TEXT-BOOK OF Normal Histology: # INCLUDING AN ACCOUNT OF THE DEVELOPMENT OF THE TISSUES AND OF THE ORGANS. BY GEORGE A. PIERSOL, M.D., PROFESSOR OF ANATOMY IN THE UNIVERSITY OF PENNSYLVANIA. WITH FOUR HUNDRED AND NINE ILLUSTRA TIONS, OF WHICH THREE HUNDRED AND FIFTY-EIGHT ARE FROM ORIGINAL DRAWINGS BY THE AUTHOR. FOURTH EDITION. PHILADELPHIA: J. B. LIPPINCOTT COMPANY, i 896. Copyright, 1893, BY J. B. Lippincott Company. Printed by J. B. Lippincott Company Philadelphia PREFACE TO FOURTH EDITION. The favorable reception accorded the “Histology” has necessi- tated the printing of a fourth edition so soon after the first appear- ance of the book that but few changes have been made at the present time ; these consist, for the most part, of slight alterations of the text and the illustrations. The opinions expressed by those most capable of passing judgment lead the author to hope that the preparation of the work has not been without gain to both teacher and student. G. A. P. October i, 1895. III PREFACE. In the preparation of these pages the aim of the author has been to present descriptions which should include the salient features of the various structures with sufficient fulness to impress important details without wearying minutiae : many years of teaching have con- vincingly shown that too great conciseness of statement, on the one hand, and too great elaboration of detail, on the other, are alike unsatisfactory to the student in his efforts to gain an adequate and lasting knowledge of minute anatomy. The recognition of the underlying morphological relations of the tissues alone can bring the appreciation of the broad principles requisite for the elevation of histology from a maze of barren details to a study full of interest and suggestion. In order that these wider bearings may become apparent, a brief account of the embryological processes and the histological differentiation concerned in the de- velopment of the tissues and the organs has been added to the descriptions of the adult structures. The desirability of keeping the size and scope of the volume within the limits adapted to its primary purpose of text-book has forbidden the systematic consideration of embryological data, and much of interest relating to the earlier stages of development has been necessarily omitted. In adopting the character of the illustrations choice has been influenced by the reflection that the mission of such drawings is instruction, and that the illustrations best accomplishing that end are of most value for the object at hand. With the exception of those taken from other, duly acknowledged sources, the drawings have been made by the author in nearly all cases with the aid of the camera lucida or from photo-micrographs. While sufficiently dia- grammatic to be efficient aids in the comprehension of the text, the drawings are faithful likenesses of the original preparations; the latter as far as possible have been taken from human tissues. PREFACE. VI For manifest reasons, references and bibliography have been omitted, except in connection with statements where mention of the name of the authority has seemed desirable. The author wishes to express his obligation to the writings of Kolliker, Ranvier, Schwalbe, Waldeyer, Retzius, Stohr, Flemming, O. Hertwig, Schaefer, Golgi, Ram6n y Cajal, and others, as well as to many papers found in the Archiv fiir mikroskopische Anatomie and other journals. University of Pennsylvania, Philadelphia, September 30, 1893. G. A. P. CONTENTS. CHAPTER I. The Cell and the Tissues 11-25 The elementary tissues; The typical cell; Protoplasm—arrangement and structure ; The nucleus and the nucleolus ; The paranucleus; The centrosome and the attraction-spheres; Vital manifestations of the cell ; Direct cell-division ; Indirect cell-division—karyokinesis ; Maturation and fecundation of the ovum ; Segmentation of the ovum ; The tissues—cellular and intercellular constituents ; The blastodermic layers and their derivatives. PAGES The Epithelial Tissues 26-34 Varieties of epithelium ; Squamous epithelium; Columnar epithelium; Modified epithelium; Glandular epithelium ; Neuro-epithelium ; Endothelium ; Development of epithelium; Development of endothelium. CHAPTER II. CHAPTER III. The Connective Tissues 35-57 Forms of connective tissue ; Cellular elements; Intercellular constituents—white fibrous and yellow elastic tissue; Mucoid tissue; Tendon; Elastic tissue ; Development of fibrous and elastic tissue; Adipose tissue; Hyaline cartilage ; Elastic cartilage; Fibro-cartilage; Development of cartilage; Bone—spongy and compact; Structure of compact bone; VII viii CONTENTS. The periosteum; The marrow of bone; Development of bone ; Endochondral bone; Periosteal bone; Summary of bone-development. PAGES The Muscular Tissues 58-68 Non-striated muscle—distribution and structure; Striated muscle-structure; Arrangement of muscle-fibres; Cardiac muscle; Development of muscular tissue. CHAPTER IV. The Nervous Tissues 69-82 Nerve-cells; Nerve-fibres—structure ; Medullated nerve-fibres ; Non-medullated nerve-fibres; Nerve-trunks—structure; Supporting tissues of nerve-centres ; Ganglia—structure; Development of nervous tissues. CHAPTER V. CHAPTER VI. The Peripheral Nerve-Endings 83-93 Terminations of sensory nerves; Special sensory nerve-endings ; Tactile cells; Tactile corpuscles; End-bulbs; Nerve-endings in non-striated muscle ; Nerve-endings in striated muscle ; Nerve-endings in tendon; Nerve-endings in blood-vessels; Nerve-endings in glands; Neuro-epithelium of the sense-organs. CHAPTER VII. The Circulatory System 94-114 The arteries ; The veins; The capillary blood-vessels; The heart; Development of the blood-vessels; Development of the heart; The blood ; The colorless blood-cells; The colored blood-cells; Effect of reagents on human blood ; Blood-crystals; Development of the blood-corpuscles. CONTENTS. IX CHAPTER VIII. The Lymphatic System ii5-t35 The lymphatic spaces; The lymphatic vessels; The lymphatic tissues; Simple lymph-follicles; Compound lymph-glands; The spleen; The thymus body; The serous membranes; The synovial membranes; Development of the lymphatic system ; Development of the spleen ; Development of the thymus body. PAGES CHAPTER IX. Mucous Membranes and Glands 136-143 Mucous membranes—structure; Glands—varieties; Tubular glands; Saccular glands ; Glandular epithelium; Glandular ducts; Mucous glands; Serous glands ; Changes due to functional activity ; Development of glands. CHAPTER X. The Digestive Tract 144-190 The oral cavity ; The teeth ; Development of the teeth ; The tongue; The papillie of the tongue ; The salivary corpuscles ; The tonsils; The pharynx; The oesophagus; The stomach; The glands of the stomach ; The intestines; The intestinal villi; The intestinal glands ; The glands of Lieberkiihn ; The glands of Brunner ; The solitary glands; The agminated glands, or Peyer’s patches ; The liver; The gall-bladder; The accessory digestive glands ; The parotid gland ; X CONTENTS. The submaxillary gland; The sublingual gland ; The pancreas ; Development of the digestive tract; Development of the accessory digestive glands. PAGES CHAPTER XI. The Urinary Organs 191-206 The kidney; The renal sinus and the ureter; The urinary bladder ; The urethra ; Development of the urinary organs. CHAPTER XII. The Male Reproductive Organs 207-223 The testicle; Spermatogenesis; The epididymis ; The semen ; The penis ; The prostate gland; The glands of Cowper. The Female Reproductive Organs 224-245 The ovary ; The ovum; The escape of the ovum ; The parovarium; The paroophoron; The oviduct; The uterus; The vagina; The genitalia; The glands of Bartholin ; The mammary glands; Milk ; Development of the reproductive organs. CHAPTER XIII. CHAPTER XIV. The Respiratory Organs 246-260 The larynx ; The trachea ; The bronchi; The lungs; The pleura ; The thyroid body; Development of the respiratory organs ; Development of the thyroid body. CONTENTS. XI CHAPTER XV. The Skin and its Appendages 261-281 The skin ; The nails; The hair; The sebaceous glands; The sweat-glands; Development of the skin and its appendages. PAGES CHAPTER XVI. The Central Nervous System 282-335 The membranes of brain and cord ; The spinal cord; The medulla; The pons; The crura cerebri; The cerebellum ; The cerebral cortex; The hippocampus major; Tne fascia dentata; The fimbria ; The septum lucidum; The corpus striatum; The optic thalamus; The corpora quadrigemina; The olfactory lobe; The white matter of the cerebrum; The pituitary body; The pineal body; The suprarenal body ; Development of the nervous tissues. The Eye and its Appendages . 336-376 The cornea ; The sclera; The choroid; The ciliary body ; The iris; The irido-corneal angle; The retina; The optic nerve and entrance ; The crystalline lens; The vitreous body; The blood-vessels of the eye; The lymphatics of the eye; The nerves of the eye ; The eyelids ; The Meibomian glands ; The conjunctiva; The lachrymal apparatus ; The capsule of Tenon ; Development of the eye. CHAPTER XVII. xii CONTENTS. The Organ of Hearing 377-401 The external ear; The tympanic membrane; The middle ear; The ear-ossicles; The Eustachian tube; The internal ear; The saccule and the utricle; The semicircular canals ; The cochlea; The ductus cochlearis; The scala vestibuli; The scala tympani; The ductus endolymphaticus; The development of the ear. CHAPTER XVIII. PAGES CHAPTER XIX. The Nasal Mucous Membrane . . 402-406 The respiratory region; The olfactory region ; Development of the nasal fossae. APPENDIX. The Most Useful Histological Methods 407-429 Fixation of the tissue ; Fixation reagents ; Preservation of the tissue; Staining; Staining solutions; Embedding; Interstitial embedding; Paraffin method; Cellcidin method; Section-cutting; Cutting ribbon-series; Fixing sections to the slide ; Mounting sections ; Finishing, labelling, and storing slides ; Outline of standard method ; Weigert’s staining method; Golgi’s silver method; Golgi’s gold method; Silver staining; Staining chromatin filaments; Injecting capillary blood-vessels. Index 431 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Colorless blood-cell n 2. Typical cell—ovum of cat 12 3. Structure of the cell 13 4. Cells exhibiting the paranucleus (Platner) 14 5. Segmenting ova of ascaris megalocephala (Boveri) 14 6. Direct cell-division of colorless blood-corpuscle 15 7. Karyokinesis—diagram of close skein (Rabl- Schiefferdecker) 16 8. Karyokinesis—diagram of loose skein [Rabl-Schiefferdecker) 16 9. Karyokinesis—diagram of polar field {Rabl-Schiefferdecker) 17 10. Karyokinesis—diagram of migration of segments {Rabl) 18 11. Karyokinesis—epidermal cells from larva of newt 19 12. Segmenting ova, showing centrosomes and attraction-spheres {Boveri) . 20 13. Large marrow-cell with multiple nuclei 20 14. Maturation and fecundation of ovum (O. Hertwig) 22 15. Blastodermic layers of rabbit embryo 24 16. Squamous epithelium 28 17. Stratified squamous epithelium in section 28 18. Isolated cells of stratified squamous epithelium 28 19. Prickle-cells from epidermis 29 20. Simple columnar epithelium 29 21. Stratified columnar epithelium 29 22. Ciliated epithelium 30 23. Isolated elements of ciliated epithelium 30 24. Goblet-cells 31 25. Pigmented epithelium 31 26. Glandular epithelium 32 27. Rod-epithelium {Heidenhain and Schiefferdecker) 32 28. Isolated neuro-epithelium 32 29. Endothelium 33 30. Endothelium showing stomata 34 31. Young connective-tissue cell 36 32. Embryonal connective tissue 36 33. Subcutaneous areolar tissue 37 34. Special connective-tissue elements 37 35. Pigmented connective-tissue cells 37 36. Pigment-cell 38 37. Plate-like connective-tissue cells 38 I38. Cell-spaces of dense connective tissue 38 39. Branched connective-tissue cells 38 40. White fibrous tissue 39 41. Elastic fibres isolated {Schiefferdecker) 39 42. Young connective-tissue cells 40 43. Tendon in transverse section 41 xiii XIV LIST OF ILLUSTRATIONS. FIG. PAGE 44. Primary bundles of tendon 41 45. Primary tendon-bundles in section 42 46. Elastic fibres in transverse section 42 47. Elastic fibres forming fenestrated membrane 42 48. Subcutaneous tissue with fat-cells 43 49. Hyaline (costal) cartilage 44 50. Hyaline cartilage with perichondrium 45 51. Elastic cartilage 46 52. Fibro-cartilage 46 53. Transverse section of dried bone 47 54. Longitudinal section of dried bone ... . . 48 55. Lacunae and canaliculi of dried bone 48 56. Bone-cell within lacuna 49 57. Fragments of bone, showing Sharpey’s fibres 49 58. Cells of bone-marrow 50 59. Primary embryonal cartilage 51 60. Developing bone—centre of ossification 52 61. Developing bone—zone of calcification 53 62. Developing bone—trabeculae of endochondral bone 54 63. Developing bone—conversion of osteoblasts into bone-cells 54 64. Developing bone—periosteal and endochondral bone . . ...... 55 65. Developing bone—longitudinal section of embryonal phalanx 55 66. Developing bone—showing Howship’s lacuna 56 67. Isolated involuntary-muscle cells 59 68. Involuntary-muscle cells 59 69. Involuntary muscle in transverse section 60 70. Involuntary muscle in longitudinal section 60 71. Voluntary-muscle fibres 61 72. Voluntary-muscle fibres in section 62 73. Voluntary-muscle fibres 62 74. Diagram of arrangement of contractile substance 64 75. Muscle-fibres, showing Cohnheim’s fields 65 76. Voluntary muscle in transverse section 65 77. Branched fibres of voluntary muscle 65 78. Heart-muscle, showing branched fibres 66 79. Heart-muscle fibres in section 67 80. Injected voluntary muscle 67 81. Developing voluntary muscle 68 82. Nerve-cell from cerebral cortex . 70 83. Nerve-cell isolated from spinal cord 71 84. Nerve-cell of first type 72 85. Nerve-cell of second type 72 86. Basket-work around Purkinje’s cell 72 87. Nerve-cell from sympathetic (Retzius) 73 88. Medullated nerve-fibres ... 74 89. Ultimate fibrillae of axis-cylinder 74 90. Medullated nerve-fibres treated with osmic acid . 75 91. Silvered nerve-fibres 75 92. Non-medullated nerve-fibres 76 93. Section of nerve-trunk 77 94. Section of single funiculus of nerve 78 95. Supporting tissues of nerve-centres 79 96. Longitudinal section of spinal ganglion . 80 97. Section of portion of spinal ganglion 80 98. Ganglion nerve-cell, showing spiral fibre (Schiefferdecker) 81 LIST OF ILLUSTRATIONS. FIG. PAGE 99. Termination of sensory nerve fibres 83 100. Termination of sensory nerve fibres within the epidermis 84 101. Special nerve-endings within the epidermis (Ranvier) 84 102. Tactile corpuscles—simple and compound 85 103. Tactile corpuscle of Meissner (Schiefferdecker) 85 104. Simple spherical end-bulb (Krause) 86 105. Genital corpuscle (Krause) 86 106. Simple cylindrical end-bulbs (Schiefferdecker) 86 107. Corpuscle of Vater, or Pacinian body (Ranvier) 87 108. Herbst’s corpuscle 87 109. Nerves of involuntary muscle 89 no. Nerves of voluntary muscle 90 in. Motor end-plate of voluntary muscle 90 112. Golgi’s corpuscle, or tendon-spindle {Ciaccio) 91 113. Nerve-fibres accompanying a small artery 92 114. Nerves ending in glands 93 115. Section of human artery 94 116. Endothelium of artery of frog 94 117. Cell-spaces of intima of human aorta 95 118. Fenestrated membrane of intima of human aorta 95 119. Muscle-cells from human artery 96 120. Section of aorta of child 96 121. Small arteries and capillary 97 122. Section of human vein 98 123. Capillary blood-vessels 99 124. Section of human heart, showing endocardium 100 125. Section of human heart, including valve 101 126. Section of human heart, including pericardium 102 127. Developing capillary blood-vessels 103 128. Section of developing heart 104 129. Human colorless blood-cells 105 130. Human blood-cells 107 131. Red blood-cells of man and of amphiuma 109 132. Human blood-cells, showing effects of reagents 109 133. Human blood, showing blood-platelets, fibrin, etc no 134. Hsemin crystals from human blood in 135. Lymph-spaces within fibrous tissue in profile 115 136. Lymph-spaces in surface view 116 137. Lymph-capillary 116 138. Lymphatics of silvered diaphragm 116 139. Perivascular lymphatic enclosing an artery 117 140. Section of human thoracic duct 117 141. Elements of adenoid tissue . 118 142. Diffuse adenoid tissue 119 143. Simple lymph-follicle 119 144. Section of lymph-gland 120 145. Section of lymphatic gland, including cortex 120 146. Section of lymphatic gland, including medulla 121 147. Section of lymphatic gland, showing details of structure 121 148. Section of spleen 123 149. Section of spleen, showing trabeculae and reticulum 123 150. Section of large trabecula of spleen 123 151. Section of human spleen cutting a Malpighian corpuscle 124 152. Portion of channel within splenic pulp 125 153. Diagram of relations of splenic vessels to pulp-tissue 126 XV XVI LIST OF ILLUSTRATIONS. FIG. PAGE 154. Section of human thymus body 127 155. Portion of periphery of follicle of thymus body 127 156. Portion of same follicle, showing Hassall’s corpuscles 127 157. Peritoneal endothelium 129 158. Section of peritoneum 130 159. Section of synovial membrane 131 160. Section of ten-day rabbit embryo 133 161. Diagram of typical mucous membrane 136 162. Cells of basement-membrane 137 163. Diagram illustrating form of glands . . 137 164. Tubular glands 138 165. Section of racemose gland 139 166. Section of human parotid gland 139 167. Serous acini of parotid gland 140 168. Mucous acini of sublingual gland 140 169. Section of lingual glands .* 141 170. Serous and mucous acini of glands 141 171. Developing salivary gland 142 172. Section of human oral mucous membrane 144 173. Longitudinal section of molar tooth 146 174. Section of dried tooth, including enamel and dentine 146 175. Interglobular spaces of dentine 147 176. Section of enamel 147 177. Section of dried tooth, including cementum and dentine 148 178. Section of young tooth and pulp 149 179. Section of jaw of rabbit embryo with early dental ridge 149 180. Model of embryonal jaw {Rose) 150 181. Section of jaw of rabbit embryo—dental ridge 150 182. Section of jaw of rabbit embryo—enamel organ 150 183. Section of jaw of cat embryo—dental papilla 151 184. Section of jaw of cat embryo with four developing teeth 151 185. Section of enamel organ of cat embryo 152 186. Section of developing tooth of cat embryo 153 187. Section of human tongue, showing conical papillae . 154 188. Section of human tongue with fungiform papilla 154 189. Section of circumvallate papilla from child’s tongue 155 190. Section of taste-bud from circumvallate papilla 155 191. Salivary corpuscles from human saliva 156 192. Section of tonsil of dog 157 193. Section of tonsil of child 157 194. Section of tonsil of child, showing structural details 158 195. Section of human oesophagus 161 196. Section of human stomach 163 197. Peptic gland from stomach of dog 163 198. Transverse sections of gastric glands of dog 164 199. Portion of peptic gland of dog 164 200. Pyloric glands from human stomach 165 201. Section of pyloric region of human stomach 165 202. Section through pylorus of child’s stomach 165 203. Section of injected stomach of cat 166 204. Nervous plexuses of human stomach {Stohr) 167 205. Longitudinal section of human small intestine 168 206. Tubular glands of large intestine of dog 169 207. Transverse section of follicles of large intestine 169 208. Longitudinal section of villus of dog’s intestine 170 LIST OF ILLUSTRATIONS. xvii FIG. PAGE 209. Transverse section of villus of dog’s intestine 170 210. Longitudinal section of large intestine of child 171 211. Section of duodenum of cat 171 212. Section of human large intestine 172 213. Peyer’s patch from small intestine of cat 172 214. Section of small intestine of child 173 215. Section of injected small intestine of cat 174 216. Section of liver of hog 176 217. Section of human liver 177 218. Diagram of structure of liver 177 219. Section of injected human liver 178 220. Hepatic cells from human liver 178 221. Section of uninjected human liver 178 222. Section of centre of lobule of human liver 179 223. Section of liver of frog 179 224. Section of rabbit’s liver, showing bile-capillaries 180 225. Section of dog’s liver, showing interlobular vessels 180 226. Transverse section of large bile-duct 181 227. Section of human parotid gland 183 228. Acini of human parotid gland 183 229. Section of human sublingual gland 184 230. Section of human pancreas 185 231. Human pancreas, showing area of immature cells 186 232. Section of developing gut of rabbit embryo 187 233. Sagittal section of nine-day rabbit embryo 188 234. Longitudinal section of human kidney (Henle) 191 235. Section of human kidney, showing general arrangement 192 236. Section of partially-injected human kidney 193 237. Diagram of the kidney 194 238. Uriniferous tubules of human kidney 196 239. Section of kidney of amphiuma 197 240. Constituents of medulla of human kidney 197 241. Section of medulla of human kidney 198 242. Section across papilla of human kidney 199 243. Section of injected kidney of dog 200 244. Transverse section of human ureter 201 245. Section of human bladder 202 246. Developing kidney of rabbit embryo 204 247. Developing kidney of rabbit embryo 205 248. Developing kidney of cow embryo 205 249. Diagram illustrating structure of testicle 207 250. Section of human testicle 208 251. Section of human seminiferous tubule 209 252. Spermatogenesis in testicle of dog 209 253. Spermatogenesis in testicle of musk-rat 210 254. Spermatogenesis in testicle of musk-rat 210 255. Spermatogenesis in testicle of musk-rat 211 256. Human testicle, showing interstitial cells 211 257. Section of tubule of human epididymis 212 258. Section through epididymis of child 213 259. Human spermatozoa 215 260. Human spermatozoa, highly magnified 215 261. Section of penis of child 217 262. Erectile tissue of human penis 217 263. Section of human prostate gland 220 xviii LIST OF ILLUSTRATIONS. FIG. PAGE 264. Human prostate gland, showing muscle 221 265. Section of ovary of cat 224 266. Human ovary with Graafian follicle . . . 225 267. Section of cortex of cat’s ovary 226 268. Ovum from ovary of cat 227 269. Section of medulla of human ovary 228 270. Section of corpus luteum of rabbit 229 271. Portion of tubules of parovarium ...... 230 272. Transverse section of human oviduct 231 273. Section of human uterus 232 274. Section of uterine cervix of child 233 275. Active human mammary gland 238 276. Acini of active human mammary gland 239 277. Atrophic human mammary gland 240 278. Human milk and colostrum-corpuscles 242 279. Wolffian bodies and sexual glands of rabbit embryo 242 280. Indifferent sexual gland of rabbit embryo 243 281. Section of developing ovary of kitten 244 282. Diagram illustrating development of sexual organs 245 283. Longitudinal section of larynx of child 247 284. Longitudinal section of epiglottis of child 248 285. Section of trachea and oesophagus of child 249 286. Section of human bronchus 251 287. Diagram of air-passages of lung 251 288. Section of human lung 252 289. Section of silvered lung of kitten 253 290. Section of injected and inflated lung of cat 254 291. Section of human pleura 256 292. Section of thyroid body of child 257 293. Acini of human thyroid body 258 294. Developing pulmonary tube of rabbit embryo 259 295. Developing lungs of rabbit embryo 259 296. Developing thyroid body of rabbit embryo 260 297. Developing lateral thyroid area of rabbit embryo 260 298. Section of human skin 261 299. Epidermis of human skin 262 300. Section of epidermis of human skin 262 301. Section of negro’s skin 264 302. Section of child’s finger, including nail 266 303. Section of human scalp 267 304. Human hair 268 305. Hair-follicle from human scalp 269 306. Transverse sections of hair-follicles 270 307. Section of hair-follicle near its mouth 270 308. Section of hair-follicle, highly magnified 271 309. Section of sebaceous gland from human scalp 273 310. Section of human sweat-gland 275 3x1. Developing skin from human foetus 277 312. Developing skin from foetal kitten 279 313. Developing hair from foetal kitten 279 314. Section of skin of foetal kitten 279 315. Degenerating hair-follicle from human scalp 280 3x6. Section of skin of human foetus 281 317. Brain-membranes of child 282 318. Section of human spinal cord from cervical region 285 LIST OF ILLUSTRATIONS. XIX FIG. PAGE 319. Diagram of fibre-tracts of spinal cord 286 320. White matter of spinal cord 287 321. Section of human spinal cord from thoracic region 288 322. Section of human spinal cord from lumbar region 289 323. Central canal and commissures of spinal cord of calf 289 324. Anterior horn of gray matter of spinal cord of man 290 325. Anterior horn of gray matter of spinal cord of calf 291 326. Diagram of cells and fibres of spinal cord (Lenhossek) 292 327. Neuroglia cells of embryonal spinal cord (_.Lenhossek) 294 328. Section of injected human spinal cord 295 329. Diagram of decussations of medulla (Testut) 296 330. Diagram of decussating tracts of medulla (Testut-Duval) 297 331. Diagram of medulla through olivary bodies {Testut-Duval) 297 332. Diagram of sensory decussation of medulla (Testut-Duval298 333. Diagram of medulla through olivary bodies (Testut-Duval) 298 334. Section of medulla of child 299 335. Section through human pons (Testut-Stilling) 302 336. Section through human cerebral peduncle (Krause) 303 337. Section of human cerebellum 305 338. Diagram of nerve-cells of cerebellum 306 339. Section of human cerebellum 307 340. Section of cerebellar cortex of dog (Relzius) 309 341. Section of human cerebral cortex 312 342. Section of silvered human cerebral cortex 313 343. Nerve-fibres of human cerebral cortex 314 344. Section across cornu Ammonis (Henle) 316 345. Diagram of constituents of cornu Ammonis {K. Schaffer) 317 346. Section across optic thalamus {Schwalbe-Meynert) 320 347. Section across corpora quadrigemina {Quain-Meynert) 322 348. Section of human olfactory bulb {Henle) 324 349. Section of olfactory bulb of rabbit {Retzius) 325 350. Diagram of cerebral association fibres {Schaefer-Meynert) 326 351. Section of human pituitary body 328 352. Section of pineal sense-organ of lizard embryo 329 353. Section of human pineal body 330 354. Corpora amylacea from human brain 330 355. Section of human suprarenal body 331 356. Section of rabbit embryo, showing open neural tube 332 357. Section of rabbit embryo, showing closed neural tube 333 358. Primary wall of neural tube {His) 333 359. Germ-cells and neuroblasts {His) 333 360. Germ-cells and spongioblasts {His) 334 361. Spongioblasts from neural tube {His) 334 362. Section of human cornea 337 363. Fibrous tissue of cornea of ox 338 364. Corneal corpuscles of calf 338 365. Corneal spaces of calf 339 366. Corneal spaces of calf 339 367. Plexus of corneal nerves 340 368. Section of walls of human eyeball 341 369. Section of human choroid 342 370. Human choroid from surface 343 371. Section of human ciliary processes 344 372. Section through ciliary region of human eye 345 373. Section of iris and lens of human eye 347 XX LIST OF ILLUSTRATIONS. FIG> PAGE 374. Injected iris from eye of dog ,43 375. Irido-corneal angle of human eye 350 376. Diagram of retinal elements (Kallius after Ramon y Cajal) 352 377. Section of human retina 333 378. Human retina at macula lutea (Max Schultze) 357 379. Human retina at ora serrata 358 380. Transverse section of human optic nerve 359 381. Section of part of human optic nerve 339 382. Longitudinal section of human optic entrance 360 383. Portions of human crystalline lens 362 384. Fibres of human crystalline lens 362 385. Section through anterior segment of human eye 363 386. Section of human eyelid 368 387. Section of human lachrymal gland 37! 388. Primary optic vesicle of rabbit embryo 373 389. Developing lens and optic cup of rabbit embryo 373 390. Choroidal fissure in developing eye of rabbit embryo 373 391. Developing eye of rabbit embryo 373 392. Section through developing eye of rabbit embryo 376 393. Section of human external auditory canal (Riidinger) 378 394. Human tympanic membrane and malleolus (.Riidinger) 379 395. Section of human Eustachian tube ( Testut) 382 396. Section of membranous labyrinth of cat 384 397. Section of utricle of rabbit, showing otoliths 385 398. Section of semicircular canal of cat 386 399. Membranous semicircular canal of cat 387 400. Longitudinal section of cochlea of guinea-pig 388 401. Section of cochlea of cat 402. Section of Corti’s organ of guinea-pig 392 403. Diagrammatic view of Corti’s organ (Testut) 393 404. Section through auditory pit of rabbit embryo 398 405. Section through otic vesicle of rabbit embryo 398 406. Section through developing ear of rabbit embryo 399 407. Section through developing cochlea of rabbit embryo 400 408. Section of human respiratory nasal mucous membrane 402 409. Section of olfactory mucous membrane of child 403 NORMAL HISTOLOGY. CHAPTER I. THE CELL AND THE TISSUES. Histology, literally, the science of tissues, represents that part of general morphology which treats of the structural elements of organisms, by the various arrangement of which the textures and organs of the body are formed. The term is, evidently, equally applicable to the structural components of plants as well as to those of animals; “histology,” however, is usually accepted as relating especially to animal tissues, “vegetal histology” expressing the extension of the study to the tissues of plants. At first sight apparently complex and numerous, the structures composing the animal economy are really made up of but few elementary tissues ; these latter may be divided into four funda- mental groups: Epithelial tissues; Connective tissues; Muscular tissues; Nervous tissues. Each of these tissues may be further resolved into the compo- nent morphological constituents, the cells and the intercellular substances. All animal cells are the descendants of the embryonal elements derived from the division of the primary parent cell—the ovum; the intercellular substances, on the other hand, are formed through the more or less direct agency of the cells. The animal cell may exist in either the embryonal, matured, or metamorphosed condition. The embryonal cell, as represented by the early generations of the direct offspring of the ovum, or by the lymphoid or colorless blood-cells of the adult, is a small irregularly round or oval mass of finely granular gelatinous substance — the protoplasm or cell-contents —in some part of which a smaller and often indistinct spherical body—the nucleus—lies embedded. In the embryonal condition, Fig. i. Colorless blood-cell exhibiting amoeboid movement. 11 12 when the cell is without a limiting membrane and composed al- most entirely of active living substance, the outlines are frequently changing, these variations in shape being known as amoeboid movements, from their similarity to the changes observed in the outline of an active amoeba, one of the simplest forms of animal life. As the embryonal cell advances in its life-history, the surrounding conditions to which it is subjected induce, with few exceptions, further specialization. Among the earliest of such effects is the condensation of the peripheral zone of the cell, whereby the reten- tion of a definite form is greatly favored; such peripheral condensa- tion may progress to the production of a distinct limiting membrane —the cell-wall. This structure is very frequently wanting; when present, however, it is usually so thin that its optical expression is a single delicate line. The cell-wall is to be regarded as a product of the specialization of a portion of the protoplasm, rather than as an essential part of the cell. The adult cell consists of the protoplasm, or cell-contents, possibly limited by a cell-wall, en- closing a nucleus, which latter, in turn, often con- tains one or more minute spherical bodies, the nu- cleoli. The more or less definite and characteristic forms which the elements of the various tissues possess on reaching their full development, depend largely upon the changes effected by growth and dif- ferentiation in the proto- plasm during the younger condition of the cells. The protoplasm of which the greater part of cells is composed, using the term in its broadest application and as sy- nonymous with cell-contents, usually appears as a finely granular semi-fluid or gelatinous substance, in which darker and coarser granules or other particles of extraneous matters are often embedded. The structure of protoplasm is now recognized as far more com- plicated than was formerly supposed, comprising a highly elastic and extensible portion—the spongioplasm—and an interstitial, NORMAL HISTOLOGY. Fig. 2. Typical cel!,—ovum of cat: a, protoplasm ; b, nucleus ; c, nuclear membrane; d, nucleolus ; e, true cell-wall, closely applied to the surrounding secondary envelope, the zona pellucida. THE CELL AND THE TISSUES. 13 seemingly less active substance—the hyaloplasm. The active contractility which has been generally credited to the spongioplasm has been recently questioned (Schaefer), since the characteristic amoeboid movements of living cells are by some attributed to the changes taking place within the hyaloplasm. The arrangement of these constituents of the protoplasm is vari- able. When they exist closely and uniformly intermingled, the customary finely granular appearance of the cell-contents is produced; not infrequently, however, the spongioplasm is disposed as a more or less well-defined reticulum. In living cells this reticulation is transient, and, to a certain degree, acci- dental, since it often depends upon an unequal distribution of the hyaloplasm induced by the presence of vacuoles or of particles of foreign substance, as se- cretion within glandular epithelium. Chemically, protoplasm consists of various albuminous substances in com- bination with a special nitrogenous pro- teid, plastin, together with water and salts. It is probable that in the albu- minous substances alone the property of contractility resides; the plastin, on the other hand, offers great resistance to those reagents, as acids, gastric juices, or trypsin, which dissolve the albuminates. The amount of plastin present within the fibrils forming the intercellular reticulum is not constant, but subject to considerable variation. In addition to the hyaloplasm, the meshes of the spongioplasm frequently contain particles of foreign substances; the latter may be fatty matters, pigment granules, particles of secretion elaborated within the cell itself, or extraneous material. The nucleus is limited by a distinct wall, the nuclear membrane, and is traversed by a variably elaborate framework of nuclear fibrils, between which lies an interfibrillar, probably semi-fluid, sub- stance, the nuclear matrix. In recognition of the marked affinity for certain dyes possessed by these threads, the substance composing the fibrils is often termed chromatin, while the but slightly staining nuclear matrix is designated achromatin. The nuclear fibrils, how- ever, contain an additional constituent, linin, which is achromatic and holds in place the deeply dyed particles of chromatin. Suspended within the nuclear net-work, lying often in close rela- tion with the fibrils, one or more minute spherical bodies may be seen ; these are the nucleoli, regarding whose true significance, at Fig. 3. Structure of the cell : a, spongio- plasm, arranged as reticulum, hyalo- plasm lies within the latter; b, cell- wall ; c, chromatin filaments, between which lies nuclear matrix ; d, nuclear membrane; e, nucleolus. NORMAL HISTOLOGY. 14 present, little is definitely established. The nucleolus is highly refracting, and, when subjected to appropriate stains, takes on a color differing from both nucleus and protoplasm, suggesting, at least, a distinct chemical condition. This body lies closely approxi-, mated to, but separated from, the nuclear fibrils, being an indepen- dent member of the cell; this fact is especially evident in such ele- ments as ganglionic nerve-cells, or ova, where the nucleolus appears with exceptional distinctness. Its disappearance during the division of the nucleus, and its subsequent reappearance within the newly- formed nuclei, lend weight to the supposition that the nucleolus plays but a subordinate rdle in the life-history of the cell; its true value, however, has yet to be determined. In addition to the parts of the cell generally recognized, recent investigators have described the occasional presence of an irregularly spherical body, lying within the protoplasm in the vicinity of the A, cell from pancreas of salamander: n, nucleus; p, paranucleus. B, sexual cell of leech : n, nucleus; p, paranucleus ; c, centro- some. (After Plainer.) Segmenting ova of ascaris megalocephala : n, nucleus ; a, centrosome, surrounded by attraction-sphere; p, po- lar body. (After Boveri.) nucleus, to which the name accessory nucleus, or paranucleus (.Nebenkern of the Germans), has been applied. According to Plat- ner, the paranucleus is an extrusion of the nucleus, and is subject to great variation in size and appearance; the nature and function of this body are at present still obscure, and need further investigation. Likewise, the presence of a very small, round, highly-refracting body—the centrosome, or pole-corpuscle—has been established in sexual cells, and also in many other elements. The centrosome is itself surrounded by an area named the attraction-sphere. While these bodies have been shown to exist during the condition of rest, it is especially in connection with the changes incident to the division of the nucleus that their most conspicuous features have been ob- served; much, however, remains to be determined regarding these constituents of the cell. THE CELL AND THE TISSUES. 15 THE VITAL MANIFESTATIONS OF THE CELL. The characteristics which distinguish the structural units of living organisms from those of the inorganic world, may be conveniently grouped as—Vegetative, Metabolism, Growth, Reproduction; Ani- mal, Irritability, Motion. Metabolism is that process by which the cell selects and assimi- lates, from the surrounding food-materials, those substances adapted to the particular needs for its nutrition and function, so changing and incorporating into its own substance the materials so acquired that they become an integral part of the cell. By a still further exercise of this process the assimilated materials are converted into new substances, which may be retained within the cell, or, as is frequently the case, given up as the various secretions of the body. Growth, the natural sequence of assimilation, may affect the cell equally in all parts, thereby producing a uniformly enlarged ele- ment; such normal or typical increase is, as a rule, hindered by the impression of neighboring elements, such limitations resulting in many local alterations of form, as conspicuously seen in epithelial tissues. It is, however, the principle of unequal growth that exerts the greatest influence in producing specializations of form, as exam- ples of which the cells of muscle, the crystalline lens, or connective tissue are familiar. Reproduction, the culminating phenomenon of the life-history of the cell, occurs by two modes : a. By direct division—without karyokinesis. b. By indirect division—with karyokinesis. Direct division, by which a cell rich in protoplasm, as the white blood-corpuscle, constricts, cuts off, and sets free a portion of itself, while undoubtedly taking place in the multiplication of the simplest organisms, or of the least dif- ferentiated elements of higher types, is no longer regarded, as formerly, as the most important and usual mode of cell reproduction; the observations of the last decade have shown that its occurrence must be accepted rather as exceptional than as customary. Indirect division, preceded by the complicated cycle of nuclear changes collectively termed karyokinesis, is now recognized as being the usual mode of the reproduction of cells of all kinds, in pathological as well as in normal conditions. The recognition and elucidation of these important phenomena have been largely due to Fig. 6. Direct cell-division of colorless blood-corpuscle. 16 NORMAL HISTOLOGY. the brilliant investigations of Flemming, Strasburger, v. Beneden, Schleicher, Rabl, and others, who, by the employment of improved optical appliances and methods of investigation, have added much to the accurate knowledge of the life-history of the cell. When the cell undergoes a complete and typical mitotic division, the following changes occur: (i) The nucleus becomes larger, and, at the same time, the chro- matin greatly increases, the fibrils becoming con- torted to form a dense convolution, whose twisted threads run generally transverse to the long axis of the nucleus and parallel to the plane of the future cleavage; these fibrils constitute the (2) Close skein, or spirem. The chromatin fibrils further thicken, be- coming less convoluted, and forming irregularly- arranged loops, known as the (3) Loose skein. The question whether these skeins are composed of the contortions of one long fibre, or whether they con- tain several shorter ones, has, as yet, not been defi- nitely determined; observa- tions made on the cells of lower forms, however, ren- der it not improbable that a single thread constitutes the entire convolution. The fibrils of the loose skein now separate at their peripheral turns, so that a number—about twenty- four (Flemming, Rabl)—of distinct loops are formed; the closed ends of these are directed towards a common centre, around which, but removed some little distance, they become arranged. The enclosed clear space is the polar field. During the formation of the skeins the nuclear membrane disappears, its former position being marked for some time longer by a clear zone or halo surround- ing the nucleus and defining the boundary of the latter from the cell-contents. Coincidently with the formation of the loose skein, a very important phenomenon takes place. Within the achromatin Fig. 7. Fig. 8. Close skein,—diagram of nuclear fibrils: A, seen from the side; B, from the polar field, P; C, from anti-pole, GP. (After Rabl- Schiefferdecker.) Loose skein : nuclear spin- dle has appeared in polar field, P. (After Rabl-Schief- ferdecker.') THE CELL AND THE TISSUES. 17 delicate striae make their appearance, so disposed that together they present a double cone, whose apices are directed towards the poles of the future new nuclei, and whose bases are placed centrally and occupy the polar field; these achromatin figures constitute the nu- clear spindle. The chromatin fibrils grow thicker and, at the same time, shorter, and arrange themselves so that the closed ends of the loops encircle the polar field, giving rise, when seen from its surface, to the wreath; seen from the side, however, the loops or V’s appear as radiating fibrils, and constitute the (4) Mother-star, or aster: the apparent differences, therefore, between the wreath and the aster depend upon the point of view, and not upon variations in the arrangement of the fibres. Another very important change is now observed. (5) Each of the loops undergoes longitudinal cleavage, split- ting up into double the number of segments: these are now entirely rearranged, the first step being (6) A rapid separation into two groups, passing towards the poles of the future new nuclei, as indicated by the foci of the nuclear spindle. Around these points as centres, a deli- cate radial marking—the polar striation—ap- pears. The halves of the longitudinally-cleft fibrils are so disposed that one of each pair of sister-segments passes along the guiding lines of the achromatin spindle to each of the groups, thus insuring an accurate and equal division of the original chromatin between the new nuclei. The chromatic segments, becoming further aggregated about the equator of the nuclear spindle in their migration, form a compressed mass, known as the (7) Equatorial plate.* As the newly-grouped fibrils pass outward towards their respective poles, the free ends of the receding segments become united by delicate threads of achromatin—the connecting filaments—which stretch between the corresponding limbs of the separating seg- ments. With the completion of migration the cardinal features of the division of the nucleus have been established, since the subsequent stages are but repetitions, in inverse order, of the changes already instituted. Following the stage of the equatorial plate, the fibrils group themselves about the poles of the spindle and form Fig. 9. Rearrangement and cleavage of V-segments : A , from the side; B. from the polar field, P; GP, anti-pole. (After Rabl- Schieffer decker.) * The term “ equatorial plate” has been employed by some authors to indicate the later phases of the aster stage. 18 NORMAL HISTOLOGY. (8) The daughter-stars, or diaster, each of these corresponding to a new nucleus. About this time the cell-protoplasm, which until now has been almost passive, begins to exhibit a constriction of its body, which impression now steadily progresses until the protoplasm of the cell completely separates into the portions destined to become the bodies of the cells, enveloping the new nuclei. The karyokinetic cycle is completed by each (9) Daughter-wreath or star in turn assuming (10) The stage of the daughter-skeins, at first loose and afterwards close; on obtaining nuclear membranes and the nucleoli reappearing, the new nuclei finally pass into the stage of rest. Fig. 10. Diagram illustrating the migration and redisposition of the segments of chromatin, guided by the achro- matic lines : A, mother-star; B and C, stage of equatorial plate ; D, daughter-stars. (After Rabl.) In recapitulation, the above changes may be tabulated as follows: Resting Mother-Nucleus: the inauguration of the changes leading to division are marked by increase of chromatin, resulting in the formation of 1. The Mother-Skein (Spirem): a. Close skein,— Disappearance of nucleoli. Disappearance of nuclear membrane. b. Loose skein,— Separation of skein into segments. Appearance of polar field. Rearrangement of segments around polar field to form 2. The Mother-Wreath, or Aster: Appearance of nuclear spindle. Longitudinal cleavage of chromatin segments. 3. Migration of Segments (Metakinesis): Segments pass towards the poles of the new nuclei. Equatorial plate produced by massing of migrating seg- ments. THE CELL AND THE TISSUES. 19 Separation of segments into polar groups. Appearance of connecting filaments. 4. Daughter-Wreaths, or Asters : Beginning division of cell-protoplasm. 5. Daughter-Skeins: a. Loose skein. b. Close skein. Completion of new nuclei. Acquisition of nuclear membranes. Reappearance of nucleoli. Completed separation of cell-protoplasm Resting Daughter-Nuclei. Fig. ii. Cells from the epidermis of very young larva of newt: A, resting nucleus ; B, close skein ; C, loose skein ; D and E, mother-stars, seen from the polar field and appearing as the wreath stage ; F, mother- star from the side ; G, migration of segments ; H, daughter-stars ; /and J, segments grouped about new polar fields (in J the protoplasm exhibits constriction); K, daughter-skeins,—division of nucleus complete with slight constriction of cell-body; L, completed division of nucleus and protoplasm. As closely connected with the division of the ovum, and probably, also, with that of many other cells, the behavior of the minute extra- nuclear bodies—the centrosomata (Boveri), or pole-corpuscles (v. Beneden), and their surrounding attraction-spheres — has attracted the attention of recent investigators. The centrosome 20 NORMAL HISTOLOGY. during the resting stage is single, but its multiplication early takes place in the dividing nucleus and anticipates the establishment of the poles of the new nuclei; the apices of the nuclear spindles coincide with the attraction-spheres, which are, probably, potent factors in determining the exact position of the spindles and, consequently, the plane of division. Fission of the nucleus is ordinarily followed by cleavage of the protoplasm, the resulting new cells being entirely distinct elements. A deviation from this usual procedure is, however, some- times encountered where the division of the nucleus has not been followed by cleavage Fig. 12. Fig. 13. Segmenting ova of ascaris megalocephala : A , cell contains nucleus, two centrosomes (c), surrounded by attraction-spheres, and adherent polar body (p); B, beginning polar striation around the centrosomes and attraction-spheres; C, cell viewed from polar field, the striation proceeding from the centrosome ; D, cell seen from the side, apices of nuclear spindle correspond with centrosomes. (After Boveri.) Large marrow-cell : the nu- cleus has undergone repealed division without cleavage of the protoplasm. of the cell-protoplasm, the latter remaining undivided even after the repeated division of the nuclei. Examples of such ‘ ‘ endogenous’ ’ formation are seen in the multinucleated giant marrow-cells. These complicated phenomena can be satisfactorily observed only in suitable preparations and with adequate optical appliances; the dividing-cells of the surface epithelium of very young larval newts (ten to twenty millimetres long) supply admirable views of all stages of karyokinesis. In order to obtain permanent preparations, how- ever, these transient changes must be ‘ ‘ fixed’ ’ by powerful reagents, insuring the instantaneous death of the tissue (see Appendix); otherwise the cycle, which occupies only from two to three hours, and often even less time, will have been completed, and all trace of the figures lost. The command of at least five hundred diameters, with unexceptionable definition, is likewise essential for the careful study of these changes. While most favorably seen in fixed and stained'preparations, the karyokinetic figures may be observed in living cells, thus proving that they in no wise depend upon reagents for their existence. THE CELL AND THE TISSUES. 21 The foregoing vital manifestations, being chiefly concerned in the mere existence and perpetuation of the cell, are appropriately termed vegetative ; irritability and motion, on the contrary, are the ex- pressions of a higher and more individual existence, and hence are called animal. It is to be remarked that the term “animal,” as here employed, must not be regarded as indicating distinctions be- tween plants and animals; for this purpose such manifestations are inadequate, since the elements of certain plants (Mimosese, Dionaea) possess irritability, and the protoplasm of others (Myxomycetes, Volvocineae) exhibits motion in a marked degree. Irritability is that property of living matter by virtue of which external influences are responded to by changes within the cell; these changes may, in turn, induce secondary phenomena. Instances of such impressions are frequent among the lower forms, where surface elements, or, as among the still simpler unicellular protozoa, the pe- ripheral zone of the protoplasm common to the entire animal, exhibit susceptibility to external stimuli. Among the higher animals irri- tability is manifested by nerve-cells, which, through their processes, influence other tissues. Concerning the exact nature of the intimate changes taking place within the cell, the sum of which we call nervous phenomena, little is known; it is probable, however, that the al- buminous constituents of the protoplasm are the particular seat of these obscure molecular changes. Motion, more or less pronounced, is a characteristic of all ani- mal cells—and, likewise, of very many vegetal ones—during some portion of their existence. The development and specialization of the adult cell usually result in limitation of the activity of the protoplasm, by reason both of its decrease and of its intimate relations with the surrounding tissues; the cells exhibiting motion in the adult condition are those which retain, to a certain degree at least, their embryonic type: such are the lymphoid and connective-tissue cells. Motion may be exhibited by elements devoid of, as well as by those provided with, special appendages. The lowest degree of this vital manifestation is encountered in the streaming of the protoplasm within cells, as in plants, enclosed within limiting membranes which do not permit such motion to affect the exterior of the cells. Con- spicuous examples of the more marked effects of protoplasmic streaming are familiar in the changes readily observed in amoebae or in the colorless blood-cells of higher types. In these latter elements, however, the motion is manifested rather in change of form than by marked variation in position. The highest expression of motion is displayed by those cells whose protoplasm has undergone specialization, resulting either in the pro- duction of a peculiar tissue, as that of the voluntary muscle fibre, or 22 NORMAL HISTOLOGY. of external appendages, as the cilia of many unicellular organisms or of the epithelial elements of the higher animals. Since every cell is derived from a pre-existing cell, it follows that all the cells of the organism are the descendants of the parent ele- ment—the ovum. The ripe mammalian egg, while small in com- parison with many other ova, is among the largest histological elements, measuring about .2 millimetre in diameter, and, further, possessing all parts of the typical cell. Before the ovum is capable of uniting with the male sexual element to carry out the changes attendant upon fecundation, it passes through a cycle of preparatory stages collectively known as maturation. These changes consist in the repeated very unequal division of the ovum, resulting in the expulsion of minute portions of its proto- plasm, the polar bodies ; of these latter, usually two are extruded. Fig. 14. Maturation and fecun lation in ova of ascaris megutocephala. : I, n, nucleus of ovum before matura- tion ; s, entering spermatozoon ; //, nucleus («) has passed to periphery of cell preparatory to di- viding ; s, spermatozoon now within the ovum ; III, nucleus dividing into first polar body (/); m, male pronucleus resulting from spermatozoon ; IV, p,p', first and second polar bodies, the last still in process of formation ; m, male pronucleus; V, p, p', polar bodies ; f and m, respectively female and male pronuclei, in contact but not yet fused ; c, centrosomes, indicating poles of nuclear spindle ; VI, pronuclei now fused ; striation proceeds from centrosomes preparatory to division of ovum. (After O. Hertwig.) The nucleus which appears within the ovum after the formation of the polar bodies is the female pronucleus. Upon the completion of these phenomena, maturation has taken place and the ovum is prepared for the reception of the male sexual element. Under THE CELL AND THE TISSUES. 23 favorable conditions the spermatozoa reach the ovum, when a single element penetrates the envelopes of the egg and is received within the protoplasm of the female cell. The entrance of the spermatozoon causes a new disturbance within the ovum, resulting in the formation of the male pronucleus. Subsequently the latter joins with the female pronucleus, the fusion of the two pronuclei being followed by a temporary disappearance of all nucleus within the ovum. Shortly afterwards the new nucleus of segmentation appears, so called from the fact that within this body cleavage of the ovum is first established. The process of segmentation following the fertilization of the ovum is essentially one of indirect cell-division, in which the stages, although modified in certain details, are essentially the same as those already described. The mammalian ovum undergoes a total segmentation; although the resulting segments are, strictly regarded, not quite equal in size, yet, as a matter of simplicity, they may be regarded as such, and the division characterized as total equal segmentation. The repeated cleavage of the segmentation-spheres into which the ovum is divided soon produces a mass of innumerable cells con- stituting the blastoderm; the latter, by continued division and further differentiation, subsequently gives place to a cell-area, in which at first two layers, an outer and an inner, and later a third middle stratum, of cells appear. These more or less imperfectly defined tracts constitute the important primary blastodermic layers, the ectoderm, mesoderm, and entoderm, from which are derived all the tissues of the body. The reader must be referred to the various text-books of embryology for a detailed account of the complicated and often obscure processes of maturation, fertilization, segmentation, and blastulation, of which only the most salient points have been indicated above. THE TISSUES Every tissue is composed of two parts,—the cellular elements and the intercellular substance. Upon the first of these depends the vitality of the tissue, while its physical properties are determined by the character of the second. The physical condition of the inter- cellular substances includes a wide latitude, varying from that of a fluid, as blood or lymph, through all degrees of density, until, by the additional impregnation of calcareous matters, the well-known hardness of bone or dentine is attained. The proportion between the cellular elements and the intercellular substance of mesodermic tissues varies with age and development, the intercellular substance in the early stages being scanty and very NORMAL HISTOLOGY. 24 yielding, while with adolescence they may become tough and re- sistant. Accompanying the growth of the tissue, an increase of the intercellular substance usually takes place through the direct or indirect participation of the cells, these latter, in consequence, suf- fering marked reduction in number and size. The younger the mesodermic tissue, the richer is it in cells and the poorer in intercellu- lar substances; conversely, the older the tissue, the more prominent the intercellular substance and less conspicuous the cellular elements. A marked example of this law is presented by tendon, where, in the embryonic condition, the cells constitute the greater bulk of the tissue, while in the adult the intercellular fibrous tissue so overwhelms the cellular elements that reagents are frequently necessary to satis- factorily demonstrate their existence. While increase of the intercellular substance usually accompanies the growth of the mesodermic tissues, those derived from the ecto- and entoderm present a marked contrast. In these latter tissues the intercellular constituent is represented by the very scanty cement substance, increase in which occurs only as necessitated by the growth of the surrounding cells, the proportion between the two elements being practically constant throughout life. Instances of this constant relation are seen in the varieties and modifications of the epithelial tissues. The primary blastodermic layers—ectoderm, mesoderm, and entoderm—early exhibit histological differences which suffice to distinguish the one from the other, and especially to indicate, at least in a general manner, the tendency of the outer and inner layers to Fig. 15. Blastodermic layers of rabbit embryo: a, ectoderm ; b, entoderm ; c, entodermal cells destined to form notochord; m, mesoderm. form epithelial structures in contrast to the less compact and more reticular formations of the mesoderm. The epithelia of the genito- urinary tract, however, are marked exceptions in their origin, being derived, as well as the connective and muscular tissues, from the mesoderm, in this respect constituting conspicuous specializations. THE CELL AND THE TISSUES. 25 Derivatives of the Primary Blastodermic Layers. From the ectoderm are derived— The epithelium of the outer surface of the body, including that of the conjunctiva and anterior surface of the cornea, the external auditory canal, together with the epithelial append- ages of the skin, as hair, nails, sebaceous and sweat glands (including the involuntary muscle of the latter). The epithelium of the nasal tract, with its glands, as well as of the cavities communicating therewith. The epithelium of the mouth and of the salivary and other glands opening into the oral cavity. The enamel of the teeth. The tissues of the nervous system. The retina; the crystalline lens. The epithelium of the membranous labyrinth. The epithelium of the pituitary and pineal bodies. From the mesoderm are derived— The connective tissues, including areolar tissue, tendon, cartilage, bone, dentine of the teeth. The muscular tissues, with the exception of the muscle of the sweat-glands. The tissues of the vascular and lymphatic systems, including their endothelium and circulating cells. The sexual glands and their excretory passages, as far as the termination of the ejaculatory ducts and vagina. The kidney and ureter .(but not the bladder). From the entoderm are derived— The epithelium of the digestive tract, with that of all glandular appendages except those portions derived from ectodermic origin at the beginning (oral cavity) and termination of the tube. The epithelium of the respiratory tract. The epithelium of the urinary bladder and urethra. The epithelium of the thyroid and thymus bodies, the atrophic primary epithelium of the latter being represented by Hassall’s corpuscles. 26 NORMAL HISTOLOGY. CHAPTER II. THE EPITHELIAL TISSUES. The free surface of the skin and of the various mucous membranes is covered by epithelium, which affords protection to the more delicate parts lying beneath. In this tissue the intercellular con- stituent is reduced to a minimum, being represented alone by the scanty cement-substance between the cells; the latter, in consequence of this relation, form practically an unbroken sheet. The epithelia are best grouped under two chief heads—squamous and columnar. The designation as tessellated or pavement is not distinctive, since either variety may present a mosaic when viewed from the free surface. These tissues may be classified in several divisions as below indicated. VARIETIES OF EPITHELIUM I. Squa?nous. a. Simple—consisting of a single layer—a. Simple. b. Stratified—consisting of several layers—b. Stratified. II. Columnar. III. Modified. a. Ciliated; b. Goblet; c. Pigmented. IV. Specialized. a. Glandular epithelium; b. Neuro-epithelium. The epithelium contains no blood-vessels, the nutrition of the tissue being maintained by the absorption of the nutritive juices conveyed by means of the intercellular clefts within the cement- substance. The nervous supply of epithelium is likewise ordinarily very scanty, the existence of nerve-fibrils within the epithelium in many localities being doubtful; in certain regions possessed of high sensibility, as the corneal or tactile surfaces, the termination of nerve- fibres among the epithelial elements may be regarded as definitely established. The epithelial cells usually rest upon a basement- membrane, or membrana propria, a modification of the subjacent connective tissue of which it is part. The principal distributions of the various forms of epithelium follow. THE EPITHELIAL TISSUES. 27 Simple squamous epithelium occurs in but few places: Partially lining the tympanic cavity, including the mastoid cells; parts of the membranous labyrinth; the infun- dibula and alveoli of the lungs ; the posterior surface of the anterior capsule of the crystalline lens; parts of ■ ducts of glands ; the capsule of the Malpighian body and the descending limb of Henle’s loop in the kidney; choroid plexuses and parts of brain-ventricles. Stratified squamous epithelium occurs widely distributed, cover- ing— The skin and its extensions, as the external auditory canal, conjunctival sac, and cornea; the mouth, lower part of pharynx, and oesophagus; the epiglottis and upper part of larynx, together with the false and true vocal cords; the pelvis of kidney, ureter, bladder, beginning and end of male and entire female urethra; the vagina. Simple columnar epithelium occurs: a. Non-ciliated, in— The digestive tract, from the oesophageal opening of stomach to anus, as well as in the larger ducts of the glands com- municating with this tube; ducts of mammary glands; seminal vesicles and ejaculatory ducts; membranous and penile portions of urethra. b. Ciliated, in— Oviduct, uterus, and part of canal of cervix; greater part of brain-ventricles and canal of spinal cord. Stratified columnar epithelium occurs: a. Non-ciliated, in — Terminal part of the vas deferens; olfactory part of nasal fossae. b. Ciliated, in— The Eustachian tube and parts of tympanic cavity; lachry- mal passages; respiratory part of nasal fossae, with com- municating sinuses; ventricle of larynx, trachea, and branchiae; epididymis and first part of vas deferens. Squamous Epithelium. When occurring as a simple layer, the flattened, polyhedral, nucleated plates form a regular mosaic; such epithelium is found but seldom in the human body, the lining of the air-sacs of the lung, the posterior surface of the anterior capsule of the crystalline lens, the membranous labyrinth, and a few other localities being its principal seats. 28 NORMAL HISTOLOGY. A far more usual arrangement is as several layers, constituting the stratified squamous variety. The isolated cells of such epi- thelium differ greatly in form, size, and appearance according to the layer from which they are taken. The cells com- posing the deepest stratum are not scaly, but irregularly columnar, resting, with slightly expanded bases, upon the sub- jacent membrana propria. The irregular borders of these cells join with neighboring elements in such a manner that minute intercellular clefts are formed; these are occupied by the yielding cement-sub- stance, and allow the passage of the nutrient juices, as well as of the migratory leucocytes, or wandering cells. The nuclei of the columnar elements are oval, and often situated nearer the outer ends of the cells. Passing from the basement-membrane towards the free surface, the form of the cells undergoes a radical change. The pronounced columnar type belongs to the deepest layer alone; the cells next become irregularly polyhedral, Fig- i6- Squamous epithelium from frog’s skin, viewed from the free surface. Fig. 18. Fig. 17. Stratified squamous epithelium in section, from the cornea : the deepest cells are columnar; the superficial are scaly plates. Isolated cells of stratified squamous epithelium : a, surface-cell; 6 and c, cells from middle layers ; d, from deepest stratum. then gradually expand in the direction parallel to the free surface, and become, finally, converted into the large thin scales so characteristic of the outer layers of stratified squamous epithelium. The cells constituting the middle strata are irregularly polyhedral, and not infrequently seem to be mutually connected by means of delicate processes, which bridge the intervening intercellular clefts and establish a direct continuity between the neighboring cells ; when such elements are isolated, the delicate threads are broken and the disassociated cells appear as if beset with minute spines: these con- THE EPITHELIAL TISSUES. 2g stitute the prickle-cells. During the journey to the free surface the character of the protoplasm also alters, the cells losing in vitality and becoming keratose or horny to a greater or less degree. The extent to which these changes occur depends upon the external conditions affecting the tissue: on mucous surfaces kept con- tinually moist by secretions the cells retain their plasticity and nuclei; where, on the contrary, they are exposed to the desiccating influences of the atmosphere, they lose their nuclei and become-dry and horny, as conspicuously seen in the superficial cells of the epidermis. Fatty granules and small oil-drops, sometimes, also, adherent masses of bacteria, are common in the superficial cells. As the young growing cells of the deeper layers increase in size and numbers, they push those of the super- imposed strata towards the free surface, where the older superficial cells become loosened and gradually set free, constituting the physio- logical desquamation continually taking place. In certain localities, as in the urinary bladder, the columnar cells of the deep layer rapidly assume the scaly character of the superficial strata; such epithelium possesses relatively few layers, and, from the facility with which the type of the cells changes, is often described as “transitional.” It is to be remembered that such epithelium constitutes not a distinct variety, but only a modification of the stratified scaly group. Columnar Epithelium. The columnar epithelium, when occur- ring as a single layer of cells, constitutes the simple columnar variety, which, however, enjoys a much wider distribution than the corresponding squamous group. The taller or shorted columnar cells rest upon the membrana propria with their bases, and join their neighbors with more or less accuracy. The free or outer ends of the cells in some localities, as, conspicu- ously, in the intestine, are char- acterized by the presence of a narrow marginal zone, or basal border: this exhibits a vertical striation, which, on the addition of a reagent, as water, often breaks up into a series of rods, resembling very robust cilia. When the single layer of these epithelial cells is replaced by several, as in the stratified columnar Fig. 19. Prickle - cells from middle strata of the epidermis. Fig. 21. Fig. 20. Simple columnar epi- thelium from intestine: the free ends of the cells present a peculiar striated border - zone. Highly magnified. Stratified columnar epi- thelium from vas defer- ens : the deepest layer consists of small cells, between which the co- lumnar cells extend. 30 NORMAL HISTOLOGY. variety, the outermost cells alone are distinctly columnar; these are usually modified at their outer ends, becoming pointed, forked, or club-shaped, in order to fit between the irregularly polyhedral and pyriform elements of the deeper strata. The nucleus is situated about the middle of the columnar surface cells, and somewhat eccentrically nearer the basement-membrane in the deeper cells. The protoplasm of columnar epithelium often contains particles of mucous secretion, indicating the beginning of those changes which result in the produc- tion of the goblet-cells. Modified Epithelium. The free surfaces of the epithelium in particular localities, as noted in detail in the foregoing summary, are armed with minute hair-like processes, or cilia ; these, by their constant active vibration, create a current, which serves to free the mucous membranes from accumulation of mucus and of- fending foreign or irritating sub- stances. Cilia are specializations of the protoplasm, with which they are probably directly and intimately connected ; widely dis- tributed and attached to the various forms of epithelium in the lower animals, in man and the higher mammals cilia are limited to columnar cells. The exact number of individual cilia attached to the free surface of a single cell varies, but there are, probably, between one and two dozen such appendages usually present. Their length likewise differs with locality, those lining the human epididymis being about ten times longer than those of the trachea. When analyzed, by careful observation of favorable cells in not too rapid vibration, the motion will be seen to consist of two parts—a rapid primary move- ment, directed to correspond with the general current, and a slower secondary return to the original position, the free end of the cilium describing a course resembling that of a whip-lash. The vibrations, whose rate has been estimated at about ten per second, do not occur simultaneously in all the cells, but exhibit a progression, one cell after the other taking part in the motions, whereby a series of distinct waves of ciliary motion is produced ; in addition, a certain periodicity or rhythm often characterizes the vibrations. When favorable conditions obtain, including a sufficient supply of moisture, oxygen, and heat, ciliary motion may be maintained for Fig. 22. Fig. 22. Fig. 23. Fig. 23. Ciliated epithelium from trachea : g, a cell filled with mucus about to be discharged. Isolated elements of ciliated columnar epi- thelium from trachea: o, m, i, cells from sur- face, middle, and deep- est strata. THE EPITHELIAL TISSUES. many hours, and even for days; the cells of cold-blooded animals in general continue to vibrate longer than those of mammals. The rapidity of the ciliary motion is readily influenced by tem- perature and reagents. While the application of gentle heat stimu- lates, the motion is temporarily arrested by a reduction to 50 C., and permanently impaired by an elevation above 50° C. Increased motion is at first produced by the addition of weak alkalies or acids, followed, however, by a permanent suspension after the prolonged action of these reagents. Cold, chloroform, etc., on the contrary, effect a prompt reduction and, finally, stoppage of the vibrations. On surfaces clothed with columnar epithelium, certain cells are distinguished by unusually clear protoplasm and exceptional size; these are the goblet-cells, whose peculiar elliptical or chalice form results from the accumulation of mucoid substance elaborated within their protoplasm. When the dis- tention becomes too great, the cell bursts in the direction of least resistance, evidently towards the free surface, and the secretion is poured out on the surface of the mucous membrane. Goblet-cells occur on all surfaces covered by columnar epithelium, but with especial profusion in the large intestine. These elements may be regarded as corresponding to the unicellular glands of the lower animals ; in the large mucous glands, as the mucous acini of the submaxillary and sublingual, the majority of the secreting elements are in a condition similar to that of the goblet-cells. The protoplasm of epithelial cells often becomes invaded by par- ticles of foreign substances; thus, granules of fatty and proteid matters are very commonly encountered, while the presence of granules of eleidin in certain cells of the epidermis characterizes the stratum granulosum. When these invading particles are colored, as when composed of melanin, the pro- toplasm of the affected cell acquires a brown or black tint, and is then known as pigmented epithelium ; such cells are constant in the deeper layers of the epidermis, especially of certain races, and in the outer layer of the retina. Specialized Epithelium. Reference has been made to the goblet-cells as being, tempo- rarily at least, sufficiently specialized to represent unicellular glands; when the elements become permanently modified to engage in the elaboration of secretion they are recognized as glandular epithelium. Fig. 24. Goblet - cells from large intestine con- taining mucous secre- tion. Fig. 25. Pigmented epithelium from outer layer of ret- ina : the nuclei («) still uninvaded. 32 NORMAL HISTOLOGY. The cells lining the ultimate divisions of glands are the modified extensions of the epithelial investment of the adjacent mucous membrane, of which they are the direct outgrowths. Glandular epithelium varies in form from columnar (pancreas) to spherical (parotid) and polyhedral (liver). The protoplasm of such cells is generally more or less filled with particles of secretion, upon whose quantity and arrangement the apparent condition of the protoplasm largely depends. Sometimes the latter is almost entirely displaced by fatty matters, as in the sebaceous glands or in the active mammary acini, or, again, is so encroached upon by particles of secretion that a reticulation of the protoplasm is very conspicuous. The elements lining parts of certain glands exhibit more or less stria- tion, on account of which peculiarity such cells are known as rod-epi- thelium ; examples of this are seen in the ducts of the salivary glands, and in the irregular and, to a less evident degree, the convoluted portions of the uriniferous tubules of the kidney. The epithelial coverings of those areas towards which the terminations of the nerves of special sense are particularly directed undergo high Fig. 26. Fig. 27. Glandular epithelium: small acinus from a serous racemose gland. Rod-epithelium : a, b, c, isolated epithelial cells from uriniferous tubules of rat (after tieidenhain) ; d, rod-epithelium from submax- illary duct of dog. (After Schiefferdecker.) specialization, resulting in the production of perceptive elements, to which, as a group, the name neuro-epithelium has been applied. The rod- and cone-cells of the retina, the hair-cells of Corti’s organ and other parts of the membranous labyrinth, the olfactory cells of the nasal fossae, and the taste-cells of the taste-buds, are all familiar examples of such specialized epithelium. In these elements two parts are present—an inner, containing the nucleus, and corresponding to the usual proto- plasm of the cells, and an outer, peripherally- directed segment, which is highly specialized, and not infrequently terminates in stiff, rigid, hair-like processes. The outer segment re- ceives the stimuli from external impressions, while the inner, centrally-directed, segment stands in close anatomical relation with the nerve-fibres. Fig. 28. Isolated neuro-epithe- lium from nose : o, olfac- tory cells; s, sustentacular elements. THE EPITHELIAL TISSUES. 33 ENDOTHELIUM. Although endothelium is intimately related to the connective tissues, being but modifications of the cells of this group, it is con- venient to describe this tissue in the present place. Endothelium forms a covering of the free surface of those spaces not directly communicating with the external atmosphere, including, therefore, the lining of the various serous cavities, as the pleura, pericardium, and peritoneum (disregarding the communication established through the oviduct), of the synovial surfaces of joints, of the heart and blood-vessels, as well as of the numerous lymphatic spaces and vessels. These cells occur normally as a single layer of thin, irregularly poly- hedral plates of variable size and of great delicacy; they possess an oval, sometimes kidney-shaped, nucleus; they never overlap, and usually unite with neighboring cells by serrated and tortuous lines of cement-substance. The endothelial plates covering the serous membranes are, in general, polyhedral, re- sembling in outline the simple scaly epithelium; those lining the blood-vessels are elon- gated, irregular spindles, while those found in the lymphatic vessels are often still more unsym- metrical, being limited by very tortuous boundaries. For the satisfactory study of endothelium resource to silver staining (see Appendix) must be had, by which method the inter- cellular cement-substance is colored deeply brown or black, appear- ing as dark, frequently-interrupted boundary-lines. In such prepara- tions the points of union common to several cells are often marked by small, deeply-stained areas—the stigmata, or pseudo-stomata. These figures are regarded by some as minute openings filled by silver-stained albuminous substances; according to Klein, however, many of these stigmata are the protruding stained processes of con- nective-tissue cells. In addition to these areas of questionable import are true distinct openings, the stomata, which establish direct com- munication with the adjacent lymphatic channels; the diaphragm. Fig. 29. Endothelium from peritoneal surface of diaphragm, stained with silver: n, nucleus of endothelial plate ; s, one of the intercellular clefts or stigmata. 34 NORMAL HISTOLOGY. and especially the septum separating the peritoneal sac from the abdominal lymph-cavity of the frog, exhibit well these pores. The larger stomata are lined by several small granular guard- cells, whose expansion and contraction largely influence the size of the openings. The development of epithelium is intimately associated with the exten- sions of the great ecto- and entodermic tracts, since, with the exception of the epithe- lium of the greater part of the genito-urinary organs, the epithelia are the direct descendants of the outer and inner embryonic layers. The cells lining the passages con- nected with the sexual glands, as well as the urinary tract as far as the bladder, are derived from those of the Wolffian body and duct, and hence have, with these latter, a common mesoblastic origin. The simple arrangement of the cells in the earlier stages gradually gives place to the more com- plex disposition of the mature tissue. The development of endothelium forms part of the history of the changes taking place within the extensive mesodermic areas; from the specialized sheet, or mesothelium, bounding the primary body-cavity of the young embryo, the endothelium of the pleural, pericardial, and peritoneal cavities directly descends, while the lining cells of the vascular and lymphatic channels trace their origin to the differentiation of certain of the mesodermic elements. Fig. 30. Endothelium from the septum cisternae of frog, stained with silver: a, one of the true stomata, lined with guard-cells; b, intercellular cleft; n, nucleus. THE CONNECTIVE TISSUES. 35 CHAPTER III. THE CONNECTIVE TISSUES. The important group of connective substances—the most widely- distributed of all tissues—is the direct product of the great meso- blastic tract, axial as well as peripheral; the several members of this extended family are formed by the differentiation and specialization of the intercellular substance, wrought through the more or less direct agency of the mesoblastic cells. The variation in the physical characteristics of these substances is due to the condition of the intercellular constituents of the tissues. Taken during the period of embryonal growth, they are represented by a semi-gelatinous, soft, plastic mass; a little later, the still soft, but already definitely formed, growing connective tissue exists, which is soon replaced by the yielding, though strong, adult areolar tissue. Grouped as masses in which the white fibrous tissue predominates, the marked tough- ness of tendon is reached ; or where large quantities of yellow elastic tissue are present, great extensibility is secured. A further conden- sation of the intercellular substance produces the resistance of the matrix of hyaline cartilage, with the intermediate gradations pre- sented by the fibrous and elastic varieties; the ground-substance becoming additionally impregnated with calcareous salts, the well- known hardness of bone or dentine is attained. In all these varia- tions in the density of the intercellular substance the cells have undergone but little change—the connective-tissue corpuscle, the tendon-cell, the cartilage-cell, and the bone-corpuscle being morpho- logically identical. The principal forms in which connective tissue occurs are,— 1. Mucous tissue, as in the jelly of Wharton of the umbilical cord. 2. Growing, immature tissue, as in very young animals or in old embryos. 3. Areolar tissue, as in the subcutaneous and intermuscular tissues. 4. Dense mixed fibrous and elastic tissue, as in the sclera, fasciae, etc. 5. Dense white fibrous tissue, as in tendon, cornea. 6. Dense elastic tissue, as in the ligamenta subflava. 7. Cartilage—fibrous, elastic, and hyaline varieties. 8. Bone. 9. Dentine. 10. The reticulum of adenoid tissue. 36 NORMAL HISTOLOGY. 11. The supporting connective tissue of the nervous system. 12. The supporting and uniting framework of the various organs. 13. Adipose tissue. The cellular elements of the connective tissues are usually de- scribed as of two kinds—the “fixed” or connective-tissue cells proper and the migratory or “wandering” cells. The former, in their typical and unrestrained con- dition, are flattened stellate pro- toplasmic plates, each with a nu- cleus occupying the thicker part of the body of the cell, from which branched processes extend; in some instances the protoplasm extends in several planes as thin, plate-like wings. The nuclei are limited by distinct membranes, and frequently contain well-marked nucleoli. While possessing in its early condition the plate-like form in a greater or less degree, the ordinary connective-tissue cell, owing to its participation in the formation of the intercellular tissue, suffers greatly during the later stages of its history; the expanded cell-body soon gives place to smaller outlines, while the protoplasm diminishes unti the once large element is reduced to the inconspicuous spindle-cells oi adult areolar tissue, in which only thin envelope of protoplasm sur rounds the nucleus. The connective tissue cells, when rich in protoplasm and under favorable conditions, are capable of exhibiting amoeboid move- ments, the variations being, however limited to alterations of form brought about by the extension or retraction of the protoplasmic processes. Associated with the flattened, plate- like elements of connective tissue, in many places are found the highly- vacuolated plasma-cells of Waldeyer. These are of uncertain form often irregular, extended, or spindle, and consist of soft protoplasm which, owing to the numerous vacuoles contained, presents an appear- ance in marked distinction to that of the ordinary branched cell. The plasma-cells probably bear a somewhat constant relation to young tissues in which the formation of new blood-vessels is still progressing. Fig. 31. Connective-tissue cell from young subcu- taneous tissue : w, wing-like expansion seen in profile. Fig. 32. Embryonal connective tissue: the inter- cellular substance is only slightly differen- tiated. THE CONNECTIVE TISSUES. 37 In addition, occasional peculiar granule-cells must be recognized. These elements, entirely distinct constituents of connective tissues, often appear spherical in form, and are distinguished by the con- spicuous granularity of their protoplasm, the granules possessing a strong affinity for eosin and many aniline stains. The granule-cells occur in especial pro- fusion in the vicinity of blood-vessels, and seem to be intimately connected with the formation of adipose tissue. In contrast to these larger connective-tis- sue elements, irregu- larly round or ovoid smaller cells are often present, which, from their ability to change their position as well as form, are termed the wandering cells. These consist of small, nucleated masses of active protoplasm, characteristic of the lymph or colorless blood-cells with which they are identical, usually being really leucocytes which have passed out of the vessels into the surrounding tissues, through which they wander as transient guests. The protoplasm of the fixed cells sometimes exhibits accumula- tions of dark particles, the elements then appearing as the large, irregularly branched pigment-cells, which form con- spicuous ob- jects in the con- nective tissues of many of the lower animals; in man, such cells occur prin- cipally within the choroid and iris, and in certain parts of the pia mater. The pigment-cells vary in shape and size; usually stellate and of mod- erate extent in the higher vertebrates, they assume the most elabo- rate and grotesque forms and reach enormous dimensions within the tissues of the lower animals. Fig. 33. Subcutaneous areolar tissue ; c, c, some of the connective-tissue corpuscles ; w, migratory cells; v, plasma-cell; e, elastic fibres. Fig. 34. Fig. 35. Special connective-tissue elements : p, vacuolated plasma-cells; g, granule- cells. Pigmented connective-tissue cor- puscles from the choroid. 38 NORMAL HISTOLOGY. The immediate vicinity of the blood-vessels is a favorite locality for pigment-cells, their arborescent processes often forming a net- work completely enclosing the vessel. The supporting stroma of various organs of many of the lower animals frequently contains such cells, the liver constantly presenting con- spicuous groups of deeply-pig- mented elements. Pigment- cells are capable of spontaneous movement, the changes in- cluding not only alterations or retractions affecting the pro- cesses—phenomena directly influenced by the action of the light— but likewise decided alterations in position and location of the cells. The granules of the dark-brown pigment are usually regarded as composed of melanin derived from the coloring-matters of the blood; recent investigations, however, render it probable that, while appar- ently the same, the dark pigment found within the various tissues is by no means always identical in composition. The isolated particles when examined with high amplification are but slightly colored, the charac- teristic tint appearing only when the pigment-granules are massed. Whether the colored particles are taken up by the cells as pre-existing pigment-granules, or whether they are produced within the proto- plasm of the cell, is still undecided ; the evidence, however, seems to favor the conclusion that the particles possess an extra-cellular origin. The arrangement of the connective-tissue cells varies with the age and density of the tissue. Where the cells retain the stellate type, a pro- toplasmic net-work extending through- Fig. 36. Pigment-cell from newt’s skin. Fig. 38. Fig. 39. Fig. 37. Plate - like connective- tissue cells found in ten- don. Cell-spaces of dense con- nective tissue in which the cells lie: silvered ground- substance ; from the cornea. Connective - tissue (corneal) corpus- cles : these cells occupy the spaces within the ground-substance. out the tissue is formed by the union of the processes; examples of such disposition are seen in young mucoid tissues, the cornea, and other connective substances rich in cells. Parallel rows of closely- THE CONNECTIVE TISSUES. placed quadrate elements are seen in tendon, while sheets of flattened endothelioid plates characterize basement-membranes and envelop the bundles of fibrous tissue. In the denser structures the cells occupy spaces within the ground-substance ; these spaces usually communicate directly with one another by means of minute channels, or canaliculi, and form a complicated system of “juice-canals” through the entire tissue. Within these tissue-spaces, or lacunce, lie the connective-tissue corpuscles, generally only partially filling the cavities, and being usually especially applied to one wall of the space after the manner of an endothelial covering. These interfascicular clefts within the ground-substance may be regarded as the radicles of the lymphatic system, in some localities, as in the peritoneum, standing in close relation with both the lymphatic and the blood channels. The intercellular or fibrous constituents of connective tissue are of two kinds—white fibrous and yellow elastic tissue. White fibrous tissue ordinarily occurs as wavy bundles of varying thickness, com- posed of silky fibrils of such fineness that, under ordinary amplifica- tion, they present no appreciable thickness ; these bundles sometimes run parallel, as in tendon, but more frequently interlace, forming coarser or finer mesh-works, as seen in the omentum and subcu- taneous tissues. When examined after teasing, the ultimate fibrils of the white fibrous tissue appear as a confused mass of delicate interlacing lines, but in their undisturbed relation they lie parallel, whatever may be the general disposition of the bundles. Fibrous tissue yields gela- tin on boiling in water, and swells up and becomes in- distinct on treat- ment with acetic acid. Yellow elastic tissue, on the con- trary, occurs usually as a net-work of dis- tinct fibres lying among the bundles of the white fibrous tissue. Examined in detail, the elastic fibre appears highly refracting and 39 White fibrous tissue: one end of the bundle has been teased to display the component fibrillse. Elastic fibres isolated ; from the ad- ventitia of the aorta. (After Schief- fer decker.) 40 NORMAL HISTOLOGY. homogeneous, and possesses a definite width throughout its length, although the several fibres forming the same net-work may vary in thickness; not infrequently slight triangular thickenings are found at the points marking the union of several fibrils. Loosened from their attachments, the elastic fibres assume a wavy, bent or coiled condition, highly characteristic. Elastic fibres do not yield gelatin when boiled, but contain elastin, which is probably enclosed within a sheath of great delicacy, but of considerable resistance towards reagents. The most immature, and morphologically the youngest, form of connective tissue is mucous tissue, a typical example of which is found in the jelly of Wharton, in the umbilical cord. Here the stellate cells still retain their embryonal characters, and, by the union of their processes, form a protoplasmic net-work throughout the tissue; the meshes of this net-work are occupied by a semi-gelatinous, indifferent, and but slightly differentiated intercellular substance, containing few fibres and occasional wrandering cells. All gradations of density between the immature mucous and the more resistant areolar tissue are supplied by the various stages of development. Ordinary connective or areolar tissue, as found be- neath the skin and in many other localities, comprises both white fibrous and elastic tissue. The former usually occurs as wavy bundles, which interlace to form a felt-work of varying compactness; it is probable that the bundles are confined by a delicate sheath, strengthened by trans- versely and spirally wrapped fibrils, whose positions are marked as con- strictions, after the treatment of the bundles with acetic acid. The indi- vidual fibrils composing the bundles lie embedded within and held together by a soft homogeneous ground-sub- stance, securely uniting them ; in the denser tissues the ground-substance contains intercommunicating cell- spaces and canaliculi, the surrounding areas appearing as a homo- geneous matrix. The elastic fibres, in varying number and size, form a net-work throughout the tissue. The fixed connective-tissue corpuscles lie embedded among or directly applied to the surface of the bundles of white fibrous tissue, forming, in such cases, an im- perfect wrapping or covering ; within the interfascicular clefts are the wandering cells. The density of the tissue depends largely upon the amount and Fig. 42. Connective-tissue cells from young um- bilical cord: processes of cells unite to form protoplasmic net-work ; fibrous ele- ments slightly developed. THE CONNECTIVE TISSUES. 41 arrangement of the white fibrous element, while its extensibility is determined by the proportion of elastic tissue present. When the former occurs in well-defined bundles, felted together into interlacing lamellae, dense and resistant structures result, as fasciae, the cornea, etc.; in such structures the cement- substance within the interfas- cicular clefts is usually hol- lowed out to form the spaces occupied by the connective- tissue cells and their pro- cesses. Tendon represents a dense connective tissue, composed almost entirely of white fibrous tissue arranged in parallel bundles of varying thickness. The primary bundles, made up of the ultimate fibrillae, are held together to form larger secondary ones, which latter are enveloped in a delicate sheath -covered by endothelial plates; the secondary bundles are bound together and grouped by connective-tissue septa, which are extensions of the thick external sheath wrapping the entire tendon. The larger septa support the blood and lymphatic vessels. The flattened connective-tissue corpuscles, or tendon-cells, occur in rows within the clefts, between the primary bundles, upon and between which the thin, plate- like bodies and wings of the tendon-cells expand. Seen from the surface, these cells appear as quadrate bodies, whose oval nuclei are frequently so disposed that those of two neighboring cells are in close proximity, lying near the ad- jacent ends of the cells, from which arrangement it follows that each pair of nuclei is sep- arated by the greater part of the length of two cells. Viewed in profile, the tendon-cells show as narrow, irregularly rectangular bodies; while when examined in transverse section the same cells appear as stellate bodies, whose extended arms, passing often in several planes, represent the sections of the wing-plates. Each cell Fig. 43. Peripheral part of a tendon in section : a, external fibrous investment sending partitions between the secondary groups (6) of the tendon-fibres; the small stellate figures represent the stained contents of the interfascicular clefts. Fig. 44. Primary bundles of white fibrous Rendon) tissue, on and between which the flattened tendon-cells lie : at j these are seen from the surface; at o and p, oblique and profile views. 42 NORMAL HISTOLOGY. occupies a corresponding space within the cement-substance, just as do the cells of other dense forms of connective substances. Elastic fibres are almost, if not en- tirely, wanting in tendon. Elastic tissue, as usually encountered as an element of areolar tissue, occurs in slender fibres; where, how- ever, the elastic tissue be- comes the dominating con- stituent, as in the ligamentum nuchse or ligamenta subflava of man, the fibres assume much greater size, becoming coarse and of considerable diameter. On transverse sec- tion of such tissue the robust individual elastic fibres appear as irregularly angular or polyhedral areas ; these are of variable size and held together by a small quantity of areolar tissue. The fibres of elastic tissue may become broad and flattened out, and so closely placed that they assume the form of a reticulated elastic membrane, as Heiile's fenestrated membrane of the larger arteries; again, the tissue may assume the form of a continuous elastic sheet, as Des- cemet’s membrane of the cornea. The development of the white fibrous tissue is still a Fig. 45. Primary tendon-bundles in section : b, the tendon- tissue ; s, interfascicular clefts occupied by granular material and the tendon-cells (a) applied to the bundles. Fig. 47. Fig. 46. Elastic fibres in transverse section : from the ligamentum nuchas : a, are- olar tissue separating the groups of the elastic fibres; b, the individual elastic fibres in section. Elastic fibres closely placed, form- ing the fenestrated membrane; from the aorta. subject of much uncertainty. It may be regarded, however, as established that it is through the agency of the cells, indirect although their influence may be, that the fibres of connective tissue originate. Two methods are recognized in the production of the fibres. The doctrine of the direct mode assumes the transformation of the cell THE CONNECTIVE TISSUES. 43 protoplasm into the white fibrillae, the greatly elongated cell-body becoming the fibres. While such conversion does probably occur, it is certain that the indirect mode, whereby the fibres originate within an indifferent matrix, is the more usual; the production of the matrix or ground-substance itself, however, must be attributed to the cellular elements. Regarding the development of the elastic fibres, strong evidence supports the view that the fibres are produced without the direct action of the cells, but result from the fusion of longitudinally-disposed rows of minute particles, which appear within the indifferent intercellular matrix. Adipose tissue must be regarded as a member of the group of connective substances, since the accumulation of oily matters within the protoplasm of connective-tissue cells is responsible for the highly Fig. 48. Fat-cells embedded in subcutaneous areolar tissue : f, fat-cells ; n, nucleus; c, connective-tissue corpuscles; w, migratory cells; e, elastic fibres; b, capillary blood-vessel. characteristic appearance of the tissue. Whether the fat-cells are developed from elements especially set apart for this role, or whether they are but modified ordinary connective-tissue cells, is still a dis- puted point; there are, however, strong reasons for holding the latter view as correct. Examined after the usual preparatory manipulations, and in places where the cells maintain their individual forms, as in the omentum, adipose tissue is seen to be made up of relatively large, clear, oval or spherical sacs. The transparent contents are limited by a delicate envelope, composed of cell-membrane and an extremely thin layer of protoplasm ; on one side of the sac a local accumulation marks the position of the nucleus. Fat-cells occur usually in groups, supported and held together by areolar tissue, through which ramifies a rich, vascular net-work. In localities possessing considerable masses of adipose tissue, as beneath the scalp and the skin, the cells are grouped into lobules, and these 44 NORMAL HISTOLOGY. again into larger masses, or lobes; where aggregated and closely- pressed together, the normal spherical shape of the individual fat- sacs gives way to a polyhedral form. Adipose tissue possesses a rich vascular supply, an arteriole passing to each lobule, there to break up into capillary net-works, which surround the individual sacs. The development of adipose tissue is probably not confined to any particular kind of connective-tissue cell, but may involve any of the corpuscles. The granule-cells, however, seem to bear a close relation to the production of fat-tissue. In those elements about to become fat-cells, a few oil-drops appear within the protoplasm ; these increase in size, coalesce, and gradually encroach upon the cell-contents, pushing the nucleus towards the periphery. This displacement progresses with the increasing volume of the accumulating oil, until, finally, the once slender cell is trans- formed into a distended vesicle, whose protoplasm is expanded to an almost invisible layer immediately beneath the cell-wall, containing, at one side, the flattened and displaced nucleus, which now appears, in profile, as an attenuated crescent. Observations on starved animals show that after the withdrawal and disappearance of the fatty matters, the cells are capable of resuming the usual appearance and properties of connective-tissue corpuscles. CARTILAGE. Cartilage represents a dense connective tissue in which the inter- cellular substance has undergone great condensation. Depending upon the variation in the character of the matrix between the cells, three varieties of cartilage are recognized — hyaline, elastic, and fibrous. Regarded in their re- lationship to the denser connective tissues, the order of enumeration should be reversed, the fibrous variety standing next and differing but little from tendon. Since by ‘ ‘ cartilage’ ’ the typi- cal hyaline variety is usually understood, that form first claims attention. Hyaline cartilage, so named from the transparent, apparently homogeneous character of the intercellular matrix, enjoys a very Fig. 49. Hyaline cartilage from the rib : the cells lie embedded within the lacunse, either singly, in pairs, or in groups; matrix exhibits differentiation around the cell-spaces as more deeply staining areas. THE CONNECTIVE TISSUES. 45 wide distribution, occurring as the articular cartilage of bones, costal cartilages, the larger cartilages of the larynx, trachea, or bronchi, nose, Eustachian tube, etc.; in the embryo the entire skeleton, with the exception of the vault of the cranium, the bones of the face, and the greater part of the lower jaw, is mapped out by primary hyaline cartilage. The homogeneity of the hyaline matrix is only apparent, since, as long ago pointed out by Leidy, the intercellular substance may be resolved into bundles of fibrous connective tissue, which, however, are so closely united and intimately blended by the cementing ground- substance that the presence of the fibres is, ordinarily, not evident. After prolonged boiling, cartilage matrix yields chondrin. Embedded within the hyaline matrix lie the cartilage-cells; these are irregularly oval or angular nucleated protoplasmic bodies, which, during life, almost fill the spaces, or lacunae, which they occupy. In adult tissue usually two or more cells share the same compartment, the original occupant of the space having undergone division, so that two, four, or even more daughter-cells form a single group. The matrix immediately sur- rounding the lacuna is specialized as a layer of different density, thereby as- suming the appearance of a distinct limit- ing membrane, described as the capsule. A further differentiation of the ground- substance is seen in the greater intensity with which the more recently formed matrix enveloping the cells stains; such re- sulting figures constitute the cell-areas. It is to be remembered that the cartilage- cells are but connective-tissue cells, and that the lacunae correspond to the lymph- or cell-spaces found in other dense connective tissues. Since it is usual to find these cell-spaces in communication through minute channels, or canaliculi, their absence and the apparent isolation of the lacunae in cartilage are to be regarded as deviations from the typical arrangement; among some lower forms, however, such a communication exists, the minute canaliculi passing between the neighboring lacunae. The free surface of the cartilage is covered by an envelope of dense connective tissue, the perichondrium ; this consists of an external Fig. 50. Hyaline cartilage with perichon- drium (/)) attached : y, zone of youngest cartilage-cells; m, hyaline matrix enclosing the lacunae contain- ing the cartilage-cells ; /, space from which the cell has been lost. 46 NORMAL HISTOLOGY. or fibrous layer of dense fibro-elastic tissue and an inner, much looser stratum, between the fibres of which are numerous connective- tissue cells. This inner portion is intimately concerned in the pro- duction of new cartilage, and is known as the chondrogenetic layer. The cells of the latter arrange themselves in rows parallel to the surface, and gradually assume the characteristics of the cartilage corpuscles, being at first spindle-shaped, but gradually assuming the more spherical form. The new cells soon become surrounded by the recently-formed matrix, which, at first small in amount, soon in- creases so that the groups of cartilage-cells become separated by more extensive tracts of intercellular substance ; as the nests of cells formed by the division of the original single occupant of the lacuna recede from the perichondrial surface they lose their primary parallel disposition and become irregularly arranged and further separated. Sometimes in those portions most removed from the perichondrium the ground-substance appears granular; this feature is intensified when a deposition of calcareous matter takes place, which not infre- quently happens in old subjects. Elastic cartilage is distinguished by the presence of elastic fibres within the intercellular substance. The typical hyaline matrix is confined to areas of limited extent immedi- ately surrounding the cell-nests, while the in- tervening matrix is penetrated by net- works of elastic fibres extending in all directions. The cells within the lacunae, in the midst of the hy- aline areas, resemble closely the usual ele- ments of hyaline car- tilage. Elastic cartilage has a much less general distribution than the hyaline variety, occur- ring principally in the cartilages of the ex- ternal ear, part of the Eustachian tube, epi- glottis, arytenoid cartilages, cartilages of Wrisbergand of Santorini. This tissue presents an opaque, yellowish tinge in contrast to the Fig. 51. Fig. 52. Elastic cartilage from the epi- glottis: c, cartilage-cells sur- rounded by a very limited area of hyaline matrix (h); the remaining part of the intercellular substance is penetrated by net-works of elastic fibres (e), cross-sections of which appear as minute points. Fibro-cartilage from the knee-joint: c, cartilage- cells surrounded by very limited areas of hyaline matrix (h); the space be- tween these areas is occu- pied by the fibrous tissue, THE CONNECTIVE TISSUES. 47 opalescent, bluish tint of the hyaline variety. It is covered by a perichondrium of the usual description. Fibro-cartilage, as implied by its name, is largely composed of interlacing bundles of fibrous connective tissue, embedded in which the round or oval cartilage-cells lie, singly or in groups, immediately surrounded by a narrow zone of hyaline matrix. The number of the cells and the proportion of fibrous tissue present differ in various specimens. Fibro-cartilage is found in comparatively few localities: around the margin of articular surfaces and within certain joints, the sym- physes and the intervertebral disks, constitute its chief distribution. The tissue is closely akin to tendon, presenting a white, tough, re- sistant but pliable tissue. A proper perichondrium is wanting. The development of cartilage proceeds directly from the ele- ments of the mesoderm. The primary close aggregation of the embryonal cells, which early indicates the position of the future cartilage, subsequently gives way to a looser disposition of the cells, resulting from the appearance of the young matrix. After the formation of the perichondrium, the cartilage grows by the addition of new layers beneath the membrane. Bone is a dense form of connective tissue impregnated with lime salts. Composed of the same histological elements as other compact connective tissues, bone differs from these in having a deposit of calcareous matter within the interfascicular cement-substance, to which peculiarity the well-known hardness of the tissue is due. The microscopical appearance of bone varies with the character of the prepara- tion, especially as to whether the earthy matter has been removed before sectioning, or whether thin plates of dried bone are examined; it is in sections of dried bone that the classical pictures of this tissue are seen. Dependent upon the arrangement of the matrix, two varieties of bone are recog- nized—spongy and compact. Although the spongy bone is, as we shall see, the fundamental form, yet the compact variety alone presents all the structural peculiarities of the tissue. A transverse section of the compact osseous tissue constituting the shaft of one of the long bones presents a number BONE. Fig. 53. Transverse section of dried bone : h, one of the Haversian canals, about which the lamellae are con- centrically disposed, constituting the Haversian systems; g, the ground or interstitial lamellae. 48 NORMAL HISTOLOGY. of round or oval openings—the Haversian canals—each sur- rounded by a broad band or zone composed of concentrically- disposed lamellae; the canal and the surrounding lamellae form an Haversian system. Seen in longitudinal sections, the Haversian canals appear as extended channels, some closely corresponding in their course with the general axis of the bone, while others run obliquely and es- tablish free communication between the adjacent canals. The concentric bone lamellae in such sections appear as parallel bands bordering the large channels. The Haversian canals communicate with the central marrow-cavity, of which they are really continuations; variable in width and length, each canal contains an extension of the bone-marrow, comprising a delicate connective-tissue reticulum, rich in cells, blood-vessels, and lymphatics. The areas between the Haversian systems are filled out by osseous lamellae, disposed without regard to the concentric systems; these are the interstitial or ground lamellae, and represent the older parts of the bone, being the remains of the primary spongy net-work of periosteal bone. The concentric lamellae constituting the Haversian systems are secondarily deposited within the enlarged spaces of the bony reticu- lum. In addition to the lamellae already mentioned, superficial os- seous strata encircle the bone on both its outer and inner (medul- lary) free surfaces; these are the outer and inner circum- ferential or fundamental la- mellae. Between the bundles of the ground - matrix spindle - shaped spaces—the lacunae—are seen, from which minute channels—the canaliculi—radiate in all direc- tions ; these dark, stellate figures with their minute lateral canals form a system ot intercommunicating lymph-spaces within the bone; the canaliculi belonging to the same space or to the adjoining lacunae of the same Haversian system anastomose with one another, but not with the canals of different svstems. Fig. 54. Longitudinal section of dried bone : h, Haversian canals opened lengthwise and bordered by the longitudinally-cut lamellae. Fig. 55. The lacunas and canaliculi of dried bone under high amplification. THE CONNECTIVE TISSUES. 49 In dried bone the spaces are filled with air, the lacunae and cana- liculi consequently appearing dark and sharply defined when viewed by transmitted light. The lacunae, sometimes improperly called “bone-cells,” in dried preparations are empty, or, at most, contain the remains of the soft, protoplasmic bodies, the true bone-cells, which during life partially fill the spaces; these, like the cells of other dense connective tissues, lie within the lymph-spaces of the ground- matrix. In sections of young, well-stained, decalci- fied fresh bone, after the usual manipulations, the bone-corpuscles are seen as nucleated, stellate, protoplasmic bodies, whose processes extend into the canaliculi; in adult and old bones, however, the cells become reduced in size and very inconspicuous. The lacunae being lenticular, they present different figures according to the direction in which they are sectioned: cut transversely, they appear as short, narrow ovals ; opened longitudinally, but not parallel to the lamellae, they are seen as long, narrow, elliptical figures; while when cut longitudinally, and at the same time parallel to the lamellae, they present a broad, oval surface, sometimes almost circular; the canaliculi, extending in all planes, appear much the same in all sections. The periosteum, an envelope of vascular connective tissue, closely invests the outer surface of all bones except the articular facets. This important structure is composed of two portions—an outer, dense, protective, fibrous layer, and an inner, much looser stratum, rich in cells and blood-vessels, which, from its intimate relations to the formation of bone, is known as the osteogenetic layer. This latter contains within its meshes numerous round or spindle cells, many of which later be- come bone-forming ele- ments—the osteoblasts. If a decalcified bone be sectioned parallel to the superficial lamellae, especially if these be of a spongy bone, or if the outer lamellae be forcibly torn off, a number of transverse or perpendicular fibres of Fig. 56. A bone-cell lying within the lacuna of the osseous matrix: decalcified and stained. Fig. 57. Fragments torn from the surface of a decalcified bone : A, surface ; B, oblique view ; s, Sharpey’s perforating fibres ; l, the lacunae. 50 NORMAL HISTOLOGY. more or less delicacy will be exposed; these are the perforating fibres of Sharpey, and represent periosteal fibres which have failed to undergo calcification; of these Kolliker recognizes two kinds— those entirely soft and uncalcified, the most numerous and, at the same time, the smallest; and those partly calcified and of larger size, which, in fact, are bundles of fibrous tissue. Sharpey’s fibres are most numerous in the superficial lamellae of spongy bones, although found in the interstitial lamellae of other bones, pinning together the lamellae which they transfix. The perforating fibres, being derived from the periosteum, never occur in the lamellae of the Haversian systems, since the latter, it will be found, are not directly produced by the periosteum, but as secondary deposits. Additional elements of the bone-matrix are the elastic fibres, which are found in the outer fundamental lamellae, as well as occasion- ally in the deeper interstitial lamellae; these elastic fibres are generally associated with the uncalcified Sharpey’s fibres ; not infrequently the elastic fibres are contained within the uncalcified bundles of fibrous tissue composing the large perforating fibres. Marrow of Bone. The cavities within bones, as well as the elaborate intercommunicating nutrient channels extending through- out the osseous tissue, are filled with the highly vascular marrow, which genetically is an extension of the osteogenetic layer of the periosteum, since the primary marrow is a direct ingrowth and ex- tension of this latter tissue. The marrow of all bones in very young animals is red in color; after a certain time, however, that con- tained within the shafts of the tubular and the spaces of some other bones assumes a lighter tint, finally becoming of a straw color, owing to the accumulation of fat within the marrow-cells. Depend- ing upon this difference, two varieties—the red marrow and the yellow marrow— are recognized: it is to be remembered that the red marrow is genetically the older and represents the primary condition. The elements of the red marrow comprise a delicate connective- tissue reticulum supporting a rich vascular distribution, composed of arterioles breaking up into numerous capillaries, which, in turn, give place to venous radicles of large size and extremely thin walls. The Fig. 58. Elements of the bone-marrow: g, multinucleated giant-cell, or myeloplax ; m, marrow-cells ; n, granule- cell. THE CONNECTIVE TISSUES. 51 meshes of the tissue contain great numbers of soft, plastic connective- tissue elements, the marrow-cells ; many of these, in actively- growing bone, become the osteoblasts. In yellow marrow the majority of the marrow-cells have undergone transformation into fat-cells. Additional huge, irregular, multinucleated, protoplasmic masses are occasionally encountered; these are the giant-cells, or myeloplaxes (Robin), and are of interest as being elements es- pecially concerned in the absorption of osseous tissue, being iden- tical with the osteoclasts (Kolliker). These cells, with their nuclei, offer an example of what formerly was described as the endogenous mode of cell-formation. Dentine is analogous to bone, although differing in details of arrangement, since it is derived from embryonal connective tissue. The matrix becomes calcified, and contains, embedded within the ground-substance, numerous long, parallel, partly-branched tubes, the dentinal tubules. These correspond with the lacunae of bone, enclosing in some places delicate processes, the dentinal fibres. A more extended account of the structure and development of den- tine will be found in connection with the structure of the teeth. Development of Bone. With the exception of the bones of the vault of the cranium, of the face, and of part of the lower jaw, the skeleton is mapped out, in its fcetal condition, by solid cartilages which correspond in form more or less closely with the future bones. The primary embryonal cartilage is of the hyaline variety, being extremely rich in cells, many of which are engaged in division; the cell-groups are separated by a relatively small amount of inter- cellular substance, and the outer surface of these solid cartilages is closely in- vested by an important membrane, the primary periosteum. When bone is formed at the centres of ossification within the cartilage, it is termed endochondral bone; when formed directly from and beneath the periosteum, periosteal bone. While quite complicated in its sequence of changes, it must be remembered that endochondral development results in the formation of structures which are largely temporary, and which finally, for the most part, suffer absorption. The permanent bones of the skeleton are, chiefly, the products of Fig. 59. Primary embryonal cartilage repre- senting one of the carpal bones : ft, perichondrium, or primary periosteum ; n, nutrient canals extending from the periphery. 52 NORMAL HISTOLOGY. the periosteum; where bone is developed directly from the periosteum, and without being mapped out by primary cartilage, the process is spoken of as intermembranous bone-formation, although differ- ing in no important respect from that producing the periosteal bone. Endochondral Bone. The first indications of the future pro- found changes within the solid cartilage correspond in position to the so-called centres of ossification, and consist in an increase in the size of the embryonal cartilage-cells, as well as in the amount of intercellular sub- stance separating the cell-nests, followed by a characteristic rearrangement of the enlarged cells into vertical rows or columns; in the matrix between and around these columnar groups a cal- careous deposit subsequently takes place. These enlarged cartilage-cells, surrounded by the calcified matrix, are the primary areolae of Sharpey. Simultaneously with the changes noted the osteogenetic tissue of the periosteum has increased and sent processes from a number of points into the solid cartilage towards the centre of ossification ; the progress of the periosteal ingrowth is accompanied by the absorption of the cartilage until the focus of central calcification is reached, when the greatly enlarged cartilage-lacunae are opened up and the spaces brought into direct communication with the primary marrow- cavities. The fate of the cartilage-corpuscles has been the sub- ject of discussion; it may be assumed as established that these cells undergo degeneration and play no part in the formation of the new bone. This periosteal ingrowth constitutes the vascularization of the cartilage. The process of breaking down the cartilage-cells and opening up the large lacunae goes rapidly forward, resulting in the extension of the primary marrow-cavity; the primary marrow, filling this latter space, is, as already pointed out, the direct deriva- tive of the inner layer of the periosteum. The primary marrow-cavity, or medullary space, soon becoming of considerable size, is bordered by the zone of calcifying cartilage ; this area includes the columns of flattened cells and enlarged lacunae, which pass into the broken and partly-absorbed larger lacunae, the secondary areolae, opening into the primary marrow-cavity. While the horizontal matrix septa between the transversely ex- panded lacunae disappear, the vertical partitions lying between the Fig. 6o. Developing bone—centre of ossifica- tion in a carpal bone : z, area of enlarged cartilage-lacunae and calcified matrix; c, young cartilage-cells. THE CONNECTIVE TISSUES. 53 columns of the cells suffer much less reduction, and, as a result, remain and project into the marrow-cavity as irregular trabeculae of calcified cartilage. The marrow-cells rapidly multiply and arrange themselves as a layer upon the surface of the cartilage- trabeculae ; now called osteoblasts, they busy themselves in enveloping these with a covering of true osseous tissue. Si- multaneously with the deposition of the bone the calcified cartilage within the trabeculae undergoes absorption, so that the amount of cartilage en- cased by the new bone gradually diminishes and finally disappears, the entire net-work of anas- tomosing trabeculae being now composed of true os- seous tissue. This newly- formed net-work consti- tutes the central primary spongy bone, a structure which, in the shafts of the long bones, is but temporary, after- wards entirely disappear- ing, except at the ends of the bones, where it per- sists as the cancellous tissue of the extremities. It will be noticed that in the changes above described the cartilage is not directly converted into bone, ossification being a process of substitution, the new bone replacing the primary cartilage. Starting near the middle of the long bones, the process of calci- fication and absorption of the cartilage and the formation of the primary spongy bone proceed towards the extremities, the original cartilage gradually disappearing, the loss being made up by incre- ments of new cartilage deposited on the surface beneath the peri- chondrium. Fig. 6i. Developing bone—from the end of a long bone : a, area of rearranging cartilage-cells ; e, area of enlarged lacunae ; c, zone of calcified matrix ; m, primary marrow-spaces contain- ing the osteogenetic tissue ; b, trabeculae of new bone cover- ing the remains (r) of the calcified cartilage-matrix. 54 NORMAL HISTOLOGY. Periosteal Bone. Simultaneously with the formation of the central spongy endochondral bone the cells of the osteogenetic layer of the periosteum are actively engaged in likewise producing osseous tissue, the trabeculae of which unite to form the peripheral net-work of periosteal bone, this in many Fig. 62. Fig. 63. Developing bone—trabecula of endochondral bone : a, the new bone; b, bone-cells; c, still unabsorbed remains of calcified cartilage-matrix. Developing bone—the surface of portion of bone-trabecula, exhibiting the conversion of the osteoblasts into the bone-corpuscles: b, lacuna with young bone-cell; o, osteoblasts arranged on the surface of the newly-formed osseous matrix (m); at l an osteoblast just being isolated. places forming an outer envelope closely embracing the central endo- chondral bone. The details of the process by which the osteoblasts are converted into the bone-cells are the same in both the intracartilaginous and the periosteal formation. The bone-matrix, deposited through the agency of the cells, gradually accumulates around the osteoblast, until this lies completely surrounded by the young matrix, when, after its isolation from the marrow-cavity, it becomes the bone-corpuscle. At first the canaliculi are wanting, as are, also, calcareous matters; these later appear. The conversion of the original spongy into compact bone depends upon the development of additional lamellae within the meshes of the primary osseous net-work. As an initial step, a local absorption takes place, resulting in the enlargement of the pri- mary medullary spaces contained between the trabeculae of the periosteal net-work; these osseous bands are thus reduced to thin bony partitions between large oval cavities, the Haversian spaces. A new growth of bone subsequently takes place within these spaces, THE CONNECTIVE TISSUES. 55 the osteoblasts depositing new bone upon the walls of these cylin- drical cavities, layer upon layer, until only a small central channel— the Haversian canal—remains as the representative of the large Haversian space. The outer boundary of the Haversian system, therefore, corresponds to the limits of the Haversian space, while the remains of the primary bone-trabeculae constitute the older interstitial lamellae of the adult tissue. Osseous tissue, wherever developed, is formed through the agency of the osteoblasts, the deriva- tives and descendants of the special- ized mesoblastic cells of the embryo; whether in endochondral or periosteal formation, the bone-producing elements Fig. 65. Fig. 64. Developing bone—both periosteal and endochondral: /, outer fibrous, o, inner osteogenetic layer of perios- teum ; /, trabeculae of periosteal bone covered by the osteoblasts; e, endochondral bone; m, primary marrow- cavities. Developing bone—longitudinal section of embryonal phalanx : e, the primary cartilage of the extremities of the bone; a, zone of enlarged and vertically-dis- posed cartilage-lacunae ; c, zone of calcifi- cation ; t, trabeculae of calcified cartilage covered with new bone; m, marrow- cavity ; b, periosteal bone formed directly beneath the overlying periosteum,/. arrange themselves over the surfaces of the cartilage-trabeculae or the periosteal fibres respectively, and soon are surrounded by osseous 56 NORMAL HISTOLOGY. matrix ; this gradually thickens and encloses the osteoblasts, which now lie within minute bays or recesses, the entrances to which become gradually contracted, until the opposed edges join and the cells lie within lacunae completely surrounded by the bone-matrix : the osteoblasts have now become the bone- cells. The matrix is deposited as lamellae, especially marked in the bone formed in the later stages of foetal life ; between these are included the lacunae. The matrix is at first soft and possessed of a distinct fibrillated structure in which the subsequent deposit of lime salts—principally the phosphate and carbonate—takes place. When, on the contrary, bone or carti- lage is absorbed, it is through the agency of the giant-cells, the osteoclasts, or chon- droclasts (Klein); these large multinucleated elements usually lie upon the surface of the bone-trabeculae within larger or smaller pits which have been excavated by them ; these are Howship’s lacunae. In recapitulation, the following summary of the phases of de- velopment during the growth of a tubular long bone may be noted : 1. Solid embryonal cartilage. 2. Enlargement and rearrangement of cartilage-cells and lacunae and calcification of matrix at centre of ossification. 3. Penetration of periosteal tissue to the focus of calcification ; vascularization of the cartilage. 4. Formation of medullary spaces by the breaking down of lacunae surrounded by the zone of calcifying cartilage. 5. Covering of the surface of calcified cartilage trabeculae by the layer of osteoblasts and the production of an enveloping sheath of true bone. 6. Resulting central net-work of endochondral bone, with gradual absorption of encased cartilage trabeculae. 7. Absorption of central spongy bone in shaft and formation of central marrow-cavity. 8. Formation, meanwhile, of peripheral periosteal net-work of spongy bone. 9. Conversion into compact bone by partial absorption of tra- beculae to form Haversian spaces; secondary deposit of concentric lamellae within these spaces forming Haversian systems of compact bone. Fig. 66. Developing bone—portion of trabecula undergoing absorp- tion : 6, bone-cells; c, osteo- blasts ; m, bone-matrix; o, multi- nucleated osteoclast lying within the absorption-pit, or Howship’s lacuna. THE CONNECTIVE TISSUES. 57 10. Absorption of inner lamella of compact bone as the shaft increases in diameter by the deposition beneath the periosteum ; production of enlarged medullary cavity. 11. Continued absorption of endochondral central bone until the latter is found alone in the epiphyses, where it continues to be pro- duced at the expense of the intermediate cartilage during the entire future growth of the bone. 58 NORMAL HISTOLOGY. CHAPTER IV. THE MUSCULAR TISSUES. Contractility is possessed, to a certain degree, in common by all cells rich in active protoplasm; the distinguishing characteristic of muscular tissue, however, is that this property is so conspicuously developed in highly specialized structures, and that the contractions take place along definite lines in limited directions alone. Con- tractile tissue or muscle occurs in two principal forms : (1) as the non-striated, smooth, or vegetative muscle, usually beyond the control of the will, and hence called involuntary, and (2) as the striated, striped, or animal muscle, which, being influenced by volition, is known as voluntary. The sharp differences separating the two groups of muscle in man and the higher animals cannot be regarded as fundamental, since in the embryonal condition of these higher forms temporarily, and in the adult form of the lower types permanently, the striped and non- striated varieties of muscle depend upon the degree of specialization rather than upon inherent differences. It is a suggestive fact that long before the cells forming the embryonal heart show indications of differentiation into muscle-tissue the contractions of the organ have commenced. The association of the striped fibres with response to the will and, on the contrary, of the plain tissue with involuntary action must be, likewise, only provisionally accepted, since in some animals the development of marked striae never takes place in the voluntary fibres. Standing between and connecting the extremes of these groups is the cardiac muscle of the higher vertebrates, in which the fibres are striated, although beyond the control of the will. NON-STRIATED OR INVOLUNTARY MUSCLE. Non-striated, smooth, or involuntary muscle, while never occurring in large individual masses, enjoys a wide distribution ; its principal localities are— 1. The Digestive Tract: the muscularis mucosae from oesophagus to anus and the delicate bundles of mucosa and villi; muscular tunic from the lower half of oesophagus to anus. 2. The Accessory Digestive Glands: in the large excretory ducts of liver, pancreas, and some salivary glands ; also in the gall-bladder. 3. The Urinary Tract: in the capsule and the pelvis of kidney, ureter, bladder, and urethra. THE MUSCULAR TISSUES. 59 4. The Male Generative Organs: in epididymis, vas deferens, vesiculae seminales, prostate body, Cowper’s glands, cavernous and spongy bodies of penis. 5. The Female Generative Orga?is: in oviducts, uterus, and vagina ; in the erectile tissue of external genitals ; in broad and round ligaments, and in erectile tissue of nipple. 6. The Respiratory Tract: in the posterior part of trachea ; encircling bands in bronchial tubes, and bundles within pleura. 7. The Vascular System: in the coats of arteries, veins, and larger lymphatics. 8. The Lymphatic Glands: in the capsule and the trabeculae of spleen ; sometimes in the trabeculae of lymphatic glands. 9. The Eye : in iris and ciliary body, and in eyelids. 10. The hitegument: as the arrectores pili connected with the hair-follicles ; in sweat and some sebaceous glands; in skin covering the scrotum and parts of the external genitals. Involuntary muscle is composed of delicate spindle, often rib- bon-like, fibre-cells ; these vary greatly in size, measuring 75-225 /x* long and 4-8 u wide. The cells found in arteries are short Fig. 68. Fig. 67. Isolated involuntary-muscle cells from intestine of man. Involuntary-muscle cells from mesentery of newt: n, nuclei; f axial fibre; m, transverse markings on surface of cell; B, muscle-cell with forked extremity. and flat, being but 25-45 t1 long and 9-12 ;j. wide; the largest ele- ments are found in the gravid uterus, where they reach a length * i n (micron) — the ioooth part of a millimetre. 60 NORMAL HISTOLOGY. of over 500 // and a breadth of 20 /j.. Occasional cells with bi- furcated ends are encountered, especially among the lower verte- brates. The spindle muscle-cell is invested with a very delicate, homo- geneous, hyaline sheath, closely resembling elastic tissue, and corresponding to the sarcolemma of the striated fibre; within this envelope lies the soft, semi-fluid, contractile protoplasm, embedded in which, near the centre of the cell, lies a characteristic, narrow, rod-shaped nucleus. Delicate longitudinal fibrillae sometimes can be made out extending the entire length of the cell; these are re- garded by many histologists as representing the actively contractile parts of the cell, the surrounding protoplasm being largely passive. Transverse markings are also often seen ; these correspond in posi- tion to local variations in the diameter of the cell, and are probably due to corrugations in the enveloping mem- brane. The individual spindle-cells are closely fitted together and united by an albu- minous cement-substance; they are dis- posed in groups or bundles, which, on cross-section, are made up of rounded polygonal areas of varying size, the larger possessing round nuclei, while the smaller have none. Since these areas are the sections of nucleated spindle- cells, the large nucleated fields corre- spond to sections passing through the nucleus of the cell, while the small ones are sections of the cell fall- ing near the pointed ends. The bundles of muscle-cells are arranged to form layers or sheets, as in the digestive tract, or net-works, as in the eye, pleura, etc. Examined in longitudinal sec- tion, or in considerable masses, it is difficult to distinguish the individual component fibre- cells, the involuntary muscle in such cases closely resembling fibrous connective tissue; how- ever, the numerous more or less regularly disposed rod- shaped nuclei, and the absence of the delicate wavy fibres, together with the impression of greater density, usually suffice to establish the identity of the muscle. Fig- 69- Involuntary muscle in transverse section : portions of three bundles are represented, separated by areolar tissue (a): the nucleated areas are sections of the muscle-cells through their nuclei; the smaller figures repre- sent sections of the cells cut nearer the ends. Fig. 70. Involuntary muscle in longitudinal section : the muscle-cells are often cut obliquely, and hence appear shorter than when isolated. THE MUSCULAR TISSUES. 61 The connective tissue uniting the larger bundles of muscle-cells supports' the blood-vessels and nerves. The larger blood-vessels break up into capillary net-works, which pass between the muscle- cells. The nerves, derived principally from the sympathetic system, likewise penetrate the intercellular spaces and terminate between the cells in the manner more fully described in the chapter devoted to nerve-endings. Lymphatics occur, as in parts of the digestive tract, closely associated with the muscular tissue. STRIATED OR VOLUNTARY MUSCLE. Striated or voluntary muscle, in addition to the extensive system attached to the skeleton, supplies the special muscles connected with many organs, including the tongue, pharynx, middle ear, larynx, upper half of the oesophagus, diaphragm, generative organs, etc. This form of muscle is composed of long, irregularly cylindrical fibres, each of which represents the high specialization resulting from the development of the single original embryonal cell; the fibre is, therefore, the structural unit of the striated muscular tissue, and corresponds to the spindle fibre-cell of the involuntary variety. The fibre of striped muscle comprises (a) the sarcolemma, (b) the muscle- nuclei, and (c) the muscle-substance. Each fibre is closely invested by a clear, homogeneous, elastic sheath—the sarcolemma—which, ordinarily, so tightly adheres to the enclosed muscle-substance that the two are optically blended together; in favorable positions, as where breaks in the sarcous substance occur, or after the action of water, the sarcolemma is separated from the muscle-substance, and is then seen in profile as a delicate line spanning the break in the continuity of the fibre. The sar- colemma forms a closed sac completely envel- oping the contractile substance of the fibre. Immediately beneath the sarcolemma, lying within minute depressions on the surface of the muscle-substance, are the muscle-nuclei. These are oval or fusiform, usually placed parallel to the long axis of the fibre, and sur- rounded, especially at their ends, by a small amount of granular protoplasm. These accumu- lations represent the meagre remains of the indifferent protoplasm which has not undergone conversion into the highly specialized muscle-substance of the fibre. In mammalian muscle the nuclei lie always upon the surface of the sarcous substance of the fibre and immediately beneath the sarco- Fig. 71. Voluntary-muscle fibres, som#what broken after treatment with water, showing the sarcolemma (s) in several places. 62 NORMAL HISTOLOGY. lemma; in the majority of other vertebrates, however, the nuclei are distributed irregularly throughout all parts of the contractile substance. These differences are well shown in the accompanying figures. The muscle-fibres present alternate light and dark transverse markings, or striae, to which the tissue owes its characteristic appear- ance. The highly specialized contents of the sarcolemma are composed of two substances pos- sessing different refractive prop- erties, that forming the dark bands being doubly refracting, or anisotropic, while that of Fig. 73. Fig. 72. Voluntary muscle, portions of two fibres show- ing the characteristic transverse markings; the lighter band is divided by the row of minute beads constituting the intermediate disk : a, termination of muscular substance and attachment of adjoin- ing fibrous tissue; », nuclei of muscle-fibres. Fibres of voluntary muscle in section: A, human fibres, with nuclei upon the surface and beneath the sarcolemma ; B, fibres from frog, with nuclei embedded within the muscle-substance. the light striae is singly refracting, or isotropic. When fresh or well-preserved mammalian muscle is examined under high am- plification it is seen that the dark striae, or transverse disks, are not unbroken homogeneous bands, but that each is composed of a number of minute prismatic elements placed side by side and sep- arated from one another by a thin layer of a substance corresponding to and continuous with that forming the light zone. This latter, in addition, is divided transversely by a delicate interrupted line or row of dark dots—the intermediate disk, or membrane of Krause. That part of the light zone between the dim intermediate and trans- verse disks constitutes the lateral disk. The explanation of these appearances has caused many and pro- longed discussions, and even at present, notwithstanding the careful study bestowed upon the subject, the exact structure of voluntary muscle must be regarded as still unsettled. Heretofore two promi- nent and opposed views have prevailed: the one regards the fibre as composed of parallel longitudinal rows of minute prisms forming fibrillse (as rows of bricks placed end to end); the other considers the fibre as built up by the apposition of their disks, whose diameter corresponds to that of the entire fibre (as cheese-boxes piled one THE MUSCULAR TISSUES. 63 upon the other). After treatment with alcohol, the fibres of striped muscle readily split up lengthwise into delicate bundles, which, with care, may be subdivided to such an extent that the resulting threads embrace in their width only a single row of alternating light and dark elements. These ultimate fibrillae were formerly considered by Kolliker as the normal elements of the fibre; the dark prisms of these fibrillae correspond to the sarcous elements, which were regarded by Bowman as the component units of muscular tissue. The transverse cleavage of the fibre following the action of diluted mineral acids, on the other hand, has been upheld as representing the natural division. According to Krause, the fibre is divided through the light bands by a number of transverse partitions con- tinuous with the sarcolemma; these assumed septa appear as delicate broken lines—the membranes of Krause—and are identical with the intermediate disks already mentioned. Adopting this view, the fibre is composed of numerous thin zones or contractile disks, each of which embraces the dark dim band in its centre and half of the light stripe at either end. Each contractile disk is further sub- divided by vertical partitions extending between the neighboring membranes of Krause, thus forming in every disk a row of com- partments or muscle-caskets. The portion of the dim band con- tained within each muscle-casket has been regarded as itself being composed of a series of thin prisms of contractile substance—the muscle-rods. After renewed critical study of the subject, Rollett has presented a view regarding the structure of voluntary muscle which not only offers the most plausible solution of this difficult problem, but is, likewise, in harmony with the history of the development of the tissue. According to this theory, the muscular tissue is composed of the highly specialized, darker, anisotropic contractile substance, and the relatively passive, lighter, semi-fluid, isotropic sarcoplasm. The contractile substance is arranged as delicate spindles, the appo- sition of whose thicker parts produces the dim transverse disk seen under medium amplification; at either end the spindle is prolonged as an extremely thin thread, which terminates in a minute sphere or bead; the apposition of these beads in the transverse row gives rise to the appearance of the interrupted line constituting the inter- mediate disk, or Krause's membrane. The darker anisotropic sub- stance forms, therefore, numbers of continuous contractile fibrillae, which extend in parallel bundles the entire length of the fibre; all the remaining interfibrillar space within the sarcolemma is filled with the lighter sarcoplasm, which appears faintly granular in preserved tissue, but is, probably, almost fluid during life. On comparing this description with the usual appearances presented by striated muscle, 64 NORMAL HISTOLOGY. it will be seen that the lateral apposition of the thicker parts of the contractile jibrillce produces the dark band, or transverse disk, while the row of minute spherical masses appears as the interrupted dark line bisecting the light zone, or intermediate disk. The threads bridging between these beads and the chief mass of the fibrillae are too delicate to be appreciated under ordinary powers, and that portion of the fibre corresponding to the lateral disk consequently appears as if made up of the lighter sarcoplasm alone. In certain forms of invertebrate muscle a more complicated arrangement exists, since on either side of the intermediate disk a row of dark granules crosses the light lateral disk, forming a dim secondary disk ; these gran- ules are connected with the intermediate and transverse disks by delicate bridges of con- tractile substance, along which they occur as local thickenings. The dim transverse disk sometimes contains a central lighter band, the median disk of Hensen, which is due, probably, to diminished thickness of the con- tractile fibrils. The contractile fibrillae, however, are not uniformly distributed throughout the fibre, but are aggregated into bundles—the muscle- columns—each of which is enveloped in a thicker layer of the sarcoplasm than the partitions separating the individual fibrillae. In transverse section each muscle-fibre presents a number of small, polygonal, dark areas, enclosed by lighter lines, which areas, under high amplification, exhibit minute punctations. These areas are sections of the muscle-columns and correspond to Cohnheim’s fields, the dots being sections of the individual constituent fibrillae; the lighter intervening and surrounding substance is the sarcoplasm, thicker layers of which surround and separate the larger groups into which the muscle-columns are further collected. The individual muscle-fibres, which usually are not circular in cross-section, but rather irregularly polygonal with rounded angles, are held together by a small amount of areolar tissue, the endo- mysium. They are grouped into primary bundles, which latter are enveloped and separated from other primary bundles by the thicker bands of connective tissue constituting the perimysium. The primary bundles are united to form larger secondary groups or fasciculi, upon the width and arrangement of which the coarseness Fig. 74. A, diagram of arrangement of the contractile substance according to the view of Rol- lett: the granular figures rep- resent the contractile elements, the intervening light areas the sarcoplasm ; B, small muscle- fibre of man ; the correspond- ing parts in the two figures are indicated : t, i, /, respectively the transverse, intermediate, and lateral disks; n, muscle- nuclei. THE MUSCULAR TISSUES. 65 or the fineness, macroscopically appreciable, of the muscle largely depends. The entire muscle is invested in a fibrous sheath, the epimysium, derived from the denser layers of the interfascicular connective tissue. When contraction takes place, the entire muscle becomes shorter and, at the same time, broader; the striae also participate in the changes, becoming narrower. These phenomena, however, affect only a limited part of the fibre at one time, consecu- tive portions being influenced in regular se- quence, so that the changes pass along the fibre as a contraction wave; after the passage of the wave the muscle resumes its previous condition. In short muscles the individual fibres quite frequently extend the entire length; in long ones, on the contrary, the fibres are shorter than the muscle, being generally some 30-45 mm. long; sometimes, however, the fibres reach a length of 120 mm. by 10-50 mm. in width (Felix). The fibres, as a whole, are gen- erally somewhat spindle-shaped, being slightly larger in the middle; the ends of the fibres are more or less pointed, although blunted or club-shaped, and, more rarely, branched, extremities are not un- common. Branched and anas- tomosing fibres frequently occur (Gage), especially in the tongue Fig. 75. Muscle-fibres in transverse section, highly magnified : A , portion of human muscle : the small, irregular areas are the fields of Cohnheim (c); B, semi-diagrammatic view show- ing the groups of muscle-col- umns composing Cohnheim’s fields; n, nucleus; m, groups of muscle-columns. Fig. 76. Fig. 77. Voluntary muscle in transverse section: the irregular polyhedral areas (_/") are the individual muscle-fibres in section, held together by the en- domysium (e); the primary bundles of the fibres are enclosed by the denser perimysium (/>). Branched voluntary-muscle fibres from the tongue. and ocular muscles. When the individual fibres do not extend the length of the entire muscle, the sarcous substance terminates in 66 NORMAL HISTOLOGY. pointed or rounded extremities, while the sarcolemma is united with the endomysium of the surrounding fibres. The muscle-substance is never directly continuous with adjacent tissues, but is always enclosed within the sac of the sarcolemma; the union between the fibres and other structures is effected by the blending of the endo- mysium of the muscle-fibres with the connective tissue of the attach- ments, whether these be tendon, periosteum, perichondrium, or subcutaneous tissue; the sarcolemma closely invests the sarcous contents, being simply received into the connective tissue without becoming directly continuous. CARDIAC MUSCLE. The muscular tissue of the heart, as well as of the cardiac ends of the large veins, forms an intermediate group of contractile tissue, standing in its development between the simple spindle non-striated cell on the one hand and the highly differentiated striped fibre on the other. Among the lower vertebrates (fishes, amphibians) the cardiac muscle is composed of nucleated spindle-cells possessing distinct transverse striations and often branched ends; in man and the higher vertebrates these spindle- cells give place to short, striated, cylindrical fibres, provided with lateral processes. By the apposition of these richly-branched cells a close, narrow-meshed net-work is formed, the juncture between the individual elements being indicated by transverse lines of cement-sub- stance. The peculiarities of heart-muscle are— 1. The absence of the sarcolemma, the transversely striated and more faintly longitudinally marked muscular tissue being naked. 2. The situation of muscle-nuclei within the sarcous substance, usually near the centre of the cell. 3. The characteristic arrangement of the contractile fibrillae, since these are so placed that the peripheral fibrillae are grouped into flat, ribbon-like muscle-columns, somewhat radially disposed about the circumference of the fibre; the remaining central portion is occupied by prismatic bundles of fibrillae, together with the nuclei and the associated protoplasm (Ranvier, Kolliker). The small masses of pro- toplasm which surround the muscle-nuclei usually contain minute fat-drops and pigment-granules. The amount of pigment normally present varies with age, increasing from the tenth year (Maass). Fig. 78. Heart - muscle, showing several joined branched fibres: around the poles of the nuclei are aggregations of pigment-granules. THE MUSCULAR TISSUES. fiy Sometimes, in preserved tissue, the position of the nucleus is occu- pied by a clear vacuole. Ranvier has called attention to certain differences in the muscles of the rabbit, describing two varieties—the red or especially dark (semitendinosus, soleus) and the white or pale (adductor magnus). The red muscles are char- acterized by slow response to electrical stimulus, less regular transverse striation, greater dis- tinctness of longitudinal markings, and great number of round nuclei. The blood-vessels of striated muscle are very numerous. The larger vessels, together with the nerve-trunks and, less frequently, the lymphatics, are contained within the perimy- sium, where they give off numerous smaller branches; these, in turn, extend between the primitive bundles and break up into extremely thin capillaries, which form a characteristic rectangular-meshed net-work around the indi- vidual muscle-fibres. The longer sides of the meshes correspond with the axis of the fibre. At various points along the course of these vessels peculiar dilatations, or ampullae, occur, the object of which is, probably, the relief of sud- den temporary interference with the circulation during muscular contractions. The relation be- tween the capillary blood-vessels and the muscu- lar fibres of the heart is very intimate; in many places the vessels lie embedded within or even en- tirely surrounded by the muscular tissue (Meigs). Lymphatic vessels occur in striated muscle in small numbers, but are entirely wanting in many small muscles; when distinct lymphatic vessels do occur, they are confined to the larger and looser masses of the perimysium (Kolliker). The nerves supplying the striated muscle include the principal trunks which run within the perimysium, where they subdivide into smaller groups of medullated fibres, in order to reach the individual muscle-fibres ; these latter receive their nervous supply at certain points only, the nerves passing to the muscle to end in the special end-plates in the manner described more fully in connection with the peripheral nerve-endings. The development of all varieties of muscular tissue is closely related to the mesoderm, of which they are the direct descendants. The plain or non-striated muscle is formed by the differentiation, Fig. 79. Heart-muscle fibres in sec tion: the peripheral zone is composed of radially-ar- ranged groups (/) of muscle- columns ; a zone (c) of less differentiated sarcoplasm surrounds the nucleus. Fig. 8o. Injected voluntary mus- cle : the capillaries form rectangular-meshed net- works enclosing the indi- vidual fibres. 68 NORMAL HISTOLOGY. within certain areas, of the irregular mesodermic elements into the elongated fusiform fibre-cells. In suitable preparations all gradations between the ordinary embryonal connective-tissue cells and the muscular elements may be observed, emphasizing the common ancestry of the two forms of tissue. Voluntary muscle, representing a higher specialization, is de- rived from definite areas constituting the inner layer of the muscle- plates, which are referable to the early stages of the primary segmenta- tion into somites. The cells of the muscle-plate soon elongate, with pro- liferation of the nuclei, to become the primitive muscle-fibres. These at first consist of greatly extended ele- ments, possessed of numerous nuclei and composed of granular indifferent protoplasm. After a time the fibre exhibits a differentiation into longi- tudinal striae, which, later, are supple- mented by the transverse markings characteristic of voluntary muscle. The sarcolemma appears about the time the longitudinal markings are seen. The striations are limited, at first, to one side of the fibre, then extend over the entire periphery, but still for some time do not reach the centre of the fibre, an inner zone of undiffer- entiated sarcoplasm occupying the middle. Later, this area also becomes converted into striated tissue, while the once numerous nuclei are reduced to the few collected beneath the sarcolemma. Cardiac muscle, likewise, develops from the mesoderm immediately surrounding the primary heart-tubes, the contractions of the cells being displayed even before the histo- logical differentiation becomes apparent. In its development it represents an intermediate stage, since the original spindle-cells be- come converted into protoplasmic fibres containing a central area which always remains less differentiated and nearer its primary con- dition of indifferent sarcoplasm than the peripheral portions of the fibre. The fibres of Purkinje, found in the hearts of certain rumi- nants, represent muscular fibres in which the sarcoplasm remains in part still undifferentiated. Fig. 8i. Developing voluntary muscle: A, young muscle-cells; a, very young spindle-cell; b, older element, exhibiting indications of future striation on one side; the remaining part of the cell is composed of the undifferentiated sarco- plasm ; B, embryonal muscle-fibres pos- sessing many nuclei and traces of striae; C, developing muscle-fibres in section ; in the larger fibres a differentiated peripheral zone of striae (d) is seen in section; an area of still indifferent sar- coplasm occupies the centre of the fibre and surrounds the nucleus («). THE NERVOUS TISSUES. 69 CHAPTER V. THE NERVOUS TISSUES. The nervous system is composed of three principal parts—the tissues originating nervous impulse, the nerve-cells ; the structures serving to transmit such impulses, the nerve-fibres ; and the tissues uniting and supporting the nervous elements, the. neuroglia and connective-tissue framework. The nerve-cells are the primary elements, being older in the development of the individual as well as in the evolution of the nervous system. In certain inverte- brates both generation and transmission of the impulse are performed by the same cell, the peripherally situated protoplasm serving to convey and expend the force originating within the more centrally lying parts of the cell. Such simplicity, however, is unusual, the nerve-cell soon becoming specialized and separated from the pe- ripheral area with which it is connected. NERVE-CELLS. Nerve- or ganglion-cells of man and other vertebrates differ greatly in form and size, since they may be either spherical (Gasserian, spinal, or other ganglia), ellipsoidal (spinal cord), pyriform (cere- bellum), pyramidal (cerebrum), or stellate (spinal cord), and vary from io to 100 p. in size ; the huge cells of the spinal cord are among the largest elements of the body. In general the cells of motor areas are largest, those found in the convolutions bordering the central fissure and in the anterior cornua of the spinal cord being of conspicuous size. The ganglion-cells are composed of granular or striated proto- plasm, containing a large round or oval vesicular nucleus within which lies a prominent nucleolus ; after certain stains the protoplasm, nucleus, and nucleolus present distinct tints. Many nerve-cells are deeply colored, owing to the presence of considerable quantities of pigment-granules around the nucleus; a certain amount of pigment within the protoplasm is almost constant. The protoplasm of every nerve-cell is prolonged into at least one and usually several processes, dependent upon the number of which it is customary to speak of nerve-cells as unipolar, bipolar, or multipolar. Since an apolar nerve-cell is, evidently, functionally useless, it is doubtful whether such cells ever normally exist; apolar cells are frequently seen in preparations, but the absence of the 70 NORMAL HISTOLOGY. processes is only apparent, being due either to mutilation or to the process lying without the plane of the section ; where processes are really wanting, an immature or pathological condition must be suspected. The processes of nerve-cells are of two principal kinds—the protoplasmic (dendrits) and the axis-cylinder (neurits) pro- cesses. When a cell possesses but one, this is always an axis- cylinder process. The proto- plasmic processes rapidly undergo dichotomous di- vision, splitting up and sub- dividing until the resulting branches form rich net-works or arborizations of slender threads, which frequently interlace, but probably never actually join, with similar fibrils of adjacent cells. Nerve-cells, in one sense, are but nucleated local accumula- tions of the interfibrillar pro- toplasm, which latter may be termed neuroplasm (Kol- liker); the large striated multipolar ganglion-cells may be regarded as switch-boards for the redistribution of the numerous ultimate fibrillae continued into the axis-cylin- ders. The fibrillae pass off in divergent paths, along the several processes of the cell, to form new combinations and relations. The peculiarities formerly supposed to constitute the distinguish- ing characteristics of the axis-cylinder processes are no longer suf- ficient in the light of recent advances in our knowledge regarding the structure of the nervous system. The investigations of Golgi and others have shown that, in addition to greater delicacy and a straighter course, the axis-cylinder processes present variations which separate ganglion-cells into two groups—cells of the first and cells of the second type. Nerve-cells of the first type include elements, as those of the motor areas, possessing the characteristic axis-cylinder processes Fig. 82. Nerve-cell from the cerebral cortex, exhibiting the striations of the protoplasm and the conspicuous char- acter of the nucleus and the nucleolus: p, pigment- granules ; a, axis-cylinder process ; b, l, apical and lateral protoplasmic processes. THE NERVOUS TISSUES. directly continuous with the axis-cylinder of the nerve-fibre. While these processes, when compared with the richly-divided protoplasmic, may be regarded as unbranched, the existence of delicate lateral off- shoots, or collateral fibrils, has been established ; these delicate branches pass backward towards the gray matter, within which they end. 71 Fig. 83. Nerve-cell from the spinal cord, isolated by maceration and teasing ; the numerous branched pro- toplasmic processes are somewhat displaced and distorted, owing to manipulation : a, axis-cylinder process. Nerve-cells of the second type are distinguished by the be- havior of the axis-cylinder process ; this, instead of passing into the white matter to become the centre of a nerve-fibre, never leaves the gray matter in which the ganglion-cell lies, but, after a longer or shorter course, rapidly undergoes division and subdivision in the production of a terminal arborization of delicate fibrillae; these ramifications are limited entirely to the gray matter, their exact manner of ending and their relations to other cells varying in dif- ferent parts. The free division of the axis-cylinder process does not curtail the branching of the protoplasmic extensions, which are often very conspicuous, notwithstanding the numerous bifurcations of the former. In some instances the axis-cylinder processes of cells of this type split up into fibrils which enclose the bodies of other nerve-cells within basket-like net-works; a notable ex- ample of this arrangement exists in the cerebellum around the cells of Purkinje. 72 NORMAL HISTOLOGY. The axis-cylinder processes usually are directed towards the nearest mass of white matter, since the axis-cylinder of the nerve- fibre becomes continuous with that of the cell. Exceptional arrangements are sometimes encountered, as where one process of a bipolar cell becomes wound about the remaining straighter fibre, con- stituting a spiral process; such cells are comparatively frequent in the sym- pathetic ganglia of the frog. Ganglion-cells lie within peri-cellu- lar lymph-spaces, which appear with greater or less distinctness ac- cording to the condition of the Fig. 84. Fig. 85 Fig. 86. Basket-work, formed by the extensions of the branched axis-cylinder process of a nerve-cell, surrounding the body of one of the ganglion-cells of Purkinje: p, base of branched process of 1'ur- kinje’scell; n. fibrils con- stituting basket-work. Nerve-cell of second type— from cerebellum: p, branched protoplasmic processes ; c, cell- body ; a, axis-cylinder process breaking up into arborization (n), but entirely confined to gray matter. Golgi staining. Nerve-cell of first type—from cere- bral cortex: p', p, protoplasmic pro- cesses directed respectively towards the free surface and laterally ; a, axis-cylin- der or nerve-process giving off collateral branches, c, c. Golgi staining. protoplasm of the enclosed cell; when this is contracted and shrunken the space is, obviously, more conspicuous than when almost entirely filled by the cell. These lymph-spaces are limited by a delicate, elastic, hyaline membrane, and lined with nucleated endothelial plates; on the exit of the axis-cylinder a delicate prolongation of this sheath accompanies the fibre as the neuri- lemma. NERVE-FIBRES. Depending upon the character of the investing coats, nerve-fibres appear as two kinds—the medullated, or white, and the non- medullated, or gray. These do not, however, constitute two sharply defined and distinct classes, but depend upon variations in the condition of fibres, which often represent both varieties at dif- THE NERVOUS TISSUES. 73 ferent portions of their course. Every medullated nerve-fibre loses its white substance of Schwann and becomes non-medullated before reaching its ultimate distribution. The majority of nerve-fibres constituting the great cerebro-spinal tract may be classed as med- ullated, although numbers of gray fibres likewise occur here; the non-medullated fibres are especially numerous in the sympathetic system, where they predominate, as well as in certain of the cranial nerves, as the olfactory. While the character of the fibre, as to whether it is motor or sensory, bears no relation to its size, the length of the fibre seems to directly influence its diameter, since fibres having long courses possess greater width than those extending for much shorter distances. A typical medullated nerve-fibre consists of the following parts: 1. The axis-cylinder, surrounded, possibly,, by its sheath, or axilemma (Kiihne). 2. The medullary stibstance, or white matter of Schwann. 3. The neurilemma, or sheath of Schwann, with the nerve-corpuscles. Perfectly fresh, uninjured, medullated nerve-fibres, when examined by transmitted light, appear as homogeneous, hyaline cylinders, with dark contours and no appreciable structure ; seen by reflected light, the fatty character of the medullary substance is indicated by the glisten- ing appearance of the fibres, and their dull white color when viewed in masses. Shortly after death the fibres exhibit characteristic double contours, enclosing an apparently structureless centre; later, the fibres become mottled by irregular spherical masses, derived from the dis- torted medullary substance. The axis-cylinder appears, in fresh nerves or in those fixed with osmic acid and teased, as an inconspicuous, clear, delicate rod extending along the central part of the fibre, or, perhaps, projecting beyond the outer sheaths at the broken end. The longitudinal striations occasionally seen, under high amplification, in carefully fixed preparations, are indi- cations of the ultimate fibrillse of which the axis-cylinder is com- posed ; these fibrillse are cemented together by a finely granular, interstitial substance, or neuroplasm (Kolliker). According to Kiihne, the axis-cylinder is enveloped by a special, delicate, elastic Fig. 87. Nerve-cell from a sym- pathetic ganglion of frog, showing the tortuous course and terminal net-work of the spiral fibre: n, neuri- lemma continued as a deli- cate sheath. (After Ret- zius.) 7 4 NORMAL HISTOLOGY. sheath—the axilemma (Kiihne); other authorities regard this appearance as an artificial production. Since every axis-cylinder is connected with the corresponding process of a nerve-cell, axis-cylinders may be regarded as direct continuatio?is of the ganglion-cells, their component fibrillae forming uninterrupted paths which con- nect the periphery with the presiding nerve-centres. On approaching its ter- mination, the axis-cylinder splits into smaller bundles of component fibrillae; these groups subsequently divide, until, finally, the naked ner- vous threads, singly or in small groups, reach their ultimate destination. The nerve-fibrillae not in- frequently exhibit nu- merous minute fusiform enlargements or vari- cosities along their course, giving to the fibrils a characteristic beaded appearance, es- pecially after gold-stain- ing. The medullary sub- stance, or white matter of Schwann, surrounds the axis-cylinder, and forms the most con- spicuous investment of the fibre. The existence of a narrow lym- phatic cleft described as lying between the medullary substance and the axilemma is still uncertain. The medullary substance consists of two parts : one of these occurs as a delicate reticulated frame- work, composed of a resistant material probably resembling neuro- keratin (Kiihne and Ewald) ; the other fills the interstices of the reticulum and appears as a semi-fluid, highly refracting, fatty sub- stance—the myelin—which affords protection to the enclosed axis- cylinder. Other authorities regard the reticulated framework as the effect of reagents, citing the variability in the appearances of the net-work as opposed to its presence as a normal constituent of the coat. At regular intervals along the medullated nerve-fibres well-marked annular constrictions occur; these are the nodes of Ranvier, and mark the interruption of the white substance of Schwann at certain points. Fig. 88. Fig. 89. Medullated nerve-fibres: A, teased in salt solution, x, shortly after death ; y, a node of Ranvier; 2, post-mortem distortions of med- ullary substance. B, an isolated stained fibre ; a, axis-cylinder ; r, node of Ranvier; m, medullary substance ; n, neurilemma, beneath which a nerve-corpuscle is seen in the lower segment. Gold-stained axis- cylinder (a), showing component fibrillae; 6, varicose nerve- fibrillae near their termination. THE NERVOUS TISSUES. 7 5 Owing to the absence of the middle coat in these positions, the outer sheath, or neurilemma, is brought into contact with the continuous axis-cylinder. The portions of the fibre included be- tween two constrictions—the internodes, or mternodal segments—vary in length with the size of the fibre, being longer (about i mm.) in large and much shorter in thin fibres. Each internode pos- sesses a single nerve-cor- puscle, usually about its middle, and probably elon- gates during the growth of the nerve. The neurilemma is not broken by the nodes into segments, but forms a continuous sheath. When a medullated nerve - fibre branches, the bifurcation cor- responds in position to a node of Ranvier. After treatment with silver nitrate the positions of the nodes of Ranvier are rendered conspicuous by the appear- ance of minute dark-brown crosses ; the transverse arm is formed by the stained, internodal albuminous sub- stances, forming an annular disk, sometimes called the constricting band (Ranvier), while a stained portion of the axis-cylinder con- tributes the less distinctly marked vertical lines of the cross. Closely-placed transverse markings, known as Frommann’s lines, as well as bi-conical swellings, occasionally are noted along the axis- cylinder after treatment with silver; their significance, however, is still undetermined. The medullary substance is very prone to post-mortem change, the coagulated or partly disintegrated myelin producing various grotesque distortions in the contour of the nerve-fibre. After treat- ment with osmic acid and other reagents, the white substance of Schwann displays oblique markings which are, apparently, clefts or incisions involving the middle coat; relying upon these appearances, many regard the medullary substance as made up of elongated pieces, the Schmidt-Lantermann segments, several of which are in- cluded within each internode. Fig. 91. Fig. 90. Silvered nerve-fibres: A, small bundle of medullated fibres displaying the silver crosses atseveral nodes ; B, node of Ranvier under high power: the horizontal limb of the cross is produced by the stained intersegmental cement-substance; the ver- tical limb is formed by the colored axis-cylinder; C, silvered axis-cylinder show- ing a bi-conical enlarge- ment and the transverse markings or lines of From- mann. £ fel m SUsi Medullated nerve- fibres after treatment with osmic acid, from frog : A , fibre displays the incisions of the me- dulla, or Schmidt-Lan- termann segments; B, the medullary substance exhibits a reticulated appearance. 76 NORMAL HISTOLOGY. The neurilemma, sheath of Schwann, or primitive sheath, the outer covering of the nerve-fibre, is a delicate, homogeneous, elastic membrane, closely investing the medullary substance, and resembling the sarcolemma. On its inner surface, placed at regular intervals corresponding to the position of the nodes of Ranvier, are the nerve-corpuscles, meagre accumulations of protoplasm sur- rounding the oval nuclei. The medullated fibres of the white matter of the brain and spinal cord, as well as those composing the optic and acoustic nerves, are noteworthy as being without a neurilemma, the surrounding neuroglia in these positions assuming the support and covering of the fibres. The non-medullated, pale, or Remak’s fibres, as indicated by the first name, are devoid of medullary substance, consisting of the axis-cylinder and the more or less modified neuri- lemma ; such fibres, when aggregated, appear as grayish, semi-transparent bands. While every medullated nerve-fibre, before reaching its pe- ripheral distribution, loses the white substance of Schwann and becomes sooner or later a non- medullated fibre, the nerves constituting the sympathetic system especially represent this group, and evince the distinctive tendency to give ofif branches, which unite to form the char- acteristic plexuses. The presence of both va- rieties of fibres, however, in nerve-trunks is quite usual; a conspicuous example of this asso- ciation is found in the vagus of the dog, where large bundles of both kinds are included within a common sheath. The fibrillae constituting the axis-cylinders of non-medullated fibres are especially distinct, this feature being probably due to the generous amount of neuroplasm separating the fibrillae ; not infrequently local accumulations of this interfibrillar substance occur, producing the conspicuous varicosities seen along the course of the fibres. The nerve-nuclei are far more numerous than in medullated fibres ; they are, however, irregularly distributed, lying upon the surface of the fibre and beneath the outer delicate sheath. This enveloping sheath —the attenuated representative of the neurilemma—is often difficult or impossible to distinguish, being very thin and closely adherent to the fibre. The smallest nerve-fibrils are probably without this coat, the fibrillae continuing as naked bundles, with the exception of the imperfect covering afforded by the numerous overlying nerve-nuclei. Non-medullated nerve-fibres are prone to form rich plexuses, the Fig. 92. Non-medullated nerve- fibres from the sympa- thetic system: the nucle- ated fibres join to form a plexus. THE NERVOUS TISSUES. yy junction of several fibres being frequently marked by characteristic triangular areas, in which a number of the nerve-nuclei are often collected. THE NERVE-TRUNKS. The nerve-fibres already described are associated in bundles—the funiculi—which, in turn, may be grouped to constitute the large macroscopic nerve-trunks. The funiculi differ greatly in diameter, a number of varying size being usually included within the nervous cord; in very small nerves, however, a single funiculus may suffice to form the entire trunk. While both kinds of fibres are grouped in bundles, the nerves composed principally of medullated fibres present the more typical arrangement. On transverse section of such trunks the individual nerve-fibres appear as small, round, nucleated cells, whose somewhat eccentrically placed nuclei are the axis-cylinders in section, while the contours of the cell-like areas are formed by the sections of the neurilemma; the shrunken granular or concentrically marked masses within the apparent cell-walls are the remains of the medullary substance. These sec- tions of the fibres are held in place by a delicate connective tissue—the endoneurium—extending among and surrounding the individual fibres. When the nerve- bundle, or funiculus, is small, the nerve-fibres are uniformly dis- tributed, and it is spoken of as simple ; when large, however, the fibres are usually divided into irregular groups by stronger fibrous trabeculae, thus forming a compound funiculus. The individual nerve-fibres vary greatly in diameter (from 2 to 20 ,a), even adjoining fibres often exhibiting marked differences. In general, the cerebro- spinal nerves possess the largest fibres, the sympathetic much the smallest (2 to 4 m), while the components of many of the cranial nerves occupy an intermediate position. Each funiculus is invested by a robust connective-tissue sheath— the perineurium—between the fibrous lamellae of which are seen Fig. 93. Section of portion of a nerve-trunk including three bundles, or funiculi, surrounded by the perineurium (/); the funiculi, together with the blood-vessels and adipose tissue, are united by the more general epineurium (e); the sections of the individual nerve-fibres are held in place by the endoneurium ; f, fat-cells, near which are the sections of blood-vessels. yg NORMAL HISTOLOGY. the nuclei of the endothelioid plates lying within the interlamellar lymphatic spaces. The endoneurium is directly continuous with the perineurium, of which it is the intrafunicular extension. Where a nerve-trunk comprises several funiculi, these are held together and enveloped by a loose general connective tissue—the epineurium—which supports the blood- vessels and lymphatics, and often contains masses of adipose tissue ; the external layer of the epineurium is usually somewhat condensed. When the funiculus divides, the new bundles receive a prolongation of the peri- neurium, the investment becoming thinner with each successive division. On nearing their final destination, the funiculi break up into small groups or single fibres, which are covered by an attenuated extension of the formerly robust perineurium ; this invest- ment constitutes the sheath of Henle, and consists of a delicate fibrous envelope lined with endothelioid plates ; in some cases these latter alone represent the entire sheath. The larger blood-vessels enclosed within the epineurium give off branches, which surround the funiculi and break up into capillaries passing within the endoneurium among the fibres. The lymphatics are represented by irregular clefts within the endoneurium, which are connected with the interlamellar spaces of the perineurium ; from these the lymph is taken up and carried off by the more definite lymphatic channels running within the epineu- rium. The nerves of the larger trunks—the nervi nervorum—are dis- tributed within the epineurium, and are said to terminate, in many cases, in special bodies which resemble in general type the spherical end-bulbs of Krause. Fig. 94. A single funiculus more highly magnified; the apparent small nucleated cells are sections of the nerve-fibres and their axis-cylin- ders : a, axis-cylinder; w, med- ullary substance ; n, neurilemma ; e, endoneurium ; p, perineurium ; b, connective-tissue cells of same. THE SUPPORTING TISSUES OF THE NERVE-CENTRES. The essential constituents of the nervous system, the cells and fibres, when associated in large masses, as in the cerebro-spinal tract, are held in place and supported by two varieties of sustentacular tissue. On examining suitably prepared sections of these organs, the cells and fibres appear everywhere to be embedded within a finely reticulated ground-substance, whose composition is especially complex in the gray matter. The basis of this reticulum is the THE NERVOUS TISSUES. neuroglia, a peculiar form of ectodermic tissue, with, therefore, close relations to the neurogenetic tract. Neuroglia consists of extremely branched elements, or glia-cells, whose numerous pro- cesses break up into brush-like bundles of delicate fibrils, which pass in all directions among the nervous ele- ments, filling more or less completely all inter- stices. The body of the glia-cells is frequently stellate, possessing a nu- cleus and staining in- tensely with certain dyes. The demonstration of these neuroglia elements is very striking in Golgi silver preparations, where they appear as dark, spider-like figures which send out delicate fibrils in all directions. In the gray matter the ground-reticulum is composed of the minutely ramifying ter- minal threads of the processes of the nerve-cells, the axis-cylinders of the nerve-fibres, together with the extensions of the neuroglia elements. The groundwork surrounding the nerve-fibres within the white matter serves the purpose of covering as well as of support, and replaces the neurilemma. In addition to the dense reticulum formed by the neuroglia, con- stituting the special sustentacular tissue of the nervous system, pro- longations from the enveloping pia mater likewise penetrate within the nervous masses and contribute connective-tissue trabeculae, which form a supporting framework throughout the organs. These connective-tissue ingrowths constitute the septa, which in many places, as conspicuously in the spinal cord, separate the nervous matter into distinct tracts and areas. The finer ramifications of these partitions fade away in delicate extensions which mingle with the fibrils of the neuroglia-cells. It is evident, therefore, that the sup- porting tissue of the nervous system can no longer be regarded simply as a form of connective tissue, since, in addition to the un- doubted connective tissue present, the larger part is contributed by the peculiar ectodermic structure, the neuroglia. Fig. 95. Supporting tissues of nerve-centres : A, extensions of the peripheral connective tissue of the pia mater ; B, neuroglia- cells, one of which is seen in profile (j). Golgi staining. THE STRUCTURE OF GANGLIA. Along the course of certain nervous cords, such as those consti- tuting the sensory roots of the spinal nerves, the trunks of many of 80 NORMAL HISTOLOGY. the cranial nerves, and especially of the sympathetic system, groups of nerve-cells occur associated with the nerve-fibres in the form of ganglia; these may be large and conspicuous masses, as the Gasserian Fig. 96. Spinal ganglion, in longitudinal section, from cat: the groups of nerve-cells lie embedded among the bundles of the nerve-fibres. ganglion of the trifacial nerve, or their size may be microscopic, as many of the interstitial ganglia connected with the distribution of the sympathetic fibres. The outer covering of the ganglion consists of a fibrous envelope, a condensation of the adjacent epineurium in many cases, from which prolongations extend among the nervous elements, where they break up into delicate bundles of connective tissue, which serve for the union and the support of the cells and the fibres. Some of the nerve-fibres pass through the ganglion on their way to more distant points with- out joining any of the nerve- cells, while many others end in or take origin from these elements. The presence or absence of the medullary coat depends upon the char- acter of the component fibres of the nerve-trunk; before joining a nerve-cell, however, the medullary substance disappears, while the neuri- lemma of the fibre continues and becomes the nucleated capsule Fig. 97. Section of spinal ganglion more highly magnified: g, the nerve-cells, cut in various planes, surrounded by the nucleated sheath (c); a, the medullated nerve- fibres, on which several nodes of Ranvier are seen ; b, cells of the supporting connective tissue. THE NERVOUS TISSUES. 81 enclosing the individual nerve-cells. These latter possess, in general, a spherical form, and are usually provided with one or two, seldom more, processes; in the bipolar cells the processes frequently pass from opposite poles to become continuous with the afferent and efferent fibres. In the ganglia of some of the lower vertebrates bipolar cells occur in which one process becomes invested by the turns of the other or spiral fibre. Unipolar cells exist in which the single process divides into T-branches extending almost at right angles ; such cells occur also in man. The development of all nerve-fibres and nerve-cells must be referred to the elements derived from the invaginated ectoderm forming the neural tube. Without entering upon an exhaustive account of the process, many details of which are still uncertain, it may be accepted that the primary neural ectoderm differentiates into two varieties of cells—the neuroblasts and spongioblasts. The nerve- fibres are formed as outgrowths from the primi- tive nerve-cells or neuroblasts. This may take place either in one direction alone, from the centre towards the periphery (centrifugally), as in the formation of the efferent fibres of the motor-nerve roots of the spinal cord ; or, as in the production of the afferent (sensory) nerves, the neuroblasts may be somewhat removed from the central nervous mass, occupying the position of the spinal ganglia, and send out fibre-pro- cesses in two directions, one set growing into the nerve-centre (centripetally), while a second group of fibres extends towards the periphery (centrifugally). In all cases the nerve-fibres are formed as outgrowths from the primary nerve-cells; in later stages the cells concerned in extending the nervous path may disappear after the establishment of the tract. The spongioblasts, on the other hand, are especially concerned in the production of the neuro- glia-cells, these ultimately becoming transformed into the close reticu- lar formation supporting the nervous elements. The nerve-fibres are at first pale and possess neither medullary substance nor neurilemma. The acquisition of the white substance of Schwann occurs much later, the exact mode of its production, however, being by no means certain ; whether the medullary sub- stance owes its formation to the influence of the axis-cylinder, or its origin must be referred to the more or less direct agency of the ele- Fig. 98. Ganglion nerve-cell with spiral fibre from the sym- pathetic of frog: Sp F, the spiral fibre surrounding the straight process (Cf) and dividing at a node of Ran- vier ( Th); s, neurilemma. (After Schiefferdecker.) 82 NORMAL HISTOLOGY. ments represented by the nerve-nuclei, future investigation must determine. If traced to the axis-cylinder, the sheath must be classed as ectodermic tissue; as mesodermic, on the other hand, if referred to the nerve-corpuscles. The period at which the medullary coat appears in the various groups of nerves is variable, but constant for given tracts ; account has been taken of this fact with great advantage in the laborious investigations of tracing the path of many nerve- tracts composed of medullated fibres. The neurilemma may be regarded as certainly derived from the differentiation of surrounding mesodermic cells, as likewise the more general connective-tissue envelopes constituting the endoneurium, the perineurium, and the epineurium of the nerve-bundles. THE PERIPHERAL NERVE-ENDINGS. 83 CHAPTER VI. THE PERIPHERAL NERVE-ENDINGS. Terminations of Sensory Nerves. A medullated nerve, in passing to its ultimate distribution, first loses the medullary sub- stance, or white matter of Schwann, which ends abruptly at some bifurcation of the nerve corresponding in position to a node of Ran- vier. The fibre continues for a variable distance non-medullated, being covered with the neuri- lemma and the nerve-cor- puscles ; these coats become reduced gradually until the neurilemma disappears, the nerve-nuclei then alone re- maining as an imperfect in- vestment of the axis-cylinder. The nuclei soon occur less frequently, until finally they disappear and the bundles of nerve-fibrillae, by this time greatly reduced owing to re- peated division, continue as naked axis-cylinders; these unite to form a widely-meshed ground-plexus, possessing characteristic triangular, nu- cleated nodal points where the bundles of fibrillse meet. The axis-cylinders sooner or later break up into their component primitive fibrillse, which frequently interlace to form rich net-works, or terminal plexuses, within the connective tis- sue of the organ supplied ; these net-works quite often are situ- ated immediately beneath the epithelium ; in immediate proximity with the basement membrane fine fibrillse emerge from the plexus, enter the epithelium, and terminate in pointed or club-shaped free endings between the epithelial cells. The nerves of common sen- sation frequently end in this manner, including, probably, many nerves of the skin, cornea, and mucous membranes. Many sensory nerves, however, terminate in special endings of Fig. 99. Termination of sensory nerve fibres; portion of the plexuses occupying the anterior layers of the cornea; gold preparation : n, n, nodal points of the coarser ground-plexus; b, small bundle of nerve-fibrils which breaks up into the terminal arbor (t) of ultimate fibrillae; z>, fibrils showing varicosities. 34 NORMAL HISTOLOGY. varying complexity: of such specialized structures over a dozen forms have been described; since a number of these occur only among the lower vertebrates, the more important types alone will be here considered. The special sensory nerve endings may be grouped as— 1. Tactile Cells. 2. Tactile Corpuscles. 3. End-Bulbs. The tactile cells are found within the deeper layers of the epi- dermis or the adjacent stratum of the corium, and may be either simple or compound; the former are oval nucleated elements, 5-12 u in size, and resemble ganglion-cells. The centrally-directed portion of the cells is embraced by a peculiar crescentic expansion—the tactile meniscus—with which the nerve-fibre is probably connected. Where two or more such cells are associated to receive the nerve-fibre, a compound tactile cell results ; the corpuscles of Grandry and of Merkel, found respectively in the epidermis of birds and of mammals, are examples of such structures. The medullated nerve-fibre, on meeting the cells, loses its neurilemma and Henle’s sheath, these coverings becoming fused with the connective- tissue capsule of the corpuscle ; the axis-cylinder passes between the cells, to become lost within an intercellular flattened tactile disk; the medullary substance terminates at the point where the axis-cylinder enters the disk. The dark stellate figures sometimes seen in gold prep- arations of the epidermis, lying between the epithelial cells, and known as the cells of Langerhans, do not rep- resent nerve-endings, as for- merly claimed, but are prob- ably migrated wandering cells. The more elaborately ar- ranged compound tactile cells and the simpler tactile cor- puscles, such as the spherical end-bulbs of the conjunctiva, are closely related, their differences being but slight; the various Fig. ioo. Termination of sen- sory nerve-fibres within the epidermis ; gold prep- aration : e, deeper layers of epidermis; c, subja- cent connective tissue ; n, nerve-fibrillac pene- trating among the epi- thelial cells. Fig. ioi. Special nerve-endings within the epidermis; gold preparation: N, nerve-fibre entering the epithelium and dividing into the fibrils which are connected with the tactile disks (in); upon these latter rest the tactile cells, c. (After Ranvier.) THE PERIPHERAL NERVE-ENDINGS. 85 tactile corpuscles present increasing degrees of complexity of struct- ure, the most highly specialized ending of this class being the tactile corpuscle of Meissner, found in the skin of the palmar surfaces of the fingers and of the toes. The corpuscles of Meissner are oval elliptical bodies, 45-140 long and 35-55 wide, situated usually at the apices of the papillae of the corium; they possess nu- merous transversely-placed nuclei, which, with the edges of the indis- tinctly defined tactile cells, produce the characteristic transverse or some- what spiral markings. Each cor- puscle is supplied with one or two, sometimes three or four, medullated nerve-fibres, which are invested with Henle’s sheaths; the fibres may undergo numerous windings before entering the corpuscle, the sheath of Henle, together with the neurilemma, becoming continuous with the fibrous envelope of the corpuscle. The nerve- fibres retain their medullary substance for a short distance, but later lose this sheath and break up into a number of non-medullated fibres ; these latter subdivide into fibrillse, which pursue a spiral course throughout the corpuscle, being connected here and there with terminal disks. The com- pressed tactile cells themselves are usually indis- tinctly defined, the transversely-placed nuclei and the outlines of the cells producing the transverse markings. A large number of the nuclei seen, however, belong to the superficial layers con- tributed by the connective-tissue coverings. As to the exact course and mode of termination of the nerve-fibrillae within these tactile corpuscles, much uncertainty still exists. The spherical end-bulbs of the conjunctiva and of other mucous membranes, as well as the genital and the articular nerve-corpuscles, must be included in this class of nerve-endings; these bodies are all formed on the same general plan, the differences in their structure being limited to the details of arrangement. The End-Bulbs. The third group of special nerve-terminations embraces the nerve-endings of a cylindrical type in contrast to the Fig. 102. Tactile corpuscles from the bill of duck: A , simple, B, compound, corpuscle ; t, tactile cells; a?, tactile disks; ft, medullated nerve- fibres entering the nucleated capsules (s) into which the neurilemma continues. Fig. 103. Tactile corpuscle of Meissner from the skin of human toe : N, the nerve entering the complicated group of tactile cells com- posing the corpuscle; Bl, blood-vessel accompany- ing the nerve-fibre. (After Schiefferdecker.) 86 NORMAL HISTOLOGY. spheroidal form of those already considered. Just as in the preceding group, so here the simpler endings lead from the tactile cells to the more highly specialized structures; the cylindrical end-bulbs of the conjunctiva of the calf are the simplest members of this group, while the corpuscles of Vater, or the Pacinian bodies, are its most highly special- ized representatives. The nerve-endings of this class are composed of three parts—the cap- sule, the inner bulb, and the nerve-fibre. Upon the arrangement and development of these the differences distin- guishing the individual endings chiefly depend. In the simpler forms of end-bulbs, as those found in the conjunctiva and the oral mucous membrane of certain mammals, the body is borne upon a stalk, which contains the medullated nerve-fibre and, possibly, a minute blood- vessel enveloped in connective tissue. The sheath of Henle invest- ing the nerve is prolonged into the nucleated capsule. The latter encloses a conspicuous cylindrical mass of granular or faintly striatec pale substance—the inner bulb— within which the free axis-cylinder lies, terminating often in a slightly marked knob-like expansion. The medullary substance ends where the nerve-fibre enters the inner bulb. Further complexity in the struct- ure of the end-bulbs is largely due to elaboration of the capsule; this latter becomes laminated and very thick, while the inner bulb likewise exhibits new details of structure. Since the intermediate forms in the series of end-bulbs do not occur in man, the highly specialized corpuscles of Vater, or the Pacinian bodies, may at Fig. 104. Fig. 105. Simple spherical end- bulb from the human con- junctival mucous mem- brane : n, the medullated nerve-fibre which disap- pears within the capsule. (After Krause.) Genital corpuscle from the human clitoris; this ending represents a group of partly fused simple spherical end- bulbs : n, nerve-fibres entering the capsule. (After Krause.) Fig. 106. Simple cylindrical end-bulbs from tbe scleral conjunctiva of calf: n, nerve-fibre passing into the inner bulb (b); K, neurilemma which, with perineurial sheath ( C), continues as the capsule, C'. (After Schiefferdecker.) THE PERIPHERAL NERVE-ENDINGS. g^ once be considered. These structures, widely distributed in man and mammals, are elliptical, semi-transparent bodies, some 2-3 mm. long and half as broad, which occur along the nerves supplying the skin, especially of the hands and feet, the external genitalia, the joints of the extremities, the periosteum of certain bones, the peri- toneum, and many other localities. Of the three component parts of the typical end-bulb, the capsule has undergone the greatest development in the corpuscles of Vater, being composed of 25-50 concentric connective-tissue lamellae, each of which possesses an outer transverse and an inner longitudinal layer of fibres, and is lined by a single layer of endothelial cells; the nuclei of these plates are seen in profile throughout the capsule. The individual lamellae Fig. 107. Fig. 108. Corpuscle of Vater, or Pacinian body, from the mesentery of cat: N, nerve-fibre enclosed within the perineurial sheath, with which the lamella of the capsule (Kps) are connected; K, nuclei of the endothelial plates of same; Jk, inner bulb enclosing the axis-cylinder {ax), which at Thp di- vides into the terminal branches. (After Ranvier.) Herbst’s corpuscle from the bill of duck: m, medullated nerve-fibre passing into the interior of the capsule, where the axis-cylinder lies within the granular inner bulb (i) surrounded by a row of nuclei; the spindle nuclei appear between the outer and less closely placed lamella of the capsule. are separated by a clear serous fluid, which is largest in amount be- tween the peripheral layers, since the lamellae immediately surround- ing the inner bulb are thinner and more closely placed. The lamellae of the capsule are often united along a longitudinal area—the intra- capsular ligament—which corresponds to the course by which the nerve-fibre gains entrance to the inner bulb; occasional trabeculae 88 NORMAL HISTOLOGY. also pass between the adjacent lamellae. After silver stainings the corpuscles of Vater appear to be completely invested with a mosaic of endothelial plates; these markings are due to the cells which line the inner surface of the outer lamellae. The core of the corpuscle is occupied by a light granular or faintly striated cylindrical mass—the inner bulb—composed, seemingly, of an almost homogeneous tissue, closely resembling protoplasm, in which nuclei and indistinct fibrils sometimes are seen. Within and corresponding to the axis of the inner bulb lies the free axis- cylinder, ending frequently in a slightly expanded terminal knob; the medullary substance surrounds the axis-cylinder as far as the inner bulb, where it disappears. The small artery usually accompany- ing the nerve-fibre within the stalk of the corpuscle gives off fine branches to be distributed to the outer layers of the capsule. The corpuscles of Herbst, found in birds, closely resemble the Vaterian corpuscles of mammals, possessing, however, a less devel- oped capsule and an inner bulb beset with a single or double row of nuclei. From the foregoing sketch it will be seen that, taking the tactile cells as a ground-form, the special nerve-endings are developed along two lines: one type is represented by the spherical tactile cor- puscle, composed of winding nerve-fibres bearing tactile disks placed between tactile cells and enveloped within a capsule; the other by the cylindrical end-bulb, in which the central nerve- fibre lies within a cylindrical inner bulb, enveloped by a capsule developed to a greater or less degree. As the highest representative of the first group stand the complex tactile bodies of Meissner; of the second group, the corpuscles of Vater. The following table indicates the relations of some of the principal forms of special sensory nerve-endings : Simple Tactile Cells. ( Ground-Form.) Epidermis of man and mammals. Compound Tactile Cells. Grandry's Corpuscles: Epidermis of birds. Merkel's Corpuscles: Epidermis of mammals. Tactile Corpuscles. (Spherical.) Spherical End-Bulbs: Conjunctiva and mucous membranes of man. THE PERIPHERAL NERVE-ENDINGS. 89 Ley dig's Corpuscles : Skin of amphibians and reptiles. Genital Corpuscles: Clitoris, penis, etc., of man, etc. Articular Corpuscles: Phalangeal joints of man, etc. Tactile End-Btilbs: Skin of bill, lip, etc., of birds. Meissner's Corpuscles: Cutis of hands, toes, etc., of man. End-Bulbs. ( Cylindrical.) Cylindrical End-Bzilbs: Conjunctiva and mucous membranes of mammals. End-Capsules: Buccal glands of hedgehog ; tongue of elephant. Herbsf s Corpuscles: Skin and mucous membranes of birds. Key-Retzius Corpuscles: Skin of bill of birds. Voter's Corpuscles: Cutis and many other situations in man and mammals. Non-Striated Muscle. The sympathetic nerves supplying this tissue are composed of bundles of non-medullated nucleated fibres, and are enveloped by a thin perineurium ; these fibres are associated as small bundles and unite to form the ground-plexus, in the nodal points of which ganglion-cells usually occur. From this net-work NERVE-ENDINGS IN MUSCLE AND OTHER ORGANS. Fig. 109. Nerves of involuntary muscle from the plexus of Auerbach of intestine of dog ; gold preparation : g, nodal points of plexus containing ganglion-cells ; n, bundles of non-medullated nerve-fibres ; from these the small branches (/) extend which give off the fibres directly supplying the muscular tissue. small branches are given off, which join to make up the intermediate plexus; fine bundles of intramuscular fibrillae further extend directly to the contractile tissue. The fibrillae pass between the 90 primary bundles of the muscle-cells, and probably terminate in finely pointed or slightly thickened free ends ; the direct connection be- tween the nerve-fibrillae and the nuclei of the muscle-cells is, at best, extremely doubtful. Striated Muscle is supplied with both motor and sensory nerves ; the latter are distributed as a loose net-work, the fibrillae of which appar- ently terminate between the individual muscle- fibres. The medullated nerve- fibres composing the motor supply of a vol- untary muscle unite to form an intramuscular plexus, from which small bundles of nerve-fibres spring, and subsequently divide in such manner that a single medullated axis-cylinder passes to each muscle-fibre. At the point where the nerve pierces the sarcolemma the medullary substance abruptly ends, while the neurilemma, blended with the sarcolemma, joins the peri- neurial (Henle’s) sheath in forming the telolemma, or the sheath investing the end-organ. The axis-cylinder, now beneath the muscle-sheath, continues upon the surface of the sarcous sub- stance, and, later, breaks up into a number of somewhat tortuous ultimate fibrillae, which irregularly unite and end in thickened bulbous extremities. The terminations of the nerve are embedded in a flattened nucleated mass—the sole-plate—composed of soft faintly granular protoplasm, which resembles sarcoplasm and is closely applied to the surface of the muscular substance; this mass, together with the embedded nerve-fibrillae, constitutes the motor disk, or end-plate. Each muscle-fibre possesses usually but a single end-plate; in exceptional cases, however, there may be two or more; likewise, NORMAL HISTOLOGY. Fig. iio. Nerves of voluntary muscle of rabbit; gold preparation: n, small bundle of medullated motor nerve-fibres, from which fibres pass to the individual muscle-fibres (m) and bear the motor end-plates; s, some of the sensory nerve-fibres sup- plying the muscle. Fig. hi. Motor end-plate of voluntary muscle from rabbit: n, medullated nerve-fibre passing to muscle (m'l, on the surface of which the axis-cylinder ends in the dark arborescent figure; the latter lies em- bedded within the nucleated sole-plate (s) composed of granular protoplasm. several nerve-fibres instead of a single one may supply the end-plate. The nerve-endings in the voluntary muscle of amphibians and bony fishes differ from the foregoing in the absence of the granular protoplasmic disk, and in the more diffuse disposi- tion of the terminal nerve- fibres. The axis-cylinders, in these cases, branch into fibrillae which extend for some distance parallel to the axis of the muscle- fibre and end in slight bulbous expansions ; gran- ular pyriform nuclei also occur along the course of these fibrillae. The muscle-spindles described by Kiihne, and considered by some (Kerschner) as special sensory nerve endings, appear to be transient developmental structures connected with the cleav- age of the muscle-fibres (Kolliker). Tendon. In addition to the sensory end- plates of tendon, studied by Kolliker, Rollett, Sachs, Golgi, and others, which consist of an intri- cate net-work of pale non - medullated fibres, Golgi has described pe- culiar nerve-endings in tendon to be found in the immediate vicinity of the union with the muscle. THE PERIPHERAL NERVE-ENDINGS. 91 Fig. i 12. Golgi’s corpuscle or tendon-spindle from the human tendo Achillis; gold preparation : N, nerve-fibres surrounded by the perineurial sheath (Fs) spreading out into the reticular ramifications (Ev) of the axis-cylinder; A, the tendon- bundles, one of which is separated at b; Mf, the muscle- fibres; R, node of Ranvier. (After Ciaccio.) 92 NORMAL HISTOLOGY. These tendon-spindles appear as sharply-defined, greatly-elon- gated, elliptical masses (in the rabbit .25-75 mm. long and .02-01 mm. broad), one end of which extends upon the tendon, while the muscular pole is usually, although not always, continuous with the adjoining muscle-fibres. The tendon-spindle is composed of a distinct connective-tissue capsule, which, embracing two or more of the primary bundles of the tendon, becomes united with the sheath of the latter ; the inner surface of the spindle is covered with endo- thelial plates. Medullated nerve-fibres to the number of two, three, or four join the organ near its widest part, sometimes, however, at one end; after repeated division as medullated fibres, the nerves spread out on the surface of the tendon as pale non-medullated fibres, whose axis-cylinders unite to form a richly but irregularly meshed arborescent figure ; the ultimate fibrillae, in addition to the net-work, present numerous knobbed free ends. Blood-Vessels. The blood-vessels are accompanied by nerve- fibres derived from the sympathetic system ; in addition to the pale fibres, a few medullated ones usually take part in the production of the irregular net-work surrounding the larger vessels. From this plexus fine branches are given off, which ultimately end between the muscu- lar bundles of the media and within the fibro-elastic tissue of the adven- titia. The capillaries are accom- panied and partly surrounded by delicate non-medullated nerve- fibres. The muscular tunics of the large lymphatic trunks are supplied with nerves in a manner similar to the blood-vessels; the delicate, thin-walled lymphatics are probably without nerves. Glands. A detailed account of the nervous supply of the larger glands will be given in connection with the consideration of the several organs ; it may be mentioned here, in general, that the more important glands are provided, in addition to the medullated nerves often found passing through the substance of the gland in their course to the contiguous skin or mucous membrane, with nervous bundles in which non-medullated fibres predominate, but in which some medullated ones also occur. These bundles form an interlobular plexus, rich in ganglion-cells, which accompanies the larger excretory ducts and blood-vessels, and gives off a few branches to be distributed to the muscular coats of these tubes. Thin bundles of pale fibres bear the smaller ducts company as far as the primary groups of acini, and there break up into minute bundles of free axis-cylinders passing Fig. i 13. Nerve-fibres accompanying a small artery (■v), from the mesentery of rabbit; gold prep- aration. THE PERIPHERAL NERVE-ENDINGS. 93 between the acini. The nerve-fibrillae may be traced readily to the membrana propria of the acini, around which a net-work is spun ; regarding their ultimate distribution and relation to the secreting cells much uncertainty still exists, notwithstanding many elaborate investigations and positive statements. The exact mode in which the nerves terminate within the acini is still doubtful; it is probable, however, that the fibrillae end between, or in apposition with, the ends of the secreting cells directed towards the basement-membrane; proof of direct con- nection between the nerve-fibrillae and the se- creting cells, as often described, is wanting. Likewise the mode of termination of the med- ullated fibres, which, as already stated, con- tribute to form the interlobular net-work, is uncertain; in some glands, as in the pancreas of the cat and the buccal glands of the hedge- hog, they terminate in special nerve-endings resembling the corpuscles of Vater. The perceptive apparatus connected with the termination of the nerves of special sense include the highly specialized epithelial struct- ures made up of the neuro-epithelium ; the rod- and cone-cells of the retina, the hair-cells of the internal ear, the olfactory cells of the nasal fossae, and the gus- tatory cells of the taste-buds are important examples of such tissue. In these structures the specialized epithelium forms the apparatus for the reception of the external stimuli, while the nerve-fibres provide for the further transmission of the impressions so appreciated. The relation between the receptive cells and the conducting nerve-fibres must be, evidently, very intimate; a direct anatomical continuity between the two, however, must be regarded as extremely doubtful in the light of recent research. Fig. 114. Nerves ending in glands, from the parotid of dog; gold preparation : s, group of secreting cells of single acinus; n, nerve-fibre lying outside the membrana pro- pria and giving off twigs which enclose the acinus within a net-work of ter- minal nerve-fibrillse. NORMAL HISTOLOGY. 94 CHAPTER VII. THE CIRCULATORY SYSTEM. The circulatory apparatus comprises the channels for the con- veyance of the blood-stream, the vessels, and the dilated and special- ized portion of the vascular tube, constituting the heart, for the pro- pulsion of the current. In development and structure the several parts of the vascular system possess much in common, although variations in the details of the walls of the blood-channels suffice to distinguish the different portions. The blood-vessels occur in three forms, as arteries, veins, and capillaries, the latter constituting an expanded system of thin- walled tubules, intimately related to the organs, and especially de- signed to facilitate the interchanges be- tween the nutritive current which they carry and the tissues through which they pass. The arteries possess three coats—the inner, or intima, the middle, or media, and the external, or adventitia. Since these coats vary in relative thickness and THE BLOOD-VESSELS. Fig. 115. Fig. 116. Section of human artery of medium size : /, the intima, consisting of the endothelium (e), the sub-endothelial tissue (s), and the internal elastic membrane (jr); M, the media, com- posed of the involuntary muscle and the bundles of elastic tissue (y); A , the adventitia, containing irregular elastic trabeculae (2). Endothelium of artery of frog: the vessel has been treated with silver, hence the boundaries of the endothelial plates are indicated by the dark lines of stained cement-substance. Sev- eral pseudo-stomata appear as minute dark areas between the cells. in details of structure with the size of the vessel, it is usual to classify THE CIRCULATORY SYSTEM. 95 arteries as small, medium, and large. The first group includes the terminal branches near transformation into capillaries, the second, all the named arteries of the body, except those which, as the aorta or the pulmonary artery, are recognized as belonging to the third group of large arterial vessels. The inner coat, or intima, as seen in a typical artery of medium size, comprises three layers: (a) an endothelial lining, made up of long, lanceolate, nucleated plates, united by a sinuous line of cement-substance and placed parallel to the axis of the vessel; (6) a sub-endothelial layer of delicate fibrous connective tissue, with branched corpuscles; (c) a band of elastic tissue—the internal elastic membrane—which forms the most prom- inent part of the intima, appearing in sections of medium-sized ar- teries as a clear, glistening, and usually corrugated band separating the tissue of the inner coat from that of the media. The sub-endothe- lial tissue, which separates the en- dothelium from the internal elastic membrane, is wanting in the smaller arterioles, but appears in vessels of medium size as a longitudinally disposed layer, becoming more conspicuous with the increased calibre of the artery. In tubes of large diameter, as in the aorta, the sub-endothelial tissue appears as a stratum composed of layers made up of fibrous tissue, elastic net- works, and flattened connective-tissue cells. Likewise, the elastic tissue of the intima increases in amount and in complexity, in the large arteries the broad elastic fibres becoming fused together to form an almost continuous sheet—the fenestrated membrane of Henle. The middle coat, or media, is the muscular tunic, and consists principally of circularly disposed bundles of non-striated muscle-cells ; these elements, when isolated, appear as broad, nucleated, irregular spindle-cells, presenting ragged outlines. In many arteries, con- spicuously the subclavian, the inner portion of the media con- tains additional muscle-cells longitudinally disposed. In the smaller arteries the muscular tissue constitutes almost the entire media, but an insignificant amount of intermuscular fibrous connective tissue Fig. ii8. Fig. 117. Portion of the elastic tissue of the intima of the human aorta; the fibres are so broad and so closely grouped that they constitute an elastic sheet — the fenestrated membrane of Henle. Portion of the intima of the human aorta, silver stained : the larger stellate figures are the cell-spaces in the ground-substance be- tween the elastic bundles and contain the connective-tissue corpuscles. 96 NORMAL HISTOLOGY. being present; with the increase in the size of the vessel, however, the quantity of such tissue becomes greater, in addition to which bands of elastic tissue also make their appearance between the muscle-bundles. In the large vessels the fibro-elastic tissue forms a considerable portion of the media; in the aorta the elastic tissue occurs as robust circularly arranged bands, supplemented by oblique and longitudinal tra- beculae of similar nature ; these elastic fibres, together with the accompanying fibrous tissue, constitute the predominating structure, the muscle being less conspicuous in places than the intermuscular fibro-elastic strata. Owing to this generous admixture of fibrous tissue, the large arteries, while possessed abso- lutely of a greater amount of elastic tissue, have walls relatively less contractile than those of the smaller arteries, whose media is composed of almost pure muscular tissue. The external coat, or adventitia, is the most resistant tunic of the vessel, its characteristic strength being due to the generous amount of component fibro-elastic tissue. The fibrous tissue is arranged as closely- felted bundles, irregularly placed and intermingled with longitudinal bands of elastic tissue; numerous flattened con- nective-tissue cells lie between the bundles applied to the fibrous trabeculae. The mesh-work is closer and the amount of elastic tissue greater next the media than towards the outer surrounding con- nective tissue into which the adventitia insensibly blends. In the larger arteries the middle and outer coats are separated by a band of condensed elastic tissue— the external elastic membrane. Cer- tain arteries present peculiarities in their coats ; as examples of such varia- tions may be noted the slight develop- ment of the sub-endothelial tissue of the intima of the external iliac, renal, mesen- teric, and coeliac arteries, the appear- ance of longitudinal muscle-cells within the intima of the aorta, and the presence of longitudinally dis- Fig. i 19. Muscle-cells isolated from the media of human artery. Fig. 120. Section of aorta of child: /, M, and A, respectively intima, media, and adventitia. The thick stratum of sub-endothelial tissue and the layer of longitudinally disposed bundles of muscle (b) are peculiarities of the inner coat. THE CIRCULATORY SYSTEM. gj posed muscular tissue within the adventitia of other vessels (su- perior mesenteric, splenic, renal, and iliac arteries). In passing from medium-sized ar- teries towards smaller vessels, the coats become reduced in thickness, the media being earliest affected. The intima of the smallest ar- terioles consists of an endothelial layer alone, the middle coat in- cludes but a single layer of muscle- cells, while the external tunic is re- duced to a few longitudinal bundles. The vessels intermediate between small arteries and true capillaries no longer possess a complete layer of muscle-cells, the media being represented in such arterioles by scattered groups of circularly placed spindle-cells, forming an imperfect muscular sheet, which partially en- circles the vessel. The nuclei of these circular muscle-cells are trans- versely placed, while those of the endothelial plates are usually longi- tudinal or parallel with the axis of the vessel. Fig. 121. A, small human artery, in which the coats are reduced each to a single layer of cells; the media here consists of only one layer of muscle-cells (nt), which are seen in optical section : i, intima ; a, adventitia ; e, nuclei of the endothelial plates. B, an arteriole just before becoming a capillary; the vessel still possesses muscle-cells {m), but these are now arranged as irregular groups C, true capillary vessel, consisting of only an endothelial coat, the other tunics having disappeared; the nuclei are those of the endothelial plates. The veins possess the same tunics as the arteries, but, in general, are characterized by thinner walls and a preponderance of connective over the muscular and elastic tissues. There is, further, less regu- larity and constancy in the structure of the coats. The inner layer of the intima consists of a single layer of endo- thelial cells, rather broader and more polyhedral in form than those lining the arteries, the spindle shape being best marked in the smaller veins. The subendothelial tissue contains numerous con- nective-tissue corpuscles, and, in the larger veins, is arranged in distinct lamellae. An inner elastic membrane is generally present, in some cases taking the form of a fenestrated layer. The media consists of circular bundles of muscle-cells, associated with lamellae of fibro-elastic tissue in the larger veins. This coat is best developed in the veins of the inferior extremities, less so in those of the upper limbs. The muscle-tissue of the veins is sub- ject to many variations, both in amount and in arrangement, that THE VEINS. ng NORMAL HISTOLOGY. of the media is very scant or altogether wanting in a number of veins, including the thoracic part of the vena cava, the internal and external jugular veins, the veins of the pia and dura, of the retina, of bone, and of the corpora cavernosa. Certain veins possess longi- tudinal muscular bundles in the inner part of the media ; such are the mesenteric, umbilical, iliac, and femoral. The adventitia, often the thickest coat of the vein, consists ol stout net-works composed of bands of fibro-elastic tissue; in some veins additional bundles of plain muscle occur within this tunic. Among the venous trunks possessing well- marked, longitudinally arranged muscu- lar tissue in the external coat are the abdominal cava, azygos, hepatic, portal, splenic, axillary, superior mesenteric, renal, spermatic, and external iliac veins. The veins of the gravid uterus contain muscular tissue in all the coats, the prin- cipal bundles running longitudinally. The valves with which many veins are provided consist of crescentic folds of the inner tunic of the vessel, strengthened by additional fibro-elastic tissue; in some instances the muscular bundles extend for a short distance into the valve. The base or the attached margin of the valve is often its thinnest part, the free edges being somewhat thickened. The striated car- diac muscular tissue is continued for a short distance in the walls of those parts of the venae cavae and of the pulmonary veins immediately adjoining the heart; the explanation of this fact is found in the derivation of these portions of the vessels from the tissues of the primitive heart- tube. FlG. 122. Section of human vein of medium size: /, M, and A, respectively intima, media, and adventitia. THE CAPILLARIES. The capillaries establish the only communication, with few excep- tions, between the arteries and the veins, and, further, provide the intimate anatomical relation betwreen the nutritive current and the tissues of the body necessary for the maintenance of the integrity and functional activity of the various organs. As exceptions to the usual intervention of the capillaries between the arterial and venous radicles, the direct communication between these vessels existing in the erectile tissue of the genital organs, in the spleen, and in some parts of the peripheral circulation, as in the tips of the fingers and toes and of the nose, may be mentioned. THE CIRCULATORY SYSTEM. 99 The capillaries form rich net-works in almost all tissues and organs, the principal localities where these vessels are wanting being epi- thelium, the hairs, the nails, teeth, cartilage, the cornea, the crys- talline lens, and certain parts of the nervous system. The capillary net-works vary in the size both of the meshes and of the constituent vessels. The average diameter of the capillaries is 7-10 ,«; the smallest are found in the brain, retina, and muscle; the largest in bone-marrow, dentinal pulp, and the liver. The closest meshes are found in the air-vesicles of the lungs, the choroid, the liver, and other glands ; the widest in the serous membranes, tendon, etc. Young tissues are more richly supplied than old ones. The capillaries consist of a single layer of endothelial cells, united by intercellular cement-substance; they are, consequently, protoplasmic tubes of high vitality, admirably designed to facilitate the interchanges constituting nutrition. After staining with silver the endothelial plates are seen as extended spindle-cells, united by irregular lines of darkened cement-substance; at the points where the vessels branch, irregular triangular cells are not infrequently seen. In such preparations, likewise, along the lines of union or at the juncture of several plates, irregular darkened areas—the stigmata—may be observed; these are probably minute spaces occupied by stained albuminous sub- stances ; these areas are supposed to aid the diapedesis or trans- migration of the blood-cells. Some capillaries are invested by an imperfect adventitious coat, formed by a net-work of surrounding branched connective-tissue cells, and resembling the reticulum present in lymphoid tissue. The intimate relation existing between the endothelium of the vessels and the surrounding connective-tissue corpuscles is well exhibited in young growing tissues, as the omentum. The peculiarities distinguishing the capillaries from the small ‘ ‘ capillary’ ’ arteries or veins consist not so much in the size of the vessels—for the capillaries may have absolutely the greater calibre— as in the character of their walls. The true capillary possesses no muscle-cells, these first appearing in irregular groups beyond the limits of the capillary vessel; in those cases where, as in certain veins, muscular tissue is wanting, the character of the adventitia of the vein will aid in determining the character of the vessel. Small blood-vessels—the vasa vasorum—provide for the nutri- Fig. 123. Capillary blood-vessels from mesentery of young dog: n, the capillaries, with the nuclei of the endothelial plates, lying within the connective tissue (g). NORMAL HISTOLOGY. tion of the walls of the medium- and large-sized arteries and veins. These vessels arise some distance from the area which they supply, frequently coming from a different branch or, as in the case of the veins, from a neighboring arterial stem. The nerves of blood-vessels are mainly derived from the sym- pathetic system, and hence are principally of the non-inedullated kind ; a few medullated fibres, however, are usually present. The nerves accompanying the blood-vessel give off branches, which form surrounding plexuses; from these minute bundles pass, whose com- ponent fibrillae are distributed to the media and the adventitia. The capillaries are accompanied by correspondingly delicate fibres. Lymphatic clefts and vessels are found in the external coat of the larger vessels. In many places, as in the nerve-centres, including the organs of special sense, in the peritoneum, etc., the lymphatic clefts of the adventitia unite to form a large ensheathing circular sinus—the perivascular lymph-space—which separates a portion of the adventitia from the remainder of the vessel; as a result of this arrangement, the blood-vessel seemingly lies within the lymph-space. Perivascular lymphatics may be readily observed in the peritoneum of the frog. The heart-walls consist of three layers—the endocardium, the muscular layer, and the pericardium. The endocardium forms the serous lining of all parts of the organ, becoming continuous with the inner tunic of the blood-vessels at the several cardiac orifices. The inner free sur- face of the heart is covered with a single layer of polyhedral nucleated endothelial cells. These latter rest upon the substance proper of the endocardium, a stratum composed of fibrous connective tissue mingled with a felt- work of elastic fibres; the elastic net-works are especially well developed in the auricles, in certain parts of which the broad fibres join to form fenestrated membranes. The outer connective-tissue layer of the endocardium is continuous with the perimysium of the muscular tissue. The heart-valves are formed bv duplica- tures of the endocardium strengthened by bands of fibrous tissue enclosing numerous elastic fibres. The endocardial layer of the auricular side of the THE HEART. Fig. 124. Secti n of human heart show- ing endocardium : a, endothe- lium ; b, subendothelial con- nective-tissue stroma in outer layer (c), containing net-work of elastic fibres (e); d, trans- versely-cut bundles; ft mus- cular tissue. THE CIRCULATORY SYSTEM. 101 auriculo-ventricular valves is thicker than that of the ventricular surface. The roots or attached portions of these valves possess thickenings — the annuli fibrosi — composed of supplementary- masses of fibro-elastic tissue. The auricular muscle is continued into the valves for about one-third of their width, following closely the general contours of the fold. Within the larger chordae tendineae the papillary- muscles extend for some distance, in addition to which isolated muscle-bundles are also sometimes present. The semilunar valves possess a thin elastic layer on the arterial surface, aug- mented by a thick stratum of connective tissue, the bundles ex- tending parallel with the margin ■ of the valve; increased strength is secured by a fibro-elastic nodule, or corpus Arantii, which occupies the middle of each leaflet. Beneath the ventric- ular endocardium, in many animals (deer, sheep, calf, pig, horse, goat, dog, certain birds, etc.), but not in man, peculiar bands— the fibres of Purkinje —occur; these are muscular fibres whose transverse striations are limited to the pe- ripheral zone, while their centre is occupied by a large continuous mass of nucleated pro- toplasm. The fibres of Purkinje represent an embryonal condition of the muscular tissue, since the peripheral part of the fibre alone has undergone differen- tiation, while the central portion has remained indifferent protoplasm. Among some lower vertebrates, as fishes, a similar condition of the muscle-fibres is constant. Fig. 125. Section of the heart, including a leaflet of the semilunar valve of the pulmonary artery of child: a, a, cardiac, b, b, arterial, surface; c, recess behind the valve (_/"), constituting part of a sinus of Valsalva; d, free border of valve; et thickening near edge of valve corresponding to a corpus Arantii; g, endothe- lium, h, intima, i, media, k, adventitia, of the pulmonary artery ; the adventitia is continuous with the principal fibrous layer of the endocardium; m, cardiac muscle ; n, areolar tissue. 102 NORMAL HISTOLOGY. The muscular tissue of the heart possesses the peculiarities already described in Chapter IV.: it is composed of short, branched, nucleated fibre-cells, devoid of a sarcolemma, which unite to form an intricate net-work. The naked muscle-fibres are enveloped within a perimysium and are grouped into primary and secondary bundles, which are associated to form lamellae disposed in a very irregular and complex manner. The muscular tissue of the auricles is arranged in general as an outer transverse and an inner longitudinal layer, many small ad- ditional bundles deviating from the principal disposition to pursue independent courses in various directions. The muscle-bundles of the ventricles have a very intricate arrangement, the majority extending in an irregular oblique or spiral direction, some, in fact, describing a figure- of-eight in their course. The pericardium, which invests the exterior of the heart, and by reflection forms the pericardial sac, resembles the endocardium in possessing a single layer of endothelial plates covering its free surface, and a stratum of fibro-elastic con- nective tissue beneath. The parietal pericardium is distinctly thicker than the visceral, all the constituent layers being better developed. The subpericardial tissue covering the heart is continuous with the intermuscular connective tissue of the outer muscular layer; in this posi- tion numerous fat-cells lie between the bundles of the fibrous and the muscular tissue. The blood-vessels supplying the muscle of the heart are derived as branches of the coronary arteries. The principal trunks are situated in the larger interlamellar masses of connective tissue, within which they divide into numerous twigs giving origin to the capillaries; the latter penetrate the primary muscle-bundles, among and parallel to which they run. The relation between the individual muscle-fibres and the capillaries is more intimate than usually supposed, since, as shown by Meigs, the blood-vessels deeply impress the fibres, and in many places are surrounded completely by the muscular tissue. The extraordinary demands made upon the nutrition of the heart- tissue as the result of its remarkable functional activity explain the Fig. 126. Section of human heart, including pericardium: a, endothelium of pericardial surface; b, subendo- thelial fibrous tissue ; c, net-works of elastic fibres ; d, subpericardial areolar tissue containing fat-cells embedded between pericardium and muscle (e); v, blood-vessel. THE CIRCULATORY SYSTEM. necessity for such close arrangement. The deeper fibrous layers of the pericardium and of the endocardium receive numerous capil- laries, a few being also found within the chordae tendineae and the valves. The lymphatics of the heart are very numerous. They form a comprehensive system, embracing the lymph-spaces occupying the clefts between the muscle-fibres and the rich net-works of more definite channels extending within the pericardium and endo- cardium, including the valves. These two sets of lymph-radicles communicate but sparingly and pursue largely independent courses. Lymphatic vessels also accompany the branches of the coronary arteries. The rich nervous supply of the heart is derived from the coro- nary plexuses, and includes numerous medullated fibres coming from the pneumogastric, as well as the non-medullated sympathetic fibres proceeding from the cervical ganglia. Numerous microscopic gan- glia are found along the course of the larger nerve-trunks accom- panying the branches of the coronary arteries, especially in the longitudinal interventricular and in the auriculo-ventricular furrows. Many additional small groups of ganglion-cells occur within the muscular tissue associated with the fibres supplying the intimate structure. The nerves and the blood-vessels are covered by the visceral pericardium. The development of all parts of the circulatory apparatus takes place within the mesoderm; while possessing a common origin, the blood-vessels and the heart, however, develop in- dependently, and, for a time, are distinct and dis- connected. The earliest blood-vessels appear near the periphery of the vascu- lar area, outside the limits of the proper body of the embryo ; later and second- arily they extend centrally and unite with the primitive heart and those parts of the large trunks which have been formed coincidently within the embryo. The mesodermic elements within certain tracts near the periphery of the vascular area undergo proliferation, which results in the pro- duction of deeply staining densely nucleated areas known as the blood-islands of Pander; these are the direct progenitors of the 103 Fig. 127. Developing capillary blood-vessels within the omentum of young rabbit: a, a, elongated protoplasmic processes connecting the walls of the newly-formed capillary (c) with the angioblastic connective-tissue corpuscles (3). NORMAL HISTOLOGY. earliest blood-vessels and the first blood-cells. The blood-channels appear within the nucleated “islands” as spaces which follow the partial breaking down of the inner portions of the areas. The peripheral zone of the nucleated cell-mass becomes the endothelium of the future blood-vessel, while, probably, certain of the enclosed mesodermic elements persist as the primary blood-cells. After a time the mesoderm surrounding the newly-formed endothelial tube differentiates into the muscular and other tissue of the remaining coats. The endothelium is, therefore, genetically the oldest part of the vessel, although its characteristic appearance, as seen in silvered adult tissue, is not visible until further differentiation has taken place. The blood-channels are further extended by the fusion of elongated mesoblastic cells with those of the walls of the primary vessels, the lumina of the latter gradually entering the solid processes, which are thus converted into tubes. After the development of the earliest vessels in the manner indicated, the formation of all new vessels subsequently, in pathological processes as well as in normal ones, is associated closely with the connective-tissue cells, since solid protoplasmic processes of the united cells become later the walls of the young vessel. The development of the heart resembles that of the extra- embryonic vessels in so far that the part first formed —the primary endothelial tube—originates by the differentiation of the mesodermic cells and the hollowing out of the tissue lying enclosed. In its very early stage the mammalian heart exists as two distinct and widely-separated tubes, which later unite to form a single sac. Outside the primary endothelial heart the mesoderm differ- entiates into the muscular tissue of the cardiac wall, but for some time the endo- thelial and muscular layers continue as independent tubes, the inner endothelial lining appearing as a shrunken cast repro- ducing the contours of the larger muscular organ. The two tunics are connected by numerous bridging bands, which increase in number and size with the progress of the development of the organ; these primary tra- beculae are represented in the adult organ by the columnae carneae and musculi pectinati. The pericardium originates as the special- ized layer of mesoderm—the mesothelium—forming the immediate boundary of the general primary body-cavity, of which the peri- cardial sac is only a constricted portion. Fig. 128. Section of a part of the develop- eleven days: ,, the endothelial tube, within which lie several of cells (*); m, the slightly differ- entiated mesobiastic ceils, which later become the muscular tissue. THE CIRCULATORY SYSTEM. 105 THE BLOOD. While, when physiologically considered, the blood is regarded, with Bernard, best as an internal medium of exchange, histologically it may be classed as a mesodermic tissue possessing a fluid inter- cellular substance, the liquor sanguinis; in the latter float the cellular elements—the blood-corpuscles. The morphological constituents of the blood are of two kinds, the colorless or white corpuscles and the colored or red cells ; to these must be added a third variety, the blood-platelets or blood-plaques, which are probably constant and independent ele- ments. The colorless blood-cells, or leucocytes, are not peculiar to the blood, since they originate in lymphoid tissues and are carried by the lymphatic trunks into the blood-current, in which fluid they usually are observed. These cells represent a widely-distributed element, whose names are as various as are the localities in which it is encountered. The “lymph-corpuscle,” “lymphoid cell,” “adenoid cell,” “white blood-cell,” “leucocyte,” “ leucoblast,” “wandering cell,” etc., are but different names for the same morphological element. The colorless blood-cell consists of a minute nucleated mass of active protoplasm, when at rest, presenting a round or spherical form and measuring about io /x in diameter. In its usual condition, however, the outline of the corpuscle is undergoing continual variation, these changes being known as amoeboid on account of their similarity to those exhibited by the amoeba. Under moderate amplification the protoplasm of the leucocyte appears faintly granular and includes a single nucleus, rarely multiple, which is ordinarily somewhat obscured by the overlying cell-contents. Additional coarse granules are of frequent occurrence, especially within the protoplasm of particular cells ; these latter Ehrlich clas- sifies, according to the affinity of their granules for certain stains, into a-cells (acidophile, eosinophile), ft-cells (amphophile, indulinophile), y-cells (mast-cells), 5-cells (basophile), and e-cells (neutrophile). The exact nature and significance of such cells, however, are still uncertain. Under high amplification the protoplasm of the white blood-cell often displays an imperfect reticulation as a transient THE COLORLESS CELLS OF THE BLOOD. Fig. 129. Colorless blood-cells of man, highly magnified: r, cor- puscle in condition of rest, as a spherical mass of proto- plasm ; the other cells are actively moving and exhibit a hyaline apparently struct- ureless substance in the most advanced parts of the cells. 106 NORMAL HISTOLOGY. structure, as well as nuclear fibrils. Pole-corpuscles and at- traction-spheres have been described by Flemming as constant constituents of the white blood-corpuscle. Division of these elements in many instances undoubtedly is accompanied by the regular cycle of karyokinesis; very commonly, however, it is equally certain, the colorless corpuscles are reproduced by direct, amitotic division. Examination of actively moving cells under high amplifi- cation emphasizes a distinction in the character of the protoplasm, that part of the cell constituting its most advanced portion seem- ing more homogeneous than the remainder of the body of the cell. The colorless cells of human blood are larger than the red corpuscles, but are much fewer in number, the ratio between the two kinds of elements being, under normal conditions, about three hundred and fifty red cells to one white corpuscle. The actual number of white cells present, however, depends upon various cir- cumstances, since during digestion the number of colorless elements is increased, while fasting greatly reduces the proportion of the leucocytes; in general these cells are more numerous in venous than in arterial blood. The colorless blood-cells must be regarded as playing a double rdle: in addition to maintaining an ever-available store of reserve active protoplasm with which to meet and to repair the destructive processes taking place normally as well as in disease, they are actively engaged in the absorption of solid and fatty matters, being capable of taking up and carrying away injurious debris. Certain of these cells—the phagocytes of Metschnikoff—seem especially aggressive in their attacks against offending foreign substances, within a limited degree including possibly the waging of a successful warfare on obnoxious microbes. The adult mammalian red blood-cell represents a condition of retrogression, since in its development it has suffered the loss of its nucleus and a profound metamorphosis of its protoplasm, changes of such importance that some authorities dispute the propriety of regarding the mammalian red blood-corpuscles as true cells. The presence or absence of the nucleus within the colored corpuscle, together with its general form, furnishes a basis for a division of all vertebrate bloods into— A. Those having nucleated, oval red corpuscles : including fishes (except cyclostomata, which have round, discoidal cells, as the lamprey), amphibians, reptiles, and birds. B. Those having non-nucleated, round, discoidal red cor- THE COLORED CELLS OF THE BLOOD. THE CIRCULATORY SYSTEM 107 puscles: including man and other mammals, except the camel family, which have oval, non-nucleated red blood-cells. Since an oval corpuscle on being subjected to certain reagents may present a circular outline, the presence or absence of a nucleus offers the most reliable means of differential diagnosis between mammalian and other bloods. The human colored blood-cell is a small round disk, measuring about 8 ii in diameter, and exhibiting individually a faint greenish- yellow tinge. The well-known color of the blood appears only when great numbers of these corpuscles are massed ; the term “ red” conventionally applied to these elements is, strictly regarded, incorrect and less appro- priate than “colored.” The two surfaces of the blood-disk are not perfectly flat, the centre of the corpuscle being slightly biconcave, while its edges are rounded, biconvex, and somewhat thickened : in consequence of this peculiar “biscuit” form, all planes of the corpuscle are not seen accurately focused at one time, the centre usually appearing either darker or lighter than the marginal parts of the cell, depending upon the focal adjustment. The structure of the colored blood-corpuscles is still a subject of discussion. According to the generally accepted view, the cor- puscles consist of two parts : (a) the transparent, colorless, apparently homogeneous, and plastic stroma, extensible and pliable to a high degree, and (6) the coloring matter, or haemoglobin, which is held within, and uniformly distributed throughout, the former. This conception of the corpuscle assumes the presence of a uniform though highly flexible stroma-mass of definite form, colored by the imbibition of the soluble haemoglobin. On the other hand, the behavior of these elements when treated with water, upon the addition of which the corpuscles swell, lose the discoidal form, and become globular, as well as the suggestive appearances following the staining with aniline of such bleached corpuscles, the outlines of the cells then showing as distinct rosy rings, offers strong arguments, in the opinion of not a few, for the belief that the red corpuscles are minute sacs, consisting of a limiting membrane and the colored fluid contents. The nuclei of the red cells, when present, lie embedded within the colored stroma; in perfectly fresh or circulating corpuscles they are made out with great difficulty, since they possess a refractive index almost identical with that of the other parts of the cell. After reagents, or after the expiration of some minutes, the nuclei become Fig. 130. Human blood-cells : w, color- less corpuscle, surrounded by red cells ; those at r exhibit a partially-formed rouleau. 108 NORMAL HISTOLOGY. very evident, and correspond in appearance and structure with those of other cells, one or more nucleoli often being visible. In fresh blood the red corpuscles within a few minutes arrange themselves in rows or piles by the apposition of their broader sur- faces, thus forming figures which, from their resemblance to rolls of coin, are termed rouleaux. The cause of this phenomenon is still uncertain, although it is not improbable that it is to be attributed to the presence, in the fresh corpuscles, of a film of a nature repelling the liquor sanguinis and favoring the adhesion of the disks; the rouleaux are only temporary, the corpuscles later spontaneously separating and remaining apart. It is of interest to note that only discoidal corpuscles of mammalian bloods (including, however, the discoidal cells of the lamprey) run together to form these figures, the projecting nuclei and the slight biconvexity of the oval nucleated cells affording surfaces evidently unfavorable for adhesion. The average diameter of the red corpuscles in the various races of mankind is identical, being between 7 and 8 or about 1-3200th of an inch. The size of the animal bears no relation to that of its red blood- cells, as shown by the following measurements of some mammalian bloods, based on the observations of Gulliver : Millimetre. Millimetre. Millimetre. Elephant. . . .0092 Guinea-pig . . .0071 Pig ... . Sloth . . . . . .0086 Dog . . . . Horse . . . . .0059 Whale . . . . .0080 Rabbit . . . Cat.... . . .0058 Man . . . Bear . . . . Sheep . . Beaver . . . . .0076 Mouse . . . Goat . . . Monkey . . . .0074 Ox . . . . . .0048 Muskdeer . . .0024 The largest corpuscles are those of the amphibians, the red cells of the frog measuring .0016 mm. in breadth by .022 mm. in length, those of the triton, .019 by .029, and those of the proteus, .035 by .058. The maximum size is reached in the huge red cells of the amphiuma, which are no less than .046 mm. wide by .075 mm. long, and are readily distinguishable by the unaided eye. The number of colored cells normally present in one cubic millimetre of human blood, as determined by the haemacytometer, is about five millions; these figures are modified by sex, the male subject usually having more corpuscles than the female. The number of red corpuscles varies in different animals : the carnivora possess a greater number of cells in a given quantity of blood than do the herbivora; in birds the proportion is still larger ; while in the sluggish amphibians the number of the huge red cells is reduced to thousands. Effect of Reagents applied to Human Blood. No elements THE CIRCULATORY SYSTEM. 109 are more sensitive to changes in environment or to the effects of reagents than are the cells of the blood. An appreciation of the alterations referable to external causes is important as guarding against unwarranted conclusions as to the existence of pathological conditions, since not infrequently ap- pearances which lead the tyro to infer disease may be ascribed to influences acting on the corpuscles outside the body. If fresh blood be exposed to a current of air, subjected to undue pressure or to other disturbing influences, alterations of the corpuscles at once take place. One of the most common distortions affects the exterior of the red corpuscles, and results in the formation of a number of minute projections, or spines, pro- ducing a condition known as crenation. Saline Solutions. The application of a weak saline solution or of urine is attended with similar effect; if the strength of the reagent be gradually increased, a corresponding progressive degree in the distortion is observed, until, finally, upon the addition of a concentrated brine, a shrivelled, shapeless mass replaces the former discoidal red corpuscle. The reaction is less marked upon the colorless cells, weak salines pro- ducing no perceptible change, while a slight shrinkage is noticeable after the stronger solutions. Water. Upon the application of water the colored cells swell up, lose the discoidal form, and become spherical, and at the same time part with their coloring matter, the haemoglobin; the latter, being dissolved, leaves the bleached and colorless stroma to form the ‘ * ghost. ’ ’ That the red corpuscles are not destroyed by the water, as sometimes stated, may be demonstrated by the addition of a suitable aniline dye, when the presence of the bleached corpuscles is made evident by the colpred rings which mark their outlines. The action of water upon the living color- less blood-cells is somewhat different. These corpuscles cease their amoeboid movements, retract their Fig. 131. Red blood cells of man and of am- phiuma, magnified to the same extent to show the size of the human cor- puscles in comparison with that of the largest known blood-cell. FlG" 132 Reactions of human blood- cells with various reagents: A, effect of treatment with water upon the white (w) and the colored cell (r); B, red cells after the addition of saline solutions, crenation following the application of weak solu- tions, great shrinking and dis- tortion (s) succeeding the action of the concentrated reagent; C, action of dilute acetic acid on the colorless cell (w) and on the red cor- puscle (r); D, red blood-cell after the addition of one-per- cent. solution of tannic acid. 110 NORMAL HISTOLOGY. processes, become round, and swell up into larger spheres; mean- while, the protoplasm resolves itself into a number of sharply-cut granules, which, owing to their suspension within a fluid of less density than the blood-plasma, exhibit the active dancing or oscil- latory movements which constitute the “ Brownian motion,” a phenomenon entirely physical in nature. The nuclei of the colorless cells after treatment with water appear as clear or slightly granular areas among the vibrating particles. After a time the distention of the corpuscle becomes too great, and rupture takes place, followed by the escape of the particles of disintegrated protoplasm. Acids. Upon the addition of weak acetic acid the red cells become rapidly decolorized, at the same time losing the discoidal form and approaching the spherical. The protoplasm of the color- less corpuscles clears up entirely, the nuclei coming very con- spicuously into view. Upon subsequent treatment of acid prepara- tions with aniline, the nuclei of the white cells appear deeply stained, while the red cells are outlined by faintly-colored rings. Tannic acid, when applied to the red corpuscles in weak (one- half to one per cent.) solutions, produces a peculiar effect: the coloring matter of the corpuscle is coagulated as it escapes from the cell and becomes conspicuous as a minute accumulation adhering to one edge of the corpuscle. Where strong solutions of tannic acid are employed, the haemoglobin is coagulated within the corpuscle before it has had an opportunity to escape, producing appearances which have been mistaken for nuclei and other details of cell-structure. The Blood-Platelets. If human blood be drawn directly into a drop of osmic acid solution (one per cent.) or of a three-fourths per cent, solution of sodium chloride, covered at once, and examined with a high power, numbers of small, colorless, circular disks will be seen on careful observation; these are the blood-platelets of Bizzozero, sometimes called the third corpuscular ele- ments of the blood. They are very unstable, prone to disintegration, and are variable in size, possessing an average diameter of about one-third of that of the red cells; they occur singly, but show a marked disposition to run together in groups preparatory to breaking up into the minute particles long known as the granules of Max Schultze. Unless great precaution is taken to insure the immediate action of the preserving fluids, the blood-platelets will not be seen in their normal form. Fig. 133. A, human red blood-cells and blood-platelets (/); g, minute fatty (?) particles, which occur isolated or in masses; B, fibrin filaments, among which lie par- tially disintegrated blood-plate- lets. THE CIRCULATORY SYSTEM. 111 These bodies may be recognized in the circulating blood, as ob- served by Osier and others, and are constant, although numerically variable, elements of mammalian bloods. The peculiar elongated elliptical “blood-spindles” found in the blood of other vertebrates are probably to be regarded as the homologues1 of the blood-plaques of mammals. While the presence of the blood-platelets as distinct, constant, and normal constituents of the human blood is now gener- ally recognized, authorities are far from accord as to their significance. The evidence at present seems to point to a close relation between these bodies and the process of coagulation, in view of their probable active role in the production of the factors in the formation of fibrin. Fibrin filaments are to be observed in a drop of blood mounted in the usual manner for microscopical examination and allowed to stand for some time in a moist chamber; they appear as very delicate straight interlacing threads which occupy the interspaces between the corpuscles and frequently radiate from a common centre, con- taining a group of partially broken-down blood-platelets. Additional minute particles are to be seen in human blood, regarding the nature, source, and significance of which much has been surmised and but little definitely established. These include the small colored disks, the microcytes or the hsematoblasts of Hayem, according to whose authority they constitute an important source of the red corpuscles; by others they are regarded as sep- arated portions of the ordinary red cells. Other minute, colorless, often highly refracting, granules are encountered floating in the liquor sanguinis; such are the elementary particles of Zimmer- mann and the granules of Max Schultze. These particles differ in nature as well as in source; some probably are derived from the disintegration of the white corpuscles and of the blood-platelets, others from that of the endothelial plates of the vascular channels, while many represent fatty granules absorbed during digestion or taken up, possibly, in the course of pathological processes. Blood-Crystals. The coloring matter of the blood—the haemoglobin—readily crystallizes in man and most mammals as elongated, rhombic prisms ; the haemo- globin crystals of the squirrel and of the guinea-pig, however, are respectively hexagonal plates and rhombic tetrahedra. These blood- crystals, of a deeper or lighter red color according to their size, often form in preparations of blood which have been sealed and allowed to stand after the addition of a few drops of water; the blood of the rat is especially favorable for their production. If dried blood be treated and thoroughly mixed with glacial acetic acid (the addition Fig. 134. Haemin crystals from dried human blood. NORMAL HISTOLOGY. of a few granules of common salt being advantageous in the case of old clots), on slightly heating until bubbles appear, numbers of dark-brown irregular rhombic prisms form. These are the haemin crystals of Teichmann, which are positive indications of the presence of blood, but have no value in the determination of its source. They vary greatly in size and considerably in form, the peculiar unequally-notched ends presented by the larger crystals being quite characteristic. DEVELOPMENT OF THE BLOOD-CORPUSCLES The origin of the colorless blood-cells must be referred to the lymphoid tissues, since these elements are identical with those occurring within the lymph with which they are poured into the blood-current. The colorless corpuscles appear later than the red cells, the first ones probably entering the circulation as migratory mesodermic elements. The lymphatic or adenoid tissues, however, undoubtedly constitute the principal sources of the colorless blood- corpuscles, which are produced by the division of the numberless masses of active protoplasm contained within the various aggrega- tions of lymphoid tissue throughout the body. The multiplication of existing colorless cells which takes place normally, but which is especially active under the stimulus of patho- logical conditions, accounts for the origin of a certain number of white corpuscles ; the division of the cellular elements of connective tissue is regarded by some as an additional source of these blood- cells. The efferent lymph-streams passing from the lymphatic tissue, as well as the blood contained in the splenic vein, are richer in color- less cells than are the corresponding afferent currents, showing that the augmentation is due to the new elements contributed by the lymphoid tissues through which the currents pass. The origin of the colored blood-cells is usually considered as taking place during two epochs—before and after birth. It must be remembered, however, that such division is conventional and largely arbitrary, since the period at which the primary embryonic processes of such formation cease and are replaced by those maintained throughout life is uncertain and variable; in man and mammals born in a condition of advanced development the production of blood-corpuscles within the marrow is instituted before the termina- tion of intra-uterine life. Before Birth. The first blood-cells originate outside the body of the embryo, within the angioblastic cells of the mesodermic tract of the vascular area. Certain cells of this layer increase in size and undergo proliferation of their nuclei, forming multinucleated areas known as the blood-islands of Pander. These subsequently THE CIRCULATORY SYSTEM. unite into an irregular net-work, the nodal points of which are dis- tinguished by an active production of new nuclei. Some of these acquire protoplasm and later become the endothelium of the blood- vessel, while others, more centrally situated, are converted into the primary blood-corpuscles, the intervening tissue undergoing liquefaction to constitute the blood-plasma. These earliest blood- cells, although destined to become the red corpuscles, are at first colorless masses of active protoplasm, provided with nuclei and exhibiting amoeboid movements. After a time the protoplasm gradually acquires the characteristic tinge and assumes a discoidal form, the elements then constituting the nucleated red blood-disks of the embryo. The earliest red cells unquestionably increase by the division of the primary corpuscles, the reproduction being attended by the changes of karyokinesis. This multiplication of the early red corpuscles probably ceases in man long before the end of gestation, the embryonal colored corpuscles meanwhile becoming smaller and losing their nuclei, so that at birth all the nucleated red cells have disappeared. The exact details of the metamorphosis from the embryonal to the adult form are still uncertain. There is no evidence at present to establish the descent of the red corpuscles from the colorless cells, the two being distinct elements having independent origins. The liver must probably be reckoned among the situations in which the formation of blood-cells takes place during embryonal life ; in this same category is included the spleen by some authorities, indeed, Bizzozero regards it as a post-natal source. After Birth. Of the many suggested sources for the post-natal production of the red blood-cells, of which great numbers must be formed constantly to replace those continually undergoing destruction, the red marrow of bones is undoubtedly the most important. Among the more common elements of the red marrow, cells usu- ally are to be observed which strongly resemble the embryonal red blood-corpuscles, being distinguished from the ordinary marrow- cells by their haemoglobin-colored protoplasm, smaller size, and unstable nuclei. These cells, often called the erythroblasts, are undoubtedly transitional forms of red blood-corpuscles, the nuclei disappearing and the protoplasm assuming the usual appearance of such elements. As to the source of the erythroblasts, however, -whether they are transformed colorless marrow-cells or distinct elements, the descend- ants of the red corpuscles of the embryo, much uncertainty still exists. There are strong reasons for regarding the latter supposition the true indication of their nature and origin, the production of the red corpuscles both before and after birth being thus closely related. Direct transformation of the colorless cells, production within the 113 114 NORMAL HISTOLOGY. spleen, and growth from the blood-platelets, or haematoblasts of Hayem, have been advanced from time to time as additional sources of origin of the red blood-corpuscles. Without entering upon a detailed critical consideration of the evidence supporting these views, it may be stated that, at present at least, they all lack the conclusive proof of unimpeachable direct observation. Concerning the rela- tions of the “haematoblasts” much confusion exists in consequence of the application of the term to different objects by various writers. The exceptionally small red corpuscles, or “microcytes,” together with those of unusually large diameter, may be regarded as ex- pressing the extremes of variation in size to which all morphological elements are subject. The formative processes within the red bone- marrow may be regarded, in the light of our present knowledge, as the most important source, if, indeed, not the sole authentic one, of the new red blood-corpuscles produced throughout life. Mention may be made in this place of the problematic organs the so-called arterial glands, which include the coccygeal and carotid glands. The first of these, the glandula coccygea, or Luschka’s gland, occurs near the tip, in front of the apex, of the coccyx, associated with the middle sacral artery, which contributes the blood-vessels largely forming its pea-sized mass. The carotid gland lies at the bifurcation of the common carotid artery, frequently between the resulting branches, and appears as a somewhat flattened ovoid nodule. These peculiar bodies are identical in structure, both consisting of dense arterial net-works surrounded by irregular groups of granular polyhedral cells, whose presence suggested the once supposed glandular nature of the organs. The entire plexiform mass is invested by connective tissue, from which fibrous septa pene- trate between the vascular structures. Numerous non-medullated nerve-fibres are also present. The true nature and function of these rudimentary organs are en- tirely unknown, and probably will remain so until the embryology of these bodies is better understood. THE LYMPHATIC SYSTEM. 115 CHAPTER VIII. THE LYMPHATIC SYSTEM. The lymphatic system consists of two parts—the lymph-channels and their contents the lymph, and the lymphatic tissue. The former may be represented by irregular interfascicular clefts between the bundles of fibrous tissue or by vessels with well-defined walls, while the latter may exist as diffuse adenoid tissue, the simple lym- phatic nodule, or the complicated compound lymph-gland. The lymphatic spaces, the radicles of the more distinct vessels, are almost universally present, since they exist in almost every locality where connective tissue abounds, forming intercommuni- cating systems of greater or less perfection throughout the various organs. The relation between the connective tissue and the lymph-radicles is very intimate, and it may be assumed that all interfascicular clefts are directly or indirectly con- nected with the lymphatics. In loose areolar tissues, as the sub- cutaneous, the lymph-spaces are ill-defined clefts, irregular in form and size, which are bounded by the neighboring bundles of fibrous tissue and lined by an imperfect layer of endo- thelioid connective-tissue cells. In the denser forms of fibrous tissue, as the central tendon of the diaphragm, cornea, etc., the lymph-spaces are more limited and form well-defined intercommuni- cating systems of canals, or “juice-channels of such the corneal spaces and the bone-lacunae are familiar examples. These spaces are filled incompletely by the connective-tissue cor- puscles, which usually are applied to one wall of the cavity to form a partial lining. The number of cells occupying a single space varies : sometimes several lie side by side (kitten’s cornea) united by lines of cement-substance; in such cases, after silvering, the cells present the appearance of endothelial plates. The large serous THE LYMPH-CHANNELS. Fig. 135. Lymph-spaces between bundles of fibrous tissue seen in profile, from the human cornea : b, 6, bundles of fibrous tissue ; s, lymph-spaces containing flattened connective-tissue cells. 116 NORMAL HISTOLOGY. cavities, as the peritoneal or pleural sacs, are, in principle, but greatly-dilated lymph-spaces, lined by modified connective-tissue cells, the en- dothelial plates, which by mutual press- ure become polygonal in outline; in- Fig. 137. Fig. 136. Lymph - capillary from silver-stained mesentery of frog: a number of lymph- corpuscles occupy the deli- cate endothelial tube which constitutes the vessel. Lymph-spaces of cornea, surface view: a, the spaces within the ground-substance (c) con- nected by the minute canals (£), or canaliculi. stead of a few cells sufficing for the formation of a lining membrane, as in the case of the minute lymph-space, innumerable ele- ments are required to clothe the large serous cavity. The lymphatic spaces within the connective tissue join to form definite channels at the margins of the fibrous tissue, the lymph being carried by the lymphatic vessels from the organs to the adjacent masses of adenoid tissue, the lym- phatic glands. The lymph- vessels immediately succeed- ing the spaces may be regarded as the lymphatic capil- laries, being protoplasmic tubes of great delicacy, com- posed of a single layer of en- dothelial plates. The contours of the lym- phatic vessels are not uniform, but present numerous dilatations and constrictions, which indicate the positions of the imperfect valves: Fig. 138. Lymphatics of silvered diaphragm of rabbit: s,s, lymph-spaces lying within the deeply-stained ground- substance ; l, l, lymphatic vessels lined with endo- thelium and possessing valves (v) and corresponding dilatations. THE LYMPHATIC SYSTEM. XIy these latter consist of a fold of endothelium, strengthened often by a minute quantity of elastic tissue. The relation of the lymph-spaces to the capillary blood-vessels on the one hand and to the lymphatic vessels on the other is very inti- mate ; in certain localities, as in the omentum, indirect communication between the blood-vessels and lymphatics is established by means of the spaces of the grcfundwork of the dense connec- tive tissue (Klein). Many nerve-trunks are en- closed by perineurial lymphatic channels, into which the lymph-spaces of the surrounding tissue open. The blood-vessels of the central nervous system, especially of the retina, likewise are surrounded by distinct perivascular lymph- sheaths, formed by the enlargement and con- fluence of the clefts within the adventitia of the vessels. In some membranous structures, notably the amphibian mesentery, the vessels lie encased within distinct endothelial tubes. Lymphatic vessels of large size have walls of considerable thickness, resembling those of the veins. In such vessels three coats are recog- nizable—the inner, or endothelial, the middle, or muscular, and the outer, or connective tissue. The thoracic duct possesses a well-developed intima, composed of a considerable layer of subendothelial connective tissue con- taining a net-work of longitudinally dis- posed elastic fibres. The muscular tissue of the media is supplemented by bundles of involuntary muscle extending length- wise within the outer coat, which in the vessel under consideration is particularly robust. The lymph contained within the lym- phatic vessels, like the blood, consists of two parts — the clear, straw-colored plasma, or liquor lymphse, and the cellular elements, the lymph-cor- puscles. The cells of the lymph are small nucleated masses of active proto- plasm, when at rest presenting a spherical form and measuring about .01 mm. in diameter; in their usual condition of activity, however, their outlines Fig. 139. Perivascular lymphatic (b) enclosing a small ar- tery (a), from the silvered mesentery of frog: c, branching lymphatic cap- illary. Fig. 140. Transverse section of human tho- racic duct: i, m, and o, respectively the inner, middle, and outer tunics ; x, endothelial lining, beneath which lies the fibrous stratum containing net-work of longitudinal elastic fibres (j/); z, longitudinally disposed bun- dles of muscular tissue within adven- titia ; v, capillary blood-vessels. 118 NORMAL HISTOLOGY. are continually undergoing the changes effected by amoeboid move- ment. These elements, in short, possess all the peculiarities of the colorless blood-corpuscles with which, in fact, they are identical. In addition to the lymph-corpuscles, numerous fatty granules are usually present within the plasma; in the lymphatic vessels of the intestinal tract the absorption of fatty matters is made conspicuous by the presence of the chyle, an emulsion occupying the so-called lacteals, or chyle-vessels; these latter are not distinct tubes, but only those portions of the lymphatic net-work which convey the milky-looking chyle during certain stages of digestion. The sources of the lymph-corpuscles are those already con- sidered in connection with the colorless cells of the blood, the lym- phoid or adenoid tissues of the body being unquestionably the most important and prolific seats for the production of these elements. The presence of a few cells within the lymph-radicles, between their commencement and the first masses of adenoid tissue occurring on their course, is due to the entrance within the vessels of migratory cells from the surrounding connective tissue; only after the lymph- stream has passed through considerable masses of lymphoid tissue do the corpuscles appear with profusion. Lymphatic, lymphoid, or adenoid tissue usually occurs as circumscribed masses known as lymphatic nodules or “glands;” in certain localities, however, as in parts of the mucous membranes of the larynx, the pharynx, the stomach, the intestines, etc., ill-defined masses of diffuse lymphatic tissue occur. These are recognized as aggregations of small round cells, fading away among the surround- ing structures. Lymphatic tissue, wherever found, is com- posed structurally of two elements—the deli- cate connective-tissue reticulum, on the surface of the fibres of which plate-like, often stellate, connective-tissue corpuscles are applied, and the small round cells contained within the reticulum. These elements—the lymphoic or adenoid cells—become the lymph-cor puscles and the colorless blood-cells on their escape from the denser reticulum into the lymph-current and their subsequent entrance into the blood. The variations in the compactness with which the cells are lodgec THE LYMPHATIC TISSUES. Fig. 141. Elements of adenoid tissue from partially brushed sec- tion of lymphatic gland of child : a, fibres of reticulum ; b, lymphoid cells; c, ex- panded connective - tissue plate. THE LYMPHATIC SYSTEM. 119 within the net-work constitute the denser or looser forms of adenoid tissue found in the lymphatic nodules; ordinarily the cells are so closely placed that the reticulum is greatly masked, satisfactory views of the latter being obtained only in sections of great thinness or after the cells have been removed by brushing or by violent agitation. The reticulum of lymphoid tissue consists of intertwining and anastomosing bundles of connective tissue ; along the fibrous trabeculae, especially at the nodal points, flattened plate-like or stellate connective-tis- sue cells are applied after the manner of an imperfect endothelial investment. In parts of many adenoid struct- ures the delicate re- ticulum seems to be formed by the union of the protoplasmic pro- cesses of the branching connective-tissue cells themselves; this ar- rangement, however, is usually only seem- ing, the cells really being applied to the surface of the fibres and not constituting an integral part of the reticulum. It is probable that in the splenic pulp and in a few other localities the processes of the stellate cells do unite to form protoplasmic net-works. Diffuse adenoid tissue represents the least specialized form of the lymphoid structures; the mucosae of the digestive and of the respiratory tracts afford good illustrations of the presence of such tissue. Simple lymphatic nodules, or solitary follicles, stand next in differentiation; these are found in almost all mucous membranes (those of the bladder and of the sexual organs excepted), while they occur in great numbers in the respiratory and digestive tracts, the solitary glands of the latter being important examples of these structures. The simple nodules consist of oval masses of adenoid tissue, limited by a delicate connective-tissue wall or capsule, com- posed of fibrous lamellae. The adenoid tissue of such simple follicles presents no considerable variations in its arrangement, that occupy- ing the more central portions of the nodule, however, being fre- Fig. 142. Fig. 143. Diffuse lymphoid tissue occu- pying deeper layers of mucosa of human stomach : the lym- phoid cells infiltrate the fibrous tissue between the glands with- out being definitely limited. Simple lymph-follicle from conjunctiva of dog: a, lym- phoid tissue, limited by the fibrous capsule (6); c, sur- rounding connective tissue. 120 NORMAL HISTOLOGY. quently somewhat less closely packed than the tissue at the periphery. The afferent lymph-vessels conveying the lymph to the simple follicles break up at the periphery of the nodule into branches, which distribute the lymph to the adenoid tissue; corresponding efferent vessels carry off the fluid returned from the lymphoid tissue and unite to form larger lymphatic trunks. Compound lymphatic follicles, the lymphatic glands of gross anatomy, are formed by the aggregation and partial fusion of a Fig. 144. Section of lymph-gland from child, showing general arrangement of lymphoid tissue and lymph- sinuses: a, capsule from which trabeculae (b, b) extend ; c, masses of dense adenoid tissue composing the cortical follicles ; d, the same, of the medullary cords ; e, lymph-sinuses. number of simple nodules. These structures enjoy a wide distri- bution, and are represented by the numerous chains of deep and superficial lymph-glands, of which the axillary and inguinal glands are familiar instances. The periphery of these lymph-glands is occupied by a firm capsule composed of fibrous connective tissue, inter- mingled with which, in the largest glands, bundles of involuntary muscle are sometimes present. At the position of entrance and exit of the larger blood-vessels and the efferent lymphatic trunks, usually opposite the most convex surface of the organ, the capsule dips deeply into the interior of the gland and forms the hilum. The space included within the capsule is subdivided into a peripheral zone, the cortex, and a centrally situated part, the medulla, which at the hilum reaches Fig. 145. Section of lymphatic gland of child, including portion of cortex at periphery : c, capsule ; s, loose tissue of the lymph- sinus ; l, denser lymph-tissue of the cortical follicle. THE LYMPHATIC SYSTEM. the exterior. The details of arrangement distinguishing these portions of the gland depend primarily upon the distribution of the trabeculae which continue the tissue of the capsule into all parts of the organ. The trabeculae, composed of stout bundles of fibrous tissue, ex- tend from the inner surface of the capsule towards the hilum and divide the cortex into a number of imperfect spherical compartments which enclose masses of adenoid tissue, the cortical follicles, which correspond to simple lymph-follicles. The continuations of the tra- beculae towards the centre of the gland unite at much more frequent intervals and form throughout the medulla a series of incomplete par- titions which separate imperfect compartments occupied by elongated masses of adenoid tissue, the med- ullary cords. These latter and the cortical follicles constitute one continuous mass of dense lymphoid Fig. 147. Fig. 146. Section of lymphatic gland of child, in- cluding portion of medulla: t, part of tra- becula, on either side of which narrow lymph-sinuses are seen, bounded by denser structure of medullary cords (/). Portion of human lymph-gland, showing de- tails of structure : a, lymph-sinus ; b, adenoid tissue; c, trabeculae; d, coarser reticulum of lymph-sinus; e, expanded connective-tissue plate applied to fibres; f, lymphoid cells. tissue, which follows the contours of the spaces occupied, but does not completely fill the compartments formed by the fibrous trabeculae. The spaces included between the fibrous trabeculae and the masses of dense adenoid tissue are occupied by a very loose reticulum and sparingly distributed lymphoid cells ; these channels are the lymph- sinuses, into which the lymph brought by the peripherally-situ- ated afferent vessels is poured and through which it finds its sluggish course, thus securing the opportunity of taking up numerous new cells in its journey through the organ. The lymph-sinuses form a freely intercommunicating system of canals throughout the gland, beginning at the periphery, where they receive the afferent lymph- vessels, and ending in the hilum, where the lymph is collected and carried off by the efferent trunks. 122 NORMAL HISTOLOGY. The trabeculae all along their course give off numerous ramifi- cations ; each of these breaks up into still finer bands, until the final divisions of the fibrous tissue terminate in the delicate reticulum con- stituting the supporting framework in whose meshes the lymphoid cells are held. In the areas of denser tissue the cells are so closely placed that the supporting reticulum is almost completely masked. The surfaces of the fibrous bundles and partitions, especially those directed towards the lymph-sinuses, support numerous plate-like connective-tissue cells, in places these elements constituting almost an endothelial covering. The blood-vessels supplying the lymphatic glands are arranged as two groups : the one set gains entrance at the periphery and is distributed principally to the capsule and larger trabeculae ; the other group enters at the hilum, the majority of the arterial branches pass- ing directly into the lymphoid tissue, while a few follow the course of the larger septa; these, following the latter course, give off numerous twigs to the surrounding adenoid tissue, the terminal branches con- tinuing to the capsule, where they finally are distributed. The cap- illaries derived from the breaking up of the arterial twigs entering at the hilum especially ramify through the denser adenoid tissue, avoid- ing the loosely reticulated lymph-sinuses. The distribution of the nerves passing to the compound lymphatic glands is uncertain, the supply including bundles of both the medullated and the pale fibres. In addition to the numerous well-developed compound lymphatic follicles, many of which, as the mesenteric and the bronchial glands, reach conspicuous dimensions, certain organs present special modifi- cations of adenoid tissue ; such are the spleen and the fully-developed thymus body, which therefore may be included with propriety in the account of the lymphatic structures. THE SPLEEN. The spleen may be regarded as a specialized compound lymphatic gland, modified by the arrangement of its blood-supply. The organ is invested by a firm capsule, composed of a dense felt-work of bundles of fibrous tissue, with which are mixed numerous elastic fibres. The outer surface of the capsule, with the exception of a limited area, is covered by the serous coat of the peritoneum, the union between the two being very intimate. On the inner surface the capsule is continuous with numerous prolongations, the trabeculae. These penetrate deeply into the interior from all sides, and by the free union of their processes form a spongy connective-tissue framework throughout the organ, enclosing an elaborate system of intercommunicating spaces occupied by the lymphoid tissue. THE LYMPHATIC SYSTEM. 123 In certain animals (dog, cat, hog) the capsule contains bundles of involuntary muscle ; these are only exceptionally present in man. Fig. 148. Section of spleen of dog, showing general structure: a, capsule, from which trabeculae extend ; sec- tions of these latter are seen in several places, as at d; b, tissue of splenic pulp ; c, c, Malpighian corpuscles ; e, sections of blood-vessels. Likewise, bundles of muscular tissue are constituents of the trabeculae in many mammals, including man to a limited degree; the muscle- cells are distin- guishable from the surrounding connective tissue by their rod- shaped nuclei. The stoutest tra- beculae are found at the hilum, which corre- sponds to the position at which the larger blood- vessels enter and leave the organ. The lymphoid tissue filling the intertrabecular spaces exists in two forms—as the loose adenoid tissue which, together with the Fig. 149. Fig. 150. Transverse section of large trabecula of human spleen : a, fibrous tissue, containing a few groups of plane muscle-cells (b); c, extension of trabecula into fibrous reticulum; d, lymph- corpuscles. Section of human spleen, showing trabeculae (a) and fibrous reticulum (6) continued into the surrounding splenic pulp ; c, lymphoid cells 124 NORMAL HISTOLOGY. intimately related vascular channels, forms the splenic pulp, and as the cylindrical or spherical masses of dense adenoid tissue ensheath- ing the arteries, constituting the Malpighian corpuscles. The largest trabeculae support the branches of the splenic artery ; on entering at the hilum, these twigs receive a strong fibrous invest- ment, or adventitious sheath, which accom- panies the vessel and becomes gradually reduced as the arte- ries diminish in size ; finally, this sheath blends with the con- nective-tissue frame- work of the paren- chyma. Many of the smaller branches of the splenic ar- tery are deflected from the trabec- ulae and enter the surrounding tissue, where they become ensheathed at irregu- lar intervals by cy- lindrical or spherical masses of dense adenoid tissue and constitute the Malpighian corpuscles. The artery usually pierces the mass somewhat eccentrically, sometimes, however, passing near the centre. Numerous small twigs are distributed to the tissue composing the corpuscles; after forming a net-work they eventually open into the channels of the pulp ; the main artery of the Malpighian corpuscle has a similar destination. The form of these ensheathing masses of adenoid tissue varies in different animals ; in some (guinea-pigs) the arteries are accompanied throughout their entire course by a layer of lymph-cells, while in others (man, cat) the investment is limited to irregularly spherical masses ; between these extremes numerous intermediate forms exist. The peripheral zone of the Malpighian corpuscle is usually denser than the central part, an arrangement favoring the sharp demarca- tion of the body from the surrounding looser parenchyma ; in man the corpuscles are less clearly defined than in many lower animals. The splenic pulp, which makes up the larger part of the bulk of the organ, consists of a loose net-work of slender bands and imperfect septa, composed of delicate fibres and broad plate-like connective- Fig. 151. Section ot human spleen cutting transversely a Malpighian cor- puscle : a, section of the somewhat eccentrically situated artery; b, capillaries distributed to the tissue of the corpuscle; l, the sur- rounding lymphoid tissue of the splenic pulp. THE LYMPHATIC SYSTEM. tissue cells. The processes of the latter unite with one another to form imperfect partitions ; in young animals inultinucleated plates are frequently encountered. Ad- hering to the delicate reticulum, partially occluding the channels throughout the pulp, are numerous lymphoid cells or leucocytes, which are largely the offspring of the ele- ments forming the adenoid tissue. The spaces of the splenic pulp are additionally occupied by num- berless colored blood-cells, brought by the arteries which open directly into the channels within the pulp ; the dark-red appearance of the organ is thus explained. As a re- sult of the breaking down of numer- ous worn-out red blood-cells,—in which process of destruction the leucocytes may take an active part, — pigment - granules, both free and within the lymph-cells, are con- stantly encountered. The splenic pulp, in addition to giving origin to numerous leucocytes, in common with other lymphoid tissues, is regarded by some histologists as the birthplace, as well as the “grave- yard,” of a certain number of colored blood-cells; the evidence, however, upon which such views rest is far from conclusive. The blood-vessels of the spleen form an important part of the organ. After entering at the hilum, the splenic artery gives off tra- becular branches which rapidly diminish in size by repeated division. As already described, many of the smaller arteries leave the septa and become ensheathed by the Malpighian corpuscles, to which they contribute with capillary net-works. A certain number of the arteries extend the entire length of the trabeculae, and hence never become encased within the masses of adenoid tissue ; both these latter and those bearing the corpuscles eventually open into the spaces of the pulp, pouring their streams of blood into the parenchyma. The pulp-spaces communicate, on the other hand, with a wide-meshed net-work of venous channels; the latter unite to form a number of large veins, which pass out at the hilum in company with the principal arteries. All the blood conveyed by the smaller arteries finally reaches the spaces of the splenic pulp, whether directly or indirectly after having first passed through the tissue composing the Malpighian corpuscle; 125 Fig. 152. Portion of channel within splenic pulp from human spleen : a, endothelioid connective-tis- sue plates of the imperfect wall of the space; b, red blood-corpuscles; c, lymphoid cells ; d, larger amoeboid elements, containing pigment- granules ; e, large multinucleated cell. 126 NORMAL HISTOLOGY. the blood then slowly traverses the partially obstructed channels within the pulp and is collected by the venous spaces and passed on to the larger veins, by which it escapes from the organ. The retarded current within the splenic pulp is favorable to the removal and destruction of the worn-out red cells and to the acquisition of additional leucocytes. Within the pulp, while passing from the arteries to the veins, the blood is probably not confined to channels provided with defi- nite walls, but comes into direct relation with the lymphoid tissue. The lymphatics of the spleen are limited to the connective-tissue framework of the organ, in which they form a superficial plexus in the deeper layers of the capsule, and a deeper plexus within the trabeculae. The lymphatic clefts within the adventitia of the arteries communicate with the deeper lymphatics of the trabeculae; regarding the definite relations of the deeper lymphatics our knowledge is incomplete. The nerves of the spleen are composed mostly of non-medullated fibres, although a few of the medullated variety are present; they are distributed to the walls of the blood- vessels ; also ganglion-cells have been ob- served along the nerve-trunks. Fig. 153. Diagram of the relations of splenic vessels to the tissue of the pulp: a, v, small arterial and venous branches of splenic vessels within trabecula (/, t); one twig of artery is diverted and becomes ensheathed by tissue of the Mal- pighian corpuscle, M; the remain- ing part of the artery follows the trabecula and passes directly into the spaces of the pulp—in either case the arterial branches termi- nate in the spaces (p, p) within the pulp surrounded by the lym- phoid tissue (/, l); the venous radicles take up the blood and carry it from the spaces of the pulp into the larger venous trunks. The thymus body is included among the lymphatic tissues on ac- count of the histological characteristics of the fully-developed organ ; in its early stages, however, the bulk of the organ is epithelial in nature, being derived from the endodermic cells and closely resem- bling many glands in its earliest growth. The rapid invasion of mesodermic tissues, at a later period, so changes the character of the organ that tissues of a lymphoid type predominate, while the original epithelial structures are reduced to mere rudimentary remains. The entire organ usually consists of two lateral lobes, more or less intimately united, composed of numbers of lobules, held together by the interlobular areolar tissue and enveloped within the general fibrous capsule of the organ. The irregularly ovoid lobules, 5-10 mm. in diameter, are further divided by connective-tissue septa into compartments, each of which includes several smaller secondary THE THYMUS BODY THE LYMPHATIC SYSTEM. 127 lobules ; these, in turn, are made up of groups of the primary alveoli or follicles. The latter closely resemble lymph-follicles in structure, being limited by a fibrous envelope giv- ing off slender tra- beculae, which are soon lost in the deli- cate reticulum of connective tissue pervading all parts of the follicles. The meshes of the re- ticulum are occupied by numerous lym- phoid cells, among which many capillary blood - vessels run. The adenoid tissue of the peripheral zone, or cortex, of the follicles is more closely packed with cells than that occupying the centre, or medulla, in consequence of which variation the medulla appears lighter than the denser cortex. Scattered throughout the follicles Fig. 154. Section of human thymus body, showing general arrangement of follicles : a, fibrous tissue enveloping lymphoid tissue and sending septa (a') between the follicles (b); d, interfollicular tissue, contain- ing blood-vessels (c). Fig. 155. Fig. 156. Portion of the periphery of one of the folli- cles of the foregoing section, more highly magnified: a, fibrous tissue; b, lymphoid tissue, containing numerous capillaries (c). Portion of the same follicle, showing corpuscles of Hassall (a), which represent the original epithelial constituents of the organ. round or oval bodies are seen, which vary greatly in number and size (20-175 /j), usually stain but faintly, and present an irregularly concen- 128 NORMAL HISTOLOGY. trie striation, with occasional nuclei; these bodies are the corpuscles of Hassall, or the concentric corpuscles. They represent the re- mains of the epithelial structures which, as already stated, in the early stages of the thymus constitute the principal tissue of the organ. The larger blood-vessels of the thymus run within the inter- lobular connective tissue, giving off branches which penetrate the follicles and break up into a rich capillary net-work supplying the adenoid tissue of cortex and medulla. As may be inferred from the character of the organ, the lym- phatics occur in large numbers. The radicles coming directly from the follicles are received by the interlobular vessels, which, in turn, communicate with the superficial net-work occupying the surface of the organ. Bundles of nerve-fibres accompany the ramifications of the arteries and veins, to the coats of which they seem principally to be distributed. The thymus body reaches its highest development about the second year, after which time it gradually diminishes, undergoing retrogressive changes and absorption, until, by the eighteenth to the twenty-first year, the characteristic tissues have disappeared or have been replaced by fibrous connective tissue and fat. THE SEROUS MEMBRANES. The serous membranes are intimately related to the lymphatic system, since the cavities which they enclose form parts of the gen- eral lymph-tract of the body; when considered in their widest sig- nificance they include the lining of all cavities clothed with endothe- lial cells and cut off from atmosphere. Regarded in a more limited and critical sense, such cavities may be separated into certain groups, following which the connective-tissue linings may be divided into : a. The serous membranes proper, as the peritoneum, the pleura, and the pericardium. b. The synovial membranes, including the synovial capsules of the joints, the synovial sheaths of tendon, and the synovial bursae placed between opposed movable surfaces to reduce friction. c. The endothelial lining of the vascular system, comprising that of the heart, of the blood-vessels,' and of the lymphatics. d. The lining of various spaces developed within the connective tissues; such spaces are usually small and provided with very rudi- mentary linings; they may be, however, of considerable size, as in the case of the perilymphatic spaces of the internal ear. The serous membranes proper, represented by the peritoneum, the pleura, the pericardium, and the tunica vaginalis, are all derived as constrictions from the originally single pleuro-peritoneal cavity THE LYMPHATIC SYSTEM. 12g first formed. In the closed sacs constituted by the serous mem- branes a parietal and a visceral layer are always distinguishable; the connection of these with the subjacent structures is slight or intimate according to the character and amount of the subserous tissue. Every organ which projects beyond the wall of the serous pouch into its cavity must be enveloped by the serous membrane to a greater or less degree. When the organ remains closely attached to the wall of the body-cavity, as does the kidney, it obtains only a partial serous investment; where, on the other hand, the organ leaves the parietes and encroaches upon the cavity, the serous invest- ment becomes almost complete, as in the case of the small intestines. In all cases the viscera lie outside the serous sac, the membrane which constitutes the lining of the space being pushed before the encroaching organ to form a serous covering more or less complete. The serous cavity of greatest extent—that of the peritoneum—in the female presents an exceptional arrangement in possessing outlets at the orifices of the oviducts; in this connection, however, it must be remembered that the oviduct is the persistent Mullerian duct, which is only one of a number of tubes formed during early foetal life by evagination of the primary serous membrane, thus establishing communication with surfaces exterior to the serous cavity. While such tubes in the higher animals are only transient, in the lower types they may remain as permanent structures. The serous membranes are sufficiently thin and transparent to permit the color of the underlying parts to be seen readily through them; moderate strength, extensi- bility, and elasticity are among their physical properties. These membranes consist of the endothelium covering their free surface and resting upon the connective-tissue stroma, which constitutes the chief substance of the membrane ; external to this layer a variable amount of subserous tissue usually is pres- ent. The endothelium comprises a single layer of the large, thin, irregularly - polyhedral connective - tissue plates already described and figured in Chapter II. In addition to the minute deeply-stained intercellular areas, or pseudo-stomata, true openings, or stomata, also exist in the sev- eral serous membranes. These orifices are especially well seen in silver preparations from the posterior wall of the frog’s peritoneal Fig. 157. Peritoneal endothelium of dog, silver-stained; several pseudo- stomata are seen as dark areas among the cells. 130 NORMAL HISTOLOGY. cavity (Fig. 30); but they may be demonstrated also in the tissues of man and of the higher animals : the central tendon of the dia- phragm, on which they were first discovered in the peritoneum by von Recklinghausen, offers a favorable place for their study. In addition to the stomata occurring in the peritoneum covering the diaphragm, similar apertures have been observed in the omentum, the pleura, and the pericardium. The stomata, either directly or through minute canals, lead into the sub- jacent lymphatic vessels and are surrounded by cuboidal or spherical guard-cells. The stroma of the serous membranes con- sists of interlacing bundles of white fibrous tissue, mingled with elastic fibres, which are especially numerous in the more superficial parts, where they frequently form a reticular layer. The interstices between the fibrous bundles are occupied by the ground-sub- stance ; the latter after a time in some cases, as in the omentum, suffers local absorption, interfascicular orifices then partially taking its place. The serous membrane, which in its earlier condition forms a continuous sheet, may become riddled with apertures, and is said to be fenestrated. Where the ground-substance and stroma are well developed and of considerable thickness, particularly in the vicinity of folds, adipose and sometimes lymphoid tissue occur in addition to the blood-vessels and lymphatics. The ground-substance in places where dense is penetrated by an intercommunicating system of lymph-spaces opening into the lymphatic vessels of the serous membrane. Branched connective-tissue cells are also frequently seen with processes extending between the endothelial plates of the free surface; such processes when stained with silver probably form the pseudo-stomata already mentioned; other protoplasmic exten- sions of the cells may come into relation with the walls of the blood- vessels or of the larger lymphatics. The subserous layer, where well developed, is composed of loosely-arranged bundles of fibro-elastic tissue, between which blood- vessels and lymphatics, with migratory leucocytes, are situated. The blood-vessels of serous membranes contribute wide-meshed net-works both to the layer of proper stroma and to the subserous tissue; in positions where tracts of adipose or of lymphoid tissue exist, the capillaries form net-works enclosing the fat-sacs or the lymphoid masses. The lymphatics of serous membranes are very numerous, and are represented by the definite lymphatic vessels and the lymph-spaces Fig. 158. Peritoneum in section from dog: p, peritoneum proper, consisting of endothelium of free surface and subendothelial fibrous stroma containing net- work of elastic fibres ; s, sub- peritoneal vascular connective tissue. THE LYMPHATIC SYSTEM. 131 within the ground-substance ; by means of the stomata and the minute passages leading from them the lymphatics communicate with the serous cavities, while, on the other hand, they join with the wide, irregular lymph-channels within the subserous tissue. The nerves supplying these membranes are limited, those which are present being largely derived from the sympathetic system, com- posed of pale, non-medullated fibres destined chiefly for the blood- vessels. The few fibres passing into the substance of the membrane form a loose reticulum throughout its deeper layers, from which finer fibrillse extend beneath the surface. The synovial membranes, which constitute a second group of serous membranes, include the lining of the clefts developed within the connective tissue (mesoderm) surrounding opposed movable surfaces, embracing the capsules enclosing the articulating surfaces of the various joints, the synovial sheaths in which the tendons glide, and the bursal sacs interposed between surfaces ; these varieties of synovial membranes are known respectively as the articular, the vaginal, and the vesicular. Synovial membranes differ from the serous in the character of their secretion ; that of the former— the synovia—is a glairy, viscid fluid, resembling the white of egg, well adapted for the lubrication of the opposed parts, and contains fat particles, lymphoid cells, and degenerated endothelial plates. Fig. 159. Section of synovial membrane at edge of articular surface : s, s, tissue of synovial membrane bearing villous projections (z>, v) ; x, position at which tissues of membrane become continuous with those of periphery of cartilage ; f, group of fat-cells ; p, fibrous tissue constituting peripheral zone of cartilage (c). The secretion moistening serous membranes is thinner, watery, and less suited to the reduction of friction. The articular synovial membranes surround the joints, tightly embracing the bones and enclosing them within their sacs, but do not extend over the articulating surfaces, which are composed of 132 NORMAL HISTOLOGY. naked cartilage, over whose surfaces of contact not even the imper- fect endothelial covering is continued; tendons or other structures traversing the joint-cavity receive an investment of the synovial membrane. The marginal zone, embracing the attachment of the membrane to the cartilage, is marked by the gradual alteration of the tissues of the synovial membrane to assume the characters first of fibro-cartilage, and finally of the typical articular cartilage of which the membrane then seems a part. The synovial sacs, originating as clefts within the mesoderm surrounding the extremities of the young bones, exhibit a structure corresponding to slightly condensed connective tissue. The mem- brane is composed chiefly of closely-felted bundles of fibrous tissue, mingled with elastic fibres, containing the usual connective-tissue elements; the free surface of the membrane possesses an imperfect covering of connective-tissue cells, which, when closely placed, as in the younger tissue, present the characters of an endothelium; when less densely arranged, they retain their processes and appear as branched elements, resembling those of other dense fibrous tissues; in the vaginal membranes the cells are often elongated to correspond with the axis of the sheath. Cleft folds of the synovial membrane project into the serous cavity as the Haversian fringes ; they are free processes of the membrane containing vascular loops and, in the larger ones, fat; the smaller secondary fringes, or villi, often present as finger-like processes attached to the edges of the larger folds, contain no blood-vessels, but consist principally of small, irregularly-round cells, separated by a scanty intercellular substance. In some cases these villi enclose a denser core, which consists of fibrous bundles; occasionally the entire villus is formed of fibro-cartilage, the superficial round cells being wanting. Blood-vessels are quite numerous within the synovial mem- branes, as well as in the subjacent tissues, nearly all parts of the joints being generously supplied. Many of the Haversian fringes contain vascular tufts, while the termination of the blood-vessels around the margin of the cartilages is marked by vascular loops possessing greatly dilated terminal arches. The nerves of the synovial membranes, by no means numerous, form a loose plexus beneath the free surface ; in connection with the joints, peculiar special nerve-endings, the articular end-bulbs of Krause, have been found attached to the nerve-fibres; Pacinian corpuscles have likewise been observed in relation with the synovial membranes. The serous surfaces lining the blood-vessels and the lymphatic THE LYMPHATIC SYSTEM. 133 channels and spaces have been considered in connection with their respective systems. The development of the lymphatic system in all its parts involves the mesoderm alone. Very early in the life-history of the embryo, shortly after the appearance of the three blastodermic layers, the mesoderm undergoes cleavage into two leaves, the separation affecting the mesodermic layer on either side as far as the lateral margin of the uncleft axial band. The resulting sheets of meso- dermic tissue become the parietal layer (somatopleuric) and vis- ceral layer (splanchnopleuric) ; the former clings to the ectoderm to become the future wall of the body-cavity, while the latter adheres to the entoderm to form the wall of the digestive tube. The space included between these leaves is the primitive body- cavity, or coelom, and the mesodermic tissue forming its imme- diate wall becomes dif- ferentiated into a special lining — the mesothe- lium—whose elements are the ancestors of the later endothelium. The fully-formed se- rous membranes, represented by the peri- toneum, the pleura, the pericardium, and the tunica vaginalis, are all derived as constric- tions from the common pleuro - peritoneal sac, or body - cavity, first formed, the subdivision of which into the above- mentioned special serous compartments occurs secondarily and at a much later period. Bearing in mind the origin of the primary lining of the serous membranes, the claims of endothelium to near kinship with con- nective tissue must be admitted ; likewise, the reasons for regarding endothelium as distinct in nature from epithelium will be appreciated. Inasmuch as the epithelium of the genito-urinary tract is derived indirectly from the mesoderm, it is related genetically to the endo- thelium of the abdominal serous membrane. In the course of the differentiation and growth of the fibrous con- nective tissue, clefts appear within the ground-substance between the bundles of young tissue, which become the lymph-spaces of the Fig. 160. Transverse section of ten-day rabbit embryo, showing the cleavage of the mesoderm and the formation of the primary body-cavity: E, ectoderm; M, M, the letters occupy the body-cavity and have the parietal (p) and visceral (v) layers of the cleft mesoderm respectively above and below them ; the immediate lining of the cavity constitutes the mesothelium; En, entoderm; N, neural canal; c, notochord; s, s, cavities within the somites—really parts of the body-cavity; a, one of the paired primitive aortae. !34 NORMAL HISTOLOGY. maturer stages. The formation of the lymphatic vessels takes place in a manner very similar to that by which the blood-channels are produced. The protoplasmic net-works established by the united processes of the connective-tissue corpuscles are at first solid ; sub- sequently they acquire a lumen and become converted into a series of protoplasmic tubes, the nuclei of whose endothelial plates are de- rived from the proliferated nucleus of the original elements. The earliest lymph-corpuscles are, probably, migrated mesoblastic cells which have entered the young vessels. The additional coats of the larger lymphatic trunks are derived from the condensation and differentiation of the surrounding young connective tissue. The development of the lymphoid tissue occurs at a rela- tively late period. The position of the future lymph-gland is indi- cated by a cleft or fissure which appears within the mesoderm and completely isolates the gland-area on all sides except that destined to become the future hilum, where the tissue devoted to the produc- tion of the gland and the surrounding mesoderm are continuous. The development of the lymphoid tissue is marked by increased numbers and greater compactness of the mesodermic elements ; the supporting reticulum, the capsule, and other details of the adenoid tissue appear later. The development of the spleen begins about the commence- ment of the third month, some time after the pancreas has become defined; a condensation of the mesodermic cells, lymphoid in character, within the primitive omentum, or the mesogastrium, in the near vicinity of the pancreas, is the earliest indication of the future organ. The lymphoid aggregation first established is sup- plemented by the elements lying beneath the peritoneum, which differentiate into elongated spindle-cells especially devoted to the formation of the trabeculae and connective-tissue framework. Numerous blood-vessels soon grow into the splenic tissue, the sub- sequent accumulations of lymphoid cells within the tissue around some branches of the arteries giving rise to the Malpighian cor- puscles. The history of the development of the thymus body demon- strates an origin markedly at variance with the character of the fully- formed organ, since, notwithstanding the pronounced lymphatic type of the tissue constituting almost the entire body when most complete, its structure in the earliest stages corresponds entirely to embryonal epithelium which is derived as the direct outgrowth of the ento- derm. The first trace of the thymus body appears as a cylindrical bud of entodermic tissue springing on either side from the third pharyngeal pouch, or inner visceral furrow. The epithelial nature of the early thymus is for some time very evident, the original ceil- THE LYMPHATIC SYSTEM. i.35 mass appearing also similar to the earliest stage of a glandular area ; repeated division rapidly converts the at first simple cylindrical aggre- gation into a complex figure, in which an elongated main part bears numerous lateral branches. At a later period the surrounding meso- derm becomes richer in cells and more compact and grows into the original epithelial structure, the result of which invasion is the final complete atrophy and disappearance of the epithelial constituents, with the exception of the inconspicuous but constant corpuscles of Hassall, which alone bear witness to the primary epithelial nature of the organ. NORMAL HISTOLOGY. CHAPTER IX. MUCOUS MEMBRANES AND GLANDS All passages and cavities directly or indirectly communicating with the exterior of the body and the atmosphere are lined by mucous membranes. These structures consist of two parts: the connective-tissue stroma, or tunica propria, and the epithelial covering; the outer surface of the connective-tissue layer is quite usually special- ized to form an extremely delicate basement-membrane, or mem- brana propria, which thus separates the epithelium from the under- lying tissue and forms a third constituent of the mucous membrane. The basement-membrane is often scarcely demonstrable as a distinct layer, while in certain organs, as many glands or the hair-follicles, it is highly developed. The epithelium of mucous surfaces varies both in character and in arrange- ment, as already described in Chapter II. The proper substance or stroma of the mucous membrane consists of a felt-work of bands of fibrous connective tissue together with net-works of elastic fibres ; these latter may be so plentiful that an especial elastic layer is formed, as in parts of the respiratory tract. Numerous con- nective-tissue cells lie between or upon the fibrous bundles, the flattened plate- like cells forming in many places par- tial linings for the interfascicular lymph- spaces found throughout this layer. Not infrequently the surface of the connective-tissue stroma is beset with numerous elevations or papillae, over which the epithelium extends. Such irregularities, when slight, may be present without impressing the free surface of the mucous membrane, since the epithelial layer completely fills the depres- sions between the elevations : when very pronounced, the papilke or folds of the connective tissue produce such conspicuous sculpt- urings of the surface as the papillae of the tongue or the rugae of the vatrina. Fig. 161. Diagram of a typical mucous mem- brane : e, epithelium of free surface con- tinuing into the glandular depression to become the secreting cells ; b, base- ment-membrane separating epithelium and connective-tissue stroma; s, s, fibro-elastic tissue of tunica propria; v, blood vessels forming net-works beneath epithelium and around gland. MUCOUS MEMBRANES AND GLANDS. Mucous membranes may be invaded to a greater or less degree by lymphoid cells, as in many localities in the digestive tract; sometimes, as in the villi of the small intestine, the tissue assumes still more closely the lymphoid type, a delicate connective-tissue reticulum supporting the lymphoid cells. The membrana propria, or basement-membrane, usually appears as a delicate homogeneous line beneath the epithelium. It must be regarded as a modification of the connective tissue, and when well developed, after suitable staining with silver, appears as a more or less complete covering of flattened, endothelioid cell-plates. The deeper layers of the mucous mem- brane fade away into the surrounding areolar tissue or into the adjacent submucosa ; sometimes, how- ever, the mucosa is limited by a delicate zone of involuntary muscle, the muscularis mucosae, consisting often of two distinct, although delicate, layers of muscle-cells. Mucous membranes are usually provided with glands, which in their simplest type are depressed portions of the general mucous surface, lined with modified epithelium—the secreting cells. A single cell may constitute an entire gland, instances of such arrangement being found in the unicellular glands of the lower forms ; the familiar goblet-cells are, in fact, such structures ; it is, however, the more developed forms of secreting apparatus which the term ‘ ‘ gland’ ’ usually represents. Glands are of two chief varieties, tubular and saccular, each of these occurring as simple and compound. Simple tubular 137 Fig. 162. Plate-like endothe- lioid connective-tissue cells constituting base- ment-membrane. Fig. 163. n Diagram illustrating the forms of glands: A, simple tubular; B, compound tubular; C, modified (coiled) tubular; D, simple saccular ; K, compound saccular, or racemose. glands are frequent, the peptic glands and the mucous follicles of the intestines being well-known examples. Compound tubular glands vary in complexity, from a simple bifurcation of the fundus, 138 NORMAL HISTOLOGY. as in many pyloric or uterine glands, to the intricate arrangement of the tubules of the kidney or the testicle. Simple saccular glands do not occur in the higher animals, but are conspicuous in the lower types, as in the integument of am- phibians. Compound saccular, or racemose, glands, on the other hand, are represented in man and mammals by such important organs as the pancreas and the salivary glands. In the least complex type of gland, the simple tubular, the two fundamental parts of all glands are distinguishable in their primi- tive form : these are the deeper actively secreting portion, the fundus, and the superficial division, or duct, through which the products of the secreting cells escape. Dilatation of the fundus of the primitive type produces the simple saccular gland ; division of the fundus and of part of the duct originates the compound tubular variety ; repeated cleavage and subdivision of the duct, with accompanying expansion of the associated terminal tracts, lead to the production of the com- pound saccular, or racemose, type. The tubular glands may exist as perfectly straight cylindrical depressions; more usually, however, the tubes are somewhat wavy or tortuous : when the torsion of the fundus reaches its highest expression, such modi- fications as the coiled sweat-glands result. Glandular epithelium is the direct de- rivative of the cells covering the adjacent mucous membrane, so modified and special- ized as to adapt it to the requirements of the several parts of the gland. In simple tubular follicles the cells of the adjacent free surface pass into those lining the neck of the gland with little change ; cells of the increased size and spherical form become more pronounced towards the fundus, where the elements assume the characters of se- creting epithelium. The cells lining the upper part of the duct of such giands not infrequently exhibit a distinctly imbricatec arrangement; this is well seen in the peptic glands. The greater complexity of the racemose glands resulting from the system of freely branching excretory tubes renders the recognition of several parts desirable. These are, towards the ducts, proceeding from the ter- minal compartments, the alveoli or acini, the intercalated or intermediate tubules, the intralobular tubes, the interlobular Fig. 164. Tubular glands : A, simple tubu- lar crypt from human small intes- tine; B, compound tubular gland from pyloric end of human stomach. MUCOUS MEMBRANES AND GLANDS. j 39 ducts, and the excretory ducts, which latter usually unite to form a single common duct of large size. At the open end of the acinus the lining cells of the latter become flattened or cuboidal, and, together with the basement-membrane, are directly continuous with the similar structures forming the walls of the narrow intermediate tubule; the latter succeeds the acinus as the continuation of the narrow intercellular clefts of several adjacent acini, and, after a longer or shorter course as a delicate narrow- lumened canal, passes into the intralobular tube. The distinctive characters of the latter are its larger lumen and the columnar epithe- lium, many cells of which exhibit a distinct vertical striation through- out the peripheral zone next the basement-membrane. The branch- ing intralobular tubes, on emerging from the lobular tissue, join to form the interlobular duct which occupies the connective tissue lying between and holding together the divisions of the glandular sub- Fig. 165. Fig. 166. Section of racemose gland showing relation of glandular tissue to origin of duct: x, acini lined with secreting cells which are directly continuous with those of the intermediate tubule (2); y, interlobular connective tissue. Section of the human parotid gland showing the interlobular tissue: s, s, se- creting cells of surrounding acini; d, inter- lobular duct; v, blood-vessels within the fibrous tissue ; g, group of ganglion-cells. stance. The interlobular ducts are clothed with simple columnar cells, which form a passive lining to the canal for the conveyance of the secretions of the more active parts of the gland. Towards the free surface of the mucous membrane the interlobular ducts unite to form the chief, often single, excretory duct of large lumen, whose walls for a variable distance from the point of discharge are covered with epithelium similar to that covering the adjoining mucous surface; this is soon replaced, however, by the columnar cells which' then continue into the smaller tubes. In the large ducts the subepithelial tissue is strengthened by net-works of elastic fibres. The saccules or alveoli are limited by a basement-membrane NORMAL HISTOLOGY. 140 upon which rests a single layer of irregularly spherical or polygonal secreting cells ; these latter do not entirely fill the acinus, but leave an intra-cellular cleft, in which the system of tubes for the conveyance of the secretions commences. Glands are often divided into serous and mucous, a differentia- tion depending upon the peculiarities of the cells lining the acini as well as upon the character of their secretion. The cells of the serous glands are distinguished by being distinctly granular, generally spherical in form, readily and deeply stained with carmine, and by having conspicuous nuclei situated near the centre of the cells ; the elements of the mucous glands, on the contrary, are distended, Fig. 168. Fig. 167. Serous acini of human pa- rotid gland; the deeply-stain- ing granular cells are sur- rounded by the basement- membrane. Mucous acini of human lingual gland: the secreting cells (a), being loaded with the slightly-staining secretion, appear clear and transparent; c, c, crescentic masses of granular cells—the demi-lunes of Heiden- hain ; b, interacinous connective tissue. very clear and transparent, slightly stained with carmine, and have the nuclei displaced to the outer edge of the cells, not infrequently immediately beneath the basement-membrane. In the embryonal pre-functionating condition these two kinds of glands are identical, both as to mode of origin and histological characteristics ; the varia- tions and the conspicuous differences subsequently appearing depend on differences of physiological function and character of secretions, and not on structural differences in the original cells. Fluids elaborated by the serous glands are thin and watery, appear- ing within the protoplasm of the secreting cells as minute dark gran- ules ; the general appearance of the cells depends upon the number of these granules stored up within their protoplasm. When a serous gland is in a condition of rest, the cells are loaded with granules, and consequently they appear larger, darker, and more granular ; after active secretion the cells are exhausted and contain fewer granules, appearing, therefore, smaller, clearer, and less granular. MUCOUS MEMBRANES AND GLANDS. 141 The mucous glands secrete a clear, viscid, homogeneous sub- stance, or mucine, having little affinity for carmine, but staining deeply with haematoxylin. During rest the cells of such glands become loaded and distended with the mucoid secretion, while the nuclei are crowded to the periphery of the cells ; under these condi- tions the cells lining the acini appear clear with well-defined outlines, and, on the sides next the basement-mem- brane, present a thin zone containing the displaced nuclei and granular protoplasm. After prolonged secretion the exhausted cells contain relatively little mucoid sub- stance ; hence the threads of the protoplasm are no longer widely separated, but are more closely placed; in consequence of these changes the cells assume appearances resembling those of the elements of the serous glands, being smaller, darker, and more granular than the cells of the quies- cent mucous gland. In the acini of mucous glands small crescentic groups of granular, deeply-staining cells are often seen lying between the clearer elements and the basement-membrane ; these are the crescents of Gia- nuzzi, or the demi-lunes of Heidenhain, the significance of which has caused extended discussion. These crescents represent, most Fig. 169. Lingual glands from tongue of cat: a, b, the serous and the mucous acini containing respec- tively the granular and the clear cells. Fig. 170. A and B, serous and mucous acini in different stages of functional activity: r, condition of rest, the cells being gorged with secretion ; a, condition of exhaustion after great activity : following the discharge of the secretion the elements of the protoplasm become more closely placed, producing an appearance of increased granularity. probably, groups of quiescent or exhausted cells which have been displaced and crowded to the periphery of the acinus by the dis- tended more centrally situated active cells. The view regarding the crescents as composed of young cells destined to replace those de- stroyed by active secretion is opposed by the absence of partially disintegrated cells as well as by that of all manifestations of cell division. The vascular supply of glands is always rich. The larger blood- vessels, conveyed by the submucosa, send off branches into the 142 NORMAL HISTOLOGY. mucosa to break up into capillaries which enclose the tubules and acini in close net-works, lying outside but in intimate relation with the basement-membrane, an arrangement favoring the passage of substances from the blood into the protoplasm of the secreting cells, which are thus placed between the blood-current on the one hand and the lumen of the gland on the other. Numerous lymphatic spaces are contained within the connective tissue surrounding the acini and the tubules, some of the clefts being immediately beneath the membrana propria and in close relation with the gland. The nerve-supply of glandular structures is often very rich. The nerve-trunks accompany the larger blood-vessels in the submucous tissue and give off numerous small bundles which follow the smaller arteries in their distribution to the mucosa, where they form delicate plexuses about the acini and the tubules immediately outside the basement-membrane. The exact mode of the final termination of the nerves and their relation to the individual secreting cells are still matters for investigation ; whether the fibres pierce the basement- membrane to terminate among the glandular epi- thelium, while probable, must be regarded as still unproved. The development of glands proceeds from the epithelial tissue of the young mucous mem- brane, the penetrating cylinder of epithelium rep- resenting ectodermic or entodermic tissue, except in those cases where the glands are formed in connection with the parts of the genito-urinary tract derived entirely from the mesoderm. The first trace of the glands consists of a cylindrical ingrowth of the epithelium into the subjacent mesodermic tissue, both the tubular and the saccular glands alike starting as simple epithelial processes. Where, however, the struct- ure is destined to become a gland of the racemose type, the branching cords of epithelial elements early indicate the nature of the future gland as distinguished from one of the compound tubular variety ; since, in this case, the terminations of the epithelial masses soon become markedly ex- panded and club-shaped, from which dilatations the ultimate divisions or primary alveoli of the racemose glands are extended secondarily. The epithelial cords, at first solid, later acquire a lumen which extends as far as the terminal compartments of the gland. Sometimes, as conspicuously instanced by the liver, Fig. 171. Developing salivary gland from fifteen-day rabbit embryo. The ectodermic ingrowth has divided into secondary branches which termi- nate in slightly expanded club - shaped ends: e, epithelium of oral sur- face ; m, young connec- tive tissue of future tunica propria into which the epithelium grows. MUCOUS MEMBRANES AND GLANDS. the primary arrangement of the gland is modified by subsequent changes to such a degree that the original plan of its structure is recognized with difficulty. The sexual glands are so highly special- ized that in their development they deviate materially from the mode of the formation of the typical secretory organs. Ordinarily the elaborating glandular cells are ectodermic and entodermic in origin, while the basement-membranes and supporting tissues are meso- dermic. 144 NORMAL HISTOLOGY. CHAPTER X. THE DIGESTIVE TRACT. The mucous membrane of the oral cavity consists of the epi- thelial covering and the connective-tissue stroma or tunica propria; the deeper layers of the latter fade insensibly into the subjacent tissues which unite the mucous membrane with the surrounding deeper parts. The epithelium lining the entire oral cavity is of the stratified squamous variety, continuous with the epidermis on the one hand and with the covering of the pharynx on the other. The tunica propria is composed of interlacing bundles of fibrous connective tissue containing elastic net- works, and possesses numerous simple papillae which encroach on the epithelial layer, but do not appear on the free surface of the mucous membrane. The latter is broken in many places by the openings of the ducts of the numerous glands which occupy the submucosa and deeper parts of the mucosa. In the transition of the skin on the lips, where the skin passes into the mucous membrane, the epithelium is greatly thickened, while the connective- tissue layer decreases in thickness ; the subepithelial papillae here become very prominent. The hair-follicles disappear, but the sebaceous glands still are present, especially near the angles of the mouth and in the upper lip. The mucous membrane covering the cheeks adheres tightly to the bucci- nator muscle, and possesses small papillae ; that covering the gums is dense, and contains numerous well-marked papillae beneath the epithelium, the submucous tissue being closely united with the peri- osteum. The portion covering the hard palate is thin and firmly united to the periosteum, while that investing the soft palate, the uvula, and the fauces is much thicker, less dense, possesses numerous mucous glands, and, in many places, is so densely crowded with lym- phoid cells that the entire mucous membrane assumes the appearance of adenoid tissue. THE MOUTH. Fig. 172. Section of oral mucous membrane of child; the surface of the fibrous tunica propria is broken by minute papillae, which contain the endings of the blood- vessels and the nerves. The papillae are covered by the stratified squamous epithelium. THE DIGESTIVE TRACT. 145 The oral mucous membrane is thickly beset with small mucous racemose glands in nearly all parts. These are especially well marked on the lips, the cheeks, the under surface of the tongue, and the soft palate, constituting, respectively, the labial, the buccal, the lingual, and the palatine glands ; on the gums and the hard palate such structures are absent or present in very limited numbers. The acini are situated within the deeper layers of the mucosa, while the ducts pierce the superficial layers to open on the free surface. The squamous epithelium of the latter is continued within the duct usually as far as its first division. Small lateral isolated groups of acini, constituting accessory mucous glands, sometimes open into the long narrow excretory duct of the main glandular mass during its journey to the free surface. The larger blood-vessels supplying the oral mucous membrane lie within the submucous tissue and give off branches which extend through the deeper layers of the mucosa to the superficial portions -of the connective-tissue stratum ; on reaching the outer boundary •of the latter the arteries break up into rich subepithelial capillary net-works, or, where papillae are present, enter the minute elevations to supply their apices with terminal capillary loops. The capillaries likewise enclose the acini of the oral glands. The lymphatics begin in the irregular net-work of interfascicular spaces between the connective-tissue bundles of the tunica propria ; these spaces unite to form definite lymphatics in the deepest layers of the mucosa, which in turn are taken up by the larger lymph- vessels of the submucous tissues. Nerve-fibres, largely of the medullated variety, accompany the blood-vessels, and form a subepithelial plexus ; special terminations —the end-bulbs—are found in the apices of some of the papillae, while additional numerous tactile corpuscles occur on the lips. In principle, and among many of the lower animals in fact as well, the teeth may be regarded as hardened papillae of the oral mucous membrane. The teeth are firmly retained within their appropriate sockets by the close attachment afforded by the alveolar periosteum which holds together the alveolus and the root of the tooth. The perios- teum lining the alveolus is composed of dense fibrous tissue, whose fibres have a general transverse disposition : elastic tissue is almost wanting, nerves and blood-vessels being, however, numerous. At its neck the tooth is especially embraced by the thickened perios- teum, which then becomes continuous with the periosteum covering the alveolar process of the jaw and with the gum. THE TEETH. 146 NORMAL HISTOLOGY. The tooth comprises the dentine, the enamel, and the cemen- tum. The dentine, or ivory, principally contributes the bulk and the characteristic form of the tooth, completely enclosing a central pulp- cavity, except where the narrow nutrient canal, admitting the blood- vessels and nerves to the pulp, pierces the apex of the fang. The dentine is composed of a matrix or ground-substance, which, as that of bone, must be regarded as modified connective tissue, formed of bundles of fibrous tissue intimately united and subsequently impregnated with calca- reous salts. Piercing the ground- substance and appear- ing under low amplifica- Fig. 173. Fig. 174. Section of dried human tooth showing portions of enamel and dentine : a, ground-substance of dentine; b, branching dentinal tu- bules ; c, terminal zone of tubules within the enamel (d). Longitudinal section of molar tooth of kitten : a, pulp-cavity, continued by canals (f) to apices of roots; d, dentine; e, en- amel ; c, cementum; p, alveolar periosteum; n, neck of tooth; b, osseous tissue of jaw. tion as a radial striation, the dentinal tubules extend the entire thickness of the dentine as minute channels ; they are seen espe- cially well in sections of the dried tooth in which the canals are filled with air. Starting from the pulp-surface with a diameter of 20-26 /*, the dentinal tubules pass in a slightly wavy and spiral course through the dentine, to terminate in irregular clefts, the interglobular spaces, situated at the juncture of the dentine with the enamel or the cementum. THE DIGESTIVE TRACT. l^y The tubules give off numerous secondary canals along their course, by which means the adjacent tubules communicate ; on approaching the enamel or the cement the tubules undergo repeated division, the resulting smaller secondary channels corresponding in their general direction with the larger canals. The marked parallel curves described by the dentinal tubules pro- duce optical effects which are appreciated as a coarse striation con- centric with the outline of the pulp-cavity; these appearances, known as Schrager’s lines, may be seen in sections with the unaided eye. That part of the dentinal matrix immediately surrounding the tubules is especially dense and resistant, and constitutes the so-called den- tinal sheaths which may be isolated by acids. Within the tubules lie the delicate dentinal fibres, which are the modified processes of the connective-tissue cells forming the peripheral layer of pulp- cells. When cut across the tubules appear circular or slightly oval, and contain a minute dot, the dentinal fibre in transverse section. Want of uniformity in the calcification of the outer zone of dentine gives rise to the incremental lines of Salter. The interglobular spaces are irregular stellate intercommuni- cating clefts situated at the margin of the dentine, into which open on the one hand a number of dentinal tubules and on the other hand the Fig. 175. Fig. 176. Interglobular spaces of dentine from dried human tooth: i, i, spaces into which certain dentinal tubules (d) open. Section of enamel from dried human tooth : a, b, longitudinal and trans- verse views of enamel rods. spaces or the lacunae of the cementum. Each space contains a pro- toplasmic body, the connective-tissue cell, the processes of which unite with the dentinal fibres. The enamel covers the exposed parts of the softer underlying dentine, and is composed of irregular 4-6-sided columns, the enamel prisms, closely placed and generally vertical to the surface of the dentine. After suitable isolation the enamel prisms appear slightly varicose in outline, the minute concavities producing the irregular dark bands often seen traversing the prisms. The prisms are held together by a delicate layer of cement-substance and grouped into NORMAL HISTOLOGY. bundles which cross one another, producing the alternate dark and light radial bands seen in the enamel. The additional dark lines extending more or less parallel to the free surface of the tooth—the stripes of Retzius—are probably due to inequalities in growth and density. At birth, and for a variable time thereafter, the outer sur- face of the enamel is covered by a delicate but resistant cuticle, the membrane of Nasmyth, composed of keratose epithelial plates, the remains of the enamel organ. This cuticle is soon worn away after the teeth are actively used. Next the dentine numerous clefts exist for a short distance between the enamel prisms ; they com- municate with the interglobular spaces and thus indirectly with the dentinal tubules. The cementum, or crusta petrosa, invests the fang of the tooth and closely resembles in structure ordinary bone ; the lamellae extend parallel to the dentine, as do likewise the long axes of the bone lacunae. Where the cementum reaches a considerable thickness, as at the apex of the root of the tooth, Ha- versian canals may exist, although usually these are wanting ; the outer layers of the cement contain fewer and smaller lacunae. The lacunae communicate with the dentinal tubules, while the protoplasmic processes of their contained bone-cells may come in contact with the filaments of the odonto- blasts lying within the dentinal tubules. The pulp consists of a matrix of soft embryonal connective tissue, in which nu- merous stellate and spindle cells form pro- toplasmic net-works by their anastomosing processes. At the periphery the connective- tissue elements are arranged as layers of elongated cylindrical cells perpendicular to the inner surface of the dentine, in contact with which they lie; these cells are the odontoblasts, being the representatives of the cells which were actively engaged in producing the dentinal matrix. The protoplasm of many of these cells is prolonged peripherally as delicate threads into the dentinal tubules, the processes becoming modified to form the stiff elastic dentinal fibres ; centrally, the odontoblasts frequently are connected with the stellate connective-tissue cells. The pulp is richly supplied with blood-vessels and nerves. The arteries run in the long axis of the tooth, breaking up into capillary net-works which are closest in the periphery. The nerves accom- Fig. 177. Section of human tooth at the junction of the dentine and the cementum: D, dentine with its tubules, which communicate with interglobular spaces (B) and with lacunae of cementum (C). THE DIGESTIVE TRACT. 149 pany the larger blood-vessels as medullated fibres ; these give off filaments which pass to the layers of odontoblasts, among which they extend as pale fibres. The ulti- mate distribution of these latter is still unsettled ; the assertion that fine fibrillae accompany the dentinal fibres into the tubules lacks confirmation. Distinct lymphatic vessels have not been demonstrated within the pulp, although the clefts within the matrix between the connective-tissue fibres represent the lymph-spaces and are in close relation with the adjacent lym- phatic channels. DEVELOPMENT OF THE TEETH. The teeth of man and the higher animals are really exaggerated papillae, the peripheral parts of which have become specialized and have under- gone calcification. The ectoderm con- tributes the enamel, while the dentine, cementum, and pulp are derived from the mesoderm. A linear thickening of the primitive oral epithelium marks the earliest indication of the formation of the teeth ; in man this band appears before the end of the sixth week (Rose), and is adherent to the under surface of the epithelial layer. Following the ex- pansion of this ectodermic thickening a con- tinuous lateral projection, the dental ridge, grows obliquely into the mesodermic tissue. The dental ridge continues tq grow back- ward towards the mandibular articulation, forming an unbroken arch of ectodermic tissue connected with the under side of the oral epithelium. The line of this attach- ment is later marked on the oral surface by a longitudinal furrow, the dental groove, which has been long known, and which was formerly regarded as the initial step in the dental development. While the dental ridge constitutes a shelf-like common epithelial invagination, the position and further development of the individual teeth are marked by local thickenings along the under surface of Fig. 178. Section of young tooth of child, show- ing peripheral portion of pulp and ad- joining dentine: b, pulp-cells, some of which send processes (a) within dentinal tubules ; c, stroma of delicate connective tissue; d, blood-vessels. Fig. 179. Section of jaw of rabbit embryo, showing thickening of ectodermic epithelium (ec) from which dental ridge (e) begins its growth into mesoderm (>n). 150 the ridge. These secondary aggregations are the first indications of the enamel organs of the temporary teeth. After the establish- NORMAL HISTOLOGY. Fig. 180. Fig. 181. Model of jaw of human embryo of 40 mm. : r, r, arch of increased epithelium constituting dental ridge; /, local thick- enings corresponding to positions of future enamel sacs. (After Rose) Section of jaw of rabbit embryo, showing dental ridge cut across : ec, oral ectoderm ; e, epithelial outgrowth corre- sponding to future enamel organ ; m, mesodermic tissue. ment of these structures the ectodermic tissue composing the dental ridge atrophies and eventually disappears in the intervals between the individual teeth. The enamel sacs of the permanent teeth are formed at a later date from the remains of the dental ridge, those for the three permanent molars being derived from a special extension of the dental ridge which grows independently of ectodermic attachments. The primitive enamel organ which grows from the dental ridge at first con- sists of a solid cylindrical process of epi- thelial tissue; soon, however, the ex- tremity becomes club-shaped and slightly tortuous, and later distinctly expanded and flask-shaped. Coincident with these changes the surrounding mesoderm be- gins to exhibit proliferation and conden- sation of its elements, this differentiation marking the earliest stage in the forma- tion of the important mesodermic dental papilla, which very soon becomes a conical mass of closely-aggregated meso- dermic elements. Along with the growth of the latter the now expanded end of the ectodermic plug becomes indented Fig. 182. Section of jaw of rabbit embryo, showing later stage of enamel organ, which now exhibits differentiation into outer (6) and inner (e) cells : m, mesodermic tissue which at a has undergone already some condensa- tion ; ec, oral ectoderm. THE DIGESTIVE TRACT. or invaginated to form an epithelial cap, which embraces the meso- dermic dental papilla, and, from its future important function, is known as the enamel organ. The impression of the dental papilla upon the overlying enamel organ is probably not to be attributed to mechanical obstruction op- posed to the advancing ecto- dermic tissue, but has its cause in more deeply lying laws of ex- pansion along lines of unequal growth. As the invagination of the enamel organ progresses, more and more of the dental papilla becomes covered, until about two-thirds of the meso- dermic cone are embraced within the sides of the ectodermic cap. The enamel organ itself under- goes a differentiation into three distinct layers : the outer layer, directly continuous for a long time with the ectodermic cells of the oral cavity, is composed of one or two layers of low columnar or polyhedral cells ; at the point where they are reflected to form the inner, invaginated part of the original epithelial sac, the cells become elongated and i51 Fig. 183. Section of jaw of cat embryo ; the dental papilla is seen as a projecting conical mass (/) of con- densed mesoderm, whose summit is enveloped by the invaginated enamel organ (e); ec, oral epi- thelium, still attached by the atrophic isthmus (a) with the enamel organ, whose outer (b), middle (c), and inner (d) layers are differentiated ; e', beginning of enamel organ for permanent tooth. Fig. 184. Section of jaw of cat embryo with four developing teeth slightly farther advanced than in the pre- ceding stage : ec, oral epithelium; a, dental groove; e, enamel organ; p, p, dental papillae; m, mesodermic tissue; b, b, bone. distinctly columnar, constituting the inner layer of the enamel organ containing the beautiful enamel cells. The outer and inner layers NORMAL HISTOLOGY. 152 of the enamel organ are separated at first by the narrow zone of epithelial elements of the middle layer ; the cells of the latter soon undergo characteristic changes, owing to an accumulation of fluid, resulting in the complete transformation of the cells, which become pressed together and reduced to thin plates, the tissue appearing" as if composed of irregularly anastomosing connective-tissue fibres rather than of epithelial elements. The enamel organ retains for a considerable time its connection with the epithelium of the oral cavity, a thin atrophic cord of cells indicating the position of the former robust stalk. At the side of this attachment a lateral cylin- drical projection early marks the beginning of the development of the second enamel organ for the permanent tooth. The columnar cells of the inner layer alone are concerned in the production of the enamel. This process consists essentially of a gradual deposition on the inner side of the enamel cells—that is, next the new dentine—of homo- geneous prisms arranged verti- cally t'o the surface of the inner layer of the enamel organ. The layer of enamel increases by the addition of increments deposited from within out, the latest-formed enamel always lying immediately internal to the inner layer of the enamel organ. During the later stages the inner and outer layers are approximated at the expense of the intervening middle layer, which finally becomes reduced to an attenuated stratum, the other coats of the enamel sac coming almost in actual contact. During the changes described in the enamel organ the central dental papilla is actively engaged in producing the dentine. The top and sides of the papilla are covered by a layer of elongated, columnar or pyriform connective-tissue cells, the odontoblasts, which are the immediate agents in causing the deposition of the dentinal matrix, the formative process being similar to that producing bone. The dentine is first formed at the apex of the papilla, and appears as a thin lamina of homogeneous matrix into which the delicate processes of the odontoblasts extend, becoming the dentinal Fig. 185. Section of developing tooth from cat embryo : tn, mesodermic tissue condensed in dental pa- pilla (/), at whose summit osteoblasts (d) are forming young dentine (c); inner layer (a) of enamel organ is engaged in producing layer of young enamel (6); e, middle, h, outer layer of enamel organ. THE DIGESTIVE TRACT. 153 fibres ; the canals left within the matrix to maintain the nutrition of the tissue constitute the dentinal tubules, the homologues of the lacunae and canaliculi of bone. With the continued growth the sides of the papilla as well as the apex be- come covered by the layer of newly- formed dentine ; the central part of the dental papilla remains, after all the dentine has been formed, as the pulp-tissue, into which the blood- vessels and nerves grow at a later period. At first both dentine and enamel are soft, the impregnation with lime salts occurring subsequently; the layer of the soft, most recently formed matrix is readily distin- guished in stained sections from the older calcified tissue. The cemen- tum, wanting during foetal life, is produced by the alveolar periosteum. THE TONGUE. The bulk of the tongue is com- posed of variously-disposed bundles of striated fibres of the lingualis, together with those of the accessory muscles, over the unattached surfaces of which the oral mucous membrane is reflected. The muscular tissue of the organ is arranged in bundles extending in three planes : (i) vertically and slightly radially (genio-hyoglossus, vertical fibres of lingualis and hyoglossus) ; (2) transversely (trans- verse fibres of lingualis) ; (3) longitudinally (lingualis superior and inferior, and styloglossus). A vertical median partition, the septum lingualse, divides the muscular tissue into two halves ; the inter- fascicular spaces are filled by delicate connective tissue and fat, in which lie embedded numerous small lingual glands. Many of the muscle-fibres find insertion in the deeper layer of the mucosa, into which their sarcolemma fades. Branched striped muscle-fibres are of common occurrence in the tongue. The mucous membrane forms the most conspicuous part of the organ. That covering the sides and inferior surfaces of the tongue is thin, containing small papillae and numerous mucous glands : on reaching the superior surface the mucous membrane greatly increases in thickness, and presents additional conspicuous irregularities, the Fig. 186. Section of developing tooth from cat em- bryo, portion of preceding figure more highly- magnified : m, mesodermic elements consti- tuting pulp-tissue; /,layer of odontoblasts en- gaged in producing dentine (K); a and b, cells of middle layer, c and d, cells of inner layer of enamel organ ; e, zone of young enamel. 154 NORMAL HISTOLOGY. papillae. The papillae are of three kinds : the filiform or conical, the fungiform, and the circumvallate. The conical papillae are widely distributed, occurring on all parts of the upper surface of the tongue. They consist of a conical or cylindrical elevation of the connective tissue of the mucosa, .5-2.5 mm. in height, covered with a thick layer of epithelium, the cells of which, as the most exposed part of the papillae, are partially removed by abrasion, the remaining epi- thelium presenting a ragged sur- face. The fungiform papillae are likewise found on all parts of the tongue, but they are fewer in number, lower, and broader than the conical, appearing as isolated but distinct red points. The connective-tissue stalks of these papillae are composed of a dense felt-work of fibrous tissue, and bear secondary papillae on their upper surface, the epithelium completely enveloping the entire connective-tissue core. The circumvallate papillae, usually eight to ten in number, are placed in two rows forming a /\ at the posterior part of the dorsum of the tongue. Each consists of a large flattened fungiform papilla surrounded by a deep furrow and a secondary encircling ridge or wall—an arrangement which has suggested the name. The upper surface of the mucosa is beset with minute secondary elevations, which, however, are not apparent on the free surface, being hidden by the thick stratum of covering epithelium. Lying altogether within the epithelium lining the sides of the deep circular furrow, the taste-buds appear as inconspicuous oval bodies Fig. 187. Section of human tongue showing conical papillae: a, connective tissue of mucosa, which forms core of papillae; b, b, partially abraded epithelium; c,masses of epithelial cells filling interpapillary recesses. Fig. i88. Section of tongue of child, showing a fungiform papilla; the connective-tissue stroma is covered by the epithelium. THE DIGESTIVE TRACT. occupying almost the entire thickness of the epithelium. Additional taste-buds are found in the folds in the vicinity of the circumvallate papillae, as likewise on some fungiform papillae. At the sides of the 155 Fig. 189. Section of circumvallate papilla from tongue of child: a, main central elevation, surrounded by the annular ridge (b) and the intervening deep furrow; c, taste-buds within the epithelium; d, ducts of neighboring glands e, blood-vessels. tongue, just in front of the anterior pillars of the fauces, are groups of parallel folds containing a number of taste-buds; these folds con- stitute the papillae foliatae, which are highly- developed in some of the lower animals, as in the rabbit. The taste-buds are oval, flask-shaped bodies, embedded within the epithelium, occu- pying usually the entire thickness of the latter, with their long axes placed in general vertically to the free surface of the epithelium. Each taste-bud consists of an enveloping layer of greatly-elongated epithelial cells, the cortical or tegmental cells, which form a complete covering, except over a small area correspond- ing to the superficial pole of the bud; at this point a minute canal, the taste-pore, connects the interior of the bud with the surface of the mucous membrane. Within the epithelial capsule lies a group of highly-specialized elements, the gustatory cells. These neuro-epithelial elements appear as spindle, rod-like, or forked cells, each being possessed of an oval nucleus situated about the centre of the elon- gated body. The peripheral or outer ends of these cells are usually prolonged with fine pointed extremities, some of which terminate in stiff hair-like processes projecting within the taste-pore almost as Fig. 190. Taste-bud from circumval- late papi la of child. The oval structure is limited to the epithelium (e) lining the furrow, encroaching slightly upon the adjacent connec- tive tissue (/); o, taste-pore through which the taste-cells communicate with the mucous surface. 156 NORMAL HISTOLOGY. far as the free surface. The inner or central ends of the gustatory- cells are prolonged as slender, sometimes forked, processes ; the minute swellings or varicosities which these extensions often exhibit are supposed to indicate the direct connection of the neuro-epithelial cells with the fibres of the nerve of the special sense of taste. It must be remembered, however, that no such continuity has been or is likely to be demonstrated. The submucous and interfascicular tissue of the tongue contains numerous glands, both of the mucous and of the serous type. The mucous glands resemble those of other parts of the oral cavity, being small racemose clusters of acini more or less filled with clear mucoid secretion. They are situated in the deeper layers of the submucous tissue, as well as between the bundles of the muscle-fibres, principally in the posterior part of the tongue, although a group of small mucous glands (Nuhn’s) is found near the tip. The ducts of those at the root of the tongue are some- times lined by ciliated epithelium. The serous glands are limited to the immediate neighborhood of the circumvallate and of the foliate papillae. The acini appear darkly granular and pour out a thin watery secretion well adapted to aid in producing gustatory impressions. The mucous membrane covering the root of the tongue contains also much adenoid tissue, which occurs either as diffuse masses or as circumscribed irregularly spherical lymph-follicles, 1-5 mm. in diameter. The position of these follicles is fre- quently indicated by slight elevations of the mucosa, in the centre of which a minute pit leads into the interior of the lymphatic crypt. The epithelium lining such recesses is completely in- filtrated with lymphoid cells, while the surround- ing diffuse adenoid tissue contains several minute spherical masses of denser structure. Among the formed elements observed in the saliva the so-called salivary corpuscles are conspicuous. These are spherical bodies, some- what larger than the leucocytes, and possess a distinct nucleus and minute granules within the cell-contents ; under high amplification these gran- ules exhibit the agitation characteristic of the molecular or Brown- ian motion. The salivary corpuscles are derived from the adenoid tissue of the mouth, and are really escaped lymphoid cells, which, in consequence of the action of the saliva, become swollen by the imbibition of a fluid less dense than the tissue-juices ; they thereupon exhibit a reaction similar to that seen when the colorless blood-cell is treated with water. Fig. 191. Salivary corpuscles from human saliva: x, group of corpuscles near epithelial cells; y, cor- puscle which has burst, allowing granules to es- cape ; z, salivary cor- puscle highly magnified, showing granules and nucleus. THE DIGESTIVE TRACT. The blood-supply of the tongue is very rich, the vessels forming a superficial net-work in the mucosa, from which minute twigs as- cend within the papillae to terminate at the summit in close capillary plexuses. The acini of the various glands are surrounded by capillaries, as are also the lymph-follicles through the adenoid tissue of which many minute vessels extend. The capillary net-works supplying the mus- cular tissue follow the general arrangement and direction of the mus- cular fibres, surrounding the latter by the characteristic rectangular- meshed net-works. The lymphatics of the tongue are numerous; they are arranged as a superficial plexus within the submucous tissue, which re- ceives the lymphatics from the bases of the papillae; the latter vessels, in turn, take up the smaller trunks having their x 5 7 Fig. 193. Fig. 192. Section of tonsil of dog : a, epithelium of mucous membrane passing into central recess (A), where it becomes infiltrated with lymphoid cells (e); c, lymph-nodules embedded within diffuse adenoid tissue; d, neighboring mucous glands. Section of tonsil of child ; the epithelium of adja- cent surface passes into the deep pits which extend into the adenoid tissue. origin in the numerous interfascicular lymph-spaces within the cen- tral papillary connective tissue. The lymph-follicles at the root of the tongue are well provided with lymphatics, which surround the follicles and give off radicles to the adenoid tissue. 158 NORMAL HISTOLOGY. The nerves supplying the mucous membrane—the glosso-pharyn- geal and the lingual branch of the trifacial—end either beneath the epithelium in the usual manner, or in close relation with the organs of special sense—the taste-buds. Numerous microscopic ganglia also occur along their course, especially in connection with the fibres of the glosso-pharyngeal nerve. The tonsils represent compound lymphatic glands, while con- siderable variation exists as to form and size, each organ consisting of an aggregation of from ten to eighteen lymph-follicles, closely resembling those found at the root of the tongue embedded within the sur- rounding diffuse adenoid tissue. The entire mass is separated from the adjacent structures on the attached borders by a fibrous capsule, and is covered with a reflection of the oral epithelium on the mucous surface, in- cluding the deep central pit on which the lymph-follicles abut. The epi- thelium covering the folds and de- pressions of these surfaces is com- pletely infiltrated with lymphoid cells, so that the demarcation between the epithelium and the subjacent adenoid tissue is often obscure. Numerous mucous glands occupy the immediate vicinity of the tonsils, into the crypts of which the glands pour their secretion to mingle with the shed epithelium and lymphoid cells occupying the recesses. Great numbers of the escaped lymphoid cells pass into the oral cavity to become salivary corpuscles, of which the tonsils are a most important source. Blood-vessels and lymphatics occur in large numbers within the adenoid tissue ; venous and lymphatic plexuses surround the organ receiving the radicles issuing from the interior. Lymph- channels encircle the individual follicles, and afterwards communicate with the larger peripheral vessels. Regarding the ultimate distribution of the nerves little is defi- nitely known; fibres have been traced into the subepithelial plexus. THE TONSILS. Fig. 194. Section of child’s tonsil, showing the details of the epithelium and part of the lymphoid tissue from preceding figure under higher amplification. THE DIGESTIVE TRACT. 159 THE PHARYNX. The pharynx consists essentially of a fibrous tunic, within which lies the mucous membrane with the submucous tissue, while without are arranged the fibres of the constrictor and other muscles ; three coats, the mucous, the fibrous, and the muscular, are recognized, therefore, as forming its walls. The histological differences distin- guishing the upper, or respiratory, from the lower, or digestive, portion of the sac depend largely upon variations within the mucosa, especially as to the character of the epithelium. The upper, respiratory division of the pharynx is clothed with stratified ciliated columnar epithelium containing numerous goblet- cells, while the part situated below the level of the soft palate is cov- ered with stratified squamous cells similar to those lining the oral cavity. The tunica propria, or stroma of the mucosa, is formed of a felt-work of fibrous bundles, together with a variable, in certain parts large, quantity of elastic tissue. The subepithelial surface of the mucosa, where covered by the squamous cells, is beset with numerous small papillae; these, however, are wanting beneath the ciliated epithelium. Small mucous pharyngeal glands occur in many places ; they are especially numerous in the deepest layers of the mucosa in the immediate vicinity of the orifices of the Eustachian tubes, occurring less frequently towards the lower part of the pharynx. The mucous membrane contains a considerable quantity of adenoid tissue ar- ranged as numerous lymph-follicles in the upper part of the cavity; these follicles closely resemble those found at the root of the tongue, existing isolated or in groups. A conspicuous aggregation of such structures lies on the posterior wall of the pharynx between the openings of the Eustachian tubes, constituting the pharyngeal tonsil, appropriately so named in view of the similarity of its struct- ure to that of the palatine organs of like name. Some of the mucous glands here also open into the central crypt. The submucous tissue unites the mucous membrane with the fibrous coat, whose dense felt-work of fibro-elastic bundles forms a structure frequently termed the pharyngeal aponeurosis. Its pos- terior part is greatly thickened and forms the raphe to which the constrictor muscles are attached. The muscular coat is formed of the striped fibres constituting the constrictor and other muscles, with whose general arrangement the disposition of the muscular tissue agrees. External to the muscular coat an irregular investment of areolar tissue attaches the pharynx to the surrounding structures. The larger blood-vessels, lymphatics, and nerve-trunks take 160 NORMAL HISTOLOGY. their course within the submucous tissue, and send off branches to supply the mucosa in a similar manner as in the mouth. The lym- phatics are exceptionally numerous in the vicinity of the lymph- follicles, around which they form net-works continuous with those of the nasal cavity, the oesophagus, and the larynx. The nerves supplying the pharynx, derived from the cranial and sympathetic trunks taking part in the formation of the pharyngeal plexus, contain both medullated and non-medullated fibres, associ- ated with minute ganglia. Small twigs are given off from the larger branches to terminate in the subepithelial tissue and among the acini of the mucous glands and the lymphatic follicles. The walls of the digestive tract, from the oesophagus to the anus, are composed of four tunics—the mucous, the submucous, the muscular, and the fibrous or serous. The muscular coat, usually thickest and most rigid, is the most essential structure in maintaining the form of the tube. The mucosa is distinguished by the highly-specialized secreting apparatus which it contains, as well as by the variations and the modifications of its surface ; the difference between the several divisions of the digestive tract is dependent largely upon the changes in the character of this tunic. The submucosa loosely connects the mucous coat with the mus- cular, and affords space for the larger blood-vessels, the lymphatics, and the nerves, as well as for some few glandular structures and lymphoid masses. The fibrous coat gives additional strength to the walls of the digestive tube, and presents a smooth external serous surface in those parts of the tract which receive a reflection from the peritoneum. The walls of the oesophagus comprise four coats—the mucous, the submucous, the muscular, and the fibrous. The mucous membrane is a continuation of that of the pharynx, and corresponds closely with the latter in structure. The stratified squamous epithelium rests upon the connective- tissue matrix, the tunica propria, the inner surface of which bears numerous small papillae completely hidden by the thick overlying epithelium. The deeper layers of the mucosa are separated from the submucous coat by longitudinal bundles of involuntary muscle, the muscularis mucosae; these muscular bundles, absent in the upper part of the oesophagus, first appear as irregular and inter- rupted groups, which become more numerous until, from the middle of the tube on, they form a continuous longitudinally-disposed layer. The submucous coat is composed of loosely-united connective tissue, serving for the conveyance and support of the larger blood- THE OESOPHAGUS. THE DIGESTIVE TRACT. 161 vessels, lymphatics, and nerves. Within the submucosa are placed likewise the acini of the mucous glands; these are rather more numerous on the anterior surface, their ducts piercing the mucosa and opening on the free surface of the mucous membrane, being lined throughout the greater part of their length by columnar epi- thelium. In the lower portion of the oesophagus, particularly about Fig. 195. Section of human oesophagus: a, squamous epithelium of surface resting upon fibrous tissue of mucosa, the deeper part of which is occupied by muscularis mucosae (6); c, submucous coat, con- taining glands (h)\ d, e, respectively circular and longitudinal muscular tunics; e', e', bundles of striped muscle-fibres. the cardiac orifice, the mucous glands are very plentiful and lie within the mucosa. The muscular tunic consists of two layers, an inner circular and an outer longitudinal, whose component bundles are held ■together by the connective-tissue septa which pass between the fas- ciculi in all directions. The character of the muscular tissue varies in the several portions of the tube. That contained within the wall of the upper third of the oesophagus is entirely of the striated variety, while the muscular tissue of the lower third is exclusively 162 NORMAL HISTOLOGY. non-striped or involuntary in character ; in the middle third both kinds exist, the striated fibres gradually disappearing as the non-striped fibres increase. The latter extend highest in the circular coat and somewhat farther in the anterior than on the posterior wall. The last traces of voluntary muscle appear as short, isolated striped fibres among the surrounding fasciculi of non-striated tissue. The fibrous coat envelops the muscular tunic externally, strength- ening the tube and affording attachment to the surrounding areolar tissue connecting the oesophagus with neighboring organs. Con- siderable elastic tissue is found in this coat, the elastic fibres forming net-works intimately connected with the bundles of involuntary muscle. The larger blood-vessels penetrate the outer coats and ramify within the submucous tissue, from which branches pass to supply the muscular and mucous tunics, the capillaries within the latter ending as net-works within the inner part of the tunica propria. The lymphatics of the deeper layers of the mucosa terminate in the larger vessels of the submucosa. Numerous nerve-fibrillae pass from the submucous tunic into the mucosa to end beneath the epithelium. The stomach must be regarded as a dilated and specialized portion of the general digestive tube, its walls consisting of the four coats common to the other parts of the tract—namely, the mucous, the submucous, the muscular, and the serous or fibrous tunic. The mucous membrane is covered by a simple columnar epithelium, the squamous cells of the oesophagus abruptly ter- minating at the cardiac orifice to be replaced by the columnar ele- ments of the gastric epithelium, many of which are goblet-cells. The free inner surface of the stomach presents, in addition to the conspicuous folds or rugae, minute inequalities and pits, which mark the openings of the gastric glands; the mouths of the latter show as minute depressions, between which the intervening por- tions of the mucosa extend as apparent elevations. The gastric glands are of two kinds—the peptic glands, situ- ated in the middle and cardiac thirds, and the pyloric glands, found in the pyloric third of the stomach. Both varieties are limited to the mucosa, extending in length the entire thickness of this coat. The peptic glands are slightly wavy, simple tubular depressions, in which a duct, a neck, and a fundus are recognized. In excep- tional cases the fundus is divided, while in nearly all it is tortuous or spiral, its extremity being often sharply bent at right angles to the general axis of the tube. The columnar epithelial cells of the ad- THE STOMACH. THE DIGESTIVE TRACT. 163 jacent gastric mucous membrane pass into the ducts of the glands with little change, becoming imbricated, and, towards the neck, shorter and more spherical in outline. At the neck, the narrowest part of the tube, the cells are more cuboidal, and assume a columnar or pyramidal form as they approach the fundus. The chief or central cells bound the lumen Fig. 197. Fig. 196. Peptic gland from stomach of dog: a, wide mouth and duct which re- ceive the terminal divisions of the gland; b, c, neck and fundus of the tubes ; e, central or chief, d, parietal or acid, cells. Section of human stomach, showing general arrangement of its coats : a, mucosa containing the tubular peptic glands; e, muscularis mucosse separating the layer of glands from the underlying submucous coat (b); h, blood-vessels; c, c', respectively the circular and longi- tudinal muscular layers; d, the fibrous tunic covered with the peri- toneum. of the gland and form the bulk of the glandular epithelium. Each cell contains a spherical nucleus embedded within the granular pro- toplasm, whose exact condition depends upon the state of functional activity. In addition to the chief or central cells, a second variety, the parietal or acid cells, exists in the peptic glands. As indicated by their name, the parietal cells are situated in the periphery of the gland immediately beneath the basement-membrane, usually separated from the lumen by the intervening central cells. Minute lateral 164 NORMAL HISTOLOGY. intercellular clefts or canals in many places afford direct commu- nication between the parietal cells and the lumen of the tube. The parietal cells are irregularly distributed from the fundus to the Fig. 198. Transverse sections of peptic glands from stomach of dog: A, plane of section passes through ducts near free surface ; a, lumen of glands ; b, surrounding fibrous stroma of mucosa ; B, plane of section passes through fundi near terminations of tubules ; the sections of the latter are arranged in groups separated by connective tissue. neck of the gland ; but they are especially numerous in the vicinity of the neck. These cells are larger than those lining the lumen, polygonal or triangular in outline, and possessed of a pale, faintly granular protoplasm surrounding a round or oval nucleus. In preparations of human stomach, the parietal cells are not infrequently the most con- spicuous and best defined, since the central cells are prone to disintegrate. On approaching the pyloric ring, the simple tubular peptic glands are gradually replaced by the compound glands, until, near the intestinal opening, these alone are present. The pyloric glands are characterized by their relatively long, wide ducts into which the severa divisions of the body open; the tubular com- partments are wavy and tortuous, and frequently end in slightly expanded extremities. The duct is lined by tall columnar epithelium, the cells be- coming lower and broader as they approach the neck and towards the fundus. The cells contain finely granular protoplasm, and do not secrete mucus, but a thin albuminous fluid. Parietal or acid cells do not occur in the pyloric glands, being confined to the true peptic glands. The gastric glands, while very uniformly dis- tributed through all parts of the stomach, are arranged in groups, the individual tubules of which are separated by very delicate partitions of the connective Fig. 199. Portion of peptic gland of dog, highly magnified: a, a, the central or chief cells next the lumen (c) ; b, b, the parietal or acid cells connected with the lumen of the tube by short lateral branches which extend to the cells. THE DIGESTIVE TRACT.. 165 tissue, thicker layers of fibrous tissue enveloping the entire group. Numbers of lymph-cells are intermingled with the fibrous tissue of the mucosa; in the vicinity of the pylorus considerable patches of diffuse adenoid tissue lie around and among the ends of the gastric follicles and constitute the lenticular glands. The muscularis mucosae oc- cupies the deepest layer of the Fig. 200. Fig. 2oi. Sectiou of pyloric glands from human stomach : a, mouth of gland leading into long, wide duct (/>), into which open the terminal divisions; c, connective tissue of the mucosa. Section of pyloric region of human stomach, showing irregular mass of adenoid tissue lying between the gastric tubules (g, g) constituting a lenticular gland; s, submucous tissue. tunica propria, and is composed of an inner circular and an outer longitudinal layer of non-striped muscle; the tissue of the muscu- Fig. 202. Longitudinal section of child’s stomach passing through pyloric orifice : S', I, the gastric and the in- testinal surface; p, pyloric glands, which gradually extend into the submucosa to become Brunner’s glands (b); a, simple follicles of the intestinal mucosa; s, submucosa ; t, the greatly thickened layer of circular muscle constituting the pyloric ring ; l, longitudinal muscular tunic. laris mucosae extends within the interglandular septa, often as far as the free surface of the mucous membrane, beneath which the muscle- cells disappear. The submucosa is a coat of considerable thickness, composed of a felt-work of fibro-elastic bundles of varying size, but so loosely interwoven that the mucosa may be shifted readily within con- 166 NORMAL HISTOLOGY. siderable latitude upon the underlying muscular tunic. The large prominent folds, or rugae, of the stomach involve both the mucous and the submucous coat, the latter forming the connective-tissue frame-work of the elevation over which the mucosa with its glands is reflected. Within the mesh-work of connective-tissue bundles are supported the larger blood-vessels, lymphatics, and nerves. The muscular tunic comprises two principal sheets of involun- tary muscle, disposed as an inner circular and an outer longitudinal layer; towards the cardiac end of the stomach irregular bundles of oblique fibres constitute an imperfect third layer. The pyloric orifice is guarded by a fold of mucous membrane supported by the submucosa and strengthened by a conspicuous local annular thickening of the inner circular layer of muscle; the outer longi- tudinal muscular layer and the serous coat pass over into the intes- tinal wall without partici- pating in the formation of this gastro-duodenal valve. The serous coat is composed of bundles of fibrous connective tissue, together with rich net- works of elastic fibres, while the peritoneal sur- face is covered with a single layer of the charac- teristic endothelial plates. The narrow areas included between the folds of the peritoneum along their lines of reflection are, of course, devoid of the serous covering ; at these points the vessels and the nerves pass to and from the stomach. The larger arteries, after penetrating the outer coats, divide within the submucosa into smaller branches, one set of which pierces the muscularis mucosae to be distributed to the mucous membrane, while the other enters the muscular and serous tunics. The vessels supplying the mucosa form a rich subepithelial capillary net- work, as well as mesh-works surrounding the gastric glands, the cap- illaries lying immediately beneath the basement in close proximity to the glandular epithelium. The branches distributed to the outer Fig. 203. Section of injected stomach of cat: a, rugse consisting of the mucosa and a core of submucous tissue (i); c, d, the circular and longitudinal layers of muscle; all the dark lines represent the blood-vessels filled with the carmine- gelatin mass ; the larger trunks break up in the submucosa, sending twigs into the mucous and muscular tunics. THE DIGESTIVE TRACT. 167 layers form long-meshed capillary net-works, from which the muscle- bundles and fibrous tissue derive their supply. The larger lymphatic trunks accompany the blood-vessels and form a coarse plexus within the submucous tissue; a much closer net-work of smaller lymphatics occupies the deeper part of the mucosa, from which radicles ascend between the glands to end beneath the epithelium in slightly dilated blind extremities. Peripherally-situated lymph-vessels drain the masses of adenoid tissue. In addition to the lymphatics of the mucosa, the larger vessels of the submucosa take up those from the muscular coat. The nerves of the stomach, after piercing the serous coat, take up a position between the circular and longitudinal muscular layers, in which situation they form a rich plexus, consisting of both medul- Fig. 204. Surface views of nervous plexuses of stomach of young child. A, Auerbach’s plexus : g, groups of ganglion-cells ; r, underlying muscular tissue. B, Meissner’s plexus : g, groups of ganglion-cells ; b, blood-vessel. (After Stohr.) lated and pale fibres ; at the nodal points of this net-work numerous microscopic ganglia are situated, the whole forming the intramuscular ganglionic plexus of Auerbach. From this plexus fibres are distributed to the serous coat and to the longitudinal layer of muscle, as well as to the outer part of the circular layer. The intramuscular net-work is continued by numerous small bundles of fibres, which, after piercing the inner layer of cir- cular muscle, and giving off lateral twigs to the inner part of the same, enter the submucosa to form there a second ganglionic plexus similar to the one lying between the muscular layers: this is the plexus of Meissner. The submucous plexus sends off numerous fibres into the mucosa, which are distributed beneath the epithelium 168 NORMAL HISTOLOGY. and to the gastric glands; the exact mode of termination of these nerve-fibrillae within the mucosa, however, is still undetermined. THE INTESTINES. The four coats of the stomach are continued, with little modifica- tion, into the mucous, the submucous, the muscular, and the serous tunics of the intestinal wall; the variations characterizing the several divisions of this tube are dependent largely upon modi- fications and specializations of the mucous membrane. The free inner surface of the small intestine is studded over with small cylindrical elevations—the villi—projecting into the intestinal lumen and bathed in the juices of the canal. In addition to the villi, which are found through the whole extent of the small intestine, the mucous membrane is thrown into transverse or oblique per- manent folds — the val- vulae conniventes — which extend partially around the tube, and are most marked in the duo- denum and the jejunum ; these folds increase the area of the mucous sur- face, and are beset with villi the same as the sur- rounding parts of the mucosa. These projections, the villi and the valvulse conniventes, are peculiar to the small intestine and serve to distinguish it from the large. The mucosa is covered by a single layer of columnar epithelium resting upon the basement-membrane. The prismatic cells contain finely granular protoplasm and oval nuclei, the latter being usually situated within the inner half of the cell. The outer free ends of the cells are invested by a peculiar cuticular zone, or basilar border, a well-defined continuous band exhibiting, in suitably preserved specimens, a fine vertical striation. The significance of these mark- ings is still uncertain, especially in view of the fact that, after the action of such reagents as water, the border breaks up into rods resembling very coarse cilia; the striation is regarded by others as the expression of fine parallel canals. Fig. 205. Longitudinal section of human small intestine, showing general relation of the folds constituting the valvulae conni- ventes to the mucosa and submucous coat; the latter con- tributes the fibrous core over which the mucosa with its villi and glands extends. THE DIGESTIVE TRACT. lQg Goblet-cells are numerous, many epithelial elements having be- come distended with mucoid secretion : in carmine preparations the cells appear as clear, oval breaks in the contour of the epithelium. While occur- ring throughout the entire digestive tube, the goblet-cells are especially numerous in the large intestine, where not infrequently the majority of the epithelial elements are in this condition. During certain stages of digestion the protoplasm of the epithelium may contain oil-drops taken up from the intestinal contents. Migratory leuco- cytes are also found in the intercellular clefts. The epithelium rests upon a mem- brana propria—the endothelium of Debove —composed of flattened connective-tissue plates. The villi consist entirely of the tissues of the mucosa, the epithelium extending over the projecting portions of the tunica propria to form a complete investment of the finger-like processes. The centre of each villus is occupied by the absorbent chyle-vessel, or lacteal, a slightly club- shaped lymphatic radicle, which ends blindly near the apex of the villus and whose walls are composed of a single layer of endothelium. The tissue surrounding the lacteal and forming the bulk of the projection approaches in character quite closely adenoid tissue, con- sisting of a fibrous reticulum holding many lymphoid cells within its meshes. The central lacteal lies enclosed within a capil- lary net-work, extending through the greater part of the villus and connecting the afferent arteriole and efferent veins. Imme- diately surrounding the lacteal, and in inti- mate relation with it, numerous delicate vertical bundles of non-striped muscle, derived from the underlying rmfscularis mucosae, ascend towards the tip of the villus. The components of the villus are held together by the common adenoid tissue, in whose interstices lie many lymphoid cells and, during certain stages of digestion, numberless fatty granules. At Fig. 206. Simple tubular glands of large intestine of dog: the epithelial elements lining the follicles have become very largely converted into goblet-cells. Fig. 207. Transverse section of follicles of large intestine of dog : the individual tubules are separated by the fibrous stroma of the mucosa. !y0 NORMAL HISTOLOGY. such times the contents of the lacteals appear milky, in consequence of the emulsion formed by the ab- sorbed oil; during the intervals of digestive inactivity the lacteal con- tains the clear, straw-colored fluid usually found within lymphatic ves- sels. The villi disappear abruptly at the ileo-caecal valve and are not present in the large intestine. Among the structures of the in- testinal wall usually included as “glands” two distinct groups must Fig. 208. Fig. 209. Longitudinal section of villus from intestine of dog, highly magnified: a, columnar epithelium containing goblet- cells (b) and migratory leucocytes (h); c, basement membrane ; d, plate-like connective-tissue elements of core; e, e, blood-vessels; f, absorbent radicle or lacteal. Transverse section of villus from intestine of dog: a, a, blood-vessels; b, lacteal. be recognized—the true and the false glands, the latter being simple or compound lymph-follicles. These structures therefore fall under the appropriate headings : Intestinal True-Glands. Intestinal Lymph-Follicles. Glands of Lieberkiihn. Solitary glands. Glands of Brunner. Agminated glands. The follicles, crypts, or glands of Lieberkuhn are very nu- merous, forming an almost continuous layer of simple tubular de- pressions throughout the intestines, large as well as small. They occupy nearly the whole depth of the mucosa, their wavy extremities approaching the muscularis mucosae. The columnar epithelium of the free surface passes directly into the tubules to become the spherical secreting cells, many of which undergo mucoid distention and THE DIGESTIVE TRACT. lnI conversion into goblet-cells. Lieberkiihn’s glands lie between the bases of the villi, but are found upon the valvulae conniventes, since the latter depend on the elevation of the submucosa for their formation, the mucosa being reflected over the pro- jecting underlying tunic. In the lower part of the large in- testine the glands of Lieber- kiihn increase in size, becom- ing longer and possessing wider mouths, their orifices appearing as minute pits. The duodenum, especially in its upper part, possesses an additional layer of true secreting structures in the glands of Brunner. These are the direct continuations and higher specializations of the pyloric glands of the stomach. In passing from the stomach into the intes- tine these tubules undergo repeated division, and, at the same time, sink deeper and deeper into the mu- cosa; finally reaching below the limits of this layer to take up a position within the submucosa of the duo- denum, beneath the over- lying layer of the follicles of Lieberkiihn within the mucosa. Brunner’s glands, or the duodenal glands, appear as groups of short, wide, tubular acini, dis- posed about long, slender ducts which pass from the submucous tissue through the mucosa to open on the intestinal surface between Fig. 210. Longitudinal section of large intestine of child : a, a, simple tubular glands ; b, submucous tissue; c and d, circular and longitudinal layers of muscle. Fig. 21i. Section of duodenum of cat: a, mucosa containing the villi (/) and the follicles of Lieberkiihn (7), and pierced by the ducts (g) of the glands of Brunner (h) within the sub- mucosa (c); b, muscularis mucosae ; d, d', circular and lon- gitudinal layers of muscle; e, fibrous tunic. ly2 NORMAL HISTOLOGY. the orifices of the follicles in the depressions between the bases of the surrounding villi. The glands, owing to the rapid branching of their tubules, more closely approach the racemose type than the compound tubular to which they really belong, as shown in their direct deriva- tion from the com- pound tubular pyloric crypts. The secretion of these duodenal glands is serous and not mucous, the cells being filled with dark granules. The solitary glands are isolated lymph - follicles scat- tered through the entire intestine ; they are, however, most abundant in the lower part of the ileum and in the first portions of the large intestine. They are situated primarily within the mucosa, al- though they frequently lie also within the submucous coat; when well Fig. 212. Section of human large intestine, containing solitary gland : a, mucosa; b, submucosa ; c, c', circular and longitudinal layers of muscle ; d, serous coat. Fig. 213. Section of small intestine of cat, showing a Peyer’s patch (d, d) cut crosswise : a, b, c, respectively mucous, submucous, and muscular coats. developed, they encroach upon the mucosa to such an extent that their inner pole slightly projects upon the free surface of the intestine. The lymphoid tissue is somewhat denser in the periphery of the fol- licle, beneath its limiting capsule, than towards the centre; but the THE DIGESTIVE TRACT. lymphoid cells are everywhere so closely packed that the support- ing reticulum of connective tissue is masked. In the upper part of the duodenum numerous ill-defined masses of adenoid tissue occupy the mucosa between the follicles and represent the lenticular glands of the stomach. The agminated glands, or Peyer’s patches, are large, oval groups of closely aggregated lymph-follicles, held and blended to- gether by diffuse adenoid tissue. These patches vary in size and number, and are usually limited to the lower two-thirds of the small intestine, reaching their highest development in the ileum, where they may attain a length of 9-11 cm. ; between twenty and thirty patches generally are present, while they are relatively better devel- oped in young than in old subjects. The agminated glands appear first within the mucosa, but later encroach largely upon the submucous tissue. The lymph-follicles of which these patches are composed become somewhat pyramidal, owing to pressure, and lose much of their individuality, the demarca- tion into separate follicles being best preserved along the outer ly^ Fig. 214. Section of small intestine of child, including a portion of a Peyer’s patch : a, b, and c, mucosa, sub- mucosa, and muscular coats; d, villi; e, e, atrophic follicles of the mucosa. boundary, occupying the submucosa, within the mucosa the out- lines of the follicles being lost in the general adenoid mass. Where the summits of the follicles impinge against the inner layer of the mucosa, the positions of the follicles are indicated by corresponding elevations of the mucous surface, at which points the villi are fre- quently pushed aside and the gland-layer more or less completely interrupted. In the vermiform appendix of some animals, and !74 NORMAL HISTOLOGY. in some cases also in man, the follicles form a continuous zone of adenoid tissue. The muscularis mucosae, like that of the stomach, occupies the deepest part of the mucosa and marks the outer boundary of the mucous layer. The muscular tissue comprises longitudinally-dis- posed bundles of muscle-cells, supplemented in some places by a more or less complete additional internal layer of circularly-placed cells. The submucosa of the intestinal wall corresponds to the similar coat of the stomach, consisting of loosely-united bundles of fibro- elastic tissue, which support the larger vascular and lym- phatic trunks, as well as a rich nervous plexus. The muscular coat con- sists of two well-developed layers—the thicker inner cir- cular and the less robust outer longitudinal stratum. These are separated by a thin layer of connective tissue, which externally becomes continuous with the envel- oping areolar tissue and passes into the outer fibrous tunic of the serosa. In parts of the large intes- tine—as the caecum and the colon—the circular muscular coat is relatively thin, while the longitudinal layer is in- complete, the fibres of the lat- ter being collected into three flat bands, 10-15 rnm. wide; these longitudinal bands are much shorter than the other layers of the intestinal wall, which arrangement results in the characteristic sacculation of the large intestine. In the ower part of the rectum the circular muscular layer becomes thick- ened to form the internal anal sphincter, composed of involuntary nuscle; the bands of longitudinal fibres spread out, and towards the ower end of the rectum form a thick, uniform layer. Fig. 215. Section of injected small intestine of cat: a, b, mu- cosa ; g, villi; i, their absorbent vessels; h, simple follicles; c, muscularis mucosa;; d, submucosa; e, e', circular and longitudinal layers of muscle; f, fibrous coat. All the dark lines represent blood-vessels filled with the injection mass. THE DIGESTIVE TRACT. j The blood-vessels supplying the intestines follow the general arrangement of those of the stomach. The larger vessels pierce the serous and muscular coats, giving off slender twigs to supply the tissues of these tunics; upon reaching the submucosa the vessels form a wide-meshed net-work. Numerous branches then pass through the muscularis mucosae to be distributed to the deeper as well as to the more superficial parts of the mucosa; narrow capil- laries form net-works which surround the tubular glands, while be- neath the epithelium wider capillaries encircle the mouths of the follicles. From this superficial capillary net-work the veins arise and, passing between the follicles, join the deeper venous plexus, which in turn empties into the larger veins of the submucosa. In those parts of the intestine where villi exist, special additional arteries pass directly to the bases of the villi, where they expand into capillary net-works which run beneath the epithelium and around the central lacteal as far as the ends of the villi. These capillaries terminate in venous stems which descend almost perpendicularly into the mucosa, in their course receiving the superficial capillaries encircling the glandular ducts. Brunner’s glands and the solitary and agminated follicles are supplied from the submucosa by vessels which terminate in capillary net-works distributed to the acini of the glands and to the interior of the lymph-follicles. The lymphatics of the intestinal tract are very abundant. They begin as blind canals, whose slightly-dilated ends lie within the mu- cosa between the tubular follicles ; in those parts of the intestine where villi exist, the centre of these projections is occupied by a lymphatic radicle, the chyle-vessel, or lacteal. All these vessels de- scend to join a rich plexus of lymphatic trunks situated within the deeper layers of the mucosa. Within the submucosa an addi- tional net-work of still larger channels exists, the two sets of vessels freely communicating through numerous anastomoses. The accumu- lations within these net-works are carried off by lymphatic trunks which pierce the muscle and pass off between the two layers of the peritoneum into the adjacent mesenteric glands, in their course taking up the vessels carrying the lymph collected from the mus- cular tissue. Many vessels of the submucous net-work, as well as the larger lymphatic trunks, are provided with valves, whose position is usually indicated by dilatations in the contour of the vessel. The nerves distributed to the intestines are arranged almost iden- tically as those of the stomach; they are composed largely of non- medullated fibres, derived from the trunks which pass within the mesentery from the large abdominal sympathetic plexuses. After giving off branches to the serous coat, the nerves pierce the longitu- dinal muscular tunic to form the rich intramuscular plexus of Auer- 176 normal histology. bach. This is composed of a rich net-work of delicate, pale fibres, at the nodal points of which microscopic ganglia exist; after supply- ing the longitudinal and outer part of the circular muscular coats, the fibres obliquely pierce the latter tunic to gain the submucous tissue, where they form the plexus of Meissner, which closely resembles Auerbach’s nervous net-work within the muscularis, possessing, how- ever, smaller ganglia and somewhat closer meshes. From the plexus of the submucous tunic fibres pass into the mucosa to form net-works about the glands and to send fibrillae into the villi. The ultimate distribution of these fibres must be regarded as still undetermined. THE LIVER. Although the liver in its development corresponds to a compound tubular gland, a type which is permanently retained in many lower vertebrates, in the adult condition of the mam- malian organ this char- acter is largely masked in consequence of the fusion of the tubes in the formation of the cords of cells. The fibrous tissue enveloping the exterior of the liver is prolonged into the interior of the organ through the transverse fissure, in company with the blood-vessels and the bile - ducts. The de- marcation of the indi- vidual lobules depends upon the development of this interlobular connective tissue, known as the capsule of Glisson; when well developed, as in the liver of the hog, the lobules are defined with great distinctness, being completely surrounded and separated from their neighbors by the connective tissue. In the human liver, on the contrary, the interlobular connective tissue is very scanty, this defici- ency producing poorly-defined lobules, the boundaries of which are scarcely indicated by the irregular areas of connective tissue occupy- ing the spaces between the approximated surfaces of three or more hepatic lobules. The arrangement of the blood-vessels is so important in de- termining the general construction of the lobule that an early con- Fig. 216. Section of liver of hog, showing very diagrammatically the lobules : a, interlobular connective tissue ; b, c, branches of por- tal vein and of hepatic artery ; d, bile-ducts; e, intralobular vein. THE DIGESTIVE TRACT 177 Fig. 217. Section of human liver, showing general arrangement of lobules : a, interlobular (portal) vein ; b, intralobular (hepatic) vein ; c, hepatic artery; d, bile-duct; the boundaries of, the lobules are imper- fectly defined by the irregular areas representing the poorly-developed capsule of Glisson. sideration of the vascular supply is necessary to an understanding of the structure of the lobule. The interlobular vessels, situated between the lobules at their periphery, are continuations of those passing through the transverse fissure; they are the portal vein, the hepatic ar- tery, and the bile-duct. The portal vein, the largest of the interlobular vessels, gives off nu- merous branches, which enter the lobule at the periphery and break up into twigs, forming a rich, freely anas- tomosing intralobular capillary net-work. The meshes of this net- work are somewhat elongated and trapezoidal in form, the smaller end of the spaces being directed towards the centre of the lobule, an arrange- ment produced by the convergence of the capillary net-work to the centrally placed intralobular vein, a branch of the hepatic. The meshes of this lobular capil- lary net-work are occupied by the Fig. 218. Fig. 218. Diagram of the structure of the liver: P. V., the portal or interlobular vein, which breaks up into the capillary net-work of the lobule; H. V., central intralobular vein, a branch of the hepatic; H. A., he- patic artery, supplying nutrition to the in- terlobular structures and terminating in the lobular capillary net-work ; B.D., the inter- lobular bile-duct which takes up the bile- capillaries at the periphery of the lobule. Xy8 NORMAL HISTOLOGY. secreting hepatic tissue, comprising the liver-cells, the bile-capil- laries, the minute channels through which the bile elaborated within Fig. 219. Section of injected human liver, the capillaries having been filled from the central vein (a) ; b, branches of portal vein. the lobule is carried off, together with lymph-radicles and a very small amount of delicate areolar tissue. The liver-cells are irreg- ular polyhedral elements (17— 25 m) in whose finely granular protoplasm, devoid of cell-mem- brane, one or more round nuclei lie embedded. Numerous oil- drops of various sizes, as well as pigment - granules, very com- monly are present within the Fig. 221. Fig. 220. Hepatic cells isolated from human liver : a, oil-drops; b, slight concavity produced by blood-vessels Section of uninjected human liver: a, cords of liver-cells lying between the blood-channe's (b). protoplasm. The variations in the apparent granularity of the cells depend, as in other glands, upon the condition of functional activity: THE DIGESTIVE TRACT. the nearer complete exhaustion, the more emphasized are the granules. The meshes of the capillary net-work are usually only suffi- ciently wide to accommodate a few liver-cells, in consequence of which arrangement almost every hepatic ele- ment is bounded directly on at least one side by a capillary blood-vessel, a relation con- ducive to free interchange between the blood and protoplasm of the cells. With few excep- tions every liver-cell exhibits a slight con- cavity on one border, which denotes the position of contact and impression by the blood-vessels. In uninjected organs the hepatic tissue ap- pears made up of irregular, branching, and anastomosing cords of cells, which form close net-works, the intervening dear clefts being the lumina of the blood-capillaries. According to Disse’s studies, the liver-cells do not lie immediately in contact with the capillaries, but are separated from the latter by delicate perivascu- lar lymphatic channels which envelop the blood-capillaries. In livers still retaining their primitive type of the tubular gland the bile-capillaries appear as minute ducts placed centrally within the cords of the hepatic cells, thd biliary passages representing lumina of tubular acini lined with secreting glandular epithe- lium. In man, however, the liver-cells are usually bordered on all sides, ex- cept on that lying next the blood- vessels, by the delicate bile-canaliculi, the latter never interposing between the cells and the blood-channels. The bile-capillaries exist as narrow (i—2 i-L) clefts between adjacent liver- cells, maintaining about the same diameter throughout the lobule ; at the periphery the intercellular channels pass into the larger, though still small, interlobular bile- ducts. The hepatic cells between which the bile-capillary takes its course become replaced at the periphery of the lobule by the low epithelium of the bile-duct, the basement-membrane present in the latter fading away into the delicate connective tissue holding together the cords of liver-cells. 179 Fig. 222. Section of centre of lobule of human liver: a, intralobular vein, into which the capillaries (b) converge; c, hepatic tissue. Fig. 223. Section of liver of frog, exhibiting tubu- lar character of gland : a, blood-channels containing corpuscles; b, lumina ofhepatic cylinders which correspond to bile-capil- laries ; c, pigment-cell. 180 NORMAL HISTOLOGY. The existence of a distinct independent wall to the bile-capillaries has been the subject of much conflicting testimony; according to some, these vessels are with- out distinct walls of their own, while other authorities regard the existence of a deli- cate special wall consisting of a homogeneous structureless membrane as established. The presence of a distinct membranous wall seems ques- tionable ; when it is recalled that the bile-capillaries really represent lumina of modi- fied tubular glands, there seems to be no greater neces- sity for or probability of the existence of a membrane to limit the lumen of the bile- tubule than in the case of other glands. The direct transforma- tion of the secreting hepatic cells into the epithelium of the bile- duct at the margin of the lobule further opposes the assumption of such limiting membrane, while examination of livers in which the tubu- lar type of the acini is re- tained fails to show such structures within the lumina of the tubes. Emerging from the lobule at the periphery to pass into the adjacent interlobular con- nective tissue, the small bile- ducts empty into the larger ones, which bear the branches of the hepatic blood-vessels company. The interlobular bile-vessels gradually in- crease in size, owing to the repeated union of the smaller tubes, until the larger trunks unite to form the hepatic duct. While the walls of the smaller bile-ducts consist of columnar epithelium strengthened by fibrous con- nective tissue mixed with elastic fibres, those of the large vessels Fig. 224. Section of rabbit’s liver in which the bile-capillaries (b) have been injected and appear as dark lines between the cells : c, blood-channels. Fig. 225. Section of liver of dog, including portion of lobule and interlobular connective tissue (a); b, portal vein; c, hepatic artery; d, bile-ducts ; e, small peripheral bile- vessel ; g, blood-channels ; h, hepatic tissue. THE DIGESTIVE TRACT. 181 comprise an outer fibrous adventitia and an inner mucous membrane. The latter, in addition to the columnar epithelium, consists of the tunica propria, containing many elastic fibres and some delicate bundles of involuntary muscle, irregu- larly disposed as circular and longitudi- nal bundles. Small mucous glands also occur within the mucosa of the larger canals and of the hepatic duct. The inter- lobular bile-ducts may be distinguished from blood-vessels of the same size by their lining of columnar epithelium. The blood-vessels of the liver, as already described, are of primary impor- tance in determining the arrangement of the hepatic tissue. The blood brought by the interlobular branches of the portal vein passes into the lobule at the periphery by the numerous twigs these, on entering the lobule, form a closely anastomosing intra- lobular capillary net-work, which converges to a central intra- lobular vein. The central vessel is vertically placed with regard to the general plane of the capillary net-work, and empties into the ad- jacent sublobular veins, which are branches of the hepatic vein, lying within planes generally at right angles to those of the portal vessels. ' The hepatic artery has directly nothing to do with the elabora- tion of the especial products of the organ, its particular province being to supply the blood for the nutrition of Glisson’s capsule and of the interlobular structures, including the blood-vessels and the bile-ducts. Minute arterial twigs are distributed to the walls of these tubes, where they end in delicate capillary net-works, which, in turn, at the periphery of the lobule, pour their contents into the intralobular net-work of the portal vein. The lymphatics of the liver constitute a superficial and a deep system. The superficial lymphatics accompany the branches of the arteries supplying the capsule, and form a close-meshed sub- serous net-work within the capsule. The interlobular blood-vessels are accompanied by numerous lym- phatics, whose ramifications and anastomoses constitute the deeper plexus. The presence of lymphatics within the parenchyma of the liver is still a matter of dispute. According to Disse, the lym- phatic channels exist throughout the lobule as perivascular canals, surrounding the capillaries and separating them from direct contact with the secreting cells. Fig. 226. Transverse section of large bile-duct from human liver : a, epithelial lining; b, fibro-muscular coat; c, surrounding areolar tissue. 182 NORMAL HISTOLOGY. The main nerve-trunks of the liver enter at the transverse fissure in company with the blood-vessels and the lymphatics. The fibres consist largely of the non-medullated, together with a smaller number of the medullated variety. These nerves run within the interlobular connective tissue in company with the hepatic artery. They may be traced with certainty to the periphery of the lobule; regarding the exact mode of their ultimate distribution, however, nothing is definitely known. Minute ganglia occur along the interlobular trunks. The gall-bladder, or bile-sac, possesses walls composed essen- tially of the same tissues as those of the larger bile-ducts, these consist- ing of a mucous membrane supplemented by oblique bands of invol- untary muscle and an outer fibrous coat. The mucosa is thrown into minute folds or rugae, which unite and interlace to form a net- work of ridges and give to the surface of the mucous membrane a reticulated appearance. The blood-vessels, the lymphatics, and the nerves form net-works within the mucosa, which usually terminate in the superficial or inner layers of the tunica propria. THE ACCESSORY DIGESTIVE GLANDS. These include the salivary glands—the parotid, the submaxillary, and the sublingual—and the pancreas. While in their quiescent, immature condition all are similar, after full functional development is attained the variation in the character of their secretions leads to the recognition of two groups—the serous and the mucous sali- vary glands. Those of the serous type, regarded as the true sali- vary glands, are represented in man and mammals by the parotid gland and the pancreas. The mucous glands are best represented in man and many animals by the sublingual, although the presence of serous acini places this organ, strictly considered, within the cate- gory of the mixed glands. The muco-serous or mixed glands are exemplified by the sub- maxillary of man and many mammals (as apes, guinea-pig, etc.); in other animals (as dog or cat) this gland is entirely mucous, while in certain others (as the rabbit) it is a true serous gland. The parotid is a compound saccular or racemose gland, en- veloped in a general fibrous capsule from which stout connective- tissue septa penetrate the organ, dividing the gland into lobes. These latter are subdivided by fibrous partitions into numerous lob- ules, each of which, in turn, is composed of groups of the ultimate saccules or acini. THE SALIVARY GLANDS. The large excretory duct of the parotid gland, or Stenson’s duct, contained within the interlobular connective tissue, is composed of a fibro-elastic tunica propria, lined by a simple low columnar epithe- lium, and strengthened externally by fibrous tissue. Passing into the smaller ducts, the salivary tubes, the cylindrical epithelium becomes slightly taller, and exhibits a distinct vertical radial striation in its outer zone. On entering the intralobular divisions of the ducts, or interme- diate tubes, the columnar epithelium is replaced by low flattened cells, which finally pass into the dilated terminal compartments, be- coming directly continuous with the secreting cells lining the acini. The acini are limited by the basement-membrane, the prolonga- tion of that of the smaller ducts, and almost completely filled by the irregularly polyhedral glandular epithelium, the narrow intercellular cleft which remains representing the commence- ment of the lumen of the system of ducts. The appearance of the cells of the acini varies with the stages of secretion; when quies- cent and filled with the serous secretion, the cells appear larger, clearer, and less granular, while after functional activity and in the ex- hausted condition they are smaller, darker, and more granular, the granules of the protoplasm lying closely packed, and not, as when the gland is at rest, sepa- rated by the intervening particles of stored-up secretion. The sublingual gland possesses the general arrangement already considered in connection with the parotid gland, its peculiarity being the absence of the intermediate division of the duct, the intralobular or ‘ ‘ mucous’ ’ tubes passing at once into the acini. The cells lining the saccules are encountered in all stages of secre- tion. During rest the majority are clear, being filled with homo- geneous viscid mucus. After the discharge of this secretion, fol- THE DIGESTIVE TRACT. i«3 Fig. 227. Svction of human parotid gland, exhibiting general ar- rangement of lobules (a) ; b, interlobular connective tissue containing large ducts (c) and blood-vessels (v) ; d, intra- lobular ducts. Fig. 228. Section of human parotid gland, including several acini: d, cut intralobular duct. jg4 .normal histology. lowing prolonged activity, the cells appear smaller, dark and granular, and closely resemble the elements of the serous glands, since the mucoid substance separating the particles of the cell pro- toplasm has been removed, thereby allowing the displaced proto- plasmic granules once more to approach closely. Not all the cells in the resting acini are in the same secretory condition, since quite usually certain cells, have failed to participate in the activity of their neighbors, and, in consequence, appear as crescentic groups of granular cells lying immediately next the basement-membrane at the periphery of the acinus, where they have been crowded by the larger mucus-filled elements. These cres- centic groups constitute the demilunes of Heidenhain or the crescents of Gia- nuzzi, and are aggregations of cells which have not participated in secretion. The ex- cretory tube of the sublingual gland, or the duct of Bartholin, consists principally of a fibro-elastic tunica propria, within which is a single layer of low columnar cells, while out- side extends a supplementary layer of fibrous tissue. ■ The submaxillary gland is a mixed gland, certain lobules being composed of acini of the serous type, while neighboring divisions contain those of the mucous variety. The excretory channel, or the duct of Wharton, resembles that of the parotid gland, dividing into the smaller tubes lined by striated V rod” epithelium, passing thence into the intermediate tubules, with low cuboidal cells, which lead into the serous acini filled with dark granular cells on the one hand, or into those filled with mucous cells and granular crescents on the other. The vascular supply of the salivary glands is very rich ; while the arrangement of the blood-vessels in the several glands presents unimportant differences, their distribution is according to the same general plan. The larger arteries accompany the excretory ducts of the glands within the interlobular fibrous septa, where they give off branches which pass between the lobules and later penetrate the tissue of the lobules to end in rich capillary net-works enclosing the acini. The capillaries lie immediately outside the basement-membrane in prox- imity to the secreting cells. The veins follow the general course taken by the arteries. The lymphatics are represented by indefinite interfascicular clefts between the acini, which are taken up by definite lymph-vessels Fig. 229. Section of human sublingual gland: among the clear cells lining the mucous acini are nests (g, g) of granular ele- ments which constitute the demilunes of Heidenhain. THE DIGESTIVE TRACT. 185 situated within the interlobular connective tissue, the larger trunks accompanying the main blood-vessels. The nerves distributed to the salivary glands constitute a rich supply, composed of both medullated and pale fibres. From the larger trunks of the interlobular net-works, along the course of which minute ganglia occur, smaller branches enter the lobules and extend between the acini. Regarding the ultimate distribution of the many fibres passing to the glandular tissue little is definitely known, not- withstanding the laborious investigations undertaken with a view to solve this difficult problem. The nerve-fibres may be traced to the basement-membrane of the acini, around which net-works are formed ; as to the further fate of the fibrillse, however, little can be regarded as proved. While an intimate relation between the nerves and the secreting cells may be assumed as undoubtedly existing, no direct continuity between these structures has been established, not- withstanding the already-published assertions and elaborate descrip- tions of such connections. THE PANCREAS. The pancreas is, as aptly described by its German name, “ Bauch- speicheldriise,” the abdominal salivary gland, belonging to the serous type, and closely corresponding in structure and in the nature of its secre- tion to the parotid gland. The connective-tissue framework of the organ divides the glandular tissue into lobes, which are subdivided by septa into the lobules, these, in turn, being com- posed of groups of acini. The laminated fibrous connective tissue constituting the walls of the pancreatic duct is clothed by a single layer of columnar epithelium. The branches of the main duct divide at once into the long intermediate tubules, the intralobular ducts, or salivary tubes, being wanting ; it follows that the vertical striation of the epithelium lining these tubes, so conspicuous in sections of the parotid gland, is absent in the pancreas. The cylindrical cells of the larger ducts gradually pass into the lower cuboidal and flattened plates lining the intermediate tubules. The acini of the pancreas are more tubular than those of the parotid gland, while the secreting cells suggest more strongly the cylin- drical or pyramidal type than those of the salivary gland ; these cells are further characterized by the presence of a zone, next the Fig. 230. Section of human pancreas, in- cluding several acini and two ducts : the cells present a central granular and a peripheral clear zone. 186 NORMAL HISTOLOGY. lumen of the acinus, containing numbers of highly refracting par- ticles, while the peripheral outer half of the cells contains the nucleus and is comparatively free from the granules. The relations, how- ever, between the clear and granular zone of the pancreatic cells are not constant, but vary with the condition of functional activity. During the earliest stages of digestion, when the cells are filled with secretion, the clear zone occupies almost the entire cell, the granules being confined to a narrow belt immediately around the lumen; towards the close of a period of functional activity, on the contrary, the granules occupy the greater part of the cell, while the clear zone is reduced to a narrow peripheral area ; during fasting the clear and the granular zone about equally divide the cells. On examining sections of pancreas under low amplification, certain round or oval areas appear lighter and less dense than the ordinary tissue of the organ. These peculiar areas, or bodies of Langerhans, under high magnification prove to be composed of groups of small, imperfectly-developed acini, among and about which ramify rich capil- lary net-works, whose frequently tortuous course and lobulated ar- rangement recall somewhat the glomeruli of the kidney. These areas probably represent groups of imperfectly-developed acini; they are well seen in the pancreas of man and most mammals. The blood-vessels of the pan- creas are distributed very similarly to those of the salivary glands. The larger arterial branches run within the interlobular connective tissue, sending off vessels which pass between the lobules and supply the glandular parenchyma with twigs. These latter enter the lobules and form net-works which en- close the individual acini within the capillary reticulum. The capil- laries lie beneath the basement-membrane in close relation with the glandular epithelium. The veins accompany the arterial trunks within the connective tissue. The lymphatic vessels also accompany the arteries, lying be- tween the lobules and receiving as tributaries the lymph-radicles originating within the lobule between the acini. The larger nerve- trunks are confined to the connective tissue between the divisions of the gland, in which situation many accompanying microscopic ganglia also are found. The ultimate termination of the nerve-fibres, Fig. 231. Section of human pancreas, exhibiting one of the areas (a) of immature gland-cells; b, the usual acini. THE DIGESTIVE TRACT. 187 as in the case of the salivary glands, is still undetermined; the fibrillae are traceable to the basement-membrane of the acini, but their further accurate disposition remains undecided. The development of the digestive tract and its appendages in- volves all three blastodermic layers, the mesoderm and the ento- derm, however, being the ones participating to the greatest extent. The epithelium of the mucous membrane, together with that of the glandular structures connected therewith, is the direct derivative of the entoderm, with the exception of that lining the oral cavity ante- rior to the fauces and the salivary and oral glands, the epithelium of which parts originates from the ectodermic invagination. For a short distance within the anus, likewise, the ectoderm contributes the cells lining the gut. As already pointed out, the enamel and the dentine are also products respectively of the ectoderm and of the mesoderm. The formation of the gut-tract consists essentially of a process of folding off and closing together of the ventral body- plates, which are composed of the entoderm united with the visceral layer of the meso- derm. The tube thus formed begins in the cephalic region of the embryo as a blind, somewhat dilated pouch, the primitive pharynx, which for a short time is separated from the primary oral recess, or stomodseum, by a parti- tion, the pharyngeal plate, consisting of the opposed ectoderm and entoderm ; after the rupture of this plate the gut-tract communicates directly with the exterior through the oral cavity. A somewhat similar process takes place at the lower part of the primitive digestive tube, whereby the anus becomes established. For a con- siderable time the gut communicates with the cavity of the umbilical vesicle through its duct. The several divisions of the primary diges- tive tube, its wall consisting of epithelial lining and supplementary mesodermic tissue, undergo differentiation and acquire distinctive characters, which, however, depend largely upon the differentiation of the embryonal epithelial layer. The division of the tube into particular regions begins with the stomach, which as early as the fourth week in the human embryo is distinguishable as a spindle-shaped enlargement. With the sub- sequent rapid increase in the size of the organ, the tissues constituting Fig. 232. Transverse section of nine-day rabbit embryo, show- ing formation of primitive gut (g) by approximation of ventral plates composed of visceral layers of mesoderm and entoderm (e); m, m, body-cavity bounded by parietal and visceral sheets of mesoderm; n, neural canal. 188 NORMAL HISTOLOGY. its walls also become augmented by many new elements, the meso- dermic cells differentiating into a narrow looser zone next the ento- derm, which later becomes the submu- cosa, and a broader, more compact stratum, representing the future mus- cular tunic. The entodermic cells, at first arranged as a single layer, soon undergo local proliferation, the resulting groups of cells disposing themselves as minute cylindrical masses, which are the earliest traces of the peptic glands. These increase in length and later en- croach upon the underlying mesoderm. In the young gland six to eight tubular divisions communicate with a single duct, but as development advances the ducts divide, with a corresponding dimi- nution in the number of terminal com- partments connected with each. The pyloric glands appear about the same time as do the peptic, or at about the tenth week of foetal life, the cells ac- quiring their characteristic form and appearance during the later stages. At first and during a considerable period the cells lining the peptic glands are all of the same variety ; later certain elements become distinguished by the accumulation of coarse granules within their protoplasm ; these constitute the acid or parietal cells, usually appearing towards the close of the fourth month of foetal life. The intestinal divisions of the primitive gut also depend for their distinctive characters on the differentiation of the entodermic epithelium and of the adjoining mesoderm, which together constitute the mucosa. The villi, distinguishable by the tenth week, are at first relatively short and thick and less numerous than later, when additional projections are developed. It is of interest to note that in the early stages villi appear in both the large and the small intes- tine, these structures subsequently atrophying and disappearing in the large gut while they increase in size and importance in the re- maining parts of the tube. Coincidently with the formation of the villi the entodermic epithelium sends outgrowths into the mesoderm between the villous projections ; these, at first solid, cylinders repre- sent the early stages of the simple tubular glands ; with the gen- Fig. 233. Sagittal section of nine-day rabbit embryo : B, B', neural canal and brain vesicles; m, ectodermic invagination which contributes the lining of anterior part of future oral cavity ; p, primitive pharynx, the blind upper end of the primitive gut (g) lined with entoderm, in this stage separated from ectoderm by septum ; U, umbilical duct connect- ing gut with umbilical vesicle ; h, h', arterial and venous segments of young heart ; delicate endothelial tube seen lying within primitive muscular walls. THE DIGESTIVE TRACT. 189 eral increase in the thickness of the young mucosa these structures lengthen and obtain their lumen. The lower ends of the glands throughout the period of their growth are the seats of active cell proliferation and the points at which the division of their fundi com- mences in the production of the compound tubules. The endothe- lium covering the serous surfaces of the intestinal tract is the direct descendant of the differentiated mesoderm, the mesothelium, lining the body-cavity. The development of the accessory glands of the digestive tube, including the liver, the pancreas, and the salivary glands, follows the same general plan. The epithelial covering of the primitive mucous membrane sends cylindrical masses of entodermic or ectodermic elements, as the case may be, into the surrounding mesoderm ; the originally single cord of cells very soon undergoes division, a richly-branched system of epithelial tubes early represent- ing the future gland. The liver originates as a ventral outgrowth of the intestinal epithelium into the septum transversum ; very soon this branches, the two hepatic diverticula following so closely upon the stage of the single outgrowth that the latter is sometimes overlooked. The walls of the distal ends of the diverticula soon become greatly thickened, which areas of entodermic epithelium represent the earliest traces of the hepatic tissue. Regarding the details of the further stages in the growth of the more complicated livers opinions do not agree; it is probable, however, that the hepatic cords of the mammalian organ are attributable to the same general plan of development as are other tubular glands, the com- plicated arrangement of the secreting tissue resulting from incomplete separation and subsequent fusion of the cell-cords. The invasion of the epithelial areas by the blood-vessels breaks up the entodermic tissue into the cell-nests which occupy the intercapillary spaces. Two forms of liver-cells are present during the greater part of foetal life, large polyhedral elements, and small round cells, the latter disappearing shortly after birth ; the relation between the two varieties is not clearly established, but the small cells are probably younger stages of the larger. Multinucleated cells of considerable size also occur within the blood-vessels of the embryonal liver; these are regarded as connected with the production of red blood-corpuscles before birth. The lining of the bile-vessels and of the interlobular bile-ducts, together with the hepatic cells, is a derivative from the entoderm, while the connective tissue and blood-vessels, as well as the tissues of the walls of the bile-vessels other than the epithelial lining, are contributions from the mesoderm. The pancreas appears shortly after the liver as a dorsal diver- ticulum, which extends from the gut into the primitive omentum, !QO NORMAL HISTOLOGY. or mesogastrium, sending out hollow buds and lateral branches. The organ first lies parallel to the sagittal axis of the body, afterwards changing its position so as to lie transversely, the former anterior ex- tremity passing to the left. In many mammals ventral diverticula appear in addition to the dorsal outgrowth : to what extent these are formed in man, and to which portions of the organ they contribute, is still uncertain. The presence of more than one pancreatic duct in cer- tain animals is explained by the persistence of the embryonal condition. The tubular acini of the organ are developed in a manner similar to that in which those of the other salivary glands are formed : the cylinders of entodermic cells send off branches, which, in turn, give rise to secondary buds, the lumen of the original diverticulum ex- tending into the terminal compartments of the gland. The ingrowth of the surrounding mesoderm establishes the division into lobules and supplies the interlobular connective tissue. THE URINARY ORGANS. 191 CHAPTER XI. THE URINARY ORGANS. THE KIDNEY. The kidney is a highly-developed compound tubular gland, com- posed of pyramidal lobules which correspond in number with the renal papillae and Malpighian pyramids: in the adult, however, their distinctness is lost, since they become blended to- gether. On laying open the fresh organ by a longitudinal section, two regions are ap- preciable, the cortex and the medulla. The cortex is readily distin- guished as the periph- eral granular zone em- bracing the outer third, while the medulla ap- pears radially striated and occupies the re- maining two-thirds of the gland. The inner surface of the medulla, next the pelvis, presents a num- ber of eminences, or papillae, at whose apices open the large terminal uriniferous tubules or excretory duqts. Each renal papilla is the cul- minating point of a sys- tem of dividing and sub- dividing tubules, which collectively form a pyramidal mass, the base of which corresponds to the surface of the organ, while its apex is the papilla. These pyramidal tracts constitute the lobules of which the kidney is com- Fig. 234. Longitudinal section of human kidney, exhibiting general relations of macroscopic details: A, renal artery; U, ureter; C, one of the calices into which a papilla projects; i, cortex containing labyrinth (/) and medullary rays (m) ; 2, medulla; M, Malpighian pyramids, some obliquely cut at 3 ; b, bound- ary layer; B, columns of Bertini ; 4, masses of adipose tissue; 5, branches of renal artery. (After Henle.) 192 NORMAL HISTOLOGY. posed. In the adult human organ all traces of such divisions have usually disappeared; during foetal life, however, the lobules are dis- tinctly seen, a condition which is permanently retained in many of the lower animals. The medulla is occupied by 8-18 striated conical Malpighian pyramids, the apices of which correspond to the papillae, while their bases occupy the line of juncture between the cortex and the medulla. Each pyramid exhibits alternating light and dark striae, these markings being respectively the uriniferous tubules and the blood-vessels. The masses of the organ ex- tending between the sides of the Malpighian pyra- mids as far as the pelvis constitute the columns of Bertini, and are trav- ersed by the large blood- vessels. At certain points along their bases the striae of the Malpighian pyramids are continued into the cortex as slender, tapering bun- dles of parallel tubules, which form the medul- lary rays, or pyramids of Ferrein. By the penetration of these bun- dles the cortex is sub- divided into the med- ullary rays and the labyrinth, the latter ap- propriately so named on account of the great tortuosity of the component uriniferous tubules. The dark-red points irregularly studded over the labyrinth indicate the position of the Malpighian bodies. In sections parallel to the free surface the medullary rays appear as groups of tubules sur- rounded by the labyrinth on all sides. The blood-vessels of the labyrinth are enveloped in connective tissue, which latter represents the interlobular tissue of other glands and the boundaries of the primary lobules. The secreting Fig. 235. Section of human kidney, including cortex and portion of medulla, showing general arrangement of tissues. Cortex (C) is imperfectly subdivided by bundles of parallel tubules constituting the medullary rays (m); between these lies the labyrinth (/) containing the Malpighian bodies (x) ; in places (jr') the glomerulus has fallen out, leaving the empty capsule; b and v, sections of blood-vessels. THE URINARY ORGANS. parenchyma of the organ is held in place by the interstitial con- nective tissue ; this is present between the tubules in most parts of the kidney in very small quantities,—the immediate vicinity of the Malpighian bodies and the papillary region of the medulla being exceptions, since considerable amounts of the interstitial tissue are present in these localities. The connective tissue of the kidney be- comes condensed at the periphery of the organ, where it forms a fibrous investment, over which, in addition, the special capsule extends. The Malpighian bodies are situated exclusively within the cortex, and are limited to the labyrinth. They consist of two parts—a spherical mass of convo- luted capillary blood-ves- sels, the glomerulus, or the Malpighian tuft, and the surrounding expanded extremity of the uriniferous tubule, the capsule of Bowman. The glomer- ulus is supplied by an afferent artery, which divides into several branches; each of these breaks up into numerous capillaries, which are united by delicate con- nective tissue into groups or lobules. The blood ■escapes from the convo- luted capillaries of the glomerulus by the effer- ent vessel, which passes out by the side of the en- tering artery. The glomerulus, as usually seen in sections, seems to lie within the capsule, the blood-vessels having apparently pierced the latter to gain entrance. The vessels, however, really are outside the cavity of the capsule, since one surface of this structure has been pushed in before the advancing tuft during its development. The masses of convoluted capillaries are closely invested by the reflected portion of the capsule, which likewise dips in between the vascular lobules of the glomerulus. The invaginated portion becomes continuous 193 Fig. 236. Fig. 236. Section of human kidney partia’ly injected : a, interlobu- lar artery giving oft afferent twig (b); c, efferent vessel passing into intertubular capillaries (d); e, convoluted capillaries of glomerulus ; ft outer layer of Bowman’s cap- sule, the nuclei of whose cells show at g; h, uriniferous tubule in transverse section, t, in oblique section. 194 NORMAL HISTOLOGY. Fig. 237. C OR TEX * LABYRINTH ' MED. R A Y LABYR. ME DUL\LA Pelvis Diagram of the kidney, showing the course of the uriniferous tubules and of the blood vessels; for convenience the medulla is represented as greatly shortened. The various divisions of the tubule— Bowman’s capsule, neck, proximal convoluted, spiral, descending and ascending limbs and loop of Henle’s loop, irregular, distal convoluted, arched collecting, straight collecting, and excretory duct— are indicated by their initial letters : a, e, and c, respectively the afferent, efferent, and capillary blood-vessels ; s, stellate vein ; v r, vasae rectae. THE URINARY ORGANS. l9S with the outer layer of the capsule at the stalk of the glomerulus, at which point the vessels and the capsule are intimately united. Each uriniferous tubule begins within the labyrinth as the dilated capsule of Bowman. A greatly constricted neck, situated at the pole of the Malpighian body opposite the position of the vascular stalk, leads into the first or proximal convoluted tubule, which is characterized by its considerable size and tortuous course. Leaving the labyrinth, to which it has thus far been confined, the tubule enters the medullary ray and passes towards the medulla as the slightly wavy spiral portion ; on reaching the medulla a marked diminution in the size of the tubule takes place, the reduced tube passing into the medulla as far as the papillary zone as the descend- ing limb of Henle’s loop, the narrowest part of the entire urinifer- ous tubule. The spiral tubule is practically the beginning of the descending limb of Henle’s loop, and takes the place of this arm in the medullary ray, into the constitution of which, strictly regarded, it does not enter. Just before reaching the loop itself the tubule becomes slightly larger, obtaining a diameter which is retained throughout the loop and the ascending limb ; on again reaching the cortex, the ascend- ing limb enters the medullary ray as its second constituent until it once more enters the labyrinth, to become, for a short distance, the conspicuous irregular tubule. The succeeding second or distal convoluted portion resembles very closely the proximal part of like name, possessing a similar size and tortuous course. The uri- niferous tubule finally leaves the labyrinth as the arched collecting tubule, to enter, for the third time, the medullary ray as the straight collecting tube. In consequence of the frequent union of canals of smaller size, the collecting tubes rapidly increase in diameter as they traverse the medulla, until, in the papillary layer, the narrow tubules have become the large excretory ducts, or tubes of Bellini, whose orifices on the free surface of the papillae are recognizable by the unaided eye. A certain number of tubules probably do not form loops of Henle, but pass directly to become the collecting canals (Rose). From the foregoing it will be seen that the Malpighian bodies—glomeruli and cap- sules ; Constricted necks of tubules ; Proximal convoluted tubules ; Irregular tubules ; Distal convoluted tubules ; Arched collecting tubules. Labyrinth contains: 196 NORMAL HISTOLOGY. Medullary ray contains : Spiral tubules ; Ascending limbs of Henle's loops ; Straight collecting tubules. Medulla contains : Descending limbs of Hcnle's loops ; The loops ; Ascending limbs of the loops ; Collecting tubules of all sizes. While the labyrinth is characterized by the irregular and tortuous course of its tubules, the medullary ray and the medulla are dis- tinguished by the longitudinal, generally parallel arrangement of their components. The wall of all parts of the tubule consists of the basement-mem- brane and the lining epithelium; the variations in the character of the latter are so numerous that it is desirable to consider each portion of the tubule in detail. 1. The capsule, the expanded and invaginated blind termination of the uriniferous tubule, is lined with a single layer of large, flattened epithelium, resembling endothelial plates. This covers, likewise, Fig. 238. Portions of the various divisions of the uriniferous tubules drawn from sections of human kidney: A, Malpighian body ; x, squamous epithelium lining the capsule and reflected over the glomerulus ; y, z, afferent and efferent vesse s of the tuft; e, nuclei of capillaries; n. constricted neck marking passage of capsule into convoluted tubule ; B, proximal convoluted tubule; C, irregular tubule ; D and F, spiral tubules ; E, ascending limb of Henle’s loop ; G, straight collecting tubule. the portion reflected over the glomerulus. In ordinary preparations the presence of the cells is indicated by the delicate spindle nuclei seen in profile; the numerous nuclei seen within the tissues of the THE URINARY ORGANS. lgy glomerulus include those of the walls of the blood-vessels and of the interstitial tissue, as well as those of the capsular epithelium. 2. At the neck the flattened epithelium abruptly becomes cuboidal and rapidly assumes the character of the lining of the convoluted tubule. The existence of ciliated epithelium at the neck or within the capsule in the mammalian kidney has been asserted, but not satis- factorily established; in many of the lower animals, however, as in the amphibians, the presence of cilia is readily demonstrated, as is like- wise the existence of tubules opening directly into the peritoneal cavity. Such trumpet-shaped orifices—the nephrostomata—represent a partial persistence of the primitive type of excretory organ, in which the tubules pass directly from the body-cavity to the outer surface. 3. The proximal convoluted tubule is clothed with low co- lumnar or cuboidal cells, whose granularity and transparency vary with the stage of secretion, as do likewise the thickness of the epi- thelium and the size of the lumen of the canal. The outer zone of Fig. 239. Fig. 240. Section of kidney of amphiuma : the peritoneal surface (b, b) exhibits one of the nephrostomata (, nucleus ambiguus ; q, root-fibres of pneumogastric nerve ; r, s, hypoglossal and vagus nuclei; t, nerve-cells of posterior funiculus ; u, posterior medullary velum closing in fourth ventricle, IV. the interior of the nucleus. The wavy zone of gray matter is composed of neuroglia, in which lie numerous small multipolar ganglion-cells. Two additional small areas of gray substance are seen in 300 NORMAL HISTOLOGY. close proximity to the corpus dentatum : these are the dorsal or outer and the mesial or inner accessory olivary nuclei, the first of which lies behind the olivary nucleus, near and parallel to its wavy band, while the second lies almost across the open end of the corpus dentatum. Attention has already been directed to the tract of large nerve- cells which lies near the median line and represents the nucleus of the hypoglossal nerve. In the lower part of the medulla, before the central canal opens out into the ventricle, a group of numerous smaller cells lies close but dorsally to the nucleus just mentioned; as the central canal approaches the surface the tissues forming its former dorsal border become gradually laterally displaced, in consequence of which this group of nerve-cells then comes to lie outside of the hypoglossal nucleus. These cells form a continuous column throughout almost the length of the medulla, constituting a common nucleus of the spinal accessory, pneumogastric, and glosso-pharyngeal nerves. The four principal tracts of the medulla are made up chiefly of the continuations of the columns of the cord; without entering into a detailed account of these structures, a brief outline of the most important of the constituents of the tracts may here find place. 1. The anterior pyramid is composed of two sets of fibres : the continuation of the direct pyramidal tract of the anterior column of the cord, which does not take part in the decussation of the pyra- mids, and the continuation of the crossed pyramidal tract of the lateral column. The proportion of the crossed to the uncrossed fibres varies greatly ; while usually from three to ten per cent, of the pyramidal fibres pass directly into the anterior columns of the spinal cord, total decussation of these fibres takes place in about eleven per cent., in such cases the anterior pyramidal tract being evidently wanting. In only about sixty per cent, is a symmetrical disposition of the two pyramidal tracts on each side observed. 2. The lateral tract claims all the fibres of the lateral column not included in the crossed pyramidal and the direct cerebellar tract, together with the external anterior or ground-bundle, since the latter really is a part of the adjacent tract of the lateral column. The antero-lateral fibres enter beneath and at the side of the anterior pyramid and pass under the olivary body and the arcuate fibres to take part in making up the formatio reticularis ; the sensory fibres derived from the posterior columns after crossing in the sensory decussation pass brainward and aid in forming the fillet. 3. The restiform body contains constituents from a number of sources ; these may be arranged in two groups,—those derived THE CENTRAL NERVOUS SYSTEM. 301 from the cord and those arising from the medulla. The first group comprises : (a) The direct cerebellar tract of the lateral column. (,b) The fibres of the postero-external (Burdach’s) column. (c) The fibres of the postero-median (Golfs) column ; the further prolongation of the posterior columns within the restiform body is chiefly by fibres from the nucleus gracilis and caudatus. Those arising within the medulla are : (d) The superficial arcuate fibres issuing from the anterior median fissure. (e) The deep arcuate fibres crossing within the raphe. (/) Fibres contributed by the olivary body. 4. The posterior pyramid is the upward prolongation of the postero-median column of the cord. On approaching the lower angle of the fourth ventricle, this column, or the funiculus gracilis, exhibits the pronounced thickening of the clavus with its contained nucleus, and then, diverging from its fellow of the opposite side, tapers into the restiform body. THE PONS. The pons, as may be inferred from the mutual relations of the sev- eral divisions of the brain which it connects, consists very largely of bundles of nerve-fibres ; in addition to these, areas of gray mat- ter, the pontine nuclei, supplement the nerve-fibres in making up its mass. On section the pons exhibits two portions, the dorsal and the ventral. The latter contains the principal commissural tracts connecting the hemispheres of the cerebellum, and constitutes a robust mass of transverse fibres ; through this the longitudinal bundles of the anterior pyramids of the medulla force their way in their course to the cerebrum. In the lower half of the pons the pyramidal fibres are collected into two closely-packed groups of bundles, one on either side of the mid-line, which are enveloped in front and behind by a layer of transverse fibres; higher up, above the middle of the pons, the pyramidal tracts become separated by the penetrating transverse bundles into a number of fasciculi. Among the transverse tracts, therefore, are recognized the ventral or super- ficial bundles, the dorsal or deep bundles, and the middle or penetrating bundles. Small multipolar cells are found widely distributed in the ventral region of the pons within the gray matter which occupies the interfibrillar interstices. The dorsal portion of the pons consists largely of structures rep- resenting the continuation of parts already encountered below, espe- cially of the formatio reticularis and of the dorsal tracts of gray substance. In addition to the gray matter scattered throughout •502 NORMAL HISTOLOGY. the reticulum, other localizations represent important nuclei of cranial nerves. The sheet of gray matter lying in the lower half of the ven- tricular floor is continued over the pons, and there gives rise to nuclei connected with the V, VI, VII, and VIII nerves. While the details of the sections must vary with each plane, the general dis- position of the structures is shown in sections passing through at about the middle of the fourth ventricle. In such sections the dorsal Fig. 335. Section through upper part of human pons : i, fourth ventricle ; 2, valve of Vieussens lined with ependyma; 7.', white matter of anterior medullary velum; 2", gray matter of lingula ; 3, descending root of trifacial nerve ; 4, substantia ferruginea; 5, posterior longitudinal bundle ; 6, formatio reticu- laris ; 7, groove indicating boundary between tegmentum and ventral part of pons; 8, superior cere- bellar peduncle; 9, mesial fillet; 9', lateral fillet; 10, transverse fibres of pons; 11, longitudinal fibres ; 12, raphe ; V, trifacial nerve. (After Testut-Stilling.) or tegmental portion of the pons bears a resemblance to the me- dulla, the gray dorsal stratum giving rise to fibres which pierce the reticulum in their course to the free surface. At a somewhat higher level, lateral groups of pigmented nerve- cells occupy the floor of the fourth ventricle ; these cells are so dark that they collectively present an area visible to the unaided eye, the substantia ferruginea ; seen through the stratum of white fibres forming the immediate floor of the ventricle, this area appears of a bluish-gray or slate-color and constitutes the locus cceru- leus. Close to this pigmented area, lying to its mesial side and near the raph6, an angular tract, known as the posterior longitudinal bundle, extends beneath the gray matter of the ventricle, just at the dorsal border of the reticular formation. This fasciculus, also prom- THE CENTRAL NERVOUS SYSTEM. inent at higher levels, is the continuation of fibres from the anterior ground-bundle of the cord. THE CRURA. The crura cerebri, or cerebral peduncles, resemble the pons in general arrangement, since they consist of a ventral portion, the crusta pedunculi, or the cerebral peduncle proper, made up Fig. 336. Section through human cerebral peduncles at point of emergence of oculo-motor nerve: C, crusta, separated from tegmentum ( Tg) by sub- stantia nigra (5) ; R, raphe dividing formatio reticularis ; F, longitudinal bundles of latter ; O, groups of nerve-cells connected with origin of oculo- motor fibres ( Om) ; Tf, cells connected with origin of trifacial nerve ; A, aqueduct of Sylvius ; CQ, anterior corpora quadrigemina. (After Krause.) exclusively of ascending and descending fibre-tracts, and of a dorsal portion, the tegmentum, which contains the prolongation of the formatio reticularis and of the dorsal stratum of the gray substance 304 NORMAL HISTOLOGY. of the medulla and the pons. On transverse section of the crura, it is seen that the tegmental halves are united, while the two peduncular pprtions are widely separated and are attached to the tegmentum alone ; the oblique line of this juncture is indicated within the section by a deeply pigmented area, the substantia nigra. The crusta is hemi-cylindrical in section, but the encroachment of the substantia nigra reduces the area devoted to the ascending and descending fibres to a narrow crescent, whose convexity corre- sponds to the external outline of the peduncle, while the concavity embraces the dark field. Since the tracts of the ascending fibres of the peduncle greatly exceed the pyramidal bundles of the pons, it is evident that many additional fibres have arisen within the peduncles. On reaching the cerebral hemispheres in their course upward, the tracts of the crusta become continuous with the fibres constituting the internal capsule. The substantia nigra, separating the crusta and the tegmentum, forms a tract of gray matter extending from the upper border of the pons forward as far as the mammillary bodies ; while it gradually diminishes in its forward course, the mesial edge of the mass becomes thickened in the vicinity of the oculo-motor groove. The area owes its exceptional color to irregular groups of deeply pigmented multipolar cells embedded within a finely granular ground-sub- stance. The tegmentum forms only part of the great nuclear tract continued through the dorsal portion of the oblongata, the pons, and the peduncle into the subthalamic region ; as in the other localities, so here, the stratum of gray matter lying beneath the floor of the neural tube and the formatio reticularis are its principal constit- uents. In addition to the gray matter distributed throughout the reticulum, groups of nerve-cells are situated along the floor of the Sylvian aqueduct; some of these are of importance as the nuclei of the bundles of the oculo-motor and the pathetic nerve. Near the middle of the formatio reticularis, on either side of the raph6, lies a conspicuous group of large pigmented nerve-cells, the tegmental or red nucleus, so called on account of its brown or reddish hue. The formatio reticularis of the tegmentum differs little from the similar structure at lower levels. In general, the fibres contained within the crusta pass to the striatum and to the cerebral cortex, while those of the tegmentum usually terminate in or about the thalamus. THE CEREBELLUM. The cerebellum consists of a peripheral or cortical layer of gray substance which encloses the various tracts of nerve-fibres composing THE CENTRAL NERVOUS SYSTEM. 305 the white matter of the medulla, together with certain additional gray nuclei embedded within the latter. On section, each leaflet of the cerebellum is seen to be made up of (1) a central core of white medullary substance, which blends into (2) the granule layer, characterized by its “rust-color,” external to which follows (3) the Fig. 337. Section of human cerebellum, slightly magnified to show general arrangement: w, white matter of medulla ; g, o, granule and molecular or outer layer, between which lies layer of Purkinje’s cells (/). outer or molecular stratum ; between the latter and the granule layer lies (4) the single row of ganglion-cells which constitutes the layer of the cells of Purkinje. The granule layer forms a zone conspicuous on account of the great number of small deeply-staining cells which it contains. It varies in thickness, being broadest at the summit of the laminae and narrowest at the bottom of the fissures. Towards the outer layer the zone is sharply defined, but it fades away on the median side into the medullary substance. The nerve-cells of the granule layer are of two kinds,—the small and the large ganglion-cells. The former are small (6-7 /1) round elements, stain deeply, but possess so little protoplasm that the greater part of the cell is formed by the nucleus. These cells, the principal elements of this layer, are arranged in irregular groups ; they are multipolar, and have, according to recent investigations, 306 NORMAL HISTOLOGY. branched protoplasmic as well as nervous or axis-cylinder processes ; while the former ramify among the cells of the granule layer, the delicate nervous processes extend into the outer, molecular layer, where they usually end by dividing into longitu- dinal T-branches which stretch horizontally parallel with the boun- daries of the zone. The processes of these cells are so delicate, as well as so masked by the surrounding elements, that their existence has been established only after the introduction of the recent methods of Golgi, the results of whose investigations have been confirmed by Ram6n y Cajal, Kolliker, and others. Other nervous elements of the granule layer are the sparingly-distributed multipolar cells, much larger than the ones just considered, which resemble in struct- ure and size the cells of Purkinje, and, like them, possess richly- branched protoplasmic processes extending within the molecular Fig. 338. Diagram representing cellular constituents of cerebellar cortex ; Golgi’s silver staining: IV, white matter ; O, G, outer and granule layers of gray matter; a, large cell of granule layer confined to gray substance ; b, b', small nerve-cells of granule layer (exaggerated for convenience), also limited to gray matter; c, cell of Purkinje. sending axis-cylinder into granule layer and richly-branched processes towards periphery ; e, similar cell seen in profile; f, small nerve-cell of outer layer, limited to gray matter; g, nerve-cell of outer layer, whose axis-cylinder process forms basket-works (d, d') around body of cells of Purkinje; at inner border of outer zone numerous horizontally ramifying branches of nerve-fibres are seen. layer ; they differ in the distribution and form of the axis-cylinder processes. The latter are directed towards the medulla, but, instead of passing into the granule layer to become continuous with nerve- fibres, the processes in question divide and subdivide into an arbor- ization of great richness. The ramifications of the two varieties of nerve-cells of the granule layer, therefore, are distributed in a manner directly opposed, the nervous processes of the small cells terminating THE CENTRAL NERVOUS SYSTEM. 307 Fig. 339. Section of human cerebellum : IV, white matter sending fibres into granule layer lG) ; 0, outer or molecular layer; P, cells of Purkinje, sending axis cylinder processes (x) into granule layer and protoplasmic processes towards periphery; n, small nerve cell of outer layer; v, blood-vessel from pia (/). 308 NORMAL HISTOLOGY. within the outer layer, while those of the larger cells divide within the granule layer ; in both cases, it will be remarked, the axis-cylinder processes terminate entirely within the gray matter, thus identi- fying their possessors as nerve-cells of the second type. In addition to the nervous elements, a few flattened cells, with feebly- developed processes, are scattered throughout the granule zone ; these are to be regarded as belonging to the supporting frame- work. The interstices between the numerous nerve-cells are partly occupied by the plexus of medullated nerve-fibres which are derived from the bundles of parallel fibres continued from the medul- lary tracts ; some of these fibres pass beyond the nuclear layer to end within the molecular zone. The cells of Purkinje form the thinnest but, at the same time, the most characteristic layer of the cerebellar cortex. These ele- ments, among the largest ganglion-cells in the body, are disposed as a single row at the junction of the nuclear and the molecular layer, and present pyriform or flask-shaped bodies, 60-70 /* in their longest diameter, placed vertically to the plane of the zone, with the larger, rounded end resting on the outer margin of the nuclear layer, while the smaller end is directed towards the periphery. Each cell pos- sesses a large nucleus (15 //.) as well as a nucleolus, and differs from other ganglionic elements in containing little or no pigment. The central pole is prolonged as the axis-cylinder process, which, after giving off collateral fibres, passes on to become the axis-cylinder of a medullated nerve. The most distinctive feature of these cells, however, is the distribution of their protoplasmic processes. A thick tapering process, usually single, but occasionally double, extends from the small end of the flask-shaped body towards the periphery ; this stem very soon divides into two, the branches run- ning horizontally, sometimes almost at right angles to the parent stalk before turning towards the surface ; the peculiarity of the rich ramification which follows is the dominating vertical direction of the larger branches. While the pictures presented by the cells of Pur- kinje in successfully-stained sections have always been among the most striking, it was not until the introduction of Golgi’s silver method that a full appreciation of the remarkable richness of these ramifications became possible. In such preparations the molecular layer is occupied to its extreme periphery by the intertwining but ununited fibrils of the branching processes. The extent and breadth of these apparent net-works, however, vary with the point of view, for the cells send out their branches especially in a direction at right angles to the long axis of the convolution or the medullary tract, while in a plane parallel to this axis the branches are limited to a narrow zone, scarcely wider than the body of the cell: it follows THE CENTRAL NERVOUS SYSTEM. 309 that in order to display Purkinje’s cells to the best advantage the tissue should be sectioned across, and not parallel with, the axis of the convolutions. These cells, further, are not placed at uniform distances throughout the row which they form, but are more numerous Fig. 340. Section of outer portion of cereb liar cortex of young dog, stained after Golgi’s silver method: P cell of Purkinje, exhibiting profuse arborization of protoplasmic processes; p, its axis cylinder process; B, B, cells of outer layer whose axis-cylinder processes form basket-works around bodies of Purkinje’s cells; C, small ganglion-cells limited to outer layer. (After Ketzius.) and more closely arranged at the summit of the convolutions, at the bottom of the fissures being more widely separated; these variations correspond with the areas of greatest and least development of the nuclear layer. The molecular or outer layer consists of a ground-substance of finely-reticulated supporting neuroglia, in which extend the elab- orate arborizations of Purkinje’s cells, together with certain nervous elements belonging to this zone. These latter are of two kinds : small multipolar cells whose branched protoplasmic pro- cesses extend towards the periphery, while the nervous process is directed centrally, but probably is confined to the molecular layer, and larger elements distinguished by the remarkable termination of their axis-cylinder or nervous processes. While the protoplasmic 310 NORMAL HISTOLOGY. processes ramify within the outer part of the molecular layer, the axis-cylinder process, after a short course, passes horizontally near the margin of the large-celled layer, and, at various intervals, sends off lateral branches which subdivide to form net-works of fibrils, the fibre-baskets, or basket-works, around the bodies of Purkinje’s cells. In addition to the nervous elements, cells belonging to the neuroglia are scattered throughout the zone. As already stated, certain of the nerve-fibres entering the granule zone continue into the outer layer ; these fibres, after penetrating for a short distance, divide into terminal branches, many of which extend horizontally, parallel to the boundaries of the zone, to end free, in close relation but without direct continuity with the nerve-cells. In addition to the peripheral cortical layer, the cerebellum possesses other masses of gray matter, the central nuclei, embedded within the medullary substance of the vermiform process and of the adjacent parts of the hemispheres. The central nuclei are two : the nucleus dentatus, situated within the hemisphere of each side, and the nuclei of the roof, within the worm ; the nucleus emboliformis and nucleus globosus, sometimes described as separate centres, are really parts of the complicated dentate nucleus. The dentate nucleus consists of a greatly plicated pouch-like sheet of gray substance, .3-5 mm. in thickness, situated within the fibre-tract of the superior peduncle of the cerebellum. The nerve- cells contained within the band are of moderate size (25-35 A4) and pigmented to a variable degree ; the loosely-packed cells possess branched processes extending outward, and an axis-cylinder process directed towards the medulla. Numerous nerve-fibres pass between the cells and connect the white core within the nucleus with the surrounding medullary substance. The nuclei of the roof consist of irregularly ovoid areas of gray substance (6-8 mm. in length) situated within the vermiform process, almost in contact along the mesial line. The masses contain large pigmented multipolar ganglion-cells (45-80 mm.) and numerous nerve-fibres, some of which are exceptionally large. The medullary or white substance of the cerebellum em- braces the numerous bundles of nerve-fibres which maintain the intricate and far-reaching communications of this division of the brain. The cerebellar fibres are arranged in three principal tracts, the cerebellar peduncles; the lower of these corre- sponds to the corpus restiforme, the middle to the pedunculi pontis, and the upper to the processus cerebelli ad corpora quadrigemina. The fibres of the white matter are disposed in thin flat bundles, THE CENTRAL NERVOUS SYSTEM. 311 which diverge from the chief stem as the primary and secondary medullary branches ; these form the ‘ ‘ arbor vitae. The blood-vessels supplying the cerebellum, principally branches of the vertebral and basilar arteries, after repeated division within the pia, send small branches vertically into the molecular layer as far as its inner boundary; a rich vascular net-work surrounds the cells of Purkinje : while capillaries are wanting within the pe- ripheral zone of the molecular layer, they are well represented in the granule layer and among the nerve-bundles of the me- dulla, where the blood-vessels run between the fibres and form elon- gated meshes which correspond to the disposition of their tracts. THE CEREBRUM. The cerebral hemispheres consist of a thin outer sheet of gray- matter, the cortex, which everywhere covers in the white matter of the medulla, accurately following the intricacies of the convoluted surface of the brain ; in addition to the cortex, large special masses of gray matter lie within the medullary substance and take part in the constitution of the nucleus caudatus, the nucleus lenticu- laris, the thalamus, the corpus subthalamicum, and the minor collections of lesser importance. The cerebral cortex forms a dark, peripheral zone, 2-4 mm. in thickness, which is best developed in the ascending frontal and the paracentral convolution, being thicker at the summit of the gyri than in the fissures ; the gray stratum appears least conspicuous in the posterior part of the occipital lobe. The arrangement of the elements of the cortex in layers is indi- cated by the stratification which the vertically-cut surface of the cortex presents even to the unaided eye ; in favorable situations three bands are distinguishable, an outer white, a middle gray, and an inner yellowish red; in certain regions, as the superior frontal, the precentral, and the occipital convolutions, the layers are increased to six by the addition of the stripes of Baillarger. These markings, however, do not accurately represent the structure of the cortex, which can be studied adequately only in successfully- stained sections cut vertically to the free surface of the convolution and parallel with the general course of the nerve-fibres. In such preparations five zones are recognizable, which, however, are not sharply defined from one another, but are often blended. 1. The first or outer layer, next the pial surface, about .25 mm. in thickness, is composed essentially of neuroglia, together with numberless delicate terminal ramifications of the protoplasmic processes of nerve-cells situated within the deeper layers, and a few tangential nerve-fibres ; the protoplasmic threads contributed 312 NORMAL HISTOLOGY. Fig. 341. by the cells are so plentiful and closely interwoven that they con- stitute no inconsiderable part of the fine ground reticulum of this layer. Immediately beneath the surface of the nervous matter, the sub-pial zone forms a narrow stratum (10-25 /f) composed al- most entirely of neuroglia, in which lie numbers of spider or Deiters’s cells. The nerve-fibres of this layer extend parallel with the free surface. 2. The second layer (.25 mm.) is characterized by the profusion of its closely-packed small triangular or pyramidal nerve-cells, the branched pro- toplasmic processes of which extend in various directions to- wards the periphery, while their axis-cylinder processes termi- nate within the gray matter, often ending in T-branches which are directed almost at right angles to the main process. 3. The third layer, the for- mation of the cornu Ammonis, is the thickest stratum of the cerebral cortex, reaching in places a breadth of 1 mm., and contains the most characteristic nervous elements of the cerebrum, the large pyramidal ganglion- cells. This layer is not sharply defined from the preceding, since the small cells of the latter are grad- ually replaced by the larger pyram- Section of human cerebral cortex stained with sodium carminate : A, outer layer, poor in nerve- cells, rich in neuroglia ; B, layer of numerous small nerve-cells ; C, layer of large pyramidal gan- glion-cells ; D, layer of irreeular numerous but small nerve-elements ; p, pial tissue ; v, v', blood- vessels. THE CENTRAL NERVOUS SYSTEM. 313 idal elements, which become more widely separated and of greater size on approaching the deeper parts of the zone; in this situation their basal diameter may reach 40-50 n. The pyramidal cells, in addition to the general outlines of their bodies, are distinguished by the arrange- ment of their processes ; the pro- toplasmic ramifications are disposed as the principal apical processes, which extend towards the periphery as far as the sub-pial zone (Retzius) and by repeated division form a rich arborization within the outer layers of the cortex, and as lateral basal processes, which pass obliquely from the base and break up into rich net- works of delicate terminal protoplas- mic threads ; in addition, numerous smaller lateral processes are given off from the sides of the cell. Not- withstanding the profusion of the fibrils resulting from the subdivision of the protoplasmic processes of these cells, it is highly probable that the fibrils terminate without uniting with one another. From the blunt, central end of the cell the axis-cyl- inder process extends into the white matter, where it becomes continuous with a nerve-fibre. These axis-cyl- inder prolongations give off recurrent collateral processes, which bend towards the periphery. The pyrami- dal body of the cell contains a large round or oval nucleus, with a dis- tinct nucleolus, embedded within a finely granular protoplasm, masses of brownish pigment almost always occupying the base of the cell. The larger pyramidal cells are surrounded by pericellular lymph- spaces, which probably communicate with the extensions of the subarachnoidean space continued with the prolongations of the pia accompanying the blood-vessels within the cerebral tissue. 4. The fourth layer embraces a closely-packed zone (.3-4 mm.) composed of small, irregular, oval or angular nerve-cells, 7- Fig. 342. Section of cerebral cortex (motor area) of child stained by Golgi’s silver method : A, layer of neuroglia-cells; B, layer of small pyramidal ganglion-cells ; C, layer of large pyramidal cells; D, layer of ir- regular smaller cells. 2!4 NORMAL HISTOLOGY. 14 /* in diameter; among the smaller elements a few larger pyramidal cells are often encountered, as well as radiating bundles of med- ullated nerve-fibres. The cells of this layer resemble those of the second, since their axis-cylinder processes are confined to the gray matter, the elements be- ing, therefore, cells of the second type. 5. The fifth layer indicates the proximity of the white matter by the large areas occu- pied by bundles of radiating nerve-fibres directly contin- uous with the medullary tracts within the interspaces between the nerve-bundles lie the small and medium - sized cells, spindle to pyramidal in form, which characterize this layer. While these cells are arranged generally parallel with the fibre- bundles, sometimes, especially at the bottom of the fissures, they are placed at right angles thereto, in the latter case as- suming a pronounced spindle type. The nerve-fibres enter- ing the gray cortex are ar- ranged in bundles, from which arise net - works variously situated and arranged. The radial bundles proceed as such through about half the entire thickness of the cortex ; beyond this level they rapidly separate into the component fibres which take their way between the ganglion - cells. The fibres given off during the course of the bundles form net- works at all depths occupying the interfascicular portions of the layers traversed ; within the deepest part of the fourth layer, how- ever, the nervous fibrillae are especially numerous, and constitute a conspicuous reticulum in preparations stained by Weigert’s Fig. 343. Section of human cerebral cortex stained by Weigert’s method, exhibiting groups of nerve-fibres; part of white matter and inner layers of gray sub- stance shown : F, white matter from which radiating bundles of nerve-fibres (n) extend into gray matter ; C, D, and E, third, fourth, and fifth zones of gray matter: cells are faintly stained. THE CENTRAL NERVOUS SYSTEM. 315 method. In the deeper parts of the broad third layer a similar well-marked net-work occurs, the interlacing fibres of which sur- round the nerve-cells of the layer. Beyond this plane nervous reticulations occupy the third and second layers and extend into the outer zone. A limited number of the nerve-fibres terminate within the outer layer as axis-cylinders which run parallel with the free surface, as do also the terminal ramifications of the axis-cylinder processes of some of the ganglion-cells. The recent investigations of Golgi and others have shown that many fibres end without demonstrable direct anatomical continuity with the nerve-cells, although a close relation between the cells and the fibres undoubtedly exists. While the arrangement just described may be regarded as typical for the greater part of the cortex, a few localities are distinguished by modifications which materially affect the histological details. These changes depend upon either an arrested development of the cortex, as in the septum lucidum, or an increased complexity of the cortical arrangement, as in the hippocampal convolution. Less conspicuous variations, affecting one or more layers, are frequently encountered; thus, the paracentral convolution contains the largest pyramidal cells, the “giant pyramids” (Betz), the entire third layer participating in the increase of size. The occipital cortex is especially differentiated by subdivisions of the third and fourth layers into eight layers (Meynert), while the gyrus cinguli has the third layer separated into an outer group of small and an inner zone of larger cells, the intervening space appearing radially striated on account of the apical processes which cross it; within the parietal lobes an additional stratum of small pyramidal cells exists between the third and fourth cortical layers. The involuted cortex of the hippocampal region, including the cornu Ammonis, or hippocampus major, and the fascia dentata, presents considerable complexity. On observing a section of this region with low amplification, it will be seen that the cornu Ammo- nis consists of a central gray zone bounded both internally and externally by a stratum of white substance; the gray zone corresponds to the cortex of other parts, and is continuous with the thickened gray mass constituting the fascia dentata above, and with the cortex of the hippocampal convolution below. The medullary substance of the latter becomes greatly reduced in its passage over the cornu Ammonis, the attenuated stratum of fibres being known as the alveus which is prolonged into the thicker fimbria. The white layer enclosing the gray zone on the mesial surface is a conspicuous thickening of the peripheral zone of the hippocampal convolution. 3t6 NORMAL HISTOLOGY. The several structures composing- the cornu Ammonis, examined from the ventricle towards the outer surface, are— 1. The alveus, an attenuated layer of medullated nerve-fibres, homologous with the medullary substance of the typical convolu- Fig. 344. Section across cornu Ammonis, fascia dentata, and fimbria : Gh, hippocampal convolution ; Fd, fascia dentata, separated from preceding by hippocampal fissure ; Fi, fimbria, composed of transversely cut longitudinal nerve-fibres ; i, 2, medulla of hippocampal convolution con- tinued over cornu Ammonis (C), as alveus, into fimbria; 3, layer of large pyramidal cells; 4, stratum radiatum ; 5, stratum lacunosum; 6, stratum moleculare; 7, lamina medullaris involuta ; m, termination of this lamina in longitudinal fibres ; «, nucleus fasciae dentatae ; g, stratum granu'osum; r, reticulated neuroglia-layer covered by thin sheet of nerve-fibres. (After Henle.) tion. The fibres, while pursuing a course generally parallel to the ventricular surface, run somewhat obliquely ; on approaching the fimbria the layer increases in thickness and the nerve-fibres assume a disposition less oblique, until, within the fimbria, their direction almost coincides with the long axis of the cornu Ammonis. THE CENTRAL NERVOUS SYSTEM. 317 2. The stratum oriens, representing the fifth layer of the cortex, and containing among the bundles of nerve-fibres numbers of spindle- form cells, whose processes extend parallel with the free surface. 3. The stratum cellularum pyramidalium, which corresponds to the deeper portions of the third cerebral layer, and is conspicuous on account of the large pyramidal ganglion-cells. The latter, moderate in size (30-40 /;.), are arranged in several closely-packed rows, and send their axis-cylinder processes into the adjoining medullary substance of the alveus, while their long apical proto- plasmic processes pass towards the periphery and give to the outer part of the third layer a vertical striation, which has received recognition as 4. The stratum radiatum. This layer consists almost entirely of the long, tapering processes of the pyramidal elements, Fig. 345. Diagram of the constituents of the cornu Ammonis, Golgi staining : H, hippocampal convolution ; C, cornu Ammonis ; F, fascia dentata; i, fusiform, 2, 3, small and irregular, 4, 5, pyramidal, and 6, small, cells of respective layers ; 7, 8, nerve-cells of fascia dentata ; al, collaterals of pyramidal cells; course of axis-cylinder processes shown by fine lines. (After Karl Schaffer.) which often show a disposition to divide into numerous branches before reaching the border of the zone. 5. The stratum lacunosum, composed principally of axis-cyl- inders, which extend parallel to the fibre-layer of the alveus, to- gether with the collateral processes from the neighboring nerve-cells. 6. The stratum moleculare, which contains sparingly distributed fusiform or pyramidal ganglionic elements, whose protoplas- 318 normal histology. mic processes extend vertically into the outer part of the zone of pyramidal cells, as well as laterally within the molecular layer; their axis-cylinder processes, on the contrary, are directed towards the peripheral nerve-fibres, among which they end. 7. The lamina medullaris involuta, constituting the outermost layer of the convolution, lying next the fascia dentata, from which it is separated by the intervening hippocampal fissure and its pial fold. This layer corresponds to the greatly thickened outer zone of the usual cortex, and is largely made up of tangential nerve-fibres which proceed from the gyrus hippocampi, together with numerous terminal fibrillae derived from the processes of ganglion-cells situated in neighboring strata. The fascia dentata must be regarded as the projecting thickened and specialized free edge of the cortical gray matter, lodged within the hippocampal fissure, which it almost fills. The divisions recognizable in this structure, from within outward, are— 1. The nucleus fasciae dentatae, which comprises an oval area containing nerve-fibres continued from the alveus and numerous ganglion-cells. The latter include three varieties of irregularly- disposed elements, the pyramidal cells proper, the representatives of the similar conspicuous constituents of the cornu Ammonis, the polymorphous cells, possessing very numerous processes, and the fusiform cells. 2. The stratum granulosum, distinguished as a conspicuous band of brilliantly staining small pyriform nervous elements, whose protoplasmic processes extend towards the periphery, while the axis-cylinder fibrils in general pass centrally. 3. The stratum moleculare, consisting of a broad reticulated zone of neuroglia, which contains numerous capillary blood-vessels, a few scattered cells, and the extensions of the processes of the nerve- cells ; it almost completely encloses the stratum granulosum, and is itself covered by 4. The stratum marginale, an extremely thin sheet of medul- lated nerve-fibres representing the outer medullary layer of the cornu Ammonis, of which it is the direct, although attenuated, con- tinuation. The fimbria receives the fibres constituting the alveus, and is com- posed entirely of bundles of medullated nerve-fibres, together with the intervening connective-tissue septa ; the thick fibre-bundles extend longitudinally, and are continued into the tracts of the posterior pillar of the fornix. The septum lucidum represents a rudimentary cortex due to the arrest of development following the isolation of this part of the wall of the cerebral vesicle by the growth of the corpus callosum. THE CENTRAL NERVOUS SYSTEM. 319 Since the so-called fifth ventricle is really a cut-off portion of the great longitudinal fissure, those surfaces directed towards the cleft correspond to the free surface of the hemispheres ; the rudimentary layer of gray matter, therefore, forms the immediate boun- daries of this space, and is homologous with the cortex of other regions, while the thin white stratum next the lateral ventricles represents the medulla. The mesially-placed gray cortex of the septum lucidum con- tains a thin superficial stratum of medullated nerve-fibres next the interseptal cleft ; following this lies a layer of gray matter con- taining many small pyramidal cells (16-18 /-*), the apical processes of which are directed towards the surface homologous with the pe- riphery of the hemispheres ; the deeper zone of the gray matter exhibits spindle-cells. The white substance of the hemisphere is represented by the thin stratum of medullated fibres interposed between the gray layer and the ependyma of the lateral ventricle. The blood-vessels supplying the cerebral cortex, after a short course within the pia almost parallel to the free surface, enter the nervous tissue vertically ; the larger arteries pierce the gray matter and enter the medulla, while those of smaller size break up within the gray cortex into capillary net-works. The law, appli- cable to all parts of the nervous system, that regions rich in large nerve-cells are plentifully supplied with blood-vessels, is illustrated by the distribution of the capillaries within the cortex, where a rich capillary net-work exists within the layer of large pyramidal cells, while the outer cortical zones, on the contrary, possess only a meagre capillary supply : the net-works within the deepest layers of the cortex are intermediate in the closeness of their meshes. The blood- vessels are surrounded by perivascular lymph-spaces, the pial tissue accompanying the vessels as a delicate sheath attached to the adventitia and enclosing a prolongation of the subarachnoidean space. The corpus striatum consists of the special masses of gray matter, the nucleus caudatus and the nucleus lenticularis, and their associated tracts of nerve-fibres. The ventricular surface of the nucleus caudatus is covered by a well-developed layer of ependyma, beneath which lies the zone of gray substance, containing nerve-cells of two kinds : large multipolar cells (25-30 f), and much more numerous smaller ganglionic elements, whose size is about half that of the former. The outer surface of the caudate nucleus, directed towards the internal capsule, is broken up by numerous bundles of fibres, which penetrate deeply into the gray mass and produce the characteristic white striae exhibited on section. 320 NORMAL HISTOLOGY. The outer division, or the putamen, of the nucleus lenticu- laris closely resembles, both in color and in structure, the caudate nucleus, with which, indeed, it anteriorly becomes continuous. The paler colpr of the inner segments, the globus pallidus, depends Fig. 346. Section across anterior end of thalamus, striatum, and insula: th, anterior end of thalamus : st t., stria terminalis ; n.c., nucleus caudatus ; n.L, outer segment of nucleus lenticularis ; l.tn., t >u.', ex- ternal and internal medullary lamina receiving fibres (x) from caudate nucleus ; c.i., internal capsule ; c.e , external capsule; cl, claustrum; co, cortex of island of Reil; co.a., anterior commissure; g, central gray matter of third ventricle, a, its commissure; c.f., section of anterior pillar of fornix; b, c, d, e, elements of subthalamic region; e', stratum zonale of thalamus ; o, portion of optic tract. (After Schwalbe-Meynert.) not only upon the presence of greater numbers of medullated fibres, but also upon the lighter tint of the yellowish pigment contained within the multipolar nerve-cells. The nerve-cells contained within the claustrum are principally fusiform elements whose long axes correspond in direction with the neighboring free surface. The optic thalamus is composed chiefly of gray matter, through which extend various tracts of nerve-fibres. The surfaces directed towards the ventricles are sharply defined, the upper or dorsal aspect being covered by a layer of medullated nerve-fibres, the stratum zonale, about .8 mm. in thickness, which fade away on the mesial surface ; the outer and ventral borders of the thalamus, on the contrary, are invaded by fibres from the respectively adjacent internal capsule and the subthalamic region. The gray matter of the thalamus is divided by tracts of fibres into a shorter median segment, the inner nucleus, and a longer THE CENTRAL NERVOUS SYSTEM. 321 external division, the outer nucleus, the fore-segment of the thala- mus containing the anterior or upper nucleus. The gray sub- stance composing these segments is traversed in many places by bundles of medullated nerve-fibres ; in the outer nucleus the nar- row fibre-bundles and the zones of gray substance alternate, fusi- form ganglion-cells (20-30 /x), arranged parallel to the course of the fibres, occupying the gray bands. In addition to the compara- tively large cells found within the anterior nucleus, thp bundle of Vicq d’Azyr, reflected from the mammillary body below, enters the anterior ventral border of the segment and contributes numerous fibres to its mass. The multipolar cells of the anterior nucleus, as well as those of the posteriorly situated pulvinar, are of large size. The central gray matter of the third ventricle is the direct continuation of that lining the Sylvian aqueduct and other parts of the neural tube. The middle or gray commissure of the ventricle contains transverse fibres, in addition to numerous pigmented ganglion-cells; posteriorly it is intimately blended with the gray substance of the thalamus, while anteriorly it is sepa- rated from the latter by a medullary layer, the inferior stalk of the thalamus. The corpus subthalamicum, situated within the region of simi- lar name, is composed of a very close net-work of fine medul- lated fibres, among which are distributed moderate-sized multi- polar nerve-cells ; the capillary net-work of this nucleus is remarkable for the closeness of its meshes. The continuation of the area of pigmented cells forming the substantia nigra within the cerebral crus separates the subthalamic region from the fibre-tracts of the crusta. The corpora quadrigemina, the homologues of the optic lobes and corpora bigemina of the lower animals, comprise a posterior and an anterior pair of eminences. The posterior quadrigeminal bodies consist in great part of gray substance which forms a lenticular nucleus on either side, and contains numerous small multipolar cells (16-18 /x), as well as a few elements of larger size ; the nuclei of the two sides are united by a gray commissure. A thin superficial lamina of medullated nerve- fibres, the stratum zonale, covers in the gray matter. The anterior quadrigeminal bodies differ from the posterior in the complexity of their structure brought about by the presence of the root-tracts of the optic nerve. In transverse section these bodies present four layers, which, from the upper or dorsal surface towards the Sylvian canal, are— 1. The stratum zonale, enveloping the superficial portions of 322 NORMAL HISTOLOGY. the bodies ; in man and apes this layer is unusually well developed, reaching a thickness of 30-40 p; the interlacing fibres, derived from the optic tract, form a lamina rather than distinctly-grouped bundles. 2. The stratum cinereum, a cap-like mass of gray matter, em- bracing the subjacent optic fibres, and containing numerous nerve- cells of varying size, the larger ones occupying the deeper parts of the layer. 3. The stratum opticum, consisting of the continuation of the preceding gray matter, through which extend the bundles of optic nerve-fibres entering by the superior brachium ; posteriorly the fibres are fine, while anteriorly they become robust to take part in the Fig. 347. Section across superior corpora quadrigemina and adjacent part of thalamus : 1, Sylvian aqueduct; gr, gray matter of aqueduct; c.q.s., quadrigeminal body, con- sisting of, l, stratum lemnisci, o, stratum opticum, c, stratum cinereum; Th, thala- mus (its pulvinar) ; c.g.i., c.g.e., internal and external geniculate bodies ; br.s, br.i., superior and inferior brachia ; f, upper fillet; posterior longitudinal bundle ; r, raphe ; III, third nerve ; «.///, its nucleus ; l.p.p., posterior perforated space; s.n., substantia nigra, above this tegmentum with circular nucleus ; cr, crusta ; //, optic tract; M, medulla of hemisphere; n.c., nucleus caudatus; st, stria terminalis. (After Quain-Meynert.) constitution of the optic tract. Among the profusely distributed nerve-cells are elements of considerable size whose axis-cylinder processes extend largely within the underlying zone. 4. The stratum lemnisci, including gray matter as well as nerve- fibres, many of the latter being continuations of tracts connecting lower levels. The geniculate bodies, lateral and mesial, are closely associated THE CENTRAL NERVOUS SYSTEM. 323 respectively with the anterior and the posterior corpora quadrigemina by means of the corresponding brachia. The lateral or external geniculate bodies exhibit a character- istic structure, consisting of alternate layers of white and gray matter ; the white striae are composed largely of fibres derived from the optic tracts, the gray zones probably also receiving collateral con- nections from the retina. The nerve-cells of the gray matter, many of which are large and deeply pigmented, send axis-cylinder processes as far as, possibly, the occipital cortex. The mesial or inner geniculate bodies contain numerous nerve-cells of medium size, together with fibres seemingly connected with the mesial root of the optic tract; an intimate relation between these bodies and the optic fibres, however, is questionable. The masses of gray matter forming the structures along the fore part and the floor of the third ventricle, including the lamina cinerea, tuber cinereum, infundibulum, and posterior per- forated space, contain scattered ganglion-cells, together with certain special bundles of nerve-fibres. The corpora mam- millaria are composed of bundles of fibres, and contain gray nuclei within the superficial layer of white matter. The hollow coni- cal infundibulum bears at its lower extremity the pituitary body, with part of which the funnel-shaped extension of the cerebral vesicle is at one time continuous. The structures described in human anatomy as the olfactory nerves represent the rudimentary olfactory lobes, which in many of the lower animals constitute conspicuous divisions of the brain. This lobe, as found in man, comprises three parts,—the tuber olfac- torium, the tractus olfactorius, and the bulbus olfactorius. The tuber olfactorium does not here call for attention, since its adequate consideration lies without the purpose of these pages. The tractus olfactorius, on transverse section, exhibits zones of white and gray matter, together with a central flattened area of neuroglia, indicating the position of the obliterated lumen, which in the embryonic condition temporarily, and in the lower animals permanently, existed as a continuation of the ventricular cavity. The gray substance is richest in the dorsal part of the tract, where it forms an oval area surrounded by medullated nerve-fibres ; these latter become continuous at the lateral angles with the thick medullary fibre-layer occupying the ventral zone, the juncture between the two being marked by a thickening of the medullary THE OLFACTORY LOBE. 324 NORMAL HISTOLOGY. stratum. Enclosed within the continuous ring of fibres lies the flat- tened gelatinous or neuroglia zone corresponding to the area of the obliterated former lumen of the tract. Outside the fibre-layer, a sheet of gray substance, extremely thin on the ventral surface, represents the cortex of the convolution. The Bulbus Olfac- torius. While the layers present in the tract are continued into the terminal olfactory bulb, the greater devel- opment of the ventral zone considerably mod- ifies the structure of this division of the olfactory lobe. In the bulb the area of the obliter- ated ventricular cav- ity lies eccentrically, closely approaching the dorsal surface, from which it is separated by a thin layer of fibres and the greatly attenu- ated gray cortical stratum, here reduced to a delicate lamina. The ventral layers, on the contrary, are greatly developed, and culminate in the nerve-fibres which pass through the cribriform plate as the true olfactory nerves. In transverse section, the dorsal portion of the olfactory bulb is occupied by (i) the central neuroglia, the atrophic field representing the obliterated lumen of the lobe ; this area is enclosed by (2) the flattened ring of medullary substance, consisting of closely- placed bundles of longitudinal nerve-fibres. Next the ventral portion of the medullary ring lies (3) the stratum granulosum, a thick zone of gray matter containing numerous ganglion-cells of different size. Some of these are small irregular elements, and immediately beneath the ring constitute a dense aggre- gation. The most conspicuous elements of the gray matter, how- ever, are the large pyramidal or mitral cells (30-50 u) which occupy the deeper parts of this stratum. Fig. 348. Section of portion of human olfactory bulb : i, 3, bundles of transversely-cut nerve-fibres, enclosing central neuroglia (2); 4, 5, 6, granule layer; 7, zone of olfactory glomeruli (g); 8, layer of olfactory nerve-fibres, bundles (0) of which pass from olfactory mucous membrane. (After Henle.) THE CENTRAL NERVOUS SYSTEM. 325 The protoplasmic processes of the mitral cells extend ven- trally and terminate in two ways, as shown by the investiga- tions of Golgi, Ramon y Cajal, Retzius, and others. In addition to the apical fibrils, lateral processes diverge and terminate in free arborizations within the ventral por- tion of the gray mat- ter. The apical processes take part in the formation of (4) the olfactory glomeruli, peculiar masses (.05-3 mm.) composed of dense interlacements of terminal ramifications of the apical pro- cesses sent out by the mitral cells and of the olfactory nerve- filaments. The axis-cylinder pro- cesses of the mitral cells pass dorsally and become continuous with nerve-fibres of the medullary ring ; during their course they give off recur- rent collateral branches. The layer of olfac- tory glomeruli is followed by (5) the stratum of olfac- tory nerve - fibres. These are non-med- ullated, and arise in the olfactory cells of the Schneiderian membrane, whence they pass into the cranium and end in arborizations included within the olfactory glomeruli, to whose formation they thus con- tribute. The exterior of the olfactory bulb is invested by a layer of pia broken by the passage of the nerve-fibres and the blood- Fig. 349. Sagittal section of part of olfactory bulb of young rabbit, stained after Golgi’s silver method : m, mitral ganglion-cells from which pass axis cylinder processes (a), sending off recurrent collateral branches (r) and protoplasmic processes (J>); h, horizontal processes extending tangentially ; g, glomeruli from whose com- plex of nerve-fibrils pass olfactory nerves (o); n, filaments as- cending from mucous membrane. (After Retzius.) 326 NORMAL HISTOLOGY. THE WHITE MATTER OF THE CEREBRUM. The parts of the cerebrum thus far considered have included areas composed chiefly of gray substance; it remains to notice briefly the complex mass of nerve-fibres forming the conspicuous medulla. It has already been stated that the nerve-fibres constituting these central masses are mostly medullated, but devoid of a neuri- lemma ; the fibres vary in diameter, those pursuing an extended course connected with the large motor cells possessing, in general, a greater diameter than those related with sensory areas. The accurate determination of the arrangement of the various nerve-tracts included within the medulla is a labor of great difficulty and one still far from satisfactory completion ; notwithstanding the great advances made in this field of investigation since the introduc- Fig. 350. Diagram of association fibres of cerebrum: j, short fibres connecting adjacent gyri; f.l.s., superior longitudinal, f.l.i., inferior longitudinal, f.u , uncinate, and f.p., perpendicular fasciculus ; ci, cingulum ; fo, fornix ; fi, fimbria ; v.d’A ., bundle of Vicq d’Azyr. (After Schaefer-Meynert.) tion of the improved methods (Weigert’s) for tracing medullated fibres, much remains to be learned regarding the course and the distribution of many tracts connecting the central nuclei with the cerebral cortex. The great mass of the cerebral medulla is composed of fibre- tracts, which may be grouped into three systems: 1. The association fibres, connecting parts of the same hemi- sphere. 2. The commissural fibres, uniting parts of the two hemispheres, THE CENTRAL NERVOUS SYSTEM. and represented by the fibres of the corpus callosum and of the anterior commissure. 3. The projection fibres, streaming- from the entrance of the brain-stalks, or cerebral peduncles, secondarily also from the basal nuclei, to spread out in the various parts of the cerebral cortex and thus constitute the conspicuous corona radiata. The association fibres consist of bundles of various length, which unite : (a) adjoining convolutions, passing from the medulla of one, beneath the intervening fissure, into the white matter of the neighboring gyrus; ib) adjacent convolutions, but not immedi- ately adjoining; (c) more distant parts of the hemisphere. The most important of these longer tracts are : 1. The fasciculus uncinatus, connecting the inferior frontal convolution with the uncinate gyrus of the temporal lobe. 2. The fasciculus longitudinalis inferior, connecting the an- terior part of the temporal with the apex of the occipital lobe. 3. The fasciculus longitudinalis superior, connecting the middle of the frontal partly with the occipital and partly with the apex of the temporal lobe. 4. The cingulum, extending along the corpus callosum within the cingulate convolution. 5. The fasciculus perpendicularis, connecting the inferior parietal with the fusiform lobe. 6. The fornix, connecting the uncinate process of the hippo- campal convolution with the thalamus by means of the continuations effected by the fimbria behind and the bundle of Vicq d’Azyr, from the mammillary body to the thalamus, in front. The majority of the commissural fibres, which connect similar regions on the two sides, take part in the formation of the great transverse bridge, the corpus callosum; these fibres, the pro- longations of the axis-cylinder processes of the cortical ganglion- cells or of the collateral processes derived from the projection fibres, pass to all parts of the cerebral surface, with the exception, probably, of the anterior portions of the temporal lobes and the olfactory tracts, which parts are connected by the fibres of the anterior commissure. On either side of the closely-packed bundles con- stituting the immediate bridge the fibres spread out in a fan-like course to reach their destination. The projection or peduncular fibres include many of the most important tracts by means of which communication between the presiding cortical centres and the more deeply lying nuclei and paths is established. The bundles of the crusta on reaching the sub- thalamic region become continuous with the internal capsule and spread out into the conspicuous corona radiata. The fibres which 327 328 NORMAL HISTOLOGY. gain the cortex, however, do not correspond exactly with those enter- ing the cerebrum as the peduncular bundles, since some of the latter are deflected and pass to the caudate and the lenticular nucleus from the internal capsule; on the other hand, the peripherally-streaming bundles are augmented by fibres which come from the thalamus and the subthalamic region. The peduncular tracts continued to the cortex consist principally of (a) the pyramidal fibres, (b) the fibres from lateral tracts, sensory paths to the temporo-occipital (?) region, and (c) the fibres from the pontine nuclei and the cerebellum. The tracts of the tegmentum largely contain fibres related to the connections of the thalami, the cerebellum, and the corpora quadrigemina ; regarding the exact course and communications of these bundles much still remains to be determined. Two small but remarkable organs, the pituitary and the pineal body, are closely associated in their genetic relations with the cere- brum, since the first of these bodies originates partly and the second entirely as a diverticulum from the cavity of the primary inter-brain. THE PITUITARY BODY. The pituitary body, or hypophysis cerebri, consists of two portions, the large anterior oral and the small posterior cerebral division. These are entirely distinct both in structure and in de- velopment, since the anterior lobe is derived as a diverticulum from the primitive oral cavity, and, as such, is lined with the oral ectoderm, while the posterior lobe descends as an outgrowth from the floor of the primary inter- brain, its stalk remaining as the infun- dibulum. In the embryo temporarily, and in many lower vertebrates permanently, the tissues composing the posterior lobe assume a distinctly nervous type ; in the higher animals, however, this character is lost, the lobe remaining small and rudi- mentary and its cavity undergoing obliter- ation ; the primary nervous character of the cerebral lobe disappears as the in- growth of the connective tissue and the blood-vessels takes place. The re- mains of the immature nervous elements are sometimes recognized in the branched and spindle pigmented cells found in this part of the pituitary body, as well as in the partially-preserved cavity lined with ciliated columnar cells. Fig. 351. Section of human pituitary body : C, portion of posterior or nervous lobe; P, portion of anterior or glandular lobe, consisting of tubular acini (a); s, conneclive-tissue septa ; v, blood-vessels. THE CENTRAL NERVOUS SYSTEM. 329 The anterior lobe, larger and darker than the preceding, for some time remains connected by its tubular ectodermic stalk with the primitive oral cavity ; later the tube becomes atrophic and finally disappears, the end of the oral diverticulum then lying iso- lated and separated from the buccal cavity. The single primary tube undergoes repeated division, producing compartments which appear in the adult organ as slightly convoluted tubular acini. The tubules are held together by vascular connective tissue, and contain polyhedral epithelial cells, with spherical or oval nuclei, irregularly disposed and often almost filling the alveoli; the lumina of the tubules are sometimes occupied by colloid masses resembling those of the thyroid gland. The pineal body, epiphysis, or conarium, since the compara- tively recent investigations of Spencer and of de Graaf, although known and described previously for centuries, is now regarded as a rudimentary sense-organ. These investigators independently demonstrated that the structure seen in man and the higher animals is the rudiment of what was a functionating sense-organ in the extinct reptiles, and even in certain living members of the same class strongly resembles an imperfect invertebrate eye in its early embryonal condition. In the light of our present knowledge, therefore, this pe- culiar body must be looked upon as representing an im- perfect organ of special sense, whether as an additional visual structure—the “pineal eye” —or as an organ for the percep- tion of warmth still remains to be determined. In man and other mammals the pineal body, instead of oc- cupying its morphologically normal position on the superior surface of the brain, is covered over by the greatly developed cerebral hemispheres, so that its final position is well towaids the base of the encephalon. The organ at no time in the higher ani- THE PINEAL BODY. Fig. 352. Sagittal section through part of head of lizard embryo, showing so-called pineal eye: P, special- ized isolated extremity of pineal diverticulum from brain-vesicle (B); b, c, so-called retinal and len- ticular areas of its walls; a, ectoderm ; d, remains of diverticulum undergoing division into tubules id'); f, blood-vessels ; e, mesodermic tissue. 330 NORMAL HISTOLOGY. mals assumes the characters of a sense-organ to the extent seen in the lower types. The adult human pineal body is composed of a number of tu- bular compartments or alveoli, which are separated by septa of connective tissue and lined by polyhedral epithelial cells ; in many places the tubules are almost oc- cluded by epithelium, together with aggre- gations of gritty calcareous matter, the so-called “ brain-sand.” The brain-sand, or acervulus cerebri, consists of irregu- larly round mammillated or mulberry-form concretions of variable size, composed of animal matter combined with earthy salts (calcium carbonate and phosphate with magnesium and ammonium phosphate). These deposits are not limited to the in- terior of the pineal body, but are encountered on its exterior and on the peduncles, as well as in the choroid plexus and in other parts of the brain-membranes ; the concretions occur at all ages, even before birth, and within the perfectly normal organ. Other bodies, the corpora amylacea, occur as round discoidal masses, and exhibit a distinct concentric striation ; they are regarded as amylaceous in nature, since they respond to the tests for such substances, staining violet with iodine and sulphuric acid. These bodies are almost constant within both the gray and the white matter consti- tuting the walls of many parts of the brain-cavities ; the olfactory tract is a particularly favorite situation, along this region the amylaceous corpuscles occurring with especial profusion. Fig. 353. section of human pineal b jdy: a, a, acini lined and partially filled with epithelium and cal- careous concretions (s) ; f, inter- tubular fibrous tissue. Fm. 354. Corpora amylacea from lateral ventricles of human brain. THE SUPRARENAL BODY. The close relations of this organ with the nervous system, as evidenced by its early history, the profusion of its nervous elements, and the results of pathological processes, entitle the suprarenal body to place, provisionally at least, within the present chapter. The parenchyma of the organ, composed of a peripheral zone, the cortex, and a central area, the medulla, is invested by a fibrous capsule of considerable thickness. From this envelope numerous connective-tissue septa penetrate deeply into the soft cellular substance, which is thus broken up into cylindrical masses. The cortex consists of aggregations of irregularly rounded or THE CENTRAL NERVOUS SYSTEM. 331 polygonal cells (13-17 /*), whose granular protoplasm frequently contains fat-particles in addition to clear nuclei. The arrangement of the cortical elements varies at different levels, the resulting disposition giving rise to the three divisions of the cortex recognized as the zona glomerulosa, the zona fas- ciculata, and the zona reticularis. The cells forming the first of these are grouped as oval masses, those of the middle layer are disposed as long cylindrical groups, and those of the third stratum are irregularly arranged as anastomosing cords supported within a reticulum of connective tissue. The zona reticularis is distinguished from the other cortical layers by the pigmented condition of its cells. The various groups of cellular elements are separated from one another by delicate fibrous septa, continuations from the outer connective-tissue envelope; the larger septa support the capillary net- works which surround the groups of cells. The medulla contains granular, fre- quently deeply-pigmented, polygonal cells arranged as cords and irregular net - works within a framework of highly vascular connective tissue. Nu- merous ganglion - cells occupy the central portions of the medulla, along with a rich net-work of non-medullated nerve-fibres and the con- spicuous venous channels. The blood-vessels of the organ divide within the capsule into numerous smaller branches, which enter the parenchyma along the fibrous septa ; capillary net-works surround the cell-groups of both cortex and medulla. The veins of the medulla are of large size and unite to form trunks which make their exit at the central hilus ; the larger radicles are accompanied by longitudinal bundles of non-striped muscle. The nerves of the suprarenal body are remarkable for their num- ber and size; they bear the arteries company within the septa to reach the medulla, where they form an intricate plexus composed chiefly of non-medullated fibres. Ganglion-cells occur along Fig. 355. Section of human suprarenal b"dy : a, fibrous capsule; b, zona glomeru- losa ; c, zona fasciculata; d, zona reti- cularis ; e, medullary cords; f, venous channel; g, ganglion-cells. NORMAL HISTOLOGY. rhe course of the nerve-trunks, and are found within the medulla in considerable numbers. The lymphatics are represented by delicate canals within the fibrous septa which communicate with the intercellular clefts of both cortex and medulla on the one hand, and with the larger lymph-ves- sels within the capsule on the other. THE DEVELOPMENT OF THE NERVOUS TISSUES. The consideration of the general changes involving the primary neural tube and its cephalic expansions, the brain-vesicles, by which are produced the various portions of the cerebro-spinal axis, belongs to embryology, and lies without our present purpose ; an account of the histogenesis of the nervous tissues, however, is of much interest in indicating the true relations of the structural components of the great nervous masses. The essential parts of the nervous system, including the nerve- cells, the nerve-fibres, and the neuroglia, are developed from the ectoderm alone, and all result from the differentiation and speciali- zation of the walls of the primary neural tube. This canal is formed by the gradual closure of the early furrow, the medullary groove, of the invaginated ectoderm along the dorsal line ; by the approxi- mation of the upper or dorsal edges of the involution the furrow is converted into a tube, the sides, or medullary plates, of the extreme ante- rior and posterior segments of which are the last to unite. Even before the closure of the groove has been completed a differentiation of two impor- tant regions is indicated; these are the areas giving rise to the segmental ganglia and to the general axial nerves. The area for the latter is represented by the lining of the neural tube, that for the former by the inconspicuous cell-groups lying on either side of the line of closure. The primary wall of the neural tube consists of a single layer of columnar epithelial cells, whose nuclei occupy the middle third of the elements, leaving an outer and an inner free zone ; within the latter appears very early a second variety of cell, which is distinguished by its large spherical form and conspicuous nucleus. The Fig. 356. Section of nine-day rabbit embryo, showing open neural tube : e, ectoderm invaginated and thickened within neural canal («) ; m, mesoderm ; b, body- cavity ; g, still open gut, lined by entoderm (h). THE CENTRAL NERVOUS SYSTEM. 333 round cells invading the inner zone represent the ancestors of the nervous elements—both cells and fibres—and are the germ-cells, while the columnar cells produce the neuroglia and are the spongio- blasts. The development of the nerve-cells proceeds from the germ-cells, which, as shown by the karyokinetic figures within their nuclei, undergo ac- tive proliferation, the resulting progeny being the neuroblasts, from which the nerve - cells are directly derived. The germ-cells are confined to the zone next the brain-cavity, which thus indicates the position of greatest formative energy. The neuroblasts at first occupy the innermost zone, next the cerebral cavity, but soon migrate towards the outer boundary of the wall, at the same time becoming pyriform and elongated. The young nerve-cells for a long time possess but a single process, Fig. 357. Section of ten-day rabbit embryo, showing closed neural tube : n, neural canal; s, area from which segmental gan- glia develop; m, mesodermic tissue ; g, gut-tube; v, v, primitive aorta;; p, pleuro-pericardial cavity. Fig. 358. Fig- 359- Primary wall of neural tube composed of single layer of epithelium (a) ; b, b, germ-cells occupying inner zone. (After His.) portion of inner zone of wall of neural tube in which round germ-cells («, «) and partially-developed neuroblasts («', n') lie among the surrounding spongio- blasts. (After His.) which grows out to become a nerve-fibre, and therefore represents the axis-cylinder, or nerve-process; the protoplasmic pro- 334 NORMAL HISTOLOGY. cesses, whose ramifications later present such striking pictures, are subsequently acquired, after the lapse of considerable time. Fig. 360. i Portion of wall of neural tube, exhibiting germ-cells (g) among the differentiating spongioblasts (s). (After His.) The development of the neuroglia depends upon the special- ization of the columnar elements, the spongioblasts. The epithelial cells elongate, their protoplasm at the same time undergoing vacuolation and partial absorption, resulting in the production of an elongated framework of connected slender trabeculae. The extremities of the changed epithelial elements, or spongioblasts, greatly dif- fer ; the inner ends of the cells extend to the inner boundary, where they are united to form a continuous sheet, the membrana limitans interna, and the outer processes break up into irregular branches, which ultimately form a close reticulum. The early spongioblasts ex- tend the entire thickness of the neural wall, but with the subsequent increase in this structure their attachments become broken, the spongioblasts then lying free among the surrounding nervous elements. The general growth of the tissues is ac- companied by great extension and sub- division of the neuroglia fibres, which eventually become the nucleated masses of fine, bristle-like processes Fig. 361. Spongioblasts from neural tube; their expanded upper ends unite to constitute the internal limiting mem- brane next the brain-cavity; their outer ends break up into reticulum. (After His.) THE CENTRAL NERVOUS SYSTEM. 335 constituting Deiters’s or spider cells. The spongioblasts im- mediately around persistent parts of the neural canal retain their inner connection and form a continuous layer of lining elements, which later constitute the ciliated columnar epithelium of the ependyma. The development of the nerve-fibres includes the origin of two sets of primary fibres—those derived from the nerve-cells of the medullary tube and those growing out from the cells of the ganglia. All nerve-fibres are formed as the direct exten- sions and continuations of the processes of the neuroblasts. In the case of those proceeding from the neural canal the fibres grow peripherally and the cells remain attached to their central ends, thus early establishing the relations afterwards existing between the motor cells and the fibres; those originating from the ganglia, on the other hand, grow in two directions, towards the periphery and towards the nervous axis, representing the sensory paths. The early nerve-fibres consist for some time of the axis-cylin- der alone, the neurilemma and the medullary substance being not only much later acquisitions but also contributions from the meso- derm. The neurilemma first envelops the ectodermic axis-cylin- der as a delicate sheath, and subsequently within this envelope the myeline of the white matter of Schwann is deposited. The ap- pearance of this coat is often very late, and takes place at different times for the various tracts of nerve-fibres, although the period at which the several groups acquire their medullary substance is con- stant and definite for each set. The young fibres soon collect into groups, which represent the early nerve-trunks, whose further growth proceeds in a straight path corresponding with the general direction of the component axis-cylinders ; a course once established is maintained until arrested by some obstacle or modified by changes in the position of the parts with which the nerve has formed attach- ments. The terminations of the growing nerves are abrupt, the finer ramifications appearing only after the trunk has undergone repeated branchings. 336 NORMAL HISTOLOGY. CHAPTER XVII. THE EYE AND ITS APPENDAGES. THE EYEBALL. The bulbus oculi consists of three coats: 1, the external fibrous tunic, comprising the sclerotic and the cornea, upon which devolves the maintenance of the form of the organ ; 2, the middle vascular tunic, made up of the choroid, the ciliary body, and the iris, to which the principal vascular supply of the eye is distributed; 3, the inner nervous tunic, the retina, which receives the termi- nal expansion of the optic nerve and contains the specialized neuro- epithelium concerned in the perception of the visual stimulus. The aqueous humor, the crystalline lens, and the vitreous body are en- closed by these coats, and represent the refractive media of the eye. Referred to their embryonic origin, the several parts of the eye may be grouped under two headings,—those developed from the ectoderm and those derived from the mesoderm. The mem- bers of the first group may be subdivided into (a) structures derived directly from the ectoderm, including the lens and its anterior epi- thelium, and the epithelium of the cornea and of the adjacent scleral surface, and ib) structures derived secondarily from the ectoderm through the optic vesicles protruded from the involuted ectoderm of the cerebral vesicles ; to this class belong the primary retinal tissues, including the pigment-layer, as well as the atrophic retinal layers continued over the posterior surface of the ciliary body and the iris. All other parts of the eyeball, comprising the remaining portions of the sclera, the cornea, the iris, the ciliary body, the choroid, and the vitreous body, as well as the connective-tissue ingrowths of the retina, are developed from the mesoderm. THE CORNEA. The cornea consists of five layers: 1. The anterior epithelium. 2. The anterior limiting membrane. 3. The substance proper. 4. The posterior limiting membrane. 5. The posterior endothelium. The anterior epithelium, the only part of the cornea derived from the ectoderm, all others being mesodermic, is stratified squa- THE EYE AND ITS APPENDAGES. 337 mous in character; it is thinnest over the central part of the cornea, where its six to eight layers of cells together measure about 45 /*, at the periphery reaching almost double that thickness. The deepest cells are columnar in form with their outer ends somewhat rounded off, while their bases are slightly expanded and conform to the sur- face of the basement-membrane upon which they rest. Succeeding the deepest layer the elements become broader and polyhedral, many pos- sessing the protoplasmic threads char- acteristic of prickle-cells. The su- perficial strata are composed of flattened cells which lie parallel to the surface and contain oval nuclei. The anterior limiting mem- brane, membrane of Bowman, or lamina elastica anterior, corre- sponds to a highly-developed base- ment-membrane, being continuous with the tissue of the substantia pro- pria, of which it is a special conden- sation. The structure is especially conspicuous in the human cornea, where it appears as a seemingly ho- mogeneous glassy layer, about 20n in thickness in the middle of the cor- nea, gradually diminishing towards the periphery. The resolution of this lamina into delicate connective- tissue fibrillae after treatment with suitable reagents demonstrates its true nature as a specialized portion of the substantia propria. The fibrous stroma, or the sub- stantia propria, constitutes the chief bulk of the cornea, and is made up of parallel lamellae composed of in- terlacing bundles of fibrous connective tissue. The exact num- ber of corneal lamellae is inconstant, since this depends upon the extent to which the artificial separation of the tissue is carried. The interlacing bundles of the white fibrous tissue composing the lamellae are united by the interfibrillar cement substance, and cross one another obliquely at various angles, the adjacent bundles being intimately united by bands, the fibrae arcuatae, which pass from one bundle to the other ; the arcuate fibres are especially prominent Fig. 362. Section of human cornea: a, anterior epithelium; c, anterior limiting membrane ; b, b, fibrous substantia propria, containing corneal corpuscles (_/") lying within cor- neal spaces; d, posterior limiting mem- brane ; e, endothelium lining anterior chamber. 238 NORMAL HISTOLOGY. in the anterior lamellae. The substantia propria resembles the matrix of cartilage in yielding chondrin on boiling, therein differ- Fig. 363. Interlacing bundles of fibrous tissue constituting substantia propria from cornea of ox ; interstitial injection with silver nitrate. ing from the sclera, which, like the usual connective tissues, produces gelatin. The cellular elements, the corneal corpuscles, are plate-like or stellate connective-tissue cells, whose branched processes unite Fig. 364. Corneal corpuscles from calf; gold preparation. with those from adjacent cells to constitute a protoplasmic reticu- lum throughout the tissue. The corneal cells and their processes lie within a system of intercommunicating lymphatic spaces THE EYE AND ITS APPENDAGES. 339 hollowed out within the cement-substance, which consists of the large lacunae or corneal spaces between the lamellae and the small canaliculi extending from the former as fine branching tubes. The corneal corpuscles are usually applied to one wall of the spaces, which they by no means completely fill, while their processes extend within the branching canaliculi. In addition to the corneal cor- puscles, wandering cells, together with the tissue- juices, occupy the spaces and canaliculi. The posterior limiting membrane, membrane of Descemet, or poste- rior elastic lamina, ap- pears as a clear homoge- neous band at the inner boundary of the cornea, sharply defined from the deepest layers of the substantia propria and clothed on its inner surface by endothelium. The membrane differs from the cor- responding anterior lamella in its less intimate attachment with the Fig. 365. Corneal spaces from calf; silver preparation. Fig. 366. Corneal spaces from calf; exhibited spaces in positive picture after interstitial silver injection. fibrous stratum and in possessing the greatest thickness (10-12 fi) at the periphery. After prolonged maceration or treatment with suit- able reagents the resistant lamina is separable into a number of thin 340 NORMAL HISTOLOGY. homogeneous layers, which sometimes exhibit a delicate longi- tudinal striation. The posterior corneal endothelium, or endothelium of Des- cemet’s membrane, consists of a single layer of regular poly- hedral plates, whose oval nuclei project slightly beyond the bodies of the cells. Blood-vessels are absent in the cornea, except within a narrow zone, about 1 mm. in width, at the limbus or margin; in the foetus, however, the vessels extend well towards the centre and form the precorneal capillary net-work. The lymphatics of the cornea are represented by the system of intercommunicating spaces and canaliculi; these clefts open into lymphatic radicles at the periphery, which, in turn, communi- cate with the larger anterior lymphatic vessels. Perineurial lymph- channels enclose the larger nerve-trunks, which they accompany for a variable distance into the corneal tissue; these lymphatic channels communicate directly with the corneal spaces at frequent intervals. The nerves of the cornea are very numerous, and are distributed largely within the anterior layers. They enter at the corneal limbus as some sixty radially-disposed twigs, each of which includes from Fig. 367. Subbasilar plexus of corneal nerves from rabbit; gold preparation. three to twelve fibres; the latter almost at once, within .5 mm. of the limbus, become non-medullated. Within the substantia propria the nerve-fibres form a coarse ground-plexus at a level corresponding to about the middle third of the corneal tissue; from this net-work twigs are sparingly given oft to supply the deepest layers, while others pass towards the THE EYE AND ITS APPENDAGES. 341 anterior lamellae, in which they form net-works. Immediately beneath the anterior elastic membrane the smaller fibres form the dense subbasilar plexus, while under the epithelium the finest fibrillae constitute the subepithelial plexus, from which delicate naked axis-cylinders ascend and enter the epithelium, to end between the cells as the intra-epithelial plexus. THE SCLERA. The sclera is composed of the same elements as is the substantia propria of the cornea, but they are less regularly disposed and lack the remarkable transparency of the latter. The ground-substance is made up of interlacing bundles of gelatin-yielding fibrous tissue mingled with elastic fibres; the fibrous bundles are arranged as two principal sets, those extending longi- tudinally or meridionally and those running transversely or equatorially. The interfascicular interspaces are occupied by the stellate connective-tissue plates, which correspond closely to the corneal corpuscles; in addition, a few small wandering cells are usually present. The sclerotic and cho- roid coats are united by a layer of loose connective tissue, the lamina suprachoroidea, the extensive interfascicular clefts of which form part of the sub- scleral lymph-space. The suprachoroidal tissue consists of many imperfect la- mellae composed of a fibro- elastic groundwork support- ing irregular groups of flattened endothelioid connective-tissue plates ; the broad trabeculae join one another at various angles, and include the imperfectly sep- arated compartments of the gen- eral lymph-space. The larger partitions convey the numerous vascular and nervous trunks in their course to and from the choroid. The deeply-pigmented tissue of the innermost layer of the sclera, next the subscleral space, constitutes the lamina fusca, and is covered with the endothe- Fig. 368. Section of human eyeball taken midway be- tween equator and posterior pole: S, sclera ; p, lamina fusca and lamina suprachoroidea ; P, peri- scleral tissue ; C, choroid; R, retina with its layers indicated by figures. 342 NORMAL HISTOLOGY. lial lining of the lymph-cavity. The outer surface of the sclera throughout a large part of its extent takes part in bounding the episcleral space, where it is likewise covered with endothelium. The blood-vessels distributed to the tissue of the sclerotic coat are meagre, although the tunic is pierced by numerous trunks related with the supply of the underlying parts; such small vessels as are present break up into capillaries passing among the bundles of fibrous tissue. The lymphatics are represented by the intercommunicating cell- spaces which connect with the larger lymph-cavities. The nerves terminating within the sclera constitute fine twigs given off from the larger trunks passing between the sclerotic and choroid coats ; they break up into fibrillae which end as naked axis- cylinders between the bundles as an interfascicular plexus. THE CHOROID. The choroid consists of a connective-tissue stroma supporting numerous blood-vessels. Dependent largely upon the size and arrangement of the blood-vessels, certain layers are distinguished, these being, from without inward : 1. The layer of choroidal stroma containing large blood-vessels. 2. The layer of dense capillary net-works—the choriocapillaris. 3. The homogeneous glassy lamina, or vitreous membrane. The stroma-layer, with its large blood-vessels, constitutes the greater part of the choroid. Within a supporting tissue made up Fig. 369. Section of human choroid : a, retinal pigment adhering to vitreous mem- brane (b); c, capillary layer, or choriocapillaris ; d, e, large blood-vessels of stroma-layer (/); g, lamina suprachoroidea; h, tissue of sclera. of closely united connective-tissue lamellae, elastic fibres, and branched pigmented cells, the freely branching arterial and venous trunks take their course, appearing as lighter-colored channels within the darker surrounding matrix. The blood-vessels and the stroma THE EYE AND ITS APPENDAGES. 343 are so intimately united that they constitute a layer of considerable consistence. The largest vessels occupy the most superficial part of the stratum, those next in size the middle, while the smallest approach the capillary zone. The most conspicuous of the large superficial blood-channels are the four venae vorticosae, with their whorls of tributaries. These veins occupy positions around the equator at points about equidistant, towards which the smaller vessels converge from all directions, returning the blood not only from the choroid but also from the ciliary body and the iris. The veins of the choroid are often surrounded by perivascular lymph-sheaths. Many of the larger arteries, in addition to the well-marked circu- larly-disposed muscle with which they are provided, are accompanied by external longitudinal bundles of involuntary muscle. The innermost part of the stroma-layer, next the choriocapil- laris, forms a narrow stratum (10 fj. in width) which is devoid, or nearly so, of pigment, and constitutes the boundary zone. In the eyes of many animals (horse, cow, sheep) this layer possesses wavy bundles of connective tissue, to whose peculiar arrangement Fig. 370. Human choroid seen from its inner surface, exhibiting surface view of cap- illary net-work, or choriocapillaris (c, c) ; b, b, large blood-vessels of stroma- layer beneath ; a, a, intervening stroma-tissue. is due the metallic reflex sometimes seen from such eyes ; this shining structure is known as the tapetum fibrosum, as distinguished from the iridescent tapetum cellulosum of the carnivora which is dependent upon the presence of several layers of plate-like cells containing innumerable small crystals. The capillary layer, or choriocapillaris, consists of a narrow 344 NORMAL HISTOLOGY. zone, about io n in width, at the inner part of the choroid, composed of a structureless, apparently homogeneous, matrix, in which lie embedded the close capillary net-works derived from the terminal branches of the short ciliary arteries. The vitreous lamina, or glassy membrane, forms the most internal layer of the choroid and supports the retinal pigment. The membrane presents a delicate homogeneous stratum (2 n in thick- ness), ordinarily without appreciable structure, and is very intimately associated with the adjoining layer of the choroid ; to its inner sur- face patches of retinal pigment frequently adhere on removal of the retina. The nerves of the choroid, non-medullated fibres distributed to the blood-vessels, are derived from the plexus formed within the suprachoroidal tissue by branches given off from the long and short ciliary nerves in their transit through the subscleral space. THE CILIARY BODY. This structure includes that portion of the uveal tract situated be- tween the termination of the choriocapillaris, opposite the ora serrata behind and the ciliary or outer margin of the iris in front. Within this important territory three areas may be distinguished : i, the ciliary ring; 2, the cili- ary processes; 3, the ciliary muscle. The ciliary ring, or orbicu- lus ciliaris, is a circular tract about 4 mm. in breadth, situated immediately in front of the ora serrata and extending to the posterior ends of the ciliary pro- cesses. This zone differs from the choroid in the absence of the choriocapillaris and in the presence of muscular tissue prolonged from the mass of the ciliary muscle ; the character of the stroma also changes, its bulk being here made up of fibrous connective bundles instead of elastic lamellae. I he ciliary processes consist of an annular series of some seventy prominent radial vascular folds which project from the inner surface of the ciliary body and arise from the confluence of several of the low ridges on the ciliary ring ; after attaining a height of about 1 mm., they abruptly end at the base of the iris, sinking Fig. 371. Section of human ciliary processes ; I, in- terstitial connective-tissue stroma, covered by retinal layers (R); i, o, inner clear and outer pigmented layers of cells; f, fibrous tissue of processes. THE EYE AND ITS APPENDAGES. to the level of the underlying ciliary muscle. The stroma of the processes is a continuation of the connective tissue of the orbicular zone, this layer being the true prolongation of the choroid, since the muscular tissue must be regarded as an intercalation between the sclerotic and choroid coats. The vitreous lamina is continued as a delicate homogenous membrane, 3 to 4 in thickness, over the inner surface of the ciliary processes. Inside this layer the internal face of this entire region, including the ciliary ring and the ciliary body, as well as the iris, is covered by the deeply-pigmented rudimentary layers of the pars ciliaris retinae, consisting of an inner small row of tall columnar elements and an outer sheet of low pig- mented epithelium. Since these layers represent the rudimentary folded anterior laminae of the ectodermic optic vesicle, the ciliary processes and the iris consist of two genetically distinct parts, the Fig. 372. Section through ciliary region of human eye : A, cornea; a, b, c, its epithelium, substantia propria, and endothelium ; C, scleral conjunctiva, terminating at d\ B, sclera/ e, sclero-corneal juncture ; D, iris ; E, ciliary body covered by pigment-layer, l; k, fibrous stroma of ciliary processes; f, bands of pectinate ligament; g, spaces of Fontana; s, canal of Schlemm; v, venous channels; m, n, o, meridional, radial, and circular (Miiller’s) fibres of ciliary muscle ; r, subscleral space bridged by fibrous bands. mesodermal connective-tissue stroma, containing blood-vessels and muscle-fibres, and the inner deeply-pigmented ectodermal stratum. 346 NORMAL HISTOLOGY. The ciliary muscle presents a conspicuous thickening for about i mm., which extends from the orbicular zone to the base of the iris and bears on its inner surface the connective-tissue stroma of the ciliary processes and the orbicular ring. In meridional sections its mass appears as a triangular area, the cross-section of a three-sided annular band of muscle entirely encircling the eyeball. The triangle thus formed closely approximates a right angle whose sides are unequal; the shorter anterior side extends from the sclero- corneal juncture towards the ciliary processes, and the longer inner border is prolonged to meet the outer side or hypothenuse at an acute angle at the anterior border of the choroid. The mass of the ciliary muscle consists of interlacing bundles of involuntary muscle, the interspaces between which are filled by connective tissue. The muscular fasciculi are arranged as three sets, the meridional, the radial, and the circular. The meridional fibres lie generally parallel to the sclera, and form a compact layer attached in front at the sclero-corneal junction, near the anterior margin of Schlemm’s canal, and behind at the fore margin of the choroid, where, in common with many of the radial fibres, it finds insertion ; in recognition of this attachment, the meridional and radial fibres were named the tensor choroideae. The radial bundles spread out fan-like from their anterior attach- ment, the most external fibres running nearly parallel to the meridio- nal bundles, with which they become continuous, while the anterior pass off at a considerable angle. The circular fibres, the ring-mus- cle of Muller, constitute a distinct group of equatorially-disposed bundles, which occupy the internal angle of the ciliary muscle and extend at right angles to the preceding bundles. The blood-vessels of the ciliary body are especially concerned in supplying the ciliary muscle, to which minute arterial twigs pass from the imperfect vascular circle lying behind the arterial circle of the iris. The numerous nerves of the ciliary muscle are derivatives of the ciliary trunks, which on entering the muscle form a plexus within its substance; from this plexus fibres pass internally to the iris, outwardly to the cornea, while others are distributed to the ciliary muscle itself. Small ganglion-cells also occur, singly or in small groups. THE IRIS. The iris constitutes the anterior segment of the uveal tract, and consists of a principal stroma-layer covered in front by a reflection of the corneal endothelium and behind by the continuation of the deeply-pigmented rudimentary retinal layers—the pars iridica THE EYE AND ITS APPENDAGES. 347 retina. The various components of the iris and their morphological relations may be grouped as follows : 1. Anterior endothelium. 2. Anterior boundary layer, 3. Vascular stroma-layer, 4. Posterior boundary layer, Continuation of the tissues of the uveal tract proper, constituting the stroma-zone. ' a. Anterior layer of pig-" merited spindle - cells representing outer layer b. Posterior layer of pig- mented polygonal cells representing inner , layer 5. Pigment-layer, Of OPTIC VESICLE. The anterior endothelium consists of a single layer of thin nu- cleated polygonal plates, the direct prolongation of the corneal endothelium. The protoplasm of the cells is finely granular, but always free from pigment. The anterior boundary layer is formed by modification of the foremost stratum of the iris-stroma; the connective tissue consti- Fig. 373. Section through part of iris and lens, from human eye: I, iris; a, anterior endothelium; b, anterior boundary layer; c, vascular stroma; d, posterior boundary layer; e, pigment-layer continued as far as g on pupillary margin (P); f, cut circular muscle-bundles of sphincter; L, surface of crystalline lens; h, anterior lens capsule, with anterior epithelium beneath; z, tissue of lens. tuting this layer resembles the reticular tissue of lymphatic struct- ures, comprising several layers of net-works within the interspaces of which lie lymphoid cells in greater or less profusion. The vascular stroma constitutes the chief mass of the iris, and, in addition to its numerous blood-vessels, contains involuntary 348 NORMAL HISTOLOGY. muscle and nerves. The stroma consists of loose spongy re- ticular connective tissue greatly strengthened by the radially- disposed blood-vessels and nerves, around which the delicate stroma forms ensheathing masses of considerable density. The clefts situated between these adventitious sheaths and the included vessels and nerves form a system of lymphatic channels through- out the iris which communicate with the anterior chamber through the lymph-spaces at the irido-corneal angle. The arteries of the iris spring from the anterior part of the cir- culus arteriosus iridis major, situated at the ciliary border, and pass towards the centre of the iris as radially-disposed, freely-anasto- mosing twigs ; about i mm. from the inner edge of the iris these vessels unite to form a second delicate vascular ring, the circulus arteriosus iridis minor, which marks the division of the iris into its pupillary and ciliary zones, which are respectively i mm. and 3-4 mm. in breadth. From this circle the arterioles continue their course towards the pupillary bor- der, and end in the capillary net- work distributed to the sphincter muscle. Capillary reticula exist also within the anterior and pos- terior layers of the stroma. All the capillaries are tributary to the radiating veins which pass to the ciliary border, where they join those of the ciliary processes and finally empty into the radicles forming the venae vorticosae. Bundles of involuntary muscle occupy the pupillary border and the posterior zone of the stroma-layer ; these are arranged as two sets,—the annular bundles encircling the pupillary margin of the iris and constituting the sphincter of the pupil, a muscular zone about 1 mm. in width, and the few scattered radially-disposed bundles extending from the pupil towards the ciliary margin and forming an incomplete, by no means continuous, layer, the dilator pupillae. The posterior boundary layer, or vitreous lamella, appears as a glassy structureless membrane, about 2 /i. in thickness, which stretches over the posterior surface of the stroma and supports the pigment-layer: in the nature of its substance this structure closely approaches elastic tissue. The pigment-layer, or pars iridica retinae, is usually so densely Fig. 374. Injected iris from eye of dog: P, pupillary margin, around which capillary net-work is formed by vessels proceeding fronj lesser ar- terial circle. THE EYE AND ITS APPENDAGES. 349 packed with deeply-colored particles that its real constitution is masked. This stratum is composed of two layers, an anterior and a posterior. The anterior or outer layer is formed of radially- arranged spindle-cells which pass without interruption from the ciliary border of the iris to the pupillary margin ; at the ciliary border the cells change their form and arrangement, becoming polyhedral and circularly disposed and continuous with the low pigmented elements constituting the corresponding layer of the ciliary processes. * The posterior layer presents a thicker zone (30-35 of pig- mented cells, in which the colored particles are so densely packed that the cell-boundaries and the nuclei are completely masked, the entire layer appearing as one continuous mass of pigment. The pigment-layer covers the entire pupillary margin, and often ends as a somewhat thickened free edge slightly in advance of the plane of the iris ; at this border, which represents the free anterior lip of the embryonic secondary optic cup, both ’strata of the pigment-layer become continuous. The posterior surface of the pigment-layer is covered by a very delicate cuticular membrane, the membrana limitans iridis, which is continued from the similar structure extending over the cili- ary processes ; it appears first at the ora serrata as a new formation, since a true membrana limitans interna, in the sense of a distinct cuticle, does not exist over the retina proper. The marked variation in the color of the iris is largely dependent on the amount and position of its pigment. In blue eyes the stroma of the iris is entirely free from pigment, the latter being confined to the posterior pigment-layer, from which position it is seen through the superimposed iridal strata. With the darker color of the iris its stroma-cells also acquire pigment; in light gray eyes this is small in amount, in brown eyes greater, while in the darkest eyes the colored particles are very numerous and sometimes appear as almost continuous pigmented areas ; in albino eyes, on the other hand, even the retinal portion of the iris is devoid of pigment. The nerves of the iris, derived from the intra-muscular ciliary plexus, enter the more superficial part of the stroma-layer as med- ullated fibres. Within the iris the nerve-fibres soon lose their med- ullary sheath and form one or two irregular net-works, the most constant of which is a circular plexus in the vicinity of the sphinc- ter muscle; from this net-work pale fibres are distributed to the substance of the latter muscle. The principal plexus lies anterior to the plane of the chief vascular net-work, the posterior zone of the iris being poorly supplied with nerves. The irido-corneal angle, marking the junction of the cornea, the 250 NORMAL HISTOLOGY. sclera, the iris, and the ciliary muscle, constitutes one of the most important regions in the eye, not only with regard to its anatomical details, but also in view of its practical clinical significance. As already described, the substantia propria of the cornea passes directly into the ground-substance of the sclera ; in consequence of the rearrangement of the tissue-elements of the two structures taking place soonest in the superficial planes of the cornea, the line of transformation becomes oblique, thereby producing an apparent overlapping of the sclera in front, and a corresponding extension of the cornea behind. The posterior elastic membrane, on reaching the corneal mar- gin, splits up into a number of stiff homogeneous fibres, many of which become attached to the base of the iris and constitute the liga- mentum pectinatum iridis. By the union of the processes from the iris and Descemet’s membrane with the elastic fibres derived from the anterior attachment of the ciliary muscle and a few bands from the sclera, a reticulum of thin trabeculae is formed, which occupies the angle between the cornea and the iris. This spongy tissue constitutes an annular mass enclosing a system of intercom- municating cavities, the spaces of Fontana. These clefts, lined Fig. 375. Section through irido-corneal angle of human eye, highly magnified : a, substantia propria of cornea; b, posterior limiting membrane, splitting at corneal margin into delicate lamellx (d); c, endothelium continued over iris (t) ; f, elastic lamellx separating Schlemm’s canal (5) from spaces of Fontana (s, s) and giving attachment to fibres of ciliary muscle (A). by an imperfect layer of endothelium, are more conspicuous in the eyes of some of the lower animals (horse, ox, pig, sheep), where they are far better developed than in man. Within the sclera, close to its inner border and the corneal juncture, THE EYE AND ITS APPENDAGES. 351 lies a flattened annular channel, the canal of Schlemm ; the inner wall of this canal is formed by intersecting delicate lamellae whose loose disposition suggests an incomplete isolation of the channel from the adjacent spaces of Fontana. The nature of the canal of Schlemm, whether a venous or a lymphatic channel, has long been a subject of discussion ; the weight of evidence warrants re- garding it as a venous canal, between which and the lymph-clefts represented by the spaces of Fontana free communication un- doubtedly exists. The inner nervous tunic of the eyeball includes the retina alone, which extends from the optic entrance throughout the posterior seg- ment of the ball and as far forward as the pupillary margin of the iris. This extensive tract, corresponding in its morphological limits to the secondary optic vesicle, falls into three divisions : (i) the pars optica retinae, including the entire posterior segment and end- ing at the ora serrata; (2) the pars ciliaris retinae, covering the posterior surface of the ciliary zone and processes and extending from the ora serrata to the base of the iris ; and (3) the pars iridica re- tinae, passing over the posterior surface of the iris from the base to the anterior edge of the pupil, where it terminates as a slightly-thick- ened margin, which corresponds to the free lip of the double-layered optic cup. The retina proper, or pars optica retinae, consists of an inner and an outer lamina, which correspond to the inner and outer layers of the optic vesicle ; the outer lamina includes the pigment-layer alone, while the inner lamina embraces the remaining layers of the retina. The inner lamina permits further subdivision of its structures into the neuro-epitlielial and the cerebral layer. The relations of these divisions to the individual retinal layers may be expressed : THE RETINA. I. Outer layer of optic vesicle. Pigment-layer. A. Pigment layer. Layer of rods and cones; Limiting membrane; Outer nuclear layer; B. Neuro-epithelial layer. II. Inner layer of optic vesicle. Outer reticular layer ; Inner nuclear layer; Inner reticular layer; Ganglion-cell-layer; Nerve-fibre-layer. C. Cerebral layer. The retinal structures consist of two parts,—the nervous ele- ments and the supporting neuroglia. The supporting tissues contribute a considerable part of the entire retina, but differ in their amount in the several layers. The most conspicuous constituents of the supporting framework are long neuroglia-fibres, the radial 352 NORMAL HISTOLOGY. fibres of Muller, which extend through the entire thickness of the retina. The expanded inner ends of the supporting fibres are so closely applied that they produce a seemingly continuous mem- brane, the so-called membrana limitans interna. The radial fibres rapidly diminish in diameter beyond their bases, and are con- tinued as narrow irregular stalks giving off lateral branches in pro- fusion to the reticular layers ; within the inner nuclear layer each fibre presents an irregular nucleated enlargement, and gives off lateral processes for the support of the nervous elements of the inner nuclear layer, as well as to each of the succeeding layers. At the Fig. 376. Diagram illustrating the relation of the retinal elements. A, layer of rods and cones ; B, limitans externa; C, outer nuclear layer; E, outer reticular layer (between the two (D) Henle’s fibre-layer) ; E, inner nuclear layer ; G, inner reticular layer; H, layer of ganglion-cells ; I, fibre-layer ; K, limitans interna, a, supporting fibres of Muller; b, c, rod- and cone-visual cells; d, bipolars belonging to rod-cells ; e-i, bipolars belonging to cone-cells ; k-m, horizontal nerve-cells; n, centrifugal nerve-fibres ; o-t, ganglion-cells connected with optic fibres; a-e, spongioblasts or amacrines arranged in layers ; d, diffuse amacrines ; 17, nervous amacrine. (Kallius after Ramon y Cajal.) inner border of the rods and the cones the expanded ends of the neu- roglia-fibres form the external limiting membrane, delicate pro- cesses extending from the latter between the bases of the rods and the cones, which they surround and embrace as the “ fibre-crates.” In addition to the long radial fibres, richly-branched neuroglia- cells occur within the outer reticular layer to the fibre-complex of which they contribute. Within the meshes of the framework just described the ner- vous elements of the retina are distributed in a manner charac- teristic for each layer : a brief consideration of these is therefore necessary. THE EYE AND ITS APPENDAGES. 353 The nerve-fibre-layer contains the continuations of the optic fibres which, after having lost their medullary substance in their passage through the lamina cribrosa, radiate as naked axis-cylin- ders to all parts of the retina as far as the ora serrata. The fibre- layer is thickest at the edge of the optic disk and thinnest at the extreme retinal periphery. Sooner or later the fibres forsake their peripherally-directed course, and, bending sharply, pass almost per- pendicularly to the ganglion- layer and other strata. The ganglion-cell-layer consists of a single row of large multipolar nerve- cells (15 to 30 /x), whose axis-cylinder processes are directed towards the fibre- layer ; their branched pro- toplasmic processes, when well developed, pass into the inner reticular zone, to meet the arborizations of the cone- bipolars. The ganglion- cells in the central part of the retina are densely packed in the macula, constituting overlying rows, but towards the periphery they are less plentiful, and at the ora ser- rata infrequent. The inner reticular layer presents a characteristic retic- ulated tissue composed of neuroglia net-works and the rich arborizations of various nerve-cells ; the processes origi- nate from both the elements of the ganglion-layer and the cells of the adjacent nuclear stratum. The internal nuclear layer includes a number of distinct ele- ments, and presents two subdivisions : (a) an inner layer of nerve- cells, the spongioblasts, or amacrines, and (b) an outer layer of the rod- and cone-bipolars forming the ganglion retinae. The “spongioblasts” are not concerned in the production of the sus- tentacular tissue, as their name—given under erroneous ideas regard- ing their function—would imply, but are nervous elements whose branched protoplasmic processes are resolved within the inner reticular layer into arborizations. The cone-bipolars send their Fig. 377. Section of human retina: a, internal limiting mem- brane formed by apposition of expanded basis of Mul- ler's fibres (y) ; b, fibre-layer; c, layer of ganglion-cells (2) ; d, e, inner reticular and inner nuclear layer ; /, g, outer reticular and outer nuclear layer; h, outer limit- ing membrane ; i, layer of rods and cones ; k, portion of pigment-layer ; v, x, b!ood-v«ssels. 354 NORMAL HISTOLOGY. axis-cylinder processes into the inner reticular layer, to end at various levels in arborizations in relation with the terminal filaments of the ganglion-cells ; their protoplasmic processes extend as far as the outer reticular layer, where they terminate in ramifications beneath the cone-cells. The protoplasmic processes of the rod- bipolars end beneath the rod-cells, their axis-cylinder processes penetrating the inner reticular stratum, to end in close relation with the ganglion-cells. The outer reticular layer appears as a narrow zone made up of an intricate net-work of fine fibres with sparingly distributed nuclei. The fibrillae are derived from the neuroglia and from the processes of nerve-cells, among which are the horizontal cells whose axis-cylinder processes extend horizontally within the layer, often for considerable distances, to end beneath the visual cells. The outer nuclear layer and the layer of rods and cones, the remaining strata of the inner lamina of the retina, together constitute the neuro-epithelium. Since the rods and the cones and the outer nuclear layer are parts of a single lamina of tall neuro-epithelial elements, the visual cells, of which they are respectively the outer and inner segments, these strata must be regarded as subdivisions of the one broad zone, and not as independent retinal layers. The outer and inner segments are sharply separated by the intervening membrana limitans, through the openings in which the rods and the cones protrude. The constituents of the neuro-epithelium are, therefore, the rod-visual cells and the cone-visual cells, supported by the sustentacular tissue. The rod-visual cells are composed of two parts, the one situ- ated without the limitans, including the non-nucleated and highly- specialized segments, the rods, and the other within the limitans, consisting of slender varicose elements, the rod-fibres, provided with fusiform enlargements, the rod-spherules, which contain the nuclei of the visual cells. The rods are slender cylindrical struct- ures, about 60 fj. in length and 2 ij. in breadth, composed of two chemically and optically distinct parts, the outer and inner seg- ments. The outer segments of the rods are cylindrical, apparently homogeneous, highly-refracting bodies, which, after certain reagents, exhibit a disposition to break up into thin transverse disks. The outer segments of the rods are further distinguished as being the exclusive seat of the peculiar visual purple or rhodopsin. The inner segments of the rods are slightly broader and less regularly cylindrical, and present a finely granular appearance, the parts of the segments nearest the membrana limitans possessing a peripheral longitudinal striation. THE EYE AND ITS APPENDAGES. 355 The inner segments of the rod-visual cells include the rod- fibres and their nucleated expansions, the rod-granules. The rod-fibres are slender, greatly extended, and often varicose, and reach from the membrana limitans into the external zone of the outer reticular layer. Each rod-fibre represents the greatly attenu- ated protoplasmic body of a visual cell, the situation of whose nucleus is indicated by the ellipsoidal enlargement. These enlarge- ments, the rod-granules, vary in position, sometimes lying near the outer end, at other times close to the middle or the inner extremity of the fibres. The granules are almost entirely occupied by the nuclei of the visual cells, which are covered by an extremely thin layer of the cell-protoplasm. The nuclei of the cells are oval in form, about 6 in length, and characterized by a remarkable differentiation of their substance into lighter and darker transverse bands. The cone-visual cells consist also of two parts, the outer di- visions, the cones, situated beyond the membrana limitans, and the inner portions including the cone-fibres and their nucleated cone- granules. The cones, like the rods, present inner and outer seg- ments, which in physical and chemical properties resemble the corresponding parts of the rods ; the cones, however, are little more than half (32-36 /*) the length of the rods. The inner segments of the cones are much wider than their outer divisions, and appear as truncated conical bodies whose sides are not absolutely straight, but slightly convex. The outer part of these segments is occupied by an ellipsoidal group of fine longitudinal fibrillae, the fibre-body, which corresponds with the similar structure sometimes present within the rods. The inner segments of the cone-visual cells, representing the bodies of the elongated cells, include the cone-fibres and their granules. The cone-fibres differ from the rod-fibres in being broader at the inner ends and more regular in their general contour; the cone-granules always lie, except in the macular region, next the membrana limitans. The distribution of the two kinds of visual cells varies in the different retinal regions ; the arrangement prevailing throughout the greater part of the retina is such that the adjacent cones are separated by three or four rods, the latter far outnumbering the cones. On approaching the macula the number of cones increases, the cones being so closely placed that they are separated by only a single row of rods; within the fovea itself the rods entirely disappear, the entire percipient layer being composed of cones alone. On the other hand, towards the periphery the number of these visual cells diminishes, and at the ora serrata the cones are widely separated, while the relative number of rods is very large. The 356 NORMAL HISTOLOGY. conclusion inferable from the distribution of these elements in the hu- man retina, that the cones are the essential perceptive instruments, is not applicable as a generalization, since in many of the lower animals the cones are in the minority or even entirely wanting (hedgehog, shark, sturgeon), and the rods predominate; it seems, however, probable that the highest acuity of vision requires the presence of cones. The entire number of cones in the human retina has been computed at something over three and one-half millions (Salzer), while the rods are supposed to aggregate one hundred and thirty millions. The pigment-layer represents the outer lamina of the embry- onal optic vesicle, and consists of a single layer of polyhedral epi- thelial cells containing pigment-granules in varying amount. These cells (12-18 ix) are usually six-sided, but may have fewer or more borders; the cells in the vicinity of the ora serrata are of exception- ally large size and dark color. The elements of the pigment-layer exhibit a differentiation into an outer zone next the choroid, free from pigment and containing an oval nucleus, and an inner zone loaded with pigment-granules. The inner part of the pigment-cells includes protoplasmic pro- cesses directed towards the layer of neuro-epithelium, between the rods and the cones of which they extend for a variable distance ; the depth to which the pigment-granules penetrate along the pro- cesses between the cells depends upon the influence of light, since under strong illumination the granules wander along the protoplas- mic processes as far as the inner segment of the rods and the cones, while in eyes kept in the dark for some time before death the intercel- lular processes remain uninvaded. The structural details above described represent the construction of the retina throughout the greater part of its extent: two regions, however, present such marked variations from the typical arrange- ment as to call for brief special mention ; these are the macula lutea and the ora serrata. The macula lutea and the contained fovea centralis corre- spond to the posterior pole of the visual axis, and are distin- guished physiologically by the acuity of vision, which here attains its highest degree. The macula lutea is characterized, in addition to its yellow color, by a distinct thickening of certain of the retinal layers and by the absence of the rod-visual cells within its area. The distinctive color of the macula depends upon the presence of diffuse yellowish pigment within the layers internal to the visual cells, the latter elements remaining colorless; in consequence of this arrangement the fovea, in which the neuro-epithelium alone exists, is devoid of pigment, and therefore appears as a light spot within the colored area. THE EYE AND ITS APPENDAGES. 357 The increased thickness of the retina at the macular margin depends almost entirely upon the extraordinary development of the layer of ganglion-cells, which progresses until a stratum from seven to nine cells deep replaces the usual single row. The fovea, on the other hand, is produced by the hollowing out of the centre of the macula consequent upon the gradual thinning, to almost suspension, of the retinal layers lying internal to the Fig. 378. Diagrammatic section of the human fovea. Magnified 375 diameters. (Golding-Bird and Schafer.)—2, ganglion layer; 4, inner nuclear layer; 6, outer nuclear layer, the cone-fibres forming the so-called external fibrous layer of Henle; 7, cones; v, section of a blood-vessel; M, membrana limitans externa; og, ig, outer and inner granules (cone-nuclei and bipolars) at the centre. outer nuclear zone ; the centre of the foveal depression, the fun- dus foveae, consists chiefly of the neuro-epithelium. Within a central area the fovea is devoid of blood-vessels. The ora serrata marks the termination of the optical part of the retina and the transition into its anterior continuations, the pars ciliaris and the pars iridica. The ora is distinguished, in addition to its irregular serrated border, by the abrupt diminution in the thickness of the retina, brought about by the sudden termina- tion at this point of many of its layers. The regular diminution in the retinal thickness proceeds gradually from the fundus towards the periphery, when, on reaching a point near the ora serrata, many layers end abruptly, the ciliary continuation measuring only about one-third of the thickness of the adjacent retina. 358 NORMAL HISTOLOGY. The nerve-fibre and the ganglion-cell layer having already ended before reaching the ora, the sudden reduction is caused principally by the abrupt termination of the two reticular strata. The Fig. 379. Section of human retina through ora serrata : A, B, visual and ciliary portion of retina; a, vacuoles; b, robust fibres of Muller; c, remains of nuclear layers; d, termination of supporting fibres ; e, transformation of inner nuclear layer into colum- nar cells within continuation of pigment-layer. region of the ora serrata is also noteworthy on account of the re- markable development of the radial fibres of Muller, which here occur not only in unusual numbers but also of exceptional strength. Beyond the ora serrata the retinal laminae are continued as the pars ciliaris and the pars iridica retinae. These prolongations consist of an outer and an inner lamina. The outer layer is the direct and only slightly modified extension of the retinal pigment; the inner lamina, the attenuated representative of the remaining retinal layers, consists of a single row of slender colum- nar cells, which originate at the ora by the transformation of the elements of the inner nuclear layer. A delicate cuticle, the limitans interna, extends over the posterior surface of both the ciliary body and the iris ; this membrane is a true cuticular for- mation, and begins at the ora as a new structure not present within the optical part of the retina. THE OPTIC NERVE. The optic nerve corresponds to a highly-developed single fu- niculus, enveloped by stout connective-tissue sheaths, which are prolongations of the brain-membranes. Externally the optic nerve is invested by a robust fibrous membrane, the dural sheath, de- rived directly from the dura ; this covering extends the entire length of the nerve, and on the entrance of the latter into the eyeball be- comes continuous with the outer part of the sclera. The surface of the optic nerve is closely invested with the pial sheath, an extension of the pia, while between the latter and the dural covering lies a delicate partition from the arachnoid, constituting the arach- noidean sheath. The clefts included between these sheaths con- THE EYE AND ITS APPENDAGES. 359 stitute the subdural and the subarachnoidean lymph-spaces of the optic nerve, which communicate with the corresponding inter- cranial cavities. On reaching the eye- ball the tissue of the dural sheath passes uninterruptedly into the outer two-thirds of the sclera ; the greater part of the pial sheath blends with the inner third of the sclera, some of its fibres, however, joining the choroid. The arachnoidean sheath unites with the dural, in consequence of which arrangement the subdural and sub- arachnoidal spaces be- c o m e continuous at their ocular extremities. The trunk of the optic nerve, about 3 mm. in diameter, consists of a great number (almost 800) of bundles of medullated nerve-fibres separated by intervening fibrous partitions, offshoots from the pial sheath. Each bundle is composed of small medullated fibres (2 fi), which are without neu- rilemma. On reaching a level corre- sponding with the confluence of the sheaths of the nerve with the sclera, the optic fibres pass through the sieve - like lamina cribrosa and lose their medullary coat, contin- uing to their retinal distribution as naked axis - cylinders. Occasionally the medullated fibres retain their medullary substance after their passage through the lamina cribrosa, such conditions presenting very striking ophthalmoscopic appearances. Fig. 380. Transverse section of human optic nerve : d, dural sheath : a, arachnoidean sheath; /, pial sheath; rt, bundles of nerve- fibres separated by fibrous septa (e). Fig. 381. Section of human optic nerve under higher magnu fication : b, bundles of nerve-fibres enveloped in con- nective-tissue sheaths (x); n, neuroglia nuclei; x, nuclei of interfascicular connective tissue (*); v, blood- vessels. 360 NORMAL HISTOLOGY. The lamina cribrosa consists of five to eight lamellae, composed of transversely extending fibrous trabeculae, the direct pro- longations of the scleral tissue. These bands bridge across what otherwise would be a canal, and unite in such manner that the openings Occupied by the nerve-bundles present less area than the intervening fibrous tissue. The fibrous lamellae, additionally con- nected with one another by vertical bands, pass from the margins of the scleral ring to the connective tissue supporting the blood-ves- sels within the optic nerve. The lamina cribrosa marks the nar- Fig. 382. Longitudinal section through optic entrance of human eye : a, a, bundles of optic fibres, which spread over retina at a', a'; b, layers of retina terminating at edge of optic papilla ; c, choroid ; d, sclera, continued across optic nerve as lamina cribrosa ; e,g, i, respectively pial, arachnoidean, and dural sheaths, including subdural and subarachnoidean lymph-spaces ; /, l', retinal vessels cut longi- tudinally. rowest diameter of the optic nerve, the loss of the medullary substance, together with the decrease in the neuroglia, reducing the size of the nerve about one-half. On arriving at the margin of the optic papilla, the bundles of nerve-fibres bend over its edges, con- stituting a thick layer, which rapidly thins away during its radial distribution over the retinal area. The centre of the optic papilla not infrequently presents a funnel- shaped depression, at the bottom of which the retinal vessels enter; this depression, variable in size and form, but always retain- ing sloping walls, is known as the physiological excavation, as distinguished from those possessing the vertical or overhanging walls indicative of grave pathological change. At some distance (15-20 mm.) from the eyeball the retinal blood- vessels pierce the exterior of the optic nerve to take up a central THE EYE AND ITS APPENDAGES. 361 position, surrounded by connective tissue, which they maintain until their final branching on the papilla. The blood-vessels of the retina constitute an independent sys- tem composed of end-arteries ; the only communication between the retinal and ciliary vessels is established within the sclera, close to the optic nerve, by means of minute scleral and choroidal branches. The larger retinal vessels are situated within the inner part of the fibre-layer and supply twigs to the cerebral division alone, the epithelial portion being non-vascular and deriving its nutrition from the adjacent choriocapillaris. The capillaries are arranged as two net-works, an inner and an outer. The inner net-work lies within the fibre-layer, is wide- meshed and derived directly from the division of the retinal vessels; the outer net-work, situated within the inner nuclear layer, is dependent upon the former, since its capillaries are derived from the branches given off from the inner vascular reticulum. The retinal arteries and veins are surrounded by adventitious sheaths, the spaces included between these sheaths and the walls of the vessels constituting perivascular lymph-clefts. The crystalline lens comprises two genetically distinct portions, the lens-substance and the lens-capsule. The lens-substance consists of the epithelium of the lens and the lens-fibres—both epithelial structures directly derived from the invaginated ectoderm. The epithelium of the lens, the representative of the anterior wall of the primary lens-vesicle, consists of a single layer of low polyhedral cells, about 20 /-* in diameter, whose granular proto- plasm contains an oval nucleus, also often vacuoles. These cells lie immediately beneath the anterior capsule and extend backward as far as the equator, at which point the epithelial cells are trans- formed into the lens-fibres. A thin subcapsular stratum of albuminous substance exists as a connecting medium between the epithelium and the capsule, the same substance being continued be- tween the posterior lens-capsule and the lens-fibres behind. Be- neath the epithelium a subepithelial stratum of somewhat simi- lar albuminous substance unites the epithelium and the lens-fibres and occupies the cleft representing the remains of the original cavity of the lens-vesicle; sometimes a few drops of fluid—the liquor Morgagni—occupy this subepithelial stratum. The lens-fibres are greatly elongated modified epithelial cells, whose ancestors constituted the posterior wall of the lens-sac, but whose more recently formed fellows result from the transforma- THE CRYSTALLINE LENS. 262 NORMAL HISTOLOGY. tion of the peripherally situated anterior epithelium at the equator. They are elongated compressed six-sided prisms varying in size with their position ; those at the periphery of the lens are the largest (12 mm. in length by 10-12 m in breadth), their size decreasing towards the centre. In the young lens all the fibres contain oval nuclei, but in the adult organ only those recently formed lying in the vicinity of Fig. 383. Fig. 384. Portions of human crystalline lens: A, section through periphery at equator; a, anterior capsule ; b, anterior epithelium converted into lens-fibres (l) at equator (2); n, nuclei of young lens-fibres. B, fragment of anterior capsule with adherent epi- thelium, viewed from under surface; h, capsule; e, epithelial cells. Fibres of human crystalline lens : A, portions of young isolated fibres; B, fibres in transverse section. the equator possess these. The fibres constituting the softer cortical zone have smooth straight contours, while those of the central part display a finely-serrated outline and are with- out nuclei. The lens-fibres are united by albuminous cement- substance, which, after suitable maceration, is dissolved, so that the fibres may be readily isolated ; since the amount of the cement-sub- stance is less between the broader than between the narrow surfaces of the fibres, after suitable maceration the lens evinces a disposition to separate into concentric lamellae, somewhat after the fashion of an onion. The apposition of the ends of the fibres takes place along definite lines which appear on the anterior and posterior surfaces of the lens as stellate figures, the lens-stars. In the simpler con- ditions of the new-born child, as well as in most mammalia, each star consists of three rays, one of which in the anterior star is directed upward, while the others are disposed at an angle of 120° down and outward ; in the posterior star the rays form an angle of 6o° with those of the anterior surface, so that the figures of both surfaces THE EYE AND ITS APPENDAGES. 363 together constitute a six-rayed star. In the adult lens, however, the typical arrangement of the rays is greatly complicated by the addition of secondary lines which obscure the figures. The capsule of the lens is a strong transparent elastic mem- brane completely enclosing the lens and, at the periphery, intimately uniting with the suspensory fibres of the zone of Zinn. The an- terior capsule covering the front lens-surface is thicker (11—15 //.) than the corresponding posterior capsule (5-7 //), the maximum thickness being at the centre of the anterior lens-surface and the minimum at its posterior pole. The zone of Zinn, zonula ciliaris, or suspensory ligament of the lens, is the radially plicated, modified anterior continuation of the hyaloid membrane of the vitreous body. At the ora ser- rata the hyaloid becomes intimately united to the posterior surface of the ciliary body as far as the ciliary processes, from whose sum- Fig. 385. Section through anterior segment of human eye, including cornea, sclera, iris, ciliary body, and lens : a, b, substantia propria of cornea (C) and of sclera (S); c, sclero-corneal juncture; d, conjunc- tival tissue; e, stroma of iris (I); f, connective tissue of ciliary processes (g); h, canal of Schlemm; k, trabeculae connecting sclera and ciliary body; l, section of blood-vessel; m, tt, o, meridional, radial, and circular fibres of ciliary muscle; p, continuation of hyaloid membrane into ligament (r) of lens (L); s, spaces of Fontana; t, muscular tissue of pupillary sphincter; u, pigment-layer mark- ing termination of retinal layers at pupil. mits thickened bands bridge across the intervening space and become attached principally to the anterior surface and to the periphery of the lens. Owing to the plication of the ciliary body over which the 264 NORMAL HISTOLOGY. hyaloid is reflected, its surface is marked by radiating folds, which at the edge of the ciliary processes become converted into the stiff fibres distinguishing the free part of the zonula. These fibres form two series, the one comprising the fibres springing from the sum- mit of the ciliary processes, the other consisting of those fibres which take their origin in the depressions between the ciliary pro- cesses ; the fibres extending from the valleys pass to the anterior surface of the lens, where they blend with the outer lamella of the anterior capsule, while those springing from the summits of the processes are inserted into the periphery and the immediately ad- joining parts of the posterior capsule. The narrow annular cleft, triangular in section, bounded in front by the zone of Zinn, mesially by the lens, and behind by the mem- brane of the vitreous body, constitutes the canal of Petit. Owing to the constrictions produced by the shorter bridging fibres, the canal presents a series of alternate constrictions and dilatations, which, on inflation, map out the position of the canal by a ring of bead-like enlargements. The vitreous body occupies the space between the lens in front and the retina behind ; it consists of the vitreous substance en- closed by the glassy hyaloid membrane, which in front, where it supports the lens within the patellar fossa on its anterior surface, comes in direct contact with the posterior capsule. The substance of the vitreous body is remarkable, in addition to its beautiful transparency, for its great fluidity, consisting of 98.6 per cent, of water, the remaining small portion being made up of solids, including its organized parts. Histologically, the adult vitreous substance corresponds to connective tissue containing an enormous watery infiltration whose fixed elements have undergone degener- ation. In its embryonal condition the vitreous body is composed of delicate gelatinous or mucoid mesodermic tissue containing numerous frail stellate cells. The formed elements of the vitreous are of two kinds, fibres and cells. The fibrous elements occur in the superficial part of the vitreous, in the vicinity of the ora serrata, as fibrillae of extreme delicacy, which take part in the formation of the zone of Zinn. Other fibrous structures are present as the remains of the minute blood-vessels permeating the vitreous in its embryonal condition. The cells of the vitreous body belong to the category of wan- dering corpuscles or leucocytes, the fixed connective-tissue cells being wanting in the matured organ. In the central part of the vitreous body, the central or hyaloid THE VITREOUS BODY. THE EYE AND ITS APPENDAGES. 365 canal extends from the optic papilla to the vicinity of the posterior lens-capsule ; during foetal life it transmits the hyaloid artery, and afterwards contains the remains of the supporting connective tissue, and, rarely, the atrophic artery itself. The canal is defined by a thin membranous wall, the continuation of the hyaloid membrane. The existence of other additional small lymphatic spaces has been demonstrated within the periphery of the vitreous body. The minute arrangement and ultimate distribution of the blood- vessels in the various parts of the eye have already been described in connection with the individual structures ; it here remains to out- line briefly the general relations of the larger trunks. The blood-vessels of the eyeball belong to two distinct systems, the retinal and the ciliary, which are connected by meagre anasto- moses only around the optic nerve entrance, otherwise they remain entirely separate. The retinal system is formed by the ramifications of the reti- nal artery and vein, which constitute the permanent circulation within the nervous layer. During foetal life an additional transient supply, represented by the hyaloid artery, is distributed to embryo- nal structures which disappear before birth. The ciliary system consists of the ramifications of the short, the long, and the anterior ciliary arteries and their complementary veins, and furnishes the blood-supply to the bulbar conjunctiva, the sclera, the choroid, the ciliary body, and the iris, and indirectly aids in maintaining the nutrition of the cornea, the lens, and the epithelial division of the retina. The short ciliary arteries supply principally the choroid, and form the choriocapillaris, at the same time giving off twigs, before piercing the sclerotic coat, to the posterior segment of the sclera and to the dural sheath of the optic nerve. The long ciliary arteries pierce the sclera and pass in the horizontal meridian between the scleral and choroid coats as far forward as the ciliary body, in which they form the larger arterial circle of the iris ; additional recurrent twigs are given off to the choroid and the ciliary muscle. The larger arterial circle sends branches to the ciliary processes and to the iris, as well as a few twigs to the choroid. The anterior ciliary arteries pass to the anterior segment of the ball, and pierce the sclera near the corneal margin to gain access to the ciliary muscle behind the canal of Schlemm. Before entering the eyeball they send branches to the anterior segment of the sclera, to the scleral conjunctiva, and to the corneal limbus. From the branches which pierce the eyeball twigs communicate with the larger arterial circle of the iris, and supply the ciliary muscle and the fore part of the choroid. 366 NORMAL HISTOLOGY. The venous vessels of the eyeball culminate in two principal sets, the posterior and anterior ciliary veins. The former, or the venae vorticosae, collect the blood from the iris, the ciliary processes, part of the ciliary muscle, and the choroid, and on emerging from the sclera receive also the episcleral veins ; they, therefore, drain the entire territory supplied by the ciliary arteries, except a part of the region nourished by the anterior ciliary arteries. The lymphatics of the eyeball constitute the anterior and pos- terior lymph-tracts, which do not comprise definite lymphatic vessels, but a series of intercommunicating lymph-spaces varying in size from the microscopic tissue-spaces to the anterior chamber. The anterior lymph-tract includes : 1. The systems of the lymph-spaces within the cornea and the sclera. 2. The anterior chamber of the eye, containing the aqueous humor, which possesses in small number the usual histological ele- ments of lymphatic fluid, the leucocytes. The anterior chamber com- municates with the posterior chamber through the cleft between the iris and the lens, and indirectly, by means of the spaces of Fontana, with the canal of Schlemm. 3. The canal of Petit, connected by means of the interfascicular clefts with the posterior chamber, and thus indirectly with the ante- rior, these three spaces standing in close relation. The posterior lymph-tract includes two groups, the lymphatics of the retina and of the vitreous body and those of the pericho- roidal space. The constituents of the first group are : 1. The hyaloid canal of the vitreous, which empties into the lymph-clefts of the optic nerve. 2. The perivascular lymph-channels surrounding the retinal vessels, which likewise pour their contents into the lymph-spaces of the nerve. 3. The lymph-clefts of the optic nerve, terminating within the subarachnoidean space of its sheaths. The perichoroidal space, lying between the scleral and the choroid coat, drains the choroid and communicates with the sac enclosed by Tenon’s capsule ; the perivascular lymphatics sur- rounding the venae vorticosae lead from the perichoroidal cleft into Tenon’s space, from which channels connect with the supra-vaginal space, embracing the optic nerve ; finally, communications exist between this space and the great intercranial lymphatic cavities. Connections between the lymph-clefts of the optic nerve and the perichoroidal space probably also exist in the vicinity of the optic entrance. THE EYE AND ITS APPENDAGES. 367 The nervous supply of the several parts of the eye has already been considered in detail; it remains to add a short description of their general relations. The long and short ciliary nerves pierce the sclerotic coat in the vicinity of the optic nerve and pass between the sclera and the choroid, giving off branches for the supply of the latter, and unite to form the ciliary ganglionic plexus on the outer part of the ciliary body. From this plexus twigs pass to the tissues of the ciliary muscle, the iris, and the cornea, to be distributed in the manner already described. THE APPENDAGES OF THE EYE. THE EYELIDS. The eyelids are protecting folds which include between their tegumental and mucous surfaces connective tissue, muscular and glandular structures. The constituents of the eyelids are arranged as general layers from without inward, these being : (i) the integu- ment and subcutaneous tissue, (2) the muscular layer, (3) the median connective tissue, (4) the tarsal plate, and (5) the con- junctiva. The skin covering the external surface of the eyelid is thin, thrown into folds, and beset with fine hairs and small sweat-glands ; the corium possesses slightly-developed papillae, except at the edge of the lid, where the fibrous tissue is denser and displays more conspic- uous elevations. The constant occurrence of pigment-cells within the corium is a noteworthy peculiarity. The loose subcutaneous tissue is rich in elastic fibres, but fat is wanting, or, if present, is found only in meagre amount. At the outer border of the margin of the lid large stiff hairs, the cilia, ex- tend obliquely outward ; they are arranged as two or three rows, their hair-follicles extending deeply into the corium and being sup- plied with small sebaceous glands. The life of the cilia is short, being about four months in duration ; as a result, hairs in all stages of growth are usually included among the eyelashes. The muscular bundles of the orbicularis palpebrarum constitute the layer next the subcutaneous tissue. At the lower margin of the lid the muscle-bundles are divided by the outer structures occupying this region ; an especially robust bundle separated by the lashes lies near the posterior margin of the lid-edge and constitutes the ciliary or marginal muscle of the lid. The succeeding connective-tissue layer is composed largely of the fibrous extensions of the tendon of the levator palpebrae, which are partly inserted into the areolar tissue—fascia palpe- 368 NORMAL HISTOLOGY. bralis—and partly attached to the upper edge of the tarsus; the tarsal portion contains bundles of non-striped muscle, which col- lectively form the lid-muscle of Muller. The tarsus consists of a semilunar plate of dense fibrous tissue Fig. 386. Section of human eyelid : a, a, skin; b, subcutaneous tissue; c, cilium ; d, median connective tissue; e, tarsal plate containing Meibomian glands (h); f, tunica propria of conjunctiva covered by its epithelium (g); i, duct of Meibomian glands ; j, Moll’s glands; m, m, cut fibres of orbicular muscle ; m', marginal bundle of same; n, sections of sweat-glands ; o, hairs ; t, anterior boundary of tarsus. lying immediately in front of the conjunctiva, and extending as a firm but elastic lamina from the sharply-defined palpebral border deeply into the substance of the lid. The tarsus is composed of closely-felted bundles of dense fibrous tissue, whose tough THE EYE AND ITS APPENDAGES. 369 resistant mass gives form and support to the softer tissues of the lids and partly covers the Meibomian glands embedded within its sub- stance. The Meibomian or tarsal glands constitute a series of about thirty elongated tubulo-acinous structures embedded within the substance of the tarsal plate, nearer the anterior than the poste- rior surface. Each gland consists of a long vertical duct, whose general course is perpendicular to the margin of the lid ; into this canal numerous short lateral tubular acini open. Since the ex- tremities of the glands occupy the outer arched border of the tarsus, these structures are longest in the middle of the lid and progressively shorten towards either end. The ducts open on the straight pal- pebral border as a row of minute orifices situated parallel to, but at some little distance from, the sharply-defined inner palpebral border. In their histology the Meibomian glands so closely re- semble the sebaceous follicles of the skin that they must be re- garded as modifications of these structures ; their secretion consists of a fatty substance similar to the sebum lubricating the integu- ment. The ducts of these glands, about. i mm. in diameter, are lined by an epithelium possessing the character of the surrounding epi- dermis, while the acini (.08-. 15 mm.) contain several layers of poly- hedral cells, most of which are in various stages of fatty degen- eration. In the upper part of the tarsus, especially in the nasal half, additional branched tubular glands lie partially surrounded by the fibrous tissue ; these structures correspond in composition to the tear-glands, and are known as the accessory lachrymal glands. The conjunctiva constitutes the innermost layer and surface of the lid, being continuous at the base of the lid with the bulbar con- junctiva and at its palpebral border with the integument. The con- junctiva consists of the epithelium covering the free surface and the connective-tissue matrix, or tunica propria. The epithe- lium covering the inner surface of the lid is stratified columnar; at the margin of the lid the columnar epithelium passes over into the squamous cells of the epidermis. The surface of the conjunctiva covering the tarsal plates is smooth, but beyond its epithelium forms irregular pockets, which in section somewhat resemble glands. Numerous lymphoid cells within the reticulated tunica propria, in certain localities, strongly suggest the presence of diffuse aden- oid tissue; the amount of such lymphoid tissue is subject to much individual variation ; it is, however, usually best marked in the retrotarsal portions of the conjunctiva. Circumscribed lymph- follicles are occasionally observed, although these structures are less constant in man than in many of the lower animals—dog, cat, sheep, or ox. 370 NORMAL HISTOLOGY. Additional minute lymphoid nodules and mucous glands occur within the conjunctival fornix. The ocular conjunctiva pre- sents no marked differences until near the corneal margin, where the epithelium loses its columnar character and assumes the stratified squamous type in its reflection over the cornea. The edge of the lid presents two borders, the outer, rounded off and tegumental in character, and the inner, distinguished by its sharply-defined margin and dense fibrous structure. In addition to the orifices of the Meibomian glands, the palpebral border is pene- trated by the ducts of the glands of Moll, structures properly regarded as modified sweat-glands. The vertical fold of conjunctiva occupying the inner canthus, the plica semilunaris, represents the third eyelid, or membrana nictitans, of the lower animals. In exceptional cases the base of the fold contains a minute plate of hyaline cartilage ; a small race- mose gland, the homologue of the Harderian gland, is also some- times present at the base of the semilunar fold. The lachrymal caruncle within the inner canthus is an isolated and modified island of skin, possessing an epithelium, a corium, and subcutaneous tissue similar to the adjacent integument; the epithelium, however, is without the stratum corneum. The caruncle contains adipose tissue, fine hairs with relatively large hair-follicles, and modified sweat-glands closely resembling the glands of Moll. A small amount of involuntary muscle usually exists in the car- uncle, and sometimes a few additional fibres of striped muscle. The blood-vessels of the eyelids pass from the outer and inner angles towards the centre of the lid, forming an arch, the arcus tarseus, along the edge of the lid, and a second anastomosis, the arcus tarseus externus, at the upper margin of the tarsal plate ; from these arterial bows smaller twigs are given off, which, in addition to supplying the integument, the Meibomian glands, and the glands of Moll, form the conjunctival capillary net-work ; additional branches pass to the fornix conjunctivae and to the conjunctiva of the eyeball. The lymphatics of the lid are arranged as two sets : the close- meshed conjunctival net-work within the tarsal mucous membrane, and the wide-meshed peritarsal net-work on the front of the tarsus at its upper border. The first set include the lymphatics running near the palpebral border, as well as the narrow channels surrounding the Meibomian glands. The conjunctival lymph-vessels communi- cate with the peritarsal net-work by means of the coarse reticulum within the tarsus surrounding its glands, as well as by direct connec- tions established by the twigs which pierce the tarsus to join the net-work within the conjunctiva. The peritarsal lymphatics possess valves. THE EYE AND ITS APPENDAGES. 371 The nerves of the eyelids form the rich marginal plexus close to the palpebral border ; the trunks taking part in the formation of this plexus before their union give off branches to the orbicular muscle and the skin, as well as additional twigs for the supply of the conjunctiva. From the plexus itself fibres are distributed to the hair-follicles of the cilia, the Meibomian glands, the tarsal conjunc- tiva, and the tissues of the edge of the lid. The ultimate nervous distribution includes the formation of subepithelial net-works of fine non-medullated fibres, together with the special endings, the spherical end-bulbs, occurring within the bulbar conjunctiva. THE LACHRYMAL APPARATUS. The lachrymal apparatus includes the lachrymal gland and the system of canals carrying off the fluid secreted under usual con- ditions. The lachrymal gland represents the serous racemose type, closely resembling the true salivary glands in structure ; the organ differs from the usual racemose gland in the independent course and the number of its ducts, of which about a dozen are usually present. It appears, therefore, more accurate to regard the lachry- mal gland as a group of closely-placed small individual racemose glands rather than as a single organ. The ducts of the lachrymal gland are lined by simple columnar epithelium. The structure of the acini and the relations of their groups corre- spond to those of the serous salivary glands, the secreting cells possessing similar spherical forms and granular pro- toplasm. The blood-vessels of the lachrymal gland form the usual capillary net-works supplying the acini and their secreting cells. The nerves distributed to the glandu- lar tissue pass between the acini and form net-works beneath the basement-mem- brane ; their ultimate relations to the secreting cells are uncertain. The lachrymal canals or canaliculi consist of three coats—the epithelium, the tunica propria, and the muscular tissue. The epithelium is stratified squamous, and forms a layer about .12 mm. in thickness, in which the deepest cells are columnar and the superficial greatly flattened. The tunica propria is composed of bundles of fibrous tissue among which lie especially rich circularly- disposed elastic net-works. Outside the tunica propria the lachry- Fig. 387. Section of human lachrymal gland: a, acini, limited by basement-mem- branes (m) and lined by secreting cells (g); i, interacinous connective tissue. 372 NORMAL HISTOLOGY. mal canals are surrounded by a layer of striped muscle derived from that part of the orbicularis known as Horner’s muscle ; this tissue is arranged as small bundles, which possess a general longitu- dinal course parallel with the axis of the greater part of the lachry- mal canals. The vertical papillary division of the tube, however, lies at right angles to the muscle-bundles, which, consequently, seem to enclose this part of the canal within circular or sphincter fibres ; some of these occupy the edge of the lid and surround the puncta with muscular loops. The mucous membrane of the lachrymal sac and of the naso- lachrymal duct is connected with the periosteum of the neighbor- ing bony surfaces by loose areolar tissue, within which is lodged a rich venous plexus. The mucous membrane of the lachrymal sac and of the duct partakes largely of the nature of lymphoid tissue, consisting of a connective-tissue reticulum infiltrated with lymphoid cells. From the tear-sac to the nasal termination of the duct the lining epithe- lium is stratified columnar in character, with the occasional pres- ence of cilia within the lower part of the tube. The eyeball is separated from the surrounding structures within the orbit by the intervention of a fibro-elastic membrane or fascia, the capsule of Tenon, covered by a continuous layer of endothe- lial plates ; the enclosed episcleral space, or space of Tenon, communicates with the perichoroidal space on the one hand and with the supra-vaginal cleft on the other. In effect, the capsule of Tenon corresponds to a synovial sac, whose lubricated surfaces of contact facilitate the movements of the eyeball. DEVELOPMENT OF THE EYE. The earliest indication of the visual organ is the optic vesicle, a large diverticulum extending on either side from the primary anterior brain-vesicle, and later becoming connected by a constricted stalk with the interbrain, or thalamencephalon. In the early stage the optic vesicle lies in contact with the ectoderm reflected over the prominently protruding optic diverticulum, the sur- rounding mesoderm at first showing no differentiation. Shortly after the optic vesicle has reached the surface ectoderm the latter exhibits proliferation and thickening opposite the external pole of the vesicle. This ectodermic area, the earliest trace of the future crystalline lens, soon becomes depressed, the invagination progressing until the pit- and the cup-stage give place to the closed vesicle, which finally separates from the ectoderm and lies beneath the surface as the lens-sac. Simultaneously with the progress of these changes in the ectoderm, THE EYE AND ITS APPENDAGES. 373 the anterior segment of the primary optic vesicle undergoes an important invagination, whereby the front wall of the sac is pushed into the cavity of the vesicle until eventually the anterior and posterior walls are in apposition and the included cavity is largely obliterated. The new space within the indented anterior walls of the sac constitutes the second- ary optic vesicle and corre- sponds to the later vitreous chamber. These important changes probably are not en- tirely attributable to the me- chanical influence exerted by the developing lens-sac on the closely-applied optic vesicle, but must be referred also to deeply-lying formative forces. The invagination of the optic vesicle is not confined to the anterior pole, but takes place likewise along the under side of the sac as well as along the optic stalk ; in consequence the vesicle is imperfectly closed below, the cleft, or choroidal fissure, thus established affording an entrance for the surrounding mesodermic tissue which takes part in the production of the primary vascular structures oc- cupying the vitreous chamber. The relations of the parts to the fissure are well shown in frontal sections, where the cleft appears as a conspicuous break in the continuity of the walls of the vesicle. The Retina. The layers of the optic vesicle very soon ex- hibit marked difference in their rate of growth, since the an- terior depressed lamina rapidly overshadows the posterior layer by. its much greater thickness and more active proliferation. The posterior wall becomes reduced in thickness, owing to the increase in the size Fig. 388. Fig. 389. Section through head of ten-day rabbit em- bryo, exhibiting primary optic vesicle (O) pro- truding from fore-brain (B) and coming in con- tact with surface ecto- derm (e) : m, surround- ing mesoderm. Section through develop- ing eye of eleven-day rab- bit embryo : B, fore-brain connected by stalk with optic vesicle (o), whose anterior wall is partly in- vaginated; /, thickened and depressed lens-area. Fig. 390. Sagittal section through developing eye of eleven- and-a-half day rabbit embryo, exhibiting choroidal fissure (C) through which mesodermic tissue (m) reaches interior of secondary optic cup: o, i, outer and inner layers of optic vesicle ; l, lens-sac. 374 NORMAL HISTOLOGY. of the sac, and later is distinguished by the appearance of deeply- pigmented granules, which mark the beginning of the pigment- layer of the retina, to the formation of which the posterior lamina of the optic vesicle is entirely devoted; the pigment is first seen in the vicinity of the lip of the cup, from which point the colored par- ticles spread towards the posterior pole. The invaginated anterior lamina becomes greatly thickened and differentiates into the remaining highly-specialized layers of the retina. The process by which these are formed corresponds in the main points with the differentiation of the nervous centres, the re- sulting tissues being of two kinds, the supporting neuroglia and the nervous elements. The retinal lamina early presents a narrow inner zone, dis- tinguished by its meagre nuclei as contrasted with the richly-nu- cleated broad outer division; this latter, next the pigmented lamina, with many strata of nuclei, differentiates into an outer layer characterized by small, deeply-staining nuclei, and an inner layer of larger elements. The outer layer subsequently divides into three strata, the outer nuclear, the outer reticular, and the inner nuclear, while the inner layer produces two zones, the inner reticular and the ganglion-cell. The rods and cones appear later as minute hemispherical eleva- tions on the outer surface of the external limiting membrane, and at first possess their inner segments alone, the outer members later growing out from the inner. At birth in many animals (as cats, rabbits, etc.) the rods and cones are wanting, and even in man they are rudimentary; the macula at birth is still undifferentiated. The nerve-fibres of the retina are derived probably from two sources, from the neuroblasts of the retina itself and from those of the interbrain. The hollow optic stalk becomes solid and con- verted into the primary optic nerve, which acquires its nerve-fibres from the ingrowing and outgrowing processes of the retinal and the cerebral elements. I he retinal blood-vessels develop within mesodermic tissue, which spreads over the inner surface of the nervous layer at a com- paratively late period ; the vessels first appear around the optic nerve and spread peripherally. They are not connected primarily with the central vessels of the retina, but with branches entering at the periphery of the nerve (O. Schultze). The crystalline lens proceeds from the ectodermal vesicle already noted. 1 he walls of this sac very early exhibit marked va- riation in thickness, the anterior lamina being relatively thin and composed of a single layer of cuboidal cells, which persist as the flattened polyhedral epithelium of the anterior lens-capsule. THE EYE AND ITS APPENDAGES. 375 T he posterior wall of the lens-sac plays the active role in the formation of the lens-substance, since the production of the lens- fibres is entirely due to the transformation of its greatly-elongated cells. After the obliteration of the original cavity of the sac has been completely effected by the apposition of the enormously-thickened posterior wall and the anterior lamella, the lens further increases in size by the addition of new fibres at the equator, where the metamorphosis of the epithelial elements into the lens-fibres is continually taking place. The anterior and posterior cap- sules of the lens are genetically dis- tinct from the lens-substance, since they are mesoblastic in origin ; for a time they are closely associated with the tran- sient lamellae of vascular mesodermic tissue which invest the surfaces of the lens and constitute the tunicse vas- culosae. The development of the fibrous tunic—the sclera and the cor- nea—proceeds from the surrounding mesoderm, which undergoes condensation immediately around the ectodermic structures representing the retina and the lens. The mesodermic tissue at the sides of the anterior segment grows be- tween the epidermis and the lens, and constitutes a layer of consid- erable thickness ; subsequently this sheet becomes unequally divided by the appearance of a cleft, the primary anterior chamber, into two laminae of unequal thickness ; of these the anterior and thicker becomes the cornea and the posterior and thinner the connective tissue of the iris and the transient vascular tunic of the lens. The mesodermic corneal stratum undergoes specialization into the substantia propria, the anterior and posterior limiting mem- branes, and the endothelium, the anterior epithelium alone being ectodermic. The choroid and the iris are closely associated in their origin with the mesodermic tract producing the fibrous tunic, the rich vascular net-works characterizing the choroid appearing relatively late. The iris does not grow forward until the anterior chamber begins to form, when it proceeds as a blunt continuation of the choroidal tract; while the stroma of the iris is contributed by the mesoderm, the pigment-layer is derived from the extension Fig. 391. Section through developing eye of eleven-and-a-half-day rabbit embryo : B, fore-brain connected with optic vesi- cle (o) nearly effaced by apposition of invaginated anterior segment (r) with posterior wall (p) ; l, lens-sac, com- pletely closed and separated from ecto- derm ; t, tissue within secondary optic cup derived from surrounding meso- derm (m). 376 NORMAL HISTOLOGY. of the rudimentary portions of the optic cup, whose double-layered lip corresponds in position with the pupillary margin. The vitreous humor is derived from the mesodermic tissue occu- pying the interior of the optic cup. This tissue appears very early, in consequence of the ingrowth of the mesoderm through the cho- roidal fissure ; the early vitreous possesses delicate branched cells as well as numerous blood-vessels, and corresponds to soft embry- onal connective tissue ; later the corpuscles and blood-vessels dis- appear and the mass assumes its characteristic semi - fluid almost structureless condition. The pe- ripheral zone of the vitreous un- dergoes condensation and forms the hyaloid membrane, which in the ciliary region becomes thick- ened and constitutes the suspen- sory ligament of the lens, or the zone of Zinn. The eyelids develop as folds of integument above and below the corneal area ; these grow towards one another and finally fuse, all epidermal demarcation for a time disappearing. Shortly b e fo r e birth the centre of the epithelial layer undergoes degeneration and the lids become permanently separated. 1 he epithelium of both the tegumentary and conjunctival sur- faces is derived from the ectoderm, as are also such epidermal appendages as the hairs and the glands, the Meibomian glands corresponding to sebaceous follicles in their formation. Fig. 392. Section through developing eye of thirteen- day rabbit embryo : e, ectoderm; l, lens, con- sisting of anterior nucleated division repre- senting thin front wall of lens-sac and greatly thickened posterior division, completely filling cavity of sac by elongated fibres whose nuclei present crescentic zone («); p, posterior pig- mented layer; r, specialized anterior retinal layer; i, point where layers of optic vesicle be- come continuous ; n, extreme peripheral section of tissue of primitive optic nerve connected with vascular tunic (v) occupying posterior surface of lens; m, surrounding mesoderm, which at t grows between lens and retina. THE ORGAN OF HEARING. 3 77 CHAPTER XVIII. THE ORGAN OF HEARING. The complicated organ of hearing of man and the higher animals, reduced to its essential factors, consists of two parts,—the system of intercommunicating epithelial tubes, certain parts of whose walls are differentiated into special structures for the perception of the sound- waves, and the elaborate conducting apparatus for the transmis- sion, direct and indirect, of the sound-impulses to the perceptive structures. THE EXTERNAL EAR. The external ear, including the pinna and the external audi- tory canal, possesses a bony or cartilaginous basis over which extend the integument and a layer of subcutaneous tissue. The cartilage is of the yellow, elastic variety, forming a thin, tough, yielding plate, displaying the various depressions and elevations seen on the outside ; the lobule, however, contains no cartilage, but only tough fibrous tissue and fat. The skin covering the pinna corresponds with the surrounding integument; within the auditory canal, however, it presents some change. The skin covering the cartilaginous division of the meatus, together with part of the roof of the bony division, is char- acterized by its thickness, the subcutaneous tissue also constituting a layer of considerable depth, which includes some fat and many bundles of dense fibrous tissue. Fine hairs, with relatively very large sebaceous glands, occur in all parts of this surface, as do also the ceruminous glands, which constitute conspicuous structures and closely correspond to the glands of Moll within the eyelid, being, like them, modified sweat-glands. Their long, narrow ducts during early life open with the sebaceous glands into the hair-follicles, but later acquire independent orifices. The ceruminous glands pos- sess a well-marked basement-membrane, within which lies a single layer of cuboidal epithelial cells, with a thin, longitudinal stratum of non-striped muscle-cells interposed. The secreting cells contain numerous brown particles, but the presence of fat is question- able, the fatty constituents of the cerumen being probably contributed by the adjoining sebaceous glands. The coiled masses of the gland- tubes are situated within the subcutaneous tissue, where they some- times reach as far as the cartilage or the bone. 378 NORMAL HISTOLOGY. The skin covering the greater part of the bony canal, on the contrary, is very thin and intimately united to the periosteum. Hairs and glands are want- ing in this part of the canal, as they are also in the integument reflected over the external sur- face of the tympanic membrane. The membrana tympani consists of three layers : (i) the central ground-stratum, or lam- ina propria, composed of fibrous connective tissue, (2) the cutaneous layer reflected over the external surface of the drum, and (3) the mucous layer covering the inner side of the membrane as the repre- sentative of the lining of the tympanic cavity. The tegumental layer con- sists of the usual epidermis and connective-tissue corium, the latter being only about half as thick as the epithelial layer. The central connective-tissue ground-plate, or lamina pro- pria, constitutes the fibrous basis of the tympanic membrane and represents its mesodermic portion. This layer consists of closely- felted bundles of fibrous tissue arranged as two strata, the outer or radial fibre-layer, composed of fibrous bundles, which in their general course radiate from the periphery of the tympanum towards the point of attachment of the head of the malleus, and the inner or circular fibre-layer, consisting of concentrically-disposed bundles, whose greatest development is at the periphery in the vicinity of the annular attachment of the membrana tympani. The mucous layer is a part of the general lining of the middle ear, and consists of a thin connective-tissue tunica propria or groundwork, composed of delicate bundles of fibro-elastic tissue, upon which rests the epithelium; the latter consists of a single layer of low cuboidal polyhedral cells without cilia. The blood-vessels supplying the tympanic membrane are derived from two sources, the one set proceeding from the branches of the external auditory canal to end in capillaries which ramify within the Fig. 393. Section of bony portion of human external audi- tory canal: s, cutaneous layer closely united with periosteal fibrous tissue ; o, osseous tissue of wall. (After Rudinger.) cutaneous layer, the other group coming from the vessels of the tympanic cavity to break up into the net-works distributed to the mucous layer. The lymphatics of the tympanum correspond in their arrange- ment with the principal strata of the membrane. In the corium of the skin-layer lies a close net-work of capillary lymphatics ; these increase in size towards the periphery, where they are collected into larger trunks, which in turn empty into the lymphatic channels of the THE ORGAN OF HEARING. 379 Fig. 394. Section through human malleus and tympanic membrane : i, bony tissue of manubrium, containing medullary canal (2) ; 3, hyaline cartilage of malleus; 4, 5, lamina propria of tympanic membrane attached to malleus ; 6, cutaneous layer; 7, mucous membrane covering hammer ; 8, blood-vessel; 9, fragment of fibro-cartilage. (After Rudinger.) external auditory canal. Within the mucous stratum a much less important lymphatic net-work exists, which communicates at the periphery with the lymphatics of the mucosa of the tympanic cavity. Suitable silver staining shows the existence of lymph-spaces in certain places, in both the fibrous layer and the mucous membrane. The nerves of the membrana tympani follow the blood-vessels in their distribution so far that they also comprise two sets destined for the cutaneous and mucous layers. The nerves of the cutaneous stratum, chiefly derived from the tympanic branch of the auriculo- temporal, pass behind the manubrium of the malleus to divide at the 28o NORMAL HISTOLOGY. lower third of the process into two terminal twigs. In addition to these central nerves, small stems enter the drum-membrane at various points at the periphery, both sets of twigs taking part in the formation of a wide-meshed ground-plexus. From the latter fine pale fibres pass to the blood-vessels which they surround, while other fibres extend to the superficial part of the layer, where, be- neath the epidermis, they constitute a subepithelial plexus. The nerves of the mucous layer originating in the tympanic plexus are largely distributed to the lymphatics as well as to blood-vessels ; an additional subepithelial plexus bears close relations to the epi- thelium ; a few fibres extend into the fibrous tissue of the lamina propria. THE MIDDLE EAR. The middle ear, the entodermic division of the auditory appa- ratus, comprises the tympanic cavity, with its extension into the mastoid cells, and the Eustachian tube, together with the series of minute ear-ossicles. The walls of the tympanic cavity consist of the surrounding bony structures with their periosteum, over which is reflected the mucous lining, indirectly continuous with that of the pharynx. The mucous membrane, closely united with the underlying periosteum, not only covers the inner surface of the membrana tympani, but is also reflected over the ear-bones and their ligaments as well as over the nerves and blood-vessels crossing the cavity. The mucosa con- sists of a thin fibrous tunica propria (50-60 n) which in places resembles the reticulum of adenoid tissue and includes leucocytes ; the mucous layer is intimately blended with the denser fibrous struct- ure of the periosteum. Connected with the trabeculae of the mu- cosa peculiar oval bodies are occasionally encountered, which are composed of an axial band and concentric lamellae of connective tissue ; these bodies are normal but probably not constant constit- uents of the middle ear. The epithelial lining (18-21 /*) of the tympanic cavity differs in character in the several regions'; over the ear-ossicles, the tympanic membrane, and the promontory, as well as within the mastoid cells, the epithelium consists of a single layer of low cuboidal po- lygonal cells without cilia ; over the remaining parts of the mid- dle ear a layer of ciliated columnar cells exists. In those places where nerve-trunks or blood-vessels are covered, the greatly-thick- ened mucosa forms local ridges, within which the trunks are en- closed. Small tubular glands, about .1 mm. in length, occur in the mucous membrane of the anterior part of the tympanic cavity ; they are sparingly distributed and subject to individual variation. The mucous lining of the antrum and the mastoid cells, clothed THE ORGAN OF HEARING. 381 by a single layer of low polyhedral cells, is very thin and inti- mately united with the delicate periosteal layer ; numerous fibres, trabeculae, or lamellae pass between neighboring surfaces and partially occlude the spaces within the bone, thereby reducing the lumina and still further adding to the complexity of the mastoid cells. The secondary tympanic membrane, closing the fenestra ro- tunda, consists of three layers, a central fibrous lamina propria, which is covered on the tympanic surface by a reflection of the mu- cous membrane, and on the other side by the extension of the lining of the vestibular perilymphatic space. The lamina propria, the unossified part of the wall of the labyrinth, is composed of ra- dially-disposed bundles of fibrous tissue passing from the indented point of its base towards the periphery. The mucous stratum is formed of a thin fibrous tunica propria invested by a single layer of flattened non-ciliated polyhedral epithelial cells, similar to those covering the neighboring promontorium. The innermost stratum of the membrane is composed of a thin layer of sub- endothelial fibrous tissue, over which extends the single layer of endothelial plates. The larger blood-vessels supplying the mucous lining of the tympanum lie within the deeper periosteal layer of the mucosa and give off smaller branches, which pass superficially to form a capillary net-work beneath the epithelium. The vessels distributed to the mucosa covering the promontorium are remarkable for the absence of anastomoses, the arteries dividing into twigs possessing rela- tively large lumina ; the terminal arterioles pass very rapidly into venous radicles, so that intervening capillaries scarcely exist, in places being entirely wanting. The lymphatics of the tympanic mucous membrane form a sys- tem of channels within the deep periosteal layer, where the lymph- vessels are supplemented by spherical enlargements and lateral dila- tations. The reticular connective tissue of the mucosa exhibits local accumulations of lymphoid cells, which strongly suggest the presence of lymphatic nodules. The principal nerves of the tympanum, derived from the tym- panic plexus, run within the periosteal layer of the mucosa, and are composed almost entirely of medullated fibres. From the deeper trunks fine twigs pass towards the surface and form a wide-meshed plexus, which contributes delicate bundles of pale non-medullated fibres to a subepithelial net-work. Along the course of the larger trunks and their immediate branches groups of ganglion-cells occur in exceptional cases, these being found in proximity with the epithelium. The ear-ossicles consist of compact bone, in which Haversian 282 NORMAL HISTOLOGY. canals and concentric lamellae are present in' the thicker parts, as the head and the base of the short process of the malleus. All surfaces of contact, including the articular facets, are invested by hyaline cartilage. The cavity of the ambo-malleal articulation is sub- divided by a minute intra-articular plate of fibrous cartilage. An investment of cartilage covers the malleus on all parts of the sur- face of its attachment to the tympanic membrane, the perichondrium becoming firmly united with the fibrous tissue of the lamina propria. The entire base of the stapes also is covered with a plate of car- tilage directly applied to the fenestra ovalis; the space intervening between the stapes and the margin of the oval window is occupied by the ring of fibrous tissue constituting the annular ligament. The Eustachian tube consists of two parts,—the supporting framework, composed partly of bone and partly of cartilage, and the mucous membrane. Neither the osseous nor the cartilaginous tissue of the canal constitutes a com- plete wall, since the tube is imperfect, being completed by the fibrous and other tissue which bridges the cleft left by the insufficient hard parts. Within the canal formed by the os- seous, cartilaginous, and fibrous tissues the soft tube of mucous membrane lies, its lower division supplemented by a stratum of submucous tissue, its upper part closely united with the periosteum of the bony walls. The epithelium lining the Eu- stachian tube is ciliated stratified columnar in type, the cells clothing the pharyngeal division of the tube being tall columnar elements, while those lining the upper bony part are low cuboidal, although ciliated, and resemble the epithelium of the tym- panum. The tunica propria presents a stratum of loose connective tissue, rich in cells and defined from the sub- mucous tissue by a denser layer; in many places the reticular connec- tive tissue is infiltrated with lymphoid cells and constitutes an adenoid structure. The profusion and distribution of this lymphoid Fig. 395. Section through oartilaginous portion of human Eustachian tube : i, bent plate of cartilage with its hook (i'); 2, fibrous tissue with fat (3); 4, tubo-pharyngeal fascia; 5, dilator tubse muscle ; 6, mucous membrane of tube with prominent fold (6') below; 7, mucous glands; 8, lumen of tube expanding above into so-called safety-tube (8'); 9, connective tissue uniting tube with base of skull; io, le- vator palati muscle. (After Testut.) THE ORGAN OF HEARING. 383 tissue vary greatly with age ; in early childhood it is present almost in all parts of the tube, but in adolescence it is plentiful only in the lower third, in the upper third being entirely wanting and in the middle third very sparingly distributed. Small mucous glands are also present, and open on the surface of the mucosa within the depressions between the longitudinal folds ; these glands may exist throughout the length of the tube, but they occur with constancy only towards its pharyngeal end. The submucous layer is well developed in the cartilaginous division of the tube, particularly in the outer membranous wall; it consists of loosely-arranged fibro-elastic tissue, which supports the mucous glands and the larger vessels and nerves, and often contains a considerable mass of fat. The blood-vessels supplying the tubal mucous membranes are derived from the pharynx and from the tympanum; the larger longi- tudinal stems run within the submucosa or the deep periosteal layers and send twigs into the mucosa to form capillary net-works. The nerves derived from the pharyngeal and tympanic plexuses occupy the deeper layers of the mucosa; the twigs given off from the larger trunks form a plexus within the superficial parts of the tunica propria, fine non-medullated fibrillae passing to the epithelial structures ; ganglion nerve-cells are found at the nodal points within the plexus. THE INTERNAL EAR. The internal ear in its fully-developed condition consists of two concentrically arranged parts, the bony and the membranous labyrinth, separated by an intervening space containing the peri- lymph. While the bony labyrinth in the cochlea and the semicircular canals quite closely repeats the general arrangement of the corre- sponding parts of the enclosed membranous tube, the central divis- ion of the osseous capsule, the bony vestibule, differs somewhat in its details from the enclosed membranous compartments. These are two almost completely separated vesicles of un- equal size, the anterior and smaller sacculus and the posterior and larger utriculus; the compartments communicate indirectly with each other by means of the ductus endolymphaticus, while the saccule connects additionally with the cochlear division of the membranous labyrinth through the narrow canalis reuniens, the utricle directly opening into the semicircular canals. The bony wall of the vestibule is lined by a very thin perios- teum, composed of a felt-work of resistant fibrous tissue, in which pigmented connective-tissue cells are not infrequent. From THE SACCULE AND THE UTRICLE. 284 NORMAL HISTOLOGY. this peripheral lamella trabeculae extend across the intervening perilymphatic space to the fibrous wall of the membranous laby- rinth. The endothelium of the inner surface of the periosteum invests the fibrous trabeculae as well as the outer or perilymphatic surface of the membranous labyrinth. The walls of the saccule and the utricle consist of the con- nective-tissue lamella, composed of the bundles of fibrous tissue and the delicate epithelium. At the positions where the filaments of the auditory nerve enter the maculae cribrosae and acusticae the fibrous stratum is best developed and densest, forming a layer . 15 to .20 mm. thick. Within other parts of the vestibule, especially in the roof of the utricle, the thickness of this layer may be reduced to 5-6 //. The lining of the saccule and the utricle consists everywhere, except at the maculae acusticae, of a single layer of thin flattened polyhedral cells. Over the regions receiving the terminations of Fig. 396. Section through membranous labyrinth of cat, showing specialized areas within ampulla (A) and utricle (B): a, surrounding bony wall separated from membranous tube (6) by layer of areolar trabeculae (d); c, crista acustica covered with specialized epithelium (_/"); e, e', bundles of ordinary epithelium; h, layer of otoliths overlying neuro-epithelium of macula acustica (/); i, blood-vessel; k, fibrous layer ; /, adipose tissue. the nerve-fibres, the maculae acusticae, on the contrary, the epi- thelium undergoes marked alteration, changing from the indifferent covering cells into the highly-specialized neuro-epithelium. At the margin of these areas the cells are at first cuboidal, then low THE ORGAN OF HEARING. 385 columnar, and progressively increase in length until they measure 30-35 ;i in contrast with their usual height of 3-4 [i. The character and arrangement of the cells of the acoustic areas in the saccule and the utricle are the same, including two kinds of elements, the sustentacular or fibre cells and the hair-cells. The sustentacular cells are elongated irregularly cylindrical, and extend the entire thickness of the epithelial layer to rest upon the well-developed basement-membrane by their expanded or divided basal processes. The oval nuclei are frequently wider than the average diameter of the cells, and produce corresponding enlarge- ments in the contour of the elements ; the nuclei occupy various levels within the inner half of the cells, but are never situated beyond ; the cell-contents appear faintly granular, and contain yellowish pigment-particles. The hair-cells are broader but shorter than the sustentacular ele- ments, and reach from the surface only to about the middle of the epithelium, where they terminate in rounded margins ; these cells possess large spherical nuclei, which usually lie well towards the slightly-expanded inner ends. The protoplasm of the hair-cells is granular, and contains yellow pigment; the outer part, next the free surface, exhibits a differentiation into a cuticular zone, cov- ering the outer ends of the cells. From the free border of each cell a seemingly single stiff robust hair (20-25 /1 long) projects into the •endolymph ; this conical process, how- ever, is resolvable into a number of agglutinated finer hairs or rods. The .free surface of the neuro- •epithelium within the saccule and the utricle is covered by a remark- able structure, the so-called otolith membrane. This consists of num- berless small crystalline bodies, the otoliths, or ear-stones, embedded within a soft gelatinous ground-sub- stance. The otoliths are minute crystals of calcium carbonate, 1-15 in length, usually six-sided prisms with slightly-rounded angles. The nerve-fibres proceed to the acoustic areas and unite beneath the epithelial layer in a plexus, from which fine bundles of fibres pass towards the surface ; the nerve-fibres usually lose their medullary substance in their transit through the base- ment-membrane and enter the epithelium as naked axis-cylinders. Fig. 397. Section of wall of utricle through mac- ular region, from rabbit, showing otoliths (o) embedded within granular substance (g): h, hair-cells with processes (/) ex- tending between sustentacular elements (s); n, nerve-fibres within fibrous tissue (/) passing towards hair-cells and be- coming non-medullated at basement- membrane (m). 2§5 NORMAL HISTOLOGY. After ascending about half-way to the free surface the fibres break up into their fibrillse, many of which are distributed to the hair-cells, with which they probably stand in close relation, while others pass as free axis-cylinders between the epithelial elements to a higher level. The blood-vessels of the saccule and the utricle form a wide- meshed capillary net-work within the fibrous wall of the membranous sacs, the vascular supply being especially rich within the maculae acusticae. The inner surface of the bony capsule constituting this part of the osseous labyrinth is lined by a thin periosteum similar to that of the vestibule already described. Along the line of attachment of the membranous canal this layer sends off numerous connecting bundles of fibrous tissue ; in other parts of the circumference of the canal only widely- separated, occasional trabeculae bridge across the perilymphatic space to aid in maintaining the position of the membranous tube. The inner surface of the periosteum, the tra- beculae, and the outer face of the fibrous tunic of the membranous canals are invested by the endothe- lium which forms the immediate lining of the perilymphatic space. The walls of the membranous semicircular canals closely re- semble those of the saccule and the utricle, being made up of an outer fibrous lamella and an inner epi- thelial lining. The fibrous coat is further differentiated into an external layer of felted connective- tissue bundles, containing many cells, and an inner, more compact, almost homogeneous layer, which corresponds to a highly-developed basement-membrane. The epithelium of the semicircular canals, supported by the outer fibrous coat, consists throughout the greater part of its extent of a single layer of flattened polyhedral cells (12-18 /j.) similar to those lining the saccule and the utricle. The areas receiving the terminal filaments of the auditory nerve, the cristae acusticae, are distinguished by specialization of the THE SEMICIRCULAR CANALS Fig. 398. Section of wall of cat’s semicircular canal: a, epithelial lining of canal; b, basement-membrane; c, fibrous tunic united with osseous lamella (_/■) by tra- beculae (d, d); e, blood-vessel. THE ORGAN OF HEARING. epithelium to constitute the perceptive apparatus of the semicir- cular canals. These specialized areas are limited to the floor of the ampullae, in which position the fibrous wall of the canal is distinguished by a local thickening forming the transverse ridge, or septum transversum. On approaching the base of the crest the epi- thelial cells become more columnar, being much taller and narrower than those of the general sur- face. The specialized cells crowning the sum- mits of the cristae acusticae, like other examples of neuro-epithelium, consist of elements of two kinds, the sustentacular or fibre cells and the hair-cells. The sustentacular elements resemble those of the maculae of the saccule and the utricle, extending the entire thickness of the epithelial layer and presenting an elongated narrow irregular cylindrical body, with prominent projecting oval nucleus. The hair-cells, or auditory cells, reach only part-way to the basement-membrane and bear on their free surfaces enormously long hair-processes, the auditory hairs, which project at least as far as the middle of the lumen of the ampulla. The auditory hairs spring from the ends of the cells by minute conical expansions, and under high amplification are resolvable into a number of finer com- ponent hairs. The nuclei of the auditory cells usually lie within dilated rounded inner extremities of the cells, with which also the terminations of the auditory nerve come in close relations. In sections of the ampullae in tissue preserved with approved reagents the inner free ends of the auditory hairs are embedded within a peculiar dome-like structure, the cupola, regarding whose nature, and even existence during life, opinions greatly differ. As usually seen in well-preserved tissue, this structure appears as a faintly-striated cuticular formation covering in the ends of the hair-processes. The fibres of the auditory nerve pass into the septum trans- versum, where they unite into net-works from which finer diverging fibrillae pass into the overlying epithelium after losing their medul- lary substance. Small groups of naked axis-cylinders extend be- tween the epithelial cells and separate into the individual fibrillae, some of which are applied to the bases of the hair-cells, while others apparently seek their ultimate distribution at higher levels. The blood-vessels supplying the semicircular canals comprise those destined for the bony capsule and those distributed to the Fig. 399. Surface view of mem- branous semicircular canal of cat: x, fibrous tissue supporting single layer of polyhedral epi- thelial cells (y). 388 NORMAL HISTOLOGY. membranous structures. These vessels form a wide-meshed capillary net-work within the fibrous tunic of the canals and the ampullae, which supplies both the endo- and perilymphatic surfaces. THE COCHLEA. The cochlea consists of the tapering bony tube wound spirally about its axis and containing the highly-specialized but much smaller epithelial canal, the ductus cochlearis. This latter tube, Fig. 400. Longitudinal section of cochlea of guinea-pig: a, bony capsule; b, central shaft or modiolus ; c, lamina spiralis; d, canal of modiolus containing bundles of nerve- fibres (e); /, terminal bundles ; g, basilar membrane ; h, spiral ligament; i, limbus ; j, membrane of Reissner ; l, Corti’s organ ; m, spiral ganglion ; n, blood-vessels ; V, T, M, respectively scala vestibuli, tympani, and media. triangular in transverse section, is attached along its base to the outer wall of the bony tube, and along its narrow opposite border to the projecting osseous spiral lamina ; in consequence of this arrange- THE ORGAN OF HEARING. 389 ment the perilymphatic space, instead of constituting a single cavity in which the epithelial tube is suspended, is divided into the scala vestibuli above and the scala tympani below, which com- municate respectively with the vestibule and the tympanum. The ductus cochlearis, or scala media, consists, like other por- tions of the membranous labyrinth, of the epithelial tube, the oldest part of the cochlea representing the specialized outgrowth from the primary ectodermic otic vesicle, and the supporting fibrous tunic derived from the differentiated surrounding mesoderm. The ductus cochlearis is triangular in section; its base or ex- ternal wall is attached to the outer wall of the bony capsule, its apical border is joined to the end of the osseous spiral lamina, and the converging sides are formed by the delicate membrane of Reissner above and the basilar membrane below, which separate respectively the scala vestibuli and the scala tympani from the scala media. The vestibular wall of the cochlear duct is formed by Reiss- ner’s membrane, an extremely fragile partition dividing the duct from the scala vestibuli ; the membrane begins on the vestibular border of the lamina spiralis, about .2 mm. inside the free edge of the crista, and extends at an angle of about forty-five degrees until it meets the outer bony wall. Reissner’s membrane consists of three layers: an extremely thin central, almost homogeneous connective-tissue stratum, one side of which is covered by the endothelium of the vestibular surface and the other by the epi- thelium of the cochlear duct. Notwithstanding the extreme thin- ness of this layer, the presence within it of sparingly distributed capillary blood-vessels has been demonstrated. The vestibular endothelium consists of a single layer of delicate plates, which here and there enclose pigment. The surface towards the duct is cov- ered by the general ectodermic lining of the canal, represented by a single layer of flat polyhedral epithelial cells. The three layers contribute equally to the 3 y. representing the entire thickness of the membrane. The outer wall of the cochlear duct rests against a greatly thick- ened crescentic cushion of connective tissue, whose convex border is intimately united with the bony wall, and whose generally concave margin looks towards the cochlear duct. This area of con- nective tissue, the ligamentum spirale, extends both above and below the boundary of the cochlear duct, its two horns forming part of the outer walls of the adjacent vestibular and tympanic canals. The concave surface of the ligamentum spirale is interrupted opposite the level of the tympanic wall of the cochlear duct by a pro- jecting ridge, the crista basilaris (Schwalbe), to which the basilar 390 NORMAL HISTOLOGY. membrane or tympanic wall of the duct is attached. Near the base of the basilar crest the outer wall of the cochlear duct is marked by an additional smaller projection, the prominentia spiralis, or accessory spiral ligament, distinguished usually by the presence Fig. 401. Section of single turn of cat’s cochlea: SV, SM, ST, scala vestibuli, media, and tympani; a, osseous tissue projecting as spiral lamina (6) ; c, basilar membrane attached to spiral liga- ment (