UM53T 616; Columbia ®nit)erj^ttp CoUesc of ^fjpgiciang anb burgeons Hibrarp A TEXT-BOOK OF HISTOLOGY BY FREDERICK R. BAILEY. A. M., M. D. THIRD REVISED EDITION PROFUSELY ILLUSTRATED NEW YORK WILLIAM WOOD AND COMPANY M D C C C C X Copyright, 1910 By WILLIAM WOOD AND COMPANY /'rill ted by The Maple Press y„rk. Pa. PREFACE TO THE THIRD EDITION. The very gratifying approval which the first and second editions of the Text-book received has made it seem unwise to attempt any change in the general plan and scope of the work as outlined in the preface to the first edition. The text has been thoroughly revised, some parts of it practically rewritten. Some figures have been replaced by new ones and a considerable number of new figures, some original, others borrowed, have been added. For the latter the writer wishes to acknowledge his obligations. To Mr. A. M. Miller the writer is indebted for many valuable criticisms and suggestions. The chapter on the nervous system has been rewritten by Dr. Oliver S. Strong. For Dr. Strong's careful and painstaking work on this chapter, for his thoroughly original treatment of his subject, and for the original drawings and photographs in this chapter, the author wishes to express his most grateful appreciation. Dr. Strong wishes to renew the acknowledgment, made in the preface to Bailey and Miller's Embryol- ogy, of indebtedness to Dr. Adolf Meyer for many ideas and terms found valuable in the preparation of the chapter on the nervous system. There may be mentioned here the terms segmental and supraseg- mental to denote an imjjortant distinction between certain }:)arts of the nervous system. in Digitized by tine Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/textbookofhistol1910bail PREFACE TO THE FIRST EDITION. The primary aim of the writer in the preparation of these pages has been to give to the student of medicine a text-book of histology for use in connection with practical laboratory instruction, and espe- cially to furnish to the instructor of histology a satisfactory manual for classroom teaching. With these objects in view, the text has been made as concise as possible consistent with clearness, and the writer has attempted to make the more essential elements stand out somewhat from the necessarily accompanying details. It has been impossible to accomplish this without some sacrifice of uniformity of treatment and of logical sequence. This is espe- cially noticeable in the chapter on the nervous system, which has been made much fuller and more "practical" than is usual. The author's reason for the method of treatment there adopted and for the consider- able amount of anatomy which this chapter contains being the apparent success the method has met with in the teaching of this always ditiicult subject to students. The chapter on general technic is intended to furnish the student with only the more essential laboratory methods. For special and more elaborate methods the student is referred to the special works on technic mentioned at the close of the chapter. The special technic given in connection with the different tissues and organs is in most cases such as can be conveniently used for the preparation of class sections. The original illustrations are from drawings by Mr. A. M. ^SLiller, to whom the writer is greatly indebted for his careful and accurate work. The uselessness of redrawing perfectly satisfactory illustra- tions has led the writer to borrow freely from various sources, each cut being duly accredited to the work from which it has l^een taken. For vi PREFACE. all of these the author wishes to express his appreciation and obligation. He is also deeply indebted to Dr. O. S. Strong for his careful review and criticism of the chapter on the nervous system and for his super- vision of the drawing of Figs. 263 and 264; to Dr. G. C. Freeborn, his predecessor as Instructor of Histology at the College of Physicians and Surgeons, for many valuable suggestions; and to Dr. T. Mitchell Prudden for his careful and critical review of the author's copy. CONTENTS. PART I.— HISTOLOGICAL TECHNIC. CHAPTER I. PAGE General Technic. General Considerations, 3 Examination of Fresh Tissues, 3 Dissociation of Tissue Elements, 4 Teasing, 4 Maceration 4 Preparation of Sections, 5 Fixation, 5 Hardening, 8 Preserving, 9 Decalcifying, . 9 Embedding, 10 Celloidin Embedding, 11 Paraffin Embedding, 12 Section Cutting, 13 Celloidin Sections, 14 Paraffin Sections, 14 Staining, 15 Nuclear Dyes, 15 Plasma Dyes 17 Staining Sections, 18 Staining in Bulk 19 Mounting, 20 Staining and Mounting Paraffin Sections, 21 Injection, 22 CHAPTER II. Special Staining Methods. Silver Nitrate Method of Staining the Intercellular Substance, . . 26 Chlorid of Gold for Demonstrating Connective-tissue Cells, .... 26 Weigert's Elastic Tissue Stain, 26 Golgi's Chrome-silver for Staining Secretory Tubules, 26 Mallory's Phosphomolybdic Acid Haematoxylin Stain for Connective Tissue 27 vii CONTENTS. PAGE Mallory's Phosphotungstic Acid Hsematoxylin Stain for Connective Tissue, 27 Mallory's Aniline Blue Stain for Connective Tissue 28 Osmic Acid Stain for Fat, 28 Jenner's Blood Stain, 28 CHAPTER III. Special Neurological Staining Methods. Weigert's Method of Staining Medullated Nerve Fibres, 29 Weigert-Pal Method, 30 Marchi's Method for Staining Degenerating Nerves, 31 Golgi Methods of Staining Nerve Tissue, 32 Slow Method, 32 Rapid Method, 32 Mixed Method 32 Formalin bichromate Method, ^^ Bichloride Method, 33 Golgi-Cox Method, 33 Cajal's Method 34 Nissl's Method, 35 General References on Technic, 35 PART II.— THE CELL. CHAPTER I. The Cell, 39 General Structure, 39 Structure of a Typical Cell, 39 The Cell Body, 40 The Cell Membrane, 42 The Nucleus, 42 The Centrosome, 44 Vital Properties of Cells, 45 Metabolism, 45 Function, 46 Irritability, 46 Motion 46 Amoeboid, 46 Protoplasmic, 47 Ciliary, 47 Reproduction 47 Direct Cell-division, 47 Indirect Cell-division, 48 Fertilization of the Ovum, 52 Technic, 57 References for further study, 58 CONTENTS- PART III.— THE TISSUES, CHAPTER I. PAGE Histogenesis- — Classification 6i Tissues Derived from Ectoderm, . • 6i Tissues Derived from Entoderm, 6i Tissues Derived from Mesoderm, 6i CHAPTER IT Epithelium (Including Mesothelium and Endothelium), 63 Histogenesis, 63 General Characteristics, 63 Classification, 64 Simple Epithelium, 64 Simple Squamous, 64 Simple Columnar 65 Pseudostratified, 65 Stratified Epithelium, 66 Stratified Squamous, 66 Stratified Columnar, 67 Transitional, 67 Modified Forms of Epithelium, 68 Ciliated Epithelium, 68 Pigmented Epithelium, 69 Glandular Epithelium, 69 Neuro-epithelium, 69 Mesothelium and Endothelium, 70 Technic, 71 CHAPTER III. The Connective Tissues, 73 Histogenesis, 73 General Characteristics, 73 Classification, 74 Fibrillar Connective Tissue, 74 Areolar Connective Tissue, 77 Formed Connective Tissue 78 Development, 78 Elastic Tissue 78 Technic for Fibrillar and Elastic Tissue, 80 Embryonal and Mucous Tissue, 81 Technic, 83 Reticular Tissue, 83 X CONTEXTS. PAGE Lymphatic Tissue, 84 Technic for Reticular and Lymphatic Tissue, 85 Fat Tissue, 85 Technic, 89 Cartilage, 89 Hj^aline, 90 Elastic, 91 Fibrous, 91 Technic, 92 Bone Tissue, 92 Technic, 94 Xeuroglia, 94 CHAPTER IV. The Blood, 95 Red Blood Cells, 95 White Blood Cells, 96 Blood Platelets, 98 Blood Dust, 99 Development, 99 Technic, 100 CHAPTER V. Muscle Tissue, loi Involuntary Smooth Muscle, loi Voluntary Striated Muscle, 102 Involuntary Striated Muscle, 106 Development of Muscle Tissue 108 Technic, 109 CHAPTER VI. Nerve Tissue, iii The Neurone, m General Structure, iii The Cell Body, 1 1 1 The Nucleus, 112 The Cytoplasm, 112 Neurofibrils, 113 Perifibrillar Substance, 113 Chromophilic Bodies, 113 The Dendrites, 116 The Axone, 116 Non-medullated Axones (Non-meduUated Nerve Fibres), . . . 117 Medullated Axones (MeduUated Nerve Fibres), 117 Theorie."; as to Physiology of the Neurone 121 Significance of Degenerative Changes in the Neurone, 122 Neuroglia, 125 Technic, 127 General References, 127 CONTEXTS. XI PART IV.— THE ORGANS. CHAPTER I. PAGE The Circulatory System, 131 The Blood-vessel System, '. 131 General Structure 131 Capillaries, 131 Arteries, 133 Veins, 137 Technic, 139 The Heart, 140 Technic, 142 Development of the Circulatory System, 142 The Lymph-vessel System, 143 Lymph Capillaries, 144 Lymph Spaces, 144 Serous Membranes, 144 Technic, 145 The Carotid Gland, 146 The Coccygeal Gland, 146 Technic, 146 General References on Circulatory System, 146 CHAPTER n. Lymphatic Organs, 147 The Lymph Nodes, 147 Technic, 150 Haemolymph Nodes, 151 Technic 153 The Thymus, 153 Technic, 155 The Tonsils, 155 The Palatine Tonsils, 155 The Lingual Tonsils, 157 The Pharyngeal Tonsils, 157 Technic, 158 The Spleen, 15S Technic, 163 General References, 163 CHAPTER III. The Skeletal Syste.m, 164 The Bones, 164 Bone Marrow, 168 Red Marrow 168 Yellow Marrow 170 CONTENTS. PAGE Technic 171 Development of Bone, 172 Intramembranous Development, 172 Intracartilaginous Development 175 Subperiosteal, 177 Growth of Bone 179 Technic, 179 The Cartilages, 180 Articulations, 180 Technic, 181 General References 182 CHAPTER IV. The Muscular System. A Voluntary Muscle, 183 Tendons, 184 Tendon Sheaths and Bursse, 184 Growth of Muscle, 184 Technic, 185 CHAPTER V. Glands and the General Structure of Mucous Membranes .... 186 Glands — General Structure and Classification 186 Tubular Glands 188 Alveolar Glands, 190 General Structure of Mucous Membranes, 191 CHAPTER VL The Digestive System, 192 Anatomical Divisions, 192 The Headgut, 193 The Mouth, 193 The Mucous Membrane of the Mouth, 193 Glands of the Oral Mucosa, 193 Technic, 196 The Tongue, 196 Technic, 200 The Teeth 200 Development of the Teeth, 206 Technic, 2x2 The Pharynx, 212 Technic, 213 The Foregut, 213 The CEsophagus 213 Technic, 215 General Structure of the Walls of the (Jastro-intestinal Canal, . 215 CONTENTS. xm PAGE The Stomach, 217 Technic, 223 The Midgut, 223 The Small Intestine, 223 Peyer's Patches, 22S The Endgut, 231 The Large Intestine, 231 The Vermiform Appendix, 232 The Rectum, 234 The Peritoneum, Mesentery, and Omentum, 235 Blood-vessels of the Stomach and Intestines, 236 Lymphatics of the Stomach and Intestine, 237 Nerves of the Stomach and Intestine, 23 8 Secretion and the Absorption of Fat, 239 Technic, 241 The Larger Glands of the Digestive System, 242 The Salivary Glands, 242 The Parotid, 243 The Sublingual, 243 The Submaxillary, 244 Technic, 247 The Pancreas, 247 Technic, 252 The Liver, 253 Excretory Ducts of the Liver, 260 The Gall-bladder, 261 Technic 261 Development of the Digestive System, 262 General References, 263 CHAPTER VII. The Respir.\tory System, 264 The Nares, 264 The Larynx, 266 The Trachea, 266 Technic, 269 The Bronchi, 269 The Lungs, 273 Development of the Respiratory System 279 Technic, 2 So The Thyreoid, 2S0 The Parathyreoids 283 Technic, 2S5 General References, 285 CHAPTER VIII. The Urinary System, 286 The Kidney, 2S6 COx\TEXTS. PAGE The Kidney Pelvis and Ureter, 297 The Urinary Bladder, 298 The Suprarenal Gland, 299 Technic, 302 General References, 302 CHAPTER IX. The Reproductive System 303 Male Organs, 303 The Testis, 303 The Seminal Ducts, 309 The Epididymis, 309 The Vas Deferens, 310 The Seminal Vesicles and Ejaculatory Ducts, 311 Rudimentary Structures Connected with the Development of the Genital System, 311 The Spermatozoon, 313 Development of the Spermatozoon, 314 Technic, 316 The Prostate Gland, 317 Cowper's Glands, 319 Technic 319 The Penis, 319 The Urethra, 319 Technic, 321 Female Organs, 322 The Ovary, 323 The Graafian Follicle, 324 The Corpus Luteum, 329 The Oviduct, 334 Technic, 335 The Uterus, 336 The Mucosa of the Resting Uterus, 337 The Mucosa of the Menstruating Uterus, 338 The Mucosa of the Pregnant Uterus, 340 The Placenta 341 The Vagina, 345 Development of the Urinary and Re]jroductive Systems 346 Technic, 349 General References, 3 5° CHAPTER X. The Skin and its Ai'pendages, 351 The Skin, 35^ Technic, 3 55 The Nails, 3S(> Technic, 3 5^ CONTEXTS. XV PAGE The Hair, 35S Technic, 364 Development of Skin, Nails, and Hair 366 The Mammary Gland, 368 Technic, 371 General References, 372 CHAPTER XL The Nervous System, 373 Histological Development and General Structure, 373 Membranes of the Brain and Cord 378 Technic, 380 The Peripheral Nerves, 380 Technic, 382 The Afferent Peripheral Neurones, 382 The Cerebro-spinal Ganglia, 382 The Peripheral Processes of the Cerebro-spinal Ganglion Cells, 3S5 The Central Processes of the Cerebro-spinal Ganglion Cells, . 390 The Sympathetic Ganglia, 390 Technic 394 The Efferent Peripheral Cerebro-spinal Neurones ^q^ The Spinal Cord, 396 Origin of the Fibres which make up the White Matter of the Cord, 396 (i) The Spinal Ganglion Cell and the Origin of the Posterior Columns, 397 (2) Cells Situated in Other Parts of the Central Nervous System which Contribute Axones to the White Columns of the Cord, 397 (3) Root Cells — Motor Cells of the Anterior Horn, .... 398 (4) Column Cells, 398 (5) Cells of Golgi Type II, 399 Technic, 399 Practical Stud}^ 400 General Topography of the Cord, Cell Groupings, Arrangement of Fibres and Finer Structure,) 401 Practical Study of Sections through Lumbar Enlargement, 401 General Topography, 401 Cell Groupings, 403 Arrangement of Fibres, 405 Finer Structure, 405 Blood-vessels, 406 Variations in Structure at Different Levels 407 Practical Study, 40S Section through the Twelfth Thoracic Segment. . . . 408 Section through the Mid-thoracic Region, 408 Section through the Cervical Enlargement 408 Fibre Tracts of the Cord 408 Ascending Tracts 413 I. Long Ascending Arms of Dorsal Root Filires, . .413 CONTENTS. PAGE II. Spino-thalamic Tract, 414 III. Dorsal Spino-cerebellar Tract, 415 IV. Ventral Spino-cerebellar Tract 415 Descending Tracts 416 I. The Pyramidal Tracts, 416 II. The Tecto-spinal Tract, 217 III. The Tract from the Interstitial Nucleus of Cajal, 417 IV. The Rubro-spinal Tract, 417 V. Tracts from Deiters' Nucleus, 418 VI. The Fasciculus of Thomas, 418 VII. Helweg's Tract, 418 VIII. The Septo-marginal Tract, 418 IX. The Comma Tract of Schultze, 419 Fundamental Columns or Ground Bundles, 419 A Two-neurone Spinal Reflex Arc, . . . - 420 A Three-neurone Spinal Reflex Arc, 421 A Cerebellar Arc, 421 A Cerebral or Pallial Arc, 421 Technic, .421 The Brain, 424 General Structure, 424 Segmental Brain and Nerves, 425 Suprasegmental Structures, 427 The Hindbrain or Rhombencephalon, 429 The Medulla Oblongata or Bulb, 429 The Pons, 431 The Cerebellum (also p. 455), 431 Technic, 431 Practical Study, 43 1 1. Transverse Section of the Medulla through the Decussation of the Pyramidal Tracts (Motor Decus- sation), 431 2. Transverse Section of the Medulla through the Decussation of the Fillet or Lemniscus (Sensory Decussation), 43. S 3. Transverse Section of the Medulla through the Lower Part of the Inferior Olivary Nucleus, . . .43 7 4. Transverse Section of the Medulla through the Middle of the Olivary Nucleus, 439 5. Transverse Section of the Medulla through the Entrance of the Cochlear Root of Nerve VIII, . . . 439 6. Section through the Hindbrain at Level of Junction of Pons and Cerebellum and Entrance of Vestibular Nerve, 44 7 7. Transverse Section of tlic Hindln-ain through the Roots of Nerves VI (Abducens) and VII (Facial), . 4S° 8. Transverse Section of the Hindbrain through the Roots of Nerve V (Trigeminus) 452 Tlic Cereljcllum, 455 The Cerebellar Cortex, 455 The Isthmus 462 CONTENTS. XVll PAGE Practical Study, 462 9. Transverse Section through the Isthmus at the Exit of Nerve IV (Trochlearis) , 462 The Midbrain or Mesencephalon, 464 Practical Study, 465 10. Transverse Section through Midbrain at Level of Anterior Corpora Quadrigemina and Exit of Nerve III (Oculomotor), 465 The Forebrain or Prosencephalon, 468 The Interbrain (Diencephalon or Thalamencephalon), . . 468 Practical Study, 470 11. Transverse Section through the Junction of Midbrain and Thalamus 470 12. Section through the Interbrain at the Level of the Optic Chiasma, 472 The Endbrain or Telencephalon, 477 The Rhinencephalon, 477 The Corpus Striatum, 478 The Pallium, 478 Practical Study, 481 13. Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus, . 481 The Cerebral Cortex, 481 Technic, 488 The Pituitary Body, 489 The Pineal Body, 490 Technic, 490 Table of Cranial and Spinal Nerves, 492 General Reference for Further Study, 491 CHAPTER XII. The Organs of Special Sense, 494 The Organ of Vision, 494 The Eyeball, 494 The Cornea, 494 The Chorioid, 49.6 The Ciliary Body , 498 The Iris, 500 The Retina, 501 The Optic Nerve, 505 The Relations of Optic Nerve to Retina and Brain 506 The Lens, 510 The Lacrymal Apparatus 512 The Eyelid 513 Development of the Eye 515 Technic, 517 The Organ of Hearing 518 The External Ear 51S The Middle Ear, 519 xviii CONTENTS. PAGE The Internal Ear, 520 The Vestibule and Semicircular Canals, 520 The Saccule and Utricle, 521 The Semicircular Canals, 522 The Cochlea 522 Development of the Ear, 529 Technic, 529 The Organ of Smell, 530 Technic, 532 The Organ of Taste, 532 Technic, 533 General References 533 Index, 535 PART I. HISTOLOGICAL TECHNIC. CHAPTER I. GENERAL TECHNIC. Certain body fluids, e.g., blood, urine, etc., may be examined by simply placing them on a slide under a cover glass. A few tissues, e.g., thin membranes, such as the omentum and the mesentery, may be examined fresh in some such inert medium as blood serum or normal salt solution (o. 75-per-cent. aqueous solution sodium chlorid). For such examination the tissue is immersed in the salt solution on a slide and covered with a cover-glass. Most tissues and organs, how- ever, require much more elaborate preparation to render them suitable for microscopic examination. Tissues too dense and thick to be readily seen through with the microscope must be so treated as to make them transparent. This is accomplished either by pulling the tissue apart into fine shreds, teasing, or by cutting it into thin slices, sec- tion cutting. Some tissues admit of teasing in a fresh condition; others can be satisfactorily teased only after they have been subjected to the action of a chemical w^hich breaks down the substance holding the tissue elements together, jnaceration. Fresh tissue can rarely be cut into sections sufficiently thin for microscopic examination. It must first be treated in such a manner as to preserve as nearly as pos- sible the living tissue relations, fixation. If too soft for section cutting it must next be put through a process known as hardening. If, how- ever, as in the case of bone, the tissue is too hard, it must be soft- ened by dissoh'ing out the mineral salts, decalcification. If very thin sections are to be cut, it is further necessary to impregnate the tissue with some fluid substance which will harden in the tissue and give to the mass a firm, even consistency. This is known as embedding. Furthermore, most tissue elements have refractive indices which are so similar that their differentiation under the microscope is often extremely difficult. To overcome this difficulty, recourse is had to staining the tissue with dyes which have an aflinity for certain only of the tissue elements, or which stain dift'erent elements with dift'erent degrees of intensity. This is known as differential or selective staining. 3 4 HISTOLOGICAL TECHNIC. The final step in the process is the mounting of the specimen, after which it is ready for microscopic study. Only the more common procedures used in the preparation of tissues for microscopic study are described in this section. At the end of each section are given the technical methods most satisfactory for the demonstration of the tissues described in that section. For other methods the student is referred to special works upon micro- scopic technic. Dissociation of Tissue Elements. This method of preparing tissues for microscopic study has only a limited application, most specimens being preferably fixed, and cut into thin sections. Certain of the structural features of such tissues as nerves, muscle, and epithelium, which have but little inter- cellular substance may be well demonstrated by dissociation. This is accomplished by (i) teasing, or (2) maceration, or both. (i) Teasing. — This consists in pulling apart fresh or preserved tissues by means of teasing needles. Instructive specimens of such tissues as muscle and nerve may be obtained in this way. (2) Maceration. — This is the subjecting of a tissue to the action of some chemical which breaks down the substance uniting the tissue elements, thus allowing them either to fall apart or to be more easily dissociated by teasing. The most commonly used macerating fluids are: (a) Ranvier^s Alcohol (33-per-cent., made by adding 35 c.c. of 96-per-cent. alcohol to 65 c.c. of water). — Bits of fresh tissue are placed in this fluid for from twenty-four to forty-eight hours. The cells may then be easily separated by shaking or by teasing. Ran- vier's alcohol is an especially satisfactory macerating fluid for epithelia. {h) Formaldehyde, in very dilute solutions (0.2- to 0.4-per-cent. commercial formalin^). — Tissues should remain in the formaldehyde solution from twenty-four to forty-eight hours. This is also es- pecially useful for dissociating epithelial cells. (c) Sodium or Potassium Hydrate (30- to 35-per-cent. aqueous solution). — From twenty minutes to an hour is usually sufficient to cause the tissue elements to fall apart or to be readily pulled apart with the teasing needles. If it is at any time desirable to stop the action of the caustic alkali, this may be accomplished by neutralizing * Commerical formalin is a 40-per-cent. solution of formaldehyde gas in water. GENERAL TECHNIC. 5 with glacial acetic acid or by replacing the alkali with a 6o-per-cent. aqueous solution of potassium acetate. The specimens may then be preserved in the potassium-acetate solution, in glycerin, or in 50-per- cent, alcohol. This dissociating fluid is largely used for muscle cells and fibres. {d) Nitric acid (10- to 20-per-cent. aqueous solution). — This is especially useful for dissociating involuntary and voluntary muscle. After any of the above procedures, the macerating fluid containing the tissue elements should be placed in a long tube, allowed to stand for a time and the fluid decanted. Water is then poured into the tube, the tissues allowed to settle and the water poured off, this being re- peated until all trace of macerating fluid is removed. The tissue elements may then be preserved or mounted in glycerin or in glycerin Jelly. It is frequently advisable to stain the tissues. For this purpose alum-carmin (p. 17) is especially satisfactory. (For details see technic I, p. 109 and technic 2, p. 109). After staining and washing, the tissues may be preserved or mounted in glycerin, eosin glycerin, or glycerin jelly. It frequently happens that on examining dissociated tissue elements after mounting, the bits of tissue are stiU too large. This may be remedied by gently tapping on the cover-glass with a lead pencil. PREPARATION OF SECTIONS. I. Fixation. Fixation is the first step in the preparation of sections of tissues for microscopic study. Its object is to so preserve the tissues that they retain as nearly as possible the same structure and relation which they had during life. Fixation of such a thin film of tissue as a blood smear may be accomplished by heating. A blood smear or even small pieces of tissue may be fixed by exposure to certain chemical vapors as, e.g., the vapor of formalin or of osmic acid. Fixation is, however, usually accomplished by means of chemicals in solution, the solution being known as a fixing agent or fixative. Pieces of tissue are immersed in the fixative and allowed to remain there until fixation is complete. The length of time required depends upon the character of the tissue and upon the fixative used. The pieces of tissue should be small, and large quantities of the fixative should be used. It may be neces- sary to change the fluid a number of times, in order to keep it up to the proper strength. 6 HISTOLOGICAL TECHXIC. Organs and even bodies may be fixed in toto by injecting the fixa- tive through an artery and allowing it to escape through the veins. After the injection, the whole specimen should be placed in a large quantity of the same fixative. This method fills the entire vascular system including the capillaries with the fixative, thus bringing the latter into very prompt and close contact with the tissue elements. The result is a very rapid and accurate fixation which is especially valuable where it is necessary to preserve the topographic relations of various parts of an organ or a body. A mercuric chlorid solution (p. 7) followed immediately by strong alcohol makes a very good injection fixative. Satisfactory fixation is largely dependent upon the freshness of the tissue when placed in the fixative. The following are the fixatives in most common use: (i) Strong Alcohol (96-per-cent.). — This is a rapid fixative and should be used on small pieces of tissue. The time required is from six to twenty-four hours, though tissues may remain longer without injury. The alcohol should be changed after two or three hours. This fixative should not be used where fine histological detail is desired, since it causes some shrinkage. One advantage in its use is the fact that tissues are hardened and ready for embedding at the end of fixation. (2) Dilute Alcohol (30-per-cent. to 80-per-cent.). — This, as a rule, gives unsatisfactory results, causing much shrinkage of the tissue elements. (3) Formaldehyde (2-per-cent. to lo-per-cent. aqueous solution). — Formalin is rapid in its action and probably has better penetrating qualities than any other fixative. For general purposes a 4-per-cent. solution (i part commercial formalin to 9 parts water) should be used. This fixes in from six to twenty-four hours. The results after formal- dehyde are not always good, owing to the fact that it has little harden- ing power, and the subsequent action of alcohol is Hkely to cause some distortion of the tissues. It acts better when combined with other fixatives than when used alone. (See Orth's fluid.) (4) Mailer's Fluid. Potassium bichromate, 2.5 gm. Sodium sulphate, i.ogm. Water, loo.o c.c. This fluid gives very good results, but is extremely slow in its action, requiring from a week to several months. Fairly large pieces of tis- GENERAL TECHXIC. 7 sue may be fixed, but in all cases large quantities of the fixative should be used and frequently renewed. (5) Orth's Fluid. ]\Iuller's fluid (double strength), \ _ „,,,,„ ) Equal parts, rormaldehyde, 8-per-cent., J This is one of the best general fixatives. Its action is similar to that of Muller's fluid but much more rapid, fixation being accomplished in from twenty-four to forty-eight hours, though specimens may remain in the fluid several days without disadvantage. Fairly large pieces of tissue may be fixed with good results. The fixative should be changed after a few hours. Fixation with Orth's fluid gives an excellent basis for a haematoxylin-eosin stain (see (i), p. 18.) The fixative should always be freshly prepared. It is convenient to keep the 8-per-cent. formaldehyde solution and the double-strength Muller's fluid in stock. Orth's fluid is then prepared by simply taking equal parts of each. (6) Osmic Acid. — This, in a i-per-cent. aqueous solution, is a quick and excellent fixative of poor penetrating power. Xtry small pieces of tissue must therefore be used. They should remain in the fluid from twelve to twenty-four hours. Osmic acid stains fat and myelin black and is consequently useful in demonstrating their pres- ence in tissues. Fixation should take place in the dark. (7) Flemming' s Fluid. Chromic acid, i-per-cent. aqueous solution, 25 c.c. Osmic acid, i-per-cent. aqueous solution, 10 c.c. Glacial acetic acid, i-per-cent. aqueous solution, 10 c.c. Water, 55 c.c. Flemming's fluid is one of the best fixatives for nuclear struc- tures, and is of especial value in demonstrating mitotic figures. \'ery small pieces of tissue should be placed in the fixative, where they remain for from twenty-four hours to three days. The solution should be freshly made as required, or a stock solution without the osmic acid may be kept, and the latter added at the time of using. (8) Mercuric Chlorid. — This may be used either in saturated aqueous solution or in saturated solution in o. 75-per-cent. salt solu- tion. Fixation is complete in from twelve to twenty-four hours, and is usually very satisfactory. A saturated solution of mercuric chlorid in 5-per-cent. aqueous solution of glacial acetic acid also wives good results. 8 HISTOLOGICAL TECHNIC. . (9) Zenker'' s Fluid. Potassium bichromate, 2 . 5 gm. Sodium sulphate, i.ogm. Mercuric chlorid, 5 . o gm. Glacial acetic acid, 5.0 c.c. Water, 100. o c.c. This fluid should be freshly made, or the salts may be kept in solu- tion and the acetic acid added at time of using. Zenker's fluid is a good general fixative, but usually causes some shrinkage of the tissue elements. Fixation requires from six to twenty- four hours. The most serious drawback to Zenker's fluid is the fact that the mercuric chlorid sometimes produces dark, irregular precip- itates in the tissues. This may be remedied, however, by the use of iodine and iodid of potassium in the hardening process (see Harden- ing, p. 9). (10) Picric acid is an excellent fixative for cytoplasm. It may be used in: {a) Saturated aqueous solution; (b) saturated solution of picric acid in i-per-cent. aqueous solution of acetic acid; (c) saturated solution of picric acid in 2-per-cent. aqueous solution of sulphuric acid. II. Hardening. Most fixatives are also hardening agents if their action is prolonged. This is, however, often detrimental. It is, therefore, customary, after fixation is complete, to carry the specimens, with or without washing, through successively stronger grades of alcohol for the purpose of hardening the tissues. For general histological purposes the specimens may be transferred directly to 70-per-cent. or 80-per-cent. alcohol, which should be changed once or twice. In the case of delicate tissues the first grade of alcohol should be 40-per-cent. or 50-per-cent., the second 70-per-cent., and the third 80-per-cent. The specimens should remain in each grade from twelve to twenty-four hours. Washing the tissues after fixation is not a matter of indifference. In some cases water should be used, while in other cases water is liable to undo the action of the fixative, in which cases alcohol must be used for washing. After fixation in alcohol no washing, of course, is necessary. Speci- mens fixed in strong alcohol are embedded immediately (see Embed- ding, p. to), or preserved (see Preserving, p. 9). After fixation in dilute GENERAL TECHNIC. 9 alcohol the specimens are passed through the graded alcohols up to 8o-per-cent. After fixation in formaldehyde solutions the specimens are passed directly through the graded alcohols without washing in water. Speci- mens fixed in any solution containing picric acid should not be washed in water, but passed directly through the alcohols; and it is usually necessary to change each grade in order to wash out the picric acid. Specimens fixed in osmic acid or any solution containing osmic acid should be washed in running water before being passed through the graded alcohols. After solutions containing potassium bichro- mate the specimens should be washed in water suflficiently to remove the excess of bichromate, though too prolonged washing seems to be detrimental. A precipitate forms in the alcohols, but this apparently does no harm. After mercuric chlorid or Zenker fixation the washing may be done either in water or in alcohol. To avoid precipitates in the tissues add a small quantity of an iodin solution (equal parts tincture iodin and lo-per-cent. aqueous solution potassium iodid) to any of the grades of alcohol. As the alcohol becomes clear more of the solution is added until the alcohol remains slightly tinged. III. Preserving. Hardened tissues are usually preserved in 8o-per-cent. alcohol. Formalin in aqueous solutions of i-per-cent. to lo-per-cent. is also used as a preservative. In either case, when it is necessary to pre- serve the specimens for a considerable length of time (several months or longer), the tissues are likely to lose their staining qualities to a certain extent. Preserving the specimens in equal parts of strong alcohol, glycerin, and distilled water is successful as a partial remedy for this. IV. Decalcifying. Tissues containing lime salts, like bones and teeth, must have the lime salts dissolved out before sections can be cut. This process is known as decalcification. Tissues to be decalcified must be first fixed and hardened. Fix- ation in Orth's fluid and hardening in graded alcohols gi\'e good results. After hardening, the tissue is washed in water and placed in one of the 10 HISTOLOGICAL TECHNIC. following decalcifying fluids. The quantity of fluid should always be large and the fluid frequently changed. The completion of decalcifi- cation can be determined by passing a needle through the specimen or by cutting it with a scalpel. The time required varies with the size and hardness of the specimen and the decalcifying fluid used. (i) Hydrochloric Acid. — This may be used in aqueous solutions of from o. 5-per-cent. to 5-per-cent. A very satisfactory decalcifying mixture is that known as Ebner's hydrochloric-salt solution. It con- sists of: Sodium chlorid, saturated aqueous solution, i part. Water, 2 parts. Hydrochloric acid, sufficient to make a from 2-per-cent. to S-per- cent. solution. The addition of the salt prevents swelling of the tissue. This fluid is slow in acting and should be changed every day. When decalcifi- cation is complete, the specimen is washed in sufficient changes of water to remove all trace of acid. This may be quickly accomplished by the addition of a little ammonium hydrate to the water. The speci- men is then carried through graded alcohols. (2) Nitric Acid. — This is less used than the preceding. The strength should be from i-per-cent. to lo-per-cent. aqueous solution. Weak solutions (i-per-cent. to 2-per-cent.) wiU decalcify smaH foetal bones in from three to twelve days. For larger bones stronger solutions and longer time are required. (3) Small bones may be satisfactorily decalcified in Zenker' s fluid (see Fixatives, page 8), or in the following: Picric acid. I part. Chromic acid, I part. Glacial acetic acid. V. Embedding. 5 parts. Most hardened tissues are still not firm enough to be cut into the thin sections suitable for microscopic study. In order to support the tissue elements and render them more firm for section cutting, recourse is had to embedding. This consists in impregnating the tissues with some substance which is hquid when the tissues are placed in it, but which can be made to solidify throughout the tissues. In this way the tissue elements are held firmly in place. The embedding substances most used are cefloidin and paraffin. GENERAL TECHXIC. 11 Celloidix Embeddixg. (i) Alcohol-ether Celloidix. — Two solutions should be made. Solution No. 2. Thick celloidin — a 5-per-cent. solution of celloidin in equal parts 96-per-cent. alcohol and ether. Solution No 1. Thin celloidin — made by diluting solution Xo. 2 with an equal volume of equal parts of alcohol and ether. The hardened tissues are placed successively in : Strong alcohol (96-per-cent.) twelve to twenty-four hours, to dehy- drate. Equal parts alcohol and ether, twelve to twenty-four hours. Thin celloidin, twenty-four hours to several days. Thick celloidin, twenty-four hours or longer. The exact time tissues should remain in the different grades of celloidin depends upon the character of the tissue, the size of the speci- men, and the thinness of section desired. Many tissues may be advan- tageously left for weeks in thin celloidin. The specimen must now be "blocked" and the celloidin hardened. By blocking is meant fastening the specimen impregnated with celloidin to a block of wood or other suitable material which may be clamped in the microtome (see Section Cutting, p. 13). The specimen may be taken from the thick celloidin (considerable of the latter adhering to the specimen,^, quickly pressed upon a block of wood or vulcanized fibre, allowed to harden five to ten minutes in air, and then immersed in 80-per-cent. alcohol. The alcohol gives an even hardening of the celloidin, attaching the specimen firmly to the block. Another method, and one by which very even-shaped blocks may be obtained, is to place the specimen from the thick celloidin into a little paper box (made by folding paper over a wooden block), slightly larger than the specimen, and covering with thick celloidin. The celloidin should dry slowly under a bell-jar for from two to twelve hours, according to the amount of celloidin, after which it should be immersed in 80-per-cent. alcohol and the paper pulled off. Such a block may be cut into any desired shape. It is attached to the wooden or vulcanized block by dipping for a moment in thick celloidin, and tlien pressing firmly down upon the block. After five to ten minutes' drying in air it is transferred to 80-per-cent. alcohol. Old, hard, celloidin-embedded specimens are sometimes difticult to attach to blocks. This usually may be accomplished by first thor- oughly drying the specimen and then dipping it in equal parts alcohol 12 HISTOLOGICAL TECHNIC. and ether for a few minutes. This softens the surface of the celloidin, after which the specimen is dipped in thick celloidin and blocked. Embedded or blocked specimens can be kept in 8o-per-cent. alcohol. iVfter several months, however, the celloidin is likely to become too soft for good section cutting. In that case the specimens can be readily re-embedded by dissolving out the old celloidin with alcohol and ether and putting them again through the regular embedding process. (2) Clove-oil Celloidin. — A more rapid impregnation of the tissue may be obtained by means of what is known as clove-oil cel- loidin. Celloidin, 3° g^- Clove oil, 100 c.c. Ether, 4°° c.c. Alcohol, absolute, 20 c.c. The celloidin is first placed in a jar and the clove oil and ether added. From two to four days are required for solution of the celloidin. Dur- ing this time the jar should be shaken several times. After the celloidin is dissolved the absolute alcohol is added and the solution is ready for use. The specimen must be thoroughly dehydrated, placed in alcohol and ether or pure ether for a few hours, and then transferred to the clove-oil celloidin. From six to twelve hours is sufficient to impreg- nate small pieces of tissue. The tissue is now taken from the cel- loidin, placed directly upon a wooden or vulcanized block, and im- mersed in chloroform. The celloidin hardens in about an hour, and is then ready for sectioning. The specimen is very firm, and very thin sections can be cut. A disadvantage in clove-oil celloidin is that neither the blocks nor the sections can be kept permanently in alcohol, as can those embedded in alcohol-ether celloidin. They may, however, be kept for several weeks in pure chloroform. Paraffin Embedding. For paraffin embedding a thermostat or paraffin oven is necessary in order that a constant temperature may be maintained. The tem- perature should be about 56° C. Pure paraffin, the melting-point of which is from 50° to 55° C, is used. In very warm weather it may be necessary to add to this a little paraffin, the melting-point of which is 62° C. GENERAL TECHXIC . 13 The hardened tissue is first put in 96-per-cent. alcohol for from twelve to twenty-four hours, and then completely dehydrated by put- ting in absolute alcohol for the same length of time, or less for small specimens. It is then transferred to some solvent of paraffin. Some of the solvents used are xylol, oil of cedarwood, chloroform, and toluol. Of these the best are perhaps xylol and oil of cedarwood. The tissue should remain in either of these for several hours, or until the tissue becomes more or less transparent. It is then placed in melted par- affin, in the paraffin oven, for from one to three hours, according to the size and density of the specimen. This allows the tissues to become impregnated with the melted paraffin. The paraffin should be changed twice. In case of very delicate tissues it is well to transfer them from the absolute alcohol to a mixture of equal parts absolute alcohol and xylol for a short time before putting them into the pure xylol. In the same way a mixture of equal parts xylol and paraffin may be used before putting the tissues into pure paraffin. For hardening the paraffin in and around the tissue a very con- venient apparatus consists of a plate of glass and several L-shaped pieces of iron or lead. Two of these are placed on the glass plate in such a manner as to enclose a space of the desired size. Into this are placed the specimen and sufficient melted paraffin to cover it. Both glass and irons should be smeared with glycerin to prevent the paraffin from adhering, and should be as cold as possible, so that the paraffin may harden quickly. The same paper boxes described under celloidin embedding may also be used for paraffin. Another good method for small pieces of tissue is to place the specimen in paraffin in an ordinary watch-glass which has been coated with glycerin. Both paper-box and watch-glass specimens are immersed in cold water as soon as the surface of the paraffin has become hard. After the paraffin has hardened any excess may be cut away with a knife. Paraffin-embedded specimens may be kept indefinitely in air. For section cutting, the block of paraffin is attached to a block of wood or of vulcanite or to the metallic block-holder of the microtome. This is done by heating the block-holder, pressing the paraffin block firmly upon it, and then dipping the whole into cold water. VI. Section Cutting. The older method of making free-hand sections with a razor has been almost completely superseded by the use of a cutting instru- 14 HISTOLOGICAL TECHXIC. ment known as the microtome. This consists essentially of a clamp for holding the specimen and a microtome knife or razor. The two are so arranged that when knife and specimen meet, a section of any desired thickness may be cut. The technic of section cutting differs according to whether the specimen is embedded in celloidin or in paraffin. In cutting celloidin sections the knife is so adjusted that it passes obliquely through the specimen, as much as possible of the cutting edge being used. The knife is kept flooded with 8o-per-cent. alco- hol and the specimens are removed by means of a camel's-hair brush to a dish of 8o-per-cent. alcohol where they may be kept for some time if desired. When celloidin sections tear or when very thin sec- tions are desired, it is often of advantage to paint the surface of the block, after cutting each section, with a coat of very thin celloidin. Celloidin sections are usually not thinner than io,«, although under favorable conditions sections 5«^ or even 3« in thickness may be obtained. In cutting parafi&n sections the knife is kept dry and is passed not obliquely but straight through the specimen, only a small part of the cutting edge being used. An exception is made in the case of very large parafi&n sections, where an oblique knife is used. Sec- tions are removed from the knife by a dry or slightly moistened brush. If not desired for immediate use the sections may be conveniently kept for a short time on a piece of smooth paper. If sections curl they may be flattened by floating on warm 30-per-cent. alcohol or on warm water. Paraffin sections may be so cut that the edges of the sections adhere. Long series or "ribbons" of sections may thus be secured. This is of decided advantage when serial sections are desired. Failure of the sections to cut evenly or to adhere in ribbons, is usually due to the paraffin being too hard and brittle, which of course is due to its low temperature. If much section cutting is to be done, the operator will find himself amply repaid by having the room-temperature fairly high. In case paraffin of a melting-point of 50° to 55° C. is used, a room temperature of 73° to 75° F. will greatly facilitate the work. Where it is not possible to have a high room temperature, recourse may be had to coating the surface of the block with a paraffin of lower melting-point than that used for the embedding. A similar effect '/< = micromillimeter or micron = xihn "^ ^ millimeter = microscopic unit of measure = about ., '\-,r, of an inch. GENERAL TECHXIC. 15 may be obtained by holding a heated metal plate or bar near the block until the paraffin is slightly softened. This process may be repeated as often as necessary. In addition to the fact that ribbon series can be cut in paraffin, this embedding substance also has the advantage over celloidin that thinner sections can be obtained. On the other hand, celloidin em- bedding produces less shrinkage of the tissues than paraffin embedding with the accompanying heat. VII. Staining. This is for the purpose of more readily distinguishing the differ- ent tissue elements from one another by their reactions to certain dyes. Based upon their action upon the different tissue elements, stains may be classified as (i) nuclear dyes, which stain nuclear structures; and (2) plasma dyes, which stain the cell body or cytoplasm. Plasma dyes, also, as a rule, stain the intercellular tissue elements, and are therefore known as diffuse stains. The dyes most frequently used for staining tissues are: I. Nuclear dyes: (a) Haematoxylin and its active principle, hasmat- ein; (b) carmine and its active principle, carminic acid; (c) basic aniline dyes. II. Plasma dyes: (a) Eosin; (b) neutral carmine, (r) picric acid; (d) acid aniline dyes. I. Nuclear Dyes. — (a) H.^matoxylin. 1. Gage^s Hematoxylin. Ammonia or potash alum, 7.5 gm. Distilled water, 200.0 c.c. Boil for 10 minutes to sterilize; cool and add the following solution: Haematoxylin crystals, o.i gm. Alcohol 95-per-cent., 10. o c.c. Four grams chloral hydrate are then added to the mixture to prevent growth of germs. This dye may be used in full strength or diluted with alum water. It stains in from two to five minutes. 2. Delapehfs HcFmatoxylin. Hiematoxylin crystals, i gm. Alcohol, 6 c.c. Ammonia alum, saturated aqueous solution, 100 c.c. 16 HISTOLOGICAL TECHNTIC. The hsematoxylin should be first dissolved in the alcohol and then added to the alum solution. The mixture should next be allowed to stand in the light for from seven to ten days to ripen. It is then filtered, and to the filtrate are added : Glycerin, 25 c.c. Wood naphtha, 25 c.c. The mixture is again allowed to stand for from two to four days and filtered. It may be used full strength or diluted with equal parts of water. It stains in from two to five minutes. 3. H eidenhain'' s Hematoxylin. Hsematoxylin crystals, i gm. Water, 100 c.c. Sections are first placed for from four to eight hours in a 2. 5-per-cent. aqueous solution of ammonium sulphate of iron. They are then washed in water and transferred to the hsematoxylin solution until they are intensely blue or black (usually several hours). The sections are now washed in water and differentiated by again placing in the iron solution till they have a light grayish color. After this they are thor- oughly washed in water. 4. Mayer^s Hcemalum. Haematein, i gm. Alcohol, 50 c.c. Ammonia alum, 5-per-cent. aqueous solution, 1,000 c.c. The haematein is first dissolved in the alcohol, after which the alum is added. This dye does not require any ripening, and is thus avail- able for immediate use. It is a rapid nuclear dye usually requiring not more than from three to five minutes. 5. A combination of Gage's and Mayer's formulae makes a very satisfactory nuclear dye. Haematein, 5 gm- Alcohol, 50 c.c. Chloral hydrate. 20 gm. Ammonia alum, 5-per-cent. aqueous solution (steril- ized), 1,000 c.c. The haematein is first dissolved in the alcohol and then added with the chloral hydrate to the alum solution. This solution is used in full strength and stains in from three to five minutes. GENERAL TECHNIC. 17 6. Weigert's HcBmatoxylin. Two stock solutions should be made up as follows : A. i-per-cent. haematoxylin, in 96-per-cent. alcohol. B. Hydrochloric acid (sp. gr. 1.126), 10 c.c. Ferric chloride, 30-per-cent., 40 c.c. Distilled water, 950 c.c. For use, mix equal parts of A and B. The mixture will keep two or three days. This is a rapid stain usually requiring not more than a minute. A more brilliant nuclear stain may be obtained by over- staining and then decolorizing. After the stain wash the sections in water and then decolorize to the proper degree in weakly acidulated water. To stop the action of the acid the sections should be dipped in water made slightly alkaline with ammonia. This is an excellent stain and gives brilliant results. It is especially good in cases where the material has become old and lost its affinity for ordinary haematoxylin stains. {h) Alum-carmine. Carmine, 2 gm. Alum, 5 gm. Carbolic acid, 2 gm. Water, 100 c.c. The alum is first dissolved in the 100 c.c. of warm distilled water, after which the carmine is added. This mixture is then boiled for twenty minutes, allowed to cool, and filtered. The carbolic acid is then added. This is a slow-acting pure nuclear dye. (c) Basic Aniline Dyes — gentian violet, methyl violet, methyl green, methyl blue, toluidin blue, fuchsin, thionin, safranin, etc. These are best kept in stock in saturated alcoholic solutions. When desired to use, a few drops of the alcoholic solution are added to distilled water. No rule as to exact proportions can be given, as these depend upon the material, the fixation, and the intensity of stain desired. II. Plasma Dyes. — {a) Eosin. This is prepared as follows: Water-soluble eosin is dissohed in water to saturation. It is then precipitated by hydrochloric acid and the precipitate washed with water upon a filter until the filtrate is tinged with eosin. After drying, the precipitate is dissolved in strong alcohol, i gm. of eosin to 1,500 c.c. of alcohol. This is a rapid plasma stain. 18 HISTOLOGICAL TECHNIC. (b) NEUTILA.L Carmine. Carmine, i gm. Liquor ammonii caustici, 5 c.c. Distilled water, 50 c.c. The last two ingredients are first mixed, and tlie carmine then added. This solution is allowed to remain in an open vessel for about three days, or until the odor of ammonia has disappeared, after which it is filtered. (c) Picric Acid — used mainly as the plasma-staining element of such a staining mixture as picro-acid-fuchsin. (d) Acid Aniline Dyes. — Of these, acid fuchsin, erythrosin, and orange G are most used. They may be prepared and kept in stock in the same manner as the basic aniline dyes (see above). Erythrosin is of especial value for sections which take the eosin stain poorly. Staining Secticns. It is often of advantage to stain the different tissue elements dif- ferent colors. This may be accomplished either by staining suc- cessively with several dyes, or by a single staining with a mixture of dyes. The following are the methods in most common use: (i) Staining Double w^ith Hematoxylin and Eosin. — Sec- tions are first washed in water. They are then stained with hsema- toxylin (solutions i, 2, 4, 5, or 6, pp. 15-17) from one to five min- utes. After being thoroughly washed in water, they are dehydrated in strong alcohol and transferred to the alcoholic eosin solution (page 17). Most sections stain in from two to five minutes. By this method nuclei are stained blue or purple, cell bodies and intercellular substances red. Very often a more brilliant staining may be accomplished as fol- lows: Overstain in hasmatoxylin and wash thoroughly in water; de- colorize in water slightly acidulated (8 or 10 drops of hydrochloric acid to 100 c.c. of water) until only the nuclei retain the stain; wash in water which has been made slightly alkaline with ammonium hy- drate; then stain with eosin as usual. (2) Staining with Picro-acid-euchsin. Acid-fuchsin, i-per-cent. aqueous solution, 5 c.c. Picric acid, saturated aqueous solution, 100 c.c. This solution usually stains in from one to three minutes. Occa- sionally a longer staining is required. Cell bodies including muscle GENERAL TECHXIC. 19 cells and fibres are stained yellow by the picric acid, connective- tissue fibres red by the fuchsin. After staining, the sections are washed in distilled water. (3) Triple Staining with H.ematoxylin and Picro-acid- FUCHSIN. — This is the same as the preceding except that before stain- ing with picro-acid-fuchsin, the sections are overstained in haema- toxylin (solutions i, 2, 4, 5, or 6, pp. 15-17). The usual purple of haematoxylin-stained nuclei is changed to brown by the action of the picric acid. Care should be taken that the sections do not remain too long in the picro-acid-fuchsin, or the haematoxylin may be com- pletely removed. After staining, sections are washed in distilled water and transferred to 96-per-cent. alcohol. If sections overstain with fuchsin, the staining solution may be diluted with water; if sections are understained with fuchsin, more fuchsin may be added. If the picric-acid stain is not sufficiently intense, the 96-per-cent. alcohol should be tinged with picric acid. (4) Staining with Picro-carmine. Ammonium carminate, i gm. Distilled water, 35 c.c. Picric acid, saturated aqueous solution, 15 c.c. The ammonium carminate is first dissolved in the water, after which the saturated aqueous solution of picric acid is added with constant stirring. The mixture is then allowed to stand in an open vessel for two days, when it is filtered. This fluid stains nuclei and connective tissue red, cell protoplasm yellow. Staining in Bulk. By this is meant the staining of blocks of tissue before cutting into sections. The method is much less used than formerly. It is slower than section staining and more difficult to control. Blocks of the hardened tissue are transferred to the stain from water or alcohol according to the solvent of the stain. Alum-carmine and borax- carmine are the most used general bulk stains. (i) Alum-carmine. Carmine, 0.5 to i gm. Ammonia alum, 4-per-cent. aqueous solution, 100 c.c. After mixing the ingredients, the solution should be boiled fifteen minutes, and after cooling, enough sterile water added to replace that lost by evaporation. The time reciuircd for staining depends upon 20 HISTOLOGICAL TECHNIC. the size of the specimen. There is, however, Httle danger of over- staining. After washing out the excess of stain with water the speci- men is dehydrated and embedded in the usual way. (2) Borax- CARMINE, Alcoholic Solution. Carmine, 3 gm. Borax, 4 gm. Water, 93 c.c. After mixing the above, add 100 c.c. 70-per-cent. alcohol, allow the mixture to settle, then filter. About twenty-four hours is required to stain blocks 0.5 cm. in diameter. Larger blocks require longer staining. VIII. Mounting. It is usually desirable to make permanent preparations or '' mounts " of the stained specimens. The most satisfactory media for mounting specimens are glycerin and Canada balsam. (i) Glycerin. — Sections may be transferred to glycerin from either water or alcohol. In the case of double-stained specimens — haematoxylin-eosin — the glycerin should be tinged with eosin, as the pure glycerin abstracts the eosin from the tissues. In many cases satisfactory eosin staining may be obtained by simply placing the haematoxylin-stained specimens in glycerin strongly tinged with eosin (eosin-glycerin). The specimen in a drop of glycerin is transferred to the glass mounting slide, the excess of glycerin removed with filter paper or with a pipette, and a cover-glass put on. Glycerin mounts, as a general rule, are unsatisfactory. Further- more, they must be cemented to exclude the air. This can be done by painting a ring of gold-size, or a thick solution of gum shellac in alcohol to which a little castor oil has been added, around and over the edge of the cover-glass. Both cover-glass and slide must be cleaned free from glycerin before the cement is applied. A camel's-hair brush moistened with alcohol is the best means of removing the excess of glycerin. Glycerin jelly is a more satisfactory mounting medium than pure glycerin. It can be purchased from firms dealing in microscopical supplies and needs merely the application of heat to make it fluid. Specimens can then be mounted in it in the usual manner, and after being allowed to cool, do not require cementing. GENERAL TECHNIC. 21 (2) Balsam. — This is the most satisfactory general mounting medium. It has an advantage over glycerin in drying down perfectly hard and thus needing no cement, and in preserving colors more per- manently. Its disadvantage is that its refractive index is so high that it sometimes obscures the finer details of structure, especially of unstained or slightly stained specimens. Specially prepared Canada balsam is dissolved either in xylol or' in oil of cedar, the solution being made of any desired consistence. Xylol-balsam dries much more quickly than does the oil-of-cedar balsam. Preparatory to mounting in balsam, stained sections must be thoroughly dehydrated and then passed through some medium which is miscible with both alcohol and balsam. This medium, which at the same time renders the section transparent, is known as a clearing medium. For celloidin specimens the most satisfactory are: (i) Oil of origanum Cretici. (2) Carbol-xylol (xylol, 100 c.c, carbolic-acid crystals, 22 gm.), followed by pure xylol. (3) Xylol and cajeput oil, equal parts, followed by pure xylol. After clearing, the section is transferred by means of a section- lifter to a glass mounting slide. In case oil of origanum is used, it is then blotted firmly with filter paper to remove the excess of oil. Care must be taken to have the filter paper several layers thick in order that the oil may be completely removed. The specimen should also be blotted firmly, giving the oil time to soak into the paper. These two precautions are necessary to prevent the section adhering to the paper instead of to the slide. After blotting, a drop of balsam is placed upon the centre of the specimen and a cover-glass put on. When xylol is used, blotting is not necessary. Drain off the excess of oil, put a drop of balsam on the specimen and put on a cover-glass. Paraffin Sections. — The technic of staining and mounting paraffin sections differs from that of celloidin sections. This is due mainly to the fact that while celloidin is transparent and may remain per- manently in the specimen, paraffin is opaque and must be dissolved out before the section is fit for microscopic study. Bulk staining with carmine (page 19) is frequently used for speci- mens which are to be embedded in paraffin. Sections may be counter- stained if desired. The following are the steps to be followed in staining and mounting paraffin sections: 22 HISTOLOGICAL TECHNIC. 1. To attach sections to slide: Place a drop of egg albumen (equal parts white of egg and glycerin to which a little carbolic acid may be added for preserving) on a slide, and spread it out thin with the finger. Place a few drops of distilled water on the slide. Float sections on the water. Warm gently to allow sections to flatten — must not melt paraffin. Pour off excess of water, holding the ends of the ribbons to prevent them floating off. Stand slides on end a few hours to allow water to evaporate. 2. To remove parajfin: Place slide in xylol three to five minutes. 3. To stain sections: Place slide in fresh xylol three minutes. Transfer to absolute alcohol. Transfer to go-per-cent. alcohol. Transfer to 80-per-cent. alcohol. Transfer to 50-per-cent. alcohol. (May be omitted.) Transfer to water. Stain with an aqueous stain. Wash in water. Transfer to 50-per-cent. alcohol. (May be omitted.) Transfer to 80-per-cent. alcohol. Transfer to 90-per-cent. alcohol. Transfer to absolute alcohol. Transfer to xylol. Transfer to fresh xylol. Mount in xylol-balsam. If an alcohol stain is used instead of an aqueous one, the carrying down and up through the graded alcohols is omitted. If it is desired to stain double with eosin-haematoxylin (page 18) use the above technic in staining with haematoxylin ; then the alcoholic eosin stain before final transfer to absolute alcohol. IX. Injection. For the study of the distribution of the blood-vessels in tissues and organs, it is often necessary to make use of sections in which tlic blood-vessels have been injected with some transparent coloring matter. The injecting fluid most commonly used is a solution of colored gelatin. GENERAL TECHNIC. 23 The gelatin solution is prepared by soaking i part gelatin in from 5 to lo parts water — the proportion depending upon the consistence desired — and when soft, melting on a water-bath. Various dyes are used for coloring the gelatin, the most common being Prussian blue and carmine. Prussian blue gelatin is prepared by adding saturated aqueous solution Prussian blue to the gelatin solution, the proportions depend- ing upon the depth of color desired. Both solutions should be at a temperature of 60° C. After thoroughly mixing, the blue gelatin is filtered through cloth. Carmine gelatin is prepared by first dissolving i gm. carmine in 30 c.c. distilled water. To this is added ammonia until the mix- ture becomes a dark cherry red. A lo-per-cent. aqueous solution of acetic acid is next added, drop by drop, with constant stirring until the mixture becomes neutral. The carmine and gelatin solutions both being at about 60° C, are now mixed in the desired proportions. If the carmine injection mass is alkaline, it diffuses through the walls of the vessels; if acid, there is a precipitation of the carmine which may interfere with its free passage through the capillaries. If, how- ever, the alkaline carmine and gelatin be first mixed, and the 10- per-cent. acetic acid solution be then added as directed above, the precipitated granules are so fine, even with an acid reaction, that they readily pass through the capillaries. The precipitation of the car- mine in the shape of coarser granules is of advantage when it is desired to have an injection mass which will fill the arteries or veins only, without passing over into the capillaries. The in'ecting apparatus consists of a vessel which contains the injection mass, and some means of keeping the latter under a con- stant but easily varied pressure. With the vessel is connected a tube ending in a cannula, through which the injection is made. A very simple apparatus consists of a shelf which can be raised and lowered, and upon which the vessel stands. The tube connect- ing with the cannula may be attached to a faucet in the vessel or to a bent glass tube which passes into the top of the vessel and acts on the principle of a siphon. In a somewhat more elaborate apparatus the injection mass is placed in a closed vessel, and this is connected with a second vessel containing air compressed by means of an air pump. Accurate regulation of the pressure may be obtained by connect- ing the injection vessel with a manometer. 24 HISTOLOGICAL TECHXIC. If the injection is to occupy considerable time, a hot-water bath in which the gelatin may be kept at an even temperature is also necessary. Whole animals or separate organs may be injected. For inject- ing a whole animal, the animal, which is usually a small one such as a guinea-pig, rat, mouse, or frog, is chloroformed, the tip of the heart is cut away and a cannula is inserted through the heart into the aorta. This is first connected with a tube leading to a bottle containing warm normal saline solution. Pressure is obtained in the same manner as above described for the injection mass. By this means the entire arterial and venous systems are thoroughly washed out until the return flow^ from the vena cava is perfectly clear. The cannula is next connected with the tube from the vessel containing the injection mass, the pressure being only sufficient to keep the liquid flowing. When the injection mass flows easily and freely from the ^■ena cava, the vessel is tied and the pressure is increased slightly and continued until the color of the injection mass shows clearly in the superficial capillaries. The aorta is now tied and the animal immersed in cold water to solidify the gelatin. After the gelatin becomes hard, the desired organs are removed and fixed and hardened in the usual way. Sections of injected material are usually cut rather thick, that the vessels may be traced the greater distance. Better results are frequently obtained by injecting separate organs. This is accomplished by injecting through the main artery of the organ (e.g., the lungs through the pulmonary, the kidney through the renal). The injection is best done with the organ in situ, although it may be accomplished after the organ has been removed. The method is the same as given above for injecting an animal in toto. The so-called double injection by means of which an attempt is made to fill the arteries with an injection mass of one color (red), while the veins are filled with an injection mass of another color (blue), often gives pretty, but usually inaccurate pictures, it being, as a rule, impossible to confine each injection mass to one system. Double injection is accomplished by first washing out the vessels with normal saline and then connecting the artery with the red gela- tin, the vein with the blue gelatin, and injecting both at the same time, the pressure dri\ing the saline out of the vessels into the tis- sues. The difficulty is that either the arterial injection carries over into the veins, or the venous injection carries over into the arteries. A somewhat more accurate method is first to inject the veins with an GENERAL TECHXIC. 25 injection mass in which the coloring matter is in the form of granules too large to pass through the capillaries, and then to inject the arte- ries and capillaries in the usual manner. This method is especially useful in demonstrating the vessels of the kidney, liver, and gastro- intestinal canal. CHAPTER II. SPECIAL STAINING METHODS. Of these only the more common will be described. (1) SiLVER-KITRATE METHOD OF StAINING InTERCELLUI AR SUB- STANCE.— After first washing, the tissue, e.g., omentum or cornea, is placed in a from 0.2- to i-per-cent. solution of silver nitrate, or better, protargol, where it is kept in the dark for a half-hour or more according to the thickness and density of the tissue. The specimen is then washed in water, transferred to 40-per-cent. alcohol and placed in the direct sunlight until it assumes a light brown color. It is then placed in fresh 80-per-cent. alcohol for preservation. (2) Chlorid of gold in i-per-cent. aqueous solution is used in the same manner for demonstrating connective-tissue cells and their finer processes. (3) Weigert's Elastic-tissue Stain. — This is prepared as follows: Fuchsin, 2 gm. Resorcin, 4 gm. Water, 200 c.c. These are boiled for five minutes, during which 25 c.c. of liquor ferri sesquichlorati are stirred in. The result is a precipitate which should be filtered out after the liquid has become cool. After drying, 200 c.c. of 95-per-cent. alcohol are added to the filtrate and boiled until the latter dissolves. Lastly, 4 c.c. of hydric chlorid are added to the solution. Sections should remain in the stain thirty minutes, after which they are washed in alcohol until the stain ceases to be given off. (4) GoLGi's Chrome-silver Method for Demonstrating Secretory Tubules. — Small pieces of perfectly fresh tissue, e.g., liver, are placed in the following: Potassium bichromate, 4-per-cent. aqueous solution, 4 vols. Osmic acid, i-per-cent. aqueous solution, i vol. After three days they are transferred without washing to a 0.75-per- cent, aqueous solution of silver nitrate, which should be changed as 26 SPECIAL STAINING METHODS. 27 soon as a precipitate forms. The specimens remain in the second silver solution from two to three days, after which they are rapidly de- hydrated, embedded in celloidin, and cut into rather thick sections. (5) Mallory's Phosphomolybdic Acid H.ematoxylin Stain for Connective Tissue. — Thin sections are placed for from two to ten minutes in a lo-per-cent. aqueous solution of phosphomolybdic acid. They are then washed in distilled water and tranferred to: Phosphomolybdic acid, ic-per-cent. aqueous solution, 100. o c.c. Distilled water, 200.0 c.c. Haematoxylin crystals, i . 75 gm. Carbolic-acid crystals, 5- 00 gm. The phosphomolybdic acid and water are first mixed, after which the haematoxylin and carbolic acid are added. After staining from ten to twenty minutes the sections are washed in distilled water, placed for five minutes in 50-per-cent. alcohol, then in strong alcohol, cleared in xylol and mounted in xylol-balsam. This stain works best after Zenker's fluid fixation. (6) Mallory's Phosphotungstic Acid H^ematoxyiin Stain for Connective Tissue. Haematoxylin (or htematein ammonium), 0,1 gm. Water, 80. c.c. Phosphotungstic acid (Merck), lo-per-cent. aqueous solution, 20. c.c. The haematoxylin should be dissolved in a little water with the aid of heat and, when cool, added to the rest of the solution. Allow to ripen for several weeks or months. Or, if it is desired to use immediately, the solution can be ripened by the addition of 10 c.c. of a 1/4-per-cent. solution of potassium permanganate. Tissues should be fixed in Zenker's fluid (p. 8). 1. Treat sections with iodin solution to remove mercury precipi- tate, 5 to 10 minutes (p. 9.) 2. Several changes 95-per-cent. alcohol. 3. Water. 4. One-fourth-per-cent. aqueous solution potassium permanganate, 5 to 10 minutes. 5. Wash in water. 6. Five-per-cent. aqueous solution oxalic acid, 5 to 10 minutes. 7. Wash thoroughly in water. 8. Phosphotungstic acid haematoxylin solution, twelve to twenty- four hours. 28 HISTOLOGICAL TECHNIC. 9. Dip for a few seconds in 95-per-cent. alcohol. 10. Clear in carbol-xylol and xylol and mount in xylol-damar. (j) Mallory's Aniline Blue Stain for Connective Tissue. — Tissues should be fixed in Zenker's fluid (p. 8.) 1. Stain thin sections in a o. 2-per-cent. aqueous solution of acid fuchsin for five to ten minutes. (This step may be omitted.) 2. Stain in the following solution for twenty minutes: Aniline blue (soluble in water), 0.5 gm. Orange G., 2 .0 gm. Phosphomolybdic acid, i-per-cent. aqueous solution, 100. o cc. 3. Decolorize in several changes of 95-per-cent. alcohol. 4. Clear in carbol-xylol and xylol and mount in xylol-damar. (8) OsMic-ACiD Stain for Fat. — For this purpose osmic acid is used in a i-per-cent. aqueous solution. The method is especially useful for demonstrating developing fat, fatty secretions (mammary gland), and fat absorption (small intestine). Very small bits of the tissue are placed in the osmic-acid solution for from twelve to twenty- four hours. They are then hardened in graded alcohols, embedded in celloidin, and the sections mounted in glycerin. (9) Jenner's Blood Stain. Water-soluble eosin (Griibler), i-per-cent. aqueous solution, 100 cc. Methylene blue — pure (Griibler), i-per-cent. aqueous solution, 100 cc. Mix, and after standing 24 hours, filter. The filtrate is dried at 65° C, washed, again dried and powdered. To make the staining solution, 0.5 gm. of the powder is dissolved in TOO cc. pure methyl alcohol. Blood smears stain in from two to five minutes. They are then washed in water, dried, and mounted in balsam. This solution acts as a fixative as well as a stain. CHAPTER III. SPECIAL NEUROLOGICAL STAINING METHODS. Weigert's Method of Staining Medullated Nerve Fibres. In preparing material for the Weigert method, two points are to be kept in mind: ist, proper fixation and preservation of the myeHn sheaths; 2d, treatment (mordanting) with a reagent which enters into combination with the myelin, the result being that the myelin sheaths stain specifically with haematoxylin. Formalin fulfils the first requirement, the bichromates the first and second. Consequently the material may be fixed and hardened in bichromate, and, if not to be used immediately, is best kept in formalin to avoid overhardening. Or the material may be fixed and kept in formalin and impregnated with the bichromate before using, the latter being done before dehy- drating in alcohol. Further mordanting, which is usually done, espe- cially when the material has been kept for some time in formalin or alcohol, is for the purpose of intensifying the stain. Material is fixed in one of the following fluids: (a) Muller's fluid (page 6). (b) Potassium bichromate, 5-per-cent. aqueous solution. (c) Formalin, lo-per-cent. aqueous solution. (d) Formalin, i volume; potassium bichromate, 5-per-cent. aque- ous solution, 9 volumes. In Muller's fluid or in plain potassium-bichromate solution a hardening of two days to four weeks is required ; in formalin or formalin- bichromate from a week to ten days is sufficient. All material is better kept until used in 5-per-cent. to lo-per-cent. formalin solution than in alcohol. The specimens are then hardened in graded alcohols, em- bedded in celloidin, and sections cut in the usual way. Material fixed in formalin should be placed for several days in the following: Chrome alum, '' i gm. Potassium bichromate, 3 gm. Water, 100 c.c. before hardening in alcohol. Sections from material fixed in any of the chrome-salt solutions are placed for from twch'e to twenty-four liours in a saturated aqueous 29 30 HISTOLOGICAL TECHNIC. solution of neutral cupric acetate diluted with an equal volume of water. The cupric acetate forms some combination with the tissues which intensifies their staining qualities, thus acting like the chrome salts as a mordant. iVfter mordanting, the sections are washed in water and trans- ferred to the following staining fluid: Haematoxylin crystals, i gm. Alcohol, 95-per-cent., 10 c.c. Lithium carbonate — saturated aqueous solution, i c.c. Water, 90 c.c. This solution must either be freshly made before using, the haema- toxylin being dissolved first in the alcohol, or the hasmatoxylin may be kept in lo-per-cent. alcoholic solution, the lithium carbonate in saturated aqueous solution, and the staining fluid made from these as needed. Sections remain in the haematoxylin solution from two to twenty- four hours, the longer time being required for staining the finer fibres of the cerebral and cerebellar cortices. They are then washed in water and decolorized in the following: Potassium ferricyanid, 2 . 5 gm. Sodium biborate, 2.0 gm. Water, 300.0 c.c. While in the decolorizer, sections should be gently shaken or moved about with a glass rod to insure equal decolorization. In the decolor- izer the sections lose the uniform black which they had on removal from the haematoxylin. They remain in the decolorizing fluid until the gray matter becomes a light gray or yellow color, in sharp con- trast with the white matter which remains dark. Sections are then washed in several waters to remove all traces of decolorizer, and de- hydrated in alcohol. Weigert-Pal Method. — In this modification of the Weigert method, material hardened in formalin should be further hardened in potassium bichromate 5-per-cent. for two weeks, or in copper bichro- mate 3-per-cent. for about a week, after which it may be cut and stained without further mordanting. Sections from material hard- ened in the other above-mentioned ways are mordanted in a 3- to 5- per-cent. aqueous solution of potassium bichromate instead of in the copper-acetate solution. After rinsing in water the sections are stained in haematoxylin as in the ordinary Weigert method. The SPECIAL NEUROLOGICAL STAINING METHODS. 31 lithium carbonate may, however, be omitted. They are then washed and transferred to a 0.25-per-cent. solution of potassium permanganate, where they remain from one-half to two minutes, after which they are again washed and placed in the following: Oxalic acid, i gm. Potassium sulphite, i gm. Water, 200 c.c. In this solution differentiation takes place, the medullary sheaths remaining dark, while the color is entirely removed from the rest of the tissue. If the section is still too dark, it may again be carried through the permanganate and oxalic-acid solutions, rinsing in water between changes, until sufficiently decolorized. All formalin-fixed material is best stained by the Weigert-Pal method. An intensification of the stain, especially of the A'ery fine fibres, may sometimes be obtained by placing the sections for a minute in a o. 5-per-cent. aqueous solution of osmic acid before decolorizing. Marchi's Method for Staining Degenerating Nerves. Small pieces of tissue are fixed and hardened for from seven to ten days in Mtiller's fluid. Thin slices of the tissue are then transferred to a solution of one part i-per-cent. osmic acid and two parts Miiller's fluid, in which they remain from two to seven days. After embedding and sectioning in the usual manner, sections are mounted, usually without further staining, in xylol-balsam. The treatment with Miiller's fluid so affects the normal medullary sheaths that they will not take the osmic-acid stain, but appear yellowish-brown, while the degenerating sheaths (probably fatty) stain black. The result is a positive picture of stained degenerating fibres in contrast with the stained normal and unstained degenerated fibres as seen after Weigert staining. Another advantage of the Marchi method is that, as the picture is a positive one, an early or slight degeneration may be recognized which would escape notice in material stained by Weigert's method; on the other hand, in a long-standing degeneration when the medullary sheaths ha\'e completely disappeared and their places have been taken by connective tissue, there being no degenerated myelin remaining, the Marchi method is inapplicable. Busch's modification of the Marchi method gives sharp pictures and has the advantag;e of allowing; formaldehvd fixation and harden- 32 HISTOLOGICAL TECHNIC. ing. Tissues thus treated are placed for from five to seven days in a solution of one part osmic acid, three parts iodate of sodium, and 300 parts water. They are then embedded, cut, and mounted as usual. GoLGi Methods of Staining Nerve Tissue. The Golgi methods in most common use at present are the fol- lowing: (i) GoLGi Silver Methods. — (a) Slow Method. — Blocks of tis- sue are placed for several months in a 3-per-cent. aqueous solution of potassium bichromate. Small pieces of the tissue are then trans- ferred immediately to a o. 75-per-cent. aqueous solution of silver nitrate. This is changed several times or until no more precipitate is formed. In the last silver solution they remain for from one to three days. The only method of determining whether the tissue has been sufficiently long in the bichromate is to try at intervals small bits of the tissue in the silver solution until a satisfactory result is secured. Sections should usually be from 50 to 8o/< thick and are mounted in balsam without a cover-glass. (b) Rapid Method. — Small pieces of tissue, 2 to 4 mm. thick, are placed in the following solution for from two to six days, the time depending upon the age and character of the tissue, the temperature at which fixation is carried on, and the elements which it is desired to impregnate: Osmic acid, i-per-cent. aqueous solution, i part. Potassium bichromate, 3. 5-per-cent. aqueous solution, 4 parts. As a rule, the longer the hardening the fewer are the elements stained, but these few are clearer. The tissue is next transferred to silver nitrate as in the slow method. Pieces of tissue should be tried each day until a satisfactory result is obtained. The pieces may be kept in silver nitrate some time, but not in alcohol, and are better cut with- out embedding, the pieces being simply washed in 95-per-cent. alcohol several hours, then gummed to the block with celloidin, cut in 95- pcr-cent. alcohol, and mounted as in the slow method. (c) Mixed Method. — Specimens are placed in the bichromate solution for about four days, then from one to three days in the osmium- bichromate mixture (see Rapid Method), after which they are trans- ferred to the silver solution (see Slow Method). SPECIAL NEUROLOGICAL STAIXIXG METHODS. 33 (d) Formalin-bichromate Method. — Tissues are placed for from two to six days in the following solution: Formalin, lo to 20 parts. Potassium bicliromate, 3-per-cent. aqueous solution, 90 to 80 parts. Subsequent treatment with silver is the same as in the previously described method. The results resemble those of the slow method. The specimens may be kept in strong alcohol. The method is satis- factory only for the adult cerebrum and cerebellum. (2) GoLGi BiCHLORiD METHOD. — Material, which need not be cut into small pieces, remains for several months in the potassium- bichromate solution (see Slow Silver Method), after which it is trans- ferred to a 0.25-per-cent. to i-per-cent. aqueous solution of mercuric chlorid for from four to twelve months or longer, the solution being changed as often as discolored. The degree of impregnation must be determined by frequently testing the material, but is usually indicated by the appearance of small white spots on the surface of the tissue. A modification of the bichlorid method, known as the Cox-Golgi Method, often gives good results. The following fixing solution is used : Potassium bichromate, 5-per-cent. aqueous solution, 20 parts. Mercuric chlorid, 5-per-cent. aqueous solution, 20 parts. Distilled water, 40 parts. After mixing the above, add Potassium chromate, 5-per-cent. aqueous solution, 16 parts. Tissues remain in this iiuid for from two to five months. In the Golgi silver methods the result of the treatment, first with bichromate and then with silver nitrate, is that a precipitate is formed in the tissue, a chromate or some other silver salt, which in favorable cases is largely confined to certain of the nerve cells and their processes. It must l)e remembered that only a few of the cells and processes are stained, these often only partially, and that other irregular precipita- tions arc usually present. In the mercury methods, the bichromate of potassium and the bichlorid of mercury may be used coml)incd in the same solution. There are other modifications of the Golgi methods, in which similar precipitates of other metallic salts are secured. Golgi specimens should be dehydrated and embedded as rapidly as possible. This is especially true of specimens treated by the rapid and the mixed methods. Those treated bv tlie slow sih'er method 34 HISTOLOGICAL TECHNIC. and by the bichlorid method are more permanent, and more time may be taken with their dehydration. Sections should be cut thick (75 to loo/i) and mounted in xylol-balsam. After the rapid method, it is safer to mount without a cover- glass; after the slow method, specimens may be mounted with or without a cover. The balsam should be hard, and melted at the time of using. (See Mounting, page 21.) Cajal's Methods for Staining the Neurofibrils in the Nerve-cells. In these methods, besides the neurofibrils, the cell processes and especially the axis cylinders are often beautifully displayed, the stain giving a picture in this respect much more general than that of the Golgi methods, but much more specific, and sharper than that of the ordinary stains. The methods consist mainly of two steps: (i) The staining of the tissue in a solution of silver nitrate; (2) the further reduction of the silver stain with a weak photographic developer. Three methods, or variations, are here given: (i) Pieces about 0.5 cm. thick are placed in a liberal quantity of from i-per-cent. or i . 5-per-cent. (new-born or embryonic mammalian material) to 5-per-cent. (adult material) solution of silver nitrate and kept at a temperature of 32° to 40° C. for two to five days. When properly stained (shown by a yellowish or light brown coloration of freshly cut surfaces) the pieces are very briefly rinsed in distilled water and placed in: pyrogallol (or hydroquinone) i gram, distilled water 100 c.c, formalin 5 to 10 c.c, for twenty-four hours or more. They are then washed a few minutes in water and transferred to 95-per-cent. alcohol, which is changed when discolored, and where they may often be kept for some time without injury. They may then be embedded in celloidin or parafhn and sections cut, usually 15-25// in thickness. Different depths of the blocks of tissue usually vary in stain, the most favorable being intermediate between the surface and centre of the block. Celloidin sections usually keep well in 95-per-cent. alcohol. They may be cleared in carbol-xylol, rinsed in xylol, and mounted in xylol-balsam or xylol-damar in the usual way. In delicate objects (study of pathological changes in neurofibrils) it may be best to abbre- viate the dehydration, and block and cut without infiltration with celloidin. (2) Pieces are first placed in 95-per-cent. alcohol or in absolute alcohol (32° to 40° C.) for twenty-four hours. For neurofibrils it is SPECIAL NEUROLOGICAL STAINING METHODS. 35 better to add from 0.25 c.c. to i c.c. of ammonium hydrate to each 10 c.c. of the alcohoh They are then treated with silver nitrate i-per- cent. or 1.5-per-cent., as in (i). This method gives better pictures of the cell processes and axis cylinders and a better fixation of the cells. (3) Pieces are first placed in distilled water 100 c.c, formalin 20 c.c, ammonia i c.c, for twenty-four hours at 32°-4o° C, washed in water twelve to twenty-four hours, and then treated with silver nitrate i-per-cent. or 1.5-per-cent., as in (i). This method gives pictures of the terminations of nerve fibres on the periphery of nerve- cells and their dendrites (end-feet or end-buttons of Auerbach). In general it is best to a^'oid, in the above methods, any excessive exposure to the light while the pieces are in the silver bath (especially when the pieces are very small), though they may be brought into the light for examination and while being transferred to the reducing fluid. Nissl's Method. This method is useful for studying the internal structure of the nerve cell. It depends upon a rapid fixation of the tissue, its subse- quent staining with an aniline dye, and final decolorization in alcohol. The aniline dye most commonly used is methylene blue. There are many variations and modifications of Nissl's method. The fol- lowing is simple and gives uniformly good results: Specimens are first fixed in mercuric-chlorid solution (page 7), in formalin (lo-per-cent. aqueous solution), or in absolute alcohol, and embedded in celloidin. Thin sections are stained in a i-per-cent. aqueous solution of pure methylene blue (Griibler). The sections are gently warmed in the solution until steam begins to be gi^'en oft". They are then washed in water and differentiated in strong alcohol. The degree of decolor- ization which gives the best results can be learned only by practice. Several alcohols must be used, and the last alcohol must be perfectly free from methylene blue. The sections are cleared in equal parts xylol and cajeput oil and mounted in xylol-balsam. A contrast stain may be obtained by having a little eosin or erythrosin in the last alcohol. General References for Further Study of Technic. Freeborn: Histological Technic. Reference Handbook of ^ledical Sciences, vol. iv. Lee: The ^Microtomist's \'ade-mecum. Mallorv and Writrhl: Patholosjical Technic. PART II. THE CELL. CHAPTER I. THE CELL. In the simplest forms of animal life the entire body consists of a little albuminous structure, the essential peculiarity of which is that it possesses properties which we recognize as characteristic of living organisms. This albuminous material basis of life is known as pro- toplasm, while the structure itself is known as a cell. Within the cell is usually found a specially formed part, the nucleus. Peripher- ally some cells are limited by a distinct cell wall or cell membrane. 2 3 4- 5 6 7 Fig. I. — Diagram of a typical cell, i, Cell membrane. 2, Metaplasm granules. 3, Karyosome or net-knob. 4, Hyaloplasm. 5, Spongioplasm. 6, Linin network. 7, Nucleoplasm. 8, Attraction-sphere. 9, Centrosome. 10, Plastids. 11, Chro- matin network. 12, Nuclear membrane. 13, Nucleolus. 14, Vacuole. An actively multiplying cell contains a minute structure associated with the reproductive function and known as the centrosome. A typical cell thus consists of the following structures (Fig. i): (i) The cell body; (2) the cell membrane; (3) the nucleus; (4) the centrosome. Of these the cell body is the only one present in all cells. Most animal cells have no cell membrane. A few cells con- tain, in their fully developed condition, no nuclei. In many mature cells it is impossible to distinguish a centrosome. All plants and animals consist of cells and their derivatives, and if an attempt be made to resolve any of the more complex li\ing struc- tures into its component elements, it is found that the smallest possible 39 40 THE CELL. subdivision still compatible with life is the cell. The cell may there- fore be considered as the histological element or unit of structure. I. The Cell Body. — This consists of a viscid semi-fluid substance, belonging to the general class of albumens. It is of complex chemical composition containing the elements carbon, hydrogen, oxygen and nitrogen in quite constant proportions, and smaller variable quanti- ties of phosphorus, sulphur, iron and other substances. It contains a peculiar nitrogenous proteid, plastin. Structurally it can be differ- entiated into a formed element, spongioplasm, and a homogeneous Q element, hyaloplasm. Dis- tributed along the spongio- plastic network are minute granules, microsomes. The exact relations which these elements bear to one another and to the cell as a whole have been the subject of much investigation and speculation. The earlier cytologists con- cerned themselves with the question as to whether proto- plasm was homogeneous {i.e., a mere solution or at most a mixture of various substances) or had a definite structure. The theory of a structureless protoplasm having been long Fig. 2. — Diagram Illustrating Theories of Protoplasmic Structure, a, Fibrillar theory'; b, granule theon,-; c, "foam" theory. (The gen- eral structure of cell body and nucleus cor- responds.) since abandoned, the question as to the character of the protoplasmic structure still remains unanswered. Altmann'.s granule theory considers protoplasm as composed of fine granules embedded in a gelatinous intergranular substance. Altmann believed these granules the ultimate vital elements, and for this reason gave them the name of bioblasts (Fig. 2, b). According to Butschli, |jrotoplasm is a foam or emulsion, the microscopic ap- jjearancc of which can be simulated by artificial emulsions. He ascribes the appearance of a reticulum to the fact that each little foam space forms a complete cavity filled with fluid, the cut walls of these spaces giving a reticular appearance on section (Fig. 2, c and Fig. 3). Other investigators consider protoplasm as made up of (i) a fibrillar element, either in the form of a network of anastomosing fibrils (cytoreticulum) or of a felt- work of independent fibrils (fiilar mass or miton), and (2) a fluid or semi-fluid sub- THE CELL. 41 stance which fills in the meshes of the reticulum or separates the fibrils (interfilar mass or paramiton) (Fig. 2, a). That the question as to the ultimate structure of protoplasm still remains unanswered is dependent mainly upon the extreme technical difficulties which have con- fronted the cytologist. Living protoplasm has a homogeneous glossy appearance, showing even under the highest magnification rarely more than a granular structure. It is usually only after death of the cell and the use of chemical fixatives and stains, that the so-called "structure" of protoplasm becomes visible. How closely the picture presented by such chemically treated protoplasm corresponds to the structure of -living protoplasm is as yet undetermined. It is quite possible that the structure of protoplasm is not entirely uniform. It certainly differs somewhat both as to struc- ture and chemical composition in dift"erent cells. It also differs in the same cell under different functional conditions. These varia- tions may account, in part at least, for the lack of uniformity of results of different observers. Protoplasm is thus probably best con- sidered as the material basis of cell activity, i.e., of life, rather than as a substance having fixed and definite chem- ical or morphological characteristics. It is convenient to use the term pro- toplasm to mean the entire substance of the cell, karyoplasm to designate the protoplasm of the nucleus, and cytoplasm the protoplasm of the cell body exclusive of the nucleus. Peculiar bodies known as plastids (Fig. i) are of frequent occurrence in vegetable cells, and are also found in some animal cells. They are apparently to be regarded as a dift'erentiation of the cytoplasm, but possess a remarkable degree of independence, being capable of subdivision and in some cases of existence outside of the cell. Fi -Foam or emulsion structure of protoplasm according to Biitschli (Biitschli). A, Epi- dermal cell of the earthworm. B, Peripheral cytoplasm of sea urchin's egg. C, Artificial emul- sion of olive oil, sodium chloride and water. 42 THE CELL. In addition to the granules which are apparently an integral part of the protoplasmic structure, other granules and various cell "inclu- sions" occur, to which the term metaplasm {paraplasm, deutoplasm) granules, has been applied (Fig. i). Some of these are intimately associated with the cell activities and represent either food substances in process of being built up into the protoplasm of the cell or waste products of cell metabolism. Others, such e.g., as the glycogen granules of the liver cell or the mucous granules of the mucous cell, are specific secretion products. Still others are fat droplets, pigment granules, and various excrementitious substances. When the protoplasm of a cell can be differentiated into a cen- tral granular area and a peripheral clear area, the former is known as endo plasm, the latter as exo plasm. When the exoplasm forms a distinct limiting layer, but blends imperceptibly with the rest of the protoplasm, it is known as the crusta. In some cells minute channels or canals are present in the cytoplasm (Fig. 4), These channels may contain branching proc- esses from other cells, forming what is known as a trophospongium,. Some in- FiG. 4 —Intracellular canals tracellular canals are apparently secretory (trophospongium) of a gan- . . . glion cell (E. Holmgren). m character and may communicate with fine intracellular secretory channels. In this way the secretion of such cells as the serous cells forming the demilunes of mucous tubules (p. 195), or of the parietal cells of the stomach glands (Fig. 136), is carried to the lumen. 2. The Cell Membrane (Fig. i). — This is present in but few ani- mal cells, and is a modification of the peripheral part of the protoplasm. In most vegetable cells the membrane is the most conspicuous part of the cell and was responsible for the name "cell" which the seven- teenth-century botanists, overlooking the importance of the enclosed protoplasm, gave to the little spaces or cavities of which they thought plants composed. When a membrane surrounds the cell, it is known as the pellicula; when cells lie upon the surface, and only the free surface of the cells is covered by a membrane, it is known as the cuticula. 3. The Nucleus (Fig. i). — This is a vesicular body embedded in the cytoplasm. The typical nucleus, like the typical cell, is spheroidal, but the shape of the nucleus varies for different cells and corresponds THE CELL. 43 somewhat to the shape of the cell body, e.g., the rod-shaped nucleus of the elongated smooth muscle cell. It may also be modified by intra- cellular pressure as, e.g., in the mucous cell and in the fat cell. The position of the nucleus is usually near the center of the cell. It may, however, be eccentric. Such eccentricity may be due to pressure of cell contents as, e.g., in the mucous cell and in the fat cell. Considered by earlier cytogists an unessential part of the cell, the nucleus is now known to be most intimately associated with cellular activities. It is not only essential to the carrying on of the ordinary metabolic processes of the cell, but is an active agent in the phenomena of mitosis, which in most cases determine cell reproduction. As a rule, each cell contains a single nucleus. Some cells contain two nuclei (quite common in the liver cell, rare in the ovum and in the nerve cell). A few cells contain many nuclei, e.g., the multinuclear "giant" cells of the spleen, bone-marrow, and certain tumors. Some cells, such as the human red blood cell and the respiratory epithelium, are, in their mature condition, non-nucleated. All non-nucleated cells, however, contained nuclei in the earher stages of their develop- ment. Non-nucleated cells, while capable of performing certain functions, are wholly incapable of proliferation. The non-nucleated condition must, therefore, be regarded as not only a condition of matur- ity, but of actual senility, at least so far as reproductive powers are concerned. In some of the lowest forms of animal life, the nuclear material instead of being grouped to form a definite body or nucleus, is more or less evenly distributed as granules through the cytoplasm. Chemically the nucleus is extremely complex, being composed of the proteids nuclein, paranuclein, linin, paralinin, and amphipyrenin. Morphologically also the nucleus is complex, much of the apparent structural differentiation being determined by the staining reactions of the different elements when treated with certain aniline dyes. The nuclear structures and their relations to the chemical constituents of the nucleus are as follows: (a) The nuclear membrane {amphipyrenin). This forms a limit- ing membrane separating the nucleus from the cell protoplasm. It is doubtful whether the nuclear membrane is different either chemic- ally or morphologically from the nucleoreticulum. It is wanting in some nuclei. When present it appears from its staining reactions to be structurally continuous with, and chemically identical with, in some cases, the linin, in others, the chromatin of the intranuclear network. 44 THE CELL. It may be complete, or fenestrated allowing free communication between the cytoplasm and the nuclear contents. (b) The intranuclear network, or nuclear eticulum, consists of a chromatic element (nuclein or chromatin) and of an achromatic ele- ment {linin). The linin constitutes the groundwork of the reticulum along which the chromatin granules are distributed. At nodal points of the network there are often considerable accumulations of chro- matin. These nodal points, at first thought to be nucleoli, are now known as false nucleoli^ or karyosomes. Instead of a distinct network there may be disconnected threads or simply granules of chromatin. Chromatin is the most characteristic of the chemical constituents of the nucleus, the only one which contains phosphoric acid, and also, apparently, the only nuclear substance which is always transmitted from parent to daughter cell in cell-division. Fine granules have been described as occurring in the linin, differentiated from chromatin by the fact that they are most susceptible to acid dyes, while chromatin takes basic dyes. (c) The micleolus or plasmosome (paranuclein, pyrenin) is a small spherical body within the nucleus. Not infrequently there are several nucleoli. Similar cells vary as to the number of nucleoli they contain. The same cell may vary as to the number of its nucleoli under varying functional conditions. The nucleolus stains intensely with basic dyes. Its function is unknown. id) Karyoplasm {nucleoplasm, nuclear fluid, nuclear sap). This is the fluid or semi-fluid material which fills in the meshes of the nucleo- reticulum. While the nucleus is a perfectly distinct structure capable in some animal and in some vegetable cells of moving about more or less actively in the cytoplasm, and is usually separated by a membrane from the rest of the cell, a marked simflarity exists between the structure of nucleoplasm and cytoplasm. This similarity is emphasized by the absence in some resting cefls of any nuclear membrane, by the apparent direct continuity in some cases of nucleoreticulum and cytoreticulum, and by the continuity of karyoplasm and cytoplasm in all cells during ceIl-di\'ision. 4. The centrosoine (Fig. 5) is a small spheroidal body found some- times in the nucleus, or more commonly in the cytoplasm near the nu- cleus. In actively dividing cells the centrosomc is frequently double, this being apparently in preparation for the succeeding cell-division. In some cases the centrosome is triple or even multiple. It was first THE CELL. 4o found in the ovum and described as peculiar to that cell. It is now be- lieved to occur in most, if not in all, animal cells. It usually consists of (i) a minute central granule or granules — the cenlriole, which stains intensely with iron-hasmatoxylin, and outside of which is (2) a clear zone, the attraction sphere. From this centre, radiations extend outward into the cytoplasm. There is much confusion of terms in connection with the cen- trosome, the term centrosome being by some applied to the entire structure including the radiating fibrils, by others to the central granule only, by still others to the central granule plus ^ , . Fig. 5. — Spermatogonium the surrounding clear area. By some the radia- from frog (Hermann), tions are beheved to be composed of a different ^^^^^ ora^uTaTtio^n substance than the general cytoplasm, which sphere or aster. Nucleus . . ■ T contains a plasmosome. is designated archoplasm. Ihe main signm- cance of the centrosome is in connection with cell-division, under which head it will be further considered (page 48). Vital Properties of Cells. It has already been noted that the essential peculiarity of the cell is that it possesses certain properties which are characteristic of life. By this is meant that a cell is able: i. To nourish itself and to grow — metabolism. 2. To do work — function. 3. To respond to stimula- tion— irritability. 4. To move — motion. 5. To produce other cells — reproduction. In the simplest forms of animal life, where a single cell constitutes the entire individual, all of these functions are performed by the one cell. In all higher, that is, multicellular animals, there are not only many cells but many kinds of cells, and this morphological differen- tiation corresponds to a physiological differentiation, each group of cells developing along certain well-defined lines for the performance of its own special function. I. Metabolism. — This term is used to designate those cellular acti\"ities which have to do with the nutrition of the cell. A cell is able (i) to take up from without substances suitable for its nutrition and to transform these into its own peculiar structure, and (2) to dis- pose of the waste products of intracellular acti\ities. The former is known as constructive metabolism or aimhoIis)n. the latter as destruc- tive nuiabolism or kalabolism. 46 THE CELL. 2. Function. — This is the special work which it is the part of the cell to perform. It varies greatly for different cells. Some cells, as, e.g., the surface cells of the skin, appear to act mainly as protec- tion for more delicate underlying structures. Other cells — gland cells — in addition to maintaining their own nutrition, produce specific substances (secretions), which are of great importance to the body as a whole. Still other cells, e.g., nerve cells and muscle cells, have the power to store up their food substances in such a way as to make them available in the form of energy. This appears to be accom- plished by the building up within the cell of highly complex and, con- sequently, unstable molecular combinations. By reduction of these un- stable combinations, molecules of greater stability and less complexity are formed. This results in the .transformation of potential into kinetic energy, and the expenditure of this energy is expressed in function. 3. Irritability is that property which enables a cell to respond to external stimuli. Cells vary in respect to their irritability, the most -A^ m^^ ''"^^^i-. :■^5^-^^'■^''■ . .-, ",'-iJ'''"~;>, -^^^|!^'.-. Fig. 6. — Amoeboid Movement. Successive changes in shape and position of fresh- water amoeba. markedly irritable cells in higher animals being those of the neuro- muscular mechanism. Stimulation may be mechanical, electrical, thermal, chemical, etc. The response of the cell to certain forms of chemical stimulation is known as chemotaxis. Some substances attract cells (positive chemotaxis) ; others repel cells (negative chemo- taxis). Stimuli other than chemical possess similar properties, as indicated by the terms thermotaxis, galvanotaxis, etc. Some cells are so specialized as to react only to certain kinds of stimulation, e.g., the retinal cells only to light stimuli. 4. Motion. — This is dependent wholly upon the protoplasm of the cell, and is exhibited in several somewhat different forms. (a) Amoeboid Movement. This consists in the pushing outward by the cell of processes (pseudopodia). These may be retracted or may draw the cell after them. In this way the cell may change both its shape and position (Fig. 6). THE CELL. 47 (b) Protoplasmic Movement. This occurs wholly within the limits of the cell, changing neither its shape nor position. It occurs in both plant and animal cells, and consists of a sort of circulation or "stream- ing" of the protoplasm. It is evidenced by the movement of minute granules present in the protoplasm, by changes in the position of the nucleus, etc. (c) Ciliary Movement. This is the whipping motion possessed by little hair-like processes called cilia, which project from the surfaces of some cells. Certain cells which are specialized for the particular purpose of motion as, e.g., muscle cells, possess such powers of contraction that they are able to move not only themselves but other parts with which they are connected. This power of contractility is dependent upon the spongioplasm, the hyaloplasm playing a more passive role. In muscle cells the highly developed contractile powers appear to be due to the excessive development and peculiar arrangement of the spongio- plasm. 5. Reproduction.— The overthrow of the long-held biological fal- lacy of spontaneous generation was soon followed by the downfall of a similar theory regarding the origin of cells. We now know- that all cells are derived from cells, and that the vast num- ber and complex of cells which together form the adult human body are all derived from a single primitive cell, the ovum. Reproduction of cells takes place in two ways, by direct cell division or amitosis, and by indirect cell division or mitosis. In both amitosis and Fig. 7.— Epithelial Cells from Ovary of Cockroach, rr,,-f^o,V fU J- • • r .1 1. Showing Nuclei Dividing Amiloticallv. (Wheeler.) mitosis the division of the cell body is preceded by division of the nucleus. Direct Cell-division— Amitosis (Figs. 7 and 8).— In this form of cell-division the nucleus di\idcs into two daughter nuclei without any apparent preliminary changes in its structure. The division of the nucleus may or may not be followed by di\-ision of the cell body, in the latter case resulting in the formation of polynuclear cells. This form of cell-diA-ision is uncommon in higher animals where Flemming consid- 48 THE CELL. ers it a degenerative phenomenon rather than a normal method of cell- increase. It is a common method of cell-division in the protozoa. Indirect Cell-division — Mitosis (Figs. 9, 10). — In this form of cell-division also, the nucleus divides into two daughter nuclei, and the cell into two daughter cells, but only after they have passed through certain characteristic and complicated changes. These changes occur as a continuous process, but it is convenient for clearness of description to arbitrarily divide them into stages or phases. Thus we recognize in mitosis : (a) the prophase; (b) the metaphase; (c) the anaphase; (d) the telophase. The prophase is the stage of preparation on the part of the nucleus for division; the metaphase, the actual separation of the nuclear elements; the ana- phase, the formation of the two daughter nuclei; the telophase, the reconstruc- ///, fibrils tion of the two daughter resting nuclei and the divi- sion of the cytoplasm. (a) The Prophase (Fig. 9, B, C, D) is marked by the following changes: I. The centrosome, if single, divides into two daughter centrosomes. In most actively dividing cells, however, the centrosome is at this stage already double (Fig. 9, A) having divided as early, frequently, as the anaphase of the preceding mitosis. The two daughter centrosomes, each surrounded by its attraction sphere, now move apart but remain connected by fibrils, probably deri\-ed from the linin (Fig. 9, B). These fibrils form the central or achromatic spindle. Two other sets of fibrils radiate from each centro- some— one, known as the polar rays, passes out toward the periphery of the cell; the other, known as the mantle fibres, extends from the centro- some to the chromosomes (Fig. 9, C). The two centrosomes with their fibres constitute the amphiaster. M Fig. 8. — Epithelial cell from bladder showing ami- totic division of its nucleus. (Nemileff.) / Cytoplasm; //, two daughter nuclei uniting daughter nuclei. THE CELL. 4U 2. During or immediately following the formation of the amphiaster, important changes take place in the nucleus. It increases in size and loses the reticular appearance of the resting nucleus, its chromatic ele- ments becoming arranged in a long spireme-thread or in several shorter threads, the closed skein or closed spireme. This next becomes thicker and more loosely arranged, thus forming the open spireme. That some Fig. 9. — Diagrams of Successive Phases of Mitosis. A, Resting cell, with reticular nucleus and true nucleolus; c, attraction sphere with two centrosomes. B, Early prophase. Chromatin forming continuous thread — the spireme; nucleolus still present ;'a, amphiaster; the two centrosomes connected by fibrils of achromatic spindle. C, Later prophase. Segmentation of spireme to form the chromosomes; achromatic spindle connecting centrosomes; polar rays; mantle fibres; fading of nuclear membrane. D, End of prophase. Monaster — mitotic figure complete; ep, chromosomes arranged around equator of nucleus; fibrils of achromatic spindle connecting centrosomes; mantle fibres passing from centrosomes to chromosomes. (E. B. Wilson, "The Cell," The ]Mac- niillan Co ) chemical as well as morphological change has taken place in the trans- formation of the reticulum of the resting nucleus into the spireme is shown by the marked increase in staining intensity, the spireme taking a much darker stain than the reticulum. Late in the prophase the nucleolus and nuclear membrane disappear. The cytoplasm and karyoplasm then become continuous and both spireme and amphiaster lie free in the general cell protoplasm (Fig. 9, C). 4 50 THE CELL. 3. The spireme next breaks up into a number of segments — chro- viosomes (Fig. 9, C). These are usually rod-shaped at first, later they may become U's or V's or may even become spheroidal. The chromosomes now arrange themselves regularly around the equator of the nucleus, their closed ends being directed centrally. The details of the transformation of the reticulum into chromosomes vary. In some cases a single spireme-thread is formed. In others Fig. 10. — Diagrams of Successive Phases of Mitosis. E, Metaphase. Longitudinal cleavage; splitting of chromosomes to form daughter chromosomes, ep; n, cast-off nucleolus. F, Anaphase. Daughter chromosomes passing along fibrils of achromatic spindle toward centrosomes; division of centrosomes; if, interzonal fibres or central spindle. G, Late anaphase. Formation of diaster; beginning division of cell body. H, Telophase. Reappearance of nuclear membrane and nucleolus; two complete daughter cells, each containing a resting nucleus. (E. B. Wilson, "The Cell," The Mac- millan Co.) the spireme-thread first splits longitudinally into two threads before segmenting into chromosomes. Again the spireme-thread may show segmentation into chromosomes from the beginning. In still other cases the chromosomes apparently form directly from the reticulum without the intervention of the spireme stage. It is most important to note that while the number of chromosomes varies for different species of plants and animals, it is fixed and characteristic for a given species. THE CELL. 51 Thus in Ascaris megalocephala (much used for study on account of its small number of chromosomes) the number is 4, in the mouse 24, in man, estimated by some as 16, by others 24. This means that when- ever mitosis occurs in Ascaris, the spireme-thread invariably segments into 4 chromosomes. Chromosomes and amphiaster now constitute the mitotic figure which at this stage is known as the monaster, its for- mation marking the end of the prophase. (b) Metaphase (Fig. 10, E). This marks the beginning of actual division of the nucleus. Each chromosome splits longitudinally (longitudinal cleavage) into two daughter chromosomes, each containing exactly one-half the chromatin of the parent chromosome. U- and \- shaped chromosomes always begin to split at the apex, from which point the separation extends to the open ends. (c) Anaphase (Fig. 10, F,G). — An equal number of daughter chromo- somes now travels along the fibrils of the achromatic spindle — appar- ently under the influence of the mantle fibres — toward each daughter centrosome around which they become grouped. In this way are formed two daughter stars, the mitotic figure being known at this stage as the diaster (Fig. 10, G). These daughter stars are at first connected by the fibrils of the achromatic spindle. In this stage may also occur beginning division of the cell body. In actively dividing cells each centrosome frequently undergoes division at this stage, resulting in four centrosomes to the cell. (d) Telophase (Fig. 10, H). — This is marked by division of the cell protoplasm and consists of a cycle of changes, by means of which each group of daughter chromosomes is transformed into the chromatin network of a resting nucleus. These changes are the same as those described in the prophase, but occur in the reverse order, the chromo- somes uniting to form the spireme, and the spireme becoming trans- formed into the nuclear network. The result is the formation of two daughter cells. The nuclear membrane reappears, as does also the nucleolus. Each daughter cell is thus provided with a resting nucleus. The fact that the number of chromosomes which enter into the formation of the chromatic reticulum of the nucleus of each daughter cell is the same as the number into which the spireme of the parent cell divided, has suggested the hypothesis that the chro- mosomes maintain their identity even during the resting stage. The time required for the mitotic process is usually from one-half to three-quarters of an hour. Exceptionally it is prolonged to several hours. 52 THE CELL. The parts which the several cell structures play in mitosis have been the subjects of much study and are as yet not fully determined. As to the behavior of the chromatic portion of the mitotic figure little doubt exists. It originates in the chromatic portion of the nuclear reticulum of the parent cell and its destination is the chromatic portion of the reticulum of the daughter cells. The role of the centrosome in mitosis is not so clear. It has been called the "dynamic centre" of the cell because in most cases it appears to be the active agent in initiating and probably further directing the mitotic process. The origin of the astral fibres is not always the same. In Infusoria the centrosome is found within the nucleus and both amphiaster and chromosomes are of nuclear origin. In some of the higher plants the amphiaster is derived wholly from the spongioplasm. In the egg cells of Echinoderm, part of the amphiaster (central spindle) is of nuclear, the remainder (asters) of cytoplasmic origin. That the centrosome is not always the active factor in mitosis is shown by the fact that in the higher plants no centrosome can be demonstrated during any stage of mitosis, and also that in some cases the chromo- somes divide without previous division of the centrosome. Between mitotic periods the centrosome with or without its aster may remain as an integral part of the resting cell. It may, on the other hand, entirely disappear during the resting stage. It is through the above-described process of cell-division that new cells are produced to replace those worn out as a result of their labors or destroyed by injury. It is through the same process that the vast number of cells which make up the adult body are developed from one original cell — the ovum. Such powers of evolution are not, how- ever, inherent in the ovum itself, but, in sexual reproduction, are ac- quired only after its union with germinal elements from the male. This union of male and female germinal elements is known as fertilization of the ovum. Fertilization of the Ovum. Prior to and in preparation for fertilization, both male and female cells must pass through certain changes. These are known as matu- ration of the spermatozoon on the male side (p. 315) and of the ovum on the female (p. 328). The spermatozoon (Fig. 11) is developed from a cell of the seminifer- ous tubule of the testis. The nucleus of this cell so divides its chro- mosomes that each spermatozoon contains just one-half the number of THE CELL. chromosomes characteristic of cells of the species. These are contained in the head of the spermatozoon, which thus represents the nucleus of the male sexual cell, the middle piece probably containing the centro- some, the tail piece the remains of the protoplasm. The nucleus of the ovum or germinal vesicle also passes through a series of changes by which it loses one-half its chromosomes. The germinal vesicle or nucleus of the ovum first under- goes mitotic division with the usual longitudinal cleavage of its chromosomes and the formation of two daughter nuclei. One of these and its centrosome are extruded from the cell as the first polar body. The remaining nucleus and centrosome again divide mitotically, only in this second division, instead of the usual longitudinal cleavage of chromosomes, by which each daughter nucleus is provided with the same number of chromosomes as the mother nucleus, the chromosomes simply separate, one-half going to each daughter nucleus. One of the daughter nuclei and its centrosome are now extruded as the second polar body. The polar bodies ultimately disappear, as does also the centrosome remaining within the egg. This leaves in the now matured ovum a single nucleus, which is known as the female pronucleus, and which contains one-half the number of chromosomes charac- teristic of cells of the species. During this process in some animals — in others after its completion — the spermatozoon enters the ovum. The head of the spermatozoon becomes the male pronucleus, the middle piece becomes a centrosome, while the tail is, in some instances at least, left behind as the spermato- zoon enters the egg. The chromatin of the male next becomes ar- ranged as chromosomes. Male and female pronuclei now lose their limiting membranes and approach each other, their chromosomes intermingling. ^4^ each pronucleus contained one-half the number, the monaster thus formed contains the full number of cliromosomcs charac- teristic of tJie species. Meanwhile the male centrosome, formed from the body of the spermatozoon, divides into two daughter centrosomes. These with their radiating fibrils have the same arrangement relative lo the monaster of mingled male and female chromosomes, alrcadv described under mitosis. Bv lonijitudinal clca\-aom Gerrish, after van Beneden.) a, Two-cell stage resulting from first division of fertilized ovum; ^, four-cell statue; c, d, e, later stages. A, Differentiation into inner and outer cells; 5, Formation of segmenTa- tion cavity; C", Embryonic vesicle, showing two primary germ lavers. Outer cells, ectoderm; inner cells, entoderm. 56 THE CELL. By similar mitotic processes ttiese two cells become four, the four cells become eight, etc. This is known as segmentation of the ovum. The earlier generations of these cells are morphologically alike and are known as hlastomeres. Soon, however, these cells become spread out and at the same time differentiated into two primary germ layers. The outer of these is known as the ectoderm or epiblast, the inner as the entoderm or hypoblast. Between these two layers and derived from them a third layer is formed, the mesoderm or mesohlast. These three la vers constitute the blastoderm. e^6c„ s Fig. 14. — The Two Primary Germ Layers; from transverse section through primitive groove of a chick of 27 hours' incubation, a, Ectoderm (outer germ layer) ; b, entoderm (inner germ layer) ; c, mesoderm (middle germ layer) ; d, anlage of notochord. As to what determines and controls fertilization, comparatively little is known. As in ordinary mitosis, the origin of the centrosome is obscure. In some forms, at least, the centrosome of the spermatid enters into the formation of the middle piece of the spermatozoon. The male centrosome thus enters the ovum. It is also known that in some eggs the egg-centrosome disappears soon after the extrusion of the second polar body, and that the centrosome of the fertilized egg develops in close relation to the middle piece of the spermato- zoon. These facts point to the male centrosome as the centrosome of fertilization. The o\'um and spermatozodn are apparently brought together by a definite attraction on the part of the ovum toward the spermatozoon. The nature of this attraction is unknown. It is possibly chemical, and is exerted only between ova and spermatozoa of the same species. This has been proved for lower forms by mixing ova of one species and spermatozoa from se^•eral species in an inert medium when only spermatozoa of the same species will attach themselves to the ova. 'i'hat the attractive force lies in the cytoplasm is shown by the fact that small pieces of the egg protoplasm free from nuclear elements will exert sufficient attractive powers to cause spermatozoa to enter them. THE CELL. 57 As to the point of entrance of the spermatozoon, some eggs may be entered at any point, others are permeable at but one point. One spermatozoon only is required for fertilization, and when this spermatozoon has entered, the egg apparently loses its power of at- tracting spermatozoa, or else develops some actual defense against further entrance of spermatozoa. TECHNIC. 1. Fresh cells may be studied by gently scraping the surface of the tongue, transferring the mucus thus obtained to a glass slide and covering with a cover-glass. 2. Red blood cells from the frog are prepared as follows: After killing the frog the heart is opened and the blood allowed to drop into a tube containing Hayem's fluid (sodium chlorid i gm., sodium sulphate 5 gm., mercuric chlorid 0.5 gm., dis- tilled water 100 c.c). After shaking, the cells are allowed to settle for from twelve to twenty-four hours. The fixative is then replaced by water, the tube again shaken, the cells allowed to settle, and the water is replaced with 80-per-cent. alco- hol tinged with iodin. After from twelve to twenty-four hours the alcohol is de- canted and the tube partly filled with alum-carmine solution (page 17). About twenty-four hours usually suffices for staining the nuclei. The alum-carmine is then poured off and the cells well shaken in water. After settHng, the water is replaced bv glycerin, to which a small amount of picric acid has been added. In this the cells may be permanently preserved. The nuclei are stained red by the carmine, the cytoplasm yellow by the picric acid. 3. Surface cells from the mucous membrane of the bladder. The bladder is removed from a recently killed animal, pinned out mucous membrane side up on a piece of cork and floated, specimen side down, on equal parts ]\Iiiller's fluid and Ranvier's alcohol (technic 4, p. 6, and a, p. 4) for from twenty-four to forty-eight hours. The specimen is then washed in water and the cells removed by gently scraping the surface. These may then be stained and preserved in the same manner as the preceding. Cells from the different layers should be studied; also the appearance of the large surface cells seen on flat and on edge, showing pitting of under surface by cells beneath. 4. Amoeboid movement may be studied by watching fresh-water amoeba? or white blood cells. A drop of water containing amoebae is placed on a slide, covered, and a brush moistened with oil is passed around the cover to prevent evaporation. The activity of the amcebas may be increased by slightly raising the temperature. An apparatus known as the warm stage is convenient for demonstrating amoeboid movement. A drop of blood, human, or better from one of the cold-blooded animals, may be used for the study of amoeboid movement in the white blood cells. It should be placed on a slide, covered, and immediately examined on the warm stage. 5. Ciliary movement is conveniently studied by removing a small piece of the gill of an oyster or mussel, teasing it gently in a drop of normal salt solution and covering. The cilia being very long, their motion may be easily studied, especially after it has become slow from loss of vitalitv. 58 THE CELL. 6. Mitosis. The salamander tadpole and the newt are classical subjects for the study of cell-division. The female salamander is usually full of embryo tad- poles in January and February. The embryos are removed and fixed in Flem- ming's fluid (technic 7, p. 7), after which they may be preserved in equal parts of alcohol, glycerin, and water. Mitotic figures may be found in almost any of the tissues. Pieces of epidermis from the end of the tail, the parietal peritoneum, and bits of the gills are especially satisfactory. If the newt's tail is used, it should be fixed in the same manner, embedded in parafiin and cut into thin sections. These are stained with Heidenhain's haematoxylin, technic 3, p. 16. Certain vegetable tissues, such as the end roots of a young, rapidly growing onion or magnolia buds, are excellent for the study of mitosis. The technic is the same as for animal tissues. General References for Further Study of the Cell. Conklin, E. G.: Karyokinesis and Cytokinesis. Jour. Acad. Nat. Sci. oj Phila., vol. xii, 1902. Harper, E. H.: The Fertilization and Early Development of the Pigeon's Egg. Amer. Jour, oj Anat., vol. iii, No. 4, 1904. Hertwig, O.: Die Zelle und die Gewebe, 1898. Hertwig, R. : Eirife, Befruchtung u. Furchungsprozess. In Hertwig's Hand- buch d. vergleich. n. experiment. Entwickelungslehre der Wirbcliiere, Bd. I, Teil I, 1903. Lillie, F. R.: A Contribution toward an Experimental Analysis of the Kary- okinetic Figure. Science, New Series, vol. xxvii, 1908. McMurrich: The Development of the Human Body. Minot: Human Embryology. A Laboratory Te.xt-book of Embryology. Sobotta, J.: Die Befruchtung u. Furchung des Eies der Maus. Arch. f. mik. Anat., Bd., xlv, 1895. Wilson, E. B.: The Cell in Development and Inheritance, 2d ed., 1900. PART III. THE TISSUES. CHAPTER T. HISTOGENESIS— CLASSIFICATION. Ectoderm, mesoderm, and entoderm (see page 56) are known as the primary layers of the blastoderm. They differ from one another not only in position, but also in the structural characteristics of their cells. The separation of the blastomeres into these three layers represents the first morphological differentiation of the cells of the developing embryo. By further and constantly increasing differentia tion are developed from these three primary layers all tissues and organs, each layer giving rise to its own special group of tissues. The tissue derivations from the primary layers of the blastoderm are as follows: Ectoderm. — (i) Epithelium of skin and its appendages — hair, nails, sweat, sebaceous and mammary glands, including smooth mus- cle of sweat glands. (2) Epithelium of mouth and anus, of glands opening into mouth, and enamel of teeth. (3) Epithelium of nose and of glands and cavities connected with nose. (4) Epithelium of external auditory canal and of membranous labyrinth. (5) Epithelium of anterior surface of cornea, of conjunctiva, and of crystalline lens. (6) Epithelium of male urethra, except prostatic portion. (7) Epithelium of pineal bodies and of pituitary body. (8) Entire nervous system, including retina. Entoderm. — (i) Epithelium of digestive tract excepting mouth and anus, and of glands connected with digestive tract. (2) Epithelium of respiratory tract and of its glands. (3) Epithelium of bladder except the trigonum, of female urethra, and of prostatic portion of male urethra. (4) Epithelium of tympanum and of Eustachian lube. (5) Epithelium of thyreoid and of Hassall's corpuscles of thymus. Mesoderm. — (i) All the connective or supporting tissues except neuroglia. ^ (2) Lymphatic organs with the exception of Hassall's corpuscles and reticulum of the thymus. (il 62 THE TISSUES. -'' (3) Blood cells and bone-marrow. ^^. (4) Striated, cardiac and smooth muscle (with the possible excep- tion of smooth muscle of sweat glands). '^ (5) Endothelium lining blood-vessels and lymphatics. (6) Mesothelium lining serous membranes — pleura, pericardium, and peritoneum. ^ (7) Epithelium of genito-urinary system with the exception of the urethra and a large part of the bladder. In all but the lowest forms of animal life the body consists of an orderly arrangement of many kinds of cells. From the cells is de- veloped a substance which lies outside the cells and is known as inter- cellular substance. This may be small in amount, just sufficient to unite the cells, as in epithelium, or it may so predominate as to deter- mine the character of the tissue, as in some forms of connective tissue. It does not always completely separate the cells which may be connected across the intercellular substance by extensions of their protoplasm, as in the "intercellular bridges" of epithelium or the anastomosing processes of connective-tissue cells. Less commonly cells are united in such a multinuclear continuum that their boundaries are almost or wholly lost. Such a structure is known as a syncytium. The association of a particular type of cell with a particular type of intercellular substance is known as a tissue. The character of a tissue depends upon the character of its cells, of its intercellular substance, and their relations to each other. Further differentiation of cells and intercellular substance within a particular tissue gives rise to various sub-groups of the tissue. The association of tissues to form a definite structure for the performance of a particular function is known as an organ. The physiological association of organs constitutes a system. A scientific classification of the tissues is at present impossible. The foregoing list of tissue deri\'ations shows how unsatisfactory is any attempt at classification on the basis of histogenesis, many tissues which are morphologically similar being derived from two or even all three of the blastodermic layers. The following is the usual classification of adult tissues: (i) Epithe- lial tissues; (2) connective tissues; (3) blood; (4) muscle tissue; (5) ner\-e tissue. Of these, epithelium and connective tissue may be regarded as the more elementary tissues, being common to both plants and animals. Blood is sometimes classified among the connective tissues. Muscle and nerve are the most highly specialized tissues and are peculiar to animals. CHAPTER II. EPITHELIUM (INCLUDING MESOTHELIUM AND ENDOTHELIUM;. General Characteristics. — Epithelium is derived from all three germ layers. It consists almost wholly of cells. The intercellular substance is merely sufficient to attach the cells to one another and is, consequently, known as cement substance. In some instances the protoplasm of adjacent epithelial cells is seen to be even more closely associated, the intervening cement substance being bridged over by delicate processes of protoplasm which pass from one cell to another and are known as ^intercellular bridges''^ (see Fig. 19, p. 66). It seems probable that the minute spaces between the processes serve as channels for the passage of food (lymph) to the cells. The surface cells of epithelium are united by continuous cement substance in which there are apparently no spaces. In this way escape of lymph is prevented. Epithelial cells vary in size and shape, the element of pressure being a frequent determining factor as regards shape. Their proto- plasm may be clear, finely or coarsely granular, or pigmented. Each cell usually contains a single well-defined nucleus. Two or more nuclei are sometimes present. Some epithelial cells are, when fully matured, non-nucleated, e.g., respiratory epithelium of lung. When epithelium rests upon connective tissue, it is usually sepa- rated from the latter by a thin, apparently homogeneous membrane known as the ha sal membrane or membra na propria. Authorities differ as to whether this membrane is of connective-tissue or of epithe- lial origin. Surface epithelial cells frequently have thickened free borders or cuticiilcE, which unite to form a continuous membrane, the ciiticiilar membrane. Striations extend from the cytoplasm into the cuticulae. A still greater specialization of the surface of the cell is seen in the ciliated cell. In this cell fine hair-like projections — cilia — extend from the surface of the cell. Some epithelial cells show important changes dependent upon their functional activities. An example of this is seen in the mucous 03 (U THE TISSUES. cell in which there is a transformation of the greater part of the cyto- plasm into, or its replacement by, mucus. Epithelia are devoid, as a rule, of both blood- and lymph-vessels. An exception to this is the stria vascularis of the cochlea. Nerves, on the other hand, are abundant. Classification. — Epithelia may be classified according to shape and arrangement of cells as follows: (i) Simple Epithelium. — (a) Squamous; (b) columnar. (2) Stratified Epithelium. — (a) Squamous; {b) columnar. (3) Mesothelium and Endothelium. Specializations of the above-mentioned types are known as : (a) Ciliated epithelium; (b) pigmented epithelium: (c) glandular epithe- lium; (d) neuro-epithehum. Pigment may occur in any type of epithelium. Cilia are found only in the simple columnar and stratified columnar forms. I. Simple Epithelium. In simple epithelium the cells are arranged in a single layer. (a) Simple squamous epithelium consists of fiat scale-like cells which are united by an extremely small amount of intercellular sub- FiG. 15. — From Section of Cat's Lung, stained with silver nitrate, showing outlines of the Simple Squamous Epithelium Lining the Air Vesicle, a, Two epithelial cells; h, the wavy staincfl intercellular substance; c, foetal cells; d, connective tissue. Stance. The edges of the cells are smooth or serrated. Seen on flat, they present the appearance of a mosaic. Seen on edge, the cells appear fusiform, being thickest at the centre, where the nucleus EPITHELIUM. 65 is situated, and thinning out toward the periphery. Simple squa- mous epithehum has but a limited distribution in man. It occurs in the lungs as non-nucleated respiratory epithelium, in Bowman's capsule of the renal corpuscle, in the descending arm of Henle's loop of the uriniferous tubule, in the retina in the form of pigmented cells, and on the posterior surface of the anterior lens capsule. iS -m ■j ■'*■ ■ -H d :iiiiBifii:#PII Fig. i6. — Simple Columnar Epithelium from the Human Small Intestine, a, Mucous (goblet) cell; b, basement membrane; c, thickened free border (cuticula); d, leucocyte among the epithelial cells; e, replacing cell. (b) Simple columnar epithelium consists of a single layer of elon- gated cells. The bases of the cells are usually separated from the underlying connective tissue by a basement membrane. The nu- cleus is, as a rule, in the deeper part of the cell, near the basement membrane. Many of these cells have prominent thickened free borders or cuticulae. This form of epithelium is often ciliated. The height of the cell varies greatl3^ there being all gradations from high columnar to low cuboidal. Simple columnar epithelium lines the gastro- intestinal canal, the uriniferous tubule (excepting the descending arm of Henle's loop), simple tubular glands, the ducts of some compound tubular glands, the smaller bronchi, the mem- branous and penile portions of the male uretha, and the gall-bladder. In simple columnar epithelium, in addition to the single row of epithelial cells, there are found lying near the basement membrane, between the bases of the epithelial cells, small, spherical, or irregular cells, which frequently show mitosis and which are known as replacing cells. They appear to develop into columnar epithelial cells as they are needed to replace older cells. The so-called pseudo-stratijied 5 Fig. 17. — Diagram of Pseudosiraliried Epithelium, showing Nuclei situated at Different Levels. 66 THE TISSUES. epitJielium is a form of simple columnar epithelium, in which, from crowding of the cells, the nuclei have come to lie at different levels, thus giving the appearance of stratification (Fig. 17). 2. Stratified Epithelium. In stratified epithelium the cells are arranged in more than one layer. Flat surface cells ;^^S'":r;: .<^' % Polyhedral cells '. <^_i2 1^ ,«si, ^ 'm r^ m -^w Cuboidal cells Fig. 18. — Stratified Squamous Epithelium from Cat's (Esophagus. (a) Stratified squamous epithelium is developed from simple epithe- lium by the growth of new cells between the old cells and the under- lying connective tissue. It consists of several f)^^j^-ai$^ (ji'^-^^t^^ layers of cells which vary greatly in size and shape. The surface cells are large and flat. Beneath these are several layers of polyhedral cells, with often very distinct protoplasmic intercellular connections ("intercellular bridges," see also page 63). The deepest cells are columnar or cuboidal. It is thus seen that in stratified squamous epithelium only the surface cells are sc[uamous. This from ihe Stratum Spinosum f^j-j^ Qf epithelium rcsts upou a more or less of the Human Epidermis showing "Intercellular distinct basement membrane, which is fre- monowifz ) ^°° ^'^" 'I'-'^^tly thrown up into folds l)y papillae of the underlying connecti\e tissue. Stratified squamous epithelium forms the surface of the skin and of mucous membranes of cavities opening upon it, mouth and cesoph- u "M V\<:. ig. — Epithelial Ctlls EPITHELIUM. 6/ agus, conjunctiva, external ear, vagina, and external sheath of hair follicle. (b) Stratified Columnar Epithelium. — Only the surface cells are columnar, the deeper cells being irregular in shape. The surface cells frequently send long processes down among the underlying cells. The free surface is often marked by a well-developed cuticula. CD %^ mM Fig. 20. — Transitional Epithelium from the Human Bladder. Some epithelia of this type are ciliated. Stratified columnar epithe- lium is found in the larynx, nose, palpebral conjunctiva, largest of the gland ducts, the vas deferens, and part of the male urethra. Stratified epithelium composed of only from three to six layers of cells is sometimes designated '"transitional epithelium^ This type of epithelium usually rests upon a basement membrane free from Fig. 21. — Stratified Columnar Epithelium from the Human Male I'rethra. X400. papillse. The surface cells are. large and frequently contain two or three nuclei. Their free surfaces are flat, while their under surfaces show depressions due to pressure from underlying cells. The deeper cells arc polygonal or irregularly cuboidal. Tliis form of epithelium lines the bladder, ureter, pehis of the kidney, and prostatic portion of male urethra. 68 the tissues. Modified Forms of Epithelium. (a) Ciliated Epithelium. — In this form of epithelium, fine hair-like processes — cilia — extend from the surface of the cell. These cilia vary from twelve to twenty-five for each cell and may be short as in the Si;' ^- c a; ^ -'^ '^' W ^ #i ^ n Fig. 22. — Stratified Columnar Ciliated Epithelium from the Human Trachea. A mucous (goblet) cell also is present. trachea or long as in the epididymis. There is usually a well-defined cuticula from which the cilia appear to spring. According to Apathy, the cilia extend through the cuticula, giving to the latter a striated Fig. 23. — Isolated Ciliated Cells and Goblet Cells from Dog's Trachea. X700. appearance (Fig. 24). Just beneath the cuticula each cilium shows a swelling — the basal granule. Lenhossek considers these granules centrosomes. The intracellular extensions of the cilia converge EPITHELIUM. 69 toward the nucleus, and are continuous with the reticular or fibrillar structure of the cell body. The motion of cilia is wave-like, the wave always passing in the same direction. Various explanations of ciliary motion have been given. The most plausible is that it is due to the contractile powers of the spongioplasm. Cilia are confined to the surface cells of simple columnar and stratified columnar epithelium. Simple columnar cihated epithelium occurs in the smaller bronchi, uterus, Fallopian tubes and central canal of the spinal cord. Stratified clumnar ciliated epithelium occurs in large bronchi, trachea, larynx, nose. Eustachian tube, vas deferens and epididymis. (b) Pigmented epithelium consists of cells the cytoplasm of which contains brown or black pig- FiG. 24. Fig. 25. Fig. 24.— Ciliated Epithelial Cell from Intestine of Mollusk (Engelmann), showing, a, cuticula, b, basal granules, and c, intracellular extensions of cilia. Fig. 25.— Pigmented Epithelial Cells from the Human Retina (X350), showing dif- ferent degrees of pigmentation. The clear spots in the centres of the cells represent the unstained nuclei. ment. It is usually present in the form of spherical or rod-like granules. Examples of it are seen in the pigmented epithelium of the retina and in the pigmented cells of the deeper layers of the epidermis in colored races (Fig. 25). (c) Glandular Epithelium.— This forms the essential or secreting element of glands and is mostly of the simple cylindrical \ariety. The dift'erent kinds of glands and their epithelia will be described among the organs. (d) Neuro-epithelium. — This is a highly specialized form of epithe- lium which occurs in connection with the end organs of ner^■es. under which heading; it will be described. 70 THE TISSUES. 3. Mesothelium and Endothelium. While recognizing the present tendency toward considering those tissues formerly classified as endothelium, as simple squamous epithe- lium, the correctness of the newer classification still remains sub judice Fig. 26. — Mesothelium from Omentum of Dog Treated according to Technic 7, p. j?. X350. Black wavy lines indicate the intercellular cement substance. The mesothelial cells cover the strands of connective tissue, the fibres of the latter being visible through the transparent cell bodies. and, so long as this is the case, we prefer to retain the certainly much more convenient classification of Minot, which coincides with his subdivision of the mesoblast. According to this classification, for those tissues which resemble epithelium in structure and which are derived from the sublayer of the mesoderm which he designates the Fig. 27. — The J'.ndothcliuin of a Small Ijlood-vessel. Silver nitrate stain. X350. mesenchyme, the term endothelium is retained. The term mesothelium is used for those tissues which rescm1)Ie epithelium but arc derived from a subdi\'ision of the mesoderm which he designates tiie meso- thch'al layer. EPITHELIUM. / 1 Mesothelium and endothelium are similar in structure. Each consists of thin flattened cells with clear or slightly granular proto- plasm and bulging oval or spherical nuclei. The edges of the cells are usually wa\ y or serrated. The cells are united by an extremely small amount of intercellular "cement" substance, which is usually indistinguishable except by the use of a special technic. Endothelium forms the walls of the blood and lymph capillaries and lines the entire blood-vessel and lymph-vessel systems. Mesothelium lines the body cavities — the pleura, the pericardium, and the peritoneum. TECHNIC. 1. Simple Squamous Epithelium. — That of the lung may be demonstrated by injecting with silver solution (technic i, p. 26) through a bronchus and then im- mersing the tissue in the same solution. The lungs of young kittens furnish espe- cially satisfactory material. 2. Simple Columnar Epithelium. — A piece of small intestine, human or animal, is pinned out flat on cork and fixed in formaHn-^NIiiller's fluid (technic 5, p. 7). Sections are cut perpendicular to the surface, stained with hematoxylin and eosin (technic i, p. 18) and mounted in glycerin tinged with eosin (page 20). Little processes known as villi project from the inner surface of the intestine. These are covered by a single layer of columnar epithelial cells. The cuticulte and cuticular membrane are usually well shown. Among the simple cylindrical cells are seen large clear or slightly blue-stained cells. These are known from their secretion as mucous cells, from their shape as goblet cells, and are classed as modified epithelium of the glandular type. These should be studied in their va- rious stages of secretion, from the cell in which only a small amount of mucus is present near the outer margin, to the cell whose protoplasm is almost wholly re- placed by mucus. Some cells will be found in which the surface has ruptured and the mucus can be seen pouring out of the cell. 3. Stratified Squamous Epithelium. — The cornea furnishes good material for the study of stratified squamous epithelium. An eye is removed from a freshly killed animal and the cornea cut out and fixed in formalin-Miiller's fluid. Sections are cut perpendicular to the surface, and treated as in the preceding. The cells are laid down in from six to eight layers. The cesophagus may be used instead of the cornea, its mucous membrane being lined by a somewhat thicker epithelium. 4. Transitional Epithelium. — This is conveniently studied in the mucous mem- brane of the bladder. Technic same as 2, above. 5. Stratified Columnar Epithelium. — A portion of trachea from a recently killed animal is treated according to the same technic. The surface cells arc ciliated so that this specimen also serves to demonstrate that type of modified ej)ithclium. Isolated cells or clumps of cells may be obtained from the trachea in the manner described in technic 3, p. 57. 6. Pigmented Epithelium. — Fix a freshly removed eye in lormalin-Muller's fluid (page 7). After hardening, cut transversely and remove the vitreous and 72 THE TISSUES. retina. The pigmented cells remain attached to the inner surface of the chorioid, and may be removed by gently scraping. They may be preserved and mounted in glycerin. 7. Mesothelium. — Part of the omentum of a recently killed animal is removed and washed in water, care being taken not to injure the tissue in handling. The water is then replaced by a i to 500 aqueous solution of silver nitrate. After half an hour the specimen is removed from the silver, washed in water, transferred to 80-per-cent. alcohol and placed in the sunlight until it becomes light brown in color. It is then preserved in fresh 80-per-cent. alcohol. The nuclei may be stained with haematoxylin (stain 5, p. 16). The specimen should be mounted in glycerin. Wavy black lines indicate the intercellular cement substance. The nuclei of the mesothelial cells are stained blue, those of the underlying connective- tissue cells a paler blue. It must be borne in mind in studying this specimen that the strands or trabeculas of the omentum are not composed of mesothelium, but of fibrous connective tissue, and that the flat mesothelial cells merely lie upon the surface of the connective-tissue strands. 8. Endothelium may be demonstrated by removing the bladder from a recently killed frog, distending it with air and subjecting it to the same technic. By this means the intercellular substance of the endothelium of the blood-vessels of the bladder wall is stained and the outlines of the cells are thus shown. CHAPTER III. THE CONNECTIVE TISSUES. General Characteristics. — All of the connective tissues, with the single exception of the connective tissue peculiar to the nervous system (neuroglia), are developed from the mesoderm. This consists at first wholly of round or polygonal cells united by a small amount of inter- cellular cement substance. As development proceeds the cells gradu- ally become more and more separated from one another by an increased amount of intercellular substance. This intercellular substance is a product of the cells and is at first homogeneous or granular. The appearance presented at this stage is that of irregular, branching, anastomosing cells, lying in a semi-fluid ground substance. This is embryonic connective tissue. With further changes in both cells and intercellular substance, but mainly in the latter, embryonic connective tissue differentiates to form the adult types of connective tissue. The most prominent characteristic of the connective tissues, with the exception of adenoid tissue and fat, is the predominance of the intercellular substance. In this respect the connective tissues differ markedly from epithelial tissues and this difference in structure cor- responds to difference in function. The role which the connective tissues play is mainly passive, and the cells instead of taking, as in epithelium, muscle and nerve tissues, the most active part, serve mainly for the maintenance of the nutrition of the more important inter- cellular substance. Moreover, with the exceptions mentioned above, it is the intercellular substance and not the cells which determines the physical character of the tissue. The denser forms of connective tissue, such as bone and cartilage, are supportive in function; somewhat less dense tissues, such as tendons, serve to attach muscles to bones; while more loosely arranged connective tissue forms the frame-work of the various organs. Connective tissue differs also from epithelium as re- gards its blood supply, being usually though not always very vascular. A further characteristic of the connective tissues is the apparent ease with which one type may become transformed into another. The division of connective tissue into its various sub-groups is also based upon structural dift'erences in the intercellular substances. 73 74 THE TISSUES. Classification. — The connective tissues may be classified as follows, although embryonal tissue and mucous tissue are essentially develop- mental forms. 1. Fibrillar connective tissue, including areolar tissue. 2. Elastic tissue. 3. Embryonal tissue and mucous tissue. 4. Reticular tissue. c;. Lymphatic or adenoid tissue. 6. Fat tissue. [ (a) Hyaline. 7. Cartilage. \ (b) Elastic. [ (c) Fibrous. 8. Bone tissue. 9. Neuroglia. I. Fibrillar Connective Tissue. Fibrillar connective tissue, also known as white fibrous tissue or connective tissue proper, consists of cells and fibres lying in a basement or ground substance. The elements of fibrillar tissue may be classified as follows: [ (a) The ordinary connective-tissues cells. [ I. Fixed \ (b) Plasma cells. Cells ] [ (c) Mast cells. 2. Wanderino;. Intercellular substance. ^., white or fibrillated, (a) Fibres ,, , .. ^ ^ yellow or elastic. [ (b) Ground or basement substance. Connective-tissue Cells.— (a) The ordinary fixed connective-tissue cellh often the only connective-tissue cell seen in ordinary sections. It is a flat, irregularly stellate cell with many branches (Fig. 30). The nucleus lies in the thickest part of the cell. The cytoplasm is usually clear or slightly granular. Each cell lies in a cell space or lacuna. From the cell spaces minute channels {canaliculi) extend in all direc- tions to unite with canaliculi from adjoining spaces (Fig. 29). Delicate cell processes extend into the canaliculi and there anastomose with processes from other cells (Fig. 30). Owing to the extreme sensitive- ness of the protoplasm of the connective-tissue cell to most fixatives, its usual appearance is that of a minute amount of cytoplasm shrunken down around a nucleus. THE CONNECTIVE TISSUES. (b) Plasma Cells. — These cells occur mainly near the smaller blood- vessels. Their protoplasm is finely granular and stains with basic aniline dyes. They frequently contain vacuoles. Small plasma cells are about the size of leucocytes, which they closely resemble. Large plasma cells are larger than leucocytes and richer in protoplasm. Some consider them as derived from leucocytes, others as a modified form of the ordinary fixed connective-tissue cell. (c) Mast cells are spherical or irregular-shaped cells, found like the preceding in the neighborhood of the blood-vessels. Their proto- FiG. 28. — Fibrillar Connective Tissue (Areolar Type) from Subcutaneous Tissue of Rabbit (technic 2, p. 80). X500. a, Fixed connective-tissue cell; b, fibrillated fibres; c elastic fibre with curled broken end; d, elastic fibres showing Y-shaped branching. plasm contains coarse granules which stain intensely with basic aniline dyes. They are believed by some investigators to be connected with the formation of fat; by others to represent a stage in the development of the fixed connective-tissue cell. Connecti\'e-tissue cells may be pigmented (Fig. 31). In such cells the cytoplasm is more or less filled with brown or black pigment granules. In man pigmented connecti^■e-tissue cells occur in the skin, chorioid and iris. The so-called icandering cells are not properly a part of connective tissue, being merely amoeboid white blood cells (see page 98) which ha\-e passed out from the \essel into the tissues. Tliey are not peculiar to conncctiN'c tissue, being found in other tissues, Ci^.. in ejiitlKlium. 76 THE TISSUES. The Intercellular Substance. — (a) Fibres. White or fibrillated ^^f^"^ are bundles of extremely fine fibrillse (o. 5 /« in diameter) (Fig. 28). The fibrillae lie parallel to one another and are united by a small amount of cement substance. The fibrillae do not branch. The fibre bundles, *?KJ''?^<3?S ;- ■ •■-, ■■ ■ .^cT^' ■'. ' ■■ "^ V. ■ ■■,■•-■-.,.1-::/:; ; -^^ ■■".■. .:;;::.3:# Fig. 29. — Section of Human Cornea cut Tangential to Surface. X 350. (Technic g, p. 81.) Connective-tissue Cell Spaces (Lacunae) and Anastomosing Canaliculi, white; whole Intercellular Substance (Ground Substance and Fibres), dark. on the other hand, branch dichotomously and anastomose. White fibres, on boiling, yield gelatin. Yellow or elastic fibres are apparently homogeneous, highly re- fractive fibres, varying in diameter from i to lo/i (Fig. 28). They ~W.,??"'T".' '\ / /■■'" Fig. 30. — Section of Human Cornea cut Tangential t© Surface. X 350. (Technic 8, p. 81.) Connective-tissue Cells with Anastomosing Processes, stained; Intercellular Substance (Ground Substance and P'ibres), unstained. branch and anastomose, forming networks. The smaller fibres are round on cross section, llie larger flattened or hexagonal (Figs, t,^ and 34j. Their elasticity is easily demonstrated in teased specimens by curling of the broken ends of the fibres (Fig. 28). On boiling THE COXXECTI\-E TISSUES. they yield elastin. Although, when subjected to the usual technic, elastic fibres appear homogeneous, they are probably composed of a thin sheath or membrane, enclosing the more granular elastin. The latter stains intensely with magenta, the sheath remaining unstained. In addition to the white v^^::--:"-.:;. Wa^SS^ Fig. 31. — Pigmented Connective-tissue Cells from Chorioid Coat of Human Eye. X 350. (Technir 7, p. 81.) fibres and elastic fibres above described, so-called "reticular" fibres are fre- quently present in fibril- lated connective tissue. (See p. 83.) (b) Basement or ground substance occurs in ex- tremely minute amounts between the individual fibrilte of the white fibres, where it acts as a cement substance. The same material also forms the basement or ground substance in which the connective-tissue cells and fibres lie (Fig. 29). Difficulty in seeing this ground substance is due to its transparency. It may be demon- strated by staining with silver nitrate. (See technic 9, p. 81.) Much variation exists in regard to the proportions of the different elements. This gives rise to variations in the physical characteristics of the tissue. When fibres predominate over cells and ground sub- stance, the tissue is dense and '^^^x^v ^ _' ^* ,.-^^, hard and is known as dense ■ -''' (S^ 'S::^ ^ 'it )) !/'fi vVv /«--. ^"^^v- : -A- 3^ f'S mm -lis Fig. 44. — Hyaline Cartilage from Head of Frog's Femur. X350. (Technic i, p. c,2.) Groups of cartilage cells in apparently homogeneous matrix. Cartilage is subdivided according to the character of its intercel- lular substance into three varieties: (i) Hyaline, (2) elastic, (3) fibrous. I. Hyaline Cartilage (Fig. 44). — The cells occur singly or in groups of two or multiples of two. An entire group of cells frequently lies in one lucuna surrounded by a single capsule. Such a group of cells has developed within its capsule from a single parent cell. In other cases delicate hyaline partitions separate the cells of a group. The cells are spherical or oval, with flattening of adjacent sides. The nucleus is centrally placed, and has a distinct intranuclear network and membrane. The cytoplasm is finely granular, and may contain drop- lets of fat, of glycogen, or of both. Toward the perichondrium the arrangement of the cells in groups is less distinct. Here the cells are fusiform and parallel to the surface. THE CONNECTR'E TISSUES. 91 The intercellular matrix, when subjected to the usual technic, appears homogeneous. By the use of special methods, such, e.g., as artificial digestion, this apparently structureless matrix has been shown to be made up of bundles of ■^ ^^'\iy>r^ -^wr^^i ^ da^ >>^' fibres, quite similar to those found in fibrous connective tissue. Hyaline cartilage forms the articular cartilages of joints, the costal cartilages, and the carti- lages of the nose, trachea, and bronchi. In the embryo a young type of hyaline cartilage, known as embryonal cartilage, forms the matrix in which most of the bones are developed. 2. Elastic cartilage (Fig. 45) resembles hyaline, but differs from the latter in that its hyaline matrix contains a large number of elastic fibres. These vary in size, many being extremely fine. The elastic fibres branch and run in all directions, forming a dense net- work of interlacing and anastomosing fibres. W^^' '^.u Fig. 45. — Elastic Cartilage from Dog's Ear. X350. (Technic 2, p. Q2.) Groups of cartilage cells in fibre-elastic matrix. =-~ >^^ l^iQ 46.— Fibrous Cartilage from Dog's Intervertebral Disc. X350. (Technic 3, p. 92.) Groups of cartilage cells in matrix of fibrillar connective tissue. Elastic cartilage occurs in the external ear, the Eustachian tube, the epiglottis, and in some of the laryngeal cartilages. 3. Fibrous cartilage (Fig. 46) is composed mainly of fibrillar 92 THE TISSUES. connective tissue. The fibres may have a parallel arrangement, or may run in all directions. Cells are few, and are usually arranged in rows of from two to six, lying in elongated cell spaces between the fibre bundles. Fibrous cartilage occurs in the inferior maxillary and sterno- cla^"icular articulations, in the symphysis pubis, and in the interver- tebral discs. Cartilage, except where it forms articular surfaces, is covered by a membrane, the perichondrium. This is composed of fibrillar con- nective tissue, and blends without distinct demarcation with the super- ficial layers of the cartilage. Like the other connective tissues, cartilage develops from mesoderm. It is at first wholly cellular. Each cell forms a capsule around itself, and by blending of these capsules are formed the first elements of the intercellular matrix. This increases in quantity and assumes the structural characteristics of one of the forms of cartilage. The white fibres of fibro-cartilage and the yellow fibres of elastic cartilage develop in the same manner as in fibrillar and elastic tissue. TECHNIC. (i) Hyaline Cartilage. — Remove a frog's femur and immediately immerse the head in saturated aqueous solution of picric acid. Cut sections tangential to the rounded head, keeping knife and bone wet with the picric acid solution. As bone must be cut, a special razor kept for the purpose should be used. Cut sections as thin as possible. The first sections consist wholly of cartilage. As bone is reached, the cartilage is confined to a ring around the bone. Mount in the picric-acid solu- tion, cementing the cover-glass immediately. {2) Elastic Cartilage. — Remove a piece of cartilage from the ear and fix in formalin-Miiller's fluid (technic 5, p. 7). Stain sections strongly with hasmatoxylin, followed by picro-acid-fuchsin (technic 3, p. 19). Clear in carbol-xylol and mount in balsam. The capsules around the cartilage cells are thick and, as they usually retain some hsematoxylin, can be readily seen. Note also the flattened cartilage cells near the surface, and the perichondrium. (3) Fibro-cartilage. — Fix pieces of an intervertebral disc in formalin-Miiller's fluid. Sections are stained either with haematoxylin-eosin or with haematoxylin- picro-acid-fuchsin and mounted in balsam. 8. Bone Tissue. Bone is a form of connective tissue in which the matrix is ren- dered hard by the deposition in it of inorganic matter, chiefly the phosphate and the carbonate of calcium. These salts are not merely THE COXXECTRE TISSUES. 93 Fig. 47. — Bone Tissue showing Lacunae and Canaliculi. X700. (Technic i, p. 94.) deposited in the matrix, but are intimately associated and combined with its histological structure. The intimacy of this association of the organic and inorganic constituents of bone is shown by the fact that, though the salts compose two-thirds of bone by weight, it is impossible to distinguish them by the highest magnifi- cation. Furthermore, if either the lime salts are dis- solved out by means of acids (decalcification) or the or- ganic matter removed by heating (calcination), the histological structure of the bone still remains. Like the other connec- tive tissues, bone consists morphologically of cells and intercellular substance. Bone cells or hone corpuscles lie in distinct cell spaces or lacunce. From the lacuna pass off in all directions minute canals — canaliculi — which anastomose with canaliculi of neighboring lacunae (Fig. 47). At the surface of bone these canaliculi open into the periosteal lymph- atics. A complete system of canals is thus formed, which traverse the bone and serve for the passage of nutri- tive fluids. The bone cells themselves (Fig. 48) are flat, ovoid, nucleated cells, with numerous fine processes, wdiich extend in all directions into the canaliculi. In young de- veloping bones the processes of adjacent cells anastomose. In adult bone the processes ex- tend but a short distance into the canaliculi. and probably do not anastomose. The basement substance or matrix has a fibrous structure, closely resembling that of fibrillar connective tissue, and it is in this fibrillar matrix that the lime salts are deposited. The fibrils are held together by cement substance into bundles. In most bone the bundles are fine and arranged in layers or lamcllcr. Less commonly the fibre bundles are coarser and ha^"C an irregular arrangement. Fig. 48. — Bone Cell and Lacuna. (After Joseph.) At a the cell body has shrunken, allowing the outline of the lacuna to be seen. 94 THE TISSUES. TECHNIC. (i) For the study of the minute structure of bone a section of undecalcified or hard bone is required. Part of the shaft of one of the long bones is soaked for sev- eral days in water and all the soft parts are removed. It is then placed in equal parts alcohol and ether to remove all traces of fat and thoroughly dried (the handle of a tooth or nail brush frequently furnishes good material and is already dried). Thin longitudinal and transverse sections are now cut out with a bone saw. One surface is next ground smooth, first on a glass plate, using emery and water, then on a hone. The specimen is now fastened polished side down on a block of wood or glass by means of sealing wax, and the other side polished smooth in the same manner as the first, the bone being ground as thin as possible. The sealing wax is removed by soaking in alcohol and the specimen looked at with the low power. If not thin enough, it is gently rubbed on a fine hone. It is then soaked in equal parts alcohol and ether, dried thoroughly, and mounted in hard balsam. This is accomplished by placing a small bit of hard balsam on a slide, melting, pushing a bit of the bone into the hot balsam, covering and cooling as quickly as possible. The object of the hard balsam and quick cooling is to prevent the balsam running into the lacunae and canaliculi and obscuring them by its transparency. The air imprisoned in the lacunae and canaliculi causes them to appear black when viewed by reflected light. (2) The structure of the bone cell is best studied in sections of decalcified bone which has first been carefully fixed. (See technic i, p. 171.) 9. Neuroglia. This peculiar form of connective tissue is confined entirely to the central nervous system and is most conveniently studied in connec- tion with nervous tissue (see page 125). CHAPTER IV. THE BLOOD. Blood is best considered as a tissue, the intercellular substance of which is fluid. This fluidity of the intercellular substance allows the formed elements or cells to move about freely, so that there is not the same definite and fixed relation between cells and intercellular sub- stance as in other tissues. The fluid intercellular substance or plasma is slightly alkaline in reaction. It consists of serum albumen, globulin, fibrinogen and inorganic salts, chiefly the chlorid, carbonate, and bicarbonate of soda. Its specific gravity ' . ff •«*««« is about i.o^o, while that of the whole blood ♦ ♦ i*«*«»«« IS about 1.060. The bulk of the plasma is ' about equal to that of the red and the white ^ gj^ g^ ^^^ cells. , " ^P ^P The formed elements of the blood are: (i) Red blood cells (red blood corpuscles, '.'{.it*: iSSi?; erythrocytes); (2) white blood cells (colorless '■ ;• •*f%v...- corpuscles— leucocytes) ; (3) blood platelets Fig. 49.— Cells from Human (thrombocytes); (4) blood dust (hama- Blood x6oo^ (Technic 2 . , P- 100.) I, Red blood cell tokonia). seen on flat; 2, red blood cell I. Red blood cells (erythrocytes) (Fig. rdis S,™lf iotie^'x:*"' 49, 1,2,3) are in man non-nucleated circular small and large lymphocytes; J. 1 rni • T • 1 ^' niononuclear leucocyte; 6, discs. iheir average diameter is about transitional leucocyte; 7, 7.5,«, their thickness 2n at the thin centre. Polymorphonuclear leuco- ' "" ' _ ' cyte, containing neutrophile They are biconcave, with rounded edges. granules; 8, polynuclear leu- o ,1 n , ,1 ^•rc • 1 • 1 cocyte, containing eosino- been on the flat, the dinerence in thickness phiie granules; q, mononu- between centre and periphery is evidenced r'^'^*" , leucocyte containing ^ ^ "^ basophile granules. by the difference in refraction (Fig. 49, i). Seen on edge, the shape resembles that of a dumb-bell (Fig. 49, 2). Singly or in small numbers, red blood cells have a pale straw color. Redness of the cells is apparent only when they are seen in large numbers. If fresh blood be allowed to stand for a moment, the red 'Some observers describe the red blood ceil as bell- or cup-shaped. (Lewis: Jour. INIed. Research, X. S., vol. v, IQ04.) 9.) 96 THE TISSUES. cells are seen to adhere to one another by their flat surfaces, forming rows or rouleaux (Fig. 49, 3). Subjected to the usual technic, the red blood cell appears homo- geneous. By the use of special methods, this apparently homogeneous substance can be separated into (a) a color-bearing proteid — hcemo- globin, and (b) a stroma, the latter representing the protoplasm of the cell. It is the haemoglobin which gives color to the corpuscles. Haemo- globin is a complex proteid, and is held in solution or in suspension in the stroma, which can be resolved into a globulin and a pigment, hamatin. The red blood cells are soft and elastic, and are easily twisted to accommodate themselves to the smallest capillaries. The red blood cell is extremely susceptible to changes in the plasma. Thus even slight evaporation of the plasma results in osmosis between the now denser surrounding fluid and the contents of the cell. This causes fluid to leave the cell, with the result that the latter becomes spheroidal and irregularly shrunken, with minute knob-like projec- tions from its surface. This is known as crenation of the red cell. The addition of water to blood, thus decreasing the specific gravity of the plasma, has the opposite effect, resulting in swelling of the cell. It also causes solution of the haemoglobin, which leaves the cell, the latter then appearing colorless. This separation of the haemoglobin from the corpuscle is also caused by freezing and thawing, by heat (60° C), by the addition of dilute acids, ether, or chloroform. Dilute alkaline solutions and bile first cause the red corpuscles to swell and become spherical, and then to dissolve. This is known as haemolysis, and may also be effected by mixing the blood of one species with that of another. Dilute acetic acid causes swelling and fading of the red cells, with the formation of prismatic crystals of haemoglobin. The red blood cells number from 4,500,000 to 5,000,000 per cubic millimetre of blood. 2. White blood cells (leucocytes) (Fig. 49, 4 to 9 inclusive) are colorless nucleated structures which have a generally spherical shape, but which are able to change their shape on account of their powers of amoeboid movement. They have a diameter of from 5 to io/<, and are much less numerous than the red cells, the proportion being about one white cell to from three hundred to six hundred red cells. This proportion is, however, subject to wide variation. Leucocytes may be classified as follows: (a) Lymphocytes; {b) THE BLOOU. 97 mononuclear leucocytes; (c) transitional leucocytes; (d) polymor- phonuclear or polynuclear leucocytes. {a) Lymphocytes (Fig. 49, 4). — These vary in diameter from 5 to 8/i, and are sometimes subdivided into small lymphocytes and large lymphocytes. The nucleus is spherical, stains deeply, and almost completely fills the cell, the cytoplasm being confined to a narrow zone around the nucleus. Lymphocytes constitute about 20-per-cent. of the white blood cells. (b) Mononuclear leucocytes (Fig. 49, 5 and 9) are of about the same size as large lymphocytes. The nucleus, however, stains more faintly and is smaller, while the cytoplasm is greater in amount. From 2-per-cent. to 4-per-cent. of the white cells are mononuclear leucocytes. {c) Transitional leucocytes (Fig. 49, 6) occur in about the same numbers as the preceding, and are of about the same size. There is relatively more cytoplasm, and the nucleus, instead of being spherical, is crescentic or horseshoe or irregular in shape. These cells represent a traditional stage between the mononuclear and the polymorpho- nuclear and polynuclear varieties. {d) Polymorphonuclear and polynuclear leucocytes (Fig. 49, 7, 8) constitute about 70-per-cent. of the white blood cells. Their size is about the same as that of the mononuclear form, but they are somewhat more irregular in shape. The appearance of the nucleus is characteristic. In the polymorphonuclear form the nucleus con- sists of several round, oval, or irregular nuclear masses connected with one another by cords of nuclear substance. These cords are frequently so delicate as to be distinguished with diflficulty. The polynuclear form is derived from the polymorphonuclear by breaking down of the connecting cords, leaving several separate nuclei or nuclear segments. The protoplasm of about 70-per-cent. of leucocytes is granular, and these granules present very definite reactions when subjected to certain aniline dyes. Aniline dyes may be divided into acid, basic, and neutral, accord- ing to whether the coloring matter is an acid, a base, or a combina- tion of an acid and a base. Upon the basis of their reaction to these dyes, Ehrlich divides these granules into five groups, which he designates by the first five letters of the Greek alphabet. a-Granules {acidophile, or, because the most common acid dye 7 98 THE TISSUES. used is eosin, eosinophile — Fig. 49, 8). These are coarse, sharply defined granules which stain intensely with acid dyes. Eosinophile cells are mainly of the polynuclear and polymorphonuclear types. More rarely transitional forms contain eosinophile granules. They are actively amoeboid. Eosinophile cells constitute from i-per-cent. to 4-per-cent. of the leucocytes of normal blood. Under certain pathological conditions the number of eosinophile leucocytes is greatly increased. /^-Granules {amphophile) . These are very fine granules, which react to both acid and basic dyes. /?- Granules are not found in nor- mal human blood. They are found in the blood cells of some of the lower animals. ^-Granules (basophile) are small granules which stain with basic dyes. They occur in the so-called Mastzellen (p. 75), which are of rare occurrence in normal blood. They are present in certain pathological conditions, and are found normally in the blood cells of some of the lower animals, and in some of the cells of connective tissue. 8-Granules (basophile) are small granules, which stain with basic dyes (Fig. 49, 9). They are found mainly in the mononuclear leuco- cytes. c- Granules (neutrophile) react to mixtures of acid and basic dyes. £- Granules are the most common of all granules, occurring in most of the polynuclear and polymorphonuclear forms, being thus present in about 68-per-cent. of all white blood cells (Fig. 49, 7). Through their powers of amoeboid movement leucocytes are able not only to pass through the walls of the vessels — diapedesis — and out into the tissues, but to wander about more or less freely in the tissues. Both inside and outside of the vessels the leucocytes have an important function to perform in the taking up and disposal of waste or superfluous materials and foreign particles. This is known as phagocytosis, and the cells thus engaged are known as phagocytes. Phagocytosis plays an extremely important role both in normal and in certain pathological processes. 3. The blood platelets (thrombocytes) are minute round or oval bodies about 2// in diameter. They are clear (colorless), and are described by some as containing chromatin granules, by others as having distinct nuclear structures. They may be separated by the action of a lo-per-cent. saline solution into two elements — one hya- line, the other granular. They are said to possess amoeboid properties THE BLOOD. 99 and to be concerned in the coagulation of the blood. They number about 200,000 per cubic millimetre. 4. The blood dust (hasmatokonia) occurs in the form of small refractive granules. In the blood of the lower mammals and in herbivorous animals small droplets of fat derived from the chyle are found. They are known as elementary granules and are not present in normal human blood. Development of the Blood. At an early stage of embryonic development certain mesodermic cells of the area vasculosa, which surrounds the embryo, become arranged in groups known as hlood islands. It is from these "islands" that both blood and blood-vessels develop. The peripheral cells arrange themselves as the primitive vascular wall, within which the central cells soon become free as the first hlood corpuscles. In this way vascular channels are formed, inside of which are developing blood cells. These channels, which are at first unconnected, anastomose and give rise to a network of channels which are the earliest capil- laries. As regards the origin of the later vessels both within the embryo and in the extraembryonic area, two views are held; (i) that they are outgrowths from the earliest capillaries; (2) that they arise in situ in the same manner as the earliest capillaries and unite secondarily to form networks. The weight of evidence at present favors the latter view. These networks of capillaries at first consti- tute the entire vascular system. As development proceeds some of the channels enlarge to form the arteries and veins, the smooth muscle and connective tissue of their walls dift'erentiating from the surrounding mesoderm. At first the formed elements of blood consist almost wholly of nucleated red cells. These undergo mitotic division and multiply within the vessels. Two views are held in regard to the manner in which the embryonic nucleated red cell gets rid of its nucleus in becoming the non-nucleated red cell of the adult. Accord- ing to one the nucleus is absorbed within the cell; according to the other the nucleus, as a whole, is extruded. In early embryonic life especially active proliferation of red cells occurs in the blood-vessels of the liver. This has led to the consider- ing of the liver as a blood-forming organ. The liver cells themselves, however, taken no actual part in the formation of blood cells, the blind pouch-like venous capillaries of tlic li\"er, willi their slow-mo\ing l:)lood 100 THE TISSUES. currents, merely furnishing a peculiarly suitable place for cellular pro- liferation. Before birth the splenic pulp and bone marrow become blood-forming organs. In the adult the bone marrow is probably, under normal conditions, the main if not the sole seat of red-cell formation. During fcetal life the number of nucleated red cells constantly diminishes while the number of non-nucleated red cells increases. At birth there are usually but few nucleated red cells in the general circulation, although even in the adult they are always found in the red bone-marrow. The earliest embryonic blood contains no white cells. The origin of the leucocytes is not well understood. It seems probable that the earliest leucocytes are derived like the red cells from the cells of the blood islands of the area vasculosa. Later they are formed in widely distributed groups of cells, lymph nodules, which are found in various tissues and organs. These cells enter the circulation as lymphocytes. According to some, the mononuclear, transitional, polymorphonuclear, and polynuclear forms are later stages in the development of these cells. According to others, the poly- morphonuclear and polynuclear forms are derived from the myelocytes of bone marrow. The origin of the blood platelets is not known. It is possible that they represent extrusion products of the blood cells. TECHNIC. (i) Fresh Blood. — Prick a finger with a sterile needle. Touch the drop of blood to the centre of the slide and cover quickly. For immediate examination of fresh blood no further preparation is necessary. Evaporation may be prevented by cementing, or by smearing a rim of vaseline around the cover-glass. (2) Blood Smears. — From the same or a second prick take up a drop of blood along the edge of a mounting slide. Quickly place the edge against the surface of a second slide and dravi^ the edge across the surface in such a manner as to leave a thin film or smear of blood. Allow the smear to become perfectly dry and stain by technic 9, p. 28. By this method the acidophile granules are stained red, baso- phile granules purple, and neutrophile granules a reddish-violet. Good results may also be obtained by fixing the dried smear for half an hour in equal parts alcohol and ether and staining first in a strong alcoholic solution of eosin, then in a rather weak aqueous solution of methylene blue. CHAPTER V. MUSCLE TISSUE. While protoplasm in general possesses the property of contrac- tility, it is in muscle tissue that this property reaches its highest de- velopment. Moreover, in muscle this contractility is along definite Fig. 50. — Isolated Smooth Muscle Cells from Human Small Intestine. X400. (Tech- nic I, p. 109.) Rod-shaped nucleus surrounded by area of finely granular protoplasm; longitudinal striations of cytoplasm. directions, and is capable of causing motion, not only in the cell itself, but in structures outside the cell. Muscle may be classified as: (i) Involuntary smooth muscle; (2) voluntary striated muscle; (3) involuntary striated muscle or heart muscle. I. Involuntary Smooth Muscle. — This consists of long spindle- shaped cells (Fig. 50). The length of the cell varies from 30 to 200/'., C"^-i^ B Fig. 51. — Apparent Intercellular Bridges of Smooth Muscle. A, From longitudinal section of intestine of guinea-pig; B, from transverse section of intestine of rabbit. X420. (2, Nerve cell; h, end of muscle cell. (Stohr.) its width from 3 to 8/(, except in the pregnant uterus, where the cells frequently attain a much greater size. At the centre of the cell, which is its thickest portion, is a long rod-shaped nucleus surrounded by an 101 102 THE TISSUES. area of finely granular cytoplasm. The rest of the cytoplasm shows delicate longitudinal striations, which probably represent a longitu- dinal arrangement of the spongioplasm. The cells are united by a small amount of cement substance. Intercellular "bridges" similar to those connecting epithelial cells have been described (Fig. 51). Smooth muscle cells may be arranged in layers of considerable thickness, the cells having a definite direction, as in the so-called "musculature" of the intestine (Fig. 52). In such masses of smooth muscle the cells are separated into groups or bundles by connective tissue. Smooth muscle cells may be arranged in a sort of network, the cells crossing and interlacing in all directions, as in the wall of the Fig. 52. — Smooth Muscle from Longitudinal Section of Cat's Small Intestine, show- ing Portions of Inner Circular and Outer Longitudinal Muscle Coats with Intervening Connective Tissue. X350. (Technic 3, p. 109.) a, Transversely cut cells of inner circu- lar layer; in comparatively few has the plane of section passed through the nucleus; b, longitudinally cut cells of outer longitudinal layer. In many of the cells the plane of sec- tion has not passed through the nucleus; c, intermuscular septum (connective tissue); d, small artery. frog's bladder. Again, they may be scattered in small groups or singly among connective-tissue elements, as in the villi of the small intestine. 2. Voluntary Striated Muscle. — This consists of cylindrical fibres from 50 to 130 mm. in length and from 10 to 100// in diameter. Each muscle fibre consists of (a) a delicate sheath, the sarcolemma, enclosing {b) the muscle substance proper, in which lie (c) the muscle nuclei. The sarcolemma is a clear, apparently structureless, membrane, which adheres so closely to the underlying muscle substance as to be indistinguishable in most preparations. In teased specimens it may frequently be seen at the torn ends of the fibres (Fig. 53). The muscle substance consists of ergastoplasm {Jibrilla) and sarco- MUSCLE TISSUE. 103 plasm, and shows two sets of striations (Fig. 54), longitudinal striations and cross striations. The longitudinal striations are due to parallel running ultimate fibrillcC, of which the muscle fibre is composed. These fibrillte are united by a minute amount of interfibrillar cement sub- stance. The transverse striations appear in the unstained fibre examined by reflected light as alternate light and dark bands (Figs. 54 and 55). The light band is composed of a singly refracting (isotropic) substance, the dark band of a doubly refracting (anisotropic) substance. Through the middle of the light band runs a fine dark (aniso- tropic) line {Kraiise's line), while an even finer light (isotropic) line {Hensen's line) runs through the middle of the dark band. As both dark and light substances run through the entire thickness of the fibre, they in reality constitute discs of muscle substance (Fig. 55). By means of certain chemicals these discs may be separated, the separation taking place along the lines of Klrause. Each "muscle disc'' thus consists of that portion of a fibre included between two adjacent lines of Krause and is composed of a central dark disc, and on either side one-half of each adjacent light disc. A muscle fibre is thus seen to be divisible longitudinally into ultimate fihrillcB, transversely into muscle discs. What is known as the sarcous element of Boicmaii is that portion of a single fibrilla which is included in a single disc, i.e., be- Fig. 53.— Semidiagramma- , ^ J. ^ ,. ^ T' /T-- N ^^^ Drawing of Parts of tween two adjacent Imes of Krause (rig. 55). The sarcoplasm is not evenly distributed throughout the fibre. On cross section irregular trabeculae of sarcoplasm are seen extending in from the sarcolemma (Fig. 56). two Muscle Fibres which have been broken, show- ing the relations between Muscle Substance Proper and Sarcolemma. (Ran- vier.) m, a. Retracted These separate ^"^1^ of muscle substance; , . . between which is seen the the fibrillce mto bundles, the muscle columns oj sarcolemma with several KolUker. A transverse section of one of these -^^herent muscle nuclei: 3. thm layer of muscle columns presents the appearance of a network of substance which has ad- , , ^ . ^, ... , hered to the sarcolemma: sarcoplasm and of mterfibrillar cement substance „, muscle nucleus: 5, sar- enclosing the fibrillce. This appearance is known ^-o'emma: p space be as Colinhcim's field (Figs. 55 and 56). tween sarcolemma and muscle substance. 10^ THE TISSUES. The contractile element of the fibre, the fibrilla, is anisotropic, the sarcoplasm isotropic; the former, therefore, appears dark, the latter light by polarized light. Upon this is based Rollet's theory of the structure of the striated muscle fibre (Fig. 57). According to this theory, each fibrilla consists of a number of rod-shaped segments joined end to end. Each segment consists of a thicker central portion, I d ► Fig. 54- I'IG. Fig. 54. — Portion of Striated Voluntary Muscle Fibre. X350. (Technic 4, p. log.) The fibre is seen to be marked transversely by alternate light and dark bands. Through the centre of the light band is a delicate dark line (Krause's line); through the centre of the dark band a fine light line indicates Hensen's line. The black line outlining the fibre repre- sents the sarcolemma. a, Fibrillse; h, muscle nucleus; c, Krause's line; d, Hensen's line. Fig. 55. — Diagram of Structure of a Muscle Column of KoUiker. The appearance presented by the cross-cut muscle column = Cohnheim's field, a, Muscle fibrillae, h, sar- cous element; c, Krause's line; d, Hensen's line; e, Cohnheim's field; /, muscle disc. which tapers almost to a point where it joins the next adjacent segment. The point of union is marked by a minute globular swelling. Between the fibrillae is the semi-fluid sarcoplasm. In the formation of a fibre similar parts of each fibril-segment lie in the same transverse plane. The thicker portions lying side by side form the dark disc in which there is comparatively little sarcoplasm. The attenuated portions, with their relatively large amount of sarcoplasm, form the light disc. The row of globular swellings forms the line of Krause. MUSCLE TISSUE. 105 Two varieties of striated voluntary muscle fibres are distinguished, white fibres and red fibres. The difference between the two is due to the amount of sarcoplasm — the red fibres being rich in sarcoplasm, the white fibres poor. Red fibres contract less rapidly than white, but are less easily fatigued. In man white fibres are in the large majority, red fibres never occurring alone, but mingled with white fibres in some of the more active muscles, such as those of respiration and mastication. In some of the lower animals are found muscles made up wholly of red fibres. H H H H H H I* ^ Muscle fibres ending within the ■■■■■■ J substance of a muscle have pointed extremities. Where muscle fibres join tendon, the fibre ends in a rounded Fig. s6. Fig. Fig. 56. — Semidiagrammatic Drawing of Transverse Section of a Voluntary Muscle Fibre, showing Sarcolemma; sarcoplasm separating iibrils into bundles, each bundle con- stituting a muscle column of Kolliker and the appearance of its cross-cut end being Cohn- heim's field, a, Sarcoplasm: h, Cohnheim's fields; c, sarcolemma. Fig. 57.— Diagram representing Rollet's Theory of the Structure of a \'oluntary Mus- cle Fibre, a, Dark disc; 6, light disc; c, sarcoplasm; rf, fibrilla; e, Krause's line. or blunt extremity, the sarcolemma being continuous with the tendon fibres (Figs. 58 and 59). Muscle fibres are usually unbranched. In some muscles — e.g., those of the tongue and of the eye— anastomosing branches occur. When muscle fibres end in mucous membranes — e.g.. the muscle fibres of the tongue — their terminations are often branched. Muscle fibres are multinuclear, some of the large fibres containing a hundred or more nuclei. In the white fibres the nuclei are situated at the periphery just beneath llic sarcolemma. In red fil)rcs they are centrally placed. 106 THE TISSUES. 3. Involuntary Striated Muscle (Heart Muscle). — This occu- pies an intermediate position, both morphologically and embryo- logically, relative to smooth muscle and to striated voluntary muscle. Like the former, it is composed of cells. Like the latter, it is both longitudinally and transversely striated. Heart- muscle cells are short, thick cylinders. These are joined end to end to form long fibres. By means of lateral branches the cells of one fibre anastomose with cells of adjacent fibres. Each heart-muscle cell usually contains one nucleus; some cells contain several nuclei. While there is no distinct sarcolemma, the sarcoplasm is more abundant at the surface of the cell, thus giving much the appearance of an enclosing membrane. The amount of sarcoplasm throughout the cell is large. Around the nucleus is an area of sarco- ])lasm free from fibrillae. This area often ex- tends some distance toward the ends of the cell. The striations of heart muscle are less dis- tinct than are those of voluntary muscle. According to McCallum, they represent very similar structures. The longitudinal striations indicate fibrillcE- united by cement substance. From the central mass of sarcoplasm which surrounds the nucleus, strands radiate tpward the periphery. These strands, anastomosing, separate the fibrillae into columns, the muscle columns of Kolliker. In cross section these pre- sent the appearance described under voluntary Fig. 58.— Semidiagram- muscle as Cohnheim'' s fields. The disposition of matic Illustration of ,. i j- .li Endings of Muscle Fi- the sarcoplasm, cxtcndmg outward irom the bres vvithin a Muscle j-egion of the nuclcus like the spokes of a wheel, and in 1 endon. (Cjage.) ° _ ... a, Tapering end of fibre gives to the cross section a characteristic radiate ':::^^:1^Sl appearance (Fig. 61). The transverse markings of the central fibre represent, as in voluntary muscle, alternate light shows the same method , , , ,. rr^, , , • ^ ^^ r i of termination; c, c, and dark dtscs. 1 hrough the middle or the each fibre terminates j| i^ ^^^^ ^^^ -^^ ^^^^ ^-^^ membrane of Krause. above in pointed intra- " -' muscular ending, below McCallum describes Krause's membrane as nected with tendon ' belonging not Only to the fibrillar element, but MUSCLE TISSUE. 107 also to the sarcoplasm. The latter he describes as further subdivided by membranes, which are transversely continuous with Krause's membranes, into minute discs. The centre of the cell around the nucleus is wholly composed of these little discs of sarcoplasm. McCallum describes two appearances which the lines of union between the muscle cells present. In one each fibrilla shows a thick- enincT at the cement line, from which one or more delicate filaments I I I I S T'S. Fig. 59. Fig. 60. Fig. 59. — Two Muscle Fibres from Upper End of Human Sartorius, to show connection of muscle and tendon. X350. (Gage.) ?», Muscle fibres: /, tendon fibres. Fig. 60. — Muscle Cells from the Human Heart (technic 6, p. no), showing lateral branches and lines of union between cells. X soo. cross the cement to unite with similar filaments from an opposite fibrilla. In the other form of union the cement substance is crossed by intercellular bridges similar to those described under epithelium. Recent investigations tend to prove that what have been described as heart-muscle cells are not separate units, but that heart muscle is a syncytial tissue, each cell representing only a groicth segment of the whole muscle fibre. The occurrence of non-nucleated segments and the fact that the longitudinal fibrillae are described by some ob- servers as passing uninterruptedly through the "intercellular" cement substance favor this view. On the other hand, the ease with which heart muscle may be separated into cells, especially in young animals 108 THE TISSUES. and in the lower vertebrates, and the definite staining reaction which intercellular substance gives when subjected to the action of silver nitrate are in favor of a cellular structure. Development of Muscle Tissue. In the higher animals muscle tissue, with the single exception of the sweat-gland muscles (page 62), is derived wholly from mesoderm. The smooth muscle cell shows the least differentiation. In be- coming a smooth muscle cell the embryonal cell changes its shape, - ^ C5i Fig. 61. — Section of Heart Muscle. X350. (Technic 7, p. no.) a, Cells cut longi- tudinally; b, cells cut transversely (only three nuclei have been included in the plane of section); c, cells cut obliquely; d, connective-tissue septum. becoming greatly elongated, while at the same time its spongioplasm is arranged as longitudinally disposed contractile fibrils. A voluntary muscle fibre is a highly differentiated multinuclear cell or syncytium. Each fibre is developed from a single cell (myo- blast) of one of the embryonic muscle segments or myotomes. These cells, which are at first spherical, become elongated and spindle- shaped. The nucleus is at this stage centrally placed, and the spongio- plasm occurs in the form of a reticulum. Regular arrangement of the spongioplasm first appears around the periphery, while the central portion of the cell is still occupied by reticular spongioplasm and the nucleus. The fibrils extend toward the centre until they fill the entire cell, which has now become a muscle fibre. During this process of fibrillation the nucleus has been undergoing mitotic MUSCLE TISSUE. 109 division. In the white fibres these nuclei migrate to the surface and come to lie just beneath the sarcolemma. The cement substance which unites the fibrils, as well as the larger masses of sarco- plasm, represents the remains of still undifferentiated protoplasm (hyaloplasm). McCallum describes the development of heart muscle in the pig as follows: In embryos lo mm. long the heart muscle consists of closely packed spindle-shaped cells, each containing an oval nucleus. The spongioplasm is arranged in the form of a network, no fibrils being present. In embryos 25 mm. long the shape of the cell re- mains unchanged, but on cross section there can be seen around the periphery a row of newly formed fibril bundles which have developed from the spongioplasm. From the periphery fibril bundles spread toward the centre. In embryos 70 mm. long the heart-muscle cell has assumed its adult shape and structure. Attention has already been called (page 47) to the spongioplasm as the contractile element of protoplasm. It is to be noted that in the development of muscle no new element appears, the contractile fihriUcE representing nothing more than a specialization of the already contractile spongioplosm. TECHNIC. (i) Isolated Smooth Muscle Cells. — Place small pieces of the muscular coat of the intestine in o.i-per-cent. aqueous solution of potassium bichromate, or in 30-per- cent, alcohol for forty-eight hours. Small bits of the tissue are teased thoroughly and mounted in glycerin. Nuclei may be demonstrated by first washing the tissue and then staining for twelve hours in alum carmine (page 17). This is poured off, the tissue again washed in water and preserved in eosin-glycerin, which gives a pink color to the cytoplasm. (2) Potassium hydrate in 40-per-cent. aqueous solution is also recommended as a dissociater of smooth muscle cells. Pieces of the muscular coat of the intestine are placed in this solution for five minutes, then transferred to a saturated aqueous solution of potassium acetate containing i-per-cent. hydric acetate for ten minutes. Replace the acetate solution by water, shake thoroughly, allow to settle, pour off water, and add alum-carmine solution (page 17). After twelve hours' staining, wash and transfer to eosin-glycerin. (3) Sections of Smooth Muscle. — Fix small pieces of intestine in formalin- Miiller's (technic 5, p. 7) or in Zenker's fluid (technic 9, p. 8). Thin transverse or longitudinal sections are stained with ha^motoxylin-eosin (technic i, p. 18), and mounted in balsam. As the two muscular coats of the intestine run at right angles to each other, both longitudinally and transversely cut muscle may be studied in the same section. (4) Striated \'oluntary Aluscle Fibres. — One of the long muscles removed from no THE TISSUES. a recently killed animal is kept in a condition of forced extension while a i-per- cent. aqueous solution of osmic acid is injected into its substance at various points by means of a hypodermic syringe. Fixation is accomplished in from three to five minutes. The parts browned by the osmic acid are then cut out and placed in pure glycerin, in which they are teased and mounted. (5) Sections of Striated Voluntary Muscle. — Fix a portion of a tongue in forma- lin-Miiller's fluid or in Zenker's fluid (page 8). Thin sections are stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 19) and mounted in balsam. As the muscle fibres of the tongue run in all directions, fibres cut transversely, longitudi- nally, and obliquely may be studied in the same section. The sarcolemma, the pointed endings of the fibres, and the relation of the fibres to the connective tissue can also be seen. (6) Isolated heart-muscle cells may be obtained in the same manner as smooth muscle cells. (See technic i, p. 109.) (7) Sections of Heart Muscle. — These are prepared according to technic 3 (above). By including the heart wall and a papillary muscle in the same section, both longitudinally and transversely cut cells are secured. The stain may be either haematoxylin-eosin (technic i, p. 18), or hasmatoxylin-picro-acid-fuchsin (technic 3, P- 19)- CHAPTER VI. NERVE TISSUE. The Neurone. In most of the cells thus far described the protoplasm has been confined to the immediate vicinity of the nucleus. In the smooth muscle cell was seen an extension of protoplasm to a considerable distance from the nuclear region, while in the connective-tissue cells of the cornea the protoplasmic extensions took the form of distinct processes. Processes, often extending long distances from the cell body proper, constitute one of the most striking features of nerve- cell structure. Some of these processes are known as nerve fibres; and nerve tissue was long described as consisting of two elements, nerve cells and nerve fibres. With the establishment of the unity of the nerve cell and the nerve fibre, the nerve cell with its processes was recognized as the single structural unit of nerve* tissue. This unit of structure is known as a neurone. The neurone may thus be defined as a nerve cell with all of its processes. In the embryo the neurone is developed from one of the ectoder- mic cells which constitute the wall of the primitive neural canal. This embryonic nerve cell, or neuroblast, is entirely devoid of proc- esses. Soon, however, from one end of the cell a process begins to grow out. This process is known as the axone (axis-cylinder process, neuraxone, neurite). Other processes appear, also as outgrowths of the cell body; these are known as protoplasmic processes or dendrites. Each adult neurone thus consists of a cell body, and passing oft" from this cell body two kinds of processes, the axis-cylinder process and the dendritic processes (Fig. 62). I. The Cell Body. — Like most other cells, the nerve cell body consists of a mass of protoplasm surrounding a nucleus (Fig. 63). Nerve cell bodies vary in size from very small cell bodies, such as those found in the granule layers of the cerebellum and of the olfac- tory lobe, to the large bodies of the Purkinje cells of the cerebellum and of the motor cells of the \'cnlral horns of the cord, wliich are among the largest in the body. There is as much \ariation in shape 111 112 THE TISSUES. as in size, and some of the shapes are characteristic of the regions in which the cells are situated. Thus, many of the bodies of the cells of the spinal ganglia are spheroidal; of most of the cells of the cortex cerebri, pyramidal; of the cells of Pur- kinje, pyriform; of the cells of the ventral horns of the cord, irregularly stellate. According to the number of processes given off, nerve cells are often referred to as unipolar, bipolar, or multipolar. /, w b ~ Fig. 62. Fig. 6- FiG. 62. — Scheme of Lower Motor Neurone. (Barker.) The cell body, protoplasmic processes, axone, collaterals, and terminal arborizations in muscle are all seen to be parts of a single cell and together constitute the neurone, c, Cytoplasm of cell body containing chromophihc bodies, neurofibrils, and perifibrillar substance; n, nucleus; n', nucleolus; d, dendrites; ah, axone hill free from chromophilic bodies; ax, axone; sf, branch (collateral); m, medullary sheath; n R, node of Ranvier where branch is given off; si, neurilemma (prob- ably not present in central nervous system); m', striated muscle fibre; tel, motor end plate. Fig. 63. — Large Motor Nerve Cell from Ventral Horn of Spinal Cord of Ox, showing Chromophilic Bodies. (From Barker, after von Lenhossek.) a, Pigment; b, axone; c, axone hill; d, dendrites. The NUCLEUS of the nerve cell (Fig. 63) differs in no essential from the typical nuclear structure. It consists of (i) a nuclear mem- brane, (2) an intranuclear network of linin and chromatin, (3) an achromatic nucleoplasm, and (4) a nucleolus. The CYTOPLASM of the nerve cell consists of two distinct elements: XER\'E TISSUE. 113 (i) Neurofibrils, and (2) perifibrillar substance. In most nerve cells a third element is present, (3) chromophilic bodies. (i) The neurofibrils are extremely delicate fibrils which are con- tinuous throughout the cell body and all of its processes. Within the body of the cell they cross and interlace and probably anastomose (Figs. 64 and 65). ^ /V'l } Fig. 64. — Ganglion Cells, Stained by Bethe's Method, showing Neurofibrils. A, Anterior horn cell (human); B, cell from facial nucleus of rabbit; C, dendrite of human anterior horn cell showing arrangement of neurofibrils. (Bethe.) (2) The perifibrillar substance (Fig. 64) is a fluid or semifluid substance which both in the cell body and in the processes surrounds and separates the neurofibrils. It is believed by some to be like the fibrils, continuous throughout cell body and processes, by others to be interrupted at certain points in the axone (see page 121). (3) The chromophilic bodies (Fig. 63) are granules or groups of 114 THE TISSUES. granules which occur in the cytoplasm of all of the larger and of many of the smaller nerve cells. They are best demonstrated by means of a special technic known as the method of Nissl (page 35). When subjected to this technic, nerve cells present two very different types of reaction. In certain small cells the amount of cytoplasm is ex- tremely small and only the nuclei stain. Such cells are found in the granule layers of the cerebellum, olfactory lobe, and retina. They are known as caryochromes, and apparently consist wholly of neurofibrils Fig. 65. — Body of Large Pyramidal Cell from Cortex of Cat. Silver Method of Cajal. Shows nucleus pale and arrangement of neurofibrils within the cell; a, axone; b, main or apical dendrite. (Cajal). and perifibrillar substance. Other cells react, both as to their nuclei and as to their cell bodies, to the Nissl stain. These cells are known as somatochromes. Taking as an example of this latter type of cell one of the motor cells of the ventral horn of the cord and subjecting it to the Nissl technic, we note that the cytoplasm is composed of two distinct elements: {a) a clear, unstained ground substance, and, scat- tered through this, {h) deep-blue-staining masses, the chromophilic bodies (Fig. 63). These bodies are granular in character and differ in shape, size, and arrangement. They may be large or small, regular NER^'E TISSUE. llo or irregular in shape, may be arranged in rows or in an irregular manner, may be close together, almost filling the cell body, or quite separated from one another. Presenting these variations in different types of cells, the appearance of the chromophilic bodies in a partic- ular type of cell remains constant, and has thus been used by Nissl as a basis of classi- fication.' It is important to note in studying the nerve cell by this method that somato- chrome cells of the same type frequently show marked variations in staining in- tensity. This appears to depend upon the size and closeness of arrangement of the chromophilic bodies, and this again seems dependent upon changes in the cytoplasm connected with functional activity. In cells stained by Nissl's method the cytoplasm between the chromophilic bodies remains unstained and apparently struc- tureless, and it is this part of the cytoplasm that corresponds to the neurofibrils and perifibrillar substance. The relation which the appearance of the Nissl-stained cell bears to the structure of the living protoplasm is still undetermined. According to some investigators the Nissl bodies exist as such in the living cell. Others believe that they are not present in the living cell, but represent precipitates due either to postmortem changes or to the action of fixatives. The significance of the Nissl picture from the standpoint of pathology lies in the fact that when subjected to a given technic, a particular type of nerve cell always presents the same ap- pearance, and that this appearance furnishes a norm for comparison with cells showing patho- logical changes, and which have been subjected to the same Icchnic Fig. 66. — Pyramidal Cell from Human Cerebral Cortex. (Golgi bichlorid method. See 2, p. 33.) Golgi cell type I. a, Cell body; h, main or apical dendrite showing gem- mules; c, lateral dendrites showing gemmules; d, a.xone with collaterals. Only part of a.xone is included in drawing. Many nerve cells contain more or less brownish or yellowish pig- . ment (Fig. 63). This pigment is not present in the cells of the new- born, but appears in increasing amounts with age. Its significance is not known. 'For this classification, the significance of which is somewhat doubtful, the reader is referred to Barker, "The Nervous System and Its Constituent Neurones." p. 121. 116 THE TISSUES. In addition to its characteristic structure, the nerve cell may con- tain many elements found in other cells (p. 39). Golgi, Holmgren, and others have also demonstrated a network of canals v^ithin the nerve cell similar to that found in other cells (p. 42, Fig. 4). II. The Protoplasmic Processes or Dendrites. — These have a structure similar to that of the cell body, consisting of neurofibrils, perifibrillar substance, and, in somatochrome cells, chromophilic bodies (Figs. 63 and 64). Dendrites branch dichotomously, become Fig. 67. — Golgi Cell Type II. from Cerebral Cortex of Cat. (KoUiker.) x, Coarse protoplasmic processes with gemmules easily distinguishable from the more delicate, smoother axone, a. The latter is seen breaking up into a rich plexus of terminal fibres near its cell of origin, practically the entire neurone being included in the drawing. rapidly smaller, and usually end at no great distance from the cell body (Figs. 66 and 67). III. The Axone. — This differs from the cell body and dendrites in that it contains no chromophilic bodies (Fig. 63), consisting wholly of neurofibrils and perifibrillar substance. Not only is it entirely achromatic itself, but it always takes origin from an area of the cell body, the axone hill (Fig. 63), which is free from chromophilic bodies. It is as a rule single, and while usually arising from the body of the cell may be given off from one of the larger protoplasmic trunks. Some few cells have more than one axone, and nerve cells without NERVE TISSUE. 117 axones have been described. In Golgi preparations the axone is distinguished by its straighter course, more uniform diameter, and smoother outHne (Fig. 66). It sends off few branches {collaterals), and these approximately at right angles. Both axone and collaterals usually end in terminal arborizations. In most cells the axone ex- tends a long distance from the cell body. Such cells are known as Golgi cell type I. (Fig. 66). In others the axone branches rapidly and ends in the gray matter in the vicinity of its cell of origin — Golgi cell type II. (Fig. 67). As they leave the cell body the neurofibrils of the axone converge to a very narrow portion of the axone, where the perifibrillar sub- stance is much reduced in amount, or according to some, entirely interrupted. Beyond this the fibrils become more separated and the perifibrillar substance more abundant. Some axones pass from their cells of origin to their terminations as "naked" axones, i.e., uncovered by any sheath. Other axones are enclosed by a thin membrane, the neurilemma or sheath oi Schwann. Still others are surrounded by a sheath of considerable thickness known as the medullary sheath. Depending upon the presence or absence of a medullary sheath, axones may thus be divided into two main groups — medullated axones and non-mediillated axones. 1. NoN-MEDULLATED AxoNES (non-mcdullatcd nerve fibres) (Fig. 68). These are subdivided into non-medullated axones without a neurilemma and non-medullated axones with a neurilemma. {a) Non-medullated axones without a neurilemma are merely naked axones. Present in large numbers in the embryo, they are in the adult confined to the gray matter and to the beginnings and endings of sheathed axones, all of the latter being uncovered for a short dis- tance after leaving the nerve cell body, and also just before reaching their terminations. (6) Non-medullated axones with a neurilemma— Jibres of Remak (Fig. 68) . In these the axone is surrounded by a delicate homogeneous, nucleated sheath, the neurilemma or sheath of Schwann (see p. 119). These axones are described by some writers as ha^■ing no true neuri- lemma, but merely a discontinuous covering of flat connective-tissue cells, which wrap around the axone and correspond to the endoneu- rium of the nerve trunk (see page 382). 2. Medullated Axones (medullated nerve fibres). — These, like the non-medullated, are subdivided according to the presence or ab- lis THE TISSUES. sence of a neurilemma into medullated axones with a neurilemma and medullated axones without a neurilemma. (a) Medullated axones with a neurilemma constitute the bulk of the fibres of the cerebro-spinal nerves. Each fibre consists of (i) an axone, (2) a medullary sheath, and (3) a neurilemma. (i) The axone is composed of neurofibrils continuous with those of the cell body, and like them lying in a perifibrillar substance or neuro- FiG. 6S. Fig. 69 Fig. 68. — Non-medullated Nerve Fibres with Neurilemma, only the nuclei of which can be seen. X300. Fig. 69. — A, Fresh nerve fibre from sciatic nerve of rabbit teased apart in normal salt solution, showing broad unshrunken axone and comparatively thin medullary sheath. B, Showing crenation of medullary sheath which occurs soon after placing fibres in salt solution. C, Same after fixation and staining with picro-acid-fuchsin, showing shrunken axone and broad medullary space. The latter usually contains irregular clumps of myelin, a. Node of Ranvier; b, incisures of Schmidt; c, nucleus of neurilemma. plasm (Fig. 72). In the fresh condition the axone is broad, and shows faint longitudinal striations corresponding to the neurofibrils, or ap- pears homogeneous (Fig. 69, A). Fixatives usually cause the axone to shrink down to a thin axial thread, whence its older name of axis- cylinder (Fig. 6g,C). A delicate membrane has been described by some as enveloping the axone. It is known as the axolemma or peri- axial sheath (Fig. 70). XER\-E TISSUE. 119 (2) The medullary sheath (Figs. 69 and 72) is a thick sheath com- posed of a semifluid substance resembling fat and known as mye- lin. In the fresh state the myelin has a glistening homogeneous ap- f pearance. It is not continuous, but , ^... is divided at intervals of from 80 to 6oo/( by constrictions, the nodes or constrictions of Ranvier. That por- tion of a fibre included between two nodes is known as an internode (Fig. 70). The length of the internode is usually proportionate to the size of the fibre, the smaller fibres having the shorter internodes. In fresh ' j , 1 fv specimens the medullary sheath of an internode appears continuous (Fig. 69, ^4), but in fixed specimens it is broken up into irregular seg- ments, Schniidt-Lantermann segments, by clefts which pass from neurilemma to the axolemma or axone, and are known as the clefts or incisures of Schmidt-Lantermann (Fig. 69, C). On boiling medullated nerve fibres in alcohol and ether a fine network is brought out in the m^edullary sheath, the neurokeratin network. Owing to the resistance of neurokeratin to the action of trypsin, it has been con- sidered as possibly similar in compo- sition to horn. (3) The neurilemma or sheath of Schwann (Figs. 70, B, and 72) is a delicate structureless membrane Fig. 71 which encloses the myelin. At the nodes of Ranvier the neurilemma dips into the constriction and comes in contact with the axone or axo- lemma. Against the inner surface of the neurilemma, usuallv about midway l^ctwcen two nodes, is an o^"al-shaped nucleus, the nucleus of Fig. 70. Fig. 71. Fig. 70. — Diagram of Structure of a Med- ullated Nerve Fibre of a Peripheral Nerve showing two different views as to relations of neurilemma and axolemma and their behavior at the nodes of Ranvier. (Szymonowicz.) a, Neuro- tibrils; /), cement substance; r, axone; d, incisure of Schmidt; e. nucleus of neurilemma; /, medullary sheath; g, sheath of Schwann; h, a.xone; /, a.xo- lemma; f, sheath of Schwann; k, node of Ranvier. -Piece of ^fedullaled Nerve from Human Radial Nerve. X400. Osmic-acid fixation and stain. (Szymonowicz.) a. Medullary sheath; /), a.xone; c, sheath of Henle; ig mechanism of the neurone. The alreadv referred to observations of Bethe regarding the intcrrup- 122 THE TISSUES. tion of the perifibrillar substance at the constricted portion of the axone and at the nodes of Ranvier, thus making the neurofibrils the only continuous structure, are obviously in favor of this view?. The neurofibrils are probably a differentiation of the spongioplasm, while the perifibrillar substance and chromophilic bodies are specializations of the hyaloplasm. As to the manner in which neurones are connected, there are two main theories, the contact theory and the continuity theory. Fig. 73. — A, B, C, Three cells of the Ventral Cochlear Nucleus of Rabbit, showing terminals of fibres of cochlear nerve and their relations to the cell bodies (Cajal). a, a, a. Fibres of the cochlear nerve, which break up into terminal arborizations upon the cells; b, c, terminal rings. The point of contact between the terminals of the one neurone and the cell body of the other neurone constitute a " synapsis." According to the contact theory each neurone is a distinct and separate entity. Association between neurones is by contact or contiguity of the terminals of the axone of one neurone with the cell body or dendrites of another neurone, and never by continuity of their protoplasm. This theory, which is known as the "neurone theory" and which received general acceptance as a result of the work of Golgi, His, Forel, Cajal, and others, has been recently called in question by such promi- nent neurologists as Apathy, Bethe, Held, and Nissl, on the ground that in some cases the neurofibrils are continuous throughout a series of neurones. Based upon the contact theory is the so-called "retraction theory," which held that a neurone being associated with other neurones only by contact was able to retract its terminals, thus breaking the association and throwing itself, as it were, out of circuit. The NERVE TISSUE. 123 point of contact between two neurones is called a synapsis (Fig. 73), and the concep- tion that there is some kind of interruption or discontinuity in neural circuits in- volving more than one neurone has proved useful to physiologists. It affords an explanation of certain differences between conduction through a circuit of two or more neurones and through a nerve fibre. For example, an impulse takes longer to traverse the circuit than to traverse a nerve fibre of equal length. Also a stimulus may pass in either direction along a nerve fibre, but cannot be "reversed" along a circuit. According to the continuity theory, while the perifibrillar substance is interrupted as above described, the neurofibrils are continuous. According to this theorv the neurofibrils, which form a plexus or network within the cell body and dendrites, Fig. 74. — A, Normal Nerve Fibres from Sciatic Nerv-e of Rabbit, osmic acid fixation and stain; each fibre shows node of Ranvier. B, Two fibres from distal part of rabbit's sciatic five days after cutting the nerve; shows segmentation of myelin; C, three fibres from distal part of rabbit's sciatic three weeks after cutting nerve; most of the myelin has been absorbed and onlv traces of the axones remain. are connected with a pericellular network — the Golgi net — which closelv invests the cell body and its dendrites. Externally the Golgi net is further connected with the neurofibrils of the axones and collaterals of other nerve cells. This connection is either direct, or, as some believe, through another general (diffuse) extracellular network. The neurofibrils are thus, according to this theory, continuous and form two or possibly three continuous networks: (a) an intracellular network, (b) a pericellular network (Golgi), and (c) a more diffuse extracellular network, lying between the cells. It seems probable that the Golgi network is either non- nervous or an artefact. The main point at issue is whether the neurofibrils, in such pericellular terminals as are illustrated in Fig. 73, are continuous with the neurofibrils within the cell enveloped or are separate from the latter. The individuality of the neurone and the interdependence of its various parts are strikingly shown by its behavior when injured. That degenerative changes, which progress to complete disappearance of the nerve structures, take place in 124 THE TISSUES. the distal part of a nerve when that nerve is cut across has long been accepted as one of the fundamental laws of neuropathology (law of Wallerian degenera- tion). In terms of the neurone concept this would mean that an axone cut off from its cell of origin degenerates and disappears and this behavior of the axone would accord with the already stated fact that the cell body is the trophic center of the neurone. The degenerative changes in the axone are best shown in their earlier stages by an osmic acid (p. 7) or by a Marchi (p. 31) stain. Later, when the degenerated fibres have been largely replaced by connective tissue, the Weigert method is most satisfactory, especially in the central nervous system. Fig. 75. — Two Motor Cells from Ventral Horn of Dorsal Cord of Rabbit; fifteen days after cutting major sacro-sciatic nerve. A, Cell in which the chromophilic bodies appear disintegrated and nucleus eccentric; B, cell showing more advanced chromatolysis, the chromophilic substance being present only in the dendrites and around the nucleus in the form of a homogeneous mass; nucleus causes bulging of surface of cell. The degeneration is not progressive from the point of injury distal ly, but takes place throughout the entire cut off portion at approximately the same time. All parts of the nerve fibre are affected. In the medullary sheath the changes con- sist in segmentation of the sheath, breaking up of the segments into granules and finally complete absorption (Fig. 74). While undergoing these physical changes chemical changes are also taking place in the myelin which result in its break- ing down into simpler fatty substances which give the fat reaction to the Marchi stain. At the same time the neurofibrils become irregular and granular and the axis-cylinder finally disappears. The neurilemma of peripheral nerve fibres is pjeculiar in that it does not degenerate; instead of this its nuclei proliferate and apparently play an important role in the disintegration and absorption of the myelin as well as in any regenerative changes which may occur later. The same rule holds good for dendrites as for axones as far as it has been possible to de- NERVE TISSUE. 125 termine, namely, that cut off from their cells of origin they undergo complete degeneration. At the time the law of Wallerian degeneration was established it was believed that the central portion of the nerve and the cell bodies remained intact after division of the nerve. More recently the method of Marchi (for nerve fibres) and the method of Nissl (for neurone bodies) have shown marked degenerative changes in the parts proximal to the lesion. The extent and rapidity of these changes are dependent mainly upon two factors (i) the type of neurone — some neurones being apparently more resistant than others to injury — and (2) the character and location of the injury — e.g., pulling out the nerve causing the greatest reaction; cutting the nerve, a reaction of less intensity; pinching the A B Fig. 76. — A, Neuroglia Cell — Spider Type — Human Cerebrum. B, Neuroglia Cell — Mossv Type — Human Cerebrum. nerve, the least degree of reaction; an injury near the body of the cell causing more effect centrally than one near the periphery. In the proximal stump of the nerve the changes are similar to those which take place in the distal stump, but these changes take place more slowly. In the cell body there is an initial turgescence, followed by disintegration and disappearance of the chromophilic bodies beginning near the center of the cell, and a displacement of the nucleus toward the periphery. This reaction on the part of the cell body to injury to its axone — " central chromatolysis " and " nuclear eccentricity" — is sufBciently characteristic to have led to the designation " axonal degeneration" (Fig. 75). The importance of these degenerations from the standpoint of anatomy lies in the fact that, combined with such methods as Nissl, Weigert, and Marchi, they enable one to trace the connections between cell bodies and nerve fibres through- out the nervous system. Neuroglia. This is a peculiar form of connective tissue found only in the central nervous system. Unlike the other connective tissues, neu- roglia is of ectodermic origin, being developed from the ectodermic cells which line the embrvonic neural canal. These cells, at first 126 THE TISSUES. morphologically identical, soon differentiate into neuroblasts or future neurones, and spongioblasts or future neuroglia cells, the latter most probably being in the form of a syncytium. Later this spongioblastic syncytium differentiates fibres, the neuroglia fibres, which, according to Weigert and others, may be entirely separate from the cells (Fig. 77). The structure of neuroglia would thus be analogous to that of fibrous connective tissue, i.e., composed of cells, the neuroglia cells, and a fibrillar intercellular substance, the neuroglia fibres. The Golgi method apparently reveals a great variety of neuroglia cells which may be divided into cells with straight radiating unbranched processes, Fig. 77. — Neuroglia Cells and Fibres from the White Matter of the Human Cerebellum stained by Weigert's neuroglia stain. A, Neuroglia cell; B, blood-vessel cut longitudinally, and C, blood-vessel cut transversely, showing enveloping neuroglia fibres; a, neuroglia fibres; b, cytoplasm of neuroglia cell. (Cajal.) Spider cells, and rough thick branching cells, mossy cells. It seems probable that in the former both cells and fibres are stained while in the latter only portions of the cells or syncytium, this method not differentiating between cells and fibres. Some authorities still maintain that the fibres are to be regarded as a part of the cell in the adult. The neuroglia cells and fibres are of marked pathological significance. Spider cells occur chiefly in the white matter, mossy cells in the gray matter in connection with blood-vessels (Fig. 76 A and Bj. NERVE TISSUE. 127 TECHNIC. (i) Pieces of the cerebral cortex are stained by one of the Golgi methods. If the rapid or mixed silver method is used, sections must be mounted in hard bal- sam without a cover; if the slow silver or the bichloride method is used, the sections may be covered. Sections are cut from 75 to ioo,u in thickness, cleared in carbol- xylol or oil of origanum and mounted in balsam. This section shows only the ex- ternal morphology of the neurone. It is also to be used for studying the different varieties of neuroglia cells as demonstrated by Golgi's method (see page 126). (2) Thin transverse slices from one of the enlargements of the spinal cord are fixed in absolute alcohol. Thin sections (5 to 10^) are stained by Nissl's method (page 35) and mounted in balsam. This section is for the purpose of studying the internal structure of the nerve cell and processes as demonstrated by the method of Nissl. (3) Medullated Nerve Fibres (fresh). — Place a small piece of one of the sciatic or lumbar nerves of a recently killed frog in a drop of salt solution and tease longi- tudinally. Cover and examine as quickly as possible. Note the diameter of the axone and of the medullary sheath and the appearance of the nodes of Ranvier. An occasional neurilemma nucleus can be distinguished. (4) Medullated nerve fibres — fibres from the cauda equina (this material has the advantage of being comparatively free from fibrous connective tissue) — are fixed in formaHn-Miiller's fluid (technic 5, p. 7), and hardened in alcohol. Small strands are stained twenty minutes in strong picro-acid-fuchsin solution (technic 2, p. 18), washed thoroughly in strong alcohol, cleared in oil of origanum, thoroughly teased longitudinally and mounted in balsam. General References for Further Study of Tissues. Barker: The Nervous System. Bethe: AUgemeine Anatomic und Physiologie des Nervensystem. Cabot: A Guide to the Clinical Examination of the Blood for Diagnostic Pur- poses. Ewing: Clinical Pathology of the Blood. Hertwig: Die Zelle und die Gewebe. KoUiker: Handbuch der Gewebelehre. Prenant, Bouin et Maillard: Traite d'Histologie. Ranvier: Traite Technique d'Histologie. Van Gehuchten: Le Systeme nerveux de I'homme. Wood: Laboratory Guide to Clinical Pathology. PART IV. THE ORGANS, CHAPTER I. THE CIRCULATORY SYSTEM. The circulatory apparatus consists of two systems of tubular structures, the blood-vessel system and the lymph-vessel system, which serve, respectively, for the transmission of blood and lymph. THE BLOOD-VESSEL SYSTEM. This consists of (a) a central propelling organ, the heart; (b) a series of efferent tubules — the arteries — which by branching constantly increase in number and decrease in calibre, and which serve to carry the blood from the heart to the tissues; (c) minute anastomosing tubules — the capillaries — into which the arteries empty and through the walls of which the interchange of elements between the blood and the other tissues takes place; {d) a system of converging tubules — the veins — which receive the blood from the capillaries, decrease in number and increase in size as they approach the heart, and serve for the return of the blood to that organ. The entire system — heart, arteries, veins, capillaries — has a com- mon and continuous lining, which consists of a single layer of endothe- lial cells. Of the capillaries this single layer of cells forms the entire wall. In the heart, arteries, and veins, the endothelium serves simply as the lining for walls of muscle and connective tissue. Capillaries. It is convenient to describe these first on account of their simplicity of structure. A capillary is a small vessel from 7 to 16 ii in diameter. Its wall consists of a single layer of edothelial cells. The cells are somewhat elongated in the long axis of the vessel. Their edges are serrated and are united by a small amount of intercellular substance (p. 70), whicli can be demonstrated by the sil\"er nitrate stain. In cer- tain capillaries — those of the early embryo, of the kidney glomeruli, of the chorioid coat of the eye, of the liver — no cell boundaries can be made out. In these capillaries the endothelium appears to be of the nature of a syncytium (p. 62). Capillaries branch without diminution in 131 132 THE ORGANS. calibre, and these branches anastomose to form capillary networks, the meshes of which differ in size and shape in different tissues and organs (Figs. 78, 79, 80). The largest meshed capillary networks are found in Fig. 78. — ^\''ein and Capillaries. Silver-nitrate and ha?matoxylin stain (technic 7, p. 72), to show outlines of endothelial cells and their nuclei. the serous membranes and in the muscles, while the smallest are found in the glands, as, e.g., the liver. As to calibre, the largest are found in the liver, the smallest in muscles. / e d Fig. 79. — Diagram of Capillaries, Small Artery, and Vein, showing their structure and relations, a, Capillaries; h, nuclei of capillary endothelium; c, precapillary arteries; d, arteriole; c, small vein;/, small artery. In such thin membranous parts as the web of the frog's foot, or the wall of the frog's bladder, the blood may be observed as it flows through the arteries, capillaries, and veins. The current is seen to be fastest in the arteries, and faster in the center of the vessel than al its periphery. It is slower in the veins and slowest in the capil- THE CIRCL'LATORY SYSTEM. 133 laries. In the case of the frog's bladder, the mere exposure to the air acts as a suffi- cient irritant to cause slight inflammatory changes, and the leucocytes maybe seen adhering to the walls of the capillaries and passing through them into the tissues. The capillary, both from the thinness of its wall and from the slowness with which the blood passes through it, is peculiarly adapted for the interchange of material between the blood and the tissues, and it is probable that it is in the capillary that all such interchange takes place. Arteries. The wall of an artery consists of three coats: (i) An inner coat, the intima. (2) A middle coat, the media. (3) An outer coat, the adventitia. The intima consists of a single layer of endothelial cells, continuous with and similar to that forming the walls of the capillaries, or, in ar- FiG. 80. — Capillar)' Network from Human Pia Mater, showing also an arteriole in "optical section" and a small vein. X350. (Technic i, p. 139.) a, \'ein; h, arteriole; c, large capillary; d, small capillaries. teries of considerable size, of this layer plus more or less connective tissue. The middle coat consists mainly of smooth muscle, the outer of connective tissue. The structure of these three coats varies according to the size of the artery, and while the transition between them is never abrupt, it is convenient, for purposes of description, to distinguish {a) small arteries, {b) medium sized arteries, and (r) large arteries. Small Arteries. — Passing from a capillary to an artery, the first 134 THE ORGANS. change is the addition of a thin sheath of connective tissue around the outside of the endothehal tube. A little farther back isolated smooth muscle cells, circularly arranged, begin to appear between the endothelium and the connective tissue. Such an artery is known as a precapillary artery. The next transition is the completion of the muscular coat, the muscle cells now forming a continuous layer. Such an artery, consisting of three distinct coats, the middle coat composed of a single continuous layer of smooth muscle cells, is known as an arteriole (Fig. 79, d; Fig. 80, h). Fig. 81. — From Cross-section through Walls of Medium-sized Artery and its Accom- panying Vein. X75. (Technic 3, p. 140.) A, Intima of artery; a, its endothelial layer; h, its intermediary layer; c, its elastic layer; B, media of artery; C, adventitia, the upper part belonging to the artery, the lower to the vein; within the adventitia are seen the vasa vasorum; D, media of vein; E, intima of vein; i, its intermediary layer; j, its endothelial layer. Medium-sized Arteries. — This group comprises all the named arteries of the body with the exception of the aorta and the pulmo- nary. Their walls arc formed of the same three coats found in the arteriole, but the structure of these coats is more elaborate. I. The INTIMA consists of three layers (Fig. 81). (a) An inner endothelial layer already described. ih) A middle layer, the intermediary layer of the intima. This is composed of delicate white and elastic fibrils and connective-tis- sue cells. THE CIRCULATORY SYSTEM. 135 (c) An outer layer, the elastic layer of the intima, or memhrana elastica interna — a thin fenestrated membrane of elastic tissue. This membrane is intimately connected with the media and marks the boundary between the latter and the intima. In the smallest of the medium-sized arteries the intermediary layer is often wanting, the endothelial cells resting directly upon the elastic membrane. Owing to the extensive amount of elastic tissue in their walls, there is a post- d^--.--: Fig. 82. — From Transverse Section of Dog's Aorta. X60. (Technic 4, p. 140.) a, In- tima; h, media; c, adventitia; d, vasa vasorum; e, elastic tissue;/, endothelium. mortem contraction of arteries which results in the intima being thrown up into folds. For this reason the elastic membrane presents, in transverse sections of an artery, the appearance of a wavy band (Fig. 81). 2. The MEDLA. is a thick coat of circularly disposed smooth mus- cle cells (Fig. 81, 5). Its thickness depends largely upon the size of the vessel, though \-arying somewhat for different vessels of the same size. A small amount of fibrillar connecti\'e tissue supports the muscle cells. Elastic tissue is present in the media, the amount 136 THE ORGANS. being usually proportionate to the size of the vessel. In the smaller of the medium-sized arteries, the elastic tissue is disposed as delicate fibrils among the muscle cells. In larger arteries many coarse fibres are intermingled with the fine fibrils. When much elastic tissue is present the muscle cells are separated into more or less well-defined groups. In such large arteries as the subclavian and the carotid, elastic tissue occurs not only as fibrils but also as circularly disposed plates or fenestrated membranes. ma Fig. 83. — From Transverse Section of Dog's Aorta, to show Elastic Tissue. X6o- (Technic 7, jj. 140.) Elastic tissue stained black, a, Intima; h, media; c, adventitia. 3. The ADVENTITIA (Fig. 81, C) is composed of loose fibrous connec- tive tissue with some elastic fibres. Occasionally there are scattered smooth muscle cells. Both smooth muscle cells and elastic fibres are arranged longitudinally. The adventitia does not form a definitely outlined coat like the media or intima, but blends externally with the tissues surrounding the artery and serves to attach the artery to these tissues. In some of the larger arteries the elastic tissue of the ad- ventitia forms an especially well-defined layer at the outer margin THE CIRCULATORY SYSTEM. 137 of the media. This is known as the membrana elastica externa. In general, it may be said that the thickness of the adventitia and the amount of elastic tissue present are directly proportionate to the size of the artery. Large arteries like the aorta (Fig. 82) have the same three coats as small and medium-sized arteries. The layers are not, however, so distinct. This is due mainly to the excessive amount of elastic tis- sue in the media (Fig. 82), which makes indistinct the boundaries between intima and media, and between media and ad^'entitia. The walls of the aorta are thin in proportion to the size of the vessel, in- creased strength being obtained by the decided increase in the amount of elastic tissue. Of the intima, the endothelial cells are short and polygonal; the intermediary layer similar to that of a medium-sized artery; the elastic layer less distinct and often broken up into several thin layers. The media consists mainly of elastic tissue arranged in circular plates or fenestrated membranes. Between the elastic- tissue plates are groups of smooth muscle cells and some fibrillated connective tissue. The adventitia resembles that of the medium-sized artery. There is no external elastic membrane. Certain arteries have structural peculiarities. The arteries of the brain and cord are thin-walled in proportion to their calibre, the inner elastic membrane is especially well defined, and there are few elastic fibres in the media. In the renal, coeliac, mesenteric, and external iliac arteries there is little or no connective tissue separating the endothelium from the media. In the subclavian, the media contains longitudinal muscle cells. Longitudinally running muscle cells occur in the adventitia of the umbilical arteries, in the iliac, splenic, renal, superior mesen- teric and dorsalis penis. The radial, femoral and coeliac arteries have compara- tively little elastic tissue, while in the common iliac, carotid, and axillary the elastic tissue is in excess of the muscular. Veins. The walls of veins resemble those of arteries. There are the same three coats, intima, media, and adventitia, and the same elements enter into the structure of each coat (Fig. 81). \'enous walls are not, however, so thick as those of arteries of the same calibre, and the coats arc not so distinctly differentiated from one another. The transition from capillary through the precapillary vein to the small \cin is similar to that described under arteries (page 133). Unlike the artery, the thickness of the wall of a vein and its structure are not directly propor- tionate to the size of the vessel, but depend also upon other factors such 138 THE ORGANS. as the position of the vein and the support given to its walls by surround- ing structures. Of the INTIMA the endothelial layer and the intermediary layer are similar to those of the artery. The elastic layer is not always present, is never so distinct, and is not wavy as in the artery (Fig. 8i). The result is a lack of demarcation between intima and media, the connective tissue of the intermediary layer of the intima merging with the mixed muscle and connective tissue of the media. Project- ina; at intervals from the inner surface of the wall of some veins are valves. These are derived entirely from intima and consist of loose fibrous and elastic tissue covered by a single layer of endothelium. Valves are especially large and strong in the larger veins of the lower limbs. They are absent in the veins of the brain and cord and their membranes, in the veins of bones, in the umbilical vein, and in most of the \isceral veins with the exception of some branches of the portal. The MEDIA of veins is thin as compared with that of arteries of the same size. It consists of fibrous and elastic tissue and smooth muscle cells. The amount of muscle is comparatively small and the cells are arranged in groups through the connective tissue. The ADVENTiTiA is well developed in proportion to the media. It consists of mixed fibrous and elastic tissue and usually contains along its inner margin small bundles of longitudinally disposed smooth muscle cells. The media is thickest in the veins of the lower extremities and in the veins of the skin. In the veins of the head and abdomen the media is very thin, while in the subclavian and superior vena cava and in the veins of bones, of the pia mater, dura mater, and retina, there is an almost entire absence of media. Arteries are as a rule empty after death, while veins contain blood. The absence of much elastic tissue in the walls of the veins prevents any such extensive post-mortem contraction as occurs in the arteries. Veins tend to collapse after death, but are usually prevented from doing so by the presence of blood in them. In the iliac and femoral veins, longitudinally disposed muscle occurs in the inner part of the media. The umbilical vein, like the corresponding artery, has three distinct muscular coats. Longitudinal muscle fibres are present in the adventitia of the superior vena cava (hepatic and abdominal portion) and of the portal and hepatic veins. In the upper portion of the inferior vena cava, in the superior vena cava, the jugular, innominate, and subclavian, there is little muscle tissue in any of the coats, while in the veins of the brain and its membranes, the retina, the placenta and the bones, no muscle is present. THE CIRCULATORY SYSTEM. 139 Vasa Vasorum. — Medium and large arteries and veins are supplied with small nutrient vessels — vasa vasorum. These vessels run in the ad\'entitia, small branches penetrating the media (Figs. 8i and 82). Lymph channels are found on the outer surface of many blood- vessels. Some of the smaller vessels are surrounded by spaces lined by endothelium — perivascular lymph spaces. These communicate with the general lymphatic system. Nerves. — The walls of the blood-vessels are supplied with both medullated and non-medullated fibres. The latter are axones of sympathetic neurones. As these nerves control the calibre of the vessels they are known as vasomotor nerves. They form plexuses in the adventitia, from which are given off branches which pene- trate the media and terminate on the muscle cells. The medul- lated fibres are the peripheral arms of spinal or cranial ganglion cells. The larger fibres run in the connective tissue outside the adventitia. From these are given off branches which enter the media, divide re- peatedly, lose their medullary sheaths, and terminate mainly in the media, although some fibres have been traced to their terminations in the intima. TECHNIC. (i) Capillaries, Arterioles, Small Arteries, and Veins. — Fix an entire brain, or slices about an inch thick from its surface, in formalin-Muller's fluid for twenty- four hours (technic 5, p. 7). Remove the pia mater, especially the thinner parts which lie in the sulci between the convolutions, and harden in graded alcohols. Select a thin piece, stain with hsematoxylin (lightly) and eosin (strongly) (technic I, p. 18), and mount in balsam or in eosin-glycerin. The veins, having thin walls and being usually well filled with blood, appear distinct and red from the eosin- stained red cells. The arteries, having thicker walls, in which are many haemo- globin-stained nuclei, have a rather purple color. Between the larger vessels can be seen a network of anastomosing capillaries with their thin walls and bulging nuclei. Some are filled with blood cells; others are empty with their collapsed walls in apposition. Note the appearance of an arteriole, first focusing on its upper surface, then focusing down through the vessel. In this way what is known as an "optical section" is obtained, the artery appearing as if cut longitudinally. Trace the transition from arteriole to precapillary artery and the breaking up of the latter into the capillary network. Similarly follow the convergence of capillaries to form a small vein. (2) Instructive pictures of the relations of arteries, capillaries, and veins in liv- ing tissues may be obtained by curarizing a frog, distending the bladder with nor- mal saline introduced through a small catheter or cannula, opening the abdomen and drawing out the bladder, which can then be arranged upon the stage of the microscope. The passage of the blood from the arteries through the capillary net- work and into the veins is beautifully demonstrated. 140 THE ORGANS. (3) For studying the structure of the walls of a medium-sized artery and vein remove a portion of the radial artery, or other artery of similar size, and its accom- panying vein, together with some of the surrounding tissues. Suspend the vessels, with a small weight attached, in formaUn-Miiller's fluid (technic 5, p. 7). Sec- tions should be cut transversely, stained with haematoxylin-eosin (technic i, p. 18), or with haematoxylin-picro-acid fuchsin (technic 3, p. 19), and mounted in balsam. The vessels of the adventitia — vasa vasorum — are convenient for studying the structure of arterioles and small veins. (4) Fix a piece of aorta in formalin-Miiller's fluid, care being taken not to touch the delicate endothelial lining. Stain transverse sections with hasmatoxylin- eosin or with h£ematoxylin-picro-acid fuchsin and mount in balsam. (5) The outlines of the lining endothelial cells may be demonstrated as follows: Kill a small animal, cut the aorta, insert a glass cannula and, under low pressure, thoroughly wash out the entire vascular system with distilled water. Follow the water by a one-per-cent. aqueous solution of silver nitrate. Remove some of the smaller vessels, split longitudinally, mount in glycerine, and expose to the direct sunlight. After the specimen has turned brown examine with the low power. The outlines of the cells should appear brown or black. (6) The endothelium of the smaller vessels and capillaries may also be demon- strated in the specimen described under technic 8, p. 72. (7) The elastic tissue of the blood-vessels is best demonstrated by means of Weigert's elastic-tissue stain. Prepare sections of medium-sized vessels and of the aorta, as above described (3), and stain as in technic 3, p. 26. The Heart. The heart is a part of the blood-vessel system especially differ- entiated for the purpose of propelling the blood through the vessels. The main mass of the heart wall consists of a special form of muscle tissue already described as heart muscle (page 106). This constitutes the myocardium. On its inner and outer sides the myo- cardium is covered by connective-tissue membranes lined, respectively, with endothelium and mesothelium and known as the endocardium and epicardium. The MYOCARDIUM varies in thickness in different parts of the heart, being thickest in the left ventricle, thinnest in the auricles. A ring of dense connective tissue, the auriculo-ventricular ring, com- pletely separates the muscle of the auricles from that of the ven- tricles. The auricular muscle consists of an outer coat common to both auricles, the fibres of which have a transverse direction, and of an inner coat, independent for each auricle, the fibres of which are longitudinally disposed. Between the two coats bundles of muscle fibres are frequently found which run in various directions. The disposition of the muscle tissue of the ventricles is much THE CIRCULATORY SYSTEM. 141 more complicated. It is usually described as composed of several layers, the fibres of which run in different directions. The meaning of these fibre layers becomes apparent when we study the arrange- ment of the fibres in embryonic hearts in which the connective tissue has been broken down by maceration. Thus dissected, the muscle of the ventricles is seen to consist mainly of two sets of fibres, a super- ficial set and a deep set. These run at right angles to each other. Both sets of fibres begin at the auriculo-ventricular rings. The superficial fibres wind around both ventricles in a spiral manner, be- coming constantly deeper, to terminate in the papillary muscles of the opposite ventricle. The deeper fibres pass from the auriculo-ven- tricular ring around the ventricle of the same side, through the inter- ventricular septum and terminate in the papillary muscles of the opposite ventricle. The ENDOCARDIUM covcrs the inner surface of the myocardium and forms the serous lining of all the chambers of the heart. At the arterial and venous orifices it is seen to be continuous with and simi- lar in structure to the intima of the vessels. It consists of two lay- ers: {a) an inner composed of a single layer of endothelial cells, cor- responding to the endothelial lining of the blood-vessels; and {h) an outer composed of mixed fibrous and elastic tissue and smooth muscle cells. Externally the endocardium is closely attached to the myocardium. Strong fibrous rings (annul i fibrosi), composed of mixed fibrous and elastic tissue, surround the openings between auricles and ven- tricles. Similar but more delicate rings encircle the openings from the heart into the blood-vessels. The heart valves are attached at their bases to the annuli fibrosi. They are folds of the endocardium, and like the latter consist of fibrous and elastic tissue continuous with that of the rings and covered by a layer of endothelium. The EPICARDIUM is the visceral layer of the pericardium. It is a serous membrane like the endocardium, which it resembles in struc- ture. It consists of a layer of mixed fibrous and elastic tissue cov- ered over by a single layer of mesothelial cells. Beneath the epicar- dium there is usually more or less fat. Blood-vessels. — Blood for the nutrition of the heart is supplied through the coronary arteries. The larger branches run in the con- nective tissue which separates the bundles of muscle fibres. From these, smaller l)ranches pass in among the indi\idual fibres, where 142 THE ORGANS. they break up into a rich capillary network with elongated meshes. From the myocardium, capillaries penetrate the connective tissue of the epicardium and endocardium. The auriculo-ventricular valves are supplied with blood-vessels, while in the semilunar valves blood- vessels are wanting. Lymphatics. — Lymph channels traverse the epicardium and endo- cardium and enter the valves. Within the myocardium minute lymph vessels have been demonstrated between the muscle fibres and accompanying the blood-vessels. Nerves. — These are derived from both cerebro-spinal and sym- pathetic systems, and consist of both medullated and non-medullated fibres. Sympathetic ganglion cells are distributed in groups through- out the myocardium. Among these cells the nerve fibres form plex- uses from which both motor and sensory terminals are given off to the muscle. (For nerve endings in heart muscle see page 393.) TECHNIC. (i) The Heart. — Cut pieces through the entire thickness of the "wall of one of the ventricles, care being talcen not to touch either the serous surface or the lining endothelium. Fix in formaUn-MiJller's fluid (technic 5, p. 7). Cut transverse and longitudinal sections; stain with hasmatoxylin-eosin (technic i, p. 18) and mount in balsam. (2) Treat the entire heart of a small animal {e.g., guinea-pig or frog) in the same manner as the preceding, making transverse sections through both ventricles. (3) An entire heart, human or animal, may be fixed in the distended condition by filling with formahn-Miiller's fluid under low pressure and then tying off the vessels. The entire heart thus distended is placed in a large quantity of the same fixative. Development of the Circulatory System. The blood-vessels and the heart begin their development separately and afterward become united. Both arc derived from mesoderm. The earliest vessels to be formed are the capillaries. These make their appearance in the mcsodermic tissue near the periphery of the area vasculosa which surrounds the developing emljryo. Here groups of cells known as "blood islands^' differentiate from the rest of the mesodermic cells. The superficial cells of these islands become flat- tened to form the endothelium, while the remaining cells form the erythroblasts. These represent the earliest blood-vessels. These chan- nels, which are at first unconnected, anastomose and give rise to a net- THE CIRCULATORY SYSTEM. 143 work of channels which are the earliest capillaries. In regard to the origin of the later vessels in the extraembryonic area, and those within the embryo, there are two views : (i) that they represent outgrowths from the original capillaries; (2) that they arise in situ in the same manner as the earliest capillaries and unite secondarily to form networks. The weight of evidence at present favors the latter view. The entire vascu- lar system is at first represented by a network of capillaries. As develop- ment proceeds some of the channels enlarge to form the arteries and veins, the smooth muscle and connective tissue of the walls being differentiated from the surrounding mesoderm. The heart first appears as an endothelial tube, the primitive endocardium, at a very early age of embryonic life. The origin of the cardiac endothelium is not definitely known. It is believed by some to be of entodermic origin, by others of mesodermic, by still others to be partly derived from each of these layers. Around this endothelial tube, but separated from it by a space, there develops from mesoderm an entirely distinct muscular tube, the primitive myocardium. These two tubes are at first united only in places by bands of connective tissue. Later they unite so that the inner tube, the endocardium, becomes a lining for the outer tube, the myocardium. The union of the two heart tubes occurs very early. In the human embryo of 2 to 3 mm. the heart is a single, slightly coiled tube con- nected at its cephalic end with the ventral aorta and caudally with the omphalo-mesenteric veins. The epicardium, as the visceral layer of the pericardium, has a separate origin, being constricted off from that portion of the mesoderm which lines the primary body cavity. THE LYMPH-VESSEL SYSTEM. The larger lymph vessels are similar in structure to veins. Their walls are, however, thinner than those of veins of the same calibre and they contain more valves. They are capable of great distention, and when empty collapse so that their thin walls are in apposition. The largest of the lymph vessels, the thoracic duct, has three well- defined coats: an intima consisting of the usual lining endothelium resting upon a subendothelial layer of delicate fibro-elastic tissue, the outermost elastic fibres having a longitudinal arrangement; a fairly thick media of circularly disposed smooth muscle cells; and an ad- ventilia which is strengthened I)y Inmdles of longitudinal smooth muscle. 144 THE ORGANS. Lymph capillaries resemble blood capillaries in that their walls are composed of a single layer of endothelial cells. The cells are rather larger and more irregular than in blood capillaries, the capil- laries themselves are larger, and, instead of being of uniform diameter throughout, vary greatly in calibre within short distances. In certain tissues dense networks of these lymph capillaries are found. Cleft-like lymph spaces — perivascular lymph spaces — partially surround the walls of the smaller blood-vessels. Lymph spaces without endothelial or other apparent lining also occur. Examples of these are the pericellular lymph spaces found in various tissues and the canal iculi of the cornea and of bone (pages 74 and 93). Septum. Trabecula of cells in cross- section. Distended blood capil- laries. Efferent vein. Fig. 84. — Section of human carotid gland. X ]6o. (Schaper.) Similar in character to lymph spaces are the body cavities, peri- toneal, pleural, and pericardial, with their linings of serous membranes. These cavities first appear in the embryo as a cleft in the mesoderm ■ — the ca'lom, body cavity, or pleiiro peritoneal cleft. This cleft is lined with mesothelium beneath which the stroma is formed. These mem- branes not only line the cavities, but are reflected over most of the viscera of the abdomen and thorax. They consist of a stroma of mixed fibrous and elastic tissue, covered on its inner side by a layer of mesothelium, the two being separated by a homogeneous basement meml.)rane. The stroma contains numerous lymphatics. These have been described as communicating with the free surfaces by means of THE CIRCULATORY SYSTEM. 145 openings — stomata. Recent observations, however, would seem to indicate that these stomata are artefacts. That the lymph-vessels form a definite and closed system of channels and are not in direct communication with the lymph spaces has been clearly demonstrated. The lymphatics apparently originate from the vascular system, starting as four buds, two from the veins of the neck, and two' from veins in the inguinal redon. M «^^ ..- ,-,'^— «»> .^•., iS^' Fig. 85. — Section through coccygeal gland (Walker), i. Blood space; 2. epithe- lium; 3. connective tissue. TECHNIC. (i) Remove a portion of the central tendon of a rabbit's diaphragm. Rub the pleural surface gently with the finger or with a brush to remove the mesothe- lium. Rinse in distilled water and treat with silver nitrate as in technic 7, p. 72. Mount in glycerin. If the silver impregnation is successful, the networks of coarser and finer lymphatics can be seen as well as the outlines of the endothelium of their walls. If care has been taken not to touch the peritoneal surface, the peritoneal mesothelium and the stomata are frequently seen. (2) The Thoracic Duct. — Remove a portion of the thoracic duct, fi.x in forma- lin-Miiller's fluid (technic 5, p. 7), and stain sections with h;cmato.\ylin-eosin (technic i, p. 18). 146 the organs. The Carotid Gland. This is a small ductless gland, about the size of a rice grain, which lies at the bifurcation of the carotid artery. It is composed of a vascu- lar connective tissue supporting spheroidal groups of polyhedral epithe- lial cells which are closely associated with tufts of capillaries. Some of the gland cells take a brownish stain with chromic acid similar to the medullary cells of the adrenal (Fig. 84). The Coccygeal Gland. This is also a ductless gland similar in structure to the preceding, but with much more irregularly arranged groups of cells. According to some observers, the cells of the coccygeal gland differ from those of the carotid in that they do not give the chromaffin reaction, the gland conforming more to the structure of lymphoid tissue (Fig. 85). TECHNIC. Technic same as for Thyreoid Gland, page 285. General References for Further Study of the Circulatory System. Kolliker: Handbuch der Gewebelehre des Menschen, vol. iii. Stohr: Text-book of Histology. Schafer: Histology and Microscopic Anatomy, in Quain's Elements of Anat- omy, tenth edition. CHAPTER IT. LYMPHATIC ORGANS. The Lymph Nodes. Lymph nodes are small bodies, usually oval or bean-shaped, which are distributed along the course of the lymph vessels. In some regions Ihey are arranged in series forming "chains" of lymph nodes as, e.g., the axillary and inguinal. Each lymph node is surrounded by a capsule of connective tissue which sends trabeciilce or septa into the organ. The capsule and septa ■A}!^-_^^ ?^A/:'^ h g f Fig. 86. — Section through Entire Human Lymph Node, including Hilum. X15. (Technic i, p. 150.) Dark zone, corte.x; hght central area, medulla, a, Lymph nodule of cortex; /), germinal centres; c, trabecule containing blood-vessels; d, capsule; e, hilum; /, lymph sinus of medulla; g, lymph cords of medulla; h, lymph sinuses of medulla and cortex. constitute the connective-tissue framework of the node, and serve as a support for the lymphatic tissue (Fig. 86). The capsule is composed of fibrous connective tissue arranged in two layers. In the outer, the fibres are loosely arranged and serve, 147 14S THE ORGANS. like the fibres of the arterial adventitia, to attach the node to the surrounding tissues. The inner layer of the capsule consists of a more dense connective tissue and contains some smooth muscle cells. At one point, known as the hilum (Fig. 86), there is a depression where the connective tissue of the capsule extends deep into the sub- stance of the node. This serves as the point of entrance for the main arteries and nerves, and of exit for the veins and efferent lymph vessels. The connective-tissue septa, which extend from the capsule into the interior of the node incompletely di\'ide it into irregular inter- communicating compartments. In the peripheral portion of the node these compartments are somewhat spheroidal or pear-shaped. Toward the centre of the node the septa branch and anastomose freely, with the result that the compartments are here narrower, more irregular, and less well defined. This arrangement of the connective tissue allows the division of the node into two parts, an outer peripheral part or cortex and a central portion, the medulla (Fig. 86). Within the compartments formed by the capsule and the septa is the lymphatic tissue (for structure see page 84). In the cortex where the compartments are large and spheroidal or pear-shaped, the lymph- atic tissue is of the compact variety, and is arranged in masses which correspond in shape to the compartments. These are known as lymph nodules (Fig. 86). In the centre of each nodule is usually an area in which the cells are larger, are not so closely packed, and show marked mitosis. As it is here that active proliferation of lymph- oid cells takes place, this area is known as the germinal centre (Figs. 86 and 87). Immediately surrounding the germinal centre is a zone in which the lymphoid cells are more closely packed than elsewhere in the nodule (Fig. 87). This is apparently due to the active pro- duction of new cells at the germinal centre and the consequent pushing outward of the surrounding cells. In stained sections the centre of the nodule is thus lightly stained, while immediately surrounding this light area is the darkest portion of the nodule (Fig. 87). From the inner sides of the nodules strands of lymphoid tissue extend into the medulla. These are known as lymph cords, and anastomose freely in the small irregular compartments of the medulla. In both cortex and medulla the lymphoid tissue is always separated from the capsule or from the septa by a distinct space — the lymph sinus — which is bridged over by reticular tissue containing comparatively few lymphoid cells (Fig. 87). These sinuses form a continuous system of anastomosin" channels throughout the node. LYMPHATIC ORGANS. 149 The reticular connective tissue (page d>T)), which forms a part of the lymphatic tissue proper, is continuous with the fibrous connec- tive-tissue framework of the organ in such a manner that it is im- possible to determine any demarcation between the two tissues. In the lymph nodules, and wherever the lymphoid cells are densely packed, the underlying reticular network is almost completely obscured. Crossing the sinuses, especially those of the medulla, and in specimens Fig. 87. — Section through Cortex and Portion of Medulla of Human Lymph Node. (Technic 2, p. 150.) a, Capsule; b, lymph sinus; c, trabecula; d, closely packed cells at outer border of lymph nodule; e, germinal centre; /, lymph cords in medulla. in which the cells have been largely washed out or removed by macera- tion, the reticular structure is well shown. The lymphoid tissue proper, as represented by the lymph nodules and anastomosing lymph cords, is thus, as it were, suspended in the meshes of a reticulum which is swung from the capsule and trabec- ulae. As both nodules and cords are everywhere separated from cap- sule and trabeculse by the sinuses, and as these latter serve for the passage of lymph through the node, it is seen that the lymphatic tis- sue of the node is broken up in such a manner as to be bathed on all sides by the circulating lymph. In addition to the definitely formed lymph nodes and the well- defined collections of lymph nodules, such as those of the tonsil or of Peyer's patches, small nodules or groups of lymphoid cells have a wide distribution throughout the various organs. While many of these collections of lymphatic tissue are inconspicuous, still the ag- gregate of lymph tissue thus distributed is by no means inconsider- 150 THE ORGANS. able. The most important will be described in connection with the organs in which they occur. Blood-vessels. — Those which enter the hilum carry the main blood supply to the organ. Most of the arteries pass directly into the lymphatic tissue, where they break up into dense capillary networks. Some of the arteries, instead of passing directly to the lymphatic tissue, follow the septa, supplying these and the capsule, and also sending branches to the surrounding lymphatic tissue. A few small vessels enter the capsule along the convexity of the organ and are distributed to the capsule and to the larger septa. Lymphatics. — The afferent lymph vessels enter the node on its convex surface opposite the hilum, penetrate the capsule, and pour their lymph into the cortical sinuses. The lymph passes through the sinuses of both cortex and medulla, and is collected by the efferent lymph vessels which leave the organ at the hilum. Within the node the lymph comes in contact with the superficial cells of the nodules and of the lymph cords. These cells are constantly passing out into the lymph stream so that the lymph leaves the node much richer in cellular elements. Nerves are not abundant. Both medullated and non-medullated fibres occur. Their exact modes of termination are not known. TECHNIC. (i) Remove several lymph nodes from one of the lower animals (ox, cat, dog. rabbit), fix in formalin-Miiller's fluid (technic 5, p. 7), and harden in alcohol,. Cut thin sections through the hilum, stain with haematoxylin-eosin (technic i, p. 18), or with haematoxylin-picro-acid-fuchsin (technic 3, p. 19), and mount in balsam. (2) Expose a chain of lymph nodules {e.g., the cervical or inguinal of a recently killed dog or cat). Insert a small cannula or needle into the uppermost node and inject formalin-Miiller's fluid until the node becomes tense. By now slightly increasing the pressure the fluid may be made to pass into the second node, and so through the entire chain. The nodes are then carefully dissected out and placed for twenty-four hours in formalin-Miiller's fluid, then hardened in alcohoL Sections are cut through the hilum, stained with haimatoxylin-eosin or with hsema- toxylin-picro-acid-fuchsin and mounted in balsam. Near the centre of the chain arc usually found nodes in which the lymph sinuses are properly distended. The most proximal nodes are apt to be overdistcnded, but for this very reason are often excellent for the study of the reticular tissue from which most of the cells have been washed out, especially in the medulla. (3) Human lymph nodes maybe treated by cither of the above mcthofls. Ow- ing to the coalescence of their cortical nodules their structure is not so easily demonstrated as that of the lymph nodes of lower animals. LYMPHATIC ORGANS. 151 Haemolymph Nodes. These are lymphoid structures which closely resemble ordinary Ivmph nodes, but with the essential difference that their sinuses are blood sinuses instead of lymph sinuses. Each node is surrounded by a capsule of varying thickness, com- posed of fibro-elastic tissue and smooth muscle cells. From the cap- sule trabecula of the same structure pass down into the node, forming its framework (Fig. 88). Beneath the capsule is a blood sinus, which may be broad or narrow, and usually completely surrounds the node. Less commonly the sinus is interrupted by lymphoid tissue extending Fig. 88. — Section through Human H;emolymph Node, including Hikim, showing capsule, trabeculse, sinuses filled with blood, and lymph nodules. (\\'arthin.) out to the capsule. From the peripheral sinus branches extend into the interior of the node, separating the lymphoid tissue into cords or islands. The relative proportion of sinuses and lymphoid tissue varies greatly, some nodes being composed almost wholly of sinuses, while in others the lymphoid tissue predominates. There is usually a fairly distinct hihim. In many glands no differentiation into cortex and medulla can be made. Where there are a distinct medulla and cortex the peripheral lymphoid tissue is arranged in nodules as in the ordinary lym])h node. Reticular connective tissue crosses the sinuses and supports the cells of the lymph nodules and cords (Fig. 89). 152 THE ORGANS. The cellular character of the lymphoid tissue has led to the sub- division of hasmolymph nodes into splenolymph nodes and marrow- lymph nodes. In the splenolymph node the lymphoid tissue resembles that of the ordinary lymph node of the spleen. In the marrow- lymph node, which is the much less common form, the lymphoid tissue resembles red marrow. There are no distinct nodules, and there is a quite characteristic distribution of small groups of fat cells. The most numerous cells are eosinophiles and mast cells (see page 98). Polynuclear leucocytes and large leucocytes with a single Fig. 89. — Section through Superficial Portion of Human Hffimolymph Node (Marrow- lymph Node). (Warthin.) Capsule, trabecuiae, and parts of two adjacent nodules; sinuses filled with blood; among the lymph cells are large multinuclear cells resembling those of marrow, nucleated red blood cells, etc. lobulated nucleus are less numerous. The very large multinuclear cells of red marrow are also found, but usually in small numbers. Large phagocytes containing blood pigment and disintegrating red blood cells are found in both forms of haemolymph nodes, but are most numerous in the splenolymph type. In nodes which have a brownish color when fresh, these phagocytes frequently almost com- pletely fill the sinuses. Further classification of haemolymph nodes has been attempted, but is unsatisfactory, owing to the large number of transitional forms. Thus many nodes are transitional in structure between the haemo- lymph node and the ordinary lymph node, between the splenolymph LYIMPHATIC ORGANS. 153 node and the marrowlymph node, and between the splenolymph node and the spleen. Under normal conditions the hsemolymph nodes appear to be concerned mainly in the destruction of red blood cells; possibly also in the formation of leucocytes. Under certain pathological con- ditions they probably become centres for the formation of red blood cells. Blood-vessels. — An artery or arteries enter the node at the hilum, and break up within the node into small branches, which communi- cate with the sinuses where the blood comes into intimate association with the lymphoid tissue. From the sinuses the blood passes into veins, which leave the organ either at the hilum or at some other point on the periphery. The course which the blood takes in pass- ing through the haemolymph node is thus apparently similar to that taken by the lymph in passing through the ordinary lymph node. The relation of the haemolymph node to the lymphatic system is not known, and like ignorance exists as to its innervation. TECHNIC. Same as for lymph nodes (technic i, p. 150). The nodes are found in greatest numbers in the prevertebral tissue, and are often difficult to recognize. Fixing the tissues in 5-per-cent. formalin aids in their recognition as it darkens the nodes while bleaching the rest of the tissues. The Thymus. The thymus is an organ of fcetal and early extra-uterine life; reaching in man its greatest development at the end of the second year. After this age it undergoes a slow retrograde change into fat and connective tissue, until by the twentieth year scarcely a vestige of glandular tissue remains. The thymus originates in the entoderm and begins its foetal exist- ence as a typical epithelial gland. Into this epithelial structure meso- dermic cells grow and differentiate into lymphatic tissue. This almost completely replaces the epithelial tissue, only rudiments of which remain. Morphologically tlie fully developed thymus consists of lobes and lobules (Fig. 90). The whole gland is surrounded by a connective- tissue capsule, and the lobes are separated from one another by strong extensions of capsular tissue. Smaller connective-tissue septa extend 154 THE ORGANS. into the lobes, subdividing them into lobules. From the perilobular connective tissue, septa extend into the lobule, incompletely sepa- rating it into a number of chambers. Each lobule consists of a cor- tical portion and a medullary portion. The cortex consists of nodules of compact lymphatic tissue similar to those found in the lymph node. These occupy the chambers formed by the connective-tissue septa. The medulla consists of a more diffuse lymphatic tissue with no connective-tissue septa. It is common for the medullary tissue to extend to the surface of a nodule at one or more points and to be there FlG. 90. — From Seclion of Human Thymus, showing parts of five lobules and interloljuiar septa. X20. (Technic, page 155.) a, Corte.x; b, medulla; c, interlobular septum. continuous with the medullary substance of an adjacent lobule. In the medulla are found a number of spherical or oval bodies composed of concentrically arranged epithelial cells. These are known as HassaWs corpuscles (Fig. 91), and represent the only remains of the original glandular epithelium. They are characteristic of the thymus. The central cells of the corpuscles are usually spherical and contain nuclei, while the peripheral cells are flat and non-nucleated. As the entire corpuscle takes a bright red stain with eosin-hiemat(;xylin, the corpuscles stand out sharply from the surrounding bluish lymphatic tissue. With low magnifications they are apt to be mistaken for blood- vessels. LYMPHATIC ORGANS. 1.55 Unlike the other lymphatic organs, the lymph nodules of the thymus contain no germinal centres. Mitosis can, however, usually be seen in the lymphoid cells. Nucleated red blood cells also occur in the thymus. The thymus must therefore be considered one of the sources of lymphoid cells and of red blood cells. ^-,:./ Blood-vessels. — The larger arteries run in the connective- tissue septa. From these, smaller intralobular branches are given off, which break up into capillary netw^orks in the cortex and me- dulla. The capillaries pass over - into veins. These converge to ''^*"^; j-f^ form larger veins, which accom- -- _^ ^j, pany the arteries. ^^'^- 9^- — Hassall's Corptiscle and Small „. , , 1 ,. - 1 Portion of Surrounding Tissue. X6oo. Of the lymphatics of the (See Technic below.) thymus little is known. They appear to originate in indefinite sinuses within the lymphoid tissue, whence they pass to the septa where they accompany the blood-vessels. Nerves. — These are distributed mainly to the walls of the blood- vessels. A few fine fibres, terminating freely in the lymphatic tissue of the cortex and of the medulla, have been described. TECHNIC. Fix the thymus of a new-born infant in formalin-Miiller's fluid (technic 5, p. 7), and harden in alcohol. Stain sections with htematoxylin-eosin (technic i, p. i8), or with haimatoxylin-picro-acid-fuchsin (technic 3, p. 19), and mount in balsam. The Tonsils. The Palatine Tonsils or True Tonsils. — These are compound lyDipJiatic organs, essentially similar in structure to tlic Ivmphatic organs already described. The usual fibrous capsule is present only over the attached surface, where it separates the tonsil from surround- ing structures. From the capsule, connective-tissue trabeculcc extend into the substance of the organ and branch to form its framework. The free surface of the tonsil is covered by a reflection of the stratified squamous epithelium of the pharynx (Fig. 92). The epithelium is sepa- 156 THE ORGANS. rated from the underlying lymphatic tissue of the tonsil by a more or less distinct basement membrane. At several places on the surface of the tonsil deep indentations or pockets occur. These are known as the crypts of the tonsil (Fig. 92), and are lined throughout by a continuation of the surface epithelium which becomes thinner as the deeper part of the crypt is reached. Passing off from the bottoms and sides of the main or primary crypts are frequently several secondary crypts, also lined with the same type of epithelium. / -":^' M. ■im V Fig. 92. — ^Vertical Section of Dog's Tonsil through Crypt. X15. (Szymonowicz.) a, Lymph nodule; b, epitheh'um of crypt; c, blood-vessel; d, crypt; e, connective-tissue cap- sule;/, mucous glands; g, epithelium of pharynx. Beneath the basement membrane is the lymphoid tissue of the tonsil. This consists of diffuse lymphatic tissue in which are found nodules of compact lymphatic tissue similar to those in the lymph node. Each nodule has a ^erm centre, where active mitosis is going on, and a surrounding zone of more densely packed cells. The nodules have a fairly definite arrangement, usually forming a single layer beneath the epithelium of the crypts. At various points on the surface of the LYMPHATIC ORGANS. 157 tonsil, and especially in the crypts, occurs what is known as lymphoid infiltration of the epithelium (Fig. 93). This consists in an invasion of the epithelium by the underlying lymphoid cells. It varies from the presence of only a few lymphoid cells scattered among the epithehal, to an almost complete replacement of epithelial by lymphoid tissue. In this way the latter reaches the surface and lymphoid cells are dis- charged upon the surface of the tonsil and into the crypts. These cells probably form the bulk of the so-called salivary corpuscles. In the connective tissue adjacent to the tonsil are numerous mucous glands, the ducts of which empty into the tonsillar crypts. The Lingual Tonsils — Folliculi Linguales. — These are small lymphatic organs situated on the dorsum and sides of the back part of the tongue between the circumvallate papillae and the epiglottis. Fig. 93. — Vertical Section through \\"all of Crypt in Dog's Tonsil, showing lymphoid infiltration of epithelium. X150. (Bohm and von Davidoff.) a, Leucocytes in epithe- lium; b, space in epithelium filled with leucocytes and changed epithelial cells; c, blood-ves- sel; d, epithelium beyond area of infiltration; e, basal layer of cells. They are similar in structure to the true tonsils. Each lingual tonsil has usually one rather wide-mouthed deep crypt (the foramen cceciim lingui) which may be branched and which is lined with a continuation of the surface stratified squamous epithelium. Into these crypts fre- quently open the ducts of some of the mucous glands of the tongue. The Pharyngeal Tonsils. — These are lymphatic structures which lie in the naso-pharynx. They resemble the lingual tonsils, except that they are, as a rule, not so sharply circumscribed. Hypertrophy of the pharyngeal tonsils is common especially in children, gi\"ing rise to what are known as adenoids. The tonsils make their first appearance toward tlic end of the fourth month of intra-utcrinc life. The earliest of the tonsillar lymph- oid cells are white blood cells which have migrated from the ves- 158 THE ORGANS. sels of the stroma of the mucosa and have infiltrated the surrounding connective tissue. Further development of the tonsil is by prolif- eration of these cells. The crypts are at first solid ingrowths of sur- face epithelium. These later become hollowed out. The blood-vessels and nerves have a distribution similar to those of the lymph nodes, but enter the organ along its entire attached side and not at a definite hilum. Of the lymphatics of the tonsil little is known. TECHNIC. Normal human tonsils are so rare, owing to the frequency of inflammation of the organ, that it is best to make use of tonsils from one of the lower animals (dog, cat, or rabbit). Treat as in technic i, p. 150, care being taken that sections pass longitudinally through one of the crypts. The Spleen. The spleen is a lymphatic organ, the peculiar structure of which appears to depend largely upon the arrangement of its blood-vessels. The surface, except where the organ is attached, is covered by a /^' e Fig. 94. — Section through Porti(jn of Cat's Spleen, to sliow general tojaography. X 15. (Technic i, p. 163.) a, Capsule; /;, .sei)ta containing blood-vessels; c, germinal centres; d, septa; e, lymph nodules. serous membrane, the periloneum (page 235). Beneath this is a cap- sule of fibrous tissue containing numerous elastic fibres and smooth muscle cells. From the capsule strong connective-tissue septa, simi- lar to the capsule in structure, extend into the interior of the organ. These branch and unite with one another to form very incomplete LY.MPHATIC ORC.AXS. 159 anastomosing chambers. The capsule and septa form, as in the lymph node, the connective-tissue frameu^ork of the organ (Fig. 94). The chambers incompletely bounded by the connective-tissue septa are filled in with tissue resembling lymphatic tissue, composed of reticular connective tissue, lymphoid cells, and other varieties of cells described on p. 161. This tissue constitutes the substantia propria of the organ and is everywhere traversed by thin-walled vas- cular channels, the tissue and vascular channels together constituting the splenic pulp (Fig. 95). Compact lymphatic tissue occurs in the spleen as spherical, oval, or cyHndrical aggregations of closely packed Fig. 95. — Section of Human Spleen, including portion of Malpighian body with its artery and adjacent splenic pulp. X300. (Technic 2, p. 163.) a, Malpighian body; b, pulp cords, c, cavernous veins; h and c together constituting the splenic pulp. lymphoid cells. These are known as Malpighian bodies or splenic corpuscles (Figs. 94 and 95) and are distributed throughout the splenic pulp. Each splenic corpuscle contains one or more small arteries. These usually run near the periphery of the corpuscle; more rarely they lie at the centre. Except for its relation to the blood-vessels, the splenic corpuscle is quite similar in structure to a lymph nodule. It consists of lymphoid cells so closely packed as completely to obscure the underlying reticulum. In a child's spleen the centre of each corpuscle shows a distinct germinal centre (see page 148). In the adult luiman spleen germ centers are rarely seen. The blood-\"essels of the spleen have a very characteristic arrangement, wliicli must l)e described Ijefore considerinsji; further the minute structure of ihe organ. 160 THE ORGANS. The arteries enter the spleen at the hihim and divide, the branches following the connective-tissue septa. The arteries are at first ac- companied by branches of the splenic veins. Soon, however, the arteries leave the veins and the septa, and pursue an entirely separate course through the splenic pulp. Here the adventitia of the smaller arteries assumes the character of reticular tissue and becomes infil- trated with lymphoid cells. In certain animals, as, e.g., the guinea- Spleen sinus XX Sheathed artery Pulp artery Pulp vein Beginningof in- terlobular vein Capillary net- work of nodule TrabeculcB Penicillus Central artery \Lobul Hilus Reticulum Spleen nodule Capsule Fig. g6. — Scheme of Human Spleen, x, Oijening of arterial capillaries into spleen sinus; xx, interruption of closed blood course at ends of arterial capillaries, at margin of nodule, xxx. For sake of clearness, sinus is placed Um far from margin of nodule. (Stohr). pig, this infiltration is continuous, forming long cord-like masses of compact lymphoid tissue. In man, the adventitia is infiltrated only at points along the course of an artery. This may take the form of elongated collections of lymphoid cells — the so-called spindles — or of distinct lymph nodules, the already mentioned splenic corpuscles. Although usually eccentrically situated with reference to the nodules, these arteries are known as central arteries. They give rise to a few capillaries in the spindles, to a larger number in the nodules. Beyond the latter the arteries divide into thick sheathed terminal arteries— LYMPHATIC ORGANS. 161 1^/ If m J ellipsoids — which do not anastomose, but He close together Hke the bristles of a brush or penicillus. The terminal arteries break up into arterial capillaries which still retain an adventitia, and which empty into broader spaces — sinuses or ampullcE — which in turn empty into the cavernous veins of the splenic pulp (Fig. 95 ). The Splenic Pulp. — The anastomosing cavernous veins break up the diffuse lymphatic tissue of the spleen into a series of anasto- mosing cords similar to those found in the medulla of the lymph node. These are known as pulp cords (Fig. 95), and with the cavernous veins constitute, as ■ "■:3> ^ already mentioned, the splenic pulp. The pulp cords consist of £' /<^k a delicate framework of reticular connective tissue, in the meshes of which are found, in addition to lymphoid cells, the following (Fig. 97) : (i) Red blood cells. (2) Nucleated red blood cells. (3) White blood cells. (4) Mononuclear cells, the so- called spleen cells. These are rather large, granular, spherical, or irregular cells. From the fact that blood pigment and red blood cells in various stages of disintegration are found in their cytoplasm, these cells are believed to be concerned in the destruction of red blood cells. (5) Multinuclear cells. These are most common in young ani- mals. Each cell contains a single large lobulated nucleus, or more frequently several nuclei. These cells resemble the osteoclasts of developing bone and the multinuclear cells of bone-marrow. In macerated splenic tissue or in smears from the spleen, there are found, in addition to the above varieties of cells, long spindle- shaped cells with bulging nuclei. These come from the walls of the cavernous veins. F Fig. 97. — Isolated Spleen Cells. X700. (Kolliker.) A, Cell containing red blood cells; b, blood cell; k, nucleus; B, leucocyte with polymorphous nucleus; C, "spleen"' cell with pigment granules; D, lympho- cyte; E, large cell with lobulated nucleus (megalocyte) ; F, nucleated red blood cells; G, red blood cell; U, multinuclear leuco- cyte; J, cell containing eosinophile granules. Two views are held regarding the vascular channels of the splenic pulp. Accord- ing to one, these channels have complete walls, the arterial capillaries passing over into venous capillaries in the usual manner; according to the other, the arterial capillaries open into spaces, the cavernous veins or spleen sinuses, which have 162 THE ORGANS. fenestrated walls, thus allowing the blood to come into direct contact with the surrounding tissues. From these open-walled sinuses, the veins proper take origin. These uniting form veins which enter the septa and ultimately converge to form the splenic veins which leave the organ at the hilum. According to Mall, the spleen, like the liver, is composed of a large number of lobules, which may be considered its anatomical units (Fig. 98). Each lobule is separated from its neighbors by several (usually three) connective-tissue septa (interlobular septa). Each interlobular septum gives off about three secondary septa (intralobular septa) which pass into the lobule and, anasto- mosing, divide it into about ten chambers, which are filled with splenic pulp. As the splenic pulp of neighboring chambers anasto- moses, cord-like structures are formed which Mall designates pulp cords. It will be seen that the pulp cords of Mall are alto- gether different from the pulp cords previously mentioned. An artery passes through the centre of each lobule, giving off a branch to each of its chambers. These branch repeatedly in the pulp cords of Mall and end in small dilatations, the ampullae of Thoma. The ampullae pass over into minute veins which converge and empty into the interlobular veins. Mall believes the walls of the ampullae and beginning venous plexuses to be very porous, "allowing fluids to pass through with great ease, and granules only with difficulty." He further states that "in life the plasma constantly flows through the intercellular spaces of the pulp cords, while the blood corpuscles keep within fixed channels." d -f Fig. 98. — Diagram of .bplenic Lobule, according to Mall, a, Capsule; b, intralobular venous spaces; c, intralobular vein; d, ampulla of Thoma; e, pulp cord; /, interlobular vein; g, intralobular vein; h, Malpighian body; f, intralobular tra- becula; j, interlobular trabecula; k, intralobular artery; /, artery to one of the ten compartments; m, intralobular trabecula. Lymphatics are not numerous. In certain of the lower animals large lymph vessels occur in the capsule and septa. These are not well developed in man. Lymph vessels are present in the connective tissue of the hilum. They probably do not occur in the splenic pulp or in the splenic corpuscles. Nerves. — These are mainly non-medullated, although a few medullated fibres are present. Among the latter are dendrites of sensory neurones whose cell bodies are situated in the spinal gan- glia. They supply the connective tissue of the capsule, septa, and blood-vessels. The non-medullated fibres — axones of sympathetic LYMPHATIC ORGANS. 163 neurones — accompany the arteries, around which they form plexuses. From these plexuses terminals pass to the muscle cells of the arteries, to the septa, to the capsule, and possibly also to the splenic pulp. The exact manner in which both medulla ted and non-medullated fibres terminate is as yet undetermined. TECHNIC. (i) The spleen of a cat is more satisfactory for topography than the human spleen, as it is smaller, contains more connective tissue, and its Malpighian bodies are more evenly distributed and more circumscribed. Fix in formalin-Miiller's fluid (technic 5, p. 7), and harden in alcohol. Cut sections through the entire spleen. Stain with hjematoxylin-eosin (technic i, p. 18), or with haematoxylin- picro-acid-fuchsin (technic 3, p. 19). (2) Human Spleen. — Small pieces are treated as in technic (i). (3) Human Spleen (Congested). — Congested human spleens are usually easy to obtain from autopsies. Treat as in technic (i). The cavernous veins being dis- tended with blood, the relations of the veins to the pulp cords are more easily seen than in the uncongested spleen. The contrasts are especially sharp in sections stained with hasmatoxylin-picro-acid-fuchsin. (4) The cells of the spleen may be studied along the torn edges or in the thin- ner parts of any of the spleen sections. Or a smear may be made in a manner similar to that described in technic (page 100), by drawing the end of a slide across a freshly cut spleen surface and then smearing the tissue thus obtained across the surface of a second slide. Dry, fix in equal parts alcohol and ether (one-half hour), stain with haematoxylin-eosin and mount in balsam. Or the cut surface of the spleen may be scraped with a knife, the scrapings transferred to Zenker's fluid, hardened in alcohol, stained with alum-carmine (pages 17 and 57) and mounted in eosin-glycerin. General References for Further Study. Kolliker: Handbuch der Gewebelehre des [Menschen, vol. iii. Szymonowicz and MacCallum: Histology and ^Microscopic Anatomy. Warthin: Haemolymph Glands (with bibliography). Reference Handbook of the Medical Sciences, vol. iv. Mall: Lobule of the Spleen. Bui. Johns Hopkins Hospital, vol. ix. — Archi- tecture and Blood-vessels of the Dog's Spleen. Zeit. f. Morph. u. Anth., Bd. ii. Oppel: Ueber Gitterfasern der menschlichcn Leber und ^Slilz. Anat. Anz., 6 Jahrg., S. 165. CHAPTER III. THE SKELETAL SYSTEM. The skeletal system consists of a series of bones and cartilages which are united by special structures to form the supporting frame- work of the body. Under this head are considered: (i) bones, (2) marrow, (3) cartilages, (4) articulations. The Bones. A bone considered as an organ consists of bone tissue laid down in a definite and regular manner. If a longitudinal section be made through the head and shaft of a long bone, the head of the bone and also part of the shaft are seen to be composed of anastomosing bony trabeculae enclosing cavities. This is known as cancellous or spongy V\c,. 99. — Section of Spongy Bone. X75. (Technic 3, p. 172.) a, Marrow space; /), group of fat cells; c, blood-vessel; d, trabeculse of bone. bone. The shaft of the bone consists of a large central cavity sur- rounded by spongy bone, which, however, passes over on its outer side into a layer of bone of great density and known as hard or compact hone. Spongy bone forms the ends and lines the marrow cavities of the long bones, and occurs also in the interior of short bones and fiat 1()4 THE SKELETAL SYSTEM. 165 bones. Compact bone forms the bulk of the shafts of the long bones and the outer layers of the fiat and short bones. In compact hone the layers or lamellae of bone tissue have a defi- nite arrangement into systems, the disposition of which is largely de- pendent upon the shape of the bone and upon the distribution of its blood-vessels. In spongy hone (Fig. 99) there is no arrangement of the bone tissue into systems. The trabeculae consist wholly of bony tissue laid down Fig. 100. — Longitudinal Section of Hard (Undecalcified) Bone: Shaft of Human Ulna. X 90. (Szymonowicz.) Haversian canals, lacuna;, and canaliculi in black. in lamellae. These trabecules anastomose and enclose spaces which contain marrow and which serve for the passage of blood-vessels, lymphatics, and nerves. On examining a longitudinal section of compact bone (Fig. 100) there are seen running through it irregular channels, the general direction of which is parallel to the long axis of the bone. These channels anastomose by means of lateral branches, and form a com- plete system of intercommunicating tubes. They are known as Haver- sian canals, contain marrow elements, and serve for the transmission of blood-vessels, Ivmplialics, and nerves. They anastomose not only with one another, but arc in communication willi the surface of the 166 THE ORGANS. bone and with the central marrow cavity. Between the Haversian canals most of the lamellae run parallel to the canals. In a cross section through the shaft of a long bone (Fig. loi), three distinct systems of lamellae are seen. These are known as Haversian lamellcB, interstitial lamellce, and circumferential lamellce. . . ~ ^ - '^ Fig. ioi. — Cross-section of Hard (Undecalcified) Bone from Human Metatarsus. X 90. (Szymonowicz.) Haversian canals, lacuna;, and canaliculi in black, a, Outer circumferential lamellae; b, inner circumferential lamella.-; c, Haversian lamellae; d, in- terstitial lamellae. (i) Haversian Lamella (Fig. 102). — These are arranged in a concentric manner around the Haversian canals. Between the lamellae, their long axes corresponding to the long axes of the Haversian canals, are the lacunae with their inclosed hone cells (page 93). The lacunae of adjacent lamellae are usually arranged alternately. In a section of or- dinary thickness the lacunae arc not nearly so numerous as the lamellae, and are seen onlv between some of the lamellae. The lacunas of a THE SKELETAL SYSTEM. IG- Haversian system communicate with one another and with their Haversian canal by means of the canalicidi. In Haversian systems the fibres of the matrix (see page 93) run in some lamellae parallel to the canal, in others concentrically. Adjacent fibres thus frequently cross at right angles. (2) INTERSTITLA.L (Intermediate or Ground) Lamella. (Figs, loi and 102).— These are irregular short lamellse, which occupy the spaces left between adjacent Haversian systems. (3) Circumferential Lamella (Fig. loi). — These are parallel lamellae which run in the long axis of the bone, just beneath the periosteum and at the outer edge of the central marrow ca\dty. Oc- casionally circumferential lamellae are absent, the Haversian systems abutting directly upon periosteum. Channels for the passage of blood-vessels from the peri- osteum to the Haversian canals pierce the circumferen- tial lamellae. They are known as V olkmann^ s canals, and are not surrounded by concentric lamellae as are the Haversian lamellae, but are mere chan- nels through the bone. Similar canals pass from the inner Haversian canals into the marrow cavity. The Periosteum. — This is a fibrous connective-tissue membrane which covers the surfaces of bones except where they articulate. It is firmly adherent to the superficial layers of the bone and consists of two layers. The outer layer is composed of coarse fibrillated fibres and contains the larger blood-vessels. The inner layer consists of fine white fibres and delicate elastic fibres which support the smaller blood-vessels. From the periosteum distinct bundles of white fibres, with often some elastic fibres, pierce the outer layers of the Ikjuc. These are Fig. 102. — Transverse Section of Compact Bone from Shaft of Humerus. X 1 50 and slightly re- duced. (Sharpey.) (Technic i, p. 171.) Three Haversian canals with their concentric lamellse and lacunas; canaliculi connecting lacunsE with each other and with Haversian canal. Between the Haversian systems of lamelke are seen the interstitial lamellfe. 168 THE ORGANS. known as the perforating fibres of Sharpey. When tendons and Kga- ments are attached to bone, their fibres are prolonged through the periosteum into the bone as perforating fibres. Bone Marrow. Bone marrow is a soft tissue which occupies the medullary and Haversian canals of the long bones and fills the space between the trabeculse of spongy bone. Marrow occurs in two forms — red marrow and yellow marrow. Red marrow is found in all bones of embryos and of young ani- mals, also in the vertebras, sternum, ribs, cranial bones, and epiphyses of long bones in the adult. In the diaphyses of adult long bones the marrow is of the yellow variety. The difference in color between red marrow and yellow marrow is due to the much greater proportion of fat in the latter, yellow marrow being developed from the red by an almost complete replacement of its other elements by fat cells. Red marrow is of especial interest as a blood-forming tissue, being in the healthy adult the main if not the sole source of red blood cells, and one of the sources from which the leucocytes are derived. The blood-forming function of marrow must be borne in mind in study- ing the various forms of marrow cells. Red marrow (Fig. 103) consists of a delicate reticular connective tissue which supports the following varieties of cells: (i) Marrow Cells — Myelocytes. — These resemble the mononu- clear and some of the transitional forms of leucocytes. The nucleus is large and may be lobulated. It contains a comparatively small amount of chromatin and therefore stains faintly. The cytoplasm is finely granular and stains with neutrophile dyes. Myelocytes are not present in normal blood, but occur in large numbers in leukaemia. It is from the myelocytes that those leucocytes, which are of bone- marrow origin, are derived. {2) Nucleated Red Blood Cells. — These are divisible into erythro- blasts and normoblasts. The former represents an earlier, the latter a later stage in the evolution of the non-nucleated adult red blood cell. The erythroblast, the younger of the two, has a well-formed nu- cleus with a distinct intranuclear network. The protoplasm contains Init little haemoglobin. In the normoblast the intranuclear network has disappeared and the protoplasm has become much richer in THE SKELETAL SYSTEM. 169 hasmoglobin. The normoblast is converted into the adult red blood cell either by extrusion of its nucleus or by the disintegration of the nucleus within the cell body. (3) Non-nucleated Red Blood Cells. — These are the same as are found in the blood (page 95). (4) Multimiclear Cells — Myeloplaxes. — These are large cells with abundant protoplasm. Each cell may contain a single large spheri- t ... # M t*# f* ^^ m ^m --:---.d *^ g f Fig. 103. — Section of Red Bone-marrow from Rabbit's Femur. X 700. (Technic 4> p. 172.) a, Red blood cells; b, myeloplax; c, fat space; d, nucleated red blood cells; e, myelocytes; /, reticular connective tissue; g, leucocytes. cal nucleus or a much lobulated nucleus or several nuclei. Myelo- plaxes are probably derived from leucocytes, and are closely related to, if not identical with, the osteoclasts of developing bone. (5) Leucocytes of all kinds are found in marrow. They ha\"c the same structure as in blood (page 96). (6) Mast cells may be present. They are usually not numerous. (For description see page 98.) (7) Fat Cells. — These are usually round and rather c\"cnly distrib- uted throughout the marrow. 170 THE ORGANS. Yellow marrow (Fig. 104) consists almost wholly of fat cells, which have gradually replaced the other marrow elements. Under certain conditions the yellow marrow of the bones of the old or greatly emaciated undergoes changes due for the most part to the absorp- tion of its fat. Such marrow becomes red-dish and assumes a some- what gelatinous appearance. It is known as ^^ gelatinous marrow^ f Fig. 104. — Yellow Marrow from Rabbit's Femur. X560. (Technic 4, p. 172.) a, nucleated red blood cells; h, myeloplax, c, fat cells; d myelocytes; e, reticular connective tissue;/, leucocytes. The large marrow cavities, such as those of the shafts of the long bones, are lined by a layer of fibrous connective tissue, the endosteum. Blood-vessels. — The blood-vessels of bone pass into it from the periosteum. Near the centre of the shaft of a long bone a canal passes obliquely through the compact bone. This is known as the nutrient canal and its external opening as the nutrient foramen. This canal serves for the passage of the nutrient vessels^ — usually one artery and two veins — to and from the medullary cavity. In its passage through the compact bone the nutrient artery gives off branches to. THE SKELETAL SYSTEM. 171 and the veins receive branches from, the vessels of the Haversian canals. Each of the flat and of the short bones has one or more nutrient canals for the transmission of the nutrient vessels. In addition to the nutrient canals the surface of the bone is every- where pierced by the already mentioned (page 167) Volkmann's canals, which serve for the transmission of the smaller vessels. In compact bone these vessels give rise to a network of branches which run in the Haversian canals. In spongy bone the network lies in the marrow spaces. Branches from these vessels pass to the marrow cavity, and there break up into a capillary network, which anasto- moses freely with the capillaries of the branches of the nutrient artery. The capillaries of marrow empty into wide veins without valves, the walls of which consist of a single layer of endothelium. So thin are these walls that the veins of marrow were long described as pass- ing over into open or incompletely walled spaces in which the blood came into direct contact with the marrow elements. These veins empty into larger veins, which are also valveless. Some of these con- verge to form the vein or veins which accompany the nutrient artery; others communicate with the veins of the Haversian canals. Lymphatics with distinct walls are present in the outer layer of the periosteum. Cleft-like lymph capillaries lined with endothelium accompany the blood-vessels in Volkmann's and in the Haversian canals. The lacunce and canalicuU constitute a complete system of lymph channels which communicate with the lymphatics of the perios- teum, of Volkmann's and the Haversian canals, and of the bone- marrow. Nerves. — Both medullated and non-medullated nerves accompany the vessels from the periosteum through Volkmann's canals, into the Haversian canals and marrow cavities. Pacinian bodies (page 388) occur in the periosteum. Of nerve endings in osseous tissue and in marrow little definite is known. TECHNIC. (i) Decalcified Bone. — Fix a small piece of the shaft of one of the long bones — human or animal — in formalin-^Iuller's fluid (technic 5, p. 7), and decalcify in hydrochloric or nitric acid solution (page 10). After decalcifying, wash until all traces of acid are removed, in normal saline solution to which a little ammonia has been added. Dehydrate, and embed in celloidin. Transverse and longitudinal sections are made through the shaft, including periosteum and edge of marrow 172 THE ORGANS. cavity. Stain with hasmatoxylin-eosin (technic i, p. i8) and mount in eosin- glycerin. (2) Hard Bone. — Transverse and longitudinal sections of undecalcified bone may be prepared as in technic i, p. 94. (3) Spongy Bone. — This may be studied in the sections of decalcified bone, technic (i), where it is found near the marrow cavity. Or spongy bone from the head of one of the long bones or from the centre of a short bone may be prepared as in technic (2). (4) Red Marrow. — Split longitudinally the femur of a child or young animal, and carefully remove the cylinder of marrow. Fix in formalin-Muller's fluid and harden in graded alcohols. Cut sections as thin as possible, stain with haema- toxylin-eosin, and mount in balsam. (5) Marrow: fresh specimen. — By means of forceps or a vice, squeeze out a drop of marrow from a young bone, place on the centre of a mounting slide, cover and examine it immediately. (6) Place a similar drop of marrow on a cover-glass and cover with a second cover-glass. Press the covers gently together, slide apart and fix the specimen by immersion for five minutes in saturated aqueous solution of mercuric chlorid. Wash thoroughly, stain with hsematoxylin-eosin, and mount in balsam. Development of Bone. The forms of bones are first laid down either in cartilage or in embryonic connective tissue. The bones of the trunk, extremities, and parts of the bones of the base of the skull develop in a matrix of cartilage. This is known as intracartilaginous or endochondral ossification. The fiat bones, those of the vault of the cranium and most of the bones of the face, are developed in a matrix of fibrillar connective tissue — intramembranous ossification. A form of bone development, similar in character to intramembranous, occurs in con- nection with both intramembranous ossification and intracartilaginous ossification. This consists in the formation of bone just beneath the perichondrium — subperichondrial ossifiication — or, as with the de- velopment of bone perichondrium l^ecomes periosteum — subperiosteal ossifiication. There are thus three forms of bone development to be considered: (i) Intramembranous, (2) intracartilaginous, and (3) subperiosteal. I. Intramembranous Development (Fig. 105). — In intramem- branous ossification the matrix in which the bone is developed is connective tissue. T'he process of bone formation begins at one or more points in this matrix. These are known as ossification centres. Here some of the bundles of white fibres become calcifiied, i.e., become impregnated with lime salts. There is thus first established a centre THE SKELETAL SYSTEM. 173 or centres of calcification. Between the bundles of calcified fibres the connective tissue is rich in cells and vascular, and from its future role in bone formation is known as osteogenetic tissue (Fig. 105). Along the surfaces of the calcified fibres certain of the osteogenetic cells arrange themselves in a single layer (Figs. 105 and 106). These are now known as osteoblasts or "bone formers.'" Under the influence of these osteoblasts a thin plate of bone is formed between them- FiG. 105. — Intramembranous Bone Development. Vertical section through parietal bone of human foetus. Xi6o. (Technic i, p. 179.) a, Osteoblasts; b, bone trabecular; c, osteoclasts lying in Howship's lacunae; d, internal periosteum; e, bone cells;/, calcified fibres; g, osteogenetic tissue; h, external periosteum (pericranium). selves and the calcified fibres. This plate of bone at first contains no cells, but as the lamella of bone grows in thickness, the layer of osteo- blasts becomes completely enclosed by bone. The osteoblasts are thus transformed into bone cells (Fig. 106), the spaces in which they lie becoming bone lacimce. The bone cell is thus seen to be derived from the embryonic connective-tissue cell, the osteoblast being an intermediate stage in its development. In this way irregular anas- tomosing trabeculce of bone are formed enclosing spaces (Fig. 105). The bony trabeculae at first contain remains of calcified connective- tissue fibres, while the spaces, which are known as primary marrow 174 THE ORGANS. Spaces, contain blood-vessels, osteogenetic tissue, and developing marrow. The osteoblasts ultimately disappear and the spaces are then occupied by blood-vessels and marrow. The connective-tissue membrane has now been transformed into cancellous or spongy hone (Fig- 99)- The bone thus formed is covered on its outer surface by a layer of connective tissue, a part of tjbe membrane in which the bone was formed, but which from its position is now known as the periosteum, or, in the case of the cranial bones, as the peri- or epicranium (Fig. 105). In this form of bone development, occurring as it does in the bones of the skull, provision must be made for increase in the size of the a b Fig. 106. — Intramembranous Bone Development. Vertical section through parietal bone of human foetus. X 350. (Technic i, p. lyg.) a, Osteoblasts; b, calcified fibres; c, osteogenetic tissue; d, osteoclast lying in Howship's lacuna; e, bone lacunas; /, bone. cranial ca\ity to accomodate the growing brain. This is accomplished in the following manner: Along the surface of the bone, directed toward the brain, large multinuclear cells — osteoclasis or " bone breakers^' — make their appearance (Fig. 106). The origin of these cells is not clear. Similar cells are conspicuous elements of adult marrow. They have been variously described as derived from leucocytes, from osteo- blasts, or directly from the connective-tissue cells. A recent theory holds that they are derived by a process of budding from the endothelial cells, which form the walls of the capillaries. These osteoclasts appar- ently possess the power of breaking down bone. They are found mainly along its inner surface, and can be seen lying in little depres- sions— Howship^s lacuna (Fig. io6) — which they have hollowed out in the bone. Between the outer surface of the bone and the pericra- nium is a layer of osteogenetic tissue, the innermost cells of which are THE SKELETAL SYSTEM. 175 arranged as osteoblasts along the outermost osseous lamellae. Here they are constantly adding new bone beneath the pericranium. This new bone is laid down, not in flat, evenly disposed layers, but in the form of anastomosing trabeculas enclosing marrow spaces. It is thus seen that subperiosteal bone, like intramembranous, is at first of the spongy variety, and that with the develop- ment of the cranium the original intramem- branous bone is entirely absorbed, together with much of the subperiosteal. 2. Intracartilaginous Development. — In this form of ossification an embryonal type of hyaline cartilage precedes the forma- tion of bone, the cartilage corresponding more or less closely in shape to the future bone (Fig. 107). Covering the surface of the cartilage is a membrane of fibrillar connec- tive tissue, the peridiondriiim or primary periosteum. In most of the long bones the earliest changes take place within the cartilage at about the centre of the shaft (Fig. 107). Here the cartilage cells increase in size and in number in such a way that several en- larged cartilage cells come to lie in a single enlarged cell space, and the cartilage assumes the character of hyaline cartilage. The cell groups next arrange themselves in rou's or columns, which at first extend outward in a radial manner from a common centre, but later lie in the long axis of the bone. During these changes in the cells there is an increase in the intercellular matrix and a deposit there of calcium salts. In this way the cartilage becomes calcified, the area involved being known as the calcification centre. Further growth of cartilage at the calcification centre now ceases and, as growth of cartilage at the ends of the bone continues, the central portion of the shaft appears con- stricted. The changes up to this point seem to be preparatory to actual bone formation. Fig 7. Inli'vK a: ::..!.,;..' 'US Bone iJevelopment. Longi- tudinal section of one of the bones of embr\-o sheep's foot, showing ossification centre. X20. (Technic 2, p. 179.) a, Periosteum; b, blood-ves- sels; c, subperiosteal bone; d, intracartilaginous bone; e, osteogenetic tissue: /, carti- lage; g, ossification centre: h, calcification zone. 176 THE ORGANS. Ossification proper begins by blood-vessels from the periosteum^ pushing their way into the calcified cartilage at the calcification centre, carrying with them some of the osteogenetic tissue from beneath the periosteum. These blood-vessels with their accompanying osteo- genetic tissue are known as periosteal buds (Fig. io8). Osteoblasts now develop from the osteogenetic tissue and appear to dissolve the calcified cartilage from in front of the advancing vessels. In this way the septa between the car- tilage cell spaces are broken down, the cartilage cells disap- pear, and a central cavity is formed — the primary marrow cavity. From the region of the primary marrow cavity blood- vessels and osteogenetic tissue push in both directions toward the ends of the cartilage which is to be replaced by bone. These break down the transverse septa between the cell spaces, while many of the longitudinal septa at first remain to form the walls of long anastomosing channels, the primary marroiv spaces (Fig. 109). As in intramembranous bone, these contain blood-vessels, embryonal marrow, and osteo- blasts, all of which are derived from the osteogenetic tissue brought in from the periosteum by the periosteal buds. The osteo- blasts next arrange themselves in a single layer along the remains of the calcified cartilage, where they proceed to deposit a thin layer of bone between themselves and the cartilage (Fig. no). As this increases in thickness some of the osteoblasts are enclosed within the newly formed bone to become hone cells, while the remains of the cartilage dimishes in amount and finally disappears. The calcification centre has now become the ossification centre, and its anastomosing b ^ ■■'^' Fig. 108. — Intracartilaginous Bone Develop- ment. X350. Showing osteogenetic tissue pushing its way into the cartilage (periosteal bud) at the ossification centre, a, Perios- teum; b, cartilage cell spaces; c, periosteal bud; d, blood-vessel; e, cartilage cells; /, car- tilage matrix. 'The term "periosteum" is admissible from the fact that the first bone actually formed is beneath the perichondrium, which thus becomes converted into periosteum. THE SKELETAL SYSTEM. V osseous trabecule, with their enclosed spaces containing osteogenetic tissue and marrow, constitute primary ^p&ngy bone. At either end of the ossification centre the cartilage presents a special structure. Nearest the centre the cell spaces are enlarged, flattened, arranged in rows and contain shrunken cells. Some of the walls break down and irregular spaces are formed. The ground substance is calcified. Passing away from the ossification centre, the cell spaces become less flat- tened, still arranged in rows, the contained cells larger, and there is a lesser degree of calcification. This area passes over into an area of hyaline cartilage which blends without distinct demarcation with the ordinary embryonal cartilage of the rest of the shaft. The area of calcified cartilage at either end of the ossification centre is known as the calcification zone and every- c where precedes the formation of Fig. 109.— Intracartilaginous Bone Develop- ment. Same specimen as Fig. 107 ( X350), showing osteogenetic tissue pushing its way into the cartilage and breaking it up into trabeculse; also formation of primary mar- row spaces and disintegration of cartilage cells, a. Disintegrating cartilage cells; b, cartilage trabecula; c, osteogenetic tissue in primary marrow space; d, blood-vessels; e, cell spaces; /, cartilage cells. true bone (Fig. 107). 3. Subperiosteal or subperi- chondrial development (Fig. 107) has already been largely de- scribed in connection with intra- membranous ossification, and dift'ers in no important respect from the latter. It always accom- panies one of the other forms of ossification. Bone appears beneath the perichondrium somewhat earlier than within the underlying cartilage. Beneath the perichondrium is a layer of richly cellular osteogenetic tissue. The cells of this tissue nearest the cartilage become osteoblasts and arrange themselves in a single layer along its surface. Under their influence bone is laid down on the surface of the cartilage in the same manner as in intramembranous ossification. Intracartilaginous and subperiosteal bone can be easily differen- tiated by the presence of cartilaginous remains in the former and their absence in the latter. All hone is at first of the spongy variety. When this is to be converted into compact bone, there is first absorption of bone by osteoclasts, 178 THE ORGANS. with increase in size of the marrow spaces and reduction of their walls to thin plates. These spaces are now known as Haversian spaces. Within these new bone is deposited. This is done by osteoblasts which lay down layer within layer of bone until the Haversian space is reduced to a mere channel, the Haversian canal. In this way are formed the Haversian canals and the Haversian systems of lamellce. Some of the interstitial lamellae are the remains of the spongy bone which was not quite removed in the enlargement of the primary marrow spaces to form the Haversian spaces; other interstitial lamellae appear to be early formed Haversian lamellae which have been more or less replaced by Haversian lamellae formed later. Fig. no. — Intracartilaginous Bone Development. Same specimen as Fig. io7(X3So), showing bone being deposited around one of the trabeculse of cartilage, a, Blood-vessel; b, bone; c, cartilage remains; d, bone cell; e, cartilage cell space; /, osteoblasts; g, osteogenetic tissue; h, lamella of bone; i, connective-tissue cells; j, cartilage cell. While these varieties of ossification have been described, we would emphasize the essential unity of the process. The likeness between intramembranous and subperiosteal ossification has been already noted. The differences observed in intracartilaginous ossification are more apparent than real. In intracartilaginous ossification the bone is developed in cartilage but not from cartilage. As in intra- membranous and in subperiosteal ossification, intracartilaginous bone is developed from osteogenetic tissue. This osteogenetic tissue is a differentiation of embryonal connective tissue, in this case car- ried into the cartilage from the periosteum in the periosteal buds. In intramembranous ossification the bone is developed within and directly THE SKELETAL SYSTEM. 179 from the embryonal connective tissue of which the membrane is com- posed. In intracartilaginous ossification there is the same embryonal connective-tissue membrane, but within this membrane the form of the bone is first laid down in embryonal cartilage. Surrounding the cartilage there remains the embryonal connective tissue of the mem- brane, now perichondrium. It is from tissue which grows into the cartilage from this membrane — embryonal connective tissue — that the bone, although developed in cartilage, is formed. Growth of Bone. The growth of intramembranous bone by the formation of suc- cessive layers beneath the periosteum has been already described (page 174). Intracartilaginous bones grow both in diameter and in length. Growth in diameter is accomplished by the constant deposition of new layers of bone beneath the periosteum. During this process, absorption of bone from within by means of osteoclasts leads to the formation of the marrow cavity. The hard bone of the shaft of a long bone is entirely of subperiosteal origin, the intracartilaginous bone being completely absorbed. Growth in length takes place in the following manner: Some time after the beginning of ossification in the shaft or diaphysis, independent ossification centres appear in the ends of the bone (epiphyses). So long as bone is growing, the epiphyses and diaphysis remain distinct. Between them lies a zone of growing cartilage, the epiphyseal or inter- mediate cartilage. Increase in length of the bone takes place by a constant extension of ossification into this cartilage from the ossifi- cation centers of the epiphyses and diaphysis. After the bone ceases to grow in length, the epiphyses and diaphysis become firmly united. TECHNIC. (i) Developing Bone — Intramembranous. — Small pieces are removed from near the edge of the parietal bone of a new-born child or animal. These pieces should include the entire thickness of bone with the attached scalp and dura mater. Treat as in technio i, p. 171, except that the sections which are cut perpendicular to the surface of the bone should be stained with ha?matoxylin-picro-acid-fuchsin (technic 3, p. 19) and mounted in balsam. (2) Developing Bone — Intracartilaginous and Subperiosteal. — Remove the forearms and legs of a human or animal embryo by cutting through the elbow, and ISO THE ORGANS. knee-joints. (Fcetal pigs from five to six inches long are very satisfactory.) Treat as in technic (i). Block so that the two long bones will lie in such a plane that both will be cut at the same time. Cut thin longitudinal sections through the ossi- iication centres, stain with hasmatoxylin-picro-acid-fuchsin, and mount in balsam. Cut away the ends of one or two of the embedded bones, leaving only the ossifica- tion centres. Block so as to cut transverse sections through the ossification centre. Stain and mount as the preceding. In the picro-acid-fuchsin stained sections of developing bone the cartilage is stained blue; cells, including red blood cells, yellow; connective tissue from pale pink to red, according to density; bone a deep red. The Cartilages. The costal cartilages are hyaline. They are covered by a closely adherent connective-tissue membrane, the perichondrium. Where cartilage joins bone there is a firm union between the two tissues and the perichondrium becomes continuous with the periosteum. The articular cartilages are described below under articulations. The other skeletal cartilages, such as those of the larynx, trachea, bronchi, and of the organs of special sense, are more conveniently considered with the organs in which they occur. Articulations. Joints are immovable (synarthrosis) or movable (diarthrosis). In synarthrosis union may be cartilaginous (synchondrosis), or by means of fibrous connective tissue (syndesmosis). Synchondrosis. — The cartilage is usually of the fibrous form except near the edge of the bone, where it is hyaline. The interver- tebral discs consist of a ring of fibro-cartilage surrounding a central gelatinous substance, the nucleus pulposus, the latter representing the remains of the notochord. Syndesmosis. — Union is by means of ligaments. These may consist wholly of fibrous tissue, the fibres and cells being arranged much as in tendon, or mainly of course elastic fibres separated by loose fibrous tissue. In such syndesmoses as the sutures of the cranial bones, the union is by means of short fibrous ligaments between the adjacent serrated edges. Diarthrosis. — In diarthrosis must be considered '(a) the articular cartilages, (b) the glenoid ligaments and interarticular cartilages, (c) the joint capsule. (a) Articular cartilages cover the ends of the bones. They are THE SKELETAL SYSTE^L l8l of the hyaline variety/ being the remains of the original cartilaginous matrix in which the bones are formed. Xext to the bone is a narrow strip of cartilage in which the matrix is calcified. This is separated from the remaining uncalcified portion of the cartilage by a narrow so-called "striated" zone. The most superficial of the cartilage cells are arranged in rows parallel to the surface; in the mid-region the grouping of cells is largely in twos and fours as in ordinary hyaline cartilage (page 90); while in the deepest zone of the uncalcified carti- lage the cells are arranged in rows perpendicular to the surface. (b) The glenoid ligaments and interarticular cartilages conform more to the structure of dense fibrous tissue than to that of cartilage. (c) The joint capsule consists of two layers, an outer layer of dense fibrous tissue intimately blended with the ligamentous structures of the joint and known as the stratum Jibrosum, and an inner layer, the stratum synoviale or synovial membrane, which forms the lining of the joint cavity. The outer part of the stratum synoviale consists of areolar tissue with its loosely arranged white and elastic fibres inter- lacing in all directions and scattered connective-tissue cells and fat cells. Nearer the free surface of the membrane the fibres run parallel to the surface and the cellular elements are more abundant. The cells are scattered among the fibres and are stellate branching cells like those usually found in fibrous connective tissue. On the free surface, however, the cells are closely packed and although in places often several layers deep, are probably of the nature of endothelium. From the free surfaces of synovial membranes, processes {synovial villi — Haversian fringes) project into the joint cavity. Some of these are non-vascular and consist mainly of stellate cells similar to those of the synovial membrane. Others have a distinct core of fibrous tissue containing blood-vessels and covered with stellate con- nective-tissue cells. From the primary villi small secondary non- vascular villi are frec^uently given off. TECHNIC. (i) Joint Capsule and Articular Cartilage. — Remove one of the small joints — human or animal — cutting the bones through about one-half inch back from the joint. Treat as in technic i, p. 171, making longitudinal sections through the en- tire joint. ' In the acromio-clavicular, sterno-clavicular. costo-vertebral, and maxillary articula- tions the cartilage is of the fibrous form. The same is true of the cartilage covering the head of the ulna, while the surface of the radius, which enters into the wrist-joint, is covered not bv cartilage, but bv dense fibrous tissue. 182 THE ORGANS. (2) Synovial Villi. — Remove a piece of the capsular ligament from near the border of the patella and cut out a bit of the velvety tissue which lines its inner surface. Examine fresh in a drop of normal salt solution. Fix a second piece of the ligament in formahn-Muller's fluid (technic 5, p. 7), make sections perpendic- ular to the surface, stain with haematoxylin-eosin (technic i, p. 18), and mount in balsam. General References for Further Study. KoUiker: Handbuch der Gewebelehre, vol. i. Stohr: Text-book of Histology. Schafer: Histology and Microscopical Anatomy, in Quain's Elements of Anatomy. CHAPTER IV THE MUSCULAR SYSTEM/ The voluntary muscular system consists of a number of organs — the muscles — and of certain accessory structures — the tendons, tendcni sheaths, and hursa. A VOLUNTARY MUSCLE coHsists of Striated muscle fibres arranged in bundles or fascicles and supported by connective tissue. The entire muscle is enclosed by a rather firm connective-tissue sheath or capsule — the epimysium (Fig. in). This sends trabeculae Fig. I II. — From a Transverse Section of a Small Human Muscle, showing relations of muscle fibres to connective tissue, a, Epimysium; h, perimysium; c, muscle fibres; d, arteries; e, endomysium. of more loosely arranged connective tissue into the substance of the muscle. These divide the muscle fibres into bundles or fascicles. Around each fascicle the connective tissue forms a more or less definite ' Definite arrangements of smooth muscle, such as are found in the stomach and intes- tines, also the muscle of the heart, are properly a part of the muscular system. They are, however, best considered under tissues and in connection with the organs in which they occur. 183 184 THE ORGANS. I, v// /y v/1 .' ■• ( I, ,1 \l y^'>: 'h ''I ' '^f ft. "■■' 'i' envelope, the perifasckidar sheath or perimysium. From the latter delicate strands of connective tissue pass into the fascicles between the individual muscle fibres. This constitutes the intrafascicular connective tissue or endomysium, which everywhere completely separates the fibres from one another so that the sarcolemma of one fibre never comes in contact with the sarcolemma of any other fibre. It should be noted that these terms in- dicate merely location; epi-, peri-, and endo-mysium all being connective tissue grad- ing from coarse to fine, as it passes from without in- ward. The structure of the muscle as an organ is thus seen to conform to the struc- ture of other organs, in that it is surrounded by a con- nective-tissue capsule, which sends septa into the organ, dividing it into a number of compartments and serving for the support of the essen- tial tissue of the organ, the Fig. ii2.-From a longitudinal Section through muscle fibres or parenchvma. J unction of Muscle and Tendon. X 150. (Bohm and Davidoff.) a, Tendon; h, line of union show- The Structure of tendon has ing increase in number of muscle nuclei; ., muscle. ^^^^ described (see page 78) . Tendon sheaths and bursoe are similar in structure, consisting of mixed white and elastic fibres. Their free surfaces are usually lined by fiat cells, which are described by some as connective-tissues cells, by others as endothelium. At the junction of muscle and tendon, the muscle fibre with its sarcolemma ends in a rounded or blunt extremity (Fig. 58, p. 106). Here the fibrils of the tendon fibres are in part cemented to the sar- colemma, and in part are continuous with the fibres of the endo- and peri-mysium. Along the line of union of muscle and tendon the muscle nuclei are more numerous than elsewhere (Fig. 112, h), and it has been suggested that there is here a zone of indiiTerent or formative tissue which is capable of developing on the one hand into muscle, on the other into the connective tissue of tendon. Growth of muscle takes place mainly at the ends of the fibres t0kif0mlM^M0^f'M THE MUSCULAR SYSTEM. 185 where the nuclei are most numerous. In addition to the growth incident to increase in size of the individual or of the particular muscle, there is a constant wearing out of muscle fibres and their replacement by new fibres. This is accomplished as follows: The muscle fibre first breaks up into a number of segments (sarcostyles), some of which contain nuclei while others are non-nucleated. The sarcostyles next divide into smaller fragments, and finally completely disintegrate. This is followed by a process of absorption and complete disappearance of the fibre. From the free sarcoplasm new muscle fibres are formed. In the early stages of their development these are known as myoblasts. The latter develop into muscle fibres in the same manner as described under the histogenesis of muscle (p. loSj. Blood-vessels. — The larger arteries of muscle run in the perimy- sium, their general direction being parallel to the muscle bundles. From these, small branches are given ofl' at right angles. These in turn give rise to an anastomosing capillary network with elongated meshes, which surrounds the indi\"idual muscle fibres on all sides. From these capillaries, veins arise which follow the arteries. Even the smallest branches of these veins are supplied with valves. In tendons blood-vessels are few. They run mainly in the connec- tive tissue which surrounds the fibre bundles. Tendon sheaths and bursae, on the other hand, are well supplied with blood-vessels. The lymphatics of muscle are not numerous. They accompany the blood-vessels. In tendon definite lymph vessels are found only on the surface. Nerves. — The terminations of nerves in muscle and tendon are described under nerve endings (page 388). TECHNIC. (i) A Muscle. — Select a small muscle, human or animal, and, attaching a weight to the lower end to keep it stretched, fix in formaHn-Miiller's fluid (technic 5, p. 7), and harden in alcohol. Stain transverse sections with ha?mato.\ylin-])icro-acid- fuchsin (technic 3, p. 19) and mount in balsam. (2) Junction of Muscle and Tendon. — Any muscle-tendon junction may be selected. Fix in formalin-MuUer's fluid, keeping stretched by means of a weight attached to the lower end. Cut longitudinal sections through the muscle-tendon junction, stain with hicmatoxylin-picro-acid-fuchsin, and mount in balsam. The gastrocnemius of a frog is convenient on account of its small size, and because by bending the knee over and tying there, the muscle can be easily put on the stretch and kept in that condition during fixation. Place the entire preparation in the fixative removing the muscle-tendon from the bone after fixation. CHAPTER V. GLANDS AND THE GENERAL STRUCTURE OF MUCOUS MEMBRANES. Glands — General Structure and Classification. Attention was called in describing the functional activities of cells (page 46) to the fact that certain cells possess the power of not only carrying on the nutritive functions necessary to maintain their own existence, but also of elaborating certain products either neces- sary for the general body functions (secretions) or for the body to eliminate as waste (excretions). Such cells are known as gland cells or glandular epithelium, and an aggregation of these cells to form a definite structure for the purpose of carrying on secretion or excretion is known as a gland. A gland may consist of a single cell, as, e.g., the mucous or goblet cell on the free surface of a mucous membrane or the unicellular glands of invertebrates. Such a cell undergoes certain changes by which a portion of its protoplasm is transformed into or replaced by a substance which is to be used outside the cell itself. The appearance which this cell presents depends upon the stage of secretion. It is thus possible to differentiate between a "resting" and an "active" cell or between an "empty" and a "loaded" cell. The mucous secreting cell of the intestine is one of the simple columnar cells which constitute the epithe- lium of the mucous membrane. It is distinguishable as a mucous or goblet cell only after secretion begins. The resting cell is granular and takes a rather dark cytoplasmic stain. As the cell becomes active, part of the cytoplasm is transformed into, or is replaced by, a clear sub- stance which does not stain like cytoplasm, but reacts to hgematoxylin. The mucus collects first in the free end of the cell, and gradually increases in amount until the entire cell is filled, with the exception of a small area at the base, where a little unchanged protoplasm surrounds a flattened nucleus. The cell at this stage is much larger than in the resting state, and finally ruptures on the free surface and pours out its secretion. Opinions differ as to the further behavior of this cell. Ac- cording to some, its life history is now ended, and its place is taken by 186 GLANDS. 187 other cells which pass through the same process. This undoubtedly takes place in the sebaceous glands, the cells of which disintegrate to form the secretion. Others belie\-e that in most cases the cell is reconstructed from the nucleus and unchanged cytoplasm, and again passes through the process of secretion. How many times a cell may repeat the secretory process is not known. In stratified epithelium secretion may begin while the cell is still deeply situated, but is com- pleted only as the cell reaches the surface, where its mucus is to be discharged. Most glands are composed of more than one cell, usually of a large number of cells, and these cells, instead of lying directly upon the sur- face, line more or less extensive invaginations into which they pour their secretions. In the simplest form of glandular invagination all the cells lining the lumen are secreting cells. In more highly developed glands only the deeper cells secrete, the remainder of the gland serving merely to carry the secretion to the surface. This latter part is then known as the excretory duct, in contradistinction to the deeper secreting portion. In both the duct portion and secreting portion of a gland the epithelium usually rests upon a more or less definite basement membrane or mem- brana propria (page 63). Beneath the basement membrane, separat- ing and supporting the glandular elements, is the connective tissue of the gland. This varies greatly in structure and quantity in different glands. When the secreting portion of the gland is a tubule, the lumen of which is of fairly uniform diameter, the gland is known as a tubular gland. When the lumen of the secreting portion is dilated in the form of a sac or alveolus, the gland is known as a saccular or alveolar gland. Intermediate forms have been described as tubulo-alveolar glands. A gland may consist of a single tubule or saccule, or of a single system of ducts leading to terminal tubules or saccules — simple gland. A gland may consist of a number of more or less elaborate duct systems with their terminal tubules or saccules — compound gland. A few glands. e.g., the thyreoid and thymus, have no ducts, and arc known as ductless glands. All compound glands are surrounded by connective tissue which forms a more or less definite capsule. From the capsule connective- tissue septa or trabeculce extend into the gland. The broadest septa usually divide the gland into a number of macroscopic compartments 188 THE ORGANS. or lobes. Smaller septa from the capsule and from the interlobar septa divide the lobes into smaller compartments usually microscopic in size — the lobules. A lobule is not only a definite portion of the gland separated from the rest of the gland by connective tissue, but represents a definite grouping of tubules or alveoli with reference to one or more terminal ducts. The glandular (epithelial) tissue is known as the parenchyma of the gland, in contradistinction to the connective or interstitial tissue. The relations of the glandular tissue proper to the connective tissue are best understood by reference to development. All glands, simple and compound, origi- nate as simple evaginations from a surface lined with epithelium. The epithelial evagination grows down into the underlying connective tissue. In a. compound gland this invagination tubule becomes the main excretory duct. As the tubule grows, it divides and subdivides to form the larger and smaller ducts and finally the secreting tubules or alveoli. During the development of the gland tubules, the con- nective tissue is also developing, but is being largely replaced by the mere rapidly growing tubules. The gland tubules do not develop irregularly, but in definite groups, each group being dependent upon the tubule (duct) from which it originates. Thus the invagination tubule (main excretory duct) gives rise to a few large branches (lobar ducts), each one of which gives off the subdivisions which constitute a lobe. From each lobar duct there arise within the lobe a large number of smaller branches (lobular ducts) each one of which gives rise to the subdivisions included in a lobule. As the lobe groups and lobule groups of tubules develop, the largest strands of connective tissue are left between adjacent lobes (interlobar connective tissue), smaller strands between lobules (interlobular connective tissue), and the finest con- nective tissue between the tubules or alveoli within the lobule (intralobular con- nective tissue). Glands may thus be classified according to their shape and ar- rangement as follows: 1. Tubular glands. i straight. (a) Simple tubular -^ coiled. ' branched. (b) Compound tubular. 2. Saccular or alveolar glands. (a) Simple saccular. (b) Compound saccular or racemose. 3. Ductless glands. J. Tubular Glands. ~(a) Simple tubular glands are simple tubules which open, on the surface, their lining epithelium being con- tinuous with the surface epithelium. All the cells may be secreting cells or only the more deeply situated. In the latter case the upper GLANDS. 189 portion of the tubule serves merely as a duct. In. the more highly developed of the simple tubular glands we distinguish a mouth, opening upon the surface, a neck, usually somewhat constricted, and a fundus, or deep secreting portion of the gland. Simple tubular- glands are divided according to the behavior of the fundus, into (i) straight, (2) coiled, and (3) branched. {1) A straight tubular gland is one in which the entire tubule runs a straight unb ranched course, e.g., the glands of the large intestine (Fig. 113, i). (2) A coiled tubular gland is one in which the deeper portion of the tubule is coiled or convoluted, e.g., the sudoriferous glands of the skin (Fig. 113, 2). Fig. 113. — Diagram Illustrating Different Forms of Glands. Upper row, tubular glands; i, 2, and 3, simple tubular glands; 4, compound tubular gland. Lower row, alveolar glands; la, 2a, and ^a, simple alveolar glands; 40, compound alveolar gland. For description of la, 2a, and 3a, see simple alveolar glands in text. (3) A forked or branched tubular gland is a simple tubular gland in which the deeper portion of the tubule branches, the several branches being lined with secreting cells and opening into a superficial portion, which serves as a duct. Examples of slightly forked glands are seen in the cardiac end of the stomach, and in the uterus. Other tubular glands show much more extensive branching, the main duct giving rise to a number of secondary ducts, from which are given oiT the termi- nal tubules. The mucous glands of the mouth, oesophagus, trachea. 190 THE ORGANS. and bronchi are examples of these more elaborate simple tubular glands (Fig. 113, 3)- (b) Compound tubular glands consist of a number, often of a large number, of distinct duct systems. These open into a common or main excretory duct. The smaller ducts end in terminal tubules. Many of the largest glands of the body are of this type, e.g., the sali- vary glands, liver, kidney, and testis (Fig. 113, 4). In certain compound tubular glands, as, e.g., the liver, extensive anastomoses of the terminal tubules occur. These are sometimes Called reticular glands. 2. Alveolar Glands. — (a) Simple Alveolar Glands. — The sim- plest form of alveolar gland consists of a single sac connected with the surface by a constricted portion, the neck, the whole being shaped like a flask (Fig. 113, i a). Such glands are found in the skin of certain amphibians; they do not occur in man. Simple alveolar glands, in which there are several saccules (Fig. 113, 2 a), are represented by the smaller sebaceous glands. Simple branched alveolar glands, in which a common duct gives rise to a number of saccules (Fig. 113, 3 a), are seen in the larger sebaceous glands, and in the Meibomian glands. {b) Compound Alveolar Glands. — These resemble the com- pound tubular glands in general structure, consisting of a large num- ber of duct systems, all emptying into a common excretory duct. The main duct of each system repeatedly branches, and the small terminal ducts, instead of ending in tubules of uniform lumen, as in a tubular gland, end in sac-like dilatations, the alveoli or acini (Fig. 113, 4 a). The best example of a compound alveolar gland is the mammary gland, although the lung is constructed on the principle of a compound alveolar gland. Certain structures remain to be considered which are properly classified as glands, but in which during development the excretory duct has disappeared. Such glands are known as ductless glands. The ovary is a ductless gland, the specific secretion of which, the ovum, is under normal conditions taken up by the oviduct and carried to the uterus. This is known as a dehiscent gland. Other ductless glands, such as the thyreoid and adrenal, are known as glands of internal secretion, their specific secretions passing directly into the blood or lymph systems. A few glands, e.g., the liver and pancreas, have both an internal secretion, and an external secretion. GENERAL STRUCTURE OF MUCOUS MEMBRANES. 191 General Structure of Mucous Membranes. The alimentary tract, the respiratory tubules, parts of the genito- urinary system, and some of the organs of special sense are lined by mucous membranes. While differing as to details in different organs, the general structure of all mucous membranes is similar. The essen- tial parts are (i) surface epithelium, (2) basement membrane, and (3) stroma or tunica propria. The epithelium may be simple colum- nar, as in the gastro-intestinal canal; ciliated, as in the bronchi; stratified squamous, as in the oesophagus, etc. The epithelium rests upon a basement membrane or membrana propria which, like the same membrane in glands, is described by some as a product of the epithe- lium, by others as a modification of the underlying connective tissue. Beneath the basement membrane is a connective-tissue stroma, or tunica propria. This usually consists of loosely arranged fibrous tissue with some elastic fibres. It may contain smooth muscle cells and lymphoid tissue. In addition to the three layers above described there is frequently a fourth layer between the stroma and the underlying connective tissue. This consists of one or more layers of smooth muscle, and is known as the muscularis mucosce. A mucous membrane usually rests upon a layer of connective tissue rich in blood-vessels, lymphatics, and nerves — the siibmucosa. CHAPTER VI. THE DIGESTIVE SYSTEM. The digestive system consists of the alimentary tract and certain associated structures such as glands, teeth, etc. The alimentary tract is a tube extending from lips to anus. Dif- ferent parts of the tube present modifications both as to calibre and as to structure of wall. The embryological subdivision of the canal into headgut, foregut, midgut, and endgut admits of further subdivision upon an anatomical basis as follow^s: I. Headgut: (a) Mouth, including the tongue and teeth. {h) Pharynx. II. Foregut: {a) CEsophagus. {h) Stomach. III. Midgut: Small intestine. IV. Endgut: {a) Large intestine. {h) Rectum. The entire canal is lined by mucous membrane, the modifications of which constitute the most essential difference in structure of its several subdivisions. Beneath the mucosa is usually more or less connective tissue, which in a large portion of the canal forms a definite suhmucosa. Muscular tissue is present beneath the submucosa throughout the greater part of the canal. In most regions it forms a definite, con- tinuous, muscular tunic. The upper and lower ends of the tube — mouth, pharynx, oesoph- agus, and rectum — are quite firmly attached by fibrous tissue to the surrounding structures. The remainder of the tube is less firmly attached, lying coiled in the abdominal cavity, its surface covered, except along its attached border, by a serous membrane, the visceral peritoneum. 192 THE DIGESTIVE SYSTEM. 193 I. THE HEADGUT. The Mouth. The Mucous Membrane of the Mouth.— This consists of stratified squamous epithelium lying upon a connective-tissue stroma or tunica propria. The latter is thrown up into papilla, which do not, however, appear upon the free surface of the epithehum. The submucosa is a firm connective-tissue layer with few elastic fibres. The thickness of the epithehum, the character of the stroma, and the height of the papillae vary in different parts of the mouth. There is no muscularis mucosas. x\t the junction of the skin and mucous membrane (red margin of the lips) the epithelial layer is much thickened, the stroma is thinned, and the papillae are very high. At this point the stratum corneum of the skin passes over into the softer nucleated epithehum of the mouth, while the stratum lucidum and stratum granulosum of the skin terminate (see skin, page 354). The mucous membrane of the gums has prominent, long, slender papillae, the summits of which are covered by a very thin layer of epithelium. This nearness of the vascular stroma to the surface accounts for the ease with which the gums bleed. That portion of the gums which extends over the teeth is devoid of papillae. The submucosa of the gums is firmly attached to the underlying periosteum. The mucous membrane lining the cheeks has low, small papillae, and the submucosa is closely adherent to the muscular fibres of the buccinator. Covering the hard palate, the mucous membrane is thin and the short papillae are obliquely placed, their apices being directed ante- riorly. The submucosa is firmly attached to the periosteum. Over the soft palate the papillae of the mucous membrane are low or even absent. They are somewhat higher on the uvula, the poste- rior surface of which shows a transitional condition of its epithe- lium, areas of stratified squamous alternating with areas of stratified columnar ciliated epithelium. Throughout the mucous membrane of the soft palate, u\ula, and fauces, the stroma and submucosa contain diffuse lymphatic tissue. In some places the lymphoid cells are so closely placed as to form distinct nodules. Glands of the Oral Mucosa.^ — Distributed throughout the oral mucosa are small l^ranched tul)ular glands. Only in those parts ' For description of the larger salivary glands see page 242. 194 THE ORGANS. of the mucous membrane which are closely attached to underlying bone, as on the gums and hard palate, are mucous glands few or entirely absent. While the deeper portions of the glands are in the submucosa, some of the tubules usually lie in the stroma of the mu- cous membrane. The ducts open upon the surface and are lined with a continuation of the surface stratified squamous epithelium as far as the first bifur- cation. Here the epithelium becomes stratified columnar, and this, as the smaller branches are approached, passes over into the simple columnar type. Not infrequently ducts of small secondary glands empty into the main duct during its passage through the mucosa. According to the character of their secretions, the oral glands are divided into: {a) Mucous glands, which secrete a mucin-containing fluid (mucus) ; (b) Serous glands, which secrete a serous (albuminous) fluid; (c) Mixed glands, the secretion of which is partly mucous and partly serous. Morphologically, also, a similar distinction can be made in regard to the glandular epithelium which lines the terminal tubules, the tubules of mucous glands being lined with "mucous" cells, those of serous glands with "serous cells," while of the mixed glands the cells of some tubules are mucous, of others serous. In certain tubules both mucous and serous cells occur. The appearance which these cells present depends largely upon their secretory condition at the time of death. Serous cells when resting have a slightly granular protoplasm, which in the fresh condition is highly refractive, giving the cells a transparent appearance. With the beginning of secretion the granules increase in number and the cells become darker. Stained with hsematoxylin-eosin, serous tubules have a purplish color. The nuclei are spherical or oval, and are situated between the centre and base of the cell (Fig. 159, p. 245). Mucous cells are in the quiescent state rather small cuboidal or pyramidal cells, with cloudy cytoplasm and nuclei situated at the base of the cell. When active the mucous cells are much larger, with clear cytoplasm and with nuclei flattened against the basement membrane. The protoplasm of the fresh unstained mucous cell is less highly refractive than that of the serous cell. It consequently appears darker and less transparent. Mucous tubules are larger and more irregular in shape than serous tubules, and when stained with hsematoxylin-eosin THE DIGESTIVE SYSTEM. 195 either remain almost wliolly unstained or take a pale blue haematoxylin stain (Fig. 159, p. 245). Many mucous tubules have in addition to the mucous cells a peculiar, often crescentic-shaped group of cells on one side of the tubule, between the mucous cells and the basement mem- brane. These cells are granular and stain very much like serous cells with haematoxylin-eosin, thus resembhng the latter in appearance. On account of the shape of the groups, they are known as the crescents of Gianuzzi or demilunes of Heidenhain (Fig. 159, p. 245). The cells of the crescents are connected with the lumen by means of secretory canals, which pass between the mucous cells and end in branches within the protoplasm of the crescent cells. It is quite possible that some of the crescents are not serous cells but mucous cells in the non-active condition which have been pushed away from the lumen by the more active cells. Such cell groups are not connected with the lumen of the gland by intercellular secretory canals. Peculiar irregular branching cells have been described, extending from the basement membrane in between the mucous cells. They are known as "basket" cells and are supposed to be supportive in character. The cells of both mucous and serous tubules rest upon a membrana propria, outside of which, separating the tubules, is a cellular connect- ive-tissue stroma. Of the small glands of the mouth, a group near the root of the tongue are of the mucous variety, some "lingual" glands in the region of the circumvallate papillae are serous, while the remainder are of the mixed type. Blood-vessels. — The larger vessels run mainly in the submucosa. The arteries of the submucosa give off one group of branches to the tunica propria, where they break up into a dense subepithelial capillary network, sending capillary loops into the papillae. A second group of arterial branches pass to the submucosa, where they give rise to capillary networks among the tubules of the mucous glands. From the capillaries veins arise which accompany the arteries. Lymphatics. — The larger lymph vessels lie in the submucosa. These send smaller branches into the tunica propria, where they open into small lymph capillaries and spaces. Nerves. — Medullated nerve fibres form plexuses in the submucosa and deeper parts of the mucosa. From these plexuses, branches are given off which lose their medullary sheaths and form a second plexus of non-medullated filires just Ijeneath the epithelium. From 196 THE ORGANS. this subepithelial plexus, branches pass in between the epithelial cells to terminate in end brushes or in tactile corpuscles. The nerves belong to the cerebro-spinal system, and are dendrites of sensory ganglion cells. Axones of sympathetic neurones are also present in the oral mucosa, destined mainly for the muscle-tissue of the blood-vessels. TECHNIC. (i) The superficial cells of the oral mucous membrane may be prepared for examination as in technic i, page 57. (2) For the study of the mucous membrane of different parts of the mouth, fix small pieces in formalin-Miiller's fluid (technic 5, p. 7), cut sections perpendicular to the surface, stain with haematoxylin-eosin (technic i, p. 18), and mount in balsam. (3) Small mucous and serous glands of the mouth may be studied in the pre- ceding sections. The Tongue. The tongue is composed mainly of striated muscle fibres, supported by connective tissue and covered by a mucous membrane. While the Fungiform papillae Fig. 114. — Surface View of Tongue showing filiform papilla: and three fungiform papillae (Spalteholz). bundles of fibres interlace in all directions, three fairly distinct planes can be differentiated. (i) Vertical and somewhat radiating fibres — hyoglossus, genio- glossus, and vertical fibres of the lingualis. (2) Transverse fibres — transverse fibres of the lingualis. THE DIGESTIVE SYSTEM. 197 (3) Longitudinal j&bres — the styloglossus and longitudinal (superior and inferior) fibres of the lingualis. The connective tissue which supports the muscle fibres and sepa- rates them into bundles contains mucous glands and fat. A strong band of connective tissue, the septum lingucF, extends lengthwise through the middle of the tongue, dividing it into right and left halves. The suhmucosa of the tongue is not well developed, the stroma of the mucosa resting directly upon the underlying muscle. £'.-M5 V ,;>•:;/• ^}. Fig. 115. — ^Vertical Section through Two Filiform Papillte from Human Tongue. X 80. (Szymonowicz.) a, Horny epithelium; h, stroma; c, epithelium; d, secondary papilla. The mucous membrane of the tongue resembles that of the mouth, but differs from the latter in that in addition to the low papillae, such as are found in the oral mucosa, the upper surface of the tongue is studded with numerous and much larger papillae or villi. These pro- ject from the surface and give to the tongue its characteristic roughness. Three forms of papillae are distinguished. Filiform, fungiform, and circumvallate. (i) Filiform Papill.e (Fig. 115). — These are the most numerous and are distributed over the entire dorsum of the organ. Each con- sists of a central core of conncctixe tissue containing ehistic fibres, 198 THE ORGANS. which is long and slender, and is covered by stratified squamous epitheHum. From the summit of each papilla are given off several secondary papilla. The epithelium covering the papillae is hornified and often extends from the surface as a long thread-like projection — hence the name, filiform. (2) Fungiform Papilla (Fig. 116). — Scattered irregularly over the entire dorsum among the filiform papillae, but fewer in number, are larger papillae of somewhat different structure known as fungiform papillae. Their summits are rounded instead of pointed and their m- Fig. 116. — Vertical Section through Fungiform Papilla of Human Tongue. X 45. (Szy- monowicz.) a, Secondary papilla; b, epithelium; c, muscle fibres. bases are narrowed. Secondary papillae are given off not only from the summit, but from the sides of the papilla. The epithelial cov- ering is comparatively thin and is not hornified. The connective- tissue core of these papillae contains but few elastic fibres. (3) The Circumvallate Papilla (Fig. 117). — These are from nine to fifteen in number, and are grouped on the posterior surface of the dorsum of the tongue. They resemble the fungiform papillae, but are much larger. Each lies rather deep in the mucous membrane, surrounded by a groove or trench and wall (whence the name circum- vallate). The wall is somewhat lower than the papilla, thus allowing the latter to project slightly above the surface. Secondary papillae THE DIGESTIVE SYSTEM. 199 are confined to the upper surface of the papilla, the sides being free from secondary papillae. The surface of the papilla and the borders of the groove and wall are covered by stratified squamous epithelium. Lying in the epithelium of the side wall and sometimes of the opposite trench wall are oval bodies, the so-called taste buds, which serve as organs for the nerves of taste (see nervous system). Into the trench surrounding the circumvallate papilla open the ducts of serous glands (Ebner's glands). ar(\c^ Fig. 117. — ^\"ertical Section through a Circumvallate Papilla of Human Tongue. X 37. (Szymonowicz.) a, Secondary papilla; b, wall; c, trench; d, epithelium of tongue; e. stroma; /, submucosa; g, Ebner's glands. The lymph follicles of the tongue have been already described (page 157) under the head of the lingual tonsils. For glands of the tongue see page 195. The larger blood-vessels run in the connective-tissue septa. These give off smaller branches, which break up into capillary net- works surrounding the muscle fibres and forming a ple.xus just be- neath the epithelium. From the latter are given oft" capillaries to the papillae. The capillaries converge to form veins, which in general follow the course of the arteries. Fine lymph spaces occur in the papillae and open into a plexus of small lymph capillaries just beneath the papilla?. These communi- 200 THE ORGANS. cate with a deeper plexus of larger lymphatics, which increase in size and number as they pass backward and form an especially dense lymphatic network at the root of the tongue in the region of the lingual tonsils. Nerves. — Sympathetic fibres pass mainly to the smooth muscle of the blood-vessels and to the glands. Medullated motor nerve fibres supply the lingual muscles. Medullated sensory nerves in- clude those of the special sense of taste as well as those of ordi- nary sensation. They end freely among the epithelial cells or in connection with special end-organs — the taste buds mainly in the circumvallate papillae, and the end-bulbs of Krause in the fungiform papillae. TECHNIC. Remove pieces of the dorsum of ihe tongue, selecting parts that will include the dififerent forms of papillae and cutting well into the underlying muscular tissue. Treat as in technic 2, p. ig6, or sections may be stained with hsematoxylin-picro- acid-fuchsin (technic 3, p. 19). In sections from the back part of the tongue good examples of mucous and serous glands are usually found. In small sections of the tongue the muscle fibres are seen arranged in bundles, surrounded by connective tissue and interlacing in all directions. For the study of the arrangement of the different planes of muscle, complete transverse sections should be made at intervals through the entire tongue. The muscle and connective- tissue relations are best brought out by the haematoxylin-picro-acid-fuchsin stain. The Teeth. A tooth is a hard bone-like structure, part of which projects above the surface of the jaw as the crown, while the deeper portion, the root or fang, is buried in a socket of the alveolar margin (Fig. 118). The junction of the root and crown is known as the neck. A tooth consists of a soft central core, the pulp cavity, surrounded by dentine (Figs. 118 and 119). The latter constitutes the main bulk of the tooth. The exposed portion of the dentine is covered by a thin layer of extremely hard substance, the enamel (Fig. 118, i), while the alveolar portion of the dentine is covered with cementum (Fig. 118, 3). Of these the dentine and cementum are of connective-tissue origin, the enamel of epithelial. The pulp cavity occuyjies the central axis of the tooth (Figs. 118 and 119). In the root it is known as the root canal. At the apex of the THE DIGESTR'E SYSTEM. 201 root it communicates with the underlying tissue by means of a minute opening, through which blood-vessels and nerves enter the pulp cavity. The dentinal pulp consists of loose connective tissue of an embryonal type, composed of many fusiform and stellate cells and comparatively dehcate fibrils not joined to form bundles. The pulp is richly supplied with blood-vessels and nerves wdiich are found only in this part of the tooth. Along the dentinal surface of the pulp the connective tissue cells are arranged as a single layer of columnar cells, the odontoblasts. These cells are closely allied to osteoblasts. Their nuclei lie toward their inner ends. Each cell sends out an inner process, which is usually single and passes into the den- tinal pulp, several lateral processes which interlace with and probably anastomose with similar processes from other cells, and one or more outer fibre- like processes which enter the dentine, where they form the dentinal fibres. These frequently extend entirely through the dentine. Dentine (Figs. 119 and 120, D) is somewhat harder than bone which it resembles in structure. According to Fig. nS. von Bibra its chemical composition is: Organic matter, 28.01 Calcium phosphate and fluorid, 66. 72 Calcium carbonate, 3-36 Magnesium phosphate, i . 18 Other salts, o-73 Vertical Section of Tooth hi situ. X 15. (Waldeyer.) c, Pulp cavity, the letter being at about the junction of crown and root; i, enamel showing radial and longi- tudinal markings; 2, dentine showing dental canals; 3, cementum (con- taining bone corpuscles); 4, dental periosteum; 5, bone of lower jaw. Dentine constitutes the bulk of the tooth and is peculiar in that it contains canaliculi, dental canals (Figs. 119 and 120, Dk). but no lacunas or bone cells. The latter are represented by the odontoblasts of the pulp, which, as already noted, lie at the inner side of the dentine, into the canaliculi of which they send the dentinal fibres. Dentine is non-vascular. The dental canals begin at the dental pulp, where they have a calibre of 2 to 5/!. They pass outward radially, taking a somewhat cur\-ed course, to the limit of the dentine, and, while taking 202 THE ORGANS. different directions in different parts of the dentine, are essentially parallel. In a longitudinal section of the dentine of the crown, lines are seen running parallel to the surface of the tooth. They are known as the incremental lines of Schreger. They are apparently due to regularity of curvature in the dental tubules. In their passage through the dentine the main canals gradually grow smaller until their diameter K ' jt » * ^ ^ •^~ *•- ',* -•.'''' ::?ifi « * ■ ''" >X,^„i<"'" ,J' !''.'. ^ l»f-'v'^''Z .*' Fig. iig. — Cross Section through Root of Human Canine Tooth (X 25) (Sobotta), showing relations of pulp cavity, dentine, and cementum. P, Pulp cavity; D, dentine; C, cementum; A', Tomes' granular layer. is from 0.5 to 1/7. They give off minute side branches from 0.3 to 0.6/j! in diameter, which leave the main tubules at almost right angles, but soon turn slightly outward. They anastomose with similar branches from other canals. This anastomosis takes place not only between branches of adjacent canals, but also between branches of canals some distance apart. The main canals terminate either in blind extremities, or form loops by anastomosing with neighboring THE DIGESTR'E SYSTEM. 203 tubules. Some of the tubules have quite extensive terminal branchings, other tubules have only two or three end branches. A few tubules run slightly beyond the limits of the dentine into the enamel. They do not pass into the cementum. The arrangement of the dental canals and their branches differs in dift"erent parts of the tooth. In the crown there are few large branches and the main canals are quite straight, most of them ending blindly in brush-like branchings just under the enamel, but some continuing over into the enamel for from lo to 4.o/(, where they lie in the cement betw^een the prisms. In KH Dk Fig. 1 20. — From Longitudinal Section through Root of Human jSIolar Tooth ( X 200) (Sobotta), showing junction of dentine and cementum. C, Cementum; D, dentine; K, Tomes' granular layer; Dk, dental canals; KH, lacunae of cementum. the root the canals have more large branches and are more uneven. They do not pass over into the cementum, but end at the granular layer. The dentine immediately around a dental canal is more dense and hard than elsewhere and forms a sort of sheath for the canal — Neumann^ s dental sheath. Between the dental canals is a calcified ground sub- stance, in which are connective-tissue fibres running in the long a.xis of the tooth. Spaces which probably represent incomplete calcification of the dentine occur in the peripheral portion of the dentine of the crown. These are known as interglobular spaces (Fig. 121, Jg). They are filled with a substance resemblinc: uncalcified dentine. The interglobular 204 THE ORGANS. spaces do not interrupt the dental canals which pass through them with no break in their continuity. In the outer part of the dentine of the root are similar spaces which are smaller and more closely placed. These form the so-called Tomes' granular layer (Fig. 120, A'). In the root of the tooth this layer is quite thick, separating the cementum from the dentine. Its spaces or lacunae send off tiny canaliculi which run in all directions and anastomose with one another, with the dentinal tubules, and with the lacunae of the cementum. This layer, with its small closely placed spaces and fine irregularly running canaliculi, contrasts sharply on the one side (Fig. 120) with the dentine and its straight parallel tubules; and on the other side, with the cementum and its large and more widely separated lacunae. Over the crown of the tooth this layer is much thinner and as the apex is approached, becomes lost completely,, the dental tubules extending to, and some of them entering slightly, (see above) the enamel. The ENAMEL covers the exposed part or crown of the tooth and is the hardest substance in the body. It contains little more than a trace of organic substance, its chemical composition being, according to von Bibra : Organic matter, 3 . 59 Calcium phosphate and fluorid, 89.82 Calcium carbonate, 4-37 Magnesium phosphate, i-34 Other salts, 0.88 It consists of long six-sided prisms 3 to 6//. in diameter — enamel fibres or enamel prisms (Fig. 121, Sp) — which take a slightly wavy course through the entire thickness of the enamel. The prisms are at- tached to one another by a small amount of cement substance, and are grouped into bundles, the prisms of each bundle being parallel, but the bundles themselves frequently crossing one another at acute angles. In the human adult the prisms are homogeneous; in the embryo they show a longitudinal fibrillation. Rather indistinct parallel lines (the lines of Retzius) cross the enamel prisms. They probably represent the de- position in layers of the lime salts, although they are considered by some as artefacts. The enamel is covered by an apparently structure- less membrane, the culicula dentis. The CEMENTUM (Fig. 120, C) covers the dentine of the root in a manner similar to that in which the enamel covers the dentine of the crown (Fig. 118, i and 3). It forms a thin layer at the neck, but in- THE DIGESTR'E SYSTEM. 205 creases in thickness as the deeper part of the root is reached. Cemen- tum is bone tissue. It contains lacunce and boiie cells. From the lacunae radiate canaHcuH. but there is no distinct lamellation and no Haversian systems or blood-vessels, excepting in the large teeth of the larger mammalia, and in the teeth of the aged, where they may be present. Channels, similar to \'olkmann's canals in bone, not sur- rounded by concentric lamellae, but serving for the passage of blood- vessels, are quite frequent in the thicker portions of the cementum. Sr Dk ■k S Fig. 121. — From Longitudinal Section of Crown of Human Premolar (X 2co) (Sobotta), showing junction of enamel and dentine. 5, Enamel; Z?, dentine; 5/), enamel prisms; Dk, dental canals; Jg, interglobular spaces. A few dentinal fibres are seen passing beyond the limits of the dentine into the enamel. The oblique dark bands in the enamel are the lines of Retzius. The ground substance of the cementum is continuous with that of the dentine and many canaliculi of the former open into the interglob- ular spaces of the latter. Many uncalcified Sharpey's fibres penetrate the cementum. The union between the root of the tooth and the alveolar peri- osteum is accomplished by a reflection of the latter over the root, where it forms the dental periosteum, or peridental membrane (Fig. ii8, 4). This membrane consists of dense fibrous connecti\"e tissue. At the neck of the tooth it l)lcnds with the submucosa of the gum to form the so-called circular dcntoid lii^auu'iil. an annular band of dense 206 THE ORGANS. fibrous tissue, which extends around the tooth. The peridental mem- brane is formed of fibrillar connective tissue free from elastic fibres. Its fibres are directly continuous with Sharpey's fibres of the cementum. Blood-vessels of teeth are confined entirely to the pulp cavity. One or two small arteries reach the pulp cavity from the underlying connective tissue, through the foramen in the apex of the root. These break up into a capillary network in the dental pulp. Lymphatics have as yet not been demonstrated in the dental pulp. Nerves. — Medullated fibres accompany the blood-vessels through the apical canal. In the pulp they break up into a number of non-medul- lated branches, which form a plexus along the outer edge of the pulp, beneath the odontoblasts. From this plexus branches are given off which pass in between the odontoblasts, some terminating there, while others end between the odontoblasts and the dentine. Development. — The enamel of the teeth is of ectodermic origin, the remainder of mesodermic. The earliest indication of tooth forma- tion occurs about the seventh week of intra-uterine life (embryos 12 to 15 mm.). It consists in a dipping down of the epithelium cover- ing the edge of the jaw into the underlying connective tissue (mesoderm) where it forms the dental shelf, or common dental germ. Soon after the formation of the dental shelf, a groove appears along the margin of the jaw where the ingrowth of epithelium occurred. This is known as the dental groove. The epitheHum of the dental shelf is at first of uniform thickness. Soon, however, at intervals along the outer side of the den- tal shelf, the cells of the shelf undergo proliferation and form thicken- ings, ten in the upper and ten in the lower jaw, each one corresponding to the position of a future milk tooth. These are known as special dental germs, and remain for some time connected with one another and with the surface epithelium by means of the rest of the dental ridge. Into the side of each special dental germ there occurs about the end of the third month (embryos of about 40 mm.) an invagination of the underlying connective tissue. In the upper jaw the invagination takes place on the upper and inner side, in the lower jaw on the lower and inner side, of each dental germ. Each invagination forms a dental papilla (Fig. 124), over which the tissue of the special dental germ forms a sort of cap, the latter being known from its subsequent function as the enamel organ, the papilla itself giving rise to the pulp and dentine. The dental germs are at this stage connected with each other by remaining portions of the dental shelf, and with the surface epithelium by remains THE DIGESTIVE S^'STEM. 207 OMlSJSiLL '"»«: °o- Fig. 122. Fig. 12^. Fig. 124. Fig . 1 2 s . Figs. 122, 123, 124, 125. — Four Stages in the Development of a Tooth (from lower jaw of sheep embryo). (Bohm-DavidolT.) Fig. 122, Beginning of enamel organ showing con- nection with epithelium of mouth; Fig. 123, Later stage showing same with iirst trace of papilla; Fig. 124, Later stage showing papilla well formed, the ditlerentiation of the enamel pulp and of the inner and outer enamel cells can be seen; odontoblast appearing along periphery of the papilla; Fig. 125 shows also beginning enamel organ of permanent tooth; Figs. 122, 123, 124, X no, Fig. 125, X 40. a, Epithelium of mouth; b, its basal layer; c, superficial ceils of enamel organ; d, enamel pulp; p, dental papilla; s, enamel cells; c, odontoblasts; S, enamel organ of permanent tooth just beginning to differentiate; v, remains of enamel ledge of milk tooth; u, surrounding ronnecti\c tissue. 208 THE ORGANS. of the original invagination. The next step is the almost complete separation of the special dental germs and ridge from the surface epithe- lium (Fig. 125), and the formation around each special dei tal germ of a vascular membrane, the dental sac. The attenuated strand of epithe- lial cells, which still maintains a connection between the dental germs and the epithelium of the gums, is known as the neck of the enamel organ and it is from this that an extension soon occurs to the inner side of the Fig. 126.— Developing Tooth from Three-and-one-half-months' Human Embryo. X 65. (Szymonowicz.) a, Epithelium of gums; h, neck of enamel organ; c, dental germ of permanent tooth; d, bone of lower jaw; e, dental papilla;/, inner enamel cells; g,^snamel pulp; Ji, outer enamel cells. dental germs of the milk teeth, to form the dental germs of the perma- nent teeth (Fig. 126, c). Into the latter, connective-tissue papillae extend as in the case of the milk teeth. There are thus present as early as the fifth month of foetal existence the germs of all milk and of some perma- nent teeth. f'*^ The ENAMEL is formed by the enamel organ. At the stage represented in Fig. 128, it consists of three layers: (i) The outer enamel cells, somewhat flattened; (2) the inner enamel cells, high columnar epithelium; (3) a layer of enamel pulp, situated between the other THE DIGESTIVE SYSTEM. 209 layers, and consisting of stellate anastomosing cells with considerable intercellular substance (Figs. 128 and 129). A membrane, the cuticular membrane, is first laid down between the inner enamel cells and the dentine. Each of the inner enamel cells now sends out a process, Tomes'' process, from its inner end. The processes are separated by a considerable amount of cement, and are the beginnings of the enamel prisms. Calcification now takes place both in the prisms and in the Epithelium of mouth enamel cells Dental sac Bone of jaw- Blood-vessel Papilla Fig. 127. — Longitudinal section of a developing tooth of a new-born pupp_v (Bonnett). late stage. cement substance, beginning in the ends nearest the papilla. As this proceeds outward, the prisms become much elongated and the cement substance reduced in amount. Further growth in thickness of enamel occurs by lengthening of the enamel prisms. During the formation of the enamel, the enamel pulp and the external enamel cells disappear. The formation of enamel in the milk teeth begins about the end of the fourth month and continues until the eruption of the teeth. The extent of the enamel organ is considerably greater than that of the 14 210 THE ORGANS. enamel, the former covering the entire tooth, both root and crown, with the exception of the base of the papilla where the latter is connected with the underlying connective tissue. As the tooth develops, the enamel organ disappears over the root, remaining to form the enamel only over that part of the tooth which is to be subsequently exposed. The function of the enamel pulp is not known. It disappears as the tooth growls. It has been suggested that it may furnish nutrition or serve as an avenue throush which nutrition reaches the non-vascular Enamel Dentine I Enamel prisms Outer Inner J ■ enamel cells ^-i — Enamel pulp Cuticle \ of enamel cells Basal memb. J Fig. 128. — Section through border of a developing tooth of a new-born puppy. (Bonnett). enamel organ. It may serve as an area of least resistance through which the tooth grows. The DENTINE is the first of the dental tissues to become hard. Both dentine and pulp develop, as noted on p. 206, from the meso- derm of the dental papilla. When the latter is first formed it is of the same structure as the surrounding mesoderm with which it is contin- uous, except that it is somewhat more dense; later it assumes more the character of embryonal connective tissue, blood-vessels and nerves growing into it from the underlying connective tissue. The most periph- eral cells of the pulp, those lying nearest the enamel organ, differen- tiate from the rest of the pulp to form a single layer of columnar or THE DIGESTIVE SYSTEM. 211 pyramidal cells — the odontoblasts. The outer end of each cell is broad, while the inner end which contains the nucleus narrows to a point from which a delicate process extends into the pulp and probably anasto- moses with other cell processes. These cells are analogous to the osteo- blasts of developing bone and Hke them appear to determine the de- position of lime salts. The lime salts are first laid down in a mem- brane-like structure — the membrana preformativa — which the odonto- blasts apparently form between themselves and the enamel. From this membrane the transformation of the connective tissue into dentine progresses inward toward the pulp, additional dentine continuing to be laid down in layers, each new layer in- ternal to the preceding. In this way the dental papilla is reduced in size to form the pulp cavity. In the outer part of the dentine spaces remain in which no lime salts are deposited. These are the interglobular spaces. As calcification proceeds the odon- toblasts do not become enclosed within the dentine as do the osteo- blasts within bone. They leave merely long slender processes, the dental fibres, lying in minute chan- nels through the dentine, dental canals, while the bodies of the cells form a single layer along the inner margin of the dentine. There are thus no lacunae and no cells within the dentine. This relation of odontoblasts to dentine, and probably the original odontoblasts, per- sist, not only throughout embryonic but through adult life. While the tooth lies within the gum, the somewhat condensed con- nective tissue which surrounds it constitutes the dental sac. As the germs of the milk teeth develop, the dental shelf broadens by extending inward toward the tongue. Along this inner margin appear the germs of the permanent teeth, the de\-elopment of the various teeth structures from the germs Ijeing identical with the process de- scribed in the case of the milk teeth. Fig. 129. — From Cross-section through a Developing Tooth. X 720. (Bohm and von Davidoff.) Note close relationship between odontoblasts and tissue of dental pulp, a, Dental pulp; b, odontoblasts; c, dentine; d, inner enamel cells; e, enamel pulp. 212 ' THE ORGANS. Twenty of the permanent teeth correspond in position to the twenty milk teeth, while twelve new molar germs, six in the upper and six in the lower jaw, represent the true molars of the adult. The first perma- nent tooth germ to appear is that of the first molar, about the beginning of the fifth month (embryos of about i8o mm.). It lies just behind the second molar of the milk dentition. The germs of the incisors and canines appear about the end of the sixth month, those of the premolars, which replace the milk molars, in the beginning of the eighth month. The germs of the second and third (wisdom teeth) molars do not ap- pear until after birth, those of the former at about six months, of the latter during the fifth year. The CEMENTUM is developed by ossification of that part of the dental sac which covers the root, its development being similar to subperiosteal bone formation (p. 177), without the formation of Haversian systems. TECHNIC. (i) Teeth are extremely diificult organs from which to obtain satisfactory ma- terial for study. Sections of hard (undecalcified) and of decalcified teeth may be prepared in the same manner as sections of bone — technics i, p. 171; 2, p. 172. The decalcified tooth should include if possible the alveolar margin of the jaw, so that in longitudinal sections the mode of implantation and the relation of the tooth to the surrounding structures can be seen. (2) For the study of developing teeth, embryo pigs, sheep, cats, dogs, etc., are suitable. For the early stages foetal pigs should be five to six inches long; for the intermediate, ten to twelve inches. The later stages are best obtained from a small new-born animal, e.g., kitten or small pup. The jaw — preferably the lower — or pieces of the jaw are fixed in formalin-Miiller's fluid (technic 5, p. 7), hardened in alcohol, and decalcified (page 9). Subsequent treatment is the same as for de- veloping bone (technic i, p. 179). The Pharynx. The wall of the pharynx consists of three coats — mucous, muscular, and fibrous. I. The mucous membrane has a surface epithelium and an un- derlying stroma. The EPITHELIUM is stratified squamous except in the region of the posterior nares, where it is stratified columnar ciliated, continuous with the similar epithelium of the nasal mucosa. The STROMA, or tunica propria, consists of mixed fibrous and elastic tissue infiltrated with lymphoid cells. In certain regions these cells form distinct lymph nodules (see pharyngeal tonsils, page 157). THE DIGESTR'E SYSTEM. 213 Beneath the stratified squamous epitheHum the stroma is thrown up into numerous low papillcp. These are absent in regions covered by ciliated cells. Bounding the stroma externally is a strongly developed layer of longitudinal elastic fibres, the elastic limiting layer, which separates the stroma from the muscular coat and sends stout bands in between the muscle bundles of the latter. 2. The muscular coat lies beneath the elastic layer and is formed of very irregularly arranged muscle fibres belonging to the constrictor muscles of the pharynx. 3. The fibrous coat consists of a dense network of mixed fibrous and elastic tissue. It has no distinct external limit, and binds the pharynx to the surrounding structures. The distribution of blood-vessels, lymphatics, and nerves is similar to that in the oral mucosa. Small, branched, tubular, mucous glands are present in the stroma, and extend down into the intermuscular connective tissue. They are most numerous near the opening of the Eustachian tube. TECHNIC. For the study of the structure of the walls of the pharynx, material should be prepared as in technic 2, p. ig6. II. THE FOREGUT. The (Esophagus. The walls of the oesophagus are continuous with those of the pharynx and closely resemble the latter in structure. They consist of four layers, which from within outward are mucous, submucous, muscular, and fibrous (Fig. 130). 1. The mucous membrane resembles that of the pharynx except that beneath the stroma is a well-developed muscularis mucosce com- posed of smooth muscle cells arranged longitudinally. The muscularis mucosae forms a complete coat only in the lower part of the oesophagus. The epithelium is stratified squamous and rests upon a papillated stroma. 2. The submucosa is composed of loosely arranged fibrous and elastic tissue. It contains mucous glands, the larger blood-vessels, lymphatics, and nerves. 3. The Muscular Coat. — In tlic upper portion of the oesophagus this coat is composed of striated muscle fibres; in the middle portion. 214 THE ORGANS. of mixed striated and smooth muscle. In the lower portion there are two distinct layers of smooth muscle, an inner circular and an outer longitudinal. The latter is not continuous. 4. The fibrous coat consists of bundles of white fibrous tissue with many elastic fibres. It serves to connect the oesophagus with the surrounding structures. Two kinds of glands occur in the oesophagus. Fig. 130. — Transverse Section through Wall of Dog's CEsophagus. X 18. (Bohm and von Davidoff.) a, Epithelium; b, stroma; c, muscularis mucosae; d, submucosa; e, circular muscle layer;/, longitudinal muscle layer; g, fibrous layer. (i) Mucous Glands. — These are of the same structure as those of the tongue, but much smaller. They lie in the submucosa and are distributed throughout the entire oesophagus, though most numerous in its upper third. The ducts pass obliquely downward on their way to the surface. Just before entering the muscularis mucosae the duct widens out to form a sort of ampulla. Beyond this it again becomes narrow and enters the epithelium in the depression between two adjacent papillae. A small lymph nodule is usually attached to the duct as it passes through the tunica propria. (2) Simple Branched Tubular Glands. — These resemble the glands of the cardiac end of the stomach, but branch much more THE DIGESTIVE SYSTEM. 215 profusely. Some contain both chief and acid cells, others only chief cells (see stomach, page 219). They lie in the tunica propria, and are for the most part confined to a narrow zone at the lower end of the oesophagus and to the level of the fifth tracheal ring. Scattered groups also occur in other regions. The distribution of blood-vessels, lymphatics, and ner\'es in the oesophagus is similar to their distribution in the mouth (p. 195). Be- tween the two muscular coats the nerve fibres form plexuses in which are many sympathetic nerve cells. These plexuses are analogous to the plexuses of Meissner in the stomach and intestine. TECHNIC. Remove a portion of the wall of the oesophagus, wash carefully in normal salt solution, and pin out, mucous-membrane side up, on a piece of cork. Fix in formalin-Miiller's fluid and harden in alcohol (technic 5, p. 7). Transverse or longitudinal sections should be cut through the entire thickness of the wall. If the details of the muscular coat are to be studied, sections from at least three dif- ferent levels should be taken: one near the upper end, one at about the middle, and the other in the lower third. Stain with hasmatoxylin-eosin or haematoxylin-picro- acid-fuchsin (technic i, p. 18; 3, p. 19) and mount in balsam. General Structure of the Walls of the Gastro-intestinal Canal. The walls of the stomach and intestines are made up of four coats (Fig. 131). These from the lumen outward are mucous, submucous, muscular, and serous. I. The mucous membrane (Fig. 131) consists of surface epithe- lium, glands, stroma, and muscularis mucosae. The surface epithelium is simple columnar and rests upon a distinct basement membrane. The arrangement of the glands and the nature of the gland cells differ in different parts of the tract. The stroma is a loosely arranged, richly cellular connective tissue, which in many places is so infiltrated with lymphoid cells as to constitute dilTuse lymphatic tissue. In other places it contains circumscribed masses of lymphatic tissue, lymph nodules. The amount of stroma depends upon the closeness with which the glands are packed. The muscularis mucoscc consists of smooth muscle cells, which have a generally longitudinal arrangement. Where, however, the muscularis mucosae is thick there are usually two distinct layers — an inner circular and an outer longitudinal. Folds of con- siderable extent occur in the mucous membrane. Those of the 216 THE ORGANS. Stomach are known as rugcE, and are not constant, depending upon the degree of distention of the organ. Those of the small intestine are much more definite, and are known as valvules conniventes. 2. The submucosa (Fig. 131) is a loose connective-tissue structure. It contains the larger blood-vessels, lymphatics, and nerves. Fig. 131. — Diagram of Structure of Wall of Gastro-intestinal Canal. A, Mucous membrane; a, glands; b, epithelium; c, goblet cells; d, stroma; e, inner circular;/, outer longitudinal layers of g, muscularis mucosae. B, Submucosa. C, Muscular coat; h, its inner circular layer; ], its outer longitudinal layer; i, intermuscular connective-tissue septum. D, serous coat; k, its connective-tissue layer; /, its endothelial layer. 3. The muscular coat (Fig. 131) consists of two layers of smooth muscle, which in the intestine are sharply differentiated into an inner circular and an outer longitudinal. In the stomach the direction of the layers of the muscular coat is less definite. A narrow layer of connect- ive tissue separates the two layers of muscle. From this, septa extend into the muscle tissue, separating it into bundles. THE DIGESTIVE SYSTEM. 217 4. The serous coat (Fig. 131) is the visceral layer of the peritoneum. It consists of a thin layer of connective tissue covered by a single layer of mesothelium. Along the attachment of the mesentery the serous coat is wanting. The subdivisions of the gastro-intestinal canal differ from one another mainly in regard to the structure of their mucous membranes, and especially in regard to the structure of the glands of the mucous membrane and submucosa. »■ cd Fig. 132. — Section through Junction of (Esophagus and Stomach of Man. X 121 (Schafer.) Oe, (Esophagus; M, stomach; cd, cardiac glands; uylorus. Here the inner and middle layers are thickened to form the sphincter pylori. In the connective tissue which separates the groups of muscle cells are collections of THE DIGESTIVE SYSTEM. 223 sympathetic nerve cells and fibres which while much less distinct are analogous to Auerbach's plexus of the intestine. 4. The serous coat consists of a layer of loosely arranged con- nective tissue covered by a single layer of mesothelium. TECHNIC. (i) Remove a human stomach (not more than two or three hours after death) or that of a recently killed dog. Open along the lesser curvature, and carefully re- move the excess of mucus by washing with normal saline. Cut pieces through the entire thickness of the wall, one from the fundus and one from the pylorus; pin out, mucus membrane side up, on pieces of cork, fix in formalin-Miiller's fluid (technic 5, p. 7) or in Zenker's fluid (technic 9, p. 8), and harden in alcohol. Sec- tions are cut as thin as possible, care being taken that the plane is such that the glands are cut longitudinally, stained with haematoxylin-eosin (technic i, p. 18), and mounted in balsam. (2) Instead of removing pieces of stomach and pinning them out on cork, as suggested in the preceding technic, the entire stoma(^ may be filled with the fixa- tive, the ends being tied, and then placed in a large quantity of the fixing fluid. After fixation, pieces are removed and hardened in graded alcohols. If this method is used, great care must be taken not to overdistend the organ, only very moderate distention being desirable. Further treatment is the same as in the pre- ceding technic (i). (3) For comparison of resting with active gastric cells, preparations should be made from the stomach of an animal that has been for from twenty-four to forty- eight hours without food, and from a stomach during active digestion. Fix in Zenker's fluid as in technic (i), above. Examine unstained sections and sections stained with haematoxylin-eosin. (4) Sections through the junction of oesophagus and stomach and through the junction of stomach and duodenum furnish instructive pictures. They should be prepared as in technic (i). (5) For the study of the distribution of the blood-vessels sections of an injected stomach should be made. This is best accompUshed by selecting a small animal, such as a rat or guinea-pig, and injecting in tola through the ascending aorta, or by injecting only the hind part of the animal through the abdominal aorta. Tech- nic, p. 22. III. THE MIDGUT. The Small Intestine. On passing from stomach to small intestine the rugcT of the former disappear, but are replaced by much more definite foldings of the mucosa, the valvidcc coiuiivcntcs (Fig. 140). These folds involve the entire thickness of the mucous membrane and part of the submucosa. They are in general parallel to one another, and pass in a circular or 224 THE ORGANS. oblique manner, partly around the lumen of the gut. The entire surface of the intestine, including the valvulae, is studded with minute projections just visible to the naked eye, and known as villi (Figs. 141 and 142). These involve only the epithelium and stroma, although they also contain some muscular elements derived from the muscularis mucosae. The villi differ in shape in the different parts of the small intestine, being leaf-shaped in the duodenum, rounded in the jejunum, club-shaped in the ileum. The valvules conniventes and the villi are characteristic of the small intestine. It is important to note that Fig. 139. — Section through Junction of Pylorus and Duodenum. (Klein.) v, Villi of duodenum; d, stomach, showing gastric pits; b, apex of a solitary lymph nodule; c, crypt of Lieberklihn; s, secreting tubules of Brunner's glands; g, pyloric glands; t, tubules of B runner's glands in submucosa of stomach; m, muscularis mucosa. while the pits of the stomach are depressions in the mucous membrane, the intestinal villi are definite projections above its general surface (Fig. 139). The wall of the intestine consists of the same four coats described as constituting the wall of the stomach, mucosa, submucosa, muscularis, and serosa. i. The mucosa, as in the stomach, is composed of a lining epithe- lium, stroma, f^lands, and muscularis mucosa'. Of these the epithelium, the stroma, and cells from the muscularis mucosae are concerned in the formation of the villi. The VILLUS consists of a central core — a fold of the stroma — of THE DIGESTIVE SYSTEM. 225 mixed fibrous and reticular tissue infiltrated with lymphoid cells, and of a covering epithelium. The epithelium is of the simple columnar type. The cells are high and have thickened striated free borders (Figs. 143 and 144). These contiguous thickened free borders unite to form a distinct membrane, the cuticular membrane (Fig. 144, c). Scattered among the columnar cells are numerous mucous or goblet cells (Figs. 143 and 144, b). The goblet cells are derived from the columnar cells, and Fig. 140.— Vertical Longitudinal Section of Human Jejunum (X i6) (Stohr), includ- ing two valvulas conniventes. a, Villi, in many of which the stroma has shrunken away from the epithelium leaving a clear space, X X . Lying free in the lumen of the gut are seen sections of villi cut in various directions, b. Epithelium; c, stroma; d, crj-pts of Lieberktihn; X, solitary lymph nodule with germinal centre; e, tissue of subriiucosa forming centre of one of the valvule conniventes;/, submucosa; g, inner circular layer of muscle; //, outer longitudinal layer of muscle; /, Auerbach's ple.xus; j, serous coat. vary in appearance according to the amount of secretion which thev contain. A cell at the beginning of secretion contains onlv a small amount of mucus near its free border. As secretion increases the mucus gradually replaces the cytoplasm until the latter is represented only by a crescentic mass containing a flattened nucleus and pressed against the basement membrane. The cell now discharges its mucus upon the free surface. The goblet cells possess no thickened border, appearing, when seen from the surface, as openings surrounded on all 226 THE ORGANS. sides by the cuticulse of the adjacent columnar cells. Small spherical cells with deeply staining nuclei are found in varying numbers among the epithelial cells. These are so-called wandering cells, migratory Mouths of crypts. Lymph nodules Villi. Fig. 141. — Surface View of Small Intestine near upper end, showing villi and one solitary lymph nodule. X 12. (Spalteholz). / - .j-m^'---"'' Fig. 142. — Vertical Section through Mucous Membrane of Human Jejunum. X 80. (Stohr.) a and b, Artifacts due to shrinkage; c, intestinal crypts (Lieberklihn); d, oblique and transverse sections of crypts; e, stroma;/, e])ithe]ium; ,^, tangenlialiy cut villi; h, muscuiaris mucosa;; i, submucosa. THE DICiESTR'E SYSTEM. 227 leucocytes, from the underlying stroma (Figs. 143, /z, and 144, /). Other cells with dark-staining nuclei, ''replacing cells,'' are found be- tween the bases of the columnar cells (pages 65 and 220J. In addition to the connective-tissue and lymphoid cells, which constitute the main bulk of the villus core (Figs. 143 and 144), isolated smooth muscle cells derived from the muscularis mucosae occur, run- Fig. 143. Fig. 144. Fig. 143. — Longitudinal Section of Villus from Small Intestine of Dog. (Piersol.) a, Columnar epithelium; b, goblet cells; h, leucocytes; c, basement membrane; d, core of villus; e, blood-vessels; /, lacteal. Fig. 144. — Cross-section of a Villus of Human Small Intestine. X 530. (Kolliker.) The stroma of the villus has shrunken away from the epithelium, b, Goblet cell; c, cuticula showing striations; e, columnar epithelial cell; gm, basement membrane with nuclei; /, leucocyte in epithelium; /', leucocyte just beneath epithelium; «2, large leucocyte in stroma; ch, central chyle vessel; g, blood-vessel. ning in the long axis of the villus. A single lymph or chyle vessel (Fig. 143, /,• 144, ch) with distinct endothelial walls traverses the centre of each villus, ending at its tip in a slightly dilated blind extremity. As it is usually seen collapsed, it appears as two closely approximated rows of flat cells with bulging nuclei. The capillaries of the villus lie for the most part away from the chyle \-essel, just beneath the basement membrane (Fig. 143, e; 144, "'). From the depths of the depressions between the villi. sim])k' tulnilar 228 THE ORGANS. :l Q '■^ . glands — glands or crypts of Lieberkiihn (Figs. 142 and 145) — extend down through the stroma as far as the muscularis mucosas. These crypts are lined with an epithelium similar to and continuous with that covering the \i\\\. The cells are, however, lower, and there are fewer goblet cells. In addition to these cells there are also found in the depths of the crypts of Lieberkiihn peculiar coarsely granular cells, the cells of Paneth (Fig. 145, k). They are found in man and in rodents, but do not occur in the carnivora. They probably pro- duce a specific secretion, the nature of which is unknown. The stroma, besides forming the cen- tres of the villi, fills in the spaces between the crypts of Lieberkiihn and between the latter and the muscularis mucosas. In places the lymphoid cells are closely packed to form distinct nodules or "solitary follicles," such as are found in the stomach (see page 221). Peyer's Patches (agminated folh- cles) (Fig. 146). — These are groups of lymph nodules found mainly in the ileum, especially near its junction with the jeju- num. They always occur on the side of the gut opposite to the attachment of the mesentery. Each patch consists of from ten to seventy nodules, so arranged that the entire patch has a generally oval shape, its long diameter lying lengthwise of the intestine. The nodules of which a patch is composed lie side by side. Their apices are directed toward the lumen and project almost through the mucosa, being uncovered by villi, a single layer of columnar epithelium alone separating their surfaces from the lumen of the gut. The bases of the nodules arc not confined to the stroma, but usually spread out in the submucosa. The relation of the patch to the stroma and sub- mucosa can be best appreciated by following the course of the mus- cularis mucos£e. This is seen to stop abruptly at the circumference of the patch, appearing throughout the patch as isolated groups of smooth muscle cells. The nodules rarely remain distinct, but are confluent Fig. 145. — Longitudinal Section of Fundus of Crypt of Lieberkiihn. X530. (Kollilier.) 5, Goblet cell showing mitosis; e, epithelial cell; k, cell of Paneth; /, leucocyte in epithelium; m, mitosis in epithelial cell. Surrounding the crypt is seen the stroma of the mucous membrane. THE DIGESTR'E SYSTEM. 229 Fig. 146. — Transverse Section of Cat's Small Intestine through a Peyer's Patch. (Stohr.) a, Villi; b, crypts; c, longitudinal muscle layer; d, circular muscle layer; e, lymph nodules;/, muscularis mucoste; g, submucosa. Solitary lymph nodules Peyer's patch Fig. 147. — Surface View of Mucous Membrane of Small Intestine (Ileum"), showing Peyer's patch (Spalteholz). ?:30 THE ORGANS. with the exception of their apices and bases. It should be noted that both sohtary nodules and Peyer's patches are structures of the mucosa, and that their presence in the submucosa is secondary. The muscularis mucosae (Figs. 142 and 148) consists of an inner circular and an outer longitudinal layer of smooth muscle. 2. The submucosa (Figs. 140, 142, 148) consists, as in the stomach, of loosely arranged connective tissue and contains the larger blood-vessels. It is free from glands except in the duodenum, where it contains the glands of Brunner (Fig. 148). These are branched tubular glands lined with a granular columnar epithelium similar to that of the pyloric glands. The ducts are also lined with simple columnar epithelium. They pass through the muscularis mucosae and empty either into a crypt of Lieberkiihn or on the surface between the villi. Brunner's glands frequently occur in the pylorus, and it is not uncom- mon for the pyloric glands to extend downward somewhat into the duodenum. Meissner^s plexus of nerve fibres, mingled with groups of sympathetic ganglion cells, lies in the submucosa (see page 238). 3. The muscular coat (Figs. 140 and 148) consists of two well- defined layers of smooth muscle, an inner circular and an outer longi- tudinal. Connective-tissue septa divide the muscle cells into groups or bundles, while between the two layers of muscle is a connective- tissue septum which varies greatly in thickness at different places and contains a plexus of nerve fibres and sympathetic ganglion cells known as the plexus of Auerbach (see page 238). 4. The serous coat consists as in the stomach of loose connective tissue covered by a single layer of mesothelium. ' •■•■••'■-••■•"••■-.^••j;_ Fig. 148. — From Vertical Longitudinal Section of Cat's Duodenum to show Brunner's Glands. (Larrabee.) a, Villus; h, epithe- lium; c, stroma; d, glands; e, muscularis mucosae; /, Brunner's glands; ,§•, submucosa; h, circular muscle layer. THE DIGESTR'E SYSTEM. 231 IV. THE ENDGUT. The Large Intestine. The wall of the large intestine consists of the same four coats which have been described as constituting the walls of the stomach and small intestine, mucous, submucous, muscular, and serous. Fig. 149. Fig. 150. Fig. i4g. — From Vertical Longitudinal Section of Cat's Large Intestine. (Larrabee.) a, Epithelium; h, stroma; c, fundus of gland; d, muscuiaris mucosa^; e, submucosa;/, cir- cular muscle layer; g, longitudinal muscle layer; h, serous coat; /, Auerbach's plexus. Fig. 150. — From Vertical Longitudinal Section of the IMucous Membrane of the Human Large Litestine. (Technic i, p. 241.) a, Mucous (goblet) cells; h. fundus of a gland cut obli(|uely; c, muscuiaris mucosa;; d, lumen of a gland cut longitudinally; e, stroma between the glands;/, leucocytes in the epithelium; g, stroma between fundi of glands and muscuiaris mucosa;. I. The mucous membrane has a comparatively smooth surface, there being neither pits as in the stomach nor villi as in the small intestine (Fig. 149). The glands are of the simple tubular variety, 232 THE ORGANS. are considerably longer than those of the small intestine, are almost straight, and extend through the entire thickness of the stroma. Owdng to the closeness with which the gland tubules are packed, the amount of stroma is usually small. The surface cells (Fig. 149, a) are very high and narrow, with small, deeply placed nuclei, and are not usually intermingled with goblet cells. Passing from the surface down into the glands, the cells become somewhat lower and goblet cells become numerous (Fig. 150, a and d). Both superficial and deep cells rest upon a basement membrane similar to that in the small intestine. The stroma also, though less in amount, is similar in struc- ture to the stroma of the small intestine. The MuscuLARis MUCOS.5: (Fig. 150, c) consists of an inner circular and an outer longitudinal layer of smooth muscle. 2. The submucosa (Fig. 149, e) consists of loosely arranged con- nective tissue. It contains large blood-vessels and the nerve plexus of Meissner (see page 238). Solitary lymph follicles occur throughout the mucous membrane of the large intestine. While properly con- sidered as structures of the stroma from which they originate, the follicles lie mainly in the submucosa. (For details of structure see page 148.) 3. Of the muscularis (Fig. 149) the inner circular layer only is complete, the muscle tissue of the external longitudinal coat being arranged mainly as three strong, flat, longitudinal bands, the lineae coli. Between these bands the longitudinal muscular coat is either very thin or entirely absent. In the connective tissue, lying to the outer side of the circular muscle coat, is the nerve plexus of Auerbach. (For details see page 238.) 4. The serous coat consists, as in the stomach and small intestine, of loose connecti\'e tissue covered by a single layer of mesothelium. The Vermiform Appendix. The vermiform appendix is a diverticulum from the large intestine. Its walls arc continuous with those of the latter, and closely resemble them in general structure. There are the same four coats, mucous, submucous, muscular, and serous. T. The mucous membrane (Fig. 151) consists of epithelium, glands, stroma, and muscularis mucosae. The epithelium resembles that of the large intestine. The glands vary in number, but are usually much less closely packed than in the large intestine. They are most THE DIGESTWE SYSTEM. 233 numerous in the appendices of infants and children. The gland tubules (Fig. 151, /') are usually rudimentary, but in most cases have the same structure as the intestinal glands, and are evidently functional as they contain mucous cells in all stages of secretion. In consequence of the wider separation of the tubules the stroma is more abundant than in the large intestine, but has the same structure. The muscularis K^ ^ ^ ^%\ i ^v Fig. 151. — Transverse Section of Human \'ermiform Appendi.x. (Teclinic 2, p. 241.) a, Mesoappendix; h, serous membrane (serosa); f, outer longitudinal muscle layer; d, inner circular muscle layer; e, submucosa;/, groups of fat cells in submucosa; g, blood-vessels in submucosa; h, lymph nodules; /, stroma; ], glands opening into lumen and cut in various planes; muscularis mucosae not present. mucosa is usually fairly distinct as a thin circularly disposed band of smooth muscle cells just beneath the stroma. It is not always present. In some cases the mucosa as such is practically absent, being replaced by fibrous tissue. This condition is especially common after middle age, and may or may not be associated with obliteration of the lumen. 2. The submucosa (Fig. 151, e) is similar to that of the intestine. 234 THE ORGANS. 3. The muscular coat varies greatly, both as to thickness and as to the amount of admixture of fibrous tissue. The inner circular layer (Fig. 151, d) is usually thick and well developed. The outer longitudinal layer (Fig. 151, c) differs from that of the large intestine in having no arrangement into lineas, the muscle tissue forming a con- tinuous layer. Less commonly a more or less marked tendency to an arrangement of the cells of the longitudinal coat into bundles, between which the outer coat is thin or wanting, is observed. 4. The serosa has the usual structure of peritoneum. The lymph nodules (Fig. 151, h) constitute the most conspicuous feature of the appendix. They lie mainly in the submucosa. In children and young adults the nodules are oval or spherical; in later life somewhat flattened. The nodules may be entirely distinct, or may be arranged as in a Peyer's patch with distinct apices and bases, but with their central portions confluent. The muscularis mucosae either passes through the superficial portions of the nodules, or, where they are separated from the lumen, passes over them. The distribution of blood-vessels, lymphatics, and nerves is similar to that in the large intestine. The Rectiun. 1. The mucous membrane of the rectum has a structure similar to that of the large intestine. The glands are longer and the mucosa is consequently somewhat thicker. In the lower part of the rectum definite longitudinal foldings of the mucosa occur, the so-called col- umnce rectales. A change in the character of the mucous membrane begins at the upper end of the columnse rectales. Here the simple columnar epithelium of the gut passes over into a stratified squamous epithelium, beneath which is a papillated stroma. The glands con- tinue for a short distance beyond the change in the epithelium, but soon completely disappear. At the anus there is a transition from mucous membrane to skin similar to that described as occurring at the margin of the lips (page 193). 2. The submucosa is similar in structure to that of the large intestine. The muscularis of the rectum differs from that of the large in- testine in that the longitudinal layer is continuous and thick. The serous coat is absent in the lower part of the rectum, being replaced l;y a fibrous connective-tissue layer, which connects the rec- tum with the surrounding structures. THE DIGESTIVE SYSTEM. 235 The Peritoneum, Mesentery, and Omentum. The peritoneum (see also p. 144) is a serous membrane which Hnes the walls of the abdomen (parietal peritoneum) and is reflected over the contained viscera (visceral peritoneum). It consists of two layers, a connective-tissue stroma and mesothelium. The stroma consists of loosely arranged connective-tissue bundles, which interlace in a plane parallel to the surface. There are numerous elastic fibres, espe- cially in the deeper layer of the parietal peritoneum. There are comparatively few connective-tissue cells. The mesothelium consists of a single layer of flat polygonal cells with bulging nuclei. The cells have irregular wavy outlines, which are easily demonstrated with silver nitrate (Fig. 26). The shape of the cells varies considerably accord- ing to the direction in which the tissues are stretched. Over some parts, e.g., the liver and intestine, the peritoneum or serosa is thin and very closely attached. In places where the peritoneum is freely movable, a considerable amount of loose connective tissue, rich in elastic fibres and containing varying numbers of fat cells, connects the peritoneum with the underlying tissue. This is known as the "subserous tissue.''' The peritoneum is well supplied with blood-vessels and lymphatics. The former give rise to a rich capillary network. The mesentery is a sheet of loosely arranged connective tissue cov- ered with peritoneum. It is reflected from the post-abdominal wall to the viscera, and serves to carry to these organs their blood-vessels, lymphatics, and nerves. In the case of the stomach, duodenum, and large intestine, the mesentery is comparatively short, and the organs are therefore ciuite firmly fixed to the abdominal wall. In the case of the small intestine the mesentery is long and the intes- tine, therefore, freely movable. The mesentery is richly supplied with lymph nodes and there is usually a considerable amount of fat. From the mesentery, the peritoneum passes over to and envelops the viscera. The omentum (Fig. 26) is a sheet of tissue which passes from the liver to the lesser curvature of the stomach (gastro-hepatic omentum) to which it is attached, and from the greater curvature of the stomach to the transverse colon (greater omentum). It is similar in structure to the mesentery, and contains usually much fat and many lymph nodes. Its connective-tissue Ijundles are arranged in networks, the strand and meshes of which vary greatly in size and sha])c. The strands are covered by a single layer of mesothelium. 236 THE ORGx\NS. ElOOD- VESSELS OF THE StOMACH AND INTESTINES. The arteries reach the gastro-intestinal canal through the mesen- tery and pass through the muscular coats to the submucosa, where they form an extensive plexus of large vessels (Heller's plexus) (Fig. 152, c). Within the muscular coats the main arteries give off small branches to the muscle tissue. From the plexus of the submucosa two main sets of vessels arise, one passing outward to supply the mus- FiG. 152. — Scheme of B load-vessels and Lymphatics of Stomach. X 70. (Szy- monowicz, after Mall.) a, Mucous membrane; h, muscularis mucosae; c, submucosa; d, inner circular muscle layer; e, outer longitudinal muscle layer; A, blood-vessels; B, structure of coats; C, lymphatics. cular coats, the other inward to supply the mucous membrane (Fig. 152). Of the former the larger vessels pass directly to the intermuscu- lar septum, where they form a plexus from which branches are given off to the two muscular tunics. A few small branches from the larger recurrent vessels also supply the inner muscular layer. Of the branches of the submucosa plexus which pass to the mucous membrane, the shorter sui>j>ly the muscularis mucostfi, while the longer branches pierce the latter to form a capillary plexus among the glands of the stroma. From the capillaries small veins take origin which pierce the muscularis THE DIGESTR'E SYSTEM. 237 mucosae and form a close-meshed venous plexus in the submucosa (Fig. 152). These in turn give rise to larger veins, which accompany the arteries into the mesentery. In the small intestine the distribution of the blood-vessels is modified by the presence of the vilH (Fig. 153). Each villus receives one small artery, or, in the case of the larger villi, two or three small arteries. - A i i Fig. 153. — Scheme of Blood-vessels and Lymphatics of Human Small Intestine. (From Bohm and von Davidoff, after Mall.) a, Central lacteal of villus; &, lacteal: c, stroma; d, muscularis mucosae; e, submucosa;/, plexus of lymph vessels; ^, circular muscle layer; h, plexus of lymph vessels; /, longitudinal muscle layer; ], serous coat; k, vein; /, artery; m, base of villus; «, crypt; o, arten,- of villus; />, vein of villus; q, epithelium. The artery passes through the long axis of the villus close under the epithelium to its summit, giving off a network of fine capillaries, which for the most part lie just beneath the epithelium. From these, one or two small veins arise which lie on the oposite side of the villus from the artery. Lymphatics of the Stom.a.ch .\nd Intestine. Small lymph or chyle capillaries begin as blind canals in the stroma of the mucous membrane among the tulnilar glands (Fig. 152). 238 THE ORGANS. In the small intestine a lymph (chyle) capillary occupies the centre of the long axis of each villus, ending in a blind extremity beneath the epithelium of its summit (Fig. 153). These vessels unite to form a narrow-meshed plexus of lymph capillaries in the deeper part of the stroma, lying parallel to the muscularis mucosas. V'essels from this plexus pass through the muscularis mucosae and form a wider meshed plexus of larger lymph vessels in the submucosa. A third lymphatic plexus lies in the connective tissue which separates the two layers of muscle. From the plexus in the submucosa, branches pass through the inner muscular layer, receive vessels from the intermuscular plexus, and then pierce the outer muscular layer to pass into the mesentery in company with the arteries and veins. Nerves of the Stomach and Intestines. The nerves which supply the stomach and intestines are mainly non-medullated sympathetic fibres. They reach the intestinal walls through the mesentery. In the connective tissue between the two layers of muscle, these fibres are associated with groups of sympathetic ganglion cells to form the plexus myentericus or plexus of Auerbach. The dendrites of the ganglion cells inter- lace, forming a large part of the plexus. The axones are grouped together in small bundles of non-med- uUated fibres, which pass into the muscular coats, where they form intricate plexuses, from which are given off club-shaped termi- nals to the smooth muscle cells. From Auerbach's plexus fibres pass to the submucosa, where they form a similar but finer-meshed, more delicate plexus, also associated with groups of sympathetic gan- glion cells, the plexus of Meissner. Both fibres and cells are smaller than those of Auerbach's plexus. From Meissner's plexus delicate fibrils pass to their terminations in submucosa, muscularis mucosae, and mucous membrane. Fig. 154. — Section through Glands of Fundus of Hunaan Stomach in Condition of Hunger. X 500. (Bohm and von Davidoff.) a, Stroma; b, parietal cell; c, lumen; d, chief cell. the digestive system. 239 Secretion and Absorption. The secretory activities of epithelial cells have already been men- tioned (page i86). The epithelium of the gastro-intestina! tract must be considered as ha^•ing two main functions: (i) The secretion of substances necessary to digestion; and (2) the absorption of the products of digestion. I (i) Secretion. — The production of mucus takes place in the mucous or goblet cell, which, as already mentioned, probably represents a differentiation of the ordinary columnar epithelial cell. The chief i^^'i :m/'} ""^V, /^ , / »§« Fig. 155. — Section through Glands of Fundus of Human Stomach during Digestion X 500. (Bohm and von Davidoff.) a, Lumen; b, stroma; c, chief cell; d, parietal cell cells, "peptic cells," of the stomach glands are large and clear during fasting, become granular and cloudy with the onset of digestion, and smaller with loss of granules during the digestive process. As activity of the chief cells (Fig. 155) is coincident with an increase in the pepsin found in the gastric mucosa, it is probable that these cells pro- duce pepsin, and that the granules represent some stage in the elabor- ation of the ferment. As their name of "acid cells" would indicate, the parietal cells were considered the source of the liydrocliloric acid of the stomach. While doubt still exists as to the function of these cells, recent investigations make it probable that it is not the secretion of hydrochloric acid. The cells of Brunner's glands undergo changes during digestion, which are quite similar to those described as occurring in the chief cells of the stomach glands, and are probablv also con- 240 THE ORGANS. cerned in the production of pepsin. The only function of the intestinal crypts which has yet been determined is the secretion of mucus. The possibility that certain cells of the crypts of the small intestine produce a specific secretion has been mentioned (page 228). A .1-^-^^^^Q.J O CD { O '^0 -^ 1 ; O O " r I ^ O 0 >'--- O ^ ^^ ^^ o o :--- ^ O pi.' u Fig. 156. — Fat Absorption. Longitudinal section of villus of cat's small intestine, three hours after feeding. X 350. Osmic acid, a, Fat droplets in epithelial cells; b, fat droplets in leucocytes in stroma; c, fat droplets in leucocytes within lacteal; d, fat droplets free in lacteal; e, capillary containing blood cells;/, central lacteal of villus. (2) Absorption of Fat. — While various other products of diges- tion are aljsorbcd l)y the intestine, the absorption of fat is the one most easily observed. After feeding fat, fatty acids, or soaps, fat globules are found to have penetrated the intestinal mucosa, and may be seen in (a) the epithelial cells, (b) the leucocytes, and (c) the lacteals of the villi (Fig. 156). Fat globules are never seen in the thickened free borders of the cells. Hence it seems probable that the fat before passing through THE DIGESTR'E SYSTEM. 241 this part of the cell becomes split up into glycerin and fatty acids which are united again to form fat within the protoplasm of the cell. Leuco- cytes containing fat globules are seen throughout the stroma. Within the lacteals are found fat-containing leucocytes and free fat droplets of various size. It would thus seem probable that the process of fat absorption consists in: (i) The passage of glycerin and fatty acids through the cell borders; (2) their reunion in the cell to form fat; (3) the transference of these, fat globules to leucocytes; which (4) carry them to the lacteals. In the lacteals the fat is probably set free by disinte- gration of the leucocytes. TECHNIC. (i) The technic for the small and large intestines and rectum is the same as for the stomach. Accurate fixation of the vilH is difficult, there being usually some shrinkage of the connective tissue of the core away from the epithelium. A longitudinal section should be made through the junction of small and large intestine, showing the transition from the villus-covered surface of the former to the comparatively smooth surface of the latter. To show Brunner's glands a section of the duodenum is required. To show the varying shapes of the villi in the different regions, sections should also be made of the jejunum and ileum. Solitary follicles may usually be seen in any of the above sections. A small Peyer's patch, together with the entire thickness of the intestinal wall, should be removed, treated as above, stained with haematoxylin-eosin (technic i, p. 18), or with hasmatoxylin-picro-acid-fuchsin (technic 3, p. 19), and mounted in balsam. (2) A vermiform appendix, as fresh as possible, should be cut transversely into small pieces, fixed in formalin-AIuller's fluid (technic 5, p. 7), and hardened in alcohol. Thin transverse sections are made through the entire wall, stained with haematoxylin-eosin or hasmatoxylin-picro-acid-fuchsin, and mounted in balsam. (3) Fat Absorption. — For the purpose of studying the process by which fat passes from the lumen of the gut into the chvle vessels, an animal should be killed at the height of fat absorption. A frog fed with fat bacon and killed two days later, a dog fed with fat meat, or a cat with cream and killed after from four to eight hours, furnishes good material. Usually if the preparation is to be successful, the lumen of the intestine will be found to contain emulsified fat and the lacteals of the mesentery are seen distended with chyle. Extremely thin slices of the mucous membrane of the small intestine are fixed in i-per-cent. osmic acid or in osmium bichromate solution (5-per-cent. aqueous solution potassium bichromate and 2-per- cent, aqueous solution osmic acid — equal parts) for twelve to twenty-four hours, after which they are passed rather quickly through graded alcohols. Sections should be ihin and mounted, either unstained or alter a slight eosin stain, in glycerin. (4) The blood-vessels of the stomach arc best studied in injected specimens. (See page 22.) 16 242 THE ORGANS. The Larger Glands of the Digestive System. The smaller tubular glands which form a part of the mucous mem- brane and submucosa of the alimentary tract have been already de- scribed. Certain larger glandular structures, the development of which is similar to that of the smaller tubules but which come to lie wholly without the alimentary tract, connected with it only by their main excretory ducts, and which are yet functionally an important part of the digestive system, remain to be considered. These are: r (a) The parotid. 1. The salivary glands -l {b) The sublingual. ( (c) The submaxillary. 2. The pancreas. 3. The liver. The Salivary Glands. The salivary glands are all compound tubular glands. In man the parotid is serous; the sublingual and submaxillary, mixed serous and mucous (page 194). Only the general structure of these glands is here described, the minute structure of mucous and serous glands having been described on page 194. Each gland consists of gland tissue proper and of a supporting connective-tissue framework. The framework consists of a connect- ive-tissue capsule which encloses the gland, but blends externally with and attaches the gland to the surrounding structures. From the capsule trabeculce pass into the gland, subdividing it into lobes and lobules. The gland tissue proper consists of systems of excretory duels opening into secretory tubules, all being lined with one or more layers of epithelial cells. Each gland has one main excretory duct. This divides into branches — interlobar ducts — which run to the lobes in the connective tissue which separates them. The interlobar ducts give rise to branches which, as they pass to the lobules in the inter- lobular connective tissue, are known as interlobular ducts. From the latter, branches enter the lobules — intralobular ducts — and split up into terminal secreting tubules which constitute the bulk of the lobule. As the smaller inlrahjbular ducts are lined with cells of a secretory type, and probably take part in the elaboration of the secretion of the gland, they ha\'e been called salivary or secreting tubules. These, in the submaxillary and ])ar()tid glands, o])cn into tubules which have a THE DIGESTR'E SYSTEM. 24.3 narrow lumen and are lined with low or flat epithelium; lying between the secreting tubules and the terminal tubules, they are known as intercalated or intermediate tubules. From the interlobular connective tissue delicate extensions pass into the lobules, separating the gland tubules. The glandular tissue is known as thQ parenchyma of the gland in contradistinction to the connecti\e or interstitial tissue. The parotid gland in man, dog, cat, and rabbit is a purely serous gland. Its duct system is complex. The main excretory duct (Stenoni) is lined by two layers of columnar epithelium resting upon a («ei (^, r-i Fig. 157. — Diagrams to illustrate the Structure of the Salivarj- Glands. (Stohr.) .1, Parotid; B, sublingual; C, submaxillary, a, Excretory duct; b, secreting tubule; c, intermediate tubule; d, terminal tubule. distinct basement membrane. The main duct divides into numerous branches, which in turn gi\'e rise to the 'secreting or salivary tubules. These are continuous with the long narrow intermediate tubules, from each of which are given ofT a number of short terminal tubules (Figs. 157, A and 158). The two-layered epithelium of the main duct becomes re- duced in the smaller ducts to a single layer of columnar cells. The sali- vary tubules are lined with high columnar epithelium, the bases of the cells showing distinct longitudinal striations. In the intermediate tubule the epithelium is flat, sometimes spindle-shaped. The terminal tubules are lined with serous cells (page 194). The connecti\-e tissue usually contains a considerable number of fat cells. The sublingual gland is a mixed gland in man, dog, cat, and 244 THE ORGANS. rabbit. The duct system is less complex than in the parotid. The main duct (Bartholini) sends off branches which are continuous with tubules, showing a few secretory mucous cells. These open directly into the terminal tubules which art convoluted and vary greatly in diameter (Fig. 157, 5). The excretory duct is like that of the parotid gland, lined with a two-layered columnar epithelium resting upon a basement membrane. In the smaller ducts the epithelium is reduced to a single layer of columnar cells. There are no intermediate tubules. ~ Intercalated tubule Fat cells Terminal tubule Fig. 158. — Section of Human Parotid Gland. X 252 (Stohr). The narrow lumina of the terminal tubules do not sho v in this figure. The terminal tubules are lined with both serous and mucous cells (page 194). The crescents of Gianuzzi (page 195) are numerous and large. The connective tissue of the gland contains many lymphoid cells (Tig 159J. Near the sublingual gland is a group of some 5 to 20 simple tubular glands. Their terminal tubules are lined almost wholly with mucous cells. This grou]> of tubules has been designated the "sublingualis minor." The submaxillary gland is also a mixed gland in man, dog, cat, and rabbit. In complexity of its duct system it stands between the parotid and the sublingual (Fig. 157). The main duct (Wharton's) has not only a two-layered epithelial lining resting upon a basement THE DIGEST RE SYSTEM. 245 membrane, but is distinguished by a richly cellular stroma and a thin layer of longitudinally disposed smooth muscle. Branches of the main duct open into long secreting tubules which communicate with the terminal tubules by means of short narrow intermediate tubules (Fig. 157, C). The secretory tubules are lined as in the parotid with colum- nar cells whose bases are longitudinally striated. These cells usually contain more or less yellow pigment. The intermediate tubules have a low cuboidal or fiat epithelium. Most of the end tubules contain y r%^ ^^ ©1 Fig. 159. — Section of Human Sublingual Gland. X 252. (Stohr). a, E.xcretory duct; b, lumina of serous and mucous tubules; c, mucous tubule: (/.demilune; e, serous tubule; /, cross section mucous tubule; g, interstitial connective tissue. serous cells only (page 194). The crescents of the mucous tubules (page 195) are less numerous and smaller than those in the sublingual, consisting as a rule of only from one to three cells (Fig. 160). Blood-vessels. — The larger arteries run in the connective-tissue septa with the ducts, gi\'ing off branches which accompany the di\'isions of the ducts to the lobules, where they break up into capillary networks among the tubules. These gi\e rise to \"eins wliich accompany the arteries. The lymphatics Ijegin as minute capillaries in the connective tissue separating the terminal tubules. These empty into larger lymph vessels which accompany the arteries in the septa. 246 THE ORGANS. The nerves of the saHvary glands are derived from both cerebro- spinal and sympathetic systems, and consist of both medullated and non-medullated fibres. The medullated fibres are afferent, probably the dendrites of cells located in the geniculate gangHon. Small bundles of these fibres accompany the ducts. Single fibres leave the bundles, lose their medullary sheaths, and form a non-medullated subepithelial plexus, from which delicate fibrils pass to end freely among the epithehal cells. Efferent impulses reach the gland through the sympathetic. The fibres are axones of cells situated in small Fig. i6o. — Section of Human Submaxillary Gland. X 252. (Stohr.) a, Mucous tubule; b, serous tubule; c, intermediate tubule; (/, "secretory" tubule; e, demilune;/, lumen; g, interstitial connective tissue. peripheral ganglia; the cells sending axones to the submaxillary lying upon the main excretory duct and some of its larger branches; those sending axones to the sublingual being situated in a small ganglion — • the sublingual — lying in the triangular area bounded by the chorda tymjjani, the lingual nerve, and Wharton's duct; those supplying the parotid probably being in the otic ganglion. Axones from these cells enter the glands with the excretory duct and follow its branchings to the terminal tubules, where they form plexuses beneath the epithelium. From these, terminals pass to the secreting cells. It is probable that the salivary glands also receive sym})athetic fibres from cells of the superior cervical ganglia. THE DIGESTR'E SYSTEM. 247 TECHNIC. (i) The salivary glands should be fixed in Flemming's fluid (technic 7, p. 7), or in formalin-Miiller's fluid (technic 5, p. 7). Sections are cut as thin as possible, stained with hasmatoxyhn-eosin (technic i, p. 18), and mounted in balsam. (2) For the study of the secretory activities of the gland cells, glands from a fasting animal should first be examined and then compared with those of a gland the secretion of which has been stimulated by the subcutaneous injection of pilo- carpine. Fix in Flemming's or in Zenker's fluid (technic 9, p. 8). Examine some sections unstained and mounted in glycerin, others stained with hasmatoxylin-eosin and mounted in balsam. (3) The finer intercellular and intracellular secretory tubules are demon- strated by Golgi's method. Small pieces of absolutely fresh gland are placed for three days in osmium-bichromxate solution (3-per-cent. potassium bichromate solu- tion, 4 volumes; i-per-cent. osmic acid, i volume), and then transferred without washing to a o . 75-per-cent. aqueous solution of silver nitrate. Here they remain for from two to four days, the solution being frequently changed. The processes of dehydrating and embedding should be rapidly done, and sections mounted in glycerin, or, after clearing in xylol, in hard balsam. Pancreas. The pancreas is a compound tubular gland. While in general similar to the salivary glands, it has a somewhat more complicated structure. A connective-tissue capsule surrounds the gland and gives off trabeculae which pass into the organ and divide it into lobules. In some of the lower animals, as for example the cat, these lobules are well defined, being completely separated from one another by con- necti\'e tissue. In this respect they resemble the lobules of the pig's liver. A number of these primary lobules are grouped together and surrounded by connective tissue, which is considerably broader and looser in structure than that separating the primary lobules. These constitute a lobule group or secondary lobule. In the human pancreas the division into lobules and lobule groups is much less distinct, although it can usually be made out. This is due to the incompleteness of the connccti\-e-tissue septa, the human pancreas in this respect resembling the human li\cr. Rarch- tlie human pancreas is distinctly lobulatcd. The gland has a main excretory duct, the pancreatic duct or duct of Wirsung. In many cases there is also a secmidary excretory duct, the accessory pancreatic duct or duct of Santorini. Both open into the duodenum. The main duct extends almost the entire length of the gland, giving off short lateral branches, one of which enters the centre of each lobule group. Here it sphts up into branches which ])ass to the 248 THE ORGANS. primary lobules. From these intralobular ducts are given off long, narrow, intermediate tubules, which in turn give rise to the terminal secreting tubules (Fig. i6i). The excretory ducts are lined with a simple high columnar epithelium which rests upon a basement membrane. Outside of this is a connect- ive-tissue coat, the thickness of which is directly proportionate to the size of the duct. In the pancreatic duct goblet cells are present, and the accompanying connective tissue of the main duct and of its larger branches contains small mucous glands. As the ducts decrease in size, the epithelium becomes lower until the intermediate tubule is reached where it becomes fiat. The terminal tubules themselves are most of them very short, frequently almost spherical. This and the fact that several terminal tubules are given off from the end of each intermedi- ate tubule have led to the description of these tubules as alveoli, and of the pancreas as a tubulo- alveolar gland, although there is no dilatation of the lumen. The terminal tubules are lined with an irregularly conical epithelium resting upon a basement membrane (Figs. 162 and 163). The appearance of these cells depends upon their functional condition. Each cell consists of a central zone bordering the lumen, which contains numerous granules known as zymogen granules, and of a peripheral zone next to the basement membrane, which is homogeneous and contains the nucleus (Fig. 163). The zymogen granules are quite large granules and as they are highly refractive stand out distinctly even in the fresh, unstained condition and under low magnification. The relative size of these zones depends upon whether the cell is in the active or resting state (compare Fig. 164, A and B). During rest (fasting) the two zones are of about equal size. During the early stages of activity (intestinal digestion) the granules largely disappear and the clear zone occupies almost the entire cell. During the height of digestion the granules arc increased in number to such an extent that they almost fill the cell, while after prolonged secretion they are again almost absent. The cell Fig. 161. — Diagram to illus- trate Structure of Pancreas. (Stohr.) a, Excretory duct; h, intermediate tubule; c,c, terminal tubules. THE DIGESTIVE SYSTEM. 249 now returns to the resting state in whicli the two zones are about equal. The increase and disappearance of the granules are marked by the appearance of the fluid secretion of the gland in the lumen. Fig. 162. — Section of Human Pancreas. X 112. (KoUiker.) av. Alveoli; a, inter- lobular duct surrounded by interlobular connective tissue; L, islands of Langerhans; v, small vein. It would thus seem probable that the zymogen granules are the Intracellular representatives of the secretion of the gland. In sections of the gland there are seen within the lumina of many of the secreting tubules one or more small cells of which little but the nucleus can usually be made out. These cells lie in contact with the secreting cells, and resemble the flat cells which line the intermediate tubule. They are known as the centro- acinar {centro-iiihidar) cells of Langerhans (Fig. 163, f). Their significance is not definitely known. Langerhans believed " , '-.it.— that they were derived from p^^ la^.-From Section of Human Pancreas. X 700. (Koiliker.) a, Gland cell; b, base- ment membrane; s, intermediate tubule; ;:»#-5 O O, ^? c >' :©To' © U^^CfC;..>'ir 6 Fig. 1 66. — Island of Langerhans and few surrounding Pancreatic Tubules. (Bohm and von Davidoff.) a, Capillary; b, tubule. packed, being small, others being large and vesicular. Some of the islands are quite sharply outlined by delicate fibrils of connective tissue containing a few elastic fibres (Fig. i66). Others blend with the surrounding tissues. The origin, structure, and function of these islands have been subjects of much controversy. For some time they were considered of lymphoid origin. They are now believed to be epithelial cells having a developmental history similar to the cells lining the secreting tubules. Each cell-island consists of, in addition to the cells, a tuft or glomerulus of broad tortuous anastomosing capillaries, which arise from the network of capillaries which surround the secreting tubules. The close relation of cells and capillaries and the absence of any ducts have led to the hy- pothesis that these cells furnish a secretion — mtcr>ial secretion — which passes directly into the blood-vessels. In a recent publication Opie reviews previous work upon the histology of the pancreas and adds the results of his own careful researches. He concludes that the 9n0 THE ORGANS. cell-islands of Langerhans are definite structures "formed in embryological life," that ''they possess an anatomical identity as definite as the glomeruli of the kidney or the Malpighian body of the spleen, and that they subserve some special function." He calls attention to the similarity which Schafer noted between these cell-islands and such small ductless structures as the carotid and coccygeal glands and the parathyreoid bodies. From his study of the pancreas in diabetes, Opie concludes that the islands of Langerhans are concerned in carbohydrate metabolism. Fig. 167. — From SecUon of Pancreas, the blood-vessels of which had been injected (Kiihne and Lea), showing island of Langerhans with injected blood-vessels, surrounded by sections of tubules. Zymogen granules are distinct in inner ends of cells. Blood-vessels. — The arteries enter the pancreas with the main duct and break up into smaller arteries which accompany the smaller ducts. These end in a capillary network among the secreting tubules. From this, venous radicles arise which converge to form larger veins. These pass out of the gland in company with the arteries. Lymphatics. — Of the lymphatics little is known. Nerves. — The nerves are almost wholly from the sympathetic system, and are non-medullated. Some of them are axones of cells in sympathetic ganglia, outside the pancreas; others, of cells situated in small ganglia within the substance of the gland. They pass to plexuses among the secreting tubules, to which and to the walls of the vessels they send delicate terminal fibrils. TECHNIC. (i) The general technic for the pancreas is the same as for the salivary glands (page 247). (2) Zymogen granules may be demonstrated by fixation in formalin-Mliller's fluid (technic 5, p. 7), and staining with picro-acid-fuchsin (technic 2, p. 18), or with Heidenhain's iron hfematoxylin (technic 3, p. 16). (3) The arrangement of the blood-vessels in the i.slands of Langerhans may be studied in specimens in which the vascular system has been injected (page 22). THE DIGESTR'E SYSTEM. 253 The Liver. The liver is a compound tubular gland, the secreting tubules of which anastomose. There are thus, strictly speaking, no "terminal tubules" in the liver, the lumina and walls of neighboring tubules anastomosing without any distinct line of demarcation. The li\'er is surrounded by a connective-tissue capsule, the capsule of Glisson. At the hilum this capsule extends deep into the substance of the liver, gi^'ing off broad connective-tissue septa, which di\'ide the Fig. i68 — Section of Lobule of Pig's Liver X 60 (technic i, p. 261), showing lobule completely surrounded by connective tissue, a, Portal vein; b, bile duct; c, hepatic artery; d, portal canal; e, capillaries;/, central vein; g, cords of liver cells; h, hepatic vein. organ into lobes. From the capsule and from these interlobar septa, trabeculas pass into the lobes, subdi\iding them into lobules. In some animals, as for example the pig, each lobule is completely invested by connective tissue (Fig. i68). In man, only islands of connective tissue are found, usually at points where three or more lobules meet (Fig. 169). The lobules are cylindrical or irregularly polyhedral in shape, about i mm. in I^rcadth and 2 mm. in length. Excepting just beneath the capsule, wlicrc they arc fre(|uent]y arranged with their 254 THE ORGANS. apices toward the surface, the Kver lobules have an irregular arrangement. The lobule (Fig. i68) which may be considered the anatomic unit of structure of the liver, consists of secreting tubules arranged in a definite manner relatively to the blood-vessels. The blood-vessels of the liver must therefore be first considered. Ik '^':7^>^^ i*^"'* ^'^'= P K ./: -•.• *'( 'v", ' -^ ^ , i', >a I, -¥, >wv:i S P H Fig. 169. — Section of Human Liver. X 80. (Hendrickson.) P, Portal vein; H, hepatic artery; B, bile duct. P, H, B constitute the portal canal and lie in the connective tissue between the lobules. The BLOOD SUPPLY of the liver is peculiar in that in addition to the ordinary arterial supply and venous return, which all organs possess, the liver receives venous blood in large quantities through the portal vein. There are thus two afferent vessels, the hepatic artery and the portal vein, the former carrying arterial blood, the latter venous blood from the intestine. ]3oth vessels enter the liver at the hilum and divide into large interlobar branches, which follow the connective- tissue septa between the lobes. From these are given off interlobular branches, which run in the smaller connective-tissue septa between the THE DIGESTR'E SYSTEM. lobules. From the interlobular branches of the portal vein arise veins which are still interlobular and encircle the lobules. These send off short branches which pass to the surface of the lobule, where they break up into a rich intralobular capillary network. These intralobular capillaries all converge toward the centre of the lobule, where they empty into the central vein (Fig. i68). The central ^■eins are the smallest radicles of the hepatic veins, which are the efferent vessels of the liver. Each central vein begins at the apex of the lobule as a small vessel little larger than a capillary. As it passes through the centre of the long axis of the lobule the central vein constantly receives capil- laries from all sides, and, increasing in size, leaves the lobule at its base. Here it unites with the central veins of other lobules to form the sublohular vein which is a branch of the hepatic (Fig. 176). The hepatic artery accompanies the portal vein, following the branchings of the latter through the interlobar and inter- lobular connective tissue, where its finer twigs break up into capillary networks. Some of these capillaries empty into the smaller branches of the portal vein; others enter the lobules and anastomose with the intralobular portal capillaries. The M.A.IN EXCRETORY DUCT — hepatic duct — leaves the liver at the hilum near the entrance of the portal \-ein and hepatic artery. Within the liver the duct di\ides and subdivides, giving off interlobar, and these in turn interlobular branches. These ramify in the connective tissue, where they always accompany the branches of the portal vein and hepatic artery. These three structures— the hepatic artery, the portal vein, and the bile duct, which always occur together in the connective tissue which marks the point of separation of three or more lobules — together constitute the portal canal (Fig. 170). From the interlobular ducts short branches pass to the surfaces of the lobules. From these are given oft" extremely narrow tubules, which enter the lobule as intralobular secreting tubules. The walls of the ducts consist of a single layer of epithelial cells Fig. 170. Portal Canal. X315. (Klein and Smith.) a, Hepatic artery; 1', portal vein; 6, bile duct. 256 THE ORGANS. resting upon a basement membrane and surrounded by connective tissue (Fig. 170). The height of the epithelium and the amount of connective tissue are directly proportionate to the size of the duct. In the largest ducts there are usually a few scattered smooth muscle cells. The walls of the secreting tubules are formed by the liver cells. The LIVER CELLS (Fig. 171) are irregularly polyhedral in shape. They have a granular protoplasm which frequently contains glycogen, pigment granules, and droplets of fat and bile. Each cell contains ■r > s- ) '^ ^ ^T. f^ C- . ^D ' A- p^ I'b ? ■^ ^^a^ r - 7 0^^ e. ,--' ^ c* ^ ^ - r^ - " V.J. f. '^ V. Tj *~ Fig. 171. — Part of Lobule of Human Liver, showing capillaries and anastomosing cords of liver cells. X 350. a, Liver cells; b, capillaries. one or more spherical nuclei. Like other gland cells, the granularity of the protoplasm depends upon its functional condition. Within the cells arc minute irregular canals, some of which can be injected through the blood-vessels, while others are apparently continuous with the secreting tubules (Fig. 173, A and B). The capillaries of the portal vein, as they anastomose and converge from the periphery to the centre of the lobule, form long-meshed capil- lary networks. In the meshes of this network lie the anastomosing secreting tubules. On account of the shape of the capillary network, the liver cells, which form the walls of these tubules, are arranged in THE DIGESTI\-E SYSTEM. 257 anastomosing rows or cords, known as hepatic cords or cords of liver cells (Fig. 171). The secreting tubules (Fig. 172) are extremely minute channels, the walls of which are the liver cells. A secretory tubule always runs Fig. 172. — Part of Lobule of Human Liver, Golgi Method (technic 3, p. 261), to show relations of bile duct to intralobular secretory tubules and of the latter to the liver cells. a, Bile duct; b, cords of liver cells; c, blood capillaries; d, central vein; e, secretory tubules. between two contiguous liver cells, in each of which a groove is formed. The hlood capillaries, on the other hand, are found at the corners where three or more liver cells come in contact. It thus results that bile tubules and blood capillaries rarely lie in contact, but are regularly A B Fig. 173. — A, Cell from human liver showing intracellular canals (Browicz); r, intracellular canal; n, nucleus. B, From section of rabbit's liver injected through portal vein, showing intracellular canals (continuous with intercellular blood capillaries). (Schiifer.) separated by part of a liver cell. Exceptions to this rule sometimes occur. While most of the secretory tubules anastomose, some of them end blindly either between the liver cells or, in some instances, after extending a short distance within the cell protoplasm (Fig. 173, A). 17 258 THE ORGANS. At the surface of the lobule there is a modification of some of the liver cells to a low cuboidal type, and these become continuous with the lining cells of the smallest bile ducts, the secretory tubule being con- tinuous with the duct lumen. Special methods of technic have demonstrated a connective-tissue framework within the lobule. This consists of a reticulum of ex- tremely delicate fibrils which envelop the capillary blood-vessels, and of a smaller number of coarser fibres which radiate from the region of the central ^'ein — radiate fibres (Fig. 174). Fig. 174. — Liver Lobule, to show Connective-tissue Framework. (Mall.) Special technical methods also show the presence of stellate cells — cells of Kupfi'er — within the lobule. These are interpreted by Kupffer as belonging to the endothelium of the intralobular capillaries. Comparing the liver with other compound tubular glands, it is seen to present certain marked peculiarities which distinguish it and which make its structure as a compound tubular gland difficult to understand. The most important of these are the following: (Figs. 175 and 176). The extremely small amount of connective tissue; in the human liver not enough interlobular connective tissue to outline the lobules, THE DIGESTIVE SYSTEM. 259 while intralobular connective tissue demonstrable by ordinary staining methods is wholly absent. There is thus no connective tissue seen separating the cells of one tubule from those of another as, for example, in such a gland as the submaxillary. The result is that cells of neigh- boring tubules lie side by side, and back to back as it were, with no intervening connective tissue. The fact that unlike the tubules of other glands, the liver tubule consists of only two rows of cells, between which lies the lumen. The latter is thus ne\'er in touch with more than two cells. Fig. 175 Fig. 175. — Scheme of an Ordinary Compound Tubular Gland. In lobule 3 only the ramifications of the excretory duct, without endpieces, are shown. (Stohr). a, Branches of excretory duct; b, artery; c, vein; d, terminal tubules; e, capillaries. Fig. 176. — Scheme of Liver. In lobule i, only the direction of the endpieces is shown; in lobule 2 only their branching; in 3 only the excretory ducts. (Stohr.) a, Branches of excretory duct; b, portal vein; c, terminal tubules (hepatic cords); d, capillaries; e, vein (central and sublobular). The cnd'to-cnd anastomosis of the sccreti)ig tubules, there being no true terminal tubules; anastomosis of neighboring tubules by means of side branches; the arrangement of the bile capillaries in such a manner that a single liver cell abuts upon more than one capillary. TJw more intimate relation of the liver cell to the blood capillaries. Thus most gland cells have one side on the lumen, one side only in contact with a capillary blood-vessel, the remaining sides being in contact with other cells of the same tubule. A liver cell, on the 260 THE ORGANS. Other hand, may and usually does come in contact with several blood capillaries. The arrangement of hath blood-vessels and tubules ivithin the lobule. In the submaxillary, for example, the terminal tubules are convoluted and run in all directions. In the liver the terminal tubules are straight and run in a definite direction from the periphery of the lobule toward the center. Again, while in other glands both intra- lobular arteries and ducts are distributed outward from the centre of the lobule, and the blood is returned through veins which pass to the periphery of the lobule, in the liver the interlobular ducts pass to the periphery of the lobule and give off secreting tubules which pass in toward the centre of the lobule. The afferent vessels also (portal veins) take the blood to the periphery of the lobule and distribute it to a capillary network which converges to an efferent vessel (hepatic vein) at the centre of the lobule. The veins are also peculiar in that they do not follow the arteries in leaving the liver but pursue an en- tirely independent course. Blood-vessels. — These have been already described. Lymph vessels form a network in the liver capsule. These com- municate with deep lymphatics in the substance of the organ. The latter accompany the portal vein and follow the ramifications of its capillaries within the lobule as far as the central vein. The nerves of the liver are mainly non-medullated axones of sympathetic neurones. The nerves accompany the blood-vessels and bile ducts, around which they form plexuses. These plexuses give off fibrils which end on the blood-vessels, bile ducts, and liver cells. Three main ducts, all parts of a single excretory duct system, are concerned in the transportation of the bile to the intestine, the hepatic, the cystic, and the common. Their walls consist of a mucous membrane, a submucosa, and a layer of smooth muscle. The mucosa is composed of a simple columnar epithelium resting upon a basement membrane and a stroma which contains smooth muscle cells and small mucous glands. The submucosa is a thin layer of connective tissue. Hen- drickson describes the muscular coat as consisting of three layers, an inner circular, a middle longitudinal, and an external oblique. At the entrance of the common bile duct into the intestine, and at the junction of the duct of Wirsung with the common duct, there are thickenings of the circular fibres to form sphincters. In the cystic duct occur folds of the mucosa — the Heisterian valve — into which the muscularis extends. THE DIGESTIVE SYSTEM. 261 The Gail-Bladder. The wall of the gall-bladder consists of three coats— mucous, muscular, and serous. The mucous membrane is thrown up into small folds or ruga, which anastomose and give the mucous surface a reticular appearance. The epithelium is of the simple columnar variety with nuclei situated at the basal ends of the cells. A few mucous glands are usually found in the stroma. The muscular coat consists of bundles of smooth muscle cells which are disposed in a very irregular manner, and are separated by considerable fibrous tissue. A richly vascular layer just beneath the stroma is almost free from muscle and corresponds to a submucosa. It frec[uently contains small lymph nodules. The serous coat is a reflection of the peritoneum. TECHNIC. (i) Before taking up the study of the human liver, the liver from one of the 'lower animals in Avhich each lobule is completely surrounded by connective tissue should be studied. Fix small pieces of a pig's liver in formalin-Miiller's fluid (technic 5, p. 7.) Cut sections near and parallel to the surface. Stain with hsem- ato.xylin-picro-acid-fuchsin (technic 3, p. 19) and mount in balsam. In the pig's liver the lobules are completely outlined by connective tissue and the yellow picric- acid-stained lobules are in sharp contrast with the red fuchsin-stained connective tissue. (2) For the study of the human liver treat small pieces of perfectly fresh tissue in the same manner as the preceding, but stain with haematoxylin-eosin (technic i, p. 18). (3) The secretory tubules and smaller bile ducts may be demonstrated by tech- nic 4, p. 26. A light eosin stain brings out the liver cells (4) For the study of the blood-vessels of the Uver, inject the vessels through the inferior vena cava or portal vein. If the vena cava is used, it is convenient to inject from the heart directly through the right auricle into the vena cava. Sec- tions should be rather thick and may be stained with eosin, or even lightly with hajmatoxylin-eosin (technic i, p. 18), and mounted in balsam. (5) For demonstrating the intralobular connective tissue, Oppel recommends fixing fresh tissue in alcohol, placing for twenty-four hours in a 0.5-per-cent. aqueous solution of yellow chromate of potassium, washing in very dilute silver nitrate solution (a few drops of o. 75-per-cent. solution to 50 c.c. of water) and then trans- ferring to o. 75-per-cent. silver nitrate solution, where it remains for twenty-four hours. Embed quickly in celloidin. The best tissue is usually found near the surfaces of the blocks. A similar result is obtained by fixing fresh tissue in 0.5- per-cent. chromic-acid solution for three days, then transferring to 0.5-per-cent. silver nitrate solution for two davs. 262 the organs. Development of the Digestive System. In the development of the digestive system all the layers of the blastoderm are involved. Mesoderm and entoderm are, however, the layers most concerned, as the ectoderm is used only in the formation of the oral and anal orifices. The primitive alimentary canal is formed by two folds which grow out from the ventral surface of the embryo and unite to form a canal, in a manner that is quite similar to the formation of the neural canal. In this way the primitive gut is lined with cells which previously formed the ventral surface of the embryo, i.e., entoderm. A portion of the mesoderm accompanies the entoderm in the formation of the folds. This is known as the visceral layer of the mesoderm. The primary gut is thus a closed sac or tube. It is connected with the umbilical vesicle, but has no connection with the exterior. These connections are formed later by oral and anal invagi- nations of ectoderm which extend inward and open up into the ends of the hitherto imperforate gut. The ends of the alimentary tract, including the oral cavity and all of the glands and other structures connected with it, are of ectodermic origin. The epitheHal lining of the gut and the parenchyma of all glands connected with it are derived from entoderm. The muscle, the connective tissue, and the mesothelium of the serosa are developed from mesoderm. The mesodermic elements show little variation throughout the gut, the peculiarities of the several anatomical divisions of the latter being dependent mainly on special differentiation of the entoderm (epithe- lium). Beneath the entodermic cells is a narrow layer of loosely arranged tissue which later separates into stroma, muscularis mucosae, and submucosa. Outside of this a broader mesodermic band of firmer structure represents the future muscularis. The stomach first appears as a spindle-shaped dilatation about the end of the first month. Its entodermic cells, which had consisted of a single layer, increase in number and arrange themselves in short cylindrical groups. These are the first traces of tubular glands. They increase in length and extend downward into the mesodermic tissue. For a time the cells lining the peptic glands are all apparently alike, but at about the fourth month the differentiation into chief cells and parietal cells takes place. In the intestines a proliferation of the epithelium and of the under- lying stroma results in the formation of villi. These appear about the tenth week, in both small and large intestines. In the former THE DIGESTIVE SYSTEM. 263 they increase in size, while in the latter they atrophy and ultimately disappear. The simple tubular glands of the intestines develop in a manner similar to those of the stomach. The mesothelium of the serosa is derived from the mesodermic cells of the primitive body cavity. The development of the larger glands, connected with the digesti^•e tract, takes place in a manner similar to the formation of the simple tubular glands. All originate in extensions downward of entodermic cords into the underlying mesodermic tissue. From the lower ends of these cords, branches extend in all directions to form the complex systems of tubules found in the compound glands. The salivary glands being developed from the oral ca^•ity, originate in similar invaginations of ectodermic tissue. In the case of the pancreas a portion of the gland has an independent origin in the epithelium of the ductus choledochus. This portion ultimately unites with the main mass of the gland and its duct. The duct of Santorini sometimes remains patent, but in many cases atrophies so that the entire pancreatic secretion usually reaches the intestine through the pancreatic duct. The liver begins as a ventral downgrowth of the intestinal epithe- lium into the mesoderm of the transverse septum. This almost immediately divides into two hepatic diverticula. About the ends of these diverticula active proliferation of entodermic cells takes place, and this represents the first appearance of liver tissue. General References for Further Study. Oppel: Lehrbuch der vergleichenden mikroskopischen Anatomie. Kolliker: Handbuch der Gewebelehre des Menschen. Opie: The Pancreas. Stohr: Salivary Glands, in Te.\t-book of Histology- CHAPTER VII. THE RESPIRATORY SYSTEM. The respiratory apparatus consists of a system of passages — nares, larynx, trachea, and bronchi, which serve for the transmission of air to and from the essential organ of respiration, the lungs. The Nares. The nares, or nasal passages, are divided into vestibular, respira- tory, and olfactory regions, the differentiation depending mainly upon the structure of their mucous membranes. The VESTIBULAR REGION marks the transition between skin and mucous membrane (page 193). Its epithelium is of the stratified squamous variety and rests upon a basement membrane, which is thrown into folds by papillae of the underlying stroma. The latter is richly cellular and contains sebaceous glands (page 362) and the follicles of the nasal hairs. The RESPIRATORY REGION is much larger than both the vestibular and olfactory regions. Its epithelium is of the stratified columnar variety. The cells of the surface layer are ciliated and are inter- spersed with goblet cells. The stroma is distinguished by its thickness (3 to 5 mm. over the inferior turbinates) and by the presence of net- works of such large veins that the tissue closely resembles erectile tissue. It contains considerable diffuse lymphoid tissue and here and there small lymph nodules. In the stroma are small simple tubular glands lined with both serous and mucous cells. There is no sub- mucosa, the stroma being connected directly with the periosteum and perichondrium of the nasal bones and cartilages. The mucous membrane of the accessory nasal sinuses is similar in structure to that of the respiratory region of the nares, but is thinner and contains fewer glands. The OLFACTORY REGION Can be distinguished with the naked eye by its brownish-yellow color, in contrast with the reddish tint of the 264 THE RESPIRATORY SYSTEM. 265 surrounding respiratory mucosa. The epithelium is of the stratified columnar type, and is considerably thicker than that of the respiratory region. The surface cells are of two kinds: (i) sustentacular cells, and (2) olfactory cells. (i) The sustentacular cells are the more numerous. Each cell consists of three parts: {a) A superficial portion, which is broad and cylindrical, and contains pigment, and granules arranged in longi- tudinal rows. The cells have well-marked, striated, thickened free borders, which unite to form the so-called memhrana limitans olfactoria. (b) A middle portion which contains an oval nucleus. As the nuclei of these cells all lie in the same plane, they form a distinct narrow band, which is known as the zone of oval nuclei, (c) A thin filamentous process which extends from the nuclear portion down between the cells of the deeper layers. This process is irregular and pitted by pressure of surrounding cells. It usually forks and apparently anastomoses with processes of other cells to form a sort of protoplasmic reticulum. (2) The olfactory cells lie between the sustentacular cells. Their nuclei are spherical, lie at different levels, and are most of them more deeply placed than those of the sustentacular cells. They thus form a broad band, the zone of round nuclei. From the nuclear portion of the cell a delicate process extends to the surface, where it ends in several minute hair-like processes. From the opposite pole of the cell a longer process extends centrally as a centripetal nerve fibre. The olfactory cell is thus seen to be of the nature of a ganglion cell (see also page 382). Between the nuclear parts of the olfactory cells and the basement membrane are the basal cells. These are small nucleated elements, the irregular branching protoplasm of which anastomoses with that of neighboring basal cells and of the sustentacular cells to form the peculiar protoplasmic reticulum already mentioned. The basement membrane is not well developed. The stroma consists of loosely arranged white fibres, delicate elastic fibres, and connective-tissue cells. Embedded in the stroma are large numbers of simple branched tubular glands, the glands of Bow- man. Each tubule consists of a duct, a body, and a fundus. The secreting cells are large and irregular and contain a yellowish pigment, which with that of the sustentacular cells is responsible for the peculiar color of the olfactory mucosa. These glands were long described as serous, but are now ])clie\'ed to be mucous in character. They fre- quently extend beyond the limits of the olfactory region. 266 THE ORGANS. The Larynx. The larynx consists essentially of a group of cartilages united by strong fibrous bands and lined by mucous membrane. The epithelium covering the true vocal cords, the laryngeal surface of the epiglottis, and the anterior surface of the arytenoid cartilages is of the stratified squamous variety with underlying papillae. With these exceptions the mucous membrane of the larynx is lined with stratified columnar cihated epithelium similar to that of the respiratory portion of the nares. Numerous goblet cells are usually present, and the epithelium rests upon a broad basement membrane. On the posterior surface of the epiglottis many taste buds (see Fig. 268 and page 532) are embedded in the epithelium. The stroma is especially rich in elastic fibres. The true vocal cords consist almost wholly of longitudinal elastic fibres covered by stratified squamous epithelium. Lymphoid cells are present in vary- ing numbers. In some places they are so numerous that the tissue assumes the character of diffuse lymphoid tissue. Distinct nodules sometimes occur. Owing to the absence of a muscularis mucosae the stroma passes over with no distinct line of demarcation into the submucosa. This is a more loosely arranged, less cellular connective tissue, and con- tains simple tubular glands lined with both serous and mucous cells. Externally the submucosa merges into a layer of more dense fibrous tissue which connects it with the laryngeal cartilages and with the surrounding structures. Immediately surrounding the cartilages the connective tissue forms an extremely dense layer, the perichondrium. Of the cartilages of the larynx, the epiglottis, the middle part of the thyreoid, the apex and vocal process of the arytenoid, the carti- lages of Santorini and of Wrisburg are of the yellow elastic variety. The main body of the arytenoid, the rest of the thyreoid and the cricoid cartilages are hyaline. After the twentieth year, more or less ossi- fication is usually found in the cricoid and thyreoid cartilages. The Trachea. I'he walls of the trachea consist of three layers — mucosa, submu- cosa, and fibrosa (Fig. 177). The mucosa is continuous with that of the larynx, which it closely resembles in structure. It consists of a stratified columnar ciliated epithelium, with numerous goblet cells, resting upon a broad base- THE RESPIRATORY SYSTEM. 267 ment membrane, and of a stroma of mixed fibrous and elastic tissue containing many lymphoid cells. The submucosa is not distinctly marked off from the stroma on account of the absence of a muscularis mucosae. It is distinguished from the stroma by its looser, less cellular structure, by its numerous large blood-vessels, and by the presence of glands. These are of the. simple branched tubular variety and are lined with both serous and mucous cells. Some of the mucous tubules have well-marked cres- .' -.'1 -J Fig. 177. — From Longitudinal Section of Human Trachea. X 40. (Technic 3, p. 269.) a, Epithelium; b, stroma; c, cartilage; d, fibrous coat; e, serous tubules; /, mucous tubules; g, glands in submucosa; //, ducts. cents of Gianuzzi. The glands are most numerous between the ends of the cartilaginous rings, where they frequently penetrate the muscle and extend into the fibrosa. The fibrosa is composed of coarse, rather loosely woven connect- ive-tissue fibres embedded in which are the tracheal cartilages. These are incomplete rings of hyaline cartilage shaped like the letter C (Fig. 178). They are from sixteen to twenty in number and encircle about four-fifths of the tul)e, l^eing open posteriorly. The openings between the ends of the cartilaginous rings are bridged over bv a 268 THE ORGANS. thickened continuation of the fibrous coat, strengthened by a layer of smooth muscle (Fig. 178, m). The bundles of muscle cells run mainly in a transverse direction, and extend across the intervals between adjacent rings as well as between their open ends. There are frequently a few bundles of obliquely disposed cells and, most external, some few that run longitudinally. , __jMsr«=«'^-^=^«=E5B>nj^^ Fig. 178. — Transverse Section of Human Trachea through One (;f the Cartilage Rings. X 8. (KoUiker.) E, EpitheHum of (s) mucous membrane; dr, glands; as, gland duct; ad, adenoid tissue; K, cartilage; ?n, smooth muscle cut longitudinally, extending across between ends of cartilage ring. Outside the fibrous coat proper is a looser, more irregular con- nective tissue, which serves to attach the trachea to the surrounding structures. Blood-vessels, lymphatics, and nerves have a similar distribution in larynx and trachea. The larger vessels pass directly to the sub- mucosa. From these, smaller branches pass to the different coats, where they break u]) into capillary networks. THE RESPIRATORY SYSTEM. 269 Lymphatics form plexuses in the submucosa and mucosa, the most superficial lying just beneath the subepithelial capillary plexus. The nerves of the larynx and trachea are derived from both cere- bro-spinal and sympathetic systems. The cerebro-spinal nerves are afferent, the dendrites of spinal ganglion cells. They form a sub- epithelial plexus from which are given off fibrils which pass into the epithelium and terminate freely among the epithelial cells. Other afferent fibres of cerebro-spinal nerves pass to the muscular coat of the trachea. Sympathetic nerve fibres form plexuses which are interspersed with minute groups of ganglion cells. Axones from these ganglion cells have been traced to the smooth muscle cells of the trachea. Sympathetic axones also pass to the glands of the trachea and larynx. On the under surface of the epiglottis small taste buds are found. TECHNIC. (i) For the study of the details of structure of the walls of the nares and larynx, fix small pieces of perfectly fresh material from different regions in formalin-Mlil- ler's fluid (technic 5, p. 7), harden in alcohol, stain sections with hsematoxylin- eosin (technic i, p. 18), and mount in balsam. (2) The general relations of the parts can be studied by removing the larynx, upper part of the trachea, and corresponding portion of the oesophagus of an ani- mal or of a new-born child, fixing and hardening as above, and cutting longitudinal sections through the entire specimen. (3) Trachea. — Remove a portion of the trachea and treat as in technic (i). Both longitudinal and transverse sections should be made; the longitudinal includ- ing at least two of the cartilaginous rings; the transverse being through one of the rings. The Bronchi. The primary bronchi and their largest branches hax'e essentially the same structure as the trachea except that the cartilaginous rings are not as complete. Bronchi branch at acute angles and also give off small side branches. As they decrease in calibre, the following changes take place in their walls (Figs. 179 to 182). (i) The epithelium gradually becomes thinner. In a bronchus of medium size (Fig. 179) it has become reduced to three layers of cells, which Kolliker describes as an outer "basal" layer, a middle "replac- ing" layer, and a surface layer of ciliated and goblet cells. In the smaller bronchi (Figs. 181, 182) the epithelium is reduced to a single layer 270 THE ORGANS. of ciliated cells. These are at first high, but become gradually lower as the bronchi become smaller, until in the terminal branches the epithehum is simple cuboidal and non-ciliated. Among the ciliated cells are varying numbers of mucous or goblet cells. (2) The stroma decreases in thickness as the bronchi become smaller. It consists of loosely arranged white and elastic fibres. This layer with the epithelium is folded longitudinally (Fig. 182.) There is considerable diffuse lymphatic tissue, and in some places small Fig. 179. — Transverse Section through two Medium-size Bronchi of the Human Lung. X 15. (Technic 2, p. 280.) In the fibrous coat are seen the bronchial arteries and veins, a, Epithelium; b, stroma; c, muscularis mucosae; d, lung tissue; e, fibrous coat;/, plates of cartilage. nodules occur, over which there may be lymphoid infiltration of the epithelium (see Tonsil, page 157). Near the root of the lung many small lymph nodules are found, which show different degrees of pigmentation. (3) With decrease in thickness of the epithelium and of the stroma, the thickness of the mucosa is maintained by the appearance of a layer of smooth muscle. In the larger bronchi this is a continuous layer of circularly disposed smooth muscle, and lies just external to the stroma, forming a muscularis mucosae (Fig. 180). It reaches its greatest thickness relative to the size of the bronchus in the bronchi of medium size. As the bronchi become smaller it becomes thinner, then discontinuous, and in the smallest bronchi consists of only a few scattered muscle cells. These continue into the walls of the alveolar ducts, but are absent beyond this point. (4) The submucosa decreases in thickness with decrease in the THE RESPIRATORY SYSTEM. 271 calibre of the bronchi. It consists of loosely arranged connective tissue. Mixed glands (Fig. i8o) are present until a diameter of about I mm. is reached, when they disappear. They lie in the submucosa and frequently extend through between the cartilage plates into the fibrous coat. The ducts pass through the muscular coat and open into pit-like depressions lined with a continuation of the surface ciliated epithelium. Muscle ." r "' ' '' "' ' .-■;•. Epithelium Stroma coat ':■• ■ - —-:r- Alveoli ■^vii^ /^- Nerve Blood-vessel' i:^^ '''"■' l- W-^!^^ Cartilage Excretory duct Fig. i8o. — Cross Section of Human Bronchus (of a child) of 2 mm. diameter. X 30. (Stohr.). (5) The cartilages, which in the trachea and primary bronchi form nearly complete rings, become gradually smaller, and finally break up into short disconnected plates (Figs. 179 and 180). They are frequently fibrocartilage rather than hyaline. These plates decrease in size and number, and are absent after a diameter of i mm. is reached. Cartilage and mucous glands thus disappear at about the same time, although it is common for glands to extend over into smaller bronchi than do the cartilage plates. The bronchi down to a diameter of from 1.5 to i mm. are inter- lobular, and belon<2; to the duct svstcm down to a diameter of about 272 THE ORGANS. 0.5 mm. From the small interlobular bronchi are given off the lerminal bronchi. These are respiratory in character and are described with the luncjs. S.-' V^f '•I ■:;\ Fig. 181. — Transverse Section of Small Bronchus from Human Lung. X 115. (Technic 2, p. 280.) a, Stroma; h, epithelium; c, muscularis mucosae; d, fibrous coat. 3^^.. .,-„„. 'ftp ^ -> «*.^ fe*^ // ----t^vr''' "^^ • ■■' I'/ • -<■ ■ -'- ■aa> 'lOft ^ r*v >:T1' Fig. 182. — Transverse Section through small Bronchus of Human Lung. (Sobotta.) Simple columnar ciliated epithelium; no cartilage; no glands; mucosa folded longitudi- nally; elastic tissue stained with Weigert's elastic tissue stain. In studying the Ijronchi it is convenient to arljitrarily divide them into large, medium-sized, and small bronchi. Large bronchi have es.sentially the same structure as the trachea except for some- what thinner wails. Medium-sized hronchi (Fig. lyg) have an epithelium about three layers deep, THE RESPIRATORY SYSTE:\I. 273 disconnected plates of cartilage, a continuous layer of smooth muscle disposed cir- cularly as a muscularis mucosae, tubular glands. Small bronchi have a single layer of ciliated epithelium, a thinner muscular coat, no glands, and no cartilage. (Figs. i8i, 182.) The Lungs. The lung is built upon the plan of a compound alveolar gland, the trachea and bronchial ramifications corresponding to duct systems, the air vesicles to gland alveoli. The surface of the lung is covered by a serous membrane — the pulmonary pleura — which forms its capsule, and which at the root of the lunjy, or hilum, is reflected upon the inner surface of the chest wall as the parietal pleura. It consists of fibrillar connective tissue containing Fig. 183. — From Lung of an Ape. The bronchi and their dependent ducts and alveoli have been filled with quicksilver. X 15. (KoUiker, after Schulze.) b, Terminal bronchus; a, alveolar duct; /, alveoli. fine elastic fibres which are more numerous in the visceral than in the parietal layer. From the capsule broad connective-tissue sepia pass into the organ, dividing it into lobes. From the capsule and interlobar septa are given off smaller septa, which subdivide the lobes into lobules. The human pulmonary lobule (Fig. 184) is irregularly pyramidal, and has a diameter of from i to 3 cm. The amount of interlobular connective tissue is so small that no distinct separation into lobules can usually be made out. The pulmonary lobule constitutes the anatomic unit of lung structure in the same sense that the liver lobule constitutes the anatomic unit of that organ. The most superficial lobules are arranged with their bases against the pleura. Elsewhere in the lung the lobules have an irrcu;ular arrangement. 274 THE ORGANS. ■JvAJ-" The apex of each lobule is the point of entrance of a terminal or respiratory bronchus (Figs. 183 and 184) which is about 0.5 mm. in diameter. From each terminal bronchus open from three to six narrow passages — alveolar passages or alveolar ducts (Figs. 183, 184 and 185). The alveolar passages open into wider chambers— a/i'eo/ar sacs or infundibula. The latter are irregularly pyramidal, their bases being directed away from the alveolar ducts. From the sides of the terminal bronchi, the alveolar ducts, and the alveolar sacs, are given off the alveoli — air vesicles or air cells (Figs. 183 and 185). Bronchial arterv Pulmonary vein Pulmonary artery ^HL. Respiratory bronchus Pleural capillaries Fig. 184. — Scheme of a Pulmonary Lobule and its Blood Supply (Stohr). The two main branches of the pulmonary vein are seen lying in the interlobular connective tissue. According to Miller a further subdivision of the alveolar duct can be made. He describes the terminal bronchus as about 0.5 mm. in diameter, and as opening into from three to six narrow tubules, the vestibula. Each vestibulum is about o . 2 mm. in diameter, and opens into several larger, nearly spherical chambers, the atria. Each atrium communicates with a number of very narrow — 0.14 mm. — air-sac passages from which open the air sacs. From the latter are given off on all sides the air cells or alveoli. Alveoli are not, however, confined to the periphery of the air sacs, but are given off in small numbers from the terminal bronchus, and in constantly increasing numbers from the alveolar ducts and infundibula. The terminal bronchus. The proximal portion of the terminal or respiratory bronchus is lined by a simple columnar ciliated epithelium, THE RESPIRATORY SYSTEM. 275 resting upon a basement membrane. Beneath this is a richly elastic stroma containing bundles of circularly disposed smooth muscle cells. The epithelium becomes gradually lower and non-ciliated, and near the distal end of the terminal bronchus there appear small groups or islands of flat, non-nucleated epithelial cells — respiratory epithelium. The alveolar duct. Here the cuboidal epithelium is almost com- pletely replaced by the respiratory. Beneath the epithelium the walls Pleura Alveoli Alveolar sacs • Blood-vessel Respiratory bronchus Alveoli Small bronchus Fig. 1S5. — Section of Cat's Lung (Szymonowicz) surface lobule; respiratory bronchus opening into alveolar duct from which are given off two alveolar sacs. have a structure similar to those of the distal end of the terminal bronchus, consisting of delicate fibro-elastic tissue with scattered smooth muscle cells. The basement membrane is extremely thin. The alveolar sac. The epithelium of the alveolar sac consists of two kinds of cells, respiratory cells and so-called "/a'/a/" cells (see Development, page 279). 276 THE ORGANS. The respiratory cells (Fig. i86) are some of them large, flat, non- nucleated plates, while others are much smaller, non-nucleated ele- ments. The absence of nuclei and the extremely small amount of intercellular substance render these cells quite invisible in sections stained by the more common methods. The cell boundaries are best demonstrated by means of silver nitrate (technic i, p. 71). The '' fcetaV cells are granular, nucleated cells which are scattered among the respiratory cells. Their position appears to be less super- ficial than that of the respiratory cells, the foetal cells lying in the meshes Fig. 186. — From Section of Cat's Lung Stained with Silver Nitrate. (Klein. ) (Tech- nic I, p. 71.) Small bronchus surrounded by alveoli, in which are seen both flat cells (respiratory epithelium) and cuboidal cells (foetal cells). of the capillary network, the respiratory cells covering the capillaries. In the embryonic lung and in the lungs of a still-born child the air passages and alveoli contain only this type of cells, the small flat plates apparently resulting from a flattening out of the cuboidal cells due to pressure from inspiration, and the large flat plates to union of a number of small plates. The alveolus is similar in structure to the alveolar duct, its walls consisting mainly of delicate elastic fibrils supporting respiratory and ffi'tal cells. Around the opening of the alveolus the elastic fibres are more numerous, forming a more or less definite ring. The dis- position of elastic tissue in the wafl of the alveoli is undoubtedly of importance in determining the contraction and expansion of the THE RESPIRATORY SYSTEM. 277 alveoli under varying conditions of pressure. It has been estimated that on forced inspiration an alveolus can expand to three times its resting capacity. Each alveolus communicates not only with its alveolar sac, alveolar duct, or respiratory bronchus, by means of a broad opening, but alveoli are connected with one another by minute I openings in their walls. The iiiteralveolar connective tissue, while extremely small in amount, serves to separate the alveoli from one another. Somewhat thicker Fig. 1S7. — Section Through Three Alveoli of Human Lung. X 235. Weigerl's elastic-tissue stain (technic 3, p. 26) to show arrangement of elastic tissue, a, .\lveolus cut through side -walls only; b, alveolus cut through side walls and portion of bottom or top; c, alveolus in which either the bottom or top is included in section. connective tissue separates the alveoli of one alveolar passage from those of another. Still stronger connecti\'e-tissue bands separate adjacent lobules. Blood-vessels. — Two systems of vessels distribute blood to the lungs. One, the hroncJiial system, carries blood for the nutrition of the lung tissue. The other, the much larger piihnanary system, carries blood for the respiratory function (Fig. 184). The hroncJiial artery and the pulmonary artery enter the lung at its hilum. "Within the lung the vessels branch, following the branchings of the bronchi, which they accompany (Fig. 184). The pulmonary vessels are much the larger and run in the connective tissue outside the bronchial walls. The bronchial vessels lie icilliiii the fibrous coat of the l)ronchus. A section of a bronchus thus usuallv shows the large 278 THE ORGANS. pulmonary vessels, one on either side of the bronchus, and two or more small bronchial vessels in the walls of the bronchus (Fig. 179). The pulmonary lobule forms a distinct "blood-vascular unit." A branch of the pulmonary artery enters the apex of each lobule close to the lobular bronchus, and almost immediately breaks up into branches, one of which passes to each alveolar passage (Fig. 184). Fig. 188. — Parts of Four Alveoli from Section of Injected Fluman Lung. X 200. (Technic 5, p. 280.) a, Wall of alveolus seen on flat; c, same, but only small part of alveolar wall in plane of section; b, alveoli in which plane of section includes only side walls; alveolar wall seen on edge. From these are given off minute terminal arterioles which pass to the central sides of the alveolar passages and alveoli, where they give rise to a rich capillary network. This capillary network is extremely close- meshed, and invests the alveoli on all sides (Fig. 188). Similar networks invest the walls of the respiratory bronchi, the alveolar ducts, and their alveoli. All of these capillary networks freely anastomose. There are thus interposed between the blood in the capillaries and the air in the alveoli only three extremely thin layers: (i) The thin endothelium of the capillary wall; (2) the single layer of flat respiratory epithelial plates; and (3) the delicate basement membrane upon which THE RESPIRATORY SYSTEM. 279 the respiratory epithelium rests together with an extremely small amount of fibrous and elastic tissues (see diagram, Fig. 189). The veins begin as small radicles, one from the base of each alve- olus (Fig. 184). These empty into small veins at the periphery of the lobule. These veins at first run in the interlobular connective tissue away from the artery and bronchus. Later they empty into the large pulmonary trunks which accompany the bronchi. The bronchial arteries break up into capillary networks in the walls of the bronchi, supplying them as far as their respiratory divisions, beyond which point the capillaries belong to the pulmonary system. Fig. 189. — Diagram of Tissues Interposed Between Blood and Air in Alveolus, a Respiratory epithelium; 6, basement membrane and small amount of fibro-elastic tissue c, endothelium of capillary. The bronchial arteries supply the walls of the Ijronchi, the bronchial lymph nodes, the walls of the pulmonary vessels, and the pulmonary pleura. Of the bronchial capillaries some empty into the bronchial veins, others into the pulmonary veins. Lymphatics. — The lymphatics of the lung begin as small lymph spaces in the interalveolar connective tissue. These communicate with larger lymph channels in the interlobular septa. Some of these empty into the deep pulmonary lymphatics, which follow the pulmo- nary vessels to the lymph glands at the root of the lung. Others empty into the superficial pulmonary lymphatics, which form an extensive subpleural plexus connected with small subpleural lymph nodes, whence by means of several larger vessels the lymph is carried to the lymph nodes at the hilum. Nerves. — Bundles of medullated and non-medullated fibres accom- pany the bronchial arteries and veins. Small sympathetic ganglia are distributed along these nerves. The fibres form plexuses in the fibrous layer of the bronchi, from which terminals pass to the muscle of the bronchi and of the vessel walls and to the mucosa. Free end- ings upon the epithelium of bronchi, air passages, and alveoli have been described. Development of the Respir.a.tory System. The epithelium of the respiratory system develops from entoderm, the connective-tissue elements from mesoderm. The first differ- 280 THE ORGANS. entiation of respiratory system appears as a dipping down of the entoderm of the floor of the primitive pharynx. The tubule thus formed divides into a larger and longer right branch, which subdivides into three branches corresponding to the three lobes of the future right lunsf, and a smaller and shorter left branch, which subdivides into two branches corresponding to the two lobes of the future left lung. By repeated subdivisions of these tubules the entire bronchial system is formed. The last to develop are the respiratory divisions of the bronchi with their alveolar ducts, alveolar sacs and alveoli. The epithelium of the alveolar sacs and alveoli is at first entirely of the fcetal-cell type, the large flat respiratory plates appearing only after the lungs have become inflated. The foetal and respiratory cells of the adult lung have therefore the same embryonic origin. During the early stages of lung development the mesodermic tissue predominates, but with the rapid growth of the tubules the proportion of the two changes until in the adult lung the mesodermic tissue becomes restricted to the inconspicuous pulmonary framework and the blood-vessels. TECHNIC. (i) The technic for the largest bronchi is the same as for the trachea (technic 3, p. 269). The medium size and small bronchi are studied in sections of the lung. (2) Lung and Bronchi. — Carefully remove the lungs and trachea (human, dog, or cat) and tie into the trachea a cannula to which a funnel is attached. Distend the lungs moderately (pressure of two to four inches) by pouring in formalin-Miil- ler's fluid (technic 5, p. 7), and then immerse the whole in the same fixative for twenty-four hours. Cut into small blocks, using a very sharp razor so as not to squeeze the tissue, harden in alcohol, stain thin sections with hsematoxylin-eosin (technic i, p. 18), and mount in balsam or in eosin-glycerin. The larger bronchi are found in sections near the root of the lung. The arrangement of the pulmo- nary lobules is best seen in sections near and horizontal to the surface. Sections perpendicular to and including the surface show the pulmonary pleura. (3) Respiratory Epithelium (technic i, p. 71). (4) Elastic Tissue of the Lung (technic 3, p. 26). (5) Blood-vessels. — For the study of the blood-vessels, especially of the capil- lary networks of the alveoli, sections of injected lung should be made. A fresh lung is injected (page 22) with blue gelatin, through the pulmonary artery. It is then hardened in alcohol, embedded in celloidin, and thick sections are stained with eosin and mounted in balsam. The Thyreoid. The thyreoid (Fig. 190) is a ductless structure built upon the general principle of a compound alveolar gland. There are usually two THE RESPIRATORY SYSTEM. 281 lateral lobes connected by a narrow band of glandular tissue, the "isthmus.'' Each lobe is surrounded by a connective-tissue capsule, from which septa pass into the lobe, subdividing it into lobules. From the perilobular connective tissue finer strands extend into the lobules, separating the alveoli. The latter are spherical, oval, or irregular in shape. They vary greatly in diameter (40 to 120a) and are as a rule non-communicating. At birth most of the alveoli are empty, but soon become more or less filled with a peculiar substance known as "colloid." The aheoli are lined with a single or double layer of cuboidal epithelial cells. Two types of cells are recognized. ., ,,. ._^ Fig. iqo. — Section of Human Thyreoid. Most of the alveoU contain colloid. One of these is actively secreting colloid and is known as a secret nig or colloid cell. The other contains no colloid and is known as a resting or reserve cell. It is probable that their names indicate the relation of these cells to each other and that the reserve cell ultimately becomes a colloid-secreting cell. The colloid cell appears in some cases simply to pour out its colloid secretion into the lumen, after which it may assume the character of a resting cell; in other cases the cell appears to be completely trans- formed into colloid, its place being taken by proliferation of the resting cells. In certain alveoli which are much distended with colloid the lining epithelium is flattened. 282 THE ORGANS. That the thyreoid exerts a decided influence upon general body metabolism is shown by the symptoms resulting from congenital absence of the gland (congenital myxoedema or cretinism) and by the effects of complete removal, the latter giving rise to a train of symptoms known as the "cachexia strumipriva." The blood supply of the thyreoid is extremely rich, the vessels branching and anastomosing in the connective tissue and forming dense capillary networks around the alveoli. Lymphatics accompany the blood-vessels in the connective tissue. Nerves are mainly non-medullated fibres which form plexuses around the blood-vessels and in the connective tissue surrounding the alveoli. Terminals to the secreting cells end in club-like dilatations Fig. 191. — Section of Human Parathyreoid; showing mainly "clear," " principal", or "chief" cells. (Pool.) against the bases or between the epithelial cells. A few afferent medullated fibres are found in the plexuses surrounding the blood- vessels. Development. — The thyreoid originates as a diverticulum from the entoderm of the primitive pharynx. It first appears in embryos of 3 to 5 mm. and grows ventrally into the mesoderm of the ventral wall of the neck. Here it forms a mass which Hes transversely across the neck. It is composed of soh"d cords of cells which become hollow THE RESPIRATORY SYSTEM. 283 to form the alveoli of the gland. At first the gland is connected with the surface by the thyreo-glossal duct. This either disappears entirely or is represented in the adult by such rudimentary structures as the so- called prehyoid, suprahyoid, and accessory thyreoid glands. The gland at first consists of solid cords of cells. Ingrowth of connective tissue divides these into groups or lobules, and at the same time breaks up the long tubules into short segments. Dilatation of the alveoli occurs with the formation of colloid. The Parathyreoids. These are small ductless glands which usually lie upon the posterior surface of the lateral lobes of the thvreoid. There are commonlv two Fig. 192. — Section of Human Parathyreoid showing groups of oxyphile cells. (Pool.) pairs, a superior and an inferior, on each side. The number is, however, subject to variation. Each gland is from 6 to 8 mm. long, about half that in breadth, and 2 mm. thick. Small groups of cells having the structure of the parathyreoids ha\e been found below the thyreoid and within the thvreoid and thvmus. 284 THE ORGANS. The parathyreoid is surrounded by a thin connective-tissue capsule which sends a variable amount of connective tissue into the gland as septa. When the amount is considerable the gland shows a sub- division into lobules. The stroma consists largely of reticular tissue and is very vascular. The number and arrangement of the cells vary. The gland may be almost wholly cellular with very little connective tissue, the groups of cells may be widely separated by interstitial tissue, Fig. 193. — Section of Human Parathyreoirl, showinjj lumina indicating tubular or tutjulo-alveolar structure. (Pool.) or there may be any intermediate condition. The cells are arranged in irregular groups or cords (Figs. 191, 192) sometimes around tubules, sometimes having a distinctly alveolar structure (Fig. 193); in the latter case colloid may be present in the lumen. Colloid has also been found between, and colloid and glycogen within, the cells. The cells themselves are spheroidal, cuboidal or pyramidal, with basal nuclei. The appearance of the cells varies sufficiently to have caused two or three types to be distinguished. All, however, probably represent different functional conditions of the same cell, (i) Chief or clear cells (Fig. 191). These are the more numerous. Their bodies are THE RESPIRATORY SYSTE^E 285 small and clear, and the cytoplasm does not stain readily. The nuclei are large in proportion to the cell and are clear and vesicular with loosely arranged pale chromatin network. ( 2 ) Oxyphile cells (Fig. 192 ). These ' are larger, their cytoplasm is more granular and takes a strong eosin stain. The nucleus is small, its chromatin network closely arranged and takes a dark stain. Compact groups of these cells occur especially just beneath the capsule. They are also found throughout the gland, arranged as cords, as single cells, or as small groups among the clear cells. Intermediate types have been described. It is probable that the clear cells represent the resting, the granular cells the active secreting condition of the same cell. The parathyreoids originate as epithelial evaginations from the third and fourth branchial grooves, and develop wholly independently of the thyreoid. The powerful influence which these minute organs exert is shown both clinically and experimentally. Fatal tetany, resulting from the earlier operations for complete removal of the thyreoids, has been shown by animal experimentation to be due not to the removal of the thyreoids, but to coincident removal of the parathyreoids, the removal of the latter only, in animals, giving the same results.* TECHNIC. The thyreoid and parathyreoid glands are best fixed in formalin-Miiller's fluid. Sections may be stained with haematoxylin-eosin or ha;matoxylin-picro-acid-fuchsin and mounted in balsam. General References for Further Study. Miller, W. S.: Das Lungenlappchen, seine Blut- und Lymphgefasse. Councilman: The Lobule of the Lung and its Relations to the Lymphatics. Kolliker: Handbuch der Gewebelehre des Menschen. Pool: Tetany Parathyreopriva. Annals of Surgery, October, 1907. * For much of the description of the parathyreoids and for the photograph the writer is indebted to Dr. Eugene H. Pool. Ax > CHAPTER Vni. THE URINARY SYSTEM. The Kidney. The kidney is a compound tubular gland. It is enclosed by a firm connective-tissue capsule, the inner layer of which contains smooth muscle cells. In many of the lower animals and in the human foetus septa extend from the cap- sule into the gland, dividing it into a number of lobes or renculi. In some animals, e.g., the guinea- pig and rabbit, the entire kidney consists of a sirgle lobe (Fig. 194.). In the adult human kid- ney the division into lobes is not complete, the peripheral parts of the different lobes blending. Rarely the foetal division into lobes persists in adult life, such as kidney being known as a "lobulated kidney." On the mesially directed side of the kidney is a depression known as the hilum (Fig. 194). This serves as the point of en- trance of the renal artery and of exit for the renal vein and ureter. On section, a division of the organ into two zones is apparent to the naked eye (Figs. 194 and 195;. The outer zone or cortex has a granular appearance, while the inner zone or medulla shows radial striations. This difference in appearance between cortex and medulla is mainly due, as will be seen subsequently, to the fact that in the cortex the kidney tubules are convoluted, while in the medulla 286 Fig. 194. — Longitudinal Section Through Kidney of Guinea-pig, including hilum and beginning of ureter. X 5. (Technic i, p. 302.) a, Pelvis; 6, papilla; c, wall of pelvis; d, ureter; e, ducts of Bellini; /, cortical pyramids; g, medullary rays; h, cortex; i, medulla; j, renal corpuscles. THE URIX.^J^Y SYSTEM. 2S7 they run in parallel radial lines alternating witii straight blood-vessels. The medullan- portion of the kidney projects into the pelvis, or upper expanded beginning of the ureter (Figs. 194 and 195) in tlie form of Fig. iQs. F:.. ioc. Fig. 1 05. — Longitudiriai Section of Kiduer ThroiigJa MUlnuim. a, CorticaS pviaaiaid: h. medullar}- rav; r, medulla; if, cortex; t. renal ca.lj5;_/", Mlum; g, Tureter; A, rertal i'T- 7, obliquely cut tubules of medulla; 7 and t, renal arches; 7, coIuttiti of Benim; w,, cor.- , tissue and fat surrounding renal vessels; w, medulla cui obliquelj; p. papilla; ^, mec—.-r- p}-Tamid. (Merkel-Henle.) Fig. 106. — Scheme of Urimferotis Tubule aud of tie Blood-Tessels of ibe Kidner sho-w-ing their relation to eaci otter and to the different parts of the tjdner. f-. :* ^'^v Fig. 201. — Cross Section Through Cortex of Human Kidney. X 60. (Technic 2, p. 302.) a, Convoluted tubules of cortical pyramid; h, interlobular artery; f, medullary rays; d, Malpighian bodies. of the high columnar type. The cytoplasm of these cells contains comparatively few granules, thus appearing transparent in contrast with the granular cytoplasm of the ascending arms of Henle's loops and of the convoluted tubules. Location in tiidney Cortical labyrinth Portion of tubule Epithelium I Bowman's capsule. , Flat with bulging nuclei. Neck Cuboidal granular. I First convoluted, . . . Pyramidal, granular; large cells I with granules in rows, J ■ giving striated appearance; striated free border; indis- tinct cell outline. Second convoluted Similar to preceding, but cells not so distinctly striated and more regular in shape. THE URINARY SYSTEM. 293 Location in kidney Portion of tubule Epithelium Arched (passing from Rather clear cuboidal cells. labyrinth to ray) Part of ascending arm Cuboidal, granular, regular. Medullary ray in cortex . i Henle's loop Collecting tubule. . . Cuboidal or columnar, clear; varying in height with diam- eter of tubule. Decendingarm,Hen- Clear flat cells with bulging le's loop nuclei. Henle's loop Usually like descending, rarely like ascending arm. Part of ascending Cuboidal, granular. arm Henle's loop Collecting tubule. . . Cuboidal or columnar, clear; varying in height with diam- eter of tubule. Papilla Ducts of Bellini .... Clear, cuboidal or columnar cells according to diameter of tubule. Medulla . The epithelium of the uriniferous tubule rests upon an apparently structureless basement membrane. Riihle describes the basement membrane as consisting of delicate longitudinal and circular connect- ive-tissue fibrils. He regards the fibrils as merely a more regular arrangement of the interstitial connective tissue. According to Riihle the epithelium simply rests upon the basement membrane, being in no way connected with it. In the cortex the tubules are closely packed and the amount of interstitial connective tissue is extremely small. In the medulla the connective tissue is more abundant. Of the function of the different parts of the uriniferous tubule our knowledge is extremely limited. The water of the urine is secreted in the Malpighian body, some specific action of the cells covering the glomerulus, allowing the water, normally free from albumen, to pass from the capillaries into the lumen of the tubule. The urinary solids are secreted mainly or wholly by the cells of the convoluted tubule and of the ascending arm of Henle's loop. Blood-vessels (diagram, Fig. 203). — The blood supply to the kidney is rich and the blood-vessels come into intimate relations with the tubules. The renal artery enters the kidney at the hilum. and immediately splits up into a numl)er of branches — the interlobar arteries (Fig 203, g). These give otT twigs to the calyces and to the capsule, then 294 THE ORGANS. without further branching pass between the papillae through the medulla to the junction of medulla and cortex. Here they bend sharply at right angles and following the boundary line between cortex and medulla, form a series of arches, the artericE arciformes or arcuate arteries (Fig. 203, d). From the arcuate arteries two sets of vessels arise, one supplying the cortex, the other the medulla (Figs. 196 and 203). The arteries to the cortex spring from the outer (Fig. 203, h) sides of the arterial arches, and as interlobular arteries pursue a quite straight ^ ^^ /0\., /^ ^) ^^ / i^^'v c. O -' I J -^ ^ \j ^yis Fig. 202. — Cross Section through Medulla of Human Kidney. X 465. (Technic 2, p. 302.) a, Capjillaries; h, collecting tubule; c, ascending arms of Henle's loops; d, descend- ing arms of Henle's loops. course through the cortical pyramids toward the surface, about mid- way between adjacent medullary rays. From each interlobular artery are given off numerous short lateral branches, each one of which passes to a Malpighian body. Entering a Malpighian body as its afferent vessel, the artery breaks up into a number of small arterioles, which in turn give rise to the groups of capillaries which form the glomerulus. Each group of glomerular capillaries arising from a single arteriole is separated from its neighbors by a rather larger amount of connective THE URINARY SYSTEM. 295 tissue than that which separates the individual capillaries. This gives to the glomerulus its lobular appearance. From the smaller glomerular capillaries the blood passes into somewhat larger capillaries, which unite to form the efferent vessel of the glomerulus. As afferent Fig. 203. — Diagram to Illustrate (left) the Course of the Uriniferous Tubule: (right) the Course of the Renal Vessels. (Szymonowicz.) A, B, C, D, form the kidney lohidcs; a, afferent vessel; e, efferent vessel of glomerulus; i, Bowman's capsule; 2, first convoluted tubule; 3, descending arm of Henle's loop; 4, ascending arm of Henle's loop; 5, second con- voluted tubule; 6 and 7, collecting tubules; 8, duct of Bellini; b, interlobular artery; c, inter- lobular vein; d, renal arch (arcuate artery above and arcuate vein below) ; /, interlobar vein ; g, interlobar artery; h, medulla; i, medullary ray; 7, cortex. and efferent vessels lie side by side, the glomerulus has the appearance of being suspended from this point (Figs. 196, 198) . The en/ ire vascular system of the glomerulus is arterial. After leaving the glomerulus, the efferent vessel breaks up into a second system of capillaries, which form a dense network among the 296 THE ORG.\NS. tubules of the cortical pyramids and of the medullary rays. The mesh corresponds to the shape of the tubules, being irregular in the pyramids, long and narrow in the rays. In these capillaries the blood gradually becomes venous and passes into the interlobular veins (Fig. 203, c). These accompany the interlobular arteries to the boundary between cortex and medulla, where they enter the arcuate veins, which accompany the arcuate arteries (Fig. 20T,,d). The main arteries to the medulla arise from the inner concave sides of the arterial arches. They pass straight down among the tubules of the medulla and are known as arterice rectce. Branching, they give rise to a long-meshed capillary network which surrounds the tubules. This capillary network is also supplied by (i) efferent vessels from the more deeply situated glomeruli (false arteriae rectse) and (2) by medul- lary branches from the interlobular arteries. The veins of the medulla arise from the capillary network and follow the arteries back to the junction of medulla and cortex, where they empty into the arcuate veins (Fig. 203, d). In addition to the distribution just described, some of the inter- lobular arteries extend to the surface of the kidney, where they enter the capsule and form a network of capillaries which anastomose with capillaries of the suprarenal, recurrent, and phrenic arteries. A further collateral circulation is established by branches of the above- named arteries penetrating the kidney and forming capillary networks within the cortex, even supplying some of the more superficial glomeruli. The most superficial of the small veins which enter the interlobular are arranged in radial groups, having the interlobular veins as their centres. These lie just beneath the capsule, and are known as the stellate veins of Verheyn. In addition to capillary anastomoses, direct communication between arteries and veins of both cortex and medulla, by means of trunks of considerable size, has been described. The lymph vessels of the kidney are arranged in two systems, a superficial system which ramifies in the capsule, and a deep system which accompanies the arteries to the parenchyma of the organ. Little is known of the relation of the lymphatics to the kidney tubules. Nerves. — These are derived from both cerebro-spinal and sym- pathetic systems. The medullated fibres appear to pass mainly to the walls of the blood-vessels which supply the kidney capsule. Plex- uses of fine non-medullated fibres (sympathetic) accompany the arteries to the glomeruli. Delicate terminals have been described as THE URINARY SYSTEM. 297 passing from these plexuses, piercing the basement membrane and ending freely between the epithelial cells of the tubules. The Kidney-Pelvis and Ureter. The kidney-pelvis, with its subdi\"isions the calyces, and the ureter constitute the main excretory duct of the kidney. Their walls consist of three coats; an inner mucous, a middle muscular, and an outer fibrous. The mucosa is lined by epithelium of the transitional type. There are from four to eight layers of cells, the cell outlines are usually well defined, and the surface cells instead of being distinctly squamous are only slightly flattened. Less commonly large flat plate-like cells, each containing several nuclei, are present. The cells rest upon a base- ment membrane, beneath which is a stroma of delicate fibro-elastic tissue rich in cells. Difl'use lymphatic tissue frequently occurs in the stroma, especially of the pelvis. Occasionally the lymphatic tissue takes the form of small nodules. Mucous glands in small numbers are found in the stroma of the pelvis and upper part of the ureter. There is no distinct submucosa, although the outer part of the stroma is sometimes referred to as such. The muscularis consists of an inner longitudinal and an outer circular layer. In the lower part of the ureter a discontinuous outer longitudinal layer is added. The fibrosa consists of loosely arranged connective tissue and contains many large blood-vessels. It is not sharply limited exter- nally, but blends with the connective tissue of surrounding structures, and serves to attach the ureter to the latter. The larger blood-vessels run in the fibrous coat. From these, branches pierce the muscular layer, give rise to a capillary network among the muscle cells, and then pass to the mucosa, in the stroma of which they break up into a rich network of capillaries. The veins follow the arteries. The lymphatics follow the blood-vessels, being especially numer- ous in the stroma of the mucosa. Nerves. — Plexuses of both medullated and non-medullated fibres occur in the walls of the ureter and pelvis. The non-medullated fibres pass mainly to the cells of the muscularis. Medullated fibres enter the mucosa where they lose their medullary sheaths. Terminals of these fibres have been traced to the lining epithelium. 298 THE ORGANS. The Urinary Bladder. The walls of the bladder are similar in structure to those of the ureter. The mucous membrane is thrown up into folds or is comparatively smooth, according to the degree of distention of the organ. The epithelium is of the same general type — transitional epithelium (see page 67) — as that of the ureter. The number of layers of cells and the shapes of the cells depend largely upon whether the bladder is ''M^Mi/:^ '<-<£ai\> Fig. 204. — ^Vertical Section through Wall of moderately distended Human Bladder. X 60. (Technic 5, p. 302.) a, Epithelium, h, stroma, of mucous membrane; c, sub- mucosa; d, inner muscle layer; e, middle muscle layer;/, outer muscle layer. full or empty. In the collapsed organ the superficial cells are cuboidal or even columnar, their under surfaces being marked by pit-like depressions caused by pressure of underlying cells. Beneath the superficial cells are several layers of polygonal cells, while upon the basement membrane is the usual single layer of small cuboidal cells. In the moderately distended bladder the superficial cells become flatter and the entire epithelium thinner (Fig. 204). In the distended organ there is still further flattening of the superficial cells and thinning of the entire epithelium. The stroma consists of fine loosely arranged THE URINARY SYSTEM. 299 connective tissue containing many lymphoid cells and sometimes small lymph nodules. It merges without distinct demarcation into the less cellular suhmucosa (Fig. 204, c). The three muscular layers of the lower part of the ureter are con- tinued on to the bladder, where the muscle bundles of the different layers interlace and anastomose, but can be still indistinctly differ- entiated into an inner longitudinal, a middle circular, and an outer longitudinal layer (Fig. 204, d, e,f). The fibrous layer is similar to that of the ureter, and attaches the organ to the surrounding structures. The blood- and l5miph-vessels have a distribution similar to those of the ureter. Nerves. — Sensory medullated fibres pierce the muscularis, branch repeatedly in the stroma, lose their medullary sheaths, and terminate among the cells of the lining epithelium. Sympathetic fibres form plexuses in the fibrous coat, where they are interspersed with numerous small groups of ganglion cells. Axones of these sympathetic neurones penetrate the muscularis. Here they form plexuses, from which are given off terminals to the individual muscle cells. For development of urinary system see page 346. The Suprarenal Gland. The suprarenal is a ductless gland situated on the upper and ante- rior surface of the kidney. It is surrounded by a capsule and consists of an outer zone or cortex and a central porton or medulla. The cortex and medulla are sharply differentiated both in general appearance, and in histological structure. The former is of rather firm consistency, its cells are arranged in rows with the blood-vessels between them, giving the zone a striated appearance. Its cells contain fat droplets and peculiar granules known as lipoid granules which give the cortex a yellowish tint. In contrast the medulla is soft, vascular, has a dark reddish appearance, and its cells contain granules known as chromaffin or pJiceochrome granules. The CAPSULE (Fig. 205, ,4) is composed of fibrous connective tissue and smooth muscle. In the outer part of the capsule the connective tissue is loosely arranged and merges with the surrounding fatty areolar tissue. The inner layer of the capsule is more dense and forms a firm investment for the underlying glandular tissue. From the capsule trabeculae extend into the orran forming; its framework and outlining 300 THE ORGANS. compartments, which contain the glandular epithelium. This con- nective tissue is reticular in character. The CORTEX (Fig. 205, B) is subdivided into three layers or zones: (a) A narrow, superficial layer, the glomerular zone; (b) a broad middle layer, the fascicular zone; and (c) a narrow deep layer, the reticular zone. The names of the layers are indicative of the shape of the connective-tissue- enclosed compartments and of the contained groups of gland cells. In the glomerular zone (Fig. 205, a) the high, irregularly columnar epithelium is arranged in spherical or oval groups. The protoplasm of the cells is granular, and their nuclei are rich in chromatin. In the fascicular zone (Fig. 205, h) polyhedral cells are arranged in long columns or fascicles. The cytoplasm is granular and usually contains some or many fat droplets. The nuclei contain less chromatin than those in the glomerular zone. In the reticular zone (Fig. 205, c) similar though some- what more darkly staining cells form a coarse reticulum of irregular anas- tomosing cords. The MEDULLA (Fig. 205, C) con- sists of spherical and oval groups and cords of polygonal cells. After alcohol or formalin fixation these cells take a renal." (Merkcl-Henle.) .4 , cl'psule ; P^l^^ ^tain than those of the COrtCX. 5, cortex; C, medulla; a, glomerular After fixation in solutions Containing zone; b, fascicular zone; c, reticular ... zone; v, vein in medulla. chromic acid or chrome salts the cells of the medulla assume a peculiar characteristic deep brown color, which cannot be removed by washing in water and which is due to chromaffin granules which they contain. Scattered in irregular groups among these cells are many sympathetic ganglion cells. Blood-vessels. — The arteries supplying the suprarenal first form a THE URINARY SYSTEM. 3U1 poorly defined plexus in the capsule. From this are given off three sets of vessels — one to the capsule, one to the cortex, and one to the medulla. The first set breaks up into a network of capillaries, which supply the capsule. The vessels to the cortex break up into capillary networks, the shape of the mesh corresponding to the arrangement of the connective tissue in the different zones. The vessels to the medulla pass directly through the cortex without branching and form dense capillary networks among the groups of medullary cells. The relations of the capillaries to these gland cells are extremely intimate, especially in the reticular zone and medulla, where the cells in many cases im- mediately surround the capillaries in much the same manner as the glandular cells of a tubular gland surround their lumina. From the capillaries of both cortex and medulla small veins arise. These unite to form larger veins which empty into one or two main veins situated in the centre of the medulla. Lymphatics. — These follow in general the course of the blood- vessels. The exact distribution of the suprarenal lymph system has not been as yet satisfactorily determined. Nerves. — The nerve supply of the suprarenal is so rich and the nerve elements of the gland are so abundant as to have led to its classification by some among the organs of the nervous system. Both medullated and non-medullated fibres — but chiefly the latter — form plexuses in the capsule, where they are associated with groups of sympathetic ganglion cells. From the capsular plexuses fine fibres pass into the cortex, where they form networks around the groups of cortical cells. The nerve terminals of the cortex apparently do not penetrate the groups of cells. Bundles of nerve fibres, larger and more numerous than those to the cortex, pass through the cortex to the medulla. Here they form unusually dense plexuses of fibres, which not only surround the groups of cells, but penetrate the groups and sur- round the individual cells. iVssociated with the plexuses of the medulla, less commonly of the cortex, are numerous conspicuous groups of sympathetic ganglion cells. Development. — The cortex and medulla have entirely dift'erent developmental histories. In the lower vertebrates (fishes) the two parts of the gland continue separate throughout life. In the ascending mammalian scale, the two parts become more and more closely united until in mammals they form a single organ. The cortex develops from mesoderm, first appearing in embryos of about five to six mm. At about the level of the cephalic third of the mesonephros the mesothelium 302 THE ORGANS. sends outgrowths into the mesenchyme. These outgrowths soon lose their connection with the main mass of mesothehum and constitute the anlage of the suprarenal cortex. The medulla has an entirely independent origin, being derived from ectoderm, as part of the peripheral sympathetic nervous system. The cells of some of the sympathetic ganglia differentiate into sympathoblasts and phaochromo- blasts, which give rise to the sympathetic cells and chromaffin cells respectively. These cells soon separate from their ganglia of origin and come to lie first near, then within, the developing cortex, thus forming the medulla. TECHNIC. (i) Fix the simple kidney of a rabbit or guinea-pig in formalin-Miiller's fluid (technic 5, p. 7). Make sections through the entire organ including the papilla and pelvis, stain with haematoxylin-eosin (technic i, p. 18), and mount in balsam. This section is for the study of the general topography of the kidney. (2) Fix small pieces from the different parts of a human kidney in formalin- Miiller's fluid or in Zenker's fluid. Thin sections should be made, some cutting the tubules longitudinally, others transversely, stained with haematoxylin-eosin and mounted in balsam. (3) Blood-vessels. — For the purpose of demonstrating blood vessels of the kidney the method of double injection is useful (page 24). (4) Ureter. — Cut transversely into short segments, fix in formalin-Miiller's fluid (technic 5, p. yj, and stain transverse sections with hsematoxylin-eosin (tech- nic I, p. 18), or with haematoxylin-picro-acid-fuchsin (technic 3, p. 19). Mount in balsam. (5) Bladder (technic i, p. 223, or technic 2, p. 223). By the latter method any •desired degree of distention may be obtained. (6) Suprarenal. Technic same as (2) above. Thin vertical sections should include both cortex and medulla. General References for Further Study. Kolliker: Handbuch der Gewebelehre, vol. iii. Gegenbauer: Lehrbuch der Anatomie des Menschen, vol. ii. Henle: Handbuch der Anatomie des Menschen, vol. ii. Johnston: A Reconstruction of a Glomerulus of the Human Kidney. Johns Hopkins Hosp. Bui., vol. xi., 1900. Miiller: Ucber die Ausscheidung des Methylenblau durch die Nieren. Deut.sches Archiv f. klin. Med., Bd. 63, 1899. Flint: The Blood-vessels, Angiogenesis, Organogenesis, Reticulum and Histology of the Adrenal. Contributions to the Science of Medicine, John Hopkins Pre.ss, 1900. Pfaundler: Zur Anatomie der Nebenniere. Anzeiger Akad. Wien, 29, 1892. Nagel: Ueber die P^ntwickelung des Urogenitalsystem des Menschen. Arch, f. Mik. Anat., Bd. xxxiv. CHAPTER IX. THE REPRODUCTIVE SYSTEM. I. MALE ORGANS. The Testis. The testes are compound tubular glands. Each testis is enclosed in a dense connective-tissue capsule, the tunica albuginea (Fig. 206, a). Outside the latter is a closed serous sac, the tunica vaginalis, the visceral layer of which is attached to the tunica albuginea, while the parietal layer lines the inner surface of the scrotum. Posteriorly the serous sac is wanting, the testis lying behind and outside of the tunica vaginalis. As the latter is derived from the peritoneum, being brought down with and invagi- nated by the testes in their descent to the scrotum, it is lined by mesothelial cells. To the inner side of the tunica albuginea is a layer of loose connective tissue rich in blood-vessels, the tunica vasculosa. Posteriorly the tunica albu- ginea is greatly thickened to form the corpus Highmori, or mediastinum testis, from which strong connective-tissue septa radiate (Figs. 206, w and 207, b). These septa pass completely through the organ and blend with the tunica albuginea at various points. In this way the interior of the testis is subdi- vided into a number of pyramidal chambers or lobules, with bases directed toward the periphery and apices at the mediastinum (Figs. 206 and 207). Behind the testis and outside of its tunica albuginea is an elongated body — the epididymis (Figs. 206, c and 207, c). consisting of con\"olutcd 303 Fig. 206. — Diagram illustrating the Course and Relations of the Seminif- erous Tubules and their E.xcretory Ducts. (Piersol.) a. Tunica albu- ginea; b, connective-tissue septum between lobules; in, mediastinum; /, convoluted portion of seminiferous tubule; s, straight tubule; r, rate testis; e, vasa efferentia; c, tubules of head of epididymis; te, vas epi- didymis; vd, vas deferens; va, vas aberrans; p, paradidymis. 304 THE ORGANS. '•??/ 1' tubules continuous with those of the mediastinum. The epididymis is di\ided into three parts: an expanded upper extremity, the head or globus major (Figs. 206 and 207, c); a middle piece, the body (Fig. 207, (/); and a shghtly expanded lower extremity, the tail or globus minor. From the last named passes off the main excretory duct of the testis, the vas deferens (Fig. 206, vd). All of the tubules of the epididymis are continuous on the one hand with the tubules of the testicle, and on the other with the vas deferens. They thus constitute a portion of the complex sys- tem of excretory ducts of the testicle. The seminiferous tubule may be divided with reference to structure and location into three parts, (i) A much convo- luted part, the convoluted tubule, which begins at the base and occupies the greater portion of a lobule of the testis (Fig. 210, a). As they approach the apex of a lobule several of these convoluted tubules unite to form (2) the straight tubule (Fig. 206, s, 210). This passes through the apex of the lobule to the mediastinum, where it unites with other straight tubules to form (3) the irregular network of tubules of the medias- tinum, the rete testis (Fig. 210, c). I. The Convoluted Tubule. — This, which may be considered the most important secreting portion of the lobule, since it is here that the spermatozoa are formed, has a diameter of from 150 to 250/L The tubules begin, some blindly, others by anastomoses with neigh- boring tubules, near the periphery of the lobule, and pursue a tortuous course toward its apex (Fig. 210, a). The wall of the convoluted tubule (Fig. 208) consists of three 1 Fig. 207. — Longitudinal SectiJfe \~''X' '^.flV^f^V'^- i,;lJ^- m .<^ l Fig. 210. Fig. 210. — Passage of Convoluted Part of Seminiferous Ti bules into Stra'ght Tubules and of these into the Rete Testis (Milhall-iowicz.) a, Convoluted part of tubule; b, fibrous stroma continued from the mediastinum testis; r, rete testis. Fig. 211. — Spermatoblast with some Adjacent Sperm Cells, from Testis of Sparrow. (From Kolliker, after Etzold.) ^1/, Basement membrane; s, nucleus of Sertoli cell; sp. spermatogones; sc, spermatocyte; st^ and st.2, spermatids lying along the surface of the Sertoli cell, s' and 5^3; at st^ are seen the nearly mature spermatozoa; /, tuft-like arrange- ment of bodies of spermatids around free end of Sertoli cell, with two mature spermatozoa. of the seminiferous tubule. They are the direct progenitors of the spermatozoa. (For details of spermatogenesis see page 314.) In the actively secreting testicle spermatozoa are frequently found either free in the lumen of the tu!)ulc or with their heads among the superficial cells and their tails extending out into the lumen (Figs. 208, sP and 211). There are also found in the lumen many small 308 THE ORGANS. cells with dark nuclei. These are spermatids which have become free and which degenerate without forming spermatozoa. Separating and supporting the convoluted tubules is a small amount of interstitial connective tissue in which are the blood-vessels and nerves. Among the usual connective-tissue elements are found 4 '/ x/ * - /,- i- //^ " s J <«>--«rt c 2^^ VC V , rf»urf**»w -r-> ^ \ \ - ; ' r s r t ^ ' \> x.-^' •^ ^-»^" ..--r-^ /' .'l' >' '' Fig. 212. — From Section through Human Mediastinum and Rete Testis. X 96. (Kolliker.) A, Artery; F, vein;Z, lymph space; C, canals of rete testis; 5, cords of tissue projecting into the lumina of the tubules and so cut transversely or obliquely; 5^, section of convoluted portion of seminiferous tubule. groups of rather large spherical cells with large nuclei — interstitial cells. They are believed to represent remains of the Wolffian body (Fig. 209, c). 2. TuK Straight Tubule. — With the termination of the con- voluted portion, the spcrmatogenic tissue of the gland ends, the remainder of the tubule constituting a complex system of excretory ducts. The straight tubule is much narrower than the convoluted, THE REPRODUCTIVE SYSTEM. 309 having a diameter of from 20 to 40/'.. It is lined by a single layer of cuboidal cells resting upon a thin basement membrane. At the apex of the lobule the straight tubules become continuous with the tu- ■^ bules of the rete testis. 3. The Tubules of the Rete Testis. — These are irreg- ular canals which vary greatly in shape and size. They are lined with a single layer of low cuboidal or flat epithelial cells (Fig. 212, C). The Seminal Ducts. — While the already described straight tubules and the tubules of the rete testis must be regarded as part of the complex excretory duct system of the testis, there are certain structures which are wholly outside the testis proper, which serve to transmit the secretion of the testis, and are known as the seminal Fig . — Part of a Cross Section through a \"as Eflterens of the Human Epididymis. X 140. (Kolliicer.) F, High columnar ciliated epithelium; d, lower non-ciliated epithelium, presenting appearance of a gland; d', the same cut obliquely. Fig. 214. — From Cross Section through Head of Ejiididymis. X 35. (KoUiker.) b. Interstitial connective tissue; c, sections through tubules of epididvmis, showing two- layered columnar epithelium; g, blood-vessel. ducts. On lea^■ing the testis these ducts form the epididymis, after which they converge to form the main excretorv duct of the testis, the vas deferens. The Epididymis. — From the tubules of the rete testis arise from eight to lifteen tubules, the vasa efferent ia. or efferent ducts of the 310 THE ORGANS. testis (Fig. 206, e). Each vas efferens pursues a tortuous course, is separated from its fellows by connective tissue, and forms one of the lobules of the head of the epididymis. The epithelium of the vasa efferentia consists of two kinds of cells, high columnar ciliated cells (Fig. 213, F), and, interspersed among these, low cuboidal non-ciliated cells (Fig. 213, d). Occasionally som^e of the high cells are free from cilia and some of the cuboidal cells may bear cilia. The cuboidal cells lie in groups between groups of the ? ' . , ,, . higher cells, often giving the ap- ' '^ pearance of crypt-like depressions. "" "■ " - , , - • ' These have been referred to as in- traepithelial glands. They do not, however, invaginate the underly- ing tissues. The epithelium rests upon a basement membrane, be- neath which are several layers of circularly disposed smooth muscle cells. T^,^ ,• .• 1 c .• .V, u v^r n ( The vasa effcrcntia convcrgc to riG. 215. — Vertical Section through Wall of o " 1 Connective-tissue and smooth muscle cells; Here the epithelium IS of the Stratl- e, basal layer of epithelial cells; /high col- fig^ variety, there being two or umnar epithelial cells; p, pigment granules _ in columnar cells; c, cuticula; h, cilia. three rOWS of Cclls. The SUrface cells are narrow, high, and ciliated, and their nuclei are placed at different levels (Fig. 215). The cilia are long and each cell has only a few cilia. The deeper cells are irregular in shape. The basement membrane and muscular layers arc the same as in the vasa efferentia. As the vas deferens is ap- proached the muscular coat becomes thickened, and is sometimes strengthened by the addition of scattered bundles of longitudinally disposed cells. Thk Vas Deferens. — The walls of the vas deferens consist of four coats — mucosa, submucosa, muscularis, and fibrosa (Fig. 216). The mucosa is folded longitudinally, and is composed of a stroma and a lining ejjithelium. The epithehum is of the stratified columnar type with two or three rows of cells, being similar to that lining the vas epididymis. The extent to which the epithehum is ciliated varies greatly. In some cases the entire vas is ciliated, in others only the upper portion, in still others no cilia are present beyond the epididymis. The epithelium rests upon a basement membrane beneath which is a THE REPRODUCTIVE SYSTEM. 311 fibro-elastic cellular stroma. The stroma merges without distinct demarcation into the more vascular suhmucosa. The muscularis consists of two strongly developed layers of smooth muscle, an inner circular and an outer longitudinal (Fig. 216), which together constitute about seven-eighths of the wall of the vas. At the beginning of the vas deferens a third layer of muscle is added ^' -- c -- d -- ^"^^ Fig. 216. — Cross Section of Human Vas Deferens. X 37. (Szymonowicz.) a. Epithelium; b, stroma; c, submucosa; d, inner circular muscle layer; p, outer longitudinal muscle layer;/, fibrous layer; g, blood-vessels. composed of longitudinal bundles, and situated between the inner circular layer and the submucosa. T\it jihrosa consists of fibrous tissue containing many elastic fibres. Near its termination the vas dilates to form the ampulla, the walls of which present essentially the same structure as those of the ^"as. The lining c])ithelium is, howe^■er, frequently markedly pigmented and the mucosa contains l)ranched tulnilar glands. The Seminal Vesicles and Ejaculatory Ducts. — The seminal vesicles. The walls of the seminal vesicles are similar in structure to those of the ampulla. The ei)ithclium is ])seudo-stratilicd with two or three rows of nuclei and contains a yellow pigment. When the vesicles are distended the epithelium flattens out and the nuclei lie more in one plane, thus giNing the ap])earance of an ordinarv simple columnar epitheliimi. Beneath the eiMthclitu-n is a ihin stroma, out- 312 THE ORGANS. side of which is an inner circular and an outer longitudinal layer of smooth muscle, both layers being much less developed than in the vas. The seminal vesicles are to be regarded as accessory genital glands. The ejaciilatovy ducts are lined with a single layer of columnar cells. The muscularis is the same as in the ampulla except that the inner circular layer is thinner. In the prostatic portion of the duct the muscularis is indistinct, merging with the muscle tissue of the gland. The ducts empty either directly into the urethra or into the urethra through the vesicula prostatica. Rudimentary Structures Connected with the Development of the Genital System. — Connected with the testicle and its ducts are remains of certain foetal structures. These are: (i) The paradidymis, or organ of Gir aides, situated between the vessels of the spermatic cord near the testis. It consists of several blind tubules lined with simple columnar ciliated epithelium. (2) The ductus aberrans Halleri, found in the epididymis. It is lined with simple columnar ciliated epithelium and opens into the vas epididymis. Instead of a single ductus aberrans, several ducts may be present. (3) The appendix testis (stalked hydatid or hydatid of Morgagni)| in the upper part of the globus major. It consists of a vascular con nective tissue surrounding a cavity lined with simple columnar ciliate epithelium: (4) The appendix epididymidis, a vascular structure, not always present, lying near the appendix testis. It resembles the latter in structure. The paradidymis and ductus aberrans Halleri probably represent remains of the embryonal mesonephros. The appendix testis and the appendix epididymidis are believed by some to be derived from the j primitive kidney, by others from the embryonal duct of Muller. Blood-vessels. — Branches of the spermatic artery ramify in the mediastinum and in the tunica vasculosa. These send branches into the septa of the testicle, which give rise to a capillary network among the convoluted tubules. From the ca|jillaries arise veins which accompany the arteries. Lymph capillaries begin as clefts in the tunica albuginea and in the connective tissue surrounding the seminiferous tubules. These connect with the more dermile lymph vessels of the mediastinum and of the spermatic cord. THE REPRODUCTIVE SYSTEM. 313 Nerves. — Non-meduUated nerve fibres form plexuses around the blood-vessels. From these, fibres pass to plexuses among the semi- niferous tubules. Their exact method of termination in connec- tion with the epithelium has not been determined. In the epididy- Acrosome Head< Neck Body ^ End ring ' ■• Galea capitis Main segment ^ of tail Anterior end knob Posterior end knob Spiral fibers .Sheath of axial thread mis are found small sympathetic ganglia. The walls of the vasa efferentia, vas epididymis, and vas deferens contain plexuses of non- medullated nerve fibres, which give oft" terminals to the smooth muscle cells and to the mucosa. The Spermatozoa. — The spermatozoa are the specific secre- tion of the testicle. Human sperm- atozoa are long, slender flagellate bodies, from 50 to ~]oa in length, and are suspended in the semen, which is a secretion of the acces- sory sexual glands. The general shape is that of a tadpole; and by means of an undulatory motion of the tail, the spermatozoon is capa- ble of swimming about freely in a suitable medium. It has been esti- mated that the human spermatozoa average about sixty thousand per cubic millimetre of semen. The human spermatozoon con- sists of (i) a head, (2) a middle piece or body, and (3) a tail or flagellum (Fig. 217). The head, from 3 to ^ix long and about half that in breadth, is oval in shape when seen on flat, pear- shaped when seen on edge. It con- sists mainly of chromatin derived from the nucleus of the parent cell. Enveloping the nuclear material of the head is a thin layer or delicate membrane of cytoplasm, the galea eapitis. The front of the head Axial thread -Capsule Fig. 217. — Diagram of a Human Sperma- tozoon. (Meves, Bonnet.) 314 THE ORGANS. ends in a sharp edge, the apical body or acrosome. In some lower forms the acrosome is much more highly developed than in man and extends forward as a pointed or barbed spear, the perforatorium. The acro- some is differentiated from the nuclear portion of the head by taking an acid dye, the chromatin, of course, taking a basic stain. The body is cylindrical, about the same length as the head, and consists of a fibrillated central core, the axial thread, surrounded by a protoplasmic capsule. A short clear portion, the neck, unites the head and body. Just behind the head the axial thread presents a bulbous thickening, the anterior end knob, which tits into a depression in the head. At the junction of neck and body are one or several posterior end knobs to which the axial thread is attached. The latter leaves the body through a perforated ring, the end ring or end disk. Delicate fibrils — spiral fibres — run spirally around the body portion of the axial thread. The tail consists of a main segment, from 40 to 6o/< in length, and a terminal segment having a length of from 5 to 10//. The main seg- ment has a central fibrillated axial thread which is continuous with the axial thread of the body. This is enclosed in a thin cytoplasmic membrane or capsule continuous with the capsule of the body. This membrane, inconspicuous and apparently structureless in man, is remarkably developed in some lower forms, e.g., the membrana undulata of birds. The terminal segment consists of the axial thread alone. The motility of the spermatozoon depends entirely upon the flagellate movements of the tail. In many of the lower animals the spermatozoon has a much more complicated structure. Of the above-described parts of the spermatozoon only the head and tail can usually be differentiated, except by the use of special methods and very high-power objeectives. Development of the Spermatozoa. — As already noted in describing the testicle, the spermatozoa are developed from the epithe- lial cells of the seminiferous tubules. The most peripheral of the tubule cells, the spermatogones (Fig. 208, sp and Fig. 209, sp) are small round cells with nuclei rich in chromatin. By mitosis the s])ermato- gone gives rise to two daughter cells, one of which remains at the periphery as a spermatogone, while the other takes up a more central position as a spermatocyte (Fig. 209, sc and Fig. 211, sc). The latter are rather large spherical cells, whose nuclei show very distinct chro- matin networks. By mitotic di\ision of the si^ermatocytes of the innermost row are formed the spermatids (Fig. 209, st and Fig. 211, st). THE REPRODUCTIVE SYSTEM. 315 These are small spherical cells, which line the lumen of the tubule and are the direct progenitors of the spermatozoa. In the transformation of spermatocyte into spermatid an extremely important change takes place in the nucleus. This consists in a reduction of its chromosomes to one-half the number specific for the species (page 52). The transfor- mation of the spermatid into the spermatozoon differs somewhat in different animals and the details of the process must be regarded as not Hea. Anterior end kncli "~m^-^ Posterior end knob ^ Body End ring Tai r- Head Anterior end knob Posterior end knob End ring Nucleus Cytoplasm '.^ Proximal centroscme Distal centroscme Fig. 218. — Transformalion of a Spermatid into a Spermatozoon (human). Schematic. (Aleves, Bonnet.) yet definitely determined. The nucleus of the spermatid first becomes oval in shape, and its chromosomes become condensed into a small homogeneous mass, which forms the head of the spermatozoon. During their transformation into the heads of the spermatozoa, the nuclei of the spermatids arrange themselves in tufts against the inner ends of the cells of Sertoli. This compound structure, consisting of a Sertoli cell and of a group of de\"eloping spermatozoa attached to its 316 THE ORGANS. central end, is known as a spermatoblast (Fig. 211). The body or middle piece of the spermatozoon is described by most investigators as derived from the centrosome, while the tail is a derivative of the cytoplasm. The details of the transformation of the spermatid into the spermatozoon are illustrated in Fig. 218. The centrosome either divides completely, forming two centrosomes, or incompletely, forming a dumb-bell-shaped body. The nuclear material becomes very compact and passes to one end of the cell, forming the bulk of the head. Both centrosomes help to form the body. They first become disk- shaped. The one lying nearer the center of the cell becomes attached to the posterior end of the head as the anterior end-knob, the other takes a position a little more posteriorly as the posterior end-knob and from it grows out the axial filament. Some of this centrosome passes to the posterior limits of the body and then becomes the end ring. As the two parts of this centrosome separate the delicate cytoplasm between them forms the spiral fibres. During these changes the axial filament has been growing and projects beyond the limits of the cell. Most of the cytoplasm of the spermatid is not used in the formation of the spermatozoon, but is cast off and degenerates. A small amount is used for the sheath of the body, and for the galea capitis. The sheath of the main part of the filament appears to develop from the filament itself. The significance of the different parts of the spermatozoon has been brought out in describing its development. From this it is seen that the spermatozoon, like the mature ovum, is a true sexual element with one-half the somatic number of chromo- somes. The head and body, containing the chromatin and the centrosomes, are the parts of the spermatozoon essential to fertilization. The acrosome is an acces- sory which in some forms at least aids the spermatozoon in attaching itself to and in entering the ovum. The tail is an accessory structure which provides motion, enabling the spermatozoon to move about freely in the semen and in the fluids of the female generative tract. Considering ther minuteness, their speed is consider- able, having been estimated at from 1.5 to 3.5 mm. per minute; enough to allow them to ascend through uterus and oviduct against the adverse action of the cilia. When in a favorable environment, such as the fluids of the female generative tract, the spermatozoon is capable of living for some time after leaving the testicle. Living spermatozoa have been found in the uterus or tubes one to three and a half weeks after coitus. TECHNIC. (i) For the study of the general topography of the testis, remove the testis of a new-born child, make a deep incision through the tunica albuginea in order to allow the fixative to penetrate quickly, and fix in formalin-Miiller's fluid (technic 5, p. 7). Antero-po.sterior longitudinal sections through the entire organ and in- cluding the epididymis should be stained with haematoxylin-picro-acid-fuchsin Ctechnic 3, p. 19) or with hicmatoxylin-eosin (technic i, p. 18) and mounted in balsam. (2) The testis of a young adult is removed as soon after death as possible, is THE REPRODUCTIVE SYSTEM. 317 cut into thin transverse slices, which include the epididymis, and is iixed in fornia- lin-Mliller's or in Zenker's fluid (technic 9, p. 8). Select a slice which includes the head of the epididymis, cut away the anterior half or two-thirds of the testis proper in order to reduce the size of the block, and, after the usual hardening and embedding, cut thin sections through the remaining posterior portion of the testis, the mediastinum, and epididymis. Stain with hasmatoxylin-eosin (technic i, p. 18) and mount in balsam. (3) For the study of spermatogenesis fix a mouse's testis in chrome-acetic- osmic mixture (technic 7, p. 7). Harden in alcohol and mount thin unstained sections in balsam or in glycerin. (4) Spermatozoa. — Human spermatozoa may be examined fresh in warm nor- mal saline solution or fixed in saturated aqueous solution of picric acid and mounted in glycerin. Mammalian spermatozoa may be obtained from the vagina after intercourse, or by incision into the head of the epididymis. Technic same as for human. (5) A portion of the vas deferens is usually removed with the testis and may be subjected to technic (2) above. Transverse sections are stained with hsematoxylin- eosin and mounted in balsam. The Prostate Gland. The prostate is described by some as a compound tubular, by others as a compound alveolar gland. It is perhaps best regarded as a collection of simple branched tubular glands with dilated termi- nal tubules. These number from forty to fifty, and their ducts con- verge to form about tw^enty main ducts, which open into the urethra. The gland is surrounded by a capsule of fibro-elastic tissue and smooth muscle cells, the muscle cells predominating. From the capsule broad traheculcE of the same structure as the capsule pass into the gland. The amount of connective tissue is large. It is less in the prostate of the young than of the old. The hypertrophied pre state of age is due mainly to an increase in the connective-tissue elements. The tubules have wide lumina and are lined with simple cuboidal epithelium of the serous type, resting upon a delicate basement mem- brane (Fig. 219). Less commonly the epithelium is pseudo-stratified. The ducts are lined with simple columnar epithelium until near their terminations, where they are Hned with transitional epithelium similar to that lining the urethra. Peculiar concentrically laminated bodies. crescentic corpuscles, or corpora amylacea, are frequently present in the terminal tubules (Fig. 219, c). They are more numerous after middle life. Through the prostate runs the prostatic portion of the urethra. Within the prostate is found the vcsiciila prostatica (ittriciihis 318 THE ORGANS. prostaticus — uterus masculinus). It represents the remains of a foe- tal structure, the MiiUerian duct and consists of a blind sac with folded mucous membrane lined with a two-rowed ciliated epithelium which dips down to form short tubular glands. The prostatic secretion is serous. The blood-vessels of the prostate ramify in the capsule and tra- beculae. The small arteries give rise to a capillary network which surrounds the gland tubules. From these arise small veins, which accompany the arteries in the septa and unite to form venous plexuses in the capsule. a • jr / . '- >', b/----' " - ^ Fig. 219. — Section of Human Prostate. X 150. (Tcchnic i, p. .^ig.) a, Epithelium of tubule; h, interstitial connective tissue; c, corpora amylacea. The lymphatics begin as blind clefts in the trabecute and follow the general course of the blood-vessels. Nerves, — Small groups of sympathetic ganglion cells are found in the larger trabeculae and beneath the capsule. Axones of these cells pass to the smooth muscle of the traljeculae and of the walls of the blood-vessels. Their mode of termination is not known. Timofeew describes afferent medullatcd fibres ending within capsular structures of flat nucleated cells. Two kinds of fibres pass to each capsule: one a large medullated fibre which loses its sheath and gives rise within the capsule to several flat fibres with serrated edges, the other small medullated fibres which lose their sheaths and split up into THE REPRODUCTIVE SYSTEM. 319 small varicose fibrils which form a network around the terminals of the large fibre. Cowper's Glands. The bulbo-urethral glands, or glands of Cowper, are small, com- pound tubular glands. Both tubules and ducts are irregular in di- ameter so that some of them have more the character of alveoli than of tubules. They are lined with mucous cells. The smaller ducts are lined with simple cuboidal epithelium. They unite to form two main excretory ducts which open into the urethra and are lined with stratified columnar epithelium consisting of two or three layers of cells. In the main duct, as well as in its branches, smooth muscle occurs. TECHNIC, (i) Fix small pieces of the prostate of a young man in formalin-Miiller's fluid (technic 5, p. 7). Stain sections with htematoxylin-eosin (technic i, p. 18) and mount in balsam. (2) The prostate of an old man should be treated with the same technic and compared with the above. (3) Cowper's glands. Same technic as prostate (i). The Penis. The penis consists largely of three long cyhndrical bodies, the corpus spongiosum and the two corpora cavernosa. The latter lie side by side, dorsally, while the corpus ^ spongiosum occupies a medial ventral position (Fig. 220). All three are enclosed in a common connective-tissue capsule which is loosely attached to the o\ev- lying skin. In addition each corpus has its own special cap- sule or tunica albuginea, about a millimetre in thickness, and composed of dense connective tissue containing many elastic fibres. The corpus spongiosum and corpora cai'crnosa have essentially the same structure, being composed of so-called erectile tissue (Fig. 221). This consists of thick trabeculae of intermingled fibro-elastic tissue and bundles of sm.ooth muscle cells, which anastomose to form a coarse Fig. 220. — Tran.s\er>e Scnion through Human Penis, a. Skin: b, subcutaneous tissue; c, fibrous tunic; d, dorsal vein; e, corpora cav- ernosa: /, corpus spongiosum; g, urethra. 320 THE ORGANS. meshed network, the spaces of which are hned with endothehum. The spaces are known as cavernous sinuses, and communicate with one another, and with the blood-vessels of the penis. In the flaccid condition of the organ these sinuses are empty and their sides are in apposition. In erection these sinuses become filled with venous blood. The arteries have thick muscular walls and run in the septa. A few of them open directly into the venous sinuses. Most of them give rise to a superficial capillary network beneath the tunica albu- ginea. From this capillary plexus the blood passes into a plexus of \-- ■' ■' !£"■■:- r/ ■M^'^ Fig. 221. — Erectile Tissue of Corpus Spongiosum of Human Penis. X 60. a, Trabecule of connective tissue and smooth muscle; b, cavernous sinuses; c, groups of leucocytes in sinus. broader venous channels in the periphery of the erectile tissue, and these in turn communicate with the cavernous sinuses. The usual direct anastomoses between arterial and venous capillaries also occur. The blood may therefore pass either through the usual course — arte- ries, capillaries, veins — or, under certain conditions, may pass through the cavernous sinuses. This determines the flaccid or the erect con- dition of the organ. The veins arise partly from the capillaries and partly from the cavernous sinuses. They pass through the tunica albuginea and empty into the dorsal vein of the penis (Fig. 220). In the corpus spongiosum there is probably no direct opening of arteries into sinuses. Both trabeculse and sinuses are also smaller. Of the lymphatics of the jjenis Httle definite is known. The nerve endings, according to Dogicl, consist of: (a) free sensory endings, (b) deeply situated genital corpuscles, (c) Pacinian corpus- THE REPRODUCTI\E SYSTEM. 321 10 f- cles and Krause's end-bulbs in the more superficial connective tissue, and (d) Meissner's corpuscles in the papillae. (For details see pages 387 and 388.) The glans penis consists of erectile tissue similar in structure to that of the corpus cavernosum, except that the venous spaces are smaller and more regular. The mucous membrane is very closely at- tached to the fibrous sheath of the ^ underlying erectile tissue. A few , ' small sebaceous glands, uncon- I nected with hairs — the glands of Tyson — are found in the mucous membrane of the base of the glans penis. The prepuce is a fold of skin which overlies the glans penis. Its inner surface is lined with mucous membrane. ff.&S3 ^■■> - d Fig. Fig. 223. Fig. 222. — From Transverse Section of Urethra and Corpus Spongiosum, including Mucous Membrane and part of Submucosa. X 15. The dari<. spots represent the cavernous veins. Fig. 223.^ — Vertical Section through Portion of Wall of Human Male Urethra. X 350. A, Mucous membrane; B, submucosa ; a, epithelium; b, stroma; c, cavernous veins: d, con- nective tissue of submucosa. The Urethra/ The MALE URETHRA is divided into three parts — prostatic, mem- branous, and penile. The wall of the urethra consists of three coats ' The female urethra, while not so distinctly divisible into sections, presents essentially the same structure as the male urethra. The epithelium begins at the bladder as stratified squamous of the transitional type, changes to a two-layered stratified or pseudostratified, and finally passes over into stratified squamous near the urethral opening. Glands of Littre are present, but are fewer than in the male. 322 THE ORGANS. — mucous, submucous, and muscular. The structure of the wall differs in the different parts of the urethra. The mucous membra?ie (Fig. 223) consists of epithelium and stroma. The epithelium of the prostatic part is stratified squamous (transi- tional), resembling that of the bladder. In the membranous part it is stratified columnar or pseudostratified. In the penile portion it is pseudostratified up to the fossa navicularis, where it changes to stratified squamous. The epithelium rests upon a basement mem- brane, beneath which is a thin stroma rich in elastic fibres and hav- ing papillae which are especially prominent in the terminal dilated portion of the urethra, the fossa navicularis. The stroma merges without distinct demarcation into the submucosa. The submucosa consists of connective tissue and, in the penile portion, of more or less longitudinally disposed smooth muscle. It contains a dense network of veins — cavernous veins — which give it the character of erectile tissue (Fig. 223). The muscular coat is thickest in the prostatic and membranous portions. Here it consists of a thin inner longitudinal and a thicker outer circular layer. A definite muscular wall ceases at the beginning of the penile portion, although circularly disposed smooth muscle cells are found in the outer part of the submucosa of the penile urethra. Throughout the mucosa of the entire urethra, but most numerous in the penile portion, are simple branched tubular mucous glands, the glands of Littre. They are lined with columnar epithelium and the longer extend into the submucosa. TECHNIC. ii) For the study of the general topography of the penis, remove the slcin from the organ and cut into transverse slices about 0.5 cm. in thickness. Fix in forma- lin-Miiller's fluid (technic 5, j). 7), cut rather thick sections across the entire penis, stain with hsematoxylin-picro-acid-fuchsin (technic 3, p. 19) or with hiema- toxylin-eosin (technic i, p. 18) and mount in balsam. (2) For the study of the structure of the penile portion of the urethra and of the erectile tissue of the corpus spongiosum, cut away the corpora cavernosa, leav- ing only the corpus spjongiosum and contained urethra, and treat as above. Sec- tions should be thin and stained with haematoxylin-eosin. (3) The same technic is to be used for the membranous and prostatic portions of the urethra. THE REPRODUCTRE SYSTEM. 323 II. FEMALE ORGANS. The Ovary. The ovary is classed as one of the ductless glands. Its specific secretion is the ovum. The ovary has no duct system which is directly continuous with its structure. In place of this it is pro\ided with what may be considered to be a highly specialized disconnected excretory duct — the oviduct or Fallopian tube — which serves for the transmission of its secretion to the uterus. On one side the ovary is attached by a broad base, the hilion, to the broad ligament. Elsewhere the surface of the ovarv is covered Fig. 224. — Ovary opened by Longitudinal Incision. Ovum has Escaped through Tear in Surface. Cavity of follicle filled with blood clot (corpus haemorrhagicum) and irregular projections composed of lutein cells. (Kollmann's Atlas.) by a modified peritoneum. At the hilum the tissues of the broad ligament pass into the ovary and spread out there to form the ovarian stroma. This consists of fibrous connective tissue rich in elastic fibres and containing many smooth muscle cells. In the deeper central portion of the organ, stroma alone is found. Here it contains many large blood-vessels, and constitutes the medulla or zmia vasculosa of the ovary (Fig. 225, 2). From the medulla the stroma radiates toward the surface of the o\"ary and becomes interspersed with glandular elements forming the ovarian cortex (Fig. 225, 3, 3'). At the surface of the ovary, just beneath the peritoneum, the stroma forms a rather 324 THE ORGANS. dense layer of fibrous tissue, the tunica albuglnea. At the margin of the peritoneal surface of the ovary the connective tissue of the perit- oneum becomes continuous with the stroma of the ovary, while the flat mesothelium of the general peritoneum is replaced by a single layer of cuboidal cells, which covers the surface of the ovary and is known from its function as the germinal epithelium (Fig. 226, he). The parenchyma or secreting portion of the ovary consists of peculiar glandular elements, the Graafian follicles. '^^'^^:£kl /m m M '^m^m ■"-^KjJ'f: ~^^%:\ ,x;^o>A\v.V Fig. 225. — Semidiagrammatic Drawing of Part of Cortex and Medulla of Cat's Ovary. (From Schron, in Quain's "Anatomy.") i, Germinal epithelium, beneath which is 3, the tunica albuginea; 2, medulla, containing large blood-vessels, 4; 2,2', fibrous stroma, arranged around mature Graafian follicle as its theca foUiculi; 3', stroma of cortex; 5, small (primitive) Graafian follicles near surface; 6, same deeper in cortex; 7, later stage of Graafian follicle, beginning of cavity; 8 and 8', still later stages in development of follicle; 9, mature follicle; a, stratum granulosum; b, germ hill; c, ovum; d, nucleus (germinal vesicle); e, nucleolus (germinal spot). The structure of the Graafian follicle can be best appreciated by studying its development. The follicles originate from the germinal epithelium during foetal life. At this time the germinal epithehum is proliferating, and certain of its cells differentiate into larger spherical cells — primitive ova (Fig. 226, op) . The primitive ova pass downinto the stroma accompanied ]jy a considerable number of the undifferentiated cells of the germinal epithelium. A cord-like mass of cells is thus formed, extending from the surface into the stroma. This is known as PJlilger's egg cord (Fig. 226). Each cord usually contains several ova. In some cases the differentiation of the ova cells does not occur uijon the surface but in the cords after they have extended THE REPRODUCTR E SYSTEM. 325 down from the surface. The connection of the cord with the surface epithehum is next broken so that each cord becomes completely sur- rounded Ijy stroma. It is now known as an eggnest (Fig. 226). During this process, proliferation of the epithelial cells of the cords and nests has been going on, and each ovum surrounded by a layer of epithelial cells becomes separated from its neighbors (Fig. 226, fp). This central ovum surrounded by a single layer of epithelial cells (follicular cells) is the primitive Graafian follicle (Fig. 226, fp, Fig. 227, and Fig. 228, a). Rarely a follicle may contain more than one ovum, of which, however, „-, at - i» % ^ , 10 ^^ o -.» .- -is X ^ ■ ^^ . Fig. 226. — From Transverse Section of Ovary of New-born Child. X 280 (Sobotta). Shows primitive ova in germinal epithelium; Pfliiger's egg cords and nests of cells; c, capillaries; he, germinal epithelium; sir, stroma; fp, primitive follicles; op, primitive ova. only one goes on to maturity, the others degenerating. The follicle increases in size, mainly on account of proliferation of the follicular cells, which soon form se\'eral layers instead of a single layer, but also partly on account of growth of the ovum itself (Fig. 228). The latter now lea^"es the centre of the follicle and takes up an eccentric position. At the same time a cavity (or several small ca\'ities which later unite) appears near the centre of the follicle (Fig. 228, e and Fig. 225, 7). This is filled with fluid which seems to be in part a secretion of the follicular cells, in part a result of their disintegration. The cavity is known as the follicular cai'ily or anlnnn. the fluid as the liquor foil iciili. 326 THE ORGANS. Lining the follicular cavity are several rows of follicular cells with granular protoplasm — the stratum granulosum. With increase in the liquor folliculi the ovum becomes still further pressed to one side of the folhcle, where, surrounded by an accumulation of folHcular cells, it forms a distinct projection into the cavity (Fig. 230, and Fig. 225, ^ Fig. and von d, ovum; follicle. 227. — Vertical Section thn;ujrh C(jrtex of Ovary of Young Girl. X igo. (Bohm Davidoff.) a, Germinal epithelium; h, tunica albuginea; c, follicular epithelium; e, primitive Graafian follicles in ovarian cortex; /, granular layer of large Graafian 8 and cj). I'his is known as the germ hill {discus proligcrus — cumulus ovigerus). The cells of the germ hill nearest the ovum Ijecome col- umnar and arranged in a regular single layer around the ovum — the corona radiata (Fig. 231). The ovarian stroma immediately sur- rounding the (jraafian follicle becomes somewhat modified to form a sheath for the follicle — the the ca folliculi (Fig. 229). This consists of two layers,' an outer more dense fibrous layer, the tunica fibrosa, THE REPRODL'CTIVE SYSTEM. 327 and an inner more cellular and vascular, the tunica vasculosa. Be- tween the theca folliculi and the stratum granulosum is an apparently structureless basement membrane. While these changes are taking place in the follicle, the ovum is also undergoing development. The ovum of the primitive follicle is a spherical cell, having a diameter of from 40 to yo/t and the structure of a typical cell. The nucleus or germinal vesicle (so called on account of the part it takes in reproduction) is about half the diameter of the cell, and is spherical and centrally placed (Fig. 228). It is surrounded i-'.- 1::. V :. v'' -■- Fig. 228. — From Section through Cortex of Ape's Ovary. X 150. (Szymonowicz.) a, Primitive follicle; h, ovum, with nucleus and nucleolus; c, zona pellucida; d, follicular epithelium; e, follicular cavity;/, ovarian stroma; g, blood-vessel in stroma. by a double-contoured nuclear membrane, and contains a distinct chromatic network and nucleolus or germinal spot. The cytoplasm is quite easily differentiated into a spongioplasm network and a homo- geneous hyaloplasm. Such ova are present in all active ovaries, i.e., during the childbearing period, but are especially numerous in the ovary of the infant and child (Fig. 227.) With the development of the follicle the ovum increases in size and becomes surrounded by a clear membrane, the zona pellucida, belie\"ed by some to be a cuticular formation deposited by the egg cell, by others to be a product of the surrounding follicular cells. Minute canals extend into the zona pellucida from its outer surface. These contain processes of the cells of the corona radiata. A narrow cleft, the periviteUinc space, has been described as se])arating the o\'um from the zona jjellucida. During the growth of the o\um its cyto])lasm 328 THE ORGANS. becomes coarsely granular from the development of yolk or deutoplasm granules (Fig. 231). Immediately surrounding the nucleus, and just beneath the zona pellucida, the egg protoplasm is fairly free from yolk granules. The further maturation of the ovum, which is necessary before the egg cell is in condition to be fertilized, consists in changes in the chromatic elements of the nucleus, v^hich result in the extrusion of the polar bodies, and apparently have as their main object the reduction in number of chromosomes to one-half the number characteristic of the species. This process has been described (page 53). In many ;'>A . '^>!^ • .^ i'* Kj^" "l''s^- -*5ii % ''^V; ' '. '*^^. ^ ■:^M0^' — / Fig. 22Q. — Section through Graafian Follicle of Ape's Ovary. X 90. (Szymonowicz.) Later stage of development than Fig. 228. a, Germ hill ; b, ovum with clear zona pellucida, germinal vesicle, and germinal spot; d, follicular epithelium (membrana granulosa); e, follicular cavity;/, theca folliculi; g, blood-vessel. of the lower animals maturation of the ovum is completed outside the ovary. In man and the higher animals the entire process takes place within the o\'ary, the second polar body being extruded just before the escape of the o\'um from its follicle. The youngest of the Graafian follicles are found just under the tunica albuginea near the germinal epithelium, from which they originate (Fig. 225, 5). As the follicle matures it passes deeper into the cortex. With complete maturity the follicle usually assumes macroscopic proportions — 8 to 12 mm. — and often occupies the entire thickness of the cortex, its theca at one point touching the tunica albuginea. Thinning of the follicular wall nearest the surface of the ovary next takes place dig. 232), while at the same time an increase in THE REPRODUCTRE SYSTEM. ;^29 the liquor tolliculi determines increased intrafoUicular pressure. This results in rupture of the Graaiian follicle and the discharge of its ovum, together with the liquor folliculi and some of the follicular cells. An escape of blood into the follicle from the torn vessels of the theca always accompanies the discharge of the ovum. The follicle again becomes a closed cavity, while the contained blood clot becomes Fig. 2.:;o — Graafian Follicle and Contained Ovum of Cat: directly reproduced from a photograph of a preparation by Dahlgren. X 235. (From "The Cell in Development and Inheritance," Prof. E. B. Wilson; The Macmillan Company, publishers.) The ovum is seen lying in the Graafian follicle within the germ hill, the cells of the latter immediately surrounding the ovum forming the corona radiata. The clear zone within the corona is the zona pellucida, within which are the egg protoplasm, nucleus, and nucleolus. Encircling the follicle is the connective tissue of the theca folliculi. organized by the ingrowth of vessels from the theca, to form the corpus hcEmorrhagkum (Fig. 233), which represents the earliest stage in the development of the corpus hitcuni. The corpus lutcum (Fig. 234), which replaces the corpus haemor- rhagicum, consists of large yellow cells — lutein cells — and of connective tissue. The latter with its blood-vessels is derived from the inner layer of the theca. The origin of the lutein cells is not clear. They are described bv some as deri\"ed from the connccti\"e-tissue cells of 330 THE ORGANS. the theca; by others as the result of prohferation of the cells of the stratum granulosum. The cells have a yellow color from the presence of fatty (lutein) granules in their protoplasm, and it is to these granules that the characteristic yellow color of the corpus luteum is due. A definite cellular structure with a supporting connective-tissue framework thus replaces the corpus hasmorrhagicum, remains of which are usually Fig. 231.— From a Scciion of a Human Ovum. Seclion taken from the ovary of a 12 year old girl. The ovum lies in a large mature Graafian follicle and is surrounded by the cells of the "germ hill " (the inner edge of which is shown in the upper left-hand corner of the figure). Photograph. (Bailey and Miller). present in the shape of orange-colored crystals of hsematoidin. By degeneration and subsequent absorption of its tissues the corpus luteum becomes gradually reduced in size, loses its yellow color, and is then known as the corpus albicans. This also is mostly absorbed, being finally represented merely by a small area of fibrous tissue. Corpora lutea are divided into true corpora lulea (corjjora lutea vera or corpora lutea of pregnancy) and false corpora lutea (corpora THE REPRODUCTIVE SYSTEM. 331 r^¥// Germinal einthelium ^xi*---; Tunica albugmea p/ ' Germ hill Theca foUicu h tv.V:^*^ with o\-um (vascular lai ers) 2t_. »\\ *{« ^v ^- wj I Theca folliculi (fibrous la\ er"* - — '[, 'M S^i Stratum granulosum Y\G.i^2. From Section of Human Ovar\-, showing mature Graatian follicle ready to rupture. (KoUmann's Atlas.) Lutein cells >^^-^,,;>e"^. ^jii^^^^u^'^'%~-2^ Point ot luptur Corpus hEemorrhagicum- Blood vessel of theca 'n ' -Cavitv of foUicle -Theca folliculi ,rr&- :anan stroma Stratum granulosum Fig. 233. — From Section of Human Ovary, showing early stage in formation of Corpus Luteum. (KoUmann's .Atlas.) 332 THE ORGANS. lutea spuria). The former replace follicles whose ova have under- gone fertilization, the latter, follicles whose ova have not been ferti- lized. The structure of both is similar, but the true corpus luteum is larger, and both it and its corpus albicans are slower in passing through their retrogressive changes, thus remaining much longer in the ovary. While the function of the curpus luteum is not known, the recent experiments of Fraenkel seem to be confirmatory of the theory ad- Point of rupture. Blood-vessel of theca Connective tissue Remnant of corpus hasmorrhagicum Lutein cells Connective tissue from theca Theca folliculi Blood-vessels of theca Fig. 234. — P'njm Section of Human Ovary, showing later stage of Corpus Luteum than Fig. 233. (Kohmann's Atlas.) \'anccd by Jiorn, that the corpus luteum is a gland having an internal secretion, which ajjpears to have some influence u])on the attachment of the fecundated ovum to the uterus and upon its nutrition during the first few weeks of its develo])ment. According to Fraenkel the cor]jus luteum is a perioflically rejux'enated ox'arian gland, which gives to the uterus a cyclic nutritional iminilse, which ];repares it for THE REPRODUCTR'E SYSTEM. 333 the implantation of the o\'um or favors menstruation whenever the ovum is not fertihzed. Of the large number of ova — estimated at seventy-two thousand in the human ovaries — only comparatively few, according to Henle about four hundred, reach maturity. The majority undergo, together with their follicles, retrogressive changes known as atresia of the follicle. The nucleus of the ovum, as well as the nuclei of the fol- licular cells, passes through a series of chromatolytic changes, or in some cases apparently simply atrophies. The cell bodies undergo fatty or albuminous degeneration and the cell becomes reduced to a homogeneous mass, which is finally absorbed, leaving in its place a connective-tissue scar, probably the remains of the theca folliculi. Blood-vessels. — The arteries, branches of the ovarian and uterine, enter the ovary at the hilum and ramify in the medulla. From these are given off branches which pass to the cortex and end in a capil- lary network in the tunica albuginea. In the outer layer of the theca folliculi the capillaries form a wide-meshed network, which gives rise to a fine-meshed network of capillaries in the inner layer of the theca. From the capillaries veins arise which form a plexus in the medulla and leave the ovary at the hilum. Lymphatics. — These begin as small lymph spaces in the cortex, which communicate with more definite lymph vessels in the medulla, the latter leaving the organ at the hilum. Nerves. — Medullated and non-medullated fibres enter the ovary at the hilum and follow the course taken by the blood-vessels. Many of the fibres end in the vessel walls; others form plexuses around the follicle and end in the theca folliculi. Some describe fibres as pass- ing through the theca and ending in the follicular epithelium. Others claim that nerve fibres do not enter the follicle proper. Grou];)s of sympathetic ganglion cells occur in the medulla near the hilum. As is the case with the testicle, certain rudimentary organs, the remains of foetal structures, are found connected with the ovary. The poroophoron consists of a number of cords or tubules of epi- thelial cells, sometimes ciliated, sometimes non-ciliated. It is found in the medulla, or, more commonly, in the connective tissue of the hilum. The cpoop/ioroii is a similar structure found in the folds of the broad ligament. Its tuljulcs open into a duct known as Gartner's duct. In man this duct ends Ijlindly. In some of the lower animals it opens into the \agina. Both paroophoron and epoophoron are 334 THE ORGANS. remains of the embryonal mesonephros, the former of its posterior segment, the latter of its middle segment. The Oviduct. The oviduct or Fallopian tube is the excretory duct of the ovary, serving for the transmission of the discharged ovum from ovary to uterus. Although there is no sharp demarcation between them, it is convenient to divide the tube into three segments: (i) The isthmus, beginning at the uterus and extending about one-third the length of ^ f A — d .^'^ Fig. 235. — Cross Section of Oviduct near Uterine End. a, Mucous membrane; h, circular muscle coat; c, longitudinal muscle coat; d, connective tissue of serous coat, (Orthmann.) the tube; (2) the ampulla, about twice the diameter of the isthmus, and occupying somewhat more than the middle third; and (3) the fimbriated or ovarian extremity. The walls of the oviduct consist of three coats: (i) Mucous, (2) muscular, and (3) serous (Figs. 235 and 236). The mucous membrane presents numerous longitudinal foldings. In the embryo four of these folds can usually be distinguished, and these are known as primary folds. In the adult many secondary folds have developed upon the primary, especially in the ampulla and fimbriated extremity where the folds are high and complicated (Fig. 236). The epithelium lining the tube is of the simple columnar cih'ated type, and completely covers the foldings of the mucous mem- THE REPRODUCTIVE SYSTEM. 335 brane. The ciliary motion is toward the uterus. The stroma con- sists of a cellular connective tissue, quite compact in structure in the isthmus, where the folds are low, more loosely arranged in the high folds of the ampulla and fimbriated extremity. The muscular coat consists of an inner circular and an outer longi- tudinal layer. The latter is a comparatively thin layer in the isthmus, consists of discontinuous groups of muscle cells in the ampulla, and in the fimbriated extremity is frequently absent. >%^ v^^- Img. 236. — Cross Section of Oviduct near Fimbriated Extremity, showing complicated fold- ings of mucous membrane. (Orthmann.) The serous coat has the usual structure of peritoneum. The larger blood-vessels run in the stroma along the bases of the folds. They send off branches which give rise to a dense capillary network in the stroma. Of the lymphatics of the tube little is known. The nerves form a rich plexus in the stroma, from which branches pass to the blood-vessels and muscular tissue of the walls of the tube and internally as far as the epithehal lining. TECHNIC. (i) Child's Ovary.— Remove the ovary of a new-born child, being careful not to touch the surface epithelium, fix in Zenker's fluid (technic 9, p. S), and harden in alcohol. Cut sections of the entire organ throtigh the hilum. Stain with htematoxylin-eosin (technic i, p. 18) and mount in balsam. (2) For the purpose of studying the Graatian follicle in the different stages of 336 THE ORGANS. its development remove an ovary from an adult cat or dog and treat as above. Technic (i). These sections also, as a rule, are satisfactory for the study of the cor- pus luteum. (3) The adult human ovary is little used for histological purposes on account of the few follicles it usually contains and its proneness to pathological changes. Its study is, however, so extremely important, especially with reference to the pathology of the ovary, that if possible a normal human ovary should be obtained from a young subject for purposes of comparison with the above. Technic (i). (4) For studying the egg cords of Pfluger and their relation to the germ epithelium, ovaries of the human foetus, and of very young cats, dogs, and rabbits are satis- factory. Technic (i). (5) Sections of the fimbriated end of the oviduct are usually found in the sections of ovary. For the study of other parts of the tube, cut out thin pieces from different regions, fix in formalin-Miiller's fluid, stain transverse sec- [:.;d tions with hfematoxylin-eosin, and mount in balsam. The Uterus. The wall of the uterus consists of three coats which from without inward are serous, muscular, and mucous. The serous coat is a reflection of the peritoneum, and has the usual structure of a serous membrane. The muscularis consists of bundles of smooth muscle cells separated by connective tissue. The muscle has a general arrangement into three layers, an inner, a middle, and an outer, which are distinct in the cervix, but not well defined in the body and fundus. The inner layer — stratum submucosum — is mainly Fig. 237.— Muscle longitudinal, although some obliquely running Ijun- dles arc usually present. The middle layer — called from the large \'cnous channels which it contains, the stratum vasculare — is the thickest of the three layers, forming the main bulk of the muscular wall. It consists mainly of circularly disposed muscle bundles. The outer layer — stratum supravasculare — is thin and consists partly of circular bundles, partly of longitudinal. The latter pre- dominate and form a fairly distinct layer just beneath the serosa. b cells from (a) non- jjregnant uterus; h, pregnant uterus; drawn to same scale. (Sellheim.) THE REPRODUCTIVE SYSTEM. 337 The muscle cells of the uterus are long spindle-shaped elements, some having pointed, others blunt, branched, or frayed ends. In the \-irgin uterus they have a length of from 40 to 60/1. During pregnancy the muscular tissue of the uterus is greatly increased. This is due partly to increase in the number, partly to increase in the size of the muscle cells. At term the muscle cells frequently ha\e a length of from 250 to 600/i. (Fig. 237.) The mucous membrane. As the mucosa presents marked \ariation in structure, dependent upon the functional condition of the organ, it is necessary to describe: ,.--- b 1. The mucosa of the resting uterus. 2. The mucosa of the men- struating uterus. 3. The mucosa of the preg- nant uterus. I. The Mucosa of the Rest- ing Uterus. This is from i to 2 mm. thick, and consists of a stroma, glands, and a lining epithelium (Fig. 238). The stroma resembles embryonal connective tissue, consisting of fine fibrils and long, irregular branching cells which form a sort of network, the meshes of which are filled in with lymphoid cells and leuco- cytes. The epithelium is of the simple high columnar ciliated variety, the ciliary motion being toward the cervix. A basement membrane separates the epithelium from the underlying stroma. The glands are simple forked tubules lined by a single layer of columnar ciliated cells resting upon a basement membrane and continuous with the surface cells. The glands extend completely through the stroma. Near the surface they run a comparatively straight course. Deeper in the stroma their course is more tortuous, while the fundus is fre- quently turned at right angles to the rest of the tulnile. In the cervix the stroma is firmer and less cellular, and the mu- FiG. 238. — From Uterus of Young \\'oman. (Bohm and von Davidoff; preparation by Dr. J. Amann.) X 34. a, Mucous membrane: h, surface epithelium; c, gland; r, muscle. 338 THE ORGANS. cous membrane is thicker and presents numerous folds — the plicce palmatcE. The epithehum is higher than in the body of the organ. In addition to glands like those found in the body of the uterus, the cervical mucosa contains peculiar short, sac-like invaginations, lined with a continuation of the surface epithelium, which secrete a glairy i mucus. Closure of the mouths of some of these sacs frequently r~ -.-^..trvjai-.'.J^S^ occurs, leading to the formation ,/^;- . of retention cysts, the so-called ~'\x ,'^ ovula Nabothi. At about the a .'- - ._. .-- - V ''t; junction of middle and lower b ,'r . ->i - d thirds of the cervical canal a W^%^\ ' • change takes place in the epi- m thelium. Here the simple .- 'iv , - "■ .-"■ * columnar ciliated epithelium ^' - "' h of the upper part of the Fig. 23Q.-From Section of Dog's Cervix. Cervix gradually pasSCS OVer X4- (Technic 2, p. 34Q.) a, Cervical canal; into a Stratified squamoUS epi- b, mucosa; c, folds of mucosa (plicae pal- , ,. mate); d, muscle layers of cervix; e, epithe- thehum. Near the external OS Hum of vagina and vaginal surface of cervix; p^piH^ appear, the Vaginal ./, vagmal epithehum; g, vagmal mucosa; li, " >- -rr y o submucosa and muscularis of vagina; /, surface of the cervix being blood-vessels. , . , , , • r i covered with a stratified squamous epithelium with underlying papillae similar to and con- tinuous with that of the vagina. Near the external os the epithelium changes over into the strati- fied squamous epithelium with underlying papillae, similar to that of the external surface of the cervix. 2. The Mucosa of the Menstruating Uterus. This consists of the same structural elements as the mucosa of the resting uterus: stroma, glands, and lining epithelium. These, however, undergo certain changes which may be conveniently divided into three stages: (a) The stage of preparation. (b) The stage of menstruation proper. (c) The stage of reparation. (a) The Stage of Preparation. — This begins se\eral days before the actual flow of blood, and is marked by an intense hyper- a^mia determining a swelling and growth of the entire mucosa. The THE REPRODUCTIVE SYSTEM. 339 d --^ It blood-vessels, especially the capillaries and veins, become greatly distended, thus contributing largely to the increase in thickness of the mucosa. There are also proliferation of the connective-tissue cells, an increase in the number of leucocytes, and a growth of the uterine glands. The surface at the same time becomes irregular, the glands opening into deep pits or depressions, and the glands them- selves become more tortuous and their lumina more widely open. The mucous membrane has now reached a thickness of about 6 mm., and is known as the de- cidua menstriialis (Fig. 240). {h) The Stage of ■ Men- struation Proper. — This is marked by the escape of blood from the engorged vessels and the appearance of the external phenomena of menstruation. The blood escapes partly by rupture of the vessel walls, partly by diapedesis. The hemorrhage is at first subepithelial, but the epithelium soon gives way and the blood escapes into the ca\'ity of the uterus. Much difference of opinion exists as to the amount of epithelial destruction during menstruation, some claiming that the entire epithelium is de- stroyed with each menstrual period, others that the epithelium remains almost intact. Complete destruction of the epithelium is hardly com- patible with the restoration of the epithelium which always follows menstruation. While there is undoubtedly destruction of most or all of the surface epithelium and of the glands to some considerable depth, the deeper portions of the glands always remain to take part in the succeeding regenerative phenomena. (f) The Stage of Repartition. — After from three to five days the bleeding from ihe uterine mucosa ceases and the return to the resting condition begins. This is marked by disappearance of the ■ '■.•.■-p?*»o;, ■■ . Fig. 240. — Section through Mucous Alem- brane of Virgin Uterus during First Day of Menstruation. X 30. (Schaper.) a. Sur- face epithelium; h, disintegrating surface; c, pit-like depression in mucous mem- brane; d, excretory duct; e, blood-vessels; g, gland tubule; h, dilated gland tubule; m, muscularis. 340 THE ORGANS. congestion, by decrease in thickness of the mucosa and in the size of the glands, and by restoration of the surface epitheHum. 3. The Mucosa of the Pregnant Uterus. This is known as the decidua graviditatis, and presents changes in structure somewhat similar to those which occur during menstrua- tion, but more extensive. It is divided into three parts: (a) The decidua serotina or decidua basalis- that part of the mucosa to which the ovum is attached. {h) The decidua reflexa or decidua capsularis — that part of the mucosa which surrounds the ovum. (r) The decidua vera — which consist of all the remaining mucosa. The development of the decidua vera resembles the changes which take place in the mucosa during menstruation. There is the same thickening of the mucosa, and this thickening is due to the same factor, i.e., distention of the blood-vessels and proliferation of the tissue elements. These changes are, however, much more extensive than during menstruation. The superficial part of the stroma be- tween the mouths of the glands becomes quite dense and firm, form- ing the compact layer. The deeper part of the stroma contains nu- merous cavities, which are the lumina of the now widely distended and tortuous glands. This is known as the spongy layer. Within the stroma, especially of the compact layer, develop the so- called decidual cells. These are peculiar typical cells derived from con- nective tissue. They are of large size (30 to ioo/(), vary greatly in shape, and in the later months of pregnancy have a rather characteristic brown color, which they impart to the superficial layers of the decidua vera. They are mostly mononuclear, although polynuclear forms occur. Luring the latter half of pregnancy there is a gradual thinning of the decidua vera, due apparently to pressure. The necks of the glands in the compact layer disappear, and the gland lumina in the spongy layer are changed into elongated spaces, which lie parallel to the muscular layer. The decidua reflexa and decidua serotina have at first the same structure as the decidua \'era. The decidua reflexa undergoes hyaline degeneration during the early part of pregnancy, and by the end of gestation has either completely disappeared (Minot) or has fused with the decidua vera (Leopold). The decidua serotina undergoes changes connected with the develojjment of the placenta. THE REPRODUCTIVE SYSTEM. 341 The Placenta/ The placenta consists of two parts, one of which is of maternal origin — placenta uterina — the other of foetal origin — the placenta foe talis. As it is in the placenta that the interchange takes place between the maternal and the foetal blood, the relations between the maternal and foetal parts of the placenta are extremely intimate. This relation consists essentially in the growing out from the foetal placenta of linger-like projections — villi — which penetrate the maternal placenta, the latter being especially modified for their reception. The Placenta Fcetalis. — This is a differentiated portion of the chorion. On the surface directed toward the foetus the chorion after the third month is covered by a delicate foetal membrane, the placental portion of the amnion. This consists of a surface epithelium, resting upon a layer of embryonal connective tissue which attaches it to the chorion. The chorion consists of {a) a compact layer — the membrana chorii — composed at first of embryonal, later of fibrous, connective tissue, and containing the main branches of the umbilical vessels and {b) an inner ^•illous layer, which gives rise to finger-like projections which extend down from the the foetal into the maternal placenta and serve to connect the two. The chorionic villi first appear as short projections composed entirely of epithelium. Each of these primary villi branches dichoto- mously, giving rise to a number of secondary villi. As they develop, the central portion of the original solid epithelial structure is replaced by connective tissue. Septa of connective tissue from the maternal placenta pass down among the villi and separate them into groups or cotyledons. The main or primary villi run a quite straight course from the chorion into the maternal placental tissue, apparently serving to secure firm union between the two. They are thus known as roots of attachment or fastening villi (Fig. 241). The secondary villi are given off laterally from the primary villi, end freely in the spaces between the latter — intervillous spaces (Fig. 241) — and are known as free, ter- minal or floating villi (Fig. 241). The chorionic villus thus consists of a central core of connective tissue covered by a layer of epithelium. The connective tissue is of the mucous type and serves for the transmission of numerous blood- vessels. In the \"illi of early pregnancy the epithelium consists of an '■ For many facts as to the structure of the placenta, the writer is indebted to the excellent chapter on the subject added by Prof. .Alfred Schaper to the tifth edition of Stohr's "Textbook of Histolosv." 342 THE ORGANS. THE REPRODUCTR'E SYSTEM. 343 inner layer of distinctly outlined cells and an outer layer of fused cell bodies — a syncytium (Fig, 243, A, a) — containing small scattered nuclei. The villi of the later months of pregnancy have no definite epithelial covering, but are surrounded by a delicate hom.ogeneous membrane, probably the remains of the syncytium. At various points on the surface of the villus are groups of nuclei. These stain intensely, are surrounded Ijy a homogeneous protoplasm, and form knob-like pro- jections above the general surface of the villus. They are known as "Giant" cell Syncytium Trophodi mass Fig. 242. — Section of Chorion of Human Embryo of one month (9 mm.). (Grosser.) cell patches, or more properly as nuclear groups (Fig. 243, f), and repre- sent remains of the nuclei of the ejjithelium of the younger villus. Between the nuclear groups the \"illus is covered only by a thin homo- geneous membrane. Small villi usually resemble more closelv in structure the younger villus, being frequently covered by a nucleated syncytium. Portions of the syncytium, especially of older villi, some- times become changed into a peculiar hyaline substance containing numerous channels. This is known as canalized fibrin, and mav form dense layers ui)on the surface of the chorion. (Fig. 242.) The Placenta Uterixa. — This de^■elops from the decidua sero- tina. The latter becomes much thinner than the rest of the decidua 344 THE ORGANS. (decidua vera), but still shows a division into a deeper spongy portion containing gland tubules, and a superficial compact portion in which are large numbers of decidual cells. From the superficial portion connective-tissue septa — placental septa — grow into the foetal placenta, as described above, separating its villi into cotyledons. Near the margins of the placenta these septa pass to the chorionic membrane and form beneath it a thin mem- brane, the subchorionic placental decidua. At the edge of the pla- centa, where decidua serotina passes over into the thicker decidua vera, there is a close attachment of the chorion to the former. As the placenta serves as the place of interchange of materials between the maternal and the foetal circulations, the arrangement of the placental blood-vessels is of especial importance. Arterial branches from vessels of the uterine muscularis enter the serotina. In the very tortuous course which these vessels take through the serotina (Fig. 241) their walls lose their muscular and connective-tissue elements and be- come reduced to epithelial tubes. These branch in the placental septa and finally open into the intervillous spaces along the edges of the cotyledons. The veins take origin from these spaces near the centres of the cotyledons. The maternal blood thus passes through the intervillous spaces from periphery to centre, and in its course comes into direct contact with the freely terminating chorionic villi. Jt is to be noted that the Ijlood-vessel systems of the mother and of the f(x,-tus are both closed systems, and that consequently there is no direct admixture of maternal and fcEtal blood. Interchange of materials must therefore always take place through the capillary walls and through the walls of the chorionic villi. (Fig. 241.) Blood-vessels. — The arteries enter the uterus from the broad hga- ment and j;ass to the stratum vasculare of the muscularis, where they undergo extensive ramification. From the arteries of the stratum vasculare branches pass to the mucosa and give rise to capillary net- FlG. 2 43-- Cross Sections of Human Chorionic Villi at End of Pregnancy. X 250. (Schaper.) A, Small villus; B, larger villus, a, Protoplasmic coat (syncytium); h, epithelial nucleus; c, nuclear groups; d, small artery; e, small vein;/, capillaries. THE REPROULXTIX'E SVSTE.\L 345 works, which surround the glands and are especially dense just beneath the surface epithelium. From these capillaries the blood passes into a plexus of veins in the deeper portion of the mucosa, and these in turn empty into the venous plexuses of the stratum vasculare. Thence the veins accompany the arteries, leaving the uterus through the broad ligament. Lymphatics. — These begin as minute spaces in the stroma and empty into the more definite lymph channels of the muscularis, which are especially well developed in the stratum vasculare. These in turn communicate with the larger lymph vessels in the subserous connective tissue. Nerves. — Both medullated and non-medullated nerve fibres occur in the uterus. The latter are associated with minute sympathetic ganglia and supply the muscular tissue. The medullated fibres form plexuses in the mucosa, from which are given off fine fibres which terminate freely between the cells of the surface epithelium and of the uterine glands. The Vagina. The wall of the vagina consists of four coats, which from without inward are fibrous, muscular, sulDmucous, and mucous. The fibrous coat consists of dense connective tissue with many coarse elastic fibres. It serves to connect the vagina with the sur- rounding structures. The muscular coat is indistinctly divided into an outer longitudinal and an inner circular layer. The latter is usually not well developed and may be absent. The suhmucosa is a layer of loose connective tissue, especially rich in elastic fibres and blood-vessels. Numerous large venous channels give to the submucosa the character of erectile tissue. The mucous membrane consists of a papillated connective-tissue stroma of mixed fibrous and elastic tissue. The stroma usually con- tains dift'use lymphoid tissue and more rarely solitary nodules. Cover- ing the stroma is a stratified squamous epithelium, the surface cells of which are extremely thin. The surface of the mucosa is not smooth, but is folded transversely, forming the so-called rugcr. Most authorities agree that glands are wanting in the vagina, the mucus found there being derived from the glands of the cervix. Blood-vessels. — The larger Ijlood-vcssels run in the sul)mucosa, giving; oft" l^ranchcs which break ui) into cainllarv networks in the sub- 34:6 THE ORGANS. mucosa, muscularis, and stroma. The vascular networks have a general direction parallel to the surface. The capillaries empty into veins which form a plexus of broad venous channels in the muscularis. The Lymphatics. — These follow in general the distribution of the blood-vessels. Nerves. — Nerve fibres from both cerebro-spinal and sympathetic systems are found in the vagina. Medullated (sensory) fibres, the dendrites of spinal ganglion cells, form plexuses in the mucosa, from which are given off delicate non-medullated terminals to the epithelial cells. Non-medullated sympathetic fibres supply the muscularis and the muscle of the vessel walls. Along these nerves are small sympathetic ganglia. In the vestibule the epithelium gradually takes on the structure of epidermis. Here are located small mucous glands — glandiilcE vestih- ulares minores — especially numerous around the clitoris and opening of the urethra. Larger mucous glands — glandula vestibulares majores, or glands of Bartholin — analogous to Cowper's glands in the male, are also found in the walls of the vestibule. The clitoris consists mainly of erectile tissue similar to that of the corpora cavernosa of the penis. It is covered with a thin epithelium with underlying papillas, and is richly supplied w^th nerves having highly specialized terminations. Development of the Urinary and Reproductive Systems. The development of the genito-urinary system is com.plicated by the appearance, and disappearance for the most part, of two sets of urinary organs, and the final formation of the permanent set. The three sets, in the order of their appearance, are the pronephroi, mesonephroi, and metanephroi. The first two sets, which are ])resent only in the embryo in the higher animals, are the representatives of organs that function in the adult in the lower vertebrates. They are also inti- mately concerned in the development of the efferent duct system of the male reproductive organs in higher animals. The metanephroi, generally known as the kidneys, are the functional urinary organs in the majority of re];tiles and in all birds and mammals. The pronephroi are represented in the human embryo of 3 to 5 mm. by one fjr two small, condensed masses of mesoderm just lateral to the primitive segments on each side in the cervical region. These masses, which are probably derived from the mesothelial lining of the THE REPRODUCTIVE SYSTEM. 347 body cavity, may or may not become hollow, but do not connect with the pronephric duct, and soon disappear. The pronephric duct appears about the same time as a derivative of the mesoderm just lateral to the primitive segments. It extends from the cervical region to the caudal region of the embryo where it bends mesially and opens into the gut. These ducts persist and become the ducts of the mesonephroi. The mesonephroi begin to develop almost as soon as the proneph- roi and just caudal to them. Condensations appear in the mesodermal tissue lateral to the primitive segments and become more or less tor- tuous. Lamina appear in these condensations, thus forming tubules which then connect at one end with the pronephric duct (now the mesonephric or Wolffian duct). The tubules develop progressively from before backward and finally form a series extending from the cervical region to the pelvic region of the embryo. At the distal end of each tubule a glomerulus, containing branches from the aorta, develops. The tubules increase in length and number and come to form a pair of large structures which project into the dorsal part of the body cavity. These are often spoken of as the Wolffian bodies. They reach the height of their development during the fifth or sixth week. During the period of their existence in the embryo in higher animals, the mesonephroi functionate as urinary organs, not only through the agency of the glom.eruli but also by means of the epithelium of the tubules themselves, among which numerous branches of the posterior cardinal veins ramify. From the sixth week on, and coincident with the development of the metanephroi or kidneys, the mesonephroi atrophy, leaving finally only certain parts which differ in the two sexes. In the male some of the tubules in the cephalic portion persist as the vasa eft'erentia, while a few in the caudal portion remain as the paradidymis and vasa aberrantia; the duct persists as the vas epididym.idis, vas deferens, and ejaculatory duct. In the female the mesonephric tubules dis- appear for the most part, only a few remaining to form the epo- ophoron and paroophoron, while the duct persists in part as Gartner's canal. Each kidney begins in embryos of 5 to 6 mm. as a hollow bud on the dorsal side of each mesonephric duct near its opening into the gut. This bud grows dorsally, and then turns cranially in the mesoderm between the vertebral column and ihc mesonei)hros. The prtiximal portion remains more slender as the ureter, white the di>tal end hu- 348 THE ORGANS. comes dilated to form the primitive renal pelvis. From this dilated end a number of secondary evaginations grow out to form the papillary ducts and straight collecting tubules. The mesodermal tissue surrounding the outgrowths from the primitive renal pelvis becomes condensed in places and gives rise to the convoluted portions of the uriniferous tubules and to Henle's loops. A glomerulus develops in connection with the distal end of each con- voluted tubule. The portion of each tubule derived from the meso- dermal tissue unites secondarily at the arched (junctional) tubule with the portion derived from the renal pelvis. Thus the kidney tubules are derived in part directly from undifferentiated mesoderm (convoluted tubules and Henle's loops) and in part from an outgrowth from the mesonephric duct (straight collecting tubules and papillary ducts) . During foetal life the kidneys are distinctly lobulated, but after birth the surface becomes quite smooth. The genital gland on each side appears on the mesial surface of the mesonephros as a thickening of a narrow band of the mesothelial lining of the body cavity. The cells in the band become differentiated into two kinds — small cuboidal cells which stain rather intensely, and larger spherical cells with clearer cytoplasm and vesicular nuclei. The latter are the sex cells which are destined to give rise to the ova in the female or the spermatozoa in the male. The whole thickened band is known as the germinal epithelium. The cells of the germinal epithelium increase in number by mitosis and soon become differentiated into two layers — a superficial layer which retains its epithelial character and contains the sex cells, and a dee]jer layer which is destined to give rise in part to the stroma of the genital gland. The elevation caused by the increased number of cells projects into the body cavity as the genital ridge. From the superfi- cial layer containing the sex cells a number of plugs or columns of cells grow down into the deeper layer carrying some of the sex cells with them. Thus far (up to about the fifth week) the changes are common to both sexes, and only later does the sex differentiation occur. After aljout the fifth week certain changes occur in the genital ridge which differ accordingly as the ridge is to become an ovary or a testicle. In the case of the ovary a layer of loose connective tissue grows in between the surface epithelium and the cell columns or plugs mentioned above. The cell columns are thus pushed farther from the surface and constitute the medullary cords. These ultimately disap- THE REPRODUCTIVE SYSTEM. 349 pear. The surface epithelium again sends plugs of cells (Pfliiger's egg cords) down into the underlying tissue. These cords are made up for the most part of epithelial cells which give rise to the follicular cells, but contain also a considerable number of sex cells (primitive ova). The egg cords then become broken up into smaller masses each of which contains a single primitive ovum (rarely more) and constitutes a primitive Graafian follicle. The sex cell grows in size and becomes the primary occyte (and finally the mature ovum), while the epithelial cells around it give rise to the stratum granulosum and germ hill of the mature Graafian follicle. During these processes the stroma also increases in amount, while the original germinal epithelium becomes reduced to a single layer of cuboidal cells. The formation of egg cords is usually completed before birth. (See also p. 324.) In the case of the testicle a layer of dense connective tissue, the tunica albuginea, develops between the germinal epithelium and the sex cords. The epithelium becomes reduced to a single layer of fiat cells. The sex cords which first grew into the underlying tissue and which contain the sex cells, are destined to give rise to the con^•oluted seminiferous tubules. This phase of development dift'ers from that in the ovary inasmuch as in the latter case the first formed sex cords (medullary cords) disappear, Pfliiger's egg cords being formed later and having no homologue in the testicle. The sex cords of the testicle become more and more convoluted and the sex cells (spermatogonia) proliferate rapidly. Beginning after birth and continuing up to the time of puberty, lumina appear in the sex cords and they thus give rise to the convoluted seminiferous tubules. The supporting cells (of Sertoli) are probably derived from the undifferentiated epithelial cells of the sex cords. (See also p. 305.) TECHNIC. (i) A human uterus — if possible from a young adult — or, if this cannot be ob- tained, the uterus of a cat or dog, is cut transversely into slices about i cm. thick and fixed in Zenker's fluid (technic 9, p. 8) or in formalin-Miiller's fluid (technic 5, p. 7). For topography these slices are cut in half through the middle of the uterine cavity and sections made through the entire half organ. These are stained with haematoxylin-picro-acid-fuchsin (technic 3, p. iq) and mounted in balsam. For details of the mucous membrane, cut away most of the muscle from around the half slice, being careful not to touch the mucous surface; make thin sections, stain with hasmatoxylin-eosin (technic i, p. 18), and mount in balsam. (2) Sections of the cervix may be prepared in the same manner as the preceding. (3) Placental tissue may be cut into small cubes and treated with tlie same technic (i). 350 THE ORGANS. (4) If a human or animal uterus with the placenta in situ is obtainable it should be cut into thin slices and fixed in formalin-Miiller's fluid. The blocks of tissue should be so arranged that sections include the utero-placental junction. They may be stained with haematoxylin-eosin or with haematoxylin-picro-acid-fuch- sin (see above). (5) Treat pieces of the human vagina according to technic i, p. 223. General References for Further Study. Ballowitz: Weitere Beobachtungen iiber den feineren Bau der Saugethier- Spermatozoen. Zeit. f. wiss. Zool., Bd. Hi., 1891. Hertwig: Lehrbuch der Entwickelungsgeschichte des Menschen und der Wir- belthiere, Jena, 1896. Kolliker: Handbuch der Gewebelehre des Menschen. Nagel: Das menschliche Ei. Arch. mik. Anat., Bd. xxxi., 1888. Ruckert: Zur Eireifung der Copepoden. Anat. Hefte, I. Abth., Bd. iv., 1894. Schaper: Chapter on the Placenta in Stohr's Text-book of Histology, 5th ed. Sobotta: Ueber die Bildung des Corpus luteum bei der Maus. Arch. f. mik. Anat., Bd. xlvii., 1896. — Ueber die Bildung des Corpus luteum beim Kaninchen. Anat. Hefte, I. Abth., Bd. viii., 1897. CHAPTER X. THE SKIN AND ITS APPENDAGES. The Skin. The skin or cutis consists of two parts: (i) The derma, corium, or true skin, and covering this, (2) the epidermis or cuticle. The derma is a connective-tissue derivative of the mesoderm, the epider- mis an epithelial derivative of the ectoderm. The Derma. — This is divided into two layers which blend with- Fig. 244. — Vertical Section of Thin Skin, Human. X 60. (Technic 2, p. 356.) a. Epidermis; b, pars papillaris of derma; c, papilla:; d, pars reticularis of derma; e, duct of sweat gland;/, sweat gland; g, subcutaneous fat. out distinct demarcation. The deeper is known as the pars reticu- laris, the more superficial as the pars papillaris (Fig. 244). The pars reticularis is made up of rather coarse, loosely arranged 352 THE ORGANS. white and elastic tibres with connective-tissue cells in varying num- bers. The fibres run for the most part parallel to the surface of the skin. The pars papillaris is similar in structure to the preceding, but both white and elastic fibres are finer and more closely arranged. Externally this layer is marked by minute folds which are visible to the naked eye, and can be seen intersecting one another and enclos- ing small irregular areas of skin. In the thick skin of the palms and soles these furrows are close together and parallel, while between them are long corresponding ridges. In addition to the furrows and A B\ C Fig. 245. — Thick Vertical Section through Skin of Finger Tip. (Merkel-Henle.) A, Epidermis; B, derma; C, subcutis. a, Stratum corneum; b, duct of sweat gland; c, stratum lucidum; d, stratum germinativum; e, papilla of derma;/, derma; g, blood-vessel; h, sweat gland; i, fat lobule; p, sweat pore. ridges the entire surface of the corium is beset with minute papillae. These vary in structure, some ending in a single point — simple pa- pillce — others in several y^oints — compound papilla'; some containing blood-vessels — vascular papilla'; others containing special nerve terminations — nerve papilla'. (Fig. 246). Smooth muscle cells occur in the corium in connection with the sweat glands. In the skin of the scrotum^ — -tunica dartos — and of the nipple, the smooth muscle cells are arranged in a network parallel to the surface. In the face and neck striated muscle fibres j^enctrate the corium. Beneath the corium is the siibculaneous tissue. This consists of THE SKIN AND ITS APPENDAGES. 353 vertically disposed bands of connective tissue — the retinaculce cutis — which serve to unite the corium to the underlying structures and enclose fat lobules. In some parts of the body this subcutaneous fat forms a thick layer — the panniculus adiposus. The Epidermis. — This is composed of stratified squamous epi- thelium. In the comparatively thin skin of the general body surface the epidermis is divided into two sub-layers: (i) One lying just above the papillary layer of the derma, and known as the stratum germina- FiG. 246. — From Vertical Section through Skin of Human Finger Tip. X 2co. (Schafer.) a. Stratum corneum; b, stratum lucidum; c, stratum granulosum; d, stratum germinativum. To the left a vascidar papilla; to the right a nerve papilla containing tactile corpuscle. tivum (stratum mucosum — stratum Malpighii); (2) the other con- stituting the superficial layer of the skin — the horny layer or stratum corneum. In the thick skin of the palms and soles two additional layers are developed; (3) the stratum granulosum; and (4) the stratum lucidum (Fig. 245). (i) The stratum gcnninaliviDJi consists of several layers of cells. The deepest cells arc columnar and form a single layer (stratum cylindricum), which rests u]jon a basement membrane separating it 354 THE ORGANS. ® (^' ■Q t «tD from the derma. The membrane and cells follow the elevations and depressions caused by the papillae. The rest of the stratum germi- nativum consists of large polygonal cells. These cells have well- developed intercellular bridges, which appear as spines projecting from the surfaces of the cells. For this reason the cells are sometimes called "prickle" cells, and the layer, the "stratum spinosum." The spines cross minute spaces between the cells, which are believed to communicate with the lymph spaces of the derma (Fig. 247, c). The cells of the stratum germinativum are usually in a state of active mitosis. (2) The stratum granulosum is well developed only where the skin is thick. It consists of from one to three layers of flattened polygonal cells. The protoplasm of these cells contains deeply staining granules — keratohya- line granules — which probably repre- sent a stage in the formation of the horny substance — keratin — of the corneum cells. The nuclei of these cells always show degenerative changes, and there is reason for be- lieving that this karyolysis is closely associated with the formation of the keratohyaline granules (Fig. 247, h). (3) The stratum hicidmn is also best developed where the skin is thickest. It consists of two or three layers of flat clear cells, the outlines of which are frequently so indistinct that the layer appears homogeneous. The transparency of the cells is due to the presence of a substance known as eleidin, and derived from the keratohyaline granules of the stratum granulosum (Fig. 247, a). (4) The stratum corneum varies greatly in thickness, reaching its greatest development in the skin of the palms and soles. The cells ^ ( ^j t wf; t\^ W'V Fig. 247. — From Vertical Section through Thick Skin. (Mcrkel- Henle.) a, Stratum lucidum; /;, stratum granulosum; c, stratum germinativum, showing intercel- lular briflges. THE SKIN AND ITS APPENDAGES. 355 are flattened and horny, especially near the surface. Some appear homogeneous, others have a lamellated appearance. They contain pareleidin, a derivative of the eleidin of the stratum lucidum. Nuclei are lost, but in many of the cells can be seen the spaces which the nuclei once occupied. Constant desquamation of these cells goes on, cells from the deeper layers taking their place. The color of the skin in the white races is due to pigmentation of the deeper layers of the epidermis. In certain parts of the body pig- mentation of the connective-tissue cells of the derma also occurs. In the dark races all cells of the epidermis are pigmented, although there is less pigment in the surface cells than in the cells more deeply situated. Two kinds of glands occur in the skin — sebaceous glands and sweat glands. Sebaceous Glands.— These are usually associated with the hair follicles, and will be described in that connection. Sebaceous glands unconnected with hair occur along the margin of the lips, in the glans and prepuce of the penis, and in the labia minora. Sweat Glands (glandulcE sudor ipavcp). — These are found through- out the entire skin with the exception of the margin of the lips, the inner surface of the prepuce, and the glans penis. They are simple coiled tubular glands. The coiled portion of the gland usually lies in the subcutis, although it may lie wholly or partly in the deeper portion of the pars reticularis. The excretory duct runs a quite straight course through the derma, and enters the epidermis in one of the depressions between the papillae. In the epidermis the duct takes a spiral course to the surface, where it opens into a minute pit just \dsible to the naked eye — the sweat pore (Fig. 245 , p) . The coiled portion of the gland is lined with a simple cuboidal epithelium, having a granular protoplasm. In the smaller glands the epithelium rests directly upon the basement membrane. In the larger glands a longi- tudinal layer of smooth muscle cells separates the glandular epithelium from the basement membrane. The walls of the ducts consist of two or three layers of cuboidal epithelial cells, resting upon a delicate basement membrane, outside of which are longitudinally disposed connective-tissue fibres. On reaching the horny layer the epithelial wall of the duct ceases, the duct consisting of a mere channel through the epithelium. (Fig. 245.) TECHNIC. (i) Fix the volar half of a finger-tip in formalin-M tiller's fluid (technic 5, p. 7) or in absolute alcohol. Curling may be prevented by pinning to a piece of 356 THE ORGANS. cork. Sections are cut transversely to the ridges, stained with haematoxyhn-picro- acid-fuchsin (technic 3, p. 19), and mounted in balsam. Thick sections should be cut for the study of the coil glands with their ducts; thin sections for cellular de- tails of the layers. (2) Prepare in the same manner and for contrast with the preceding, sections of thin skin from almost any part of the body. (3) Prepare a piece of negro skin in the same manner and note the position of the pigment. The Nails. The nails are modified epidermis. Each nail consists of: (a) a body, the attached uncovered portion of the nail; (6) 2i free edge, the anterior unattached extension of the body; [c) the nail root, the pos- terior part of the nail which lies under the skin (Fig. 248). e d Fig. 248. — Longitudinal .Section through Root of Human Xail and Nail Bed. X 10. (Schaper.) a, Body of nail; b, free edge; c, root of nail; d, epidermis; e, eponychium; /, stratum germinativum of nail; g, folds in derma of nail bed; h, bone of finger; k, hyjjrjnyrhium. The nail lies ujjon a specially modified jjortion of the corium, the nail bed, which beneath the nail root and somewhat forward of the root is known as the matrix. The nail bed is bounded on either side by folds of skin, the nail wall, while between the nail wall and the nail bed is a furrow, the nail groove (Fig. 249). 'J'he nail bed consists of corium. Its connective-tissue fibres are THE SKIN AND ITS APPENDAGES. 35/ arranged partly horizontal to the long axis of the nail, partly in a vertical plane extending from the periosteum to the nail. Papillae are not present, but in their place are minute longitudinal ridges, Fig. 24q. — Transverse Section of Nail and Nail Bed. (Rannie.) n. Nail; a, epidermis; p, nail wall, to inner side of- which is the nail groove; /, folds of derma; d, nail bed. which begin at the matrix and, increasing in height as they pass for- ward, terminate abruptly at the end of the nail bed, beyond which are the usual papillae of the derma. ifc '•<=»' 'v-i?* au.y ,*' sir ^v^jijy &,, v3?^ Fig. 250. — \'ertical Transverse Section through Nail Binl\ . X 2S0. ^S/.ymono\\ icz.) a, Nail; b, stratum germinativum; c, ridge of nail bed; d, derma; e, blood-vessel. The nail itself consists of two parts^ — an outer harder part or true nail, and an under softer part. The outer portion is hard and horny, is developed from the stratum lucidum, and consists of several layers ^ Another division of nail and nail bed considers the nail as composed of the hard part only, the soft stratum germinativum being considered a part of the nail bed. 358 THE ORGANS. of clear, flat, nucleated cells. These layers overlap in such a manner that each layer extends a little farther forward than the layer above. The under softer portion of the nail corresponds to the stratum germinativum of the skin and, like the latter, consists of polygonal "prickle" cells and a stratum cylindricum resting upon a basement membrane. In the matrix where the process of nail formation is going on, this layer is thicker than elsewhere and is white and opaque from the presence of keratohyalin. The convex anterior margin of this area can be seen with the naked eye and is known as the lunula. At the junction of nail and skin, in the nail groove, the stratum corneum extends somewhat oxtr the nail as its eponychium. A simi- lar extension of the stratum corneum occurs on the under surface of the nail where the nail becomes free from the nail bed. This is known as the hyponychium (Fig. 248). Growth of nail takes place by a transformation of the cells of the matrix into true nail cells. In this process the outer hard layer is pushed forward over the stratum germinativum, the latter remaining always in the same position. TECHNIC. (i) Remove two or more distal phalanges from the fingers of a new-born child and fix in absolute alcohol or in formalin-MuUer's fluid (technic 5, p. 7). After fixing, the bone should be carefully removed. Both longitudinal and transverse sections are made, stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 19), and mounted in balsam. In cutting the sections it is usually best so to place the block that the knife passes through volar surface first, through nail last. (2) The cellular elements of nail do not show well in sections. For demon- strating the nail cells, boil a piece of nail in concentrated potash lye or warm it in strong sulphuric acid, scrape off cells from the softened surface, and mount in glycerin. The Hair. The hair, like the nail, is a development of the epidermis. The hair itself consists of a shaft, that portion of the hair which projects above the skin, and a root, that portion embedded within the skin. At its lower end the root presents a knob-like expansion, the hair bulb, in the under surface of which is a cup-like depression, which receives an extension of corium. This is known as the papilla. Enclosing the hair root is the hair follicle. The Hair. — This is composed of epithelial cells arranged in three layers, which from within outward are medulla, cortex, and cuticle (Fig. 252 j. (i) The medulla occupies the central axis of the hair. It is absent THE SKIX AND ITS APPENDAGES. 359 in small hairs, and in the large hairs does not extend throughout their entire length. It is from i6 to 2o/.< in diameter, and consists of from two to four layers of polygonal or cuboidal cells with finely granu- lar, usually pigmented protoplasm and rudimentary nuclei. (2) The cortex makes up the main bulk of the hair and consists of several layers of long spindle-shaped cells, the protoplasm of which shows distinct longitudinal striations, while the nuclei appear atrophied. As these striations give the hair the appearance of being composed of fibrillse, the term "corti- cal fibres" has been applied to them. In colored hair, pigment, granules and pigment in solution are found in and between the cells of this layer. This pigment determines the color of the hair. In the root the cortical cells are less flattened than in the shaft. (3) The cuticle has a thickness of about i/(, and consists of clear scale- like, non-nucleated epithelial cells. These overlap one another like shingles on a roof, gi^'ing to the sur- face of the hair a serrated appear- ance (Fig. 252.) The Hair Follicle. — This is also a modification of the skin. In the formation of the follicles of the finer (lanugo) hairs the epidermis alone is concerned. The follicles of the larger hairs contain both epidermal and dermal elements. The latter form the connective-tissue follicle, while the epidermis forms the root sheaths. (i) The root sJieath consists of two sub-layers — the i>i>!cr root sheath and the outer root sheath (Figs, 253, 254, and 255). (a) The inner root sheath consists of three layers, which from within outward are the cuticle of the root sheath, Huxley's layer, and Henle's layer. The cuticle of the root sheath lies against the cuticle of the hair and Fig. 2 Longitudinal Section of Hair and its Follicle from Vertical Section of Scalp. (Ranvier.) a, Shaft of hair; b, derma; c, arrector pili muscle; d, sebaceous gland; e, outer root sheath; /, inner root sheath; g, con- nective-tissue follicle: h, vitreous membrane; /, hair bulb: 7. papilla; s, epidermis. 360 THE ORGANS. is similar to the latter in structure. It consists of thin scale-like over- lapping cells, nucleated in the deeper parts of the sheath, non-nucleated nearer the surface (Figs. 253, 254 and 255, c). Huxley's layer lies immediately outside the cuticle of the root sheath, constituting the middle layer of the inner root sheath. It consists of about two rows of elongated cells with slightly granular protoplasm containing eleidin. In the deeper portion of the root these cells contain nuclei. Nearer the surface ° ^ ^ the nuclei are rudimentary or absent (Figs. 253, 254 and 255, d). Henle's layer is a single row of clear flat cells. In the bulb these cells may contain nuclei; elsewhere they are non- nucleated (Fig. 255, e) . (b) The outer root sheath is derived from the stratum germinativum to which it corresponds in structure. Next to the \'itreous membrane is a single layer of columnar cells (stratum cylindricum). Inside of this are several layers of "prickle" cells (Figs. 253, 254 and (2) The connective-tissue follicle con- sists of three layers — an inner vitreous membrane, a middle vascular layer, and an outer fibrous layer. (a) The vitreous or hyaline membrane is a thin homogeneous structure of the nature of an elastic membrane. It lies next to the outer root sheath and corresponds to the basement membrane of the derma (Figs. 253, 254 and 255,^). (b) The middle or vascular layer is composed of fine connective- tissue fibres, the general arrangement of which is circular. Cellular elements are quite abundant, while elastic fibres are, as a rule, absent. As its name would indicate, this layer is especially rich in blood- vessels (Figs. 253, 254 and 255, f). (c) The outer or fibrous layer consists of rather coarse, loosely woven bundles of white fibres, which run mainly in a longitudinal direction. Among these are elastic fibres and a few connective-tissue cells. In the deeper portion of the root, -some little distance above the Fig. 252. — Longitudinal Section of Hair. X 350. (KoUiker.) a, Medulla; b, cortex; c, cuticle. THE SKIN AND ITS APPENDAGES. 361 bulb, all the layers of the hair and its follicle can be distinctly seen. The differentiation of the layers becomes less marked as one passes in cither direction. At about the level of the entrance of the ducts of the sebaceous glands (see p. 362) the inner root sheath disappears, and the outer root sheath passes over into the stratum germinativum of the skin, while between the outer root sheath (now stratum germi- FlG. 2 (Kolliker.) inner root membrane: 53. — Longitudinal Section of Lower End of Root of Hair, including a, Root of hair; b, cuticle of hair; c, cuticle of root sheath; d, Huxley's sheath; e, Henle's layer of inner root sheath;/, outer root sheath; g, ; ;', connective-tissue follicle; k, bulb of hair; p, papilla. Papilla, layer of vitreous nati\um) and the hair are interposed the outer layers of the skin, stratum granulosum and stratum lucidum, when present, and stratum corneum. x^ll of these are continuous with the same layers of the skin. In the region of the bulb the outer root sheath first becomes thinner, then disappears, while the layers of the inner root sheath retain their identity until the neck of the papilla is reached, at which point the different layers coalesce. The arrector pill j}u{sclc (Fig. 251, c) is a narrow band, or bands, of smooth muscle connected with the hair follicle. It arises from the 362 THE ORGANS. outer layer of the derma on the side toward which the hair slants, and is inserted into the wall of the follicle at the junction of its middle and lower thirds, the sebaceous gland being usually included between the muscle and the hair (see below). The contraction of the muscle thus tends to straighten the hair and to compress the gland. The sebaceous glands are with few exceptions connected with the hair follicles. They are simple or branched alveolar glands. The Fig. 254. — Transverse Section through Root of Hair and Hair Follicle. (Kolliker.) a, Hair; b, hair cuticle; c, cuticle of root sheath; d, Huxley's layer; e, Henle's layer;/, outer root sheath; /, connective-tissue follicle. size of the gland bears no relation to the size of the hair, the largest glands being frequently connected with the smallest hairs. The glands are spherical or oval in shape and each gland is enclosed by a connective-tissue capsule derived from the follicle or from the derma. Beneath the capsule is a basement membrane continuous with the vitreous membrane of the follicle. The wide excretory duct empties into the upper third of the follicle and is lined with stratified squamous epithelium continuous with the outer root sheath and stratum ger- minativum. The lower end of the duct opens into several simple or branched alveoli, at the mouths of which the epithelium becom.es reduced to a single layer of cuboidal cells. In the alveoli them.selves THE SKIX AND ITS APPENDAGES. 363 J A B ►^sar ■■£^^ the cells are spheroidal or polyhedral, and usually fill the entire alveolus. These cells, like those lining the duct, are derivatives of the outer root sheath. The secretion of the gland — an oily substance called sebum — appears to be the direct product of disintegration of the alveolar cells, which can usually be seen in all scares of the process of transformation of the cell into the secretion of the gland. The most peripheral cells show the least secretory changes, containing a few small fat droplets. The central cells and these in the lum.en of the duct show the most marked changes, their protoplasm being almost wholly converted into fat, their nuclei shrunken or disintegrated or lost. In the middle zone are cells showing in- termediary stages in the process. Shedding of hair occurs in most mammalia at regularly recurring periods. In man there is a constant death and replacement of hair. In a hair about to be shed, the bulb becomes cornified and splits up into a number of fibres. The hair next becom.es detached from the papilla and from the root sheath and is cast off, the empty root sheaths collaps- ing and forming a cord of cells between the papilla and lower end of the shed- ding hair. If the dead hair is to be re- placed by a new one, there sooner or later occurs a proHferation of the cells of the outer root sheath in the region of the old papilla. From this "hair germ" the new hair is formed in a manner similar to embryonal hair formation, the new hair growing upward under or to one side of the dead hair, which it finally replaces. As to the manner in which growth of hair lakes place, two \-iews are held. According to one of these, the hair, cuticle, and inner root sheath are replenished by proliferation of the epithelial cells surrounding the papilla. These parts thus grow from below toward Fig. 255. — From Lungiludinal Sec- tion through Hair and Hair Follicle. Enlarged to 800 diame- ters. (Schafer.) A, Hair. a, Cortex of hair; /), cuticle of hair. B, Inner sheath, c, Cuticle of root sheath; d, Hu.xley's layer; e, Henle's layer; /, outer root sheath ; ^§^, vitreous membrane; i, connective-tissue follicle; »;, fat cells. 364 THE ORGANS. the surface. The oldest cells of the outer root-sheath, on the other hand, lie against the vitreous membrane, so that growth of this sheath takes place from without inward. According to the second view, the various parts of the hair and its follicle are direct derivatives of the different layers of the skin, and their growth takes place by a contin- uous process of invagination. Thus the most peripheral cells of the outer root-sheath— stratum cylindricum — pass over the papilla and turn upward to form the medulla of the hair; the deeper cells — stratum spinosum — of the outer root-sheath become continuous with the cortex of the hair; the stratum lucidum, with the sheath of Henle, which turns up on the hair as its cuticle; Huxley's layer, with the cuticle of the inner root-sheath. According to this \dew growth of hair is accomplished by continuous growth downward from the surface, and turning up into the hair, of these layers. TECHNIC. Pin out small pieces of human scalp on cork and fix in absolute alcohol or in formalin-Mliller's fluid (technic 5, p. 7). From one block cut sections perpen- dicular to the surface of the scalp and in the long axes of the hair and follicles. From a second block cut sections at right angles to the hair follicles, i.e., not quite parallel to the surface of the scalp but a little obliquely. By this means not only are transverse sections secured, but if the block be sufficiently long the follicles are cut through at all levels. Sections are stained with haematoxylin-picro-acid- fuchsin (technic 3, p. 19) and mounted in balsam. Blood-vessels of the skin. From the larger arteries in the subcu- taneous tissue branches penetrate the pars reticularis of the derma, where they anastomose to form cutaneous networks. The latter give off Ijranches, which pass to the papillary layer of the derma and there form a second series of networks, the subpapillary, just beneath the papillae. From the cutaneous networks arise two sets of capillaries, one supplying the fat lobules, the other supplying the region of the sweat glands. From the subpapillary networks are given off small arteries which break up into capillary networks for the supply of the ]ja]jillce, sebaceous glands, and hair follicles. The return blood from these capillaries first enters a horizontal plexus of veins just under the papillce. This communicates with a second plexus just beneath the first. Small veins from this second plexus pass alongside the arteries to the deeper part of the corium, where they form a third ]j]exus with larger, more irregular meshes. Into this ])lexus pass most of the veins from the fat lobules and sweat glands, although one or two small veins from the sweat glands usually follow the duct and THE SKIN AND ITS APPENDAGES. 365 empty into the subpapillary plexus. The blood next passes into a fourth plexus in the subcutaneous tissue, from which arise veins of considerable size. These accompany the arteries. Small arteries from the plexuses of the skin and subcutis pass to the hair follicle. The larger arterioles run longitudinally in the outer layer of the follicle. From these are given off branches which form a rich plexus of small arterioles and capillaries in the vascular layer of the follicle. Capillaries from this plexus also pass to the sebaceous glands, the arrectores pilorum muscles, and the papillae. The lymphatics of the skin. These begin as clefts in the papillse. which open into a horizontal network of lymph capillaries in the pars papillaris. This communicates with a network of larger lymph capillaries with wider meshes in the subcutaneous tissue. The latter also receives lymph capillaries from plexuses which surround the seba- ceous glands, the sweat glands, and the hair follicles. The nerves of the skin. These are mainly sensory. Efferent sympathetic axones supply the smooth muscle of the walls of the blood- vessels, the arrectores pilorum, and secretory fibres to the sweat glands. The medullated sensory nerves are peripheral processes of spinal gang- lion cells. The larger trunks lie in the subcutis, giving off branches which pass to the corium, where they form a rich subpapillary plexus of both medullated and non-medullated fibres. From the subcutaneous nerve-trunks and from the subpapillary plexus are given off fibres which terminate in more or less elaborate special nerve endings (see page 386) . Their location is as follows: (i) In the subcutis: Vater-Pacinian corpus- cles, the corpuscles of Ruffini, and the Golgi-Mazzoni corpuscles of the finger-tip. The first two forms are most numerous in the palms and soles. (2) In the derma: Tactile corpuscles of Meissner and Wagner. These are found in the papillae, especially of the finger-tip, palm, and sole. Krause's end bulbs — usually in the derma just beneath the papillae, more rarely in the papillae themselves. (3) In the epithelium: Free nerve endings among the epithelial cells. Branches of the cutaneous nerves supply the hair follicles. As a rule but one nerve passes to each follicle, entering it just below the entrance of the duct of the sebaceous gland. As it enters thefolhclc the nerve fibre loses its medullary sheath and divides into two branches, which further subdivide to form a ring-like plexus of fine fibres encir- cling the follicle. From this ring, small varicose fibrils run for a short distance up the follicle, terminating mainly in slight expansions on the vitreous membrane. 366 THE ORGANS. TECHNIC. For the study of the blood-vessels of the skin inject (technic p. 22) the entire hand or foot of a new-born child. E.xamine rather thick sections either mounted unstained or stained only with eosin. Development of the Skin, Nails, and Hair. The epidermis is, as already noted, of ectodermic origin. It con- sists at first of a single layer of cuboidal cells. This soon differen- tiates into two layers — an outer, the future stratum corneum, and an inner, the future stratum germinativum. The stratum granulosum and stratum lucidum are special developments of the stratum germi- nativum. The corium is of mesoblastic origin. It is at first smooth, the papillae being a secondary development. The nail first appears as a thickening of the stratum lucidum. This spreads until the future nail bed is completely covered. During development the stratum corneum extends completely over the nail as its eponychium. During the ninth month (intra-uterine) the nail begins to grow forward free from its bed and the eponychium disap- pears, except as already noted. The hair also develops from ectoderm. It first appears about the end of the third foetal month as a small local thickening of the epidermis. This thickening is due mainly to proliferation of the cells of the stratum mucosum, and soon pushes its way down into the underlying corium, forming a long slender cord of cells — the hair germ. Differentiation of the surrounding connective tissue of the corium forms the follicle w^all, while an invagination of this connective tissue into the lower end of the hair germ forms the papilla. The cells of the hair germ now differentiate into two layers: a central core the middle portion of which forms the hair, while the peripheral portion forms the inner root sheath; and an outer layer which becomes the outer root sheath. The sublayers are formed from these by subsequent differentiation. The hair when first formed lies wholly beneath the surface of the skin. As the hair reaches the surface its pointed extremity pierces the surface epithelium to become the hair shaft (Fig. 256). The sebaceous gland develops as an outgrowth from the outer root sheath. This is a flask-shaped and at first solid mass of cells, which later differentiate to form the ducts and alveoli of the gland. The sweat glands first appear as solid ingrowths of the stratum germinativum into the underlying corium. The lower end of the ingrowth becomes thickened and convoluted to form the coiled por- THE SKIN AND ITS APPENDAGES. 367 III Fig 256 —Five stages in the development of a human hair. (Stohr ) <;Pinill-v & arrector pih muscle; c, beginning of hair shaft; ^, point whe^ ha r shaf throws hrough epidermis; ., anlage of sebaceous gland; /, hair' germ; e "air haft rH?r's r/tl-' ?"■''' 3 U 3 3 5 ^ it resembles in structure, the vertebras having their own .separate periosteum. The outer surface of the spinal dura is covered with a single layer of flat cells, and is separated from the periosteum bv the epidural space, which contains anastomosing venous channels Iving in an areolar tissue rich in fat. 380 THE ORGANS. The pia mater closely invests the brain and cord, extending into the sulci and sending prolongations into the ventricles. It consists of fibro-elastic tissue arranged in irregular lamellse, forming a spongy tissue, the cavities of which contain more or less fluid. The outer lamellse are the most compact, and are covered on the dural surface by a single layer of flat cells. It is this external layer of the pia which is frequently described as a separate membrane, the arachnoid. The inner lamellae of the pia are more loosely arranged, are more cellular and more vascular. Especially conspicuous are large, irregular cells with delicate bodies and large distinct nuclei. They lie upon the connective-tissue bundles partially lining the spaces. The Pacchionian bodies are peculiar outgrowths from the outer layer of the pia mater cerebralis, which are most numerous along the longitudinal fissure. They are composed of fibrous tissue, and frequently contain fat cells and calcareous deposits. Blood-vessels. — The spinal dura and the inner layer of the cere- bral dura are poor in blood-vessels. The outer layer of the cerebral dura, forming as it does the periosteum of the cranial bones, is rich in blood-vessels which pass into and supply the bones. The pia is very vascular, especially its inner layers, from which vessels pass into the brain and cord. TECHNIC. For the study of the structure of the membranes of the brain and spinal cord, fix pieces of the cord with its membranes, and of the surface of the brain with mem- branes attached, in formalin-Muller's fluid (technic 5, p. 7) and stain sections with haematoxylin-eosin (technic i, p. 18). THE PERIPHERAL NERVES. The peripheral nerves are divided into spinal nerves and cranial nerves, the former being connected with the cord, the latter with the brain. Each spinal nerve consists of two parts — a motor or efferent part and a sensory or afferent part. Of the cranial nerves some are purely efferent, others purely afferent, while still others consist like the spinal nerves of both efferent and afferent fibres. The efferent fibres of the spinal nerves are axones of cell Ijodies situated in the anterior horns of the cord (see p. 404, and Figs. 279 and 289) and axones of symjjathetic ganglion cells. The former leave the cord as separate bundles, which join to form the motor or efferent root. The THE NERVOUS SYSTEM. 381 afferent fibres are peripheral processes of cell bodies situated in the spinal ganglia (p. 385 and Figs. 279, 264). These leave the ganglion and join with the fibres of the motor root to form the mixed spinal nerve (Fig. 279, /). The connection of the ganglion with the cord is by means of the central processes of the spinal ganglion cells, which enter the cord as the posterior root. Among the afferent fibres of the posterior root are also found, in some animals, a few eft'erent Fig. 262. — From Transverse Section of Human Nerve Trunk. (Osmic acid fixation.) (Quain.) ep, Nerve sheath or epineurium surrounding the entire nerve and containing blood-vessels {v) and small groups of fat cells (/); per, perifascicular sheath or perineurium surrounding each bundle or fascicle of nerve fibres; end, interior of fascicle showing sup- porting connective tissue, the endoneurium. fibres (Fig. 279, c), processes of cells in the cord. Some fibres from the spinal ganglion and from the efferent roots form the white ramus communicans to the sympathetic ganglia. Fibres from the sympa- thetic ganglia form the gray ramus communicans to the mixed spinal nerve (Figs. 261 and 273). For further details regarding cranial nerves see pp. 425, 426. The peripheral nerve consists of nerve fibres supported by con- nective tissue (Fig. 262). Enclosing the entire nerve is a sheath of dense connective tissue, the epineurium. This sends septa into the nerve which divide the fibres into a number of bundles or fascieles. 3S2 THE ORGANS. Surrounding each fascicle the connective tissue forms a fairly distinct sheath, the perifascicular sheath or perineurium. From the latter, delicate strands of connective tissue pass into the fascicle, separating the individual nerve fibres. This constitutes the intrafascicular con- nective tissue or endoneurium. In the connective-tissue layers of the perineurium are blood-vessels, and lymph spaces lined with endo- thelium, which communicate with lymph channels within the fascicle. When nerves branch, the connective-tissue sheaths follow the branch- ings. When the nerve becomes reduced to a single fibre, the connect- ive tissue still remaining constitutes the sheath of Henle. On emerging from the central nervous system, the root fibres receive an in- vestment of connective tissue as they pass through the pia mater. This is reinforced by additional connective tissue, as the nerve passes through the dura mater. For description of medullated and non- meduUated nerve fibres see Chap. VL TECHNIC. Fix a medium-sized nerve, such as the human radial or ulnar, by suspending it, with a weight attached to the lower end, in formalin-Miiller's fluid (technic 5, p. 7). Stain transverse sections in haematoxylin-picro-acid-fuchsin (technic 3, p. 19), or hgematoxylin-eosin (technic i, p. 18), and mount in balsam. Pieces of nerve should also be fixed in osmic acid imbedded, cut, and mounted without further staining. Pieces of fresh nerve may be teased and examined without staining. THE AFFERENT PERIPHERAL NEURONES. These consist, as already stated, of the bodies and processes of the cerebro-spinal ganglion cells, probably some of the sympathetic ganglion cells, certain cells of the retina and certain cells of the olfactory mucous membrane. The two last named are described in connection with the organs of special sense. The Cerebro-spinal Ganglia. The cerebro-spinal ganglia are groups of nerve cells connected with the afferent roots of the cerebro-spinal nerves. Each ganglion is sur- rounded by a connective-tissue capsule which is continuous with the perineurium and epineurium of the peripheral nerves. (Fig. 261.) From this capsule connective-tissue trabeculae extend into the ganglion, forming a connective-tissue framework. Within the ganglion the nerve cells are separated into irregular groups by strands of connective tissue THE NERVOUS SYSTEM. 383 and by bundles of nerve fibres. Each ganglion cell contains a centrally located nucleus and a distinct nucleolus, and is surrounded by a capsule of fiat, concentrically arranged cells which are probably derived from the neural plate (p. 375) and are often called amphicytes or satellite cells (Fig. 265). Stained by Nissl's method the cytoplasm is seen to contain rather small, finely granular chromophilic bodies, which show a tendency to concentric arrangement around the nucleus. Pigmentation is common, the granules usually forming a group in the Fig. 263. — Longitudinal Section through a Spinal Ganglion. X 20. (Stohr.) a, Ventral nerve root; b, dorsal nerve root; c, mixed spinal nerve; d, groups of ganglion cells; e, nerve fibres;/, perineurium; g, fat; h, blood-vessel. vicinity of the point of origin of the main process of the cell. The majority of the cells of the spinal ganglia have one principal process which, at some distance from the cell body, divides into peripheral and central branches (p. 376). These cells are usually called unii)oIar cells. The principal process usually becomes medullated soon after emerging from the cell capsule (Fig. 264). Among these cells a number of forms have been distinguished by Dogiel: (a) Cells with only a principal process. This process may pass almost directly from the capsule, but often takes several turns around the cell body, forming a "glomerulus," before emerging from the capsule (Fig. 264, i and Fig. 265, ,1). This form is usually represented as the typical cerebro-spinal ganglion cell, but constitutes only a minority of these cells, (b) The principal process gives off collat- erals. These lie within the capsule or are given off outside the capsule and termin- ate in other parts of the ganglion and its covering, either in terminal arborizations 384 THE ORGANS. or in terminal enlargements or bulbs. The collaterals may branch. Some of the terminations may be around the capsules of other cells. There may be one or several collaterals, sometimes a number of very short intracapsular collaterals. This last variety of cell may have more than one main process. (Fig. 264, 2 and 3, Fig. 265, B). (c) Cells with split processes. Here the main process divides into a number of fibres which reunite; this may be repeated. The splitting may be intracapsular or extracapsular. A variation of this is where there are two to six processes from the cell which form a complicated intracapsular network, finally uniting to form the single main process (Fig. 264, 4 and 5 and Fig. 265, C and D). (d) Cells with a number of dendrite-like processes which divide, forming an intra- FiG. 264. — -Various Types of Cells and Nerve Terminations found in a Spinal Ganglion. Schematized from Dogiel. g.r., gray ramus communicans; sy.c, sympa- thetic cell; w.r., white ramus communicans. a. Spinal ganglion; b, dorsal or afferent root; c, ventral or efferent root; d, sympathetic ganglion; g, spinal nerve. For further explanation see text (pp. 383-385). capsular network which finally fuses into the main process (Fig. 264, 6). (e) Cells whose principal process divides as usual, but the peripheral branch terminates by arborizations or bulbs in the ganglion and in its covering, or in the neighborhood of the dorsal root (Fig. 264, 7). (f) Bipolar cells (Fig. 264, 8). (g) Multipolar cells with a number of intracapsular dendrites and amain process (Fig. 264, 10; Fig. 265, E and F). (h) Cells with a principal process which probably enters the dorsal root and a number of processes which may be dendrite-like in character but also become meduUated in places, and which branch and terminate in arbor- izations or bulbs in various parts of the ganglion. These latter apparently collect- ively represent the peripheral process which here ends in the ganglion (Fig. 264, 9). The various endings in the ganglion of collaterals and other processes of gang- lion cells often have capsules and resemble the terminations in receptors in other THE NERVOUS SYSTEM. 385 parts of the body (corpuscles of Pacini, end bulbs, etc.) and, together with their enve- lopes, may represent the receptors of the ganglion itself and of the connected nerves. Sympathetic fibres enter the ganglion and form a plexus within it from which fibres pass and terminate within the capsules of the various ganglion cells. a.f. Fig. 265. — Cerebro-spinal Ganglion Cells and their Capsules. (Cajal.) A (adult man), Unipolar cell with single process forming a glomerulus; B (man), cell with short process ending in intracapsular bulb and main process giving off intracapsular collateral; C (dog), "fenestrated" cell with several processes uniting to form main process; D (ass), more complicated form of the same; E (man), cell with short bulbous dendrites; F (man), cell with Inilhous dendrites and enveloped with pericellular arborizations (/>.a.) of fibres ((/./.) terminating around cell; r, collateral; (/, dendrite; p, principal process; s.p., short process. (Cajal's silver slain.) The peripheral processes of the cerebro-spinal ganglion CELLS arc the ajfcrciil fibres of the cerebro-s])inal nerxx's (p. ^Si). The modes of termination of these peri|)heral processes are extremclv \-aried 386 THE ORGANS. and complicated. These peripheral terminations are always free, in the sense that, while possibly sometimes penetrating cells, they probably never become directly continuous with their protoplasm. In the skin, and in those mucous membranes which are covered with squamous epithelium, the nerve fibres lose their medullary sheaths in the subepithelial tissue, and, penetrating the epithelial layer, split up into minute fibrils which pass in between the cells and terminate there, often in little knob-like swellings (Fig. 266). In addition to such comparatively simple nerve endings, there are also found in the skin Fig. 266. — Free Endings of Afferent Nerve Fibres in Epithelium of Rabbit's Bladder. (Retzius.) 0, Surface epithelium of bladder; bg, subepithelial connective tissue; n, nerve fibre entering epithelium where it breaks up into numerous terminals among the epithelial cells. and mucous membranes, especially where sensation is most acute, much more elaborate terminations. These may be classified as (i) tactile cells, (2) tactile corpuscles, and (3) end bulbs. A simple tactile cell is a single epithelial cell, the centrally di- rected end of which is in contact with a leaf-like expansion of the nerve terminal, the tactile meniscus. In the corpuscles of Grandry, found in the skin of birds, and in Merkel's corpuscles, which occur in mammalian skin, several epithelial cells are grouped together to receive the nerve terminations. These are known as compound tac- tile cells, the axis cylinder ending in a flat tactile disc or discs between the cells. THE XER\-OUS SYSTEM. 387 Of the tactile corpuscles (Fig. 267) those of IMeissner, which occur in the skin of the fingers and toes, are the best examples. These corpuscles lie in the papillae of the derma. They are oval bodies, surrounded by a connective-tissue capsule and composed of flattened ^-^^^7^ ' ' v\. ^ ?l\ \\>y'~'^Y) Fig. 267. Fig. 267. — Tactile Corpuscle of Meissner, tactile cell and free nerve ending. (Merkel- Henle.) a, Corpuscle proper, outside of which is seen in the connective-tissue capsule, h, fibre ending on tactile cell; c, fibre ending freely among epithelial cells. Fig. 268. — Taste Bud from Circumvallate Papilla of Tongue. (Merkel-Henle.) a, Taste pore; h, nerve fibres entering taste bud and ending upon neuro-epithelial cells. On either side fibres ending freely among epithelial cells. cells. From one to four medullated nerve fibres are distributed to each corpuscle. As a fibre approaches a corpuscle, its neurilemma becomes continuous with the fibrous capsule, the medullary sheath disappears. Fig. 269. — End Bulb from Conjunctiva. (Dogiel.) a, Medullated nerve fibre, a.xone of which passes over into dense end skein. and the filjrills pass in a spiral manner in and out among the epithelial cells. Of the so-called end bulbs, the simplest, which are found in the mucous membrane of the mouth and conjunctiva, consist of a central 3S8 THE ORGANS. core formed by the usually more or less expanded end of the axis cylinder, surrounded by a mass of finely granular, nucleated proto- plasm— the inner bulb — the whole enclosed in a capsule of flattened connective-tissue cells. More complicated are the Pacinian bodies found in the subepithelial tissues of the skin and in many other or- gans of mammalia. The Pacinian bodies (Fig. 270) are laminated, elliptical struc- tures which differ from the more simple end bulbs already described, mainly in the greater development of the capsule. The capsule is formed by a large number of concentric lamellse, each lamella consisting of connective-tissue fibres lined by a single layer of flat connective-tissue cells. The lamellae are separated from one another by a clear fluid or semifluid substance. As in the simpler end bulbs there is a cylin- drical mass of protoplasm within the cap- sule known as the inner bulb. Extending lengthwise through the centre of the inner bulb, and often ending in a knob-like ex- tremity, is the axis cylinder. In voluntary muscle afferent nerves thelioid cells lying between terminate in Pacinian corpitsclcs, in end laminae of capsule; n, nerve in 1 • ^• . ^ 1 fibre, consisting of axis cyiin- 0^^^-^' and m complicated end organs called muscle spindles, or neuromuscular bundles. The muscle spindle (Fig. 271) is an elongated cylindrical structure within which are muscle fibres, connective tissue, blood-vessels, and medullated nerves. The whole is enclosed in a connective-tissue sheath which is pierced at \'arious points by nerve fibres. A single spindle contains several muscle fibres and nerves. According to Ruflmi, there are three modes of ultimate terminations of the nerve fil^rils within the s])indles: one in which the end fibrils form a series of rings which encircle the indi\irlual muscle fibre, termed annular lerminalions; a second in which the nerve fibrils wrap around the muscle fibres in a spiral manner — spiral lerminalions; a third in which the terminations take the form of delicate exjjansions on the muscle fibre — arborescenl lerminalions. At the junction of muscle and tendon are found the Fig. 270. — Pacinian Body from Mesentery of Cat. (Ranvier.) c, Lamina of capsule; d, epi der surrounded by Henle's sheath, entering Pacinian body; /, perineural sheath; m, inner bulb; n, terminal fibre which breaks up at a into irregular bulbous termi- nal arborizations. THE NERVOUS SYSTEM. 389 Fig. 271. — ^Middle Third of Muscle Spindle from Striated \\)luntary Muscle P'ibre of Cat. (From Barker, after Ruffini.) A, rings; 5, spirals; J^, arborescent branchings. Fig. 272. — -Tendon of Muscle of Eye of O.x. (Ciaccio.) Two muscle-tendon organs of Golgi, each showing ring-like and brush-like endings, gli, sheath of Henle; sr, node of Ranvier. 390 THE ORGANS. elaborate afferent terminal structures known as the muscle -tendon organs of Golgi (Fig. 272). It is evident from the above that the nerve terminations are only stimulated through the intermediation of surrounding cells w^hich may form quite elaborate structures. These cells constitute the receptors and probably render the various nerve terminations they envelop more or less inaccessible to all but one par- ticular kind of stimulus. The above receptors are scattered throughout head and body {general or common senses) as distinguished from those which are con- centrated into the organs of the special senses (smell, sight, hearing, taste) pres- ent only in the head (pp. 424, 425 and Chap. XII). The various stimuli received by them may give rise to sensations of light pressure or touch (tactile cells and corpuscles?), temperature and pain (diffuse terminations in epithelium and con- nective tissue?), muscle-tendon sense of movement and position (muscle spindles and possibly end bulbs, etc., in muscles, tendon organs of Golgi?). These may be roughly grouped into general cutaneous or superficial sensation and deep sen- sation (muscle-tendon and other bodily sensations) . All receptors, both of general and special senses, may be classified (Sherrington) as those receiving stimuli from the external world (extero-ceptors, of superficial sensation, smell, sight, and hearing), those concerned with visceral reactions (intero-ceptors, including taste) and those giving information of bodily changes {proprio-ceptors, deep sensation). The central processes of the cerebro-spinal ganglion CELLS enter the central nervous system as the fibres of the afferent root, the entire bundle of afferent root fibres of a single nerve consisting of all the central processes of the corresponding ganglion. Having entered the central nervous system, the central processes divide into ascending and descending arms, as already mentioned (p. 376). THE SYMPATHETIC GANGLIA. The sympathetic system of the neck and trunk consists of a series of vertebYal or chain ganglia, lying ventro-lateral to the vertebrae and connected by longitudinal cords, and of gangliated prevertebral plexuses connected with the vertebral ganglia and also connected with ill- defined peripheral ganglia in the walls of the viscera {e.g., plexuses of Auerbach and Meissner). The sympathetic ganglia of the head are the ciliary, sphenopalatine, otic, and submaxillary . The peripheral processes of certain spinal ganglion cells pass via the white rami communicantes to the vertebral ganglia and thence to visceral receptors. Axones of splanchnic efferent spinal neurones pass irom the s[jinal cord via the ventral roots and white rami com- municantes to the vertebral ganglia and terminate in various sympa- thetic ganglia. The first thoracic to the second lumbar spinal nerves THE NERVOUS SYSTEM. 391 Smooth muscle Type II I \ Somatic efferent Somatic afferent Splanchnic cerebrospinal afferent Splanchnic cerebrospinal efferent Sympathetic - Dorsal root Spinal ganglion Dorsal ramus ^'entral ramus Blood vessel Smooth muscle Prevertebral ganglion Afferent sympath. neurone White r. com. •-- Gray r. com. •- Vertebral or chain gang. Gland — Periph. gang. - - B lood vessel Sens, ending Pacinian corpuscle Fig. 273. — Diagram of the Sympathetic System and the Arrangement of its Neurones in a Mammal. On the left are shown the typical elements of a trunk segment including the sympathetic system. On the right are shown only the somatic afferent and efferent neurones of the spinal nerve. Of the sympathetic system are shown the white and grav rami, three ganglia of the chain, one prevertebral ganglion and one peripheral ganglion. The symbols used are explained in the figure. In most respects the diagram follows one of Ruber's figures. (Johnston.) It will be noted that the sympathetic outflow from any particular cord segment may ultimately reach other segments of the bodv. 39: THE ORGANS. are thus connected with the sympathetic. The second, third, and fourth sacral nerves and visceral branches of the vagus, glossopharyn- geus, and facialis have similar connections with the prevertebral plex- uses. Some cerebro-spinal (especially vagus) fibres may pass directly to visceral structures. The sympathetic ganglia of the head are con- nected with certain of the cranial nerves (see p. 426). Some of the sympathetic cells are probably afferent but the ma- jority are efferent and send their axones via the gray ramus com- FiG. 274. — Sympathetic Nerve Cells (woman of 36 years). (Cajal.) A and B, cells whose dendrites (b) form a pericellular plexus. C, cell witli long dendrites, a, axone; cand d, terminal fjart of a dendrite. The capsular cells are faintly indicated. (Cajal's silver stain.) municans (to spinal nerves), longitudinal cords and efferent rami to the various visceral effectors (see below). The sympathetic is thus in the main composed of additional neurones intercalated in the visceral efferent jjath. The fibres of the white rami communicantes are mostly fine and medullated. The axones of the sympathetic cells are fine and non-medul!atcd or thinly medullated. For further details see Fig. 273. The larger ganglia resemble the sjjinal ganglia in having a connec- tive-tissue capsule and framework. The cells are smaller and often densely pigmented. Fach cell is surrounded by a capsule of cells similar to tho.se surrounding the spinal ganglion cells. Often two or three THE NER\'OUS SYSTEM. 393 cells and their interlocked dendrites are enclosed within a common capsule I Fig. 275, A). The typical sympathetic nerve cell is a multipolar cell with short branching dendrites confined to the capsule of the cell and often inter- locked with the dendrites of adjacent cells, forming glomeruli. Some- times a dendrite will pass some distance from the cell, arborize, and Fig. 275. — Sympathetic Nerve-Cells and their Capsules. (Cajal). .1, Two-celled glomerulus; B, cell surrounded with the pericellular terminal arborizations of two fibres («./.) passing to the cell; a, axones; d, fibre, probably dendritic, with bulbous termina- tion. (Cajal's silver slain.) interlock with a similar dendritic arborization of another cell. Another form of sympathetic cell has long slender dendrites often indistinguish- able from axones. These cells are more frequent in the peripheral ganglia. Some of these cells have been considered to be afferent sym- pathetic cells. (Figs. 274 and 275.) The sympathetic cells receive fibres which form arborizations around and within their capsules and also around the long dendrites. 394 THE ORGANS. Many of these are terminations of the visceral cerebro-spinal efferent neurones (Figs. 273 and 275, B). The axones of the efferent sympathetic cells terminate in heart muscle, in smooth muscle of viscera (viscero-motor), of blood-vessels (vaso-motor) and of hairs (pilo-motor), and in glandular epithelia (secretory). In heart muscle (Fig. 276) and in smooth muscle (Fig. 277) the nerves of the sympathetic system end in fine feltworks of fibres, which are in relation with the muscle cells. Satisfactory dift'erentia- tion between efferent terminals and aft'erent terminals in heart and in smooth muscle has not yet been made. In organs whose parenchyma is made up of so-called glandular epithelium, the sympa- thetic nerves terminate mainly in free endings which lie in the cement substance between the cells, thus coming in contact with, though not penetrating, the epithelial cells. Fig. 276. — Nerve Endings on Heart Muscle Cells. (From Barker, after Huber and De Witt.) A. Fig. 277. — Nerve Endings on Smooth Muscle Cells. (From Barker, after Huber and De Witt.) a, Axis cylinder; b, its termination; n, nucleus of muscle cell. TECHNIC. (i) Fix spinal and sympathetic ganglia in formalin-Miiller's fluid (technic 5, p. 7). Stain sections with haematoxylin-eosin (technic i, p. 18), or vi^ith ha^niatoxylin- picro-acid-fuchsin (technic 3, p. 19). (2) Fix spinal and sympathetic ganglia in absolute alcohol or in lo-per-cent. formalin, and stain sections by Nissl's method (technic, p. 35). (3) See also technic i, p. 399. (4) Ganglia should also be prcjjarcd by Cajal's silver method, using the al- cohol fixation (j). 34). THE NER\'OUS SYSTEM. 39.5 EFFERENT PERIPHERAL CEREBRO-SPINAL NEURONES. It has been seen that the bodies of these neurones lie in the ventral part of the neural tube where they form a part of the ventral gray column in the cord and the "motor" nuclei of cranial nerves in the segmental brain. The axones of these cell bodies emerge as the efferent roots and usually either pass {via the white ramus com- municans, in spinal nerves) to various sympathetic ganglia to terminate there, or proceed as the efferent fibres of the peripheral nerves to ter- minate in the striated voluntary muscles of the body and head. In the Fig. 278. — Motor nerve-endings in abdominal muscles of a rat. Gold preparation. X 170. (.Szymonowicz). spinal nerves these fibres pass beyond the spinal ganglia and then join the afferent fibers; in some cranial nerves they pass out with the afferent fibres. On their way to the muscles the motor axones may bifurcate several times, thus allowing one neurone to innervate more than one muscle fibre. In the perimysium the nerve fibres undergo further branching, after which the fibres lose their medullary sheaths and pass to the individual muscle fibres. Here each fibre breaks up into several club-like terminals which constitute the motor cud plate. The location of the end plate, whether within or without the sarcolemma, has not been determined. As a rule each muscle fibre is supplied with a single end plate, though in large fibres there may be several. (Fig. 278.) 396 THE ORGANS. THE SPINAL CORD. The spinal cord encased in its membranes lies loosely in the ver- tebral canal, extending from the upper border of the first cervical vertel^ra to the middle or lower border of the first lumbar vertebra. It is cylindrical in shape and continuous above with the medulla oblongata, while below it terminates in a slender cord, the filum ter- minale. At two levels, one in its cervical and one in its lumbar region, the diameter of the cord is considerably increased. These are known, respectively, as the cervical and lumbar enlargements. The spinal nerve roots leave the cord at regular intervals, thus indicating a division of the cord into segments, each segment extending above and below its nerve roots one-half the distance to the next adjacent roots. There are 31 segments corresponding to the 31 spinal nerves; 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and i coccygeal. Origin of the Fibres which Make up the White Matter of the Cord. It has already been observed that the white matter of the cord is composed mainly of meduUated nerve fibres, most of which run in a longitudinal direction. From the study of the neurone it follows that each of these fibres must be the axone of some nerve cell. These cells, the medullated axones of which form the white matter of the cord, are situated as follows: A. Cells oulside the spinal cord. (Extrinsic cells.) ii. Cells situated in the fi;ray matter of the cord. (In- trinsic cells.) (i) Cells outside the central nervous system (spinal ganglion cells). (2) Cells in other parts of the central nervous sys- tem (the brain). (3) Root cells, such as those of the anterior horn, whose axones form the ventral root (efferent peripheral neurones). (4) Intermediate neurones, whose axones enter into formation of the fibre columns of the cord (column cells.) (5) Cells of Golgi,type II, the axones of which ramify in the gray matter. (These cells do not give rise to fibres of the white matter, but are conveniently mentioned here among the other cord cells.) THE NER\'OUS SYSTEM. 397 (i) The Spinal Ganglion Cell and the Origin of the Posterior Columns. It has already been seen that the central processes of these cells enter the cord as dorsal root fibres and split into ascending and descend- ing longitudinal arms composing the greater part of the dorsal funic- ulus and the zone of Lissauer. They are described more in detail later (pp. 413 and 414)- Fig. 279. — Transverse Section through Spinal Cord and Posterior Root Ganglia of an Embryo Chick. (\'an Gehuchten.) a, Spinal ganglion, its bipolar cells sending their peripheral processes outward to become fibres of the mixed spinal nerve (/), their central processes into the dorsal columns of the cord as the dorsal root fibres (&) ; within the poste- rior columns these fibres can be seen bifurcating and sending collaterals into the gray matter of the posterior columns, one collateral passing to the gray matter of the opposite side. The few efferent fibres of the dorsal root (c) are disproportionately conspicuous. The large multipolar cells of the ventral horns are seen sending their axones (d) out of the cord as the ventral root fibres (e) which join the peripheral processes of the spinal ganglion cells to form the mixed spinal nerve (/); col, collateral from axone of ventral horn cell. Dendrites of the anterior horn cells are seen crossing the median line in the anterior com- missure. About the centre of the cord is seen the central canal; dorsal and ventral to the latter some ependymal cells stretching from the canal to the periphery of the cord. (Golgi Method.) (2) Cells Situated in Other Parts of the Centr.4l Nervous System which Contribute Axones to the White Columns OF the Cord. These cells are situated in the motor areas of the cortex of the pallium, in the midbrain, cerebellum!?), and Mirious ])arts of the segmental brain. The axones of these cells ])ass down the cord, forming the descending fibre tracts of the cord (]). 416). Collaterals and terminals of these fibres enter the gray matter of the cord to ter- minate there. 398 THE ORGANS. (3) Root Cells — Motor Cells of the Anterior Horn. The course of the axones of these cells has been described (p. 395). The bodies are described on pp. 404 and 405. (4) Column Cells. These are cells which lie in the gray matter of the cord and send their axones into the white columns or funiculi (see p. 403) where they either bifurcate or turn up or down, becoming longitudinal fibres. Some of the cells send their axones into the white matter of the same ventral dorsal Fig. 280. — Cross Section through Spinal Cord of Embryo Chick of Eight Days' Incubation. (Cajal, Golgi's method.) A, Hecateromeric cell with axone sending off side fibril to gray matter and then dividing, one branch passing to the white matter of the same side, d, the other through the ventral commissure to the white matter of the opposite side, a and d. B and C, Hecateromeric cells of the dorsal gray matter; the axones divided, one branch, a, passing to the dorsal white columns of the same side, the other, c, through the anterior commissure to the opposite side of the cord. D, Tautomeric cell, the axones branching, but all h)ranches passing to the gray matter or white matter of the same side of the cord. E, Tautomeric cell of the ventral horn with axone dividing into two branches, a and d, in the white matter of the same side. The importance of the hecateromeric cells is exaggerated in the figure. A, B and C without side processes a and d give a good representation of heteromeric cells. side of the cord. These are known as tautomeric cells (Fig. 280, D, E). Others send their axones as fibres of the anterior commissure to the white matter of the opposite side of the cord — heteromeric cells. The axones of a few cross in the dorsal white commissure. In a few others the axone divides, one branch going to the white matter of the same side, the other to the white matter of the opposite side — hecateromeric cells (Fig. 280, A,B,C). THE NER\'OUS SYSTEM. 399 The axones of many of these cells are short, constituting the short fibre tracts (fundamental columns — ground bundles) of the cord (see page 419; others are long, passing up through the cord to the brain (see page 413). Terminals and collateral branches of these longitudinal axones, especially of the short ones are con- stantly re-entering the gray matter to end in arborizations around the nerve cells (Fig. 281). (5) Cells of Golgi Type II. The axones of these cells do not leave the gray matter, but divide rapidly and terminate in the gray matter near their cells of origin, some crossing to terminate in the gray matter of the opposite side. TECHNIC. (i) For the purpose of studying the spinal ganglion cell with its processes and their rela- tions to the peripheral nerves and to the cord, the most satisfactory material is the embryo chick of six days' incubation, treated by the rapid silver method of Golgi (technic b, p. 32). Rather thick (75/t) transverse and longitudinal sections are made and mounted in balsam without a cover-glass. Owing to the uncer- tainty of the Golgi reaction several attempts are frequently necessary before good sections are obtained. (2) The root cells of the anterior horn with their axones passing out of the cord and join- ing the peripheral processes of the spinal ganglion cells to form the spinal nerves, can usually be seen in the transverse sections of the six-day embryo chick cord prepared as above, technic (i). (3) For studying the column cells of the cord, embryo chicks of from five to six days' incubation should be treated as in technic (i). Owing to the already mentioned uncertainty of the Golgi reaction, it is usually necessarv to make a large number of Fig. 281. — From Longitudinal Section of Spinal Cord of Embryo Chick. (Van Gehuchten.) .4,\\'hite column of cord; B, gray matter. The cells of the gray matter (column cells) are seen sending their a.xones into the white matter, where they bifurcate, their ascending and descending arms becoming fibres of the white column. The dendrites of these cells are seen ramifying in the gray matter. To the left are seen fibres (posterior root fibres) entering the white matter and bifurcating, the ascending and descending arms be- coming fibres of the white column. From the latter are seen fibres (col- laterals and terminals) passing into the gray matter and ending in arbor- izations. (Golgi r^rethodj 400 THE ORGANS. sections, mounting only those which are satisfactorily impregnated. It is rare for a single section to show all types of cells. Some sections contain tautomeric cells, some contain heteromeric, while in very few will the hecateromeric type be found. Sections containing fewest impregnated cells frequently show collaterals to best advantage. These are seen as a fringe of fine fibers crossing the boundary line between gray matter and white matter. (4) The silver method of Cajal, using the alcohol fi.xation (technic 2, p. 34), mav also be advantageously used to display many of the above details of struc- ture in embryo chicks. It is also capricious. "Whole neurones with long axones obviously cannot be directly demonstrated in single sections. To demonstrate these serial sections and indirect methods (degeneration, etc.) are used (see Fibre Tracts of Cord). PRACTICAL STUDY. Transverse Section of Six-day Chick Embryo (Technic i, p. 399). — Using a low-power objective, first locate the cord and determine the outlines of gray mat- ter and white matter. Observe the spinal ganglia lying one on either side of the cord (Fig. 279, a). One of the ganglia will probably show one or more bipolar cells, send- ing one process toward the periphery, the other toward the spinal cord. Note that the peripheral process is joined, beyond the ganglion, by fibres which come from the ventral region of the cord (fibres of the anterior root). In some specimens the latter can be traced to their origin in the cells of the anterior horn (Fig. 279, d). The union of the peripheral processes of the spinal ganglion cells and the anterior horn fibres is seen to make up the mixed spinal nerve (Fig. 279,/). Observe the central processes of the spinal ganglion cells entering the dorsal column of the cord and bifurcating (Fig. 279, b). As these branches pass up and down the cord, only a short portion of each can be seen in a transverse section. Note the fibres (collaterals) passing from the white matter into the gray matter. Note in some of the sections a little round mass just ventral and to the inner side of the spinal ganglion, in which nerve cells may be seen, and some fibres passing into or out of it. This represents the beginning of the sympathetic system with its chain of ganglia. Note the rela- tion which this bears to the spinal cord and spinal ganglia. In the same or other transverse sections study the column cells of the cord, carefully distinguishing between the dendrites and axone. This is not always easy, but the axone can usually be distinguished as being more slender, with smoother outline and more uniform size throughout its course. A.xones may come off from dendrites as well as from the cell bodies. At least one tautomeric and one heteromeric column cell should be found and studied (Fig. 280). Study also the collaterals if they are stained. Remember that only a few of the ele- ments present are stained in Golgi preparations and that there are apt to be pre.sent irregular silver precipitates without any significance. Capillaries often appear as a coarse brown-stained meshwork. Study also any spongioblasts that may be stained. Those with nuclei near the central canal give a fair reprei^entation of the ependyma cells of ihc adult cord, except the rcll floes not usually in the latter extend entirely through the wall of the neural lube. THE XER\'OUS SYSTEAL 401 Longitudinal Section of Six-day Chick Emtryo (Technic i, p. 399). — Using a low-power objective locate gray matter and white matter and identify plane of section relative to transverse section above described. Note in the white matter longitudinally-running fibres from which branches pass off into the gray matter (Fig. 281). Those of the posterior columns are the ascending and descending branches of the central processes of the spinal ganglion cells, and the branches pass- ing into the gray matter are their collaterals and terminals. If the section happens to include the entering fibers of a posterior root, these can be seen branching in the posterior columns into ascending and descending arms (Fig. 281). The longitudi- nal fibres of the lateral and anterior columns are axones of column cells and of cells situated in higher centres (see pages 397 and 398). These also send collaterals and terminals into the gray matter. General Topography of the Cord, Cell Groupings, Arrangement of Fibres and Finer Structure. The further description of the cord is best combined with the practical study of sections of the cord, taking sections through the lumbar enlargement as a type. PRACTICAL STUDY OF SECTIONS THROUGH LUMBAR ENLARGEMENT. General Topography (Figs. 282 and 283). — The general features of the sec- tions can be best seen with the naked eye or with a low-power dissecting lens. Notethe shape and size oi ihe cord, and that it is surrounded by a thin membrane, the pia mater spinalis; the deep anterior median fissure, into which the pia mater extends; the posterior median septum consisting principally of neuroglia, over which the pia mater passes without entering; and the postero-lateral grooves or sulci at the entrance of the posterior root fibres. The gray matter is seen in the central part of the section (stained more lightly in the Weigert preparation, on account of the presence of numerous unstained cell bodies and dendrites) and arranged some- what in the form of the letter H. Dorsally the gray matter extends almost to the surface of the cord as the dorsal gray columns (posterior horns or cornua). The ven- tral gray columns (anterior horns) are, on the other hand, short and broad, and do not approach the surface of the cord. Surrounding the gray matter is the while matter (stained deep blue in the Weigert preparation). This is divided by the posterior horn into two parts, one lying between the horn and the posterior median septum, the posterior funiculus (dorsal white column); the other comprising the remainder of the white matter, the antero-lateral funiculus (antcro-lateral white column). This latter is again divided by the anterior horn and anterior nerve roots into a lateral funiculus (lateral white column) and an anterior jioiiculus (ventral white column). In the concavity between the anterior and posterior horns some processes of the gray matter extend out into the white matter where they interlace with the longi- tudinallv running fibres of the latter to form the reticular process (not well marked in the lumbar cord). 26 402 THE ORGANS. For the stud}' of further details the low-power objective should be used. Gray ^Iatter. — In the cross portion of the H is seen the central canal, usually obliterated in the adult and represented only by a group of epithelial cells. The central canal divides the gray matter connecting the two sides of the cord into a ventral gray commissure and a dorsal gray commissure. Immediately surrounding the epithelial cells is a light granular area composed mainly of neuroglia and known .2 T3 "S s 0:s ft o, CO o j2 o rt ,s^ £ ^ s u 3 6 S as the central gelatinous substance. Toward the surface of the cord the posterior horn expands into a head or caput, external to which is an area similar in general appearance to that surrounfiing the central canal, the gelatinous substance of Rolando. The head is connected with the ba.se of the dorsal horn by a narrower wec/e or cervix. External to the gelatinous sub.stance of Rolando is a thin zone containing a plexus of fine modullatcd fibres (Wcigert stain) known as the marginal zone or zona span- THE NERVOUS SYSTEAI. 403 giosa, and external to this, occupying the space between it and the periphery, is a zone composed of fine longitudinal medullated fibres rather sparsely arranged and therefore staining more lightly with the Weigert method. This is the zona tenni- nalis or zone of Lissauer. It belongs obviously to the white matter of the cord (see page 405 ). The portion of the gray matter connecting the dorsal and ventral horns may be termed the intermediate or middle gray. Note the well-defined groups of large nerve cells in the anterior horns and the fibres passing out from the anterior horns to the surface of the cord, ventral {anterior) nerve roots. (Figs. 282 and 283.) White Matter. — Note the general appearance of the white matter and the disposition of the supporting strands of neurogha tissue (light in the Weigert, usually darker in other stains). The neuroglia is seen to form a fairly thick layer just be- neath the pia mater from which trabeculae pass in among the fibres, the broadest strand forming the posterior median septum. If the section has been cut through a dorsal (posterior) nerve root, a strong bundle of dorsal root fibres can be seen entering the white matter of the cord along the dorsal and mesial side of the posterior horn. Just ventral to the anterior gray commissure is a bundle of transversely-running medullated fibres — the anterior white commissure. (Fig. 283.) It is composed of the axones of various heteromeric column cells and of decussating terminals of various fibres. In the dorsal part of the dorsal gray commissure are also a few fine trans- versely-running medullated nerve fibres — the dorsal white commissure. It consists of collaterals of fibres in dorsal funiculi and axones of heteromeric column cells. Cell Groupings. — The positions and groupings of the various cell bodies should now be studied. (Nissl, Haematoxylin-Eosin, Cajal, Figs. 282 and 283.) (For convenience, their general arrangement throughout the cord is here given; also the course of their axones, though this is usually only seen in Golgi preparations.) (A) Cells of the Dorsal Horn. — (a) Marginal cells arranged tangentially to the border of the gelatinous substance of Rolando, (b) Cells in the gelatinous substance of Rolando, arranged radially. The Golgi method indicates that the axones of (a) and (b) principally enter the adjoining lateral column, (c) Large stel- late cells in the apex of the caput, most of the axones of which go to the lateral columns, but some cross in the ventral white commissure, (d) A central group in the central part of the dorsal cornu, some of the axones of which may cross in the ventral commissure, (e) Basal cells in the base of the horn and in the processus reticularis, the axones of which usually go to the lateral column but may cross. (f) Dorsal (thoracic) nucleus or cells of Clarke's column in the mesial part of the base of the dorsal horn. These are tautomeric cells, the axones of which form the dorsal spino-cerebellar tract (see page 415). Clarke's column is mainly confined to the dorsal or thoracic cord, but is also present in the first lumbar segment. (B) Cells of the Intermedlvte Gr.a.y. — (a) Middle nucleus whose cells may send their axones across in the ventral commissure or uncrossed to the lateral column; (b) various small cells including the accessory nucleus near Clarke's column; (c) intermcdio-lateral group which in parts of the cord forms a projection of the gray known as the lateral horn. These are probably root cells whose axones pass into the sympathetic system. This nucleus is more conspicuous in the thoracic cord, but extends from the eighth cervical to about the third lumbar segment and is also present in the sacral cord (especially the third segment). 404 THE ORGANS. From the above it is seen that all the cells of the dorsal and intermediate gray, except the intermedio-lateral group, are column cells. (C) Cells of the Ventral Horn. — These fall into two categories: (i) Tauto- meric column cells, sending axones to the adjoining white matter, and heteromeric column cells, the axones of which cross in the ventral commissure. Among the latter may be a well-marked group in the dorso-mesial part of the horn (commissural nucleus). {2) Root cells. Two main divisions may be distinguished: (a) Mesial group, present throughout the cord above the fifth sacral segment (Bruce). This group probably innervates the striated voluntary (.somatic) muscles of the trunk. The me.sial group is in part of the cord subdivided into a ventro-mesial and dorso- mesial group, the latter being present in the first, sixth and seventh cervical, thoracic (except first), and first lumbar .segments, (b) Lateral group, present in the cervical, first thoracic, and lumbo-sacral regions. This group innervates the muscles of the THE NERVOUS SYSTEM. 405 * extremities and exhibits the following subdivisions: An antero-lateral (C4 to C8, L2 to S2), a postero-lateral (C4 to C8, L2 to S3) and a post-postero-lateral (C8 to Thi, Si to S3). There is also a central group (L2 to S2) and a small anterior group (Li to L4). The exact muscle groups innervated by these cell groups respectively have not yet been definitely determined. Other special cell groups are the phrenic group (C4), centrally located, cilio-spinal and other cells (C8 to Th2, via the sympathetic to dilator pupilte and blood-vessels of head), and the spinal accessory (Ci to C6). The latter is located laterally and innervates the sterno-mastoid and trapezius muscles. For the determination of the destination of the axones of the efferent root cells the method of studying the changes in the cell body (Nissl stain) in definite lesions of the peripheral fibres (axonal degeneration) is used. Arrangement of Fibres (Fig. 283). — With the low- and high-power objectives the course of the transverse {i.e., longitudinally cut) nerve fibres should be carefully studied in Weigert and Cajal preparations. These fibres pass from gray to white matter or vice versa, and are in general (a) root fibres entering or leaving the cord; (b) either axones of column cells in the gray passing out into the white there to become longitudinal fibres by turning or splitting, or they are the collaterals and terminals of the fibres of the white matter entering the gray to terminate there. The arrangement of these fibres should be carefully- studied in all parts of the section (Weigert and Cajal), taking one field at a time. In the dorsal part of the cord, the dorsal roots can be seen entering. From their lateral portion fine fibres detach themselves and enter the zone of Lissauer, the fibres of which are largely composed of their short ascending and descending arms. Most of the fibres of the root passes along the dorsal and mesial side of the dorsal horn forming the zone oj entry of the dorsal roots. By bifurcating (not visible in the preparation) they be- come the longitudinal fibres of the dorsal funiculus. From the entering root fibres and fibres of the dorsal funiculus, bundles of fine fibres (collaterals and terminals) pass radially through the gelatinous substance of Rolando or sweep around its mesial side and enter the gray. Some of these terminate in the gelatinous substance of Rolando (Golgi preparations), some form part of the dense plexus of fibres in the caput and terminate there, others can be traced to the intermediate gray and, in some cases, some ("direct reflex collaterals") can be traced to the ventral horn. It will be noted that not many come from the mesial part of the dorsal funiculus. Collaterals from the zone of Lissauer enter the gelatinous substance of Rolando. In the middle part of the cord there is a similar interchange of fibres between the plexus in the gray and the adjoining white matter. In the ventral part of the cord a similar interchange takes place, but here besides these fine fibres are seen the coarse fibres of the ventral roots gathered from various parts of the ventral horn to form bundles which leave the ventral side of the horn, pass through the white matter and emerge as the ventral root fibres. The larger bundles of fibers in the ventral horn separate the cell groups, but between individual cells are seen numerous fine medullated fibres (principally terminals of fibres from the white funiculi ). Trace as far as possible the course of the fibres of the ventral and dorsal white commissures. Finer Structure. — Study with the high power the general histological structure of the gray and white matter. In the gray matter note (Cajal, Xissl, H.-E.), besides the nerve cells and their processes, the neuroglia nuclei. Note also the structure 406 THE ORGANS. and size of the medullated nerve fibres (Weigert). In tlie wliite matter note tl:e appearance of the cross-cut medullated nerve fibres in Weigert, Cajal and H.-E. preparations. With the neuroglia stains study carefully the neuroglia cells and neuroglia fibres, including the neuroglia zone forming the margin of the cord. (Fig. 284.) Note also the pia mater and the connective-tissue septa entering the cord from the pia accompanied usually by a denser aggregation of glia libres. Note carefully the number of neuroglia nuclei in some small field. Increase in neuroglia is characteristic of many pathological conditions. Study the ependyma. (Weigert, Nissl, Cajal, glia stain). Study the internal structure of the nerve-cells of various sizes present, espe- cially the amount and arrangement of the chromophilic substance (Nissl). Fig. 284. — From Transverse Section of Elephant's Cord. Neuroglia Stain, h, c, d and i, Four types of neuroglia cells; k, through several neuroglia cells; /, leucocyte. (Hardesty.) Benda's neuroglia fibre passing The smallest nerve cells of the cord have a limited amount of chromophilic sub- stance, usually either in the form of perinuclear caps or small bodies near the periphery of the cell. In the medium cells more chromophilic bodies are present. The cells of Clarke's column have a considerable number of chromophilic bodies arranged near the periphery of the cell. The root cells are richest in chromophilic suh)stance (for further details see Chap. VI). Blood-vessels (Fig. 285). — Study the arrangement and structure of the blood-vessels of the cord and pia. There are three principal longitudinal arter- ies, the anterior spinal artery given off from the vertebral arteries near their union into the basilar artery, and two posterior spinal arteries, given off from the inferior and superior cerebellar arteries. These arteries are reenforced by small arteries passing to the cord along the dorsal and ventral roots and form an arterial network in the pia mater. From the network terminal (i. c., non- ana.stomosing) branches enter the cord, supplying all parts except the ventral horn and column of Clarke. The latter are supplied by branches from the an- terior spinal artery which pass dorsally in the ventral sulcus (sulco-commissural arteries) and enter the cord alternately to right and left. They then break up THE NERVOUS SYSTEM. 40i into a rich capillary network in the ventral horn, supplying also a branch to the column of Clarke. The veins of the cord also form a plexus in the pia mater. Larger posterior median and anterior median veins can be distinguished. Por- tions of the above vessels can be seen, cut in various planes, in the pia and in the cord. The general appearance and structure of the blood-vessels, including capillaries, should be noted in the various methods of staining. Posterior spinal artery Region supplied by sulco-com- missural arterv Column cells Anterior spinal artery (giving off a sulco-commissural artery) Root cells Fig. 285. — Schematic transverse section of Cord, Showing General Distribution of Blood-vessels (left) and Nerve-cells (right) (Bing). Root-cells; i, postero-lateral group; 2, antero-lateral group; 3, antero-medial group; 4, central group; 5, postero- medial group. The broke i black lines on the surface of the cord are portions of the vascular network in the pia mater. Variations in Structure at Different Levels. While the general structure above described obtains throughout the cord, the size and shape of the cord, the size and shape of the gray matter, and the relative proportion of gray matter and while matter, vary in different parts of the cord, which must therefore be separately considered. These variations are due to: (i) \'ariations in the size of the nerves entering and leaving, which cause correspond- ing variations in the gray matter which receives the afferent fibres and contains the cells of origin of the efferent fibres. Thus the larger nerves of the extremities cause the increase in size of the gray matter of the cervical and lumbo-sacral cord with which they are con- nected, and also an increase in the dorsal funiculi. (2) A gradual increase in the white matter of the cord, as higher levels are reached, due to an increase in the number of long ascending and descending fibres to and from the brain. 408 THE ORG.AJSrS. PRACTICAL STUDY. Section through the Twelfth Thoracic Segment (Fig. 287). — Note that the cord is smaller than in the lumbar enlargement and somewhat iiattened dorso- ventrally; that the amount of gray matter and white matter is diminished; that both anterior and posterior horns are more slender, the anterior horn containing compara- tively few cells. At the inner side and base of the posterior horn may be seen the group of cells known as Clarke's column (p. 403). MeduUated fibres can be seen passing from the dorsal funiculus into Clarke's column, where they interlace among the nerve cells. These fibres are collaterals of the dorsal root fibres terminating in the nucleus. From the nucleus coarser fibres can be seen gathering at its ventral side and thence passing outward to the periphery of the cord where they bend upward forming the beginning of the dorsal spino-cerebellar tract (see p. 415). Section through the Mid-thoracic Region (Fig. 287). — Compare with the lumbar sections. Xote the change in shape and size; that the cord is more nearly round and smaller; that while the reduction in size affects both gray matter and white matter, it is the former that shows the greater decrease. The horns are even more slender than in the twelfth thoracic section, and the anterior horn contains still fewer cells. Clarke's column is present, but not so large. Section through the Cervical Enlargement (Fig. 286). — Note the marked increase in size of the cord, which affects both gray matter and white matter. De- pending upon the exact level at which the section is taken, the cord may be nearly round or flattened dorso-ventrally. The posterior horns remain slender while the anterior are much broader than the posterior horns. The reticitlar process is more prominent than in any of the previous sections. As in the lumbar cord, the cell groups of the anterior horn are numerous and well defined. A more or less definite septum divides the posterior column into an inner part, the column of GoU, and an outer part, the column of Burdach. For further variations and differences between the segments of the cord, compare Figs. 286, 287 and 288. Fibre Tracts of the Cord. The determination of the fibre tracts of the cord has been accom- plished principally by two methods: (i) The myelogenelic method, which is based upon the fact that the fibres of difi"erent systems acquire their myelin sheaths at different periods of embryonic developm,ent. Thus by examining cords from embryos of various ages and young specimens it is possible, using a myelin stain {e.g., Weigert), to dis- tinguish different tracts by the presence or absence of myelinization of their fibres. (2) The method of secondary or axonal degeneration, based ujjon the fact that a fibre separated from its cell undergoes degenerative changes and ultimately disappears and that the cell body also usually shows certain changes (see page 124). The fibres distal to the injury can be distinguished during active degeneration by THE NER\'OUS SYSTEM. 409 C.II C. Ill CIV f* ^%j*^'. C. V C. VI C. VII C. VIII Fig. 2S6. — Transverse Sections through the Cervical (II-\'III) Segments of the Cord. \\'eigcrt preparations. (Rauber-Kopsch.) 410 THE ORGANS. Th. I Th. II Th. IX Th.X Th. XI Th. XII Fig. 287.— Transverse Sections through the Thoracic (I-XII) Segments of the Cord. Weigcrt preparations. (Rauber-Kopsch.) THE XER\OUS SYSTEM. 411 ea»^ L. I L. II L. Ill L. IV S. I S. II S. Ill ^^f$St^ S. IV y ■ ^ ri-.a 5. V Fig. 288.— Transverse Sections through the Lumbar (I-\-) and Sacral (l-Y) Segments of the Cord. Weigert preparations. (Rauber-Kopsch.) 412 THE ORGANS. ^ s- • - *^ti 5J fli ti ^qcnaj ^^ H, a, 2 C ,, THE NERVOUS SYSTEM. 413 applying the Marchi stain (page 31). After their disappearance, however, a negative picture is obtained by staining the surrounding normal fibres (Weigert). The changes in the cell bodies whose axones are injured are distinguished by applying the Nissl stain (p. 35). Thus if the cord is cut at some particular level, at any level above the cut all fibres present which originate from cells below the cut will show degeneration ("ascending" degeneration), while the cell bodies of the cut fibres will show the axonal degeneration changes. On the other hand, at any level below the cut, fibres which originate from cells above the cut will show degeneration ("descending" degeneration), while the cell bodies of these fibres, located above the cut, will exhibit axonal degeneration. (3) .4//-o/>/?3' (von Gudden's method). This method is based upon the fact that extirpation of some part of the nervous system in a young animal is followed by an atrophy of parts in intimate relation therewith. This method only demonstrates grosser changes than the preceding, but on the other hand whole conduction paths involving more than one neurone relay may show changes. Other methods are the method of comparative anatomy, i.e., study of the simpler nervous systems of lower forms and the correlated development or absence of related parts of the nervous system, and the method of physiology, i.e., study of effects of stimulation or extirpation of various portions of the nervous system thereby indirectly demonstrating anatomical pathways. Ascending Tracts. A. Tracts forming parts of afferent pallial paths. I. Long Ascending Arms of Dorsal Root Fibres. (Posterior Funiculi). — The origin of these tracts — central processes of the cells of the spinal ganglia — has been described (page 390) . The distribution of the posterior root fibres to the gray matter of the cord was noted in connection with the study of the lumbar enlargement section (page 405). The general arrangement of these fibres in the dorsal funiculi remains to be noted. Fig. 289 — Diagram of the Tracts of the Cord (Cervical Region). Ascending tracts are shown on the left side, and descending tracts on the right. It will be noticed that the tracts of the cord are roughly divisible into three concentric zones: (i) A zone oc- cupying most of the posterior columns and the peripheral part of the lateral columns. This zone comprises the principal long ascending tracts (beginnings of afferent supra- segmental paths). (2) The second zone lies immediately within the tirst in the lateral columns and also occupies the peripheral part of the anterior columns. It comprises the principal long descending tracts from various parts of the brain (terminal portions of ef- ferent suprasegmental paths). (,s) The third zone borders the gray matter and includes the ground or fundamental bundles of the cord (chiefly spinal intersegmental fibres). In the fisrure, for «», Ddflzschni'ilschi read interstitial nucleus of Cajal. 4U THE ORGANS. Each successive dorsal root sends its fibres into the cord next to the dorsal horn and therefore to the outer side of those from the next root below. Thus the fibres of the lower roots as they ascend the cord are gradually pushed inward toward the median line until they finally occupy that part of the posterior column lying near the posterior septum. The separation of the posterior column by a connective-tissue septum into the column of Goll and the column of Burdach occurs only in the cervical cord (Figs. 282 and 286). Here the most median fibres, i.e.. those lying in the column of Goll, are the longest fibres of the pos- terior columns, having come from the lower spinal ganglia, while the column of Burdach (Fig. 282) consists of short and medium length fibres. The fibres of Goll's column end in the nucleus funicidi gracilis or nucleus of the column of Goll in the medulla (see p. 435 and Fig. 297). Those fibres of Burdach's which do not terminate in the spinal cord terminate in the medulla in the nucleus fujiiculi cuneati or nucleus of the column of Burdach (p. 435 and Fig. 297,). The nucleus gracilis and nucleus cuneatus — which will be seen in sections of the medulla (Fig. 297) — thus serve as terminal nuclei for the afferent root fibres in the columns of Goll and those of the column of Burdach which do not terminate within the cord. Inasmuch as many of the ascending arms are short, it is evident that only a fraction of the dorsal root fibres are represented at the higher levels. Those long arms which reach the medulla constitute the beginning of one of the principal afferent cerebral or pallial pathways. The axones of the neurones, whose bodies are the nuclei of Goll and Burdach, cross and form the tract known as the medial fillet (or lemniscus), composing the second system of this path. The fillet terminates in the thalamus and the path is completed by a third system of thalamo-cortical neurones to the cortex pallii. This path is, in brief, the long ascending arms of dorsal roots -f- fillet -f thalamo-cortical path, decussating in the medulla (Fig. 293; . II. Spino-thalamic Tract. — This arises from heteromeric cells lying probaljly principally in the dorsal horn (groups c and d, p. 403). Their axones cross in the ventral commissure and reach the opposite lateral funiculus where they ascend in a position mesial to the ventral spino-cerebellar tract (see below). This tract terminates in the thalamus whence the path is completed by thalamo-cortical neurones. The path is thus: spinal ganglif)n (short arms and collaterals of dorsal roots) + spinothalamic -F- thalamo-cortical neurones, three systems of neurones, its decussation takes place in the cord at about the level THE NER\'OUS SYSTEM. 415 of entry of the dorsal roots involved. Associated with this system may be some fibres to the tectum mesencephali (spino-tectal). (Figs. 289, 2go and 294). B. Tracts forming part of paths to the cerehellum. III. Dorsal Spino-cerebellar Tract {Tract of Flechsig. Direct or Uncrossed Cerebellar Tract). — This tract lies along the dorso-lateral periphery of the cord, being bounded internally by the crossed pyrami- dal tract (Fig. 282, and Fig. 289). The fibres of the direct cerebellar tract are the axones of the cells of Clarke's column (Figs. 289, 290 and 294). These axones cross the intervening gray matter and white matter of the same side (tautomeric column cells) and turn upward as the direct cerebellar tract. In the medulla they form part of the res- tiform body or inferior cerebellar peduncle and pass to the cere- bellum. Here they enter the gray matter of the vermis of the same or opposite side, ending in ramifica- tions among the nerve cells. Some fibres either end in, or send off collaterals to, the cerebellar nuclei. The tract first appears in the upper lumbar cord, and increases in size until the upper limit of Clarke's column has been reached (page 403 J- As already noted abo\'e, some fibres of the posterior root, or their collaterals, end in the column of Clarke. This path is composed then of two systems of neurones, spinal ganglion cells and Clarke's column cells, and is uncrossed (with the exception that some fibres are interrupted in the nucleus lateralis of the medulla) (Fig. 308). IV. Ventral Spino-cerebellar Tract.— This tract lies along the periphery of the cord, extending from the anterior limit of the direct Fig. 290. — Diagram showing Beginnings of Principal Long Ascending Tracts of Cord and Termination of Lateral Pyramidal Tract. Each group of neurones is repre- sented by one or two neurones, d.s-c, Dorsal spino-cerebellar tract; p, lateral pyramidal tract; s-t., spino-thalamic tract; v.s-c, ventral spino-cerebellar tract; v.r., ventral root. 416 THE ORGANS. cerebellar to about the exit of the ventral roots (Fig. 282 and Fig. 289). It is probably formed by axones whose cell bodies are scattered through the intermediate gray matter, possibly group a (p. 403 and Fig. 282). Some fibres come from tautomeric, others from heteromeric cells, the axones of the latter crossing in the ventral commissure. The tract first appears in the upper lumbar cord and naturally increases in size as it passes upward. The fibres of this tract also end in the vermis of the cerebellum. They reach their destination in the cerebellum by a different route, ascending considerably farther than the dorsal spino- cerebellar fibres and then turning back along the outer side of the superior cerebellar peduncle to the vermis. This path is thus also a two-neurone path, partly crossed and partly uncrossed. The ventral spino-cerebellar and spino-thalamic tracts are sometimes referred to as Gower's tract. (Figs. 294 and 308;. It seems probable that muscle-tendon sense passes up the cord by tract I (uncrossed in the cord), while pain and temperature pass up by the spino- thalamic tract (crossed). The path pursued by touch is more doubtful but it may pass up partly by tract I and partly by ascending arms of varying lengths which end in the cord around heteromeric column cells (partly uncrossed and partly crossed in the cord). This path may join the fillet in the medulla. It would seem probable that the cerebellar tracts convey sdmuli from muscle- tendon receptors. Descending Tracts. I, The Pyramidal Tracts {Tractus C or tico- spinalis, Cerehro- spinalis or Pallio-spinalis). — The cell bodies of the neurones whose axones make up this system are situated in the cerebral cortex anterior to the fissure of Rolando (precentral area, Fig. 327). Their axones converge and pass downward through the internal capsule, pes pedun- culi, pons, and medulla, sending off fibres to the efferent nuclei of the cranial nerves. In the medulla the tracts come to the surface as the anterior pyramids. At the junction of medulla and cord occurs what is known as the pyramidal decussation. Here most of the Irbres of each tract cross to the opposite dorso-lateral region of the cord and continue downward as the crossed pyramidal tract. This lies in the dorsal i>art of the lateral column (Figs. 282 and 289). It extends to the lowermost part of the cord. In the cervical and dorsal regions it is separated from the surface of the cord by the direct cerebellar tract. In the lumbar region the latter tract is no longer present and the crossed pyramidal comes to the surface. The minority of the THE NERVOUS SYSTEM. 417 fibres, instead of decussating, remain on the same side to pass down the cord along the anterior median lissure as the direct pyramidal trad, occupying a small oval area adjacent to the anterior sulcus (Fig. 289). It does not usually extend below the middle or lower dorsal region of the cord. As these tracts descend they decrease in size from loss of fibres which continually leave them to terminate in the ventral horns. The fibres of the crossed tract terminate mainly in the horn of the same side, while most of the fibres of the direct tract cross through the an- terior commissure to the opposite side of the cord. These tracts are thus mainly crossed tracts, as the great majority of their fibres cross to the opposite side of the cord. There are, however, some homo-lateral (uncrossed) fibres in the lateral pyramidal tract. The tracts are apt to differ in size on the two sides of the cord, owing to the fact that the proportion of fibres which decussate is not constant. The axones terminate in arborizations around the motor cells of the ventral horns. The pyramidal tracts or pallio-spinal system together with the spinal efferent peripheral neurone system constitutes the pallio-spino-peripheral efferent conduction path. According to some authorities the pyramidal fibres terminate around cells in the inter- mediate gray matter whose axones in turn terminate around the efferent root cells. (Figs. 293 and 294.) The pyramidal tracts convey to the cord the impulses which result in volun- tary movements, especially, probably, individual movements of parts of the limbs (foot, hand, finger, etc.). II. The tecto-spinal tract originates in the midbrain roof, decussates and descends to the cord where it lies near the ^■cntral sulcus. Its presence in the cord has been disputed. (Fig. 320. ) III. Tract from the interstitial nucleus of Cajal (located in the reticular formation of the tegmentum of the midbrain cephalad to the nucleus of the III nerve). '^ It is uncrossed and lies also near the ventral sulcus. Its fibres terminate in the ventral horn. Some fibres have been traced into the lumljar cord. (Figs. 289 and 320.) IV. The Rubro-spinal Tract {von Monakow's Tract). — This consists of axones of tlie red nucleus (nucleus rul)er) located in the tegmentum of the midbrain. These axones cross and descend to the cord, being joined l)y axones of the other cells in the reticular forma- ' The fibres in question have been variously stated to originate from the nucleus of Darkschewitsch, the nucleus of the posterior longitudinal fasciculus and the nucleus of the posterior commissure. Whether any of these nuclei is the same as the interstitial nucleus of Cajal, or the nucleus of van Gehuchten in fishes, is uncertain. 418 THE ORGANS. tion in the region of the pons. In the cord the tract lies mingled with and ventral to the lateral pyramidal tract. Its fibres terminate in the dorsal part of the ventral horn. This tract is a lower link in a three- neurone path from the cerebellum composed as follows: (a) Cells in cerebellar cortex to nucleus dentatus in cerebellum; (h) cells in dentatus via superior peduncle to the red nucleus; (c) rubro-spinal tract to {d) efferent peripheral neurones of cord. (Figs. 289, 294 and 308.) V. Tract from Deiters' Nucleus (a terminal nucleus of the vestib- ular nerve in the medulla) or vestihulo-spinal tract. — This tract occupies the ventral and mesial periphery of the cord. The more lateral fibres are uncrossed, those near the ventral sulcus come from the nuclei of both sides. Descending with these fibres are probably axones of other vestibular nuclei, and of other nuclei in the gray reticu- lar formation (reticulo-spinal fibres) of the medulla. These fibres all terminate in the ventral horn. Some fibres have been traced to the sacral cord. Deiters' nucleus receives fibres from the cerebellum, and thus this tract is a segment of a second efferent cerebellar pathway: {a) Cells in cerebellar cortex to nucleus fastigii in cerebellum; {h) cells of nucleus fastigii to Deiters' nucleus; {c) cells of Deiters' nucleus to (d) efferent peripheral neurones of cord. According to some authorities some fibres proceed from cerebellum to cord without interruption in Deiters' nucleus. (Figs. 289 and 294.) All the fibres of V are som.etimes collectively called the antero-lateral descending tract or marginal bundle of Lowenthal. Tract III and the mesial part of V constitute the major portion of the descending fibres of an important bundle in the segmental brain known as the medial longitudinal fasciculus . VI. Fasciculus of Thomas. — Besides the reticulo-spinal fibres already mentioned are fibres in the lateral column which originate in the reticular formation of the medulla and terminate in the gray of the cer\ical cord. These are known as the tract of Thomas. (Fig. 289.) VII. Helweg's tract is a small triangular bundle of fibres lying along the ventro-lateral margin of the cord, and is traceable upward as far as the olives (Fig. 289). The origin and destination of its fibres are not definitely known. VIII. Septo-marginal Tract. — This is a small bundle of fibres lying next the ]josterior septum. It appears to change its location in different levels, e.g., in the sacral cord it occupies a small dorso-median triangle, in the lumbar region it forms an oval bundle (of Flechsig) at the middle of the septum and a superficial bundle, in the thoracic and THE NERVOUS SYSTEM. 419 cervical cord its fibres are more scattered. It is probably composed of descending axones of cells in the cord. (Fig. 289). IX. The so-called "comma" tract of Schultze is a small comma- shaped bundle of descending fibres lying about the middle of the posterior column (Fig. 289). It is most prominent in the dorsal cord. Its fibres are believed by some to be descending branches of spinal ganglion cells, by others to be descending axones from cells situated in the gray matter of the cord (column cells). In general the descending tracts fall into two categories : (i) descend- ing suprasegmental tracts which originate in the cortex of the pallium (Tract I), or of the midbrain (Tract II), and (2) descending inter- segmental tracts. Of the latter some originate in nuclei lying in the segmental brain (interstitial nucleus of Cajal, nucleus ruber, nucleus of Deiters) which receive efferent suprasegmental fibres, and thus form links of descending suprasegmental paths. Other reticulo-spinal fibres come from other cells in the gray reticular formation. Other still shorter tracts (spino-spinal), from cells in the cord, form the de- scending fibres in the ground bundles (see below). Tracts \TII and possibly IX, are in this category. Many tracts contain fibres proceeding in a direction opposite to that of most of the fibres. Fundamental Columns or Ground Bundles. The ascending and descending tracts above described are known as the long-fibre tracts of the cord. If the area which these tracts occupy be subtracted from the total area of white matter, it is seen that a considerable area still remains unaccounted for. This area is especially large in the antero-lateral region, and extends up along the lateral side of the posterior horn between the latter and the crossed pyramidal tract (Figs. 282 and 289). A small area in the posterior column just dorsal to the posterior commissure, and extending up a short distance along the medial aspect of the horn, should also be included. These areas are occupied by the fundamental columns or short-fibre (spino-spinal or proprio-spinal) systems of the cord. The fibres serve as longitudinal commissural fibres to bring the dift"erent segments of the cord into communication (Fig. 292). The shorter fibres lie nearest the gray matter and link together adjacent segments. The longer fibres lie farther from the gray matter and continue through several segments. The origin of these fibres as axones of cells of the gray matter, and the manner in which they re-enter the gray matter as terminals and collaterals have been considered (pp. 39S and 399). 420 THE ORGANS. The fact, alluded to above, that the shorter fibres lie nearest, or mingled with the gray is, in a general way, true throughout the central nervous system. A result of this in the cord is the superficial position of many of the longest tracts. Attention has already been called to the concept of neural arcs which may traverse the cord or both cord and suprasegmental structures (page 378). It must be kept in mind that there are probably no isolated neural arcs and that every neural reaction involving any given arc always influences and is in- fluenced by other parts of the nervous system. Receptor Effector Fig. 291 — -Diagram illustrating a Two-neurone Spinal Reflex .\rc. Groups of neu- rones are represented by one neurone, gg, Spinal ganglion. (Van Gehuchten). From the neurones thus far studied and the tracts which their axones form, the following neural arcs may be constructed: (i) A Two-neurone Spinal Reflex Arc (Fig. 291). — {a) Peripheral afferent neurones; their peripheral processes and receptor, the spinal ganglion cells, their central processes with collaterals terminating around motor cells of anterior horn; (/;) peripheral efferent neurones, i.e., motor cells of anterior horn with axones passing to effector. Such a two-neurone reflex arc is chiefly uncrossed and in most cases in- volves only one segment or closely adjacent segments. As it involves only one synapsis (see chapter VT) fin the ventral gray) it is some- times termed a monosynaptic arc. THE XERVOUS SYSTEM. 421 (2) .4 Three-neurone Spinal Reflex Arc (Fig. 292). — (a) Peripheral afferent neurmies as in (i), but terminating around column cells of the cord. 1/)) Cord neurones (column cells) — axones in the fundamental columns with collaterals and terminals to anterior horn cells of different levels. \c) Peripheral efferent neurones as in the two-neurone reflex. Such a three-neurone or disynaptic reflex arc may involve segments above or below the segment of entrance of the stimulus and is uncrossed or crossed according as the cord neurones are tautomeric or heteromeric. The independence of the cord as a reflex mechanism is much diminished in man. (3) -4 cerebellar Arc may be constituted as follows: [a) Peripheral afferent neurones to {h) column cells in cord {e.g., Clarke's column) via spino-cerebellar tracts to cere- y. bellar cortex; (c) various associative cortical neurones; [d) axones of cortical cells to {e) dentate nucleus the axones of which (superior peduncle) pass to (/) nucleus ruber via rubro-spinal tract to [g) efferent p; peripheral neurones in cord. Another arc would consist of {a), {h) and (c) the same, {d) cerebellar cortex to nucleus fastigii in cerebellum to {e) nucleus of Deiters to (/) efferent peripheral neurones to effector (Figs. 294 and 308). (4) -4 Cerebral or Pallial Arc: {a) Peripheral afferent neurones, via long ascending arms to (b) nucleus gracilis or cuneatus, thence as medial lemniscus to (c) thalamus to cortex pallii; {d) associative neurones of cortex; (e) cortical precentral neurones via pyramid to (/) efferent peripheral neurones to effector. Another arc would invoh-e the spino-thalamic tract instead of the lemniscus. (Figs. 293 and 20^4.) Similar arcs may include eft'erent sympathetic neurones. TECHNIC. fi) Carefully remove the cord (human if possible; if not, that of a large dog) with its membranes, cut into two or three pieces if necessary, and lay on sheet cork. Slit the dura along one side of the cord, lay the folds back, and pin the 292. — Diagram illustrating Three-neurone Spinal Refle.x .Arcs of one segment and more than one segment. Groups of neurones are represented by one neurone, g. Spinal gang- lion cells; h.c.c, heteromeric column cell: t.c.c, tautomeric column cell; x'.r., ventral root. 422 THE ORGANS. dura to the cork. Care must be taken to leave the dura very loose, otherwise it will flatten the cord as it shrinks in hardening. With a sharp razor now cut the cord, but not the dura, into segments about i cm. thick. Fix in Orth's fluid (p. 7). Pieces of the cord may be cut out as wanted and em- bedded in celloidin. Sec- tions should be cut about 15K in thickness. (2) For the study of the general internal structure of the cord, stain a section through the lumbar enlarge- ment of a cord prepared according to the preceding technic (i) in haematoxylin- picro-acid-fuchsin (technic 3, p. 19) and another section through the same level in Weigert's haematoxylin (technic p. 29). Mount both in balsam. For Weigert staining, material fixed in formalin or in Orth's fluid should be further hardened in Miiller's fluid for at least a month, changing the fluid frequently at first to remove the formalin. Mallory's glia stain should also be used with material fixed in Zenker's fluid (technic, p. 27). The silver method of Cajal (alcohol-fixation) should also be used (technic, p. 34) and that of Nissl. (3) From a cord prepared according to technic i, re- move small segments from each of the following levels: (i) the twelfth dorsal, (2) the mid-dorsal, and (3) the cervical enlargement. The segments are embedded in celloidin, sections cut 15 to 20/'. thick, stained by Weigert's method (page 29), and mounted in Fig. 293- THE XER\-OUS SYSTEM. 423 balsam. Medullated sheaths alone are stained by this method and appear dark blue or black. (4) A human cord from a case in which death has occurred some time after fracture of the vertebrae with resulting crushing of the cord, furnishes valuable but of course rarely available material. If death occur within a few weeks after the injury, the method of Marchi should be used; if after several weeks, the method of Weigert (page 29). The picture in the cord is dependent upon the fact that axones cut off from their cells of origin degenerate and disappear. After a complete transverse lesion of the cord, therefore, all ascending tracts are found degenerated above the lesion, all descending tracts below the lesion. The method of ^larchi gives a positive picture of osmic-acid-stained degenerated myelin in the affected tracts. The method of Weigert gives a negative picture, the neuroglia tissue which has replaced the degenerated tracts being unstained in contrast with the normal tracts, the myelin sheaths of whose fibers stain, as usual, dark blue or black. (5) Human cords from cases which have lived some time after the destruction of the motor cortex, or after interruption of the motor tract in any part of its course, may also be used for studying the descending fibre tracts. (6) The cord of an animal may be cut or crushed, the animal kept alive for from two weeks to several months, and the cord then treated as in technic (4). The most satisfactory animal material may be obtained from a large dog by cutting the cord half-way across, the danger of too early death from shock or complica- tions being much less than after complete section. Fig. 293. — Diagram showing the Most Important Direct Paths which an Impulse follows in passing from a Receptor (5) to the Cerebral Cortex and from the latter back to an Effector (M) {e.g., muscle), also some of the cranial-nerve connections with the cerebral cortex. Groups of neurones are represented by one or several individual neurones. A, Sensory cortex: B, motor cortex; C, level of third nerve nucleus: D. level of sixth and seventh nerve nuclei; E, level of sensory decussation: F, level of pyramidal decussation; 6', spinal cord. From Periphery to Cortex. jVeurone No. i. — The Peripheral afferent Neurone: i, Spinal, cell bodies in spinal ganglia: receptor, S, peripheral arm of spinal ganglion cell: central arm of spinal ganglion cell as fibre of dorsal root to column of GoU or of Burdach, thence to nucleus of one of these columns in the medulla. Tj, Cranial (example, fifth cranial nerve, trigeminus: cell bodies in Gasserian ganglion): receptor: peripheral arm of Gasserian ganglion cell: central arm of Gasserian ganglion cell to medulla as aff'erent root of fifth nerve, thence to terminal nuclei in medulla. Neurone No. 2. — 2, Spinal connection — cell body in nucleus of Goll or of Burdach : axone passing as fibre of fillet to thalamus. ]'.,, Cranial nerve connection (trigeminal), cell body in one of trigeminal nuclei in medulla, axone as fibre of secondary trigeminal tract to thalamus. Neurone No. 3. — 3, Cell body in thalamus, a.xone passing through internal capsule to termination in cortex. (Various association neurones in cortex omitted.) From Cortex to Periphery. Neurone No. 4. — 4, Cell body in motor cerebral cortex: axone through internal capsule and pes to (a) motor nuclei of cranial nerves, (b) by means of pyramidal tracts to ventral gray of spinal cord. Neurone No. 5. — 5, Spinal, cell body in ventral gray of cord: axone as motor fibre of ventral root through mi.xed spinal nerve to effector (muscle). Neurone No. 5. — Cranial — 1'-, Cell body in motor nucleus of trigeminus: axone passing to muscle as motor fibre of fifth nerve. III3, Peripheral efferent neurone of third nerve — oculomotor. 1'/-, Peripheral efferent neurone of sixth nerve — abducens. 1'//-, Peripheral efferent neurone of seventh nerve — facial. NII^, Peripheral efferent neurone of twelfth nerve — hypoglossal. 424 THE ORGANS. (7) The cord of a human foetus from the sixth month to term furnishes good material for the study of the anterior and posterior root fibres, the plexus of fine fibres in the gray matter, the groupings of the anterior horn cells, etc. The pyram- idal tracts are at this age non-meduUated and are consequently unstained in Wei- gert preparations. The Weigert-Pal method gives the best results. (8) For the study of the course of the posterior root fibres within the cord, cut anv desired number of posterior roots between the ganglia and the cord and treat material by the Marchi or the Weigert method, according to the time elapsed between the operation and the death of the animal. BRAIN. General Structure. The principal peculiarities of the brain as distinguished from the cord depend upon two factors: certain peculiarities of the receptors and effectors of the head and the development of higher coordinating apparatus in the central nervous system of the head. Besides the receptors of the general senses (p. 390), there are in the head the highly specialized receptors of smell, sight, hearing and position (semi-circular canals), which are respectively concentrated into certain localities and form, together with certain accessory structures, the organs known as the nose, eye and ear. Each of these groups of receptors has its own special connection with the brain (nerves I, II and VIII) and its own paths within the latter (see below). The special receptors of taste show a less degree of aggregation into an organ and, together with other visceral receptors, are innervated by afferent portions of a group of nerves (VII, IX and X) which have a common continuation within the brain. The remaining somatic receptors of the general senses, scattered over the anterior part of the head, are innervated by one nerve (V) having its own central continuations. The striated voluntary muscles of the head fall into four groups; those of the eye, of the mouth, of the face, pharynx and larynx (modified branchial musculature) and of the tongue. The oral and branchial muscula- tures are usually regarded as visceral and those of the eye and tongue as somatic, a distinction shown by differences of grouping in the brain of their efferent peripheral neurones. The nose and ear have practic- ally no N'oluntary motor apparatus. The higher coordinating apparatus or suprasegmcntal structures (p- 377J ^-"f ^^^ brain arc essentially expansions of the dorsal walls of parts of the brain having manifold connections with the rest of the nervous system which are complexly interrelated by enormous num- THE NERVOUS SYSTEM. 425 bers of association neurones. The presence of these latter has prob- ably necessitated the extensive layers of externally placed neurone bodies (cortex) characteristic of suprasegmental structures. The structure of the basal part of the brain, connected with the cranial nerves (segmental brain, p. 377), is affected by both the pecu- liarities of peripheral structures mentioned above and by the presence of bundles of fibres and masses of gray forming portions of paths to and from suprasegmental structures (see below). The following summary embodies the resulting general structural features of the brain: Segmental Brain and Nerves. Afferent Peripheral (Segmental) Neurones. — (i) Visceral group comprising nerves VII (geniculate ganglion), IX (superior and petrosal ganglia) and X (jugular and nodose ganglia). The nerves of taste probably belong entirely to this group. The peripheral arms of these ganglia innervate visceral receptors and the central arms form a descending tract in the medulla, the fasciculus solitarius which has its terminal nucleus giving rise to secondary tracts. (2) General Somatic Group (common sensibility). — Semilunar gang- lion of \ . Peripheral arms to skin of anterior part of head, to mouth and probably to muscle tendon receptors. Central arms form de- scending tract in medulla, the radix spinalis \\ Terminal nucleus the continuation in medulla of dorsal horn. Secondary tract to thalamus and thence to cortex cerebri. (3) Semicircular Canal Group. — Ganglion of Scarpa of \'III. Peripheral arms to semicircular canals. Central arms constituting vestibular portion of VIII, forming descending tracts in medulla and terminating in several vestibular terminal nuclei (including Deiters). (4) Acoustic Group. — Ganglion spirale of VIII. Peripheral arms to cochlea. Central arms forming cochlear part of \'III and termi- nating in medulla in various nuclei with secondary tract (lateral lillet) to midbrain. Thence by third system of neurones to medial geniculate body and by fourth system of neurones to cortex cerebri. (5) Visual Group. — Ganglion in retina. Second neurone system beginning in retina and forming the optic tract (optic "nerve") to lateral geniculate body and by third neurone system to cortex cerebri. (6) Olfactory Group. — "Ganglion" cells in olfactory mucous membrane forming olfactorv nerve (fila olfactoria). Secondarv tracts 426 THE ORGANS. from olfactory bulb (and tertiary tracts) to parts of diencephalon and hippocampus. Efferent Peripheral (Segmental) Neurones. — (i) Visceral. — (a) Bodies more laterally placed in gray matter of hindbrain. Axones to striated voluntary muscles of jaw (V), face (VII), pharynx and larynx (IX and X). {b) Bodies more deeply placed in gray of mid- and hindbrain. Axones to glands, smooth and heart muscle, either directly or via sympathetic ganglia of head and body. (2) Somatic. — Bodies located near median line in gray matter of hindbrain (XII and VI), and midbrain (IV and III), to muscles of tongue (XII) and eye (VI, IV and III). Nerve III also contains neurones whose axones pass to sympathetic ganglia (ciliary). (3) Sympathetic. — Bodies compose the sympathetic ganglia of the head (ciliary, sphenopalatine, submaxillary and otic). These ganglia are connected with afferent and efferent portions of cranial nerves (III, V, VII, IX and X), in a manner similar to the connections between sympathetic ganglia of body and spinal nerves. They are also connected with sympathetic fibres from the body. Intersegmental Neurones. — These are represented principally by the gray reticular formation of the hindbrain and midbrain and long descending tracts external to it. The gray reticular formation is composed of neurone bodies and short intersegmental tracts inter- mingled. Among the former are certain well differentiated nuclei {e.g., nucleus ruber, nucleus of Deiters and interstitial nucleus of Cajal) the axones of which form long, principally descending, intersegmental tracts external to the gray reticular formation. Other cells in the gray reticular formation form the shorter tracts within it. The reticular formation may also contain the motor nuclei of the cranial nerves and is traversed by various fibers passing to tracts and by terminals from tracts. Afferent and Efferent Suprasegmental Patlis. — The afferent paths consist of the ascending tracts and nuclei already mentioned (spino-cerebellar, nuclei gracilis and cuneatus and medial lemniscus, terminal nuclei of the afferent cranial nerves and their secondary tracts and further continuations). The tracts occujjy, in general, positions external to the reticular formation and long intersegmental tracts. The efferent suprasegmental paths consist either of descending suprasegmental tracts which proceed without interruption to the efferent ]jeri]jhcral neurones, or of descending suprasegmental and short or long intersegmental tracts in whose nuclei the suprasegmental THE NERVOUS SYSTEM. 427 tracts terminate and which in turn terminate around the efferent peripheral neurones. The large descending tracts from the pallium markedly affect the configuration of the brain. There are two such principal descending pallial paths; one to the nuclei of the pons VaroHi and thence across to the opposite cerebellar hemisphere and one continuing down to the medulla and cord as the pyramids. These two paths are added ventrally to the segmental and intersegmenta- apparatus and form the pes pedunculi (added ventrally to the tegmenl tum of the midbrain), the pons Varolii (ventral to the tegmentum of part of the midbrain, to the isthmus and part of the hindbrain) and the pyramids (ventral to the hindbrain). SUPRASEGMENTAL STRUCTURES. These are the pallium or cerebral hemispheres, the corpora quad- rigemina and the cerebellum. They consist essentially of the end- ings and beginnings of their respective afferent and efferent paths and of their own association neurones, the bodies of which lie in their respective cortices. The corpora quadrigemina are relatively of much less importance in the human brain. In accordance with the above there are usually to be distinguished in transverse sections of the brain at various levels the following: A. Peripheral {segmental) neurones, (i) Efferent ("motor" nuclei and root fibres). (2) Central continuations of afferent neurones (afferent roots), {a) Those entering at and therefore belonging to the segment involved. {b) Those entering above or below the segment and represented in the segment by descending or ascending (overlapping) tracts. B. Terminal nuclei of (2) and the secondary tracts originating from them. These may fall under category C or D (below). C. Intersegmental nuclei and tracts oi the segmental brain, consist- ing principally of the gray reticular formation and long descending tracts (arrangement much modified in forebrain). D. Nuclei and tracts forming a/fcrcnt and efferent suprascgmental paths. E. Suprascgmental structures (not present in man}' transections). The aft'erent paths include some of the nuclei and tracts under B, and their continuations, and the efferent include some of the longer systems under C, together with cft"crcnt su]irasegmcntal tracts to them. 428 THE ORGANS. ^ S_P_H SP. c;anc, cell Fig. 294. THE NERVOUS SYSTEM. 42!) The general histology of the brain is similar to that of the cord. The largest nerve-cells (large cells of efferent or motor nuclei, cells in motor cerebral cortex and certain cells in the reticular formation) usually present a chromophilic substance similar in arrangement, etc.. to the efferent nerve cells of the cord. The Purkinje cells, however, differ from these (see cerebellum). The chromophilic substance of medium and small cells presents an appearance in general similar to corresponding cells of the cord. Many minor differences may, however, obtain between various neurone groups. The neuroglia cells and fibres also present the same general characteristics as those in the cord, with variations peculiar to certain localities (e.g., parts of the cerebellum). Hindbrain or Rhombencephalon. This includes the medulla, cerebellum, and part of the tegm.entum and pons. Its peripheral nerves are the \\ \ I, \TI, \ III, IX, X. and XII. ' The medulla oblongata or bulb is the continuation upward of the spinal cord and extends from the lower limit of the pyramidal decussa- tion below to the lower margin of the pons above." Fig. 294. — Principal afferent and efferent suprasegmental pathways (excepting the rhinopalHal connections, the eilerent connections of the midbrain roof and the olivo- cerebellar connections) . Efferent peripheral neurones of cranial nerves are omitted. Each neurone group (nucleus and fasciculus) is indicated by one or several individual neurones. Decussations of tracts are indicated by an X. ac, Acoustic radiation, from medial genicu- late body to temporal lobe; br. conj, brachium conjunctivum (superior cerebellar peduncle) : brachium pontis, from pons to cerebellum (not labeled); h.q.i, brachium quadrigeminum inferius; c.g.l, lateral or external geniculate body; c.g.m, medial or internal geniculate body; c.qiiad, corpora quadrigemina; f.cort.-sp, cortico-spinal fasciculus (pyramidal tract): /. c.-p.f, frontal cortico-pontile fasciculus (from frontal lobe); f.c.-p.t, temporal cortico- pontile fasciculus (from temporal lobe) ; f.c.-p.o, occipital cortico-pontile fasciculus (from occipital \ohe): f.c It )i, fasciculus cuneatus (column of Burdach); t.f.-b. fastigio-bulbar tract ;/.^rac, fasciculus gracilis (column of GoW) ; f.s.-l, spino-thalamic fasciculus; f.sp.-c.d. dorsal spino-cerebellar fasciculus (tract of Flechsig); f.sp.-c.v, ventral spino-cerebellar fasciculus; /e;«. /a/, lateral lemniscus or lateral fillet; lem. nied, medial lemniscus or tillet; u.coch, cochlear nerve; ii.cun, (terminal) nucleus of the column of Burdach; n.d, nucleus of Deiters; n.dent, nucleus dentatus; ;/._?rof, nucleus of the column of Goll; n.opt, optic nerve; ;;.r, nucleus ruber; n.l. nucleus tecti (or iastign); u.trig, trigeminal nerve; Ji.vest. vestibular nerve; pes. ped, pes pedunculi (crusta) ; pulv. thai, pulvinar thalami; pyr. pyramid ; rad. ant, ventral spinal root; rad. post, dorsal spinal root; rad. opt, optic radiation (from lateral geniculate body to calcarine region); sodices, bundles from thalamus to postcentral region of neopallium; sp. gang, spinal ganglion; t.f.-b. ^ tractus fastigio-bulbaris; thai. thalamus; t.n.d, tract from the nucleus of Deiters; /. rub.-sp, rubro-spinal tract (von Alonakow). (Lateral view of brain.) ' It is better probably to reckon the so-called medullary part of the XI with the X. -It would be better to include in the term medulla oblongata what here falls under pontile tegmentum of the hindbrain. 430 THE ORGANS. Externally, the medulla shows the continuation upward of the anterior fissure and posterior septum of the cord. On either side of the anterior fissure is a prominence caused by the anterior pyramid, and to the outer side of the pyramid the bulging of the olivary body may be seen. The antero-lateral surface of the medulla is also marked by the exit of the fifth to the twelfth (inclusive) cranial nerves. The posterior surface shows two prominences on either side. The more median of these, known as the clava, is caused by the nucleus Corp. mamillaria - Corp. pineale Colliculus sup. -■- Colliculus inf Eminentia med. r Olive (in A) I. Area acustica (in B) Eminentia med. Ala cinerea Clava Dec. pyramids 1 Tub. cuneatum / Tub. cinereum Fig. 297 Fig. 296 Fig. 295. — Ventral (A) and dorsal (B) views of part of Brain Stem (cerebellum removed). Structures named at the left are indicated by their reference lines running to an X. On the right are named the figures showing transverse sections through the brain at the levels indicated by the reference lines. The level for Fig. 31Q is not accurately indicated. gracilis, or nucleus of the column of Goll; the other, lying just to the outer side of the clava, is due to the nucleus cuneatus or nucleus (jf the column of Burdach. Lateral to this is a third eminence, the tuberculum cinereum, due in part to the descending root of the V, merging anteriorly with the eminence of the restiform body. The cen- tral canal of the cord continues into the medulla, where it gradually approaches the dorsal surface and opens into the cavity of the fourth ventricle. The floor of the fourth ventricle exhibits a medial eminence occupied caudally by the nucleus hypoglossi. Lateral to this is a tri- angular area, the ala cinerea, surrounded by furrows. This is partly occupied by nuclei of the vagus. Forward and laterally a broader THE NERVOUS SYSTEM. 431 triangular area with an angle directed into the lateral recess marks the area occupied by the nuclei of the acoustic nerve. Still further forward near the median line are eminences indicating the positions of the nucleus abducentis and genu facialis. The roof of the fourth ven- tricle is formed by the thin plexus chorioideus and the cerebellum. (Fig. 295.) The pons is a mass of fibres and gray matter extending across the ventral surface of portions of mid- and hindbrain. The term is often used to include the whole of the basal part of the brain thus covered by the pons. It is better, however, to restrict it to the pons itself. The part of the hindbrain dorsal to the pons, which is the continuation forward of the medulla, may be included in the term tegmentum of the hindbrain. The cerebellum is described on p. 455. TECHNIC. The technic of the medulla (and the rest of the segmental brain) is the same as that of the cord (page 421). Transverse sections should be cut through the following typical levels, stained by Weigert's method (page 29), and mounted in balsam: 1. Through the pyramidal decussation. 2. Through the sens-ory decussation. 3. Through the lower part of the olivary nucleus. 4. Through the middle of the olivary nucleus. 5. Through the entrance of the cochlear nerve. 6. Through the entrance of the vestibular nerve. 7. Through the roots of the sixth and seventh cranial nerves. 8. Through the root of the fifth cranial nerve. The methods of Nissl, Cajal and glia stains should also be used when practicable. PRACTICAL STUDY. I. Transverse Section of the Medulla through the Decussation of the Pyram- idal Tracts (Motor Decussation) (Figs. 295 and 296). The most conspicuous features of this section are the decussation of the pvra- mids, the larger size of the dorsal horn and the beginning of the gray reticular formation. Surrounding the central canal is the central gray. Efferent Peripheral Neurones,— Nuclei of first cervical spinal nerve in ventral gray, and root fibres passing out to emerge on ventral aspect. Xuclei of XI, in mesial position in central gray, or in ventral gray. A.xones pass out laterally from latter and emerge on the lateral surface. The mesial or deep nuclei are best reck(Micd with nerve X. 432 THE ORGANS. THE XER\'OUS SYSTEM. 433 Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Some afferent fibres of the first cervical spinal nerve are still entering at this level. Ascending afferent roots: The dorsal funiculus comprising the fasciculus cuneatus and fasciculus gracilis remain as in the cord. Collaterals and terminals from them can be seen entering the subjacent gray. Descending afferent roots: Some of the fine fibres between the enlarged dorsal horn and the periphery, occupying the position of the zone of Lissauer in the cord, are descending afferent root fibers of the V-cranial nerve or tractus spinalis trigemini {spinal V). Collaterals and terminals from these fibres terminate in the gelatinous substance of Rolando and also traverse it to form a plexus of meduUated fibres in its inner side very similar to the cord. The axones of the dorsal horn cells (or terminal nucleus of the V) form the secondary tracts of the V, which cannot be distinguished (p. 452 and Fig. 307). Secondary tracts, forming parts of afferent suprasegmental paths: These form a mass of fibres along the lateral periphery of the medulla which consists of (a) the dorsal spino-cerebellar, (b) the ventral spino-cerebellar and (c) the spino-thalamic tracts. Intersegmental Neurones. — The neurone bodies are, as in the cord, scattered throughout the gray. The continuation of the ventro-lateral intersegmental tracts of the cord (and the tecto-spinal tract) is the U-shaped mass of fibres around the ventral gray. This mass consists of the long descending, the shorter ascending and descending intersegmental tracts, and the tecto-spinal; i.e., (a) rubro-spinal (in lateral arm of U), (b) vestibulo-spinal (lateral and mesial), (c) tract from interstitial nucleus of Cajal (mesial), (d) tecto-spinal (mesial), (e) shorter descending and ascending tracts which may be regarded as the equivalent of the ground bundles of the cord comprising shorter reticulo-spinal and spino-reticular fibres. The shortest of these fibres, which in the cord were next the lateral gray, are now mingled with the gray, the combination constituting the gray reticular formation. Other short intersegmental tracts lie in and adjoining the dorsal horn, as in the cord. Descending suprasegmental paths include certain of the long descending inter- segmental tracts as previously explained. Besides these there are efferent supraseg- mental neurones known as the pallio-spinal or pyramidal tracts and the tecto-spinal tracts. Bundles of fibres are seen crossing {pyramidal decussation) from the an- terior pyramid of one side to the opposite dorso-lateral column, where they turn downward as the crossed pyramidal tract. In their passage through the gray matter, they cut off the ventral horn from the rest of the gray matter. These fibres, as already noted in the cord, are descending axones from motor cells situated in the precentral cerebral cortex. In the pyramidal decussation most of these fibres cross to the opposite dorso-lateral region to pass down the cord as the crossed pyramidal tract (p. 416, and Fig. 289; Fig. 293, F). A few remain in their original anterior position to continue down the cord as the direct pyramidal tract (p. 417, and Fig. 289; Fig. 293, F). A few pass to the ventral tract in the same side, thus being uncrossed fibres in the lateral tract. The bundles of fibres do not cross in a trans- verse plane, but take a downward direction at the same time. For this reason trans- verse sections show these fibres cut rather obliquely. Because of the fact that the fibres cross in alternate bundles, the number of decussating fibres seen in any one section is greater on one side than on the other (Fig. 295). 28 434 THE ORGANS. THE NERVOUS SYSTEM. 435 2. Transverse Section of the Medulla through the Decussation of the Fillet or Lemniscus (Sensory Decussation; (Figs. 295 and 297). The most conspicuous features are the appearance of the nuclei cuneatus and gracilis, the decussation and formation of the medial lemniscus or iillet, and the increase of the gray reticular formation. Peripheral Efferent Neurones. — In the lateral part of the central gray is the dorsal nucleus of the X {nucleus alee cinerece). In the ventral part of the central gray is the nucleus hypoglossi and, passing ventrally and emerging lateral to the pyramids, may be seen the axones of its cells — the root fibres of the XII. In the nucleus XII can be distinguished (Weigert stain) coarse fibres which are the root fibres, and fine fibres which are terminals of other fibres ending in the nucleus. Among these have been distinguished collaterals from secondary vago- glossopharyngeal and trigeminal tracts (three-neurone reflex). Whether pyram- idal fibres reach the nucleus directly or z'ia intercalated neurones is uncertain. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Entering afferent root fibres are usually not present. The funiculi or fasciculi cuneatus and gracilis have diminished, and internal to them have appeared large masses of gray. These are the nuclei of the columns, and are known, respectively, as the nucleus of the column of Goll or the nucleus gracilis, and the nucleus of the column of Burdach or the nucleus cuneatus. In the higher sensory decussation levels there is usually an accessory cuneate nucleus. These nuclei serve as nuclei of termination for the fibres of the posterior funiculi. Their termination in these nuclei is the ending of that system of fibres which has been traced upward from their origin in the cells of the spinal ganglia; the comple- tion of the course of the spinal peripheral afferent neurones. As the fibres of the posterior columns are constantly terminating in these nuclei, there is, in passing from below upward, a constant increase in the size of the nuclei and a correspond- ing decrease in the size of the posterior columns, until, just below the olive, the whole of the column of Goll and most of the column of Burdach are replaced by their respective nuclei. (Pp. 413, 414.) Study the plexus of fine fibres in these nuclei, formed by the terminals of the column fibres, also the coarser fibres (axones of the cells of the nuclei) gathered in the ventral part of the nuclei, whence they emerge and curve' around the central gray, cross to the opposite side ventral to it and dorsal to the pyramids, and then turn brainward forming the bundle of fibres known as the medial lemniscus or fillet. The spinal V has increased and also its terminal nucleus, the dorsal horn. The spino-cerebellar and spino-thalamic tracts occupy about the same positions. In the central gray dorsal to the central canal is a nucleus representing a union of the caudal ends of the terminal nuclei of the fasciculi solitarii (see next section) — the nucleus commissuralis. Fibres of the fasciculi solitarii also de- cussate here. Intersegmental Neurones. — The rubro-spinal tract, tracts from Deiters' nucleus and interstitial nucleus of Cajal occupy about the same positions. The 'Fibres having a transverse curved or arched course are in general termed arcuate fibres. If they are deeply located, they are inlernal arcuate fibres, if near the per- iphery, they are superficial or external arcuate fibres. Obviously the same fibre may be, in different parts of its course, internal arcuate, e.xternal arcuate, and longitudinal. 436 THE ORGANS. oi > o o u O a; x; THE NERVOUS SYSTEM. 437 reticular formation has increased, the whole of the ventral horn and intermediate gray containing bundles of longitudinal fibres. The formation is also traversed by transverse fibres, representing the beginnings or terminations of various longi- tudinal fibres. Efferent Suprasegmental Neurones. — The decussation of the pyramids has now nearly or entirely ceased. The lateral pyramidal tracts are no longer in the lateral columns but are part of the anterior pyramidal tract which forms two large masses of fibres on each side of the ventral sulcus. The tecto-spinal tract occupies the same position. 3. Transverse Section of the Medulla through the Lower Part of the Inferior Olivary Nucleus (Figs. 295 and 298). The central canal has opened into the fourth ventricle, the central gray (includ- ing the central gelatinous substance) now being spread out on its floor. The roof of the ventricle is formed by its chorioid plexus. The most con- spicuous new feature is the olive. Efferent Peripheral Neurones. — The nucleus of the XII is large and occupies a swelling in the floor of the ventricle each side of the median line, known as the "emi- nentia" hypoglossi. The root fibres of the XII pass lateral to the medial lemniscus, between the olive and pyramid, and then emerge at the groove between olive and pyramid. The dorsal nucleus of the X occupies a swelling lateral to the preceding and known as the ala cinerea. Some of the root fibres of the X are axones from this nucleus. They probably innervate (via the sympathetic) some, at least, of the smooth muscles, heart and glands, innervated by the vagus (X). The bodies of another group of peripheral eft'erent neurones form the nucleus amhiguus, often diffi- FiG. 299. — Diagram of Origin of Cranial Nerves XandXII. (Schafer.) /)_vr, Pyramid; o, olivary nucleus; r, restiform body; d.V, spinal root of fifth nerve; n.XII, nucleus of hypoglossal; XII, hypoglossal nerve; d.n.X.XI, dorsal nucleus of vagus; n.amh, nucleus ambiguus; f.s., solitary fasciculus (descending root of vagus and glosso- pharyngeal) ; f.s.n, nucleus of solitary fasciculus ; A', motor fibre of vagus from nucleus ambiguus; g, ganglion cell of sensory root of vagus sending central arm into solitary fasciculus {f.s.) and collateral to its nucleus {f.s.n.); f.s.n, cell of nucleus of solitary fasciculus sending axone as internal arcuate fibre to opposite side of cord (secondary vagus and glosso-pharyngeal tract.) This course of the secondarv tract is doubtful. cult to distinguish, in the reticular formation. Their axones pass obliquely, dorsally and mesially, join the other root fibres of the X, and then bending abruptly, pass with them to the lateral surface of the medulla. Some of them probably innervate the striated muscles of the larynx. (Fig. 290). 438 THE ORGANS. Afferent Peripheral Roots, their Terminal Nuclei and Secondary Tracts. — Other root fibres are the afferent fibres of the X which form a common root with the preceding. Sometimes they can be seen joining the fasciculus soHtarius of which they form a part. Some fibres or collaterals may enter the adjacent gray {terminal nucleus of the X) (see page 437, Fig. 299). If the roots of the X do not show well in the section, defer their study until the following section where the IX shows similar relations. The spinal V is partly pierced and partly covered by transverse fibres, princi- pally olivo-cerebellar fibres (see below). Its terminal nucleus is less conspicuous. Two new bundles of descending root fibers have appeared; one is the fasciculus solitarius composed of the afferent root fibres of the X, IX (including gustatory fibres), and, higher up, of the VII. A small mass of gray of a gelatinous appear- ance near it is its terminal nucleus. The course of the secondary tract cannot be made out and is not accurately known. The other bundle is the descending vestib- ular root. It lies lateral to the fasciculus solitarius. Accompanying it are cells which constitute its terminal nucleus. Occupying the floor of the ventricle lateral to the dorsal nucleus of the X is another terminal nucleus of the vestibular nerve, the nucleus medialis (triangular or chief nucleus). Internal arcuate fibres emerg- ing from these regions may represent secondary tracts (probably reflex) from these nuclei. (P. 441; Fig. 302.) The nucleus gracihs has disappeared. The nucleus cuneatus may be present, much diminished, and give rise to some internal arcuate fibres to the medial lemnis- cus. The lemniscus is now a tract which has become built up on each side of the median line. This latter is known as the raphe (i.e., "seam," stitched by the decus- sating fibres). The ventral spino-cerebellar tract and spino-thalamic tract are in about the same lateral position, but the dorsal spino-cerebellar tract has moved dorsally and together with olivo-cerebellar fibres (see below) begins to form the restiform body (see below). In the lateral part of the reticular formation, between spinal V and olive are seen the nuclei laterales. In these nuclei some of the spino-cerebellar fibres end. The axones of these nuclei partly enter the restiform body on the same side and partly cross to the opposite restiform body (p. 455; Fig. 308). They form some of the ventral external arcuate fibres seen in the section. The lateral nuclei are thus partial interruptions in the spino-cerebellar path. The nuclei arcuati are well marked. Other Afferent Cerebellar Neurones. — A new and important convoluted mass of gray is the inferior olivary nucleus, forming a bulging in the lateral surface of the medulla known as the olive. Near it are the dorsal and medial accessory olives. The axones of the olivary cells are the olivo-cerebellar fibres. They cross through the fillets, pass through or around the opposite olivary nucleus, thence proceed dorso- laterally, being gathered into more compact bundles, traverse or surround the spinal V and dorsal to it bend longitudinally, forming a great part of the restiform body. The latter produces an eminence on the dorso-lateral surface of the medulla. The restiform Ijody, thus formed by these spino-cerebellar and olivo-cerebellar fibres, together with certain others, passes into the cerebellum higher up, forming the major part of the inferior cerebellar arm or peduncle. (Comp. p. 455.) I'ibres appearing on the external surface of the olivary nucleus are the termina- THE NERVOUS SYSTEM. 439 tion of a large tract descending to the olivary nucleus, the central tegmental tract. Its origin in higher levels is not accurately known, but is possibly the nucleus lentic- ularis of the endbrain. Intersegmental Neurones. — The reticular formation is now still more extensive. The original U-shaped mass of intersegmental tracts (and the tecto-spinal tract) has now become widely separated into two parts. The lateral part consisting principally of the rubro-spinal tract and uncrossed fibres from Deiters' nucleus lies mesial to, or partly mingled with, the spino-thalamic and ventral spino-cerebellar tracts. The mesial part of the U, consisting principally of crossed and uncrossed fibres from Deiters' nucleus and other nuclei in the reticular formation, and of fibres from the interstitial nucleus of Cajal, now forms the medial longitudinal fasciculus dorsal to the fillet. Near this bundle or united with it, is the tecto-spinal tract (predorsal tract). When these tracts have passed down to below the formation of the fillet and the olives, they assume the positions noted in the lower levels of the medulla. Efferent Suprasegmental Neurones. — The pyramids are the same. Small bundles of more lightlv stained fibres present in the fillet here and in higher levels (Weigert stain, not indicated in the figures) are efferent pallia! fibres detached from pes or pyramids. They are aberrant fibres which rejoin the pyramids or are fibres innervating motor cranial nuclei. The tecto-spinal tract (see above). 4. Transverse Section of the Medulla through the Middle of the Olivary Nucleus. Such a section is so similar to 3 and 5 that its detailed description may be omitted. The nucleus cuneatus has disappeared; the fillet increased somewhat; fasciculus solitarius and descending vestibular root have increased; also their terminal nuclei. The olivary nucleus, olivo-cerebellar fibres, and the restiform body have greatly increased. The formatio reticularis has increased in extent. 5. Transverse Section of the Medulla through the Entrance of the Cochlear Root of Nerve VIII. (Figs. 295 and 300.) Efferent Peripheral Neurones. — The dorsal vagus nucleus is not present, but the nucleus ambiguus is usually present and probably sends some axones to nerve IX, passing out with the aft'erent fibres (see below). The nucleus XII has disap- peared and also its root fibres. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Usually the afferent root fibres of nerve IX are present. They enter on the lateral aspect of the medulla ventral to the restiform body, traverse the spinal V, and pass to the fasciculus soHtarius or its terminal nucleus. The fasciculus solitarius is smaller, and just above the entrance of the IX consists of only a comparatively few descend- ing afferent root fibres of the VII. The fibres of the cochlear nerve enter the extreme lateral angle of the medulla, where many, or, according to some, all of them terminate in two masses of cells enveloping externally the restiform body and known as the ventral (or accessory) and 440 THE ORGANS. /C, o-r; 2S ^° THE NERVOUS SYSTEIM. 441 dorsal (or lateral) cochlear nuclei. Most of the axones of the dorsal nucleus pass across in the floor of the ventricle {strice medullar es) to form a part of the opposite secondary tract {lateral lemniscus) of the cochlear nerve. The axones of the ventral nucleus also decussate, but by a more ventral route, and also form a part of the lat- eral lemniscus. This latter decussation takes place at a higher level (see next section). The auditory nerve is divided into two parts: the cochlear nerve (ganglion spirale) and the vestibular nerve (ganglion of Scarpa) . The fibres of the cochlear root enter at a lower level than those of the vestibular. Some of them enter the ventral cochlear nucleus; the remainder pass dorsalward to the dorsal cochlear nucleus, or nucleus of the acoustic tubercle. According to some authorities, some root fibres pass to the superior olivary and trapezoid nuclei. The axones of the cells of the ventral and dorsal nuclei form the secondary cochlear tract (lateral fillet). These fibres decussate (trapezius) and send collaterals to, or are partially interrupted in, the nucleus olivaris superior, trapezoideus, nucleus of lateral fillet and posterior corpus quadrigeminum. According to some authorities all the fibres of the lateral lemniscus terminate in the posterior corpus quadrigeminum. From the posterior corpus quadrigeminum, the path is formed by the arm or brachium of the latter to the medial geniculate body and thence to the temporal cortex cerebri. It is thus not possible to state definitely how many neurone systems are involved, but the principal ones are: (i) ganglion spirale, (2) dorsal and ventral nuclei and (decussa- tion) lateral lemniscus, (3) posterior corpus quadrigeminum and its brachium, (4) medial geniculate body of the thalamus and geniculo-cortical fibres. If the lateral lemniscus fibres be regarded as simply passing by the posterior corpus quadrigemi- num, giving collaterals to it (Cajal), the path might in part consist of three neurone systems analogous to those of the paths from the cord, trigeminus and eye. (Figs. 294, 301.) The fibres of the vestibular root enter higher and mesial to those of the cochlear root, passing dorsally along the inner side of the restiform body to four terminal nuclei, which cannot all be clearly seen in any one section; (a) Deiters' nucleus (lateral vestibular nucleus) situated at the end of the main bundle of root fibres, just internal to the restiform body; {b) von Bechterew's nucleus (superior vestibular nucleus) situated somewhat dorsal to Deiters' nucleus in the lateral wall of the fourth ventricle; (c) the median or principal nucleus of the vestibular division — a large triangular nucleus, occupying a considerable part of the floor of the fourth ventricle; {d) the descending vestibular nucleus which accompanies the descend- ing fibres of the vestibular root (spinal eighth). Fibres also pass to the cerebellum. The axones of the cells of the terminal vestibular nuclei form the secondary vestib- ular tracts, some axones going to (a) the cerebellum (?), (b) the midbrain, espe- cially to the nuclei of nerves III and IV {via Deiters and von Bechterew, the former by the medial longitudinal fasciculus), (c) the medulla and cord, probably to vari- ous motor nuclei, via the medial longitudinal fasciculus, lateral tract from Deiters' nucleus and other tracts in the reticular formation. (Figs. 294, 302 and 308.) The descending vestibular root is large, as is also its terminal nucleus and the medial terminal vestibular nucleus, in the present section. The spinal V is unchanged, its terminal nucleus being rather indistinct. Second- ary trigeminal tracts cannot be distinguished — such fibres probably either join the 442 THE ORGANS. L«mniscu5 Lemniscus. . /,- ; Nu. Urrj. l h-1 h THE NERVOUS SYSTEM. 40/ guishable as a bundle dorsal to the dorsal edge of the medial lemniscus. At this level, then, the afferent paths from cord to pallium have practically united. In the anterior corpus quadrigeminum are terminations of the optic tract (so-called optic nerve) (see below). The central tegmental tract is displaced dorsally by the red nucleus. Intersegmental Neurones. — The reticular formation is smaller. The rubro- spinal tract (not distinguishable) is emerging from its nucleus of origin, the nucleus ruber. Just below this level its fibres decussate {ventral decussation of Forel) and pass to the location they have been seen to occupy in preceding levels. The nucleus ruber or red nucleus is very conspicuous, occupying a large part of the reticular for- mation. It gives rise to rubro-bulbar as well as rubro-spinal fibres. The medial longitudinal fasciculus is diminished, some of its descending fibres having been formed from reticular formation nuclei below this level. Many of its fibres, both ascending and descending, send collaterals and terminals to the cells of the oculomotor nucleus. Efferent Suprasegmental Neurones. — The ventral part of the brain is com- posed of a mass of efferent pallial fibres, the pes peduncidi. The pyramidal fibres from the precentral areas of the cerebral hemisphere (pallio-spinal and some pallio- bulbar fibres) occupy about the middle three-fifths, but are also scattered through other parts of the pes, especially the mesial part. The leg fibres are probably more numerous laterally, the arm fibres in the middle, and the face fibres mesially. In the lateral part of the pes are pallio-pontile fibres from the occipital and temporal lobes of the cerebral hemispheres (occipito-temporal pallio-pontile fibres). In the mesial part are efferent palHal fibres from the frontal lobe, in part to the pons nuclei (frontal pallio-pontile) and in part possibly from the lower frontal region to the motor nuclei of cranial nerves VH and XII. There is also the bundle already referred to (p. 464) which in its downward course passes from the lateral part of the pes to the mesial part of the medial lemniscus and is variously stated to contain fibres from cortex cerebri or from thalamus to the medulla(p. 470). Dorsal to the pes and constituting the remainder of the basis pedunculi is a mass of gray matter which, on account of the pigmentation of its cells, is known as the substantia nigra. This receives many collaterals and terminals of efferent pallial fibres and possibly of fillet fibres. The destinations of the axones of its cells are not definitely known. They are stated to join the pes. The superior cerebellar peduncle has completed its decussation below this level and its fibres are seen surrounding or within the nucleus ruber which is their ter- minal nucleus, as well as the nucleus of origin of the rubro-spinal tract. Internal arcuate fibres from the gray matter of the anterior corpus quadrigemi- num pass through the reticular formation, and form an oblique decussation. This decussation is the dorsal, or jountain-like decussation of Meynerl. The fibres origi- nate from cells in the anterior corpus quadrigeminum (tectum opticum), and after decussation form the descending tectospinal (prcdorsal) tract or ventral langitudinal fasciculus (see also below). The Anterior Corpus Quadrigeminum or Superior Colliculus. — In this four principal lavers mav be distinguished besides the usual covering of neuroglia cells and fibres: (i) An outer while layer, stratum zoialc. This consists of fine nerve fibres coming from the superior brachium, possibly fibres from the optic 468 THE ORGANS. tract and cerebral cortex. Among them are small nerve cells, mostly horizontal and with tangential or centrally directed a.xones. (2) A gray layer, the stratum cinereitm. This consists of radially arranged nerve cells with their larger dendrites proceeding outward, and their axones inward. The largest cells lie deepest. In this layer the optic fibres principally terminate. (3) The stratutn opticuni con- sists principally of optic fibres which send their terminals mostly into the pre- ceding layer, but also into the deeper layers. It also contains cells whose axones pass into the next layer. (4) Deep gray-white layer, or stratum leninisci, because it is stated to contain fibres from the medial lemniscus which terminate in the superior colliculus (denied by some). This layer contains large and medium stellate cells whose axones, together with axones from cells in the more super- ficial layers, either pass across to the opposite colliculus or sweep ventrally around the central gray, decussate in the raphe and proceed caudally as the tecto-spinal tract. The above relations have been principally ascertained by the Golgi method. The superior colliculus also possibly receives fibres from the lateral lemniscus and spino-thalamic tract. It also receives fibres from the occipital and temporal cortex cerebri. Belonging to the midbrain is the posterior commissure (not in the section) the fibres of which cross in the roof just anterior to the superior colliculus. Its fibres originate from collicular cells (in turn receiving optic fibres), decussate and termi- nate in the interstitial nucleus and other nuclei in the reticular formation. The colliculus thus consists essentially of (a) afferent fibres from the retina (optic tract), the pallium and possibly other parts of the nervous system, and {b) efferent neurones to other parts of the brain and cord brought into various relations with each other in the colliculus either directly or by (c) the association cells of the colliculus, the axones of which do not leave the latter. Forebrain or Prosencephalon, Interbrain (diencephalon or thalamencephalon). In the interbrain or diencephalon, three parts may be distinguished; the thalamus, epithalamus, and hypothalamus. The epithalamus consists principally of the pineal body, the habenulse, and stride thalami. The hypothalamus consists mainly of the structures in the ventral expansion of the interbrain, such as the corpora mamillaria, tuber cinereum,, in- fundibulum and jxjsterior lobe of the hypophysis. The epithalamus and hypothalamus are principally connected with olfactory paths (see p. 477 and Fig. 321). Certain extensions forward of the tegmentum are also termed subthalamic (t^,^^, corjjus subthalamicum or cor])us Luysii). The thalamus comprises the great bulk of the interljrain. It con- sists of a number of nuclei forming links in afferent and efferent jiallial paths and of other nuclei connected with the corpora striata. There is much difference of opinion both as to the number of the nuclei and THE NER\-()IS SYSTEM. 409 their connections. According to some authorities the thalamus may be regarded as di\ided into internal and external segments (usually sepa- rated by the lamina medullaris medialis). The internal segm.ent con- sists of an anterior nucleus, median nucleus, the "median center" or nucleus of Luys, and a nucleus arcuatus. The external segment con- sists of a dorso-lateral, an external ventro-lateral, an internal ventro- lateral, and a ventral nucleus. To the external segment should be added the pulvinar and lateral and medial geniculate bodies (metathala- mus). The various nuclei of this external segment receive the fibres of the afferent pallial paths and complete the paths by sending fibres to the cortex pallii. These paths are (i) the medial lemniscus, spino- thalamic, and secondary trigeminal tracts (general sensory from body and face) to the ventro-lateral nuclei and thence to the cortex of the central region of the pallium; (2) the lateral fillet or brachium of inferior colliculus (hearing) to the medial geniculate body, and thence to the temporal region of the pallium; (3) the optic tract to the lateral genicu- late body and thence to the occipital pallial cortex; (4) part of the supe- rior cerebellar peduncle (also said to be distributed to nuclei of inner segment). The visceral (including gustatory) and vestibular paths to the pallium are not definitely known. As the olfactory, nerve belongs to the endbrain, its path to the pallium does not traverse the thalamus. Besides giving rise to the above thalamo-cortical fibres, the external thalamic segment in all probability receives many descending fibres from the cortex pallii. The various fibres connecting thalamus and cortex constitute the thalamic radiations. In general the anterior parts of the cortex are connected with the anterior part of the external thalamic segment, the middle with the middle, and the posterior with the posterior. It is also probable that the thalamo-cortical fibres from the various lateral nuclei are arranged dorso-ventrally, so that the fibres from the ventral parts pass to the ventral part of the central region of the pallium, dorsal to dorsal, etc. (E. Sachs.) The nuclei of the internal segment of the thalamus do not appear, according to some recent researches, to have direct connections with the cortex pallii. The anterior nucleus receives the bundle of \'icq d' Azyr (mamillo-thalamic tract) and probably sends fibres to the nucleus caudatus (see p. 478). It thus belongs to the olfactory apparatus. The median nucleus is also probably connected with the nucleus cau- datus. The median center of Luys and the nucleus arcuatus send fibres to adjoining thalamic nuclei, especially the lateral nuclei, and appear to be thus association nuclei. Other authorities afiirm that 470 THE ORGANS. ascending tracts are received by some of these internal nuclei and that they have direct cortical connections. Descending tracts coming directly from the thalamus have not been definitely demonstrated. The ventral nucleus appears to send fibres to the nuclei of the cranial nerves V, VI, \TII, and X, via the medial longitudinal fasciculus. (E. Sachs.) PRACTICAL STUDY. II. Transverse Section through the Junction of Midbrain and Thalamus. (Figs. 295 and 318.) The most conspicuous change from the last section is the appearance, or in- crease, of the geniculate bodies and pulvinar, and the thalamic radiations. Efferent Peripheral Neurones. — The nuclei and root fibres of the III nerve are still present. Afferent Roots, their Terminal Nuclei, Secondary Tracts, and Tertiary Neurones. — The fibres of the optic tract (optic "nerve") are seen entering the ventral surface of one of their terminal nuclei, the lateral or external geniculate body. Other optic fibres (not entirely traceable) enter the pulvinar thalami and the superior colHculus {Stro) (see also preceding section). On the dorso-lateral surface of the lateral geniculate body, a bundle of fibres accumulates which represents the beginning of the geniculo-calcarine tract to the occipital cortex, thereby com- pleting the visual path. Internal to the lateral geniculate body is the medial (internal) geniculate body which now contains the terminals of the brachium of the inferior colliculus. Fibres from its cells gather on its lateral surface. These represent the beginning of the geniculo-temporal tract to the temporal cortex, thereby completing the auditory path. Internal and ventral to the medial geniculate body are the medial lemniscus (bulbo-thalamic) and spino-thalamic tracts about to terminate in the ventro-lateral thalamic nuclei whose axones complete the general sensory path by passing to the central cortex. The central tegmental tract can hardly be distinguished. Intersegmental Neurones. — The nucleus ruber is still large, but the remainder of the reticular formation has nearly disappeared. The medial longitudinal fascic- ulus is much diminished as its ascending fibres terminate in the nucleus of nerve III and many of its descending fibres originate from cells below this level. Efferent Suprasegmental Neurones. — The pes pedunculi occupies the same position, and dorsal to it is the diminished substantia nigra. Along the ventro- mesial border of the pes a bundle of fibres can sometimes be distinguished (Fig. 318, Lmp), which at lower levels comes to lie mesial to the medial lemniscus (ah)errant peduncular fibres, comp. p. 467). Descending pallial fibres (not dis- tinguishable) also probably form part of the thalamic radiations (pp. 469, 472). Fibres of the superior cerebellar peduncle may be seen within and around the nucleus ruber. Some of these terminate in the latter, some pass further for- ward to end in the thalamus (compare pp. 455, 469). THE NERVOUS SYSTEM. 471 l/vJSL ■ V •■^^^^ ^? . ,,. . ^1 I ■"-' 71 :/: tfi ^ c/: C --"^ C Tr" P 3 O C 3 .2 £ — ci: ii ^ '^ 2 ~ rt ^ IT. y: p — C fi^ tfl in Cfi c<^ b '^ -? ■3 ^^ o cd O S !fl ^ 'O "^ " •f' O S C^ i"^. o G o — ;^ •--] Ji •- -h ■^5 MS M-5 g'f.SDg c p a y -^ C K^ •60-^ c > .= - S ^ =" 3 § s i ~.:^ - ^ to -n^ =J — •- r- "-^-i: 3 T "d " cj ° c b"'-^ P « o ""^ 3 S2; :; ^ »~^^' fi^ 474 THE ORGANS. Corte;i Fig. ^20. THE NERVOUS SYSTEM. 475 EXPLANATION OF FIG. 320. Fig. 320. — Diagram of the Optic (II) Nerve and some of its Principal Connections. A, Level of nerves II and III; B, level of nerve IV; C, level of nerves VI and VII; D, spinal cord. Neurone groups are represented by one or several individual neurones. The rods and cones (receptors) and the bipolar cells ( = Neurone No. i) of the retina are not indicated. Neurone N^o. 2. — 2 a, Axones of ganglion cells in temporal part of retina pass to pulvinar of thalamus of same side; 2 b, axones of ganglion cells in temporal retina pass to anterior corpus quadrigeminum of same side; 2 c, axones of ganglion cells in temporal retina pass to external geniculate body of same side; 2 e, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to external geniculate body of opposite side; 2/. axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to anterior corpus quadrigeminum of opposite side; 2 g, axones of ganglion cells in nasal side of retina cross in optic chiasma and pass to pulvinar of thalamus of opposite side. Macular fibres are partly crossed and partly uncrossed. Neurone No. 3. — 3 a, Axones of cells in pulvinar to cortex of occipital lobe of cerebrum (this connection is disputed); 3 b, axones of cells in external geniculate body to cortex of occipital lobe of cerebrum; 3 a and 3^ constitute the primar}^ optic radiation; t, c, t, d and 3 e, axones of cells in middle layer of tectum (roof) of anterior corpus quadrigeminum decussate ventral to medial longitudinal fasciculus (dorsal tegmental decussation or decussation of ]Meynert) and form the tractus tecto-bulbaris et spinalis or predorsal bundle. (Tr. tectobulb. et spin.) to bulb (medulla) and anterior column of cord, innervating by collaterals and terminals, directly or indirectly, chiefly the nuclei of III, IV, \I, and ^TI cranial nerves and motor nuclei of spinal nerves. 3 / and 3 g (possibly another neurone intercalated between these and optic terminals), axones of cells in interstitial nucleus of Cajal (Nu. fasc. long, post.) form part of medial longitudinal fasciculus and descend on same side to anterior column of cord next to anterior median fissure, innervating nuclei of III, IV, and VI cranial nerves and motor nuclei of spinal nerves. Neurone No. 4. — •x\xones of cells in above-mentioned motor nuclei. Axones from cells in median nucleus of nerve III (Nu. med. Ill N.) and possibly in Edinger-Westphal nu- cleus, probably innervate the intrinsic muscle of eveball (ciliary and pupillary reflex path). Pallio-tectal fibres, by means of which the quadrigeminal reflex centre is brought under the control of the cerebral cortex, are not indicated. It is evident from the diagram that the cerebral pathway of the optic nerve is via the external geniculate body (and pulvinar of thalamus ?), and the reflex pathway is via the superior colliculus (anterior corpus quadrigeminum). 476 THE ORGANS. lateral nucleus and median center of the thalamus (see also p. 469). The intersti tial nucleus of Cajal falls in the level between this and the preceding section. Efferent Suprasegmental Neurones. — The pes pedunculi now lies partly between the thalamus and nucleus lenticularis (see p. 478) constituting the greater part of the internal capsule. The parts of the internal capsule as shown in horizon- tal sections of the hemispheres are shown in figure 322. The most dorsal part is here passing into the corona radiata (p. 478) of the cerebral hemispheres (not included in the section). The part present in this level is the most posterior part of the capsule (occipito-temporal pallio-pontile fibres (see p. 479). Fig. .321. — Diagram of Olfactory Paths (von Bcchterew.) A', Root fibres of vagus; ca, commisura anterior; cm, corpus mammillare; cp, fibres from nucleus habenuke to posterior commissure; /G, tract from corpus mammillare to Oudden's nucleus; fi, fasciculus mammillo-thalamicus;/, fasciculus longiludinalis medialis; fr, fornix; fid, fibres of fornix longus; gh, nucleus habenulse; gi, ganglion inlerpedunculare; gp, gyrus pyriformis; /, lemniscus medialis; m, fibres from Oudden's nucleus to substantia reticularis grisea; na, nucleus anterior thalami; nG, Oudden's nucleus; nt, nucleus tegmenti (v. Oudden); nX, nucleus molorius nervi vagi; pcE, pedunculus corporis mammillaris from fillet; qa, corpora quadrigemina; r, fibres from nucleus tegmenti (v. Oudden) to nuclei of cranial nerves; re, radix lateralis tractus olfactorii; rf, fibres of tractus olfaclorius to trigonum olfactorium; ro, radix medialis tractus olfactorii; s, fibres from ganglion inlerpedunculare to nucleus tegmenti; so, area of trigonum olfactorium; Ih, thalamus; Iro, tractus olfactorius; It, tienia thalami; :x:, fasciculus retroflexus. THE NERVOUS SYSTEM. 477 Dorsal to the mesial part of the pes is the corpus sublhalamicum which has replaced the substantia nigra. It receives collaterals from the pes and is said to contribute fibres to the latter. Superior cerebellar peduncle (see Intersegmental Neurones above). Thalamus. — At this level the vcnlro-lateral )iitcleiis, the nucleus arcuatus, and the median center oj Luys can usually be distinguished. At the outer border of the thalamus, fibres accumulate forming the lateral medullary lamina. These fibres continue outward as thalamic radiations, entering the internal capsule which they may follow a distance, or cross obliquely and enter the corona radiata. Epithalamic and Hypothalamic Structures and their Connections. — The ganglia habenulce are two small masses of gray matter occupying eminences on the mesial walls of the thalamus. A bundle of fibres near each is the stria medul- laris (near the tcenia thalami) consisting of fibres from the olfactory bulb and trigonum and representing afferent olfactory connections (p. 478). The ganglion habenulae contains a mesial small-celled and a lateral large-celled nucleus. Their axones form the fasciculus retroflexus of Meynert to the interpeduncular ganglion, situated more caudally (Fig. 321). There is also a commissura habenularis connect- ing the two ganglia. The stria terminalis {stria cornea), another olfactory connec- tion, lies in the groove between the ventricular surfaces of nucleus caudatus and thalamus. The tuber cinereum is seen projecting ventrally. Dorsal to this are seen the corpora mammiUaria containing lateral and mesial nuclei. The mammillary body receives the fibres of the jornix (from the rhinopallial cortex, see below) and also fibres from the medial fillet. It gives rise to the bundle of Vicq d'Azyr (mammillo-thalamic tract) to the thalamus, and mammillo-tegmental fibres to the nucleus of Gudden (Fig. 321) and red nucleus. The fibres entering the mammillary body from the fillet (and other sources) constitute its peduncle. ( See also Endbrain) . On the right is seen the posterior part of the nucleus lenticularis of the corpus striatum. The Endbil4in or Telencephalon. The endbrain consists of pallium (dorsal expanded part), corpus striatum, and rhinencephalon. Two principal parts of the j^allium may be distinguished; the olfactory pallium or rhinopallium (archipallium) , including principally the cornu ammonis and gyrus dentatus; and the neopallium including the greater part of the cerebral hemispheres. The rhinencephalon^ includes the olfactory nerves and bulb, the trigonum olfactorium, the tuberculum olfactorium or anterior per- forated space, and the gyrus hippocampi, in part at least (pyriform lobe). The olfactory nerve is composed of axones of cells in the olfactory mucous membrane which terminate in the olfactory bulb. They there form synapses with the dendrites of the mitral cells, the axones of which constitute the secondary tract, part of which decussates in the ' The term rhinencephalon is often used to include also the olfactory pallium. 478 THE ORGANS. pars olfactoria of the anterior cerebral commissure. A secondary tract ("lateral root") proceeds, with tertiary tracts, to the cortex of the gyrus hippocampi and thence to the cornu ammonis. Efferent axones of cornu ammonis cells are collected in the fimbria and descend by the fornix to the mammillary body, the further caudal connections of which have been de- scribed (p. 477). Fibres of the fimbria also cross, forming the commissure of the fornix (olfactory pallial commissure). Secondary olfactory tracts also pass to the trigonum, whence tertiary neurones pass as the stria medullaris to the ganglion habenulse (see p. 477 and Fig. 321). The principal com.missure of the rhinencephalon is the anterior cerebral commissure. The corpus striatum consists of the nucleus caudatiis and nucleus lenticularis, the con- nections and significance of which are ob- FiG. 322.— Scheme of Gen- scurc. They receive collaterals from the de- irintemafcTpsuie^ ^vo'r scending pallial fibres which pass by them Bechterew.) I, II, III, and also apparently send out fibres to join The three parts of the lenticular nucleus; tic, nu- the latter. They also have connections with cleus caudatus; Fh, thala- mus; gp, globus pallidus; pt, putamen; i, fibres of rhinenencephalon and thalamus. The pallium consists of an extensive ex- anterior thalamic peduncle; , 1 1 , 1 1 . r j^j. / 1 2, fibres of medialffrontal) ternal convoluted sheet of gray m^atter {cortex pons system; ,3, fibres of w//f or coi'tex cerebri) and of white matter motor cranial nerves; 4, pyramidal fibres; 5, pyra- underlying the gray. In the white m.atter rhLt',f[SaSMt* -"ay be distinguished the corona radiala sory) path; 6, fibres of the composcd of the afferent and efferent pallial lateral pons system. The _, . , ... • 1 1 various systems are not fibres connecting the pallium With Other parts sharply marked off as in- qJ ^j^g j^j-^j^ (projection fibres). The remain- dicated, but are more or / j ./ / less intermingled. ing fibres of the white matter are association fibres of the pallium and are either crossed or com.missural, connecting the two hemispheres {corpus callosum and fornix commissure), or arc uncrossed. The term association fibres is often restricted to the latter. The afferent connections of the neoj^allium (p. 469) and rhino- palh'um fpfj. 477, 478) have been summarized and also the efferent connections of the rhinopallium (p. 478). The following are the princijjal descending or efferent connections THE NERVOUS SYSTEM. 479 of the neopallium: (i) The pyram.idal or pallio-spinal tract. This is composed of the axones of the giant cells (of Betz) of the arm, body, and leg precentral motor areas. They descend in the corona radiata, the posterior limb of the internal capsule, middle part of the pes, and thence through pons and medulla to the cord. Their decussation and further course has been described. (2) The descending tracts to the motor nuclei of the cranial nerves originate from precentral cells of the various areas controlling the muscles in question and pass down in the vicinity of the genu of the internal capsule. Their path is not so well known but they apparently do not pass down in the pes throughout their course (pp. 470, 467, etc.). (3) The pallio-pontile system to the pons (continuation to opposite cerebellar hemisphere). This originates in various parts of the cortex. The fibres from the occipital and temporal regions pass down in the extreme posterior part of the internal capsule and lateral part of the pes, those from the frontal region pass down in the anterior limb of the internal capsule and mesial part of the pes. (4) Pallio-tectal fibres to the midbrain roof. (5) Fibres to the substantia nigra and corpus subthalamicum. (6) Fibres to the red nucleus. (7) Pallio-thalamic fibres (see p. 469). (8) Fibres, or collaterals, to the corpora striata. (Fig. 322.) The crossed association fibres of tJie neopallium (corpus callosum) connect principally corresponding parts of the hemispheres. The long uncrossed association fibres (furthest from the gray matter) form certain more or less well-defined bundles among which are the following: (i) The cingulum, a longitudinal bundle near the corpus callosum; also contains projection fibres and belongs to the olfactory part of the brain as well as to the neopallium,. (2) The superior longitudinal fasciculus or fasciculus arcuatus; connects frontal with occipital and part of temporal lobes. (3) The inferior longitudinal bundle connecting temporal and occipital lobes. It may, however, be a projection bundle. (4) The uncinate fasciculus connecting frontal and temporal lobes. (5) The perpendicular fasciculus of Wernicke connecting inferior parietal and fusiform lobules. Projection fibres may form portions of these bundles. Shorter association fibres nearer the gray matter connect adjoining convolutions (fibras propriae of Meynert) and in the grav matter itself are fibres which fall under this category. 480 THE ORGANS. PiTh Coa X Vli ti Roi)Fli Fig. 323. — Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus. Weigert preparation. (Dejerine.) //, Tractus opticus; Alent, ansa lenticularis; c, sulcus centralis (Rolandicus); C'a, gyrus centralis anterior; Cell, corpus callosum; Ce, capsula externa; CFo, columna fornicis; Cia, crus anterior capsul. int.; Cig, genu capsul. int.; CI, claustrum; dim, sulcus callosomarginalis; Cng, cingulum; Coa, commissura anterior; Cp, gyrus centralis posterior (ascending parietal convolution); CR, corona radiata; Far, fasciculus arcuatus; Fli, fasciculus longitudinalis inferior; Frn, gyrus fornicatus; fs, .sulcus frontalis superior; Fs, gyrus fnjntaiis superior; Fu, fasciculus uncinatus; Fus, gyrus fusiformis; glp, globus pallidus (inner segment); glp[, globus pallidus (outer segment); /, insula; Ime, lamina medullaris externa nuclei lenti- THE NERVOUS SYSTEM. 481 PRACTICAL STUDY. 13. Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus. (Fig. 7,2^ ) First distinguish in general (i) the pallium, its cortex and white matter, (2) the corpus striatum and its divisions, i.e., the caudate nucleus and the putamen and globus pallidus (two subdivisions of the lenticular nucleus) and (3) the thala- mus and other structures of the interbrain. Afferent Roots, their Terminal Nuclei, Secondary Tracts, and Tertiary Neurones. — The optic tract here forms a part of the ventral surface of the brain. The geniculo-cortical portion of the optic path forming a part of the optic radiation may be seen. Other afferent pallial connections are hardly distinguishable among the fibres connecting thalamus and pallium. Efferent Suprasegmental Neurones. — A great part of the pes has now entered the corona radiata. The part now about to enter the corona is the anterior limb of the internal capsule (Fig. 322). The ansa lenticularis is, according to some authorities, composed of fibres or collaterals from the pes to the lenticular nucleus. The fibres of the anterior peduncle of the thalamus are evident. Thev are con- sidered by some as composed of cortico-thalamic fibres. The anterior pillars oj the fornix (efferent rhinopallial) are shown cut through twice, the upper section shows its earlier course emerging from the fimbria, the lower section is near its termina- tion in the mammillary body. For other structures of thalamus, epithalamus, and hypothalamus see Fig. 323. The Pallium. — In the white matter distinguish as far as possible the corona radiata, the corpus callosum, and the long association bundles {inferior longitudinal fasciculus, fasciculus uncinatus, and fasciculus arcuatus) (see p. 479). Note the nucleus amygdaliformis, the anterior perforated space and the anterior commissure, belonging to the rhinencephalon. Other details shown in figure 323 should be studied. The Cerebral Cortex. — The following types of cells are found in the cerebral cortex: (i) Pyramidal cells. This is the prevailing type and is characterized by a long apical dendrite usually directed toward the surface of the brain. This dendrite gives off branches, and usually reaches the outer cortical layer, there to break up into a number of branches. From the cell body are also given off' a number of basal cularis; Ime', supplementary lamina of the outer segment of the globus pallidus: /;«/, lamina medullaris interna nuclei lenticularis; nip and lus, sulcus circularis (Reili): XA, nucleus amygdaliformis; A"c, nucleus caudatus; OpR, operculum; oli, sulcus occipitotem- poralis inferior; pCR, pes coronae radiate; PiTh, pedunculus inferior thalami; prs, sulcus pra;centralis; Pu, putamen; rcc, stratum reticulum corona; radiata^; Rop. radiatio optica; 5, fissura Sylvii (posterior branch); Sgc, substantia grisea centralis; sM, sulcus Monroi; ^o-f, substantia grisea subependymalis; ssc, stratum subcallosum; ^/r", stratum zonale thalami; Tbc, tuber cinereum; Tli, thalamus opticus; li, sulcus temj)oraiis in- ferior; 7"/, gyrus temporalis inferior; t>ii, sulcus temporalis medius; Tm, gvrus temporalis medius; ts, sulcus temporalis superior; Ts, gyrus temporalis superior; Tte, ta?nia lecta; U, uncus; 17, ventriculus lateralis; 17/, ventriculus lateralis (cornu inferius); vsl, pedunculus anterior thalami; A", pedunculus putaminis; CM, Commissure of ^leynert. 482 THE ORGANS. dendrites. By the Golgi and Ehrlich methods, gemmules can be demonstrated on the dendrites. The axone proceeds from the base of the cell (opposite to the apical dendrite) and usually passes into the white matter. It gives off several collaterals on its way to the white matter. (2) Stellate cells. These have dendrites passing in various directions. Many, especially the smaller (granules), m^ay have short axones (Golgi's second type). (3) Polymorphous cells of a triangular or spindle shape are usually found in the deepest layers of the cortex and send their axones into the white matter. (4) Horizontal cells (of Cajal), found in the outer layer, with long horizontal dendrites and ax- ones confined to the outer layer. (5) Inverted pyramidal cells (of Martinotti) with axones directed toward the surface. (Fig. 325.) The largest cells of the cortex (giant cells of Betz) are very rich in chromophilic substance arranged similarly to that in the efferent root cells of cord and brain. The medium and small cells have fewer chromophilic bodies which are often in the shape of irregular masses, either near the periphery of the cell or around the nucleus. The neurofibrils vary in their arrangement according to the shape of the cell. The neuroglia cells and fibres are in general similar to those in other parts of the nervous system. The cells of the cortex are arranged in layers which have the same general character throughout, but in various regions exhibit variations such as suppression, diminution, enlargement, or subdivision of certain layers. The cell layers of the cortex are: (i) Molecular layer (zonal layer, plexiform layer of Cajal). This contains the horizontal cells, other cells with short axones, and also receives the axones of the Mar- tinotti cells. Besides this, it contains the terminal branches of the apical dendrites of the pyramidal cells. (2) External granular layer, very often termed the layer of small pyramids. The dendrites of the cells of this layer mostly enter the first layer, their axones pass downward into the white matter. (3) Pyramidal layer, often called the layer of superficial medium and large pyramids. This is composed principally of typical pyramids sending dendritic branches into the first layer and axones into the white matter. The larger cells are in the deeper part (sublayer of large pyramids). This layer also contains many granule cells with short axones and cells of Martinotti. (4) Internal granular layer. Here the predominating elements are stellate cells, the larger usually sending their axones into the white matter. Among these are many short axone granules, the axones of which end in the THE NERVOUS SYSTEM. 483 same layer or in more superficial layers. (5) Ganglionic layer or deep layer of large and medium sized pyramids. These send their axones into the white matter. Mingled with them are short axone and Marti- notti Cells. (6) Multiform layer or layer of polymorphous cells. These / / r ■■ * •*. '' « Fig. 324. — ^Vertical Sections of Calcarine .\rea of Adult Human Cortex. Left, W'eigert preparation showing fibre arrangement. Right, Arrangement of cells. (Campbell.) G", Line of Gennari; R, radiary layer; 5, supraradiary layer; Z, layer of superficial tangential fibres in molecular layer, i, molecular layer; 2, external granular (small pyramid) layer; 3, pyramid layer; 4 (large granules) and 5 (small granules), internal granular layer; 6, ganglionic layer (containing solitary cells of Meynert); 7, multiform layer. usually send their axones into the w^iite matter. Mingled with them are short axone and Martinotti cells. (Figs. 324, 325 and 326.) The cells of the cortex obviously fall into two classes: efferent projec- iion cells and association cells. Which cells are projection cells is not definitely known, except in the case of the preccntral motor cortex 484 THE ORGANS. Ill Fig. 325. where it has been estab- lished that these cells are the cells of Betz, the axones of which form the pyra- midal tract. An examina- tion of this area shows that the association cells must enormously outnumber the efferent projection cells in the cortex. The association cells comprise the short axone cells and cells the Fig. 325. — Vertical section of Calcarine Area of Cortex of an Infant 15-20 days old. (Cajal, combined from three, /, // and ///, somewhat overlapping figures. The multiform layer is not in- cluded). Golgi's method. /. — A, Molecular layer; B, external granular layer (of small pyramids); C, pyramidal layer (of medium pyramids); a, descending axones; h, ascending collaterals; c, apical dendrites of large pyramidal cells in ganglionic layer. II. — A, Sublayer of large stellate cells; B, sublayer of small stellate cells; C, outer part of ganglionic layer; a. crescentic stellate cells; h, and/, horizontal, spindle-shaped stellate cells; c, medium-sized pyramidal cells; e, stellate cells with arched axones; g, triangular stellate cells with stout, arched collaterals; h, pyra- midal cells with arched axones. III. — A, Part of internal granular layer; B, sublayer of small pyramidal cells with arched as- cending axones; C, sublayer of large pyramids; a, large pjyramidal cell; b, medium-sized pyramidal cell with lo.g descending axone; c, small pyramidal cell with arched ascending axone; d, pyramidal cell with axone split into two arched ascending branches; e, pyramidal cells whose axone sends out various ascending branches; f,g,h, stellate cells with ascending axones which branch in B and in the sublayer of small stellate cells; i,j,k, pyra- midal cells with arched ascending axones which send branches into the ganglionic layer. THE NERVOUS SYSTEM. 485 fibres of which enter the white matter, but terminate in some other part of the cortex forming the association fibres of the white matter. (Compare p. 479.) It is thus evident that every part of the cortex contains terminations of association fibres. The areas containing the terminations of aft'er- ent projection fibres are those which receive the thalamocortical con- tinuations of the afferent pallial paths and the continuations of the olfactory paths. From observations made with the Golgi method it seems probable that the latter are represented by coarse fibres which may ramify throughout the greater part of the thickness of the cortex, but are confined mainly to the third and fourth layers. The principal areas of the cortex receiving the afferent projection fibres are the olfactory hippocampal area, the calcarine (visual) area (fibres from lateral geniculate body), the transverse temporal gyri of Heschl (auditory fibres from medial geniculate body), and the pre- and post- central areas (postcentral only, according to some authorities — area of general sensation from body and head — fibres from ventrolateral thalamic nuclei). (Fig. 327.) The medullated fibres of the cortex consist of radially, obliquely, and tangentially running fibres. The radial fibres enter the cortex from the white matter in bundles known as the radiations of Meynert which extend a variable distance toward the periphery, diminishing until they end usually in the third layer. They consist mainly of the axones of the adjoining cells passing to the white matter. Their fibres are of varying caliber, the coarsest originating from the largest cells. The oblique fibres form a dense plexus of coarse and fine fibres between the radial fibres, the interradiary plexus. Toward the surface (in the second and third cell layers) they form a delicate plexus of fine fibres. This latter plexus lies principally superficial to the radiations of Meynert and is the siipraradiary plexus. A denser aggregation of irregular fibres constitutes the line or stria of Baillarger located in the layer of superficial large pyramids. It is probable that this represents a layer especially rich in terminals of fibres from the white matter. Other striae are also described. Besides representing the terminals of such fibres the oblique fibres in general are also com- posed of medullated collaterals of axones of pyramids and possibly ar- borizations of short axone cells. A few coarser fibres ascending to the molecular layer are ascending fibres from Martinotti cells. The deep tangential fibres, most marked on the sides of the con\"olutions and in the sulci, are considered short association fibres belonging to the THE ORGANS. -/•:' r.A- i.^^.v •I w 'i ■ i- • i ■ /■i' Fig. 326. — Vertical Sections of Precentral or Motor Area of Adult Pluman Cortex. Left, Weigert preparation .showing fibre arrangement. Right, Arrangement of cells. (Campljell.j B, Position of line of Baillarger, its position obscured by surrounding wealth of fibres; R, radiary layer; .V, supraradiary layer; Z, layer of superficial tangential fibres in molecular layer, flense and well defined; i, molecular layer; 2, external granular layer (small pyramids), 3 (medium-sized) and 4 (large), pyramid layer; 5, internal granular layer, indistinct and with scatterefl granule or stellate cells; 6, ganglionic layer (large deep pyra- mids); 7, multiform layer. THE NERVOUS SYSTEM. 487 ,' P,.«n^..! posl'"' ^.P0'"=- ■psychic Visuo-xnsorjf ^uditO'sensory ■X'.t^^" // / II Fig. 327. — Diagram (orthogonal) showing Cortical Areas as determined bv the Arrangement and Distribution of Fibres and Cells (A. W. Campbell). Large portions of important areas are concealed within fissures, e.g. the calcarine or visual {visuo-sen- sory, within the calcarine fissure) precoitral (motor) and postcentral (within the fissure of Rolando) and especially the acoustic {aitdito-sensory) which is almost completely hidden within the Sylvian fissure. A, B and C, parts of the limbic lobe. 488 THE ORG.^NS. fibrffi propriae of Meynert. In the molecular layer are the superficial tangential fibres consisting of the axones of the horizontal cells and the terminals of the axones of Martinotti cells. (Figs. 324 and 326.) The cortex is divided into various areas by various investigators, the areas being distinguished (a) by the time of medullation (myelo- genetic method of Flechsig), {h) by the number and arrangement of the meduUated fibres (myeloarchitecture), especially the number, thickness, and distinctness of the striae, such as that of Baillarger, formed by them and (c) by the number and arrangement of the cells (cy toarchitecture) . Many such areas have been thus distinguished by different investigators with results agreeing in many respects but differing in others. The areas most clearly defined and concerning which there is perhaps the most general agreement are the various sensory (afferent projection) areas already enumerated (p. 485) and the motor (efferent projection) precentral area (Fig. 327). These areas myelinate first (at or soon after birth), next areas adjacent to them, and last areas occupying a considerable portion of the human pallium but much less extensive in other mammals. There is much difference of opinion as to the extent to which these last myelinating areas are supplied with pro- jection fibres. xA.ccording to some authorities the areas myelinating last have no projection fibres and are consequently entirely composed of association cells and fibres. Perhaps the two best-marked of the various areas are the motor and visual areas the structure of which is shown in figures 324, 325 and 326. The motor precentral cortex is characterized by the presence of the giant cells of Betz, by an almost complete absence of an internal granular layer and by a great wealth of fibres. The calcarine or visual area is characterized by a strongly marked line of Gennari ( = Baillarger) and by a double or triple internal granular layer containing large granules in place, apparently, of the superficial large pyramids. The line of Gennari is probably partly composed of the terminals of the optic path. TECHNIC. (i) The general structure of the cerebellum is well brought out by staining sections of formalin-MuUcr's fluid-iixed material with hiematoxylin-picro-acid- fuchsin ftechnic 3, p. 19), and mounting in balsam. (2) The arrangement of the cell layers of both cerebellum and cerebrum, as well as certain details of internal structure of the cells, can be studied in sections of alcohol- or formalin-fixed material stained by the method of Nissl (technic, P- 35 J- THE NERVOUS SYSTEM. 489 (3) The distribution of the medullated nerve fibres of either the cerebellar or cerebral cortex is best demonstrated by fixing material in Miiller's fluid (technic 4, p. 6) or in formalin-Miiller's fluid, and staining rather thick sections by the Wei- gert or Weigert-Pal method (technic, p. 29). (4) The external morphology of the cerebellar and cerebral neurones and the relations of cell and fibre can be thoroughly understood only by means of sec- tions stained by one of the Golgi methods (technic, pp. 32 and ^^). Especially in the case of the cerebellum, sections should be made both at right angles, and longi- tudinal to the long axis of the convolution. Golgi preparations from embryonic material and from the brains of lower animals furnish instructive pictures. (5) The silver method of Cajal should be used, especially with alcohol fixa- tion (technic p. 34, No. 2), both for the neurofibrils and for the external mor- phology of the neurones. It is especially successful with the cerebellum. (6) Neuroglia stains should also be used. The Pituitary Body. The pituitary body or hypophysis cerebri consists of two lobes which are totally different both in structure and in origin. The Anterior Lobe. — This is the larger, and is glandular in character. It is of ectodermic origin, developing as a diverticulum from the primitive oral cavity. Its mode of development is that of a compound tubular gland, the single primary diverticulum undergoing repeated division to form the terminal tubules. The original diver- ticulum ultimately atrophies and disappears, leaving the gland en- tirely unconnected with the surface. The gland is enclosed in a connective-tissue capsule, from which trabeculae pass into the organ forming its framework. The gland cells are arranged in slightly convoluted tubules and rest upon a basement membrane. Between the tubules is a vascular connective tissue. Some of the gland cells are small cuboidal cells with nuclei at their bases and a finely granular basophile protoplasm {chief cells). Others, somewhat less numerous than the preceding, are larger polygonal cells with centrally placed nu- clei and protoplasm containing coarse acidophile (eosinophile) granules (chromophile cells). While presenting different appearances and usu- ally described as two kinds of cells, it is probable that chromophile cells and chief cells represent merely different functional conditions of the same cell. Some alveoli in the posterior portion of the lobe fre- quently contain a colloid substance similar to that found in the thyreoid. As in all ductless glands, the blood supply is rich and the rela- tions of capillaries to gland cells are extremely intimate, dense net- works of capillaries surrounding the alveoli on all sides. 490 THE ORGANS. The Posterior Lobe. — This, like the anterior, is surrounded by a connective-tissue capsule which sends trabeculse into its substance. In the human adult the lobe consists mainly of neuroglia with a few scattered cells, which probably represent rudimentary ganglion cells. In the human embryo, and in many adult lower animals, the nervous elements are much more prominent and more definitely arranged. Thus Berkley describes the posterior lobe of the pituitary body of the dog as consisting of three distinct zones: (i) An outer zone of three or four layers of cells resembling ependymal cells, connective-tissue septa from the capsule separating the cells into irregular groups; (2) A middle zone of glandular epithelium, some of the cells of which are arranged as rather indefinite alveoli which may contain colloid. This is termed the pars intermedia. The epithelial cells are derived from the oral invagination. (3) An inner layer of nerve cells and neuroglia cells. These react to the Golgi stain, the nerve cells having axones and dendrites. Most of the axones appeared to pass in the direction of the infundibulum, but could not be traced into the latter. The posterior lobe is of ectodermic origin, developing as a diverticulum from the floor of the third ventricle. The remains of the diverticulum con- stitute the infundibulum. The Pineal Body. The pineal body originates as a fold of the wall of the primary brain vesicle. It lies upon the dorsal surface of the inter- and mid-brain, ?jeing connected with the former. The pineal body is ap- parently of the nature of a rudimentary sense organ, being some- times referred to as the median or pineal eye. In man it is surrounded by a firm connective-tissue capsule, which is a continuation of the pia mater. This sends trabeculse into the organ, which anastomose and divide it into many small chambers. The latter contain tubules or alveoli lined with cuboidal epithelium. This may be simple or strati- fied, and frequently almost completely fills the tubules. Within the tubules are often found calcareous deposits known as "brain sand." TECHNIC. The general structure of the pituitary body and of the ])ineal body can be studied by fixing material in formalin-Muller's fluid (technic 5, p. 7) and staining sections with ha;matoxyIin-eosin ("technic t, p. t8). •r- ■^ nx OXl 0) (ij 2 y. P X o o 0 o o >> ji; p x • ni 1§ o ^^^ O^ 'm O "i O -^^3 S O O di v.> ■ — I tj ..H O ^ w /v^ 3 rt 3 o 3 M 3 M^ ■2S. .!2 a ""ago £"■■ ■£ 3 u " E 3'3' (U nl be OJ CD dj -* D. u o 3 0 n1 3 3 (1) & 0 3 b ■^ 3 a3'5 4) E " 3 3.3 iJ E 3-S ID p - 3 "- :2 oEy o > v; . 3 C3 4J3:: u^ 2 ^ 23 M a p p 3 3 rt 2 O >3 ~ ~ ~ no •3 a.-- u c g-3 ^ " 5 0) o oj aE" c 3 ^C 3 O a> c a (<; c a a.™ g , S H :S o o e cs oi C t4 VH d) fn ■K ^ 0) r CS o OJ g 1 43 rt +-> ^ O OJ O w 3 3 •Z fr^ 'Z O oi E o c o'C E => E-3 cS '^ O •a o SO " o C l_ (1) ^ Ah . gSnJ 3.0 8x ^-S 3 a^ o O rt Em 1-0) C u c^ ^^«° la's o.3'S E_gS o S 0) ^S +-■ 3 ^2 i.l 3 J- Is . 8^ si's E o-s"^ c B 3+e S 2 rt // THE NERVOUS SYSTEM. 493 General References for Further Study. Bailey and Miller: A Text-book of Embryology, Xew York, 1909. Chaps. XVII and XVIII. Barker: The X'ervous System and its Constituent Neurones, New York, 1899. Dejerine: Anatomie des centres nerveux, Paris, 1895. Edinger, L.: Vorlesungen iiber den Bau der nervosen Zentralorgane des Menschen und der Tiere, Leipsig, 1904. Van Gehuchten: Anatomie du systeme nerveux de I'homme, Louvaine, 1906. Golgi: Untersuchungen iiber den feineren Bau des centralen und peripher- ischen Nervensystems, Jena, 1894. Johnston, J. B.: The Nervous System of Vertebrates, 1906. Kolliker: Handbuch der Gewebelehre des Menschen, Leipsic, 1896. Von Lenhossek: Der feinere Bau des Nervensystems im Lichte neuester For- schungen, Berlin, 1895. Marburg: Atlas des menschlichen Centralnervensystems, Leipzig and Wien, 1910. Meyer, Adolf: Critical Review of the Data and General Methods and De- ductions of Modern Neurology. Joiini. of Comp. Neurol., Vol. VIII. X'os. 3 and 4, 1898. Obersteiner: Anleitung beim Studieren des Baues der nen-osen Centralorgane, Leipsic. Quain's Elements of Anatomy, Vol. Ill, Neurology, Parts i and 2, 1908. Ramon yCajal: Beitrag zum Studium der Medulla Oblongata, etc., Leipsic, 1896.^ — Les nouvelles idees sur la structure du systeme nerveux chez I'homme et chez les vertebres, Paris, 1894. — Textura del sistema nervioso del hombre v de los vertebrados, Madrid 1899-1904 (contains many illuminating figures). — Studien iiber die Hirnrinde des Menschen, Leipsig, 1900. Spalteholz, W. : Handatlas of Human Anatomy (trans, by L. F. Barker) Vol. Ill, 1903. CHAPTER XII. THE ORGANS OF SPECIAL SENSE. The Organ of Vision. The eyeball and optic nerve constitute the organ of vision. To be described in connection with them are the eyelid and the lacrymal apparatus. The Eyeball or Bulbus Oculi. — This is almost spherical, although slightly flattened antero-posteriorly. It consists of a wall enclosing a cavity filled with fluid. The wall of the eyeball consists of three coats: (a) An external fibrous coat — the sclera and cornea; (b) a middle vascular — the cho- rioid; and (c) an internal nervous — the retina (Fig. 328). The Sclera (Figs. 328 and 329). — This consists of dense fibrous tissue with some elastic fibres. The fibres run both meridionally and equatorially, the tendons of the straight muscles of the eyeball being continuous with the meridional fibres, those of the obUque muscles with the equatorial fibres. The few cells of the sclera lie n distinct, very irregular cell spaces, and frequently contain pigment granules. Pigmented cells in considerable numbers are regularly present near the corneal junction, at the entrance of the optic nerve, and on the inner surface of the sclera. Where the optic nerve pierces the sclera, the continuity of the latter is broken by the entering nerve fibres, forming the lamina cribosa (Fig. 337). The pigmented layer of the sclera next the chorioid is known as the lamina fusca, and is lined internally by a single layer of flat non-pigmented endothelium. Anteriorly a loose connective tissue attaches the sclera to the scleral conjunctiva. The Cornea (Figs. 330 and Zo2>)- — This is the anterior continua- tion of the sclera so modified as readily to allow the light to pass through it. It is about i mm. thick and consists of five layers, which from before backward are as follows (Fig. 330) : ii) Anterior epithelium. {2) Anterior elastic membrane or memljrane of Bowman. (3) Substantia propria corneas. 494 THE ORGANS OF SPECIAL SENSE. 495 (4) Posterior elastic membrane or membrane of Descemet. (5) Posterior endothelium or endothelium of Descem.et. (i).The anterior epithelium (Fig. 330J, i) is of the stratified squa- mous type and consists of from four to eight layers of cells. The deepest cells are columnar and rest upon the anterior elastic mem- brane. The middle cells are polygonal and are connected by short intercellular bridges. The surface cells are fiat. Along the margin Fig. 328. — Diagram of Eyeball showing Coats. (Merkel-Henle.) a. Sclera; b, chorioid; c, retina; d, cornea; e, lens;/, iris; g, conjunctiva; h, ciliary body; /, sclero-corneal junction and canal of Schlemm; j, fovea centralis; tz, optic nerve. of the cornea the epithelium is continuous with that of the conjunc- tiva (Fig. zZo)- (2) The anterior elastie membrane (Fig. 330, 2) is a highly de\elopcd basement membrane, its anterior surface l^eing pitted to receive the bases of the deepest epithelial cells. It is apparently homo- geneous, and while called an elastic membrane, does not conform chemically to either fibrous or elastic tissue. By means of special technic, a fibrillar structure has l)cen demonstrated. 496 THE ORGANS. (3) The subslaniia propria (Fig. 330, 3) constitutes the main bulk of the cornea. It consists of connective tissue the fibrils of which are doubly refracting and are cemented together to form bundles and lamellae. In the human cornea the lamellae are about sixty in number. The lamellae are parallel to one another and to the sur- face of the cornea, but the fibres of adjacent lamellae cross one another at an angle of about twelve degrees. The lamellae are united by cement substance. Fibres running obliquely through the lamellae from posterior to anterior elastic membranes hold the lamellae firmly together. They are known as perforating or arcuate fibres. ^iAraMj.^v>YtoaHt,ig-iy.fc^:^iF^^^ c Fig. 329. — Vertical Section through Sclera, Chorioid, and Pigment Layer of Retina. (Merkel-Henle.) A, Sclera; B, chorioid; C, pigment layer of retina; d, lamina supra- chorioidea; e, Haller's layer of straight vessels;/, choriocapillaris; g, vitreous membrane. Between the lamellae are irregular flat cell spaces which commu- nicate with one another and with the lymph spaces at the margin of the cornea by means of canalicuh. Seen in sections vertical to the surface of the cornea, these spaces appear fusiform. In the spaces are the connective-tissue cells of the cornea or corneal corpuscles. These are fiat cells corresponding in shape to the spaces and sending out processes into the canaliculi (Figs. 331 and 332). (4) The posterior elastic membrane or membrane of Descemet (Fig. 330, 4) resembles the anterior, but is much thinner. Like the anterior, it docs not give the chemical reaction of elastic tissue. (5) The posterior endothelium or endothelium of Descemet (Fig. 330, 5j consists of a single layer of flat hexagonal cells, the nuclei of which frequently project slightly above the surface. The cornea contains no blood-vessels. The Chorioid. — This is made up of four layers which from without inward are as follows (Fig. 329J : THE ORGANS OF SPECIAL SENSE. 497 (i) The lamina suprachorioidea. (2) The layer of straight vessels — Haller's layer. (3) The capillary layer — choriocapillaris. (4) The vitreous membrane — lamina citrea — membrane of Bruch. (i) The lamina suprachorioidea (Fig. 329, d) is intimately con- nected with the lamina fusca of the sclera and consists of loosely arranged bundles of fibrous and elastic tissue among which are scat- tered pigmented and non-pigmented connective-tissue cells. Numerous lymph spaces are found between the bundles of connective tissue and be- tween the lamina suprachorioidea and lamina fusca. The latter are known as the perichorioidal lymph spaces (Fig- ZZZ)- (2) The layer of straight vessels (Fig. 329, e) consists of tibro-elastic tissue containing numerous pigmented and non-pigmented cells, supporting the large blood-vessels of the layer. The latter can be seen with the naked eye, and, running parallel straight courses, give to the layer a striated appearance. The arteries lie to the inner side. The veins which are larger than the arteries converge toward four points — vencB vorticos(2 — one in each quadrant of the eyeball. A narrow boundary zone, rich in elastic fibres and free from pigment, limits this layer internally. It is much more highly developed in some of the lower animals than in man. Formed of connective- tissue bundles in ruminants and horses, it is known as the tape turn fibrosum, while in the carnivora its structure — several layers of fiat cells — gives it the name of the ta petit m cellulosum. (3) The choriocapillaris (Fig. 329, /) consists of connective tissue supporting a dense network of capillaries, which is most dense in the region of the macula lutca. This layer is usually described as free from pigment, although it not infrequently contains some |)iu;mentcd cells. Fig. 330. — ^\'ertical Section of Cornea. (Merkel-Henle.) i, Anterior epi- thelium; 2, anterior elastic mem- brane; 3, substantia propria corner; 4, posterior elastic membrane; 5, posterior endothelium. 498 THE ORGANS. (4) The vitreous membrane (Fig. 329, g) is a clear, apparently struc- tureless membrane about two microns thick. Its outer surface is grooved by the capillaries of the choriocapillaris, while its inner surface is pitted by the retinal epithelium. ' ^ >*i' ,-?.•' V.^ -^if-f- ■^ M: 12^ Fig. 331. — Section of Human Cornea cut Tangential to Surface— X350 (technic 9, p. 81) — showing corneal cell spaces (lacunae) and anastomosing canaliculi. The Ciliary Body. — This is the anterior extension of the chorioid and consists of the ciliary processes and the ciliary muscle (Fig. 333). It extends from the ora serrata (a wavy edge which marks the anterior ■/ - •*^ *>>j?^'''"*"'' *' "■' / ?' .•'<, f-t: '"''Zl.x^ Fig. 332. — Section of Human Cornea cut Tangential to Surface — X350 (technic 8, p. 81) — shcjwing corneal ceils and their anastomosing processes. limit of the nervous elements of the retina — see Retina) to the margin of the iris (see below). The ciliary processes (Fig. 2)2)2)) y f^^-*"^ seventy to eighty in number, THE ORGAXS OF SPECIAL SEXSE. 499 are meridionally-running folds of the chorioid from which are given off numerous irregular secondary folds. The processes begin low at the ora serrata, gradually increase in height to about i mm., and end abruptly at the margin of the iris. The ciliary processes consist of connective tissue containing many pigmented cells and supporting numerous blood-vessels. Invaginations lined with clear columnar epithelium have been described as ciliary glands. The ciliary folds f i III! I|i 'ill! P ''■ Anterio? chamber — S, . /';'-, Canal of Schlemm — i^ Spaces of Fontana Conjunctiva Iris Pars iridica retiose ^^ Ciliary process ^^jV; Ligamentum ; ,^, . ~r^ pectinatum iridU ' iSS%-^-^! Circular fibres of ciliary muscle Radial fibres of ciliary muscle Pars ciliaris retinas Pericliorioidal lymph space. Fig. 3^^. — Vertical Section through Human Sclero-corneal Junction. (Cunningham.) are covered by the vitreous membrane, and internal to the latter is a continuation forward of non-nervous elements of the retina — pars ciliaris retina (Fig. ^2>2>)- This consists of two layers of columnar epithelial cells, the outer layer being pigmented, the inner non- pigmented. The ciliary muscle (Fig. t^t^t^) is a band of smooth muscle which encircles the iris. It lies in the outer anterior part of the ciliary body, and on cross section has a generally triangular shape. It is divisible into three groups of muscle cells: {a) An inner circular group near the base of the iris — circular muscle of Midler; [b) an outer meridional 500 THE ORGANS. ^^S53 group lying next to the sclera and known as the tensor chorioideae, and (c) a middle radial group. The meridional and radial groups both take origin in the posterior elastic lamina of the cornea, the former passing backward along the margin of the sclera to its insertion in the ciliary body near the ora serrata, the latter radiating fan-like to a broad insertion in the ciliary body and processes. The ciliary body is closely attached to the sclero-corneal junction by the ligamentum pectinatum (Fig. 333), a continuation of the posterior elastic lamina of the cornea. Within the ligament are spaces {spaces of Fontana) lined with endothelium. These are apparently lymph spaces, and communicate with each other, with similar spaces around the canal of Schlemm, and with the anterior chamber. The canal of Schlemm (Fig. 333) is a venous canal which encircles the cornea, lying in the sclera close to the corneal margin. Instead of a single canal there may be several canals. The Iris (Fig. 334). — This rep- resents a further continuation for- ward of the chorioid. Its base is attached to the ciliary body and ligamentum pectinatum. From this point it extends forward as a dia- phragm in front of the lens, its centre being perforated to form the pupillary opening. It is deeply pig- mented, and to its pigment the color of the eye is due. Four layers may be distinguished, which from before backward are as follows : fi) The anterior endothelium. (2) The stroma. (3) The vitreous membrane. (4) The pigmented ei^ithelium.. ({) The anterior endothelium is a single layer of pigmented cells continuous with the posterior endothelium of the cornea (Fig. 334, a). (2) The stroma is divisible into two layers: an anterior reticular layer, containing many cells, some of which are ])igmented, and a vascular layer, the vessels of which are ])eculiar in that their walls con- FlG. 334. — Vertical Section through Iris. (Merkel-Henle.) a, Anterior endothelium; b, stroma or substantia propria; c, vitreous membrane; d, pigment layer; v, blood-vessel. THE ORGANS OF SPECIAL SENSE. 501 tain almost no muscle, but have thick connective-tissue sheaths. In the posterior part of the stroma are bundles of smooth muscle. Those nearest the pupillary margin encircle the pupil forming its sphincter muscle, while external are scattered radiating bundles forming the dilator muscle. (3) The vitreous membrane is continuous with, and has the same structure as the membrane of Bruch. (4) The pigmented epithelium (Fig. 334, d) consists of several layers of cells and is continuous with the pars ciliaris retinae. Except in albinos, both layers are pigmented. The Retina. — The retina is the nervous tunic of the eye. It lines the entire eyeball, ending only at the pupillary margin of the iris. Its nervous elements, however, extend only to the or a serrata, which marks the outer limit of the ciliary body (Fig. ^,^5). The nervous part of the retina is known as the pars optica retina, the non-nervous extension over the ciliary processes as the pars ciliaris retina, its further continuation o^•er the iris as the pars iridica retina. Modifications of the optic portion of the retina are found in the region of the macula lutea and of the optic nerve entrance. The Pars Optica Retina. — This is the only part of the retina directly concerned in the reception of impulses, and may be regarded as the extremely complex sensory end-organ of the optic nerve. It is divisible into ten layers, which from without inward are as follows (Fig- roS) ■ (i) Layer of pigmented epithelium. (2) Layer of rods and cones. Layer of neuro-epithelium. Ganglionic lavcr. The layer of pigmented epithelium (Fig. 335, B, i) consists of a single layer of regular hexagonal cells (Fig. 25, p. 69). The nuclei lie in the outer part of the cell, while from the inner side thread-like projections extend down lu'twecn the rods and cones of the layer next internal. The pigment has the form of rod-shaped granules. Its (3) Outer limiting membrane. (4) Outer nuclear layer. (5) Outer molecular layer. (6) Inner nuclear layer. (7) Inner molecular layer. (8) Layer of nerve cells. (9) Layer of nerve fibres. (10) Inner limiting membrane. 502 THE ORGANS. distribution seems to depend upon the amount of light being admitted to the retina. When Httle or no Hght is being admitted, the pigment is found in the body of the cell, the processes being wholly or almost free from pigment; when the retina is exposed to a bright light, some of the pigment granules pass down into the processes so that the pig- ment becomes more evenly distributed throughout the cell. Fig. 335. — A, Scheme of retina as shown by the Golgi method. B, Vertical section of retina to show layers as demonstrated by the hjematoxylin-eosin stain. (Merkel-Henle.) B. — I, Layer of pigmented epithelium; 2, layer of rods and cones; 3, outer limiting layer; 4, outer nuclear layer; 5, outer molecular layer; 6, inner nuclear layer; 7, inner molecular layer; 8, layer of nerve cells; 9, layer of nerve fibres; 10, inner limiting layer. A. — i, Pigment layer; 2, processes of pigmented epithelial cells extending down between rods and cones; 3, rods; 4, rod-cell nuclei and rod fibres; 5, cones; 6, cone fibres; 7, bipolar cells of inner nuclear layer; 8, ganglion cells of nerve-cell layer; 9, larger ganglion cells of nerve- cell layer; ID, fibres of optic nerve forming layer of nerve fibres; 11 and 12, types of horizon- tal cells; 13, 14, 15, and 16, types of cells the bodies of which lie in the inner nuclear layer; 17, efferent optic-nerve fibre ending around cell of inner nuclear layer; iS, neuroglia cells; 19, Miiller's fiVjre; 20, rf)f]-bipf)lar cell of inner nuclear layer. The layer of rods and cones and the outer nuclear layer (Fig. 335, B, 2, 4) are best considered as subdivisions of a single layer, the neuro- epithelial layer. This consists essentially of two forms of neuro- epithelial elements, rod visual cells and cone visual cells. These, with supporting connective tissue, constitute the layer of rods and cones and the outer nuclear layer, the separation into sub-layers being due THE ORGAXS OF SPECIAL SENSE. 503 to the sharp demarcation between the nucleated and non-nucleated parts of the cells, and the separation of the two parts by the perforated outer limiting membrane. The rod visual cell (Fig. 335, A, 4) consists of rod, rod-fibre, and nucleus. The rod (Fig. 335, A, 3) is a cylinder from 30 to 40/4 in length and about 2« in diameter. It is divisible into an outer clear portion, which contains the so-called "visual purple" and an inner granular portion. At the outer end of the latter is a fibrillated ellip- soidal body, much more distinct in some of the lower animals, the ellipsoid of Krause. At its inner end the rod tapers down to a fine fibre, the rod fibre, which passes through a perforation in the outer limiting membrane into the outer nuclear layer, where it expands and contains the nucleus of the rod visual cell. These nuclei are situated at various levels in the fibre and constitute the most conspicuous element of the outer nuclear layer (Fig. t^^^,^, B, 4). The cone visual cell (Fig. 335, A, 5, 6) consists of cone, cone-fibre, and nucleus. The cone (Fig. t^t,^, A, 5) is shorter and broader than the rod, and like the latter is divisible into two parts. The outer part is short, clear, and tapering, the inner part broad and granular, and like the rod contains a fibrillated ellipsoid body. The cone fibre (Fig. ;^^^, A, 6) is much broader than the rod fibre, passes completely through the outer nuclear layer and ends in an expansion at the margin of the outer molecular layer. The nucleus of the cone cell usually lies just beneath the outer limiting membrane. The remaining layers of the retina must be considered in relation on the one hand to the neuro-epithelium, on the other to the optic nerve. The inner nuclear layer (Fig. 335, B, 6) and the layer of nerve cells (Fig. 335, B, 8) are composed largely of nerve-cell bodies, while the lico molecular layers (Fig. 335, B, 5, 7) are formed mainly of the rami- fications of the processes of these cells. In the inner nuclear layer are two kinds of nerve elements, rod bipolar cells and cone bipolar cells. The bodies of these cells with their large nuclei form the bulk of this layer. From the rod bipolars (Fig. 335, A, 20) processes (dendrites) pass outward to ramify in the outer molecular layer around the termi- nations of the rod fibres. From the cone bipolars (Fig. 335, .4, 7) similar processes (dendrites) extend into the outer molecular layer where they ramify around the terminations of the cone cells. Two other forms of nerve cells occur in the inner nuclear layer. One is known as the horizontal cell (Fig. 335, A, 12). Its processes ramify almost whollv in the outer molecular laver. The other lies alonsf the 504 THE ORGANS. inner margin of the inner nuclear layer and sends its dendrites into the inner molecular layer (Fig. 335, A, 13, 14, 15, and 16). Many of these latter cells appear to have no axone and are consequently known as amacrine cells. The outer molecular layer is thus seen to be formed mainly of terminations of the rod and cone visual cells, of the dendrites of the rod and cone bipolars, and of the processes of the horizonal cells. From the cone bipolar a process {axone) extends inward to ramify in the inner molecular layer, while from the rod bipolar a process {axone) passes inward through the inner molecular layer to terminate around the cells of the nerve-cell layer. The layer of nerve cells (Fig. 335, B, 8) consists for the most part of large ganglion cells whose dendrites ramify in the inner molecular layer, and whose axones pass into the layer of nerve fibres and thence into the optic nerve. Some small ganglion cells are also found in this layer, especially in the region of the macula lutea (see page 505). The inner molecular layer is thus seen to be composed mainly of the processes {axones) of the rod and cone bipolars and of the den- drites of the ganglion cells of the nerve-cell layer. The layer of nerve fibres (Fig. 335,5, g) consists mainly of the axones of the just-described ganglion cells, although a few centrifugal axones of brain cells (Fig. 335, A, ly) are probably intermingled. The outer and inner limiting layers or membranes (Fig. 335, B, 3, 10) are parts of the sustentacular apparatus of the retina, being connected with the cells ot fibres of Miiller (Fig. 335, A, 19 and Fig. 336). The latter form the most conspicuous elements of the supportive tissue of the retina. They are like the nerve elements proper, of ectodermic origin and are elongated cells which extend through all the retinal layers, excepting the layer of rods and cones and the pigment layer. The inner ends of th-e cells, which are conical and fibrillated, unite to form the inner limiting membrane (Fig. 336, 10). Through the inner molecular layer the cell takes the form of a narrow stalk with numerous fringe-like side fibrils (Fig 336, 7). This widens in the inner nuclear layer, where cup-like depressions in the sides of the Miiller's cell are caused by the pressure of the surrounding nerve cells (Fig. 336, b). This wide portion of the cell in the inner nuclear layer contains the nucleus (Fig. 336, a). In the outer molecular layer the cell again becomes narrow (Fig. 336, 5) and in the outer nuclear layer broadens out into a sponge-like reticulum (Fig. 336, 4), which supports the rod and cone bipolars. At the inner margin of the layer of rods and THE ORGANS OF SPECIAL SENSE. 505 cones the protoplasm of the Muller's cells spreads out and unites to form the so-called outer limiting membrane (Fig. 336, 3), from which delicate fibrils {fibre baskets) pass outward between the rods and cones. In addition to the Muller's cells, which are neuroglia ele- ments, spider cells also occur in the retina (Fig. 2>C)S^ -4, 18). The retina of the maeiila lutea presents certain peculiarities. Its name is derived from the yellow pigment which is dis- tributed diffusely through the inner layers, extending as far out as the outer molecular layer. The ganglion-cell layer and the inner nuclear layer are thicker than in other parts of the retina . In the layer of rods and cones there is a gradual reduction in the number of rods, while the number of cones is correspondingly increased. In the centre of the macula is a de- pression, \\\t fovea centralis. As the retina approaches this area it becomes greatly thinned, little remaining but the layer of cone cells and the somewhat thickened layer of pigmented epithelium. At the ora serrata the nervous elements of the retina cease. The non-nervous re tinal extension over the ciliary body {pars ciliaris retince) and over the iris {pars iridica retmce) have been described in connection with the ciliary body and iris. The Optic Nerve. — The optic nerve (Fig. 337, d) is enclosed by two connective- tissue sheaths, both of which are extensions of the brain membranes. The outer dural sheath (Fig. 337, a) is continuous with the dura mater of the brain posteriorly, while anteriorly it blends with the sclera. The inner pial sheath (Fig. 337, &) is an extension of the pia mater and is separated from the outer sheath by the subdural space (Fig. 337, c). The pial sheath is divisible into two sub-layers: an outer fibrous layer (the so-called arachnoid), and an inner vascular layer. These two layers are separated by a narrow space, the sitbaracJinoid space. The o|)tic nerve fibres, in passing through the sclera and cho- FiG. 336. — Two Muller's Fibres from Retina of Ox showing Relation to Layers of Retina. (Ramon y Cajal.) 3, Outer limiting layer; 4, outer nu- clear layer; 5, outer molecular layer; 6, inner nuclear layer; 7, inner molecular layer; 8, layer of nerve cells; 9, layer of nerve fibres; 10, inner limiting layer; a, nucleus; b, cup-like depression caused by pressure from surrounding cells. 506 THE ORGANS. f a rioid, separate the connective-tissue bundles so that they form a lattice- work, the already mentioned lamina crihrosa (Fig. 337, h). The optic nerve fibres are medullated, but have no neurilemma. Ks they pass through the lamina cribrosa the medullary sheaths are lost, the fibres reaching the retina as naked axones. Relations of Optic Nerve to Retina and Brain. The rod and cone visual cells are the neuro-epi- thelial beginnings of the visual tract (Fig. 335, A, 3, 4, 5, and 6). By their expanded bases in the outer molecular layer, the rod and cone cells communicate with the neurone system No. I. of the optic tract. This comprises {a) rod neurones, (&) cone neurones, (c) horizontal neurones. Neurone System No. I. — {a) Rod neurones. The cell bodies of these neurones (Fig. 335, A, 20) lie in the inner nuclear layer. Their dendrites enter the outer molecular layer where they form networks around the expanded bases of the rod cells. Their axones pass through the inner molecular layer and end in arborizations around the bodies and dendrites of cells of the nerve-cell layer (neurone system No. 11.) . (b) Cone neurones (Fig. 335, A, 7). These have their cell bodies in the inner nuclear layer. Their dendrites ])ass to the outer molecular layer where they form networks around the expanded bases of the cone cells. Their axones pass only into the inner molecular layer where they end in arborizations around the dendrites of neurones whose cell bodies are in the layer of nerve cells (neurone system No. 11.) . (c) Horizontal neurones (Fig. 335, ^4, 11 and 12. These serve as associa- FiG. 337. — Section through Entrance of Optic Nerve into Eyeball. (Merkel-Henle.) a, Dural sheath; h, pial sheath, inner and outer layers; c, space between inner and outer layers of pia mater; d, optic nerve; e, central artery of retina; a', sclera; /, chorioid; g, retina; h, lamina cribrosa. THE ORGANS OF SPECIAL SENSE. 507 tion neurones between the visual cells and may be divided into rod association neurones and cone association neurones. The cone associa- tion neurones are the smaller and more superficial and both dendrites and axones end in the outer molecular layer around the terminal expan- sions of the cone visual cells (Fig. 335, A, iij. The rod association neurones are larger, more deeply seated, and behave in a similar manner toward the rod visual cells (Fig. 335, A, 12). Some of these cells send processes to the inner molecular layer. Neurone System No. II. — This has been already partly de- scribed in connection with the axone terminations of neurone system No. I. The cell bodies of the second neurone system (Fig. 335, A, 8, 9j are in the layer of nerve cells and are, as above noted, associated either directly or by means of their dendrites with the axones of the first neurone system. Their axones pass into the layer of nerve fibres and ultimately become fibres of the optic nerve (Fig. 335, .4, 10). The optic nerves (Fig. 338, No) unite at the base of the brain to form the optic decussation or chiasma (Fig. 338, CM). Here the axones from the mesial part of the retina cross to the optic tract of the opposite side, while those of the lateral part of the retina remain in the optic tract of the same side. The axones of the optic tract (Fig. 338, Tro) terminate in the thalamus, in the lateral geniculate body, and in the anterior corpus quadrigeminum (Fig. 338). Neurone System No. III. — The neurones of this system have their cell bodies in the thalamus, lateral geniculate body, and anterior corpus cpiadrigeminum (Fig. 338). The axones of the two former terminate in the cortical visual centers in the occipital lobe (Fig. ^^^, Fig. 338. — Diagram showing Main Rela- tions of Optic Tract. (Testut.) R, Re- tina; Xo, optic nerve; C.V, optic decussa- tion or chiasma; Tro, optic tract; Tho, thalamus; Cgl, lateral geniculate body; Qa, anterior corpus quadrigeminum; Rd, fibre of optic tract passing directly to cortex; Sm, third neurone system of optic tract (excepting Rd) connecting thalamus, lateral geniculate body, and anterior corpus quadrigeminum with the cortex, Co. 508 THE ORGANS. Coi-te '■\': ,''","'"V--";--../harynx by means of the Eustachian tube. Its walls arc formed by the surrounding bony structures covered by periosteum. It is lined with mucous membrane and contains the ear ossicles and their ligamentous and muscular attachments. The epithelium is of the simple low cuboidal type. In places it may be ciliated and not infre- quently assumes a pseudostratified character with two layers of nuclei. Beneath the epithelium is a thin stroma which contains some ditluse lymphoid tissue and blends with the dense underlying ])cri()stcum. Small tubular glands are usually present, es])ecially near the ojiening of the Eustachian tul)e. The fenestra rotunda is covered l)y the secondary tympanic irem- brane. This consists of a central lamina of connecti\"e tissue coxcred 520 THE ORGANS. on its tympanic side by part of the mucous membrane of that cham- ber, on the opposite side by a single layer of endothelium. The ossicles are composed of bone tissue arranged in the usual systems of lamellae. The stapes alone contains a marrow cavity. Over their articular surfaces the ossicles are covered by hyaline cartilage. The Eustachian Tube. — This is a partly bony, partly cartilaginous canal lined with mucous membrane. The epithelium of the latter is of the stratified columnar ciliated variety consisting of two layers of cells. In the bony portion of the tube the stroma is small in amount and intimately connected with the periosteum. In the cartilaginous portion the stroma is thicker and contains, especially near the pharyn- geal opening, lymphoid tissue and simple tubular mucous glands. Ampunac The Internal Ear. The internal ear consists of a complex series of connected bony walled chambers and passages containing a similar-shaped series of membranous sacs and tubules. These are known, respectively, as the osseous labyrinth and the membranous labyrinth. Between the two is a lymph space, which contains the so-called perilymph, while within the membranous labyrinth is a similar fluid, the endolymph. The bony labyrinth consists of a central chamber, the vesti- bule, from which are given off the three semicircular canals and the cochlea. The vestibule is separated from the middle ear by a plate of bone in which are two openings, the fenestra ovalis and the fenestra rotunda. Just after leaving the vestibule each canal presents a dilatation, the ampulla. As each canal has a return opening into the vestibule and as the anterior and posterior canals have a common return opening (the canalis communis), there are five openings from the vestibule into the semicircular canals (Fig. 347). The bony labyrinth is lined with periosteum, covered by a single layer of endothelial cells. The Vestibule and the Semicircular Canals. — In the vestibule the membranous labyrinth is subdivided into two chambers, the sac- Amjjidla Fig. 347. — The Bony Labyrinth mann.) X3. (Heitz- THE ORGANS OF SPECIAL SENSE. 521 cule and -the utricle, which are connected by the iitriculo-sacciilar duct. From the latter is given off the endolymphatic c/z^c/ which communicates, through the aqueduct of the vestibule, with a subdural lymph space, the endolymphatic sac. The saccule opens by means of the ductus reuniens into the cochlea, while the utricle opens into the ampullae of the semicircular canals. The saccule and utricle only partly fill the vestibule, the remaining space, crossed by fibrous bands and lined with endothelium, constituting the perilymphatic space. Fig. 34S. — Diagram of the Perilymphatic and Endolymphatic Spaces of the Inner Ear. (Testut.) Endolymphatic spaces in gray; perilymphatic spaces in black, i, Utricle; 2, sac- cule; 3, semicircular canals; 4, cochlear canal; 5, endolymphatic duct; h, subdural endo- lymphatic sac; 7, canalis reuniens; 8, scala tympani; g, scala vestibuli; 10, their union at the helicotrema; 11, aqueduct of the vestibule; 12, aqueduct of the cochlea; 13, perios- teum; 14, dura mater; 15, stapes in fenestra ovalis; 16, fenestra rotunda and secondary tympanic membrane. Saccule and Utricle. — The walls of the saccule and of the utricle consist of fine fibro-elastic tissue supporting a thin basement membrane, upon which rests a single layer of low epithelial cells. In the wall of each chamber is an area of special nerve distribution, the macula acustica. Here the epithelium changes to high columnar and consists of two kinds of cells, sustentacular and neuro-epithelial. The sustentac- ular cells are long, irregular, nucleated cylinders, narrow in the middle, widened at each end, the outer end being frecpiently split and resting upon the l^asement membrane. The ncuro-cpitJiclial cells or "hair cells" arc short cvlindcrs which extend onlv about halfwav ihrouEfh .522 THE ORGANS. the epithelium. The basal end of the cell is the larger and contains the oval nucleus. The surface of the cell is provided with a cuticular margin from which project several long hair-like processes, the auditory hairs. Small crystals of calcium carbonate are found on the surfaces of the hair cells. These are known as otoliths and are embedded in a soft substance, the otolithic membrane. The hair cells are the neuro- epithelial end-organs of the vestibular division of the auditory nerve and are, therefore, closely associated with the nerve fibres. The latter Fig. 34Q. — Diagram of the Right Membranous Labyrinth. (Testut.) i, Utricle; 2 superior semicircular canal; 3, posterior semicircular canal; 4, external semicircular canal; 5, saccule; 6, endolymphatic duct; 7 and 7', canals connecting utricle and saccule respectively with the endolymphatic duct; 8, endolymphatic sac; 9, cochlear duct; 9', its vestibular cul-de-sac; 9", its terminal cul-de-sac; 10, canalis reuniens. on piercing the basement membrane lose their medullary sheaths and split up into several small branches, which form a horizontal plexus between the basement membrane and the bases of the hair cells. From this plexus are given off fibrils which end freely l^etween the hair cells. Semicircular Canals. — The walls of the semicircular canals are similar in structure to the walls of the saccule and utricle; they also bear a similar relation to the walls of the bony canal. Along the concavity of each canal the e]Mthelium is somewhat higher, forming the raphe. In each ampulla is a crista acustica, the structure of which is similar to that of the maculae of the saccule and utricle. With the adjoining high columnar cells, this forms the so-called semilunar fold. As in the case of the maculae the hair cells have otoliths upon their surfaces, the otolithic membrane here forming a sort of dome over the hair cells known as the cupula. The Cochlea. The bony cochlea consists of a conical axis, the modiolus, around which winds a spiral Ijony canal. This canal in THE ORGANS OF SPECIAL SENSE. .523 man makes about two and one-half turns, ending at the rounded tip of the cochlea or cupola. Projecting from the modiolus partly across the bony canal of the cochlea is a plate of bone, the bony spiral Uuuiiia (Fig. 351, x). This follows the spiral turns of the cochlea, ending at the cupola in a hook-shaped process, the hamulus. Along the outer side of the canal, opposite the bony spiral lamina, is a projection of thickened periosteum, the spiral ligament (Fig. 351, h). A connective- tissue membrane, the membranous spiral lamina (Fig. 351, 5), crosses Fig. 350. — The Membranous Labyrinth from the Right Internal Ear of a Human Embryo at the Fifth Month; seen from the Medial Side. (After Retzius, from Barker.) 1-5, Utricle; 2, utricular recess; 3, macula acustica of utricle; 4, posterior sinus; 5, superior sinus; 6, 7, 8, superior, lateral, and posterior ampullfe; g, 10, 11, superior, posterior, and lateral semicircular canals; 12, widened mouth of lateral semicircular canal opening into the utricle; 13, saccule; 14, macula acustica of the saccule; 15, endolymphatic duct; 16, utriculo- saccular duct; 17, ductus reuniens; 18, vestibular cul-de-sac of cochlear duct; ig, cochlear duct; 20, facial nerve; 21-24, auditory nerve; 21, its vestibular branch; 22, saccular branch; 23, branch to inferior ampulla; 24, cochlear branch; 25, distribution of cochlear branch within the bony spiral lamina. the space intervening between the spiral ligament and the bony spiral lamina, thus completely dividing the bony canal of the cochlea into two parts, an up])er, scala vestibuli (Fig. 351, /) and a lower, scala tympani (Fig. 351, k). These are perilymphatic spaces, the scala vestibuli communicating with the perilymph space of the vestibule, the scala tympani communicating with the perivascular lymph spaces of the \eins of the cochlear duct. The scala vestibuli and the scala tym])ani communicate with each other in the cu])()la l)y means of a minute canal, the Jielicoirema. The Cochlear Duct {Membranous Cochlea or Scala Media). — This is a narrow, membranous tube lying near the middle of the bony cochlear canal and following its s]Mral turns from the vcstiljule. 524 THE ORGANS. where it is connected with the saccule through the canalis reuniens, to its bhnd ending in the cupola. It is triangular in shape on trans- verse section, thus allowing a division of its walls into upper, outer, and lower (Fig. 351, Dc). The upper or vestibular wall is formed by the thin membrane of Reissner (Fig. 351, h) which separates the cochlear duct from the Fig. 351. — Section through a Single Turn of the Cochlea of a Guinea-pig. (Bohm and von Davidoff.) a, Bone of cochlea; I, scala vestibuli; Dc, scala media or cochlear duct; k, scala tympani; b, membrane of Reissner; d, membrana tectoria or membrane of Corti; /, spiral prominence; g, organ of Corti; h, spiral ligament; i, basilar membrane (outer portion — zona pectinata — covered by cells of Claudius); z, stria vascularis; v, external spiral sulcus; r, crista basilaris; s, membranous spiral lamina; x, bony spiral lamina; m, spiral limbus; n, internal spiral sulcus; o, medullated peripheral processes (dendrites) of cells of spiral ganglion passing to the organ of Corti; p, spiral ganglion; q, blood-vessel. scala vestibuli. The mcml;rane consists of a thin central lamina of connecti\'e tissue covered on its \'estibular side by the vestibular en- dothelium, on its cochlear side by the epithelium of the cochlea. The outer wall of the cochlear duct is formed by the spiral liga- ment, which is a thickening of the periosteum. The outer part of the spiral ligament consists of dense fibrous tissue, its projecting part of nifjre loosely arranged tissue. From it, two folds project THE ORGANS OF SPECIAL SENSE. 525 slightly into the duct. One, the crista basilar is (Fig. 351, r), serves for the attachment of the membranous spiral lamina; the other, the spiral prominence (Fig. 351, /), contains several small veins. Be- tween the two projections is a depression, the external spiral sulcus (Fig. 351, V). That part of the spiral ligament between the spiral prominence and the attachment of Reissner's membrane is known as the stria vascularis (Fig. 351, z). It is lined with granular cuboidal epithelial cells, which, owing to the absence of a basement membrane, are not sharply separated from the underlying connective tissue. For this reason the capillaries extend somewhat between the epithe- lial cells, giving the unusual appearance of a vascular epithelium. The lower or tympanic wall of the cochlear duct has an extremely complex structure. Its base is formed by the already mentioned bony and membranous division-wall between the scala media and the scala tympani (bony spiral lamina and membranous spiral lamina). The bony spiral lamina has been described (page 523). The membranous spiral lamina consists of a substantia propria or basilar membrane, its tympanic covering, and its cochlear covering. The basilar membrane (Fig. 351) is a connective-tissue mem- brane composed of fine straight fibres which extend from the bony spiral lamina to the spiral ligament. Among the fibres are a few connective-tissue cells. On either side of the fibre layer is a thin, apparently structureless membrane. The tympanic covering of the basilar membrane consists of a thin layer of connective tissue — an extension of the periosteum of the spiral lamina — covered over by a single layer of flat endothelial cells. The cochlear covering of the basilar membrane is epithelial. Owing to the marked difference in the character of the epithelium, the basilar membrane is divided into an outer portion, the zona pectinaia (Fig. 351, /) and an inner portion, the zona tecta (Fig. 351, s). The epithelium of the former is of the ordinary columnar type; that of the latter is the highly differentiated neuro-epithelium of Corti's organ. The Organ of Corti. — The spiral organ or the organ of Corti (Fig. 351, g, and Fig. 352) is a neuro-epithelial structure running the entire length of the cochlear canal with the exception of a short distance at either end. It rests upon the membranous portion of the spiral lamina, and consists of a complex arrangement of four dift'erent kinds of epithe- lial cells. These are known as : ( i ) pillar cells, (2) hair cells, (3) Deiter's cells, and (4) Hensen's cells (Fig. 352). (i) The pillar cells arc di^•^ded into outer pillar cells and inner 526 THE ORGANS. pillar cells. They are sustentacular in character. Each cell consists of a broad curved protoplasmic base which contains the nucleus, and of a long-drawn-out shaft or pillar which probably represents a highly specialized cuticular formation. The end of the pillar away from the base is known as the head. The head of the outer pillar presents a convexity on its inner side, which fits into a corresponding concavity on the head of the inner pillar, the heads of opposite pillars thus "artic- ulating" with each other. From their articulation the pillars diverge, limhvLt memhraTUi tectoria outer hair-cells iierve Jihres inner rod vas basilar outer ceUs of Deitera iqdralc membrane rod Fig. 352. — Semidiagrammatic Representation of the Organ of Corti and Adjacent Structures (Merkel-Henle.) a. Cells of Hensen; b, cells of Claudius; c, internal spiral sulcus; X, Nuel's space. The nerve fibres (dendrites of cells of the spinal ganglion) are seen pass- ing to Corti's organ through openings (foramina nervosa) in the bony spiral lamina. The black dots represent longitudinally-running branches, one bundle lying to the inner side of the inner pillar, a second just to the outer side of the inner pillar within Corti's tunnel, the third beneath the outer hair cells. SO that their bases which rest upon the basilar membrane are widely separated. There are thus formed by the pillars a series of arches known as Corti's arches, enclosing a triangular canal, Corti's tunnel. This canal is filled with a gelatinous substance and crossed by delicate ncr\c llljrils. As the outer pillar cells are the larger, they are fewer in number, the estimated number in the human cochlea being forty- five hundred of the outer cells and about six thousand of the inner cells. (2) The hair cells or auditory cells lie on either side of the arches of Corti, and are thus divided into inner hair cells and outer hair cells. Both inner and outer hair cells are short, cylindrical elements which do not extend to the basilar membrane. Each cell ends below in u point, while from its free surface are given off a number of fine stiff hairs. THE OR(iANS OF SPECIAL SENSE. 527 The inner hair cells lie in a single layer against the inner side of the inner pillar cells, one hair cell resting upon about every two pillars. The outer hair cells lie in three or four layers to the outer side of the outer pillar cells, being separated from one another by susten- tacular cells, the cells of Deiter, so that no two hair cells come in contact. (3) Deiter' s Cells (Fig. 352). — These like the pillar cells are sus- tentacular. Their bases rest upon the basilar membrane, where they form a continuous layer. Toward the surface they become separated from one another by the hair cells. The long slender portions of the Deiter's cells, which pass in between the hair cells, are known as pha- langeal processes. Between the innermost of the outer hair cells and the outer pillar is a space known as N'ttePs space (Fig. 352, :v). (4) Hensen's Cells (Fig. 352, a). — These are sustentacular cells, which form about eight rows to the outer side of the outermost Deiter's cells. These cells form the outer crest of Corti's organ and conse- quently have a somewhat radial disposition, their free surfaces being broad, their basal ends narrow. They decrease in height from within outward, and at the end of Corti's organ become continuous with the cells of Claudius (Fig. 352, h), the name given to the cochlear epithelium covering the basal membrane to the outer side of Corti's organ. The phalangeal processes of the Deiter's cells are cemented to- gether and to the superficial parts of the outer pillars in such a man- ner as to form a sort of cuticular membrane, the lamina reticularis, through which the heads of the outer hair cells project. This mem- brane also extends out as a cuticula over the cells of Hensen and of Claudius. The Membrana Tectoria. — This is a peculiar membranous structure attached to a projection of the bony spiral lamina known as the spiral limbiis (Fig. 352), the concavity beneath its attachment being the internal spiral sulcus Fig. 352, c). The membrane is non-nucleated and shows line radial striations. It bridges over the internal spiral sulcus and ends in a thin margin, which rests upon Corti's organ just at the outer limit of the outer hair cells. Blood-vessels. — The arteries consist of two small branches of the auditory — one to the bony labyrinth, the other to the membranous labyrinth. The latter divides into two branches — a vestibular and a cochlear. The vestibular artery accompanies the branches of the audi- tory nerve to the utricle, saccule, and semicircular canals. It supplies these parts, giN'ing rise to a capillary network, which is coarse meshed except in the crista? and macula?, where the meshes are tine. The 528 THE ORGANS. cochlear artery also starts out in company with the auditory nerve, but accompanies it only to the first turn of the cochlea. Here it enters the modiolus where it gives off several much coiled branches, the glo- merular arteries of the cochlea. Branches from these pierce the vestibular part of the osseous spiral lamina and supply the various structures of the cochlear duct. The veins accompany the arteries, but reach the axis of the modiolus through foramina in the tympanic part of the bony spiral lamina. Lymphatics. — The scala media contains endolymph and is in communication with the subdural lymph spaces by means of the endo- lymphatic duct, the endolymphatic sac, and minute lymph channels connecting the latter with the subdural spaces. The perilymph spaces — scala tympani and scala vestibuli — are connected with the pial lymph spaces by means of the perilymphatic duct. Lymph spaces also surround the vessels and nerves. These empty into the pial lymphatics. Nerves. — The vestibular branch of the auditory nerve divides into branches which supply the saccule, utricle, and semicircular canals, where they end in the maculae and cristas as described on page 521. The ganglion of the vestibular branch is situated in the internal audi- tory meatus. The cochlear branch of the auditory nerve enters the axis of the modiolus, where it divides into a number of branches which pass up through its central axis. From these, numerous fibres radiate to the bony spiral laminae, in the bases of which they enter the spiral ganglia (Fig. 351, ^). The cells of the spiral ganglia are peculiar, in that while of the same general type as the spinal ganglion cell they maintain their embryonic bipolar condition (see page 376) throughout life. Their axones follow the already described course through the modiolus and thence through the internal auditory meatus to their terminal nuclei in the medulla (.see page 441). Their dendrites become medullated like the dendrites of the spinal ganglion cells and ]jass outward in bundles in the bony spiral laminae (Fig. 351, 0, and Fig. 352). From these are given off branches which enter the tympanic portion of the lamina, where they lose their medullary sheaths and ])ass through the foramina nervosa (minute canals in the tympanic part of the spiral lamina) to their terminations in the organ of Corti. In the latter the fibres run in three bundles parallel to Corti's tunnel. One bundle lies just inside the inner jjillar beneath the inner row of hair cells (Fig. 352). A second Ininfllc runs in the tunnel to the outer side of the inner pillar THE ORGANS OF SPECIAL SENSE. 529 (Fig. 352). The third bundle crosses the tunnel (tunnel-tibres) and turns at right angles to run between the cells of Deiter beneath the outer hair cells (Fig. 352). From all of these bundles of fibres are given off delicate terminals which end on the hair cells. Development of the Ear. The essential auditory part of the organ of hearing, the membranous labyrinth, is of ectodermic origin. This first appears as a thickening followed by an invagination of the surface ectoderm in the region of the posterior cerebral vesicle. This is known as the auditory pit. By closure of the lips of this pit and growth of the surrounding mesodermic tissue is formed the otic vesicle or otocyst, which is completely separated from the surface ectoderm. Diverticula soon appear passing off from the otic vesicle. These are three in number and correspond respect- ively to the future endolymphatic duct, the cochlear duct and the membranous semicircular canals. Within the saccule, utricle, and ampullae special differentiations of the lining epithelium give rise to the maculag and cristse acusticse. Of the cochlear duct the upper and lateral walls become thinned to form Reissner's membrane and the epithelium of the outer wall, while the lower wall becomes the basilar membrane, its epithelium undergoing an elaborate specialization to form the organ of Corti. Of the cochlea, only the membranous cochlear duct develops from the otic vesicle; the scala vestibuli, scala tympani, and bony cochlea developing from the surrounding mesoderm. The mesodermic con- nective tissue at first completely fills in the space between the coch- lear duct and the bony canal. Absorption of this tissue takes place, resulting in formation of the scala tympani and scala vestibuli. During the differentiation of the above parts a constriction appears in the body of the primitive otic \'esicle. This results in the incomplete septum which divides the utricle from the saccule. The middle ear is formed from the upper segment of the pharyngeal groove, the lower segment giving rise to the Eustachian tube. The external ear is developed from the ectoderm of the first branch- ial cleft and adjacent branchial arches. The tympanic membrane is formed from the mesoderm of the first branchial arch, its outer cover- ing being of ectodermic, its inner of entodermic origin. TECHNIC. (i) For the study of the general structure of the pinna and walls of the exter- nal auditory meatus, material may be fi.xed in formalin-^Miiller's fluid (technic 5, 34 530 THE ORGANS. p. 7) and sections stained with ha^matoxylin-eosin (technic i, p. 18). In sections of the wall of the cartilaginous meatus the ceruminous glands may be studied, material from children and from new-born infants furnishing the best demonstra- tions of these glands. (2) For the study of the inner ear the guinea-pig is most satisfactory on account of the ease with which the parts may be removed. Remove the cochlea of a guinea-pig with as much as possible of the vestibule and semicircular canals and fix in Flemming's fluid (technic 7, p. 7). A small opening should be made in the first turn of the cochlea in order to allow the fixative to enter the canal. After forty-eight hours the cochlea is removed from the fixative and hardened in graded alcohols (page 8). The bone is next decalcified, either by one of the methods mentioned on page 9 or in saturated alcoholic solution of picric acid. If one of the aqueous decalcifying fluids is used, care must be taken to carry the material through graded alcohols. Embed in celloidin or parafiin, cut sections through the long axis of the modiolus, through the utricle and saccule, and through the semicircular canals. Stain with hsematoxylin-eosin and mount in balsam. (3) The neurone relations of the cristae, maculae, and cochlear duct can be demonstrated only by means of the Golgi method. The ear of a new-born mouse or guinea-pig furnishes good material. The cochlea together with some of the base of the skull should be removed and treated by the Golgi rapid method (page 32). Sections should be thick and must of course be cut through undecalcified bone. Good results are difficult to obtain. The Organ of Smell. The olfactory organ consists of the olfactory portion of the nasal mucosa. In this connection it is, however, convenient to describe briefly the olfactory bulb and the olfactory tract. The Olfactory Mucosa. — This has been described (page 264). The peculiar olfactory cells there described are not neuro-epithelium but are analogs of the spinal ganglion cell, being the only example in man of the perij^herally placed ganglion cell found in certain lower animals. Each cell sends to the surface a short dendrite which ends in several short, stiff, hair-like processes. From its opposite end each cell gives off a longer centrally directed process (axone), which as a fibre of one of the olfactory nerves passes through the cribriform plate of the ethmoid (Fig. 353, ethm) to its terminal nucleus in the olfactory bulb (Fig. 353). Ill/' Olfactory Bulb. — This is a somewhat rudimentary structure an- alogous to the much more prominent olfactory brain lobe of some of the lower animals. It consists of both gray matter and white matter arranged in six fairly distinct layers. These from below upward are as follows: (a) The layer of olfactory fibres; (b) the layer of glomeruli; (c) the molecular layer; (d) the layer of mitral cells; (e) the granule THE ORGANS OF SPECIAL SENSE. ry.n layer; (/) the layer of longitudinal fibre bundles. Through the centre of the last-named layer runs a band of neuroglia which represents the obliterated lumen of the embryonal lobe. The relations of these layers to the olfactory neurone system are as follows: The layer of olfactory fibres (Fig. 353, a) consists of a dense plexi- form arrangement of the axones of the above-described olfactory cells. Fig. 353. — Diagram of Structure of Olfactory Mucosa and Olfactory Bulb. (Ramon y Cajal.) be. Bipolar cells of olfactory mucosa; svi, submucosa; ethm, cribriform plate of ethmoid; a, layer of olfactory fibres; og, olfactor}- glomeruli; vie, mitral cells; ep, epithe- lium of olfactorv ventricle. From this layer the axones pass into the layer of olfactory glomeruli where their terminal ramifications mingle with the dendritic terminals of cells lying in the more dorsal layers, to form distinctly outlined spheroidal or oval nerve-fibre nests, the olfactory glomeruli (Fig. 353, og). The latter mark the ending of neurone system No. I. of the olfactory conduction ])ath. The molecular layer contains both small ncr\e cells and large nerve cells. These send their dendrites into the olfactory glomeruli. The smaller cells belong to Golgi Type II. (see page 117) and appear to be association neurones between adjacent glomeruli. The axones of the larger cells, the so-called l)rush cells, become fil)res of the oljac- orx trad. 532 THE ORGANS. Of the mitral cells (Fig. 353, me), the main dendrites end in the olfactory glomeruli, while their axones, like those of the brush cells, become fibres of the olfactory tract. In addition to the fibres which pass through it (axones of mitral and of brush cells), the granular layer contains numerous nerve cells. Many of these are small and apparently have no axones (amacrine cells.) Their longer dendrites pass toward the periphery, their shorter dendrites toward the olfactory tract. Larger multipolar cells, whose axones end in the molecular layer, also occur in the granular layer. The layer of longitudinal fibre bundles consists mainly of the cen- trally directed axones of the mitral and brush cells. These fibres run in distinct bundles separated by neuroglia. Leaving the bulb they form the olfactory tract by means of which they pass to their cerebral terminations. The brush cells and mitral cells with their processes thus constitute neurone system No. II. of the olfactory conduction path. TECHNIC. (i) Carefully remove the olfactory portion of the nasal mucosa (if human material is not available, material from a rabbit is quite satisfactory). This may be recognized by its distinctly brown color. Fix in Flemming's fluid (technic 7, p. 7), or in Zenker's (technic 9, p. 8). Stain thin vertical sections with htemat- oxylin-eosin (technic i, p. 18) and mount in balsam. (2) For the study of the nerve relations of the olfactory cells material should be treated by the rapid Golgi method (page 32). The Organ of Taste. The organ of taste consists of the so-called taste buds of the lingual mucosa. These have been mentioned in connection with the papillae of the tongue (page 199) and under sensory end-organs (Fig. 268). The taste buds are found in the side walls of the circumvallate papillae (page 199), of some few of the fungiform papillae, in the mucosa of the posterior surface of the epiglottis, and especially in. folds (foliate papillae) which occur along the ])Ostero-lateral margin of the tongue. The taste bud (Fig. 354) is an ovoid epithelial structure embedded in the epithelium, and connected with the surface by means of a minute canal, the gustatory canal (Fig. 354, a), the outer and inner ends of which are known, respectively, as the outer and inner taste pores. Each taste bud consists of two kinds of cells, neuro-epithelial cells THE ORGANS OF SPECIAL SENSE. 533 or gustatory cells and sustentacular cells (Fig. 354). The gustatory cells are long, delicate, spindle-shaped cells which occupy the centre of the taste bud, each ending externally ^ in a cilium-like process, which usually projects through the inner pore. The inner end of the cell tapers down to a fine process, which may be single or branched. The sustentacular cells are long, slender cells which form a shell several cells thick around the gustatory cells. Sensory terminals of the glosso- pharyngeal nerves (Fig. 354, b) end within the taste buds in a network of varicose fibres — intrageminal fibres. Other sensory terminals of the same nerve end freely in the epithelium be- tween the taste buds. These are finer and smoother than the intrageminal fibres and are known as inter geminal fibres (Fig. 354) Fig. 354. — Taste-bud from Side Wall of Circumvallate Papilla. (Merk 1-Henle.) a, Taste-pore; b, nerve fibres, some of which enter the taste-bud — intra- geminal fibres; while others end freely in the surrounding epi- thelium— intergeminal fibres. TECHNIC. (i) The general structure of the taste buds is shown in the sections of tongue (technic, p. 200). (2) For the study of the nerve terminals the method of Golgi should be used (page 32). General References for Further Study. Kolliker: Handbuch der Gewebelehre des Menschen. McMurrich: The Development of the Human Body. Ramon y Cajal: La retine des vertebres. La Cellule, i.x., li Schwalbe: Lehrbuch der Anatomie der Sinnesorgane, 1887. INDEX. Abducens (nerve Yl), 448, 492 Aberrant peduncular fibres, 470 Absorption, 239 of fat, 240 Accessorius (nerve XI), 431, 493 Accessory nasal sinuses, 264 olivary nucleus, 438 Achromatic element of intranuclear network, 44 spindle, 48 Acid aniline dyes, 18 cells, 219, 239 Acidophile granules, 97 Acini, 190 Acoustic group of segmental neurones, 425 Acrosome, 314 Acusticus (nerve VIII), 425, 431, 441, 492 Adelomorphous cells, 219 Adenoids, 157 Adipose tissue, 85 Adrenal, see Suprarenal Adventitia of arteries, 136 of lymph vessels, 143 of veins, 138 Afferent peripheral nerves, 376 segmental neurones, 425 paths, 426 pallial, 413, 414, 416,421, 422, 428, 469, 470 suprasegmental neurones, 378 Agminated follicles, 228 Air cells, 274 passages, 274 sacs, 274, 275 vesicles, 274 Ala cinerea, 430, 437 Alcohol, as a fixative, 6 dilute, as a fixative, 6 for hardening, 8 Ranvier's, 4 strong, as a fixative, 6 Alcohol-ether celloidin, 1 1 Alimentary tract, 192 development of, 266 endgut, 231 foregut, 213 headgut, 193 midgut, 223 Altmann's granule theory of proto- plasmic structure, 40 Alum-carmine, i 7 for staining in bulk, 19 Alveolar ducts, 274, 275 glands, 187, 190 passages, 274 sacs, 274 Alveoli, 190, 274 i Amacrine cells, 504 1 Amitosis, 47 Amoeboid movement, 46 technic for, 57 Amphiaster, 48 Amphicytes, 383 Amphipyrenin, 43 Amphophile granules, 98 Ampullae, 520 of Thoma, 162 Anabolism, 45 Anaphase, 51 Aniline dyes, acid, 18 basic, I 7 Anistrophic line, 103 substance, 103 Annular terminations, 388 Annuli fibrosi, 141 Ansa lenticularis, 48 1 Anterior corpus quadrigemini, 466, 465, 467, 471 cerebral commissure, 4 78, 481 horns, 39S, 401, root or motor cells of, 380, perforated space, 477, 481 pyramids, 416 white commissure, 48 1 Antrum, ^2>, 535 536 INDEX. Anus, 234 Aorta, 137 Apathy, concerning cilia, 68 Appendix epididymidis, 312 testis, 312 vermiformis, 232 Aqueductus Sylvii, 374, 462, 464 Arachnoid membrane, 380 Arbor vitae, 455 Arborescent terminations, 388 Arborizations, terminal, 112 Arc, neural, 377, 420 Archipallium, 477 Archoplasm, 45 Arcuate fibres, 435, 438, 447 internal, 438, 448 Area tegmenti, 472 acustica, 43 i Areolar (loose) connective tissue, 77 Arrector pili muscle, 361 Arrectores pilorum, 365 Arteriae arciformes, 294 rectae, 296 Arteries, 133 adventitia of, 136 anterior spinal, 406 aorta and other large, 137 arcuate, 294 arteriole, 134 bronchial, 277, 279 coats of, 133 coronary, 141 development of, 142 elastic tissue of, 135, 137 greater arterial circle of iris, 511 hepatic, 254 interlobar, 293 interlobular, 294 intima of, 134 large, like the aorta, 137 lesser arterial circle of the iris, 5' ' lymjjh channels (jf, 139 media of, 135 medium-sized, 134 nerves of, 139 phrenic, 296 posterior spinal, 406 precapillary, 134 pulmonary, 277 recurrent, 296 renal, 286, 293 Arteries, small, 133 structural peculiarities of some, 137 sulco-commissural, 406 suprarenal, 300 technic of, 139 vasa vasorum, 139 Arteriole, 134 Articular cartilages, 180 Articulations, 180; see Joints diarthrosis, 180 synarthrosis, 180 synchondrosis, 180 syndesmosis, 180 technic of, 181 Arytenoid cartilages, 266 Ascending degeneration, 413 fibre tracts of spinal cord, 413 direct cerebellar, 415 Gowers', 416 long arms of dorsal root fibres, 413 posterior columns, 414 funiculi, 413 spino-tectal, 415 -thalamic, 414 tract of Flechsig, 415 tracts forming parts of afferent pallial paths, 413 to cerebellum, 415 tractus spino-cerebellaris dorsalis, 415, 426 ventralis, 415, 426 uncrossed cerebellar, 415 Association fibres; see also Inter- segmental neurones of brain, 425, 426 of cerebellum, 461 of cord, 399, 419 of endbrain, 478, 479, 483, 485 of isthmus, 464 of interbrain, 470, 472 of midbrain, 467 Asters, 52 Atresia of follicle, 333 Atria, 274 Atrophy method of determining fil)re tracts of the cord, 413 Attraction sphere, 45 Auditory canal, 518 cells, 526 hairs, 522 INDEX. 537 Auditory nerve, 441, 443 cochlear branch of, 439, 441, 443 vestibular branch of, 425, 438, 441- 445. 448 path, 425, 441, 442, 443, 470, 485 Auerbach, end-buttons of, 35 end-feet of, ^s plexus of, 225, 230, 232, 238 Auricle, 518 muscle of, 140 Auriculo- ventricular ring, 140 Axis cj'linder, iii, 118; see also Axone Axolemma, 118 and neurilemma, relation of, 120 Axonal degeneration, 124 Axone, the, iii, 116, 375 Bethe's views of, 121 Cajal's views of, 120 collaterals of, 117 degenerative changes in, 123 development of, 375 fibres of Remak, 117 medullary sheath of, 119 naked fibres, 117 non-medullated, 117 terminal arborizations of, 117 Axone-hill, 1 16 Baillarger, line of, 485 Balsam, Canada, for mounting, 21 Barker, concerning the neurone, 112, 115 Bartholin, glands of, 346 duct of, 243 Basal granule, 68 Basic aniline dyes, 17 Basis pedunculi, 464, 465 Basket cells, 195 Basophile granules, 98 Bellini, duct of, 289 Berkley, concerning ]ntuitary body, 490 Bertini, columns of, 28S Bethe, concerning continuity of ax- olemma and neurilemma, 120 Betz, cells of, 479, 482 Bioblasts, 40 Bipolar nerve cells, 112 Bladder, urinary, 298 Blastoderm, 56 Blastomeres, 56 Blocking, 11 Blood, 95 corpuscles, 95, 99 crenation of red cells, 96 development of, 99 diapedesis, 98 dust, 99 erythrocytes of, 95 granules, elementary, 99 granules of Ehrlich, 97 hseinoglobin of, 96 haematin, 96 hccmatokonia, 95 haemolysis, 96 Jenner's stain for, 28 leucocytes of, 96 phagocytosis, 98 plasma of, 95 platelets, 98 red cells of, 95 crenation of, 96 technic for, 57 smears, technic of, 100 stroma of, 96 technic of, 100 thrombocytes, 98 vascular unit, 278 white cells of, 96 Blood-islands, 99, 142 Blood-sinuses of hsemolymph nodes, 151 Blood-vascular unit, 278 Blood-vessel system, 131 arteries, 133 capillaries, 131 development of, 142 heart, 140 lining of, 131 technic of, 139, 142 veins, 137 Blood-vessels, 131 lymph channels of, 139 nerves of, 139 technic of, 139 Body cavities, 144 Bone, breakers, 174 decalcification of, 10 formers, 173 tissue, 92 calcination of, 93 538 INDEX. Bone tissue, cells of, 93 cementum, 212 corpuscles of, 93 decalcification of, 3, 9, 93 intercellular substance of, 93 lacunae and canaliculi of, 93 lamellae of, 93 technic of, 94 Bone-marrow, 168 blood-vessels of, 170 red, 168 cells of, 168 erythroblasts of, 168 fat cells of, 169 leucocytes, 169 marrow cells, 168 mast cells of, 169 multinuclear cells of, 169 myelocytes of , 168 myeloplaxes of, 169 non-nucleated red blood cells of, 169 normoblasts, 168 nucleated red blood cells of, 168 technic of, 172 yellow, 170 endosteum, 170 gelatinous, 170 Bones, 166 blood-vessels of, 170 cancellous or spongy, 164 cells of origin of, 173 circumferential lamella of, 166 development of, 172 intracartilaginous, 175 intramembranous, 172 subperichondral, 177 subperiosteal, 177 growth of, 179 hard or comijact, 165 Haversian canals of, 165 lamellae, 166 Howship's lacunae, 174 intermediate lamellae, 167 interstitial lamellae of, 167 lacunae, origin of, 173 lymphatics of, 171 marrow, 168 red, 168 yellow, 170 nerves of, 171 Bones, nutrient canal, 170 foramen, 170 vessels, 170 osteoblasts, 173 osteoclasts, 174 osteogenetic tissue, 174 perforating fibres, 168 perichondrium, 175 pericranium, 174 periosteal buds, 176 periosteum of, 167, 174 Sharpey's fibres, 168 technic of, 171 developing bone, 179 Volkmann's canals, 167, 171 Bony spiral lamina, 563 Borax-carmine, alcoholic solution, 20 Born's theory of corpus luteum, 332 Bowman, capsule of, 288, 290 glands of, 265 membrane of, 495 sarcous elements of, 103 Brachia conjunctiva, 450 Brachium of posterior corpus quad- rigeminum, 465 pontis, 449 Brain, the, 424 cerebral cortex of, 481 contrasted with spinal cord, 424 development of, 373 endbrain (telencephalon), 477 corpus striatum, 477, 478 pallium, 477, 478 rhinencephalon, 477 forebrain (prosencephalon) , 468 diencephalon (thalamence pha- lon), 468 epithalamus, 468 hypothalamus, 46S thalamus, 468 interbrain, 468 general histology of, 429 structure of, 424 higher coordinating ajiparatus of, 424 hindbrain (rhombence phalon) , 429 cerebellum, 455 isthmus, 462 medulla, 429 nerves of, 429 pons, 43 I tegmentum, 43 r INDEX. .'):!!) Brain, membranes of, 378 arachnoid, 380 blood-vessels of, 380 cerebral dura, 378 dura mater, 378 pia mater, 380 relation of optic nerve to, 506 technic of, 380 midbrain (mesencephalon) , 378 aqueductus Sylvii, 464 basis pedunculi, 464 corpora quadrigemina, 464, 465 iter, 464 pes pedunculi, 464 substantia nigra, 464 tegmentum, 464 Pacchionian bodies of, 171, 380 pineal eye, 490 pituitary body, 489 relation of, to optic nerve, 506 sand, 490 segmental brain and nerves, 377, 425 suprasegmental structures, 427 technic of, 431, 488, 490 ventricles, 373, 374 vesicles, 373 Bridges, intercellular, 3, 66, 102 Bronchi, 269 blood-vessels of, 277 cartilages of, 271 development of, 279 lymphatics, 279 nerves of, 279 primary, 269 respiratory, 274 structure of walls of, 269 technic of, 280 terminal, 272, 274 Bruce concerning root cells, 404 Bruch, membrane of, 497 Brunner's glands, 230, 239 Bulb, 429; see Medulla Bulbus oculi, 494; see Eyeball Bundle of Lowenthal, 4 i 8 of Vicq d'Azyr, 469, 477 inferior longitudinal, 471), 481 Burdach, column of, 408, 414 nucleus of, 414, 430 Bursas, 184 Busch-Marchi staining inethod, 31 Butschli's theory of protoplasm struc- ture, 40 diagram of, 41 Cachexia strumipriva, 282 Cajal, cells of, 482 concerning the neurone, 114, 120, 121, 123 interstitial nucleus of, 417, 426 methods for staining neuro- fibrils in nerve cells, 34 Cajeput oil for clearing sections, 21 Calcarine area, 488 Calcification centre, 173, 175 zone, 177 Canada balsam, 20 Canal, gastro-intestinal, 215 lacrymal, 513 of Cloquet, 511 of Petit, 511 of Schlemm, 500 portal, 255 Canaliculi of bone, 93 of connective tissue, 74 Canalis communis, 520 Canalized fibrin, 343 Cancellous bone, 164, 174 Capillaries, 131 chyle, 237 development of, 142 technic of, 139 Capillary endothelium, 132 network, 132 Capsule of Bowman, 288, 290 internal, 476 of ganghon cells, s&3, 39^ of Glisson, 253 of Tenon, 512 Carbol-xylol for clearing specimens, 21, 28 Carmine alum, 17, 19 borax, 20 gelatin, 23 neutral, 18 picro-, 19 Carotid gland, 146 Cartilage, 89 arytenoid, 266 cells, 89 chondrin, 89 classification of, 90 540 INDEX. Cartilage, cricoid, 266 development of, 92 elastic, 91 embryonal, 91 epiphyseal, 179 fibrous, 91 hyaline, 90 intercellular matrix of, 92 intermediate, 179 of developing bone, 175 perichondrium of, 92 Santorini's, 266 technic of, 92 thyreoid, 266 tracheal, 267 Wrisburg's, 266 Cartilages, articular, 180 costal, 180 skeletal, 180 Caryochromes, 114 Caudate nucleus, 472, 478, 481 Cavernous sinuses, 320 Cavity of embryonic vesicle, 55 Cedarwood oil as solvent, 13 Cell, the, 39 amitosis, 47 body of, 40 centrosome of, 44 crusta of, 42 cytoplasm of, 41 deutoplasm granules of, 42 division, direct, 47 indirect, 48 endoplasm of, 42 exoplasm of, 42 function of, 46 hyaloplasm of, 40 intranuclear network of, 44 irritability of, 46 islands of Langerhans, 250 function of, 25 r Opie's theory of, 251 origin of, 251 structure of, 251 technic of, 252 karyoplasm of, 4 i linin of, 44 membrane of, 42 metabolism of, 45 metaplasm granules of, 42 microsomes, 40 mitosis, 48 Cell, motion of, 46 nuclear membrane of, 43 nuclein of, 44 nucleolus of, 44 nucleoreticulum, 44 nucleus, 42 origin of word, 42 patches, 343 paraplasm granules of, 42 plastids, 41 plastin, 40 primary germ-layers of, 56 protoplasm of, 39 Altmann's granule theory of, 40 Biitschli's foam or emulsion theory of, 40 fibrillar theory of, 40 reproduction of, 47 secretory process of, 187 space (lacuna), 74 spongioplasm of, 40 technic for study of, 57 trophospongium, 42 typical, 39 diagram of, 39 structure of, 39 vital properties of, 41 Celloidin, alcohol-ether, 1 1 clove-oil, I 2 embedding, 1 1 sections, 14 clearing of, 2 1 mounting of, 2 1 ■Cells, acid, 219, 239 active, 186 adelomorphous, 219 air, 274 amacrine, 504, 532 auditory, 526 basal, 265 basket, 195, 457 blood, 95 bone, 93 brush, 53 I capsule, 375 cartilage, 89, j 77 centro-acini, of Langerhans, 249 centro-tubular, 249 chief, 2 19, 284, 489 chromophile, 489 ciliated, 269 IXDEX. .541 Cells, clear, 284 colloid, 281 compound tactile, 386 connective-tissue, 74 corneum, 354 crescents of Gianuzzi, 195 daughter, 51 decidual, 340 Deiter's, 527 delomorphous, 21Q demilunes of Heidenhain, 195 empty, 186 endothelial, 131 eosinophile, 98, 152 epithelial, 63 erythroblasts, 168 extrinsic, 396 fat, 86, 169 fibroblasts, 78 fixed, 74 foetal, 275, 280 ganglia, 382 gland, 186 goblet, 186, 227, 269 Golgi, Type I., 117 Type II., 117, 482, 531 granule, 482 gustatory, 533 hair, 521, 526 hecatomeric, 398 Hensen's, 527 heteromeric, 398 interstitial, 308 intrinsic, 396 Kupffer's, 258 Langerhans', 249 leucocytes, 96, 169 Leydig's, 513 liver, 256 loaded, 186 lymphoid, 84, 148 lutein, 329 marrow, 168 "last, 75, 98, 152, 169 megalocyte, 161 migratory leucocytes, 226 mitral, 530 mononuclear, 161 mossy, 126 mucous, 186, 194, 227 multinuclear, lOi inuscle, 101 Cells, myelocytes, 168 myeloplaxes, 169 nerve, 11 1; for classification see Nerve cells neurilemma, 375 neuroblasts, iii, 126, 374 neuro-epithelial, 522 neuroglia, 125 non-nucleated red blood, 95, 169 normoblasts, 168 nucleated red blood, 161, 168 odontoblasts, 201, 206, 211 of Claudius, 527 of oral glands, 194 olfactory, 265 osteoblasts, 173 osteoclasts, 161, 174 ovum, 47, 327 oxyntic, 219 oxyphile, 283, 285 Paneth's, 228 parietal, 219 peptic, 219, 239 phaeochromoblasts, 302 phagocytes, 98, 152 pigmented, 42, 64, 77 pillar, 525 plasma, 75 prickle, 354, 3 5S primitive ova, 324, 348 Purkinje, 456 red blood, 95 replacing, 65, 220, 227 reserve, 281 respiratory, 276 resting, 49, 186, 281 secreting, 187, 281 serous, 194 SertoH's, 305, 315, 340 sex, 348 signet-ring, 86 simple tactile, 386 single primitive, 47 smooth muscle, 102 spermatids, 307, 314 spermatocytes, 307, 314 spermatogenic, 305 spermatogones, 306, 314, 34g spider, 126 spleen, i6i supporting, 305 sustentacular, 250, 265, 305. 521 512 INDEX. Cells, sympathoblasts, 302 tactile, 386 tautomeric, 398 thrombocytes, 98 wandering, 75, 226 white blood, 96 Cementing glycerin mounts, 20 Cementum, 200, 204, 212 Centres of calcification, 173 Central canal, 402 chromatolysis, 124 gelatinous substance, 402 gray, 431 nervous system, 373 neurones, 376 spindle, 48 tegmental tract, 439, 447, 454, 464, 470 Centriole, 45 Centro-acinar cells of Langerhans, 249 Centrosome, 44, 48, 56 archoplasm, 45 attraction sphere, 45 centriole, 45 daughter, 48 of fertilization, 56 Centro-tubular cells of Langerhans, 249 Cerebellar arc, 421 connections, 415, 417, 418, 421, 428, 447, 449, 455 cortex, 450, 455 peduncles, 450 Cerebello-olivary fibres, 438, 439, 447 Cerebellum, 455 arbor vitae, 455 ascending paths to the, 415 association cells of, 461 basket cells of, 457 cells of, 455, 456, 457 climh)ing fibres of, 460 cortex of, 450, 455 dentate nucleus of, 418, 450, 455 development of, 374 fibres of, 458, 460 climbing, 461 mossy, 460 of Bergmann, 462 parallel, 458 general histology of, 455 granular layer, 455 gray matter of, 455 Cerebellum, heinispheres of, 449, 455 internal nuclei of, 450, 455 laminae of, 455 middle peduncle of, 455 molecular layer, 455 neuroglia of, 462 nuclear layer, 455 nucleus dentatus, 450 emboliformis, 450, 455 globosus, 450, 455 tecti or fastigii, 418, 450, 455 peduncles of, 455 Purkinje cells of, 429, 455 technic of, 488 vermis, 415, 449, 455 Cerebral arc, 421 cortex, 481 hemispheres, 477 development of, 374 membranes, 378 peduncles, 464 vesicle, 373 Cerebro-spinal ganglia, 375, 382 technic of, 394 nervous system, 373; see Nervous system {cerebro- spinal) neurones, efferent peripheral, 395 Ceruminous glands, 518 Cervical enlargement of cord, 396 segments of cord, 396 Cervix, 337 epithelium of, 338 external os, 338 of posterior horns, 402 ovula Nabothi, 338 plicae palmatae, 338 technic of, 349 Cheeks, mucous membrane of, 193 Chemotaxis, 46 Chiasma, optic, 504 Chief cells, 219, 489 Chloride of gold for staining connect- ive-tissue cells, 26 Chloroform as solvent, 13 Choledochus ductus, 263 Choriocapillaris, 497 Chorion, 341 Chorionic villi, 341 Chorioid, the, 496 choriocapillaris of, 497 ciliary body of, 498 INDEX. 543 Chorioid, fissure, 516 Haller's layer of, 407 iris, 500 lamina citrea, 497 suprachorioidea, 497 perichorioidal lymph spaces of, 497 plexus, 431, 437. 45° tapetum cellulosum of, 497 fibrosum of, 497 venae vorticosse of, 497 vitreous membrane of, 49S Chromaffin granules, 299 Chromatic element of intranuclear network, 44 Chromatin, 44 Chromatolytic changes, 333 Chromatolysis, 124 Chrome-silver method of Golgi, 26 Chromophilic bodies, 113, 114 significance of, 115, 121 Chromosomes, 50 Chyle vessels', 237 Cilia, 47, 63, 68 Ciliary artery, 511 body, 498 blood-vessels of, 5 1 1 canal of Schlemm, 500 ligamentum pectinatum, 500 muscles of, 499 ora serrata of, 498, 501 pars ciliaris retinae, 499 processes of, 498 spaces of Fontana, 500 vitreous membrane of, 499 ganglion, 390 movement, 47 technic for, 457 muscle, 499 plexus, 512 processes, 498 Cingulum, 479 Circulatory system, 131 blood-vessel system, 131 carotid gland, 146 coccj'geal gland, 146 development of, 142 general references for further study, 146 lymph-vessel system, 143 Circumferential lainellae, 167 Circum vallate papillae, ig7 Cirl, concerning fibres of internal capsule, 472 Clarke's columns, 408, 415 Claudius, cells of, 527 Clava, the, 430 Clearing specimens before mounting, 2 I Clefts of Schmidt-Lantermann, 119 Climbing fibres, 404 Clitoris, 346 Closed skein (spireme), 49 Cloquet's canal, 511 Clove-oil celloidin, 12 Coagulum sheath, 120 Coccygeal glands, 143 segments of spinal cord, 396 Cochlea, 522 bony spiral lamina of, 523 cupola of, 523 hamulus of, 523 helicotrema, 523 membranous spiral ligament of, 523 modiolus of, 522 scala tympani, 523 vestibuli, 523 spiral ligament of, 523 Cochlear duct, 523 basilar membrane of, 525 crista basilaris, 525 external spiral sulcus, 525 membrane of Reissner, 524 organ of Corti, 525 spiral prominence of, 525 stria vascularis, 525 zona pectinata, 525 tecta, 525 nerve, 439, 441 tracts, 450, 453, 465 Coelom, 144 Cohnheim's field, 103 Collaterals, 117 CollicuH, 364, 365, 374 Colloid, 281, 284 Colostrum corpuscles, 370 Column cells, 398 hecateromeric, 3 98 heteromeric, 398 tautomeric, 398 technic of, 399 of Burdach, 408, 414 of GoU, 40S, 4 1 4 544 INDEX. Columnas rectales, 234 Columns of Bertini, 2 88 of Sertoli, 305 Comma tract of Schultze, 419 Commercial formalin, 4 Commissura habenularis, 477 Commissural fibres, 478 Compact bone, 165 Compound tactile cells, 386 Conduction path, 377 afferent pallial, 413 afferent and efferent supraseg- mental, 426 auditory, 441 descending suprasegmental, 433 pallio-spino-peripheral efferent, 417 to cerebellum, 415 Cone association neurones, 507 fibres, 503 -visual cell, 503 Cones, layer of rods and, 502 Conjunctiva, 513 end bulbs of, 387 Connective tissue, 73 adipose or fat, 85 areolar, 77 basement substance of, 77 bone, 92 canaliculi of, 74 cartilage, 89 cells of, 75 characteristics of, 73 chlorid-of-gold method for dem- onstrating cells of, 26 classification of, 74 dense fibrous, 77 elastic, 78 elastin of, 77 embryonal, 73, 8r, 172, 179 fat, 85 fibres of, 76 elastic, 76 fibrillated, 76 reticular, 76 white, 76 yellow, 76 fibrillar, 74 fibroblasts, 78 fixed cells of, 74 formed, 78 gelatin of, 76 Connective tissue, histogenesis of, 73 intralobular, 188 interalveolar, 277 intercellular substance of, 76 interlobar, 188 interlobular, 87, 89, 188 intrafascicular, 184, 382 lacunae of, 74 lymphatic, 84 loose, 77 Mallory's stain for, 28 mast cells of, 75 mucous, 81 neuroglia, 94, 125 pigmented cells of, 77 plasma cells of, 75 reticular, 83 retinaculse cutis, 353 staining cells of, 26 technic for, 80, 83, 85, 89, 92, 94 theories of development of fibres of, 78 wandering cells of, 75 Constrictions of Ranvier, 119 Contact theory of neurones, 122 Continuity theory of neurones, 122 Convoluted tubules, 288, 289, 290 Cord, spinal; see Spinal Cord Corium, 351; see Derma Cornea, the, 494 anterior elastic membrane of, 495 epithelium of, 495 corpuscles of, 49 j endothelium of Descemet of, 496 layers of, 494 membrane of Bowman of, 495 of Descemet of, 496 perforating or arcuate fibres of, 496 posterior elastic membrane of, 496 substantia propria of, 496 Corneal corpuscles, 496 Cornu ammonis, 477, 478 Cornua of cord, 401 Corona radiata, 326, 478, 479, 481 Coronary arteries, 141 Corpora amylacea, 3 i 7 cavernosa, 3 j 9 lutea of pregnancy, 330 spuria, 332 vera, 330 INDEX. 545 Corpora mammillaria, 477, 478 quadrigemina, 374, 464 anterior, 464, 465 development of, 374 posterior, 464, 465 striata, 373 Corpus albicans, 330 callosum, 478, 479, 481 dentatum, 450 hsemorrhagicum, 329 Highmori, or mediastinum testis, 303 luteum, 329 theory of, 332 Luysii, 468 quadrigeminum, anterior, 464, 465, 467, 510 spongiosum, 319 striatum, 477, 478, 481 caudate nucleus, 481 putamen, 481 subthalamicum, 468, 477 trapezoideum, 450 Corpuscles, blood, 95 colostrum, 370 corneal, 496 crescentic, 317 Golgi-Mazzoni, 365 Grandry's, 386 Hassall's, 154 Meissner's, 321, 365, 387 Merkel's, 386 Pacinian, 365, 388 renal, 288 Rufifini's 365 salivar}^ 157 splenic, 159 tactile, 365 Vater-Pacinian, 365 Wagner, 365 Cortex cerebelli, 450, 455; see also Cerebellum cerebri, 481; see also Cerebntin areas of, 4S8 association fibres of, 485 barren or molecular layer of, 48 2 cells of, 481 of Betz, 484 of Cajal, 4S2 of Golgi, Type II., 482 of Martinotti, 482 Cortez cerebri, cells of, pyramidal, 481 commissural fibres, 478 corona radiata of, 478, 481 deep tangential fibres of, 485 external granular layer, 482 ganglionic layer, 483 internal granular layer, 482 interradiary plexus, 485 layer of polymorphous cells, 483 of pyramidal cells of, 481 line of Baillarger, 48 5 molecular la^'-er, 482 multiform layer, 483 plexiform layer of Cajal, 482 projection fibres, 485 radiations of Meynert, 485 superficial, tangential fibres of, 482 supraradiary plexus of, 485 Cortical labyrinths, 288 P3^ramids, 288; see also Kidney Corti's arches, 526 organ, 525; see also Organ of Corti tunnel, 526 Cotyledons, 344 Cowper's glands, 319 technic of, 319 Cox-Golgi method of staining, 33 Cranial nerves, 380, 492; see also Xerves, cranial Crenation of red blood cells, 96 Crescentic corpuscles, 317 Crescents of Gianuzzi, 195, 244, 267 Cretinism, 282 Cricoid cartilage, 266 Crista acustica, 522 basilaris, 525 Crossed pyramidal tracts, 416, 433 Crura cerebri, 465 Crusta (exoplasm), 42 Crypt of Lieberkiihn, 228, 230 Cumulus ovigerus, 326 Cupola, 523 Cupula, 522 Cuticle, 353; see Epidermis Cuticula, 42, 63 dentis, 204 Cuticular membrane, 63, 20Q, 225 Cystic duct, 260 Cytoarchitecture, 488 546 INDEX. Cytoplasm, 41 of nerve cells, 112 Cytoreticulum, 40 Darkschewitsch, nucleus of, 417 Daughter cells, 5 i centrosomes, 48, 53 chromosomes, 51 stars, 5 1 Decalcification, 3, g Decalcifying, 9 fluids, 10 Decidua basalis, 340 capsularis, 340 graviditatis, 340 menstrualis, 339 placentalis subchorialis, 344 reflexa, 340 serotina, 340 vera, 340 Decidual cells, 340 Decolorizing fluid for Weigert's hsematoxylin, 30 Decussation of fillet, 435 optic, 507 of Forel, 467 of Meynert, 467 of pyramids, 416, 429, 431 sensory, 435 Degenerating nerves, Marchi's method for staining, 3 i Dehiscent glands, 190 Deiter's cells, 527 nucleus, 418, 426, 448 descending tract from, 418 Delafield's hsematoxylin, 15 Delomorphous cells, 219 Demilunes of Heidenhain, 195 Dendrites, the, iii, 116, 375 Dental canals, 201, 21 r germ, 206 jjapilla, 206 periosteum, 205 ridge, 208 sac, 208 sheath, Neumann's, 203 shelf, 206 Dentate nucleus, 418 Dentinal fibres, 201 jmlp, 20 r Dentine, 201, 210 chemical com]K;sitif)n of, 201 Dentine, development of, 210 Derma, or corium, 351 corpuscles of Meissner, 387 muscle cells of, 352 papillae, compound, 352 nerve, 356 simple, 352 vascular, 352 pars papillaris, 352 reticularis, 351 pigmentation of, 355 Descemet, endothelium of, 496 membrane of, 496 Descending degeneration, 413 fibre tracts of the spinal cord, 416; see Fibre tracts of spinal cord {descending) Deutoplasm, 42, 328 Development of bone, 172 technic of, i 79 Diapedesis, 98 Diaphysis of bone, 179 Diarthrosis, 180 articular cartilages, 180 glenoid ligaments, 181 interarticular cartilages, 181 joint capsule, 181 Diaster, 50, 51, 54 Diencephalon, 373, 468 Digestive system, 192 alimentary tract of, 192 development of, 262 endgut, 231 foregut, 2 13 gall-bladder, 261 general references for further study, 263 headgut, 193 large intestine, 231 larger glands of, 242 liver, 253 mesentery, 235 midgut, 223 mouth, 193 oesophagus, 213 omentum, 235 jjancreas, 247 ])ancreas, 247 jjharynx, 212 peritoneum, 235 rectum, 234 salivary glands, 242 IXDKX. .'a: Digestive system, small intestine, 223 stomach, 217 teeth, 200 tongue, ig6 vermiform appendix, 232 Direct cerebellar tract, 415 pyramidal tract, 417 Discus proligerus, 326 Dissociation of tissue elements, 4 Distal convoluted tubule, 287 Disynaptic arc, 421 Dogiel's end plates, 320, 512 theory of structure of spinal gan- glion, 383 Dorsal accessory olivary nucleus, 438 decussation of Meynert, 465 graj?^ commissure, 402 root fibres of white matter, 403 spino-cerebellar tract, 414 white commissure, 403 Duct systems of glands, igo Ducts, aberrans Halleri, 311 alveolar, 274 Bartholini's, 244 Bellini's, 289, 292 choledochus, 263 cochlear, 523 coinmon, 260 excretory, of glands, 190 cystic, 260 endolymphatic, 521 ejaculatory, 311 excretory, 1S7 Fallopian tube, 334 Gartner's, 333 hepatic, 255 mesonephric, 347 Miillerian, 3 18 nasal, 513 of Miiller (embryonal), 312 of sweat glands, 355 oviduct, 334 pancreatic, 247 pronephric, 347 reuniens, 521 Santorini's, 247, 266 secondary pancreatic, 247 seminal, 309 Stenoni's, 243 thoracic, 143 thyreo-glossal, 283 utriculo-saccular, ^21 Ducts, vas deferens, 304, 310 epididymis, 309 vas efferentia, 309 Wharton's, 244 Wirsung's, 247 Wolffian, 347 Ductus aberrans Halleri, 3 1 1 reuniens, 521 Duodenum, 230 Brunner's glands, 241 technic of. Dura mater, 378 blood-vessels of, 380 cerebral, 378 spinal, 379 technic of, 380 Dyes, basic aniline, i 7 nuclear, 1 5 plasma, 17 Dynamic centre of cell, 52 Ear, development of, 521; drum, 518 external, 518 auricle, 518 blood-vessels of, 519 ceruminous glands of, 518 ear drum, 518 external auditory canal, 518 lymphatics of, 519 nerves of, 519 pinna, 518 tympanic membrane, 578 internal, 520 ampulla, 520 blood-vessels of, 527 canalis communis, 520 cochlea, 520 ducts of, 521 ductus reuniens, 521 endolymph of, 520 endolymphatic duct, 521 sac, 521 fenestra ovalis, 520 rotunda, 520 lymphatics of, 5 28 membrana tectoria. 527 membranous labyrinth, 520 nerves of, 528 organ of Corti, 325 osseous labyrinth of, 520 perilymph of, 520 )48 INDEX. Ear, internal, saccule, 521 semicircular canals, 522 utricle, 521 utriculo-saccular duct, 521 vestibule, 520 middle, or tympanum, 519 fenestra rotunda of, 519 ossicles of, 520 stapes, 520 wax, 518 Ebner's glands, 199 Ectoderm, 56 tissue, derivations from, 61 Edinger-Westphal nucleus, 465 Effectors, 375, 424, 426 Efferent pallial paths, 413, 414, 416, 421, 422, 428, 469, 470 Egg cords, Pfiiiger's, 324 technic of, 336 nests, 325 Ehrlich, granules of, 97 Ejaculatory ducts, 311 Elastic cartilage, 91 tissue, 78 Weigert's stain for, 26 Elastin, 77 Eleidin, 354 Ellipsoid of Krause, 503 Ellipsoids of spleen, 161 Embedding, 10 celloidin, 1 1 paraffin, 12 Embryonal tissue, 73, 81 fat tissue, 86 Eminentia hypoglossi, 437 medialis, 430 Emvilsion theory of protoplasmic structure, 40 Enamel, 200, 206, 208 cells, 209 chemical composition of, 204 development of, 206, 208 fibres, 204 organ, 206, 208 prisms, 204, 209 lines of Retzius of, 204 Endbrain {telencephalon) , 373, 477 corpus striatum, 478 pallium, 478 neopallium, 478, 470 olfactory pallium, 477, 478 rhinence]jhalon, 477 Endbrain, anterior perforated space, 477 gyrus hippocampi, 477 olfactory bulb, 477 nerve, 477 pyriform lobe, 477 trigonum olfactorium, 477 tuberculum olfactorium, 477 End-buttons, 456 of Auerbach, 35 -feet of Auerbach, 35 -bulbs, 385, 387 of Krause, 321 Endgut, 231 large intestine, 231 mesentery, 235 peritoneum, 235 omentum, 235 rectum, 234 vermiform appendix, 232 Endocardium, 141 Endochondral ossification, 172, 175 Endolymph, 520 Endolymphatic duct, 521 sac, 521 Endomysium, 184 Endoneurium, 117, 382 Endoplasm, 42 Endosteum, 170 Endothelial tube, 143 Endothelium, 70 of Descemet, 496 Engelmann, showing ciliated epithe- lial cell, 69 Entoderm, 56 tissue derivations fron-1,61,262,279 Eosin, 17 -glycerin, 20 -hsematoxylin stain, 18 Eosinophile granules, 98, 152, 489 Epiblast, 56 Epicardium, 141 Ejjicranium, 174 E])idermis (or cuticle), 353 eleidin, 354 keratin, 354 keratohyaline granules, 354 mitosis of cells of, 354 pareleidin, 355 pigmentation of, 355 jmckle cells of, 354 stratum corneum of, 354 INDEX. 549 Epidermis stratum corneum of, cylin- dricum of, 353 germinativum of, 353 granulosum of, 354 lucidum of, 354 Malpighii of, 353 mucosum of, 353 spinosum of, 354 Epididymis, 309 cells of, 309 vas epididymis of, 309 vasa efferentia of, 309 Epidural space, 338 Epiglottis, 266 Epimysium, 183 Epineurium, 381 Epiphyseal cartilage, 179 Epiphysis of bone, 179 Epithalamus, 468, 477 Epithelium, 63 basal membrane of, 63 ciliated, 68 classification of, 64 cuboidal, 65 cuticular membrane of, 63 endothelium, 70 follicular, 326, 327, 31,3 general characteristics of, 63 germinal, 324, 348 glandular, 69, 186 histogenesis of, 63 intercellular bridges of, 63 lens, 510 membrana propria of, 63 mesothelium, 70 neuro-, 69 pigmented, 69 pseudo-stratified, 65 replacing cells of, 65 respiratory, 275 simple, 64 columnar, 65 pseudo-stratified, 65 squamous, 64 stratified, 66 columnar, 67 squamous, 66 transitional, 67 surface, of mucous membranes. 191 syncytium, 343 tactile cells of, ^ technic of, 7 1 86 Eponj^chium, 358 Epoophoron, 333 Erectile tissue, 319, 346 Ergastoplasm, 102 Erythroblasts, 168 Erythrocytes, 95 Erythrosin, 18 Eustachian tube, 519, 520 Excretory ducts, 187 substances in cells, 42 Exoplasm, 42, 78 External arcuate fibres, 435, 438, 447 ear, 518; see Ear, external OS, 338 spiral sulcus, 525 Extero-ceptors, 390 Eye, the, 494; see Organ of vision pineal, 490 Eyeball (or bulbus oculi), 494 blood-vessels of, 511 chorioid of, 496 ciliary body of, 498 cornea of, 494 development of, 515 iris of, 500 lens, 510 lymphatics of, 512 nerves of, 501, 505, 512 retina of, 501 sclera of, 494 technic of, 517 Eyelashes, 513 Eyelid, the, 513 blood-vessels of, 514 conjunctiva of, 513 epidermis of, 513 glands of, 513 of Mall, 513 lymphatics of, 514 Meibomian glands of, 514 muscles of, 514 nerves of, 514 tarsus of, 513 technic of, 518 Facialis (nerve VII), 425, 492 Fallopian tube, 334; see Oviduct ampulla of, 334 blood-vessels of, 335 coats of, 334 development of, 346 fimbriated extremity of, 334 isthmus of, 334 550 INDEX. Fallopian tube, lymphatics of, 335 nerves of, 335 ovarian extremity, 334 technic of, 335 False corpora lutea, 330 Fascicles of muscle, 183 of nerves, 381 Fasciculi, 362 Fasciculus arcuatus, 479, 481 of Thomas, 418 medial longitudinal, 418 posterior longitudinal, 43Q, 467 perpendicular of Wernicke, 479 retroflexus of Meynert, 477 solitarius, 438, 439 superior longitudinal, 479, 481 uncinate, 479, 481 Fastigio-bulbar tract, 450 Fat, absorption of, 240 technic of, 241 blood supply of, 89 development of, 86 technic of, 89 gobules, 240 osmic-acid stain for, 28 subcutaneous, 353 tissue, 85 histogenesis of, 86 technic of, 89 Fat-droplets in cells, 43, 86 Fat-lobules, 86 Fauces, mucous membrane of, 193 Female genital organs, 323 pronucleus, 53 Fenestra ovalis, 520 rotunda, 519 Fenestrated membrane, 79 Ferrein, pyramids of, 288 Fertilization of the ovum, 52 Filjrae jjropriae of Meynert, 479, 488 FiVjre baskets, 505 systems, 377 efferent, 380 main motor, 380 shf^rt, 399, 4 I 9 jjrojjrio-spinal, 4J9 spino-spinal, 419 tracts of cord (ascending), 413 direct cerebellar, 4 1 5 Gowers', 4 1 6 long ascending arms of dor- sal root filjres, 4 1 3 Fibre tracts of cord (ascending), of spinal cord, 40S posterior columns, 401 spino-tectal, 415 spino-thalamic, 414, 433 tract of Flechsig, 4 1 5 tractus spino-cerebellaris dorsalis, 415, 426, 433 ventralis, 415, 426, 433 (descending), 416 anterior marginal bundle of Lowenthal, 418 anterior pyramids, 416 antero-lateral, 41S cerebro-spinalis, 416 comma tract of Schultze, 419 crossed pyramidal, 416 descending tract from Dei- ter's nucleus, 41S direct pyramidal, 417 fasciculus of Thomas, 418 from the interstitial nucleus of Cajal, 417 fundamental, 399, 419 Helweg's, 418 marginal bundles of Low- enthal, 418 origin of tracts, 396 oval bundle of Flechsig, 418 pallio-spinalis, 416 pyramidal, 416 rubro-spinal, 417 septo-marginal, 418 tecto-spinal tract, 4x7 tract of Tlirck, 417; see Di- rect pyraniidal tractus cortico-spinalis, 416 vestibulo-spinal, 418 Von Monakow's tract, 417 Fil)res, afferent nerve, 3 7() arcuate, 43 5 association of iJuUium, 478, 479, 4«5 calcified, 1 72 cartilage, 9 1 commissural, 478 cone, 503 connective-tissue, 76 development of, 78 cortical, 359 dentinal, 2 1 1 efferent root, 375 INDEX. 551 Fibres, enamel, 204 external arcuate, 435 genioglossal, 196 heart muscle, 106 intergeminal, 533 internal arcuate, 435 interzonal, 50 intrageminal, 533 involuntary striated (heart) muscle, 1 06 lens, 510 Mallory's method of staining con- nective-tissue, 27 mantle, 48 Mliller's, 504 nerve, 113; see also Nerve fibres meduUated, 117 non-medullated, 117, neuroglia, 126 of areolar tissue, 78 of bone, 93 of developing muscle, 108 of formed connective tissue, 78 of Remak, 117 of Sharpey, 168 olfactory, layer of, 530 perforating or arcuate, of cornea, 496 projection, 469, 470, 478, 479 radiate, 258 reticulo-spinal, 418 rod, 503 styloglossal, 197 superficial arcuate, 435 tendon, 77 tunnel, 529 voluntary muscle, 102, 105 Weigert's method for staining elastic, 26 method for staining nerve, 29 white or fibrillated, 76 yellow or elastic, 74, 76 Fibrillar connective tissue, 74 theory of protoplasmic structure, 40 Fibroblasts, 78 Fibrous cartilage, 91 Fila olfactoria, 425 Filar mass, 40 Filiform papillae, 197 Fillet (or medial lemniscus), 414, 42(1, 435, 436, 440, 442, 44(1, 45 1 Filum terminale, 396 Fimbria, 478, 481 Fissure, anterior median, 401 chorioid, 516 Fixation, 5 by injection, 6 in toto, 6 Fixatives, 6, 7, 8 Flechsig, oval bundle of, 418 myelogenetic method of, 40S, 488 tract of, 415 Flemming concerning cell-division, 47 Flemming's fluid, 7 Foam theory of protoplasm structure, 40 Foetal cells, 275, 280 structures, appendix epididy- midis, 312 of genital system, 312, ^^^ testis, 312 ductus aberrans Halleri, 311 organ of Giraldes, 311 paradidymis, 311 Foliate papillae, 532 Follicle, Graafian, 324; see also Graafian follicle Follicles, agminated, 228 solitary, 221, 228 Follicular cavity or antrum, 325 FoUiculi linguales, 157; see Tonsils Fontana, spaces of, 500 Foramen caecum lingui, 157 Foramina nervosa, 528 papillaria, 289 Forebrain (prosencephalon) , 373, 46S diencephalon [thalamencephalon) , 468 epithalamus, 468 hypothalainus, 468 thalamus, 468 interbrain, 468 section through junction of mid- brain and thalamus, 470 Foregut, the, 213 general structure of walls of the gastro-intestinal canal, 215 oesophagus, 213 stomach, 2 i 7 Forel, decussation of, 4(17 tield of, 472 INDEX. Formaldehyde, as a fixative, 6 for macerating, 4 -bichromate method, 7,t, ForinaHn, commercial, 4 Formalin-Miiller's fluid (Orth's), 7 Fornix, 477, 47S, 481 anterior pillars of, 481 commissure, 478 Fossa navicularis, 322 Fountain-like decussation of Meynert, 467 Fourth ventricle, 430, 435, 447 Fovea centralis, 505 Fraenkel's theory of corpus luteum, 332 Free endings of sympathetic nerves, 394 Fuchsin, i 7 Function of cells, 46 Fundamental columns of spinal cord, 399 Fundus glands, 219 Fungiform papillae, 197 Funiculus cuneatus, 414 gracilis, 414 posterius, 413 Gage, showing muscle fibres, 106, 107 Gage's hsematoxylin, 15 Gall-bladder, 261 coats of, 261 mucous membrane of, 261 rugse of, 261 Galvanotaxis, 46 Ganglia, 382 amphicytes, 383 cerebral, 382 cerebro-spinal, 375, 382 chain, 390 Gasserian, 393 habenularis, 477 of Corti, 38 7 of vScarpa of VIII., 425 satellite cells, 383 spinal, 382 spiral, 528 spirale of VIII., 425 structure of, 382 sympathetic, 375, 390 technic for, 394 vertebral, 390 Ganglion cells, 375 capsule of, 383, 392 development of, 375 Gartner's canal, 347 duct, 333 Gasserian ganglion, 454 Gastric crypts, 218 glands, 218 pits, 218 Gastro-hepatic omentum, 235 Gastro-intestinal canal, general struc- ture of the walls of, 215 Gelatin, 76 carmine for injecting, 23 Prussian blue, for injecting, 23 Gelatinous marrow, 170 substance of Rolando, 402 Gemmules, 456 Geniculate body, 465, 470 ganglion of, VII., 425 Genio-glossal fibres, 196 Genital gland, 348 ridge, 348 system, 303; see also Reproduc- tive system development of, 346 rudimentary structures con- nected with development of, 3ii> 333. 346 Genito-urinary system, see Urinary system, 286, and Reproduc- tive system, 303 Gennari, line of, 488 Gentian violet, 17 Genu-facialis, 431, 448 Germ hill, 326 layers, 56 tissues derived from, 61 Germinal epithelium, 324, 348 spot, 327 vesicle, 53, 327 Giant cells of Betz, 482, 484 Gianuzzi, crescents of, 195, 244, 267 Giraldes, organ of, 311, 347 Gland cells, 186 Glands, 186 acini of, 1 90 alveolar, 187, J90 compound, 188, 190 simple, 188, s()0 alveoli of, j 87, j 90 Bartholin's, 346 INDEX. 553 Glands, Bowman's, 263 Brunner's, 230 carotid, 146 cells of, 1S6, 1S7 ceruminous, 518 ciliary, 499 classification of, 188 coccygeal, 146 compound, 187 corpus luteum, 332 Cowper's, 319 epithelium of, 187 excretory ducts of, 187 dehiscent, 190 development of, 188 ductless, 1S7, 190 Ebner's, 199 fundus, 219 gastric, 218 general structure of, 186 genital, 348 haemolymph, 141 internal secreting, 186 interstitial tissue of, 188 intraepithelial, 309 kidney, 286 lacrymal, 512 large, of digestive system, 242 Lieberkiihn's, 228, 230 lingual, 195 Littre's, 321, 322 liver, 253 lobes of, 1 88 lobules, iSS lymph, 147 Mall's, 513 mammary, 368 Meibomian, 190, 514 mixed, 194 mucous, 194 of internal secretion, 190 of the oral mucosa, 193 ovary, 323 pancreas, 247 parathyreoids, 283 parenchyma of, 18S parotid, 243 peptic, 219 pineal, 490 prehyoid, 283 prostate, 317 pyloric, 219, 220 Glands, racemose, 188 reticular, 190 saccular, 187, 190 compound, 190 simple, 190 salivary, 242 sebaceous, 355, 362 secreting portions of, 187 serous, 194 simple, 187 spleen, 158 sublingual, 243 submaxillary, 244 sudoriferous, 189 suprahyoid, 283 suprarenal, 299 sweat, 355 tarsal, 514 thymus, 153 thyreoid, 280 accessory, 283 tonsils, 155 tubular, 187, 188 compound, 188 simple branched, 188 simple coiled, 188 simple straight, 188 tubulo-alveolar, 187 Tyson's, 321 uterine, 337 Glandulse sudoriparse, 355 vestibulares majores, 346 minores, 346 Glandular epithelium, 69, 186 Glans penis, 319 Glenoid ligaments, 181 Glisson, capsule of, 253 Globus major, 304 minor, 304 pallidus, 472, 481 Glomerulus of kidney, 288 blood-vessels of, 295 olfactory, 531 Glosso-pharyngeal (IX. nerve), 425, 439, 493 Glycerin for mounting specimens, 20 jelly, 20 Glycogen granules, 256 Goblet cells, 186 Gold chlorid for staining connective- tissue cells, 26 Gold-size for glvcerin mounts, 20 .5.54 INDEX. Golgi cell. Type I., 115, 117 cell, Type II., 115, 117, 399, 531 method, bichlorid, 33 chrome-silver, 26 Cox modification of, ^^ formalin bichromate, ^;i mixed, 32 rapid, 32 silver, for nerve tissue, 32 slow, for nerve tissue, 3 2 muscle-tendon organs of, 390 net, 122 organs of, 390 Golgi-Mazzoni corpuscles, 365 GoU, column of, 408, 414 nucleus of, 414, 430 Gowers' tract, 416 Graafian follicles, 324 antrum of, 325 corona radiata, 326 cumulus ovigerus, 326 development of, 324, 349 discus proligerus, 326 egg nest, 325 epithelium of, 324 follicular cavity of, 325 germ hill of, 326 liquor folliculi, 325 nerves of, 333 ovum of, 325 Pfltiger's egg cords, 324 primitive Graafian follicle, 325 ova, 324 rupture of, 329 stratum granulosum, 326 technic of, 335 theca folliculi, 326 tunica fibrosa, 326 tunica vasculosa, 327 Graded alcohols, 7 Grandry, corpuscles of, 386 Granule theory of protoplasmic struc- ture, 40 Greater omentum, 235 Gray matter, 377, 402 Gray rami communicantes 381, 392 Gray reticular formation, 419, 426 Ground bundles of spinal cord, 399, 4.3.3 Griibler's methylene blue, 28, 35 water-soluVjle eosin, 28 Gums, mucfjus membrane of, r93 Gustatory canal, 533 Gyrus dentatus, 477 hippocampi, 477 HABENUL.i', 468 Haemalum, Mayer's, 16 Hasmatein, 15, 96 Hasmatoidin, crystals of, 330 Hasmatokonia, 95 Hsematoxylin, 1 5 and eosin, for staining double, 18 and picro-acid fuchsin, iq Delafield's, 15 Gage's, 15 Heidenhain's, 16 Mallory's stain, 26 Weigert's 17, 29 Haemoglobin, 96 Haemolymph nodes, 151 blood sinuses of, 151 blood-vessels of, 153 cells of, 152 eosinophiles, 152 mast cells, 152 phagocytes, 152 function of, 153 hilum of, 151 marrow-lymph, 152 relation of, to h^mphatic system, 153 spleno-lymph, 152 technic of, i 53 Hair, 358 arrector pili muscle of the, 361 blood-vessels of, 364 bulb, 358 cells, 521, 526 cells of the, 360, 363 connective-tissue follicle of, 360 cortex of, 359 cortical fibres of, 359 cuticle of, 359 development of the, 3 58, 366 excretory duct of, 363 eyelashes, 5 i 3 follicle, 359 germ, 363 growth of the, 363 hyaline membrane, 3^)0 inner root sheath, 359 cuticle of, 359 Hcnle's layer of, 360 IXDEX. Hair, inner root sheath, Huxle^^'s layer of, 360 lanugo, the, 359 layers of the, 359, 360 lymphatics, 365 medulla of, 358 nerves of, 365 outer root sheath, 360 papilla of, 358 prickle cells, 360 root of the, 358 root sheath, 359 sebaceous glands of the, 362 sebum of the, 363 shaft of, 358 shedding of the, 363 stratum cylindricum, 360 technic of the, 364 vitreous membrane, 360 Halleri, ductus aberrans, 311 Haller's layer, 497 Hamulus, 523 Hardening, 8 celloidin-embedded specimens, 1 1 clove-oil celloidin-embedded spec- imens, 12 Hassal's corpuscles, 154 Haversian canals, 165 development of, 178 fringes, 181 lamellae, 166 spaces, 178 systems, 167 development of, 178 Hayem's fluid, 57 Head, sympathetic ganglia of, 390 Headgut, 193 mouth, 193 pharj'^nx, 2 i 2 teeth, 200 tongue, 196 Hearing, organ of, 518: see Ear Heart, 140 annuli hbrosi, 14 i auricular muscle, 140 auriculo- ventricular ring, 140 blood-vessels of, 141 coronary arteries of, 141 development of, 143 endocardium of, 141 epicardium of, 141 lymphatics of, 142 Heart muscle, 106, 140; see also In- voluntary striated muscle myocardium of, 140 nerves of, 142, 394 technic of, 142 valves of, 141 Hecateromeres, 398 Haeidenhain, demilunes of, 195 Heidenhain's haematoxylin, 16 Heisterian valve, 260 Helicotrema, 523 Heller's plexus, 236 Helweg, tract of, 418 Hemispheres of cerebellum, 449, 45 5 Hendrickson, concerning coats of liver ducts, 260 Henle's laj^er, 360 loop, 288, 290, 291 sheath, 120, 382 Hensen's cells, 527 line, 103 Hepatic artery, 254 cells, 257 cords, 257 diverticula, 263 duct, 255, 260 Hermann, showing centrosome, 45 Heteromeres, 398 Hilum of liver, 253 of kidney, 286 Hindbrain, 373, 429 bulb, 429 cerebellum, 374, 455 medulla oblongata, 374, 421; section of, through, at level of junction of pons and cere- bellum, and entrance eighth nerve, 447 through roots of VI, abducens, and VII facial nerves, 450 through roots of V, trigeminus nerve, 452 His, marginal veil of, 374 myelospongium of, 374 spongioblasts of, 374 Histogenesis, 61 Holmgren, showing trojihospongium. 42 Horizontal cells, 482, 504 Howship's lacunae, 173 Huxley's la^-er, 3(10 556 INDEX. Hyaline cartilage, 90 Hyaloid canal, 511 membrane, 511 Hyaloplasm, 40, log Hydatid of Morgagni, 312 Hj'drochloric acid for decalcifying, 10 Hyoglossal fibres, 196 Hypoblast, 56 Hypoglossal (XII nerve), 435, 437, 492 Hyponychium, 358 Hypophysis cerebri, 489; see also Pituitary body H3-pothalamus, 468, 477 IxcisuRES of Schmidt-Lantermann, 119 Incremental lines of Schreger, 202 Indirect cell division, 48; see Mitosis Inferior brachium quadrigeminum, 465 cerebellar peduncle; see Restiform body Infundibula, 274 Injection, 22 apparatus, 23 double, 24 separate organs, 24 whole animals, 24 Innervation of muscles, 388 Interalveolar connective tissue, 277 Interarticular cartilages, 181 Interbrain, 373, 468 epithalamus, 374, 468 hypothalamus, 374, 468 thalamus, 374, 468 Intercellular bridges of epithelium, 63, 66 bridges of muscle tissue, 102 substance, 62 of connective tissue, 76 silver-nitrate method of stain- ing, 26 Intero-ceptors, 390 Interfilar mass, 4! Intermediate cartilage, 179 lamellae, 167 neurones, 376 Internal arcuate fibres, 438, 448 Internal capsule, 476 Internal ear, 520; see also Ear, internal Internal nuclei of cerebellum, 450, 455 Internode, 119 interradiary plexus, 485 Intersegmental neurones, 378, 426, 433. 435- 439. 447. 449. 452, 454, 464 Interstitial lamellae, 167 nucleus of Cajal, 417, 426, 476 Intestine; see Small intestine, 223; Large intestine, 231 Intestines, development of, 262 Intima, 134 coats of, 134 endothelial layer of, 134 intermediary layer of, 134 membrana elastica interna, 135 of arteries, 134 of lymph vessels, 143 of veins, 138 Intracartilaginous bones, growth of, 179 ossification, 175 Intracellular canals, 42 Intrafascicular connective tissue, 184, 382 Intramembranous ossification, 172 Intranuclear network of typical cell, 44 Involuntary striated muscle (heart), 106 Cohnheim's field, 106 development of, in pig (McCal- lum), 109 McCallum's views, 106, 109 membrane of Krause, 106 muscle columns of Kolliker, 106 nerves of, 394 sarcoplasm of, 106 technic of, 1 10 smooth muscle, 10 r intercellular bridges of, 102 Iodine, to remove mercury, 9 Iris, the, 500 greater arterial circle, 5 i r layers of the, 500 lesser arterial circle, 511 muscles of the, 50 1 pigmentation of, 500 vitreous membrane, 50 r Irritability of cells, 46 Islands, blood, 99, 142 of Langerhans, 250 INDEX. Isolated smooth muscle cells, loi technic of, no Isotropic line, 103 substance, 103 Isthmus, 374, 462 section through, at exit of IV, trochlearis, nerve, 462 at level of optic chiasma, 472 Iter, 374, 462, 464 Tenner's blood stain, 28 Joint capsule, 181 stratum fibrosum, 181 synoviale, 181 synovial membrane, 181 Joints, 180; see Articulations Jugular ganglion of X, 425 Juxta-restiform body, 449 Karyolysis, 354 Karyoplasm, 41 Karyosomes, 44 Katabolism, 45 Keratin, 354 Keratohyaline granules, 354, 358 Kidney, the, 286 arteriae arciformes, 294 rectse, 296 blood-vessels of, 293 Bowman's capsule of, 290 capillaries of, 295 columns of Bertini, 288 convoluted tubules of, 288 cortex of, 286 cortical pyramids of, 288 development of, 301, 346 duct of Bellini, 289 epithelium of, 292 glomerulus of, 288 Henle's loop, 291 interlobar arteries of, 293 hilum of, 286 labyrinths of, 288 lobulated, 286 location of tubules in, 292 lymphatics of, 296 main excretory duct of, 297 Malpighian body, 288 pyramid, 288 medulla of, 286 niedullary (or Malpighian) pyra- mid, 288 Kidney, medullary rays, 288 nerves of, 296 papillas of, 287 pelvis of, 287, 297 pyramids of Ferrein, 288 renal artery, 286 corpuscle, 288 vein, 286 renculi or lobes of, 286 septa renis, 288 single lobe of, 286 stellate veins of Verheyn, 296 technic of, 302 ureter, 286, 297 uriniferous tubule, 288; see also Uriniferous tubule Kidney-pelvis, 297 blood-vessels of, 297 calyces of, 297 coats of, 297 development of, 346 lymphatics of, 297 nerves of, 297 technic of, 302 KoUiker, muscle columns of, 103 showing Golgi cell type II, 116 spleen cells, 161 Krause, ellipsoid, 503 end-bulbs, 200, 321, 365, 512 line of, 103 Kupffer, cells of, 258 Labia minora, sebaceous glands of, 355 Labyrinth, membranous, 520 osseous, 520 Lacrymal apparatus, 512 canal, 513 gland, 512 blood-vessels of, 513 excretory ducts of, 513 lyinphatics of, 513 nerves of, 513 technic for, 518 nasal duct of, 513 sac, 513 Lacteals, 240 Lacunae, 89, 93 origin of, 173 Lamellae, circuinferential, 1(17 Haversian, 166 intermediate, 167 558 INDEX. Lamellae, interstitial, 167 of bone tissue, 93 Lamina, bony spiral, 523 citrea, 497 cribrosa, 494, 506 fusca, 494 reticularis, 527 suprachorioidea, 497 Laminse of cerebellum, 455 Langerhans, cell islands of, 250 centro-acinar cells of, 249 centro-tubular cells of, 249 Lanugo hairs, 359 Large intestine, 231 Auerbach's plexus, 232, 238 blood-vessels, 326 coats of, 231 development of, 262 gland tubules, 232 lineae coli, 232 lymphatics, 237 nerves, 238 plexus of Meissner, 232, 238 mj'entericus, 238 technic of, 241 Larynx, the, 266 blood-vessels of, 268 cartilages of, 266 arytenoid, 266 cricoid, 266 epiglottis, 266 Santorini's, 266 thyroid, 266 Wrisburg's, 266 cells of, 266 develojjment of, 280 epithelium of, 266 lymphatics, 268 nerves, 268 perichondrium, 266 technic, 269 Lateral lemniscus, 441, 449, 450, 462 Law of Wallerian degeneration, j 22 Lemniscus, 378, 381 lateral, 441, 449, 450, 462 medial, 4/4, 426, 435, 43^'. 44°, 442, 446, 451, 452, 453 Lenhossck, concerning ciliated ejjithe- lium, 68 Lens, 510 Lenticular nucleus, 477, 478 canal of Petti t, 5 ' 1 Lenticular capsule, 510 epithelium, 510 fibres, 510 hyaloid membrane, 511 suspensory ligament, 510 zonula ciliaris, 510 zonule of Zinn, 510 Leopold, concerning pregnant uterus, 340 Leucocytes, 96 lymphocytes, 96 migratory, 226 mononuclear, 97 of milk, 370 polymorphonuclear, 97 polynuclear, 97 transitional, 97 Lewis, concerning shape of blood- cells, 95 Lieberkiihn, crypts of, 228, 230 glands of, 228 Ligament, circular dentoid, 205 glenoid, 180 spiral, 523 structure of, 78 suspensory, 510 Ligamentum nuchse, 79 pectinatum, 500 Lineae coli, 232 Line of Gennari, 488 Lines of Retzius, 204 Lingual glands, 195 tonsils, 157 Lingualis, genio-glossus fibres of, 196 hyoglossus fibres of, 196 longitudinal fibres of , 197 styloglossus fibres of, 197 transverse fibres of, 196 Linin, 44 Lipoid granules, 299 Licjuor ferri sesquichlorati, 26 folliculi, 325 Lissauer, zone of, 397, 403 Littrc', glands of, 321, 322 Liver, the, 253 blood supply of, 254 capillary network, 255 capsule of Glisson, 253 cells of, 256 of Kupffer, 258 central vein of, 255 INDEX. 559 Liver, compared with other com- pound tubular glands, 2 58 connective tissue, 253 cords of liver cells, 257 development of, 263 ducts, 260 common, 260 cj'stic, 260 hepatic, 260 glycogen granules, 256 Heist erian valve, 260 hepatic artery, 254 cords, 257 duct, 255 hilum, 2^^ intralobular secreting tubules, 255 lobes of, 253 lobules, 253 lymphatics, 260 main ducts, 260 nerves, 260 portal canal, 255 vein, 254 radiate fibres, 258 reticulum, 258 septa, 253 sublobular vein, 255 technic of, 261 tubules of, 257 Lobulated kidney, 286 Lowenthal, anterior marginal bundle of, 418 Longitudinal cleavage, 5 i fasciculus, 439, 447, 454, 4'H, 4^1 Loop of Henle, 2gi Loose (areolar) connective tissue, 77 Lumbar enlargement of spinal cord, 396 segments of cord, 396 Lungs, the, 273 air cells, 274 sacs, 274 vesicles. 274 alveolar ducts, 274, 275 sacs, 274, 275 alveoli of, 274 atria of, 274 blood-vessels of, 277 bronchial artery, 277 system, 277 capsule of, 273 Lungs, cells of, 276 development of, 279 epithelium of, 275 foetal cells of, 275, 276 infundibula of, 274 interalveolar connective tissue of, 277 lobes of, 273 lobules of, 273 lymphatics of, 279 Miller's subdivisions, 274 nerves of, 279 parietal pleura, 273 pulmonary artery, 277, 279 lobule, 273, 278 pleura of, 273 respiratory bronchi, 274 cells, 276 epithelium, 275 septa of, 273 technic of, 280 terminal bronchi of, 274 vestibula of, 274 Lunula, 358 Lutein cells, 329 Luteum, corpus, 329 Luys, nucleus of, 469 Lymph, 63 capillaries, 143 glands, 147; see LympJi nodes nodes, 147 blood-vessels of, 150 capsule of, 147 chains of, 147 connective tissue of, 147 cords of, 14S cortex of, 148 germinal centre of, 148 lymphatics of, 150 medulla of, 148 nerves of, 150 nodules of, 148 reticular connective tissue of, 149 sinuses of, 14S technic of, 150 nodule, 84, 148 germinal centre of, 147 paths of the eye, 5 1 2 spaces, 144 pericellular, 144 vessel system, 143 560 INDEX. Lymph vessel system, capillaries of, 144 developm.ent of, 145 lymph capillaries, 144 spaces, 144 relation of, to hsemolymph node, 153 technic of, 144 vessels, coats of, 143 structure of, 143 Lj-mphatic organs, 147 development of, 145 hsemolymph nodes, 151 lymph nodes, 147 spleen, 158 technic of, 150, 153, 155, 158 thymus, 153 tonsils, 155 tissue, 84, 148 Lymphocytes, 85, 96 Lymphoid cells, 84, 148 tissue, 149 Macerating fluids, 4 Maceration, 4 Macula acustica, 521 lutea, 505 fovea centralis, 505 Male genital organs, 303 pronucleus, 53 Mall, glands of, 513 concerning development of fibril- lar connective tissue, 78 concerning splenic pulp, 162 Mallory's aniline blue stain for con- nective tissue, 28 haematoxylin stain, 27 phosphomolybdic acid haema- toxylin stain for connective tissue, 27 phosphotungstic acid haematoxy- lin stain for connective tis- sue, 27 Maljjighian bodies, 159 body of kidney, 288, 290 development of, 288, 347 ];yramid, 288; see Kidney Mamillo-thalamic tract, 469 Mammary gland, 368 active, 368 alveoli of active, 369 ampulla of, 368 blood-vessels of. 370 Mammary gland, cells of, 369 colostrum corpuscles, 370 development of, 371 ducts of, 368 of nipple, 368 inactive, 368 interlobar septa of, 368 interlobular septa of, 368 lobular ducts of, 368 lymphatics of, 370 milk, 370 nerves of, 370 secretion of, 369 structure of, 368 technic of, 371 Mantle fibres, 48 Marchi's method for staining de- generating nerves, 3 1 Busch's modification of, 31 Marginal bundle of Lowenthal, 418 veil of His, 374 Marrow, 168; see Bone marrow lymph nodes, 152 Martinotti, cells of, 483 Mast cells, 75, 98 Matrix of nail, 356 Maturation, 52 of ovum, 52, 328 of spermatozoon, 52 Mayer's hsemalum, 16 McCallum, concerning heart muscle, 106, 109 Media of arteries, 135 of lymph vessels, 143 of veins, 138 Medial column, 357 eminence, 430 Median center of Luys, 477 lemniscus, 414, 426, 435 raphe, 381, 438 septum, posterior, 401 Mediastinum testis, 303 Medulla oblongata, 429 accessory olivary nucleus, 438 olives, 438 afferent cerebellar neurones, 438, 447 roots, 433, 435, 23S, 439 secondary tracts of, 433, 435. 438, 439 terminal nuclei of, 433, 435, 438, 439 INDEX. 501 Medulla oblongata, ala cinerea, 437 anterior fissure, 430 ground bundles, see Interseg- mental neurones pyramid, 430, 447 arciform (arcuate) nucleus, 378, 381, 384 arcuate fibres, 435, 438 auditory nerve, 441 central canal, 430 gelatinous substance, 437 gray matter, 431, 433, 437 tegmental tract, 439, 447 cerebellar peduncles, 447 cerebello-olivary fibres, 438, 439, 447 chorioid plexus, 437 clavia, 430 cochlear nerve, 439, 441. nuclei, 441 column of Burdach, 430, 435 of Goll, 430, 435 compared with spinal cord, 430 corpus restiforme, 436, 438, 449 crossed pyramidal tract, 416, 433 cranial nerves of, 429, 430 decussation of fillet, 435 of pyramids, 431 Deiter's nucleus, 435, 447 tract, 435 descending root of fifth nerve, 430 or spinal root of vestibular por- tion of eighth nerve, 439 suprasegmental paths, 433 tract from Deiter's nucleus, 435. 439. 447 from the vestibular nuclei, 441 development of, 374 direct cerebellar tract, 433 direct pyramidal tract, 433 funiculus, 433 horns, 431 nucleus of ninth cranial nerve, 439. 440 nucleus of tenth nerve, 430, 43 5- 437 efferent peripheral neurones, 431. 435. 437. 439 suprasegmental neurones, 433, 437. 439. 447 36 Medulla oblongata, eminentia hypo- glossi, 437 spino-cerebellar tract, 433, 435 external arcuate fibres, 435, 438, 447 fasciculus cuneatus, 433 gracilis, 433 fillet or medial lemniscus, 414, 426, 435, 436, 440, 442, 446, 447. 451 formatio reticularis, 439 fourth ventricle, 430, 437 funiculus cuneatus, 435 gracilis, 435 gelatinous substance of Rolando, 433. 437 general structure of, 429 genu facialis, 43 1 Gowers' tract, see Ventral spino- cerebellar gray reticular formation, 431 internal arcuate fibres of, 438 intersegmental neurones, 431, 435. 439. 447 lateral fillet, 441 lemniscus, 441 longitudinal fasciculus, 439 median lemniscus, 414, 426, 435, 436, 440, 442, 446, 447, 451 longitudinal fasciculus, 439, 447 raphe, 436, 438 nuclei arcuati, 438, 447 of the floor of the ventricle, 430 laterales, 438 of the thalamus, 469 of posterior columns, 430, 435 nucleus, abducentis, 431 accessory cuneate, 435 alse cinereas, 435, 437 ambiguus, 437, 439 commissuralis, 435 cuneatus, 430, 435 gracilis, 430, 435 hypoglossi, 430, 435 of acoustic nerve, 431, 430, 441 of the column of Burdach, 430. 435 of the column of Goll, 430, 435 of the fifth spinal nerve, 435 of origin of eleventh cranial (spinal-accessory) nerve, 43 1 .562 INDEX. Medulla oblongata, nucleus of origin of twelfth cranial (liypoglos- sal) nerve, 435, 437 of vagus nerve, 430, 437 olives, 437, 447 olivary nucleus, 438, 439 olivo-cerebellar fibres, 438, 439, 44 7 pallio-spinal tract, 433 peduncles of, 447 plexus chorioideus, 431 pons Varolii, 447 posterior columns of, 435 longitudinal fasciculus, 439 septum, 430 predorsal tract, 439 pyramidal decussation, 433 tracts, 433, 437 raphe, 436, 438 restiform body, 430, 436, 438, 439 reticular formation, 431, 433, 435 437. 439- 447 reticulo-spinal tract, 433 root fibres of spinal V., 433 and nucleus of origin of sixth {abdiicens) cranial nerve, 447, 448, 450 and nucleus of origin of seventh {facial) cranial nerve, 439 and nuclei of eighth {auditory) cranial nerve, 441 of ninth {glosso- pharyngeal) and tenth {vagus) cranial nerves, 439 of eleventh {spinal-accessory) cranial nerve, 43 i of twelfth (hypoglossal) cra- nial nerve, 435, 437 rubro-s]jinal tract, 435, 439, 447 secondary cochlear tract, 441 vestibular tract, 44 i sensory tract of fifth nerve, 433 section through decussation of fillet, 435 through entrance of cochlear branch of eighth, 439 through lower part of inferior olivary nucleus, 437 through middle of olivary nucleus, 439 through pyramiflal decussa- tion, 43 I Medulla oblongata, section through sensory decussation, 435 sensory decussation of, 435 and motor nuclei of the fifth nerve, 452 solitary fasciculus, 438, 439 spinal (descending) root of fifth cranial nerve, 432, 433, 435, 436, 438. 440, 441, 448, 451. 452 v., 435 spino-tectal tract, 433, 437, 439, 447 -thalamic tract, 435 striae meduUares, 441 technic of, 421, 431 tecto-spinal tract, 433, 437, 439 tegmentum, 447 terrainal nucleus of the descend- ing (sensory) root fibres of the fifth nerve, 433 tract of Gowers, see Ventral spino-cerebellar from interstitial nucleus of Cajal, 433, 435 of Helweg, 418, 432, 434 of Lowenthal, 418 tractus spinalis trigemini, 433 trapezius, 441 tuberculum cinereum, 430 ventral spino-cerebellar tract, 433 vestibular nerve, 441 vestibulo-spinal tract, 433 Von Monakow's bundle, 417 Medullary pyramid (Malpighian), 288 rays, 288 sheath, 117, 119 superior, 462 Medullated nerve fibres, Weigerts' stain for, 29 Megalocytes, j 6 1 Meibomian glands, 190, 514 Meissner, corpuscles of, 321, 365, 387 plexus of, 215, 222, 230, 238 Membrana chorii, 341 elastica externa, 137 interna, 135 limitans olfactoria, 265 preformativa, 2 i 1 propria, 63 tectoria, 527 INDEX. 563 Membrane, basal, 63 cuticular, 209 mucous, iQi of Bowman, 495 of Desceniet, 496 of Krause, 106 of Reissner, 524 peridental, 205 serous, 144 synovial, iSi Membranes of brain and cord, 378 Membranous cochlea, 523 labyrinth, 520 spiral lamina, 523 ligament, 523 Meninges, 37S Menstrualis, decidua, 339 Menstruating uterus, 338 Menstruation, 339 Mercuric chlorid as a fixative, 7 Merkel's corpuscles, 386 Mesencephalic root of fifth (trigem- inus) cranial nerve, 452 Mesencephalon, 373 Mesentery, 235 Mesoappendix, 233 Mesoblast, 56 Mesoderm, 56 tissue derivations from, 61, 262, 279 Mesonephros, 334 derivations from, 312, 334 Mesothelium, 62, 70 Metabolism of cells, 45 Metanephroi, 346 Metaphase, 5 i Metaplasm, 42 Methods for studying fibre tracts of the cord, 408 atroph}'-, 413 axonal degeneration, 408 comparative anatomy, 413 myelogenetic, 408 physiology, 413 secondary degeneration, 40S Methyl blue, i 7 green, 17 violet, 17 Methjdene blue, 35 Meynert, decussation of, 4O7 librae proprias of, 479 radiations of, 4S5 Micron, 14 Microsomes, 40 Microtome, 14 Midbrain, 373, 464 anterior corpora quadrigemina of, 464, 465, 467 aqueductus Sylvii, 464 basis pedunculi, 464, 465 brachia conjunctiva, 450 cerebral peduncles, 464, 465 coUiculi, 374 corpora quadrigemina, 374, 464 cranial nerves III. and IV., 464 crura cerebri, 465 decussation of Forel, 467 of Meynert, 467 Edinger-Westphal nucleus, 465 fourth cranial nerve, 465 geniculate bodies of, 465 inferior brachium quadrigemi- num, 465 colliculi, 464 internal arcuate fibres, 467 iter, 464 lateral lemniscus, 465, 466 mesencephalic root of fifth nerve, 465 optic nerve, 467 pes pedunculi, 464, 465 posterior corpora quadrigemina, 464 longitudinal fasciculus, 467 red nucleus of, 467 reticular formation, 467 root fibres and nucleus of origin of third (oculomotor) cranial nerve, 465 through exit of third (oculo- motor) cranial nerve, 465 spino-tectal tract, 467 substantia nigra, 464, 4(1$, 467 superior cerebellar peduncles of, 467 colliculi, 464, 465 tegmentum, 374, 464 Middle ear, 519; see Ear. middle Midgut, 223 small intestine, 223 Migratory leucocytes, 226 Milk, 370 cells of, 370 colostrum corpuscles of, 370 564 INDEX. Milk teeth, 206, 208 Miller" s theory of lung subdivisions , 274 Minot, concerning endothelium and mesothelium, 70 concerning the pregnant uterus, 340 Miton, 40 Mitosis, 48 anaphase, 51 metaphase, 5 1 method of demonstrating by Flemming's fluid, 7 prophase, 48 technic for, 58 telophase, 5 i Mitotic figure, 5 i Mitral cells, 530 Modiolus, 522 Monaster, 49, 51 Mononuclear leucocytes, 97 Monosynaptic arc, 420 Mordanting, 29 Morgagni, hydatid of, 312 Motion of cells, 46 Motor cells of anterior horns, 380 decussation, 43 i end plate, 395 nuclei, 395 path to cranial nerves, 454 peripheral nerves, 380 precentral area, 416, 479, 488 Mounting, 20 celloidin specimens, 21 in balsam, 2 i in glycerin, 20 paraffin sections, 2 i Mouth, the, 193 blood-vessels of, 195 end bulbs in mucous membrane, 387 glands of, 193 lymphatics of, 195 mucous membrane of, 193 nerves of, 195, 387 technic of, 196 Mucin, 82, J94 Mucous glands, 194 membranes, 191 basement membrane of, J91 end bulbs in, 387 general structure of, r 9 j membrana propria, 191 Mucous membranes, muscularis mu- cosse, 191 nerves of, 387 of alimentary tract, 192 stroma of, 191 surface epithelium, 191 tactile cells of, 386 corpuscles of, 386 tissue, 81 tunica propria of, 191 Mucus, 186, 194 Miiller, cells of, 504, 505 circular muscle of, 500 fibres of, 504 MuUer's fluid, 6 Miillerian ducts, 318 Multipolar nerve cells, 112, 384 Muscle, arrector pili, 361 auricular, 140 cells, 1 01 ciliary, 499 circular, of Muller, 490 columns of Kolliker, 103 discs, 103 fibrillse, 103 nuclei, 102 of sweat glands, 3 68 spindles, or neuro- muscular bun- dles, 388 tendon junction, 184 organs of Golgi in, 388 peripheral nerve terminations in, 388 tissue, 10 1 classification of, 10 1 development of, loS heart, 106 histogenesis, 107 intercellular bridges of, 102 involuntary smooth, loi striated, 106 technic of, 109 voluntary striated, 102 anisotropic substance, 103 Cohnheim's fields, 103 end bulbs of, 388 ergastoplasm, 102 Hensen's line, 103 isotropic substance, 103 Krause's line, 103 muscle columns of Kolliker, 103 INDEX. ofio Muscle tissue, voluntary striated, muscle discs, 103 nuclei, 102 spindles, 388 substance, 102 nerves of, 388 Pacinian corpuscles of, 388 Rollett's theory, 104 Ruffini's theory of nerve ter- minations in, 388 sarcolemma, 102 sarcous element of Bowman, 103 technic of, no terminations in, 388 annular, 388 arborescent, 388 spiral, 388 ultimate fibrillae, 103 white and red fibres, 105 Muscles, voluntary, 183 capsule of, 183 endomysium, 184 epimysium, 183 fascicles of, 183 growth of, 184 intrafascicular connective tissue of, I 84 perifascicular sheath, 184 perimysium, 184 Muscular system, 183 blood-vessels of, 185 bursas of, 184 lymphatics of, 185 nerves of, 185 technic of, 185 tendons of, 78, 184 tendon sheaths of, 184 voluntary muscle, 183 Muscularis mucosse of mucous mem- branes, 191 Musculature of intestine, 102 Myelin, iiq Myeloarchitecture, 488 Myelocytes, 16S Myelogenetic method for determin- ing fibre tracts of cord, 408 Myeloplaxes, i6q Myelospongium of His, 3 74 Myentericus, plexus, 23 8 Myoblast, loS, 185 Myocardiuni, 140 primitive, 143 Myotome, 108 Myxoedema, 282 Nabothi, ovula, 338 Nails, the, 356 cells of the, 358 development of, 366 eponychium of, 358 growth of, 358 hyponychium, 358 keratohyalin of, 358 lunula of, 358 matrix of, 356 prickle cells of, 358 structure of, 356 technic of, 358 Nail-bed, 356 groove, 356 wall, 356 Nares, 264 accessory nasal sinuses, 264 cells of, 265 basal, 265 olfactory, 265 sustentacular, 265 development of, 280 glands of Bowman, 265 membrana limitans olfactoria, 265 stroma of, 265 structure of, 264 olfactory region, 264 respiratory region, 264 vestibular region, 264 technic of, 269 zone of oval nuclei, 265 of round nuclei, 265 Nasal duct, 513 Nemileff", showing amitosis, 48 showing meduUated nerve fibre, 120 Neopallium, 477, 478, 479 bundles of, 479 efterent connections of, 479 Nerve cells, in; see also Neurone amacrine, 504 amphicytes, 383 anterior horn, 398 association, 483 basket, 195, 457 Betz', 47q, 482, 4S4 bipolar, 112, 384 566 INDEX. Xerve cells, brush, 531 Cajal's, 482 caryochromes, 114 cerebro-spinal ganglia, 382 column, 396, 398 cone-bipolar, 503 cone-visual, 503 efferent projection, 483 ependymal, 374 extrinsic, 396 ganglion, 382, 385, 397 giant, of Betz, 482 glia, 125 Golgi, Type I, 115, 117 Type II, 116, 117, 396, 399 hecateromeric, 398 heteromeric, 398 horizontal, 482, 503 in gray matter of cord, 396 intrinsic, 396 inverted pyramidal, 482 large granule cells, 458 Martinotti's, 482, 483 mitral, 530 mossy, 126, motor, of the anterior horn, 398 Miiller's, 504 multipolar, 112, 384 neuroblasts, iii, 126, 374 neuroglia, 374, 462, 482 of motor area of cerebral cor- tex, 483 outside the spinal cord, 397 peripheral motor, 395 sensory, 382 ]jolymorphous, 482 Purkinje, 456 pyramidal, 48 r inverted, 482 rod-bipolar, 503 rod- visual, 503 root, 396 small granule, 458 somatochromes, 1 (4 spider, 126 spinal ganglion, 390, 397 spongioblasts, 374 stellate, 457, 482 sympathetic ganglion, 390 tautomeric, 398 nnijKjlar, f i 2, 383 Nerve endings, 385; see Peripheral nerve terminations fibres, 113 afferent, 376 association, 478 climbing, 460 commissural, 478 cone, 503 deep tangential, 485 felt works of, 394 layer of, of retina, 504 medullated, 117 of cerebellum, 461 methods of staining, 29 mossy, 460 motor end plates of, 395 non-medullated, 117 of Bergmann, 462 origin of, of white matter of cord, 396 pallial, 478 pallio-pontile, 464 pallio-tectal, 479 pallio-thalamic, 479 parallel, of the cerebellum, 458 perpendicular pontile, 454 projection, 478 rod and cone, 503 superficial tangential, 488 terminations, 385 motor, 395 sensory, 385 tissue, 1 1 1 Golgi methods of staining, 32 neuroglia, 125 neurone, 1 1 1 axone, 116 cell body, 1 1 1 dendrites, 1 16 protoplasmic processes, 116 technic for, 127 Nerves, cranial, table of, 492 motor and sensory nuclei of, 377 III (oculomotor), 464, 465 oculomotor nucleus, 465 root fibres and nucleus of origin of third, 450, 465, 470 IV (trochlearis), 462, 464 root fibres and nucleus of origin of fourth, 462, 465 V (trigeminus), 425 INDEX. 567 Nerves, cranial, mesencephalic root of fifth, 452, 462, 465 motor nucleus of fifth, 452 semilunar ganglion of fifth, 425 sensory nucleus of fifth, 452 and motor root fibres of fifth, 452 spinal root of fifth, 432, 433, 435, 436, 438, 440, 441, 448, 449, 450, 451, 452 VI (abducens), 448 nucleus of origin of sixth, 447. 448, 450 abducentis, 450 root fibres of sixth, 447, 448, 450 VII (facial), 425 ganglion geniculate, 425 nucleus facialis, 447 of origin of seventh, 447 root fibres of seventh, 439, 447 VIII (auditory), 425, 431, 441 cochlear branch of eighth, 441, 439, 443 ganglion of Scarpa, 425, 441 spirale, 425, 441 nuclei of eighth, 438, 441, 450 Deiter's, 448, 449 von Bechterew's, 448, 454 root fibres of eighth, 441 vestibular branch of eighth, 425, 438, 441, 445, 448 IX (glosso-pharyngeal) , 425, 439 descending or sensory root fibres of the ninth, 439 dorsal nucleus of ninth, 439, 440 motor nucleus of ninth, 439 root fibres of ninth, 438, 439 X (vagus), 425, 437 descending or sensory root fibres of tenth, 437, 43S dorsal nucleus of tenth, 435, 437 ganglion jugular, 425 nodose, 425 motor nucleus of tenth, 438 root fibres of tenth, 437, 439 XI (spinal accessory), 43 i nucleus of origin of eleventh, 431 Nerves, cranial, root fibres of eleventh, 43 i XII (hypoglossal), 435, 436,437 nucleus of origin of twelfth, 435- 436, 437 root fibres of twelfth, 435, 437 mixed spinal, 380, 381 olfactory, 477 peripheral, 380 spinal, anterior, motor or efferent roots of, 395 sensory, or afferent portions, 382 Nervous system, the, 373 cerebro-spinal, 373 development of, 373 general structure of, 3 73 sympathetic, 373 central, 373 afferent peripheral neurones, 382 brain, 424 cerebro-spinal ganglia, 382 cranial nerves, 425, 492 development of, 373 efferent peripheral cerebro- spinal neurones, 395 general structure of, 373 histological development of, 373 membranes of brain and cord, 378 segmental part, 377, 425 spinal cord, 396 nerves, 380, 492 suprasegmental part, 377, 427 cerebro-spinal, 373 central nervous system, 373 peripheral portion, 373, 380 sy 111 pathetic, 373 development of, 375 ganglia, 373, 390 sympathetic nerves, 373, 31)0 Neumann's dental sheath, 203 Neural arc, 377, 420 cerebellar, 42 i cerebral, 42 i disynaptic, 42 i mono-synaptic, 420 ])allial, 42 I three-neurone, spinal, 421, 435, 452 568 INDEX. Neural arc, two-neurone, spinal, 420 fold, 373 groove, 373 plate, 373 tube, 373 cells of, 374 ependymal cells of, 374 marginal veil of His, 374 myelospongium of His, 374 neuroblasts, 374 neuroglia cells of, 374 spongioblasts of His, 374 Neuraxone, 1 1 1 Neurilemma, 117, 119 and axolemina, relation of, 120 -cells, 375 Neurite, 1 1 1 Neuro-epithelium, 6q cone bipolar cells, 503 visual cells, 503 rod bipolar cells, 503 visual cells, 503 Neurofibrils, 113 Cajal's method of staining, 34 importance of, in neurone, 121 Neuroglia, 73, 94, 125 mossy cells of, 126 Mailer's cells of, 504 neuroblasts, iii, 126, 374 of cerebellum, 462 spider cells, 126 spongioblasts of, 126 technic for, 127 Neurokeratin network, 1:9 Neurological staining methods, 29 Neuromuscular bundles, 388 Neurone, the, 1 1 1 axone of, 116 caryochromes, 114 cell body, 1 1 1 chromophilic bodies, 113, j 14 contact theory of, 122 continuity theory of, 122 cytoplasm of, 112 degenerative changes in, 122 dendrites of, 1 1 6 development of, i 1 1 extracellular network of, 122 functional centre of, 121 genetic centre of, 121 Golgi net, 1 22 neurofibrils of, 113, 1 sC> Neurone, Nissl, special method of technic for, 35 nucleolus, 112 nucleus, 112 nutritive centre of, 121 pericellular network of, 122 perifibrillar substance, 113 physiological significance of, 121 pigment in, 115 retraction theory of, 122 somatochromes, 114 synapsis of, 122 technic for, 35, 127 theory, 122 trophic centre of, 121 Neurones, afferent peripheral, 376, 382 segmental, 425 suprasegmental, 278, 426 associative, 421, 425 central, 375 cone association, 507 cord, 42 I cortical precentral, 421 efferent peripheral, 375, 395, 421 suprasegmental, 378, 426 peripheral segmental, 426 intermediate, 375, 376 intersegmental, 426 peripheral, 376 afferent, 376, 382, 420, 421 efferent, 375, 377, 395, 420, 421 ponto-cerebellar, 452 rod-association, 507 somatic (peripheral), 376 splanchnic (peripheral), 376 suprasegmental associative, 378 thalamo-cortical, 414 visceral (peripheral), 376 Neuroplasm, 118 Neutral carmine, 18 Neutrophile granules, 98 Nipple, 368 Nissl method for staining nerve cells, 3 5 pathological value of, 1 1 5 concerning chromophilic l)odies, Its Nitric acid for decalcifying, 10 for dissociating muscle tissue, 5 Nodes of Ranvier, 119 Nodose ganglion of X nerve, 425 INDEX. 569 Normoblasts, i68 Notochord, anlage of, 56 Nuclear dyes, 1 5 alum carmine, 1 7 basic anilin, 17 combination of Gage's and Mayer's formulas, 16 Delafield's haematoxylin, 15 Gage's haematoxylin, 15 haematoxylin, 15 Heidenhain's haematoxylin, 16 Mayer's haemalum, 16 Weigert's hasmatoxylin, 17 eccentricity, 124 fluid, 44 groups, 343 membrane, 43 sap, 44 structures, method of demon- strating by Flemming's fluid, 7 Nuclein, 44 Nucleolus of typical cell, 44 false, 44 Nucleoplasm, 44 Nucleoreticulum, 44 Nucleus, the, 42 abducentis, 450 accessory olivary, 438 amygdaliformis, 481 arciform, 477 arcuatus, 477 caudatus, 472, 478, 481 chromatin of, 44 Deiterls, 418, 426, 448 dentate, 41S, 450, 455 dorsal cochlear, 441 Edinger-Westphal, 465 emboliformis, 450, 455 facialis, 447 fastigii, 418, 450, 455 function of, 43 funiculi cuneati, 414, 426 gracilis, 414, 426 globosus, 450, 455 interstitial, of Cajal, 417, 476 karyoplasm of, 44 lenticularis, 477, 478 linin of, 44 membrane of, 43 network of, 44 nuclein of, 44 Nucleus, nucleoreticulum of, 44 nucleolus of, 44 oculomotor, 465 of acoustic tubercle, 43 i, 439, 441 of a typical cell, 42 of column of Burdach, 414, 430, 435 of Goll, 414, 430, 435 of Darkschewitsch, 417 of Luys, 469 of origin, 377 oculomotor, 465 ohvary, 430, 438, 439, 448 pontis, 464 pulposus, 180 red, 417, 41S, 465, 467 resting, 51 reticularis tegmenti, 452 ruber, 417, 418, 465, 467 tecti, 418, 450, 455 terminal, 377 trapezoid, 441, 449 triangular, 438 vestibular, 438, 441, 45° von Bechterew's, 448 Nuel's space, 527 Nutrient canal, 170 foramen, 170 vessels, of bone, 170 Oculomotor III nerve, 464, 465 nucleus, 465 Odontoblasts, 201, 206, 211 Oesophagus, the, 213 coats of, 213 glands of, 214 technic of, 215 Oil of origanum Cretici for clearing specimens, 2 i Olfactorius (I nerve) , 477, 492 Olfactory bulb, 477, 530 cells of, 530 granule layer, 530 layer of glomeruli, 530 of longitudinal fibre bundles, 531 of mitral cells, 530 of olfactory fibres, 530 molecular layer, 530 olfactory glomeruli of, 531 path, 425, 477, 478, 485 pallial commissure, 4 78 570 INDEX. Olfactory group of segmental neu- rones, 425 nerve, 425, 477 organ, 530 olfactory bulb of, 530 mucosa of, 264, 530 technic of, 532 tract, 532 Olivary nucleus, 430, 437, 439, 448 Omentum, 235 gastro-hepatic, 235 greater, 235 Opie, concerning the pancreas, 251 concerning the cell-islands of Langerhans, 252 Oppel's method of staining intra- lobular connective tissue of liver, 261 Optic chiasma, 472, 507 cup, 516 decussation, 507 depressions, 515 nerve, 425, 467, 47°' 505 arachnoid of, 505 diagram showing, with some principal connections, 509 dural sheath of, 505 lamina cribrosa, 506 pial sheath, 505 relation to retina and brain, 506 subarachnoid space, 505 subdural space, 505 technic of, 517 stalk, 5, 15 tract, 467, 470, 472, 481 vesicle, 5 i 5 Opticus (II nerve), 425, 467, 470, 492 Ora serrata, 498, 501 Oral glands, cells of, 194 crescents of Gianuzzi, 195 demilunes of Heidenhain, 195 mixed glands, j 94 mucous glands, j 94 serous glands, 194 technic of, j 96 Orange G, x8 Organ of Corti, 525 cells of Claudius of, 527 Corti's arches, 526 tunnel, 526 IJeiter's cells, 527 Organ of Corti, hair or auditory cells, 526 Hensen's cells, 527 lamina reticularis of, 527 Nuel's space of, 527 phalangeal processes, 527 pillar cells, 525 of Giraldes (paradidymis), 311, 347 of hearing, 518; see also Ear blood-vessels of, 527 development of, 529 ear, external, 518 internal, 520 middle, 519 lymphatics, 528 nerves, 528 technic of, 529 of smell, 530; see Olfactory organ olfactory bulb, 530 mucosa, 530 tract, 532 technic of, 532 of taste, 532 cells of, 532 foliate papillae, 532 gustatory canal, 532 intergeminal fibres of, 533 intrageminal fibres of, 533 taste buds, 199, 269, 387, 532 technic of, 533 of vision, 494 blood-vessels of, 511 development of, 5 1 5 eyeball or bulbus oculi, 494 eyehd, 513 lacrymal apparatus, 512 lens, 510 lymphatics of, 512 nerves of, 512 neurone systems of, 506 optic nerve, 505 technic of, 517 Orgiins, the, 129 circulatory system, 131 digestive system, 192 glands and general structure of muctjus membranes, 186 lymphatic organs, J47 muscular system, 183 nervous system, 373 INDEX. 571 Organs, reproductive system, 303 respiratory system, 264 skeletal system, 164 skin and its appendages, 351 special sense organs, 494 urinary organs, 286 of Golgi, peripheral nerve ter- minations in, 390 of special sense, 494 organ of hearing, 5 1 8 of smell, 530 of taste, 532 of vision, 494 Orth's fluid (formalin-Miiller's) , 7 Osmic acid as a fixative, 7 action on fat, 7 on myelin, 7 stain for fat, 28 Osseous labyrinth, 520 Ossicles of middle ear, 520 Ossification centres, 172 endochondral, 172 intracartilaginous, 172 intramembranous, 172 subperichondral, 172 subperiosteal, 172 Osteoblasts, 173 Osteoclasts, i 74 Osteogenetic tissue, 173 Otic ganglion, 390 vesicle, 529 Otocyst, 529 Otolythic membrane, 522 Otoliths, 522 Oval bundle of Flechsig, 418 Ovary, the, 323 blood-vessels of, 333 corpora lutea, of pregnancy, 330 spuria, 332 vera, 332 corpus albicans, 330 hasinorrhagicum, 329 luteum, 329 cortex of-, 323 egg nest, 325 epoophoron, ^;^;^ Fallopian tube, 323, 334 germinal epithelium of, 324 Graafian follicles, 324 haematoidin crystals, 330 hilum of, 323 lutein cells, 329 Ovary, lymphatics of, ^^^ medulla of, 323 nerves of, 333 ovarian stroma, 323 oviduct, 323, 334 ovum, 323, 327 paroophoron, 333 Pfliiger's egg tubes or cords, 324 primitive ova, 324 secretion of, 323 structure of, 323 technic of, 335 tunica albuginea, 324 zona vasculosa, 323 Oviduct, the, 334; see Fallopian tubes Ovula Nabothi, 338 Ovum, the, 47, 52, 323, 327 atresia of follicle, t,^^ cells of, 327 deutoplasm granules, 328 development of, 327, 348, 349 fertilization of, 52 germinal spot, 327 maturation of, 328 perivitelline space, 327 segmentation of, 56 yolk granules of, 328 zona pellucida of, 327 Oxyntic cells, 219 Oxj'phile cells, 285 Pacchionian bodies, 171, 380 Pacinian bodies, 388 corpuscles, 320, 3S5 Palate, mucous membrane of, 193 Palatine tonsils, 155; see Tonsils Pallio-pontile fibres, 464, 479 Pallial conections, 413, 414, 416, 421, 422, 428, 469, 470 Pallio-spino-peripheral efferent con- duction path, 41 7 Pallio-thalamic fibres, 469, 479 Pallio-tectal fibres, 479 Pallium, 374, 477, 478, 481 association cells of, 483, 484 cortical areas of, 488 fibres of, 478, 479, 4S5 Pancreas, the, 247 blood-vessels of, 252 cell-islands of Langerhans, 250 572 INDEX. Pancreas, centro-acinar cells of Lan- gerhans, 249 development of, 263 duct of Santorini, 247 of Wirsung, 247 epithelium of ducts, 248 excretory ducts, 247, 248 intracellular secretory tubules of, 250 lobules of, 247 lymphatics of, 252 nerves of, 252 Opie, concerning cell-islands, 252 secondarj^ excretory dvict of, 247 secretion of, 249 sustentacular cells of, 250 technic of, 252 terminal tubules of, 248 zymogen granules of, 248 Paneth, cells of, 228 Panniculus adiposus, 353 Papillae, circumvallate, 198 compound, 352 filiform, 197 fungiform, 198 nerve, 352 simple, 352 vascular, 352 Paradidymis, or organ of Giraldes, 311. 347 Paraffin embedding, 12 apparatus for, 13 oven, 12 section-cutting, 14 sections, staining and mounting of, 21 Paramiton, 41 Paranuclein, 44 Paraplasm, 42 Parathyreoids, 283 chief or clear cells of, 285 development of, 285 function of, 285 oxyphile cells, 285 Pool's theory of, 285 structure of, 283, 284 technic of, 285 Pareleidin, 355 Parenchyma of glands, 188, 243 parietal cells, 219 Paroojjhoron, 333 Parotid gland, 243 development of, 263 intercalated tubule of, 244 nerves of, 241 Stenoni's duct of, 243 technic of, 247 Parovarium, 313 Pars ciliaris retinas, 499, 501, 505 iridica retinae, 501, 505 optica retinae, 501 papillaris, 352 reticularis, 351 Peduncle, inferior, 447, 449 middle, 447 superior, 447, 450, 452, 454, 464, 47o> 477 Peduncles, cerebral, 464 Pellicula, 42 Penicillus, 161 Penis, 319 arteries of, 320 cavernous sinuses, 320 corpora cavernosa of, 319 corpus spongiosum of, 319 erectile tissue, 319 glans, 321 glands of Tyson of, 321 lymphatics, 320 nerve endings of, 320 prepuce of, 321 sebaceous glands of, 321 technic of, 322 tunica albuginea of, 319 Peptic glands, 219 Perforated space, anterior, 477, 481 Perforating fibres, 168 of cornea, 496 of Sharpey, 168 Perforatorium, 314 Periaxial sheath, 118 Pericardial cavity, 144 Perichondrium of bone, 175 of cartilage, 92 Perichorioidal lymph spaces, 497 Pericranium, 173 Peridental membrane, 205 Peri fascicular sheath, 184, 382 Perifibrillar substance, 113 Perilymph, 520 Perimysium, 184 Perineurium, 382 Periosteal buds, 176 INDEX. 573 Periosteum, 167, 174 dental, 205 primary, 175 Peripheral afferent neurones, 376, 382 cerebro-spinal ganglia, 3S2 sj^mpathetic ganglia, 390 efferent neurones, 377, 395 motor neurone system, 395 nerves, 376 aft'erent part of, 380 cranial, 380 efferent part of, 380 endoneurium of, 382 epineurium of, 381 fascicles of, 381 intrafascicular connective tis- sue of, 382 medullated fibres of, 117, 118 motor or eft'erent, 380 motor nerve terminations, 395 non-medullated fibres of, 117, 118 perifascicular sheath of, 382 perineurium of, 382 sensory or afferent, 380 sensory nerve terminations, 385 sheath of Henle, 382 spinal, 380 structure of, 380 technic of, 382 nerve terminations, 386 annular, 387 end-bulbs, 386, 388 free endings, 386, 394 in penis, 320 in mucous membrane of mouth and conjunctiva, 387 in muscle-tendon junctions, 387 in skin, 365, 386 in smooth muscle, 388, 394 in voluntary muscle, 388 Krause's end-bulbs in penis, 321 Meissner's corpuscles in pap- illae of penis, 321 muscle spindles, 388 muscle-tendon organs of Golgi, 390 neuromuscular bundles, 388 Pacinian bodies, 388 corpuscles of penis, 320 Peripheral nerve terminations, Ruf- fini's theory of, 388 spinal nerves, 380 spiral terminations, 388 tactile cells, 386 corpuscles, 386 meniscus, 386 taste buds, 387, 533 Peritoneal cavity, 144 Peritoneum, 235 parietal, 235 subserous tissue of, 235 visceral, 235 Perivitelline space, 327 Permanent teeth, 208, 211 Perpendicular pontile fibres, 453 Pes pedunculi, 374, 427, 464,465,470 Petit, canal of, 511 Petrosal ganglion of IX, 425 Peyer's patches, 228 Pfliiger's egg tubes or cords, 324, 349 Phseochrome granules, 299 Phaeochromoblasts, 302 Phagocytes, 98, 152 Phagocytosis, 98 Phalangeal processes, 527 Pharyngeal tonsils, 157; see Tonsils Pharynx, the, 212 blood-vessels of, 213 lymphatics of, 213 nerves of, 213 structure of, 212 technic of, 213 Pia mater, 380 blood-vessels of, 3 So cerebralis, 380 Pacchionian bodies, 380 spinalis, 380 technic of, iSo Picric acid as a fixative, 8 as plasma dye, 18 Picro-acid-fuchsin, 18 Picro-carmine, 19 Pigment granules in cells, 42, 355 in connective tissue, 77 in epithelium, 64 in nerve cells, 116 Pillar cells, 525 Pineal body, 490 brain sand of, 490 technic of, 290 Pineal eye, 490 574 INDEX. Pinna, 51S Pituitary body, 489 anterior lobe of, 489 Berkley, concerning posterior lobe, 490 posterior lobe of, 490 technic of, 490 Placenta, 341 blood-vessels of, 344 canalized fibrin, 343 cell patches, 343 chorionic villi, 341 cotyledons, 341 fastening villi, 341 foetalis, 341 free or floating villi of, 341 lymphatics of, 345 membrana chorii of, 341 nerves of, 345 nuclear groups, 343 septa of, 344 subchorionic placental decidua, 344 syncytium of, 343 technic of, 349 uterina, 343 villi of, 341 Plasma cells, 75 Plasma dyes, i 7 acid aniline, 18 eosin, i 7 neutral carmine, 18 picric acid, 18 Plasmosome, 44 Plastids, 41 Plastin, 40 Pleura, parietal, 273 pulmonary, 273 Pleural cavity, 144 Pleuroperitoneal cleft, 144 Plexiform layer of Cajal, 482 Plexus annularis, 5 i 2 Auerbach's, 225, 230, 232, 238,390 ciliary, 5 i 2 chorioideus, 431, 450 Heller's, 236 Meissner's, 222, 238, 390 myentericus, 238 prevertebral, 390 Plicae palmataj, 338 Pneumogastricus (vagus nerve), 425, 43 7. 403 Polar bodies, ^^ rays, 48 Polymorphonuclear leucocytes, 97 Polymorphous cells, 482 Polynuclear leucocytes, 97, 152 Pons Varolii, 427, 374, 431, 447, 449 longitudinal fibres of, 449 perpendicular fibres, 449, 453 pontile nuclei of, 449 pyramid of, 449 transverse fibres of, 449 Pool, E. H., concerning parathyreoid gland, 285 Portal canal, 255 vein, 254 Posterior columns of spinal cord, 397, 401 origin of fibres of, 396, 397 column of Burdach, 408 of GoU, 408 commissure, 468, 472 distribution of fibres of, 414 horns, 398, 401 longitudinal fasciculus, 439 median septum, 401 nerve root, 403 root fibres, 403 nucleus funiculi cuneati, 414 funiculi gracilis, 408 of the column of Burdach, 414 of the column of GoU, 414 tract, or terminal zone of Lis- sauer, 397, 403 zone of Lissauer, 397 Potassium hydrate, as a macerating fluid, 4 Precapillary artery, 134 Predorsal fasciculus, 452, 464 Prepuce, 321 Preparation of sections, 5 Preserving, 9 Prevertebral plexuses, 390 Primary germ layers, 5O renal vesicles, 348 Primitive ova, 324 Projection fibres, 469, 47°. 47^. 479 Pronephric or Wolffian ducts, 347 Prone]jhroi, 346 Prcjnucleus, female, 53 male, 53 Prophase, 48 Proprio-ceptors, 390 INDEX. oto Prosencephalon, 373 Prostate gland, 317 blood-vessels of, 318 capsule of, 317 corpora amylacea of, 317 crescentic corpuscles of, 317 epithelium of, 317 lymphatics of, 318 Miillerian duct, 318 nerves of, 318 technic of, 319 trabeculae, 317 urethra, 317 uterus masculinus, 318 utriculus prostaticus, 317, 318 vesicula prostatica, 317 Protargol, for staining intercellular substance, 26 Protoplasm, 39, 41 streaming of, 47 theories of structure of, 40 Protoplasmic movement, 47 processes, 1 1 1, 116 Proximal convoluted tubule, 290, 292 Prussian blue gelatin as an injecting fluid, 23 Pseudopodia, 46 Pulvinar radiations, 470 thalami, 470, 472 Pulmonary artery, 278 lobule, 273, 278 pleura, 273 Pulp cavit}^ 200 cords, 161 of Mall, 162 splenic, 161 Purkinje cells, 456, 461 Putamen, 481 Pyloric glands, 219 Pylorus, 230 Pyramid, cortical, 281 of Ferrein, 288 Malpighian, 288 Pyramidal cells, 482 decussation, 416, 429 tracts, 416, 479 anterior pyramids, 416, 431, 447. 454 crossed pyramidal, 416 direct pyramidal, 417 pyramidal decussation, 4 i 6 of Tiirck, direct pyramidal, 417 Pyramids, 3 74 Pyrenin, 44 Pyriform lobe, 477 Racemose glands, 188 Radiations of Meynert, 485 Radix spinalis, V, 425 Rami communicantes, gray, 351, 392 white, 381, 390, 391, 395 Ranvier's alcohol as a macerating fluid, 4 nodes or constrictions of, iig showing muscle fibres, 103 Raphe, of semicircular canals, 522 median, 436, 438 Receptors, 376, 390, 424, 425 Receptor to effector, direct path, 423 Rectum, 234 anus, 234 columnse rectales, 234 technic of, 241 Red blood cells, 95; see also Blood nucleated, 161, 168 bone marrow, 168 nucleus, 417, 418, 465, 467 Reduction of chromosomes, 315 Reflex arc, cerebellar, 421 cerebral, 421 disynaptic, 42 i monosynaptic, 420 pallial, 42 I three-neurone spinal, 421 two-neurone spinal, 420 Reissner, membrane of, 524 Remak, fibres of, 117 Renal corpuscle, 288 development of, 288, 347 Renculus, 286 Replacing cells, 65, 220 Reproduction of cells, 47 Reproductive system, 303 development of, 314, 324, 346 rudimentary structures con- nected with the, 311 female organs, 323 clitoris, 346 Fallopian tube, 334 ovary, 323 oviduct, 334 placenta, 341 urethra, 32 i uterus, ; ;6 576 INDEX. Reproductive system, female organs, vagina, 345 vestibule, 346 male organs, 303 Cowper's glands, 319 ejaculatory ducts, 311 penis, 319 prostate gland, 317 seminal ducts, 309 vesicle, 311 seminiferous tubule, 304 spermatozoa, 313 testis, 303 urethra, 321 Respiratory bronchus, 274 cells, 274 epithelium, 295 Respiratory system, 264 bronchi, 269, 274 development of, 279 general references for further study, 285 larynx, 266 lungs, 273 nares, 264 technic of, 269, 280, 285 trachea, 266 Restiform body, 430, 438, 439 Rete testis, tubules of, 304, 309 vasa efferentia, 309 Reticular formation, 419,426, 431,433, 437. 439. 447. 449. 452, 454 glands, 190 process, 401 tissue, 83 Retina, 501 blood-vessels of, 511 cells of, 501, 502, 503, 504 ellipsoid of Krause, 503 fibre-baskets of, 505 fovea centralis, 505 ganglionic layer of, 501 horizontal cells of, 503 inner limiting membrane of, 504 molecular layer, 504 nuclear layer, 503 layer of nerve cells, 504 of nerve fibres, 504 of neuro-epithelium, 502 of pigmented eijithelium, 501 of rods and cones, 502 macula lutea, 505 Retina, Miiller's cells and fibres, 504 ora serrata, 501 outer limiting membrane, 504 molecular layer, 503 nuclear layer, 502 pars ciliaris retinse, 501, 505 iridica retina, 501, 505 optica retinae, 501 relation to optic nerve, 506 rod and cone cells of, 503 visual purple of, 503 Retinaculse cutis, 353 Retrolenticular portion of internal capsule (Cirl), 472 Retzius, lines of, 204 Rhinencephalon, 477, 478 Rhinopallium, 477 Rhombencephalon, 373, 429 Ribboning paraffin sections, 14 Rod association neurones, 507 fibres, 503 Rod-visual cells, 502 Rods, layer of rods and cones, 502 Rolando, gelatinous substance of, 437 RoUett's theory of striated muscle, 104 Root canal, 200 cells, 398 Roots, afferent, 376 Ruffini, corpuscles of, 365 theory of nerve terminations, 388 Rugae, 216, 218, 345 Riihle, cohcerning the uriniferous tubule, 293 Saccular glands, 187, 190 Saccule, 521 and utricle, 521 auditory hairs of, 522 macula acustica, 521 neuro-epithelial cells of, 521 otolithic membrane of, 522 otoliths of, 522 sustentacular cells of, 521 Sachs, E., concerning thalamus, 469, 470 Sacral segments of spinal cord, 396 Safranin, 17 Salivary corpuscles, 157 glands, 193, 242 blood-vessels of, 245 capsule of, 242 IXDEX. 577 Salivary glands, development of, 263 ducts of, 242 interstitial tissue, 243 lobes of, 242 lobules of, 242 lymphatics of, 245 minute structure of, 194 nerves of, 246 parenchyma of, 243 parotid, the, 243 secretory tubules of, 242 sublingual, the, 243 submaxillary, the, 244 trabeculse of, 242 technic of, 247 tubules of, 242 Santorini, cartilage of, 266 duct of, 247, 266 Sarcolemma, 102 Sarcoplasm, 102 Sarcostyles, 185 Sarcous elements of Bowman, 103 Satellite cells, 383 Scala media, 523 tympani, 523 vestibuli, 523 Scarpa's ganglion, 425 Schafer and Opie concerning cell- islands of Langerhans, 252 Schlemm, canal of, 500 Schmidt-Lantermann, clefts of, 11 incisions of, iig segments of, 119 Schreger, incremental lines of, 202 Schultze, comma tract of, 419 Schwalbe, lymph paths of, 512 Schwann, sheath of, 116, 119 Sclera, the, 494 lamina cribrosa of, 494 fusca of, 494 Scrotum, skin of, 303 Sebaceous glands, 264, 355, 362 development of, 366 of glans penis, 355 of labia minora, 355 of margin of lips, 264, 355 of prepuce, 355 Sebum, 363 Secondary cochlear tract, 441 trigeminal tract, 433 vestibular tract, 441 Secretion, 239 Secretory tubules, 242 Golgi method of demonstrating, 26 of parietal cells of stomach, 219 Section cutting, 13 celloidin specimens, 14 paraffin specimens, 14 staining, 15 Segmental brain, 377, 425 nerves, 377, 425 Segmentation cavity, 55 of ovum, 56 Segments of Schmidt-Lantermann, 119 • of spinal cord, 396, 409, 410, 411 Semen, 313 Semicircular canals, 520, 522 crista acustica of, 522 cupula of, 522 raphe of, 522 semilunar fold of, 522 Seminal ducts, 309 epididymis, 309 vas deferens, 310 vasa efferentia, 309 vesicles, 311 ' ■ Seminiferous tubules, 304 cells of, 304 columns of Sertoli, 305 convoluted portion of, 304 development of, 349 glandular cells of, 305 rete testis, 304, 309 spermatids, 307 spermatocytes, 307 spermatogenic cells, 305 spermatogones, 306 spermatozoa, 307 straight portion of, 304 supporting cells of, 305 sustentacular cells of, 305 Semilunar ganglion of V, 425 Senses, common, 390, 425 general, 390 special, 390 Sensory decussation, 435 path, general, 390, 413, 414, 425, 452, 470, 485 peripheral nerves, 3 So Septa renis, 2 88; see Kidney Septo-marginal tract, 418 Septum linguae, 197 578 INDEX. Serial sections, 14 Serous membranes, 144 Sertoli, cells of, 305, 315 columns of, 305 Sex cells, 348, 349 Sharpey, showing bone lamellae, 167 Sharpey's fibres, 168, 205 Sheath of Henle, 120, 382 medullary, 117, 119 of Schwann, 117, 119 perifascicular, 382 Sherrington concerning receptors, 390 Signet-ring cell, 86 Silver-nitrate method of staining inter- • cellular substance, 26 Skein, closed, 49 Skeletal system, 164 articulations, 180 bone-marrow, 168 bones, 164 cartilages, 180 general references for further study, 182 technic of, 171, 179, 181 Skin and its appendages, 351 blood-vessels of, 364 color of, 355 corium, 351 corpuscles of Grandry, 386 of Meissner, 365, 387 of Ruffini, 365 of Wagner, 365 cuticle, 353 derma, 351 development of, 366 eleidin of, 354 end-bulbs in, 386 epidermis of, 353 glands of, 355 glandulae sudoriparse, 355 Golgi-Mazzoni corpuscles of, 365 hair follicles of, 359 junction of, with mucous mem- brane of mouth, 193, 264 keratohyalin granules, 354 Krause's end-bulbs, 365 lymphatics of, 365 mammary gland, 368 Merkel's corpuscles of, 386 mitosis of cells of, 354 nails, 356 nerves of, 365, 386 Skin, of scrotum, 352 Pacinian bodies of, 38S panniculus adiposus of, 353 papilla; of, 352 pareleidin, 355 pars papillaris, 352 pars reticularis, 351 peripheral nerve terminations in, 386 prickle cells of, 354 retinaculse cutis, 353 sebaceous glands of, 355 subcutaneous tissue of, 352 sweat glands (glandulse sudorip- arse), 355 pores of, 355 tactile cells of, 386 corpuscles, 365, 387 technic of, 355 for blood-vessels of, 366 Vater-Pacinian corpuscles of , 365 Small intestines, 223 agminated follicles, 228 Auerbach's plexus, 230, 238 blood-vessels of, 326 Brunner's glands of, 230, 239 cells of, 226, 227 chyle capillaries of, 237 coats of, 224 crypts of Lieberkiihn, 224, 228, 230. 239 development of, 262 lacteals of, 227, 236, 237, 240 lymphatics, 237 Meissner's plexus, 230, 238 muscle of, 230 nerves of, 238 Peyer's patches of, 228 plexus myentericus, 238 replacing cells, 227 secreting cells, 225 solitary follicles, 228 technic of, 241 valvulae conniventes of, 216, 223 villi of, 224, 237 Smooth muscle, joi; see Involuntary ^nuscle vSoflium hydrate as a macerating (iuid, 4 Solitary fasciculus, 438, 439 follicles, 221, 228 Somatic fperi]jheral) neurones, 376 INDEX. 579 Somatochromes, 114 Spaces of Fontana, 500 Spalteholz, iq6 Spermatids, 307, 314 Spermatoblast, 316 Spermatocytes, 307, 314 Spermatogenesis, 313 technic of, 317 Spermatogones, 306, 314, 349 Spermatozoa, 52, 307, 313 acrosome, 314 apical bod}^ 314 development of, 52, 314 diagram of, 53, 313 galea capitis, 313 perforatorium, 314 structure of, ^2, 314 technic of, 317 Sphenopalatine ganglion, 390 Spider cells, i 26 Spinal accessory nerve, 493 Spinal cord, 396 anterior columns of, 401 horns of, 398, 401 marginal bundle of Lowenthal, 41S median fissure, 401 nerve roots of, 403, 412 p^j^ramids, 416 white commissure of, 403 antero-lateral columns of, 401 ascending tract, 4 1 5 ; see Ven- tral s pino-cerebellar descending tract, 418 arachnoid membrane of, 380 arrangement of fibres of, 405 arteries of, 406 ascending tracts of, 413, 426 blood-vessels of, 406 cell-groupings of, 403 cells of dorsal horn, 403 cells of the intermediate gray matter, 403 of Golgi, Type II, 3()g of ventral horn, 404 central canal of, 373 gelatinous substance, 402 cervical enlargement of, 396 segments of, 396 Clarke's colunin of, 403, 40S coccygeal seginents of, ^,q(^ column of Burdach, 40S, 414 Spinal cord, column of Goll, 408, 414 cells, 398 hecateromeric, 398 heteromeric, 398 tautomeric, 398 comma tract of Schultze, 419 conduction paths of, 377, 417 cornua of, 401 crossed pyramidal tract, 416 descending paths from higher centres, 419, 427 tract from Deiter's nucleus, 418 from vestibular nuclei, 418 diagram showing tracts of, 412 direct ascending paths to higher centers, 413, 415. 423 cerebellar tract, 415 pyramidal tract, 417 reflex collaterals, 405 dorsal gray columns, 401 commissure, 402 spino-cerebellar tract, 415 white columns, 401 dura mater of, 370 ependyma of, 406 fasciculus, medial longitudinal, 418 of Thomas, 418 fibre tracts of, 408 methods of determining, 40S filum terminale of, 396 finer tructure of, 405 fundamental columns of, 399, 419 ganglion cells of, 397 gelatinous substance of Rolando, 402 general topography of, 401 Gowers' tract, 416 gray matter of, 377, 401 ground bundles of, 399. 41Q H el wag's tract, 418 interchange of fibres, 405 intermediate gray matter, 403 intermedio-lateral column, 403 lateral horn of, 403 long ascending arms of dorsal root fibres, 413 longitudinal section of six days' chick embryo, 401 lumbar enlargement of, 396 580 INDEX. Spinal cord, lumbar segments of, 396 main motor fibre systems of, 395, 420 marginal bundle of Lowenthal, 41S marginal zone, 402 medullated fibres of, 395, 406 membranes of, 378 arachnoid, 380 blood-vessels of, 380 dura mater, 379 pia mater, 380 technic of, 380 mixed spinal nerve, 425, 452, 492 motor cells of anterior horn, 398 multipolar ganglion cells of, 112, 384, 397 neuroglia cells, 406 fibres, 406 tissue, 403 neurone systems of 371, 420, 421 nucleus, Darkschewitsch's, 417 Deiter's, 418 funiculi cuneati, 414 gracilis, 414 origin of fibres of white matter, 396 of posterior columns of, 397 oval bundle of Flechsig, 418 peripheral motor or efferent neurone system, 375 395 sensory or afferent neurone system, 376, 382 pia mater, 379, 401 plexus of fine fibres, 405 posterior columns, 397 funiculus, 401, 413 horns, 398, 401 median septum, 401 nerve roots, 403 root fibres, 403 postero-lateral grooves, 401 sulci, 40 r pyramidal decussation, 416 tracts, 416, 426 reflex arcs, 420 reticular process, 401, 408 root cells, 398 rubro-spinal tract, 417 sacral segments of, 396 scheme of neurone relations of, 412 Spinal cord, section through cervical enlargement of, 408 through lumbar enlargement, 401 through mid-thoracic region, 408 through six-day chick embryo, 400 through twelfth thoracic seg- ment, 408 segments of, 396, 409, 410, 411 septo-marginal tract, 418 shape of, 396 short fibre systems of, 399, 419 size of, 396 spino-tectal tract, 415 -thalamic tract, 414 tecto-spinal tract, 417 technic of, 421 thoracic segments of, 396 tract from interstitial nucleus of Cajal, 417 tractus cerebro-spinalis, 416 cortico-spinalis, 416 pallio-spinalis, ,416 reticulo-spinalis, 419 spino-cerebellaris dorsalis, 415 spino-spinalis, 419 ventralis, 415 variations in structure at dif- ferent levels, 407 veins of, 407 ventral gray columns, 401 commissure, 402 white columns, 401 vestibulo-spinal tract, 418 von Monakow's tract, 417 white commissure, 403 matter, 377, 396 zona spongiosa, 402 terminalis, 403 zone of Lissauer, 403 Spinal ganglia, 382 amphicytes, 383 capsules of, 383 development of, 375 technic of, 394 ganghon cells, 390, 397 ascending arms from central processes of, 390 central processes of, 390 classification of, 383 INDEX. 581 Spinal ganglion cells, collaterals from, 384 descending arms from central processes of, 390 development of, 375 Dogiel's classification, 383 ectodermic origin, 373 modes of termination of per- ipheral processes of, 385 peripheral processes of, 3S5 relation to dorsal roots, 376 Ruffini's classification of termi- nations in muscle spindles, 388 satellite cells, 383 structure of, 382 technic of, 394, 399 Spinal nerves, 380 Spindle, achromatic, 48 Spino-cerebellar tract (dorsal) 415,426 (ventral), 415, 426 Spino-peripheral motor neurone sys- tem, 375, 395 Spino-tectal tract, 415, 433, 437, 439, 447, 450 Spino-thalamic tract, 414, 431, 450, 452, 453- 465 Spiral ganglion, 425, 528 ligament, 523 organ, 525 prominence, 525 terminations, 388 Spireme, closed, 49 open, 49 Spireme-thread, 50 Splanchnic (peripheral) neurones, 376 Spleen, 15S ampulte, 161 blood-vessels, 160 cavernous veins, 161 cells of, 161 central arteries of, 160 connective tissue framework, 159 cords of, 161 corpuscles of, 160 ellipsoids of, 160 germinal centres of, 159 lymphatics of, 162 Mall's theory of vascular chan- nels of pulp, 162 Malpighian bodies, i^q, nerves of, 162 Spleen, penicillus, 161 pulp of, 159, 161 cords of, 161 spindles of, 160 technic of, 163 Splenic corpuscles, 159 pulp, 159, 161 Spheno-lymph nodes, 152 Spongioblasts, 115 of His, 374 Spongioplasm, 40 Spongy bone (cancellous) 164, 174 primary, 177 Staining, 1 5 differential, 3 double with haematoxylin-eosin, 18 in bulk, 19 methods, special, 26 Golgi's chrome silver for secre- tory tubules, 26 Jenner's, for blood, 28 Mallory's aniline blue for con- nective tissue, 28 phosphomolybdic acid haem- atoxylin stain for con- nective tissue, 27 phosphotungstic acid hsema- toxylin stain for connec- tive tissue, 27 osmic acid, for fat, 28 silver nitrate, for intercellular substance, 26 Weigert's elastic-tissue stain, 26 paraffin sections, 20 sections, 18 double with htematoxylin- eosin, 18 triple with hasmatoxylin- picro-acid-fuchsin, 19 with picro-acid-fuchsin, 18 with picro-carmine, 19 selective, 3 special neurological methods, 29 Cajal's methods for neuro- fibrils in nerve cells, 34 Golgi bichlorid method, ^t, silver method, 32 Marchi's, for degenerating nerves, 3 i Nissl's method, 35 582 INDEX. Staining, Weigert's, for niediiUated nerve fibres, 2g Weigert-Pal method, 30 Stains, nuclear dj^es, 15 plasma d^^es, i 7 Stalked hydatid, 312 Stapes, 520 Stellate cells, 457, 482 Stenoni, duct of, 243 Stomach, 217 acid cells of, 21Q, 23Q adelomorphous cells of, 219 Auerbach's plexus, 225 blood-vessels of, 326 chief cells of, 219, 239 delomorphous cells of, 219 development of, 262 epithelium of, 218 fundus glands of, 219 gastric crypts of, 218 glands of, 218 pits of, 218 lymphatics of, 237 mucous membrane of, 218 muscular coat of, 221 nerves of, 238 parietal cells of, 219, 239 peptic cells of, 219, 239 glands of, 2 1 9 pyloric glands of, 219, 220 rugae of, 216, 218 secretion of, 239 solitary follicles of, 2 2 l stroma of, 220 technic of, 223 Stohr, scheme of spleen, 160 Stomata, 145 Stratum cinereum, 468 corneum, 354 cylindricum, 353 fibrosum, 181 germinativum, 354 granulosum, 354 lemnisci, 468 lucidum, 354 Mal]jighii, 353 mucosum, 353 opticum, 468 spinosum, 354 zonale, 467 synoviale, j8i Streaming of protoplasm, 47 Stria meduUaris, 477, 4 78 terminalis, 477 vascularis, 525 Strise thalami, 468 Stroma of mucous membranes, 191 of the red blood cell, 96 Styloglossal fibres, 197 Subchorionic placental decidua, 344 Sublingual gland, 243 crescents of Gianuzzi of, 244 development of, 262 duct of Bartholin of, 244 nerves of, 246 technic of, 247 Sublingualis minor, 244 Submaxillary ganglion, 390 gland, 244 development of, 262 duct of Wharton of, 244 nerves of, 246 technic of, 247 Submucosa, 191 Subperichondrial ossification, 172, 177 Subperiosteal ossification, 172, 177 vSubstantia alba, 377 grisea, 377 nigra, 464, 465, 467, 470 pro]jria corneae, 496 Sulcus, external spiral, 525 Superior cerebellar peduncles, 447, 45°. 452, 454, 464 coUiculus, 467 ganglion of IX, 425 longitudinal fasciculus, 479, 481 olive, 451 Suprarenal gland, 299 blood-vessels of, 300 chromaffin granules, 299 development of, 301 lipoid granules, 299 lymphatics, 301 nerves of, 30 j phaeochrome granules, 299 ])haeochromoblasts, 302 sympiithoblasts, 302 technic of, 302 Supraradiary i)lcxus, 4S5 Su])r£isegmenla1 brain, 424, 427 cerebral heniis])heres, 427 connections (afferent and ef- ferent), 426, 42 7 IXDEX. 5S3 Suprasegmental brain, corpora quad- rigemina, 42 7 intersegmental nuclei and tracts of segmental brain, 427 nuclei and tracts forming supra- segmental paths, 427 pallium, 427 peripheral (segmental) neu- rones, 427 terminal nuclei, 427 neurones, 378 afferent, 378 associative, 378 efferent, 378 Suspensory ligament, 510 Sustentacular cells, 250, 265, 305, 521 Sweat glands, 355 development of, 366 ducts of, 355 muscle tissue of, 368 pore, 3S5 Sympathetic ganglia, 390 cells of, 392 chain ganglia, 390 development of, 375 in Auerbach's plexus, 390 in Meissner's plexus, 390 pigmentation of cells of, 392 prevertebral plexuses, 390 structure of, 390 technic of, 394 termination of nerves, 394 vertebral ganglia, 390 nervous system, 373 Synapsis of neurones, 122 Synarthrosis, 180 Synchondrosis, 180 Syncytial cells, 374 Syncytium, 62, 108, 343 Syndesmosis, 180 Synovial membrane, t8i viUi, 181, 182 Szymonowicz, showing intercellular bridges, 66 showing medullated nerve fibre, 119 Tactile cells, 386 corpuscles, 196, 386 of Meissner, 387 of Wagner, 387 meniscus, 386 Taenia thalami, 477 Tapetum cellulosum, 497 fibrosum, 497 Tarsal glands, 514 Tarsus, 514 Taste buds, 199, 269, 387, 533 Tautomeres, 398, 404 Teasing, 4 Technic, general, 3 Tecto-spinal tract, 417, 452, 464, 467 Tectum mesencephali, 415 Teeth, 200 blood-vessels of, 206 cementum of, 200, 204, 212 circular dentoid ligament, 205 crown of, 200 cuticula dentis, 204 dental canals, 201, 21 r germ, 206 groove, 206, 209 papilla, 206 periosteum, 205 ridge, 208 sac, 208, 211 dentinal fibres, 201 pulp, 201 dentine of, 200, 201 development of, 206, 212 common dental germ, 206 cuticular membrane, 209 dental papilla, 206 ridge, 208 enamel organ, 206 special dental germ, 206 technic of, 212 Tomes' process, 209 enamel of, 200, 204, 208 cells, 2og fibres, 204, 205 organ, 206, 20S prisms, 204, 2og fang of, 200 incremental lines of Schreger, 202 interglobular spaces, 203, 2ri lines of Retzius, 204 lymphatics of, 206 milk, 200 nerves of, 206 Neumann's dental sheath, 203 odontoblasts of, 201. 206 peridental membrane, 205 ]:)ermanent, 20S, 211 pulp cavity, 200 oS4 INDEX. Teeth, root of, 200 canal of, 200 special dental germs, 206 technic of, 212 Tomes' granular layer, 204 process, 209 true molars, 212 Tegmentum, 374, 427, 431, 447 brachia conjunctiva, 465 central tegmental tract, 439, 447, 454, 464 development of, 374 fillet, 397 fourth cranial nerve, 464, 465 lateral lemniscus, 397 nucleus ruber, 465 posterior longitudinal fasciculus, 467 reticular formation, 467 superior coUiculi, 467 peduncles, 465 Telencephalon, 373 Telophase, 5 1 Tendon, structure of, 78 sheaths, 184 Tendon-muscle junction, 184 organs of Golgi in, 390 peripheral-nerve terminations in, 388 Tenon, capsule of, 512 Tensor chorioideae, 500 Terminal arborizations, 117 bronchus, 274 nucleus, 438 Terminations, nerve, 385 annular, 388 arborescent, 388 Ruffini's theory of, 388 spiral, 388 Testis, 303 blood-vessels of, 312 corpus Highmori, 303 development of, 349 ducts of, 309 epididymis of, 303 lobules of, 303 lymphatics of, 312 mediastinum, 303 nerves, 3 13 secretion of, 3 r 3 semen, 3 13 seminal ducts of, 309; Testis, seminiferous tubules of, 304 spermatozoa, 307, 313 technic of, 3 16 tunica albuginea of, 303 vaginalis, 303 vasculosa, 303 vas deferens, 304 Thalamencephalon, 468 Thalamo-cortical neurones, 414, 469 470 Thalamus, 468, 470 anterior peduncle of, 481 bundle of Vicq d'Azyr, 469 external segment of, 469 internal segment of, 469 mamillo-thalamic tract, 469 nuclei of, 468, 469 nucleus of Luys, 469 Sachs, E., concerning the, 469, 470 thalamic radiations, 469, 470, 472 Theca folliculi, 326 Thermostat, 1 2 Thermotaxis, 46 Thionin, 17 Thoma, ampulla of, 162 Thomas, fasciculus of, 418 Thoracic duct, 143 technic for, 145 Three-neurone afferent suprasegmen- tal conduction path, 378 spinal reflex arc, 421, 435, 452 Thrombocytes, 98 Thymus, 153 blood-vessels of, 155 developraent of, 153 Hassall's corpuscles, 154 lymphatics of, 155 nerves of, 155 technic, 155 Thyreoid, 280 absence of, 282 blood-vessels of, 282 cartilage, 266 cells of, 281 colloid of, 281 development of, 282 isthmus of, 281 lymjjhatics of, 282 nerves of, 282 technic, 285 INDEX. 5S.3 Tiniofeew, concerning nerve fibres in prostate gland, 318 Tissue, connective, 73; see also Con- nective tissue elements, dissociation of, 4 Tissues, 59 adipose, 85 blood, 95 bone, 92 cartilage, 89 classification of, 61 connective, 73 derivatives from ectoderm, ento- derm, mesoderm, 61 dissociation of, 4 elastic, 78 endothelium, 70 epithelial, 63 erectile, 319, 345, 346 • examination of fresh, 4 fat, 85 general references for further study of, 127 histogenesis of, 61 lymphatic, 84 mesothelium, 70 rauscle, 10 1 nerve, 1 1 1 osteogenetic, 173 subcutaneous, 352 subserous, 235 Toluidin blue, 17 Toluol, as solvent, 13 Tomes' granular layer, 202 process, 209 Tongue, 196 blood-vessels of, 199 circumvallate papillae, 198 connective tissue of, 197 Ebner's glands, 199 end-bulbs of Krause, 200 fibres of, 196 filiform papillas, 197 fungiform papillae, 198 glands of, 199 longitudinal fibres of, 197 lymph follicles of, 157, 199 spaces of, 199 muscles of, 196 nerves of, 200 papillae of, 197 septum linguae, 197 Tongue, taste buds, 199, 200, 3S7, technic of, 200 transverse fibres of, 196 vertical fibres of, 196 Tonsils, 155 blood-vessels of, 158 crypts of, 156 development of, 157 lingual; folliculas linguales, 157 foramen caecum lingui of, 157 lymphatics of, 158 l3^mphoid infiltration of epithe- lium, 157 nerves of, 158 nodule of, 156 germ centre of, 156 palatine or true, 155 pharyngeal, 157 adenoids of, 157 salivary corpuscles of, 157 technic of, 158 Trachea, 266 blood-vessels of, 2 68 cartilages of, 267 glands of, 267 lymphatics of, 269 muscle cells of, 268 nerves of, 269 technic of, 269 Tract, antero-lateral ascending — ven- tral spino-cerebellar, 415 anterio-lateral descending, 41S Burdach's, 408, 414 central tegmental, 439, 447, 454, 464 cochlear, 450, 452 comma, of Schultze, 419 cortico-spinalis, 416 crossed pyramidal, 416 descending, from Deiter's nu- cleus, 418, 435 from interstitial nucleus of Cajal, 417, 433, 435 from vestibular nuclei, 41S direct cerebellar, 415 pyramidal, 417 dorsal spino-cerebellar, 415, 426, 433. 45°. 452, 454 dorso-lateral ascending — dorsal spino-cerebellar, 415 fasciculus of Thomas, 418 586 INDEX. Tract, fundamental or ground bundles, 399. 419 Flechsig's, 415 Goll's, 408, 414 Gowers', 416 Helweg's, 418 Lissauer's, 397, 403 long ascending arms of dorsal root fibres, 413 raamillo-thalamic, 469 marginal bundle of Lowenthal, 418 oval bundle of Flechsig, 418 pallio-spinalis, 416 posterior, 414 funiculi, 413 pyramidal, 416 reticulo-spinalis, 433 rubro-spinal, 417, 433, 435, 447 secondary cochlear, 44 1 vestibular, 441 septo-marginal, 418 short fibre, 399 spinalis trigemini, 433 spino-cerebellar, ventral, 415, 426, 433, 450, 452, 453, 464 spino-tectal, 415 spino-thalamic, 414, 431, 45°, 452, 453. 465. 470 tecto-spinal, 417 Turck's, 417 uncrossed cerebellar, 415 Von Monakow's, 417 Tractus, see Tract Transitional leucocytes, 97 Transverse temporal gyri of Heschl, 485 Trapezoid nucleus, 441 Trapezium, 44 i Trigeminus (V nerve), 425, 492 Trigonum, 6r olfactorium, 477 Trochlearis (IV nerve), 462, 464, 492 Trophospongium, 42 True corpora lutea, 330 tonsils, 155 Tuberculum cinereum, 430, 477 olfactorium, 477 Tubules, arched, 29;, 293 collecting, 29;, 293 distal, 29 f, 292 first or i^roximal, 290, 292 Tubules, intercalated, 243 salivary, 242 second or distal, 291, 292 secreting, 242 seminiferous, 304 straight, 291, 293, 308 uriniferous, 288 Tubulo-alveolar gland, 248 Tunica albuginea, of ovary, 324 of penis, 319 of testis, 303 dartos, 352 propria, of mucous membranes, 191 vaginalis, 303 vasculosa, 303 Two-neurone spinal reflex arc, 420, 452 Tympanic membrane, 519 Tympanum, 519; see also Ear, middle Tyson, glands of, 321 Ultimate fibrillae, 103 Uncinate fasciculus, 479, 481 Unipolar nerve cells, 112 Ureter, 297 blood-vessels of, 297 coats of, 297 development of, 346 glands of, 297 lymphatics of, 297 nerves of, 297 technic of, 349 Urethra, female, 321 glands of Littre of, 321 male, 321 fossa navicularis, 322 glands of Littre of, 322 technic of, 322 Urinary bladder, 298 blood-vessels of, 299 cells of, 298 development of, 346 c])ithelium of, 298 fllirous layer of, 299 lymjjhatics of, 299 muscular layers f)f, 2(;() mucous membrane of, 298 nerves of, 299 system, 286 development of, 30 j, 346 IXDEX. 587 Urinary system, kidney, 286 pelvis, 297 suprarenal gland, 299 ureter, 297 urinary bladder, 298 technic of, 302 Uriniferous tubule, 288 arched tubule of, 289, 291 ascending arm of Henle's looj), 288, 291 blood-vessels of, 293 Bowman's capsule, 290 descending arm of Henle's loop, 288, 290 development of, 288 duct of Bellini, 289 epithelium of, 292, 293 first or proximal convoluted, 289, 290 foramina papillaria, 289 glomerulus, 288, 289 Henle's loop, 263, 288 location in kidney of, 292 Malpighian body, 288, 289 meinbrana propria of, 289 neck of, 290 renal corpuscle, 288 Rlihle, concerning, 293 second or distal convoluted, 289, 291 straight or collecting, 289, 291 Uterus, 336 blood-vessels of, 344 cervix, 337 coats of, 336 decidua basalis, 340 capsularis, 340 graviditatus, 340 menstrualis, 339 reflexa, 340 serotina, 340 subchorionic placental, 344 vera, 340 decidual cells of, 340 development of, 346; see also Reproductive system, develop- ment of lymphatics of, 345 masculinus, 3 1 8 mucosa of menstruating, 338 of pregnant, 340 of resting, 337 Uterus, muscle cells of, 337 stratum submucosum, 336 supravasculare, 336 vasculare, 336 nerves of, 345 placenta, 341 pregnant, 340 theories concerning, 340 stage of menstruation jjroper, 339 of preparation, 338 of reparation, 339 techriic of, 349 with placenta in situ, technic of. Utricle, 521; see also Saccule and utricle Utriculo-saccular duct, 521 Utriculus prostaticus, 317, 318 Uvula, mucous membrane of, 193 Vagina, 345 blood-vessels of, 345 coats of, 345 lymphatics of, 346 nerves of, 346 rugag of, 345 technic of, 350 Vagus nerve, 425, 437, 492 Valve, Heisterian, 260 Valves of heart, 141 of veins, 138 Valvulae conniventes, 216 Vas deferens, 309, 310 technic of, 317 epididymis, 309 Vasa efferentia, 309, 347 vasorum, 139 Vascular papillae, 352 system, 131; see also Circulatory system Vater-Pacinian cor]Hiscles, 365 Veins, 137 adventitia of, 138 arcuate, 296 central, 255 coats of, 137 development of, 142 intima of, 138 lymph channels of, 139 media of, 138 588 INDEX. Veins, musculature of, 138 nerves of, 139 perivascular lymph spaces of, 139 portal, 254 renal, 286 splenic, 160 stellate, of Verheyn, 296 sublobular, 255 technic of, 139 valves of, 138 vasa vasorum, 139 venae vorticosae, 497 Venas vorticosse, 497 Ventral spino-cerebellar tract, 414 Ventricle, fourth, 374, 430, 437, 447 chorioid plexuses of, 374 muscle of, 140 Verheyn, stellate veins of, 296 Vermiform appendix, 232 coats of, 232 lymph nodules of, 234 mesoappendix, 233 technic of, 241 Vermis of cerebellum, 415, 449, 455 Vesicle, air, 274 brain, 373 germinal, 53 optic, 515 otic, 529 seminal, 311 Vesicula prostatica, 317 Vestibular ganglion, 425 nerve, 425, 441 nuclei, 441 descending tract from, 441 Vestibule, 520 ductus reuniens of, 521 endolymphatic duct, 521 saccule of, 521 utricle of, 521 utriculo-saccular duct of, 521 Vicq d'Azyr, bundle of, 469 Villi, 224, 237, 341 development of, 262 lacteals of, 240 synovial, 18 r Visceral neurones, 376, 425 peritoneum, 192 Visual area, 488 path, 425, 470, 472, 474, 475, 485, 488 Visual purple, 503 Vital properties of cells, 45 function, 46 irritability, 46 metabolism, 45 motion, 46 reproduction, 47 Vitreous body of the eye, 511 Cloquet's canal, 511 hyaloid canal of, 511 membrane of chorioid, 498 of iris, 501 Vocal cords, 266 Volkmann's canal, 167, 171 Voluntary striated muscle, 102; see Muscle, striated, voluntary Von Bechterew's nucleus, 448, 453 Von Bibra, concerning chemical com- position of dentine, 201 Von Gudden, concerning method of determining fibre tracts of cord, 413 Von Monakow's bundle, 417 Wagner, corpuscles of, 365 Walker, coccygeal gland, 145 Wallerian degeneration, law of, 112 Wandering cells, 75 Warthin, showing haemolymph node, 151, 152 Washing after fixation, 8 Weigert's elastic-tissue stain, 26 haematoxylin, 17 method of staining medullated nerve fibres, 29 Weigert-Pal method, 30 Wernicke, perpendicular fasciculus of, 479 Wharton's duct, 244 Wheeler, showing amitosis, 47 White blood cells (leucocytes), 96 White matter, 377, 403 rami communicantes, 381, 390, 301, 395 Wilson, E. B., diagrams showing mi- tosis, 48, 49 Wirsung, duct of, 247 Wolffian body, 308, 347 bodies, 347 ridges, 346 Wrisburg, cartilage of, 266 INDEX. 589 Xylol and cajeput oil for clearing, -balsam, 21 damar, 28 Xylol-paraffin for embedding, 13 Yolk granules, 328, 330 Zenker's fluid for decalcifying, 9 for fixation, 8 Zinn, Zonule of, 510 Zona incerta, 472 pectinata, 525 pellucida, 55, 327 spongiosa, 402, 403 tecta, 525 Zone of Lissauer, 397, 40J of oval nuclei, 265 of round nuclei, 265 Zonula ciliaris, 510 Zonule of Zinn, 510 Zymogen granules, 24S technic of, 252 Y^^o^ ^Msi^Wtf^^^v