DEPARTMENT OF PHARMACOLOGY University of California Medical Schod SAN FRANCISCO, CALIF. MEDICAL Gift of Dr. Chauncey D. Leake I 9- 3. . L, 7, ^ A TEXT-BOOK OF HISTOLOGY OF PHARMACOLOGY University of California M^tk-ai SH *>, SAN FRANCISCO, CALlh\ A TEXT-BOOK OF HISTOLOGY BY HARVEY ERNEST JORDAN, A.M., PH.D. PROFESSOR OF HISTOLOGY AND EMBRYOLOGY, UNIVERSITY OF VIRGINIA WITH FIVE HUNDRED AND NINETY-FOUR ILLUSTRATIONS IN THE TEXT, AND FOUR PLATES NEW YORK AND LONDON D. APPLETON AND COMPANY 1920 COPYRIGHT, 1916, 1917, 1920, BY D. APPLETON AND COMPANY pnraTE0 nr 9KB UKITKD STAVES OF AWBRIC* PEEFACE In the preparation of this text-book of Histology we have kept fore- most in mind the needs of medical students as we have come to under- stand them in our experience as teachers of the subject. These needs have been suggested by the common difficulties we have discovered on the part of the average student. It has been our effort to lessen these difficulties in the most practical way. Our experience has demonstrated to us the efficacy of the methods here pursued. The bulk of the subject matter of Histology is now relatively stable. With respect to this nucleus our problem has largely been one of the best means of presentation. The secret of success in the acquisition of bis- tologic data is of course fundamentally an interest in the subject. In- terest can best be stimulated by a revelation of relationships to future more clearly conceived practical ends; and it can best be sustained by the possession of principles and generalizations that serve at least pro- visionally to coordinate the mass of seemingly unrelated facts. These considerations force an approach to the subject of Histology largely from the viewpoint of function. The known or believed function dependent upon the structure described is therefore briefly indicated whenever this seems desirable. With the same end in view comparative anatomic and embryologic facts are frequently presented. Since Embryology usually constitutes a separate course of the med- ical curriculum, the development of tissues and organs is discussed only to the extent deemed absolutely essential for a proper appreciation of structure. Since the anatomy of the Nervous System likewise properly constitutes a separate course, only the nervous tissues are here described, including the microscopic structure of the spinal cord, the cerebral cor- tex and the cerebellar cortex. A description of the structure of the brain stem is not included. An effort has been made to include the results of the latest investigations especially as regards cytologic data, particularly in relation to the genital glands. The organs of internal secretion are treated under a single head, and rather more fully, in view of their now evident importance, than has hitherto been the case. The illustrations have been taken from various sources, duly cred- PREFACE ited including more than 300 originals our one object having been to best elucidate the structures described. Photomicrographs of actual sections, combined with interpretive drawings and diagrams, appear to us the ideal illustrative procedure. The majority of the photomicrographs are taken from Ferguson's "Normal Histology and Microscopical Anatomy." For a number we are indebted to our friend and colleague, Professor Albert H. Tuttle of the University of Virginia. An attempt is made to adapt the book somewhat to prevailing non- uniform demands, by putting the more essential and what we regard as additionally desirable in different type. Eeferences to the recent literature are inserted for the student who may wish to consult the more important original works upon which the later developments of Histology have advanced. We gratefully acknowledge our indebtedness for illustrations and data taken from the recent and earlier literature, and our obligations to the publishers for their kindly help and courtesy. HARVEY ERNEST JORDAN JEREMIAH S. FERGUSON. PREFACE TO THE SECOND EDITION IN the preparation of the second edition I have been guided by sug- gestions and criticisms made by teachers who have used the first edition. There appears to be very general agreement that the plan and scope of the book meet the desires of teachers and students of Histology in Medi- cal Schools. The chief point' of disagreement among teachers concerns the teaching value of the photomicrographs. I have tried to compromise the conflict of opinion on this point by substituting drawings for the less satisfactory photographs of the first edition. A number of new illustra- tions have also been added. To meet the demand for a laboratory guide, in general conformity with the text, a chapter has been added on "Direc- tions for Laboratory Work." In the first edition I used freely of the illustrations and certain por- tions of the text in Ferguson's "Normal Histology and Microscopical Anatomy/' My indebtedness for this material was recognized by publi- cation under joint authorship. The present revision has made the work more exclusively my own, and I now assume sole responsibility for the book as it stands. Certain portions, however, still remain in essentials as originally published in Ferguson's excellent book, and I am under deep obligation to my friend and former colleague for this aid to my efforts to produce an acceptable textbook of Histology. To all those who have in any way helped, especially by constructive criticism and by the loan of drawings for new illustrations, to improve the book in a second edition, I desire to express my gratitude. I am also again greatly indebted to my publishers for very generous co- operation in the work of revising the text and of changing many illus- trations. HARVEY ERNEST JORDAN. LABORATORY OP HISTOLOGY AND EMBRYOLOGY, UNIVERSITY OF VIRGINIA. CONTENTS CHAPTER PAGE I. INTRODUCTION PROTOPLASM CELL ..... 1 II. EPITHELIAL TISSUES ........ 30 III. CONNECTIVE TISSUE CARTILAGE BONE . . . .49 IV. MUSCULAR TISSUE 90 V. NERVOUS TISSUES 119 VI. PERIPHERAL NERVE TERMINATIONS END ORGANS ... . 159 VII. THE BLOOD VASCULAR SYSTEM 176 VIII. BLOOD 203 IX. THE LYMPHATIC SYSTEM 225 X. Mucous MEMBRANES GLANDS ...... 251 XI. THE SKIN 262 XII. THE RESPIRATORY SYSTEM 292 XIII. THE DIGESTIVE SYSTEM . 320 XIV. THE URINARY SYSTEM 423 XV. THE REPRODUCTIVE SYSTEM 455 XVI. THE DUCTLESS GLANDS : ENDOCRIN GLANDS .... 548 XVII. THE NERVOUS SYSTEM 587 XVIII. THE EYE 626 XIX. THE EAR . 682 XX. HISTOLOGIC TECHNIC 720 LIST OF ILLUSTRATIONS FIGURE PAGE 1. Various spheroidal cells . . < 1 2. Ameba proteus in motion ....... 3 3. Paramecium caudatum ..... ... 4 4. A generalized cell ......... 5 5. A unicellular flagellate animal (Tetramitus chjlomonas) . . 6 6. Egg of a clam (Cumingia tellinoides) ..... 7 7. Primary spermatocyte of a turtle (Cistudo Carolina) ... 8 8. Spermatid of opossum in early stage of metamorphosis into a sper- mium 8 9. Cells from the newly-hatched rainbow trout, treated according to Meves' technic for the demonstration of mitochondria (plasto- somes) .......... 9 10. Two cells from the mesenchyma of the newly-hatched rainbow trout 9 11. Intracellular network, or Hrophospongium' within a Purkinje cell of the cerebellum of Strix flammea . . . . . .10 12. Diagram illustrating the various theories of protoplasmic structure 11 13. Egg of the blood starfish (Henricia sanguinolenta) in later growth period .......... 11 14. Pancreas cell .of turtle, filled with zymogenic granules ... 12 15. Motor nerve cell from the ventral horn of the spinal cord of the ox 12 16. Interstitial cell from the testis of a twenty-one year old man. Showing granular and filamentous mitochondria . . 13 17. A neuron (giant pyramidal cell, or cell of Betz) from the cerebral cortex of man, showing the neurofibrils 13 18. Developing fat cells . . . . . . . .13 19. Columnar ciliated epithelial cells. Showing canalicular apparatus 14 20. Successive stages in the movement of an ameba .... 15 21. A leukocyte from human blood in active ameboid motion . . 16 22. Three cells from the epididymis of the rabbit . . . .16 23. Ciliate and flagellate cells 17 24. Successive steps in amitotic division in tendon cell of new-born mouse 18 25. Successive stages in the amitotic division of the ciliated cells lining the vasa efferentia of the epididymis of the mouse ... 19 ix LIST OF ILLUSTKATIONS 26. Multinucleated giant cell, from the yolk-sac of a 10 mm. pig embryo 19 27. Spermatocyte of Pyeris crategi, a butterfly, showing a cilium at- tached to the centrosome ....... 20 28. Diagrams illustrating successive stages of mitosis ... 21 29. Cells from epidermis of the salamander ..... 22 30. Successive stages of mitosis in the root tip of the dogtooth violet (Erythronium americanum) ....... 23 31. Successive stages in the maturation, fertilization and segmentation of the star-fish (Asterias forbesii) egg ..... 26 32. Transverse section of a frog embryo, showing the three germ layers 27 33. Group of epithelial cells from the Malpighian layer of the skin . 30 34. A villus of the human placenta, showing a peripheral syncytmm of irregular thickness . . . . . . . .31 35. Cells from the pancreas of Necturus, containing secretory granules and basal ergastoplasmic filaments . . . . .31 36. Various forms of cells . . . . . . . .31 37. Polyhedral epithelium, from a section of the human liver . . 32 38. Goblet and columnar cells from the large intestine of the cat . . 32 39. Columnar epithelium from the pyloric region of the human stomach 33 40. 'Terminal bars' of cement substance as seen between the epithelial cells of a tubular secreting gland in the pyloric region of the human stomach 34 41. Semidiagrammatic illustration of endothelium lining a large artery 37 42. Mesothelium (surface view) from the mesentery of a rat . . 38 43. Cuboidal epithelium from the rete testis of the rabbit ... 39 44. Tip of a villus of the synovial membrane from the knee-joint of an old man .......... 39 45. Columnar ciliated epithelium from the epididymis of a rabbit . 40 46. A group of cells from a transection of an acinus of the human pan- creas; glandular epithelium . . . . . . .41 47. Goblet cells as seen in a transection of a crypt of the large intestine in man .......... 42 48. Diagram showing the arrangement of the columnar and goblet cells of figure 47 42 49. Stratified epithelium from the human esophagus ... 43 50. Epidermis of the skin of the finger tip, showing extreme keratization of the epithelium ......... 45 51. Pseudostratified columnar ciliated epithelium from a bronchial tube of man .......... 46 52. Diagram showing the manner in which all the epithelial cells of pseudostratified ciliated epithelium reach the basement mem- brane ........... 46 53.-- Transitional epithelium from a transection of the ureter of an infant 47 LIST OF ILLUSTRATIONS xi 54. Isolated cells which may appear in human urine . . .48 55. Embryonal connective tissue, early stage . . . .49 56. Embryonal connective tissue at a later stage than is represented in figure 55 . 49 57. Subcutaneous areolar connective tissue of guinea pig . ...___. 50 58. Plasma cells of connective tissue from the human breast . . 51 59. Spindle-shaped connective tissue cells from the stroma of the human ovary ........... 51 60. Pigmented cells from the choroid coat of the ox's eye ... 52 61. Granule cells from the fibrous connective tissue of the human mam- mary gland .......... 52 62. Gelatinous connective tissue from the umbilical cord of a new-born infant ........... 54 63. Reticulum of a cervical lymph node of man, from a thin section from which the lymphatic corpuscles had been partially washed out . 55 64. Dense fibrous tissue from the tendon of one of the ocular muscles of a child 57 65. Longitudinal section of tendon of human finger .... 58 66. Portion of tendon from a cow ....... 58 67. Transverse section of portion of tendon of human finger . . 59 68. Piece of tendon from tail of white mouse ..... 59 69. Isolated tendon cells 60 70. Coarse elastic fibers from the ligamentum nuchae of the ox . .60 71. Transection of a fasciculus of the ligamentum nuchae of the ox, showing the very large elastic fibers embedded in a very delicate network of collagenous fibers 61 72. Portion of ligamentum nuchae of ox 61 73. Portion of a fat lobule from the areolar connective tissue surround- ing the esophagus of a cat ....... 62 74. A group of fat cells from the subcutaneous tissue of a young rabbit 63 75. Fat cells from a teased preparation of adipose tissue of man . 63 76. Adipose tissue ......... 64 77. Developing adipose tissue from the subcutaneous tissue of an infant 64 78. Reticulum from the mucosa of the fundus region of the dog's stomach 65 79. Section through a small lymph gland of a dog .... 65 80. From a section through the medulla of a cervical lymph node of man 66 81. Transection of a plate of hyaline cartilage, from the trachea of a child 68 82. Cells and matrix of hyaline cartilage from the wall of a large bronchus of man .......... 69 83. Elastic cartilage from the human epiglottis, showing the large ovoid cartilage cells and the very delicate reticulum of elastic fibers . 70 84. Fibrocartilage, showing a group of oval cartilage cells . . .71 xii LIST OF ILLUSTRATIONS 85. Notochordal tissue ......... 72 86. Transection through the compact bony wall of a human meta- carpal bone ......... 73 87. Longitudinal section of ground bone from the shaft of the human femur 74 88. Isolated bone cell shrunk away from wall of its lacuna . . 75 89. An Haversian system, including the central canal, several lamellae, lacunae and canaliculi 75 90. Transverse section of Haversian canal, with contents ... 76 91. The primary changes in intracartilaginous bone formation . . 80 92. A longitudinal section of the two distal phalanges from the finger of a five-months' human fetus . . . . . .81 93. Reconstruction of cartilage into bone . . . . .82 94. Trabecula of primary enchondral bone, showing a central deep- staining core of calcified cartilage and a peripheral layer of osteo- blasts 83 95. Trabecula of primary bone from the finger of a human fetus . 84 96. Intramembranous bone formation in the lower jaw of a sheep fetus 87 97. Smooth muscle cells 91 98. Smooth muscle cells from the pig's stomach .... 92 99. Smooth muscle cells from the wall of the human intestine . . 92 100. Smooth muscle cells from the wall of the human intestine . . 93 101. Two stages in the histogenesis of smooth muscle, from the wall of the esophagus of a pig embryo 94 102. A group of myoblasts from the heart muscle syncytium of a 48-hour chick embryo ......... 95 103. Cardiac muscle of guinea-pig, showing several branches, cross striations (ground membranes) and a number of intercalated disks 95 104. Cardiac muscle cells from the pig's heart, isolated in equal parts of alcohol, glycerin, and water 96 105. Cardiac muscle of the human heart; the abundant branches are plainly shown . . . . . . . .97 106. The central portion of the preceding figure, more highly magnified . 98 107. Transection of a group of cardiac muscle fibers from a papillary muscle of the human heart ....... 98 108. Developing muscle fibers from the heart of a human fetus at seven months ......... 99 109. Cardiac muscle fibers ........ 100 110. Diagram of a striped muscle fiber, according to Heidenhain . . 101 111. Six-lobed nucleus from the heart muscle of Limulus, showing the continuity of the nuciear wall with the telophragmata . . 101 112. Seven nuclei, the product of amitotic division, lying in an undifferen- tiated mass of sarcoplasm, from the heart of Limulus . . 102 LIST OF ILLUSTEATIONS x iii FIGURE PAGE 113. Longitudinal section of a trabecula of Limulus (King crab) heart muscle, showing an intercalated disk separating a contracted from an uncontracted portion 103 1 14. Semidiagrammatic illustrations of various types of intercalated disks 104 115. Successive stages of skeletal muscle histogenesis in mammals . 105 116. Transverse section of a striped muscle fiber of a newly-hatched rain- bow trout, showing the process of myofibril increase by radial longitudinal splitting ........ 105 117. Striated muscle fibers ruptured by teasing, showing the sarcolemma 106 118. Isolated fragments of striated muscle fibers, unstained . . 107 119. Striated muscle fibers of the dog, seen in transection . . . 107 120. A portion of a striated muscle fiber seen in longitudinal section . 108 121. A small portion of a muscle fiber of a crab showing beginning separa- tion into fibrils 108 122. Fibrils from the wing muscles of a wasp . . . . . 109 123. Longitudinal section of a portion of a striped muscle trabecula of Limulus, showing a nucleus of serrated contour with the telo- phragmata attached to the serrations . . . . .109 124. Striated fiber from a leg muscle of the sea spider (Anoplodactylus lentus), showing the complexly striped condition characteristic of insect muscle ......... 110 125. Semidiagrammatic drawing, representing the appearance of the same fiber from the leg muscle of a beetle in ordinary and polarized light 110 126. Lateral contractive wave of Cassida equestris . . . .111 127. Striated muscle fibers of the dog 112 128. Striated muscle of a cat seen in transection . . . .113 129. Motor end-plate on an intercostal muscle fiber of a young rabbit . 115 130. Portion of a transection of a large tendon . . . . .116 131. Transverse section of tendon of tail of adult mouse . . .116 132. Portion of a muscle fiber from the tail of a 5 cm. frog tadpole . 117 133. Diagram of a neuron . . . . . . . .120 134. A unipolar ganglion cell of a frog . . . . . .122 135. Multipolar ganglion cell from the ventral horn of the gray matter of the spinal cord of the ox .122 136. Pyramidal multipolar nerve cell from the cerebral cortex of a mouse 123 137. Isolated nerve cells from the spinal cord of man .... 124 138. Various types of nerve cells of the cerebellar cortex . . .125 139. Three types of nerve cells supplying respectively cardiac (1), smooth (2) and (3) striated muscle 126 140. A nerve cell from the trapezoid nucleus in the midbrain of a rabbit 127 141. A neuron (giant pyramidal cell, or cell of Betz) from the cerebral cortex of man, showing the neurofibrils ..... 128 xiv LIST OF ILLUSTRATIONS 142. Intracellular network (trophospongium) within a Purkinje cell of the cerebellum of Strix flammea . . . . . .128 143. Golgi cell, type I 129 144. Golgi nerve cell, type II .130 145. Isolated nerve fibers from a frog 133 146. A small portion of a transection of the sciatic nerve of a dog . . 134 147. A group of large medullated fibers from a nerve in the peritracheal areolar tissue of the cat ....... 134 148. Nerve fibers 135 149. Cross and longitudinal sections of the same funiculus of non- medullated nerve fibers (turned up at the left), showing the perineurium and the relationship of the neurolemma nuclei to the axis cylinder bundles of neurofibrils ..... 136 150. Cross-section of the trunk of the human vagus nerve, some distance below the nodose ganglion, showing medullated and non-medul- lated fibers . . 137 151. Successive stages in the degeneration process exhibited by the distal stump of a medullated axon (from sciatic nerve of adult dog) fol- lowing section . . . . . . . . 138 152. Regenerative stages in the proximal stump of the cut sciatic nerve of the dog, several millimeters above the level of section . .139 153. Transection of the spinal cord of an embryo chick . . . 140 154. Transection of the spinal cord of a child, fifth lumbar segment . 141 155. Portion of gray substance from the anterior horn of the spinal cord of man, showing nerve cell bodies, dendrons, medullated and non- medullated portions of axons, and neuroglia .... 142 156. Transverse section through the white substance of the human spinal cord 142 157. Neuroglia from the spinal cord of a fetal pig .... 143 158. A long-rayed astrocyte ........ 144 159. A short-rayed astrocyte, or mossy cell 144 160. Neuroglia cell with adjacent fibers, from the pineal body of a yearling sheep 145 161. Neuroglia cells and fibers from the spinal cord of an elephant . 146 162. Transection of the sciatic nerve of a dog . . . . .147 163. Diagram of the origin and relations of the peripheral motor and sensory neurons ......... 148 164. Bipolar cell from a spinal ganglion of a fish .... 149 165. Transformation of bipolar cells into unipolar cells in the Gasserian ganglion of the pig ........ 149 166. Section through the dorsal root ganglion of the first thoracic nerve of a cat . ......... 150 167. A nerve cell from a section of a human Gasserian ganglion . .150 LIST OF ILLUSTRATIONS XV JTIGUKE PAGE 168. Schematic representation of the relations of the structures com- posing a spinal ganglion . . . . . . .151 169. Common atypical, though probably perfectly normal, nerve cells from the spinal ganglion of the dog . . . . .152 170. Sympathetic neurons ........ 154 171. The sprouting of an axon by a neuroblast from the spinal cord of a frog embryo ......... 156 172. The sprouting of an axon by a neuroblast from the spinal cord of a frog embryo 156 173. Nerve endings in the epithelium of the larynx .... 160 174. Tactile cells in the epithelium of the groin of a guinea-pig . . 160 175. Schematic representation of a taste bud ..... 161 176. Taste bud from the human tongue ...... 162 177. Tactile corpuscle of Meissner from the skin of the human toe . 163 178. Tactile corpuscle of Meissner ....... 164 179. Tactile corpuscle of Meissner . . . . . . .164 180. Ruffini's end organ 165 181. End bulb of Krause from the margin of the ocular conjunctiva . 165 182. Genital corpuscles from the clitoris of a rabbit . . . .166 183. A lamellar corpuscle from the mesentery of a cat . . . 166 184. A lamellar corpuscle from the pleura of a child . . . .167 185. Lamellar corpuscle from the mesentery of a kitten . . .167 186. A lamellar corpuscle in longitudinal section, showing a network of spiral elastic fibers . . . . . . . .168 187. Axial section of a corpuscle of Herbst from a duck's tongue . . 168 188. A papilla of the duck's tongue, containing a corpuscle of Grandry . 169 189. Golgi-Mazzoni corpuscles from the subcutaneous tissue of the tip of the finger ......... 170 190. Motor nerve endings in striated muscle ..... 171 191. A muscle spindle from the psoas magnus of man . . . 172 192. Middle third of a terminal plaque in the muscle spindle of an adult cat 173 193. Neurotendinous end organ or tendon spindle of Golgi . . . 174 194. Nerve endings in cardiac muscle, from the heart of a cat . .175 195. Nerve endings in smooth muscle, from the intestine of a cat . . 175 196: A small artery from the connective tissue of the anterior cervical region of man 177 17. The external carotid artery of a child 179 198. Transection of the wall of the aorta of a child . . . .179 199. Part of a cross-section of the femoral artery of a dog . . . 180 200. Transection of the celiac axis of man ..... 181 201. A group of small blood-vessels . . . . . . .182 202. Semi-diagrammatic illustration of small branch of pulmonary artery of ox 183 1 xv i LIST OF ILLUSTEATIONS 203. Semi-diagrammatic illustration of dividing small branch of pul- monary artery of guinea-pig . . . . . . .183 204. The capillary network connecting an arteriole and venule of the omentum of a young rabbit ....... 185 205. Capillary vessel of the frog's mesentery ..... 185 206. Two sinusoidal vessels from the medulla of the human adrenal . 186 207. Precapillary venule and arteriole ...... 187 208. Transection of an arteriole and venule . . . . .188 209. Transection of the wall of the human vena cava . . . 189 210. A 13 mm. human embryo ....... 192 211. 'Vasoformative' cells from the mesentery of a rabbit seven days old 193 212. The parietal layer of the pericardium of a child .... 196 213. The endocardium 197 214. Radial sections of the mitral valve, from the heart of a man . . 198 215. Human heart, opened from the right to show the atrioventricular bundle of His .199 216. Reconstruction of the sino ventricular system (bundle of His) of the calf's heart 200 217. Oxalated plasma of human blood clotted with thrombin, showing fibrin needles ......... 204 218. From a freshly prepared, unstained specimen of human blood . 205 219. Blood cells from a specimen of freshly drawn unstained human blood 206 220. Showing the action of water upon the red blood corpuscle . . 206 221. Five nucleated red cells (erythrocytes) from the blood of a frog . 207 222. Three nucleated red blood cells (erythrocytes) from the marrow of a human rib 207 223. A group of cells from normal human blood .... 208 224. A group of blood platelets, from the human blood . . . 209 225. Outline drawings of living polymorphonuclear leukocytes of rabbit, from a drop of blood mixed with Ringer's solution to which a small amount of hirudin had been added to prevent coagulation . 211 226. Section of bone marrow from skull of 25 mm. turtle embryo (Chely- dra serpentina), showing three main stages in the hemopoiesis . 212 227. Hemoglobin crystals 213 228. Crystals of chlorid of hematin or hemin 213 229. Wall of yolk sac of a 13 mm. human embryo (Fig. 210), showing a small blood island and several small blood vessels contain- ing erythrocytes 215 230. Large blood island from yolk sac of a 13 mm. human embryo . 216 231. Diagrammatic illustrations of successive stages in the transforma- tion of the mammalian erythrocyte to form the erythroplastid . 218 232. Successive stages in the elimination of the erythroblast nucleus, from homoplastic cultures of blood of a 32 mm. pig embryo . 218 LIST OF ILLUSTRATIONS xvii FIGtTRE PAGE 233. From a section of red marrow of a human bone .... 220 234. Types of cells from a smear preparation of the marrow of a human rib 221 235. Subcutaneous lymphatic vessel of a fetal pig .... 227 236. The growing end of a developing lymphatic vessel in the subcutane- ous tissue of a fetal pig ...... v 228 237. Lymphatic and blood vessels in the hilum of a human lymph node . 229 238. Lymphatic capillary from the spermatic cord of a dog, showing nerve endings 230 239. Transection of the pericardium of a child 232 240. Section of a vascular synovial villus from the knee joint of a child 233 241. A lymph nodule, solitary follicle, from the large intestine of man . 234 242. Diagrammatic illustration of a lymph node .... 236 243. Transection of a cervical lymph node of a dog . . . . 237 244. Transection of a mesenteric lymph node of a man . . . 237 245. Diagram of the blood vessels of a lymph node .... 238 246. Section of human hem^lymph node (" splenolymph gland ") . . 240 247. Horizontal section through the faucial tonsil of a child . . . 241 248. From a crypt of a dog's tonsil 243 249. The lingual tonsil of man ... . .244 250. Portion of spleen of cat, showing capsule (above and at left) and five splenic nodules . . . . . . ... 246 251. Diagram of a lobule of the spleen ...... 247 252. The origin of a vein in the splenic pulp .... . 248 253. Types of cells from a smear preparation of the pulp of the human spleen ... 249 254. Diagram of a mucous membrane having simple tubular glands . 252 255. Diagrams of the principal types of glands 253 256. Transection of three secreting tubules of the submaxillary gland of man ........... 255 257. Model of a reconstruction of the lacrimal gland of man . . 258 258. Reconstruction of a mucous gland from the respiratory region of the nasal mucosa of a child ....... 259 259. Reconstruction of an intralobular duct dividing into its terminal intercalary ducts and acini 259 260. Epidermis of the foot . 263 261. Section of thin skin from abdomen of negro, showing the distribu- tion of the pigment granules in dermal and epidermal cells . 264 262. Section of thin skin from abdomen of light brown mulatto . . 264 263. Skin from sole of human foot, showing spiral ducts of two sweat glands opening through the epidermis ..... 266 264. Transection of the epidermis of the foot , 267 xviii LIST OF ILLUSTKATIONS 265. Three early stages in the histogenesis of the skin . . . 270 266. From a section of the abdominal integument of an infant . . 272 267. Several coils of a sudoriparous gland of the human finger . . 273 268. Terminal phalanx of finger of human fetus .... 275 269. Transection through the margin of a finger nail .... 276 270. Longitudinal vertical section of the young nail and nail-bed of an infant . .277 271. Five stages in the development of a human hair . . . 278 272. From a section of the skin of an infant's arm, showing small im- mature hair follicles in transection 280 273. From a section of the human scalp 282 274. Transection of a hair near the middle of the root sheath . . 284 275. Regeneration of a hair 287 276. Sebaceous glands in the scalp of a child 288 277. Section of a sebaceous gland from the human scalp, through point of opening into a hair follicle (obliquely cut) .... 289 278. Cells from the central portion of figure 277, showing two successive stages in sebum formation by process of fatty metamorphosis of the cytoplasm .... .... 289 279. Reconstruction of the cutaneous blood vessels .... 290 280. Photograph of Azoux model, showing nostril, pharynx, larynx and related structures ........ 292 281. From a section of the mucous membrane of the respiratory region of the human nose ........ 294 282. The olfactory mucosa of a cat ....... 296 283. The olfactory mucous membrane ...... 297 284. Vertical section of the olfactory mucosa of a kitten . . . 298 285. Diagram of the relations of the neurons of the olfactory nerve and olfactory bulb . 299 286. A vertical section through the lateral wall of the human larynx . 301 287. Transection of the wall of a child's trachea .... 302 288. Mucus-secreting, tubulo-alveolar gland of the human tracheal mucosa .......... 303 289. A bronchus from the human lung ...... 305 290. Diagram of primary lobule of lung (lung unit) .... 307 291. From a section of a child's lung ...... 308 292. From a section of a child's lung ...... 309 293. From a section of a child's lung ...... 310 294. Diagram of the three pulmonary lobules connected with a terminal bronchiole .311 295. Two alveoli of a child's lung .... .312 296. Transection of the pleura of an infant . . . . .313 297. From a section of the pleura of man . . ' . . . .313 LIST OF ILLUSTKATIONS X J X FIQTJRE PAGE 298. From the lung of a child 315 299. From the lung of a dog whose blood vessels had been injected with a gelatinous mass, and appear black 316 300. From the central portion of figure 299 317 301. From a section through the lip of an infant . . . __,_ 321 302. Axial section of a human molar tooth . . . . . 322 303. Diagram of an axial ground section of tooth, showing the several stripes of the dentin and the enamel ..... 323 304. From a longitudinal section of the neck of a child's tooth and the adjacent alveolus 324 305. From a section of a human tooth which had been ground to extreme thinness .......... 325 306. Section of fang parallel to the dentinal tubules, human canine . 326 307. Dentin from a ground section of a human molar, showing the den- tinal tubules cut across ....... 327 308. Enamel prisms in transection ....... 328 309. A group of enamel prisms cut longitudinally, from the incisor tooth of the rat, showing their irregularly beaded character and the cross striations 329 310. From a section of a human tooth which had been ground to extreme thinness 330 311. Developing tooth from a human embryo 17 mm. long . .331 312. Dental anlages from a human fetus 40 mm. long . . . 332 313. Two stages in the early development of the teeth, from a 25 mm. pig embryo 332 314. Developing tooth from a human fetus 30 cm. long . . . 333 315. A developing tooth from an infant's jaw . . - . . 334 316. A portion of Fig. 315, near the apex of the developing tooth . 335 317. Odontoblasts and dentin of the tooth of a new-born cat . . 336 318. View of dorsum of tongue, showing the various papillae, the tonsils and the fauces 338 319. One lateral half of a coronal section of a dog's tongue . . . 339 320. Filiform papillae of the dog's -tongue . ... 340 321. A filiform and a fungiform papilla, from an injected specimen of tongue of cat ......... 341 322. Circumvallate papillae of the human tongue .... 342 323. Two foliate papillae from a rabbit's tongue, showing numerous taste buds along their lateral margins 343 324. Diagram of the alimentary canal of man 345 325. Surface view of Auerbach's intramuscular nerve plexus, from the esophagus of a cat 346 326. Photomicrograph of a transverse section through upper third of cat's esophagus . . 349 XX LIST OF ILLUSTRATIONS FIGUKE PAGE 327. Transverse section of human esophagus through lower third . . 350 328. From a section of the human esophagus 351 329. Section through the stomach wall of man (pyloric region) . . 352 330. The mucosa of the fundus region of the dog's stomach . . 354 331. Longitudinal section of the fundus glands of man . . . 355 332. Transections of three glands of the fundus region of the human stomach 356 333. A pyloric gland, from section of the dog's stomach . . . 357 334. Portion of gastric gland from the fundus region of the stomach . 357 335. Secretory capillaries of the fundus glands of the dog's stomach . 358 336. The mucosa of the pyloric region of the human stomach . . 359 337. Blood vessels and lymphatics of stomach 361 338. Termination of sympathetic nerve fibers 362 339. Schematic diagram illustrating probable relationship of sympathetic neurons in myenteric and submucous plexuses . . . 363 340. Section through the commencement of the duodenum at the pylorus 364 341. From a longitudinal section through the duodenum of a cat . 365 342. The central portion of a Peyer's patch in the ileum of a dog's in- testine ' .367 343. Diagram of small intestine, showing the topographical relationship of the intestinal glands (crypts of Lieberkuhn) to the villi . . 368 344. Longitudinal section of villus ....... 369 345. Several villi from the small intestine of the 'dog, in longitudinal section 370 346. Reconstruction model of a Brunner's gland, from the human duodenum .......... 372 347. The blood-vessels of the small intestine of a dog, drawn after an injected preparation ........ 373 348. Intestinal mucosa of a frog during the absorption of fat . . 375 349. Apex of an intestinal villus of a rabbit which had been fed with milk 376 350. Section of large intestine of dog, showing intestinal glands (crypts of Lieberkuhn) cut longitudinally ...... 378 351. Section of portion of large intestine of dog, showing the intestinal glands (crypts of Lieberkuhn) cut across, their lining including columnar and goblet cells ....... 379 352. Transection of the vermiform appendix of man .... 380 353. Semidiagrammatic representation of a small mucous gland from the oral mucosa of a rabbit ........ 382 354. Corrosion model of an interlobular duct and its branches, from the human submaxillary gland ....... 383 355. Intercalary ducts and acini of the human submaxillary gland, corro- sion model 385 LIST OF ILLUSTRATIONS XXI FIGURE PAGE 356. A group of mucous acini, from the human submaxillary gland . 386 357. From the sublingual gland of man ...... 387 358. Mucous acini of the retrolingual gland of the rat .... 388 359. Diagram of the arrangement of the cells in a mixed salivary gland 389 360. Diagrams of A, parotid gland; B, submaxillary gland; C, sublingual gland; and D, pancreas ....... r~ 389 361. From a section of the human parotid gland .... 390 362. From a section of the human submaxillary gland . . . 391 363. Reconstruction model of the sublingual gland of man . . . 392 364. Nerve endings in a salivary gland 393 365. Early stages in the development of the pancreas, illustrating condi- tions in the 5 and 7 weeks old human embryos . . . 394 366. From a section of the human pancreas, showing several lobules and the broad interlobular bands of connective tissue . . . 395 367. Drawing of an intercalary duct with three branches ending in acini to form centro-acinial cell groups ...... 396 368. Reconstruction model of the human pancreas .... 396 369. Acini of the human pancreas 397 370. Pancreatic acinus of cat cut transversely near fundus, showing the basal (prozymogen) filaments of the cells .... 398 371. Cells from pancreas of Necturus ...... 399 372. Two adjacent acini from the guinea-pig's pancreas . . . 399 373. Section of an acinus from the guinea-pig's pancreas, showing the basal mitochondrial content and the central zymogen granules . 400 374. Intercalary duct with branches, from pancreas of guinea-pig, showing highly branched tubules connected with the duct and with the islet 400 375. Pancreatic islet 401 376. From the human pancreas ....... 402 377. Section of a pancreatic islet from injected specimen of cat's pancreas to show the profuse blood supply .'.""'. . . . . 404 378. A lobule of the pig's liver; the central vein lies in the middle of the figure . .... 405 379. Diagram of liver lobules, the upper two cut transversely, the lower longitudinally ......... 406 380. From a section of the turtle's liver, showing the tubular arrange- ment of the parenchyma ....... 407 381. The reticulum of the dog's liver ...... 408 382. Stellate cells of von Kupffer in the liver of a dog . . 409 383. A lobule of the pig's liver in longitudinal section, showing the rela- tion of the central and sublobular veins and the arrangement of the hepatic cells . . . . . . . .410 384. A lobule of the human liver, seen in transection . 411 xxii LIST OF ILLUSTKATIONS 385. Section of liver tissue showing the liver cell-cords, and the sinusoids lined with endothelium . . . . . . .412 386. Diagram of four adjacent liver cells ...... 413 387. Bile capillaries of the hepatic lobule, from the liver of a cat . .413 388. Showing the connection between the intralobular and interlobular bile ducts in the cat's liver . . . . . . .414 389. Types of cells from a section of the normal human liver . . 414 390. Human liver cells, showing enlarged intracellular canaliculi, a con- dition characteristic of jaundice .. . . . . .415 391. Diagram of a portal canal, including a branch of the portal vein, hepatic artery, hepatic (bile) duct, lymphatic and non-medul- lated nerve trunks 415 392. From a section of the rabbit's liver whose blood vessels had been injected with a carmin stained gelatin mass; somewhat more than a single lobule is represented 416 393. A sublobular vein of the pig's liver 418 394. Intralobular nerve fibers in a rabbit's liver .... 420 395. From a section through the wall of a dog's gall-bladder . .421 396. Reconstruction of the wall of a dog's gall-bladder . . . 421 397. Longitudinal section of kidney 424 398. Diagram of the structure of the kidney 425 399. Reconstruction of a uriniferous tubule of an infant . . . 426 400. Diagram of uriniferous tubule of a mammal . .',- 428 401. Reconstruction of a glomerulus of the human kidney . . . 429 402. From the cortical labyrinth (pars convoluta) of the human kidney 430 403. From the cortex of the human kidney, showing a transection of a cortical ray in the lower left-hand corner .... 432 404. From a longitudinal section of a convoluted tubule of the guinea- pig's kidney ......... 433 405. Cross section of a proximal convoluted tubule from the kidney of a mouse, showing basal filaments breaking up into granules cen- trally, and the central striated border of the cells . . . 433 406. A group of tubules from a transection of a renal pyramid of the human kidney; the section passes through the boundary zone . 439 407. The distribution of the left renal artery 441 408. From the cortex of the human kidney 442 409. Nerve endings in a convoluted tubule of the frog's kidney . - . 444 410. Cast of the pelvis, infundibula and calices of the kidneys of a man . 445 411. Transection of human ureter ....... 446 412. Transitional epithelium of dog's ureter 447 413. Transection of the wall of a child's bladder . . . .449 414. The mucosa of a child's bladder in the contracted state of the organ 450 415. Transitional epithelium of dog's bladder 451 LIST OF ILLUSTRATIONS xx iii 416. Epithelial cells from the bladder of the rabbit . . . .452 417. Transection of the female urethra 453 418. Diagram of a male genitalia (adapted from Merkel) . . . 455 419. Diagram of female internal genitalia 456 420. Diagrams illustrating the metamorphoses of the indifferent urogeni- tal system into the male and female systems . . . ~.~457 421. Diagrams illustrating the process of maturation in the male and female gametes 462 422. Primary spermatocyte of a grasshopper, Hippiscus tuberculatus, showing the compact accessory chromosome among the paler mossy prophase euchromosomes, and the idiosome . . . 464 423. Chromosome groups of Schistocerca damnifica .... 473 424. Diagram illustrating the behavior of the chromosomes during the first and second maturation divisions ..... 474 425. The testicle with its system of efferent passages .... 480 426. Seminal tubule of a man in transection ..... 481 427. Sertoli cells of the human testis ...... 482 428. Portion of a transection of a seminiferous tubule from the human testis, illustrating the various stages in spermatogenesis . . 483 429. Successive stages in the metamorphosis of the spermatid into the spermatozoon ......... 484 430. Diagram of human spermatozoon . . . . . 485 431. Spermatozoa of various animals . . . . . . 486 432. Spermatozoa from the semen of man ..... 487 433 . A group of interstitial cells from the testis of a thirty-five year old negro 488 434. Interstitial cells from the human testis 489 435. A small portion of the wall of an efferent ductule of the testicle . 491 436. Efferent ductules of the rabbit's epididymis .... 491 437. Several coils of the rabbit's epididymis in transection . . . 492 438. Transection of the ductus deferens of a dog .... 494 439. From a section through the wall of a seminal vesicle of man . 495 440. Model of a reconstructed prostate gland of man .... 496 441. Several alveoli of the human prostate gland, seen in section . 498 442. Portion of prostate gland of an old man, showing the prostatic con- cretions .......... 499 443. Prostatic genital corpuscles 500 444. Reconstruction of a bulbo-urethral (Cowper's) gland of man . 501 445. From a section of the bulbo-urethral (Cowper's) gland of man . 501 446. Transection of a child's penis, just back of the glans . . . 503 447. Helicine artery in section, from the urethral bulb of man . . 504 448. The erectile tissue of the penis 505 449. Section of ovary of adult cat, showing five vesicular (Graafian) folli- cles, four with the cumulus oophorus and the enclosed ovum . 508 xxiv LIST OF ILLUSTRATIONS FIGURE PAGE 450. From the ovarian cortex of an infant, showing many ova in the primary follicular stage 509 451. Ovum, containing a yolk nucleus ('Dotterkern') at the left and above the nucleus .......... 511 452. From a section of the ovarian cortex of a new-born kitten . . 513 453. A primary ovarian follicle of the human ovary .... 514 454. A vesicular (Graafian) follicle of the human ovary, somewhat more advanced than the preceding . . . . . .515 455. A nearly ripe Graafian follicle from the ovary of a dog . .516 456. Photomicrograph of a section of cat's ovary, showing two primary follicles and one vesicular . . . . . . .517 457. Section through the peripheral portion of a corpus luteum, showing lutein cells . .520 458. Portion of corpus luteum of rabbit . . . . . .521 459. A corpus albicans, from a section of the human ovary . . 522 460. From a thick section of the ovary of a woman .... 523 461. Transections of the human oviduct ...... 524 462. From a transection of the ampulla of the oviduct, showing the struc- ture of the mucosa . . . . . . . . 525 463. Transection of the uterus of an ape ...... 527 464. Transection through the body of the human uterus . . . 529 465. From a transection of the uterine mucosa ..... 530 466. From the cervix uteri of a girl of sixteen years, showing the cervical glands in section . . . . . . . . .531 467. A gland of the human cervix uteri in longitudinal section . . 532 468. From a section of the human uterine mucosa at the first day of men- struation .......... 534 469. A group of decidual cells from the human uterus during the early stages of pregnancy ........ 535 470. Chorionic villi from the human placenta at full term . . . 536 471. Chorionic villus at various stages of development . . . 537 472. Vaginal mucosa . . . 538 473. Transection of a labium minus of an infant .... 540 474. From the actively secreting mammary gland of a woman . . 542 475. Model of a reconstruction of an intralobular duct and its acini from the active mammary gland of a woman ... . 543 476. Portions of several adjacent lobules, and a lactiferous duct, of the active mammary gland .... . 544 477. From a section of the human mammary gland in the resting condition 545 478. From a section through the human adrenal .... 550 479. Photomicrograph of suprarenal gland of dog .... 551 480. More highly magnified region of the preceding section, to show the capsule, zona glomerulosa, and a portion of the zona fasciculata . 552 LIST OF ILLUSTBATIONS xxv 481. Reconstruction of a dog's adrenal 555 482. Section of part of an accessory suprarenal (chromophil body), new- born child 556 483. From a section of the human thyroid gland .... 558 484. Diagram of pharynx of human embryo showing the origins of the anlages of the thymus, thyroid and parathyroids (epithelial bodies) 560 485. From the border of a mass of aberrant thyroid tissue of man, occur- ring in the region of the parathyroid glands .... 562 486. Human parathyroid tissue, moderately magnified . . . 564 487. A section through several lobules of the thymus of an infant . . 566 488. A thymic corpuscle from the thymus of an infant . . . 567 489. Carotid gland of an ape ........ 569 490. From a section of the coccygeal gland of man . . '. . 570 491. Median section through the anlages of the hypophysis cerebri of a 10 mm. cat embryo ......... 571 492. Sagittal view of a wax reconstruction of the hypophysis cerebri of the adult cat 574 493. Pars tuberalis, hypophysis of cat ...... 576 494. Pars infundibularis, hypophysis of cat ..... 576 495. Pars distalis, hypophysis of cat . . . . . 577 496. Section of hypophysis cerebri of dog, showing portions of pars distalis of anterior or buccal lobe, residual lumen, pars tuberalis (pars intermedia), pars neuralis, and capsule 578 497. Field from the center of a normal canine (puppy) pars anterior . 579 498. Semidiagrammatic representation of a median longitudinal section through the epiphysis of a 17 cm. sheep fetus .... 581 499. Cells from the pineal body of a 11 cm. sheep fetus . . . . 582 500. Photomicrograph of a peripheral portion of the pineal body of a 21 cm. sheep fetus, showing several cysts and vascular trabeculae, and an enormous number of intracellular melanic granules . 583 501. Photomicrograph of peripheral region of pineal body of a yearling sheep, to show the character of the parenchyma, the neuroglia cells and fibers, and the interneuroglia cells ..... 584 502. Two neuroglia and three interneuroglia cells from the pineal body of a lamb . . . . . . . . . . 584 503. Cells from pineal of yearling sheep ...... 585 504. Section of pineal body of an old sheep, showing 'brain sand' (acer- vulus) in the parenchyma ....... 586 505. Human embryo 2 millimeters long ...... 587 506. Transection through the Graf Spee embryo shown in figure 505 . 588 507. Three successive stages in the process of closure of the medullary (neural) groove to form the medullary (neural) canal and neural (ganglionic) crests ........ 588 xxvi LIST OF ILLUSTRATIONS FIGTTRE PAQB 508. Cell lining the neural canal of the newly-hatched rainbow trout, showing mitochondria in an embryonic nerve cell . . . 589 509. Section through medullary plate of a rabbit embryo . . . 589 510. Section through medullary plate of closing neural groove of rabbit embryo .......... 590 511. Section through wall of later neural tube of rabbit embryo, showing a stage in the differentiation of the ependyma cells and the forma- tion of a myelospongium ....... 590 512. Section of wall of forebrain of four-day chick embryo . . . 591 513. Diagram of a transection of the spinal cord of an early embryo, show- ing the migration of neuroblasts toward the marginal veil, and the ventral nerve root . . . . '. . . .591 514. Transection of the spinal cord of a human embryo of four weeks . 592 515. Transection of the spinal cord of an embryo chick . . . 593 516. Reconstruction of the anterior portion of the body of a chick, the head distinctly differentiated, seen from the surface . . 593 517. Transection of the spinal cord of a child, seventh cervical segment . 595 518. Diagram of the fiber paths of the spinal cord . . . .518 519. Transection of the spinal cord of a child, third sacral segment . 600 520. Transection of the spinal cord of a child, fifth lumbar segment . 600 521. Transection of the spinal cord of a child, eighth thoracic segment . 601 522. Transection of the spinal cord of a child, fourth cervical segment . 604 523. Median sagittal section through the brain . . 605 524. From a section of the cerebellar cortex of man . . . 606 525. A Purkinje cell from the human cerebellar cortex . . 607 526. A Purkinje cell from the cerebellar cortex of the rabbit . . 608 527. Diagram of the cerebellar cortex . . . . . .610 528. Left lateral surface view of cerebral cortex in man, showing the lobes, main sulci, and the larger functional areas . . . 612 529. Large pyramidal cell of the cortex . . 613 530. Scheme of the motor area of the cerebral cortex, showing the effect of various staining methods 614 531. Disposition of the nerve fibers in the cerebral cortex of man . .615 532. Human cortex cerebri, motor area 616 533. Human cortex cerebri, parietal lobe . . . 617 534. Human cortex cerebri, olfactory region . . . . .619 535. Section of the spinal cord and its membranes, from the upper thoracic region 621 536. Dissection of eyelids and lacrimal apparatus .... 626 537. Horizontal section of the right eyeball ..... 627 538. The anterior segment of a child's eye; meridional section . . 629 539. From a meridional section of the human cornea .... 630 540. Corneal corpuscles of the frog 632 LIST OF ILLUSTEATIONS FIGURE PAGE 541. Corneal cells, isolated 633 542. From a meridional section of the choroid coat . . . 637 543. The ciliary body and the adjacent structures; meridional section . 639 544. The developing eye in meridional section; diagrammatic . . 645 545. Schematic reconstruction of the developing eye .... 646 546. The retina of a child's eye; meridional section . . ... 647 547. Pigmented epithelium of the retina, viewed in transection . . 647 548. Isolated rod and cone visual cells of the pig .... 648 549. Diagram of the rod and cone visual cells, and their respective bipolar neurons .......... 649 550. A rod and cone visual cell from the fundus of the human retina, outside the macula lutea 650 551. Two cones from the human retina ...... 650 552. From the human retina . . . . . . . .651 553. Diagrams of the human retina, showing the relationships to each other of the retinal neurons, and their disposition in the different layers 652 554. From a meridional section of a child's eye, showing the layers of the retina at a point midway between the macula lutea and the ora serrata .......... 653 555. Horizontal cell from the retina of a calf ..... 655 556. Two amacrine cells from a transection of the retina of a calf . 656 557. A nerve cell of the large ganglion cell layer; from the retina of a cat . 657 558. A fiber cell of Miiller, or sustentacular cell, from the dog's retina . 658 559. Transection through the fovea centralis retinae .... 660 560. Developing rod and cone visual cells, from the retina of a 345 mm. (6 mos.) human fetus ........ 661 561. Two early stages in the development of the rod and cone visual cells in the chick 662 562. Diagram illustrating Balfour's theory to account for the inversion of the visual cells of the vertebrate retina . . - . . 663 563. Entrance of the optic nerve ....... 664 564. Lens fibers 667 565. The nuclear zone at the margin of the crystalline lens of a child's eye, showing the transition of the lens epithelium to the lens fibers and the attachment of the suspensory ligament ... 668 566. Schematic representation of the intrinsic blood vessels of the eye . 671 567. Vertical section through the upper eyelid ..... 675 568. Arterial supply of the eyelid . ... 678 569. Section through a lobule of the lacrimal gland of man . . 680 570. Portions of two adjacent lobules of the lacrimal gland of the rabbit, showing two stages in secretory activity of the tubules . .681 571. Transection of the lobule of the external ear of an infant . 683, xxviii LIST OF ILLUSTEATIONS FIGURE PAGB 572. From the external acoustic meatus of man .... 684 573. Transection of the tympanic membrane of a child . . . 687 574. Section through the margin of the tympanic membrane of a child . 688 575. The auditory ossicles 689 576. The cavity of the tympanum, viewed from above . . . 690 577. Transection of the Eustachian tube; diagrammatic . . . 692 578. The bony labyrinth 694 579. Diagram of the membranous labyrinth in lateral view . . 695 580. Diagram of the right membranous labyrinth .... 696 581. The isolated membranous labyrinth ...... 696 582. Transection of the margin of the macula acustica sacculi of a guinea- pig . . .... 697 583. Nerve endings in the macula acustica of a guinea-pig . . . 698 584. Transection of a human semicircular canal .... 700 585. Axial section through the cochlea of a fetal calf .... 701 586. Axial section through a turn of the cochlea of a guinea-pig . . 702 587. A radial section through Corti's organ in the first turn of the human cochlea 707 588. Diagram of the organ of Corti 711 589. Axial section through Corti's organ of the guinea-pig, showing the terminal nerve fibrils . . . . . . . .713 590. Scheme of the vascular supply of the internal ear ... 714 591. Scheme of the vascular terminations in the wall of the cochlear canals .......... 715 592. Semidiagrammatic illustrations of successive stages in the develop- ment of the internal ear of the chick . . . . . 718 593. Wax reconstructions of three early stages in the development of the internal ear (membranous labyrinth) of man . . . .718 594. A method of preparing a paper box for paraffin embedding . . 737 A. Successive stages in the spermatogenesis of Schistocerca damnifica . 465 B. Successive stages in the spermatogenesis of Schistocerca damnifica (continued) ......... 467 C. Successive stages in the spermatogenesis of Schistocerca damnifica (continued) . . . . . . . . 469 D. Successive stages in the growth, maturation, and fertilization of the egg of the starfish, Asterias forbesii . . . . . .471 A TEXT-BOOK OF HISTOLOGY A TEXT-BOOK OF HISTOLOGY CHAPTER I INTRODUCTION PROTOPLASM CELL INTRODUCTION Definition of Histology. Histology is the science of tissue strnc- ture^plant or animal.^ It concerns itself, therefore, chiefly with the "structural^characteristics and interrelationships of the component ele- ments of tissues. These elements are the cells, and the material con- necting or separating the cells, the intercellular substances. A tissue consists of cells associated in the perforrnance_pf a specin7r~ftmrtioii. A cell may be defined ...in a preliminaixjway as _ jhjnrl..of _ Qrgamc_structiirc' and function. The minuter details of histology involve "also cell anatomy or cytology. Here we meet with the essential substance of the cells, the protoplasm, or bioplasm, the 'material basis of life.' We also meet with the chief 'organ' of cells, the nucleus. A completer definition of a cell may accordingly be given as a circumscribed mass of protoplasm containing a nucleus (Fig. 1). A com- FIG. 1. VARIOUS plete histologic description embraces, therefore, SPHEROIDAL CELLS, details of the relationships of the component cells /> fr m ovar y ... T ,, ,. -.of a child; 2, sperm- of a tissue, and of the protoplasmic structure and a t oc yte; and 3, sperm- nuclear characteristics of the types of cells in- atid, from the testicle volved. Histology includes further the data of of . a ral ? bit - . Hema - .. . . n tein and cosin. x tissue origin and development, or hist o genesis, and 759 of cell origin and development, or cytogenesis. Cells are the 'building stones' of tissues ; tissues combine to form organs ; organs are associated into systems. Histology is accordingly a part of general anatomy; it is tissue anatomy; that part of histology which con- siders the relationships between tissues in organs is sometimes spoken of as microscopic anatomy. 2 1 2 INTRODUCTION PEOTOPLASM CELL Historical Development of Histology. Modern human histology had its origin in the work of BjdiajLJJ.771-1801). He did not employ the microscope; but his careful and extensive studies of the minute anatomy of tissues gave the impulse and general outline for later studies "by means of the microscope through which mammalian histology has grown to a relatively complete science. Great impetus was given also by the announcement of the 'cell theory' by Schleiden and Schwann in 1839, namely, the statement that all tissues areTcomposed of structural units, or cells. Other epochal steps in histologic science were the recognition of the nucleus by Eobert Brown in 1831, and of protoplasm by v. Mohl in 1846. Cytology arose almost as an incident to embryology. It traces its origin to the work of 0. Hertwig on the fertilization of the sea urchin's, egg (1875). It is the infant anatomic science, its late develop- ment being due, largely, to its dependence upon the optical and me- chanical refinements of the microscope. It deals with fundamental structures within the limits of visibility, and is destined to grow to vast proportions, as the already voluminous literature on 'mitochondria' ('plastosomes') in part foreshadows. Relation of Jpstology to Other Biologic Sciences. Histology aims to complete anatomic knowledge. It is thus the complement of gross anatomy. It furnishes also essential preliminary data for the understanding of pathology; abnormal structure and function become fully intelligible only in the light of normal histology. It is funda- mental also to physiology, the science of normal function. A certain function demands a specific structure; structure and func- tion sustain reciprocal relationships. Normal function depends upon the normal structure of the cells involved in the function; abnormal function, or disease, is associated with altered cellular structure. His- tology gains enormously in interest and value to the student who will always keep well in mind the function that a certain structure under consideration is called upon to perform. Embryology also to a consid- erable extent builds upon histologic and cytologic data. PROTOPLASM Chemical Constitution. The unit of both structure and function is the cell. The essential constituent of cells is protoplasm. Protoplasm may be thought of as a physicochemical mechanism. Chemically, it is a very complex aqueous mixture of substances, containing the elements, PEOTOPLASM 3 carbon, oxygen, hydrogen, nitrogen, and small quantities of sulphur, phosphorus, calcium, sodium, chlorin, magnesium, potassium and iron. The principal compounds of protoplasm are proteins, which furnish the main source of energy expended in function; carbohydrates; fats; and water, which constitutes about three-quarters of its weight. It is believed by one school of biologists (mechanists) that if we had "the w.v fiSsaBEsv] M^sm*:-:-/ : Sfe^i&W2^S7 ^J^i^^^S 1 / ^E^^S^SssSx g^Rf^r ^-^ c# ,.;.- : ;6:?^: /V^v FIG. 2. AMEBA PROTEUS IN MOTION. c.v., contractile vacuole; f.v., food vacuole; n., nucleus; w.v., water vacuoles. The arrows indicate the direction of the protoplasmic flow. Note the peripheral non- granular ectoplasm, and the granular endoplasm. (From Calkins' "Biology," H. Holt & Co., after Sedgwick and Wilson.) formula for the proper stereo-isomeric association of the elements and compounds of protoplasm, life could be artificially produced; another school of biologists (vitalists) assume an additional ' vital principle' as a prerequisite for life. Physical Constitution. Physically, protoplasm is a granular semi- fluid or gelatinous substance. It possesses properties characteristic of both solids and liquids. It is an aggregate of colloids and crystalloids. The physicochemical laws which govern the crystalloids and colloids underlie the properties of living matter. An organism is essentially INTRODUCTION PEOTOPLASM CELL TRiCHOtYSTS T CANAL9 an aqueous solution, holding in suspension colloidal substances of great complexity. Crystalloids are divisible into two groups: electrolytes and non-electrolytes. The one (salts, acids, bases) in solution conduct the electric current, the others (urea, sugar) do not. Colloids ex- ist in two states, a liquid or sol state, and a semi-solid or gel state. There exists no sharp line of di- vision between colloids and crystal- loids ; these terms designate phases or states rather than substances; between them lie all kinds of inter- mediate grades. Protoplasm is a sol ; and since its fluidity is due to water, it is commonly classed as a hydrosol. It passes readily into a gel condition, thus becoming a hy- drogel. In living protoplasm this metamorphosis is a reversible proc- ess. Agents which effect an irre- versible gelation of protoplasm tend to bring life to a standstill. Fixation, or killing, of tissue for microscopic study consists in a sep- aration of the more solid part of colloidal protoplasm from a more liquid part. Death is histologically such a process of coagulation. Liv- ing protoplasm may be studied to good advantage in the one-cell ani- CONTRACTILE FIG. 3. PARAMECIUM CAUDATUM. Note the peripheral cilia and the gran- ulo-alveolar character of the protoplasm. (From Calkins' "Biology," H. Holt &Co.) mal forms, Ameba (Fig. 2) or Paramecium (Fig. 3). These and other equally favorable protozoan forms are readily available from hay infusion cultures, and can be profit- ably employed for the demonstration also of the simpler modes of proto- plasmic activity, and of the changes suffered by protoplasm in passing from the living to the dead condition. Since protoplasm is commonly organized into cells, the next step demands a knowledge of a typical or generalized cell. THE CELL THE CELL General Statements. A generalized cell is of spheroidal shape (un- modified by pressure) and contains certain 'organs' and a variety of fun- damental and secondary elements (Fig. 4). A cell (or protoplast; Hanstein) is a mass of protoplasm endowed with vital propertied "The confines of such a cellular mass of protoplasm exist in a cell membrane. FIG. 4. A GENERALIZED CELL. a, exoplasm; 6, endoplasm; c, spongioplasm; d, hyaloplasm; e, microsomes; /, chromidia; g, centrosome (centriole) ; h, centrosphere, surrounded by astrosphere; i, cell membrane;,/, deutoplasmic granule; k, fluid vacuole, or oil drop; I, mitochondria or plastosomes; m, nuclear membrane; n, nucleolus; o, linin; p, karyosome; q, chromatin (net knot); r, foreign inclusions, pigment, etc. (metaplasm). This represents a differentiation product of protoplasm; when robust as in plant cells, it forms a cell wall. In certain cells, e.g., white blood- cells, it is apparently lacking; however, in these so-called naked cells the peripheral layer of protoplasm is more condensed and most probably subserves the osmotic function of a distinct membrane. In fact, the surfaces of protoplasm possess the properties of semipermeable mem- branes, probably lipoid in nature. An essential organ of the cell is the nucleus. It is trophic in function, the center of oxidation processes. In certain protozoa this is represented by scattered nuclear materials 6 INTRODUCTION PEOTOPLASM CELL or granules (Fig. 5). The shape of the nucleus is spherical; typically it has a central location, but it frequently assumes eccentric positions. It is physically denser and more elastic than the extranuclear protoplasm. Its periphery simulates, or perhaps consists of, a membrane, the nuclear watt. Whether as a membrane it be complete or reticulated, whether of nuclear, cytoplasmic or composite origin, are undecided points. Eecent investigations on the nuclear membrane indicate that it is fenestrated; such conditions would per- mit of an easy escape of nuclear material into the cytoplasm. Nucleus. The protoplasm composing the nu- cleus is known as nudeoplasm or Tcaryoplasm; that constituting the remainder of the cell, the cytoplasm. The nuclear constituents include a more fluid ground substance or nuclear sap (karyo- lymph: paralinin), throughout which extends a delicate reticulum of linin threads (Fig. 4). Upon these linin or achromatin threads are sup- ported, more abundantly at the points of inter- section of the mesh, granules (chromioles) and masses (net-knots) of a substance staining deeply in the basic dyes, the chromatin. Spheroidal net- knots are known as karyosomes. The linin is said to be achromatic. Whether it is chemically different from chromatin or simply more atten- uated chromatin is disputed. The 'chromatic' gran- ules themselves undergo changes in stainability : on the basis of reaction to acid and basic dyes, this substance is divided into oxy chromatin (Ian- tanin) and basichromatin. Liniri and chromatin are regarded by some as different phases in the elaboration of the same substance. The nucleus in- cludes, furthermore, usually one, frequently more, nucleoli. These do not grade into the nuclear sap, like the nuclear network, but are limited by a sharp line of demarcation. They may be achromatic, when they are known as plasmosomes, or they may take on chromatin, becoming chromatin nucleoli. It is uncertain whether the lat- ter are identical in all cases with the karyosomes. The difference among nucleoli is more probably one of degree of abundance of chromatin. The nucleus is the metabolic organ of the cell ; without a nucleus a cell may FIG. 5. A UNI- CELLULAR FLAG- ELLATE ANIMAL (TETRAMITUS CHILOMONAS) . Showing the nu- clear material dis- tributed as granules throughout the cell. (Redrawn from Calkins.) THE CELL continue to live for a time, but it can neither grow nor undergo progres- sive differentiation. All changes in enucleated protoplasm are regressive, leading to death. The nucleus is also largely the reproductive center, as will be described below. The nucleolus plays the role, among other pos- sible functions, of a center of storage, perhaps also elaboration, of chro- matin. Nuclear protoplasm, more especially the chromatin, is relatively rich in phosphorus. Astral System. Another organ of a typical cell is the aster, astral system or attraction sphere. Its substance is collectively known as archo- plasm. It usually lies outside of, but close to, the nucleus; in cer- tain cells it is intranuclear, e.g., spermatocytes of Ascaris. It con- sists centrally of a granule, the centrosome; in this, in certain in- stances, may be differentiated cen- trally a smaller granule, the cen- triole; when the latter appears, the more outlying portion of the centrosome is designated the cen- troplasm. The centrosome may divide into two, becoming a diplo- some, or in some instances it may become multiple, when it is FIG. 6. EGG OF A CLAM (CUMINGIA TELLINOIDES). Showing the first maturation spindle with centrosomes and chromosomes at metaphase, and the disappearing nucleolus (ri) at the right. X lOOQ. known as a pluricorpuscular cen- trosome. Surrounding the cen- trosome is a clearer, minutely granular sphere, the centrosphere; radiating from this peripherally are delicate astral rays, collectively known as the astrosphere (Fig. 6). Structurally the aster is subject to considerable variations in different cells. On account of its relation to cell division, it is regarded as the dynamic center of the cell ; viewed thus its substance is known as kinoplasm. The attraction sphere may or may not be visibly present; in all living cells it, or its analogue, is generally believed to be potentially present. Cytoplasm. The cytoplasm may be divided into a thin peripheral or cortical layer of less granular protoplasm, the exoplasm (ectoplasm), and the main central mass, the endoplasm. In certain highly differen- tiated cells the exoplasm is not discernible. In others, at certain stages in the development it contains the products of differentiations, when it is 8 INTRODUCTION PEOTOPLASM CELL FlG. 7. P R I M A R Y SPERMATOCYTE OF A TURTLE (CiSTUDO CAROLINA). Showing chromatic spherules ('chromidia') apparently in process of extrusion from the nucleus. X 1500. known as 'deuteroplasm' (Studnicka). The endoplasm is commonly described as consisting of a more fluid, finely granular ground substance, the hyaloplasm (paraplasm, interfilar mass, para- mitome, enchylema, cytolymph), containing a del- icate denser reticulum, or cytoreticulum, with polygonal or spheroidal meshes. The substance of the reticulum is called spongioplasm (mitome; filar mass). It is held by some to be continuous with the linin mesh of the nucleus. The granules of the ground substance, both free of and attached to the spongioplasm, are called microsomes. Ac- cording to one interpretation the spongioplasm arises by coalescence of microsomes. A more re- cent interpretation regards both network and granule as simply more condensed portions of the hyaloplasm. The cytoplasm may contain, besides the aforementioned fundamental constituents, nu- tritive materials including yolk granules or glo- bules (deutoplasm) ; vacuoles, foreign enclosures, e.g., bacteria, etc., and pigment (metaplasm) ; plastids (in plant cells) ; chromidia (Fig. 7), masses of chromatic gran- ules, presumably of nuclear origin, and probably the raw material for certain differentiation prod- ucts; and mitochondria or plastosomes. Mitochondria. Mitochondria are cytoplas- mic elements of very variable form and of almost universal distribution. These are destined to bulk very large in immediate cytological investi- gations. They may prove to be very important elements of the more fundamental protoplasmic structure and function. In the germ cells of vertebrates, as in undifferentiated cells generally, they are for the most part, granular (chondrio- somes) (Fig. 8) ; in the somatic differentiated cells filamentous or rod-shaped (chondriomites; chondrioconts; pseudochromosomes) (Figs. 9 and 10). Both chromidia and trophospongium (a canalicular network of the cytoplasm, prob- ably concerned with circulation of nutritive material or secretion prod- ucts) (Fig. 11) have been identified with mitochondria. Trophospon- FIG. 8. SPERMATID OF OPOSSUM IN EARLY STAGE OF METAMOR- PHOSIS INTO A SPER- MIUM. Showing granular mitochondria, m, in the cytoplasm, n, nucleus, s, archoplasmic sphere. X 2000. THE CELL gium at least is a distinct structure, and chromidia more probably also, though by some regarded as the elements from which the filamentous mitochondria are formed. Mitochondria have been credited with most I f -r > "**"" ^mm~~- a b c d FIG. 9. CELLS FROM THE NEWLY HATCHED RAINBOW TROUT, TREATED ACCORDING TO MEVES' TECHNIC FOR THE DEMONSTRATION OF MITOCHONDRIA (PLASTOSOMES). a and b, cartilage cells; c, young blood cell; d, epithelial cells from the intestine. X 2000. various functions, e.g., formation of presecretion and excretion granules, and the formation of various kinds of fibrils. M. Heidenhain regards the chondriosomes as vegetative organs of the cells subserving metabolism. FIG. 10. Two CELLS FROM THE MESENCHYMA OF THE NEWLY HATCHED RAINBOW TROUT. The one to the left (a) at late anaphase of mitosis, showing mitochondria (plasto- somes). M eves' technic. X 2000. Our knowledge is as yet too limited to speak with assurance either as to their origin, complete function, or fate. One thing only is certain, namely, that they are actual constituents of the cytoplasm of practically 10 INTKODUCTION PEOTOPLASM CELL every type of cells, at certain, perhaps all, stages of development and active function. They have been seen and studied even in living plant cells (Maximow), and in animal cells grown in artificial media they were observed by M. E. and W. H. Lewis (Amer. Jour. Anat., 17, 3, 1915) to move, to change shape, to divide into granules and again to reunite into filaments facts which render inadmissible their interpretation in fixed material as lipoid precipitation products (Faurier- Fremier), and strongly suggest their connection with FIG 11 INTRA- me tabolic activity. They have the chemical composi- CELLULAR NET- tion of a lipoid (probably a phosphatid) united to an WORK, OR 'TROPH- albuminoid base. It has been suggested that they are OSPONGIUM,' WITH- _ & IN A PURKINJE a su PP r t to, and the region of, oxidation. Certain CELL OF THE CERE- investigators (Benda, Meves, Duesberg) regard them BELLUM OF STRix as t ne cytoplasmic basis of heredity, and ascribe to FLAMMEA. . . . . Golgi's stain. (After them an im P ortant role in histogenesis. Golgi.) Schreiner's findings in developing fat cells sug- gest a nutritive significance (vide, p. 64). STRUCTURE OF PROTOPLASM Four main types of protoplasmic structure are generally recognized: (1) the homogeneous; (2) the granular; (3) the alveolar or foam type; and (4) the fibrillar, (a) reticular or sponge type and (b) filar (Fig. 12). The type of protoplasm of a particular cell may vary with the stage of development and function. In successive stages of development and differentiation the protoplasm of the same cell may pass from the apparently homogeneous, through the granular and granulo-alveolar, to a granulofibrillar type. Homogeneous protoplasm, as for example the ectoplasmic layer of ameba, is more probably to be interpreted as com- posed of minute, perhaps ultramicroscopic, colloidal granules ('colloidal biogens'). Young and undifferentiated cells commonly have a granular cytoplasm. In general it may be said that the actual fundamental type of protoplasm is the granular. This changes into the alveolar type by the appearance of spherules among the granules. The contents of the alveoli constitute the alveolar substance; the inter-alveolar sap, the hya- loplasm ; the granules, the microsomes ; and the walls of the alveoli may be identified with the spongioplasm of the reticular type of protoplasm. Another interpretation of alveolar protoplasm regards the content of the FIG. 12. DIAGRAM ILLUSTRATING THEORIES OF PROTOPLASMIC STRUCTURE. 1, alveolar structure; granules occur only at the angles formed by the alveoli. 2, filar structure, showing filar and interfilar substance. The centrosome (a diplosome) is represented in this portion; it is surrounded by a clear attraction sphere. 3, granular structure; coarse microsomes irregularly disposed. This portion contains three foreign bodies which have been included by the cell, a streptococcus, a crys- tal, and a spheroidal pigment mass. 4> the alveolar walls are formed by regularly arranged microsomes; a vacuole is shown in this section. 5, reticular structure. The cell is inclosed by a cell membrane, and contains a central nucleus in which are shown the nuclear membrane, indistinct linin fibrils, deeply stained chromatin in coarse threads and irregular masses (karyosomes), and a centrally sjtuated nucleolus or plasmosome. FIG. 13. EGG OF THE BLOOD STARFISH IN LATER GROWTH PERIOD. Showing a stage in the change of an earlier granular to a later alveolar condition jf the cytoplasm. The nucleus contains many spherical nucleoli of various sizes. The space (a) is a fixation artifact. X 1500. 11 INTRODUCTION PEOTOPLASM CELL alveoli as hyaloplasm. The microsomes may in part be closely associated with the alveolar walls, perhaps forming them and the spongioplasm by the process of coalescence. The so-called alveolar a*VJ>% protoplasm is in reality of the granulo-alveolar type. &' ** spermatozoa arise from the centro- some, and the ob- servation that in certain cells the centrosome of mi- totic spindles de- velops cilia (Fig. 27). (b) MITOSIS. This is the prevail- ing type of cell di- vision. For con- venience of descrip- tion the process, which must of course be thought of as continuous, may be divided into (1) prophase; (2) metaphase; (3) anapliase; and (4) telopliase. An alternative and preferable terminology employs the words anaphase (prophase), mesophase (metaphase) and kataphase (anaphase and telophase). These phases involve coincident changes in the nucleus and the archoplasm (attraction sphere, Fig. 28, A to J). Mitotic figures can be seen in all rapidly growing tissues. The process is an essentially similar one throughout the plant and animal kingdoms; variations relate only to details associated chiefly with the archoplasm. The most favorable locations for study of mitosis are the growing tips of roots of certain plants, e.g., onion, hyacinth, dogtooth violet; amphibian tissue (particularly skin and blood-cells), and the testes of grasshoppers. Mitosis in germ cells involves certain specialized features, and calls for additional theoretical consideration; hence the description of these maturation mitoses will be reserved for the chapter dealing with the ovaries and testes, where a complete account will follow. Among the simplest types of mitosis, and those best adapted for labora- FIG. 29. CELLS FROM EPIDERMIS OP THE SALAMANDER. Three cells are in process of division by mitosis, a, prophase; 6, anaphase. The second cell above 6, whose cell body is in process of fission, presents a stage of the telophase. (After Wilson.) FIG. 30. SUCCESSIVE STAGES OF MITOSIS IN THE ROOT TIP OF THE DOGTOOTH VIOLET (ERYTHRONIUM AMERICANUM). a, resting nucleus; 6, close spireme; c, loose spireme; d, segmented spireme; e, late prophase;/and g, metaphase; h and i, anaphase;.?-?, telophase (showing mid-body or cell plate); m and n, daughter cells, n, with resting nuclei. X 1500. 23 24: INTRODUCTION PEOTOPLASM CELL tory study, at least as an approach to the subject, is that shown in the root tip of the dogtooth violet (Fig. 30, aton). The cells and the mi- totic figures are here so large that all the major details can be easily recognized by use of the usual dry high power lenses of the microscope. (1) Propliase. This stage can again be subdivided into that (a) of the resting nucleus; (b) of the nucleus with close spireme; (c) of the nucleus with the loose spireme; and (d) of the segmented spireme. Coincident with these nuclear changes, the centrosome in animal cells divides into two (diplosome) ; these moieties move apart toward opposite poles of the nucleus and build a spindle (amphiaster) between them- selves. Meanwhile the nuclear membrane begins to disappear, only a remnant distant to "the achromatic spindle persisting at the end of the prophase. In the root tip cell of the dogtooth violet and in plant cells generally, the spindle appears in less conspicuous fashion than in animal cells. A centrosome is apparently lacking. The first indications of the spindle are the polar caps of faint radiations which grow medially to build the spindle. The resting nucleus (Fig. 30, a) is characterized by a random, granular, nuclear reticulum with net-knots and one or several nucleoli. This reticulum becomes changed into a delicate, deeply chro- matic, probably continuous, close spireme (Fig. 30, b). By process of shortening and thickening, this changes into the loose spireme stage (Fig; 30, c). The nucleoli have meanwhile contributed chromatic sub- stance to the spireme, but may persist for some time longer as achro- matic, ultimately fragmenting or dissolving, bodies. On closer inspection the close spireme is seen to consist of a series of granules (chromomeres) ; during the loose spireme stage these become split, thus giving rise to a double row of granules. The loose spireme passes into the succeeding stage (segmented spireme., Fig. 30, d) by transverse segmentation into a number of rods or chromosomes. At this stage all indication of chro- momeres is generally again lost, the chromosomes appearing as compact, deeply staining rods of various shapes. The number of chromosomes is believed to be constant for all cells of a species. This belief rests upon data of actual counts in various insects and other lower animals and certain plant forms. Here the number is rela- tively small, and the individual chromosomes are large and can in conse- quence be readily counted. (Certain qualifying statements must be made in the chapter which includes a discussion of sex determination.) Attempts have recently been made to throw doubt upon the matter of a specific chro- mosome constancy, but it is only fair to note that these attempts have dealt with relatively unfavorable material, where exact chromosome counts VITAL PEOPERTIES OF CELLS 25 are extremely difficult if not at present actually impossible. It may be noted in passing also, that the chromosomes are believed by many to be the bearers of the determiners of hereditary characters a point to be further discussed below. (2) MetapJiase. This is a relatively brief stage in mitosi?. - It includes the period when the chromosomes are arranged upon the spindle in the equatorial plate. Seen in polar view this is called the monaster stage (Fig. 30, e). In this stage the chromosomes split longitudinally. In dogtooth violet the number of chromosomes is twenty-four. A com- mon form of chromosome is the U-shaped type. The point of attachment to the spindle is the apex of the bent chromosome (Fig. 30., f). In the more rapidly growing cells the double or split condition of the chromo- somes has remained discernible since the' preceding telophase, a true resting stage having been omitted. At metaphase the already longi- tudinally split chromosomes are completely divided, and the sepa- rated moieties (daughter chromosomes) drawn toward opposite poles (Fig- 30, g). (3) Anaphase. The limits. of this phase are indefinite (Fig. 30, h to j). It may be said to include all stages between that when the separation of the daughter chromosomes, resulting from the longitudi- nally splitting of the mother chromosomes, is severally consummated, and that when the groups of daughter chromosomes drawn to either pole are still distinct. Seen in side or oblique view the later stages of this phase present a double star arrangement of chromosomes hence diaster stages (Fig. 30, i). The daughter chromosomes, an equal number at either pole, were drawn apart by activity of the outer- most of the spindle fibers (called mantle fibers) presumably by process of contraction. The inner or Interzonal fibers' constitute the central spindle. (4) Telopliase. Meanwhile a plate of granules (cell plate; mid" body) has appeared in the equatorial region of the spindle. This marks the plane of the future division (Fig. 30, j and k). In animal cells, an annular constriction appears peripherally in the cell membrane. This proceeds centrally throughout telophase until ultimately the mother-cell is divided into two daughter-cells. The constriction of the cells in divi- sion is generally interpreted as a phenomenon of alteration in surface tension. Coincidently with the steps of this process, the chromosomes and cen'trosomes (archoplasm material in plants) pass through the stages of the prophase, but in inverse order : segmented spireme, loose spireme, r.S. FIG. 31. SUCCESSIVE STAGES IN THE MATURATION, FERTILIZATION AND SEGMENTA- TION OF THE STARFISH (ASTERIAS FORBESII) EGG. o, first maturation spindle (m.s.l) and the spermatozoon (s); b, formation of the first polar body (p.b.l) and the residual substance of the nucleus (r.s.); c, second maturation spindle (m.s.2] ; d, female pronucleus ( $ ) ; e, union of male ( $ ) and female pronuclei (the male pronucleus is derived from the spermatozoon) ; /, first segmenta- tion spindle; g, two-cell stage of division, d and e are less highly magnified. 26 HISTOGENESTS 27 close spireme, resting daughter nucleus with its daughter centrosome and ultimately a nucleolus (Figs. 30, k, 1, m, n). Where a cell -plate appears, division is consummated without constriction. In certain pathological tissues, e.g., cancers, the cells divide in various atypical ways, involving the formation of tri- and multipolar spin- dles. HISTOGENESIS Every higher organism begins as a fertilized 'egg or zygote; this in- volves the fusion of a male (spermatozoon) and a female (egg) germ cell (Fig. 31, a and e). The result of the fusion is a mingling of approxi- mately equal parts of paternal and mater- nal chromatin (pre- sumably the basis of specific heredity), a large mass of mater- nal cytoplasm and nutritive substance with a small, but perhaps important mass of male cyto- plasm; and a coinci- dent stimulus to de- velopment. The fer- tilized egg divides by mitosis into two spheroidal cells the typical embryonic form or blastomeres (Fig. 31, f and g), and each of these FIG. 32. TRANSVERSE SECTION OF A FROG EMBRYO, SHOWING THE THREE GERM LAYERS. a, neural crest; 6, neural groove; c, neural plate; d> ccelom; e, ectoderm;/, mesoderm; g, entoderm; h, somite; i, notochord; j, parietal mesoderm; k, visceral meso- derm; I, yolk; central opening, the primitive intestine. (Drawing by G. A. Pagenstecker.) again into two, the segmentation process continuing until the adult organism results as an aggregation of innumer- able cells. This process of growth through cell multiplication is accom- panied by cell differentiation, which constitutes histogenesis. The first outstanding stage in the differentiation is that when the three funda- 28 INTRODUCTION PROTOPLASM ELL mental germ layers: ectoderm (ectoblast; epiblast), mesoderm (meso- blast),, entoderm (endoblast; hypoblast) have appeared (Fig. 32). Back of this of course must lie a more fundamental differentiation, perhaps already present in the unfertilized egg, a predelineation of adult struc- ture destined to develop from localized egg materials. By the process of histogenesis all the adult tissues arise from the several germ layers. This matter is summarized in the following table : ADULT TISSUE DERIVATIVES FEOM ECTODERM Epidermis and its de- rivatives : hair, nails, and epithelium of se- baceous, sweat and mammary glands. Epithelium of mouth and its derivatives : enamel, taste buds, epithelium of salivary and other b u c c a 1 glands, and anterior portion of hypophysis. Epithelium of anus, and distal portion of the male urethra. Epithelium of nos- trils and communicat- ing glands and cranial sinuses. Epithelium of con- junctiva and associated ducts and lacrimal glands. The lens, and the epithelium of the pars nervosa, ciliaris and iridica retinae. Epithelium of mem- MESODERM Epithelium of urinif- erous tubules, renal pelves and ureters. Epithelium of the seminiferous tubules and the associated ex- cretory ducts of the tes- tis; epithelium of ovi- duct and uterus; prob- ably also the sex cells. The cortex of the su- prarenal gland. All muscular tissue ; con- nective tissue ; vascular tissue (blood and lymph vessels and cells), and lymphoid organs in general. Epithelium (meso- thelium) of pleurae, pericardium and peri- toneum; of the tendon sheaths, joint cavities and bursse; and of the chambers of the eye, and the perilymph spaces of the internal ENTODERM Epithelium of diges-. tive tract (including pharynx; excluding mouth and anus) and associated glands: pharyngeal, esophageal, gastric, intestinal, pan- creas and liver, with gall-bladder. Epithelium of middle ear (tympanum) and auditory (Eustachian) tube. Epithelium of respiratory system, be- yond nostril. Epithe- lium of thyroid, para- thyroids, and the thy- mic reticulum and cor- puscles. Epithelium of female urethra, proximal part of male urethra, and of the urinary bladder. Epithelium of pros- tatic and Cowper's glands in the male, and of the glands of CYTOMORPHOSIS 29 ECTODERM branous labyrinth of internal ear, and lining of external ear. Epithelium lining the central canal of the spinal cord, and the ventricles of the brain. All neurons and neu- roglia of the nervous system. Certain ductless glands: pineal, posteri- or (nervous) portion of hypophysis, medulla of suprarenal, and the chromaffin system or paraganglia. Possibly smooth mus- cle associated with sweat glands, and in iris of eye. MESODERM ear (scalse tympani and vestibuli). ENTODERM Bartholin in the fe- male. Nuclei pulposi of vertebrae, remains" ~t>f the embryonic noto- chord. (Of ectodermal origin in the guinea pig, according to G. Carl Huber ; Anat. Kec., Vol. 14, p. 217, 1918.) CYTOMORPHOSIS From the standpoint of the individual cells of tissues, histogenesis involves progressive and regressive changes. This process may be desig- nated as cytomorphosis (Minot). The gradual acquirement of definite form by development is known as morphogenesis. Cytomorphosis in- cludes several successive steps: (a) undifferentiated or embryonal stages; (b) differentiated stage, during which the cell acquires and maintains its maximum differentiation expressed structurally by a definite shape and specific content and performs its specific function; (c) regressive, when the function gradually wanes, and finally fails (reflected in coincident protoplasmic alterations), the cell concerned suffering death and ulti- mately removal from the body. With this preliminary general view of protoplasmic organization and function (general cytology) we are prepared to approach histology proper. CHAPTER II EPITHELIAL TISSUES TISSUES A tissue in the histologic sense is a collection of similarly specialized cells united in the performance of a particular function, e.g., liver tissue. In certain tissues the cells are joined together by an intercellular cement substance, which is a secretion product of the cells themselves. Through this cement may extend the so-called 'intercellular bridges' or cytodesmata (Fig. 33), the minute intervening spaces forming delicate FIG. 33. GROUP OF EPITHELIAL CELLS FROM THE MALPIGHIAN LAYER OF THE SKIN. The intercellular bridges are very distinct. Hematein and eosin. X 1,000. canaliculi, presumably for mediating the transfer of nutritive material from cells more favorably placed with respect to the source of supply to those less favorably located, e.g., epidermis; these bridges arise through process of vacuolization in the exoplasm of adjoining cells, the walls of the original vacuoles persisting as 'bridges.' Through such bridges, fibrils may extend from cell to cell. Practically every tissue contains 30 TISSUES 31 also connective tissue elements for unification and support; also vascular and nervous constituents. Tissues in which the cell boundaries are ab- sent are known as syncytia (Fig. 34). A syncytium may obviously arise through nuclear proliferation in the ab- sence of cytoplasmic division,, or as the result of the disappearance of original cell boundaries. We may distinguish FIG. 34. A VILLUS OF THE HUMAN PLACENTA, SHOWING A PERIPH- ERAL SYNCYTIUM OF IRREGULAR THICKNESS. The connective tissue inclosed by the syncytium contains three capillary vessels. Hematein and eosin. X 500, FIG. 35.- CELLS FROM THE PANCREAS OF NECTURUS, CONTAINING SECRETORY GRAN- ULES AND BASAL ERGASTOPLASMIC FILA- MENTS. (After Matthews.) the following fundamental tissues: (a) epithelial; (b) connective; (c) muscular; (d) nervous; and (e) vascular. Lymphoid tissue may be regarded as still another fundamental tissue; FIG. 36. VARIOUS FORMS OF CELLS. a, squamous epithelium from the tongue; b, a columnar cell from the small intestine; c, a polyhedral or spheroidal cell from the liver; d, a smooth muscle cell from the mus- cular coat of the stomach. X 550. or it may be included under vascular tissue. In fact from the genetic viewpoint, vascular may be included under connective tissue, since both arise from the mesenchyma. 32 EPITHELIAL TISSUES Representatives of all of the fundamental tissues are generally found in all histologic preparations, or tissues in a general sense. Cells vary greatly both from the standpoints of shape and- contents in the various tissues both depending upon the types and phases of function. The more usual form variations include: (a) spheroidal, spherical (e.g., em- bryonic cells and egg cells, Fig. 1, chap. I), polyhedral (spherical cells modified by pressure from adjacent cells, e.g., liver cells, Fig. 37) ; (b) scalelike or squamous (e.g., super- ficial cells of mucous membrane of * mouth, Fig. 36, a) ; (c) columnar, prismatic or cylindrical (e.g., cells r lining intestine, Fig. 38, b). Col- umnar cells, when very short, are FIG. 37. POLYHEDRAL EPITHELIUM, FROM A SECTION OF THE HUMAN LIVER. The central blood capillary contains one leukocyte, and its wall contains the nucleus of a flattened endothelial cell. Hematein and eosin. X 550. FIG. 38. GOBLET AND COLUMNAR CELLS FROM THE LARGE INTESTINE OF THE CAT. A, Goblet cells; B, isolated columnar cells. X 900. usually designated cubical or cuboidal (e.g., bronchioles and rete testis^ Fig. 43) ; intermediate .lengths may be designated either tall cuboidal or short columnar; when modified by confinement in an alveolus into a pyramidal shape as in glands, they may be called pyramidal or 'glandu- lar 7 (Fig. 46). Glandular cells, moreover, are characterized also by an internal differentiation commonly expressed in the form of granules or filaments. Columnar cells may be further modified by the appearance of cilia into ciliated epithelium (e.g., trachea, bronchial tube, Fig. -52), or of mucus into goblet cells or 'unicellular glands' (e.g., intestine, Fig. 38, a) ; or as specialized receptors for stimuli of special sense they may become modified as neuro-epithelium (e.g., certain cells of eye, ear, nose and tongue). EPITHELIAL TISSUES 33 EPITHELIAL TISSUES Epithelia are cellular membranes covering the surfaces and lining the internal cavities of the body. They serve for protection, secretion, ex- cretion, and the reception of stimuli. The constituent cells may be-ef-any of the above enumerated forms. The spheroidal types, however, are found only in embryonal membranes. The term spheroidal epithelium is sometimes employed to designate masses or solid columns of spheroi- dal cells, such as appear in the sex cords of the developing testis and ovary, and in the early stages of glands. They are in general, outgrowths or evagination from embryonic or undifferentiated epithelia. An epithelium may consist of a single layer of cells, when it is called non-strat- ified or simple epithelium. A complete description, however, must include the name of the preponderating type of cell, e.g., simple columnar epithelium, or sim- ple squamous epithelium, as the case may be. Moreover, an epithelium may consist FIG. 39. COLUMNAR EPITHE- of several or many layers, when it becomes LIUM FROM E P RIC RE - J . J ' e GION OF THE HUMAN STOMACH. a complex or stratified epithelium. The (Profile view.) uppermost type of cells gives the name to Hematein and eosin. X 550. stratified epithelium; for example, in the epidermis the outermost cell is of the squamous type, though the middle cells are polyhedral, and the innermost columnar ; hence called stratified squamous epithelium (Fig. 49). In the stratified epithel.'a the superficial cells arise through cell divi- sion in the deeper layers, and if they become detached by abrasion, dis- integration, or by other physiological or pathological processes, they may be replaced by cell reproduction occurring in the lower layers. When but a single layer of cells is present, as in the simple epithelia, loss of cells over large areas will obviously become more difficult of replacement by cell division. Hence it is that repair of extensively destructive patho- logical conditions involving such epithelial tissues becomes exceedingly difficult and often impossible, as, for example, in the alveoli of the lung. Each epithelial cell is to some extent a secreting cell. Sometimes secretion is its chief function, as is the case with goblet cells, which might well be called 'unicellular glands/ and which secrete abundant mucus. The same is true of those cells which form the parenchyma of 4 34 EPITHELIAL TISSUES secreting glands salivary glands, kidney, and liver. In many epithelia secretion is a subsidiary function, protection being the primary purpose. In all epithelia a cement substance is present between the cells. This becomes especially abundant and dense between the distal ends of the cells of columnar epithelium, and is here known as terminal bars (Fig. 40). Cement substance has the pe- culiar property of precipitating sil- ver nitrate from solutions, which turns black on exposure to sunlight. This furnishes an especially favor- able technic for demonstrating cell boundaries. All epithelia, simple or stratified, rest upon a homogeneous basement membrane or membrana propria, frequently a product of the cells themselves but occasionally of connective tissue origin, and a sub- jacent connective tissue supporting membrane or tunica propria (or corium). The latter only contains blood and lymph vessels from which the epithelial cell must draw nour- ishment by process of absorption, and transfer through 'intercellular bridges/ It furnishes support also for the nerve supply. We may now consider briefly the usual types of simple and stratified epithelia. The main facts are summarized in the appended outline : FIG. 40. 'TERMINAL BARS' OF CEMENT SUBSTANCE AS SEEN BETWEEN THE EPITHELIAL, CELLS OF A TUBULAR SECRETING GLAND IN THE PYLORIC REGION OF THE HUMAN STOMACH. The columnar epithelium is seen in profile at a; at b, the free ends of the cells are seen. Hematein. X 550. CLASSIFICATION OF EPITHELIA I. SIMPLE (NON-STRATIFIED) EPITHELIA those which com- pose a membrane but one cell in thickness. Epithelial cells, usually spherical or polyhedral in shape, occur also en masse in the form of cords or clusters. 1. Squamous, composed of flattened, scale-like cells. (a) Lining closed cavities. Pavement epithelium or (1) endotkelium; heart, arteries, capillaries, veins, and lymphatic vessels. (2) mesothelium; serous membranes. (3) mesenchymal epithelium,; synovial membranes, bursae, and tendon sheaths, lining of the anterior EPITHELIAL TISSUES 35 Squamous, composed of flattened, scale-like cells. Columnar. chamber of the eye, and of the perilymph spaces of the internal ear. (b) Lining the alveoli of the lungs, some tubules of the kidney, the middle ear, and the membranous labyrinth of the internal ear. (c) As the superficial cells of stratified epithelium (vide infra'). (a) Lining the mucous membrane of the alimentary tract stom- ach, small intestines, large in- testines, gall-bladder. (b) Lining the ducts of all secret- ing glands liver, pancreas, sali- vary, lacrimal, and mammary glands, testicle, prostate, kidney, etc. (c) The deepest layer of cells in stratified epithelium is composed of columnar-shaped cells, which, however, differ in structure from the true columnar type. C (A) Plain (B) Modified (1) Ciliated (2) Pyramidal or 'glandular' (3) Goblet 4 (a) Lining the uterus and ovi- ducts. (b) Lining portions of the ventri- cles of the brain and central spinal canal of the embryo and infant. (In later life these cells lose their cilia.) The secreting cells of all tubular glands kidney, pancreas, sali- vary glands, intestinal glands, etc. - (a) Respiratory tr a c t n a s a 1, pharyngeal, tracheal, and bron- chial mucous membranes. (b) Alimentary tract stomach, small and large intestines. * Cells whose protoplasm has been converted into mucinogen. be considered unicellular, mucus-secreting glands. They may 36 EPITHELIAL TISSUES 2. Columnar. (4) Neuro-epi- thelium. (a) Eye the rod and cone cells of the retina. (b) Ear in the cristse and mac- ula of the labyrinth and in Cor- ti's organ. (c) Nose in the olfactory mucous membrane (true neuron). (d) Tongue in the taste buds. II. COMPLEX (STRATIFIED) EPITHELIA those whose cells \ form several superimposed layers. Forms the epidermis of the skin, and covers the free surface of those mucous membranes which clothe all orifices in direct con- nection therewith viz., the con- junctiva and cornea; the exter- nal auditory canal; part of the nasal mucous membrane ; mouth, pharynx, and esophagus; epi- glottis and vocal cords; anus, as high as the internal sphincter; vagina and external portion of the urethra. 1. Squamous. 2. Columnar (Pseudo- stratified columnar) Superficial cells, squamous; deeper, polyhe- dral; the deep- est, columnar in shape. Superficial cells, columnar; deeper cells, polyhedral or spindle-shaped. (a) Non-ciliated (rare) (b) Ciliated. (a) Part of ductus deferens. (b) Respiratory tract; nasal mu- cous membrane and passages connected therewith, tear-ducts, auditory tube, etc., larynx, trachea, and bronchi. Genital tract; epididymis and vas deferens. 3. Transitional. Superficial cells 1 only somewhat flattened; next deeper layer, pear-shaped; deepest layers, polyhedral. Found only in the urinary system viz., pelvis of the kidney, ure- ter, bladder, and first portion of the urethra. NON-STKATIFIED EPITHELIA 37 I. NON-STRATIFIED EPITHELIA 1. SIMPLE SQUAMOUS EPITHELIUM (Pavement Epithelium) This variety of epithelium comprehends two main groups: (1) the endotlielia, lining the vascular system, and (2) the mesothelia of the serous membranes lining the large internal closed cavities pleura, peri- cardium and peritoneum. This distinction is somewhat arbitrary but nevertheless useful, and derives justification in that endothelia arise in the first instance from syncytial mesoderm (mes- enchyme) and mesothelium from epithelial meso- derm. But according to Bremer (Amer. Jour. Anat., 16, 4, 1914), at least some of the earliest blood- vessels in man also arise from true mesothelial cells. Mesothelium lines the extra-embryonic body cavity and is reflected over the yolk-sac and body-stalk. In the latter location Bremer de- scribes ingrowths of mesothelium into the mesen- chyme, from which endothelium and blood-cells develop. This classification should include also another group of closed cavities, namely, the tendon sheaths, bursae, joint or synovial cavities, cham- bers of the eye, and the scalse tympani and vesti- buli of the internal ear. These cavities arise as splits, or by the union of isolated spaces, in the mesenchyma, the mesenchymal lining cells taking FlG ^. onepithelioid characters and arranging themselves MATIC ILLUSTRATION in the form of a membrane. In their method of OF ENDOTHELIUM LIN- T . , . ,, T, , , ING A LARGE AR- denvation these cells resemble more closely the TERY earliest endothelial anlages. The most satisfac- tory disposition of this group of epithelia seems to be to classify them as 'false' or 'mesenchymal' epithelia, as prroposed by F. T. Lewis. Such epithelia have been experimentally produced by the introduc- tion of small sheets of celloidin and masses of paraffin into the subcuta- 38 EPITHELIAL TISSUES neous tissue and cornea respectively, of laboratory animals : the connective tissue cells became changed into large flat cells, disposed in the manner of a mesothelium. These results suggest the conclusion that the meso- thelial cells of pleura, pericardium and peritoneum may be regenerated in the event of destruction from exposed connective tissue cells of the subepithelial stroma (W. C. Clarke, Anat. Eec., 8, 2, 1914). The individual squamous cells are flat plates bulging at the center where the oval nucleus is located, with serrated borders. In surface view FIG. 42. MESOTHELIUM (surface view), FROM THE MESENTERY OF A RAT. Silver nitrate and hematein. X 550. the endothelial cell is oblong, the long axis parallel with the long axis of the vessel (Fig. 41), while the mesothelial cell is polygonal in outline (Fig. 42). In sections through the nucleus, these cells in side view present a flat spindle-shaped appearance. Mesothelia exhibit small intercellular spaces, the stigmata. They have been regarded as openings between the body cavities and lymph spaces and vessels; but are more probably transient structures, perhaps artifacts. Abdominal mesothelia of lower forms, e.g., frog, contain also permanent openings, or stomata, surrounded by specialized guard cells. The mesenchymal epithelial cells of synovial membranes vary greatly in shape according to the degree of pressure to which they are subjected. They may thus be of the cuboidal or the squamous type; and they may even become pressed apart so as to expose the underlying connective tissue. (Compare Figs. 44 and 240.) t NON-STBATIFIED EPITHELIA 39 FIG. 43. CuBoroAL EPITHELIUM FROM THE RETE TESTIS OF THE RABBIT. a, epithelium; b, connective tissue. Hema- tein and eosin. X 550. 2. SIMPLE COLUMNAR EPITHELIUM (a) Plain This type of epithelium consists of columnar or cylindrical elements (Fig. 39), in transverse section presenting polygonal, frequently hex- agonal, outlines (Fig. 40). It may be tall, medium or low ^ ^ y * ^. JE ' ...O^ g^ . columnar epithelium, depend- ing upon the height of the in- dividual cell of .the particular membrane. The lower types may be designated cuboidal epithelia (Fig. 43). The phe- nomenon of polarity is partic- ularly well exhibited by a tall columnar cell, a condition inhering in a structural and functional differ- entiation between the attached, or proximal, and the free, or distal, end of the cell, dependent in a final an- alysis in larger measure upon dis- tance from source of the nutritive and oxygenative stream in the blood. The nucleus is generally located nearer the proximal end; this end, moreover, tapers to a point and is occasionally bifid; and it contains the presecretion (ergastoplasm, pro- zymogen, etc.) granules, rodlets and fibrils in secreting epithelia (Fig. 35). The distal border is frequently striated (cuticular margin, striated border, Fig. 38, b), an appearance due to the presence of minute canals, or more frequently, short pseudo- podia, mediating absorption or the elimination of secretion. Striated borders are particularly prominent in the columnar cells of the intes- tine. In the secreting cells of the FlG. 44. TlP OF A VlLLUS OF THE SYNOVIAL MEMBRANE FROM THE KNEE-JOINT OF AN OLD MAN. The core contains capillaries em- bedded in a compact, delicately fibrillar stroma. A distinct basement mem- brane appears in certain regions. The epithelium is of the low columnar or cuboidal type. 40 EPITHELIAL TISSUES kidney the marginal processes are more prominent and divergent, forming 'brush borders.' Where the cell membrane becomes greatly thickened on the free surface of a columnar cell, it is termed a crusta. Also, in neuro- epithelial cells, the free border mediates reception, the attached pole transmission, of stimuli. (b) Modified Columnar Epithelium (1) Ciliated Epithelium. Here the columnar cells carry upon their free surface a group of delicate hairlike processes called cilia, or a single flagellum (flagellate cells of lower forms), which during life are capable of a rapid vibratory or undu- latory motion, a further expression of cell polarity. The direction of this ciliary motion is constant and is such as to produce a definite current within the fluids which bathe the surface of these cells whose direction is invariably from within toward the external surface of the body. In the human body the cilia occur almost exclusively upon the free extremities of columnar cells (Figs. 45, 51 and 52). In some of the lower animals, as for example in the a epithelium; 6 connective tissue; mouth of the f dHa are found algo c, cilia. A leukocyte is seen between the bases of the columnar cells, upon polyhedral and pear-shaped cells. Hematein and eosin. X 550. In simplest form cilia are pseudopod- like extensions of the cytoplasm of the cell body, and may be regarded as modifications of its exoplasm. The ciliated border is separated from the protoplasm of the cell body by a fine chromatic line, which on higher magnification resolves into a number of knob-like segments, the basal granules (Fig. 22, Chap. I). The cytoplasm and nucleus of ciliated epithelium, except for the peculiarities dependent upon the formation of cilia, is similar to that of the simple non-ciliated columnar cells. Their cytoplasm as in other types, may contain vacuoles, pigment granules, metaplasm, and even secretory granules, e.g., epididymis. (2) Glandular Epithelium. This type derives its name from the fact of the predominance of the glandular function. This condition is FIG. 45. COLUMNAR CILIATED EPI- THELIUM FROM THE EPIDIDYMIS OF A RABBIT. NON-STKATIFIED EPITHELIA 41 structurally expressed in segregation of the presecretion bodies (gran- ules, rods, threads) in the basal end of the cell, and of the secretion products (granules, spherules, and mucous or serous fluid) in the distal end (Fig. 35). Morphologically it represents simply a modified colum- nar cell, its pyramidal shape resulting from mechanical factors due to its disposition in saccular or spherical acini, the periphery of the cen- tral lumen of which is much less than the periphery of the acinus, necessitating the modification in shape of the individual cells (Fig. 46). The cells of glandular epithelium usually lack cuticular borders. Pyram- idal or glandular epithelium is found in tubules of the kidney, salivary glands, the pancreas, in the secreting glands of the gastric and intestinal mucous membrane, in the mucous glands of the esophagus, pharynx, bronchial tubes and oral and nasal cavities, and in the secreting glands of the skin. (3) Goblet Cell Epithelium. A further important and very widespread modification of columnar cells in epi- thelia concerns the elaboration and storage of mucous secretion, giving to the loaded cells a goblet form (Figs. 47 and 48 ) . Goblet cells may occur among either the plain or ciliated columnar cells. They are most abundant in the intestinal tract but are also to be found in the stomach, bronchial tubes, trachea, nasal mucous- membrane, and in the -ducts and tubules of mucus secreting glands. In such epithelial membranes certain columnar cells, if not indeed all of these cells, are destined to secrete mucus. The cytoplasm of such cells is converted into a glairy mass of a peculiar vitre- ous appearance, which occupies an increasing proportion of the free ex- tremity of the cell. This 'mucinogen/ when acted upon by alcohol, is precipitated within the cell, and then forms fine basophilic fibrils or gran- ules which stain deeply with the muchematein and mucicarmin solutions of P. Mayer. At the base of the goblet cell, its nucleus is embedded in a minute mass of unaltered granular cytoplasm. The accumulation of mucus (mucinogen) within the cytoplasm ex- pands the cell, finally ruptures its wall in the direction of least resistance, and thus permits its mucous content to exude upon the free surface, leaving behind the small granular protoplasmic cell remnant attached to FIG. 46. A GROUP OF CELLS FROM A TRANSECTION OF AN ACINUS OF THE HUMAN PANCREAS; GLANDU- LAR EPITHELIUM. Hematein and eosin. X 550. EPITHELIAL TISSUES the basement membrane. The further history of these cell remnants is somewhat doubtful. They are possibly resorbed or removed, and finally replaced through mitotic division of adjacent cells. There is, however, some evidence to show that after function they are still capable of further FIG. 47. GOBLET CELLS AS SEEN IN A TRANSECTION OF A CRYPT OF THE LARGE INTESTINE OF MAN. Sections of five goblet cells are seen among the columnar cells which line the tubule. Muchematein and eosin. X 550. FIG. 48. DIAGRAM SHOWING THE AR- RANGEMENT OF THE COLUMNAR AND GOBLET CELLS OF THE PRECEDING FIGURE. The goblet cells are represented as being empty; their unaltered basal por- tions containing the nucleus are dis- tinctly seen. growth, whereby they may regain their original form and become again able to pass through the same stages of secretory activity. (4) Neuro-epithelium. The cells of neuro-epithelium are colum- nar elements specially differentiated to form nerve end-organs. They are usually elongated cells having a bulging nucleated center, their free extremity either projecting beyond the epithelial surface as a bundle of fine cilia or as a slender non-ciliated process which terminates within a pore-like opening directly connected with the free surface. Their at- tached extremity, tapering to a fine process, is in relation with the terminal arborization of the axis cylinder of a nerve fiber. Neuro-epithe- lium is found only in the several organs of special sense, and will be more fully described as a part of these several organs. (See chapters of the Eye, the Ear, the Olfactory Organ, the Tongue, and on the Nerve End- Organs.) STRATIFIED EPITHELIUM 43 H. STRATIFIED EPITHELIUM 1. STRATIFIED SQUAMOUS EPITHELIUM This variety of epithelium occurs as a membrane of varying thickness but always comprising several cell layers. A straight line perpendicular to its free surface would penetrate from five to thirty or more epithelial cells. But while there is a wide diversity in the thickness of the epithelial layers, the character of the cells at any given level is very nearly con- '- ' FIG. 49. STRATIFIED EPITHELIUM FROM THE HUMAN ESOPHAGUS. a, basement membrane; 6, connective tissue. Hematein and eosin. X 410. stant. Thus the deeper cells, those nearest the basement membrane, are nucleated, of soft consistence and may contain mitotic figures, indicating that it is at this level that cell reproduction is most active. Toward the surface of the membrane the cells become progressively of firmer con- sistence, so that the most superficial ones present a horny appearance as a result of the gradual keratization of the cytoplasm during the progress 44 EPITHELIAL TISSUES of the cell toward the surface. The keratization is apparently dependent upon surrounding physical conditions, for it is much more marked in the skin, which from constant and rapid evaporation is comparatively dry, than in the mouth, esophagus, or conjunctiva, where the epithelium is constantly moistened by glandular secretions; the margins of the lips, eyelids, etc., present an intermediate state of keratization. With these chemical changes in the composition of the cytoplasm there are corresponding changes in its nucleus. In the deeper cells, the nucleus is oval or spherical and highly chromatic. Toward the surface, the nucleus becomes more and more flattened and more and more obscured by the cornification of the cell protoplasm. In the most superficial cells it is usually impossible to demonstrate the nuclei, except by acting upon their protoplasm with strong reagents such as caustic alkalies, soda or potassa. But the most characteristic change in the cells of stratified epithelium is the progressive transition in shape undergone during their passage from the deeper layers to the free surface. New cells, resulting from indi- rect division of the cells in the deeper layers, are by continued reproduction gradually pushed toward the surface, whence they are constantly being desquamated in small scaly masses. The pressure exerted in this process tends to gradually flatten these cells, so that their vertical diameter, that perpendicular to the surface, becomes progressively shorter the nearer they approach the free surface ; on the other hand, their transverse diam- eter, that parallel to the surface of the epithelial membrane, is correspond- ingly increased. The deepest cells of the stratified epithelium those which rest upon the basement membrane are elongated in their vertical diameter and possess an irregularly columnar shape. Their nuclei are likewise elongated, oval or elliptical in shape. In the skin of brunettes and the dark-skinned races, and in the epithelium of the skin of the scrotum, perianal region, and areolae of the breasts, these cells contain small granules of the pigment to which the color of the cuticle is largely due. This columnar cell layer is then described as the layer of pigment epithelium. Superficial to these, but still in the deeper layers, are poly- hedral cells with spherical nuclei, which are known as priclde cells be- cause of their prominent intercellular bridges. Superficial to the prickle cells, the epithelial cells become progressively more flattened, until at the surface, they are mere scales. This gradual transition from columnar and polyhedral cells below, to thin flat scales on the surface is character- istic of all stratified epithelium. The thin superficial scales resemble very closely in shape and appear- STRATIFIED EPITHELIUM 45 ance the squamous epithelium previously described. The deeper cells have a finely granular cytoplasm and distinct nuclei except when obscured by the appearance of keratin within their protoplasm. Many of these cells contain coarse granules of eleidin and keratohyalin substances chemi- cally intermediate between the unaltered and keratized protoplasm. As stated, the formation of keratin within these cells is more-active in those membranes which are comparatively dry from exposure to the air. Consequently, it is most active in the epidermis of the skin. If stratified epithelium is at all times well moistened, as, for example, in the FIG. 50. EPIDERMIS OF THE SKIN OF THE FINGER TIP, SHOWING EXTREME KERATIZA- TION OF THE EPITHELIUM. o, keratized epithelium; 6, Malpighian or germinal layer; c, connective tissue. Hematein and eosin. X 50. mouth and esophagus, the formation of keratin is slight, and the soft polyhedral cells compose the major portion of the epithelial membrane which then has only a thin superficial covering of flattened scaly cells. In the comparatively dry epidermis, on the other hand, the flattened horny cells frequently occupy more than half the thickness of the epithe- lial layer (Fig. 50). In the superficial squamous cells of moist membranes the nucleus can ahvays be readily demonstrated, even in the keratized cells of the extreme surface. Cells of the intermediate layers, especially those just above the prickle cell layer, frequently show nuclei in process of amitotic division. This condition is presumably associated with an early stage of degeneration dependent upon a scarcity of nutriment due to the relatively greater distance of these cells from the source of supply. 46 EPITHELIAL TISSUES It will assist in the understanding of the structure and morphological characteristics of the several layers of cells to think of the superficial a squamous cells in terms of the innermost columnar cells, modified during the pas- sage to the surface by mechanical (pres- sure), physical (desiccation), and chem- ical (keratization) factors. 2. PSEUDO-STRATIFIED COLUMNAR EPITHELIUM FlG. 51. P S E U D O - STRATIFIED COLUMNAR CILIATED EPITHE- LIUM FROM A BRONCHIAL TUBE OF MAN. a, a goblet cell; 6, cilia; c, super- ficial cytoplasmic layer; d, deeper nucleated layer, the nuclei of the columnar cells are somewhat more deeply stained than those of the basal cells; e, basement mem- brane; /, connective tissue. Hema- tein and eosin. X 550. The superficial cells only of this va- riety of epithelium are columnar in shape, and except in one or two unim- portant places are always ciliated. The deeper extremities of these columnar cells taper to a point, and extend all the way to the basement membrane. Be- tween the tapering ends of these cells small spindle-shaped and spheroidal cells are closely packed. The several varieties of cells thus appear to be superimposed, though all actually rest upon the basement membrane. The 'superficial' cells of this variety extend throughout the entire thickness of the mem- brane. Hence this form of epithelium may in one sense be called 'simple' rather than 'strat- ified.' The distribution of this variety of the epithelium is practically identical with that of ciliated cells. The deeper extremities of the columnar cells are occasionally bifid or even somewhat varicose in order the more closely to fit between the spindle-shaped and spheroidal cells of the deeper portion. The nucleus of these latter cells is usually situated a little below the middle of the columnar cell, so that all the nuclei of the epithelial membrane lie within its deeper half, thus giving to this portion a more deeply chromatic appearance when observed in stained sections under low mag- nification. The superficial half of the epithelial layer contains only FIG. 52. DIAGRAM SHOWING THE MAN- NER IN WHICH ALL THE EPITHELIAL CELLS OF PSEUDO- STRATIFIED CILIATED EPITHELIUM REACH THE BASEMENT MEM- BRANE. Letters as in the pre- ceding figure. STRATIFIED EPITHELIUM 47 the cytoplasmic portion of the columnar cells with their ciliated bor- ders. This type of epithelium is frequently designated simply 'stratified columnar'; and in fact in certain instances under conditions of further modification involving a separation of the taller cells from the basement membrane, it passes, over more or less extensive areas in the respiratory and male genital tracts, into actual stratified columnar epithelium. Toward the proximal end of the male urethra the epithelium is of the true stratified columnar (non-ciliated) type. 3. TRANSITIONAL EPITHELIUM This variety resembles somewhat stratified squamous epithelium in that it is composed of several cell layers, the deeper cells of which are more nearly polyhedral but are somewhat flattened upon the free sur- face, but differs in having a smaller number of cell layers in which respect it is 'transitional' between simple and strat- ^ ^- * ified squamous epitheli- um and in the charac- ter of the superficial cells. Transitional epi- thelium is not usually ^ A A 4^ A ft mor thn frm hr more than from three to ten cells deep, four to six being the rule. The * ten cells deep, four to six * * ^ 9 % %^ % & |X^ * 53. TRANSITIONAL EPITHELIUM FROM A THAN- SECTION OF THE URETER OF AN INFANT. upon their state of ten- .^ ,. A . a, epithelium; 6, connective tissue. Hematem and sion during life; thus the eosin. X 550. transitional epithelium of the urinary bladder is much thicker when the organ is collapsed than dur- ing distension. The deepest cells are polyhedral, and these form the greater portion of the membrane. Only the more superficial layers differ therefrom. Those polyhedral cells which lie in the midregion of the epithelial layer possess a peculiar flask or pear shape, with well-rounded bodies and a 48 EPITHELIAL TISSUES broad tapering process which is embedded between the adjacent cells of the deeper layers. The rounded extremities of the pear-shaped cells fit into peculiar indentations in the deeper surface of the superficial layer of epithelial cells, producing peculiar concave facets, which are specially characteristic of the detached superficial cells of transitional epithelium. The superficial cells, while somewhat flattened, usually have a thick- ness equal to one-sixth to one-third their transverse diameter. In this respect -they differ markedly from the superficial scaly cells of stratified squamous epithelium and are easily distinguished therefrom, even in the fi FIG. 54. ISOLATED CELLS WHICH MAY APPEAR IN HUMAN URINE. A, from the vagina of a woman (stratified squamous epithelium); B, from the ureter of a child (transitional epithelium); a, cells from the deep layers; b, superficial cell. Moderately magnified. isolated condition in which they are frequently found in the urine. The concave facets on their under surface, as well as the peculiar pyri- form shape and small size of the deeper cells, are sufficient to distinguish the transitional cells from those of stratified epithelium. There is little, if any, formation of keratin in transitional epithelium. This is possibly explained by the fact that, as it occurs only in the urinary system, this form of epithelium is always well moistened. Differ- entiation of this variety of epithelial tissue, though neglected by some authors, becomes most important in the clinical examination of urine where it is necessary to determine the origin of individual cells. Transi- tional cells from the bladder are easily distinguished from the stratified squamous cells of the vagina, urethra, or epidermis. CHAPTEK III CONNECTIVE TISSUE CAETILAGE BONE CONNECTIVE TISSUE General Statements. While in the epithelial tissues the cells are developed chiefly at the expense of the intercellular elements, in the connective or supporting tissues the conditions are the reverse. The intercellular elements are here developed out of all proportion to the connective tissue cells. The cells of these tissues therefore are scanty, the ground substance considerable, and within the latter a new element, the connective tissue fiber, makes its appearance. The fibers are of three mm- FIG. 55. EMBRYONAL CONNECTIVE TISSUE, EARLY STAGE. Highly magnified. (After Mall.) FIG. 56. EMBRYONAL CONNECTIVE TIS- SUE AT A LATER STAGE THAN Is REP- RESENTED IN FIG. 55. (After Mall.) varieties : wliite or collagenous fibers, elastic fibers, and reticulum. In any given location either of these varieties may predominate to such an extent as to determine the character of the mature tissue, while in the immature forms of connective tissue it is the cellular elements which attain the greatest prominence. The minute structure of connective tissue is subject to great and important changes during its development. Beginning as it does with the primitive mesoderm, connective tissue is originally a cellular struc- ture. ThejieUs-OLL^nmitiye connective tissue, the fibroblasts, not only increase in number S_eJ division "bTTtr-alsa-^ecrete" an intercetlulaf" consistence. The fibroblasts fuse with each 5 49 50 CONNECTIVE TISSUE CAETILAGE BONE other and finally form a syncytial tissue, the mesenchyma, in which there promptly occurs a differentiation of the cytoplasm with the forma- tion of an endoplasm and an exoplasm; and within the latter the fine fibrils soon make their appearance, according to Meves, by processes of fusion and chemical alteration of mitochondria (chondrioconta). This process continues, new ground substance and fibers being con- Wz Kolf Kl Elf Plz Plb Mz Kl Kolf Elf FIG. 57. SUBCUTANEOUS AREOLAR CONNECTIVE TISSUE OF GUINEA PIG. (Maximow.) Elf, elastic fiber; Kolf, collagenous (white) fiber bundles; Fb, fibroblast (lamellar cell); Mz, mast cell; Wz, resting wandering cell (clasmatocyte) ; Plz, plasma cell; Kl, clasmatocyte ('macrophage'); Eos, eosinophil. X 1750. stantly formed at the expense of the endoplasm, until finally the remnant of the latter again forms isolated cells. The culmination of these changes results in the mature fibrillar connective tissue in which the cells are shrunken and scarce, though still apparently capable of assuming renewed activity on demand of altered conditions. The definitive fibrils result in part from a longitudinal splitting of the coarser primitive fibers, collage- nous (Mall), elastic and reticular. Embryonic connective tissue is therefore typically cellular as com- pared with the mature type; its ground substance is abundant but the fibers, whose development is as yet incomplete, are scanty. Such embry- CONNECTIVE TISSUE 51 onic connective tissue is found not only in the fetus but also in early child- hood and in the adult, especially during regeneration of destroyed areas of connective tissue, and in other more or less pathological conditions. FIG. 58. PLASMA CELLS OF CONNEC- TIVE TISSUE FROM THE HUMAN BREAST. Hematein and eosin. X 750. CONNECTIVE TISSUE CELLS Connective tissue cells not only vary in number as they approach ma- turity, but in their structure and appearance as well. The cells of em- bryonic connective tissue are comparatively large, are frequently stellate from the presence of numerous interlacing and sometimes anastomosing branches, and their cyto- 9& plasm has a typical reticular or granular appear- ance. In the later stages of their development A @ ameboid motion has been observed in such cells, and within the limits of the tissue in which they are developed, they are presumably endowed with the power of locomotion. In the neighborhood of developing blood-ves- sels plasma cells of large size and irregular shape are frequently seen. The cytoplasm of these cells is of considerable volume, is finely granular, stains readily in most dyes, especially the basic varieties, and is prolonged into broad protoplasmic branches of considerable length. Both in the cell body and in the proc- esses vacuoles are so numerous as to give the cell a typically reticular appearance, a peculiarity which is emphasized m by the removal of the contents of the vacuoles, ^* ^ gj^ as frequently happens in the preparation of microscopical specimens. Plasma cells are found in considerable numbers in the mucous membrane of the intestinal tract and in the subcutaneous tissue, where they are frequently of spheroidal form. In the denser forms of mature connective tissue, where the cells are apparently subjected to more or less compression between the firm bundles of fibers, the con- nective tissue cells lose their typical embryonal stellate form and become somewhat fusiform; they are then known as the spindle cells of connec- tive tissue. Such cells occur in great abundance in the stroma of the ovary and the mucosa of the uterus and oviduct. In the mature tissue of the adult many of the cells become more or FIG. 59. SPINDLE-SHAPED CONNECTIVE TISSUE CELLS FROM THE STROMA OF THE HUMAN OVARY. Hematein and eosin. X 550. CONNECTIVE TISSUE CARTILAGE BONE FlG. 60. PlGMENTED CELLS FROM THE CHOROID COAT OF THE Ox's EYE. Unstained; hence, only the pigment granules appear in the figure. 1, gran- ules contained within the cytoplasm; 2, free granules which have escaped from cells injured during the process of teas- ing; 3, the non-pigmented nuclei. less flattened and are often closely applied to, or even wrapped around, the fiber bundles. These lamellar cells have a small nucleus, a consider- . , able rim of cytoplasm, which fre- ^P^few- quently has a shrunken appearance, . and sometimes a few short cytoplas- /^raS m * c P rocesses - ^ ne branching stel- ^1||||| late forms, however, are characteris- ^ .. tic of the younger connective tissues. * Jpl|i ^ n certain locations a deposit of ^^^^^ * 9&iLJ$Sffi ' pigment granules occurs within the ' 8 *^^te^tl** . ment cells are usually found where L _j protection against light seems desir- able, and are most abundant in the choroid coat and iris of the eye. The pigment granules are entirely confined to the cytoplasm of the cell ; the nucleus is never invaded by the deposit. These granules belong to the melanin series of pigments. The cytoplasm of certain cells found in connective tissue contains coarse basophil granules, which stain with dahlia and similar basic dyes. This type is known as lasophil granule cells, or mast cells (Mastzellen of the German authors). The granules of other granulocytes are readily stained with acid dyes, such as eosin (eosinopliil, acidopliil or oxyphil granulocytes). According to the observa- tions of H. B. Shaw (Jour. Anat. and PhysioL, 1901), certain of the granule cells abound in those locations where fat is deposited, and have a special relation to the development of the fat cells of adipose tissue. These granulocytes of fibro-elastic connective tissue are apparently identical with those of the blood. Lymphocytes and phagocytic leukocytes are also pres- ent in connective tissue. It is a disputed point whether the granulocytes of connective tissue differentiate from fibroblasts or from lymphocytes ; the weight of evidence seems to incline to the latter position. Plasma cells seem more probably altered fibroblasts but have also been regarded by some as lymphocyte derivatives. The so-called FIG. 61. GRANULE CELLS FROM THE FIBROUS CON- NECTIVE TISSUE OF THE HUMAN MAMMARY GLAND. A, a basophile cell; B, an eosino- phile cell. Hema- tein and eosin. X 750. CONNECTIVE TISSUE 53 testing-wandering' cells, or 'clasmatocytes/ are perhaps to be regarded as varieties of basophilic granulocytes characterized principally by the presence of irregular protoplasmic processes. According to Kite (Jour. Infec. Dis., 15, 2, 1914) the 'clasmatocytes' described for the frog by Eanvier in 1891 are lymphocytes which have protruded pseudopods. Evans classifies them with the 'macrophages' of Metschnikoff. TYPES or CONNECTIVE TISSUE The proportions and character of the cells and fibers present in any given connective tissue, to a certain extent determine its character. If the collagenous fibers of connective -tissue are losely-packeUm-4eft&e h^^ comparatively insignificant in ~number, the type of connective tissue may then be said to be dense fibrous or white_flbrous tissue. . In elastic tissue on the other hand, the yellowjelagtic fibers are highly developed, the white fibers forming only insignificant and very delicate shealhs which inclose the coarser elastic fibers. Again, it is the variety of delicate connective tissue fiber known as reticulum which preponderates in reticular tissue, and if the meshes of TirHsreticular network become infiltrated by lymphocyte.^ which then mul- tiply by division until they exceed the other tissue elements, the connective tissue is then said to be of the lymphoid or adenoid variety. Large num- bers of the fixed cells of areolar connective tissue muv change into fat cells, the tissue as a whole then forming adipose tissue. In all we distinguish the following varieties of connective tissue: (1) Embryonal; (2) mucous; (3) reticular; (4) loose fibro-elastic or areolar; (5) dense fibrous; (6) dense elastic; (7) adipose; (8) adenoid; (9) cartilage; (.10) bone. Embryonal Connective Tissue. Embryonal connective tissue (Figs. 55 and 56) occurs not only in fetal and infantile life, but also during the regeneration of destroyed connective tissue areas and in pathological neoplasms. It is distinctly cellular in character. Its cells are spinclle^ shaped and stellate, are much branched^ and through their larger proc- esses they frequently anastomose. The fibers are extremely fine; they are not usually arranged in bun- dles, but form a delicate network which permeates the ground substance in every direction. In the very immature types the fibers are all of the collagenous variety ; delicate elastic fibers appear later. The fluid ground substance forms an abundant mass of tissue juice which occupies the meshes of the fibrous net. The earliest developmental stages are identi- 54 CONNECTIVE TISSUE CARTILAGE BONE cal with mesenchyma; from the viewpoint of progressive differentiation it properly heads the list of connective tissues. Mucous Connective Tissue. Mucous, gelatinous or mucoid connec- tive tissue occurs only in the umbilical cord, where it forms the ' jelly of Wharton/ and in the vitreous humor of the eye. Its semifluid ground substance is of a gelatinous consistence and forms the greater portion of the tissue; in the vitreous humor there is little else. The cells are mostly of the branched lamellar variety, are few in num- FIG. 62. GELATINOUS CONNECTIVE TISSUE FROM THE UMBILICAL CORD OF A NEW- BORN INFANT. Safranin and water blue. X 410. ber in the vitreous, but more abundant in the umbilical cord. In the vitreous humor also, there are very few fibers; those which are present are very fine and form a delicate reticulum. In the umbilical cord the fibers are more abundant, and possess a tendency to form bundles which are disposed in parallel cylindrical layers around the large blood- vessels. Elastic fibers are wanting. This type lacks also nerves, blood- vessels and lymph- vessels ; the two large umbilical arteries and the umbilical vein have no vascular connection with the mucous tissue of the cord. The essential chemical body in mucus is mucin, a glycoproteid. The most typical mucous substance is the secretory content of goblet cells. The mucus of embryonic and gelatinous connective tissue is closely CONNECTIVE TISSUE 55 similar. Less closely similar 'mucous' substances of the more compact connective tissues are properly designated, mucoids. Reticular Tissue (Reticulum). Reticular tissue occurs as the stroma of adenoid tissue in the lymphatic glands and other lymphoid or- gans, and according to Mall (Johns Hopkins Hosp. Rep., 1896), is found also in the membrana propria of the secreting tubules of the stomach, intestine, kidney, testis, and thyroid, and in the marrow of bone and the walls of the pulmonary air sacs. Like the other connective tis- sues, reticular tissue consists of cells, fibers, and ground substance ; the latter, however, is no more than a fluid tissue juice which, at least in the lymphoid organs, is identical with the lymph. The fibers are extremely fine and are arranged in slender bundles, which freely anastomose to form a deli- cate close-meshed reticulum. In- dividual fibers can be readily dem- onstrated in these bundles only after the action of alkalies, diges- lion by artificial gastric juice, or >>y other methods of dissociation, yet on careful examination indica- tions of fibrillar structure can be seen in the reticulum of fresh tis- sue and in ordinary microscopical preparations. The chemical reactions of the reticular fibers are similar to those of collagenous fibers except that the former are much less readily digested by artificial gastric juice. Flattened connective tissue cells clasp the bundles of reticular fibers ; they are mostly found at the intersections of the anastomosing bundles. This fact was accountable for the former theory, which regarded reticu- lar tissue as formed by the anastomosing branches of stellate cells. The careful investigations of Carlier (Jour. Anat. and Physiol., 1895) and others have shown the true nature of the lamellar cells and their under- lying fiber bundles. The fibers of reticular tissue very closely resemble the collagenous fibers of areolar tissue, but differ from them in having a clearer, more FIG. 63. RETICULUM OF A CERVICAL LYMPH NODE OF MAN, FROM A THIN SECTION FROM WHICH THE LYMPHATIC CORPUSCLES HAD BEEN PARTIALLY WASHED OUT. a, polynuclear lymphatic corpuscle; b, large mononuclear cell; c, connective tis- sue cells of the reticular tissue; d, fibrous bundle of the reticulum; e, small mono- nuclear lymphocyte. Hematein and eosin. X 500. 56 CONNECTIVE TISSUE CARTILAGE BONE highly refractive appearance. Their digestion in pepsin begins only after an interval of two hours, while white fibers are digested in a few minutes ; they also stain less readily than white fibers and yield reticulin, which differs somewhat from the gelatin of white fibrous tissue. The intimate histologic relation between the reticular and white fibrous tissue is shown by the fact that the two tissues are frequently continuous and exhibit similar staining reactions. Mall (Amer. Jour, of Anat., 1902) has attempted to show that reticu- lar tissue should be considered as that form of connective tissue which has been least differentiated from the embryonic mesenchymal type. He accordingly considers the cells of the reticulum as formed by the un- differentiated endoplasm, and the reticular fibers as representing the specialized exoplasm of this most primitive type of true connective tissue. In the liver, the reticulum arises from the endothelial cells of von Kuppfer instead of from mesenchyme (Mall}. Loose Fibro-elastic or Areolar Connective Tissue. Loose fibro- elastic or areolar connective tissue (Fig. 57) is the most widely distrib- uted of all the varieties; it fills all otherwise unoccupied spaces within the body, and in* all microscopical sections areolar tissue is almost in- variably to be found. It is also known as loose connective tissue in con- tradistinction to the more compact or dense varieties. This tissue con- nects the skin with the underlying structures, maintains the position and relation of adjoining muscles, surrounds the heart and its great vessels, envelops the abdominal viscera as submucous and subserous sheets, occu- pies the spaces of the , mediastinum, and fills similar intervals between the various organs in all parts of the body. Areolar tissue of course varies in the degree of its laxity or density. The ground substance of areolar tissue is a coagulable fluid, the tissue juice. Solutions of silver nitrate injected into the interstices of areolar tissue coagulate its tissue juice or ground substance and darken it slightly. It is then seen to be permeated by broad lymphatic channels, which are lined by delicate endothelioid mesenchymal cells (W. G. Mac- Callum, Arch. f. Anat., 1902; also Bull. Johns Hopkins Hosp., 1903). Both collagenous and elastic fibers occur in areolar tissue, the former being far in excess of the latter. The comparatively loose reticular ar- rangement of the fibers of fibro-elastic tissue affords a most favorable opportunity for the .study of these connective tissue elements. The collagenous or white fibers in mature tissues are invariably ar- ranged in bundles which interlace with one another to form an open net- work. Each bundle consists of a number of very fine fibers whose CONNECTIVE TISSUE 57 course is characteristically wavy or undulating. Though the individual fibers rarely branch, the fiber bundles frequently anastomose with one an- other. The white fibers are readily stained with most 'acid' dyes, and possess a special affinity for acid fuchsin. Chemically they consist of the albuminoid collagen, which on boiling in water yields gelatin, and is readily dissolved by boiling in dilute acids or alkalies. Collagen "fibers are digested by artificial gastric juice in five or ten minutes but are , FIG. 64. DENSE FIBROUS TISSUE FROM THE TENDON OF ONE OF THE OCULAR MUSCLES OF A CHILD. Hematein and eosin. X 550. scarcely altered after several hours when acted upon by solutions of pan- creatin. After boiling, however, white fibers are readily digested by pan- creatin. In dilute acids they swell and become transparent. The elastic fibers of areolar tissue, in comparison with the collagenous fibers, are few in number. They occur as isolated fibers never in bundles which frequently branch and anastomose, forming in this way a very fine net with wide meshes, within which are the interlacing bundles of white fibers. The elastic fibers exist under a certain tension during life, so that their course under favorable conditions is invariably straight. When areolar tissue is removed from the body this tension is frequently relieved and the elastic fibers then curl up, especially at their free ends. 58 CONNECTIVE TISSUE CARTILAGE BONE Under these conditions they are no longer straight, but present a grace- fully curved contour. The elastic fibers also possess a glassy, shining, or highly refractive appearance, the collagenous fibers by comparison look- ing dull and opaque. Elastic fibers stain but slightly with most dyes; they are readily colored by orcein and by WeigertV elastic tissue stain (resorcin-fuchsin), both of which serve as specific dyes for these fibers, coloring the fibers dark brown or black. Elastic fibers are not dissolved by dilute acids or alkalies even when boiled, and are only digested by artificial gastric juice after a lapse of several hours ; they are, how- ever, readily digested in faintly alkaline solu- tions of pancreatin. They consist of the al- buminoid body, elastin, which on boiling does not yield gelatin. Both collagenous and elas- tic fibers arise by a similar process involving transformation of the exoplasm of their re- spective fibroblast progenitors into a fibrillar FIG. 65. LONGITUDINAL SECTION OF TENDON OF HUMAN FINGER. Only the nuclei of the tendon cells are conspicu- ous, scattered in rows among the collagenous fibrils. The rows of nuclei mark the boundaries of the primary bundles. X 750. FIG. 66. PORTION OF TENDON FROM A Cow. conn. t. c., connective tis- sue cells (tendon cells) seen from the side and, in one case, from the surface. (From Dahlgren and Kepner's "Ani- mal Histology," Macmillan Co.) structure. Whether the fibers are deposited as such or arise by coales- cence of more fundamental granular elements is a disputed point. The cells of areolar tissue are few in number, but may include any of the several varieties, though lamellar and spindle cells together with leukocytes form the more common types. Many of the lamellar cells are CONNECTIVE TISSUE 59 closely applied to, or even wrapped around the bundles of white fibers. Fat cells occur in considerable numbers in all areolar tissue and in some places are aggregated into large groups which form lobules of fatty tissue. Dense Fibrous Tissue. In dense fibrous tissue the ground substance is comparatively deficient. Large bundles of collagenous fibers are~~ar= ranged in approximately parallel rows, and are so closely packed as to Tendon fibre Nucleus of a tendon cell, surface view Ridges on cell due to pressure' Tendon cells seen from edge FIG. 68. PIECE OF TENDON FROM TAIL OF WHITE MOUSE. Between the bundles of connec- tive-tissue fibrils are cells arranged in rows. Some are seen in surface view, and others im optical section. X 400. (From Szymonowicz-Mac- Callum, "Histology and Microscopic Anatomy.") FIG. 67. TRANSVERSE SECTION OF PORTION OF TENDON OF HUMAN FINGER. a, three-winged cell; 6, four- winged cell; c, primary bundle, completely ensheathed by the wings of tendon cells, and divisible into still smaller bundles of collagenous fibers outlined by finer processes of the wings. The individual fibers are not shown. Gold chlorid. X 1000. form a dense, firm, highly resistant tissue. Its scanty connective tissue cells are of the lamellar variety and are usually arranged in rows which occupy the interstices between the parallel fiber bundles. Dense fibrous tissue occurs typically in tendons; in these the connec- tive tissue cells often have a peculiar quadrate shape and are arranged in rows of exceptional regularity (Figs. 64-67). These should be studied in dissociated tendinous tissue. It also forms the ligaments, the fasciae, the muscular sheaths (aponeuroses), and the enveloping capsules of many of the viscera. Thus it surrounds the liver, kidney, lymphatic nodes, and other organs; it also forms the valves of the heart, the tendinous rings 60 CONNECTIVE TISSUE CAETILAGE BONE B FIG. 69. ISOLATED TENDON CELLS. A, with two 'wings'; B, with four 'wings.' (From Maximo w, after Tourneau.) X 1000. which surround the cardiac orifices, and the chordae tendineae which are attached to its valves ; and in general, it is found wherever great firmness and resistance are required. Elastic fibers in this tissue are relatively few in number and are so obscured by the dense bundles of white fibers as to be scarcely demonstrable except by means of the specific stains. Tendon will be further discussed in connection with striped muscle. Dense Elastic Tissue. I n this form of tissue the elastic fibers are de- veloped at the expense of the colla- genous fibers. The ground substance is insignificant in amount, and the connective tissue cells are scanty and are confined to the white fibrous sheaths in which the elastic fibers are enveloped. The elastic fibers are of very large size (10 to 15 ft ) as com- pared with those of other forms of Connective tissue. But except for their larger size, these fibers have the same peculiar characteristics as the elastic fibers of areolar tissue. In their straight course, frequent branches, and their glistening, highly refractive appearance, as also in their character- istic reactions to specific dyes and other reagents, these fibers are identical FIG. 70. COARSE ELASTIC FIBERS FROM THE LlGAMENTUM NuCILE OF THE Ox. Isolated by teasing. Partly dia- grammatic. X about 250. CONNECTIVE TISSUE 61 FIG. 71. TRANSECTION OF A FASCIC- ULUS OF THE LlGAMENTUM NUCH.E OF THE Ox, SHOWING THE VERY LARGE ELASTIC FIBERS EMBEDDED IN A VERY DELICATE NETWORK OF COLLAGENOUS FlBERS. Picro-fuchsin. X 550. with the elastic fibers of the other types of connective tissue. The elastic fibers are bound to- gether by delicate sheaths of very fine collagenous fibers, and are united into bundles by coarser bands of fibrous tissue. Elastic tissue is found in the ligamenta flava, the stylohyoid liga- ment and in the ligamentum nuchae ('whitleather') of quadrupeds. In these locations it occurs in consider- able quantity and has a peculiar yel- lowish color ; it is for this reason that it is frequently described as yellow elastic tissue. It occurs also as fen- estrated membranes in arteries. These are formed by a coalescence of neighboring fibers. In the process of occlusion of the postfetal ductus ar- teriosus of the pig by increase in the amount of the elastic tissue in the wall of the artery, the .new elastic fibers arise both from latent fibro- blasts and by delamination of fibers from preformed elastic tissue (J. P. Schaeffer, Jour. Exp. Med., vol. 19, 1914). Adipose Tissue (Fat Tissue). Wherever areolar tissue occurs, adipose tissue may also be found; its distribu- tion is therefore identical with that of areolar tissue. It forms a consider- able mass, panniculus adiposus, be- neath the skin of many parts. In it are embedded the kidneys, adrenals, and many lymphatic nodes. The mes- entery and omentum are freely sup- plied with fat. The same tissue is found in the grooves of the heart wall and it also occupies the spaces of the mediastinum. Adipose tissue is composed of Mules or groups of fat cells which are FIG. 72. PORTION OF LIGAMENTUM NUCH^ OF Ox. conn, t. c., connective tissue cells. (From Dahlgren and Kepner.) CONNECTIVE TISSUE CARTILAGE BONE supported by fibrous bands and septa and are abundantly supplied with small blood-vessels. The fat cells arise from the connective tissue cells by a deposit of fat droplets within the cytoplasm of the latter. These droplets continue to increase in number and fuse with each other to form globules ct.c st.c ct.c. f.c. f.c. FIG. 73. PORTION OF A FAT LOBULE FROM THE AREOLAR CONNECTIVE TISSUE SUR- ROUNDING THE ESOPHAGUS OF A CAT. cap., capillary; ct. c., nucleus of a connective tissue cell; /. c., fat cell showing nucleus; tr., trabecula of fibre-elastic connective tissue. X 500. of increasing size, until the cytoplasm finally becomes so excavated as to form a mere limiting membrane or cell wall (Fig. 74). The nucleus is pushed to one side in this process and is flattened against the cell mem- brane ; it is usually embedded in a remnant of granular cytoplasm. Be- ing thus distended with fluid fat,, the cell acquires a spheroidal shape. The routine specific stains for fat are osmic acid, which colors the CONNECTIVE TISSUE 63 FIG. 74. A GROUP OF FAT CELLS FROM THE SUBCU- TANEOUS TISSUE OF A YOUNG RABBIT. Cells a show stages in de- velopment; cell 6 is cut tan- gentially through the nucle- ated pole. X 1000. fat globules black; sudan III, which gives a red reaction; and scharlack E (fettponceau), which also stains fat red. For the successful application of these stains it is required that the tissue has not been previously subjected to treatment involving the use of alcohol or ether, since these reagents extract fat from the cells. Fat, in cooling, solidifies and pre- cipitates delicate threads, the margarin crys- tals. During periods of starvation or malnu- trition, at which time fat decreases greatly in volume, many of the fat cells return to a condition which approximates their former state. As the fat is removed the cytoplasm of the cell increases in amount, but assumes a peculiar fluid appearance and is not read- ily colored by the usual dyes. These cells, which still contain a number of fat droplets, are known as 'serous' fat cells. The origin of the fat cell is still somewhat in doubt. It was for- merly thought that it might result from a deposit of fat within any of the connective tissue cells. A second theory maintains that it arises only from a special fat-forming connective tissue cell. The demonstration of large num- bers of peculiar ovoid granular cells within areas where fat cells were undoubtedly form- ing in fetal and young sub- jects, and the demonstration of similar cells in areas show- ing fat formation in adult tis- sues, has lent support to the hypothesis that these granular cells are the only progenitors of the fat cells (Shaw, Jour. Anat. and PhysioL, 1901). According to Weiskotten and Steensland (Anat. Rec., FIG. 75. FAT CELLS FROM A TEASED PREP- ARATION OF ADIPOSE TISSUE OF MAN. X 110. 64 CONNECTIVE TISSUE CAETILAGE BONE FIG. 76. ADIPOSE TISSUE. The fat cells have been blackened by osmium tetroxid. X HO. 8, 2, 1914) fat cells can arise also by a process involving the enclosure of free fat spherules by endothe- lial cells. They suggest that fat cells may be modified endothelial cells rather than mod- ified fibroblasts. The forerunners of the original smallest fat droplets are gran- ules (Altmann, 1890). In the subcutaneous tissue of Myxine (Hag- fish) embryos, Schrei- ner (Anat. Anz. 48, 7, 1915) has described the process of fat elaboration in minute detail. The pre-fat granules originate from rod-like chromidia ('mitochondria') by process of segmentation. The chro- midia arise as nucleolar buds which wander through the nucleus and traverse the nu- clear membrane as spherical granules. These 'primary granules' elongate into rods, and subsequently segment into 'secondary granules/ which liquefy and coalesce to form the definitive fat spherules. This important investigation suggests a functional role for mitochondria in terms of a nutritive material upon which cell metabolism and differen- tiation may depend. Lymphoid Tissue (Ade- noid Tissue). Lymphoid tissue is a reticular tissue the meshes of whose network are occupied by a closely packed mass of lymphocytes, cells with FIG. 77. DEVELOPING ADIPOSE TISSUE FROM THE SUBCUTANEOUS TISSUE OF AN INFANT. The fat has been removed by immersion in alcohol and ether. The polygonal outlines of the fat cells are well shown. Within many of them is seen the finer cytoplasmic network by which the inclosed droplets of fat were in- vested; this network had not been completely replaced by the accumulation of fat. Hema- tein and eosin. Photo. X 325. FIG. 78. RETICULTJM FROM THE MUCOSA OF THE FUNDUS REGION OF THE DOG'S STOMACH. The section was made parallel to the surface and the glandular tissue removed by shaking in water. Picro-carmin. X 125. (After Mall.) Lymph sinus Cortical substance Capsule Medullary cords 1 Follicles H ilus~wiih vessels' entering Medullary substance FIG. 79. SECTION THROUGH A SMALL LYMPH NODE OF A DOG. X 20. (From Szymonowicz-MacCallum, "Histology and Microscopic Anatomy.") beculss 6 65 66 CONNECTIVE TISSUE CARTILAGE BONE a deeply staining nucleus enveloped by a narrow shell of homogeneous slightly basophilic cytoplasm. The lymph cells (lymphocytes) are so closely packed that it is almost impossible to distinguish the fine threads of the reticular stroma, except in those portions where some of the lymphatic cells have been washed out or displaced in the preparation of the specimen.* The density of the lymphoid tissue varies much, however, in dif- ferent organs and even in different portions of the same organ. The denser accumulations of lymphoid corpuscles may form either ovoid lymph nodules or follicles, or long dense trabeculaB, the lymphatic cords, which are surrounded by looser portions of lymphoid tissue. Lymphatic corpuscles are fre- quently infiltrated into the connec- tive tissue of the mucous mem- branes, where they form irregular collections, which may be termed diffuse lymphoid tissue, in contra- distinction to compact lymphoid tissue, which occurs in the lymph nodes, tonsils, thymus, and spleen, and in the aggregate and solitary nodules of the intestinal canal. Diffuse lymphoid tissue is found in the mucous membranes of (A) the respiratory tract nose, nasophar- ynx, larynx, trachea, and bronchi; FIG. 80. FROM A SECTION THROUGH THE MEDULLA OF A CERVICAL LYMPH NODE OF MAN. a, a 'cord' of dense lymphoid tissue; b } looser lymphoid' tissue of the medullary sinuses; c, the margin of a fibrous tra- becula; d, nucleus of the connective tis- sue reticulum; e, endothelial lining of the lymphatic sinus. Hematein and eosin. X475. and (B) the alimentary tract mouth, tongue, pharynx, esophagus, stomach, and intestines. In the basement membranes of certain tubular glands e.g., sweat, kidney, tear and mammary and in the peripheral portion of the large cells of the umbilical cord, Mallory (Jour. Med. Res., 1903 and 1905) * Mall's technic for this purpose consists in injecting gelatin into a fresh lymph organ (e. g., spleen), freezing the tissue, and placing thin sections into warm water when the lymphocytes are largely carried away by the dissolving gelatin leaving the reticulum free. CAKTILAGE 67 has discovered robust fibers extending also from cell to cell, resembling somewhat white fibers, but unrelated by transition elements to, and dif- fering microchemically from, collagenous fibers. They are said to be similar to the fibrils of neuroglia cells of nervous tissue and to the border or myoglia fibrils of plain muscle cells. BLOOD AND NERVE SUPPLY OF THE CONNECTIVE TISSUES The connective tissues, but especially the areolar variety, form a supporting substance through which the various blood and lymphatic vessels and nerve trunks are distributed to all portions of the body. Within the connective tissues these vessels are everywhere present, and from them the connective tissue itself receives its supply of capillary vessels and terminal nerve fibrils. The vascular supply of the connective tissues is very abundant. Small arteries, which are derived from the main trunks, form a capillary plexus throughout the tissue, the capillaries finally reuniting to form the venules. It is in this capillary plexus that the fluid portions of the blood exude into the surrounding perivascular lymphatic or tissue spaces of the con- nective tissue. The tissue juices which arise in this manner are most active agents in the physiological processes of assimilation. From the tissue juice spaces, lymph reenters the abundant capillary lymphatic vessels to be finally returned to the venous blood. This transfer is mediated by process of filtration and osmosis, the tissue spaces being generally regarded as closed spaces making no direct connection with the lymphatic terminals. Of the several varieties of connective tissue, the adipose possesses the most abundant blood supply ; the lymphoid, on the other hand, is most richly supplied with lymph. Abundant nerves are distributed to the connective tissues, some of which, the sympathetic nerves, supply its blood-vessels while others, medullated, terminate in special forms of. sensory nerve end-organs. CARTILAGE Cartilage is a dense, firm, but elastic substance, resembling connective tissue in that it is developed from similar mesodermal cells. It contains a ground substance, the cartilage matrix, and at times, fibers which may be either collagenous fibers or elastic. The presence, absence, or 68 CONNECTIVE TISSUE CARTILAGE BONE character of these fibers determines the variety of cartilage. Three varieties are thus distinguished: hyaline cartilage, in which no specific fibers can ordinarily be demonstrated within the matrix ; elastic cartilage, whose matrix is permeated by elastic fibers; and fibrocartilage, whose matrix contains collagenous fibers. Fia. 81. TRANSECTION OF A PLATE OF HYALINE CARTILAGE, FROM THE TRACHEA OF A CHILD. The margin of the fibrous perichondrium can be seen on either side of the plate of cartilage, in the upper right and lower left hand corners of the figure. Hematein and eosin. Photo. X 400. The dark bodies are shrunken cells within lacunae. Hyaline Cartilage. This is the most abundant of the three varie- ties, commonly known as gristle. It is found in the respiratory system, forming the cartilages of the nose, larynx, trachea, and bronchial tubes ; in the costal cartilages of the ribs; as articular cartilages covering the ends of long bones ; and in the fetus, where in the course of development of the bones, the entire skeleton, excepting only the flat bones of the skull and face, at first consist of hyaline cartilage. In niost of these loca- tions the cartilage occurs as platelike masses, which are invested by a vas- CARTILAGE 69 cular membrane of dense fibre-elastic tissue. This membrane is the perichondrium. The inner portion of this membrane is richly supplied with small cells, and it is from this cell layer, the chondrogenetic layer, that the cartilage is presumably developed. The cartilage blastema is essentially mesenchyma. The ehendro- genetic cells of this precartilage multiply, and deposit about themselves the structureless mass which first forms merely a capsule to the cell, but which as it increases in amount, separates the various cells by wider areas and becomes the cartilage matrix. The cells, which in the perichondrium are small and decidedly flattened, likewise in- crease in size during this proc- ess, and become more nearly spherical, so that those cartilage cells which lie near the center of the cartilaginous plates are spheroidal in shape, while those toward the surface are more and more flattened or elongated, their long axes gradually re- volving from a perpendicular position in the center of the plate to one parallel with the perichondrium at the surface. Each cartilage cell is inclosed within a small space or lacuna, The grouping in pairs and fours, and the tendency to produce a so-called 'capsule,' are especially noticeable. Hematein. X550. FIG. 82. CELLS AND MATRIX or HYALINE CARTILAGE FROM THE WALL OF A LARGE BRONCHUS OF MAN. which during life it entirely fills. Cell multiplication in carti- lage is peculiar in that cell di- vision occurs within a firm capsule and results in the formation of two daughter-cells, which at first lie within the same encapsuled space. These two cells may each again undergo division within the same space with formation of four new cells. As a result of this peculiar method of cell division the cartilage cells are arranged in groups of two, four, or even eight cells. Each of the cells in the group deposits its capsule, and thus forms a matrix about itself, so that the increasing space thus produced between the cells of a group may separate them until they become com- pletely isolated cartilage cells each within its own lacuna. In this way the matrix of the cartilage is produced. Enlargement of a cartilage plate occurs through a combination of interstitial and perichondrial growth. 70 CONNECTIVE TISSUE CAETILAGE BONE The matrix of hyaline cartilage is devoid of fibrous or cellular structure. Chemically it consists of collagen, chondromucoid and albuminoid sub- stances. Von Korff (1914) interprets hyaline matrix as being composed of matrical fibrils masked by a homogeneous cementing substance. During life, or if the tissue is examined in the fresh state, the car- tilage cell entirely fills the lacuna in which it lies. But shortly after death shrinkage of these cells begins, so that after some hours a considerable space intervenes between the cell and the wall of its lacuna. It has been supposed that this space was occupied during life by lymph. It would, however, seem more probable that it is partially the result of post-mortem shrinkage of the cell. Frequently, and especially in developing cartilage, concentric lines may be seen surrounding each lacuna. These lines have been described as the 'cell cap- sule.' They appear only to in- dicate the successive layers of material which have been de- posited by the cell, and which have fused together to form its surrounding matrix. Cartilage arises from a mes- enchymal syncytium in which the matrix is formed from the exoplasm of the syncytial tissue, the cartilage cell representing its endoplasm. The so-called capsule of the cartilage cell would accordingly represent the partially modified bor- der line between the original endo- and exoplasm, and would thus cor- respond to similar conditions which are observed in other forms of devel- oping connective tissue. Cartilage cells frequently contain small droplets of fat, and these may coalesce until the cell is completely transformed into a fat cell. Isolated masses of adipose tissue, resulting from the transformed groups of carti- lage cells, thus make their appearance within the cartilaginous plates. This fatty metamorphosis is most marked in the elastic variety of cartilage. By coloration with iodin, glycogen granules may also be demonstrated in the cartilage cells. FIG. 83. ELASTIC CARTILAGE FROM THE HUMAN EPIGLOTTIS, SHOWING THE LARGE OVOID CARTILAGE CELLS AND THE VERY DELICATE RETICULUM OF ELASTIC FIBERS. Ehrlich's triacid stain. X 550. CARTILAGE 71 Elastic Cartilage. Elastic cartilage occurs in the external ear, in the auditory tube, in the epiglottis and in the cuneiform and corniculate cartilages and the vocal processes of the arytenoid cartilages of the larynx. It is essentially hyaline cartilage the matrix of which has become permeated with delicate elastic fibers forming a dense interlacing network. The large spheroidal cartilage cell lies in a lacuna bounded by a capsule and surrounded by a layer of hyaline matrix free of elastic fibers. The plates of elastic cartilage, like the hyaline variety, are sur- rounded by a dense fibrous perichondrium. Neither blood-vessels, nerves, nor lymphatics are distributed within the matrix of elastic cartilage. Fibrocartilage This tissue forms the interarticular cartilages of the lower jaw, the clavicle, and the knee; composes the intervertebral disks and the other cartilaginous symphyses of the body; lines the tendon grooves of the bones, and forms the glenoid ligament of the shoulder and the cotyloid ligament of the hip. Fibrocar- tilage is intermediate in structure between hyaline cartilage and such very dense fibrous tissue as occurs in the tendons of , FIG. 84. FIBROCARTILAGE, FROM muscles. At the attached margins of the INTERVERTEBRAL DISK OF Ox. cartilaginous plates its tissue is continued by imperceptible gradations into the surrounding fibrous connective tis- sues. Like the other forms of cartilage, this variety is also non-vascular and devoid of nerves. . Microscopically, fibrocartilage differs from such dense white fibrous tissue as is found in the ligaments and tendons, in that the meshes of the dense fibrous tissue of fibrocartilage are everywhere permeated by a hyaline matrix in which here and there are small groups of ovoid car- tilage cells. Each cartilage cell is occasionally surrounded by a charac- teristic, concentric, lamellar appearance of the adjacent matrix, the so- called 'capsule/ Plates of fibrocartilage, unlike the other varieties, are not surrounded by a perichondrium. A peculiar sort of connective tissue of entodermal origin is found in the nuclei pulposi of the invertebral disks. It is the sole adult vestige of the embryonic axis, the notochord. According to Williams (Amer. Jour. Anat., 8, 3, 1908), who carefully studied its cytomorphosis in the pig, "It is primarily cellular and epithelial; later it becomes a 72 CONNECTIVE TISSUE CARTILAGE BONE syncytial network with a mucin-like substance in its vacuoles ; and finally it becomes cellular and closely resembles cartilage/*' The Perichondrium. The perichondrium is a dense fibrous mem- brane which surrounds each individual plate of cartilage. It is continuous with the surrounding connective tissue, and is well supplied with blood- vessels and lymphatics ; it may also contain terminal nerve fibrils. The cartilage itself is an absolutely bloodless and nerveless tissue. Neither are lymphatic channels demonstrable within the cartilage matrix. FIG. 85. NOTOCHORDAL TISSUE. A, from pig embryo of 150 mm.; ther syncy tium contains many mucin-filled spaces. X 800. B, from nucleus pulposus of an adult pig; the three cells shown are greatly vacuolated. X 452. (After L. W. Williams, Amer. Jour. Anat., 8, 3, 1908.) After long maceration or artificial digestion the matrix assumes a granu- lar or fibrous appearance, and small channels have been demonstrated within it, which have been said to connect the various lacunas; but it is evident that these appearances were possibly the result of artificial de- structive processes and could not therefore be considered as evidences of the presence of such structure in living cartilage. BONE General. Bone is a firm calcareous tissue which is found only in the skeletal system. In the flat bones it forms a double layer of dense osseous tissue between which is a narrow space, bridged across at fre- quent intervals and thus subdivided into a number of compartments, the BONE 73 marrow cavities. This central stratum presents a spongy appearance as compared with the denser periphery; it is therefore said to contain spongy or cancellous bone, while the more superficial lamellae contain compact bone. In the long bones a similar condi- tion exists in the epiphyses, which consist of a wall of compact bone within which the marrow cavity is subdivided by bony partitions into numerous compartments. The epi- physis consists, therefore,, of spongy bone. The shaft or diaphysis of the bone, however, contains a single large marrow cavity whose walls, except for a thin layer at either end, consist en- tirely of compact bone. A little spongy structure is present for some distance at either end of the shaft, in that portion which adjoins the mar- row cavity. The ends and facets of the bones are covered by a disk of hyaline car- tilage, which forms the articulating surfaces of those bones which enter into the formation of the movable joints. These articular cartilages are peculiar in that they are not covered by a perichondrium, and their deeper cells, which adjoin the bone, are so arranged that their long axes are per- pendicular to the free surface, as is the case in the central portion of free cartilaginous plates. Toward the free surface of the cartilage the long axis of the cell lies more nearly parallel to the surface, as is likewise the case at the surface of cartilaginous plates elsewhere. In the long bones of younger individuals a plate of hyaline cartilage is found also at the epiphyseal lines between the epiphysis and the diaphysis. This plate, which extends through the entire axis of the FIG. 86. TRANSECTION THROUGH THE COMPACT BONY WALL OF A HUMAN METACARPAL BONE. a, outer circumferential lamellae; 6, inner circumferential lamellae; c, Haversian canals; d, interstitial lamel- lae; e, lacunae, with delicate radiating canaliculi. From a thin section of ground bone. X 90. (After Kolli- ker.) CONNECTIVE TISSUE CARTILAGE BONE bone, becomes ossified later in life. It represents the line of growth, and is the last portion of fetal cartilage to be transformed into ;adult bony tissue. Periosteum. All those portions of the bone which are not covered by an articular cartilage are supplied with a membranous coat of fibrous tissue, the periosteum. The outermost layer of this membrane consists of interlacing bundles of dense fibrous tissue in which are the larger blood- vessels, whose branches are distributed to the underlying bone. The inner portion of this layer forms a firm fibro-elastic stratum, which in older individuals is closely attached to the surface of the bone. The periosteum of developing and growing bone, how- ever, contains a third or innermost areolar layer, in which are small blood-vessels, fine connective tissue fibrils, and numerous small osteogenic cells, the osteoblasts. After growth of the bone has ceased, the deepest layer of the periosteum contains few small blood-vessels and only occa- sional osteoblasts. These cells, how- ever, are present in sufficient numbers to accomplish the regeneration of the bone after destruction of its osseous tissue. The medullary surface of the bone is likewise supplied with an osteo- genic membrane of fibrocellular tis- sue, similar to the innermost layer of the periosteum ; it is known aa the periosteum internum, endosteum, or membrana medullaris. Compact Bone. Compact bone, such as that composing the shafts of the long bones, consists of concentric lamellce of calcified fibrous tissue which constitute the Haversian systems, together with groups of parallel laminae, which are interposed between adjacent Haversian systems and are known as the interstitial or ground lamellce. Many of the interstitial lamellae are the remains of Haversian systems which have been partially FIG. 87. LONGITUDINAL SECTION OF GROUND BONE FROM THE SHAFT OF THE HUMAN FEMUR. a, Haversian canals; 6, lacunae; c, canaliculi. X 100. (After Kolliker.) BONE 75 FIG. 88. ISOLATED BONE CELL, SHRUNK AWAY FROM WALL OF LACUNA AT I. (Schafer, after Joseph.) absorbed during the development of the bone. In a section through the shaft of a long bone the Haversian systems are found in the middle of the wall, while superficial to them and just within the periosteum are a number of lamellae which may be traced much or all of the way around the cir- cumference of the cylindrical shaft, and which are known as the external circum- ferential or periosteal lamella. On the inner surface of the compact bony wall is a similar group of parallel laminae which adjoin the marrow cavity, and are known as the internal circumferential or endos- teal lamellae. In their finer structure the circumferential lamella? are exactly similar to the cylindrical bony lamellae of the Haversian systems. HAVERSIAN SYSTEM. A Haversian system contains a small central canal (0.05-0.1 mm. dia.), which is occupied by connective tissue, marrow cells derived from the marrow cavity during the process of development, small blood-vessels, nerve fibers, and perivascular lymphatics. Concen- trically arranged around the Haver- sian canal are parallel layers of dense fibrous tissue, the Haversian lamellce. The fiber bundles of this tissue form an interlacing network whose bundles frequently cross each other at right angles and whose in- terstices are occupied by a solid cal- careous mass, consisting chiefly of the phosphates (about 80 per cent.) and carbonates of calcium. From four to twenty such calcareous lam- ellae are found in each Haversian system. The organic substance of bone consists chemically of collagen, osseomucoid and small amounts of other albuminoid bodies. Both in and between the lamellae are many small ovoid spaces which are partially filled by small flattened cells, the lone cells; these spaces are known as the lacunce. From each lacuna minute canals, the canaliculi, FIG. 89. AN HAVERSIAN SYSTEM, INCLUDING THE CENTRAL CANAL, SEVERAL LAMELLAE, LACUNAE AND CANALICULI. CONNECTIVE TISSUE CAETILAGE BONE radiate in all directions, thus placing the lacuna in open communicatio ; with its neighbors, and eventually with the lymph spaces of the cen- tral Haversian canal. The branching processes of the bone cells fro quently project for a short distance into the canaliculi. These cyto- plasmic branches are more numerous in newly formed bone, later they are retracted and the cells become more or less shriveled in appearance. The Haversian system, being developed about a central canal which marks the course of a blood-vessel, necessarily acquires a slender columnar shape, its long axis being usually disposed in a direction nearly parallel to that of the bone of which it forms a part. The Haversian canals fre- c quently branch to permit a corre- sponding division of their blood-ves- sels, and all of the Haversian canals are connected either directly or indi- rectly with the periosteum, the nu- trient foramina, or the marrow cavity thus forming a complete connected system between marrow cavity and surface from the blood-vessels of which their vascular supply is de- rived. INTERSTITIAL LAMELLA. The in- terstitial lamellae are likewise com- posed of dense interlacing bundles of calcified fibrous tissue, within and be- tween which are lacunae, canaliculi, and bone cells, all disposed in a man- ner exactly similar to their arrangement within the concentric lamella? of the Haversian systems. Coursing through the interstitial and circum- ferential lamellae are Volkmanris canals, which are similar in origin, con- tents, and function to the Haversian canals but which are not surrounded by concentric lamellae. Volkmann's canals frequently arise as branches of the Haversian canals which wander out, as it were, into the interstitial lamellae. CIRCUMFERENTIAL LAMELLAE. The circumferential lamellae do not differ in structure from the other osseous lamellae. They possess the same arrangement of laminated calcareous connective tissue, with lacunae, canaliculi, and bone cells, as in the concentric and interstitial lamellae. Even more than elsewhere, however, the outer circumferential lamellae are firmly bound together by collagenous and elastic fibers which pass from FIG. 90. TRANSVERSE SECTION OF HAVERSIAN CANAL, WITH CONTENTS. a, arteriole; v, venule; Z, lymphatic; n, non-medullated nerve fibers; c, bone cell. (After Schafer.) BONE 77 the periosteum into and through the superficial lamella?; these are known as the perforating fibers of Sliarpey. Similar fibers connect together tjie concentric and interstitial lamellae. The perforating elastic fibers are frequently surrounded by an envelope of fibrous connective tissue. Bone Marrow. Bone marrow consists of a variety of connective tissue, largely reticular, which is rich in fat cells and blood-vessels and which also contains osteogenic and hemogenic elements, the marrow cells or myelocytes. According to the relative proportion of these ele- ments marrow is said to present two types, the yellow and the red marrow. The yellow marrow consists almost entirely of fat, with only occasional bands of true marrow tissue. The red marrow contains very little fat, but is so abundantly supplied with blood and marrow cells as to closely resemble a very vascular lymphoid tissue. The embryonic medulla of all bones contains fetal red marrow, but in later life the larger masses in the medulla of the shafts of the long bones are, in man, changed to the yellow variety. The red marrow, however, persists in the epiphyses of the long bones and in cancellous bone generally; it is especially charac- teristic of the marrow cavities of the ribs, vertebrae, base of the skull, and sternum. It is the source of supply of blood-cells in the adult. BED MARROW. Bed marrow consists of fibrous and reticular tissues which are infiltrated by marrow cells and richly supplied with small blood-vessels. The smaller veins possess exceedingly thin walls, readily pervious to the blood-cells. The walls are so delicate that it becomes very difficult to determine with certainty whether or not their eiidothe- lium, as also that of the capillaries, may be occasionally absent, thus placing the blood-stream in direct communication with the pulp of the bone marrow. The hemogenic elements of marrow will be described under the subject of blood development, where red marrow must again be con- sidered. At this point it is only necessary to describe the osteogenic elements. These are (1) the osteoblasts, or bone builders, and (2) the osteoclasts, or bone destroyers. The osteogenic process as a whole is of course dependent upon the blood, with all its hemal elements. OSTEOBLASTS. These are cells which may assume various shapes depending upon their spatial relationship to the bony substance. When free they are of round or slightly oval shape; lining the marrow cavity or covering the bone as portions of the periosteum or applied to spicules of cancellous bone they may become considerably flattened. The nucleus is generally round or oval, deeply chromatic and granular. As spheroidal ceils they have an average diameter of about 8 microns. They are with 78 CONNECTIVE TISSUE CAETILAGE BONlJ difficulty distinguished from lymphocytes except when characteristically arranged as a membranous coat upon the surface of bony walls or spic- ules. They become the bone cells of compact bone. Osteoblasts and lympho- cytes are genetically closely related., both being relatively slightly differen- tiated mesenchymal cells. In the bone marrow of the turtle osteoblasts may differentiate into leukocytes. It seems probable that persistent fetal osteoblasts of adult red marrow may function as parent blood-cells. OSTEOCLASTS. These are giant multinuclear cells, often containing as many as ten to twenty or more nuclei. They are the cells by whose agency bone is destroyed during the processes of development and growth. They are similar to, but not identical with, the polykaryocytes of hemo- genic foci which are concerned with the process of blood-cell formation. The osteoclasts originate by a process of fusion of reticular cells of the marrow; the hemogenic polykaryocytes originate from lymphocytes (hemoblasts) by repeated amitotic division of the nucleus. Blood Supply. Marrow, and especially the red variety, is richly supplied with blood. The nutrient or medullary artery penetrates obliquely through the nutrient foramen to the marrow cavity of a long bone where it divides into an ascending and descending branch and supplies an abundance of small arteries to all portions of the medulla. The terminal arteries end in broad capillary vessels whose wide lumen and delicate endothelial walls determine their character as sinusoids. It was formerly thought that the endothelial walls of these vessels were here and there deficient, and although recent investigations discredit the former observations, the all-important fact remains that the endothelial walls are pervious to both red and white blood-cells. Certain of the ter- minal arteries anastomose with those of the cancellous epiphyses, and with the arteries which enter the Haversian canals of the compact bone from the periosteum. Efferent veins return the blood from the sinusoidal capillaries of the marrow. These veins, passing as companion veins to the medullary artery through the nutrient foramen, or independently through separate foramina, as also those of the bony tissue, are not supplied with valves. Outside of the bones, however, these same veins contain abundant valves. The Lymphatics. The lymphatics of bone occur in great abundance in- the periosteum, and as perivascular spaces penetrate the canals of Havers and Volkmann and thus reach the medullary cavity. The exist- ence of lymphatics within the marrow, other than in the sheaths of the blood-vessels, is doubtful. The Nerves. The nerves accompany the blood-vessels in all portions BONE 79 of the bone and marrow and form a rich perivascular plexus which is distributed to the walls of the vessels ; occasional side fibrils are also dis- tributed to the marrow. Nerve endings have not been demonstrated in compact bone nor in the articular cartilages. In the periosteum terminal nerve fibrils are supplied to the musculature of the blood- vesselsy-asd other sensory fibrils end in lamellar corpuscles. Development of Bone- Bone tissue makes its appearance relatively late in fetal life. The long bones are first mapped out by masses of hyaline cartilage. The entire skeleton., with the exception of the flat bones of the face and those of the vault of the skull, is thus primarily formed by plates of fetal cartilage. The process by which these cartilag- inous plates are transformed into bone is known as intracartilaginous or enchondral ossification. The process is essentially one of replacement of cartilage by bone, not one of change of cartilage into bone. The resulting bones are known as substitution bones. The flat bones of the face and skull (including the interparietals, parietals, frontals, squamosals, tympanics, median pterygoid plate of the sphenoid, nasals, lacrimals, malars, palatine, vomer, maxilla, and a portion of the mandible) are formed directly from the mesenchymal blastema without the intervention of cartilage. This method of bone formation differs somewhat from the above and is known as intramem- Iranous ossification. INTRACARTILAGINOUS OSSIFICATION. This process begins with the formation of plates of hyaline cartilage whose shape corresponds more or less closely with that of the future bone. This type of fetal cartilage differs from the hyaline cartilage of the adult only in the irregular form and distribution and greater abundance of its cartilage cells. Each plate of fetal cartilage is enveloped by a layer of embryonal fibrous tissue, the fetal perichondrium. The outer portion of the fibro- cellular layer is destined to become the periosteum of the future bone; its innermost portion contains many small round cells, which from their intimate relation to bone production, are known as osteoblasts. The inner portion of the perichondrium forms the osteogenic layer of the future periosteum. Centers of Ossification. Ossification of the cartilage begins at one or more points which are called centers of ossification. In the long bones, in which the process of bone formation can be most readily traced, there are usually three such centers, one near the middle of the cartilaginous plate, from which the diaphysis is formed, and one epiphysial center at each extremity. The centers for the epiphyses make their appearance 80 CONNECTIVE TISSUE CAKTILAGE BONE much later than that for the shaft of the bone, for the most part not until some months after birth, and from an extension of marrow from the primary center. Enlargement of the Cartilage Cells. The first indication of begin- ning bone formation is evidenced by an enlargement of the cartilage cells which promptly arrange themselves in rows or columns that radiate from the center of ossification (calcification). This process is accompanied by absorption of the adjacent cartilage matrix, so that the enlarged car- FIG. 91. THE PRIMARY CHANGES IN INTRACARTILAGINOUS BONE FORMATION. A, metatarsus; B and C, phalanges of human fetus. In A, the earliest enlarge- ment of cartilage cells at the center of ossification is shown. B and C are successively later stages. The bones are cut in longitudinal section. Carmin hematoxylin stain. X 27. (After Toldt.) tilage cells are contained within broad spaces or areolw. The cartilage cells now appear to undergo a gradual but progressive absorption; their cytoplasm becomes shrunken and granular and finally disappears, even the nucleus at last succumbs to the process. Calcified Cartilage. The absorption of the cartilaginous matrix pro- ceeds more rapidly in those portions which separate the individual cells in the columns than in those other portions which intervene between the adjacent rows of cartilage cells. While the former portions are entirely absorbed, remnants of the latter remain, and in them calcium salts are deposited in an irregular manner. Calcified cartilage, the most primitive of the calcareous tissues, is thus formed. FIG. 92. A LONGITUDINAL SECTION OF THE Two DISTAL PHALANGES FROM THE FINGER OF A FIVE-MONTHS' HUMAN FETUS. Kn, cartilage showing calcification and resorption; ek, enchondral bone; M, marrow cavity; pk, periosteal bone. X 15. (From Sabotta's "Histology.") 81 82 CONNECTIVE TISSUE CAETILAGE BONE car. c. Primordial Mar- row Cavities. The absorption of the cartilage matrix re- sults in the forma- tion of broad spaces into which osteo- genic buds of prim- itive marrow tissue push their way from the perichondrium. Thus the primordial marrow cavities are formed. The fetal marrow which now occupies these cav- ities is derived from the osteogenic layer of the primitive peri- osteum. The oste- ogenous tissue of this layer, containing os- teoblasts, osteoclasts, and developing blood-vessels,, grows into the cartilage in the form of budlike cords which are pre- ceded by absorption of the adjacent car- tilage matrix. This so-called 'eruptive tissue ' promptly reaches the center of ossification and bur- rows its way into the enlarged cartilage lacunae whose cells are now replaced by primary osteogenic marrow. The destruction of cartilage is initiated and maintained by agency of the osteogenic tissue, presumably through specific cells, the so-called chon- FIG. 93. RECONSTRUCTION OF CARTILAGE INTO BONE. car. c., cartilage cells in successive stages of degenera- tion; ost, osteoblasts; gi. c., giant cells (osteoclasts) ; 6, young bone; bl. c., blood cells. (From Dahlgren and Kepner.) BONE 83 droclasts, the morphological marks of identification of which are not yet known. According to some investigators (e.g., Retterer, 1900) the car- tilage cells do not disintegrate but pass into the marrow cavity where they become osteoblasts. Primary Bone. The osteoblasts which thus gain access to the pri- mary marrow cavities, now arrange themselves. along the surface of the remnants of calcined cartilage and begin the deposit at their proximal surface of the fibrous tissue and calcareous salts which compose the primary bone. The osseous matrix is commonly assumed to be the product of a transformation of the exoplasm of the osteoblasts. Many of the osteoblasts apparently be- come entangled in this newly formed tissue and form the bone cells. The fetal cartilage is thus transformed into a spongy mass of primary osseous tissue whose spic- ules are formed by a core of calci- fied cartilage upon which are de- posited successive layers of bony tissue with their included lacuna? and bone cells. In sections stained with hematoxylin and eosin, the central strand of calcified cartilage is colored blue, the primary bone, red. Axial sections of long bones at this stage of ossification show all the above changes in regular suc- cession from the fetal hyaline car- tilage at the extremities to the primary bone with its marrow cavities in the center. The process of ossification steadily progresses toward the ends of the bone, the line of enlarged cartilage cells constantly advancing farther and farther from the original center of ossification. Absorption of the Newly Formed Bone. It is at this stage, however, that the giant cell osteoclasts become most active and the absorption of the newly formed bone progresses rapidly. The osteoclasts collect along FIG. 94. TRABECULA OF PRIMARY ENCHONDRAL BONE, SHOWING A CENTRAL DEEP-STAINING CORE OF CALCIFIED CARTILAGE AND A PER- IPHERAL LAYER OF OSTEOBLASTS. Osteoblasts have become incorpora- ted within the bone as bone cells. From the finger of a human fetus. CONNECTIVE TISSUE CAETILAGE BONE the surface of the spicules of primary bone in considerable numbers and appear to sink into little recesses which they form within the bony tissue. The little bays which are thus formed in the primary bone are the lacu'nce of HowsJiip. The continued absorption soon breaks down and removes the trabeculae and partitions of spongy bone and forms a central medullary cavity, of constantly increasing size. Perichondrial Ossification. Coincident with these changes within the cartilage the osteogenic tissue which forms the inner layer of the peri- chondrium produces succes- sive layers of bony tissue upon the surface of the fetal cartilage. This process of perichondrial (periostea!) ossification proceeds in a manner similar to that by which bone is formed in membrane which is not closely applied to cartilage. Perichondrial bone for- mation is essentially of the intramembranous type. In essence there is no valid dis- tinction between endochon- dral, perichondrial and membrane bone develop- ment, since each involves calcification of a fibrillar matrix by agency of the same cell, the osteoblast. At irregular intervals the osteo- clasts collect and the pri- mary perichondrial bone is absorbed. Into these cavities buds of vascular osteogenic tissue push their way to form canals of considerable length. Upon the surface of the canals which are thus hollowed out of the perichondrial bone, the Haver- sian spaces, the osteoblasts deposit successive concentric layers of bony tissue and the Haversian systems make their appearance. Finally, upon the surface of the periosteal bone successive layers of newly formed bony tissue compose the external circumferential lamellce, while upon the wall of the medullary cavity a similar endosteal layer of bone-forming FIG. 95. TRABECULA OF PRIMARY BONE FROM THE FINGER OF A HUMAN FETUS. Three giant cells (osteoclasts) are shown at the right, two resting in Howship's lacunae. BONE 85 cells deposits the internal circumferential lamella. The Haversian canals are actually continuations of the marrow cavity, and the larger are even lined by endosteum. With the formation of the perichondrial bone the lateral expansion of the organ by endochondral bone formation necessarily ceases. Hence- forth increase in diameter of the bone is only produced by continued absorption internally of the compact bony wall and the formation of new bone beneath the periosteum by frequent repetitions of the processes of periosteal (perichondrial) ossification as already described. The rem- nants of those Haversian and circumferential lamellae which are only partially absorbed in this process form the interstitial lamella of the ma- ture bone. In the long bones and in flat cartilage bones ossification at first proceeds in the perichondrium, endochondral ossification appearing only later ; in the short bones ossification is endochondral until the carti- lage is entirely replaced by bone. Epiphysial Ossification. During the processes of endochondral and perichondrial ossification within the shaft of the bone, the epiphysial cartilages continue to grow. Finally, however, ossification begins in the epiphysis, osteogenic tissue having pushed in from the primary center of the diaphysis, and proceeding in the same manner as in the shaft, results in the formation of primary spongy bone, some of which is ab- sorbed and replaced by more compact bony tissue, as occurs in the wall of the diaphysis. In its central portions the tissue retains its spongy arrangement and but few Haversian systems are formed. It is thus that the cancellous bone of this part, as also of the ends of the diaphysis, is formed. At the point where the expanding centers of ossification of the shaft and epiphysis are about to meet, a line of unossified cartilage, the epiphysial line, persists until growth of the bone is complete. It is by growth of this cartilaginous disk, with continued formation of cartilage mainly on its inner surface, and its concomitant replacement by bone, that the bone increases its length. After ossification of this epiphysial synchondrosis at about the twenty-first year, growth in length must cease. Meanwhile the perichondrium has become periosteum. The following is a resume of the various stages of endochondral ossifi- cation : 1. Formation of the fetal hyaline cartilages from precartilage mesen- chyme blastema. 2. Enlargement of the cartilage cells with a rearrangement into radiating cell rows at the center of ossification. 7 86 CONNECTIVE TISSUE CAETILAGE BONE 3. Absorption of the cartilage matrix between the cells of the rows and finally also of the cells themselves. Calcification of per- sistent remnants of cartilage matrix between the rows of cells. 4. Eruption of the subperiosteal osteogenic tissue, invasion to center of cartilage plate, and the formation of primary marrow cavi- ties at the center of ossification. 5. Gradual extension of the above processes followed by a deposit of primary bone by the osteoblasts upon the calcified cartilage spicules. Coincident osteoblastic deposit of perichondrial bone beneath and within the perichondrium of the cartilage plate. 6. Absorption of portions of the primary bone by the osteoclasts to form the large central marrow cavity or medulla. The ab- sorption involves both the endochondral and the perichondrial bone and is accompanied by a further deposit of new bone at the periphery. In the perichondrial bone cylindrical axial channels are formed, in which the deposit of new bone pro- duces the Haversian systems of the compact bony tissue. INTRAMEMBRANOUS OSSIFICATION. This is the simpler and more direct method of bone formation. In principle it is identical with peri- chondrial ossification. Endochondral bone development differs from it only in respect of the additional processes involved in the removal and replacement of the hyaline cartilage. Membrane bones, including the flat bones of the face and the vault of the cranium, arise directly in the mesenchyma. The first indication of ossification is the enlargement and rounding up of a group (or groups) of mesenchyme cells, and their association in the form of an irregular membrane. Among the cells appear bundles of delicate collagenous fibrils, the osteogenic fibers (Sharpey), radiating beyond the limits of the cell group. The cells of this initial ossific group begin to function as osteoblasts and deposit osseous matrix among the fiber bundles. This original osseous trabecula marks approximately the center of the future bone. The surrounding loose mesenchyma has meanwhile become increasingly vascular. Vaguely outlining the peripheral limits of the definitive bone appears a relatively thick layer of denser, more cellular mesenchyma, the cells in general maintaining a fusiform shape. This represents the primitive periosteum of the forming bone. The bone takes shape internally by the appearance of numerous trabecula?, which arise in the manner described for the initial spicules and then unite into a BONE 87 bony sponge-like structure enclosing vascular mesenchyma, the primary marrow. The spicules of the cancellous bone contain numerous bone cells the representatives of original osteoblasts which have become enmeshed in their own product of osseous matrix and are covered with an epithe- lioid membrane of a single or double layer of osteoblasts, which contribute FIG. 96. INTRAMEMBRANOUS BONE FORMATION IN THE LOWER JAW OF A SHEEP FETUS. a, bone; b, primary marrow cavity; c, osteoblasts; d, growing point of the primitive bone, beyond which primary marrow is developing in the connective tissue of the mesoblast. X 300. (After Bohm and von Davidoff.) to the further growth of the bony trabeculae. The marrow includes besides osteoblasts and the specific marrow cells- somewhat less numerous than in the primary marrow of endochondral bone numerous osteoclasts under whose absorptive agency, assisted by the productive activity of the osteoblasts, the inner conformation of the growing bone continually alters its details. Peripheral osteoblasts, arising from the inner layer of the periosteum, produce the more compact external plates of the bone. 88 CONNECTIVE TISSUE CARTILAGE BONE In the flat bones of the skull, the central cancellous bone is designated diploe, the peripheral compact bone, tables. Membrane bones may con- tain typical Haversian systems (Arey, Anat. Bee., 17, 1, 1919). The conditions which determine that certain bones may arise directly in mesenchyma while others must pass through a cartilaginous stage are obscure. It is commonly believed that periosteum is essential for bone regenera- tion, and its preservation is aimed at where new growth is desired after osteotomy. But according to W. Macewen ("The Growth of Bone," Maclehose & Sons, Glasgow, 1912), who has made a comprehensive ex- perimental study of osteogenesis in regenerating bone in dogs, the perios- teum functions simply as a confining, nutritive, and protective membrane, but has no osteogenic significance. His observations lead him to conclude also that in the long bones the osteoblasts are derived from proliferating cartilage nuclei freed from the disappearing matrix. Under more favor- able conditions regeneration is said to occur through direct osteoblastic activity, under less favorable conditions a cartilaginous transition stage intervenes. He deduces from his experiments that "diaphyseal bone is re- produced by proliferation of osteoblasts derived from preexisting osseous tissue, and that its regeneration takes place independently of the perios- teum/' 7 The periosteum is conceived as being an important factor in de- termining the conformation and growth limit of bone. JOINTS Joints are divisible into two main types, the movable and the 'im- movable/ or (1) diarthroses and (2) synarthroses. These and their several modifications call for histologic description. Synarthroses include (a) syndesmoses, or joints in which the connecting substance is a dense fibre-elastic tissue joining the bones immovably as in the articulation of the skull (sutura), or where it consists of ligamentous tissue permitting slight movement as between the lower ends of the tibia and fibula; and (b) synchondroses, in which the connection is effected by cartilage, either hyaline (e.g., between the epiphyses and diaphysis of young bones) or fibrous (e.g., the inter vertebral disks of the vertebral column). In relation with diarthroses are in several instances (mandibular, lower radio-ulnar, costosternal, sternoclavicular, acromioclavicular) intra-articular menisci of fibrocartilage ; here the articular cartilages of the bones concerned are also of the fibrous variety. The semilunar car- tilages of the knee and the glenoid cartilage of the shoulder joint are also JOINTS 89 of the fibrous type. These cartilages serve to deepen the sockets in which the respective ends of the femur and humerus move and are known as adaptation cartilages or labra glenoidalia. The joint cavity of a diarthrosis is enveloped in a capsule consisting of two layers, an outer fibro-elastic continuous with the periosteum and an inner cellular layer, the synovial membrane, consisting of epithelioid cells forming a mesenchymal epithelium. The function of the synovial membrane is to secrete a lubricating fluid, the synovia, consisting of about 94 per cent, water with small amounts of mucoid substances and oil. In the large joints the synovial (serous) membrane is thrown into villus- like folds (Fig. 240). The covering cellular membrane is occasionally imperfect; the cells vary from the flattened, typical mesothelial cells, to the cubic variety (Fig. 44), and rest directly upon a vascular, frequently fatty, fibrous stroma. CHAPTER IV MUSCULAE TISSUE GENERAL CONSIDERATIONS Muscular tissue consists essentia]ly of protoplasm in which the gen- eral vital property of contractility has become predominant. However, the path of contraction is practically limited to one direction, the long axis of the cell. This phenomenon of contractility results from the dif- ferentiation of specially contractile fibrils, the myofibrils, from the pro- toplasm of embryonic muscle elements, the myollasts. The protoplasm of the muscular tissue is called sarcoplasm. Adult muscular tissue may be divided into three classes: smooth, cardiac, and striped. All three types arise from mesoderm, with the exception of the dilator and sphinc- ter muscles of the iris of the eye, and the muscle of the secretory portion of the sweat gland both of the smooth variety which are generally be- lieved to be of ectodermal origin. In lower forms, muscle tissue may be largely derived from the ectoderm and even from the entoderm. The smooth muscle is in general limited to the viscera ; it is not under the control of the will, hence also called involuntary muscle. The cardiac type is limited to the heart, and to the middle layer of the roots of the aorta, pulmonary artery, and pulmonary veins. It is striped, but like smooth muscle, controlled by the sympathetic nervous system; therefore independent of the will, hence also of involuntary type. So-called striped or skeletal muscle is practically limited to the skeleton, and subserves the function of skeletal movement. This group includes also the muscles of the eyeball, the ear, the upper third of the esophagus, diaphragm, and tongue. It is under the control of the will, hence designated volun- tary. The striped muscle of the diaphragm and the esophagus is appar- ently only partially voluntary. It is obvious from the above that there is demanded a more specific terminology: involuntary smooth (unstriped) ; involuntary striped (car- diac); and voluntary striped (skeletal). The three types pass through very similar, perhaps identical, earlier stages of histogenesis. The essen- 90 GENERAL CONSIDERATIONS 91 tial difference seems to be one of degree differentiation. In general, skeletal mus- cle is most highly differentiated, cardiac muscle being intermediate between smooth and the voluntary striped type. For a proper understanding of the structure of these three types it is neces- sary that we now consider the process of muscle histogenesis. The student should gather the several criteria by which he may distinguish between smooth, cardiac, and skeletal muscle, both in transverse and longitudinal sections. HISTOGENESIS AND STRUCTURE Smooth Muscle. As stated above, the germ layer involved in muscle histogene- sis is the mesoderm. Smooth muscle is derived chiefly (exception: arrectores pil- orum) from the visceral or splanchnic layer. This is at first an epithelial struc- ture of a single layer of cells, the prim- itive mesothelium. The cells subsequent- ly proliferate and change their shape in general to a fusiform type. Intercellular connections (cytodesmata) are either maintained or established, and the tissue is permanently more or less in a syncytial condition. These so-called intercellular bridges are particularly pronounced and can be readily demonstrated in the tunica media of the blood-vessels of the umbilical cord. It must be emphasized, however, that the outlines of the genetic units in smooth muscle are always distinct, where- as in striped, especially the cardiac type, the outlines of the original myoblasts are lost. FIG. 97. SMOOTH MUSCLE CELLS. A, an isolated cell from the cat's intestine. The nucleus is surrounded by coarsely granular sarcoplasm, continuous periph- erally with the finely granular interfibrillar sarcoplasm. The innermost myofibrils may er- roneously suggest a cell mem- brane. The fusiform element is invested by a true cell mem- brane, or sarcolemma. X 750. B, oblique transverse section of a cell from the muscularis mu- cosae of the cat's esophagus. The perinuclear sarcoplasm has contracted away from the nu- cleus leaving a clear space limited by a sharp line, external to which lies the perinuclear granular sar- coplasm. Hematoxylin and eosin. X 750. MUSCULAR TISSUE The early myoblast, of short spindle shape with central oval nucleus, contains a granular cytoplasm, limited by a delicate membrane, the sarcolemma. The granules may be called myochondria; whether identi- cal with cytomicrosomes or with mitochondria, whether of cytoplasmic FIG. 98. SMOOTH MUSCLE CELLS FROM THE PIG'S STOMACH. Isolated in equal parts of alcohol, glycerin, and water. Unstained. X 410. or of nuclear origin, are disputed points. No evidence of a distinct spongioplasm is discernible. This observation tends to invalidate the teaching of certain histologists, that the contractile fibrils (myofibrils) represent modified spongioplasmic threads arranged in rectilinear meshes. FIG. 99. SMOOTH MUSCLE CELLS FROM THE WALL OF THE HUMAN INTESTINE. Longitudinal section. Hematein and eosin. X 665. Moreover, it has been established by direct observation (McGill, et al.) that the myofibrils arise through process of alignment and subsequent fusion of the myochondria. McGill (Internat. Monatschr. Anat. u. Phys., Bd. 24, 1907) recognizes two types of myofibrils, namely, stouter periph- eral border fibrils (myoglia) which may pass beyond the limits of a cell and form intercellular bridges; and the more central, or myofibrils HISTOGENESIS AND STRUCTURE 93 proper, which are limited to the cells proper and are considerably more delicate. It is believed that border fibrils may subsequently arise by fusion of the more delicate fibrils. The function of the border fibrils is disputed, some claiming that they serve to straighten the cell following contrac- tion produced by the central fibrils, others claim- - 9 ing that they have a contractile role similar to the central myofibrils. Whatever their complete func- ^ tion may be, they certainly seem to bind, together o with the connective tissue, the individual cells into 9 ^ a compact tissue in which coordinated movement, it as in peristalsis of the intestine, becomes possi- FIG. 100. SMOOTH ble an obviously important condition. MUSCLE CELLS The oval or rod-shaped nucleus retains its cen- FROM THE WALL OF tral location, and is surrounded by a mass of gran- THE HuMAN INTES- ular, relatively undifferentiated sarcoplasm, con- ^ Transection. Hema- tammg mitochondria, lipoid, and glycogen gran- tern and eosin. X 750. ules. It changes its shape with the phase and degree of contraction, occasionally even assuming a short, spiral form. It has been shown (McGill, Amer. Jour. Anat., 9, 4, 1909) that during contraction the nucleus decreases markedly in length and increases in thickness ; and that the uniformly distributed chromatin granules stream toward the poles, where they collect in coarse strands. This structural intranuclear change is apparently unaccompanied by any change in volume. Smooth muscle cells vary greatly in size from the shortest of about 50 microns, to some of 500 microns in length in the pregnant uterus. When in the contracted condition, they show a number of broad, more deeply staining contraction bands, very conspicuous in the smooth mus- cle of the lower portion of the esophagus. As seen in transverse section these fibers vary in size from a mere point up to their maximum diame- ter, according as the section happens to pass through the end or through the middle of a fiber. Because of its central location, the nucleus is only found in the larger transections. Smooth muscle fibers may be joined together in interlacing groups as in the wall of the uterus or bladder; or they may form broad mem- branous layers as in the wall of the alimentary tract; or again, they may form small isolated bundles, as in the skin. In any case, the muscle bundles are united by a delicate network of connective tissue. 94 MUSCULAR TISSUE Smooth muscular tissue occurs chiefly in the walls of the hollow or tubular viscera. Its distribution may be classified as follows : (1) In the alimentary tract: lower portion of the esophagus, stom- ach, small and large intestines. (2) In the respiratory system: trachea and bronchial tubes. (3) In the g enito-urinary system: ureter, bladder, urethra, penis, prostate, vagina, uterus, oviduct, and ovary. (4) In the vascular system: ar- teries, veins, and the larger lymphatic vessels. (5) In the ducts of all secreting glands: gall ducts and gall bladder, and the ducts of the pancreas, sal- ivary glands, testicle, etc. (6) It is also found in the cap- sules of the spleen and lymphatic nodes, in the skin, and in the intrin- sic muscles of the eye. Small numbers of branching cells have been described in the walls of the urinary bladder and the large arteries. Cardiac Muscle. Heart muscle likewise takes origin from splanchnic mesothelium, which soon becomes modified into a loose-meshed syncy- tium, in which all trace of the orig- inal cellular element is lost. The myoblast areas of stellate and irreg- ular form contain a central oval nu- cleus and a finely granular cytoplasm. In a manner similar to that described for smooth muscle histogenesis, the myochondria form myofibrils which extend for great lengths throughout the sarcoplasmic meshwork. In car- diac muscle the myofibrilla? subse- quently acquire a cross striation. Adult muscle consists of stouter mus- cle fibers or trabecula3 joined into an intricate close meshwork, by means of less robust branches. The nuclei retain their axial position in the FIG. 101. Two STAGES IN THE HIS- TOGENESIS OF SMOOTH MUSCLE, FROM THE WALL OF THE ESOPHAGUS OF A PIG EMBRYO. A, 10 mm. stage of development. The central nucleus of the mesenchy- mal syncytium has become enlarged and is enveloped by a greater mass of cytoplasm. It represents a myo- blast; the peripheral myochondria have become aligned preparatory to fusion to form a muscle fibril. B, 21 mm. stage of development! Four adjacent myoblasts, with peripheral stouter myoglia fibrils and central more deli- cate myofibrils. X 1500. HISTOGENESIS AND STEUCTURE 95 fibers and are surrounded by an oval area of undifferentiated granular sarcoplasm. The cardiac fibers and their branches contain peripheral myofibrils, which during growth of the muscle arise by longitudinal splitting of the original fibrils and take position progressively toward the center. Cardiac muscle thus consists of a slender axial core of undiffer- entiated sarcoplasm swelling to an oval, more expansive mass where the FIG. 102. A GROUP OF MYO- BLASTS FROM THE HEART MUSCLE SYNCYTIUM OF A 48 HOUR. CHICK EMBRYO. Showing myofibrils, myo- chondria and mitochondria. The latter are the deeper staining granules. M eves' technic. X 2000. FIG. 103. CARDIAC MUSCLE OF GUINEA PIG, SHOWING SEV- ERAL BRANCHES, CROSS STRIATIONS (GROUND MEM- BRANES) AND A NUM- BER OF INTERCALATED DISKS. Zimmermann's tech- nic. X 1000. nuclei are located ; this core is surrounded by successive rows of myofibrils arranged in groups representing Cohnheim's fields in transverse section ; and the whole is invested by a delicate sarcolemma. The striations of the fiber result from the fact of a transverse alignment of identical areas in adjacent fibriUae a correspondence which indicates a definite func- tional stimulus to a structural modification (Fig. 103). The sarcoplasm contains mitochondria (Fig. 102), lipoid, albuminoid (interstitial gran- 96 MUSCULAB TISSUE ules of Kolliker), and glycogen granules. Fat granules, 'liposomes' (Bell) of probably nutritive significance, and varying greatly in amount according to the functional condition of the individual, are normally present in cardiac muscle (Bullard, Amer. Jour. Anat, 14, 1, 1912). This fatty content can be demonstrated by the several microchemical technics for lipoids. According to Meves, Duesberg, and others,- the myo- fibrils of striped muscle differentiate from the mitochondria of the myo- blasts; but since mitochondria can be demonstrated in highly developed #v FIG. 104. CARDIAC MUSCLE 'CELLS' FROM THE PIG'S HEART, ISOLATED IN EQUAL PARTS OF ALCOHOL, GLYCERIN, AND WATER. Unstained. (The nuclei are somewhat darker than they actually appear.) X 410. fibers (Fig. 116) it seems improbable that mitochondria have anything directly to do with the development of muscle fibrils. It has been claimed that heart muscle and striped muscle generally can be interpreted in terms of muscle cells, and intercellular myofibrillae, in analogy with connective tissue (Baldwin). But the presence of a con- tinuous axial core of undifferentiated sarcoplasm, lack of a definite cell membrane separating this sarcoplasm from the outlying myofibrillae, in- ability to separate such 'cells' by dissociation methods, and the extension of the telophragma to the nuclear wall, seem to render this view unten- able. The myofibrils must be further considered. No distinction between border fibrils and central fibrils, as in smooth muscle, is possible in car- diac muscle. But the myofibrils undergo greater differentiation. This HISTOGENESIS AND STKUCTUEE 97 expresses itself in an alternation of light and dark disks ('bands/ 'seg- ments/ 'stripes'), said to consist of iso tropic and anisotropic substances respectively. While the disks are conspicuous both in fresh and stained tissue, the demonstration of their physical properties under the polari- scope is a matter of difficulty. Indeed with crossed Nichols the entire FIG. 105. CARDIAC MUSCLE OF THE HUMAN HEART; THE ABUNDANT BRANCHES ARE PLAINLY SHOWN. Longitudinal section. Hematein and eosin. Photo. X 120. fiber appears lighter than the field, showing the presence of anisotropic materials (granules) scattered throughout the fiber, but a definite band- ing corresponding to the light and dark disks of fresh muscle is not apparent in all striped muscle. It seems more probable that, though anisotropic granules are more abundant in the dark disk, they are not absent in the lighter disk; moreover, they are more or less definitely aggregated in the dark disk according to the phase of contraction. The 98 MUSCULAR TISSUE lighter disk, or intermediate disk of Krause, is commonly designated by the letter J (Isotrope streife) ; the dark disk, or transverse disk of Briicke, by the letter Q (Quersclieibe). On closer inspection the J disk is seen to be bisected by a dark disk or membrane, the ground membrane of Krause, designated by the letter Z (Zwischenscheibe) . Indeed this is the most conspicuous stripe, and gives to the mus- Nuc cle, as seen under ordinary low magnification, its band- ed appearance in uncontracted fibers. The term telophragma has recently been employed by \ a en FIG. 106. THE CENTRAL PORTION OF THE PRECEDING FIGURE, MORE HIGHLY MAG- NIFIED. a, intercalated disk; en, endothelial nuclei of a capillary; Nuc, nucleus of -muscle syn- cytium. Hematein and eosin. X 500. Heidenhain for this mem- brane. The myofibrils are in- timately connected with it. Similarly, the Q disk is bi- sected by a narrow light disk, the median disk of Hensen (H), which in turn is said to be bisected by the intermediate membrane of Heidenhain, or mesophragma (M, Mittelscheibe). Both telo- and mesophragmata (inophragmata) are supposed to unite with the sarcolemma peripherally, and to be structurally similar. The telophragma is in intimate connec- tion both with the sarcolemma and the nuclear membrane. But the mesophragma, at least in striped muscle of certain forms, e.g., Limulus, is not a true membrane to which the fibrils are attached in the manner of the telophragma. Indeed it re- mains an open question whether heart muscle actually possesses a mesophragma. The portions of a fibril, or sarcostyle, included -between successive telophragmata, con- stitute structural units, the sar- comeres, or inokommata (Heidenhain, Fig. 110). In macerating fluids f I FIG. 107. TRANSECTION OF A GROUP OF CARDIAC MUSCLE FIBERS FROM A PAPIL- LARY MUSCLE OF THE HUMAN HEART. Hematein and eosin. X 550. HISTOGENESIS AND STEUCTUEE 99 fractures occur at the Z lines. These membranes extend completely across the fiber,, through the axial strand of sarcoplasm a significant fact con- troverting the cellular idea of cardiac muscle originally advanced by Apathy. The interstitial granules of Kolliker (sarcosomes of Eetzius) scattered throughout the Q and J disks in striped muscle, both cardiac and skeletal, are designated the Q and J granules re- spectively (Holmgren). INTERCALATED DISKS. A unique char- acteristic of cardiac muscle pertains to the presence of the intercalated disks, func- tional lines/ or bands of Eberth. These are barely visible in ordinary historic preparations, but can be rendered conspic- HUMAN FETUS AT SEVEN uous by the special technics of Heidenhain MONTHS. and of Zimmermann. In gross appear- Fibrillae are well developed at . , , the periphery: the undifferenti- ance they are of several sorts: straight ate d cytoplasm in the center bands, step-like forms, and serrated forms, presents a clear appearance and The bands (disks) may extend completely in some cases is Partially occu- ni -in o -i pied by the nucleus. Hematem across a fiber, or only the width of a single J^nd e0 g in> x 75a fibril (granule type) ; the step form may consist of one or many groups of steps and risers, the 'risers' being the height of one or, occasionally, several inokommata; the saw-tooth type also may be of small or greater extent, and of the height of one or several inokommata. All three types may be arranged in rings or even longer or shorter spirals. The intercalated disks are peripheral in position, extend- ing for varying depths, but never completely through a fiber, and never central to the axial sarcoplasm. They are occasionally on the same level with the nucleus. They have been found in the heart of representatives of all the animal groups to', and including, teleost fishes (Jordan and Steele). They are present sparsely and in simple form also in the heart of Limulus. They are probably a morphologic incident of the rhythmic contraction of cardiac muscle. They appear only late in fetal life, toward the end of the last week of gestation in the guinea pig. The earliest disks are all of the coarsely granular band type. Subse- quently they increase in number and complexity, the older stages being characterized by occasional saw-tooth forms. Once formed, they are evidently for the most part permanent structures, undergoing modifica- tion largely through mechanical factors. On closer inspection, under the higher powers of the" inicrescope, they 'are seen to consist of units 100 MUSCULAR TISSUE A B FIG. 109. CARDIAC MUSCLE FIBERS. A, portion of a fiber from a macerated preparation of a cat's heart, drawn according to its appearance in the opti- cal longitudinal plane. Two nuclei are shown, connected by a continuous axial strand of coarsely granular sarco- plasm. The sarcolemma ap- pears festooned between suc- cessive ground membranes. There is no evidence of a cell membrane separating the cen- tral granular from the periph- eral non- or finely-granular sarcoplasm. X 1000. B, median longitudinal section of a fiber from the ventricle of an adult white mouse. Note the continuity of the axial sarcoplasm. Hematox- ylin and eosin. X 1000. corresponding to portions of a single fibril. These units may be granular or compact. The units are bisected or bounded on one side by the Z membrane. Association of the units in transverse lines gives rise to the band forms; they may be drawn into spirals by longitudinal traction of the fibers involved; unequal transverse and oblique tractions probably produce the step forms; the saw- tooth form arises by process of longitudinal splitting of fibrils, enlargement of fibers, and the various tensions characteristic of hyper- trophying fibers. The exclusive type of hypertrophied heart muscle is the more or less complex saw-tooth type. The practically exclusive type of atrophied heart muscle is the 'comb type/ a type produced from a band type by a modification involving longitudinal tension (Fig. 114, d). In brief, the unit of structure is a modified focus on a myofibril, in essence involving an accumulation of gran- ules about the Z membrane. Such foci asso- ciated in various ways produce the various types of bands and steps, the latter in part due also to external mechanical factors, the extreme condition of such effect being saw- tooth forms. A significant point concerns the similarity between the phylogenetic and ontogenetic de- velopment of intercalated disks : that is, below birds, as in all fetal hearts, 1 only simple bands appear ; in birds as in young hearts, step forms are present; only in mammals and in old hearts do the more complex types appear. What then is the meaning of these disks ? Any interpretation must be more or less tentative at present. It is easier to say what they prob- ably are not, than what they probably are. They were originally interpreted as cell boun- daries, cr intercellular cement lines (Schweig- HISTOGENESIS AND STRUCTURE 101 ger-Seidel) ; this interpretation has recently been again supported by Zimmermann. This interpretation would mean that from a syncytium a cellular tissue has secondarily arisen by the appearance of cement lines, secondary cells having been formed in a syncytium, irrespective of the original genetic units. A number of facts render this interpretation inad- missible, chief among which are their occasional supernuclear position, and their peripheral loca- tion. A more recent interpretation conceives of them as places where the muscle fiber grows, that is, as sarcomeres in the making (Heidenhain). Among the countervailing facts to such interpre- tation are chiefly the absence of transition stages, their relative scarcity at the period of greatest growth of the heart, and their continued abun- Telophragma J granule Q-granule Mesophragma Q.S. J. S. Transverse fiber net- work Transverse fiber net- work FIG. 111. LONGI- TUDINAL SECTION OF PORTION OF WING MUSCLE FIBER OF MANTIS, AT MID-PHASE OF CONTRACTION. z, telophragma; h, Hensen's disk; j, isotropic disk; q, anisotropic disk; q. s., g-sarcosome; j. s., j-sarcosome. X 1600. FIG. 110. DIAGRAM OF A STRIPED MUSCLE FIBER, AC- CORDING TO HEIDENHAIN. The transverse fiber network may be a trophospongium. dance in full-grown, even aged, hearts. The suggestion has been made that they are related to a phase of contraction. This seems more likely. Since, once formed, disks are largely permanent, and undergo subsequent modification, they must represent an irreversible condition of the con- traction phase. The interpretation of the disks as irreversible contrac- tion bands rests upon the similarity of the simplest types and the con- traction bands of Eollet, both characterized by accumulation of dark stain- 102 MUSCULAR TISSUE ing granules about the Z membrane. In the older hearts, where they are mechanically modified, and in diseased hearts, as in hypertrophies, where probably a chemical modification is also suffered, they represent lines of weakness. These are the locations of fracture in fragmented and. segmented pathological hearts a significant point in relation to 'heart failure.' Heart muscle is syncytial in structure, and the myofibrillse pass unin- terruptedly through the intercalated disks. These facts are of special im- portance because of their bearing on the opposed theories of the origin and conduction of the stimulus to the heart beat, the myogenic and the neuro- genig. A complete cellular structure with actual cement lines, combined with the fact that the atria are apparently completely separated from the ventricles by intervening con- nective tissue, was once urged as a strong argument against the validity of the myogenic theory of heart beat the theory which proclaimed the adequacy of heart muscle to initiate and conduct the stimulus to contraction without the intervention of nerve ele- ments, that is, to beat automatically and indepen- dently of the nervous system. The neurogenic the- ory, which holds that nerve elements are essential for the conduction of stimuli for contraction, on the other hand, seemed contradicted by the observation that in the chick the heart beat rhythmically before the appearance of nerve fibers. However, there re- mained the possibility that nerve fibers were present but unc'emonstrable by the method employed; also that while nerves might be unnecessary for maintain- ing rhythmic contraction during embryonic life, they nevertheless became necessary in fetal and adult life. The discovery of the atrioventricular bundle of His (1893) at first added apparently the strongest evidence to the support of the application of the myogenic theory of heart beat in the mammalian. This is a muscular bundle which effects an inti- mate connection between the atria and the ventricles. See heart, p. 199. An important matter is the observation that the final ramifications of the bundle of His are identical with the so-called Purkinje fibers. These have long been known, especially in the sheep's heart, where they are unusually large and abundant. They are limited to a region directly under the ventricular endocardium. They are coarser, less branched, with fewer intercalated disks, almost exclusively of the band type, than are the fibers of the myocardium proper. They would FIG. 112. LONGI- TUDINAL SECTION OP PORTION OF ATRIO - VENTRICU- LAR BUNDLE OP HEART OF BEEF. MANY OF THE CELLS ARE BINU- CLEATED. X 160. HISTOGENESIS AND STEUCTUEE 103 seem to represent a younger or less highly differentiated stage of muscle fiber. In cross section they are of greater diameter, with fewer peripheral myofibrils and a far greater amount of central undifferentiated sarco- plasm, rich in glycogen. According to Lange (Arch. mikr. Anat., Bd. 84, 1914) the Purkinje fibers cannot, however, be regarded as remains of embryonic muscle cells, since they are clearly distinguishable already in very young mammalian embryos ; they constitute the non-nervous apparatus for conducting stimuli to heart beat. The myogenic theory accordingly seemed well es- tablished. It was apparently strongly supported by the experiments of Erlanger (1906), who clamped the bundle in the dog's heart and produced a condition of 'heart block' a disturbance of the coordinated rhyth- micity of the atria and ventricles without, however, interfering with the conduction of impulse, since there resulted no stoppage of contraction in the ventricles. But the subsequent discovery of abundant nerve fibers and ganglion cells (Tawara, Wilson, McGill) in the bundle, intimately related to the cardiac fibers, robbed this experiment of its specific applicability and seemed for a time to force an interpretation favoring the neu- rogenic theory. Carlson, moreover, demonstrated its validity for the Limulus heart, in which, after the removal of its ganglion, it could not be made to beat. But more recently Burrows (1911) has shown that single cells of a 14- to 18-day embryo chick heart, grown in artificial culture media, may begin to beat automatically and rhythmically an observation which would seem to settle the point that heart muscle may beat rhythmically in the absence of nerve supply or even nerve stimulus. Furthermore, Hooker (Jour. Exp. ZooL, 11, 2, 1911) showed that in the frog larvaB in which the nervous system was entirely removed, the cardiac muscle will differentiate and function normally, independently of nervous control. The myogenic theory is further supported by the fact that it has been possible to revive the excised heart of man to rhythmic activity 20 hours after death (Flack, "Further Advance in Physiology/' Ed., Hill, 1909), whereas the longest time that a nerve cell is known to survive (in the intes- tine) is 31/2 hours (Cannon and BurJcett, Amer. Jour. Phys., vol. 32, p. 347, L913). In the superior cervical ganglia, nerve cells may survive 1 hour,' while in the brain the maximum time of survival is said to be 15 minutes. FIG. 113. LONGI- TUDINAL SECTION OF A TRABECULA OF LIMULUS (KING CRAB) HEART MUSCLE, SHOWING AN IN- TERC ALATED DISK SEPARAT- ING A CONTRACT- ED FROM AN UN- CONTRACTED PORTION. Nitric acid-alco- hol fixation (Zim- mermann's tech- nic), iron-hematox- ylin stain. X 1300. 104 MUSCULAR TISSUE Any interpretation of the intercalated disks as actual intercellular struc- tures (cement substance) would seem inconsistent with the myogenic theory of the heart beat, which now seems largely to prevail. The present status of the matter seems to be that the origin of stimulus to heart beat in vertebrates is myogenic, in invertebrates probably neurogenic. The differ- ence may inhere in the absence in the hearts of invertebrates of a muscular coordinating structure analogous to the atrioventricular bundle of verte- brate /hearts. FIG. 114. SEMIDIAGRAMMATIC ILLUSTRATIONS OF VARIOUS TYPES OF INTERCALATED DISKS. a, from guinea pig's heart; 6, from chipmunk's heart; c, from monkey's heart; d, from monkey's heart; e, from guinea pig's heart;/, from chipmunk's heart. X 1200. Voluntary Striped or Skeletal Muscle. The unit of structure of skeletal muscle is essentially the same as that described for cardiac mus- cle, namely, a myofibril or sa/rcostyle. The fiber is likewise divisible into successive sarcorneres or inokommata. A difference in detail inheres in a greater defmiteness of striation, .and a greater complexity, due to the presence generally of an additional disk in the J stripe. This stripe or accessory disk of Engelmann (N line; nebenscheibe) bisects the H1STOGENESIS AND STRUCTURE 105 portion of the J disk between the Z line and the succeeding Q disk. It is interpreted more or less tentatively by Heidenhain as due to a linear arrangement of J granules. The median segment of the J disk, that is, the portion between the N line and the Z line, is called the terminal disk of Merkel (E disk; endscheibe). This complex condition of striping is conspicuous only in certain insect muscles (e.g., leg and wing muscles of wasp, etc.) where powerful activity or great rapidity of beat is required. In general, complexity of striation of conspicuous character corresponds with a relatively greater irritability and a capacity for more powerful or relatively more rapid function. Another difference between skel- etal and cardiac muscle pertains to the diameter of the fiber and the posi- tion of the nucleus. In skeletal mus- FIG. 115. SUCCESSIVE STAGES OF SKELETAL MUSCLE HISTOGENESIS IN MAMMALS. a, myoblast with fine cytoplasmic granules (myochondria), from a 13 mm. sheep embryo; 6, myoblast with homogeneous myofibrils, from a 10 mm. guinea-pig embryo; c, myoblast with cross striped fibrils, from a 8.5 mm. rabbit embryo. (From Heidenhain, after Godlew- ski.) FIG. 116. TRANSVERSE SECTION OF A STRIPED MUSCLE FIBER OF A NEWLY-HATCHED RAIN- BOW TROUT, SHOWING THE PROCESS OF MYO- FIBRIL INCREASE BY RA- DIAL LONGITUDINAL SPLITTING. Mitochondria are seen in the peripheral sarcoplasm and around the nucleus at the right. Meves' technic. X 2000. cle the fiber has a greater diameter, is more nearly circular in cross sec- tion, the myofibrils are scattered throughout its diameter, and the nuclei are peripheral, lying just beneath the sarcolemma. The nuclei are envel- 106 MUSCULAR TISSUE FIG. 117. STRIATED MUSCLE FIBERS RUP- TURED BY TEASING, SHOWING THE SARCO- LEMMA. o, ruptured end of the muscle fiber; B, a bundle of fibrils projecting from the torn end; m, a muscle fiber; n', a nucleus of the muscle cell; at p, the muscle substance has shrunken away from the sarcolemma; s, sarcolem- ma. Moderately magni- fied. (After Ranvier.) oped in a small amount of granular sarcoplasm. The sarcolemma is a homogeneous, plastic, non- nucleated membrane, the representative and product of the cell membrane of the original myoblasts. In routine laboratory preparations only the Z, Q, and J stripes are conspicuous. Meigs (Zeitschr. allg. Phys., 8, 1, 1908) recognizes at most only three different substances in striped muscle sarcostyles (fresh wing muscle of fly) : Q, Z, and M. He regards J and H as optical effects dependent upon the reflection of light by the Z and M membranes. Thulin (Arch. mikr. Anat., 86, 3, 1915) records the absence of both Z and M membranes in the wing muscles of certain insects (Coleop- tera) and the analogous (pectoral) muscles of birds and bats. The perinuclear sarcoplasm contains filar and granular mitochondria. Both the perinuclear and interfibrillar sarcoplasm contain also fat granules and globules (liposomes), interstitial granules of Kolliker and glycogen. Heidenhain (Anat. Anz., 44, 11-12, 1913) has conclusively shown that in the trout embryo the progenitor (the manner of whose origin is un- known) of the definitive myofibrillse is a single, stout, deeply staining column, close to the nuclear wall externally, which undergoes a succession of radial and concentric longitudinal divisions. This observation would seem to dispose of the assump- tion of a direct mitochondrial origin of myofibril- Ise in this form at least, in the manner of the current descriptions. And this conclusion is strengthened by the demonstration of mitochon- dria throughout the earlier development (Fig. 116). Skeletal muscles develop from a portion of the mesodermic segments or primitive somites, called the myotomes. The myoblasts pass through FIG. 118. ISOLATED FRAGMENTS OP STRIATED MUSCLE FIBERS, UNSTAINED. The one above is from the end of a fiber; that on the right shows at one end a tendency to cleavage into transverse disks. X 360. FIG. 119. STRIATED MUSCLE FIBERS OF THE DOG, SEEN IN TRANSECTION. The areas of Cohnheim are indistinctly outlined. Hematein and eosin. X 490. 107 108 MUSCULAR TISSUE the early histogenetic changes already described for cardiac muscle. In the frog, irritability was shown by Hooker to follow closely upon differen- tiation of the fibrillse, and the establishment of nervous connections. The adult skeletal muscle fiber is a multinucleated structure. Is this condition the result of fusion of distinct myoblasts, or of growth of a single myoblast accompanied by nuclear proliferation ? Both interpreta- tions have been advanced; many observational data tend to show that Q- A C FIG. 120. A PORTION OF A STRIATED MUS- CLE FIBER SEEN IN LONGITUDINAL SEC- TION. The alternate light and dark cross stria- tions are well shown. h, light line, Hensen's line, in the middle of the dark disk Q. z, dark line, Krause's membrane or Dobie's line, in the middle of the light disk. Hema- tein. X 1200. (After Bohm and von David- off.) FIG. 121. A SMALL PORTION OF A MUS- CLE FIBER OF A CRAB SHOWING BE- GINNING SEPARATION INTO FIBRILS. Drawn from a pho- tograph. X 600. (After Schafer.) a skeletal muscle fiber represents a myoblast which has elongated and multiplied its nuclei. In the trout embryo, however, considerable fusion of myoblasts occurs. The mode of nuclear division appears to be at first mitotic, and subsequently amitotic. In the tongue a small number of branched fibers have been described. Striped muscle fibers differ in the relative amounts of myofibrillar and sarcoplasmic content. When the myofibrillae are relatively preponder- ant and the interstitial granules sparse, the fiber is known as 'light'; HISTOGENESIS AND STRUCTURE 109 when the sarcoplasm, with its interstitial granules, is relatively abundant, : dark/ In the latter fibers many of the nuclei may be centrally located. FIG. 122. FIBRILS FROM THE WING MUSCLES OF A WASP. A, compressed; B, stretched; C, un- contracted. The alternate dark and light disks are prominent; the mem- brane of Krause in the light disk, and the line of Hensen in the dark disk are well shown. Very highly magnified. (After Schafer.) FIG. 123. LONGI- TUDINAL SECTION OF A PORTION OF A STRIPED MUS- CLE TRABECULA OF LIMULUS, SHOWING A NU- CLEUS OF SER- RATED CONTOUR WITH THE TELO- PHRAGMATA AT- TACHED TO THE SERRATIONS. The nucleus lies in an undifferenti- ated mass of sarco- plasm containing below a deutoplas- mic granule. The adjacent myofibrils simulate a cell mem- brane. Fleming's fluid, iron-hematox- ylin. X 2000. Most mammalian muscles contain both types of fibers, but with the 'light' greatly in excess. When a muscle consists chiefly of 'light' fibers it is a 'white' muscle; when the 'dark' fibers are abundant it may be 110 MUSCULAE TISSUE called 'red' muscle. The former variety, for example the biceps muscle, acts more energetically but is more easily fatigued; the latter, like the muscles of mastication, respiration, the eyeball, and cardiac muscle, are functionally characterized by slower activity but less ready, fatigue. The intersti- tial granules accordingly seem to be of nutritive sig- nificance. They are generally more abundant in the J than in the Q segments. The J granules are of spherical form and smaller than the oval Q granules. Striped muscle fibers contain also a trophospongium ( Holmgren ) . Adult muscle cannot regenerate, but is replaced by scar tissue. A young fiber is said to be able to regenerate, the process involving movement of proliferating nuclei toward the cut surface. Muscle growth, as with exercise, depends upon enlargement -N FlG. 124. S T R I A T E D FIBER FROM A LEG MUSCLE OF THE SEA SPIDER (ANOPLO- DACTYLUS LENTUS), SHOWING THE COM- PLEXLY STRIPED CON- DITION CHARACTER- ISTIC OF INSECT MUS- CLE. Q, anisotropic disk; J, isotropic disk; M, mem- brane of Heidenhain (mesophragma); Z, membrane of Krause (telo- phragma) ; H, median disk of Hensen; N, accessory disk of Engelmann; E, terminal disk of Merkel. X 1000. B FIG. 125. SEMIDIAGRAMMATIC DRAWING, REP- RESENTING THE APPEARANCE OF THE SAME FIBER FROM THE LEG MUSCLE OF A BEETLE IN ORDINARY AND POLARIZED LIGHT. A, appearance in ordinary light; B, appearance in polarized light. (After Meigs.) of the muscle fiber, consequent upon a multiplication of fibrillse by process of longitudinal splitting, and their individual enlargement. MUSCULAR CONTRACTION 111 MUSCULAR CONTRACTION Sarcolemma The physics and chemistry of muscle constituents are too inadequately known to permit anything like a confident description of the mechanism of contraction. It has been suggested that muscular contraction is essen- tially a reversible coagulation process. The rapidity of the process seems a fatal objection to this explanation. A plaus- ible interpretation fol- lows the analogy afford- ed by the action of cat- gut suspended in water, the temperature of which is suddenly raised by passage of an electric current, namely, a swell- ing and consequent shortening of the fiber (Engelmann). In mus- cle, the myofibrillse may be conceived of as cor- responding to the catgut of the experiment, the semifluid sarcoplasm to the water, and the nerve impulse to the electric current. The rapidity of muscular activity, however, again seems a difficulty. However, the optical changes under- gone by a contracting muscle give evidence in favor of such interpreta- tion. The fibrillae short- en and thicken in con- traction, and there is a rearrangement of the optically different substances of the fiber; the dark granules aggregate about the Z line, so that it seems to have disappeared a change which involves also the disappearance of the original Q stripe. We may be fairly certain that the explanation of muscular contraction must be sought for in physical and chemical changes in the myofibril, it being FIG. 126. LATERAL CONTRACTIVE WAVE OF CASSIDA EQUESTRIS. (After Rollet.) The formation of the contraction band is well seen, at the left, as thick black lines. (From Szymonowicz- MacCallum," Histology and Microscopic Anatomy.") 112 MUSCULAR TISSUE the essential contractile element, the sarcoplasm serving largely as a nutri- tive substance. However, it should also be mentioned that it has been held that the sarcoplasm may be the essential contractile substance. Meigs questions the pertinency of Engelmann's 'artificial muscle' in his catgut experiment, and the validity of the suggested hypothesis of con- traction as a thermodynamic phenomenon. He regards contraction as the result of a rise in osmotic pressure and the consequent imbibition of fluid caused by the breaking down of larger into smaller molecules within the sarcostyles. According to Roaf (Proc. Roy. Soc., Series B. 88, 1914) contraction can be explained on the hypothesis that lactic acid is set free, and that this combines with certain proteins to form salts, with a consequent rise of osmotic pressure. We must now consider the muscle as a whole. For this purpose we may select any well-known muscle, for example, the biceps. A muscle as seen in transverse section is enveloped in a moderately dense, fibro-elastic mem- brane, the epimysium (ex- ternal perimysium) ; this gives off septa which sep- arate the muscle into a larger or smaller number of bundles, depending upon its size, each bundle, or fascicu- lus, being again separately closely enveloped in a fibro- elastic covering, the peri- mysium (internal perimy- sium). Each fasciculus is, moreover, again subdivided into larger and smaller collections of muscle fibers, each bundle imper- fectly separated from its fellows by septa from the perimysium, the endo- mysium. The ultimate subdivisions of the endomysium completely en- velop each fiber and blend with the sarcolemma. This brings us to an individual fiber. Each fiber is enclosed in a sarcolemma. The myofibrilla3 are collected into larger and smaller bundles, Kollilcer's columns, sepa- rated from each other by semifluid, granular sarcoplasm; these collec- tions in cross section are known as the areas of Cohnheim. They repre- sent the definitive division products of the original group of fibrils (Heid- enhain). The ultimate histologic units are the myofibrillse or sarcostyles. FIG. 127. STRIATED MUSCLE FIBERS OF THE DOG. The blood-vessels have been filled by injection with a gelatinous mass and are represented in black. One whole fasciculus and one fiber from an adjacent fasciculus have been included, a, perimysium; 6, endomysium; c, a large vein seen in transection. The section was not stained. X 53. MUSCULAR CONTRACTION 113 But these may consist of still more delicate fibrils; in Limulus muscle, for example, the myofibrils may be resolved into successively finer fibrils to the limits of visibility. Heart muscle can be similarly, but less precisely divided, the endo- cardium and epicardium corresponding to the epimysium. Fasciculi and perimysium are not so readily distinguished, but the endomysium is re- ' A' " i *C %...- ' Xoi FIG. 128. STRIATED MUSCLE OF A CAT SEEN IN TRANSECTION. The blood-vessels have been injected and are black in the figure. At a an artery is contracted and empty. The heavy black vessels are veins and arterioles; the small black dots are capillaries in transection. One whole fasciculus is represented and is surrounded by a delicate perimysium of connective tissue. Between the muscle fibers is the still more delicate endomysium. The larger vessels are almost exclusively found in the perimysium. The section was not stained. X 80. lated to cardiac muscle in a manner essentially similar to that described for skeletal muscle. The student should have well in hand the several criteria for the differentiation of the three types of muscle, both in transverse and longitudinal sections; and of smooth muscle from the dense white fibrous connective tissue. In brief, cross sections of skeletal striped muscle can be readily distinguished on the basis of their greater diameter and the peripheral position of the nucleus. Both cardiac and smooth muscle 114 MUSCULAR TISSUE have a central nucleus and peripheral fibrillae; but the fibers of the cardiac muscle are more or less polygonal in outline and more constant in size, excepting occasional branches, while the smooth cells are circular in outline and of very diverse diameters, depending upon the different levels at which the section passes through adjacent fusiform cells. In longitudinal sections the cardiac muscle can be easily recognized by its branching character and the presence of intercalated disks ; smooth mus- cle by the fusiform character of its associated celjs. Smooth muscle is frequently difficult to distinguish from compact, white fibrous connective tissue. When both are present in the same section, stained with the routine hematoxylin-eosin technic, the two exhibit a slight difference in staining reaction. The smooth muscle commonly stains a deeper red; the collagenous fibers have a lighter orange tinge. Moreover, from a morphological standpoint, while portions of the white fibrous connective tissue may appear spindle-shaped, thus simulating the unit of smooth muscle structure, the associated nuclei of enveloping connective tissue cells are peripheral to the bundle, whereas the nucleus of smooth muscle is of course in the center of the analogous structure, the muscle cell. BLOOD SUPPLY The blood-vessels of voluntary striped muscle distribute their larger trunks within the connective tissue of the epimysium. The smaller branches penetrate the endomysium and supply a rich capillary plexus with long rectangular meshes. This network of capillaries surrounds the muscle fibers so completely that each fiber is placed in relation with four or five capillary vessels which run parallel with the long axis of the fiber. The blood supply of cardiac muscles is in general similar, but even more abundant and intimate, with respect to its terminal meshes. The blood supply of smooth muscle is relatively meager. Numerous lymphatics occur in the perivascular connective tissue. These lymphatic vessels are especially abundant in cardiac muscle. NERVE SUPPLY Skeletal muscle is innervated both by cerebrospinal and sympathetic nerves, supported in the connective tissue envelopes and septa. The former include both sensory and motor fibers ending in muscle spindles TENDONS 115 and motor end-organs respectively. . These endings will be further de- scribed under Peripheral Nerve Terminations. The sympathetic or 'ac- cessory fibers' (Fig. 129) relatively sparse and delicate, and in close relationship to the motor fibers and endings terminate in special 'end- plates/ close-meshed networks of generally oval outline. Boeke (AnaJ;. Anz.,44, 15-16, 1913) sug- gests that they may me- diate the maintenance of muscle tone. Cardiac muscle is sup- plied only with sympa- thetic motor fibers. These terminate on the muscle fibers in brushes of fibrils, but without highly spe- cialized endings. Sensory FIG. 129. MOTOR END-PLATE ON AN INTERCOSTAL MUSCLE FIBER OF A YOUNG RABBIT. The motor nerve fiber m is accompanied by an accessory (sympathetic) fiber, a/. (After Boeke, Anat. Anz., 44, 15, 1913.) fibrils from the vagus are distributed to the cardiac endomysium. Each smooth muscle cell, likewise, is supplied with a sympathetic fibril, ending in minute knobs or plates. According to Malone (Amer. Jour. Anat., 15, 1, 1913), the three types of muscle are innervated by three histologically distinct types of nerve cells, representing specific functional differences. The cells supplying heart muscle are from the standpoint of size and granular (chromophilic) content, intermediate between those supplying smooth and those supply* ing skeletal striped muscle. (See Fig. 139 below.) TENDONS A tendon, taken as a whole, is invested by a dense fibro-elastic membrane, the epitendineum, . or vagina fibrosa. Where tendons play in bony grooves this may be modified into a tendon sheath, the epitendineum acquiring a mucous cavity, when it becomes known as a vagina mucosa. Septa from the epitendineum penetrate the mass and divide the tendon imperfectly into irregular columns, the tertiary bundles. These are further divisible into more regular aggregations of fibrils, completely enveloped by a peritendineum, and are the ten- don fasciculi. These correspond to the muscle fasciculi enveloped by 116 MUSCULAR TISSUE FIG. 130. PORTION OF A TRANSECTION OF A LARGE TENDON. a, fibrous capsule with circular, and at 6, longitudinal bundles of connective tissue; c, d, and e, fibrous septa be- tween the fasciculi of the tendon; I, lymphatic cleft. Moderately magnified. (After Schafer.) perimysium. In the mouse, tendons con- sist of from 1 to 11 fasciculi ; in the chick from 2 to 5 (Loevy, Anat. Anz., 45, 10-11, 1913). Each fasciculus con- sists of elementary bundles of collage- nous fibrils, envel- oped by a complete sheath formed by the anastomosing processes of the ten- don cells (cells of Eanvier). The cell bodies lie between these primary bun- dles; they are connected to each other by their processes, forming an 'endotheliaF tube (Eanvier), the cells of which have a characteristic mesothelial appearance in sil- ver nitrate preparations. In the tendons of the tail of the mouse, Loevy describes the cells as flat, rectangular, and rhomboidal; they are parallel to the long axis of the tendon, two of their sur- faces extended into flat plates or wings which effect a union with ' wings' of adjacent cells. A cell may have from 2 to 4 wings. The wings or plates have been interpreted as elas- tic in nature, but do not re- FIG. 131. TRANSVERSE SECTION OF TENDON act to specific stains for elas- OF TAIL OF ADULT MousE ' tic tissue. Each cell contains Jt consists of four secondary bundles or fas- a snhprirfll or rwal s ' b ''> p - b '> P rimar y bundle; p., epiten- oval deeply dineum . cR tendon cell (cell of Ranv i er ). staining nucleus; the nuclei (After Loevy, Anat. Anz., 45, 10, 1913.) c.R. TENDONS 117 of successive cells are often so placed as to be immediately adjacent. Ac- cording to Loevy the fibrils are developed from fibroblasts ; the definitive tendon cells, which form the primary bundles, arise from cells of Ran- vier. Both come from mesenchyme cells, but the fibroblasts entirely disappear, while the cells of Ranvier persist as the characteristic winged tendon cells. Ligaments, fascia, and aponeuroses are very similar to tendon, but are less compact and contain more elastic tissue. Bursse are mesothelium-lined sacs in connec- tion with the large diarthroses and certain locations where tendons are subject to friction. Tendons are supplied with blood-vessels and sensory nerve endings, in a manner very similar to skeletal muscle. The exact manner of the attachment of striped muscle to tendon is still disputed. According to certain investigators (0. Schultze, Arch. f. mikr. Anat., Bd. 79, 1912), the myofibrils and tendon fibrils are directly continuous through the sarco- lemma. Others (Baldwin, Morph. Jahrb., Bd. 45, 1912) hold that the muscle ends sharply, remain- ing striped to its termination, and that the rounded or pointed end is completely enveloped by the sar- colemma (Fig. 132). The muscle fibers are de- scribed as being dovetailed into the tendon, the ten- don fibrils being attached to the sarcolemma. This is the more commonly accepted interpretation; but it seems probable that both types of muscle-tendon connections occur in different muscles, for in cer- tain muscles the cross striations become gradually more vague toward the tendon, and the point of transition from muscle to tendon is by no means sharply marked. More- over, the fact that certain ligaments and aponeuroses arise normally by transformation of muscle adds support to the idea of muscle-tendon con- tinuity. Baldwin distinguishes two general types of muscle termination with respect to tendon: one in which the long axes of tendon and muscle fiber coincide ; a second in which they meet at an angle. In neither type does he recognize a direct continuity between muscle and tendon fibrils. 9 FIG. 132. PORTION OF A MUSCLE FI- BER FROM THE TAIL OF A 5 CM. FROQ TADPOLE. Each cone-shaped sarcolemma process has attached to it a tendon fibril. Two of the processes de- rive fibrillae from a large fibroblastic cell situated among the tendon fibrillae. (Af- ter Baldwin.) xiooo. 118 MUSCULAR TISSUE In the first type the sarcolemma presents pointed ends, to which the ten- don fibrils are attached; in the second, the sarcolemma presents a flat surface which rests directly against the attached structure, whether fascia, periosteum, or ligaments. Lymphatics are abundant in tendons. CHAPTER V NERVOUS TISSUES GENERAL CONSIDERATIONS Nervous tissue comprehends those tissue elements which are peculiar to the nervous system. In the protoplasm of nervous tissue proper (neuroplasm) the fundamental properties of irritability and conductivity have become predominant. The nervous system includes the cerebro- spinal comprising the central (brain and spinal cord) and the periph- eral (cerebral and spinal nerves) portions and the sympathetic divi- sions. For convenience we may speak also of the central and peripheral nervous systems, the latter including the sympathetic division. The essential unit of structure, comparable to the cell of other tissues, is here the neuron, or neurocyte. A neuron is a nerve cell in the broadest sense of the term. It consists of the cell body (nerve cell of the older writers, cyton), together with all of its processes. These latter are divisible into two varieties, the axon and the dendrons (dendrites). The neurons are among the largest cells of the body. Their cell body is of variable size, in some cases extremely minute, at other times sufficiently large to be readily observed with the unaided eye. Their processes, usually of considerable number, vary in length from a milli- meter or less, up to half the height of man. It is therefore obviously impossible to study microscopically at one time the entire course of these longer processes. This circumstance renders it advisable to retain the term nerve fiber of the older writers to designate, not as was the former conception, a histological entity, but rather that portion of those long processes of the nerve cell which pursues its course, as a rule, outside of the gray matter of the central portion of the cerebrospinal division. On this basis we may divide the neuron into the nerve cell and the nerve fiber. The former term includes the cell body, or cyton, with its dendrons and the proximal portion of its axon; the distal portion of the axon forming the essential part of a long nerve fiber. The nerve cells 119 FIG. 133. DIAGRAM OF A NEURON. a h, axon hillock; a x, axon; c, cytoplasm, the Nissl granules have been stained; d, dendrons; m, myelin sheath of the nerve fiber; m f , muscle fiber; n, nucleus; n', nucleolus; n of n, nucleus of the neurolemmaof the nerve fiber; n R, node of Ran- vier; s /, collateral; s L, segment of Lantermann; tel, telodendrion or terminal arborization which here forms a motor end-plate. (After Barker.) 120 THE NEEVE CELL are found throughout the gray matter of the central portion and in the peripheral ganglia of the cerebrospinal division and in the sympathetic ganglia. Nerve fibers occur in the white matter of the central portion and in the nerve teunks and ganglia of the peripheral portions of the nervous system. In the peripheral nervous system the nervous tissues are chiefly supported by the connective tissues, but in the central portion a special form of supporting tissue, the neuroglia, is also found. This is de- scribed below. THE NERVE CELL (Cyton, Cell Body, Perikaryon, Ganglion Cell) This term,, as already stated, includes the cell body with its den^ drons and the proximal portion of its long axon. The cell bodies vary in size from 4 /* to 200 ^ in diameter. Their shape is chiefly dependent upon the number of their dendritic processes. Unipolar nerve cells, with but a single process, are flask-shaped or pyriform; bipolar cells, whose processes are usually derived from opposite extremities, are most fre- quently fusiform; multipolar nerve cells, from the considerable number of their processes, are irregularly stellate. Nucleus. The cytoplasm of the cell is finely granular and contains a large vesicular nucleus which, as a rule, is excentrically situated. The appearance of this large nucleus is quite characteristic of the nerve cell as distinguished from the cells of other tissue. The nuclear membrane is distinct and highly chromatic. The contents of the nucleus, however, except for the large spherical nucleolus which is quite constantly present, is noticeably deficient in chroma tin. Those few small karyosomes which are present are mostly adherent to the inner surface of the nuclear mem- brane. The achromatic nucleoplasm forms the greater portion of the nucleus. Occasionally the chromatin forms still finer granules, and is more equally distributed throughout the nucleus. A large, chromatic, centrally situated nucleolus is nearly always present. Cytoplasm. The finer structure of the cytoplasm of the nerve cell is the subject of considerable difference of opinion. The studies of Nissl have shown that it is divisible into a substance which is readily stained by methylene blue, thionin, etc. (the stainable substance of Nissl, tigroid of von Lenhossek), and an apparently homogeneous substance which is not so readily stained (the unstainable substance of Nissl). FIG. 134. A UNIPOLAR GANGLION CELL OF A FROG. o, cell body; 6, axon; c, dendron. Methylene blue. Highly magnified. (After von Smirnow.) FIG. 135. MULTIPOLAR GANGLION CELL FROM THE VENTRAL HORN OF THE GRAY MATTER OF THE SPINAL CORD OF THE Ox. a, axon; b, dendrons. (From Barker, after Deiters.) 122 THE NERVE CELL 123 Nissl's substance, chromophilic or tigroid substance, occurs in the form of flake-like granules of vary- ing size and irregular shape. Their disposition within the cytoplasm is subject to considerable variations in different nerve cells, but accord- ing to Nissl it is fairly constant in cells of the same location for any given species. The amount also of the chromophilic substance is sub- ject to variation depending upon the functional condition of the in- dividual. It has been shown that the substance is greatly diminished by fatigue (Dolly) and after sur- gical shock (Crile). In general also, the longer the axon the great- er the amount of chromophilic sub- stance. Chemically, it is a nucleo- proteid. There is considerable his- tologic evidence to indicate that it has a nuclear origin, appearing first in the form of 'chromidia/ and it is accordingly sometimes designated as cytochro matin. Miihl- man has shown, however, that tigroid nuclein is soluble in weak soda solutions while nucleus nu- clein is not. It has been suggested (Heidenhain) that it may perhaps have an accessory nuclear function. According to Held it is present as a diffuse continuous substance, co- agulated in the form of flakes and granules in fixed tissues. Those nerve cells in which the Nissl substance is abundant are said. to be in a pylcnomorplious, those in which it is scanty in an FIG. 136. PYRAMIDAL MULTIPOLAR NERVE CELL FROM THE CEREBRAL CORTEX OF A MOUSE. a, axon; d, dendrons; Golgi teohnic. (Barker, y Cajal.) c, collaterals, after Ram6n 124 NERVOUS TISSUES apyknomorplious condition. The Nissl granules are apparently used up during functional activity of the nerve cell. The brain-cells show a strong affinity for adrenalin, the secretion of the suprarenal glands; this fact leads Crile (1914) to strongly suspect that the Nissl substance is a volatile, extremely unstable combination of certain elements of the brain-cells and adrenalin because the suprarenal FIG. 137. ISOLATED NERVE CELLS FROM THE SPINAL CORD OF MAN. x, axon. Carmin. X 160. (After Sobotta.) glands alone do not take the Nissl stain, and the brain deprived of adrenalin does not take Mssl stain. Nissl substance disappears in case of lesion of the neuron, but re- appears in abundance after temporary injury and recovery of the cell. Such disappearance after section of the axon (axonal reaction) is accom- panied by a swelling of the neuroplasm and the peripheral migration of the nucleus, after from ten to fifteen days. Concerning the finer structure of the unstainable substance of Nissl, comparatively little is known. With varying methods of fixation this portion of the cytoplasm has been found to show very fine fibrils (neuro- THE NERVE CELL 125 fibrils, Fig. 141) ( Schultze, Flemming, Apathy, Bethe) and fine acidophil granules (neurosomes of Held; probably mitochondria). Besides these structures there remains a homogeneous ground substance or hyaloplasm, which, though of extreme physiological importance, in the usual histo- logical preparations presents no structure. Centrosomes and attraction spheres have been frequently observed in the nerve cells of the lower vertebrates, and occasionally in those of mammals. The cytoplasm of many nerve cells con- tains a characteristic brownish-yellow pig- ment, whose fine granules have a tendency to accumulate in the vicinity of the nu- cleus. Mitochondria also have been reported in ganglion cells of the rabbit ( Schirokogoroff , Anat. Anz., 43, 19 and 20, 1913). Cowdry (Amer. Jour. Anat., 17, 1, 1914) describes granular and rod-like mitochondria in the spinal ganglion cells of a number of verte- brates, including man. They are said to occur throughout the entire neuron, axon as well as dendrons. They are regarded as fundamental constituents of the neuro- plasm. It is suggested that they are con- cerned with the metabolism of the neuro- cyte. Neurons are incapable of division; de- stroyed neurons cannot be replaced; the axon, however, may regener- ate. End fibrils of other neurons have been demonstrated within the cyto- plasm of the nerve cell. Apathy has likewise demonstrated that fibrils occasionally pass from onp neuron to another, so that we no longer con- sider that a neuron, though a structural unit, is in all cases anatomically independent of all other neurons. The present status of this much dis- cussed question seems to be comparable to that of the cell, as a histological unit of structure, which though formerly- thought to exist independently of other cell units, has since been found to be frequently connected, as by the intercellular bridges of epithelium and of smooth muscle. The neurons of the nervous system therefore, while being usually related to one another by contiguity or by contact only, may occasionally be more FIG. 138. VARIOUS TYPES OF NERVE CELLS OF THE CERE- BELLAR CORTEX. 1, cell of Purkinje; the cyto- plasm contains large flakes of Nissl substance; 2 and 3, small- er nerve cells, 'granule cells/ Nissl's stain. X 1200. * l '-V , r EJ 'HI a il'l 126 THE NERVE CELL 127 directly connected by fibrillse, which pass from the processes of one neuron to the cell body or processes of a second neuron (Apathy, Bethe), or by 'concrescence/ as described by Held. The nerve cells are surrounded by a narrow interval which separates them from the surrounding tissue. This is presumably a lymphatic or tissue fluid space. Holmgren has demonstrated also the presence, within the cytoplasm of the nerve cell, of minute canaliculi which form an intra- FIG. 140. A NERVE CELL FROM THE TRAPEZOID NUCLEUS IN THE MIDBRAIN OF A RABBIT. a, axon; 6, axons of other nerve cells which terminate in relation and apparently fuse with the cytoplasm of the ceil body; c, points of fusion or zones of concrescence; d, dendrons which have been cut off close to the cell body; e, neuroglia. The cyto- plasm shows a neurofibrillar network and Nissl granules. Iron hematoxylin. Very highly magnified. (After Held.) cellular network, more abundant near the surface of the cell, and which he has termed juice canaliculi, or trophospongium. These canaliculi may possibly account for the peculiar mtracellular network which Golgi has demonstrated in the periphery of the nerve cell, by a modification of his rapid silver impregnation method. The processes of the nerve cell are of two varieties: the one, broad, granular, and rapidly dividing in the vicinity of the cell body into a number of branching subdivisions, is the dendron; the other, long, slen- der, and finely but distinctly fibrillar, arises from the cell body direct, or 128 NEKVOUS TISSUES from the base of a dendron, and passing for a considerable distance from the cell body, finally enters the nerve fiber as its axis cylinder, or termi- nates in relation to some distant JIL ^^^ nerve cell. This latter process is the ^L ja&afe&SS^R axon. Each cell body usually pos- ^^^B^B ' m^^^^ sesses a single axon and several den- IJIJijjjjff drons. Cells without an axon are *^ Sr* found in the retina and in the ol- factory bulb; except for these, all f*jffij&gjjr nerve cells in the body of man pos- sess an axon and usually but one such process. The subdivision of nerve cells into uni-, bi-, and multi- polar cells is, therefore, chiefly based upon the number of their dendrons. Dendrons (Dendrites, Protoplas- mic Processes) . The dendrons of a FIG. 141. A NEURON (GIANT PYRAMI- DAL CELL, OR CELL OF BETZ) FROM THE CEREBRAL CORTEX OF MAN, SHOWING THE NEUROFIBRILS. Bielschowsky technic. X 500. nerve cell vary from one to a consid- erable number. They arise from the cell body by a broad stem, and quickly' break into branches which can be traced for a considerable dis- tance in fact, the arborization of the dendrons is usually so extensive that it can be followed for only a short portion of its course. Occasionally dendrons do not branch until they have arrived at a considerable distance from their parent cell-body. The structure of the dendron is, to all appear- ances, similar to that of the cell-body. The chromo- philic substance is continued for some distance into the arborizing dendrons, which often possess a finely fibrillar appearance. In Golgi-stained preparations the dendrons frequently present a thorny appearance, due to the clustering along their borders of minute lateral projections, the gemmules. The terminal filaments of the dendronic arboriza- tion are frequently in relation with the cell bodies or axons of other neurons, less frequently with the dendrons of other neurons. Such contact relation- ship is known as synapsis. Dendrons are cellulipetal processes, transmitting impulses to the cyton. The Axon (Neuraxis, Neuraxon, Neurite, Axis Cylinder Process}. FIG. 142. INTRA- CELLULAR NET- WORK (TROPHO- SPONGIUM) WITHIN A PUR- KINJE CELL OF THE CEREBELLUM OF Strix flam- mea. Golgi's stain. (After Golgi.) THE NERVE CELL 129 This process in contradistinction to the dendron, is long and slender, as a rule does not arborize near its parent cell-body, is of smooth and regular contour in Golgi preparations, and contains no chromophilic substance. It arises from the cell body, or less frequently from the base of a dendron, by a conical, clear area, the axon hillock or implantation cone, which, like the process itself, is devoid of chromophilic granules. It consists of FIG. 143. GOLGI CELL, TYPE I. c, collaterals; n, axon. Golgi's stain. (After Kolliker.) a bundle of delicate neurofibrils (axon fibrils) embedded in axoplasm. During early developmental stages the fibrils increase in number by a splitting of preexisting fibrils. At some little distance from the parent cell-body the axon gives off very fine lateral branches, the collaterals., which leave the parent stem at the nodes of Eanvier at nearly right angles. These delicate branches, as also the axon proper, finally terminate by a sudden end arborization, or telodendrion, by which each axon is placed in relation with a large 130 NEEVOUS TISSUES number of neurons, or a considerable area of surface. The telodendrion may terminate in minute knobs or plates, the neuropodia. The teloden- drion is structurally apparently an efficient mechanism for mediating FIG. 144. GOLGI NERVE CELL, TYPE II. a, axon; x, dendron. (After Kolliker.) the phenomenon of 'axon reflex' (Langley) . The parent stem of the axon may be finally exhausted in its collaterals, or it may in turn end in a terminal arborization. Collaterals are said to be more frequent in the proximal than in the distal portion of the axon. The axon transmits impulses away from the cell-body ; it is a cellulifugal process. THE NERVE FIBER 131 According to the length of their axons, neurons are divided by Golgi into two types. 1. Golgi cells, Type I (Deiters' cells). 2. Golgi cells, Type II (Golgi's cells). The cells of Type I possess a long axon which passes beyond the con- fines of the gray matter in which it arises and usually becomes the axis cylinder of a nerve fiber. The cells of Type II possess a short axon which forms its terminal arborization in the vicinity of its parent cell-body. The cells of this type are usually association and commissural neurons; they place in conduc- tion relation other not very remote neurons. The cells of Type I, on the other hand, are more frequently projection neurons ; they are distributed from the nerve centers to other and perhaps very different tissues, their courses lying in the long projection tracts and nerve trunks of the nervous system. The cells of Type II are therefore most frequently intrinsic or endog- enous neurons, their whole course lying in one division of the central nervous system, e.g., the gray matter of the spinal cord. The cells of Type I are more frequently extrinsic or exogenous; they arise in one part of the nervous system to be distributed to a distant portion, e.g., they arise in the peripheral ganglia and enter the spinal cord to terminate in its gray matter, or vice versa. The size of a nerve cell is thought to bear a general relation to the length of its axon, the larger cells possessing the longer axons. The cells of Golgi's Type I are therefore larger than those of Type II. Like- wise the cells of the motor tracts, whose axons are as a rule much longer than those of the sensory tracts, are characterized by their large size as compared with the sensory cells. THE NERVE FIBER The origin of the nerve fiber and its relation to the other portions of the neuron will be appreciated by tracing the course of the axon of a motor nerve cell of the ventral horn of gray matter in the spinal cord. This process, arising in the central gray matter, is at first a naked axon. It soon leaves the gray matter to traverse the white matter and makes its exit from the spinal cord as the axis cylinder of one of the fibers of a ventral nerve root. On leaving the gray matter the axon acquires a 132 NERVOUS TISSUES cylindrical sheath of myelin substance, the medullary sheath, myelin sheath, or white substance of Schwann. On entering the ventral nerve root, which lies outside of the white matter of the spinal cord, the axon receives an epithelioid membranous sheath, the neurolemma or nucleated sheath of Schwann. The axon retains these two sheaths until near its termination, when the sheaths suddenly stop, the axon becoming again naked as it breaks into terminal fibrils. Not all nerve fibers are medullated, nor do they all possess a neuro- lemma. The axons of the central nervous system are not supplied with a neurolemma until they pierce the meninges to enter the nerve roots. Those of the gray matter also have no appreciable medullary sheath. The axons of the peripheral nerve trunks and ganglia are all supplied with a neurolemma except at their terminals, as already explained. Yet some of the peripheral axons have a medullary sheath, while others have none. An axon with its enveloping sheaths constitutes a nerve fiber, and upon the presence or absence of these sheaths nerve fibers may be classified as follows : , r T ,, , 1 f 1. With a neurolemma A. Medullated nerve fibers -K ITT -XT, , j^2. Without a neurolemma. B. Non-medullated nerve f 3. With a neurolemma fibers. [4. Without a neurolemma alemmal. 1. Medullated Nerve Fibers with a Neurolemma. Nearly all the nerve fibers of the cerebrospinal nerve trunks and ganglia and some of those of the sympathetic nerves are of this type. These nerve fibers consist essentially of three cylindrical structures : the axis cylinder, which is the continuation of the axon of a nerve cell, and which forms the cen- tral portion or core of the nerve fiber; the medullary sheath, which forms a hollow cylinder inclosing the axis cylinder, and which suffers frequent interruptions, as will be described ; and the neurolemma, which is an extremely thin investing sheath forming an uninterrupted envelope from the point where the nerve fiber leaves the central nervous system to a point near the end of the fiber where the axis cylinder breaks into its terminal fibrils. To these structures an investing sheath of con- nective tissue, the sheath of Henle, is sometimes added. It is derived from the connective tissue endoneurium in which the nerve fibers are embedded. It serves to support the capillary blood-vessels destined for the supply of the nerve fibers. THE Axis CYLINDER. The axis cylinder presents a finely fibrillar THE NERVE FIBER 133 structure. The nature of these fibrils is not well understood. They are apparently continuous with the neurofibrillar network of the cell body. In certain nerve fibers of the lower animals these fibrils have a ten- dency to collect into the center of the axis cylinder, leaving a peripheral clear zone; this distribution is especially characteristic of those fibers FIG. 145. ISOLATED NERVE FIBERS FROM A FROG. The axis cylinders, the enveloping myelin sheaths, and the nodes of Ranvier are clearly shown. Intra-vitam methylene blue stain. (Barker, after von Kollikcr.) which are not supplied with a medullary sheath. In mammals., however, the fibrillae occupy a larger portion of the axis cylinder, the clear peri- pheral area being correspondingly diminished until in man it can scarcely be recognized. The fibrils of the lower animals are also coarser. Apathy, studying chiefly the lower animals, has considered these 'ulti- mate fibrillce' to be the conducting element of the nerve fiber. Others, however, lay greater stress upon the intervening clear portion, the 134 NEEVOUS TISSUES FIG. 146. A SMALL PORTION OF A TRANSECTION OF THE SCIATIC' NERVE OF A DOG. Nerve fibers are seen in transection; their myelin sheaths are black, their neuraxes un- stained. Osmium tetroxid. Photo. X 700. neuro plasm of Schiefferdecker or axoplasm, as containing the active conducting substance of the fiber. According to Verworn, Lenhossek and E. Goldschmidt, these elemen- tary fibrillae (axon fibrils) in the axis cylinder are nothing else than skeletal substance for the support of the semi-fluid neuroplasm. The circum- stance that many of the fibrils of an axis cylinder may be sectioned without diminution of the maximum effect of stimulation favors the view that the neuro- plasm is the essential con- ducting substance. Tashiro has demon- strated that a living nerve gives off a definite amount of carbon dioxid, and that when the nerve is stimulated the amount of carbon dioxid production is increased. He conceives of the propagation of nerve impulses as a chemi- cal change, the propagation being in essence a restoration of equilibrium in the nerve fiber disturbed at the point of contact. The axis cylinder is, under certain conditions at least, found to be in- closed by an extremely delicate mem- brane, the axolemma of Kiihne. The existence of this membrane as an in- tegral part of a living axis cylinder has been denied by others. It may be simply a fixation artifact. THE MEDULLARY SHEATH (White Substance of Schwann, Myelin Sheath). The medullary sheath forms a cylindrical investment for the axis cylinder. Medullated fibers vary greatly in diameter according to the amount of myelin present. It appears to be retained in position by the neurolemma, for when the Ax FIG. 147. A GROUP OF LARGE MED- ULLATED FIBERS FROM A NERVE IN THE PERITRACHEAL AREOLAR TIS- SUE OF THE CAT. Ax, axis cylinder; Nk, neurokeratin framework; Nc, neurolemma cell and nucleus; M, medullary sheath; N, neurolemma. X 1000. THE NERVE FlBEIfc 135 latter is ruptured the myelin exudes in the form of 'myelin drops.' The myelin thus obtained possesses the physical properties of a fat. It is also capable of being blackened by osmium tetroxid. By extraction with ether the myelin can be removed, leav- ing behind a supporting framework of neurokeratin. The function of myelin is probably nutritive, though it has been regarded as an insulating substance. It is thought to be present in small amounts even in so-called non-medullated fibers. It seems rea- sonable to suppose that it may have a double function, that is, nutritive and in part insulating. At frequent intervals in the course of the nerve fiber its myelin sheath suffers complete interruption, thus forming the annular constrictions or nodes of Ranvier. At these points the neurolemma dips in until it is in con- tact with the axis cylinder. Both axis cylinder and neurolemma are contin- ued past the node without interrup- tion. The successive nodes of Eanvier divide the nerve fiber into internodal segments. Within each internodal segment the medullary sheath, on blackening with osmium tetroxid, presents clear intervals which pene- trate the myelin sheath it such man- ner as to give the appearance of obliquely disposed clear lines or inci- sions. These incisures of Schmidt FIG. 148. NERVE FIBERS. A and B, from the sciatic nerve of a rabbit, isolated by teasing, and viewed in profile; C, a group of nerve fibers in transection, from the sciatic nerve of a dog. a, axon; 6, neurolemma pro- jecting beyond the torn end of the fiber; d, nucleus; h, endoneurium or fibrous sheath of Henle; Z, Schmidt- Lantermann lines; n, nodes of Ran- vier. Osmium tetroxid. A and B t X 670; C, X 900. (Schmidt-Lantermann lines) have not been satisfactorily explained and can not be demonstrated in the living fiber, yet they present a constant form and are always present in 10 136 NEKVOUS TISSUES osmic preparations. These incisures subdivide the interannular segments of the medullary sheath into medullary segments. Schmidt originally considered them to be the optical expression of folds in the outer fibrous coats. Lantermann and others claim to have shown that they are within the neurolemma. They are believed by others to represent the limits of cones of neurokeratin. The incisures may point in different directions. They are more probably artifacts, representing fractures in the delicate myelin sheaths. In preparations of fresh nerve fibers which have been treated with silver nitrate according to the method of Ranvier, the solution is found to enter the fiber most readily at the nodes of Ranvier, so that if blackened by exposure to the sunlight, minute + -like appear- ances are seen at each node. By - prolonged maceration in weak solutions of silver nitrate the so- lution penetrates still farther FIG. 149. CROSS AND LONGITUDINAL SEC- TIONS OF THE SAME FUNICULUS (N) OF NON-MEDULLATED NERVE FlBERS (TURNED UP AT THE LEFT), SHOWING THE PERINEURIUM (P) AND THE RELA- TIONSHIP OF THE NEUROLEMMA NUCLEI TO THE Axis CYLINDER BUNDLES OF NEUROFIBRILS. From the peritracheal areolar tissue of the cat. X 1500. and the blackened axis cylinder is found to possess spiral trans- verse markings which are quite characteristic. The true mean- ing of these appearances has not been satisfactorily explained. Because of the apparent greater permeability of the fiber at these points, these peculiarities have been taken to indicate a certain relation of the annular constrictions to the nutrition of the fiber. THE NEUROLEMMA (Nucleated Sheath of Schwann). The neuro- lemma is the outermost of the nerve fiber sheaths. It is of ectodermal origin and makes its appearance prior to the medullary sheath. It forms a very delicate membrane, which incloses the myelin substance, and at each node of Ranvier comes into contact with the axis cylinder. Attached to the inner surface of the neurolemma in each internode, and usually but one for each internodal segment, is an oval nucleus. The nucleus is surrounded by a minute amount of finely granular cyto- plasm. This structure is taken to indicate that the embryonal neuro- lemma is formed by cells which became spread out over the surface of the primitive fiber, one cell, as a rule, supplying each internodal segment ; THE NEKVE FIBER 137 lio 8*3 m** and its nucleus with a minute amount of undifferentiated protoplasm is, according to this hypothesis, considered to remain as a permanent part of the neurolemma. 2. Medullated Nerve Fibers without a Neurolemma. This type of nerve fiber composes the white matter of the central nervous system. The axis cylinder does not, of course, differ in the least from those of the previous variety and will need no further description. The medullary sheath also is similar in its finer structure to that of the previous type, but since no neurolemma is pres- ent, these fibers possess no nodes of Ranvier. The medullary sheath of the fibers found in the white matter of the brain and spinal cord, is therefore uninterrupted. Its sur- face is in direct contact with the neuroglia net- work, which forms the supporting tissue of these organs, the innermost layer being condensed into a membrane which simulates a neurolemma. These fibers are accom- panied by sheath cells (Hardesty), homologues of the neurolemma cells of other fibers, which aid in the formation and maintenance of the myelin. 3. Non-medullated Nerve Fibers with a Neurolemma (Sympar thetic Nerve Fibers, Remak's Fibers}. The most of the fibers of the sympathetic division are of this type. The axis cylinder does not differ from that of the previous types. The medullary sheath is entirely ab- sent or, at most, only slightly developed in these fibers. The neuro- lemma is perhaps incomplete at times, but exhibits frequent nuclei along the course of the fiber. The neurolemma nuclei appear to be em- bedded in the superficial portion of the axis cylinder. Fibers of this type are of quite frequent occurrence also in the cerebral (cephalic; m *w '.jf' * FIG. 150. CROSS-SECTION OF THE TRUNK OF THE HUMAN VAGUS NERVE, SOME DISTANCE BELOW THE NODOSE GANGLION, SHOWING MEDULLATED AND NON-MEDULLATED FIBERS. Pyridin-silver. X 680. (After Ranson.) 138 NEEVOUS TISSUES cranial) nerves of the cerebrospinal division. Other cerebrospinal nerve fibers lose their medullary sheath and finally also their neurolemma prior to their termination. The recent work of Hanson has shown that even in the typical medul- lated spinal and cerebral nerves non-medullated fibers are very abundant. In the vagus of the dog, for example, the non - medullated fibers actually preponderate be- low the diaphragm. Hanson states that of these no consid- erable portion can be of sympa- thetic origin and that only a few represent medullated fibers which have lost their myelin distally. The non-medullated fibers of the vagus are said to comprise both , afferent and efferent fibers, the -latter arising from cells in the ganglia (jugular and nodose) connected with the vagus nerve (Anat. Kec., 24, 1, 1914). The spinal nerves also are shown to contain more non-medullated than medullated fibers (Amer. FIG. 151. SUCCESSIVE STAGES IN THE DEGENERATION PROCESS EX- HIBITED BY THE DISTAL STUMP OF A MEDULLATED AXON (FROM SCIATIC NERVE OF ADULT DOG) FOLLOWING SECTION. (1) on the second day. (2) on the fourth day, the two fibers a and b are at different stages of degenera- tion, the neurolemma can be seen bounding the unstained myelin sheath. (3) on the eighth day, the fragmented axon is surrounded by an elliptical segment of myelin (a). (4) on the nineteenth day, a, nu- cleus, 6, droplet of myelin containing fragments of axon. (Ranson, Jour. Comp. Neur., 22, 6, 1912.) Jour. Anat., 12, 1, 1911). 4. Non-medullated Nerve Fibers without a Neurolemma. These fibers are naked axis cylinders and as such are found at the cytoproximal end of the axon in the gray matter of the central nervous system, and at the cytodistal end prior to the termination of the axon in its arborization of terminal fibrils. In man nerve fibers are of this type throughout their entire course only in the olfactory nerves. All portions of the neuron, its axon and collaterals as well as its dendrons, are dependent upon the cell body for nutrition ; hence each nerve cell becomes the so-called trophic center for all of its processes. THE NERVE FIBER 139 The entire nervous system may be considered as an enormous tangle, formed by the interlacing processes of an innumer- able number of neurons whose complex fiber paths place all portions of the body in communication with all other portions. Nerve cells are un- equally distributed throughout the central division of the nervous system ; they therefore oc- cur in more or less dis- tinct groups or nuclei, from each cell of which an axon is frequently dis- tributed along the same path. The larger bundles thus formed are called funiculif fasciculi, or fiber bundles; the smaller ones tracts. Since each fiber of such a tract is dependent for nutrition upon the nerve cell from which it arises, the tract as a whole must depend upon its nucleus of origin for its nutrition. Each nu- cleus therefore becomes the trophic center for the fiber tract to which it gives origin. It may be readily demonstrated that if any such group of axons be cut or otherwise separated from its trophic center, that tract will promptly X Mill. FIG. 152. REGENERATIVE STAGES IN THE PROXIMAL STUMP OF THE CUT SCIATIC NERVE OF THE DOG, SEVERAL MILLIMETERS ABOVE THE LEVEL OF SECTION. p, toward the periphery; c, toward the center. (1) on the nineteenth day after section; o, point in the old medullated axon from which arises an extremely short branch which at once divides into two. (2) on the twenty-fifth day. (3) five protoplasmic strands down which a new axon is growing. (Ranson.) (Pyridin-silver prepara- tions.) 140 NERVOUS TISSUES degenerate. If these axons happen to be the axis cylinders of medullated nerve fibers, as is often the case, their myelin sheaths become rapidly altered in composition and acquire a tendency to disintegrate into small globular granules, which stain deeply with osmic acid when used according to the method of Marchi. For the experimental demonstration of this form of partial cell death occurring in that portion of the neuron which has been cut off from its cell of origin, we were originally indebted to the eminent English physiologist Waller; the resulting changes are there- fore called Wallerian degeneration. Obviously that portion of a neuron or of a fiber tract which, after injury or disease involving its path, still retains its connection with its c. rod. post. FIG. 153. TRANSECTION OF THE SPINAL CORD OF AN EMBRYO CHICK. c. rod. ant., axons to the ventral roots; c. rod. post., axons to the dorsal roots; col, collateral from an axon back to the gray matter; gg, dorsal root ganglion; roc. ant., ventral root; roc. post., dorsal root. (After van Gehuchten.) cell body or trophic center, will not degenerate. This part of the neuron is called its central portion, in contradistinction to its distal portion, the latter of which has been severed from its trophic center and is consequently degenerated. To the study of the various types of Wallerian degeneration we are indebted for many of the facts by means of which the intricate tangles of axons composing the various fiber tracts of the central nervous system have been partially unraveled. The contiguous relationship of different neurons within the nervous system occurs in any one of several ways. The terminal arborizations or telodendrions of one neuron may interlace with : a. the telodendrions of axons belonging to other neurons, b. the telodendrions of collaterals of other neurons, c. the dendrons of other neurons, or d. the terminal arborization may surround, basket-like, the cell body of other neurons. NEUKOGLIA 141 NEUROGLIA Both the gray and the white matter of the central nervous system contain a peculiar supporting tissue, the neuroglia, which consists of two elements, the glia cells and the glia fibers. The latter are very probably produced by the glia cells, of which they were formerly con- sidered to be processes. They consist of a substance similar to, perhaps identical with, the neurokeratin framework of myelin. The Glia Cells. The glia cells, as seen in Golgi preparations, are divisible into two distinct types, the ependyma cells and the astrocytes. The EPENDYMA CELLS may be considered as undifferen- tiated relics of the embryonal cells, from which both glia and true nerve or ganglion cells were developed. These cells line the central canal of the spinal cord and the ventricles of the brain, in which latter organ they also form the cover- ing or outer coat of the telas choroideae. The ependyma consists of long nucleated columnar cells . The central H-shaped gray substance con- wnose ree ends, in fetal and sists of nerve cell bodies, dendrons, non-medul- lated portions of axons, and neurogliar sup- porting tissue. The enveloping white sub- stance consists of medullated axons supported by neuroglia. Weigert stain. X 7. FIG. 154. TRANSECTION OF THE SPINAL CORD OF A CHILD, FIFTH LUMBAR SEGMENT. early life, carry a tuft of cilia ; in adult life they are usually non-ciliated. The attached ends of these cells are embedded in the surrounding gelatinous tissue, and are frequently prolonged for some distance as a fine branched process. In this way the ependyma of the spinal cord enters into the formation of the central gelatinous sub- stance, in which the branched processes of its cells ramify in a glia-like manner. In the fetus the filamentous processes extend from the central canal all the way to the periphery of the spinal cord. In the adult the ependyma cells are prone to so multiply as to almost occlude the central canal; their processes have apparently become shorter, and now reach the surface of the spinal cord only at its dorsal median sulcus. The ASTROCYTES, when stained by the Golgi method, apparently con- sist of a small cell body and an innumerable number of long slender FIG. 155. PORTION OP GRAY SUBSTANCE FROM THE ANTERIOR HORN OF THE SPINAL CORD OF MAN, SHOWING NERVE CELL BODIES, DENDRONS, MEDULLATED AND NON-MEDULLATED PORTIONS OF AXONS, AND NEUROGLIA. (From Salinger, after Kolliker.) FIG. 156. TRANSVERSE SECTION THROUGH THE WHITE SUBSTANCE OF THE HUMAN SPINAL CORD. The dark oval bodies are cross-cut axis cylinders; the surrounding light halos represent the myelin sheath. The coarser trabeculse are connective tissue, contii>- uous with the finer neuroglia framework. (After Salinger.) 142 NEUEOGLIA 143 processes. Two varieties of these cells are recognized: the spider cell or long-rayed astrocyte, with a small cell body and very many exception- ally long and slender processes; and the mossy cells or short-rayed astrocytes, whose processes are shorter and somewhat thicker but de- cidedly more varicose than those of the long-rayed type. Recent investigations by means of the staining methods of Weigert, Mallory, and Benda, have demonstrated that the astrocytes^ as seen in 144 NERVOUS TISSUES the Golgi preparations, probably include two distinct structures, the glia cells and the glia fibers. Glia cells, as seen in sections prepared according to these methods, appear as small cells with large and deeply staining nuclei. In the small glia cells the cytoplasm is so slight as to form scarcely more than a mere rim about the nucleus;. in the larger cells the cytoplasm is more FIG. 158. A LONG-RAYED ASTROCYTE. Golgi's stain. Highly magnified. (After Berkley.) abundant and the processes larger and more numerous. The presence of cytoplasmic processes gives the cell an irregularly stellate appearance. In Golgi preparations these processes can not be distinguished from the dense net- work of glia fibers with which they are surrounded. The Glia Fibers. The glia fibers com- prise numerous filiform fibrils which occur as a dense network around the glia cells, from which they radiate in all directions. They pass alongside of, over, or under the glia cells; their filaments have even been described as passing entirely through the cytoplasm of the cell. Nevertheless they appear at all points to be anatomically dis- tinct from the cell body. The relation of the glia cells to the fibers of neuroglia is perhaps comparable to the arrangement in fibrous or reticular tissue. The fibers FIG. 159. A SHORT-RAYED ASTROCYTE, OR MOSSY CELL. Golgi's stain. Highly mag- nified. (After Berkley.) NEUROGLIA 145 of each of these tissues appear to be ontogenetically derived either di- rectly or indirectly from its cells, yet when fully formed they often exist as anatomically distinct elements. Occurrence of Neuroglia. Neuroglia cells and fibers occur in both gray and white matter of the central nervous system, though perhaps more abundant in the latter. The fibers radiate for considerable dis- tances from their glia cells, and thus form a supporting tissue for the nerve elements. They are frequently in intimate relation with the blood-vessels, on the walls of which many of the glia fibers, particularly FIG. 160. NEUROGLIA CELL WITH ADJACENT FIBERS FROM THE PINEAL BODY OF A YEARLING SHEEP. X 1500. the thicker or mossy variety, terminate in expanded plates, which, in some parts, form an almost complete outer membranous coat of the vessel. The astrocytes are ontogenetic derivatives of the embryonic epen- dyma cells. From their point of origin around the neural canal they wander to all portions of the central nervous system, and even into the optic and olfactory tracts, which are embryonic outgrowths from the fetal cerebral vesicles. Thus neuroglia occurs throughout the brain 146 NERVOUS TISSUES and spinal cord, and also in the olfactory nerves, the optic chiasm, and the retina of the adult. The supporting tissues of the central nervous system include, besides the neuroglia, numerous bands or trabeculse of fibrous connective tissue, FIG. 161. NEUROGLIA CELLS AND FIBERS FROM THE SPINAL CORD OF AN ELEPHANT. The letters indicate various types of neuroglia cells. /, a leukocyte. Benda's stain. X 940. (After Hardesty.) which push inward from the pia mater, carrying with them the vascular branches for the supply of the nervous tissues, and which penetrate deeply into the substances of the spinal cord and brain. NERVE TRUNKS Structure. The nerve fibers of the peripheral nervous system are united into bundles to form the nerve trunks or nerves. Each nerve is surrounded by a heavy connective tissue sheath, the epineurium, which sends trabecula-like septa into the nerve. 'These septa subdivide the nerve trunk into smaller bundles of nerve fibers, the funiculi. The funiculus forms a compact bundle of nerve fibers, and is in turn invested with a sheath of dense connective tissue, the perineurium. Hence the NEEVE TRUNKS 147 perineurium stands in the same relation to the funiculus as does the epineurium to the whole nerve trunk. From the inner surface of the perineurium, septa pass into the funiculus and break up to form a fine connective tissue framework, the endoneurium., for the support of the individual nerve fibers. On sepa- rating the fibers of a funiculus with needles a portion of this fibrous endoneurium remains adherent to the surface of the nerve fiber and gives the appearance of an outermost fibrous sheath, the so-called con- nective tissue sheath of Henle. Nerve trunks fre- quently branch, the branches being formed either by an individual funiculus or by groups of funiculi. In the smaller nerve trunks the funiculi are further subdivided. It is by anastomosis of the funiculi that most of the nerve plexuses are formed. Individual nerve The fat cells and the mye]in sne aths of the nerve fibers of the medullated fibers have been blackened by osmium tetroxid. a, type do not generally fat cells; 6 ' 6 '' blood vessels > that at b ' Ues within a . J funiculus; c, epineurium; d, perineurium; e, coarser branch except in those bands of the endoneurmm. Osmium tetroxid. Photo, portions which are naked X 30. axis cylinders, viz., at the cytoproximal portion of the axon by means of collaterals, and at the cytodistal portion by means of end arborizations. Occasionally also col- laterals arise at a node of Eanvier. Vascular Supply. The nerve trunks are well supplied with blood- vessels. The larger of these are found in the epineurium, and from them branches of considerable size enter the septa to be distributed throughout the perineurium to the funiculi. The coarser septa of the endoneurium contain minute arterioles and venules. A capillary network with elongated meshes occupies the finer divisions of the endoneurium, its vessels being thus brought into contact with the nerve fibers. Perivascular lymphatic vessels abound in the epineurium and its septa, and lymphatic tissue spaces are found throughout the connective tissue of the nerve trunk. Where the cerebrospinal nerve trunks pene- FIG. 162. TRANSECTION OF THE SCIATIC NERVE OF A DOG. 148 NEEVOUS TISSUES FIG. 163. DIAGRAM OF THE ORIGIN AND RELATIONS OF THE PERIPHERAL MOTOR AND SENSORY NEU- RONS. A cylindrical section of the spinal cord, with its ventral and dorsal nerve roots, is shown, a, nerve cell of the ventral horn whose axon passes through the ventral nerve root, 6, to its peripheral termina- tion, c; at d is a unipolar sensory nerve cell in the dorsal root ganglion; its process immediately divides into a peripheral and a central branch. The central branch enters the spinal cord and at e divides into an ascending, /, and a descending, g, branch from both of which numerous collaterals, h, enter the gray matter and terminate in fine end brushes. The peripheral branch of the spinal ganglion cell enters a spinal nerve and finds its way to its termination which is here represented in the skin; it terminates partly by free endings among the epithelial cells, i, and partly in connection with a sensory end organ, k, in this case a tactile corpuscle of Meissner. (After von Lenhosse'k.) trate the meninges these lymphatic vessels are said to be continuous with the similar vessels of the- dura mater. Minute nerve fiber bundles, nervi nervorum, are also found in the epi- neurium; their fibers are mostly, if not entirely, distributed to the blood- vessels. GANGLIA A ganglion may be described as a group of nerve cells occurring in the course of a peripheral nerve trunk. The larg- est of the ganglia form fusiform swellings in the course of the nerve, which are visible to the naked eye. The smallest, on the other hand, contain not more than half a dozen nerve cells, and these must be sought with the aid of the micro- scope and can only be found by the most care- ful observation. Whatever may be their size, all ganglia appear to have a similar struc- ture, except for those differences which charac- terize the sympathetic as GANGLIA 149 distinguished from the cerebrospinal type. The essential elements of structure are the nerve cells, nerve fibers, and a supporting framework of rather dense fibre-elastic connective tissue. Many of the nerve cells of the adult mammal are unipolar in the cerebrospinal ganglia, but are usually multipolar in the sympathetic. The spinal ganglia of the lower vertebrates and of the embryo mammal, FIG. 164. BIPOLAR CELL FROM A SPINAL GAN- GLION OF A FISH. (Barker, after Corti.) FIG. 165. TRANSFORMATION OP BIPOLAR CELLS INTO UNIPOLAR CELLS IN THE GASSERIAN GAN- GLION OF THE PIG. (Barker, after van GehucK- ten.) however, contain bipolar ganglion cells. In mammals the two processes of the embryonal neuron fuse to form a single one which branches in a Y- or T-like manner soon after leaving the parent cell body. In the ganglia of the acoustic nerve the- primitive bipolar condition of the neuron is retained; and the cell body is not surrounded by a capsule. The nerve cells of all other ganglia are surrounded by a capsule of flat epithelioid cells which form a complete investment for the nerve 150 NERVOUS TISSUES FIG. 166. SECTION THROUGH THE DORSAL ROOT GANGLION OF THE FIRST THORACIC NERVE OF A CAT. The ganglion cells contain a large vesicu- lar nucleus, with nucleolus, and are en- veloped by a nucleated capsule. Several medullated fibers appear among the gan- glion cells. (From Barker, after Hodge.) frequently proximally convoluted and,, after branching in T-shape fashion, passes out of the ganglion to become the axis cylinder of a medullated nerve fiber,, and (2) cells with a slender axon which breaks up within the ganglion and whose terminal arborizations form a pericapsular plexus around the cell capsule; from this plexus fine end branches penetrate the capsule to form a pericellular arborization about the nerve cell itself. The cells of this lat- ter type suggest association neurons cell and its processes, being con- tinuous with the neurolemma. The capsule is not, however, as a rule, closely applied to the cell, but leaves a narrow inter- val which is occupied by lymph or 'tissue juice/ In their structure the gang- lionic neurons do not appear to differ in any way from other neurons. The large vesicular nucleus with its distinct nucleo- lus readily distinguishes these cells from those of neighboring tissues. The studies of the ganglion cells by Dogiel, Eanvier, and Cajal have done much to ex- plain the relations of these cells to each other, especially in the sympathetic system, where they were formerly but little under- stood. In the spinal ganglia Dogiel (Anat. Anz., 1896) de- scribed two types of ganglion cells: (1) a unipolar cell in which the axon is thick and FIG. 167. A NERVE CELL FROM A SECTION OF A HUMAN GAS- s E R i A N GANGLION. C, capsule. Nissl's stain. X 500. GANGLIA 151 within the ganglion. Nerve fibers from the sympathetic ganglia also enter the spinal ganglia and form pericellular arborizations about the cells of the second type. Dogiel found also that multipolar ganglion cells occur in the spinal ganglia of the adult as well as of the embryo. The more recent work of Cajal (1905), Dogiel (1908), and Ranson (1912) has re- vealed a third distinct type of cell formerly apparently included under Dogiel's Type I : smaller, pyriform, uni- polar, cells with non-medul- lated axon, rarely convoluted, dividing into a central and a peripheral branch, the ex- act terminations of which are unknown ; but having accord- ing to Ranson apparently the same distribution as the coarser medullated fibers of Type I, and probably affer- ent in nature (Jour. Comp. Neur., vol. 22, 1911). In cat and rat Ranson esti- mates the number of these cells at two-thirds that of the total number. These are the cells which contribute the bulk of the very numerous non-medullated fibers of the spinal nerves, only a small portion of which are believed to arise in sympathetic gang- type of ganglion cells. (After Dogiel.) lia. With the exception of relatively few cells of bi- and multipolar form, all of the nerve cells of the spinal ganglia are unipolar in the adult condition. In the case of the larger cells, the medullated aron before FIG. 168. SCHEMATIC REPRESENTATION OF THE RELATIONS OF THE STRUCTURES COMPOSING A SPINAL GANGLION. A and B, ventral and dorsal spinal nerve roots; Cj a spinal nerve; D and E, its ventral and dorsal divisions; F, its ramus communicans. a, nerve cells of the first type, whose neuraxes divide and form the axis cylinder of a peripheral and a central nerve fiber; 6, nerve cells of the second type, whose neuraxes, n, end in a felt work about the cells of the first type; s, sym- pathetic nerve fibers which terminate in a bas- ket work about the cell bodies of the second 152 NERVOUS TISSUES leaving the capsule is more or less extensively convoluted over the cell body forming in the extreme condition a so-called 'glomerulus.' These same cells are variously modified by the presence of short, coarse and fine intra- and extracapsular processes (both dendrons and collaterals) frequently terminating in 'end disks' (Huber). Such processes may fuse more or less extensively forming the 'fenestrated' variety of cells. The FIG. 169. COMMON ATYPICAL, THOUGH PROBABLY PERFECTLY NORMAL, NERVE CELLS FROM THE SPINAL GANGLION OF THE DOG. a and 6, cells with collaterals ending in 'end bulbs'; c, a multipolar cell; d and e, 'fenestrated' cells. (Ranson, Jour. Comp. Neur., 22, 2, 1912.) axon, prior to its division, may split at one or several points, for longer or shorter distances, into two or many portions, and reunite again into a single fiber; rarely also the ax on may have two or more points of origin, probably the result of fusions of collaterals with the cell body. These more complex atypical forms are said to predominate in man (Ranson, Jour. Comp. Neur., 24, 6, 1914). Ranson regards them to some extent at least as transient modifications, which may return to the simpler unipolar condition. Nageotte (1907) has suggested that THE SYMPATHETIC DIVISION OF THE NERVOUS SYSTEM 153 the phenomena of end disks and f enestrations signify regenerative activity. They are relatively more abundant in regenerating transplanted ganglia. But they are abundant also in pathological ganglia (Nageotte, 1906), and in fetal ganglia (Huber, 1913). No conclusive evidence has yet been presented that these modified forms signify functional derangement. THE SYMPATHETIC DIVISION OF THE NERVOUS SYSTEM The sympathetic division of the nervous system (autonomic system) consists essentially of three sets of ganglia: (1) the ganglionated cords (sympathetic trunks; vertebral ganglia); (2) the prevertebral plexuses; and (3) the visceral or terminal plexuses, including chiefly the my enteric and submucous plexuses of the alimentary canal. The ganglia of the ganglionated cords are segmentally arranged, and interconnected trans- versely (caudally) and longitudinally by plexuses of non-medullated fibers. They are connected also with the spinal nerves by the white and gray rami communicant es. Homologous ganglia in the head region, less definitely related to the cerebral nerves, are the ciliary, sphenopalatine, submaxillary, sublingual, parotid and otic ganglia. The prevertebral plexuses develop from non-segmentally arranged anlages which originate as outgrowths at certain levels of the early ganglionated cord. These plexuses contain fewer and smaller cells, with a preponderance of fibers, e.g., cardiac, celiac (semihmar; solar), hypogastric and pelvic plexuses. The myenteric and submucous plexuses are located in the muscle and submucous layers re- spectively of the esophagus, stomach and intestine. Here the cells are still smaller and less numerous than in the prevertebral plexuses, and the fiber- network is less dense. A plexus is a network of nerve fibers with few cells ; where the nerve cells are relatively abundant, the plexus is known as a ganglion. The embryonal cells (neuroblasts) which develop into sympa- thetic neurons have migrated from the neural crest, possibly in part also from the wall, of the primitive spinal cord. Langley employs the term 'autonomic nervous system' for -all that portion of the peripheral nervous system not included among the cerebro- spinal nerves, commonly designated as the 'sympathetic system/ This comprises four components: (1) the sympathetic proper, including the autonomic fibers arising from the thoracicolumbar regions of the spinal cord, together with the associated vertebral ganglia and their postgang- lionic neurons; (2) sacral autonomic, preganglionic fibers included in the roots of the second, third and fourth sacral nerves, together with the asso- ciated postganglionic neurons; (3) cranial autonomic, a group of fibers arising from the midbrain and the medulla (this component is separated from the sympathetic proper by the whole extent of the cervical region 154 NERVOUS TISSUES of the spinal cord, which region lacks white rami communicantes) ; (4) enteric, including the myenteric and submucous plexuses of the digestive tube. Langley proposes also the term 'parasympathetic' to designate the sacral and cranial autonomic fibers, since in many parts of the body they overlap the distribution of the sympathetic proper. In the sympathetic (or autonomic) ganglia Dogiel (Anat. Anz., 1896) likewise recognized two cell types, in general smaller than those of the FIG. 170. SYMPATHETIC NEURONS. A, in myenteric plexus, ileum of cat; B and C, in myenteric plexus, ileum of dog; D, E, F, in submucous plexus, ileum of dog; a, axon. A corresponds to Dogiel' s Type I, a motor neuron; B and C correspond to Dogiel's Type II, probably sensory neurons. (After Kuntz, Jour. Comp. Neur., 23, 3, 1913.) spinal ganglia: (1) small multipolar fusiform or stellate nerve cells with 5 to 20 dendrons and an axon which enters the nerve trunks as a non- medullated fiber, but may later acquire a thin medullary sheath motor neurons; (2) larger spheroidal nerve cells with 1 to 16 dendrons and a THE SYMPATHETIC DIVISION OF THE NERVOUS SYSTEM 155 single axon which also enters the nerve trunk as a non-medullated nerve fiber, but may later acquire a very thin medullary sheath, perhaps^ sen- sory neurons. The dendrons of the second type are distinguished from those of the first by being very long and slender and also by entering the nerve trunks, to pass, presumably, to neighboring ganglia. The dendrons of the first cell type on the other hand, are shorter, thicker, and end in relation with other cells within the same ganglion. Carpen- ter and Conel report also intermediate types in the cat. In certain rodents (rabbit and guinea pig) many of the neurons of the vertebral and pre vertebral autonomic ganglia are bi-nucleate (Car- penter and Conel, Jour. Comp. Neur., 24, 3, 1914). The ganglionic cell group is excentrically placed as regards the axis of the nerve trunk, some funiculi apparently passing the ganglion with- out being in any way connected with its nerve cells. The sympathetic differ from the cerebrospinal ganglia chiefly in the relative preponderance of non-medullated nerve fibers in the former and of the medullated type in the latter. Just as the cerebrospinal ganglia receive a few non-medullated sympathetic fibers, so also the sympathetic ganglia receive, through the medium of the white rami communicant es, a certain number of medullated nerve fibers from the cerebrospinal sys- tem. Moreover, with the intense staining method of Weigert, very thin medullary sheaths may now be demonstrated where formerly they were not suspected. The sensory and motor neurons of the cerebrospinal division show characteristic differences in their chromophilic substance. In the cere- bral and spinal ganglia the cell bodies of the sensory neurons contain fine Nissl granules evenly distributed throughout the cytoplasm. The motor cell bodies from the spinal cord contain fewer and much larger chromophilic flakes. The sympathetic neurons likewise present a charac- teristic and constant appearance : the chromophilic granules are inter- mediate in size and generally massed toward the periphery (Malone; Carpenter and Conel). The ganglia are supplied with blood vessels and lymphatic vessels in a manner similar to the nerve trunks in whose course they occur. The earlier conception of the nervous system interpreted the nerve fiber (axon) as the fusion product of a chain of cells extending from its proximal to its distal end. The axis cylinder fibrils were regarded as differentiation products of the cytoplasm (Schwann; Apathy; et al.). The view which now prevails interprets the axon as the outgrowth of the cell body to which it is attached (His; Cajal; et al.). The tissue culture 156 NEBVOUS TISSUES v,!/ FIG. 171. THE SPROUTING OF AN AXON BY A NEU- ROBLAST FROM THE SPI N A L CORD OF A FROG EMBRYO. From a live spec- imen grown in lymph; the cell body is filled with yolk granules; the protoplasmic proc- ess (axon) is of hyaline appearance and undergoes ame- b o i d movements. (Harrison.) work of Harrison and others has established the out- growth view upon a firm basis of observational data. By growing small pieces of the embryonic spinal cord of frogs in lymph, Harrison could observe the cells sprouting an axon process (Figs. 171 and 172). He describes the beginning of a nerve fiber as an outflow of hyaline protoplasm from cells which were situated within the central nervous system. The experiments of Harrison upon frog larvae demonstrate further that the sheath cells of the neurolemma of motor and sensory fibers have their origin in the ganglionic crest, there- fore ectodermal, and that they are unessential to the formation of the fibrils of the axis cylinder. He ex- cised the dorsal half of the cord, including the neural crest, and observed that in such larvae the fibers of the motor roots did not acquire sheath cells. On the con- trary, when he excised the ventral half of the cord, dorsal root fibers developed normally with a neuro- lemma, but the sheath cells which migrated to the lo- cation where the ventral fibers normally appear were unable to produce these fibers in the absence of neuro- blasts in the ven- tral half of the cord. (Anat.Rec., 2, 9, 1908.) The influence which guides the nerve along its proper path is apparently exerted by the tissue which is to be innervated. The essential factors comprising this influence are obscure; they may be of a chemotropic nature. It must be emphasized, however, that the con- nection between a particular nerve and its tissue terminal is made rela- tively early, that is, while the two elements are still spatially relatively closely associated. Probably mechan- ical stimuli, inducing thigmotropic reactions, also play an important role in determining the path of a nerve FIG. 172. THE SPROUTING OF AN AXON BY A NEUROBLAST FROM THE SPINAL CORD OF A FROG EMBRYO. Two views of the same nerve fiber, grown in lymph, taken twenty-five minutes apart. (Harrison.) NEURONE THEORY 157 fiber. The earlier relations are of course modified during growth; the definitive relation between nerve and end-organ are acquired by mutual adjustment. Recently Harrison has contributed further experimental -evi- dence in support of the view that the growing axon is guided through a stereotropic response (Jour. Exp. Zool., 17, 4, 1914). By cultivating sympathetic neurons from pieces of the intestine of the embryo chick in saline solutions, W. H. and Margaret R. Lewis (Anat. Rec., 6, 1, 1912) have been able to demonstrate that here also the fibers arise as outgrowths of nerve cells. NEURONE THEORY The work of Harrison, the Lewises and many others, including both experimental and morphological investigations, leave scarcely any fur- ther doubt of the accuracy of the Neurone Theory of Waldeyer (1891), which simply applies the Cell Theory of Schleiden and Schwann (1838- 39) to the nervous system. It holds that the unit of structure is the neuron (neurocyte), consisting of cell-body (cyton) and processes, in- cluding one axon, and one or several dendrons. The nervous system consists therefore of innumerable associated neurons. Neurons arise each from a single embryonic cell, the neuroblast, retain their independ- ence throughout life, and make connection with each other in general only by contact, which, however, is sufficiently intimate to insure func- tional continuity. A neuron exhibits a structural and functional po- larity; and constitutes a trophic unit for the maintenance of whose metabolic activity a nucleus is necessary. Further confirmation of the outgrowth interpretation as opposed to that of autogenesis of the axon has recently been furnished by the experi- ments of Clark (Jour. Comp. Neur., 24, 1, 1914) on the domestic fowl. By prolonged exclusive feeding of polished rice he induced degeneration in the peripheral medullated nerves. On return to an adequately nutritive diet regeneration, accompanied by a return to normal locomotion and func- tion, followed. The material thus gave opportunity for a microscopic study of the steps in the nerve degeneration and regeneration. When the degen- erative process had not been excessively prolonged a new axis cylinder grew down the old medullary sheath, which returned to normal; when greatly prolonged the myelin disappeared and the nuclei of the neurolemma multi- plied, giving an appearance very similar to that of embryonic nerve fibers ('bandfasern' stage). Clark concludes that the function of these excessive sheath cells is the removal of the degenerating myelin, a new medullary 158 NEEVOUS TISSUES sheath being supplied probably by joint influence of the new axis cylinder and the neurolemma cells. Mitochondria of granular and rod forms are abundant in the neuro- blasts. Meves, Duesberg and others have claimed that these differentiate into neurofibrils. The recent work of Cowdry (Amer. Jour. Anat., 15, 4, 1914) on chick embryos proves the untenability of this view. Cowdry shows that the neurofibrils arise as a differentiation of the ground substance of the neuroblast ; and that mitochondria persist in undiminished numbers throughout the period of neurofibril-development. Moreover, it is now known that mitochondria are present also in adult neurons. They are ap- parently fundamental cytoplasmic constituents of a metabolically active cell. Spinal ganglion cells of certain adult mammals (cat and rabbit) have been kept alive in tissue cultures for as long as twenty days (Minea, Anat. Anz., 46, 20, 1914). The cells remain apparently normal, augment their amount of chromophilic substance, produce new neurofibrils, develop short processes with end-plates (neuropodia) and become fenestrated, but do not proliferate. CHAPTER VI PERIPHERAL NERVE TERMINATIONS: END ORGANS All peripheral nerve fibers end either as terminal fibrils or in rela- tion to a highly specialized end organ. The function of these latter bodies is apparently included in the changing of ordinary stimuli mechanical, thermal, chemical, etc. into a nerve impulse, or, vice versa, the changing of a nerve impulse to a cell stimulus which results in motion, secretion, etc., according to the nature of the tissue cells which are thus stimulated. Some of the nerve end organs are connected with efferent (motor) fibers, others with afferent (sensory) fibers. Nerve endings are found in nearly all the tissues of the body with the exception of cartilage and the calcareous tissue of the bones. NERVE ENDINGS IN EPITHELIUM Intra-epithelial nerve fibrils are derived from the nerve fiber plexuses in the subjacent connective tissue; the epithelium usually receives a very abundant nerve supply. The following types of intra-epithelial nerve endings have to be considered. 1. End Fibrils. This form of nerve termination has been demon- strated in all the varieties of epithelium. Terminal nerve fibers enter the epithelial tissue as naked fibrils, often somewhat varicose, which form a delicate plexus between the epithelial cells. The terminal fibrils of this plexus frequently end in minute knoblike enlargements which are in contact with the surface, but rarely, if ever, penetrate the interior of the epithelial cells. The 'trefoil plates of Bethe represent unusually large end knobs. 2. Tactile Cells (Merkel). These are modified epithelial cells, with clear cytoplasm and a slightly vesicular nucleus, which are found in the deeper layers of the stratified epithelium of the epidermis and in the root sheaths of hairs. These cells are recognized by their vesicu- 159 160 PERIPHERAL NEKVE TEEMINATIONS : END OEGANS .FiG. 173. NERVE ENDINGS IN THE EPI- THELIUM OF THE LARYNX. On the left a taste bud; on the right, nerve endings in the stratified epithelium are rep- resented. (After Retzius.) lar character and by the fact that they occur most abundantly in the interpapillary portions of the epidermis. The deeper surface of the tactile cell rests in a cuplike ex- pansion of a terminal nerve fibril which is known as the tac- tile meniscus. 3. Neuro-epithelium. The cells of some types of neuro- epithelium, e.g., the olfactory cells, are true nerve cells ; others are modified epithelial cells, in relation to which the nerves ter- minate by intercellular end fi- brils. The neuro-epithelium of the eye and the ear will be de- scribed in the chapters devoted to these organs, that of the gustatory organ forms typical nerve end organs, the taste buds. TASTE BUDS (Gustatory Organ}. These end organs appear to be concerned with the special sense of taste. They occur in the stratified epithelium of the base of the tongue, uvula, soft palate, and epiglottis. Disse has also found similar structures in the nasal mucous membrane. They are most abundant on the lateral surfaces of the cir- cumvallate papillae of the tongue and on the walls of the sulci in the foliate pa- pillae which are most highly developed in the rabbit. They are occasionally found on the fungiform papillae of the tongue, where they occur in considerable numbers in fetal life but mostly disappear be- fore birth, and in the lateral walls of the sulci about the circumvallate papillae. Taste buds are ovoid, ellipsoidal, or spheroidal masses which occupy almost the entire depth of the epithelial layer. Their broad base rests FIG. 174. TACTILE CELLS IN THE EPITHELIUM OF THE GROIN OF A GUINEA-PIG. a, tactile cell; c, epithelial cell; m, tactile men- iscus, at the end of a nerve fibril; n, nerve fiber. Chlorid of gold. Highly magnified. (After Ranvier.) NERVE ENDINGS IN EPITHELIUM 161 Cone. Supporting cett. N 'euro-epithelial cell. -Rod cell. upon the basement membrane, their narrower apex extends nearly to the surface of the epithelium. The apex of the bud is thus covered by the superficial squamous epithelial cells except for a narrow tubular opening which overlies the superficial pole of the end organ. This canal presents an external and an internal ostium, respectively desig- nated the outer and inner taste pore. The inner taste pore leads into a goblet-shaped depression in the apex of the taste bud, into which the cuticular processes of the gustatory cells project. Composite buds with two and three pores are common in the foliate papillae of the rabbit; Heidenhain (Anat. Anz., 45, 16, 1914) re- ports also buds with four, five and six pores. The taste buds con- sist essentially of two varieties of cells, the gustatory and the sus- tentacular. The latter include the broad outer sustentacular or teg- mental Cells at the SUr- "EE^^^^^^Z^^J^/. -Nerve fibrils. face of the bud, the in- ner sustentacular cells within, and the basal cells which lie near the basement membrane. The Gustatory Cells. The gustatory cells are slender neuro-epithe- lial structures whose nucleus causes a fusiform enlargement near their center or toward the basal end. Their cytoplasm is finely granular; their nucleus stains deeply and is ovoid or rod-shaped. The distal end of the cell carries a delicate, highly refractive cuticular process which projects beyond the apices of the sustentacular cells and as far as the inner taste pore. Their proximal end is often bifid, forked, or so flat- tened as to form a footlike extremity which is connected with the basal cells by fine processes. Sapid substances in solution enter the pore and stimulate the taste cells through the hair processes. Sustentacular Cells. The outer and inner sustentacular cells are elongated epithelioid cells, having an ovoid or spheroidal vesicular FIG. 175. SCHEMATIC REPRESENTATION OF A TASTE BUD. (After Hermann, from Bohm and von Davidoff.) 162 PERIPHERAL NERVE TERMINATIONS: END ORGANS nucleus which causes no bulging of the protoplasm, and a coarsely retic- ular and frequently vacuolated cytoplasm. The distal ends of the cells taper to blunt points which collectively form the lateral wall of a goblet-shaped cavity at the apex of the taste bud. The proximal end \v pa.. CI ^*" Gz Bz Bz FIG. 176. TASTE BUD FROM THE HUMAN TONGUE. A, in longitudinal section; B, transection through the deeper third; C, transection through the base. Bz, basal cells; Bz, extra-bulbar cells; Gz, gustatory cell; L, leuko- cytes, in A one of these is seen in the pore; Pg, perigemmal space; Sg, subgemmal spaces; Sp, connective tissue of the tunica propria; Sz, sustentacular cells; x, cells of the adjacent epithelium. (After Graberg.) is broad, often blunt or serrated, and, like the gustatory cells, it is con- nected with the basal cells by protoplasmic processes. The Basal Cells. The basal cells are flattened bodies with small ovoid vesicular nuclei and a relatively small amount of cytoplasm which is prolonged into numerous processes that appear to be continuous with the sustentacular and gustatory cells. These cells have been considered as having a similar function to the sustentacular cells. NERVE ENDINGS IN CONNECTIVE TISSUE 163 The Fibers. The nerve fibrils of the taste buds are derived from a sub-epithelial plexus which distributes terminal fibrils to the gustatory and sustentacular cells, intragemmal fibers, and to the intervening portions of the stratified epithelium, inter gemmal fibers, where they terminate in end fibrils. Von Lenhossek (Anat. Anz., 1892) states that the intragemmal and intergemmal fibers are never derived from the same nerve fiber. Circumgemmal fibers, distributed as varicose fibrils to the surface of the taste bud, may, however, arise from the same nerve fiber as the intragemmal branches. Those nerve fibers which enter the taste buds form fine varicose fibrils which are closely applied to, but are not continuous with, the gustatory and the sustentacular cells. The terminal twigs of these fibrils end by minute end knobs which are scarcely distinguishable from the varicosities (Fig. 175). NERVE ENDINGS IN CONNECTIVE TISSUE The nerve fibers form extensive plexuses in the connective tissues from which terminal branches are distributed to the epithelium (free sensory endings), the walls of the blood and lymphatic vessels (sympathetic vasomotor endings), and to the numerous sensory end organs (encapsulated endings) which occur in abundance in most of the connective tis- sues. Nerves also terminate in connective tissue by free end fibrils some of which, as in the epithelial tissues, possess minute end knobs. Free nerve endings of this nature occur in the tendons, the lungs, the stom- achal and intestinal mucous membranes, the meninges, and in the superficial layer of the corium of the skin and the hair follicles. The following types of nerve end organs are found in connective tissue : 1. Tactile Corpuscles (Touch Cor- puscles of Meissner). These organs are formed by the terminal expansion of a nerve fiber, which forms a varicose plexus inclosed within a delicate con- nective tissue sheath. The nerve fiber, or its primary branches, prior to FIG. 177. TACTILE CORPUSCLE OF MEISSNER FROM THE SKIN OF THE HUMAN TOE. Bl, blood-vessel; N, medul- lated nerve fiber. Highly magnified. (After Schieffer- decker.) 164 PEEIPHERAL NERVE TERMINATIONS: END ORGANS its ultimate division makes several spiral turns about the corpuscle. The course of the nerve fiber gives the corpuscle a peculiar spirally striated appearance. Within the corpuscle the nerve fiber breaks into a plexus of varicose fibrils, many of which end in knobbed extremities. The cor- FIG. 178. TACTILE CORPUSCLE MEISSNER. OF 6, epithelioM cells; c, nerve endings; e, connective tissue capsule. (Maxi- mow, after Van de Velde.) FIG. 179. TACTILE CORPUSCLE OF MEISSNER. a, nerve fibrils which enter the corpuscle and supply its nerve skein. Methylene blue. Very highly magnified. (After Dogiel.) puscles also contain many flattened or cuneiform epithelioid cells which are interspersed among the nerve fibrils. Tactile corpuscles occur in largest numbers in the cutaneous papilla of the finger tips, where there may be as many as twenty to the square millimeter. They are found in considerable abundance also in other highly sensitive regions, including especially the corium of the toe tips, the lips, nipple, conjunctiva, glans penis and clitoris. The cutaneous senses comprehend four different qualities of sensation : pressure, warmth, cold and pain. These are mediated 'by two distinct groups of sensory fibers ending in the skin: the one conveys the impulses for pain and extremes of temperature (protopathic sensibility), the other for light pressure and small changes of temperature (epicritic sensibility). The NEKVE ENDINGS IN CONNECTIVE TISSUE 165 it FIG. 180. RUFFINI'S END ORGAN. A single nerve fiber breaks up to form the tangle of nerve fibrils within the organ. gH, medullary sheath; il, terminal fibrils of the axis cylinder; L, connective tissue capsule. (Af- ter Ruffini.) ally at its end. Now and tributed to several of these various subcutaneous endings mediate sub- cutaneous sensibility to pressure and move- ment. 2. Ruffini 's End Organs. These bodies, also known as terminal cylinders, resemble the tactile corpuscles in structure but possess a definite, though thin, connec- tive tissue sheath within which the ter- minal arborization of the nerve fiber is embedded in a granular core. They occur FIG. 181. END BULB OF KRAUSE FROM THE MAR- GIN OF THE OCULAR CONJUNCTIVA. The axon forms a dense skein within the en- capsulated bulb. Methylene blue. Highly mag- nified. (After Dogiel.) in the deeper part of the true skin near its junction with the subcutaneous tissue and in the connective tissue septa of the latter, whereas the tactile corpuscles are found in the papillary layer of the skin. Ruffini (Arch. ital. de biol., 1894) states that they occur in large numbers in the skin of the finger tips, where they rival in number the rather more deeply placed Pacinian cor- puscles. The Ruffini organs are cylindrical in shape and their nerve fibers usually enter at the side of the organ, though occasion- then a single branching nerve fiber is dis- end organs. 166 PERIPHERAL NERVE TERMINATIONS: END ORGANS 3. End Bulbs (Krause). These structures (bulbous cor\ together with those which follow, form the true so-called encapsulated nerve end organs. In the end bulbs the nerve forms a terminal arboriza- FIG. 182. GENITAL CORPUSCLES FROM THE CLITORIS OF A RABBIT. A single axon from the nerve plexus enters each corpuscle. Methylene blue. Highly magnified. (After Retzius.) tion of the varicose and knobbed fibrils which freely anastomose (Dogiel, Euffini). The bulb is invested with a distinct connective tissue capsule. On entering the bulb the nerve fiber loses its sheaths and the perineu- rium, now represented by Henle's sheath, becomes continuous with the FIG. 183. A LAMELLAR CORPUSCLE FROM THE MESENTERY OF A CAT. A, a nearly axial section; B, a transection. Hematein and orange G. X 410. capsule of the bulb. Within the capsule the nerve fibrils are embedded in a granular inner bulb. The end bulbs vary much in both size and shape. They may be either spheroidal, ovoid, twisted or convoluted, branched or compound, or cylindroid. They are abundantly found in the conjunctiva, but also NERVE ENDINGS IN CONNECTIVE TISSUE 167 occur in the corium of the skin. Similar, though more highly developed, end bulbs form the so-called genital corpuscles which are found in con- siderable numbers in the connective tissue of the glans penis, prepuce, and clitoris. In some of the smaller (cylindrical) end bulbs found in the tendons, the mucous membranes, and in certain portions of the skin, \ FIG. 184. A LAMELLAR CORPUSCLE FROM FIG. 185. LAMELLAR CORPUSCLE FROM THE PLEURA OF A CHILD. THE MESENTERY OF A KITTEN. a, lamellae; 6, nerve fiber; c, nerve. Methylene blue. Moderately magnified. (After Dogiel.) The nerve fiber shows lateral proc- esses, many of which are knobbed. Methylene blue. Moderately magni- fied. (After Sala.) the nerve fiber fails to divide but ends near the distal extremity of the bulb in a small fusiform end knob. 4. Lamellar Corpuscles (Pacinian Corpuscles, Vater's Corpuscles, Vater-Pacinian Corpuscles). These are among the largest of the nerve end organs. In the mesentery of the cat they are of macroscopic size, varying in length from two to three millimeters. They assume the form of a large ovoid thickening, placed upon the end of a nerve fiber. The 12 168 PERIPHERAL NERVE TERMINATIONS: END ORGANS Pacinian corpuscle consists of a thick lamellated connective tissue coat, and a central granular protoplasmic core which is pierced by the nerve fiber. The medullated nerve fiber enters the axis of the corpuscle, its Henle's sheath becoming continuous with the superficial capsule of con- FIG. 186. A LAMELLAR CORPUSCLE IN FIG. 187. AXIAL SECTION OF A COR- LONGITUDINAL SECTION, SHOWING A NETWORK OF SPIRAL ELASTIC FIBERS. Weigert's elastic tissue stain. Highly magnified. (After Sala.) PUSCLE OF HERBST FROM A DUCK'S TONGUE. a, medullated nerve fiber; b, naked axial nerve fiber with a bulbous end; c, nuclei of the core; d, inner concentric capsule; e, nuclei of the outer lamellated capsule. X 380. (After Sobotta.) nective tissue. The nerve fiber on entering the core loses its medullary sheath, and after traversing a greater or less portion of the core divides into two to five branches which end near the distal pole in a disk-like expansion. In its course through the core, the nerve fiber gives off fine lateral twigs (Sala, Eetzius). The connective tissue sheath consists of a granular protoplasm which NEEVE ENDINGS IN CONNECTIVE TISSUE 169 is permeated by densely felted spiral fibers (Sala) and is divided into ten to fifty concentric lamellae by lines of flattened connective tissue cells and fibers. According to Schwalbe, however, these cells form an endothelioid coat on either surface of each lamella. Lamellar corpuscles are occasionally compound,, two or more adjacent corpuscles being sup- plied by branches of the same nerve fiber. Lamellar corpuscles are found in large numbers in the subcutaneous tissue of the finger tips and of the penis, as well as in the skin of other parts, in the mesentery and the connective tissue of neighboring organs, e.g., the pancreas, in the prevertebral connective tissue of the abdomen and mediastinum, near the walls of the large blood-vessels, in the serous membranes, pleura, peri- cardium, peritoneum in the periarticular connective tissue and periosteum, in the sheaths of the larger nerve trunks, and in the connective tissue of the thyroid gland and of the skeletal muscles. 5. The Corpuscles of Herbst (Key- Retzius Corpuscles). The corpuscles of Herbst are similar in structure to the lamellar corpuscles except that the core which surrounds the axial nerve fiber con- tains cuboidal tactile cells. They occur only in the cere of aquatic birds. 6. The Corpuscles of Grandry (Merle el's Corpuscles) . The corpuscles of Grandry, also found only in aquatic birds, contain several tactile cells of ecto- blastic origin similar to those found in the epidermis. Each cell is in relation with a ring or meniscus formed by the expanded end of a nerve fiber. The whole is included within ^a thin connective tissue capsule and may be regarded as a compound tactile cell occurring in connective tissue. 7. The Golgi-Mazzoni Corpuscles. The Golgi-Mazzoni corpuscles described by Euffini (Arch. ital. de biol., 1894) somewhat resemble the Pacinian corpuscles in that they possess a lamellar, though relatively very thin, connective tissue sheath and a central granular core. The core, however, is relatively excessive in size, and the entering nerve fiber FIG. 188. A PAPILLA OF THE DUCK'S TONGUE, CONTAINING A CORPUSCLE OF GRANDRY. The corpuscle contains four large cells, between which are the tactile menisci of the nerve end- ing, n, nerve. Highly magnified. (After Merkel, from Kolliker.) 170 PERIPHERAL NERVE TERMINATIONS: END ORGANS breaks into a number of branches with discoid terminal expansions simi- lar to those found in the nerve endings of Golgi in the tendons. They Ik , ; v '..'* FJG. 189. GOLGJ-MAZZONI CORPUSCLES FROM THE SUBCUTANEOUS TISSUE OF THE TIP OF THE FINGER. (After Ruffini.) occur also in the corium of the skin in certain regions (e.g., the external genitalia, finger tips), and in the conjunctiva and the periosteum. NERVE ENDINGS IN MUSCLE AND TENDON 171 NERVE ENDINGS IN MUSCLE AND TENDON A. VOLUNTARY STRIPED MUSCLE 1. Motor End Plates. These organs form the intramuscular end- ings of motor axons whose cell bodies are found in the ventral horns of the gray matter of the spinal cord. The efferent fibers reach the muscle through the many cerebrospinal nerve trunks. On entering the muscle FIG. 190. MOTOR NERVE ENDINGS IN STRIATED MUSCLE. A, from a lizard; B, from the guinea-pig; C, from the hedgehog. A and C are sur- face views; in B the end plate is seen in profile, a, muscle fiber; 6, nerve fiber; c, nerve ending in the form of a 'brush'; d, the sole plate; e, sarcolemma. A, X 160; B, X 700; C, X 1200. (After Bohm and von Davidoff.) these nerves form a plexus in the perimysium from which nerve fibers are distributed within the muscle bundles. Here they form an abundant plexus of branching nerve fibers within the endomysium, the ultimate branches being of sufficient number to supply one or more terminal nerve fibers to each muscle fiber. 172 PERIPHERAL NERVE TERMINATIONS: END ORGANS At the surface of the muscle fiber the nerve fiber loses its medullary sheath, its neurolemma becomes continuous with the sarcolcmma of the muscle cell, and its naked axis cylinder divides into two to five branches, which end, often after repeated subdivision, in flattened terminal disks, distributed in mammals over a limited, in amphibians over a broad area, but which never completely encircle the cylindrical muscle cell. The terminal expansions of the axon rest upon a granular, slightly raised sole plate which contains many ovoid muscle nuclei, the sole nuclei. 2. Muscle Spindles (Neuromus- cular Spindles, N euromuscular End Organs). These are sensory nerve endings which are concerned witli the so-called muscle sense. They are especially numerous in the extrinsic muscles of the tongue, in the small muscles of the hand and foot, and in L 1^' ^"f** the intercostal muscles (Huber, Amer. Jour, of Anat., 1902). They have 5L -J not been found in the muscles of the diaphragm. A detailed description of the developing neuromuscular spin- dle in the extrinsic eye muscles of the pig has recently been given by Sutton (Am. Jour. Anat., 18, 1, 1915). He describes a coarsely granular 'plaque,' different from both muscle and nerve, which he inclines to regard as an 'intermediary structure/ perhaps a receptor substance analogous to the sole plate of motor endings. A muscle spindle contains from five to twenty striated muscle fibers of small size, and an almost equal number of nerve fibers. The whole is inclosed within a connective tissue capsule of considerable thickness. The bundle of intrafusal muscle fibers is again surrounded by a delicate axial sheath of connective tissue which is united to the capsule by bands of fine fibrous tissue which span the broad periaxial lymphatic space. The larger of these fibrous bands support the nerve fibers, on their way to the intrafusal muscle cells, together with several small blood-vessels. The muscle spindles form long fusiform bodies (from 1 to 5 milli- meters in length) whose muscle fibers at the pole of the spindle may be FIG. 191. A MUSCLE SPINDLE FROM THE PSOAS MAGNUS OF MAN. 1, intrafusal muscle fibers; 2, nerve fibers; 3, axial sheath; 4, connective tissue capsule; 5, muscle fibers of an adjacent fasciculus; 6, peri-axial lymphatic spaces; 7, blood-vessel. Hematein and eosin. X 470. NKKVK ENDINGS IN ANP TKNPON 173 connected with the tendon, or they may join other muscle fiber bundles. The muscle spindles are usually found in the fibrous septa of the peri- invsium. Compared with the adjacent muscle fibers, the intrafu- have a smaller diameter, are less distinctly but more coarsely striped, and contain some centrally located nuclei. Either one or several nerve minks enter the spindle, usually near its equator rather than at its poles. The nerve tibers branch repeatedly , I'u;. U>_. Munn.K THIKD OK v TKKMINAL PLAQVK IN THK. MISCI.K SPINDLE OF AN Am i.r t/ .1. rings; b\ dendritic branchings: S t spirals. Chlorid of gold preparation. Highly magnified. v Aft or Uiimiii.) ill the intracapsular coniuvtivo tissue, and tinally pierce the axial sheath as naked processes which form a rich arborization of terminal fibrils about the intrat'usal muscle tlbers. RutVmi distinguishes three types of terminal nerve fibrils: ( I) annulur. which form rings around the muscle fibers; ^M N/ /><*/. which are spirally twisted about the intrat'usal fibers: and (X) > 1 ' IP omewhat thic mains consta vessels). O large relative a PH 02 J hH i y I 1 ?1 A3 11 i 1 2 S J3 | 1^ i || ll g "d CO ll rrt ! o Q H fll f'S VM ftO & g 02 ^ 02 'o 'E Jy p ^ d O EH |1 1 -tj W 'a ^ ^ co.b "rt o < d . CQ Q S 1 Q "C ^ *"O o3 K/ 1 g > ^ h-3 03 H -, C5 ^"^ 11 H ^ IS *i gg | ||p| 1| || J^^g si I ( o3 d > HH ^ w ^ HI till g.2.d^ III . - w s f^^l d c3 111 ^" O 02 i jf . . .5 II 1 1 MEDIA (ELASTIC TISSUE AND SMOOTH MUSCLE). Large amount. O ^ ^ ^_^ rtji 1 - 1 if i ii H 4 * 1 ij | 1 i a ^ j/2 OT'^.'B'yT I ' J H 1 g <5 > pazis-uinipaj\[ HEART 195 HEART The wall of the heart consists of interlacing bundles of cardiac mus- cle fibers, the myocardium, which are covered externally by the epicar- dium, a serous membrane which forms the visceral layer of the peri- cardium. Internally the muscular wall of the heart is lined by the endocardium, wjiich resembles the serous membranes in that it consists of pavement epithelium supported upon a layer of fibro-elastic connective tissue. The endocardium lines all the cavities of the heart, and its endo- thelium is directly continuous with that of those arteries and veins which are connected with the cavity of the heart. Thus the entire vascular system heart, arteries, capillaries, lymphatics, and veins may be said to be lined by an uninterrupted sheet of pavement epithelial cells, the endothelium. Myocardium. The muscle cells of the myocardium are so disposed as to form long fibrous bundles which by their figure-of-8 arrangement are interwoven with one another to form a dense interlacing mass of mus- cle bundles. (For detailed description see Mall, Amer. Jour. Anat., 11, 3, 1911.) The structure of these cardiac muscle fibers has already been described. Because of the irregularity of their disposition, transections of the cardiac wall present sections of muscle fibers which have been cut in every conceivable direction. Between the muscle fibers is a very delicate framework of fibrous connective tissue, the endomysium, which surrounds the muscle fibers and supports the abundant capillaries, arterioles, and venules, with which they are supplied. The proportion of connective tissue in the normal myocardium as compared with the muscle is, nevertheless, very small. In certain portions of the myocardium connective tissue is more abun- dant. Thus it is slightly increased in the vicinity of the endocardium, in the papillary muscles, and near the bases of the cardiac valves. At the surface of the heart, beneath the epicardium, especially in the various grooves on the surface of the heart, the connective tissue is still more abundant, and may contain groups of fat cells. It is through these accumulations of connective tissue that the larger blood-vessels are dis- tributed to the myocardium. Epicardium. The epicardium, like the other serous membranes, consists of a layer of pavement cells, so joined edge to edge as to form a complete mesothelial coat. Here and there the mesothelium presents 196 THE BLOOD VASCULAK SYSTEM small openings at the angles between its cells; these stomata are sur- rounded by minute, finely granular cells and are perhaps connected with the lymphatic vessels. The mesothelium of the epicardium is supported upon a thin layer of dense areolar tissue in which are many small blood-vessels and lymphatics. Fibers from the deeper surface of this layer are prolonged into the myocardium to become continuous with its endomysial connec- tive tissue. The larger of these connective tissue trabeculae accompany the branches of the larger arteries and veins which are distributed to the muscular wall of the heart. Endocardium. The endocardium consists of a lining membrane of polygonal endothelial cells supported upon a thin layer of delicate fibrous connective tissue, of en- dothelial origin (Mall). In this membrane is a network of elas- tic fibers, and a small amount of smooth muscle. The en- dothelium of this membrane is continuous with that of those blood-vessels which open from 212 THE PARIETAL LAYER OF THE fhe cavit ies of the heart. Its PERICARDIUM OF A CHILD. connective tissue also forms a o, mesotnelmm; 6, connective tissue. Hematein and eosin. Photo. X 500. continuous layer with that of the tunica intima of these ves- sels; in fact, the three coats of the cardiac wall endocardium, myocar- dium, and epicardium might well be compared with the corresponding three coats of the arterial and venous walls the intima, media, and ad- ventitia. In either organ, the inner coat consists of a lining membrane of endothelium, and a supporting membrane of connective tissue ; muscle in large part composes the middle coat, while the outer coat is typically a connective tissue layer. Valves. At the cardiac orifices the entire thickness of the endocardium is folded upon itself to form a double layer, between the folds of which an intervening stratum of dense fibro-elastic tissue is inserted. These endocardial folds form the cardiac valves. The number and shape of their cusps are dependent upon the location. The semilunar valves of the aortic and pulmonary orifices consist of three crescentic endocardial folds; at the auriculo ventricular orifices the tricuspid valve consists of three, the bicuspid or mitral of two, folds. The margin of the valvular cusp or fold is extremely thin ; just within HEART 197 the margin, however, the central mass of dense fibrous tissue is somewhat thickened to form, in each cusp, a dense rim which during valvular closure secures the firm and accurate approximation of the free margins of adjacent cusps. At the apex of the valvular cusp, where the adjacent fibrous margins of the valve meet, the dense connective tissue, particu- larly in the semilunar valves, is considerably thickened to form a nodule, the corpus arantii. These corpora or noduli, in the aged, are frequently subject to calcareous infiltration. Muscular fibers are continued from the adjacent cardiac wall into the dense fibrous tissue at the base of the valve, except in the case of the semilunar valves of the pulmonary and systemic aortse. This muscle FIG. 213. THE ENDOCARDIUM. From the ventricular wall of the heart of man. Hematein and eosin. Photo. X 469. is generally non-striped, and probably functions as a sphincter. The base of the valve is also surrounded by a ring of fibrous tissue, the annulus fibrosus, whose interlacing bundles are so closely packed as to give them an almost cartilaginous feel. At the auriculoventricular orifices, these fibrous rings are continuous with the auriculoventricular septum, from which the muscle bands of the myocardium take their origin. Chordae Tendineae. These are firm, unyielding cords, composed of parallel bundles of dense collagenous fibers, with a few elastic fibers, and covered with a very thin endocardium continuous with that of the ven- tricular wall and cardiac valve. These fibrous bands unite the apices of the papillary muscles to the ventricular surfaces of the mitral and tri- cuspid valves. At the apex of the papillary muscle the fibrous bundles of the chordae intermingle with the muscle fibers, and are continued into the endomysial connective tissue, which is especially abundant in those 198 THE BLOOD VASCULAR SYSTEM portions of the myocardium. At their valvular attachment the fibrous bundles of the chordae tendineae turn almost at right angles, and spread out, in a somewhat radial manner, to become continuous with the dense fibrous tissue which forms the interior of the valve. FIG. 214. RADIAL SECTIONS OF THE MITRAL VALVE, FROM THE HEART OF A MAN. A, from the base of the valve showing the extension into it of cardiac muscle fibers from the wall of the heart; B, from the mid-region of the valve, a, auricular endo- cardium; b, muscle fibers; c, dense fibrous tissue; d, ventricular endocardium. Hema- tein and eosin. Photo. X 800. Columnae Carnese. The columnae carneae are columelliform projec- tions of the myocardium into the ventricular cavity. They consist of cardiac muscle fibers, largely of the Purkinje fiber variety, which are dis- posed in their long axis, and are covered by reflections and reduplications of the endocardium. The irregular contour of the ventricular cavities appears to be entirely due to the projecting columnae carneae. HEART 199 These muscular columns may present any one of three modes of attachment to the myocardium: (1) they may be attached along their entire extent; (2) they may be attached only at their two ends, the rnid- portion being free; (3) they may be attached to the myocardium at one end only, the other end projecting into the ventricular cavity as a papil- lary muscle, from whose apex chordae tendineae pass to the auriculoven- Main Bundle of His Pulmonary Veins Fossa Ovalis Inf. Cava Coronary Sinus Reticulum Artery to Bundle Tricuspid Valve Right Ventricle Chordae Tendinece Papillary Muscle Sup. Cava Aorta Pulmonary Artery ( Muscular Fibers Streaming to \ the Reticulum Right Auricle Pars Mem. Septi 5 Bifurcation of MainBundle into \ Right and Left Sept. Branches Right Septal Branch Moderator Band FIG. 215. HUMAN HEART OPENED FROM THE RIGHT TO SHOW THE ATRIOVEN- TRICULAR BUNDLE OF His. The illustration shows also a heart valve, the chordse tendineae, and the pap- illary muscles. (After Curran, Anat. Rec., 3, 12, 1909.) tricular valves. Either of the last two forms may, in transections of the ventricles, appear as isolated islands of muscular tissue surrounded by endocardium and lying apparently free within the cavity of the ventricle. Columnae carneae which span the ventricular cavity constitute moderator bands. One such band is frequently present in the right ventricle near the apex, and occasionally one appears in the left ventricle. Atrioventricular Bundle. The atrioventricular bundle of His was discovered in the human heart by His, Jr., in 1893. Previously in the same year it had been noted by Kent in the heart of a number of mam- mals. It has since been seen in every species of mammal investigated. 200 THE BLOOD VASCULAR SYSTEM It consists of a dense meshwork of cardiac muscle fibers rich in sarco- plasm, in the form of a band taking origin in scattered fibrils in the posterior wall of the right atrium near the septum in the atrioventricular groove (sinus region; hence, sino ventricular conducting system, Eetzer, 1908), and coursing forward in the interatrial septum into the upper anterior portion of the interventricular septum, where it divides into two limbs which branch profusely and spread out in a complicated system of terminal branches, the subendocardial Purkinje fibers. Macroscopically the bundle has a grayish appearance ; where it passes from the intera- trial to the interventricu- lar septum (pars mem- branacea septi) it ex- pands into the so-called node. In man the right limb is much smaller than the left. The bundle of the calf's heart has been re- constructed by De Witt (Anat. Eec., 3, 9, 1909). Curran (Anat. Eec., 3, 12, 1909) has described a constant bursa or lubri- cating mechanism in re- lation with the bundle, furnishing protection against friction during contraction of the heart. He describes "its connection with all parts of both auricles through three large trunks and a number of smaller twigs, and not, as was once thought, merely arising in the right auricle only." Tawara (1906) first carefully described the histology of this bun- dle in several mammals, including man. De Witt more recently (1909) has extended the study in this same field. "In the sheep and calf, where the fibers are most typical and most clearly differentiated from the myocardial fibers, the fibers are much larger than the myocardial fibers, with fewer fibrils and much more sarcoplasm." She describes the bundle as a muscular syncytium. "Connective tissue and especially elastic fibers FIG. 216. RECONSTRUCTION OF THE SINOVEN- TRICULAR SYSTEM (BUNDLE OF His) OF THE CALF'S HEART. (De Witt, Anat. Rec., 3, 9, 1909.) HEAET 201 are much more abundant than in the myocardium." The bundle con- tains abundant ganglion cells and nerve fibers. It is also very rich in glycogen. In the ox it is distinctly cellular (Fig. 112). It would seem, on the basis of its constancy of presence and structure, and its probably independent blood and nerve supply, that the atrioventric- ular bundle has a function independent of the myocardium, probably of the nature of a neuromuscular end-organ (Retzer; DeWitt), providing for the conduction of the impulse to contraction, and the coordination of the atrial and ventricular rhythm. A muscle bundle of closely similar structure intimately related with the vagus and sympathetic nerves, the f sino-atrial node/ has been described by Keith and Flack (1907) at the juncture of the sinus venosus and the atrium. It is believed to be the place of origin of the impulse to the heart beat, from which it is transmitted to the atrioventricular bundle. Laurens (Anat. Rec., 2, 8, 1913) has described an analogous muscular connection between auricles and ventricles in certain reptiles, where it assumes the form of an inverted funnel-shaped tube. Development of Heart. The anlage of the heart arises from the fusion of a pair of parallel endothelial tubes in the paramedial angioblast, each surrounded by primitive mesenchyma. The endothelium of the result- ing sac differentiates into the endocardium of the definitive heart, while the connective tissue and muscle develop from the mesenchyma in a man- ner essentially similar to that described for the blood-vessels. Blood-Vessels. The heart is supplied with blood through the coro- nary arteries. The larger branches of these vessels pursue their course beneath the epicardium in the superficial grooves of the cardiac wall. From these large arteries, smaller branches are distributed to the epi- cardium and to the muscular wall, the latter vessels penetrating as far as the endocardium, in whose connective tissue they form a meager capillary plexus. The capillaries of the myocardium are extremely abundant. They form elongated meshes between the muscle fibers, the circumference of each muscle fiber being in relation with several capillary vessels. The veins return the blood from these rich capillary plexuses and pursue a course similar to that of the arteries, the larger veins being always found in the broader connective tissue septa. In the right atrium certain small veins, the v ence minimce, empty directly into the cavity of the heart. The lymph supply is very abundant and intimate. The lymph ves- sels form two superficial plexuses, the endocardial and the epicardial, both draining into the larger lymph vessels at the base of the heart. 14 202 THE BLOOD VASCULAE SYSTEM Nerve Supply. The nerve supply of the vascular system is by means of fine branches from the cerebrospinal and sympathetic systems. In the heart these minute nerve trunks end in the various cardiac ganglia, most of which are found in the connective tissue of the heart, e.g., the coronal plexuses about the orifices of the aorta and pulmonary artery. From these ganglia sensory nerve fibers are distributed to the endocar- dium and epicardium, and motor fibers to the myocardium. The most of the former are connected with the vagus, the latter with the sympathetic trunks. Through both the vagus and the sympathetic trunks are dis- tributed also efferent cerebrospinal fibers : the accessory nerve through the vagus contributes 'inhibitory' fibers, the cervical spinal nerves through the inferior cervical ganglia contribute 'acceleratory' fibers. From the cardiac ganglia branches pass to form a coarse plexus in the connective tissue between the muscle bundles, the perimysial plexus, from the branches of which a fine plexus is distributed to the endo- mysium. The terminal branches end in relation with the surface of the muscle fibers. The pericardium contains numerous encapsulated nerve endings (corpuscles of Golgi and Mazzoni). According to Martynoff (Arch, mikr. Anat, vol. 84, 1914) unencapsulated endings also are present, of three types : coils, dendriform terminal ramifications, and modified den- driform endings. He describes also naked terminal filaments ending on the bases of the mesothelial covering cells. The blood-vessels are similarly supplied, minute ganglia occurring here and there in the adventitia or adjacent connective tissue. From these ganglia sensory branches are distributed to the adventitia and in- tima and motor branches to the tunica media. Naked nerve fibrils can be traced to the smallest blood-vessels, and even in the capillaries terminal fibrilla3 are found in relation with the endothelial wall. CHAPTER VIII BLOOD Blood may be regarded as a tissue in which the intercellular material is a fluid, the plasma. The plasma contains fibrin in solution (fibrino- gen) ; on exposure to air, as in case of cuts, the fibrin is precipitated in the form of delicate needle-like crystals (Fig. 217), leaving a clear straw- colored liquid, the serum. The active element in causing this precipita- tion is thrombin (or prothrombin), probably liberated by the blood- platelets on disintegration under the influence of air or the stimulus of a roughened area on the wall of the blood-vessels. The blood-cells become entangled in this fibrin net forming a clot or thrombus. Plasma is very similar to, but not identical with, the lymph of the lymph vessels. The formed elements, or blood-cells, are of two main classes, red and white, or erythroplastids (erythrocytes) and leukocytes. The red blood corpuscles or plastids owe their characteristic color to the presence of hemoglobin. Single corpuscles have a light yellowish-green color, in masses they appear red. The function of the hemoglobin is to carry oxygen in weak combination as oxyhemoglobin for transportation through the blood-vessels from the lungs to the tissues, where it is employed in the oxidation processes upon which life depends. THE RED BLOOD CELL The erythroplastid is a circular biconcave disk, of 7.5 microns (a micron is 1-1000 of a millimeter) diameter. They are present in prac- tically all histologic preparations, hence they serve well as a ready scale for approximate determination of size of cellular elements in the same microscopic field. Every section; blood preparation, as of a drop under a cover-slip; and even the blood-vessels of living mammals, con- tain also a larger or smaller number of cells of saucer shape (cup shape; 203 204 BLOOD bell shape, etc.). These are believed by some (Weidenreich, Lewis, Mi- not, and others) to be the more normal in shape; however, there remains FIG. 217. OXALATED PLASMA OF HUMAN BLOOD CLOTTED WITH THROMBIN, SHOWING FIBRIN NEEDLES. X 512. (After Howell, Amer. Jour. PhysioL, 35, 1, 1914.) little doubt that the circular biconcave disk shape is the normal (origi- nal), the saucer or cup shapes, the derived forms, the result of modifi- THE BED BLOOD CELL 205 cation by extrinsic factors. The erythroplastid is of course a very delicate structure, and slight tractions or tensions incident to passage through exiguous confines, or osmotic currents set up in fixing fluids, or the coagulation of its protoplasm, would produce modifications and distortions. Almost any conceivable mechanical modification would of FIG. 218. FROM A FRESHLY PREPARED, UNSTAINED SPECIMEN OF HUMAN BLOOD. Three leukocytes, an eosinophil, a polynuclear, and a lymphocyte, are represented. Many red blood corpuscles (erythroplastids), some on the flat, some in rouleaux and in profile, are also shown. X 1200, but reduced somewhat in reproduction. (After Schafer.) necessity change a circular biconcave disk into some sort of cup-shaped element. The red blood element lacks a nucleus, hence the propriety of in- sistence on the term 'plastid' or 'corpuscle' in preference to cell (cyte). The red elements of all mammals are non-nucleated. The lower forms have nucleated elements (erythrocytes), frequently of ellipsoidal form. Mammalian red elements of greatly divergent sizes (2.5 ft in musk ox; 9.4 ft in elephant ; dog, 7.5 ft ) are all of circular outline, except those of the camelidse (llama, camel, etc.) which are elliptical. The erythro- plastids are generally believed to be enclosed by a delicate membrane, and 206 BLOOD to contain a very delicate stroma (spongioplasm) and fluid matrix (hyal- oplasm, with hemoglobin) . The red corpuscles have a tendency when ex- posed, as in a drop mounted under a cover-slip, to arrange themselves in rows, like coins in a pile, concave surfaces apposed, forming rouleaux. Drop preparations show after a short time also an increasing number of plastids with puckered or spiny surfaces, crenated corpuscles. This condi- tion results from evaporation producing a medium of greater density than that of the normal blood plasma. Any me- dium of density equal to that of the plasma of any particular blood is spoken of as an isotonic solution for that blood. The solu- tion in most common use for human blood is a 0.9 per cent, solution of sodium chlorid in distilled water. Solutions of higher den- sity are hypertonic; these produce exosmotic currents causing destruction leading through crenation. Solutions of lower density, for example, water, are Jiypotonic; they produce destruction (hemolysis) through endosmosis causing swelling, a stage of which shows a saucer-shaped corpuscle. This is accom- panied by laking, or extraction of hemoglo- bin, giving rise to blood shadows; and final bursting, leav- ing a debris called FIG. 220. SHOWING THE hemokonia. The number of red corpuscles FIG. 219. BLOOD CELLS FROM A SPECIMEN OF FRESHLY DRAWN UN- STAINED HUMAN BLOOD. A, red blood corpuscles, deep focus, showing a light center and dim margin; B, the same with a higher focus; the center, being slightly out of focus, is dim while the margin is light; C, crenated red corpuscles from the mar- gin of the preparation; a, deep focus; 6, higher focus; D, two polymorphonuclear leukocytes; E, large mono- nuclear leukocyte. X 750, O ACTION OF WATER UPON THE RED BLOOD COR- PUSCLE. in to constant varia- tion between wide a, the corpuscle in pro- the blood is subject file; b-e, various stages in the transformation which leaves only a 'shadow' e; diagrammatic. (After limits. Many Schafer.) physiologic condi- tions influence their total number, as well as the. relative proportion of red elements to the white. The average number of red corpuscles in the adult male is about 5,000,000 per cubic millimeter. In young robust persons the number may be considerably higher. The number may also be much reduced by considerable hemorrhages or by the imbibition of large quantities of fluid. Profuse perspiration tends to produce THE WHITE BONE CELLS 209 by great increase of nuclear and cytoplasmic constituents, gives rise to the so-called GIANT CELLS. These are of two sorts, depending upon whether the nucleus is single or multiple, the megakaryocyte and the polykaryocyte. The latter represents a modification of the former, and must not be confused with the osteoclasts of developing bone. Megakaryocytes are practically limited to bone-marrow. They fre- quently show long and numerous pseudopodia. These, as also the cell- body proper, show a differentiation of the cytoplasm into a superficial hyaline layer and a central basophilic granular core. According to Wright, constriction and segmentation of these IMII^M pseudopods give rise to the BLOOD-PLATELETS (plates ; plaques) . These commonly hold positions at the center of masses of converging fibrin fibrils in blood clots, in consequence of which they are supposed to be the essential elements, probably FIG. 224. GROUP OF liberating 'thrombin', in clotting, hence their JHREE SMALL AND ^, . . Two LARGE BLOOD synonym, thrombocyte. This term, however, is ill- PLATELETS. X2000. chosen, for these elements contain no nucleus. What simulates a nucleus is the central spheroidal mass of basophilic granules. Blood-platelets are capable of ameboid motility. They vary in diameter from 2 to 4 microns, and in number per cubic millimeter from 200,000 to 800,000. Analogous elements of avian, reptilian and ichthyoid bloods are nucleated spindle cells, or true thrombocytes. Wright's observations on mammalian megakaryocytes furnish at present the best data for the genetic interpretation of blood-platelets. How- ever, almost every conceivable mode of origin, notably from extruded nuclei of erythrocytes, and fragmenting leukocytes have had, and still claim, prominent supporters. Granulocytes., The granulocytes comprise three varieties distin- guished on the basis of their cytoplasmic granules: (1) neutrophils; (2) eosinophils (oxyphils) and (3) basophils, or mast leukocytes. The nu- cleus is of the polymorph type, perhaps occasionally, polynuclear. This nuclear condition consists in a commonly crescentic chain of nuclear masses connected by frequently extremely delicate nuclear strands. The NEUTROPHILS are characterized by their fine cytoplasmic gran- ules, having a neutrophil staining reaction (lilac color) in mixtures of acid (eosin) and basic (methylene blue) dyes. They range in size from 7.5 //, to 10 /* in diameter. They are predominantly phagocytic. They constitute in normal adult life about 70 per cent, of the total white- cell blood content. 210 BLOOD The EOSINOPHILS contain larger spheroidal cytoplasmic granules which show .a special affinity for eosin. They are slightly larger than the neutrophilic granulocytes, and comprise about 4 per cent, of the leuko- cytes of normal blood. The BASOPHILS are characterized by an extremely variable polymor- phous nucleus, but especially by their spheroidal and irregular basophilic granules of greatly varying sizes. They occur to the extent of only about 0.5 per cent, in the circulating blood. They are of approximately the same size as the eosinophils. Their significance and genetic relationship is uncertain, but they are usually interpreted as degenerating granulo- cytes, the granules being variously regarded as products of nuclear frag- mentation, and as cytoplasmic products of mucoid degeneration. Maxi- mow (Arch. mikr. Anat., 83, 1, 1913), however, regards the f mast cells* of the blood as specialized kinds of granulocytes, distinct from similar cells of the tissues, and without sign of degeneration. Cowdry has demonstrated mitochondria, by vital staining with janus green, in all types of leukocytes, except mast leukocytes, including plate- lets. The lymphocytes and polymorph neutrophils contain them abun- dantly. They are only sparsely present in eosinophils. Mitochondria are said to be totally absent in the erythroplastids of normal adult human blood (Intern. Monatschr. Anat. u. Physiol., 31, 4, 1914). P. Ehrlich in a series of communications announced that by coloring the leukocytes with various stains he was able to distinguish by their reaction, several types of granules. These he called ( a ) oxyphil or acido- phil, which were deeply stained by eosin, acid fuchsin, etc.; (ft) amphophil, which were stained both by eosin, and by dahlia and like dyes ; ( y ) baso- phil, which were stained deeply by dahlia, thionin, etc. ; ( 8 ) certain cells which neither after staining with eosin, etc., nor with dahlia, etc., could be made to show any granules other than the nodes of cytoreticulum ; ( c ) neutrophil, which can be stained only by a due admixture of acid and basic dyes, as of fuchsin and methylene blue, or the so-called 'triacid mixture' of Ehrlich. The demonstration of these characteristics presupposed a division of dyes ; nto three primary classes : 1. Acid e.g., eosin, orange-G, acid fuchsin, aurantia, erythrosin. 2. Basic e.g., methylene blue, dahlia, thionin, hematin. 3. Neutral which are only formed by the interreaction of ex- amples of each of the two preceding classes ; the neutral dye is supposed to arise de novo in such mixtures, as a result of chemical reaction. HEMOGLOBIN The application of such a classification of stains to other tissues than the blood has, however, been found to present considerable difficulties. According to Kite the cytoplasm of the polymorphonuclear leukocytes has nothing of the nature of a cell membrane, but they are completely naked, nor do they contain a spongioplasm and hyaloplasm. "The cyto- plasm is a jelly in which are embedded large numbers of globules/' The structures usually termed cytoplasmic granules are of the nature of separation products ; they do not grade into the surrounding cyto- plasm. All leukocytes un- dergo also certain definite structural transformations,, characterized by the appear- ance of pseudopods chang- ing into vibratile cilia. Kite suggests that the protoplas- mic processes may be prom- inently concerned in phago- cytosis. Under certain con- ditions erythroplastids may be made to protrude similar processes (Jour. Infect. Dis., 15, 2, 1914). FIG. 225. OUTLINE DRAWINGS OF LIVING POLY- MORPHONUCLEAR LEUKOCYTES OF RABBIT, FROM A DROP OF BLOOD MIXED WITH RINGER'S SOLUTION TO WHICH A SMALL AMOUNT OF HIRUDIN HAD BEEN ADDED TO PREVENT COAG- ULATION. HEMOGLOBIN In the course of half an hour the cells develop retractile undulatory processes, a, hyaline-sur- face phase; x, hyaline layer; 6 and c, ciliated phase; d, flagellated phase. Leukocytes of all classes of vertebrates undergo similar changes. (After Kite, Jour. Infectious Dis., 15, 2, 1914.) Hemoglobin is a very complex chemical compound of iron with a globulin; it gives the characteristic color to the blood. It combines readily with oxygen to form oxyliemoglobin, a loose chemical combina- tion by which the oxygen is carried from the lungs to the tissues, and which gives the brighter red color to the arterial as compared with the venous blood. The hemoglobin is held either in solution or in unstable chemical union by the cytoplasm of the erythroplastids. It escapes from these corpuscles after rupture, or it may be extracted by ether, and is 212 BLOOD then prone to crystallize on evaporation in the form of minute brownish- yellow prisms. The crystals of the various species of any genus belong to a crystallographic group (Keichert) but generic differences are fre- quently striking; thus, in human blood they are long rhombohedra (Fig. 227), in guinea pig, tetrahedra, in squirrel, hexagonal plates, in rat, FIG. 226. SECTION OF BONE MARROW FROM SKULL OF 25 MM. TURTLE EMBRYO (CHELYDRA SERPENTINA), SHOWING THREE MAIN STAGES IN THE HEMOPOIESIS. L, hemoblast (lymphocyte), differentiating in the mesenchyma and passing into a blood-vessel; Me, blood mother cell (hemoblast); Mg, megaloblast; Eb, erythro- blast; EC, erythrocyte; E, endothelial cell; Mes, mesenchymal cell; PC, pigmented cell. Helly fixation; Giemsa stain. X 700. elongated six-sided plates, in hamster (a rodent), rhombohedra, and in dog they appear as rhombic prisms which are diamond shaped in cross- section. Various other crystalline, and also amorphous hemoglobin derivatives may occur as decomposition products. The iron of the coloring matter of the hemoglobin may be thus obtained 4n 'the form of hematin, a soluble, amorphous protein compound of a brownish-red color. If he- matin is combined with hydrochloric acid the chlorid of hematin, kemin, is produced. Hemin occurs in deep brownish-red crystals, known also as Teichmann's crystals, which differ somewhat according to the animal HEMOGLOBIN 213 species from which they are obtained; those of human blood take the form of triclinic plates (Fig. 228). Hemin crystals derive a certain importance as a forensic test for the presence of blood, and they may be obtained from old and dried-up specimens as readily as from fresh blood. The procedure for testing a suspected stain consists in heating to the boiling point in a drop of acetic acid, a drop of a nor- mal salt solution of the speci- men. From fresh blood the crystals may be produced by heating together on a slide a drop of blood, a grain of so- dium chlorid, and a drop of acetic acid. Hemin crystals prove only that hemoglobin is present, but give no precise in- FIG. 227. HEMOGLOBIN CRYSTALS. a and b, from human blood; c, from the cat; d, from the guinea-pig; e, from the hamster; /, from the squirrel. (After Ranvier.) FIG. 228. CRYSTALS OF CHLORID OF HEMATIN OR HEMIN. (After Ranvier.) formation as to the species from which the blood in question came, for the crystals obtained from many mammals are apparently identical. Hematin may decompose also into hemosiderin,, an iron-containing hemoglobin derivative, in the form of light-brown granules, frequently found in phagocytic leukocytes as the product of cellular digestion of effete erythroplastids. Hemosiderin granules thus appear in the spleen and bone-marrow. Iron in the tissues, and ferruginous pigments gen- erally, can be recognized by application of the ferrocyanid test, which stains the iron blue. 214 BLOOD When extravasations of blood occur within the tissues of the body, as, for example, in the corpus hemorrhagicum of the ovary, the hematin is frequently deposited as hematoidin., an iron-free derivative of hemo- globin which forms stellate groups of yellowish needle-like crystals.- He- matoidin is apparently identical with bilirubin, a bile pigment. COAGULATION According to Howell's theory of coagulation, prothrombin, probably produced at least in part by disintegrating platelets, in the presence of calcium is converted into thrombin. The latter precipitates from the fibrinogen of the blood plasma the fibrin needles which mass to produce a mesh in which the corpuscles become aggregated to form the thrombus or clot. Howell has recently studied with the ultramicroscope the process of coagulation in oxalated blood plasma and solutions of fibrinogen to which a solution of thrombin was added. He describes clotting as pro- ceeding after the manner of crystal formation; the crystals begin as granules which adhere to form threads. Clotting is f an aggregation of the invisible particles (amicrons) to visible particles and then the further consolidation of these particles into rigid looking needles' (Amer. Jour. Physiol., 35, 1, 1914). HEMOPOIESIS A consideration of blood development involves the questions of the sources of primary origin and the genetic relationship of the different varieties of cells. The blood-forming or hemopoietic organs comprise (1) the yolk-sac; (2) body mesenchyme and endothelium of primitive blood- vessels of the embryo; (3) liver and spleen; assisted by lymph organs in the production of lymphocytes, and finally (4) marrow of fetal and adult bone. These several sources function in development in the order enumer- ated, though the successive stages overlap to some extent. With the assumption of function on the part of bone-marrow the earliest foci being the scapular, pelvic, vertebral and costal elements the other blood-forming organs cease activity. However, in certain diseases the spleen may possibly be stimulated to renewed function; and lymph organs function throughout life in the production of lymphocytes. From the standpoint of the germ layers hemapoiesis is more probably limited to the mesoderm (mesenchyme). This is certainly true largely at least, probably completely, of sources outside of the yolk-sac. HEMAPOIESIS 215 The yolk-sac wall is the earliest locus of blood-cell origin. This struc- ture is a vesicle lined by columnar entodermal cells, invested by mesen- chyme, the surface cells of which become arranged in the form of a mem- brane, the mesothelium. The first anlage of blood appears in the shape of irregular masses and cords of cells next the entoderm. These cords anastomose at the same time that they acquire lumina through a process which involves the peripheral differentiation of cells into endothelium and the appear- ance of plasma in which the central cells are suspended. The very intimate spatial re- lationship of these blood- islands to the entoderm has led many to incline to an en- todermal origin of the earliest blood anlages. The layer in which blood first appears in the yolk-sac has been named angioblast, a term which does not commit one to any view as to primitive ger'm layer origin. However, the impor- tance of the matter of origin seems great only when one believes in the strict specifi- FIG. 229. WALL OF YOLK-SAC OF A 13 MM. HUMAN EMBRYO (FiG. 210), SHOWING A SMALL BLOOD ISLAND (B. I.) AND SEVERAL SMALL BLOOD VESSELS CONTAINING ERY- THROCYTES. The wall consists of stratified columnar and cuboidal entodermal cells (to the right), a middle mesenchymal layer (angioblast), and a covering of mesothelium. X 240. city of germ layers. Such position is hardly tenable in view of the fact that origin- ally there is only one germ layer, the ectoderm, and the origin of the mesoderm from the primitive streak, a phase of fusion of ectoderm and entoderm. More- over, in certain forms, e.g., Tarsius, a primate, it is claimed by Hubrecht that mesoderm has a double origin from ectoderm and entoderm, as well as from primitive streak. Again in subsequent early embryonic stages it is believed by many that blood-cells arise by process of differentiation of body mesenchyme. In the further discussion of hemopoiesis we may take for granted the origin of blood exclusively from mesenchymal elements. Regarding the question of the original blood mother-cell two sharply contrasting views are held, expressed in the monophyletic (unitarian) and polyphyletic (dualistic) theories of blood-cell origin. The former holds that all types of blood elements, red and white, have their origin directly in one and the same primordial blood cell ('hemagonium/ hemoblast) ; the 16 BLOOD several types are conceived to arise through process of differentiation along distinct lines. The polyphyletic theory holds to a dual origin, from geneti- cally distinct mother-cells, for red and white cells; or to a multiple origin, tracing erythrocytes and the several varieties of leukocytes each along different lines of differentiation to different mother-cells. The later investigations into this problem of blood-cell origin done with an admit- tedly superior technic tend to show that the monophyletic view is correct. This was first well supported by the work of Saxer, who gave the name at' FIG. 230. LARGE BLOOD ISLAND FROM (FIG. 210), A 13 MM. HUMAN EMBRYO. a 1 and a 2 , mesenchymal cells; a 1 is differentiating into an endothelial cell; a 3 , endo- thelial cells becoming separated from wall to form hemoblasts ('lymphocytes'); 6 1 and 6 2 , hemoblasts (mesameboids) ; c, megaloblasts; d 1 and d 2 , normoblasts; R, binucleated giant cell. X 750. 'primary wandering cell' to the common blood-mother cell; more recently by the work of Maximow, who applies the name 'lymphocyte' (primary lymphocyte) to this cell, on the ground of its resemblance to the large lymphocyte of adult blood. This lymphocyte (hemoblast) is according to Maximow originally a cell of the general mesenchyma, the earliest differen- tiation stages of which are characterized by increase of basophily of the cytoplasm, separation from the general syncytium, and assumption of ameboid properties. These steps involve blood-island stages. We may now trace the steps through which the primitive blood-cell passes in the process of attaining the adult condition of the various types of cells. Of two daughter-cells one differentiates into an erythroblast ; the other may remain as a primitive blood-cell to further function as a mother- cell; at the same time primitive blood-cells may continually differentiate In adult life from the mesenchyme of bone-marrow. An alternative view BONE-MARROW 217 accepted by some, e.g., Minot, and still in accord with the monophyletic teaching regards the later embryonic, fetal and adult hemopoietic^ organs as simply foci of proliferation and differentiation of primordial blood-cells (hemoblasts) originally derived from the angioblast and subsequently carried by the blood stream to these locations (red marrow, etc.) where they persist throughout life. We will follow first the several steps in the developmental process of the red element, to which taken together the name 'erythrocyte' may be appro- priately applied. According to Maximow, in the forms studied by him, there are two lines of erythrocytes, a primitive and a definitive line; the primitive line dies out after a brief span, arid, since the definitive line differs only in the matter of smaller size of cells, we may confine our atten- tion to that line. The several stages in the differentiation process re : (1) megaloblast; (2) norm.dblasi ; (3) erythroblast and (4) erythroplastid ('erythrocyte')- All except the last may multiply by mitosis. The megaloblast differs from its progenitor, the hemoblast, in having somewhat more cytoplasm, and a small amount of hemoglobin, which gives it a light reddish-brown color. Its nucleus is relatively large and vesicular and contains a delicate gran- ular network; this stage corresponds to the ichthyoid stage of Minot; a seemingly appropriate terminology, since it calls attention to the similarity between the adult (phylogenetic) stage of a lower form, and the embryonic (ontogenetic) stage of a higher form. The normoblast is a daughter-cell of the megaloblast and is of somewhat smaller size, containing more hemo- globin, with a somewhat denser nucleus, with larger net-knots; this corre- sponds to the sauroid stage of Minot, the significance of which term is the same as explained above for ichthyoid. The erythroblast differs from the normoblast in having an increased amount of hemoglobin, but mainly in its smaller, more compact nucleus. This cell becomes an erythroplastid through loss of nucleus. (It conduces to simplicity and clearness to regard the megaloblast and normoblast as two developmental phases of the erythro- blasts.) The manner by which the nucleus disappears has been much disputed. It has been held to be absorbed (karyolysis) ; Howell has de- scribed its extrusion either as a single or fragmented body (karyorrhexis) in the cat; Emmel (Amer. Jour. Anat., 16, 2, 1914) has recently de- scribed for pig a process by which the red corpuscle arises through budding, leaving a nucleated remnant. Certain erythroblasts may be smaller than the average, others larger; these dimentionally atypical forms are termed respectively microcytes and megalocytes. The large lymphocyte of the blood is believed to be derived from the primitive blood-cells by slight differentiation; perhaps it represents a po- tential hemoblast. The small lymphocyte is simply a daughter-cell of a large; a large, a grown small lymphocyte. 218 BLOOD The primitive blood-cell may by slight differentiation become a leuJco- Nast; according to whether the cytoplasm elaborates neutrophilic, acido- philic, or basophilic granules, it becomes a polymorphonuclear neutrophil, eosinophil, or basophil ('mast-cell') leukocyte. The transition stage, from FIG. 231. DIAGRAMMATIC ILLUSTRATIONS OF SUCCESSIVE STAGES IN THE TRANS- FORMATION OF THE MAMMALIAN ERYTHROCYTE (a) TO FORM THE ERYTHRO- PLASTID (/). 6 and c, by extrusion of the nucleus as a whole; d and e, by extrusion of the frag- mented nucleus. the standpoint of the nucleus, between the leucoblast with spherical nucleus and the granulocyte with polymorphous nucleus, is the large mononuclear leukocyte (transitional leukocyte). According to Kyes (1915), certain large mononuclear leukocytes are derived from the reticulum of lymph nodes. Both the reticular cells and their leukocyte derivatives are said to be phagocytic. Giant cells are derived from the leukoblast, or perhaps primitive blood- cell, along a separate line of differentiation, characterized by absence of cy- p FIG. 232. SUCCESSIVE STAGES IN THE ELIMINATION OF THE ERYTHROBLAST NU- CLEUS, FROM HOMOPLASTIC CULTURES OF BLOOD OF A 32 MM. PlG EMBRYO. This is regarded by Emmel as a 'somewhat imperfect case of constriction/ but it illustrates the fundamental similarity between erythroplastid formation by ex- trusion of nucleus (Howell) and by cytoplasmic constriction. (After Emmel, Amer. Jour. Anat., 16, 2, 1914.) toplasmic division, excessive growth, and giant or multiple nucleus. The multinucleate condition is attained apparently by both mitotic and amitotic division of the nucleus. From the megakaryocyte pseudopodia are derived the blood-platelets as explained above. Blood development in marrow passes through the same phases, the hemoblast stage here being generally known as the myeloblast, and characterized by a considerable finely granular neutro- philic cytoplasmic content. The above is given in outline in the following scheme, adapted largely from the work of Maximow on hemopoiesis in the rabbit embryo. (Arch. mikr. Anat., Bd. 73, 1909.) 219 220 BLOOD BONE-MARROW The red variety of bone-marrow, found in the flat bones generally and in the epiphyses of long bones, functions as the sole hemopoietic or- gan of later fetal and adult life. According to certain authorities it is assisted in this function to some extent by the spleen. Besides a hemo- poietic function, red bone-marrow possesses also the capacity of de- "V^lpwl^iF \^4^i$?^ } V- 1 .jg~ r *^^ J|afK^~ \ ^;* i-:' fP^fcll- , ^rm>WIt^ \ ^^^-^-^MA-l.., ^f FIG. 233. FROM A SECTION OF RED MARROW OF A HUMAN BONE. a, giant cell; b, leukocytes; c, nucleated red blood cells; d, mitosis in a marrow cell; e, outline of a fat cell; /, reticulum; g, mitosis in a giant cell. X 680. (After Bohm and von Davidoff.) stroying worn-out red blood-corpuscles. In this process it is assisted greatly also by the spleen and lymphoid organs generally. The phago- cytic leukocytes of these locations are largely active in this destructive process and in the transportation of the hemoglobin debris to the liver where as hematoidin it is apparently appropriated in the manufacture of the bile pigment, bilirubin. The generic name for the specific hemogenic cells of marrow is myelocyte. This includes the parent blood-cell (hemoblast) and the intermediate developmental stages of both white (lymphocytes, mono- nuclear leukocytes leukoblasts) and red (megaloblast, normoblast, erythroblast stages) cells. Besides these, there are of course abundantly present also the adult forms of blood -cells both white (eosinophil, neu- trophil, and basophil polymorphonuclear leukocytes, and lymphocytes) and red (erythroplastids). Potential osteoblasts and osteoclasts are BONE-MARROW 221 also present, but indistinguishable from the lymphocytes and hemogenic polykaryocytes respectively unless specially stained. A description of the cells in their order of hemogenic cytomorphosis follows : 1. Myeloblast (Premyelocyte; HemoUast, Mesameboid Cell; Prim- itive Blood Cell; 'Lymphocyte'); This is the parent blood-cell of bone- FIG. 234. TYPES OF CELLS FROM A SMEAR PREPARATION OF THE MARROW OF A HUMAN RIB. 1, red blood corpuscles; 2, nucleated red blood cells, erythroblasts; 3, small lympho- cytes; 4, large mononuclear cells with neutrophil granules; 5, polynuclear neutro- phil; 6, eosinophil cells; 7, a basophil cell. Eosin and methylene blue. Nocht's stain. X 1200. marrow. It produces by proliferation and slight differentiation the more direct ancestors of both the white (leukoblast) and red (megaloblast- erythroblast) cells. Mother- and daughter-cells are similar in their relatively large granular nuclei, well developed cell-body, ameboid capac- ity, and proliferative activity. The cytoplasm of the myeloblast is homo- geneous or very finely granular and slightly basophilic; likewise the leukoblast. The megaloblast may be distinguished by its slightly oxyphil character due to the presence of a small amount of hemoglobin. 222 BLOOD 2. Lymphocytes. These include larger and smaller varieties. They are similar to, and in part identical with, the above-described cells, the myeloblasts. They include both cells differentiated from marrow and cells transported by the blood stream. They are indistinguishable from the osteoblast. In short, these several cells of similar characteristics may be identical in their capacity to function as progenitors of blood-cells, and may represent the parent blood-cell. 3. Large Mononuclear Leukocytes. These cells are distinguished from the myeloblasts and lymphocytes only by their slightly modified nucleus, and by the occasional presence of a small number of fine neu- trophilic granules. The nucleus is oval or bean-shaped. It represents a transition phase between the less differentiated lymphocyte stage and the granulocytes. It contains a centrosome located usually in that por- tion of the cytoplasm lying in the concavity of the bent nucleus. This cell may proliferate extensively. 4. Polymorphonuclear Neutrophil Granulocytes. These include a graded series of stages from the standpoint of shape of nucleus, and amount of granular content. The younger stages contain a less com- plicated nucleus, generally bean-shaped with centrosome and relatively fewer (some basophilic, f unripe 7 ) granules; these have a slight prolifera- tive capacity. Older stages have progressively more complicated nuclei, characterized by a lobulated chain (with from two to five segments) of dense chromatin, and are no longer capable of mitotic division. 5. Eosinophil Granulocytes. These cells resemble the neutrophils in their graded series of nuclear forms, ending in a lobulated poly- morphous nucleus ; but differ in the matter of size and staining capacity of the cytoplasmic granules. These granules are of fairly uniform size and of spherical shape, but much larger than the neutrophilic granules, and are strongly acidophilic, showing special affinity for the cytoplasmic stain, eosin. In the younger forms the granules are basophilic. The older types are identical with those of the blood. Again, only the less differentiated, that is, those with oval nucleus and a centrosome, may divide mitotically. The commonly accepted view of the origin of neutrophilic and oxy- philic cytoplasmic granules is that they arise intracellularly under nuclear influence, perhaps from less differentiated nuclear extrusions or chro- midia. Originally these granules are basophilic ('unripe'). -However, Weidenreich regards the eosinophil granules as the ingested hemoglobin particles of degenerating and fragmenting erythroplastids. Their con- siderable presence in red marrow and spleen where blood-cell disintegra- BONE-MARROW 223 tion is extensive, lends weight to this hypothesis. Likewise Badertscher (Amer. Jour. Anat, 15, 1, 1913) finds that eosinophil leukocytes and free eosinophilic granules are very abundant in the vicinity of degenerat- ing muscles in salamanders during the time when the gills atrophy, and believes that the eosinophilic granules are ingested fragments of degener- ated muscles and red blood-corpuscles. Eosinophils do undoubtedly in- crease greatly in number during certain infectious diseases characterized by extensive tissue disintegration, e.g., trichiniasis ; but disproof of intra- cellular origin a teaching more in accord with our knowledge of cyto- plasmic granule origin through protoplasmic activity demands direct evidence of extensive granular ingestion, which is lacking. Moreover, the microchemical nature of eosinophil granules differs from that of hemoglobin; also, they differentiate from basophilic granules and in hemogenesis in the turtle, for example, eosinophils appear before hemo- globin-containing cells are present. The free eosinophil granules in degenerating tissues are more likely derived from disintegrating eosino- phils, abundant in such regions. 6. Basophil Granulocytes (Mast cells). These are identical with the mast leukocytes of the blood, and possibly also with the cells of this name in connective tissue, the latter perhaps representing degenerating phases of the former. They are characterized by a variable polymor- phous nucleus, apparent lack of centrosome, extremely slight prolifera- tive capacity, and presence of spheroidal non-uniform basophilic cyto- plasmic granules. They are numerically increased in marrow and the circulating blood and in the spleen in certain diseases. 7. Giant Cells or Myeloplaxes. These are relatively enormous cells (of 30 to 100 microns diameter). They may have either a single large, frequently lobulated annular, nucleus (megakaryocyte) or several, even many, nuclei (polykaryocyte). The megakaryocyte is a derivative of the lymphocyte (myeloblast). The polykaryocyte represents a later modification of the megakaryocyte, the lobulated nucleus having become broken up into separate nuclei. These polykaryocytes ('hemogenic giant- cells') have been erroneously regarded as identical with the multinucle- ated osteoclasts. They are in fact potential erythroblasts, comparable to 'blood-islands/ with only slight or no phagocytic capacity. After Wright's technic the cytoplasm of these cells presents fine purple gran- ules. Osteoclasts do not contain such 'metachromatic' granules. The polykaryocyte of the yolk-sac of the 10-millimeter pig embryo can be seen to differentiate into erythrocytes, a hemoglobin-containing area developing about the several nuclei, the whole finally breaking up into an equal num- 15 224: BLOOD ber of red cells. Giant cells are characteristic elements of all hemopoietic organs. The megakaryocytes protrude long pseudopodia which segment into blood-platelets (Wright), abundantly present in red marrow. 8. Erythrocytes. These cells include the several developmental forms of red cells : (a) megaloblast, (b) normoblast, (c) erythroblast and (d) erythroplastid. The megaloblast is very similar to the leukoblast ex- cept that the cytoplasm contains a slight amount of hemoglobin, and therefore gives an oxyphilic staining reaction. The nucleus is granular, with a delicate chromatic network. This is the so-called ichthyoid stage of Minot. Normoblast and erythroblast are closely similar stages (sauroid stage of Minot), characterized by the relatively smaller and denser more chromatic nucleus, and a relatively more extensive shell of cytoplasm with increasingly more hemoglobin. The erythroplastid develops from the erythroblast through loss of nucleus, generally by extrusion. Evans (Amer. Jour. Physiol., 37, 2, 1915) has directed attention anew to the phagocytic large mononuclear leukocytes, the 'macrophages' of Metsch- nikoff (1892). On the basis of a specific response to vital azo dyes he iden- tifies them as a group distinct from the large mononuclear elements and lymphocytes of the blood, and includes among them certain endothelial and reticular cells and the 'clasmatocytes' (Ranvier) or 'resting wandering cells' (Maximow) of connective tissue, all of which may become free macrophages. These cells in mammals, of round or elongated shape, range in diameter from about 10 to 30 microns; they contain a stoutly cresentic nucleus of irregular contour and excentric position; the cytoplasm is weakly baso- philic, covered with delicate pseudopods of various sizes, and frequently filled with large vacuoles. As endothelial cells they may line the capillaries and venules of the liver (Von Kuppfer cells'), spleen, red bone-marrow, hemal glands, and the lymphatic sinuses of lymph nodes. They include also reticular cells of lymph nodes, spleen pulp and bone-marrow, and the clasmatocytes of connective tissue. As free cells they occur in the serous cavities, lymph sinuses of lymph nodes, spleen and hepatic capillaries. They are abundant in transudates and exudates in serous cavities. Only under pathological conditions do they appear in the peripheral blood stream. Weidenreich identified them with the large mononuclear leukocytes of the blood and lymph. Evans' experiments, on the contrary, give no indication that leukocytes are converted into these cells. They originate chiefly from endothelia. They are phagocytes and are active also in the normal physio- logical processes. They handle the blood and bile pigments, and fats and lipoids. The protoplasm of the macrophages is characterized especially by its response to finely particulate matter. Macrophages share the func- tion of phagocytosis with the polymorphonuclear elements of the blood. CHAPTEE IX THE LYMPHATIC SYSTEM The lymphatic series includes a system of lymphatic channels which collect the lymph from the various tissues of the body and return it to the large veins of the neck, where it mixes with the blood. In the course of this lymph vascular system are various aggregations of lym- phoid or adenoid tissue which occur in the form of lymph nodules or follicles, lymph glands or nodes, and the lymphoid organs. These or- gans are the tonsils, thymus, and spleen. The lymphatic vessels also stand in intimate relation if not in direct communication with the serous and synovial membranes and the bursae. LYMPH Like the blood, the lymph may be considered as a primary tissue whose intercellular elements are entirely of a fluid nature. In most portions of the body, lymph is a colorless fluid which is scantily provided with corpuscular elements, the lymphatic corpuscles. The lymphatic corpuscles are identical with the leukocytes of the blood. In the lymph most of these cells are of the monoiiuclear form, the small mononuclears or lymphocytes being the most abundant. Lymph also contains a small proportion of polymorphonuclear cells, which not only are derived from the lymphoid tissues, but as wandering cells find their way into the lymphatic vessels from the tissues generally. Blood-platelets are not present in lymph (Howell) ; prothrombin is liberated by the lymphocytes. The cells of lymph, predominantly of the small lymphocyte type, are derived from the numerous lymphoid masses (nodes and nodules) through which the lymph passes on its way from the tissues to the sub- da vian veins. According to Davis and Carlson (Amer. Jour. Physiol., vol. 25, 1909) the number of lymphocytes contributed to the blood daily may be more than the total permanently present in the blood. Since the number in a cubic millimeter remains fairly constant, a number must 225 226 THE LYMPHATIC SYSTEM be daily consumed in the body equal to the number added to the blood. Many of course actually suffer destruction, but it seems probable that a considerable number also first undergo differentiation into granulocytes, and perhaps as potential hemoblasts may function as parent cells of erythrocytes. In addition to the leukocytes lymph contains fat globules and glyco- gen. These are mostly the products of absorption from the intestinal tract, in which process the lymphatic vessels play an important role. In the lymphatic vessels of the intestine during absorption fat globules are so abundant as to impart to the lymph a milky white color ; this variety of lymph is termed the chyle. These fat globules are rapidly removed by the lymphoid organs, since even in the presence of abundant chyle only comparatively few fat globules escape into the general blood current.. The lymph of other portions of the body than the abdominal region,, therefore, contains relatively little fat. The lymph, unlike the blood, circulates in but one direction, viz., toward the heart. It must therefore be formed in the tissues generally. The blood plasma constantly escapes through the walls of the capillary vessels into the surrounding lymphatic spaces of the tissues. It is these tissue spaces which have been considered as forming the beginning of the lymphatic system. Recent evidence, however, goes to show that the tissue spaces are not directly connected with the lymphatic vessels, but that just as the plasma exudes into the tissue spaces by processes of secretion, osmosis, and filtration, so the tissue juices, as the predecessors of lymph, enter the lymphatic vessels by similar processes of secretion, osmosis, and filtration. Lymph is also formed by absorption, which oc- curs chiefly in the alimentary tract. Under favorable conditions the lymph will coagulate, though more slowly than blood, the fibrin forming a firm, colorless clot in which the leukocytes are entangled. Because of their tendency to adhere to the; sides of the vessel thus circulating at the periphery of the current the lymph cells are most likely to be found at the periphery in those; post-mortem clots which occur within the lymphatic vessels. LYMPHATIC VESSELS (Lymphatics) The lymphatic vessels vary in size from that of the smallest capillary vessels up to that of the thoracic duct. The smaller vessels, lymphatic LYMPHATIC VESSELS 227 capillaries, form anastomosing meshes in all tissues where blood capil- laries are found. They are most' abundant in the peri vascular connective tissues, where they form a dense plexus about the wall of the blood-ves- 3. The wall of the lymphatic capillary, like that of the blood capillary, FIG. 235. SUBCUTANEOUS LYMPHATIC VESSEL OF A FETAL PIG. At the right is a small blood-vessel. Hematein and eosin. Highly magnified. (After MacCaUum.) consists of a single layer of endothelium. This endothelium probably forms a complete lining for the lymphatic capillary and is continuous through larger and larger vessels with that of the veins, from which, according to Sabin (Amer. Jour. Anat., 1902), the lymphatics are originally developed. The relation of the lymphatic capillaries to the tissue spaces is not as yet definitely settled. It was formerly thought that these spaces were 228 THE LYMPHATIC SYSTEM continuous with the lymphatic capillaries, but the more recent observa- tions, represented by those of MacCallum (Johns Hop. Hosp. Bull., 1903), seem to show that the capillaries of the lymphatic system, like those of the blood vascular system, form a series of branching channels which are open only toward the veins. According to this conception, therefore, the tissue juices, formerly also considered as lymph, are con- tained within a separate series of channels, the tissue spaces and lym- phatic canaliculi, and they enter the true lymphatics only by processes of osmosis and the secretory activity of the lymphatic endothelia. FIG. 236. THE GROWING END OF A DEVELOPING LYMPHATIC VESSEL IN THE SUB- CUTANEOUS TISSUE OF A FETAL PIG. The lumen of the vessel has been filled with a dark injection mass. Highly mag- nified. (After MacCallum.) The lymphatic capillaries are of rather irregular caliber, generally greater than that of blood capillaries, and possess frequent sinus-like dilatations, which peculiarity is also characteristic of the larger lym- phatic vessels. The lymphatic capillaries soon acquire an adventitial sheath of fibro- elastic tissue and pass into the smaller lymphatic vessels. On attaining a size of from 0.2 to 0.8 millimeter their wall is differentiated into the same three coats which are found in the veins. Except for the fact that they contain lymph instead of blood, these vessels closely resemble the small veins, and like some of the latter vessels they possess frequent valves. LYMPHATIC VESSELS 229 The tunica intima of the lymph vessel consists of an endothelial lining with a thin delicate fibro-elastic membrane. The tunica media is thin and contains circular smooth muscle fibers. The adventitia is the thickest coat of the lymph vessel. It consists of fibro-elastic connective tissue and longitudinally disposed bundles of smooth muscle fibers. The wall of the lymph vessels is supplied with small blood-vessels and nerves, in the same manner as the veins. The nerves form a plexus in FIG. 237. LYMPHATIC AND BLOOD VESSELS IN THE HILUM OF A HUMAN LYMPH-NODE. a, lymph vessels; 6, blood-vessels. Hematein and eosin. Photo. X 160. the adventitia from which branches are distributed to the media and intima. Kytmanof (Anat. Anz., 1901) has traced the fine nerve fibrils to the smallest lymphatic capillaries, where, as in the intima of the larger vessels, they end in close relation to the endothelial cells. To summarize: the lymphatic capillaries arise by one of three methods : 1. As lymphatic plexuses in all connective tissues ; the most abun- dant of these are the perivascular lymphatics. 2. As dilated' pouches having blind extremities, as in the villi of the small intestine, where they are known as lacteals. 230 THE LYMPHATIC SYSTEM 3. By direct communication with the stomata of the serous mem- branes. The presence of true stomata in the serous membranes of man with the exception of possibly certain portions of the peritoneum is dis- puted. The lymph is derived from the tissue juices and by absorption from the alimentary tract, and is conveyed by the lymphatic capillaries to PIG. 238. LYMPHATIC CAPILLARY FROM THE SPERMATIC CORD OF A DOG, SHOWING NERVE ENDINGS. and alon g which NODE. the peripheral lymph sinuses are continued into the substance of the node to partially surround its lymph nodules. Each lymph nodule is thus surrounded, except at its central pole, by a peripheral lymph sinus, into which the afferent lymphatic vessels pour their contents. The lymph on entering the gland is thus permitted to enter the spaces of the reticulum and percolate through the lymph nodules of the cortex before it can reach the looser portions of the medulla. Each of the nodules of the cortex contains a germinal center in which lymphocytes are actively formed by mitosis, and from which the lymphocytes readily escape along the lymph channels of the reticu- lum into the more open meshes of the medulla. Medulla. The medulla occupies the center of the gland, and at one point, the hilum, it reaches the surface. At this point a considerable mass of fibrous trabeculae enters the medulla, carrying with it the larger blood-vessels to be distributed to all portions of the gland. The finer Efferent Lymph Vessel THE LYMPH NODES 237 ramifications of these medullary trabeculae are continuous with those of the cortex. FIG. 243. TRANSECTION OF A CERVICAL LYMPH NODE OF A DOG. The denser portions of lymphoid tissue are light in the figure, a, medullary cord of dense lymphoid tissue; 6, looser lymphoid tissue of the cavernous medulla; c, capsule; F, dense lymph nodule of the cortex; HF, fibrous tissue containing the large vessels of the hilum; s, peripheral lymphatic sinus; V, blood-vessel. Magnified several diameters. (After Ranvier.) The lymphoid tissue of the medulla is divisible into the denser branch- ing lymph cords, in which the lymphocytes are closely packed, and the FIG. 244. TRANSECTION OF A MESENTERIC LYMPH NODE OF A MAN. Hematein and eosin. Photo. X 38. intervening pulp spaces, in which lymphocytes are less numerous, and the reticulum of which is continuous with that of the cortical nodules. 238 THE LYMPHATIC SYSTEM The pulp spaces are broad channels, which are occupied by a reticu- lum whose meshes are partially filled with lymphocytes. They are bounded by a layer of endothelioid cells which everywhere incloses the denser lymph cords. The function of these cords would seem to be comparable to that of the peripheral lymph nodules. The pulp spaces are open toward the cortex, whence they receive the afferent lymph after it has percolated through the nodules, but toward the hilum the spaces are continued into the efferent radicles of the lymph vessels which, in the connective tissue of this part, unite into larger trunks, and finally form several efferent lymph vessels of considerable size. The reticulum of the lymph gland is a close-meshed network of interlacing fi- brillar bundles, which are here and there clasped by flattened endothelioid connec- tive tissue cells. Reticulum is but poorly stained with either acid or basic dyes, is destroyed by acids and bases, but is not digested by pancreatin. After prolonged action of Weigerf s specific stain for elas- tic tissue it is but slightly colored. Lymph Cells. The great majority of these cells are of the small mononuclear or lymphocyte type. Large mononuclear cells with a considerable cytoplasmic body are also very numerous. Polymorphonu- clear neutrophil leukocytes, though of frequent occurrence, are less abundant than the previous varieties. Eosinophil cells are present in small numbers, and large basophilic mast-cells are occasionally seen, though according to Carlier (Jour. Anat. and Physiol., 1893) they are mostly confined to the connective tissue. Drummond (Jour. Anat. and Physiol., 1900) also found large multinuclear giant cells, megakaryocytes, similar to those of the bone-marrow; these were, however, very rare. Many of these cells, after proper fixation, show mitotic figures. This FIG.. 245. DIAGRAM OF THE BLOOD-VESSELS OF A LYMPH NODE. A composite section of three follicles and the medullary cords of a mesenteric lymphatic node of the dog. A, a*rtery; B, medul- lary artery; C, follicular vein; E, artery going to the capsule; F, capillaries in the periphery of a cord; G, medullary vein; //, fol- licular artery; 7, arterial capil- laries in a follicle; J, vein from capsule; K, cord; L, trabecula; 7, vein. X 501. (After Calvert.) HEMOLYMPH NODES 239 mitosis has been most frequently observed in the large mononuclear type, and is most abundant in the germinal centers of the nodules. The small mononuclear and polymorphonuclear types have also been shown to be capable of cell reproduction by indirect division. Eeproduction by direct division of leukocytes appears to be rare, if indeed it ever actually occurs. The mononuclear as well as the polymorphonuclear forms appear to be phagocytic. Among the inclusions which have been found within these cells are fat globules, pigment granules, red blood corpuscles in partial disintegration, insoluble pigments, such as carbon granules, etc., and bacteria. The cells of the reticulum are also believed to be phago- cytic. Blood-vessels. The arteries enter the lymph node at its hilum, and, following the trabeculae within which they lie, are distributed to all por- tions of the organ. In the medulla branches are distributed to the lymph cords, in which they form a wide-meshed capillary plexus. The terminal branches of the primary divisions of the afferent artery are distributed to the nodules of the cortex. A single nodular branch (Calvert, Anat. Anz., 1897) enters the nodule and passes straight toward its center, where it breaks into a plexus of divergent capillaries which unite at the surface of the nodule to form small venous radicals. The veins follow the internodular trabeculae in their course toward the medulla, where they enter the medullary trabeculae, are augmented by venous radicals from the capillary plexuses of this portion of the gland, and thence follow the trabeculae to the hilum, where they unite to form the efferent vein. Certain of the arteries also pass from the medulla through the inter- nodular trabeculae to the capsule of the gland, to which they supply a capillary plexus. The blood is returned through veins which retrace the course of the arteries and enter the large veins of the medullary trabeculae. The spleen contains no lymphatics beyond the capsule. HEMOLYMPH NODES (Hemal Nodes) These structures, which closely resemble the lymph nodes, were first described by H. Gibbes (Quart. Jour. Mic. Sc.), in 1884. He found them in the connective tissue between the renal artery and vein, in the human subject. They have since been found in the prevertebral connec- tive tissue, and in the mediastinum and mesentery. They are larger and 16 240 THE LYMPHATIC SYSTEM more numerous in the ruminants, ox, sheep, etc., than in man. Their size varies from that of a millet seed to that of a pea. In color they closely resemble a minute extravasation of blood- These organs are essentially lymphatic structures in which the lym- phoid tissue is arranged in the form of cords rather than in nodules. The s. B. s. FIG. 246. SECTION OF HUMAN HEMOLYMPH NODE ("SPLENOLYMPH GLAND"). C.j capsule; T., trabecula; P. B. S., peripheral blood sinus; L. T. y lymphoid tissue, largely in the form of cords; S. B. S., secondary blood sinus; H, hilus. (After Warthin.) node is inclosed by a fibrous capsule, beneath which is a broad sinus filled with blood. In this fact lies the chief distinguishing feature of these glands. The peripheral Hood sinus, which is analogous to the peripheral lymph sinus of a lymph node, sends into the interior of the organ a greater or less number of secondary sinuses. Based largely upon the abundance of these secondary sinuses, the hemolymph nodes have been divided into two varieties, named by Warthin (Jour. Bost. Soc. Med. Sc., 1901) the 'splenolymph glands' and the 'marrowlympli glands' HEMOLYMPH NODES 241 In the splenolymph type, which is the more abundant, the node is of small size and is well filled with secondary blood sinuses. The lymphoid tissue is supported by a similar reticulum, and contains the same varieties of lymph cells as in the lymph nodes. In the marrowlymph nodes a somewhat similar structure is found. The blood sinuses are less numerous and lymph nodules do not occur (Vincent, Warthin). The eosinophil leukocytes are more numerous than in the splenolymph type, and the marrowlymph nodes as a rule are the larger. Huntington (Amer. Jour. Anat., 16, 3, 1914) has suggested that FIG. 247. HORIZONTAL SECTION THROUGH THE FAUCIAL TONSIL OF A CHILD. Semi-diagrammatic, a, stratified epithelium; 6, crypts; c, lymph nodule; d, mucus- secreting gland. Hematein and eosin. X about 20. some of the structures described as hemolymph nodes may be post-natal hemopoietic foci, in which erythrocytes develop from the endothelium of the lymph channels. They probably function as accessory spleens hav- ing a combined lymphopoietic and phagocytic activity. Intermediate types between the lymph nodes and the splenolymph type (Vincent, Jour. Anat. and Physiol., 1897) on the one hand, and be- tween the splenolymph node and the spleen and marrowlymph type on the other hand, are of frequent occurrence. Blood Supply. The afferent artery, according to Drummond (Jour. Anat. and Physiol., 1900), enters the hilum with the connective tissue, and through the trabeculae reaches all parts of the node. In the lymphoid 242 THE LYMPHATIC SYSTEM tissue its branches form a capillary plexus whose vessels open into the blood sinuses. All the sinuses, peripheral and secondary, communicate with each other, and from them the blood is ultimately collected into two or more thin-walled veins. In the center of the gland these vessels unite to form an efferent vein which passes out at the hilum. DEVELOPMENT OF LYMPH NODES Lymph nodes arise through the invasion of primary lymphatic capillary plexuses by lymphocytes. The first lymph nodes arise in the regions of the axilla and groin during the third month of development. Such areas be- come circumscribed by the development of a capsule from the surrounding mesenchyma. The capsular tissue is continued into the developing node in the form of trabeculse, terminating in a dense network of delicate reticu- lar fibers. Hydrostatic conditions probably determine the formation of a peripheral lymph sinus. The retention of certain channels ( intermodular and medullary sinuses) between the peripheral sinus and the efferent lym- phatics at the hilum is likewise probably determined mainly by the opera- tion of like factors, brought into play through the appearance of cortical nodules. These nodules arise as regions of proliferative activity of lympho- cytes. The node has meanwhile early become invaded at a point which becomes the hilum by a vascular and nerve supply. Nodules arise as ac- cumulations of proliferating lymphocytes about the cortical arterial twigs. Hemolyrnph nodes apparently arise in a manner similar to the origin of ordinary lymph nodes, and become only secondarily modified. The reticu- lar tissue of lymph nodes may in part arise from the capillary eudothelium. The function of lymph nodes is the production of lymphocytes, which become phagocytic leukocytes. Besides having a leukopoietic role, lymph nodes probably function also as centers for the dissolution of worn-out blood elements, in which process phagocytosis predominates, the lympho- cytes being in part assisted by the endothelial cells of the capillaries. Lymphoid aggregations also serve as 'lymph filters/ the phagocytes removing from the lymph bacteria and other noxious products. THE TONSILS The Faucial Tonsils (Palatine Tonsils; Amygdala). The tonsils consist of a mass of lymplioid tissue which projects slightly from either side into the cavity of the fauces, and is covered by a layer of stratified epithelium continuous with that which lines the oral and pharyngeal THE TONSILS 343 cavities. The lymph nodules which compose the tonsil immediately underlie the epithelial coat, and are embedded in areolar connective tissue. The epithelial coat here and there penetrates the substance of the organ in the form of invaginated funnel-shaped depressions, the crypts ('follicles' of the tonsils). The direction of the crypts in the upper third of the tonsil is downward and outward (C. P. Johnson). The ducts of many mucous glands open into the recesses of these branching crypts. The mucus-secreting glands lie in the loose connective tissue which sur- . : ;'& ^^Ms^^SKKSSSfS&mL^ FIG. 248. FROM A CRYPT OF A DOG'S TONSIL. a, stratified epithelium; 6, basal margin of the epithelium; c, infiltration of the epi- thelium by leukocytes; d, spaces in the epithelium filled with leukocytes and epithelial cells; e, blood-vessel;/, lymphoid tissue.' X 150. (After Bohm and von Davidoff.) rounds the tonsil on all but its faucial surface. The crypts are lined throughout by a layer of stratified epithelium, which is continuous with that on the free surface of the tonsil, but which becomes progressively thinner as it recedes into the deeper recesses of the crypts. Many of the lymph cells migrate into the intercellular spaces of the epithelial layer, and even penetrate to the free surface; thus they find their way into the oral cavity, where they are found in large numbers in the saliva, as 'salivary corpuscles/ If such salivary corpuscles are ex- amined in a drop of saliva, freshly prepared, the fine intracellular gran- ules of the polymorphonuclear leukocytes will be seen to undergo an active dancing movement, Brownian motion. The salivary corpuscles are derived not only from the faucial tonsils but from the other lymphoid tissue which is in relation with the oral mucous membrane, e.g., the lingual and pharyngeal tonsils. The passage of leukocytes through the epithelial surface of the faucial 244 THE LYMPHATIC SYSTEM tonsil is so very active that at times the epithelium becomes completely filled with these cells, and it is then difficult to distinguish it from the adenoid tissue beneath. The normal tonsils atrophy after puberty. The Lingual Tonsil. A collection of lymph nodules is also found at the base of the tongue in the median line, between the circumvallate papillae and the epiglottis. This, because of its similarity in appearance and in structure to the faucial tonsil, is called the lingual tonsil. In the lingual tonsil, however, the nodules are grouped about a FIG. 249. THE LINGUAL TONSIL OF MAN. a, a crypt; b, von Ebner's glands. Hematein and eosin. X 45. single wide-mouthed crypt, the foramen caecum lingui This crypt is frequently branched, and into it the many mucous glands of the neighbor- ing lingual mucosa pour their secretion. The Pharyngeal Tonsil. The posterior wall of the nasopharynx is supplied with a similar accumulation of lymph nodules, the pharyngeal tonsil. It lies in the median line and extends downward from between the orifices of the auditory (Eustachian) tubes for a distance of three centimeters (Klein). It contains a considerable number of lymph nod- ules and several small crypts. The lateral extensions in the vicinity of the tubal orifices are sometimes known as the tubal tonsils. The pharyngeal tonsil is prone to hypertrophy in youth, in which THE SPLEEN 245 case it forms the adenoid growths which are so common in strumous children. Viewing the several tonsils and the associated lymphoid tissue as a whole, it will be perceived that they constitute a lymphoid ring at the gateway to the alimentary and respiratory tracts. The function of the lymphoid tissue is to produce phagocytic leukocytes for the protection of the body against bacteria and other noxious products. The tonsillar crypts offer favorable foci for the lodgment, invasion and attack of such harmful .elements. The location of this annular mass of lymphoid tissue is signifi- cant; it is placed where it can apparently best perform a necessary func- tion. When called upon to increase its functional activity lymphoid tissue responds by hypertrophy; this of itself may cause inconvenience by ob- structing the channels employed in respiration and phonation. But when unable to respond adequately and thus successfully cope with the infecting material, the tonsils become diseased. This is commonly considered to call for removal of the involved lymphoid masses. But it would seem that the excision of the tonsils would result in handicapping the organism in its perpetual combat with bacteria, by depriving it of a means of defense; moreover in the case of removal of the faucial tonsils, proper phonation may also be interfered with. However, excised tonsillar tissue is probably largely compensated for by regeneration and hypertrophy of other non- involved lymphoid tissue. Nevertheless it has been suggested that much could be gained through prophylactic means consisting largely perhaps in the promotion of nose breathing and the prevention of chronic nasal cold in infants. THE SPLEEN Structure. The spleen is the largest lymphoid organ of the body. It is located to the left and dorsally between the stomach and diaphragm, has an irregular oval outline, measures about five inches in length and three inches in thickness, and weighs about seven ounces. It is subject to great variations in size and shape. It is enveloped by a thick fibro- elastic capsule,, or tunica albuginea, containing smooth muscle in its inner portion. External to this is also a peritoneal investment, or tunica serosa. The capsule of the spleen of the ox is especially robust, and rich in smooth muscle. At one point, the liilum, the capsule projects into the spleen as a large mass of trabecular tissue. Over the entire surface also other trabeculse project from the capsule into the paren- chyma of the organ. These trabeculse are of similar structure to the capsule. The supporting tissue of the parenchyma is a delicate reticu- 246 THE LYMPHATIC SYSTEM lum. This is continuous with the fibro-elastic terminal processes of the fibromuscular trabeculae. The primary trabeculae divide the parenchyma imperfectly into roughly pyramidal compartments about one millimeter in diameter, with three trabeculae for each lobule. This lobulation is faintly indicated by surface markings. According to Mall this unit of structure, the splenic lobule, is further subdivided into about ten smaller compartments by FIG. 250. PORTION OF SPLEEN OF CAT, SHOWING CAPSULE (ABOVE AND AT LEFT) AND FIVE SPLENIC NODULES. Between the nodules can be seen vascular trabeculae continuous with the capsule. X60. anastomosing septa, continuous with the primary trabeculaa. The divi- sion of the spleen into lobules, and their subdivision into lobular com- partments has structural significance also from the viewpoint of the blood supply. A knowledge .of the microscopic structure of the spleen is dependent upon an understanding of the distribution of the blood- vessels. Blood-vessels. The splenic artery enters at the hilum, associated with the splenic vein. The larger arterial branches are located within the coarser trabeculae continuous with the connective tissue of the hilum and still accompanied by the larger tributaries of the splenic vein. The THE SPLEEN 247 smaller arteries part company with the veins when they leave the tra- beculse and pass into the apices of the lobules. At the point of entrance into the lobule the adventitia of the intralobular artery becomes in- filtrated with lymphocytes, forming thus a spherical or fusiform lymphoid mass, the splenic nodule (Malpighian cor- puscle) character- istic of the spleen. This arterial ves- sel gives off nu- merous branches to the splenic nodule, some of which pass beyond the confines of the nodule into the splenic pulp. Some of these nodules contain germ centers. In infancy all of the nodules are said to contain germ centers. The artery usually passes excentri cally through the nod- ule. The nod- ules are fre- quently situated FIG. 251. DIAGRAM OF A LOBULE OF THE SPLEEN. A, artery lying in the center of the lobule; Am, a terminal ampulla of the artery; C, intralobular vein; L, a splenic cor- puscle; P, venous plexus within the pulp of the spleen; TV, fibromuscular trabecula within the lobule; V, interlobular vein, lying in a large trabecula. (After Mall.) at the point where the artery branches and in consequence contain two arterial vessels. Beyond the splenic nodules, the intralobular artery breaks into a number of twigs, one for each lobular compartment. Within each compartment, the arteriole divides into a brush of delicate precapil- lary arterioles, the penicilli of Euysch. On these appear an ellipsoidal condensation of reticular tissue forming the so-called splenic ellipsoids. These arterioles (six to ten microns in diameter) are known as sheathed arteries. The splenic pulp of the lobules can be divided into somewhat 248 THE LYMPHATIC SYSTEM denser cords of uncertain outline, the splenic pulp cords, and slightly looser intercordal pulp, the venous sinuses, corresponding to the sinuses of the medulla of lymph nodes. The penicilli are located in the pulp cords. Terminally these penicilli expand into dilatations, the ampullae of Thoma. The exact method of passage of the blood from the terminal arterioles to the initial venules is uncertain and disputed. It is certain only that blood passes freely at this point into the splenic pulp. This gives the adenoid tissue a deeply red or purple color, in contrast to the light pink color of ordinary lymph nodes, due to the presence of innumerable ery- throplastids. It seems prob- able that the walls of the con- necting capillaries and the ini- tial venules (cavernous veins, venous sinuses) are fenestra- ted, permitting the free pas- sage of blood from these vessels into the spleen pulp. j^ ' "^ ^fe* According to some au- b '-^ thorities the blood passes FIG. 252. THE ORIGIN OF A VEIN IN THE by two routes from the SPLENIC PULP. arterioles to the venules; a, venous endothelium; 6, leukocytes; c, (1) through the arterial red blood corpuscles (appearing rather too ampu]la3 directly into the dark in the reproduction); d, a mesh of the r splenic pulp. Highly magnified. (After venules (venous ampullae) ; Bannwarth.) and (2) from other ter- minal arterial twigs into the spleen pulp, from where it is collected by the venules. The veins thus begin as wide sinusoidal channels (pulp veins) within the splenic pulp. At first, and for a considerable distance, they follow an independent course through the pulp, receiving at the same time frequent accessions of blood from other venous radicals. Finally, how- ever, the veins enter the larger trabeculae, but are still devoid of more complete coats than the thin membrane of fibro-elastic tissue which sur- rounds the endothelial tube, but which is now ensheathed by the trabecu- lar tissue. Henceforth the path of the veins lies within the trabeculse (interlobular veins), and is directed toward the hilum. On approaching the hilum the larger veins acquire the usual venous coats. Having ar- rived at the hilum, they form several efferent vessels wfiich, in the out- lying connective tissue, form by their union the splenic vein. THE SPLEEN 249 Differentiation of Spleen from Lymph Node. The spleen in sec- tion can be readily differentiated from a lymph node : it lacks a_dennite subdivision into cortex and medulla characteristic of the lymph node, its pulp contains a preponderating number of red blood corpuscles, and its capsule is relatively very robust and contains a greater amount of smooth muscle. The spleen may be thought of as a congested lymph node, con- sisting wholly of medulla, throughout which are scattered lymph nodulea, the splenic nodules. It might quite properly be described as a huge hemo- lymph node. Splenic Cells. Besides erythroplastids and occasional erythrocytes, the pulp contains also the several varieties of leuko- /f cytes: lymphocytes, granu- locytes, a few megakaryo- cytes, and blood-platelets. The leukocytes of the spleen are largely of the large mononuclear type. These are notably phagocytic, fre- quently containing erythro- plastids, fragments of cells, pigment and other granular debris. They are in a sense specific for the spleen, hence called splenic cells. Mega- karyocytes are abundant in the fetal spleen during its period of erythropoietic function, but rare in the adult spleen. Lymph Supply. The lymph supply of the spleen is relatively scanty. The capsular is independent of the parenchymal system, which latter consists . chiefly of perivascular lymph spaces and vessels draining toward the hilum. Nerve Supply. The innervation includes both medullated and non- medullated fibers. The latter predominate, and are distributed to the smooth muscle of the capsule, trabeculae, and blood-vessels. Functions. The spleen functions as a leukopoietic organ and as a blood filter. This is indicated both by the direct evidence of lymphocyte FIG. 253. TYPES OF CELLS FROM A SMEAR PREP- ARATION OF THE PULP OF THE HUMAN SPLEEN. o, lymphocytes; b, polymorphonuclear neutro- phil leukocytes whose granules are not stained by the method used; c, large mononuclear leuko- cyte; d, eosinophil cells; e, basophil cell; /, red blood corpuscles. Hematein and eosin. X 1200. 250 THE LYMPHATIC SYSTEM proliferation and the ingestion of erythroplastid debris by the splenic and endothelial cells, and by the fact that the proportion of lymphocytes to erythroplastids in the splenic vein is very much greater than in the splenic artery. After severe hemorrhage or certain anemias the spleen may resume its fetal erythropoietic function. Numerous small super- numerary spleens, of varying size but usually about the size of a pea, are frequently found in the vicinity of the spleen. Besides the production of lymphocytes and the destruction of senile red corpuscles, the normal adult spleen has been credited also with an eryth- ropoietic role, and with a function concerned with the metabolism of iron. That the spleen does not, however, have any specific function absolutely essential to life is proved by the fact that it may be removed without serious consequence. Obviously its function may be taken over by some other organs. Such compensatory role is usually attributed to the hemo- lymph nodes and the red marrow. However, splenectomy in dogs is not followed by increase in the number or size of the hemolymph nodes, nor by a production of accessory spleens (Meyer, Jour. Exp. Zool., 16, 2, 1914). Removal of the spleen in dogs seems to exert a stimulating effect upon the formation of red cells in bone-marrow (Krumbhaar and Musser, Jour. Exp. Med., 20, 2, 1914). Pearse and Pepper (Jour. Exp. Med., 20, 1, 1914) found that splenectomy caused a transformation of yellow into red marrow. The result is interpreted as showing that in the absence of the spleen the mar- row may take on the function of storing and elaborating the iron of the blood pigment for future utilization by new red cells. The experiments of Austin and Pearse (Jour. Exp. Med., 20, 2, 1914) on the contrary lead them to conclude that the spleen does not exert a constant and important function on iron metabolism. The complete function of the spleen appar- ently remains largely unknown. Like the thymus it is sometimes classified among the organs of internal secretion. Development. The anlage of the spleen appears at the beginning of the second month as a condensation and swelling in the mesenchyma on the left border of the dorsal mesogastrium. The overlying mesothelium proliferates extensively and its cells invade the mesenchyma obliterating the line of demarcation. The early histogenesis is obscure. The mesen- chyma is potentially capable of producing all the definitive elements of the spleen: connective tissue capsule and framework, and lymphocytes. The mesothelium, genetically closely related to mesenchyma, probably aids in the general process. Probably also the bulk of the later lymphocytes invade the spleen from without. CHAPTER X MUCOUS MEMBRANES GLANDS The histologic structures which are necessary for the formation of a secretion include an epithelial surface, and a tunica propria of con- nective tissue which supports the requisite blood and lymphatic vessels and the controlling nerve supply. These structures may either form smooth membranous surfaces or apparent epithelial invaginations. The former are found on the surface of the mucous membranes, the latter are the secreting glands. MUCOUS MEMBRANES The mucous membranes may be said to include all those secreting surfaces which are directly or indirectly connected with the surface of the body, hence their epithelial clothing is continuous with that of the skin. The mucous membranes form the lining coat of the respiratory and alimentary systems, together with the ducts of their secreting glands ; in the nose this membrane is continuous through the tear ducts with the conjunctiva of the eye and through the auditory (Eustachian) tubes with the lining membrane of the middle ear. The broad expanse thus formed is known as the gastro pneumonic mucous membrane. A second membranous sheet, the genito-urinary mucous membrane, clothes the organs of the genital and urinary systems ; it thus forms the lining mem- brane of the vagina, uterus, and Fallopian tubes, of the urethra, bladder, ureters and pelvis of the kidney, of the ducts and tubules of the prostate gland, the testis, and the smaller secreting glands which are connected with the genital system. A mucous membrane consists of a superficial layer of epithelium of varying type, which rests upon a basement membrane (membrana propria) and is in turn supported by an investment of connective tissue, the tunica propria, or corium. The tunica propria is richly supplied with small blood-vessels and lymphatics; its nerve fibrils are not only distributed to the walls of the blood-vessels but in many cases send ter- 251 252 MUCOUS MEMBKANES GLANDS minal filaments which enter the epithelial layer and terminate in contact with the secreting cells. The mucous membranes are mostly contained within hollow organs which are subject to alternate col- lapse and distention; hence the membranes are frequently much folded. The deeper portion of the mucous membrane usually contains a more or less well-defined layer of smooth muscle fibers, the muscularis mu- cosce. The mucous membranes, as their name indicates, are nearly all moist- ened by a mucus-containing secre- tion. The relative amount of mu- cus which its secretion contains, and consequently the viscidity of the secretion, bears a close physio- logic relation to the intensity of the mechanical irritation to which the membrane is subjected. Thus the mucus-secreting goblet cells of the gastropneumonic membrane are here and there reenforced by numerous mucus-secreting glands of considerable size; these are es- pecially abundant in the oral cav- ity, pharynx, and esophagus, and in the nose, trachea, and bronchi; in the urinary system even the goblet cells are absent. The basement membranes, upon which the epithelium of the mu- cous membranes and the secreting glands is supported, are connective tissue structures. They are some- a V FIG. 254. DIAGRAM OF A Mucous MEM- BRANE HAVING SIMPLE TUBULAR GLANDS. o, artery; 6-6, basement membrane; o, connective tissue; d, duct of the gland, lined by cuboidal cells; E, epithelium of the free surface with clear, columnar cells; G, lumen of the fundus of the gland, lined by granular, serous secreting, col- umnar cells surrounded by secretory capillaries; V, veins. The arteries are striped, the capillaries and veins, black. Nerves are not represented. times formed by interlacing bun- dles of delicate collagenous fibers intermingled with numerous elastic fibers. Frequently, however, they consist of reticular tissue. Basement membranes of this nature have been demonstrated by Mall and his pupils in the mucous membranes and glandular tubules of the stomach, intes- tine, liver, salivary glands, kidney, testis, and thyroid. Occasionally basement membranes are homogeneous or hyaline in structure and pre- sent a more or less clear or glassy appearance. GLANDS 253 GLANDS The secreting glands may be quite properly considered as invagina- tions of the epithelial surfaces of the mucous membranes. They appear as such in the embryo. Their earliest anlage is formed by a solid or funnel-shaped process of epithelium, in which a distinct lumen soon appears, and which grows into the surrounding mesoblast, carrying with it its embryonal tunica propria. CLASSIFICATION OF GLANDS The form of the glandular invaginations is subject to great variation. They may be straight and simple, more or less branched and compound, convoluted or coiled, or the tubules may terminate in minute ampullary FIG. 255. DIAGRAMS OF THE PRINCIPAL TYPES OF GLANDS. T, simple tubular (intestinal glands) ; Ts, slightly coiled tubular (intestinal glands and small sweat glands) ; Tss, more extensively coiled tubular (larger sweat glands) ; Tr, branched tubular (gastric glands); Tc, compound tubular (kidney, lacrimal glands); As, simple alveolar or saccular (small sebaceous glands); Ar, branched alveolar (larger sebaceous glands, tarsal glands of eyelid); Ac, compound tubulo- alveolar or racemose (A) (salivary glands) ; compound saccular (B) (mammary gland). In major part after Prenant, Bouin and Maillard. enlargements, the acini or alveoli. The invaginations may also be dis- tinctly tubular and of approximately equal diameter throughout, or they may form pouch-like saccules. According to the form of the organ, it is thus possible to distinguish the following 254 MUCOUS MEMBEANES GLANDS HISTOLOGIC TYPES OF GLANDS I 1. Simple. 2. Convoluted. I. Tubular J 3. Branched. 4. Compound. 5. Compound tubulo-acinar (alveolar) ; racemose. 1. Simple. II. Saccular r , . J 2. Branched, (alveolar) | 6. Compound. III. Ductless glands; endocrine glands. Glands of the tubular and saccular types contain an actively secreting portion or fundus and a duct. Such externally secreting glands are also known as 'exocrine' glands. In the ductless ('endocrine') glands the duct is absent. The duct, though its epithelium may take some part in the formation of the glandular secretion, primarily serves to convey the secretion of the fundus to the free surface of the mucous membrane. The epithelium of the duct, as a rule, more or less closely resembles that of the mucous membrane upon whose surface it .opens. The epi- thelium of the fundus, on the other hand, usually differs from that of the duct and varies according to the nature of its secretion. In many of the glands the epithelium is typically mucus secreting; others produce a clearer, watery, and less viscid serous secretion. Hence it is possible to distinguish the following PHYSIOLOGIC TYPES OF GJLANDS I I. Serous glands. II. Mucous glands. III. Glands which are both mucous and serous (mixed glands). IV. Glands which are neither mucous nor serous. This physiologic classification is not in any way the equivalent of the histologic gland types mentioned above. Thus both serous and mucous glands, in different locations, form almost every variety of tubular gland. DESCRIPTION OF PHYSIOLOGIC TYPES The glands of the fourth type are too varied in their structure to be considered collectively to advantage. The reader is referred to 6LANDS 55 the several chapters in which they are described in detail. This type includes the testis, the prostate, the ceruminous glands, many_pf jthe ductless glands, and also some authors describe the ovary and the lungs as comforming to the glandular type of structure. The mixed glands include some tubules which are characteristically mucous, while others are typical serous secreting. Occasionally both types of secreting cells are contained within the same tubule. Mucus-secreting 1 cells possess the general characteristics which have been previously recited under the head of goblet cells ( Chapter II ). When void of secretion the cytoplasm of mucous cells is granular, their nucleus centrally situ- ated, and their shape more or less columnar. The pre-secre- tion accumulates in the cen- tral portion of the cell and occupies an area, adjacent to the glandular lumen, which .steadily increases in size until FIG 256 ._ TBANSECTION OF THREE SECRETING -,the greater part of the cyto- TUBULES OF THE SUBMAXILLARY GLAND OF plasm has been replaced; the MAN. nucleus is pushed to the proxi- A * a serous tubule ' B > a mucous tubule; C, ,, a mucous tubule with a demilune, d. Hematem ; ma i or attached end of the cell; an( j eosm >< QQ .and the whole cell often be- . comes swollen and distended to more than double its original size. Finally the cell membrane ruptures and the mucus pours out upon the free sur- face of the membrane. At the base of the mucus-secreting cells, and between them and their basement membrane, are groups of epithelial cells having a finely granular cytoplasm, which form crescentic cell masses, the demilunes tubular lumen is consequently diminished in size or even occluded. Finally the period of secretory activity arrives, and the secretion is poured into the glandular lumen; the cells become shrunken and the lumen of the tubule correspondingly dilated. The cytoplasm returns to its former condition; if the secretion is of a granular character the cell becomes clearer, but if watery the cytoplasm acquires a finely granu- lar appearance. The nucleus resumes its former central location and the cell enters upon a second period of constructive and accumulative activity. GLANDS 2-57 Many of the serous-secreting cells contain minute miracellular canals which connect with a network of intercellular passages about the^ cell. The intercellular canaliculi may, on the one hand, open into the glandu- lar lumen, or they may communicate with the tissue spaces of the tunica propria. This system of intracellular and intercellular canaliculi may thus serve either as a system of nutrient channels or as a network of secretory capillaries by which the secretion is conveyed from the interior of the secreting cells to the lumen of the gland or even to the duct system. Nutrient and secretory canaliculi of this nature have been demonstrated in the secreting cells of the liver, cardiac glands of the stomach, salivary glands, pancreas, adrenal, and epididymis, but they are not by any means confined to the actively secreting cells, for they have been found in the cells of bladder epithelium (Holmgren) and. are highly developed in the nerve cells (Holmgren, Golgi, et al.). DESCRIPTION OF HISTOLOGIC TYPES Simple Tubular Glands. Simple tubular glands occur in the: mir- cous membrane of the small and large intestine as the crypts of Lfeber- ktihn or intestinal glands. In shape these glands resemble a test-tube. They form straight tubules which open on the free surface of the mem- brane, are of approximately equal caliber throughout, and at their deeper end terminate in a blind extremity. The tubules are lined with epi- thelium and are embedded in a thin vascular tunica propria. Their epithelium includes the usual columnar and goblet cell types, the latter being more abundant near the mouth of the gland. Near the blind extremity are certain granular cells, the granules of some of which are slightly basophilic: other cells possess coarse granules which are highly acidophil, as demonstrated by Kultschitsky (Arch. f. mik. Anat, 1897) in the intestinal glands of the dog, an observation which is easily cor- roborated for the simple tubular glands in the small intestine of man. Convoluted Tubular Glands. Convoluted tubular glands occur as the sweat glands of the skin, the ceruminous glands of the ear, and the glands of Moll in the eyelids. The above are typical simple coiled glands. Certain other glands, which are less typically coiled but are more or less convoluted near their blind extremities and are frequently branched, are also to be included under this type. Such glands are the pyloric glands of the stomach, and the small mucous glands of the oral and nasal cavities, pharynx, larynx, trachea, bronchi, and esophagus. Some of these glands, and especially those of the pyloric end of the ; 17 258 MUCOUS MEMBEANES GLANDS stomach,, present terminal acinar dilatations, hence they also resemble to some extent a small tubulo-acinar type of gland. The typical coil glands consist of a duct whose epithelium resembles an attenuated layer of the stratified epithelium upon which they open, and a fundus or secreting portion which is lined by columnar epithelium of the glandular type. They also possess a connective tissue basement membrane and a vascular tunica propria. Branched Tubular Glands. Branched tubular glands include the cardiac and pyloric glands of the stomach and the glands of the uterine mucous membrane. These glands possess a duct whose epithelium corresponds in type with that of the surface upon which they open. Several secreting tubules open into this duct by means of a short constricted por- tion, the neck. The fundus or secreting portion, after a typically spiral course, ends with a blind extremity which is often curved or hooked. This portion of the gland is clothed with columnar or glandular epi- thelium and invested with a thin basement membrane and tunica propria. Compound Tubular Glands. Com- pound tubular glands include the kidney, testis, lacrimal gland, and liver. The finer structure of the glands of this type is so pe- culiar that the reader must be referred to the several chapters in which they are more fully described. Compound Tubulo-alveolar Glands (Tubulo-acinar or Racemose Glands). This is the most widely distributed of all the types of se- creting glands. It includes the parotid, the submaxillary, the larger mucous and serous glands of the oral cavity, and of the nose, pharynx, trachea, bronchi, and esophagus, the duodenal (Brunner's) glands, the pancreas, bulbo-urethral (Cowper's) glands, urethral (Littre's) glands, and the large mucous glands of the cervix uteri. The form of these glands may be likened to a much branched tree, whose stem as the main excretory duct opens upon the free surface of a mucous membrane, and the branches and twigs as the larger and smaller interlobular ducts reach out in all directions to finally end in minute alveolar dilatations, the secreting acini. FIG. 257. MODEL OF A RE- CONSTRUCTION OF THE LAC- RIMAL GLAND OF MAN. The tubular duct divides into the terminal secreting tubules, forming a compound tubular gland. X 170. (After Maziarski.) GLANDS 259 Except for the ducts of certain mucous glands whose epithelial coat resembles that of the mucous membrane to which they are attached^ the ducts of this type of secreting gland are lined by columnar cells whose cytoplasm fre- quently presents a rodded appearance at the deeper end of the cell. The acini contain typical serous or mucous, secreting epithe- lium. Occasionally the secreting cells are also found for some distance beyond the acinus in the lining membrane of the small- est ducts. The tubules and acini of these glands are invested with a basement mem- brane and a delicate tunica propria. The acini are united by the connec- tive tissue into small groups which inclose a central duct of the smallest type, the intercalary (or inter- mediate) duct. These acinar groups are again united into the lobules of the gland by fine bands of connective tissue, and broader bands of loose connective tissue ce- ment the many lobules into one glandular mass. The intercalary ducts by union within the lobule form numerous small intralobular ducts which approach the periphery of the lobule and at its margin open into the inter- lobular ducts; the latter are found in the broader septa of connective tissue between the lobules. The interlobular ducts by union with one another result in progressively FIG. 258. RECONSTRUCTION OF A Mucous GLAND FROM THE RESPIRATORY REGION OF THE NASAL MUCOSA OF A CHILD. The duct passes directly into the secreting alveoli. A typical small tubulo-alveolar gland. X 200. (After Maziarski.) FIG. 259. RECONSTRUCTION OF AN INTRALOBULAR DUCT DIVIDING INTO ITS TER- MINAL INTERCALARY DUCTS AND ACINI. The terminal divisions of a large compound tubulo-aci- nar gland. The model was made from serial sections of the human pancreas. X 344. (After Maziarski.) larger branches which finally form the main excretory duct of the gland. Simple Saccular Glands. Simple saccular glands occur as the 260 MUCOUS MEMBRANES GLANDS smallest sebaceous glands of the skin. These are small glandular pouches with a short duct, a constricted neck, and a dilated fundus which, in- stead of having a single coat of epithelium as in most of the tubular glands, is more or less completely filled with a mass of epithelial cells. The cells as they approach the duct of the gland show progressive stages of degeneration and disintegration which culminate in the formation of a thick, viscid, fatty secretion. Since these cells form their secretion by disintegration they are obviously capable of passing through the various stages of secretory activity but once, and hence they must be renewed by the repeated mitotic cell division which occurs at the periph- ery of the saccule. The epithelium of the secreting saccule rests upon a distinct base- ment membrane and is invested with a very vascular tunica propria. Branched Saccular Glands Branched saccular glands include the larger of the sebaceous glands of the skin, in which several saccules pour their secretion into a common duct, and the tarsal (Meibomian) glands of the eyelids in which a considerable number of saccules open into an axial canal by which the secretion is conveyed to the terminal duct. The structure of each glandular saccule of this type is identical with that of a simple saccular gland. Compound Saccular Glands. This type includes only the mam- mary gland. It consists of a system of tubular ducts which possess ampullary dilatations and many branches. Its ducts terminate in small saccular alveoli which have a thin epithelial lining. During the period of their inactivity the lining epithelial cells are much flattened and the acini appear shrunken. The epithelium of the lactating gland, on the other hand, is cuboidal or columnar, the height being more or less dependent upon the accumulation of secretion within the cell. The secretion is formed in the same manner as in the tubular glands with an additional process of fatty infiltration by which fat droplets are formed within the cytoplasm. These droplets collect in the central portion of the cell and are finally discharged into the lumen of the acinus with apparent rupture of the cell membrane and the escape of a portion of its superficial cytoplasm. The epithelium is thus capable of repeated secretion. The mammary glands may be considered as offering an inter- mediate type between the branched saccular and the tubulo-acinar types. Ductless Glands (Endocrine Glands}. Under the head of glands it is necessary to consider certain structures which apparently contain GLANDS 261 secreting epithelium and which present a more or less distinct tubular arrangement. These bodies are the adrenals, thyroids, parathyroids, carotid glands, coccygeal gland, paraganglia, hypophysis cerebTi, ~and epiphysis cerebri (pineal gland). These are also known as organs of internal secretion. While these glands do not possess an excretory duct, nevertheless some of them certainly, and the others probably, form certain products which find their way into the blood or lymph as so-called 'internal se- cretions? The epithelium of the glands may form either alveoli, tubules, or solid cell columns, which are supported by very delicate connective tissue tunics. Many blood-vessels, often of the thin walled sinusoidal type, are found within these tunics and are thus brought into intimate relation with the epithelial parenchyma. In some instances lymphatics are distributed in a similar manner within the gland. The property of internal secretion is not peculiar to the ductless glands. It has long been ascribed to the liver cells in connection with their influence upon nitrogenous and carbohydrate metabolism, and, in fact, many secreting glands, even though not of vital importance, are nevertheless found to influence the economy in certain ways which can not be accounted for by the properties of their external secretions. Finally, it must be emphatically stated that the types of secreting glands, as above described, are not bound by hard and fast lines, but many forms will be found which might well be placed under either of two or more types. Hence any classification of secreting glands be- comes more or less arbitrary; nevertheless such a classification is of extreme importance as serving to establish in the mind of the student certain typical pictures with which individual glands may be compared, and important structural details will thus be noticed which might other- wise escape observation. CHAPTER XI THE SKIN The skin, or integument, consists of an outermost, ectodermal layer of stratified squamous epithelium, the epidermis (cuticle), and a sub- jacent mesodermal layer of dense connective tissue, the corium or derma (derma vera, cutis vera), homologous with the tunica propria of the mucous membranes. The corium contains the nerves and the nerve end organs of special sense, and rests upon a subcutaneous layer of areolar and adipose connective tissue which*, blending with fascia or periosteum, unites the skin to the underlying muscles and bones. The skin serves a composite function : protection, regulation of body temperature, tactile sensation, and excretion. Its excretory role is in fact accessory to the kidney. The skin is typically a stratified organ, and for convenience of description may be divided into the following layers : 1. Scaly layer. 1 2. Flattened cell layer. I Horny layer; 3. Eleidin-containing layer ; f stratum corneum. stratum lucidum 4. Granular layer; stratum] SKI1ST. -I granulosum. 5. Prickle cell layer. I. Epidermis.- I Germinal, Mal- > pighian or mu- cous layer. 6. Cylindrical cell layer. 1. Papillary layer. 2. Reticular layer. III. Subcutaneous tissue. II. Derma. THE EPIDERMIS The epidermis (cuticle) or scarf skin serves for the protection of the more sensitive corium or 'true skin/ It is formed by a dense layer of stratified epithelium and varies in thickness in different portions 262 THE EPIDERMIS 263 of the body, being thickest upon those surfaces which are exposed to the greatest mechanical violence, e.g., the palms of the hands and soles^of the feet; and thinnest in the least exposed portions, e.g., inner sides of the arms and the back. The layer of stratified squamous epithelium composing the epidermis differs from that of the mucous mem- branes in that its superficial cells con- tain an abundance of keratin, a peculiar horny material. The production of keratin in the cells of stratified epi- thelium appears to be more or less de- pendent upon the desiccation which oc- curs in those cells which form the com- paratively dry cutaneous surface. The cornification can scarcely be demon- strated in the stratified squamous epi- thelium of the moistened mucous mem- brane of the mouth, esophagus, etc. ; it is present though not pronounced in the partially moistened margins of the eyelids, lips, labia minora, glans penis, etc. In the epidermis, however, corni- fication is pronounced and characteris- tic in all portions of the body. The thickness of the cornified lay- er's appears to be in proportion to, if not entirely dependent upon, the amount of mechanical violence to which the cutaneous surface is subjected. Ac- cordingly the increased thickness of the epidermis covering the palms and soles is found to be due almost entirely to an increase in the superficial horny portion of the epidermis, the germinal layers being no more pronounced than in other portions of the body. The epidermal tissue is divisible into a superficial horny portion consisting of flattened, desiccated, cornified cells the stratum corneum or horny layer and a deeper protoplasmic, so-called 'mucous' portion, which consists of polyhedral and cylindrical cells stratum germinati- vum, or mucpsum, rete mucosum, rete Malpighii, FIG. 260. EPIDERMIS OF THE FOOT. a, flattened cells; 6, stratum luci- dum; c, granular layer; d, germinal layer; e, cylindrical cell layer; /, derma. Picrocarmin. Moderately magnified. (After Ranvier.) 2G4 THE SKIN Cylindrical Cell Layer (Stratum CyUndricum).The deepest cells of the stratum mucosum are elongated in a direction nearly perpendicu- ^S^^^^SP^^^^^^^^^^^^^^^. te - * =;%fe^^ sfe8?l ">^ ^J->H/>^^v-,::;;:V : ;,. W&M^. ^^^^0^^m^m^Mm^^^'^^^*A r3SB$&ii '.^,.,:^:;i> ' : p^f;S!s::> H tei ISm ,ti)' :v ;s ll 9' FIG. 261. SECTION OF THIN SKIN FROM ABDOMEN OF NEGRO, SHOWING THE DISTRI- BUTION OF THE PIGMENT GRANULES IN DERMAL AND EPIDERMAL CELLS. X 750. lar to the basement membrane upon which they rest; they are thus irregularly cylindrical in shape. It is these cells which in the pig- mented portions of the body, i.e., areo- Ia3 of the nipples, s c r o t u m, circu- manal region, etc., and in the skin of brunettes and the colored races con- tain the pigment which gives rise to the darkened color of the skin. In the skin of the negro, the pig- ment (m e 1 a n i c) granules, while most abundant in the basal cylindrical cells, are nevertheless present, but in progressively smaller number, in all of the outer cell layers. In the FIG. 262. SECTION OF THIN SKIN FROM ABDOMEN OF LIGHT BROWN MULATTO. X 750. THE EPIDERMIS 265 various shades of mulatto skin, there is a close correspondence between the degree of color and the number of pigmented layers of epidermal .cells. Certain connective tissue cells of the corium of pigmented skins ;also contain abundant melanin granules; in the darker skins such cells .are numerous. However, the dermal pigmented cells are not correctly regarded as the sources of supply of melanic granules for the epidermal tcells, as has been maintained. The latter can produce their own .granules; both dermal and epidermal pigmented cells owe their condi- tion to the same underlying cause (Jordan; Amer. Nat., vol. 45, 1911). 'The pigment granules are said to arise as a differentially staining nuclear ;substance ( c pyrenoid substance') which passes through the nuclear mem- brane into the cytoplasm, where it gradually acquires the character of pigment granules (Meirowsky: "On the Origin of the Melanotic Pig- ment in the Skin and the Eye/' Leipzig, 1908). The nuclear origin of the pre-pigment granules is, however, disputed by some. The more re- cent investigations indicate that the physiology of melanic pigment for- mation involves the interaction of a nuclear oxidase (tyrosinase) with ;an extranuclear, perhaps cytoplaemic, chromogen (tyrosin). The processes of mitotic ' cell division are very active in these 'columnar cells, and they, with the adjacent portion of the prickle cell layer, form the stratum germinativum of Flemming, in which the re- generation of the epidermis occurs. The cylindrical cells are firmly united to the basement membrane by delicate cytoplasmic fibrils, the intercellular bridges. Their nuclei are ovoid in shape, and vesicular in appearance. Prickle Cell Layer (Stratum Spinosum). Superficial to the cylin- drical cells is a stratum of polyhedral epithelium which extends inward between the adjacent papillae of the corium (interpapillary region of the epidermis), and is therefore thick in these portions, but is rela- tively much thinner over the apices of the dermal papillae (suprapa- pillary portion of the epidermis). The polyhedral cells of this layer contain a soft granular cytoplasm and a very chromatic, though vesicular, spheroidal nucleus. They are. separated from one another by narrow intercellular spaces which are bridged across by innumerable delicate cytoplasmic fibrils. These fibrils, connect adjacent cells and are frequently continued without interruption through one, two, or even three or four neighboring cells. Their course is characteristically curved, the convexity being directed toward the nucleus. Those portions of the numerous cytoplasmic fibrillae which span the intercellular spaces form the so-called intercellular bridges. 266 THE SKIN It is because of the resulting spinous appearance that the polyhedral cells have been termed prickle cells (Schultze). In the thinner portions of the epidermis the prickle cells are imme- diately covered by several layers of hard flattened cells whose nuclei have partially or wholly disappeared, and whose cytoplasm has been changed into a horny, keratin-containing mass. The flattening and desiccation of these cells becomes more pronounced as they approach the surface. In the thin portions of the epidermis the change from the prickle cell layer to the horny lay- er is abrupt. In the thicker por- tions of the epidermis, as in the palms of the hands, the change is more gradual, and re- sults in the appearance of two additional cell layers, in the cytoplasm of whose cells are in- termediate products of chemical metamorpho- sis, keratohyalin and eleidin, which may be considered as the prede- cessors of the keratin or par eleidin which is peculiar to the cells of the horny portion. Granular Layer (Stratum Granulo- sum). In the thicker parts of the cuticle the most superficial prickle cells become slightly flattened, and coarse granules appear within their cytoplasm. These cells form the granular layer (stratum granulosum), a double cell layer which occupies the superficial portion of the rete .muco&um. FIG. 263. SKIN FROM SOLE OF HUMAN FOOT, SHOW- ING SPIRAL DUCTS OF Two SWEAT GLANDS OPENING THROUGH THE EPIDERMIS. The stratum germinativum is represented by the dark, wavy line; below this is the stratum papillare of the derma, above the stratum lucidum. THE EPIDERMIS 267 The cells of the granular layer are flattened and angular. They possess an indistinct, apparently degenerating nucleus, and theif eyto- plasm contains large plate-like granules of keratohyalin (eleidin of Ranvier), which are strongly basophil and stain readily with most nuclear dyes. Eleidin-containing- Layer (Stratum Lucidum). The granule cells are abruptly transformed into the shiny cells of the stratum lucidum, which is the deepest layer of the horny portion of the epidermis. The cells of this layer possess an indistinct nucleus, are irregularly flattened and angular in shape, are more or less fused together at their adjacent margins, and contain a smooth, highly refractive, glassy cytoplasm which reacts feebly to most staining reagents, but is deeply colored by safranin. The stratum lucidum is so named because of its highly refractive appear- ance; it is usually about two cells thick. Its cyto- plasm contains eleidin,, a substance which is prob- ably intermediate in chem- ical composition between the keratohyalin of the stratum granulosum and the keratoid pareleidin of the horny cells. Flattened Cell Layer and Scaly Layer (Stra- tum Corneum and Stra- L FIG. 264. TRANSECTION OF THE EPIDERMIS OF THE FOOT. a, superficial scaly layer; 6, layer of flattened cells, the inner and outer portions of which have been characteristically blackened by osmium tet- roxid; c, stratum lucidum; d, granular layer; e, prickle cells; /, cylindrical cells; g, papillary layer of the derma. Osmium tetroxid, carmin. Moder- ately magnified. (After Ranvier.) turn Disjunct-urn of Ran- vier). Above the stratum lucidum the horny layer consists of flattened corni- fied cells which are closely packed and somewhat fused and blended with each other at their faintly serrated margins. Intercellular bridges and spaces have almost entirely disappeared. The nuclei of the cells are no longer demonstrable, and their cytoplasm has been changed into a dry, shiny, highly refractive mass of 'keratin (pareleidin) which responds but slightly to ordinary 268 THE SKIN stains. Its superficial layers stain deeply in osmic acid, indicating a considerable fatty alteration. If, however, these cells are acted upon by solutions of strong alkalies, soda, potassa, etc., the outlines of the degenerated nuclei reappear. As the cells are pushed nearer the free surface, by the process of cell division in the deeper layers and the coincident desquamation of cells from the free surface, they become more and more flattened and desiccated and more completely and firmly fused together until at the surface they form the partially detached cell masses or scales scaly layer, stratum squamosum which are eventually removed by continued desquamation. It is the thicker portions of the epidermis only, which possess all the characteristic layers above described. In other portions of the body the horny layer is much thinner (Fig. 262). In these thinner parts the cuticle of the epidermis consists of a prominent rete mucosum which is covered by a relatively very thin layer of horny cells. The stratum granulosum, in such portions, is not usually demonstrable, the stra- tum lucidum is absent or indistinct, and the entire horny layer consists only of flattened cornified cells, the more superficial of which form a very thin scaly layer. THE DERMA The derma or corium (derma vera, cutis vera) forms a connective tissue bed or matrix upon which the epidermis lies. It is divisible into two strata, a deeper reticular layer in which coarse fiber bundles inter- lace to form a loose connective tissue network, and a superficial papil- lary layer in which the finer bundles of connective tissue form a more closely meshed network. The Papillary Layer (Stratum Papillare). The surface of the papillary layer presents numerous conical elevations, the papillce of the corium, which project into corresponding cup-shaped cavities in the under surface of the epidermis. Many of the connective tissue papillae contain tactile end-organs (touch corpuscles of Meissner), and terminal filaments of the nerve fibers. They may therefore be regarded as the special organ of tactile sensation. Other papillae contain no touch cor- puscles but are richly supplied with capillary blood-vessels. Two types are thus distinguished, the tactile papillce and the vascular papillce. Papillae are most abundant in the palms of the hands and the soles of the feet, where they are mostly arranged in rows which are responsible for the fine lines and ridges visible to the naked eye. In other portions SUBCUTANEOUS TISSUE 269 of the body they are less numerous and are often less regularly disposed. The papillary layer consists entirely of white fibrous and elastic con- nective tissues which form a supporting membrane for the finer branches of the cutaneous blood-vessels and nerves. The elastic tissue supplies a rich network of fine fibrils to all portions of the papillary layer, and just beneath the epidermis it forms a delicate elastic membrane whose fibers intermingle with the hyaline cuticular deposit of the columnar epidermal cells to form a firm resistant basement membrane. Many of the elastic fibers of the papillae, especially the more superficial ones, pursue a peculiar archiform course from the base to the apex of the conical papillae. In this way they surround and inclose the centrally situated capillaries and the tactile corpuscles of the papillae. The Reticular Layer (Stratum Reticulare). The deeper portion of the corium consists of interlacing bundles of connective tissue fibers which form a dense meshwork. These bundles are much coarser than those of the papillary layer with which they are imperceptibly blended. The reticular layer contains the larger blood-vessels of the corium, many small nerve trunks, the ducts and parts of the secreting portions of the sweat glands, the more superficial sebaceous glands, and many of the smaller hair follicles. Lamellar corpuscles and nerve end-organs of Euffini are also found in this layer. The skin of the face contains many striated muscle fibers which are derived from the insertions of the mimetic muscles. The corium of the scrotum (where it forms the tunica dartos), of the penis, perineum, and areola of the nipple contain much smooth muscle, intermingled with which is a considerable amount of elastic tissue. SUBCUTANEOUS TISSUE The subcutaneous tissue (tela subcutanea, subcutis) consists of bands and septa of fibrous connective tissue which extend from the deeper margin of the derma to the underlying fasciae of the muscles, the peri- osteum of the bones, etc. The direction of these fibrous bundles is very variable. The more nearly parallel to the cutaneous surface the fiber bundles are, and the looser the meshes which they J[orm, the greater is the mobility of the skin. The meshes of the subcutaneous network are occupied by lobules of adipose tissue. When abundant the subcutis is termed panniculus .adiposus. The subcutaneous tissue contains the main nerve trunks and 270 THE SKIN larger blood-vessels of the skin, the larger sudoriparous and sebaceous glands, and the coarser hair follicles. It also, together with the deeper part of the derma, contains the nerve end-organs of Pacini, Euffini, and the G olgi-Mazzoni corpuscles (see Chapter VI). Small bundles of smooth muscle fibers which form the arrectores pilorum muscles take origin from the deeper surface of the corium and are inserted into that portion of the hair follicle which is embedded in the subcutaneous tissue. These fusiform or columnar muscle bundles are found in connection with all the hairs, but in the scalp they are most highly developed and lie most deeply in the subcutaneous tissue. At the level of the vascular plexuses between the corium and the tela subcutanea, Hagg- quist (Anat. Anz., 45, 2, 1913) has described a thick bundle of smooth muscle not previous- ly recognized. It lies directly beneath a cold spot, and is not found in skin lacking cold spots. The muscle is believed to contract reflexly when a cold object is placed on the skin and constrict the local blood supply. DEVELOPMENT AND * s GROWTH OF THE SKIN The skin may be said to arise with the first differen- tiation of the embryo into its three germ layers. The ectoblast, which is at first a single cell layer, becomes a double layer by the end of the first month. It continues to increase in thickness until by the end of the second month it can be differentiated into two layers, a superficial periderm (epitrichium) , and a deeper germinal layer. FIG. 265. THREE EARLY STAGES IN THE HlSTOGENESIS OF THE SKIN. A, single-layered epidermis from dorsal body wall of a 5 mm. human embryo; B, two- layered epidermis, from dorsal body wall of 13 mm. human embryo; C, multiple-layered epidermis from nose of 21 mm. pig embryo. Per, periderm (epitrichium) ; Epi, epidermis; Mes, mesenchyma differentiating into the derma. X 750. CUTANEOUS APPENDAGES 271 The periderm forms a layer of peculiar dome-shaped cells with flat- tened margins and a vesicular center. It continues to form the super- ficial layer of the epidermis until about the sixth month, when it is lost by desquamation. The germinal layer consists of a deep stratum of cylindrical cells and one or two superficial strata of spheroidal vesicular cells. The latter are known as the stratum intermedium. By the fifth or sixth month cell differentiation has advanced in the intermediate portion until cornification can be distinguished in its superficial cells. Further development is analogous to the growth of the mature epi- dermis; new cells are rapidly formed in the deeper portion, stratum germinativum, and are steadily pushed toward the surface, their migra- tion being either accompanied by slight, or later by more pronounced cornification, which in the latter case gives rise to the stratum granulo- sum, stratum lucidum, and horny layer, but in the former produces only relatively slight flattening of the superficial cells without the ap- pearance of keratin or the disappearance of the nucleus. The derma arises from the superficial layers of the mesoblast as ordinary connective tissue, in which the appendages of the skin make their appearance as ingrowths from the epidermis. Certain mesenchymal cells form the smooth muscle fibers of the arrectores pilorum muscles and of the derma of those locations where muscle is present in the mature skin. Other mesenchymal cells produce the fat lobules of the subcu- taneous tissue. Papilla appear during the fourth or fifth month but do not attain their completed development until much later. CUTANEOUS APPENDAGES The cutaneous appendages include the sudoriparous glands, the nails, the hairs, and the sebaceous glands. SUDORIPAROUS GLANDS (Glandules Sudoriparce, Sweat Glands) The sudoriparous glands occur in all portions of the skin, but more abundantly in certain locations, e.g., palms of the hands and soles of the feet, where their number has been estimated at between two and three thousand to the square inch axillae, groin, and circumanal re- gion. Over the back, where they are least numerous, their number is said to be less than five hundred to the square inch. They are long, coiled or convoluted, tubular glands whose secreting portions lie in the THE subcutaneous tissue and in the deeper part of the corium; ftieir ducts extend through the corium to the under surface of the epidermis where the lining epithelium of the duct becomes continuous with the cells of the interpapillary portion of the stratum germinativum. In its further course through the epidermis the duct of the gland forms only a FIG. 266. FROM A SECTION OF THE ABDOMINAL INTEGUMENT OF AN INFANT. Beneath a, and a', sweat glands are seen; the secreting portion of a' is detached from its duct; b, b, epidermis; c, c, derma; d, d, panniculus adiposus. Hematein and eosin. Photo. X 65. tortuous spiral cleft or passage whose wall is formed only by the con- centrically placed cells of the various epidermal layers through which it passes. The glands of the axilla and circumanal region are branched. The secreting or coiled portion of the gland (fundus) consists of a secretory epithelium resting upon a delicate hyaline membrana propria in whose outer portion are concentrically disposed connective tissue fibers. The inner portion of this membrane contains many longitudinal CUTANEOUS APPENDAGES 73 fusiform fibers whose nature is somewhat doubtful, though they have been most frequently considered to be smooth muscle fibers. These fibers are frequently branched,, their processes often extending between the cells of the secreting epithelium nearly to the lumen of the gland. The secreting epithelium of the fundus consists of tall columnar cells which possess a large spheroidal chromatic nucleus and a finely granular cytoplasm. The basal portion of their cytoplasm is often slightly rodded and the cells are so closely pressed together that it is frequently impossible to distinguish their outlines. The secreting cells are disposed in a single layer and, except after active secretion, are so !TfS& *k ;.>: >J ^^a^%^i% * ** \ -1 . ; S/YA - *^ .aw / ^HhevM. ^^^- r, , W^^.Kjl BF^^esyJe^SUs^ slilSi fas8 FIG. 267. SEVERAL COILS OF A SUDORIPAROUS GLAND OF THE HUMAN FINDER. a. secreting portions, their lumen containing traces of secretion; b, ducts; m, muscle cells. Hematein and picrofuchsin. X 550. tall as to leave only a very narrow, central, glandular lumen. During secretory activity the cells become shrunken and their cytoplasm more granular. After a period of rest the cytoplasm again becomes clear and vesicular in appearance and the cells are much distended. The secretion reaches the lumen through intra- and intercellular canaliculi. The ducts are lined by a double, occasionally triple, layer of some- what flattened epithelial cells, which rest upon a delicate membrana propria continuous with that of the secreting portion. The gross di- ameter of the duct is much less than that of the secreting portion of the gland, yet the lumen of the duct may be larger. That portion of the duct which is lined by the thin stratified epithelial layer pursues a spiral course through the subcutaneous tissue and the derma. It finally 18 274 THE SKIN reaches the epidermis, which it enters in the interval between the dermal papillae (interpapillary portion of the epidermis). Its lining epithelium is continuous with that of the stratum germinativum, and in its course through the epidermis the wall of the duct consists solely of the sur- rounding epidermal cells. The stratum granulosum and adjacent portion of the horny layer in the immediate neighborhood of the duct is in- vaginated into the stratum mucosum, which is thus considerably thinned by the passage of the duct. The sweat glands are abundantly supplied with capillary blood- vessels and small non-medullated nerves, which form plexuses about the walls of the coiled portion of the gland, and from which terminal fibrils penetrate the basement membrane and end in contact with the secreting cells. Development. The sudoriparous glands first appear in the embryo during the fifth month as solid columnar ingrowths from the stratum germinativum of the epidermis. These processes grow inward through the primitive corium to its junction with the looser subcutaneous tissue. Here the cell columns become thickened and convoluted, and at about the same period their lumen appears. The glandular lumen is not at first connected with the free surface, but as the cells of the germinal layers of the epidermis gradually replace those which are more super- ficial the epidermal portion of the duct is formed. At about the seventh month the lumen of the duct opens upon the epidermal surface. The membrana propria of the fundus and dermal portion of the duct is derived from the surrounding connective tissue elements of the mesenchyma. THE NAILS The nails are produced by a peculiar modification of the epidermis by which the stratum lucidum becomes greatly thickened (Bowen, 1899) while the horny layer (eponychium of the embryonic nail) is at the same time wanting. The nail is divisible into the nail body and nail root; the former comprising the exposed, the latter the hidden portion of the organ. The root of the nail is overhung by a fold of the skin, the thickened horny layer at the margin of which forms an adherent border, the eponychium of the adult nail. The nail groove or sulcus is included between the overhanging skin and the root of the nail. It is deep at its proximal end but is shallow at the lateral margins of the nail. The distal or free border of the nail projects over the skin at the tip of the finger and the thickening of CUTANEOUS APPENDAGES 275 the horny layer of the subjacent epidermis forms the so-called hypony- chium. Finer Structure. The nail consists of two layers, the superficial stratum lucidum and the deeper germinal layer. These are continuous at the border of the nail with the corresponding layers of the epidermis which lines the nail groove. At the distal border, however, the nail FEN H 268. TERMINAL PHALANX OF FINGER OP HUMAN FETUS. Showing A", nail; F, nail fold; H, hyponychium; E, eponychium; developing sudoriparous glands, and developing bone. proper or thickened stratum lucidum ends in a free margin. The finer structure of these two layers does not essentially differ from that of the corresponding layers of the epidermis. The stratum lucidum in the body of the nail is very thick and its cells are so completely blended with each other through the excessive eleidin production that it is impossible to distinguish their outlines. By maceration in alkaline solutions, however, the outlines of both cells and nuclei may be caused to reappear. In the nail root the stratum lucidum increases rapidly in thickness as it grows distalward; in the body of the nail this layer is not very materially thickened as it ap- proaches the distal or free margin. The stratum germinativum is of nearly equal thickness in all por- tions of the nail body. In the nail root it is somewhat thicker and 276 THE SKIN forms the nail matrix of Kanvier. In this portion also is a distinct stratum granulosum, a layer which is absent or rudimentary beneath the body of the nail. It is the presence within this layer of numerous keratohyalin granules which renders the root of the nail opaque and thus forms the dull white lunula which contrasts with the transparent, eleidin-containing, stratum, lucidum, which latter layer alone covers the germinal layer of the nail body (Unna). The NaH Bed. The nail rests upon a very vascular co- rium or nail bed (matrix of authors) which is continuous; with the corium or derma of the skin. The nail bed is some- times regarded as active in nail formation,, but this process is now generally believed to be limited to the matrix of the root. The nail bed at the margins of the nail is provided with papillae as in other por- tions of the skin, but beneath the body of the nail its surface is raised into longitudinal ridges which possess only very On the left is the skin, on the right the nail. . a, a', horny layer; 6, V, germinal layer; c, c>, mmute . secondary papillae, corium; d, margin of the nail; s, nail sulcus. Nail Growth. The growth Moderately magnified. (After von Brunn.) o f t } le na ji occurs i n the matrix of the nail root. The cells of the stratum germinativum of this portion, having been once formed by active mitosis push obliquely forward and outward toward the nail body. It is thus that the more advanced are constantly carried onward toward the free border. The growth of the nail occurs at the rate of about one thirty-second of an inch per week (Schafer). Development. In the fetus the nail appears as a direct formation of the epidermis, which is very early evidenced by a thickening of the stratum lucidum in the nail area. The nail is therefore at first covered by the superficial peridermal cells of the cuticle. The nail groove is rapidly formed by an invasion of the mesoblast by the epidermal FIG. 269. TRANSECTION THROUGH THE MAR- GIN OP A FINGER NAIL. THE HAIR 277 cells which become piled up at the margin of the groove to form an excessive horny layer, the definitive representative of the embryonic eponychium. At the distal extremity of the nail the superficial cells are also accumulated into a considerable mass which forms a promi- nent hyponychium. Further growth of the nail pushes its distal margin forward over the hyponychium so that the border becomes free shortly prior to birth. The peridermal cells are then shed and the nail body finally presents, at about the time of birth, its naked stratum lu- cidum. THE HAIR Development. The structure of the hair will be most readily appreciated if preceded by a brief introductory sketch of its development. THE HAIR GERM. The hairs 'arise at any time after the third month of fetal life, their earliest anlage appearing as a slightly increased proliferation of the cells of the germinal layer of the epidermis. The further multiplication of the cylindrical cells produces a solid colum- nar ingrowth of the epidermis which pene- trates into, and sometimes through, the primi- tive derma. The spheroidal cells of the inter- mediate layer of the epidermis increase in size, assume a vesicular character, and finally by fatty degeneration form the epidermal hair canal through which the future hair reaches the surface. THE HAIR COLUMN. The columnar epi- dermal ingrowths, hair columns or hair pegs, come into early relation with the anlage of the hair papilla which is formed by a proliferation of the mesenchymal cells at the tip of the hair column. Further development of the papilla FIG. 270. LONGITUDINAL VERTICAL SECTION OF THE YOUNG NAIL AND NAIL-BED OF AN IN- FANT. n. r., nail root or lunula; n., nail; n. b., nail bed; e., eponychium; h., hy- ponychium. (From Dahl- gren and Kepner.) S V - 9 -^A*"* rW $w r 4 L' V* * T " " j S-5 ; ^~;-^ CK"^ '-._>-- ^.- y/ ipp||^^^ g^ v i ' IV . Lif' -sm * \ . '^' *t&& ^ \ l^^^i^^ ^\^%m$ Z P ^ * : %a$l >.',i > -> i ^^5? J^r-^^V^ *$i&??^^ - --- - -m- ^Jj^^" ^?^''iJk*\'* ij ^ '*>* FIG. 271. FIVE STAGES IN THE DEVELOPMENT OF A HUMAN HAIR. a, papilla; 6, arrector pili; c, the line is directed toward the primordial shaft; d, cells which form the hair canal; e, sebaceous gland; /, hair germ in epidermis; 7, hair shaft; h, Henle's layer; i, Huxley's layer; k, cuticle of the root sheath, { inner root sheath; m, outer root sheath in tangential section; n, outer root sheath in longitudinal section; a, dermal root sheath; P, epithelial bed. X 460. (After THE HAIR 279 produces an indentation of the advancing hair column and gives rise to a true dermal papilla of considerable size. THE HAIR BULB. Coincident with the formation of the "papilla there is an increased proliferation of the cells of the hair column by which it is surrounded, and which therefore represents the future hair bulb. Two other swellings appear in the hair column; one, the more superficial, forming the anlage of the sebaceous gland, and the other, the deeper, forming the so-called epithelial bed or matrix of the hair which stands in close relation with the growth and future regeneration of the hair. This second swelling is sometimes interpreted as simply offering a point of attachment for the arrector pili muscle. The development of the hair papilla produces a slight evagination of the epithelium of the hair bulb, which is just sufficient to redirect the growth of central cells of the hair column toward the cutaneous surface. It is thus that the younger cells which arise by mitosis in the germinal layers of the hair bulb are pushed outward along the axis of the hair column where they form the shaft of the future hair. The growth of the hair from the germinal cells of the hair bulb is accom- panied by beginning cornification of the newly formed cells of the primi- tive hair shaft and of the intermediate cells of. the hair column. The growth of the shaft is, however, preceded by enlargement, vesiculation, and fatty degeneration of the central cells of the hair column, thus producing a central canal through which the hair may grow, and which later becomes continuous with the hair canal of the epidermis. THE HAIR FOLLICLE. At this stage the hair column has become differentiated into a peripheral follicle, the primitive root sheath, and a central hair. Continued multiplication of the cells in the germinal layer of the bulb pushes the advancing tip of the hair nearer and nearer the surface until it forces its way into the epidermal hair canal. Finally the thin cuticular covering is ruptured and the eruption of the hair shaft occurs. Further differentiation of the cells of the epidermal root sheath and the formation of a mesenchymal or dermal sheath of connective tissue completes the development of the hair follicle. This process is fre- quently repeated and results in the formation of new hairs not only during fetal life, but also, in constantly decreasing numbers, through- out childhood and adult life. The Mature Hair. Its development teaches that the hair follicle, being formed as it were by an invagination of the epidermis, contains a dermal and an epidermal sheath and that the outer portion of the 280 THE SKIN latter, being identical with the deeper portion of the epidermis, must possess a close structural resemblance to the rete mucosum, while its inner portion, like the horny layer of the skin, is more or less cornified. There is thus an outer and an inner epidermal root sheath corre- FIG. 272. FROM A SECTION OF THE SKIN OF AN INFANT'S ARM, SHOWING SMALL IMMATURE HAIR FOLLICLES IN TR/JSTSECTION. a-a, epidermis; b-b, derma; c-c, subcutaneous connective tissue; d-d, muscle. Hematein and eosin. Photo. X 95. spending respectively to the mucous and horny layers of the epidermis; the cornified portion, inner root sheath, becomes progressively thinner toward the hair bulb. The hair, on the other hand, represents an ex- cessively developed horny layer whose rete mucosum is found in the germinal layer of the hair bulb. The mature hair is divisible into a hair shaft or free portion, and a hair root or concealed portion. The latter is inclosed within an epi- THE HAIR 281 dermal and a dermal root sheath which together form the hair follicle. THE HAIR SHAFT. Sections of the hair shaft present a thin cuticle which consists of delicate horny scales whose free edges are imbricated upward, viz.,, toward the tip of the hair. Within the cuticle the hair may consist solely of a hair cortex formed by flattened and very much elongated horny epithelial cells, which frequently retain the remnant of a nucleus, and whose keratized cytoplasm is often much pigmented; or the axis of the coarser hair may contain enlarged angular cells in which eleidin granules and much pigment are found. In the latter case the hair is said to possess a medulla. The medulla is seldom if ever present throughout the entire length of the hair. When present it sometimes contains numerous air bubbles which, together with the paucity of pigment, produce the lighter shades of hair peculiar to certain individuals. In the light of its development it is obvious that the several layers of the hair shaft are comparable to the homologous layers of the horny epidermis, the cuticle, cortex, and medulla of the hair being respectively homologous with the scaly layer, the flattened cell layer, and the eleidin- containing layer or stratum lucid um of the epidermis. THE HAIR BOOT. The root of the hair, except for the fact that it is immediately invested with a hair follicle, does not in any way differ in structure from the hair shaft. It possesses the same three layers, the medulla, however, being very irregularly developed. The imbricated cells of its cuticle interdigitate with the similar cells of the cuticle of the inner root sheath in the deeper half of the follicle; in its superficial half, viz., above the opening of the sebaceous gland, a narrow space intervenes between the cuticle of the hair and that of the root sheath. The axis of the hair root is always inclined at an angle to the epi- dermis; it therefore makes with the epidermis an obtuse angle on one side and an acute angle on the other. The arrector pili muscle is always found on the side of the obtuse angle ; it therefore, by drawing the hair follicle and its inclosed hair root nearer the perpendicular, causes the erection of the hair. The sebaceous gland is included in the angle be- tween the arrector muscle and the hair follicle. Contraction of the muscle may aid also in the expulsion of the sebum. Extreme contraction of the arrectores pilorum muscles may result from fright, causing the hair to 'stand on end' ; in a similar manner cold air may effect, through the pilomotor fibers, the erector muscles of the small hairs distributed over the body and cause the so-called 'goose-flesh.' Rfl FIG. 273. FROM A SECTION OF THE HUMAN SCALP. Ap, arrector pili muscle; c, corium; ep, epidermis; fp, epidermal root sheath; Gap, muscular aponeurosis; gls, sudoriparous gland; glse, sebaceous gland; KH, so-called 'club-hairs' in various stages of molting and regeneration; pp, papilla of the hair; Re, fibrous band in the subcutaneous tissue; Rp, hair root; Sp, hair shaft; ts, subcutaneous adipose tissue. Hematoxylin and eosin. X 15. (After Sobotta.) 282 THE HAIR 283 THE EPIDERMAL BOOT SHEATH. The epidermal root sheath con- sists of an inner and an outer portion, each of which at about the "mid- level is divisible into three layers corresponding to the three similar layers of the horny and the mucous portions of the epidermis. In those portions of the follicle and in those individual hairs in which the process of cornification is less advanced these subdivisions cannot all be demon- strated, and it is only in the most highly developed hairs that they are typically found. This is in accordance with the structure of the epidermis, in which the subdivisions of its horny and mucous portions are typically found only in the more highly developed portions, e.g., the palms and soles. Inner Root Sheath. The cuticle of the inner root sheath consists of thin horny epithelial scales which are imbricated downward, viz., toward the hair bulb, and which interdigitate, in the deeper portion of the fol- licle, with the similar scales of the hair cuticle. The direction of the imbrication explains the removal 'of the epidermal root sheath when the hair is artificially extracted. The mid-layer of the inner root sheath, layer of Huxley, one or two cells thick, consists of horny cells which are somewhat flattened, and in which the semblance of a nucleus is sometimes present. It corresponds to the flattened cell layer of the epidermis. The outer layer of the inner root sheath, layer of Henle, is frequently wanting or imperceptibly blended with the preceding layer. Its cells are clear and highly refractive and their nuclei can but rarely be demon- strated in the usual microscopical preparations. The layer is seldom more than one cell deep. It is homologous with the stratum lucidum of the epidermis. Outer Root Sheath. The outer root sheath is continuous with the stratum mucosum of the epidermis and therefore contains similar cell types. The granular layer, as in the epidermis, is frequently absent or rudimentary, but can be readily demonstrated in hematein-stained sec- tions of the more highly developed hair follicles. It rests upon a layer, several cells deep, of spheroidal prickle cells. The outermost layer of the outer root sheath is formed by a basal layer of cylindrical cells. It grows progressively thinner toward the hair bulb, where its cells become mingled with the germinal cells of the hair matrix. DERMAL ROOT SHEATH. The dermal root sheath presents three layers, an innermost basement membrane or glassy layer, a layer of circular connective tissue fibers and a similar layer of longitudinal fibers. These layers are obviously homologous with the basement mem- 284 THE SKIN brane, the papillary layer, and the reticular layer of the derma, respec- tively. The dermal root sheath is, however, devoid of papilla. FIG. 274. TRANSECTION OF A HAIR NEAR THE MIDDLE OF THE ROOT SHEATH. o, dermal root sheath; 6, outer margin of the epidermal root sheath the light space is the glassy membrane, the polyhedral cells form the outer root sheath; &, Henle's layer of the inner root sheath; d, Huxley's layer; e, cuticle of the root sheath; /, cuticle of the hair; g, cortex of the hair shaft. Highly magnified. (After Kolliker.) The glassy membrane is a peculiarly thick homogeneous membrane which is chiefly mesoblastic in origin, but whose innermost portion is formed as an exoplasmic product of the adjacent epithelium. This THE HAIR 285 membrane is highly refractive and contains very few connective tissue cells or fibers. The circular filters of the dermal root sheath contain interlacing bundles of connective tissue fibers, which are mostly disposed in a ring- like manner. Elastic fibers are absent. Within this layer is a dense anastomosing plexus of capillary blood-vessels, together with a rich subepithelial plexus of non-medullated nerve fibers. The longitudinal filters of the connective tissue root sheath also form interlacing fiber bundles, most of which are somewhat obliquely disposed. The bundles are coarser than those of the preceding layer and contain a few elastic fibers. This portion of the root sheath contains many small blood-vessels and nerves which supply the plexuses of the circular layer. The hair follicle in transverse section varies in different races from circular to elliptical form. In the Chinese race the diameter of the fol- licle and the hair is 100 X 7? to 85, in the European 100 X 62 to 72, and in the Negro 100 X 40 to 60. The more elliptical the form of the follicle the greater the curl of the hair. ATYPICAL PORTIONS or THE HAIR FOLLICLE. As already indicated, the hair follicle presents some structural differences at various levels. The typical arrangement is found only in the mid-portion of the fol- licle. In its superficial portion the hair lies free in the follicular lumen, the interval between it and the inner root sheath being only partially occupied by the fatty secretion of the sebaceous gland which enters the lumen of the follicle at the deeper portion of its middle segment. At this level also, the root sheaths of the hair offer a gradual transition from their typical structure to that of the dermal and epidermal layers with which they are continuous. The hair bulb likewise differs prominently from the typical struc- ture of the hair root. In this portion the germinal layers are very highly developed at the expense of the horny layers. It is, therefore, in this portion that growth is most active. The cells of this region are often deeply pigmented. The increased size of the germinal layer, more- over, produces a distinct bulging of the follicle, which incloses the hair papilla and results in the peculiar bulbous shape of the extremity of the hair follicle. THE HAIR PAPILLA. The structure of the hair papilla is identical with that of the vascular papillae of the derma except that it is con- structed upon a much larger scale. It consists of a conical or club- shaped elevation of connective tissue which indents the extremity of 286 THE SKIN the hair bulb. It contains an abundant plexus of capillary blood-vessels ^ and a rich supply of non-medullated nerves. It also contains an undue j proportion of connective tissue cells. KEGENERATION or THE HAIR. Hairs are being continuously shed and regenerated, the average life of a hair of the scalp being stated as 1 sixteen hundred days (Stohr). The shedding of a hair is first heralded by an atrophy of its papilla and a cornification of its bulb. Growth ; ceases, and the hair, firmly adherent to its root sheath, is gradually j carried, by the continued growth of the latter, nearer and nearer the ; surface of the skin. Its excursion leaves behind a narrowed cell column which still unites j the hair with its former papilla. From this rudiment a new hair germ may form (Unna), a new papilla develop, and the resulting hair grow toward the surface in the path of , the molting hair, its eruption being preceded by the falling of its pred- ecessor. The formation of the new hair germ very probably occurs at a . point nearly corresponding with the insertion of the arrector pili muscle, where there is a swelling of the root sheath which has been already men- tioned as the matrix of the hair follicle. This matrix appears very ( early in the development of the hair, but remains quiescent until regen- eration becomes necessary, when the cells are said to proliferate and grow downward filling the space between the atrophic hair and the new bulb. In infancy and youth shed hairs are also compensated for by new j formation from hair germs appearing at the germinal border of the epi- dermis, the process proceeding in the manner already described for the ; development of the hair. THE SEBACEOUS GLANDS These are branched saccular glands which may be subdivided into two classes, (1) those whose ducts open into the hair follicles, and (2) those whose ducts open upon the free surface of the epidermis. The former are by far the more numerous; the latter occur in the skin of the face, red margins of the lips, labia minora, glans penis and prepuce, and the tarsal glands of the eyelids. With the above exceptions the dis- tribution of the sebaceous glands is coextensive with that of the hairs. They are therefore absent from the palms of the hands and soles of the feet. A sebaceous gland consists of a dilated saccular fundus^ a constricted NERVE SUPPLY 291 vessels. Branches from this plexus pass to the papillary layer, where they form a second (subpapillary) plexus from which terminal arteries are distributed to the capillaries of the papillae. The distribution of the veins is similar to that of the arteries. The primary plexus is found in the papillary layer; occasionally a second plexus immediately underlies the first, and from these, venules pass to the deeper part of the corium, whence after free anastomosis they pro- ceed to the subcutaneous tissue, collecting on the way the venules return- ing from the hair follicles and secreting glands, and from the subcuta- neous connective tissue. The very rich capillary network in the papilla of the hair bulb is worthy of special mention. The lymphatic vessels of the skin begin as a terminal lymphatic plexus in the corium, which collects the lymph from the tissue spaces of both derma and epidermis. The vessels of this plexus communicate with a subcutaneous lymphatic plexus of larger vessels which follow the course of the blood-vessels on their way to reach the neighboring groups of superficial lymph glands. NERVE SUPPLY The skin is abundantly supplied with large nerve trunks, both sympa- thetic and cerebrospinal (sensory), which find their way along the sub- cutaneous fat and send branches directly to the larger blood-vessels, the hair follicles, the sebaceous and sudoriparous glands, to the cor- puscles of Pacini, Ru$ini, and Golgi-Mazzoni, and to the end bulbs of Krause, which lie in the connective tissue. In the cutis vera the nerve trunks form a plexus of delicate fiber bundles in the reticular layer, with a secondary, more closely meshed plexus of finer nerve bundles in the papillary layer. From these plexuses fibrils are distributed to the smaller blood-vessels and to the papillas, where many end in tactile corpuscles. Other fibrils penetrate the epidermis, terminating as naked fibrils or on tactile cells. In the region of the hair follicle small branches form a network of fibrils in the dermal root sheath. Branches (pilomotor nerves) are also distributed to the arrectores pilorum muscles. In the sudoriparous glands the nerves form a fine plexus about the membrana propria (epilamellar plexus), from which naked axis cylin- ders penetrate the basement membrane and terminate between the secreting cells. CHAPTER XII THE RESPIRATORY SYSTEM The respiratory system may be said to comprise a true respiratory organ, the pulmonary alveoli, in which the interchange of gases between the air and the blood occurs, and a system of duct-like passages leading Superior turbinate bone Olfactory nerve Frontal sinus - I / Sup - Nasal sinus. Mid. turbinate Mid. meat Inf. turbinate Hypoglossal nerve Larynx Thyroid cart. Sphenoidal sinus Nasal nerve Auditory tube opening Anterior pillar Tonsil Uvula Post, pillar of fauces " * Pharynx Epiglottis Ventricle Vocal cords Esophagus FIG. 280. PHOTOGRAPH OF Azoux MODEL, SHOWING NOSTRIL, PHARYNX, LARYNX AND RELATED STRUCTURES. 292 THE NASAL CAVITY 293 thereto, which, beginning with the nasal cavity, successively includes the nasopharynx, larynx, trachea, and bronchi of gradually diminishing caliber, and which finally ends in the terminal bronchioles leading to the air sacs and pulmonary alveoli. The arrangement of these several portions of the respiratory system has been frequently compared to the structure of the tubulo-acinar glands. From this point of view the larynx and trachea form the duct stem of the gland, the bronchi form the branching interlobar and inter- lobular ducts, and the terminal bronchioles (intercalary ducts) end in the numerous acinar air saccules of the lung. Development. The original anlage of the respiratory system, begin- ning with the larynx, is a short linear ventromedial evagination from the cephalic end of the primitive esophagus. This tracJieal groove be- comes separated, distally, thus forming a tube, which grows backward, meanwhile dividing distally into the bronchi which undergo further division to form the successively finer branches of the pulmonary system. The respiratory epithelium is thus of entodermal origin, and becomes enveloped in connective tissue of mesodermal origin. THE NASAL CAVITY This cavity is bounded by a cartilaginous and bony wall and is lined by a mucous membrane which, according to the nature of its epithelium, may be divided into three portions: (1) the vestibule, (2) the respira- tory portion, and (3) the olfactory portion. Its external and internal openings are the nares and clioance respect- ively. Communicating with the nasal chambers are the sphenoidal, maxil- lary, frontal, and palatal accessory sinuses, and the ethmoidal air cells. Their lining membrane is continuous with, and histologically essentially like, though thinner than, that of the respiratory portion of the nostril. THE VESTIBULE The vestibule of the nose corresponds very closely to the cartilaginous portion of the nasal wall. Its mucous membrane is continuous anteriorly with the skin and posteriorly with the mucous membrane of the res- piratory portion. The vestibule is lined by stratified squamous epithe- lium, which offers a gradual transition from the moist respiratory epithe- lium to the dense horny epidermis of the skin. Near its external ori- 294 THE RESPIRATORY SYSTEM fice are numerous coarse stiff hairs, vibrissce, connected with which are many sebaceous glands. The vibrissse have no associated arrectores pilorum muscles. Some of the glands also open directly upon the surface of the mucous membrane. The fibrous tunica propria of the vestibule is continuous with the corium of the skin, and in it are embedded the deeper portions of the vibrissae and the secreting portions of the sebaceous glands. By its deeper surface the tunica propria is closely attached to the perichon- drium of those plates of hyaline cartilage which form the septum and alse of the nose. THE RESPIRATORY, PORTION The respiratory portion of the nasal mucous membrane (Schneider- ian membrane) clothes the middle and inferior meatus of the nose. It is continuous anteriorly with the mucous membrane of the vestibule, FIG. 281. FROM A SECTION OF THE Mucous MEMBRANE OF THE RESPIRATORY REGION OF THE HUMAN NOSE. a-a, ciliated epithelium; 6, 6, mixed mucoserous glands; c, blood vessel. Hema- tein and Congo red. Photo. X 185. THE NASAL CAVITY 295 above with the olfactory mucous membrane, and posteriorly with that of the nasopharynx. The respiratory region is lined by columnar ciliated epithelium of the pseudo-stratified type, which also contains" many mucus-secreting, goblet-cells. The epithelium rests upon a distinct basement membrane which reacts to the specific stains for elastic tissue. The tunica propria consists of a very vascular connective tissue ; it varies much in thickness in different portions. It is thinnest in the accessory sinuses and is thick- est where it covers the turbinal bones and the adjacent portions of the nasal septum. The tunica propria is richly supplied with both mucous and serous glands. The smaller ones, in the thinner portions of the mu- cous membrane, are somewhat convoluted; the larger and more numer- ous are tubulo-acinar glands. Many of the latter are mixed glands con- taining both mucous and serous acini. They produce an abundant secre- tion. The Schneiderian membrane is in all portions extremely vascular, many of its vessels having very thin walls. The thicker portions over the turbinals and the septum are typically erectile. The dense connective tissue of these portions is permeated with broad venous channels which are surrounded by bands of smooth muscle. Other muscular bundles are longitudinally distributed. The small arteries are contained within the fibromuscular stroma. The subepithelial portion of the tunica propria contains fine inter- lacing bundles of connective tissue and many capillary blood-vessels. Here and there it is also infiltrated with lymphocytes and occasional very minute solitary nodules are found. The lymphatics of the Schnei- derian membrane lead posteriorly to the lymph nodules of the naso- pharynx. THE ORGAN OF JACOBSON Associated with the respiratory portion is the rudimentary vomero- nasal organ (of Jacobson). In the embryo of one month it appears as a tubular extension, one on either side, into the corium of the median septum, opening anteriorly. In transverse section it has a semicircular outline, with its convexity mesial. In lower forms, e.g., Amphibia, it persists in the adult as a functionally important organ concerned with smell. It is innervated by fibers from the olfactory nerves, vomero- nasal nerves, and by the nervus terminal-is f which is present also in man (Johnston, Anat. Eec., 8, 4, 1914). In the cat the tall columnar ciliated epithelium (pseudo-stratified) includes true sensory cells, similar to the 296 THE KESPIRATOEY SYSTEM olfactory cells. The nervus terminalis contains some medullated fibers, and may contain both afferent and efferent components (Johnston). In the rabbit it innervates in addition to Jacobson's organ, also a wide area of the nasal septum (Huber and Guild). THE OLFACTORY PORTION The olfactory portion of the nasal mucous membrane, the olfactory organ, lines the superior meatus, a portion of the superior turbinate bone and a corresponding portion of the median septum, arid its irregular border here and there invades the upper portion of the middle meatus. It 'consists of a fibrous tunica propria and a clothing of FIG. 282. THE OLFACTORY MUCOSA OF A CAT. a, epithelium; 6, basement membrane; c, corium; d, cuticle; e, sustentacular cell layer; /, olfactory cell layer; g, basal cells; h, blood-vessel; i, a tubule of Bowman's glands; k, bone. Hematein and picrofuchsin. Photo. X 270. neuro-epithelium. The tunica propria contains elastic as well as col- lagenous fibers, and many small tubulo-acinar serous glands, the olfac- tory glands of Bowman. Beneath the epithelium is an indistinct base- ment membrane. THE NASAL CAVITY 297 The neuro-epithelium contains three intermingled cell types, the sustentacular, olfactory, and basal cells. The SUSTENTACULAR CELLS are columnar ciliated epithelial cells which possess a distinct cuticular margin. Their nuclei are ovoid, and, since they lie at the same level, they form a continuous superficial zone of oval nuclei. The deep ends of the cells are often branched; they interlace with one another and with the processes of the olfactory and FIG. 283. THE OLFACTORY Mucous MEMBRANE. a, duct of gland; 6, basal cells; c, cuticular border with olfactory cilia; /c, nuclei of the sustentacular cells; k', nuclei of the olfactory cells; r, layer of olfactory cells; s, corium, containing connective tissue cells and nerve fibers; st, sustentacular cells. X 465. (After Kolliker.) basal cells. The cytoplasm of the sustentacular cells is finely granular and contains a yellow pigment. The OLFACTORY CELLS occupy a unique position among neuro-epithe- lial cells in that they are true nerve or ganglion cells. They possess a small cytoplasmic body and two processes, a distal and a central. Their nuclei are spherical and are disposed in several rows beneath the nuclear zone of the sustentacular cells ; thus they form a broad zone of spherical nuclei. The distal process of the olfactory cell projects as a slender filament whose free end, carrying several fine cilia, reaches the surface of 298 THE EESPIEATOEY SYSTEM the membrane through a pore-like opening in the cuticular membrane which is formed by the cuticle of adjacent sustentaciilar cells. The central process of the olfactory cell penetrates to the tunica propria and becomes a non-medullated nerve fiber of one of the many rami of the olfactory nerve ; it passes to the olfactory bulb, where its terminal arbori- zation with the dendrites of the mitral cells forms the olfactory glom- eruli. The BASAL CELLS are flattened cells which form the deepest nuclear zone of the olfactory neuro-epithelial layers. Their cytoplasm is finely granular and their nu- clei are ovoid or flat- tened. Frequently they send a short process be- tween the branched ends of the sustentacular cells. Many small nerve trunks occur in the tunica propria. The great majority of these are non-medullated and are formed by the cen- tral processes of the ol- factory cells, which proc- esses are true axons. Several of the smaller superficial fiber bundles unite in the deeper part of the tunica propria to form one of the small olfactory nerves. Taken together, about twenty in number on each side, these form two olfac- tory nerves, unique among cerebrospinal nerves in being collectively non-medullated. A few medullated fibers, derived from the trigeminus, are also found in the tunica propria ; they distribute their terminal vaso- motor branches to the blood-vessels, and, by fine sensory filaments which end between the epithelial cells, supply the neuro-epithelial layer. The trigeminus fibers supply a similar innervation to the respiratory mucosa. The blood vessels of the olfactory mucous membrane are abundant. Their capillary plexuses form several layers in the tunica propria, and their veins mostly empty, through the ethmoidal veins, into the superior FIG. 284. VERTICAL SECTION OF THE OLFACTORY MUCOSA OF A KITTEN. a, deep, and 6, superficial border of the epithe- lium; c, olfactory cells; d, sustentacular cells; g, ol- factory nerve fibers in the corium of the mucosa. Golgi stain. X 325. (After Kolliker.) THE NASOPHAKYNX 299 longitudinal sinus a most significant fact. On the other hand the veins of the respiratory region return their blood to the internal maxil- lary vein, while some of those of the vestibule anastomose with the radi- cals of the facial vein which supply the adjacent skin. FIG. 285. DIAGRAM OF THE RELATIONS OF THE NEURONS OF THE OLFACTORY NERVB AND OLFACTORY BULB. olf. c., olfactory nerve cells, located in the olfactory region of the nasal mucosa, whose axons enter the olfactory nerve, olf. n., and terminate in relation with the dendrites of the mitral cells, me., in the olfactory glomeruli, gl. The axons of the mitral cells, a., enter the olfactory tract, where they make a sharp bend and pass toward the cerebrum giving off frequent collaterals. At n' a nerve fiber appears to end by a free ramification among the mitral cells of the olfactory bulb. (After Schafer.) The lymphatics of the olfactory region can be readily injected from the sub-dural spaces of the meninges. They form a network in the con- nective tissue of the tunica propria. THE NASOPHARYNX This cavity, like that of the nose, is limited by a bony wall. Its mucous membrane is continuous anteriorly with that of the respiratory portion of the nose, and posteriorly with that of the oropharynx. The 300 THE KESPIKATOBY SYSTEM structure of its mucous membrane resembles that of the Schneiderian membrane, but its dorsal wall, in addition to the ciliated epithelium, the thin-walled blood-vessels, and the numerous secreting glands, contains many small nodules of lymphoid tissue. These nodules form a consider- able mass, the pharyngeal tonsil. The ciliated epithelium of the nasopharynx is also continuous with the lining epithelium of the auditory (Eustachian) tube. The tunica propria is firmly adherent to the bony wall of the dorsal surface, but is more loosely attached laterally and ventrally to the pharyngeal and palatine muscles. THE LARYNX The wall of the larynx is formed by several large plates of hyaline cartilage thyroid, cricoid, and arytenoid cartilages which are firmly united by ligamentous bands of fibrous tissue; the cartilaginous wall in- closes a mucous membrane of considerable thickness. The larger cartilages are of the hyaline variety and are prone to ossify in adult life. The tips of the arytenoids, the cornicula of San- torini, the cuneiform cartilages of Wrisberg, and the epiglottis are of the elastic variety of cartilage, and, though frequently much infiltrated with fat, are not, like the hyaline cartilages, subject to ossification. The median portion of the thyroid cartilage is also generally elastic in char- acter; this portion does not generally ossify in women. In the lateral hyothyroid ligaments are the minute triticeous cartilages, generally hyaline -but occasionally fibrous in character. The intrinsic muscles of the larynx, taking origin from these car- tilages, pursue their course beneath the mucous membrane. The upper portion of the larynx, including the greater part of the epiglottis, as far as the false vocal cords is lined by stratified squamous epithelium which is continuous with that of the pharynx. The epithelium of the vocal cords and that covering the anterior surface of the arytenoids is also of the stratified squamous variety. The remaining portions of the larynx, including the base of the epiglottis on its laryngeal surface, the ventricle, and the entire portion below the level of the true vocal cords, are lined by columnar ciliated epithelium of the pseudo-stratified type. The ciliary motion is directed toward the pharynx. The epithelium rests upon a basement membrane which is less highly developed than in other portions of the respiratory tract. The tunica propria consists of fibre-elastic connective tissue in which THE LAEYNX 301 are many small tubulo-acinar mucous glands. These are most abun- dant in the region of the ventricle and the false vocal cords. Jji_this region also there is much diffuse lymphoid tissue, and the lateral and dorsal wall contains several solitary nodules which are so constant in their appearance as to warrant designating them 'laryngeal tonsil' (Frankel). Occasionally, however, lymph nodules are not present in the mucous membrane of the human larynx. The deeper portion of the tunica propria in certain parts, e.g., in the false vocal cords, contains a few striped muscle fibers in addition to those of the named muscles of the larynx. The false vocal cords and the aryteno-epi- glottic folds contain loose fibrous tissue and frequently much fat. The true vocal cords are formed by dense bands of elastic and a few col- lagenous fibers which are covered by a mucosa clothed with stratified squamous epithelium. Their free margin is sharp- ly defined; at their attached margin, however, they blend indistinctly with the tunica propria. The free margin of the vocal cords has no connective tissue papillae on the surface of the tunica propria, but toward the trachea super- ficial papillae of connective tissue pro- ject into the deeper surface of the stratified epithelium. The mucous membrane of the larynx is freely supplied with blood-vessels and lymphatics. The latter terminate in the deep cervical lymph nodes. The nerve fibers form an abundant plexus in the laryngeal mucosa, from which motor fibers are distributed to the muscles and sensory fibers to the epithelium. The latter end in fine fibrils between the cells of the lining epithelium. In the stratified squamous epithelium, especially that of the epiglottis, small taste buds are also found; none, however, occur on the vocal cords. FIG. 286. A VERTICAL SECTION THROUGH THE LATERAL WALL OF THE HUMAN LARYNX. a, cartilage; b, laryngeal mucosa, clothed with ciliated epithelium; c, transection of the vocal cord, in this region the mucosa is clothed with stratified epithelium; d, ducts of mucous glands; e, lymphoid nodule; /, muscle; g, mucous glands; h, submucosa; i, blood- vessel; V, ventricle of the larynx. Hematein and eosin. Photo. X 8. 302 THE KESPIKATORY SYSTEM THE TRACHEA The trachea proper extends from the lower border of the cricoid car- tilage to the point where it bifurcates into the two primary bronchi, a distance of about four and one-half inches. The wall of the trachea somewhat resembles that of the larynx. It consists of three layers : 1. The mucous membrane, or mucosa. 2. The submucosa. 3. The fibrocartilaginous coat, or adventitia. The mucous membrane presents slight longitudinal folds, and is lined by columnar ciliated epithelium, like that of the larynx, which III- IV FIG. 287. TRANSECTION OF THE WALL OF A CHILD'S TRACHEA. 7, mucosa; II, submucosa; ///, cartilage; IV, outer fibrous coat; a, columnar ciliated epithelium; 6, tunica propria; c, layer of elastic fibers; d, mucous glands; e, perichondrium. Hematein and eosin. Photo. X 90. rests upon a delicate basement membrane. The tunica propria includes a thin inner layer of connective tissue which is richly supplied with small blood-vessels and infiltrated by many lymphocytes, and an outer THE TEACHEA 303 layer of elastic tissue most of whose fibers are longitudinally disposed. The elastic layer begins in the region of the vocal cords in the-laxynx and is continuous below with the similar layer of the bronchial mucous membrane. Elastic fibers are more numerous in the trachea of the lower mammals than in that of man. A muscularis mucosa, a characteristic structure of the mucosa of the digestive tube, is lacking in the trachea. The elastic mem- brane occupies the position held by the mus- cularis mucosa in other organs. The submucosa consists of loose areolar tissue which contains many small tubulo-acinar mucous glands. The ducts of these glands penetrate the mucosa and open upon the free surface of the trachea. They supply an abun- dant mucous secretion. This coat also contains the larger blood-vessels and nerves which are destined for the supply of the mucosa. The fibrocartilaginous coat is formed by the C-shaped 'ring cartilages' of the trachea (from sixteen to twenty in number) which are firmly united to one another by ligamentous membranes of fibrous tissue continuous with the perichondrium of adjacent cartilage plates. The cartilages are of the hyaline variety and are subject to more or less ossification as age advances. They rarely overlap each other, so that but a single plate of cartilage forms the a f , demilu ^ nn * e A T~ struction. X 200. (After wall at any given point. Iheir borders are Maziarski.) irregular, and horizontal sections near the upper or lower margin of the cartilage frequently pass through several projections, which, unless properly interpreted, would lead one to infer that the cartilaginous plate was interrupted. The interval between the ends of the C-shaped cartilage plates is occupied by a membrane of smooth muscle whose transverse fibers unite the adjacent ends of the cartilages. The muscle fibers are inserted into the perichondrium of the cartilages. Many of the fibers are obliquely, and a few of the outermost are longitudinally, disposed. This muscular portion of the tracheal wall forms the so-called trachealis muscle. The mucous membrane and submucosa of this portion of the trachea are un- usually thick and their mucous glands are exceptionally large. The FIG. 288. MUCUS-SE- CRETING, TUBULO - AL- VEOLAR GLAND OF THE HUMAN TRACHEAL MU- COSA. The terminal dark areas 304 THE KESPIKATORY SYSTEM loose fibrous tissue which invests the outer surface of the cartilaginous coat contains many small sympathetic nerve trunks and ganglia. THE LUNG At the root of the lung the trachea divides into a primary bronchus for jjach_ lung. By repeated subdivisions the earliest branches being given off at acute angles, the later ones at more obtuse angles the smaller and smaller bronchi finally end in minute terminal bronchioles which lead through the alveolar ducts to the pulmonary air sacs and alveoli. One may thus distinguish between primary, secondary, and tertiary bronchi, and bronchioles. The mode of division for the main series of bronchi is monopodial, of the smaller bronchi a mixed dichot- omy and monopody (Miller, Jour. Morph., 24, 4, 1913). According to Piersol the bronchioles are the branches of the fourth or fifth order. BKONCHI The wall of the primary bronchi is similar in structure to that of the trachea, but in bronchial tubes which are one or two divisions removed (secondary and tertiary bronchi) from the primary bronchi the plates of cartilage are no longer C-shaped, and a complete muscularis mucosae, within the cartilages, forms the outermost boundary of the mucous membrane. In such tubes typical bronchi the wall, as in the trachea, comprises: 1. A mucosa. 2. A submucosa. 3. A fibrocartilaginous coat. The mucosa, a continuation of that of the trachea, is lined by tall, columnar, pseudo-stratified, ciliated epithelium which rests upon a distinct elastic basement membrane. The epithelium is thrown into wavy longitudinal folds. . The tunica propria is extremely vascular; it possesses an abundant supply of thin-walled veins of small caliber, to- gether with many lymph vessels. Its connective tissue forms a delicate fibrous reticulum in the meshes of which are many lymphocytes. The outer portion of the tunica propria contains bundles of fine longitudinal elastic fibers, which form a complete layer about the tube. This elastic layer is thickest opposite the ridges and thinnest opposite the troughs of the epithelial waves. The outer boundary of the mucous membrane contains a well- THE LUNG 305 FIG. 289. A BRONCHUS FROM THE HUMAN LUNG. a, lining epithelium; 6, duct of a mucous gland; c, muscularis mucosse; d, accu- mulated mucus, etc., bathing the surface of the epithelium; e, mucous glands; /, hyaline cartilage; g, outer fibrous coat; h, pulmonary alveoli. Hematein and eosin. Photo. X 34. developed muscularis mucosce composed of interlacing bundles of circular smooth muscle fibers. This layer forms a complete muscular coat which 20 306 THE KESPIKATOKY SYSTEM is here and there pierced by the ducts of mucous glands whose secreting portions lie in the submucosa. The submucosa, by its broad-meshed areolar tissue, loosely unites the mucous membrane to the cartilage plates. This coat contains the larger blood-vessels, nerves, and lymphatics which are distributed to the mucosa. It also contains the secreting portions of many tubulo-acinar mucous glands, which occur in groups that in the larger bronchi almost completely surround the tube. The number and size of these glands is in direct proportion to the size of the bronchus. The efferent ducts of the mucous glands penetrate the muscularis mucosae and open upon the free surface in the interval between adjacent folds of the epithelial lining. In the tunica propria the ducts possess ampullary dilatations which are lined by ciliated cells and contain portions of the mucous secretion. The fibrocartilaginous coat is formed by a dense fibrous membrane in which the cartilages are embedded. The plates of hyaline cartilage vary much in number and size, being more or less highly developed in proportion to the size of the bronchial tube. They possess at all times a somewhat crescentic shape. In the larger bronchi three or four car- tilage plates with overlapping edges encircle the entire tube. In the lower mammals, e.g., the pig, these overlapping cartilages are so highly developed that the plates often lie three or four deep; in man they are rarely more than one or two deep. As the bronchi diminish in size by division, the cartilage plates are no longer of sufficient size to completely encircle the wall but leave broad intervals in which this coat is only represented by fibrous tissue. In tubes of a diameter of 0.8 to 1 milli- meter, bronchioles, the cartilages entirely disappear, and in these or some- what smaller bronchioles the mucous glands are, likewise, no longer found. According to Cutore (Anat. Anz., 47, 13/1914), the cartilage plates of the intrapulmonary bronchi contain elastic fibers, and are in fact true elastic cartilages. The outer surface of the cartilages is invested with a clothing of loose fibrous tissue of varying thickness sometimes known as the outer fibrous coat in which the branches of the pulmonary artery and veins and also many nerve trunks and ganglia are found. In the larger bronchi the two vessels, pulmonary artery and pulmonary vein, are found on opposite sides of the tube ; in the bronchioles only one vessel, the artery, is in relation with the tube, the vein pursuing an independent course within the pulmonary tissue. Near the root of the lung many small lymph nodes are found in the outer fibrous coat. In the smaller bronchi these are represented by THE LUNG 307 solitary nodules which, it is important to note, are always found in the fibrous connective tissue which forms the outer portion of the bronchial wall. The bronchial lymph nodes and nodules are deeply pigmented, the volume of the pigment being dependent upon the age and occupation of the individual. It is apparently derived by absorption from the surface of the bronchi and is therefore absent in infancy, defi- cient in youth, abundant in adult life, and especially abundant in those individuals whose occupations have necessitated the inhalation of a dusty atmosphere. THE BRONCHIOLES ^x The bronchioles possess neither cartilage, mucous glands, nor lymph nodules. Their epithelium, though still ciliated, is low short columnar, or, in the smaller bronchioles, cuboidal. The tunica propria contains many lym- phocytes and the elastic tissue forms an almost complete layer of longitudi- nal elastic fibers. The muscularis mucosae is relatively more highly devel- oped than in the larger bronchi ; it completely encircles the wall and is in- vested with an ad- ventitious layer of fibrous tissue which contains the small arteries, nerves, lymphatics a capil- FIG. 290. DIAGRAM OF PRIMARY LOBULE OF LUNG lary plexus with (LUNG UNIT). elongated meshes ^.r., respiratory (terminal) bronchiole; d. al., alveolar , duct; a, atria; s. al., alveolar or air sac; a.p., pulmonary alveoli; art., pulmonary artery; v., pulmonary vein. (After ules. Miller.) The fibrous coat of the bronchiole here and there blends with the fibrous bands of inter- lobular tissue and is in contact with the adjacent pulmonary alveoli. 308 THE KESPIKATORY SYSTEM Each bronchiole enters the apex of a pulmonary lobule (secondary) and divides into terminal so-called respiratory bronchioles (terminal bron- chioles). FIG. 291. FROM A SECTION OF A CHILD'S LUNG. B, bronchiole in transection, with its adjacent pulmonary artery, A; TB, bron- chiole, ending in a terminal bronchiole, from which are derived the alveolar duct, 7, the atrium, A, and the pulmonary alveoli, PA. In the center of the figure a pul- monary alveolus, PA, is seen in transection; many similar ones are shown. Hema- tein and eosin. Photo. X 45. The respiratory bronchiole bears alveoli which become more numer- ous toward its distal end. It thus contains a variable epithelial lining consisting proximally of a low columnar, sometimes ciliated, epithelium, with distally a more flattened non-ciliated type, and in the alveoli a typical greatly flattened respiratory epithelium including non-nucleated THE LUNG 309 plates. It has a diameter of 0.5 millimeter or less. The epithelium rests upon a thin fibromuscular coat, the continuation of the mucous membrane of the bronchioles. The muscle still forms an almost com- plete though very thin investment of circular fibers; the muscle fibers, FIG. 292. FROM A SECTION OF A CHILD'S LUNG. a, a small tertiary bronchus; 6, bronchioles; c, bronchioles ending in terminal bronchioles, alveolar ducts, etc.; d, terminal bronchioles in transection, they have a more regular contour and thicker wall than the alveoli; e, pulmonary arteries; /, a bronchial artery; g, a bronchial vein; h, interlobular fibrous septum. Hematein and eosin. Photo. X 62. however, are not continued into the wall of the pulmonary air sacs. The elastic fibers, derived from the elastic layer of the bronchioles, pass over to the alveolar walls in which they form a delicate network. The respiratory bronchioles are short branching tubules leading to 310 THE BESPIRATORY SYSTEM broader spaces, the alveolar ducts (infundibula of Schultze), which are surrounded by air saccules with pulmonary alveoli. According to Miller the alveolar ducts bear numerous alveoli, and are lined with flattened respiratory epithelium, and contain scattered bundles of smooth muscle which end in a delicate sphincter where the duct passes into the non- FIG. 293. FROM A SECTION OF A CHILD'S LUNG. A, -atrium; B, bronchioles ending in terminal respiratory bronchioles, TB; PA, pulmonary artery; PV, pulmonary vein. Hematein and eosin. Photo. X 50. muscular atria, from three to six for each alveolar duct. Each atrium, of more or less circular outline, opens into a variable number (two to five) of irregular and variable alveolar saccules (air sacs). Each sac- cule bears on its periphery numerous pulmonary alveoli. The epithelium of the entire bronchial tree, including the nucleated respiratory cells, con- tains abundant mitochondria (Meves and Tsukaguchi, Anat, Anz v 46, 1, 1914). THE LUNG 311 THE PULMONARY ALVEOLI The pulmonary alveoli are minute air cells, open toward the alveolar ducts, whose extremely thin wall consists of a capillary network, a deli- FIG. 294. DIAGRAM OF THE THREE PULMONARY LOBULES CONNECTED WITH A TERMINAL BRONCHIOLE (TB). The middle lobule is stippled. BR, bronchiole;. RB, respiratory bronchiole of the first order; TB, respiratory bronchiole of the second order (terminal bronchiole); /, //, ///, alveolar ducts; /, 2, and 3 a, b, c and d, atria; As, alveolar sacs with pulmonary alveoli or air cells. (Adapted from Miller, Jour. Morph., 20, 4, 1913.) cate nbro-elastic reticulum, and a lining epithelium. The alveoli are so densely clustered about the alveolar duct that the capillary plexus, in the form of a reticulated membrane of wide capillary vessels, is exposed to the air of two adjacent alveoli, being separated therefrom only by its own endothelium and the epithelial lining of the alveolus. 312 TH RESPIRATORY SYSTEM The lining epithelium of the alveoli, continuous through the alveolar ducts with that of the respiratory bronchioles, consists of flattened cells and broad protoplasmic non-nucleated plates. These cells are narrower A B and thicker (cubi- _ cal) in the prenatal &' **i < ^ iuug anc ^ w ^ en t ne ^ :<& w$ iuug * s c ii a p seo ^ ? S*,i %* broader and thin- ' *e A-: * *Vj **i " ""V\ tract, and enter the epithe- \ lium in solution (White- head., Amer. Jour. Physiol., 24, 2, 1909). Here they \*l ,' ... * ".. are again synthesized into * \ \J'/ 9 " neutral fat by the activity M| of the epithelium (Pfliiger, Arch. f. d. ges. Physiol., FIG. 349. APEX OF AN INTESTINAL VILLUS OF A RABBIT WHICH HAD BEEN FED WITH MILK. )0 ) ^uch a process ac- counts for the abundance of fat within the distal por- tions of the cells. The drop- lets are then secreted into the intercellular and subja- cent tissue spaces. The second phase of absorption includes the transference of the fat particles to the lacteal. This process appears to depend partially, at least, upon the activity of the leukocytes, as suggested by Schafer, the particles of fat thus finding their way through the diffuse lymphoid tissue. According to Eeuter (Anat. Hefte, 1902), fat droplets are found in the tissue spaces as well as in the lymph corpuscles of the diffuse lymphoid tissue, a fact which would seem to indicate that other agencies aid in the transit of the fat from the epithelium to the lacteal than are accounted for by the purely mechanical theory of Schafer. The third phase includes the secretion of the fat into the lumen of the lacteal ; this is, at least partially, accomplished by the disintegra- The fat droplets have been blackened by fixa- tion with picric acid and osmium tetroxid. The figure shows the distribution of fat during certain stages of absorption. .Alum carmin stain. High- ly magnified. (After R. Heidenhain, from Oppel.) THE ALIMENTARY CANAL 379 rectum and in the vermiform appendix. In the latter the nodules are more or less confluent, a condition which is not found elsewhere in the large* intestine. In the appendix the greater portion of the mucous membrane is invaded by lymphoid tissue, and the glands are much diminished in both riumber and size (Fig. 352). The Vascular and Nerve Supply, The vascular and nerve supply of the large intestine is identical in its arrangement with that of the small intes- tine. The mucous mem- brane contains a capillary plexus of blood and lym- phatic vessels in the corium about the glands. The nerves of the large intestine include both meclullated and non-medullated fibers. The latter supply its muscular coats and blood-vessels. The former end in naked vari- cose or knobbed fibrils be- neath and upon the epithe- lium of the glands. The usual myenteric (Auer- bach's) and submucous (Meissner's) plexuses ap- pear in the large intestine with the same structure and location as elsewhere in the alimentary canal. In the rectum the lining epithelium is continuous at the anus with the stratified squamous epithelium of the skin. In this region also, the circular fibers of the inner layer of the muscular coat are much thickened to form the internal rectal sphincter. Lymphoid tissue abounds in the rectal mucous membrane. The glands are less numerous but larger than in the colon proper, and the mucosa is thicker. In the lower portion of the rectum the mucosa is thrown into a number of longitudinal folds, the rectal columns, at which level the columnar epithelium changes to stratified squamous type. The Heocecal (Colic) Valve. The ileocecal valve, which guards the orifice by which the small intestine opens into the large, is formed FIG. 351. SECTION OF PORTION OF LARGE INTES- TINE OF DOG, SHOWING THE INTESTINAL. GLANDS (CRYPTS OF LIEBERKUHN) CUT ACROSS, THEIR LINING INCLUDING COLUMNAR AND GOBLET CELLS. 380 THE DIGESTIVE SYSTEM by a reduplication of the mucous membrane, which is strengthened by a thickening and overlapping of the circular muscular layers of both small and large intestines. The valve itself consists of an upper and lower segment enfolding the slit-like orifice; laterally the two folds unite to form the frenulum, which encircles the colon and marks the boundary between it and the cecum. The outer longitudinal muscular layer is continued directly from the wall of the ileum to that of the cecum, and therefore pursues a relatively shorter course than either the internal muscular layer or the mucous membrane. Section of only the outer layer of the muscular coat FIG. 352. TRANSECTION OF THE VERMIFORM APPENDIX OF MAN. The tela submucosa contains three lymph nodules (at the right), two of which have pushed beyond the scattered bundles of the lamina muscularis mucosse into the lamina propria of the tunica mucosa, a mass of diffuse lymphoid tissue (at the left), and some adipose tissue. Xll. permits one to straighten the fold of the intestinal wall and thus oblit- erate the valve. In other words, the outer muscular layer is not included in the valvular reduplication. The muscularis mucosce is slightly thickened at the margin of the valve. At this point also, the villi become shorter and at the margin of the cecal surface of the valve they entirely disappear. The following tabular statement of the more important characteris- tics of the several portions of the alimentary tract may be of assistance to the student in the identification of microscopic sections of these organs. g 1-8 n -8 & . o5 ^ 1 &" ia 1 5 b 5 cu l| it 'o'c ^ .i| O2 3 1 gl fl O 03 02 02 o HH as a s ILEUM AND JEJUNUM. Columnar and goblet. Columnar and goblet. || Mucosa. Solitary and agminated. Smooth. -iJ DUODENUM. Columnar and goblet. Columnar, muco- albuminous. Branched tubulo-acinar. Mucosa and submucosa. I 8 Smooth. -(-j PYLORIC STOMACH. Columnar. Columnar, muco- albuminous. Convoluted tubular. Mucosa. ! ! OJ 1 gg | 11. la o3 ! rd a5 gi | 1 l|| ii pq^ 1 1 8 cc CARDIAC STOMACH. Columnar. Muco-albumi- nous, few parietal cells. Branched tubular. Mucosa. Many, medium. Smooth. 1 ESOPHAGUS. Stratified squamous. Columnar, mucous. Branched tubulo-acinar. Mucosa and submucosa. 1 Striated or smooth. 1 a a 3 o ^ a 13 & K .j H 8 & | | '-2 o3 b 3 DD ,13 i^H 1 I T3 1 1 1 ^ 3 o O ^ j j> 381 382 THE DIGESTIVE SYSTEM THE SALIVARY GLANDS General Considerations. The salivary or oral glands include the smaller glands of the oral cavity and three pairs of large compound tubulo-acinar glands, the parotid, submaxillary, and sublingual glands. The three large pairs are commonly designated as the salivary glands proper, the smaller as the accessory salivary glands. All these are of the tubulo-acinar type, but certain ones secrete a mucous fluid while others pro- duce an al l)ii 11 ii nous secretion which ^conTamsjLD-Jiiucus. The former are known collectively as the mucous, the latter as the serous salivary glands. Still other salivary glands secrete a fluid which is intermediate in compo- sition, and as these glands contain cer- tain alveoli which resemble those of the mucous, and others which are sim- ilar to those of the serous glands, this type is known as mixed salivary glands. The salivary glands may therefore be subdivided |nto: I. Mucous (glands; glands on the anterior surface of the hard and soft palate (palatine glands) and the mu- cous glands of the-* margins and root of the tongue. II. Mixed glands^ subrjaasrfl lary, sublingual, molar, buccal, labial and the anterior lingual glands (of Nuhn). lands; parotid, and von Ebner's glands at the base FIG. 353. SEMIDIAGRAMMATIC REP- RESENTATION OF A SMALL Mu- cous GLAND FROM THE ORAL MUCOSA OF A RABBIT. a, mucous alveoli; e, epithelium of the oral mucosa; m, mouth of the glandular duct. X 70. (After Koffiker.) III. Serous of the tongue The form of the salivary glands will be appreciated by examination of the accompanying diagram (Fig. 353) which represents one of the smaller glands of this type. The larger ones are constructed in the THE SALIVARY GLANDS 383 same manner, the larger number of their secreting alveoli or acini arising through a more complex duct system. The larger ducts of the gland are lined by columnar cells, which, M 'VO FIG. 354. CORROSION MODEL OF AN INTERLOBULAR DUCT AND ITS BRANCHES, FROM THE HUMAN SUBMAXILLARY GLAND. C, interlobular duct; D, large intralobular duct; E, small intralobular duct; F, inter- calary duct. X 12. (After Flint.) as they approach their termination, become superposed and thus offer a gradual transition to the stratified squamous epithelium upon whose surface they open. The epithelium rests upon a basement membrane 384 THE DIGESTIVE SYSTEM which, in the larger ducts, is invested with a fibro-elastic coat containing a few longitudinal smooth muscle fibers. The ducts divide and subdivide in an arborescent manner, the larger branches lying in the connective tissue which invests the lobules into which the gland is subdivided, while the smaller branches are found within the lobule. The duct system is thus divisible into interlobular and intralobular ducts; the latter include generally an excretory por- tion continuous with the interlobular duct, a modified 'salivary' se- cretory portion, and a constricted intercalary or intermediate portion connecting with the acinus. In the smaller glands of the mouth the number of subdivisions of the duct system is relatively small, but in the larger salirary glands the small ducts are practically innumerable. Thus, in the submaxillary gland, Flint (Amer. Jour, of Anat., 1902) found that the interlobular duct system formed 1,500 terminal branches, each of which entered a lobule and was further subdivided into intralobular and intercalary ducts before terminating in the secreting acini. The larger glands may therefore be said to bear to the smaller ones represented in Fig. 353, a relation which is comparable with that of a full-grown tree to the youngest sapling. The larger salivary glands are enveloped by continuous with the adjacent areolar_Jtissji. Fro trabeculaB enter to divide the gland into groups The lobules are invested by more delicate septa trabeculae. The ducts of the interlobar connective nated interlobar ducts to distinguish them from between the lobules. These are the excretory due fibre-elastic capsule this capsule coarse >f lobules, the lobes, from the interlobar tissue may be desig- le interlobular ducts s of the system; the 'salivary' intralobular ducts have a secretory function. The glandular tissue is known as the parenchyma, the connective tissue as the inter- stitial tissue of the gland. The smaller intralobular ( 'salivary*) ducts are lined by columnar epithelium whose cells contain two zones, one on eithfcj* side of the centrally situated nucleus. The distal zone or free extremity of the cell is finely granular, the proximal zone or base presents a character- istic striated appearance which is apparently due to a fibrjllar structure of the cytoplasm in this portion of the cell. The bas|d fibrillae are probably, in part at least, the mitochondria which are Jpresent in all functionally active cells. The epithelium is easily detached from its basement membrane by the artificial contraction of the tissues during fixation and hardening. THE SALIVARY GLANDS 385 (nd secondarily, through a ito the secreting alveoli or The lumen of the ducts is of considerable ydiameter and contains the reticulated or granular particles of the secr/tion. The largerjlucts lie in the connective tissue septa which inveyst the lobular groups of acini. Each of these groups is derived' from the ramifications of the terminal branch of an interlobular duct Vhich enters the lobule to divide into numerous intralobular ducts, short intermediate or intercalary portion, acini. The intercalary ducts are lined bj low cuboidal epithelium and are the small* tubules of the gland. As the duct passes into ' the acinus the tubule is increased jn size, and its secretory epithelium becom/es taller. The tubular acinus is more or less tortuous and possesses a sacculated or alv^o- lar appearance. The epithelium differs^ accordingly as it secretes a mucous or a serous ..flujol. Thus the acini are either mucous or serous secret- --- FIG. 355. INTERCALARY mg. """"THE" SEROUS ACINI. The serous acini contain pyramidal epithelial cells of sufficient" height to almost completely fill the tubule; hence the lumen is very narrow. The form of the secreting cells is somewhat irregular ? .intercalary duct; tf; acini. , , ' Highly magnified (After a fact which apparently depends upon 'their Flint.) crowded condition* within the acinus. The nucleus is situated in the ceritrai portion or in the proximal end of the cell, and is spheroidal in sfiapej The cytoplasm is finely gran- ular, the granules being more promment in the distal portion of the cell. DUCTS AND ACINI OP THE HUMAN SUBMAXIL- LARY GLAND, CORROSION MODEL. F, small intralobular duct, The epithelium rests upo beneath the bases of the secre 'basket-cells' which here and cells of the secretory epithelium and thus provide cup-like depressions which receive the bases of the of these 'basket-cells' is not at nized by their deeply stained within the thin cytoplasmic c< cells, destined to replace worn-o it secretory cells. The appearance of the s i a basement membrane within which, ing epithelial cells, are certain flattened there send short processes between the secreting cells. The function and origin present known. They are readily recog- nd flattened nuclei which are contained 11 body. They may be immature acinal creting epithelium varies with its ac- 386 THE DIGESTIVE SYSTEM tivity. During rest the granular until the non-granular zone is r extremity and the nucleus is obsc The cell becomes therefore much i obliterated. During activity the the lumen; the cell shrinks and more distinct, and the granular (ecretion accumulates within the cell, duced to a narrow rim at its basal red and pushed somewhat basalward. \vollen and the alveolar lumen almost y mo gen granules are discharged into >ecomes clearer; the nucleus appears zone becomes progressively narrower, the basal non-granular zone being copespon- dingly increased in breadth. In this basal zone elongated gran- ules have been demon- strated, which in part are to be regarded as prozymogen i ('basal filaments' of Solger), in part (repealed by special technic) as mitochondria! The seroiite cells are provided nth sys- FIG. 356. A GROUP OF Mucous ACINI, FROM THE HUMAN SUBM AXILLARY GLAND. a-a, interlobular connective tissue. Hematein and eosin. Photo. X 510. terns of secrc naliculi whic ning at the | lumen, invesl tory ca- , begin- andular the cell with a network of canals which lie in the intercellular substance and may even send short offshoots or intracellular canaliculi into the body of the cell itself. These canaliculi are considered to be character- istic of the serous acini and are not found in relation with the' cells of the mucous acini (Fig. 358). THE Mucous ACINI. The mucous acini may contain only mucus- secreting epithelium, or they may also include ceEtara-iinely granular acidophil cells which resemble the epithelium of the serousglands. The former variety of acinus is found in the mucous glands at the base of the tongue and in the soft palate; the latter in the sublingual gland, in the lingual glands of Nuhn, and in the mucous glands of the lips and cheeks. THE SALIVARY GLANDS 387 The serous appearing cells of the latter form of mucous acinus are frequently arranged as i cj^c_entic ^raups -bordering upon the adjacent mucous cells. Such groups are known as the demilunes (of Heidenhain) or crescents (of Gianuzzi; Fig. 357"). They occur at the periphery of the acinus, their base being applied to the membrana propria, their inner margin sometimes reaching the glandular lumen, but more fre- quently separated therefrom by the overlapping of the adjacent mu- FIG. 357. FROM THE SUBLINGUAL GLAND OF MAN. a, salivary intralobular duct; e, acinus whose cells contain no mucus; s, mucous acini, at s' with a demilune; sz, mucous cells in .the duct. X 500. (After Kolli- ker.) cous cells. The demilunes are frequently found at the blind ex- tremity of the secreting" acinus, but they may also occur along its sides. The nature of the demilunes is the subject of considerable discussion. Heidenhain (Arch. mikr. Anat., 1869) first advanced the theory that the mucous cells were destroyed during secretion, and that the function of the demilunes was therefore to replace the disintegrated mucinous cells. This theory has been practically abandoned, for no one has yet demonstrated active cell division in the demilunes, a process which would necessarily be concomitant with the rapid development of mucus from demilune cells. Hebold (1879) is responsible for the theory, strongly supported by 388 THE DIGESTIVE SYSTEM Stohr, that the demilunes represent an inactive, the mucous cells, an active phase of mucus secretion. The easy demonstration of inter- mediate stages in many mucu^-secreting glands lends strong support to this theory, and in the present! state of our knowledge it seems beyond doubt that such a process actually occurs in at least some of the mucus- secreting glands. A third theory, more recently, advanced by Solger (Anat. Anz., 1894) and stoutly supported by Krausb (Arch. mikr. Anat., 1895, 1897, and 1901) and others, considers the demilunes to be true secreting cells which form a serous secre- tion and are therefore functionally independent of the irjucinous cells. This theory receives strong confirmation in the fact first observed by Cajal (u.889) and since that time repeatedly demonstrated, that the demilunes, like the true serous eelll, are provided with a system of in- tercellular \secretory canaliculi by which they are placed in relation with the glandular lumen. Moreover Iwause was able to demonstrate that granules of! sodium indigo sulphate were se- creted by these .cells, as also by the true serous cells and the striated epithelium of the intra- lobular ducts. The mucus-secreting cells examined in the fresh state present a clear, highly refractive appearance. TKey closely resemble the typical goblet cells, FuTTnstead of being isolated among the granular serous cells, they may invest the entire^icinus, _ox~eyen the whole of a small lobule may contain only mucus-sejcj^tiug-eells. After the customary preparation by fixation and staining, the mu- cous cells present a coarse basophilic reticulum which occupies the distal portion of the cell. Coarse granules, with proper fixa ion and in fresh tissue as well, can be demonstrated within the meshes of the reticu- lum. These granules are readily colored by the so-cilled specific mu- cus stains (Mayer's muchematein and mucicarmin, sairaipn, and thio- nin). ; v In the rrfucous cells the nucleus is crowded to the base or proximal end of the tell and flattened against the basement membrane. It is sur- rounded by a small remnant of finely granular cytoplasm, which, after FIG. 358. Mucous ACINI OF THE RETROLINGUAL GLAND OF THE RAT. The ducts and secretory capillaries have been black- ened, r, demilunes with secretory capillaries; s, mucous cells. Golgi meth- od. X500. (After Kolli- ker.) FIG. 359. DIAGRAM OF THE ARRANGEMENT OF THE CELLS IN A MIXED SALIVARY GLAND. a, intralobular duct; a', intercalary duct; b, serous-secreting tubules; c, mucus- secreting tubules; d, demilune. (After Krause.) A B CD FIG. 360. DIAGRAMS OF A, PAROTID GLAND; B, SUBMAXILLARY GLAND; C, SUB- LINGUAL GLAND; AND D, PANCREAS. (After Sobotta.) c^, inter lob ular duct; a^ intralobular duct; sr, salivary intralobular d''<3t; s, inter- calary duct; hm % demilune; ts, serous alveolus; tm, mucous alveolus 389 390 THE DIGESTIVE SYSTEM the discharge of the mucus during secretion, is presumably capable of reloading the cell with its mutinous content. We will now consider the more important peculiarities of each of the larger salivary glands. The Parotid Gland. This is the largestof the salivary glands and in man, sheep, dog, cat and rabbit is purely a serous-secreting organ. However, in sheep, dog and cat it contains also a variable number of mucous alveoli during Sri > *fr~^*N the fit geners disapj seconc Traut st year ; these de- te and entirely ear during the year (Brock and iann, Anat. Anz., 47, 17, 1914). It is situated^ in front of the external ear^ and overlaps^ in- feriorly both faces of the upper portion of the ramus of the mandible. A variable number of small accessory lobules, including mucus-pro- ducing alveoli, lie along the course of the main parotid ( Stenson's) The latter opens into the mouth at the level of the second upper The parotid is invested by a dense fibro-elastic sheath, septa from which divide the gland into lobes and lobules. The lobes and lobules are firmly united by the dense but narrow bands of connective tissue ; these contain the larger ducts, bloodvessels, lymphaticsTand a few small gan- glia. The secreting acini are relatively longhand tortuous; they are fre- quently hninched or forked. Because of the relatively low ,height_jif__. their serous-secreting cells the acini appearjslender and their lumen is kreg^Ikr^Jini^ narrow. The 'basket cells' upon which the secreting cells rest are highly developed in the parotid and often form a complete investment for the acinus. The acini of the parotid are all of one type. The only other tubules within the lobules of this gland are the intercalary and the intralobular FIG. 361. FROM A SECTION OF THE HUMAN PAROTID GLAND. I, lumen of a serous acinus; sch, intercalary duct; sr, intralobular duct; T, secreting acini. Hematoxy- lin and eosin. X 280. (After Sobotta.) duct. THE SALIVARY GLANDS 391 or salivary ducts. The former are characterized by their very narrow caliber and low epitheliunl They are slender tubules which openjm the one side from the acini g(rid on the other into the branched terminals of the more spacious salivairy ducts. The 'salivary' (secretory) portion of the intralobular duct l|as a somewhat greater diameter also than the excretory portion, whicjh. is histologically similar to the interlobular ducts. In the parotid the salivary ducts are relatively short as com- pared with the intermediate ducts, but are readily recognized by their striated columnar epithelium, which is deeply colored bjr acid dyes (eosin, etc.), and are; thus sharply distinguished from the secreting cells of the abini, which stain poorly with these dyes. The Submaxillary Gland. In man and in most mam- mals this organ is a jmxejLsal- ivary gland ; that of the\ bear and dog contains the largest, that of man and the apeij the smallest proportion of mucous acini (Krause, Arch. mikr. Anat., 1897). In man the proportion of mucous to serous In the center is a small salivary duct> just above which are three mucous acini, the uppermost one possessing a demilune. The other acini are serous. Hematein and eosin. Photo. X 370. FIG. 362. FROM A SECTION OF THE HUMAN SUBMAXILLARY GLAND. alveoli is about one to five (Piersol). JThe submaxillary is situated in the floor of the mouth under the posterior por- tion of the mandible. The main submaxillary (Whartoirs) duct_oj)eiis_ into the mouth one on either side of the f renulum of the tongue, some- times joined by the duct of the sublingual gland. The serous acini of the submaxillary are shorter and less typically tubular than those of the parotid, and they are lined by taller secreting cells. The diameter of the acinus is therefore slightly greater in this gland than in the parotid. Its mucous acini contain a relatively large proportion of demilunes. The intercalary ducts are considerably shorter than in the parotid^ while the salivary intralobular ducts are much longer in the submaxil- lary. The interlobular connective tissue is not quite so delicate as in 392 THE .DIGESTIVE SYSTEM the parotid. It contains many sympathetic ganglia of relatively large size. Small lamellar corpuscles of simple construction are occasionally found in the interlobular connective tisime (Krause). The Sublingual Gland. This is the smallest of the three pairs of large salivary glands. It is not invested by a distinct capsule. It lies under the floor of the mouth anteriorly, one on either side of the frenu- lum. It is a. mixer! idfjffd in man, dog, cat and raooit. It differs from the submaxillary, how- ever, in that all of its acini are mucous. It be- comes 'mixed' by reason of the presence of Tery many demilunes (serous cells). Although iso- lated sections which pass through the larger collections of demilune cells may appear as sec- tions of serous-secreting tubules, if examined in longitudinal section or by reconstruction the true mucous character of each lobule is ap- parent. Many of the terminal acini of the sublingual gland, however, j although much branched, contain no demilunes. The 'basket cells 7 are readily recognized ii$ the acini of this gland though they are less highly developed here than in the parotid. Its intralobular duct system lacks intermediate duets, and is largely of the 'salivary' type. Accompanying the sublinguajl gland proper, or larger component, are a variable number (5 to 20) of accessory sublingual stands of various sizes. They consist largely of mucous alveoli, each gland opening into the mouth by an independent duct./ The duct of the sublingual proper (duct of Bartholin) opens at the sid* of the frenulum. Blood Supply. The salivary glands., possess a nc]i_JjTonj supply. The arteries accompany the glandular ducts within the interlobar and mterlobular_ connectiy^^Jbissue, and thus reach all the lobules of the gland. Small arterial twigs enter the lobule from all sides and form a rich capillary plexus in the delicate connective tissue coats of the acini. The capillaries are thus brought into intimate relation with the secreting cells, from which they are only separated by the basement membrane of the acinus. The veins return by a similar course, the smallest venules passing out of the lobule into the connective tissue septa in which they retrace the course of the arteries. FIG. 363. RECONSTRUC- TION MODEL OF THE SUBLINGUAL GLAND OF MAN. An intralobular duct terminating in salivary ducts and acini. X 285. (After Maziarski.) THE SALIVARY GLANDS 393 Lymphatics. Lymphatics are relatively few and are for the most part confined to the interlobularjsepta. where they form cleft-like spaces which lead to true lymphatic vessels and so on to the lymph nodes of the cervical region. Nerve Supply. The salivary glands are abundantly supplied with nerves, which are derived from both sympathetic^ arid They are distributed to the walls of, the blood-vessels and ducts, and to the secreting cells of the__acini. The nerve trunks are found in the interlobular connective^ tissue where they are supplied with small ganglia which are most abundant in the sub- maxillary and least numerous in the parotid gland. The sympathetic fibers vate the blood-vessels proc which inner- >ed from the They are FIG. 364. NERVE ENDINGS IN A SALIVARY GLAND. H, demilune; I, secreting acini; n, nerve fibrils. Highly magnified. (After Retzius, from Rauber.) superior cervical ganglion believed to be vasodilator jfibers. The cerebral nerves, which suprply fibers to the gland cells, are the facial and the glossopharyngeal. The parotid gland is supplied by the glossipharyngeal ; the fibers (secretory and vasoconstric- tor) passing by way of the tympanic nerve (nerve of Jacobson),pnall superficial petrosal, otic (sympathetic) ganglion, and the auriculatemporal branch of the inferior maxillary division of the trigeminal nerve. The submaxillary and sublingual glands receive their secretory fibers and vasoconstrictor fibers from the trigeminal nerve, by way of Ithe chorda tympani nerve and the submaxil- lary (sympathetic) ganglion. The fibers beyond the otic and submaxil- lary ganglia are believed it be true postganglionic fibers; that is, axons of cell bodies situated in these ganglia. Sensory fibers are said to pass to the ducts. Delicate fiber bundles from the mjterjobularjnerve trunks^ enter the lobules ana form a plexus of naked fibrils afyoijti ^ p walla nf j-hp anim^ known as the epilemmal plexus, from which terminal fibrils pierce the basement membrane and as liypolemmal fibers end in contact with and between the. secrating cells. Small lerminal expansions, varicosities, or end knobs are found in the course ol the hypolemmal fibers. 394 THE DIGESTIVE SYSTEM THE PANCREAS The pancreas is a flalreleBgate body lying behind the stomach be- tween the loop of the duodenum and the spleen. It arises as two anlages : a smaller from the common bile duct (ventral pancreas), and a larger from the duodenum (dorsal pancreas). These subsequently fuse to form a single organ drained mainly by the duct of the Central component, which has meanwhile made an anastomosis with thatj'of the dorsal com- ponent, the latter proximal to the anastomosis nevertheless commonly FIG. 365. EARLY STAGES IN THE DEVELOPMENT OF THE PANCREAS, ILLUSTRATING CONDITIONS IN THE 5 AND 7 WEEKS OLD HUMAN EMBRYOS. Dp, dorsal pancreas; Vp, ventral pancreas; Pd, pancreatic (Wirsung's) duct; Apd, accessory pancreatic (Santorini's) duct; Dch, duct us choledochus; cd, cystic duct; Hd, hepatic duct. (Adapted from Kollman.) remaining pervious and functional. The pancreas! bears a close struc- tural resemblance to the salivary glands, more especially the parotid. It is a compound tubulo-acinar gland which contains an immense num- ber of small lobules associated into lobes and which pours its secretion into the lumen of the duodenum by means of the chief (Wirsung's) and accessory (Santorini's) pancreatrut directly connected ,obules these include the great majority and in connection with either ^cini or ducts or both ; and (4) islets unconnected with either ducts or aci^ii, both in the inter- lobular connective tissue and in the acinar parenchyma. THE PANCREAS 403 The tubules (intercalary ducts) are said to branch and anastomose freely and to be capable of differentiating either into acini, or islets, or both. They may produce also small mucous glands which open into the tubules ; and single islet cells may jbe formed along the ducts. Notwith- standing their intimate developmental relationship the islets remain isolated from the general exocrin parenchyma., since the lumen of the connecting tubules does not penetrate its substance. There is no satis- factory evidence to show that islet tissue may be increased or diminished, or that acinus tissue may change into islet tissue, or vice versa, concom- itant with experimentally induced alterations in nutritive and func- tional conditions, as has been repeatedly claimed ; nor is there evidence of a transition between islet and aciijar cells. Islets and acini have a com- mon embryonic origin, but once differentiated they are not capable of transformation one into the other (Bensley). The islets appear in the human pancreas about the time the embryo attains a length of 50 milli- meters (Lewis). The islets are believed to constitute a group of internally secreting organs. The evidence for such conclusion is derived from a series of experiments by Opie and many others. When the pancreas is removed, a form of diabetes follows, characterized by the appearance of sugar in the urine. When the pancreatic duct is simply ligated, the flow of pan- creatic juice is checked and atrophy of the acinar tissue ensues, but no disturbance in carbohydrate metabolism results, nor is any alteration produced in the islet tissue. Moreover, in cases of death following diabetes mellitus, Opie demonstrated gross degenerative changes. Ho- mans' recent experiments (Jour. Med. Eesearch, March, 1914) have ex- tended the evidence in support of the conclusion .that the islets are con- cerned in the mrtnholinm of nrr^vr" WrICh more than three-fourths of the pancreas was removed in cats, the diabetes occasionally followed. Microsi opic examination of the islets revealed the following conditions : ( 1 ) i did not follow the operation, the islet ce indicated in part by a disappearance o subjects which died from diabetes follow ain duct being left intact, fatal i those instances where diabetes Is showed signs of over-activity secretory granules; (2) in the ng the operation, the islet cells showed degenerative changes, the acinbus tissue having remained un- altered. Blood Supply. The large blood-vessels of the pancreas accompany the interlobular ducts, but after repeated subdivision these vessels part company, and the smaller arteries pursue a separate course through the Tnterlolniiar connective tissue. Thus they reach all portions of the 26 404 THE DIGESTIVE SYSTEM gland and supply capillaries to the intralobular connective tissue about the acini. Certain arterial branches also enter -the islets and form a specially"rich plexus of broad capillaries^ ( sinusoids ) within these cell groups. The veins return the islet blood by a similar course. The Lymphatics. The lymphatics are' mostlyjconfined to the interlobular tissue; where they are in re- lation with the blood- ves~sels7"~ The Nerves. The nerves are derived from the sympathetic system, and occur as small trunks within tnlTlnterlobular con- nective tissue. . Nu- merous small ganglia occur in their course. As in the salivary glands the nerves sup- ply the vascular walls. About_JJije~secreting acini they form a del- icate network of naked fibrils, from which end branches penetrate Hre"~~base- ment membrane and terminate upon, the secreting cells. Lam- ellar corpuscles are oc- casionally found in the interlobular connective tissue of the pancreas. Resume. Finally the attention of the student should be specially directed to the presence of the pancreatic islets, the centro-acinar cells, the very distinct inner granular and outer fibrillar zones of the secreting cells, the thick walls of the interlobular ducts, the absence_ofjfl.tralobular ducts except of the intercalary type, and the loose character of the inter- lobular tissue as the distinguishing characteristics of the pancreas. FIG. 377. SECTION OF A PANCREATIC ISLET FROM INJECTED SPECIMEN OF CAT'S PANCREAS TO SHOW THE PROFUSE BLOOD SUPPLY. The capillaries were accurately drawn by aid of a camera lucida. Only the nuclei of the islet cells are indicated; one acinus is shown at the left. X 500. THE LIVER 405 THE LIVER The liver is the largest gland of the body, and may be classed as a peculiar form of compound tubular gland whose cells resemble the serous- FIQ. 378. A LOBULE OF THE PIG'S LIVER; THE CENTRAL VEIN LIES IN THE MIDDLE OF THE FIGURE. a, capsule of Glisson. Hematein and eosin. Photo. X 115. secreting type. The organ is invested with a ^connective tissue sheath, the greater portion of which is clothed with peritoneal epithelium. From this connective tissue capsule, fibrous bands or septa are continued into the substance of the organ and permeate to all its portions. These processes jDJLjcaqngctive tissue, collectively forming the rnpsule of Glisson, are most abundant at thTtTangverse fissure where they contain the large" 406 THE DIGESTIVE SYSTEM blood-vessels and hepatic ducts this fissure serving as a hilum for the organ. The liver is very irregular in outline and shape, and comprises five so-called lobes of very unequal size. In the adult its weight is Central vein ictiL _rT~ a \.Porfa! FIG. 379. DIAGRAM OF LIVER LOBULES, THE UPPER Two Cur TRANSVERSELY, THE LOWER LONGITUDINALLY. The portal veins are blue striped; the hepatic veins (central with branches, and sublobular) are solid blue; the hepatic arteries are red; and the bile canals black. (After Merkel.) Cells not indicated. about one-fortieth that of the body, about 3 to 3% pounds. The parenchyma of the organ arises from a tubular outgrowth of the embry- onic duodenum, hence entodermal; the interstitial tissue develops iri part from the mesenchyma of the caudal layer of the primitive dia- THE LIVER 407 phragm (septum trans ver sum) and of the intervening ventral mesentery throughout which the entodermal tubules ramify, and in part (the-retre- ulum) from endothelium of the original venous sinusoids (Mall). The liver js dependent for its structural characteristics upon the peculiar disposition of the connective tissue of Glisson's of the blood-vessels whose, branches it contains; for by these tissues the substance of the liver is ex- tensively subdivided into mi- nute collections of hepatic cells, each group forming an anatomic unit, the hepatic lobule, which in addition to the hepatic cells contains a connective tissue reticulum and the smaller blood-vessels andjsecretory capillaries ( bile canaliculi). The hejaatic^ lobules are of cylindrical shape7"aT>out 2 millimeters in length and 1 millimeter in diameter (Bai- ley). In transverse section they present a polygonal (hexagonal or pentagonal) outline. In the dog they are short polyhedra about 0.7 millimeter high, and 0.7 millimeter in diameter; the entire liver con- taining 480,000 (Mall, Amer. Jour. Anat., 5, 3, 1906), They are anal- ogous to the lobules of compound tubulo-acinar glands, inasmuch as they contain the secreting parenchyma of the organ, but are very different from the latter in the arrangement of the secreting cells which, in the human liver, do not present either a tubular or acinar structure, but form solid cell columns. Thus in the human liver the tubular character of the gland is scarcely apparent, yet in the liver of many of the lower animals, notably in that of the turtle and frog, the cells form typical tubules within the indistinct hepatic lobules. The bile^f ormed by the liver cells is conveyed to the duodenum by an excretory system, beginning^trir-innUmerable interlobular bile due which receive the intralobular secretory capillaries, and, leaving the lobule "from all its sides, find their way through the interlobular con- FIG. 380. FROM A SECTION OF THE TURTLE'S LIVER, SHOWING THE TUBULAR ARRANGE- MENT OP THE PARENCHYMA. a, blood capillary, partially filled with clotted blood; 6, vascular endothelium; c, darkened central portions of the hepatic cells; d, periph- eral portion of the hepatic cells. Osmium tetroxid; carmin. X 400. (After Shore and Jones.) 408 THE DIGESTIVE SYSTEM nective tissue of the capsule of Glisson and unite with their fellows to form larger and larger bile ducts, which finally - join to form the main excretory or liepalic duct; the latter unites with the cystic duct of the gall-TxTadder "to form the common bile duct through which the bile reaches the intestine. The gall-bladder which is in principle a diver- ticulum from the hepatic duct is simply a reservoir for the storage of bile; it is absent in some animals, for example, the horse and the ele- phant. In all their course the bile ducts are in close relation with the radicals of the portal vein and of the hepatic artery the group of vessels which, together with their supporting (interlobular) connec- tive tissue and the in- cluded nerve and lymph channels, form the so- csilledjiortal canals. __ The Hepatic Con- nective Tissue. T h e hepatic connective tis- sue, or the supporting tissue of the liver, in- cludes the capsule of the organ and the cap- sule of Glisson the lat- ter forming a frame- work throughout the liver and inclosing its hexagonal lobules together with the more delicate intralobular reticu- lum. These tissues convey the blood-vessels, lymphatics, nerves, and bile ducts. The fibrous framework, which forms both the outer fibrous capsule of the liver and the capsule of Glisson, contains both collagenous and elastic tissue, the latter being fairly abundant a fact which sharply contrasts with the complete absence of elastic fibers from the interior of the hepatic lobules. The intralobular connective tissue is extremely delicate, and consists of very fine fibrils and stellate cells (of von Kupfer) which form a deli- cate reticulum, in which the capillary blood-vessels and columns of liver FIG. 381. THE RETICULUM OF THE DOG'S LIVER. a, central \nein; b, capsule of Glisson at the margin of the lobule. Gold chlorid. X 120. (After Bohm and von Davidoff.) THE LIVER 409 cells are suspended. The anastomosing strands of reticulum^ converge from the periphery toward the center of the lobule, thus following the course of the blood capillaries and cell columns. This reticular tfssue (Mall) exists in so small a quantity and is so extremely delicate that although it can be readily studied after removal of the liver cells, as by artificial digestion, in ordinary preparations except those of extreme thinness it can scarcely be discovered in the minute clefts between the cell columns and the blood capillaries. It differentiates, at least in part, from the endothelium of the original venous sinusoids; the stellate cells seen in Golgi preparations also represent endothelial elements. The endothelial cells of the intralobular capillaries are actively phagocytic. The volume of the interlobular connective tissue which forms Glis- son ? s capsule varies greatly in different animals. In the liver of the pig and the camel this tissue is very extensive, and forms a complete investment for each lobule. In man it is very limited in amount and is con- fined to minute areas between the adjacent angles of the lo- bules, with an occasional frag- FIG. 382. STELLATE CELLS OF VON KUP- FEB IN THE LlVEB OF A DOG. g, capillary blood-vessel; I, hepatic cells; st, stellate cells. Gold chlorid. X 200. (After Kolliker.) ment separating the lateral sur- faces of neighboring lobules. It is in the latter portions, viz., be- tween the opposed surfaces of the lobules, that the branches of the hepatic veins (sublobular veins) are found. The interlobular veins, the subdivisions of the portal vein, together with the bile ducts and the branches of the hepatic artery are found at the angles of adjacent lobules; hence the portal canals, which contain these vessels, should always be sought in this location, while the sublobular veins, which run alone and form no part of the portal canals, will be found between the opposed surfaces of the lobules. The capsule of Glisson also contains many lymphatic vessels and non- medullated nerve fibers. The Hepatic Lobule. The lobule is the structural unit of the liver and consists chiefly of hepatic cells which are arranged in radiating cords. In shape the lobule is an irregularly hexagonal pyramid, the exact number of its faces being extremely variable. The periphery of the lobule is outlined by the connective tissue of Glisson's capsule which 410 THE DIGESTIVE SYSTEM either completely invests each lobule, as in the pig's liver, or forms only a very incomplete investment, as in the liver of man. Blood enters the lobule from the vessels of the portal canals and finds its way, through converging capillaries, from the periphery to the center of the lobule. Here it enters the intralobular or centra! vein, FIG. 383. A LOBULE OF THE PIG'S LIVER IN LONGITUDINAL SECTION, SHOWING THE RELATION OF THE CENTRAL AND SUBLOBULAR VEINS AND THE ARRANGEMENT OF THE HEPATIC CELLS. a, sublobular vein; 6, capsule of Glisson. Hematein and eosin. Photo. X 68. which occupies the axis of the lobule and conveys the blood thence to the sublobular -vem&r-whicli again lie in the interlobular connective tissue of Glisson's capsule. The hepatic cells occupy the meshes of the intralobular capillaries (sinusoids) and are arranged in cords which radiate from the central vein toward the periphery. The frequent anastomoses of the capillaries THE LIVER 411 as they approach the central vein produce great irregularities in the arrangement an44engtbrof the cell cords. Each cord, however, reaches the periphery of the lobule after a more or less tortuous course, and it is here~ that the secretory bile capillaries, which are found within the cell cords, become continuous with the minute bile ducts of the portal canals.~~ FIG. 384. A LOBULE OF THE HUMAN LIVER, SEEN IN TRANSECTION. It is outlined by three small portal canals and contains a single central vein. Hematein and eosin. Photo. X 50. THE BILE CAPILLARIES. The bile capillaries occur as secretory canaliculi between the opposed surfaces of the hepatic cells. They are thus found within the cell cords and stand in the same relation to the hepatic cells as though each cell cord formed a tubule whose capillary lumen, the bile canaliculus, was surrounded by only two secreting cells, whereas in other tubular glands a larger number of cells encircle the lumen of the secreting tubule. Hence the bile canaliculi and the blood capillaries are never in contact, but are always separated by at least one-third to one-half the diameter of a hepatic cell. The bile canaliculus occurs on that surface of the hepatic cell which is in contact with other 412 THE DIGESTIVE SYSTEM cells within the cord; the blood capillary, on the other hand, is in rela- tion with that surface of the hepatic cell which forms the periphery of the cell cord. The blood capillaries are suspended in the fine meshes of the delicate reticulum which has already been described as the intralobular connective tissue, and which also invests the cords of hepatic cells. This reticular connective tissue is of relatively insignificant volume. The bile capillaries are true secretory canaliculi by which the bile, after secretion by the hepatic cells, finds its way along the anastomos- ing cell cords to some point at the periphery of the lobule, where the cell cord becomes continuous with a minute bile duct, the secreting cells within the lobule presenting a rapid transition to the very low columnar or flattened epithelium of the interlobular bile duct. The immediate lining of the intralobular bile canaliculus is a del- icate cuticular mem- brane, probably the prod- uct of the hepatic cell. The Hepatic Cells. These are large poly- hedral cells which possess one, or very frequently two 7 spherical nuclei and a coarsely granular cytoplasm. A true cell membrane may be re- garded as being absent, yet there is often a sharply defined exoplasm which forms the surface of the cell and simulates a true membrane. The nuclei of the hepatic cells are rich in chromatin, and stain deeply. They are situated well within the cell, but usually in an eccentric position. Frequently they contain a distinct nucleolus. The cytoplasm of the hepatic cells is finely reticular, the meshes being filled with coarse granules of irregular size. Many of these are undoubtedly glycogenic granules, and show a decided color reaction when FIG. 385. SECTION OF LIVER TISSUE OF CAT, SHOWING THE LIVER CELL-CORDS, AND THE ARTI- FICIALLY ENLARGED SINUSOIDS LINED WITH ENDO- THELIUM. X 375. THE LIVER 413 treated with LugoFs solution of iodin after alcoholic fixation. The amount of glycogeri present varies with the diet. After digestion and absorption of a carbohydrate meal it is greatly increased, but disappears during fasting. Even when glycogen is quite deficient, the hepatic cells still present a granular appearance from the presence of other substances, possibly zymogens. FAT GLOBULES. Fat glo- bules occur in hepatic cells in limited numbers; and appear to be a normal constituent. The (Adapted from Merkel. ) globules vary much in size, but are all very small. Their number is also dependent upon diet and diges- tion. During absorption of a fatty meal, fat globules occur in consider- able numbers, and are most numerous in those hepatic cells which are Blood capillary Bile capillary Lymph space Nucleus FIG. 386. DIAGRAM OF FOUR ADJACENT LIVER CELLS. FIG. 387. ISOLATED LOBULES OP THE PIG'S LIVER. X12.5 (After F. P. Johnson; Am. Jour. Anat., Vol. 23, 1918.) at the periphery of the lobule. They are not normally found in the vicinity of the central vein. The hepatic cells also frequently contain brown or yellowish-brown 414 THE DIGESTIVE SYSTEM FIG. 388. SHOWING THE CONNECTION BETWEEN THE INTRALOBULAB AND INTER- LOBULAR BILE DUCTS IN THE CAT'S LIVER. a, interlobular vein; 6, interlobular bile duct; c, intralobular bile capillaries- Golgi stain and hematein. Highly magnified. (After Geberg.) granules of ferruginous PIGMENT, which are more prone to occur in the interior of the lobule near the central vein. When present in con- siderable amount this pigment can no longer be considered a normal constit- uent of the hepatic cell. Mitochondria have also been described in the hepatic cell (Policard, Compt. Rend, de Soc. et Biol., 1, 66, 1909). An intracellular canalicular apparatus can also be dem- onstrated within the cytoplasm. By some these channels have been inter- preted as intracellular terminals of the intercellular (intralobular) bile canal- iculi, by others as fixation artifacts, and still others regard them as a tro- phospongium. Schaffer (1902) inter- prets them in part as channels in rela- tion with the blood sinusoids. Such in- tracellular canaliculi undoubtedly ap- pear under certain functional conditions, but it is uncertain whether they are definite preformed channels, or simply transient secretory canals. FIG. 389. TYPES OP CELLS FROM A SECTION OF THE NORMAL HUMAN LIVER. A, the usual type of liver cell; B, fatty, and C, pigmented cells. Types B and C were very scarce. Hematein and eosin. X 900. THE LIVEB 417 longitudinal muscle fibers also appear in the walls of the excretory ducts, and so accumulate in the wall of the gall-bladder and common bile duct as often to form a distinct layer. As has been pointed out by Mall (1906) the hepatic lobule cannot be regarded as the homologue of the lobule of other glands. For the portal canal with its bile duct, the excretory duct of the glandular unit, should be axial to the functional lobular unit. Moreover, an interlobular bile duct drains not the whole of a particular lobule, but portions of a num- ber of adjacent lobules. The functional unit (portal lobule}, in contra- distinction to the structural unit (hepatic lobule), is accordingly that portion^)]: liver TlfifWP (pflrti^rTf 1 of a number of hepatic lobules) plied by an ultimate branch of the vessels of a portal canal. Thus a hepatic lobule is drained by several bile ducts, and conversely, a single ultimate bile tluci drains portions of several hepatic lobules. Blood Supply. The liver is supplied with blood from two inde- _jpendent sources, the hepatic artery and the portal vein. That supplied by the artery is of minor importance and is destined only for the nutri^. I tion of the connective tissue framework of the organ. On entering the liver at the transverse fissure, the HEPATIC ARTERY gives off numerous capsular branches which ramify, in the capsule of the liver and supply capillaries to its connective tissue/ Other branches, the : direct continuation of the hepatic artery, enter the portal canals and by repeated division form the interlobular arteries, which '"ramify in the tissue of Glisson's capsule, and whose vaginal branches supply capillaries to this connective tissue. _,-; These capillaries, as well as those from the capsular branches, become continuous, at the periphery of the lobule, with the intralobular capillaries which are derived from the branches of the portal vein. tAfrjU**> THE PORTAL VEIN. The portal vein likewise enters_at the transverse fissure, bringing to the liver the blood corrected from the^capillaries of the organs of digestion and absorption. It divides into numerous branches which follow the portal canals, in which they are known as the interlobular reins, and in this way reach all portions of the organ. The interlobular veins throughout all their course give off small branches which at once enter the periphery of the hepatic lobules and immediately break into a brush of capillary vessels. These intralobular A capillaries (caj^Uiform^ sinusoids, Lewis) converge toward the center of the lobule and anastomose to form a capillary network, in the elongated meshes of which are the cords of hepatic cells. These capillaries approach the center of the lobule where they unite to form the intralobular or 418 THE DIGESTIVE SYSTEM kJ 9 ^ ' central vety. The central vein frequently begins in the form of a Y, its two or more branches finally uniting to form a single vessel which pursues its course through the axis of the lobule. The central vein makes its exit at the periphery of the lobule and enters the interlobular connective tissue where it unites with its fellows to form larger sub- lobular veins. The sublobular are easily distinguished from the inter- lobular veins by their thicker walls and by the fact that the former p.v.~ FIG. 393. A GROUP OF SURFACE LOBULES OF THE PIG'S. LIVER. p. v. t portal vein; a. v., sublobular (hepatic) vein; c. v., central (hepatic) vein. X10. (After F. P. Johnson.) pursue an independent course through the tissue of Glisson's capsule, being nowhere in relation with either artery or duct. The sublobular veins are, as a rule, vessels of considerable size, and by frequent union with their fellows become constantly larger. In their general direction they tend toward the dorsal surface of the liver and finally make their exit as four or five large hepatic veins which enter the inferior vena cava. The blood supply of the liver, 30 per cent, of which can be ac- counted for in the hepatic artery, 60 per cent, in the portal vein (Mac- THE LIVER 419 leod and Pearce, Amer. Jour. Physiol., vol. 35, 1914), is peculiar in that: (1) the greater portion of the blood has already passed through the capillaries of the digestive organs before entering the liver; (2) its arterial supply is relatively meager and supplies only the connective tissue framework, intermingling with the portal blood at the periphery of the lobule; (3) its intralobular capillaries are extremely abundant and are in intimate relation witty the hepatic cells, each cell coming into contact with four to six capillary vessels. s The course of the blood .through the vessels of the liver will be readily appreciated by reference to the following table which indicates the succession of the hepatic blood-vessels: 1. Portal vein 1. Hepatic artery 2. Interlobular veins 2. Interlobular arteries 3. Branches to lobule 3. Vaginal branches and capillaries in Glisson's capsule 4. Intralobular capillaries 5. Central vein (intralobular) 6. Sublobular veins 7. Hepatic veins 8. Vena cava inferior The three classes of veins, interlobular, central, and sublobular are readily differentiated by the fact that the two latter lie alone, while the interlobular veins are always in company with the ducts and arteries within the portal canals. Moreover the central vein has almost no con- nective tissue wall until near its exit from the lobule, where it passes into the sublobular branches; the sublobular veins, on the other hand, possess a relatively thick connective tissue wall and even some smooth muscle, except in the very smallest, which are to be regarded as mere interlobular continuations of central veins which soon unite to form the larger sublobular vessels. Lymphatics. The lymphatics of the liver may be considered as con- sisting of a superficial set which supplies the hepatic peritoneum and the capsule of the liver, and which is continuous with a deeper set in Glisson's capsule. The lymphatics of the deep set begin as perivascular spaces within the lobule, from which the lymph enters larger lymphatic vessels in the interlobular connective tissue, which follow the vessels of the portal canals to their exit from the liver ; the larger lymphatics pass to the abdominal lymph nodes. Other lymphatics follow the sublobular and hepatic veins and pass to the mediastinal lymph nodes. 420 THE DIGESTIVE SYSTEM Nerves. The nerves of the liver are mostly of the non-medullated variety. They follow the portal canals and are distributed to the walls of the blood-vessels, the walls of the bile ducts, and to the capsule of the liver. Naked fibrils from these trunks also enter the lobules and form a plexus among the hepatic cells (Korolkow, Anat. Anz., 1893), in relation with which they form fine terminal brushes and varicose -end knobs (Berkley, Johns Hop. Hosp. Rep., 1895). The liver secretes the bile a fluid which aids in the digestion and absorption of fats and in fact probably serves also an excretory role. Bile has a greenish-yellow color, due in part to the FIG. 394. _ INTRALOB- presence of the pigment bilirubin, which is be- ULAB NERVE FIBERS lieved to be identical with the hematoidin elab- IN A RABBIT'S LIVER. orated from the hemoglobin of the erythroplas- a, hepatic cells; 6, ^ g disintegrating in the spleen, and carried to nerve fiber. Golgi stain. . . Highly magnified. (Af- the liver by way of the splenic and portal veins. ter Berkley.) Under certain pathological conditions known as jaundice, bile finds its way within the liver di- rectly into the blood stream where it produces hemolysis of the red corpuscles. In addition to its bile-producing activity, the liver functions also as an organ of internal secretion, in the conversion of glycogen into sugar and the elaboration of urea. Experimental evidence indicates that the liver is concerned greatly also with the production of fibrinogen (Whipple, Amer. Jour. Physiol., 1914). THE GALL-BLADDER The wall of the gall-bladder consists of three coats: (1) mucous; (2) muscular; (3) fibroserous. The mucous membrane is markedly folded or corrugated, the irregularly polygonal depressions being rela- tively broad at the f undus but becoming narrower toward the neck of the organ. The lining epithelium is of the tall columnar variety, with spheroidal or ovoid nuclei which lie near the base of the cell. The free extremity of the epithelial cells presents an indistinct cuticular border. The epithelium follows all the folds of the mucosa and lines the inter- vening depressions. The corium of the mucosa consists of delicate connective tissue and contains a few smooth muscle fibers derived from the muscular coat. THE GALL-BLADDER 421 FIG. 395. FROM A SECTION THROUGH THE WALL OF A DOG'S GALL-BLADDER. a, epithelium; b, lymph nodule; c, serous coat. X 80. (After Sudler.) It is connected with the muscularis by a thin layer of denser connective tissue which contains blood and lymphatic vessels and which simulates a submucosa. FIG. 396. RECONSTRUCTION OF THE WALL OF A DOG'S GALL-BLADDER. a, vein; b, artery; c, lymphatic vessel; d, epithelium. X 60. (After Sudler.) The gall-bladder possesses a distinct muscular wall, consisting of numerous interlacing smooth muscle bundles the most of which are cir- cularly disposed. Occasionally they form fairly distin^ circular and longitudinal layers. 422 THE DIGESTIVE SYSTEM The fibroserous coat consists of loose areolar tissue, which contains the larger blood-vessels with which the organ is abundantly supplied. The free surface of the gall-bladder also receives a peritoneal investment. Occasional mucous glands occur in the mucosa of the gall-bladder. These are mostly of small size and widely separated, but toward the neck of the organ they increase in both number and size. They form short, branched, convoluted tubules. The blood-vessels form a plexus just outside the muscular coat, from which branches are distributed to the peritoneal coat and to a plexus in the depth of the mucosa from which capillaries are supplied to the muscu- lar layers and to a subepithelial plexus. The nerves are distributed to the blood-vessels and to the muscular wall. Minute ganglia occur in the muscular coat. The gall-bladder is lacking in many species of vertebrates includ- ing the lamprey, pigeon, rat, htfrse, certain ruminants and the porpoise. In these instances a well-differentiated cystic anlage suffers regressive changes in later embryonic life and ultimately disappears. With the disappearance of the gall-bladder, the biliary system generally under- goes compensatory enlargement. Dilatation of the bile ducts is fre- quently associated with congenital absence of the gall-bladder in the human subject, and commonly follows cholecystectomy (see Scammon, Anat. Rec., 10, 8, 1916). CHAPTER XIV THE URINARY SYSTEM This system includes the kidneys, which are two large bean-shaped glands, together with their excretory passages, the ureters, which conduct the urine to the urinary bladder, whence it is voided through the urethra. THE KIDNEY Each kidney is a gland of the compound tubular_tye. measuring about four and one-half inches irTTength, two and one-half inches in width, and one and one-half inches in thickness. Its secretion, the urine, is produced by the uriniferous or renal tubules, which are long tortuous canals beginning near the surface of the kidney and finally ending at the hilum of the organ where they pour their secretion into the calyces of the renal pelvis. The uriniferous tubules are in intimate relation with the renal blood-vessels which supply rich capillary plexuses to the entire extent of the tubules. Each uriniferous tubule consists of both tortuous and straight portions, and these are so regularly disposed as to produce macroscopical variations in the appearance of the different por- tions of the renal parenchyma according as the tortuous or the straight portions of the tubules predominate. These variations result in the fol- lowing topographical subdivisions. TOPOGRAPHY OF THE KIDNEY If the kidney be divided parallel to its long axis by an incision extend- ing from its convex surface to the hilum, the cut surface shows that the parenchyma is divisible into a superficial cortex and a central medulla. The slit-like Jiilum of the organ opens into a deep excavation, the rp.n(\], sinus^ which is occupied by the renal pelvis and its subdivisions, the infundibula and calyces, ^i^w^^i_^^ m Achilla pj^ts in the form of several conical pyramids. The pelvis of the kidney, the expanded 423 424 THE URINARY SYSTEM f unnel-f orrn Jbeginning of jhe ureter, toward the renal parenchyma divides into two, sometimes three, infundibula, which in turn subdivide into several calyces, each of which incloses the conical apex of a projecting medullary or renal (Mai pig Man) pyramid. The Medulla, The medulla of the kidney consists of a number of these conical renal pyramids, (usually twelve to fifteen) each of whose apices, as already stated, is received into the extremity of a renal calyx. Occasionally a calyx may receive two papillae. The base of each pyramid is embedded in the adjacent renal cortex, and that portion of the cor- tex which is interposed between the bases of adjacent pyramids, and thus brought into relation with the fi- brous and adipose tissue which en- velops the pelvis and calyces at the hilum of the organ, composes the cortical renal columns (of Bertini). Each renal pyramid may be sub- divided into a central free portion, the apical or papillary zone of the medulla, which is received into a calyx, and an outer or basal portion, which is embedded in the renal cor- tex and is known as the boundary FIG. 397. LONGITUDINAL SECTION OF KIDNEY. 1, cortex; 1', cortical rays; 1", laby- rinth; 2, medulla; 2', papillary portion of medulla; 2", boundary layer of me- dulla; 3, transverse section of tubules in the boundary layer; 4, fat of renal sinus; 5, artery; *, renal column of transverse medullary rays; A, branch of renal artery ; C, renal calyx; f/, ureter. (After Tyson and Henle, from Hill.) zone of the medulla. These two por- tions of the medulla, the papillary and boundary zones, can be readily distinguished, since the latter con- tains only narrow tubules and is highly vascular, while the former, relatively deficient in blood-vessels, contains the broad terminations of the uriniferous tubules, the papillary ducts (of Bellini) which converge toward the apex of the pyramid where they open into the calyces. The Cortex. The cortex of the kidney, on careful observation, pre- sents numerous dark lines or delicate columns which radiate from the base of the pyramids outward toward the surface of the organ. These radiating columns are the medullary rays (pyramids of Ferrein) or THE KIDNEY 425 pars radiata of the cortex. They contain straight portions of the urinif- erous tubules ; these are continuous with the similar tubules ki the boundary zone of the medulla. These columns lie within the cortex and not, as their name might be taken to indicate, in the medulla. They are termed 'medullary rays' because of their peculiar rela- tion to the medulla, from which they extend outward in a radial direction. It would seem more proper to desig- nate them ' cortical rays.' That portion of the cortex which invests the cortical rays and which includes all the remaining cortical por- tions of the organ, consists of extremely tortuous tubules, and is characterized by the presence of small globular bodies, each of which con- tains a tuft of capillary ves- sels. These are the renal corpuscles (Malpighian bod- ies) which are characteristic of the kidney. The portion of the cortex in which they occur includes the entire cor-- tical substance with the ex- ception of the cortical rays, and is known as the rejial labyrinth or pars convoluta. The "labyrinth is divided into: (1) the renal columns, FIG. 398. DIAGRAM OF THE STRUCTURE OF THE KIDNEY. a, papillary zone, and 6, boundary zone of the medulla; c, cortex; 1, apex of a renal pyramid; 2, capsule; 3, tubules of the me- dulla; 4) vasa rectse; 5, vascular arcades; 6, a cortical ray; 7, labyrinth; 8, interlobular ar- tery; 9, renal corpuscle; 10, 'cortex corticis.' already mentioned; (2) the (After Testut.) intercolumnar portions, or labyrinth proper, which includes that portion of the labyrinth which invests the cortical rays, and which, in sections cut parallel to these columns (longitudinal sections) appears as a portion of cortex inserted between the adjacent rays; (3) a narrow boundary zone of the cortex, ' cortex corticis' of Hyrtl, which is included between the fibrous capsule 426 THE URINARY SYSTEM of the organ and the tips of the cortical rays, and in which the renal corpuscles, though present, are relatively few in number. The Renal Lobule. In fetal and infantile I life the kidney is dis- tinctly lobed. This condition is permanent in slome animals e.g., rep- tiles, birds, porpoise, ox, bear each lobe consisting of a renal pyramid with its related portion of \cortical substance. In man, after the first year, \ the renal lobes com- pletely fuse and eventually \leave scarcely a trace of the early lobed condition. \ Internally this fetal lo]i)ea condition is in part indicated by the definitive renal pyramids, from eight to eighteen in number, each representing the product of fusion of frorft two to nine primitive lobes. In certain mamnfials e.g., mouse, rabbit, cat, guinea pig neither a primitive nor definitive internal or external lobar arrangement appears. Such kidneys are designated unipyramidal or uni- papillary, in contradistinction to the multipapil- lary type. In certain ox the perissodactyls (ele- phant, hors.e) no distinct pyramids are present. The term renal lobule or renculus, as applied to the adult human kidney, refers to a still smaller subdivision of the organ, one which includes a single cortical ray together with that portion of the cortical labyrinth by' which it is immediately invested. Ijs_peripheral boundary is marked by the interlobular blood-vessels. This lobule is the anatomical unit of the kidneyltha is thus co'mparable to the portal and to the pulmonary lobule, except that its arterial supply enters at the periphery. The tortuous secreting portions of its uriniferous tubules are contained in the laby- rinth at the periphery of the lobule, while its straight conducting por- tions lie in the cortical ray in the axis of the lobule. The larger inter- lobular arteries and veins lie at the periphery, where they supply \ ^branches to several adjacent lobules. THE RENAL CONNECTIVE TISSUES The kidney is enveloped by afibrous capsule, the tunica fibrosa, which is loosely attached to the substance of the organ and contains the usual proportion of jskstic^jibers together with a little smooth muscle. A fatty layer, the tunicaja^^^a, invests the capsule. At the hilum of FIG. 399. RECON- STRUCTION OF A URI- NIFEROUS TUBULE OF AN INFANT. (After Stoerk.) THE KIDNEY 427 the organ the capsule is continuous with the connective tissue which envelops the renal pelvis, infundibula, and calyces, and which^m the intervals between adjacent calyces, comes into relation with the cortical substances of the renal columns. This connective tissue of the hilujn is of the^areolar variety and contains much adiposQissue. It supports the large__arteries and veins as they pass along the surface of the renal pelvis on their way to and from the renal columns, where they enter or leave the parenchyma. Sympathetic nerve fibers and a few small ganglia are also found in this region. The connectj_ve_tissue of the interior of the organ, interstitial is ver^jiscjyity, and in most parts consists only of isolated fibrils which invest the blood-vessels and the renal tubules. It forms a very delicate reticulumjjv which the walls of the uriniferous tubules are loosely ur'^ted 11 the epithelium of these tubules is removed, a ^ delicate fibrous network remains pEhisnetwork incloses a homogeneous basement membrane upon which the lining epithelium ordinarily rests. Elastic fibers scarcely occur arn_ojijhejubules_ oj^ the kidney. The interstitial tissue is slightly increased in amount afyout the larger blood-vessels, the renal corpuscles of the cortex, and the femall blood-vessels of the boundary zone of the medulla. At the apex ojf the renal pyramid it invests the large papillary ducts in considerable quantity. THE URINIFEROUS OR EENAL TUBULES The urinif erous tubules begin in the cortical labyrinth as the capsules pprpnsnTps. Assuming a tubular form they then pursue a tortuous course through the pars convoluta and filially enter the boundary zone~ot~the medulla, where, much reduced in size, they form the loop of Henle, which consists of a short, descending, thin limb, a U-shaped loofi, ancTa long, ascending or thick limb. This last division, after recrossing the boundary zone of the medulla, enters a pars radiata and returns to the region of its origin, where it becomes again convoluted. A short arched lultuJe connects this convoluted portion witli a straight collect UK/ tubule of the cortical ray. The collecting tubules traverse the whole length of the ray, uniting with their fellows and receiving other arched tubules along their entire course. They then cross, the boundary zone of the medulla, and finally, in the papillary zone, having meanwhile received numerous accessions of straight collecting tubules, form the 428 THE URINARY SYSTEM large terminal tubules, the papillary ducts, which pour the urinary secretion into_the renal calyces. Each uriniferous tubule may thus be subdivided into several portions Intercalary (dis- ) tal convoluted) > segment } Proximal convo- luted segment Renal corpuscle Vasa arciformia Stout, clear por- } tion of loop of > Henle ) Interlobar] blood-vessels ( Stout, cloudy por- ", tion of loop of Henle Slender, clear~) portion of loop > of Henle } Henle's loop Large, straight collecting tu- bule Arched collecting tubule Vas afferens Vas efferens Glomerulus Cortex 5 Straight, collect- l ing tubule Interlobular vein Interlobular artery Arteriola recta Outer stripe 5 Outer zone of ( medulla Inner stripe 5 Inner zone of ( medulla Papilla FIG. 400. DIAGRAM OF URINIFEROUS TUBULE OF A MAMMAL. (Adapted from Peter and Merkel.) which differ from each other, not only in their location, but also in the character of their lining epithelium. The successive portions which compose a single uriniferous tubule may be enumerated as follows : THE KIDNEY 429 I 1. Capsule of the renal corpuscle. 2. Neck of the tubule. 3. Proximal convoluted portion. 4. Descending limb of Henle's loop. 5. Loop of Henle. Medullary loop. 6. Ascending limb of Henle's loop. 7. Distal convoluted portion. 8. Arched collecting tubule. 9. Straight collecting tubule. 10. Papillary duct. It should be borne in mind that all of these several portions form only successive parts of a single uriniferous tubule. Those portions of the urine which are secreted into the capsule of the renal corpuscle must therefore find their way through each of these successive portions before it can reach the excretory passages of the renal calyces, pelvis, and ureter. 1. The Renal Corpuscle (Malpighian Body). A renal corpuscle consists of a spherical tuft of capillary vessels, the glomerulus, which in the course of its development is invaginated into the end of the uriniferous tubule, and thus comes to be enveloped by a double layer of flattened,, epi- thelial cells known as the glom- erular capsule (of Bowman). The inner visceral layer of the capsule closely invests the entire surface off the glomeru- lus, except at tha^ point where" the afferent and efferent vessels enter and leave the capillary tuft; at this point the visceral \ FlG " 401 .-RECONSTRUCTION OF A GLOMER- ... ,. ULUS OF THE HUMAN KlDNEY. epithelium is reflected outward .. .,, a. afferent ^rteriole: 6, efferent artenole: and becomes continuous with c> capi iiaries. I X 444. (After Johnston.) the parietal layer. The sur- faces of these two layers are almost in apposition; the narrow interval between them which results from the slightly eccentric position of the glomerulus forms the first portion of the lumen of the uriniferous tubule. At that pole of the renal corpuscle which is opposite the entrance of its 430 THE URINARY SYSTEM blood-vessels the capsule opens, through a narrow neck, into the first or proximal convoluted portion of the uriniferous tubule. The glomerulus is a true arterial ._rete_jnirabile, since it receives an afferent artery, which, after forming the capillaries of the glomerulus, FIG. 402. FROM THE CORTICAL IABYRINTH (PARS CONVOLUTA) OF THE HUMAN KIDNEY . A large renal corpuscle is in the center of the figure. At its upper border are several sections of distal convoluted tubules. The great majority of the tubules shown are from the proximal ccnvoluted portions, a, a portion of a glomerulus; b-b, parietal layer of the capsule; c, proximal convoluted tubules; d, just within this point is a transection of , junctional tubule having relatively low and clear epithelium and a broad lumer.. Hematein and eosin. Photo. X 135.- passes out as an efferent artery to again enter a capillary plexus about the neighboring tubules of the renal cortex. The afferent vessel is of 'somewhat larger caliber than the efferent a noteworthy fact because of its relation to the ihtraglomerular blood pressure. ?HE KIDNEY 431 On entering the glomerulus the artery divides into two vessels which immediately subdivide with the formation of five branches (Johnston). Each of these branches forms a series of anastomosing capillary^ loops whose convexity is directed away from the entering artery. The capillary loops reunite/ in a similar manner,, to form the efferent vessel, which leaves the glomerulus in company with the afferent; but, once out, they soon part company, the efferent vessel breaking into a second capillary plexus about the neighboring tubules. Within the glomerulus the capil- laries are united by a very delicate but scanty connective tissue containing no elastic fibers. 'The visceral layer of the capsule is firmly adherent to the walls of the glomerular capillaries. It consists of a single layer of flat epithelial cells which are intimately blended with each other and with the endothe- lium of the capillaries. The epithelial cells possess a clear cytoplasm and a flattened ovoid nucleus, which, being thicker than the body of the cell, produces a considerable bulging. In fetal and infantile life the shape of the cells of this layer is cuboidal or even low columnar, but becomes more and more flattened as development progresses, until the epithelium finally simulates a layer of endothelial cells. The epithelium of Ijhe parietal layer is also cuboidal in fetal life, but during development becomes nearly as much flattened as that of the visceral layer. Its single layer of finely granular cells forms a complete lining for the capsule. It rests upon a homogeneous basement membrane which is invested by a thin layer of connective tissue. This fibrous layer is rather more highly developed about those renal corpuscles which lie near the medulla than about those of the more peripheral portions of the cortex. 2. The Neck of the Tubule. In this portion of the tubule the flattened epithelium of the glomerular capsule rapidly changes to the low" columnar te 1 of llieiUiial (fl>fnmlill.wl orl.imT This section is extremely short; it fopns a constricted portion which marks the begin- ning of the tortuous jmbule. This constriction is more apparent than real, since the caliber . of the tubule in the neck is as great as in the succeeding portion wh<>se external diameter is, however, much increased by the increasing height of the epithelial cells. This portion of the tubule, being in relation with the renal corpuscle, is necessarily found in the cortical labyriiith. 3. The Proximal Convoluted Portion. This is commonly the longest and broadest portion of the uriniferous tubule. Collectively the convoluted tubules" form the greater part of the cortical labyrinth, in 432 THE URINAKY SYSTEM which region only they occur. This portion of the tubule is remarkable for the irregularity of its course, it being twistedand bent upon itself in a most tortuous manner. Arising at the renal corpuscle, it at first passes toward the surface of the organ, but soon turns about and runs toward w/ FIG. 403. FROM THE CORTEX OF THE HUMAN KIDNEY, SHOWING A TRANSECTION OF A CORTICAL RAY IN THE LOWER LEFT-HAND CORNER. a, glomerulus; 6, glomerular capsule; c, proximal convoluted tubule; d, collecting tubule; e, ascending limb of Henle's tubule;/, spiral tubule; g, blood-vessel; h, distal convoluted tubule. Hematoxylin. X 200. (After Schaper, from Stohr.) the medulla, at first with extreme convolutions, but later pursuing a rather spiral course (spiral tubule). On reaching the border of the medulla the tubule becomes sharply constricted and entejrejthje_medullary boundary zone at the thin descending limb of Henle's loop. THE KIDNEY 433 The epithelium of the convoluted tubule is of the columnar or pyram- idal type, its cells having broad, firmly united bases and eonicaL-free apices. The lateral at the base as to res( by impregnation with extremely irregular a each cell interdigitati argins of the cells are often so intimately blended nble a syncytium. When isolated, or if outlined silver salts, the borders of the epithelial cells are d are deeply fluted or serrated, the serrations of with those of its neighbors. The deep fluted ser- FIG. 404. FROM A LONGITUDINAL SECTION OF A CONVOLUTED TUBULE OF THE GUINEA-PIG'S KIDNEY. The cell outlines have been blackened by the Golgi method. Very highly magnified. (After Landauer.) FIG. 405. CROSS SECTION OF A PROXIMAL CONVOLUTED TUBULE FROM THE KIDNEY OF A MOUSE, SHOWING BASAL FILAMENTS (PROBABLY LARGELY MITOCHON- DRIAL) BREAKING UP INTO GRAN- ULES CENTRALLY, AND THE CEN- TRAL STRIATED BORDER OF THE CELLS. Meves' imtochondrial X 1000. technic. rations of the interlocked epithelium gives many of its cells a coarsely striated Appearance/ the striation being more prominent beneath the centrally situated nfucleus than in the apical portion of the cell. Other longitudinal striatiins in the proximal or basal portion of the cell are the result of a linear arrangement of granules and filaments which occur in this part. These! appearances often give the epithelium of the convo- luted tubules a peculiar striated or 'roddecF character. The granules and filaments are probably largely mitochondlrial in nature. The apices of the epithelial cells are [very easily destroyed, but when 434 THE UEINAKY SYSTEM perfectly preserved often present a delijcately striated, cuticular border (' brush border'). The remaining portions of the cytoplasm are finely granular. The nuclei of the epithelial (tells of the convoluted tubules are spheri- cal in shape, and do not stain veky deeply with nuclear dyes as compared with the more distinct and deeply staining nuclei of the collecting tubules. Thus they appear as ij partially clouded by the granular cyto- plasm, an appearance which is greatly exaggerated with the onset of acute inflammatory processes, which] on attacking the kidney, are prone to involve the convoluted tubules, j The chromatin is quite evenly distributed throughout the nucleus and the nuclear membrane is not easily demon- strated. The lumen of the convoluted portion of the uriniferous tubule is of variable caliber ; it presents frequent slight dilatations. The caliber also depends, to some extent, upon the secretory activity of the epithelium, whose cells become shrunken, and ttie lumen correspondingly dilated, during active secretion. The diameljer varies from 40 to 60 microns. The convoluted tubules are most actively engaged in the secretion of uriney but the further changes accompanying their secretion have not yet been satisfactorily demonstrated. It is generally believed that, the water and salts of the urine are secreted in the glomerular capsule, the urea in the convoluted tubules. 4. Descending Limb of Henle's Tubule (The Thin or Narrow Tu- bule of Henle). In this portion, which is, typically, located in the boundary zone of the medulla, the uriniferQusjtubule becomes very much iflrrnw^rl (8-15 ft ), but the decreased diameter is the_result of dimin- ished height of the lining epithelium rather than of any change in the caliber of the tubule. The length of this portion of the tubule is very variable; typically it corresponds very nearly with the breadth of the medullary boundary zone. The lining epithelium of the descending limb is of a peculiar flattened shape. Its cells possess an ovoid nucleus which, being thicker than the surrounding portions of the cell, projects slightly into the lumen of the tubule. The bulging nuclei of opposite sides of the tubule are not in apposition but interlock with 'one another, the nuclei of one side of the tubule being opposed to the cell margins of the opposite side. The lumen of longitudinal sections through the axis of the tubule thus ac- quires a sort of zigzag outline. The nuclei stain deeply but possess an evenly distributed chromatin. The cytoplasm of the epithelium is very finely granular, and although its cells are intimately adherent at their THE KIDNEY 435 lateral margins they do not present the typical striations which are characteristic of the preceding portion. 5. The Loop of Jlenle. As the descending limb enters the loop of Henle, the tubule iriakes an abrupt turn and returns toward the cortex. The location of the loop, being dependent upon the variable location in the cortex of the renal corpuscle and the variable length of the thin seg- ment, may be in any portion of the medulla except the extreme tip of the pyramids; its most sequent site, however, is near the junction of the boundary and papillary zones. The structure jDfJhe Jpo may be that of either the descending or the ascending limb of .the typical loop. It is also subject to great variations, since the change in structure from the narrow to the broad type, though it typically occurs just prior to the formation of the loop, is frequently delayed until well into the ascending limb. As a rule, the change in type occurs earlier when the loop lies in the boundary zone, and later when it occurs nearer the apex of the renal pyramid ; the thick ascending limbs do not occur in the papillary zone of the medulla. 6. The Ascending Limb of Henle 's Loop (The Broad or Thick Limb), This portion of the tubule returns through the boundary zone of the medulla and enters a cortical ray, its course being parallel to that of the descending limb. Tt then passes toward the surface of the kidney, but finally leaves the ray and enters the labyrinth to reach that renal corpjiscle (close to the vas efferens) from which, the uriniferous tubule took origin, and in relation to which the tubule again acquires a tortuous course (distal convoluted portion). Within the boundary zone of the medulla this portion of the tubule is much broader than the preceding division, but it becomes_somewhat reduced in size in its course through the cortical ray. The epithelium of the ascending limb is of a short cuboidal form. Its cytoplasm resembles that of the lining epithelium of tne convoluted" portion, although the nuclei in the tubule of Henle are rather more dis- tinct. Basal striations are also less distinct than in the convoluted tu- bule, the lateral serrations less deep, and the cell outlines sharper. The cells of this portion frequently possess a slightly imbricated arrange- ment. The recent careful comparative studies of mammalian kidneys by Peter ("Untersuchungen iiber Bau and Entwickelung der Niere," Jena, 1909) have revealed certain important details. Peter divides the medullary portion of the renal pyramid into an outer and inner zone; these portions have no precise correspondence with the boundary and papillary zones above 436 THE URINARY SYSTEM mentioned. In the outer zone Peter describes an outer and an inner stripe, the line of demarcation being the very definite level where the proximal convoluted tubule passes into the thin, clear, narrow limb of Henle's loop (Fig. 400). The point of transition from outer to inner zone is marked by a change in the character of the epithelium lining the distal limb of the long Henle's loops; the epithelium becomes cloudy (granular) and thicker. In view of the great variability in the character of the epithelium at the vari- FIG. 406. A GROUP OF TUBULES FROM A TRANSECTION OF A RENAL PYRAMID OF THE HUMAN KIDNEY; THE SECTION PASSES THROUGH THE BOUNDARY ZONE. a, collecting tubule; 6, ascending limb of Henle's loop; c, descending limb; d t loop of Henle. Hematein and eosin. Photo. X 275. ous levels in loops of different lengths, and the recognition by Peter of a thicker cloudy segment, it seems better to speak of thin clear, thick clear, and thick cloudy segments of the loop. In kidneys of the dog and cat there are no really short loops, none lying within the outer zone. In the kidney of the pig many loops lie in the cortex; such loops lack entirely the thin, clear narrow limb. 7. The Distal Convoluted Portion (Intercalary or Intermediate Portion). This portion of the uriniferous jubule begins close to the vascular pole of the renal corpuscle, and, after several irregular contor- THE KIDNEY 437 tions which are confined to the region of the cortical labyrinth, enters an arched collecting tubule. The distal is much shorter than the proxi- mal convoluted portion. Its caliber is subject to great irregularities, so much so that its early turns have been characterized as the irregular or zigzag portion of the uriniferous tubule. The epithelium of this section resembles that of the proximal convoluted portion but is lower, more cubc4dl,^id - sjjiatioG8--are indistinct. This portion terminates the typically secretory portion of ,the urinif- erous tubule. Beyond here the tubule possesses more the function of a duct, hence its epithelium shows a decided change in character. Hith- erto it has possessed the peculiar character, the typically granular cyto- plasm, of a secreting type of cell. Beyond this section the epithelium is no longer so granular but possesses a characteristically clear appearance. The secretory and excretory portions of the renal tubule have a separate embryonic origin, and only secondarily unite to form a continuous duct. 8. The Arched Collecting Tubule (Junctional Tubule). This is a short portion of the uriniferous tubule which connects the distal con- voluted portion in the cortical"labyrinlh" with the straight collecting tu- bules of the_cortical rays. Its course is characteristically arched. The epithelium of the arched tubule consists of clear^cuboidal cells with distinct outlines and deeply stained, sharply defined nuclei. The chromatin of the nucleus is irregularly distributed forming numerous karyosomes, and th> nuclear menjibrane is distinct The cytoplasm is relatively devoid of Igra nules, and imlike that/of the secreting epithelium does not possess a ^trobg affinity for the acid dyes, 9. The Straight Collecting Tubules. These portions__JL~ihe tu- bules begin in the cortical rays, where they receive the arched tubules, and, proceeding to the medulla, become considerably increased in size. They penetrate the boundary zone of the medulla, all pursuing a parallel or slightly convergent course, and occasionally uniting with each other. On entering the ^papillary zone, by frequent union they become rapidly larger, and in the apex oi each pyramid finally form about a score of large terminal pa^nia^y^ducts. Though shorter than the convoluted tubules, the straight collecting portions, because of their direct course, traverse a broader area of the renaFtissiie ; beginning near the peripheral end of the cortical rays, in these,j^aLumns they cross nearly the whole breadth of the renal cortex__and_eniering the medulla extend from base to apex of the re Throughout their whole course they progressively increase in size and caliber. A corresponding progressive increase in the height of their epi- 438 THE URINARY SYSTEM thelial cells likewise occurs, so that the lumen of the straight tubules of the medulla is not only actually greater than that of those of the cortical rays, but the walls of the former tubules are also considerably thicker. The extreme of this progression is found in the broad lumen and tall epithelium of the papillary ducts. The epithelium of the straight tubules, like that of the arched, possesses a clear cytoplasm, distinct and deeply staining chromatic nuclei, and well defined cell outlines. Beginning in tfie cortical rays with a low columnar type, it gradually increases in height in the course of the tubule until, in the papillary zone, the epithelium acquires a tall columnar form. The clear cytoplasm and distinc of the epithelium of the collecting tubules stand ou the granular cytoplasm and the evenly distribut ; nuclear membranes ; in sharp/ contrast to id chromatin in the nuclei of the lining cells in the secreting portions of the; uriniferous tubules. / 10. Papillary Ducts. (Ducts of Bellini). These are the wide mouths of the urinifejous tubules which are formed by the dichot- omous union of from ten to thirty collecting tubules, andjvhich empty their secretion into the renal c'a'lytte'fe at the aftex oi"the renal pyramids. They attain aT diameter Q J i'romtwo hundred to three hundred microns. They are lined by tall columnar cells with an exceptionally clear cyto- plasm which has an affinity for the basic in preference to the acid class of dyes. The nuclei are spheroidal or ovoid in shape and lie in the basal portion of the cell. At their termination several papillary ducts frequently open into a common depression or foveola which is lined by an involution of the layer of transitional epithelium, derived from that of the renal calyx, by which the free papillary portion of the renal pyra- mid is clothed. It is obvious that the entire renal tubule, from the glomerular capsule to the papillary duct, is a continuous canal whose epithelial wall, sup- ported by a thin homogeneous basement membrane varies in character in each succeeding portion. Thus the proximal and distal convoluted portions and the typical ascending limbs of Henle's loops possess a granular, rodded or striated, acidophil, secreting epithelium ; the capsule has thin cells of an endothelioid type ; the typical descending limb and loop of Henle are lined by flattened finely granular and faintly acidophil epithelium; the curved and straight collecting tubules and papillary ducts possess a clear columnar epithelium. It should also be noticed that the several portions of the renal tubule occur in different topographical sub- THE KIDNEY 439 divisions of the kidney and that, therefore, each of the subdivisions contains only certain characteristic portions of the renal tubule.^ Thus are found in the CORTICAL LABYRINTH. (Pars Convoluta) CORTICAL RAYS . . (Pars radiata) BOUNDARY ZONE OF THE MEDULLA . . PAPILLARY ZONE OF THE MEDULLA. , Renal corpuscle. Neck of the tubule.-. Proximal convoluted portion. Distal convoluted portion. Arched collecting portion. f Spiral portion of the convoluted tubule. J Ascending limb of Henle's tubule. [Straight collecting portion. Descending limb of Henle's tubule. Ascending limb of Henle's tubule. Straight collecting portion. Descending limb of Henle's tubule. Loop of Henle's tubule. Straight collecting portion. Papillary duct. MEDULLA^ The following outline, based upon the investigations of Peter, gives a more detailed classification of the different portions of the renal tubule in the medulla : 1. Outer stripe (a) straight collecting- tubules. (b) proximal convoluted tubules. "A. Outer Zone (c) thick ascending limbs of Henle's loops, including both (x) thick cloudy portions, and (y) thick clear portions. Inner stripe (a) thin clear descending limbs of Henle's loops. (b) thick cloudy portion of ascending limbs of Henle's loops. (c) short loops of Henle, of thick cloudy type of epithelium. B. Inner Zone (a) straight collecting tubules. (b) long loops of Henle, entirely of thin clear type of epithelium. 28 440 THE URINAEY SYSTEM The following tabular resume may be of service by emphasizing the more important peculiarities of the several portions of the uriniferous tubule. PORTION OF TUBULE. EPITHELIUM. LOCATION. Renal corpuscle. Flattened, endothelioid. Cortical labyrinth. Neck. Changing from flattened Cortical labyrinth. to low columnar. Proximal convoluted. Low columnar, granular, Cortical labyrinth. and rodded. Spiral portion of above. Low columnar, granular, Cortical rays. and rodded. Descending limb. Low cuboidal or flat- Medulla (boundary and Loop. tened, granular. Varies; like either the papillary zones). Medulla (boundary and preceding or following. papillary zones). Ascending limb. Cuboidal or low colum- Boundary zone of me- nar, granular, imbri- dulla and cortical rays. cated. Distal convoluted. Low columnar or pyram- Cortical labyrinth. idal, granular, and rod- ded. Arched collecting. Cuboidal, clear cyto- Cortical labyrinth. plasm, dark nucleus. Straight collecting. Cuboidal, changing to Cortical rays and both columnar. zones of medulla. Papillary duct. Cuboidal, tall columnar. Papillary zone of medulla. RENAL BLOOD-VESSELS, LYMPHATICS AND NERVES Blood Supply. The kioWyrp^pivpa ifs blood supply from the renal artery, which, as it enters the hilum7divides into twojsets of principal branches, of which the ventral set supply three-fourths T the dorsal set one-Jourjj3,,of thcjenal substagce. . These principal branches, the arterice propriw renales, or interlobar arteries, are embedded in the connective tissue of the sinus and follow the walls of -tn/fe infundibula and calyces, upon which they lie, thus reaching the renal/ /columns between the renal calyces. Here they enter the cortical substances and divide, each branch passing in a curved or arched manner beneath the base of the adjacent pyramids. These vessels form an incomplete arterial arcade (arciform artery) which lies in 'the margin of the cortex at the outer border of the medullary boundary zone. From the arterial arcade, branches a^e given to the medullary tissue of the pyramids on the one hand, and on the other to the cortical sub- stance. Those branches which enter the medulla are slender vessels which THE KIDNEY 441 pursue a characteristically straight course between the paralMJubules of this region and are known as the arteriolce rectos^ They branch freely at acute angles and form a rich capillary plexus in the boundary zone, the longest of the vessels reaching beyond the limits of this region to supply a less abundant plexus to the papillary zone of the medulla. The cortical branches of the arterial arcades are vessels of considerable size which enter the labyrinth between the cortical rays and as interlobular arteries. (cortical art orioles) pass toward the surface of the organ, a few of the longest branches reaching the^ fi- brous capsule_wiili_jvhose vessels thej_ Anastomose. Throughout their whole course the interlobular, ar- teries give off numerous short branches, which leave the parent stem at a wide angle, and pass di- rectly to a renal corpuscle as the affergnLaztery (ar- teriole; Vas afferens) to its glomerulus. Here it sup- plies the capillary plexus in the manner already de- scribed (page 431). Certain of the afferent arterioles are peculiar in that (1) they give off small branches which supply capillaries directly to the convoluted tubules of the cortical labyrinth; and (2) they occasionally form a small rete mirabile before they reach the glomerulus. By far the greater portion of the branches of the interlobular arteries, however, pass directly to the glomeruli. The capillaries of the glomerulus reunite to form an efferent FIG. 407. THE DISTRIBUTION OF THE LEFT RENAL ARTERY. Of the six arteriae propriae renales, five enter in front of the renal pelvis, and, lying upon the wall of the calyces, are distributed from the arterial arcade to both cortex and medulla, a, ureter; 6, renal artery; c, arteriae propriae renales; d, the dark border is the cortex, within which is the lighter medulla. (After Brodel, from Szymonowicz and MacCallum.) 442 THE UKINABY SYSTEM vessel which, after leaving the corpuscle, promptly breaks into a second capillary plexus about the adjacent tubules. The efferent vessels of those glomerul^i which lie in that portion of the cortex adjoining the medulla, frequently pass to the boundary zone of the latter region where they form the arteriaa rectae spurise which, though lacking a true arterial structure they possess no circular mus- FIG. 408. FROM THE CORTEX OF THE HUMAN KIDNEY. The blood-vessels have been injected and appear dark. A cortical ray and the adjacent labyrinth is included, a, cortical ray; 6, a glomerulus in the labyrinth. Moderately magnified. (After Disse.) cle fibers pursue the same course as the true arteriolse rectse and assist the latter in supplying the capillary plexus of, the medulla. The medul- lary capillaries, like those of the rays N in the cor.tex, form a network with elongatexi meshes which surround the parallel tubules of this region. The capillaries of the cortical labyrinth, being distributed among irregu- lar tortuous tubules, possess a more polygonal mesh. The interlobular veins begin near the surface of the organ where THE KIDNEY 443 small venules* derived from the cortical capillaries and from occasional anastomoses with the capsular vessels, unite to form broad thin-walled venous spaces, the stellate veins of Verheyn, just beneath the capsule. From these subcapsular vessels the interlobular veins arise, and pass toward the medulla in company with the interlobular arteries, throughout their course collecting the minute venules which return the blood from the capillaries about the tubules of the renal cortex. Arriving at the border of the medulla, but still embedded in the cortical substance, the interlobular -veins turn sharply, at nearly right angles to their former course, and by free anastomoses form a venous arcade (arciform vein), which receives the venulce rectce coming from the capillary plexuses of the medulla, and at the border of the renal pyra- mid enters a renal column to unite with similar vessels coming from the borders of the adjacent pyramids. The union of these vessels forms large venous trunks which leave the renal columns, in company with the arteries, as the vence pro price renales (interlobar veins). They enter the connective tissue of the hilum, traverse the wall of the calyces, in- fundibula, and renal pelvis, and finally unite to form the renal vein. TABLE SHOWING THE COURSE OP THE RENAL CIRCULATION Renal artery Renal vein \ t. ! renales Venae propri i f Arterise propriae renales Venae propriae renales Arterial arcade Venous arcade \ Interlobular arteries Arteriolae rectae - Venulae rectae Interlobular veins XX f \ nnri11nrip>a r\f morJnlla * Capillaries of medulla Afferent artery to renal corpuscle Venae stellatae X f Capillaries of the cortical labyrinth Capillaries of the glomeruli \ Efferent vessel from renal corpuscle 444 THE UEINAEY SYSTEM The venulae rectse are peculiar in their typically straight course, and in the fact that the cells of their endothelium are extremely long so long indeed, as frequently to present a somewhat fibrous appearance, the elongated axis of the cell being parallel to the long axis of the vessel. The capsule of the kidney is supplied by branches of the lumbar, phrenic, and suprarenal arteries, which form a rich, capillary plexus. These vessels anastomose with the terminal branches of the interlobular arteries of the kidney in the manner above described a fact which acquires surgical importance from its relation to the establishment of a collat- eral circulation. The table on page 443 may be useful as a resume of the more important paths in the course of the renal circulation. In it the names of the several vessels are arranged in order and the arrows indicate the direction of the -blood current. Lymphatics. The renal lymphatics con- sist of a superficial set which forms plexuses in the perirenal fat and deeper layers of the capsule, and a deep set which supplies the parenchyma of the organ. These two sets of lymphatic vessels are in communication by frequent anastomoses. The vessels of the superficial set convey their lymph to the neighboring lymph glands of the lumbar region. The deep renal lymphatics accompany the arteries and veins throughout their course. They form narrow cleftlike vessels of irregular caliber in the scanty interstitial tissue between the urinifer- ous tubules of both cortex and medulla. These vessels are relatively few in number. They converge to tjie hilum of the organ where they pass to the nearby lymphatic glands. Nerves. The nerves of the kidney include botji nledullated and non-medullated fibers. The sympathetic fibers forin a ganglionated plexus in the connective tissue about the renal pelvis. From this plexus fibers are distributed to the blood-vessels of the capsule and to the parenchyma of the organ. The latter accompany the blood-vessels, dis- FIG. 409. NERVE ENDINGS IN A CONVOLUTED TUBULE OF THE FROG'S KIDNEY. a, nerve fiber; 6, blood-ves- sel; c, secreting epithelium. Methylene blue; cochineal. Very highly magnified. (Af- ter von Smirnow.) EENAL PELVIS AND URETERS 445 tributing their fibers to the walls of the arteries and veins and to the uriniferous tubules of both cortex and medulla. The parenchymal branches form an 'epilemmal plexus' in the In- terstitial connective tissue about all portions of the uriniferous tubule. From this plexus fibrils pierce the membrana propria and anastomose to form a 'hypolemmal plexus' about the base of the epithelial cells. Ter- minal fibrils penetrate between the epithelial cells where they form minute end knobs. RENAL PELVIS AND URETERS The excretory passages of the kidney include the renal pelvis, the ureters, the urinary bladder and the urethra. All of these portions possess certain common characteristics. They have three coats, mucous^ muscular, and fibrous, and are lined by a common type of epithelium, FIG. 410. CAST OF THE PELVIS, INFUNDIBULA AND CALICES OF THE KIDNEYS OF A MAN. Showing the expansion of the ureter, the subdivision of the pelvis, and the con- cave facets by which the calices fit over the apices of the renal pyramids. (After Hauch.) transitional, which extends from the renal pelvis to the prostatic por- tion of the urethra. The mucosa of the renal pelvis and ureter is lined by transitional epithelium which rests upon a fibrous tunica propria. The epithelium consists of several cell layers, of which the superficial is formed by broad 446 THE UEINARY SYSTEM cuboidal cells, or thick flattened plates, whose form varies with the state of distention of the organ, and whose deep surfaces are indented by the rounded ends of the pear-shaped cells which form the deeper layers. The deepest cells are smaller and are irregularly spheroidal in shape. They FIG. 411. TRANSECTION OF HUMAN URETER. Hematein and eosin. Photo. X 113. are firmly attached to the underlying connective tissue which here and there projects into the epithelial layer carrying with it the most super- ficial capillary vessels. The cells of the deeper layers divide by karyo- kinesis and push toward the surface to replace the more superficial cells which are gradually desquamated. Direct cell division occurs in the BENAL PELVIS AND UEETEES 447 large plate-like cells of the superficial layer. The thin transitional epi- thelium of the calyces is continuous with the columnar epithelium- o the papillary ducts. The tunica propria, continuous with the renal interstitial tissue, contains both col- lagenous fibers and elastic fibers. 1 1 is indistinctly divisible into a superficial denser portion, and an open-meshed deep portion whose fi- brous bands loose- ly attach the mu- cous membrane to the muscular coat. This deep layer is analogous to t h e submucosa of the alimentary tract. The mucous mem- brane is thrown into numerous deep folds or rugae which in the ure- ter have a longi- tudinal direction; this condition gives to the canal in transverse sec- tion a stellate ap- pearance. Irregu- lar folds or inva- ginations of the epithelium occur in the renal pelvis and have been described as glands, but true secreting glands are not found. Occasional lymphocytes occur in the mucosa, and small lymph nodules have also been found but cannot be regarded as of constant FIG. 412. TRANSITIONAL EPITHELIUM OF DOG'S URETER. A, in the contracted condition; B, in the distended con- dition; a, basal layer of cubic cells; 6, middle layer of polyg- onal cells; c, superficial layer of rectangular and ovoid cells. (R. W. Harvey, Anat. Rec., 3, 5, 1909.) X 750. 448 THE URINARY SYSTEM occurrence. The mucosa becomes gradually thinner as it is traced through the infundibula and calyces and is reflected upon the surface of the renal pyramids. The muscular coat of the ureter consists of a well-defined .layer of circular, fihers.within which are many discrete bundles of longitudinal, smooth muscle. In the lp^erji_alf.....o-the^ ureter a third layer, whose fibers also have a longitudinal direction, is found outside the circular fibers. In this portion ^o^the__jir^ter^_theref ore, the muscular coat consists of three layers, an inner longitudinal, middle circular, ami outer longitudinal: In the renal pelvis and calyces the .muscular coat becomes pro- gressively thinner toward the renal substance, and at the apex of the pyramid consists chiefly of circular fibers which are slightly thickened to form a 'sphincter' about the papilla. The outer fibrous coat of the ureter consists of areolar tissue which blends with that of the surrounding parts? In the renal pelvis this coat becomes continuous with the connective tissue capsule of the kidney. (For development of excretory passages, see next Chapter.) Blood Supply. The larger blood-vessels lie in the outer fibrous coat and distribute branches to the muscular coat and to the mucous membrane. In the latter they form a superficial capillary plexus which is in unusually intimate relation with the deeper cells of the lining epithelium. The Lymphatics. The lymphatics begin in an intramuscular plexus and, by scanty vessels, in the deeper part of the mucosa. They pass to larger vessels in the outer coat, which possess valves, and convey the lymph to the neighboring lymph nodes. The Nerves. The nerves form a coarse plexus in the outer fibrous coat which contains many small ganglia. From this plexus motor fibers are distributed to the muscular layers, and sensory fibers to the mucosa. The latter form a plexus beneath the epithelium from which terminal fibers pass to end by minute end brushes in the con- nective tissue and by varicose fibrils between the deep cells of the epi-' thelium. THE URINARY BLADDER The wall of the urinary bladder closely resembles that of the ureter. It consists of mucous, muscular, and fibro-serous coats. Its mucous membrane is lined by transitional epithelium like that of the ureter THE UBINAKY BLADDER 449 and renal pelvis. It is therefore impossible to determine from which of these portions detached epithelial cells are derived when fouiuTby microscopical examination of the urine. Epithelium may become de- :&: ^ ;: '-' ^'*9**m ^^::lKM|C &*&'. J(W H^v t. Wv- ** iA>j ^w/;^- * ^i> !p. FIG. 413. TRANSVERSE SECTION OF URINARY BLADDER OF DOG. a, b and c, compose the tunica mucosa; d, e and/, the tunica muscularis; g, tunica serosa; a, transitional epithelium; 6, lamina propria mucosae; c, deep portion ol tunica mucosa, corresponding to a tela submucosa; d, internal longitudinal muscle layer; e, middle circular muscle layer; /, external longitudinal muscle layer. The tunica mucosa is thrown into larger and smaller folds. X 20. tached from any portion of the excretory passages, and as a result of inflammatory changes not only the superficial cells but also the deeper pear-shaped cells may be desquamated. The character of the epithelium varies with the collapse and dis- tention of the organ. When empty the mucous membrane is thrown 450 THE URINARY SYSTEM into deep folds *or ruga9, whose surface also presents secondary folds of irregular direction. The epithelial layer is relatively thick, and is thicker on the sides of the folds than upon either their apices or bases where the folding of the epithelium increases the tension of its cells. When the organ is distended, the folds are more or less completely obliterated, the epithelial layer is much thinned, often until it appears a b FIG. 414. THE MUCOSA OF A CHILD'S BLADDER IN THE CONTRACTED STATE OF THE ORGAN. a-a, and a'-a', transitional epithelium; 6, fibromuscular tissue of the mucosa. Hematein and eosin. Photo. X 216. to consist of no more than a double layer of cells, and its cells be- come much broader and relatively much thinner than in the collapsed or empty condition. Hence the transitional variety of epithelium, by which the organ is lined, may be said to be remarkable for the extreme elasticity of its cells. The mucous membrane, except near the urethral orifice, contains no glands. In this location, however, the bladder of adult man contains a few small mucus-secreting glands, lined by columnar cells (Kol- THE UEINAEY BLADDER 451 liker). These are not present in infancy. They have been interpreted as vestigial prostatic tubules. The tunica propria resembles that of the ureter and is loosely united to the muscular coat. This latter layer is formed by interlacing bundles of smooth muscle. It varies much in thickness according to the condition of the organ, being relatively thick when the viscus is B FIG. 415. TRANSITIONAL EPITHELIUM OP DOG'S BLADDER. A, in the contracted condition; B, in the distended condition. (R. W. Harvey, Anat. Rec., 3, 5, 1909.) empty and very thin when it is completely distended. In most portions three indistinct layers can be observed, a middle thick layer of circular fibers,, and an inner and outer longitudinal layer. The outer longitudinal muscle is most distinct on the anterior and posterior surfaces of the organ. The outermost coat of the bladder consists of areolar tissue, having very broad meshes; in the lower portion of the bladder this coat blends with the connective tissue of the adjacent organs. Over the fundus of 452 THE UEINAEY SYSTEM the organ its outer fibrous coat is covered by the peritoneum, the con- nective tissue of which is indistinguishable from that of the outer coat of the bladder ; it is clothed by the peritoneal mesothelium. Vascular and Nerve Supply. The vascular and nerve supply of the bladder is exactly similar to that of the ureter. The larger blood- J5 FIG. 416. EPITHELIAL CELLS FROM THE BLADDER OF THE RABBIT. A, as seen from the under surface, showing the depressions made by the under- lying polygonal and pyriform cells (P); B, side view of similar cell. (After Klein, from Schafer.) vessels occur in the outer fibrous coat, whence they distribute branches to the muscular coat and mucous membrane. Many small ganglia are also found in the outer coat, and motor and sensory nerves are dis- tributed to the musculature and to the epithelium and connective tissue of the mucosa, as in the ureter. THE URETHRA THE FEMALE URETHRA The mucosa of the female urethra is lined by a variable type of epithelium. Being continuous with the mucosa of the bladder, the epithelium of the urethra is at first of the transitional variety, but as it approaches the meatus this is changed to a stratified squamous type which is continuous with that of the vestibule. In certain indi- viduals the superficial cells of the midportion of the urethra are much elongated and assume an irregular stratified columnar type. The epithelium rests upon a tunica propria of dense areolar con- nective tissue whose outer portion blends with a looser connective THE TJBETHRA 453 tissue which contains many broad venous channels and forms a sort of spongy erectile_tissjie. This erectile coat is surrounded by_a_ihm muscular coat whose innermost fibers, longitudinal in direction, are continued outward to the meatus, and whose outer circular fibers, de- FIG. 417. TRANSECTION OF THE FEMALE URETHRA. d, gland-like diverticulum; e, urethral epithelium; L, urethral lumen; m, striated fibers of the urethral muscle; s, erectile tissue of the tunica propria, containing many venous spaces and smooth muscle fibers. X 10. (After Kolliker.) ficient toward the meatus, are often slightly thickened near the neck of the bladder" to form an indistinct sphincter urethras muscle. This coat contains occasional striated muscle fibers. The female urethra is scantily supplied with urethral' glands which open near the meatus and supply a meager mucoid secretion. 454 THE URINARY SYSTEM THE MALE URETHRA The male urethra conducts the urine from the bladder to the sur- face, its course being through the axis of the corpus spongiosum of the penis (Fig. 446). It forms the terminal segment also of the genital canal and conducts the semen; it accordingly performs a double func- tion, serving both the urinary and the male reproductive system. The character of its epithelium is variable, not only in its successive por- tions, but it is also subject to great individual variation, and like that of the bladder and ureter changes its appearance according as the canal is collapsed or distended. In the first or prostatic portion of the urethra the epithelium is of the transitional type ; in the membranous and penile portion its superficial cells are elongated so that the epithelium usually acquires a stratified columnar form; near the meatus the type is again exchanged for a stratified squamous epithelium which is continuous with that of the glans penis. The tunica jgropria of the urethra consists of^areolar_ti^ssue in which are embedded the small branched tubulo-acinar urethral_^lands (of Littre) lined by columnar, mucus-secreting cells. These glands are more abundant along the upper surface. The mucosa also contains frequent lacunar invaginations of the epithelium, and is thrown into longitudinal rugae, its lumen being obliterated except when distended by the passage of urine. The tunica propria, and especially that portion which is distant from the urethral canal, is permeated by the thin-walled broad venous spaces of the erectile tissue of the corpus spongiosum. The septa between these venous spaces, in addition to the dense areolar tissue of which they chiefly consist, contain many bundles of longitudinal smooth muscle. In the deeper portions of the urethra circular muscle fibers are also found in the outer part of this coat, and near the orifice of the bladder they are somewhat thickened to form the sphincter urethrse Moreover, the corpus spongiosum is invested with a thick sheath of dense fibrous connective tissue. At the apex of the prostate circularly disposed striped muscle fibers form a second urethral sphincter, the external vesical sphincter. Nerve Supply. The nerve supply of the urethra includes both sensory and motor spinal fibers, and sympathetic fibers for the smooth muscle and the blood-vessels. CHAPTER XV THE REPRODUCTIVE SYSTEM GENERAL CONSIDERATIONS The reproductive system differs structurally, widely in the two sexes. The several organs involved, however, are in general strictly homologous in the male and female organism. . The differences result from a divergent specialization concomitant with a division of labor in the reproductive act and process. The specialization involves both progressive and regressive differentiation, the latter producing certain Sinus prostaticus Corpus Cavernosa penis Urethral gland." Urethra Corp. cav. urethrae Prepuce Glans penis Fossa navicularis Ampulla of ductus defer ens Seminal vesicle Ejaculatory duct Prostate gland Bulbourethral gland Bulbus urethrae Ductus deferens Paradidymis Ductuli efferentes Ductulus abberans Ductus epididymis Rete testis Tubuli recti ^Tubuli contorti A ppendix epididymis FIG. 418. DIAGRAM OF MALE GENITALIA. (Adapted from Merkel.) vestigial appendages, of different origin in the opposite sexes. The MALE ORGANS OF REPRODUCTION include a primarily and essentially in- ternal group of genital organs, namely, the testis, and its associated 29 455 456 THE REPRODUCTIVE SYSTEM ducts, glands and appendages; and an external genital organ, the penis (Fig. 418.) The FEMALE ORGANS OF REPRODUCTION likewise include an internal group of genitalia, namely, the ovary, with its ducts and the Oviduct Paroophoron Epoophoron Ampulla Fimbriae Ostium abdominale Hydatid of Morgagni Fimbria ovarica FIG. 419. DIAGRAM OP FEMALE INTERNAL GENITALIA. associated glands and appendages; and a group of external genitalia and glands (Fig. 419). The essential sex organs are the testis and ovary, respectively. DEVELOPMENT The primary anlages are the same in both sexes. They include essen- tially: a pair of undifferentiated sex-glands or gonads located on the mid- ventromesial side of each fetal kidney (Wolffian body or mesonephros) ; a double pair of parallel canals, the fetal Wolffian (mesonephric) ducts and the Miillerian ducts. An embryo at this stage of development (13 milli- meters, Fig. 210, about 40 days) is said to be in the indifferent sexual stage. The ducts communicate terminally with the gonads and the exterior. Sub- sequently they undergo a different development, as do also the terminal associates (gonads, and urogenital sinus, respectively) in the two sexes. For the details of this process reference must be made to a text-book of Embryology. But briefly, the primitive gonads in the female develop into the ovaries, in the male into the testes. In the female the Miillerian ducts become the oviducts, and fuse proximally to form the uterus and vagina; in the male, this duct suffers regressive changes and persists only as ves- tiges: the appendix testis, and the sinus pocularis. On the contrary the Wolffian duct becomes vestigial in the female, persisting as the appendix fimbria (hydatid of Morgagni) and the canal of Gartner; while in the male, it develops into the main portion of the definitive duct system : ductus epi- didymis, and ductus deferens. The tubules of the fetal kidney (Wolffian DEVELOPMENT 457 body) also contribute to the excretory duct system. In the male, they per- sist in part (10 to 15) to form the ductuli efferentes; several may become vestigial forming thus the ductuli aberrentes, the paradidymis and Ihe appendix epididymis. In the female, these ducts early disappear for the Kidney Mullerian duct Genital gland Mesonephros ~ Ureter Inguinal ligament Mesonephric duct * ------ Mullerian duct ---Apex of bladder Bladder Opening of ureter (Opening of mesoneph- \ ric duct {Opening of Mullerian duct Rectum Urogenital sinus Cloaca Genital tubercle Genital ridge Opening of cloaca FIG. 420. DIAGRAMS ILLUSTRATING THE METAMORPHOSES OF THE INDIFFERENT (A) UROGENITAL SYSTEM INTO THE MALE (B), AND FEMALE (C) SYSTEMS. (From Polak, after Hertwig.) most part a variable number persisting as vestigial structures : the epo- ophoron and the parob'phoroii. The seminiferous tubules (including the recti and rete tubuli) of the testis arise as solid cords of cells continuous with the peritoneal epithelial covering of the gonad, and apparently as derivatives of this so-called 'ger- minal epithelium.' These cords subsequently acquire a lumen, and connect with the efferent tubules. In the ovary likewise such cell cords (sex-cords) appear, continuous with the peritoneal (germinal) epithelium; from these develop the ovarian follicles with their ova, as will be described below. The relation of the extra-regional 'primordial germ-cells' to the germinal epithelium and to sex-gland derivatives, discussed in the next section, Kidney - Appendage of testicle - A pex of bladder Bladder Opening of ureter Urethra {Opening of ejaculatory duct Prostate Epididymis Testis Paradidymis Deferent duct Mullerian duct Gubernaculum testis Ureter Seminal vesicle Deferent duct Epididymis *-- Gubernaculum testis Kidney Hydatid Oviduct ) (fimbrice) J Epoophoron Ovary Paroophoron Mesonephric \ duct J Oviduct..^-. Epodphoron "-"--.. Ovary ...^^^^^ Ovarian hga- ) ment ) Uterus Round ligament Vagina --Apex of bladder DEVELOPMENT 459 remains in doubt. Felix (Keibel and Mall's "Human Embryology," vol. 2) inclines to the view that in man the primordial germ-cells (primitive geni- tal cells) disappear, and that they bear no genetic relationship to the primi- tive germ-cells (secondary genital cells) of the gonad. Table of the Adult Male and Female Derivatives of the Fetal (Indifferent) Reproductive System. (The vestigial structures are given in italics, synonyms in parentheses.) INTERNAL GENITALS. Indifferent stage. Male. Female. Genital ridge. Testis. Gubernaculum (mesor- chium). Fimbria ovarica. Ovary Ovarian ligament (meso- varium). Round ligament. Wolffian body (tubules) (Mesonephros). Globus major of epididy- mis (ductuli efferen- tes). Paradidymis. Ductuli aberrentes. Appendix of epididymis (stalked hydatid) . Epoophoron (Parovarium, organ of Rosenmuller) . Paroophoron. Wolffian duct (Mesonephric duct). Duct and globus minor of epididymis. Ductus deferens. Seminal vesicle. Ejaculatory duct. Chief vesicular appendage (Hydatid of Morgagni). Collecting tubule of epo- ophoron. Canal of Gartner. Mullerian duct. Appendix testis (sessile hydatid). Sinus pocularis (Uterus or vagina masculina, or utriculus prostaticus) . Oviducts (Uterine or Fal- lopian tubes). Uterus. Vagina. EXTERNAL GENITALS. Urogenital sinus. Genital tubercle. Genital folds. Genital swellings. fProstatic and membra-1 nous portions of male > [ urethra. J Penis. Scrotum. Urethra. Vestibule. Clitoris. Labia minora. Labia majora. 460 THE BEPEODUCTIVE SYSTEM The outline on the preceding page summarizes in tabular form the main facts regarding the development of the reproductive system. In both sexes the Wolffian duct proximally sprouts a duct which dilates and subdivides distally, meanwhile separating from the parent duct and making a secondary connection with the developing bladder. The subdi- visions elongate to form the collecting portion of uriniferous tubules, and unite with ontogenetically distinct tubules (the secretory portions) to form the complete tubules. The dilatation and earlier subdivisions become the pelvis and calyces, and the proximal portion of the original anlage persists as the ureter. GAMETOGENESIS The study of the sex or genital glands, the ovary and the testis, is perhaps best approached by way of a consideration of the mechanism by which they perform their respective specific functions, namely the production of ova and spermatozoa ripe for union. The common process is known as g am eto genesis. The end products are the male and female gametes, or sperm and ova. In the male the process is known as spermato genesis, in the female, odgenesis. The act of subsequent union .of the gametes is called fertilization, and the fertilized egg is the zygote. The result of gametogenesis is the preparation of a primary germ- cell for union with a gamete from the opposite sex. The essence of the process is known as maturation, and involves prominently mitotic cell division. However, the method of division is not of the simple homeotypic type, where a chromosome simply divides longitudinally into two daughter chromosomes, but is of the type called Jieterotypic, the chief characteristic of which is the formation of tetrads. The latter are of various sorts, all, however, characterized by a four-lobed condition representing a quadripartite double or bivalent chromosome (Plate B, figs. 15-18, page 467). Both sperm and egg trace their ancestry back to primordial germ cells, indistinguishable in the sexually undifferentiated organism, ex- cept for a difference in chromosome content, which difference is com- monly indiscernible. In certain instances, e.g., Ascaris, the germ cell can be distinguished from the soma cell at the two-cell stage. In a number of vertebrates, e.g., dogfish, turtle, etc. (Allen), the primordial germ-cell has been traced from a position among the entoderm cells lining the gut of the young embryo through a migration into the dif- ferentiating gonad } ovary or testis. Swift (Amer. Jour. Anat., 15, 4, GAMETOGENESIS 461 1914) has traced the primordial germ-cells in the chick from a crescen- tic area in the entoderm in front of the head end of the primitive streak, through their migration to the gonad by way of the developing blood- vessels. The earlier anatomists, on the other hand, derived the primordial germ-cells from the mesothelial covering (germinal epi- thelium) of the germ gland region of the primitive kidneys, the meso-- nephroi. This mode of origin, either exclusively or in part, is still supported by certain investigators. Whatever the actual source of origin whether entodermal or meso- dermal, whether specific or otherwise and whenever the time of dif- ferentiation, whether early or relatively late, the primordial germ-cells in the gonad undergo extensive proliferation, increasing greatly in numbers. The earlier generations may be inclusively designated pri- mary spermatogonia or oogonia respectively, the final generation taking the term secondary gonia. The question of the origin of the primordial germ-cells bears upon the hypothesis of the continuity of germ-plasm. Spermatogenesis. Since gamete production is easier to follow, though apparently more specialized, in the male, spermatogenesis is advantageously first described. For the purpose we may employ the active testis of Schist ocerca damnified, a common grasshopper. Grass- hopper material of many species is peculiarly favorable for a demon- stration of the process of spermatogenesis. This scheme will apply with slight qualification to most grasshoppers. The reduced number of chromosomes is the same, namely, 12 ; moreover, this material shows peculiarly well the accessory or sex chromosome, and thus serves as a splendid basis for a discussion and comprehension of the salient facts and recent theories touching the problems of the determination, control, and inheritance of sex. The grasshopper testis is a long tubular structure subdivided into a number of compartments, or cysts, each filled with cells of ap- proximately the same stage of development, the successive stages in spermatogenesis being represented in successive cysts from distal to proximal pole. For purpose of readiest study there is required, then, a median longitudinal section of ah entire testis in active condition. As is obvious, such material is useful also for illustrating mitosis but, with the exception of the spermatogonial and second maturation mitoses, cannot be regarded as typical; but for application and extension of a preliminary knowledge of simple mitosis it is perhaps unexcelled. It must be emphasized that this peculiar type of cell division, charac- terized chiefly by the phenomena of synapsis and tetrad formation, in- '9 * 1 F 3 9 r S I ? ; 3> i ^ w B &2 ^_^ H .FH - I 1 | I o & 'g J ^ h s " ^ f k 5 I 1 1 c g 02 1 O' - I GAMETOGENESIS 463 volving a reduction as well as an ordinary equation division, is limited to the maturation mitoses occurring in active testes and ovaries^ The appended scheme will serve to show graphically the steps in spermato- genesis and oogenesis, and the correspondences and differences between the two. We may begin the detailed description with the last generation of spermatogonia, the secondary spermatogonia. In the early resting stage the nucleus is poly-vesicular, each vesicle representing one or several chromosomes (Plate A, Fig. 1). Subsequently the separate vesicles fuse to form a greatly lobulated nucleus. In Fig. 2 one vesicle is shown still separated ; this probably represents the accessory or sex chromosome ; sub- sequently it also fuses with the main nucleus and remains generally indistinguishable from the other nuclear elements until early stages of the succeeding generation of cells, the primary spermatocytes (Fig. 8). Figs. 3, 4, 5, 6 and 7 represent successive stages in the karyo- kinetic division of the secondary spermatogonia. In the telophase of the spermatogonial division and the earliest prophase of the primary spermatocytes, as well as subsequently, the sex chromosome (allosome; heterochromosome) does not pass through the diffuse and spireme phases of the ordinary chromosomes (autosomes; euchromosomes) but remains chromatic and relatively compact. Subsequently it condenses consid- erably more (Fig. 8) and thereafter persists until late spermatid stages as a compact, deeply chromatic body, generally of oval or bilobed form, and usually close to or on the nuclear wall (Figs. 10 ; plate B, 14 and 20 ; and plate C, 27). Following the early growth stages, when the nucleus is in the resting or diffusive chromatic stage (Fig. 8), a series of changes occur known as synapsis. The most characteristic phase is the one in which the spireme becomes closely aggregated and polarized (synizesis) and subsequently segmented, the segments becoming looped and attached by their open ends to the nuclear wall over a constricted area, generally in the vicinity of the sex chromosome. The loops now free one attached end and unite in pairs by their free ends forming taller loops, oriented similarly to the smaller loops (bouquet figure, synaptenic nucleus). The number of loops is now half the original number, which represented the full number of chromosomes. This chromosome pairing is the essence of synapsis. An end to end union as here described is termed telosynapsis (metasyndesis) ; side by side union, described in certain forms, parasynapsis (parasyndesis). The important point is the reduction to half (plus one, the accessory) the original number (diploid) of chromosomes; the reduced number is 464 THE EEPEODUCTIVE SYSTEM termed the Jiaploid group. The loops now lose their polarized condi- tion (postsynaptic phase) and become scattered throughout the nucleus, which has meanwhile grown in size, and become more or less closely united into a more or less continuous thread. This soon shows evidences of a longitudinal split (Fig. 12), the diplotene phase; subsequently this undergoes transverse segmentation into half the original number (23) of chromosomes, irrespective of the ac- cessory; thus there are 11 segments. The nucleus contains in addition a pale-staining plasmosome, and the deep-staining accessory chromosome in its characteristic location. Figs. 11 to 14 are designed to show these two structures in varying relationships. Fig. 15 illustrates a late prophase condition, in which the euchromosomes (McClung) are less compact than the heterochromosome (accessory), and in the typical tetrad or quadripartite condition. At this point we may recapitulate our facts regarding the chromosomes. The sper- matogonium contains 23 (Fig. 423 A). With one exception the chromosomes can be grouped in pairs; such grouping gives 11 pairs. Moreover, the chromosomes can be arranged in a progressive series from the viewpoint of size; the unpaired, or accessory chromosome (lOx) takes rank between the 9 and 11 chromosomes. These chromosomes represent the original contribution from father and mother when the egg was fertilized preceding development. At synapsis the chromosomes actually pair and fuse; and obviously in the manner indicated in (A), for the chromosomes can again be ar- ranged in a graded series, corresponding approximately in size and range to the pairs indicated. Since the accessory has no mate the reduced number of chromosomes is 12. FIG. 422. PRIMARY SPERMA- TOCYTE OF A GRASSHOPPER, HlPPISCUS TUBERCULATUS, SHOWING THE COMPACT ACCESSORY CHROMOSOME (x) AMONG THE PALER MOSSY PROPHASE EUCHRO- MOSOMES, AND THE IDIO- SOME (l). The cytoplasm contains many granular and filamen- tous mitochondria. Flem- ming fixation; iron-hematox- ylin stain. X 1800. It is now quite generally believed that the chromosomes of a pair rep- resent paternal and maternal contributions; thus from the viewpoint of inheritance, assuming that the chromosomes are the vehicles of certain hereditary characters and qualities, the offspring of a pair of parents inherit PLATE A. 1 """) / L* 6 9 V-A;- 5< M$S ^v* *& PLATES A, B, AND C. SUCCESSIVE STAGES IN THE SPERMATOGENESIS OF SCHISTO- CERCA DAMNIFICA. rc, accessory or sex chromosome; p, plasmosome; i, idiosome. 1 to 7, spermato- gonia; 8 to 19, primary spermatocytes ; 20 to 26, secondary spermatocytes; 27 to 30, spermatids, differentiating (31 to 35) into the spermatozoon (36). 10, synapsis stage; 18 and 24, metaphase plates of chromosomes; 6 a, diagram of a longitudinally split univalent chromosome from the metaphase plate of a spermatozonial mitosis; 16b, diagram of a bivalent chromosome, in tetrad condition, from the metaphase plate of the first maturation spindle, undergoing the reduction division; 23c, longi- tudinally split univalent chromosome entering the second maturation spindle for the equation division. (Flemming fixation; iron-hematoxylin stain.) X 1750. 465 PLATE B. " 13 *s I 19 e * 14 15 % * *:> 17 18 21 ***** c 24 PLATES A, B, AND C. SUCCESSIVE STAGES IN THE SPEBMATOGENESIS OF SCHISTO CERCA DAMNIFICA. 467 PLATE C. 31 32 33 34 36 PLATES A, B, AND C. SUCCESSIVE STAGES IN THE SPERMATOGENESIS OF SCHISTO- CERCA DAMNIFICA. 469 PLATE D PLATE D (Fics. 1 TO 16). SUCCESSIVE STAGES IN THE GROWTH (1 AND 2, X 2100, AND 3, X 700), MATURATION (4, 5, 6, 7, 8, 9, 10, 11, 12 AND 13, X 2100, AND 14, X 700), AND FERTILIZATION (15 AND 16, X 700) OF THE EGG OF THE STAR- FISH, Asterias forbesii. In the ripe egg (3) the chromosomes are aggregated in a group connected with the nucleolus by a chromatic thread. In 4 the nucleolus is fragmenting (c) and the chromosomes (a and 6) are taken onto the first maturation spindle (5). In 13 two polar bodies are present, and the female pronucleus is forming. In 14, the mature egg, three polar bodies are present. In 15 and 16, the male and female pronuclei are fusing to contribute their chromosomes to the first segmentation spindle. 30 471 GAMETOGENESIS 473 oy way of chromosomes only from their grandparents* Poi* their parents are simply, as it were, the 'guardians' of their grandparental chromosomes which, following the synapsis phase of growth (the final stage of the pre- FIG. 423. CHROMOSOME GROUPS OF SCHISTOCERCA DAMNIFICA. (A) Diploid chromosome group of dividing spermatogonium of Schistocerca, showing the 11 pairs of chromosomes, and the unpaired (10-x) accessory chromo- some. (B) Haploid group of primary spermatocyte, showing 11 bivalent chromo- somes and the accessory (10-x). The 11 bivalents fall into a series with the same size relationships as the pairs of chromosomes in the diploid group. (C and D) Chromosome groups of secondary spermatocytes, one (D) with 11 univalent chromosomes, like those of the diploid pairs, the other (C) with similar chromo- somes, plus the accessory (x). All mature eggs presumably contain 12 chro- mosomes, homologous with C. vious fertilization which resulted in the parents) become redistributed in various combinations in the maturing germ cells from which the offspring develop, and determine their hereditary possibilities. In other words the germ-plasm is a continuous substance handed on from generation to gen- 474 THE BEPRODUCTIVE SYSTEM eration. Its 'elements' are the same, except in so far as the complement is halved during maturation and restored at fertilization by addition of an equal quota from another line of descent. This halving and doubling is the physical mechanism which underlies the phenomenon of variation in- volved in heredity. Offspring thus resemble parents because both have had origin largely from the same germ-plasm; lack of resemblance results from the fact mainly that the germ-plasm of parents and offspring of neces- sity differ in certain elements since different additions are made at fertili- zation in the case of successive generations. We may now return to the metaphase (Fig. 16) and follow the further course of the chromo- somes. The chromosomes are in typical tetrad condition. The first division most probably sep- arates the chromosomes that paired at synapsis. The unpaired, odd, or accessory chromosome (x) passes undivided and in advance of the other chromosomes to one pole of the spindle. Thus one of the resulting daughter-cells, prespermatid or secondary spennaiocyte, receives 11 chromo- somes, the other, 11 plus the accessory, or 12 chromosomes. Fig. 17 is a side view and Fig. 18 a polar view (metaphase plate), of the metaphase group, showing all 12 chromosomes. The acces- sory is still recognizable at telophase, Fig. 19. The chromosomes of the resulting secondary sper- matocytes do not pass into a diffuse stage but re- main more or less compact and chromatic a con- dition characteristic of the later segmented spi- reme stage of mitosis. In half the cells the acces- sory appears in its typical shape, staining reac- tion, and position, as shown in Fig. 20. This shows also a remnant of the original spindle. Attention should here be called to the fact that in* many forms chromatic particles have been de- scribed passing from the nucleus into the cyto- plasm, during the growth stages of the primary spermatocyte, particularly at the synapsis phase. Similar conditions have occasionally been de- scribed also for the spermatogonia, and in several instances even for the secondary spermatocytes. These extrusion products are called chromidia, FIG. 424. DIAGRAMS ILLUSTRATING THE BEHAVIOR OP THE CHROMOSOMES DUR- ING THE FIRST (A) AND SECOND (B) MATURATION DIVI- SIONS. The bivalent chro- mosome a + 6 enters the first maturation spindle and suffers a reductional division, chromosome a being separated from chro- mosome 6. In the sec- ond maturation divi- sion, each chromosome undergoes an ordinary longitudinal division. This division is already foreshadowed during the first or heterotypic division, producing the tetrad condition of the bivalent chromosome. GAMETOGENESIS 475 and are regarded by some as the raw material for mitochondria, to which latter certain investigators have attached hereditary significance. The further description of spermatogenesis will be confined to that half of the secondary spermatocytes containing the accessory or sex chromosome; conditions in the other half are closely similar except for the absence of the odd chromosome. Figs. 21 and 22 illustrate suc- cessive prophase stages. Figs. 23 and 24 illustrate side and polar views of the metaphase of the second maturation mitosis; in the latter all 12 chromosomes are shown. Plate C, Figs. 25 and 26 show anaphase and telophase stages respectively. Fig. 27 is the early spermatid stage, the accessory still conspicuous. A conspicuous cytoplasmic element of these stages is the idiozome (i), apparently derived from a portion of the disappearing spindle. Later stages of the metamorphosis of the spermatid are shown in Figs. 28 to 31. The idiozome divides into two moieties (Fig. 29). These elongate (Fig. 30) and spread out over a filament (Fig. 31), which has grown out from a dark-staining granule on the outer surface of the nucleus. The cytoplasm also concurrently flows backward around this filament. In conformity with what is known of a similar granule with a similar function in various forms, this granule represents the centrosome of Fig. 23, but its origin and history are obscure. Further steps in the metamorphosis are illustrated in Figs. 32 to 36. The mature spermatozoon consists of a long, rela- tively lightly chromatic, lance-shaped head, a deeply-staining thimble- shaped middle piece (body), and a long gradually tapering tail. The latter consists of the central filament ensheathed by the products of the idiozome. The entire spermatozoon is furthermore enveloped in a close-fitting sheath of cytoplasm. The head originated from the nucleus, and the middle piece presumably from the centrosome and a small contribution of cytoplasm. Determination of Sex. Anticipating somewhat, we may say that the process of oogenesis is essentially similar to that described for spermato- genesis. The cardinal difference relates to a numerical inequality between the functional descendants of a spermatogonium and an oogonium: in the former case the four resulting cells are all functional, and from the stand- point of their numerical chromosome content of two sorts; in the latter, three of the resulting cells are abortive (polocytes), only one remaining functional. Of the two classes of spermatozoa, one has an even number of chromosomes, the other an odd number. All mature eggs have the same number of chromosomes as that of the spermatozoa with the greater num- ber. This description applies strictly only to those instances where an odd 476 THE EEPKODUCTIVE SYSTEM or accessory chromosome appears in the male, or where its representative is an odd group. Cases are known among insects, where the homologue of the accessory may have a mate of smaller size (or possibly even equal size, but of typical allosome behavior idiochromosomes), or where the accessory representative may be an even group, in which instances the numerical rela- tionships are different, but the principle of sexual relationship remains the same. Confining our attention then to the simplest case, as illustrated in our specimen of the grasshopper, we may note that all the cells of the female, including the ob'gonia, contain 24 chromosomes. Two of these correspond in size to the accessory of the male group, which group in all soma cells and all spermatogonia comprises 23 chromosomes. We recall that one-half of the spermatozoa received 11 chromosomes, the other half 11 plus the accessory, or 12 chromosomes. All mature eggs contain 12 chromosomes. When therefore a spermatozoon with 11 chromosomes fertilizes an egg with 12 chromosomes, the resulting organism will have 23 chromosomes, and become a male; when, on the contrary, a spermatozoon with 12 chromo- somes fertilizes an egg with 12 chromosomes, the organism will have 24 chromosomes and become a female. An essentially similar relationship is now known to obtain in many forms, chiefly insects, but including also various vertebrates, even mam- mals and man. According to Winiwarter (Arch, de Biol. T. 27, 1912), the white human male produces spermatozoa of two sorts, one with 23 chro- mosomes, the other with 24 ; the male cells containing 47 chromosomes, the female 48. There is some uncertain evidence to indicate that the negro has half the number of chromosomes possessed by the white man, the mulatto consequently possessing numerically intermediate groups. With regard to the function of the accessory chromosome in relation to sex determination, two obvious possibilities present themselves: (1) it may qualitatively determine sex, that is, carry the determiner upon which sex depends; (2) it may act in a quantitative way, that is, sexual differ- ence may depend upon differences in the amount of chromatin. On the contrary the presence of one accessory in the male, and two (homologues) in the female may be simply accompaniments of sex in a sense somewhat similar to secondary sexual characters (e. g., combs and spurs in poultry), these as well as other sexual differences being in common dependent upon some more recondite fundamental sex 'determiner/ Experimental evidence tends increasingly to show that all organisms carry the potentialities of both sexes, and that sex differences are largely differences in degree of dif- ferentiation ; in other words, that femaleness represents an inhibited male. The greater amount of chromatin in the female, due to the presence of two sex chromosomes as against one in the male may be conceived to inhibit the higher grade of differentiation demanded by maleness. Attempts have frequently been made recently somehow to bring sex GAMETOGENESIS 477 determination in line with our knowledge of inheritance in general, and more particularly with the principles of Mendelian inheritance. This_ ac- cordingly would seem to be the logical place for a brief presentation of the main facts of Mendelism, and the hypothesis that attributes to the chromo- somes the function of vehicles, or determiners for hereditary qualities. Barring the sexual numerical differences above discussed, the specific num- ber of chromosomes is constant; that is, all cells of the white human male have 47, of the female 48 chromosomes. The more variable, superficial, or more recently acquired characters are believed by many to depend upon the presence in the developing egg consequently the cells of the organism of certain chromosomes, the so-called 'determiners' The more fundamental characters are believed to inhere in the cytoplasm of the egg. For exam- ple, the vertebrate condition of a man is thus believed to result from the constitution of the egg; the color of his eyes from the presence or absence of some determiner or factor in a particular chromosome. To elucidate the matter further, we must regard an organism as a congeries of almost numberless characters; of these some are like those of the mother and her ancestors, others like those of the father and his an- cestors, others apparent blends. In Mendelian phraseology these characters are called unit characters, and their material basis is located in particular chromosomes. We may select ^ single pair of unit characters in order further to present the principles of Mendelian inheritance. Suppose we consider the coat color of gray and white guinea pigs. When individuals characterized by these coat colors are crossed, the resulting offspring, first generation, are all gray in color. The gray color is said to be dominant, the white recessive. However, the potentialities or determiners for white coat color are still present in the gray hybrid, for if such hybrids are in- bred, there result gray and white offspring in the proportion, of 3 of the former to 1 of the latter. The latter if inbred produce only white off- spring, hence called pure recessives, but of the 3 gray only one if bred with its own type (pure dominants) will produce gray; two out of the three if inbred produce again 3 gray to 1 white. The proportion of 1 pure domi- nant to 2 hybrid dominants to 1 pure recessive is the well-known Mendelian formula for one pair of unit characters in cross-breeding of specific varie- ties. The character of this formula depends upon the fact that no germ cell carries both determiners of a pair of unit characters, but only one or the other. It thus follows that from the standpoint of a single pair of unit characters (e. g., color of iris) there are two categories of eggs and likewise of sperm. According to the laws of chance there will then be one chance for the determiner of either character (e. g., gray and white coat) to meet its like, to two chances that it may meet its opposite, hsnce the formula 1 :2 :1 : The central concepts of Mendelian inheritance thus are (1) unit char- acters; (2) the phenomenon of dominance; and (3) the principle of seg- 478 GENERAL CONSIDEEATIONS regation, producing a purity of the germ cells. The unit chaiacters are external features, the hypothecated determiners for these are provision- ally located in the chromosomes. Sex is of course inherited in the sense that an individual resembles either its father or its mother in respect of sex. Moreover, considering a species as a group, the two sexes are approximately numerically equal. This is the ratio expected when a Mendelian pure recessive (homozygote) is crossed with a hybrid dominant (heterozygote) for again by the laws of chance, the chances are equal that a determiner shall meet with its oppo- site or its like. On the basis of these considerations mainly, coupled with the data of sex-linked inheritance (see Morgan, "Heredity and Sex." 1915), sex is conceived by some to be inherited in Mendelian fashion, the indi- vidual with a single accessory (male) being regarded as the heterozygote, that is, digametic; that with two accessories, the homozygote, that is, the homogametic. However, such serious contradictions and difficulties arise when the matter is thus strictly interpreted that a simple Mendelian view of sex ratios must probably be abandoned. In any event the interpretation that regards the heterochromosome, when present in the duplex condition, as in the female, as an inhibitor to male sex development from the viewpoint of secondary sex characters, accords well with much of the experimental evidence from vertebrates. The bulk of ,the best evidence tends to show that sex is determined at fertilization, and that it cannot thereafter be altered by control of environmental conditions. Relatively much chro- ma tin may be thought to underlie the anabolic condition characteristic of femaleness, relatively less chromatin the katabolic conditions which sub- serve the high differentiations of maleness. Oogenesis. We must now return to a detailed description of the oogenesis. Splendid material for the presentation of the facts con- cerning this process is the egg of Ascaris, or that of certain molluscs, or the more easily available egg of some echinoderm, e.g., starfish. This description will confine itself to the starfish egg. The essential points can be well illustrated with this material (Plate D, Figs. 1-16). In con- trast to the spermatozoon, which is relatively minute, highly motile, and with a minimum amount of cytoplasm, the egg on the other hand grows enormously in size (ratio of sperm to egg 1 : 500,000 ), is non-motile, and has a large amount of cytoplasm heavily laden with yolk material. This phase of the oogenesis is known as the growth period; during the early portions of this period synapsis occurs, and subsequently chro- matic material (chromidia) appears in the cytoplasm, presumably ex- truded by the nucleus, and in part at least changed into yolk. This generation is the primary oocyte, the preceding generation being MALE OEGANS OF REPRODUCTION 479 oogonial, primary and secondary, characterized by extensive prolifera- tion. The primary oocyte divides in a manner similar to the primary spermatocyte, that is by heterotypic mitosis, but the daughter-cells are very unequal in size, each, however, with the same number of chromo- somes (18) ; the small abortive cell is called the polar body, or better polocyte, while the large cell becomes a secondary oocyte. Both polocyte and secondary oocyte again divide, the end result being three similar polar bodies and one large ootid or mature ovum., now ready for fertili- zation. It must be recalled that the maturation process is in essence a process of eliminating one-half of the original quota of chromosomes, and a quantitative reduction of its original amount of chromatin to one-fourth. At fertilization, by fusion of egg and sperm, the specific number of chromosomes is reestablished differing in many cases in male and female of the same species and the chromosomes by growth regain or even exceed their original size. With this brief consideration of the function of the germ glands in general, we may proceed to a description of the structure of the male organs of reproduction and subsequently of the female organs. MALE ORGANS OF REPRODUCTION These organs include the penis and the testes, together with their accessory glands, and the excretory ducts which connect the testes with the urethral canal. The excretory ducts include the epididymis, ductus (vas) deferens, seminal vesicles, and ejaculatory ducts, and with their termination in the urethra there are connected the ducts of the prostate gland and the bulbo-urethral glands (of Cowper), whose secretion mixes with that of the testes to form the semen. The male urethra serves the double function of a uro genital canal. INTERNAL GENITAL ORGANS Testis The testis is to be regarded as a gland with a double function; it produces cells (spermatozoa), hence a cy to genie gland; and an internal secretion, hence, in part an endocrin gland. In connection with the testis must be considered also its excretory duct system, and the various glands accessory to it. The relationship of these various structures is shown in diagram, Fig. 418. 480 THE EEPEODUCTIVE SYSTEM Lobuli epididymidis Ductuli efFerentes testis Ductus epi- didymidis .Ductulus aberrans superior The testis is encased in a robust fibre-elastic capsule, the tunica albuginea, the innermost layer of which is of looser texture and very vascular, hence called ike* tunica vasculosa. External to the albuginea is a double-layered sac of peritoneum, the tunica vaginalis, its visceral layer closely adherent to the capsule. The human testis measures about one and one-half inches in length, one and one-quarter inches in width and one inch in thickness. Septa continuous with the capsule divide it into a -- Ductus deferens nu m b e r of com- partments or lo- bules, pyramidal in shape, the apices converging to an anterodorsal mass of dense connective tissue, the medias- tinum testis (cor- pus High mori), corresponding to a hilum. The lobules contain each sev- eral, frequently two, extensively convo- luted tubules, the seminiferous tu- bules (tubuli con- torti). When un- coiled they measure from one to two feet. The entire testis contains several hundred lobules; the number has been variously estimated at from one hundred to four hundred. Bremer (Amer. Jour. Anat., 11, 4, 1911) describes the tubuM con- torti of man as tubules that may be single, ending blindly, may branch or may anastomose. Huber and Curtis (Anat. Eec., 7, 6, 19f3), on the contrary, state that in the adult rabbit the seminiferous / tubules "present no blind ends, diverticula or nodular enlargements." Their simplest form is that of an arch beginning and ending in a tubulus rectus. The two limbs may lie in adjacent lobules. Complex tubules are also described, resulting from the linkage of from three to twelve Corpus epi- didymidis 'Ductulus aberrans (inferior) FIG. 425. THE TESTICLE WITH ITS SYSTEM OF EFFERENT PASSAGES. Natural size. (After Toldt.) MALE ORGANS OF REPRODUCTION 481 simple arched tubules. According to Curtis (1915) branches and anastomoses of seminiferous tubules are infrequent in the mouse -testis, more frequent in dog, and most frequent in rabbit. The testis is lodged in the scrotum. The wall of the scrotum is essentially like the general integument, except that it may be more ^Pfeif. M ; ^^ mm!*_ FIG. 427. SERTOLI CELLS OF THE HUMAN TESTIS. A, showing a crystalloid (of Charcot) and lipoid granules and spherules; below to the right a spermatogonium with a crystalloid (of Lubarsch). B shows a crys- talloid and two accessory rods, and lipoid granules and droplets. C shows at the base a crystalloid and one accessory rod, and at the summit two accessory rods. X 600. (Wimwarter.) tutes a so-called spermatoblast of von Ebner. The sustentacular cell contains one or several crystalloids. Their origin and function is un- known. They were regarded by Montgomery as probably sustentacular cell determinants, having been traced by him from the common mother- cells of both trophocyte and spermatogonium into the former, the latter being said to lack these elements. However, Winiwarter (Fig. 427) re- ports similar crystalloids in both trophocytes and spermatogonia. Tro- phocytes more probably have an origin distinct from that of the germ-cells, according to certain investigators arising from the Bowman r s capsule of the Wolffian tubules of the mesonephros. Every section of a tubule of an active testis contains several, some- times all the stages of sperrnatogenesis. Since the spermatogenetic process generally travels in waves, a longitudinal section is most favor- 483 484 THE REPRODUCTIVE SYSTEM able for a study of the continuous process. From wall to lumen of tubules can be seen, at successive levels, spermatogonia, primary sperma- tocytes, secondary spermatocytes (prespermatids), spermatids, and sper- matozoa attached to the tips of the sustentaeular cells. Mitoses can frequently be seen in spermatogonia, primary and secondary spermato- FIG. 429. SUCCESSIVE STAGES IN THE METAMORPHOSIS OF THE SPERMATID INTO THE SPERMATOZOON. (Schematic, from Bonnet, after Meves.) cytes. The primary spermatocytes are the largest of the germ cells; the secondary spermatocytes are approximately half the size and lie distal, while the spermatids are approximately one-fourth the size; the spermatids may be at any stage of a continuous process of metamorpho- sis involving nuclear, cytoplasmic, and general form changes. The spermatogonia are of various sizes, frequently approximating that of the primary spermatocytes, but always lie peripherally, on the basement membrane between sustentacular cells. The above named marks con- stitute the grosser criteria for identifying the several generations of MALE ORGANS OF REPRODUCTION 485 Acrosome Head\ Neck Body End ring - Main segment of tail Galea capitis Anterior knob | Spiral fibers Sheath of axial thread cells involved in spermatogenesis. As concerns details, the criteria above given for the grasshopper testis, can be applied. The Spermatozoon. Spermiohistogenesis, however, differs some- what. The spermatid nucleus becomes progressively more compact, chromatic and smaller. At the same time it moves toward one pole of the cell, the cytoplasm flowing back- ward, except for a thin envelope (galea capitis) which terminally forms a covering for the perfora- torium which represents a portion (acrosome) of the original idiosome. The idiosome is the germ-cell repre- sentative of the archoplasmic com- plex. Concurrently with the changes outlined for the nucleus it liberates a centrosome, and centrosome and acrosome pass to opposite poles of the nucleus. The centrosome di- vides into two moieties, a proximal, which becomes attached to the nu- clear membrane, and a distal, which moves peripherally at the same time sprouting a flagellum, the central filament. Meanwhile a peripheral portion separates from the more central portion of the distal centro- some and moves backward over the central filament enclosing an envel- oping column of cytoplasm, the mid- dle piece, in which mitochondria have aggregated. The mitochondria fuse to form a filament, the spiral filament, which becomes spirally ar- ranged about the central filament, grow distally beyond the final locus of the ring centrosome, and together with an envelope of cytoplasm constitutes the tail of the spermatozoon. A short terminal naked portion of the central filament is known as the terminal filament. Summarizing the above: The nucleus becomes the head of the Axial thread L _ Capsule Terminal flament FIG. 430. DIAGRAM OF HUMAN SPER- MATOZOON. (After Bonnet.) The central filament continues to 486 THE EEPEODUCTIVE SYSTEM spermatozoon. This is tipped with an aerosome, a derivative of the idiozome, and covered by the galea capitis, the two comprising the per- foratorium. The other constituents of the idiozome contribute the central filament. The extent between distal and proximal centrosomes constitutes the neck; the portion between distal centrosome and ring cen- trosome the connecting or middle piece (body), with its spiral filament derived from mitochon- dria. The tail or flagellum consists of central filament enveloped by cytoplasm. In many forms a spiral fin develops on the body. Also, there is endless variation with reference to the shape of the head. But the development and morphology of spermatozoa of all vertebrate forms is essen- tially the same. At fertilization only head and middle piece nucleus and centrosome and mito- chondria enter into the egg, the tail being gen- erally left behind. The human spermatozoon has a length of from fifty to sixty microns. Of the total the head con- stitutes about 5 /A, the middle piece about 5 ju. and the end-piece about 10 p.. Seen en face the head has an ovoid shape ; in profile it appears pyrif orm, due to a thinning along its terminal margin. Oc- casionally atypical or monstrous, double-headed, multiple-tailed, and giant spermatozoa appear ; but these are probably non-functional. Though flagel- late, the spermatozoa are non-motile until they reach the epididymis, where they become bathed in a secretion. Their motility, however, is not pro- nounced until they reach the place in the excretory passage where the secretion of the seminal vesicles, prostate and bulbo-urethral glands are present. The mixture of these secretions with the sperm forms the semen. The spermatozoa can withstand considerable variations 4n tempera- ture, but they are very susceptible to acid solutions; they survive best in slightly alkaline media. Under favorable conditions spermatozoa may be kept alive for as long as eight days; it is probable that they remain alive for even a longer time in the female genital tract; but FIG. 431. SPERMATO- ZOA OF VARIOUS AN- IMALS. A, from the badger; B, from the bat. Re- drawn after Ballowitz. X 1200. C, from the rat. Hematein and eo- sin. X 1200. MALE ORGANS OF REPRODUCTION 487 B it is uncertain whether they can maintain their functional virility for this length of time. The number of spermatozoa per cubic millimeter has been estimated at 60,876; and the total in an average ejaculation at 200,000,000 (Lode, 1896). The tubuli convoluti pass into short tubuli recti and the latter into the rete testis of the mediastinum. The rete testis is the beginning of the excretory duct, and consist of a meshwork of irregular tubules lined with a single layered cuboid non-ciliated epithelium. They connect with a group of greatly coiled tubules, the duc- tuli efferentes, from ten to fifteen in number, each terminally winding so as to assume a coni- cal shape, hence coni vasculosi which tubules empty into the distal portion of a common duct the ductus epididymis. This portion of the duct, together with the efferent ductuli, consti- tute the globus major of the epididymis. Con- sideration of the duct system will follow below. Interstitial Cells of Leydig. Here should be considered the intertubular connective tissue of the testis. Fibro-elastic connective tissue acts as a supporting stroma for the testicular paren- chyma of the seminiferous tubules. Scattered throughout this stroma, between the adjacent tubules, are peculiar large polygonal cells, inter- stitial cells of Leydig, either scattered or ar- ranged in masses. The condition of the nucleus varies from pale vesicular to dense deeply chro- matic. Division figures are exceedingly rare among them, though an occasional cell may, in young testes, be seen in mitosis. In older testes an occasional nucleus may be seen in what appears to be amitotic division. A number of the cells also are bi- and multinucleate. However, multiplication of specific interstitial cells is evidently exceptional. These cells arise directly through modification in shape, and growth from the fusiform and irregular connective tissue cells of the stroma. Moreover, this process seems to be reversible. They thus represent a transient phase in the life of certain cells of the inter- tubular connective tissue. Their cytoplasm contains crystalloids (mito- 31 FIG. 432. SPERMATO- ZOA FROM THE SEMEN OF MAN. A, usual type; B, 'giant' double sperma- tozoon. Hematein and eosin. X 1200. 488 THE EEPEODUCTIVE SYSTEM chondria) arid fatty granules. The nature of the lipoid granules is dis- cussed by Whitehead (Amer. Jour. Anat., 14, 1, 1912). These cells increase in number during senile atrophy of the testis, but later entirely disappear. They are rela- tively larger and more abun- dant also in the testes of tuberculous patients where /there is considerable degen- eration and a general in- fantile condition of the seminal epithelium. Vari- ous functions have been ascribed to them that is, those obviously dependent upon the internal secretion of the testes prominently, origin and maintenance of secondary sexual characters, and basis of sexual in- stinct. Obviously other cells of the testis might conceiv- ably subserve these func- tions, namely, the susten- tacular cells and the sex- cells. However, observa- tions on testes of mules (in- fertile) and cryptorchid horses, and the findings of Whitehead (Anat. Eec., 2, 5, 1908) in a third abdom- FIQ. 433. A GROUP OF INTERSTITIAL CELLS FROM THE TESTIS OF A THIRTY-FIVE YEAR OLD NEGRO. The outlines of three adjacent seminiferous tubules are shown. Among the interstitial cells are three nuclei of ordinary connective tissue. The interstitial cells are filled with lipoid spher- ules and granules. Formalin fixation, iron- hematoxylin stain. X 1500. inal testicle of a horse where only interstitial cells in great abundance persisted in normal condition, would seem to rele- gate these functions to the interstitial cells. Kingsbury's careful studies (Amer. Jour. Anat., 16, 1914) of the inter- stitial cells of the ovary of the cat (elements presumably homologous with the interstitial cells of the testis) strongly support the interpretation of these cells in terms of absorbers of degenerating and disintegrating ma- terials. In the ovary all of the earliest and many of the later primary oocytes disintegrate. In the vicinity of such atretic follicles interstitial MALE OBGANS OF REPRODUCTION 489 cells laden with lipoid debris are abundant. Their presence, however, is transient, and their abundance reciprocal to the abundance of degenerat- ing follicles. This observation, reasoning by analogy, would explain their abundance in the testes of cryptorchid horses, mules, and in the third abdominal testicle ; for here also degenerative processes are going on among the sex cells. Testicles of cryptorchid horses and mules are characterized by an unusual abundance of interstitial cells, and degeneration of sex cells. These animals experience heat, but are infertile; this was true also of the stallion from whom two testicles had been removed and in whom subse- quently the third abdominal testicle was discovered. These observations ABC D FIG. 434. INTERSTITIAL CELLS FROM THE HUMAN TESTIS. A, B, and C, from a twenty-five year old man; A shows the idiosome, bacillary centrosomes, lipoid granules and crystalloids; B, with two nuclei; C, with four nuclei (probably the result of amitotic division) and eight centrosomes. D, from a 41-year-old man, showing large and small crystalloids. Highly magnified. (Wini- warter, Anat. Anz., 41, 11, 1912.) would seem to indicate that the sexual instinct depends upon an internal secretion on the part of the interstitial cells. The two results can be har- monized on this basis: The internal secretion may actually be formed by the cells of the seminiferous tubules; when these degenerate the products are removed by the interstitial cells; since even in healthy testes there is some degeneration of sex cells, all testes contain a few interstitial cells; these are more abundant at puberty ; the secretory products of degenerating sex cells are included among the debris removed by the interstitial cells from which they may be passed into the capillaries of the testicular stroma. The suggestion that t\ie interstitial cells are in some way connected with secondary sex characters seems disproved by various castration experi- ments. For example, the spayed hen takes on male secondary sexual charac- ters; if these were dependent exclusively upon the interstitial cells of the testis, they could not appear in the absence of both ovaries and testes. 490 THE EEPEODUCTIVE SYSTEM However, the experiments of Steinach (Zentralblatt f. Physiol., 27, 14, 1913) cannot be ignored in tliis connection. Steinach made reciprocal transplantations of testes and ovaries in young male and female rats and in guinea pigs, and claims to have succeeded in changing a potential female, from the view point of secondary sex characters (both physical and temperamental), into a male, and vice versa. A male is said to have devel- oped functional mammary glands, and even to have suckled young. Stein- ach interprets his results to mean that not only do the secondary sex char- acters of the male and female depend upon the presence of the 'pubertal gland' (interstitial cells), but also the fact whether the undifferentiated gonad shall develop into a testis or an ovary. Microscopic examination of the transplant showed that everything suffered degeneration except Ihe in- terstitial tissue, which underwent extensive hyperplasia. For further in- formation on this and related subjects reference should be made to Mar- shall's "The Physiology of [Reproduction," 1910. The Duct System Tubuli Recti. At the apex of the testicular lobule the tortuous seminiferous tubules pass into the rete testis of the mediastinum. At this point the tubule becomes straight and is abruptly narrowed. Thus the short straight tubules, tubuli recti, are formed. In the straight tubules the stratified epithelium of the tortuous portions is abruptly' exchanged for a very low columnar or flattened type of epithelium with which the Sertoli cells of the tortuous tubules seem to be continuous. The straight tubules are very short and are soon transformed into the irregular anastomosing canals of the rete testis. Rete Testis. The connective tissue of the mediastinum is perme- ated by a network of irregular channels of varying diameter which present frequent dilatations and often have the appearance of broad cleft-like spaces. These are the canals of the rete testis which form a dense network of anastomosing channels. On the one hand they receive the straight tubules, and on the other they pass into the ductuli ef- ferentes, which convey the secretion onward to the globus major of the epididymis. The canals of the rete testis are lined by cuboidal or flattened epi- thelium, which rests upon a delicate basement membrane. This in turn is supported by the connective tissue of the mediastinum. The broad but irregular lumen of the canals is occupied by the secretion from the seminiferous tubules and contains many spermatozoa. Ductuli Efferentes, As the tubules of the rete testis leave the MALE OEGANS OF KEPRODUCTION 491 mediastinum they are abruptly transformed into peculiar efferent ducts, 10 to 15 in number, which pass into the globus major of the epidldymis and by means of spiral windings form conical masses, coni vasculosi, whose apex projects into the glo- bus major. The epithelium of these tubules is peculiar in that it contains two varieties of cells, and in that it is thrown into many prominent longitudinal folds or rugae. In the lining epithelium there are short columnar cells which FIG. 435. A SMALL PORTION OF THE WALL OF AN EFFERENT DUCTULE OF THE TES- TICLE. rest upon the basement mem- brane and carry upon their free ends a tuft of short cilia. These d, 'glands' in longitudinal section; d r , the same in oblique section; F, ciliated epithe- cells have an ovoid nucleus and a ii um . x 140. (After Kolliker.) very finely granular, eosinophil cytoplasm. Between and among the ciliated cells are many broad col- umnar or polyhedral cells, having remarkably clear cytoplasm, which FIG. 436. EFFERENT DUCTULES OF THE RABBIT'S EPIDIDYMIS. Hematein and eosin. Photo. X 250, 492 THE REPRODUCTIVE SYSTEM chiefly occur between and at the base of the rugae, and are frequently arranged in small groups simulating minute secreting glands. These clear cells have spheroidal nuclei and their cytoplasm is filled with large coarse granules. They are quite characteristic of this portion of the excretory tubules of the testis. The coni vasculosi form a considerable FIG. 437. SEVERAL COILS OF THE RABBIT'S EPIDIDYMIS IN TRANSECTION. The lumen of the tubules contains groups of spermatozoa. Hematein and eosin. Photo. X 178. portion of the globus major of the epididymis. The epithelium rests upon a well-developed basement membrane, outside of which is a robust layer of circularly disposed, smooth muscle cells with intermingled elastic fibers. The nuclei of the lining cells can frequently be seen in some stage of amitotic division. Epididymis. The ductus epididymis forms a long coiled tubule whose convolutions, by their regular cylindrical form and their tall ciliated epithelium, are sharply distinguished from those of the ductuli MALE OKGANS OF KEPBODUCTION 493 efferentes, which have much thinner walls. The lining epithelium of the epididymis is of the tall, ciliated, simple columnar type with Elon- gated ovoid nuclei, a finely granular cytoplasm, and a group of non- motile cilia which often adhere together to form a peculiar tuft or cluster ('brush border 7 ). At the base of the ciliated cells is an in- complete layer of basal epithelium, whose flattened cuboidal elements are wedged between the bases of the tall columnar cells. The two- layered epithelium is thus of the pseudo-stratified type. The cells ap- parently multiply largely by amitosis. The epithelium rests upon a cellular basement membrane, which is supported by a connective tissue tunic of varying thickness. In addi- tion to many elastic fibers, this coat contains a few smooth muscle cells. The coils of the epididymis are firmly united into a solid mass by means of the dense intervening connective tissue. They form the whole of the globus minor and a considerable portion of the globus major. When unraveled the duct of the epididymis measures about twenty feet in length. The Ductus Deferens (Vas Deferens). This duct is a continua- tion of the epididymis, whose course now becomes relatively straight. It measures about eighteen inches in length. In this portion of the excretory duct of the testis the lining epithelium soon loses its cilia, and the basal cells become more prominent. Hence in the greater portion of its course the ductus deferens is lined by tall, columnar, non-ciliated epithelium, with low basal cells between the attached ends of the columnar cells. The epithelium rests upon a fibro-cellular basement membrane, which is supported by a fibrous tunica propria. This, in turn, passes almost insensibly into the muscular coat which consists of two layers, an inner circular and an outer longitudinal, both of which are highly developed. In the lower portions of the ductus deferens, a thin internal layer of longitudinal muscle fibers is also found. The fibers of the internal and middle circular layers are frequently less regularly ar- ranged, in which case their oblique bundles interlace with one another in a most intricate manner. The very thick, smooth muscular coat and the relatively 'narrow lumen give this portion of the duct a firm consistence and a cord-like feel. In its ampulla the dilated portion near its prostatic termination the mucous membrane of the ductus deferens is more loosely attached and the folds or rugae, which elsewhere are few in number, are here very pronounced. The lumen of the ampulla is broad, but elsewhere 494 THE REPRODUCTIVE SYSTEM the lumen of the duct is narrow, as compared with its exceptionally thick muscular wall. As elsewhere in the excretory canal of the testicle, the lumen of the ductus deferens contains many spermatozoa. THE SPERMATIC CORD. The spermatic cord in its scrotal portion, FIG. 438. TRANSECTION OF THE DUCTUS DEFERENS OF A DOG. Hematein and eosin. Photo. X 104. in addition to the ductus deferens, contains a mass of connective tissue in which are embedded the smooth muscle fibers o*f the internal cre- master muscle, the spermatic artery, veins, and nerve plexus, and the vessels of the pampiniform plexus. Closely -associated with these con- stituents is also the striated cremaster muscle proper. The whole is invested by a reflection of the tunica vaginalis.- The pampiniform plexus is a considerable group of venous spaces, usually completely collapsed after death, which are characterized by MALE OKGANS OF BEPRODUCTION 495 very thick, firm, fibromuscular w'alls. The vessels are embedded in dense connective tissue, and the whole plexus in general appearance somewhat resembles the erectile tissues. FIG. 439. FROM A SECTION THROUGH THE WALL OF A SEMINAL VESICLE OF MAN. a, mucosa; 6, muscular coat; c, fibrous coat. Hematein and eosin. Photo. X 185. The Seminal Vesicles. The walls of the seminal vesicles consist of a thin outermost coat of connective tissue in which are many small ganglia, a muscular coat similar to that of the ductus deferens but 496 THE EEPEODUCTIVE SYSTEM much thinner, and a characteristic mucosa. The tunica propria of the mucous membrane is a thin layer of delicate cellular connective tissue which loosely attaches the lining epithelium to the muscular coat. The surface of the mucosa presents numerous folds which not only form longitudinal rugae but also pos- sess an intricate network of secondary ridges which are both longitudinal and transverse in direction. This peculiar arrangement re- sults in the appearance of diverticula of various forms and sizes which, except that their epithelium does not differ from that of the surface, might often be interpreted, when seen in section, as representing sec- ondary secreting glands within the mucosa. Slender processes of the corium extend into all the folds of the mucous membrane. The lining epithelium of the seminal vesicles is of the columnar type and usually contains but a single layer of cells. Occa- sionally basal cells are also found in the deeper part of the epithelial layer; in such case the epithelium may be said to be of the pseudo-stratified type. This variation may possibly be partly dependent upon the dis- tention or relaxation of the vesicles. The cells of the epithelium contain peculiar granules of yellowish pigment which are present in considerable numbers and are quite characteristic of the organ. The su- perficial cells appear to be readily desquam- ated, and together with coarse granules of secretion form the chief contents of the lumen. Occasional small concretions of ir- regular form, and homogeneous or lamellar structure, similar to those of the prostate gland, are also found. The seminal vesicles usually contain but few spermatozoa. Occasionally these are present in large numbers; at other times they may be entirely absent. Their chief function is now thought to be secretory. FIG. 440. MODEL OF A RECONSTRUCTED PROS- TATE GLAND OF MAN. The figure includes one lobule. The narrow duct expands and terminates in a large number of alveoli of very varied size and form. X 40. (After Maziarski.) MALE OKGANS OF EEPEODUCTION 497 The Ejaculatory Ducts. These ducts are formed by the union of the ampulla of a ductus deferens and the duct of a seminal vesicle and are similar in structure to the ampullae of which they are the continua- tion. The ejaculatory ducts, however, possess a thinner wall and their mucosa presents the same folded condition as in the seminal vesicles, but to a lesser degree. In its prostatic portion the musculature of the ductus deferens blends with the muscular stroma of the prostate, so that in the ejaculatory duct the smooth muscle no longer forms a distinctly lamellated coat. On approaching the urethra, the epithelium of the ejaculatory ducts presents a gradual transition to the stratified epithelium of the urethral canal. Associated Glands The Prostate Gland. This is a compound tubulo-ajveolar gland consisting of from 30 to 50 lobules investing the urethra and the ejacula- tory ducts. It pours its serous secretion, which has a characteristic odor, into the neighboring portion of the urethra by means of two large and many (15 to 30) small ducts. These open either directly into the urethral canal or indirectly through the utriculus prostaticus (sinus pocularis). The secreting alveoli are embedded in a very dense fibro- muscular stroma which, at the surface of the organ, forms an unusually thick capsule in which interlacing bundles of smooth muscle are most prominent. This portion of the stroma also contains intrinsic striated muscle fibers in limited numbers. Broad bands of fibromuscular tissue pass inward from the capsule and form a network of thick septa in the meshes of which are the glandular alveoli. These septa converge toward the urethra, which penetrates the ventral portion of the organ, their muscular fibers finally blending with the sphincter fibers of the prostatic portion of this canal. The stroma consists of smooth muscle and connective tissue; their fibers are intimately blended. The muscle and connective tissue con- stitute each about one-fourth of the organ. The muscle cells form either groups or bundles of variable size, or are frequently isolated within the meshes of the connective tissue. Their extreme abundance in some parts exceeding the connective tissue in volume is char- acteristic of the prostatic stroma. The connective tissue, which is sparingly supplied with elastic fibers, is rich in cells. Near the se- creting alveoli the muscle fibers are absent and the cellular connective tissue becomes more prominent. The lining cells are of the tall columnar type, sometimes forming 498 THE REPRODUCTIVE SYSTEM a single, sometimes a multiple cell layer. They possess spherical or ovoid nuclei which lie in their deepest third. Their cytoplasm is finely granular and often contains small yellowish granules. The epithelium rests upon a prominent membrana propria, composed of peculiar coarse collagenous fibers, 'B-collagenous' fibers (Ferguson, Anat. Eec., 5, 12, 1911) somewhat resembling, but not identical with, reticulum. This FIG. 441. SEVERAL ALVEOLI OF THE HUMAN PROSTATE GLAND, SEEN IN SECTION. Hematein and eosin. Photo. X 160. type of fiber is present also throughout the stroma and distinguish- able by the Bielschowsky technic from the usual collagenous ('A-col- lagenous') fibers. The epithelium is remarkably folded upon itself, the narrow interval between the two layers of the epithelial folds being always occupied by delicate extensions of the connective tissue stroma. The prominence of the folds varies greatly in different tubules, some showing scarcely MALE ORGANS OF REPRODUCTION 499 any such, the lumen of others being subdivided by deep rugae into numerous anastomosing compartments. The amount of the folding also varies in different species, being more highly developed in some of the lower mammals, e.g., the dog, than in man. The lumen of the prostatic tubules is broad, and is beset with numer- ous alveolar dilatations and shallow diverticula. It is usually broader FIG. 442. PORTION OF PROSTATE GLAND OF AN OLD MAN, SHOWING THE PROSTATIC CONCRETIONS. In the upper right hand concretion the concentric lamellae are clearly discernible. near the blind extremity and diminishes somewhat in diameter toward the duct. The caliber of the lumen also varies greatly in different tubules and is possibly dependent in part upon the state of secretory activity. The contents of the lumen include the granular albuminous secretion, desquamated epithelial cells, and, as age advances, many so- called prostatic concretions (' corpora amylacea'; 'prostatic calculi'). The concretions vary greatly in size (10 /* to 1 mm. in diameter), and may be homogeneous, but more frequently present a distinctly 500 THE EEPEODUCTIVE SYSTEM lamellated appearance. Prostatic concretions may occur at all ages but increase both in number and size in later life. Occasionally they attain a large size and may become encysted. The prostatic ducts are lined by either a single or a pseudo-stratified layer of columnar epithelium, and, except for their narrower caliber and more regular contour, they closely resemble the secreting tubules. As the ducts approach their termination their epithelium increases the number of its cell layers. The larger ducts, just prior to their FIG. 443. PROSTATIC GENITAL CORPUSCLES. a, axial nerve fiber; 6, peri-axial nerve fiber. . Methylene blue. Moderately magnified. (After Timofejew.) termination, are lined by transitional epithelium similar to that of the urethra, into which they open. BLOOD AND LYMPH SUPPLY. The prostate gland possesses a rich blood supply. Its larger vessels are found in the capsule, whence they send branches into all portions of the fibromuscular stroma, and form a rich capillary plexus in the connective tissue layer about the epithelium of the secreting alveoli, and a second plexus in the substance of the stroma itself. The prostate is abundantly supplied, also, with lymphatic vessels, which are connected with the deep pelvic lymph nodes. The capsule of the prostate, as also the neighboring connective tissue, both in relation with this organ and with the adjacent seminal vesicles, contains many nerve trunks, chiefly sympathetic, and small ganglia. The latter are especially numerous. In this region a peculiar variety of special nerve ending is found. It was formerly regarded as a Pa- MALE OEGANS OF REPEODUCTION 501 cinian corpuscle, but differs somewhat from these bodies. It perhaps more nearly resembles the genital corpuscles. These bodies are. dis- tinctly lamellated and possess a broad axial nerve fiber which some- what resembles that of the end bulbs of Krause. This nerve fiber is, '^^'.^"^ty-i 1 *;- FIG. 445. FROM A SECTION OF THE BUL- BO-URETHRAL (CoWPER's) GLAND OF MAN. a, duct; 6, tubules; c, stroma. (After Braus.) X 130. however, accompanied by another and finer fiber which, as Timofejew (Anat. Anz., 1896) has shown, breaks into a close network of fine fibrils surrounding the axial nerve fiber in a peculiar basket-like manner. The Bulbo-urethral Glands (Cowper's Glands}. These are two small tubulo-acinar mu- cus-secreting glands which are divisible into numerous small lobules. The lobules are sep- arated by connective tissue septa containing both smooth and striated muscle fibers. The latter are continuous with the adjacent compressor urethras muscle. The se- creting acini are' lined by columnar cells, some of which are finely granular and stain with eosin and acid dyes, while others are apparently filled with mucous secretion and react to the specific dyes for mucin. Certain other tubular alveoli are lined by low cuboidal or flattened epi- thelium. The epithelium rests upon a distinct cellular basement mem- brane. FIG. 444. RECON- STRUCTION OF A BUL- BO-URETHRAL (COW- PER'S) GLAND OF MAN. The tubular ducts are closely surrounded by the expanded alve- oli. X 100^ (After Maziarski.) 502 THE REPRODUCTIVE SYSTEM The interlobular and the smaller intralobular ducts are also lined by a single layer of columnar cells. Their wall is supplied with smooth muscle, most of whose fibers have a longitudinal direction. The two ducts of Cowper's glands open into the bulbous portion of the urethra. Associated Vestigial Structures The vestigial structures associated with the male reproductive system include the appendices testis and epididymis, the superior and inferior duc- tuli aberrantes, the paradidymis and the sinus pocularis. Of these it may be said in general that they are more or less variable with respect of gross and microscopic structure and even with respect of presence; that they resemble histologically the structures with which they are homologous, and that they tend to become cystic. The APPENDIX TESTIS is the least variable of the vestigial associates. It is present in about ninety per cent, of cases. It is a small spherical, fre- quently pedunculated, sac attached to the superior pole of the testis ; it is covered with tunica vaginalis and lined with simple columnar epithelium, sometimes ciliated, and represents the end of the degenerated fetal Miil- lerian duct. The APPENDIX EPIDIDYMIS is a very similar pedunculated structure, much less frequently present. It is situated on the globus major of the epididy- mis. It is supposed to represent a degenerated Wolffian tubule. By some it is regarded as the atrophic end of the Wolffian duct. The DUCTULI ABERRANTES are blind tubules, the remnants of meso- nephric tubules which failed of inclusion among the ductuli efferentes of the globus major. The superior ductule opens into the epididymis below the globus major; the inferior opens at the globus minor; both lie between the testis and the epididymis. They are lined with a single layer of colum- nar epithelium, sometimes ciliated. The inferior ductule is the more gen- erally present, and has a length of about five centimeters. The PARADIDYMIS (organ of Giraldes) lies within the spermatic cord between the head of the epididymis and the pampiniform plexus. It con- sists of a variable number of irregular branching tubules, blind at both ends, and lined with a single layer of columnar, ciliated epithelium. These tubules also represent persisting rudimentary mesonephric tubules. The SINUS POCULARIS (sinus prostaticus) represents the remnant of the proximally fused degenerated fetal Miillerian ducts. It is the homologue of the vagina in the female. It is a shallow, blind pocket opening into the floor of the prostatic portion of the urethra ; it may be bifid distally, varies in length from six to twelve millimeters, and is lined with a columnar epithelium which may be locally ciliated. MALE ORGANS OF REPRODUCTION 503 EXTERNAL GENITAL ORGAN The Penis. The penis consists of three masses of erectile tissue, the two corpora cavernosa penis and the corpus spongiosum or corpus cavernosum urethra, which are firmly united by dense fibrous and areolar FIG. 446. TRANSECTION OF A CHILD'S PENIS, JUST BACK OP THE GLANS. The two corpora cavernosa (fused in the median line) and the corpus spongiosum, the latter containing the urethra, are well shown. The cutaneous surface is not included. Hematein and eosin. Photo. X 8. connective tissue. The outer cutaneous surface is loosely attached over the body of the organ; its structure does not differ materially from that of the skin of other parts. The subcutaneous tissue is remarkable for the extreme looseness of its areolas and the absence of fat. In the glans penis the epithelial covering, which is continuous with the pre- pucial epidermis, is firmly united to the underlying erectile tissue. 32 504 THE BEPKODUCTIVE SYSTEM Each corpus cavernosum is invested with a thick sheath of very dense fibro-elastic tissue, the tunica albuginea, divisible into an inner circular and an outer longitudinal layer of fibers, and imperfect between the two corpora cavernosa penis where it forms the pectiniform septum. From the inner surface of this fibrous coat connective tissue septa, the trabec- ula?, pass in all directions and form a reticular framework whose fibrous bands contain many smooth muscle fibers. In the meshes of this framework are characteristic broad venous sinuses which possess no true wall other than their endothelial lining. In the flaccid condition of the organ the blood sinuses are com- pletely collapsed, their walls are in contact, and their lumina almost obliterated, which gives them the appearance of mere slits in the *' dense connective tissue of the caver- nous body. When distended by in- jection,, or, in the erectile condition of the organ, by blood, these spaces become widely dilated and form true blood sinuses of broad caliber. The blood supply of the erectile tissue is peculiar. The arteries ter- minate either (1) in capillaries, (2) hy direct anastomosis with the ven- ules, or (3) by opening directly into FIG. 447. HELICINE ARTERY IN SEC- TION, FROM THE URETHRAL BULB OF MAN. a, lumen of a helicine artery; 6, fibrous bands of the erectile tissue; c, c, lumen of a venous blood space; m, open- ing of the helicine artery into the blood space; ms, muscular coat of the artery. Hematoxylin and eosin. X 180. (Af- ter Kolliker.) the venous sinuses, in which case the minute terminal arterioles have a peculiar looped appearance and were described by J. Miiller (1835) as helicine arteries. The capillaries form a superficial plexus beneath the tunica albuginea, which opens into a deeper plexus of broader vessels from which the venules take origin. Blood following this course through the capillaries and into the venules may not enter the venous sinuses a direction which is assumed by the greater portion of the blood in the flaccid condition of the organ. The deeper venous plexus communicates freely with the venous sinuses so that the least obstruction to the usual venous outflow diverts the circulation through these channels. The helicine arteries are confined to the corpora cavernosa penis, MALE OKGANS OF EEPKODUCTION 505 where they are most abundant near the root of the organ. The ar- terioles from which they are derived end by arborization in the con- nective tissue framework, their terminal twigs entering fibrous processes which project into the venous sinuses and are frequently bound down by delicate fibrous bands which, unite their extremity to the wall of the sinus and produce the characteristic looped condition when the villus- like projection is distended by the injection of its arteriole. When partially injected the helicine arteries appear to end blindly, but when :^^f-:~::^'-" FIG. 448. THE ERECTILE TISSUE OF THE PENIS. c r, peripheral capillary plexus; t a, tunica albuginea; v s, venous spaces; z, bands of smooth muscle and vascular connective tissue. X 30. (After Kolliker.) completely distended they pour their contents into the venous sinuses. The venous spaces at the periphery of the erectile body are relatively narrow and the intervening trabecula? are thick; toward the axis of the body the sinuses become broader and occupy a relatively greater portion of the tissue. Here, also, their long axis, except in the corpus spongi- osum, frequently lies in the transverse axis of the penis. Both the arteries and the veins of the erectile tissue possess very thick muscular walls, and in both, the intima becomes locally thickened by accumula- tions of longitudinal muscles and elastic tissue bulging into the lumen ; these modifications are less pronounced in the veins than in the arteries. 506 THE EEPEODUCTIVE SYSTEM The tunica albuginea and trabeculse of the corpus spongiosum are formed by less dense connective tissue than is found in the corpora cavernosa penis, and their venous spaces are not so broad. The broad anterior end of the corpus spongiosum forms almost the entire body of the glans penis, being only indented beneath the corona by the conical anterior ends of the corpora cavernosa penis which in this part are blended together to form a single median mass. The urethral canal occupies the axis of the corpus spongiosum from its bulb forward to the urinary meatus at the tip of the glans penis. This canal has already been described in the preceding chapter. It should be recalled that its lining epithelium differs in the several segments,, being transitional in its prostatic and membranous portions, and stratified columnar through- out the greater length of the spongy portion, changing to stratified squamous in the dilated fossa navicularis of the glans penis. The skin of the glans penis is peculiar in the relatively moist char- acter of its epidermis and the consequent imperfect development of its superficial horny layer. Its dermal papillae are conspicuously developed. In the region of the corona the derma contains a ring of large sebaceous glands, the preputial glands, which open on the free epithelial surface. Their secretion forms the smegma, a peculiarly odoriferous sebum. The so-called glands of Tyson are shallow, non-glandular epithelial pockets opening near the frenulum preputii (Lewis). The medullated sensory nerves (dorsal nerves of the penis; branches of the pudic) are abundantly supplied with special nerve end-organs. In the skin they form free varicose endings among the epithelial cells, and are connected with tactile corpuscles of Meissner in the dermal papilla. Deeper in the skin are many end bulbs of Krause, while still deeper are the peculiar genital corpuscles. Naked fibrils pass to the mucosa of the urethra. Pacinian corpuscles are also found in the loose connective tissue and in the tunica albuginea of the corpora cavernosa. Sympathetic nerve fibers are abundantly supplied to the walls of the blood-vessels and to the smooth muscle of the erectile tissue. Branches from the third and fourth sacral nerves also enter the penis as the nervi erigentes, supposed to convey the impulse to erection as vasodilator fibers. The lymphatics of the penis form an abundant superficial set in the subcutaneous tissue; these follow the larger blood-vessels and empty into the inguinal lymph glands. A less abundant deep set of lymphatics in the erectile tissue, also, accompanies the blood-vessels of these parts, but is distributed to the pelvic lymph glands. THE FEMALE REPRODUCTIVE ORGANS 507 THE FEMALE REPRODUCTIVE ORGANS This system includes the ovaries, oviducts, uterus, vagina, and ex- ternal genitals. All of these organs are concerned in the reproductive function, the ovary producing the germ cell or ovum, and the oviduct providing a suitable site for its maturation and fertilization and the uterus for the later development of the resulting embryo. INTERNAL GENITAL ORGANS The Ovary The ovary also is properly regarded as a gland with a double func- tion, namely cytogenic and endocrin. The specific cells involved in the production of the internal secretion (exclusive of the lutein substance) are in doubt. As in the testis, the possibilities include the germ cells, the general connective tissue and the interstitial cells, homologues of the interstitial cells of Leydig of the testis. The. genetic and functional relationship between the several types of cells are similar to those described for the testis. The ovary, however, apparently does not contain a homologue of the Sertoli cells. As in the testis the internal secretory activity of the ovary sustains a reciprocal relationship to other internally secreting glands and in some manner, directly or indirectly, underlies normal development and the sexual instinct. The ovary, moreover, periodically elaborates still another internal secretion. The organ involved is a transient structure, the corpus luteum of pregnancy. This will be described below. Its function pertains to an inhibition of ovulation during pregnancy (Loeb; Pearl), and a stimulation to secretory activity of the mammary glands (Bouin et Ancil; Ott and Scott), and apparently in part also to the preparation of the uterine mucosa for proper implantation and normal develop- ment of the fertilized ovum. As shown by extirpation experiments the internal secretions of both ovary and testis influence also the nervous system, and seem essential to normal nervous function. The ovary is a solid ovoid body, about one and one-half inches long, three-quarters of an inch wide and one-half inch thick. It is attached to the margin of the broad ligament posteriorly by a short, thick connective tissue pejdicle, the mesovarium, which transmits the blood-vessels with which the ovary is richly supplied. At its ovarian attachment the mesovarium becomes continuous with the connective 508 THE KEPKODTJCTIVE SYSTEM tissue stroma of the ovary. The indentation which is thus produced is known as the hilum. The substance of the ovary is divisible into a central medulla which reaches the surface only at the hilum',, and a peripheral cortex which invests all other portions of the medulla and is in turn clothed by a layer of germinal epithelium, a continuation of the peritoneal epithelium, whose cells in this area are peculiar in that they possess a typically cuboidal shape, and are thus sharply distinguished from the flattened mesothelial cells of the surrounding portions of the peritoneum. Corpus ' teum Pfimnry orortan follicle An tnan folliculi 'of Graafiaa follicle iobphorus Germinal 'epithelium fwith blood-ressety FIG. 449. SECTION OP OVARY OF ADULT CAT, THROUGH HILUS, SHOWING FIVE VESICULAR (GRAAFIAN) FOLLICLES, WITH THE CUMULUS OOPHORUS AND THE ENCLOSED OVUM. The antrum folliculi is filled with a granular material, a coagulum of the liquor folliculi. Note that the ovarian follicles are in the cortical portion, the medulla containing no follicles. X7. The Medulla. The medulla of the ovajy consists of a fibromuscular stroma and large numbers of blood-vessels. Its arteries are characterized by their spirally tortuous course and thick muscular walls; its veinsjjje - numorous- and large, and their endothelium rests almost directly upon the nbromuscular strojna. This portio~n of the ovarian stroma consists of fibrous connective tissue in which are elastic fibers and considerable numbers of smooth muscle cells. The connective tissue is richly sup- plied with cellular elements, most of which are ovoid or fusiform in shape. The Cortex. The cortex of the ovary likewise contains a vascular gtroma and also large numbers of ova which are in all stages of develop- THE FEMALE KEPKODUCTIVE OKGANS 509 merit, from the genetic cells of the germinal epithelium up to the more mature germ cells, contained within epithelial sacs and known asTuvaiian follicles. I)uringjjie_jneji strual. epoch the ovaries also contain peculiar yellowish bodies, corpora lutea, resulting from the rupture of the largest FIG. 450. FROM THE OVARIAN CORTEX OF AN INFANT, SHOWING MANY OVA IN THE PRIMARY FOLLICULAR STAGE. The portion above and to the right is near the free surface; that below and to the left adjoins the medulla. Hematein and eosin. Photo. X 200. follicles, a phenomenon, which marks the climax of the process of ovula- tion. The stroma of the ovarian cortex is a connective tissue structure which contains relatively few elastic fibers and, except near the medulla, very little if any smooth muscle. It is, however, abundantly supplied with connective tissue cells of large size, most of which are ovoid, fusi- form, or even considerably elongated in shape. Many of these cells 510 THE KEPKODUCTIVE SYSTEM closely simulate smooth muscle on superficial examination, but are readily distinguished by careful study, especially if specimens are pre- pared by the various differential staining methods. In the vicinity of the follicles the stroma is specially rich in cellular elements and is otherwise modified to form a concentric coat for each of these bodies. This coat, the theca follicnli, consists of (a) an outer layer, or tunica externa, composed chiefly of connective tissue whose interlacing bundles are concentrically disposed, (b) an inner layer, tunica interna, which is peculiarly rich in large ovoid cells, and (c) an innermost membrana propria, upon which the epithelial cells of the follicle directly rest. At the surface of the ovary the cortical stroma forms a dense layer of fine connective tissue fibers whose delicate bundles interlace in a close-meshed network. This layer, which immediately underlies the germinal epithelium at the surface of the ovariai\ cortex, is known as the tunica albuginea. It differs greatly in thickness in different mam- malian species, in different individuals of the saine species, and even in different portions of the same ovary. Its deeper surface blends in- sensibly with the underlying stroma of the cortex. The general appearance of the ovary varies according to the number, size, and stage of development of its ova and their follicles. At birth the cortex is packed with large numbers of newly formed ova, all of which are in approximately the same stage of development. Their number has been estimated at between 30,000 and 70,000. No new ova are formed after birth. Since in the normal sexual cycle from puberty to the menopause (from about the thirteenth to the forty-fifth year), a period of atyout 32 years, only about 400 eggs are liberated, the vast majority of potential ova must degenerate. This process of degeneration is especially active after the climacteric. During childhood the formation of larger follicles goes forward at an unequal rate, some ova rapidly approaching maturity, others ap- parently remaining almost stationary, and still others undergoing retro- grade development, so that at the age of puberty the ovary contains germ cells and follicles in all stages of development. After puberty the ripe follicles successively rupture and result in the formation of many corpora lutea which promptly degenerate, and are finally replaced by dense connective tissue in the form of small scar-like masses known aa the corpora albicantia. Hence throughout the menstrual epoch the ova- rian cortex contains many corpora lutea and corpora albicantia in addition to ova and follicles in various earlier stages of development. After the THE FEMALE REPRODUCTIVE ORGANS 511 climacteric the remaining follicles degenerate and the process of ovulation gradually ceases. We shall now discuss the structure of the ovum or female germ cell and shall then successively trace its development and maturation, the formation of its vesicular (Graafian) follicle, the rupture of the follicle, and the subsequent history of the corpus luteum. The Ovum. The ovum is a spherical cell of large size (200 to 300 /*). When fully developed it is surrounded by a thick layer of exoplasm, the zona pellucida, which is probably derived from the cytoplasm of the follicular epi- thelium by which the j ovum is closely invested. The ovum itself consists of a mass of cytoplasm, the vitellus, and a large vesicular nucleus or germinal vesicle, within which is frequently a single prominent nucleolus or germinal spot. The cytoplasm of the- mature ovum is inclosed by a very delicate cell membrane, known as the vitelline membrane^w^ch is noT demonstrable in the prim- itive ova of the younger follicles. The CYTOPLASM of the ovum at first appears finely reticular, but as its development advances a fatty material is deposited within its nfeshes, usually in the form of minute irregular spheroids, by the accumulation of which the reticulan cyto- plasm is in great part replaced by a granujpfatty mass .of faint yellowish color known as laeuto- plasm. Frequently this metamorphosis is not quite complete, a remnant of the original cytoplasm persists ij^neath the vitelline membrane and in the vicinity of the nucleus. Numerous cytoplasmic structures have been described in, tjhese cells, chief among which are the accessory nucleus (Nebenkern), and the yolk nucleus (Dotterkern). The accessory nuclei, occasionally chromatinic and therefore basophil, more frequently stain with cytoplasmic dyes and are at times attached, at other times separate from the true nucleus. They are more probably remnants of mitotic spindles. The yolk nuclei of mammalian ova most frequently take the form of crescentic masses of lightly staining chromatin ('chrornidia') which partially surround the nucleus, forming a so-called nuclear cap. They are often found in various stages of disintegration, and the fragments may be transported to the peripheral portions of the cytoplasm, or may be irregularly scat- FIG. 451 .OVUM, CON- TAINING A YOLK NUCLEUS (DOTTER- KERN) AT THE LEFT AND ABOVE THE NU- CLEUS. The peripheral nuclei are derived from the adjacent stroma. Iron- hematoxylin. Highly magnified. (After von Skrobansky.) 512 THE EEPEODUCTIVE SYSTEM tered as small round chromatic granules, which occur throughout the cytoplasm. The physiological interpretation of these bofiies is uncertain. The NUCLEUS of the ovum is a large spheroidal vesicle, the volume and distribution of whose chromatin is subject to great {variation. Chro- matin is present in greatest amount during the period qf most active cell growth, in which the cytoplasm of the ovum is enormously increased in volume. At this time the nucleus often appears as a solid mass of chro- matin. Later the chromatin is diminished in volume, portions of its sub- stance being possibly extruded into the surrounding cytoplasm; the nucleus then acquires a characteristic vesicular appearance. The nuclear membrane is sharply defined and is at most times prominent, except, as in other cells, during mitosis, a process which marks the final maturation of the germ cell. The nuclear matrix or nuclear sap abounds in the vesicular type of nucleus and the chromatin is scattered in small particles which adhere to the inner surface of the nuclear wall or to the delicate achromatic linin threads. Each ovum as a rule contains a single nucleus (germinal vesicle), though occasionally two nuclei occur. The latter condition is presumed to arise either by the fusion of two ova within a single follicle or from incomplete cell division during development. Each nucleus, during its vesicular stage, usual nucleolus (germinal spot), which forms a spherical situated, like the nucleus itself, eccentrically rather staining properties of the nucleoli vary remarkably] contains a single mass of chromatin, lan centrally. The Usually they take the basic (nuclear) dyes to a greater or less depth; occasionally they exhibit an affinity for the acid (cytoplasmic) dyes; Still other nuclei take a metachromatic or irregular tint with the ordinary nuclear stains. Many nuclei even in the absence of mitosis contain no nucleolus. In the development of the ovum from the germinal epithelium, whose cells from their homology with the spermatogoi}ia have been termed oogonia, there occur several mitoses which result in so-called oocytes; these later develop into the mature ovum. At jdbout the time of its extrusion from the ripe follicle a final series of mftoses occur, which dis- tinguish the maturation of the ovum. In this process there is a series of two mitoses which result in the appearance of the polar bodies 'and pro- duce a reduction in the number of chromosomes (to one-half the number which is characteristic of the somatic cells. By the first mitosis the cell produces what may be termed a* daughter ovum, together with the first polar body, a minute cell of insignificant /size. A second mitosis ensues giving origin to the mature ovum or oojtid and the second polar THE FEMALE EEPRODUCTIVE ORGANS 513 body. The egg now contains only one-half the specific number of chromo- somes ; the full specific number is restored at fertilization by the addition of the male reduced complement carried by the sperm. Development of the Ovarian Follicle. The development of the ovarian follicle goes hand in hand with that of the ovum and can be readily followed in ovaries from individuals of different ages, children Str K FIG. 452. FROM A SECTION OF THE OVARIAN CORTEX OF A NEW-BORN KITTEN. K, Pfliiger's tubes; Ke, germinal epithelium; m, mitosis; Str, ovarian stroma; Ub, primitive follicles. Moderately magnified. (After KoUiker.) and adults, the ripe follicles and corpora lutea appearing only after puberty. The process begins in the germinal epithelium in whichjcertam cells_so_increase"m size_tn^t~trrery"may be readily distinguished as future ova. As noted above, it is still an open question whether the primordial germ cells are genetic derivatives of the germinal epithelium or specific sex cells which have wandered intp and become mingled with the cells of the peritoneal epithelium. More (frequently, however, the earliest step in the process consists in the growth of solid cell columns from the layer of germinal epithelium into thi cortical stroma of the ovary. These cell columns are known as Pftugers tubes, though, except occasionally at the extreme surface of the organ, they lack a true tubular form and possess no vestige of a lumen. /Felix, however, concludes that Pfliiger's cords do not occur in the human female. 514 THE EEPKODUCTIVE SYSTEM Certain cells in these columns, by their increased size and prominent nucleus, become very early distinguishable as the primitive ova; their differentiation is rapidly followed by tlu i constriction of the columns, through the activity of the surrounding tissue of the stroma, in such manner that one, rarely two or more ova, and several undifferentiated epithelioid cells are included in each portion whose connection with the FIG. 453. A PRIMARY OVARIAN FOLLICLE OF THE HUMAN OVARY. From within outward are seen the germinal spot, germinal vesicle, vitellus, vitel- line membrane, zona pellucida, granule cell layer, membrana propria, and theca folliculi. The ovarian stroma forms the border of the figure. Hematein and eosin. Photo. X 575. layer of germinal epithelium is thus severed. In this way Jhe_ primitive follicles ('egg nests') are formed. In the ovary of the new-born hun- dreds of such primary follicles occur in all portions of the cortex (Fig. 450). They are also found persistent in large numbers in the ovary of the adult. Many follicles never go beyond this primary stage of development, but after a time undergo retrograde metamorphosis either by gradual atrophy or by a process, known as atresia of 1 the follicle, in which the 1 THE FEMALE KEPRODUCTIVE OEGANS 515 chromatolysis in the ovum and its surrounding follicular cells is followed by growth and organization of the theca folliculi, the connective tissue which is thus formed finally replacing the atretic follicle. FIG. 454. A VESICULAR (GRAAFIAN) FOLLICLE OF THE HUMAN OVARY, SOMEWHAT MORE ADVANCED THAN THE PRECEDING. The accumulation of liquor folliculi has separated the granular cells into a discus proligerus and a membrana granulosa. The membrana propria is very sharp, and the theca folliculi is almost divisible into an inner cellular and an outer fibrous layer. Hematein and eosin. Photo. X 460. After remaining stationary for a long period, often for years, certain of the primitive follicles enter upon a period of rapid growth. This process first affects the ovum and results in the appearance of the deuto- plasm, zona pellucida, and other accessory structures, as already de- scribed. Cell multiplication now occurs in the surrounding epithelial cells, so that, instead of the single row of epithelium which surrounds the ovum of the primitive follicle, the ripening follicle soon acquires 516 THE REPRODUCTIVE SYSTEM a layer of follicular epithelium several cells deep. This may be desig- nated the mantle-layer. The rapid ImiTtfiplication of the epithelial cells is soon followed by ative secretion, resulting in the formation of a clear fluid by which the cells are more and more separated, and the cytoplasm of adjacent a FIG. 455. A NEARLY RIPE GRAAFIAN FOLLICLE FROM THE OVARY OF A DOG. a, fibrous, and 6, cellular layer of the theca folliculi; c, membrana propria; d, mem- brana granulosa, which, as a result of contraction during hardening, has retracted from the membrana propria, leaving a broad artificial space; E, liquor folliculi; f, discus proligerus; g, zona pellucida; h, vitelline membrane; i, vitellus; k, the ger- minal spot, lying within the germinal vesicle. Hematein and eosin. Photo. X 150. cells is then readily seen to be firmly joined together by numerous deli- cate processes which maybe regarded as intercellular bridges. Similar processes unite the neighboring cells to the zona pellucida which has already formed about the ovum. The accumulation of the fluid liquor folliculi within the follicle soon appears to tear apart certain of the epithelial cells, and a fluid-filled space, THE FEMALE REPEODUCTIVE ORGANS 517 the atitrum follicuU, is thus formed. Such a follicle is known as a vesicu- lar OT_Graafian follicle. Follicles intermediate between the primary folli- cles with a single or double layer of mantle cells and vesicular follicles may for convenience be called intermediate or growing follicles. The epithelial cells are separated by the antrum into two layers: the one, FIG. 456. PHOTOMICROGRAPH OF A SECTION OF CAT'S OVARY, SHOWING Two PRI- MARY FOLLICLES AND ONE VESICULAR FOLLICLE. adherent to the membrana propria of the follicle, is known as the mem- brana or stratum granulosum; the other, adherent to the zona pellucida of the ovum, is designated the discus proligerus. The two layers remain in contact at one point, and as the liquor folliculi increases in volume, the attached discus proligerus with its contained ovum comes to occupy a more and more eccentric position, and the cells of the stratum granulo- sum, where the two layers are in contact, appear to pile up about the 518 THE KEPEODUCTIVE SYSTEM ovum in the form of a hillock, the so-called cumulus oopliorus. The latter term is now generally used exclusively in a sense to include and displace the term discus proligerus. The cells of the cumulus, which adjoin the zona pellucida, become somewhat elongated and in this way they form a radiate investment con- sisting of one or two rows of columnar cells which surround the zona pellucida of the ovum and are known! as the corona radiata. With the increase of the liquor folliculi the cupiulus with its contained ovum is soon separated from its attachment t the stratum granulosum and the development of the folliculi is complete. During this period of rapid growth and development the follicle has increased in size from a diameter which scarcely exceeded that of its ovum (about 300 JA) to such a size that it occupies the entire breadth of the ovarian cortex. It is now ready for the final steps in the maturation of its ovum and for the rupture of the follicle coincident with the ap- proach of the menstrual period. The forces which lead to the rupture of the follicle are not fully determined. They are undoubtedly varied, and, in addition to the gradual attentuation of the layer of cortical stroma which covers the free surface of the follicle and is known as the stigma, they include the gradual accumulation of liquor folliculi under increasing tension, the marked congestion of the ovary at the approach of the menstrual period, which is accompanied by the determination of an undue proportion of blood to the theca of the ripe follicle (Clark), and possibly the contrac- tion of the smooth muscle contained in the stroma of the deeper part of the cortex and adjacent portions of the medulla. In any event, as a result of the independent or combined action of these, or other unknown forces, the follicle ruptures in the direction of least resistance, viz., at the attenuated stigma, and the liquor folliculi gushes forth, carrying with it the detached ovum invested with its cumulus. The ovum is now free to enter the oviduct and prepare itself for fertilization and the development of the future embryo. The ovum finds its way from the abdominal cavity into the oviduct by aid of currents established in the direction of the orifice by the cilia of the cells covering the fimbriae ; perhaps assisted by a direct grasping activity on the part of the fimbriae. Since spermatozoa are free to wan- der out of the oviduct into the abdominal cavity, and since an egg may occasionally fail to reach the opening of the oviduct, the possibilities are offered for an abdominal pregnancy, a result occasionally consummated. Likewise an egg may fail to become expelled from its follicle and may THE FEMALE REPRODUCTIVE ORGANS 519 then become fertilized within the ovary; or an egg fertilized in the ab- dominal cavity may perhaps subsequently become implanted in an empty follicle. Development proceeding under such conditions results in an ovarian pregnancy, at least seven well authenticated cases being now on record (Bryce, Kerr and Teacher, "An Early Ovarian Pregnancy/' 1908). Ovarian pregnancies cannot proceed normally, and may early call for surgical intervention. Occasionally a follicle may contain more than one ovum, bi- and triovular follicles being common. Such follicles apparently present the possibilities for ordinary twinning and multiple births. Arnold (Anat. Rec., 6, 11, 1912) records follicles in the ovary of a negress also with from four to ten oocytes, and one each with eleven and thirteen oocytes. The following table is offered for the benefit of the student as a resume of the several structural layers of the ripe Graafian follicle. The structures are enumerated in order from without inward : f tunica externa 1. Theca folliculi J tunica interna [membrana propria. 2. Stratum (seu membrana) granulosum. 3. Liquor folliculi occupying the antrum folliculi. 4. Discus proligerus cumulus oophorus. 5. Corona radiata. 6. Zona pellucida (seu striata). 7. Perivitelline space (possibly an artifact). 8. Vitelline membrane. 9. Vitellus egg cytoplasm. 10. Nucleus or germinal vesicle. 11. Nucleolus or germinal spot. The Corpus Luteum. The rupture of the follicle is accompanied by sudden relief of the intrafollicular tension and consequent hemorrhage from the thin-walled capillaries of the theca folliculi. Thus the cavity of the follicle is filled with blood; the ruptured follicle is then known as a corpus hemorrhagicum. This is the first stage in the formation of the corpus luteum. Promptly succeeding the formation of the corpus hemorrhagicum,, lutein cells appear at the periphery of the body. They are large, ovoid or polyhedral cells having a clear finely granular cytoplasm and a peculiar yellow color due to the presence of a pigment known as lutein. More- over, the cytoplasm of the lutein cells becomes very rapidly infiltrated with droplets of fat, likewise deeply colored by the lutein pigment which 33 520 THE KEPEODUCTIVE SYST.EM is apparently held in solution. The origin of these cells is still a matter of controversy. By certain observers they have been thought to result from the growth and multiplication of those cells of the stratum granu- losum which remain after the rupture of the follicle (Bischoff, Pfluger, Sobotta) ; by others they are derived from the connective tissue cells in the tunica interna of the theca folliculi (Kolliker, His, Palladino). Teacher (1908) interprets his preparations to 'indicate quite clearly that, FIG. 457. SECTION THROUGH THE PERIPHERAL PORTION OF A CORPUS LUTEUM, SHOWING LUTEIN CELLS. o, the fibrous coat of the corpus luteum; 6, lutein cells with bands of newly formed connective tissue; c, central blood clot, partially organized. Moderately magnified. (After Williams.) whatever the source of the cells may be in lower animals, they do not in man arise from the membrana granulosa/ Lutein cells may multiply by mitosis. The lutein cells increase rapidly both in number and in size, and gradually encroach upon the margin of the blood clot whose progressive absorption precedes the advance of the lutein cells. But not only does the lutein mass grow centralward, it also, and especially in the event of fertilization of the discharged ovum with the consequently increased THE FEMALE REPRODUCTIVE ORGANS 521 vascularity of the reproductive organs, grows at the periphery and in this way greatly increases the diameter of the corpus luteum. Minute vascular sprouts of embryonic connective tissue now pene- trate the lutein mass from the adjacent stroma of the theca folliculi, and growing centralward in septa-like processes,, finally penetrate as far as the central blood clot. Hence the corpus luteum at this stage presents a more or less radiate structure. The central ends of the embryonic connective tissue septa frequently unite to inclose the remnant of the central blood clot, or by further proliferation they may entirely re- place the clot by a mass of newly formed gelatinous connective tissue. The absorption of the blood clot usually proceeds slowly. Eemnants of the disintegrating blood in the form of a central stellate mass, which often contains hematoidin crystals, frequently persist until the corpus luteum has become well or- ganized with connective tissue. The formation of new connec- tive tissue is followed by its con- traction. That this process occurs very early in the connective tissue first formed at the periphery of the body, may possibly be held to ac- count for the fatty infiltration and final degeneration of the lutein cells, because of the consequent interference with their vascular supply. By continued development the entire mass of lutein cells is gradually replaced by connective tissue, which, by further contraction, finally pro- duces a dense white fibrous scar, no longer containing lutein pigment, known as a corpus albicans. This body persists for a long period, but undergoes progressive contraction until only a minute scar of almost microscopical size remains to mark the site of the ruptured corpuscle and the highly developed corpus luteum. Such scars persist for years in the stroma of the ovarian cortex. Finally it must be stated that there are no recognizable histological differences, other than those of size and duration, between the corpora lutea vera of pregnancy and the corpora lutea spuria whose formation FIG. 458. PORTION OF CORPUS LUTE- UM OF RABBIT. The lutein cells contain peripherally many fat vacuoles. The stroma con- sists of fibrous connective tissue. X 500. 522 THE EEPKODUCTIVE SYSTEM accompanies the extrusion of the* unfertilized ovum. The true corpora lutea are of relatively large size and persist for many months, the spurious are somewhat smaller and persist for little more than one month; yet both pass through the same histological process of develop- ment and degeneration and both leave their scars in the substances of the ovarian stroma. Ovarian scars also arise through atresia of the larger follicles, the degeneration of whose epithelium is followed by an ingrowth of tissue derived from the theca folliculi, and the gradual development, organiza- .*/* ' ' / /> ' * ' i ' VHiC r ' T fcVu i a FIG. 459. A CORPUS ALBICANS, FROM A SECTION OF THE HUMAN OVARY. X 75. (After Williams.) tion, and final contraction of the connective tissue, forming, as it were, a minute but imperfect corpus albicans, in the center of which is often the shrunken degenerating remains of the ovum. Certain small acido- philic homogeneous bodies, the so-called Call-Exner bodies, of uncertain significance are also occasionally present in the stratum granulosa of the ovarian follicles. Blood Supply. The blood-vessels of the ovary are derived from the branches of the ovarian and uterine arteries. These vessels enter the ovary through the mesovarium and divide into numerous branches which pursue a peculiar spiral or corkscrew course through the stroma of the medulla, and finally enter the cortex. They possess thick muscular walls containing bundles of longitudinal smooth muscle fibers. In the cortex they supply capillaries to the stroma, and in the theca folliculi of the Graafian follicles they form rich plexuses of broad capillaries and THE FEMALE REPRODUCTIVE ORGANS 523 thin-walled venules. As the follicle ap- proaches maturity these plexuses be- come enormously de- veloped and appar- ently bear an impor- tant relation to the rupture of the fol- licle and the rapid development of the corpus luteum. The veins, which take origin from the ven- ules of these capil- lary plexuses, con- verge toward the medulla, where they form a plexus of large thin-walled vessels, the plexus venosus ovarii or pampintform plexus, which is embedded in the connective tissue of the medul- la, the mesovarium, and the adjacent portions of the broad ligament. FIG. 460. FROM A THICK SECTION OF THE OVARY OF A WOMAN. The blood-vessels have been in- jected. A, a, and a', arteries; b, cor- pus luteum, partially organized; c, point where rupture of the follicle occurred; d, tangential section of a follicle; e, corpora lutea which have organized and are already retrogres- sive. (After Clark.) FIG. 461. TRANSECTIONS OF THE HUMAN OVIDUCT. A, uterine; B, isthmic; and (7, ampullar portions. X 15. (After Williams.) 524 THE FEMALE REPRODUCTIVE ORGANS 525 The lymphatics arise in the cortical stroma by anastomosing canals and capillaries of irregular caliber, which are especially abundant in the walls of the Graafian follicles. These vessels converge toward the medulla,, where they enter lymphatics which are supplied with valves, and find their way to the lymph nodes of the pelvic and lumbar regions. The nerves are chiefly derived from the ovarian sympathetic plexus. They enter the hilum and are distributed to the walls of the blood- vessels, and to the stroma of the ovary; here they form a rich terminal plexus in the walls of the follicles. Whether or not the naked fibrils are distributed to the epithelial cells within the follicle has not been satis- factorily determined. The small ganglia of the medulla include also pheochrome cells (Winiwarter) ; and certain sensory fibers are said to end in lamellar corpuscles. The Oviduct The oviduct, uterine tube, or Fallopian tube is a narrow duct about 4V2 inches long, leading from the ovary to the cavity of the uterus. It consists of a broad, funnel - shaped, fringed or fimbri- ated extremity (or infundibulum), a constricted neck, an intermediate ampulla of consid- erable diameter, and a slender isth- mus by which the oviduct communi- cates with the ute- rine cavity. Throughout the entire tube its wall, which becomes gradually thinner from isthmus to infundibulum, consists of three coats mucous, mus- cular and serous but the character of its mucous membrane differs FIG. 462. FROM A TRANSECTION OF THE AMPULLA OF THE OVIDUCT, SHOWING THE STRUCTURE OF THE MUCOSA. "X 280. (After Williams.) 526 THE EEPEODUCTIVE SYSTEM somewhat in its several portions. In the isthmus it is relatively smooth and usually presents four longitudinal ridges which have few secondary or accessory folds ; in the ampulla the mucosa is greatly folded, the pri- mary ruga? possessing small secondary folds which extend in all direc- tions, and by their very complexity nearly obliterate the otherwise broad lumen. In the fimbriated portion the folds of the mucosa are continued into the fimbrias, at the margin of which the columnar ciliated epithe- lium of the oviduct becomes directly continuous with the serous mesothe- lium of the peritoneum investing the outer, surface of the tube. The mucosa is lined by columnar epithelium, arranged either in a simple or pseudo-stratified manner, the greater portion of whose cells are provided with cilia. The ciliary motion is directed toward the uterus. The epithelial layer covers all the folds of the mucosa and, extending deeply into the crevices, forms invaginations which, in transections of the tube, simulate glandular structures. There are, however, no true secreting glands in the oviduct. Here and there groups of non-ciliated cells with clear cytoplasm occur among the more numerous ciliated cells of the mucosa. The epithelium rests upon a thin homogeneous basement membrane beneath which is a tunica propria consisting of a cellular type of connec- tive tissue. Many of the connective tissue cells are of fusiform shape, and, unless specially stained or carefully examined, they closely resemble smooth muscle cells. The mucosa, however, contains no muscle except at the bases of the largest folds, into which occasional fibers from the adja- cent muscular coat penetrate. The muscular wall of the oviduct is formed .by two layers of smooth muscle a broad inner circular layer, and an outer longitudinal coat, which is very unequally developed at different portions of the cir- cumference, but is relatively thin in all parts, and is entirely wanting at frequent intervals. The outer layer is usually broadest at the free mar- gin of the oviduct and at its opposite side where the tube is attached to the broad ligament. The inner circular fibers are more or less obliquely disposed, and, toward the mucosa, the muscular bundles fuse insensibly with the cellular connective tissue of the mucous membrane. In gen- eral the inner circular layer is thickest at the isthmus and thinnest at the infundibulum, while the longitudinal layer is thickest toward the fimbriated end. The serous coat of the oviduct is continuous with the peritoneum. It consists of an outermost layer of mesothelium which rests upon a sub- epithelial layer of connective tissue, by which it is firmly united to the THE FEMALE EEPRODUCTIVE OEGANS 527 muscular wall. This portion of the serous coat contains the larger vessels and nerves, which are distributed to the inner coats. FIG. 463. TRANSECTION OF THE UTERUS OF AN APE. a, mucosa; 6, circular muscle; c, longitudinal muscle; d, serous coat; e t lateral ligament; /, Wolffian tube; g, blood-vessels. X 4. (After Sobotta.) Blood Supply. The arteries of the oviduct are derived from uterine and ovarian vessels. The larger divisions find their way through the connective tissue of the serosa whence they send smaller branches inward to form a plexus between the layers of the muscular wall and among the bundles, of. circular muscle fibers. Eronxthis plexus capillaries 528 THE EEPEODUCTIVE SYSTEM are distributed to the muscular coat, and to the mucous membrane in which they form a rich subepithelial capillary plexus. The veins follow a similar course, and like the arteries, form an extensive plexus in the muscular coat. The abundance of vessels in the muscular wall of the oviduct has led to the description of this coat as the vascular layer of the organ. The lymphatics arise by anastomosing plexuses in the mucosa, from which vessels pass to the serous coat and enter valved lymphatics by which the lymph is conveyed to the lymph nodes of the lumbar region. The nerves, which are chiefly sympathetic, are distributed from a plexus in the serous coat, to the muscular wall, and to the mucosa, in which they form a terminal subepithelial plexus. The Uterus The uterus is a hollow pear-shaped organ, divisible into a deepest or fundus portion, the body, and the cervix ~, about 3 inches in depth, 1% in width and 1 in thickness. The cervix opens into the body through the internal os, into the vagina through the external os. Its wall consists of a mucous membrane, a muscular coat, and an outermost serous coat which is derived from the peritoneum and invests the body of the organ. The cervix uteri projects into the vaginal canal and the serous coat is there replaced by a reflection of the vaginal mucosa. The serous coat, or perimetrium, of the uterus consists of meso- thelium which rests upon a thin subepithelial layer of connective tissue. It presents no peculiarities. The muscular coat, or myometrium, of the uterus consists of smooth muscle whose fibers are of large size (40 to 60 ju. in length) and which are disposed in interlacing bundles. In the lower mammals these form quite regular layers an outer longitudinal, a thick inner layer, most of whose fibers are circular, and an innermost, but less distinct, submucous portion containing oblique and longitudinal fibers. The outer longitudinal and circular layers are separated by a fibromuscular stratum containing a rich plexus of large blood-vessels. In the human uterus the arrangement of muscle fibers is much less regular, but follows a similar plan, though there is no distinct sub- division into layers. Nevertheless, careful examination reveals three indis- tinct strata which are intimately blended with one another. The outer- most of these indistinct layers consists of irregularly disposed longitudi- nal fibers, the stratum supravasculare. This layer is in most parts very THE FEMALE EEPEODUCTIVE OEGANS 529 thin, and is best developed opposite the margin of the lateral ligament and in the cervix uteri. Within this is a broad layer of interlacing bundles of more or less circular fibers, which, from the slight obliquity of their course, frequently cross each other at acute angles. Inter- mingled with these circular bundles are many large blood-vessels, from which both the mucous and muscular coats are supplied. This broad middle layer is therefore known as the stratum vasculare. The inner FIG. 464. TRANSECTION THROUGH THE BODY OF THE HUMAN UTERUS. g, blood-vessels; I, lumen; II, broad ligament; Im, longitudinal muscle; ra, circular muscle (the fibers are mostly oblique); s, serous coat; tp, mucosa. Hematoxylin and eosin. X 2. (After Sobotta.) portion of this second layer passes insensibly into a thin innermost stratum submucosum, which again contains many longitudinal fibers, and upon which the mucosa directly rests. The uterine mucosa, or endometrium, is of considerable thickness (1 to 3 mm.). It is clothed with epithelium, and its tunica propria con- tains numerous tubular glands. The epithelium is of the ciliated columnar type, and consists of a single row of cells. Apparently not all of its cells are provided with cilia, areas of ciliated alternating with groups of non-ciliated epithelium. The epithelial layer is continuous with the epithelium of the uterine glands ; in the region of the external os uteri it is replaced by the strati- fied squamous epithelium of the vaginal mucosa. Ofttimes, and espe- 530 THE KEPEODUCTIVE SYSTEM cially in multipart, the stratified squamous epithelium of the vagina is continued for some little distance within the canal of the cervix uteri; it never clothes more than the lower one-half to two-thirds of the cervical canal. The current resulting from the vibration of the intra-uterine cilia is directed toward the vagina. The tunica propria of the mucosa consists of a peculiar embryonal type of connective tissue, similar to that of the oviducts, which contains very few collagenous and no elastic fibers, but which is richly supplied, FIG. 465. FROM A TRANSECTION OF THE UTERINE MUCOSA. X 16. (After Williams.) in fact is literally packed, with cellular elements. These cells are ovoid or fusiform in shape, and many of them are branched ; their nuclei, also, are ovoid and somewhat vesicular. Many lymphocytes are found in the tunica propria, but these mostly occur in the vicinity of the lymphatics and smaller blood-vessels with which the uterine mucosa is abundantly supplied. In the mucosa of the cervix uteri the development of the con- nective tissue appears to be more advanced, the cellular elements being relatively fewer; it also contains many fine fibers which appear to form a delicate network, At the external os uteri the tunica propria is con- THE FEMALE REPRODUCTIVE ORGANS 531 tinuous with the similar, though still more fibrous., layer of the vaginal mucosa. The uterine glands are divisible into two types those of the body of the organ, and those of its cervix. The former are, perhaps, to be re- garded as tubular imaginations of the lining epithelium, whose function FIG. 466. FROM THE CERVIX UTERI OF A GIRL OF SIXTEEN YEARS, SHOWING THE CERVICAL GLANDS IN SECTION. a-a, lining epithelium. Hematein and eosin. Photo. X 102. is one of epithelial regeneration rather than of glandular secretion. The tubules of the cervix uteri are true mucus-secreting glands. The uterine glands proper, those of the body of the organ, are slightly branched or forked tubules which traverse the entire breadth of the mucosa, presenting a characteristic spiral or corkscrew course; their blind extremities are often bent or turned to one side, apparently from the proximity of the adjacent muscular coat. The glandular epithelium 532 THE EEPEODUCTIVE SYSTEM is of the columnar type and, like that of the free surface, is frequently provided with cilia, especially near the mouth of the gland. The epithe- lium rests directly upon the connective tissue of the tunica propria. The cervical glands (glandulce uterince cervicales) resemble those of the body of the organ in their tubular form and the columnar shape of their epithelium, but here the re- semblance ceases. The cervical glands are much branched, and their tubules present frequent dilatations, some of which, apparently from oc- clusion of their outlet, attain a macroscopic size, and are then known asNabothian follicles (ovula Nabothii) ; they are filled with a tenacious mucous secretion. The glandular epithelium near the crypt- like ducts is usually ciliated, like that of the surface, but in the secreting portions it consists of tall, clear, columnar cells which are in Jf^'lt^^te . v ^^% ( ' '\> '**/** various stages of secretory activity, ^v^^^/^^l^?^' their P roduct bein S a viscid S lail T \''Vl$^A^ mucus, strings and granules of ' A'ft//$['j^ '"*#%+ * : %K^\ '\? which are found within the lumen i * { , ,''/'"- ;^&fe; i'J T% v;,^t:L- .. ... _ of the glands, as well as within the canal of the cervix uteri. The uterine cavity is a relative term. In the virgin, the mucosa is considerably folded and its surfaces are almost in apposition, being only separated by a very limited amount of desquamated epithelium and cel- lular debris, to which, in the canal of the cervix uteri, the mucous secretion is added. During pregnancy, the development of the fetus within the uterine cavity distends its walls and so dilates the canal that it at last forms a sac of sufficient size to contain the fetus, which floats within the amniotic fluid inclosed by its membranes. The blood-vessels of the uterus enter through the folds of the lateral ligament and find their way, through the subepithelial connective tissue FIG. 467. A. GLAND OP THE HUMAN CERVIX UTERI IN LONGITUDINAL SECTION. X 90. (After Williams.) THE FEMALE REPRODUCTIVE ORGANS 533 of the serous coat and the muscular wall, to all portions of the organ. In the vascular layer of the muscular coat they form an extensive ^plexus from which branches are distributed to the musculature and to the mu- cosa, the branches to the latter penetrating nearly to the surface, where they form rich, subepithelial, capillary and venous plexuses. The uterine arteries, like those of the ovary, possess a peculiar, spirally tortuous course. The veins accompany the arteries, but are less tortuous. The lymphatics of the uterus arise by anastomosing channels in the mucous and muscular coats. They form a vascular plexus in the serous coat and lead outward, through the lateral ligaments and pelvic connec- tive tissue, to the lower lymph nodes of the lumbar region. The nerves of the uterus are very numerous. They include both spinal and sympathetic fibers. They enter the serous coat from the ganglionic pelvic plexus, and are distributed to the vascular layer of the muscular coat. They there form a rich plexus, from which sympa- thetic motor fibers are distributed to the musculature and to the walls of the blood-vessels. The distribution of nerves within the mucosa has not yet been thoroughly worked out. According to von Gawronsky (Arch. f. Gynakol., 1894) and Kostlin (Fortschr. d. Med., 1894) sensory nerve fibrils pene- trate nearly to the surface and form a scanty subepithelial plexus, whence are derived fibrils which terminate between the epithelial cells. Since the uterus is subject to extensive structural variations dependent upon its functional phase and condition, it becomes important to recog- nize the differential marks of the menstruating and of the pregnant uterus. Besides general histologic alterations in the wall, especially in the mucous and muscular layers, additional structures involved in the pregnant uterus are the decidual cells and chorionic villi; these are diagnostic of pregnancy. Only the histology of these structures will be here described ; for a consideration of their embryologic significance and relationship reference must be made to a text-book of Embryology. The Menstruating Uterus The appearance of the phenomena of menstruation is accompanied by decided alterations in the structure of the uterine mucosa. In spite of the difficulty of obtaining sufficiently fresh and well preserved material, certain changes which characterize the menstruating uterus are now definitely known. These chiefly consist in increased vascularity, hypertrophy of the elementary tissues of the mucosa, epithelial desquamation, and rupture of the blood-vessels, with consequent hemorrhages. These changes are fol- 534 THE REPRODUCTIVE SYSTEM lowed by a process of regression and later of regeneration, by which the uterine mucosa rapidly returns to its former condition. The first or hypertrophic stage involves the epithelium, whose cells are elongated, and the tunica propria, in which many of the connective tissue cells undergo multiplication and enlargement. Thus the mucous mem- brane becomes greatly thickened; its glands, also, are increased in both length and breadth, becoming at the same time even more tortuous than before. The glandular hypertrophy involves both the uterine and the cervical glands; the secretion of the latter is much increased. FIG. 468. FROM A. SECTION OF THE HUMAN UTERINE MUCOSA AT THE FIRST DAY OF MENSTRUATION. e, epithelium; d, disintegrating layer; g, gland; v, blood-vessel; m, muscular coat. X 44. (After Minot.) At the same time, the blood-vessels become widely dilated, especially those near the surface, and broad thin-walled sinuses are formed beneath the epithelium. Finally these vessels rupture and hemorrhages occur into the substance of the mucosa as well as into the cavity of the organ; des- quamation and disintegration of the superficial portions of the mucosa result. The menses which are thus formed contain blood, epithelium, con- nective tissue cells, and many leukocytes, which wander out from the blood- vessels of the mucosa in large numbers. The greatly thickened' and hemor- rhagic mucosa is known as the decidua menstrualis. Regression and regeneration follow rapidly upon one another, the mucosa gradually regaining its former condition. During this process fat droplets appear in many of the connective tissue cells. The epithelium is rapidly regenerated, the new cells arising from the epithelial remnants at THE FEMALE REPRODUCTIVE ORGANS 535 the mouths of the uterine glands. In the course of a few days the mucosa regains its former quiescent condition. The complete cycle includes 28 days, of which the menstrual process occupies about seven. The Gravid Uterus In the event of conception the uterine changes are more pronounced than during menstruation. These alterations include the same processes of hypertrophy and thickening as occur in the decidua menstrualis; they in- volve the musculature as well as the mucosa but are not followed by regres- sive changes, hemorrhage, desquamation, etc. until parturition occurs. The muscular wall undergoes an enormous increase both in the num- ber and size of its fibers. The rela- tively short (30 to 60 /,) smooth muscle fibers of the uterine wall gradually increase in size to as much as eleven times their former length and two to five times their breadth (Kolliker). The connective tissue of the muscular coat also increases in volume and becomes more distinctly fibrous. After parturition, fat drop- lets appear within the muscle cells, and the muscular wall by gradual atrophy returns to its former condi- tion. In the mucosa the formation of a decidual membrane goes forward in a manner similar to the develop- ment of the decidua menstrualis, but the process is exaggerated. The tu- nica propria soon becomes divisible into two distinct, though not sharply defined, layers, a deeper cavernous portion which is permeated by broad vascular channels together with the atrophied remains of the uterine glands, and a superficial compact layer in which the vascular channels, except for the thin-walled venous spaces, are smaller and the connective tissue cells more closely packed. Many of the connective tissue cells attain a large size and their nuclei are frequently multiple, or they may acquire an irregular polymorphonu- clear form. Giant cells are thus produced in the compact layer of the mucosa of the gravid uterus; they are highly characteristic of this tissue and are known as decidual cells. Though it is frequently asserted that 34 FIG. 469. A GROUP OP DECIDUAL CELLS FROM THE HUMAN UTERUS DURING THE EARLY STAGES OF PREG- NANCY. One of the cells contains two nuclei and a number of fat vacuoles. Three nuclei of the connective tissue stroma are also shown. X 750. 536 THE REPRODUCTIVE SYSTEM similar cells occur in the decidua menstrualis, this is denied by Minot (1903), who states that in a considerable number of menstrual decidua examined, no such cells were ever found. The superficial epithelium is soon desquamated and the tunica propria comes into contact with the fetal chorion. The glandular epithelium is also partially degenerated, often becoming flattened and of irregular shape. It is frequently desquamated into the glandular lumen; this lumen is thus FIG. 470. CHORIONIC VILLI FROM THE HUMAN PLACENTA AT FULL TERM. Hematein and eosin. Photo. X 114. reduced to a narrow crevice, which is so elongated during the dilatation of the uterine wall that the axis of the glandular remnant becomes nearly parallel to the surface of the decidua. The decidual membrane which is thus formed is divisible into three portions, according to its relation to the tissues of the embryo: 1, that portion upon which the developing ovum directly rests, which is known as the decidua serotina or decidua basalis but later forms the placenta uterina or maternal portion of the placenta; 2, at the margins of the THE FEMALE REPRODUCTIVE ORGANS 537 implanted ovum the decidual tissues close up over the ovum which is thus surrounded by the so-called decidua reflexa or decidua capsnlavis, which, after the early months of pregnancy, is gradually obliterated by the increasing growth of the fetus, and is finally replaced, its functions being progressively usurped by the newly formed placental tissues; 3, all the remaining portions of the decidual mucosa, those which line the greater part of the uterine cavity, collectively form the decidua vera, with whose surface, in the later months of pregnancy, the fetal chorion is intimate in relation. The mucosa of the cervix uteri meanwhile becomes greatly hyper- A B C FIG. 471. CHORIONIC VILLUS AT VARIOUS STAGES OF DEVELOPMENT. A, chorionic villus at third week; B, at fourth month; C, at term. (After Wil- liams.) X 225. trophied and its glands much enlarged. This portion of the uterine mucosa does not, however, enter into the formation of the decidua vera; the changes occurring in its tissues, though similar, are much less pro- nounced. The Chorionic Villi. These innumerable processes form the greater portion of the placental tissues. They vary in size from the broad main stems to the very slender terminal branches of the floating villi. They consist of a core of mesoderm covered with a variable layer of ectoderm. In the early condition of the placenta (fourth or fifth month of pregnancy) the villi are clothed with a double epithelial layer, of which the superficial takes the form of a syncytium (plasmoditrophoblast), while the deeper consists of a cellular layer, the cells of Langhans (cytotrophoblast). At later periods (seventh month to full term) the syncytium is found to have undergone a peculiar alteration, having become much thinner, and having even completely disappeared from considerable portions of the villi, it being replaced by canalized fibrin; at other points the syncytial cytoplasm is much thickened and the nuclei appear to be bunched or grouped within the thickened portions; these areas are known as cell-knots or proliferation islands. Here and there the degenerated cell-knots have been replaced by 538 THE REPRODUCTIVE SYSTEM canalized fibrin. Wherever the main stems are inserted into the decidua the epithelium which formerly covered their tips appears to have also degenerated into a peculiar hyalin border zone. Towards the end of preg- nancy the cytotrophoblast becomes converted into plasmoditrophoblast. Within its syncytium the substance of the villus consists of the super- ficial cells of Langhans with their large ovoid nuclei, and a core of con- nective tissue of a delicate embryonic type, in which are the fetal blood- vessels. Even the smallest villi contain capillary loops of broad caliber, which are supplied by fetal arteries, derived from the umbilical arteries, which distribute their branches throughout the chorionic connective tissue. The fetal veins accompany the arteries. The Vagina The vagina is a fibromuscular sheath whose wall is divisible into three coats mucous, muscular, and fibrous. The mucous membrane is clothed by a layer of stratified squamous epithelium, and is thrown into numerous folds or rugae. The epithelium FIG. 472. VAGINAL MUCOSA. X 90. ep., epithelium; p., papilla; c.t., connective tissue. (After Williams.) rests upon a fibrous basement membrane. The tunica propria is formed by a close-meshed areolar tissue which, in its deeper and looser portion, is permeated by vascular channels of considerable size. This deep vascular layer is frequently described as a submucosa; it rests directly upon the muscular wall. The surface of the mucosa presents numerous conical papillae which project well into the epithelial layer. The musculature of the vagina contains smooth or involuntary fibers, and is divisible into an inner circular and an outer longitudinal layer. The muscle fibers are long and slender. Considerable connective THE FEMALE EEPEODUCTIVE OEGANS 539 tissue is distributed among the muscle bundles. The latter are arranged in more or less parallel layers which are united by the delicate bands of connective tissue. The outer fibrous coat consists of dense areolar tissue which is well supplied with elastic fibers. It loosely unites the vaginal wall to the surrounding tissues. In this coat is a plexus of blood-vessels and lymphatics, from which branches pass to the muscular coat, and to the mucosa, in which they form an abundant plexus. An extensive nerve plexus, including spinal and sympathetic fibers, among which are many small ganglia, is also found in the fibrous coat; it distributes motor branches to the muscular wall and to the blood-vessels, and sensory fibers to the mucosa, in which they end in relation with the cells of the lining epithelium. The vaginal mucosa is reflected upon the outer wall of the cervix uteri, and at or near the external os it is continuous with the mucosa of the uterine cavity. Though occasional glands have been found in the vaginal mucous membrane, lined either by mucus-secreting or by ciliated cells, these glands would seem to be properly considered as anomalies, since they are usually absent, the mucoid secretions of the vaginal canal being chiefly provided by the abundant supply of mucus from the cervical glands of the uterus. The vaginal mucosa is continuous below with that of the vestibule. The Vestigial Structures The vestigial structures associated with the female reproductive system include the vesicular appendage (hydatid of Morgagni), the epoophoron, and the paroophoron. The same general statements made concerning the male vestigial structures hold likewise for those of the female. The VESICULAR APPENDAGE is attached to the fimbriated end of the ovi- duct, its stalk being continuous with the collecting duct of the epoophoron. It is a globular pedunculated structure of small size (three to six milli- meters diameter) ; it is lined with cuboidal epithelium, and may contain fluid. It represents the atrophic end of the degenerated Wolffian duct. There are besides a variable number of smaller accessory vesicular appen- dages attached to the broad ligament. The EPOOPHORON (parovarium ; organ of Rosenmiiller) lies between the layers of the broad ligament in the triangular area between the ovary and the ampulla of the oviduct. It consists of a variable number of tubules (eighteen to twenty), the homologues of the ductuli efferentes of the male. These tubules may be blind at only one or at both ends ; they may be lined with ciliated columnar epithelium, or their lumina may become obliterated. 540 THE REPKODUCTIVE SYSTEM Those blind only at one end connect with a longitudinal duct, a variable remnant of the Wolffian duct. The PAROOPHORON homologue of the male paradidymis is a more mesial collection of similar tubules of like structure and genetic signifi- cance. It is said to be present only in infants. The further extension mesially of the longitudinal duct, either in continuation with the collecting duct of the epoophoron or paroophoron, or as an isolated blind duct, usually in the wall of the uterus and vagina, is known as the canal of Gartner. It is the vestige of the lower portion of the fetal Wolffian duct. THE EXTERNAL GENITALS The vestibule is supplied with a mucosa which offers a gradual tran- sition from the vagina, on the one hand., to the skin on the other. Its stratified squamous epithe- lium becomes in this way gradually more and more like that of the skin, eleidin granules first, and keratin later appearing on the outer surface of the labia minora. The epithelium of the labia majora is identical with that of the skin. The labia minora or nymphae form the lateral walls of the vestibule and consist of a fold of the mu- cosa which is provided with exceptionally tall papillae. Small sebaceous glands open directly urjon the sur- face of the stratified squam- ous epithelium. There are no hair follicles in relation with these glands, and the labia minora contain no adipose tissue. They are richly supplied with blood-vessels, and with sensory nerve endings. The labia majora are formed by similar folds whose inner surface resembles the adjacent portion of the labia minora, but whose outer FIG. 473. TRANSECTION OP A LABIUM MINUS OP AN INFANT. a, labium minus; b, border of the labium majus; c, adipose tissue of the latter. Hematein and eosin. Photo. X 12. THE FEMALE BEPKODUCTIVE OEGANS 541 surface is cutaneous and is supplied with sebaceous and sudoriparous glands and with numerous hair follicles. The subepithelial~areolar tissue is very dense and its deeper portion contains much fat. The clitoris consists of a mass of erectile tissue, homologous with the corpora cavernosa and glans penis of the male; it is covered by a fold of the mucosa. It is well supplied with nerves, which terminate in tactile corpuscles, end bulbs, and genital corpuscles. In this vicinity also, as well as in the region of the labia, Pacinian corpuscles are occa- sionally found. The hymen is formed by a reduplication of the vestibular mucosa. Its inner surface is similar to that of the labia minora and vagina; its outer is like that of the cutaneous surface, except that it contains no hair follicles. The glandulae vestibulares minores are a group of small mucus- secreting glands, similar in structure to the urethral glands of Littre in the male, which occur in the vestibular mucosa in the vicinity of the meatus urethrse. The glandulae vestibulares majores (glands of Bartholin) form a paired tubulo-alveolar mucus-secreting gland which opens by a narrow duct into the groove between the hymen and labium minus. The tubular alveoli are lined by columnar mucus-secreting cells ; the ducts are clothed with columnar epithelium, which, as they approach their termination, becomes double-rowed, and finally changes to a stratified squamous epi- thelium similar to that of the surface upon which they open. These ducts frequently present saccular dilatations. THE MAMMARY GLANDS From a strictly histogenic standpoint the mammary glands should be considered as appendages of the skin, and as such should more properly have been considered in the chapter devoted to that subject. Yet these glands are so closely related to the reproductive functions, attaining their full development only in the lactating female, that it seems equally proper to consider them at this time as accessory reproductive organs. The mammary glands may be regarded as modified sweat glands. Though producing a fatty secretion they show no resemblance to seba- ceous glands. The mammae undergo the same slight but progressive de- velopment in both sexes until the time of puberty when they suffer regressive changes in the male, persisting thereafter only in rudimentary condition. In the female they continue to grow, but become functionally 542 THE EEPEODUCTIVE SYSTEM active only in the event of pregnancy. Upon the hemispherical corpus mammce can be distinguished the central raised nipple or mammilla, and the surrounding roughened and pigmented circular area, the areola. Each mammary gland consists of fifteen to twenty lobes, each of which is of itself a branched saccular gland whose lactiferous duct opens 6 FIG. 474. FROM THE ACTIVELY SECRETING MAMMARY GLAND OF A WOMAN. Several lobules are included, a, interlobular duct; 6, interlobular connective tissue. Hematein and eosin. Photo. X 52. on the surface of the nipple near its apex. The main lactiferous or lobar ducts subdivide in an arborescent manner into many interlobular ducts, about which are clustered the groups of secreting alveoli, each group forming one of the many lobules included in a lobe of the gland. The structure of the lobule, as well as the general appearance of microscopi- cal sections of the gland, varies according to the stage of development and the condition of activity of the organ. THE FEMALE KEPEODUCTIVE ORGANS 543 The Active Gland. During lactation the glandular alveoli are so numerous as to form by far the most prominent portion of the-gland. Each lobule consists of a cluster of saccular alveoli which open by short alveolar ducts into the interlobular ducts of the connective tissue which invests the lobules of the gland. The alveoli are closely packed within the lobule. The actively secreting alveoli are lined by cuboidal or low columnar cells which vary much in height even within the same alveolus, and are often considerably flattened. Fat drop- lets accumulate within the distal portion of their cytoplasm. The droplets in- crease,, in size as well as in number, until they finally occupy the greater part of the distal end of the cell and are separated from each other by only a narrow interval of albuminous cyto- plasm. Finally the fat droplets are discharged into the broad lumen of the alveolus, where they apparently still re- tain a thin albuminous envelope which prevents their cohesion and consequent fusion, and thus permits their suspen- sion in the albuminous, fluid portion of the milk. The milk may include also cytoplasmic and nuclear debris. The spherical nuclei of the secret- ing cells during this process are crowd- ed to the base of the cell, and after the discharge of the secretion the shrunken but nucleated cell remnants remain in situ; after a period of rest the cells apparently resume their secretory function. It appears probable that each cell in its life history may repeatedly pass through the cycle of secretory changes, though the exact number of such cycles which an individual cell may present obviously does not admit of dem- onstration. As a rule, the active epithelium consists of a single row of cells, though here and there they appear as if piled upon one another to form a double layer. During pregnancy many of these cells may be seen in mitosis. The actively secreting cells contain basal (ergastoplasmic) fila- ments. According to Hoven (Anat. Anz., 39, 1911) these filaments break up into granules from which minute fat spherules develop. The FIG. 475. MODEL OF A RECON- STRUCTION OF AN INTRALOB- ULAR DUCT AND ITS ACINI FROM THE ACTIVE MAMMARY GLAND OF A WOMAN. X 200. (After Maziarski.) 544 THE REPRODUCTIVE SYSTEM epithelium rests upon a reticular or homogeneous basement mem- brane, within which are occasional basket cells. These have been in- terpreted as smooth muscle cells, similar to those of the secretory por- tions of the sweat glands. The alveoli of the active gland are so closely packed that a connective tissue tunica propria is no more than scarcely demonstrable. The thin tunica propria is, however, richly supplied with blood capillaries, lymphatic vessels, and nerve fibers. The ducts of the mammary gland are lined by either a single or double row of low columnar cells. They possess a relatively broad lumen. Their membrana propria is supported by a thin connective tissue wall, PUfc * FIG. 476. ACTIVE MAMMARY GLAND OF RABBIT (22 DAYS AFTER FECUNDATION.) The alveoli are filled with milk containing fat droplets. X416. (After L. Schil.) The alveoli are filled with a stainable milk coagulum in which are many fat spher- ules. containing both circular and longitudinal elastic fibers but no muscle. The elastic fibers of the smaller ducts are poorly developed, but in suit- able specimens the longitudinal fibers are readily seen even in very small branches. Beyond the lactiferous sinus the duct epithelium changes to a stratified squamous variety which is continuous with that of the cutaneous surface of the nipple. The glandular lobules are firmly united by strong septa derived from the dense areolar tissue in which they are embedded. In the deeper parts of the gland occasional lobules of fat are found in this tissue. Within the nipple and beneath the adjacent portions of the areola, smooth THE FEMALE KEPEODUCTIVE OKGANS 545 muscle fibers are also found. These are arranged in circular bundles at the base of the nipple, with longitudinal fibers within its substance which, at the base of the mammilla, di- verge in radiating bundles into the sub- cutaneous tissue of the areolar zone. Con- traction of these fibers elevates and hard- ens the nipple, thus simulating the action of the erectile tissues. According to Liperovsky (Anat. Anz., 45, 20, 1914) elastic fibers are more abun- dant than was formerly recognized in the walls of the alveoli and in the interalveo- lar connective tissue, frequently in inti- mate association with smooth muscle cells. Such 'elastico-muscular apparatus' is well developed in the peripheral portion of the glands, in the subpapillary layer of the skin, and especially in the vicinity of the nipple. The elastic fibers, which are super- ficially distributed, appear embedded at their deeper ends in the muscle fibers. This peculiar arrangement of elastic fibers in relation to smooth muscle probably aids in the expulsion of the secretion. Embedded in the subcutaneous tissue of the areola are also a number of small accessory lactiferous glands known as the Glands of Montgomery (Areolar Glands of Duval). The nipple and areola contain also abundant sebaceous glands; and sweat glands are present in the periphery of the areola. The Resting Gland. With the cessa- tion of lactation the glandular alveoli un- dergo a rapid atrophy, and are replaced by connective tissue derived from the inter- lobular stroma. The ducts contract and the epithelium piles up to form a two- rowed, or even thicker, layer. The alveoli are reduced to mere buds from the terminal ducts, and their lumen is almost obliterated; their FIG. 477. FROM A SECTION OF THE HUMAN MAMMARY GLAND IN THE RESTING CONDITION. a, remnants of the glandular alveoli; 6, duct; c, connective tissue; d, adipose tissue. Hem- atein and eosin. Photo. X 10. 546 THE KEPRODUCTIVE SYSTEM epithelium is similarly massed into a double layer of small cells. The lobules are reduced in size and consist only of a few shrunken alveoli clustered about the termination of an interlobular duct. The lumen of the alveoli, if any, contains no secretion, and that of the ducts, except for a little granular albuminous material and an occasional leukocyte, is empty. The connective tissue stroma is much increased in volume, and in places shows a marked infiltration with fat. The alveolar tissue of the mammary gland at all times contains wandering leukocytes, and many granule cells, both acidophil and basophil in character. With the appearance of pregnancy the gland promptly reenters a state of activity; its alveoli multiply; its connective tissue becomes rela- tively diminished in volume; its lobules are reformed and their alveoli finally begin ' secretion, a process which is heralded by the formation of a granulo-fatty colostrum, a rather serous fluid in which are suspended large numbers of colostrum corpuscles, large spheroidal cells, resembling leukocytes in their general form and in the character of their nuclei, but which possess a broad rim of cytoplasm often containing numbers of fat globules of varying size. Their cytoplasm has also been shown to contain neutrophil granules of Ehrlich similar to those of the polymorphonuclear leukocytes (Michaelis, Arch. mikr. Anat., 1898). Colostrum discharge precedes and follows the period of lactation for a few days ; it appears also in both sexes for several days after birth when it is commonly known as 'witch's milk/ The origin of the colostrum corpuscles is still somewhat in doubt, though modern technic has gradually discredited the theory of their origin from desquamated remnants of the alveolar epithelium, and shows them to be more probably enlarged leukocytes which have wandered through the alveolar wall and have thus found their way into the lumen, where they take on a phagocytic activity and continue their growth. The following facts may be mentioned in support of this theory : a, leukocytes can be readily found between the cells of the alveolar epithelium as well as in the lumina of the saccules; I, the colostrum corpuscles examined in a fresh condition on a warmed slide have been repeatedly shown to possess the property of ameboid motion; c, the colostrum corpuscles, when stained, present the same granular and non- granular varieties as do the leukocytes of the blood; d, finally, the colostrum corpuscles have been shown to undergo mitotic cell division (Bizzozero and Ottolenghi), a phenomenon which we should hardly ex- pect to find in degenerated and desquamated epithelial cell remnants. THE FEMALE REPRODUCTIVE ORGANS 547 The blood-vessels of the mammary gland are specially abundant. They form rich capillary plexuses about the walls of the active "alveoli. Many of the venules coming from these plexuses converge toward the areola, where they form an incomplete venous circle ( circulus venosus of Hatter) from which the efferent veins take their origin. The lymphatics of the mammary gland are also numerous. They take origin from broad channels among the alveoli and enter a rich plexus about the interlobular ducts. From here several vessels pass to the lymph nodes of the axilla. The nerves of the mammary gland include both spinal (sensory) and sympathetic fibers. The latter are distributed to the vascular walls, to the smooth muscle of the areola and nipple and to the alveolar epithe- lium. The sensory fibers supply the connective tissue of the nipple and areola where they occasionally terminate in tactile and Pacinian cor- puscles. Among the secreting alveoli the nerve fibers form an epilemmal plexus beneath the membrana propria, from which fibrils penetrate be- tween the epithelial cells, upon which they end in minute granular varicosities (Arnstein, Anat. Anz., 1895). Milk. Milk, secreted by the active mammary gland, consists of an emulsion, in which fat droplets, varying in size from two to twenty microns or more, are suspended in a watery albuminous fluid. Water constitutes about eighty-six per cent, of the secretion; the protein con- stituent (three per cent.), which is largely nuclein, is derived in part from degenerating nuclei. Milk contains also a small amount of sugar (five per cent.) and a trace of salt. Each fat droplet is presumably invested with a thin coat of casein, derived from the cytoplasm of the secreting epithelium. Occasionally leukocytes occur in the milk, but never in large numbers, and like the similar colostrum corpuscles, they are mostly confined to the earlier periods of lactation. CHAPTEE XVI THE DUCTLESS GLANDS ENDOCEIN GLANDS (Organs of Internal Secretion) Under this heading it will be convenient to consider the suprarenal, thyroid, parathyroid, thymus, carotid, and coccygeal glands, the hypoph- ysis cerebri, the epiphysis cerebri and the paraganglia. This group of organs properly includes also the pancreatic islets, the interstitial cells of the testis and the ovary, and the corpora lutea already described. I. THE SUPRARENAL GLANDS THE SUPRARENAL GLANDS (bodies or capsules; also called adrenals) are two flattened irregular glandular masses situated close to tiie cranial extremity of eacn_kidriey, but without Organic or genetic relationship with the renal system. On section the adrenal is seen to be readily divisible into a bright yellow or brownish-yellow cortex and a more vascular, and hence darker and somewhat reddish, medulla j ^w'hose central portion transmits several large veins which make their exit from an indentation in the anterior surface of the organ, known as the hilum. Development and Function. The suprarenal gland is a composite structure formed b*y the intimate association of /two embryologically dis- tinct anlages: oixe, the cortical component, of ^lesodermal, the other or medullary component,' of ectodermal origin. The/ cortical component arises as a series of buds from the celomic epithelium Covering the 'medial upper surface of the cephalic third of the Wolffian bj>dy; the medulla develops from cells which have migrated from the abdominal sympathetic plexus (celiac plexus), and which elaborate peculiar gjranules having an affinity for chromium, hence called chromaffin (or plieoclirome) cells. The onto- genetic process recapitulates almost precisely the phylogenetic history of the suprarenals: In fishes the homologues of the two mammalian com- ponents remain separate, and the cortical representative includes a series of bodies, the interrenal bodies or organs; the medullary representative, 548 THE SUPKARENAL GLANDS 549 also comprising a series of structures, is known throughout, due to its close topographical relationship to the kidney, as the adrenal. The cortex of the mammalian suprarenal gland thus represents the product of fusion of the ichthyoid interrenals. In the groups between fishes and mammals, the association of interrenals (cortical component) and adrenals (medul- lary component) becomes progressively more intimate. The suprarenal is absolutely essential to life; removal promptly results in death. According to Crile (1914) the brain is intimately dependent upon the suprarenals; when both glands are excised in the rabbit death follows in eighteen hours, the brain cells meanwhile exhibiting loss of chromophilic substance. The two portions are believed to have, in part at least, a different function, both inhering however in an internal secretion. The cortex is generally believed to elaborate an antitoxic secretion for neutralization of harmful products of destructive metabolism. The func- tion of the medulla is dependent upon the adrenalin (adrenin; epinephrin) of the pheochrome granules, probably having to do with maintaining the proper tonus of the muscle of the heart and blood-vessels, thus underlying blood pressure. Minute amounts of epinephrin in the blood effect a sensi- tization of the vasoconstrictor nerve endings so that the efferent impulses discharged cause the muscular coats of arterioles to contract vigorously, the result being an increase in blood pressure. The most conspicuous diseases of the suprarenals involve hypersecretion, perhaps inducing to arteriosclerosis; and hyposecretion, frequently the result of tuberculous lesions, producing a clinical complex known as Addison's disease. (Refer- ence should be made to Vincent's "Internal Secretions and the Ductless Glands," Longmans, 1912.) The organ is enclosed by a connective tissue capsuje^of considerable thickness. From the inner surface of the capsule delicate fibrous trabecu- lae pass inward and subdivide the epithelial parenchyma of jthe organ into cell groups and columns, which vary in their appearance according to the distribution of the connective tissue trabeculae. The suprarenal paren- chyma is exceptionally prone to post mortem changes. In the medulla the connective tissue presents an irregular areolar arrangement; the more regular, though varying form of the areolae in the cortex, subdi- vides this portion of the organ into threp more or less distinct layers, which were first described by Arnold (Arch. f. path. Anat., 1866) as the zona glomerulosa, zona fasciculata, and zoria reticularis. In the zona glomerulosa the connective tissue trabeculae subdivide the epithelium into spheroidal groups of cells, many of which are con- tinuous with the cell columns of the adjacent zona fasciculata. The glomerulate layer is relatively thin and lies close beneath the capsule. 550 THE DUCTLESS GLANDS ENDOCRIN GLANDS The stroma of the zona f asciculata is continued inward from the glomerulosa, but is so drawn out as to form elongated areolae, inclosing Y\ Us* FIG. 478. FROM A SECTION THROUGH THE HUMAN ADRENAL. a, central vein; b, capsule; c, zona glomerulosa; d, zona, fasciculata; e, zona reticu- laris; /, medulla; g, peri-adrenal adipose and areolar tissue. Hematein and eosin. Photo. X 45. cell cojvjrn rip ^^considerable length, which are disjjased in a radial man- lier. This is the broadest of the three cortical zones and is interposed between the glomerulosa and reticularis. THE SUPRARENAL GLANDS 551 At the inner border of the zona fasciculata the connective^ tissue bundles pass insensibly from the regular columnar arrangelnenT of this layer into a reticular maze. The resulting cell groups are of very irregu- lar form and compose the innermost cortical layer, the zone reticularis. This layer is the thinnest and least d is t i n c t of the three zones of the c o r t e x. It can often be more read- ily distinguished by the highly Ipig- mented condition of its cells, than by the mere form of its cell columns. In man it ; passes almost insensibly into the medulla ; in many animals e.g., the dog, cat, and pig there is a sharp demarcation between the zona reticularis and the medulla, produced by a thin mem- branous layer / of connective tjssue which apparently results from It h e fusion of the , cen- tral ends of j the fibrous bands $1 the cortical stroma. Such a membranous septum is usually wanting in the human adrenal. The connective tissue stroma of the adrenal consists of a delicate vascular network, which in the cortex contains very few if any~elastic fibers. Flint (1900) has shown that this connective tissue is, in large part, at least, ^reticular tissue. The capsule consists of dense bundles 35 FIG. 479. PHOTOMICROGRAPH OF SUPRARENAL GLAND OP DOG. Magnification X 30. A, loose areolar connective tissue of outer portion of capsule containing two large (G) and several small gan- glia; c, capsule proper; g, zona glomerulosa; F, zona fascic- ulata; R, zona reticularis of cortex; M, medulla, showing the large central vein (F). 552 THE DUCTLESS GLANDS ENDOCiftN GLANDS of white fibrous tissue among which are many elastic fibers. The stroma of the medulla is also richly supplied with elastic tissue. The epithelium of the zona glomerulosais arranged in spheroidal groups or in hooked or slightly coiled columns which are continuous with the straight columns of the fascicular zone. The cells of the zona glomerulosa are closely packed within the connective tissue meshes and FIG. 480. MORE HIGHLY MAGNIFIED REGION OF THE PRECEDING SECTION, TO SHOW THE CAPSULE (C), ZONA GLOMERULOSA (0), AND A PORTION OF THE ZONA FASCICULATA (F). the cell outlines are very indistinct. Wherever their outlines can be readily distinguished the cells are seen to be of columnar shape and are arranged in slender columns whose cells are often grouped about an indistinct central lumen. The cytoplasm of finely granular and stains readily with acid dyes droplets appear in the innermost cells of the gioup, but these are never so abundant as in the more internal portions o in this zone are spheroidal in shape and rich in frequent mitoses (Canalis, 1877), but these are cells of this zone is Occasional minute fat the cortex. The nuclei 'hromatin; they present more abundant in early life than in the adult. The cells of the zona fasciculata are highly characteristic. They THE SUPRAKENAL GLANDS 553 are arranged in long straight columns which extend from the_zona glomerulosa inward to the zona reticularis. The cells are columnar or polyhed^ral_in_ shape ; many of them contain minute fatty droplets in great abundance. This fat is readily blackened by osmic acid. Arnold (1902), by extraction with ether, obtained crystals of palmatin and stearin from the suprarenal gland. Plec of the body. Each columnar columns of this zone may b ik (1902), however, considers that the adrenal fat diifers in its ultimate chemical properties from the other fat roup consists of cells which are, as a rule, in approximately the same silage of fatty metamorphosis, and the cell divided into those which are distinctly acidophil and those which ark distinctly fatty, though between these extremes there are many intermediate stages. The acidophil cells are ovoid or polyhedral elements which possess one or two highly chromatic spheroidal nuclei and a finely granular cytoplasm. On careful examination with high magnification, extremely minute fat droplets may often be" demonstrated even in the most char- acteristic of these cells; with lower magnification these are frequently invisible. The fatty cells possess a spheroidal nucleus which is usually vesicular in character ; occasionally it is/ highly chromatic. Frequently the ap- parent chromatolysis seems to progress in exact ratio to the accumulation of fat; those cells in which thd fatty metamorphosis is more advanced present the more typically vesicular nucleus. With the progress of the fatty metamorphosis the cell outlines are again lost and the granular acidophil cytoplasm gradually remaced. The presence of fat in the broad zona fasciculata is partially responsible for the bright yellow color of the cortex of the organ. The cells of the zona reticularis are similar to those of the zona fasciculata, though the fatty metamorphosis is less pronounced. In one particular, however, the cells of this layer are remarkable. They con- tain an abundance of a peculiar brownish-yellow pigment which occurs both in the form of coarse granules and as a diffuse coloration of the cytoplasm. The spherical nuclei, highly chromatic or only slightly vesicular in characte^, are not invaded by the pigmentation. The vol- ume of pigment varies greatly in different individuals; it is usually absent in young persons, but is, as a rule, present after the twentieth year of life (Maass, 1B89). In the suprarenal of the mouse many of these cells can be seen i^ process of amitotic division. The epithelial cells of the medulla are ovoid elements .with-ona, or two spherical nuclei, which in many cases possess a vesicular char- 554 THE DUCTLESS GLANDS ENDOCRIN GLANDS acter; in other cells they consist of a dense, almost solid, mass of chro- matin. The shape of the cell groups in the medulla varies greatly; usually they form small spheroidal masses or short columns. The cells are frequently arranged in~a~more or less tubular form but without a distmcjJ*H*reii. Frequently they surround a minute" capillary vessel. The medullary cells presumably pour their secretion into the blood- vessels, whose broad sinusoidal capillaries permeate the delicate connec- tive tissue bands which inclose the cell groups. Felicine (Arch. f. mikr. Anat., 1904) claims to have demonstrated the presence of minute intra- and intercellular secretory canaliculi which open directly or indirectly through broader lacunce, into\ the blood-vessels. The cell groups of the mecTtifWlike those of the cortex, are divisible into the acidophil and the fatM types; the former are the more abun- dant, but the fatty metamorphosis is scarcely ever so advanced as in the cortex. There is, however, great variation in the size of the medullary cells. The larger ovoid elements form the typical groups; between these groups are narrow cell colLmns consisting of much smaller and less highly acidophil cells, which \ are arranged in slender columns and scattered irregular masses. The striking feature of the mequllary cells is their granular content. These chromaffin granules have a Special affinity for chromic acid and its salts, and stain a light brown or yellow. Their staining capacity in chromium solutions is due to the l presence of adrenalin (Kingsbury, Anat. Rec., 5, 1, 1911). The granules are very readily soluble in acids. In the vicinity of the central vlins, small nerve trunks are found, and occasional minute ganglia or isolated nerve cells occur along their course. These are not to be confusedlwith the large ovoid epithelial cells of the medulla. Blood Supply. The arteries which^supjply .tha^uprarenal glands form a p]eW_pJ_^esselsin the_capsulejof the organ and in the neighbor- ing connective tissue. Some of the 'smaller branches of this plexus, the capsular arteries, supply the capsule itself, others enterjhe organ and are distributed to the cortex and to the medulla^ The blood supplied to the oapsnlar arteries, after traversing the capillaries, enters small venules which are tributary to the lumbar and phrenic veins. The course of the cortical and medullary vessels has been exhaustively studied by Flint (Proc. Bost. Soc. of Nat. Hist, 1900). The cortical arteries enter the zona glomerulosa where they abruptly break up to form a capillary plexus which occupies the connective tissue between the cell columns. Capillary vessels are continued from this THE SUPRARENAL GLANDS 555 plexus through the intercellular reticulum of the zona fasciculata, where they are in intimate relation with the epithelial cells, and reach the~zona reticularis. Here the capillaries are collected into thin-walled venules or sinusoids. These vessels, after some anastomoses, form venous stems which are continued, without further anastomosis, through the medulla to the central veins. The venules of the cortex possess no walls other than their endothelium. The medullary arteries are also derived from the capsular plexus. They penetrate the cortex, and at the border of the medulla abruptly FIG. 481. RECONSTRUCTION OF A DOG'S ADRENAL. a, arteries; v, vein. X 25. (After Flint.) terminate in a plexus of capillary vessels which lie in the connective tissue stroma and come into intimate relation with the medullary cells. These vessels possess extremely thin walls, their endothelium often being in direct contact with the adjacent epithelium, whose cells frequently im- pinge upon the lumen of the capillary vessel (see Fig. j|81). The capil- lary plexus pervades the entire medulla, its vessels, being here and there collected into small venules which unite to form the central veins. These form two, or sometimes four, main stems (Flint) wEich make their exit at the hilum and enter the lumbar or renal vein, or, on the right side, enter the inferior vena cava. All of the efferent veins of the adrena\ distribution of their smooth muscle fibe abundance, but are nearly all disposed are characterized by a peculiar s, which occur in considerable in the axis of the vessel; the 556 THE DUCTLESS GLANDS ENDOCRIN GLANDS circular muscle fibers are confined to a very thin coat beneath the endo- thelium, or are often entirely absent. Frequently, and especially in the central veins of the adrenal, the coarse bundles of longitudinal muscle fibers project into the lumen of the vessel in a somewhat rugose manner. Whenever two veins unite to form a larger vessel, and at the junction of a central vein with any of its branches, these protuberant muscular bundles are especially prominent. Moreover, Ferguson (Amer. Jour. Anat., 5, 1, 1905) describes anomalous vessels of a venous nature which arise in the medulla, penetrate the cortex, and enter the venous plexus of the capsule; and in these in- stances the same peculiar distribu- tion of the muscle has? been observed in the veins of the capsular plexus. "^BP%r f -|| j Lymphatics. The 1 lymphatics of the suprarenal gland, according j*ifc to Stilling (1887), form rich plex- uses in the zona glomerulo_sa and in the medulla; elsewherethey are lessL. abundant. They follow the course of the blood-vessels and are espe- cially well developed in the vicinity of the central veinsT" Nerves. The adrenal is well supplied with small sympathetic nerve trunks from the_solar plexus. They form a plexus in the capsule from which branches are distributed to the cortex and to the medulla. In the cortex they invest the blood- vessels with a delicate plexus, but have not been found within the epithe- lisdcell columnar In the medulla they are also distributed to the blood- vessels and are supplied with occasional small ganglia. Passing from the plexus of sympathetic nerve fibers which invests the groups of medul- lary epithelium, Dogiel (1894) demonstrated delicate fibrils, supplied with minute varicosities, which penetrate between the epithelial cells and terminate in a manner very similar to that which is characteristic of the epithelial parenchyma of other secreting glands. Accessory Suprarenals. These include bodies of three types of structure: (1) bodies consisting exclusively of cortical substance; (2) FIG. 482. SECTION OF PART OF AN AC- CESSORY SUPRARENAL (CHROMO- PHIL BODY), NEW-BORN CHILD. chr, a chromophil cell. (Sohafer, " Quain's Anatomy.") THE THYKOID GLAND 559 broken into minute spherules. In general, the' ratio of colloid content within the follicle, roughly stated, is in proportion to the age of the in- dividual. The follicles at the periphery of the lobes of the gland are less fully distended than those in the in Embedded in the colloid mass withir ently normal thyroid, red blood corpuscles and desquamated follicular epithelium are frequently found, but m cytes are of less frequent occurrence an erior. .the follicle, even in the appar- ver in large quantity. Leuko- are more rarely found in the human thyroid than in that of the lower\mammals. The FOLLICULAR EPITHELIUM is typically cuboidal in shape; in young individuals it is somewhat taller than broad. In those follicles which are distended with colloid secretion the epithelium is relatively short ; in those which are empty it is tallep. Each cell contains a single spheroidal nucleus which lies in the center of the cell, or somewhat toward its basal extremity. This orderly! disposition causes the nuclei, when seen in sections of the follicle, to appear as a continuous row in the wall of the alveolus, a disposition wjiich is noticeable for its ex- ceptional regularity. The cytoplasm of the epithelium is acidophilic. It usually contains some fatty droplets, which generally occupy the nute spheroidal granules which give the also found in the cytoplasm of the epithelial cells. Hiirthle (Arch. f. d. ges. PhysioL, 1894), by staining with the Biondi-Ehrlich mixture, suc- ceeded in differentiating two types of cell, one lightly staining, the 'chief cells/ the other a darker colloid-containing type which he designated as 'colloid cells/ These variations probalbly only represent different stages of secretion in the same epithelial cell type. Minute intercellular canaliculi occur at the angles between adjacent cells. In the thyroid of the opossum Bensley (Anat. Rec., 8, 9, 1914) also describes two types of cells, namely, the usual epithelial cells and ovoid cells. The latter hold -a parietal position ip. the follicle; they are filled with fine eosinophilic granules which give t6 these cells a character strik- iely granular and decidedly ?e granules and very small [tremities of the cells. Mi- >lor reactions of colloid are ingly similar to that of the acidophil cells o physis cerebri. He describes also large nee< the epithelial cells similar to those in the the testis. ' the anterior lobe of the hypo- ile-shaped crystalloid bodies in aterstitial and Sertoli cells of v The epithelium rests upon a very delicate reticular basement mem- brane and is inTcIose. Delation with the capillaries and lymphatic vessels. 560 THE DUCTLESS GLANDS ENDOCEIN GLANDS of the interf ollicular stroma. Colloid material, similar to that within the follicles, KalTbeen repeatedly found within the lymphatic \jessels (Baber, Langendorf, Hiirthle) and may be readily demonstrated irj of the doubted not, ho sent the most sections hyroid. Un- this does repre- entire 'inter- tion' of the FIG. 484. DIAGRAM OF PHARYNX OF HUMAN EMBRYO, SHOWING THE ORIGINS OF THE ANLAGES OF THE THYMUS, THYROID AND PARATHYROIDS (EPITHELIAL BODIES). Thy 1 , earliest position of thyroid anlage; Thy 2 , sec- ondary position of thyroid anlage; Tgd, thyroglossal duct; Fc, foramen cecum; LThy, lateral thyroid an- lages; Pthy, parathyroid bodies; pb, postbranchial body; T, tonsil; /, II, III, IV, V, pharyngeal pouches; a, b, first and second branchial grooves ('gill clefts'); 1, 2, 3, first, second and third aortic arches in the cores of the branchial arches. The complete wall of the pharynx is only shown at the right, above. (Adapt- ed from Kohn.) nal seen gland. Blood Supply. - The arteries form a rich~~plexus in and about the capsule of the 'thyroid, from * V which numerous branches penetrate the organ, lying in the connective tissue trab- eculae between the lo- bules; they are dis- tributed to all parts of the gland. They supply a rich capillary plexus in the walls of the follicles. The veins retrace the course of the arteries. The w a 1.1. s of the smaller venules consist only of endothelium, with a very^thin coat of nbro-elastic connec- tive tissue. Lymphatics. The thyroid is very abun- dantly supplied with lymphatic vessels. These form a plexus of very broad lacunar capillaries in the interfollicular__onnective tissue, where they stand in intimate relaTion~with the f ollicular epithelium. From this plexus vessels pass to the interlobular connective tissue, in which they form a second plexus, whence lymphatic vessels pass out of THE THYROID GLAND 561 the thyroid in company with the blood-vessels and enter the deep cer- vical lymph nodes. Nerves. The nerves of the thyroid are derived from the sympa- thetic system and are mostly non-medullated. They g ^ nr npnny tb Q QT *- teries and form a delicate terminal plexus in the walls of the follicles. The finer fibrils of this plexus end in contact wit.h t.hp ppithplinTfl. Berkeley (1895) describes also occasional fibrils which apparently pene- trated between the epithelial cells. Development. The thyrtoid develops from three anlages, a median and a pair of lateral outgrowths from the primitive pharynx. The median anlage sprouts from the flo6r of the pharynx at the level of the first pharyngeal pouches. Its sitte of origin is marked in the adult by the foramen cecum of the tonguel It forms the isthmus and pyramidal lobe (process) of the definitive thyroid. The lateral anlages grow down from the ventral border of the fourth pharyngeal pouches ; they form the lateral lobes of the gland. The pyramidal lobe represents the remnant of the embryonic thyroglossal duct; viduals; it may even retain a t varies greatly in length in different indi- partial lumen which may be filled with col- loid, but it does not open upon the surface. The primary anlages consist of solid cords of cells; the cords subsequently acquire a lumen, and become broken up into lobules and follicles through the invasion of connective tissue. According to Grosser tjhe definitive thyroid arises exclusively from the median anlage. Function. Removal of thb thyroid is followed by serious symptoms and frequently fatal results, particularly in the case of the carnivora. Its internal secretion is apparently necessary for normal metabolism and devel- opment. It governs the conditions favoring tissue oxidation (Crile). The essential secretion is probably something apart from the colloid and iodin content of the gland. According to Crile the active constituent, thyroiodin, is iodin in a special proteid co also an antitoxic role is based tomized animals are extremely experiments with frog tadpoles entiation but inhibits growth into diminutive frogs a result is fed. An enlarged thyroid is may be due to increase in the colloid parenchyma ; one results tion of hyposecretion, associate cretinism an apparently herec tion, associated with exophtha animals the thyroid shows a seas bination. The belief that the thyroid has chiefly on the observation that thyroidec- iable to certain infections. Gudernatsch's show that a thyroid diet accelerates differ- the tadpoles metamorphosed prematurely he opposite of that obtained when thymus mmonly known as a 'goitre/ Enlargement amount of the connective tissue or of the in atrophy of the parenchyma and a condi- with myxedema and in extreme cases with tary defect; and the other in hypersecre- nic goitre (Grave's disease). In man and nal enlargement related to the sexual cycle. 562 THE DUCTLESS GLANDS ENDOCRIN GLANDS ACCESSORY OR ABERRANT THYROIDS These bodies, first described by Zuckerkandl (1879), are widely dis- tributed through the connective tissue of the cervical region. They are most frequently found in the course of the embryonic thyroglossal FIG. 485. FROM THE BORDER OF A MASS OF ABERRANT THYROID TISSUE OF MAN, OCCURRING IN THE REGION OF THE PARATHYROID GLANDS. Hematein and eosin. Photo. X 204. duct and in the immediate vicinity of the lateral lobes of the thyroid. They present the appearance of embryonal remnants of thyroid tissue, but are found in nearly all individuals. The colloid follicles of the aberrant thyroids are usually small, though, in the larger specimens of these bodies, they may attain as great THE PARATHYROID GLANDS 563 a size as those of the thyroid itself. The cell columns without colloid are more numerous than in the thyroid gland, giving to the aberrant bodies a decidedly cellular appearance. Each aberrant mass is usually inclosed by a very thin connective tissue capsule which sends delicate processes between the cell groups. The epithelial cells retain all the characteristics of those of the thyroid gland, and can be readily distin- guished from the epithelium of the parathyroid glands with which the accessory thyroid bodies have been frequently confused. They are also much less vascular than the parathyroids. III. THE PARATHYROID GLANDS The PARATHYROIDS, or epithelial bodies, are small glandular bodies of irregular distribution, usually found in relation with the posterior margin of the lateral lobes of the thyroid gland. They arise as buds from the dorsal pockets of the third and fourth pharyngeal pouches. Typi- cally four parathyroids are present, the superior pair situated near the lateral, the inferior near the median margin of the lateral lobes of the thyroid. The inferior pair in the adult develops from the anterior pair of anlages, and frequently becomes embedded in the thyroid tissue. They may occur also in relation with the tracheal or laryngeal wall and may be found as high as the hyoid bone or as low as the border of the thymus. They also vary greatly in size and shape, but usually are of ovoid form and about three to five millimeters in diameter. Each parathyroid is invested by a thin capsule of dense connective tissue and consists of a mass of epithelial cells supported by a delicate fibrous reticulum. The epithelial cells are of two chief types, desig- nated by Welsh (Jour. Anat. and Physiol., 1898) as the 'principal' and the 'oxyphiF or acidophil cells. The principal cells are the more abundant. They are ovoid or spheroidal elements, with a clear vesicular cytoplasm, a distinct cell membrane, and a large spherical nucleus, whose chromatin is irregularly distributed and often gives the nucleus a somewhat vesicular character. The acidophil cells are of similar shape but are provided with a small spherical nucleus, which is very rich in chromatin, and a granular acidophilic cytoplasm. The acidophil are less numerous than the principal cells. The distribution of the epithelial cells is subject to considerable vari- ation. Most frequently they form an almost solid epithelial mass, in 564 THE DUCTLESS GLANDS ENDOCHIN GLANDS which capillary vessels are here and there found, the larger blood-vessels occupying the coarser bands of the fibrous stroma. In such glands the two cell varieties are either intermingled irregularly, or the acidophil cells may occur in scattered groups which are interspersed among the more numerous principal cells. In certain instances the epithelial cells are arranged in small alveo- FIG. 486. HUMAN PARATHYROID TISSUE, MODERATELY MAGNIFIED. Several blood-vessels are included. Capillary vessels can scarcely be recognized with this magnification. Hematein and eosin. Photo. X 300. lar groups which are surrounded by a network of capillary vessels. This arrangement appears to be more frequent in young individuals. The cell groups in this type of gland frequently form branching columns. Occasionally, epithelial cells surround a central lumen, in which are small masses of an acidophil substance which resembles colloid in its reactions. This colloidal material is less abundant in the human para- THE THYMUS 565 thyroid than in that of the lower mammals. Likewise the cystic _ducts, lined by columnar or ciliated columnar epithelium, which have been de- scribed by Kohn (1897), though of frequent occurrence in the lower mammals are rarely, if ever, found in the human parathyroid. The parathyroids and the thyroid are frequently in anatomical con- tinuity; this area is characterized by transitional histologic conditions. Moreover, after thyroidectomy the parathyroids may alter their structure so as to resemble very closely thyroid tissue. The parathyroids are gen- erally regarded as having a function absolutely essential to life. How- ever, the experimental evidence touching this point is conflicting. But in certain forms, e.g., cat and dog, extirpation produces tetany which results in death. Vincent interprets the experimental data as indi- cating that the thyroid and the parathyroids constitute parts of the same apparatus, to which the pars intermedia of the hypophysis cerebri may have to be added as a third constituent. The connective tissue of the gland is of variable quantity. It forms a thin but dense capsule; occasionally trabecula3 extend inward and partially outline indistinct lobules. In many instances a hilum trans- mits the larger blood-vessels by means of vascular trabeculse which radiate to all portions of the organ. A delicate fibrous or reticular stroma invests the individual cells, or the cell groups, when these are present. Occasionally the cells are so closely packed- that the stroma is scarcely demonstrable. The blood supply of the parathyroid is exceedingly rich. Ar- teries enter from the capsule, or at the hilum, and rapidly break up into a plexus of broad capillary or sinusoidal vessels which follow the fibrous bands of the stroma and are in intimate relation with the epi- thelium. They are collected into thin-walled venules which retrace the course of the arteries. IV. THE THYMUS The THYMUS was formerly regarded as an organ of fetal and in- fantile life, and was believed to attain its maximum development during the~sec6TTd year of childhood. After this time it was thought to be- come gradually replaced by adipose tissue, its retrograde metamorphosis reaching completion at about the age of puberty. However, Waldeyer (1890) showed that the thymus may persist even in advanced age as a body of considerable size, and apparently functional. The more recent 56G THE DUCTLESS GLANDS ENDOCRIN GLANDS studies of Hammar (1906) and others indicate that the thymus nor- mally continues its development until about the time of puberty. Subsequently it loses slowly in weight, but a true atrophy of the parenchyma with cessation of function, does apparently not occur until about the age of fifty. It is still uncertain whether the thymus should be classified with FIG. 487. A SECTION THROUGH SEVERAL LOBULES OF THE THYMUS OF AN INFANT. a, loose fibrous septum between the lobules; b, cortex, and c, medulla of the lobule; d, blood-vessels in the connective tissue. Hematein and eosin. Photo. X 60. lymphoid organs or with endocrin glands. Eecent investigations in- creasingly indicate that it is essentially of lymphoid character but func- tions, in part at least, as an organ of internal secretion, related to normal growth and sexual development. At its maximum the thymus forms a large lymphoid mass, embedded in areolar ^rmonfi^o nobi-io +Ks^TV'flhppTilao r>+ whnnh fMyiHfTlthft organ into several ^bes, and innumerable minute lobules. Each lobule is surrounded by a thin^fibpusjcapsuTe, by which it is loosely united 1&. its neighbors. The lobule consists of a rriass^of lymphoid tissue, which is dense at the periphery but looser in the centra*! portion/ It is thus divisible into a dense^corte and a loose medulla^ both composed essentially of lymphoid tissue, but between which, because of the difference in density, there is a sharp line of demarcation. Frequently, at some point on THE THYMUS 567 its circumference, the medulla reaches the surface of the lobule^ and at such locations a narrow column of medullary lymphoid tissue connects it with the adjoining lobule. A close-meshed reticulum, within the narrow meshes of which are closely packed lymphoid cells, composes the lymp > Md^jfeue T> oJJJb^.loJmle. That of^Tte ' Coftex ancfthe medulla is alike, except for the fact that the meshes of the reticulum in the cortex are much more crowded with 'lymphocytes' than are those of the medulla. The medulla of each lobule is also charac- terized by thejpresence of several groups of concen- trically arranged epitheli- oid cells, the thymic cor- puscles (concentric corpus- cles of H assail). Each thymic corpuscle^ consists of a large central cell or group of cells, which is surrounded by two to, five layers of concentrically ar- ranged flat epithelioi_d cells. These groups *Tr cell nests FlG 4gg _ A THYMIC CORPUSCLE FROM THE are strongly acidophil in THYMUS OF AN INFANT. their staining reactions, Hematein and eosin. X 665. and therefore stknd out in marked contrast to the basophilic nuqlei of the surrounding lymphoid tissue. A substanie simulating the colloid of the thyroid may be present among the cells, &ince no similar structure occurs elsewhere in the body, the concentric corpuscles are absolutely characteristic of the thymus lobule. The nature of the tSWrnic corpuscles is not satisfactorily understood. According to one hypot has been obliterated by theory regards them as the organ arose in the of hypertrophied cells o lesis they represent blood-vessels whose lumen proliferation of its endothelial cells. Another remains of the epithelial columns from which imbryo. They are also interpreted as masses the reticulum. The types of lymphoid corpuscle which are found in the thymus are similar to those of the lymph nodes, though polymorphonuclear leukocytes are rather more frequent here, and giant cells, mononuclear or multinuclear in form, may be readily found in the medulla of this 36 568 THE DUCTLESS GLANDS ENDOCKIN GLANDS organ. Pappenheimer regards the specific cells of the thymus as epi- thelial in character, simulating, but not identical with, lymphocytes. Blood Supply. The larger arteries of the thymus are distributed within the interlobular connective tissue. They supply branches to the lobule which penetrate to the medulla, where they form a plexus of sinusoidal capjllarie's with elongated meshes, and also distribute radi- ating capillaries to the cortical portion of the lobule. These sinusoidal vessels are highly characteristic of the medulla of the thymus lobule. They unite to form"venpus radicals of considerable caliberpwhich leave the lobule to jomj:he interlobular veins in the loose connective tissue between the lobules. Lymph vessel a_are of frequent occurrence in the interlobular con- nective tissue, and their branches occasionally penetrate to the medulla of the lobule7^But~there is nothing corresponding to the lymph sinuses of lymph nodes. Small non-medullated_nerve trunks are also found in the interlobular connective tissue, but seem to be chiefly distributed to the walls of the larger blood-vessels. Development and Function of the Thymus. The thymus arises as a pair of tubular outgrowths fr6m the ventral aspect of the third pharyn- geal pouch, which subsequently Become solid and fuse to form a median, flat, bi-lobed organ lying in the root of the neck and the upper portion of the thorax. According to certain investigators (Stohr, et al.) the lymphoid cells of the definitive organ are derived from the entodermal cells of the initial anlages. The recent work of Maximow (Arch. rnikr. Anat., 74, 3, 1909), however, seems to demonstrate that these cells are true lymphocytes and that they have their origin in the -surrounding mesenchyme, from the cells of which they differentiate, and from which location they migrate into the thymus anlages a conclusion confirmed by the still more recent work of Badertscher (Amer. Jour. Anat., 171 4, 1915). The original lymphocytes are said to be of the large variety ; these give rise through proliferation with- in the thymus to the smaller types. An^ong the invading leukocytes are also a small number of polymorphs. The original epithelial anlages continue to grow for a time, and gradually becom^ differentiated into the definitive reticulum of the thymus. Hassall's corpuscles are believed to represent non- reticular remnants of the original entodermal anlages. According to Ham- mar, and Bell, these concentric corpuscles are derived from hypertrophic reticular cells. They first appear early in fetal life, and continue to form and increase in size during infancy. They are interpreted by some as thy- mic elements of internal secretion. The thymus is commonly regarded as a hemopoietic organ, but its activity is limited to the formation of lympho- cytes and possibly a small number of granulocytes. Beard (1900) views it THE CAROTID GLAND 569 as the original source of leukocytes. Th portions of the lobules; the medulla doi ters of lymph nodules, but is probably The thytnus does not seem to be an org* dogs and guinea pigs is not followed by evidence indicates a reciprocal function and certain of the organs of internal s This suggests a secretory role; a cone the recent experiments of Gudernatsch 5 proliferative foci are the cortical ! not correspond to the germ cen- an area of leukocyte dissolution, n essential to life. Extirpation in death. However, the experimental 1 relationship between the thymus cretion, especially the sex-glands, ision supported by the results of (Amer. Jour. Anat., 15, 4, 1914) in which thymus fed to frog tadpoles, Accelerated growth, but suppressed differentiation. V. THE CAROTID GLAND This body was first carefully described by Luschka (1862) and, from its intimate relation to the blood-vessels and nerves, is also known as the glomus caroticum or ganglion inter car oticum. It is about the size of a rice grain. It consists of scattered masses of epithelioid cells, usually grouped in small spheroidal clumps or 'cell balls/ embedded in the connective tissue at the point of bifurcation of the '^ % M common carotid artery. Am Kohn (Arch. f. Mik. 1\?/ Anat., 1900) has de- connect ive tissue -the type found in man septum. Portions of two adjacent lobules are in- consists of scattered cell eluded in the figure. X 200. (After Kohn.) groups ; in the rabbit they are even more diffuse. The carotid gland of a cat consists of a single cell mass, while that of the ape is intermediate between that of the cat and man. The glandular elements are derived from embryonal sympathetic ganglion cells (Kohn). They are ovoid cells with finely granular cytoplasm and a spheroidal, somewhat vesicular nucleus. Many of them contain chromamn granules. Because of its genetic relationship and 570 THE DUCTLESS GLANDS ENDOCRIN GLANDS histologic similarity to a sympathetic ganglion, Kohn proposes the name, paraganglion caroticum. Its function is obviously similar to that of the other organs of the chromaffin system, and probably chiefly dependent upon the presence of adrenalin. The carotid gland is richly supplied with capillary blood-vessels and small non-medullated nerve trunks. The capillaries are in intimate relation with the glandular epithelium. VI. THE COCCYGEAL GLAND This small body 2.5 mm. in diameter (Eberth) was discovered by Luschka in 1860. Its structure closely resembles that of the carotid gland. It usually con- sists of several minute groups of epithelioid cells which are in re- lation with the ter- minal branches of the middle sacral artery. It is richly supplied with broad capillaries or sinusoids and hence is also known as the glomiis coccygeum. The parenchymal FIG. 490. FROM A SECTION OF THE COCCYGEAL GLAND cells of the organ are OF MAN. ovoid elements which Highly magnified. (After Sertoli.) are closely packed about the walls of the blood-vessels in groups or short columns inclosed by delicate sheaths of connective tissue. The origin and function of these cells are un- known. No chromamn cells have yet been certainly demonstrated. The organ is embedded in the dense connective tissue at the tip of the coccyx. Its general structure only suggests an internal secretory function. According to Stoerck (Arch. mikr. Anat., Bd. 69, 1906), the cells of the coccygeal body do not give the pheochrome reaction at any period of life. They are believed to bear no genetic relation to the sympa- thetic nervous system. Schumacher regards the epithelioid cells as a transformation of the smooth muscle cells of the arterial vessels; PAEAGANGLIA 571 others interpret them as modified endothelial cells of the blood-vessels. Vincent suggests that this body may functioji as a 'safety-valve' in the course of the peripheral circulation; he regards the balance of evidence as against any function involving an internal secretion. VII. PARAGANGLIA The so-called chromaffin system includes besides the medulla of the suprarenal and the carotid gland, other larger and smaller groups of chromaffin cells ('pheochrome organs') derived from embryonal sym- pathetic cells and scattered along the entire sympathetic nervous system, the PARAGANGLIA. They are therefore present in many organs which receive a consider- able sympathetic innervation, e.g., kidney, liver, uterus, testis, etc. Chromaffin tissue composed of widely scattered cells is present also in the sympathetic trunks, particularly the various plexuses, among the definitive neurons of this system. The largest paraganglia (or- gans of Zuckerkandl) occur in close relation to the inferior mes- enteric artery, one on either side. Other smaller groups are found in relation to the abdominal aorta. Busachi has recently (1912) described chromaffin tis- sue in the human heart; and Thulin (Anat. Anz., 46, 22, 1914) reports a paraganglion in the striped muscle of the upper portion of the esoph- agus of man. In typical paraganglia the polygonal cells are arranged in irregular cords which form a fine-meshed network, the areolae of which are occupied by sinusoidal blood-vessels, upon which the cells rest directly without the intervention of a basement membrane. The spher- ical nucleus is poor in chromatin; the reticular cytoplasm contains the pheochrome granules. The internal secretion of the paraganglia de- FIG. 491. MEDIAN SECTION THROUGH THE ANLAGES OF THE HYPOPHYSIS CEBEBRI OF A 10 MM. CAT EMBRYO. (Bonnet.) Di, diencephalon; Te, telencephalon; M, mouth; R, Rathke's pouch (pituitary anlage); /, infundibulum. X 50. 572 THE DUCTLESS GLANDS ENDOCRIN GLANDS pends upon the adrenalin content of the granules. Adrenalin produces a contraction of involuntary muscle. Its function appears to be to maintain the proper tonus of the blood-vessels. VIII. HYPOPHYSIS CERE-BRI (Pituitary Body) The HYPOPHYSIS is a gland of the internally secreting (endocrin) or ductless type. The secretion passes directly from the specific cells into capillaries. This gland is a composite structure, arising from the association of two originally distinct anlages; one taking origin as a median dorsal diverticulum (Kathke's pouch) from the primitive mouth, the other as a medial ventral evagination from the second cerebral vesicle (third ventricle) or diencephalon. Both components are thus ectodermal. The cerebral flask-shaped element invaginates the buccal element and modifies the original bulbar shape of the latter into that of a cup. The buccal connection disappears, while the cerebral connection persists as the infundibular stalk of the definitive struc- ture. The hypophysis is present as an essentially similar organ in all craniates. In man it measures about 12 mm. in the transverse, about 7 mm. in the sagittal, and about 5 mm. in the vertical diameter. It has an inverted mushroom shape, the stalk or infundibulum, a hollow funnel-shaped structure lined with ependyma, attaching it to the brain, while its head is lodged in the sella turcica of the skull. The hypo- physis was known to the early anatomists as the pituitary gland (Vesal, 1553), and was supposed to function as an excretory organ in the elimi- nation of mucus (pituita) from the brain by way of the nose. The term pituitary gland is now generally applied only to the larger anterior component or epithelial lobe; the posterior component or neural, lobe is designated the infundibular process; the term hypophysis is applied to the associated elements. But hypophysis and pituitary body are still often used synonymously. The neural and glandular tissues are said to be connected with each other by means of nerve fibers, probably sym- pathetic, connective tissue, and blood-vessels. Certain investigators have described accessory pituitary bodies. Thus Haberfeld (1909) finds in man at all ages a 'pharyngeal pituitary/ a solid cord of neutrophilic cells about 5 mm. in length immediately behind the vomer. This is frequently the initial seat of tumors. HYPOPHYSIS CEEEBRI 573 Function. Very little is definitely known concerning the function of the hypophysis, notwithstanding extensive investigation. Much of- the recorded clinical and experimental data is contradictory. However, the presence of the gland would seem to be essential to health, perhaps indis- pensable to the maintenance of life. . It appears also that its two funda- mental components (anterior and posterior lobes) subserve different func- tions, and that pathological alterations in the gland induce marked changes in some or all of the other ductless glands, which glands appear to sustain reciprocal functional relationships to each other. Absence of the hypophysis (apituitarism) in mammals is associated with grave nutri- tive disturbances usually leading to death. Disordered function (dyspitui- tarism) is accompanied by inordinate enlargement of the extremities (acromegaly) and gigantism, mental derangements, disturbances in the sexual organs and improper metabolism of sugar. Reduced secretion ('hypopituitarism') of the posterior lobe, leads to an acquired high toler- ance for sugars with the resultant accumulation of fat (Gushing; "The Pituitary Body and Its Disorders," 1910). Increased secretion may be responsible for some types of emaciation and for certain mental distur- bances. Deficient secretion of the anterior lobe appears to inhibit com- plete development of the skeleton, resulting in dwarfism and infantilism; conditions .of superabundant secretory activity (hyperpituitarism) are ac- companied by acromegaly and gigantism (Gushing). Injection of extract of the posterior lobe (pituitrin) into the blood-vessels produces a rise of blood pressure accompanied by diuresis, believed to be due to a stimulation of the smooth musculature of the blood-vessels. In its effect upon smooth muscle, particularly of the uterus, which it stimulates to sharp and pro- longed contraction, it resembles very closely the secretion of the suprarenal gland. Pituitrin is now extensively used in practical medicine and espe- cially in obstetrics. The anterior lobe is believed by some to be physiologically inactive, except in cases of defective secretion of the gland, while others hold that the removal of the posterior lobe gives rise to no symptoms, but that ex- cision of the anterior lobe produces ' all the ill effects of total pituitary extirpation. The contradictory results of experimental work may be due to failure or inability to completely separate the several portions; more- over, certain elements of the supposed symptom complex accompanying hypophysectomy may be due to mechanical injury to associated parts of the brain. Histologic Structure. The hypophysis is usually Described as consisting of three portions: (1) the anterior lobe or glandular portion; (2) the intermediate portion or boundary zone; and (3) the posterior lobe., or nervous portion. The recent more critical investigation of 574 THE DUCTLESS GLANDS ENDOCRIN GLANDS Tilney (Internationale Monatschrift f. Anat. u. Phys., Bd. 30, 1911), including careful reconstructions of a number of mammalian hypophyses, shows that this description is incomplete. The form, extent, relation and genetic significance of the pars intermedia had apparently hitherto not been fully appreciated, though Herring (1908) had already called attention to the fact that it comprised two histologically different areas. Tilney divides the hypophysis into: (I) pars neuralis (pars nervosa) ; 11 13 15 FIG. 492. SAGITTAL VIEW OF A WAX RECONSTRUCTION OF THE HYPOPHYSIS CERE- BRI OF THE ADULT CAT. 1, third ventricle; 9a and b, eminentia saccularis; 6, area premammillaris; 5, cor- pora mammillaria; 12, pars tuberalis; 8, recessus infundibuli; 10, inf undibulum ; 13, pars infundibularis; 16, recessus processi infundibuli; 11, processus infundibuli; 15, lumen residuale; 14, pars distalis; 17. recessus tuberis; 3, optic chiasm. (Tilney, Internat. Monatschr., Bd. 30, 1913.) and (II) pars buccalis. The pars neuralis consists of three distinct elements: (a) the eminentia saccularis of the tuber cinereum; (b) the inf undibulum ; and (c) the infundibular process. The pars luccalis consists of two elements: (a) the pars juxta-neuralis in close relation with and investing the neural portion; and (b) the pars distalis. A cleft, the residual lumen, remnant of the original cavity in the buccal diverticulum, appears between the two. Further analysis of the juxta- neural portion of the gland reveals two histologically different parts: (1) the pars infundibularis completely investing the inf undibulum HYPOPHYSIS CEKEBEI 575 and the infundibular process; and (2) the pars tuberalis, which sur- rounds the eminentia saccularis. The following outline presents this analysis in resume: HYPOPHYSIS CEREBRI I. Pars Neuralis: (Posterior lobe). (1) Eminentia Saccularis. (2) Infundibulum. (3) Processus infundibuli. II. Pars Buccalis seu Glandularis: (1) Pars juxtaneuralis. (Intermediate portion.) (a) Pars tuberalis. (b) Pars infundibularis. (2) Pars distalis. (Anterior lobe.) "The pars infundibularis makes its appearance almost immediately after the anlage of the buccal portion of the hypophysis is formed. The pars tuberalis arises as a relatively late structure. It has its origin in two secondary diverticula or sprouts from the body of the pituitary sac (buccal evagination). These sprouts, the lateral processes, ultimately fuse with each other across the median line, displace the body of the pituitary sac ventrad" and thus secondarily assume their juxtaneural position. The juxtaneural portion of the hypophysis is intimately connected with the neural portion by means of nerve fibers, blood-vessels and connective tissue processes. Attempted separation consequently necessarily includes laceration of neural elements" (Tilney). Accordingly complete removal of the pars buccalis would seem to be impossible without the attendant re- moval of portions of the pars neuralis. This must be taken into account in interpreting symptoms following removal of the anterior lobe of the hypo- physis. I. PARS NEURALIS : INFUNDIBULUM. This consists of fusiform and multipolar neuroglia cells and fibers. A substance resembling colloid is also occasionally present. Small nerve cells have been reported, but their presence seems doubtful. The glia cells generally have a single nucleus, but two or more may appear. The protoplasm is finely granu- lar, and occasionally contains pigment granules, which seem to be more abundant in aged specimens. Delicate and coarse glia fibers extend through the protoplasmic portions of the cells, frequently terminating the cell processes (Fig. 496). II. PARS BUCCALIS SEU GLANDULARIS: (1) Pars juxtaneuralis. 576 THE DUCTLESS GLANDS ENDOCRIN GLANDS FIG. 493. PARS TUBERALIS, HYPO- PHYSIS OF CAT (Tilney.) (a) Pars tuber alls. The cells of this portion are arranged in large tubular acini; the lumina may contain colloidal material. The cells are of the cuboidal type, sometimes ciliated, frequently two layers deep. The cytoplasm of the cells is sparse, homogeneous and deeply basophilic. The nuclei are large and vesicular. Blood-vessels (sinusoidal capillaries) are numerous, and are said occa- sionally to contain colloid. (b) Pars infundibularis. Here the cells are arranged in a dense stratum several layers deep. Occa- sional acini (cysts) occur, but they are smaller than in the pars tuber- alis and are lined with only a sin- gle layer of low cuboidal cells. These cells contain large vesicular centric nuclei surrounded by a con- siderable layer of faintly basophilic granular cytoplasm. Blood-vessels are very meager and thin-walled. (2) Pars distalis. This portion is surrounded by a robust fibrous capsule continuous posteriorly with a thinner investment for the neural lobe. Delicate trabeculse traverse the parenchyma and support the very abundant plexus of arterio- venous sinusoidal blood spaces. The peripheral cells of the pars distalis constitute a narrow zone of almost exclusively basophilic cells. The cells of the main central portion are arranged in dense convoluted anas- tomosing cords. The cords com- prise axial and parietal cells. The parietal cells are the larger, of polygonal shape, and with a gran- ular cytoplasm deeply acidophilic. These are the 'eosinophilic' or acid- ophilic ( c chromophiF) cells. The axial cells are smaller; their cyto- plasm contains 'neutrophilic' granules. These are the 'chief,' or 'chrom- ophobe' cells. The so-called 'chromophils' and 'chromophobes' of the FIG. 494. PARS INFUNDIBULARIS, HY- POPHYSIS OF CAT. (Tilney.) HYPOPHYSIS CEREBEI 577 earlier terminology (Flesch, 1880), which was based simply upon_a difference in the degree of tingibility, probably represent merely different functional phases of the same cell type. The classification of the cells as acidophils and basophils, a distinction based upon a more precise micro- chemical difference, accords with a topographical segregation and more probably designates an actual functional division. True chromophobes, cells which stain only faintly and with difficulty in either acid or basic dyes an( j probably functional phases of acidophil and basophil cells appear first only in the higher sau- ropsids (Tilney). The nuclei of the several cell types are large and deeply staining. The cells in great part rest directly upon the endothe- lial walls of the capillaries. The blood-vessels are said to contain colloid. Comparative histologic studies indicate that the pituitary gland has a twofold activity, one part de- pendent upon the basophils, the other upon the acidophils. The residual lumen is lined only with basophils, and occasionally contains colloid. Comparative morphology indicates that the neural portion may represent an original special sense organ. It has been suggested by Tilney (Mem. Wistar Inst., Biol. and Anat., No. 2, 1911) that the basophils contribute their secretion by way of the residual lumen and the infundibular process to the cerebrospinal fluid, and the acidophils directly through the blood spaces. According to Dandy and Goetsch (Amer. Jour. Anat., 11, 2, 1911), the pars distalis receives from eighteen to twenty small arteries which converge to it from the circle of Willis. Within the lobe no true arteries or veins occur. The blood channels are sinusoidal in nature; they drain into a venous circle overlying the circle of Willis. The above description of the hypophysis is based largely upon dog and cat material. Our knowledge of the human hypophysis is less complete ; but as far as is known, the structural elements are essentially closely similar. The nerve supply to the hypophysis in the dog is, according to Dandy (Amer. Jour. Anat., 15, 3, 1913), derived from the carotid plexus of the FIG. 495. PARS DISTALIS, HYPOPHY- SIS OF CAT. (Tilney.) >78 THE DUCTLESS GLANDS ENDOCB1N GLANDS sympathetic system, which connects with the oculomotor and optic nerves. The innervation of the anterior lobe is described as abundant, that of the FIG. 496. SECTION OF HYPOPHYSIS CEREBRI OF DOG, SHOWING PORTIONS OF PARS DISTALIS (D) OF ANTERIOR OR BUCCAL LOBE, RESIDUAL LUMEN (R), PARS Tu- BERALIS (PARS INTERMEDIA) (7 1 ), PARS NEURALIS (N), AND C, CAPSULE. Note the abundant large blood sinuses in the glandular or buccal portion, and the tubules in the pars tuberalis (referring to the portion next the tuber cinereum). The pars neuralis is largely of neurogliar composition. posterior lobe as very scanty. The fibers are thought to be secretory in character. Dandy (ibid.) described also a small spherical body which he names paraliypophysis in connection with the under central surface of the hypo- EPIPHYSIS CEKEBRI , 579 physis, resting in a depression in the floor of the sella turcica. This was found in over eighty per cent, of dogs examined. It is interpreted as a remnant of the embryonic Rathke's pouch. It is said to vary greatly in size and histological character, occasionally extending to the 'pars inter- media/ and to have an individual blood supply and possibly also a nerve supply. A parahypophysis has not yet been reported for man. 1 Tjt FIG. 497. FIELD FROM THE CENTER OF A NORMAL CANINE (Puppy) PARS ANTERIOR (DISTALIS). Note the excess of eosinophils lining the sinuses. No basophils in the field, as they are more in evidence in the glandular periphery. The central elements in the cell columns are neutrophilic. (Gushing.) In the hypophysis of the ox, Wulzen (Anat. Kec., 8, 8, 1914) has described a cone-shaped structure in connection with the 'pars intermedia' resembling in cellular constitution the pars glandularis but differing in having finer connective tissue septa and smaller acini. IX. EPIPHYSIS CEREBRI (Pineal body) The EPIPHYSIS (pineal Tsody, pineal gland or conarium) may be provisionally regarded as an endocrin gland. Its glandular function may be of importance only during late fetal and infantile (or pre- puberal) life, when it may control normal growth. The mammalian 580 THE DUCTLESS GLANDS ENDOCRIN GLANDS epiphysis has been known to anatomists as a probable gland since the time of Galenus (200). It has excited interest and been the subject of much speculation for centuries. Descartes (1649) regarded it as the seat of the 'soul/ Notwithstanding much study, it is still but im- perfectly understood, and even its histological makeup is to some extent in dispute. Development. The epiphysis cerebri arises as an ependymal diver- ticulum from the roof of the diencephalon. The apical cells proliferate and undergo differentiation into neuroglia and interneuroglia cells. The latter may perhaps assume secretory activities, but no convincing cy to- logical evidence appears indicative of such function apart from numerous melanic granules, which, however, are present in all of the cells in early stages of development and certain lipoid granules and spherules. In the sheep numerous cysts, lined by tall columnar (ependymal) cells appear at half term (21 cm. stage). The cysts contain a coagulum. The cysts pro- gressively disappear, apparently through pressure from proliferating cells without, only a few persisting to birth and occasionally after. In the sheep the cysts arise through accumulations of fluid, compelling a cellular arrangement simulating acini, not as tubular outgrowths from the original lumen, as is apparently the case in birds. Both melanic granules and alveoli (cysts) are more probably to be interpreted as of ancestral signifi- cance. As far as is known an epiphysis is present in all vertebrates, with the exception of Myxinoides and Crocodilia. No trace of an epiphysis is said to appear even in embryos of Crocodilia. The pineal eye of certain reptiles is commonly regarded as the homologue of the pineal body of mammals. However, in the lower groups of animals the true homologue is generally a double structure, the pineal eye developing terminally on a secondary anterior evagination from the base of the primary one. This secondary process is not developed in mammals. In Hatteria (a New Zealand 'lizard') the pineal eye comes to the surface in the middle of the head, and consists of an optic cup with a lens covered by transparent epi- dermal scales, forming a cornea. It is believed to function only as a light or warmth perceptive organ. An additional structure, the paraphysis, found in certain lower forms and in the marsupialia, anterior to the pineal body, and arising similarly as an ependymal evagination, is commonly regarded as a choroid plexus, evaginated instead of being invaginated, as is usually the case in mammals. In sheep the pineal undergoes its greatest development during the first year of life, approximately fivefold. The pineal body is attached to the posterior portion of the roof of the third ventricle by means of the pineal stalk. This contains the pineal recess, a cavity continuous with the ventricle. The anterior wall EPIPHYSIS CEREBRI 581 /.AC t. fee. ft. of 7 C*,. of the stalk is limited basally by the habenular commissure, the posterior wall by the posterior commissure. The pineal proper thus lies under- neath the posterior end of the corpus callosum and rests on the anterior pair of the corpora quadrigemina. It is a solid cone-shaped structure enveloped by a cap- sule of pia mater, and more or less distinctly lobed (Fig. 498). It measures in man 7 mm. in length by 5 mm. in transverse diameter. The mam- malian pineal is per- haps rather a meta- morphosed than a ru- dimentary organ. Its size bears no relation to the size of the FlG 493. SEMIDIAGRAMMATIC REPRESENTATION OF A brain or the size of MEDIAN LONGITUDINAL SECTION THROUGH THE EPI- the body; it is no PHYSIS OF A 17 CM. SHEEP FETUS. larger in large than in small dogs, for example, and no larger in horse than in dog; that of sheep is about the size of that of man. Function. Injection of pineal extract of sheep into the blood of cer- tain mammals produces only very slight effects. These, however, are fairly uniform, but very transient. The most conspicuous effect is a slight fall of blood pressure. There is also a slight improvement in the beat of the isolated cat's heart, a transient diuretic effect (in rabbit) and a slight ir- regular respiratory effect (in dog, cat and sheep Jordan and Eyster, Amer. Jour. PhysioL, 1911). These results are, to some extent, the reverse of those obtained with extracts of the hypophysis, and suggest a compensatory or antagonistic regulatory functional relationship. Extirpation experi- ments have not yet yielded perfectly satisfactory results. The evidence, however, indicates a functional role which is negligible or nil. Clinical evidence combined with autopsy findings indicate symptom complexes asso- ciated with pineal tumors or other structural alterations in general like, but of reverse relationship, to tnose present in case of hypophyseal altera- tion; i.e., the symptoms of hypopinealism are in general those associated with hyperpituitarism, and those of hyperpinealism like those accompany- ing hypopituitarism. The symptoms associated with morbid conditions of the pineal may possibly be the inherent result of pressure (or undue relaxa- 582 THE DUCTLESS GLANDS ENDOCBIN GLANDS tion) upon the hypophysis transmitted through the third ventricle with which both pineal and hypophysis are connected. Certain symptoms may also be due to a blocking of the aqueduct, and to pressure upon the corpora quadrigemina. A conservative estimate of all the evidence indicates very meager, if any functional activity, probably never essential to life. The practical absence of the pineal body in the opossum adds further support to this conclusion; as also its occasional disappearance, except for a mere shell, in apparently normal cats. Biedl (1910), however, arrives at the conclusion that the pineal body is an organ of internal secretion with metabolic significance limited to the young. Its general histological struc- ture, and profuse vascularity, certainly suggest a glandular function. On the basis of feeding experiments with bullocks' pineals on certain labora- tory animals and mentally defective children, Dana and Berkeley (Med. Rec., May, 1913) venture to suggest a relationship of pineal function in the young to bodily nutrition, including the development of the genital organs, the deposit of subcutaneous fat, general growth and mental prog- ress. The pineal is a common seat of cysts and tumors, frequently glio- mata. Histologic Structure. The epiphysis cerebri is divided imper- fectly into lobes by trabeculse of fibro- ,^ ^ elastic connective tissue provided by the > + . * . pia mater capsule. These support the , larger blood-vessels. The finer trabeculse shade into the fundamental reticular tis- sue which supports the parenchyma and the capillaries. The larger vessels are surrounded by considerable spaces, per- haps lymphatics. The parenchyma is ;>, ...j ; ** /i*\ I x aggregated into uncertain follicular -J masses and consists of two distinct types of cells: neuroglia and interneuroglia a (secretory cells?). The abundant neu- . nn ~ roglia fibers appear to blend intimately FIG. 499. CELLS FROM THE ri- NEAL BODY OF A 11 CM. SHEEP with the connective tissue framework of FETUS. the structure. The interneuroglia cells a, two neuroglia and one inter- are oval and polygonal, with a vesicular reurogHa cell; 6 various forms and nuc i eus and apparently lacking a cell sizes of mtracellular melamc gran- * * ules. X 1500. membrane. The neuroglia cells are fusiform or stellate, with denser, more deeply staining nuclei, and glia fibers in the cytoplasm. Transition EPIPHYSIS CEREBRI 583 forms are abundant. In young sheep both types of cells are Charac- terized by pigment granules; in aged specimens the pigment is for the most part present in the shape of extracellular clumps. Also in these stages the connective tissue is relatively much more abundant, FIG. 500. PHOTOMICROGRAPH OF A PERIPHERAL PORTION OF THE PINEAL BODY OF A 21 CM. SHEEP FETUS, SHOWING SEVERAL CYSTS AND VASCULAR TRABECUL^J, AND AN ENORMOUS NUMBER OF INTRACELLULAR MELANIC GRANULES. X 250. and the interneuroglia cells rare. The interne uroglia cells are prob- ably less highly differentiated neuroglia cells, both types originally arising from ependyma. The interneuroglia cells also contain granules and vacuoles of a lipoid nature, perhaps degenerative in significance; no pheochrome granules are demonstrable. Involution begins about the age of 7 years in man and ends about the age of puberty, though regressive histological changes proceed throughout life. Adult corpora pinealia present very variable histologic pictures. 584 THE DUCTLESS GLANDS ENDOCRIN GLANDS FIG. 501. PHOTOMICROGRAPH OF PERIPHERAL REGION OF PINEAL BODY OF A YEARLING SHEEP, TO SHOW THE CHARACTER OF THE PARENCHYMA, THE NEUROGLIA CELLS AND FIBERS, AND THE INTERNEUROGLIA CELLS. In older specimens large lamellated masses of brain sand (acervulus cerebri) appear, consisting chiefly of calcium carbonate and magnesium phosphate. These specimens are also characterized by large edematous V V .X \ FIG. 502. Two NEUROGLIA AND THREE INTERNEUROGLIA CELLS FROM THE PINEAL BODY OF A LAMB. X 1500. EPIPHYSIS CEREBEI 585 areas. The interneuroglia cells have been variously designated as 'pineal cells/ 'epithelial cells/ 'lymphoid cells' and 'secretory cells/ ~ Cer- tain workers claim to have distinguished, in certain mammals, two types of cells, basophils and acidophils. Occasionally smooth and striped muscle fibers have also been described in a few forms (e.g., ox). No nerve cells are demonstrable. Sympathetic nerves accompany the blood- vessels; and a few of the medullated type are found near the attached FIG. 503. CELLS FROM PINEAL OF YEARLING SHEEP. A, neuroglia cell and fibers; B, interneuroglia cell with melanic pigment granules. X 1500. portion, probably taking origin from the habenular and posterior com- missures. The blood supply is very profuse. It takes origin from the pial vessels which are in union with the blood-vessels of the tela choroidea anteriorly. The vessels follow the ramifications of the pial septa. In the sheep the capillaries frequently terminate in the form of tangled loops or 'glomeruli' within spaces surrounded by more com-* pact parenchyma ('follicles'). Cross sections of the coarser trabeculae show a pair of vascular comites. The dependence of normal liver function upon the integrity of the ductless glands is indicated by derangements following their removal (Whipple and Christian, Jour. Exp. Med., 29, 3, 1914). Many other 37 586 THE DUCTLESS GLANDS ENDOCEIN GLANDS organs (e.g., brain, and digestive system) no doubt hold a similar de- pendence upon the secretions (hormones) of the ductless glands. More- over, the ductless glands themselves seem to sustain interdependent relationships among themselves and with the genital glands. FIG. 504. SECTION OF PINEAL BODY OF AN OLD SHEEP, SHOWING 'BRAIN SAND' (ACERVULUS) IN THE PARENCHYMA. (For recent reviews of literature on the epiphysis cerebri see Kidd, "The Pineal Body : a Keview," Ke. Neur. Psyc., Jan. and Feb., 1913 ; and Jordan, "Besults of Kecent Studies of the Mammalian Epiphysis Cerebri," Trans. Amer. Micr. Soc., 31, 4, 1912.) CHAPTER XVII THE NERVOUS SYSTEM Am. THE DEVELOPMENT OF THE NERVOUS SYSTEM The first external evidence of the beginning of the nervous system is the appearance of a median longitudinal furrow in the dorsal (neural) ectoderm of the very young embryo (about the fifteenth day in man). This is known as the neural or medullary groove. Still earlier stages, as revealed in sections, in- clude first an increase in height of the ectodermal cells to form an axial plate of cuboidal cells, the neural plate; and secondly the conversion of this simple layered plate into a struc- ture of several strata of cuboidal cells, which structure meanwhile forms the floor and lateral walls of the developing neural groove. The neural plate apparently grows more rapidly laterally causing thus a pro- gressive elevation of its borders, the neural folds, and producing in con- sequence a gradually deepening neu- ral groove (Fig. 506). According to Glaser (Anat. Rec., 8, 12, 1914), the inequality in growth . between neural plate and the adjacent ectoderm which causes folding is due to great- er water absorption on the part of the plate. By continuation of this process of unequal growth medially and laterally, the folds finally meet in the dorsal mid-line and fuse to form the neural tube. On both sides of the line of fusion a group of ectodermal cells appears, the neural or gan- glionic crest, from which arise the neurons of the dorsal and sympathetic 587 FIG. 505. HUMAN EMBRYO 2 MILLI- METERS LONG. (Graf Spee.) X 30. Am, amnion; C, chorion; C.V., chori- onic villi; Bs, body stalk (embryo- phore); Mg, medullary groove; Nc, neu- renteric canal; Ps, primitive streak; Ys, yolk-sac. (From Williams.) 588 THE NEEVOUS SYSTEM ganglia, and the neurolemma (sheath) cells of both the afferent and efferent fibers of the dorsal and ventral roots respectively. The neural tube (canal) is originally of approximately uniform caliber throughout; but shortly the mes FIG. 506. TRANSECTION THROUGH THE GRAF SPEE EMBRYO SHOWN IN THE PRECEDING FIGURE. Am, amnion; ect, ectoderm; mes, mesoderm; ent, entoderm; M, medullary groove. (From Williams.) cephalic end, by a series of processes involving dilatations, constrictions, foldings and fusions, develops into the brain ; while the caudal portion gives rise to the spinal cord. _...- Ganglionic ridge ,.>. Ectoderm Medullary plate Ectoderm Ganglionic ridge - Myotome FIG. 507. THREE SUCCESSIVE STAGES IN THE PROCESS OF CLOSURE OF THE MED- ULLARY (NEURAL) GROOVE TO FORM THE MEDULLARY (NEURAL) CANAL AND NEURAL (GANGLIONIC) CRESTS. (From Barker, after M. von Lenhossek.) THE DEVELOPMENT OF THE NERVOUS SYSTEM 589 These gross developmental changes more appropriately considered in detail in works of Embryology involve histogenetic processes in Ihe^walls of the canal. The earlier of these processes are essentially alike in both cerebral and spinal portions of the canal. Originally the ectodermal layer, which subsequently forms the wall of the tube, consists of a single layer of cuboidal cells similarly undifferentiated. Shortly spherical and oval ger- minal cells appear among the primitive ependymal cells. These indifferent cells migrate peripherally so that two layers can now be distinguished in the wall ; an inner nuclear and an outer non-nuclear or .marginal layer. Cell boundaries have meanwhile become obliterated and the entire wall is now essen- tially a dense syncytium. Still later the syncytium assumes a looser texture, forming the myelospon- gium, and the indifferent germinal cells give rise to two types of cells: the neuroblasts, from which differentiate the neurons; and the spongioblasts, from which the neuroglia cells and fibers develop. Shortly after the time when neuroblasts and spongioblasts first become distinguishable the wall of the neural tube can be conveniently divided into three zones: (1) the ependymal layer, forming in- ternally also a limiting membrane; (2) the inter- mediate, middle, nuclear or mantle layer, containing neuroblasts, spongio- blasts and indifferent cells; and (3) the outer non-nuclear or marginal layer, bounded externally by an outer limiting membrane. A glia frame- work pervades the entire width of the wall. Neuroglia cells differentiate from spongioblasts by a process involving chiefly the formation of glia fibers in their exoplasm. These fibers apparently arise from spongioplasmic fibrils of the pro- toplasm. Many glia fibers subse- quently become disposed extracel- lularly. The primitive ependymal cells become greatly modified into stout, slightly branching fibers ex- tending through the width of the walls of the neural tube. The main body, of columnar shape, with its nucleus and a distal tuft of cilia, retains its cen- tral position and forms an ependymal lining for the spinal canal and the ventricles .of the brain. The several neuroglia constituents (astrocytes and FIG. 508. CELL LIN- ING THE NEURAL CANAL OF THE NEW- LY-HATCHED RAIN- BOW TROUT, SHOW- ING MITOCHONDRIA IN AN EMBRYONIC NERVE CELL. Meves' technic. X 2000. FIG. 509. SECTION THROUGH MEDULLARY PLATE OF A RABBIT EMBRYO. The cells include a large round ger- minal cell. (After His.) 590 THE NERVOUS SYSTEM glia fibers) form a loose-meshed syncytium, the myelospongium, for the support of the developing neuroblasts. Peripherally it is of denser texture and relatively free of nuclei, and is designated the marginal velum. Coin- cident with this process of neuroglia histogenesis, the neuroblasts enlarge, proliferate extensively and differentiate into neurons, their processes axon and dendrons arising as sprouts from the originally spherical neuroblasts. The motor neurons remain located in the ventral horn of the spinal cord (Fig. 515), their .axons (efferent fibers) passing out through the ventral root to unite with the fibers of the dorsal root FIG. 510. SECTION THROUGH MEDULLARY PLATE OF CLOSING NEURAL GROOVE OF RABBIT EMBRYO. Two germinal cells are visible. (After His.) FIG. 511. SECTION THROUGH WALL OF LATER NEURAL TUBE OF RABBIT EM- BRYO, SHOWING A STAGE IN THE DIFFERENTIATION OF THE EPENDYMA CELLS AND THE FORMATION OF A MYELOSPONGIUM. (After His.) to form a spinal nerve. The sensory neurons arise external to the cord from neuroblasts of the linear neural crests, and migrate some distance ventrolaterally, where they aggregate into metameric oval masses which differentiate into the spinal ganglia. The differentiation process includes both capsule and neuron formation. The sensory neuron located in the dorsal (spinal) ganglia is also originally spherical or oval; subsequently it becomes bi-polar; and finally, by a process involving the retraction of the cell body (cyton) from its two processes and the fusion of these processes proximally, it; becomes a unipolar cell. The definitive axon thus divides in the manner of a T or Y. Its central process, or axon proper, passes into the dorsolateral portion of the cord as an afferent fiber and here makes THE DEVELOPMENT OF THE NEBVOUS SYSTEM 591 contact through a telodendrion either directly or indirectly (through asso- ciation neurons) with the dendrons of motor neurons. Its peripheral process is to be regarded as the dendron, modified structurally so as to be essentially identical with the axon. The sympathetic may be regarded as a cenogenetic addition to the cerebrospinal nervous system. Ontogenetically it appears in man at the FIG. 512. SECTION OF WALL OF FORE- BRAIN OF FOUR-DAY CHICK EMBRYO. A, apolar neuroblast; B, bipolar neuro- blast; a, the beginning of the sprouting of the axon; c, axons, showing the terminally expanding growth area ('cone of growth'). Cajal's silver technic. (Heidenhain, after Cajal.) FIG. 513. DIAGRAM OF A TRANSECTION OF THE SPINAL CORD OF AN EARLY EMBRYO, SHOWING THE MIGRA- TION OF NEUROBLASTS TOWARD THE MARGI- NAL VEIL, AND THE VENTRAL NERVE ROOT. a, neural canal; 6, ven- tral root. (After His.) beginning of the fifth week (7 mm. embryo). Phylogenetically, it first appears in cyclostomes. Kuntz (Jour. Comp. Neur. and Psyc., 29, 3, 1910) traces the neurons of the sympathetic trunks and the prevertebral plexuses to neuroblasts in both the neural crest and the medullary wall, from whence they migrate by both the dorsal and ventral nerve roots. The neurolemma cells, as also the capsular elements of the sympathetic neurons, arise from 'indifferent' cells which migrate from these same locations and proliferate and differentiate along their course. The cardiac plexus and the sympa- thetic plexuses in the walls of the visceral organs (terminal plexuses ; Vagal sympathetic' plexuses Kuntz) have their origin, according to Kuntz, in nervous elements which migrate from the hind-brain and the vagus gan- 592 THE NEEVOUS SYSTEM glia along the fibers of the vagus nerves. Kuntz believes that 'indirect embryological and anatomical evidence warrants the conclusion that the sympathetic excitatory neurons arise from cells which migrate from the neural tube along fibers of the motor nerve roots, while the sympathetic sensory neurons, wherever such neurons exist, arise from cells which migrate from the cerebrospinal ganglia/ However, according to Carpenter (1914) the autono- mic neurons show no distinct differences from the standpoint of their chromophilic content. Since such differences are con- spicuous between the sensory and motor neurons of the cere- brospinal system (Malone), this observation arouses skepticism regarding the presence of sen- sory sympathetic ganglion cells. The primary cerebral divi- sions (Fig. 516) and their ma- jor adult derivatives are enu- merated in the appended out- line. The microscopic anat- omy of their definitive condi- tion lies outside the scope of this book. These matters are more appropriately treated in special text-books of Neurology (e.g., Quain's "Anatomy," Vol. Ill, 1908; and Villiger's "Brain and Spinal Cord," translation FIG. 514. TRANSECTION OF THE SPINAL CORD OF A HUMAN EMBRYO OF FOUR WEEKS. The central canal is immediately sur- rounded by ependyma cells. The peripheral nerve cells are shown on the left of the figure. The ventral nerve roots are already pushing outward from the primitive cord, d, dorsal; v, ventral nerve roots. (After His.) by Piersol, 1912) and constitute the work of a separate course. Likewise the developmental history is more advantageously considered in a special course in Embryology. This work will include only the histology of sections of the spinal cord; and that of the cerebellar and cerebral cortex. FIG. 515. TRANSECTION OF THE SPINAL CORD OF AN EMBRYO CHICK. c. rod. ant., axons to the ventral roots; c. rad. post., axons to the dorsal roots; col, collateral from an axon back to the gray matter; gg, dorsal root ganglion; me. ant., ventral root; rac. post., dorsal root. (After van Gehuchten.) Head fold" Neuropore ./ Pharynx Vitellin vein Medullary groove nterior cerebral vesicle, or pros' encephalon Middle cerebral vesicle, or mes- encephalon Posterior cerebral vesicle, or rhomb- encephalon Heart Mesodermic somite FIG. 516. RECONSTRUCTION OF THE ANTERIOR PORTION OF THE BODY OF A YOUNC CHICK EMBRYO, SEEN FROM THE DORSAL SURFACE. (After Kollmann.) DERIVATIVES OF THE NEURAL TUBE A. MYELON (spinal cord) I. RHOMBENCEPHALON (rhomboid brain) 1. Sacral region 2. Lumbar region 3. Thoracic (dorsal) region 4. Cervical region (a) MYELENCEPHALON f 5. Medulla (after brain) T Oblongata (b) METENCEPHALON f 6. Pons Varolii II. MESENCEPHALON (mid brain) (hind brain) (c) MESENCEPHALON (mid brain) III. PROSENCEPHALON (fore brain) (d) DIENCEPHALON (interbrain or 'tween brain) (e) TELENCEPHALON (end brain) C 6. I V. Cerebellum 8. Region of the crurso cerebri (brain stem) ; corpora quadrigemina 9. Region of the optic thalami (basal nuclei) 10. Optic tract (primitive optic evaginations) 11. Posterior lobe of hypophysis cerebri 12. Pineal body '13. Cerebral cortex (pallium) 14. Corpora striata 15. Olfactory lobes 594 (f "v^; : "x - ; 'T*>V -'lv^ V' v ?^r^ stfwvuO J\J^p^^^vi^!' : ^ ; '*^'t? V| ipuvjoy vsouijnpB vijuvisqn 596 THE NEKVOUS SYSTEM THE SPINAL CORD STRUCTURE The spinal cord (Fig. 517) consists of a considerable mass of central gray matter which is surrounded by a layer of medullated nerve fibers, the white matter. The gray matter comprises two lateral portions united by a central commissure (gray commissure) in transverse section resembling the letter H. Each lateral portion includes a ventral and a dorsal horn with an intervening deeper portion, the central mass or 'intermediate zone' of Golgi. Each horn or cornu ends in a head or caput united with the intermediate portion or basis cornu by a neck or cervix cornu. The ventral is somewhat broader than the dorsal horn. Its cells supply axons, which, after uniting into bundles, pass ventralward through the white matter to form the ventral (anterior) nerve roots. The spinal cord may be considered as consisting of ontogenetic segments whose number corresponds to the number of the spinal nerves. Hence each segment contains the ventral horn cells whose axons form the ventral root of the corresponding spinal nerve. In an entirely similar manner the dorsal horns of the gray matter receive a large portion of the incoming fibers of the dorsal roots, which in large part form end brushes around the cells of the dorsal horns and the intermediate zone (Fig. 518). The dorsal roots enter through a distinct longitudinal groove, the dorsolateral sulcus. At the exit of the ventral roots there is, how- ever, only a broad, shallow indentation, these roots making their exit in isolated bundles distributed through a vertical plane of considerable width. The dorsal root fibers of each segment, on the other hand, enter in a single compact mass. The gray matter consists of a dense tangle of nerve cells and non- medullated fibers, both axons and dendrons, together with neuroglia and blood-vessels. The fibers of a given area are derived not only from nerve cells in their immediate vicinity, but also include many processes which come from very distant regions. The gray reticulum is thus supplied from fibers of the ventral and dorsal nerve roots, to- gether with innumerable collaterals, not only from the root fibers, but more especially from those fibers which collectively form the many large tracts passing up and down the spinal cord and placing each seg- 597 598 THE NERVOUS SYSTEM ment in communication with many other levels of both the spinal cord and brain. The center of the gray commissure contains the central canal which lies in the axis of the spinal cord and is continuous above with the ven- tricles of the brain. It represents the remains of the fetal neural canal ; and in the young subject is still patent, filled with cerebrospinal fluid, and lined by columnar cells which are frequently ciliated. In older subjects the cells of the lining epithelium have usually lost their cilia, and the lumen of the canal is more or less filled by cell proliferation which involves not only the lining epithelium but also the surrounding glia cells. The central canal is immediately surrounded by a peculiar gelatinous tissue in which are many glia cells. This mass is called the substantia gelatinosa centralis. A similar area of gelatinous tissue occurs near the extremity of the dorsal horns, and is called the substantia gelatinosa Rolandi. The latter, however, is said to contain only a scanty supply of neuroglia. The white matter forms a covering or shell -around the central gray mass. It increases in thickness from below upward. This pecu- liarity is the result of the progressive influx of centripetal fibers, and a corresponding contribution of centrifugal fibers, through the spinal nerves of each successive segment. The dorsal median septum extends inward from the shallow sulcus on the dorsal surface of the spinal cord to the central gray commissure, and divides the dorsal mass of white matter into two white columns, lying on either side of the median line, and bounded laterally by the dorsal horns of gray matter and the dorsal nerve roots. The ventral median sulcus in a similar manner, splits the ventral portion of white matter into the two ventral white columns. This sulcus, however, does not penetrate all the way to the gray commissure but leaves an interval of white matter containing many transverse and obliquely disposed nerve fibers. The ventral or white commissure thus formed connects the two ventral columns of white matter. The spinal cord is thus divided into two lateral and symmetrical halves by a 'plane passing through the ventral and dorsal median sulci and the central canal. Each lateral half includes a central curved mass of gray matter completely surrounded, except at the gray commissure, by the white matter. The latter is subdivided into a ventral, lateral, and dorsal column, each of which extends the entire length of the spinal cord and is apparently (to the naked eye only) continuous above with a similar column in the medulla oblongata. THE SPINAL COED 599 The ventral white column is included between the ventral jmedian sulcus and the ventral gray horns and nerve roots; the lateral columns extend from the ventral roots in front, around the lateral surface of the spinal cord, to the dorsal roots; the dorsal columns are included between the dorsal horns of gray matter and dorsal nerve roots, and the dorsal median septum. Each of these columns of white matter is again subdivided by neurogliar septa of variable size and number, which extend inward from the pia mater for a considerable distance. Such septa may even penetrate all the way to the central gray matter. One of these septa, the paramedian septum, more constant than the others, subdivides the dorsal column into two portions, a dorsomedial and a dorsolateral column, or the funiculus gracilis (column of Goll), and the funiculus cuneatus (column of Burdach) of the upper portions of the cord. In the ventral white column two chief fiber tracts are recognized: a narrow median, the anterior or direct pyramidal tract; and a more lateral larger anterior ground bundle. In the lateral column four main tracts are recognized: three lateral, including an upper crossed pyram- idal tract, a middle direct cerebellar tract, and a lower or Grower's column; and a large medial lateral ground bundle (Fig. 518). These tracts do not possess sharp boundaries. Moreover, the ground bundles at least are composite. The several elementary tracts contain fibers passing either from the periphery to the brain along the cord (ascending fibers) or from the brain through the cord to the periphery (descending fibers), or fibers connecting brain and cord or different segments of the cord. For a detailed description of the constitution of these fiber tracts reference must be made to the systematic text-books on the nervous system. The larger blood-vessels are distributed along the fibrous septa, taking their origin from the vessels of the pia mater; the most of them are distributed to the white matter, but to some extent they also supply the gray matter. The entire surface of the spinal cord presents, just beneath the pia mater, a thin superficial layer or marginal veil of glia tissue. In the brain this layer is somewhat exaggerated in thickness'. THE REGIONS OF THE SPINAL CORD The varying number of fibers which are given off at different levels of the spinal cord results in considerable differences in size in its sev- eral portions. By means of these peculiarities, as well as by the spinal 600 THE NERVOUS SYSTEM FIG. 519. TRANSECTION OF THE SPINAL CORD OF A CHILD, THIRD SACRAL SEGMENT. Weigert stain. X 7. nerve roots to which they give origin, we distinguish a sacral, lumbar, thoracic, and cervical region. Each of these regions presents certain more or less important morphological characteristics. In the sacral region the investment of white matter is very thin, the gray matter though actually less in amount than in the more cephalad regions ap- pearing large by comparison. Both the ventral and dorsal horns of gray matter are short and thick. The substantia gelatinosa Eolandi is of considerable volume. The cell groups in the ventral horns of this region are a ventromedial and a dorsolateral. The cord as a whole is small and its transection nearly circular in outline. The five segments of this region contain the neuron centers for the urinary bladder, the anus, some of the musculature of the lower limbs, and the sensory reflexes of the perineum and genito- urinary organs. Below the sacral region the spinal cord tapers rapidly (conus medullaris) and is con- tinued downward for a consid- erable distance as the filum terminale. The surrounding leash of lumbar and sacral nerve roots forms the cauda equina. The fibrous mem- branes which surround the cord continue even farther downward in the medullary canal to form the central liga- ment, which is finally attached to the sacrum or coccyx. In the lumbar region there is a distinct enlargement, chiefly in- volving the gray substance, which here includes the immense number of cells of the ventral horns whose 'motor' fibers enter the large lumbar nerve trunks for the supply of the lower limbs. These nerve trunks also supply to the cord a great number of centripetal or sensory fibers FIG. 520. TRANSECTION OF THE SPINAL CORD OF A CHILD, FIFTH LUMBAR SEG- MENT. Weigert stain. X 7. THE SPINAL COKD 601 which enter the dorsal and, later (through secondary neurons ) x the lateral columns; thus both of these columns are of large size in and above the lumbar region. The dorsomedial column in this region at- tains an appreciable size, and a distinct pial septum marks its lateral boundary. The spinal cord is now nearly circular in transection, its ventro- dorsal being perhaps slightly less than its transverse diameter (10 mm.). The gray commissure lies very near the middle of the spinal cord, and the ventral me- dian fissure is, therefore, quite as deep as the dorsal median sep- tum. Both the ventral and dorsal gray horns are long and thick. Each dorsal horn contains a large apical area of gelatinous substance, is somewhat longer on its lateral than on its medial side, and reaches nearly to the dorsal surface of the spinal cord, opposite the dorsolateral sulcus. The dorsal nerve roots entering at this level are apparently directed toward the middle of the tips of the dorsal horns of gray matter; once within the spinal cord they pass around to the mesial side of the dorsal horns. The ventral horns, somewhat larger than the dorsal, present two short and broad protuberances, the one at the ventromesial, and the other and more prominent at the ventrolateral angle. A similar though less prominent protuberance is seen at the base of the ventral horn, on its lateral aspect. Each of these projections contains a more or less well-defined group of motor nerve cells. The cell groups of the ventral horns in the lumbar region are therefore a ventromedial, ventro- lateral, and dorsolateral, together with an ill-defined central group oc- cupying the deeper 'intermediate zone' of gray matter. The nerve centers contained in the lumbar region control the re- flexes and musculature of the lower limbs and the lower part of the abdominal wall. A transection of the spinal cord in the thoracic region Is of small 38 FIG. 521. TRANSECTION OP THE SPINAL CORD OF A CHILD, EIGHTH THORACIC SEGMENT. Weigert stain. X 7. 602 THE NERVOUS SYSTEM diameter (eight millimeters), and is very nearly circular in outline. The white matter, since it contains the many nerve fibers going to and coming from the lumbar enlargement, is much more voluminous than the gray matter. The latter is reduced to a comparatively small central mass. The dorsomedial column attains a considerable size in this region, and is distinctly marked off from the adjacent dorsolateral column by a fibrous septum derived in part from the pia mater, in part being of neuroglia composition. The dorsal and the lateral columns, having been much augmented by the influx of fibers from the large dorsal roots of the lumbar nerves, form the larger part of the white matter. The gray matter consequently appears to be pushed forward, its gray commissure lies considerably ventrad of the center of the spinal cord, the ventral median fissure is shorter than the dorsal median septum, and the tips of the dorsal gray horns are far removed from the surface, being only connected with the dorsolateral sulcus by the slender dorsal nerve roots. In fact, the dorsal horns of gray matter in this region are reduced to a minimum size; they are short and slender and contain comparatively few nerve cells. At the base of each dorsal horn, on its mesial side, there is a dis- tinctly outlined cell group whose transection is of oval or circular out- line. Indeed, this cell group, the cell column of Clarke, begins in the second or third lumbar segment, and is continued upward to the second or third thoracic at times even into the lowermost cervical segments at which level it has dwindled to a relatively insignificant group. In the lower lumbar region an ill-defined group of cells occupying a similar position and having the same function is known as the nucleus of Stilling. The ventral gray horns are very short and narrow, and their cells cannot be subdivided into groups as in the other regions of the spinal cord. In the upper part of the thoracic region a distinct protuberance makes its appearance at the base of the ventral horn, on its lateral aspect. This is the precursor of the larger lateral horn of the cervical region. In the upper thoracic region it contains a small cell group, the dorsolateral. The nerve centers of the thoracic segments control the upper abdom- inal region, the thorax, and the viscera. In the lower half of the cervical region the spinal cord presents a distinct enlargement, within the gray matter of which are the nuclei for the upper limbs. The spinal cord in this region is somewhat flat- THE SPINAL CORD 603 tened, its transverse diameter (14 mm.) considerably exceeding its anteroposterior. The major portion of the white matter is stilF con- tained within its dorsal rather than its ventral portion, the gray com- missure appearing to lie somewhat ventrad of the center. The dorsal median septum dips inward for a much greater distance than does the ventral median fissure. The dorsal columns are decidedly larger than the ventral, and a distinct groove, the pa'ramedial sulcus, from which a fibrous septum is continued inward, separates the dorsomedial from the dorsolateral column. The dorsal gray horns are long, relatively slender, and more divergent than in the lower levels. They do not reach the surface of the spinal cord, but are connected therewith by the long, slender dorsal nerve roots. The gray matter of the dorsal horns in this region is more or less invaded by bundles of nerve fibers derived from the lateral and dorsal columns; the tips of the dorsal horns are thus almost cut off from the deeper portions of gray matter. The ventral horns are both long and broad. They present three noticeable promontories or processes a medial (ventromedial), a ventral (ventrolateral), and a lateral. The lateral, because of its special promi- nence, is frequently called the lateral horn; it is one of the noticeable characteristics of the cervical region. Each of these processes contains a corresponding cell group; hence we distinguish in the cervical enlargement a mesial, a ventral., and a lateral group, together with a small intermediolateral, which is partially or completely detached from the dorsal portion of the lateral group. There is also a small disseminated central group of nerve cells occupying the deeper portion of the ventral horn. The nuclei of the segments included in the cervical enlargement contain the centers for the musculature and sensory reflexes of the upper limbs. The partial control of the pupillary movements in the eye is also located in the lowermost segments of this region. In the upper half of the cervical region a transection of the spinal cord, except for its larger size, resembles very closely that of the thoracic region. The larger size is due to an increase in the white matter of the dorsal and lateral columns, consequent upon the acquisition of new fibers which enter the subjacent segments from the nerves sup- plying the upper extremities, together with an increased number of centrifugal fibers from the cerebrum which are distributed to the gray matter of this region. C04 THE NERVOUS SYSTEM The ventral columns are also much increased in size by the addi- tion of many fibers coming down from the medulla and cerebellum, which place the nerve centers of the spinal cord in close relation with those of the cranial nerves and with the association centers of the cere- bellum. In addition to the large size of its white columns, a noticeable char- acteristic of the upper cervical region is the "prominence of its lateral horns of gray matter. Just dorsal to the lateral horns is also a peculiar reticular for- mation which results from an invasion of the adjacent portions of the lateral white columns by bands of gray matter. The gray matter thus forms a coarse net- work whose meshes inclose isolated bun- dles of longitudinal nerve fibers. The ventral horn cells of this region are scarcely divisible into groups, but a large and distinct cell group, the intermediolateral cell column, occupies the so-called lateral horn. The nuclei of the upper cervical region innervate the skin and muscles of the neck and shoulder, they also supply the diaphragm. The nerve cells of this region not only supply the cervical spinal nerves, but they also send root bundles to the spinal accessory or eleventh cerebral nerve. FIG. 522. TRANSECTION OF THE SPINAL CORD OF A CHILD, FOURTH CERVICAL SEGMENT. Weigert stain. X 7. THE CEREBELLUM The cerebellum consists of an irregular core of white substance, the medulla, and a thick mantle of gray matter, the cortex. It com- prises two hemispheres or lobes connected by a third lobe, the vermis. Each lobe is a combination of lobules which include a variable number of transverse convolutions or folia. Each folium contains a medullary eos 606 THE NEEVOUS SYSTEM core and is covered by a cortical layer. The central medulla contains four paired masses of gray substance, the internal nuclei. THE CEREBELLAR CORTEX The cortex is divisible into an inner granular layer, and an outer molecular layer. The granular layer in a fresh section appears rust- colored; in a stained preparation it appears much darker and more granular than the molecular layer due to the abundance of nuclei; it is thickest over the summit of the folium, and thinnest in the depth of the sulcus between adjacent folia. FIG. 524. FROM A SECTION OF THE CEREBELLAR CORTEX OF MAN. a-a, pia mater; b-b, molecular layer; c-c, granular layer; d-d, white matter of the medulla. Nisei's stain. Photo. X 38. THE CEREBELLUM 607 The most conspicuous and distinctive elements of the cerebellar cortex are the Purkinje cells, situated along a line marking the division between molecular and granular layers. They are regarded as belong- ing to the molecular layer. The remaining cellular elements of the cortex cannot be studied in detail in ordinary histologic preparations. They must be identified mainly by their relative size and position. Our FIG. 525. A PURKINJE CELL FROM THE HUMAN CEREBELLAR CORTEX. Moderately magnified. Photo. (After Berkley.) knowledge of their finer structure and their interrelationships has been gained by use of special staining technics, chiefly the Golgi process. By use of this technic two other . types of cells can be distinguished in the molecular layer: the small cortical ceils, and the large cortical or basket-cells. The Purkinje cells are large flask-shaped elements, with typically a robust dendron and a delicate axon at opposite poles. The axon passes centrally to contribute to the white matter 'of the medulla. It gives off numerous collaterals, some of which turn back into the molec- ular layer and terminate in relation with neighboring cells of Purkinje. The dendron passes toward the surface dividing almost immediately 608 THE NEEVOUS SYSTEM into two coarse branches, which each divides dichotomously into a suc- cession of increasingly more delicate branches forming peripherally a dendronic field of extreme profusion of non-anastomosing fibrils. The dendron viewed as a whole constitutes a fan- shaped structure. Its expansion is in a plane at right angles to the long axis of the convolution. . In sections parallel with the long axis of the convolution the dendronic field is very narrow, and never wider than the diameter of the cell-body. The basket-cells, or large cortical cells, are multipolar elements with relatively short robust branching dendrons, and a long axon which passes horizontally in the same plane in which the dendronic expan- sion of the Purkinje cells are placed. Along its course it gives off five or six collaterals which, as also the post-collateral portion of the axon itself, pass centrally toward the Purkinje cells where each breaks up into a profuse terminal arborization which invests the cell-body in the manner of a 'basket.' The basket cells occupy the middle and outer portions of the molecular layer; they are apparently of the nature of association neurons, perhaps coordinating the function of a number of Purkinje cells. The small cortical cells are distributed throughout the molecular layer but are more abundant in the outer half. They are mul- tipolar and vary considerably in size, some being almost as large as the basket cells. They possess from two to five delicate den- drons distributed for the most part in the same plane as those of the Purkinje cells. Their short slender axon, which is horizon- tally placed, is frequently characteristically The axon of some of these cells has numerous collat- FIG. 526. A PURKINJE CELL FROM THE CEREBELLAR CORTEX OF THE RABBIT. Highly magnified. (After Nissl.) looped, erals. The nuclear or granular layer also contains three distinct types THE CEREBELLUM 609 of cells: the granule cells, the large stellate cells and the so-called soli- tary cells. The latter are small fusiform elements, but of their signifi- cance and relationships little is definitely known. The GRANULE CELLS have a general distribution throughout the granular layer. They are of relatively small size, multipolar, and pos- sess short dendrons (frequently four) which end in claw-like processes. These latter are in close relationship with the so-called eosin bodies, small spheroidal finely granular masses having a special affinity for eosin. The eosin bodies are believed by some to represent synapses between the dendrites of the granular cells and the telodendria of the centripetal mossy fibers contributed by the medulla. The axon of the granule cell is unique among the cerebellar cortical neurons in that it passes toward the surface; here it divides in T-fashion, the branches passing parallel to the long axis of the convolutions thus coursing at right angles to the dendronic expansion of the Purkinje cells, in relation with which they probably end. The LARGE STELLATE CELLS have a limited distribution in the granu- lar layer; they are more or less closely confined to the boundary zone between the molecular and granular layers. They are large multipolar cells, with a profuse dendronic contribution to the molecular layer, and an almost equally profuse short axonic and collateral contribution to the granular layer where the telodendria end in relation to the granule cells. These are typical Golgi cells of the second type. THE MEDULLA The medulla next the cortex contain three important types of medul- lated fibers: the climbing fibers, the mossy fibers, and the axons of the Purkinje cells. The latter are centrifugal fibers passing to the internal nuclei. The climbing fibers are so designated by reason of their end- ing in profuse telodendria which are closely associated with the dendrons of the Purkinje cells, apparently 'climbing' over them. The mossy fibers owe their designation to the nodular mossy character of their terminal branches within the granular layer. Their end-arborizations are in- timately related to those of the dendrons of the granule cells prob- ably contributing to the formation of the eosin bodies. Both cortex and medulla contain an abundant neurogliar supporting substance. The astrocytes of the cortex are largely of the short-rayed (mossy) type; those of the medulla are exclusively of the long-rayed Axon Telodendrion Large of cell of of collateral Basket stellate granular climbing cell cell layer fiber \ V iJlnilf ^e S 1 -2 o s >1 jdfivj j,3flvi > .Cl .0 _ 73 '!*' *J CQ Q 1 o> ^ "5* o ^ -% o ^ "^ !ll 2^ ^ .3 Q ^ " 111 ,S -5 -o ^ gW bC 02 rt ll S III bc'Sb^ d d O II a r^? 1 "3 H --W) ^ ^ o l^ ^"^ |!1 t*.& . FIG. 542. FROM A MERIDIONAL SECTION OP THE CHOROID COAT. a, membrane of Bruch; b, the inner margin of the vascular layer. Between a and b is the capillary layer or choriocapillaris; c, venule containing blood cor- puscles; d, fibrous layer of the choroid or lamina suprachoroidea. Highly magni- fied. (After Cadiat.) ana), which contains an exceedingly close-meshed capillary network. This network is specially dense near the macula lutea at the posterior pole of the eyeball. Its inner surface forms a very thin homogeneous mem- brane, the lamina basalis, lamina vitrea or membrane of Bruch, which increases somewhat in thickness as age advances. The inner surface of the lamina basalis is indented by the bases of the adjacent pigment cells of the retina. Anteriorly the vessels of the choriocapillaris, like those of the vascular layer, become continuous with the vessels of the ciliary body and iris. Between the pars vascularis and the choriocapillaris may be distin- guished a narrow dense fibre-elastic boundary zone, free of pigment. In ruminants this layer becomes pronounced, due to the presence of ro- bust connective tissue fibers, and is known as the tapetum fibrosum. This layer gives to the eyes of ruminants their characteristic metallic luster. In the eyes of carnivora and certain fishes the tapetum is com- posed of rectangular epithelioid cells, filled with peculiar glistening 638 THE EYE crystals giving to these eyes an iridescent sheen, and is known as the tapetum cellulosum. THE CILIARY BODY The ciliary body (corpus ciliare] represents the thickened anterior border of the choroid coat. It is, therefore, of annular shape and occu- pies a zone' whose posterior border blends with the choroid at a point opposite the ora serrata of the retina, and whose anterior margin is continued into the iris opposite the sclerocorneal junction. It may be said to consist of three structures arranged in layers of vary- ing thickness: 1, the ciliary muscle; 2, the fibrous layer with its ciliary processes; and 3, that portion of the pigmented epithelium of the retina which constitutes the pars ciliaris retince or ciliary epithelium, and covers the inner surface of the ciliary body. The suspensory ligament of the crystalline lens is attached to the inner surface of the retinal epithe- lium of the ciliary processes and grooves. The ciliary muscle consists of an annular mass of non-striated fibers which arise from the inner surface of the sclera near the sclerocorneal junction, and are inserted into the entire breadth of the fibrous mass of the ciliary body as far back as the anterior margin of the choroid. The muscle fibers are divisible into three sets, according to the direction of their long axis; these are the meridional, the radial, and the circular. The meridional fibers form the outer and greater portion of the mus- cle. They begin just posterior to the cornea! margin, taking their origin from the inner surface of the sclera, and radiate backward in a meridional direction for a variable distance, to be finally inserted into the fibrous bundles of the posterior half of the ciliary body (ciliary ring), the longest fiber bands passing as far back as the choriociliary junction, where they are attached to the anterior margin of the choroid. The radial fibers simulate the meridional fibers in that they radiate from the corneal margin. They pursue, however, a shorter course. From their origin they pass backward with a sharp inward curve to assume a direction which approaches that of the radii of the ocular globe (hence their name) ; they are inserted into the anterior half of the fibrous layer of the ciliary body. Their radial disposition becomes progressively more apparent toward the axial margin of the ciliary body. These fibers are far less numerous than the meridional. The circular fibers comprise numerous small non-striated muscle bundles which are interspersed among the bundles of radial fibers. 640 THE EYE They are disposed in a circular direction about the axial margin of the ciliary body on its outer surface, and hence are in relation with the inner surface of the sclerocorneal junction and the outer margin of the base of the iris. The circular muscle fibers are also interspersed among the fibers of the ligamentum pectinatum, which pass in a radial manner from the margin of the posterior homogeneous membrane of the cornea to the base of the iris and anterior margin of the ciliary body. The circular fibers are said to be deficient or even absent in myopic eyes, but are exaggerated in hyperm.etropic eyes. The disposition of the ciliary muscle fibers is such that during con- traction the fibrous ciliary body and the base of the iris are drawn forward, the choroid is made tense, and the suspensory ligament of the lens is relaxed. The lens then becomes more nearly spherical because of its own elasticity. The fibrous layer of the ciliary body consists of connective tissue, and connects the fibrous portion of the choroid to the similar tissue of the iris. It is formed by a reticulum of -the fine fibers in the meshes of which are numerous lamellar and a few pigmented cells. Buried within the outer portion of this fibrous mass and intermingling with its fibers are the fiber bundles of the ciliary muscle. Into the inner portion of the fibrous layer a vascular plexus is continued from the vascular and capil- lary layers of the choroid; branches of the ciliary arteries communicate with this plexus. Appended to the inner surface of the fibrous layer are numerous meridionally disposed folds of connective tissue which radiate from the base or outer margin of the iris to the margin of the choroid opposite the ora serrata. These are the ciliary processes. Their inner or free surface is covered by the pigmented retinal epithelium, and within these proc- esses are contained the greater portion of the pigmented connective tissue cells of the ciliary body. Each fold is much deeper (about 1 mm.) toward its axial margin and becomes progressively diminished in height toward the choroid. The pigmented epithelial layer is here and there invaginated into the fibrous tissue of the ciliary processes to form ampullate recesses (the ciliary glands), which somewhat resemble true secreting glands. These so-called glands have been supposed to be concerned in the secre- tion of the aqueous humor. They are probably not true secreting glands, but represent mere invaginations of the epithelium. The ciliary epithelium (pars ciliaris retina) consists of a double layer of epithelial cells, continuous posteriorly with the retina, and in THE MIDDLE COAT THE VASCULAR TUNIC 641 front with the pars iridica retinae. The superficial (innermost) cells present a clear or slightly granular cytoplasm with a centrally situated nucleus. Their cytoplasm is but slightly pigmented, and ofttimes is indistinctly rodded or fibrillated. In shape, these cells are of the low columnar type, but they become progressively flattened toward the iris, where they are continuous with the pars iridica retinse. They represent a continuation of the sustentacular cells of the retina. The cells of the deeper (outer or anterior) layer vary in height from a low columnar at the ora serrata to a somewhat flattened cell near the iridal margin, and are continuous with the pigmented cell layer of the retina. This cell layer is deeply pigmented, the entire cytoplasm being filled with the dark brown pigment granules. The nucleus, however, as in the pigmented cells of the choroid, contains no pigment, and therefore, in unstained preparations, appears under the microscope as a clear opening in the dark background of pigmented cytoplasm. THE IRIS The iris (Figs. 558 and 543) forms an annular curtain which pro- jects from the anterior margin of the ciliary body toward the axis of the eye. It presents a central circular opening, the pupil, which lies in the visual axis. The iris is suspended in the aqueous humor, its pupillary margin resting gently upon the anterior surface of the lens, its base or ciliary margin being separated from the lens by an interval, the posterior chamber, which is also filled by the aqueous humor. The iris may be said to consist of three layers: 1, the external epithelium; 2, the fibrous stroma; 3, the internal epithelium. The external epithelium (endothelium of the iris) is a mesen- chymal epithelium continuous at the margin of the anterior chamber with the posterior epithelial layer of the cornea, which appears to be reflected upon the anterior surface of the iris. At the pupillary border it is also continuous with the internal epithelium of the iris (pars iridica retinse). The cells of the anterior or external epithelium are very much flattened and almost endothelioid in appearance; at occasional intervals the epithelium is incomplete. These intervals occur either near the pupillary or the ciliary margin, and correspond to recesses which open directly into the fibrous stroma of the iris and become continuous with its lymphatic interstices. To the naked eye the anterior surface of the iris presents an uneven 642 THE EYE appearance, which is apparently due to the presence of slight meridional ridges, with shallow intervals, which extend from the pupillary margin of the iris to its outer border. The lighter radial and circular markings are due to the blood-vessels. The fibrous stroma of the iris (pars choroidalis iridis, pars uvealis iridis) consists of a loose spongy connective tissue of an almost embry- onal type. Its fibers are scanty and are gathered into small bundles, which interlace somewhat, but which are for the most part disposed in a meridional direction. This disposition is especially noticeable near the ciliary margin. The fibrous stroma is very rich in connective tissue cells, which are mostly stellate and branch and interlace freely. They contain more or less brownish pigment, which is most abundant near the posterior (in- ner) surface. The color of the iris, when viewed with the naked eye, is dependent upon the depth of pigmentation in these connective tissue cells, as well as in the cells of the internal epithelial layer, and to the relative transparency of the stroma. In dark blue and black eyes the stroma pigment is scanty, and the very dark epithelial pigment shows through the more anterior clear layers of the iris. In the brown eye the stroma pigment is dense and opaque. A gray color is produced by a scanty stroma pigment clouded by a rather dense fibrous stroma. In the eyes of albinos the iris lacks pigment altogether; the pink color of the iris is due to the blood in the numerous vessels of the stroma. Embedded in the fibrous stroma, near its pupillary margin, is a small bundle of non-striated muscle fibers, which are circularly disposed, to form the so-called sphincter muscle of the iris. Its fibers are distrib- uted in a plane parallel to the surface of the iris, and within the inner (posterior) part of its fibrous stroma. They are most abundant near the pupillary margin, and become progressively thinner toward the base of the iris. Internally to the sphincter muscle, and in contact with the basement membrane of the internal epithelium, is an incom- plete layer, more distinct toward the ciliary margin of the iris, which contains radially disposed smooth muscle fibers, the dilator muscle of the iris, apparently of ectodermal origin. Mydriatics (e.g., atropin) pro- duce an enlargement of the pupil, presumably through inhibition of the constrictor and stimulation of the dilator muscles; miotics (e.g., morphin) produce a contrary effect. The stroma of the iris is exceedingly vascular, the arteries and veins being meridionally disposed, the capillaries forming an irregular plexus. Near the pupillary margin the vessels form a rich capillary anastomosis, THE POSTERIOR CHAMBER 643 the circulus minor. The entering arteries likewise form a circulus major by anastomoses at the ciliary margin of the iris. The internal epithelium (posterior epithelium, pars iridica retinae) resembles that of the ciliary body or pars ciliaris retinae, with which it is continuous. The innermost (superficial) layer of epithelial cells, in the iridal epithelium, is deeply pigmented and somewhat flatter than in the ciliary body. The pigmentation is so deep that in the adult iris it is scarcely possible to distinguish the two epithelial layers. These dan, however, be readily seen in the fetal eye, and even in that of the child, THE ANTERIOR CHAMBER The anterior chamber is bounded in front by the posterior (internal) surface of the cornea, and behind by the anterior surface of the crystal- line lens and the anterior (external) aspect of the iris; it contains the aqueous humor presumably largely a filtration product from the numer- ous blood-vessels of this region, in part a secretion product of the ciliary epithelium. Its anterior boundary is convex, its posterior concave, and its circular margin is limited by an area which is known as the irido- corneal angle. At this angle the mesenchymal epithelium is reflected from the poste- rior surface of the cornea upon the anterior surface of the iris. The latter portion of the epithelial layer is incomplete, since it presents nu- merous openings which communicate with the lymphatic spaces between the fibers of the ligamentum pectinatum and ciliary muscle. These lym- phatic recesses are the spaces of Fontana. The ligamentum pectinatum consists of fibers which arise from the margin of Descemet's membrane, and pass backward and inward, in a radial direction, to the fibrous stroma of the iris and ciliary body. Viewed from the cavity of the anterior chamber the fibers of this liga- ment, with the intervening spaces of Fontana, present a toothed appear- ance; the ligament derives its name from this peculiarity. THE POSTERIOR CHAMBER The posterior chamber is an annular cavity, somewhat triangular or trapezoidal in transection, whose lumen, like that of the anterior cham- ber, is occupied by aqueous humor, suspended in which are the fibers of the suspensory ligament of the crystalline lens. 644 THE EYE It is limited anteriorly by the internal surface of the iris, and an- tero-externally by the ciliary processes. Its postero-internal boundary is formed by the marginal portion of the lens, together with the adjacent portion of the hyaloid membrane, which incloses the vitreous humor. THE INTERNAL COAT THE NERVOUS TUNIC The internal coat of the eyeball is divisible into three portions: 1, the pars optica retinae or retina proper; 2, the pars ciliaris retina?, and 3, the pars iridica retinae. The last two portions, though morphologically continuous with the pars optica retinaa, differ therefrom in their physiological function ; they respectively form the innermost layer of the ciliary body and iris. As such they have already been described. GENERAL CONSIDERATION OF THE RETINA The retina (pars optica retince) may be said to be formed by the radial expansion of the fibers of the optic nerve which enter the eye at the inner side of its posterior pole, piercing the sclera and choroid and spreading out over the inner surface of the eyeball. These nerve fibers arise from groups of nerve cells which are disposed in layers to form the optic and retinal ganglia (ganglion nervi optici .and ganglion retince). The association of nerve cells and fibers with their supporting tissues forms the inner, cerebral, or neural portion of the retina. The dendritic arborizations of many of these nerve cells lie within the outer half, or neuro-epithelial portion of the retina. The retina may be said to extend forward from the entrance of the optic nerve (optic disk) as far as the posterior margin of the ciliary body, where it apparently ends abruptly with an indented border, the ora serrata. From this border the retina is continued farther forward, but only as the dark pigmented layers of the ciliary processes and iris. In the usual preparations these layers contrast intensely with the opaque white color of the true retina. Like all the other tissues which are placed in the optical axis of the eye, the retina, during life, with the ex- ception of its pigment layer, is perfectly transparent, but becomes opaque immediately after death or local injury. The retina presents on its inner surface a slightly elevated yellow spot, the macula lutea, which lies exactly at the posterior pole of the THE INTERNAL COAT THE NERVOUS TUNIC 645 visual axis. The favea centmlis is the slight depression in the center of the macula lutea, and is the result of an apparent thinning of _ the retinal layers at this point. The papilla optica, or entrance of the optic nerve, also forming a slight elevation with a central depression, the physiologic excavation,, is placed 3.5 to 4 mm. to the nasal side of the macula lutea, and at a slightly lower horizontal plane. DEVELOPMENT OF THE EYE A word as to the development of the organ will make clearer the description of the several layers of the retina. The retina is developed as an evagination of the first cerebral vesicle, and is, therefore, to be re- garded as a detached lobe of the cerebrum itself. The evagination or optic vesicle grows forward in the embryo, and soon forms a flask-shaped process whose expanded extremity is early infolded in a cup- like manner forming the optic cup. The peripheral layer of the cup becomes the pigmented layer, the lining layer differentiates into the neural layers, of the retina. The inferior surface of this optic cup and the connecting optic stalk at first presents a slit-like deficiency, the choroidal fissure, into which grows the mesoblastic tis- sue which ultimately forms FIG. 544. THE DEVELOPING EYE IN MERID- IONAL SECTION; DIAGRAMMATIC. A, early; B, later stage. E, E, ectoderm; L, lens; M, M, mesoblast; a,, optic vesicle, pro- truding from, 6, the first cerebral vesicle; c, a thickening of the ectoderm, anlage of the tens; o, constricted pedicle of the optic cup; p, outer coat of the optic vesicle, anlage of the retinal epithelium; r, inner wall of the vesicle, anlage of the neural portion of the retina. (After Fuchs.) the vitreous humor and conveys the central artery of the optic nerve. The indented extremity of the optic cup is soon occupied by the developing lens, which arises, under the influence of the optic cup itself (Lewis, Amer. Jour, of Anat., 1904), but is formed from the overlying area of the epidermal ectoderm. 646 THE EYE According to certain investigators (Szily, 1908, et al.) the definitive vitreous humor is preceded by a primitive structure in the form of a loose- meshed reticulum of protoplasmic fibers (cytodesmata) derived from, and in continuity with, both the lens and the retina, hence ectodermal in origin. This primitive vi- treous body is subsequently invaded by vascular mesenchyma, the two tissues uniting to produce the definitive vitreous. The hyaloid membrane, surrounding the vitreous body, and the fibers of the suspensory ligament of the lens are thought by some to represent the persistent peripheral FIG. 545. SCHEMATIC RE- remains of the original ectodermal stroma. CONSTRUCTION OF THE Others, among them most recently Baldwin DEVELOPING EYE. (Arch. mikr. Anat., vol. 80, 1912), describe the a,opticcup; S ,choroidal ori ^ in of the suspensory ligament from mesen- fissure; s,, optic nerve; L, chyme. developing lens. (After Failure of closure on the part of the chor- Fuchs.) oidal fissure gives rise to various degrees of a defect known as coloboma. This condition may appear simply as a shallow cleft in the iris, or as a more or less extensive fissure in the retina or even the optic nerve, which seriously interferes with vision. LAYERS OF THE RETINA The retina may be said to consist of ten layers, which from without inward are : 1. The pigment epithelium. 2. The layer of rods and cones. 3. The external limiting membrane. 4. The outer nuclear layer. 5. The fiber layer of Henle. 6. The outer reticular layer. 7. The inner nuclear layer. 8. The inner reticular layer. 9. The ganglion cell layer. 10. The nerve fiber layer. To these several layers an additional one, the internal limiting mem- brane, is frequently added. The first five of these layers are contained within the neuro-epithelial portion of the retina, the last five form its cerebral or neural portion. 1. The pigment epithelium (layer of pigmented cells) consists of THE INTEKNAL COAT THE NERVOUS TUNIC 647 a single layer of columnar epithelial cells whose bases, of generally hex- agonal outline, rest upon and are firmly adherent to the inner surface of the choroid coat, and from whose free borders irregular processes extend inward between the ele- ments of the rod and cone layer. These epithelial cells have a finely b granular cytoplasm. Their nucle- us is oval, somewhat flattened, and placed near the base of the cell; it is, however, obscured or even entirely hidden by the mass of dark pigment granules (fuscin) by which the cytoplasm of the cell is more or less completely filled. The disposition of the pigment within the epithelial cell appar- ently corresponds to, and is de- pendent upon, the effect of light upon the retina. In an eye ex- FIG. 546. THE RETINA OF A CHILD'S EYE; MERIDIONAL SECTION. a, nerve fiber layer, the broad bases of Muller's fiber cells show distinctly; b, posed to the action of light at the blood-vessel; c, large ganglion cell layer; instant of death, the pigment d > inner reticular layer; e, inner nuclear ., layer; /, outer reticular layer, with a granules accumulate in the irreg- promine nt fiber layer of Henle; g, outer ular processes of the cells which nuclear layer; h, layer of rods and cones; surround the rods and cones, the *> la y. er of Panted epitheUum; k, choroid coat. Hematem and eosm. outer or basal portion of the cell photo. X 225. being relatively free from pig- ment. In an eye which is shaded from the light, or in one removed in comparative darkness, the pigment has apparently retracted until it lies entirely with the body of the cell. Even under these conditions the extreme base of the cell fre- quently presents a narrow zone which is rela- tively free from pigment. Similar changes in the disposition of the pigment undoubtedly occur in the living eye under the influence of exposure to varying degrees of light. The function of this pigment and of the pe- culiar changes in its disposition is still somewhat speculative, but it may, without doubt, be safely asserted that these phenomena are concerned with the renewal of the visual purple of the outer segments of the rods after the same has FlG. 547. PlGMENTED EPITHELIUM OF THE RETINA, VIEWED IN TRANSECTION. X 500. (After Fuchs.) 648 THE EYE been bleached by exposure to light. Possibly these changes possess a stimulant action upon the neuro-epithelial elements. 2. The rod and cone layer (bacillary layer) consists of a series of columnar elements which are disposed in a palisade-like manner, and whose narrow extremities are embedded in the surface of the layer of pig- ment epithelium. The rod and cone layer contains elements of two dis- tinct types, the rods and the cones, specialized receptors of the rod and cone visual cells,, and very similar to each other in their structure. Each FIG. 548. ISOLATED ROD AND CONE VISUAL CELLS OF THE PIG. o-e, cones; f-i, rods; za, outer segment; zi, inner segment of the cone, the latter consisting of an ellipsoid, ze, and a more or less elongated neck, ra; zk, cone nucleus; zi, cone fiber; sa, outer, and si inner segment of the rod; sk, rod nucleus; sf, rod fiber. (After Kolliker.) rod and each cone visual cell consists of two distinct portions, the outer of which, alone, lies in the bacillary layer; the inner portion is included in the outer nuclear layer of the retina. The outer portion is cytoplas- mic, and its broad base rests upon the external limiting membrane; the inner portion is narrow, nucleated near its center, and extends entirely through the outer nuclear layer. THE EODS. The outer, cytoplasmic, or bacillary portion of each rod visual cell consists of a somewhat thickened spheroidal base, the inner segment, and an outer filamentous extremity, the outer segment. These two segments are quite as distinct in fresh unstained tissue as in fixed THE INTERNAL COAT THE NEKVOUS TUNIC 649 and stained preparations, the distinction being due to the fact that the inner segment of each rod, while finely granular and easily stained, is also singly refractive ; the outer homogeneous segment, on the other hand, not only stains with difficulty but is doubly refractive or anisotropic. The outer, therefore, under all conditions appears bright and lustrous as com- pared with the isotropic inner seg- ment. The outer segment is said to be covered by a delicate sheath of neurokeratin, and to consist of a sub- stance chemically very similar to that of the myeliri of medullated nerve fibers. The outer segment contains the visual purple or rhodopsin which, during life, is rapidly bleached by exposure to light, and is as rapidly renewed through the agency of the pigment epithelium. Both segments, but especially the inner, under favorable conditions, present slight longitudinal striations. These striations, when present, are most distinct in the outer half of the inner rod segment where they form the so-called ellipsoid; they are gen- erally interpreted as due to linear surface grooves in the outer segments, and to fibrils in the ellipsoid. The outer filamentous segment of each rod sometimes exhibits transverse mark- ings, possibly indicating a minute structure which is comparable to a series of superposed disks. The rods have an average length of 60 microns, and an average diameter of 2 microns; of the entire extent, the outer cylindric seg- ment and the inner spheroidal segment contribute approximately equal portions. The inner or nucleated portion of each rod, the rod fiber, is found 41 FIG. 549. DIAGRAM OF THE ROD AND CONE VISUAL CELLS, AND THEIR RESPECTIVE BIPOLAR NEU- RONS. (Schwalbe.) m.l.e., external limiting membrane; n.l.e., external nuclear layer; r.l.e., external reticular (molecular) layer; n.l.i., internal nuclear layer (layer of rod and cone bipolars). 650 THE EYE in the outer nuclear layer and is continued as a fine filament, which, having penetrated the external limiting membrane, extends as far as the border line between the outer nuclear and outer reticular layers, at which a FIG. 550. A ROD AND A CONE VISUAL CELL FROM THE FUN- DUS OF THE HUMAN RE- TINA, OUTSIDE THE MACULA LUTEA. a, outer segment; 6, inner seg- ment; c, rod- or cone-fiber; d, nucleus; e, rod- or cone-foot; f, ellipsoid; g, myoid (of cone); h, external limiting mem- brane. (Schafer, Greeff.) X 1000. FIG. 551. Two CONES FROM THE HUMAN RETINA. A, from close to the ora serrata; B, from near the margin of the forea centralis. The fiber (f ) is cut short beyond the nucleus. Be- tween the ora and the fovea cones of inter- mediate lengths are found, forming a series from A to B. In the central part of the fovea, the cones are still longer than B, the increase being due mainly to an elongation of the outer segments (os) to about twice the length in B. (Greeff.) X 1000. THE INTERNAL COAT THE NEEVOUS TUNIC 651 level the rod filament ends in a knob-like expansion. Similar knobs may appear along its course. At some point in its course through the-Treclear layer the rod fiber presents a nucleated enlargement, which, under some conditions, shows one to three alternate light and dark transverse stria- tions, the optical expression of the distribution of the chromatin in the FIG. 552. FROM THE HUMAN RETINA. 1, outer nuclear layer; #, outer reticular layer; 3, inner nuclear layer; 4, inner reticular layer; a, external limiting membrane; b, rod cell nuclei; c, cone cell nuclei; d, cone bipolars; e, rod bipolars; /, an exceptionally long process of a rod bipolar. Methylene blue. Highly magnified. (After Dogiel.) form of bands. The nuclei of the rod fibers are placed at various levels in the nuclear layer, and collectively occupy nearly its entire thickness. Its outer border, however, contains relatively few rod nuclei. THE CONES. The cones resemble the rods in structure, but their cytoplasmic portion is shorter, the inner segment of which is several times as broad (35/u,by7/x). The outer anisotropic segment is espe- cially short and does not contain visual purple, while the isotropic basal segment, whose fibrillated ellipsoid occupies a somewhat greater propor- 652 THE EYE tion of its length than is the case with the homologous rod segment, rests directly upon, and may even project through the external limiting mem- brane. The inner or nucleated portion, therefore, begins as a broad nucleated mass, equal in diameter and continuous with the bacillary por- tion of the cone element, to which it is ofttimes united by a slightly con- stricted neck. In this inner portion, just within the external limiting ADJACENT TO THE CHOROID ADJACENT TO TMC VITREOUS FIG. 553. DIAGRAMS OF THE HUMAN RETINA, SHOWING THE RELATIONSHIPS TO EACH OTHER OF THE RETINAL NEURONS, AND THEIR DISPOSITION IN THE DIF- FERENT LAYERS. (From Fox's "Ophthalmology.") membrane, is the cone nucleus; it differs from the rod nucleus in that it stains less deeply, presents no transverse striations, and frequently incloses a distinct nucleolus. From its nucleated portion the cone fiber is continued as a rather broad cytoplasmic filament straight inward to the border of the nuclear layer, where it terminates in an expanded portion or cone foot, from the flattened inner surface of which the fine filaments penetrate the margin of the outer reticular layer. The outer segments of both rods and cones are embedded in the cells of the pigment layer, whose delicate filamentous processes project between the rods and cones, frequently extending almost to the external limiting THE INTERNAL COAT THE NERVOUS TUNIC 653 membrane. Both rods and cones are hexagonal or. nearly circular in transection. The slight intervals between the neighboring elements and the processes of the pigment epithelium are occupied by a homogeneous fluid, probably a somewhat modified lymph. The rods far outnumber FIG. 554. FROM A MERIDIONAL SECTION OP A CHILD'S EYE, SHOWING THE LAYERS OF THE RETINA AT A POINT MIDWAY BETWEEN THE MACULA LUTE A AND THE ORA SERRATA. 1, pigment layer; 2, bacillary layer; 8, external limiting membrane (indistinct); 4, outer nuclear layer; 6, outer reticular layer; 7, inner nuclear layer; 8, inner reticular layer; 9, ganglion cell layer; 10, nerve fiber layer; 11, internal limiting membrane; a, choroid coat. Hematein and eosin. Photo. X 400. the cones; three to four rods generally appear between two successive cones. The total number of cones in the human retina has been esti- mated at 70,000,000 ; that of rods at 130,000,000. 3. The external limiting membrane (membrana Umitans externa) consists of the flattened and amalgamated extremities of the sustentacular cells (Muller's fibers), which form the chief supporting tissue, the neu- 654 THE EYE roglia element, of the retina, and which extend from the extreme inner surface outward to the external limiting membrane. It will therefore be more convenient to defer further description of this membrane until the remaining layers have been described, and the Miillerian fibers can be considered in their entirety. 4. The outer nuclear layer (outer granular layer) consists, for the most part, of the nucleated portions of the rod and cone elements. The outermost zone of this layer contains only cone nuclei; the inner portion, comprising about three-fourths of its thickness, contains only rod nuclei. The former, with occasional exceptions (Stohr), are situated in only one relatively narrow plane ; the latter are distributed at various levels, though they occur more abundantly in the mid-region of the nuclear layer. In addition to portions of Miiller's fibers which serve for the support of the nucleated elements, this layer contains the terminal filaments of the distal processes of some of the small bipolar nerve cells of the inner nuclear layer. 5. The fiber layer of Henle is formed by that portion of the cone fibers which is internal to the layer of rod nuclei. It is a thin layer and' only acquires importance in the neighborhood of the macula lutea, where the cones are most abundant. In this portion of the retina it is easily distinguished from the outer reticular layer by the somewhat radial disposition of its fibers, the fibers of the reticular layer having an irregu- larly meridional direction. 6. The outer reticular layer (outer molecular layer; outer plexi- form layer) presents a dense tangle of neural tissues consisting of sup- porting neuroglia fibers and interlacing processes from the horizontal and bipolar nerve cells of the inner nuclear layer. Terminal fibrils from this network intermingle in the fiber layer of Henle with terminal fibrils from the cone feet; more externally they are in intimate relation with the end knobs of the rod fibers. This arrangement permits the transmis- sion of stimuli from the neuro-epithelium to the retinal ganglion. 7. The inner nuclear layer (inner granular layer, ganglion retina, outer ganglionic layer) contains a mass of nerve cells, together with the nucleated portion of the sustentacular fiber cells of Miiller. The nerve cells may be described as corresponding to one of three types, which, from the plane in which they are distributed may be termed the outer, middle, and inner. THE OUTER NERVE CELLS. The outer nerve cells (horizontal cells, basal cells) possess pyramidal, stellate, or flattened cell bodies whose dendrons are distributed to the horizontal plexus of the outer reticular THE INTERNAL COAT THE NERVOUS TUNIC 655 layer. These cells vary in size; the dendritic or distal processes of the smaller cells on reaching Henle's layer are in relation with the ierminal fibrils of the cone feet; those from the larger nerve cells are in relation with the terminal knobs of the rod fibers. The axis cylinders or central processes from all these cells after traveling horizontally viz., in a plane parallel to the layers of the retina for a greater or less distance, turn inward and pass to the inner reticular layer, where they come into FIG. 555. HORIZONTAL CELL FROM THE RETINA OF A CALF. o, cell body; 6, axon; c, terminal arborizations of the axon. Golgi's stain. X 150. (After Marenghi.) relation with the dendrons of the large nerve cells of the ganglion cell layer. Other processes, mostly from the smaller cells, terminate in the outer reticular layer, probably serving the purpose of association neu- rons. THE MIDDLE TYPE. The nerve cells of the middle type are usually of bipolar form, and are the most abundant elements of the inner nuclear layer. The one set of their processes is directed outward (periph- eral ward) ; they pass to the outer reticular layer where they eventually come into relation with either the rod fibers or the cone fibers. Hence those cells which are in relation with the visual rods are classified as 656 THE EYE rod bipolars, those in relation with the visual cones as cone bipolars (Fig. 553). The terminal fibrils of the cone bipolars are horizontally, those of the rod bipolars radially, disposed. The central processes, axons, of the bipolar cells are directed inward (central ward), and on entering the inner reticular layer terminate in an end brush which is in relation with the dendritic processes from the large ganglion cell layer. THE INNER NERVE CELL TYPE. The inner nerve cell type (amac- rine cells of Cajal) are large nerve cells which occupy a narrow zone at the inner margin of this nuclear layer. These are large stellate cells whose dendritic processes extend into the inner reticular layer and take part in the formation of the dense felt work of which that layer consists. The course of the axis-cvlinders of these cells is still a matter of some FIG. 556. Two AMACRINE CELLS FROM A TRANSECTION OF THE RETINA OF A CALF. Golgi's stain. X 260. (After KoUiker.) doubt. Eamon y Cajal, believing these cells to possess no axon, desig- nated them 'amacrine cells' and subdivided them according as their den- drons were distributed in either one of several horizontal planes (the number varying in different species) or diffusely throughout the inner reticular layer. Some of the amacrine cells, however, send an axon in a horizontal direction to the inner reticular layer, and are also in relation with the terminal arborizations of centrifugal nerve fibers which enter from the nerve fiber layer. These have been regarded by some observers as 'dis- located nerve cells' of the ganglion cell layer ; Cajal named them 'associa- tion amacrinsf 8. The inner reticular layer (inner molecular layer, inner plexi- form layer) is a densely tangled network of nerve-cell processes, a neuro- spongium. To these are added a much branched portion of Miiller's fibers, which form the chief supporting tissue of this layer. The cell proc- esses entering into this formation are derived from the cells of the inner nuclear and ganglion cell layers, and it is here that the processes of these cells interlace so closely as to permit the transmission of impulses from the one neuron to the other. Their terminal arborizations are, for the THE INTEENAL COAT THE NEEVOUS TUNIC 657 most part,, disposed in horizontal planes, though a few spread throughout the entire thickness of the reticular layer. 9. The ganglion cell layer (ganglion nervi optici, inner ganglionic layer, layer of large nerve cells') is of variable thickness. Its greatest depth is in the region of the macula lutea, where it consists of five or six superposed ganglion cells. Toward the equator of the eye it becomes progres- sively thinner, until near the ora serrata its single layer of cells only forms an in- complete stratum. The cells comprising this layer are mostly large, stellate, pyriform, or sphe- roidal nerve cells, from whose peripheral border dendrons pass to the inner reticu- lar layer, and from whose central border an axon passes to the nerve fiber layer to eventually become the axis cylinder of a fiber of the optic nerve. These cells, though varying much in size and shape, possess a body of the usual structure with neurofibrils and chromo- philic granules, and a pale vesicular nu- cleus with a distinct chromatic nucleolus. Intermingled with the nerve cells are many fine branches of the sustentacular cells which here form an open-meshed network within whose spaces the nerve cells are inclosed. 10. The nerve fiber layer, in inti- mate relation with the preceding, forms the innermost of the retinal zones. It consists of naked axis-cylinders passing from their origin in the ganglionic layers to their immediate destination, the optic nerve. They are, therefore, mostly if not wholly centripetal fibers. A few centrifugal fibers have been demonstrated in this layer, but they would appear to be probably vaso- motor in function, a few possibly ending in relation to the amacrine cells. The nerve fibers of this layer converge from all portions of the retina, follow a meridional course through the open meshes of the network of branching sustentacular cells, and converge toward the optic papilla, the. FIG. 557. A NERVE CELL OF THE LARGE GANGLION CELL LAYER; FROM THE RETINA OF A CAT. n, n, axon; c, c, collaterals. X 325. (After 658 THE EYE entrance, or rather the point of exit, of the optic nerve. Hence the nerve fiber layer, being augmented by the constant acquisition of new axons from the ganglion cells, becomes pro- gressively thicker toward the posterior pole of the eye, and is thickest at the margin of the optic papilla, where it is so highly de- veloped as to almost exclude the other retinal layers. The course of these non-medullated nerve fibers is not straight; on the contrary, they interlace to form a delicate fibrillar network. At the margin of the papilla op- tica the nerve fibers bend outward with a sharp curve almost at right angles to their former course. At this point also they grad- ually acquire a medullary sheath and, unit- ing into many bundles, penetrate the nu- merous openings of the lamina cribrosa of the sclerotic and choroid coats to form the optic nerve. THE SUPPORTING TISSUES OF THE RETINA FIG. 558. A FIBER CELL OF MULLER, OR SUSTENTACU- LAR CELL, FROM THE DOG'S RETINA. 1, nerve fiber layer; 2, gan- glion cell layer; 3, inner retic- ular layer; 4, inner nuclear layer; 5, outer reticular layer; 6, outer nuclear layer; a, a process extending into the inner reticular layer; 6, nu- cleus of the cell; m.l.e., exter- nal limiting membrane; m.l.i., internal limiting membrane. Golgi's stain. Highly mag- nified. (After Cajal.) The nucleus of the fiber layer. These consist of a gliaform reticulum distributed throughout the cerebral portion of the retina., and of a special supporting tissue, Miiller's fibers, which may also be re- garded as glia tissue, though they are com- mon to both the neural and epithelial por- tions. The fibers of Miiller (radial fibers; sustentacular cells') comprise numerous large glia cells whose processes begin with an ex- panded base at the inner surface of the nerve fiber layer, and can be traced all the way through the retina to the membrana limitans externa, which is likewise formed by the terminal expansions of these cells, cell lies in the mid-region of the inner nuclear THE INTEENAL COAT THE NEEVOUS TUNIC 65U The expanded and flattened bases or inner extremities of these glia cells are so closely approximated to one another as to form a complete investment for the inner surface of the retina, which is known as the internal limiting membrane (membrana limitans interna) and is fre- quently classed as the innermost layer of the retina. Under low magnifi- cation it appears as a continuous membrane, but under higher powers it is readily resolved into the broad, conical, basal expansions of which it consists. From these initial expansions the glia cells may be traced outward through the nerve fiber and ganglion cell layers by means of the numerous coarse processes or glia fibers. The glia fibers then pass in a fairly straight course through the inner reticular layer. In this portion numerous short, fine, lateral off- shoots from the main stem support the neurospongium of the reticular layer. Continuing through the inner nuclear layer the glia substance is somewhat thickened; it sends off fewer but coarser lateral processes, and in the mid-region of this layer presents an enlargement which is almost entirely occupied by the large ovoid nucleus. The fiber cell, somewhat narrowed, may then be traced through the outer reticular to the outer nuclear layer, where its processes form a dense network about the nucleated segments of the rod and cone visual cells. The terminal processes of the fiber cells become again flattened, some- what after the manner in which the internal limiting membrane is formed, and are so closely approximated as to form an external limiting membrane, a distinctly membranous structure which derives a reticular appearance from being pierced by each of the innumerable rod and cone elements. From the outer surface of the expanded ends of the Miillerian fiber cells which form the external limiting membrane, minute fibrils are continued between the bases of the non-nucleated portions of the rod and cone cells to form shallow sockets, the rod and cone sockets, into which the bacillary portions of these elements are fixed. The neuroglia supporting tissue includes also the ordinary long- and short-rayed astro- cytes, limited, however, to the cerebral portion of the retina. THE MACULA LUTEA / (Yellow Spot) The macula lutea being apparently the most highly developed portion of the retina, deserves some special consideration. The macula is a cir- THE EYE cular elevation about 2 mm. in diameter, in the center of which is a marked depression, the fovea, centralis. It is yellow in color, due to the presence of a pigment. The elevation results from an in- creased thickness of all the ret- inal layers, but especially of the ganglion cell layer, which in this portion of the retina is five or six cells deep. The reticular layers are also much thickened in this area. In the bacillary layer, within the area of the macula, the cones are far more numerous than elsewhere, especially when considered in relation to the rods, which are greatly diminished peripherally and absent centrally. The cones of the macula are al- most twice as long as those of the equatorial region of the re- tina, the increased length being due to elongation both of the in- ternal and external segments, but mainly the latter; they are also somewhat more slender, and their nuclei may be placed some distance beyond the external lim- iting membrane. Toward the fovea centralis the inner layers of the retina be- come very much thinned, until at its center the nerve tissues are merely represented by scattered cells of the inner nuclear and ganglion cell layers. Eod ele- ments are not found in this area ; the bacillary layer consists en- tirely of elongated, slender cones. THE INTERNAL COAT THE NERVOUS TUNIC 661 The much elongated nuclear portion of the cones deviates in a slanting direction toward the margin of the macula, and the cone nuclei are further removed from the external limiting membrane than elsewhere in the retina. The pigment of the epithelial layer is much diminished and may even be absent at the fovea. Because of the diminution in the number of ganglion cells in this area the nerve fiber layer is greatly diminished in thickness on approaching the margin of the fovea, and toward its center entirely disappears. The fovea centralis lies at the posterior pole of the anteroposterior axis. The light stimulus here meets least obstruction in passing to the neuro-epithelial elements; this is the point of acutest vision. Since the fovea centralis contains no rods, it lacks the visual purple; and since vision is sharpest at this point, the visual purple would seem unessential to sight. The visual purple is said to be absent in the eyes of the | pigeon, the hen, some rep- Jj ^ tiles and some bats. It is supposed to enhance the irritability of the rods in dim lights. In the eyes of birds and reptiles the cones out- number the rods; and in certain reptiles, e.g., liz- ards, rods are entirely lacking. In sharks and rays, most nocturnal ani- mals, and the owl, cones are either absent or few in number or rudimentary in structure. According to the view of Kreis (1895) A! FIG. 560. DEVELOPING ROD AND CONE VISUAL CELLS, FROM THE RETINA OF A 345 MM. (6 MOS.) HUMAN FETUS. M, diplosome in a Miiller's fiber at the level of the external limiting membrane; C, diplosome in a cone cell; F, fiber growing out from the cone diplo- some; R, diplosome in a rod visual cell. (Seef elder.) X 1000. the cones function in the perception of color, the rods are sensitive only to light and darkness. In color blindness the cones are defective; in night-blindness the rods are affected. Development Of Rods and Cones. The rods and cones arise and by an essentially identical mode of differentiation from apparently similar em- bryonic cells. A protoplasmic bud is pushed beyond the external limiting membrane towards the layer of pigmented cells. The cone bud is from 662 THE EYE four to five times as stout as that of the rod cell. These processes carry apically a diplosome, surrounded by a lighter cytoplasmic halo. From each member of the diplosome a fiber-process grows out, the one passing towards the elongating distal, the other towards the proximal, pole of the cell. The distal process thus becomes enveloped by an extension of the cell cytoplasm, the two constituents, fiber and investing cytoplasm, uniting in the forma- z. -.>>> t * fiV C P - *M \. % S v,-;MrV.! FIG. 561. Two EARLY STAGES IN THE DEVELOPMENT OF THE ROD AND CONE VISUAL CELLS IN THE CHICK. A, from 12-day embryo, showing the visual cell buds containing mitochondria; L.e., external limiting membrane; C.V., visual cell; R.e., external reticular layer; C.p.j bipolar cells; B, from 1-day-old chick, showing two complete rods and one cone, both elements containing a large lipoid spherule (G) and mitochondria. The cone contains an ellipsoid (E); seg. int., internal segment; seg. ext., external segment. (Leplat.) X about 2000. tion of the outer segment of the rods and cones. Seefelder ("Atlas zum Entwickslungsgeschichte des Menschlichen Auges," Leipzig, 1914) has re- cently confirmed the essential points in the earlier descriptions of rod and cone differentiation by Leboucq (1909) and by Magiott (1910). Leplat (Anat. Anz., 45, 8, 1913) has investigated the role of the mitochondria in connection with this process. He describes the migration of the. 'plasto- somes' into the external segment where they become chemically altered and disappear as such in their contribution to the homogeneous cytoplasmic sheath of the centrosomal filament of the external segment. Leplat inclines to regard the transverse striation of this segment and its cleavage into THE INTEENAL COAT THE NERVOUS TUNIC 663 discs by maceration as the expression of its mode of construction from mitochondria. It remains uncertain whether the longitudinal fibrillation of the internal segments is likewise the probable result of an arrangement of mitochondria in the developing cell ; the complete history of these mito- chondria has not yet been traced. The Inversion of the Retina. The receptive cells of the vertebrate retina exhibit, in contrast to all other neuro-epithelial cells, a reversed polarity with respect to the source of their special stimulus, the ether waves; the transmitting end of the cells is nearer the source of the light than the percipient end. The human retina is appropriately described as Visual cell Retinal layer Pigment layer Infuniibulum Neural ectoderm Neural canal (forebrairi) Epidermal ectoderm t Optic cup Optic stalk Rathke's pouch Buccal cavity FIG. 562. DIAGRAM ILLUSTRATING BALFOUR'S THEORY TO ACCOUNT FOR THE INVERSION OF THE VISUAL CELLS OF THE VERTEBRATE RETINA. Transverse section through the head of a hypothetical vertebrate embryo, to show the morphological relations of the surfaces of the ectoderm of the integument, the neural tube, and the forming retina. In each of these situations a single sense cell is indicated. (After Parker, Amer. Nat., 42, 501, 1908.) an inverted sense-organ. The most plausible theory yet proposed in ex- planation of this inversion of the visual cells seems to be the one outlined by Balfour (1881), expressed in terms of the ancestral history of the verte- brate eye. (Among the invertebrates an inverted retina is known only in certain mollusca, e.g., Pecten, and in certain spiders and the scorpion.) 'According to this view the vertebrate retina originated on the outer sur- face of the ancestral vertebrate in much the way that the eyes of many invertebrates have been produced. The primitive retinas thus formed were implanted in that portion of the surface of the animal from which the central nervous system was destined to develop, and when this was infolded these retinas were carried in with it and came thus to be involved in the central organ. If the morphological position of a sensory cell, such as may have existed in the primitive external retina, is supposed to have been thus retained as this organ was carried from its superficial location into the central nervous system and out again almost to the external sur- 664 THE EYE face, the resulting retina would be composed of inverted elements (Fig. 562). Thus this theory at once offers an explanation for the two most striking features of the vertebrate retina, namely, its formation as an apparent outgrowth from the central nervous system and the inverted con- dition of its receptive cells' (Parker). The difficulties of this theory are s~? discussed, and an alternative Jj theory, based upon the direc- tion eyes of Amphioxus, is presented by G. H. Parker (Amer. Nat., 42, 501, 1908). THE OPTIC NERVE The optic nerve is a large nerve trunk, com- posed, like the white mat- ter of the brain of which it is an ontogenetic portion, of medullated nerve fibers without a neurolemma, sup- ported by a neurogliar net- work containing long-rayed astrocytes. It receives an investing sheath from each of the cerebral membranes, septa from the pia mater enveloping the several funi- culi. These sheaths are continued as far forward as the eyeball, at which point they become contin- uous with the sclera. Though the choroid corre- sponds to the pia mater, the two are not apparently in direct continuity. Lying in the axis of the nerve, the arteria centralis relince with its accompany- ing vein enters the eye and appears on the inner sur- THE OCULAR CONTENTS 665 face of the retina at the porus options (physiologic excavation; optic disk) in the center of the optic papilla. Here it divides, its two -branches at first pursuing a meridional course between the hyaloid membrane and the retinal surface; soon they pierce the latter to supply the cerebral portion of the retina. No vessels penetrate the neuro-epithelial portion of the retinal layers ; these are nourished by the choroid. The vena cen- tralis retince pursues a course exactly similar to that of the artery. THE ORA SERRATA At the ora serrata (Fig. 538) the typical layers of the retina, already much thinned, abruptly cease. The first elements to disappear are the rods and cones ; the cones, which become much shorter, practically lacking the outer segment, extend farther toward the ora than the rods. The remaining layers are continued forward only as the double layer of epithelial cells belonging to the pars ciliaris retinae, the inner stratum of which appears to be analogous to and continuous with the cerebral portion of the retina, apparently the sustentacular cells ; while the outer, deeply pigmented layer apparently represents the pigmented layer of the retina. For some distance toward the ora serrata, the retina becomes modified by the presence of large vacuoles, probably lymph spaces. THE OCULAR CONTENTS Within the ocular globe, whose walls are formed by the three coats of the eye, are certain structures which may be collectively considered as its contents. They are : 1. The aqueous humor. 2. The crystalline lens. 3. The vitreous humor. 4. The hyaloid membrane. 5. The suspensory ligament. THE AQUEOUS HUMOR The aqueous humor is a fluid, closely allied to lymph, which occupies the anterior and posterior chambers of the eye. Microscopically it is structureless. Occasional leukocytes, migrants from adjacent lymph channels, may be encountered. 42 666 THE EYE THE CRYSTALLINE LENS The crystalline lens with its suspensory ligament forms a sort of diaphragm which separates the ocular cavity into two compartments, of which the anterior is occupied by the aqueous humor, the posterior by the vitreous humor. The lens is a biconvex transparent body having a somewhat greater convexity on its posterior than on its anterior surface; its curvature is greater at its margin than toward its center. It has a transverse diameter of from 8 mm. to 9 mm., and an anteroposterior diameter of from 3.5 mm. to 4 mm. according to the degree of accommodation. It consists of a capsule, epithelium, and a substantia lentis. The capsule of the lens is a homogeneous membrane which covers its entire surface and receives the attachment of the suspensory liga- ment. It presents faint meridional striations and may sometimes be separated into several lamellae' (Berger, 1893) ; this lamellation may be purely artificial, but appears to be somewhat dependent upon the attach- ment of the fibers of the suspensory ligament to the surface of the lenticular capsule. The capsule is about twice as thick over the anterior as over the posterior surface of the lens. On the former surface it is in relation with the lenticular epithelium, but on the posterior surface the capsule rests directly upon the substantia lentis. The anterior surface of the capsule is in gentle contact with the free margin of the iris. The lenticular epithelium consists of a single layer of cells which covers the entire anterior convexity of the lens, extending as far back as its equator. The height of these cells varies with the age of the indi- vidual. In fetal life they are distinctly columnar, in youth short colum- nar or cuboidal, in adult life low cuboidal or flattened. Toward the mar- gin of the lens the epithelial cells become progressively lengthened, and at its equator are transformed directly into the fibers of the lenticular substance. This definitive structure of the lens recalls its manner of development from the original lens vesicle; the hollow vesicle becomes solid by the elongation of the cells of the posterior wall. The substantia lentis is, therefore, the product of the epithelium of the lens, whose cells become greatly elongated to form slender hexag- onal prisms, known as the lens fibers. When it is first formed each prism exhibits a nucleus which persists for some time, but gradually disappears as in the process of growth the older fibers become farther and farther removed from their source of nutrition, the lymph and the THE OCULAE CONTENTS 667 aqueous humor in which the surface of the lens is bathed. This change is accompanied by a hardening or cornification and slight shrinkage of the lens fibers, so that those prisms which come to occupy the center of the lens form a dense, hard mass of non-nucleated fibrous cells with faintly serrated margins; the peripheral fibers retain their smooth edges and their nuclei, and form a protoplasmic mass of much softer con- sistency. The hardened central mass is the so-called nucleus of the lens. Any opacity of the lens or its capsule is known as a cataract. The nuclei of the lens fibers remain in the neighborhood of the equator, where they are first formed, and are thus con- tained within a narrow, superficial, equa- torial zone, the nuclear zone. Each lens fiber is disposed along a meridian of the lens, and extends from its anterior to its posterior hemisphere; the fibers are so arranged that they abut upon one another, end to end, along V-- shaped lines which radiate from either pole. This union is often quite firm, and thus are formed long fibrous bands which can be traced from the anterior to the posterior hemispheres of the lens. These bands are distributed in a peculiar man- ner. Near each pole along the line of abutment, the band may be said to bend upon itself with a sharp curve making an angle of about 60 degrees whose convexity is directed toward the pole, the parallel fibers being so arranged as to form a sector whose apex is also directed toward the pole. The corresponding sectors of opposite poles overlap one another so that the fibrous bands are continued from one side of one polar mass to the reverse side of the overlapping sector and so back, on the farther side, to the adjacent sector of the former hemisphere. By teasing, fibrous bands can sometimes be traced suc- cessively through all of the polar sectors and thus back to a sector be- neath -that from which the start was made. Obviously no individual lens fiber is of sufficient length to extend from pole to pole of the lens. FIG. 564. LENS FIBERS. 1, in profile, from the crystal- line lens of the ox's eye; 2, in transection, from the human crys- talline lens. X 350. (After Koffiker.) 068 THE EYE The polar figures formed by the lines of terminal union of the lens fibers are known as lens starts; these are at first three-rayed, but later become six- and even nine-rayed. During the earlier stages of the development the lens is invested FIG. 565. THE NUCLEAR ZONE AT THE MARGIN OF THE CRYSTALLINE LENS OF A CHILD'S EYE, SHOWING THE TRANSITION OF THE LENS EPITHELIUM TO THE LENS FIBERS AND THE ATTACHMENT OF THE SUSPENSORY LIGAMENT. a, lens fibers; 6, lenticular epithelium; c, capsule of the lens; d, suspensory liga- ment. Hematein and eosin. X 273. by a delicate vascular membrane, the tunica vasculosa lentis (designated pupillary membrane in front), supplied by the hyaloid artery continued from the central artery of the retina. This tunic normally disappears before birth; its atypical persistence in whole or in part seriously inter- feres with vision. THE OCULAR CONTENTS 669 THE VITREOUS HUMOR The vitreous humor (vitreous body) is a soft jelly-like mass which fills the entire cavity of the eye behind the line of the ora serrata and crystalline lens. About 98 per cent, of its composition is water. It is completely invested by the hyaloid membrane. Its anterior excavation which holds the posterior convexity of the lens is known as the hyaloid or patellar fossa. The vitreous humor appears to be a peculiarly deli- cate form of very loose gelatinous connective tissue whose scanty fibers present a somewhat concentrically lamellated arrangement and are so very delicate as to be recognized under ordinary conditions only with the greatest difficulty. Occasionally stellate and fusiform cells, remarkable for their large vacuoles and varicose processes, have been demonstrated in small num- bers within the vitreous body. Small rounded cells somewhat resembling leukocytes are also found, but for the most part they are flattened against the hyaloid membrane; they occur in very limited numbers. These various cells, as well as occasional filamentous remnants of the original mesenchymal constituents of the vitreous humor, may cast shadows upon the retina within the visual field. Such shadows, called rmiscce volitantes, on account of their 'flitting* motion when the eyes are moved, are seen when looking at a bright light, or frequently in looking through the microscope. In advanced age crystals may form in the vitreous which are observed to settle to the bottom of the eye when the eyes are held still. THE HYALOID MEMBRANE The hyaloid membrane is a very thin structure which surrounds the vitreous humor and unites it to the inner surface of the retina and the crystalline lens. It consists of delicate glassy fibers so disposed as to form an extremely thin reticular membrane. It passes forward over the inner surface of the retina, to which it is loosely united, until at the ora serrata its fibers leave the retinal surface and pass inward to the margin of the lens to become inserted into the lenticular capsule. THE SUSPENSORY LIGAMENT (Zonula Ciliaris) Certain fibers from the hyaloid membrane pass forward from the ora serrata and are firmly adherent to the ciliary processes, or become 670 THE EYE attached in the grooves between the processes. From the sides of these processes fibers diverge at frequent intervals and pass to the margin of the lens, where they are attached on either side of the equator, spread- ing over a zone which is somewhat narrower posteriorly than anteriorly. These fibers form the suspensory ligament of the crystalline lens (Figs. 543 and 565). They occupy an annular zone which is included between the ciliary processes and the margin of the lens, and which is known as the zonula ciliaris (of Zinn). The glassy fibers of this ligament take origin from the sides of the ciliary processes along which they are firmly attached, becoming free only near the apices of these processes. They pass thence to the margin of the lens and spread out upon the surface of the capsule to which they are intimately adherent. The fibers arising more posteriorly are said to be attached to the lens anteriorly to the equator, those arising from the more anterior portions of the ciliary processes becoming attached posteriorly to the equator of the lens. The most anterior of these fibers form a somewhat plicated but in- complete membrane which serves as the anterior boundary of an annular series of connecting lymphatic spaces collectively .forming the spatia zonularis (canals of Petit). This irregularly sacculated, annular canal is bounded posteriorly by the hyaloid membrane, anteriorly by the in- complete membranous wall of the posterior chamber through which the aqueous humor readily diffuses, internally by the margin of the crystal- line lens, and antero-externally by the ciliary processes. Besides sup- porting the lens, the suspensory ligament assists in the accommodation of the lens to far and near vision, a process involving a change of con- vexity, and dependent upon the activity of the ciliary muscle. BLOOD-VESSELS OF THE EYE The circulation of blood in the globe of the eye is maintained through four sets of vessels: 1. The arteria and vena centralis retinae. 2. The short ciliary arteries and venae vorticosae. 3. The long ciliary arteries. 4. The anterior ciliary arteries and veins. 1. The arteria centralis, destined for the supply of the retina, en- ters the optic nerve about midway between the optic commissure and the ocular globe, and arriving at the center of the nerve runs in its axis BLOOD-VESSELS OF THE EYE 671 to the papilla optica, at which point it divides into two branches, an inferior and a superior branch, which, by rapid dicho- tomous division, radi- ate from the optic papilla to all parts of the, retinal surface, thereby forming a plexus of small ar- teries within the nerve fiber and gan- glion cell layers. From this plexus ca- pillaries are distrib- uted to all the cere- bral layers of the re- tina. No blood-vessels are found within the neuro-epithelial lay- ers. The retinal ar- teries, like those of the brain, do not anas- tomose with one an- other; they are ter- minal arteries. The retinal veins follow a course exact- ly similar to that of FIG. 566. SCHEMATIC REPRESENTATION OF THE INTRINSIC BLOOD-VESSELS OF THE EYE. Arteries in outline, veins in solid black. A, choroid; a, central artery, and c&i, vein of the retina; B, conjunctiva; 6, retinal arteries; b lt retinal veins; c, c, short ciliary arteries; d, long ciliary artery; e, e 1} anterior ciliary arteries and veins; /, chorio- capillaris; g, capillaries of the ciliary body; //, cornea; h, circulus major of the iridal arteries; i, arteries, and i\ t veins of the iris; k, circulus minor of the iridal arteries; L, crystalline lens; I, vense vorticosse; m, anastomosis of ciliary and an- terior ciliary veins; N, retina; n, canal of Schlemm; 0, optic nerve; o, posterior conjunctiva! artery, and 01, vein; p, anterior conjunctival vessels; q, vascular loops at the margin of the cornea; R, internal rectus muscle; S, sheath of the optic nerve; Sc, sclera. (After Leber.) 672 THE EYE the arteries ; they converge to form a single efferent vessel, the vena cen- tralis retinae. The retinal veins are peculiar in that their walls contain no muscle. The optic nerve is supplied with small branches from the arteria and vena centralis retinae in their passage through its substance. In the fetus a small branch, the hyaloid artery, apparently the direct continuation of the arteria centralis retinae, passes forward through the vitreous humor to the posterior surface of the lens, whence capillary vessels pass around the margin of the lens and are connected with the anterior ciliary vessels at the margin of the iris. The hyaloid artery supplies blood to the fetal tunica vasculosa lentis for the nutrition of the developing lens. Before birth these vessels disappear; the hyaloid artery remains for a time as a delicate fibrous strand, occupying the persistent canalis hyaloideus or canal of Stilling, which lies almost in the visual axis and extends from the papilla optica to the posterior surface of the lens. The hyaloid canal (also called the canal of Cloquet) establishes a channel between the aqueous humor and the lymphatic spaces of the retina. In adult life both the vitreous humor and the crystalline lens are bloodless tissues. 2. The short ciliary arteries, twelve to fifteen in number, enter the globe of the eye in a circle (circle of Zinn) which surrounds the optic nerve. They supply branches to the meningeal sheaths of the optic nerve and to the sclera, their main stems penetrating this coat to enter the choroid. Here they subdivide to form the plexus of arteries in the lamina vasculosa from which the vessels of the choriocapillaris are supplied. The capillaries of the last-named layer unite to form small venous radicals which converge toward the equator of the eye, where they unite in a whorl-like manner to form the four or five vena vorticosce, which pass obliquely backward through the sclera, receiving additional branches from this coat, and finally emerging from the eye to empty into the ophthalmic vein. The vessels of the choroid communicate posteriorly with those of the optic nerve, and anteriorly, by a free anastomosis, with those of the ciliary processes. 3. The long ciliary arteries, two in number, enter peripherally at the circle of Zinn on either side of the optic nerve, and pass horizontally forward upon the outer surface of the choroid to the ciliary muscle. Near the base of the iris they divide, and by anastomosis with each other and with the anterior ciliary arteries form a vascular circle, the circulus major, about the base of the iris. From this circle recurrent branches supply the ciliary body and THE LYMPHATIC SYSTEMS OF THE EYE 673 anastomose with the vessels of the choroid; other branches pass into the iris and, converging toward the visual axis form, just outside 4he pupillary margin, a second circle of anastomosis, the circulus minor. The veins of the iris and ciliary body follow closely the distribution of the arteries, the greater portion of their blood returning through the veins of the choroid and the venae vorticosaa. Some, however, is returned by means of anastomoses with the anterior ciliary veins. 4. The anterior ciliary arteries, derived from the muscular and lacrimal branches of the ophthalmic, distribute branches to the con- junctiva and sclera, and within the latter membrane, about 2 mm. outside of the corneal margin, pass to the circulus iridis major and partially supply the iris and ciliary body as already described. The anterior ciliary veins follow the course of the corresponding arteries. They empty into the vessels of the ocular conjunctiva. THE LYMPHATIC SYSTEMS OF THE EYE The lymphatic systems of the eye include very few true lymphatic vessels, but consist rather of a series of channels which may be arbitrarily considered as an anterior and a posterior set of intercommunicating spaces. The former set includes the lymphatic spaces of the cornea, the spaces of Fontana, the anterior and posterior chambers, the lymphatic clefts of the ciliary muscle and iris, and the zonular spaces or canals of Petit. The posterior set includes the subdural and subarachnoid spaces in the sheath of the optic nerve, the capsule of Tenon, the lymphatic spaces of the lamina suprachoroidea, the perivascular spaces of the choroid and retina, the irregular clefts between the pigmentary and bacillary layers of the retina, the similar clefts of the ganglion cell layer, the lymphatic spaces of the hyaloid membrane, the hyaloid canal, and the interstices of the vitreous humor. These two sets of lymphatic channels communicate with each other by means of the perivascular spaces of the two outer tunics, as well as through that portion of the hyaloid membrane which forms the posterior, wall of the spatia zonularis, through the clefts of which and the hyaloid canal the lymph of the vitreous body communicates freely with the aqueous humor of the spatia zonularis and posterior chamber. Conse- quently, if the cornea be penetrated either accidentally or otherwise, and the anterior and posterior chambers be emptied, their aqueous humor is rapidly replaced, not only from the adjacent spaces of the 674 THE EYE anterior set of lymphatic vessels, but from the vitreous humor and posterior set as well. It is also important to note that the posterior set of lymphatic spaces is directly connected through the meningeal sheaths of the optic nerve with the subdural and subarachnoid spaces of the cerebral meninges. THE NERVES OF THE EYE The nerves of the eye, in addition to the optic, are the long and short ciliary branches of the ophthalmic nerve. The former, two or three in number, and the latter, six to ten, after supplying a vasomotor branch to the arteria centralis retinae, pierce the sclera in company with the corresponding ciliary arteries and pass meridionally forward on the inner surface of the sclera, supplying branches to this tunic and to the vessels of the choroid, and finally reaching the ciliary muscle, where their branches form an annular plexus containing a few ganglion cells. From this plexus fibrils are supplied to the blood-vessels and mus- cular tissues of the ciliary body and iris, and to the cornea. The corneal branches pass to the annular plexus at the sclerocorneal junction, whence they are distributed to the corneal tissues, as already described (page 633). APPENDAGES OF THE EYE The appendages of the eye include the eyelids, conjunctiva, and lacrimal glands. THE EYELIDS The eyelids are developed in the embryo through an invagination of the skin, which, leaving a slit-like aperture (the palpebral fissure) between its involuted margins, covers the inner surface of the lid to form the palpebral conjunctiva, and is reflected over the globe of the eye as the ocular conjunctiva and anterior corneal epithelium. The lids, therefore, may be said to consist of two membranous por- tions, the cutaneous (outer or anterior) and the conjunctival (inner or posterior). Between these two portions the orbicularis palpebrarum forms a septum of striated muscle fibers. The cutaneous portion of the eyelid differs from other portions APPENDAGES OF THE EYE 675 of the skin only in that its subcutaneous tissue contains no fat. The derma is loosely connected with the muscle by a wide-meshed afeolar tissue. Fine hairs are distributed over the cutaneous surface, their follicles extending well down through the derma. Small sebaceous FIG. 567. VERTICAL SECTION THROUGH THE UPPER EYELID. a, sweat glands; ai, primary tarsal arch; as, secondary tarsal arch; 6, conjuncti- val epithelium; c, eyelashes; co, rugated portion of the conjunctiva; d, epidermis; e, fine hairs; /, process of the levator palpebrae superioris which is inserted into the skin; g, Meibomian gland; h, internal angle of the margin of the lid; I, levator palpe- brae superioris; m, duct of a Meibomian gland; o, orbicularis palpebrarum muscle; p, superior palpebral muscle of Miiller; r, ciliary muscle of Riolan; s, glands of Moll; t, fibrous tissue of the tarsus; v, external angle of the margin of the lid; w, posterior tarsal glands; z, sebaceous glands. (After Fuchs.) 676 THE EYE glands open into the hair follicles and occasional sudoriparous glands pour their secretion upon the" epidermal surface. At the margin of the lid its cutaneous portion is reflected inward, and at its inner angle becomes directly continuous with the palpebral conjunctiva. The free margin of the lid presents, therefore, an outer angle, an inner angle, and an intermediate surface. Two or three rows of large stiff hairs, the eyelashes or cilia, project from tbe outer angle, and large sebaceous glands open into their follicles. Other smaller sebaceous glands open directly upon the free surface. These sebaceous glands are sometimes called the glands of Zeiss. The intermediate surface of the margin of the lid retains the char- acter of the skin, though no hairs are found in this portion. Peculiar sweat glands, the glands of Moll, occur in the derma of this part. At the inner angle of the lid the epidermis abruptly changes its char- acter to that of the conjunctiva, the derma of the cutaneous surface being continuous with the submucous connective tissue of this membrane. At the inner angle also, are the openings of the peculiar large sebaceous glands, the tarsal glands (of Meibom), their orifices forming a con- tinuous punctate row of pores barely visible to the naked eye. The tarsal (Meibomian) glands are long compound saccular glands, about thirty in the upper, and about twenty in the lower lid, whose secreting saccules open into a common, axially placed duct which extends the whole length of the gland. Each saccule is filled with cells in various stages of fatty degeneration and is exactly similar in structure to the saccules of the ordinary sebaceous glands. The glands are em- bedded in the connective tissue of the conjunctiva and are so large as to form projecting ridges on its surface, which are disposed in vertical lines radiating from the row of glandular orifices at the margin of the lid. At their blind extremities the glands are often slightly bent or curved upon themselves, and this portion is embedded in a dense mass of fibrous tissue known as the tarsus. The tarsus in each eyelid forms a very dense plate-like mass of areolar connective tissue which is so dense and resistant as to erroneously suggest a cartilaginous structure. It is inserted between the conjunctiva and the orbicularis muscle. It is thickest toward the free margin of the lid, but becomes progressively thinner in the opposite direction, until, as a mere fibrous membrane, the palpebral fascia, it is continued to the margin of the orbit. The conjunctiva! portion of the lids, the palpebral conjunctiva, consists of a peculiar stratified epithelium and a thin connective tissue APPENDAGES OF THE EYE 677 corium. Its epithelium comprises four or five layers of cells, the deeper of which are small and spheroidal, and the superficial elongated oT crmi- cal, their blunt ~ ends forming the free surface of the conjunctiva, their pointed extremities buried between the cells of the deeper layers. The bases of these elongated cells become somewhat expanded and broader from the increased tension of the conjunctiva when the lids are closed; they retract and become narrower when the lids are separated and the conjunctiva relaxed. The cells of the superficial layer are often so distinctly elongated as to possess a columnar form. They may, however, be spheroidal or even somewhat flattened, in which case they very closely resemble the ordinary type of stratified squamous epithelium. The epithelial layer rests almost directly upon the connective tissue corium, the basement membrane being imperfectly developed. The corium of the conjunctiva is thin. With the aid of a thin layer of submucous areolar tissue it unites the epithelium to the tarsus and to the fibers of the orbicularis muscle; near the margin of the lid its submucous tissue incloses the Meibomian glands. Opposite the plane at which the blind ends of the Meibomian glands are embedded in the free margin of the tarsus, the conjunctival surface is thrown into eight to twelve horizontal folds, beneath which, in the connective tissue, are a few minute tubulo-alveolar glands, the posterior tarsal glands (glands of Waldeyer; glands of Henle). Their ducts open upon the free surface of the conjunctiva near the fornix conjunctivse. At the attached base of the lid a narrow band of smooth muscle Ex- tends from the levator palpebraa and inferior oblique muscles into the body of the lid. These fibers have been described by H. Miiller (1858) as the superior and inferior palpebral muscle of the upper and lower lid, respectively, and have come to be known as the muscle of Miiller. The fold by which the palpebral conjunctiva is reflected upon the globe of the eye to become continuous with the ocular portion of the membrane is known as the fornix conjunctive. The extremely loose attachment of the conjunctiva of the fornix to the underlying connective tissue and intra-orbital fat permits the great freedom of motion which is characteristic of the ocular globe. The small accessory lacrimal glands (glands of Krause) open into the margin of the fornix conjunctivas. The superior fornix contains from eight to twenty, the inferior from two to five. In this region, also, occasional goblet cells occur in the superficial layers of the epithelium. The ocular conjunctiva is likewise very loosely attached to the sclera. 678 THE EYE The scleral portion of the conjunctiva is nearly identical in structure with the palpebral portion already described. Near the margin of the cornea the superficial cells of the epithelium become at first spheroidal and then, as the cornea is approached, they are progressively flattened, so that, just outside of the corneal margin, the conjunctival epithelium conforms to the stratified squamous type which forms the anterior epi- thelium of the cornea. The blood supply of the eyelids is derived from the internal palpe- bral artery which furnishes a branch to each lid; these two branches anastomose at the external angle with the lacrimal branch of the ophthalmic, through the external palpebral artery. These trans- verse ARTERIAL VESSELS (primary tarsal arches) lie near the free margin of the lids between the tarsus and the orbicularis palpe- brarum muscle. Each tarsal arch sends pre-tarsal twigs forward to supply the muscle and integu- ment, and post-tarsal twigs back- ward to supply the tarsal fascia, the tarsal glands and the conjunc- tiva. At the basal end of the tar- sus, just back of the levator pal- pebraB muscle, a second arch (the (secondary tarsal arch) may be present, more commonly in the upper lid; this second arch represents a larger branch of the palpebral artery, and it anastomoses freely with the primary arch. The VEINS follow an essentially similar course. The LYMPHATICS likewise include a pre- and post-tarsal set, which unite to form larger tarsal tributaries which drain along the facial vein toward the sub maxillary lymph nodes. The nerve supply includes sensory, motor and sympathetic fibers. The main trunks lie between the tarsus and the orbicularis palpebrarum muscle. The sympathetic fibers supply the blood-vessels, the smooth muscle of Miiller, and the glands. Motor fibers contributed by the facial nerve, supply the annular sheet of striped muscle; the oculomotor nerve contributes motor fibers to the levator palpebrse muscle. The sensory fibers, which are derived from the trigeminal nerve, end in naked fibrils among the cells of the external integument and the internal con- FIG. 568. ARTERIAL SUPPLY OF THE EYE- LID. (After Fox.) APPENDAGES OF THE EYE 679 junctiva, and in the subcutaneous connective tissue and the tarsus. Numerous sensory fibrils end also in special end-bulbs, especially along the inner margin of the lids ; similar endings occur in the palpebral and bulbar conjunctiva. THE LACRIMAL LAKE In the -shaped area of the internal canthus, the lacus lacrimalis, lies a roughened irregular reddish mass of delicate modified skin, the caruncle (caruncula lacrimalis) . It is embedded in fat and contains a number of very delicate hairs, large sebaceous glands and a few sweat glands. Where the borders of the lake pass into the palpebral margins there appears a slightly raised papilla lacrimalis, each of which contains an apical opening, the puncta lacrimalia, leading into the two canaliculi lacrimaleSj which conduct the excess of lacrimal secretion or tears into the nasolacrimal duct. To the outer side of the lacrimal lake there appears a vertical crescentic fold of delicate skin, the plica semilunaris, which is the homologue of a functional third eyelid (nictitating mem- brane) of birds and reptiles. THE LACRIMAL GLAND The lacrimal glands are two flattened, lobulated, glandular masses situated at the upper and outer angle of the orbit, one in relation with each eye. They secrete a clear watery fluid, the tears. These glands are somewhat molded to conform to the shape of the orbit and the globe of the eye, between which they are inserted. Each lacrimal gland is a secreting gland of the compound tubular type (Marziarski) (Fig. 257, page 258), and consists of eight to twelve small lobules which open into the fornix conjunctiva? by about as many minute ducts. The lobules are aggregated into two fairly distinct lobes, separated by a denser fascia, the superior lobe or orbital portion and the inferior lobe or palpebral portion, and are united by thin fibrous fasciae which contain the larger ducts. Each lobule of the gland contains many serous-secreting acini and numerous small intralobular ducts. The secreting acini are lined by tall, columnar cells, resting upon a thin basement membrane, which is supplied with 'basket cells' and is invested with a delicate fibrous tunica propria. The appearance of the secreting epithelium differs some- 680 THE EYE what according to its state of activity. After a period of rest, and in the ordinary condition of relative inactivity, the epithelium becomes distended with secretion and is either clear in appearance or at most is only very finely granular, the nuclei are crowded to the base of the swollen cells, and the lumen of the acinus is very small. After a period of excessive activity the secreting cells become shrunken and more dis- FIG. 569. SECTION THROUGH A LOBULE OF THE LACRIMAL GLAND OF MAN. a, small duct branching within the lobule; 6, intercalary ducts; c, connective tissue; /, fat cells. A, transection of an interlobular duct. Hematoxylin and eosin. X 112. (After Kolliker.) tinctly granular, and the lumen of the acinus appears much dilated. The secreting acini empty into narrow intercalated ducts which lie within the lobule, have a considerable lumen, and are lined by tall columnar cells resting upon a second incomplete layer of small, some- what flattened basket cells. These intralobular ducts unite at the margin of the lobule to form the larger interlobular ducts, which are contained in the interlobular connective tissue. Here the duct is lined by low columnar or even somewhat flattened cells, at first disposed in a single, but later in a APPENDAGES OF THE EYE 681 double layer. As the duct approaches the conjunctiva! surface the number of cell layers increases until their lining epitheliumfinally comes to resemble the stratified epi- thelium of the con- junctiva with which it is continuous. Minute collec- tions of diffuse lymphoid tissue and even small lymph nodules are occasionally found just beneath the epithelium of t h e conjunctiva in the neighborhood of the lacrimal glands of the f ornix ; occa- sionally the lym- phoid tissue is quite abundant. In animals pos- sessing a membrana nictitans ('third lid') a small, mu- cus-secreting gland occurs at the inner angle of the orbit; this is known as the gland of Harder. It is well developed also in some mam- mals, e.g., rabbit, but in man and other primates it is usually absent, though in an extremely vestigial condition it may occa- sionally be found within the basal portion of the plica semilunaris. FIG. 570. PORTIONS OF Two ADJACENT LOBULES OF THE LACRIMAL GLAND OF THE RABBIT, SHOWING Two STAGES IN SECRETORY ACTIVITY OF THE TUBULES. Magnifica- tion X 100. In the upper lobule the cells are filled with minute, in the lower lobule with large, secretion spherules. 43 CHAPTER XIX THE EAR This organ may be subdivided for description into the external, the middle, and the internal ear. The first two portions serve for the collection and transmission of sound waves, the last for the transforma- tion of the sound waves into nerve stimuli which are then transmitted through the path of the acoustic nerve to the cerebrum. THE EXTERNAL EAR The external ear includes an auricular or free portion and an ex- ternal acoustic (auditory) meatus. THE AURICLE The auricle, or pinna, contains a thin cartilaginous plate of peculiar form which is covered on both sides by the skin. The cartilage is of the elastic variety, but differs from the similar cartilages of other parts in the abundance of its large cartilage cells; in occasional areas the elastic reticulum is deficient. This reticulum is closely connected with the fibrous perichondrium, beneath which it forms a complete layer. The extrinsic muscles of the ear are inserted into the perichondrium and the fibrous tissue by which it is surrounded. The skin of the external ear does not essentially differ from that of other parts. It is supplied with fine hairs and with many large sebaceous glands; sweat glands also occur on the outer surface. The derma is united to the underlying cartilage by connective tissue; on the concave surface this union is very firm and permits but little motion. The subcutaneous tissue, except in the lobule, contains but little fat. The lobule does not contain cartilage. 682 THE EXTERNAL EAR 683 THE EXTERNAL ACOUSTIC MEATUS The external ACOUSTIC MEATUS (or auditory canal) is divisible into an outer cartilaginous and an inner bony portion; the walls of the two FIG. 571. TRANSECTION OF THE LOBULE OF THE EXTERNAL EAR OF AN INFANT. o, cartilage; 6, skin; c, adipose connective tissue. Hematein and eosin. Photo. X20. portions, except for this difference, are quite similar in structure. The cartilage is continuous with that of the auricle, and is of the cellular elastic variety. The skin of this portion contains large stiff hairs and both sebaceous and ceruminous glands. The former, as in the auricle, 684 THE EAE open either upon the free surface of the skin or into the adjacent portion of the hair follicles. THE CERUMINOUS GLANDS The ceruminous glands resemble in structure the sweat glands of other portions. They are coiled tubular glands which open upon the surface of the skin by means of a narrow duct. The coils of their se- creting portion are lined by columnar cells with spheroidal, basally FIG. 572. FROM THE EXTERNAL ACOUSTIC MEATTTS OP MAN. a, sebaceous gland; 6, ceruminous gland; c, cartilage. Hematoxylin and eosin. X 15. (After Sobotta.) situated nuclei and a clear cytoplasm containing many small brownish granules of pigment and a few fatty particles. The cytoplasm is often diffusely colored by the brownish pigment. Between the lining cells and the basement membrane appear slender fusiform elements re- sembling similarly placed elements in the sweat glands, and likewise interpreted as muscle cells. The secretion of these glands, the cerumen, in addition to the pigmented and fatty secretion of the glands, contains THE MIDDLE EA^fc ^ 685 sebum,, desquamated epithelial cells and occasional fine hairs, together with foreign particles of a very varied sort. In the bony portion of the meatus the corium or derma is firmly adherent to the periosteum of the bone, and all the layers of the skin are much reduced in thickness. The scanty hairs are very fine, and, with the glands, are continued inward to the tympanic membrane only in the superior portion of the wall of the canal. Papillse are present as far as the margin of the tympanic membrane. Upon the surface of this membrane, which closes the inner end of the external acoustic meatus and separates it from the cavity of the middle ear, the skin is reduced to an extremely thin cutaneous coat, devoid of hairs, glands, and papilla. THE MIDDLE EAR GENERAL CONSIDERATIONS The middle ear or tympanum ( 'ear-drum' ) is an irregular cavity, broad above and behind, narrow below and in front, which lies just within the external acoustic meatus. Its outer wall is largely formed by the tympanic or drum membrane, its inner by the osseous wall of the internal ear. The contour of the tympanum is very irregular, its cavity being encroached upon by numerous bony elevations which are most pro- nounced on its internal wall. Externally the tympanic membrane is attached to a bony and fibrocartilaginous ring, the annulus tympanicus, which projects somewhat into the tympanic cavity. In front, the orifice of the auditory (Eustachian) tube is marked by a slight cartilaginous projection near the floor of the cavity. Above and behind, the tympanic cavity is prolonged into a deep recess, the epitympanic cavity, in the upper part of whose posterior wall are the orifices of the mastoid cells. The upper portion of the cavity contains the rounded heads of the malleus and incus, the two largest of the auditory ossicles. The internal wall of the tympanum presents anteriorly a bulging prominence which is known as the promon- tory, and which indicates the position of the first or broadest turn of the spiral canal of the cochlea. Beneath this prominence is a recess leading to a bony 'window/ the fenestra cochleae (or fenestra rotunda), which, in life, is closed by a delicate membrane, the secondary tympanic membrane. Behind the promontory and at a slightly higher level a 686 THE EAR deep recess, the pelvis ovalis, leads inward to the fenestra vestibuli (or fenestra ovalis), which is closed by the base of the stapes; the body of this ossicle is contained entirely within the pelvic recess, and near its mouth the stapes articulates with the orbicular extremity of the long process of the incus. The superior portion of this deep recess is en- croached upon by the projecting wall of the aqueductus Fallopii which transmits the facial nerve, and posteriorly, near the point where it merges with the general tympanic cavity, a low, conical, bony projection known as the pyramid transmits the stapedius muscle. The canal of the tensor tympani muscle contained within a still more prominent, conical, bony projection, the processus coM ear if or mis, is found near the antero- internal angle of the tympanic cavity just above and parallel to the auditory tube. The narrowest portion of the tympanum is, perhaps, almost its very center, and is included between the promontory on the inner and the tympanic membrane on the outer side. Extending from this narrowed central portion upward, backward, and inward, are expanded recesses which are partially occupied by the three auditory ossicles ; the remaining portions of the tympanum are filled by air which gains access to the cavity through the auditory tube. THE TYMPANIC MUCOSA The tympanic mucosa consists of a thin but dense tunica propria which is firmly attached to the underlying periosteum and softer parts by loose connective tissue, and is clothed with a layer of flattened ento- dermal epithelium, which, in the vicinity of the origin of the auditory tube, is of low columnar form and is provided with cilia, but in most other portions of the tympanum is squamous in character and of the tessellated type, closely resembling endothelium. The floor of the tym- panum and the lower portions of its anterior, internal, and posterior walls also possess a partial clothing of low ciliated cells (Kessel). Occa- sional gland-like folds of the mucosa occur near the orifice of the audi- tory tube, though the true glandular character of these folds is very questionable. THE MASTOID CELLS The mastoid cells (cellulce mastoidece sen pneumaticce) are numer- ous small spaces situated within the mastoid process of the temporal bone; they are lined by a continuation of the tympanic mucosa, which is everywhere clothed by flattened epithelium. The corium is closely attached to the periosteum of the bony wall, the periosteum also serving THE MIDDLE EAR 687 as a vascular layer of the mucosa in the mastoid cells, as well as in the general tympanic cavity. THE TYMPANIC MEMBRANE The tympanic membrane is a thin delicate partition which is formed by a reflection of the cutaneous layer of the external acoustic meatus on the one hand, the tympanic mucosa on the other, and between these two membranes a layer of dense fi- brous tissue whose tendinous bands are disposed in radial and circular directions. The margin of the tympanic membrane is in- serted into a fibrocartilaginous ring which rests upon a. bony ele- vation, the annulus tympanicus. The slender manubrium or handle of the malleus projects from the superior margin of the ring and is inserted between the folds of the tympanic membrane, extending downward to about the center of the membrane, at which point is the deepest part of its concavity, its umlo. The bony handle of the malleus, lying be- tween the cutaneous and mucous layers of the tympanic membrane, is covered by a thin cartilaginous layer, and receives the insertions of the tendinous fibers. These fibers are divisible into an outer radial layer which extends from the fibrocartilaginous ring at the periphery inward to the manubrium mallei, and an inner circular lay? r whose thickest portions are found close to the manubrium and near the FIG. 573. TRANSECTION OF THE TYM- PANIC MEMBRANE OF A CHILD. a, a', fibrocartilaginous ring; 6, 6', bone; c, c', skin of the external acoustic meatus; d, d r , tympanic mucosa; e, cutaneous layer of the tympanic membrane; /, fibrous layer, obliquely cut at/'; g, layer derived from the tympanic mucosa; h, handle of the malleus; i, blood-vessels. Hematoxy- lin and eosin. X 11. (After Kolliker.) 688 THE EAB periphery of the membrane; between these points the circular layer of fibers is partially or entirely deficient. Just within the fibrocartilagi- nous ring at the periphery of the membrane the cir- cular layer of fibers abruptly ends. The cutaneous layer of the tympanic membrane forms a very thin coat, its epidermis consisting of a germinal layer one* or two cells deep, which is cov- ered by several flattened non-nucleated cells of the horny portion. The derma or corium is very thin, contains no papillae, and is intimately adherent to the fibrous layers of the membrane ; it contains neither glands nor hairs. The mucous layer of the tympanic membrane is even thinner than the cutaneous. It consists of a flattened entodermal epithelium which rests al- most directly upon the layer of circular fibers. A few connective tissue fibers pass irregularly from the mucous, through the fibrous, to the cu- taneous layer, thus firm- ly uniting the several lay- Ji'liip ''~**-&jS: FIG. 574. SECTION THROUGH THE MARGIN OP THE TYMPANIC MEMBRANE OP A CHILD. a, fibrocartilaginous ring; 6, bone; c, derma of the external auditory canal; d, tympanic mucosa; e, e', epidermis; /, radial fibers, and /', circj^ar fibers of the tympanic membrane; g, mucosa of the membrane; h, epithelium of the tympanum; i, blood-vessels. X 55. (After Kolliker.) ers into a compact mem- brane. In the upper quadrant of the tympanic membrane, above the at- tachment of the malleus, the fibrous layers are wanting; the mucous and cutaneous layers are therefore in contact, and the membrane presents THE MIDDLE EAR 689 a flaccid appearance in comparison with the tense condition of its other parts. This portion is known as the membmna flaccida or Shrapnell's membrane. THE AUDITORY OSSICLES These are three in number, the malleus, the incus, and the stapes ; they form a continuous bony chain, extending from the insertion of the manubrium mallei in the tympanic membrane to the fenestra vestib- FIG. 575. THE AUDITORY OSSICLES. 7, ossicular chain of the left ear; 1, malleus; 2, incus; 3, stapes. //, ossicular chain of the right ear; 1, malleus; 2, processus graciUs; 8, manubrium; 4, long process of the incus; 5, short process of the incus; 6, stapes. (After Riidinger.) uli, with whose margin the foot of the stapes articulates. The ossicles consist of compact bony tissue containing loosely packed Haversian sys- tems; they are united with each other by firm fibrocartilaginous ar- ticulations. With the exception of the stapes, none of the ossicles contain a marrow cavity. The manubrium of the malleus is firmly fixed in the tympanic mem- brane, as already described, the head of the bone articulating with the head of the incus in the epitympanic recess. The long process of the incus, circular in transection, extends downward along the tympanic wall in a course nearly parallel to that of the manubrium mallei, being, in a portion of its course, contained within a recess in the osseous wall of the tympanum. Finally, at the level of the stapes it makes a sharp bend, almost at right angles with its former course, to articulate, by means of a rounded end or orbicular process, with the head of the 690 THE EAE stapes. This latter bone is deeply placed within the recess of the pelvis ovalis, and continues the bony chain to the fenestra vestibuli, where the foot plate of the stapes is in relation, by its inner surface, with the vestibular perilymphatic space of the internal ear. The course of the chain of ossicles is such that they form a lever; the long process of the incus being shorter than the manubrium mallei, the vibrations of the tympanic membrane in response to sound waves are transmitted to the internal ear diminished in amplitude but ex- aggerated in intensity. sf The com bi n ed activity of the tympanic membrane and the auditory ossicles is said to reduce the am- plitude as much as sev- enty-six times, and to increase the force of the vibrations thirty times. Two muscles and sev- eral ligaments are con- nected with the ossicles. The tensor tympani muscle is mostly con- tained within a canal which is parallel to and lies just above the audi- tory tube, and from its bony wall the muscular fibers arise. The wall of the canal forms a conical projection known as the processus cochleariformis, which projects well into the cavity of the tympanum, being directed toward the neck of the mal- leus. Leaving its canal at the apex of this conical process the tendon of the muscle bends sharply over the margin of the processus cochleari- formis and passes directly to its insertion into the neck and the adjoin- ing part of the manubrium of the malleus. Hence the naked tendon of the muscle lies within the tympanic cavity. The stapedius muscle is similarly contained within the cavity of the pyramid, from whose bony wall its fibers take origin. Passing for- FIG. 576. THE CAVITY OF THE TYMPANUM, VIEWED FROM ABOVE. i, the body of the incus; I, ligamentous fold of the mucosa; l.a.m., anterior ligament of the malleus; Le.m., external ligament of the malleus; Li., pos- terior ligament of the incus; M, mastoid cell; m, head of the malleus; ra.ra., mucous membrane; n, chorda tympani nerve; pr. o., orbicular process of the incus articulating with the stapes in the depth of the cavity; R, beneath this space is the flaccid por- tion of the tympanic membrane; s.l.m., cut end of the superior ligament of the malleus; sp, spina tym- panica anterior; st, st f , tendon of the stapedius muscle; tt, tendon of the tensor tympani muscle. X 4. (After Schafer.) THE MIDDLE EAR 691 ward, the muscle makes its exit at the apex of the pyramid, and is di- rectly inserted into the neck of the stapes close to the articulation of the orbicular process of the incus. , The ligaments of the malleus are the anterior, the external, and the superior. The anterior ligament firmly attaches the head of the malleus to the margin of the Glaserian fissure in the anterior wall of the tympanum. The processus gracilis of the malleus is inclosed by the fibers of this ligament. It is also in close relation with the chorda tympani nerve, which, being clothed by the tympanic mucosa, trav- erses this portion of the tympanic cavity and enters the iter chordae anterius. The external ligament connects the neck of the malleus with the upper portion of the external wall of ^he tympanum. It is somewhat fan-shaped. The space lying between the external ligament of the malleus and the membrana flac'cida is known as Prussak's space. The superior ligament is a looser fibrous band which passes from the head of the malleus to the superior wall of the tympanum. The ligament of the incus is decidedly fan-shaped, its straight, coarse, fibrous bands radiating from the short process of the ossicle to the ad- jacent portion of the posterior wall of the tympanum. The articulation of the malleus with the incus, as also that of the latter bone with the stapes, is supplied with a delicate capsular liga- ment. The annular ligament of the stapes connects the margin of the foot plate of this bone with the adjacent portions of the cartilaginous and bony wall of the vestibule at the margin of the fenestra vestibuli. The articulation which is thus inclosed is directly formed by an an- nular plate of cartilage investing the margin of the oval foot of the stapes, and a similar annular plate of hyaline cartilage which lines the borders of the foramen vestibuli. The fibers of the annular ligament are continuous with those of the perichondrium and adjacent periosteum. THE AUDITORY (EUSTACHIAN) TUBE The auditory or Eustachian tube connects the cavity of the tym- panum with that of the nasopharynx. Its first portion, including about one-third of its entire length, is surrounded by a bony wall; be- yond this it is supplied with a cartilaginous plate ; its pharyn^eal ostium is entirely membranous. The mucosa consists of an entodermal epithelium, which is of the G92 THE EAE columnar ciliated variety, continuous with, and similar to the respiratory epithelium of the nasopharynx, together with a fibrous membrana propria which is loosely connected with the surrounding bony, car- tilaginous, and muscular walls. The lower portions of the tube are richly supplied with mucus-secreting, tubulo-acinar glands, and toward FIG. 577. TRANSECTION OF THE EUSTACHIAN TUBE; DIAGRAMMATIC. 1, cartilaginous plate; S, lateral or hooked end of the cartilage; 3, 'dilator tuba?' (tensor palati); 4> levator palati; 5, fibrocartilage at the base of the skull; 6 and 7, mucous glands; 8, adipose tissue; 9, 11, lumen of the tube; 10, 12, connective tissue. Low magnification. (After Riidinger.) its pharyngeal end the mucosa is much infiltrated with lymphoid tissue, thus forming the tubal tonsil of Gerlach. The cartilage of the auditory tube is firmly adherent to the bony wall. At the point of attachment it has a hyaline structure, the fibers of the perichondrium penetrating only the surface of the cartilaginous plate. Lower down the cartilage becomes infiltrated with fibers and conforms to the typical elastic variety. Like the cartilage of the auricle it is rich in cellular elements. Its transection presents a peculiar hook- like form, by means of which the posterior surface, the superior margin, and the upper portion of the anterior surface are invested by cartilage, THE INTEKNAL EAR 693 while the remaining portions of the anterior surface and the_ whole of the inferior margin are entirely membranous. Closely associated with the auditory tube are the tensor and levator palati muscles, lying laterally and ventrally to the tube. These muscles serve also as dilators for the tube. VASCULAR SUPPLY The Blood-vessels. The mucosa of the middle ear is richly sup- plied with blood-vessels,, the larger of which lie in the deeper part of the membrane and supply capillary vessels to the tunica propria. The blood- vessels of the auditory tube are especially numerous. In the tympanic membrane the arteries and veins form an annular plexus at the margin; and a group of similar vessels surrounds the manubrium mallei, lying in the deeper layers of the cutaneous portion of the membrane. The mucosa of the tympanum is peculiar in the relative deficiency of capillary vessels (Prussak, 1869) ; the veins are numerous. The veins of the auditory tube empty into the internal jugular; they also communicate with the cavernous sinus by a trunk of considerable size (Bench, 1895). The lymphatics of the middle ear form plexuses in the connective tissue of the mucosa and in a general way follow the course of the smaller veins. They lead in part to the lymphatic nodes behind the ear, and in part to the parotid group (Kolliker). They also communicate with the perilymphatic spaces of the internal ear. THE INTERNAL EAR GENERAL CONSIDERATIONS The internal ear includes a series of membranous structures together with the terminal fibers of the acoustic nerve ; these are contained within a series of connected cavities hollowed out of the petrous portion of the temporal bone, and in relation with the mesial wall of the tympanum. The central portion of this bony cavity, an ovoid space, is known as the vestibule; its outer wall presents the orifice of the fenestra vestibuli which leads to the tympanum, but during life is closed by the base of the stapes. Opening from the vestibule, at one end, are the bony cavi- ties occupied by the three semicircular canals which, in a general way, 694 THE EAR project from the posterodorsal aspect of the vestibule; at the other end the bony cochlea containing its series of spiral canals projects anteriorly from the vestibule. Collectively these spaces, with several diverticula, form the bony labyrinth, and within them in life are contained a num- ber of membranous sacs whose general form corresponds more or less FIG. 578. THE BONY LABYRINTH. 1, round window; 2, osseous lamina spiralis; 3, osseous cochlear canal; 4, floor of internal acoustic meatus; 5, vestibule; 6, 7, 8, 9, semicircular canals. The figures are placed at that portion of the margin which is nearest the structure indicated. (After Rudinger.) closely to that of the bony cavity; these sacs collectively form the mem- branous labyrinth. The vestibule contains two of these membranous sacs, the sacculus and the utriculus, which are connected by means of the slender utriculo- saccular canal, from which a much prolonged diverticulum enters the aqueductus vestibuli to penetrate to the posterior surface of the petrous bone where it comes into relation with the cerebral meninges ; this di- verticulum is known as the ductus endolymphaticus. Its dilated terminal portion constitutes the endolymphatic sac, which lies upon the dura and probably opens into the subdural space. The utricle and saccule, as also all other portions of the membranous labyrinth, contain a watery fluid, THE INTERNAL EAR 695 the endolympli; they do not entirely fill the bony cavity of the labyrinth in which they lie, the intervening space being occupied by a retiform connective tissue with broad interstices which are permeated by an aqueous fluid, the perilymph. THE SACCULE The saccule is a rounded membranous cavity which is connected, on the one hand, by means of the slender canalis reuniens, with the cochlear duct or scala media, and on the other hand with the ductus endolymphaticus and utricle, as already stated. Its wall consists of an FIG. 579. DIAGRAM OF THE MEMBRANOUS LABYRINTH IN LATERAL VIEW. A, ampullae; U, utricle; S, saccule; SM, scala media or cochlear duct; CR, canalis reuniens; C, crus, or common canal; SE, saccus endolymphaticus; DE, ductus endo- lymphaticus; dss, dsp, dsl, superior, posterior and lateral (external) semicircular ducts. The utricle and saccule are connected by the utriculosaccular duct. The cochlear duct terminates in the ceca vestibulare and cupulare. (After Gray.) ectodermal epithelium, a membrana propria and a fibrous coat. The epi- thelium consists of flattened squamous cells; it completely lines the cavity. The epithelial surface is somewhat irregular from the papillary elevations of the fibrous coat. On the antero-inferior surface of the sac- cule the epithelium is peculiarly altered so as to form a layer of col- umnar cells, many of .which are provided with stiff cilia. This neuro- epithelium is distributed over an oval area (3 mm. by 2 mm. in extent) beneath which the fibrous coat is much thickened by the entrance of many fibers derived from the vestibular portion of the acoustic nerve. FIG. 580. DIAGRAM OF THE RIGHT MEMBRANOUS LABYRINTH. l t utricle; 2, superior semicircular canal; 3, posterior semicircular canal; 4, ex- ternal semicircular canal; 5, saccule; 6, endolymphatic duct; 7 and 7', canals con- necting utricle and saccule respectively with the endolymphatic duct; 8, endo- lymphatic sac; 9, cochlear duct; 9', its vestibular cul-de-sac (cecum vestibulare) ; 9", its terminal cul-de-sac (cecum cupulare); 10, canalis reuniens. (After Testut.) FIG. 581. THE ISOLATED MEMBRANOUS LABYRINTH. 1, utricle; 2, saccule (opened); 8, location of the macula acustica sacculi; 4, am- pulla of a semicircular canal; 5, canalis communis. Low magnification. (After Rudinger.) 696 THE INTERNAL EAR 697 This elevation with its neuro-epithelial covering is known as the macula acustica sacculi. The neuro-epithelium contains two varieties of cells, the sustentacu- lar and the hair cells. The former, fiber cells of Retzius, form a layer, two or three cells deep, which rests upon the basement membrane, and whose broad basal portion contains a spheroidal nucleus. Beyond the nucleated portion the cytoplasm of the sustentacular cell is con- FIG. 582. TRANSECTION OF THE MARGIN OF THE MACULA ACUSTICA SACCULI OF A GUINEA-PIG. a, otolithic membrane; b, hairs; c, cuticular membrane; d, hair cells; e, susten- tacular cells; /, epithelium of the saccule; g, tunica propria; h, nerve fibers; i, bone. Hematoxylin and eosin. X 325. (After Kolliker.) tinued inward between the bodies of the hair cells to the surface of the epithelial layer, this portion of the cell being relatively slender. The hair cells occupy the superficial part of the epithelial layer by their broad nucleated portions, which carry upon their free extremity a single tuft of long stiff cilia, having the appearance of a delicate hair- like process which projects into the endolymphatic cavity. That por- tion of the endolymph which immediately overlies the macula, and into which the hair-like processes project, though not essentially dif- ferent in microscopic appearance in fresh tissues, appears to possess a somewhat gelatinous consistence, and in it are suspended various forms of minute crystals of calcium carbonate which are known as otoconia or f otoliths/ The free surface of the neuro-epithelium is also provided 44 698 THE EAK with a reticulated cuticular membrane which presumably is formed by the amalgamation of the free ends of the sustentacular cells. Through the openings in this reticular membrane the ciliary tufts of the hair cells project. The central ends of the hair cells, beneath the nucleated enlarge- ment which is found near the middle of the epithelial layer, are pro- longed outward between the nucleated portions of the sustentacular cells and frequently terminate in small knobbed extremities. This portion of the cells is in intimate relation with the terminal fibrils of the vestibu- lar portion of the acoustic nerve, which, coming from a nerve plexus in the fibrous wall of the saccule, forms an intra-epithelial plexus of delicate varicose fibrils. Fre- quently the epithelial coat con- j tains coarse granules of a brown- ish pigment which, at times, also produces a diffuse coloration of the cells. FIG. 583. NERVE ENDINGS IN THE The lining epithelium of the MACULA ACTJSTICA OF A GUINEA-PIG. a, epithelium; 6, tunica propria; c, three terminal nerve fibers. Golgi stain. X about 200. (After Retzius.) saccule rests upon a thin homo- geneous basement membrane and is further supported by a delicate fibrous coat or tunica propria. The connective tissue of this coat forms interlacing bundles the most of which are distributed in a circular manner about the wall of the ovoid sacculus. At the macula this coat is much thickened by the en- trance of the fibers from the vestibular nervp. It also contains the minute blood-vessels which supply the organ. As is the case with the other divisions of the membranous labyrinth, the fibrous wall of the saccule is in contact on one aspect of its surface with the periosteum which lines the osseous labyrinth; elsewhere it is separated from the periosteum by the perilymphatic cavity. THE UTRICLE The utricle is somewhat larger than the saccule. It lies behind and somewhat above the saccule, is of a very irregular oblong form, and receives the insertions of the semicircular canals. Its anterior portion is provided with a macula and the structure of its wall differs in no wise from that of the saccule; both of these mem- THE INTERNAL EAE 699 branous sacs are contained within the irregular cavity of the vestibule. The structure of the utriculus, therefore, requires no further description. THE SEMICIRCULAR CANALS The semicircular canals or ducts are three in number, the posterior, superior, and lateral. The last is also horizontal in its position; the first two are vertical, but are so placed as to form a right angle with one another. The posterior lies in the long axis of the petrous bone and its plane is therefore more nearly sagittal, while that of the superior canal is more nearly coronal. Each canal forms something more than half a circle, its two ends opening separately into the cavity of the vesti- bule, with the exception of the posterior and superior canals whose in- ternal ends open by a common orifice, the canalis communis. The un- joined orifices of the posterior and superior canals, as also the outer extremity of the lateral canal, present a marked dilatation at their termination in the vestibule. These dilatations are known as the am- pulla. They .lodge the neuro-epithelial patches, the cristaa acusticae. The osseous and membranous canals are of similar shape; the latter is, of course, contained within the former. The membranous semicircular canals open into the utricle. They do not entirely fill their bony canal, but, like the utricle and saccule, lie in contact with the periosteum at one surface only, this surface being that of the outer wall or periphery of the semicircle, while iPthe re- maining portion of the circumference of the cylindrical bony duct, the membranous canal is loosely united to the periosteum of the osseous wall by a retiform connective tissue whose loose meshes are filled with perilymph and lined with mesenchymal epithelium. The wall of the membranous canal is similar in structure to that of the saccule and utricle and consists of an ectodermal epithelium, a membrana propria, and a fibrous tunic. Each of the three ampullae presents a marked differentiation of the epithelial lining, which is there raised in the form of a prominent crescentic fold, inappropriately termed by the older anatomists the crista acustica, from its supposed connection with the auditory function. Like the maculae of the saccule and utricle, the cristae are supplied by the vestibular nerve and are concerned with the function of equilibration. The utricle and saccule represent the original anlage of the ear the otocyst from which the canals and cochlea arise as evaginations ; they correspond most closely also to the f ear' of certain invertebrates, 700 THE EAR e.g., Crustacea, which is simply an equilibrating organ. They are be- lieved to function as static organs of equilibration, giving information as to position at rest or during progressive movements ; the semicircular canals on the other hand are commonly conceived of as dynamic or- gans of equilibration and are thought to furnish information regarding the direction and extent of rotatory movements. FIG. 584. TRANSECTION OF A HUMAN SEMICIRCULAR CANAL. 1, bone; 2, retiform connective tissue membranes; 3, at this point a band of con- nective tissue joins the periosteum; 4> membranous semicircular canal; 5, liga- mentous attachment of the canal; 6, at this point the membranous and osseous canals are in contact. Moderately magnified. (After Riidinger.) The cristae are clothed with tall columnar cells which, though some- what taller, are otherwise similar in structure to those of the maculae, and are similarly divisible into sustentacular cells and hair cells. They are also covered by a gelatinous cuticular formation, containing otoliths, which is here known as the cupola. The vibratory stimulus is trans- mitted from the endolymph to the hair cells through the medium of the otolithic membranes. THE INTEENAL EAR 701 THE COCHLEA The cochlea, like the vestibular portion of the internal ear, consists of a bony case which incloses a membranous organ. Structure. The bony cochlea possesses a peculiar flat pyramidal shape. The base of the pyramid is in contact with the anterior aspect of the vestibule; its apex or cupola is directed forward, outward, and slightly downward. The pyramid is hollow and contains in its axis a conical bony support, the modiolus, which tapers from a broad base to a pointed apex beneath the broader, blunt, and rounded cupola of the outer bony wall. The modiolus con- tains a broad canal which receives the cochlear division of the acoustic nerve as it enters from the internal meatus. The outer surface of the modio- lus supports a bony shelf, the lam- ina spiralis ossea, which winds in a spiral manner from its base to its apex, and ends in a hook-like proc- ess, the Jiamulus. This shelf only partially spans the interval between the modiolus and the outer wall of the cochlea. In life the remaining interval is completed by a firm fi- brous membrane, the basilar membrane (lamina spiralis membranacea) . Thus the cylindrical canal of the cochlea, which is wound spirally around the modiolus making two and one-half turns from the base to the apex, is subdivided into two parallel longitudinal divisions, which are respectively known as the scala vestibuli and the scala tympani. They are so dis- posed that in a given turn of the canal the former is always nearer the apex, the latter nearer the base of the cochlea. According to Wieder- sheim (1893) the human cochlea has nearly three turns, the pig four, the cat three, the rabbit two and one-half, the ox three and one-half, and cetacea one and one-half turns. The osseous lamina spiralis presents a grooved margin or sulcus, from the basal or tympanic lip of which the lamina basilaris is con- tinued to the opposing surface of the bony wall. The lamina spiralis ossea is hollowed out in a diploic manner for the transmission of the FIG. 585. AXIAL SECTION THROUGH THE COCHLEA OF A FETAL CALF. a, internal acoustic meatus in which is the cut end of the cochlear nerve as it enters the modiolus. X 6. (After Kol- liker.) 702 THE EAR branches of the cochlear nerve, which are continuously given off all the way from the base to the apex of the osseous spiral lamina, and which pass outward through the foramina nervosa upon the basilar membrane FIG. 586. AXIAL SECTION THROUGH A TURN OF THE COCHLEA OF A GUINEA-PIG. a, bone of the outer wall of the cochlea; 6, membrane of Reissner; d, membrana tectoria; DC, cochlear duct or scala media; /, stria vascularis; g, organ of Corti; h, spiral ligament; i, cells of Claudius; fc, scala tympani; I, scala vestibuli; ra, vestibular lip of the limbus spiralis; n, internal spiral sulcus; o, nerve fibers of the cochlear nerve, contained within one of the radiating canals within the osseous spinal lamina; p, nerve cells of the spiral ganglion; q, blood-vessel; r, external spiral sulcus, upon which open Shambaugh's glands; s, prominentia spiralis, containing the vas prom- inens. X 90. (After Bohm and von Davidoff.) to be distributed to the epithelium of the spiral organ (of Corti). This organ is a peculiar spiral group of neuro-epithelial cells which extends the whole length of the basilar membrane from the base to the cupola of the cochlea. THE INTEENAL EAR 703 The margin of the osseous spiral lamina is much thickened by the fibrous and epithelial tissues by which it is invested, so that a mem- branous sulcus of considerable depth is formed between the two Tips (vestibular and tympanic lips) of the bony sulcus spiralis internus. This is further thickened by a marked elevation of fibrous tissue cov- ered by columnar cells, from the outer margin of which a delicate mem- brane,, the membrana tectoria, extends outward and overhangs the epi- thelium of Corti's organ. From the inner margin of the elevation of fibrous tissue, the limbus spiralis, which is supported by the vestibular lip of the bony lamina, a delicate membrane, the vestibular membrane (of Reissner), passes obliquely outward to the bony wall of the cochlea, and in transections appears to cut off a corner of the scala vestibuli, thus marking off a triangular space whose base is formed by the outer wall of the cochlea, its sides by the membrane of Eeissner and the basilar membrane upon which rests the organ of Corti; its blunt apex is found at the sulcus spiralis internus. Since these membranes ex- tend the entire length of the bony spiral canal of the cochlea, the space which is thus apparently cut off from the scala vestibuli must form a spiral canal, included between the scala tympani on the one side and the scala vestibuli on the other; this canal is the scala media or cochlear duct. The scala media is an endolymphatic canal. At the apex of the cochlea it ends in a blind extremity which is known as the lagena or cecum cupulare; its opposite end forms a blind pouch between the fenes- tra cochleae and the fenestra vestibuli, at the base of the cochlea, which is termed the cecum vestibular e. The scala media is connected with the saccule and utricle by means of the canalis reuniens, as described above. The scala tympani and scala vestibuli, on either side of the scala media, extend spirally from the base to the apex of the cochlea. At the apex they are united by the helicotrema, a continuation of these canals which curves around the hamulus. At the base of the cochlea the two canals diverge, the scala tympani ending abruptly at the fenestra cochleae, which is closed by a fibrous membrane, clothed on its tympanic surface by the flattened epithelium of the tympanic mucosa, and on its cochlear surface by the epithelium of the scala tympani. This secondary tympanic membrane serves for the relief, of tension in the cochlea when the perilymph is set into motion by the stapes. The scala vestibuli, on the other hand, is continued backward into the vestibule, where it is in relation with the external surface of the saccule and utricle, and, 704 THE EAE since it is in contact with the outer wall of the bony vestibule., this por- tion of the scala vestibuli receives the opening of the fenestra vestibuli, which is closed by the foot plate of the stapes. Corresponding to the relative positions of the fenestra vestibuli and fenestra cochleae, the scala vestibuli in the first turn of the cochlea lies above the scala tympani, and being somewhat the longer it also extends farther backward. Having traced the general form and relations of the several portions of the cochlea, we are now in a position to study more carefully the finer structure of its more important parts. The membranous wall of the scala tympani and scala vestibuli is clothed by a mesenchymal epithelium of flattened endothelioid cells, which rest upon a double layer of fibrous tissue. Thus the tunica propria also serves as a periosteum for the inner surface of the bony wall of the cochlea, and conveys the blood and lymphatic vessels. The scalse are perilymphatic canals. They communicate with the subdural space through the aqueductus vestibuli and the aqueductus cochlea?. The latter opens from the scala tympani near its beginning at the fenestra coch- leae, and passes below the pyramid to the dura transmitting a small vein. The vestibular membrane (of Reissner) is an extremely delicate structure which consists of a thin central substantia propria, covered on either surface by epithelium, that on the one surface being con- tinuous with the mesenchymal epithelium of the scala vestibuli, that on the other with the ectodermal epithelium of the scala media. It is non-vascular in the adult. The outer wall of the scala media is lined by a continuation of the epithelium in that portion which adjoins the membrane of Eeissner, and this rests upon a fibrous membrane similar to that which forms the walls of the other scalae. Toward the attachment of the membrana basilaris, however, the tissue of the outer fibrous wall of the scala media is much thickened, and forms a dense ligamentous structure, triangular in shape as seen in a longitudinal section of the cochlea, which receives the insertion of the membrana basilaris at its apex, and being, like the basilar membrane, continued from the base to the apex of the cochlea, is known as the spiral ligament. Its dense fibrous bands radiate from the attachment of the basilar membrane to all portions of the ligament, and are firmly attached to the bony wall of the cochlea, with whose periosteum the deeper fibers of the spiral ligament are blended. The surface of the spiral ligament, which forms the outer wall of the scala media, slopes gradually away from the attachment of the basilar THE INTEENAL EAE 705 membrane; that which impinges upon the scala tympani slopes more abruptly. The greater portion of the spiral ligament, therefore, is con- tained within the scala media. Here it is lined by low columnar or cuboidal epithelium whose cells blend, without demarcation, with the underlying vascular connective tissue, so that the minute blood-vessels frequently appear as if lying within the epithelial layer, although they probably are always contained within the connective tissue processes which project into the attached surface of the epithelial layer. This very vascular subepithelial portion of the spiral ligament is known as the stria vascularis. A short distance above the point of attachment of the basilar membrane to the spiral ligament appears a prominent spiral ridge, the prominentia spiralis, the intervening groove constituting the sulcus spiralis externus. The larger blood-vessel (venous) within the prominence is the vas prominens. From the ex- ternal sulcus there extend into the subjacent ligamentous tissue numer- ous large clear clumps of epithelioid cells. These have been variously interpreted as neuro-epithelial elements and as smooth muscle cells. But Shambaugh (Archives of Otology, 37, 6, 1908) has shown that they contain tubules which open into the sulcus, and that they are in reality branched tubular glands. He ascribes to them the function of pro- ducing at least a portion of the endolymph of the scala media; the stria vascularis is probably also an important source of endolymph. The tympanic wall or floor of the scala media presents for examina- tion several structures, which, from within outward (viz., from the modiolus to the ligamentum spirale), are the limbus spiralis, membrana tectoria, sulcus spiralis internus, basilar membrane, and the organ of Corti which rests upon the basilar membrane (Fig. 587). The vestibular lip of the limbus spiralis presents a distinct eleva- tion, which is formed by a peculiar cellular variety of connective tissue, and is covered by columnar epithelium, whose cells are not sharply de- nned from those of the underlying connective tissue. The surface of the epithelium presents a distinct cuticular formation of considerable thickness, which seems to be prolonged outward from the margin of the vestibular lip, and forms the membrana tectoria. The surface of the limbus spiralis, when viewed from the scala media, presents slight elevations which, at the margin of the vestibular lip, are prolonged into prominent ridges whose indented borders over- hang the sulcus and are known as the auditory teeth (of Huschke). The Membrana Tectoria (Membrane of Corti). This is an exo- plasmic or cuticular tissue, formed by the epithelium of the inner or 706 THE EAR limbus portion of the embryonic cochlear duct. It has a gelatinous fibrillar structure, but lacks nuclei ; and, unlike the otherwise very simi- lar otolithic membranes of the maculae and cristaB, it contains no calca- reous products. Its free margin overhangs, or rests lightly upon, the hair cells of Corti's organ. The tectorial membrane has been very carefully studied by Hardesty (1908) in the pig, from whose cochlea he has been able to remove it entire. It is said to measure about 30 mm. in length, to occupy the four turns of the cochlea, to be about five times as wide and five times as thick in the apical turn as at the basal end, and to have a section area in the apical turn approximately twenty-one times and a volume ninety-five times the area and volume of its basal end. It is described as consisting 'of a hyaline matrix, probably keratin, in gelatinous form, in which are embedded the very numerous fine fibers or threads of uniform size' (Anat. Rec., 8, 2, 1914). The membrane has a slight amount of elasticity, is of a semi-solid character and possesses 'marked adhesiveness'; its specific gravity is said to be but little greater than that of the endolymph. None of the fibers extend the entire width of the membrane, none are attached at both ends, and the greater number are attached at neither end (Hardesty, Amer. Jour. Anat., 8, 2, 1908). Hardesty describes a stripe (Hensen's stripe) on the under surface of the tectorial membrane opposite the row of inner hair cells, which he explains 'as a line of intercrossing ends of fibers of the under surface resulting from the process by which the growth of the mem- brane terminates/ He describes also a thin, exceedingly delicate, 'acces- sory tectorial membrane' along the under surface of the outer portion of the chief membrane; only its outer edge is attached to the latter, and it is bounded internally by Hensen's stripe, thus covering only the outer hair cells. The Sulcus Spiralis Interims. This is a deep groove included be- tween the vestibular lip of the limbus and the basilar membrane which is attached to the tympanic lip. The sulcus is lined by flattened epi- thelial cells, which are apparently continuous with those of the vestib- ular lip, and like them are not readily distinguished from the under- lying connective tissue. The epithelium is continued outward upon the basilar membrane to the margin of Corti's organ, with the innermost cells of which it is continuous. The Basilar Membrane (Membrana basilaris) . This is a thin but resistant membranous structure, upon which rests the epithelium of Corti's organ. Hardesty (1908) describes it as a 'flat tendon . . . whose purpose is merely to strengthen the floor of the ductus cochlearis and J g g 1 1*1 s il'S* H PI e ? - - U ^ ^T'o o o B & jz ir.a s 5- *! V* 9 gi.!.JS P C d ^ S||| *!ii H .^ag ^ o w fc S M 5^^ S "3 fe 'i I mil 5 HI H4 CJ) 5 1 a 1 Ss " 'l'i-1 M c3 18 *" fe * . a o U r i LJ s alS 19 i QJC 708 THE EAR the position of the organ of Corti, and the fibers of which are too rigid and firmly associated to allow of resonant vibration.' Its tympanic sur- face is clothed by a continuation of the lining membrane of the scala tym- pani, consisting of a mesenchymal epithelium, resting upon a very thin and delicate connective tissue layer. The substantia propria of the bas- ilar membrane consists of tendinous bands which, being radially disposed, span the interval between the margin of the tympanic lip of the osseous spiral lamina and the opposed margin of the spiral ligament. Because of the great breadth of the modiolus at the base, and its rapid diminution in thickness toward the apex of the cochlea, this interval is relatively narrow at the beginning of the first turn of the spiral scala media, but progressively widens as the apex of the cochlea is approached. Consequently, the shortest tendinous fibers of the basilar membrane are found at the base of the cochlea, the longest at its apex. The shortest fibers are also the coarsest. It has been estimated that there are 24,000 distinct fibers or 'auditory strings' in the basilar membrane. The substantia propria is covered upon that surface which faces the scala media by a thin homogeneous membrane, a cuticular forma- tion or exoplasmic derivative, upon which rests the epithelium of the organ of Corti. The Organ of Corti. This organ consists of a highly differenti- ated neuro-epithelium whose specialized cells are disposed according to a very regular arrangement. The flattened epithelium of the sulcus spiralis internus is continued for a short distance upon the basilar mem- brane. Suddenly, at the margin of Corti's organ, it alters its char- acter. Here the epithelium becomes abruptly changed to a tall columnar variety, the first cells, known as the inner sustentacular cells, being ap- parently piled upon one another and resting against the inner hair cells, which form a single row of neuro-epithelium; these, like all the suc- ceeding rows of cells, can be traced as a continuous line in the spirally wound scala media, from the base to the apex of the cochlea. The inner auditory or hair cells have a broad body which is con- fined to the superficial third of the epithelial layer and which is nu- cleated at its deeper end. Its free surface forms an expanded oval plate from which about twenty stiff cilia project through a cuticular membrane toward the cavity of the scala media. These end plates interdigitate with the phalanges of the inner pillar cells, which are to be shortly described. The bases of the inner hair cells are thin and slender, and are in relation with a nerve plexus of fine fibrils derived THE INTERNAL EAR 709 from the terminal processes of the cochlear nerve. These nerve fibrils make their exit in small bundles from the bony spiral lamina, through the foramina nervosa and passing outward upon the basilar membrane are distributed in a plexus beneath the epithelium, some of their naked processes almost immediately penetrating the epithelial layer to end between the bases of the inner hair cells. The inner hair cells rest against the inner pillar cells, or rods, of Corti's arch. This arch is formed by two rows of highly specialized cells, the inner and the outer pillars, which are widely separated where their bases are attached to the basilar membrane, but are in contact at their free ends ; in fact, the free extremity of the inner pillar is pro- longed into a broad flattened plate-like process whose inner margin in- terdigitates with the head plate of the inner hair cells, as stated above, and whose outer margin is so prolonged as to almost, though not com- pletely, cover the rounded head of the outer pillar. The head of the outer pillar, being similarly flattened, expanded, and prolonged outward beyond the margin of the head plate of the inner pillar cell, comes into contact with the phalanges of Deiters' cells and with the cilia of the outer hair cells which lie next without; they leave a space between the outer pillars and the outer hair cells which is known as Nuel's space, filled by a semi-fluid intercellular substance. The inner pillar cells are rather more numerous than the outer in the entire length of the scala media, according to Retzius, there are 5,600 of the former to 3,850 of the latter so that about three of the expanded head plates of the inner pillars overlap two of the rounded heads of the outer pillar cells. The arch formed by the opposed pillar cells, being succeeded by similar arches of successive pillars, forms a continuous tunnel, triangular in transection, which extends the whole length of the scala media, and is known as the canal of Corti. This canal is also filled with a semi-fluid substance. Each pillar cell is differentiated into two portions, the pillar proper and the basilar cell, the latter containing the nucleus. The pillar presents a fibrillar appearance, the fibrils being disposed in the long axis of its body. This portion of the cell reaches from the basilar membrane to the free surface of the neuro-epithelium. The basal part of the cell, the basilar cell, probably represents the undifferentiated portion of the primordial pillar cell. It consists of a clear, finely granular cytoplasm and contains the spheroidal nucleus. It lies on that side of the pillar which faces the canal of Cor!i, the bases of the opposed cells being expanded until they meet, thus forming a 710 THE EAE cuticular floor for the tunnel. This undifferentiated basilar portion occupies only the deeper half of the pillar cell. The outer hair cells form three to five rows of ciliated cells which are similar in structure to the inner hair cells, and which are sup- ported by the sustentacular cells of Deiters. Their cylindrical cell bodies occupy the superficial third of the epithelial layer and at the deeper extremity present a nucleated enlargement, beyond which they are continued only as an extremely slender basal process. The free ends of the outer hair cells present an expanded oval surface from which the hairs project. The outer hair cells are about five times as numerous as the inner, that is, there are about 3,600 of the inner to 18,000 of the outer (Waldeyer). According to certain authorities the hair cells lack the delicate basal process in the adult condition. The outer sustentacular cells (Deiters' cells) are cylindrical cells whose expanded bases rest upon the basal membrane and whose distal portions extend toward the surface between the outer hair cells. The superficial portion of these cells, being encroached upon by the broad outer hair cells, is very slender; the broader basal portion occupies the deeper two-thirds of the neuro-epithelium, the spheroidal nuclei being found at the level of the middle third. Each sustentacular cell contains a cuticular filament (fiber of Retzius) which begins in contact with the cuticle of the basal membrane, and extends through the axis of the cell to its free border, where it expands to form a broad flattened plate of peculiar shape, known as the phalangeal process. These cuticu- lar processes surround and overlie the margins of the head plates of the hair cells, thus forming a reticular layer through the openings of which the cilia of the hair cells project. The cells of Deiters are succeeded by the sustentacular cells of Hensen. These are tall columnar cells about eight rows broad, the inner- most of which equal in height the tall cells of the preceding type, but which at their outer border become abruptly shortened. Here they pass into the cuboidal cells of Claudius, and are thus continued outward to the spiral ligament. The nuclei of the cells of Hensen are found in their superficial third, those of the cells of Claudius in the center of the cell. Beneath Hensen's cells other small nucleated elements are occasionally found; they give to this layer somewhat the appearance of a two-rowed epi- thelium and are known as the cells of B ditcher. Both the cells of Hensen and those of Claudius are provided with a cuticular margin which, with the similar cuticle of the cells of Deiters, THE INTEKNAL EAE 711 forms a continuous, membranous, cuticular layer known as the lamina reticularls. The inner portion of this cuticular membrane is pierced by the cilia of the three to five rows of outer hair cells, as already described. A fibrillar axial core is a common feature of all of the sustentacular elements of Corti's organ. This fiber of Eetzius becomes progressively A FIG. 588. DIAGRAM OF THE ORGAN OF CORTI. A, surface view, from the direction of the scala media; B, as seen in section, profile view, a, the vestibular lip of the lamina spiralis; b, margin of same; c, sulcus spiralis internus; d, inner sustentacular cells; e, inner hair cells; /, pillar cells; g, outer hair cells and phalanges of Deiters' cells; h, cells of Hensen; i, cells of Claudius. Very highly magnified. less pronounced in passing from the pillar cells, where it is very highly developed, to the cells of Claudius, where it is barely discernible. In the above description we have directed attention to the appear- ance of transections of the organ of Corti. In the study of this organ in the fresh condition, and occasionally in fixed and stained preparations, it is possible to obtain a surface view of this organ from the direction of the scala media. In such preparations the polygonal outlines of the columnar cells of the limbus spiralis, beneath which are the auditory teeth, are seen on the outer side of the attachment of Keissner's mem- THE EAE brane. Beneath the overhanging vestibular lip of the limbus the mosaic of large polygonal epithelial cells of the internal sulcus comes into view. At the margin of the organ of Corti these are exchanged for the broader cell ends of the inner sustentacular cells and the adjacent single row of inner hair cells. The flattened rectangular head plates of the inner pillar cells form the next row, the heads, of the outer pillars projecting from beneath,, and extending beyond the heads of the inner pillar cells. These are fol- lowed by the interdigitating phalanges of the cells of Deiters, which enter into the formation of the reticular membrane, through the f enestra of which the cilia of the three to five rows of outer hair cells project. This cuticular membrane is continued outward, and beneath it are successively seen the ends of the cells of Hensen, and of the cells of Claudius. THE ACOUSTIC NERVE The acoustic nerve presents two distinct divisions both of which are sensory, but which differ greatly as regards their central termination. They likewise differ in their peripheral distribution. Within the inter- nal acoustic meatus the nerve divides, each branch consisting of numer- ous bundles. The vestibular (superior or anterior) division is supplied with a ganglion of considerable size, the vestibular ganglion (of Scarpa), beyond which the nerve separates into three branches which supply, re- spectively, the macula of the utricle, and the cristaB of the superior and lateral semicircular canals, in the neuro-epithelium of each of which their terminal fibrils end in relation with the bases of the hair cells (Figs. 582 and 583). The remaining nerve fibers which are distributed to the vestibule are derived from a branch of the cochlear (inferior or poste- rior) division, and they supply in a similar manner the macula of the saccule and the crista of the posterior semicircular canal. According to Streeter (Amer. Jour. Anat, 1907), the vestibular nerve contributes also the innervation to the posterior canal and to the saccule, the cochlear nerve supplying only the cochlea. The cochlear branch proper, cochlear nerve, enters the modiolus, where it becomes abruptly narrowed by giving off numerous fine branches which pass outward between the layers of the bony spiral lamina. Here they form a continuous spiral succession of small nerve trunks, supplied with many bipolar ganglion cells, which collectively form the spiral ganglion (Fig. 586). They penetrate the margin of the bony sulcus through the foramina nervosa,, a succession of perforations, in the tym- THE INTERNAL EAR 713 panic lip of the sulcus. Here the nerve fibers lose their medullary sheath and come almost at once into relation with the inner "hair cells. From this point the path of the non-medullated fibers varies, most of them passing for some distance along a spiral course through the organ of Corti. One such spiral bundle is found on the inner, and another on the outer side of the inner pillars, the latter lying within the canal of Corti. Still other fibers, the tunnel fibers., cross the canal of Corti to form a spiral plexus beneath the outer hair cells and the cells FIG. 589. AXIAL SECTION THROUGH CORTI'S ORGAN OF THE GUINEA-PIG, SHOWING THE TERMINAL NERVE FIBRILS. B, cells of Bottcher; D\ D z , D 3 , three rows of Deiters' cells; H, cells of Hensen; i.b., inner border cell; i.h., inner hair cell; i.p., inner pillar cell; n, terminal branch of the cochlear nerve; o.h.-l, 2, 3, three rows of outer hair cells; o.p., outer pillar cell; p., phalangeal process of the outer sustentacular process. Very highly magnified. (After Held.) of Deiters. Terminal fibrils from these spiral plexuses end in relation with both the inner and the outer hair cells. , The relation of the nerve cells of the spiral ganglion and the vestibu- lar ganglion to the termination of the nerve fibrils about the hair cells of the organ of Corti, the maculae, and the cristaa, is essentially the same. The ganglia contain the cell bodies of the peripheral sensory neurons of the eighth cerebral or acoustic nerve. These are bipolar cells, of which the central process or axon enters a medullated nerve fiber of the acoustic nerve, while the peripheral process is distributed to the hair cells of the several areas of specialized neuro-epithelium, as above de- scribed. 45 714 THE EAE THE VASCULAR SUPPLY Blood Supply. The internal ear is supplied by the internal audi- tory artery, a branch of the basilar artery, which enters the labyrinth along with the acoustic nerve, and at once divides into two main stems the vestibular and the cochlear (arteria cochlearis communis, Sieben- FIG. 590. SCHEME OF THE VASCULAR SUPPLY OF THE INTERNAL EAR. C 1 , first turn of the cochlea; S, saccule; Sup.S.C., Ext.S.C., and Post.S.C., superior, external, and posterior semicircular canals; U, utricle. The arteries are in heavy black, the veins somewhat lighter; a, central vein, and 6, central artery of the cochlea; c, vestibular artery; d, vestibulocochlear artery; e, arteria propriae cochleae; /, vena aqueductus cochleae; g, vena aqueductus vestibuli. mann) . The vestibular artery accompanies the branches of the vestibular nerve to the saccule, utricle, and semicircular canals, supplying these structures in the posterior portion of the vestibule, and forming a rich plexus in the connective tissue of the maculae and cristse, and a more scanty network in the remaining portions of the membranous labyrinth. The cochlear division of the internal auditory artery, according to Siebenmann, promptly subdivides into the cochlear artery proper, which appears as the continuation of the vessel, and the vestibulocochlear THE INTERNAL EAR 715 artery, which supplies the macula sacculi, the posterior ampulla, and the adjacent portions of the utricle and posterior semicircular canat. "This vessel also supplies the early portion of the first turn of the spiral cochlea. The true cochlear artery enters the modiolus and supplies a branch to the remaining portion of the first cochlear turn, and a terminal branch which passes as far as the apex of the cochlea, distributing its FIG. 591. SCHEME OF THE VASCULAR TERMINATIONS IN THE WALL OF THE COCH- LEAR CANALS. c, capillary vessels in the spiral ligament; DC, cochlear duct or scala media; d, capillaries in the limbus spiralis; /, scala tympani; g, arteriole; h t spiral ganglion; i, vena spiralis inferior; v, scala vestibuli; j, vena spiralis superior. (After Bohm and von Davidoff.) branches to the last two turns. All of these vessels are characterized by their peculiarly tortuous course. They distribute terminal branches to the limbus spiralis and to the connective tissue of the membranous scala vestibuli, extending as far around this canal as the spiral ligament. No vessels cross in the basilar membrane. The veins collect the blood from the limbus spiralis and the wall of the scala tympani and form venous trunks within the modiolus, which correspond more or less closely with the arteries. Two of the cochlear radicals of the venous tributaries are important by reason of their posi- tion and relative size: the vas prominens of the prominentia spiralis of the stria vascularis, and the vas spiralis beneath the organ of Corti. 716 THE EAE Those veins coming from the wall of the scala tympani unite to form superior and inferior spiral veins in the inner wall of the scala tympani. These vessels chiefly empty into the vena aqueductus cochleae which finds its way through the aqueduct to the internal jugular vein. Other branches from the interior of the cochlea unite to form the central vein of the cochlea, which becomes the chief radical of the internal auditory vein, and thus enters either the transverse or inferior petrosal sinus. The veins from the utricle and semicircular canals mostly enter the vena aqueductus vestibuli, which follows its aqueduct to the superior petrosal sinus. It will be perceived that the blood has three chief avenues of exit from the labyrinth: I, by the vena aqueductus vestibuli; 2, by the vena aque- ductus cochleae; and, 3, by the internal auditory vein. The greater portion of the blood pursues the second course and thus finds its way to the internal jugular vein, the smaller remainder entering the petrosal sinuses by one of the other two avenues. Lymphatics. The internal ear contains relatively few lymphatic vessels but is richly supplied with broad lymphatic spaces. Anastomosing vessels are found in the periosteum and membranous wall of the laby- rinth. These communicate with the perilymph spaces between the peri- osteum and the membranous wall in the vestibule, and with the vestibular and tympanic scalae in the cochlea. The perilymphatic spaces are con- nected with the subdural space of the meninges by means of lymphatic channels in the aqueductus cochleae. The perilymph of the vestibule also communicates with the subdural space through vessels which follow the sheaths of the nerves. The endolymph cavities of the several divisions of the membranous labyrinth communicate freely with one another ; by means of the ductus endolymphaticus a connection is also established though the aqueductus vestibuli with the subdural space, the blind terminal saccule of this canal, the saccus endolymphaticus lying upon the posterior surface of the petrous bone and in contact with the dura mater. FUNCTION OF THE COCHLEA The cochlea is the essential organ of hearing. The fundamental struc- ture concerned in audition is the spiral organ of Corti. The physiology of sound perception involves proximally the stimulation of the hair cells of Corti's organ by the tectorial membrane. This membrane is thrown DEVELOPMENT OF THE EAE 717 into synchronous vibrations by the undulations in the endolymph of the cochlear duct transmitted through the vestibular membrane from_the peri- lymph. The latter receives the sound waves through the foot-plate of the stapes. The Helmholtz (1896) theory of tone perception, until recently widely accepted, postulated sympathetic vibrations, in resonance with atmospheric waves, on the part of the fibers of the basilar membrane. This view has been shown to be untenable, notably by Shambaugh (Archives of Otology, 37, 6, 1908) and by Hardesty (Amer. Jour. Anat., 8, 2, 1908). The basilar membrane contains only a little more than half as many fibers as the maximum number of vibrations (40,000 double vibrations per second) com- monly audible; moreover, it does not possess the physical and histologic properties demanded by the 'resonance theory' of tone perception. Har- desty has shown that the tectorial membrane on the contrary does answer the requirements of an alternative theory, a modification of the earlier 'telephone theory' of Rutherford (1886) ; and he has succeeded in construct- ing an apparatus which simulates the cochlea, and imitates its presumed functional activity at least in the lower ranges of the tone scale. Hardesty suggests 'that notes up to a certain pitch throw the entire natural tectorial membrane into vibrations of corresponding frequencies and that sensations of pitch are determined by the frequency of impinge- ment of the membrane upon the auditory hairs, intensity being determined by the amplitude and quality by the quality of the wave motion imparted. Further, that the highest notes within the range of the auditory apparatus throw, according to their frequency, only varying extents of the smaller, basal end of the tectorial membrane into vibration, being so damped out in passing toward the apex of the cochlea, overcoming friction, the inertia of the endolymph and that of the membrane itself, as not to produce vibra- tions in the heavier, apical portions.' (Anat. Rec., 8, 2, 1914). In essence, the tectorial membrane is conceived to respond in its sev- eral parts in the manner of a physical resonator to tones of different pitch (Shambaugh). According to this conception, tone analysis is accomplished peripherally, the specific stimulus being carried to the brain by the special sets of cochlear nerve fibers. DEVELOPMENT OF THE EAR The external ear develops in connection with the first branchial fur- row: the meatus from the deepened groove, the auricle through the fusion of definite tubercles on the adjacent branchial arches. The middle ear and auditory tube arise from the corresponding pharyn- geal pouch, the process involving a ventral elongation of the groove to form FIG. 592. SEMIDIAGRAMMATIC ILLUSTRATIONS OF SUCCESSIVE STAGES IN THE DE- VELOPMENT OF THE INTERNAL EAR OF THE CHICK. A, 45 hour embryo; B, 60 hour embryo; C, 5 day embryo; D, 7 day embryo; M, wall of neural canal at level of metencephalon; E, epidermal ectoderm; op, otic pit; oc, otocyst (otic vesicle); DE, ductus endolymphaticus; SE, saccus endolymphaticus; cd, cochlear duct; u, utriculus; dss, superior semicircular duct; dsl, lateral semi- circular duct. c? FIG. 593. WAX RECONSTRUCTIONS OF THREE EARLY STAGES IN THE DEVELOP- MENT OF THE INTERNAL EAR (MEMBRANOUS LABYRINTH) OF MAN. A, lateral view, from a 6.6 mm. embryo; B, lateral view from an 11 mm. embryo J C, front view from a 20 mm. embryo (A and B correspond approximately to stages C and D of the preceding figure). DE, ductus endolymphaticus; SE, saccus endo- lymphaticus; VP, vestibular pouch; cp, cochlear pouch; ap, absorption focus; c, cms; s, sacculus; u, utriculus; cd, cochlear duct; dss, dsp, and dsl, superior, poste- rior, and lateral semicircular ducts. (After Streeter.) 718 DEVELOPMENT OF THE EAR 719 the tube and a subsequent dorsal dilatation, expanding as the tympanum to include the auditory ossicles which have meanwhile taken form in the adjacent mesenchyma. The mastoid cells are formed by a late erosion and invasion of the bone by the mucous membrane of the tympanum. The internal ear develops from a thickening in the epidermal ectoderm at the level of the third primary cerebral vesicle. This auditory anlage becomes invaginated to form an otic pit, the aperture of which subse- quently closes and thus separates an auditory vesicle or otocyst from the overlying parent ectoderm. At the point of closure medially a dorsal evagination arises to form the endolympJiatic duct. At the upper pole of the vesicle appear the semicircular ducts through a process involving the elevation of three circular folds, the lateral walls of which fuse proximally, and subsequently disappear leaving a peripheral duct dilated at one end to form an ampulla. The cochlea arises at the opposite pole as a tubular evagination which becomes spirally disposed. The original vesicle persists as the utricle; on its anteromedial border is formed an alveolar evagina- tion, the saccule, which remains connected by a constricted duct, the utric- ulosaccular canal, to which is attached the ductus endolymphaticus. Each ampulla differentiates an elongated patch of neuro-epithelium, the cristce; in both utricle and saccule a similar oval patch appears, the maculfE; the spiral organ of the cochlea develops in like manner through a specialization of the ectoderm along the floor of the membranous duct. The bony labyrinth develops from the mesenchyma originally envelop- ing the membranous labyrinth. The mesenchyma immediately surround- ing the membranous labyrinth becomes converted into a mucoid tissue which eventually disappears, leaving the perilymphatic spaces; these are bounded by periosteum, the innermost layer of which becomes modified into a mesenchymal epithelium. The scala tympani and scala vestibuli of the cochlea are formed by the coalescence and subsequent dilatation of small mesenchymal tissue spaces in two distinct regions: one between the saccule and the oval window, the other between the saccule and the round window. From these two areas the two great scalae of the cochlea proceed in a definite and constant direction to their definite position and condition (Streeter, Proc. Amer. Assoc. Anat., 1916). CHAPTER XX HISTOLOGIC TECHNIC The satisfactory examination of the tissues with the aid of the mod- ern microscope requires certain preparatory steps which are in certain cases very simple, in others very complicated. The present chapter deals briefly with the more important and simpler methods, and the general principles upon which they are based. THE EXAMINATION OF FRESH TISSUES Certain tissues may be examined immediately after they have been removed from the body. This method is applicable to blood, lymph, scrapings from the spleen, liver, uterus, and similar organs, small frag- ments of muscle, connective tissue, etc. A small drop of blood may be collected upon the under surface of a cover glass, which is then quickly dropped upon a glass slide and examined at once. The glass must be thoroughly cleaned, otherwise a thin preparation can not be obtained. Slides and covers should be washed in very dilute hydrochloric acid (about 10 per cent.), then washed in running water for several hours, and finally rinsed in 95 per cent, alcohol. Ordinarily slides and cover glasses can be sufficiently cleaned by simply dipping in alcohol and drying with a linen cloth. Scrapings from the epithelium of the mouth, or from similar mucous membranes, may be prepared in the same manner as blood, and ex- amined while still suspended in their own fluids. Most tissues, however, are not sufficiently well moistened for examination after this manner; the preparation must then be diluted with some inert fluid. Normal saline solution may be used for this purpose; the following formula is recommended : Sodium chlorid 0.75 to 0.9 grm. Distilled water 100 c.c. 720 THE DISSOCIATION OF TISSUES 721 Some histologists regard a 0.6 per cent, solution preferable. Normal salt solution has become quite generally displaced by Ringer's Solution, which corresponds more nearly to blood plasma and is less likely to produce distortion. This is prepared, adjusted to tissues of warm- blooded animals, according to the following formula: Sodium chlorid 90.0 Potassium chlorid 4.2 Calcium chlorid (anhydrous) . 2.4 Potassium bicarbonate 2.0 Distilled water 10000.0 In order to render the nuclei more conspicuous a drop of a 5 per cent, aqueous solution of acetic acid may be added. This, however, will dissolve the collagenous fibers and albuminous granules. The nuclei may be stained by the addition of a drop of a 1 per cent, aqueous solu- tion of methylene blue. A 40 per cent, solution of glycerin in distilled water also is useful as an examining medium; better still, the tissues may be suspended in a mixture of equal parts of 95 per cent, alcohol, glycerin, and dis- tilled water. This mixture is especially useful, for in it tissues may be kept for a long period without deterioration. Amniotic fluid obtained from pig fetuses is also valuable as a medium for the examination of fresh tissues. Hogan (Jour. Amer. Med. Assoc., 64, 9, 1915) has devised a normal- salt-gelatin mixture with a colloidal constitution like that of blood serum, which serves as a favorable indifferent medium for the examination of fresh blood and other delicate tissues. For the method of preparation reference should be made to the original article. THE DISSOCIATION OF TISSUES It is frequently desirable to dissociate tissues to a certain extent into their component elements prior to microscopical examination. This is accomplished by teasing, or by the solvent action of relatively strong acids or alkalies. Isolation by the latter method is known as maceration. For teasing, minute fragments of tissue are separated by the aid of needles or scissors and placed on a clean slide, where they are to be kept always moistened with normal, saline solution or, other isotonie 722 HISTOLOGIC TECHNIC fluid. For their further manipulation a dissecting microscope is useful, though not essential. Two sharply pointed needles, mounted in wooden handles, are to be used. The fragment of tissue is pinioned with the needle held in the left' hand, and with that in the right the tissue is gently torn by a rhythmic combing motion, being very careful to avoid squeezing the tissue between the needle and the slide. With a little practice bundles of fibers, groups of cells, etc., are readily isolated sufficiently to be studied under moder- ate magnification. During the teasing, the fragments of tissue should be kept well moistened, and are to be frequently inspected under low magnification to determine the progress of the operation. When satis- factorily prepared, a cover lass may be applied, and the preparation examined under higher magnification. In applying a cover glass care should be taken to permit one edge of the cover to first touch the slide while being held at an angle of 30 degrees to 40 degrees. If the cover is then gently lowered into place, the air is forced out before the advance of the fluid, and the many air bubbles which would otherwise be included are not found in the prepara- tion. The method of teasing is particularly applicable to the study of the connective, and peripheral nervous tissues. Collagenous fibers, elastic fibers, fat cells, and nerve fibers are readily isolated in this way. If desired, they may be stained by the addition of a drop of a solution of methyl green, picrocarmin, etc. Chemical Dissociation. It is necessary to dissociate many tissues by chemical means, either because of the firm union of the elements composing the tissue or because they may be too delicate and fragile to withstand the mechanical teasing. Epithelial cells, nerve cells, and muscle fibers are readily macerated in this way. For the dissociation of muscle fibers small cubes (0.25 to 0.5 c.c.) are placed for ten to thirty minutes in the following solution : Strong nitric acid 100 c.c. Potassium chlorate, sufficient to saturate. The fragments of tissue should be handled with glass rods. After some minutes they begin to disintegrate at the surface, and should then be transferred to running water, where they are left to wash from three to twelve hours. The pieces of tissue are then transferred to a mixture of equal parts of alcohol, glycerin, and water, and thoroughly shaken. FIXATION 723 Muscle fibers isolated in this way may be kept for months or even years. Caustic potash (potassium hydroxid) is also a valuable dissociator for muscle tissue. The solution commonly used is made by dissolving 40 grams of the potash in 60 c.c. of water. Cardiac muscle is suffi- ciently macerated in twenty minutes. The tissue must then be thor- oughly washed, and may be preserved in the alcohol-glycerin mixture. Epithelium may be dissociated by teasing or by the action of a 40 per cent, solution of potassium hydroxid, or by means of a 10 or 20 per cent, aqueous solution of 'lysol/ and preserved, if desired, in the mixture of alcohol, glycerin, and water. Nerve cells may be isolated from the anterior horns of the spinal cord or other gray matter of the central nervous system by teasing. They may also be isolated by immersion in a 0.2 per cent, aqueous solution of formalin in normal salt solution for two to twenty-four hours, or in a 0.2 per cent, aqueous solution of potassium bicarbonate, two to five days. Afterward they are transferred to a normal saline solution or to a mixture of alcohol, glycerin, and water, and isolated by shaking, assisted, if necessary, by gentle teasing. Similar preparations may also be made by placing, small fragments of tissue in 30 per cent, alcohol (Ranvier's alcohol; 2 parts water, 1 part 95 per cent, alcohol) for two days or more; then shake thoroughly, allow the debris to se'ttle, remove a drop of the fluid with a pipette, and examine. For the isolation of the elements of the epidermis, hair and nails, stronger solutions are required. Concentrated sulphuric acid may be employed. After maceration is complete, the cells must be thoroughly washed in water and may then be preserved in the alcohol-glycerin mixture. A caustic potash solution, acting for two to three hours, is also serviceable with these tissues. Any of the above preparations may be stained by the addition of a small drop of a solution of eosin, picrocarmin, or methyl green to the fluid in which they are examined. FIXATION For the preservation of tissue, and as a preparation for further manipulation, most tissues require to be 'fixed/ Innumerable formulas have been advocated for this purpose, many of them having as their 724 HISTOLOGIC TECHNIC object the demonstration of certain structural features by the after ap- plication of special staining methods. The action of the fixing fluids is in most cases dependent upon the combination of the reagent with the chemical elements of which the tissue consists; very elaborate compounds are thus formed. The distinction between a physical and a chemical combination of a dye with particular tissue elements must be emphasized. The following reagents are recommended for general use. The choice of a fixative is in great measure determined by the staining method which is to be afterward applied. Alcohol. This is especially useful for the fixation of the glandular organs. Tissue may be placed directly in 95 per cent., or in absolute alcohol. The fluid is to be changed in twenty-four hours, and again in five to seven days. This method of fixation is desirable for after-staining of the nervous tissues with methylene blue and for the demonstration of glycogen in the hepatic cells, cartilage, etc. Alcohol causes consider- able distortion of the internal architecture of the cell by its rapid and forceful diffusion from the surface toward the center of the tissue, the cytoplasmic granules often being in this way forced to one side of the cell. This result may be partially avoided by the use of 'graded alcohol/ viz., 67 per cent, alcohol for three to twelve hours, 82 per cent, for twenty-four hours, and finally 95 per cent, alcohol, which should be once changed after a few days. Glycogen, however, is partially dissolved by the action of the dilute alcohols. The graded alcohols may be derived from the 95 per cent, stock supply by the following dilutions: 95 per cent, alcohol, 1 part; distilled water, 1 part=48 per cent. 95 per cent, alcohol, 3 parts; distilled water, 1 part=67 per cent. 95 per cent, alcohol, 5 parts; distilled water, 1 part=82 per cent. For practical purposes these grades may be regarded as 50, 70 and 80 per cent, respectively; they may be derived also by the use of the fol- lowing formula: 265 c.c. 95% ale. + 235 c.c. aq. dist. = 50% ale. 370 c.c. 95% ale. + 130 c.c. aq. dist. = 70% ale. 425 c.c. 95% ale. + 75 c.c. aq. dist. = 80% ale. The distortion from the use of strong alcohol, as well as the disso- ciation which follows the use of the weaker strengths, may also be par- FIXATION 725 tially avoided by the addition of a little iodin to the stronger alcohol, or by combination with acetic acid, thus: Glacial acetic acid 5 c.c. 95 per cent, alcohol 60 c.c. Distilled water 35 c.c. After fixation for three to twenty-four hours the tissues are washed, and hardened by immersion for twenty-four hours in each strength of graded alcohol (50, 70, 80 and 95 per cent.), and may be kept indefinitely in 80 per cent, alcohol. Tissues should always be stored in 80 per cent, alcohol. Lower grades tend to macerate, higher grades to harden unduly. Carnoy recommends also a mixture of glacial acetic acid and abso- lute alcohol in the proportion of 1 to 3 ; this gives excellent results with muscular tissues. Tissues for fixation by these or any subsequent method are pref- erably cut into small cubes; a size not exceeding 0.5 to 1 cm. is most desirable. If larger pieces of tissue are necessarily used, the reagents will each require increased time to insure complete penetration. Mercuric Chlorid (Corrosive Sublimate). This salt is to be used in saturated aqueous solution. As it dissolves with difficulty in cold water, the use of a hot, normal saline solution hastens the operation. Mercuric chlorid is an excellent fixative for cytoplasm, but gives still better results when combined with a nuclear fixative such as acetic acid. The following is an excellent method for general use: Mercuric chlorid 7 grm. Sodium chlorid 0.75 grm. Distilled water 95.0 c.c. Just prior to use add 5 c.c. of acetic acid. Small pieces of tissue remain in this corrosive acetic mixture for two to twenty-four hours, and are then transferred to graded alcohol, beginning with 70 per cent, and remaining twenty-four hours in each strength. Most dyes will act perfectly on tissue fixed in this way. If, however, the presence of mercury interferes with the action of a dye, this salt can be readily removed by the addition of a few crystals of iodin to the higher grades of the graded alcohols, renewing the iodin if necessary until it is no longer decolorized. Formalin. Tissues may be fixed in formalin (formol), a 40 per 726 HISTOLOGIC TECHNIC cent, solution of formaldehyd gas, which is to be used in strengths vary- ing from 5 to 20 per cent. Small pieces should be left in the weaker solutions (5 to 10 per cent.) from 'six hours to two days, not longer. In the stronger solutions (10 to 20 per cent.) tissues should remain for only two to six hours. More prolonged immersion in the fixative causes considerable swelling. As a rule, the stronger solutions are pref- erable; this is especially true for the fixation of the cells of lymphoid tissue. The 10 per cent, formalin solution is valuable for the fixation of human embryos. For the hardening of nervous tissues, also, the 10 per cent, solution is very serviceable. After fixation., the tissues are transferred directly to 80 per cent, alcohol, with one or several changes. This method gives excellent re- sults with lymphoid and epithelial tissues, but does not bring out the finer details of cytoplasmic structure. Potassium Bichromate (Dichromate) . This salt has been used in all sorts of combinations; those which follow may be specially recom- mended. Muller's solution is employed for the fixation of the tissues of the central nervous system, and must be used when fixation is to be followed by any of the Weigert hematoxylin staining methods. Applied to the fixation of other organs, Miiller's fluid is apt to produce some maceration and better results are usually obtained with Miiller-formol or with Tellyesniczky's solution. For the special fixation of cytoplasmic granules, and also for after- staining with Mallory's connective tissue stains, Zenker's solution yields better results. Miiller's Solution: Potassium bichromate 2.5 grm. Sodium sulphate 1.0 grm. Water 100.0 c.c. Pieces of tissue are left in the fluid for one to six weeks; large pieces of the spinal cord or brain require four to six weeks. If left too long the tissues will become brittle. After fixation, the tissue is washed thoroughly in running water for twelve to twenty-four hours, and hardened in graded alcohol. A relatively large volume of the fixing fluid should be used, and it should be frequently changed. It should not be allowed to become turbid nor to deposit crystals : this is avoided by frequent changes and by keeping the jars in the dark, or at least in such a position that they are not exposed to a bright light. FIXATION 727 Tellyesniczky's Fluid: Potassium bichromate 3 grm. Water 100 c.-c. i Glacial acetic acid (added just before use) 5 c.c. Pieces of tissue are placed in a considerable volume of the fixing fluid and left for three to seven days. They are then washed in run- ning water for twelve . to twenty-four hours, and hardened in graded alcohol. (Where tissues are first washed in water, the graded alcohols must begin with the 50 per cent, strength.) This fluid yields excellent re- sults with muscular and glandular tissues, and is particularly serviceable where pieces of considerable size must be used, e.g., whole embryos. Orth's Fluid (Muller-formol) : Muller's fluid 95 to 90 c.c. Pure formalin 5 to 10 c.c. This is an excellent fixative for general use, for by it most tissues are well preserved. Small pieces of tissue are left in a considerable volume of the fluid for one to five days, washed thoroughly in running water for twelve to twenty-four hours, and hardened in graded alcohol. The washing should be so thorough as to remove all excess of the chromium compounds, otherwise difficulty will be experienced in obtain- ing satisfactorily stained preparations. Chrom-Acetic-Formalin Mixture. This makes an excellent fixing fluid for general embryologic work. Mix and keep in stock the following solution: 1 per cent, chromic acid 16 volumes Glacial acetic acid 1 volume At the time of using add to 2 volumes of the stock mixture, 1 volume of formalin. Wash in water from twenty minutes to two hours according to size. Zenker's Solution: Potassium bichromate 2.5 grm. Sodium sulphate 1.0 grm. Mercuric chlorid 5.0 grm. Distilled water 100.0 c.c. Just prior to use add 1 c.c. of glacial acetic acid to each 20 c.c. of the fluid. 728 HISTOLOGIC TECHNIC Small pieces of tissue, only, should be used. They remain in a con- siderable volume of the solution for three to twenty-four hours, after which they are thoroughly washed in running water for twelve to twenty-four hours, and hardened in graded alcohol. The corrosive sublimate deposits crystals in the tissue. These may be removed by the addition of a crystal of iodin (or tincture of iodin) to the stronger alco- hols until decolorization no longer occurs. If the mercury is not thus removed it will be difficult to obtain well stained specimens, and the presence of the crystals may cause much confusion. In obstinate eases LugoPs solution is recommended in place of tincture of iodin (alcoholic solution of iodin), to be added until the alcohol has a 'port wine' color. (See page 758.) This is probably the best fixing fluid for general routine histologic work. The presence of the acetic acid, however, dissolves the more deli- cate albuminous granules. To avoid this, formalin may be substituted in the same proportion, and added in the same way, for the acetic acid. This modified Zenker's fluid (KELLY'S FLUID) is particularly valuable for the preservation of tissues where it is desired to investigate the granular cytoplasmic contents, e.g., blood cells in embryos, etc. Flemming's Fluid (strong solution) : Chromic acid, 1 per cent, aqueous solution 15 parts Osmic acid, 2 per cent, aqueous solution 4 parts Glacial acetic acid 1 part The mixture should be newly made, from stock solutions of the ingredients, immediately before using. The stock mixture undergoes a deteriorating chemical transformation. Pieces of tissue should not be more than 2 to 3 mm. in thickness and should be left in the solution and kept in the dark for one to twenty- four hours, according to the results desired. For mere fixation a short immersion is sufficient ; for blackening fat and the myelin of medullated nerve fibers the longer period is necessary. After fixation the tissues are to be washed in running water for three to twenty-four hours, and hardened in graded alcohol. This fluid gives splendid results for the fixation of the finer cyto- logical elements of glandular epithelium; and for the demonstration of nuclear constitution, chromosomes, and mitotic figures, it is unex- celled. It serves also to demonstrate the presence of fat and myelin, which are blackened by the osmium tetroxid. When used for the dem- onstration of fat in sections, chloroform should be substituted for FIXATION 729 xylol in the embedding process. It does not penetrate the tissues very readily, and the surfaces of the piece are usually destroyed by over- fixation. Kleinenberg's Fluid: Saturated aqueous solution of picric acid 99 c.c. Sulphuric acid 1 c.c. Shake well, filter and dilute the filtrate with 200 c.c. of distilled water. Small pieces of tissue should be left in the fluid for about three hours and then transferred to 70 per cent, alcohol, which is changed two or three times during the first day. Hardening is continued in 80 and 95 per cent, alcohols, which are to be frequently changed. The picric acid will be slowly dissolved by the alcohol, but will not be en- tirely removed even after a considerable time; a trace does no harm. This fluid gives excellent results with small pieces of embryonic tissue, and possesses the additional advantage of removing the calcareous salts from partially calcified bone; it is not, however, a strong decalcify- ing reagent. When a fixative is employed which contains picric acid, the tissue must never be washed in water, but is transferred directly to 70 per cent, alcohol. Bouin's Fluid: Picric acid, saturated aqueous solution 75 c.c. Formalin 20 c.c. Glacial acetic acid 5 c.c. Small pieces of tissue are fixed from three to twelve hours, trans- ferred to 70 per cent, alcohol, and then passed through the higher grades. This is an excellent fluid for the fixation of embryonic ma- terial. For chromosome studies it is probably surpassed only by Flem- ming's fluid. Van Gehuchten's Fluid (Carney's fluid, No. I) : Absolute alcohol 60 c.c. Chloroform 30 c.c. Glacial acetic acid 10 c.c. Very small pieces of tissue should be used. They should be left in the fluid three to twelve hours, and the vessel tightly closed to prevent evaporation of the volatile fluid. The tissues are then transferred to 46 730 HISTOLOGIC TECHNIC several changes of absolute alcohol to remove the fixing fluid and com- plete the dehydration. This fluid is especially valuable for the fixation of lung tissue. It can be advantageously applied only to perfectly fresh, viz., living tissues. Gilson discovered that this fluid could be advantageously modified for cytologic purposes by saturation with mercury bichlorid. About 20 grams should be added to the above mixture. As thus modified the solution is commonly known as Carnoy's fluid, No. II. Its penetrating properties are very high and the degree of fixation delicate. Tissues should not be left in the fluid for more than an hour. The after- treatment is the same as with the simple solution, except that tincture of iodin must be used with the alcohols. Gilson 's Fluid (Mer euro-nitric mixture) : Bichlorid of mercury 5 grm. Nitric acid (approximately 80 per cent.) 4 c.c. Glacial acetic acid 1 c.c. Alcohol (70 per cent.) 25 c.c. Distilled water 250 c.c. Fix small pieces of tissue from three to twelve hours, transfer directly to 70 per cent, alcohol, and pass through the higher grades, with the addition of tincture of iodin. This fluid has a high degree of penetration, and produces faithful and delicate fixation. Heat. This is a useful agent for the fixation of blood, marrow cells, and scrapings from glandular and other organs, which are not to be afterward stained with methylene blue or its compounds. For this purpose smears made upon glass slides or cover glasses are quickly dried in the air, heated to 110 degrees C. for twenty to thirty minutes, and are then ready for immediate staining. The smears are made in the following manner: Slides or cover glasses should be thoroughly cleaned with a final rinsing in equal parts of absolute alcohol and ether. A small drop of blood or other fluid is collected by quickly touching the center of a cover glass to a drop of ordinary size. This cover glass is then immediately dropped upon the surface of a second one, and the two are drawn apart by a rapid sliding motion, the two surfaces being maintained parallel to one another during the motion. The success of the maneuver depends upon its rapidity, and to obtain very thin preparations some little dexterity is required. Fairly good smears are more easily made with slides. A drop of blood is collected upon the end of one glass slide whose edge must have been DECALCIFICATION 731 ground. The end of the slide with the drop of blood is then touched to the middle of a second slide, the drop spreads out between the two7 and the first slide is rapidly drawn over the surface of the second, while being held at an angle of about 45 degrees. A broad smear is thus left upon the surface of the second slide,, some portions of which are sufficiently thin, other portions too thick for use. Like the former maneuver the success of this depends upon rapidity, cleanliness, and the use of a sufficiently small drop of fluid. Fixation by Vapors. Smears of fluids or very thin pieces of tissue may be fixed by a very brief exposure to the vapor of osmium tetroxid, formalin, etc. This method is only useful in occasional instances. Os- mium tetroxid vapor has been shown to preserve faithfully the mito- chondria in young cells from tissue cultures (M. R. and W. H. Lewis, Amer. Jour. Anat., 17, 3, 1915). In all methods of fixation where pieces of tissue are immersed in the fixing and hardening fluids, it is desirable to prevent the distortion of the object from the pressure of contact with the glass container. This is accomplished by suspending the object by means of a thread, or by resting the tissue upon a thin layer of cotton placed in the bottom of the jar. DECALCIFICATION Tissues containing bone or other calcareous material require decalci- fication before they can be sectioned for examination. If the calcareous deposit is limited in amount, as in early fetal tissues, this can be ac- complished and the tissue fixed at the same time by the use of Kleinen- berg's fluid, a saturated aqueous solution of picric acid, or a 5 to 10 per cent, aqueous solution of sulphurous acid, the tissues being per- mitted, to remain in the decalcifying fluid until a needle or slender scal- pel can be readily pushed to the most central portions without producing great resistance or any grating sensation. For well developed and mature bones the above methods are in- sufficient, and stronger acids must be relied upon. Nitric acid is the one most generally used for this purpose. The tissue should have been previously fixed, Miiller-formol, Zenker's fluid, or mercuric chlorid being the preferable fixatives. The fixed and washed tissues are placed in 2 to 5 per cent, nitric acid, and the fluid changed daily until decalcifi- cation is complete. They are then thoroughly washed in running water for twelve to twenty-four hours and hardened in graded alcohol. 732 HISTOLOGIC TECHNIC INJECTION Injection is used either for the rapid dissemination of fixing fluids through whole organs, embryos, etc., or for the demonstration of blood or lymphatic vessels. For the former purpose mercuric chlorid is the most useful fixative, since it may be immediately followed by the in- jection of a hardening fluid, alcohol, by which the remaining mercury is dissolved out of the tissue before overfixation occurs. For the latter purpose a colored fluid, either aqueous or gelatinous, is forced into the blood or lymphatic vessels. Berlin blue, carmin, vermilion, and lamp- black are the coloring matters most frequently used. The last two merely require suspension in a gelatinous or an aqueous menstruum; the preparation of Berlin blue and carmin is somewhat more com- plicated. Berlin Blue Gelatin Mass: Saturated aqueous solution (1 to 20) of Berlin blue (Griibler's) 100 c.c. Pure French gelatin (in sheets) 5 to 10 grm. The gelatin should be quickly washed to remove dust, etc., and then placed for several hours in a very little distilled water until it becomes swollen and soft. The superfluous water is then poured off, and the gelatin melted over a water bath. The warmed solution of Berlin blue may now be added, a little at a time, and continuously stirred. Finally, the mixture is filtered through cotton flannel (or flannel) which has been previously wrung out of hot water. If the mass is not to be used at once, a few crystals of thymol may be added as a preservative, or, after cooling, a little methylic alcohol may be floated upon the surface of the solidified mass. It is better to use it at once. Carmin Gelatin Mass: Carmin (Griibler's) 3 grm. Ammonium hydrate, strong 6 c.c. Pure French gelatin 7 grm. Distilled water 80 c.c. The gelatin is prepared and melted as above, 50 c.c. of the water being used, and the evaporation replaced. The carmin is rubbed up in a mortar with the remaining 30 c.c. of the water, and the ammonia is added to render the carmin soluble. The mixture is now permitted to HARDENING 733 stand for two hours, after which it is neutralized by the gradual addi- tion of 4 to 6 c.c. of glacial acetic acid, the mixture being conBtantly stirred, and the latter portions of the acid diluted with four volumes of distilled water, and added drop by drop. The acid soon changes the color of the mixture from a purplish carmin to a bright crimson. Care should be taken not to add too much acid. When properly prepared, the sense of smell should detect both ammonia and acetic acid, and the fluid should have a dark crimson color (the addition of too much acid produces a brighter crimson). Should the mixture be slightly over- acidified a few drops of diluted ammonia will restore the proper con- dition. The carmin solution is now added to the gelatin mass, a little at a time and with constant stirring, and the whole is filtered through cotton flannel wrung out of hot water. The gelatin mass may be kept for a short time by being covered with methylic alcohol, but is better used at once. The pressure required for injection may be obtained by the gentle use of a hand syringe; by the displacement of the confined air in a large bottle or carboy by tap water; or much better by the use of a water blast, of which the small glass type is relatively inexpensive and will furnish a pressure for injection about equal to 180 mm. of mercury. The air outflow of the water blast is connected by rubber tubing with a glass canula of proper size to fit the vessel injected, a Wolff bottle containing the warm injection mass being interposed. I,f a manometer is connected, by means of a T -canula, on the proximal side of the Wolff bottle, a relatively even and accurately measured pressure is assured. The amount of pressure should be at first low (20 to 40 mm. of mercury), and should be gradually increased up to, but not much beyond, the normal blood pressure in the vessel injected. The injected organ is cooled rapidly in a refrigerator, or by being packed in ice or immersed in ice water. After solidification small pieces are immediately placed in 95 per cent, alcohol for fixation, dehydration, and hardening. HARDENING After proper fixation nearly all tissues require to be further hardened before satisfactory sections can be cut. This is accomplished by im- mersion in alcohol until dehydration is complete. The process requires from a day to a week, according to the size of the tissue and the volume and strength of the fluid. Various strengths of alcohol are advised. For 734 ' HISTOLOGIC TECHNIC general use the procedure recommended by Gage ("The Microscope") is found to be very satisfactory. The tissues after fixation are suc- cessively placed for one or two days in each of the following strengths of alcohol 70, 80 and 95 per cent. They are then returned to 80 per cent, alcohol, where they may remain indefinitely, but it is generally safer to embed for sectioning without great delay; this is especially true of tissue which has been fixed with Zenker's solution. Most fixing fluids also harden; the terms 'fixing* and 'hardening' are consequently often used synonymously. EMBEDDING Thick sections may be obtained from the firmer tissues by freehand sectioning with a razor, but for the satisfactory preparation of thin sec- tions a microtome is a necessity and the tissues must have been pre- viously embedded to render them sufficiently firm. This is accomplished by infiltrating the tissue with celloidin or paraffin, either of which yields a firm, waxy consistence. Embedding in Celloidin. Make a saturated solution of a little cel- loidin (Schering's) in a mixture of equal parts of alcohol and ether. The alcohol should contain no trace of copper sulphate. This solution is for convenience known as number III and should have a very thick, syrupy consistence. A small portion of number III is mixed with three to five times its volume of the alcohol and ether mixture, to obtain number II, which should have a somewhat viscid consistence. A second small portion of number III is diluted with ten to fifteen times its volume of the alcohol and ether, to produce celloidin number I, which should have a thin, watery consistence. Small pieces of tissue which have been thoroughly hardened in 95 per cent, alcohol are treated as follows : 1. Dehydrate in absolute alcohol, six to twenty-four hours. 2. Place in the absolute alcohol and ether mixture, twelve to twenty- four hours. 3. Place in celloidin number I, twelve to twenty-four hours. 4. Place in celloidin number II, twelve to twenty-four hours. 5. Place in celloidin number III, twenty-four to forty-eight hours, or longer. Pieces of tissue of considerable size may be satisfactorily embedded EMBEDDING 735 in celloidin, but should be passed through the successive solutions in a much more leisurely manner. Thus an eye requires two -4o- three weeks, a large piece of the central nervous system three to four weeks for proper embedding. The tissue should now be fastened to a wooden block and the celloidin hardened. Ordinary wood yields its resins to the alcohol in which the blocks are to be kept ; the white pine blocks which are commercially known as 'deck plugs' contain very little resin and are admirably adapted for the purpose. Vulcanized fiber blocks are still better, and glass blocks are also serviceable. The piece of tissue should be so oriented upon the block that the future sections may be cut nearly parallel to the block surface. Thick celloidin is then poured over the tissue; a few moments' exposure to the air cements it firmly to the block. After the celloidin has become firm by partial drying in air, the block may be stored indefinitely in 70 per cent, alcohol (stronger alcohol is apt to soften the celloidin)-, where the hardening is completed. If it is desired to harden the celloidin rapidly for early cutting, the block may be floated in a jar of chloroform, tissue down, for two or three hours. The final step in the embedding process may be accomplished ad- vantageously by allowing the tissue to remain in the slowly hardening thick celloidin in a shallow dish. Care must be taken to begin this step with sufficient celloidin to keep the tissue covered until the harden- ing is completed. The tissue may then be cut from the firm celloidin and blocked. Embedding in Paraffin. If sections thinner than 15 ft are de- sired, paraffin embedding must be used; it is impossible to cut celloidin sections with any great degree of certainty thinner than 10 /; to 15 //,. Paraffin sections are readily cut at 5, and may be cut as thin as 2 //,. The paraffin method is also to be selected for the rapid preparation of tissues for sectioning, but it is only applicable to small pieces of tissue. For large pieces better results will be obtained with celloidin. Many methods for the employment of paraffin have been extolled ; the following can be recommended. The tissue, after fixation, should have been hardened in alcohol. 1. Dehydrate in absolute alcohol twenty-four to forty-eight hours; v^ery small pieces of tissue (1 to 2 mm.) may be completely dehydrated in three to six hours. 2. Place in equal parts of absolute alcohol and xylol (xylene), one to three hours. 3. Place in pure xylol until clear and translucent, one-half to two 736 HISTOLOGIC TECHNIG hours. (For relatively large pieces of tissue cedar-wood oil or pure anilin oil may be substituted for the xylol). 4. Place in melted paraffin containing a little xylol (or cedar oil) ; that which has been previously used for embedding does very well. 5. Transfer to pure melted paraffin. 6. Transfer to a second dish of pure melted paraffin (with melting point of about 54). The object of these changes is to replace the xylol (or oil) with pure paraffin. If the xylol is not completely re- moved the tissue will contain bubbles and satisfactory sections cannot be made. 7. Embed in a paper box or a watch glass. If glass is used the surface should be smeared with the least trace of glycerin to prevent adhesion. The box should be filled with pure melted paraffin, the tissue handled with warmed forceps, and placed with proper orientation so that it is completely covered with the melted paraffin. The paraffin is now rapidly cooled by immersion in cold water ; in summer months ice water must be used. If a paper box is used it can be left to float on the water until the paraffin is thoroughly congealed. The manner of preparing these boxes is shown in Fig. 594. Considerable depends upon the choice of a proper grade of paraffin. That which melts at 58 to 65 is most desirable for use in temperate climates during the warmer months; during the winter months paraffin of 54 to 56 is preferable. If too hard, the paraffin cracks; if too soft, it fails to retain its form during sectioning. The former condition may be improved, if necessary, by the proximity of a small flame during the sectioning process, or by breathing upon the knife blade and tissue block ; the latter fault may be remedied by placing the tissue for a short time in the refrigerator, just prior to cutting. SECTIONING The cutting of free-hand sections is so simple an operation as to scarcely require description. A small, inexpensive hand microtome and a sharp razor whose surfaces are ground flat, not concave, are all that is necessary. For more precise sectioning a stationary microtome is a necessity. Many types of these instruments are on the market. The Thoma type of instrument is specially adapted for celloidin work, but may also be used for paraffin sections. The Schanze instrument is very useful for SECTIONING 737 celloidin sections and may also be used for small paraffin sections. The Minot and the new Spencer rotary microtomes are specially adapted for the production of serial sections in paraffin. These instruments of them- selves suggest the manner in which they are to be used, and the technic is easily acquired. Like all delicate instruments, they must be kept well cleaned and properly oiled, to do good service. Much depends upon the choice and care of the knife. The micro- c I d A b' c' d' D II /' e' f e/ FIG. 594. A METHOD OF PREPARING A PAPER Box FOR PARAFFIN EMBEDDING. 7. A slip of paper, A, B, D, C, is folded, both ways, on the lines a-a', b-b f , c-c', and d-d'. Then being folded into the form shown in //, it is laid flat, the section o, a', b f , 6, shown in 7, being uppermost, and the paper is creased on the lines e-e' and /-/'. It is then opened, folded in the shape of a box, /, e, /', e', forming the bot- tom, and is secured by folding down the ends after creasing the paper on the lines e-f and f-e'. tome knives of Jung are of excellent quality and should be kept in good condition by the frequent use of the hone and strop. The h one should be of fine Belgian stone. It should be well moist- ened with water, the addition of a little fine soap being a distinct ad- vantage. The edge of the knife, carefully applied to the hone, should first be drawn obliquely from heel to toe and toward the operator, being held at a constant angle and drawn the whole length of the stone. The knife is then turned over and the motion is reversed, the knife being held obliquely at an angle equal to the previous one, the edge directed 738 HISTOLOGIG TECHNIC away from the operator, and the knife pushed from heel to toe, the whole length of the stone. The motion being repeated, a sharp edge is gradually acquired, which can be finished by the use of the strop. In the use of the strop the motions are the reverse of those with the hone, the back of the knife in this case preceding its edge as it is drawn along the leather, and the draw should be from the toe to heel of the knife. The angle, however, between the knife and the hone and the knife and the strop should always be a constant one, and should be such that the microscopical 'teeth' which are thus formed on the edge of the knife should be directed obliquely toward its heel. In sectioning, the knife should be so placed in the microtome that its edge crosses the paraffin-embedded object at right angles, and for ribbon sectioning the paraffin block should be so trimmed that it forms a perfect rectangle. In sectioning celloidin-embedded objects, the knife should cross the object at as acute an angle as possible. With paraffin, also, the stroke should be sharp and quick; with celloidin, somewhat slower and rhythmic. The knife should remain dry when used with paraffin; with celloidin both the knife and the object should be at all times well moistened with 70 per cent, alcohol. STAINING The sections, having been cut, are at once ready for staining, pro- vided they were embedded in celloidin. If paraffin was used for em- bedding, the sections have first to be fastened to the slide. This is accomplished in the following manner. The paraffin sections are properly arranged upon the surface of a clean slide, a few drops of water from a pipette are allowed to flow between the slide and the sections, so that the -latter float upon the surface of the water; and the slide is gently heated over a small flame. Thus the paraffin sections are straightened; care should be used not to melt them. The excess of water is now carefully drained off and the slide placed in an oven and heated to about 40 C. for several hours, until thoroughly dried. Most tissues will now adhere firmly to the slide. If, however, the tissue was fixed with solutions containing bi- chromate of potassium the sections are liable to come off the slide, a misfortune which may be avoided by the use of a celloidin adhesive, with or without the previous use of Mayer's albumin. As a precautionary STAINING 739 measure against the loss of sections, the routine use of albumin ad- hesive is recommended. . Mayer 's Albumin : White of egg, chopped with scalpel or scissors and filtered 10 c.c. Glycerin 10 c.c. Thymol a small crystal A drop of this fluid is rubbed evenly upon the surface of a clean slide. Distilled water is added with a pipette and the paraffin sections arranged upon the slide by the method detailed above. In the process of drying the albumin is coagulated and the tissue becomes firmly at- tached to the slide. The drying may be accomplished without special heating, but requires at least twelve hours at ordinary room tempera- tures. Celloidin Adhesive. The sections having been fastened to the slide and dried, the paraffin is removed by dipping the slide into one or two changes of pure xylol, the xylol removed by washing with absolute al- cohol, and a few drops of very thin solution of celloidin (made by diluting thin celloidin, number I (see page 734) with eight or ten vol- umes of the alcohol and ether solvent) are poured over the sections. The solution of celloidin should be so thin as to scarcely leave an appreci- able film on the slide. The excess of celloidin is drained off and the film hardened by first flooding the slide with 70 per cent, alcohol, and after a few minutes transferring it to water. The sections will not now be removed from the slide except by mechanical violence. In handling serial sections of embryos, time may be saved, and the same end ac- complished, by simply spreading by means of a soft brush a thin film of the very thin celloidin solution over the dried paraffin sections. The sections may then be manipulated for staining in the usual way. Celloidin sections do not need to be fastened to the slide before being stained; they are sufficiently firm to be gently handled with a needle. Staining 1 in Bulk. It is occasionally desirable to stain tissue in bulk so that sections once cut can be immediately mounted. This is best accomplished by the use of a single stain applied to small blocks of tissue immediately after fixation and hardening, and is usually done in aqueous media. Borax carmin is the most useful dye for the purpose, and is used as described below, except that it will require two or three 740 HISTOLOGIC TECHNIG days to penetrate the tissue. Delafield's hematoxylin also gives brilliant results in bulk staining. Regressive Staining". Staining in bulk is necessarily a regressive process, viz., the tissue is first overstained and then partially decolorized. With borax carmin the decolorization is accomplished by acid alcohol (hydrochloric acid 1 c.e., 70 per cent, alcohol 100 c.c.). Since the stain is removed more rapidly from the cytoplasm than from the nucleus, a differentiation is thus produced. Progressive Staining. In progressive staining the dye, having been once taken up by the tissue, is not removed, the differentiation of nucleus and cytoplasm being accomplished by the selective affinity of the dye. Thus, certain dyes are nuclear (basic), others are cytoplasmic (acid). The former possess a special affinity for the nucleus, the latter stain both nucleus and cytoplasm. Certain dyes may be used either progressively or regressively ; in the former case care must be exercised that the section be not over- stained; in the latter case overstaining is impossible, but decolorization must be watched with care. Classification of Dyes. Dyes may be classified according to their affinity for certain granules or other portions of the cytoplasmic struc- ture. A classification of this kind was advanced by Ehrlich through his pupil, G. Schwartze (Inaug. Dissert., 1880), and has been greatly elaborated by Pappenheim (Grundriss der Farbechemie, 1901). Such a classification is very incomplete and unsatisfactory, but in a very general way serves a useful purpose. The following is sufficient for our present needs : 1. Basic dyes, those which color the chromatin of the nucleus (largely nucleic acid) and the so-called basophil granules. Hematein, methylene blue, methyl green, safranin, and basic fuchsin are examples. 2. Acid dyes, those which are usually cytoplasmic dyes, and have an affinity for the acidophil (i.e., basic) granules. Such are eosin, Congo red, orange G, methyl blue, and acid fuchsin. 3. Neutral dyes, which result from a due admixture of acid and basic colors, and which gives a specific tint to the so-called neutrophil or azure granules. Such dyes are Ehrlich's triacid mixture, eosinated methylene blue, etc. 4. Specific dyes, which result from the due admixture of dyes with certain reagents, other dyes or chemicals, and which have a selective affinity for particular tissues. This is an indefinite class which includes Weigert's elastic tissue stain, Mallory's connective tissue stain, Sudan III STAINING 741 for fat, the intra vitam staining of nerve tissues with methylene blue, etc. Mordants. The successful application of a dye requires tTiat~the tissue shall have a chemical affinity for the stain. This affinity may be either natural or artificial, e.g., eosin will color nearly all tissues under any ordinary conditions without the aid of any other reagent ; the colora- tion is, however, largely a physical phenomenon, due to inhibition of the dye; hematoxylin, on the other hand, stains ordinary tissue but slightly, but its action is much enhanced by first acting upon the tissue with alum or a similar reagent. The alum, in this case, serves as a mordant. A mordant should have a strong affinity for both the stain and the tissue. Hence it is that, after a tissue has been once stained by the aid of a mordant, it may be decolorized, partially or completely, by the second application of the same or another mordant of equal strength. SINGLE STAINS WITH NUCLEAR DYES Hematein. Hematein is the active principle of the dye, hema- toxylin, obtained by extracting logwood. Hematein is derived from the solutions of hematoxylin by oxidation, either by chemical reagents or by prolonged exposure to the air. As a dye it must be combined with a mordant, which, most frequently, is some form of alum. The following formulas are recommended: Alum Hematein (Mayer) : Alum hematein (Griibler's) 0.2 grin. 95 per cent, alcohol 5 c.c. Saturated aqueous solution of ammonium alum 100 c.c. About 10 grm. of alum are required for this solution. The hematein should be first dissolved in the alcohol, with gentle heat if necessary, and afterward added to the warm solution of alum. The fluid is ready for use in two or three days, but will increase in strength for several weeks ; it then requires dilution. Bohmer's Hematoxylin: Hematoxylin 0.5 grm. Absolute alcohol 5 c.c. Saturated aqueous solution of potassium alum 100 c.c. 742 HISTOLOGIC TECHNIC The ingredients should be mixed as above and allowed to stand for eight to ten days in an open bottle and exposed to sunlight. Filter. Eipening will continue for some weeks, and the dye will require dilution from time to time with a saturated solution of potassium alum. Delafield's Hematoxylin : Hematoxylin 4 grm. 95 per cent, alcohol 25 c.c. Saturated aqueous solution of ammonium alum 400 c.c. Mix as above and permit the fluid to stand in an open vessel ex- posed to the air and sunlight for three or four days. Add: Glycerin 100 c.c. Methylic alcohol 100 c.c. Expose to light in a cotton-plugged bottle. After two days filter. After several days filter again, and keep in tightly stoppered bottle. The fluid will ripen for several weeks and may require dilution with a saturated aqueous solution of alum, containing glycerin and methylic alcohol in the above proportions. This is a very deep nuclear stain, in fact it is so deep that, while the nuclei are sharply differentiated, the intranuclear structure is nearly obliterated. The stain is somewhat improved in this particular by the slight regressive action of very weak acids e.g., hydrochloric, picric, or dilute acetic acid. This is a very useful dye for the toto staining of small embryos. Mann's Acid Hematein (Ehrlich's Hematoxylin) : Hematein (Griibler's) or hematoxylin crystals 2 grm. Absolute alcohol 100 c.c. Glycerin 100 c.c. Distilled water 100 c.c. Potassium alum 10 grm. Glacial acetic acid 10 c.c. The hematein (or hematoxylin) is dissolved in the acetic acid, with 25 c.c. of the alcohol; the glycerin and the remainder of the .alcohol are then added. The alum is dissolved in water by the aid of heat, and the warm solution is poured slowly while stirring into the solution of hematein. The fluid keeps indefinitely and is an excellent hematein stain for general use. Hematoxylin solutions may be ripened immedi- ately by the addition of small amounts of hydrogen peroxide (about 1 c.c. to 200 c.c. of stain, neutralized by a few grains of sodium chlorid). STAINING 743 Application of the Hematein Stains. All of the above solutions are used in a similar manner. Sections, either free or attached to the slide, are taken from water and immersed in the dye for three to five minutes; they are then thoroughly washed in water. The stained sec- tions are at first of a reddish-purple color, but soon become a deep blue from the slight alkalinity of the tap water used for washing. If necessary this alkalinity may be increased by the addition of one or two drops of ammonia to 500 c.c. of the water used for washing. Hematoxylin solutions require air and sunlight for ripening. After ripening is complete the solution should be kept tightly stoppered. The solutions should be filtered before using. Ehrlich's hematoxylin is prob- ably the best nuclear dye for general routine work. Better differentia- tion is usually obtained if the sections are over-stained (5 to 10 minutes) and then, after thorough washing, partially decolorized in a 1 per cent, solution of hydrochloric acid in 70 per cent, alcohol (a few seconds is usually sufficient), washed, and the color restored in an ammoniacal solution (1 per cent, aqueous solution of ammonium hydroxid). The sections must again be thoroughly washed, and the best differentiation is secured if the sections are now placed in very .dilute watery eosin for ten to twelve hours. Methylene Blue. This dye is a derivative of thionin, and may be similarly used. For staining fresh tissues the dye is used in 2 per cent, aqueous solution. Its preparations are not very permanent. The chief uses of this dye are in combination with eosin as a stain for blood; as a stain for nerve cells according to the method of Nissl; and as applied to living organs as a specific stain for nerve tissues after the method of Ehrlich. These methods will be described below. Methyl Green. This dye is preferable to methylene blue as a stain for fresh tissues. It is used in 2 per cent, aqueous solution and applied as a progressive stain. It also enters into the composition of Ehrlich's triacid mixture. It is strongly basic. Carmin. This valuable dye is derived from the cochineal bug, and is used either as a progressive or a regressive stain. For the former, picro-carmin or alum carmin are recommended; for the latter, borax carmin is preferable. Borax Carmin: Borax 4 grm. Distilled water (boiling) 100 c.c. ; cool, filter, and add Carmin , 3 grm. ; when dissolved, add 70 per cent, alcohol 100 c.c. 744 HISTOLOGIC TECHNIC Mix the ingredients in the above order, and after twenty-four hours filter. It may be necessary to use a drop or two of ammonia to complete the solution of the carmin. This is again removed by evaporation. Tissues are to be overstained in the carmin solution, and differen- tiated in acid alcohol (70 per cent, alcohol containing 0.5 to 1 per cent, of hydrochloric acid) until the red color is no longer removed in clouds. The sections are then well washed with several changes of 95 per cent, alcohol, cleared and mounted. This is a most excellent dye for staining embryos in toto. Alum Carmin: Potassium alum 5 grm. Distilled water (hot) 100 c.c. Carmin 1 grm. Mix in the order given, boil for twenty minutes, and when cold filter. Picro-Carmin: Ammonium hydrate 5 c.c. Distilled water 50 c.c. Carmin 1 grm. ; when dissolved, add Saturated aqueous solution of picric acid 50 c.c. Expose to light and air for two days; filter. Picro-carmin is used as a progressive stain. Since the picric acid is soluble in alcohol, dehydration should be rapid, or a crystal of picric acid should be added to the alchohol used for dehydration. Aceto-Carmin. This is an invaluable fluid for certain purposes, especially where it is desired to study nuclear structure, chromosomes and spindles in fresh and uncut (teased) tissue. The tissue is simul- taneously fixed and stained in this mixture, and remains preserved in the fiuid in a condition favorable for study for a considerable time. The stained tissue, e.g., insect testes, etc., is teased out on a slide in the fluid, covered with cover glass, and may then be studied under an oil immer- sion lens. The mitotic figures are almost as clear as in sectioned ma- terial. The solution is made by dissolving to saturation carmin in boil- ing acetic acid of 45 per cent, strength, and filtering. Safranin. This dye is a coal-tar derivative; it is an excellent nuclear stain. Like carmin, safranin yields a deep red color. Safranin (Griibler's) 1 grm. Distilled water . 100 c.c. STAINING 745 1. Tissues taken from water are stained five minutes. 2. Wash in water. 3. Dehydrate rapidly in absolute alcohol. The alcohol removes some of the safranin, giving a regressive effect. SINGLE STAINS WITH CYTOPLASMIC DYES Eosin. This dye is a coal-tar derivative. There are no less than seventeen varieties of the dye on the market, of which five are in general use. These are: (1) yellowish alcoholic; (2) bluish alcoholic; (3) yellowish watery; (4) bluish watery; (5) pure French eosin. The most reliable of these dyes are manufactured by Grubler. The first and fifth varieties are to be recommended as blood stains, the first and fourth are the best for general use. Two distinct methods are based upon this choice of dyes. Method I: Yellowish alcoholic eosin 1 grm. 70 per cent, alcohol 100 c.c. This stock solution is usually diluted with four to ten volumes of 70 to 95 per cent, alcohol just before using. The stain is preferably preceded by the use of a nuclear dye, after which the sections should be dehydrated in 95 per cent, alcohol. 1. Stain in the diluted eosin, one to five minutes, or until the sec- tions become a bright red color. 2. Wash quickly in absolute alcohol, clear, and mount. The color is dissolved out during this process, producing some differentiation by regression. Method II: Bluish watery eosin 1 grm. Distilled water 100 c.c. The stock solution should be diluted with one to four volumes of distilled water before using. 1. Tissues are taken from water and placed in the dilute eosin, one to five minutes. 2. Wash quickly in water to remove the excess of the dye. 3. Pass radially through graded alcohol, clear, and mount. Slight differentiation may be obtained by prolonging the washing in water, otherwise the stain is progressive. Much greater differenti- 47 HISTOLOGIC TECHNIC ation is possible with either eosin method by making the stain very dilute and staining for twenty-four to forty-eight hours. Congo Red: Congo red 1 grm. Distilled water 100 c.c. A few drops of dilute acetic acid should usually be added to the above; the bright red color is then exchanged for a dull bluish red, and in this neutralized condition the stain usually gives the highest differen- tiation. The dye should be used in the same manner as watery eosin (see Method II, above). Congo red gives especially good results when applied to fetal and young tissues. Orange G. There are many varieties of orange. The orange G and the aurantia of Griibler will be found satisfactory. As a cytoplasmic stain the former is preferable. It should be used in the same manner as alcoholic eosin (see Method I, above). Fuchsin. Two distinct dyes, the one of acid, the other of basic, properties, pass under this name. Acid fuchsin is a cytoplasmic dye, but when used in acid solution has a slight selective affinity for the nuclei. Basic fuchsin is chiefly useful in bacteriology. It is also used in preparing Weigert's elastic tissue stain. It is recommended that euparal be used as a mounting medium with this group of stains. The tissue may be mounted directly from 90 per cent, alcohol. DOUBLE STAINING Hematein and Eosin : 1. Stain with one of the hem- atein solutions, preferably Mann's or Ehrlich's for general use, five minutes. 2. Wash well in water. 3. Stain in watery eosin, one to ten minutes. Or 3. Dehydrate in 95 per cent. alcohol. 4. Wash quickly in water. 4. Stain in alcoholic eosin, one to five minutes. 5. Dehydrate in absolute al- 5. Dehydrate quickly (one to cohol. five minutes) in absolute alcohol. 6. Clear and mount. 6. Clear and mount. STAINING 747" Methyl Blue and Safranin. Methyl blue is a very different dye from methylene blue but is practically identical with water blue~(Wasser blau). It is an acid or cytoplasmic stain. 1. Stain with a 2 per cent, aqueous solution of methyl blue, three minutes. 2. Einse in water. 3. Stain in a 1 per cent, aqueous solution of safranin, five minutes. 4. Wash in water, 5. Differentiate and dehydrate quickly in absolute alcohol, till the sections become again blue. 6. Clear and mount. This method gives a permanent stain which yields excellent results with certain tissues e.g., the skin. Other nuclear and cytoplasmic dyes may be combined in a similar manner to the above methods. SPECIAL STAINING METHODS Iran Hematoxylin (Heidenhain) : I. Mordant: Ferric alum (iron ammonium sulphate violet crystals) 2 grm. Distilled water 100 c.c. II. Stain: Hematoxylin 1 grm. 95 per cent, alcohol 10 c.c. Distilled water 100 c.c. 1. Mordant the sections four to twelve hours in I. 2. Einse in water. 3. Stain twelve to twenty hours in II. 4. Wash well in water. The sections should become very black; a drop or two of ammonia to one-half liter of water often improves the color. 5. Decolorize in the mordant, watching each section under the microscope, and stopping the decolorization at the proper time by 6. Wash thoroughly in slowly running water, or in several changes of still water. 7. Counter-stain if desired, dehydrate, clear, and mount. 748 HISTOLOGIC TECHNIC This method gives an excellent stain for the finer nuclear structure, mitosis, etc. Muchematein (Mayer) : Hematein (Griibler's) 0.2 grm. Glycerin 40 c.c. Aluminium chlorid 0.1 grm. Distilled water 60 c.c. Mix the ingredients in the order given, rubbing the hematein with the glycerin in a mortar. One or two drops of nitric acid added to the final mixture will sharpen its properties as a nuclear stain. This dye is used as a specific stain for mucinous tissues. It is used in the same manner as hematein, and stains rapidly (three to ten minutes) . Mucicarmin (Mayer) : Carmin 1 grm. Aluminium chlorid 0.5 grm. Distilled water 2 c.c. 50 per cent, alcohol 100 c.c. Mix in the order given ; heat over a small flame till the fluid darkens (two minutes) ; after twenty-four hours, filter. For use, dilute with five to ten volumes of 50 per cent, alcohol. Like muchematein, mucicar- min is a specific stain for mucus-containing cells. It also stains rapidly. Weigert-Pal Stain for Medullated Nerve Fibers. The tissues must have been previously fixed in Mutter's -fluid or in 10 per cent, formalin, washed in water, hardened in alcohol, and sectioned. I. Stain: Hematoxylin 1 grm. Absolute alcohol 10 c.c. Distilled water 90 c.c. Saturated aqueous solution of lithium carbonate (1 to 80) 1 c.c. II. Differentiating Solution: Potassium permanganate 0.25 grm. Distilled water 100 c.c. STAINING 749 III. Decolorizing Solution: 1 per cent, aqueous solution of oxalic acid 50 c.c. 1 per cent, aqueous solution of potassium sulphate 50 c.c. This last solution should be freshly prepared by mixing the two stock solutions just prior to use. 1. Stain sections (six to twenty-four hours) until black. 2. Wash well in water. A few drops of lithium carbonate solution added to the water may improve the color, which should become a deep blue-black. 3. Differentiate until the gray matter becomes brown (one-quarter to two minutes). 4. Einse in water. 5. Decolorize until the white matter becomes a steel blue, the gray matter a light brown (one-quarter to one minute), watching each section with ca/e. 6. Wash thoroughly in several changes of water, or in running water. 7. If desired, counter-stain with alum carmin, and wash in water. 8. Dehydrate, clear, and mount. Methylene Blue, for non-medullated nerve fibers (Intravitam Method) : I. Stain: Methylene blue (Griibler's 'rectif. nach Ehrlich') 0.1 grin. Distilled water 100 c.c. Dissolve with heat, cool, and filter. II. Fixing Solution (Bethe's) : Ammonium molybdate 1 grm. Distilled water 20 c.c. Hydrochloric acid, C. P 1 drop. The solution should be freshly made and kept at or near C. 1. The method is only applicable to living tissues, by injecting the blood-vessels with the stain, or by partially immersing in the staining fluid small pieces of tissue, freshly removed from the living animal. 2. After ten to thirty minutes, rinse in normal saline solution, and place in the cold fixing solution for two to six hours, according to the size of the pieces. The tissue should be kept cold. 3. Wash well in distilled water. 750 HISTOLOGIC TECHNIG 4. Dehydrate quickly in 95 per cent, and absolute alcohol, kept at or near C. 5. Embed in paraffin. At a convenient time, cut and mount. The stain is rather unstable, but may be kept fairly well if mounted in glycerin or in neutral balsam. This technic is invaluable for the demonstration of the terminal nerve supply of tissues. Methylene Blue, for chromophilic (tigroid) granules in cyton and dendron (Nissl's Method) : I. Stain : Methylene blue (Grubler's 100 c.c. II. Fixative: Phosphomolybdic acid 1 grm. Distilled water 100 c.c. III. Counter-Stain: Anilin blue (soluble in water) 0.5 grm. Orange G- (Griibler's) 2 grm. Oxalic acid 2 grm. Distilled water 100 c.c. 1. Stain in the fuchsin solution, three to twenty minutes. The sections should become a bright red. 2. Wash in water. 3. Fix in the phosphomolybdic acid solution, one minute. This 'fixes' the fuchsin and prevents decolorization. 4. Wash well in water. 5. Counter-stain in the anilin blue solution (No. Ill), five to twenty minutes. The section should become decidedly blue. 6. Wash in water. 7. Dehydrate, clear, and mount. Eosinate of Methylene Blue (Basting's Method for Blood). For the somewhat complicated method of preparing the stain the reader is re- ferred to the original article, Johns Hop. Hosp. Bull., 1904, vol. xv, p. 122. The stain is applicable to smears of blood, marrow, splenic cells, etc. When used with smears which contain traces of fat, a preparatory treatment with a 2 per cent, aqueous solution of sodium metaphosphate, which probably serves as a mordant, improves the staining properties. Otherwise the stain is applied without previous fixation. 1. Stain for one minute. 2. Dilute the stain with several volumes of distilled water, and STAINING 755 continue the stain for five minutes, or until satisfactorily differentiated. 3. Wash with distilled water. 4. Dry and mount. The Giemsa and the Wright blood stains (other modifications of the original Eomanowsky stain) may be used for the same purposes, and are applied in a similar manner. Eosin and Methyl Blue Mixture (Mann) : 1 per cent, aqueous solution of methyl blue 35 c.c. 1 per cent, aqueous solution of bluish watery eosin 45 c.c. Distilled water 100 c.c. 1. Mordant the sections in water, leaving them till all alcohol has been replaced (five to thirty minutes). 2. Stain in the above mixture, five to ten minutes. 3. Wash well and differentiate in water; the effect may be varied by the duration of the washing (ten to forty minutes). 4. Dehydrate in alcohol, clear, and mount. When a sharper nuclear dye is desired the stain may be used as a counter-stain after hematein. In this case the methyl blue solution should be allowed to act only three to five minutes. The methyl blue used in this method is a cytoplasmic dye and should not be confounded with methylene blue. This is a valuable stain for differentiating cyto- plasmic granules. Triacid Stain (Ehrlich) : Saturated aqueous solution of orange G 13 c.c. Saturated aqueous solution of acid f uchsin .... 7 c.c. Distilled water 15 c.c. 95 per cent, alcohol 15 c.c. Saturated aqueous solution of methyl green .... 12.5 c.c. 95 per cent, alcohol 10 c.c. Glycerin 10 c.c. Be certain that the solutions of the dyes are saturated, and mix in the order given. The following formula by Mayer may be substituted : Distilled water 45 c.c. Glycerin 10 c.c. 95 per cent, alcohol 25 c.c. Acid fuchsin 3 grm. 756 HISTOLOGIG TECHNIG Orange G 2 grm. Methyl green 1 grm. Mix the fluids and dissolve the dyes in the order given. When used for staining sections., either of these formulas should be diluted with five to ten volumes of the following mixture : Glycerin 10 c.c. Distilled water 15 c.c. 95 per cent, alcohol 25 c.c. 1. Stain five to ten minutes in the diluted solution. (Use full strength for blood smears.) 2. Einse in water. 3. Dehydrate in absolute alcohol. (Smears are dried in the air.) 4. Clear and mount. The best results are obtained with tissues fixed in a sublimate mix- ture. It is a delicate cytologic stain, and is serviceable in differentiating between chromatin (chromosomes) and plastin (plasmosomes and linin) ; also in differentiating cytoplasmic granules. Sections should not be over 3 p in thickness. The stain is more or less capricious. Auerbach's Fuchsin-Methyl-Green Stain. This stain does well only after a sublimate fixative. It is serviceable in cytologic work for differentiating between 'active' and 'resting 5 chromatin. When suc- cessfully manipulated it stains the chromosomes green and the plasmo- somes and linin red. Keep in separate bottles the following stock solutions: I. Acid fuchsin 1 part, distilled water 1,000 parts. II. Methyl green 1 part, distilled water 1,000 parts. Mix when ready to use in the following proportions : 2 parts of I, acidulating every 50 c.c. with 1 drop of a 10 per cent, solution of glacial acetic acid. 3 or 4 parts of II. Fix tissues (e.g., insect testes, etc.) in the sublimate acetic mixture, transfer direct to 80 per cent, alcohol (removing the mercury with tinc- ture of iodin), dehydrate, and embed in paraffin. Sections should not be cut over 3 p thick. Stain for fifteen minutes, and pass directly to 95 per cent, alcohol. When the green stain no longer leaves the sections in clouds, pass STAINING 757 rapidly through absolute alcohol and xylol, and mount in balsam. The sections may also be mounted directly from 95 per cent, alcohol in euparal. Mitochondria] Technics. Mitochondria (chondriosomes, plasto- somes) are readily destroyed by acids, hence these must be eliminated from fixing solutions, or greatly reduced. I. MEVES' METHOD. This is the simplest method for demonstrating mitochondria, and in many instances it is quite satisfactory. Very small pieces of tissue are fixed for eight days in Flemming's strong solution in which the acetic acid has been reduced to 3 or 4 drops in the simplest formula (see page 728). The tissue is then embedded in paraffin in the usual way, and the sections cut at 3 to 6 ft are stained by the iron-hematoxylin method. The nuclei and cytoplasm are a uni- form pale gray; the mitochondria appear as deep black granules, rods or filaments. II. BENDA'S METHOD. This is a more complicated technic, but gives excellent results, and is claimed to be specific. 1. Fixation in strong Flemming's fluid (acetic acid reduced to 3 drops) for eight days. 2. Wash in water one hour. 3. Place in a mixture of equal parts of pyroligneous acid and 1 per cent, chromic acid for twenty-four hours. 4. Transfer to a 2 per cent, solution of potassium bichromate for twenty-four hours. 5. Wash, dehydrate and embed in paraffin and fix on slide in the usual way. 6. Mordant sections on slide in 4 per cent, solution of ferric alum for twenty-four hours. 7. Rinse with water, and place in an amber-yellow aqueous solu- tion of Kahlbaum's suphalizarinate of soda (prepared by dropping 1 c.c. saturated alcoholicL solution in 100 c.c. of water) for twenty-four hours. 8. Rinse with water, flood slides with a solution of crystal violet (1 vol. sat. sol. of dye in 70 per cent, ale.; 1 vol. 1 per cent. HC1 in 70 per cent. ale. ; 2 vols. anilin water) and warm until vapor is given off. 9. Rinse, differentiate one or two minutes in 30 per cent, acetic acid solution. 10. Wash in running water five to ten minutes. 758 HISTOLOGIC TECHNIC 11. Dry with blotting paper, drop in absolute alcohol, pass through bergamot oil into xylol and mount in balsam. According to Wildman (Jour. Morph., 24, 3, 1913) the slides may be passed from the alizarin solution after rinsing, 1, into a 3 per cent, solution of crystal violet (3 c.c. anilin stain in 100 c.c. distilled water) for ten minutes; 2, rinse and place in 80 per cent, alcohol for five sec- onds; 3, pass through 95 per cent, and 100 per cent, alcohol, where the differentiation is watched; 4, clear and mount in the usual way. III. EEGAUD'S METHOD (Arch. d'Anat, Micr., t. 11, 1910, p. 296). This constitutes a very satisfactory routine technic. 1. Fix small piece of tissue for twenty-four hours in a mixture of Potassium bichromate, 3 per cent, aqueous solution . . 80 parts Formalin ' 20 parts 2. Transfer to a 3 per cent, aqueous solution of potassium bichro- mate. This acts as a mordant. The tissue should be left in this solu- tion for from 1 to 4 weeks, with several changes of fluid. 3. Wash, harden, embed in paraffin, and cut sections of from 3 to 5 microns. 4. Stain with iron-hematoxylin. Mallory's Phosphotungstic Acid Hematoxylin for Neuroglia and may be used to advantage with the connective tissues) (Jour. Exp. Med., Vol. 19, No. 5, 1900). Tissues should be fixed in Zenker's fluid or in Kelly's fluid. It stains neuroglia, myoglia and fibroglia fibrils and fibrin and nuclei blue; collagen fibrils are stained- reddish-brown; other intercellular substances pink. Hematoxylin 0.1 gram Water 80.0 c.c. Phosphotungstic acid (Merck), 10 per cent. aqudous solution 20.0 c.c. Dissolve the hematoxylin by heating in a small amount of water; when cool add to rest of solution. Allow to ripen several weeks. The solution can be ripened for immediate use by addition of 10 c.c. of 14 per cent, solution of potassium permanganate, or by the addition of 0.2 gram peroxid of hydrogen. 1. Pass sections through graded alcohols to water. 2. Place in 14 per cent, aqueous solution of potassium perman- ganate for five to ten minutes. HISTOLOGIC TECHNIC 759 3. Wash in water. 4. Place in a 5 per cent, aqueous solution of oxalic acid for "five to ten minutes. 5. Wash thoroughly in water. 6. Place in phosphotungstic acid hematoxylin solution for twelve to twenty-four hours. 7. Dip for a few seconds in 95 per. cent, alcohol. 8. Clear in carbol-xylol and xylol and mount in xylol-damar. Vance's Method for Staining Bile Canaliculi (Anat. Anz., Vol. 44, No. 17, pp. 412-413). 1. Fix tissues in Helly's fluid; or in a mixture of equal parts of 10 per cent, formalin and 5 per cent, mercuric chlorid. 2. Embed in colloidin. 3. Place sections in a dilute solution of iodin in 95 per cent, alco- hol for five to fifteen minutes. 4. Remove excess iodin by washing in several changes of 95 per cent, alcohol. 5. Stain in Mallory's phosphotungstic acid hematoxylin for twenty- four hours. 6. Transfer directly to 95 per cent, alcohol. 7. Clear in carbol-xylol or in oil of origanum, and mount in balsam. Jenner's Blood Stain. This solution acts as a fixative as well as a stain. Blood smears are immersed in the stain for two to five minutes, washed in water, dried and mounted in balsam. Water soluble eosin (Grublers), 1 per cent, aque- ous solution 100 c.c. Methyl ene blue (Griibler's), 1 per cent, aqueous solution 100 c.c. Mix the above solutions ; after twenty- four hours, filter. Dry the filtrate at 65 C., wash the dry filtrate, and again dry and powder. Dis- solve 0.5 gram of the powder in 100 c.c. pure methyl alcohol. The result constitutes the stock solution. Zimmermann's Technic, for demonstrating the intercalated disks of cardiac muscle. 1. Small pieces of tissue are fixed for twenty-four hours in a solu- tion of 90 parts absolute alcohol and 10 parts 25 per cent, nitric acid. 2. Wash in several changes of 95 per cent, alcohol, or until it re- mains neutral to litmus paper. 760 HISTOLOGIC TECHNIC 3. Pass through graded alcohols to distilled water. 4. Transfer to solution of 1 gram hemalum (Griibler's) to 10 c.c. water, where it remains for eight to ten days. 5. Wash in distilled water, embed in paraffin, and section. The intercalated disks are conspicuously stained a dark blue, which contrasts sharply with the light-blue background. These disks can be well demonstrated also by fixation in Carnoy's fluid, followed by iron-hematoxylin stain. For the same purpose Heiden- hain employs vanadium hematoxylin after sublimate fixation (see Lee's Vade Mecum). Gage's Method, for Demonstrating Glycogen. Glycogen granules are readily soluble in aqueous media. Tissue which is to be tested for this substance should be fixed in 95 per cent, alcohol. 1. Embed in paraffin in the usual way. 2. Flatten sections on slide by use of the following iodin solution (Lugol's solution), which is subsequently used for staining: Iodin 1.5 grm. Potassium iodid 3.0 grm. Sodium chlorid 1.5 grm. 50 per cent, alcohol 300 c.c. 3. Stain, dehydrate, dissolve paraffin with xylol, mounted in melted vaselin, and seal cover glass with shellac. The glycogen grains are stained mahogany red. Osmic Acid Technic for Fat. Free fats and lipoids are soluble in ether, chloroform, absolute alcohol, xylol, benzene, and essential oils. Since these must be employed in both paraffin and celloidin embedding, fat cannot be very satisfactorily demonstrated by these technics. Frozen sections (made with a freezing microtome) of fresh or formalin-fixed tissues are therefore preferable (in some instances indispensable) ; these may be satisfactorily stained with Sudan III (red), Sharlach E (red), Nile blue (blue) ; they must be mounted in glycerin jelly. Osmic acid, however, renders certain fats resistant to solution in oils, and tissue so fixed may accordingly be treated by the paraffin tech- nic; but thick cedar oil must be substituted for xylol or chloroform, and absolute alcohol should be avoided: 1. Fix small pieces of fatty tissue in Flemming's fluid for two days. isg ris - ri ap. ri t If Iff fill i^iH^! 5B *1Sri!i ^owfiiBa' U S-t g&H - H 8 -a JQ [3 7^B l1 d 'P! J&S'S b's ,/ pn-8 1*1 ^^j >rt *| 3 a '? 3 3.S.J2 3 ^^^-g a fe ) W3 p) ' I? "31 liiifl HHHO ccTt< iocot>lo6 Or-tcqco-* PH ;=s d 1 F T^ ^ ** -^ o o o .8*3 1 ^^ CQ 784 DIRECTIONS FOB LABORATORY WORK IX. THE BLOOD VASCULAR SYSTEM (A) ARTERIES. (1) MEDIUM-SIZED ARTERIES (the larger peripheral arteries, e.g., the carotids, femoral, radial, popliteal, etc.). Study the vessel as a whole, in a stained transverse section, noting its circular outline, the generally empty lumen, the corrugated inner surface, and the thickness of the wall relative to the diameter of the lumen. Make a l.p. sketch (142) of entire vessel, indicating the three tunics: (a) tunica interna (intima) ; (b) tunica media, and (c) tunica externa (adventitia). Sketch h.p. (143) a narrow segment of the wall, noting the fundamental tissues of the several layers. Identify the internal and external elastic (fenestrated) membranes, and the vasa and nervi vasorum of the tunica externa. Which is the thickest tunic and what tissue elements does it include ? (2) LARGE ARTERIES (e.g., the aorta). Study the wall of the aorta in stained section. Sketch h.p. (144) a segment of the wall. Compare with wall of medium sized artery. Note the relatively wider tunica interna, the much narrower tunica externa, the apparent absence of the internal and external elastic membranes, and the great abundance of elastic tissue among the muscle cells of the tunica media. Study also a similar section stained with some specific elastic tissue stain (e.g., resorcin-fuchsin), and note the elastic fibers in the three tunics. Com- pare aorta with some other large (conducting) artery, e.g., subclavian and common iliac. (B) VEINS. (1) MEDIUM-SIZED VEINS .(the larger peripheral veins, e.g., exter- nal jugular, basilic, median, femoral, etc.). Study the vessel as a whole, in stained transverse section, and note its relatively thinner wall as compared with a medium-sized artery, its generally collapsed condi- tion, and its lumen frequently filled with blood. Compare in detail with a medium-sized artery, noting differences in composition and rel- ative thickness of the several tunics. Sketch l.p. (145) the entire vessel, indicating the three tunics. Sketch h.p. (146) a narrow segment of the wall, noting the fundamental component tissues in each tunic. Note the delicate character of the internal and external elastic membranes. (2) LARGE VEINS, (a) The Inferior Vena Cava. Study the wall in a stained section. Compare with aorta. Note great thickness of DIRECTIONS FOR LABORATORY WORK 785 tunica externa. Sketch h.p. (147) a segment of the wall, noting the fundamental tissues in each tunic. Note the longitudinal musele-bun- dles in the tunica externa. Compare with portal vein; with hepatic vein. (b) The Superior Vena Cava. Sketch h.p. (148) a segment of a section stained with a specific elastic tissue stain noting the distribution of elastic fibers. Compare with pulmonary vein. (3) VALVES. Study a longitudinal stained section of a large peri- pheral vein of the lower extremities (e.g., long saphenous vein). Iden- tify the valves, and note their distribution, position with respect to tributaries, and their gross and minute structure. Sketch (149) a seg- ment of the wall, including a valve. How many cusps are generally included in a valve ? Why are valves less numerous in the veins of the upper than lower extremities, and why are they entirely lacking in the abdominal and thoracic veins, and in veins of smaller calibre than 2 mm.? What specific structural differences obtain among the following veins: venae cavae, mesenteric, external jugular, pulmonary, umbilical and cranial? What is the functional significance of these differences? (C) SMALL VASCULAR COMITES. (1) SMALL ARTERIES AND VEINS. Many sections will contain pairs of smaller vessels. Study a larger pair in a stained section, and make detailed comparison with respect of relative gross size and form, con- dition of lumen, thickness of wall relative to diameter of bore, and rela- tive thickness of the several tunics. What can you determine regarding the presence of elastic membranes? Make (a) l.p. sketch of pair (150) ; (b) h.p. sketch of segment of wall of each (151). (2) ARTERIOLES AND VENULES. (a) Find a pair of vascular comites of this size in almost any stained section. Make h.p. sketch (152), not- ing especially relative thickness of the several tunics in the vessels of the pair. (b) In a stained toto mount of the mesentery or the pia mater find a typical pair of comites and trace from arteriole and venules through precapillary twigs into the capillaries. Sketch (153) such a system,, noting the differential marks between arteriole and venule, precapillary artery and vein; and between the foregoing and capillaries. What are sinusoids? Eetia mirabilia? How do they differ from capillaries? (c) In stained sections identify precapillary arteries and veins and capillaries. Sketch (154). What is the chief differential feature be- tween, a precapillary artery and vein, and between these and a capillary ? 786 DIRECTIONS FOB LABORATORY WORK (d) In a toto mount of mesentery, treated with silver nitrate, sketch (155) the endothelial lining of one of the smaller vessels. (e) Sketch (156) in longitudinal or oblique section a capillary, and a precapillary vein in the bone marrow of a developing membrane bone, e.g., jaw of pig fetus of 35 mm. (At this stage the student should review his slides, noting in each section: (a) the type of epithelium present; (b) the type of connective tissue; (c) muscle; (d) nerves; and (e) types of blood vessel. Several laboratory periods may be spent to advantage on this exercise.) (D) HEART (Cor). (1) Make gross study of heart (human, hog, dog or beef) and note in intact organ: (a) ventricles, (b) atria, (c) auricular appendages, (d) pericardium (visceral and parietal portions), and (e) the roots of the several large arteries and veins (aorta, pulmonary artery, inferior and superior vena cava,*and the pulmonary veins). Open the several chambers and note in the ventricles : (a) columns carnese, (b) papillary muscles, (c) chordae tendinese, attached to the valves guarding the atrio- ventricular orifices, (d) the moderator band of the right ventricle, and (e) the semilunar valves guarding the aortic and pulmonary orifices. In the atria note the (a) pectinate muscles of the auricular appendages and (b) the crista terminalis. Note also the inter atrial and the interven- tricular septa, and expose the atrio-ventricular bundle (bundle of His) in the anterior septal portion of the heart. (2) Study a stained transverse section of the heart of a mouse (or other small mammal) cut through the ventricles. Note (a) epicardium, (b) myocardium, (c) endocardium, (d) papillary muscles, (e) columns carnese. Sketch under l.p. (157). (3) Study a stained section through the ventricular wall of the human heart. Note the several layers, and compare with the three tunics of a large blood vessel. Make l.p. sketch (158). (4) From stained sections make h.p. sketches of (a) epicardium (159), (b) endocardium (160), (c) Purkinje fibers, in transverse sec- tion (most conspicuous in rabbits and sheep's heart) (161), and a cusp of an atrio-ventricular valve, including base (annulus fibrosus) and apex, noting in each case the component tissue elements (162). (5) Make drawings from demonstration preparation of the node of the sino-ventricular bundle, showing the constituent muscular and con- nective tissue elements (163), and the nerve and blood supply (164). Structure of sino-ventricular bundle of His? of the moderator band? DIRECTIONS FOR LABORATORY WORK 787 (6) Sketch small area of myocardium from injected specimen of cat's heart to show vascular supply (165). (7) Sketch from a demonstration slide (stained with methylene blue) a small area of the myocardium, showing the manner of the nerve supply (166). X. BLOOD (A) THE RED BLOOD CORPUSCLES (Erythroplastids, 'Erythro- cytes). (1) Sterilize tip of finger or lobe of ear with an alcohol-ether solu- tion, and allow to dry thoroughly. Sterilize a needle in the flame of a Bunsen burner. Prick finger tip or lobe of ear, and mount a consider- able drop of blood under a cover-glass. Examine quickly. Compare color as viewed macroscopically and under the microscope. Note shape and form of red corpuscles, as seen en face and in profile. Note the formation of rouleaux. Sketch (167). After a time crenated corpuscles appear near the edges due to evaporation of the blood plasma which in consequence has become hypertonic. Sketch (168). Note also the white glistening leucocytes, and the clumps of platelets. (2) Place on a slide a small drop of Ringer's solution (an isotonic solution). Prick finger tip and touch a small drop of blood to the Ringer's solution. Mount the mixture under a cover-glass supported by a hair (or prepare the mixture on the cover-glass and mount as a hanging 'drop over a hollow ground slide, sealing the cover-glass with vaselin). Follow a red corpuscle as it floats and turns in a current, noting carefully its form. What is the shape of the red blood corpuscle ? Is it nucleated? Sketch (169). What is the significance of the cup- shaped corpuscles? Sketch (170). Do rouleaux appear in the circu- lating blood? Note also the white glistening round and irregular leu- cocytes, and the granular masses of platelets. (3) Make successive mounts of blood in Ringer's solution and add to (1) a drop of a one per cent solution of acetic acid; to (2) a drop of a one per cent solution of tannic acid; to (3) a drop of a solution of bile. Note the results and explain. How is the third result related to the morbid condition known as jaundice or icterus? (4) Mount a drop of blood in distilled water (a hypotonic solution). Follow the successive changes undergone by the red corpuscles. Sketch (171). Explain. Recognize the final steps when blood shadows and blood dust (hemokonia) appear. 788 BISECTIONS FOE LABOKATOKY WOKK (B) THE WHITE BLOOD CELLS (Leucocytes; Amelocytes). (1) Mount a drop of blood in Toison's solution under cover-glass. Note the several types of white blood cells: (a) small mononuclear; (b) large mononuclear; (c) polymorphonuclear ; and (d) note also a group of blood platelets (plaques). Are the latter elements capable of ameboid motility? What is their derivation? Are they true cells? What is their most probable function? (2) Study carefully with oil immersion lens a blood smear stained with Wright's (or Basting's, Ehrlich's triacid, or Jenner's) stain. Identify and sketch (172) the various types of leucocytes: (a) lympho- cytes (large and small) ; (b) large mononuclear and 'transitional' non- granular leucocytes; (c) polymorphonuclear neutrophil leucocyte; (d) eosinophil leucocyte; (e) basophil leucocyte (mast cell); and (f) blood platelets. Note relative size, shape of nuclei, granular cytoplasmic con- tent, and relative abundance of each type. Make similar study of rab- bit's blood. (Here the neutrophils are replaced by similar cells with fine eosinophilic, 'special', granules. In amphibia and reptiles the neu- trophils are replaced by eosinophils with ellipsoidal granules.) (3) Study the contents of a vein in a stained section. Note the vari- ous shapes of the red corpuscles, and identify the white blood cells present. (4) Make mount in Toison's solution of frog's blood. Note the ellipsoidal nucleated red corpuscles, the white blood cells (a, small, large and 'transitional' non-granular leucocytes; b, polymorph neutrophils; c, eosinophils; and d, basophils) and the small spindle-shaped throm- bocytes. Sketch (173). Watch a large leucocyte in ameboid progres- sion. Sketch several steps (174). (5) Make similar mount and study of bird's or turtle's blood. How does it differ from frog's blood? How do sauropsid bloods differ from mammalian? rabbit's blood from human blood? (C) FIBRIN. Place a small drop of blood on slide and spread out thin, allow to coagulate slowly. Breathe on the coagulum at intervals through a period of about a quarter of an hour. Add a drop of methylene blue, and allow it to act for several minutes. Rinse in water; dry thoroughly and mount under a cover-glass in balsam. Examine the thinner portions of the coagulum for the fibrin net. Sketch (175). Study and sketch (176) also a small area of the fibrin net from a stained section of a blood clot. Difference between plasma and serum? DIRECTIONS FOE LABORATORY WORK 789 (D) HEMOGLOBIN CRYSTALS. Place large drop of blood on slide in a small drop of distilled water. Allow to dry slowly. Mount when dry in balsam under cover-glass. Ex- amine h.p. for crystals. Note shape, size and color. Sketch (177). (E) HEM IN CRYSTALS. Place a drop of blocd on slide. Add 2 or 3 grains of sodium chloride and a drop of glacial acetic acid. Heat slowly over flame of a Bunsen burner until bubbles begin to appear. Mount in balsam. Examine h.p. for crystals. Note shape, size and color. Sketch (178) several isolated crystals and several groups. (F) BLOOD DEVELOPMENT (Hemopoiesis). (1) IN EMBRYO. Sketch (179) a few blood cells from a young mammalian embryo, noting the several types of embryonic erythrocytes : (a) hemoblasts, (b) erythroblasts, (e) normoblasts. (2) IN ADULT BONE MARROW. Study with oil immersion lens smear preparation or a section of red bone marrow (e.g., of femur of rabbit or guinea pig), stained in Wright's solution. Identify and sketch (180) the several types of myelocytes: (a) myeloblast, (b) leucoblast, (c) erythroblast ; also (d) myeloplaxes (giant cells), (e) erythrocytes, (f) the several types of granular' myelocytes : eosinophil, basophil and 'spe- cial' (neutrophil of higher mammals), and (g) origin of platelets from pseudopods of megakaryocytes. Note cells in process of division, both mitotic and amitotic. Note also the framework of reticular connective tissue (reticulum) ; and the process of extrusion of the nucleus of a normoblast in the formation of an erythroplastid. Compare with sec- tion of red marrow of femur of frog. Differences? Make similar study of fragment of marrow of femur of rabbit or guinea pig, preserved in aceto-carmin, and teased in glycerin. XI. THE LYMPHATIC SYSTEM (A) LYMPH. Study a smear preparation of lymph stained with Wright's stain from the thoracic duct of a dog. Sketch (181) the several types of leucocytes. Which type predominates? Do blood platelets occur in lymph ? Eed blood corpuscles ? Origin of lymph ? Consider the coagu- lation of lymph. 790 DIRECTIONS FOB LABORATORY WORK (B) LYMPH VESSELS (Lymphatics). (1) LYMPH CAPILLARIES. Identify a lymph capillary in a stained section (e.g., lymph node, subcutaneous tissue, submucosa of trachea or large intestine) and sketch (182). How does it differ from a blood capillary? a venule? a sinusoid? (2) LACTEAL. Sketch (183) the terminal lymph capillary in a villus of a stained section of the small intestine. (3) THORACIC DUCT. Sketch (184) a segment of the wall of a stained section of the thoracic duct (Weigert's elastic tissue stain, and picric-acid-fuchsin). Note the several layers, and the fundamental tis- sues of each. How can you distinguish between a large lymphatic and a vein of the same calibre? Do lymphatics contain valves? (C) SEROUS MEMBRANES (Peritoneum, Pleura, and Pericar- dium). Sketch (185) h.p. a small area of a stained toto mount of a frag- ment of the peritoneum treated with silver nitrate. Note the character of cells and intercellular stomata. Sketch (186) also a short segment of a stained section of same. Note shape and character of cells; inter- cellular bridges, striated border, basement membrane, and the submeso- thelial fibre-elastic corium. (D) LYMPH NODULES (Lymph follicles). Sketch (187) l.p. a small lymph nodule from the submucosa of a stained section of large intestine, stomach, or vermiform appendix. (E) LYMPH NODE (Lymph gland). Study a stained section of a mesenteric lymph node. Sketch (188) l.p. Sketch (189) h.p. small area including a medullary cord and the adjacent lymph sinus. What is the predominant cell type ? Note divid- ing cells in germinal center of cortical nodules. Examine in Ringer's solution scrapings from a lymph node of cat, after addition of drop of methylene blue. Examine also a glycerin mount of aceto-carmin pre- served material. (F) LYMPHOID ORGANS. (1) HEMOLYMPH NODE. (Hemal Node). Study a stained sec- tion of a hemolymph node of sheep. Sketch (190) h.p. a small area including the denser and looser parenchyma. Differential characteris- DIRECTIONS FOB LABORATORY WORK 791 tics of lymph, hemal and hemolymph nodes? Blood and lymph supply of each? Giant cells? (2) TONSILS (Fancied tonsil). Study a stained section of the faucial tonsil. Make h.p. sketch (191) of the infiltrated stratified squamous epithelium lining a crypt. Function of tonsils? (3) SPLEEN (Lien). (a) Study a stained section of spleen. Note (1) robust fibromuscular capsule ; (2) similar trabeculae ; and (3) splenic (Malpighian) corpuscles with central or subcentral arteriole. Sketch segment under l.p. (192). Study under h.p. the spleen pulp, noting dif- ference between pulp cords and intercordal pulp (venous sinuses). Sketch (193) a small area of this portion of the parenchyma, noting types of cells and character of the terminal arterioles. Study injected specimen of spleen, noting vascular terminals within the lobule. (b) From a smear preparation of spleen, stained with Wright's blood stain, sketch (194) the several types of parenchyrnal cells: (1) lymphocytes; (2) polymorphonuclear neutrophil leucocytes; (3) large mononuclear leucocytes ('splenic cells') ; (4) eosinophil leucocytes; (5) basophil leucocytes; (6) erythrophages ; (7) erythroblast ; (8) giant cells. 'Make similar study of an aceto-carmin preparation mounted in glycerin. What are the chief criteria for distinguishing the several lymphoid organs from lymph glands, and from each other? Function of spleen? XII. SKIN AND APPENDAGES (A) THE SKIN (Integument). (1) Study under l.p. a vertical stained section of thick skin (from palm of hand or sole of foot). Note epidermis (cuticle), dermis (cor- ium, cutis), and the tela subcutanea. Distinguish between the stratum corneum and the stratum germinativum ; between the pars papillare and the pars reticulare. Note the spiral epidermal portion of the ducts of the sweat glands. Note also the panniculus adiposus of the subcu- taneous tissue. Sketch a narrow segment through entire thickness (195). Make h.p. sketch (196) of several adjacent cells from each of the six distinct layers of the epidermis. What are the several factors (physical, chemical and mechanical) which operate upon a cell of the cylindrical cell layer during its metamorphosis into an element of the scaly cell layer? (2) Study under l.p. a vertical stained section of thin skin (e.g., from abdomen). Sketch (197). What are the chief structural differ- 792 DIRECTIONS FOR LABORATORY WORK ences between thick and thin skin? Between thin skin and the mucous membrane lining the mouth? Note the delicate hairs. (3) Make h.p. sketches (198, a, b, c) from different portions of a section through the head of a 20-mm. pig fetus, illustrating three earlier stages in the development of skin : one consisting of a single layer of cells (with superjacent periderm or epitrichium), one of two or three layers, and one of a larger number of layers. (4) Compare sections of thin skin of: (a) dark negro; (b) light mulatto; (c) brunette; (d) blond. Note quantity and distribution of the pigment (melanin) granules in the several specimens of skin. (B) SWEAT GLANDS (Sudoriparous glands}. (1) In a stained vertical section of skin identify and study a sweat gland (complete if possible). Note: (a) its duct (excretory) including the spiral 'mouth' within the epidermis; (b) its coiled fundus (secre- tory). Under h.p. draw a cross-section of the dermal (or subcutaneous) duct and fundus (199). Note the longitudinally disposed smooth mus- cle cells between the secretory cells of the fundus and the membrana propria. (C) THE NAILS (Unguis; Onyx). (1) Examine a finger-nail and note: (a) body; (b) root; (c) lun- ula; (d) nail sulcus; (e) nail fold or vallum; (f) eponychium; (g) hyponychium. Sketch (200). Explain the opacity and color of the lunula. (2) Study l.p., a stained transverse section of the finger tip includ- ing the nail. Note the homologous layers of nail and adjacent skin. What differences obtain? What layer of the epidermis does the nail body represent ? How does the pars papillare differ in the two regions ? Sketch (201). (3) Make l.p. sketch (202) of a stained vertical longitudinal section of the unguinal phalanx. Note nail matrix and nail bed. What is the epidermal homologue of the nail matrix? The relation of the matrix to the lunula? Sketch (203) h.p. a small area through the body of the nail including its corium. (D) THE HAIR (Pilus; Thrix). (1) Mount in water under cover-glass a freshly extracted hair. Note: (a) hair bulb; (b) root (radix); (c) shaft (scapus) ; (d) me- dulla; (e) cortex; (f) pigment granules; and (g) cuticle. Explain DIRECTIONS FOE LABORATORY WORK 793 differences in color of hair. Sketch under h.p. a portion of the shaft (204). (2) In a stained vertical section of the scalp examine, under the low power, a hair follicle with its included hair in longitudinal section. Note: (a) hair papilla; (b) hair bulb (pigmented?) ; (c) hair root; (d) epidermal root sheath; (e) arrector pili muscle; (f) sebaceous gland. Sketch (205). Make h.p. sketch (206) of the hair and its follicle, cut longitudinally, at the level of the sebaceous gland (about the middle third of the root), noting the several layers of the inner and outer root sheaths (the epidermal sheath), and of the dermal sheath. What is the function of the arrectores pilorum muscles? Make h.p. sketch (207) of the region of transition from root and follicle to the hair bulb. (3) Sketch h.p. (208) a transverse section of a hair follicle at or near the level of the middle third, showing the several layers of the follicle as above. What layers are homologous in hair follicle and skin? Explain the homologies in terms of development. Explain the cause of the curling of certain types of hair. (E) SEBACEOUS GLAND. (1) Study a sebaceous gland in a stained vertical section of the scalp. Note its relation to the hair follicle. Note also its duct and saccules. Sketch l.p. (209). Make h.p. sketches (210 a, b, c) of cells of the sebaceous gland at three successive stages of their metamorphosis into the sebum 'secretion'. How many glands to a hair follicle? (F) THE BLOOD SUPPLY. (1) Study in stained section injected specimens of skin and scalp. Sketch (211). Note the precise location of the several vascular plexuses of the derma and the tela stibcutanea ; and the blood supply of the sweat glands, hair follicles, sebaceous glands, and the fat lobules of the tela. XIII. THE RESPIRATORY SYSTEM (A) THE NASAL CAVITY. (1) Study in a stained section the lining mucous membrane of the vestibule. Note transition from external skin to internal mucous mem- brane ; note, also the vibrissae and the associated sebaceous glands. Make h.p. sketch of a narrow segment (212). (2) Study mucosa of respiratory portion ( Schneiderian membrane). 794 DIRECTIONS FOB LABORATORY WORK Note the pseudo-stratified ciliated columnar epithelium, goblet cells, the extensive vascular corium, and the sero-mucous glands. Sketch h.p. a narrow segment (213). (3) Make h.p. sketch (214) of olfactory mucous membrane, noting: a, sustentacular cells; b, olfactory cells; c, basal cells; and d, serous glands of Bowman. In what respect is the olfactory cell unique ? (4) In a section of the cat's nose, note the vomero-nasal organ of Jacobson in the median septum, one on either side of the cartilaginous plate. Sketch a narrow segment. (B) THE LARYNX. (1) Study a stained vertical section through lateral wall of the larynx. Note vocal cords (true and false) and ventricle. Sketch (215), indicating the types of epithelium and the contents of the corium at different levels. What are the fundamental tissues comprised in the true vocal cords? (C) THE TRACHEA. (1) Study a stained transverse section of the trachea. Note: a, character of the lining epithelium; b, goblet cells; c, elastic layer in tunica propria; d, muscularis mucosse? e, tela submucosa, with its mucous glands with demilunes; f, fibre-cartilaginous tunic. Trace the duct of a mucus-secreting gland to the surface. Note its apulla in the corium. What is the character of the cartilage of the adventitia? Ex- plain the trachealis muscle. Sketch h.p. (216) a narrow segment of the complete width of the wall. (D) THE BRONCHI. (1) Study a stained transverse section of a primary bronchus. Com- pare with trachea. In what points do they differ ? (E) THE LUNG (Pulmo). (1) Make h.p. sketch (217) of narrow segment of wall of sec- ondary or tertiary bronchus. Note : a, muscularis mucosse ; b, lymphoid tissue (solitary nodules?); c, mucous glands; d, character of lining epithelium. How are the pulmonary artery and vein, and the bronchial artery and vein, related to these bronchi? (2) Sketch h.p. (218) a bronchiole. What are the chief structural differences between a bronchiole and a tertiary bronchus? Relation of pulmonary artery and vein to bronchiole? Structure of terminal (res- piratory) bronchiole ? DIRECTIONS FOB LABORATORY WORK 795 (3) Make l.p. sketch (219) of a primary lobule (histologic and functional unit), including: a, transition from respiratory (terminal) bronchiole to b, alveolar duct ; c, atria ; d, pulmonary sacs ; e, pulmonary alveoli. Study the several portions of the primary pulmonary lobule under the high power and note the structural differences. At what point in the lobule does the smooth muscle end? Where do the lymphatics end? Relative abundance of the elastic fibers? Relation of pulmonary artery and vein to the primary lobule? Define a secondary pulmonary lobule (anatomic unit). (4) Make h.p. sketch (220) of wall of pulmonary alveolus (air cell)* cut tangentially. Note the flat respiratory cells and the expansive non- nucleated respiratory plates. (5) Sketch (221) several adjacent alveoli from a section of an in- jected specimen of the cat's lung. (6) Make h.p. sketch (222) of small extent of the visceral pleura. (7) Consider the distribution of the lymphatics, the lymph nodes, lymph nodules and the more diffuse lymphoid tissue in the lung. Func- tion of lymphoid tissue in the lung? Relation to inhaled dust? to infecting bacteria? to tuberculosis^ Consider also the distribution and relative abundance of smooth muscle and of elastic fibers. Significance ? Relation to asthma? to anaphylaxis, e.g., in guinea pig? Explain ab- sence of bronchial veins beyond the bronchioles. XIV. THE ALIMENTARY TRACT (Digestive Canal) (A) THE LIP. (1) Study l.p. stained section (vertical) of lip. Distinguish cuta- neous and mucous surfaces. Note : a, bundles of striped muscle fibers of orbicularis oris muscle in transverse section; b, mucous glands; c, hair follicles; d, sweat glands; e, sebaceous glands; f, coronary (labial) ar- tery. Sketch (223). (B) THE TEETH. (1) Study a stained axial section of a decalcified molar tooth within its alveolar socket. Note: a, crown; b, neck; c, root; d, fangs; e, pulp cavity; f, root canal; g, foramen apicis dentis. Sketch h.p. (224) small area of a, enamel ; b, dentin ; c, cementum ; d, dental pulp reticulum ; e, granular layer of Tomes; f, an odontoblast with its Tomes' fibril within 796 DIRECTIONS FOR LABORATORY WORK a dentinal tubule. Sketch l.p. (225) a portion of the area of contact between root of tooth and alveolar process. Note : a, alveolar bone ; b, periosteum and pericementum ; c, circular dental ligament; d, epithelial remnants of dental lamina ; e, fibers of Sharpey ; f , cementum cells. (2) Study l.p. an axial ground section of tooth. Note in the enamel, the contour lines of Eetzius and the prism stripes of Schreger; in the dentin, the dentinal tubules, the contour lines of Owen, and the incre- mental lines of Schreger. Make h.p. sketches (226) of small areas of the enamel showing the prism bundles cut transversely and longitudi- nally; also (227) of small areas of the dentin showing the tubules cut transversely and longitudinally. Sketch also (228) a small area of the cementum; and (229) of the granular layer of Tomes in the region of the neck of the tooth. Describe the complete form, structure, content and significance of the dentinal tubules. (3) Sketch (230) several successive early stages in the development of the deciduous teeth in a stained vertical section of the jaw of some mammalian fetus {e.g., of 25 mm. pig embryo). Note: a, labiodental strand and groove ; b, labial lamina ; c, dental lamina ; d, enamel organ ; e, dental papilla. (4) Study a stained axial section of an infant's tooth some time before eruption. Note: a, enamel organ (enamel, inner enamel epithe- lium, outer enamel epithelium, and enamel pulp) ; b, dental pulp, odon- toblasts and the strata of older and younger dentin; c, the dental sac; d, anlage of the permanent tooth; e, remnant of the dental lamina. Make l.p. sketch (231). Sketch h.p. (232) a small area near the apex of the crown, showing from without inward : a, enamel pulp ; b, layer of ameloblasts ; c, Tomes' processes ; d, enamel ; e, membrana perf ormativa ; f, stratum of older basophilic dentin; g, stratum of younger acidophilic (only slightly calcified) dentin with Tomes' fibrils; h, layer of odonto- blasts ; i, dental pulp. (C) THE TONGUE (Lingua). (1) Make diagram l.p. (233) of transverse section of a cat's tongue to show the disposition of the striped muscle. Note the bilaterally sym- metrical structure of the tongue; the extent of the medial septum lin- guae; the difference between the mucous covering on the dorsum and ventrum of the tongue. Note also the superficial longitudinal muscles, the genio-glossus ventrally, and the deep transverse and vertical fibers. Note further the glands imbedded in the muscle. (2) Sketch from stained sections of the human tongue the several DIRECTIONS FOB LABORATORY WORK 797 types of lingual papillae: filiform (conical) (234); fungiform (235); circumvallate (vallate) (236); and foliate (237). Note the taste-buds in the latter two types of papillae. (3) Study an injected specimen of cat's tongue and note the blood supply of the muscle, and of the several papillae. (4) Study a transverse section of the foliate papillae of the rabbit's tongue. Note the numerous taste buds. (D) THE PALATE (Palatinum). (1) Study a stained longitudinal section of the palate (including the uvula, the soft palate, and a portion of the hard palate), noting the character of the epithelium on the nasal and oral surfaces. Sketch (238). (E) THE PHARYNX. (1) Study l.p. a stained section of the oropharynx. Note: a, type of epithelium of the mucosa; b, character of the tunica propria; c, lymphoid tissue; d, boundary of fibre-elastic tissue, the representative of both the tela submucosa and the muscularis mucosa of other portions of the digestive tube (the elastic fibers are mostly longitudinally dis- posed) ; and e, the outermost layer of obliquely disposed striped muscle fibers, among the connective tissue of which are embedded many tubo- acinar mucous glands. Sketch (239). (F) THE ESOPHAGUS (Gullet). (1) Study l.p. a stained transverse section through the lower third of the esophagus. Note the four tunics from within outward : a, tunica mucosa; b, tela submucosa; c, tunica muscularis; and d, tunica adven- titia (fibrosa). In the tunica mucosa, note the type of the epithelium, the character of the corium (lamina propria mucosae), and the lamina muscularis mucosae. (Near the cardiac orifice the muscularis mucosae consists of an incomplete inner layer of circularly disposed smooth mus- cle cells, and a more extensive outer stratum of longitudinally arranged cells; elsewhere in the esophagus generally the latter stratum only occurs; below the esophagus both layers occur.) In the tela submucosa note the character of the tissue and the mucous glands. Do the glandu- lar alveoli contain demilunes? (Mucus-secreting glands are absent in the esophagus of the cat, very abundant in that of the dog, and very variable in man. They are generally more abundant towards the oral end.) In the tunica muscularis, note the two strata of 'smooth muscle cells, the outer longitudinal and the inner circular (at the cardiac orifice 798 DIRECTIONS FOR LABORATORY WORK an additional innermost oblique layer may appear). Is a mesothelial layer present in the tunica adventitia? Sketch l.p. (240) a narrow seg- ment through the complete wall. (2) Compare the above section with sections through the middle and upper third of the esophagus. What differences obtain at these levels in the several tunics? (3) Compare a section through the upper third of the esophagus with one through the pharynx. (A longitudinal section through the area of transition from pharynx to esophagus is preferable.) What takes the place in the pharynx of the muscularis mucosse of the esoph- agus ? Note that in the upper end of the esophagus a third innermost stratum of oblique or longitudinal striped muscle fibers may occur in the tunica muscularis. (4) Study sections through the upper cardiac glands (superficial esophageal glands). Sketch (241). (5) Sketch small portions of the myenteric (242), and the sub- mucous nerve plexuses (243). (G) THE STOMACH (V entriculus ; Gaster). (1) Study l.p. a stained longitudinal section through the area of transition from the esophagus to the cardiac portion (pars cardiaca) of the stomach. Note the tunica serosa of the cardia. Make sketch (244) including the four tunics. Sketch h.p. (245) a cardiac gland. (2) Study a section through the pars fundica of the stomach (fun- dus ventriculi). Sketch l.p. (246). Examine carefully under h.p. a gastric gland of this region and note: a, mouth; b, foveola (crypt, pit) ; c, neck (cervix) ; d, body (corpus) ; and e, fundus. Note the different types of epithelium occurring in the several regions. Sketch h.p. (247) a complete gland, noting especially the chief and parietal cells. Sketch h.p. (248) a transverse section through the body of a gastric gland. Look for a 'lenticular gland' (solitary lymph nodule) in the tunica mucosa. (3) From a demonstration slide prepared by the Golgi technic sketch h.p. (249) a portion of a longitudinal section of the secretory region of the fundic gland to show the main lumen and the system of inter- and intracellular secretory canaliculi. (4) Study a section of the pars pylorica of the stomach. Sketch l.p. (250). Sketch h.p. (251) a gastric gland of this region. In what features do the pyloric and fundic glands differ? How do they differ functionally as indicated by these features? Enumerate four salient DIRECTIONS FOR LABORATORY WORK 799 differential characteristics between the pars fundica and the pars pylorica of the stomach. (5) Study an injected specimen of cat's stomach showing the blood supply of the several tunics. Sketch (252). .(6) Study demonstration preparations (methylene blue technic) of cat's stomach showing the nerve supply. Note especially the myenteric and the submucous plexuses. Sketch (253). (H) THE SMALL INTESTINE (Intestinum t&nue). (1) Study l.p. a longitudinal section through the area of transition from the pylorus to the duodenum. Compare part for part. In the duodenum note: a, villi; b, intestinal glands (crypts of Lieberkiihn) ; c, duodenal (Brunner's) mucous glands; and d, the plicae circulares (valvulae conniventes). Sketch (254). Enumerate the chief criteria for distinguishing between the duodenum (small intestine) and the pyloric portion of the stomach. (2) Sketch h.p. : a, a villus in transverse and longitudinal sections (255) ; b, intestinal gland (256). Identify a cell of Paneth. (3) Study l.p. a stained transverse section through the jejunum or the ileum. Sketch (257) at the level of a Peyers patch (agminated nodule). Difference in shape of villi in the three segments of the small intestine? Distribution of lymphoid tissue? Presence of glands in the tela submucosa? (I) THE LARGE INTESTINE (Intestinum crassum). (1) Draw l.p. (258) a portion of a stained section of the colon, including the four tunics. How does it differ in grosser features from the small intestine ? In the relative abundance of goblet cells ? of sol- itary lymph nodules? of granule cells of Paneth? Note the tenia3 coli of the tunica muscularis. (J) THE VERMIFORM APPENDIX (Processus vermiformis) . (1) Study a transverse section of a normal human appendix. Note the relative abundance of the crypts of Lieberkiihn and the solitary lymph nodules; and the differences in the lamina muscularis mucosaB and the tunica muscularis, as compared with the colon. Sketch (259). (K) CAECUM. Study a longitudinal section through the area of transition (caecum) from the ileum to the ascending colon. Note the iliocolic valve. Note 800 DIRECTIONS FOE LABORATORY WORK also in the colon the haustra, the plicae semilunares and the appendices epiploica?. Sketch l.p. (260). (L) RECTUM. Sketch l.p. (261) the area of transition from the rectum to the anus to show the structural differences in the several tunics. Compare the rectum with the colon. Enumerate the grosser differential marks among colon, appendix and rectum. Note rectal valves, rectal columns, and anal valves. Study an injected specimen of the small intestine of the cat. Sketch the blood supply of a villus and of a crypt of Lieberkiihn (262). XV. THE LARGE GLANDS OF THE DIGESTIVE CANAL (A) THE LARGER SALIVARY GLANDS. (1) THE PAROTID GLAND. Study under the low power of the micro- scope a stained section of the parotid gland. Sketch (263), show-' ing the division into lobes and lobules, and the location of the inter- lobular and intralobular excretory ducts. Note the distinction between the parenchyma and the stroma or interstitial tissue. Sketch 'h.p. (264) an alveolus (acinus) in connection with its inter- calary duct. Note the shape, manner of distribution within the alveolus, stage of secretion as indicated by the cytology, staining reaction and relation to the 'basket cells' of the constituent secretory cells of the acinus. Sketch h.p. (265) also transverse sections of the secretory (sal- ivary) and the excretory portions of the intralobular duct. (2) THE SUBLINGUAL GLAND. Study a stained section of the chief sublingual gland (of man, dog, cat or rabbit). Sketch h.p. (266) an alveolus with a large demilune ( f crescent' of Gianuzzi). Explain the functional significance of the difference in staining reaction between the cells lining the alveolus and those of the demilune. Probable function of the demilune cells? Chief difference in duct systems of parotid and sublingual glands? (3) THE SUBMAXILLARY GLAND. Compare the submaxillary with the parotid and sublingual glands. Enumerate the chief differen- tial characteristics among these three glands. Sketch h.p. (267) two adjacent alveoli, one of the mucous the other of the serous type. From an injected specimen of cat's submaxillary gland sketch (268) the intra- DIRECTIONS FOR LABORATORY WORK 801 lobular blood supply. Nerve supply of the salivary glands? Note dif- ferences between the resting and active (or stimulated) gland. ~ (B) THE PANCREAS. (1) Study a stained section of the pancreas. Make l.p. sketch (269) to show the arrangement into lobes and lobules, and the location of the ducts. Sketch h.p. (270) an acinus with its intercalary duct (inter- mediate duct), and a centro-acinal cell group. Note the polar differen- tiation of the zymogenous acinar cells ; and the different staining reaction of the zymogenous and the centro-acinal cells. Significance? What is the relationship between the distal granular zone of the zymogenous cells and their basal filar zone ? the mitochondria ? the 'nebenkern' ? How do these structures vary according to the phase of functional activity? (2) Sketch h.p. (271) a pancreatic islet (island of Langerhans). From a specially prepared demonstration slide of the pancreas sketch (272) cells of the A and B types. Number, form, distribution, staining reaction, and significance of the islets? (3) Sketch h.p. (273) a cross-section of a large interlobular duct. From an injected specimen of a cat's pancreas sketch (274) the blood supply of the zymogenous parenchyma and of the islets. Signifi- cance of the relatively great vascularity of the islet? Eelation of the cap llaries to the cords of islet cells? Enumerate the differential marks between the pancreas and the parotid gland. (C) THE LIVER (Hepar). (1) Study l.p. a stained section of the liver: a, of pig or camel; b, turtle or frog; c, cat or human. Enumerate differences. From sections a and c sketch l.p. (275) a hepatic lobule including the capsule of Glis- son. From section b, sketch h.p. (276) a small extent of the liver tubule, noting the definite structural polarity of the constituent cells.- Functional significance of the latter phenomenon? Similarity between embryonic mammalian liver and the adult reptilian and amphibian livers ? Significance ? What is the shape of the liver lobules as revealed by reconstructions? Difference between a hepatic lobule and a portal lobule ? (2) Study h.p. a stained section of the human or cat's liver. Sketch h.p. (277) a small area, noting the hepatic cell-cords (trabeculse), the interstitial reticulum, the capilliform sinusoids (intralobular capil- laries), the relation of the latter to the cell cords, and of the stellate cells (of von Kupfer) to the endothelial lining cells and to the liver 802 DIRECTIONS FOR LABORATORY WORK cells proper. From a demonstration preparation according to the Golgi technic or the Vance technic, draw h.p. (278) a cell cord with its inter- and intracellular secretory canaliculi. In a section prepared according to Mallory's technic for connective tissue, study the character of the inter- and intralobular connective tissue. (3) Sketch h.p. (279) several types of liver cells: a, granular (with zymogenic and glycogenic granules); b, fatty; c, pigmented; d, binu- cleated. Consider the questions regarding the presence, character, sig- nificance and relation to the bile capillary (ductule) and to the capillary blood supply (in health and in disease, e.g., in jaundice) of the intra- cellular bile canaliculi. (4) Sketch h.p. (280) a portal canal (interlobular vein, artery, lymphatic and bile duct) ; a central (intralobular) vein (281) ; a sub- lobular vein (282). Sketch h.p. (283) the area of transition from an intralobular to the interlobular bile duct. (5) From an injected specimen of the cat's liver sketch (284) a small area of the parenchyma. Trace the path of the portal and the hepatic blood through the liver. Diagram (285). (D) THE GALL BLADDER (Vesica felled). (1) Make h.p. sketch (286) through the wall of gall-bladder, noting the constituents of the several tunics. XVI. THE URINARY SYSTEM (A) THE KIDNEY (Ren). (1) From a gross specimen of an adult human kidney divided in the median longitudinal plane, make a sketch (287) to show the general topography. Note : capsule (tunica fibrosa, tunica adiposa) ; cortex (pars radiata, pars convoluta) ; medulla (renal pyramids, renal columns, renal papillae) ; renal sinus (in hilus), containing the renal pelvis (with infundibula or major calyces and the minor calyces). Note also the ureter, and the distribution of the branches of the renal artery and vein in relation to this duct. (2) Study l.p. a stained longitudinal section of the kidney of the mouse or rat (or other small mammal). Sketch (288). In capsule note: mesothelium; tunica albuginea; smooth muscle? Define a renal lobule (renculus). How does the adult human kidney differ from that of the fetus and the infant? from that of reptiles? from that of mouse and cat? from that of horse? from that of pig? DIRECTIONS FOR LABORATORY WORK 803 (3) Study a stained longitudinal (sagittal) or radial section of the mammalian kidney. Make l.p. sketch (289) (diagrammatic) far show the finer topographic relationship of the several divisions and their larger constituents. Note cortex corticis, pars radiata, pars convoluta, renal corpuscle, renal column, and boundary and papillary zones of the medulla. Sketch h.p. (2$0) a renal corpuscle. Note: glomerulus; capsule (visceral and parietal layers) ; arterial pole (with afferent and efferent arterioles) ; uriniferous pole (neck). Sketch h.p. (291) portions of the several segments of a renal (urinif- erous) tubule: proximal convoluted tubule; descending limb of Henle's loop; ascending limb of Henle's loop; distal convoluted tubule; arched collecting tubule; straight collecting tubule; papillary duct. Note the difference in staining reaction of the two portions of the tubule, divided at the point where the distal convoluted joins the arched collecting tubule. Significance with respect of function? of origin? (4) From a demonstration slide prepared according to Huber's tech- nic for isolating the renal tubules (Anat. Rec., vol. 5, 1911), sketch a complete tubule (292). (5) Study a stained coronal (tangential) section of the kidney through cortex. Identify the several constituents of the pars radiata and the pars convoluta. Make a similar study of a similar section through the medulla. (6) Study a longitudinal section of the kidney stained with Mai- lory's connective tissue stain. Note the types and distribution of the connective tissue. (7) From a demonstration slide of the kidney prepared by Meve's mitochondrial technic, sketch (293) a portion of the proximal convo- luted tubule. Shape of the mitochondria? Function? (8) Study a radial section of an injected cat's kidney. Sketch (294) the blood supply. Note interlobar arteries and veins; arciform (arcu- ate) vessels; interlobular vessels; afferent and efferent glomerular ar- terioles; the glomerular rete mirabile; the capillary supply of the con- voluted tubules; the stellate veins; the arteriolae and venulae rectse (verae and spurise) of the medulla. (B) THE URINARY BLADDER (Vesica urimria). Study l.p. a stained section of the urinary bladder. Make l.p. sketch (295) of segment of complete wall; h.p. sketch (296) of tunica mucosa. How does the mucosa vary with the degree of distension ? How does the 804 DIRECTIONS FOR LABORATORY WORK tunica muscularis differ from that of the digestive tube? Character, origin and relationship of the trigonum vesicse? (C) THE URETER. Make similar study and sketches (297, 298) of ureter cut trans- versely. (D) THE URETHRA. Make similar study and sketches (299, 300) of female urethra. (The male urethra should be studied in connection with the penis.) XVII. THE MALE ORGANS OF REPRODUCTION (Male Genital Apparatus) (A) THE TESTIS (Orchis). (1) Study a gross fresh specimen of a mammalian testis, with its associated duct system. Identify: a, globus major (caput), globus minor (cauda) and corpus of epididymis; b, ductuli efferentes (and coni vas- culosi) ; c, ductuli abberentes (superior and inferior) ; d, ductus def- erens; e, paradidymis; and f, the appendices testis and epididymis. Sketch (301). (2) Study with hand lens a median longitudinal section of a mam- malian testis. Identify: a, capsule; b, hilus; c, mediastinum (corpus Highmori) with rete testis; d, lobular compartments (lobules of testis) ; e, semniferous tubules (tubuli contorti) ; and f, tubuli recti. Sketch (302). (3) Study under h.p. a stained section of an active mammalian testis (e.g., mouse, guinea-pig, human). Sketch (303) a segment (or seg- ments) of the wall showing besides the several phases of Spermogenesis (viz.: a, spermogonium ; b, primary and c, secondary spermocytes both resting and in mitosis d, spermatids in several stages of metamorphosis into e, spermium or spermozoon), a Sertoli cell (sustentacular cell or trophocyte) with attached spermia, forming a 'spermoblast'. Note also: a, tunica vaginalis; b, tunica albuginea; c, tunica vasculosa; and d, interlobular septa. Sketch h.p. (304) a portion of wall of : a, a tubulus rectus; and b, (305) the rete testis. What is the form of a seminiferous tubule? Its relation to the lobule of the testis? How may the testis be classified among glands? (4) Study h.p. a section of the active testis of a grasshopper. Iden- tify the successive stages in spermogenesis. Count the number of DIRECTIONS FOB LABORATORY WORK 805 chromosomes in the metaphase plates of : a, the dividing spermogonium ; b, the primary spermocyte ; c, the secondary spermocyte. Note the acces- sory or sex chromosome. What is its position and behavior in: a, the resting primary spermocyte; b, the dividing primary spermocyte; c, the resting and dividing secondary spermocytes ; d, the spermatid ? What is its probable significance with respect of sex determination ? sex control ? How do the maturation mitoses differ from ordinary somatic cell di- vision? Significance of this difference from the standpoint of inher- itance? Probable significance of the chromosomes? (5) THE SPERMATOZOON (spermozoon; spermium; sperm). Sketch (306) from a stained section under the oil immersion lens a human spermatozoon. Compare with a cover-glass preparation of preserved sperm. Identify: a, head (with perforatorium, consisting of acrosome and galea capitis) ; b, neck; c, middle piece or body (with distal cen- trosome, end ring, central filament and spiral filament) ; d, tail; and e, terminal filament. Compare with spermium of dog and mouse, and some other mammal (e.g., opossum). From what constituents of the spermatid are the several above- enumerated portions of the fully developed spermium derived? Origin, function and fate (in fertilization) of: a, the head; b, the middle piece (with its centrosome and mitochondrial spiral filament) ; and c, the tail? (6) THE INTERSTITIAL CELLS (of Leydig). Identify among the in- ter tubular connective tissue (testicular stroma) the interstitial cells. Sketch (307) several adjacent cells. Origin, structure, probable func- tion and fate? Condition of the interstitial cells, and of the seminal (epithelium, in cryptorchism ? (B) THE DUCT SYSTEM OF THE TESTIS. (1) TUBULI EECTI. Sketch (308) h.p. a segment of the wall of a tubulus rectus at the point of transition to the tubulus contortus. With what cells of the latter tubule are the lining cells of the tubulus rectus homologous? (2) RETE TESTIS. Study l.p. the mediastinum of the testis with its included rete tubules. Sketch (309) a small area. Make h.p. sketch (310) of the lining epithelium. (3) DUCTULI EFFERENTES. Study l.p. a stained section through the efferent ductules of the epididymis. Sketch (311) a small .area. Eelation of the ductuli efferentes to the coni vasculosi and to the 806 DIRECTIONS FOR LABORATORY WORK globus major (caput epididymis). Make h.p. sketch (312) of a segment of the wall, showing the character of the epithelium (with its groups of ciliated and non-ciliated columnar cells), the basement membrane, and the muscular tunica propria. (4) THE EPIDIDYMIS. Make l.p. sketch (313) of a stained sec- tion of this extremely convoluted duct; and a h.p. sketch (314) of a segment of the wall, noting the character of the epithelium, the base- ment membrane, and the muscular tunica propria. How can the duct of the corpus epididymis be distinguished in section from the ductuli efferentes of the caput epididymis? (5) THE DUCTUS DEFERENS (Vas deferens). Study l.p. a stained section of the ductus deferens. Note the tunica mucosa, the lamina pro- pria mucosas and the tunica muscularis. Sketch (315). Make h.p. sketch (316) through the mucosa. How does the epithelial lining differ in the upper, middle and lower portions? How does the tunica muscularis of the lower portion differ from that of the remainder of the duct ? Difference between tunica mucosa and tunica muscularis of the ureter and the ductus deferens? (6) THE SEMINAL VESICLES. Sketch l.p. (317) a segment of the entire wall; and h.p. (318) a small extent of the mucosa. (7) THE EJACULATORY DUCTS. Sketch l.p. (319) a segment of the complete wall; and h.p. (320) a portion of the mucosa. (C) THE SPERMATIC CORD. Study l.p. a stained transverse section of the spermatic cord. Note the following constituents : a, fibrous stroma ; b, ductus deferens ; c, ex- ternal cremaster (striped) muscle; d, internal cremaster (smooth) mus- cle; e, spermatic artery and veins, including f, the pampiniform plexus. Sketch (321). (D) THE SCROTUM. Study l.p. a stained section of the wall of the scrotum. Note rela- tively abundant pigmented cells in stratum germinativum of epidermis; loose corium (derma) with a considerable number of elastic fibers and smooth muscle cells (constituting the dartos). In what respects does the skin of the scrotum differ from ordinary integument ? Sketch (322) . (E) THE GLANDS ASSOCIATED WITH THE MALE GENITAL SYSTEM. (1) THE PROSTATE GLAND. Study l.p. a stained section of the human prostate. Sketch (323) a small area showing several alveoli DIEECTIONS FOB LABOBATOBY WOBK 807 and the abundant fibre-muscular stroma. Contents of the alveoli? Make h.p. sketch (324) of a small extent of the mucosa. Significance of the alveolar content? (2) THE BULBO-URETHRAL GLANDS (Cowpers glands}. Make h.p. sketch (325) of several adjacent alveoli and included fibro-muscular stroma. Character of the muscle content? Sketch h.p. (326) a portion of the alveolar lining. (F) THE PENIS (Phallus). (1) Study a stained transverse section of the human penis, through its middle portion. Note: a, corpus spongiosum (corpus cavernosum urethra) with the urethra; b, urethral glands (glands of Littre) ; c, the two corpora cavernosa (penis) ; d, the enveloping tunica albuginea, and c, the peripheral cutaneous envelope. Sketch (327). Sketch h.p.: a, the lining of the urethra including a urethral gland (328) ; b, a small area of erectile tissue from one of the corpora (329) ; and e, a helicine artery (330). How does the epithelium of the urethra differ in its sev- eral segments, namely the prostatic, the membraneous, the penile, and the glans (fossa navicularis) portions? Sketch l.p. (331) portion of a preputial gland from a section through the corona. Location and significance of Tyson's glands? Note char- acter of the skin of the glans penis. Trace the course of the blood through the penis in the flaccid, and in the erect condition. Nerve supply and nerve end-organs? XVIII. THE FEMALE ORGANS OF REPRODUCTION (Female Genital System) (A) THE OVARY (Ovarium). (1) Study l.p. and h.p. a stained section of the ovary of a full-term fetus or young infant. Note: a, the peripheral 'germinal epithelium', with its larger spherical primordial germ cells (ova) ; b, egg tubes of Pflueger : c, 'egg nests' ; and d, a typical primitive ovarian follicle with its central ovum (primary oocyte) and its enveloping mantle layer. Sketch each under the h.p. (332 a, b, c and d). Derivation of the ovarian germ cells? (2) Study l.p. a stained section of the ovary of an adult. Note: a, the enveloping peritoneal layer ('germinal epithelium') ; b, the periph- eral cortex with its ovarian follicles and the superficial tunica albuginea ; 808 DIRECTIONS FOB LABORATOEY WORK and c, the medulla, coining to the surface at the hilus. Sketch (333). In the cortex note also the corpora lutea and the corpora albicantia. What is their significance? Make h.p. sketches (334, 335) of portions of each. Make h.p. sketch (336) of a typical area of the medulla. Sketch h.p. also: a, a primary ovarian follicle (337) ; b, a follicle at an intermediate stage pf growth (338) ; and c, an older vesicular (Graaf- ian) follicle (339), noting all the constituent elements. Manner of derivation of the secondary oocyte from the primary oocyte of the ves- icular follicle? ootid from secondary oocyte? (3) Study an injected specimen of a cat's ovary. What is remark- able about the arteries? Sketch (340) an area of the medulla, and an adjacent vesicular follicle. (4) Oogenesis. Sketch the several salient stages of oogenesis from a series of demonstration slides of eggs of mouse or starfish: a, unripe (small) ovarian egg (primary oocyte) (314) ; b, ripe (full-grown) primary oocyte (342); c, first maturation spindle (343); d, secondary oocyte with first polocyte (344) ; e, second maturation spindle (345) ; f, ootid or mature ovum with second polocyte (346). (B) THE UTERINE TUBE (Oviduct; Fallopian tube; Salpinx). Make a comparative study of transverse sections of the uterine tube at: a, the isthmus; b, the ampulla; c, the infundibulum, noting differ- ences in the size of the lumen, and the character and constituents of the three tunics (tunicas mucosa, muscularis and serosa). Diagram (347). Sketch h.p. (348) a segment of the mucosa from the isthmic region. Differential features among uterine tube, ductus def erens and the ureter ? (C) THE UTERUS (Metra; Hystera). Compare vertical sections through the body and cervix of the uterus. Sketch l.p. (349) a narrow segment of the entire wall from both regions. Enumerate the chief differences. Sketch h.p. : a, the mucosa of the body (350); b, a cervical gland (351). Significance of non-ciliated areas? How many strata in the tunica muscularis ? Which is the stratum vas- culare? How does the resting uterus differ from the menstruating uterus? from the pregnant uterus? How does the human uterus differ in gross form and with respect of the muscular tunic from that of cer- tain common mammals, e.g., dog, cat, etc.? (D) THE VAGINA. Study l.p. a section from the middle portion of the vagina. Note: a, the tunica mucosa; b, tunica muscularis; c, tunica fibrosa. Sketch BISECTIONS FOB LABOEATOEY WOEK 809 (352). Type of lining epithelium? Are glands present? Whatsis the striking common characteristic of the tunica propria of the entire duct system of the female genital apparatus? (E) THE EXTERNAL GENITALS. These include: a, the vestibule; b, the labia minora; c, the labia ma jora ; d, the clitoris ; e, the hymen ; f , the minor vestibular glands, and g, the major vestibular glands. Enumerate the histologic characteristics of each. What are the male homologues for d, f, and g? The major vestibular glands (glands of Bartholin) open into the groove between the hymen and the labium minus. Sketch h.p. several adjacent alveoli of this gland (353). (F) THE MAMMARY GLAND (Mamma). (1) Study l.p. a section of the active (lactating) gland. Note the arrangement into lobes and lobules. Sketch (354), showing the inter- lobar lactiferous duct and the inter- and intralobular ducts. Make h.p. sketch (355) of an alveolus, noting especially the cytoplasmic contents of the lining cells. Significance of the basket cells? of the basal stria- tions of the secretory cells? (2) Make l.p. study of a resting mammary gland. Sketch (356). (3) Examine a cover-glass preparation of fresh milk stained with methylene blue. Note especially the leucocytes and fat droplets. Sketch (357). Derivation of the leucocytes? (4) Enumerate the histologic features of: a, the mammilla; b, the areola; c, the glands of Montgomery, and d, the integument of the mammae. Character and significance of 'witches milk'? XIX. DUCTLESS GLANDS (Endocrine Glands; Organs of Internal Secretion; Cryptorhetic Glands) (A) THE SUPRARENAL GLANDS (Adrenals). (1) Examine a divided fresh suprarenal gland of some mammal. Note the yellowish coloration of the cortex, and the red color of the medulla; note also the capsule and the hilus. Cause of difference in color between cortex and medulla? (2) Study l.p. stained vertical sections of the human and the dog's adrenal. Chief differences? Sketch (358) a narrow segment from cap- sule to center of medulla. Make h.p. sketches (359) of groups of cells 810 DIRECTIONS FOB LABORATORY WORK from the zonae glomerulosa, fasciculata and reticularis. Enumerate the chief differences in gross arrangement of the cells and in their cyto- plasmic contents. Sketch also (360) a group of cells from the medulla. What is the chief granular content of these cells ? Significance? Sketch also (361) a segment of the central vein. What is remarkable about this vessel ? (3) Study a stained horizontal section through the zona glomerulosa. Arrangement of the glomerular cells? Function of the adrenal? Rep- tilian and ichthyoid homologues? (4) Study an injected specimen of cat's adrenal. Diagram the ar- terial blood system (362). Relation of the cells to the venous capil- laries ? Significance of the separate blood supply of the cortex and the medulla ? (B) THE THYROID GLAND. Study l.p. a stained section of the human thyroid gland. Note : cap- sule, lobules, follicles and colloid content of latter. Sketch (363) a small area including the capsule and the interfollicular stroma. Sketch h.p. (364) a folicle, noting especially the two types of lining cells: a, chief cells; b, colloid cells. What are the striking characteristics of the colloid content of the follicle ? Cellular inclusions in the colloid ? From an injected specimen of the cat's thyroid diagram the blood supply of a lobule (365). What morbid condition is associated with atrophy of the thyroid? with hypertrophy ( hyper seer etion )? with arrested development? Nor- mal function? How does aberrant or accessory thyroid tissue differ from that of the thyroid proper? What are the salient differential characteristics of sections of the thyroid, active mammary gland and prostate gland? (C) THE PARATHYROID GLANDS. Make h.p. sketch (366) of a small area of a stained section of a parathyroid (epithelial body). Note the 'acidophiF and 'principal' types of cells. Does colloid material occur? Spatial and functional relation- ship between thyroid and parathyroids. Function of the parathyroids? (D) THE THYMU8 GLAND. Study l.p. a stained section of the human thymus. Note: capsule, lobules (with cortex and medulla) and interlobular connective tissue DIRECTIONS FOB LABOBATOEY WOEK 811 septa. Sketch (367) several adjacent lobules. Note continuity of medullary substance between adjacent lobules. Make h.p. sketch of a lobule (368) ; also of a thymic corpuscle (concentric corpuscle of Has- sall) (369). Where are the latter located? Origin and function of the thymic corpuscles? Is the thymus a lymphoid organ or an endocrin (cryptorhetic) gland? Ontogenetic history of the thymus? From an injected specimen of cat's thymus, sketch (370) the blood supply of a lobule. t (E) THE CAROTID GLAND. Location, origin and probable function" of this gland ? Sketch h.p. (371) a small area. Compare with the Coccygeal Gland and the Para- ganglia. Origin and function of the latter? (F) THE HYPOPHYSIS CEREBRI (Pituitary Gland). In a stained sagittal section of the hunlan hypophysis identify: a, pars neuralis (posterior lobe) ; b, pars juxtaneuralis (intermediate por- tion) ; c, residual lumen; d, pars distalis (anterior lobe). Make h.p. sketches (372) of a small area from each of these portions. In the pars distalis note the cords of central neutrophilic and parietal eosinophilic cells. Significance of these two types? Over the periphery of the pars distalis and lining the residual lumen note the basophilic cells. Signifi- cance? What are the 'chromophobe' cells? Origin and probable function (normal and morbid) of this gland? Blood and nerve supply? (G) THE EPIPHYSIS CEREBRI (Pineal Body). Study l.p. a stained sagittal section of the adult sheep's pineal body. Note : pineal stalk with its pineal recess ; capsule, 'lobes', trabeculae, and acervulus (brain sand). Compare with the human pineal body. Sketch h.p. (373) a small area near the periphery. Note the two chief types of cells: neuroglia and 'interneurogliar'. Significance of the two types? Note also the intercellular clumps of pigment, and the lamellated and conglomerate types of acervulus, and the larger edematous areas. Com- pare with a similar section of this gland from a young sheep. In the latter note the intracellular pigment granules and the intercellular cysts. Origin and probable function (normal and morbid) of this gland? Blood and nerve supply? Significance of the melanic pigment? of the cysts ? of .the brain sand ? 812 DIRECTIONS FOB LABORATORY WORK XX. THE EYE (Bullus Oculi, and Appendages: Palpebrce and Lacrimal Glands) (A) MACROSCOPIC STUDY. (1) With the aid of a mirror, sketch your eye (374). Note: ar, supercilium (eye-brow) ; b, superior and inferior palpebrae ( eye-lids ), with their cilia (eye-lashes) at the base of which internally may be seen a row of whitish pits, the orifices of the tarsal (Meibomian) glands; c, the medial and lateral anguli. oculi and the canthi (commissures) ; d, in the depth of the medial angle (the lacus lacrimalis) a reddish rough- ened area, the caruncula lacrimalis ; e, at the base of this angle the plica semilunaris (homologue of nictitating membrane or third eye-lid of birds, reptiles, etc.), and f, at either basal angle of the internal canthus an elevated papilla lacrimalis, with its punctum lacrimalis leading to the canaliculus lacrimalis and the naso-lacrimal duct. Note also the fine hairs on the caruncula. Eetract the lower lid and note the con- junctive palpebrae; its continuity with the conjunctiva oculi across the fornix conjunctivas can readily be observed. Within the rima palpebrarum, the opening between the lids, note:: a, the pupil; b, the iris; and c, the overlying transparent cornea, con- tinuous peripherally with the opaque sclera. What is the pinguecula? 1 (2) In a formalin-preserved specimen of the eye-ball of sheep or ox,, dissect out carefully the six extrinsic eye-muscles. Note the point and manner of insertion of the muscles and the relation of the superior and 1 inferior obliques to the superior and inferior recti muscles respectively.. The levator palpebrae superioris muscle may also have remained intact; also a muscle attached by a stout tendon to the well-developed carti- laginous plica semilunaris, the musculus plicae. In the eyes of ruminants an additional muscle occurs, not present in the human eye, the retractor: bulbi ; this large stout muscle completely envelops the optic nerve. Sep- arate the retractor bulbi muscle from the optic nerve down to the point: of exit of the nerve from the bulb. By noting the position of the point: of exit with respect to the posterior pole of the bulb, and the relation of this point to the plica semilunaris and levator palpebrae muscle, deter- mine which eye you have been dissecting. Sketch anterior and posterior poles (375). (3) Bisect the eye-ball in the horizontal meridian. (This should be done with a sharp razor, proceeding from the posterior pole, first divid- DIRECTIONS FOB LABORATORY WORK 813 ing the optic nerve.) Examine under water. Note: cornea, iris, crys- talline lens, suspensory ligament (zonula ciliaris of Zinn) of4errs, an- terior and posterior chambers with their content of aqueous humor; the ciliary body with the ciliary folds and processes; the ora serrata; the vitreous humor (body) ; the retina; the tunica choroidea; the sclera; the lamina fusca and the lamina suprochoroidea between the separated por- tions of the choroid and sclerotic coats ; the macula lutea with the f ovea centralis; the optic papilla (disc) with the central depression (physio- logic excavation); hyaloid canal; and the hyaloid membrane. Sketch (376). Significance of the hyaloid canal (canal of Cloquet; canal of Stilling) ? Eemove the lens. Note the difference in curvature of its anterior and posterior surfaces. Eemove the anterior lens epithelium, and the lens capsule. Remove the more superficial layer of lens fibers and free the central non-nucleated lens 'nucleus'. Explain the arrangement of the lens fibers, with relation to each other, the 'nucleus', and the poles of the lens (lens 'stars'). Sketch (377). How is the lens held in place ? Describe the action of the lens. What is a cataract of the eye? Difference between the anatomic axis of the bulb and the visual axis? Embryonic origin of the several tunics and humors of the bulbus oculi, and of the lens? What is the capsule of Tenon? (B) MICROSCOPIC STUDY. (1) Study l.p. a stained horizontal section of the human eyeball (or that of cat, dog or monkey). (The bulb should have been preserved in Perenyi's fluid,* which softens the lens). Sketch (378). Note the difference in curvature between the cornea and the posterior scleral seg- ment of the bulbus oculi; also between the anterior and posterior sur- faces of the lens. Identify the several chambers, angles and tunics of the bulb, and the several laminae of each tunic. Note the disposition of the nuclei in the lens. Enumerate and identify the intrinsic muscles of the eye. Explain their nerve supply. Explain the action of the ciliary body. Function of the aqueous chambers and the canal of Schlemm (sinus venosus sclerse) ? Relation to glaucoma? Define astigmatism. (2) Study h.p. the sclero-corneal junction. Identify all of its con- * Perenyi 's 'fluid : 4 parts 10 per cent nitric acid 3 parts alcohol (95%) 3 parts 0.5 per cent chromic acid. Fix 5 to 10 hours, and pass directly into 70% alcohol. 814 DIRECTIONS FOR LABORATORY WORK stituent elements, and note its relationship to adjacent structures. Sketch (379). Function of ligamentum pectinatum and the included spaces of Fontana? (3) Sketch h.p. (380) the cornea. Identify its five layers. Inner- vation of the corneal conjunctiva? Function of the membrane of Des cemet? Changes in cornea after death or fixation? (4) Sketch h.p. (381) the sclera. Location and character of the lamina fusca? (5) Sketch h.p. (382) the tunica choroidea. Note the several lam- inae: suprachoroidea, vasculosa, tapetum fibrosum (cellulosum), chorio- capillaris, and lamina basalis (vitrea). Function of the choroid? (6) Sketch h.p. (383) the equatorial margin of the crystalline lens, including the zonula ciliaris of Zinn (suspensory ligament) with its spacia zonularis (canals of Petit). Note the lens capsule, lenticular epithelium, and the substantia lentis composed of nucleated lens fibers., with serrated margins. (7) Sketch h.p. (384) a segment of the iris. Note: a, its external layer of mesenchymal epithelium ('endothelium') ; b, its fibrous stroma, with its pigmented cells; c, the internal epithelial layer (pars iridica retinae); d, the stromal recesses; e, the dilator and sphincter muscles; and f, the circuli major and minor. Origin and function of the iridal muscles? Relation of pigmented stroma to 'color of eye'? Effect of certain drugs upon the size of the pupil? (8) Sketch h.p. (385) the pars optica retina, or the retina proper, at a point about midway between the macula lutea and the ora serrata. Relation to pars ciliaris retinae and the pars iridica retinae? Identify the eleven different layers. Compare this portion of the retina with that near the fovea centralis and that near the ora serrata. Probable function of: a, the pigment (fuscin) of the pigmented layer; b, the visual purple (rhodopsin) of the rod visual cells; and c, the myoid element of the rods and cones ? Explain the inversion of the retina. Explain coloboma and scotoma. (9) In a Golgi preparation of the retina identify the different neu- rons of the several layers and determine their interrelations (synapses). Construct a diagram (386) showing the interrelationships of the ele- ments of the different layers, including the neuroglia supporting cells of Miiller. (10) (a) Study an injected specimen of cat's eye. Note the dis- tribution of the branches of the central artery of the retina, the short ciliary arteries, the long ciliary arteries, and the anterior ciliary arteries, DIRECTIONS FOE LABORATORY WORK 815 especially with respect to the uvea or complete middle tunic. Do these several arterial systems anastomose ? Note also the radicles of the Tena vorticosae. Sketch (387) lateral half of bulbus oculi. (b) Trace out also the lymphatic systems of the eye. (C) THE OCULAR APPENDAGES. (1) THE PALPEBILE (eye-lids). Study l.p. a stained vertical section of the human superior eye-lid. Note: cutaneous surface, rimal margin and the conjunctival surface; the orbicularis palpebrarum mus- cle, insertion of the musculus levator palpebrae superioris, the superior palpebral muscle of Muller, the ciliary muscle of Kiolan, and the tarsal fascia (tarsus) ; the cilia (eye-lashes), sebaceous glands of Zeiss, glands of Moll, tarsal glands of Meibon, and the glands of Waldeyer (posterior tarsal glands) ; the primary and secondary tarsal (arterial) arches. Sketch (388). Make h.p. sketch (389) of the palpebral conjunctiva near the level of the fornix conjunctivas. (2) THE LACRIMAL GLAND. Study a stained section of the lacrimal gland. Sketch l.p. (390) a lobule, showing the ducts and the secre- tory tubules. Make h.p. sketches (391, a, b, etc,) of sections of tubules at several stages of secretory activity. Where are the accessory lacrimal glands of Krause? Location and significance of the gland of Harder? XXI. THE EAR (A) THE EXTERNAL EAR. (1) AURICLE (Pinna). Study l.p. a stained section of the lobule of the ear, including adjacent portion of the pinna. Note the absence of cartilage in the lobule proper, and the abundant adipose tissue. Sketch (392). Make h.p. sketch (393) of portion of the elastic carti- lage of the pinna. Note the relatively large number of cells. (2) Study l.p. a stained section of the cartilaginous portion of the External Acoustic Meatus. Note the abundant large stiff hairs, seba- ceous glands, and ceruminous glands. Sketch (394). Sketch h.p. (395) a transverse section of the secretory portion of the coiled, simple tubular, ceruminous gland. Enumerate differences and resemblances between the ceruminous glands, the sudoriparous glands, and the mammary glands. (B) THE MIDDLE EAR (Tympanum). (1) Study l.p. a stained radial section of the tympanic membrane. Note: annulus tympanicus; umbo, with attached manubrium of the 816 DIEECTIONS FOE LABOEATOKY WOEK malleus; membrana flaccida (Shrapnell's membrane)? and cutaneous layer, mucous layer, and intermediate radial and circular collagen fibers. Sketch (396). Sketch (397) h.p. a small segment of the tympanic membrane near the umbo* Function of tympanic membrane? Relation to the auditory ossicles? Structure of the ossicles? Relation of tym- panum to the mastoid 'cells'? to the auditory tube? (2) THE AUDITORY TUBE (Eustachian Tube). Study a stained transverse section of the cartilaginous portion of the tuba auditive Note character of the lining epithelium, and the fibro-cartilaginous wall. Relation of cartilage plate to medial and lateral surfaces, and to the tensor and levator palati muscles. Sketch (398). Function of the auditory tube ? Position and significance of the tubal tonsil ? Function of the musculus tensor palati in relation to the auditory tube? (C) THE INTERNAL EAR (1) VESTIBULE (Utriculus and Sacculus). Study a stained sec- tion of the wall of the utricle or saccule through the macula. Note char- acter of the lining epithelium. Sketch h.p. (399) a segment of the macula. Function of the otolithic membrane? (2) SEMICIRCULAR DUCTS (Canals). Study h.p. a stained trans- verse section of a membraneous semicircular duct. Compare with a section through the ampulla. Sketch segments of the lining epithe- lium (400), and of the crista ampulla? acustica including the overlying cupola (401). (3) COCHLEA. Study l.p. a stained axial section of the cochlea of some mammal (cat; rabbit). Sketch a complete vertical section of a turn of the cochlea (402). Identify and label all the included parts. Function of the vestibular (Reissner's) membrane? tectorial membrane? basilar membrane ? Shambaugh's glands ? the stria vascularis ? vas prom- inens ? Make h.p. sketch (403) of the spiral organ (of Corti), identifying the various cells involved. Study an injected specimen of the cat's cochlea. Diagram (404) the blood supply of a turn of the cochlea. Describe the nerve supply of the membraneous labyrinth of the in- ternal ear. Where is the internal acoustic meatus? Function of the utriculus? sacculus? semicircular canals? cochlea? DIRECTIONS FOB LABORATORY WORK 817 XXII. THE NERVOUS SYSTEM (A) THE SPINAL CORD (Medulla spinalis). (1) Study l.p. a stained (preferably by the Weigert-Pal technic) transverse section of the spinal cord in the lower cervical region, includ- ing a spinal ganglion. Compare with the upper cervical and thoracic levels. Note the enveloping membranes (spinal meninges) : dura mater, arachnoidea, and the pia mater; and their continuity with the connective tissue envelopes of the spinal nerve. "Sketch (405) a segment through these membranes with the included lymph spaces. Note the greater vascularity of the pia mater spinalis. Note further the compressed oval shape of the lower cervical section, and its division into symmetrical halves by the dorsal longitudinal septum and the ventral longitudinal fissure; also the central canal; the central gray matter arranged in the form of an H with its dorsal, ventral, and lateral horns and their con- stituent cell groups ; the peripheral white substance, divisible into dorsal, lateral, and ventral columns; the gray and white commissures; the sub- stantia gelatinosa centralis, and the gelatinous substance of Rolandi ; the formatio reticularis; the cell column of Clarke; nucleus of Stilling; the connective tissue and neuroglia septa of the white substance; and the dorsal and ventral roots of the spinal nerves. Sketch (406). Describe the structural differences between gray, white, gelatinous, and reticular substances. Compare the lower cervical with the lumbar and sacral sec- tions. Differences? Development of the spinal medulla? Define tract (fasciculus), column (funiculus), and nucleus (of origin and termina- tion) of the spinal cord. Define: conus medullaris, filum terminate, and the cauda equina. Describe the relations to each other of the sensory, motor and association neurons in the spinal cord. (2) Study l.p. an injected specimen of the spinal cord of the cat. Note the five longitudinal arterial trunks in the pia mater spinalis : the larger antero-median artery, and the more slender postero-lateral longi- tudinal arterioles (arranged anteriorly and posteriorly to the line of entrance of the posterior nerve roots). Note their further penetration and distribution within the gray and white substances. (Sketch (407). Location, structure and function of the septum posticum, the liga- menta denticulata and the subarachnoid spaces? origin of cerebrospinal fluid? Trace the course of the lymph circulation through the central nervous system. 818 DIRECTIONS FOR LABORATORY WORK (B) THE CEREBELLUM. (1) Study l.p. a stained parasagittal section of a hemisphere of the human cerebellum. Note the arrangement into lobules composed of folia. Note also that each folium comprises a central medulla and a peripheral cortex. In the cortex identify the superficial molecular layer, the deeper nuclear or granular layer, and between the two the inter- mediate layer of Purkinje cells (commonly classified as the innermost stratum of the molecular layer). Sketch (408). Make h.p. sketch (409) of a Purkinje cell. i (2) From a specially prepared demonstration slide of the cerebellum (Golgi technic) make a diagram (410) showing the location and inter- relations of the constituent cells and fibers: In the molecular layer, the large ('basket') and small cortical cells; the Purkinje cells; and in the nuclear layer, the granule cells, eosin bodies, large stelate cells, and the solitary cells. In the medulla note the axons of the Purkinje cells, the mossy fibers and the climbing fibers. What is the significance of the eosin bodies? Eelation between granule cells and mossy fibers? How does a sagittal section differ in appearance from a frontal section, with respect to the Purkinje cells and the large cortical (basket) cells? Note also the types and disposition of the neuroglia cells. (C) THE CEREBRAL CORTEX (Pallium). (1) Study l.p. a stained (hematoxylin and eosin) vertical section of the human cerebral cortex in the motor area (precentral gyrus). Note the arrangement of the cells into five tangential strata : molecular layer, outer polymorphous cell layer, small pyramidal cell layer, large pyramidal cell layer, and inner polymorphous cell layer. Sketch (411). Compare with similar sections from the parietal lobe (a sensory area), the visual area of the occipital lobe, the auditory area of the temporal lobe, and the olfactory area of the hippocampal gyrus. Note also the cerebral meninges (dura mater, arachnoidea, and pia mater) and the enclosed lymph spaces. Describe the granulationes arachnoidales (Pacchionian bodies; arachnoidal villi). (2) From a Golgi preparation of the motor cortex, sketch (412) a segment showing the character and arrangement of the neurons and the neuroglia elements. Identify a cell of Betz; a cell of Martinotti. Sig- nificance of these cells? Distribution of the 'solitary cells' of Meynert? (3) From a Weigert-Pal preparation of the motor cortex sketch (413) the arrangement of the nerve fibers. Note the bands of radial DIRECTIONS FOR LABORATORY WORK 819 fibers alternating with the interradial feltwork; the supraradial felt- work; and the tangential fiber strata: the stratum zonale, the stripe of Bechterew, and the outer and inner stripes of Baillarger. Relation of the tangential stripes to the cell strata ? Compare with a similar prep- aration of the visual cortex. Define the stripe of Gennari. INDEX Abiogenesis, 17 Absorption, bone, 83 Accessory suprarenal, 556 Accessory thyroid, 562 Acervulus, 584 Aceto-carmin stain for tissues, 744 Acid dyes, 740 Acidophil cells, of small intestine, 371 of pituitary body, 576 Acoustic meatus, external, 683 Acoustic nerve, 712 cochlear branch of, 712 spiral ganglion of, 712 vestibular branch of, 712 vestibular ganglion (of Scarpa),712 Acromegaly, 573 Adenoid tissue, 84 Adipose tissue, 61 * Adrenal (suprarenal) gland, blood sup- ply of, 554-555 cells of medulla, 553 chromaffin granules in, 554 lymphatics and nerves of, 556 zona fasciculata of, 552 zona glomerulosa of, 552 zona reticularis of, 553 Adrenalin (epinephrin), 549-554 Adrenin, 549 Agminate nodules of small intestine (Peyer's patches), 366 Alcohol for fixation of tissues, 724 Alimentary canal, fibroserous coat of, 345 glands of, (crypts of Lieberkiihn), 366 ileocecal valve of, 379 intestinal villi in, 366 intestine, large, 377 appendices epiploicaa of, 377 Alimentary canal, intestine, large, lymphoid tissue and lymph nodes of, 378 mucous membrane of, 377 plicae semilunares of, 377 rectum of, 379 sacculations or haustra of, 377 teniae (linese) coli, 377 vascular and nerve supply of, 379 vermiform appendix of, 378 intestine, small, 375 absorption from, 375 agminate nodules of (Peyer's patches), 366 blood supply of, 373 corium of, 366 duodenal glands of (Brunner's glands), 365-372 granules of Paneth in, 370 intestinal glands of, 370 intestinal villi in, 368 lacteals of, 369-374 lining epithelium of, 369 lymphatics of, 374 mucous membrane of, 366 nerve supply of, 374 Peyer's patches in, 366 solitary nodules of, 366 structure of, 363 submucosa of, 364 valvulae conniventes of, 364 mucous membrane of, 347 muscular coat of, 346 submucous coat of, 346 stomach, blood supply of, 360 cardiac glands of, 360 fundus glands of, 354 lenticular glands of, 360 lymphatics of, 362 821 822 INDEX Alimentary canal, stomach, mucous coat of, 353 muscular coat of, 353 nerve supply of, 362 oxyntic or delomorphous cells of, 356 peptic or adelomorphous cells of, 355 parietal cells of, 356 pyloric glands of, 357 secretion of, 357 serous coat of, 352 submucous coat of, 353 Alimentary tract, tabular statement of characteristics, 381 Alum carmin for staining tissues, 744 Alum hematein (Mayer) for staining tissues, 741 Amebpid mobility, 16 Ameloblasts (adamantoblasts), 333 Amitosis, 19 Anaphase, 25 Anatomy, microscopic, 1 Angioblast, 215 Annulus tympanicus of ear, 685 Anterior chamber of eye, 636 Appendix epididymis, 502 Appendix testis, 502 Appendix vermiformis, 378 Aqueductus Fallopii, relations of, to ear, 686 Aqueous humor of eye, 665 .Arachnoid membrane, 620 granulations, 621 villi, 621 Arciform veins of kidney, 443 Areas of Cohnheim in muscle, 112 Areolar connective tissue, 56 Arterial wall, general characteristics of, 180 Arteries, 176 anterior ciliary, of eye, 673 anterior spinal, 624 arciform, of kidney, 440 arteria centralis of retina, 664, 670 arterise propriss renales, 440 arteriae rectge of kidney, 441 Arteries, atypical, structure of, 182 auditory, 714 bronchial, 317 central, of retina, 664, 670 circle of Willis, of brain, 624 circle of Zinn, of eye, 672 cortical, of cerebrum, 625 external elastic membrane of, 179 fissural, of brain, 625 helicine, of corpus cavernosum penis. 504 hyaloid, of eye, 672 interlobular, of kidney, 441 of liver, 415 internal elastic membrane of, 178 large, structure of, 181 large and small compared, 184 long ciliary, of eye, 672 medium-sized, coats of, 177 medullary, of cerebrum, 625 middle cerebral, 625 pulmonary, 314 structure of, 183 precapillary, structure of, 182 short-ciliary, of eye, 672 small, structure of, 181 tunica adventitia of, 177-178 tunica intima of, 177 tunica media of, 177-178 umbilical, structure of, 182 vasa vasorum of, 180 Arterioles, structure of, 181 Atrioyentricular bundle of His, 102 Auerbach's fuchsin methyl green stain, 756 Auditory area of cerebral cortex, cell layers of, 618 Auditory canal, 683 Auditory ossicles, 689 Auditory tube of ear, 691 Auerbach's stain for chromatin, 756 Auricle or pinna of external ear, 682 Axis cylinder, the, 132 Axon, the, 128 Balsam (Canada) for mounting sec- tions, 760 INDEX 823 Bartholin's glands of the vulva, 541 Basic dyes, 740 Basket cells of cerebellum, 608 Basophilic cells of pituitary body, 576 Bellini's ducts of kidney, 438 Benda's technic for demonstrating mitochondria, 757 Berlin blue gelatin mass for injection of tissues and organs, 732 Betz cells of cerebral cortex, 613 Bibliography of histologic technics, 761 Bioplasm, 1 Bladder, urinary, 448 Blastomeres, 27 Blood, 203 angioblast, 215 coagulation of, 214 development of, 24 fibrin of, 203 hematin of, 212 hematoidin, 214 hemin, 212 hemoglobin of, 203-211 hemokonia of, 206 hemolysis of, 206 hemosiderin, 213 hypertonic solution for, 206 hypotonic solution for, 206 isotonic solution for, 206 leukoblasts in, 218 mononuclear leukocytes in, 218 oxy hemoglobin, 211 prothrombin, 203' Teichmann's crystals in, 212 thrombin, 203 Blood cells, basophils, 210 crenation of, 206 dyes for, 210 eosinophils, 210 erythroblasts, 217 erythrocytes, 203-217 erythroplastids, 203-217 giant, 209-218 granulocytes, 209 hyaloplasm of, 206 ichthyoid stage, 217 leukocytes, 203 Blood cells, lymphocytes, 207 megaloblasts, 217 megalocytes, 209 neutrophils, 209 non-granular leukocytes, 208 normoblasts, 217 plastids, 203 plates (plaques), 209 polykaryocytes, 209 primitive, derivatives from, 219 red, 203 rouleaus, 206 sauroid stage, 217 spindle cells, 209 spongioplasm of, 206 thrombocytes, 209 transitional leukocytes, 208 white, 207 Blood clot, 203 Blood corpuscles, 203 crenated corpuscles, ,206 Blood crystals, 212 Blood plasma, 203 Blood platelets, 203-209 Blood serum, 203 Blood shadows, 206 Blood stain, eosinate of methylene blue, Hastings' method, 754 Blood vascular system, 176 Blood vessels, circulus major of iris, 672 circulus minor of iris, 673 development of, 192 essential structure of, 194 fetal, 538 of brain and spinal cord, 619-624- 625 of bronchi, 317 of eye, 636-643-664-670-673 of eyelids, 678 of heart, 201 of internal ear, 714 of kidney, 440-444 of large intestine, 379 of liver, 415-417 of lungs, 314 of lymph nodes, 242 824 INDEX Blood vessels, of mammary gland, 547 of middle ear, 693 of olfactory mucosa, 298 of ovary, 522 of oviduct, 527 of pancreas, 403 of parathyroid glands, 565 of penis, 503-504 of prostate gland, 500 of renal pelvis and ureter, 448 of salivary glands, 392 of skin, 290 of small intestine, 373 of spleen, 246 of stomach, 360 of suprarenal gland, 554 of thymus gland, 568 of thyroid gland, 560 of tongue, 343 of urinary bladder, 452 of uterus, 532 Bohmer's hematoxylin for staining tis- sues, 741 Bone, 72 absorption of, 83 cancellous, 85 canalieuli of, 75 circumferential lamellae of, 76 compact, 74 development of, 79 diploe of, 88 endochondrial ossification, 85 epiphyseal line, 85 epiphyseal ossification, 85 external circumferential lamellae of, 84 general considerations of, 72 Haversian canals of, 85 Haversian lamellae of, 75 Haversian spaces of, 84 Haversian systems of, 75-84 internal circumferential lamellaB of, 85 interstitial lamellae of, 76-85 intramembranous ossification, 86 lacunae of, 75 lacunae of Howship, of, 84: Bone, lymphatics of, 78 nerves of, 78 osteoblasts of, 83 osteoclasts of, 77 periosteum of, 74 primary, 83 red marrow of, 77 tables of, 88 Volkmann's canals of, 76 Bone marrow, 77-220 Bone marrow cells, basophil granulo- cytes, 223 eosinophil granulocytes, 222 erythrocytes, 224 giant cells, 223 hemoblasts, 221 large mononuclear leukocytes, 222 lymphocytes, 221-222 mast cells, 223 mesameboid cells, 221 myeloblasts, 221 myelocytes, 220 myeloplaxes, 223 polymorphonuclear neutrophil gran- ulocytes, 222 premyelocytes, 221 primitive blood cells, 221 Bony labyrinth of the internal ear, 694 Borax carmin stain for tissue, 743 Bouin's fluid for fixation of tissues, 729 Bowman's elastic membrane of cornea, 631 Box for embedding in paraffin, 737 Brain sand (acervulus), 584 Bronchi, 304 structures of, 304 Bronchial arteries, 317 Bronchioles, 307 Brownian movement, 17 Bursae, 117-233 CajaPs method for demonstrating neurofibrils, 750 Calcified cartilage, 80 Canal of Stilling (canalis hyaloideus), 672 INDEX 825 Capillary plexuses, 187 Capillary wall, stigmata of, 186 Capillaries, structure of, 184 Cardiac glands of the stomach, 360 Cardiac muscle, 94, 99 Cardiac muscle, intercalated discs of, Zimmerman 's method of staining, 758 Cardiac Purkinje fibers, 102 Cardiac plexus, origin of, 591 Carmin gelatin mass for injection of tissues and organs, 732 Carmin stain for tissues, 743 Carney's fluid No. 1 for fixation of lung tissue, 729-730 Carotid gland, the, 569 Cartilage, 67 blastema of, 69 calcified, 80 cells, enlargement of, 80 elastic, 71 fibro-, 71 hyaline, 68 matrix of, 69 origin of, 70 primordial marrow cavities in, 82 Cell, definition of, 1 Cell layers of motor area of cerebral cortex, 615, 616 Cell plate (midbody), 25 Celloidin, adhesive for sections, 739 embedding of tissues in, 734 Cells, acidophil, of parathyroid, 564 acidophilic, of pituitary body, 576 acidophil, of small intestine, 371 amitosis of, 19 anaphase of, division of, 25 archoplasm of, 7 astral rays of, 7 astral system of, 7 . basophilic, of pituitary body, 576 basket, of salivary glands, 385 basophil granules of, 52 Betz, of cerebral cortex, 613 cartilage, enlargement of, 80 cementum, of teeth, 337 Cells, of cerebral cortex, Golgi's, types 1 and 2, 613 centro-acinal, of Langerhans, 396 centriole of, 7 centrosome of, 7 centroplasm of, 7 centrosphere of, 7 chondrioconts of, 8 chondriomites of, 8 chromaffin of small intestine, 371 chromaffin, of suprarenal gland, 548 chromidia of, 8 chromophobe, of pituitary body, 576 chromosomes of, 24 cilia of, 16 close spireme, 24 connective tissue, 51 cytology of, 1 cytoplasm of, 7 decidual, of gravid uterus, 535 deutoplasm of, 8 diplosome of, 7-24 division of, 18 eosinophil granulocytes, 52 ependyma, of nervous system, 589 general statements in regard to, 5 hepatic, 412 pigment in, 414 goblet, of small intestine, 370 granular, of cerebellum, 609 granule of Paneth in small intestine, 370 kinoplasm of, 7 lamellar, 52 of Langhans, of chorionic villi of gravid uterus, 537 large stellate, of cerebellum, 609 loose spireme of, 24 lutein, of ovary, 519 of Martinotti of cerebral cortex, 617 mastoid, of ear, 686 metaphase, of division, 25 metaplasm of, 8 microsome of, 8 mitochondria of, 8 mitosis of, 22 nucleus of, 6 826 INDEX Cells, neuroglia, of nervous system, 589 osteoblasts, 77 osteoclasts, 78 parietal (oxyntic or delomorphous), of stomach, 356 peptic or adelomorphous, of stom- ach, 355 pheochrome, of suprarenal gland, 548 pigment, 52 plasma of, 51 plastosomes of, 8 prickle, 44 pseudochromosomes of, 8 Purkinje, of cerebellum, 607 pyramidal, of cerebral cortex, 613 reproduction of, 17 resting nucleus of, 24 segmented spireme of, 24 Sertoli, of seminiferous tubules, 481 small cortical, of cerebellum, 608 solitary, of Meynert of cerebral cor- tex, 613-618 spindle, 51 telophase of, division of, 25 tendon, 117 interstitial, of Leydig of testis, 487 trophospongium of, 8 varieties of, 32 vital properties of, 14 Cement substance, intercellular, 30 nitrate of silver stain for, 752 terminal bars of, 34 Cementoblasts, 337 Cementum, development of, 336 Centers of ossification, 79 Central artery of retina, 664 Central nervous system, blood supply of, 624 veins of, 625 Central spindle, 25 Cerebellum, 603 basket cells of, 608 climbing fibers of medulla of, 609 cortex of, 606 granule cells of, 609 Cerebellum, large stellate cells of, 609 medulla of, 609 Purkinje cells of, 607 resume of structures of, 611 small cortical cells of, 608 Cerebral cortex, 611 auditory area, cell layers of, 618 cell layers of motor area, 615, 616 cells (solitary) of Meynert, 613-618 fiber tracts of, 618 Golgi's types of cells of, 613 motor area of, 615 motor area, cells of Martinotti of, 617 nerve cells of, 612 olfactory area, cell layers of, 618 stripes of Baillarger, 619 stripe of Bechterew, 619 visual area, cell layers of, 617 Cerebrum, cortical arteries of, 625 lobes of, 611 medullary arteries of, 625 meninges and blood supply of, 619- 620-621 middle cerebral artery of, 625 Cervical glands of uterus, 532 Ceruminous glands of ear, 684 Chondrioconts, 50 Chondroclasts, 82 Chordae tendineae of heart, 197 Chorionic villi, 537 Choroid coat of eye, 636 lamina basalis of, 637 lamina capillaris of, 636 membrane of Bruch, 637 suprachoroid layer of, 636 tapetum cellulosum of, 638 tapetum fibrosum of, 637 tunica Euyschiana of, 637 vascular layer of, 636 Choroidal fissure of the eye, 645 Chromaffin cells of small intestine, 371 of suprarenal gland, 548 Chromaffin granules, technic for dem- onstrating, 759 Chromaffin system, 571 Chromatin, Auerbach's fuchsin methyl- green stain for, 756 INDEX 827 Chromidia, in spermatogenesis, 474 Chromo-acetic formalin mixture for fixation of tissues, 727 Chromophobe cells of pituitary body, 576 Chromosomes, 24-476 sex, in spermatogenesis, 475 in spermatogenesis, 464 Chyle, 226 in small intestine, 374 Cilia, 16 Ciliary body of eye, 638 fibrous layer of, 640 Ciliary epithelium of eye, 640 Ciliary glands of eye, 640 Ciliary motility, 16 Ciliary processes of eye, 638 Ciliary muscle of eye, 638 Circle of Willis, 604 Circle of Zinn, 672 Circulus venosus of Haller, 547 Ciliated epithelium, 40 Clarification of sections, 759 Classification of dyes, 740 Clitoris, 541 Coagulation of blood, 214 Coccygeal gland, 570 Cochlea of internal ear, accessory tec- torial membrane of, 706 basilar membrane of, 706 cochlear artery of, 714 function of, 716 hamulus of, 701 Hensen's stripe of tectorial mem- brane in, 706 limbus spiralis of, 705 membrana tectoria (membrane of Corti), 705 membranous wall of scala tympani and scala vestibuli, 704 organ of Corti, 708 varieties of cells of, 708-709 prominentia spiralis of, 705 spiral ligament of, 704 stria vascularis of, 705 structure of, 701 sulcus spiralis externus of, 705 Cochlea of internal ear, sulcus spiralis internus of, 706 vestibular membrane of Eeissner, 704 Collagen, 57 Colloid in thyroid gland, 557 Coloboma, eye, 646 Colostrum, 546 Columnar carnse, 198 Comparison of artery with vein of cor- responding size, 79 Concentric corpuscles (Hassall) of thymus, 567 Congo red for tissue stain, 746 Conjunctiva, ocular, 674 palpebral, 674 Conarium (pineal body), 579 Connective tissue, Mallory's stain for, 753 Conus medullaris, 600 Corium of skin, 268 Cornea of eye, 630 anterior homogeneous membrane in, 631 Bowman's elastic membrane of, 631 endothelium of, 633 epithelium of, 631-633-674 layers of, 631 membrane of Descemet, 632 posterior epithelial layer of, 633 posterior homogeneous membrane of, 632 vascular and nerve supply of, 633 Corneal corpuscles, 631 Corneal epithelium, 631-674 Corneal substance, 631 Corpora albicantia, of ovary, 510 Corpora lutea spuria, 521 Corpora lutea vera of pregnancy, 521 Corpus cavernosum penis, 503 helicine arteries of, 504 pectiniform septum of, 504 tunica albuginea of, 504 Corpus hemorrhagicum of ovary, 519 Corpus Highmori testis, 480 Corpus luteum of ovary, 519 Corpus spongiosum penis, 503-506 828 INDEX Corpuscles, Pacinian, 174 Cover glass, how to apply, 722 Cowper's glands of urethra, 501 Cretinism, 561 Crusta petrosa of teeth, 329 Crystalline lens of eye, 666 capsule of, 666 lenticular epithelium of, 666 nuclear zone of, 667 nucleus of, 667 substantia lentis, 666 suspensory ligament of, 669 Cutaneous appendages, 271 Cutis vera, 268 Cytogenesis, 1 Cytologic stain, Ehrlich triacid, 755 Cytology, 1 Cytolymph, 8 Cytomorphosis, 29 Cytoplasm, 7 Cytoplasmic dyes, 745 Cytoplasmic granules, eosin and methyl blue mixture (Mann), stain for, 755 Cytoreticulum, 8 Dartos of scrotum, 481 Decalcification of tissues, 731 Decidua reflexa of the gravid uterus, 537 Decidua vera of the gravid uterus, 537 Delafield's hematoxylin for staining tissues, 742 Dendrons (dendrites), 128 Dental cement, 329 Dental cuticular membrane, Dental enamel, layers of, 335 Dental groove, 330 Dental lamina, 330 Dental papilla, 330-336 Dental pulp, 323 odontoblasts in, 324 Dentin, 325 contour lines of Owen in, 327 globules in, 326 granular layer of Tomes in, 327 incremental lines of Schreger in, 327 Dentin, interglobular spaces of, 326 sheaths of Neumann in, 327 tubules in, 326 Derma, 268 Descemet's membrane of cornea, 632 Determiners, chromosomal in sex hered- ity, 477 Digestive system, 320 alimentary canal, character of wall in, 344 fibroserous coat of, 345 mucous membrane of, 347 muscular coat of, 346 submucous coat of, 346 esophagus, coats of, 348 glands of, 351 mucous coat of, 350 muscular coat of, 350 ileocecal (colic) valve, 379 intestinal glands (crypts of Lieber- kiihn), 366 intestine, large, 377 appendices epiploica? of, 377 lymphoid tissue and lymph nodes of, 378 mucous membrane of, 377 plicae semilunares of, 377 rectum, 379 sacculations or haustra of, 377 teniae (lineae) coli, 377 vascular and nerve supply of, 379 vermiform appendix, lymphoid tis- sues in, 378 intestine, small, absorption from, 375 agminate nodules of (Peyer's patches), 366 blood supply of, 373 corium of, 366 duodenal glands (Brunner's), 365- 372 granule cells of Paneth in, 370- 371 intestinal glands of, 370 intestinal villi in, 368 lacteals of, 369-374 INDEX 829 Digestive system, intestine, small, lin- ing epithelium of, 369 lymphatics of, 374 mucous membrane of, 366 nerve supply of, 374 Peyer's patches, 366 solitary nodules of, 366 structure of, 363 submucosa of, 364 valvulae conniventes of, 364 liver, bile capillaries of, 411 blood supply of, 417 capsule of Glisson of, 408 cells of, 412 course of blood through, 419 gall-bladder, 420 hepatic connective tissue in, 408 hepatic lobule, 409 interlobular arteries and veins of, 415 interlobular bile ducts, 415 lymphatics of, 419 nerves, of, 420 pigment cells in, 414 portal canals, 408-415 structure of, 405 mouth, 320 lymphoid tissues in mucous mem- brane of, 321 mucous membrane of, 320 secreting glands in mucous mem- brane of, 321 pancreas, 394 acini of, 396 blood supply of, 403 nerve supply of, 404 resume, 404 pharynx, 347 portal vein, 417 salivary glands, 382 blood supply of, 392 general considerations of, 382 mucous acini of, 386 nerve supply of, 393 parotid gland, 390 serous acini, 385 sublingual gland, 392 Digestive system, salivary glands, sub- maxillary gland, 391 stomach, 360 blood supply of, 360 cardiac glands of, 360 fundus glands of, 354 lenticular glands of, 360 lymphatics of, 362 mucous coat of, 353 muscular coat of, 353 nerve supply of, 362 peptic or adelomorphous cells of, 355 parietal cells (oxyntic or delomor- phous cells) of, 356 pyloric glands of, 357 serous coat of, 352 secretion of, 357 submucous coat of, 353 teeth, 337 ameloblasts (adamantoblasts) ofj 333 cementoblasts of, 337 cementum, 329 development of, 336 deciduous, 331 papilla of, 336 pulp of, 323 dentin of, 325 development of, 330 enamel of, 328 enamel germ of, 330-331 enamel layers of, 335 enamel organ of, 330 layers of, 331 Tome's processes in, 333 odontoblasts, 324 odontoclasts, 337 permanent, 331 structure of, 323 tongue, 337 blood vessels of, 343 foramen cecum of, 343 lymphatics of, 344 mucous membrane of, 337 muscle fibers of, 337 nerve supply of, 344 830 INDEX Digestive system, tongue, papillas of, 339, 340, 341 taste buds of, 341 tonsil of, 343 Diploe, of bone, 88 Diploid number of chromosomes, 363 Diplosome, 24 Double staining of tissues, 746 Ductless glands, 260-548 hormones of, 586 Ductuli aberrentes, mesonephric, 502 Duodenal glands (Brunner's), 365- 372 Dura mater, 620 Dyes, 740 acid, 740 basic, 740 classification of, 740 cytoplasmic, 745 mordants for, 741 neutral, 740 specific, 740 Dyspituitarism, 573 Ear, 717 development of, 717-719 external, 682 acoustic meatus of, 683 auricle of, 682 ceruminous glands of auditory ca- nal, 684 pinna of, 682 internal, 712 acoustic nerve, 712 cochlear branch of, 712 vestibular ganglion of Scarpa, 712 aqueductus vestibuli of, 694 auditory artery, 714 blood supply of, 714 bony labyrinth of, 694 canalis communis of semicircular canals, 699 cochlea, 706 accessory tectorial membrane of, 706 basilar membrane of, 706 Ear, internal, cochlea, cecum vestibu- lare of scala media, 703 cochlear artery, 714 cochlear nerve, spiral ganglion of, 712 cupola of, 701 development of, 719 foramina nervosa of, 702 function of, 716 hamulus of, 701 helicotrema of, 703 Hensen 's stripe of tectorial mem- brane in, 706 lagena or cecum cupulare of scala media in, 703 limbus spiralis of, 705 membrana tectoria, membrane of Corti, 703-705 membranous wall of scala tym- pani and scala vestibuli, 704 organ of Corti, 708, 709-710 organ of Corti, cells of, 708-709 prominencia spiralis of, 705 scala media, or cochlear duct, 703 scala tympani, 701 secondary tympanic membrane of, 703 scala vestibuli of, 701 spiral ligament of, 704 spiral ligament, stria vascularis of, 705 spiral organ of Corti, 702 structure of, 701 sulcus spiralis externus, 705 sulcus spiralis internus, 703-706 vestibular membrane of Eeissner, 703-704 vestibular artery, 714 cupola of semicircular canals in, 700 ductus endolymphaticus, 694 endolymph of, 695 endolymphatic sac, 694 fenestra vestibuli, 693 general considerations of, 693 internal auditory vein, 716 INDEX 831 Ear, internal, cochlea, lymphatics of, 716 macula acustica sacculi, 697 macula acustica utriculi, 698 membranous labyrinth, 694 neuro-epithelium of saccule, 695 neuro-epithelium, varieties of, 697 organ of equilibration, 700 otoliths in, 697 otocyst, 699 perilymph of, 695 saccule of, 694 * sacculus of vestibule in, 694 semicircular canals of, 699 ampullae of, 699 utricle of, 698 utriculosaccular canal of vestibule, 694 utriculus of vestibule, 694 veins of, 715-716 vestibulei of, 693 middle, 686 aqueductus Fallopii, relations of, 686 annulus tympanicus, 685-687 auditory canal, 683 auditory ossicles of, 689 auditory tube (Eustachian tube), 691 blood vessels of, 693 epitympanic cavity of, 685 Eustachian tube, 691 fenestra ovalis of tympanum, 686 fenestra rotunda of tympanum, 685 general considerations of, 685 incus, 689 orbicular process of, 689 lymphatics of, 693 malleus, 689 ligaments of, 691 mastoid cells, 686 membrana flaccida of, 689 Prussak's space, 691 Shrapnell's membrane, 689 stapedius muscles of, 690 stapes, 689 tensor tympani muscle, 690 Ear, middle, tympanic membrane, 687 tympanic mucosa, 686 tympanum, 685 pelvis ovalis of, 686 processus cochleariformis of, 690 promontory of, 685 tubal tonsil of, 692 umbo of tympanic membrane in, 687 Ectoderm, 28 Ehrlich's triacid stain, 755 Ejaculatory ducts, 497 Elastic fibers, 57 Elastic tissue, Weigert's stain for, 753 Elastin, 58 Eleidin, 45 Embedding, in paraffin, box for, 737 of tissues, 734 in celloidin, 734 in paraffin, 735 Embryonic connective tissue, 50-53 Enamel, 328 contour lines of Eetzius in, 329 prism, 328 radial lines or prism stripe of Schreger in, 329 Enchylema, 8 Endocardium, 195-196 Endocrin glands, 260-548 hermones of, 586 Endomysium, 195 Endothelium, 37 Enlargement of cartilage cells, 80 Entoderm, 28 Eosin, as a tissue stain, 745 and methyl blue mixture (Mann), stain for cytoplasmic gran- ules, 755 Eosinate of methylene blue stain ( Hasting 's method) for blood, 754 Ependyma cells, 141-589 Epicardum, 195 Epicritic sensibility, 164 Epidermis, 262 Epididymis, 492 832 INDEX Epidural space, 620-623 Epinephrin (adrenin), 549 Epiphysis cerebri, 579 blood supply of, 584 development of, 580 function of, 581 habenular commissure of, 581 histologic structure of, 582 nerves of, 584 posterior commissure of, 581 Epithelia, classification of, 34 Epithelial tissues, 30-33 Epithelium, 40 ciliated, 40 endothelial, 37 glandular, 40 goblet cell, 41 mesothelial, 37 modified columnar, 40 neuro-, 42 non-stratified, 37 pavement, 37 plain, 39 pseudo-stratified columnar, 46 simple columnar, 39 simple squamous, 37 stratified, 43 stratified squamous, 43 transitional, 47 varieties of, 33 Epitympanic cavity of ear, 685 Epoophoron (parovarium), of female genitalia, 539 Equatorial plate, 25 Eruptive tissue, of developing bone, 82 Esophagus, 348 coats of, 348 glands of, 351 mucous coat of, 350 muscular coat of, 348 submucous coat of, 348 Euparal, for mounting sections, 74 Eustachian tube, 691 Exophthalmic goitre, 561 External ear, 682 External genitals, female, 540 clitoris, 541 External genitals, female, glandulso vestibulares m a j ores (Glands of Bartholin), 541 glandulae vestibulares minores, 541 hymen, 541 labia majora, 540 labia minora, 540 vestibule, 540 External genitals, male, 503 penis, 503 corpora cavernosa of, 503-504 corpus spongiosum urethras of, 506 helicine arteries of, 504 lymphatics of, 506 nerves of, 506 nervi erigentes of, 506 pectin if orm septum of, 504 preputial glands of, 506 tunica albuginea of corpus caverno- sum, 504 Tyson's glands of, 506 Eye, anterior chamber of, 636-643 appendages of, 674 aqueous humor of, 665 blood supply of sclerocorneal junc- tion, 636 blood vessels of, 670 canalis hyaloideus, or canal of Stil- ling, 672 canal of Schlemm (sinus venosus soleras), 635 capsule of Tenon, 627 choroid coat of, 636 lamina basalis of, 637 lamina capillaris of, 636 membrane of Bruch, 637 .tapetum cellulosum of, 638 tapetum fibrosum of, 637 tunica Euyschiana of, 637 vascular layer of, 636 ciliary arteries, anterior, 673 long, 672 short, 672 ciliary body of, 638 fibrous layer of, 640 ciliary epithelium, 640 ciliary glands, 640 INDEX 833 Eye, ciliary muscle, 638 ciliary processes, 640 circle of Zinn, 672 coats of, 628 conjunctiva, 674 ocular, 674 palpebral, 674 contents of, 665 cornea, 630 anterior epithelium of, 674 anterior homogeneous membrane of, 631 Bowman's elastic membrane of, 631 corpuscles of, 631 endothelium of, 633 epithelium of, 631 layers of, 631 membrane of Descemet, 632 posterior epithelial layer of, 633 posterior homogeneous membrane of, 632 substance of (substantia propria), 631 vascular and nerve supply of, 633 crystalline lens of, 666 capsule of, 666 lenticular epithelium of, 666 nuclear zone of, 667 nucleus of, 667 substantia lentis, 666 suspensory ligament of, 669 development of, 645 choroidal fissure, 645 optic cup, 645 optic stalk, 645 optic vesicle, 645 external or fibrous tunic of, 629 general considerations of, 626 gland of Harder, 681 glaucoma, 635 hyaloid artery of, 672 hyaloid membrane of, 669 iridocorneal angle, 643 iris, 641 circulus major of, 672 circulus minor of, 673 coloboma of, 646 49 Eye, iris, color of, 642 dilator muscle of, 642 endothelium of, 641 fibrous stroma of, 642 internal epithelium of, 643 layers of, 641 sphincter muscle of, 642 lacrimal gland, 679 lacrimal lake, 679 ligamentum pectinatum of, 635-643 lymphatic systems of, 673 middle coat of, vascular tunic, 636 muscae volitantes, 669 nerves of, 674 optic nerve, 664 optical or visual axis, 628 ora serrata, 665 pineal, 580 posterior chamber of, 636-643 retina of, 644 cell types of inner nuclear layer of, 654-655-656 central artery of, 664-670 cones of, 651-652-653 external limiting membrane of, 653 fibers of Miiller of, 658 fiber layer of Henle, 654 fovea centralis of, 660 ganglion cell layer of, 657 general considerations of, 644 inner nuclear layer of, 654 inner reticular layer of, 656 inversion of, 663 layers of, 646 macula lutea of, 659 nerve fiber layer of, 657 optic papilla of, 665 optic disk of, 665 outer nuclear layer of, 654 outer reticular layer of, 654 pigment epithelium of, 646 porus opticus of, 665 rods of, 648-649-650 rods and cones, development of, 661-663 layer of, 647 supporting tissues of, 658 834 INDEX Eye, retina of, visual purple of, 647- 649 yellow spot of, 659 sclerocorneal junction of, 635 sclerotic coat of, 634 corpuscles of, 634 lamina fusca of, 634 lamina cribrosa of, 634 pinguecula of, 634 substantia propria of, 634 spaces of Fontana of, 635-643 spatia zonularis of (canals of Pe- tit), 670 suprachoroid layer of, 636 uvea of, 636 uveal tract of, 636 vascular tunic of, 636 venae vorticosae of, 672 vitreous humor of, 669 zonula ciliaris of Zinn, of, 670 Eyelid, 674 accessory lacrimal glands of Krause of, 677 blood supply of, 678 cutaneous portion of, 674 conjunctival portion of, 676 glands of Moll of, 676 glands of Zeiss of, 676 lymphatic and nerve supply of, 678 palpebral muscle of Miiller, 677 posterior tarsal glands of Waldeyer, 677 tarsal (Meibomian) glands, 676 tarsus of, 676 Fallopian tube, 525 blood supply of, 527 mucosa of, 526 lymphatics and nerves of, 528 muscular wall of, 526 serous coat of, 526 structure of, 525 Female reproductive organs, 507. See also Eeproductive system. Fat, osmic acid test for, 758 stains for, 62, 759 Fat tissue, 61 Fibers, collagenous, 49 elastic, 49-57 white, 49 Fiber tracts of cerebral cortex, 618. "See also Nervous system. Fibrocartillage, 71 Filar mass, 8 Filum terminale of spinal cord, 600. See Nervous system. Flagellate motion, 16 Flagellum, 16 Flemming's fluid for fixation of tis- sues, 728 Fontana, spaces of, 635. See also Eye. Formalin, for fixation of tissues, 725 Fovea centralis of retina, 660. See also Eye. Fresh tissues, examination of, 720 Fuchsin for tissue stain, 746 Gage's method for demonstrating glycogen, 750 Gall-bladder, 420. See also Digestive system. Gametogenesis, 460. See also Eepro- ductive system. Ganglia, 148. See also Nervous sys- tem. Gastric glands, 354. See also Digestive system. Gigantism, 573 Gilson's fluid for fixation of tissues, 730 Glands, 253 accessory lacrimal, of Krause, 677 adrenal, 548 lymphatics and nerves of, 556 anterior lingual (of Nuhn), 342 branched saccular, 260 branched tubular, 258 bulbo-urethral, 501 cardiac, of stomach, 360 carotid, 569 ceruminous, of ear, 684 ciliary, of eye, 640 cervical, of uterus, 632 classification of, 253 INDEX 835 Glands, coccygeal, 570 compound saccular, 260 compound tubular, 258 compound tubulo-alveolar, 258 convoluted tubular, 257 Cowper's, of urethra, 501 ductless, 260-548 hormones of, 586 duodenal (Brunner's), 365-372 endocrin, 260-548 hormones of, 586 histologic types of, 254 internal secretion of, 261 intestinal (crypts of Lieberkiihn), 366 of small intestine, 370 lacrimal, 679 lenticular, of stomach, 360 mammary, 541 active, 543-545 blood vessels of, 547 milk, 547 of Montgomery (areolar glands of Duval), 545 Marchand's (Marchand's adrenal), 557 Meibomian (tarsal), of eyelid, 676 mucous, 255 mucus of, 256 of Bartholin ( Bartholin 's glands of vulva), 541 of esophagus, 351 of eyelid (glands of Moll), 676 of Harder, 681 of tongue, 342 of Tyson, of penis, 506 of von Ebner, 343 of Zeiss, of eyelid, 676 parathyroid, acidophil cells of, 564 blood supply of, 565 structure of, 562 parotid, 390 physiologic types of, 254 pineal, 579 posterior tarsal, of Waldeyer, 677 preputial, 506 prostate, 497 Glands, pyloric, 357 racemose, 258 secreting, of mouth, 321 suprarenal, 548 chromaffin granules in, 554-555 development and function of, 548 lymphatics and nerves of, 556 medulla, cells of, 553 zona glomerulosa of, 552 zona reticularis of, 553 zona fasciculata of, 552 salivary, 382 serum-secreting cells of, 256 simple saccular, 259 simple tubular, 257 submaxillary, 391 thymus, 565-567 thyroid, blood supply of, 560 colloid in, 557 development of, 561 follicles of, 557 follicular epithelium of, 559 function of, 561 lymphatics of, 560 nerves of, 561 structure of, 557 urethral (Littre's), 454 uterine, 531 Glandulas vestibularis, majores (glands of Bartholin), 541 minores of vulva, 541 Glandular epithelium, 40 Glaucoma, 635 Glia cells, 141 Glomus caroticum, 569 Glomus coccygeum, 570 Glycerin jelly, for mounting sections, 760 Glycogen, Gage's method for demon strating, 758 Goblet cell epithelium, 41 Goitre, 561 exophthalmic, 561 Gold chlorid for demonstrating nerve plexuses and nerve endings (Kanvier's method), 752 836 INDEX Golgi's cells of cerebral cortex, types 1 and 2, 613 Golgi's stain for nerve cells, 751 Graafian follicle, 517 Graafian follicle, Kesume of structures, 519 Graves' disease, 561 Guard cells, 38 Gum-damar, for mounting sections, 760 Hair, 277 development of, 277 regeneration of, 286 root of, 281 root sheaths of, 283 shaft of, 281 structure of, 277 Haploid group of chromosomes in spermatogenesis, 464 Hardening of tissues, 733 Hasting 's method for blood (eosinate of methylene blue stain), 754 Haversian spaces in bone, 84 Haversian systems of bone, 75-84 Heart, 195 annulus fibrosus of, 197 atrioventricular bundle of, 199 blood vessels of, 201 chordae tendinese of, 197 columnae carnae of, 198 development of, 201 endocardium of, 196 endomysium of, 195 epicardium of, 195 moderator bands of, 199 myocardium of, 195 nerve supply of, 202 valves of, 196 venae minimae of, 201 Heat, fixation of tissues by, 730 Kelly's fluid for fixation of tissues, 728 Hemal nodes, 239 Hematein, for staining tissues, 741 stains, application of, 743 and eosin stains for tissues, 746 Hemoglobin, 211 Hemolymph nodes, 239 Hemopoiesis, 214 Heterozygote, 478 Histogenesis, 1-27 Histologic technic, 720 bibliography of, 761 Histology, definition of, 1 historical development of, 2 relations of, to other biological sci- ences, 2 Homozygote, 478 Hone for microtome knife, 737 Howship, lacunae of, 84 Hyaline cartilage, 68 Hyaloid artery, of eye, 672 Hyaloid membrane, of eye, 669 Hyaloplasm, 8 Hydatid of Morgagni, 539 Hymen, 541 Hypopituitarism, 573 Hypophysis cerebri, 572-573-574-575 function of, 573 histologic structure of, 573-574-575 pars buccalis seu glandularis, 575 pars neuralis; infundibulum, 575 parts of, 575 Eathke's pouch, 572 Ileocecal (colic) valve, 379 Incus, of ear, 689 Injection of tissues, 732 pressure required for, how obtained, 733 with Berlin blue gelatin mass, 732 with carmin gelatin mass, 732 Intercalated discs of cardiac muscle, 99 Internal ear, acoustic nerve, 702 aqueductus vestibuli of, 694 auditory artery of, 714 blood supply of, 714 bony labyrinth of, 694 canalis communis of, 699 cecum vestibulare of scala media, 703 cochlea, accessory tectorial membrane of, 706 basilar membrane of, 706 function of, 716 foramina nervosa of, 702 INDEX 837 Internal ear, cochlea, hamulus of, 701 helicotrema of, 703 Hensen's stripe of tectorial mem- brane, 706 limbus spiralis of, 705 membrana tectoria (membrane of Corti) of, 703-705 membranous wall of scala tympani and scala vestibuli, 704 organ of Corti, cells of, 708-709 prominentia spiralis of, 705 spiral ligament of, 704 structure of, 701 sulcus spiralis externus of, 705 sulcus spiralis internus of, 703 vestibular membrane (of Keiss- ner), 704 cochlear artery, 714 eochlear nerve, spiral ganglion of, 712 ductus endolymphaticus of, 694 endolymph of, 695 endolymphatic sac of, 694 fenestra vestibuli of, 693 general considerations of, 693 helicotrema, 703 internal auditory vein, 716 lagena or cecum cupulare of scala media, 703 lymphatics of, 716 macula acustica sacculi, 697 macula aeustica utriculi, 698 membranous labyrinth of, 694 neuro-epithelium, of saccule, 695 varieties of, 697-708-709 organ of equilibration, 700 otocyst, 699 otoliths in, 697 perilymph of, 695 saccule of, 695 sacculus of vestibule, 694 scala media or cochlear duct of, 703 scala tympani of cochlea, 701 scala vestibuli of cochlea, 701 secondary tympanic membrane of cochlea, 703 Internal ear, semicircular canals of, 699 ampullas of, 699 spiral organ of Corti, 702 utricle of, 698 utriculosaccular canal of vestibule, 694 utriculus of vestibule, 694 veins of, 715-716 vestibular artery, 714 vestibular ganglion (of Scarpa), 712 vestibular membrane (of Eeissner), 702 vestibule of, 693 Internal genital organs, female, 507. See also Reproductive sys- tem. Interrenal bodies, 548 Interstitial granules of Kolliker, 99 ' Interfilar mass, 8 Intestine, large, 377 appendices epiploicge of, 377 lymphoid tissue and lymph nodes of, 378 mucous membrane of, 377 plicge semilunares of, 377 rectum, 379 sacculations or haustra of, 377 taenise coli (lineaa coli), 377 vascular and nerve supply of, 379 vermiform appendix of, 378 Intestine, small, absorption from, 375 acidophil cells of, 371 agminate nodules of (Peyer's patches), 366 blood supply of, 373 chromaffin cells of, 371 chyle in, 374 corium of, 366 duodenal glands (Brunner's), 365- 372 glands of, 370 glands (crypts of Lieberkiihn) of, 366 granule cells of Paneth, 370-371 lacteals of, 369 838 INDEX Intestine, small, lining epithelium of, 369 lymphatics of, 374 mucous membrane of, 366 myenteric ganglionic plexus (Auer- bach's), 375 nerve supply of, 374 Peyer's patches of, 366 solitary nodules of, 366 structure of, 363 submucosa of, 364 submucous plexus (Meissner's), 375 valvulee conniventes of, 364 villi of, 366-368 Intracartilaginous ossification, 79 Intra vitam method of staining non- medullated nerve fibers with methylene blue, 749 Iridocorneal angle, 643 Iris, 641 cireulus major of, 672 circulus minor of, 673 color of, 642 dilator muscle of, 642 endothelium of, 641 fibrous stroma of, 642 internal epithelium of, 643 layers of, 641 sphincter muscle of, 642 Iron hematoxylin, for staining tissues, 747 Joints, 88 adaptation cartilages of, 89 diarthroses, 88 intra-articular menisci, 88 labra glenoidalia, 89 sutura, 88 synarthroses, 88 synchondroses, 88 syndesmoses, 88 synovia, 89 synovial membrane, 89 Karyon, 18 Keratin, 45 Kidney, 423 ' Kidney, arched collecting tubule of (junctional tubule), 437 arciform arteries of, 440 arciform veins of, 443 arteries, interlobular, of, 441 arteriolae rectae of, 441 ascending limb of Henle 's loop, 435 Bellini's duct of, 438 blood vessels, lymphatics and nerves of, 440 capsule of Bowman, 429 corpuscles in, 425 cortex of, 424 descending limb of Henle 's tubule, 434 distal convoluted portion of tubule in, 436 divisions of the renal tubule, 429 glomerulus of, 429 loop of Henle, 435 lymphatics and nerves of, 444 medulla of, 424 neck of tubule of, 431 papillary ducts of, 438 proximal convoluted portion of tu- bule of, 431 renal connective tissue, 426 renal corpuscles (Malpighian body), 425-429 renal lobule, 426 renal pelvis and ureter, 445 blood supply, lymphatics and nerves of, 448 mucosa of, 445 muscular coat of, 448 tunica propria of, 447 straight collecting tubules of, 437 table of divisions of renal tubule, 439 table showing the course of the renal circulation, 443 topography of, 423 tubules, peculiarities of, 440 urinif erous, or renal tubules of, 427 venae propriae renales, 443 veins, stellate of, 443 Kleinenburg 's fluid, for fixation of tis- sues, 729 INDEX 839 Knife, microtome, hone for, 737 Kolliker's muscle columns, 112 venulae of, 443 Labia majora, of pudendum, 540 Labia minora, of pudendum, 540 Lacrimal gland, 679 Lacrimal lake, 679 Lacteals, of small intestine, 369-374 Lactiferous duct, of mammary gland, 542 Lamellae, external circumferential, of bone, 84 internal circumferential, of bone, 85 interstitial, of bone, 85 Larynx, 300 structures of, 300 vocal cords of, 301 Lenticular glands, of stomach, 360 Ligamentum denticulatum, of spinal cord, 621-624 Ligamentum pectinatum, of eye, 643 Ligaments, 117 circular dental, 329 Lingual glands, anterior, 342 Lingual tonsil, 244-343 Littre's glands, urethral, 454 Liver, 405 bile capillaries of, 411 blood supply of, 417 capsule of Glisson, 408 cells of, 412 connective tissue of, 408 course of blood through, 419 interlobular arteries and veins in, 415 interlobular bile ducts of, 415 lobule of, 409 lymphatics of, 419 nerves of, 420 pigment in, 414 portal canals of, 408-415 portal vein of, 417 structure of, 405 Loose spireme, of cell nucleus, 24 Lumbar region, of spinal cord, 600 Lung, 304 atria of, 310 blood supply of, 314 lobule of, 314 lymphatics of, 318 nerve supply of, 319 pulmonary alveoli of, 311 tissue fixation of, 730 Lutein cells, of ovary, 519 Lymphatic system, 225 amygdalae (faucial tonsils), 242 bursae, 233 cisterna chyli, 230 chyle, 226 hemolymph, or hemal nodes, 239 lymph, 225 lymph cells, 235-238 lymph corpuscles, 225 lymph follicles, 233 lymph glands, 235 lymph nodes, 235 blood vessels of, 239 development of, 242 structure of, 236 lymph nodules, 233 lymph sinus, 236 lymph vessels, development of, 230 lymphatic capillaries, 227 lymphatic vessels, 226 lymphatics, of adrenal gland, 556 of bone, 78 of eye, 673 of eyelid, 678 of internal ear, 716 of kidney, 444 of large intestine, 378 of liver, 419 of lung, 318 of mammary gland, 547 of middle ear, 693 of penis, 506 of salivary glands, 393 of small intestine, 374 of stomach, 362 of thyroid gland, 560 of tongue, 344 lymphoid tissue, 64 840 INDEX Lymphatic system, marrowlymph glands, 240 spleen, 245 blood vessels of, 246 development of, 250 differentiation of, from lymph node, 249 functions of, 249 pulp veins of, 248 splenic cells, 249 splenic nodule (Malpighian cor- puscle), 247 splenolymph glands, 240 thoracic duct, 230 tonsils, faucial, 242 lingual, 244 palatine, 242 pharyngeal, 244 salivary corpuscles of, 243 tubal tonsil (of Gerlach), 692 Malleus, of ear, 689 ligaments of, 691 Mallory's connective tissue stain, 753 Mammary glands, 541 active gland, 543 blood vessels of, 547 circulus venosus of Haller, of, 547 colostrum, 546 lactiferous duct of, 542 lymphatics and nerves of, 547 milk, 547 resting gland, 545 Mann's acid hematein (Ehrlich's Hematoxylin) stain, for tis- sues, 742 Marchand's glands (Marchand's adre- nal), 557 Marginal velum, 590 Marrow, of bone, 77 blood supply of, 78 cavities, primordial, in, 82 primary osteogenic, 82 red, 77 Mastoid cells, of ear, 686 Maturation, of germ cells, 460 Mayer 's albumin, for fastening sections to slide, 739 Medullary sheath, 134 Meibomian glands, of eyelid, 676 Meissner's submucous plexus, 347 Membrana propria, 34 Membrane, mucous, of large intestine, 377 mucous, of small intestine, 366 mucous, of mouth, 320 synovial, 89 Membranous labyrinth, of internal ear, 694 Meninges and blood supply of cere- brum, 619-620-621 Mendelism, 477 Mercuric chlorid, for fixation of tis- sues, 725 Mercuro-nitric mixture, for fixation of tissues, 730 Mesenchyma, 50 Mesoderm, 28 Mesothelium, 37 Mesovarium, 507 Metabolism, 14 Metaphase, of mitotic cell division, 25 Methyl blue and safranin stain, for tissues, 747 Methyl green stain, for tissues, 743 Methylene blue, for chromophilic (tigroid) granules in cyton and dendrons, 750 stain for non-medullated nerve fibers (intra vitam method), 749 tissue stain, 743 Meves' method for staining mitochon- dria, 757 Microtome, for sectioning tissues, 736 Microtome knife, for sectioning tis- sues, 737 hone for, 737 use of strop for, 738 Middle ear, 685 blood vessels of, 693 general considerations of, 685 lymphatics of, 693 Milk, 547 INDEX 841 Mitochondria, 8-50 Mitochondrial technics, Meves' method, 757 ' Benda's method, 757 Mitome, 8 Mitosis, 22 Montgomery 's glands, of mammae, 545 Molecular motility, 17 Monaster stage of mitotic cell division, 25 Mordants, for dyes, 741 Morphogenesis, 29 Motor area, of cerebral cortex, 615 Mounting sections, 759 euparal for, 761 glycerin jelly for, 760 gum-damar for, 760 neutral balsam for, 760 xylo-balsam for, 760 Mouth, 320 lymphoid tissue of, 321 mucous membrane of, 320 secreting glands of, 321 Muchematin stain, for tissues, 748 Mucicarmin stain, for tissues, 748 Mucous membranes, 251 of alimentary canal, 347 of mouth, 320 structure of, 251 Mucous tissue, 54 Mucus, 256 formation of, 41 Miiller's fibers, of retina, 658 Miiller's solution, for fixation of tis- sues, 726 Miillerian ducts, 456 Muscse volitantes, 669 Muscle, striped, 112 areas of Cohnheim, 112 as a whole, 112 blood supply of, 114 cardiac (involuntary striped), 90-94, 99 atrio-ventricular bundle of His, 102 intercalated discs in, 99, 104 intercalated discs (Zimmermann's technic for demonstrating), 758 Muscle, cardiac, nerve endings in, 175 Purkinje fibers of, 102 contraction bands of, 93 endomysium of, 112 epimysium of, 112 fasciculus of, 112 histogenesis and structure of, 91 inokommata of, 98-104 interstitial granules of Kolliker, 99 Kolliker's columns, 112 mesophragma of, 98 myoblasts, 90 myofibrils, 90 myochondria of, 92 myofibrils of, 104 myoglia of, 93 myotomes, 106. nerve endings in, 114-115, 171, 175 of heart, 94-99 perimysium of, 112 red, 110 sarcolemma of, 92 sarcomeres of, 98-104 sarcoplasm of, 90 sarcosomes, of Retzius, 99 sarcostyle of, 104 skeletal, 90 smooth (involuntary; unstriped), 90, 91 histogenesis, and structure of, 91 myofibrils, 93 nerve endings in, 175 situation of, 94 structure of border fibrils (myog- lia) in, 93 telopragma of, 98 types of, 90 unstriped, 90 voluntary striped, 104 (skeletal), 90 Muscle spindles, 172 Muscular contraction, 111 Myelospongium, of developing spinal cord, 509 Myenteric plexus, and ganglion, 346 Myoblasts, 90 S42 INDEX Myocardium, 195 Myofibrils, 90 Myxedema, 561 Nabothian follicles, of uterus, 532 Nails, 274 structure of, 275 Nasal cavity, 293 blood vessels of, 298 neuro-epithelium of, 297 olfactory portion of, 296 respiratory portion of, 294 vestibule of, 293 vomero-nasal organ of Jacobson, 295 Nasopharynx, 299 Nerve cell, 121 apyknomorphous condition of, 124 association neurons, 131 axis cylinder process of, 128 axon, 125 axon hillock, 129 axoplasm, 129 cellulifugal process of, 130 chromophilic substance of, 123 collaterals of, 129 commissural neurons, 131 cytochromatin of, 123 cytoplasm of, 121 dendrons (dendrites) of, 127-128 end arborization of, 129 ganglia, 148 Golgi's cells, type 1 (Deiter's cells), 131 type 2, 131 implantation cone of, 129 mitochondria in, 125 neuraxis, 128 neuraxon, 128 neurite, 128 neuro-epithelium, 42 neurons, 125 Nissl's substance of, 123 nucleus of, 121 origin of, 131 processes of, 127 projection neurons, 131 protoplasmic processes of, 128 Nerve cell, pyknomorphous condition of, 123 size of, 131 spinal ganglia, 150 telodendrion of, 129 tigroid substance of, 123 Nerve endings, 159 circumgemmal fibers in taste buds, 163 in connective tissue, 163 corpuscles of Grandry, 169 corpuscles of Herbst, 169 end bulbs of Krause, 174 end fibrils, 159 epicritic sensibility of, 164 epithelial cells in, 159 genital corpuscle, 166 Golgi end organs, 174 Golgi-Mazzoni corpuscles, 169 gustatory cells, 161 gustatory organs, 160 intergemmal fibers, 163 intragemmal fibers, 163 Key-Retzius corpuscles, 169 lamellar (Pacinian) corpuscles, 167 Merkel's corpuscles, 169 motor end plates, 171 muscular, 171 muscle spindles, 172 neuro-epithelium, 160 neuromuscular end organs, 171 neuromuscular spindles, 172 neurotendinous end organs, 174 Pacinian corpuscles, 167-174 peripheral end organs, 159 protopathic sensibility of, 164 Ruffini's end organs, 165 sole plate, 172 sustentacular cells, 161 tactile cells, 159 tactile corpuscles, 163 tactile meniscus, 160 taste buds in, 160 tendon in, 171 tendon spindles, 174 terminal cylinders, 165 touch corpuscles of Meissner, 163 INDEX 843 Nerve endings, trefoil plates, 159 Vater's corpuscles, 167 Vater-Pacinian corpuscles, 167 Nerve fibers, 132 axis cylinder of, 132 axolemma of Kiihne, 134 axon fibrils, 134 axoplasm of, 134 endoneurium of, 132-147 fiber bundles, 139 funiculi of, 139 glia fibers, 144 incisures of Schmidt, 135 internodal segments of, 135 medullary segments of, 136 medullary sheath of, 132-134 medullated, 132 with a neurolemma, 132 without a neurolemma, 137 myelin of, 135 sheath of, 132-134 neuroglia, occurrence of, in, 145 neurolemma of, 136 neurokeratin of, 135 neuroplasm of, 134 nodes of Eanvier, 132 non-medullated, 132 with a neurolemma, 137 without a neurolemma, 137 nucleated sheath of Schwann of, 136 Remak's fibers, 137 Schmidt-Lantermann lines in, 135 sheath cells of, 137 sheath of Henle, 132-147 sympathetic nerve fibers, 137 telodendrions of, 140 terminal arborizations of, 140 tracts, 139 trophic center of neuron, 137 ultimate fibrillse of, 133 Wallerian degeneration of, 140 white substance of Schwann, 132- 134 Nerve trunks, funiculus of, 146 lymphatic vessels of, 147 nervi nervorum, 148 nervi erigentes, of penis, 506 Nerve trunks, perineurium of, 146 structure of, 146 vascular supply of, 147 Nervous system, 587 blood supply of, 624 cerebellum, 603 basket cells of, 608 climbing fibers of medulla, 609 cortex of, 606 granule cells of,- 609 large stellate cells of, 609 medulla of, 609 Purkinje cells of, 607 resume of structures of, 611 small cortical cells of, '608 cerebral cortex, 611 arteries of, circle of Willis, 624 cortical, 625 fissural, 625 medullary, 625 middle cerebral, 625 terminal, 625 auditory area of, cells of, 618 cell layers of, 612 cells, Betz, of, 613-616 giant pyramidal, 613 Golgi's, types 1 and 2, 613 solitary, of Meynert, 613 frontal lobe of, 617 granule cell layer of, 617 inner large pyramidal cell layer of, 617 inner polymorphous cell layer of, 617 molecular layer of, 617 occipital lobe of, 617 outer large pyramidal cell layer of, 617 outer polymorphous cell layer of, 617 outer stripe of Baillarger, 619 parietal lobe of, 617 radial fibers of, 618 small pyramidal cell layer of, 617 temporal lobe of, 617 thickness of, 612 visual area of, 617 844 INDEX Nervous system, cerebrum, 611 and spinal cord, meninges and blood supply of, 619 basal ganglia of, 611 corpus callo.sum, 611 gyri of, 611 hemispheres of, 611 insula of, 612, 617 medulla of, 611 occipital lobe of, 611 pallium of, 611 parietal lobe of, 611 sulci of, 611 Sylvian fissure of, 612 temporal lobe of, 612 veins of, 625 development, 587-595 of cardie plexus, 591 of ependyma cells, 589 of marginal velum, 590 of myelospongium, 589 of neural groove (medullary groove), 587 of neural or ganglionic crest, 587 of neural plate, 587 of neural tube, 587 derivatives of, 594 of neuroblasts, 589 of neuroglia cells, 589 of neurons, 587 of spongioblasts, 589 of sympathetic plexus, 591 meninges, 620 arachnoid, 620 granulations of, 621 Pacchionian bodies of, 621 subarachnoid space, 621 villi of, 621 dura mater, 620 epidural space of, 620-623 subdural space of, 620 pia mater, 622 choroid plexus of, 623 telsB choroideae of, 623 motor area of cerebral cortex, 615 cells of Martinotti, 617 Nervous system, inner polymorphous cell layer of, 616 layers of, 615 marginal velum, 615 molecular layer, 615 outer polymorphous cell layer, 615 outer polymorphous cell layer of, 618 outer stripe of Baillarger of, 618 pyramidal cell layer, 616 small pyramidal cell layer, 615 small pyramidal cell layer of, 618 solitary cells of Meynert of, 618 stratum zonale, 615 stripe of Bechterew of, 619 supraradial felt work, 619 tangential fiber layer, 619 spinal cord, 596 anterior spinal artery of, 624 basis cornu, 596 caput cornu, 596 cell column of Clarke, 600 central canal of, 598 central commissure, 596 cervical region, lower half, 602 upper half, 613 cervix cornu, 596 columns of, 598 conus medullaris, 600 filum terminale, 600 gray matter of, 596 intermediate zone of, 596 ligamentum denticulatum, 621 lumbar region of, 601 nerve roots of, 596 nucleus of Stilling, 602 regions of, 599 sacral region of, 600 segments of, 596 septum posticum, 621 structure of, 596 thoracic region of, 607 veins of, 625 white matter of, 598 INDEX 845 Nervous system, sympathetic division of nervous system, 153 cranial autonomic, 153 sacral autonomic, 153 sympathetic ganglia, 154 sympathetic proper, 153 tissues of nervous system, 119 axon, 119 cell body, 119-121 cyton, 121 dendrites, 119 dendrons, 119 epineurium of, 146 ganglion, 121 ganglion cell, 121 glomerulus of, 152 general considerations of, 119 nerve fiber, 119 neuroglia (glia tissue), 121 astrocytes (neuroglia cells), 141 long rayed or spider cells, 143 short rayed or mossy cells, 143 ependyma, 141 neuron, 119 neurone theory, 157 perikaryon, 121 stains of, 748 CajaPs method for demonstrat- ing nerve fibrils, 750 gold chlorid, Kanvier's method, for nerve plexuses and nerve endings, 752 Golgi's, 751 methylene blue, for non-medul- lated nerve fibers (intra vitam method), 749 NissFs method for staining chromophilic (tigroid) gran- ules in cyton and dendron with methylene blue, 750 Weigert's, for medullated nerve fibers, 748 Neurone theory, 157 Neutral balsam, for mounting sections, 760 Neutral dyes, 740 Normal salt solution, 720 Nucleus, basic chromatin of, 6 chromatin, 6 chromatin nucleoli, 6 chromioles, 6 karyolymph, 6 karyoplasm, 6 karyosomes, 6 linin, 6 net knots in, 6 nucleoplasm, 6 nuclear sap, 6 oxychromatin of, 6 paralinin, 6 plasmosomes, 6 Ocular conjunctiva, 674 Ocular contents, 665 Odontoblasts, 324 Odontoclasts, 337 Olfactory area, of cerebral cortex, 618 Oogenesis, 460-478 oogonia, in ovary, 512 ootid, or mature ovum, 479 primary oocyte, 478 polar body, 479 secondary oocyte, 479 Optic cup, 645 Optic nerve, 664 Optic stalk, 645 Optic vesicle, 645 Optical axis, of eye, 628 Ora serrata of eye, 665 Orange G. tissue stain, 746 Organ, of Corti, of ear, 708-709 of equilibration, 700 of hearing, 716 of Jacobson, 295 of Eosenmuller, 539 Orth's fluid, for fixation of tissues, 727 Osmic acid stain, for fat, 758 Ossification, centers of, 79 endochondral, 85 epiphyseal, 85 intramembranous, 86 intracartilaginous, 79 perichondrial, 84 Osteoblasts, 77-83 846 INDEX Osteoclasts, 78 Otoliths, 697 Ovarian follicle, 518 atresia of, 514 corona radiata of, 518 cumulus ob'phorus, 518 development of, 513 discus proligerus, 517 liquor folliculi, 514 Pfliiger's tubes, 513 resume of structures, 519 stigma of, 518 stratum granulosum, 517 vesicular, or Graafian follicle, 517 Ovarian scars, 522 Ovary, 507 blood supply of, 522 corpora albicantia, of, 510-521 corpora lutea of, 509 spuria, 521 vera, of pregnancy, 521 corpus hemorrhagicum, 519 corpus luteum, 507-519 cortex of, 508 hilum of, 508 lutein cells of, 519 lymphatics of, 525 medulla of, 508 nerves of, 525 ova, 510 ovarian follicles, 509 pampiniform plexus of, 523 plexus venosus of, 523 primitive follicles of (egg nests), 514 tunica albuginea of, 510 Oviduct, blood supply of, 527 lymphatics and nerves of, 528 mucosa of, 526 muscular wall of, 526 structure of, 525 Ovum, the, 511 accessory nucleus of, 511 cytoplasm of, 511 deutoplasm of, 511 nuclear cap of, 511 nucleus or germinal vesicle of, 511- 512 Ovum, the, oocytes, 512 obtid, 512 vitelline membrane of, 511 vitellus, 511 yolk nucleus of, 511 zona pellueida of, 511 Oxyhemoglobin, 211 Pacchionian bodies, 621 Pacinian corpuscles, 174 Palpebral conjunctiva, 674 Palpebral muscle, of Miiller, 677 Pampiniform plexus, of ovary, 523 of spermatic cord, 494 Pancreas, 394 centro-acinal cells, of Langerhans, of, 396 general considerations of, 394 islands of Langerhans, 399 nerve supply of, 404 pancreatic islets, 399 resume of, 404 Papillae, circumvallate, of tongue, 341 conical, of tongue, 340 filiform, 340 foliate, of tongue, 341 fungiform, of tongue, 340 lenticular, of tongue, 342 Paradidymis, organ of Giraldes, 502 Paraffin, embedding of tissues in, 735 Paraganglia, 571 Paraganglion caroticum, 570 Parahypophysis, 578 Paramitone, 8 Paraplasm, 8 Parasynapis, 463 Parathyroid gland, 562 acidophil cells of, 564 blood supply of, 565 structure of, 562 Parietal lobe, of brain, 617 Parob'phoron, 540 Parovarium, 539 Parotid gland, 390 Stenson's duct of, 390 Pectinate ligament, of eye, 635 INDEX 847 Penis, 503 corpus cavernosum, pectiniform sep- turn of, 504, corpus spongiosum of, 503-506 glands of Tyson of, 506 helicine arteries of, 504 lymphatics of, 506 nerves of, 506 nervi erigentes of, 506 preputial glands of, 506 tunica albuginea of, 504 Peptic glands, 354 or adelomorphous glands, of stom- ach, 355 Pericementum, of teeth, 329 Perichondrial ossification, 84 Perichondrium, 72 Periodontium, 329 Periosteum, 74 Peyer's patches, 366 Pharyngeal tonsil, 244 Pharynx, 347 Pheochrome cells, of adrenal, 548 Pheocrome organs, 571 Pia mater, 622 Picrocarmin stain, for tissues, 744 Picro-fuchsin (Van Gieson's) stain, for connective tissue, 752 Pigment cells, 52 Pineal body (conarium; epiphysis cere- bri), 579 blood supply of, 584 development of, 580 function of, 581 habenular commissure of, 581 posterior commissure of, 584 structure of, 582 Pineal eye, 580 Pineal recess, 580 Pineal stalk, 580 Pinna, 682 Pituitrin, 573 Pituitary body, 572 pars buccalis of, 575 pars neuralis of, 575 Rathke's pouch, 572 structure of, 573-575 Placenta, 536 Pleura, 312 Pleural pores, 313 Polarity, of cells, 39 Portal vein, 417 Posterior chamber of eye, 636-643 Potassium bichromate, for fixation of tissues, 726 Preputial glands, 506 Prickle cells, 44 Prophase, 24 Prostate gland, 497 blood supply of, 500 concretions of, corpora amylacea, 499 structure of, 497 Protopathic sensibility, 164 Protoplasm, 1 alveolar, 13 ameboid motility of, 16 chemical constitution of, 2 ciliary motility, 16 circulatory movement of, 17 colloidal biogens of, 10 colloids, 4 contractility of, 15 crystalloids, 4 emulsoid of, 14 gel state of, 4 granular, 13 irritability of, 15 metabolism of, 14 molecular motility of, 17 muscular movement, 17 physical constitution of, 3 sol state of, 4 structure of, 10 Prussak's space, of ear, 15 Pseudopodium, 16 Pseudo-stratified columnar epithelium, 49 Pulmonary artery, 314 Pulmonary veins, 317 Pyloric glands, of stomach, 357 Pyramidal cells, of brain, 613 Racemose glands, 258 Ranvier's method of staining nerve plexuses and endings, 752 848 INDEX Rathke's pouch, of hypophysis cerebri, 572 Rectum, 379 Renal pelvis and ureters, 445. See also Kidney. Reproductive system, 479 development of, 456 female organs of generation, 507 external organs of, 540 clitoris, 541 hymen, 541 glands of Bartholin, 541 glandulse vestibularesminores,541 labia majora, 540 labia minora, 540 vestibule, 540 mammary gland, 541 active, 543 blood vessels of, 547 colostrum, 546 glands of Montgomery, 545 lactiferous duct of, 542 lymphatics of, 547 milk of, 547 resting, 545 internal organs of generation, 507 Fallopian tube, 525 lymphatics of, 528 mucosa of, 526 muscular wall of, 526 serous coat of, 526 structure of, 525 Graafian follicle, resume, of structures, 519 liquor folliculi of, 514 stratum granulosum of, 518 Pniiger's tubes, 513 primitive follicles (egg nests), 514 ovarian follicles, 509 atresia of, 514 corona radiata of, 518 cumulus oophorus, 518 development of, 513 discus proligerus of, 517 Graafian follicle, 517 ovarian scars, 522 Reproductive system, female organs of generation, internal organs of generation, ovary, 507 blood supply of, 523 corpora lutea spuria, 521 corpora lutea vera, of preg- nancy, 521 corpus albicans of, 510-521 corpus hemorrhagicum of, 519 corpus luteum of, 507-519 cortex of, 508 hilum of, 508 lutein cells of, 519 lymphatics of, 525 medulla of, 508 mesovarium, 507 nerves of, 525 ova, 510 pampiniform plexus of, 523 tunica albuginea of, 510 oviduct, 525 blood supply of, 527 lymphatics of, 528 mucosa of, 526 muscular coat of, 526 structure of, 525 ovum, 511 accessory nucleus of, 511 cytoplasm of, 511 deutoplasm of, 511 nucleus of (germinal spot, germinal vesicle), 511-512 nuclear cap, 511 oocyte, stage of, 512 oogonial, stage of, 512 ootid, stage of, 479-512 vitelline membrane of, 511 vitellus, 511 yolk nucleus of, 511 zona pellucida of, 511 uterine cavity, 532 uterine glands, 531 * uterus, 528 blood vessels of, 532 . cells of Langhans of chorionic villi, in, 537 INDEX 849 Beproductive system, female organs of generation, internal organs of generation, uterus, cer- vical glands of, 532 chorionic villi of pregnant uterus, 537 decidua menstrualis of, 534 decidua reflexa of, 537 decidua serotina of, 536 decidua vera of, 537 decidual cells of, 535 fetal blood vessels in, 538 gravid, the, 535 lymphatics and nerves of, 533 menstruating, 533 mucosa, or endometrium, of, 529 muscular coat of, or myo- metrium, 528 Nabothian follicles of, 532 placenta uterina, 536 serous coat of, or perimetrium, 528 structure of, 528 vagina, 538 mucosa of, 538 musculature, 538 outer fibrous coat of, 539 structure of, 538 vestigial structures, 539 epoophoron (parovarium, organ of Bosenmiiller), 539 hydatid of Morgagni (vesicu- lar appendage), 539 paroophoron, 540 fetal, derivatives of, 459 gametogenesis, 460 chromosomes in, 464 determination of sex, 475 chromosomes in, 476 diploid number of chromosomes, 463 gonad, 460 haploid chromosome group, 464 heterotypic division, 460 maturation, 460 50 Beproductive system, gametogenesis, Mendelian inheritance, 478 Mendelism, 477 determiners in, 477 homozygote, 478 heterozygote, 478 spermatocytes, primary, in, 463 synapsis phenomena, in, 461 synizesis in, 463 telosynapsis in, 463 tetrads, formation of, in, 460 unit characters in heredity, 474 zygote, 460 general considerations of, 455 gonads, or sex glands, 456 male organs of generation, 501 external organ, 503 penis, 503 corpora cavernosa of, 503 corpus spongiosum of, 503-506 erectile tissue of, 503-505 ^ glands of Tyson, 506 helicine arteries of, 504 pectiniform septum of, 504 lymphatics of, 506 nerves of, 506 nervi erigentes of, 506 preputial glands of, 506 tunica albuginea of, 504 internal organs, 479 Cowper's glands, 501 ejaculate ry ducts, 497 prostate gland, 497 blood supply of, 500 structure of, 497 prostatic concretions, 499 scrotum, dartos of, 481 seminal vesicles, 495 testis, 479 corpus Highmori (mediasti- num) of, 480 duct system of, 490 ductuli efferentes of, 490 epididymis of, 492 functions of, 479 histogenesis and structure of, 485 850 INDEX Eeproductive system, male organs of generation, internal organs, testis, interstitial cells of Leydig, of, 487 lipoid granules in, 488 pampiniform plexus of, 494 rete of, 490 semen, 486 spermatic cord, 494 spermatid, 484 spermatoblasts, 482 spermatozoon, human, 486 structure of, 479 trophocytes (Sertoli cells) of, 482 tubules of, 487 tubuli recti of, 490 tunica albuginea of, 480 tunica vaginalis of, 480 tunica vasculosa of, 480 vas (ductus) deferens of, 493 vestigial structures, associated with, 502 appendix epididymis, 502 appendix testis, 502 ductuli aberrentes, 502 paradidymis (organ of Gi- raldes), 502 sinus pocularis (sinus prostati- cus), 502 Miillerian ducts, 456 oogenesis, 460-478 parasynapsis in, 463 polar body in, 479 primary oocyte in, 478 secondary oocyte in, 479 sex chromosome in, 475 spermatogenesis, 460 chromidia in, 474 idiozome in, 475 prespermatid in, 474 spermatogonia in, 463 spermatozoon in, 475 Wolffian body, or mesonephros, 456 Wolffian ducts, 456 Besting nucleus, 24 Respiratory system, 292 development of, 293 larynx, the, 300 structures of, 300 vocal cords of, 301 lung, the, 304 alveolar ducts of, 310 alveoli of, 311 atria of, 310 blood supply of, 314-319 bronchi of, 304 bronchial arteries, 317 bronchioles of, 307 lobule of, 314 lymphatics of, 318 pleura of, 312 pleural pores of, 313 pulmonary artery, 314 pulmonary veins, 317 nasal cavity, 293 blood vessels of, 298 neuroepithelium of, 297 olfactory portion of, 296 organ of Jacobson, of, 295 respiratory portion of, 294 vestibule of, 293 nasopharynx, 299 trachea of, 302 Eetia mirabilia, of kidney, 187 Reticulum, 49-55 Retina, arteria centralis of, 664-670 cell types of inner nuclear layer of, 654 cones of, 651 external limiting membrane of, 653 fiber layer of, 654 fibers of Muller, 658 fovea centralis of, 660 ganglion cell layer of, 657 general considerations of, 644 inner nuclear layer of, 654 inner reticular layer of, 656 inversion of, 663 layers of, 646 macula lutea of, 659 nerve fiber layer of, 657 optic disc of, 665 INDEX 851 Eetina, optic papilla of, 665 outer nuclear layer of, 654 outer reticular layer of, 654 pigment epithelium of, 646 porus opticus of, 665 rod and cone layer of, 647 rods of, 648-658 and cones, development of, 661- 663 supporting tissues of, 658 visual purple of, 647-649 yellow spot of, 659 Einger's solution, 720 Safranin stain, for tissues, 744 Salivary corpuscles, 243 Salivary glands, 390 blood supply of, 392 lymphatics of, 393 nerve supply of, 393 parotid gland, 390 sublingual gland, 392 submaxillary gland, 391 Santorini's duct, 394 Sarcoplasm, 90 Sarcosomes, of Eetzius, 99 Scala media, of cochlea, 703 Scala tympani, 703 Scleral corpuscles, 634 Sclerotic coat of eye, 634 Scrotum, 481 Sectioning of tissues, 736 microtome for, 736 Sections, colloidin adhesive, for, 739 clarification of, 759 Mayer's albumin for, 739 mounting, of, 759 in euparal, 761 in glycerin jelly, 760 in gum damar, 760 in neutral balsam, 760 in xylol-balsam, 760 Segmentation process, 27 Segmented spireme, 24 Semen, 486 Semicircular canals, of ear, 699 Seminal vesicles, 495 Septum posticum, 621 Serous membranes, 232 mesothelia of, 232 stomata in, 232 tunica propria of, 232 Sertoli cells, 482 ShrapnelFs membrane, of ear, 689 Sinus prostaticus, 502 Sinusoids, vascular, 187 Skin, 262 blood supply of, 290 capillary layer of, 268 cylindrical cell layer of, 264 derma, the, 268 eleidin layer of (stratum lucidum), 267 epidermis, the, 262 flattened cell layer and scaly layer (stratum corneum) of, 267 granular layer or stratum granu- losum, of, 266 growth and development of, 270 hair, development of, 277 mature, 279 regeneration of, 286 root of, 281 sha-ft of, 281 sheaths of, 283 structure of, 277 keratin of, 267 layers of, 262 nails, 274 growth and development of, 276 structure of, 275 nerve supply of, 291 prickle cell layer (stratum spinosum) of, 265 reticular layer of, 269 sebaceous glands of, 286 stratum germinativum, 265 sudoriparous glands of, 271 Small intestine, 374. See Digestive system. Smears, method of making, 730 Solution, Hogan's, 721 normal saline, 720 Einger's, 721 852 INDEX Spaces, of Fontana, 643 Specific dyes, 740 Spermatic cord, 494 Spermatid, and spermatozoon, 484 Spermatocytes, primary, 463 Spermatogenesis, 460 idiozome in, 475 prespermatid in, 470 Spermatogonia, 463 Spermatozoon, 475 histogenesis, and structure of, 485 human, 486 Spinal cord, 596 basis cornu of, 596 caput cornu of, 596 cell column, of Clarke, 602 central -canal of, 598 central commissure of, 596 cervical region, lower half of, 602 cervical region, upper half of, 603 cervix cornu of, 596 columns of, 598 filum terminale of, 600 gray matter of, 596 intermediate zone of, 596 lumbar region of, 600 nerve roots of, 596 nucleus of Stilling, of, 602 regions of, 599 segments of, 596 sacral region of, 600 thoracic region of, 601 white matter of, 598 Spiral organ, of Corti, 702. See also Ear. Spireme, loose, 24 Spleen, 245 blood vessels of, 246 development of, 250 differentiation of, from lymph nodes, 249 functions of, 249 pulp veins of, 248 Splenic cells, 249 Splenic nodules, 247 Spongioblasts, 589 Spongioplasm, 8 Staining methods, special, 747 Benda's mitrochondrial technic, 75? blood, Hasting 's eosinate of methy- lene blue, for, 754 chromaffin granules, technic for, 759 chromophilic (tigroid) granules in cyton and dendron, with methylene blue, 750 connective tissue, Mallory's, 753 with picro-fuchsin (Van Gieson's), 752 cytoplastic granules, eosin and methylene blue mixture, Mann's, 755 Ehrlich's triacid, 755 elastic tissue, Weigert's, 753 ' fat, osmic acid for demonstrating, 758 glycogen, Gage's method for demon- strating, 758 medullate nerve fibers, Weigert-Pal method for demonstrating, 748 mitochondria, 757 mucinous tissues, specific, 748 nerve cells, Golgi's, 751 nerve plexuses and endings, gold chlorid (Ranvier's) for, 752 neurofibrils, Cajal's, 750 nitrate of silver, for cement sub- stances, 752 Staining of tissues, 738 double, 746 in bulk, 739 progressive, 740 regressive, 740 Stains, aceto-carmin, 744 alum carmin, 744 Auerbach 's f uchsin-methyl-green, 756 borax carmin, 743 carmin, 743 Congo red, 746 Delafield's hematoxylin, 742 eosin, 745 fuchsin, 746 hematein, 743 and eosin, 746 iron hematoxylin, 747 INDEX 853 Stains, Mann's acid hematein (Ehr- lich's hematoxylin), 742 methylene blue, 743 methyl blue and safranin, 747 methyl green, 743 muchematein, 748 mucicarmin, 748 orange G, 746 picro-carmin, 744 safranin, 744 single, with cytoplasmie dyes, 745 with nuclear dyes, 741 Stapedius muscle, 690 Stapes, 689 Stenson's duct, 390 Stigmata, 38 Stomach, 353 blood supply of, 360 cardiac glands of, 360 fundus glands of, 354 lenticular glands of, 360 lymphatics of, 362 mucous coat of, 353 muscular coat of, 353 nerve supply of, 362 parietal cells (oxyntic or delomor- phous cells) of, 356 peptic, or adelomorphous cells, of, 355 pyloric glands, 357 secretion of, 357 serous coat of, 352 submucous coat of, 353 Stomata, 38 Stratified epithelium, 43 Stratified squamous epithelium, 43 Stripe of Bechterew, 619 Stripes of Baillarger, 619 Strop, use of, 738 Subarachnoid space, 621-624 Subcutaneous tissue, 269 Subdural space, 620-623 Sublingual gland, 392 Submaxillary gland, 391 Submucous plexus (Meissner's), of small intestine, 375 Suprarenal glands, 548 accessory, 556 Suprarenal glands, chromaffin gran- ules in, 554 blood supply of, 554-555 development and function of, 548 lymphatics and nerves of, 556 medulla, cells of, 553 zona fasciculata, 552 zona glomerulosa, 552 zona reticularis, 553 Sweat glands, 271 Sympathetic plexus, 461-463 Synapsis phenomena, 461-463 Syncytia, 31 Synizesis phase of gametogenesis, 463 Synovia, 89 Synovial membranes, 232 Tapetum cellulosum, of eye, 638 Tapetum fibrosum, of eye, 637 Tarsus of eyelid, 676 Technic, for demonstrating chromaffin granules, 759 Teeth, 333 ameloblasts (adamantoblasts), 333 cementoblasts, 337 cementum, 329 development of, 336 deciduous, 331 dental enamel, layers of, 335 dental papilla, 336 dental pulp, 323 dentin of, 325 development of, 330 enamel germ of, 330-331 enamel organ, of, 330 layers of, 331 Tomes' processes, 333 odontoclasts, 337 permanent, 331 root membranes of, 329 root canals of, 336 Sharpey's fibers in, 329 structure of, 323 Telge choroidese, 623 Tellyesniczky 's fluid, for fixation of tissues, 726 Telophase of mitotic cell division, 25 854 INDEX Telosynapsis, 463 Tendon, 115 nerve endings in, 171 peritenoneum of, 115 vagina fibrosa, 115 vagina mucosa, 115 Tendon cells, 117 Tendon fasciculi, 115 Tendon spindles, 174 Testis, 479-480 corpus Highmori of, 480 duct system of, 490 ductuli efferentes of, 490 epididymis, 492 human, 486 interstitial cells, of Ley dig, 487 lipoid granules in, 488 mediastinum of, 480 rete of, 490 semen, 486 spermatic cord of, 494 spermatid, and spermatozoon, 484 spermatoblasts, 482 spermatozoon, histogenesis and structure of, 485 trophocytes of, 482 tubules of, 487 tubuli recti of, 490 tunica albuginea of, 480 tunica vaginalis of, 480 tunica vasculosa of, 480 vas (ductus) deferens of, 493 Thoracic duct, 230 Thrombin, 203 Thymus gland, 565 blood supply of, 568 development and function of, 568 lymphoid corpuscles of, 567 Thyroid gland, 557 accessory, 562 blood supply of, 560 colloid in, 557 development of, 561 follicles of, 557 follicular epithelium of, 559 function of, 561 lymphatics of, 560 Thyroid gland, nerves of, 561 structure of, 557 Thyroidin, 561 Thyroglossal duct, 343 Tissue adenoid, 64 adipose, 61 bone, 72 compact, 74 general consideration of, 72 bone marrow, 77 cartilage, 67 blastema of, 69 elastic, 71 fibro-, 71 hyaline, 68 matrix of, 69 origin of, 70 connective, acidophil granulocytes, 52 areolar, 56 basophil granule cells, in, 52 blood and nerve supply of, 67 cells of, 51 dense elastic, 60 dense fibrous, 59 elastic fibers of, 57 embryonal, 50-53 eosinophil granulocytes of, 52 fibrocartilage, 71 general statements, 49 lamellar cells of, 52 loose fibro-elastic, 56 marrow, blood supply, 78 mucous, 54 oxyphil granulocytes in, 52 perichondrium, 72 periosteum, 74 plasma cells of, 51 red marrow, 77 reticular, 55 spindle cells of, 51 types of, 53 yellow elastic, 61 ectoderm, 28 entoderm, 28 epithelia, classification of, 34 INDEX 855 Tissue, epithelial tissue, 30-3 epithelium, transitional, 47 varieties of, 33 eruptive, 82 fat, 61 stains for, 62 intercellular cement substance, in, 30 lymphatic corpuscles, 66 lymphoid, 31-64 mesoderm, 28 mucous, 54 muscular, 90 Tissue fixation, by, 723 alcohol, 724 Bouin's fluid, 729 Carnoy's fluid (lung), 729 chromo-acetic-formalin mixture, 727 Flemming's fluid, 728 formalin, 725 Gilson's fluid, 730 heat, 730 Kelly's fluid, 728 Kleinenberg 's fluid, 729 mercuric chlorid, 725 mercuro-nitric mixture, 730 Miiller's solution, 726 Orth's fluid, 727 potassium bichromate, 726 Tellyesniczky 's fluid, 726- vapors, 731 van Gehuchten's fluid, 729 Zenker 's solution, 727 Tissue fixation, of human embryos, 726 lung tissue, 730 Tissue juice, 55 Tissue staining, in bulk, 739 progressive, 740 regressive, 740 Tissue stains, 744 aceto-carmin, 744 alum carmin, 744 alum hematein (Mayer), 741 Bohmer's hematoxylin, 741 borax carmin, 743 carmin, 743 double staining, 746 Tissue stains, Congo red, 746 Delafield's hematoxylin, 742 eosin, 745 fuchsin, 746 hematin, 741-743 and eosin, 746 iron-hematoxylin, 747 Mallory's, for connective tissue, 753 Mann's hematein (Ehrlich's hema- toxylin), 742 methyl green, 743 methyl blue, and safranin, 747 methylene blue, 743 muchematein, 748 mucicarmin, 748 mucinous, specific stain, 748 orange G, 746 picro-carmin, 744 picro-fuchsin, for connective tissue (Van Gieson), 752 safranin, 744 single, with nuclear dyes, 741 Weigert's, for elastic tissue, 731 Tissues, 30 box for embedding for sectioning of, 737 decalcification of, 731 dissociation of, 721 chemical, 722 embedding of, 734 in celloidin, 734 in paraffin, 735 hardening of, 733 injection of, 732 maceration of, 721 sectioning of, 736 staining of, 738 teasing of, 721 Tongue, 337 blood vessels of, 343 foramen cecum of, 343 glands of, 342 lymphatics of, 344 mucous membrane of, 337 nerve supply of, 344 papillae of, 339 856 INDEX Tongue, serous glands of, 343 taste buds of, 341 Tonsil, faucial (palatine), 242 lingual, 244, 343 pharyngeal, 244 tubal, 692 Trachea, 302 structures of, 302 Tunica propria, corium of skin, 34 Tympanic membrane, 687 Tympanum of ear, 685. See also ear Tyson's glands, in penis, 506 Umbo, of tympanic membrane, 687 Unit characters, in Mendelian inheri- tance, 477 Urinary system, 423 kidney, 423 areiform veins of, 443 arteries, arteriae proprise renales, 440 areiform, 440 arterige rectaB, 441 interlobular, 441 Bellini's ducts of, 438 capsule of Bowman, of, 429 corpuscles (renal) of, 425 cortex of, 424 glomerulus of, 429 lymphatics and nerves of, 444 medulla of, 424 papillary ducts of, 438 pelvis and ureter of, 445 blood supply of, 448 mucosa of, 445 muscular coat of, 448 tunica propria of, 447 renal circulation, table showing course of, 443 renal connective tissue, 426 renal corpuscles (Malpighian bod- ies), 425-429 renal lobule, 426 topography of, 423 tubules of, arched connecting (junctional tubule), 437 Urinary system, kidney, tubules of, ascending limb of Henle 's loop, 435 descending limb of Henle 's loop, 434 distal convoluted portion of, 436 divisions of renal tubule, 429 loop of Henle, 435 neck of, 431 peculiarities of, 440 proximal convoluted portion of, 431 straight collecting, 437 table of divisions of renal tu- bule, 439 uriniferous, or renal, 427 urethra, female, 452 structure of, 452 urethra, male, structure of, 454 urinary bladder, 448 mucous membrane of, 448 muscular coat of, 451 vascular and nerve supply of, 452 veins of, stellate, 443 venae propriaa renales, 443 venulae rectae of, 443 Uterus, the, 532 blood vessels of, 532 cells of Langhans of chorionic villi of, 537 cervical glands of, 532 chorionic villi of pregnant uterus, 537 decidua menstrualis of, 534 decidua reflexa of, 537 decidua serotina of, 536 decidua vera of, 537 decidual cells of, 535 fetal blood vessels, 538 glands of, 531 gravid, 535 lymphatics and nerves of, 533 menstruating, 533 mucosa, or endometrium, of, 529 muscular coat, or myometrium, of, 528 INDEX 857 Uterus, Nabothian follicles of, 532 placenta uterina, 536 serous coat, or perimetrium, of, 528 structure of, 528 Utricle, of ear, 698 Vagina, mucosa of, 538 musculature of, 538 outer fibrous coat of, 539 structure of, 538 Valves, of heart, 196 Valvulee conniventes, 364 Vas (ductus) deferens, 493 Vascular sinusoids, 187 Vascular tissue, 176 Veins, 187 arciform, of kidney, 443 atypical, 189 auditory, 716 circulus venosus, of Haller, 547 cranial, 190 larger, structure of, 188 of central nervous system, 625 of ear, 715 of kidney, 443 of liver, 415 of ovary, 523 of sclera, 635 of spermatic cord, 494 portal, 417 pulmonary, 190-317 small, 188 smaller and larger, comparison of, 190 Veins, valves in, 191 vena cava, 190 vena comes, 191 venae vorticosae, of eye, 672 Velum, marginal, 590 Venous spaces, 190 Venules, precapillary, 187 Vermiform appendix, 379 Vesicular appendages, 539 Vestibule, of pudendum, 540 Vestigial structures, associated with reproductive organs, 539- 540 Visual area, 617 Visual axis, 628 Vitreous humor, 669 Vocal cords, 301 Weigert-Pal stain, for medullated nerve fibers, 748 Weigert's elastic tissue stain, 753 Wirsung's duct, 394 Wolffian body, or mesonephros, 456 Xylo-balsam, for mounting sections, 760 Zenker's solution, 727 Zimmermann's technic, for intercalated discs of cardiac muscle, 758' Zonula ciliaris, of eye, 670 Zygote, 27-460 (4) QM551 Jord J82 1920 c2 H.I. Teict-book of ed.a D4501 histology. ,ARY