuming both kidneys together weigh from 3850 to 7700 grains, more than 770 grains of blood would pass through these glands in one minute. But taking only 770 grains, the blood would require but '07 per cent to contain -57 grains of urea. And recollecting that part of the urea is retained by the albumen of the blood, and lost by precipitation with nitric acid, we shall not be surprised to find that only traces of it are discoverable. 958. In the urine of man and carnivora the quantity of uric acid is small. In the herbivora, its presence is exceptional, and its amount still smaller. But a large quantity is found in the mixed faeces and urine of birds and snakes. When treated with oxidizing substances, it is con- verted into other organic compounds allied to it. The action of nitric acid produces alloxan, alloxanthin or parabanic acid, and urea ; treatment with potash and ferrocyanide of potassium, carbonic acid, allantoin and urea ; and finally, with peroxide of lead, carbonic and oxalic acids, allan- toin, and urea. We shall hereafter find that many of these metamor- phoses occur in the living body ; and that the urea possibly thus origi- nates in the blood. 959. Hippuric (or, as it was formerly called, uro-benzoic) acid may 290 HIPPURIC ACID. [CHAP. xi. not only be found in the urine of the horse and cow, but also in that of the human subject at all ages. In certain morbid circumstances, as for instance in many cases of diabetes or chorea, its quantity is greatly increased. And after the ingestion of benzoic acid, cinnamic acid, oil of bitter almonds, or benzoic aether, these substances are found in the urine as hippuric acid. 960. If hippuric acid be boiled for some time with dilute sulphuric, hydrochloric, nitric, or oxalic acid, or with soda or potash, it is decom- posed into benzoic acid and glycocoll or glycin ( 319). The latter sub- stance we have also found appearing in the after-changes of the cholic acid of the bile (926). 961. The presence of lactic acid in the urine was formerly deduced from the fact that the salts of zinc gave a crystalline precipitate. But Heintz and Pettenkofer found that this deposit contains kreatin and kreatinin ( 319) ; the same compounds which may be obtained from broth, and from the cold watery extract of striped muscle (Tab. IV. Fig. 54). The later researches of Heintz indicate that kreatin is only an after product, and is not originally present. 962. What are called the extractive matters of the urine form a variable mixture of different compounds. The colouring matter which may be separated from them by acetate of lead contains, according to Scherer, 56-6 to 61 '3 per cent of carbon, 4-1 to 6*2 per cent of hydrogen, 6 '3 to 7* per cent of nitrogen, and 33- to 2 5 -5 per cent of oxygen. 963. The nature and amount of the salts are necessarily subject to great variety, since many soluble substances of this kind which are introduced with the food pass, changed or unchanged, into the urine. If proper saline solutions be injected into the blood, this phenome- non generally occurs still more rapidly. For instance, Vierordt and Wellzien introduced 1375 grains of salt, dissolved in 18-6 cubic inches of water, into the jugular vein of a horse, in the course of 25 mi- nutes. Thirty minutes later, 6-1 cubic inches of urine contained 11- grains of salt; an hour later, 10-92 grains; and l\ hours later, 11-98 grains. While 6-1 cubic inches of healthy urine only offered -15 to -23 grains. 964. The most careful analysis of the ashes of the food will not fully explain the circumstances now under consideration. For it is probable that the meroxygenous ( 295) constituents of the food and urine are unequal in quantity. The general appellation of animal or vegetable food is evidently yet more uncertain. One can, at most, but conjecture, from reasons hereafter to be mentioned, that the quantity of sulphates and phosphates may be increased by a diet of meat or eggs. This opinion is supported by some experiments instituted by Lehmann on himself. He found as follows : CHAP. XI.] SALTS OF THE URINE. 291 Average daily Quantities. Proportions per cent. Absolute amount in Grains. Accompanying circumstances. Phos- phate of Soda. Phos- phates of the Earths. Sulphates of the Alkalis. Total Quantity of Urine. Phos- phate of Soda. phates of the Earths. Sulphates of the Alkalis. None but necessary food and drink for 14 days. Two 347 104 664 163367 56-73 16-94 108-51 hours' daily move- ment in the open air Purely animal food during 1 4 days . J \ 451 "296 865 18571-4 83-72 55-01 160-6 965. Bodily exercise increases the alkaline phosphates of the urinary residuum. The quantity of earthy phosphates in the urine of fasting- men is but small. According to Lehmann, it is increased in this state, according to Bence Jones, it is not. The urine of carnivora contains much phosphates; while that of herbivora contains but little, sometimes mere traces. But on the other hand, the urine of the latter presents more carbonates. In the urine of children, the quantity of the alkaline phosphates is also diminished. 966. Oxalate of lime (Tab. I. Fig. 3) is frequently found in the urine of perfectly healthy persons. Its quantity may be increased by the use of vegetable substances containing oxalates, as well as by many peculiar and unusual changes of the uric acid ( 958) of the blood or other tissues. According to Donne", champagne produces the same effect. 967. Sugar taken in moderate quantities does not re-appear in the urine. But if a larger amount be introduced into the stomach or the blood, part of it is found in the urine. In saccharine diuresis or diabetes mellitus, the urine, which is secreted in abnormal quantity, contains sugar. This kind of sugar, which formerly received the special appellation of diabetic sugar, corresponds in all respects with grape- sugar. 968. Different methods have been used to show the saccharine con- tents of the urine. Rotation of the plane of polarization ( 173, 256) can, at most, only indicate the fact, and not decide it. The fermen- tation-test formerly adopted, can only be practised when a large quantity of sugar is present. Besides this, it is very deceptive, since carbonic acid, or carbonate of ammonia, may be given off by other consti- tuents of the urine. In applying this test, the urine, mixed with yeast, is introduced into a flask a (Fig. 156), from which passes off a tube, b, into a vessel, c, partially filled with lime-water. This is connected by a second tube, d, with an open flask, e, also containing lime-water. The lime in e attracts to itself the carbonic acid of the atmosphere ( 795), so as to u 2 292 SUGARY URINE. [CHAP. XL maintain the lime-water contained in c in a state of purity. On bringing the whole apparatus into a warm place, a lively fermentation is soon set up. The carbonic acid set free at a, passes through b towards c, causing a white precipitate of carbonate of lime to be thrown down here. FIG. 156 FIG. 157. When sugary urine undergoes fermentation, numerous yeast-plants (Tab. II. Fig. 30) are frequently found in it. But the presence of this vegetable, fungus is no infallible sign that the urine contains sugar. When sugary urine is boiled with a solution of caustic potash, the whole acquires a red-brown colour. The application of nitric acid then produces an agreeable odour of treacle. Trommer's test depends upon the fact that, during or after boiling, grape-sugar reduces oxide of copper which has been precipitated from a solu- tion of its sulphate by an alkali. We thence get a yellowish or reddish colour of the precipitate. According to Fehling, this test may even be used to determine the quantity of sugar contained in the urine. A vessel (Fig. 157) having a gra- duated capacity, and provided with a tube of outlet, is filled with a watery solution, 100 cubic inches of which con- tain 1012 grains of sulphate of copper, 4050 grains of tar- trate of potash, and 14,172 grains of soda ley of sp. gr. 1-12. Another similar tube receives a portion of the urine, diluted with 9 to 19 times its quantity of water. We now pour one cubic inch of the copper solution into four cubic inches of water, boil the mixture, and drop urine from the second tube, until no red precipitate is produced. A cubic inch of this copper solution corresponds to 146 grains of grape- sugar. Larger quantities of sugar may be extracted by alcohol. 969. Urine containing sugar may often be distinguished by its high specific gravity (1030 to 1060), and by the large quantity (6 to 12 per cent) of its solid residuum. The quantity of sugar in the urine is CHAP. XI.] ALBUMINOUS URINE. 293 increased by a diet of hydrates of carbon ( 303). Hence it has fre- quently been attempted to feed diabetic patients upon meat only. But, as a rule, this treatment is unsuccessful. The exclusively meat-diet becomes unbearable after a time. Besides this, the urine never loses the whole of its sugar, probably because grape-sugar can be produced from the nitrogenized compounds. 970. The urine of many consumptive patients frequently contains large quantities of fat : and, on standing, often deposits oil-globules on its surface. Something similar to this occurs in patients suffering from other diseases; where considerable quantities of fat may also be discharged with the faeces. 971. In persons of weak constitution or debauched habits, boiling the urine often throws down, not only a whitish powder of carbonate of lime, but also flocculi of albumen, which do not disappear on the application of nitric acid. This fact is best observed in the urine evacuated some time after a meal. And in a variety of diseases such as inflammation, dis- eased heart, and dropsy the urine sometimes contains large quantities of albumen. Hence this phenomenon forms no exclusive sign of that affec- tion of the kidney which is usually designated by the name of Bright's disease. In this disorder, the urine generally contains, not only albumen, but also minute masses of fibrine, which in shape resemble sausages.* When but little albumen is present, no flocculi are deposited by boil- ing, especially if the urine is either originally alkaline ( 972) or has acquired this reaction from its spontaneous decomposition. And since white salts of lime are also precipitated by boiling, the urine should always be mixed with a small quantity of nitric acid, and carefully heated. 972. Fresh human urine has generally an acid reaction. But the nature of the food may give rise to different results. We shall see that the constituents of many kinds of food (especially of vegetables) reappear in the urine as alkaline carbonates. If these are present in large quan- tity, the urine is originally alkaline. When the acid urine undergoes spontaneous decomposition, it also becomes alkaline, from the gradual conversion of its urea into carbonate of ammonia ( 327). Hence, in diseases of the spinal cord, where the urine is passed involuntarily, we often find that the soiled linen of the patient is soaked with a fluid of an alkaline reaction, and ammoniacal odour. But even in these in- stances, the fresh urine is acid. The urine of carnivora is acid, while that of well-fed herbivora is alkaline. The urine of the horse is mixed with a great number of minute crystalline globules (Tab. II. Fig. 20). These, which may be collected in pounds on a filter, also occur in the urine of the cow, the pig, the rat, and sometimes the mouse and rabbit. They consist chiefly of * These are casts of the urinary tubules. EDITOR. 294 REACTION OP THE URINE. [CHAP. XI. carbonate of lime and magnesia, united with a small quantity of an organic substance. But if an herbivorous animal be made to fast, its urine becomes acid, since it only consumes its own flesh. Hence the alkaline character of the urine depends on the nature of the food. A fact observed by Liebig may serve to indicate the cause of the acid character of the urine of man and carnivora. We have seen ( 965) that the urine of these animals contains a considerable quantity of alkaline phosphates compounds of which only traces are found in the urine of herbivorous animals. Pure uric acid is only soluble in water with ex- treme difficulty. According to Bensch, one part requires from 1800 to 1900 parts of boiling water, and from 14,000 to 15,000 at 68. While a solution of bibasic phosphate of soda takes up hippuric acid even when cold, and uric acid when warm. It thus acquires an acid reaction, if sufficient uric acid be present. On subsequently cooling, a part of the uric acid is again precipitated. We shall soon find that the urine fre- quently offers a similar phenomenon. 973. The researches of Woehler, and the corrections since furnished by himself and Frerichs, accurately inform us which of the substances taken into the alimentary canal are transferred changed or unchanged to the urine. We have already ( 959) learnt what substances reappear as hippuric acid. When the neutral salts of the vegetable acids are introduced into the blood, either immediately, or through the stomach, they are converted into carbonates. They undergo combustion at the expense of the oxygen of inspiration ( 270). Oxalic, citric, malic, tartaric, and succinic acid, reappear in the urine in this form, in union with bases. Tannic acid is converted into gallic and pyrogallic acids, and substances allied to humus. The following substances reappear in the urine : indigo, gamboge, the colouring matter of madder, logwood, beet-root, bilberries; the odorous principle of valerian, asafoetida, garlic, castor, saffron, and turpentine; certain compounds of opium; the narco- tizing principle of toadstools; together with carbonate, chlorate, nitrate, and sulphocyanate of potash; ferrocyanide, and sulphocyanide of potas- sium; borax, chloride of barium, silicate of potash, and potassio-tartrate of nickel. But this is not the case with the compounds of alcohol, ether, camphor, Dippel's animal oil, resins, the colouring matter of chloro- phyll, litmus, alkanet, and cochineal. Sulphuret of potassium reappears, partly in this form, partly as sulphate of potash. Iodine is converted into alkaline iodides : ferridcyanide, into ferrocyanide, of potassium. Quinine frequently passes off in the urine, but caffein ( 343) does not. Salicine is probably metamorphosed into spiric or spireeic acids; and phlorrhizin into hippuric acid, and oxalate of lime. Oil of bitter almonds containing no hydrocyanic acid, is converted into benzoic acid, and thence into hippuric acid. By taking urate of- potash or urate of ammonia, the quantity of urea is increased. Hence the uric acid is CHAP. XI. J TRANSIT OF SUBSTANCES INTO THE URINE. 295 probably converted into urea by oxydation ( 958). Rhodallin (or am- moniated oil of mustard) furnishes sulphocyanide of ammonium, as it does after being heated with soda and lime. Most of the metals such as gold, silver, iron, lead, tin, bismuth, arsenic, and mercury, are occasionally carried off in greater or smaller quantities with the urine. 974. The experiments instituted by Stehberger and Erichsen on persons with extroversion of the bladder ( 939) show that traces of the sub- stances introduced with the food soon appear in the urine. Under the most favourable circumstances, ferrocyanide of potassium only required an interval of one minute ; while the colouring matter of indigo or madder demanded a quarter of an hour. This velocity is explained by the rapidity of the circulation ( 717), and by the way in which suit- able solutions transude porous animal membranes ( 146). When the stomach is filled with food, the ferrocyanide of potassium appears in the urine much more slowly. 975. Urine left to itself sooner or later deposits a sediment. The mucus and epithelium mixed with it frequently separate during cooling. The fresh or concentrated urine subsequently precipitates uric acid (Tab. I. Fig. 2), oxalate of lime (Tab. I. Fig. 3), carbonate of lime chiefly in the form of fine granules, and ammoniaco-phosphate of mag- nesia (Tab. I. Fig. 17, ik I). Sometimes this deposit occurs spontane- ously ; sometimes only after the application of hydrochloric acid, or other reagents. In the urine of diseased subjects large quantities of such deposits are frequently found. The red brick-dust sediment of feverous subjects chiefly consists of uric acid, generally in the form of minute tables (Tab. I. Fig. 2) ; and of alkaline and earthy urates. The colour of the whole is produced by a peculiar red matter, of the nature of which little is known. In other cases these salts of lime and magnesia are mixed with a large number of mucous corpuscles (Tab. II. Fig. 31, d), blood-corpuscles (Tab. II. Fig. 24, a), pus-corpuscles (like those in Tab. II. Fig. 23, c), and spermatozoa (Tab. V. Fig. 79). 976. Urinary calculi may be produced in any part of the urinary organs. Most frequently they arise in the bladder. For since the urine remains in it for some time, solids which are but little soluble are most easily deposited here. The presence of a solid nucleus of any kind greatly favours their deposition. A lump of mucus, a grain of sand, a small stone, a straw, or a piece of metal which has accidentally penetrated from without, may form the kernel around which new layers successively arrange themselves. 977. The lithic or uric calculi chiefly consist of uric acid and the inso- luble urates. The oxalic calculi consist in great part of oxalate of lime ; and the phosphatic, of combinations of phosphoric acid with lime and magnesia. With these other substances are frequently mixed. The 296 SUPPRESSION OF URINE. [CHAP. XI. nucleus often consists of uric acid, or urate and oxalate of lime ; while the external rind is formed of earthy phosphates. 978. The absence, degeneration, or removal, of one kidney does not necessarily give rise to any important disturbance. But dogs, cats, and rabbits, from whom both have been excised, die at latest in a few days. The operation is followed by fever, loss of appetite, depression, and sometimes diarrhoea. Death is frequently preceded by convulsions. The quantity of urea contained in the blood is probably much increased. Most of the secretions appear to be more watery than usual ; and frequently contain compounds of ammonia. The fluid exsudations sometimes met with in the abdominal and thoracic cavity generally contain large quan- tities of urea or carbonate of ammonia. CHAPTER XII. THE VASCULAR GLANDS. 979. THE class of glands indicated by the above name includes the spleen (g, Fig. 75, p. 132), the supra-renal capsules (a b, Fig. 154, p. 285), the thyroid (e, Fig. 101, p. 187), and the thymus (d, Fig. 100, p. 186), glands. Many observers, such as Ecker, add to these the pituitary body. But this class includes organs of very different structure and functions. The supra-renal capsule, the thyroid, and the thymus, at least so far correspond to each other as that they all consist of closed tubes, which otherwise resemble the ducts of the glands, and contain cells, nuclei, and other solid structures. They may therefore, with a certain degree of correctness, be called glands without ducts. But the anatomy of the spleen is so different, as to assign it quite a different office. Still the functions of all the structures now reckoned amongst the vascular glands remain almost unknown. 980. The strong fibrous membrane that clothes the spleen is pro- longed into its interior : giving off sheaths, which surround the branches of the arteries (see Fig. 158), and unite with each other so as to form a network. These enveloping structures contain unstriped muscular fibres (Tab. IV. Fig. 69), together with the ordinary areolar and elastic fibres. FIG. 159. According to Ecker, the muscular fibres in the human trabecular tissue consist of fibre-cells, having nuclei which are appended laterally, as shown in Fig. 159. By the aid of the rotatory electro-magnetic appa- ratus, R. Wagner, Koelliker, and Ecker, have succeeded in producing contractions of the surface of the spleen in the dog and c at : while Har. less has obtained the same result in an executed criminal. 981. The branches of the arteries distributed in the interior of the spleen are occupied by peculiar roundish vesicles, the Malpighian or 298 THE SPLEEN. [CHAP. XII. splenic corpuscles. The situation of these, as seen under a low magnify- ing power, is represented in Figs. 160 and 161. They contain a colourless fluid, which rarely coagulates in the air, together with cells and free nuclei. They frequently collapse after death. On this account they FIG. 160. FIG. 161. are sometimes missing, especially if the vessels of the spleen are not tied before its removal. Their connection with the vessels and ab- sorbents has not yet been definitely made out. 982. According to Ecker, the small arteries which ramify in the neighbourhood of the splenic corpuscles, or elsewhere, are afterwards continued into a delicate capillary network distributed in the spleen- pulp. But, hitherto, he has not been able to observe the transition of these capillaries into veins. Many of the latter vessels have distinct dilatations. They form meshes, which somewhat resemble those in the corpus cavernosum of the penis. Finally, as regards the absorbents, we are as yet only acquainted with those larger vessels which emerge from the spleen, or are distributed upon its surface. Their relations in its interior have not been observed. 983. The spleen-pulp is the more or less red and pulpy substance which is seen on cutting up the organ. According to Ecker, it contains nuclei; cells; single or aggregated blood-corpuscles (Tab. II. Fig. 24); cells which enclose blood-corpuscles; and yellow or colourless granules and masses, either free, or surrounded by cells. 984. It was formerly conjectured that new blood-corpuscles were produced in the spleen. This view was especially supported by the fact, that the absorbents of oxen killed during the digestive process are filled with a reddish lymph, in which blood-corpuscles are revealed by the microscope. And some observers consider that the cells just mentioned as filled with blood-corpuscles ( 983) furnish additional support to this theory, the blood-corpuscles being produced in these cells, and afterwards set free by their solution. But since other cells include solid structures, which gradually lose their definite form, many believe that precisely the reverse process obtains that the blood-corpuscles undergo destruction in the spleen. They suppose that clusters of them are enclosed in cells, are next converted into these yellow masses, then break up into granules, and finally disappear. CHAP. XII.] THE SUPEA-BENAL CAPSULES. 299 985. According to the concurrent statements of many anatomists, the cells which enclose blood-corpuscles, as well as the other transitional forms just mentioned, are found, not only in the spleen, but also in abnormal effusions of blood in the brain, liver, and kidneys. Hence their presence is not an appearance peculiar to the spleen. It may even be questioned whether it is normal, or whether it is the direct result of accidental injury to the delicate parenchyma of the organ. At present a decision of the whole question is impossible. 986. Recently, J. Beclard has stated, that the blood of the splenic vein in the dog and horse contains a much smaller quantity of corpuscles than that of the subclavian vein. According to him, the former contains from 8-18 to 16-14 per cent of blood-corpuscles, while the latter has from 9-83 to 18-51 per cent. If these differences were not suspiciously large, they might be regarded as proving the decay of blood-corpuscles in the spleen. 987. The spleen has often been noticed to exhibit an extraordinary- increase of volume some hours after feeding. Dogs live unhurt a long time after the extirpation of this organ. The voracity, emaciation, and alteration of the sexual impulses, which have been remarked in such cases by some observers, have not been found by others. 988. The supra-renal capsules are distinguished by their comparatively large size in the new-born infant (r, Fig. 119, p. 208), and their still greater size in the earlier embryo. According to Ecker, they consist chiefly of closed tubes; which lie close to each other, contain a fluid, nuclei, and a number of fine granules, and are surrounded by a dense capillary network. The cortical substance of older human subjects con- tains bodies such as are represented by Tab. V. Fig. 67. An extra- ordinary number of nerves enters this organ. But many only pass through their tissue, to be distributed to other structures. It is probable that certain compounds secreted from the blood undergo a peculiar elaboration in these tubes. But the object of this function is as yet unknown ; nor are we even acquainted with the means by which the altered substances are rendered useful to the other organs of the body. At present the theories formerly advanced that the function of the supra-renal capsules is closely related to that of the cerebral, urinary, or sexual, organs rest upon no secure basis of facts. 989. The thyroid gland contains a great number of lobules, sepa- rated from each other by areolar tissue. Imbedded in these we again find tubes, containing a fluid, and nuclear structures, which, according to Ecker, are apposed to the limitary membrane in the form of a pavement, as represented in Fig. 162. We may sometimes see simple cells, as indicated in Fig. 163; and in the dog these are often double. A rich network of capillaries surrounds the several lobules of the gland. 300 THE THYROID GLAND. [CHAP. XII. 990. The degeneration of the thyroid so frequently met with, and the safety with which it may be extirpated, plainly forbid us to assign it any function essential to life. Dogs sustain the simultaneous loss of thyroid and spleen without any perceptible mischief. FIG. 162. FIG. 163. 991. The anatomy of the thyroid gland indicates that certain sub- stances are deposited and changed in its tubes. But all the exertions of its numerous investigators have been unable to substantiate anything beyond this. It is frequently stated that the thyroid enlarges after violent screaming, parturition, or coitus. But these facts are not only exceptional, but inconclusive. And the theory that the thyroid is closely related to the brain, to the organs of voice, or of respiration, is devoid of all satisfactory foundation. 992. The wen or goitre is a degeneration of this vascular gland. Its tubes frequently become distended with foreign substances, or gelatinous deposits, which greatly increase its total bulk. The vesicles thus pro- duced often form the greater part or the whole of the organ. In these we often find cells, which are distended with this gelatinous substance; together with fat globules, and crystals of cholesterine, which are visible to the naked eye as glittering points or scales, and are exhibited, highly magnified, in Fig. 164. In many cases the blood-vessels are themselves enlarged, so as to form globular dilatations such as we shall again meet with in treating of the function of nutrition. In others, we find effusions of blood, exsudations, and deposits of cholesterine, or earthy substances. Finally, considerable lengths of the smaller vessels, or walls of the vesi- cles, may become more or less calcified : the first of these appearances is represented five times magnified (after Ecker) in Fig. 165. We may judge of this vast increase of size by the fact, that while many wens CHAP. XII.] THE THYMUS GLAND. 301 weigh more than 2 pounds, the healthy thyroid body weighs but f to gths of an ounce. FIG. 164. FIG. 165. 993. The thymus attains a large size in the embryo; but reaches its greatest bulk in the earlier years of childhood. It then becomes sta- tionary; and is subsequently converted into a peculiar fatty mass, in which form it sometimes persists to the end of manhood. If we cut through the areolar tissue and vessels which pass between its lobules, each half may be unrolled like a spirally coiled ribbon, as shown (after Ecker) in Fig. 166. Each consists of a number of small vesicles or tubes seated upon a central canal; the cavity of which is immediately con- tinued into their interior. Thus this organ offers a greater resemblance to the arborescent glands than the supra-renal capsules or the thyroid. FIG. 166. FIG. 1 67. 994. The vesicles of the thymus contain a clear fluid, with nuclei, and rarely cells. Ecker found peculiar concentric structures, enclosing a fatty substance. Similar appearances are seen in the fat of embryos which have been kept in alcohol. And in fibrinous coagula of the heart Hassall has noticed similar substances (Fig. 167), which offer a concentric lamination. 995. We have seen that the final destiny of the thymus is a meta- morphosis into fat. According to Ecker, this change may occur in new- 302 THE THYMUS GLAND. [CHAP. XII. born infants, or children under two years of age. Thus, while under a high magnifying power, the margin of a lobule of healthy thymus offers the appearance represented in Fig. 168, fragments of the altered organ look like Fig. 169. Masses of aggregated fat-globules make the whole still more opaque, and almost black by transmitted light* FIG. 168. FIG. 169. 996. We may suppose that the thymus also forms a kind of laboratory for the elaboration of matter. Its great development in early life indicates that its chief use belongs to this age. From this we might conjecture, though not conclude, that its function is related to the milk- diet of the suckling. t Young mammalia in whom Restelli had extirpated the thymus showed an extraordinary voracity, and many unusual cravings for food. They emaciated rapidly, and died much more quickly than other animals, who had suffered equally considerable wounds without the removal of this organ. * The medical reader should, however, remember that this is not the change finally under- gone by the healthy thymus. Here we have not merely fatty molecules, but fat-cells, or adipose tissue. EDITOR. f A conjecture which, as pointed out by Mr. Simon, is sufficiently refuted by his dis- covery of the thymus in birds and reptiles. EDITOR. CHAPTER XIII. NUTRITION. 997. THE phenomena of nutrition may be considered under three chief sections : a morphological; relating to the several changes which are undergone by the tissues, and are visible with or without the aid of the microscope: a numerical;* which concerns those changes of weight experienced by the whole body, and the several organs: and, finally, a chemical; which treats of the changes of combination gradually undergone by the constituents of the organism. Since the state of these factors during the advance or retrogression of the organized being forms the history of development, the study of nutrition only requires us to consider it as merely maintaining its own existence. 998. Morphological Phenomena of Nutrition. Since the absorbents constantly transmit lymph (Tab. II. Fig. 22, a) and blood-corpuscles (Tab. II. Fig. 24, a) to the blood, this fluid would gradually become overladen with solid structures, if other of its mechanical elements were not destroyed. We are therefore entitled to infer, that the balance is maintained by a continuous cycle in the development of blood- corpuscles. The colourless corpuscles are gradually converted into coloured ones (Tab. II. Fig. 24, b c) ; while the oldest of these disappear by solution, or otherwise. 999. The way in which this occurs has not yet been established. We have seen that there is no sufficient foundation for the views according to which the liver ( 928) or the spleen ( 984) would form the site of this solution of the older blood-corpuscles. Since extirpation of the spleen produces little or no disturbance of the vital functions, we are justified in conjecturing that this vascular gland does not, at any rate, exclusively fulfil the important office of diminishing the number of blood-corpuscles. It may rather be supposed, that the changes under- gone by the blood in the different channels of the circulation themselves maintain the counterpoise. The balance would thus be maintained in the blood itself, instead of being exclusively connected with any special organ. Under these circumstances, respiration and nutrition * Here, as well as in 1078, the above word has been substituted for " statistical ;" a modern term, which ought to be limited to what it connotes, viz. matters directly relating to the state. See Dr. Guy's able article " VITAL STATISTICS," in the " Cyclopaedia of Anatomy." EDITOR. 304 VARYING NUMBER OF BLOOD-CORPUSCLES. [CHAP. XIII. would form the chief conditions for the development of blood-corpuscles. The statement that those of the frog (Tab. II. Fig. 23, a I) are dis- solved by alternately transmitting oxygen and carbonic acid through the blood, appears to be incorrect. 1000. The proportion of colourless corpuscles (Tab. II. Fig. 23, c, Fig. 24, b c) varies with the condition of the body, and with the charac- ter of the blood. According to Bonders and Moleschott, 35 ) their number is increased some time after a meal. Eleven hours and a half after the last meal, the blood contained, on an average, 5'1 lymph-corpuscles (Tab. II. Fig. 24, b c), to 2000 coloured ones (Tab. II. Fig. 24). While three hours after the midday meal, their number rose to 6-2. According to Remak, when an animal loses large quantities of blood in suc- cession, the paler blood contains a disproportionately large number of colourless corpuscles. According to Virchow, in the human subject this obtains to such an extent, that it may be mistaken for pus in the blood. As regards the retrogression of blood-corpuscles, it would seem that those which contain distinct nuclei (Tab. II. Fig. 23, b) at the highest point of their development, lose them before being destroyed. 1001. The blood which circulates in the vessels is a mechanical mix- ture of homogeneous liquor sanguinis with the varieties of blood-corpuscle (a c d, Fig. 114, p. 200) just mentioned. The coagulation seen in the blood taken from a vein, consists in the fact, that a certain quantity of solid fibrine separates itself from the liquor sanguinis, and entangles the blood-corpuscles which, owing to their greater specific gravity, are falling to the bottom. In this way we get a leathery substance, the clot or crassamentum, and a yellowish green or reddish fluid, the serum, of the blood. The former is therefore a mixture of blood-corpuscles and fibrine, soaked in blood ; while the latter is what was formerly liquor sanguinis, deprived of its fibrine. Since the large amount of albumen contained in the serum gives it a certain viscidity, blood-corpuscles which have acci- dentally escaped entanglement in the fibrine, may remain floating in it. 1002. The chief cause of the red colour of the blood consists in the great number of its coloured corpuscles ( 658). Under the microscope, which certainly diminishes colour, the liquor sanguinis is at most of a faint yellow. The fibrine separated by coagulation is yellowish-white or yellow. The intense red colour of the clot is explained by its me- chanically enclosing the blood-corpuscles during coagulation. 1003. When the blood taken from a vein is allowed to rest, two pro- cesses occur. The heavier blood-corpuscles gradually descend. The specific gravity and viscidity of the liquor sanguinis or serum can but delay this movement. Besides this, solid fibrine is deposited from all points of the fluid. If these two occurrences tolerably coincide in point of time, the fibrinous masses will everywhere enclose blood-cor- puscles. The whole of the clot will therefore have a red colour. But CHAP. XIII.] NATURE OF THE CLOT. 305 if, on the other hand, any cause delay coagulation, the corpuscles may sink before all the fibrine is deposited in a solid form. Hence it is only the deeper portions of fibrine which enclose many corpuscles j so that the clot is red below and in the middle, but yellow above. 1004. The upper yellow layer has been designated the buffy or inflam- matory coat, because it is found in blood removed by venesection in inflammatory disorders. But experience teaches that it is often absent during inflammation, and present under other circumstances as, for in- stance, during pregnancy. From what has been just mentioned, it is ob- vious that, by delaying coagulation, it may be produced artificially. A protraction of the process of cooling such as may be produced by re- ceiving the blood into warm wooden vessels ( 202) or the addition of a certain quantity of potash, soda, carbonate of soda, or sulphate of mag- nesia, is always sufficient to produce a bufiy coat. And should any cause diminish the specific gravity or viscidity of the serum, while the blood-corpuscles retain their ordinary number and specific gravity, it is obvious that a similar result will follow. 1005. At first, the clot forms almost the whole mass of the blood. Afterwards, it gradually contracts, and thus loses part of the serum which was mechanically united with it. Hence the separation of the coagulated blood into a large solid mass, and a fluid, is the result of an after process. And if the blood forms a thin layer, so as to offer a comparatively large surface for evaporation, this distinct separation does not occur. When large quantities of blood are left to themselves, the coagulum subsequently again softens, and finally becomes intimately mixed with the greater part of the putrefying serum. 1006. Under the microscope, the coagulated fibrine is a yellow homoge- neous mass; but is often disposed in folds, or broken up into streaks, so as to exhibit deceptive appearances of a fibrous structure (Tab. II. Fig. 25, a). When small fragments of it FlG - W- are torn up in serum, we sometimes see flat scales (Fig. 1 70), such as are designated by Nasse, fibrine- flakes. The corpuscles represented in Fig. 167, occur very rarely. Now and then the fibrine is mixed with granules, oil-globules, and very minute struc- tures which glitter like crystals ( 176) in the dark field of the polarizing microscope ( 1 72). 1007. The coloured blood-corpuscles of mamma- lia are originally circular flat discs, with excavated surfaces (Tab. II. Fig. 24, a). Hence, standing on their edges, they look like a small ribbon tapering at its middle (Tab. II. Fig. 24, above and to the right of a). The colourless corpuscles are more or less globular (Tab. IV. Fig. 24, b c); so that they offer much the same shape in all positions. When the liquor sanguinis is diluted with water, so as to x 306 BLOOD-STAINS. [CHAP. XIII. diminish its density and the proportion of its saline contents, the coloured corpuscles 'take up water, and give off part of their colouring matter to the surrounding fluid. They thus become swoln, pale, and globular (Tab. II. Fig. 25, b c). The proper application of salt some- times restores their previous flattened form. 1008. Since the liquor sanguinis is thinned by the separation of its fibrine, some of the blood-corpuscles it contains frequently exhibit a globular form. Others, which retain their flat form, are often ap- posed to each other by their surfaces, like a rouleau of coins (Tab. II. Fig. 24, c d\. On examining fresh blood from an incised wound of the finger, we sometimes see that lively currents spring up in the fluid, and tear up these rouleaus into fragments, while threads of half-coagulated fibrine are drawn out from between them. The blood-corpuscles are easily wrinkled into radiating folds; and by more complete drying, they become star-shaped. 1009. The medical jurist frequently has to determine whether red stains on linen, furniture, or cutting instruments, are due to human blood. Where but small quantities are concerned, chemical exami- nation cannot afford any trustworthy results; since the blood does not contain any characteristic and peculiar substances, and its small amount of iron furnishes no indication. A microscopic examination is often equally inconclusive. If the blood-stain is to be extracted with a watery fluid, we must not select pure water, but a liquid which, like a solution of salt or sugar, does not aifect the forms of the blood-corpuscles. If these exhibit oval forms (Tab. II. Fig. 23, a b) it will follow that the blood does not come from man, or any domestic animal, but from a bird, fish, or reptile. If their shape be spherical, they may belong to some fishes, the domestic mammalia, or man. But there are generally insuper- able difficulties in deciding whether they are those of a man or a mammal. It has certainly been maintained that the medical jurist may be guided by the smallness of the structures remarked under the microscope. But apart from the circumstance that smallness of diameter may be due to previous desiccation, we must remember that the average diameter of the human blood-corpuscle equals l-3560th of an inch, while that of the dog is l-3330th, the cat 1 -4400th, the pig 1-422 Oth, the horse l-4720th, the ass l-4010th, the cow 1 4320th, the sheep 1 -5310th, and the goat l-6350th. Hence, even under the most favourable circumstances, we could only recognise the blood of some of the mammalia, such as the domestic ruminants : and should never be really entitled to affirm that we had human blood before us. 1010. We shall hereafter see that it is the blood which sustains the phenomena of nutrition ; that it sets apart the substances necessary to the tissues, and takes up compounds which have become unfit for their use, either directly, or by means of the absorbents ( 534). It is therefore CHAP. XIII.] NUTRITION OF THE TISSUES. 307 FIG. 171. obvious, that all parts of the organism require blood-vessels for their maintenance and growth. 1011. The n on- vascular tissues, such as the epidermis (Tab. IV. Fig. 62, a b), the nails, the hairs, the epithelium, and, generally, all the horny tissues, contain in their interior neither blood-vessels nor nerves. Hence they may be wounded without exciting haemorrhage or pain. But the vascular parts are permeated by numerous vessels ; and, for the most part, by nerves also. Hence their mechanical injury is followed by haemorrhage, and frequently by pain. 1012. The substances required by the non- vascular tissues, are furnished by their matrix, i. e., by the vascular tissues in their immediate neighbourhood. While, conversely, the vascular structures derive them immediately from the blood-vessels which run in their own mass. Still this difference is, in many respects, less important than might at first sight appear. 1013. For instance, on examining a villus of the small intestine, such as is represented by the diagram (Fig. 171), we see that the nutritional substances required by the non-vascular cylinders of epithelium (a) will be supplied by the remote capillary network of the matrix (d). While the remainder of the substance of the villus, which belongs to the tis- sues called vascular, possesses no other capillaries, than those indicated at d. Hence all parts of it not in imme- diate contact with the vessels (d) are compelled to derive the com- binations they require from the nu- tritional fluid, which is everywhere present, and is constantly renew- ed by the blood. Since even the finest vessels surround a certain number of primitive nerve-fibres, a quantity of areolar tissue, &c., the same observations will apply to these structures. In all the tissues, the nutritional fluid in which they are soaked forms the path by which the necessary substances reach them, and others leave them to enter the blood. 1014. Since the vascular tissues are surrounded on all sides by a net- work of capillaries, and are thoroughly steeped in nutritional fluid, this path will, in their case, be shorter, and the mixture more uniform. But in the thicker horny tissues this is not the case. Since the matrix of the hair (Tab. IV. Fig. 63, d) occupies the bottom of the cavity of skin from which it grows, the compounds which are to reach the point of the hair have to pass through a considerable distance. Let 1, 2, 3, 4, (Fig. 172) represent the separate layers of the epidermis; 1 being the most superficial, and 4 the deepest, which is immediately bounded by x 2 308 NUTRITION OF THE NON-VASCULAR TISSUES. [CHAP. XIII. the matrix. Here it is evident that only those substances rejected by 4, can reach 3, and only those refused by 3, can arrive at 2 and so on. Hence every layer has a more limited selec- FIG. 172. tion, according to its distance from the matrix. But it is obvious that something of the same kind must occur, to a less extent, with the vascular tissues. 1015. The denser horny tissues such as the cuticle, the nails, and the hairs, are subject to a constant and more or less active integral renovation, even in adults. Let us suppose that the epidermis existing at any given time, consisted of the layers, 1, 2, 3, Fig. 172; the upper- most stratum of which is usually set free in the form of scales (Tab. IV. Fig. 62, c). These are so small, that their desquamation generally eludes the naked eye. But if a finger be kept many weeks bandaged in linen, a mealy-looking mass will be found, which consists solely of desqua- mated epidermis. While 1 thus disappears, the new layer, 4, is produced below ; so that the cuticle now consists of the layers 2, 3, 4. It therefore loses none of its absolute thickness. And as the process is continually repeated, the person has, after some time, a totally different skin from that which he formerly possessed. 1016. If we investigate these changes with the aid of the micro- scope, we see that the soft layer of cuticle which immediately limits the corium, developes nuclei, enclosed in delicate cells. These after- wards lose their albuminous contents, and become flattened, while their coats are partially hornified. They then acquire an appearance resembling that represented in Tab. II. Fig. 33. They are finally converted into very thin horny scales, which some- FIG. 173. times appear elevated in the region of the thicker / a. (k ut now clear) nucleus, b, Fig. 173. If the hornify- -;5^r p rocegs i s carr ied still further, the nucleus be- comes indistinct, or disappears (Tab. II. Fig. 32). The oldest horny scales of epidermis are at length dismissed from the organism by the process just mentioned ( 1015). During this development of the horny cells, the desquamation of the epidermis by layers is constantly pressing them forward towards the surface ; so that the oldest cells of the cuticle are found in this situation (Tab. IV. Fig. 62, a c\ while the youngest occupy the region of the rete Malpighii which adjoins the corium (Tab. IV. Fig. 62, b). 1017. Pressure acting on the skin exerts a visible influence upon the thickness of the epidermic strata, and the degree in which they become horny. The difference between the rough hand of a smith, and the tender one of a lady, is in part thus explained. But it would be wrong to apply this theory universally. It is true that the thickest cuticle CHAP. XIII.] SHEDDING OF THE EPITHELIA. 309 occupies the heel (Tab. IV. Fig. 62), which is also the place where the weight of the body rests in standing. But this part of the cuticle is thicker even in the embryo ; a fact which proves that the peculiarity essentially depends upon the original plan of organization. 1018. Many pavement-epithelia, for instance, those of the tongue and mouth also consist of various layers, from which the older and more superficial are constantly being shed. This explains why every drop of saliva contains a number of thin horny scales (Tab. II. Fig. 31, a b). The same phenomena are exhibited in a less remark- able degree by other more delicate varieties of pave- FlGt ] ^ 4< nient-epithelium such as the conjunctiva of the eye (Tab. II. Fig. 23), the nuclei of which are larger and more granular, while their walls become less horny. Younger cells (a, Fig. 174) also lie under those ciliated columns (b, Fig. 174 and Tab. II. Fig. 36), which we meet with in the respiratory pas- sages. But this desquamation is not so regular as that which occurs in the pavement-epithelia above mentioned. 1019. Under the microscope, a thin section of the free margin of the nail shows an indistinctly granular grey mass, which is traversed by irre- gular, and frequently zig-zag, lines of fission (Tab. II. Fig. 37). If the whole be soaked for some time in sulphuric acid, or boiled in a solution of caustic potash or soda, we find that the semi-fluid mass contains a number of transparent horny scales (Tab. II. Fig. 38). And a careful examination teaches us that, however much the substance of the horny nail appears, at first sight, to differ from the cuticle, it is still entirely composed of horny cells, united to each other by a solid cement. The lines mentioned above are merely clefts, which have been produced by cutting away the brittle mass; they rarely indicate true super-imposed layers. 1020. The matrix of the nail lies underneath its surface. It forms elevations and depressions, which resemble those seen in the skin at the end of the finger, but take a straighter course. Large vascular loops (comp. Fig. 110, p. 199, and Tab. IV. Fig. 52, e) run in the interior of these moulds, which essentially correspond with the rows of tactile pa- pillae (Tab. IV. Fig. 62, d e) on the rest of the skin. From hence are secreted the younger nail-cells. Still the substance of the nail is thinner at the root than further forwards. Its white spots sometimes show that it grows slowly forwards from the root, so that the oldest segment finally projects beyond the neighbouring soft tissues. 1021. The horny shaft of the hair (Tab. IV. Fig. 63, c) consists of three parts. Around its outer surface lie thin epithelial scales, which overlap each other like the tiles of a roof, so that their margins form 310 GROWTH OF THE HAIR. [CHAP. XIII. transverse lines (Tab. II. Fig. 39, a). Occasionally, some of them are partially stripped off; and project from it, either alone, or in connection with oil-globules, and accidental impurities (Tab. II. Fig. 39, e). Under the thin cuticle (b) is the striated cortical substance, which forms the greater part of the hair (Fig. 39, b). This first tears up into fibrous bands; and may then, with the aid of sulphuric acid, be separated into small, thin, horny scales. Finally, in the middle is often seen the medullary canal, filled with pigment (Fig. 39, c). But in some places it is frequently absent, or, at any rate, does not enclose pigment (Fig. 39, between c and d). Here and there paler patches of pigment also occur (Fig. 39, d). 1022. In grey hair, the colour of the cortical substance is whitish grey; in flaxen hair, it is yellow; in red hair, it varies from yellowish red to reddish; and in brown or black hair, it is light or dark brown. The uniform colour of this cortical part always has its effect. Still many of the hairs appear darker to the naked eye ; either on account of the large quantity of pigment enclosed in their highly developed medul- lary canal, or from numerous small deposits of pigment in their cortical substance, or from both of these circumstances together. Such scattered patches of pigment are the chief cause of the change of flaxen hair into brown as adult age advances : since it is only later that the cortical sub- stance alters in colour. While, on the other hand, the alteration which makes the hair grey appears to attack this part of it first. 1023. That segment of the horny part of a hair which is concealed by the skin (Tab. IV. Fig. 63, c c?) is fixed into a peculiar sac, the hair-bulb (efg). Its sides are surrounded by a double coat; the outer and inner root-sheath of Henle (e and/). These are special processes, sent inwards from the deeper and superficial layers of the epidermis (a b). The middle layer of the hair-bulb contains, according to Koelliker, un striped muscular fibres (Tab. IV. Fig. 59), which pass round it in circles. 1024. That segment of the shaft which is fixed in the hair-bulb frequently enlarges at its lower extremity (Tab. IV. Fig. 63, d) ; while, in other instances, it is more or less pointed. The elements of the cortical substance here experience a transition into horny cells, which are younger, the deeper they are followed; and finally, they merge into nuclei and cell-formations, which are secreted from the matrix i. e., from the blood of the vessels which here surround the hair-bulb. In this way the hair is pushed upwards from below. The subsequent cells apply to their further development the materials which have been left by their predecessors. 1025. What has just been stated sufficiently shows, that many of the most important phenomena of nutrition and growth depend on the cha- racter of the matrix of these dense horny tissues. Thus, while cutting the hair not only does not injure it, but may even, from obvious reasons, CHAP. XIII.] DEVELOPMENT OF FAT AND PIGMENT. 311 further its growth, destruction of its lower germinal part or matrix leads to baldness. Such an injury may be produced by general internal diseases, by cutaneous eruptions, or by the growth of fungi in the bulbs. 1026. It cannot be doubted that the vascular tissues undergo a thorough change in the course of time. But few, if any, of them exhibit rapid and regular changes. Many indeed appear to remain stable during a very long time. 1027. The quantity of fat varies greatly with the state of nutrition. In corpulent persons, the masses of fat which lie beneath the skin, in the mesentery, on the surface of the heart and great vessels, between the muscles, and in the neighbourhood of the nerves, are considerably increased. All these deposits consist of the ordinary fat-cells (Tab. II. Fig. 27). And, conversely, in sick persons who are much emaciated, we sometimes find beneath the skin nucleated cells, which only contain one or more oil-drops. Similar alterations are also found in dropsical subjects. But many masses of fat which have an important relation to muscular actions such as the fat of the orbit or the cheek do not disappear in the most emaciated subject. We shall presently see that, even in starvation, the fatty substances of the brain and spinal cord, and probably those of the nerves, are retained. 1028. The pigments are closely related to the fats. In the choroid of the eye, they have a definite physiological use. Thus it is only in albinos like the white rabbit that these cells of the choroid (Tab. II. Fig. 29) contain no black molecules of pigment (Tab. II. Fig. 28). Here they are of a greyish white colour. In such persons there is a general absence of dark pigment. Hence their white or whitish flaxen hair, their clear blue irides, and pale yellow faces. 1029. Under various normal or abnormal conditions molecules of pigment are frequently deposited, either free or enclosed in cells. Thus in the bodies of men and animals, we sometimes find branched pig- ment-cells (Tab. II. Fig. 30) occupying the neighbourhood of the blood- vessels, the nerves, the ganglia, and the cerebral and spinal dura mater; while, in other instances, they are absent from all these points. And in various kinds of tumours, considerable quantities of pigment are often produced, giving rise to black discolorations, which are called melanoses. 1030. Fat and pigment are frequent supplementary products of the hornifying process. The deposits of pigment met with in the hairs ( 1022), and the dark tint of the skin (which chiefly depends upon the epidermis), are partially referable to this cause. The black colour of the negro depends exclusively on the epidermis, and chiefly on its youngest and deepest layers. Here Krause has found pigment-cells, together with a large quantity of dark brown nuclear structures. 1031. Just as the quantity of adipose tissue is increased in corpulence, 312 NUTRITION OF THE FIBROUS TISSUES. FIG. 175. [CHAP. xin. so muscles, strengthened by good food and exercise, increase in size, and especially in diameter. But microscopic research teaches that the striped fibres (Tab. IV. Fig. 54) of a strong labourer, and those of a weak ema- ciated girl, do not essentially differ in thickness : or, at any rate, not suffi- ciently so to explain the enlargement of the total muscular mass as due to that of its several elements. From this it follows that, when a person becomes more muscular from continuous labour or gymnastic exercise, new muscular fibres are gradually produced.* We are justified in con- jecturing that something similar occurs with the nerve-fibres. Still the thickness of the nervous trunks does not experience so remarkable an increase as that of the muscles. 1032. Many vascular organs of the adult possess certain structures, which appear to indicate earlier stages of development. The denser sub- stance of the crystalline lens of the eye consists of peculiar fibrous structures of various size, called lenticular fibres (Tab. IV. Figs. 56 and 57). A semi-fluid mass, the liquor Morgagni, is found between the solid lens and the capsule which encloses it. It contains a number of globules (Tab. IV. Fig. 55) resembling the elements of the first deposits of lenticular substance in the embryo. Many cartilages contain simple carti- lage-corpuscles at their circumference, and com- pound ones towards their middle. The striped muscular fibres (Tab. IV. Fig. 54, b) possess a membrane, the myolemma or sarcolemma, on the surface of which may be remarked numerous nu- clei. When the bundles of areolar tissue (Tab. III. Fig. 40) are treated with acetic acid, the whole becomes gelatinous, transparent, and homogene- ous (Tab. III. Fig. 41, a). At the same time, there appear certain peculiar fibres (Tab. III. Fig. 41, b) much akin to the elastic fibres (Tab. III. Fig. 42, 44). These sometimes encircle the bundle of areolar tissue, in the manner represented, after Henle, in Fig. 175. But some of them are imper- fect, and seem to consist of a number of elongated nuclei, which are arranged in longitudinal series at certain distances from each other. This may be illustrated by the upper half of Fig. 1 75. * But the bulk of a limb is sometimes increased by exercise so rapidly, that we may doubt whether there has been time for that development of a complex muscular and tendinous apparatus which every new fibre would imply. And although the great range of diameter in the fibres of the same subject obscures calculation, still when a very slight difference is multiplied by the enormous number of these in the muscular thickness of a limb, it seems sufficient to account for the altered bulk. It is, however, by no means unlikely that the original number of fibres differs greatly in different individuals. EDITOR. CHAP. XIII.] DEVELOPMENT OF BONE. 313 Similar nuclei and cells are found in the earlier development of the embryo. Hence the occurrence of these structures in the adult admits of two interpretations. We may either regard them as indicating a continuous integral renovation of substance j or may view them as organs which, from collateral circumstances, are less completely developed than their neighbours. The latter idea seems nearest the truth. 1033. In various healthy or morbid conditions, large quantities of ashy constituents, and especially of calcareous salts, are deposited in the cartilaginous or fibrous tissues. The process of ossification i.e., of transition into true bone consists, not merely in the copious deposit of these inorganic compounds, but also in the simultaneous occurrence of definite structural changes. Where the hardened mass is devoid of true osseous structure, the process is often distinguished by the special name of calcification, or earthy transformation. 1034. True bone is produced from cartilage. This consists of a basis of intercellular substance (Tab. III. Fig. 45, a), in which are imbedded the cartilage-corpuscles (bed). They form simple (c) or compound cells (b c d) ; the walls of which are frequently thickened, and form parent-cells enclosing smaller ones. The latter frequently possess cavi- ties (e), the dark margins of which make them look like isolated oil- globules at first sight. 1035. According to H. Meyer, 36 ) cartilage ossifies in two ways. In all those cartilages which are bones in the adult, the intercellular substance becomes earthy before the corpuscles. But where centres of ossification are deposited in the nasal, thyroid, and costal cartilages, or in the fibro- cartilages, the reverse of this process obtains. 1036. When we examine a thin section of an ossifying long bone for instance, the tibia of an infant under a low magnifying power, the corpuscles of the cartilage in the neighbourhood of the bony substance (Tab. IV. Fig. 51, b c) are seen to be aggregated in clusters (a b), which are often arranged lengthwise in rows, that have a direction (d e) iden- tical with that of the osseous partitions already produced (be). A higher magnifying power shows that the several groups of cartilage- corpuscles correspond with secondary cells, which are contained in parent-cells. This circumstance, however, is not essential to the ossify- ing process : many of the embryonic cartilages contain only simple cartilage-cells. 1037. The calcareous salts are first deposited in the intercellular substance that extends between the cells of the cartilage. Although they are chemically united with this substance, still we frequently see a few fine calcareous molecules, which agglomerate in many places, and finally become inseparably confused with the rest of the osseous mass. In this way are produced balks or partitions of bone (Tab. IV. Fig. 51, b c) ; which, at one end, are connected with each other, and at the other, 314 FORMATION OP BONE. [CHAP. XIII. protrude their simple or forked branches (b e) into the neighbouring cartilage. They enclose between them the simple or compound cells of the cartilage. Where medullary cavities (/) are produced, these cells gradually disappear, together with the substance by which they are surrounded. But, when not thus removed, the greater part of their substance also becomes earthy. There remains only the central cavity (corresponding to e, Tab. III. Fig. 45), which sends branches into the thickening walls, and becomes converted into a corpuscle or lacuna (Tab. III. Fig. 47, e, and Fig. 50, b c d c) of the permanent bone. 1038. In the second mode of deposit just mentioned, the calcareous granules are deposited in the walls and interior of the cartilage-cells. Frequently they also become fused into a continuous mass. If the inter- cellular substance is converted into bone, the fine granular deposit is repeated on the outer surface of the cell-wall (compare Tab. III. Fig. 45, b c d e). 1039. On examining a thin section of perfectly developed bone under a low magnifying power, we find that its compact tissue (Tab. III. Fig. 46, a) consists of a number of cavities (b) the medullary or Ha- versian canals and numerous small structures (c), the corpuscles or lacunae. Under a powerful lens, such a transverse section from the human femur shows the round or oval orifices of the Haversian canals which have been cut across (Tab. III. Fig. 47, b). Many of these, which pass obliquely downwards (c), are also seen in a part of their length. A longitudinal section of the bone frequently affords a side view of these cavities (Tab. III. Fig. 47, a). In the compact or cortical substance which forms the outer surface of most bones, they possess a smaller diameter. While, conversely, in the internal spongy or medullary tissue, these canals are larger than their intervening osseous parti- tions. 1040. Transverse sections of the compact tissue show that the Ha- versian canals are encircled by concentric layers of bone (Tab. III. Fig. 47, d) -, while the thinner bony partitions of the cancellated tissue are devoid of this arrangement. The lacunae (Tab. III. Fig. 47, e, Fig. 48, b c), which, in the first of these substances, are also placed con- centrically, give off on all sides (Tab. III. Fig. 48, d) minute tubes or canaliculi; which unite with each other, and, in some places, form an independent network. Although they often appear black by trans- mitted, and white by reflected light, still during life they usually contain a fluid, without calcareous granules. In some of the lacunae, how- ever, the latter may occur. Still, when a few lacunae are included in a very thin lamina of bone, they are generally clear and transparent : so that we may easily convince ourselves of the absence of all mechanical deposit of calcareous salts. 1041. The medullary cavities are clothed by the medullary membrane, CHAP. XIII.] FORMATION OF BONE AND TEETH. 315 and possess blood-vessels, which pervade the interior of the bone. Where they form large cavities, the additional space thus acquired is occupied by a deposit of fat-cells (Tab. II. Fig. 27), the aggregation of which forms what is called the marrow. But these contents do not en- tirely fill the medullary cavity. It may therefore be conjectured that, in the living animal, the vacant space is occupied by watery vapour, and probably by other elastic fluids. And just as the surface of these internal cavities of the bone is covered by the medullary membrane, so its outer surface is clothed by another fibrous tunic, the periosteum. 1042. When cartilage is converted into bone by the process just ( 1036) mentioned, none but cancellated or spongy substance is at first produced. In very young bones this extends to their surface. The pro- cess is subsequently completed in two ways. New bone is secreted beneath the periosteum. And the several rods of bone are also enlarged by the deposit of additional strata. Upon this fact the concentric arrangement of its layers chiefly depends. 1043. Many have supposed that the bones and teeth are subject to an uninterrupted integral renovation. This opinion is based upon the results of feeding animals with madder. Thus if the food of a young pigeon or pig be mixed with madder, after some time the bones will be found of a rose-red colour. But if the food mixed with madder, and the ordinary food, are given during alternate weeks, the bones are afterwards found to contain some red layers, and other white ones. Many ob- servers have therefore assumed that these portions of the bone were new productions, gradually formed by constant integral renovation: that the red layers corresponded to the times during which the food had been mixed with the madder; and the white, to the intervals of simpler food. They supposed that the new bone was chiefly deposited by the periosteum; and the old absorbed by the medullary membrane. Hence the renovation of the osseous substance proceeded from without inwards. But in the teeth it took the reverse course. That part of the true dentine which lies internally, and is apposed to the vascular and nervous tooth-pulp (Tab. III. Fig. 49, a) was the youngest; and that (6) which bordered on the enamel (b c) the oldest. Hence it was only the true dentine (Tab. III. Fig. 49, a d b) provided with tubes and bony cement, which took the red colour : the enamel did not do so. But such observations do not prove a rapid and continuous change of these hard -tissues. The colouring matter of the madder enters the blood; the liquor sanguinis of which thus acquires a red hue. It easily combines with salts of lime ; as may be shown by artificial experiments. Hence those parts of the bones and teeth which are in immediate propinquity to the blood-vessels become coloured red. Micro- scopic research supports this proposition. When the coloured food is stopped, the red portions resume their yellowish colour. And those parts 316 INFLAMMATION. [CHAP. XIII. which lie next to the vessels are again situated most favourably for the occurrence of the change. And in point of fact, a microscopic examination shows, that the whiter strata do not consist of new layers, but of portions which have lost their previous colour. The varying colour is best seen in young animals whose bones have not yet finished their growth. 1044. It cannot be doubted that slow changes occur in the bones and teeth, as in the other parts of the body. In old people the propor- tion of cancellated tissue is increased. Chossat fed poultry upon grain unmixed with gravel, and found that the bones became gradually thinner, and hence more brittle and flexible. If a rodent, as for instance, a mouse, be imprisoned for months without a possibility of gnawing any- thing, his incisor teeth grow out, become curved, and even penetrate the jaw-bone by their extremities. 1045. The intercellular substance of young cartilage appears almost homogeneous. But in many of the articular and other cartilages of the adult, it exhibits a granular or fibro-granular structure (Tab. III. Fig. 45, a). The fibro-cartilages lead us a step further. Here the inter- cellular substance consists exclusively of fibres, in the intervals of which lie the cartilage-corpuscles. In the later years of life it sometimes happens, that not only the intercellular substance, but also most of the cells, of the previous true cartilage, are in certain places converted into fibres. And conversely, the whole sometimes softens, and either remains gelatinous, or finally disappears, so as to give rise to the forma- tion of cavities. 1046. Congestion and inflammation are caused by certain disturb- ances of circulation and nutrition. It is supposed that a part in a state of active congestion is permeated by too large a quantity of blood, while in passive repletion this fluid becomes permanently aggregated in its interior. The cerebral haemorrhage which causes the apoplectic stroke is intimately connected with such morbid conditions of the circulation. 1047. Hitherto the microscopic phenomena of commencing inflamma- tion have chiefly been investigated in the web of the frog's foot ( 651). These animals together with many other reptiles and fishes offer the advantage of allowing their circulation to be examined without the inflic- tion of any injury of importance. Still such observations only afford very imperfect results; since in these creatures, inflammation takes a more sluggish course, and the subsequent exsudation is but sparing in quantity. Warm-blooded animals, such as bats, are better suited to such microscopical examinations. 1048. If any point of the extended frog's web be burnt with a hot iron, or moistened with a drop of mineral acid, the blood-corpuscles (c, Fig. 114, p. 200) in the neighbourhood of the wounded place are sometimes seen to quicken their speed. But soon afterwards the blood within a certain distance becomes completely obstructed. At first sight, the ves- CHAP. XIII.] INFLAMMATION. 317 sels seem to be dilated. But a peculiar circumstance here deceives us. The small side-space (a, Fig. 114) which was formerly the immovable layer, ( 656), and was chiefly occupied by colourless liquor sanguinis, now contains numerous blood-corpuscles; so that the whole vessel is redder, and appears wider. But no genuine increase of diameter can at first be established by the micrometer : at least not in the web of the frog's foot. 1049. In the mammalia, all the smaller vessels of the inflamed part become considerably distended with blood. This fact explains the intense red colour seen in inflammation. It is probable that when this over- distension of the vessels has lasted some time, their transverse diameter increases. It has been sometimes remarked by Hasse, Koelliker, Ecker, Harting, and myself, that in inflamed portions of brain, thyroid gland, bronchi, and ovaries, many of the vessels are dilated like those repre- sented (from a diseased ovary after Harting) in Fig. 176. Finally, the FIG. 176. observations of Hasse and A. Mueller on the smaller cerebral vessels of apoplectic subjects showed that their internal membranes had burst in many places, and had allowed blood to be effused between them and the external tunics. A series of dilatations were thus produced; which, unlike those in Fig. 176, were not formed by the whole of the vessel. 1050. The vessels filled with obstructed blood contain an unusual number of coloured corpuscles ( 658). The way in which the circula- tion sometimes returns to its normal condition has already been de- scribed ( 661). But if this fails to occur, the whole coagulates into a red granular mass ; in which the several blood-corpuscles gradually lose their distinctness. At the same time the resistance offered by this im- movable mass exposes the coats of the neighbouring vessels to a more or less increased pressure. Hence a fluid is copiously effused, and forces its 318 EXSUDATION. [CHAP. XIII. way into the neighbouring tissues. We thus get an exsudation ; which distends the tissues, and produces a swelling. 1051. But all exsudations are not produced by the phenomena of inflammation just mentioned : since they may also be caused by a very watery state of the blood, by an abnormal porosity of the vessels, too strong a pressure of the blood, or a want of the corresponding lymphatic absorption ( 534). The character of these, as well as of inflammatory, exsudations, varies with the circumstances of the particular case. 1052. The exsudations which form the majority of dropsical effusions generally maintain their liquid form both during life and after death. They usually contain more or less albumen, yellow colouring matter, salts, and sometimes urea. The fluid removed by tapping a patient with ascites rarely deposits a fibrinous coagulum. But many inflammatory exsuda- tions deposit solid matters even in the living body. These frequently form a gelatinous mass, which is easily thrown into folds, or tears irregularly. Granules of albumen, or globules of fat, may frequently be found on or between them by the aid of the microscope. 1053. The elementary constituents of other deposits of this kind exhibit a higher development. They contain what are called exsudation- corpuscles, inflammatory globules, or granule-cells (Tab. II. Fig. 25, d) : i.e. granular and spherical cells, which consist of a transparent mem- brane enclosing minute granules. Water causes many of them to burst ; and makes the nuclei of others more distinct. Under the influence of acetic acid, the nuclei become indented, so as to exhibit separate divi- sions. This phenomenon, which also occurs in other similar cells, has been designated the cleavage of the nucleus. 1054. In the fresh and pure exsudations of many animals, these corpuscles are sometimes surrounded by clear rings, or cells, which the application of water causes to burst like soap-bubbles. It is probable that this observation will never be repeated upon the exsudations of the human subject, since these are always injured by a previous mixture. 1055. Where the exsudation undergoes a further development, we re- mark longitudinal strise; which possess, either a single elongated nucleus (Fig. 177, a), or several nuclei at definite dis- FIG. 177. tances from each other (Fig. 177, b). Subse- quently these are often narrowed and elongated. They next become paler; and finally, altogether disappear. The fibrous bands become more solid ; and their most minute constituents are then formed by fibres like those of the areolar tissue (Tab. III. Fig. 40). In this way are produced the fibres of cicatrization, by means of which wounds are healed up. They are distinguished by their great strength, which enables them to unite separated parts with extreme tenacity. CHAP. XIII.] SUPPURATION. 319 1056. Pus is a peculiar degeneration of the exsudation. It consists of a fluid basis, the liquor puris; with which are mechanically mixed numerous peculiar solid structures, the pus-corpuscles. These essentially correspond with the exsudation corpuscles already described ( 1053). But they sometimes exhibit a rather yellower colour under moderate mag- nifying powers; and chemical examination indicates that they contain a larger quantity of fat. 1057. The fluid of ichor or of thin sanious pus, exhibits important dif- ferences from that of the ordinary thick, yellow or, as it is called, lauda- ble pus. It possesses more or less corrosive properties ; in consequence of which it acts injuriously upon many of the accompanying pus-corpuscles, as well as on the constituents of the tissues in which it is present. 1058. In order that a suppurating wound should heal, the suppuration itself must decrease. Hence the production of pus is a round-about way ; which involves a loss of time, and of a certain quantity of the corporeal juices. On this account, wherever circumstances will at all allow it, we strive to heal wounds by the more rapid union " by first intention" ; in which the exsudation proceeds at once to the formation of a cicatrix. 1059. A suppurating wound which is about to close first secretes a smaller quantity of liquor puris : in which the corpuscles are sometimes heaped together like a precipitate. These, either alone or mixed with the relics of the disturbed tissues, form a kind of plug. The exsuda- tion-corpuscles, which subsequently replace the previous pus-corpuscles, are frequently aggregated into luxuriant papillae : these are called gra- nulations, or, when of improper quality, " proud flesh." The remainder of the process essentially resembles that by the first intention. 1060. Mortification or gangrene constitutes another termination of inflammation. This morbid affection of nutrition is usually caused by a deficient transmission of blood to the part, or by certain poisoned states of the fluid. The parts lose their colour, and frequently become as black as coal ; and either dry up like the flesh of a mummy, or become liqui- fied by putrefaction. The microscope often shows small black granules, which have been designated gangrene-corpuscles, and which, in many respects, resemble molecules of pigment (Tab. II. Fig. 28). These are generally mixed with greasy masses of blood; fragments of the tissues; fluid and semi-fluid exsudations; and crystals, chiefly of ammoniaco- magnesian phosphates (Tab. I. Fig. 17, kit). 1061. A part attacked by gangrene may be regarded as lost to the organism. The best termination therefore consists in the separation of the dead part by the establishment of suppuration in its neighbourhood, so as to leave a healthy wounded surface. Large portions of the body may thus be removed. For instance, it not only unfrequently happens that frozen feet separate spontaneously at a line of demarca- tion, which forms a circle of suppuration : leaving the surgeon nothing 320 REPRODUCTION. [CHAP. XIIL FIG. 178. to do but to saw through the bones, in order to complete the ampu- tation. 1062. When part of an organ has been lost in consequence of a wound, one of two cases may occur. In the course of time, the part which has been removed either is, or is not, reproduced. If we limit our attention to the higher animals, we find that many tissues possess a capacity of reproduction, which others are devoid of. But many of the lower animals can, under proper circumstances, reproduce all parts alike. 1063. A tissue which does not reproduce itself heals up by means of fibres of cicatrization. For instance, if a muscle has been cut through, portions taken from the seat of injury subsequently pre- sent microscopic appear- ances like those represented at Fig. 178. Its transversely striped fibres (a) cease ab- ruptly ; and the substance of the cicatrix (b) consists of the fibres described in 1055. Something similar to this oc- curs in the skin, the mucous membranes, the cartilages, the brain and spinal cord, and most other tissues. 1064. The crystalline lens, the nerve-fibres, and the bones, belong to those parts which frequently reproduce themselves in the mamma- lia. The new tissues are here developed after laws which are essentially iden- tical with those regulating their origin in the embryo. 1065. If the crystalline lens (Tab. I. Fig. 12) of a rabbit be extracted, leaving its capsule as nearly as possible uninjured, a new lens containing the ordinary lenticular fibres (Tab. IV. Figs. 56, 57) may gradually be produced. The disease called cataract consists in a cloudiness of the crystalline lens. This causes blindness by cutting off the transmission of the rays of light like a screen. Hence we CHAP. XIII.] REGENERATION OP NERVE. 321 seek to restore the sight, either by extracting the lens, or by pushing it into a place where it will not be in the way. It is only in very rare instances that a new lens is formed. But in persons previously operated on for cataract, many observers have found a substance which, to the naked eye, very much resembled that of the lens. 1066. On dividing a nerve, its two ends (a and b, Fig. 179) retract by their own elasticity, so as to leave between them a gap of variable extent (c d). If c and d remain opposite to each other, the interval, c d, is filled up by an exsudation, which gradually undergoes a further develop- ment, and forms a lump (c d, Fig. 180) thicker than the rest of the nerve. New nerve-fibres, which unite the old ones passing from a to 6, are pro- duced at c d. They proceed from the older portions of the nerve at these points ; are at first grey, but subsequently contain true nervous matter ; and are usually distinguished by their smaller diameter. The remainder of the knot is gradually converted into fibres of cicatrization ( 1055). In the course of time, its size generally diminishes. And it may even happen that after some years, no trace of it can be found : or its site may be occupied by a constriction. The process just described may also occur when a piece has been cut out of the nerve. 1067. In order to this reproduction of nerve, the ends of the nerve- fibres must be opposed to each other, and the gap must not be too long. If one of the two portions be twisted, or otherwise so placed as not to afford any free cut surface which may originate the further growth, restoration will not occur. And even under the favourable circum- stances mentioned above, it appears to be rare for all the primitive fibres to recover their previous condition. v 1068. If the divided nerve be not reproduced, one or both of its ends may enlarge into a bulb, or may sprout into narrower portions, which are attached to neighbouring parts (compare e, Fig. 180). Many fibres of 322 REPARATION OF FRACTURES. [CHAP. xiii. the peripheral segment subsequently lose their nervous medulla, become grey and pale, and in all probability finally disappear. This renders the whole nerve thinner, and gives it a dull greyish- white appearance. The central half, which is connected with the brain or spinal cord, is much less subject to this alteration; but some of its primitive fibres are also absorbed. 1069. Experiments on the free cervical ganglion of the vagus nerve in the rabbit show that the ganglion-corpuscles (Tab. V. Fig. 72) are also capable of reproducing themselves. In rare instances of disease, numer- ous pale ganglion-corpuscles may be met with in nerves which do not usually possess any. 1070. The union of fractures depends upon the capacity of bones for regeneration. A new substance, the callus, knits up the gap. It is generally more solid than the rest of the osseous substance : so that, if not injured by a morbid softening or attenuation, the bone yields in any part of its course rather than in the callus. FIG. 181. FIG. 1 82. 1071. When the tubular bone of a limb is completely broken across, the contraction of the muscles makes the two segments more or less over- lap each other. The limb is therefore shortened. Hence it is the duty of the surgeon to restore the previous length by artificial extension, and to maintain it by splints or bandages until a callus of sufficient solidity is present. If this precaution be neglected or insufficiently carried out, the bones unite as they have overlapped; and the limb therefore remains shortened to a corresponding extent. This latter condition may be illustrated by Fig. 181. It represents CHAP. XIII.] CLOSURE OP DELIGATED ARTERIES. 323 the shin-bone of an adult, which has united with a certain amount of shortening. The upper and lower fragments (a and b) overlap each other in the callus (cd). Fig. 182 exhibits the same bone partially sawn up, so as to expose the medullary canal of the two fragments (a and b) and the dense callus (c d) which unites them. 1072. The fracture itself ruptures some of the blood-vessels which run in the periosteum and the interior of the bone. Any further displace- ment of the ends of the bone wounds the neighbouring soft tissues. Both of these circumstances cause the effusion of a considerable quantity of blood in the parts surrounding the fracture ; and subsequently this blood gradually coagulates. The effusion produced by the subsequent inflamma- tion permeates and surrounds this extravasated blood, the colouring matter of which is gradually washed out prior to its own final disappear- ance by absorption. The exsudation meanwhile becomes cartilage. Bone is then developed, partly from the fractured extremities, partly in the rest of the mass, until finally the whole callus is completely ossified. It now generally appears thicker and rougher than the rest of the bone. But in course of time it is smoothed down by absorption ; although, even after many years, it may still exhibit a greater bulk. 1073. The excretory ducts of many of the larger glands have a peculiar tendency to restore their channels. Even after the biliary duct of a living dog (between m and r, Fig. 151, p. 280) has been tied, the bile is some- times again poured into the intestine (i k). The pancreatic duct (p q) may exhibit similar phenomena. 1074. On tying an artery in the middle of its course, it becomes filled with a thrombus or plug of coagulated blood, that extends to the neighbourhood of those collateral branches through which the circulation proceeds unimpeded. Here also there is an admixture of a new exsuda- tion, which gradually produces fibres of cicatrization, while the coagulated blood in great part disappears. The portion of artery occupied by the plug finally becomes ligamentous, and is narrower than the rest of the tube. 1075. In the meantime the collateral circula- FIG. 183. tion restores the movement of the blood, which had been checked or disturbed by the applica- tion of the ligature. The diagram in Fig. 183 may explain how this takes place. Let a be the ligature which prevents all direct flow of the blood from c to d. Large side-branches or col- lateral vessels, b b, are subsequently found to arise from c, and open into e. Thus the blood takes a roundabout course (c b d), and avoids the seat of deligation (a). 1076. It is probable that these collateral vessels originate in what were Y 2 FIG. 184. 324 STRUCTURE OF STUMPS AFTER AMPUTATION. [CHAP. XIII. formerly minute anastomosing branches of the same artery. But the latter delicate tubes are not only mechanically dilated, but also acquire stronger coats, which finally correspond with those of the larger arteries. 1077. When vessels or nerves are amputated, many of these pheno- mena are partially repeated. This may be illustrated by the anatomy of a stump after amputation. The stump of an amputated limb generally undergoes a considerable emaciation, which is usually greater in the upper arm or fore-arm than in the thigh or leg. In the most favourable instances the cicatrix forms a simple straight line. But in stumps formed by the upper part of the thigh, which have healed by suppuration, and possess a certain thickness, this scar is often radiating; so as to exhibit deep furrows, between which the other soft tissues protrude. Contrac- tion of the muscles of one side may also remove it from its original place. Fig. 184 shows the stump taken from a man whose left arm had been amputated about three years before death. At a are seen the folds of skin which occupy the neighbourhood of the cicatrix. The moderately thick muscles, bb (which have undergone a partial contraction), and the ten- dons, are attached by the tissues of the cicatrix, c, to the neighbour- ing structures. The larger nervous trunks, d and e, terminate in large bulbs, / and g, which are, however, solely composed of fibrous tissue (Tab. III. Fig. 40). Some fibrous bands pass from these swellings to the neighbouring structures. These bulbs do not occur on all the nervous trunks of a stump. The arteries, h i, frequently take a more or less ser- pentine course. Their lower ends have become ligamentous as an after result of being plugged ( 1074). The medullary cavity of the bone is closed by new osseous substance, which sometimes form a bulbous and round or notched swelling. New bone also generally unites the radius and ulna (h, Fig. 127, p. 230) of the fore-arm, and the tibia and fibula (n) of the leg. CHAP. XIII.] NUMERICAL RELATIONS OF NUTRITION. 325 1078. Numerical Relations of Nutrition. In the healthy human being, the changes undergone by the whole mass are distributed over such long periods of time as to give but very small proportionate numbers for each several day. But since the quantity of food introduced, and of faeces and urine evacuated, cause great fluctuations, the weight of the body will vary in twenty-four hours within the limits thus produced. On weighing myself for many days immediately upon rising, I found that the greatest difference amounted to 164 oz. in about 116 Ib. 15 oz. of bodily weight. A breakfast may add more than 18 oz.; and a copious micturition may subtract an equal weight. 1079. Confining our attention to the direct evidence of the eye, we find that the food and drink constitute the ingesta which raise the weight of the body. While conversely, the faeces, the urine, and the cutaneous desquamation ( 1015), together with accidental evacuations of saliva and nasal mucus, form the egesta which diminish its mass. Since the latter substances are in very small quantity, it is usual in these numerical researches to regard only the excrements and urine, including their sum under the name of the " sensible " evacuations. 1080. The carbonic acid given off by the body generally weighs more than the oxygen which is at the same time taken up ( 846). And not to speak of the subordinate relations of nitrogen, we find that large quantities of watery vapour pass off by the lungs and skin. We have thus a second series of egesta, which, not being directly visible, were named by the older physiologists " insensible " evacuations, and by the moderns "loss by perspiration." Assuming that the weight of the body does not alter, the difference between its ingesta and its sensible evacua- tions will determine its loss by perspiration. 1081. An examination of the numerical relations of the author's body during three days showed that, on an average, 45,158 grains of food and drink were introduced in the 24 hours. The fasces amounted to 2950, and the urine to 22,363 grains. Supposing the weight of the body com- pletely unchanged, the loss by perspiration was, therefore, 19,846 grains. But a more careful examination proved that 588 grains were retained for the evacuations of the following days. Hence the insensible evacuations amounted to 19,258 grains. 1082. The mode of calculation just mentioned does not presuppose an hypothesis of any kind whatever. The ingesta had to the sensible evacu- ations the proportion of 45,158 to 25,313, or of 1 to *56: and to the insensible evacuations, that of 45,158 to 19,258, or of 1 to *43. About T^th of the food and drink was laid by for the following days. The sen- sible and insensible evacuations were to each other as 22,363 to 19,258, or as 1 to '76. Thus 56 per cent of the food was given off in the excre- ments, and 43 to 44 in the pulmonary and cutaneous evaporation. 1083. But a consideration of that gaseous interchange which accom- 326 INGESTA AND EGESTA. [CHAP. xni. panies evaporation requires many hazardous assumptions. The oxygen absorbed directly increases the ingesta; but its quantity has never yet been determined by a series of direct experiments. Hence we are obliged to extend the estimates made for a few minutes, and under somewhat con- strained breathing ( 824), over a larger period of time, a method which greatly multiplies the amount of their errors of observation. A second and equally unsafe method consists in analyzing specimens of the food, drink, urine, and faeces, and subtracting the carbon and hydrogen of the sensible evacuations from that of the food, so as to estimate the remaining carbon and hydrogen as carbonic acid and water. If we then subtract the oxygen contained in the fseces and urine from that of the food, and again deduct this residue from the quantity of oxygen contained in the carbonic acid and the water of combustion, we shall obtain a value which may be regarded as that of the oxygen absorbed. Barral made use of the latter method. But we have already seen ( 286, 846) that all this trouble will not afford trustworthy results. 1084. The carbonic acid of evaporation carries off the greater part of the oxygen taken up by the lungs and skin. The surplus may be applied to the oxydation of hydrogen or other bodies. 1085. Let us suppose that the author consumes 52046 grains of oxygen every hour ( 824) : this will give 12,491 for the 24 hours. The sum of the sensible and insensible ingesta will thus average 57,649 grains. But the sensible evacuations amount to 25,313 grains ( 1081); so that they claim somewhat less than half. This leaves 32,336 grains for the carbonic acid, the watery vapour, the cutaneous desquamation, and the smaller egesta. But if 14,493 grains of carbonic acid are given off ( 824), 17,843 (or more accurately 17,235) are required for the watery vapour, the cutaneous desquamation, and the small and casual excretions of saliva, nasal mucus, &c., always presupposing that the small quantity of car- bonic acid exhaled by the skin is compensated by the somewhat forced respiration which obtains in these experiments. Reducing everything to parts per cent of the total sensible and insen- sible evacuations, and adding the results deduced by Barral from his observations, we obtain as follows : AVERAGE PERCENTAGE DURING THE TWENTY-FOUR HOURS. Incomings. Evacuations. Residue for the evacua- tions of the follow- ing clay. Observers. Food and Drink. Oxygen con- sumed. Faeces. Urine. Sensible evacua- tions. Car- bonic Acid exhaled. Watery vapour. Cutaneous desqua- mation and other small losses. 74-4 25-6 34-8 30-2 34-5 5 Barral. 78-3 21-7 5-1 38-8 43-9 25-2 30- 9 The author. CHAP. XIII.] INGESTA AND EGESTA. 327 Thus the oxygen daily received amounts to ^rd ^th of the average food and drink. The fseces carry off ^th, the urine Jrd to fths, the total of the sensible evacuations more than Jrd to -fths, the carbonic acid Jth to ^ths, the formation of water about Jrd, and the cutaneous de- squamation about 2 -^th, of the whole. On an average, about ^th is reserved for the evacuations of the following day. 1086. If from the experiments made by Barral and myself, we reduce the several ingesta and egesta to fractions of the weight of the body, we find as follows : (See next page.) 1087. The objections which may be raised against the estimates in Barral's experiments have already ( 846) been stated. We will there- fore limit ourselves to the first two series of these, instituted by this chemist upon his own body. Assuming the food and drink = 100, we have as follows : Proportionate quantities of the constituents of the ingestn and egesta. Substances. Experiment. Food Sensible and Pieces. Urine. evacua- Perspiration. Drink. tions. ("Winter 100 5-3 53-6 58-9 S 62-4: hence 2 1-3 water of I combustion. Water . I Summer 100 3- 53-1 56-1 V 62* : hence 18*1 water of i combustion. \ Winter 100 4-2 4-1 8-3 91-7 Carbon . ^ Summer 100 3-4 5-2 8-6 91-4 Hydrogen S Winter { Summer 100 100 4-2 3-0 5-2 6-5 9-4 9-5 90-6 90-5 Nitrogen S Winter i Summer 100 100 10-0 6-1 38-9 46-2 48-9 52-3 51-1 477 I Winter 100 3-4 3-0 6-4 93-6 Oxygen I Summer 100 2-9 3-8 6-7 93-3 These numbers confirm many of the facts with which we have been previously made acquainted. 1088. The urine of man carries off more water than the faeces. But in animals whose excrements are very fluid, the reverse is sometimes the case. And persons suffering from violent diarrhoea also lose large quantities of water by the alvine evacuations. 1089. We have seen that, under favourable circumstances, more water is usually carried off by the urine in winter, than in summer. The esti- mates quoted above exhibit this relation, although not very decisively. The sensible evacuations excrete about as much water as the perspiration, or a little less. 1090. The water of combustion i.e. that portion of water which is not introduced as such, but is produced in the body itself from the oxy- dation of hydrogen, forms about ^th to |th of the moisture contained in 328 INGESTA AND EGESTA. [CHAP. XIII. e o e O S 5 V *5 1 _4 3 g S 1 -g i & a J 5 H PQ H -fc * 1 CO CO a .1 2 | H S, o o Cd rt . M M I _! je X 2 1 iH h 111 tx ij HI I WEIGHT III 3 H; * 1 b O a 111 111 (M CO CO CO 1 2 D o 3 ^tj -R 2 || M< CO H H --* o 1 H CO (M * |1 i 1 11 r r a o p 5 w M -i! S Cu I cc |j H5 * 3 1 1. 1 02 O rt || s S, HOUR J i HS H5 'I 1 ? en S 1 > t | 1 i I H O ^ &JO H i 5 ^ 05 ^O CO H i ^ftf ""jOl fe w H > c3 Z, bt H EH S5 I -6 i s 1 ^P & S H 1 2 1 r (M cp o -i) h te> it. S ^ A D 4-l H C? 2 ^ H O ^ H > ll f | s, - a o> 11 CO W3 CO ^ M K *J |!0 &* S I wl" 3 o og "^ | 14 -e * 1 CO CO ^ .2 2 1 00 rrj .5 !* * ^ i 5 CO CO CHAP. XIII.] DISTRIBUTION OF THE INGESTA. 329 the food. But the experiments on which this estimate is based are such as not to afford very trustworthy results ( 287). 1091. Somewhat more than ^ths of the carbon, .hydrogen, and oxygen of the food reappear in the carbonic acid and water of evaporation. The remainder is pretty equally divided between the fseces and the urine, but the latter generally obtains the greatest share. 1092. We have already ( 286) seen that such numerical observations include sources of error which prevent them from deciding the more delicate question, whether any nitrogen, or if so, how much, is given off. According to the mere figures of Barral, an equal quantity of nitrogen is given off in the insensible and sensible evacuations. The fact that the urine contributes largely ( 954) to the excretion of the nitrogen is also plainly seen in the preceding table ( 1087). 1093. The food consumed by Barral in the winter series of experi- ments consisted of meat, potatoes, vegetables, bread, milk, cheese, sugar, wine, and brandy. Calculating the quantities per cent of the elementary substances contained in its volatile constituents, we find 51 '06 per cent of carbon, 7-98 hydrogen, 3-9 of nitrogen, and 37-06 of oxygen. The cor- responding faeces gave an average of 52-09 of carbon, 7-92 hydrogen, 9-56 nitrogen, and 30-43 of oxygen : while the urine gave 40-9, 8-2, 29-3, and 21-6 respectively. Hence we see that the fseces are chiefly distin- guished from the urine by the large quantity of carbon, and small quan- tity of nitrogen, they contain. 1094. As yet our attention has been limited to numbers which are the averages of a series of observations lasting some days. But there are many collateral circumstances which speedily cause very important variations, and which may thus greatly affect any particular day. It is obvious that a casual constipation will alter the quantity of the fseces, and the copious use of drinks, that of the urine. The quantities of per- spiration also rise and fall in visible correspondence with a change of food or bodily activity. They are also capable of being affected by all the causes which increase or diminish the excretion of carbonic acid ( 808, et seq.) ; so long as these are not compensated by other collateral circum- stances. Thus they generally rise during digestion, or continuous bodily movement ; and sink during rest, and therefore, during sleep. Finally, the act of sweating is one of the chief causes of variations in these num- bers. Fasting and at rest, the author's average hourly quantity of perspi- ration amounted to 463-3 grains. By walking up hill and down dale, so as to sweat copiously, this quantity was raised to 2048-8 ; that is, to between four and five times the former amount. 1095. When the egesta of a fasting animal have used up the residue of the food previously consumed, the substances subsequently given off in the faeces, urine, and evaporation must be yielded by the body itself. Hence the weight of the animal continually decreases. An apparent 330 RESULTS OF STARVATION. [CHAP. XIII. exception occurs in the hybernating animals. This was first discovered by Sacc in the marmot ; and is confirmed by experiments which I have instituted on the hedgehog. When such an animal is plunged in its deep sleep, its bodily weight increases from day to day until faeces and urine are expelled. But the animal loses more by these evacuations, than it had gained in the preceding period of rest. 1096. The first series of experiments on the numerical relations of fasting were furnished by Chossat, a physician of Geneva. Those subse- quently instituted by Schuchardt confirm his principal results. 1097. It is first necessary to distinguish four estimates; viz., the abso- lute, and the relative, amounts of the total, and the daily, loss. An example may illustrate what these numbers mean, and how they may be calculated. We will suppose that a number of rabbits had been allowed to die of starvation. Each of them weighed 16,874 grains on the first day on which food was withdrawn; and 10,201 grains immediately after death. The difference of these two will give the total absolute loss: or 16,874 10201 = 6673 grains. The proportionate total loss is the quotient of the absolute loss divided by the original weight : or 6673-7- 16874= -4, or fths. The rabbits die on an average 9J days after the commence- ment of fasting. Thus we get 6673-r-9'33=715 grains for the absolute daily loss; and 4-f-9'33= < 043 for the relative one. 1098. It may be stated generally, that one of the higher vertebrata dies by starvation after losing about fths of its bodily weight. As might, however, be expected, the quantities for each case vary within wide limits in different animals and circumstances. But the averages deduced from numerous series of experiments all approximate to -4 as the proportionate total loss. Thus Chossat obtained from -31 to -42 for birds and small mammalia, and -41 for frogs. 1099. The daily loss varies greatly with the nature of the animal. Confining our attention to rabbits, guinea-pigs, fowls, pigeons, and other domestic birds, the average proportion is '024 *112; usually -04. Here from J to 24 weeks suffice to produce death by starvation. But frogs will go for months without any solid food. And hence if they also finally lose |ths of their weight, their proportionate daily expenditure would be only -002. 1100. Daily experience teaches us that^a fasting animal emaciates; i. stances, and 63*7 of water J 304 015 21-19 5512-5 We may first remark that the total relative loss of an animal which has perished from the use of an improperly exclusive food, tolerably corresponds to that of an animal starved to death ( 1097). Death by thirst is less speedy, because solid food always contains certain quantities of water ( 339). Since the large quantities of carbonic acid in the perspiration and especially in that of small birds require much combustible organic matter, it becomes explicable why feeding with hydrates of carbon sustains life almost thrice as long as with albu- minous substances. It is obvious that the differences exhibited by the daily relative losses are mere results of the varying durations of life, and the almost equal amount of the total relative loss. 1103. Chemical Phenomena of Nutrition. The blood is the centre of all the functions subservient to the change of substance. It receives many constituents of the food, either directly, or by means of the ab- sorbents ( 525). The oxygen which it attracts from the atmosphere effects a direct change in many of its constituents. And it is probable that a certain quantity of this gas penetrates the tissues, in order to act, either upon these, or on the fluid by which they are soaked. Since the blood, especially that of the arteries ( 623, et seq.) and capillaries, is exposed to a greater pressure than the nutritional fluid, it will allow the * Two structural details may also be indicated as affording a partial explanation of this contrast. The vascular relations of fat perhaps place it in peculiarly immediate depen- dence on the blood. While the cell-form possessed by much of the nervous centre may be conjectured to confer a greater and in some respects, a more independent vitality. EDITOR, 332 TRANSITIONAL SUBSTANCES. [CHAP. XIII. proper substances to pass through the walls of the vessels. In addition to this, the chemical difference of the neighbouring mixtures will neces- sarily excite diffusive currents ( 129, et seq.). We have already learnt the important influence exerted by the blood upon the secretions. To it are also due the maintenance and growth of the tissues. Most parts of the body are bathed in the nutritional fluid ; and are, as it were, con- stantly exposed to a permeating stream of this renovating solution. And while the absorbents carry off superfluous water, together with some substances which have become useless ( 534), the blood permits the transudation of those compounds which are necessary to the restoration or increase of the part. 1104. We will suppose the author takes, on an average, 45,158 grains of food and drink in the 24 hours, and gives off only 2950 grains of faeces. If, in spite of this, the weight of his body remains nearly the same, at least 61bs. 4f ozs. of matter daily enter his blood. But the quantity of blood in his body which weighs 1191bs. is about 231bs. 13 ozs. ( 694). Hence the fluids daily passing through the blood altogether amount to about ^th of its weight. 1105. It may probably be assumed that the various kinds of food ordinarily made use of together contain, on an average, 75 to 80 per cent of water, and 20 to 25 per cent of solid residuum. Now since human blood contains, on an average, 78 per cent of water, it follows that the proportionate quantity of transitional substances is somewhat greater for the water, than for the solid residuum. 1106. Water and dilute watery solutions are rapidly carried into the blood ( 504). Saline solutions, which are in moderate quantity, and not too concentrated, are quickly excreted again in the urine ( 944, et seq.). Substances which are more difficult of solution, and fats, are necessarily delayed some time in the alimentary canal ( 438, et seq.) before becoming the property of the blood. The important but slow changes which most of them suffer will soon occupy our attention. The blood of a person who, from time to time, consumes large meals, must therefore vary greatly. It becomes richer in salts and water shortly after taking much food, and especially after the use of liquids. But the kidneys soon obviate this condition. The other ingesta produce less considerable variations : since they reach the blood in smaller quantities. A part of them undergo important changes in the blood, so as to be at once rendered capable of excretion : while others subsequently follow. This successive play lasts for some time : so that the action of small quantities of these transitional substances is spread over large intervals of time. 1107. Beginning with the consideration of the solid food, we find that the non-azotized kinds such as the hydrates of carbon ( 303) and the fats ( 305) undergo combustion with the oxygen of the inspired air; CHAP. XIII.] CONSTITUENTS OP THE FOOD. 333 being converted into carbonic acid and water, so as to cover the carbonic acid given off by evaporation. They have therefore been distinguished by the name of respiratory food. But this term is insufficient ; since not only are other compounds subservient to the same purpose, but fats intro- duced in excess may be deposited as adipose tissue, and may perhaps combine with azotized substances. 1108. It is easy to see why animals fed exclusively on such non- azotized compounds, perish of inanition. Muscular movements, and other functions, use up a certain quantity of nitrogenous substances, the relics of which are removed from the body. The urine always contains urea, uric acid, and other compounds rich in nitrogen ; for the loss of which such food furnishes no compensation. Besides this the mixture of the blood necessarily suffers so greatly, as to injure the functions of the most important parts of the body. Hence death is induced, not so much by mere loss of substance, as by an injurious mutual action of the nutritive functions, and by delicate molecular changes of the nervous tissues. 1109. The exclusive use of nitrogenized substances, such as albumen, is equally unsuitable to nutrition. For though it is true that the azo- tized tissues can thus receive the compensation which their action de- mands, still, in order that the food should cover the quantities of carbonic acid given off by evaporation, it must enter the blood both quicker and more copiously than the digestive powers will allow. Besides this, the composition of the blood is seriously altered; so that here also the organism is finally undermined. 1110. Hence it is only a suitable mixture of non-azotized and azotized substances which can satisfy the requirements of nutrition. Such a mix- ture is prepared by Nature in the milk ( 346), and even in many natural foods ( 336). Indeed most of the pure azotized or non-azotized sub- stances (in the strict sense of these words) can only be obtained artificially. 1111. The total food of the horse, cow, and hog, contains, according to Boussingault, from 46 to 51 per cent of carbon, and 1-6 to 2-1 per cent of nitrogen. That of the turtle-dove has 47 per cent of carbon, and 34 of nitrogen : and that of fowls, according to Sacc, 48 and 2 '4 respectively. The ordinary mixed food of the human being furnishes much more carbon than nitrogen : according to Barral, about 51 per cent of carbon, and 3-9 of nitrogen. Since beef contains but 15J per cent of nitrogen for 53 per cent of carbon, this statement also holds good for carnivorous animals, though in a more limited degree : a degree which varies with the larger or smaller quantity of fat present. 1112. This fact is the natural consequence of the composition of most organic compounds. Some highly azotized substances such as urea ( 321), or allantoin ( 319) contain less carbon than nitrogen. But even in uric acid ( 321), theobromin, thein, and cafFein ( 343), the reverse of this is the case. While the albuminous substances ( 312), 334 NITROGEN OF THE FOOD. [CHAP. XIII. and the animal tissues generally ( 317), contain much less nitrogen ; and in vegetables its amount is often very small. And since the egesta of the animal are derived either from its food, its own tissues, or from both of these together, the carbon must greatly exceed the nitrogen in them also ( 1093). 1113. The preceding estimates ( 11 11) have already taught us how little nitrogen is contained in the food of the large herbivorous animals, such as the horse and cow. And even part of this is lost by passing off undigested in the food. The body of these animals therefore requires but a small quantity of nitrogen. This statement will also apply to some of the carnivora. Dogs may be kept for months upon potatoes and water : the boiled potatoes containing 46-4 per cent of carbon, 6-1 of hydrogen, 1-6 of nitrogen, and 45-6 of oxygen. In like manner these animals live more than half a year when fed upon adipose tissue. Hence the albuminous walls of the fat-cells, and their intervening tissues, contain a quantity of nitrogen ( 336) which is for a time sufficient. 1114. Many chemists have ascribed too much importance to the azotized constituents of the food. They have arranged the several kinds of food according to their percentage of nitrogen, and have regarded such tables as scales of diet; i.e., as tables, in which the nutritive character of the food rose and fell with the amount of nitrogen present. But there are many arguments which prove that the whole theory is based upon incorrect premises. The study of digestion has already taught us, that the elaboration and therefore the usefulness of the food, is not deter- mined by any single constituent, but by its total admixture ; and espe- cially by its molecular constitution. Hence the flesh of herrings, with 14 -5 per cent of nitrogen, or boiled beef with 15 per cent, or ox liver with 10-7 per cent, are not more nutritious than yolk of egg, which con- tains only 4 '9 per cent. The excess of hydrates of carbon, and especially of starch, which is met with in most parts of plants, greatly diminishes the proportion of nitrogen which they contain. Thus in the different kinds of flour, it is only 1 -4 to 2 '2 per cent ; and in potatoes, turnips, and carrots from 1-5 to 2-4 per cent. The vegetable structures which contain more albuminous substances exhibit a larger quantity of nitrogen. The podded fruits furnish 5 per cent. The fungi are also distinguished by a considerable quantity : from 3-2 to 4-6 per cent. While, in spite of this, they are much less nutritious than many other kinds of animal and vege- table food, which contain, on the whole, much less nitrogen. 1115. Since coffee and tea two drinks in universal use both contain a highly azotized alkaloid, caffein or thein ( 343), it has been thought that instinct has led us to select these two vegetables on account of the large quantity of nitrogen which their alkaloid contains. But no analysis has hitherto shown more than Jrd per cent of caflein in the coffee-beans ; or to 1 per cent of thein in tea. And even supposing rather more CHAP. XIII.] ASHES OP THE FOOD. 335 than this were contained in the plants themselves, still the infusions in ordinary use could only introduce extremely small quantities of nitrogen into the body. And, on the other hand, the theory that the irritating or poisonous qualities of these alkaloids depend on their nitrogenous constituent, is equally contradicted by facts. The poisonous effect of such alkaloids is not proportionate to the nitrogen they contain. For strychnin, the deadly alkaloid of the nux vomica, contains 5-81 per cent of nitrogen; and the comparatively harmless quinine, 8-1 per cent. 1116. The injurious effect of an exclusively albuminous diet ( 1109) shows that a mere azotized food does not favour the vital functions, but rather interferes with them. A further consideration of the excretions will hereafter inform us of other facts which lead to the same conclusion. The substances best adapted to the maintenance and growth of the various tissues are mixtures which contain much carbon and a moderate quantity of nitrogen, and which are easily overcome by the organism. The residuum of the milk, which forms the very ideal of a proper food, consists of 57' per cent of carbon, 8-2 of hydrogen, 4 '4 of nitrogen, and 30-4 of oxygen. 1117. Many of the hard tissues, such as the bones, can only be main- tained by the addition of new mineral substances in the food ( 1044). But since a fasting animal daily gives off ashy constituents, the quantity and quality of which shows that they could not have been yielded by its skeleton, it follows that the action of its soft tissues excretes a certain quantity of inorganic matters, which require replacement. In point of fact, almost every natural food, whether animal or vegetable, contains certain quantities of ashes, which undergo various destinies in the interior of the body. 1118. Many only enter the blood in very small quantity. By far the larger part of them wanders through the intestinal canal, to be expelled with the fseces. This is especially the case with the silicates, which are largely contained in the stems of plants, and form the basis of their skeletons. Thus the excrements of the horse yield numerous fragments of hay; which retain their previous form, having only undergone masti- cation and extraction. The urine and the horny tissues, such as the epidermis or hair, contain but very small quantities of silicic acid. When calcareous salts are largely taken in the food, something similar obtains. Thus the faeces of dogs who have eaten many bones are distinguished by the large quantity of lime which they contain. 1119. The more soluble mineral substances chiefly reappear in the urine. The salt so frequently used as a condiment belongs to this class of substances, as long as it undergoes a proper elaboration. But the result depends on the quantity and density of the salts. Small quantities, and dilute solutions, which easily enter the blood ( 504, et seq.\ are quickly discharged in the urine ( 944, et seq.). But larger quantities, or 336 ASHES OF THE FOOD. [CHAP. XIII. more concentrated solutions, or as is the case with the sulphates of soda and of magnesia a peculiar character of the salt itself, may give rise to diarrhoea : an action which renders the intestine their chief outlet. 1120. Attempts have often been made to explain the preference accorded to salt as a condiment on chemico-physiological principles. It was supposed that its chlorine furnished the hydrochloric acid necessary to gastric digestion ; and its sodium, the soda required for the bile ( 925). But we have seen ( 436) that under normal circumstances the acid of the gastric juice is the lactic, and not the hydrochloric. Besides, a large part of it is given off unchanged in the urine. It is therefore probable that the beneficial effects of this condiment depend upon its general cha- racters, and not upon the products of its decomposition. From observa- tions on himself and others, Plouviez believes that the copious use of salt gradually raises the weight of the body to a certain maximum. It is well known that the food of ruminants is often mixed with salt to facili- tate fattening. The experiments instituted by Boussingault on bullocks showed that their activity was thus increased, although their bodily weight was not raised to the extent anticipated. 1121. It is probable that the phosphates and sulphates of the alkalis also exert an important influence upon nutrition. We have ( 965) seen that their quantity is larger in the urine of the carnivora. The meta- morphosis, restoration, and growth, of the nitrogenous tissues are proba- bly connected with a cycle of these substances. Besides this, a solution of an alkaline phosphate is capable, both of taking up carbonic acid with great facility, and of disengaging that previously combined with it under the influence of collateral circumstances. Hence many have believed that this process obtains in the living blood. 1122. The poisonous or injurious effect produced by the ingestion of many substances is only relative. Too large a quantity of nutritious food is capable of gradually destroying the body. On the other hand, some poisons which rapidly kill one class of animals, leave another unhurt ; or lose their injurious effects when united with particular substances, or introduced into the body by particular channels. Horses bear much larger quantities of prussic acid than man and most of the smaller mam- malia. According to Fontana, snakes, tortoises, snails, and leeches, are not killed by the poison of vipers. Although arsenic is one of the most violent and insidious of poisons, yet, according to Berthold and Bunsen, rabbits sustain a solution of kakodylic acid with impunity. Many creatures eat wourali poison in tolerably large quantities without injury ; while its introduction into the blood kills most animals in a short time. The same holds good of some other poisons prepared by the natives of the American forests. 1123. That absorption of oxygen and expulsion of carbonic acid which accompany the respiration of the higher animals produce a remarkable CHAP. XIII.] COMPOSITION OP THE BLOOD. 337 change in the blood ( 724, et seq.). Since the smaller branches of the pulmonary veins and systemic arteries convey a bright-red blood, and those of the pulmonary arteries and systemic veins a dark red fluid, it follows that at least the greater part of the change of colour occurs in the capil- laries. In the fine vascular network of the lungs the blood loses part of its carbonic acid, and receives the oxygen inspired. In the systemic capillaries this change is reversed. But in all probability a twofold process occurs here. It is possible that a part of the substances directly taken into the blood from the alimentary canal undergoes an immediate combustion, and thus furnishes carbonic acid at the expense of a certain quantity of the. oxygen consumed. We may conjecture that the darker colour of the portal blood ( 921) noticed by some observers is due to this cause. And the action of the organs destroys certain consti- tuents of their substance. The effete compounds thus produced probably contain carbonic acid, or will at least produce it whenever they acquire a certain quantity of oxygen. Hence there will be a mutual exchange between the nutritional fluid and the blood at all points of their mediate contact, and especially at the vast surface presented ( 689) by the capillaries. 1124. It is obvious that the admixture of the blood will vary with the circumstances of the individual, and with the existing state of absorp- tion and excretion. This increases the difficulties of all inquiries insti- tuted upon this topic. Besides this, chemistry has hitherto failed to discover any method of analysis which gives a satisfactory account of the intimate composition of the blood. For these reasons, the numerous researches instituted on the human blood in health and disease offer a confused mass of results, containing but few trustworthy facts. 1125. According to the averages of Becquerel and Rodier, the blood of the adult male contains 77-9 per cent of water, and 22-1 of solid resi- duum. The latter consists of 14-1 parts of blood- corpuscles, 6-9 albu- men, -2 fibrine, -2 fat, and -7 of extractive matters and salts. Hence we see that the fibrine, which apparently enters so largely into the coagu- lated blood ( 1005), constitutes in reality but a small quantity of its mass. In the clot, it mechanically encloses large quantities of serum and blood-corpuscles ( 1001). 1126. The blood of the female contains, on an average, more water, and fewer corpuscles. According to Denis, that of the new-born infant has a greater density than that of the female at the end of pregnancy. 1127. When blood is repeatedly taken from the vein of an animal, the amount of its water rises after much has been lost. This change may even be produced in a few minutes, by the removal of very large quantities. It is probable that the elasticity of the walls of the vessels, and especially of the arteries ( 57), prevents these tubes from collapsing beyond a certain limit. We may conjecture that after the loss of a z 338 CHANGES IN THE COMPOSITION OF THE BLOOD. [CHAP. XIII. large quantity of blood, this fluid attracts watery solutions from wherever it can ; from the absorbents, or the nutritional fluid. Its increased quan- tity of water may thus be explained. 1128. The question, in what cases the fibrine really increases or diminishes, is met by insuperable difficulties. The presence of many substances prevents coagulation ( 1004). It will therefore first depend upon the composition of the blood, how much of this substance is set free. As Virchow has accurately observed, many fluids only deposit their coagulable contents after long exposure to the air. So that there are supplementary causes, which lead to the formation of what are called fibrinous substances. In addition to this fact, these substances have such indefinite chemical characters, and their boundary from the albu- minous compounds is so indistinct, that all basis for further conclusions is wanting. 1129. The blood-corpuscles sometimes exhibit more favourable cir- cumstances. Numerous observations teach that, in women suffering from chlorosis, their number is diminished, and that it is increased by the use of medicines containing iron. And since the greater part of the colouring matter of the blood is connected with its corpuscles ( 1002), the muddy yellow or pale green colour of the face in chlorotic girls is easily explained. According to Hannover, the subjects of this disease exhale more carbonic acid than healthy women. Hence the number of blood- corpuscles does not directly measure the quantity of respiratory pro- ducts. 1130. In some cases, the blood or its serum ( 1001) exhibits what is called a milky character. The blood of sucking animals is sometimes mixed with white streaks. In man, a white or yellowish-white serum is rarely deposited after coagulation. This often depends upon a real excess of fat : but too large a quantity of colourless corpuscles may produce a deceptive appearance of the same kind. 1131. There are many phenomena which indicate, that substances only present in the blood in very small quantity nevertheless exert an important influence on the vital functions. Such are the transfusion of heterogeneous kinds of blood, the ordinary changes of nutrition, and the influence of certain poisonous compounds. Hence it is probable that, however completely the blood may hereafter be analyzed, many important questions will only be decided by examining several pounds of this mixed fluid. 1132. Transfusion consists in injecting the fluid blood of one animal into the vessels of another. This operation has frequently been per- formed on the human subject after considerable losses of blood, such as uterine hemorrhages. In order to diminish the danger of obstruction by coagulation, the blood injected is generally deprived of its fibrine. 1133. It frequently happens that an animal dies shortly after the CHAP. XIII.] MINUTE CHANGES OP THE BLOOD. 339 injection (Tab. II. Fig. 23, a) of the blood of another species. Although frogs possess blood-corpuscles which are much larger than those of man and mammalia (Tab. II. Fig. 24, a d), still they cannot bear the trans- fusion of human blood. Hence death does not depend upon a mecha- nical obstruction of the smaller vessels, but upon more recondite causes. A fact communicated by Bischoff leads to the same conclusion. He found that mammals died after the injection of the venous blood of birds, but not after that of the arterial fluid. 1134. We have already seen ( 1106) that the less soluble parts of the food only enter the blood slowly, and in small quantities. This circum- stance explains many processes of the ordinary interchange of matter. 1135. The starch of the food is gradually converted into grape-sugar (461) or lactic acid. Hence the small quantities taken up by the blood, at short intervals of time, are thus enabled to undergo complete combustion into carbonic acid and water. We therefore find no sugar in the urine. On the other hand, after much sugar has been consumed, the blood receives more of this soluble substance than it can at once elaborate : so that part of it re-appears as such in the urine. 1136. The slow alteration and absorption of fatty matters in the small intestine ( 480) probably leads to a similar result : that, namely, of preventing the blood from being overladen with these substances. This explains why Boussingault 37 ) was unable to detect any constant difference in the fatty contents of the blood of ducks and pigeons, whether fed upon starch or albumen, or not fed at all. 1137. The phenomena which attend the metamorphosis of albuminous bodies point to the same conclusion. The ingestion of these substances has a remarkable effect in increasing the quantity of urea in the urine ( 951). But, since the blood only contains traces of this substance ( 957), it follows, that the large amount given off is gradually accumulated by a continual and repeated secretion and excretion of very small quantities. And the albuminous urine sometimes passed by weakly subjects shortly after a meal may depend on the fact, that the lax porous walls of their vessels do not afford the necessary protection ( 144). It would thus constitute a phenomenon like that which follows the ingestion of large quantities of sugar. 1138. The phenomena of infection show what an influence may be exerted by minute quantities of matter. The drop of lymph intro- duced into the vaccinated arm forms a quantity which is inconceiv- ably small in comparison with that of the blood. And since it contains but little solid residuum, the quantity of its active constituents is yet smaller. Still the action of these continues for a whole week, until the pustule of inoculation is completed, and the protective substance is reproduced in the vaccinated person. And this process, which is only paralleled by some contactive effects ( 299), leaves behind it a z 2 340 EXCRETIONS OF FASTING ANIMALS. [CHAP. xiii. permanent change. For after successful vaccination, the susceptibility for true small-pox disappears; either for ever, or, at any rate, for a number of years. 1139. The evacuations of an animal which has fasted for a long time, and has consumed most of the relics of food that remain in its intestine, are less than usual. Its fseces and urine are diminished, as well as its absolute quantity of urea : it exhales less carbonic acid, and consumes less oxygen, than a healthy animal. But up to the last moment of life, none of these evacuations are totally suppressed. 1140. The emaciation of the fasting animal shows that these evacua- tions come from the decomposition of its tissues. Since a mammal whose blood amounts to about -Jth of its weight ( 694) loses |ths before it dies of starvation, it follows that the matters gradually given off in its evacuations together make up an amount which exceeds that of the blood. But the loss of substance is chiefly caused by the various functions. The muscular contraction necessary to the action of the heart ( 575), to the respiration ( 739), to the local transference of the secretions ( 867), and to the voluntary or involuntary phenomena of movement, furnishes a series of substances undergoing metamorphosis, which are finally given off as carbonic acid, water, urea, &c. Violent exertion of the fasting animal increases its evacuations. While conversely, in the hybernating animal they sink to very small quantities. But since a torpid hedgehog, which remains for days in the same place, with scarcely any respiration, and a slow and infrequent cardiac beat, still voids both faeces and urine ( 1095), it follows that these sensible evacuations do not necessarily depend upon the ingestion of food. The metamorphoses of the different tissues furnish a certain quantity of water and organic matter, which can only be got rid of in this manner. 1141. The continuance of the heart's pulsations and of the respiratory movements, together with the final appearance of febrile phenomena, would lead us, & priori, to expect, that the starving animal gives off large quantities of carbonic acid. Boussingault found in his quantitative researches that the carbon and hydrogen given off by fasting pigeons was from ^ to Jrd of that furnished by well-fed birds of the same species. But Regnault and Reiset did not observe such a difference. Comparing their averages we obtain the following numbers : Animal. Average Quantity of Carbonic Acid in grains, for each pound of bodily weight. Animal. Average Quantity, in grains, of Carbonic Acid, for each pound of bodily weight. Fed. Fasting. Fed. Fasting. Rabbit . . Dog . . . 8-19 9-17 4-97 6'3 Fowl . . . Duck . . . 9-73 12-32 6-58 9-24 CHAP. XIII.] METAMORPHOSIS OF THE TISSUES. 341 These observers state that the fasting animal takes up rather more oxygen than it gives off of carbonic acid. In fasting rabbits, the propor- tion of the latter ( 840) amounted to -93 to -97; in dogs, -996; in the fowl, -88 to -97; and in the duck, -95. Supposing this was not caused by the mode of respiration ( 814), it would follow, that in the fasting animals, more of the oxygen consumed is applied to the combustion of hydrogen, or to the production of those organic compounds which are given off in the sensible evacuations. In two marmots, the proportion of carbonic acid in the waking state was 1-17 : while in one partially asleep it was -55. This difference must be still more strongly marked in the state of complete torpidity. 1142. Since the fasting animal consumes its own body, it behaves to some extent like one of the carnivora, even although itself belonging to the herbivora. This fact explains the acid character of the urine passed by the fasting rabbit ( 972), and the larger quantity of phosphates and sulphates which occur in that of the fasting human being. 1143. Hitherto those metamorphoses of the tissues which accompany the various functions have not received a successful chemical investiga- tion. We can only deduce a few general statements from those of the collateral circumstances with which we are best acquainted. 1144. The substances which are produced by the contraction of muscle, and which are immediately transferred to the nutritional fluid, undergo a second change in the blood, before leaving the body by the urine. Urea is found in the blood, but not in the watery extract of muscle. And the absence of lactic acid from the fresh urine would, if true, indicate the same fact j since it is present in muscle. At present, it is doubtful whether kreatin is produced by the chemical decomposition of the urine ( 961); and hence its relations are as yet uncertain. We are equally in doubt whether the oxygen of the blood does or does not pro- duce an increased quantity of sulphates and phosphates from the sulphur and phosphorus of the metamorphosed albuminous substances. 1145. We have ( 958) seen that the artificial oxidation of uric acid produces carbonic acid, urea, and other supplementary compounds. The ingestion of urates also increases the quantity of urea contained in the urine. It may therefore be conjectured that the urea secreted under ordinary circumstances is due to oxidized uric acid. Small quantities of this acid remain unchanged, and reappear as such in the urine. But the hippuric acid ( 321) is a complementary product of decomposition ; and contains more carbon and less nitrogen. 1146. The chalky deposits which stiffen the joints of gouty subjects consist chiefly of urates, and principally of the insoluble urate of soda ( 977). Hence this disease has been attributed to what is called the uric acid diathesis : i.e. to a deficient oxidation of the uric acid originally produced. 342 COURSE TAKEN BY THE INGESTA. [CHAP. XIII. 1147. Since part of the bile is absorbed in the intestine (468), Liebig has assumed that the compounds thus returned into the blood are subservient to respiration. The accompanying organic products would be given off in the urine. But considering how little bile is secreted by the liver ( 917), it is evident that all the carbonic acid given off cannot come from the absorbed biliary substances. Still it is possible that the constituents of the bile, which is itself a deposit purifying the blood, tend to undergo a final decomposition. Under the influence of alkalies, the cholio ( 926) and hippuric acids furnish glycose : the former also pro- ducing cholalic, and the latter benzoic, acid. 1148. From observations on dogs and rabbits, Frerichs 38 ) supposes that the urine of an animal which is fed on non-azotized food contains about as much urea as that of a fasting animal. And conversely, we have already seen ( 951) that the use of highly azotized food greatly increases the quantity of urea. The differences in the urine of herbivorous and carni- vorous animals ( 972) are explained by the different products of meta- morphosis derived from the ternary or quaternary ( 269) compounds of which their respective food chiefly consists. 1149. The cycle undergone by the highly azotized parts of the food has been explained in two ways. Many imagine them to be decomposed in the blood. According to this view, the constituents of the food un- dergo a simple transit, which only protects the tissues from the assaults they sustain during fasting. Others, on the contrary, suppose that the highly azotized compounds of the ingesta are converted into the elements of the tissues ; and that these give off corresponding equiva- lents of decomposed substances. But this theory involves two results which are not supported by the other vital phenomena. The several tissues would thus undergo a very rapid integral renovation ; and the carnivora would be subject to a far more rapid change of substance than the herbivora. 1150. A third hypothesis has more probability than either. According to it part of the highly azotized food taken up by the blood is decom- posed, in this fluid, into carbonic acid, uric acid, and other compounds, which are given off in the different evacuations. But since this meta- morphosis is limited by the composition of the blood itself, another part exsudes into the nutritional fluid. Here it compensates what is neces- sary; and, with the metamorphosed substances furnished by the wear and tear of the functions, returns into the absorbents, perhaps also into the veins. Subsequently it also is decomposed, and appears in an altered form in the evacuations. This view does not assume any simple passage of the food through the blood. And it has the additional advantage of not presupposing any wide contrast between the change of substance in carnivora and herbivora, 1151. Starchy substances are frequently converted into grape-sugar CHAP. XIII.] METAMORPHOSIS OF THE TISSUES. 343 or lactic acid ( 461). But they often assist in the production of fat : perhaps by means of a fatty fermentation ( 326). The well-known effect of cramming geese may be thus explained. Connected with this is the fact, that a milch cow gives off more fat in her milk than she has taken in her food. 1152. Although the combustion of the hydrates of carbon, like that of the fats, may furnish large quantities of carbonic acid, still it is by no means indifferent which of these two non-azotized classes forms the food. The fats ( 305) require more oxygen for this purpose than the starchy substances ( 304). This explains why a dog fed upon beef fat gradually becomes fat but not strong, and constantly gives off an exhalation which has a repulsive odour of the volatile fatty acids ( 309). In addition to this, it is obvious that the differences in the combustion of the hydrates of carbon and the fats will react upon the excretions generally. 1153. Many ashy constituents of the food immediately enter the urine. Others undergo a previous oxidation at the expense of the oxygen of the blood ( 973). Since the muscles contain more potash and magnesia, and the bile and blood more soda and lime ( 354 and 925), it is pro- bable that there is some approach to a separation in this respect. Under ordinary circumstances, human faeces contain more magnesia than lime. 1154. The metamorphosis of the tissues is even more obscure than that of the food. As yet chemistry is unable to investigate the minute differences of the several constituents ; or even the essential characters of the various compounds of albumen, fibrine, and cartilage. Many of the expressions made use of in this subject are base upon insufficient evidence. For instance, when we speak of a fibrine of the muscles, this name is founded upon an assumed resemblance to the fibrine of the blood. But microscopic observation contradicts this parallel (Tab. IV. Fig. 64, and Tab. II. Fig. 25, a). The albuminous substances, which are probably of at least two kinds, are easily converted into each other, as well as into other organic compounds. These slow and complicated changes, which, though successfully accomplished by Nature, remain unnoticed by the comparatively coarse tests of the chemist, lead to important differences both in external appearance, and in physical pro- perties. 1155. 1 \e few facts hitherto established relate to the gradual changes undergone by the tissues in the course of their development. When young structures rich in albumen subsequently become horny, they lose in carbon, ai d hence gain in nitrogen. The frequent deposit of fat and pigment in Lnd between the horny tissues is probably connected with this fact ( 1(30). Mucus is produced as an alkaline or saline solution of compounds which belong to the group of horny substances consti- tuted by keratxV and its congeners ( 880, et seq.). Permanent carti- lage furnishes crondriiie; but the cartilage of bone, gelatine ( 317). 344 COMPOSITION OF -BONE. [CHAP. XIII. The bones of children, or of morbidly ossified new structures, contain a smaller proportion of calcareous salts. 1156. The true dental substance (Tab. III. Fig. 49, a d b) contains 71 to 79 per cent of ash ; while the enamel (Fig. 49, b c) has from 94 to 96-4 per cent. The teeth as a whole contain 78-8 per cent. In man and the mammalia, healthy bone contains one third of organic cartilagin- ous skeleton, and two thirds of ash. Carbonate and phosphate of lime form the bulk of the fixed compounds : of these the carbonates are the smallest fraction. Mollities ossium, scrofula, and sometimes caries, are associated with a diminution of the earthy salts, and an increase of the organic compounds. The abnormal flexibility met with in the first two forms of disease is thus explained. Here the acid urine sometimes contains larger quantities of calcareous salts. 1157. None of the natural soft tissues contain such large quantities of ash as those which we have just been considering. The cartilages, which have the next greatest share, only yield ^th to j- 5 th of the quantity pos- sessed by an equal weight of bone. The other tissues have still less. The fixed constituents are generally greater in old age than in the earlier years of life. CHAPTER XIV. ANIMAL HEAT. 1158. A mammal or bird which is surrounded by an atmosphere at 50 to 68 has a temperature of about 99*5 in its mouth, rectum, or other internal parts. This difference of temperature is called animal heat j and the creatures which exhibit it are called warm-blooded ani- mals. And since the temperature of most amphibia, fishes, and inverte- brata differs but little from that of their surrounding medium, they are called cold-blooded animals. 1159. But these two classes of the animal world cannot be sharply denned. Many reptiles (such as the oviparous snakes), certain fishes (such as the tunny), and many gregarious insects, exhibit an elevation of tem- perature which, though never reaching that of the warm-blooded creatures, is still very considerable. The heat found in the interior of bee-hives chiefly depends on this cause. The hybernating animals, in their winter sleep ( 1095), behave like cold-blooded creatures ; while, in the waking state, they correspond with other mammalia. 1160. A more careful examination into the circumstances of what are called cold-blooded animals shows that they are by no means devoid of all independent heat. But the quantities which they evolve are so small as to be mostly either compensated, or overcome, by any active cause of cooling. Hence in the most favorable instances their temperature is but little raised ; while in the least favorable, it is positively lowered. Since their heat is more altered by collateral causes, and is hence more variable to the eye, Bonders and Bergmann propose to call them poekilo- thermal, or of variable temperature j and the warm-blooded animals homo-' thermal, or of uniform temperature. 1161. The animal heat has been determined in two ways: by the thermometer, and by the thermo-electric apparatus. The latter verifies fractions of degrees which surpass the powers of the most delicate thermometer. 1162. The annexed wood-cut (Fig. 185) represents a form of thermo- meter suitable to such observations. The bulb, a, and lower part of the tube, b, project out of the case, c, in which they are inserted at d. Thus a and b may be introduced into the external auditory meatus, the rectum, or the vagina of small animals. The bulb being bare, the quicksilver rises more quickly to the requisite height. And the rapidity with which 346 MODES OP DETEEMINING ANIMAL HEAT. [CHAP. XIV. FIG. 185. it rises is increased by the fact, that the diameter of the tube, b, forms but a small fraction of that of the bulb, a. The division need only be 115; since the temperature of animals is never higher. 1163. Two or more rods of a different metal, soldered together, and forming part of a closed circuit, furnish an electric current which may be recognized by the galvano- meter ( 220), so long as the temperature of the one point of union differs from that of another. Bismuth and anti- mony are best adapted for such observations. Copper and iron produce less decided effects upon the magnetic needle. The deviation of the latter increases with the difference in temperature of the points of junction. Hence this, with the warmth of one junction, will inform us of the tempera- ture of the other. 1164. These brief allusions may explain the thermo- electric apparatus used in physiological researches. The apparatus made use of by Becquerel and Breschet is re- presented in Fig. 186. The two copper needles, a and 6, are connected at c with a galvanometric apparatus, called a thermo-multiplier. A steel needle, ef, is soldered to a copper one, d e : the former is united with the end of the steel wire, g, and the latter with that of the copper wire, a. A second piece, h i, also consists of a copper wire,* soldered to a steel one. Supposing everything else removed, and h i united with g and b, a d efg h i b, and the wire of the thermo-multiplier c, form a closed circuit. Hence when the point of junction, e, becomes warmer or colder than that belonging to h i, the magnetic needle at c will deviate to a certain amount. We now plunge d e/into a receiver filled with water, I, the tempera- ture of which is shown by the thermometer, m. This receiver is placed within a second one, n, the temperature of which is notified by the ther- mometer, o; and which can receive a stream of warm water from p, through s t u, and allow the surplus to flow off from w towards x. This arrangement maintains a constant temperature of the water in I, and hence of the junction, e. If the other soldered needle be passed through the muscles, k, of the human forearm, the magnetic needle at c will deviate by a certain arc of the graduated circle. Supposing that the amount of this deviation corresponds to 3 '06 of Fahrenheit's scale, and that the thermometer, m, stands at 95, the temperature of the muscle must be 98-06. 1165. In the human subject, the thermometer averages 98-6 to 99-1 under the tongue, 98-6 to 102-2 in the rectum, 100-2 to 100-9 in the vagina, and 97 to 101-5 in the urethra. While Breschet and Becquerel CHAP. XIV.] AMOUNT OP THE ANIMAL HEAT. 347 found only 94-6 for the subcutaneous areolar tissue, and from 98-1 to 98-6 for the biceps muscle of the upper arm. The external skin, which is cooled down by the atmosphere and other bodies in contact with it, gave almost everywhere still lower temperatures. Under ordinary cir- cumstances it is between 89-6 and 97-7. FIG. 186. 1166. Many of the deeper internal parts of mammalia (and probably of men also) are warmer than the structures just mentioned. For instance, Berger found 104-5 for the brain, 106*3 for the liver, and 106-5 for the lungs, of the sheep. Most observers have found a somewhat lower temperature in the venous than in the arterial blood. Breschet and Bec- querel state that the blood of the aorta and femoral artery is from 1 -4 to 2- warmer than that of the vena cava. But Hering obtained contrary results. The venous blood of the right ventricle of the calf with ectopia already mentioned (in 633), gave 102-9 ; while the arterial blood of the left ventricle was but 101-8. Berger has found a similar difference in the sheep. The amount of animal heat may be greatly altered by a number of collateral causes, which will shortly be again adverted to. But its varia- tions are generally within narrow limits. The differences often amount but to fractions of degrees ; and are rarely more than from 1-8 to 3-6. This fact renders it very difficult to investigate the more delicate effects of external circumstances. We are constantly referred to numerical averages, which are rendered unsafe by the numerous, though tolerably uniform, causes of excitement which exist. 1167. Differences of age and race, or of elevation and climate, lead to no constant differences of animal heat, such as exceed the ordinary variations just mentioned. An old man of 80 or 90 exhibits a 348 VARIATIONS IN ANIMAL HEAT. [CHAP. XIV. temperature of 98-6 to 99-5 j just like the child only born yesterday. According to J. Davy, the sublingual region gave on an average 99-1 for the English inhabitants of Ceylon, and from 98-6 to 101-5 for the numerous coloured races of this island. It has often been asserted that the dark skin of the negroes is adapted to the hot climate to which they originally belong. But although there are numerous physical reasons which militate against this view as well as the fact that brown and red races inhabit the colder zones still it is remarkable, that the highest temperature of 101-5 was exhibited by albinos of negro race. 1168. Breschet and Becquerel found that the heat of the same young man's biceps amounted to 98-32 at Martinach in Wallis, and 98-51 on the St. Bernard: the former place being 1562, and the latter 7195, feet above the level of the sea. The observations of Eydoux and Souleyet lead them to conclude, that the average animal heat at Cape Horn, at 32 of temperature, and 59 of south latitude, is only 1 -8 less than usual. 1169. This slight alteration in the temperature of internal parts is repeated under many ordinary circumstances. When a person is in a warm bath the heat of the skin is certainly increased, since it is less cooled by the outer air, and by the evaporation of water from the blood. Internal surfaces, like that of the urethra, exhibit something similar to this. While on the other hand, in Breschet and Becquerel's researches, the temperature of the biceps only rose -72, when that of the surround- ing water amounted to 120-2. And even when the arm is kept some time in freezing water, the heat of this muscle scarcely falls -36. 1170. From reasons which may easily be conceived, an animal into whose blood a large quantity of cold water has been injected, at first offers a somewhat lower temperature. According to Bergmann, a similar depression of temperature obtains in the sublingual region of persons who have been taking cold fluids. And conversely, the act of digestion, or rather the more energetic phenomena of combustion which it pro- duces ( 822), increase its amount. Thus Gierse found that the tempe- rature beneath his tongue was 98-78 before, and 99-5 after, an early dinner. The contraction of the muscles raises their animal heat. Breschet and Becquerel obtained an increase of 1 -8 in the biceps. Davy found that continuous movement increased the temperature of the skin, the mouth, and the urine. Hence the more active metamorphosis of the tissues which then occurs reacts as much as possible upon the rest of the organism. The combustion and levelling of temperature ( 197) thus in- duced in many parts ( 330), also contribute to this effect. 1171. The thermo-magnetic researches instituted upon frogs by Helm- holtz showed that the temperature of the contracting muscles is raised much more than that of the nerves which excite the contractions. On throwing the femoral muscles of a prepared frog into continuous tetanic convulsions by means of electrical irritation of the spinal chord, the heat CHAP. XIV.] VARIATIONS IN ANIMAL HEAT. 349 of the muscular substance rose from -25 to -32; while that of the nerves was either not increased at all, or at most but from -0036 to -0054. 1172. An animal dying of starvation may exhibit greater or less amounts, under different circumstances. But, according to Chossat, the averages experience a gradual increase. The only considerable exceptions to this rule occur in the periods of time just before death. 1173. Fricke and Gierse could not detect any constant elevation of temperature in the vagina (z Fig. 9. p. 34.) of pregnant females. Nor does menstruation cause any important deviations: the vagina then having the ordinary temperature ; or, at most, but -54 more. 1174. According to Breschet and Becquerel, when the skin of a rabbit is covered with an air-tight varnish, the temperature of the dying animal sinks in an extraordinary degree. The muscles formerly at 100-4 sink in one hour to a temperature of from 684 to 76-1. 1 1 75. During the hot stage of a febrile attack, the temperature of the skin is but little increased. But here, as in many other cases, we must distinguish between the objective heat shown by the apparatus ( 1162), and the subjective feeling of warmth. The former is the necessary result of the chemical actions of the organism, and of the collateral causes which operate in conjunction with it. The latter, on the other hand, merely expresses certain molecular changes of the nervous substance; which will occur under very different circumstances, and may be indicated in a great variety of ways. The peculiar phenomena seen in intermittent fevers evidently support this view. 1176. It might be expected that a man attacked by the violent shiver- ing fit which commences an intermittent fever, would exhibit a diminished heat of skin. But the thermometer shows that exactly the opposite is the case that the temperature is higher than usual : and, according to Gavar- ret, the difference may amount to 7 - 2. The subjective character of this sensation of cold explains why the shivering does not disappear imme- diately on covering up the patient. The heat which subsequently occurs is, for the same reason, not accompanied by an elevation of temperature, although the patient thinks himself almost on fire. The feeling of the hand may also greatly deceive us. For although the sense of touch pos- sessed by our fingers seems to inform us that the skin is greatly heated, still a physical examination shows that the difference is much less than might be supposed. Finally, the sweat which concludes the whole will rather lower the temperature of the skin, by that evaporation of fluid ( 184) which it implies. 1177. We have already seen ( 209) that the animal heat is chiefly based upon that process of combustion which is continually going on in the organism. The limited elementary analysis (330) induced by the oxygen inspired, together with the metamorphosis of certain quantities of carbon and hydrogen contained in the food, and in the several parts of the 350 PRODUCTION OF HEAT IN SMALL ANIMALS. [CHAP. XIV. body, must set free a definite number of units of heat ( 208). These will first warm those structures of the body in which this process either occurs to a very small extent, or not at all ( 202). They will also replace the losses Of temperature caused by the surrounding atmosphere, by the ground beneath the feet, or by the introduction of cold food. Hence the metamorphosis of its substance constitutes the body a kind of oven, which heats both itself and the neighbouring substances; and, in the warm- blooded animals, maintains itself at as constant a temperature as possible. 1178. This uniformity of the animal heat is not so mysterious a phe- nomenon as it might seem at first sight. We may often see that the chief causes of the change of substance, the necessary quantities of food, the consumption of oxygen, the formation of carbonic acid, and the general process of combustion, all vary greatly with those collateral circum- stances which are connected with the development of heat. 1179. Other circumstances being equal, a small body cools more quickly than a large one, because it exposes a greater amount of sur- face* in proportion to its bulk to that surrounding medium by which its surplus heat is carried off. Hence in order that its temperature should remain constant, it ought to possess a more active source of heat. This rule holds good for the smaller warm-blooded animals, as far as circum- stances will permit. 1180. The quantities of oxygen consumed (and to some extent those of carbonic acid given off') afford a tolerable, though by no means perfect, measure of the amount of heat set free by that process of combustion which takes place in the body. And since a greater expenditure demands a greater income, a small animal will, as a rule, eat proportionally more than a large one; will take up more oxygen, and will exhale more car- bonic acid. If we collect a few of the averages relating to this fact, we shall find that, on the whole, they confirm this proposition. For instance, Animal. Average volume of the body in cubic inches. Average number of grains per hour, for each pound of bodily weight. Food. Oxygen consumed. Carbonic Acid exhaled. Man .... Old Dogs . Very fat Puppies . Old Rabbits Young Rabbits Mice Pigeons . . . Crossbills . 3112-43 353-35 46-08 205-66 12-27 604 19-35 1-65 15-82 114-8 4-34 8-33 7-35 5-95 8-82 76-09 9-17 76-79 5-04 8-82 77 7-21 10-08 86-31 10-78 84-21 See 869 and foot note. CHAP. XIV.] CAUSES OF ANIMAL HEAT. 351 Hence the mouse consumes a proportion of food about eight times larger than that of man. And as it takes much less fluid than we do, the relative quantity of solid matter is still greater. This constitutes one of the chief causes why it takes up from 17 to 18 times as much oxygen, and also gives off larger quantities of carbonic acid. The numbers adduced for the crossbills also teach us, that smaller animals develope more heat. And the other animals mentioned confirm this conclusion, though with less strikingly contrasted figures. On examining more carefully into the several numbers, we first of all find, that the quantities of oxygen consumed do not increase in inverse proportion to the volume of the body. Thus, although man is about ten times as bulky as a dog of average size, his relative consumption of oxygen is only one-half of that animal's. We further observe, that birds by no means possess the advantage over the mammalia which is usually ascribed to them in this respect. For instance, on examining into the temperature of the cloaca of birds i. e. of the outlet common to the urinary, sexual, and alimentary canal we generally find it about 105'8 to 109-4, or something more than in mammalia. Very small birds, which fly here and there incessantly, cer- tainly consume a comparatively large quantity of oxygen. But the mouse, which is still smaller, exhibits a more active combustion than the larger singing birds. And in the middling sized domestic birds, such as fowls and pigeons, the heat of the body scarce exceeds that of a young rabbit. 1181. It is obvious that a variety of circumstances will exert an important influence in this respect. Such are the peculiarities of the animal's tissues, the nature of its food, the mechanism of its respiration, the amount of watery vapour it gives off, and the process of cooling to which it is exposed. It is to these causes that we must attribute the fact, that very fat puppies give off proportionally less carbonic acid than older animals. 1182. A man breathing in the cold takes into his lungs a denser air ( 195). Hence, even though the volumes remain the same, he consumes a greater weight of oxygen. He can therefore effect the combustion of a larger quantity of food, and can heat his body to a greater degree. Thus the more active cooling process which accompanies the cold of winter finds its compensation in the heightened function of respiration. This also explains why remaining in the cold excites hunger, and why the appetite is so remarkably diminished by exposure to great heat. 1183. But however strongly the facts just adduced support the view, that the animal heat results from that process of combustion which occurs in the body, still we are quite unable to follow out this theory in a satisfactory manner, or to define the influence which is exerted upon it by other collateral causes. It has frequently been attempted to calculate 352 CAUSES OF VARIATIONS IN THE ANIMAL HEAT. [CHAP. XIV. the combustive heat of the animal body by the method previously men- tioned ( 209), comparing the amounts of heat thus set free with those lost by the causes of cooling. In this way round numbers have been obtained, which seem to explain the independent temperature of the animal body, and even the slowness with which cooling takes place in the interior of the dead subject. But nothing can be proved by these calcu- lations, because they are based upon a series of assumptions many of which are at least improbable, if not incorrect. We do not know what organic substances undergo combustion in each particular instance ; and therefore we cannot decide how many units of temperature are set free ( 208 et seq.). Besides, all the estimates, both for heating and cooling, are only averages : which certainly deserve an inspection, but cannot be regarded as proved. 1184. And even apart from this, we meet with many other phenomena, which the theory of combustion cannot at present explain. Many of the effects produced by the nervous system, such as nausea and fainting, lead to local changes in the temperature of particular parts of the skin, without any corresponding depression of circulation or respiration. These facts can only be explained upon other hypotheses, with which we shall be made acquainted in the study of the nervous system. Although a fasting animal gives off less carbonic acid ( 1139), still its heat generally rises rather than falls. Hence either the capacity of the animal tissues for heat (200) is altered; or, what is much more probable, substances are consumed which can furnish a greater combustive heat. An increased oxidation of hydrogen ( 208) might partly conduce to this effect. Future researches must decide whether something similar does not obtain in the hybernating animals ; who, apparently, do not always cool in proportion to the diminished quantity of carbonic acid they give off. At any rate, it cannot be indifferent to the combustive heat, whether it is fat, a hydrate of carbon, or an albuminous substance which is consumed; or in what way the metamorphosis is effected. 1185. From the ultimate destinies of the inspired oxygen ( 1123), it is probable that one part of the animal heat is produced in the blood ; and another in the tissues, or in the nutritional fluid. While conversely, those tissues which are not traversed by any blood-vessels such as the epidermis, the nails, and the hair must receive their temperature chiefly by communication from these ( 1177). And since they are bad con- ductors of heat, they will obstinately retain whatever they receive ( 203). 1186. At present we do not know why one group of vertebrata has a higher temperature than another. In both divisions, the structure of the organs is on the whole similar. Hence the more rapid metamorphosis of substance in the warm-blooded beings can only affect the minute details of the process. The muscles and nerves of the cold-blooded animals generally preserve their vital properties with greater tenacity. They are less injured by the influence of low degrees of temperature than the same CHAP. XIV.] CAUSES OF THE DIFFERENCES IN ANIMAL HEAT. 353 structures in higher vertebrata. Hence they are more constant, and less susceptible, agents of the most essential functions of life. The quicker metamorphosis of matter in the warm-blooded animals at once furnishes both more delicate substances, and those higher degrees of heat which are necessary to their action. The latter also favour the transit of fluids through the fine tubes of the different organs (110); as well as the maintenance of the normal state of admixture in the blood and other juices ( 1 104), and many chemical changes which greatly influence absorp- tion and secretion. In one word, we have a more laborious apparatus, the greater activity of which probably allows of more delicate and perfect operations, especially in the nervous system. A A CHAPTER XV. LOCOMOTION. 1 187. MANY of the movements seen in the animal tissues by the naked eye, or with the microscope, are also found in other departments of nature. Such are the diffusive currents formerly ( 129) noticed, and the molecular movement observed by Brown. But there are others which are absent from minerals and the dead organic being; so that they form the expression of certain vital functions. Still movements which are either similar, or at any rate related to these, occur in both the kingdoms of organized nature i.e., in plants as well as animals. Thus the movements of the cilia and spermatic elements, and altera- tions in the form of simple contractile substances, occur in vegetables, although less frequently. Finally, those movements which necessarily or possibly obey the rule of the nervous system, belong exclusively to man and animals. They form one of the characteristics of these, the highest members of the creation. 1188. The Brownian Molecular Movement. On looking at pigment- molecules (Tab. II. Fig. 28) mixed with water under a magnifying power of 350 to 550 diameters, almost every one of them is seen to be in con- tinual movement. Many (a, b, g, m, n, Fig. 187) vibrate here and there as indicated by the dotted lines. Others FlG * 18 ^' (as c, k, i, o, p,) describe curves of dif- c_? Qfjli .-( V; ''/*" n^jf'j ferent kinds, by means of which they ' * *' d ' ^ '* return pretty nearly to their original ("\x\""^ ) } place. Others (e } f, I, q,) proceed for C.' f- } O ( ( / \"T\ ( n -'"/ a certain distance in curves or circles. *^ *% .-.f ^ Many only turn upon their axis in r. .., .. , 'Y"&"\ .-J the way shown by the different forms "U fj '^ (*--JJ ' y ft represented in Fig. 187 j at the same 171 ' Jl " r time moving through more (a, k, I, q,) or less (g, r,) additional space. The same globule of pigment may exhibit different movements at different times. If the fluid contains a large number of them, the whole reminds one of the moving atoms of dust which are seen in a ray of the sun, or of the restlessness of the numerous minute infusoria Bacteria, or Vibriones which occupy a putrefying liquid. 1189. Similar phenomena are exhibited by the smallest fragments of CHAP. XV.] CONDITIONS OF THE MOLECULAR MOVEMENT. 355 most bodies, whether vegetable or mineral, when minutely divided and placed in a not too tenacious fluid. The liquid contents of cells some- times offer the necessary collateral conditions. Where this is not the case, the reception of water by endosmosis ( 129) may constitute the exciting cause. Hence we often find that the minute substances con- tained in some vegetable or animal tissues exhibit an active and pro- tracted molecular movement. 1190. The crystals of the otolithes of the higher vertebrata (Tab. I. Fig. IV.) are a good example of the influence which bulk exerts upon these changes of place. Those of larger size lie quiet, while the smaller are in continual vibration. When a number of different substances are mixed with water, we generally find that it is the fatty and resinous matters which exhibit the most energetic movements. Hence molecules of pigment, or finely powdered asafcetida, are particularly adapted to these observations. 1191. We have seen that this molecular movement can only be recog- nised under high magnifying powers. Its velocity is therefore very small ( 653). In point of fact, approximative estimates of time and space show that a molecule of pigment proceeds with a velocity of only r^th of an inch per second. 1192. Since the molecular movement is continued incessantly in a fluid which is to all appearance at rest, and since two neighbouring par- ticles often vibrate in the most different manner, it follows that the phe- nomenon is not due to any strong and partial current. It has been attempted to explain the movement by the slow currents which accom- pany evaporation. But although, in some instances, these may perhaps increase it, still we may easily show that they do not constitute its prin- cipal cause. If a mixture of water and black pigment be introduced into a thin and small glass tube, and if this be hermetically sealed, evaporation will soon be limited to that which is caused by a change of tempera- ture ( 191). But, in spite of this, the molecular movement continues for many hours with great activity. And it is not suspended by oil or other fluids which do not evaporate. 1193. We may rather conjecture two other causes. We are apt to believe that no mechanical agitation disturbs the observation. But a more careful examination leads to a very different conviction. The walls of every building are in almost continual vibration; since their solid contiguous parts easily propagate the influence of the various movements caused by driving, walking, hammering, or the undulations of rapid currents of liquid. The beat of the heart, and the act of respiration, which visibly displace the observer's body ( 641), render perfect quiescence impos- sible. And the small molecules which are shaken by these impulses will subsequently vibrate j during a period which is greater, the less force they can lose by collateral obstructions, or by way of communication ( 66). A A 2 356 CAUSES OF THE MOLECULAR MOVEMENT. [CHAP. XV. A second cause lies in the temperature. When the closed tube just mentioned ( 1192) lies on the object-plate of the microscope (e, Fig. 34, p. 59), certain layers of the fluid are nearer than others to the good conducting brass. The surrounding atmosphere which continually flows hither and thither, and the radiant heat of the observer's body (210), are additional causes why many of the liquid particles become warmed sooner than others. These therefore seek to rise, while the colder masses descend. A series of slow alternating currents are thus produced, which are capable of impelling the minute molecules in a variety of paths. 1194. There can be no doubt that direct mechanical agitation, and indirect thermic movements, exert an important influence on the phe- nomena we are now considering. And although the results differ with the nature of the molecules and the fluid, still the mutual physical relations may assist to determine the amount of the original displace- ment, and the duration of the subsequent vibrations. The question whether these are the sole exciting causes, or whether the molecular movement is not based upon other attractive forces, cannot at present be decided. The forces to which it is due are at any rate easily overcome by the ordinary phenomena of adhesion ( 112). Molecules of pigment which adhere to the glass wall of the tube by only a part of their surface exhibit no vibrations. But particles of camphor thrown upon water move very energetically until they are wholly or chiefly destroyed. While finely powdered salt which is mechanically diffused in a saturated solution of the same substance exhibits prolonged vibrations under the microscope. 1195. The Ciliary Movement. When a small piece of mucous mem- brane from the palate of a newly killed frog is so folded up that its margin corresponds to its former free surface, a magnifying power of 150 to 255 diameters exhibits an appearance resembling that repre- sented, on a larger scale, in Fig. 188. The ciliated epithelia are cylinders, (a, Fig. 188; and Tab. II. Fig. 36) which stand upright or obliquely toge- ther, so as to make a kind of palisade. Their cilia (c) move so rapidly that it is impossible to recognise them feiugly ; we can only see a number of them for an instant of time. They form a whirling border which occupies the margin of the fold. Hence molecules (b) which are mecha- CHAP. XV.] DISTRIBUTION OF CILIATED EPITHELIUM IN MAN. 357 nically diffused in the neighbouring fluid are hurried past with great apparent velocity in one or more directions. 1196. The ciliary movement occurs in many parts of other animals. Confining our attention for the present to the mammalia, it exists on the surface of the cerebral cavities, and where such exist, in those of the spinal cord and olfactory lobe, especially of the embryo ; in the lachrymal canals (k I, Fig. 150, p. 273), the lachrymal sac (m, Fig. 150), and the lachrymal duct (n, Fig. 150) j in the Eustachian tubes (/, Fig. 128, p. 232), and in the mucous membrane of the nose (a, Fig. 128), with the exception of its lower part ; in the supplementary cavities of the nose, such as the cavities of the ethmoid bone, the frontal sinus, and the antrum Highmorianum of the superior maxillary bone ; in the lowest part of the larynx (i, Fig. 128), the trachea (&), and the bronchial ramifications which penetrate the lungs; in the uterus and the Fallopian tubes of the adult female ; finally, on the surface of the early ovum ; and, possibly, in the capsules which surround the Malpighian tufts of the kidney ( 396). In reptiles, there are many other structures, such as the internal sur- face of the tympanum, the pericardium, the peritoneum, the mucous membrane of the mouth and oesophagus, the deciduous or permanent gills ( 725), and certain portions of the cloaca, which also exhibit the ciliary movement. The amphibia, and many invertebrata such as the snails and muscles possess numerous ciliated membranes. But in the higher insects and Arachnida, they are altogether absent. The gills of the lowest ichthyoid reptiles, the Sirens, are clothed with cilia ; while, with the single excep- tion of the Ampliioxus, those of fishes are smooth. Hence the presence or absence of the ciliary movement is not decided by the general arrange- ment of the organs, but rather by their special circumstances. 1197. Many of the infusory animalcules which form the lowest extreme of the animal kingdom possess hairy or prickly processes, which are more or less constantly in whirling motion, and are frequently governed by the volition of the animal itself. And since the ciliary movement of the higher animals is not under the control of the nervous system, this fact has been supposed to constitute an essential distinction between the two classes of structures. But the study of the simple contractile substances teaches us that this difference is less than it appears to be at first sight. 1198. Since the minute hairs of a severed fragment of ciliated mem- brane vibrate energetically ( 1195), it is obvious that the phenomenon is alike independent of the circulation of the blood, and of the influ- ence of the brain, spinal cord, or ganglia. If we scrape the surface of a ciliated membrane such as the mucous membrane of the frog's mouth the mass so obtained contains isolated ciliated cells (a and b, Fig. 189); together with aggregations of these. (Fig. 190.) The cilia of both often 358 ACTION OF THE CILIA. [CHAP. xv. continue to vibrate for some time. Hence the maintenance of their mode of attachment is the sole condition of their activity. FIG. 189. FIG. 190. 1199. The cilia are usually placed upon cylindrical epithelia (Tab. II. Fig. 36). But sometimes they occupy spherical cells, as for instance, in the choroid plexus of the brain, or on the surface of the very young ovum or embryo. They may even be planted directly on simple mem- branes. 1200. The cilia of different animals do not always exhibit the same kind of movement. They are usually either flexed and extended like a finger (Fig. 191) ; or they describe a circle (a b, Fig. 192) with their point, FIG. 191. FIG. 192. mmof (d) and a cone (a b c) with their whole length. Contiguous cilia generally exhibit the same movement. The character of the total movement, and the currents which are produced in the neighbouring fluid, depend upon the situation, nature, and succession of the motion, of the several cilia. 1201. In a level ciliated border (such as that at c, Fig. 188,) the main current generally flows in a straight line (like the arrow opposite b). But many mucous surfaces possess small hillocks or papillae, separated by valleys or furrows. Such an elevation is shown (strongly magnified) by Fig. 193. Where this is the case, we may often remark a correspond- FIG. 193. FIG. 194. ing curve of the stream (a b c d efg, Fig. 193). The conjoined action of neighbouring cilia which occupy different heights frequently leads to CHAP. XV.] CILIARY CURRENT. 359 movements such as are indicated by the arrows in Fig. 194. In this way many of the small bodies in the surrounding fluid are first impelled in curved paths towards the ciliated margin, and are then driven off in the contrary direction. 1202. Where the ciliated epithelia (a, Fig. 188) are fixed upon an immovable foundation, it is obvious that the cilia (c) can only propel the neighbouring fluid, and any small bodies (b) which may be contained in it. But if, on the contrary, a small piece of ciliated membrane be cut loose, it will often be itself displaced in the opposite direction to the main current, as shown by the lower arrows in Fig. 188. This phenomenon is a counterpart to the well-known fact of a boat going forwards when its oars propel the water backwards. Indeed, such a result is actually repeated when any very light body bears a large number of actively whirling cilia. In this way the minute ovum or embryo of many animals, as well as the mature individuals of some of the lower creatures, is enabled either to rotate continually, or to move in directions which correspond to the action of its cilia. 1203. In many ciliated membranes the main current takes a definite direction. For instance, in the mucous membrane of the rabbit's nose, molecules of ink, powdered sepia, or black pigment, are carried towards the nostrils ; while in the larynx or trachea they are propelled towards the lungs. But in the tracheal mucous membrane of a puppy, Sharpey found the contrary course. The cilia of the Fallopian tube conduct small particles towards the uterus ; or from within outwards. In the gills of Molluscs we sometimes see cilia, which have long maintained a motion towards the right, suddenly reverse their current towards the left. Sometimes this alternate play is repeated several times. In the gills of the Ascidia, J. Mueller remarked that the action of the cilia ceased, and recommenced after a certain interval of rest. 1204. The velocity of the ciliary current can only be determined approximatively, since the results are materially altered by the density of the contiguous fluid, by the weight of the small substances which it contains, by the nature of their movement, and, finally, by the activity of the cilia themselves. In the mucous membrane of the frog's mouth, blood-corpuscles and molecules of pigment are propelled with an average velocity of about j^- th of an inch per second. Their movement is therefore slower than that of the blood-corpuscles in the capillaries of the same animal's foot ( 712) ; but quicker than that of particles in a state of molecular movement ( 1191). Since these obser- vations require a high magnifying power, the velocity of the movement appears to be greater than it really is ( 653). Krause found that each cilium makes from 190 to 320 vibrations per minute. My own obser- vations would give about 77 to 152 for the gills of muscles, and for the mucous membrane and lungs of the frog. Perty estimates 240 360 INFLUENCE OF EXTERNAL CIRCUMSTANCES. [CHAP. XV. to 300 for the subsiding ciliary movement on the outer surface of the Alcyonella, a fresh- water polyp. 1205. Rarefaction of the air does not stop the ciliary movement. But higher temperatures soon suppress it; and too great a degree of cold often has the same effect. Still a ciliated membrane may be plunged for an instant into water of 178 without causing the vibrations of the cilia to cease. The ciliated membranes of warm-blooded animals are more sensitive to low degrees of temperature than those of the cold- blooded creatures. 1206. The shock of a Leyden battery does not disturb the ciliary movement on the gills of the Muscle. If a large piece of such a mem- brane be laid between the two poles of an electro-magnetic machine ( 248), many hundred shocks may be transmitted through it without producing any change in the current of most of its parts. It is only the places attacked by the electrolytic action which sets free acids or alka- lies that suffer from the caustics thus produced. If a few ciliated epithelia be placed between the opposite poles of such a machine, at a distance of about ^th of an inch from each other, after a short time their cilia cease to act. 1207. Water deprived of its air, or charged with large quantities of carbonic acid, does not destroy the ciliary movement. But a fluid which has absorbed much sulphuretted hydrogen exerts an injurious influence. j . 1208. Many soluble substances only injure the ciliary movement when they have a certain degree of density. Ordinary caustic ammonia anni- hilates it even when diluted 10,000 times; nitrate of silver, 1000 times; sulphuric eether, 100 times; and common salt, only 10 times. Hydro- cyanic acid which is quite free from alcohol and sulphuric acid, and solutions of acetate of morphine or strychnin, do not check the move- ment. The blood preserves it longer than pure water. The bile, which is so liable to decomposition ( 467), is always more injurious than saliva or undecomposed urine. 1209. In the frog, the ciliary current is often checked in a short time by the influence of cold water : this is especially FIG. 195. the cage wittl t he e pithelia ( 1198) which have been removed by scraping. Under such circumstances the form of the ciliated cells is sometimes altered as a conse- quence of the absorption of water ; so that they become surrounded with a transparent ring (a, Fig. 195). But in spite of this, their cilia often continue to vibrate. 1210. We have already ( 1018) seen that young cells destined to the replacement of those above them are now and then met with beneath the ciliated cylinders. Some mucous membranes shed their ciliated epithelia at definite intervals of time. In treating of reproduction we CHAP. XV.] INFLUENCE OF EXTERNAL CIRCUMSTANCES. 361 FIG. 196. c shall find, that the mucous membrane of the uterus loses its ciliary cover- ing at each menstruation and pregnancy, and subsequently reproduces it. Many morbid conditions lead to a similar peeling of this covering in other parts. When a person gets a vio- lent cold in the head, his nasal mucus at first contains a number of ciliated epithelia. Their form is frequently changed, as represented by Fig. 196. The secretion of mucus is disturbed; and the saline fluid which streams forth in large quantity operates like the water that enters by endosmose in the observation previously men- tioned ( 1209). But even here the cilia continue to vibrate for a certain time. 1211. The ciliary motion is often retained with great tenacity in the dead subject. In the mucous membrane of the rabbit's nose and trachea it sometimes continues 5 to 6 days after the death of the animal; in that of the mouth of the frog, 8 to 9 days; and in the oesophagus of the tortoise, 15 days. In the meantime decomposition advances so far that the whole acquires a putrefactive odour, and a somewhat mucous and diffluent consistence ; and contains a large number of infusory animalcules. If a human trachea be excised some hours after death, and laid in serum ( 1001) of not too low a temperature, the ciliary movement may generally be recognized for one or two days afterwards. And in favourable cases it may be seen that the movement of the cilia only ceases in consequence of their being either shed, or dissolved by putrefaction. 1212. Although the ciliary movement chiefly occurs in the animal kingdom, it may also be found in plants, such as the lower crypto- gamia. The seed-granules or spores of the fresh-water A Igce or Conferva;, and of the sea-wracks or Fuel, frequently whirl round actively in the water on emerging from their parent plant. After this movement has lasted some time, they sink to the bottom, to undergo their germinating process. 1213. The cilia which cause such movements may be arranged in three ways. Many tissues have but very few; e.g., two. or four. These are sometimes so delicate that they can only be recognized after the use of alcohol or tincture of iodine. This, for instance, is the case with the small Alga (Hcematococcus pluvialis) which is represented in Fig. 197; and which sometimes occurs in such quantities as to give a red colour 362 CILIARY MOVEMENT IN VEGETABLES. [CHAP. xv. to large portions of water. The sketch a was taken after the operation of alcohol ; and bed after that of tincture of iodine. FIG. 197. FIG. 1 98. FIG. 199. The second arrangement is exemplified by Fig. 198; which represents the entire spore of Ectosperma (seu Vancheria) clavata, surrounded by a continuous covering of cilia. They are so delicate that they can only be verified after their movements have been checked by the action of opium. Finally, according to Decaisne and Thuret, the cilia are sometimes sustained, not by single spores, but by the common integument of many spores or the episporium. An instance of this kind (Pelvetia caniculata) is represented, after these observers, in Fig. 199. 1214. We have already ( 1201) seen that the action of the cilia is able to rotate and propel light substances of moderate size. But it is equally capable of propelling mucus and other fluids. Hence where the gills or integu- ments of an aquatic animal possess a ciliated epithelium, new quantities of the water which contains its respiratory air ( 725) will be constantly brought into contact with its sur- face. In this way the cilia are made to assist the respiratory function. But none of these phenomena are sufficient to account for the wide distribution of the ciliated epithelia, or to form the objects to which they are chiefly subser- vient. It is probable that the disturbance produced by their movement leads to certain molecular operations at present unknown. And to these we may perhaps refer the peculiarities of their distribution. 1215. Movements of the spermatic elements. The mature semen of animals generally contains certain bodies, which are capable of numer- ous movements. These are either spontaneous, or follow the appli- cation of water. Formerly such bodies were regarded as animals related to the Infusoria or Entozoa; and were hence named seminal animal- cules or Spermatozoa. The growing conviction of modern times that they are not independent beings, but belong to the tissues, has CHAP. XV.] MOVEABLE SEMINAL CORPUSCLES. 363 led to their receiving more suitable names : such as spermatozoids, seminal filaments, seminal cilia, or moving seminal corpuscles. 1216. In the vertebrata, we meet with three chief varieties; which are represented in Fig. 200, magnified 255 diameters. The form a is found in many osseous fishes b, in some of the carti- laginous fishes, and in birds ; and c in the mammalia (Tab. V. Fig. 78). The anterior and thicker part is called the head or body; and the thinner posterior part the tail, or caudal filament. 1217. The elements of the mature semen exhibit some movement, even in the undiluted seminal fluid. But an application of water, blood-serum, or any other solution which is properly diluted, gradually increases this motion. In immature semen such an admixture must often be made, before any vibrations can be seen. And in many varieties of semen, such as that of the river craw- fish, no trace of movement has ever been found at any period of their development. 1218. Where the body and tail are sharply defined (a and c, Fig. 200), we may see that it is only the latter which induces the movement ; and that it appears to propel the former. Where the whole has a filamentous shape (b, Fig. 200), this statement only holds good when the anterior part is considerably thicker. The vibrations seem almost confined to the long and thin segments. 1219. The caudal threads of the mammalian semen (c, Fig. 200), may move like a pendulum to and fro ; may wave up and down ; may take a simple or serpentine curve ; or may combine these elementary movements. At the same time, the whole structure vacillates in various directions. It is either driven forwards in a longitudinal path, or is contained for a time within a given space, which it traverses with various turnings and wind- ings. To this are frequently added rotations of the particle around its long axis. The forms represented by b (Fig. 200) often press forwards like a corkscrew does into a cork. In all cases the head precedes the tail. The average velocity amounts to about ~th of an inch per second ; it is therefore considerably less than that of the ciliary movement ( 1204). 1220. Cold appears to injure the seminal movements of the warm- blooded, more than those of the cold-blooded, animals. A great heat annihilates them altogether. But repeated shocks of the electro-magnetic machine ( 248) do not disturb them. Acids and alkalies, alcohol, ether, and many salts, soon destroy them. Many varieties of mucus and urine do not exert this influence. Solutions of narcotic poisons which possess no chemical influence seem not to affect the activity of the seminal elements. 1221. The action of these structures lasts about as long as the ciliary 364 INFLUENCE OF EXTERNAL CIRCUMSTANCES. [CHAP. XV. movement ( 1211). The human seminal particles have been seen to vibrate 84 hours after death, and those of the frog from 5 to 6 days after. The semen remaining in the testicle is much more favourably situated in this respect than that kept in any other fluid. The semen introduced into the female generative organs of some insects retains its movements here for many months. 1222. Just as a ciliary movement is sometimes met with in the vege- table kingdom ( 1212), so the structures now under consideration repeat this distribution. What have been called seminal animalcules in the phanerogamous plants, are nothing but corpuscles of the fovilla of the pollen, which exhibit the Brownian molecular movement ( 1188). But in the antheridia (or the organs resembling these) of the ferns, mosses, and Algce, we find elements which are really seminal. For instance, if we examine a fine section of the mature antheridia of a large leaved moss under a magnifying power of 255 diameters, we see that each cell (a, Fig. 201) shelters an involuted seminal thread (b). Moistening it with water gives rise to a tremulous movement. When it emerges from the cell, the head (b) goes first. Many of these moving structures as, for instance, those of the sea-weeds, and some large-leaved mosses possess two threads, as shown at Fig. 202. FIG. 201. FIG. 202. I 1223. If we confine our attention to outward form and capacity of movement, we shall find that filaments like those of the semen are not limited to this fluid. The skin of many polyps and medusse, the contact with which excites such pain as to earn them the title of Acalephce, or sea-nettles, possesses similar structures : which are enclosed in capsules, and, when set free, exhibit serpentine curves of their tails. 1224. Simple Contractile Substances. The body of many Infusoria, Polyps, and JEntozoa, and of the young embryos of most (if not all) of the higher animals, contains a simple gelatinous substance, which possesses a certain degree of contractility, and which, in these lower animals, is generally named sarcode. In the infusoria and polyps, the action of this substance is frequently accompanied by another phenomenon: viz. the production of cavities filled with fluid (a, Fig. 203, from Loxopliyllum meleagris, DuJ). When the mass contracts, these disappear from their previous situations, and reappear at other points. It may be conjee- CHAP. XV.] SAECODE. 365 FIG. 203. tured that a portion of the fluid which soaks the mass is pressed out from it at certain times, and taken up again by it at others. In the simple contractile substance of the vertebrate embryo this phenomenon has not hitherto been seen. 1225. Siebold and Koelliker have observed a peculiar periodical movement in the cells of the ova of Planarice, at a certain stage of their de- velopment. Certain constrictions gradually pro- ceed from one end of the cell towards the other, as represented by abed, Fig. 204. This alter- nate play of contraction and relaxation may be constantly repeated for many hours. Many other simple contractile substances, such as the caudal vesicle of the embryo of some snails, also exhibit periodical contractions under the microscope. Finally, the heart of the young vertebrate embryo beats most energetically, when as yet no muscular fibres can be recognized in its substance, but only a gelatinous basis with granules, nuclei, and cells. FIG. 204. 1226. In many of the lower invertebrata, separated fragments of the simple contractile substance are also capable of active contraction. Hence contractility does not depend upon the mode in which the constituents of a tissue are connected with each other. 1227. Many of the tissues now under consideration are easily destroyed by the influence of the galvanic current. Many of the small infusoria suddenly burst under the shock of the electro-magnetic machine ( 248) ; and the same effect may be produced upon the young larva of frogs. It may be conjectured that the chief cause of this phenomenon lies in the accompanying electrolytic effects. 1228. We are justified in supposing, that there are subordinate dif- ferences in the various tissues at present numbered in this class. It is probable that the sarcode of the infusory animalcule differs essentially from that of the young frog-larva, which is composed of cells. In the latter instance, the cell-contents, and the simple substance which unites the several vesicles to each other, demand a special consideration. 366 STRIPED AND UNSTRIPED MUSCULAR FIBRES. [CHAP. XV. 1229. Contraction of Muscular Fibres. Many of the lower animals, such as the Rotifera possess muscles which consist of a simple or faintly striated substance very similar to sarcode. But in most of the inver- tebrata, and all the vertebrata, the muscle or flesh is an aggregation of peculiar fibres, called the muscular fibres. In the higher animals we meet with two chief forms : the compound, transversely striped, or animal; and the simple, smooth, flat, unstriped, or organic muscular fibre. 1230. Every striped muscular fibre (Tab. IV. Fig. 54) consists of a number of longitudinal threads lying close to each other. Its surface exhibits a series of transverse strise, which are generally very distinct, and remain visible even in alcoholic preparations. Indeed, in this state they are sometimes even more sharply defined than in the fresh con- dition. But in rare instances they are absent from some parts of the still irritable muscular fibre. They are easily deranged by putrefaction, which finally destroys them. A simple transparent membrane (Tab. IV. Fig. 54, b) called the sarcolemma or myolemma, encloses the whole, and separates one muscular fibre from another. On it are easily recognized numerous nuclei, especially after the application of acetic acid. The several fibres are collected together into bundles. These are separated by a perimysium of areolar tissue, containing vessels, nerves, and fat, and forming the non-contractile part of the muscle. 1231. On examining a thin section of unstriped muscular substance we find that a more or less decided striation indicates the direction taken by its fibres (Tab. IV. Fig. 59). Nuclei may also be remarked, espe- cially after the whole has been moistened with acetic acid (Tab. IV. Fig. 61, b). And after tearing up a small piece with needles as finely as possible, we see numerous elongated fragments (Tab. IV. Fig. 60) ; many of which exhibit a nucleus, and resemble fibre-cells ( 1055, compare Fig. 177, p. 318). These were previously joined lengthwise together to form the fibres, which are almost always smaller than the striped fibres of the same animal. 1232. The unstriped muscles are pale and yellowish to the naked eye; while the striped are distinguished by their fleshy redness in mammals and birds. But the muscles of many amphibia and fishes are white or yellowish, although their fibres offer very distinct transverse striae. And some of the muscles of fishes, which are of a whitish-yellow colour when fresh, occasionally become red by the advance of putrefaction to a certain extent. Hence the colour is not an invariable characteristic. 1233. The course of the fibres leads to the same conclusion. Most of the striped fibres run close to each other without any mutual connection (Fig. 178, p. 320). But in the auricles of the frog's heart we find that a bundle (a, Fig. 205) breaks off from one fibre (b) to be applied to another (c). This course, which is the exception in striped muscles, is the rule CHAP. XV.] DISTRIBUTION OF THE STRIPED FIBRES. 367 for the unstriped variety. The latter chiefly surround and enclose tubes, such as the alimentary canal, the gall-bladder, the urinary bladder the uterus, the Fallopian tubes, and the excretory ducts of the larger glands. Here they form longitudinal and FlG - 205 transverse layers, which cross each other at certain angles. The same arrangement is repeated in those striped muscles which occupy a similar position (kopm, Fig. 72, p. 127, from the upper part of the oesophagus). But in most of the thick muscles of the trunk and extremities it is absent. 1234. The names of animal and organic muscular fibres ( 1229) are based upon the notion that the striped variety is found in those muscles which are subject to the influence of the will; while the unstriped occurs in those involuntary- organs which chiefly minister to the interchange of matter. But later researches have effected an important alteration in this theory. We shall hereafter see that the obedience of any part to the commands of the will depends upon its nervous structures, and not upon the nature of its muscular substance. It is true that in man and the higher animals, the organs provided with unstriped muscular fibres are generally not under the immediate control of the will. But the voluntary move- ments of many invertebrate animals, such as the snail and the muscle, are effected by unstriped muscular fibres. And as regards the striped fibres, the proposition does not even hold good for the vertebrata. The heart, and the lymphatic hearts of birds and amphibia, consist of dis- tinctly striped fibres: and present red muscular substance, even when the muscles of the trunk and limbs are of a whitish yellow colour. 1235. In the human subject the following parts consist of striped fibres. All the free muscles of the head, neck, trunk, and limbs; the muscles of the eye ; the small muscles of the ear ; the tongue ; the pharynx ; the upper third of the oesophagus ; the heart; the diaphragm ; and the red contractile structures of the pelvis, and the organs of generation. Unstriped fibres are found in the following textures: in the lachrymal sac (m, Fig. 150, p. 273), and the lachrymal canals (k I) ; in the iris of the eye (c, Fig. 150), and in the tensor muscle of the choroid ; in many of the mucous membranes, in the two lower thirds of the oesophagus and the rest of the alimentary canal (qstuvy, Fig. 124, p. 227), in the gaU-bladder (I, Fig. 151, p. 280) ; and the spleen (g, and 980); the urinary bladder (x, Fig. 124); the larger gland-ducts; the trachea (at &, Fig. 101) and ramifications of the bronchi; the biliary duct (r, Fig. 151); the ureter (Fig. 154, p. 285); the seminal duct (Fig. 154) and vesicles (Fig. 154); the prostate gland (Fig. 154), the uterus (w, Fig. 124, p. 227, a?, Fig. 119, p. 208), the Fallopian tubes (yz, Fig. 119), and the vagina (z, Fig. 124); in many of the smaller glandular canals, as, for example, in some of the cutaneous glands (Tab. 368 THE PREPARED FROG. [CHAP. XV. FIG. 206. IV. Fig. 62, op); in the tissue of the corium (Tab. IV. Fig. 62, def), the dartos of the scrotum; the hair-bulbs (Tab. IV. Fig. 63); the vessels, &c. And although in many of the smaller gland-ducts no fibrous structure can be recognized, still we are justified in supposing that their transparent middle tunic (Tab. V. Fig. 65, b) possesses a certain capacity of contraction. It was formerly supposed that the ordinary areolar tissue (Tab. III. Fig. 40) was capable of active contraction; but recent observations have shown that unstriped muscular fibres are con- cealed here. 1236. The two kinds of muscular fibre are distinguished not only by their form, but by their mode of contraction. This circumstance is one of the chief causes of their distribution ; while a second may probably be found in the relations of their nutrition and organization. Since the appearances presented by the striped fibres have been most accurately investigated, they will be first considered; and will be followed by a notice of the similarities and differences exhibited by the unstriped variety. 1237. Every muscle receives a number of nervous fibres, which are capable of exciting its contractility. They are therefore named motor fibres. If we take a prepared frog's leg, i.e., the skinned leg (c, Fig. 206) of a recently killed frog, from the thigh of which all the soft tissues have been re- moved, with the exception of the sciatic nerve, a b, we may produce contraction of its muscles in two ways ; by irritating the nerve, a b, or the muscular sub- stance, c. 1238. The nerve a 6 is extremely sensitive towards mechanical irritations. The gastrocnemius muscle, c, contracts vigorously when we compress a b, or slowly cut it across. But if the section be too rapidly made, c may remain at rest. Mechanical stimuli applied directly to the mass of striped muscle, c, less frequently lead to marked contractions. 1239. Heat may either excite contraction or com- pletely destroy it. When the nerve ab is dipped in water of 86 to 100, the gastrocnemius c often con- tracts vigorously. Hence a sudden difference of tem- perature, or a quick alteration in the amount of warmth, exert an influence similar to that of a mechanical interference. In favourable instances this experiment may be repeated several times without injury. But if the whole preparation be sunk into water the temperature of which is higher than 104 to 113, its muscles become paler and more brittle, and both their force and that of the nerve is permanently lost. At most, we only see some contraction at the first CHAP. XV.] CONTBACTIONS PRODUCED BY THE ELECTRICAL CURRENT. 369 instant of applying the heat. The muscles of birds and mammals sustain a somewhat higher degree of heat; but even they will not bear hot water. This fact shows that the animal heat could not rise much higher than it appears really to be ( 1165) without inflicting a serious injury. 1240. Cold weakens contractility, and finally destroys it. And hence the activity of a prepared frog may be raised or depressed at will, by placing it either between portions of ice, or in water at 99-5. And by keeping it in water at 68 to 86, its sensibility may be preserved for long periods of time. 1241. The motor nerves and muscular substance of the newly killed animal obey no kind of irritation with such delicacy and exactness as the fluctuations of the electrical current. When the act of establish- ing a current in the animal tissues raises the tension of electricity from zero to a given height, or when breaking the current brings it down from this point to zero ( 233), contractions instantly follow. The first case occurs at the instant of closing the circuit ( 232); and the second at that of opening it. Hence in the most favourable instances we have two contractions ; one of closing, and one of opening, the circuit. If the tension of the current remain nearly unchanged during the closure of a weak galvanic circuit, and if all violent electrolytic action be absent, the muscles will remain at rest. Under these circumstances the electric currents which pass through the nerve ( 216) only alter its con- dition gradually : as we shall hereafter find in treating of the nerves. But if, on the other hand, the strength of the current varies more considerably, or if powerful electrolytic influences interfere ( 239), the muscles con- tract during the closure of the circuit. 1242. Electricity may be applied with this object in either of three ways. If we insulate ( 217) the prepared frog's leg on a glass plate FIG. 207. (Fig. 207), its gastrocnemius may be made to contract, either by placing the two conducting wires (c and d, Fig. 50, p. 80) on two different points of the nerve a b, or of the muscle c; or, finally, by bringing one wire into contact with the nerve b, and a second against the muscle c. The two first cases imply a much higher sensibility than the last. 1243. We have already ( 223) seen that the artificial or natural transverse section of the striped fibre (a b r c d, Fig. 208) is negative B B FIG. 208. rT /' 370 CONTRACTIONS PRODUCED BY THE MUSCLES THEMSELVES. [CHAP. XV. with respect to its longitudinal surface (a c, b d). We have therefore a kind of galvanic battery in the muscle itself. And since it responds to slight changes in the tension of electric currents by contractions, these may be excited by the mere influence of the animal tissues, without any metallic circuit. 1244. The simplest experiment consists in exciting II a tijjjjb contraction in the galvanic preparation by means of its own muscular substance. For example, if the nerve a b, (Fig. 206) be so bent round, that a part of it b touches the neighbouring artificial transverse section, when a is laid upon the longitudinal surface of the //' gastrocnemius, c, an electric current will pervade a b. f I + I I "** The current passes in the direction of the upper arrow in Fig. 208. The muscle c therefore con- tracts at the instant of its quitting a, or at the moment of closing or opening the circuit ( 1241) : the contraction being due to the electrical tension of its own mass tending to become equalized by means of its motor nerve. Less frequently we may succeed by bringing one part of the nerve into contact with the longitudinal surface (c) of the gastrocnemius, and another part against the expansion of the tendo Achillis which v occupies its lower extremity, and covers the natural termination of its muscular fibres (c d, Fig. 208). 1245. These facts illustrate the column invented by Matteucci. A certain number of the skinned thighs of newly killed frogs are so divided above and below as to admit of being joined to each other in the manner represented by Fig. 209. This will give us a negative;;artificial transverse surface (a b) ; and a positive longitudinal one (b c). If a prepared frog be now completely insulated ( 217) by being placed in a glass tube (a, Fig. 210), while its nerve is allowed to hang out and if a certain part of this latter be brought into contact with a b, Fig. 209, while an- other part of it touches a segment of b c, the muscles of the prepared frog (Fig. 210) will again contract at the instant of opening or closing the circuit. 1246. The source of the exciting electric current is a matter of indifference. Not only the contactive currents just mentioned, but those of chemical or thermal electricity ( 1164) are also successful. Electricity of tension or friction ( 216) acts very powerfully on the prepared frog at the instant of being equalized; because its transition from the given elevation to zero ( 232) occurs with great rapidity. 1247. When the successive closures and openings of the galvanic cur- rent succeed each other slowly, the muscles fall into a state of clonic FIG. 209. CHAP. XV.] CLONIC AND TONIC SPASM. 371 or alternating convulsion : i. e., we have first a contraction, then a re- laxation; then another contraction, followed by another relaxation; and so on. But when, on the contrary, the closure and opening of the cur- rent are repeated more quickly, we get stiff or tonic convulsions : i. e., the FIG. 210. muscles remain constantly contracted. But if this should continue too long, or if the muscular substance did not originally possess any great degree of susceptibility, alternating convulsions occur ; either in the whole muscle, or in some of its fasciculi. The electro-magnetic ( 248) and magneto-electric machines ( 252) are best adapted to a rapid and alter- nate opening and closure of the circuit. And hence with the help of such apparatus, tonic convulsions may easily be produced. 1248. Soon after excision, the vigorous muscles of frogs may exhibit tonic or rigid convulsions, with an average of only two strokes of the machine per second. But a smaller number at once gives rise to alter- nating contractions. 1249. The same difference also obtains with other irritants. If the nerve (a b, Fig. 206, p. 368) be gradually constricted by a thread, the successive irritations thus produced will sometimes excite rigidity of the gastrocnemius. Hence this kind of muscular contraction does not de- pend on a special character of the electric stimuli, but solely on the rapidity of their succession. A complete and visible relaxation of the muscular fibres requires more time than intervenes between every two stimulations. At present we are ignorant whether anything similar occurs in the continuous contraction of a muscle during life. 1250. The results of galvanic irritation are intimately connected with the condition of the motor nerves. It is this which determines whether the contractions occur only on closing or on opening the circuit, or on both of these occasions; and whether the effect is independent of all change in the direction of the current. But the further illustration of these facts belongs to the study of the nervous function. 1251. A prepared frog which has been kept some time in a rarefied space, or in an atmosphere of hydrogen gas, or in a mixture of ordinary air and sulphuretted hydrogen, is still capable of contracting vigorously under the influence of the electro-magnetic apparatus. Even water which has absorbed the latter gas does not at once destroy its irri- tability. But on a longer application it is more injurious than pure B B 2 372 INFLUENCE OF EXTERNAL CIRCUMSTANCES. [CHAP. XV. water. The vapour of acetic acid acts very injuriously: and that of ammonia is still more hurtful. 1252. Irritability is at once destroyed by alcohol, ether, or solutions of acids or alkalis which are not too dilute ; as well as by solutions of nitrate of silver. Very weak fluids of this kind are capable of exerting two effects; temporary excitement, or permanent destruction. So that here also we meet with an alternation of results like that already ( 1239) seen to be produced by heat. 1253. A. motor nerve may be brought into contact with a pure watery solution of hydrocyanic acid or strychnin without losing the influence which it exerts upon muscular contraction. But if alcohol form the solvent, or if the prussic acid contains the usual admixture of sulphuric acid which prevents its decomposition ( 299), the injurious effect soon appears. The watery solution of opium also acts injuriously. 1254. We have seen ( 1237) that a striped muscle contracts on irritating it or its motor nerve. In the latter case, a change in the molecular state of the nerve is propagated along its fibres to their termi- nations in the substance of the muscle. And this again produces that peculiar alteration in the physical condition of the muscle, of which contraction forms the outward expression. 1255. When a stimulus for instance, an electric current is applied to the muscle itself, the result admits of two explanations. We may first suppose that it directly stimulates the muscular fibres; and that these do not require the mediation of the nerves, but possess an inherent irritability or susceptibility of their own. Or secondly, we may regard the electric current as acting directly upon the nerves which are distri- buted in the interior of the muscular substance. It is thus essentially the same thing as if the nerve itself had been stimulated. The mole- cular change in the nerve produces the physical alteration in the muscle ; which latter is incapable of direct contraction, and can only accomplish it by an intermediate action of the motor nerve-fibres. We shall hereafter find reasons for conjecturing that the unstriped muscular fibres possess an independent capacity for contraction, and do not require the aid of the nerves; or, at any rate, not of those primitive nerve-fibres which possess oily contents. But at present we have no means of deciding whether the striped fibres, which are so much more obedient to the nervous influence, resemble the unstriped in this respect. 1256. Many have laid great stress upon the results obtained by the section of nerves. After the sciatic nerve (a b, Fig. 206, p. 368) or the lumbar plexus has been divided in a living frog, the . corresponding muscles for instance, the gastrocnemii are no longer capable of con- traction in obedience to the will. At first, however, the application of an electric current to the inferior segment of the nerve, or to the paralyzed muscles themselves, is followed by vigorous contractions. But if the CHAP. XV.] IRRITABILITY OF THE MUSCULAR FIBRES. 373 nerve is not reproduced ( 1066), it subsequently becomes incapable of exciting any contraction; while by applying the poles of a galvanic circuit to the muscle itself, these are constantly produced. The prepared frog ( 1237) exhibits the same results. In the gradual destruction of irrita- bility there finally comes a stage, during which the muscles respond by contractions to a direct stimulus, while irritation of the nerve is attended with no results ( 1242). From these facts it has been con- cluded, that the muscular fibres possess an irritability which is inherent to their substance, and which therefore enables them to contract after the destruction of their nerves. But a more careful study of these phe- nomena will show that they do not warrant this conclusion. In treating of the nerves we shall find, that the activity of their terminal distribution in the muscular substance endures longer than that of their trunks. When these latter no longer respond to stimuli, the branches which pass between the muscular fibres ( 1255) still pre- serve their powers. The later disappearance of irritability in the mus- cular substance therefore admits of a double interpretation ( 1242). 1257. Serious disturbances of nutrition also destroy the vital functions now under consideration. A nerve- fibre, the oily content of which has coagulated (Tab. V. Fig. 69), is no longer capable of exciting any move- ments. There is also a certain degree of decomposition of the living muscular fibres which involves the destruction of their contractile power. Under such circumstances they appear paler and softer; although they may still exhibit transverse stripes. The muscles of limbs which have been long paralyzed sometimes fall into this state. A better nutrition may again restore their powers. 1258. But these injurious changes only take place slowly. Hence interruption of the current of the blood disturbs irritability less than might be expected. Of course the circulation ceases in the galvanic preparation (Fig. 206, p. 368). But in spite of this, the contractility of the muscles, and the capacity of the sciatic nerve to excite contractions, remain many days. The whole preparation is often so exhausted by successive experiments that the effects cease. But on allowing it to rest for some time, it recovers from these effects of stimulation. So that this recovery of the nerve and muscles does not require the supply of fresh blood. 1259. On tying the abdominal aorta of a dog, the half-lamed animal drags its hind legs, to which the access of blood has been obstructed. This effect is afterwards greatly diminished. And if only one artery be ren- dered impervious, it is frequently absent. On the other hand, Brown- Sequard states that the local sensibility of the muscles of a rabbit's hind legs was lost a few hours after the ligature of its aorta, but reappeared soon after the circulation was restored. 1260. In the dead bodies of most animals, three conditions of the mus- 374 CONDITIONS OF IRRITABILITY. [CHAP. XV. cular substance succeed each other. There is first a space of time, during which the muscles retain a greater or smaller residue of their vital acti- vity, or of their capacity of contracting under the influence of proper stimuli. They next experience an extraordinary contraction, which gives rise to that peculiar phenomenon called the rigor mortis, or the stiffening of death. Finally, as putrefaction advances, the muscular substance softens, and dissolves with more or less rapidity. 1261. The muscles of amphibia are especially distinguished by the obstinacy with which they retain their contractile powers. On this account frogs are selected for galvanic and other experiments on irrita- bility. The femoral muscles of a frog frequently respond to the shocks of an electro-magnetic machine three days after death ; and sometimes even from 5 to 6 days after. The muscles of the head, trunk, and fore-legs, usually die sooner. A decapitated turtle may move its limbs in answer to an external stimulus about 14 days after death. Many fishes present a similar though less marked tenacity of irrita- bility. Hence the well known fact that pieces of a fish which has been killed many hours before may spring out of the kettle. 1262. Irritability generally disappears much earlier in the dead bodies of mammalia and birds, than in reptiles. In very young mammals, however, its duration is longer. And in persons who have been exe- cuted, or who have died of diseases which do not involve degeneration of the fluids, we sometimes find certain of the muscles remaining irritable 15 hours after death. The best means of ascertaining this fact is by galvanic currents. But the supposition that these may be used to dis- tinguish real from apparent death soon after the last breath, is obviously erroneous. 1263. These phenomena are greatly influenced by temperature. We have already noticed the effect of low degrees of heat on the irritability of the prepared frog ( 1240); and in warm-blooded animals it is still more remarkable. When the corpse of an adult bird or mammal is rapidly cooled by collateral causes, the irritability of the red muscles disappears in a few minutes. Many narcotic poisons, such as hydrocy- anic acid and wourali, lead to the same result ; especially when intro- duced in large quantities. 1264. In addition to this, the duration of irritability in some degree depends on the character of the muscular fibres. Just as these are paler in the amphibia and fishes ( 1232), so they are yellowish or less red in newly-born mammals, which retain irritability longer ( 1262) than adults of the same species. It has often been stated, that the neces- sity of respiration is inversely as the tenacity with which the muscles retain their irritability. But future researches must decide the accuracy of this proposition. Although the smaller mammals consume more oxygen ( 1180), and cool more rapidly, still their irritability does not CHAP. XV.] RIGOR MORTIS. 375 always disappear with a rapidity proportional to the amount of combus- tion which occurs during life. 1265. Irritability is, as it were, dissolved by the rigor mortis. The former is the last relic of life ; the latter, the first stage of spontaneous decomposition. The muscular substance then changes its physical pro- perties. It becomes shorter, more solid, and less extensible. A muscular fibre which has lapsed into the state of rigor mortis has for ever lost its living force of contraction. But the change does not occur simultaneously in all the muscles of the body, nor even in all the bundles of the same muscle. Hence galvanic currents which excite contractions in one part, may have no effect on another part of the same muscle. 1266. A stiffened human corpse generally offers many peculiarities of attitude. The lower jaw drops immediately after death; but during the rigor mortis, it again approaches the upper jaw. The fore-arm is bent against the upper arm; and the leg against the thigh. The strongly bent fingers partially cover the flexed and adducted thumb. Any attempt at forcible extension tears some of the muscles before bringing the limb to a straight line. 1267. We shall hereafter see, that there are other structures besides the striped muscular fibres which undergo a rigor mortis. The peculiar positions just mentioned chiefly depend upon the mechanical prepon- derance of the powerful flexor muscles of the limbs over the extensors. Cutting across the muscles restores the mobility of the limb ; a result which is not produced by severing the greater part of the articular liga- ments. 1268. In the human subject, signs of the rigor mortis may appear in the first quarter of an hour after the last breath. Under the most unfavourable circumstances, it only occurs about 18 hours afterwards. When it appears later, it frequently lasts longer. Still we often find exceptions to this rule ; as, for instance, according to Bruecke, in animals who have been poisoned by strychnin. In the corpses of men who have died suddenly the rigidity is generally very great : while in those of dropsical subjects it is but inconsiderable. Paralysed muscles always undergo a rigor mortis, unless their minute structure has suffered too much during life. 1269. The stiffness usually begins at the head and neck; and thence passes gradually downwards. But sometimes the thighs seem to lose their flexibility before the arms. 1270. The progress of putrefaction again softens the muscles, and renders them rotten. Hence they are easily torn by a moderate extension. The transverse stripes are subsequently deranged, and finally disappear. The fibres are longer recognized. Many of them present a finely granular precipitate. Finally, the nuclei of the sarcolemma (Tab. IV. Fig. 54, b) become invisible, even on the addition of acetic acid. The whole is at last 376 ZIG-ZAGS OF THE MUSCULAR FIBRES. [CHAP. XV. converted into a dirty greasy substance of a greenish or brownish colour. In exceptional cases, a large portion of the soft tissues of a corpse becomes converted into a waxy substance, which resembles fat, and is thence called adipocere. 1271. Hitherto we have only considered the changes which the striped muscular fibre exhibits to the naked eye. We have now to examine into its more minute circumstances. 1272. When a muscle of a newly killed animal is cut across, its two surfaces separate from each other, so as to leave a certain interval. If this experiment be repeated at any point of either of these portions, the same phenomenon occurs, although usually with somewhat diminished force. On examining a thin and isolated piece of muscle (such as a piece of the abdominal muscles of the frog) under a moderate magnifying power, we see that the course of its fibres is not straight, but in zig-zags. These often correspond to each other in neighbouring fibres, as is shown by Fig. 211. But in other cases, the greater independence of the several fibres produces a want of uniformity in this respect. Fig. 212 represents an example of the latter kind. FIG. 211. FIG. 212. 1273. These zig-zag bendings may diminish the length of a muscle by | to -J. Under such circumstances, the angle of the plications varies from 50 to 108. Still we sometimes meet with curves which are so slight, as scarcely to deviate from 180. In the muscles of the frog we often find folds which are almost rectangular, and which would cause a shortening of about f ths of its whole length, 1274. The forms represented above are absent from muscles in an advanced stage of spontaneous decomposition. It is probable that the intermediate rigor mortis renders their occurrence impossible. But they are often found in muscles which are no longer capable of contraction under the influence of the galvanic current. 1275. Formerly these zig-zags were generally regarded as expressing the capacity of contraction present during life. Thus, under favourable circumstances, an examination of some of the hyoid muscles of the frog proves that, according to the state of the respiratory movements, the muscular fibres are either folded up in angles of this kind, or are again CHAP. XV.] ELASTIC CONTRACTION OF MUSCLES. 377 extended. But Ed. Weber has shown that the whole phenomenon is caused by elasticity, and not by vital contraction. Thus, for instance, if a thin membranous portion of frog's muscle offering the appearance represented in Fig. 211 be thrown into a state of tetanic rigidity ( 248) by the electro-magnetic apparatus, we may sometimes see that the mus- cular fibres are extended perfectly straight. At the same time the whole mass becomes shorter, broader, and thicker. When the electrical action ceases, the muscular fibres spring back into their former situation, and the zig-zags again come into view. But in many cases this decisive result is wanting. It sometimes hap- pens that only a few of the muscular fibres really contract. These may then draw with them neighbouring fibres which have already lost their vital properties, and may thus either strengthen or level their folds. When contractility is considerably diminished, it becomes incapable of completely removing these zig-zags. Hence they become smaller, and approach 180. In one word, they only form the expression of a contest, in which elasticity overcomes the remaining traces of con- tractility. 1276. When an extended silken cord or a violin string is cut through, it is twisted round by its own elasticity. Many fibrous tissues ex- hibit the same phenomenon. In their natural situation, they undergo a certain extension. The act of division frees them from this compul- sory state. In this way are produced the wavy curves exhibited by the bundles of white fibrous tissue under the microscope (Tab. III. Fig. 40), as well as the zig-zags of the muscles (Figs. 211 and 212). Peculiar shapes are here produced by the form, size, character, and mutual con- nection of the tissues. 1277. There is reason to suppose that many other phenomena belong to the same class. Many muscular fibres exhibit deep constrictions (Fig. 213); and the transverse extremities of others offer a variety of pe- culiar forms (Fig. 214). These may even occur in muscles which are approaching the state of rigor mortis. Sometimes isolated and fresh muscular fibres curl up under water (Fig. 215), or even vibrate to and fro like a pendulum. It may be conjectured that the latter phenomenon, which has been remarked in the muscles of insects, like many undulating movements which occur in those of the river crayfish under similar circumstances, is not due to mere capillary attraction ( 119, et seq.), but to certain relics of the vital functions. 1278. Microscopic research proves that the transverse stripes of the frog's muscular fibres approach each other at the instant of contraction. FIG. 213. 378 VOLUME OF CONTRACTED MUSCLES. [CHAP. xv. But there are other important changes which we are unable to verify. Here, as in many other molecular phenomena, the main fact escapes the eye, and can only be deduced from accompanying circumstances. FIG. 21 4. FIG. 215. FIG. 216. ft .... 1279. The muscular fibre becomes shorter and thicker during con- traction; its transverse diameter being increased at the expense of its length. Supposing that its form is in both cases cylindrical, it will corre- spond to a b c d (Fig. 216) in the relaxed, and to efgh in the contracted state. Under such circumstances one of two things may occur. The contents of a cylinder are the product of its height (a d) by the area of its transverse section (a b). Now since a d is converted into the shorter e h, and a b into the larger ef, these relations may so compensate each other that the capacity of the cylinder remains un- changed. Assuming a d to be an inch, and a b a square inch, the volume of the cylinder will amount to one cubic inch. Now if, at the moment of contraction, a d is altered to e k, or | an inch, while e f enlarges to two square inches, the new cylinder efgh will still con- tain one cubic inch. Hence the muscle will only have been altered in diameter, and not in capacity. The second possibility is, that all these proportions are altered : that a condensation of the muscle leads to a diminution of its volume, or its extension to an enlargement. 1280. The way in which this question has been inquired into may be represented by Fig. 217. A certain number of frogs are first beheaded, skinned, and gutted as shown at d and then stuck upon wires, which converge to one of the two conducting wires : for instance, to /. The second conductor is continuous with g } which is free at its extremity, but is elsewhere covered with sealing-wax. The inverted bell-glass is closed by a stopper, c, which is perforated by two small tubes, a and b. Luke- warm water is poured in at a, while the compressed air escapes at b. When the fluid rises so high as to flow out of the open end of the shorter tube 6 by means of the hydrostatic law of equal pressure ( 81), the aperture at b is hermetically closed. Special care must be taken that CHAP. XV.] VOLUME OF CONTRACTED MUSCLES. 379 FIG. 217. no air -bubble of any size remains adhering to the frogs d, the wires d g, the inferior surface of the stopper c, or anywhere else: since the compressibility of gases ( 67) could thus produce a diminution of volume which might be ascribed to other causes. If / and h be now brought into contact with the two poles of a galva- nic column, or of an electro-magnetic apparatus, the muscles of the pre- pared frogs contract, and the legs are powerfully extended. If the muscles now occupy the same space which they exhibited before the contraction, the level of the column of fluid at a will be no way altered. While it will be depressed by condensation, and raised by expansion. The application of a scale at a, and an examination by means of a magnifying-glass, will inform us of the smallest change in this respect. 1281. All the observations hitherto instituted concur in the statement, that no important difference of volume can be observed. Prevost, Dumas, and Matteucci, found no alteration of capacity in the case of the frog and the torpedo. The author sometimes found variations; which, however, scarcely amounted to 10 Q 00 th of the whole capacity. Weber, who expe- rimented on an eel by means of the rotatory apparatus, remarked a trifling decrease of volume. 1282. Even after all air-bubbles have been removed, the too frequent transmission of electric shocks may easily give rise to deception, since these exercise an electrolytic action on the surrounding fluid, and cause bubbles of gas to collect on one of the conducting wires : for example, on the point of g (Fig. 217). Be that as it may, we can at any rate state that there is no important alteration of volume at the instant of muscular contraction. 1283. A contracted muscle feels harder than a relaxed one. But the accurate researches of Edward Weber have taught us that this pheno- menon depends solely upon the increased tension of the muscular fibres. Their substance itself becomes rather softer than harder. The amount of their elasticity is diminished instead of increased. We may convince ourselves of this remarkable fact in a variety of ways : but perhaps the best will be a simple experiment by means of the apparatus used by Weber, which is represented in Fig. 218. The hyo-glossus muscle of a 380 SOFTENING OF CONTRACTED MUSCLES. [CHAP. xv. frog is fastened above to the metal hook, a ; and below, it sustains the hook, b, which carries a small scale and weight : while a filament of raw silk, h i, is passed through the lower part of the muscle. This filament, which runs over two sticks, and carries a small weight at each end, serves as an index for the millimeter scale, g, placed behind it. Its situation may be read off by means of a magnifying glass. FIG. 218. One of the conducting wires, c, goes from the upper hook, a; and the second, d, from the lower one, b. This is permanently connected with one pole of the electro-magnetic apparatus (Fig. 58, p. 87). The second, d, dips into the mercury contained in e. When this also receives the second pole, /, of the electro-magnetic apparatus, the electric shocks pass through d b a c. The filament of silk, h i, is raised by the contracted muscle : and the difference between the present and former length may be read off with a magnifying glass on the scale g. Let us suppose that in the state of rest the distance from k i to a equals one inch. Under the influence of the electro-magnetic apparatus this is gradually reduced to its minimum. If the apparatus continues to act, the muscle becomes partially relaxed, and then shortens again. Subse- quently its length continually increases. But if we cease to apply the electric shocks when the distance again amounts to one inch, we shall CHAP. XV.] CHANGE OF THEIR ELECTRIC STATE. 381 find that, in spite of this, the filament hi descends to 1-05 or 1-1 inches. Now the weight which keeps the muscle extended, viz. the scale and the weight which it contains, has remained the same. And hence this considerable elongation of the muscle can only depend upon its having meanwhile become more soft and yielding. In other words, its index of elasticity ( 55) is diminished. 1284. A series of comparative researches further shows, that this phenomenon depends upon the vital contraction, and not upon any physical circumstances. Thus although when the dead muscle is loaded with a weight, it also undergoes a gradual extension, still small weights require much longer intervals of time in order to produce results equal to those which are furnished in a minute by contraction. When a dead muscle is exposed to the shocks of the electro-magnetic machine, it is not softened : a proof that this change is not produced by electrolysis. And the softening often has a visible proportion to the amount and duration of contraction in the living muscle. When this has been exhausted and softened by continual irritation, a rest which restores its contractility to any considerable extent also increases its elasticity. 1285. It is probable that, even during life, a fatigued muscle is more yielding than when fresh and vigorous. Still the feeling of fatigue does not depend upon this, but upon certain conditions of the nerves, to which we shall return in speaking of the nervous system. Here also we find circumstances which resemble those of the sensations of warmth ( 1175). A sick person, whose muscles exhibit their normal characters, may feel so prostrated as to be ready to drop FIG. 219. from exhaustion. 1286. At the instant of muscular contraction, the electric state is also changed. We have seen ( 223) that the muscular current chiefly depends on the fact, that the longitudinal surface is relatively positive, while the natural or artificial transverse section is negative. We may therefore suppose that the muscular fibres consist of molecules, which are positively electric at their sides, and negatively electric at their ends; as exhibited by the diagram in Fig. 219. If we connect b to a by means of a suitable conductor, in which a galvanometer (Fig. 43, p. 76) has been interposed, the positive current will set out in the direction of the arrow in Fig. 219. The needle of the galvanometer will also deviate in this direction, and after it has finished vibrating, will remain fixed at a definite number of degrees on the positive side of the circle. While according to Du Bois, 3 9) when the muscle, for instance, the separated gastrocnemius of a frog, falls into a state of tetanic rigidity from the sciatic nerve being exposed to a rapid succession of electric shocks, the needle recedes, 382 CHANGE OF THEIR ELECTRIC STATE. [CHAP. XV. glides past zero, and passes over a certain extent of the negative half of the graduated circle. The contractions which accompany mechanical disturbance of the spinal marrow, or are produced by mechanical, thermical, or chemical irritation of the sciatic nerve, may give rise to similar (though less marked) results. Hence at the instant of powerful contraction the muscular current is visibly diminished. And here again accurate observations show that the phenomenon does not depend upon accidental collateral circumstances, but upon the vital contrac- tion itself. 1287. This fact has been applied by Du Bois to explain what Mat- teucci has described under the name of the induced contraction. If the sciatic nerve a (Fig. 220) of one prepared frog, d, be made to rest FIG. 220. upon the femoral muscles, 6, of a second, and if the corresponding sciatic plexus, c, be galvanized with the apparatus represented in Fig. 221, which is a zinc plate, a, connected with a copper one, 6, by means of a CHAP. XV.] CHANGE OF THEIR ELECTRIC STATE. 383 twisted copper wire, c, not only the femoral muscles at I, Fig. 220, but also those of the muscles belonging to d, will contract. In susceptible preparations, this experiment may be successfully repeated several times, even when the whole lies upon the table without being insulated by a FIG. 221. glass plate, or when the nerve a is only laid along the outer surface of the femoral muscles, and c is only electrified in a short extent of its course. Since the different parts of the length of a muscle possess different electrical properties ( 223), so long as the nerve a lies upon the muscles, it will be permeated by a weak cur- rent. Electrical irritation of c ex- cites a contraction of 5, which changes its previous electric condition (Fig. 220). Thus we get a variation of current for a, which is answered by a contraction of the corresponding muscles at d. Hence the induced contraction consists in the fact, that the prepared frog is rheoscopic: i. e., that it tests the current, and indicates its negative variation by contraction. 1288. The longitudinal surface and the transverse section present a greater electrical opposition than two dissimilar points of the former only ( 223). It might hence be expected that the induced or secondary con- traction would occur more easily when one part of the nerve (a, Fig. 247) touched the longitudinal surface, and a second the transverse one of b. And Du Bois states this to be the fact. Electrical irritation of c (Fig. 220) succeeds with most certainty. But mechanical, chemical, or thermical stimuli sometimes produce the same effect. Still they fail much more frequently. This strange fact evidently depends on peculiar collateral circumstances. For the mere use of the simple circuit repre- sented in Fig. 221, which produces a simple galvanic contraction, with- out continuous tetanic rigidity, always causes the secondary contraction of any suitable preparation. 1289. It is evident that when a lamina of glass or any other insulating substance is brought between the nerve a, and the muscles b, the secondary contraction at d will cease. But a thin layer of moisture soon evokes it. Laminae of silver, gold leaf, or other conducting substances, do not destroy it. If the nerve of a third preparation be laid upon d, a tertiary contraction may be produced ; and through this, a quater- nary one : and so on. 384 TOTAL AND SUCCESSIVE CONTRACTION. [CHAP. XV. 1290. The general result of all this is, that at the instant of con- traction the cubic capacity of the muscles alters but very little. The act of contracting renders them softer, and diminishes their muscular current. These facts indicate a great change in their molecular relations. In study- ing the function of innervation we shall see how far this change of physical characters resembles, and differs from, that produced by magnetism. 1291. In the vigorous contractions of striped muscles, the change usually affects their whole length in an instant. This statement may be verified in all the free muscles of the living body ; and in vigorous excised portions of the frog's muscles, it may sometimes be seen under the micro- scope. But there are some exceptions to this rule, even in muscles which exhibit a distinct transverse striation. 1292. When the nerve (c) of the prepared frog represented in Fig. 220 is exposed to the continuous action of an electro- magnetic apparatus, we first obtain tonic spasms of the muscular substance, for instance, of the femoral muscles (6). But after this state has continued a certain time, it is interrupted by alternating convulsions of some of the muscular bundles ( 1247). Sometimes these contractions appear to be limited to certain portions of their lengths. A similar appearance is not infrequently offered by isolated and feeble portions of muscle when subjected to electric irritation under the microscope. 1293. The upper third of the human oesophagus contains very distinct striped fibres. In many animals, for instance in the rabbit, these descend to the cardiac aperture of the stomach. We have already ( 381) seen that, at the instant of deglutition, the oesophagus of living mammalia exhibits undulatory movements; which consist of a local and progressive alternation of contraction and relaxation. The descending muscular fibres do not contract at once, but in successive segments (See Figs. 71, 72, 73, p. 127). In short, we have here undulatory movements, which resemble those of most unstriped muscular fibres ( 399). But a more careful examination teaches that, in certain cases, the striped fibres of the oesophagus act more conformably to their nature, that they are only compelled by circumstances to these undulatory contractions. On breaking up the spinal cord of a mammal, no vermicular movements of the oesophagus appear. When the shocks of the electro-magnetic apparatus are transmitted through the cervical trunks of a dog or rabbit which has been killed in this way, the whole oesophagus falls into a state of tonic spasm, and becomes shorter and thicker. Indeed, we have already seen ( 383) that the continuous progress of these undulatory movements depends upon collateral causes connected with the nerves. 1294. The nerve-fibres which are distributed in the interior of a muscle enter it at certain distances of its length. Thus each of them has under its influence a certain number of molecules of muscular sub- stance, by which it is obeyed. When the entire muscle contracts, one of CHAP. XV.] PECULIARITIES OF THE UNSTRIPED FIBRE. 385 two things may occur. The stimulus may either be applied simulta- neously to the whole mass, or may be propagated with extreme velocity from one nerve-fibre to another. The latter supposes a mere succession, with extremely minute intervals. The undulatory movements of the oesophagus may be partly due to the nerve-fibres acting in a slower and more ordinate succession. The local alternate convulsions which are finally caused by the electro-magnetic apparatus are partially explained by supposing some nervous fibres to be exhausted sooner than others. 1295. Many of the essential phenomena with which we have been made acquainted in the striped muscular fibres are repeated in the un- striped variety. They exhibit an electrical antagonism of the longi- tudinal surface and the tranverse section ( 226); although it is some- what weaker. They sometimes present zigzags when cut through ( 1272) ; although this form is less frequently remarked in them than in the striped fibres. They undergo a rigor mortis (1266) as the intermediate link between the departure of the vital functions and the access of vigorous putrefaction. Finally, their contraction is also imme- diately excited by two causes ; by stimulation of the nerves, or of the muscular substance itself. 1296. The strong muscles of the alimentary canal, the urinary bladder, and the internal female organs of most mammalia, easily contract under the influence of artificial irritation. But many other parts in which microscopic research recognizes unstriped muscular fibre only respond to more favourable conditions. Hence we shall commence by considering the above structures. 1297. In the striped fibres, total contraction is the rule, and vermicular movement the exception ( 1291). But in the unstriped fibres of the alimentary canal, the bladder, the oviduct, the uterus, and the larger gland-ducts, the reverse of this obtains. On looking at the small intes- tine of a recently killed rabbit, we chiefly remark an undulation (d a, Fig. 76, p. 134), which passes onwards with more or less activity, and often recedes ( 399). But it proceeds more slowly than that of the transverse fibres of the oesophagus : so as to offer a certain deliberate tenacity, such as distinguishes the action of unstriped muscular fibre generally. 1298. The urine which descends from the kidneys gradually distends the bladder. The walls of this viscus yield by reason of their elasticity ( 938). When the phenomena are observed in a recently killed mam- mal, the volume of the bladder is seen to be greatly lessened by the evacuation of the urine. It finally forms a spherical mass, the walls of which are thicker than before, but still exhibit vermicular movements under the influence of the electro-magnetic apparatus. Hence the state of rigor mortis is not necessary to this maximum diminution of capacity. No doubt one chief cause of it is the elastic reaction due to the decrease c c 386 PECULIARITIES OF THE UNSTRIPED FIBRE. [CHAP. XV. of that tension which formerly opposed contraction. But it sometimes happens that the contracted and globular bladder subsequently appears collapsed and thinner in its coats. This change indicates a previous vital contraction of its fibres, followed by a relaxation. It therefore follows that the unstriped muscular structures are capable of maintaining a tonic contraction ( 1247) during a considerable period of time. 1299. This capacity is still more evident in the intestinal canal. When a certain portion of this is compressed in a newly-killed rabbit, it frequently becomes constricted to the remarkable extent represented by d e, Fig. 76. More limited irritation, or smaller degrees of sensi- bility, give rise to smaller constrictions; which are sometimes mere furrows of one side of the tube. All these contractions usually con- tinue a long time, and are but slowly effaced. And however active the vermicular movements of neighbouring parts, they remain undis- turbed. 1300. On irritating the sciatic nerve (a b, Fig. 206, p. 368) of a pre- pared frog, the transverse stripes of its gastrocnemius (c) at once contract. Here the stimulus and the effect follow each other so quickly, that it would require the most delicate instruments to measure the time which intervenes. But when we irritate the alimentary canal or its nerves, a considerable interval often precedes the occurrence of contraction. 1301. The differences exhibited by the two kinds of muscular fibre may be best verified in those organs which possess striped fibres in one animal, and unstriped in another. The iris (b, Fig. 150, p. 273) contains striped fibres in birds ; and unstriped in man and the mammalia. On bringing the eye of a pigeon into the circuit of an electro-magnetic apparatus, the size of its pupillary aperture (c, Fig. 150) is instantly changed. But when this experiment is repeated on a rabbit, the change is slow and gradual. And in the bird, it disappears when the operation ceases ; while in the mammal, it may continue afterwards. 1302. The oesophagus of most domestic mammalia is chiefly composed of striped fibre ; and that of birds, of the unstriped variety. Confining our attention to the phenomena presented by newly-killed animals after the destruction or removal of their medulla oblongata, we shall find that, under the influence of electric irritation, the oesophagus of mam- malia contracts totally and instantaneously ; and relaxes the moment it ceases. That of birds generally responds more slowly and locally, and retains the constrictions it has once acquired during a longer time. The same difference may often be seen in the oesophagus of many mammalia such as the cat or horse, in which a considerable layer of unstriped muscular fibres ascends from the stomach. 1303. The alimentary canal of man and most of the vertebrata con- tains unstriped fibres. Reichert found that, in the tench, (Cyprinus tinea seu Tinea chrysites) the walls of the stomach and intestine contain striped CHAP. XV.] PECULIARITIES OF THE UNSTRIPED FIBRE. 387 fibres. These parts contract with a sudden impulse. The intestine of the river cray-fish (Astacus fluviatilis), which also contains striped fibres, offers somewhat similar results. 1304. It is easy to see that these differences in the action of the two kind of fibres affect degree only. And there are many phenomena which show that, under peculiar collateral circumstances, even this distinction may altogether disappear. It is true that the maximum velocity attained by the intestinal undulations of a rabbit never equals that of the vermicular movement of the oesophagus ( 382). But, in some rare instances, the difference is much less than usual. And the action of the heart which we have hitherto purposely abstained from considering, and to which we shall return in treating of the function of innervation plainly proves that local and enduring contractions may occur in striped fibres. While there are special circumstances which cause an exception to the rule, that unstriped muscular substance only contracts a short time after irrita- tion. By irritating the medulla oblongata, the cerebrum, or the cere- bellum, the vermicular movement of the alimentary canal may be excited as quickly as that of the gastrocnemius on stimulating the sciatic nerve. 1305. The unstriped muscle usually responds to mechanical irritation of its substance with greater energy than the striped fibre; and often exhibits a more punctual obedience to such a stimulus than to an irrita- tion of its free nerve-fibres. It is probable that its substance is capable of contracting without the intervention of the nerves ( 1255). But the study of the cardiac movements will again show that many of these properties also belong to the striped fibre. In the ureter of the rabbit, the unstriped muscle sometimes does not respond to electrical irritations of its own substance : while, when these are applied to its nerves, active vermicular movements ensue. 1306. Structures having the same morphological characters as the unstriped fibre are found with the aid of the microscope in the bronchi, the vessels, many of the smaller gland-ducts, the spleen, and some portions of the external integument. But it is generally impossible to excite them to contraction by mechanical or electrical irritation. The rapid succession of electric shocks produced by the rotatory or the electro-magnetic machine is oftener, though not always, effectual. Under such circumstances, however, we obtain, not vermicular movements, but tonic constrictions; which remain some time after the cessation of the jtfimulus, and are then gradually effaced. These results are, no doiibt, determined by peculiar collateral circumstances. We must recollect that the cutis anserina, the narrowing of the vessels, the wrinkling of the scrotum, and other similar phenomena which depend upon these fibres, only occur during life under peculiar conditions such as depression c c 2 388 ACTION OF LEVERS. [CHAP. XV. of temperature. We are therefore entitled to conjecture, that the chain of causation is made up of a series of links which are at present unknown. 1307. The behaviour of the alimentary canal also leads to the con- clusion that the contraction of unstriped fibres depends upon many conditions which are not always present. The study of digestion has already taught us that the intestines rest at particular times, in spite of the irritations to which they may be subjected; while at others they become active under stimuli which are to all appearance similar. 1308. General mechanical relations of the locomotive organs. The muscles are the active organs of movement, their contractility furnishing the mechanical force which produces changes of place. In order to this, they rule over certain passive organs of locomotion; the bones, cartilages, ligaments, tendons, skin, and other parts with which they are connected. And since all these structures act as levers, which are moved by an exercise of force, we must first become acquainted with the phy- sical laws which regulate the action of levers. 1309. Let us suppose that a rod such as the beam of an ordinary scale a b (Fig. 222) rests by its middle c on a solid support or fulcrum, while its two arms, a c and c b, possess the same length and FIG. 222. c weight. Such a lever, being one of the first kind, will remain in a state of equipoise. If we hang a weight, w, at a, and a second, k, exactly equal to it at b } the equipoise will still be maintained. But if k is heavier than w, the arm c b descends, while a c and w are raised. This occurrence is commonly said to depend on the equality of the mechanical momenta. The product of the length of the arm of the lever c b, by the weight k slung vertically, is called the static or mechanical momentum. If wxa c=kxc b, we have a state of counterpoise. But if k be greater than w, and hence w x a c<^k xb c, that arm of the lever which answers to the greater burden k will preponderate. 1310. But the latter result may be attained in a different way. If c b be greater than a c, while w and k remain equal, we also find wxa c<& xbc. Hence the preponderance may be determined by three circumstances : by increase of the weight, elongation of the arm of the lever, or both. If a c be less than b c, and w greater than k, the dif- ferences may so compensate each other that wxac=kxbc; and the counterpoise may still be maintained. CHAP. XV.] ACTION OF LEVERS. 389 1311. Supposing that the lever a b (Fig. 223) is poised exactly on its middle, d, and that its two arms a d and b d correspond in weight as well as length, its equilibrium will not be disturbed. But if they differ from each other if, for instance, a d is heavier than d b, or if the fulcrum be displaced from d to c, so that a c is longer, and therefore heavier, FIG. 22S. there will be a certain preponderance of weight on this side. In this case the weight w suspended from the arm of the lever acts in common with the superfluous weight of the latter itself. Hence, in order to com- pensate this, a second or counterpoising weight n must be applied to the lever, b c. It is only when n-\-k furnishes the same force as w that equilibrium is produced. 1312. This fact explains why we distinguish between the mathema- tical, and the real or material lever. The first constitutes that passive line of movement which is distinct from all material relations, and in which we have but to consider the length of the lever's arms. While as regards the second or real lever, we must be closely acquainted with its material relations, and must estimate the weight of its several segments, before we can determine the conditions which cause equilibrium or dis- placement. 1313. The action of the one-armed lever may be easily deduced from what has been mentioned above. Let us suppose that a c (Fig. 224) is FIG. 224. a mathematical lever, having its fulcrum at c, sustaining a certain weight w at b, and a counterpoise or a force at h. This is essentially a lever having two arms which partially coincide. The arm which sustains the weight corresponds to b c, and that acted upon by the force to a c. If 390 ACTION OP LEVEES. [CHAP. XV. we assume wk, we find wxbc^kxac, because a c>&c. Hence to produce equilibrium, we must have k x a c=w x b c, consequently k= w^ : i.e., the longer its arm of the lever, the smaller may be the force k. Supposing b c to be 2 feet, ac 4 feet, and w 10 pounds, k need only amount to 5 pounds, since 5x4=2x10. Thus lengthening the arm of the mathematical lever diminishes the force necessary to counterpoise in exactly the same proportion. Such an arrangement, in which the advan- tage turns in favour of force, is called a lever of the second kind. In a lever of the third kind these circumstances are reversed. Here k forms the weight, which has to be moved by a force acting at b. Hence we lose an amount of force that corresponds to the shortening of the arm to which it is applied. 1314. When the lever (ac, Fig. 224) remains horizontal ( 1315), the masses hitherto represented as the weights and powers act by their gravity; i.e., perpendicularly. Hence w b c forms an angle of 90. But where, on the contrary, a force impinges obliquely upon its arm of the lever, part of its effect is lost. The amount of such a force may be represented in the form of a straight line. Supposing it = cf for the horizontal arm of the lever b c (Fig. 225) FIG. 225. it will not effect more than the perpendicular c d, the length of which is determined by the line df, parallel to b c. If we now describe an arc which has c for its centre, and c c? as its radius, it will be evident that cf loses an amount equal to h f. We also see that the disadvantage must increase as the line approaches to & c or eg; and must diminish as it nears c d. 1315. We shall shortly mention reasons which entitle us to assume that those attractive forces of the earth which produce the phenomena of weight, and the fall of bodies, are united in its centre. If we represent our planet under the scarcely accurate form of a perfect sphere, a body at d will be attracted in the direction of the radius d e, and one at i in that of i e. But the several molecules of every mass of moderate size lie so closely to each other that we may without perceptible error regard the corresponding spherical surface d i as a plane tangent to it, and the radii d e and i e, as parallels which are perpendicular to d i. Hence we say that the molecules of a body are attracted perpendicularly downwards towards the ground. 1316. The innumerable parallel actions of gravitation which result CHAP. XV.] CENTRE OP GRAVITY. 391 from the equally infinite number of the smallest atoms may all be re- garded as united into one point ; which is the common centre of them all. Hence this is generally called the centre of gravity. And the straight line which connects it with the centre of the earth is called the line of gravity. This will therefore cut the horizontal surface of the ground at a right angle. 1317. A sphere a d b c (Fig. 226), has its centre of gravity at e, and its line of gravity in that diameter d c, which being produced would meet the centre of the earth. The centre of gravity of every symme- trical body for example, that of a regular ellipsoid A B C D (Fig. 227) must lie in some part of the median line C D, since all the corresponding FIG. 227. f, molecules on each side offer equal mechanical momenta ( 1309). If the mass be also longitudinally symmetrical, this point will occupy the centre S, of the median axis C D. But if, on the contrary, it be arranged asymmetrically, its centre of gravity will occupy some place dependent on the form of the whole. 1318. In order that a body should remain at rest, its line of gravity must meet its surface of support. When it drops beyond this, the mass falls towards that side on which the line of gravity cuts the horizontal surface of the ground at an angle of 90. Many attitudes of the human body are thus easily explained. 1319. Setting aside the subordinate influence exerted by many asym- metrical organs such as the liver, the spleen, the pancreas the human body consists of parts which are almost uniformly repeated on both sides. Hence so long as the distribution of these masses is not rendered unequal by any special movement, such as extension of the hand, the centre of gravity will occupy its median line. Accurate research further shows, that the centre of gravity of a healthy adult lying in the horizontal posi- tion occupies a transverse plane which cuts horizontally through the last lumbar vertebra. It is thus placed somewhat higher than k (Fig. 231, p. 394). While in the newborn infant it lies considerably higher ; be- tween the navel and the lowest part of the sternum (c, Fig. 231). 1320. Let us imagine that the man represented in Fig. 228 stands upright on a horizontal surface; without any burden, and with a sym- 392 MAINTENANCE OF EQUIPOISE. [CHAP. xv. metrical disposition of his limbs. If the centre of gravity is at G, the line of gravity will descend on G G. It thus strikes upon some point of that surface of support which is furnished by the soles of the feet. And so long as this condition is satisfied, the man cannot fall. FIG. 228. FIG. 229. dTT"^ Let us now suppose his back to be burdened with a heavy pack having a centre of gravity at s, Fig. 228. This burden alone would have the line of gravity s s'; and would therefore stand safely on the horizontal ground. But when it hangs on the back of the man while he is standing upright, the centre of gravity common to both is shifted further back- wards; namely, from G to g. Hence the line of gravity g g' falls upon the ground beyond the surface which supports the soles of his feet. The man will therefore be pulled backwards. But if he bows the upper part of his body as represented in Fig. 229, the common centre of gravity g is placed further forwards. And since g g 1 descends within the surface limited by his feet, the man remains standing. 1321. Just as a person carrying a burden on his back necessarily bends the upper part of his body forwards, so when he carries a consi- derable weight before him in his hands, he will incline himself backwards. Women advanced in pregnancy assume a similar attitude, because the anterior and lower part of the belly contains the enlarged uterus, the membranes of the ovum, and the foetus; forming in all a weight of about 11 pounds. And mechanical considerations teach that a man ascending a steep path must act as if he carried a burden on his back ; that is, he must incline the upper part of his body forwards. While when we hasten down hill, it is as if we had a propulsive weight hung in front: so that we extend the trunk backwards. The lateral curva- tures produced by carrying burdens on one side, together with the some- what oblique attitudes of men who have lost the greater part of one arm, are also explained by the same laws. CHAP. XV.] VERTEBRAL COLUMN. 393 1322. Passing on to the arrangement of the skeleton, \ve find that the body (a, Fig. 230) of each vertebra offers a broad surface of support (b) a kind of block. The laminae (h) form rings which enclose the spinal marrow and its membranes. The different processes such as the spinous process (c), the transverse process (d), and the oblique or arti- cular processes (e and g) complete the whole, and serve for the attach- ment of muscles. And some of them compose the joints on which the mobility of the vertebral column chiefly depends. FIG. 230. 1323. These vertebrae, which may be divided according to their various forms and situations into cervical (6, Fig. 231), dorsal (d), lumbar, sacral, and coccygeal, are piled one upon another like a series of discs. In this way they form the rod of the vertebral column, which is somewhat curved, and is moveable within certain limits. Under the most favourable circumstances, the head (a, Fig. 231) is self-poised upon the atlas, or uppermost cervical vertebra. But it frequently loses its equilibrium, and tends to fall forwards. This tendency is checked by the action of the cervical muscles. In old age these contractile tissues become weakened ; so that the head often falls forwards. 1324. The vertebral column (b d, Fig. 231) is wedged into the pelvis (k). This therefore forms a kind of basis of support, upon which the trunk can rest upright in the sitting attitude. But when a man stands upright, the legs (I m s), which sustain the pelvis, and hence the trunk also, form two columns, in the interval of which the line of gravity descends. 1325. The thorax or chest (c), which is composed of the ribs and sternum, is suspended in front of the vertebral column. It and its contents together form a burden which tends to flex the dorsal part of 394 VERTEBRAL COLUMN. [CHAP. xv. the vertebral column (d). In old age the relaxation of the soft tissues often leads to a curvature of this part. FIG. 231. [CHAP. xv. FORMS OF BONES. 395 1326. The arms (g h i) hang like two weights from the upper part of the trunk. Hence when placed symmetrically, they will not affect the lateral equilibrium. But if one upper extremity be brought into a different situation from the other, the distribution of the masses is altered, and their centre of gravity changes its situation. Upon these circumstances depend the alternate movement of the arms in walking or running, in passing along a narrow path, or a tight-rope, or in any other feat of balancing. 1327. We have already learned the advantages of the peculiar arrange- ment of the osseous substance ( 62). The other characters of the skeleton are adapted to its special requirements with equal nicety. This may be illustrated by a few examples. 1328. Bones which are destined to enclose other parts such as those of the skull (a, Fig. 231), thorax (c), and pelvis (k), are generally flat : while those in which the circumstances better correspond with a columnar form such as most of the segments of the arm (g h i), and the leg (I m n) exhibit a more circular transverse section. The flatness of the shoulder-blade (e) is connected with the peculiar mechanism of the move- ment of the arm. 1329. The mobility possessed by numerous pieces is the chief reason why the skeleton consists of so great a number of bones. That of the adult consists of 218 pieces, besides the six small bones of the ear and the 32 teeth. Most of them are articulated with each other, so as to allow of considerable mutual movements. While many such as the sternum, the bones of the pelvis, and tarsus, are intimately united by tissues which only permit a very small amount of gliding move- ment. The sutures (above d, Fig. 66, p. 122) by which the numerous bones of the skull fit into each other only allow still smaller displace- ments. This is also the case with the teeth, which are fixed like nails into the cavities of the jaw. But all these modes of union have the advantage of permitting slight movements, which mitigate shocks, and thus assist to ward off the danger of fracture. 1330. The enlargement of many bones in the neighbourhood of joints not only increases their articular surface, but with this, the arc of their possible movement. And the tubercles and other eminences here met with often elongate the arm of a lever on which the muscles act. Assuming that the lowest of the two bones represented in Fig. 232 rotates around a centre at c, a muscle attached at d would act upon the arm of the lever c d. But if we now add the protuberance fe g, the force will be enabled to use the longer arm c e. In this way the common extensor tendon g (Fig. 233) of the leg is attached to the patella h ; and the ligamentum patellae i which comes off from this is fixed to the tuberosity of the tibia (m, Fig. 231, p. 394). Hence the arm of the lever hf, which is acted upon by the extensor tendon of the 396 IRREGULARITIES OF THE BONES. [CHAP. XV. leg, has more favourable mechanical relations than if the tuberosity of the tibia / were absent. FIG. 232. 1331. The convex or concave parts of the bones furnish extensive surfaces, which are capable of giving attachment to a larger number of muscles. We shall hereafter see that, other circumstances being equal, the strength of the muscles increases with the number of their fibres. Hence they are thus rendered capable of effecting a more powerful traction. 1332. The hardness and roughness of the osseous substance render it necessary that its articular surfaces should be provided with proper coverings, such as cartilage and other dense tissues. Where requisite, fat and solid fibrous discs are interposed, so as to form elastic cushions. The whole joint is intimately united by means of ligaments. We have already seen ( 886) that the synovia furnished by the synovial mem- brane fills the superfluous space, and diminishes the amount of fric- tion ( 80). And the way in which the external atmospheric pressure diminishes the burden of the joints has also been ( 96) explained. 1333. The various joints exhibit very different degrees of mobility. The cartilages which are interposed between the vertebra? (as shown by the median longitudinal section in Fig. 234) contain fibrous layers, which may be bent or stretched according to the attitude. When the bodies of the vertebrae a and b meet anteriorly, so that the corresponding part of the vertebral column becomes concave, / and g are folded inwards, while i and k are put upon the stretch. While bending the vertebrae backwards will give rise to the contrary result. ' 1334. Many of the more perfect joints, such as those of the articular processes (e and g, Fig. 230, p. 393), only allow of slight gliding move- ments, and very limited rolling ones. Others are essentially hinges, which allow of movement in but one direction; such as obtains in the flexion and extension of the fingers. A similarly limited function also CHAP. XV.] JOINTS. 397 occurs in the elbow-joint ; and under certain circumstances, in the knee. The former exhibits the further peculiarity that the olecranon process (c, Fig. 240) is locked into a depression of the humerus (a) ; so that, even in extreme extension, the upper and fore-arm can only form a straight line. FIG. 234. FIG. 235. 1335. The enarthrodial joints, such as those of the hip and shoulder, allow a very free movement. The spherical head of the femur (g, Fig. 1 1, p. 35) rotates through a considerable arch in the acetabulum (i). The head of the humerus (/, Fig. 235) plays even more freely in the shoulder-joint. 1336. A dislocation is produced by the rupture of part of the soft tissues of a joint, so that the head of the bone emerges, and is driven into some other place ; both by the blow which produced the injury, and by the tractile force of the muscles ( 1071). The operation of reduction is an attempt to restore it to its previous position. 1337. Most of the powerful muscles are not immediately attached to the passive organs of loco- motion over which they preside, but are connected with them by intervening tendons. We have already (61) seen that the substance of these is endowed with all the qualities necessary for such cords. 398 TENDONS. [CHAP. xv. 1338. The end of each muscular fibre is connected with a smaller tendinous bundle. Hence the entire transverse section of the tendon is much smaller than that of the muscle. This secures the advan- tage of a smaller surface of attachment, and therefore of a smaller size of bone. In this way the biceps muscle of the arm (/, Fig. 236) con- FIG. 236. verges to the small tendon seen in the figure. This circumstance, together with the double action of which the muscles are capable, explains why they are often provided with tendons at both ends. 1339. With this is often associated another advantage, which may also be illustrated by Fig. 236. Powerful flexion of the finger demands strong muscles. But these could not be applied to the fingers or the hands without rendering such parts disproportionately large, and unfit for their more delicate functions. Hence nature removes the contractile tissues to the fore-arm, and conducts them downwards as the white cords CHAP. XV.] TENDONS. 399 or tendons depicted in Fig. 236. Both the usefulness and beauty of many parts of the body depend upon similar arrangements. 1340. We have already found ( 850) that the tendons have special sheaths, which allow them to glide to and fro without much loss of power. But since the muscles cannot always be applied in the directions in which their tendons are intended to act, other structures are often interposed as pulleys. A fatty cushion (b, Fig. 237) effects this change of direction FIG. 237. for the straight muscles of the eye. While the tendon of its superior oblique muscle c passes through a cartilaginous pulley e, in order that the action of its muscle in the direction of c e may draw it in that of/ e. 1341. The force exerted by the muscles at the instant of their contrac- tion will of course vary with the state of their nutrition, and the amount of stimulus which they at the moment obey. But since many muscles are attached obliquely to their tendons, and these are not fixed at right angles to their bones, a certain quantity of force will be lost ( 1314). Hence the work really effected must be less than that performed by the muscles themselves. 1342. The diagram at Fig. 238 may serve to illustrate the chief cir- cumstances here concerned. In this figure, a I, e d, cf, g k, &c. represent the straight fibres of a muscle ; while ae, ec, gi, il, are the small intervals between the several fibres, and c g those between the bundles of fibres. Both are filled by areolar tissue ( 448), which in no way con- tributes to the muscular contractions. 1343. Every isolated muscle exhibits a natural length, which corre- 400 FORCE AND EFFECTIVE ACTION OF MUSCLES. [CHAP. XV. spends to its elasticity. But if it be suspended above, and laden below with the weight n, the elastic resistance of its fibres will be overcome, and they will be extended to a certain amount. The lengths of this artificial extension, a b, e d, &c., will ex- ceed the natural, aq, Ir, by a certain amount. 1344. When the muscle laden with n con- tracts to op, the diminution of its length amounts to 06; or, compared with its former extension, to o b-r-a b. But the weight n is raised to this height, o b. Hence the height to which a weight is raised would express the diminution in the length of any contracting muscle which consisted of straight fibres. 1345. If we suppose that the whole of the muscular fibre a b consists of particles having an equal activity, the height of elevation will be directly proportionate to the length of the fibres. If a fibre which is two inches long contracts to half this length, we get an elevation of one inch. But, other circumstances being equal, this would amount to two inches for four inches of extended length. Hence we say that the amount of contraction of a muscular fibre depends upon its length. 1346. At the instant of contraction every cylindrical filament of a muscular fibre developes a certain force, corresponding to a definite load. This force will necessarily increase with the number of filaments contained in each fibre. If we imagine two muscles to contain the same proportion of areolar tissue, and fibres of equal thickness, the number of these latter will be proportionate to the transverse sections a I, q r, b m, (Fig. 238). Hence the forces excited by corresponding degrees of con- traction are proportional to their transverse sections. We may therefore refer our estimates of force to some unit of transverse section : as, for instance, a square inch. 1347. The effective action of a muscle is due to two causes; the amount by which it is shortened in a given time, and the force which is then developed. The product of both these, reduced to an unit of time, is the mechanical momentum or dynamic unit; and it therefore forms a quantity which is a function of these two elements. For example, if a muscle four inches long contracts to half this length in one second of time, and raises a pound for each square inch of its transverse section, we have an effective action of 2 x 1=2 pounds, for one second of time, and one square inch of transverse section. 1348. The shortening is a function of the length, the force is similarly related to the transverse section, and the effective action is the product of both these magnitudes. But since the volume of a body is the product CHAP. XV.] EFFECTIVE ACTION OF MUSCLES. 401 of its length by its mean transverse section, it follows that the effect is proportional to the capacity. If the specific gravity is constant, the weights vary with the volumes. Supposing this to be the case with the muscles, other circumstances being equal, the effects will correspond to the weights. The general theory may therefore be thus stated: the length of the fibres determines the amount of shortening; the transverse section of the muscle, the force; and the weight or volume, the effective action. 1349. But the actual occurrences are far from offering an exact cor- respondence with the suppositions upon which this statement is based. In the first place, it may be questioned whether one part of a longi- tudinal filament (a 6, Fig. 238) always shortens as much as another. Under low powers of the microscope it may frequently be seen that, with heavy burdens, this is not the case. This renders the amount of shortening a variable quantity, from which no simple relation to length can be deduced. Even comparing the different effects of one and the same muscle, the number of fibres in an unit of its transverse section the fundamental condition of the force will only remain unaltered when their watery contents, their extension, and their form, are unchanged, And comparing different muscles with each other, we meet with impor- tant original differences. One muscle has a larger proportion of areolar tissue than another. Its several fibres do not always exhibit the same thickness : a condition which may again give rise to differences in the relative quantity of its surrounding tissues. And since the given trans- verse sections diminish with the extension, we have here a new source of difference. Indeed it is possible that this circumstance may interfere still more actively. Supposing every molecule of the transverse section of a fibre to exhibit the same amount of force, this would vary directly as the square of the diameter. But it may be questioned whether the fibres are not so constructed, that the forces of their peripheric atoms differ from those of their central ones ; while the relative number of both differs with the diameter of the fibres. In this case a transverse section of the same muscle would include dynamic agents of unequal value. And equal transverse sections of two different muscles always form unequal magni- tudes, which cannot be directly compared with each other. 1350. Hitherto we have supposed that the muscular fibres take a direct and parallel course (a b, e d, &c. Fig. 238), and are acted upon by the weight (ri) at right angles. But when, on the contrary, they are attached obliquely to their tendinous substance, / b c, like bd and e (Fig. 239), they everywhere form variable quantities, the values of which defy calcu- lation. Under such circumstances the degree of elevation from c is only a part of the true diminution in length, and of the force really deve- loped by b d and e ( 1314). Since the several fibres of almost all mus- cles exhibit unequal lengths, and unequal angles of attachment, the most D D \r 402 MAXIMUM ACTION AND SHORTENING OF THE MUSCLES. [CHAP. XV. careful research can only furnish vague averages. While a longitudinal or transverse unit of such a mass of muscle allows no definite comparison with a similar unit of a muscle composed of straight fibres. 1351. In practically working out such ex- periments, we are met by new difficulties. Since the action of a muscle may vary between a minimum which may be regarded as =0, and a maximum, our first object is to determine the value of the latter. But this depends, not merely on the nature and amount of the stimulus applied to the muscle, but also on the degree of its susceptibility, and on the molecular constitution of the par- ticular muscle. Hence we can never reckon upon any maximum which might not be ex- ceeded by the results of other and more favourable circumstances. 1352. In making such an attempt with the entire limb of an animal, many cir- cumstances render it impossible to calculate the result. Such are the number of the muscles, the positive and negative amounts of the antagonist contractile masses, the oblique attachment of the muscular and tendinous fibres, and finally, the changeable phenomena of their leverage. On these grounds, even the simplest investigation of this kind, which concerns only the flexors of the head, the flexors or extensors of the finger or toes, or the muscles of the calf of the leg, is utterly unsatisfactory. While when we experiment on an isolated (and straight- fibred) muscle of a frog, which owing to its tenacity of irritability is preferable to that of a mammal ( 1261), there is little doubt that its action is diminished by the previous wound, by the gradual death of the muscle, by the evaporation from its surface, and perhaps also by the abnormal influence of the air. Besides this we shall here- after see that even weights not merely extend the muscle, but are also able to withdraw a certain amount of its vital activity. 1353. Hence all these observations can only prove certain general propositions. Their details merely serve as illustrations, and not as definite numbers to which any higher significance may be ascribed. 1354. The hyoglossus of the frog, and some of its femoral muscles, such as the sartorius offer the advantage of having muscular fibres which are straight and parallel in at least the greater part of their course. Hence there is no need of correcting their results by any hypo- thetical calculations. When laden with a small weight such as half a drachm they sometimes contract under the influence of the electro- CHAP. XV.] AMOUNT OF SHORTENING OF THE MUSCLES. 403 magnetic machine so powerfully, that their length is lessened by more than fths or fths. The formation of zigzags ( 1272) does not render this great amount of contraction impossible. An excised muscle, the length of which has been diminished more than one half by these elastic foldings ( 1273), may lose an additional fourth under the influence of the galvanic current. Now since the fibres become straight during con- traction ( 1275), it follows that the shortening here amounts to jths of the original length. 1355. Most of the living muscles are incapable of contracting to their maximum ; since, prior to this, they are checked by the levers on which they act. It is only some of those which surround tubes, such as the pharynx and oesophagus (g to s, Fig. 71, p. 127) or the sphincter ani, which are more favourably situated in this respect. 1356. The amount of contraction effected by a muscle generally decreases with the increase of the weight by which it is laden. A hyo- glossus muscle which contracted an inch under a burden of 31 grains, contracted but '72 inches under one five times as heavy, and only '063 under 10 times, '024 under 15 times, and '004 under 25 times, the first weight. Hence a maximum of burden permits but a minimum of shortening. 1357. Alteration of the weights may operate in two ways. The heavier weight obviously elongates the mass of muscle. The extension ( 1343) of the hyoglossus adduced as an example amounted to 1'33 inches with 31 grains; and to 1'59 with 5 times, 1'75 with 15 times, and 1-99 with 30 times, this weight. Hence the degree of shortening ( 1344) is determined by the amount of extension and of contraction; or by the stretching, the elasticity, and the vital contraction. In addi- tion to this, much of the susceptibility is destroyed by too heavy a burden. If a heavy weight be allowed to fall on the scale represented in Fig. 218, (p. 380), so that the rapidity of its descent increases its force of traction ( 66), the contractility of the most vigorous muscle may be annihilated at once and for ever. 1 358. The amounts of force may be considered under two forms : viz., the force of counterpoise, and the maximum force. By reducing both to units of transverse section, we may perhaps compare the corresponding numbers offered by the same kind of muscle. 1359. Let us suppose that a q, Fig. 238, is the natural and original length of the muscular fibre when laden with no weight, when the weight n elongates it to a b, the increase q b is due to extension. If the muscle then contracts to the length a q, it is obvious that the shortening has removed the extension. Now the muscles become more elongated, and less shortened, the greater the forces of traction which are applied. The force of counterpoise is determined by the weight which extends the quiescent muscle just as much as it contracts under D D 2 404 FORCE OF COUNTERPOISE. [CHAP. XV. the influence of its vital activity. This mode of regarding the action of force was originally introduced by Edward Weber. 1360. In this way the hyoglossus of a frog exhibited 41bs. 3| oz. for ^ths of a square inch of transverse section, and 39 per cent as the amount of shortening; the sartorius, 2 Ibs. 6| oz., and 18*4 per cent; and finally the gastrocnemius,- the fibres of which pass obliquely (Fig. 239), 2 Ibs. 10 oz., and 10*4 per cent. Hence we see that the force of counterpoise increases as the corresponding length of extension decreases. These relations are explained by the disturbing effects of the latter. The burden which produced the state of counterpoise was from 30 to 32 times as great as the weight of the hyoglossus or sarto- rius : and 47 times as great as that of the gastrocnemius. 1361. It results from the examples already adduced ( 1356), that the weight and the amount of shortening have no simple inverse ratio to each other. The former may be greatly increased, while the latter is less decreased. Enormous weights still allow of minute contractions. So that, by taking a minimum of shortening, we get a corresponding maximum of force. Where the former is so small that it may be dis- missed without any appreciable error, it presents this advantage : that the force need not be previously reduced to an uncertain transverse section, but to units of weight ; so that it can be directly compared with the weight of the active muscle. 1362. We may assume '004 inches to be the minimum contraction of the frog's muscle made use of in such experiments, an estimate which is equivalent to about -^th Jrd per cent of its length. Under the most favourable circumstances it was found that a square line of the hyo- glossus overcame a weight of 2427*3, and the same unit of sartorius 3874'!, grains. In the first case, the weight raised amounted to 1092 times as much as the muscular mass, and 1216 times in the last. Under the most favourable circumstances, the gastrocnemius raised more than 2800 times its own weight. And if we consider that such powerful traction greatly damages the irritability of the muscles, especially of those with straight fibres, it will follow that even these extraordinary quantities are rather too small than too large. 1363. Other observations will but confirm this conclusion. Thus, if we make use of an apparatus 40 ) in which the artificial extension pro- duced by the weight is avoided, and the frog's gastrocnemius is allowed to act with its natural mode of attachment, we find that it can raise from 16 to 17 thousand times its own weight through a minute distance. Here a square line of transverse section corresponds to from 4Jrd to 5Jth pounds, and from -1 to -3 per cent of shortening. And since these muscles were separated from the rest of the animal's body, it is pro- bable that their effect was always less than what they would have accomplished in the living animal. CHAP. XV.] AMOUNT OF EFFECTIVE ACTION. 405 1364. If the amounts of shortening and the weights had a simple inverse proportion to each other, the effective action ( 1347) would always remain the same. But we have already seen ( 1361) that the weights increase more than the lengths decrease. It is true that a muscle which only sustains small weights contracts more strongly. But the greater decrease of length is incapable of compensating the diminu- tion of weight beyond a certain limit. Hence the maximum of effective action will coincide, neither with that of the shortening, nor with that of the weight, but with a certain mean of both. For instance, the hyoglossus of a frog exhibited the following results : Weights in grains. Extension in inches. Elevation in inches. Effect in grains to an inch. 30-89 1-33 1-02 202-4 154-44 59 72 717-9 308-88 75 063 125-5 463-32 87 024 70-6 61776 91 012 47-1 772-20 95 078 39-2 926-64 1-99 039 23-5 So that the maximum of effect coincides, neither with the maximum contraction of 1-02 inches, nor yet with the maximum weight of 926 grains, but with one of the intervening observations, which offers a weight of 154 grains, and a height of -72 inch. At the same time we see that the heaviest weights give the smallest results when reduced to a dynamic unit. 1365. The same conclusions may be deduced from our own actions. It is true that we can raise a heavy weight through a small space during a short time ; but fatigue renders it impossible to continue or repeat such a task. In order to use the muscular force of an animal most effec- tively, we select moderate weights, such as will not produce too great exhaustion. This not only enables us to obtain the greatest dynamic effect, but also allows of the action being prolonged for many days with out permanent exhaustion. 1366. Under abnormal circumstances, the convulsive contraction of muscles sometimes ruptures the strongest tendons ( 50). And even ordinary muscular actions presuppose very considerable force. But the effects accomplished are far short of the maxima which we have found in the dead muscles ( 1362, et seq.). This difference depends upon two causes. Even the contraction made use of to overcome heavy weights never rises to the artificial maxima which are produced by the rapid shocks of the electro-magnetic machine \ for these would exhaust irrita- bility much too rapidly. Besides this, most muscles act under circum- stances which waste a good deal of their power ( 1314) in obliquity of attachment, or in unfavorable leverage. 406 LEVERAGE OF MUSCLES IN THE LIVING BODY. [CHAP. XV. 1367. Both the shapeliness and use of the muscles require many of the fibres to be fixed obliquely to their tendons or aponeuroses. This statement also applies to the attachment of tendons to the bones and other passive organs of locomotion. And not only is a great amount of force thus lost, but, for obvious reasons, unfavourable leverages are selected. 1368. We have seen (1313) that levers of the third kind in which the force acts upon the shorter, and the weight upon the longer, arm imply the greatest exertion of force; while those of the second kind in which the contrary obtains require much less. It might thence be supposed that Nature would everywhere make use of this latter advantage. But an examination of the organs of locomotion will show that the levers in which force is thus favoured occur less frequently than those in which it meets with greater difficulties. The reason of this will be evident from the study of a single example. The centre of gravity of the fore-arm and extended hand lies at about |ths of the distance from the elbow-joint towards the point of the middle finger. But let us suppose that a man holds a weight (w, Fig. 240) in his flattened hand. The centre of gravity common to the weight, the hand, and the fore-arm, then lies at d; so that when the fore-arm moves upon the lower end of the humerus in the elbow-joint, around the centre a, that arm of the lever which corresponds to the weight (1313) amounts to d a. But the tendon of the biceps muscle (/, Fig. 236, p. 398) which bends (and, in our example, raises,) the fore-arm is attached at b : so that the shorter arm of the lever, a b, corresponds to the force. Hence the effect is reduced to the quotient of a b divided by a d ( 1313). 1369. The diagram (Fig. 241) will at once explain the reason of this. In the first place, if the muscles or the force belonged to the longer arm CHAP, XV.] LEVERAGE OF MUSCLES IN THE LIVING BODY. 407 a c, and the passive organs of locomotion or the weight to the shorter one b c, very unsuitable forms and arrangements would result. And since the arc produced by the contraction of muscles is determined by the shortening of their fibres, they would have to contract to an amount FIG. 241. corresponding with the longer line a d, in order that the centre of gravity of the weight should describe the smaller arc b c. Hence nature selects this kind of lever when the circumstance just mentioned allows force to be economized without any disadvantage. But as this is not usually the case, the arm b e is made that of the power ; with an obvious waste of a certain amount of muscular force. The quantity of this ( 1363) is, however, amply sufficient to sustain the loss. So that a muscular contraction which only amounts to be, can thus give rise to the more extensive movement a d. 1370. From all this it is evident, that the muscular fibres contain a substance which, at the instant of its shortening, furnishes a powerful, tractile force. Hence large quantities of force may be sacrificed to the perfection of a particular arrangement without destroying its efficiency as an apparatus of motion. 1371. General Movements of Man. We have already seen that the cervical muscles are generally required to keep the head upright ( 1335) ; and the dorsal, to preserve the extension of the back ( 1325). Their aid also suffices to the sitting posture, in which the pelvis furnishes the immediate basis of support. But that of standing requires additional muscular actions. The legs consist of a series of pieces united to each other; and hence they bend of themselves when any weight is placed upon them. The large glutei muscles, which occupy the outside of the pelvis (k, Fig. 231, p. 394) must therefore contract, to prevent rotation at the hip-joint; and the extensors of the leg ( 1330), to hinder flexion at the knee. And as the leg (m, Fig. 231) cannot be spontaneously retained perpendicular to the foot (s, Fig. 231), muscles are applied here also to secure the necessary solidity. This explains why a person whose muscles are exhausted by fatigue, or paralyzed by abnormal in- fluences, falls together in a heap when the support of the body is thrown upon his lower extremities. The dead body will obviously present similar phenomena. 1372. The surface limited by the two soles of the feet forms the basis of support of the human body in the standing posture. Consequently 4:08 STANDING. [CHAP. XV. the body only falls, when the perpendicular line of gravity ( 1316) drops beyond these limits ( 1318). When a person raises one leg, the surface of support is diminished by more than half its breadth; since it now consists of the sole of one foot only, while it was formerly the surfaces of both feet, together with the variable space left between them. And, in order that he should not fall, he must bend the upper part of the trunk towards the side of that leg on which he stands; otherwise the unsup- ported half of the body preponderates, and. pulls down the whole towards its side ( 1320). And this change of posture must be carried so far, as to allow the common line of gravity to fall upon that limited surface of support which is furnished by the sole of one foot. 1373. These conditions are easily satisfied. Indeed, we shall soon find that the ability to walk and run depends upon such adjustments. But since the surface of support is extremely small, and the muscles which keep the active leg extended are laden with at least twice their former weight, the attitude of standing on one leg can only be maintained for a short time. Fatigue produces oscillations, which at first favour counterpoise, but afterwards falling. 1374. In a person possessing only one leg, we find a second disad- vantage. When a man raises one of his two feet, he can hold the lower extremity in such a position, that the centre of gravity of the whole body is shifted but little upwards. But where the thigh has been amputated in the upper third of its length, the great loss of substance below this point has considerably raised the centre of gravity. Thus while in an entire corpse the horizontal plane of gravity falls between the navel and pubis, it is raised to the navel by the removal of one leg, and to the lower extre- mity of the sternum (c, Fig. 231, p. 394) by that of both. Hence in the upright attitude, the centre of gravity of the maimed person will be higher than that of the uninjured subject. But since, other circum- stances being equal, a body falls over more easily, the further its centre of gravity is removed from the basis of support, a one-legged man will fall more easily than a person who is standing on one of his legs. The difficulty of preserving the attitude will vary with the shortness of the portion of limb which remains. And though, in spite of this, such persons are often able to stand alone for some time, stih 1 we must not forget how greatly habit assists them in discovering the necessary com- pensative attitudes of the upper part of the body. 1375. In man, progression chiefly depends upon a definite and alter- nate play of both legs; one of which is always sustaining the weight of the body, while the other is moved onwards. Hence the former is called the sustaining, and the latter the progressive or swinging limb. In the course of a complete or double step, both legs take up these functions alternately. 1376. Supposing the right lower extremity forms the column of sup- CHAP. XV.] WALKING. 409 port, the pieces of bone which compose it will have the attitude repre- sented by Im s (Fig. 231, p. 394) ; while the left, which has to swing, is placed further backwards. This now raises the greater part of its sole from the ground, or as it is usually expressed, rolls it off, and thus moves from somewhere about s to o p q r. It then impinges against the surface of support only by the toes and the ball of the foot, and thus acquires a greater length, which, although diminished by a simultaneous flexion of the knee-joint (over n), still always suffices to push the left half of the upper part of the body somewhat forward, while it allows the right to remain to a certain extent behind it. If the left leg, which hangs like a pendulum from the hip-joint, now swung forwards, its length would soon make it strike against the ground. Hence a flexion of the knee and hip-joints shortens its straight length. When it has com- pleted its swing, it is extended so as to reach the ground, on which it treads by the entire sole of the foot, s, to take henceforth the part of the sustaining limb. But the arc through which it has swung has caused it to precede the other leg, so that the latter now forms the limb of progression. 1377. From the circumstances mentioned in 1371, it follows that the sustaining extremity must be extended so as to form a column. The straight line formed by the thigh and leg (I and m, Fig. 231, p. 394), is inclined backwards so long as the progressive leg remains behind, and forwards after it has advanced. Its upright posture occurs in the middle of the swing of the advancing limb. 1378. If we consider the mode in which the upper part of the body is supported in the course of a double step, we shall find that it is succes- sively sustained, (1.) by both legs with a gradual predominance of the first supporting leg; (2.) by the first supporting limb alone, while the second one swings; (3.) by both legs again; and (4.) by the second supporting leg, while the first steps forwards. But since both feet co-operate in the act of standing, 1 and 3 form the instants of standing, and 2 and 4 those of progression strictly so called. Hence, other circumstances being equal, a person will advance more slowly, the more 1 and 3 predomi- nate. While if the times occupied by 1 and 3 are diminished almost to nothing, we get movements of progression which approximate to those of running. 1379. The velocity of progression may be diminished not merely by prolonging the times of standing, but also by other means. If, for instance, the progressive leg swings further forwards than where it is subsequently deposited on the ground, a certain amount of time is lost in its backward swing. Hence those methods of walking in which this occurs are not the most advantageous. Or if a person sinks into soft ground at every step, the necessary raising of the body leads to another loss of time. From similar reasons, persons who limp advance but slowly. 410 RUNNING. [CHAP. xv. In such abnormal cases curved or slow movements of the progressive leg are sometimes associated. 1380. The act of running consists in an union of the movements of rapid walking with those of leaping. The body of the walker is always supported on one or both legs. The leaps which are connected with the act of running form intervals of time, during which the entire body is free in the air. It therefore oscillates greatly in the vertical direction. If the instants of leaping are diminished as much as possible, we get a rapid run, the steps of which are shorter than those of the leaping run. In this case the supporting leg loses as little time as possible in its action, and prepares beforehand for its swing. 1381. The leg, which is held fast in the hip-joint by the external pres- sure of the atmosphere ( 96), may be regarded as a pendulum suspended thence, and having a swing the duration of which is determined by its length. It was found by W. and Ed. Weber, that the time required for a single step in the quickest possible walking almost exactly corresponded with that of half a pendulum vibration of the leg; amounting on an average to '357, while the latter was -353, seconds. A slower walk of course gives' greater amounts of time. According to Weber, in the act of running they are also increased; and more by the leaping, than by the rapid, run. 1382. Let us suppose that the elastic rod planted upon the ground ef (Fig. 242), has been bent into the shape bed, either by a weight a, or by any other force. On removing a there will be an FIG. 242. elastic reaction (51) of the particles of b c d, which will act through d upon the ground ef with the same amount of pressure a that was formerly exerted. But since the immovable ground reacts with a force equal to that which strikes it, the rod bed springs up, as soon as these reactions overcome its gravity. Its gravity subsequently causes it to fall back, like any P other propelled body on the loss of its active projectile force. 1383. The act of leaping depends on phenomena which are essentially the same. However various its several movements, they all coincide in the fact, that a greater or smaller portion of one or both legs is flexed, and then suddenly extended. Here the rapid and powerful straightening of curves exerts an influence similar to the elastic reaction of a curved rod (b c d, Fig. 242). And what has already been stated explains why a man easily springs from the solid ground, and even more forcibly from a stretched elastic rope. A previous run furnishes an ultimate velocity, which is capable of assisting to maintain the impulse of the leap that immediately follows. In all these cases, the arms may be used as regu- lating pendulums ( 1326). CHAP. XV.] SWIMMING AND FLYING. 411 1384. In creeping and climbing, the movements of progression are assisted by the upper extremities. Here the limbs are used in a way which somewhat reminds one of the four-footed animals. In creeping, we seek to advance along a more or less horizontal surface ; and in climbing, up a perpendicular or steep one. In both, we first fix the arms, and then draw the rest of the body after them. When this has been done, the legs form the means of attachment, while the rest of the body is propelled forwards. 1385. The act of swimming depends upon the resistance which water furnishes when it is powerfully struck without being displaced to a corresponding extent. Here again progression is based upon circum- stances similar to those of the act of leaping. The sudden extension which follows flexion, and the rotation of the arms in a curve backwards, act like oars, which, striking the water with their broad surface, make the boat itself move forwards in the opposite direction. And just as an oar ought to be feathered i. e. brought back edgewise, so as not to cause an equal counter-movement so the arms of the swimmer are brought forward again, bent against each other so as to occupy the smallest pos- sible space. Since even after deep inspiration, the specific gravity of the human subject is generally greater than that of water ( 41), small nata- tory movements are required as soon as the greater part of the body is under water. 1386. Most of our fresh-water fishes use the tail as a kind of steering oar. Here the stroke upon one side urges the animal forwards and towards the opposite one. Rapid successive strokes on each side alter- nately may produce a more rectilineal movement forwards. But they often give rise to small lateral deviations or a zigzag path. The other fins rarely afford more than a subordinate help to the movements pro- duced by the tail. The gases contained in the air-bladder improve the mechanical momenta of the whole fish, as well as the mutual relations of its anterior and posterior parts. 1387. The act of flight is essentially a swimming in the elastic fluid medium of the atmosphere. The body of the bird is pervaded by many sacs, which are filled with gases and which, proceeding from the lungs, even penetrate the interior of many of the bones. Since these parts there- fore contain warm air, the specific gravity of the whole animal is much less than what it would be if they were replaced by liquids or solids. But even were the air-sacs absent from the bones, their cavities would be filled with vapour where they contain no fat. Hence the large cavi- tary bones of the bird's skeleton greatly lighten its body, while the air- sacs which occur in many of them do not do so. The act of flight itself is accomplished, not by them, but by vigorous strokes of the broad expanded wings, and by the resistance of the atmosphere. Indeed, many animals which possess the power of flight possess neither air-sacs nor 412 DYNAMOMETER. [CHAP. XV. any structure which can be compared to them. And even the larger sacs may be opened without destroying the ability to fly. And however obscure the function of these organs, still it may be conjectured that their use is more closely related to respiration than to flight. Prechtl supposes that the air which they contain recedes towards the lungs, and thus assists in ensuring the continuance of respiration under many unfa- vourable circumstances, such as prolonged singing, diving under water, or flight in the rarefied air of the higher regions. This observer further conjectures that, when greatly distended with air, they may serve as fulcra for the action of the neighbouring muscles. 1388. Actions of Man as a machine. The action of the muscles furnishes a certain force of pressure or traction, which is capable of acting on foreign substances, or on the tissues of the body itself. For technical purposes, attempts have frequently been made to determine certain averages of the strength of man and animals, and to discover, by the aid of a mathematical analysis, rules which may be applied to practice. 1389. Regnier's dynamometer, which is often used to determine the maximum pressure of the human hands, or the force of draught pos- sessed by beasts of burden consists of the spring balance represented in Fig, 243. The anterior plate which covers it has been purposely omitted FIG. 243. from the woodcut in order to allow an easier inspection of the whole. When the two hands press together the elastic spring plates, a c and b d, CHAP. XV.] TRACTILE FORCE OF MAN. 413 FIG. 244. the angular lever g h i k pushes the index I m through a corresponding arc, which is given on the scale e. And if this is also graduated with units of weight for each degree, we may at once read off the number sought. 1390. The little reliance that can be placed on most spring balances, and the dependence of the result upon the skilfulness of the grasp, the .suitableness of the muscular movements, and the rapidity of the im- pulse, make the results of these experiments but very approximative and variable. Besides this, it is evident that they only exhibit the maximum of an instantaneous effort. 1391. According to Quetelet, an adult man can exert a pressure of about 66 to 991bs. with his right hand, from 57 to 901bs. with his left hand, and from 123 to 196 with both. Under similar circumstances, women can exert a pressure of 48 to 55, 42 to 48, and 99 to llOlbs. respectively. 1392. In order to determine the tractile force of man, which is greatly affected by extension of the trunk, we make use of the arrangement represented in Fig. 244. The man treads upon a fixed iron plate a b c, from which proceeds a staff provided with hooks. One of these receives the curved part of the dynamometer a c, (Fig. 243). The second curved segment 6 d y (Fig. 243) hangs from the draught-hook ef, (Fig. 244). Hence the action here takes place in the direction a b, (Fig. 243). The index I n gives the corresponding amount of force on the second scale, /. In order to find the tractile force of a horse, the dynamometer is similarly interposed between the traces arid the waggon. 1393. According to Quetelet, an adult man averages 205 to 342 Ibs. in this way, and the female 130 to 170. Forbes ob- tained much higher numbers for strong men of various provinces of Great Britain ; Englishmen from 20 to 25 years of age gave 366 to 3841bs., Scotchmen 373 to 404, and Irishmen 397 to 41 3 Ibs. 1394. From experiments made .at Bern it resulted that powerful men addicted to gymnastics raised with both hands a weight of 364 Ibs. about 2 feet in height. 1 395. Even a person walking unladen exerts himself to a certain extent, since he sustains the weight of his own body for a certain distance at some definite average velocity. Let us suppose that he 414 EFFECTIVE MECHANICAL ACTION OF WORKMEN. [CHAP. XV. weighs 1321bs. ; and makes 125 steps, of 31*5 inches, in the minute. His velocity per second will thus amount to 125 x 31-5---60=65-6 inches. This makes about 236160 inches, or 3| miles, in the hour. If he can continue this movement about eight hours per day, it will give a daily result of 132 x 3'75 x 8=39601bs. for one mile. Under such circumstances, the French engineers estimate the ordinary result at 4826*25 of these dynamic units. 1396. When a person has to carry a burden, it, and the fatigue which it causes, consume a part of the velocity and effective action. For instance, according to Morin, a man who carries a weight of 88 Jibs, on his back, presents a velocity of 29*53 inches per second. If he works seven hours daily, we get a result of 88-25x29-53x60x60x7 = 5953248-r-12-r-3-r-1760=1036-51bs. for a mile. But since the weight of the body is 1321bs., the effective action will be (132 + 88-25) x 29-53 x 3600x7-r-63360* =2586 of these dynamic units, if the burden be borne during the whole period of movement. Hence .we only obtain an effect equal to about half that of the unburdened labourer. And, other circumstances being equal, it will be less in ascending a mountain or a flight of steps, than on level ground. 1397. The maximum of exertion may effect extraordinary results for short periods of time. For instance, while the ordinary military step ex- hibits a velocity of somewhat less than 39 inches per second, swift runners can accomplish from 5 to 9 times this speed, so as to equal or exceed the cavalry trot or gallop. The latter maximum velocity was offered by West, but was not sustained for an entire minute. In like manner, a strong man can raise a burden of 330 to 4401bs. for a very short space of time. But the exhaustion which renders a frequent repetition of this impossible prevents great effective results from being thus distributed over wider periods of time. 1398. It has been frequently attempted to estimate, in dynamic units, the average results of the different kinds of work. These are divisible into two chief classes one, in which the results consist in a horizontal movement; and another, where it is a perpendicular lifting. For in- stance, a labourer who carries 143-41bs. on his back for a certain distance, and returns empty-handed, represents an horizontal effective action of 960'51bs. for one mile, during a working-day of six hours: while a man who lifts 39'71bs. by means of a pulley, and allows the cord to fall back again, represents a perpendicular effect of 106-451bs. for a mile during the same period. But the great differences in the statements of different engineers sufficiently indicate that the estimates which we here start from are themselves subject to many variations, and are in many cases deduced from incorrect presumptions. * The number of inches in a mile. CHAP. XV.] MECHANICAL RESULTS OF LABOUR. 415 1399. The same statement applies to the formula by which it has been attempted to determine the force of labourers in particular cases. The rule laid down by Gerstner is that most frequently made use of. Here the quotient of the actual and average velocity is subtracted from 2; as is also the quotient of the times occupied by the work. The product of these two magnitudes is multiplied by the medium force in order to obtain the power for the particular case.* This method presupposes that the weights and the velocity of average effects rise and fall in inverse proportion to each other. But the accuracy of such a theory may be doubted on physiological grounds ( 1364). * Thus supposing a burden of 30 Ibs., carried 15 miles in 8 hours, to be a fair day's work, a man only walking 10 miles in that time could sustain (= 30 X (2 |4f) X ('2 f ) =) 40 Ibs. On the other hand, if he has to increase his speed so as to accomplish the 15 miles in 6 hours, the burden ought to be reduced from 30 to (30 x (2 |f) X (2 f ) =) 20 Ibs. EDITOR. CHAPTER XVI. VOICE. 1400. Those vibrations ( 157) of ponderable matter which exceed a certain minimum of strength, and propagate themselves to the ear, evoke corresponding sensations of sound. A single powerful concussion gives rise to a sound ; an irregular repetition of impulses to a noise ; and their rhythmical recurrence, to a musical sound. 1401. Two kinds of sonorous undulations are met with: curved and molecular waves. Let us imagine a b, Fig. 245, to be a string fastened FIG. 245. at both ends, and occupying, when in a state of equipoise, the position afdgb. If we take hold of it at d, pull it towards d', and then resign it to the operation of its own elasticity, it swings back towards a d b. The velocity thus attained then drives it towards the opposite side as far as d". It then returns again, and repeats these curved undulations, but with constantly diminished extents or widths of vibration, d' d", owing to the constant loss of active force by the communication of impulses to neigh- bouring bodies. When this is altogether lost, the string rests in its posi- tion of equipoise, afd g b. But the formation of sound ceases as soon as the movements lose the requisite strength and velocity. 1402. The compressive elasticity of the air ( 67) frequently leads to molecular undulations, in which particular portions of the atmosphere are alternately rarefied and condensed. For example, on blowing into a tube which is open at both ends the gas present in the middle is FIG. 246. at a particular moment condensed, while that at both ends is rarefied, as illustrated by Fig. 246. Subsequently the reverse obtains, as shown by CHAP. XVI.] HEIGHT OF NOTES. 417 Fig. 247: i. e. the greatest rarefaction is at the middle, and the condensa- tion at the ends. FIG. 247. 1403. The height of the note depends upon the number of vibra- tions which occur in an unit of time ; for instance, in a second. If we regard as unity the number of vibrations which correspond to the lowest note, the whole octave will be as follows : 9 sol Hence the next highest octave has twice, the third, e, 1^, and the fifth <7, 1 1 times, as many vibrations as the lowest note. The intensity of the musical sound is determined by the width of the vibration ( 1401). Its peculiar character or "timbre" is connected with the molecular con- stitution of the vibrating substances, and is as yet unexplained. 1404. The formation of sound in stringed instruments depends upon the vibrations of stretched elastic cords; and in the different tubular instruments, upon the concussions of the columns of air which they con- tain. In both, the height of the sound is essentially determined by the amount of tension, and the length of the active substances. The tongue of an instrument is a lamina which is partially fixed, but has a free segment that can be made to vibrate by an external impulse. The metal plate 1 1, Fig. 248, which covers the greater part of the opening abed, repeatedly approaches and recedes from this aperture, when a FIG. 248. FIG. 249. current of air is forced through it. If we fix two pieces of India rubber, or two elastic animal membranes, over the orifice of a tube, so that there remains a small fissure between them (Fig. 249), blowing into the tube causes these tongues to give out musical sounds. The application of a second tube over them produces a column of air which can greatly E E 418 THE LARYNX. [CHAP. XVI. modify the results. The pipes of an organ are the best illustration of such musical instruments. 1405. The vocal organ of man and the higher animals may be best compared with a tongued instrument. The larynx, which is represented from the front in Fig. 250, and from the side in Fig. 251, is chiefly formed by the cricoid a, and the thyroid cartilage b (Figs. 250, 1, 2). They represent the box in which the tongues are fixed. If the larynx be FIG. 250 FIG. 251, FIG. 252. regarded from above, as seen in Fig. 252, and if the epiglottis (which is represented by d in all three figures) be turned outwards, we see behind it the two arytenoid cartilages, c c, which are placed over the cricoid cartilage, a. We may further observe the two chief tongues, the inferior or proper vocal cords, e e, which extend from the thyroid cartilage, b, to the arytenoid cartilages, and leave between them a fissure, the rima vocalis, or glottis. This leads on one side into the trachea, which lies below the larynx, and on the other into the cavity of the larynx itself, which communicates with the pharynx (g, Fig. 68, p. 125), and through it with the cavities of the mouth (between c and d, Fig. 68) and nose (a, Fig. 68). Above the proper vocal cords, e e, are extended two folds, the superior or ventricular cords //, (Fig. 252), which are further from each other than the preceding. The depressions internal to these in the wood- cut are called the ventricles of the larynx, a name which explains that of the cords themselves, //. 1406. Just as the musician tunes his instrument by increasing or diminishing the tension of its vibrating strings, so something like this obtains with the tongues of the human larynx. Nature has placed here a series of small muscles, the contraction of which is capable of extending or relaxing the vocal cords. Many of them at the same time alter the width of the vocal fissure. We shall soon see that it is this arrangement CHAP. XVI.] MUSCLES OP THE LARYNX. 419 which allows of singing and speaking, and enables us voluntarily to raise or depress the sounds of the voice. The human being is thus enabled to tune his instrument at will. 1407. These small muscles of the larynx are represented of their natural size in Figs. 253 and 254. The crico-thyroidei (6, Fig. 253, a, Fig. 254) and the crico-arytenoidei postici (b, Fig. 254) extend the vocal cords in the direction of their length, and at the same time narrow the glottis. The crico-arytenoidei laterales (c, Fig. 254) and the thyro- arytenoidei (d, Fig. 254) rather relax the vocal cords. The oblique and FIG. 253. FIG. 254. transverse fibres of the arytenoideus (e and fg, Fig. 254) close the pos- terior half of the glottis. This part of the glottis is called the respiratory portion, because it remains open during ordinary breathing, but is closed during the exercise of the voice by the mutual approximation of the vocal cords. Hence in the latter case there remains but a narrow anterior fissure, which bears the name of the vocal glottis. The epiglottis (k, Fig. 254) forms a valve which can be brought over the glottis by fine muscular bundles attached at h and i ( 374). 1408. The action of the organ of voice, and the reason why it is fixed into the upper segment of the trachea, may be best explained by com- paring it with the pipe of an organ. Let us suppose tf, Fig. 255, to be the wind-tube into which the air is driven from below, b the stopper in which is placed the tongue a, and t the body-tube. We will now place such a pipe o (Fig. 256) in the wind-box, c c, and drive in air from the bellows, ffp, through t. The air throws the tongue a (Fig. 255) into a state of vibration, and passes out in undulating movements from the E E 2 420 TONGUE- APPARATUS OF THE VOCAL ORGAN. [CHAP. XVI. body-tube. It is obvious that something similar occurs in the organ of voice. The lungs, the air of which is carried upwards in the bronchi, form the bellows ; the larynx is the wind-box, in which are placed the tongues of the vocal cords ; and the tissues of the pharynx, mouth, and nose, are body-tubes of various shape, the influence of which is diffused over the tongue-apparatus. And as these parts were independently re- quisite for nutrition, respiration, and some of the senses, Nature had only to interpose the vocal box of the larynx in some fitting situation between the bellows and the body-pieces, to form a musical instrument without the addition of any new textures. FIG. 2.55. FIG. 256. FIG. 257. 1409. Many of the vocal operations may be verified in the dead larynx. For this purpose we may either fix the prepared head of a corpse as represented (after Mueller) in Fig. 257, or may cut out the larynx alone. A thread, e, which passes over a roller to a scale, is so applied to the larynx that the tension of the vocal cords can be in- creased by placing a greater weight in the scale. We thus imitate the action of the muscles ( 1406). The compressing apparatus seen in the wood-cut brings the vocal cords nearer to each other, and thus produces the requisite diminution in the width of the vocal fissure. The tube at / serves to convey the wind which throws the tongue-apparatus into action. By making use of the heads of the corresponding animals, we can imitate the voice of man, the barking of the dog, the grunting of the pig, &c. 1410. When the glottis is too widely open, we get a more or less in- CHAP. XVI.] COMPENSATIVE PHENOMENA. 421 distinct noise, and not a pure sound. The latter requires a certain nar- rowness of the glottis. This explains why the respiratory part is closed, and the vocal one narrowed, during singing and speaking aloud ( 1407.) 1411. Other circumstances being equal, the height of the notes is raised by increasing the weight which extends the vocal cords in the apparatus represented at Fig. 257. And conversely, the artificial relaxa- tion of these structures gives rise to lower notes. In this way a register of about three octaves may be produced. And even after all the parts which lie over the vocal cords (e e, Fig. 252) have been removed, these variations of sound may still occur. 1412. The strength of the wind injected at / (Fig. 257) produces two effects. It makes the sound more powerful ; and thus regulates the transition from piano through crescendo to forte. And it can also par- tially compensate an extension of the vocal cords. For instance, J. Mueller found that when these were not stretched by any weight, and the pressure of the injected air amounted to 1-97 inch of water, fis was produced ; while when the tension was six times as great, dis was sounded.* And conversely, within certain limits a more forcible extension of the vocal cords requires a smaller strength of wind to produce a note of the same height. These are called the compensative phenomena of the vocal organs. 1413. In the living animal, the lungs which drive up the air into the tongue-work of the vocal cords, may, in some cases, similarly assist to de- termine the height of the sound. But it is the vocal cords which essentially determine the production of higher or lower notes. Their clang or timbre is very different. This property of the sounds produced is greatly influenced by the vibrations of the elastic walls of the trachea and the bronchi, as well as by those of the remaining structures of the thorax and organs of respiration. 1414. The ordinary vocal sounds are produced at the instant of expi- ration. Hence the stream of air first causes the tongue-work to vibrate from below upwards (from k towards i, Fig. 128, p. 232). It always arrives here with an increased pressure. Cagniard Latour fixed a mano- meter ( 86) into a tracheal fistula of the human subject, and found that vocal sounds of medium strength gave a tension of '394 inch of quicksilver (compare 760). More powerful sounds would obviously lead to greater amounts ( 761). 1415. Expiration is not essential to the production of voice. During laughing, crying, yawning, and especially during sobbing ( 755), inspi- ration is accompanied by loud sounds. This fact may at any time be verified artificially. 1416. Experiments on the dead larynx lead to the conclusion that the * Corresponding to a rise from F sharp to D sharp, or nine semitones. Editor. 422 ACTION OF THE SEVERAL PARTS OF THE LARYNX. [CHAP. XVI. total register of the human voice can be produced by the inferior vocal cords alone. If we remove the whole of the larynx with the exception of the superior or ventricular cords and their neighbouring tissues, and if these be then mutually approximated, so as to leave but a small fissure be- tween them, they may give rise to sounds like any other tongues would do. But since in the living subject they are further from each other, and lie above the inferior vocal cords, we are justified in doubting whether they ever produce independent sounds, or whether they can in any case ex- clusively determine the height of a note. Lately Segond 41 ) has sought to prove that, in some mammals, the falsetto depends upon the superior, and the chest voice on the inferior, cords. Cats whose inferior vocal cords had been cut through at first entirely lost their voice, but eight days after were again able to mew : while injury of the ventricular cords permanently suppressed this vocal act. Dogs gave high or low cries of pain, according as their inferior or superior vocal cords had been rendered inefficient. The destruction of both led to a permanent loss of voice. But since the cats whose inferior vocal cords had been rendered useless did not immediately produce falsetto sounds, we are justified in entertaining strong suspicions of this theory, which attributes a divided action to these two kinds of vocal cords. 1417. At present we are unable to state how the ventricular cords act upon the clang of the voice ; or whether the ventricles themselves, which render the vocal cords freer, and facilitate their vibrations, play an important part in this respect. Mayer and Noeggerath state that during the highest sounds, the human epiglottis lies horizontally, with its lateral margins rolled up, Magendie and Biot believe that it permits us to swell the sound without raising it. In this case it would act like the elastic covering of Grenie, which is sometimes placed over the tongue of an organ-pipe. 1418. The cavities of the mouth and the nose form, as it were, a double or bind tube, which assists to determine the peculiar characters of the clang, This proposition is confirmed by a direct comparison of the sounds produced by the apparatus represented in Fig. 257, with those given out by the larynx alone. The difference between the ordinary and the nasal voice result from the manifold use which may be made of these two body-pieces. 1419. The vocal box formed by the larynx, frequently descends during low notes, and ascends during high ones. But in rare exceptions the reverse of this may obtain. Finally, in many cases it has no perceptible movement of any kind. Segond agrees with Dutrochet in supposing that the inferior constrictor of the pharynx (i, Fig. 71, p. 127) increases the tension of the vocal chords, when the larynx rises during the pro- duction of the higher notes. 1420. The bass, barytone, tenor, alto, and soprano voices, are distin- CHAP. XVI.] LIMITS OF THE SINGING VOICE. 423 guished from each other, not merely by the height, but by the clang, of their notes. Their corresponding heights may be best represented by bringing together the limits usually ascribed to the kinds of voice thus named. We then obtain as follows : FIG. 258. E a A f c a f f c c Bass. Barytone. Tenor. Alto. Soprano. The absent semitones form stages of transition, which are allotted to one note or another, according to their clang and intensity. It is obvious that many of the notes which limit the different ranges may be given in several kinds of voice. The lowest E mentioned above corresponds to 165 vibrations in the second, and the highest" to 2112 ( 1403). 1421. A good singing voice includes about 2J octaves. But distin- guished female singers are able to bring out an octave more. 1422. Women and children generally move in higher notes, in discant, soprano, or alto ; and adult males in lower ones, in tenor, barytone, or bass. The period of puberty, during which the boy ripens into a man, and the girl into a woman, exerts an important influence upon the voice. About this time the voice, which previously moved in higher notes, breaks. During the rapid development of the larynx, the voice loses its purity. Its notes subsequently become more resonant, deep, and powerful. If the normal process of development is destroyed, morbid conditions of the voice are subsequently met with. Hence men whose sexual development is checked retain a delicate voice. The high notes 424 VARIETIES OP VOICE. [CHAP. XVI. of eunuchs depend upon this fact. And, conversely, women with an apparently masculine build of body have sometimes a deep and powerful barytone voice. In old age, the parts lose too much of their elasticity, and hence the resonant singing voice is lost. 1423. It has hitherto been found impossible to lay down any theory capable of explaining the different methods of singing with the chest-, falsetto-, and head- voice ; or with the clear and veiled voice. But it is highly probable that the differences of the chest-voice and falsetto are greatly assisted by original diameter, and voluntary tension. We may conjecture that, in the higher notes of the head-voice, the vibrations are limited to the inner margins of the vocal cords. 1424. The strong current of air which occurs in the act of screaming will alone tend to raise its notes ( 1412). Whistling and whispering depend upon the difficult passage of masses of air driven violently through narrow fissures. 1425. The glottis, and the more or less movable pieces of the double body-tube formed by the cavity of the mouth and nose, together pro- duce the various sounds of speech ( 1427). But all these structures are present in the higher animals as well as in man. And if, in spite of this, these creatures are devoid of speech, the fact does not depend upon mere minor peculiarities of the movable parts of their vocal organs. The chief cause rather lies in the lesser development of their nervous system. It is not merely the thoughts of the human mind which are betrayed by the conventional sounds of speech. We might rather say that the mind, and the arrangements of the nervous substance which are linked with it, constitute the only material way of sufficiently expressing the sensations. The imperfect speech of cretins and other imbeciles, or of patients who have undergone apoplectic seizures, is intimately connected with these circumstances. 1426. Hitherto the attempt to determine how the various articulate sounds are produced 42 ) has only been made experimentally. Extraor- dinary patience has succeeded in constructiDg speaking machines, which deserve all possible acknowledgment from physiologists. The automaton recently prepared by Faber, and exhibited in most of the countries of Europe and in North America, may be regarded as the most perfect of all. Apart from the unnatural and unpleasant clang of its voice, the figure speaks quickly and plainly in various languages, when the corre- sponding keys are played upon. Its singing voice includes 12 notes (from Re to La), which sound very well when accompanied by a small organ. Here the difference in the height of the notes is produced by varying the width of the glottis, and not by altering the tension of the vocal cords. This was done in accordance with the opinion formerly uni- versal, and now exploded that the height of the sounds during life is determined by the condition of the glottis only. CHAP. XVI.] ARTICULATE SOUNDS. 425 1427. The vowels are the only sounds the origin of which can at present be followed on acoustic principles. Every quick repetition of impulses gives rise to the perception of a vowel. When one string which hums in low notes produces u, and another which sounds in higher notes i, the difference of the impression depends upon the simple vibrations and the height of the note. But when an air-tube is placed before a tongue-work the conditions are more complicated. According to Willis, it is the length of the tube and the size of its orifice which determine whether i, e, a, o, or u is heard. Here the height of the note de- pends upon the number of vibrations of the tongue, while the nature of the vowel is determined by the height of the note which the tube would give as an open pipe ( 1402). Hence an alteration of the tongue only affects the altitude of the note, and does not change the vowel. 1428. Many questions in comparative philology may be explained by a physiological examination of the pronunciation of the several sounds made use of in the different languages and dialects. This means of phi- lological inquiry has been made less use of than it deserves. Every dialect is based on a particular adjustment, a special education, of the organs of speech. This explains why certain series of sounds have a peculiar effect; why a particular foreign language is more easily and perfectly spoken by the inhabitants of one country than of another ; or why some of its accents sound like those of their mother tongue. Such physiological considerations frequently illustrate the destinies undergone by the same radical word in the course of time or in different allied lan- guages ; and even explain, in a surprising degree, many relations of quan- tity and metre. 1429. Many investigators have supposed that the chief means adopted by ventriloquists consists in the use of inspiratory sounds ( 1415). This agrees with the well-known fact, that the notes are at that period higher, and that, if only of moderate intensity, they appear farther off. Many ventriloquists also make use of other means of deception. They cover the face with a cloth, in order that no play of features may attract attention to the person of the speaker. They often choose the form of an alternating dialogue, in which the use of different notes facilitates the error. And in speaking with expiratory notes, they sometimes dis- tribute the air expelled at one expiration over a large space of time, and a considerable number of sounds. 1430. In the act of stuttering 43 ) the several organs of speech do not play in their normal succession, but undergo contraction in a more aimless and uncertain manner. They are thus continually interfered with by convulsive impulses and inefficient adjustments. Many of the sounds altogether fail for some time, while others are frequently repeated before the whole word is completed, generally with a sudden explosion. The cause of this defect lies almost exclusively in the nervous apparatus 426 STUTTERING. [CHAP. XVI. which rules over the organs of speech. This explains why mental em- barrassment, fright, the imitation of others, or affectation, may all lead to stuttering ; and why a powerful will may conquer it. It is at the same time obvious that the cure can only be accomplished by proper educa- tional means ; and that all mechanical apparatus for the coercion of the tongue is useless : as is also the section of the genio-hyoglossus (a, Fig. 67, p. 124). 1431. In deaf and dumb persons the organs of speech have originally no essential defects. The differences met with in these parts depend solely upon want of use. The true cause of their dumbness lies in their inability to perceive sound. The impossibility of appreciating the several sounds, and thus of gradually acquiring the proper adjustment of the organs of speech by comparing the sounds they produce, constitute the chief reason why the second infirmity is associated with the first. We sometimes find that adults who become completely deaf gradually forget their speech in the course of years, until they finally reduce it to a few words, or even lose it altogether. CHAPTER XVII. FUNCTIONS OF THE SENSES. 1432. EVERY organ of sense consists of two portions. The first re- ceives the stimuli, in order to their suitable preparation. The second, which pertains to the nervous system, gives them a peculiar elaboration, and at the same time brings about an action and reaction between them and the cerebral structures. In this way the eye consists of a series of refractile bodies which effect a suitable change in the images ; the ear, of a chain of solid and fluid parts which conduct the waves of sound ; the nose, of a ciliated mucous surface which takes up odorous particles ; the tongue, of a covering which receives sapid substances ; and the skin, of tissues which exert a definite influence upon the stimuli of temperature and pressure. While, on the other hand, the retina connected with the optic nerve of the eye, and the nerves of hearing, smelling, taste, and touch, lead to those vital reactions which succeed the impressions pre- pared by the above structures. 1433. An external object which is thus perceived produces an objec- tive sensuous impression ; since the first corresponding cause of ex- citement lies without the subject to be perceived itself. A number of men or animals may, with slight differences, simultaneously perceive the same luminous body. But when, on the other hand, the optic nerve or the part of the brain connected with it is compressed, burnt, or electrified, a person observes flashes of light which are visible to no one else. Hence we have here a purely subjective sensation. In like manner the auditory nerve is capable of responding to irritation with perceptions of sound ; and the nerves of smell, taste, and touch, with similar corre- sponding impressions. In short, the distinction of sensations into objec- tive and subjective is repeated in all the organs of sense. 1434. The several lenses of the healthy human eye contain no sub- stances capable of casting shadows in the way of ordinary vision. But in certain artificial or morbid states the retina is capable of perceiving- various substances present in the eye, like any external object. This is therefore essentially an objective perception of a body present within the organism. Still such a situation often causes the appearance just men- tioned to be regarded as a subjective sensuous impression. 1435. Every organ of sense is specially adapted to a definite cycle of influences, or, as it is usually expressed, to its suitable or corresponding 428 PURPOSES OP THE VARIOUS SENSES. [CHAP. XVII. stimuli. To other impressions it responds either not at all, or at any rate not with its proper delicacy. Thus the eye only responds to the impulses of the waves of light, and not to the more forcible vibrations of ponder- able matter. And changes of the latter, which are at most evident to the feeling of touch as a kind of trembling, give rise to the simple arid specific sensations of sound in the auditory organs. While since the act of smell necessarily presupposes the gaseous or vaporous condi- tion, the contact of any odorous liquid which completely fills the nasal cavities is only perceived by the nerves of touch, and not by the proper olfactory nerves of the mucous membrane of the nose. 1436. The five senses are incapable of appreciating all the changes of the external world. The movement of the waves of light, the impulses of ponderable substances, the hitherto unexplained action of odorous and sapid substances, and the alteration of pressure and temperature, form the chief stimuli which are distinctly recognized by the several organs of our senses. On the other hand, we have no special sensation of the influences of electricity and of magnetism. And active me- chanical or chemical interference only gives rise to pain ; i. e. to special changes in those nerves of sensation which, under different collateral circumstances, afford tactile impressions. 1437. We shall hereafter see that the nerve-fibres are much better adapted to perceive a change, than a permanency, of their state. Thus in point of fact, the eye only recognizes vibrations of the luminous eether, the ear those of ponderable matter: while the sense of touch only notices changes in the static relations, or in the balance of heat. It may be conjectured that the sense of smell also depends merely upon certain impulses ; and that of taste, upon chemical operations which vary from one moment to another. Finally, pain depends upon the sudden or gradual production of certain deeper changes of the nervous structures. These require to be roused from their quiescent state, and must be maintained in a state of internal movement, as long as the corresponding action is to continue. 1438. Sight. The eye forms a globular mass, which is attached to the optic nerve as to a handle (Fig. 259), and is imbedded with it in the fat contained in the cavity of the orbit. From the orbit the optic nerve passes backwards, through a special aperture, into the cavity of the skull. Here the two nerves unite to form an azygos middle-piece, from which two nervous trunks again proceed to the brain. These circum- stances are illustrated by Fig. 259. 1439. Since the rays which come from a luminous body follow certain prescribed paths, every telescope ought to be capable of rotation towards all parts of the sky, so as to suit every position of an object. Something similar to this occurs in the eye. In addition to the globe of the eye (c Fig. 260), fat (d), the optic and other nerves, and numerous vascular CHAP. XVII. J MUSCLES OF THE EYE. 429 trunks, the orbit contains seven muscles. Of these all but one the levator palpebrse superioris (Fig. 260, g) are directly subservient to the movement of the globe, and assist to rotate it in all directions. They may be divided into four straight, and two oblique, muscles. FIG. 259. 1440. When the internal rectus muscle (i, Fig. 259) contracts alone, the eye is turned inwards. In like manner the superior (s, Fig. 259) draws it upwards, the external (n) outwards, and the inferior, down- wards. 1441. The superior oblique muscle (o, Fig, 259, o, Fig. 237, p. 399) runs along the upper and inner part of the orbit. It is then attached to a tendon, which passes through a special pulley (e, Fig. 237), is next bent outwards and backwards, and finally expands upon the outer and hinder part of the globe (c, Fig. 259, /, Fig. 237). We may imagine these circumstances represented by a vertical transverse section (Fig. 261); in which a corresponds to this tendinous structure. The muscle will therefore tend to rotate the globe in the direction of the arrow, ee; that is, it will carry the segment cd upwards and inwards. The in- ferior oblique muscle (b, Fig. 261) arises from the lower and inner part of the orbit, and arches outwards, to be finally attached to the eye externally and posteriorly (m, Fig. 259). It will therefore rotate the segment c d outwards and downwards in the direction of the arrow, ff. 432 CHANGE OF GAZE. [CHAP. XVII. perception of the point a, which lies in the middle line, a b, and in the same plane as the centres of rotation, c and e. The axis fg is then turned inwards towards k I, and h i towards m n, -so that Ik a and n m a meet in a. Hence both globes are rotated inwards towards the middle line, a b, so as to acquire the necessary angle of convergence, cae. 1445. But if, on the other hand, we suppose that the point a lies external to the middle line, a b for instance, at o, which is nearer the right eye the right axis of vision, h i, must move outwards r s, and the left inwards towards p q, to attain the angle, coe. Here the two eyes have to a certain extent opposite actions. The one rolls inwards, and the other outwards, in order that both may be fixed on o. 1446. If the point a lies higher or lower than the horizontal plane the surface of which is represented by Fig. 262, the visual axes of both eyes must be elevated or depressed to a corresponding degree. It is obvious that this case does not permit the double contingency which we have just explained. 1447. Putting all this together, we find that looking upwards or down- wards always presupposes an uniform action of the corresponding upper or lower muscles of the eye. While the two kinds of rotation which cor- respond to the position of points lying in the horizontal plane are very different, both as regards their direction, and the muscles on which they immediately depend. The act of looking inwards (towards , Fig. 262) requires two movements, which are designated by the same word, the impulse to both being furnished by the two internal recti muscles. But if we compare them with the original and parallel direction of the axes of vision, fg and h i, we shall find that the movement is in reality unharmo - nious, and constitutes a squint, since m n and k I deviate towards the same side, towards the line a b. And, on the other hand, when we look outwards (towards o,Fig. 262), the external rectus of the nearest eye, and the internal rectus of the more remote one, act together. The former rolls outwards, and the latter inwards, so that we get opposed movements. But notwith- standing this, the movements are harmonious ; since the variations from parallelism (from/^r and hi) are directed towards the same side. 1448. The statement that a single rectus muscle suffices for the adjustment of the eye, is rarely correct. A more exact study of the mechanism of these muscles indicates that a second rectus generally assists, while an oblique muscle secures the immobility of the centre of rotation ( 1441). But in looking inwards, this complicated play is effected by similar agents ; while in looking outwards, it is done by what are in part antagonists. 1449. It is evident that when the axes of vision, k I and mn, are adjusted to the point a, their deviation from parallelism, fg and h i, will be less, the greater the distance at which a lies from the centres of rota- tion, c and e. When the distance is indefinitely increased, the external CHAP. XVII.] SQUINTING. 433 angle of their direction c a e becomes so small, that it may be regarded as=0. And as in this case ac and ae may be considered parallel, so in looking at very remote objects, the eyes are also said to be parallel. But it is obvious that this expression is in its strictest sense incorrect, since the produced axes of vision will always intersect each other in a certain point. 1450. We have already ( 1447) seen that the act of looking at a luminous point lying in the line a b (Fig. 262) requires an inharmonious movement of the eye, which we characterized as a squint. The abnor- mal and permanent squint consists in the fact that the axes of vision are not parallel even when in a state of rest. Under such circumstances one or both organs of vision deviate more or less considerably in all positions. 1451. Thus while during the act of looking into infinite space the normal axes of vision would be in/^r and hi (Fig. 263), that of the left FIG. 263. and squinting eye would be in p q. So that in the state of rest the two axes, q o and i o, intersect each other at o. When the person desires to perceive the point a with both eyes simultaneously, the left and abnormal eye need not be moved at all, while the healthy right eye must describe F F 434 SQUINTING. [CHAP. xvn. an arc, h t, which corresponds to the angle of the squint, fcp. The near object x necessitates the small angle of rotation pcy for the squinting eye, and the larger one h e z for the healthy one. In general terms, the angle of squinting affects all the movements of the eye as some constant positive or negative value. 1452. The inward squint, in which the axis of vision p q deviates from its normal position fg, may depend on insufficient length or ex- cessive contraction of the internal rectus muscle. It may also be due to paralysis of the external rectus muscle, when the internal acts more freely, from having lost the resistance naturally presented by its antago- nist. Thus for instance, we often find that the left eye of a patient suf- fering from hemiplegia of the same side of the body begins to squint inwards. But we are not justified in asserting that this phenomenon will always follow inactivity of the external rectus. For this muscle may be divided in the living animal without the instant (or even the subsequent) occurrence of an inward squint. 1453. The abnormal circumstances which have just been explained in the case of the internal rectus sometimes obtain with others of the recti muscles : so that a person may squint outwards, upwards, or down- wards, as well as inwards. Deviations upwards or downwards are, how- ever, very rare. And although the outward squint is often met with, still on the whole, it is less frequent than the inward one. The former is generally associated with serious lesions of the retina, which are followed by blindness. 1454. Immoderate contraction of one or both oblique muscles will obviously cause certain irregularities in the movements of the eye. It has been supposed that the eye then remains abnormally rotated around its antero-posterior long axis : giving rise to what is called a rotary squint. But the accuracy of this supposition must be established by further researches. 1455. The abnormal muscular shortening which causes squinting may be permanent. Here it occurs under all circumstances, and may be recognized either at once, or after a careful analysis of the phenomena. But in some instances this morbid contraction only appears under certain collateral conditions. We have seen ( 1430) that persons otherwise fluent speakers may begin to stutter in consequence of mental impres- sions. In like manner, there are many persons who only squint when embarrassed. 1456. Finally, there is a peculiar kind of squint which depends upon the fact that one eye either cannot be moved at all, or at least not beyond a certain limit. This has been distinguished from the move- able kinds previously described by the name of the fixed or immoveable squint. 1457. A ray of light ( 157) passing in the same medium always CHAP. XVII.] REFLECTION OF LIGHT. 435 FIG. 264. takes a rectilineal course. But when it meets with a medium of a diffe- rent kind, four different effects may result. Part of the light is dis- persed on all sides, while another part is reflected in a regular or pre- scribed path. If the new medium be transparent, part will pass onwards in a refracted state. Finally, part is absorbed or lost in the interior, probably by partial reflection and interference ( 165). 1458. Let us suppose mm (Fig. 264) to be a plane reflecting surface, and li an incident ray of light j the line pp f , perpendicular to the point of contact i, makes with it the angle lip, which is called the angle of incidence. The reflected ray c i furnishes the angle of reflection dp, the amount of which coincides with that of the angle of incidence lip. 1459. The angle mFm' (Fig. 265) at which the rays of light proceeding from F towards m m' strike against a concave mirror, becomes less, the further F recedes from m m r . And when the distance is greatly increased, it diminishes so much, that there is scarcely any error in regarding the rays as taking a parallel course, FIG. 265. FIG. 266. instead of proceeding from a single point. Striking in this way upon the mirror m mf, they undergo reflection, and are united in the focus, F. And conversely, rays which come from this focus are reflected parallel to each other. The place where the rays from a terminable distance converge to meet each other, or their mutual focal distance, varies with the distance of the luminous point, and the form of the mirror. It is real when it lies before the mirror, as at F (Fig. 265) ; and virtual when it lies behind it, as at v (Fig. 266). 1460. It depends upon the distance of objects, whether their several points have real or virtual foci in concave mirrors, and whether the whole is represented of natural, increased, or diminished size. Convex mirrors, such as the cornea and the anterior surface of the crystalline lens, always possess virtual foci. 1461. The angle of incidence of a ray of light ef, which passes through a transparent medium abed, (Fig. 267) will be efo ; supposing o i to be F F 2 434 SQUINTING. [CHAP. xvn. an arc, fit, which corresponds to the angle of the squint, fcp. The near object x necessitates the small angle of rotation pcy for the squinting eye, and the larger one hez for the healthy one. In general terms, the angle of squinting affects all the movements of the eye as some constant positive or negative value. 1452. The inward squint, in which the axis of vision p q deviates from its normal position fg, may depend on insufficient length or ex- cessive contraction of the internal rectus muscle. It may also be due to paralysis of the external rectus muscle, when the internal acts more freely, from having lost the resistance naturally presented by its antago- nist. Thus for instance, we often find that the left eye of a patient suf- fering from hemiplegia of the same side of the body begins to squint inwards. But we are not justified in asserting that this phenomenon will always follow inactivity of the external rectus. For this muscle may be divided in the living animal without the instant (or even the subsequent) occurrence of an inward squint. 1453. The abnormal circumstances which have just been explained in the case of the internal rectus sometimes obtain with others of the recti muscles : so that a person may squint outwards, upwards, or down- wards, as well as inwards. Deviations upwards or downwards are, how- ever, very rare. And although the outward squint is often met with, still on the whole, it is less frequent than the inward one. The former is generally associated with serious lesions of the retina, which are followed by blindness. 1454. Immoderate contraction of one or both oblique muscles will obviously cause certain irregularities in the movements of the eye. It has been supposed that the eye then remains abnormally rotated around its antero-posterior long axis : giving rise to what is called a rotary squint. But the accuracy of this supposition must be established by further researches. 1455. The abnormal muscular shortening which causes squinting may be permanent. Here it occurs under all circumstances, and may be recognized either at once, or after a careful analysis of the phenomena. But in some instances this morbid contraction only appears under certain collateral conditions. We have seen ( 1430) that persons otherwise fluent speakers may begin to stutter in consequence of mental impres- sions. In like manner, there are many persons who only squint when embarrassed. 1456. Finally, there is a peculiar kind of squint which depends upon the fact that one eye either cannot be moved at all, or at least not beyond a certain limit. This has been distinguished from the move- able kinds previously described by the name of the fixed or immoveable squint. 1457. A ray of light ( 157) passing in the same medium always CHAP. XVII.] REFLECTION OF LIGHT. 435 FIG. 264. takes a rectilineal course. But when it meets with a medium of a diffe- rent kind, four different effects may result. Part of the light is dis- persed on all sides, while another part is reflected in a regular or pre- scribed path. If the new medium be transparent, part will pass onwards in a refracted state. Finally, part is absorbed or lost in the interior, probably by partial reflection and interference ( 165). 1458. Let us suppose mm (Fig. 264) to be a plane reflecting surface, and I i an incident ray of light ; the line p p', perpendicular to the point of contact i, makes with it the angle lip, which is called the angle of incidence. The reflected ray c i furnishes the angle of reflection dp, the amount of which coincides with that of the angle of incidence lip. 1459. The angle mFmf (Fig. 265) at which the rays of light proceeding from F towards m m' strike against a concave mirror, becomes less, the further F recedes from m m'. And when the distance is greatly increased, it diminishes so much, that there is scarcely any error in regarding the rays as taking a parallel course, FIG. 265. FIG. 266. instead of proceeding from a single point. Striking in this way upon the mirror m m', they undergo reflection, and are united in the focus, F. And conversely, rays which come from this focus are reflected parallel to each other. The place where the rays from a terminable distance converge to meet each other, or their mutual focal distance, varies with the distance of the luminous point, and the form of the mirror. It is real when it lies before the mirror, as at F (Fig. 265) ; and virtual when it lies behind it, as at v (Fig. 266). 1460. It depends upon the distance of objects, whether their several points have real or virtual foci in concave mirrors, and whether the whole is represented of natural, increased, or diminished size. Convex mirrors, such as the cornea and the anterior surface of the crystalline lens, always possess virtual foci. 1461. The angle of incidence of a ray of light ef, which passes through a transparent medium abed, (Fig. 267) will be efo ; supposing o i to be F F 2 436 REFRACTION OF LIGHT. [CHAP. xvn. perpendicular to a b. If its path remained unaltered, it would pass on- wards in fg 1. But when the new medium has a greater refractile power than that out of which the ray ef comes, the latter is bent towards the perpendicular o i, so that its path fh forms a smaller angle hfi than its rectilineal prolongation fg I. A less refractile medium produces the reverse effect. When the ray re-enters the atmo- sphere, its angle of incidence ghf is less than its angle of refraction nhk. 1462. When the two media traversed by the ray remain unaltered, the quotients of the sines of the various angles of incidence and the cor- responding angles of refraction form a constant magnitude, which is designated the index of refraction. This law was first deduced by Snell FIG. 267. FIG. 2G8. 1 and Descartes, and is hence often called by their names. Thus, in Fig. 268, the sine c^/---sine t influence upon the mechanism of hearing. * The annexed woodcut (Fig. 317), represents a magnified view of these structures in situ. The head d of the malleus rests in the excavated FIG. 317. if articular surface of the body e of the incus, while the long process of this latter is united to the stapes /. The long process of the malleus d is united to the tympanum which bounds the auditory canal a. The 478 MUSCLES OF THE AUDITORY OSSICLES. [CHAP. XVII. oval aperture that bounds the foot-piece of the stapes leads to the vestibule, which contains a portion of the auditory nerve. The chain of the auditory bones is thus interposed as a solid and articulate piece, connecting the tympanum with the innermost and ; and at q are some of the sensitive nerves which come from the trigeminal trunk, vw (and s, Fig. 128). Hitherto the branches of the olfactory nerves the fibres of which are distinguished by their clear grey colour, their small size, their softness, and their ap- parent want of medullary matter have only been followed into the upper and middle part of the mucous membrane of the nose. Their exact mode of termination is unknown : and we are equally unacquainted with the way in which they bring about the olfactory sensation. 1610. Daily experience teaches that very small quantities of many odorous substances suffice to excite the sense of smell. The smell of tobacco, musk, ambergris, or phosphuretted hydrogen, remains attached to paper for years. By partially evaporating a grain of musk on a CHAP. XVII.] MINIMUM QUANTITIES OF ODOROUS SUBSTANCES. 485 hot plate, we may communicate to a whole room a smell of musk that lasts for many months. 1611. If one volume of an odorous matter be mixed with TOO of air, and one volume of the mixture again diluted with 100 of atmosphere, and this again treated in the same way, we shall finally obtain a gaseous substance containing only a certain known minimum of the particular odorous matter. Fluid solutions of odorous substances may also be diluted in the same way. This therefore affords a means of approximative^ determining the limits to the perceptions of the organ of smell. 1612. A space of air containing ^oVc^th part of bromine vapour in- stantly affords an unpleasant smell. Hence it probably requires less than -00002574 grains of bromine to evoke the peculiar olfactory sensa- tion. A volume of phosphuretted hydrogen gas amounting to not more than ss^o^th of the whole gives a distinctly garlic odour. The minimum of sulphuretted hydrogen appears to be less than one or two rnillionths ; so that the organ of smell here constitutes the most delicate of all re- agents. While on the other hand, a glass rod moistened with hydrochloric acid forms a white fog that betrays the presence of ammoniacal vapour, even when its quantity is too minute to be perceived by the human olfactory sense. 1613. The essential oils (which are often adulterated with fatty oils) are very effective in this respect. It is probable that Tstrsinroth of a grain of otto of roses suffices to give rise to its peculiar smell. After dropping T^th of a grain of oil of cloves into a balloon which contained from 3350 to 3400 cubic inches, its smell remained more than three months. 1614. But of all these substances, the most remarkable is musk. On diluting its alcoholic extract with water, it is found that as little as about TFooWfT^th of a grain can be traced by its smell. 1615. It is important to distinguish between those matters which are really odorous, and those which act upon the mucous membrane of the nose in another way. Twigs of the trifacial cerebral nerve (s, Fig. 128, p. 232,) are distributed with those of the olfactory (o) in the lining membrane of the nasal fossa. The former give rise to sensa- tions which differ from those of the other organs of touch only in their mode, and not in their nature. While in ordinary language, many of these essentially tactile perceptions are called olfactory. For example, the perceptions produced by caustic ammonia depend chiefly on its corrosive effects. And this probably explains its comparatively large minimum in the odorous scale ( 1612). 1616. Just as the construction of an organ of voice ( 1408) only required the larynx to be interposed at a suitable part of the respiratory organ, so something similar obtains with the organ of smell. The current of air which the machinery of respiration drives through the 486 . CURRENT OF OLFACTORY SUBSTANCES. [CHAP. XVII. nasal fossa only demands the addition of the mucous membrane of the nose, and the twigs of olfactory nerve that supply it. 1617. The nasal fossa is continuous with several supplementary cavi- ties j such as the antrum of the superior maxillary bone, the frontal sinus, and the ethmoidal cells. But at present the uses of these peculiar structures are unknown. Still, as no fibres of the olfactory nerve can be found in them, we have anatomical grounds for presuming that they do not effect olfactory impressions. Physiological research seems to corroborate this inference. Where disease has aiforded free access to one of the maxillary or frontal sinuses, odorous fluids or gases introduced into these cavities have not been perceived to be such. But this fact cannot be regarded as a valid proof ; since these experiments imply the absence of many collateral circumstances, which will shortly be mentioned as included in the mechanism of smell. 1618. It is probable that the ciliary movement on the surface of the mucous membrane of the nose ( 1196) takes an important share in the sense of smell, by producing small whirling currents in the air, and in the odorous particles which this contains. The irregularities of the lining membrane of the nose assist to maintain these various currents. In violent catarrh the sense of smell is always more or less impaired. This fact appears to indicate, that the conditions necessary to smell are capa- ble of being suppressed by the loss of cylindrical epithelium, together with the swelling and the altered secretion of the mucous mem- brane. 1619. If a tube traversed by odorous substances be introduced as high as possible into the nasal fossa, we shall find that the smell gra- dually diminishes, and is finally lost. Hence it is not enough that the odorous substances should pass over the mucous membrane of the nose generally. It would rather seem that, in order to give rise to the olfac- tory sensation, they must traverse a large portion of the entire path prescribed to them. The inferior turbinated bone (c, Fig. 322, p. 484) must have a peculiar effect in bending the inspiratory stream. But since we can also smell at the instant of expiration, it follows that this bend is not essential to the olfactory sense. Odorous substances which enter by the posterior riares (/, Fig. 128, p. 232) are always distinctly, though feebly, appreciated. 1620. Repeated respiratory movements greatly assist the olfactory sensation. The act of snuffing depends upon an impulsive, strong, and accelerated inspiration. The dilatation of the nostrils must at the same time allow the entry of larger quantities of air. 1621. Weber has pointed out that when a person is extended horizon- tally, with his head hanging back, we may pour in fluid at one nostril until it runs out at the other. An examination of the throat shows that, under these circumstances, the entrance into the pharynx is nearly CHAP. XVII.] DELICACY OF THE OLFACTORY SENSATION. 487 or quite closed by the soft palate and the palatine arches. This simple fact explains the above experiment. When the fluid contains odorous matters, they are not perceived to be such. Or if these mixtures be injected directly up the nose, we shall get a similar, although less marked, result. Hence we see that when the mucous membrane of the nose is covered with liquid, it loses its capacity of smell. And this inactivity even remains for a few minutes after the fluid has been allowed to run off : so that substances which, like ammonia, operate as caustics, are at first perceived either indistinctly, or not at all. 1622. The delicacy of the sense of smell varies greatly in different persons. While many fail to perceive the most penetrating odours, others can instantly remark their faintest traces. Every person diffuses a peculiar odour, which depends upon the cutaneous exhalation ( 848). This is so slight, that few notice it. But many savages can thus re- cognize the path which another person has taken. The instantaneous contact with the ground or with neighbouring substances gives off olfactory substances in quantities which, though minute, are still enough for perception. Dogs and other animals which hunt by scent exhibit a similar perfection of the olfactory organs : their sense of smell conduct- ing them more easily and accurately than that of sight or hearing. And the female animal is frequently provided with special glands, the odorous secretion of which attracts the male from a distance during the rutting season. 1623. Many substances which give an agreeable smell to most persons, seem quite inodorous to others. Mignonette and other plants of deli- cate perfume are instances of this. And it is even more frequent to find substances, the smell of which is unpleasant to some persons, but attractive to others. The smell of asafoetida is liked by many persons : and hysterical women are often fond of the odour of burnt feathers or other empyreumatic substances. And, conversely, the olfactory or- gans may be blunted by custom. Workmen who are engaged with putrefying substances, apothecaries, surgeons, and anatomists, exemplify the truth of this last proposition. 1624. A strong smell suppresses the more delicate odours present. If a drop of oil of cloves, and a drop of oil of peppermint, be let fall into a large bottle, the latter will rarely be perceptible. Here also the senses are completely blunted by a want of variety. A person who continually surrounds himself with perfumes becomes at last quite indifferent to them. The most agreeable odours may become repulsive when in excess. 1625. The smell of the odorous body often seems to survive its presence. But although we are justified in expecting that there will be a certain after-impression in the organ of smell as well as in the other senses, still this phenomenon scarcely proves such an effect; since 488 TASTE. [CHAP. xvn. it is possible that minute quantities of the odorous substance remain in the nasal fossae. 1626. Powerful odours often produce stupefaction and fainting. Many cause nausea and vomiting. These energetic effects are pro- bably explained by the intimate connection of the olfactory nerves with the brain. The voluptuous sensations which are frequently felt seem to be only mediate results, effected by the help of memory and imagination. 1627. The attitude which we have to assume in order to perceive any odorous substance constitutes our only means of ascertaining the direction from which it approaches. All those delicate differential phenomena which we have met with in the eye ( 1523), and to some extent in the ear ( 1605), are absent. 1628. If one of two faintly odorous substances be held before each nostril, the olfactory impression of either may be made to predominate at will. This phenomenon to some extent resembles the contention between the two visual fields ( 1556). 1629. Subjective olfactory sensations are on the whole very frequent. The mechanical agitation which accompanies violent sneezing may suffice to produce a peculiar smell. But galvanism does not give rise ( 1578) to any definite impression. It is true that Dupuytren found that dogs into whose blood he had injected odorous liquids began to snuff. But since the odorous matters might have been given off in their own exhalations, we are not justified in regarding this phenomenon as a mere subjective impression. 1630. TO^WQ have seen (1621) that the organs of smell are only sensible to elastic fluids. On the other hand, the liquid state is the necessary condition of taste. Insoluble substances at most only cause tactile sensations, such as the feeling of cold : they are unafole to produce true gustative impressions. Here again ordinary language fails accurately to distinguish between those perceptions which are proper to the organs of touch, and those which correspond to the true organs of taste. There are many substances to which we are in the habit of ascribing a hot, cold, acid, or caustic taste, although they chiefly excite the sensitive, and not the proper gustatory nerves. 1631. The tongue is usually regarded as the exclusive organ of taste. But an easy experiment may convince us that some parts of this organ are incapable of producing gustative impressions. When a small piece of salt, or a drop of vinegar, or solution of extract of aloes, is placed upon the upper and anterior half of the extended tongue, the pecu- liar taste of each of these substances fails to appear so long as they do not pass towards its inferior surface or root. While, in the latter situa- tion, the smallest quantity of any sapid substance is distinctly perceived. This is due to the fact, that the nerves of taste are distributed in the CHAP. XVII.] GUSTATIVE REGIONS OF THE TONGUE. 489 root of the tongue, while its anterior half is chiefly supplied by nerves of touch. 1632. The upper surface of the tongue is clothed by projections of various kinds, which are called its papillae. The circumvallate papillae I (Fig. 323) belong to its posterior part. The conical, i, and the filiform, k, between which there are often transitional forms, are found in the FIG. 323. other parts of the organ. All of these contain a rich network of capil- laries, together with nerve-fibres which effect their gustative or tactile sensations. From what has been already stated ( 1631), it follows that the circumvallate papillae occupy that region which offers the most distinct gustative impressions. 1633. It has always been a matter of much discussion whether sen- sations of taste can only be excited by the tongue, or by other parts of the commencement of the alimentary canal also. However simple the experiments decisive of this question might appear to be, many of them are frustrated by two circumstances, which ought to be constantly borne in mind. When a fluid solution is applied to any part of the palate, it easily spreads into the neighbourhood. Hence we sometimes get gustative impressions, which are wrongly ascribed to the point of con- 490 GUSTATIVE REGIONS. [CHAP. XVII. tact. While a soluble and quiescent solid produces either no sensation of taste, or a very slight one. Movement appears to be quite as essen- tial to taste, as the current form of odorous matter is to smell. Hence the substance used as the test ought to be not merely dotted on the part with a pencil, but rubbed here and there; without, however, being allowed to distribute itself over other and more ambiguous regions. Under these circumstances, it sometimes happens that the tactile impression occurs first, while the true gustative sensation only appears afterwards. 1634. Bearing this in mind, it would follow that sweet and bitter sub- stances can be tasted at other places besides the root of the tongue. We meet, however, with individual varieties ; which may depend upon the strength of the gustative sensibility, if not upon other peculiarities. 1635. The lips, the inner surface of the cheeks, the gums, the skin of the hard palate, and the greater part of the upper surface of the anterior half of the tongue, are always devoid of the sense of taste. The quickest and most energetic perception belongs to that part of the tongue which lies immediately in front of the foramen ca3cum (I, Fig. 323). The under surface of the tongue possesses, in most persons, a vigorous capa- city of taste. Positive results are also usually obtained with certain portions of the palatine arch ; as well as with the folds which connect the tongue to the epiglottis, the tonsils, the part of the pharyngeal mucous membran6 which is opposite to the root of the tongue, and, more rarely, with the soft palate and uvula. 1636. Hence the root of the tongue occupies the first gustative rank; while other parts and especially the fauces are also capable of a less delicate and rapid taste. So that the mechanism of deglutition ( 372) not only affords that movement of the soluble or dissolved sub- tances which is necessary to their being tasted, but forces the alimentary bolus through a narrow path of transit, which also possesses a gustative capacity in numerous situations. 1637. In treating of the nervous system, we shall find that some phy- siologists regard the glossopharyngeal nerve (e, Fig. 323; and w, Fig. 128, p. 232) as the nervous trunk upon which the sensation of taste exclu- sively depends : while others ascribe this capacity to both it and the trigeminal nerve (g, Fig. 323; s, Fig. 128). We need not now consider the various arguments which support the first of these views. Thus much at any rate is certain, that the root of the tongue, which exhibits the liveliest gustative sensibility, receives a large number of fibres from the glosso-pharyngeal nerve. 1638. We may often notice that there is a tendency to set down all indistinct perceptions of taste as faintly acid, bitter, or saline. This fact gives rise to numerous deceptions : and is very liable to betray us into error in examining the various regions of taste. It is to this cause that CHAP. XVII.] MINIMUM OP SAPID SUBSTANCES. 491 we may perhaps attribute the opinion, defended by so many experi- menters, that substances such as sulphuric acid, salt, or ox-gall, produce dissimilar gustative impressions, according as they are applied to various papillae or regions of the organs of taste. 1639. The repeated dilution of a dissolved sapid body finally gives us a mixture which produces no distinct gustative impression. Hence this process affords a means of accurately determining the delicacy of taste, as well as of smell ( 1611). The taste of sweet substances, such as sugar or syrup, is the first to disappear : the limit being for cane sugar about 1 to 1^ per cent. Salt somewhat goes further. While very acid or bitter substances, such as sulphuric acid, extract of aloes, or sulphate of quinine, are easily recognized in the greatest state of dilu- tion. The taste peculiar to each can be perceived with soWth to Wooth of a grain of sulphuric acid, soWth of extract of aloes, and ToWth of basic sulphate of quinine. 1640. We might expect that these results would be determined, not merely by the dilution of the fluid, but also by its absolute quantity. But we find that there is a very peculiar law in this respect. The gustative sensation can be excited by swallowing so small a quantity of a dense solution, that the absolute amount of sapid matter which it con- tains may be less than that required in a more dilute solution. Hence the greatest dilutions afford no information as to the absolute minimum of a sapid substance. 1641. The varying sensitiveness met with in the organ of smell ( 1623) recurs in that of taste. While some persons, who are in the habit of tasting wine and tea, can educate this sense to an almost incre- dible delicacy, others evince a remarkable indifference to gustative im- pressions. And the putrefying substances which are liked by one man are rejected by another. Custom and prejudice exercise a great influence in this respect. Bitter substances often leave their taste behind them for some time, on account of very small quantities being sufficient for per- ception. Many substances have an after-taste, which differs from the original one. And if two different sapid substances occupy opposite lateral halves of the root of the tongue, one of them can often be per- ceived in preference to the other. 1642. It frequently happens that a sapid substance which gives off odorous matters at the same time excites the olfactory organs. But the two senses do not really react upon each other. Nor does their distinct- ness depend on the mere fact, that elastic fluids are perceived by the nose, and liquids by the organ of taste. Their corresponding sensations rather require the addition of some unknown properties; which, by throwing the nerves of smell and taste into their states of activity conditionate their several perceptions. 1643. Like the other senses, the organs of taste are probably 492 TOUGH. [CHAP, xvn, capable of subjective sensations. But in most of the phenomena included under this head it is very doubtful whether an objective im- pression is not really present. Thus, when a sick person suffers from a bitter taste, it may be questioned whether the altered blood does not give out some bitter substance, which passes through the nutritional fluid, and thus reaches the nerves of taste. The same remark applies to those experiments in which sapid substances have been injected into the blood. And the apparently subjective gustative sensation produced by the galvanic current is perhaps due to electrolytic decomposition. 1644. Touch. We shall hereafter see that injuries of the nerves of smell, sight, or hearing, are not followed by any feelings of pain. While on the other hand, those nerves which are the agents of the sensations of touch may, under altered circumstances, give rise to pain. Hence the tactile impressions are said to be effected by sensitive, and the operations of the higher senses, by sensuous, nerves. 1645. Every free external or internal surface would be of itself adapted to touch, were it not for a peculiar nervous arrangement, which limits the number of sensitive structures. We shall hereafter find that, under ordinary circumstances, few of the intestinal nerves can excite any conscious impressions. While under abnormal conditions, they allow of pain. This explains why we are unconscious of the presence of the food and its residue in the greater part of the alimentary canal ; and of the movement of the blood in the heart and great vessels : why, in short, the greater part of the vegetative functions are executed without con- sciousness or volition. 1646. Among the parts more or less capable of distinct tactile sensa- tions, we may enumerate the whole surface of the skin; the external auditory meatus; the conjunctiva; the greater part of the cavities of the nose, mouth, and pharynx ; part of the oesophagus ; the end of the rectum ; the urethra; and the lower part of the male and female organs of genera- tion. But we must not regard all of these structures as equally endowed in this respect. The apex of the tongue, and the skin, are the most perfectly so. The remaining surfaces give rise to perceptions which are less distinct, and which easily merge into feelings of pain. Their tactile capacity is, as it were, only a collateral result of their being pro- vided with sensitive nerves ( 1644). They do not form tactile organs in the strictest sense of the word ; this implying a higher grade of the tactile function. 1647. The organs of touch enable us to perceive actions of two kinds. They indicate alterations of mechanical state, and changes of tempera- ture. But in order that the sensation should remain pure, the excite- ment must not exceed certain limits. Violent impressions of either kind at once give rise to pain. 1648. Beginning by a glance at the preparatory structures which are CHAP. XVII.] TACTILE SENSIBILITY OF DIFFERENT REGIONS. 493 found in the external integument, we see that the latter sustains an epidermis of variable thickness (Tab. IV. Fig. 62, a, b.). The tactile papillae of the corium (d, e,) form prominences, which are more or less closely followed by the layers of epidermis ; and which are best seen by the naked eye on the volar or thumb side of the terminal phalanges of the fingers, where they are very large and regular. The nerves which run in the corium effect the tactile sensations, through the intervention of the tissues of the corium and epidermis, on which the result of the impression essentially depends. 1649. Something similar to this is repeated in the other tactile sur- faces. For instance, the tongue sustains a pavement epithelium, in place of an epidermis. And its nerves run in a fibrous tissue which lies beneath this layer, and corresponds to the corium. 1650. The simplest tactile sensations are caused by the mechanical resistance which a body offers to the sensitive skin, in virtue of its own consistence, and the compression and displacement of the several cuta- neous structures. It is thus we are enabled to judge of the hardness or softness of any substance with which we come into contact. But since the thickness of the epidermis always varies in different parts of the body, the same substance will excite different impressions according to the locality it touches. The same degree of pressure which produces pain at the lips, will only cause a tactile sensation when exerted on the thick epidermis (Tab. IV. Fig. 62, a, 6) of the sole of the foot. While anyplace that has been deprived of its epidermis, which normally mode- rates irritation, is pained by the slightest causes. 1651. If two points touch an unmoved cutaneous surface, they can only be perceived separately when the distance between them exceeds a certain limit. And Weber has shown that the minimum of distance thus established varies in different parts of the skin. This experiment enables us to construct a scale of the sensibility possessed by the several tactile surfaces. For this purpose we make use of a pair of compasses, the points of which are armed with suitable pieces of cork ; finding out the smallest distance at which they are recognized as separate. A smaller distance than this gives rise to an indistinct impression of a long-drawn point : and finally, on approximating them still more closely, the per- ception becomes completely single. 1652. The absolute values thus obtained vary greatly from each other. A certain portion of skin may give four or five times as much in one person as in another. The most striking differences are generally found in those parts which themselves offer the highest absolute values. A deli- cate skin, and an active mind, seem to admit of smaller distances. 1653. The point of the tongue has a more delicate sense of touch than any other part of the body. Here the minimum distance is -0433 inch. The skin of the middle of the back gives a minimum distance 494 TACTILE SENSIBILITY OF DIFFERENT REGIONS. [CHAP. XVII. of 2-13 to 2-68 inches, and is the region where touch is dullest. Hence the extremes may differ from fifty to sixty-fold. 1654. Assuming that the average for the tongue is =1, the distance for the terminal phalanx of the index finger is 1 -2, and for that of each of the remaining fingers 1 -8. At the thumb side of the first and second phalanges, it is 3-3 ; and on the dorsal surface of the last phalanx, 44. 1655. The red part of the lips gives 3-1, and the white 4-6. This differ- ence is chiefly due to the unequal thickness of their coverings, and per- haps to their nervous relations. The remainder of the face has a still duller sense of touch. On the outer surface of the eyelids, it is 7 -9 ; on the skin of the cheeks, 9-4 to 10-9 ; and in the inferior frontal region, 12-4. 1656. The tactile sensibility of the foot is in every respect inferior to that of the hand. For example, the volar side of the terminal phalanx of the thumb gives 1-5 ; and that of the great toe, 6-7. The dorsal surface of the hand gives from 44 to 14-4 ; and that of the foot, 12-2 to 25-9. 1657. The extremities of the limbs, such as the hand and foot, have a more delicate sense of touch than their middle segments, such as the forearm and leg : while these again are more sensible than the segments connected with the trunk, such as the thigh and upper arm. The two latter belong to those parts which do not possess a high development of tactile capacity. The vicinity of the elbow and the knee-joint is more sensitive, being easily excited to pain. 1658. The face has a more accurate sense of touch than the crown of the head or the neck. The dorsal surface of the trunk is inferior to the abdominal in this respect. 1659. It would seem that, in the adult, these minimum distances alter very little by lapse of time. At least the author finds that his skin gives about the same numbers as it did eleven years ago. 1660. The friction of some parts of the skin gives rise to peculiar feelings of tickling, or to voluptuous sensations. But such parts do not necessarily rank high in the scale of tactile sensibility. Thus the axilla gives 2 6 '9, and the foreskin 10-6, as the minimum of distance. 1661. The tactile sensibility is capable of being increased by habit to an extraordinary degree. In this way some blind persons are able to recognize different colours by inappreciable differences in their grain. The Bengalese spinning women can distinguish the threads of the cocoon with a tactile delicacy which is almost incredible. And persons devoid of arms may educate the sensibility of the toes, until it corresponds with that of the fingers of an ordinary individual. 1662. In judging of the delicacy of touch, we usually take the minimum distance at which two points can be recognized as the unit from which to start. This fact explains a peculiar illusion, to which attention was first drawn by Weber. When we draw the protected points of the com- passes downwards from the cheek to the lips, it seems as if the distance. CHAP. XVII.] ESTIMATION OP WEIGHT. 495 between them gradually increased, in consequence of our thus proceeding from a less sensible part to one which is more so. 1663. At present we are unable to assign the exact causes of the degrees of tactile sensibility possessed by different parts of the skin. Here and there a varying thickness of epidermis may assist to increase or diminish the delicacy of touch. But this difference is not always parallel with that of the tactile capacity. Many physiologists have supposed that a part which has a smaller minimum distance possesses more nerves. But hitherto this supposition has not been proved : on the contrary, it may be definitely denied that the nerves of the skin of the back are 50 to 60 times less numerous than those of the apex of the tongue. 1664. The perceptions of touch appear to occur somewhat more slowly than those of the other senses. Here also, a stronger stimulus easily suppresses a weaker one. The addition of painful sensations readily destroys the tactile impression. 1665. When the eyes are bandaged, the place upon which pressure is being made by any substance is stated with more uncertainty, the less acute the feeling of the corresponding part. For the same reason we often blunder in attempting to lay hold of a definite point of the neck, the back, or the leg. When various neighbouring portions of skin are pressed at the same time, our judgment is greatly facilitated by the unequal excitement of the several parts. And it may be ren- dered still more exact by simultaneous muscular contractions or dis- placements. 1666. The sensibility of the various parts of the skin greatly influences our judgment of the peculiar forms possessed by the objects we touch. The perception which is produced by the pressure of a hollow tube or prism only becomes a distinct one, when the diameter of the apposed body exceeds the minimum distance of tactile sensibility. In like manner, a very sensitive part of the skin recognizes small roughnesses more accu- rately than one which is less so. On rubbing a woven hair- chain against the skin of the neck, we get an impression which is much less definite than when the experiment is repeated at the point of the tongue. 1667. The clearness of our perceptions is greatly increased when the tactile surface of the skin is made to glide over the body which is being examined. Our consciousness of the way in which the contractile tis- sues have to act in order to bring about the result, frequently affords a decision which must have otherwise been suspended. It is only with this assistance that the blind are able to recognize the forms of many bodies. 1668. The pressure which a quiescent body exerts on a tactile surface affords tolerable information as to its weight. Here again, if the muscles have to contract to a certain degree in order to afford the necessary re- sistance, the delicacy of the perception will be greatly increased : since our consciousness of the amount of contraction requisite for this purpose 496 ESTIMATION OF TEMPERATURE. [CHAP. XVII. facilitates a comparison. We may best convince ourselves of this fact by estimating a weight with the supported and unsupported hand suc- cessively. 1669. For example, Weber 44 found that, in the first case, weights could be distinguished which were to each other as 29 to 30. While, when wrapped up in a cloth, and suspended free, they were distinguish- able with proportions of 39 to 40. The author found that he could estimate weights about twice as delicately when he held them free in the hand, and executed the necessary movements. 1670. When a billiard-ball is allowed to roll down the cheek towards the lips, it appears to increase in weight : a deception which, though less marked, resembles that of the distances formerly mentioned ( 1662). And in general, the more susceptible parts of the skin appear to recognize smaller differences in the weights with which they have been gradually laden. Still we often find that parts (such as the hand and fore-arm) which differ greatly in their perception of distances, exhibit but very slight differences in this respect. Thinner portions of skin enjoy more advantage in this respect than in the determination of distances. 1671. The right hand appears to appreciate pressure better than the left. A mass which is very bulky, or which possesses a temperature very- different from that of the skin, is liable to be estimated by most people as heavier than it really is. 1672. When two nearly equal weights, of ecnial surface and tempera- ture, are placed upon the same part of the skin immediately after each other, the estimate is generally more correct, the shorter the interval of their application. Here, as in the other senses, the recollection gradually diminishes in delicacy. 1673. Although the tactile surfaces are capable of affording information as to temperature, still they cannot do this like a thermometer. For they give no impressions of fixed temperatures, but only of those changes which are produced by the equalization of differences in the amount of heat. A body in contact with the skin only appears to be warm or cold, in so far as it alters the temperature of the tactile surface itself. 1674. Although little is known of the mechanism of our sensations of temperature, still there are two points on which we may theorize with great probability. Since the volume of solids and fluids is altered although in a very slight degree ( 182) by changes of temperature, the molecules of the tactile organs will thus be displaced. This displace- ment perhaps re-acts upon the nerves of sensation which they enclose. But since it is chiefly the variations of temperature which we recognize, the impression that causes the perception must be transient and fluctuat- ing. This would offer a certain resemblance to the effects of that equalizing electrical curve ( 232) which obtains on making and breaking the circuit, and leads to contractions chiefly at those times ( 1241). CHAP. XVII.] ESTIMATION OF TEMPERATURE BY THE SKIN. 497 1675. The degree of heat already possessed by the skin forms the measure by which we generally estimate the temperature of other bodies. If we dip the hand in water at 104, a fluid at 89-6 will at first appear cold. But if the organ of touch have been in a liquid at 68, the same fluid will seem to be lukewarm. 1676. The conducting power and specific heat of the bodies in con- tact with our tactile surfaces will obviously exercise a great influence on the impressions of temperature which we receive. Hence, when a series of rods of the same form and bulk, but of different metals, occupy the same water-bath, the copper produces a very different impression to the lead. The feeling of cold caused by quicksilver may be explained in similar way. 1677. When a hot body touches the surface of the skin, its tissues become dry. This circumstance alone is capable of producing pain. The unpleasant impression therefore ceases, or at any rate diminishes, when the burnt portion of skin is held in cold water, until its previous state of tension is restored by transudation. But since fluids also burn, it follows that the equilibrium of the nerves may be permanently dis- turbed, not merely by this mechanical displacement of the cutaneous tissues, but also by the induction of higher degrees of temperature. And the long subsequent duration of the pain teaches us, that continuous molecular changes here occur. 1678. A person may dip his hand for an instant into a mass of molten metal without injury. This experiment, which is often performed by jugglers, essentially corresponds with that known in physics under the name of Leidenfrost's experiment. If a platinum spoon be made red hot, a drop of water flung into it will take a spheroidal form, and become rounded like a globule of mercury, without undergoing boiling. When the platinum is allowed to cool, there comes a point of time in which the water begins to boil violently. It is probable that, in the first case, the globule of water is surrounded by a layer of vapour, which prevents the necessary elevation of its temperature. In like manner, the epidermis is not only always moist, but is surrounded by other volatile substances. Hence the mass of vapour which it gives off forms a layer that is able to protect us from being burnt during a short space of time. 1679. We may easily convince ourselves that the different parts of the skin are not equally sensitive to the pain of burning. The region of the elbow is much more sensitive in this respect than some others, which can better distinguish the minimum distance of two bodies. The result seems to be chiefly determined by the thinness of the epidermis, and by the amount of nerves which the organs of touch possess. 1680. The locality of the organs of touch also determines the time after which the pain of burning appears. For example, the point of the tongue and the last joint of the index finger can remain 4 seconds, and K K 498 SENSIBILITY OF THE SKIN TO HEAT. [CHAP. XVII. that of the middle finger only 3 seconds, in water at 176. Heated substances which are placed upon more sensitive parts of the skin appear to us to be hotter. And when a large surface of the skin is dipped into warm or cold water, the fluid appears to be of a higher or lower tem- perature than if smaller tactile surfaces are used in the experiment. 1681. When the skin is penetrated by great cold, signs of numbness first appear; together with feelings of itching, creeping, formication, or stinging. The tactile sensibility suffers greatly ; so that the contact of bodies is much less distinctly felt. We experience an impression, as if some substance were interposed between them and the surface of the skin. It is probable that this feeling is chiefly due to the commence- ment of coagulation in the nervous medulla. The further effect of the low temperature is shown by a lively feeling of pain, which is not limited to the surface of contact, but shoots along corresponding distributions of the nerves. For instance, when the elbow is plunged into ice, the unpleasant, sensation sometimes extends down to the fingers. 1682. It may be stated generally, that pain is only produced by bodies which have a temperature under 50, or over 122. Hence burn- ing implies an amount which considerably exceeds that of the ordinary animal heat ( 1165), and must seriously disturb the molecular condi- tion of the nerves. 1683. The unequal temperature of the skin in different regions, which are subjected to different causes of cooling together with the different thicknesses of those bad conductors of heat which protect the organs of touch ( 204) render it impossible to construct an exact scale of the susceptibility to temperature possessed by its several parts. The former circumstance often deceives us when we attempt to determine the tem- perature of two fluids at once with both hands. While when mixtures the temperatures of which are nearly alike are examined with the same hand in immediate succession, we often come to a pretty accurate esti- mate. When no feelings of pain interfere, sensitive persons can distin- guish differences of -45 to -9. 1684. Painful feelings of temperature suppress the more delicate tactile sensations. But, under voluptuous excitement, the pain may be unnoticed, at least at the time. Persons suffering from eruptions are often seen scratching them until they bleed, with strong sensations of pleasure, which are only later replaced by more or less pain. And onanists frequently wound their genitals, in order more fully to satiate their unnatural lust. 1685. Although most of the organs of touch are symmetrical and in pairs, still two corresponding parts of skin never furnish a single impres- sion. Hence we have here no phenomenon which parallels the corre- sponding portions of the retinae. We do, however, meet with something analogous to double vision. But the double feeling manifested under CHAP. XVII.] DOUBLE FEELING. 499 certain artificial arrangements depends upon causes altogether different to those of the double images which are perceived in voluntary squint- ing, or with an unsuitable adjustment of the conducting lines ( 1549). 1686. When a ball (a Fig. 324) is rolled between the opposed sides of the index c and middle finger b, we get the ordinary single impression. But when, on the other hand, these two fingers are crossed as shown at e and/, and d is made to glide up and down, we fancy we feel two balls. The cause of this perception lies in the fact, that the evidence of FIG. 324. the active tactile surface is referred to our general judgment. The ordi- nary position b c offers two apposed concave surfaces, which our thoughts complete to a single ball. While, on the other hand, / gives a concavity which is referred outwards, and e one which is referred inwards, im- pressions which we therefore seek to complete as two more or less perfect spheres. But when we cross the thumb and little finger, this double feeling is absent; because our judgment is aided by the freer muscular movement of these parts, and by the less constrained position which this enables them to take. 1687. The subjective impressions of the organs of touch may be reduced to (or at least partially explained by) the different modes of action of which they are capable. Hence we have sensations of pressure, prick- ling, burning, shivering ( 1176), and the like. Such disturbances often give rise to errors of perception. A person suffering from partial paralysis of the soles of his feet not unfrequently feels, when standing, as if a bladder of water were placed under these organs. When a part of the upper or under lip is completely paralyzed, the patient sometimes fancies that a piece is broken out of the glass from which he is drinking. Here, as in our mental life, we are often inclined to charge upon others the subjective faults which belong to ourselves. CHAPTER XVIII. FIG. 325. INNERVAT1ON. 1688. The most important advantages enjoyed by the animal are due to the nervous system. For, in the first place, it forms an indis- pensable link in all those actions which are usually ascribed to the mental powers. Beside this, it conditionates the external sensations; induces the voluntary, and most of the involuntary, contractions; and exerts an indirect influence on all the other functions. And in particular, the contractility possessed by some of the constituents of most organs allows their adjustments to be modified by the nerves. It is on influences of this kind that most of the phenomena of self- d regulation ( 17), which impart such a complex character to the animal apparatus, chiefly or exclusively depend. 1689. The nervous system is divisible into J two chief portions; a centre, and a peri- g phery. The former appears at first sight to be constructed very differently in the vertebrate and invertebrate animals. But microscopic ob- servation teaches that the difference is not so m essential as might be thought from a mere examination with the naked eye. 1690. The annexed woodcut (Fig. 325) is a rude outline of the human nervous centre, and the commencement of its periphery, as seen from before and below. The centre consists of the brain, a b e /, and the spinal cord, s t. Each of these gives off a series of nerves, which are thence called the cerebro- spinal nerves; or the cerebral and the spinal nerves, according to their different origin. For example, c and d belong to the former group of nervous cords, and g hikl o p r to the latter ; which are subsequently distributed to the several tissues of the body. This contrast between the brain and spinal cord on the CHAP. XVIII.] NERVOUS SYSTEM OF 1NVERTEBRATA. 501 FIG. 32G. one hand, and the peripheric nerves on the other, is repeated in all the vertebrata. 1691. The nervous centre of the invertebrate animals does not consist of a continuous cerebral or spinal mass, but of a series of ganglia, which are united to each other by smaller nervous cords. But the forms which they take are very various. For ex- ample, Fig. 326 represents that generally seen in insects and most articulata. The greater part of the ganglionic cord runs near the under surface of the animal, and is therefore usually called the abdominal cord. A nervous ring, the cesophageal, surrounds the commence- ment of the alimentary canal. The ganglionic mass which lies above this, and hence in the superior half of the animal, is frequently desig- nated the cerebral ganglion. 1692. Periphery of the nervous system. Here an examination with the naked eye first distin- guishes two kinds of substance : w'z., nerves of a more or less cylindrical form, and ganglia which essentially correspond with the thickenings represented in Fig. 326. The nerves are composed of primitive nerve-fibres (Tab. V. Figs. 68 to 70). While on the other hand, the ganglia contain, in addition to these, peculiar structures called ganglion-globules or corpuscles (Tab. V. Figs. 71 to 74). The latter therefore form another important constituent of the peripheric nervous tissues. 1693. Every nerve-fibre (Tab. V. Fig. 68) consists of a thin membrane or sheath the neurilemma which encloses a peculiar oily content, the nervous marrow or medulla. The latter appears to be quite homogeneous during life. But after death, it is very apt to coagulate, especially on the access of water or other injurious fluids (Tab. V. Fig. 69). Sometimes the central part of the medulla separates from that around it; and not unfrequently coagulates in striae. A peculiar structure is thus produced, which is called the primitive band (b, Fig. 327), and which often protrudes from one end of a torn nerve-fibre. In other instances, how- ever, the central part is distinguished by its more homo- geneous characters. It has therefore been distinguished from the cortical part of the medulla by the name of the axis cylinder. 1694. A microscopic examination of the smaller and more transparent nerves will generally convince us that their several fibres run sepa- rately near each other, as shown by the diagram, Fig. 328. When a FIG. 327. 502 COURSE OF THE PRIMITIVE FIBRES. [CHAP. XVIII. branch A divides into two subordinate ones B and (7, the primitive fibres a b c d ef, which were formerly united in A, separate into two groups. The fibres abed are retained by (7, which is continuous with the chief trunk A ; while e and / pass into B. FIG. 328. FIG. 329. ft 6 C d f> f c a i V 1695. The anastomoses which unite many nervous trunks with each other depend upon similar circumstances. For instance, in the union of A and B (Fig. 329) to each other by the intervening branch JE, the fibres / and g pass from B to C, while d and e go from A to D. It is obvious that such arrangements will permit them to take a variety of courses. Other circumstances being equal, if E conduct more fibres from B to C, than from A to D, C will be larger than A. 1696. The frequent repetition of such anastomoses, and the reticular union they produce, give rise to the plexuses of the nerves. The spaces between their meshes may be filled either by ganglion-corpuscles, or by less important structures. In the latter case the plexus is called simple, and in the former, compound. In any case there is a manifold inter- change of nerve-fibres. 1697. From what has just been stated it is evident, that the branches and unions of the nerves have quite a different signification from those of the vessels. The tubes traversed by the lymph or blood really divide, and often really unite with each other. But in the nerves these appear- ances are generally deceptive. Their branches and anastomoses are usually due to a mere change in the situation of their minute elements, i.e., of their primitive fibres, which are only brought into view by the magnify- ing glass. The law of these structures obliges them to take an isolated or separate course : in short, like the coiled wires of an electric spiral CHAP. XVIII.] DIVISION OP THE NERVE-FIBRES. 503 ( 220), they are virtually distinct from each other, and only pass in company. 1698. Until lately this law was supposed to have no exception. But recent researches have led to the conviction, that true divisions do some- times occur. The commonest example of this kind is given by Tab. V. Fig. 70; where the fibre a divides into the subordinate branches b and c. Divisions into many small branches are less frequent. They have, however, been seen in the membranes endowed with sensation, in the organs of locomotion, in voluntary and involuntary organs, in the larger nervous trunks, and in the terminal distribution of the nerves. They certainly seem to be most frequent just before the nerves reach the end of their peripheric course. Still they are sometimes present in situations prior to this. 1699. Supposing that the above law always held good, and that every fibre was completely isolated throughout its whole course, the number of primitive fibres in the total peripheric system would not exceed that contained in the roots of the cerebral and spinal nerves. But the presence of these divisions augments the number of primitive fibres. Hence it must directly increase the quantity of nervous medulla. It also leads to many other physiological results, to which we shall here- after return. 1700. Those smaller branches of the nerves which run in the interior of organs generally form plexuses; which can be recognized, either with the naked eye, or under low magnifying powers. They are called the terminal plexuses of the peripheric system. It is obvious that their first effect is to mix the various primitive fibres. But there are physiological reasons for conjecturing that this is not their exclusive object. In many cases we find that a primitive fibre passes into the plexus of another branch, to return, after some time, into its former trunk. From this we might conclude that the presence of the terminal plexus subserves two other purposes. It lengthens the path which the primitive fibres have to traverse before reaching their peripheric termination; and hence in- creases the quantity of the active nervous medulla. It also enlarges the number of the mutual points of contact, and thus multiplies those collateral actions of the several nerve-fibres which they permit. 1701. Many nerve-fibres seem to lose their distinctly medullary con- tents before reaching their peripheric termination. This peculiarity, as well as the division of the medullary fibres, is better seen in the plates of the electrical organs of the torpedo than in any other structure. The oily content af (Fig. 330) disappears at 6, beyond which we see only yellowish-grey branches apparently void of medulla. The latter are enclosed in a thick membrane ; so that it remains at present undecided what are their contents, and how the transition occurs at b. Rudolph Wagner and other observers believe that something similar to this 504 TERMINATION OP THE NERVE-FIBRES. [CHAP. XVIII. is repeated in the muscles, the vascular glands, and many other organs. 1702. The way in which the nerve-fibres end in the various organs of the body has been a subject of much dispute. The opinion, that they become continuous with the several foreign tissues in their neighbour- hood, is very properly abandoned by almost all observers. At present the contest lies between two theories. One of these supposes that the nerves have a free termination, and either contain medulla to the last, or undergo a transition into those apparently marrowless fibres just men- tioned ( 1701). The other regards them as ending by curved loops, in which two fibres rifeet in the form of an arch, as indicated by Fig. 331. FIG. 331. FIG. 332. 1703. The chief difficulty of deciding the question arises from the fact, that there are very few organs in which the nerves can be unmistake- ably followed to their termination. The appearances of a free extremity are open to this objection, that it is always possible, or even probable, that their course has been but imperfectly followed. And against the looped ends it may generally be alleged, that we are perhaps only look- ing at a simple bend of a nerve-fibre, which subsequently continues onward to its true termination. But since these loops are found in small organs, such as the tooth-sacs, which can be completely and thoroughly inspected, this objection cannot always be maintained. 1704. In the mammalia, many of the smaller nerves are occupied by peculiar enlargements, which are usually called Pacinian corpus- cles. They are most frequently found in the nerves that run in the mesentery of the cat, and in the palm of the human hand, and sole of CHAP. XVIII.] PACINIAN COBPUSCLES. 505 the foot, One of the simplest forms of these bodies is represented by Fig. 332. It consists chiefly of a bulbous and stratified capsule a, in the central canal b of which runs a primitive fibre e. There is often a peduncle c at the place of its attachment. The nerve-fibre d gradu- ally undergoes an alteration in the character of its medulla. It becomes paler ; and on entering the capsule a it sometimes divides, and frequently ends by becoming very indistinct. Hence many have supposed that the Pacinian corpuscles plainly enunciate the general law of a free termina- tion of nerve-fibres. But there are several objections to this view. We may sometimes distinctly see that the nerve-fibre only passes through the Pacinian corpuscle. It may also be questioned whether the whole structure is not due to extraordinary collateral circumstances, which can finally break off its connection with the primitive fibre. While the proportionate number of nerve-fibres which enter into Pacinian cor- puscles is so small as scarcely to warrant us in deducing any general law. 1 705. So far as we know, each particular nerve-fibre has a determinate direction of activity has, so to speak, a certain one-sided mode of action, a peculiar and special energy. These influences are divisible into three kinds. The fibres of the nerves of special sense respond to the stimuli which meet them by their proper sensuous phenomena, by seeing, hearing, smelling, or tasting ; and those of the sensitive nerves, by sensations of touch or pain. On the other hand, the motor nerve- fibres excite the muscles to contraction. 1706. We shall hereafter see that every local irritation of a nerve results in a certain molecular change of its nervous medulla. Where the stimulus and its after-effects are strong enough, the excitement is pro- bably propagated along the whole course of the primitive fibre. But both in the centre and periphery, the fibre adjoins certain foreign tissues. And hence two modes of action are possible. The stimulus may excite, either certain parts of the centre, or the corresponding peripheric organs. If one of these acts as the cause, the second will appear as the effect. The three groups mentioned in the preceding paragraph differ essentially from each other in this respect. 1707. Let us suppose a b c (Fig. 333) to be sensuous, de sensitive, and fg k motor, nerve-fibres. The two first classes will conduct their im- pressions centripetally : i.e., from the peripheric organ of sense towards the nervous centre; or in the directions DC A and E B. Thus the rays of light which impinge upon the retina, or the waves of sound which strike the auditory nerve, begin the corresponding action : which ends by its respective impression being perceived at the centre. The action of the motor fibres is precisely the reverse of this. When we voluntarily contract a muscle, the change begins in a certain part of the brain. The excitement of the motor nerves then takes a peripheric 506 PEOPAGATION OF IRRITATION. Fi>. 333. [CHAP, xviii. course from E towards F: and the contraction of the muscles is its final result. 1708. Hence we may regard each of the above nervous actions as consisting originally of three processes. The stimulus excites the peri- pheric sensuous or sensitive organs; and the central motor organs. And just as the phenomena of induction translate the electric current into magnetism, and vice versd ( 250), so the first stimulus is converted into a second, which runs along the nerve- fibres. When this has been conducted centrally to the brain, or peripherally to the organs of movement, a second transfer occurs; which is the counter- part of the first, and produces the final result. 1709. It is not necessary that the original stimulus should proceed from those tissues which surround the peri- pheric ends of the centripetal fibres, or the central ends ot the centrifugal fibres. On stimulating a motor nerve in the middle of its course, the corresponding muscles contract : or repeating the experiment on a sensuous or sensitive trunk, a sensuous or painful impression is produced. 1710. That molecular change of the nervous medulla upon which the activity of the primitive fibres depends, can only be propagated when the particles retain their natural arrangement and properties. This proposi- tion, which we have already been obliged to lay down for the phenomena of movement ( 1237), explains why the action of the nervous medulla is paralyzed by section, chemical or electrolytic changes, or serious distur- bances of its nutrition. At the same time, it is obvious that the pheno- mena of the centripetal and centrifugal nerves will be, to some extent, the reverse of each other. If we suppose ef, Fig. 32 8, to be two sensuous or sensitive fibres, having their peripheric ends below, an injury at i Jc will completely paralyse I ; while m, which retains its connection with the brain, will still be capable of producing subjective sensuous perceptions or pains. And conversely, if e and / were motor, I could produce mus- cular contractions, while m would have no effect. Hence the capacity of excitement is retained by the central segment of the centripetal nerve- fibre ; and by the peripheric segment of the centrifugal one. 1711. The site of the injury must also determine the extent of the subsequent paralysis. When the section is at ik (Fig. 328) only e and/ are rendered inactive. But when it is above, at^A, the disturbance CHAP. XVIII.] STIMULATION OF PARTICULAR NERVES. 507 extends to abcdef. It is obvious that the plexuses will give rise to great complications in this respect. FIG. 334. 1712. That mixture of the several primitive fibres which their mutual anastomoses effect ( 1695), cannot be satisfactorily followed for any distance, either with the naked eye or the microscope. Hence ana- tomical research rarely furnishes trustworthy details respecting the course of a single nerve-fibre, or even of the larger bundles and roots of nerves. But physiological experiments sometimes afford more satisfactory re- sults. For instance setting aside those collateral disturbances which we shall hereafter mention the stimulation of a particular nerve always leads to the contraction of that corresponding portion of muscle, in which its fibres terminate. Something similar obtains with the tactile impressions of a given part of the cutaneous surface. Knowing the nervous injuries after which the sensibility of any part is lost, we may thence deduce the terminal distribution of its corresponding sensitive nerves. 1713. For instance, Fig. 334 shows the sciatic plexus of a frog, laid bare from behind. It contains four chief trunks : a is the inguinal or seventh spinal nerve, b is the femoral or eighth, c is the sciatic or ninth, and d the pubic or tenth. All four subsequently unite in a single trunk j which supplies the corresponding hind leg, and the neighbouring structures. 508 DISTRIBUTION OF THE DIFFERENT FIBRES. [CHAP. XVIII. 1714. On stimulating the inguinal nerve a of a newly killed frog, a great part of its femoral muscles contract : together with in excep- tional instances some muscles of its leg and toes. Stimulation of the femoral nerve b, and of the sciatic c, is responded to by the muscles of the thigh, leg, and toes. The action of the pubic nerve d is often limited to the muscles of the anus and coccyx : but may extend to some of the muscles of the hind leg, and down to the toes. 1715. When these experiments are made on a number of frogs, the details of their results often differ from each other. For although the chief regions affected are tolerably constant, still muscles which obey a particular nerve in one frog, are excited in a second animal by another. In addition to this, the same muscle often appears to be governed by several nerves. Indeed, the quadriceps extensor of the thigh is some- times excited to contraction by all four trunks of the sciatic plexus. 1716. This peculiar phenomenon may be ascribed to various causes. It often happens that primitive fibres of two or more nerves enter the same muscle. So that here the result corresponds to the fact.* But it is also possible that these appearances are deceptive. Thus we shall hereafter see that the excitement of one fibre may possibly be trans- ferred to another near it, especially in the region of the terminal plexus. In electrical irritation by means of the electro-magnetic ( 248) or rotary ( 252) machine, this is very liable to occur. Hence there is reason to doubt all statements which enter too much into details. 1717. The study of the nervous centre will teach us that muscular movements are not only produced by the motor nerves, but at least mediately by sensitive fibres. On irritating any part of the skin, the change is propagated along the sensitive fibres to the spinal cord or brain. Here the impulse may be transferred to certain motor fibres. And hence the corresponding muscles may contract as though they were thrown into action by the influence of the will. The contractions pro- duced in this indirect way are called reflex movements. They can only occur where the co-operating sensitive and motor fibres, and the cor- responding part of the nervous centre, alike retain their force. But section of the sensitive fibres removes one of these essential condi- tions. So that we have here another means of determining their terminal * Ever}' anatomist must have remarked great varieties in the distribution of the small nerves of the arm and leg. Now we cannot doubt that the function of each fibre requires that it should unite some two specific points of centre and periphery. And it is just as obvious, that the co-ordination of several fibres will imply an equally definite relation of each to all the others. It is possible that this is effected by a suitable admixture of fibres in the plexus. But, in any case, these varieties sufficiently prove, that a nerve with a given name need not and does not represent a definite collection of really identical fibres. From obvious reasons ( 1697) the exact composition of any particular branch seems to be neither constant nor essential. Beyond the plexus, at least, its several fibres may vary their routes without any alteration of their functions. And the frequency of such deviations would quite account for the diverse results obtained by stimulating what is to all appearance the same nerve iu two different animals. EDITOR. CHAP. XVIII.] PURE AND MIXED NERVES. 509 distribution in an animal which is newly killed, and still inclined to these movements. For instance, we have but to divide the various trunks of the sciatic plexus, or their sensitive roots ( 1720), and exa- mine what parts of the skin have lost their excitability to reflex move- ment. In this way Eckhard 45 ) found that the sensitive fibres of the seventh spinal nerve (a, Fig. 334) were distributed chiefly to the thigh, and those of the eighth (b) to certain parts of the skin of the whole hind leg, those of the ninth (c) to the leg and foot, and those of the tenth (d) to the neighbourhood of the anus. The sensitive and motor fibres included in one nerve, do not always run to the skin and muscles of the same part of the body. 1718. The action of the various parts of the peripheric nervous system is one of the chief problems of that part of the science of life with which we are now occupied. We have to determine what functions belong to the roots and trunks of the different nerves, and whether they are pure or mixed : i.e., whether they contain only one kind of fibres sensuous, sensitive, or motor or two or more of these together. Mere anatomical examination can decide nothing in this respect : since sensitive fibres frequently pass through the muscles; while the organs of touch often enclose contractile tissues, to which motor fibres are pro- bably given off ( 1235). Hence such decisions require the aid of phy- siological experiments. 1719. The spinal cord of the human subject (st, Fig. 325, p. 500) gives off a double series of nervous cords, which are called the roots of the spinal nerves. The annexed woodcut (Fig. 335) represents a piece of the spinal cord, a a, of natural size. Its external membrane or dura mater, which is slit up and opened out posteriorly, is indicated by b. At c is the denticulate ligament, which is covered by a fold of the arachnoid or middle membrane of the spinal cord. Finally, the pia mater or innermost sheath lies immediately on the medullary substance of the spinal marrow itself. On the left side are seen the uninjured posterior roots of the spinal cord. On the right they have been cut through ; the central part d being still attached to the spinal cord, while the peripheric portion e is severed from it. These roots subsequently enlarge into the posterior spinal ganglia h. Each anterior root / finally unites with a posterior to form the trunk of a spinal nerve. Imme- diately after leaving the intervertebral foramen, this divides into an anterior and a posterior branch, i i, which are destined to supply the corresponding parts of the body. These trunks next ramify with nu- merous anastomoses, and finally terminate in the various organs of the trunk and limbs. 1720. It is to Sir Charles Bell that we owe the discovery that, as regards the tactile surfaces of the skin, and the voluntary muscles of the trunk and limbs, the posterior roots of all the spinal nerves are 510 LAW OF BELL. [CHAP, xviii. purely sensitive, and those of the anterior purely motor.* We may easily convince ourselves of this in the frog. On laying bare the nervous FIG. 335. FIG. 336. centre of this animal from behind, and cutting through, on one side, the posterior roots of the four last spinal nerves (16, 17, 18, 19, Fig. 336), which enter into the sciatic plexus ( 1713), and supply the corresponding hind leg, the limb loses all sensibility to touch or pain. The foot may be burnt to a coal, without any sign of suffering from the animal. In spite of this, however, he moves the stump at will. And conversely, when the anterior roots are divided on the other side of the same animal, the will has no longer any influence on the limb; while tactile impres- sions are plainly perceived by its skin. The law thus discovered by Bell plainly shows, that the various primitive fibres of the spinal cord emerge with a symmetrical arrangement. The posterior are aggregations of sensitive structures, without any which can subserve to muscular motion. The anterior exactly reverse this proposition. * The complete accuracy of this statement is somewhat affected by an important experi- ment, which we owe to M. Magendie, and which establishes what he calls the " recurrent sensibility" of the anterior roots. In other words, the motor root is also sensitive to pain. And this property is derived from the corresponding sensitive trunk. For after section of the anterior root, the central of the two cut surfaces is insensible, while the peripheral is sensi- tive. And finally, cutting across the posterior root deprives the peripheral surface of all sensibility. In 1 850, the kindness of M. Claude Bernard enabled me to verify the details of this discovery on a dog, which he subjected to the above experiments with his usual admirable skill. EDITOR. CHAP. XVIII.] THE CEREBKAL NERVES. 511 1721. The trunks which arise from the fusion of the two roots are obviously mixed nerves, as are also their subsequent branches. It is doubtful whether there is any further separation of the sensitive and motor fibres in the terminal plexus ( 1718). 1722. Those parts of the nervous centre which are enclosed in the cavity of the skull, give off several pairs of nervous trunks, which are usually designated the cerebral nerves. The greater part of these, how- ever, do not proceed from the brain itself, but from its connection with the spinal cord, and brain, or from the medulla oblongata. Strictly speaking, the brain only gives off the olfactory ( 1609), and part of the optic, nerves. The common nerve of the muscles of the eye proceeds from the crura cerebri, and the remaining cerebral nerves come imme- diately from the pons varolii and the medulla oblongata. 1723. The cerebral nerves of the right side are shown, as they leave the cavity of the skull, in Fig. 337. The brain itself has been removed in order to allow a better view of the nerves within the skull. The 1st is the olfactory nerve o; the 2nd, the optic p ; the 3rd, the motor oculi, q; the 4th, the nervus trochlearis seu patheticus, r; the 5th, the trigeminate nerve, s; the 6th, the nervus abducens, or nerve to the external rectus, t; the 7th, the facial nerve, u, and the auditory nerve, v; the 8th, the glosso-pharyngeal, w, the vagus or pneumo-gastric, x, and the spinal accessory, y; and, finally, the 9th, the hypoglossal nerve, z* 1724. The olfactory nerve (o, Fig. 337), the branches of which are distributed in the mucous membrane of the nose (p, Fig. 322, p. 484), is the medium of the impressions of smell. Stimulating it gives rise to subjective olfactory phenomena ( 1629). But no pain or direct muscular contractions result. 1725. In like manner, the optic nerve (p, Fig. 337), the fibres of which are distributed over the retina, gives rise to visual impressions which are unaccompanied by any direct sensitive or motor reactions. The reason why both optic nerves are united in the chiasma (Fig. 259, p. 429) has not yet been discovered. 1726. The third nerve (q, Fig. 337) contains a large number of motor fibres, which are distributed to most of the muscles of the orbit, to the iris (b, Fig. 150, p. 273), and probably to other structures in the interior of the eye. It governs the levator palpebra3 superioris (g, Fig. 260, p. 430), * Here the enumeration of the cerebral nerves used in this country has been substituted for that given in the original. In the latter, the facial and auditory nerves form the 7th and 8th : while the glossopharyngeal, pneumogastric, and spinal accessory are the 9th, 10th and llth; and the hypoglossal completes the series of 12. Both these sets of numbers are so far objectionable that they represent as a nerve the lobe of the nervous centre from which the proper olfactory nerves arise. But that generally adopted by British authors has the graver disadvantage of confounding in its seventh number a motor (the facial), a sensuous (the auditory), and probably also a sensitive (the portio intermedia) nerve. A similar objec- tion might be made to the union of three equally diverse elements in the eighth nerve. EDITOR. 512 THE CEREBRAL NERVES. [CHAP, xviii. the superior (s, Fig. 259), internal (i, Fig. 259), and inferior rectus, and the inferior oblique muscle of the eye (m, Fig. 259); and supplies the short root of the optic or lenticular ganglion, which gives rise to the ciliary FIG. 337. nerves. These pass amongst other places to the iris, the median aperture of which is formed by the pupil (c Fig. 150, p. 273). In this way, the motor oculi nerve is enabled to assist in the production of those movements on which the enlargement or diminution of the pupil depends. In some exceptional instances two other muscles of the orbital cavity, namely, the external rectus (n, Fig. 259) and the superior oblique (e o, Fig. 259), receive, besides other nervous fibres, twigs of this cerebral nerve. 1727. Experiments on newly killed animals will easily convince us that the whole of the organs of movement supplied by this nerve are thrown into contraction when it is suitably stimulated. But it has been often disputed, whether it is a purely motor or a mixed ( 1718) nerve. For when it has been laid bare and irritated at its cerebral origin ( 1722), CHAP. XVIII.] TRIGEMINAL NERVE. 513 many experimenters have observed expressions of pain. And although these have not been noticed by others, still the great injuries implied in the observation throw doubts upon all negative results. 1728. The fourth or trochlear nerve (r, Fig. 337) passes to the supe- rior oblique muscle of the eye (o, Fig. 259, p. 429). Its motor influence may often be verified in the dead body. 1729. The fifth or trigeminal nerve (s, Fig. 337) arises by two roots, a larger (v, Fig. 322, p. 484), and a smaller (w). The greater part of the primitive fibres of the former subsequently enlarge to form the Gasserian ganglion (#). The whole nerve then divides into three chief branches ; the ophthalmic, the superior maxillary, and the inferior maxillary, divi- sions of the fifth. 1730. The two roots of the trigeminal nerve have often been com- pared to the double root of a spinal nerve ( 1720). Here the larger (v, Fig. 322) would correspond to the posterior, and the smaller (w) to the anterior, root of a spinal nerve ; the former containing only sensitive, and the latter only motor, fibres. 1731. The results observed in recently killed animals appear so far to sustain this view, as that irritation of the larger root causes no movement in those striped muscles which its branches penetrate, while stimulating the smaller root gives rise to vigorous contractions of the muscles of mas- tication ( 366). Still we ought not to forget that many structures which are moved involuntarily, and are provided with unstriped muscular fibre ( 1231), such as the ducts of the lachrymal glands ( 890), are supplied by twigs which come from the ophthalmic branch, i.e., to all appearance, from the larger portion of the root. On the other hand, vivisection fails to certify that the small root originally contains no sensi- tive fibres. 1732. Much of the tactile sensibility possessed by the organs of the senses, and by the skin of the face, is due to the trigeminal nerve. Its ophthalmic division (y, Fig. 322, p. 484) supplies the lachrymal gland; the inner part of the globe of the eye; the conjunctiva (d, Fig. 150, p. 273); a large part of the mucous membrane of the nose (Fig. 322, p. 484) and its supplementary cavities; together with the skin of the forehead, the anterior part of the skull, the upper eyelid, and a great part of the outer surface of the nose. The superior maxillary branch (z, Fig. 322, p. 484) supplies the remainder of the mucous membrane of the nose, and of its supple- mentary cavities ; a considerable extent of the mucous membrane of the Eustachian tubes, the upper part of the pharynx (/, Fig. 322), the soft palate (u), the tissues of its neighbourhood, the membrane covering the hard palate (h), the interior of the upper jaw with its gums and teeth, the skin of the lower eyelid ; a great part of the middle of the nose, and of its inferior half ; the surface of the cheeks as far as the temples, and of the upper lip. The inferior maxillary branch (a/ Fig. 322, p. 484) L L 514 FACIAL NERVE. [CHAP. XVIII. supplies the skin of the temples, of part of the external ear, of the lower part of the face and the lower lip ; the inferior surface of the cavity of the mouth; the gum and teeth of the lower jaw; and the greater part of the surface of the tongue. The fibres which pass to all these organs effect tactile sensations. The motor fibres of the third branch of the trigeminal nerve govern the more important muscles of mastication (the temporal, masseter, and pterygoids) ; together with some other muscles of the hyoid bone (the mylo-hyoid, and anterior belly of the digastric), and the tensor tympani ( 1585) of the tympanic cavity. 1733. Each lateral half of the head receives its several branches from the trigeminal nerve of the same side. The branches of the right nerve do not pass over to the left side, nor do those of the left towards the right. Hence paralysis of the right nerve only affects the correspond- ing organs of the same side. The distinction is very marked ; the loss of sensibility being limited to the right half of the upper or lower lip : a fact which shows that there is very little transit of fibres from one side to the other. 1734. The sixth nerve (t, Fig. 337, p. 512) supplies the external rectus muscle (n, Fig. 259, p. 429) with motor fibres. Occasionally, it also gives off twigs to the optic ganglion ( 1726); as well as to other muscles of the eye. 1735. While the skin of the face derives its chief sensitive fibres from the trigeminal nerve ( 1732), the movements of the features are regu- lated by the facial nerve (u, Fig. 337, p. 512). This governs the muscles of the face and external ear, the stapedius in the tympanum, and some of the muscles of the neck (the stylo-hyoid, the posterior belly of the digastric, and the platysma myoides). Filaments also pass from it to the muscles of the soft palate. Irritation of the roots of the facial nerve in the newly-killed animal excites all these parts to contraction. At present, the roots of the facial nerve cannot be proved to enclose sensitive fibres. But it certainly receives many such elements during its subsequent course. Hence its trunk, on emerging from beneath the ear, belongs to the class of mixed nerves. Still the number of its motor fibres remains greatly predominant. 1736. Each of the two facial nerves is also distributed solely to its corresponding half of the head. In man, it frequently happens that one of these nerves is seized with temporary or permanent paralysis. Under such circumstances, the corresponding muscles of the face are deprived of their action. The effect of this is to destroy all the ex- pression of the physiognomy, and the play of the features. An addi- tional influence is exercised by the still active contractile structures of the other side. The muscles of the right half of the face are antago- nized ( 1309) by those of the left. And, under normal circumstances, each side is, to a certain extent, held in check by the other; so that all CHAP. XVIII.] GLOSSO-PHARYNGEAL NERVE. 515 parts of the face are kept in lateral symmetry. The median line thus forms a straight line ; which is, as it were, a graphic that forms the sum of these positive and negative values. So that the paralysis of the right facial nerve removes an important force which formerly opposed the elastic or vital contraction of the left facial muscles. Hence the mouth is often drawn towards the left or healthy side. 1737. The auditory nerve (v, Fig. 337, p. 512) conducts the impres- sions of hearing. In it, as in the olfactory and optic nerves ( 1724), it is impossible to detect any fibres sensitive to pain. 1738. The functions of the glosso-pharyngeal nerve (w, Fig. 337) are very variously interpreted. The site of their origin and termination, and the doubtful manner in which animals express their sensations, lead to many indistinct phenomena, which we can only judge of sub- jectively. 1739. When the glosso-pharyngeal nerve of a dog or other domestic animal is examined immediately after its exit from the skull, we fre- quently get evident signs of pain. From this it has been inferred, that sensitive fibres are originally present. But the nervous trunk emerges from a very sensitive part of the medulla oblongata, the action of which may therefore be easily confounded with that of the glosso-pharyngeal nerve. At any rate, however, the glosso-pharyngeal does not belong to that class of nerves which is chiefly sensitive. 1740. On stimulating the smaller root of this part of the eighth nerve in newly-killed calves or cats, Volkmann has observed contractions of some of the pharyngeal muscles (the stylo-pharyngeus, and constrictor faucium medius). Still this motor influence cannot be compared with that of the vagus and accessory nerves. For instance, it often happens that irritation of the pneumogastric and accessory causes vigorous con- tractions, after the glosso-pharyngeal has long ceased to afford any. Hence many observers have been unable to verify the motor influence of this nerve. 1741. These facts sufficiently prove, that the greater part of the glosso- pharyngeal nerve is composed of neither sensitive nor motor elements. On the other hand, experiment shows it to be a sensuous nerve, which is destined to effect the true sensations of taste. The extent of this part of its function is at present the main object of dispute. 1742. We will regard the tongue as the special representative of the organ of taste ( 1633). This organ is chiefly supplied by branches from three nerves the trigeminal (a, Fig. 337), the glosso-pharyngeal (w), and the hypoglossal (2). All observations unite to state that the manifold movements of the tongue are due to the hypoglossal nerve (/, Fig. 323, p. 489); while the most important gustative impressions are effected by the glosso-pharyngeal. But many consider the latter the sole nerve of taste ; and regard the trigeminal nerve as only effecting L I, 2 516 VAGUS NERVE. [CHAP. XVIII. sensations of touch and pain. While others suppose it to be also capable of recognizing true sapid impressions. 1743. Repeated observations on living mammalia speak with increas- ing certainty to the fact, that the glosso-pharyngeal is the sensuous, and the trigeminal the sensitive, nerve of the tongue and most of the other organs of taste. Some instances of paralysis of the trigeminal nerve in the human subject may be understood in this sense. And we must not forget that many impressions, which are ordinarily considered sensations of taste, will be assigned to the sense of touch on a more careful ex- amination ( 1630). 1744. The vagus nerve (x, Fig. 337, p. 512) swells into a ganglion shortly after its origin. Here a number of fibres of the spinal accessory (y\ are apposed to those of the pneumogastric, and proceed onwards with the branches of the latter nerve. Hence most of the branches of the vagus contain a mixture of fibres from the pneumogastric and the spinal accessory nerves. 1745. An attempt has been made to compare the roots of these two nerves with the larger and smaller division of the trigeminal nerve ( 1730), or with the two roots of a spinal nerve. The vagus is supposed to be a purely sensitive nerve, and the spinal accessory an exclusively motor one. But recent experiments show that, when the filamentous roots of the vagus of a newly-killed animal are cut through at their origin from the medulla oblongata, their irritation can cause various muscles to contract. And some observers assert that the upper rootlets of the spinal accessory give rise to pain. Now since experiments on the living animal establish beyond all doubt that the vagus is sensitive, and the spinal accessory motor, it would seem that both of these nerves possess mixed qualities. But we shall hereafter become acquainted with some experiments which perhaps oppose this conclusion. 1746. On irritating the roots of the vagus in a rabbit, the animal shrieks with pain. When the same nerve is pinched in a newly-killed dog, vigorous contractions of the soft palate, the oesophagus, and the stomach, often follow. It may be further proved, that the roots of the pneumo- gastric nerve exert an important influence on the functions of the heart. The small muscles of the larynx are undoubtedly governed by those trunks which proceed from the union of the vagus and accessory nerves ( 1744). Many assert that stimulating the roots of the pneumogastric in recently killed animals causes these parts to contract while others make the same statement of the spinal accessory. This contradiction may be due to two causes. Some of these bundles, which lie midway between the vagus and accessory, may be assigned to either nerve, according to the judgment of the observer. In addition to this, we shall see that there are many facts which indicate that a sort of transfer here occurs: so that a stimulus applied to an anterior rootlet of the CHAP. XVIII.] VAGUS NERVE. 517 pneumogastric may excite the action of other fibres, and even of those which belong to the accessory nerve. 1747. Numerous twigs of the vagus pass to the pharygneal plexus, on which much of the sensation and motion of the soft palate and pharynx depends. The oesophageal plexuses have similar anatomical and physiological relations. So that the united actions of the vagus and accessory exert a great influence on the mechanism of deglutition. Section of the cervical descending trunks of both pneumogastric nerves (h, Fig. 101, p. 187) is soon followed by difficulty of swallowing; and fragments of the food easily pass into the larynx and trachea ( 372). The oesophagus not unfrequently becomes distended with alimentary matters. And in dogs and cats, a remarkable inclination to vomit is generally superadded, especially when the stomach is full. 1748. Each side of the larynx receives two branches from the cervical trunk of the vagus (1i, Fig. 101). The superior laryngeal branch is given off above the larynx. The inferior one comes up out of the thorax, and then ascends along the trachea (near k, Fig. 101), to enter the larynx. Both are nerves of a mixed nature; but the superior is chiefly sensitive, and the inferior chiefly motor. The former supplies but a small part of the muscular structures of the larynx and the adjoin- ing pharynx (the crico-thyroid, b, Fig. 253, p. 419, and h, Fig. 71, p. 127); while the latter is distributed to the remaining small muscles of the larynx (the crico-arytsenoideus lateralis, c, Fig. 254, p. 419; the crico-ary- teenoideus pcsticus, b; the thyro-arytsenoid, d; the arytsenoideus transver- sus, e, and obliquus/). This distribution may also be established physio- logically, by irritating these nerves in the newly killed animal. But their mere anatomy is less conclusive; for the superior and inferior laryngeal branches are so united to each other in the interior of the larynx, that it is impossible to demonstrate the course of their several fibres. The sensitive elements are chiefly distributed to the mucous membrane of the larynx, but probably also form a partial supply for that of the adjoining pharynx. 1749. The sensibility of the mucous membrane of the trachea, and the contraction which follows an artificial irritation of this tube ( 1306), are also dependent upon the cervical trunk of the vagus. The nume- rous branches which it gives off to the pulmonic plexuses in the thorax, and (through these) to the lungs themselves, exert a considerable in- fluence on the sensation and motion of these important parts of the respiratory apparatus. Hence the two vagi nerves can effect great changes in the function of respiration. 1750. When the two inferior laryngeal nerves, or the cervical trunks of the vagus, are cut through in a new-born animal, it soon dies of suffocation. But if a fistulous opening be at the same time made into its trachea ( 1414), life may be maintained for a while. Older animals 518 VAGUS NERVE. [CHAP. XVIII. can better resist this injury ; so as to live many hours or days. The above experiment proves that the speedy death of the young mammal depends upon some obstacle to respiration situate above the trachea. It is, in fact, caused by the circumstance, that on the paralysis of the laryngeal muscles, the vocal cords shut up like a valve, and thus close the glottis. This produces a mechanical obstacle to respiration ( 738), which can be obviated by a tracheal fistula. 1751. Although young animals in whom an artificial respiratory aper- ture has been made, remain alive, and older ones do so even without this assistance, still both die, at latest, a few days after the infliction of this injury on the nerves. For it is followed by a degeneration of the lungs, which gradually undermines and destroys life. Many observers conclude, with Schiff, that the paralysis of the vagi seriously disturbs the nutrition of the lungs, and causes more or less inflammatory phenomena, which are accompanied by peculiar exsudations. Others, with Traube, attribute the whole result to mechanico-chemical causes. They suppose that the food and mucus which pass from the commencement of the oesophagus into the trachea and bronchi ( 1747) irritate the lungs, and thus gradually cause their degeneration. 1752. On compressing the cervical trunk of the vagus in a newly killed mammal, some muscular bundles of the ventricles of the qui- escent heart often contract anew. This is best seen in an experiment introduced by Ed. Weber and Budge. It answers equally well in mammals, birds, amphibia, and fishes. But since it is most easily made upon frogs, we will describe the phenomena as seen in this animal. 1753. At A, Fig. 338, is the heart of a frog, turned back, and trans- fixed with a needle; and at c is the trunk of its left vagus. This gives off the large cardiac branch// which, with the one opposite, then forms a ganglionic plexus in the neighbourhood of the auricle. 1754. When the shocks of the electro-magnetic machine ( 248) are made to act upon / or c in a newly-killed (and still highly sensitive) animal, the pulsation of its heart is instantly brought to a stand-still. If the electrical action be not continued too long, the state of diastole ( 576} will last during the whole time of the experiment. But if it be continued, the heart after some time recommences beating. The roots of the vagus or accessory nerve (y, Fig. 337, p. 512) lead to what are essentially the same results. 1755. When the cervical trunk of the pneumogastric nerve of a newly killed mammal is subjected to mechanical, chemical, or galvanic irrita- tion, movements are often seen in its stomach. After section of this nerve in the living dog, it has sometimes been remarked that, although the unhappy animal does not seek after food, still it occasionally devours Jarge quantities of that which is offered to it. Hence it has been sup- CHAP. XVIII.] VAGUS NERVE. 519 posed that the paralysis of the sensitive fibres of the vagus prevents the feelings of hunger and satiety. But future researches must decide as to the accuracy of this conclusion. FIG. 338. 1756. Section of both vagi in the neck of an older ( 1750) dog or rabbit, seriously disturbs the process of gastric digestion. This is always rendered slower. Some portions of the food, such as hard meat, appear to be scarcely attacked at all. But the gastric contents may still have a strong acid reaction ; and the acidulated artificial digestive fluid which has been prepared from the stomach of an animal thus ope- rated upon, can effect the rapid solution of coagulated albumen ( 439). So that those organic contactive substances which we include under the name of pepsine ( 299), are still present. In contradiction to this, however, many physiologists deny that an acid gastric juice is secreted. According to their statements, mechanical irritation of the gastric mucous mem- brane of dogs in whose stomachs a fistula has been established, affords, under such circumstances, an alkaline fluid. 1757. The cervical trunk of the vagus of recently killed dogs or rabbits can also excite the action of the small or large intestines. This fact is explained by the circumstance, that the gastric plexus which both vagi assist to form, gives off branches that enter (either directly or by means of the solar plexus) into the trunks that supply those segments of the alimentary canal. 1758. The external branch of the spinal accessory nerve that is, the 520 SYMPATHETIC NERVE. [CHAP. XVIII. part of this nerve which does not penetrate the branches of the pneu- mogastric, but only receives a few of its sensitive fibres supplies some muscles of the neck and back (the sterno-cleido-mastoid, a b, Fig. 236, p. 398, and the trapezius, c). When both spinal accessory nerves are torn out in a cat or rabbit, so that all their roots are destroyed, the voice of the animal almost or quite disappears. But it would seem that the movements of deglutition can still be executed as vigorously as before. 1759. We have already ( 1742) seen that the ninth or hypoglossal nerve (0, Fig. 337, p. 512) is the chief agent of the movements of the tongue. All its roots appear to contain motor fibres. But it is pro- bably not altogether devoid of sensitive filaments. 1760. This cerebral nerve gives off a peculiar branch, the descendens noni, in which the superior cervical nerves take an important share. It is the motor nerve of some of the cervical muscles (the omo-hyoid, sterno-hyoid, and sterno-thyroid); and, in man, it also unites with the phrenic or diaphragmatic nerve. The latter, which is given off by the inferior cervical nerves, receives the fibres just mentioned, and governs the contractions of the diaphragm (m n o, Fig. 9, p. 34). 1761. The main trunk of the sympathetic nerve descends on each side of the body; from the head, along the neck, thorax, and ab- domen, to the coccyx. It is chiefly distinguished by its offering a double row of ganglia, which in the chest and belly are repeated at every vertebra. Its roots are formed by numerous filaments derived from all the spinal, and most of the cerebral, nerves. And in the oppo- site direction it gives off a large number of branches; which subse- quently enter into ganglia, and assist to supply the viscera of the trunk, many of the blood-vessels, numerous glands, and other textures. Similar ganglia are present in many other nerves, such as the posterior roots of the spinal nerves ( 1719), part of the nerves of the eye ( 1726), and the trunks and some of the branches of the trigeminal ( 1729), the vagus ( 1744), and other cerebral nerves. All of these swellings are often com- prehended under the name of the ganglionic system. But the sympa- thetic being especially distinguished by the number of its ganglia, many authors limit this expression to the two cords of this nerve, together with the swellings which are connected with them. 1762. These enlargements of the nerves are due to the addition of a new element, the ganglion-corpuscle ( 1692), to the nerve fibre. For example, Fig. 339 represents a ganglion from the posterior root of a cat's cervical nerve, slightly magnified. Here numerous cor- puscles are seen deposited around the bundles of primitive nerve-fibres which penetrate the ganglion. Some of these corpuscles (more strongly magnified) are represented in Fig. 340, in that isolated state in which they may often be found after a ganglion (Tab. V. Fig. 74) has been torn up. CHAP. XVIII.] STRUCTURE OF THE GANGLIA. 521 1763. Fine sections which exhibit these structures in their natural position lead directly to the conviction, that they do not lie free between the nerve-fibres, but that they everywhere possess a proper sheath, which has probably a certain physiological import. At the margin of the ganglion we frequently find a few corpuscles having the appearance represented in Fig. 341. The whole is surrounded by a concentric and apparently fibrous membrane, on which are remarked numerous oval FIG. 339. FIG. 340. FIG. 343. nuclei. These nuclear structures sometimes also cover the free surface of the ganglion- corpuscle; so as to prevent all recognition of its substance, its nucleus, or its nucleolus (Tab. V. Fig. 71, ale). This state may be illustrated by Fig. 342. Here some medul- lary primitive fibres are also seen winding between the gang- lion-corpuscles. These are termed circumferential fibres, in contradistinction to those represented in Fig. 339, which pass through the ganglion in bundles, to run in the trunk of the sympathetic or its subordinate branches. 1764. A very fine section of the ganglion and nerve of a mammal at their point of union with each other sometimes affords the view sought to be represented in Fig. 343 (com- pare Tab. V. Fig. 71). Here the ganglion-corpuscle (a) is surrounded by the sheaths (b) formerly mentioned ( 1763). But these are continued (at d) into the nerve, forming what are sometimes called vaginal processes of the ganglion-corpuscles. Between these run nerve- fibres with distinctly medullary contents (c). Hence such a fragment of a nerve at its connection with the gang- 522 LARGE AND SMALL NERVE-FIBRES. [CHAP. xvm. FlG. 344. r lion frequently shows both these elements together; the medullary primitive fibres (a, Fig. 344), and the vaginal processes provided with nuclei (6). 1765. A nerve which chiefly consists of the ordinary medullary fibres appears to the naked eye of a brilliant white. But when these are mixed with considerable quantities of such vaginal processes, the whole possesses, when fresh, a whitish-grey colour; which is soon converted into a reddish-grey hue by putrefaction. The trunk of the sympathetic is then generally softer. Hence branches of this kind have been named grey or gelatinous nerves; while the vaginal processes them- selves are frequently called organic fibres, or fibres of Remak. But their appearance indicates that they belong to the group of areolar tissues, or of investing substances. Their physiological import is at present quite unknown. 1766. On comparing a purely motor nerve for example, the anterior root of a spinal nerve with a branch of the sympathetic, the former is seen to contain a great many large nerve-fibres (Tab. V. Fig. 68, b), while the latter is chiefly composed of small ones (Tab. V. Fig. 68, c). Bidder and Yolkmann have supposed that this difference of diameter indicates two special classes of fibres : that the larger proceed from the brain and spinal cord, and may be thence called cerebro-spinal or animal nervous elements; while the smaller arise from the ganglia of the peripheric part of the nervous system, and especially of the sympathetic, and ought therefore to be regarded as special sympathetic fibres. But recent researches show that there is no absolute distinction between these two kinds of fibres. Both possess the same medullary content ; which exhibits varicose swellings (Tab. V. Fig. 68, d) as the result of injury or other abnormal circum- stances, and which coagulates sooner or later after death (Tab.V. Fig. 69). Many of their fibres are of such a medium diameter, that we may allot them to either the large or the small variety at will (Tab. V. Fig. 68, d). And although the sympathetic is chiefly composed of fine fibres, still large ones may frequently be found in it : while, conversely, small fibres are often seen in the various cerebro-spinal nerves. And the diameter of the same fibre often diminishes in its course ; so that, for instance, most of the terminal plexuses exhibit great numbers of the smaller fibres. Hence we have no right to suppose that every fine fibre met with in any particular nerve necessarily belongs to the sympathetic. 1767. On examining an entire thoracic ganglion of the sympathetic of a cat under a low magnifying power, we observe appearances like those represented in Fig. 345. Here the greater part of the fibrous CHAP. XVIII.] PROCESSES OF THE GANGLION-CORPUSCLES. 523 FIG. 345. substance of the sympathetic trunk a b descends vertically. The ganglion itself is traversed by other bundles of filaments, which come from its roots c and d, and are subsequently apposed to the fibrous substance of the sympathetic trunk, or its branches. In one word, many of the fibres only come into mediate contact with the ganglion-corpuscles. The finer fibrillation that obtains in the ganglia of fishes leads to many peculiarities, which were first recognized by Robin, Wagner, and Bidder. For example, on tearing up the Gasserian ganglion (x, Fig. 322, p. 484) of a newly killed eelpout (Gadus lota) into extremely small fragments, we often meet with appearances such as are re- presented in Tab. V. Fig. 72. The gan- glion-corpuscle (a) is connected above and below with a process (b and c) which is obviously a medullary primi- tive fibre. When putrefaction has ad- vanced somewhat further, such speci- mens are still more easily found. Here the coagulation of the contents, and the spontaneous decomposition of the re- mainder of the nerve, frequently lead to appearances such as are represented by Tab. V. Fig. 73. 1768. The appearances just described may easily be verified in many fishes. The eelpout, the pike, the eel, and most of the cartilaginous fishes, are better adapted to this purpose than the Cyprinoid genus. And the Gasserian ganglion, or that of the root of a spinal nerve, is preferable to the ganglion of the vagus ; while this is better than the solar ganglion of the abdominal viscera. In fishes, it is evident that the medullary process of nerve is never limited to one side only, but that at least two such proceed from every ganglion-cor- puscle. 1769. The facts just described may be explained by supposing a primi- tive nerve-fibre (Tab. V. Fig. 72, be) to be interrupted by a ganglion- corpuscle (a) in the middle of its course. But the appearances hitherto known are very far from clearing up the whole bearings of the facts. For example, it becomes a question what is the relation of the medul- lary content of the nerves to the very different substance of the ganglion- corpuscles ; whether it is a true proximity, or otherwise ; whether the ganglion-corpuscle floats in a modified nervous content, or whether the latter only occupies its neighbourhood. In any case it would seem that the membrane which encloses the nervous medulla dilates to receive the 524: INDEPENDENCY OR DEPENDENCY OF THE GANGLIA. [CHAP. XVIII. ganglion-corpuscle. It is also possible that the same nerve-fibre is in- terrupted by a second corpuscle at a further point of its course. 1770. In reptiles, birds, and mammals, there are great obstacles to a satisfactory examination of these structures. For instance, a fine section of a sympathetic ganglion from a mammal shows ganglion-cor- puscles (a, Fig. 346), none of which are connected with any of the true nerve-fibres (6) seen here. In some of the smaller ganglia of the frog, it may be proved that there are far FIG. 346. more corpuscles than fibres. It is therefore certain that some of the ganglion-corpuscles are not interposed between nerve-fibres. It is true that in other ganglia of the frog double processes may be detected. But no medullary content can be verified in their interior. Koelliker and others have often ob- served a medullary fibre proceeding from one side of a ganglion-corpuscle. But against these observations, however frequently confirmed, we have a right to object, that the second pro- cess may either have been torn off in preparing the specimen, or concealed by its unfavourable position. In point of fact, a prolonged search in such preparations will sometimes enable us to find it. 1771. We have seen (1761) that numerous branches connect the trunk of the sympathetic with the cerebral and spinal nerves. During more than a hundred years the import of these intervening structures has been made the subject of a number of conflicting theories. Many have sup- posed, with Haller and his school, that the primitive fibres contained in these connecting cords arise from the cerebro-spinal nerves ( 1690), pass through the ganglion, and subsequently radiate into its branches. Ac- cording to this view, the sympathetic would be essentially a cerebro- spinal nerve ; presided over by the brain and spinal cord, just as the peripheric organs are by the other nerves. Hence the physiological pecu- liarities of its branches (which we shall hereafter describe) would depend solely on the numerous ganglia that are interposed in their course. Others follow the theory started by Petit, and developed by Bichat. Accord- ing to it, the sympathetic or ganglionic system constitutes a nervous system which is independent of the brain and spinal cord. Its peculiar fibres chiefly supply the intestines, the blood-vessels, the glands, and in general terms all those tissues which are destined to subserve the phenomenon of nutrition, and the unconscious and involuntary functions. It is thus a special visceral, vegetative, or organic nervous system ; which has an authority independent of, and co-equal with, its animal counterpart. The latter is composed of the cerebro-spinal nerves, CHAP. XVIII.] MULTIPLICATION OF FIBRES IN THE GANGLIA. 525 which effect the conscious feelings of sensation and pain, and the volun- tary movements. 1772. The question must be examined from two points of view an anatomical and a physiological. Most of the ganglia ( 1767) are visibly traversed by some fibres or bundles from their roots. Indeed, in the smaller vertebrata, large fibres may be followed through a number of ganglia; for instance, through those of the trunk and visceral branches of the sympathetic in the thorax. These facts sufficiently indicate that the sympathetic does not form a perfectly independent system : a pro- position which they would indeed incontestably prove, were it possible to follow the whole course of the fibres under the microscope. 1773. We may frequently observe that the total bulk of the branches which emerge from a ganglion is much greater than that of the roots which enter it. This increase of diameter is partly referrible to the vaginal processes ( 1764), which occur in large quantity in mammals and birds. But it also occurs in amphibia and fishes, in whom the pro- portion of these supplementary structures is much smaller. Here it may be distinctly shown that the branches given off from a ganglion contain more primitive fibres than its roots. Bidder and Volkmann have especially adduced this difference of number in support of the anatomical independence of the ganglionic system ; and regard it as a proof that there are special sympathetic, organic, vegetative, or gan- glionic fibres ; which arise in the ganglia, and which pass, partly in the peripheric direction, and partly towards the cerebro-spinal centre. 1774. This real or apparent multiplication of fibres may be due to a variety of causes. Many nerve-fibres do not run straight to their termination, but rather present appearances like those with which we have already been made acquainted in the terminal plexuses ( 1700). The fibres pass for a certain distance in a given nerve, and then turning round, come back in the same nerve, to take another and a further path. This peculiar course is best exemplified by the thoracic portion of the vagus of the mouse. Other examples are now and then seen in the sympathetic trunk of the frog. It is obvious that these arched fibres will be counted twice over in the branch in which they lie. Hence the apparent multiplication of fibres which occurs in the branches of the ganglia may be partially due to this circumstance. Still all care- ful researches indicate that it only occurs to a limited extent, while the number of fibres is really increased. 1775. Koelliker, who especially supports the view of a one-sided origin of fibres, concludes from his observations that the sympathetic is partly dependent, but partly independent. The cerebro-spinal fibres which pass through the ganglia place them in certain relations with the brain and spinal cord ; while the ganglionic fibres which proceed from one side of the corpuscles form the independent elements of the sym- 526 MULTIPLICATION OF FIBRES IN THE GANGLIA. [CHAP. XVIII. pathetic and its ganglia. This view at the same time explains that increase in the number of fibres, which these swellings exhibit. 1776. But we have already seen ( 1770) that there are many con- siderations which militate against the view of a one-sided origin of fibres. In the ganglia of the fish's vagus, the greater thickness of the emergent trunk is very striking. In spite of this, however, it is only in suspicious preparations that we see one-sided processes of the cor- puscles; and even here they are by no means frequent. And recent microscopic researches suggest other explanations of the multiplication of fibres. 1777. Those ganglion-corpuscles which only give origin to an upper and a lower medullary fibre, are most frequently seen in fishes. Now, if it could be proved that one process (Tab. V. Fig. 72, 6) was connected with the brain or spinal cord, while the second, (c) took a further and peripheric course, it would be obvious that the interposition of ganglion corpuscles would allow no multiplication whatever of the fibres. But we sometimes find that the two medullary fibres do not proceed from the opposite poles (Tab. V. Figs. 72, 73), but from the same side, of the ganglion corpuscle. For example, in a ganglion of the root of the spinal cord in an eelpout, Bidder remarked two such processes entering the same nerve in a peripheric direction, as shown by Fig. 347. This would not only afford another explana- tion of the multiplication of fibres; but would also support the theory of indepen- dent ganglionic fibres. But many facts are opposed to this ex- planation. The appearances shown in Fig. 347 are extremely rare. Many observers have never been able to see them, in spite of the most industrious search : while the frequency of the supposed ganglionic fibres ought often to give rise to appear- ances of this kind. And, even apart from this, it is doubtful whether the two fibres which enter the nerve in the peripheric direction really take such a course. 1778. There are other facts which better account for the increase of fibres. It is true that in the fish we generally see but two processes proceeding from one ganglion- corpuscle. But in the dog-fish, Stannius has observed a nervous vesicle giving off one fibre on one side, and two on the other. The drawing he has given of that instance 46 ) appears to indicate a division of the primitive fibre close to the ganglion-corpuscle. Traces of division of FJG. 347. CHAP. XVIII.] ANATOMICAL DEPENDENCY OF THE SYMPATHETIC. 527 the fibres have also been seen by the author in various ganglia. These facts suggest that the multiplication of fibres may be due to their dividing in the ganglia, as well as in the free nerves. 1779. A second cause may perhaps be found in the possibility, that many of the sheaths which proceed from all sides of the ganglion-corpus- cles become filled with medullary substance. Some of the grey nerves ( 1765) for example, those which arise from the gastric ganglion of mammals occasionally exhibit no medullary contents whatever, even after treatment with caustic potash or soda. In other cases the applica- tion of these reagents causes its appearance. The neurilemma or invest- ing membrane, which is thin, clear, and transparent in the fibres of the ordinary cerebro-spinal nerves, appears thicker, less transparent, and of a greyish-white or yellowish-red colour, in many branches of the ganglia. It therefore depends upon accidental circumstances whether its medullary content can or cannot be seen. Such membranes radiate on all sides from the ganglion-corpuscles, and even sometimes bifurcate (Tab. V. Fig. 74, cde). And supposing that they sooner or later become filled with nervous medulla, this would permit any amount of multi- plication of the true nerve-fibres, either in the central or peripheric direction. 1780. Hence in the present state of histology, the complete anatomi- cal independency of the sympathetic or ganglionic system must be regarded as highly improbable. While, in spite of the most perfect anatomical dependence, the number of fibres might still undergo a considerable increase ; either by their dividing, or by some of the vaginal processes of the ganglion-corpuscles becoming filled with ner- vous medulla. At present we cannot demonstrate any special sympa- thetic fibres, which may be easily recognized as such. It is true that the stronger sheaths above mentioned ( 1779) are most frequently found in the branches of the sympathetic trunk, or their subsequent ganglia. But they are often met with in the ganglia of the cerebro- spinal nerves. And their number varies with the age, species, and perhaps even the individual peculiarities, of the animal. 1781. The main argument for a physiological independence of the sympathetic or ganglionic system lies in the peculiar relations of most of those organs which are chiefly supplied by this part of the nervous system. All tactile stimulation of the thoracic and abdo- minal viscera, the blood-vessels, the glands, and other organs exclu- sively nutritive, passes unnoticed. And the will is equally incapable of exerting any immediate influence upon these organs. Now conscious perceptions and voluntary motions alike depend directly or indirectly upon the brain. And hence it would appear that the nervous tissues which conduct the phenomena of nutrition do not extend so far as this organ. Their centres lie in the sympathetic system, or in the ganglia 528 PHYSIOLOGY OP THE SYMPATHETIC. [CHAP. XVIII. generally : that is, in parts which are remote from those which bring about the higher mental emotions. 1782. But many facts of daily experience testify against this view. Under abnormal circumstances, the intestines become the seat of acute pain; and irritations, which would otherwise pass unnoticed, give rise to the most unpleasant sensations. Hence, under certain morbid con- ditions, impressions are transferred to the organ of consciousness. So that there cannot be any insurmountable barrier between the ganglia and the brain. And, conversely, many of the stimuli which impinge on the most delicate organs of touch habitually fail to excite our con- sciousness. We scarcely ever notice the soft currents of air which play upon our skin, or the slight changes of temperature which are constantly occurring. Every instant, many delicate sensuous impressions escape our consciousness. We may therefore suppose, that the ganglia are capa- ble of furnishing some collateral condition, which, under ordinary cir- cumstances, renders conduction to the brain a matter of difficulty. And this supposition, which forms a complete explanation, is also compatible with the most perfect dependency upon the cerebro-spinal centre. 1783. Anatomy often teaches that the intestines are supplied not only by the sympathetic, but by cerebral nerves, and especially by the vagus. It is therefore not so much the independence of the sympathetic nerve, as that of the ganglionic system generally, which we have to consider. By experimenting upon those cerebral nerves which give off ganglionic branches to the viscera, we shall find that their roots, (which are chiefly connected with the medulla oblongata,) are capable of governing the motor, and even the sensitive, structures of these organs. The spinal nerves which give origin to the connections of the sympathetic ( 1761) with the spinal cord, afford what are essentially similar results. Hence the complete physiological independence of the intestinal nerves with respect to the brain and spinal cord, can only be assumed with the aid of certain arbitrary collateral hypotheses. 1784. On irritating the denuded sympathetic trunk, or its intestinal branches, in a living mammal, it sometimes happens that no sensations of pain at first occur. But on prolonging the experiment, they appear quite unmistakeably. Now and then slight irritations are not responded to at all, while more violent attacks are vigorously so. The filaments (c d, Fig. 345, p. 523) which connect the trunk of the sympathetic to the spinal nerves are generally very sensitive. The ganglia them- selves at first respond to irritation more slowly \ and branches which come from a series of ganglia, still more faintly. These facts seem quite to confirm the interpretation already given ( 1782) : namely, that the ganglion-corpuscles oppose the undisturbed conduction of a stimulus. We must, however, remember that the posterior roots of the CHAP. XVIII.] ITS INFLUENCE ON THE EYE AND HEART. 529 spinal nerves are provided with ganglia ( 1719), and yet transmit tactile impressions to the seat of consciousness with the greatest accuracy. Hence there are but two alternatives : either this capacity of obstruc- tion only pertains to particular ganglia, or the ganglion-corpuscles con- tained in the posterior roots of the spinal nerves are connected solely with those fibres which afford no tactile sensations. 1785. The number of ganglia in the cervical portion of the sympa- thetic trunk does not correspond to that of the vertebrae ( 1761). In man and many mammalia, there are but two or three in this region; the superior cervical ganglion, the middle (which is less constant), and the inferior. The first of these gives off, amongst others, a number of fibres which are distributed in the head. This arrangement is remark- ably illustrated by many physiological phenomena. 1786. In man and the rabbit, the cervical trunks of the sympa- thetic and vagus take a separate course. In the dog, they are more intimately united. On cutting through this common nerve in the upper half of a dog's neck, the aperture of the pupil (c, Fig. 150, p. 273) soon undergoes a considerable diminution in size, and remains in this state for weeks or months. The iris (b, Fig. 150, p. 273) can therefore be regulated by means of nerves from two sources. The common motor oculi ( 1726) acts upon it by means of those fibres which pass through the optic ganglion and ciliary nerves j while the upper part of the united vagus and sympathetic acts through those of its own fibres which as- cend towards the head. From experiments on rabbits, anxl from patholo- gical observations on the human subject, it would seem that the iris derives nerves from this second source even in animals in whom the vagus and sympathetic nerves do not form a common trunk. 1787. We have seen ( 1753) that the vagus nerve gives off a large number of filaments to the heart. The sympathetic also supplies nume- rous branches to the chief organ of the circulation. But comparative anatomy clearly indicates that the relations of these nerves to the viscera vary greatly in different animals. The same parts which, in higher animals, derive their branches from the sympathetic trunk, are, in the lower vertebrate, supplied by the vagus. This circumstance appears also to affect their physiological actions. 1788. However obvious the influence exerted by the vagus nerve on the heart of the frog ( 1754), it is difficult to determine the precise action of the branches which are supplied to this organ from the sympa- thetic trunk. In mammalia, however, the two nerves offer a singular antagonism. On stinmlating the cervical trunk of the vagus with the electro-magnetic machine ( 248), the action of the heart is generally arrested. While, when the experiment is successfully repeated on the trunk or branches of the sympathetic, its pulsation is always acce- lerated. The same antagonism is often repeated by the roots of the M M 530 CAUSE OF THE HEART'S RHYTHM. [CHAP. XVIII. vagus, and those of the sympathetic or the corresponding spinal nerves. Hence there is some peculiar difference, which the intervening ganglia can neither originally produce, nor subsequently remove, at least not as regards the powerful stimulus of repeated electric shocks. 1789. The heart is chiefly distinguished by the fact, that its muscular substance undergoes an alternate contraction and relaxation at each successive moment ( 576). And since these periodical variations of activity are repeated in an excised heart, it follows that they do not primarily depend on any of the nervous trunks which supply the organ. This fact has been regarded as proving the independency of the ganglionic system. The nerves which run in the interior of the (apparently single, but in reality double) auricle of the frog ( 598), present minute ganglia, as do also most of the branches which enter the heart of the mammal. The independence of these has been supposed to explain why the excised heart may retain its activity many days after the removal of the brain and spinal cord. 1790. Although physiology cannot explain all the mysterious phe- nomena exhibited by the movements of the heart, it at any rate pos- sesses sufficient facts to justify the rejection of this view. We can indeed prove that the extraordinary gulf which, on this assumption, would be interposed between the involuntary movements of the heart and those of other muscular structures, does not really exist. 1791. In the first place, alternate contraction and relaxation may also occur in the voluntary muscles of the trunk and limbs. When a frog is beheaded and skinned, the muscles of its fore-legs sometimes tremble for a long time. It is true that they do not present an uniform and definite rhythm like that of the heart. But they offer the same essential fact; viz., a repeated alternation of contraction and relaxation. And on dividing the chief nerves which supply the convulsed muscles, their contractions are often instantly arrested a fact which proves that they do not originally depend upon the contractile tissues, but on the nerves by which these are governed. 1792. Similar alternating phenomena are sometimes seen in the dia- phragm of mammalia. These, however, frequently continue after the extirpation of the trunk and branches of the diaphragmatic nerve. Now the diaphragm can be made to contract voluntarily, and there are no ganglia in the terminal plexus of its nerve. It therefore follows, that the continuous contractions of a muscle which is capable of being subjected to the will, do not necessarily require the aid of the nervous centre, or of ganglion-corpuscles. We shall also find that there are other facts which prove the same proposition with respect to the muscles of the limbs. 1793. A comparison of the lymphatic hearts ( 550) with those of the blood-vessels, will carry us a step further. In the frog there is CHAP. XVIII.] THE LYMPHATIC HEART. 531 a pair of anterior lymph-hearts (h i } Fig. 348) each of which lies on a transverse process of the third vertebra (be) within the abdominal cavity; and a pair of posterior ones (hi, Fig. 349), which are placed immediately under the skin at the junction of the thigh with the trunk, where their pulsation may be recognized in a suitable position of the uninjured animal. All four derive their motor fibres from the spinal cord ; each of the two anterior from the third nerve of its own side, and each of the two posterior probably in great part from the tenth nerve (Fig. 336, p. 510). Hitherto ganglion-corpuscles have not been found in these nerves, either in the frog, or even in the large pos- terior lymphatic hearts of some snakes. If the spinal cord of a frog be destroyed, the posterior lymphatic hearts may still continue to beat for a certain time. But each of them then divides into a number of sacculi, which contract successively. And if cut out without any further injury, it may, under favourable circumstances, continue to beat for a long time. M M 2 532 THE LYMPHATIC HEART. [CHAP. XVIII. 1794. On comparing these appearances with those presented by the blood-heart, we find that, even when excised from the body, the latter preserves its ordinary mode of contraction. Its pulsation may continue for hours, and even for 2| days; while that of the excised lymph - heart has never been known to last longer than three hours in any experiment hitherto made. 1795. Hence whatever the chief cause of the rhythmic contraction, it does not necessarily imply the co-operation of the nervous centre, or of ganglion-corpuscles. The advantages ( 1794) possessed by the vas- cular over the lymphatic heart are possibly due to its ganglia, or to other collateral causes. 1796. The excised frog's heart, which has ceased to beat of itself, may be excited to one or more contractions by a puncture with a needle, that appears only to irritate a limited locality. And to what- ever point of the ventricle (c d e, Fig. 350) we apply it, the contraction still appears to begin in the auricle (a b). The systole FIG. 350. O f t h e ventricle then follows as an after-beat. It has therefore been supposed that the heart contains some arrangement, by means of which a direct irritation of any part is so conducted, distributed, and elaborated, as to result in an uniform beat of the whole organ. But it is also possible that the whole phenomenon depends merely on collateral mechanical conditions. Any gaseous or liquid current which passes over the internal sur- face of the heart, easily gives rise to a contraction that resembles a reflex movement ( 1717). The auricles seem to be more susceptible in this respect than the ventricles. But their systole leads to a beat of the ventricle, which is intimately united with them at the transverse fissure (/Fig. 350). It is possible that the pressure exercised by the needle may displace air or blood, so as to give rise to a current, which passes through the open auriculo-ventricular aperture. Still we shall hereafter be made acquainted with some facts which are opposed to the general application of this theory. 1797. If a portion of the apex of the heart, e, be cut away, the ordinary rhythm of its pulsations may still remain. And the transverse section may even be made along cd, so as to leave but a small ring of ventricle, without causing any essential alteration. But if, on the other hand, the section be made exactly in the transverse fissure that separates the auricle a b from the ventricle cde, the latter is frequently arrested, while the former continues to beat. Still, however ener- getic the influence thus exercised by the transverse fissure, we have no right to suppose it therefore necessary to the contraction of the whole ventricle. For even after this has been separated from the auricles, it may either continue to beat spontaneously, or may be artificially CHAP. XVIII.] MOVEMENT OF THE HEART. 533 excited to a complete pulsation. But in the latter case, the excitability seems to diminish, the further we go from the apex towards the trans- verse fissure. On dividing the ventricle lengthwise, so as to begin with a small incision at the apex, and carry it towards the transverse fissure, both halves may at first continue to beat, although with unequal rhythm. But the subsequent increase of the injury arrests the action of one or both segments. 1798. However mysterious all these facts at present appear, still they suffice to show that even the physiological relations of the heart do not justify our assuming the independence of the sympathetic or gauglionic system. But there are other experiments even more deci- sively opposed to this supposition. 1799. We have already ( 1752) seen that repeated electrical irrita- tion of the roots of the vagus brings the heart of all vertebrata to a stand-still for a certain time. This state is a true diastole, having no resemblance whatever to a tonic systolic spasm. Indeed, the ten- sion of the arterial blood is considerably diminished : although it finally remains at a considerable height. On pressing a limited portion of the ventricular surface at this instant, we get a rhythmic systole ; which, as usual, begins at the auricle, and is afterwards repeated in the ventri- cle. When the electric stimulus is applied to the roots or trunk of the vagus, its action is at first local. But its effects afterwards extend to the heart, something like the way in which irritation of the sciatic nerve forces the gastrocnemii to contract. And that influence of local stimulus which was mentioned above, may perhaps be partially explained by another fact : namely, that the peripheric segments of motor nerves are more easily excited than those which lie nearer the nervous centre. 1800. Assuming the sympathetic to derive its roots from the brain and spinal cord, we might expect that, under favourable circumstances, these would exert an influence on the heart. And, in point of fact, the heart of recently killed mammals may be excited to renewed action, not only through the spinal accessory, but also through the superior cervical nerves. In experiments upon frogs, Schiff found that the beat of the heart ceased soonest, after cutting through the roots of the fifth cerebral, the vagus, and the first spinal, nerves. While, on the other hand, if but one of these connections with the brain or spinal cord was left unin- jured, the movement of the heart endured for a much longer time. Ravens died at once, on cutting through both their vagi, and tearing out the sympathetic ganglia on their brachial nerves. 1801. All this incontestably proves that the central roots of the nerves are capable of influencing the action of the heart. But the rhythmical character of the cardiac movement, and its continuance after excision of the heart, neither testify to the independence of the 534 INFLUENCE OF THE SYMPATHETIC ON THE VISCERA. [CHAP. XVIII. nervous ganglia, nor even imply a co-operation of the ganglion-corpuscles. It may be conjectured that all the circumstances just mentioned, are originally due to transfers of action, and to peculiar arrangement of muscular fibres ( 597). 1802. The lungs derive their nerves from the vagus and sympathetic. The vagus may be proved to have the capacity of exciting the con- tractions of those muscular fibres which they contain ( 1749). But it has hitherto been found impossible to determine the exact purpose fulfilled by the sympathetic fibres. 1 803. Something similar to this obtains in the oesophagus : where stimulation of the vagus again gives rise to energetic contractions ( 1746), while that of the sympathetic produces either no results, or at most very doubtful ones. 1804. All those segments of the alimentary canal which occupy the cavity of the abdomen, can contract vigorously under the influence of the corresponding divisions of the sympathetic trunk, and their branches. The thoracic portion acts upon the small intestines, and even upon some parts of the large intestine. The lumbar portion governs the lower part of the alimentary canal; together with the urethra (k I, Fig. 154, p. 285), the bladder (ra), the vas deferens (w q), the vesiculse seminales (xri), the Fallopian tubes (yz, Fig. 119, p. 208), and the uterus (#). 1805. Experiments on the several spinal roots of the sympathetic show that they can excite contraction, not only in the iris ( 1786) and the heart ( 1788), but also in the abdominal and pelvic viscera just mentioned. Such observations acquaint us with a peculiar law: viz., that the spinal roots which act upon a given organ do not lie at the same height with it, but at a certain distance above or below. Their fibres pass for some length with the ganglia and the trunks and branches connected with them, before finally radiating to their peripheric distri- bution. But this phenomenon is only a parallel to the relations of the cerebro-spinal nerves most, if not all, of which exhibit something very similar. 1806. At present we know but little of those influences which the nervous system is capable of exercising on the local phenomena of cir- culation, secretion, and nutrition. Very few of these changes occur with sufficient constancy to allow of their being artificially produced, in accordance with known preliminary conditions. Many circum- stances are partially explained by various hypotheses. But a com- plete and satisfactory theory will long remain impossible. Finally, we are ignorant whether the influences under consideration are not exerted by a special nervous tissue forming what are sometimes spoken of as nutritive fibres. 1807. It is a matter of daily experience that the colour of the face CHAP. XVIII.] INFLUENCE OP THE NERVES ON THE SECRETIONS. 535 quickly changes under the influence of mental impressions. Here the nerves are supposed to alter the diameter of the capillaries : the constric- tion of these causing paleness, while their dilatation produces the blush of shame or anger. However correct this supposition, still we must not forget that many energetic stimuli such as repeated electric shocks ( 663) effect no direct change in the capillaries. Hence this rapid alte- ration of their size presupposes collateral causes, such as are hitherto unknown. So that at present we are alike unable to decide, whether, under ordinary circumstances, the nerves determine the afflux of blood to the several organs of the body, how they do so, or what influence they exercise on the phenomena of inflammation and exsu- dation. 1808. The increased lachrymal secretion which accompanies the act of weeping is generally due to nervous excitement. The sight or recol- lection of pleasant food causes a remarkable increase of the fluids of the mouth. Anger often gives rise to effusions of bile. When the nerves which supply the interior of the kidney are tied or cut through, a blood-red and albuminous urine is often evacuated. Sexual excesses frequently cause an increased secretion of some of the sebaceous glands of the face, especially of those which occupy the neighbourhood of the alee nasi : together with a more copious desquamation of the epi- dermis of this region. And the milk of wet nurses is sometimes seriously altered by mental emotions. 1809. Since the blood-vessels and gland-ducts contain contractile tissues, we may suppose that the nerves first excite these constituents, and thus indirectly alter the porosity of the septa ( 861) themselves. But it is obvious that this theory, though in itself probably correct, only affords a very general explanation. At present, our physical obser- vations ( 861) are so few, and our chemical analysis of the blood and the secretions so imperfect, that it is impossible to enter into any details. And, in addition to this, we are ignorant whether the nerves may not be capable of effecting a direct change in the corresponding mixtures of blood, secretions, and nutritional fluid whether, in fact, they may not act like the electrolysis of a galvanic current ( 239). 1810. Paralyzed limbs sometimes undergo a gradual emaciation. But this phenomenon also occurs without disease of the nerves, provided that the parts are made little or no use of from any cause whatever. The disuse of the muscular substance leads to its atrophy. But, on the other hand, paralyzed limbs often present a certain roundness, which is chiefly due to fat; and either appear perfectly healthy, or are at most only distinguishable by the paleness of their skin. In spite of this, how- ever, we should be wrong to deny all visible influence of the nervous tissues in these respects. 1811. When the sciatic nerve of a rabbit or dog has been cut through, 536 INFLUENCE OF THE NERVES ON NUTRITION. [CHAP. XVIII. the animal soon runs lame on the paralyzed side. The superficial cutaneous structures over a larger or smaller space gradually become converted into a scab; beneath which an ulcer constantly eats inwards, sometimes even reaching the bone, and causing its destruction by caries. But this active degeneration occupies the very places that chiefly sustain the weight of the body in standing or walking : and may therefore be ascribed to the unusual pressure to which they are so frequently exposed. The remainder of the leg, however, preserves its former condition, with some exceptions which will shortly be mentioned. These facts are a direct proof that the inaction of the nerves destroys that capacity of resistance which belongs to the healthy tissues; so that agencies which, under normal conditions, would produce no disturbance, inflict serious injuries on the structures of the paralyzed limb. 1812. Similar appearances are also seen in the human subject. When a piece of the sciatic nerve has been excised for neuroma, the terminal portion of the affected limb gradually becomes bent like a club-foot : and those parts subjected to pressure are attacked by scabs, ulcers, and even caries. The paralyzed foot becomes extremely blue when exposed to cold, and is very liable to be affected with chilblains; facts which prove that it has suffered just as much in its capacity of resisting the influence of temperature, as in it ability to withstand mechanical agencies. To the same cause we may obviously ascribe the similar results seen when paralyzed or dropsical persons, whose vital energy is sunken, lie upon their heels or sacrum. And electric shocks, which stimulate the nerves ( 242), may be advantageously used as a means of cure. 1813. There are some phenomena which indicate, that these changes do not depend on the nerve-fibres that preside over voluntary move- ments. We have seen ( 1731) that the larger primitive division of the trigeminal nerve (v, Fig. 322, p. 484) includes no fibres of this kind, and therefore does not govern the movements of the muscles of the face, but only the sensibility of its skin. Still, after section of this nerve in the rabbit, scabs often form gradually upon those parts of the lips which are pressed against other solid bodies in the prehension of food. Hence here we may conclude that these changes are produced by either sensitive or nutritive nerves. 1814. The epidermis of the foot desquamates more freely after divi- sion of the sciatic nerve. The epidermis of the hand or foot of a person affected with hemiplegia often presents a remarkable smooth- ness. The other horny tissues also exhibit many extraordinary changes. The nails often appear to grow more strongly and irregularly. Rabbits in whom one trigeminal nerve has been cut through, gradually lose the tactile hairs of the corresponding lateral half of the face. In dogs, the leg paralyzed by section of the sciatic nerve often becomes bald here and there. The limbs of persons who have been long paralyzed contain CHAP. XVIII.] INFLUENCE OF THE NERVES ON NUTRITION. 537 more fat than muscle. The muscular fibres themselves are generally paler. And although the contractile tissues emaciate more quickly and remarkably; still the other soft parts, and even the bones, gradually become atrophied. 1815. The appearances which follow section of a frog's sciatic nerve, vary greatly with the external circumstances. When the animal is kept in water, the paralyzed extremity frequently undergoes considerable swelling. An extraordinary quantity of fluid collects in the lymphatic spaces : i. e., in those cavities which intervene between the skin ' and many of the muscles. While, when the frog is kept in damp moss, this change is either absent or much less marked. Hence the effusion is not so much an exsudation of liquor sanguinis, as an entry of water from without, through the paralyzed tissues of the skin. We may con- jecture that its collection is chiefly due to a change in the porosity of this septum. 1816. The paralyzed limb often suffers from emaciation, ulcers of the foot and knee, mortification of some of the toes, or desquamation of the cuticle in large coherent fragments. But it is impossible to state how these phenomena are related to each other. Besides this, similar changes sometimes occur without any special injury of the nerves, where frogs are kept in a narrow receiver, or in impure water, or in other abnormal situations. 1817. Ulcers and fractures of paralyzed limbs may heal just like those of healthy ones. Hence suppuration ( 1056), granulation ( 1059), and the production of callus ( 1072), are in themselves quite as inde- pendent of the nerves, as the first moulding of the several tissues in the embryo or adult animal. 1818. Hitherto nothing very constant or important has been observed in the temperature of paralyzed parts. Their cutaneous surface gene- rally feels cool. But the thermometer shows that the temperature of the skin is lowered in some instances, and raised in others. Nor do thermo-magnetic researches present any constant deviations, from which conclusions can safely be deduced. 1819. The great influence exerted by respiration on the process of combustion, and therefore on the animal heat ( 1177), is occasionally seen after sections of nerves also. Thus a bird whose vagi have been divided sometimes at first presents a lower heat, which is succeeded by a higher one shortly before death. 1820. The rapid decrease of cutaneous heat which is met with in sickness, nausea, or fainting, depends in the first instance on the in- fluence of the nerves. It is probable that these cause a constriction of the capillaries of the skin, which are therefore traversed by less blood. Still the phenomenon is not completely explained by this single circum- stance. And the function of respiration is not always affected in exactly 538 INFLUENCE OP PARTICULAR NERVES ON NUTRITION. [CHAP. XVIII. the same proportion as the cooling of which it is the chief cause : a fact which indicates that the nerves themselves may have a direct action of this kind : an action such as our present knowledge of Physics will certainly not allow us even to guess at. 1821. Paralysis of the olfactory, optic, auditory, or hypoglossal nerve, does not necessarily disturb the nutrition of the corresponding sen- suous organ. But the cornea and crystalline lens of the eye often become cloudy after amaurosis i.e., paralysis of the retina of long standing. Still this degeneration does not immediately depend on the paralysis of the fibres of the optic nerve, but on the inflammatory phenomena which subsequently occur. 1822. Section of the common motor nerve of the eye, or of the fourth, sixth, or facial nerve, produces no immediate change in the nutrition of the corresponding muscular structures. These, at most, only undergo a slow emaciation, like other inactive contractile substances ( 1810). Paralysis of the hypoglossal nerve is attended by similar phenomena : the surface of the tongue subsequently presenting wrinkles, such as are not normally met with. And since the animal is incapable of effecting direct changes in the position of its tongue, this organ is often injured by the teeth. Hence ulcers and scabs frequently appear. 1823. The trigeminal and vagus nerves, both of which include nume- rous sensitive fibres ( 1730 and 1746), give rise to different phenomena. When the fifth nerve of a rabbit is cut through within the cranial cavity, the pupil of the corresponding eye immediately diminishes in size. The blood-vessels soon become greatly distended : and the mucous secretion of the conjunctiva is increased. These changes are followed by a profuse suppuration; which somewhat resembles the Egyptian or syphilitic ophthalmia, or that of the new-born infant. The iris also then becomes distended with blood ; while the conjunct ival sac, the anterior chamber of the eye, and the pupil, fill with exsudations. A funnel-shaped ulcer eats into the middle of the cornea, which also becomes cloudy in the rest of its substance. From this point the destructive process may take a retrograde course. In that case, the suppuration gradually diminishes. But the opacity of the cornea remains, as do also the exsudations in the anterior chamber and pupil ; so that the sight is permanently lost. Section of the fifth nerve in dogs or cats, or disease of this trunk in the human subject, often causes the eye to burst, in consequence of such suppuration and ulceration. Its softer internal parts such as the aque- ous humour, the crystalline lens, and the vitreous body are then eva- cuated; and the whole is finally converted into an amorphous mass. When this storm has passed over, the suppuration gradually ceases. 1824. Many experimenters state that these disturbances in the nu- trition of the eye are only produced by cutting through the trige- minal nerve at the Gasserian ganglion (x, Fig. 322, p. 484). They CHAP. XVIJI.] INFLUENCE OF PARTICULAR NERVES ON NUTRITION. 539 therefore conclude that these important changes in the constituents of the organ of sight depend upon the influence of that ganglion, or of those sympathetic fibres which may be supposed to arise from it ( 1766). But in the albino ( 1028) rabbit we may easily convince ourselves that, even when the injury lies between the Gasserian ganglion and the brain, and does not implicate the whole of the nervous trunk, it is quickly followed by this vascular distension. Hence there is no absolute proof that the ganglionic tissues necessarily co-operate. 1825. Where the eye has not been evacuated, an examination of the rabbit's body after death shows that only the anterior half of the globe is attacked. While the cornea is opaque, and the anterior chamber completely filled with exsudations, these but partially occupy the pupil, and at most give off a few bands to the capsule of the lens : the sub- stance of the lens itself, together with the vitreous body, the sclerotic, the choroid, and the retina, remaining quite free from such morbid appearances. Now since this storm of inflammation proceeds from the delicate conjuiictival surface, it is possible that here again the first im- pulse is given by the destruction of that capacity for resistance which the tissues possess ( 1811); and that all the subsequent phenomena are mere consequences of this primary disturbance. 1826. We have already ( 1751) seen that the inflammation of the lungs produced by section of the vagus may be explained in two ways. Some regard it as the simple consequence of a chemical irritation, which is itself caused by a defect in the mechanism of deglutition. While others look on it as a direct effect of the nervous paralysis ; such as is seen in other organs of the body, though with somewhat varying results. And recollecting that the delicate pulmonary tissues are constantly exposed to the influence of the inspired air, and that this very circum- stance causes a large proportion of mankind to die of pulmonary con- sumption, we might conjecture, that, here again, a paralysis of the nerves of these organs has destroyed their capacity of resistance. If the gastric juice really loses its acid character after section of the vagi ( 1756), the fact may be attributed to a change in the porosity of the coats of the vessels, and the limitary membranes of the glands. 1827. If the external branch of the spinal accessory nerve ( 1758) of rabbits and cats be torn out on both sides, without any further injury of the vagi, the animals may continue to live for a long time without this pulmonary degeneration. So that here also the disturbances of nutrition are produced by the sensitive, and not by the motor ( 1813) nerve. 1828. Although the distribution of the sympathetic has led to its being regarded as the special nerve of nutrition, experiment has not hitherto succeeded in verifying any series of peculiar and unusual nutritive phenomena in connection with this part of the peripheric 540 INFLUENCE OP PARTICULAR NERVES ON NUTRITION. [CHAP. XVIII. nervous system. It is true that extirpation of the superior cervical ganglion of the sympathetic is followed after some time by inflam- mation of the conjunctiva of the corresponding eye, attended with protrusion of the membrana nictitans, and an increased secretion of mucus or tears. But such disturbances of nutrition bear no compa- rison with those which follow paralysis of the trigeminal nerve ( 1823). According to Schiff and Bernard, removal of the two superior thoracic ganglia of the rabbit is followed by increased distension of the blood- vessels of the pericardium, and the effusion of exsudatious around the heart itself. We have already seen ( 1808) that paralysis of the renal nerves is capable of changing the characters of the urine. Extir- pation of the terminal segments of a frog's sympathetic trunk fre- quently gives rise to effusions in the abdominal cavity or viscera, and to signs of hypenemia in the different tissues there. But the subsequent phenomena are so different in different animals, that it is at present impossible to establish any specific details. According to Guenther, divi- sion of the (mostly cerebro-spinal) nerves of the horse's penis causes the corpora cavernosa to become extremely distended with blood, while the sensibility of the corresponding integuments is almost lost. 1829. We must admit that most parts of the ganglionic system are so deeply placed, that in the living animal they cannot be reached without the infliction of considerable accompanying injuries. Hence many experiments fail, on account of the necessary operations being soon followed by death. In others, the subsequent inflammatory phe- nomena are mixed with the effects produced by the section of the nerves themselves. Examples of such ambiguous results may be seen in the changes which follow excision of the posterior extremity of the frog's sympathetic, or the renal nerves of the mammal. But however small the number of observations hitherto made, it is im- portant to notice that the extirpation of a considerable portion of the sympathetic has sometimes been followed by very little disturb- ance of nutrition. Even the removal of the whole cervical trunk is unattended by any important change in those tissues which form the commencement of the respiratory and digestive apparatus. And the transplantation of testicles already referred to ( 865) shows that paralysis of the corresponding nerves does not prevent the pre- paration of a seminal substance provided with normal spermatozoa ( 1215). Large pieces of the Fallopian tubes may be cut away with- out the appearance of any other phenomena than the ordinary inflam- mation and exsudation attributable to the injury. It is true that the numbers of nerves which are possessed by the vessels, glands, and other organs of secretion, as well as by various parts having little sensibility and motion, plainly indicate the existence of certain special arrange- ments, such as require the aid of nervous tissues. But these refer to CHAP. XVIII.] NECESSARY CONTINUITY OF THE NERVOUS MEDULLA. 541 delicate relations, which are not even indicated by our existing know- ledge, and which will probably long baffle all the efforts of the observer of nature. 1830. Hitherto those internal changes which accompany the action of the peripheric nerves have been chiefly investigated in the motor fibres ( 1705). For the resulting muscular contraction has the advantage of allowing the easy and accurate performance of many delicate experi- ments, and exhibiting some very definite peculiarities. 1831. It has already been stated ( 1237) that a local stimulus, which impinges upon a point e (Fig. 351) of the sciatic nerve a b of a prepared frog ( 1237), throws the gastrocnemius c into contraction. A certain molecular change is propa- gated from e to the extremities of the nerve present in c. It finally induces that change in the physical properties of the muscular substance which we desig- nate contraction. But contraction may be produced by any local irritation of the nerve whether me- chanical, thermical, electrical, or chemical. The ner- vous medulla must therefore possess a peculiar capa- city for communicating the corresponding change of any special point, from one of its parts to another, to the very end of its course. 1832. When the sciatic nerve is cut through, tied, or otherwise seriously injured in the point d, no sti- mulation of e can cause c to contract. However accurately the cut surfaces at d may be apposed to each other, e still retains this disadvantage. Hence the propagation of the physical change in the nervous medulla presupposes that its molecules possess their natural situation and properties. 1833. This fact at once explains the ordinary distinct conduction of the nerve-fibres ( 1710). However closely these lie to each other in a branch of nerve (Tab. V. Figs. 68, 69), still their medulla is iso- lated by means of the neurilemma in which it is enclosed. The change excited at a definite point (e) of the fibre may be propagated in all directions: i.e., both in its length and breadth. The filamentous form of the primitive fibre is the reason why the first of these direc- tions predominates. And since the molecules of the medulla of two neighbouring fibres are not in contact with each other, all commu- nication is rendered impossible. The rare exceptions to this rule will hereafter receive a special consideration. 1834. It has frequently been conjectured that the nervous principle or agent i. e., the force upon which the action of the nerves depends is nothing more or less than electricitv. But the fact adduced in 542 ELECTROTONIC STATE OF THE NERVES. [CHAP. XVIII. 1831 will at least prove that it is no ordinary progressive movement of a galvanic current with which we are here concerned. For since this would be able to break through the moist animal tissues in contact with each other, we could scarcely imagine an isolated conduction on the part of the fibres ( 1710). And supposing that the irritation acting upon e led to a disturbance of electric counterpoise a disturb- ance which was carried onwards through e d b, as through a conduct- ing wire, we might expect that the division at d previously men- tioned would not oppose any insurmountable obstacle to the mechanical stimulation of e, 1835. Electrical phenomena form the most delicate means of testing this view with which we are at present acquainted. The galvanometer ( 220) betrays molecular changes such as the eye cannot recognize under the highest magnifying powers. The use of the electric current points out states which are inappreciable by all other means of experi- ment. And, at the same time, the galvanic stimulus is better adapted than any other to exhibit those alternate electrical changes which obtain in the more remote parts of the nerves. 1836. Let us suppose ad, Fig. 352, to be a moistened linen thread, a wet cotton-wick, or a fresh leaf- or flower-stalk. If a and 1> FIG. 352. be connected with the two poles of a galvanometer (x and u, Fig. 43, p. 76), and c and d with those of a galvanic circuit (u and s, Fig. 52, p. 81), the current will take the shortest route through cd, without the occurrence of any change in ab. Hence the galvanometer will remain at rest during the passage of the electric current through c d. But the fresh nerves lead to very different results. Let us imagine that a, Fig. 352, is the central end of d A the nerve, corresponding to the brain or spinal cord, while d is its peripheric extremity here the central current passes from d to a, and the peripheral from a to d (\ 228). On connecting the two poles of the galvanometer with a and b } the magnetic needle pre- sents a deviation, which is due to the chemical difference of the two points of contact, to the original nervous current ( 225), or to both of these conditions together. Hence it may be central or peripheral, accord- ing to collateral circumstances. If we wait until the needle rests, and then connect c and d with the poles of the galvanic circuit, it undergoes a new deviation. And the kind of deviation always indicates that the current produced in a b has the same course as that in c d. If the galvanometer previously showed only the nervous current, this devia- tion is increased when the exciting current proceeds in a corresponding- direction. In the opposite case, it will be diminished, or altogether overcome. In all these experiments, it is at the first instant that the magnetic needle vibrates most energetically. It afterwards gradually CHAP. XVIII.] ELECTROTONIC STATE OF THE NERVES. 543 recedes, but is finally arrested at a point which always indicates the direction of the exciting current. When the circuit is broken, the needle swings vigorously backwards, and subsequently rests in some other place. 1837. The electric current which excites cd throws the remaining molecules of the nervous medulla into a state of tension or polarity ( 240) corresponding with its own. We must therefore substitute the diagram, Fig. 353, for that representation of the electric properties of the nervous molecules which was given in Fig. 47, p. 79. This continuous displacement of the smallest par- FlG> tides reminds one to some extent of the perma- nent rotation of the planes of polarization which is produced by powerful inductive currents ( 256). Hence Du Bois, who first investigated this phe- nomenon, named it the electrotonic condition of the nerve-fibres. 1838. The other tissues of the frog either exhibit no electrotonic state at all, or one which is much weaker than that of the nerves. It is true that strips of muscle also give rise to positive results. But, other circumstances being equal, these are much weaker. And we are en- titled to doubt whether such results depend on the muscular fibres, or on the nerve-fibres which run between them. The fact that other tissues provided with nerves are inferior to the muscles in this respect, would rather decide the question in favour of the muscular substance itself. 1839. The electrotonic condition of the nerves is an immediate consequence of a property already assigned to ( 1831) the nervous medulla : viz., that of propagating change from molecule to molecule. Hence this propagation is destroyed by section, deligation, and local injury, whether thermic or chemical. Under such circumstances the columnar polarization (Fig. 353) struggles, as it were, with the original peripolar arrangement (Fig. 47, p. 79, and 223). 1840. When the exciting current takes the longitudinal direction (a d, Fig. 352), the conditions are more favourable than when it passes transversely through the nerve. The movement of the magnetic needle is also favoured by an increased length of the irritated tract (cd), and by an approximation of the conducting segment (a b) to the part (cd) traversed by the galvanic current. Hence the electrotonic effect diminishes with the distance from the seat of irritation : a cir- cumstance which shows that there is a certain resistance to the propa- gation of the molecular change from one atom of the nervous medulla to another. We shall hereafter find something similar to this in the vital actions of the nerves. 1841. The more delicate properties of the nerves sometimes exercise an extraordinary influence on the results. Other circumstances being 544 SECONDARY AND PARADOXICAL CONTRACTION. [CHAP. XVIII. equal, the pale white nerves of feeble and ill-nourished frogs give a smaller deviation of the magnetic needle. Indeed, with a weak circuit, or unfavourable collateral conditions, the experiment sometimes fails. If the phenomena be investigated day after day in the dead frog, the capacity of falling into the electrotonic states will be found to last longer than that of contraction. Thus it may be present in the putrefying animal. But at a certain stage in the decomposition of the nervous medulla, it is permanently lost. 1842. These facts suffice to show, that the electrotonic action of the nerves does not quite coincide with that process on which the excite- ment of sensation or motion depends. And the circumstance that it continues even during the closure of a galvanic current, affords another distinction, which completely separates the two classes of phenomena ( 233) from each other. 1843. That columnar polarization of distant portions of nerve which is produced by the electrotonic state, as well as the changes of electric condition by which it begins and ends ( 233), may also be observed in some physiological experiments. It is to these circumstances that Du Bois attributes the secondary contraction ( 1287) which proceeds from the nerves; as well as that which he has named the paradoxical contraction. 1844. We have seen ( 1287) that a second prepared frog, the nerve of which lies upon the muscles of the first, begins to contract, when these are thrown into contraction by the (preferably galvanic) irritation of their own nerves. But the exciting prepared frog or its muscles may be replaced by a piece of nerve. The nerve of the second prepared frog is laid on the longitudinal and transverse section of the piece of nerve, or on the first of these only ; and a certain extent of the latter nerve, at a distance from the point of contact, is connected with a strong gal- vanic battery. The occurrence or cessation of the electrotonic state leads to a secondary contraction of the prepared frog. When the place excited is too far from the contracting part, the experiment sometimes fails ( 1840). 1845. The paradoxical contraction is a collateral effect of what is essentially the same phenomenon. When a motor or mixed nerve, B, (Fig. 354), is stimulated at i k or any other part of its course whether mechanically, thermically, chemically, or by a weak galvanic current we only get contractions of those muscles which are supplied by B in its further and peripheric distribution, or which lie beyond ef ( 1712). But when, on the other hand, a certain length of B is exposed to the influence of a strong galvanic circuit, so that n falls into the electrotonic state, all the muscles which are governed by A (and hence those also which obey the trunk C), contract energe- tically. This result immediately depends on the fact, that the elec- CHAP. XVIII.] NEGATIVE DEVIATION OP THE ACTIVE NERVE. 545 FIG. 354. 6 C C it trotonic condition advances, not only in the direction of the living con- duction of the primitive fibres i.e., centripetally in the sensitive, and centrifugally in the motor, fibres ( 1707) but also in the opposite course ( 1836). The columnar polari- zation which attacks n, then secondarily excites abed. Here again, the nearer the excited portion, the greater the likelihood of success. And it is evi- dent that such paradoxical contractions, produced by powerful electric cur- rents, will be capable of frustrating the chief experiment which proves the law of Bell ( 1720). Hence, for this purpose, galvanic stimuli should be used. 1846. It has already been stated ( 226) that the electro-motor proper- ties of the molecules of a fresh nerve resemble those of the muscular fibre. Hence its longitudinal surface is positive with respect to its transverse section. But just as muscular contraction is accompanied by a negative deviation of the current ( 1286), so, according to Du Bois something similar occurs during the action of nerves. 1847. Here again electrical irritation affords more favourable results than any other stimulus. An inductive apparatus which is capable of tetanizing i.e., of continuously exciting the piece of nerve, and a delicate galvanometer of the ordinary construction, will, when properly arranged, suffice to prove this negative deviation of the current. But when other irritants are employed, the galvanometers at present made use of fail to show this momentary decrease of the nervous current. Hence Du Bois made use of a very sensitive instrument, which possessed 24160 coils ( 220), and underwent a violent deflection on applying the muscular or nervous current. A frog was poisoned with strychnine, and its sciatic nerve disconnected from the muscles. On uniting the longitu- dinal and transverse sections of the nerve with this galvanometer, the magnetic needle sometimes receded from 1 to 4, at the time when the action of the poison would have produced tetanic spasms in the muscles of an uninjured animal. When, under similar circumstances, the nerve is severed from the spinal cord, the deviation does not occur. Repeated mechanical or thermical irritations are also capable of pro- ducing a negative deviation of from 1 to 3 : but chemical stimuli exercise a less marked influence. And gradually scalding the skin of the leg and foot may finally oblige the sciatic nerve to indicate, by the 'galvanometer, the change which is going on in its own interior. 1848. On testing the nerve by electricity, the amount of negative N N 546 NEGATIVE DEVIATION OF THE ACTIVE NERVE. [CHAP. XVIII. deviation will be found to increase with the change of electrical ten- sion ( 231), and with the length and proximity of the stimulated portion. Although the same conditions are requisite for the electro- tonic state ( 1836), still they do not hold equally good for both phenomena. The negative deviation is much less dependent on the distance of the excited and conductive portions, than is the columnar polarization of the nerve. But both are essentially determined by the character of the nerve ( 1841), both are suspended by tying or cutting it across ( 1847), and both are raised again by its recovery ( 1284). And in both, that transmission of the exciting current which corresponds to the transverse axis of the nerve is the condition least favourable to the negative movement of the magnetic needle of the galvanometer. 1849. According to this, the negative deviation of the nervous current accompanies that internal change which is undergone by the nerves of sensation or motion at the instant of their action. But in the centripetal and centrifugal nerve-fibres ( 1707), it may be detected pro ceeding in two directions : in the former, towards the periphery also ; and in the latter, towards the centre ( 1805). Hence the change of molecular condition extends on both sides. Still we shall hereafter see that, in spite of this, certain one-sided contractions result. It may there- fore be questioned whether the negative deviation has a definite energy in this respect, or whether its strength is only a subordinate and colla- teral effect of deeper changes in the nervous medulla. 1850. According to Du Bois, the galvanometer is capable of indi- cating many of the changes which the nervous substance undergoes in consequence of putrefaction ( 1693). When the longitudinal and trans- verse surfaces of the nerve are connected with the poles of the galvano- meter, the electric opposition diminishes, as soon as the exposed trans- verse surface begins to undergo decomposition. By making a new transverse section, we restore the previous results. And the infliction of severe injury upon even a limited spot, sometimes inverts the direc- tion of the current in the nerve, just as in the smaller muscles. The electromotor properties last very little longer than the capacity of ex- citing contraction. When they are quite absent, the nervous substance is found to be coagulated ( 1693). They never return. 1851. We have already ( 1241) seen that electric currents constitute the most delicate test which can be applied in the living body, or the prepared frog. There is probably no substance so sensitive to electrolysis ( 239), and to the other effects of weak stimuli of this kind, as that unstable compound which forms the contents of the nerves ( 1693). The results obtained by such experiments acquaint us indirectly with many internal molecular relations, which would otherwise escape our notice. 1852. The contractions produced by galvanic irritation of the nerves CHAP. XVIII.] LAW OF CONTRACTION OF THE LIVING NERVE. 547 may occur at four different times : at the instant of closing the circuit, during its closure, at the moment of opening it, and after it has been opened. Since a verbal description of the numerous differences here met with would be both prolix and confusing, it will be better to make use of a simple and summary language, such as at once shows, both the direction of the electric current, and whether it produces an instan- taneous contraction only, or a series of alternating spasms. The strength of the contraction occurring in a given experiment is expressed by advancing letters or numbers : which also show what muscular regions of the preparation have contracted. A,JB,C, D, and E, indicate that the muscles of the thigh and leg (or the whole hind leg) of a frog have contracted, A representing the smallest, and E the greatest, degree of contraction : while abode indicate that only those of the thigh are affected. The figures 1, 2, 3, 4, 5, represent the same ascending- succession for that ordinary action of the galvanic prepared frog in which the muscles of the calf (c, Fig. 351, p. 541) take the most important part. The letters 22, or z, indicate a series of alternate convulsions lasting some time : while the cipher, 0, means that no change whatever is observed in the muscular substance. The letter p is the peripheric, and c the central, current, i.e., in the first case, the positive galvanic current goes from the nervous centre towards the muscular or sensuous organs; while in the latter, it takes the reverse course. Every formula of this kind includes four values. The first corresponds to the event which occurs at the instant of completing the circuit; the second to that on breaking it ; the third to that during its completion ; the fourth to that during its interruption. It will be seen that the value which appears on breaking the circuit is put before that present during its closure. This somewhat artificial arrangement has been adopted because, in most experiments, it is chiefly the closing and opening con- tractions (1241) which we have to consider; these being often inversely proportionate to each other. The two phenomena are therefore brought together, in order that their difference may be more easily perceived. For example, the formula p = 3. 0. 0. 0. and c = 0. 3. 0. 0. means that a galvanic prepared frog contracted at the instant of closing the peripheric current, and at that of opening the central one ; but at no other time. 1853. We will first examine the phenomena in the living animal which has not sustained any important injuries. The frog is properly tied upon a board, and a metallic conducting needle is stuck into the course of the sciatic nerve shortly after its emergence from the cavity of the belly (below 6, Fig. 334), while a second is inserted into it immediately below the knee-joint. When these two needles are suitably connected with a galvanic circuit (Fig. 52, p. 81), the electric current traverses almost all the femoral portion of the sciatic nerve. On selecting a very N N 2 548 LAW OF CONTRACTION OF THE LIVING NERVE. [CHAP. XVIII. weak galvanic stimulus, the direction of the current exercises no appre- ciable influence upon the final result. Whether the positive current runs peripherally or centrally ( 228), we only get a contraction at the instant of closing the circuit. Hence the formula of the law of con- traction which appears under these simple circumstances will be p=c=A to E. 0. 0. 0. 1854. We may examine the animal from time to time during a whole day or more, without finding any essential change in the result just mentioned. Setting aside some exceptions which will presently be mentioned, this law of contraction of the living nerve holds good after narcotization with ether or chloroform, poisoning with strych- nine or opium, decapitation, and division of the sciatic plexus (abed, Fig. 334, p. 507) or nerve in the living or dead animal. Finally, under all these circumstances, it may remain unaffected as long as any sensibility lasts. And by inserting the conducting needles into the facial nerve of a dog or rabbit ( 1735), we may convince ourselves that the same law of contraction holds good in mammalia also. 1855. When the sciatic plexus or nerve of a recently beheaded frog has been pressed, pinched, or otherwise injured, a contraction usually a weak one is often added at the instant of breaking the circuit. Hence we generally have p=c=A to E. C. 0. 0. instead of p=c=A to E. 0. 0. 0. But on repeating the experiment some time after- wards, the simpler law of contraction often appears. Hence this injury produces a certain change in the nervous substance, a change which extends beyond the place immediately attacked, and lasts for some time, but is subsequently lost. Diseased and emaciated frogs often exhibit these double actions spontaneously. CHAP. XVIII.] LAW OF CONTKACTION OF THE LIVING NERVE. 549 1856. A frog which has long exhibited the ordinary law of contrac- tion, offers closing and opening contractions (p=c=A to E. A to E. 0. 0.) as soon as the weak galvanic current is replaced by a stronger one. If the latter has but a moderate strength, so as only to produce a change from zero to a moderate degree of tension ( 232), the simpler law of contraction may often be produced by gradually bringing the conducting needles closer to each other, so as to transmit the current through a constantly diminished segment of the nerve. Hence the length of the irritated tract of nerve, and the change of tension at the instant of closing or opening, are, as it were, complementary to each other. 1857. When the circuit is so powerful as to induce stronger elec- trolytic actions, alternate convulsions are added, especially during the closed state of the circuit. For example, in many cases we have p=c=C. C. zzb. 0. Previous injury of the nervous structures now and then produces the same results, even when weaker circuits are used. 1858. In decapitated frogs we frequently find that it is only a certain direction of current which produces double actions. For example, we get p=B. 0. 0. 0., and cE.E. 0. 0. Here we may already detect those molecular states or dispositions which we shall shortly find so evident in the prepared frog. Subsequently they often disappear. 1859. In exposing a galvanic prepared frog ( 1237) to these tests, it (s, Fig. 355) is placed in a glass, the bottom of which is covered with water (t). In this way we avoid| that drying of the sciatic nerve, which is so prejudicial to the experiment. The whole is hung upon a hook of horn, and the sciatic nerve (with the plexus, a b c d, Fig. 334, p. 507, when it is itself too short) is slung upon two metallic wires, which pass through the stopper (u). These are connected with an apparatus for shifting the current (/), which in its turn is united to the galvanic circuit. By means of this, we can instantly exchange a peripheric for a central current, or can make currents of similar direction immediately follow each other. 1860. If a galvanic frog be prepared with the greatest care to avoid all injury of its sciatic nerve, we may often convince ourselves, by repeated experiments, that the law of contraction of the living nerve still continues. Very weak currents, or very short tracts of nerve, afford closing contractions; while at all the other periods there is no result whatever. But ordinary prepared frogs give rise to different results, since the violence done to the nerve during their preparation produces a continuous abnormal disposition, a permanent change in their molecular state. 1861. On exposing a long tract of the sciatic nerve of a freshly pre- pared frog to the influence of moderate galvanic currents, we generally get double actions ( 1855) with each of the two directions of current (p=cl to 5. 1 to 5. 0. 0). But this result appears to depend solely 550 CONTRACTIONS OF THE GALVANIC PREPARED FROG. [CHAP. XVIII. on the amount of stimulus made use of. If this be weakened, or if we make use of a very short transit of nerve, and of very weak electric currents ( 234), we instantly get one-sided results, which vary with the path of the current, and indicate the nature of that permanent artificial disposition just mentioned ( 1860). 1862. Here we find two chief cases. The peripheric current leads to a closing, and the central to an opening, contraction (p=l to 5. 0. 0. 0., and c=0. 1 to 5. 0. 0.). Since the voluntary contraction of the muscles depends upon a peripheric propagation of the stimulus in the interior of the motor nerve, we have here a one-sided effect, which is uniform or homogeneous with the vital action. This is designated the law of Marianini. In other cases the contrary i. e., a one-sided and dissimilar or heterogeneous action may obtain. The central current then furnishes a closing, and the peripheric an opening, contraction (p=0. 1 to 5. 0. 0., and c=l to 5. 0. 0. 0.). 1863. It is easy to see that the increase of quantity which leads to the double actions, only conceals the disposition really present. But the latter sometimes so far betrays itself, that one of the two contrac- tions is stronger than the other. 1864. In the nerves of the dead animal, the degree of susceptibility gradually diminishes. This explains why a prepared frog which at first gives a double response, afterwards only affords a single one to the same stimulus. "We may confirm it by preparing one thigh of a frog imme- diately after death, and the other some hours subsequently. 1865. At the period of recovery, similar phenomena also obtain in the uninjured animal ( 1853), or in the prepared frog which exhibits no artificial disposition. Here a previous injury often gives rise to double actions, with weak currents and moderate lengths of nerve. While rest reproduces that one-sided action which is the law of contraction for the living muscle ( 1848). 1866. It has been conjectured that the one-sided and heterogeneous action precedes the homogeneous one, since it more closely corresponds to the circumstances which obtain during life. According to such a view, a perfectly vigorous prepared frog would at first give c=l to 5. 0. 0. 0., p=Q. 1 to 5. 0. 0.; and p=\ to 5. 0. 0. 0., c=0. 1 to 5. 0. 0. subse- quently. But experiment by no means confirms this supposition. We may convince ourselves that the homogeneous or heterogenous disposi- tion is originally present. Besides, the law of contraction of the living nerve differs just as much from one as from the other. 1867. At present it is impossible to say why one preparation fur- nishes a homogeneous, and another a heterogeneous, one-sided current.* * On this subject the reader may refer to some more recent observations by Prof. Valentin in Vierordt's " Archiv fuer Physiologische Heilkunde," vol. xii. p. 66. Their novelty and importance induce me to sum up their chief results. The exceptional instances of heterogeneous one-sided contraction generally occur under CHAP. XVIII.] CONTRACTIONS OF THE GALVANIC PREPARED FROG. 551 Violent compression, or any other great injury of the nerve, always causes such an essential and permanent change in its medulla, that it returns different answers to the two directions of current. The homogeneous and one-sided action, which corresponds to the law of Marianini, then occurs much more frequently than the heterogeneous one. But the particular result does not depend on any original arrangement or property of the medulla during life. For it may happen that one thigh of the same frog gives rise to homogeneous, and the other to heterogeneous, results. In rare instances, the disposition of the same nerve may even become inverted in the course of the experiment, as a consequence of the collateral circumstances of two kinds: (a) the lapse of a considerable time after death; and (6) much injury of the nerve in preparation, especially of its length. But we cannot in- tentionally produce these conditions, so as to invert the current at will. By means of changes of temperature, such an inversion can, however, be effected. A heat of 105 to 115 annihilates every trace of susceptibility to contraction : not only in those instances in which it suddenly changes the preparation into a pale, stiff, brittle mass, but also when (this physical change having been avoided by the slow induction of a nearly equal heat) the mass retains its capacity of subsequently falling into the state of rigor mortis. A moderate increase or decrease of the ordinary temperature respectively increases or decreases the force and frequency of the contractions. The degree of cold required to destroy the susceptibility is very considerable. At 14, the preparation remains active. In varying the temperature of the surrounding air, we come to a degree of heat and of cold, which renders the nerves incapable of excitement by a strong simple circuit, but leaves them still amenable to the influence of the electro-magnetic apparatus. The restoration of the previous temperature completely restores the latent susceptibility. On exposing the frog's heart to similar temperatures, all of these variations are repeated without the aid of electricity. But the most peculiar effect of cold is its reversing the ordinary disposition of the nerves. When a prepared frog, whose motor nerve follows the law of Marianini, is enclosed in a receiver surrounded with snow or ice, and cooled to a certain degree, it gives the one-sided heterogeneous result viz., a closing contraction for the central current, and an opening for the peripheric one. On applying a further degree of cold, these become latent. But they return after its removal by warming. And continuous warming destroys the reversal, and restores the original disposition : so that the peripheric current has only closing, and the central, opening, contractions. The result depends upon the altered temperature of the nerve, and not of the muscles. This is shown by an apparatus in which the latter are excluded from its influence. But since the slow and feeble contractions already alluded to are not produced, the latter pro- bably depend upon the muscular substance itself. This reversal, latency, and recurrence, all occur earlier in a central segment of nerve than in a peripheric one. Indeed the disposition may be reversed in a central portion of the nerve, at the very time that a lower or peripheric tract is still exhibiting the ordinary condi- tion. While when the current is passed through both nerve and muscle, the results become very variable and uncertain. It would seem that, although higher temperatures render latent the susceptibility for the simple circuit, they do not reverse the disposition like cooling. When the disposition is originally reversed at the ordinary temperature, neither heat nor cold will invert it. It would therefore follow that the one-sided heterogeneous action is incapable of being affected by changes of temperature ; while the ordinary homogeneous action can only be reversed by cold, and not by heat. At ordinary moderate temperatures, the one-sided results occur later in the sciatic nerves of the prepared frog which lies in situ, and is bathed by nutritional fluid, than in the isolated nerve. The same difference is repeated in the influence of cooling. The temperature at which the reversal takes place varies according to the state of the animal : preparations from animals ill-nourished, or some time dead, undergo the change at a higher temperature, or, in other words, with a smaller degree of cooling. This confirms the view which regards the heterogeneous disposition as unlike the vital state of the nerves, and as implying a lower degree of activity than the homogeneous action. EDITOR. 552 CONTRACTIONS OF THE GALVANIC PREPARED FROG. [CHAP. XVIII. influences exerted by external circumstances. But, as a rule, the dispo- sition once taken remains until the last relics of susceptibility dis- appear. 1868. We might easily conjecture, that the various dispositions which cause the law of contraction for living nerve, and the artificial one-sided action of the prepared frog, are accompanied by differences in the electrical relations of the nervous medulla. But since Du Bois has frequently examined living and dead nerves, which certainly differ greatly in this respect, we may rather suppose that the difference is not so much in the chief phenomena themselves, as in the several circumstances which determine their amounts. 1869. It has already been remarked ( 1862) that the contractions are generally absent during the closure of weak circuits; i.e., during the transit of currents of pretty uniform tension, and but moderate electro- lytic powers. Still experience shows that the actions these evoke are not the less definite. The weak electrolysis leads to permanent changes of disposition, which gradually disappear as the preparation recovers. 1870. Supposing that we have an ordinary prepared frog, which gives a homogeneous one-sided current ( p = 1 to 5. 0. 0. 0., and c = 0. 1 to 5. 0. 0.), and that we allow a peripheric electric current to pass continu- ally through it, we may carry this so far that no closing contraction occurs on introducing a new peripheric current, the latter remaining without any action whatever (>=0. 0. 0. 0., c=0. 1 to 5. 0. 0.). On allowing a central current then to traverse the same length of nerve for some time, the closing contraction returns when the peripheric cur- rent is subsequently introduced (p=l to 5. 0. 0. 0., c=0. 1 to 5. 0. 0.). This alternate phenomenon is generally called the Voltaic alterna- tive. 1871. The facts formerly ( 233) mentioned indicate that the motor nerve only produces muscular contractions when its molecular state undergoes a sudden and energetic change. "We therefore get a contrac- tion of closure or opening, when the tension of the transmitted elec- tricity rises from zero to a given height, or sinks from the latter to the former. The same result is produced by a sudden increase or decrease in the tension of any continuous stream of electricity by which it may be traversed. But, on the other hand, those slighter differences which appear during the closure of moderately strong circuits very rarely disquiet the muscles of the ordinary prepared frog. In spite of this, however, they will, to a certain extent, electrolyze and polarize ( 239 et seq.) the excited tract of nerve ; and will exercise an electrotonic influence beyond it ( 1836). Meanwhile, the nervous molecules undergo a gradual change as regards the peripheric current. On inducing a new peripheric current, no great change of the molecular state can CHAP. XVIII.] CONTRACTIONS OF THE GALVANIC PREPARED FROG. 553 occur. Hence, for the time, all susceptibility for a current in this direction is lost. But when a central current is introduced, it imme- diately seeks to coerce the atoms in its own direction. And after it has partially or completely destroyed the previous disposition, the peri- pheric current becomes again capable of producing a contraction of closure. 1872. Since the homogeneous one-sided effect is more frequent than the heterogeneous ( 1867), the special injurious influence exercised by the peripheral current on the contraction of closure is of course very marked. Hence it has been supposed that this current is capable of weakening the nervous action, while the central current can strengthen it. But experience shows that this conjecture has only arisen from the deceptive appearances generally present. If we take one of those rare preparations which offer heterogeneous one-sided results and the closing contraction of which is therefore observed under the influence of the central current (c=2. 0. 0. 0., p=0. 3. 0. 0.), we shall find that this contraction can be overcome by the continuous action of a central current, while that at the opening of the peripheric current still re- mains (c = 0. 0. 0. 0., p = 0. 2. 0. 0.). Hence the explanation given in 1871 affords a more accurate theory, presuming that the two opposite dispositions really proceed from differences in the molecular arrangements or properties of the nervous medulla. 1873. The voltaic alternative ( 1870) destroys the closing contraction more easily than the opening; both with the peripheric and central current. The opposite direction of current, which precedes the latter contraction, suffices to favour the occurrence of the positive results. 1874. Preparations the susceptibility of which has fallen very low may be so greatly depressed by continuous weak currents, as to present no contraction at all under the influence of a current in either direc- tion. When allowed to rest some time, their capacity for contraction often returns spontaneously. 1875. The use of continuous currents plainly shows how greatly that injury of the nerve which produces the artificial disposition in the prepared frog diminishes its capacity of resistance. That is to say, the nervous medulla is much more easily and seriously injured by con- tinuous electric currents in the prepared frog than in the living animal, For under such circumstances, the latter only exhibits double actions, instead of the simple law of contraction of the living nerve ( 1855), Besides this, these double actions pass off more quickly than the re- markable disposition forced upon the prepared frog ( 1871). 1876. After the disposition of the excised sciatic nerve has been altered ( 1871) by a current of definite direction, the opposite current finds so to speak a more fruitful soil for its action. So that the relations of the nervous molecules become, as it were, confused. A 554 CONTRACTIONS OF THE GALVANIC PREPARED FROG. [CHAP. XVIII. rapid change then leads to contractions at times which, without these preliminaries, would afford no such results. We will suppose that a galvanic prepared frog originally gave p=2. 0. 0. 0., c = 0. 3. 0. 0. An instantaneous change of the directions of the galvanic stimulus by means of the proper apparatus ( 1859), affords p=2. 0. 0. 0., and c=l. 3. 0. 0. Indeed, we may remark that a current which itself presents no contractions is capable of effecting that con- fusion of molecular relations which is necessary to their occurrence. For example, by rapid changes of direction, we get c = 0. 4. 0. 0., p=Q. 0. 0. 0., c=l. 4. 0. 0. 1877. We have already seen ( 1875) that the living nerve is more independent of external influences than the sciatic nerve of the prepared frog. The dependence of both, and the energetic mobility of the atoms of their nervous medulla, sometimes increases with the number of stimulations at the beginning of the experiment. The injuries to which the sciatic nerve is exposed frequently lead to such violent deviations, that contractions appear during the state of closure, or after the open- ing of the circuit : that is, during times which are otherwise periods of repose ( 1853). 1878. A careful examination of these phenomena indicates that there are two kinds of change which may occur in the nervous medulla, so as to cause the continuance of the contractions. We frequently get prepa- rations which exhibit alternating convulsions during the closure of the peripheral current, but not during that of the central. In other cases, it may happen that these, which have previously appeared sponta- neously, cease during the transmission of a definite direction of current, and recommence on opening the circuit. Hence we have one kind of disturbance of the nervous medulla, which is supported by central electric currents, and weakened or removed by peripheral ones; and another, which is exactly the reverse of this. 1879. When a portion of nerve, db, which lies near the muscles, c (Fig. 356), is exposed to a galvanic irritation, the results are gene- rally more successful than when a remote segment, a e or e d, is ex- posed to the same stimulus. We may further convince ourselves that a longer piece of nerve (even though it be somewhat nearer the centre), affords better results than a shorter one. Both of these laws, however, are subject to some exceptions. 1880. When a continuous and moderate current is transmitted through a more central piece a e, while a nearer one d b is tested with alternate currents, the actions of db do not essentially differ from those ordinarily seen. The weak electrotonic state ( 1836) of db does not noticeably alter the results. But if we reverse the experiment i. e., if we transmit the continuous current through the lower piece d b, while we expose the upper portion ae to the alternating currents CHAP. XVIII.] GALVANIC STIMULATION OF SENSITIVE NERVES. 555 FIG. 356. we obtain what are at most very weak contractions. These generally cease as soon as the continuous closure has lasted for a certain time. When this is interrupted, c contracts at the instant that a e or e d are attacked. Here the continuous galvanism of the inferior portion produces an electro- tonic state, a comparatively vigorous tension of its molecules. This is not suspended by the upper cur- rent, the action of which is connected with a certain resistance to conduction. But the removal of the obstacle at once allows the upper current its free action. 1881. Here again the nerve of the living animal possesses a greater independence than that of the ordi- narily prepared frog ; in which the fibres are, as it were, artificially thrown out of tune. When a con- tinuous and moderate current is transmitted through the inferior segment of the sciatic nerve, interference with its upper portion gives rise to actions which evidently correspond with the law of contraction of the living nerve ( 1853). 1882. The sensitive fibres give rise, on the whole, to less decided results than the motor nerves. With strong galvanic circuits it becomes evident that the pain on closing the circuit is greater than the shock on opening it. This, again, is a counterpart to the law of contraction of the living nerve. Here also an unpleasant impression often continues during the closure of the circuit, whether due to the perception of the electrotonic state ( 1836) ; or to a continual alter- nation of closure and opening, produced by the perpetual slight vibra- tions of the body ( 1193). The inductive apparatus produces violent pain on opening the circuit. But on testing the sciatic nerve of a living frog by sticking in conducting needles ( 1853), the animal frequently shrieks with pain both on closing and opening the circuit. Accord- ing to some observers, the colours of the galvanic luminous image ( 1578) are inverted in the latter case. The organs of smell and hearing respond to the electric currents either ambiguously or not at all ( 1608, 1629). 1883. From all this together there can be no doubt, that the func- tion of the nerves essentially depends on the molecular state of the nervous substance, and that this is greatly altered at the instant of their action. , Hence the nervous principle works by means of mate- rial changes. And the nervous medulla possesses a mobility in this respect, such as has hitherto been found in no other substance. In this mobility, each molecule influences its neighbour by direct contact : the loss of living force caused by this communication from point to 556 ELECTRICAL FISHES. [CHAP. XVIII. point constituting the physiological resistance to conduction. And any interruption to the natural direct contact at once destroys the con- tactive propagation which depends upon it. The extraordinary mobility or susceptibility of the nervous molecules allows any local disturbance that lasts a certain time whether me- chanical, thermical, or chemical to diffuse its influence along the whole course of the nerve-fibre ( 1831). But no stimulus alters the mole- cular state of the nervous medulla so easily and quickly as that of elec- tricity. This proposition is evidently confirmed by the electrotonic state ( 1836), by the negative deviation ( 1846), and by the influence of the continuous transmission of galvanic currents ( 1870). Here the most delicate electrolytic influences suffice to produce rotations of differently polarized molecules, or special phenomena of polarization. The molecules of the living nerve are far more elastic, and therefore more independent, than those of the dead body or the prepared frog the latter of which are coerced into a permanent artificial disposition by the previous injury. Here we have certain changes of molecular state, which betray themselves, not by direct muscular movements, but by an alteration of disposition. The recovery from this consists in the restoration of a better molecular arrangement. It does not imply the continuance of the circulation. The first repetition of the sti- muli can confuse the relations of the molecules, and visibly increase their mobility. Putrefaction destroys, first the vital actions, then the capacities for the electrotonic state and the negative deviation, and, finally, the nervous current itself. This may at last be inverted, just like the muscular current. The finer relations of the nerves and the muscular substance have what is in many respects an extraordinary resemblance to each other. 1884. The electric fishes viz., the torpedo and the gymnotus (Fig. 357), show that the nervous function is capable of generating FIG. 357. shocks of electricity. This remarkable action, which is effected by spe- cial electrical organs, will probably hereafter afford important disclosures respecting the more recondite processes of innervation. 1885. The electrical organs of the torpedo are symmetrically repeated on both sides of its body. At , Fig. 358, is shown the posterior CHAP. XVIII.] ELECTRICAL ORGANS. 557 surface of the left organ ; together with numerous large nerves, d efg, which enter its interior from the trigeminal and vagus trunks. On examining its dorsal or abdominal surface, or any sections parallel with them, we see a number of polygonal structures (Fig. 359), FIG. 359. FIG. 358. which are separated from each other by parti- tions of areolar tissue. While an inspection of the lateral surface, or its transverse section, presents appearances similar to those repre- sented in Fig. 360. There are a great num- ber of columns, which stand closely to each other, and contain transverse laminae, sepa- rated by small intervals filled with fluid. This arrangement has a very suggestive re- semblance to galvanic columns (Fig. 51, p. 80) isolated from each other by rods or walls of glass. 1886. The hundreds of columns and thousands of laminae which occur in the two electrical organs of the torpedo, make up a large total surface ; the whole of which is in contact with the fluid that occupies the intervals of the laminae. In the electrical eel, whose electric organs are similarly constructed, and form the great bulk of the animal, the total surface is much larger, amounting to many square yards. But while the columns of the torpedo run from the back towards the belly, and its laminae take a course from one side towards the other (Fig. 360) parallel to these surfaces; the columns of the gymnotus take a direction from the head towards the tail. Hence the laminae descend from the dorsal FIG. 36], FIG. 360. towards the abdominal surface, as shown in Fig. 361. And the electrical organs of the gymnotus are supplied by more than two hundred spinal 558 ELECTRIC LOBES OF THE TORPEDO. [CHAP. XVIII. nerves, while those of the torpedo are provided with cerebral nerves ( 1885). 1887. Although the nerves which penetrate the electrical organs of the torpedo are originally of large size, still the quantity of the nervous medulla is greatly increased by the numerous divisions which occur in their subordinate branches, and in the lamina of the columns ( 1885) themselves. We have already seen ( 1701) that the medul- lary fibres finally merge into others which are yellowish and apparently devoid of medulla. These again divide, and often unite into a network. The result of all this is, that the electric organs are provided with a quantity of nervous elements, such as has never yet been seen in any other organ of the body. But no ganglion-corpuscles can be verified at any point of this peripheric distribution of the nerves. 1888. The shocks which the electrical fishes can give off at will, form a weapon like the poison of venomous snakes. Neighbouring animals are stunned or killed by the electric discharges of these organs, just as though struck down by a powerful shock from an electrical machine, or a flash of lightning. 1889. All the effects which are producible by the electricity of an artificial apparatus, may also be obtained by the discharge of living electric fishes. The formation of sparks, the propagation through con- ductors, the insulation by non-conductors ( 215), the elevation of tem- perature, the deviation of the magnetic needle, and the contractions of the rheoscopic prepared frog all these unite to certify that we are here concerned with ordinary electrical phenomena. 1890. The central organs of the electric nerves of the torpedo ( 1885), are two special cerebral lobes of a lemon-yellow colour. The annexed woodcut (Fig. 362) represents the inner surface of the brain of a large Mediterranean torpedo, longitudinally divided along its middle : / is the uppermost part of the spinal cord, e the medulla oblongata, and d the right of the two electric lobes just mentioned. This lobe is characterized by the very large and distinct ganglion-corpuscles which it contains. These are seen by the naked eye as small reddish granules; and they lie closely upon each other, as shown in Fig. 363. Numerous nerve-fibres, which often form loops, pass amongst them with many windings. Many of the ganglion-corpuscles give off grey processes, the transition of which into medullary nerve-fibres is still somewhat doubtful. 1891. After its electric lobes have been excised, the torpedo can no longer communicate shocks at will. But the organs are still capable of reflex discharges, such as will shortly be mentioned. 1892. Various collateral circumstances indicate that the action of the electric nerves is very similar to that of the motor fibres. When the animal imparts its voluntary electric shock, the stimulus is propagated CHAP. XVIII.] SHOCKS OF THE ELECTRIC FISHES. 559 in a centrifugal or peripheric direction, from the electric lobes (d, Fig. 362) towards the laminae of the columns (Fig. 359), just as when the FIG. 362. FIG. 363. motor nerves compel the muscles to contract. The roots of the motor nerves are distinguished by their possessing a large number of thick fibres; and the same is the case with those of the electric nerves of the torpedo (defg, Fig. 358). When a single trunk or branch of these nerves is irritated mechanically, chemically, or thermically, a corre- sponding segment of the electric organ ( 1714) is discharged. Section of the nerves destroys all peripheric propagation of the excitement ( 1832). And cutaneous irritations, such as in other animals give rise to reflex movements ( 1717), sometimes produce reflex discharges in the electric fishes. Both of these capacities are lost with the destruc- tion of the corresponding central organ ( 1937). A weak stimulus which impinges upon the right electric lobe (d, Fig. 358) often only acts upon the right electric organ : while a stronger one also arouses the left electric lobe, and hence the whole apparatus. We shall hereafter see that something similar to this obtains in the central organ of the muscles. 1893. The positive current set in motion at the instant of discharge takes a perpendicular course through the laminae, which are traversed horizontally by the terminal branches of the electric nerves. Hence in the torpedo, this current passes from the back towards the belly; and in the electrical eel, from the head towards the tail. 1894. Although the torpedo and gymnotus stupify or kill other animals, and can produce contractions in v the muscles of the prepared frog, their own contractile tissues are unaffected at the instant of dis- charge. When one of these creatures has given off a series of shocks in rapid succession, its electrical force is, for the moment, exhausted. This state of exhaustion only disappears after a certain period of recovery, like that of the muscles. Hence the exhausted animal may be grasped with impunity. But it shows just as little appearance of stupefaction at this time, as it did of pain or any sensation at the instant of its vigorous 560 NERVOUS ACTIONS RESEMBLING INDUCTION. [CHAP. XVIII. discharge. In one word, the body of the electric fish appears to be entirely freed from both the sensitive and motor consequences of its own electric shocks. 1895. That external resemblance to a galvanic battery which is pre- sented by the several divisions of the electric organ of the torpedo and gymnotus ( 1885), has given rise to the conjecture, that the different constituents of the laminae and their intervening fluid are related to each other like the copper, zinc, and moist conductor of a galvanic pile ( 230); the large surface of contact ( 1886) forming the chief cause of the vigorous discharge. But since the shock only occurs by the aid of an instantaneous nervous act, it follows that the electric organs are not a battery which stands ready for use, only requiring its poles to be connected with each other. The vital action of the nerves is rather an essential link in the process a condition of the discharge. 1896. We have a right to suppose that the negative deviation of current ( 1846) accompanies the action of the electric nerves, just as it does that of the other primitive fibres. We might therefore con- jecture it to be the chief cause of the discharge. The extraordinary richness of these parts in nerves would cause such effects to be more powerful here than elsewhere. We might even imagine that the resem- blance with the galvanic column was only apparent, and that the whole structure of the electric organs was intended to provide a large surface for the further course of those numerous branches which result from the division of the electric nerve-fibres. The fact that the positive current passes perpendicularly to the nerve-fibres ( 1893) might per- haps be connected with the change in the electric state of the nervous molecules. The great discharge of the electric fishes would thus be no special phenomenon, but only an instance of those electric changes which generally accompany nervous actions, favoured by the collateral arrangements of the part. But it is obvious that in the absence of more delicate researches into the electric organs and their nerves, we shall only lose ourselves in a variety of uncertain conjectures. 1897. The nervous function has often been compared with electrical induction. A comparison of the two phenomena does really offer many true analogies, in addition to others which are less complete. 1898. The strength of induction increases with the length of the inductive stimulus ( 246). The muscular contraction increases with the length of nerve traversed by the exciting electrical current. The inducteous current only arises at the instant of closing or opening the inductive one ( 243). An analogous phenomenon is presented by those prepared frogs which then give double actions ( 1855), but exhibit no alternating convulsions during the closed state of the circuit. Finally, we have already been informed of the resemblances presented by the electrotonic state ( 1837). CHAP. XVIII.] NERVOUS ACTIONS RESEMBLING INDUCTION. 561 1899. But the law of contraction of the living nerve ( 1853) seriously limits the completeness of this comparison with the phenomena of induction. For the nerve then responds at the instant of closing the circuit, but not at that of opening it. While the inducteous current obtains at both these times. 1900. There is another resemblance which in itself appears much more forced, and which it is just as impossible to follow out. The inducteous closing current is opposed to the inductive one ; while the inducteous opening current corresponds with the latter ( 243). Hence the central current gives the same direction of induction on closing the circuit, as the peripheric does on opening it, and vice versd. In this respect two currents with an opposite direction resemble those prepared frogs in whom there is only a one-sided action, i e., in whom the peripheric current presents a closing, and the central an opening, contraction, or vice versd ( 1862). But since many other circumstances and results ( 1869) may possibly interfere, this resemblance is some- what arbitrary as well as imperfect. 1901. The several neighbouring coils of a long spiral wire can act upon each other by induction ( 247). Here we have a change of their own substance, which has a remote analogy to the electrotonic state ( 1836) of the nerves. But since the latter continues during the closure of the circuit, the phenomenon may rather be compared with the magnetization of an iron rod in the centre of the inductive coil ( 248), or with the rotation of the plane of polarization ( 256). Still as the electrotonic state proceeds from atom to atom for a certain distance ( 1840), it is obvious that even this parallel is incomplete. 1 902. Finally, we may observe that the special influence of rapid change of tension ( 233) holds good in the inductive, as well as in the nervous, currents. Both inductive and nervous phenomena most readily obey those changes of the electric state which are effected in the shortest pos- sible time. But in many other respects they offer important differences. The extreme sensibility and mobility of the nervous molecules lead to numerous results which cannot be noticed in the inert links of any inductive apparatus. 1903. The mode in which the motor nerves constrain the muscles to contraction is in many respects analogous to that magnetization of iron produced by an inductive spiral. Both substances the iron ( 248) and the muscular fibre then undergo a molecular change. Both gradually become softer ( 1283), and change their diameter. But the magnetism of the iron disappears immediately on the inter- ruption of the electric current; while, under favourable circumstances, the contraction of the muscles may continue subsequently. And in the muscles, the change of form is altogether different in nature, and far more marked in degree ( 1275). Exhaustion first shortens the duration o o 562 VELOCITY OF PROPAGATION IN THE NERVES. [CHAP. XVIII. of the contracted state, and finally renders the contraction itself impos- sible. And rest can again restore the previous force. All this proves that we are here concerned with a very peculiar and changeable sub- stance, such as necessarily affords a special series of inductive results. 1904. The contactive communication of the change excited in any part of the nervous medulla ( 1832) requires a certain amount of time. Here, as in other mechanical processes, there is a certain velocity of propagation. But this velocity is even greater for electricity than it is for light. It amounts to 462 millions of yards per second for the former, and 310 millions for the latter. So that if the nervous agent were identical with electricity, it would be propagated in an infinitely small space of time through the short distance which is all it has to traverse even in the largest animals. The arguments which contradict this suppo- sition ( 1834), and the physiological resistance to conduction ( 1883) which is often considerable appear to indicate that what we call the nervous principle moves far more slowly than the electric fluid. The comparative shortness of the nerves would allow of the greatest punc- tuality, even with a much smaller velocity. 1905. The nervous principle of the sensitive fibres is translated into a corresponding perception at the centre; and that of the motor fibres, into a contraction at the muscles. So that when any part of a nerve is irritated, the time which elapses between the irritation and the cor- responding action represents the period required, both for its propaga- tion along the given tract of fibre, and for the translation which ensues. 1906. On holding the nail of the index finger against a rotating cog-wheel, we can perceive one hundred distinct blows in the second. Reducing into yards the distance which has to be traversed before reaching the central organs, and recollecting that every sensation has a certain after-duration ( 1536), it will follow that the propagation and translation have a velocity of more than 100 yards per second. 1907. Helmholtz made use of a galvanic apparatus, in the circuit of which a galvanometer was interposed; and he arranged it in such a way, that its irritation impinged on the nerve at the instant the circuit was closed, while the resulting muscular contraction, on proceeding to a certain extent, itself opened the circuit. In this way the time of closure could be indirectly estimated from the amount of deviation in the mag- netic needle. But this period corresponded to the conduction of the stimulus in the primitive fibres, to its translation into muscular con- traction, and to a certain duration of the latter act. The sciatic nerve of dead frogs gave an average velocity of 35*4 yards per second. 1908. An expert pianist can flex and extend the middle finger about ten times in a second. Assuming that each separate muscular contrac- tion occupies ^th of a second, and that the distance from the brain to the flexor muscles of the fingers (p, Fig. 236, p. 398) is somewhat OHAP. XVIII.] ACTIONS OF REPRODUCED NERVES. 563 more than a yard, this will give us a velocity of rather more than 20 yards per second for the elaboration of the commands of the will, the propagation of the excitement, its conversion into muscular contraction, and the duration of this contraction. Rapid talking would probably exhibit a much greater velocity. 1909. The nerve-fibres are the links which connect two organs of transfer, a central and a peripheral ( 1905). In these acts of transfer the sensitive fibres appear to conduct their excitements only in the central direction, and the motor only in the peripheral ( 1707). But it may be questioned whether this is not solely due to the only means of elaboration being, in the former, at the nervous centre, and, in the latter, at the peripheric muscular substance whether the fibres themselves are not more indifferent, so as to propagate the change which occurs at the middle of their course in both the central and peripheral direction. We have already ( 1836 and 1849) mentioned that, with the electro- tonic state and the negative deviation, this is certainly the case. Hence it may possibly hold good for other phenomena. FIG. 364. 1910. It has been attempted to solve this question by a experiment. Supposing a c (Fig. 364) to be the central part of a sensitive nerve, and d b the peripheric end of a motor one, which have united in the swelling cd ( 1066), if the excitement proceed both centrally and peripherically, a stimulus impinging on ac will produce muscular contrac- tions. But if this be not the case, the excitement of ac will not induce contractions, while that ofdb will. The experi- ments instituted by Bidder upon the lingual branches of the trigeminal and hypoglossal nerves ( 1742) of the dog were frustrated by the fact, that the tubercle generally included more or less of the similar nervous stumps : so that it was impossible to be certain that corresponding fibres had not united. Hence we can but conjecture from the theoretical consi- derations already adduced ( 1909) that, under more favourable circum- stances, these experiments would give affirmative results. 1911. An experiment made by Flourens appears quite decisive as regards the conduction of the influence of the will by the several motor nerves. When the nerves which supply the upper surface of a cock's wing were made to unite with those which run to the lower surface, the subsequent movements presented no difference from those of a healthy animal. Irritation of the central segment led to the same contractions. 1912. We have already ( 1700) observed, that the anastomoses and plexuses of the nerves intimately mingle their fibres; but that the presence of the terminal plexus cannot be similarly explained. The latter would rather increase the quantity of the nervous medulla, and multiply o o 2 564 NERVOUS EXCITEMENT OF THE TERMINAL PLEXUSES. [CHAP. XVIII. the mutual contact of the various fibres. It might be conjectured that the several fibres here act upon another, and thus effect a mutual communication of their excitements. There are certainly many pheno- mena which indicate that, within certain limits, something of this kind may occur. 1913. The oesophagus contracts under the influence of any of the dif- ferent roots of the vagus or spinal accessory nerves. Sometimes the same portion appears to be capable of responding to the stimuli of a number of nerves at least so far as we can judge by the naked eye. If this were really the case, we might conjecture that the change of one fibre of the terminal plexus induced a molecular change of another in its neighbourhood. The puncture of any part of the heart with a needle leads to a more or less perfect beat ( 1796). And if this pheno- menon is not based upon mechanical causes ( 1796), it must be due to a communication in the terminal distribution of the nerves. In rare instances, a local irritation of the gastrocnemius gives rise to a general contraction. Supposing that this result is not caused by any deception by any propagation of the pressure to neighbouring fibres it would argue a communication in the terminal plexuses of non-ganglionic nerves also. But the observations already adduced ( 1714) show that, in the larger nervous branches, nothing of this kind occurs. Since the para- doxical contraction ( 1845) depends on the electrotonic state, and not on the negative deviation of current, the latter could only act, either by the assistance of some special apparatus present in the terminal plexuses themselves, or through a direct influence exerted on the muscular fibres by a change in the electrical condition of the nerve. But the first of these suppositions is pretty decisively contradicted by a pheno- menon which we shall now mention. 1914. The inducteous current that arises at the instant of closing the circuit, takes a direction opposite to that of the inductive ( 245) one. Now if a similar phenomenon obtained in the nerves, the peripheral excitement of the motor fibres would necessarily produce a central one in the neighbouring sensitive elements. We might therefore expect, that the irritation of a motor root which has been cut away from the spinal cord ( 1720) would give rise to extensive reflex movements ( 1717). But experience teaches us that this is not the case. From reasons which may be easily conceived, such secondary action ( 1845) can only be produced by those vigorous electric impressions that give rise to a powerful electrotonic state. It is true that the denuded muscles now and then undergo reflex movements. But these are both rarer and weaker. One can scarcely avoid suspecting that in such cases the sensitive fibres which pervade the muscles co-operate ( 1721). And since the excitement of a sensitive root which has been separated from the spinal cord does not cause any muscular movements ( 1720), it is CHAP. XVIII.] EFFECT OF DIVISION OF THE FIBRES. 565 evident that the experiments hitherto made rather support the notion of communications in the terminal plexuses, than of inductive actions. 1915. The division of a nerve-fibre ( 1698) is no physiological difficulty, so long as its branches do not supply organs which are essentially different, or regions which are endowed with independent consciousness. It is true that we are in the habit of supposing ourselves able to recognize the site of the finest puncture with a needle. But we, have already seen ( 1651) that this is not the case. And since percep- tion is indistinct over a space the size of which varies with the degree of tactile sensibility ( 1653), the branches of division might very well be distributed within this limit. It would then be almost a matter of indifference whether the excitement originally proceeded from one branch of the primitive fibre or another. The simultaneous contraction of large muscular, bundles ( 1913) allows us to apply the same conclusion to those divisions which occur in motor nerves. But, on the other hand, could it be proved that one twig of a sensitive fibre went to the point of the finger, and another to the surface of the hand, it would certainly be a matter of great mystery how we could recognize the locality of a puncture inflicted upon either of these places with ban- daged eyes. The special organs of preparation ( 1650) afford no satis- factory explanation of this phenomenon, which is so essential to the perfection of the senses. Most of the divisions hitherto observed be- long to the terminal segments of nerves, or to the nerves of the intestines, where the distinction of locality is less acute. The future must decide whether such an instance as the above, which would materially shake the doctrine of separate conduction ever really occurs. 1916. The physiological details with which we are at present ac- quainted afford no indication of the way in which the nerve-fibres begin and end. They neither imply free terminations, nor contradict looped communications, or confluent networks of similar fibres ( 1702). 1917. The physiological relations of the ganglion-corpuscles are as yet almost unknown. According to Du Bois, the ganglia of the posterior roots of the spinal cord ( 1719) offer no obstacle to the propagation of the electrotonic state ( 1836) or the negative deviation of current ( 1846) to the fibres of the sciatic plexus (ale d, Fig. 334, p. 507). But many of the fibres which enter the ganglion are uninterrupted by ganglion-corpuscles ( 1770); so that we are not justified in concluding that these latter exert no influence on those primitive fibres which really are connected with them. On the other hand, it must be recol- lected, that it is at present undecided how the nervous medulla of the double processes behaves to the ganglion-corpuscle itself or whether the ordinary conducting substance is not interrupted here, so that the excitement is necessarily propagated by the corpuscles themselves. And the fact that many of these swellings contain far more corpuscles than 566 CONSTITUENTS OF THE NERVOUS CENTRE. [CHAP. XVIII. fibres ( 1770) at any rate shows that the ganglion-corpuscles develope certain independent effects, which are only communicated to the nerve- fibres or other neighbouring tissues. 1918. Many phenomena which are generally ascribed to the action of nerves or ganglia, do not immediately depend upon these, but upon other tissues. For example, the influence of narcotic poisons is usually re- ferred to the nerves. It is thus explained, why the pupil of the eye (c, Fig. 150, p. 273) dilates when some drops of a solution of hyoscyamus or belladonna are dropped into the sac of the conjunctiva (d, Fig. 150). But comparative physiology refutes this supposition. The enlargement of the pupil occurs only in the mammalia, whose iris contains unstriped muscular fibres ; and not in birds, in whom its contractile elements are striped. While in both these classes, the membranes of the iris include numerous nerve-fibres, part of which have traversed the optic ( 1726), or other ganglia ( 1786). So that here the special character of the con- tractile tissues appeal's to be more important than the influence of the nerves. (Compare 1301.) 1919. The nervous centre formed by the brain and spinal cord con- tains two chief substances : a white, or medullary ; and a grey or reddish- grey, or cortical. For example, on looking at a median longitudinal section of a human brain (as exhibited in Fig. 365) we find the grey FIG. 365. matter on the surface of the convolutions of the cerebrum (a b c) and the cerebellum (d) ; and the white in the corpus callosum (/), the fornix (1), the septum lucidum (between / and /), the ciliary body (g), &c. While a transverse section of the spinal cord (a b, Fig. 366, p. 572) has white fibres externally, and grey matter in its centre. CHAP. XVIII.] FIBRES OF THE NERVOUS CENTRE. 567 1920. These two tissues of the nervous centre essentially correspond to the two chief constituents of its periphery ( 1689). The white substance contains primitive fibres ; and the grey, cell-structures (Tab. V. Fig. 76, a) which in many respects resemble the ganglion-corpuscles (Tab. V. Fig. 74.) 1921. An examination of the roots of the spinal cord (d e, Fig. 335, p. 510) will convince us that each of their peripheral primitive fibres (Tab. V. Fig. 68) is directly continuous with a central one. The latter possesses a medullary content, and a sheath or neurilemma, like the former. But they generally have either a smaller transverse diameter from the very first, or afterwards undergo a gradual diminution in size. And owing to their greater delicacy, and to their want of that areolar tissue (Tab. III. Fig. 40) which intervenes between the nervous bundles ( 1694) of the periphery, these central fibres, though originally cylin- drical, often become varicose (Tab. V. Fig. 68, d) as a result of compres- sion or injury. 1922. From the frequent division (Tab. V. Fig. 70) of the fibres of the peripheric part of the nervous system ( 1698), we might easily con- jecture that something similar occurs in the cerebro-spinal centre. But at present experience has not fully established the accuracy of this sup- position. Setting aside the deceptive appearances sometimes presented by different layers, there certainly are rare instances in which we find divisions of single fibres. But it is a question whether these are riot produced artificially. For the nervous medulla is easily protruded in various directions by pressure. And this lateral branching is rendered more deceptive by the fact, that the delicacy of the neurilemma is such as to prevent its being recognized without artificial assistance ; such as, for instance, the application of acetic acid. At a further stage of putrefac- tion, any compression of the medullary content in the act of preparation breaks it up into separate drops (Tab. V. Fig. 75), which are generally single, but sometimes appear to be bifurcated. 1923. While the primitive fibres of the centre are distinguished from those of the periphery by their delicacy, this is still more remarkably the case with its ganglion-corpuscles. The slightest mechanical injury so greatly disturbs their natural connection as to leave nothing visible but a finely granular and grey or reddish-grey substance, with relics of nuclei and uucleoli. This effect is greatly favoured by their want of firm intervening tissues ( 1921). 1924. The numerous grey portions of the nervous centre exhibit far greater variety of form than the ganglia of the periphery. We some- times find very large ganglion-corpuscles (Tab. V. Fig. 76), the chief substance of which (a) has a pale appearance, and is here and there extremely granular (b). The nucleus (c) contains a clear vesicle (d). The chief substance of 'other ganglion-corpuscles, which are often 568 GREY MATTER OF THE NERVOUS CENTRE. [CHAP. XVIII. smaller, consists of nothing but granules. It is the latter which decide the colour seen by the naked eye. When many of the former ganglion-corpuscles are aggregated together, the whole has a pale whitish grey colour. While larger numbers of the second variety give the mass a reddish-grey aspect. 1925. The proportion of the general contents to the nucleus also varies greatly. The latter may form either a small (Tab. V. Fig. 76) or a large fraction of the whole : a diiference which greatly affects the form and bulk of the corpuscle. Thus while the ganglion-corpuscles of the spinal cord (Tab. V. Fig. 76) and of the electric lobe of the torpedo ( 1890) are large enough to be recognized by the naked eye, others require a magnifying power of from two to three hundred diameters. Finally, we meet with some forms which cannot be reduced to the type of cell, nucleus, and nucleolus ; but present simple granules, or mere aggregations of minute globules. All of these solid structures are united to each other by a homogeneous, colourless, semifluid, and tenacious sub- stance ; which is in all probability very rich in albumen. 1926. We have already ( 1890) seen that the ganglion-corpuscles of the electric lobe of the torpedo give off grey or greyish-red processes. Something similar is repeated in the grey matter of the nervous centre in other vertebrata (Tab. V. Fig. 76 e). Since the ganglion-corpuscles of the periphery emit corresponding processes, some of which are me- dullary (Tab. V. Figs. 72, 73), and others devoid of medulla (Tab. V. Fig. 74, b c d e), it becomes a question whether the same process is not frequently repeated in the nervous centre. Now in microscopic examinations we certainly may rarely observe that a grey branch of a ganglion-corpuscle (Tab. V. Fig. 76,/) appears to undergo a tran- sition into a true medullary fibre (g]. But more exact adjustment of the focus ( 1469 et seq.), or movement of the preparation, will generally show that this appearance is deceptive; and that the medullary fibre merely lies on or near the process of the ganglion-corpuscle. Still what has already been stated of the ganglia ( 1767) will justify us in stating that the central primitive fibres are probably intimately connected with the processes of the ganglion-corpuscles. 1927. Since every primitive fibre of the root of a spinal or cerebral nerve is continued into a fibre of the centre ( 1921), the brain and spinal cord must contain representatives of all the origins of the cerebro- spinal nerves. But it has hitherto been found impossible to give even an approximative answer to many of the chief questions which here sug- gest themselves. The brain and spinal cord of the smallest vertebrata contain so large a number of microscopic constituents these are again so densely and complexly interlaced, and oppose such extraordinary diffi- culties to research, and finally, our researches themselves are always limited to such small portions that it will probably be hundreds of CHAP. XVIII.] RELATIONS OP THE FIBRES IN THE CENTRE. 569 years before we gain any clear insight into this important part of the anatomy of the nervous system. 1928. The presence of grey matter in the interior of the spinal cord ( 19 19) at once contradicts the notion that this structure corresponds to a single nerve uniting all the primitive fibres of the spinal roots. The ganglion-corpuscles endow it with a higher import, the physiological results of which will hereafter occupy our attention. But since con- scious sensations and the mandates of the will alike proceed from the brain, it becomes a question whether the central processes from the primitive fibres of the spinal nerves ascend to the brain, or whether they end in the spinal cord, and depute all further communication with the brain to other intervening tissues. 1929. Some observers assert that the central fibres of the spinal nerves terminate by free extremities shortly after their entry into the spinal cord. But this statement is probably based upon deceptive appearances. It is more likely that their transition into ganglionic processes ( 1767) permits the fibres to terminate, or allows their num- ber to decrease, always supposing that, in the latter case, there are fewer medullary fibres present near the brain than towards the end of the spinal cord ( 1927). 1930. Volkmann endeavoured to decide the question by comparing the transverse section of any given part of the spinal cord with the sum of that of the roots of all the nerves which had previously entered it. He argued that, if representatives of all the spinal nerves ascend towards the brain, we might expect that the transverse section of a piece of spinal cord which lies nearer to the brain would be at least as great as the united transverse sections of all the roots of nerves which had hitherto entered it. But this conclusion is not so safe as it appears at first sight to be. The roots of the nerves contain a large quantity of areolar tissue, which is absent from the nervous centre. The primitive fibres of the latter are smaller than those of the former ( 1921). And the difference of their surfaces of course increases in a quadratic porportion : i.e., a central fibre which has half the diameter of a peripheric one, has but one fourth of its transverse section. It is true that grey matter is added in the spinal cord. But unless it equal the difference just men- tioned, the smaller transverse section of the spinal cord will be no valid proof that the central fibres have previously terminated. While the further changes which might be introduced by the processes of the ganglion-corpuscles ( 1929) and by the divisions of the nerve-fibres, will obviously prevent any safe conclusion from being at present come to. 1931. In fishes, the spinal cord is so narrow at its continuation into the medulla oblongata (to the right of m, Fig. 365, p. 566), as to justify our supposing that it cannot contain anatomical or physiological equiva- 570 RELATIONS OF THE FIBRES IN THE CENTRE. [CHAP. XVIII. lents of all the fibres which have previously entered it from without. On the other hand, in birds and mammalia, the difference is by no means so extraordinary. At present, however, it is impossible to decide whether these animals are really similar in this respect, or whether the higher development of their brain is associated with a more complete representation of the spinal fibres. The physiological phenomena which here come into consideration will again occupy our attention. 1932. The pure white medullary substance consists solely of primitive fibres j which are densely aggregated together, or closely interwoven with each other. In certain situations, however, for example in the crura cerebri of the human subject pigment cells (Tab. II., figs. 29, 30) are interposed between these fibres, and produce a dark colour which is visible to the naked eye. The mass thus coloured is called the substantia nigra. Most of the apparently grey matter exhibits scattered primitive fibres under the microscope. When these are in large quantity, the whole mass has a light grey colour. While the uniform mixture of numbers of primitive fibres with a certain proportion of highly granular ganglion-corpuscles produces a yellow colour, such as may often be seen at the innermost margin of the grey matter covering the human cerebral hemispheres (a be Fig. 365, p. 566). The yellow colour of the electric lobe of the torpedo ( 1890) depends upon similar causes. 1933. The quantity of pure medullary substance in the brain of birds and mammals is many times greater than that contained in the roots of all the cerebral and spinal nerves. Hence many believe that there are special cerebral and spinal fibres ; i. e. fibres which only belong to the tissues of the centre, and have no direct connection with any peri- pheric nerves. But however probable this view, the difficulties which oppose anatomical research prevent its being proved in detail, and thus rendered really useful. Part of the medullary substance probably begins by the peripheric fibres being prolonged in very circuitous routes. While another part of it perhaps originates in the medullary processes of the ganglion-corpuscles, whether by merely uniting portions of the centre to each other, or by connecting them with peripheric fibres. 1934. It has often been attempted to unravel the arrangement of the cerebral and spinal fibres in the higher animals by dissecting their course with the naked eye in preparations which have been steeped in alcohol, nitric acid, a solution of creosote, or other suitable fluids. But it is impossible thus to verify those minute microscopic relations which could alone be decisive. These can only be obtained by patiently examining sec- tion after section of the brain with the microscope, and by sketching out general representations in accordance with these observations a process which has been followed in the medulla oblongata by the unwearied industry of Stilling. 1935. We have already remarked ( 1917) that the ganglion-cor- CHAP. XVIII.] REFLEX MOVEMENTS. 571 puscles of the periphery are capable of developing certain special actions, which are continued into the nerve-fibres. This distinction between generation and conduction is perhaps repeated in the nervous centre. Here the grey matter would form the special generator of force ; while the primitive fibres of the centre would not only conduct ( 1904) the excitement to or from the periphery, but would probably execute similar internuncial functions for various segments of the brain and spinal cord. 1936. Such a mutual communication forms one of the most remark- able characteristics of the nervous centre. We have seen (1914) that the excitement of a motor fibre of a peripheric nerve leaves the neighbour- ing sensitive fibre at rest. On the other hand, in the brain and spinal cord, transfers frequently occur. Reflex movements as well as the reflex sensations assumed by some physiologists are due to the con- joined excitement of dissimilar actions; while co-ordinate movements and sensations are produced by the association of similar actions. 1937. Reflex movements are due to a stimulus reacting in a cen- tripetal course towards the nervous centre, where it sets in action cer- tain motor fibres. Hence what was originally a sensitive impression is followed by muscular contractions. The laughter and involuntary de- fensive movements generally caused by tickling, are familiar examples of this kind. 1938. Reflex movements imply the co-operation of the nervous centre ; and, in all probability, of its grey matter. Supposing v (Fig. 366) to be a sensitive fibre which ends in the skin of a limb, the central excitement will proceed to the grey matter efghof the spinal cord abed. Here it indirectly excites the corresponding motor fibres v> : either alone, or together with those of the opposite side $, or even with the more remote motor fibres . The dotted double lines and arrows in the diagram represent the directions in which the excitements are propa- gated, but not their several paths. 1939. If a beheaded frog be allowed a little time to recover its irrita- bility, and if a single toe of one leg for example, the right hind leg be pinched, or stimulated with a drop of sulphuric or acetic acid, reflex movements may occur ; either in the right hind leg, or in both hind legs, or even in all four extremities. The extent of this motor reaction depends chiefly on the strength of the irritation, and on the degree of sensibility which the preparation possesses. Powerful stimuli give rise to more diffuse actions. The sensibility is generally much weaker imme- diately after decapitation. But rest subsequently restores it to such a degree, that pressure on the toes gives rise to vigorous reflex movements, in which the beheaded animal leaps violently for some distance. And on placing it on its back, and pinching the skin of its belly, it appears to protect itself, and to thrust away bodies in contact with it. Subse- 572 REFLEX MOVEMENTS. [CHAP. XVIII. quently this irritability gradually decreases, so that pressure upon a single toe of the right hind leg only gives rise to reactions in this and the other hind leg or one fore leg. During the last relics of irrita- bility, contractions only occur in the muscles of the stimulated hind leg; and they finally become limited to particular portions of this limb. 1940. Reflex movements are easiest brought about by irritations of the skin. For example, pinching the sciatic nerve of a beheaded frog forms a much less effective means of excitement. And on irritating a limited portion of the muscular mass of the hind leg, these reactions are still more frequently absent. 1941. The impression made upon the skin must always exceed a cer- tain minimum. Hence we frequently find preparations in which light pressure and mechanical excitement are quite ineffective, while a drop of acid succeeds. The frequent repetition of slight impressions, such as are made by tickling certain parts of the skin ( 1660), easily gives rise to corresponding reflex movements. 1942. We have already seen ( 1533) that the impressions of the senses have a certain after-duration. These reflex actions may last still longer. A single cutaneous irritation often produces a storm of reactive movements, which continue during a considerable period of time. Hence the communication widens the cycle of action in duration as well as in extent. But the conditions of this latter result are more CHAP. XVIII.] REFLEX MOVEMENTS. 573 limited than those of the first. It presupposes a greater mobility of the elements of the spinal cord, a mobility which is due either to a previous injury, or to an abnormal disposition. We have here a parallel to the double contractions of the living nerve ( 1855), or to the tonic spasms which follow some kinds of poisoning. 1943. The natural connection of the conducting paths with each other is just as necessary to the reflex movements as it is to the actions of the peripheric nerves. Hence division of the sensitive fibres (v, Fig. 366) renders it impossible for the corresponding portions of skin to in- duce reflex movements. While section of the motor fibre (, Fig. 366) will obviously destroy not only the reflex, but also the voluntary, con- traction ( 1710) of its corresponding muscles. 1944. The reflex movements of the beheaded frog ( 1939) have already taught us that the transverse section of the nervous centre does not prevent the reflex action of that segment of the spinal cord which still retains its natural connection. Under favourable circumstances, irrita- tion of one hind leg may still give rise to reactions in all four of the limbs ( 1939). And touching the conjunctiva of the severed head of a mammal may cause its eyelids to close : the action of the sensitive fibres of the trigeminal nerve ( 1729) being transferred in the me- dulla oblongata (h m, Fig. 365, p. 566) to those motor fibres of the orbicularis palpebrarum muscle (pq, Fig. 150, p. 273) which are given off from the trunk of the facial nerve ( 1735). 1945. The nervous centre may be cut across in several places without destroying all possibility of reflex movement. It is only those structures which lie in the immediate neighbourhood of the injured place that seem much affected. More distant organs are merely severed from each other by the transverse section, so as to diminish the extent to which the communication can occur. Thus supposing p q (Fig. 366) to be the sensitive, and aft the motor roots of the fore leg of a beheaded frog, while v w and -n are the same structures of the hind foot, transverse section of the spinal cord at I m will permit a stimulus applied to v to produce reflex movements of both hind legs ? $, but not of the fore legs a (3. The body of a snake or the tail of a lizard may be thus divided into a series of segments; each of which (owing to the sim- plicity of their nervous distribution) retains its reflex activity. 1946. Incomplete transverse sections which leave only a bridge of grey matter remaining, do not prevent all longitudinal communica- tion. For example, if the spinal cord (Fig. 366) be cut through on the right side to k x, and on the left to I y, a stimulus which passes along the sensitive fibres q of the right fore leg may excite reflex movement of all four feet through , (3, ?, and $. So that the grey matter at xy renders the interference harmless. On repeating the experiment in the living frog, the animal may gradually recover the full 574 REFLEX MOVEMENTS. [CHAP. XVHI. influence of volition over its hind legs. While on the other hand, complete transverse section of its spinal cord at n o destroys the voli- tional influence that descends from the brain to the hind legs ; and at a b, that which passes to all four extremities. Hence the nearer these injuries lie to the brain, the more extensive is the effect they produce. 1947. Those nerves which occupy the immediate neighbourhood of the complete transverse section generally lose all reflex influence : as will be the case, for example, with tu and e (Fig. 366) when the transverse section is made at Im. Hence incomplete transverse sections ( 1946) which lie immediately behind each other, may exercise the same obstacle as complete ones. 1948. When the spinal cord abed (Fig. 366) of a beheaded frog is cut longitudinally through its middle, z a' b' c', all transverse conduction is rendered impossible. But longitudinal communication can still ob- tain. Hence the application of proper stimuli to the sensitive fibres q of the skin of the right fore leg still gives rise to reflex movements in both the right legs through /3 and $, but not in those of the left side through a and 17. But the living animal can still move all four limbs at will. If the longitudinal section be made more externally, much will depend on the circumstance whether it still occupies the grey matter efgh, or the white fbcg. In the former case, a longitudinal commu- nication may still occur; while in the latter, it is destroyed. Now many of the central fibres of sensation and motion run in the white substance (fbcg, Fig. 366) in greater or less proximity to each other. Hence there is neither any direct communication of excitements by the central primitive fibres, nor any simple parallel of the paradoxical con- traction ( 1845) : but a necessary co-operation of that substance of the nervous centre, which appears grey to the naked eye. 1949. The complete removal of the spinal cord destroys its corre- sponding reflex movements; and the destruction of the medulla ob- longata and brain, those of the cerebral nerves. The peripheric primitive fibres of the nerves therefore behave just like those central fibres which have not yet reached the grey substance. Hence the com- munication can not depend on the different degrees of delicacy possessed by the structures which ensheath these two kinds of conductive tissues ( 1921). 1950. Hitherto we have only considered the reflex actions of the voluntary muscles. But similar phenomena may also be exhibited by the intestines. On pinching various parts of the alimentary canal of a beheaded frog, its limbs often move vigorously. And even when a frog has been so far narcotized with ether that the application of pressure to the toes excites no reflex phenomena in its trunk or limbs, parti- cular portions of the alimentary canal may still be made to contract by such interference : or the heart may recommence beating. Thus the CHAP. XVIII.] REFLEX MOVEMENTS. 575 ganglia neither check the advance of sensational stimuli towards the centre, nor the propagation of motor ones from it. Changes that pro- ceed from the peripheric extremities of the sympathetic may be trans- ferred to cerebro-spinal nerves which obey the mandates of the will : and, conversely, stimuli that impinge upon the tactile skin may be transferred through the spinal cord to the intestines which are governed by the sympathetic. But since, in both these respects, the ganglionic nerves furnish negative results more frequently than the cerebro-spinal nerves, we are justified in supposing that here, as in the sensations of pain, certain special conditions are present. 1951. In the heart there is a peculiar and exceptional appearance, which is sometimes repeated under other circumstances. The closure ( 1241) of a galvanic circuit sometimes disturbs the rest of the excised heart, and gives rise to a complete pulsation of its auricles (a b, Fig. 98, p. 185) and ventricle (cde, Fig. 99). On repeating the ex- periment several times, the heart often continues to beat spontaneously. But the same galvanic current then frequently has no effect in accele- rating the pulsations already present. This indifference on the part of the active heart is also sometimes seen in etherized frogs. The same pressure on the toes which aroused the quiescent heart, often loses all action after the restored pulsation has lasted for some time. 1952. Since the grey matter of the nervous centre forms a link which is essential to the phenomena of transfer ( 1948), the ques- tion suggests itself, whether that of the periphery or the ganglion- corpuscles may not independently permit of similar effects. But the facts hitherto known do not establish the possession of such a capacity by the ganglia. When the mucous membrane of the palate or oeso- phagus of a mammal is tickled with a feather, reflex movements of deglutition or vomiting are produced. But, on the other hand, after removal of the medulla oblongata these results no longer obtain; although the commencement of the peripheric course of the vagus nerve ( 1744) has a large ganglion, which contains the corresponding motor fibres. The separated loops of the intestine of a recently killed rabbit (Fig. 76, p. 134) are thrown into more vigorous and extensive undulatory movements (da, Fig. 76) when left in their natural attach- ment, or when cut out with their mesentery (hi), than when this is completely removed. Hence it has been supposed that these reflex or associated movements are produced by the corresponding ganglia of the sympathetic. But we may easily convince ourselves that the absence of those parts which contain the ganglia does not preclude the possi- bility of extensive and repeated undulatory movements. So that they would seem only to favour and assist the result. 1953. Since it is not every cutaneous irritation that is followed by a reflex movement, the transfer requires the support of certain collateral 576 REFLEX MOVEMENTS. [CHAP. XVIII. conditions. On comparing an uninjured with a beheaded animal in this respect, we find that the latter affords far more marked reflex phe- nomena as soon as its first period of exhaustion is past ( 1874). The influence of cerebral action may frequently be seen in our own persons. The laughing and reactive movements which tickling would otherwise produce, may be suppressed by the will either for a time, or altogether. Several of the reflex phenomena which will shortly be mentioned such as sneezing or deglutition are partially under our own control. Others, however, are quite involuntary. 1954. Many of our corporeal acts are based upon a mechanism of reflex and corresponding movement. Such are the closure of the eyelids when a particle of dust has fallen into the sac of the conjunctiva (890); the alteration of the pupillary aperture in light or darkness ( 1496); the sneezing which succeeds mechanical or chemical irritation of the mucous membrane of the nose ; the involuntary movements of deglutition in the pharynx and oesophagus (381); the cough which follows irritation of the internal surface of the larynx or trachea; the effect of tickling; the scream which is sometimes uttered on the unex- pected puncture of the skin by a needle with many similar phenomena. These examples show that every portion of the skin conditionates a more or less determinate variety of reflex movement. We meet with a certain co-ordination, which is effected by the exciting sensitive fibres and their central organs. 1955. This statement is confirmed by a more careful examination of those reflex movements which occur in the beheaded animal. Those motor nerves of the centre which lie near the entrance of the exciting sensitive fibres are the first to be thrown into action. Hence the reflex movement predominates in that limb the skin of which is irri- tated ( 1940). On irritating the anterior part of the skin of the belly in the decapitated frog, its fore legs are moved forwards. But on shifting our attack towards the middle of the belly, they move back- wards. In like manner, on irritating the posterior half of this surface, the hind legs are pushed forwards. While, on compressing one of the toes of the hind leg, these limbs are extended, so that the animal frequently springs forwards. The application of a stimulus to the hind legs may produce reflex movements of the fore feet, and vice versd. Hence the communication in the spinal cord may be either from before backwards, or from behind forwards. But when, on the other hand, we irritate the conjunctiva, it is only the orbicular muscle of the eye- lids (p q, Fig. 150, p. 273) which contracts; and not the other muscles of the face, which are also supplied by the facial nerve ( 1735). In like manner, tickling the soft palate acts on the commencement of the alimentary canal, but not on the heart or the lungs, which equally depend on the vagus ( 1749) nerve and the medulla oblongata. CHAP. XVIII. j REFLEX MOVEMENTS. 577 In one word, the apparatus of the nervous centre possesses certain keys, which are played upon as soon as an impulse is furnished by stimulation of the corresponding sensitive fibres. This often gives an appearance of adaptation to the reflex actions seen in the beheaded animal. But a more careful examination teaches us that there is here neither volition, nor purpose, but a definite organic action. 1956. The ducts ( 867) and receptacles ( 923) of the glands, which are provided with unstriped muscular fibres, frequently offer a reflex movement like that of the intestines. Friction of the glans penis (g 1 , Fig. 154, p. 285) leads to a reflex action of the seminal ducts (q w, Fig. 154) and the seminal vesicles (nx) ; so as to be followed by emis- sion. The copious flow of tears which succeeds irritation of the conjunc- tiva (d, Fig. 150, p. 273) is an act of reflex * secretion. To the same category also belongs that increased effusion of saliva which follows mechanical irritation of the soft palate. 1957. The question whether certain corresponding segments of the brain and spinal cord do not co-operate in all reflex secretory acts of this kind cannot at present be answered with certainty. It is true that the flow of tears ceases on the destruction of the medulla oblongata, so that the transfer would seem not to be effected by either the Gas- serian (a?, Fig. 322, p. 484) or the optic ganglion ( 1726). But we must not forget that injury of the medulla oblongata more or less destroys one essential condition of secretion ; namely, circulation. On the other hand, the observation that this copious flow of tears appears not to occur after section of the trigeminal nerve between the Gasserian ganglion and the brain, points very expressively to the influence of the nervous centre. It is true that a person suffering from disease of the spinal cord, who has paralysis of both legs, and not the least trace of sensibility during sexual intercourse, may still have seminal emissions. But here it is possible that the organic degeneration is of such a kind as to destroy conduction to the brain, without affecting the transfer to the corresponding motor fibres. 1958. We have seen ( 1707) that the vital actions of the nerve- fibres take a one-sided and centripetal or centrifugal course, while the electrotonic state ( 1836), as well as the negative deviation of current ( 1846), passes in both directions. This has already led us to con- jecture, that the latter is a mere collateral phenomenon, which accom- panies, but does not constitute, the living function of the nerves. The laws which regulate the transfer at the nervous centre confirm this conclusion. 1959. When we irritate a sensitive fibre (v, Fig. 366, p. 572), the * But this effusion sometimes follows so instantaneously, as to justify the conjecture that it begins by a reflex muscular contraction, to which the slower secreting process is only sub- sequently added. EDITOR. P P 578 REFLEX SENSATIONS. [CHAP. XVIII. excitement takes a centripetal course. The change which causes the reflex movement then runs centrifugally in u, so as not to contradict the law of a vital one-sided propagation. But when we attack the motor fibre (, Fig. 366) in the middle of its course, no reflex sensation or pain ( 1720) is produced. Yet sipce the negative deviation of current obtains equally in its more central' segment, could this alone determine a transfer, it might occur. While on the other hand, the supposition that the final result depends on a one-sided, centrifugal,^ and vital pro- pagation, at once explains the insensibility of the anterior roots of the nerves. 1960. Many have supposed that, under certain unusual circumstances, reflex sensations may occur. But all the facts thus explained are open to other interpretations. The painful sensations which follow violent muscular movements are possibly due to changes in the peripheric tissues and their nerves. It is probable that the pains which accompany con- tractions of the uterus also proceed from the terminations of its nerves. And even were this not the case, the phenomenon might be otherwise explained. The grey matter which is connected with the central motor fibres, and which evokes their action, is perhaps itself agitated so strongly, as to cause a transfer to the neighbouring representatives of the sensitive fibres. It is probable that we might thus explain the fact, that morbid muscular contractions sometimes cause such violent pain, as to persuade a thoughtless surgeon to amputate (or otherwise uselessly mutilate) a limb. 1961. Co-ordinate movements are due to the habit that certain mus- cles have of acting simultaneously, whenever any one of them gives the first impulse. Here we have an alternate play, such as is repre- sented by y $ E { , (Fig. 366, p. 572). These phenomena are sufficiently explained by the statements in 1960. 1962. Like the reflex contractions ( 1954), the co-ordinate move- ments materially assist many vital acts. Among the movements which are necessarily associated we may enumerate the simultaneous adjustment of both eyes ( 1443), the regular and successive move- ments of deglutition ( 381), the normal act of respiration ( 739), the different exceptional varieties of this movement ( 755), the abdominal pressure ( 393), and many other phenomena. Here again certain keys in the nervous centre are played upon in a prescribed manner ( 1955). The will has either no action at all, or exerts only a limitary and quan- titative influence. 1963. All the instinctive movements belong to the class of pheno- mena now under consideration. Many of them such as deglutition or respiration are executed by the new-born infant ( 742); while others such as the maintenance of equilibrium during the various movements of progression ( 1320) are only learnt gradually. Others such as, CHAP. XVIII.] ASSOCIATED MOVEMENTS AND SENSATIONS. 579 for example, the simultaneous movement of some groups of the facial or digital muscles are imperfections, which can only be gradually over- come by effort. Here the neighbouring and corresponding parts of the nervous centre influence each other so easily, that nothing but practice ever enables us to govern them singly. 1964. Consensuous impressions are caused by the simultaneous ex- citement of different sensitive fibres, (such as are represented by p q r and s, Fig. 366, p. 572). In many cases it is possible that the transfer occurs at the peripheric extremities of the nerves. The irradiation of the retina ( 1531) is an example of this kind. But in most other instances the communication only occurs at the nervous centre. The general thrill which follows the scratching or brushing a small portion of the skin belongs to this class of phenomena, and may be explained by the theory mentioned in 1960. 1965. Many of the morbid consensuous impressions occur in those fibres, the central segments of which run in the neighbourhood of the representatives of the exciting nerves ( 1939). Hence pinching the bulbous nerve of the stump of an amputated thigh sometimes gives rise to sensations of pain in the skin of the remainder of the limb, and in the neighbouring abdominal walls. In other cases, overloading the stomach produces violent pain in the foot, which disappears on the artificial production of vomiting. 1966. Since the local accuracy of sensation and voluntary motion are in themselves some of the highest perfections of the animal body, every transfer is, to a certain extent, a disturbance such as can only be excused or required by collateral considerations. This is especially true of those organic arrangements which give rise to many reflex ( 1937) and co-ordinate ( 1961) movements. Most of the instinctive movements correspond to physical laws, which cannot be completely fathomed by the most toilsome research. The explanations already given of the simultaneous movements of the eye ( 1443), and the maintenance of equilibrium ( 1320), may illustrate the truth of this statement. It seems as if Nature had determined to prevent all chance of error, and had therefore constructed an instrument capable of playing upon itself. A similar arrangement is obviously implied in these involuntary serial contractions which like deglutition (381) and parturition, can only be executed by a succession of determinate acts. 1967. The influence of practice on our movements of progression plainly shows how gradually we learn the use of the nervous instrument with which nature has equipped us. The same statement also applies to many other voluntary movements : such as the combination of various sounds ( 1425) in speaking, or of various fingers in playing music, toge- ther with many similar acts which the practised performer executes at p p 2 580 DISPOSITIONS OF THE NERVOUS CENTRE. [CHAP. XVIII. once, and without bestowing any consideration on their details. All of these actions end by becoming essentially instinctive. 1968. The continual afflux of scarlet blood is a condition very im- portant to the normal molecular constitution of the nervous centres. This proposition especially holds good with mammals and birds : but is less strictly applicable to reptiles and fishes, in whom the interchange of gases is less active ( 840), and irritability more independent ( 1261). When the blood carried to the brain of a man becomes deficient in quantity, or dark in quality, its alteration is soon followed by deceptive sensations, head-ache, fainting, unconsciousness, suffocation, convulsions, and finally death. Hence the brain of a beheaded person soon dies. Persons who are hanged, or who are suffocated in carbonic acid, or in an irrespirable gas that is not directly noxious, perish in a similar manner. 1969. Many poisons greatly change the disposition of the whole nervous centre, or of certain of its parts. When pure strychnine or one of its salts is introduced into the stomach or into a wound, the animal falls into the most violent convulsions as soon as a sufficient quantity of the poison has been absorbed, and transmitted with the blood to the nervous centre. The slightest mechanical disturbance of a frog which has been thus treated at once excites a vigorous tetanic state of vary- ing duration. A similar phenomenon is seen in many other kinds of poisoning. Frogs who have been dosed with opium, or who have had a solution of belladonna, oil of turpentine, or sulphuric ether in- jected into the rectum, sometimes exhibit general tetanic convulsions on the local application of external stimuli. But that contraction of the muscles of the limbs which succeeds the use of ether, appears not to reach the great intensity produced in poisoning by strychnine. 1970. The poison need not necessarily reach the spinal cord through the blood. If we behead a frog, excise its heart, lay bare its spinal cord, and moisten this with a solution of strychnine, we may often obtain com- plete tetanic convulsions of the muscles of the foot. 1971. After removing the brain and spinal cord, we may moisten the peripheric nerves with a solution of strychnine (even while the heart continues to beat), without producing this change of disposition ( 1969). Hence this poison only acts through the nervous centre, and chiefly through its grey matter. Of course the solution of strychnine must not contain any other substance capable of altering the contents of the nerves. Tincture of opium and ether check the action of those parts of the peripheric nerves which they thoroughly penetrate. But no general tetanic convulsions follow. 1972. One or two sixtieths of a grain of strychnine suffice to throw a mammal into violent tetanic convulsions. And since only a fraction of this quantity is carried by the blood to the spinal cord, it is evident CHAP. XVIII.] DISPOSITIONS OF THE NERVOUS CENTRE. 581 that the change of disposition must depend upon extremely minute quantities of the noxious agent. The alteration thus brought about may in course of time disappear. A rabbit may be thrown into powerful tetanic convulsions by a little strychnine, and yet be quite well on the following day. The same recovery may be seen in frogs after enemata of weak solutions of belladonna or opium, or ether. 1973. Great attention has been given to the action of ether and chlo- roform; on account of the insensibility which they produce allowing the infliction of violent injuries without pain. Opium had previously been administered to persons who were about to undergo operations; and laudanum had been injected into the blood of animals before painful experiments. But the use of this drug is far too unsafe and trouble- some to be compared with that of ether as recommended by Jackson, or with that of chloroform, which has been substituted for ether in accordance with the proposal of Simpson. 1974. The usual ether apparatus consists of a receiver, at the bottom of which lie sponges moistened with the drug. The walls of their nume- rous cavities furnish a large evaporating surface. Hence the atmospheric air which fills the remaining space of the vessel, and is frequently changed during respiration, is easily saturated with the vapour of ether. Proper connecting tubes allow the access of air. The person breathes through a mouth-piece with opposite valves, which cause the air laden with vapour of ether to enter the lungs, while the gases subsequently expired pass off into the surrounding atmosphere. The action of chlo- roform is so energetic that we have but to pour a few drops of this fluid on a handkerchief, and hold it under the nose, to produce stupefaction in a very short time. 1975. A person beginning to inhale the vapour of ether is often at- tacked by irritative cough. To this frequently succeeds a slight and pleasant intoxication, which is connected with an increased impressibility of the senses, or with deceptive sensations, and great mental hilarity. This is sometimes followed by delirium or raging frenzy. Ultimately sensation or, at any rate, the perception of sensuous impressions is altogether lost. The capacity of hearing appears to be retained longer than the other senses. The person no longer feels the most violent pain, or at least becomes incapable of perceiving it with the clearness and emphasis of reflection and memory which are present in persons not thus intoxicated. Hence an entire limb may be removed without the patient being awakened from his sleep and dreams. The act of dividing the skin and the large nerves which forms the most painful part of an operation leads to none but very inconsiderable movements, and slight or transient expressions of pain. It sometimes happens that the patient, who is wandering in his thoughts, sees the operation com- menced without evincing the least excitement. Many, however, scream 582 STUPEFACTION BY ETHER AND CHLOROFORM. [CHAP. XVIII out. But when subsequently awakened, they no longer remember the past pain. The tactile sensibility of the skin often gives rise to a pecu- liar phenomenon. There is a period of stupefaction by ether, in which the patient feels a puncture with a needle, and recognizes a stick pressed upon the skin as a blunt and broad body ( 1651), but yet undergoes cutaneous incisions without offering any resistance. 1976. At a further stage of the action of ether, the face becomes pale. The objective actions ( 1433) of the senses altogether disappear. The muscles become extremely relaxed. They lose their elasticity, and retract less when cut through ( 1272). The patient is plunged into a deep sleep, which is accompanied by snoring or stertorous breathing. The external resemblance to a dying person becomes more and more prominent. If pure air be now allowed to enter the lungs, the patient soon recovers completely. At most, he suffers some time from swimming of the head, and an ethereal odour of the breath and eructations; or occasionally from nausea, melancholy, and prostration. He now recollects his dreams during the ethereal intoxication ; and indicates what he underwent during the narcotized state in accordance with the delusions which then exclusively occupied his mind. Finally, it sometimes hap- pens that, immediately on waking, the patient continues the discourse in which he was interrupted by the stupefying effect of the ether. 1977. The effect of chloroform resembles that of sulphuric ether; but is much more powerful and rapid. Many persons on whom ether has little or no effect are soon stupefied by chloroform. The latter also more frequently gives rise to attacks of violent excitement, and the in- voluntary evacuation of the urine or freces, followed by deep sleep at- tended with snoring or stertorous breathing. The administration of chlo- roform is, on the whole, more dangerous than that of ether. The latter only kills when it is inhaled for too long a time without interruption. On suspending the experiment, the abnormal phenomena at once diminish ; and the dangerous symptoms almost always disappear after a short period of rest. But the action of chloroform frequently goes on subse- quently. The stupefaction which it produces lasts for a long time ; and may even be considerably increased after the re-admission of pure air into the lungs. Hence its effects are not quite so manageable as those of ether. 1978. Mammals and birds can only recover provided that their respi- ration, however enfeebled, continues during the period of stupefaction. But this condition does not apply to frogs. These animals often exhibit no trace of respiratory movement during a long period of time ; and yet recover their previous activity, if the pulsation of the heart has not ceased. 1979. The immediate appearance of mental disturbance, which is quickly followed by insensibility ( 1975), shows that the brain is soon OHAP. XVIII.] STUPEFACTION BY ETHER AND CHLOROFORM. 583 attacked by the ethereal vapour. The spinal cord appears to be next affected. At a later period, those muscles which it supplies can no longer be excited to action from the cord or from their nerves, whether by a mechanical stimulus, or by the electro-magnetic machine ( 248). The medulla oblongata is afterwards paralyzed. But its various actions do not all disappear at the same time. For the respiratory movements which it induces continue to occur, even after irritation of the nerves no longer causes contractions in those muscles of voluntary motion that depend upon this part of the nervous system, and injury of the me- dulla oblongata itself does not produce any pain. In mammals, the irritability of the posterior sensitive roots seems to disappear before that of the anterior motor ones. 1980. Those segments which are the last to succumb to the action of the ether, appear to be the first to recover from it. The reflex move- ments of the eyelids ( 1944) are restored before those of other parts of the face; while the latter generally contract in obedience to a cutaneous stimulus before the limbs. The fore legs generally precede the hind ones in this respect. 1981. The viscera of the chest and belly only succumb after the mus- cles of the trunk and limbs. The toes of the narcotized frog may often be pinched without any appearance of reflex movements ( 1950); while similar irritation of the stomach is more successful. The heart always continues to beat after the muscles of the body have ceased to give any answer whatever to irritation of their nerves. 1982. From these facts some have deduced the independency of the sympathetic nerve ( 1781). But there are two phenomena which decidedly contradict this view. The longer duration of irritability in the stomach and heart is due to these structures being chiefly or wholly governed by the vagus nerve (1752); and hence by the medulla oblongata, which is the last part to be paralyzed ( 1979). Besides this, the posterior lymphatic hearts (h i, Fig. 349, p. 531) continue to beat tranquilly, when reflex movements can no longer be induced by cutaneous irritation of the toes. Their action may be destroyed by electrical stimulation of ( 1793) the spinal cord, the free muscles of the body remaining perfectly quiescent. But since these structures are governed by non-ganglionic spinal nerves ( 1793), the greater resistance of the stomach and heart cannot be due to the ganglionic character of their nerves, or to the special nature of the sympathetic. 1983. The continuance of the cardiac pulsation explains why the blood traverses the capillaries of the frog's web ( 651) sometimes with an undiminished velocity even when no trace of respiratory movement is perceptible. Any serious disturbance of the gaseous in- terchange in higher animals renders their arterial blood of a dark red colour ( 826). The slow occurrence of an intense degree of narcotism 584 STUPEFACTION BY ETHER AND CHLOROFORM. [CHAP. XVIII. is attended by a considerable decrease of the animal heat, both in the rectum of the mammal and the cloaca of the bird. But this collateral effect may be suspended by the rapid appearance of phenomena endan- gering life, or by death itself. 1984. The action of the vapour of ether does not require the aid of the nervous centre ( 1971) or the peripheric nerves. On placing a prepared frog (Fig. 206, p. 368) in a space of air saturated with this vapour, its susceptibility gradually decreases. And the excised heart of a frog ( 598) after some time ceases to beat, The cilia of a separated mem- brane ( 1195) are also sooner or later arrested. The seminal corpuscles ( 1215) would probably offer similar results. In favourable instances, all of these prepared frogs, hearts, and cilia recover when exposed to the air, and may thus be repeatedly narcotized by the vapour. Similar phenomena are exhibited by chloroform. And when this is poured into a wound, the latter loses its sensibility to pain before the access of the general effects. 1985. These facts indicate that the vapours of ether and chloroform cause important changes in the molecular constitution of the tissues of the nervous centre, the nerve-fibres of the periphery (the muscular fibres?), the cilia, and probably the seminal corpuscles. Here again we have an action produced by extremely minute quantities, such as we have already become acquainted with ( 1972) in the case of other poisons. The recovery which afterwards takes place is probably due to an evaporation of the ether and chloroform ; and shows that these drugs cannot induce any collateral decompositions capable of preventing a return to the normal admixture of the parts. 1986. We have already ( 1854) seen that the changes caused by strychnine or ether in the living animal only diminish the susceptibility of the nerves, without essentially altering their disposition or tone. The same conclusion may be deduced from the etherized prepared frog. The only effect of ether is, that it finally deprives the nervous medulla of its capacity for the contactive propagation of excitement from molecule to molecule. But it neither rotates the atoms, nor alters their phenomena of polarization ( 1833) as would, for example, be the case, if a prepared frog which formerly presented one-sided homogeneous contractions ( 1862) offered heterogeneous ones after moderate narcotism. In rare instances, however, the vapour of acetic acid certainly does effect such an important revolution in the molecular state. 1987. A peculiar serous secretion the cerebro-spinal fluid fills the cerebral cavities, the aqueduct of Sylvius (h, Fig. 365, p. 567), and the spaces which intervene between the arachnoid and pia-mater ( 1719). A morbid increase of this fluid gives rise to dropsy of the brain and spinal cord. 1088. When the denuded dura mater of a rabbit is punctured between CHAP. XVIII.] CEREBRO-SPINAL FLUID. 585 the skull and the first cervical vertebra, a certain quantity of cerebro- spinal fluid streams out in a more or less curved jet. The animal then becomes incapable of maintaining its equilibrium as before. It often staggers as if intoxicated, is very liable to fall in rapid running, and moves its legs more uncertainly than usual. Magendie refers these phenomena to the absence of cerebro-spinal fluid the resulting change of pressure and friction giving rise to important disturbances of the nervous centre, such as materially damage the precision of its action. But since a considerable portion of the spinal cord or brain may sometimes be laid bare without the production of these results, it follows that, at the junction of the head with the vertebral column, certain important collateral causes are present. Longet has attempted to show that the dis- turbance of equilibrium is chiefly due to that separation of the cervical muscles which necessarily precedes exposure of the dura mater here. According to him, the crura cerebelli (near m, Fig. 365, p. 567) are, under these circumstances, disproportionately injured, and react upon the remaining parts of the nervous centre. 1989. The skull of the adult forms a hard capsule, the walls of which are almost everywhere unyielding. It is completely filled by the cerebral meninges or membranes, the cerebro-spinal fluid, the brain, the vessels, and the nerves. Something similar is the case with the vertebral column. But here the limitary walls contain a larger quantity of soft tissues, which by extending outwards, permit some increase of the abso- lute internal space. 1990. The compressive elasticity of solid and fluid bodies ( 69) is so inconsiderable, that we may at once dismiss it from our consideration with respect to the ordinary changes of pressure to which the nervous centre is exposed. The entry of more blood into the vessels of the skull and brain can only occur by way of dispossession or replacement : by its causing cerebro-spinal fluid to flow off towards the more yielding spinal canal, or blood to pass out into the numerous sinuses around this part, and into the veins of the neck. But when the slight increase of space permitted by the various intervertebral tissues has once been claimed, the blood in the cavity of the skull can receive no further addition except by being substituted for lymph, or for the bulk which the nervous substance has itself lost from some morbid cause. Hence there is a limit to congestion of the brain ( 1046) a limit which depends upon physical circumstances, and which, except under extraor- dinary collateral circumstances, is soon reached. The abnormal increase of cerebro-spinal fluid in the various dropsies of the nervous centre ( 877), is met by the same obstacles. Finally, these explain why, in beheaded persons, the cavity of the skull loses less blood than the exter- nal soft tissues of the head. 1991. The phenomena just mentioned obviously imply that the 586 MOVEMENT OF THE BRAIN AND SPINAL CORD. [CHAP. XVIII. FIG. 367. hard skull forms an unyielding case around the whole space which encloses the brain. But in very young children this condition does not hold good. The skull is then soft and cartilaginous in many places; such as the great fontanelle (d, Fig. 367) between the frontal and parietal bones (a and b), and the small fontanelle (y) between the parietal (b) and occipital bones (c). The smaller index of elasticity ( &5) possessed by these struc- tures, enlarges the limits of the afflux or efflux of fluid. And when a piece of bone has been removed from the skull of an adult by the operation of trepanning, or when the whole calvarium of an animal has been taken away, similar diffe- rences will obviously obtain. 1992. The denuded brain of a mammal exhibits two kinds of move- ment ; arterial and respiratory. The former coincides with the pulse, the latter with the breathing. The results of the latter are greater than those of the former. And deep respiratory movements especially increase its amount. In small mammalia like the rabbit, no arterial movement of the brain is at first visible. 1993. The impulse and increased arterial distention which are pro- duced by the systole of the left ventricle ( 610) raise and dilate the mass of the brain. And since the arteries which unite the internal carotids with the vertebrals pass between the lower surface of the brain and the skull, the former of these parts must then undergo a considerable eleva- tion. Deep expiration propels the arterial blood with increased force ( 625) in the peripheric direction ; at the same time that it obstructs the centripetal course of other fluids, such as the lymph and the venous blood. These phenomena explain the respiratory movement of the brain. They also illustrate, why the stream of cerebro-spinal fluid which gushes out of an opening in the uppermost part of the spinal meninges ( 1988) describes a wider curve during deep expiration, and a smaller one during energetic inspiration. By compressing the chest of a recently killed dog we may imitate these results artificially. 1994. Under circumstances otherwise equal, the movements of the denuded spinal cord are smaller than those of the brain. Its respiratory displacement is distinct, while only traces of the arterial movement can be recognized. 1995. From the facts already stated ( 1 990) we may conjecture that, in CHAP. XVIII.] SENSITIVE PARTS OF THE BRAIN. 587 the uninjured animal, these phenomena take a different form. For the afflux of new matter is so soon checked by the resistance of the limitary walls, that the change must at any rate have a narrower range of action. But since part of the cerebro-spinal fluid occupying the cavity of the skull can deviate towards the vertebral column ( 1990), it is probable that the arterial and respiratory movements are not altogether absent. Hence a man suffering from giddiness is liable to headache and dimness of sight when he exerts an unusual amount of abdominal pressure ( 393). But this is less remarkable in the adult than in the infant ( 1991) : in whom we may satisfy ourselves, both by sight and touch, that the great fontanelle ascends and descends with a rhythm corresponding to that of the respiratory movements. 1996. The slightest injury of many parts of the nervous centre gives rise to violent expressions of pain. While other portions may be pinched or torn, without any more notice on the part of the animal than if its hair or nails were being cut. 1997. Every irritation that impinges either on the posterior columns of the spinal cord, or on those parts of it which occupy the neighbour- hood of the posterior sensitive roots of the nerves (d d, Fig. 335, p. 510), gives rise to excruciating pain. On the other hand, the medullary sub-- stance of the anterior columns is as insensible as the motor roots ( 1720) which penetrate their interior. But their irritation gives rise to violent muscular contractions. The lateral columns of the spinal cord belong to the class of mixed structures. For they not only produce contractions in the corresponding muscles of the body, but also possess a certain sensibility, which is, however, weaker than that of the pos- terior columns. 1998. Among the sensitive portions of the brain and medulla ob- longata, we may enumerate the following : the surfaces of the me- dulla oblongata (m, Fig. 365, p. 567) and pons Varolii (i); the various medullary crura of the cerebellum (dy)-> * ne crura cerebri (between i and o); the deeper medullary masses of the cerebellum (g); the interior of the optic thalamus, and (partly) of the corpus striatum of the cerebral hemispheres (a c). On the other hand, we may slice away the superficial grey or mixed substance ( 1924) of the cerebral and cerebellar hemi- spheres, without any notice on the part of the animal. This fact is the more extraordinary, since it is these parts of the brain which minister to the higher acts of thought, and to the perception of sensitive impres- sions. Many other parts are devoid of all trace of sensibility to pain : such are the walls of the aqueduct of Sylvius (ti, Fig. 365), the superfi- cial segments of the corpora quadrigemina (i), the pineal gland, the boundaries of the third ventricle, the anterior and soft commissures, the corpus callosurn, (//), the fornix (I), the septum lucidum (between I and /), together with a great part of the corpus striatum and optic 588 MOTOR INFLUENCE OF THE CENTRE ON THE VISCERA. [CHAP. XVIII. thalamus, and many peripheric portions of the white substance of the cerebral (a b c), and cerebellar (d ) hemispheres. 1999. We have already seen that complete transverse section of the spinal cord destroys the sensation and voluntary motion of all the organs which derive their nerves from its posterior (or inferior) segments. When its thoracic portion is completely torn across in the human subject, the feet become paralyzed. A similar injury in the middle of the cervical portion reacts upon all four extremities. But if the grey matter of the segment below the site of the injury retain its powers, reflex movements of the paralyzed parts may still take place. 2000. Destruction of the right half of the spinal cord affects the corresponding tissues of the right side ( 1948); while those of the left still retain their vital functions. Hence there is no decussation of these in the spinal cord. 2001. Similar results are afforded by the posterior part of the me- dulla oblongata. But the further we proceed forwards from this point, the more prominent a decussating action becomes. Hence injuries of the right half of the nervous centre paralyze or weaken certain corre- sponding parts of the left side of the body, and vice versd. In the autopsy of a person affected with paralysis of the left half of the body, the region of the right corpus striatum and optic thalamus is generally found destroyed by hzemorrhage or disease. The details of these phe- nomena of decussation will occupy our attention hereafter, with reference to the compulsory movements seen in animals after certain injuries of the brain. 2002. The thoracic and abdominal viscera, which derive their (chiefly ganglionic) nerves from the vagi ( 1744) and the sympathetic ( 1787) trunks, are acted upon by many parts of the nervous centre, as well as by the roots of the spinal nerves. Such an influence is possessed, not only by the spinal cord and medulla oblongata, but also by some seg- ments of the cerebrum and cerebellum. So that the same chief portions of the nervous centre are capable of governing both the viscera and the voluntary muscles of the limbs. It is possible that corresponding fibres pass from each of these to the brain. But our imperfect knowledge of all these anatomical details ( 1927) leaves us open to the conjecture, that the chain is perhaps completed by transfers which mutually unite various portions of the nervous centre. Since the electrotonic state ( 1836) is repeated in the tissues of the nervous centre, it may have played an important part in many of those experiments which have been insti- tuted with the electro-magnetic apparatus. But as the influence exerted on the intestines by the brain and spinal cord may be verified with simple mechanical or chemical stimuli, there must at any rate be some special communication, or direct dependency. 2003. When the heart of a newly killed frog has ceased to beat, it is CHAP. XVIII.] MOTOR INFLUENCE OF THE CENTRE ON THE VISCERA. 589 just as incapable of being excited to new contractions from the spinal cord as from the sympathetic ( 1788). On the other hand, when the shocks of the electro-magnetic machine are sent through the medulla oblongata of the living animal, the pulsation of its heart soon ceases. But on a further continuance of the irritation, the heart recommences to beat, -just as after the similar treatment of a limited portion of the vagus ( 1754). 2004. We have already remarked ( 1788) that the application of a rapid succession of electric shocks to the vagus arrests the pulsation of the mammalian heart, while a similar treatment of the sympathetic is capable of increasing its frequency. The same contrast is repeated by the medulla oblongata and the upper part of the spinal cord ; the former arresting, and the latter quickening, the beats of the heart. But in fishes, the anterior and middle segments of the spinal cord are capable of stop- ping its pulsation ; probably by collateral electrotonic effects. 2005. No part of the nervous centre acts so decisively on the action of the heart as the medulla oblongata. This also retains its influence after all other segments of the centre have become powerless from exhaustion, or from the time which has elapsed since death. On laying bare the nervous centre of a living frog, and applying electrical irri- tation to the whole medulla oblongata (D to F, Fig. 368), the heart is always arrested during diastole. The rudimentary cerebellum (D) and corpora quadrigemina (in front of D) sometimes give rise to the same results ; while the optic lobes (B) only do so under circumstances of special collateral excitement. In careful experiments, however, the cerebral hemispheres (A) offer nothing of the kind. But in all these cases it may be questioned whether the results are more than apparent; whether they are not solely due to conduction through the moist tissues, and to the electrotonic state. The corpus callosum and the deeper lateral portions of the cerebral hemispheres, as well as the crura cerebri and the corpora quadru- gemina, frequently evince a most unquestionable ac- tion on the heart of the recently killed mammal. 2006. We shall hereafter see what an influence the medulla oblongata exerts on the respiratory organs. This segment of the nervous centre also controls the actions of the pharynx and oesophagus. But in dogs, cats, or rabbits, which have been killed some minutes previously, the contractions of the oesophagus are not vermicular ; but are generally either inverted or continuous ( 1302). 2007. The stomach and small intestines of the rabbit, horse, cat, and dog, may often be thrown into contraction from the nervous centre. FIG. 368. 590 INFLUENCE OF THE CENTRE ON THE LYMPH-HEART. [CHAP. XVIII. In mammals that have just been killed, the middle and upper part of the spinal cord, the medulla oblongata, the deeper portions of the cere- bellum, the crura cerebri, the optic thalamus, and the corpus striatum, frequently give rise to more or less deep constrictions of the stomach ; as well as to an active peristalsis of this organ and the duodenum. Since these results are repeated after division of the vagi and the lowest part of the oesophagus, it follows that they sometimes depend, not on the influence of these nerves ( 1755), or on a mechanical propagation of the cesophageal contractions, but on an action of the sympathetic fibres. 2008. The large intestines and the urinary bladder may be influenced by almost all the spinal cord, the medulla oblongata, and the parts of the cerebrum above mentioned ( 2007). In the recently killed rabbit, vigorous peristaltic contractions of the ureters are often produced by the upper and middle part of the spinal cord, the medulla oblongata, the optic thalamus, and sometimes also by the deeper parts of the cere- bellum. The subsequent electrical irritation of the ureter itself often causes no contractions. The vasa deferentia, Fallopian tubes, and uterus, are obedient to the whole of the spinal cord and medulla oblongata ; and sometimes to the cerebellum also. 2009. In recently killed rabbits, irritation of some parts of the centre such as the spinal cord often leaves the muscles of the body in a state of rest, while it is instantly responded to by the intestines. While birds frequently exhibit precisely the reverse of this. In other instances, the ureter presents vigorous undulations, while the alimentary canal remains at rest. In short, the various parts of the centre which govern the different organs of the periphery lose their irritability in very unequal degrees. 2010. The action of the lymphatic hearts has already ( 1793) led us to the conclusion that the rhythm of the cardiac movement does not imply the co-operation of the peripheric ganglia. The relations of these structures to the nervous centre may assist to dispose of another very similar theory. Those who assert the physiological independence of the sympathetic or the ganglia ( 1781) are obliged to explain the influ- ences exercised by the brain and spinal cord on the thoracic and abdo- minal viscera as the results of a communication : excitements of the centre being transferred, in the interior of the ganglion, to the inde- pendent ganglionic fibres. But the nerves which rule the lymphatic hearts ( 1793) are devoid of ganglion-corpuscles, and derive their fibres directly from the spinal cord. It therefore follows, that the pulsating heart does not require any special and independent nervous system, but may depend immediately on the nervous centre, like other parts of the body. 2011. On beheading a frog at the junction of its medulla oblongata and spinal cord (between E and F, Fig. 368, p. 589), the action of its CHAP. XVIII.] ITS INFLUENCE ON THE METAMORPHOSIS OF MATTER. 591 lymphatic hearts is at first often arrested. But they subsequently recover themselves, and then continue to beat for a long time. Since the anterior of these hearts is chiefly supplied by the third spinal nerve (de, Fig. 348, p. 531, and 12, Fig. 336, 510), and the posterior by the tenth (g, k i, Fig. 349, p. 531, and 19, Fig. 336), they are most plainly acted on by those segments of the spinal cord which immediately give rise to these nerves ( 1947). The destruction of those segments weakens the lymphatic hearts. During the period which immediately follows the injury, they remain at rest. But subsequently they acquire new force, and may continue to beat for many days, especially after the application of a mechanical irritation. The anterior lymph-hearts are, however, more easily overcome than the posterior. The latter exhibit the peculiarity, that they no longer contract at once, but in numerous successive divisions or sacculi. So that here the influence of the spinal cord determines the unity of the cardiac beat, but not its rhythm or continuance. 2012. On exposing the spinal cord of the frog to the influence of the electro-magnetic machine, the action of its lymphatic hearts is instantly arrested. But in the frog poisoned with strychnine, we may easily convince ourselves that, after the division of the seventh (d, Fig. 349, p. 531), eighth (e) and ninth (/) spinal nerves, these organs take no share in the general tetanic convulsions. And their alternate play may continue days or weeks after the destruction of the posterior half of the spinal cord. 2013. The influences exercised by the nervous centre on secretion, nutrition, and animal heat, are, if possible, even more obscure than those of its periphery ( 1806). Here daily experience is perhaps more in- structive than the physiological experiments which have hitherto been made. The flow of bile produced by anger, the diarrhoea sometimes caused by fright, the nearly colourless urine passed by hysterical women after violent mental impressions, the noxious milk given out by an angry wet nurse all these phenomena indicate how remarkably irrita- tion of the brain and spinal cord can react on the organs of secretion. The bed-sores which often occur in cases of palsy due to disease or injury of the spinal cord prove that here also the capacity of the tissues for resistance is depressed ( 1811). And when the retention ( 942) or incontinence ( 941) of urine from which such men suffer renders it advisable to introduce a catheter into the urethra, the trabecular tissue of the penis becomes strongly distended, while the size of the member is greatly increased. Still this priapism is less than that producible by sexual excitement. And similar erections appear to occur spontaneously ; i. e., from any accidental transitory irritation. 2014. Some have attempted to deduce the independence of the sym- pathetic system from the fact, that in frogs whose brain (A ED, Fig. 3 68, 592 COMPULSORY MOVEMENTS. [CHAP. XVIII. p. 589) and spinal cord (F F) have been destroyed, circulation, digestion, nutrition, and the secretion of urine, still continue. Where the medulla oblongata (E F] remains uninjured, the pulsation of the heart lasts much longer than where this segment of the nervous centre has been destroyed. But the continuance of the cardiac pulsation implies that of the circu- lation generally : and the passage of the blood through the glands and the other tissues will be followed by secretion ( 850) and the exsuda- tion of nutritional fluid ( 1010), as a necessary physical result. The general fact that the acts concerned in the metamorphosis of matter are not interruped under these circumstances does not suffice to imply the co-operation of nerves independent of the brain and spinal cord. While supposing the fibres which supply the ducts of the glands, the vessels, and the other tissues now under consideration, to be more or less completely governed by the nervous centre, we might expect that its destruction would give rise to some subordinate changes, in spite of the continuance of the normal cardiac pulsation. 2015. Our knowledge of the chemistry of the secretions is not sufficiently advanced to decide this question in a satisfactory manner. There is no doubt that the removal of the spinal cord of the frog de- presses the capacity of resistance in the paralyzed parts. If the animals are kept some time in dirty water, the hind legs become dropsical, and often ulcerate, or even undergo a partial putrefaction. In short, we finally observe all the phenomena described in 1811, frequently in a remarkable degree. Similar destructive changes are seen in mammalia; besides corresponding differences in their local animal heat. 2016. Many limited injuries of the nervous centre lead to what are called compulsory movements ; in other words, to the repetition by the animal of a series of certain prescribed (and hence partly involuntary) movements, the nature and direction of which depend on the character of the previous interference. The animal goes straight forwards or back- wards, rotates its recumbent body around the long axis, revolves (while standing) around one of its hind legs, or runs round and round like a horse which is being led in a circle by a cord. These movements fre- quently occur from inward impulses ; i. e., without any apparent excite- ment. In other cases, the animal remains for some time perfectly quiet. But on any attempt at a change of attitude, the prescribed act unre- lentingly interferes, like the movement of a clock-work when its catch is released ; and the animal only returns to its previous state of rest, on becoming exhausted by a repetition of such movements. 2017. Transverse section of the spinal cord does not give rise to any noticeable compulsory movements. But incisions on one side of the me- dulla oblongata sometimes produce curvatures of the trunk, and squint- ing movements of the eyes. 2018. When any part of the right half of the pons Varolii (above /, CHAP. XVIII.] COMPULSORY MOVEMENTS. 593 Fig. 365, p. 566) of a rabbit is cut through longitudinally, the animal generally rotates towards the same side, and the more vigorously, the greater the distance of the injury from the median line. But when the other side is symmetrically injured in the same manner, the mutilated animal remains quiet. Here the compulsory movement arises from a lateral disturbance of equilibrium ; and may to some extent be compared with the depression of one scale of a balance, on removing the second. According to Magendie, a longitudinal section exactly in the median line gives rise to oscillations from one side to the other, but not to the com- pulsory movements just mentioned. 2019. Section of one of the crura cerebelli (at m, Fig. 365,) also produces vigorous rotations, the direction of which depends chiefly on the locality of the injury. When the wound is situated near the medulla oblongata, the animal rotates towards the injured side. But if it be posterior and superior to this, in the medullary substance (g) of one hemisphere of the cerebellum, the result will be exactly the reverse. The eye of the same side rolls hither and thither, or is directed up- wards and backwards; while that of the opposite side looks forwards and downwards. 2020. The influence thus exercised by the situation of the nervous tissues which are divided and exposed in the experiment, seems almost equally valid in the remainder of the cerebellum. A longitudinal section of one hemisphere (d, Fig. 365) produces circular movements, which, in accordance with the site of the incision, are directed towards the injured or the uninjured side. On adding a symmetrical section of the other hemisphere, the animal sometimes goes straight forwards. While the longitudinal division of the vermiform process (and thus of the whole cerebellum) into two pretty equal lateral halves, only leads to oscillations ( 2018) which somewhat resemble those of an intoxicated animal. 2021. Many observers state that animals whose cerebellum has been removed or extensively injured involuntarily go backwards on attempt- ing to execute any change of place. But it is only in a few rare in- stances that this phenomenon is clearly observable. And even in these, it is at times scarcely to be recognized. 2022. Removal of the right half of the corpora quadrigemina (be- tween / and h, Fig. 365) causes birds and mammals to rotate towards the injured side. Under these circumstances, the right eye stares back- wards, and the left forwards. 2023. According to Schiff, division of one crus cerebri or the pos- terior part of one optic thalamus produces circular movements which are directed from the injured side; while other sections of the optic thalamus give rise to rotations towards it. This explains why a longitudinal sec- tion in the inner half of one hemisphere of the brain (a b c, Fig. 365) Q Q 594 COMPULSORY MOVEMENTS. [CHAP. XVIII. produces movements towards the opposite side. But these effects do not follow division of the corpus callosum (//) and fornix (Q in the middle line from before backwards. On subsequently removing the upper half of one cerebral hemisphere, they may, however, occur. 2024. Many observers state that animals both of whose corpora striata have been extirpated, are constantly impelled forwards. They suppose that in the uninjured brain, two forcible impulses are maintained in mutual equilibrium one which urges the animal in the anterior, and another in the posterior direction. The former is lost by removing the cerebellum, and the latter by extirpating the corpora striata. Hence, in the first case, the mutilated animal involuntarily moves backwards; while, in the latter, it is impelled forwards. But we have seen that injuries of the cerebellum are not always accompanied by this retro- grade movement ( 2021). And that far more vigorous impulse for- wards which has been observed after extirpation of the corpora striata frequently succeeds other injuries such as, for instance, division of the optic nerves. While when the corpora striata are extirpated with- out interfering with the optic nerves beneath, the movement sometimes fails to occur. 2025. The tendency to compulsory movements may continue for some days. Under such circumstances, the irritability is often morbidly in- creased. A moderate irritation, such as would be little or no excite- ment to the quiescent animal, frequently gives rise to a continuous storm of violent rotations ; by which, for example, the rabbit may be either completely buried in the hay in which it lies, or flung upon some neighbouring object. But when dogs survive this period, their compulsory movements gradually become weaker; and, finally, every- ting returns to the normal state. 2026. In man, local degenerations of the cerebral substance produce continuous rotations far more rarely than in mammals, birds, and frogs. We have already seen ( 2001) that those injuries of the optic thalamus and corpus striatum which frequently accompany apoplectic strokes only produce paralysis of the opposite limbs, and sometimes also of the facial muscles and external rectus ( 1452). They never produce movements of rotation. The paralysis itself by no means extends to all the contractile tissues of one side. The greater part of the mus- cles of respiration ( 747) generally retain their previous relations with the nervous centre. In rare instances, destruction of one crus cerebelli has been observed to be attended with repeated movements in a circle on the application of any external or internal excitement. But the destruction of a great part of one cerebellar hemisphere from abscesses is rarely followed by these phenomena. Here again paralysis constitutes the chief occurrence. The rotations of animals are perhaps mainly due to their various muscular structures being weakened, and CHAP. XVIII.] ORGANS OF THE NERVOUS CENTRE. 595 then subjected to one-sided stimuli, and to an abnormal alternation of their action. 2027. Continuous rotation of the body produces that subjective de- ception of sensibility which we designate by the name of giddiness or vertigo. During its continuance, the paths apparently taken by external objects are chiefly determined by the attitude of the head, and by the direction of movement. Many have attempted to compare these pheno- mena with the circular compulsory movements of animals. But their conjecture that only half of the brain is here specially excited has hitherto not been proved. The subsequent deception of sight, and the fall which succeeds it, rather contradict than support their theory. Finally, there is nothing to indicate that animals are affected with giddiness while undergoing such compulsory movements. 2028. Although the brain and spinal cord form one continuous whole, still they include various groups of tissues, each of which enacts a more or less definite and special function. But everything concurs to hide from the eye of the physiologist the mechanism of this the most important organ of the animal body. We have already been made ac- quainted ( 1927) with the insuperable difficulties of its anatomy. The various parts of the brain on which different names have been bestowed no way correspond to groups of tissues that possess a physiological im- port, but only to portions that are capable of being recognized externally. Hence such names often divide a single structure, and unite different ones. So that, while they facilitate the topographical description of the nervous centre, they do not map out its true organs. All phy- siological experiments rest on an insecure basis, since we are devoid of that microscopic knowledge which could alone render them decisive ( 1927). And, as we can never determine how far disease of one part has reacted upon the remainder, pathological observations on the human subject furnish no materials such as physiology can safely elabo- rate. We are thus almost exclusively referred to experiments upon animals. These offer almost equal disadvantages. For not only do we thus operate on an organ which is unlike the human nervous centre, and less perfect than it : but, after all, the sensations of these creatures cannot be satisfactorily ascertained. 2029. The application of a stimulus to some of the large peripheric nerves causes whole groups of the corresponding muscles to contract, and thus produces various general movements of the limbs. But the reflex phenomena ( 1937) expressly indicate that the aid of the grey matter of the spinal cord is required for the harmonious alternation of flexors and extensors, adductors and abductors ; as well as for that mutual play of the muscles by which many attitudes are effected. In short, the cord con- tains a mechanism, in which definite series of keys are played upon under the influence of the will, the instinct, or incident centripetal stimuli : Q Q 2 596 SPINAL CORD. [CHAP. xvm. the several keys either sounding separately, or in more or less suitable accords. It was formerly supposed that the central organs of the flexors were chiefly contained in the anterior half of the cord, while those of the extensors occupied its posterior half, or vice versd. But hitherto this conjecture has not been confirmed by experiments. 2030. Some facts easily verified in the spinal cord of the frog lead to the conclusion, that the segments nearer to the head exercise an influence different from that of the subsequent portions upon the same limbs. On cutting across several spinal cords at the first, the second, the third vertebra, &c., we see that the hind-legs are more strongly flexed, the nearer the seat of the injury is to the brain itself. On passing beyond the fifth vertebra, extension of the hind feet replaces the previous rotation at the hip-joint. But the strength of the exten- sion constantly diminishes as we pass further backwards. 2031. From hence we might at first conclude that, in the horizontal spinal cord, the central organ of the flexors is anterior, and that of the extensors posterior. But the accuracy of this opinion is at any rate very doubtful. For both the shocks of the electro-magnetic machine, and simple mechanical stimuli, give rise to vigorous extension when applied to the anterior part of the cord. We should therefore have to assume, that, in these instances, the powerful stimulus was propagated further backwards ; and that the extension was due to the facility and energy with which it was responded to by those central organs that lie close to the entrance of the nerves ( 1947). 2032. When a frog is beheaded at the first vertebra, and its limbs extended, the headless trunk generally adjusts itself after some time ; the hind legs being bent at the hip, the knee, and the ankle. But when the spinal cord is cut across between the fifth and sixth vertebra, the animal preserves its extended attitude. 2033. The energetic action exercised by the medulla oblongata on the pulsations of the heart ( 2005) and the movements of respiration ( 2034) endows it with an influence on the duration of life such as is not possessed by any other segment of the nervous centre. The cere- brum, cerebellum, and spinal cord of an animal may be removed with- out immediate death. But the destruction of the medulla oblongata of a bird or mammal destroys its life in a very few minutes. 2034. The central organs of respiration are contained in that part of the medulla oblongata which gives origin to the roots of the vagus (q, Fig. 336, p. 510), and in the segment immediately behind this. The nearer a transverse section of the spinal cord is to the medulla oblongata itself, the greater the number of muscles which it withdraws from the play of respiration. But even after the spinal cord has been cut through as far forwards as possible, the facial muscles are still capable of main- taining their respiratory movements. And conversely, injury of the CHAP. XVIII.] MEDULLA OBLONGATA. 597 pons Varolii from which the facial nerves ( 1735) emerge destroys the respiratory play of the face, but not of the trunk. While the destruc- tion of that part of the medulla oblongata above mentioned instantly annihilates all respiratory movement. 2035. But the central organ of respiration claims only part of the structures here present. The remainder govern the heart, the organs of deglutition, the abdominal viscera, and various muscles of the trunk and limbs. Many of the latter muscles never share in the movements of respiration. This arrangement may assist to explain the fact, that the muscles which share in the act of respiration become more nu- merous, the more energetically a person breathes, or the greater his danger of suffocation ( 749). 2036. In recently killed mammalia, we may often convince ourselves that the central organs of the respiratory apparatus retain their irri- tability longer than most of the neighbouring portions of the medulla oblongata. On irritating this part mechanically or chemically, we fre- quently get a deep respiration, when no further trace of convulsions appears in the muscles of the limbs, and no movement of deglutition can be produced. 2037. On cutting through this segment of the nervous centre in animals, they fall into violent alternating spasms which are soon fol- lowed by death. The transverse section of one lateral half does not necessarily give rise to this result. In like manner, we often find old effusions of blood in one side of the human medulla oblongata : while more extensive injury, or pressure on all sides, is capable of destroying life in a few minutes. 2038. The whole nervous centre of a frog may be slit up longitudi^ nally, without affecting the simultaneous play of the respiratory muscles of both sides of its body. So that longitudinal section of the medulla oblongata does not separate the symmetrical respiratory muscles of both sides into two groups capable of independent or inharmonious actions. The same phenomenon may be distinctly seen in mammalia, when the brain is removed, and the medulla oblongata cut into from before backwards. 2039. We have already observed ( 704) that, under favourable colla- teral circumstances, the heart of a recently killed animal is revived by artificial respiration. When the medulla oblongata of a living rabbit from which the brain has been removed is exposed to the powerful shocks of the electro-magnetic machine, not only the pulsations of its heart, but also its respiratory movements, cease. Here the heart is in a state of relaxation or diastole, while the respiratory muscles are con- tracted at any rate at the commencement of the experiment. But weaker stimuli of the same kind are capable of increasing the frequency of respiration. Since electrical irritation of the medulla oblongata fre- 598 MEDULLA OBLONGATA. [CHAP. XVIII. quently arrests the pulsating heart of a recently killed animal, without causing the appearance of any movement in the respiratory apparatus, or any convulsions in the muscles of its limbs, it follows that the heart possesses its own central organs, which may die even later ( 2036) than those of the muscles of respiration. While another experiment on the frog teaches us that the medulla oblongata exerts an influence on the heart, not only through the breathing (as seen in artificial respira- tion), but in some more direct way. For the pulsation of the heart con- tinues longer after extirpation of the lungs or section of the vagi, than after the destruction of the medulla oblongata. And the breathing may be suspended for some time by the action of ether without any stoppage of the heart ( 1978). 2040. It has already been remarked ( 1758) that destruction of both spinal accessory nerves injures the voice of the mammal, but not its respiration. Hence we may conjecture that the central organs of the voice, which regulate the lingual action ( 1408) of the laryngeal muscles, are not identical with those of respiration. It is probable that the peculiar origin of the spinal accessory nerve (by roots that arise low down from the cervical portion of the spinal cord) has some intimate connection with these facts. 2041. According to Magendie, injuries of the junction between the medulla oblongata and the spinal cord cause a gradual but violent in- flammation of the eyes in the domestic mammalia. Here we might con- jecture some special injury of those nerve-fibres which ascend through the superior cervical ganglion of the sympathetic towards the head ( 1786). 2042. It is on the medulla oblongata that the normal and successive movements of deglutition ( 381) essentially depend. Hence after an animal has been killed some minutes, these are no longer producible by artificial irritation ( 2006). But on the other hand, they may occur after the removal of the brain : and are not destroyed by cutting through the middle of the cervical region of the cord. 2043. From what has already been stated, it follows that the medulla oblongata can directly or indirectly govern the greater part of the striped and unstriped muscles of the thorax and of the abdominal viscera. And the addition of the pons Varolii makes up a segment of the nervous centre that forms a kind of ganglionic focus for the actions of all the spinal, and the greater part of the cerebral, nerves. 2044. Many have felt obliged to assume that painful impressions may be perceived in the medulla oblongata For even after the cerebrum and cerebellum of a mammal have been removed, it still shrieks on receiving an injury of the skin. And powerful counter-movements generally follow. But these facts do not necessitate the assumption just mentioned. We have ( 1953) seen that the removal of the brain especially favours the appearance of those reflex movements which proceed from the spinal CHAP. XVIII.] CEREBELLUM. 599 cord. The same proposition holds good of the medulla oblongata. The central fibres from the skin, or the central tissues in physiological union with them, lie close to the nervous centre of the voice a proximity which would greatly facilitate the reflex production of cries of pain ( 2040). Just as the apparent purpose of the reflex movements seen in the limbs of a beheaded frog does not prove the presence of conscious volition ( 1955), so in an animal whose brain has been removed, the start on receiving a pistol shot, or the scratching of the nostrils after the inhalation of the chemical irritant ammonia, may be immediately de- duced from the above mechanism of the central organs. 2045. The corpora quadrigemina (below /, Fig. 365, p. 566) have a remarkable reaction on the eyes. The two right ones directly govern the pupil (c, Fig. 150, p. 273) of the left eye, and vice versd. Still, owing to a transverse communication of the excitement, both pupils can at once indicate the irritation of one half. Their removal produces blindness, but does not destroy the mobility of the iris. 2046. A mammal whose cerebellum has been removed is more or less incapable of executing certain general movements. It stands less steadily than before ; and, when lying down, makes a series of useless efforts to rise, and to avoid an obstacle or a blow. Birds which have been thus mutilated tire themselves out by unusual and ineffective fluttering move- ments. Still most of the various muscles remain under the control of the will. Hence it is only their co-ordinate combination that is lost. Many physiologists have therefore supposed the cerebellum to be the organ that co-ordinates the more complex movements of the body. But how- ever plausible this view, still we may doubt whether these unsuitable muscular actions, and the useless exertions which accompany them, are not a consequence of the loss of certain local conditions of progression or flight such as, for instance, the proper fixation and adjustment of the spinal column. 2047. Phrenologists have assigned the cerebellum a special influence on the sexual organs. They have asserted that a greater develop- ment of this part of the nervous centre results in an increased acti- vity of the sexual impulse. But experience does not confirm this theory. It is true that the seminal ducts, the oviducts, and the uterus of the domestic mammalia may be thrown into contraction from the cerebellum. But the same effect can be produced through other parts of the nervous centre ( 2008). The cerebellum of geldings is quite as large as that of stallions. And Flourens found that a cock from whom this part of the nervous centre had been removed still made dis- tinct attempts at copulation. 2048. Degeneration of the human cerebellum does not necessarily give rise to imbecility or any other affection of the mental powers. Destruction of one of its hemispheres has sometimes been accompanied 600 CEREBRUM. [CHAP. XVIII. by an uncertainty of gait, a tendency to rotatory movements ( 2026), or hemiplegia usually of the opposite side ( 2001). But limited disease of the cerebellum may exist without any considerable disturbance of the action of the voluntary muscles. 2049. A longitudinal section through the centre of the corpus cal- losum (//Fig. 365, p. 566) and fornix (I) of a rabbit is unattended by either rotatory movements ( 2023) or insensibility, so long as no collateral phenomena interfere. The animal generally becomes irritable and ill-tempered ; and its heart beats very quickly. It subsequently suffers from diarrhoea, which is sometimes accompanied by vomiting : and certain parts of the intestine become so distended with gases, as to cause the belly to protrude. The rabbit now and then gnashes its teeth : and is sometimes attacked by convulsions. It finally becomes paralytic and comatose ; and dies, often during a violent convulsive fit. 2050. In recently killed mammalia, we may convince ourselves that the corpus striatum and optic thalamus of one cerebral hemisphere govern the muscles of the limbs, and the numerous thoracic and ab- dominal viscera ( 2005). The statement of some observers that the anterior parts of the cerebrum or the corpora striata act chiefly on the hind legs, and the posterior or the optic thalami on the fore legs has hitherto received no confirmation. One corpus striatum can ex- cite movement in the extremities of both sides at least in recently killed animals ( 2001). The destruction of the corpus striatum does not necessarily give rise to extensive paralysis. But when the optic thalamus is also removed, the animal falls towards the opposite side. Still, when assisted to rise, it proceeds some distance properly enough. Just as the secretions of the intestines are visibly disturbed by section of the median tissues of the brain ( 2049), so something similar obtains after injury of the optic thalami or crura cerebri. The faeces are evacuated more copiously, and have a slimy character; and are often admixed with blood. Rumbling noises in the bowels betray an increased development of intestinal gases. The appetite disappears. The previously alkaline urine ( 972) becomes acid; and, according to Schiff, sometimes contains traces of albumen. This observer also states that an acid character of the urine is a certain indication of the approach of death. In the human body we may sometimes observe how remarkably a cerebral disturbance can react on the condition of the abdominal viscera. Serious injuries of the brain are often accompanied by vomiting, diarrhoea, increased effusion of bile, and disease of the liver. 2051. In the recently-killed rabbit, irritation of the hippocampus major sometimes gives rise to contraction of the facial muscles. In this case, the decussation ( 2001) is generally distinct, 2052. According to Flourens, the transverve section of one cerebral CHAP. XVIII.] CEREBRUM. 601 hemisphere of a bird leaves behind it more permanent injury than the longitudinal one. But provided that only one hemisphere is injured, or that the two are not cut through uniformly, the mental powers may sooner or later reappear. 2053. In the dog the chief phenomena which result from the excision of one cerebral hemisphere are paralysis of the optic nerve, weakness of the muscles of the opposite side, a liability to be easily startled, and rotatory movemeijts. The latter may, however, subsequently disappear. The mental powers are not much affected. Nor are they necessarily lost by disease of the greater part of one hemisphere in the human subject. 2054. When both hemispheres of a mammal are removed in succes- sive layers, the mental functions gradually decrease in proportion to the loss of substance. On reaching the ventricles, complete unconsciousness is generally produced. 2055. A dog or a rabbit whose cerebrum has been completely re- moved generally lies as if sunk in deep sleep. Everything which does not absolutely touch its body appears to pass unnoticed. The mental elaboration of the sensations is completely annihilated. But loud sounds still rouse the blind animal ; and painful cutaneous impressions cause it to shriek and attempt to defend itself. All these phenomena, however, depend merely on the reflex actions already mentioned ( 2044). The mutilated animal does not take any food spontaneously. But food thrust into the commencement of its pharynx is swallowed and digested as usual. In this way birds deprived of their brain may be kept alive more than a year by artificial feeding. External stimuli or internal irritations (which probably proceed from the viscera) occasionally lead to some change of place. But these movements are often very peculiar and abnormal. The blindness of the animal, and its deficiency of mental power, frequently cause it to move clumsily, and strike against obstructions which it would otherwise have avoided. All such move- ments soon cease, and are again replaced by a lethargy of many hours' duration. 2056. In the human subject, limited structural disease of both cerebral hemispheres often produces weakness of intellect, imbecility, or lethargy. Here the visible disease may occupy either the anterior, middle, or posterior sections of the cerebrum, without much influencing the general results. 2057. The exsudation of large quantities of liquid into the ventricles of the brain, or a morbid increase in the amount of cerebro-spinal fluid, may produce weakness of intellect, coma, sopor, and other abnormal states of mind. Hence such appearances are often found in the brains of cretins, as well as in persons who have died of inflammation of the brain, nervous fever, or other similar disorders. But at present it cannot be proved that the abnormal phenomena exhibited during life 602 CEREBRUM. [CHAP. XVIII. depend exclusively on either the collateral causes which precede and permit the increased exsudation ( 1990), or on the pressure it produces. It is probable that these are partly due to minute disturbances in the molecular relations of the nervous tissues, such as will perhaps never be visible to our senses. 2058. It is usually stated that the brain forms a larger fraction of the weight of the body in man than in any other vertebrate animal. In the adult, it is about ^th to ^th ; and in the infant, ith to th. But many small mammals and birds exhibit a larger proportion than the adult human being. 2059. There is no doubt that an abnormal smallness of the brain is connected with idiocy. And it is very probable that persons dis- tinguished for their intellectual powers possess brains which are large, either as a whole, or in particular parts. But it is far more difficult to prove this excess, than the converse diminution. For the abnormal circumstances which precede death may themselves produce a deceptive increase of weight ( 2057). And we have a right to suppose, that the mental endowments are materially influenced, not merely by the quan- tity of the organ, but also by its quality, and relative activity. The high forehead which is frequently regarded as an external indication of mental power certainly does generally depend on a greater development of the anterior cerebral lobes, and of those parts of the skull which cover them. Still this does not justify the conclusion, that it is these segments of the cerebral mass which exclusively regulate the higher mental capacities or many of the faculties that express them; such as, for instance, eloquence. Comparative and pathological anatomy unite to testify, that the middle and posterior lobes of the brain, and many of its internal swellings (such as the pes hippocampi and pes accessorius, which lie in the posterior cornu of the lateral ventricle of each hemi- sphere) are at least as important as its anterior segments. 2060. It has often been maintained, that in men distinguished for in- tellect, the convolutions of the two cerebral hemispheres (a b c, Fig. 365, P. 566), are more numerous, and less symmetrical. But the fact itself is by no means established. And beside this, experience teaches that the advantages which are perhaps associated with the convoluted arrange- ment may be quite annihilated by internal disease. The brain of a cretin often exhibits large and complicated convolutions, while its cavi- ties are distended by copious fluid exsudations. 2061. The microscopic characters of the cerebral substance undergo important changes in the course of development. In the new-born in- fant, a fine section of its superficial layer exhibits a minutely granular basis, together with cell-like nuclei (Tab. V. Fig. 7 7) such as are met with in the pure grey matter of the adult. A similar section from the adult brain shows numerous (and distinctly medullary) primitive fibres, in addi- CHAP. XVIII.] SYMMETRY OF THE NERVOUS CENTRE. 603 tion to the preceding structures. The above peculiarities are even more distinct in the embryo, which exhibits voluntary and reflex movements at a period when no distinct white substance or oily content can be ob- served in its peripheric nerve-fibres, and when its nervous centre only includes a basis and cellular elements resembling those of pure grey matter. Hence the general actions on which these movements are founded do not exclusively depend on the peculiar forms possessed by the nervous tissues of the adult. 2062. The lateral repetition of most of the organs, and of the nerves by which they are governed, partially explains why so many segments of the centre are in pairs also. But since the same arrangement holds good in portions which are not immediately connected with peripheric organs, there must be other reasons for it. It has often been supposed that the commissures or middle portions fulfil the purpose of linking the two lateral and complementary organs in unity of action. But since the division of these median portions ( 2049) or the removal of one of the cerebral hemispheres ( 2053) does not destroy the mental powers, and since the median longitudinal section of the whole nervous centre in the frog does not disturb the simultaneous and corresponding action of the respiratory muscles ( 2038), this theory must be regarded as on the whole invalid and incorrect. 2063. The organs of sight may best illustrate the actions which depend on the arrangement of the corresponding cerebral structures in pairs. We have seen that when two complementary colours are allowed to fall upon corresponding points ( 1553) of both retinse by means of the stereoscope, they produce the impression of white ( 1564). Here we may imagine that the changes in the cerebral organs of the two optic nerves are completed to a single intermediate sensation in the brain itself. While the fact that the normal (and partially opposed) movement of gazing ( 1445) is lost after injuries of one half of the corpora quadrigemina ( 2045) or the cerebellum (2019) suggests the conjecture, that the continuity of these lateral pairs is requisite for their combined action. 2064. We have already ( 2029) represented the nervous centre under the guise of a row of keys, which are arranged suitably to their objects, and in fitting connection with each other. This forms a definite topo- graphy of structures, some of which represent the various organs of the body, while others minister to the purely cerebral functions. Such a view explains the apparent object of the reflex movements ( 1955), as well as the real purpose of the instinctive actions. And it is con- firmed by many of the phenomena of sensation. 2065. Although no sensuous impression can itself do more than fur- nish an excitement, the final translation of which must occur in the brain ( 1905), still we always refer it externally : either to the peri- pheric organ, such as the tongue or skin ; or still further outwards for 604 PEHIPHERIC REFERENCE OP IMPRESSIONS. [CHAP, XVIII. instance, to a corresponding distance from the retina or the optic nerve. Hence there must be some arrangement which obliges us to neglect our own cerebral actions, and to receive the peripheric transfer as though it were a direct impression. 2066. Under certain abnormal conditions, this external reference be- comes sometimes very remarkable. Persons who have undergone ampu- tation of a limb are good instances of this kind. A man whose thigh has been many years removed often distinctly feels his toes or foot, If the stump be surrounded with a ligature, the mutilated person feels as if the long lost foot and leg were asleep. Such deceptions can neither be overcome by the evidence of his senses, nor by his own consciousness of their error. And since they also occur in persons who have lost both legs, they are not due to the impressions applied to the existing limb being referred to that which has been removed. All of these sensations are far stronger in the hands and feet, than in the middle or proximal segments of the limb. 2067. Continuous pressure on the ulnar nerve at the elbow is attended by results allied to the preceding. The little finger first becomes " asleep," and then the ring finger and part of the middle finger ; while subsequently the pain appears to run along the course of the nervous trunk. Similar pressure on the popliteal nerve gives rise to a pricking sensation in the toes and sole of the foot. And when the sciatic plexus of a parturient woman is compressed by the child's head, the impression of pain which is produced takes a peripheric course along her leg. Many unpleasant sensations in the alimentary canal and other peri- pheric parts of the body, depend solely on the nerves being aifected either in the middle of their course, or in the centres themselves. 2068. The preceding considerations will show how slight are the foundations which physiology affords to psychology. One chief cause of this is certainly our deficient knowledge of the physiology of the nervous centre, a branch of the science which will probably never attain a very satisfactory position ( 1927). But a second is the way in which we conceive of the mental functions. We describe certain external phenomena as results of knowledge, judgment, or reason, without recol- lecting that the basis here assumed to be an unit is the result of a series of links to which we have no access. We have seen that ( 1708) the perception of the simplest sensuous impression, and the excitation of the slightest muscular movement, depend upon a transfer and con- duction of certain material changes, which mutually conditionate each other, and play into one another like the cog-wheels of a machine. Hence we are justified in conjecturing, that what is apparently the most direct mental action also proceeds from a series of mutual tensions and transfers ; and that the failure of any link of the chain leads to an error or a false conclusion. The general significance of such processes may CHAP. XVIII.] SLEEP. 605 be easily imagined from the analogies offered by these simpler phe- nomena. But the satisfactory investigation of their details the only true self-knowledge will probably for ever baffle the spirit of human inquiry. 2069. From hence alone it becomes obvious how far all phrenological systems are from the truth. Not one of the facts which constitute their foundation will survive a careful examination. The exterior of the skull is by no means an exact cast of that of the brain, but is modified by many intermediate conditions; such as the frontal sinuses, the thickness of the skull, and the form of its surfaces. And the outside of the brain is itself incapable of affording any conclusion as to the mental powers ( 2059). If to these considerations we add, that the topographical sub- division of many of the phrenological organs is based upon misinterpreted facts of comparative anatomy, or absurd psychological subdivisions, and that external circumstances can materially alter the fatalism of the organic plan the reader will clearly understand why physiologists are compelled to reject phrenology; and that only the more emphatically, the more violently it is defended by some educated persons. 2070. Numerous exact determinations of the size and weight of the skull and brain would probably furnish averages of some value for general psychology. Such cranioscopic researches have, on the whole, a more secure foundation than the theories of phrenology. But what has already been said renders it sufficiently obvious that it would still be quite impossible to determine, from such coarse material relations, the mental powers of any given individual. 2071. The restoration required by certain of the nervous organs leads to these phenomena of periodic rest which we include under the name of sleep. The time necessarily occupied by this strengthening inaction undergoes a great diminution in the course of life. The sucking child sleeps longer than it wakes. The adult is satisfied with about one-third of sleep, or even less. 2072. It is generally supposed that the whole of the nervous centre, or at least of the brain, rests during sleep. But more careful obser- vation leads to the conclusion that this is not the case. It may even be questioned whether any part of the nervous centre is condemned to complete inactivity during sleep, whether the whole phenomenon does not depend solely on the fact, that certain mutual actions, which occur during the waking state, are suspended in the sleeping animal. 2073. All the functions subservient to the metamorphosis of matter such as the pulsation of the heart, and the circulation ; the movements of respiration, and the interchange of gases; together with the mecha- nical and chemical phenomena which accompany digestion, absorption, secretion, and nutrition go on unchecked during sleep. Hence certain parts of the medulla oblongata, and probably of the spinal cord, continue 606 DREAMS. [CHAP. xvni. to act mechanically, just as in the waking state. And external irritations are capable of producing reflex phenomena without awakening the person. While, on the other hand, the occurrence of dreams teaches us that the brain is also in a peculiar state of activity. It perceives some sensuous impressions, but often gives them a different interpretation from what it would do in the waking condition. The ideas thus excited constitute the commencement of the most extraordinary phantasies, which as the dream goes on, are generally further spun out, and are repeatedly confounded with new and disconnected series of thoughts. All this indicates that the machinery of the nervous centre is differently adjusted in the sleeping and waking states. This alteration of adjustment causes many sensuous impressions to pass unnoticed, and many muscles of the body to become relaxed, while it allows others to execute the at- titudes most favourable to repose. The peculiar changes of respiration which give rise to snoring are less essential (and therefore less constant) collateral effects of the same cause. 2074. Some observers have drawn a complete parallel between the condition of a sleeping animal, and one which has been deprived of its brain ( 2055). But this comparison will not survive an exact analysis. The latter seems not to dream, and exhibits a peculiar irritability, a somewhat unusual excitability to reflex ( 1953) phenomena which is not evinced by the sleeping animal. In short, that activity which sleep only diverts into another channel, is here altogether absent. 2075. The subjective phenomena of vision, and the fantastic images seen by many persons at the instant of falling asleep, are the immediate forerunners of dreams. The question whether these only occur just after the beginning and before the end of sleep, or during its whole continuance, cannot at present be decided. The statement that there are persons who have never dreamed rests upon very doubtful founda- tion. 2076. The sleeper instinctively executes many voluntary movements. Wearied soldiers have been known to sleep on a march, and still keep time in their steps. This fact plainly indicates that sleep is chiefly cha- racterized by the isolation of consciousness from the impressions of the outer world, and not by the repose of the locomotive organs. We . may verify the same proposition in the mysterious state of somnambulism : which, in common with many other morbid conditions, proves that the keys of the nervous centre are capable of playing the most complicated movements, without the brain resuming its ordinary rela- tions to the external world. The fact that a cataleptic person can maintain the arm until waking in the constrained attitude which has been artifically impressed upon it, shows that the continuous tension of the corresponding nervous organs does not always imply the influence of the will. CHAP. XVIII.] ANIMAL MAGNETISM. 607 2077. At present we are ignorant what material changes occur in the nervous centre during sleep. It has often been conjectured that the cerebral substance contains more dark blood than in the waking state. But even should this statement be hereafter confirmed, it will still remain doubtful whether the phenomenon is the cause of sleep, or the effect of a previous change in the bulk of the nervous tissues ( 1990). 2078. The magnetic sleep, and animal magnetism generally, are some of the most equivocal phenomena which medical literature has hitherto offered to the thoughtful observer of nature. We may admit that persons suffering from nervous disorders are now and then susceptible to impressions which would pass unnoticed by a healthy individual; and that in such cases credulity, prejudice, implicit trust, and phantasy, can produce very strange results. Many nervous affections such as the catalepsy already mentioned ( 2076) give rise to a variety of mysterious phenomena, which it will be long before physiology can explain. If it be true that these persons can maintain a limb in an artificial position which would soon exhaust a waking individual, or that the discourse interrupted by the attack is resumed by them imme- diately on its cessation ( 1976), this would exhibit a persistence in the mechanism of the nervous centre, due to causes concerning which nothing is known. But most of the wonderful phenomena generally described as results of animal magnetism depend upon conscious or unconscious deception. The whole of this subject forms a parallel to that of phrenology ( 2069), an edifice which those who only work upon the foundations of natural laws have little call to aid in constructing. CHAPTER XIX. GENEKATION AND DEVELOPMENT. 2079. Modes of Generation. Since natural inquiry, strictly so called, is only concerned with what may be established by experience, it has no right to discuss the question, how the first organic inhabitants of the globe were produced. The petrifactions which we meet with in various strata furnish us with a series of halting-points; from which we are jus- tified in deducing the alteration of the several creations they represent, but not their original production. We are thus informed that the pre- vious world was inhabited by species of animals which are now no longer met with ; and that, myriads of years ago, what are now temperate zones were hotter climates, while existing continents occupied the depths of the sea. And palaeontology and geology agree in indicating, that the organic beings of the globe did not proceed from any limited portion of its surface, or from any exclusive centre of creation, but that the varying geographical distribution of plants and animals is based upon a series of equally different local causes. But the way in which this happened, and the causes which permitted or perhaps implied the first organiza- tion, elude our present means of research. 2080. The same law which connects the life of every organized being with a certain duration of time necessitates the phenomena of propaga- tion. Failing these, nature could only have avoided the depopulation of the globe by repeated new creations. Generation enables it to dispense with this requirement ; which, as we shall see, is probably at present never fulfilled. Every organized being commences as an invisible germ, which continually attracts new substances, and by their proper elabo- ration, attains a certain stage of completeness. And when the develop- ment of the plant or animal has progressed to a certain degree, its organization acquires the capacity of producing, in some part of its substance, a new germ that exhibits the same capacity of development. Since the animal which thus results is immediately produced from the constituents of that by which it is generated, this mode of propagation has been called maternal or homogeneous generation. It is obvious that the time which must elapse before the new germination can possibly take place, will cause the producing animal to be older than the pro- duced. Hence the former may arrive at its prescribed termination of life, at a period when the latter is still continuing a vigorous growth, and CHAP. XIX.] UNISEXUAL AND BISEXUAL GENERATION. 609 adding new links to the chain of creation. And thus, owing to the lapse of time which intervenes between the development of the previous germ, and the origin of the new one, it is only the individuals which pass from the stage of life : the species of the organic beings are them- selves permanent. 2081. Unfavourable external circumstances, and the predatory habits of various animals, cause many organic beings to perish before they reach that stage of development which enables them to prepare a germ. And many collateral causes may obstruct the evolution of the genera- tive organs. The various dangers which thus threaten the maintenance of the species can only be obviated, either by one individual producing a number of germs simultaneously, or by its generative function being frequently repeated in the course of life. This also permits a gradual increase in the number of individuals which people the globe. 2082. In unisexual generation, the germ furnished by a single indivi- dual can at once go through its complete course of development. The bisexual generation, on the other hand, demands two different person- alities. One of these, the female, furnishes the germ, which has generally a definite form, that of the ovum or egg. The second, or the male, prepares a special complementary fluid, which is called the semen or sperm. 2083. These two kinds of generative materials have their definite and special characteristics; which, subject to certain variations of form, are repeated in all animals. The annexed woodcuts represent the young ovum (Fig. 369 that of the pike, Fig. 370 that of the frog). It contains FIG. 370. a basic substance or yolk a, which is enclosed in a special vitelline membrane b; a vesicular nucleus, the germinal vesicle, c; and one (d, Fig. 369) or more nucleoli (d, Fig. 370) that form the simple or compound germinal spot. The essential characteristic of the efficient male generative fluid is the possession of the seminal corpuscles (Tab. V. Fig. 78, a b), already specially described ( 1215, et seq.). 2084. The ovum cannot spontaneously develope itself beyond a certain limited stage. In order that it should overstep this limit, and produce from its tissues the foetus or embryo of the new being, it must have been previously subjected to the action of the male semen, or fecundated. Hence the bisexual generation to which the vertebrata are R R 610 HERMAPHRODISM. [CHAP. XIX. exclusively restricted, demands more elaborate antecedents in order to the production of a more complex and delicate organization. 2085. The higher animals just mentioned exhibit different sexes : many individuals possessing none but male organs of generation, which prepare and extrude the semen; while others have only female organs, which produce the ovum, and further develope it to a certain degree. The same arrangement is repeated in many invertebrate animals. But, on the other hand, in some of these lower animals, the same indi- vidual includes both male and female sexual organs. This genuine an- drogynous or hermaphrodite state is to some extent the expression of a low grade of organization. The organs of the higher animals work so peculiarly and exclusively, that they are only able to produce the material substrata of one of the two sexual contrasts : while the duller and more indifferent hermaphrodites can evolve both of the elements necessary to generation in separate secreting organs. 2086. The mammalian ovum only leaves the body of the mother when the foetus produced in it is strong enough to maintain a separate life under the necessary collateral conditions. But since the action of the semen conditionates embryonal development, this must be preceded by an internal impregnation. The same conclusion holds good for other viviparous animals. The oviparous creatures offer two varieties in this respect. Birds, and most of the scaly reptiles, lay eggs which have previously undergone an internal fecundation. The evolution of their embryo is therefore entrusted to a mere external incubation, aided by favourable collateral circumstances. On the other hand, the ova of frogs and most of the osseous fishes are expelled before coming into contact with the semen. So that here the embryonal development, which takes place outside the mother's body, demands an external fertilization. 2087. The different modes of propagation which we have just been considering may be met with in various animals which are allied to each other. Although reptiles and fishes are, for the most part, oviparous, yet salamanders and sharks are viviparous. In the sword-fish and the Marsupialia the young undergo a further development in external brooding cavities or special pouches of the mother. But the arrange- ments of the two are very different. 2088. Unisexual generation obtains most extensively in the vegetable kingdom. Owing to the greater uniformity of their various groups of tissues, the more simple conditions of their nutrition, and the less complicated phenomena of their growth, the germinal structures which sooner or later shoot forth form independent plants; and pieces severed from the parent grow into distinct individuals. Their suckers and bulbs, their numerous buds properly so called, and the segments of their leaves and stalks, often possess the power of developing new shoots CHAP. XIX.] GENERATION IN PLANTS. 611 FIG. 371. which maintain and increase the species. The operations of inoculation and grafting depend on the capacity for development possessed by severed portions of the leaves and axis. Here the maternal basis is so indif- ferent, that the plant into which the foreign portion has been grafted need only possess a certain degree of relationship, in order that the sub- sequent union of the two should allow of a more vigorous development, and the preparation of a richer sap. Finally, the weaker unity of the vegetable explains how severed portions of many cryptogamic plants continue to live uninjured, cellular projections from leaves or stems become perfect buds, and similar germinal structures proceed from the margins of wounds. 2089. We have already seen ( 1222) that in many cryptogamic plants special organs the ripe antheridia enclose mobile elements, the forms and movements of which more or less correspond with those of the seminal corpuscles of the animal. Other organs produce seed-grains or spores, which subsequently yield young plants of the same species. This contrast, which may remind us of the bisexual generation of ani- mals, is often met with in sea-weeds, leafy and fleshy mosses, and ferns. But at present the mechanism of their im- pregnation has not been discovered. Other modes of propagation are also met with in these plants, as well as in the higher vegetables. 2090. The phanerogamous plants possess stamina, the anthers of which contain the grains of pollen; and pis- tils, which are continuous with the style and stigma, and enclose within their cavity the ovule or external basis of the future seed. The pollen grains (d, Fig. 371) subsequently gain the stigma (c)- } either spontaneously, or by the assistance of insects, winds, or changes in the position of the flowers. They remain sticking to the gummy covering of the style ; and subsequently emit special prolongations, the pollen tubes. These contain the active mu- cous substance called thefovilla of the pollen grains. They gradually descend through the canal of the style (as shown in 6, Fig. 371), gain the interior of the pistillary cavity, and betake themselves to the ova there present. R R 2 612 GENERATION IN PLANTS. [CHAP. XIX. 2091. The anther has long been regarded as the male, and the ovum as the female sexual apparatus of the phanerogamous plants. The fovilla, which would thus correspond to the male semen, often contains granules that exhibit the molecular movement discovered by Brown ( 1188). But it possesses no elements having characters like those of the spermatozoa of cryptogamous plants or animals. The ova have an external membrane (e, Fig. 372), and a second internal protective sheath (d), to which are sometimes added other and similar structures. In the centre of all these lies a nucleus (b), in the substance of which is imbedded the embryonal sac (c). While the ovule attaches itself to the wall of the pistillary cavity (at a, Fig. 372), a canal (/) left by corresponding gaps in the membranes the exostomium and endostomium leads to the nucleus (b). The (gene- rally elongated) embryonal sac forms a clear and transparent vesicle, which is filled with a peculiar solution. Still it cannot be compared with the germinal vesicle of ( 2083) of the animal. For its want of a germinal spot, and its different subse- quent development, prevent it from forming any satisfactory parallel with this important constituent of the animal ovum. 2092. There is no doubt that after the pollen-tube (b, Fig. 373) has passed from the stigma (a) through the canal of the style (c) into the pistillary cavity, it proceeds through the exostomium and endostomium to the nucleus. Schleiden states that this tube causes the embryonal sac (/) to be reflected on itself. New cells are then developed in its inferior extremity : they multiply, and gradually form the embryo. The radicle is subsequently directed upwards, and the attachment of the cotyle- don buds downwards. In the mean time the upper portion of the pollen-tube gradually atrophies, and disappears. According to this account, it is the pollen which produces the most important part, namely, that which at a later period of germination is developed into the new plant ; the ovum only forming a suitable protective and nutrient mass, in which this is imbedded. This process would there- fore constitute, not a bisexual generation, but only a special development from the pollen-tube of a bud, the further growth of which is assisted by its being properly implanted in the substance of the ovum. On the other hand, according to Amici, Mohl, and others, the lower end of the pollen tube does not enter the embryonal sac to be then metamorphosed into the embryo itself. The latter is produced independently, after the fertilizing fovilla has been FIG. 373. CHAP. XIX.] PROPAGATION BY FISSION OR GEMMATION. 613 conducted to the outside of the embryonal sac by the aid of the pollen- tubes. Hence the action of the fovilla on the internal structures of the germinal receptacle might to some extent resemble that of the animal semen on the unimpregnated ova. 2093. In the invertebrate animals, although, as a rule, generation is probably bisexual, even down to the polyps, still in numerous examples it is unisexual. Many of the circumstances here met with forcibly remind us of the methods by which multiplication is effected in the vegetable kingdom. 2094. Many animals in whom the whole body has a vegetative uni- formity are capable of being multiplied by normal, accidental, or arti- ficial divisions. Separate fragments of Infusoria (such as the Para- moecice), of Polyps (such as the Hydrce), and of the Trematoda (such as the Planarice), are frequently completed to form perfect animals. In many of the articulate worms as the Naiadce transverse (but not longitudinal) division gives rise to the same result. Here we have what is, to a certain extent, the highest degree of reproductive capacity ( 1062). This may, in some of the lower animals, assist to maintain the species ; and will in any case diminish the injury of accidental attacks. 2095. Many infusory animals (such as the Vorticellce or Hydrce), and some of the polyps (such as the Corallince), together with some of the Helminthce for instance the Cystica often give out projections, which either only increase the number of animals in organic connection with each other, or subsequently fall off, and live as completely inde- pendent beings. But although these are called buds, still we must re- member that it is only in their general mode of production, and not in their internal structure, that they resemble the bud of the plant. 2096. The fertilized ovum of a mammal or bird proceeds at once to the production of a developed animal. It is true that the constituents of the new creature undergo manifold changes. But we have always certain permanent characteristics, which more or less correspond with those of the adult animal. Frogs and salamanders exhibit greater dif- ferences. Here larvae are first produced ; these are devoid of extremities, and respire by gills, like most other aquatic animals. Subsequently the limbs gradually protrude ; and ultimately, lungs replace the gills. The tadpole finally loses its tail, and assumes the shape of the developed frog. And in other animals we see far greater changes of form. Thus the butterfly has to pass through the preparatory stages of caterpillar and pupa : while the Cirrhiped swims about as a marine crustacean, before acquiring its permanent form, and the attachment which condemns it to a sedentary life. 2097. That development of nursing-animals, which occurs in many of the invertebrata, and the circumstances of which have been so accu- rately elucidated by Steenstrup, frequently leads to a series of similar 614 ALTERNATING GENERATION OF THE LOWER ANIMALS. [CHAP. XIX. phenomena, as well as to many other peculiar effects. Thus instead of the new being proceeding at once along the path of development, it has to go through a prescribed succession of complex metamorphoses, before attaining the form and conditions of organization which obtain in the adult animal. And here, as in the simpler larval state, some of the organs at last become superfluous, and therefore disappear ; while various important parts only sprout forth at a later period of development. But the two most remarkable phenomena we meet with are that a single germ pro- duces not merely an individual, but a series of animals ; and that the latter are not produced directly by an ovum, although the original being from which they proceed is only formed under the influence of a bisexual generation. 2098. We have seen ( 2085) that bisexual generation is but the visible sign of more delicate conditions of development. And the numerous circuitous routes which this generation by nurses often has to follow, may be regarded from a similar point of view. The accuracy of such a statement may be illustrated by some examples. 2099. The ovum of the Medusae (or sea-jellies) first exhibits that peculiar fission (a, Fig. 374) which is often met with in other classes of FIG. 374. animals, and to which we shall return in considering the development of the embryo. According to the repeated observations of Siebold and Sars, it is then converted into a creature (b), which resembles one of the infusoria, being provided with cilia ( 1202), and capable of swimming about in the sea. This animal then becomes firmly attached to some other substance by its lower extremity, while the upper sprouts out into a constantly increasing number of branches (c). The whole is finally divided by a series of transverse sections, so as distinctly to resemble a set of cups placed one within another (d). Each segment now gives off processes (e)- } and then becoming completely separated, gradually developes itself to a perfect medusa (/). Hence we see that CHAP. XIX.] ALTERNATING GENERATION OF THE LOWER ANIMALS. 615 a single germ furnishes a series of individuals; and that each of these becomes a male or female, whose bisexual generation produces new germs which are destined to undergo the same development. So that the generation of these, and the cleavage of their progeny, play alter- nate and different parts; the first maintaining the species, the latter multiplying its numbers. 2100. Something similar to this is probably repeated in the Salpce and Tcenice or tapeworms. The young tapeworms which are possibly produced from larvee of a different form (Fig. 375), such as that called the Tetrarhynchus change the form of their body, to give rise to the first links of their chain (a, Fig. 376). They then increase in length, and multiply their joints, either by division or interposition. Each of these finally gets the sexual organs and ova necessary to further propa- gation. The dark structures (b, Fig. 377) which are met with in the FIG. 375. FIG. 376. FIG. 377. m "* mature pieces (a)' of the Bothriocephalm latus indigenous in Poland, Switzerland, and Russia, are oviducts that contain many thousand eggs packed closely together. The joints are chiefly cast off at particular times of the year, such as summer ; and are evacuated with the fseces, so as to allow the ova to undergo their further development out of the body. The rest of the head, which remains in the human alimentary canal, subsequently becomes elongated, and is then constricted so as to form new joints, which subsequently acquire sexual organs and ova. 2101. Just as a more or less perfect division is an essential link in this process, so in many polyps, there is a similar circuitous pro- cess of gemmation, which finally relapses into a sexual propagation. For example, the Campanularice shoot forth excrescences ; which subse- 616 ENDOGENOUS GENERATION. [CHAP. xix. FIG. 378. quently become free, swim about in the ocean, and only attain their sexual maturity after a certain period of development. 2102. A still more peculiar mode of multiplication, which may be called an endogenous generation, sometimes assists that by fission and external gemmation. The young produced by the bisexual generation of some of the trema- toid Entozoa at first swim free by means of their ciliated epithelium ( 1202), and are subsequently converted into a peculiar vermiform animal. Now in many land- and water-snails, we find movable worms or living germ-tubes, which are probably produced by a similar metamorphosis. Such a tube is represented in Fig. 378, after a drawing by Steenstrup. Here a b c indicates the alimentary canal. In addition to this, the worm or nurse encloses in its interior a number of independent animals or cercarice, d, e,f, g. These commence as a peculiar deposit, without any previous impregnation. They are developed within the mother ; who serves as their cavity of incuba- tion, and afterwards gradually perishes. They sub- sequently become free ; and casting off their tails, bore into other animals, perhaps sometimes at once reaching their alimentary canal, where they pass through a chrysalis state, and are finally metamor- phosed into Trematodes, which further develope double sexual organs. It is probable that those female aphides which bring living young into the world are also mere nursing-animals; in which the new beings are produced from aggregations of cells. 2103. Hitherto we have only been occupied with the different forms of maternal generation. But it has also been imagined that living beings may be produced by the direct combination of the ultimate ele- ments of their substance ( 270), or from foreign and putrefying matters. This mode of origin has been named spontaneous, equivocal, or heteroge- neous generation. Supposing such a process really obtained, it is obvious that new species, or at least new individuals, might at any time arise. 2104. The hypothesis of spontaneous generation was very favourably received in the infancy of science. It was then supposed that many of the lowest plants, and a large number of invertebrate animals, owed their immediate origin to putrefying organic substances; as did also some of the phanerogamous plants, and many species of vertebrate animals. But a more careful study of the phenomena of propagation subsequently proved that, in all the higher beings, the ordinary parental generation was the exclusive method; and that it even obtained in many lower animals, which were perhaps also producible by spontaneous CHAP. XIX.] SPONTANEOUS GENERATION. 617 generation. And the further that the history of development was traced, the more the defenders of heterogeneous propagation were obliged to limit their hypothesis. So that, finally, the only instances in which it could be defended with any appearance of correctness were those of the mould-plants, Confervce, and Infusoria, that appear in putrefying liquids; together with the various parasites, and the seminal corpuscles which were then regarded as genuine animalcules. 2105. A careful examination must withdraw even these last supports from the doctrine of non-parental propagation. The seeds of the mould- plant and the Algce, as well as the germs of infusory animalcules, are so minute and light, that they are propelled to a great distance by the weakest currents of air or water. A few threads of mould frequently deposit thousands of spores. Hence a great number of the latter may be lost without endangering the maintenance of the species. Should some of these accidentally find a suitable nidus, numerous masses of mould can grow in a very short time. And experience teaches that many of these plants nourish luxuriantly in dilute acids. Hence that acid fermentation ( 324) to which organic substances are so liable during their spontaneous decomposition, yields a favourable nidus for the development of any spore-granules which may accidentally have reached it. This character of the nidus and the food explains why mould-plants are found in the relics of digestion ( 496), and why the scabs of cutaneous eruptions show enormous quantities of mould- filaments under the microscope. The annexed figure (Fig. 379) re- FlG - 379 - presents those which occur in the ,,,., crusts of the scalled heads ot chil- dren. 2106. The lower or polygastric Infusoria present similar pheno- mena. Their germs and young are of such small size, that they easily penetrate invisible apertures. The facility with which they multiply is favoured by the peculiarities of the sarcode ( 1224) which constitutes the bulk of their corporeal substance. 2107. We may convince ourselves by experiment that these living creatures do really enter infusions from without. If distilled water be boiled for some time, and sprinkled with fragments of a plant or an animal while it is yet hot, and if the whole be now hermetically sealed, so as not to leave any air over the liquid, no mould-plants or infusoria will be developed. The continuous boiling of the water killed the germs of the minute beings which it probably contained. And supposing none to have been present in the organic matters exposed to putrefaction, 618 GENERATION OF ROTIFERA. [CHAP. XIX. their subsequent absence is also explained. And a certain quantity of air which has been just passed through a solution of potash, may be allowed to stand over the fluid infusion, without the appearance of any Infusoria. But there are obvious reasons why the presence of the ordinary atmosphere should lead to the development of these animals, as well as of the mould-plants. 2108. The rapid growth and great multiplication of these minute organisms, when received into a suitable locality, will explain many of the phenomena of contagion which are so often met with in the vegeta- ble and animal kingdoms. If a small quantity of the mould that occupies apples or pears be transferred to a puncture in a sound fruit of the same kind, this often becomes mouldy after some time. That most destructive disease of silk-worms, called the muscardine, consists in the growth of mould within the abdominal cavity of the insect; a growth which sooner or later kills it. Here again a healthy individual may be infected by an artificial transfer. The same result may some- times be brought about by inoculating them with the vegetable parasite of the scalled head, or with other vegetable parasites of the human body. In this way infection may be voluntarily produced, even with the coarse material aids at our disposal. 2109. The Rotifer a, which are closely related in structure to the Annulata, propagate by means of ova. Some of these creatures can produce many generations in the course of a few hours. This explains why we sometimes find vast numbers of them in a small quantity of water which has only been taken out of a pool the day before, and which perhaps contained but a single parent individual. 2110. Many of these Rotifera, as well as the Tardigrade animalcules which exist in the gutters of roofs, enjoy an advantage unknown to most other beings. They may be completely dried up, and subsequently soaked in water, without losing their capacity of life during this impor- tant change of state. They may thus be preserved for years without moisture. A drop of water soon restores them to their original activity. Higher degrees of heat generally destroy the vital capacity of the various animal and vegetable tissues. But such a dried-up Tardigrade may be exposed to the temperature of boiling water without the application of fluid, and still, according to Doyere, live when moistened. When boiled in water, however, it dies like any other animal. This difference reminds us of the relations of fluid albumen; which, when mixed with or dissolved in water, coagulates by heat, while when completely dried, it is capable of supporting a higher temperature without losing its solubility. 2111. The notion of a spontaneous generation derives its strongest apparent support from the external parasites (Epizoa), and from the intestinal worms which occupy the interior of the body (Entozoa). The CHAP. XIX.] GENERATION OF ENTOZOA. 619 occurrence of Cystic entozoa (Cysticercus, Coenurus) in the brain and areolar tissue, of Distomce in the liver, of Strongyli in the kidneys, and of Trichince in the muscles, appeared to be best explained by the supposition that they originated directly from the decomposed juices of the body. But the various intestinal worms the A scar is, OxyuriSj Trichocephalus, and the tape-worms (Tcenia and Bothrioce- phalus) rather countenance the supposition that they penetrate from without ; either alone, or mixed with food. Formerly, however, it was almost universally believed that the intestinal worms could only live within another organized being. Hence the more accurate opinion was either rejected or passed over in silence. 2112. Here again recent observations have deprived the hypothesis of spontaneous generation of all its probability. They have distinctly proved that these animals originate from a bisexual generation, and often have to pass through a series of remarkable conditions either as larvae or nurses ( 2099) before attaining their final form, and with it, their capacity for sexual propagation. In these states, they frequently live free and devoid of all organic nidus. Thus the ova or young leave the animal at a definite time, and, subsequently, when their stage of development again requires them to resort to a living animal, seek to return either to it or a similar one. Here a series of periodical changes, and many remarkable circuits, all conduct to the same final result : namely, that such creatures occupy the innermost recesses of another animal. This may be best illustrated by some examples. 2113. The number of germs generally increases in proportion to the dangers which oppose their development ( 2081). On this account fishes and frogs have vast numbers of eggs, a large portion of which perish without attaining their object. And since the incubation of most of the intestinal worms is exposed to innumerable accidents, the same provision is also repeated in these parasites on a very extensive scale. For example, Fig. 380 shows the chief internal organs of a Filaria or thread-worm, from a fish, the Trigla. Here the long oviduct c winds along the greater part of the intestine b. The similar oviduct of the ascaris makes still more numerous curves, and is so distended with minute ova, that a single female contains many thousands of germs. Each of the older joints of the Bothriocephalus latus (a, Fig. 377, p. 615) As- sesses a female sexual apparatus, which encloses at least many hundreds, and probably many thousands, of eggs. According to Eschricht, the different joints from time to time given off by such a tape-worm together amount to 10,000. So that a single animal can gradually develope many millions of ova. 2114. The young of many intestinal worms are developed in the interior of the animal which the parent inhabits. Indeed many Hel- 620 GENERATION OF ENTOZOA. [CHAP. xix. .a FIG. 380. minthce belong to the class of viviparous ani- mals ( 2086). But the ova or young of others appear to be only capable of thriving out of the body. Although the adult A scar is, Oxyuris, and Trichocephalus (2111) are often found in the human intestine, their developed ova have never been detected in this situa- tion. The heads of the Teenies produce new joints by prolongation and subsequent trans- verse division ( 2100). When the several joints have reached a certain stage of deve- lopment, they acquire sexual organs, in which ova are produced and developed to a certain degree. These having arrived at maturity, a segment of the tapeworm becomes spontane- ously detached, and is evacuated in the faeces. This exit of headless portions, which is really due to the normal course of development, might easily be attributed to any medica- ments that may have been made use of. But the head still left in the intestine pos- sesses the capacity of again producing new joints. 2115. The experiments instituted byEsch- richt plainly indicate a certain periodicity, such as is often betrayed by the human Bothriocephalus (2100). A fish of the Baltic Sea (the Coitus scorpio) contains a tapeworm (the Bothriocephalus punctatus) whose head grows new joints in autumn and the begin- ning of winter. The ova these contain attain maturity in the course of the spring. The joints in which they are imbedded ( 2100) are then detached, and expelled with the feeces. Hence in the summer the fish con" tains none but neuter or unisexual parasites. The ova expelled with the faeces are pro- bably set free by the gradual putrefaction of the surrounding substance of the joint : to be afterwards developed into larvae, the evolution of which continues until they can again take up their final residence in a similar fish. 2116. Circumstances connected with their CHAP. XIX.] GENERATION OF ENTOZOA. 621 nutrition or growth frequently induce the entozoa to change their resi- dence. Such circumstances may be either accidental, or due to causes essentially final. The Lumbrici frequently wander out of the rectum into the vagina. They may also migrate from one child to another. And just as many granivorous birds eat indigestible seeds, in order that these may be deposited with their fseces at a distant place the animal thus forming a living means of transport so many intestinal worms can only undergo a complete development after the voracity of another animal has transferred them to a more favourable nidus. The intestine of the stickleback (G aster osteus) contains a tapeworm (the Boihriocephalus solidus), which here developes no sexual organs ( 2100). But if its land- lord is accidentally devoured by an aquatic bird (such as one of the divers), the parasite, which resists the solvent powers of its digestive organs, gains a dwelling more favourable to its further development. Its joints then develope mature ova in their female sexual organs, giving rise to what seems a new species of tapeworm (the Bothriocephalus nodo- sus). These ova are subsequently discharged from the bird's body ; so that the resulting worms, when developed to a certain degree, can again seek out a stickleback as their preliminary residence. 2117. The wanderings which appear to constitute an important ele- ment in the life of most entozoa often require special locomotive organs or offensive weapons, by which the animal attains its new residence. Hence many of their embryos and young have a ciliated external surface ( 1202). Others possess hooks, prickles, and similar horny structures, by means of which they are enabled to bore into the interior of various animals. When this object is completely attained, these weapons often perish, or are converted into other structures more suitable to the period for example, into suctorial organs, which facilitate the absorption of the juices of the being that now serves as their residence. 2118. While the intestinal worms were formerly often looked upon as condemned for life to a definite and concealed habitation, later researches agree in indicating that most (if not all) parasitic animals spend a consi- derable part of their life in travelling. In some instances, Nature seems compelled to make use of every means which can possibly conduce to this end. 2119. The food often serves as a means of smuggling in the intestinal worms, and their eggs or larvse. Small animals akin to the vinegar- eel (Anguillula aceti) exist in mildewed corn, and in the seeds of similar grasses. They possess an extraordinary tenacity of life, being not even destroyed by desiccation ( 2110). Siebold offers the justifiable conjec- ture, that they are early stages in the development of other Hdminthae; and that when such grain is eaten, they can undergo a further develop- ment in the intestine. It is probable that aquatic animals often consume joints of teenia, or the ova and young of various parasites, along with the 622 GENERATION AND MIGRATION OF ENTOZOA. [CHAP. XIX. food which is so easily contaminated in their haunts. And we shall scarcely go too far if we deduce the production (or rather the transfer) of the tapeworm in particular localities mainly from the use of certain waters. 2120. The movement of the blood may also assist in forwarding the younger entozoa toward their destination. Sometimes many of the ves- sels of a frog contain small creatures resembling Filarioe ( 2113), In examining the circulation of the web, these may even be seen propelled with the blood of the capillaries ( 651). Similar creatures are often contained in the membranes of pupse which are imbedded in the walls of the stomach and intestines. But, on the other hand, they often occupy very concealed places ; such as, for instance, the neighbourhood of the choroid plexus of the fourth cerebral ventricle (at E F, Fig. 368, p. 589). It is probable that the pointed extremities of their bodies endow them with a power of penetrating the walls of the blood-vessels, and of sub- sequently leaving the current of blood at some distant part of the body. 2121. In fishes, the entozoa which seek to advance by such powerful mechanical means have been observed projecting from the intestines, muscles, or skin. The sand-flea (Pulex penetrans) of the torrid zone of Brazil thus nestles in the naked human foot. The Filaria medinensis probably penetrates in the same way, either in the adult state or at an earlier period of its development. We have already ( 2102) seen that the cercarise of the Distoma bore into the bodies of other animals, and are here metamorphosed into more perfect intestinal worms. The development of nurses ( 2097), and the endogenous mode of generation ( 2102), has also been noticed as sometimes occurring in the Helmin- thoid class. 2122. The external obstacles to the wanderings prescribed to the entozoa frequently lead them into by-paths, the results of which are seen in various ways. No doubt a large portion of the eggs or young perish for want of the nidus necessary to their further development. The immense number of such germs ( 2113) is thus explained. But it may also happen that, in a less suitable residence, these creatures degenerate, and are gradually converted into peculiar animals. 2123. According to Siebold, it is probable that when the young of a certain tapeworm (Tcenia crassicollis) found in the cat stray into the body of a rat or mouse, they become dropsical. If they penetrate the liver, they retain only the anterior half of their body, and are found enclosed in a cyst. And when a cat devours the mouse inhabited by the parasite, the latter, if it have not suffered too much, may possibly resume its normal course of development. Thus Siebold conjectures that the hair- worm (Trichina spiralis) sometimes found in thousands within the striped muscular fibres of men or animals, is nothing more than the larva CHAP. XIX.] THE SPERMATIC FILAMENTS. 623 of some other parasitic animal ; which has lost its way, and has therefore become crippled and sexless. 2124. From all this it is evident, that the two chief grounds on which it was thought necessary to assume the spontaneous generation of entozoa viz. their apparently residing exclusively within other animals, and their occurrence in the most concealed recesses of the organism will, in the present state of our knowledge, no longer sustain this theory. We now know that the entozoa either can or must spend one part of their lives in a state of freedom, and another within various animals. These migrations, and the organs by which they are aided, will also explain the presence of such animals in the most secret organs of the body. Besides, all that has hitherto been observed expressly indicates, that there is no departure here from the general rule of parental gene- ration nay, for the most part, not even from the rule of bisexual pro- pagation. Only circumstances often compel the adoption of very round- about methods for the production of those beings which develope the semen, the ova, or both. 2125. The fact that the entozoa are developed by a parental mode of generation exercises a certain influence on the pathology of the com- plaint which their presence produces. It renders this helminthiasis a disease produced by contagion; while the notion of the spontaneous generation of entozoa stamped it as a simple dyscrasia. We may indeed admit that a faulty admixture of the juices can perhaps favour the prosperity of many parasitic animals. But since these must always be introduced from without, any improvement in the elaboration of the juices could at most only prevent the proper development or mainte- nance of the individuals themselves. Besides, few of the entozoa produce any remarkable phenomena of disease. Some of them, as the Filaria medinensis, lead to the formation of ulcers. Tape-worms sometimes give rise to colic of variable severity. But many of the symptoms assigned to worms in medical treatises are not really due to the influence of these parasites. 2126. We shall hereafter see hat the gradual development of sper- matozoa ( 1215) in the interior of cells may be directly followed under the microscope. Could we regard these bodies as animals, and not as tissues, they would form the strongest arguments in favour of the possibility of spontaneous generation. The assumption of their animal nature was mainly founded upon the movements which they generally exhibit at the period of their maturity ( 1217). But the spermatic elements of many entozoa and crustaceans appear to be devoid of the capacity of movement. Besides, so far as we know, every animal can reproduce its like, so as to assist in maintaining its own species; while nothing at all akin to such a propagation is seen in the mobile seminal corpuscle. Finally, by classifying the spermatozoid among the tissues 624 THE SEMEN. [CHAP. XIX. we shall be better able to conceive, why similar structures occur on the cutaneous surface of some polyps ( 1223), and why the Psorospermia met with by J. Mueller in many tumours (e.g. of fishes) possess a con- siderable resemblance to seminal corpuscles, although devoid of all trace of movement. 2127. On the whole, the hypothesis of a spontaneous generation of plants or animals can only be regarded as a kind of superstition, which is constantly receding before the advance of the natural sciences. All experience concurs to indicate that species are at present only main- tained, without any new ones being created. Their original creation would perhaps demand conditions and meteorological states different from those attainable at the present day. Unfavourable collateral causes have totally destroyed some species such as the Dodo in the last few centuries. 2128. That parental generation which is alone evident to our senses may occur in a variety of ways, according to the special circumstances of different organizations. Reproduction by fission or division is based upon a close correspondence between the various segments of the exist- ing corporeal mass. Gemmation and endogenous generation are con- nected with an easy deposit of the first rudiments of the new being. While sexual propagation implies the most delicate conditions of growth. The sexual organs may themselves also be formed either directly or indi- rectly. The more homogeneous young of many of the lower animals have not acquired the capacity of producing special sexual products. Hence the evolution of the specific sexual organs may be preceded by various intervening stages; such as those of fission, gemmation, endogenous generation, and the various and mysterious kinds of growth and propagation that are connected with the formation of nurses. Finally a lower degree of organization permits of hermaphrodism ; and a higher one effects the separation of the sexes. 2129. Male sexual organs. The chief object of these organs consists in the preparation and expulsion of the semen, which is provided with moving seminal corpuscles. The semjnal matter of man and mammals originates in the tortuous seminal tubes which fill the various lobules of the testicles (s, Fig. 382, p. 625). It then passes through the rete testis, the vasa efferentia, and the coni vasculosi, towards the epididymis (t) and vas deferens or seminal duct (v, w, p, q), undergoing in the meantime a further development. Hence, under favourable circumstances, the whole development of the seminal elements may be followed in the organs of the same individual. 2130. When a person has attained the age of puberty, he acquires the capacity of secreting a fertilizing semen at all periods, a capacity which he retains until extreme old age. Animals, however, only become rut- tish at definite epochs. They then fall into a condition of increased CHAP. XIX.] DEVELOPMENT OF THE SEMEN. 625 sexual excitement. The bulk of the testicles is more or less increased, and they yield mature semen; while at other times, they are either completely at rest, or only prepare smaller quantities of a somewhat different fluid. Here again the period of rut only occurs after the attainment of a certain age. When this period has passed, no new spermatozoa are formed, and the residue of the old gradually disappear. 2131. The testicles of the human subject secrete more slowly than most of the other glands. A rapid succession of excitements and evacua- tions may spur them on to a somewhat increased activity. But since the complete development of seminal elements demands long periods of time, the fertilizing fluid evacuated under such circumstances exhibits under the microscope a constantly diminishing number of free and moving seminal corpuscles, and a corresponding increase of those in the earlier stages of development. 2132. In all the animals which have hitherto been examined the spermatozoa are developed in cells, by the destruction of which they are subsequently set free. We first observe small, simple, and roundish cells (Tab. V., Fig. 78, c), in which we afterwards perceive some separate secondary masses, or complete se- condary cells (Tab. V., Fig. 78, d). The latter form the materials for the development of the spermatozoa, which lie either separately (c, Fig. 200, p. 363) or in bundles (Fig. 381) within the parent cell. The walls of the parent cell are finally dissolved, so that the spermatozoa move freely about in the seminal fluid (Tab. V., Fig. 78, a 5). 2133. In the mature semen of man and most animals, there is scarcely anything mixed with the liquor seminis save the moving seminal filaments. This fact may be sometimes verified with the contents of the vas deferens (v, w, p, q, Fig. 382) and epididymis (t) in the bodies of chaste men. But the substance met with in the tubules of the testicle contains younger cell-forms, which are mechanically admixed with the fertilizing fluid. Loose epithelial cells, and a few scattered oil-drops, may also be present. And where the seminal evacuations succeed each other too rapidly, large quantities of the semen -cells are found in the emissions. 2134. Hitherto the quantitative composition of the pure seminal sub- stance has not been accurately determined. According to Frerichs, the fluid which surrounds the immature seminal corpuscles contains albumen, while the mature fluid contains horny substance. The development of the spermatozoa may therefore be compared to the process by which the cuticle becomes horny ( 1016). 2135. The emission of semen is generally due to a reflex action. Friction of the glans (/, Fig. 382) gives rise to reflex movements in the vasa deferentia (v, w, p, q,) and probably also in the seminal tubules of the epididymis (t) and testicles (s o). This effect may be artificially pro- 8 S 626 SEMINAL EMISSION. [CHAP. xix. duced in recently killed animals. The semen next reaches the inferior and glandular part of the vas deferens (r, Fig. 382 ; /, g, Fig. 383). It then traverses the urethra (z a! V d', Fig. 382) to gain the orifice of the glans ( g, Fig. 382) : whence it is ejaculated with a force which, in vigorous men, can expel it to a distance of many feet. FIG. 382. 2136. Sometimes a direct irritation of the nerves also leads to seminal emissions. In recently killed mammalia, mechanical or chemical irritation of the lumbar por- tion of the sympathetic has been observed to produce such violent contractions of the vesiculse seminales as to cause the expulsion of semen from the orifice of the urethra. The urethra of beheaded criminals contains a mucous fluid, in which spermatozoa may be recognized under the microscope. The emission of semen was formerly believed to be characteristic of death by hanging. But it does not always occur ; and is probably limited to those cases in which the upper part of the spinal cord has been much injured. 2137. The tubular and glandular vesiculce seminales (I, m, Fig. 383 ; x, Fig. 382) contain a mucous fluid, in which seminal corpuscles may sometimes be met with. They furnish a peculiar secretion, which is only now and then mixed with true semen. Hence they are not the necessary receptacles of the fertilizing fluid ; and thus enact a part very CHAP. XIX.] SEMINAL EMISSION. 627 FIG. 384. different from the gall-bladder ( 923), and the urinary bladder ( 938), with respect to the bile and urine. 2138. Each of the vesiculae seminales (I m, Fig. 383) unites with the corresponding vas deferens (/ g) to form a short duct (n o), which opens into the urethra on the verumontanum. In the annexed figure (Fig. 384) a represents the ureter (&, I, Fig. 382 ; a, b, Fig. 383), c the vas deferens, and b the bladder (m, Fig. 382 and e h, Fig. 383) which has been slit up, and is continuous with the similarly exposed urethra; while d indicates the aperture of the ejacula- tory duct of the semen. But since the vesiculse se- minales contract simultane- ously with the vasa defe- rentia ( 2135), it is not pure semen which enters the urethra, but a mixture of this substance with the secretion of the vesiculse ( 2137) themselves. It is probable that the prostate gland (e, Fig. 384 ; r s, Fig. 383 ; y, Fig. 382) at the same time empties its fluid secretion into the neighbour- ing part of the urethra (above d, Fig. 384). Further onwards Cowper's glands (g, Fig. 384) also pour out their contents. The semen is thus mixed with a number of foreign constituents. The resulting mixture of secre- tions deposits flocculi in the open air. It stiffens linen on which it dries, and com- municates to it a faint greenish-grey colour. 2139. Although the urethra is the common efferent canal of the urine and semen, still, under normal circumstances, the two secretions s s 2 628 ERECTION OF THE PENIS. [CHAP. XIX. are never expelled simultaneously. The bladder remains closed at the instant of emission ( 942). And we shall see that the erection of the penis materially assists to shut off the bladder from the urethra. And conversely, the mechanism which expels the urine ( 941) leaves the seminal ducts and vesicles undisturbed, so that no ejaculation then takes place. But in persons who suffer from diseased spine, or are addicted to onanism or sexual excesses, the semen is often discharged involuntarily at the end of mictiirition, without any voluptuous sensa- tion. This spermatorrhoea is by no means so dangerous as it is gene- rally supposed to be. And if such persons sometimes become hypo- chondriacal or paralytic, or even die of emaciation, it is not from the losses of substance caused by these seminal emissions, but from the collateral nervous disturbances which sooner or later ensue. 2140. The erection of the penis is not a necessary antecedent of seminal emission, but is chiefly subservient to coition, by enabling the organ to distend the tube of the vagina more completely, and thus to excite more intense voluptuous feelings in both sexes. And since it only requires the presence of the trabecular or cavernous tissue which we shall shortly describe, it often occurs without any such object for example, in new-born infants. And eunuchs, or men who have lost a portion of the penis, are still capable of more or less complete priapism. 2141. The annexed woodcut (Fig. 385) represents a transverse section of the upper part of an adult human penis, which has been inflated from the veins, and then dried. The two corpora cavernosa a and b are separated from each other by an apo- neurotic partition c d. They form the erectile or trabecular tissue of the penis. They finally fuse into each other, and terminate by becoming continuous with the erectile tissue of the glans. At e is the corpus spongiosum of the urethra /. In Fig. 384 part of the fibrous network is seen in longitudinal section. 2142. The corpora cavernosa result from an intimate union of nu- merous veins, the cavities of which thus form the meshes of a many- sided network. Hence we have here, as it were, the greatest possible concentration of anastomosis ; the most perfect rete mirabile of the whole venous system. The broader partitions which divide the several cavities enclose afferent arterial trunks ; as do also the smaller trabeculse which often traverse the larger ones. Many of the finer of these vessels take a spiral or tortuous course ; this is especially the case with those contained in the smallest partitions. These then continue directly into the adjoining venous cavities, without the intervention of any capillary network. While, on the other hand, the broader partitions which form CHAP. XIX.] MECHANISM OF ERECTION. 629 part of the root of the organ, and the whole of the remaining trabecular tissue contain reticulate vessels, that are subsequently continued into the venous cavities. The arteries nowhere terminate by blind extremities. Those called the arterice helicince, and which have been supposed to end in this way, are only twigs that have been torn off and then coiled up by their own elasticity : prior to this they occupied the smaller trabeculee, and were cut across in some part of their course by division of the corpus spongiosum. In addition to these vessels, the numerous partitions con- tain white and yellow fibrous tissue, unstriped muscular fibres, nerves, and probably absorbents. Their external limitary surface is formed by the internal coat of the veins. 2143. Erection of the penis is not always due to that sexual ex- citement which forms its most appropriate cause ; but may be produced by mechanical irritation especially friction of the glaus, by pressure on the nerves of the penis from a distended bladder, calculi, or tumours ; and by irritation of the nervous centre. After its bulk has increased to a certain extent, voluptuous feelings are generally superadded. 2144. All the external changes exhibited by the penis in a state of complete erection namely, its increased volume and hardness, and its direction upwards depend solely on extreme distention of the meshes of its erectile tissue. Thus, in the dead subject, the imitation of this distention by injection produces a state of complete erection. But the two chief causes of this change of circulation have not yet been satisfactorily determined. We are ignorant in what way the nerves of the penis permit the afflux of more blood to the cavernous tissues. And we can but imperfectly explain, why most or all of the fluid thus brought is retained in their meshes. 2145. The root of the penis first enlarges in volume (below g, Fig. 384, p. 627). The increase of bulk then advances towards the glans. Erection and hardening only occur after a certain amount of distention. Since the channel formed by the venous intervals of the fibrous network is considerably larger than that of the afferent arteries, it is obvious that the blood will traverse these cavities with a diminished velocity, and be retained in them for a longer time ( 106). But this fact will at most only explain why the erectile tissues of the relaxed penis should contain large quantities of dark blood in the state of rest. While that excessive disteution which causes erection implies that there is some decided obstacle to the expulsion of the venous blood, the afflux of the arterial fluid remaining free. Since the arteries are protected by their running in the interior, the continuance of their afferent stream is easily explained. And as regards the return of the venous blood, it has been supposed that special muscles (the ischio-cavernosi) compress the penis against the symphysis pubis (below k, Fig. 231, p. 394), and thus close the efferent venous trunks. But there is a second theory, which 630 MECHANISM OF ERECTION. [CHAP. XIX. seems more in correspondence with the truth namely, that the exit of blood is opposed or altogether prevented by part of the unstriped fibres which occupy the septa. 2146. After the seminal emission, the erection of the penis soon diminishes. The return of the organ to its normal bulk occupies less time than its previous erection. The sluices of the venous blood are now suddenly opened. The elastic reaction of the immoderately dis- tended partitions and membranes presses upon the blood in contact with them. The unstriped muscular fibres ( 2142) probably add to this propulsive force. The excess of blood is therefore returned with increased velocity from the spongy texture towards the pelvic veins. 2147. When the nervous discharge which generally accompanies seminal emission does not occur, the erection disappears much more slowly. The tissues which hinder the return of the blood then yield but very gradually. And nervous influences can subsequently produce a second erection with greater ease and rapidity. 2148. The visible increase in the bulk of the penis is chiefly due to the spongy tissues of the glans (g*, Fig. 382, p. 626) and urethra (c). Their interspaces communicate (a b, Fig. 385, p. 628) with each other, and thus under all circumstances secure the perfect access of the blood. Other venous cavities extend along the bulb (a, Fig. 382) and the con- stricted portion (z) of the urethra, as far as to the neck of the bladder (from g to e, Fig. 384, p. 627). These likewise swell, and thus secure the perfect closure of the vesical orifice. Hence a man who wakes with a complete erection of the penis can only micturate after it has to some extent subsided. 2149. Female sexual organs. While the testicles furnish the male fertilizing fluid, the ovum which constitutes the essential female gene- rative products is produced in the ovary (e, Fig. 386). The Fallo- FiG.-386. pian tube or oviduct (gf), and the uterus (a b) into which this sub- sequently opens, only add new structures to protect and nourish it after impregnation. In many of the lower animals the ovary is essentially CHAP. XIX.] OVUM OF THE MAMMAL. 631 a tubular gland ( 857), which contains ova or peculiar cells as the secre- tion of its tubes. But the mature ovary of the mammal appears, at first sight devoid of all traces of glandular structure. 2150. While the ovarian ova of birds, amphibia, fishes, and many invertebrata, are enclosed in thin capsules, those of the mammalia present a more complicated organization. They contain a certain number of vesicles, the Graafian vesicles or follicles, which project from the surface of the ovary, so as to elevate its peritoneal covering, and are imbedded by their remaining portions in its substance or stroma. Each of them in- cludes a single minute ovum. In rare instances, two or more are present. 2151. The annexed diagram (Fig. 387) represents a section of a Graa- fian vesicle from the human ovary. At a is the follicular membrane which everywhere encloses it. A peculiar homogeneous fluid fills up the greatest part of its cavity b. Almost all the inner surface of the follicular membrane a is covered by a proper membrana granulosa c, the - roundish cellular or nuclear elements of which are imbedded in a homoge- neous gelatinous substance, and are at first arranged in the form of a tesselated pavement. The extremely minute ovum/<7 h occupies the high- est point of the follicular cavity. It is everywhere surrounded by a clear ring or girdle e called the zona pellucida. Many regard this as a substitute for the vitelline membrane ( 2083), while others state that there is a special vitelline membrane in addition to this. The discus pro- ligerus d arises from a thickening and projection of the membrana gra- nulosa in the neighbourhood of the zone e. Its innermost and thickest section forms the cumulus. The ovum itself contains the yolk or mtellus /, the germinal vesicle g, and the germinal spot h ( 2083). 2152. We have seen ( 2130) that it is at the period of puberty or sexual maturity that the testicles are first endowed with the capacity of developing a true semen provided with spermatozoa. But Graafian vesi- cles and ova may be met with in new-born infants, and even in the advanced embryo. It has often been conjectured that in course of time the older vesicles are absorbed, and new ones developed in their place. But hitherto this statement has not been fully established. 2153. The ova of birds and the other lower vertebrata undergo more or less enlargement prior to their exit from the ovary. But those of the mammalia leave their original habitation as structures of extremely , minute size. A special mechanism effects this change of their resi dence. 632 EXIT OF THE OVUM FROM THE OVAKY. [CHAP. XIX. 2154. The larger and riper follicles project from the surface of the ovary. The vessels which surround the follicular membrane (a Fig. 387) of the vesicle that is about to burst become greatly distended with blood. This considerably increases the quantity of its contents b, and the dis- tention of the whole follicle. A quantity of blood is sometimes effused into the cavity of the vesicle, where it subsequently coagulates. To this is always added an exsudation, which is at first deposited at the bottom of the vesicle (c), without completely filling its uppermost part (a). The fluid contents of the follicle are hence impelled towards that part of it which projects beyond the stroma of the ovary, and is only covered by peritoneum. This becomes more and more prominent, and finally bursts at the site of its greatest projection. Since the small opening thus made (a) lies exactly in front of the ovum (f g h), this rushes out, together with the zona pellucida (e) the germinal disc (c?), which is torn off at its circumference, and the greater part of the follicular contents (b). The membrana granulosa (c) and the follicular membrane (a) remain in the ovary, to which they are firmly attached. 2155. The exsudation which causes the exit of the ovum is afterwards considerably increased. It gradually fills up the whole follicular cavity ; and sometimes even projects in the form of a wart, as in the rabbit. It then forms a dense globular mass, which has been named the corpus luteum, on account of its yellow colour in man and some mammalia. It may however be greyish white, red, violet, or brown. It subsequently diminishes in bulk, at the same time becoming denser ; and is gra- dually converted into a small roundish or oval nodule, leaving behind it a cicatrix which may finally disappear. 2156. The periodic rut of the brute mammalia ( 2130) gives rise to an energetic determination of blood towards the female sexual organs. It is only at this time that the ova project from the ovary. A bloody fluid or mucous secretion then frequently comes from the vagina. The desire for coition is almost limited to this period ; and appears to depend immediately on these phenomena of rut. 2157. The puberty of the human female is announced by the appear- ance of a flow of blood from the sexual organs. This discharge is called the menstrual flux ; because, under normal circumstances, it is repeated once every month. It continues from the access of puberty to an epoch which is generally called the turn of life. It is only during this period that a woman is capable of being impregnated. During the age of child- hood which precedes the access of puberty she is incapable of maintain- ing the species. And after menstruation has ceased, her generative organs are incapable of further fruitful action. 2158. The date at which menstruation begins and ends is liable to great variety. In our climate the menses usually appear between the age of 14 to 16 years, and cease between 40 and 45. Still very con- CHAP. XIX.] MENSTRUATION. 633 siderablc differences are met with. In the natives of tropical climates, menstruation frequently begins between 9 and 11, and ceases between 30 and 35. On the other hand, in the high northern latitudes these periods of time are often delayed beyond the date at which they occur in the more temperate regions. Still there are negresses whose menses appear as late as those of some Swedish women; e.g., for instance, at the age of from 20 to 21. 2159. In exceptional instances the menses return every one or two, or every five or six, weeks. But the ordinary time is exactly four weeks or 28 days. Extensive series of observations give this as the average. Since it is also the most frequent of all the periods, it is obviously a natural average, and not an accidental one. 2160. It has often been supposed that there is some relation between the menstrual discharge and the changes of the moon, since a single rotation of this satellite around the earth likewise demands 28 days. But this view is not based upon proofs such as are demanded by natural science. The menses appear at various days in different women, and often return a few days earlier or later in the healthiest individuals : facts which are utterly unlike the fixed phenomena presented by the revolutions of the moon. 2161. In healthy women, menstruation does not itself lead to any special disorder. But when collateral causes interfere, morbid pheno- mena are more liable to occur than in most other normal actions. The appearance of the menses is often preceded by pain in the loins, nausea, colic, prostration, and even by febrile symptoms. And their continuance is sometimes accompanied by paleness of the face, blue rings round the eyes, headache, dulness of intellect, vomiting, tympanites, and various derangements of the abdominal viscera. 2162. The access of menstruation is sometimes immediately preceded by the discharge of more mucus than usual from the vagina. But to all appearance, it often commences as a flow of blood even in healthy women. After lasting some days, it is succeeded by a watery fluid, which continually diminishes in redness. This gradually becomes more colourless and scanty, until it is converted into a mucous mixture, w r hich at last disappears. 2163. These successive changes prevent any exact estimate of the duration of menstruation. The pure sanguineous flux, and the reddened watery discharge, generally last from 4 to 6 days. But in women who menstruate but sparingly, they often last only 2 to 3 days, while in persons more inclined to haemorrhage, they remain 7 or 8. 2164. The menstrual blood always contains a large number of blood- corpuscles (Tab. II. Fig. 25, b c). Although it is more fluid than ordi- nary blood, still it is not devoid of all capacity for coagulation ( 1001). Microscopic examination sometimes shows a few masses of coagulated G34 MENSTRUATION. [CHAP. XIX. fibrine, especially in menstrual blood which is still contained in the uterus (Tab. II. Fig. 25, a). While those larger quantities of blood which flow from the genitals during and after parturition (as well as in the earlier part of the puerperal state, and in abnormal uterine haemorrhages) coagu- late in the ordinary manner : namely, in larger masses, such as are at once recognized by the naked eye. Exsudation-corpuscles ( 1053, Tab. II. Fig. 25, def) are also present; and in larger numbers, the more completely the proper sanguineous character of the discharge has disappeared ( 2162). 2165. Disease sometimes causes the internal surface of the uterus to become everted, and project from the orifice of the vagina. In cases of this kind, the menstrual blood has been distinctly seen flowing from the mucous membrane of the uterus. But the way in which it is poured out is at present unknown. 2166. When a menstruous woman dies of any disease not directly affect- ing her sexual organs, a vast number of the blood-vessels of the ovaries and uterus are found greatly distended. On opening the uterus one or more days after death, we see lumps of coagulated blood (the constituents of which are represented magnified in Tab. II. Fig. 25.) At first the blood- corpuscles (Tab. II. Fig. 25, b c) are in large numbers while the ex- sudation-corpuscles (def) are much less numerous. The coagulated fibrine (a) forms amorphous masses, which traverse the whole in various directions, and are soaked in the fluid in which the remaining solid struc- tures swim. The liquid menstrual blood discharged from the living female is very muddy ; . but on being allowed to stand in a glass, it subsequently deposits a loose precipitate, which is composed chiefly of blood-corpuscles. 2167. During menstruation, the internal surface of the uterine mucous membrane consists of a greyish- white gelatinous substance. Examined under the microscope, this presents a transparent basis (Tab. II. Fig. 2 6, a), together with granular globules heaped together in the form of a pave- ment. Besides this we observe red points or spots, containing blood- corpuscles, or even portions of coagulated fibrine. Highly distended blood-vessels are seen through many parts of the gelatinous mass. This alteration in the internal surface of the mucous membrane precedes the appearance of the menses themselves. At any rate, it has been distinctly seen in the uterus of a young woman executed a few days before the return of menstruation. 2168. Hence before the haemorrhage can itself force its way through, the lining membrane of the uterus is distended and partially loosened by an increased quantity of blood. Blood subsequently exsudes from various points ; and is found, mixed with coagula, in the uterine cavity of the dead subject. The most obvious supposition is, that many of the blood-vessels rupture, and thus extrude their contents. This would at once explain the presence of corpuscles in the menstrual blood. But CHAP. XIX.] MENSTRUATION. 635 many think it more probable that the porosity of the walls of the vessels undergoes such a change as to allow the blood-corpuscles a direct transit. 2169. When the haemorrhage has lasted some time, it is accompanied by an exsudation, which gradually supplants it. This change is indicated by a gradual increase in the number of exsudation-corpuscles, and by the colourless and highly saline characters of the fluid. The exsudation itself subsequently diminishes. Part of its mucous characters are pro- bably due to the horny substance which it dissolves in its course, and to its being mixed with the mucous secretions of the vagina. Some epithelial scales from the external genitals are always present as a foreign constituent. 2170. We shall hereafter see that the mucous membrane of the uterus sheds its innermost layers during the puerperal state. Pouchet states that something similar to this occurs at the close of menstruation. The partially dissolved fragments of this membrane which are shed during the second week form a mucous substance, the exit of which marks the conclusion of the menstrual changes. 2171. The periodic loss of blood which we have just been consider- ing is one of the most important vital actions of the fertile female. Its non-appearance at puberty, or its subsequent absence, causes a series of disorders which are comprised under the name of chlorosis or green sickness. This state is marked by a pale yellowish-green colour of the skin, blue rings round the eyes, prostration, dulness of intellect, and finally, dropsical effusion. The blood of chlorotic women contains fewer corpuscles than in health. The curative effects of the preparations of iron often used to remedy the disease are due to their obviating this fault of the blood. The healthy female gives off less carbonic acid as long as she is subject to this periodic hemorrhage ( 823). But, according to Hannover and Scharling, in chlorotic persons who do not menstruate, the quantity of carbonic acid is increased, although the number of blood- corpuscles is considerably diminished. 2172. When the os uteri (at c, Fig. 386, p. 630) is morbidly closed, the menstrual blood gradually distends the womb. The abdomen slowly acquires a bulk almost equal to that which it offers in a woman far advanced in pregnancy. But since the resistance offered by the walls of the uterus sooner or later obstructs the entry of new menstrual blood, chlorotic symptoms finally appear, together with other disorders which are immediately due to the abnormal enlargement of the uterus. On puncturing the occluded os uteri, there gushes out a large quantity of black and partially coagulated blood; which is extremely foetid from putrefaction. If the artificial orifice is kept open, the uterus gradually resumes its normal size and activity. The chlorosis then ceases spon- taneously. 636 MENSTRUATION. [CHAP. XIX. 2173. The menstrual flux is only an external index of various im- portant changes which the internal organs of generation undergo at these periods of time ( 2159). Where absent, it is sometimes replaced by other haemorrhages : for instance, from the nose, lungs, or stomach. Such " metastases " of menstruation sometimes follow the extirpation of the uterus. According to Roberts, in those Indian women who have been castrated by the removal of the ovaries, there is no trace of either menses or metastatic haemorrhage. This fact has been confirmed by many European cases in which both ovaries have been removed by sur- gical operations. But according to other medical narratives, the menses have sometimes continued in spite of this extirpation. Of course the loss of a single ovary will not prevent menstruation. 2174. Recent researches have established that the ovaries are the site of the most important of those changes which accompany the rut of the mammal. Formerly it was supposed that the discharge of the ova from the Graafian follicles ( 2154) could only be produced by the action of the semen. The corpora lutea ( 2155) were therefore regarded as a certain sign of previous impregnation. But the observations of Duver- noy, Pouchet, Raciborski, and (especially) Bischoff teach us that this is not the case. The vigorous sexual activity which obtains during the period of rut suffices to ripen one or more follicles. And this again requires the discharge of ova : and the subsequently development of corpora lutea. 2175. The periodical recurrence of rut together with that discharge of blood from the genitals which accompanies it in some mammalia have led many physiologists to suppose that the menstruation of the human female corresponds to the rut of the mammal. According to this idea, the rutting season of the human female recurs every four weeks all the year round. Although this comparison has lately received very im- portant confirmation, still there are many important differences. While the sexual appetite of the animal attains its greatest height in the rutting season, the menstruating woman generally rejects coition, and either has no period of increased sexual excitement, or at most only betrays it for a short time after the last traces of sanguineous men- struation have ceased. 2176. The notion that the menstruation of the human female forms a parallel with the rut of mammalia has led to the conjecture that one or more Graafian vesicles discharge their ova during this flow. Many ob- servers have in fact found a ruptured follicle or a recent corpus luteum in one ovary of women who have died shortly after the end of men- struation. But under similar circumstances, others have been unable to discover any evacuated follicles. Still it may be questioned whether, in these cases, death did not occur at so early a period, that the sexual excitement only sufficed to produce the menstrual flux, without attain- CHAP. XIX.] MENSTRUATION. 637 ing that intensity requisite for the expulsion of ova. Others believe that corpora lutea may arise independently of menstruation in children and old women : or that there are two varieties of these bodies ; the true, which result from menstruation, and the false, which are due to other causes. There is no doubt that diseased states of nutrition are capable of converting the follicles into watery vesicles, or of producing exsudations similar to corpora lutea and cicatrices. But at present we have no proof that true corpora lutea can be normally developed except during the age of menstruation, or that they are ever found in the ovaries before or after this period. 2177. The oviducts or Fallopian tubes of the human female (/, Fig. 386, p. 630) and most mammalia have no immediate connection with the ovaries (e). Their cavity rather opens into that of the abdomen. A row of fringes or fimbrise (eloppement destissus et des organes du corps humain. Utrecht. 1845. 4to. Pp. 47, 50, 51, 52, 73. 2. WERTHEIM in the Annales de Chimie et de Physique. Troisieme Serie. Tome XXI. 1847. P. 393, und Lehrbuch der Physiologic. Zweite Auflage. Bd. I. S. 792. 3. A. QUETELET, Des proportions du corps humain. Bulletin de 1'Academie de Bruxelles. Tome XV. Nro. VII. P. 13. 4. LEHRBUCH Bd. I. S. 793. 5. C. BRUNNER, Untersuchungen ueber die Cohaesion der Fluessigkeiten. Neufchatel. 1847. 4to. S. 32. 6. JOLLY in Henle und Pfeuffer's Zeitschrift fuer rationelle Medicin. Bd. VII. 1848. S. 83-148. 7. VIERORDT in R. Wagner's Handwoerterbuch der Physiologic. Bd. III. Abtheilung I. Braunschweig. 1849. S. 634. 8. LEHRBUCH Bd. I. S. 65, 66. 9. VIERORDT in Griesinger's Sechswochenschrift. Bd. VI. 1847. S. 672. 10. C. G. LEHMANN, Lehrbuch der physiologischen Chemie. Zweite Auflage. Bd. I. Leipzig. 1849. 8vo. S. 417. 11. FRERICHS in R. Wagner's Handwoerterbuch der Physiologic. Bd. III. Abtheilung I. Braunschweig. 1849. 8vo. S. 759. 12. BERNARD in the Archives d'Anatomie. Paris. 1846. 8vo. P. 6. 13. A. T. MIDDELDORPF, De glandulis Brunnianis. Vratislawiae. 1846. 4to. p. 26. 14. VIERORDT in Griesinger's Sechswochenschrift. 1848. S. 284. 15. F. G. NOLL, De cursu lymphae in vasis lymphaticis. Marburgi. 1849. 8vo. P. 14. Compare Henle und Pfeuffer's Zeitschrift. Bd. IX. Heidelberg. 1849. 8vo. S. 52 et seq. 16. W. CRUIKSHANK, Geschichte und Beschreibung der einsaugenden Gefaesze. Mit Anmerkungen von S. F. Ludwig. Leipzig. 1789. 4to. S. 26. 17. C. LUDWIG in Henle und Pfeuffer's Zeitschrift. Bd. VII. S. 208. 18. GUIL. KLEEFELD, De arteriarum coronariarum cordis pulsu. Berolini. 1849. 8vo. P. 13. 1 9. C. LUDWIG in Mueller's Archiv. 1847. S. 257. 20. HERING in Vierordt's Archiv fuer physiologische Heilkunde. Bd. IX. Stuttgart. 1850. 8vo. S. 13-22. 21. HERING Repertorium fuer Thierheilkunde. Bd. VIII. S. 1-19. 22. LEHRBUCH Bd. I. S. 833. 23. GUY in Todd's Cyclopsedia of Anatomy and Physiology. Vol. III. Pp. 181-194. 24. H. ABEGG, De capacitate arteriarum et venarum pulmonalium. Vratislawiae. 1848. P. 23. 25. F. A. HUETTENHEIN, Observationes de sanguinis circulatione homodromometri ope institutiae. Halis. 1846. 4to. P. 18 et seq. 26. J. HUTCHINSON, Von der Capacitaet der Lungen und von den Athmungs-Functionen mit Hinblick auf die Begruendung einer genauen und leichten Methode, Krankheiten der Lungen durch das Spirometer zu entdecken. Uebersetzt und mit Anmerkungen versehen von Dr. Samosch. Braunschweig. 1849. 8vo. S. 53 und S. 62. 672 TABLE OF AUTHORS REFERRED TO. 27. G. SIMON, Ueber die Menge der ausgeathmeten Luft bei verschiedenen Menschen und ihre Messung durch das Spirometer. Mit einem Vorwort von J. Vogel. Giessen. 1848. 8vo. S. 14. 28. LEHRBUCH Bd. I. S. 852. 29. BARRAL in the Annales de Chimie et Physique. Troisieme Serie. Tome XXV. Paris. 1849. 8vo. P. 163. 30. FRERICHS in R. Wagner's Handwoerterbuch der Physiologie. Bd. III. Abth. I. S. 463-468. 31. N. JACUBOWITSCH, De saliva. Dorpati. 1848. 8vo. P. 10. 32. J. GERLACH, Handbuch der allgemeinen und speciellen Geweblehre des menschlichen Koerpers. Fuer Aerzte und Studirende. Mainz. 1849. 8vo. S. 282 et seq. 33. F. STACKMANN, Quaestiones de bilis copia accuratius definienda. Dorpati. 1849. 8vo. Pp. 12-45. 34. C. E. LOEBELL, De conditionibns, quibus secretiones in glandulis perficiuntur. Mar- burgi. 1849. 8vo. Pp. 25-31. 35. J. VAN DEEN, DONDERS, UND MOLESCHOTT, Hollaendische Beitraege zu den anato- mischen und physiologischen Wissenschaften. Bd. I. Duesseldorf und Utrecht. 1848. 8vo. S. 369. 36. H. MEYER in Mueller's Archiv. 1849. S. 292-357. 37. BOUSSINGAULT in the Annales de Chimie. 3me Serie. Tome XXIV. Paris. 1848. 8vo. P. 463. 38. FRERICHS in Mueller's Archiv. 1848. S. 478. 39. E. DU BOIS-REYMOND, Untersuchungen ueber thierische Elektricitaet. Bd. II. Abth. I. Berlin. 1849. 8vo. S. 50. 40. LEHRB. Bd. II. Abth. I. S. 242. Fig. 125. 41. SECOND in the Archives generales. Juillet 1849. P. 1 95 et seq., and 311 et seq. 42. LEHRB. Bd. II. Abth. I. S. 394 et seq. 43. LEHRB. Bd. II. Abth. I. S. 414 et seq. 44. E. H. WEBER in R. Wagner's Handwoerterbuch. Bd III. Abth. II. S. 547. 45. ECKHARD in Henle und Pfeuffer's Zeitschrift fuer rationelle Medicin. Bd. VIII. Heidelberg. 1848. 8vo. S. 309. 46. H. STANNIUS, Das peripherische Nervensystem der Fische, anatomisch und physiolo- gisch untersucht. Rostock. 1849. 4to. Taf. IV. Fig. 11. 47. Eine ausfuehrlichere Uebersicht der Organentwickelung, s. Lchrb. Bd. II. Abthei- lung III. S. 90 et seq. 48. Compare Lehrb. Bd. II. Abth. III. S. 121 et seq. 49. J. G. F. WILL, Ueber Milchabsonderung. Erlangen. 1850. 4to. S. 9. et seq. 50. Compare Lehrb. Bd. II. Abth. III. S.]56etseq. 51. Compare Lehrb. Bd. II. Abth. III. 164 et seq. INDEX. Abdominal pregnancy, 642. pressure, during digestion, and vomiting, 131. Aberration, chromatic, 55, 453. Abnormal pregnancy, 643. Absorbents of small intestines, 1 63. Absorption, 156. cutaneous, 162. drinks, 158. fat, 160. gases, 51. of liquids, 41. mechanism of, 157. of narcotic poisons into the blood , 163. solutions, 159. Accessory nerve, spinal, 519. Acetous fermentation, 110. Aeration of the blood, 175, 225. Age, old, 669. Ages, different, average weight of body at,669. Air, pressure of, on the body, 32. residual, 235. Albumen, 118. Albuminous substances, 106. urine, 293. Alcaloids, animal, 109. Aldehyd-ammonia, 281. Allantoin, 109. Allantois, 653. Amnion, 651. Amputation, structure of stumps after, 324. Analysis, elementary, 96. Androgynous state, 610. Angle of vision, 446. Animal food, constituents of, 119. heat, 345. amount of, 347. variations in, 348 . causes of, 352. causes of , 351. causes of the differences in, 353. influence of nervous centre on, 591. membranes, evaporation through, 43. filtration through, 42. porosity of, 157. magnetism. 607. and organic functions, 5. tissues, diamagnetism of, 90. specific gravity of, 1 7. Animals, independence of vital manifestation in, 8. lower, alternating generation in, 614. small, production of heat in, 350, Anus, 136. sphincters of, 137. Aorta, origin of, 174. semilunar valves of, 181. Appendix vermiformis, 135. Aqueous humour of eye, 270. Arrangement of the bones, 23. Arterial blood, mean velocity of, 216. tension of, 191. equality of, in dif- ferent animals, 194. waves, 189. Arteries, capacity of contraction of, 196. closure of, when ligatured, 323. and veins, walls of, 175. Articulate sounds, 424. Artificial hatching, 647. Ashes of food, 335. organic constituents of, 101. Atmosphere, constituents of, 247. Atmospheric pressure, 32. effects of, on serous sacs, 34. on the joints, 34. Atomic weight, 98. Attraction, capillary, 40. organic, 9. Auditory canal, 476. ossicles, 477. muscles of, 478. nerve, 480, 515. sensations, subjective, 483. Aura seminalis, 638. Auricles and ventricles, cardiac, contraction and dilatation of, 176. Axis of vision, 455. Barometer, 30. Beat of heart, 185. excised, 183. quantity of blood expelled by each, 2 16. Benzoic acid, 110. non-azotized, 110. Bezoar stones, 281. Bile, 139, 149,277. action of, 150. X X 674 INDEX. Bile, quantity of, 270. composition of, 281. Bilin,281. Bipolar and unipolar actions, 88. Birth of human foetus, 663. Bisexual generation, 609. Bladder, gall, 132, 277, 286. urinary, 285. influence of nervous cen- tre, on 590. Blood, 15, 304. corpuscles, 15, 304. absorption of narcotic poisons into, 163. circulation of, 173. movement of, 175. renovation of, 175. circulation of, through the heart, 178. curves of, 193. quantity of, 210. expelled by each beat of the heart, 216. velocity of, 218. changes in, after death, 221. of the two circulations, mixture of, 224. aeration of, 225. dependence of secretion on, 265. composition of, 337. changes in, 338. minute changes in, 339. arterial, tension of, 191. equality of, different animals, 194. mean velocity of, 216. venous, current force of, 204. clot, nature of, 305. stains, 306. vessels, walls of, 175. state of, after death, 220. Body, human, physical properties of, 13. combustive heat of, 73. pressure of air on, 32. living, decomposition in, 113. Bone, composition of, 344. development of, 313. Bones, arrangement of, 23. forms of, 395. irregularities of, 396. Brain, 500, 600. and spinal cord, movements of, 586. sensitive parts of, 587. Breathing pressures, 193. Brown's molecular movement, 354. Brunnerian glands, secretion of, 1 48. Buccal glands, 139. Bursae mucosae, 272. Butyric acid, 118. fermentation, 111, 118. Caffein, 116. Calculi, biliary, 281. urinary, 295. Calorific and colorific spectrum, 74. Callus, 322. Camera obscura, 440. Capacity of cardiac cavities, 215. of the chest, changes in, 243. vital, of the lungs, 241. Capillaries, 175, 199. circulation in, 200. immoveable layer in, 202. obstruction of, 203. respiratory, and nutrient, 223. Capillary attraction, 40. repulsion, 41. tubes, transit of liquids through, 39. Capsules, supra-renal, 299. Carbon, hydrates of, 104. Carbonic acid in respired air, 248. proportions of, and of oxygen, 249. absolute quantities of, 252. and oxygen given off by eva- poration, 259. Carburetted hydrogen in intestines, 155. Cardiac cavities, capacity of, 215. valves, 180. action of, during systole and diastole, 181. sounds, 185. a square Cartilage, 313. Caruncula lachrymalis, 273. Casein, 109, 118. Catalepsy, 607. Cells, epidermoid, number of, in inch, 13. Cellulose, 118. Cerebellum, 559. -- section of one, or both crura of, 593. -- removal of, 599. -- supposed influence of, on sexual organs, 599. degeneration of, 599. Cerebral nerves, 500, 511. roots of, 14. Cerebro-spinal fluid, 585. nerves, 500. Cerebrum, 600. division of one crus of, 593. excision of one hemisphere of, 601. complete removal of, 601. structural disease of, 601. effusion of serum into ventricles of, 601. abnormal smallness of, 602. microscopic characters of, 602. Cerumen of the ear, 476. Chalky deposits of gout, 341. Changes in the blood, 339. after death, 221. Chemical composition of organised beings, 93. phenomena of nutrition, 331. Chemistry of digestion, 1 37- Chest, changes in capacity of, 243. Chinese tea, 116. Chitin, 109. INDEX. 675 Chloroform, action of, on the nervous system, 581. inhaling apparatus, 581. stupefaction by, 582. Chocolate, 116. Cholalic) Choleic lacid, 281. Cholic j Cholesterine, 106, 281. Chordse vocales, 422. Chorion, 651. Choroid coat, 441. tensor of, 449. Chromatic aberration, 55, 453. Chyle, 137, 152, 166. formation of, 161. Chyme, 130, 132, 146. Cilia, action of, 358. Ciliary current, 359. velocity of, 359. ligament, 449. movement, 356. in vegetables, 362. Ciliated epithelium in man, distribution of, 357 . Circuit, constant, 81. Circular polarisation, 60. apparatus for, 92. Circulation of the blood, 173. small or pulmonic, 174. great or systemic, 174. of the blood through the heart, 178. influence of respiration on, 194, 222. - in the capillaries, 200. in the veins, 204. portal, 208. placental, 658. Circulations, mixture of the blood of the two, 224. Clonicand tonic spasm, 371. Clot of blood, nature of, 305. Clothes, use of, 71. Cochlea, 480. Cocoa, 116. Coecum, 135. digestion in, 1 52. Coffee, 116. Coition, 639. Colon, 135. sigmoid flexure of, 1 36. Colorific and calorific spectrum, 74. Colors of polarisation in crystals, 62. Colostrum, 667. Combustive heat of human body, 73. Combustion, heat of, 72. of organic bodies, 94. Compass, tangential, 77. Composition of the blood, 337. changes in, 338. of bone, 344. Compressive elasticity of tissues, 25. Compulsory movements, 592. Conduction of electricity, 75. heat, 71. Congenital malformations, 663. Congestion, 316. Constituents of atmosphere, 247. food, 333. animal, 119. vegetable, 118. of the nervous centre, 566. Contactive operations, 103. Contracted muscle, volume of, 378. softening of, 380. change in electric state of, 381. Contractility of the venous parietes, 207. Contraction of arteries, capacity of, 196. elastic, of muscles, 377. of the living nerve, law of, 547. Contractions of galvanic prepared frog, 551. produced by electrical current, 369. : produced by muscles them- selves, 370. Cord, umbilical, 653. Corpora cavernosa, 628. quadrigemina, removal of half of, 593. striata, removal of one or both of, 594. Corpus callosum, section of, 600. luteum, 632. spongiosum, 628. Corpuscles of the blood, 15. moveable seminal, 363. Coughing, 233. Crystalline lens, stratified structure of, 444. Crystals, colors of polarisation in, 62. Current, ciliary, 359. velocity of, 359. electrical, contractions produced by, 369. Currents, electrical, direction of, 80. muscular, 78. of muscle and nerve, 79. of electrical induction, 85. Curve of electrical action, 82. Curves of the blood, 193. Cutaneous absorption, 162. or sweat glands, 264. transpiration, 254. Cystic duct, 132. Cystin, 109. Dal ton theory, 51. Deaf and dumb, 426. Deafness, 483. Death, changes in the blood after, 221. state of bloodvessels after, 220. Decomposition in the living body, 1 1 3. Descendens noni, 520. Development, 608. post-embryonal, 666. Deviation, negative, of the active nerve, 545. Dextrin, 110. Diastole, 179. action of cardiac valves during, 182. Diabetic urine, 291. Diamagnetism of animal tissues, 90. Diffusion of liquids, 44, 48. T- gases, 50. x x 2 C7G INDEX. Digestion, 114. chemistry of, 137. relation of fermentation to, 138. artificial gastric, 143. of flesh and milk, 145. in the caecum, 152. Dilatation of gases by heat, 69. Direction of electrical currents, 80. Dislocation, 397. Distance, estimate of, by the eye, 459. Double refraction, 58. Dreams, 606. Drink, 115. action of, 116. fermented, 116. Drinks, absorption of, 158. Ductus arteriosus, 224. communis choledochus, 1 32. Duodenum, 129, 132. Dynamometer, 412. Dyslysin, 281. Ear, external, 476. cerumen of, 476. internal, 480. middle, 479. small bones of, 477. Egesta and ingesta, 326. Elain, 105. Elastic bodies, vibrations of, 52. contractions of muscles, 377. Elasticity, index of, 22. compressive, of the tissues, 25. of vapors, 67. Electric state of contracted muscle, change in, 381. Electrical currents, direction of, 80. contractions produced by, 369. action, curve of, 82 induction, 85. - nervous actions re- sembling, 560. polarization, 84. rotation of the plane of, 91. irritation of nerve, 545. fishes, 556. Electricity, 74, 369. conduction of, 75. constant circuit of, 81. Electrodes, 84. Electro-magnetic apparatus, 87. Electro-motor properties of nervous mole- cules, 545. Electro-tonic state of nerves, 542. Elementary analysis, 96. Embryo, development of, 646, 655. Empyreumatic combinations, 94. Enarthrodial joints, 397. Endochorion, 653. Endogenous generation, 616. Endolymph, 480. Endosmometer, 46. Endosmotic equivalents, 47. Entoptic figures, 474. Untozoa, generation of, 619. migration of, 622. piglottis, 124, 418. Epithelia, shedding of, 309. Epithelium, ciliated, distribution of, in man, 357. Spizoa, generation of, 618. quivalents, endosmotic, 47. Equipoise, maintenance of, 392. Erection of penis, 628. mechanism of, 629. Ether, action of, on nervous system, 581. stupefaction by, 582. eenanthic, 116. Eustachian tube, 479. Excretions of fasting animals, 340. Expiration, 226. velocity of, 231. channels of, 232. Expired air, quantity of, 240. watery vapor of, 237. Expiratory pressure, 234. Extra-uterine pregnancy, 642. Evaporation, 254. gases of, 255. water lost by, 258. Evaporation, carbonic acid and oxygen given off by, 259. through animal membranes, 43. Eye, 273,428. constituents of, 441. capacity of adjustment of, 448. influence of great sympathetic on, 529. mechanism of, 449. refraction in, 443. Eyes, rolling of, 431. Facial nerve, 514. Falciform folds of the colon, 135. Fallopian tubes, 637. Fasting animals, excretions of, 340. Fat, absorption of, 160. development of, 311. Fatty acids, 105. bodies, 1 05. Feeling, double, 499. Fermentations, 110. relation of, to digestion, 1 38. Fibre, striped and unstriped, 366. unstriped, peculiarities of, 385. Fibres, nervous, effect of division of, 565. of nervous centre, 567. Fibrin, 118. Fibrous tissues, nutrition of, 312. Filter, 42. Filtration, 157. through animal membranes, 42. Fishes, electrical, 556. Fistula lachrymalis, 274. Fission, propagation by, 613. Flesh, digestion of, 145. Flexure, sigmoid, 136. Flight, 411. Flow of liquids, velocity of, 36. INDEX. 677 Fluctuations of the organic functions, 10. Fluids, salivary, 139. Faecal vomiting, 135. Faeces, 135, 152. constituents of, 154. Food, 115. constituents of, 333. quantity of, 120. animal, constituents of, 119. vegetable, constituents of, 118. improper, results of, 331. nitrogen of, 334. ashes of, 335. Foramen ovale, 224. Force, vital, 2. Forms of bones, 395. Formula of organic substances, 99. Fractures, reparation of, 322, Frequency of pulse, 213. Friction, index of, 27. Functions, animal and organic, 5. fluctuations of, 10. vegetable, 5. n periodicity of some, 12. of the senses, 427. Gall-bladder, 132,277,286. Gall-stones, 281. Galvanic prepared frog, contractions of, 551. stimulation of sensitive nerve, 555. Galvanometer, 75, 545. needle, deviation of, 77. tangent, 77. Ganglia, anterior spinal, 509. posterior spinal, 509. of the great sympathetic, 520. structure of, 521. independence or dependence of, 524. multiplication of fibres in, 525. Ganglion corpuscles, 520. processes of, 523. physiological relations of, 565. Gasserian, 523. Gangrene, 319. Gases, respiratory, volume and weight of,244. analysis of, 245. interchange of, in the lungs, 251. of evaporation, 255. given off in a changing atmosphere, 257. absorption of, 51. diffusion of, 50. dilatation of, by heat, 69. intestinal, 1 55. Gasserian ganglion, 523. Gastric digestion, artificial, 143. -glands, 142. juice, 138,142. Gelatin, 108. vegetable, 118. Gelatinous substances, 108. Gemmation, propagation by, 613. Generation, 608. Generation, unisexual and bisexual, 609. in plants, 611. alternating, in the lower ani- mals, 614. endogenous, 616. spontaneous, 616. of rotifera, 618. ' of entozoa, 619. of epizoa, 618. parental, 624. Gland, lachrymal, 272. parotid, 275. thymus, 301. thyroid, 299. Glands, secreting, 15, 264. different forms of, 264. gastric, 1 42. - Haversian, 272. of hair, 269. lenticular, 271. lingual, 275. Meibomian, 269. -of Naboth.,271. Peyer, 271. - salivary, 139, 275. sebaceous, 269. sub-lingual, 275. sub -maxillary, 275. sweat, 264. vascular, 297. Gland-cells, influence of, on secretion, 266. Gland-ducts, vermicular movement of, 267. Glosso-pharyngeal nerve, 490, 515. Glottis, 124, 424. Glucen, 1 09. Glycerin, 105. Goitre, 300. Graefian vesicle, 631. Grape sugar, 110, 118. Grey matter of nervous centre, 566, 568. Growth of hair, 310. Guanin, 109. Gustative regions of tongue, 489. Gymnotus, 556. Haemadynamometer, 31, 191. Hair, 308. growth of, 310. Hatching, artificial, 647. Haversian glands, 272. Hawking, 233. Heart, structure of, 1 73. a forcing and sucking pump, 1 76. contraction and dilatation of auricles and ventricles of, 176. cavities of, 177. circulation of the blood through, 178. systole and diastole of, 179. valves of, 1 80. beat of, 185. when excised, 183. quantity of blood expelled by each, 216. sounds of, 185. influence of great sympathetic on,5'29. 678 INDEX. Heart, cause of the rhythm of, 530. the lymphatic, 531. the lymphatic influence of nervous centre on, 590. movement of, 538. reflex movements of, 575. Hearing, 476. Heat, animal, 345. amount of, 347. variations in, 348. causes of, 351. causes of, 352. causes of the differences in, 353. production of, in small animals, 350. influence of nervous centre on, 591. of combustion, 72. combustive, of human body, 73. conduction of, 71. dilatation of Jiquids and solids by, 64. gases by, 69. latent, of vapours, 65. rays of, 73. specific, 70. Hepatic duct, 132, 277. Hermaphrodism, 610. Hiccough, 233. Hippuric acid, 110,289. Human body, physical properties of, 13. Hunger, 115. Hutchinson's spirometer, 241. Hydrates of carbon, 104. Hydro-carbons, polymeric, 106. Hydrostatic pressure, 29. Hygrocrocis in the intestines, 155. Hypoglossal nerve, 515. Ichor, 319. Ileo-caecal valve, 134. Ileum, 135. Images of convex lenses, 440. Impregnation, 637. Incubation of ovum, 646. Index of elasticity, 22. friction, 27. Induction, electrical, currents of, 85. Inflammation, 316. Inflection of light, 57. Ingesta and egesta, 326. distribution of, 329. course taken by, 342. Inspiration, 226. . velocity of, 231. Innervation, 500. Interference of light, 56. Interstitial pregnancy, 642. .Intestinal gases and mould, 155. mucus, 148. villus, 161. worms, generation of, 619. Intestines, large, 1 35. peristaltic movements of, 135. reflex movements of, 574. small, 133. Intestines, small, absorbents of, 163. peristaltic movements of, 133. Invertebrata, nervous system of, 501. Irradiation, 460. Irregularities of bone, 396. Irritability, conditions of, 374. of muscular fibre, 373. Irritation, nervous, propagation of, 506. Jejunum, 134. Joints, 396. effects of atmospheric pressure on, 34. Kidney, 282. structure of, 283. Kreatin, 109. Kreatinin, 109. Labial glands, 139. Labor, mechanical results of, 415. Labyrinth, 480. Lactic acid, 113, 117, 118. the free acid in the gastric juice, 142. Lactin, 110. Lachrymal canal, 273. duct, 273. fistula, 274. gland, 272. sac, 273. secretion, 272. Larynx, 418. muscles of, 419. action of, 422. Latent heat of vapours, 65. Laughing, 233. Leaping, 410. Lens, crystalline, 312. stratified, structure of, 444. Lenses, convex, 438. images of, 440. refraction in, 436. spherical, 438. Lenticular glands, 271. Levator ani, 137. Leverage of muscles, 406. Levers, action of, 388. Leucin, 109. Lieberkuehn, follicles of, 148, 270. Life, phenomena of, 1. Light, ordinary and polarized rays of, 54. interference of, 56. inflection of, 57. reflection of, 435. refraction of, 436. Ligatured arteries, closure of, 323. Lingual glands, 275. Lingualis, 123. Lippyl, 105. oxide of, 105. elaate, stearate, and margarate of, 105. Liquids, absorption of, 41. diffusion of, 44, 48. INDEX. 679 Liquids, pressure of, 29. transit of, through capillary tubes, 39. velocity of flow of, 36. and solids, dilatation of, by heat, 64. Liquor Morgagni, 312. sanguinis, 304. seminis, 625. Living body, decomposition in, 113. Lobes, electrical, of the torpedo, 558. Locomotion, 354. Long-sightedness, 449. Luminous rays, spherical aberration of, 439. Lungs, interchange of gases in, 25 1 . temperature of, 235. vital capacity of, 241. weight of, 223. Lymph, constituents of, 1 66. movements of, 169. primary, 166. quantity of, 167. uses of, 165. velocity of, 172. Lymphatic heart, 531. system, 168. Lymphatics, course of, 164. valves of, 168. Magnetism, animal, 607. Magneto-electric machine, 89. Malformation, congenital, 663. Mammal, ovum of, 631. Man, general movements of, 407. specific gravity of, 18. tractile force of, 413. Mannite, 118. Manometer, 30, 193. Margarin, 105. Mastication, 122, 138. action of tongue in, 123. Meat broth, 117. lozenges, 117. Mechanical action of workmen, 414. results of labour, 415. Mechanism of respiration, 225. Medulla, nervous, continuity of, 541. oblongata, 597. sensitive parts of, 587. action of, on the heart, 589. on respiratory organs, 596. spinalis, 501. Medullary matter of nervous centre, 566. Meibomian glands, 269, 273. Membranes, animal, nitration through, 42. evaporation through, 43. Menstruation, 632. Mesentery, 1 36. Metamorphosis of matter, influence of nerv- ous centre on, 591. the tissues, 341, 343. Microscope, polarizing, 59, 61. use of, 200. Milk, 117,666. digestion of, 145. Migration of entozoa, 622. Mitral valve, 181. Molecular movements, Brown's, 354. Mortification, 319. Motor oculi nerves, 512. Mould, intestinal, 155. Mouth, mucus of, 138. Movements, general, of man, 407. and sensations, associated, 579. Mucous bursae, 272. fermentation, 118. Mucus, 270. of mouth, 138. stomach, 142. Muscular current, 78. fibre, irritability of, 373. fibres, striped and unstriped, 366. Muscle, contracted, volume of, 378. softening of, 380. change in electric state of, 381. leverage of, 406. Muscles of respiration, 229. contractions produced by, 370. elastic contraction of, 377. force and effective action of, 400. shortening of, 402. force of counterpoise to, 404. Muscae volitantes, 473. Musical notes, 482. Myopia, 449. Naboth, glands of, 271. Nails, 308. Narcotic poisons, absorption of, into the blood, 163. Nasal fossa, 486. Nerve and muscle, current of, 79. regeneration of, 321. electrical irritation of, 545. living, law of contraction of, 547. Nerves, cerebral, 500, 511. spinal, 500. cerebro-spinal, 500. plexus of, 503. pure and mixed, 509. particular stimulation of, 507. influence of, on secretion, 535. nutrition, 536. electro-tonic state of, 542. sensitive, galvanic stimulation 555. velocity of propagation in, 562. reproduced, actions of, 563. Nerve-fibres, course of, 502. division of, 503. large and small, 522. Nervous medulla, continuity of, 541. system, 500. of invertebrata, 501. constituents of, 566. centre, dispositions of, 580. fibres of, 567. grey or cortical matter 566, 568. of, of, 680 INDEX. Nervous centre, influence of, on lymphatic heart, 590. influence of, on metamor- phosis of matter, 591. on urinary bladder, 590. on secretion, nutrition, and animal heat, 591. motor influence of, on viscera, 588. organs of, 595. relations of fibres in, 569. white o'r medullary matter of, 566. - symmetry of, 603. actions, resembling electrical induc- tion, 560. current, negative deviation of, 545. fibres, effect of division of, 565. impressions, peripheric reference to, 604. terminal plexuses, excitement of, 564. Neurilemma, 501. Nitrogen of food, 334. Non-vascular tissues, nutrition of, 308. Nose, 484. Numerical relations of nutrition, 325. Nutrient capillaries of lungs, 223. Nutrition, 303. chemical phenomena of, 331. of fibrous tissues, 312. influence of the nerves on, 536. nervous centre on, 591. of non-vascular tissues, 308. numerical relations of, 325. of the tissues, 307. Obstruction of capillary circulation, 203. (Enanthic ether, 116. (Esophagus, 127. actions of, 127. Ohm's law, 83. Old age, 669. Olfactory nerve, 484, 511. substances, current of, 486. sensations, delicacy of, 487. subjective, 488. Optic nerve, 428, 511. thalamus, division of, 593. Optometer, 447. Ordinary and polarized rays of light, 54. Organic and animal functions, 5. ashes, constituents of, 101. attraction, P. bodies, combustion of, 94. substances, formula of, 99. Organization, 1. Organized beings, chemical composition of, 93. tissues, doubly refractive, 63. Organs, electrical, 557. watery contents of, 16. Otoconia, 480. Ovarian pregnancy, 642. Ovary, 630. Oviducts, 637. Ovum, 609. of the mammal, 631. exit of, from the ovary, 632, 637. incubation of, 646. Oxygen and carbonic acid, proportions of, in respired air, 249. given off by eva- poration, 259. Pacinian corpuscles, 504. Palate, soft, 125. muscles of, 125. glands of, 1 39. action of, in swallowing, 126. Palatine arches, 126. Pancreas, 275. excretory duct of, 133. Pancreatic juice, 138, 148, 277. Paraguay tea, 116. Paralysis, 506. Parotid gland, 139, 275. Parturition, 663. Par vagum, 516. Pectic acid, 118. Pectin, 118; Penis, 627. erection of, 628. mechanism of, 629. Peripheric reference of nervous impressions, 604. Periphery of nervous system, 501. Periodicity of some organic functions, 12. Peristaltic movements of small intestines, 133. Peyer's glands, 271. Pharynx, 124. muscles of, 127. Physical properties of human body, 1 3. the tissues, 16. Pigment, development of, 311 Placenta, 653. Placental circulation, 658. Plants, generation in, 611. Plexus of nerves, 503. terminal nervous, 564. Pneumamometer, 31, 233. Pneumogastric nerve, 515. Polarization, circular, 60. apparatus for, 92. . colors of, in crystal, 62. electrical, 84. * rotation of plane of, 91. Polarized and ordinary rays of light, 54. Polarizing microscope, 59, 61. Polymeric hydro-carbons, 106. Portal blood, 164. circulation, 208. Post-embryonal development, 666. Pregnancy, 642. signs of, 644. extra-uterine, 642. Presbyopia, 449. INDEX. 681 Pressure, abdominal, during digestion and vomiting, 131. of air on the body, 32. atmospheric, effects of, on serous sacs, 34. on joints, 34. of liquids, 29. the two ventricles, 195. Prismatic spectrum, 453. Profiles of respiratory movements, 228. Propagation by gemmation, 613. Properties, physical, of human body, 1 3. tissues, 1 6. Protein, 107. compounds, 106. Ptyaline, 276. Puberty, period of, 668. Puerperal state, 666. Pulmonary artery and veins, 174. : semilunar valves of, 181. Pulmonic circulation, 174. Pulse, 197. velocity of propagation of, 198. frequency of, 213. Punctum lachrymale, 273. Pus, 319. Putrefaction, 111. Putrefactive compounds, 112. Pyloric aperture, 129, 132. valves, 134. Pylorus, 129. Rays of heat, 73. light, ordinary and polarized, 54. Receptaculum chyli, 164. Rectum, 136. movements of, 136. Reflection of light, 435. Reflex movements of voluntary muscles, 571 intestines, 574. heart, 575. muscular contractions, 577. sensations, 578. Refraction, double, 58. of light, 436. in lenses, 437. convex, 438. in the eye, 443. colored, 452. Refractive (doubly) organized tissues, 63. Regeneration of nerve, 321. Relation of fibres in nervous centre, 569. Renal vessels, 283. Renovation of the blood, 175. Reparation of fractures, 322. Reproduced nerves, actions of, 563. Reproduction, 2, 320. Repulsion, capillary, 41. Residual air, 235. Respiration, 69, 222. influence of, on the circulation, 194, 222, mechanism of, 225. muscles of, 229. Respiration, special forms of, 233. varieties of, 227 . velocity of, 231. Respired air, carbonic acid in, 248. and oxygen, pro- perties of, 249. change in bulk of, 241. warmth of, 236. Respiratory gases, analysis of, 245. volume and weight of,244. movements, alternate play of, 226. profiles of, 228. and nutrient capillaries, 223. organs, skeleton of, 229. Retina, 441, 511. Rhythm of the heart, cause of, 530. Ribs and their cartilages, 229, 393. Rigor mortis, 375. Rolling of the eyes, 431. Rotifera, generation of, 618. Running, 410. Rut, 636. Sacs, serous, effects of atmospheric pressure on, 34. Saliva, 138, 275. action of, 140. Salivary fluids, 1 39. glands, 139. Salivation, 277. Sarcina ventriculi, 155. Sarcode, 364. Sea- water, specific gravity of, 18. Sebaceous glands, 269. of the hair, 269. secretion, 269. Secreting glands, 15. Secretion, 262. dependence of, on the blood, 265. influence of gland-cells on, 266. lachrymal, 272. influence of the nerves on, 535. nervous centre on, 591. of skin, 268. Secretions, serous, 269. Semen, 362, 610, 624. Semicircular canals, 480. Semilunar valves of pulmonary artery and aorta, 181. Seminal corpuscles, moveable, 363. emission, 626. filaments, 625. Sensations, reflex, 578. and movements, associated, 579. Senses, functions of, 427. Sensitive parts of the brain, 587. spinal cord, 587. Serous jsacs, influence of atmospheric pres- sure on, 34. secretions, 269. Serum, or liquor sanguinis, 304. Sexual organs, male, 624. female, 630. 682 INDEX. Sexual organs, supposed influence of cere- bellum on, 599. Shadows, 472. colored, 464. Shedding of epithelia, 309. Shortening of the muscles, 402. Shortsightedness, 449. Sighing, 233. Sight, 428. Sigmoid flexure, 136. Singing voice, limits of, 423. Skeleton of respiratory organs, 229. Skin, 492. estimation of temperature by, 497. secretion of, 268. sensibility of, to heat, 498. Sleep, 605. magnetic, 607. Small intestines, absorbents of, 163. peristaltic movements of, 133. valves of, 134. Smell, 484. delicacy of sense of, 487. Sneezing, 233. Snoring, 233. Soaps, 105. Sobbing, 233. Softening of contracted muscles, 380. Solids and liquids, dilatation of, by heat, 64. Solidity, perception of, by the eye, 469. of the tissues, 19. Solutions, absorption of, 159. Sonorous undulations, 416. Sounds, articulate, 425. Spasm, clonic and tonic, 371. Spawning, 538. Specific heat, 70. gravity of the animal tissues, 17. blood, 216. man, 18. sea- water, 18. Spectacles, 450. Spectrum, calorific and colorific, 74. prismatic, 453. Spermatic elements, 362. Spermatozoa, 623. Speech, 424. Spherical aberration of luminous rays, 439. Sphincters of anus, 137. Spinal accessory nerve, 519. cord, 500, 596. cord and brain, movements of, 586. sensitive parts of, 587. ganglia, anterior and posterior, 509. motor, 510. posterior, sensitive, 509. nerves, 500. roots of, 509. Spine, 393. Spirometer, Hutchinson's, 241. Spleen, 297. Spontaneous generation, 616. Squinting, 433. Stains of blood, 306. Standing, 408. Starch-granules, 117. Starvation, results of, 330. Stearin, 105. Stereoscope, 470. Sternum, 229, 393. Stomach, movements of in swallowing, 129. mucus of, 142. Striped and unstriped muscular fibres, 366. Structure of the heart, 1 73. Strychnine, influence of, on the nervous sys- tem, 580. Stumps, structure of, after amputation, 324. Stupefaction by ether or chloroform, 582. Stuttering, 425. Sub-lingual glands, 139, 275. Sub-maxillary glands, 139, 275. Suffocation, 253. Sugar, grape, 110. of milk, 110. Sulphuretted hydrogen in intestines, 155. Suppuration, 319. Supra-renal capsules, 299. Surface, estimate of, by the eye, 459. Swallowing, 126.' Sweat, 268'. Sweat-glands, 264. Swimming, 411. Sympathetic, great, 520,. 527. influence of, on eye and heart, 529. on viscera, 534. Synovia, 271. Systemic circulation, 174. Systole of the heart, 179. action of cardiac valves during, 181. Tactile sensibility of different regions, 493. Tangential compass, 77. Tangent galvanometer, 77. Tartar on teeth, 155. Taste, 488. Taurin, 109, 281. Tea, Chinese, 116. Paraguay, 116. Tears, 272. Teeth, 121, 668. formation of, 315. varieties of, 121. tartar on, 155. Teething, 668. Temperature, estimation of, 496. by the skin, 497. influence of, on vapours, 67. of lungs, 235. variations of, 6*4. Tendons, 398. Tension of arterial blood, 191. equality of in dif- ferent animals, 191. of vapours, 66. Testicles, 625. Thaumatrope, 462. INDEX. 683 Thein, 116. Theobroma cacao, 116. Theobromin, 117. Thermo-electric apparatus, 345. Thermometer, 345. Thirst, 115. Thoracic duct, 164. Thorax, 229, 393. Thymus gland, 301. Thyroid cartilage, 124, 232. gland, 299. Tissues, animal, diamagnetism of, 90. specific gravity of, 17. compressive elasticity of, 25. doubly-refractive organized, 63. metamorphosis of, 341, 343. nutrition of, 307. the non-vascular, 308. the fibrous, 312. physical and chemical properties of, 16. solidity of, 19. Tongue, 123, 488. action of, in mastication, 123. apparatus of the vocal organ, 420. glands of, 139. - gustative regions of, 489. muscles of, 124. Tonic spasms, 371. Tonsils, 125. Torpedo, 556. . . electrical lobes of, 558. Touch, 492. Trachea, 419. Tractile force of man, 413. Transfusion, 338. Transpiration, cutaneous, 254. Tricuspid valve, 181. Trigeminal nerve, 513. Trochlear, 513. Tubal pregnancy, 642. Tubes, capillary, transit of liquids through, 39. influence of, on velocity, 38. Turn of life, 668. Turbinate bones, 484. Tympanum of ear, 477. Tyrosin, 109. Umbilical cord, 653. vesicle, 653. vessels, 653. Undulations, sonorous, 416. Undulatory movements, 53. Unipolar and bipolar actions, 88. Unisexual generation, 609. Unstriped fibre, peculiarities of, 385. and striped muscular fibres, 366. Urachus, 286. Urea, 109, 288. Ureters, 286. Urethra, 286. Urinary calculi, 295. Uric acid, 110, 288. Urine, 282. Urine, albuminous, 293. constituents of, 287. diabetic, 291. suppression of, 296. Uvula, 125. Vaccination, 339. Vagus nerve, 516. Valve, ileo-caecal, 134. mitral, 181. pyloric, 134. tricuspid, 181. Valves, cardiac, 180. of the lymphatics, 168. 'semilunar, 181. Valvulae conniventes, 135. Vapour, influence of temperature on, 67. watery, of expired air, 237. Vapours, elasticity of, 67. latent heat of, 65. tension of, 66. Variations of temperature, 64. Varieties of respiration, 227. Vas deferens, 627. Vascular glands, 297. Vegetable food, constituents of, 118. gelatin, 118. functions, 5. Vegetables, ciliary movement in, 362. Veins and arteries, walls of, 175. circulation in, 204. Velocity of arterial blood, 216. the blood, 218. of circulation in capillaries, 201. flow of liquids, 36. influence of tubes on, 38. of inspiration and expiration, 231. lymph, 172. of propagation in nerves, 562. - of the pulse, 198. Venae cavae, 174. Venous parietes, contractility of, 207. Ventricles of brain, effusion of serum into, 601. heart, contraction and dilata- tion of, 176. pressure of, 195. Ventricular cords, 422. Ventriloquism, 425. Vermicular contraction of intestines, 133. movement of gland-ducts, 267. Vermiform appendix, 135. perforation of, ] 35. Vertebral column, 393. Vesicle, umbilical, 653. Vesicles, seminal, 626. Vestibule, 480. Vibrations of elastic bodies, 52. Vibriones, 155, 354. Villus, intestinal, 161. Vinous fermentation, 110, 118. Viscera, influence of great sympathetic on,534. motor influence of nervous centre on, 588. 684 INDEX. Vision, adjustment of, 432. angle of, 446. -axis of, 455. 'capacity of, 448. of colors, 463. direct and indirect, 455. . erect, 457. -limit of, 446. mechanism of, 449. with both eyes, 467. Visual circle, 465, 467. Vital capacity of lungs, 241. force, 2. manifestations in animals, independ- ence of, 8. Vocal cords, 422. organ, tongue-apparatus of, 420. Voice, 416. singing, limits of, 423. varieties of, 424. Volume of contracted muscle, 378. and weight of respiratory gases, 244. Voluntary muscles, reflex movements of, 571. Vomiting, 132. faecal, 135. Vomiting, and digestion, abdominal pressure, during, 131. Walking, 409. Walls of arteries and veins, 175. Warmth of respired air, 236. Water lost by evaporation, 258. Watery contents of organs, 16. vapour and water of expired air, 237. Waves, arterial, 189. Weeping, 233. Weight, average, of body, at different ages, 669. estimation of, 495. of lungs, 223. and volume of respiratory gases, 244. Wen, 300. Workmen, effective mechanical actions of, 414. Xanthin, 109. Yawning, 233. Yeast plant in intestines, 1 55. END. Woodfall and Kinder, Printers, Angel Court, Skinner Street. RENSHAWS PTTTJT.Tr!ArTiTnivr ta'o 14 DAY USE HE gra RETURN TO DESK FROM WHICH BORROWED 2 MOOT LIBRARY By This book is due on the last date stamped below, or ON EE on I* 16 date to which renewed. Bu Renewed books are subject to immediate recall. " B? JUN 2 1966 A Die: Sai ON TH] Ev clo t-v MEMOR ta= late STNOPS, .. M - : AY201 GRAY s EL SEP 2 7 1968 Gr< &%L ^ T>) x? LECTUR tQ/^J V *^(^/f N TH! I \B^^ ON RH *H 1 1**- By \J&f^H HOOPEF M.; .^ ON THI TI x.0 7) A HIST C.V R.: J ON INF r / ri SEP 18 1968 I Wo LECTUR MA Priv LAENNEC Transla. OUTLINES OF hP D 21-40m-5,'65 cloth. Pri ^43088lO)476 General Library University of California Berkeley MR. RENSHAW'S PUBLICATIONS. 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