TEXT-BOOK OF HISTOLOGY ,^'' BY > FREDERICK R. BAILEY. A. M.. M. D. FIFTH REVISED EDITION PROFUSELY ILLUSTRATED UNI NEW YORK WILLIAM WOOD AND COMPANY MDCCCCXVl / 551 G 79493 2. 7. 5s PREFACE TO THE FIFTH EDITION The very gratifying approval which the previous 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 rewritten. Some figures have been added, for which the writer wishes to acknowledge his obligations. To Prof. H. von W. Schulte and to Prof. A. M. Miller the writer has been indebted for many valuable criticisms and suggestions. The chapter on the nervous system, which was rewritten by Dr. Oliver S. Strong for the third edition and revised by him for the fourth edition, remains unchanged. For Dr. Strong's careful and pains- taking 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 again to express his most grateful appre- ciation. Dr. Strong again desires to acknowledge his indebtedness to Dr. Adolf Meyer for many ideas and terms found valuable in the preparation of the chapter on the nervous system. m 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 especially 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 considerable amount of anatomy which this chapter contains is the apparent success the method has met with in the teaching of this always difficult subject to st ^nts. The chapter on g' ral technic is intended to furnish the student with only the more sential laboratory methods. For special and more elaborate me ds the student is referred to the special works on technic mentioned at the close of the chapter. The special tech- nic 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. Miller, 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 been taken. For all of these the author wishes to express his appreciation and obli- gation. 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 supervision of the drawing of Figs. 263 and 264; to Dr. G. C. Free- born, his predecessor as Instructor of Histology at the College of Phy- sicians 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 General Technic. Page General Considerations, 3 Examination of Fresh Tissues, 3 Dissociation of Tissue Elements, 4 Teasing, 4 Maceration, 4 Preparation of Sections, 5 Fixation, 5 Hardening, 9 Preserving, . . . lo Decalcifying, lo Embedding, n Celloidin Embedding, ii Paraffin Embedding, 13 -Section Cutting, 14 Celloidin Sections, 15 Paraffin Sections, 15 Frozen Sections, 16 Staining, 17 Nuclear Dyes, 17 Plasma Dyes 20 Staining Sections, 20 Staining in Bulk, / 22 Mounting, 22 Staining and Mounting Paraffin Sections, 24 Injection, 25 CHAPTER II Special Staining Methods. Silver Nitrate Method of Staining the Intercellular Substance, 28 Chlorid of Gold for Demonstrating Connective-tissue Cells, 28 Weigert's Elastic Tissue Stain 28 Verhoeff's Diflerential Elastic Tissue Stain, 28 Golgi's Chrome-silver for staining Secretory Tubules, 29 Mallory's Phosphomolybdic Acid Haematoxylin Stain for Connective Tissue, 29 Mallory's Phosphotungstic Acid Haematoxylin Stain for Connective Tissue, . 30 vii viii - CONTENTS Page Mallory's Aniline Blue Stain for Connective Tissue 30 Maresh's Modification of Bielschowsky's Stain for the Finer Connective- tissue Fibrils 31 Osmic Acid Stain for Fat, 31 Sudan III and Charlack R for Fat, 31 Jenner's Blood Stain, 32 CHAPTER III Special Neurological Staining Methods. Weigert's Method of Staining Medullated Nerve Fibres, ^^ Weigert-Pal Method, 34 Marchi's Method for Staining Degenerating Nerves, 35 Golgi ^Methods of Staining Nerve Tissue, 36 Slow Method, 36 Rapid Method, 36 Mixed Method, 36 Formalin Bichromate Method, 37 Bichloride Method, 37 Golgi-Cox Method,. ...'.. 37 Cajal's Method, 38 Nissl's Method, 39 General References on Technic, 40 PART II.-THE CELL CHAPTER IV \ The Cell, 43 General Structure, 43 Structure of a Typical Cell, 44 The Cell Body, 44 The Cell Membrane, 47 The Nucleus, 47 The Centrosome, 49 Vital Properties of Cells, 50 Metabolism, 50 Function, 51 Irritability, 52 Motion, 52 Amoeboid, 52 Protoplasmic, 52 Ciliarj', 53 Reproduction, 53 Direct Cell-division 53 Indirect Cell-division, 54 Fertilization of the Ovum, 59 Technic, . . ?* 63 References for further study, 65 CONTENTS ix PART III.-THE TISSUES CHAPTER V Page Histogenesis — Classification, 6g Tissues Derived from Ectoderm, 69 Tissues Derived from Entoderm, 69 Tissues Derived from ^Mesoderm, 70 CHAPTER VI Epithelium (Including Mesothelium and Endothelium), 72 Histogenesis, 72 General Characteristics, 73 Classification, 73 Simple Epithelium, 74 Simple Squamous, 74 Simple Columnar, '. 75 Pseudostratified, 75 Stratified Epithelium, 75 Stratified Squamous, 75 Stratified Columnar, 77 Transitional, 77 Modified Forms of Epithelium, 78 Ciliated Epithelium, 78 Pigmented Epithelium, 80 Glandular Epithelium, 80 Neuro-epithelium, 80 Mesothelium and Endothelium, 80 Technic, 82 CHAPTER VH The Connectr'e Tissues, 84 General Characteristics, 84 Development, 85 Classification, 86 Connective Tissue Proper 86 Embryonal Connective Tissue, 86 Fibrillar Connective Tissue, 86 , Connective-tissue Cells, 87 Intercellular Substance, 90 Areolar or Loose Connective Tissue, ... 91 Fat Tissue, 91 Formed Connective Tissue, 95 Tendons and Ligaments, 95 Elastic Tissue, 96 . Reticular Tissue, 98 , Technic, 99 Cartilage, 102 Hyaline, 102 X CONTENTS Page Elastic, 103 Fibrous, 103 Technic, 104 Bone, 104 Technic, 106 CHAPTER Vni The Blood, 107 Red Blood Cells, 107 White Blood Cells, 109 Blood Platelets, 112 Blood Dust, 112 Development, 113 Technic, 114 CHAPTER IX Muscle Tissue, 115 Involuntary Smooth Muscle, 115 Voluntary Striated Muscle, 118 Involuntary Striated Muscle (Heart Muscle), 123 Development of Muscle Tissue, 126 Technic, 129 CHAPTER X Nerve Tissue, 150 The Neurone, 130 General Structure, 130 The Cell Body, 130 The Nucleus, . . ." 131 The Cytoplasm, 132 Neurofibrils, 132 Perifibrillar Substance, 132 Chromophilic Bodies, 133 The Protoplasmic Processes or Dendrites, 135 The Axone, 136 Non-medullated Axones (Non-medullated Nerve Fibres), 136 Medullated Axones (MeduUated Nerve Fibres), 137 Theories as to Physiology of the Neurone, 146 Significance of Degenerative Changes in the Neurone, 148 Neuroglia, 149 Technic, 140 General References, 141 PART IV.— THE ORGANS CHAPTER XI The Circulatory System, i55 The Blood-vessel System, i55 CONTENTS XI Page General Structure, 155 Capillaries, iSS Arteries 157 Veins, 162 Technic, 164 The Heart, 165 Technic, 167 Development of the Circulatory System, 167 The Lymph-vessel System, 169 Lymph Capillaries, 169 Lymph Spaces, 169 Development of Lymph-vessel System, 170 Technic, 170 General References, 170 CHAPTER XII Lymphattc Organs, 171 Lymphatic Tissues. 171 The Lym.ph Nodes, ^ 171 Lymph 17S Chyle 175 Development, 176 Technic, 176 Hasmolymph Nodes, i77 Technic, i79 The Thymus, i79 Development, 181 Technic, 182 The Tonsils, 182 The Palatine Tonsils, 182 The Lingual Tonsils, 184 The Pharyngeal Tonsils, 184 Development, 184 Technic, 185 The Spleen, 185 Technic, 191 General References 192 CHAPTER XIII The Skeletal System 193 The Bones, 193 Bone Marrow, 197 Red Marrow, 199 Yellow Marrow, 201 Technic, 202 Development of Bone 202 V/ Intramembranous Development, 203 Intracartilaginous Development, 205 Subperiosteal, 207 Growth of Bone, 208 Technic, 208 The Cartilages, 209 xii CONTENTS Page Articulations, 209 Technic, 211 General References, 211 CHAPTER XIV The Muscular System. A Voluntary IMuscle, 212 Tendon Sheaths and Bursre, 213 Growth of Muscle, 214 Technic, 216 CHAPTER XV Glands and the General Structure of Mucous Membranes, 217 Glands — General Structure and Classification, 217 Duct Glands, 221 Tubular Glands 221 Alveolar Glands, 222 Ductless Glands, 222 General Structure of Mucous Membranes, 223 CHAPTER XVI The Digestive System, 224 Anatomical Divisions, 224 The Headgut, 225 The Mouth, 225 The Mucous Membrane of the Mouth, 225 Glands of the Oral Mucosa, 225 Technic, 228 The Tongue, 228 Technic, 232 The Teeth, 232 Development of the Teeth, 243 Technic, 247 The Pharynx, 247 Technic, 248 The Foregut, 248 The Oesophagus, 248 ^^ Technic, 250 General Structure of the Walls of the Gastro-intestinal Canal, .... 250 . The Stomach, 252 Technic, 259 The Midgut, 260 I The Small Intestine, 260 Peyer's Patches, 265 The Endgut, 26S The Large Intestine, 268 The Verrmform Appendix, 270 The Rectum, 271 CONTENTS xiii Page The Peritoneum, IMesentery, and Omentum,. . . 272 Blood-vessels of the Stomach and Intestine, ... ... 273 Lymphatics of the Stomach and Intestine, 275 Nerves of the Stomach and Intestine, 276 Secretion and the Absorption, 277 Technic, 280 The Larger Glands of the Digestive System, 281 The Salivary Glands, 281 The Parotid, 282 The Sublingual, 283 The Submaxillary, 284 Technic, 286 The Pancreas, 286 Technic, 292 The Liver, 292 Excretory Ducts of the Liver, 294 The Gall-bladder 301 Technic, 301 Development of the Digestive System, 302 General References, 304 CHAPTER XVTI The Respir.atory System, 305 The Xares, 305 The Larynx, 307 The Trachea, 307 Technic, 310 The Bronchi, 310 The Lungs, 3^4 Development of the Respiratory System, 321 Technic, • • 323 General References, 323 CHAPTER XVIII The Urinary System, .... 324 Ix' The Kidney, .....* 324 The Kidney-Pelvis and Ureter, 335 The Urinary Bladder, 336 Technic, 337 General References, 338 CHAPTER XIX The Reproductive System, 339 Male Organs, 339 The Testis, 339 The Seminal Ducts, 345 The Epididymis, 345 The Vas Deferens, 34^ xiv CONTENTS Page The Seminal Vesicles and Ejaculatory Ducts, 348 Rudimenlary Structures Connected with the Development of the Genital System, 348 The Spermatozoon, 349 Development of the Spermatozoon, 350 Technic, 352 The Prostate Gland, 353 Cowper's Glands, 354 Technic, 355 The Penis, . 355 The Urethra, 357 Technic, 358 Female Organs, 358 t-- The Ovary, 35^ The Graafian Follicle, 360 The Corpus Luteum, 365 Rudimentary organs connected with the Ovary, 369 The Oviduct, 37° Technic, 371 The Uterus, 372 The Mucosa of the Resting Uterus, 373 The Mucosa of the Menstruating Uterus, 374 The Mucosa of the Pregnant Uterus, 375 The Placenta, 377 The Vagina, 381 Development of the Urinary and Reproductive Systems, 382 Technic, 3^5 General References, 3^5 CHAPTER XX The Skin and its Appendages, 386 The Skin, 386 Technic, . . .- 39° The Nails, 39^ Technic, 393 The Hair, 393 Technic, ' 399 Blood-vessels of Skin, 399 Technic, 4°° Development of Skin, Nails, and Hair, 401 The Mammary Gland, 402 Technic, 407 General References, 407 CHAPTER XXI The Thyreoid and Parathyreoid, the Pituitary Body, the Paraganglia and THE Adrenal. The Thyreoid, 408 Development, 409 CONTENTS XV Page The Parulhyreoids, 410 Technic, 412 The Pituitary Body, 4^3 Paraganglia 4^5 Carotid Glands, 4^6 Coccj'geal Gland, 4^7 Adrenal Gland, 418 General References, 4^1 CHAPTER XXII The Nervous System, 422 Histological Development and General Structure, 422 Membranes of the Brain and Cord, 428 Technic, 43° The Peripheral Nerves, 43° Technic, 432 The Afferent Peripheral Neurones, 432 The Cerebro-spinal Ganglia, 432 The Peripheral Processes of the Cercbro-spinal Ganglion Cells, . . 435 The Central Processes of the Cerebro-spinal Ganglion Cells, . . . 442 The Sympathetic Ganglia, 442 Technic, 447 The Efferent Peripheral Cerebro-spinal Neurones, 447 The Spinal Cord, 448 Origin of the Fibres which make up tlie White Matter of the Cord, . . . 449 (i) The Spinal Ganglion Cell and the Origin of the Posterior Col- umns, 449 (2) Cells Situated in Other Parts of the Central Nervous System which Contribute Axones to the White Columns of the Cord, . . 450 (3) Root Cells — Motor Cells of the Anterior Horn, 45° (4) Column Cells, 45° (5) Cells of Golgi Type II, 45i Technic, 45 ^ Practical Study, 453 General Topography of the Cord, Cell Groupings, Arrangement of Fibres and Finer Structure, 454 Practical Study of Sections through Lumbar Enlargement, .... 454 General Topography, 454 Cell Groupings, 45^ Arrangement of Fibres, 458 Finer Structure, 459 Blood-vessels, 460 Variations in Structiure at Different Levels, 461 Practical Study, 461 Section through the Twelfth Thoracic Segment, 4^1 Section through the Mid-thoracic Region, 461 Section through the Cervical Enlargement, 461 Fibre Tracts of the Cord, 465 Ascending Tracts, 467 I. Long Ascending Arms of Dorsal Root Fibres, 467 xvi CONTENTS Page II. Spino-thalamic Tract, 468 III. Dorsal Spino-cerebellar Tract, 469 IV. Ventral Spino-cerebellar Tract, , 469 Descending Tracts, 470 I. The Pyramidal Tracts, 470 II. The Colliculo-spinal Tract, 471 III. The Tract from the Nucleus of the Posterior or Medial Longitudinal Fasciculus, 471 IV. The Rubro-spinal Tract, 472 V. The Deitero-spinal Tract, 472 VI. The Fasciculus of Thomas, 473 VII. Helweg's Tract, 473 VIII. The Septo-marginal Tract, 473 IX. The Comma Tract of Schultze, 473 Fundamental Columns or Ground Bundles, 474 A Two-neurone Spinal Reflex Arc, 475 A Three-neurone Spinal Reflex Arc, 475 A Cerebellar Arc, 476 A Cerebral or Pallial Arc, 477 Technic, 477 The Brain, 479 General Structure, 479 Segmental Brain and Nerves, 480 Suprasegmental Structures, 484 The Hindbrain or Rhombencephalon, 485 The Medulla Oblongata or Bulb, 485 The Pons, 488 The Cerebellum (also p. 507), 488 Technic, 488 Practical Study, 489 1. Transverse Section of the Medulla through the Decussation of the Pyramidal Tracts (Motor Decussation), 489 2. Transverse Section of the Medulla through the Decussation of the Fillet or Lemniscus (Sensory Decussation), .... 491 3. Transverse Section of the ISIedulla through the Lower Part of the Inferior OHvarj^ Nucleus, 493 4. Transverse Section of the ^Medulla through the IMiddle of the Olivary Nucleus, 497 5. Transverse Section of the Medulla through the Entrance of the Cochlear Root of Nerve VIII, 497 6. Section through the Hindbrain at Level of Junction of Pons, and Cerebellum and Entrance of Vestibular Nerve, . . . 505 7. Transverse Section of the Hindbrain through the Roots of Nerves VI (Abducens) and VII (Facial), 508 8. Transverse Section of the Hindbrain through the Roots of Nerve V (Trigeminus), 510 The Cerebellum, 5^3 The Cerebellar Cortex, S'^5 The Isthmus, S^i Practical Study, 521 CONTEXTS xvii Page g. Transverse Section through the Isthmus at the Exit of Nerve IV (Trochlearis), 521 The IMidbrain or Mesencephalon, 523 Practical Study, 523 10. Transverse Section through Midbrain at Level of Anterior Corpora Quadrigcmina and Exit of Nerve III (Oculomotor), 523 The Forebrain or Prosencephalon, 528 The Interbrain (Diencephalon or Thalamencephalon), 528 Practical Study, 53° ir. Transverse Section through the Junction of Midbrain and Thalamus, 530 12. Section through the Interbrain at the Level of the Optic Chiasma, 532 The Endbrain or Telencephalon, 538 The Rhinencephalon, 538 The Corpus Striatum, 538 The Pallium, 53-"^ Practical Study, 54i 13. Transverse Section through the Cerebral Hemispheres, Corpora Striata and Thalamus, 541 The Cerebral Cortex, 542 Technic, 549 The Pineal Body, 55° Technic, 55° Table of Cranial and Spinal Nerves, 55^ General References for Further Study, 553 CHAPTER XXIII The Organs of Special Sense, 554 The Organ of Vision, 554 The Eyeball, 554 The Cornea, 554 The Chorioid, 55^ The Ciliary Body, 558 The Iris, 560 The Retina, 561 The Optic Nerve, 565 The Relations of Optic Nerve to Retina and Brain, 566 The Lens, 57° The Lacrymal Apparatus, 573 The Eyelid, 573 Development of the Eye 575 Technic, 576 The Organ of Hearing, 578 The External Ear 578 The Middle Ear, 579 The Internal Ear, 580 The Vestibule and Semicircular Canals, 581 The Saccule and Utricle, 581 xviii CONTENTS Page The Semicircular Canals, 582 The Cochlea, 582 Development of the Ear, . . . .■ 589 Technic, 590 The Organ of Smell, 590 Technic, 592 The Organ of Taste, 593 Technic, 593 General References, 594 Index, . . . ■. 595 PART I HISTOLOGICAL TECHNIC CHAPTER I GENERAL TECHNIC Certain body fluids, such as blood and urine, may be ex- amined by simply placing them on a slide under a cover-glass. A few tissues, for example, thin membranes, such as the omentum and the mesentery, may be examined in the fresh or living condition by immersing them in some such inert medium as blood serum or normal salt solution (a 0.75-per-cent. solution of socUum chlorid in distilled water). For such examination the tissue is immersed in the salt solution on a slide and covered with a cover-glass. IMost tissues and organs, however, 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 hne shreds, leasing, or by cutting it into thin shces, section 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 which breaks down the substance holding the tissue elements together, maceration. Fresh tissue can rarely be cut into sections sufficiently thin for micro- scopic examination. It must first be treated in such a manner as to preserve as nearly as possible the living tissue relations, fixation. If too soft for section cutting it must next be put through a process known as hardening. If, however, as in the case of bone, the tissue is too hard, it must be softened by dissolving 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 know^n as embedding. Furthermore, most tissue elements have so nearly the same color, and possess refractive indices so similar, that their dift'erentiation under the microscope is often extremely difficult. To overcome this difiiculty, recourse is had to staining the tissue with dyes which have an affinity for certain only of the tissue elements, or which stain 3 4 HISTOLOGICAL TECHNIC different elements with dift'erent degrees of intensity. This is known as differential or selective staining. The final step in the process is the mounting of the specimen, after which it is ready for microscopic study. In this chapter only the more common procedures used in the preparation of tissues for microscopic study are described. 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 Certain of the structural features of such tissues as nerves, muscle, and epithelium, which have but little intercellular substance, or of the looser forms of connective tissue, may be well demonstrated by dissociation. This is accomphshed 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. In fact it is feasible to study many tissues in the same manner if tiny pieces are picked into fine shreds on the slide. This method possesses the advantage of exhibiting the tissues in their natural or living condition, whereas most of the methods subsequently described, although having distinct advantages of their own, exhibit the tissues after they|are killed and artificially colored. During the past few years extensive and important observations have been made on cells and tissues while living and actually growing. (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. ^^ GENERAL TECHNIC 5 (b) 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 espe- cially useful for dissociating epithelial cells. (c) Sodium or Potassium Hydrate (30- to 35-per-cent. aqueous solu- tion).— From twenty minutes to an hour is usually sufficient to cause the tissue elements to fall apart or to be readily pulled apart with teas- ing needles. If it is at any time desirable to stop the action of the caustic alkali, this may be accomplished by neutralizing with glacial acetic acid or by replacing the alkali with a 60-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 ofl", this being repeated until all trace of macerating fluid is removed. This washing is facilitated by using the centrifuge. The tissue elements may then be preserved or mounted in glycerin or in glycerin jelly. It is fre- quently advisable to stain the tissues. For this purpose alum-car- min (p. 19), and picro-carmine (p. 21), are especially satisfactory. (For details see technic i, p. 129 and technic 2, p. 129.) After stain- ing 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 still too large. This may be remedied by gently tapping on the cover-glass with a lead pencil. PREPARATION OF SECTIONS I. Fixation Even tissues, the structure of which admits of their being exam- ined in the fresh condition, as described on p. 3, soon undergo post-mortem changes if placed in an inert medium such as normal ' Commercial formalin is a 40-per-cent. solution of formaldehyde gas in water. 6 HISTOLOGIC.\L TECHXIC salt solution or blood serum. As a result they soon lose their char- acteristic appearance and disintegrate. A proper killing or fixation is therefore the first step in the j^ireparation of most tissues for micro- scopic study, the object being to so preserve the tissues that they retain as nearly as possible, and more or less permanently, the same structure and relation which they had during life. Fresh tissues are therefore essential for satisfactory results, as soon after death changes set in which destroy the structure and relations of the tissue elements. Tissues should also be handled as little as possible. Fixed tissue may be placed in water, salt solution, alcohol, etc., in fact may be subjected to many and varied manipulations which would destroy fresh tissues, without disturbing the relation of the tissue elements. 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 ox fixative. The tissue is immersed in the fixative and allowed to remain there until fixation is complete. The pieces of tissue should be small enough to permit rapid and thorough penetration, and large quantities of the fixative should be used; at least ten times the bulk of the tissue to be fixed. It may be necessary to change the fluid a number of times, in order to keep it up to the proper strength. The length of time required depends upon the character of the tissue and upon the fixative used. In general it should be only long enough to bring about the desired result, as prolonged immersion in the fixative may make the tissue brittle or may interfere with the subsequent treatment of the tissue, especially staining. 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 of an entire body. The following are the fixatives in most common use: (i) Absolute Alcohol. — This is a rapid fixative and should be used on small pieces of tissue. The time required is from six to GENERAL TECHNIC 7 twenty-four hours. thou[i;h 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) Formalin (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 lo-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 formalin are not always good, owing to the fact that it has little hardening power, and the subsequent action of alcohol is likely to cause some distortion of the tissues. It acts better when combined with other fixatives than when used alone. (See Orth's fluid.) (4) Formalin-alcohol (Schaffer). Formalin, 30 c.c. Alcohol, 96-per-cent., 60 c.c. Tissues remain in this fixative about forty-eight hours and are then placed in 96-per-cent. alcohol for a like period. (5) Muller's Fluid. Potassium bichromate, 2 . 5 gm. Sodium sulphate, i . o gm. Water, 100. 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 tissue may be fixed, but in all cases large quantities of the fixative should be used and frequently renewed. (6) Orth's Fluid. MuUer's fluid (double strength), ] ^ r ■,. r. . r Equal parts. Formalm, 8-per-cent., j This is one of the best general fixatives. Its action is similar to that of Miiller'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 wdthout disadvantage. Fairly large pieces of tissue may be fixed with good results. The fixative 8 HISTOLOGICAL TECHNIC should be changed after a few hours. Fixation with Orth's fluid gives an excellent basis for a hsematoxylin-eosin stain (see (i), p. 20). The fixative should always be freshly prepared. It is convenient to keep the 8-per-cent. formalin solution and the double-strength Miiller's fluid in stock. Orth's fluid is then prepared by simply tak- ing equal parts of each. (7) Osmic Acid. — This, in a i-per-cent. aqueous solution, is a quick and excellent fixative of poor penetrating power. Very 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 presence in tissues. Fixation should take place in the dark. (8) 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 structures, and is of especial value in demonstrating mitotic figures. Very 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. (9) Mercuric Chlorid. — This may be used either in saturated aqueous solution or in saturated solution in 0.75-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 gives good results. (10) Zenker's Fluid. Potassium bichromate, 2.5 gm. Sodium sulphate, i . o gm. Mercuric chlorid, 5 . o gm. Glacial acetic acid, 5 . o c.c. Water, 100. o c.c. This fluid should be freshly made, or the salts may be kept in solution 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 GENERAL TECHNIC 9 is the fact that the mercuric chlorid sometimes produces dark, irregular precipitates in the tissues. This may be remedied, however, by the use of iodine and iodid of potassium in the hardening process (see under Hardening, p. lo). (ii) Pi'mf aa(/ 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 pro- longed. 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. Specimens fixed in strong alcohol are embedded immediately (see Embedding, p. 11), or preserved (see Preserving, p. 10). After fixation in dilute alcohol the specimens are passed through the graded alcohols up to 80-per-cent. After fixation in formalin, specimens are placed directly in 96- per-cent. alcohol without washing in water. Specimens 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 sufficiently to remove 10 HISTOLOGICAL TECHNIC 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 tis- sues 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 solu- tion 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 give good re- sults. After hardening, the tissue is washed in water and placed in one of the following decalcifying fluids. The quantity of fluid should always be large and the fluid frequently changed. The completion of decalcification 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. When decalcification 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 specimen is then carried through graded alcohols. (i) Hydrochloric Acid. — This may be used in aqueous solutions of from 0.5-per-cent. to 5-per-cent. A very satisfactory decalcifying mixture is that known as Ebner's hydrochloric-salt solution. It consists of: GENER.VL TECIIXIC li Sodium chlorid, saturated aqueous solution, i part. Water, 2 parts. Hydrochloric acid, sufficient to make a from 2-per-cent. to 5- 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. (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.) will decalcify small fcctal bones in from three to twelve days. For larger bones stronger solutions and longer time are required. Transferring from the nitric solution to a 5-per-cent. alum solution for twenty-four hours before washing will prevent swelling of the tissue. Phloroglucin is some- times added to the nitric acid solution for the purpose of protecting delicate tissues or of allowing stronger solutions of the acid to be used. One gram of phloroglucin is dissolved in 10 c.c. of nitric acid. To this are added 100 c.c. of lo-per-cent. aqueous solution of nitric acid. Small pieces of tissue decalcify in a few hours. If less rapid action is desired, reduce the amount of nitric acid without changing the percentage of phloroglucin. (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, 5 parts. V. Embedding ]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, re- course is had to embedding. This consists in impregnating the tissues with some substance which is liquid 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 celloidin and paraffin. Celloidin Embedding (i) Alcohol-ether Celloidin. — Two solutions should be made. Solution No. 2. Thick celloidin — a 5-per-cent. solution of cel- loidin in equal parts 96-per-cent. alcohol and ether. 12 HISTOLOGIC.U. TECHNIC Solution No. i. Thin celloidin — made by diluting solution No. 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 dehydrate. 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 specimen, and the thinness of section desired. Many tissues may be advantageously 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 cel- loidin to a block of wood or other suitable material which may be clamped in the microtome (see Section Cutting, p. 14). 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 hard- ening 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), sHghtly larger than the specimen, and covering with thick celloidin. The celloidin should dry slowl}' 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 oft". Such a block may be cut into any desired shape. It is attached to the w^ooden or vulcanized block by dipping for a moment in thick celloidin, and then 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 difficult to attach to blocks. This usually may be accomplished by first thoroughly drying the specimen and then dipping it in equal parts alcohol 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 80-per- cent. alcohol. After several months, however, the celloidin is likely to become too soft for good section cutting. In that case the speci- GENERAL TECHNIC 13 mens can be readily re-embedded by dissolving out the old celloidin with alcohol and ether and putting them again thiough the regular embedding process. If blocked specimens are kept in equal parts alcohol and glycerin, the celloidin will retain its toughness for months or even years. (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, 30 gm. Clove oil, 100 c.c. Ether, 400 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. During 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 ss° 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. 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 14 HISTOLOGICAL TECHNIC 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 paraffin, 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 w^ell 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 par- affin from adhering, and should be as cold as possible, so that the par- affin may harden quickly. The same paper boxes described under celloidin embedding ma}^ 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- ment known ns the microtome. This consists essentially of a clamp GENERAL TECHNIC 15 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 paraflin. In cutting celloidin sections the knife is so adjusted that it passes obUquely through the specimen, as much as possible of the cutting edge being used. The knife is kept flooded with 8o-per-cent. alcohol 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 sections 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 lo//, although under favorable conditions sections 5//^ or even 3/« in thickness may be obtained. In cutting parafhn 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 parafhn sections, where an obhque knife is used. Sections 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. Parafhn sections may be so cut that the edges of the sections ad- here. Long series or "ribbons" of sections may thus be secured. This is of decided advantage when serial sections are desired. Fail- ure 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 may be obtained by holding a heated metal plate or bar near the ^ 11 =micromillimeter or micron =Taoo of ^ millimeter = microscopic unit of measure = about jsioo 16 niSTOLOGIC.\L TECHNIC block until the paraffin is slightly softened. This process may be re- peated 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. Frozen Sections. — This method, while more used in pathology where rapid diagnosis is more often important, is still sometimes extremely useful to the histologist, especially as an easy and quick method of testing material. Perfectly fresh tissues may be used or tissues which have been pre\dously fixed. Fixed tissues must have the fixative thoroughly removed before freezing. Of fixatives lo- per-cent. formalin is probably the best and the tissue should remain in the fixative from one to two hours. If time is important the tissue may be boiled for two or three minutes in the formalin solution. Ether, rhigolene and carbon dioxid are the most common freezing agents. A special freezing microtome and knife may be used, or a freezing attachment may be used with the ordinary microtome. In ether freezing the ether is vaporized by means of compressed air and the vapor carried to the under side of a metallic plate upon which the block of tissue is placed. In carbon dioxid freezing, which has now largely superseded ether freezing on account of convenience and econ- omy, the commercial cylinder of gas such as is used for charging soda fountains is used. As the liquid carbon dioxid and not the gas is required for freezing, the cyhnder should be hung valve end down, with the valve about the level of the microtome. The carbon dioxid is carried to the metallic disc of the microtome by means of a stout rubber tube and it is advisable to screw a cap with a small hole in it over the valve and to attach a longer handle to the valve lever in order to more perfectly control the flow. The piece of tissue to be frozen should not be over 5 mm. thick and the freezing should be done slowly. A proper hardness of the tissue is important and is of course determined by the amount of carbon dioxid admitted. As the frozen section is not supported by any embedding mass, special handling is required, the section being attached to a slide or cover-glass before staining. For this purpose either egg albumen or celloidin may be used. GENER.VL TECHNIC 17 Egg albumen, 50 c.c. Distilled water, 150 c.c. Saturated solution sodium salicylate (made slightly alkaline with lithium carbonate) 50 c.c. (or sufficient to completely dissolve the albumen). Sections from material previously fixed are transferred directly from the microtome to the albumen solution. Sections from fresh material are placed for three to five minutes in 5- to lo-per-cent. formalin solution before being transferred to the albumen solution. From the albumen solution sections are floated on a slide or cover-glass, the excess of solution drained off and the section firmly blotted with several thicknesses of washed cheese-cloth. The slide or cover-glass is next immersed in alcohol or in equal parts alcohol and ether to coagulate the albumen, thus fixing the section to the glass. The section may now be stained and mounted in the usual way. In the celloidin method the section is transferred from the micro- tome to water, floated upon a slide, blotted with filter paper, flooded with absolute alcohol and drained. Before the alcohol dries the slide is flooded with a very thin celloidin solution which is immediately drained off. The celloidin is then allowed to harden a moment and the slide immersed in water. The section is now fixed to the slide and may be stained and mounted as usual. VII. Staining This is for the purpose of more readily distinguishing the different 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, haematein; (b) carmine and its active principle, carminic acid; (c) basic anihne dyes. II. Plasma dyes: (a) Eosin; (b) neutral carmine, (c) picric acid; (d) acid aniline dyes. I. Nuclear Dyes. — (a) Hematoxylin. I. Gage^s Hcematoxylin. 18 HISTOLOGICAL TECHNIC 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. DelafieWs HcBmatoxylin. Haematoxylin crystals, i gm. Alcohol, 6 c.c. Ammonia alum, saturated aqueous solution, 100 c.c. The haematoxyhn 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. Heidenhain's HcBmatoxylin. Ha^matoxylin 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 haematoxylin solution until they are intensely blue or black (usually several hours). The sec- tions 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 thoroughly washed in water. 4. Mayer^s Hamalum. Haematein, i gm. Alcohol, 50 c.c. Ammoiiia alum, 5-per-cent. aqueous solution, 1,000 c.c. GENERAL TECHNIC 19 The hacmatein 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 formuL-c makes a very satisfactory nuclear dye. Hajinatein, 5 gm. Alcohol, 50 c.c. Chloral hydrate, 20 gm. Ammonia alum, 5-per-cent. aqueous solution (steril- ized), 1,000 c.c. The haimatein 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. 6. Weigerfs Hcematoxylin. 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 acidu- lated 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 afiinity for ordi- nary haematoxylin stains. ih) 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. 20 HISTOLOGICAL TECHNIC (c) Basic Aniline Dyes — gentian violet, methyl violet, methyl green, methyl blue, toluiciin 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 dissolved 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, or water- soluble eosin may be kept in saturated aqueous solution containing a trace of thymol as a preservative. This may be diluted to any desired strength at time of using. Eosin is a rapid plasma stain. (b) Neutral Carmine. Carmine, i gm. Liquor ammonii caustici, 5 c.c. Distilled water, 50 c.c. The last two ingredients are first mixed, and the 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 anihne dyes (see above). Erythrosin is of especial value for sections which take the eosin stain poorly. Staining Sections It is usually advantageous to stain the different tissue elements different 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 with Hematoxylin and Eosin. — Sec- tions are first washed in water. They are then stained with haema- toxylin (solutions i, 2, 4, 5, or 6, pp. 17-19) from one to five minutes. GENERAL TECHNIC 21 After being thoroughly washed in water, they are dehydrated in strong alcohol and transferred to the alcoholic eosin solution (page 20). Most sections stain in from two to five minutes. By this method nuclei arc stained blue or ])urj)Ie, cell bodies and intercellular substances red. Very often a more brilliant staining may be accomplished as follows: Overstain in haematoxylin and wash thoroughly in water; decolorize 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 Picric-acid-fuchsin (Van Gieson). 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 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 Hematoxylin 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- toxyHn (solutions i, 2, 4, 5, or 6, pp. 17-19). 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 haematoxyhn 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 \ -=«*. "■'N.^ 22 HISTOLOGICAL TECHNIC 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 difiicult 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, loo 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 required for staining depends upon the size of the specimen. There is, however, little 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. Vm. Mountmg 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 GENER.\L TECHNIC 23 haematoxylin-staincd specimens in glycerin strongly tinged with eosin (eosin-glycerin) . The specimen in a drop of glycerin is trans- ferred 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 deaHng 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. (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 permanently. Its disadvantage is that its refractive index is so high that it sometimes obscures the finer details of structure, espe- cially 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. 24 HISTOLOGICAL TECHNIC 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 cellodin is transparent and may remain perma- nently in the specimen, paraffin is opaque and must be dissolved out before the section is fit for microscopic study. Bulk staining with carmine (page 22) is frequently used for speci- mens which are to be embedded in paraffin. Sections may be coun- terstained if desired. The following are the steps to be followed in staining and mount- ing paraffin sections: 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 as a preservative) 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 paraffin: 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 gc-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. GENER/VL TECHXIC 25 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 20) use the above technic in staining with ha^matoxylin ; then the alcohohc 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 the blood-vessels have been injected with some transparent coloring matter. The injecting fluid most commonly used is a solution colored gelatin. The gelatin solution is prepared by soaking i part gelatin in from 5 to 10 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 aque- ous solution Prussian blue to the gelatin solution, the proportions depending 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 mixture 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 mix- ture 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 alkahne, 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, however, the alkaline carmine and gelatin be first mixed, and the lo-per-cent. 26 HISTOLOGICS TECHNIC 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 carmine in the shape of coarser granules is of advantage when it is desired to have an in- jection mass which will fill the arteries or veins only, without passing over' into the capillaries. The injecting 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 connecting 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. 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 injecting 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 sufiicient to keep the liquid flowing. When the injection mass flows easily and freely from the vena 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 GENERAL TECHNIC 27 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 gelatin, the vein with the blue gelatin, and injecting both at the same time, the pressure driving the saline out of the vessels into the tissues. The difiiculty 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 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 arter- ies 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. (i) Silver-nitrate Method of Staining Intercellular 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 live 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) Verhoeff's Differential Elastic Tissue Stain. — • Haematoxylin crystals (Griibler), 0.15 gm. Absolute alcohol, 25.00 c.c. Dissolve by heating, then add 5-per-cent. aqueous solution ammonium hydrate, i drop. Allow to stand 5 minutes or longer, then add Lugol's solution (iodin 2 parts, potassium, iodid 4 parts), 22.00 c.c. Filter, let stand in corked bottle 24 hours. 28 SPFXL\L STAINING METHODS 29 In using this stain add to each cubic centimeter required of the above solution i drop of a 7-per-cent. solution of ferric chlorid in absolute alcohol. Sections are carried from alcohol into the staining fluid where they remain one to three hours. The stain should be kept covered to prevent evaporation. Sections are next washed in water one or two minutes and examined under the microscope. If further differentia- tion is desirable, place a few seconds in i-per-cent. aqueous solution ferric chlorid. Wash in water. Counterstain in 0.2-per-cent. eosin, dehydrate in alcohol, clear and mount in balsam. Elastic tissue is stained black, nuclei take hajmatoxylin, while other connective tissues, myelin and neuroglia stain with eosin. Fixation by Zenker's fluid gives perhaps the best results although the stain may be used after other fixations. Water in the staining fluid causes precipitates. It is therefore important to see that all dishes used are perfectly dry or washed with alcohol before using. Too much ferric chlorid gives precipitates and overstaining, too little results in failure of the elastic fibres to stain. (5) GoLGi's Chrome-silver Method for Demonstrating Se- cretory Tubules. — Small pieces of perfectly fresh tissue, e.g., Hver, are placed in the following: Potassium bichromate, 4-pcr-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- <;ent. aqueous solution of silver nitrate, which should be changed as soon as a precipitate forms. The specimens remain in the second silver solution from two to three days, after which they are rapidly dehydrated, embedded in celloidin, and cut into rather thick sections. (6) Mallory's Phosphomolybdic Acid Hematoxylin 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 transferred to : Phosphomolj'bdic acid, lo-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 30 Hli^TOLOGICVL TECHXIC 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. (7) Mallory's Phosphotungstic Acid H.^matoxylin Stain for Connective Tissue. Haematoxylin (or ha^malein ammonium), o.i 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 de- sired for immediate use, the solution can be ripened by the addition of 10 c.c. of a 1/4-per-cent. solution of potassium per- manganate. 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. 10.) 2. Several changes 95-per-cent. alcohol. 3. Water. 4. One-fourth-per-cent. aqueous solution potassium permangan- ate, 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 hfematoxylin solution, twelve to twenty- four hours. 9. Dip for a few seconds in 95-per-cent. alcohol. 10. Clear in carbol-xylol and xylol and mount in xylol-damar. (8) Mallory's Aniline Blue Stain for Connective Tis- sue.— 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 solu- tion, 100. o c.c. 3. Decolorize in several changes of 95-per-cent. alcohol. SPECI.\L STAINING METHODS 31 4. Clear in carbol-xylol and xylol and mount in xylol-damar. (9) Maresh's Modification of Bielschowsky's Stain for THE Finer Connective Tissue Fibrils. 1. Very thin paraffin sections are fixed to the slide and placed for twelve to twenty-four hours in a 2-per-cent. silver nitrate solution. 2. For fifteen to thirty minutes in freshly prepared alkaline silver solution (20 c.c. of 2-per-cent. silver nitrate solution, to which have been added 3 drops of 40-per-cent. caustic soda, and the precipitate redissolved by adding ammonia drop by drop while stirring). 3. Rinsed quickly in distilled water and placed in 20-pcr-ccnt. formalin for three minutes or until black. 4. Washed in distilled water and transferred for ten minutes to acid gold bath (10 c.c. distilled water to which have been added 2 or 3 drops of i-per-cent. gold chlorid and 2 or 3 drops acid). 5. Placed one-half to one minute in 5-per-cent. jtiyposulphitc of soda to remove all unreduced silver. 6. Washed in distilled water, dehydrated, cleared in xylol and mounted in balsam. (10) 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. (11) Sudan III and Scharlack R for Fat.- — Either of these dyes may be used in concentrated solution in 70-per-cent. alcohol. Frozen sections of fresh or formalin-fixed tissue are used. They are first placed in 50-per-cent. alcohol for several minutes, then trans- ferred to this staining fluid for from fifteen to thirty minutes, washed in water or in 50-per-cent. alcohol and mounted in glycerin or in glycerin jelly. Both dyes stain fat intensely red. A nuclear stain such as hsemalum, or a plasma stain such as picric acid, or both may be used in conjunction with either Sudan III or Scharlack R. Sec- tions are first stained in hsemalum, washed in water, transferred to 50-per-cent. alcohol, then stained with either the Sudan III or Scharlack R, washed in water, transferred to an aqueous solution of picric acid, w^ashed again in water and mounted in glycerin or glycerin jelly. 32 HISTOLOGICAL TLCHXIC (12) Jenner's Blood Stain. Water-soluble eosin (Griibler), i-per-cent. aqueous solution, 100 c.c. Methylene blue — pure (Griibler), i-per-cent. aqueous solution, 100 c.c. Mix, and after standing 24 hours, filter. The filtrate is dried at 65° C, washed, again dried and powdered. To make the staining solution, o . 5 gm. of the powder is dissolved in 100 c.c. pure methyl alcohol. Blood smears stain in from two to live minutes. They are then washed in water, dried, and mounted in^balsam. This solution acts as a fixative as well as a stain. 1 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 myelin sheaths; 2d, treatment (mordanting) with a reagent which enters into combination with the myelin, the result being that the myehn sheaths stain specifically with haematoxylin . Formahn 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 formahn to avoid overharden- ing. Or the material may be fixed and kept in formalin and impreg- nated with the bichromate before using, the latter being done before dehydrating in alcohol. Further mordanting, which is usually done, especially when the material has been kept for some time in formahn or alcohol, is for the purpose of intensifying the stain. Material is fixed in one of the following fluids: (a) ]\Iuller's fluid (page 7). (b) Potassium bichromate, 5-per-cent. aqueous solution. (c) Formalin, lo-per-cent. aqueous solution. (d) Formahn, i volume; potassium bichromate, 5-per-cent. aqueous solution, 9 volumes. In ^Miiller's fluid or in plain potassium-bichromate solution a hardening of two days to four weeks is required; in formahn or formahn-bichromate from a week to ten days is suflicient. 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, embedded in celloidin, and sections cut in the usual way. Material fixed in formahn 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. 33 34 HISTOLOGICAL TECHNIC Sections from material fixed in any of the chrome-salt solutions are placed for from twelve to twenty-four hours in a saturated aque- ous 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. After 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- toxyhn being dissolved first in the alcohol, or the haematoxyhn may be kept in lo-per-cent. alcohohc solution, the hthium 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 haematoxyhn. They remain in the decolorizing fluid until the gray matter becomes a Hght 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 SPECIAL NEUROLOGICAL STAINING METHODS 35 copper-acetate solution. After rinsing in water the sections are stained in ha^matoxylin as in the ordinary Weigert method. The Hthium carbonate may, however, be omitted. They are then washed and transferred to a 0.25-per-cent. sokition of potassium permanga- nate, where they remain from one-half to two minutes, after which they are again washed and ])laced 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 sufhciently decolorized. All formaUn-lixed material is best stained by the Weigert-Pal method. An intensification of the stain, especially of the very fine fibres, may sometimes be obtained by placing the sections for a min- ute in a 0.5-per-cent. aqueous solution of osmic acid before decolorizing. *&• Marchi's Method por Staining Degenerating Nerves Small pieces of tissue are fixed and hardened for from seven to ten days in Miiller's fluid. Thin sHces of the tissue are then trans- ferred to a solution of one part i-per-cent. osmic acid and two parts Mtiller'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, w^hile 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 sHght degeneration may be recog- nized which would escape notice in material stained by Weigert's method; on the other hand, in a long-standing degeneration w^hen the medullary sheaths have completely disappeared and their places have been taken by connective tissue, there being no degenerated myehn remaining, the Marchi method is inappKcable. 36 HISTOLOGICAL TECHNIC Busch's modification of the Marchi method gives sharp pictures and has the advantage of allowing formaldehyd fixation and harden- 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 0.75-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 80 u 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-per-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 transferred to the silver solution (see Slow Method). SPECIAL NEUROLOGICAL STAINING METHODS 37 {d) Formalin-bichromate MctJiod. — Tissues are placed for from two to six days in the following solution: Formalin, lo to 20 parts. Potassium bichromate, 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 transferred 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 impreg- nation must be determined by frequently testing the material, but is usually indicated by the appearance of small white spots on the sur- face 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 fluid for from two to five months. In the Golgi silver methods the result of the treatment, first with bichromate and then w^th 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 proc- esses. It must be remembered that only a few of the cells and proc- esses are stained, these often only partially, and that other irregular precipitations are usually present. In the mercury methods, the bichromate of potassium and the bichlorid of mercury may be used combined in the same solution. There are other modifications of the Golgi methods, in which similar precipitates of other metallic salts are secured. 38 HISTOLOGIC.\L TECHNIC 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 by the slow silver method and by the bichlorid method are more permanent, and more time may be taken with their dehydration. Thick sections should be cut (75 to ioo/() and mounted in xylol-balsam. After the rapid method; it is safer to mount without a cover-glass; after the slow method, speci- mens may be mounted with or without a cover. The balsam should be hard, and melted at the time of using. (See Mounting, page 22.) 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 1.5-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 gm., 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 paraffin and sections cut, usually 15-25/i in thickness. Difi'erent 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 dehcate objects (study of pathological changes SPECIAL NEUROLOGIC.\L STAINING METHODS 39 in neurofibrils) it may be best to abbreviate 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 better to add from 0.25 c.c. to i c.c. of ammonium hydrate to each 10 c.c. of the alcohol. 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 avoid, 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 sub- sequent 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 following is simple and gives uniformly good results: Specimens are first fixed in mercuric-chlorid solution (page 8), 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 given off. They are then washed in water and dift'erentiated in strong alcohol. The degree of decolorization 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 40 HISTOLOGICAL TECHNIC 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 Medical Sciences, vol. iv. Lee: The Microtomist's Vade-mecum. Mallory and Wright: Pathological Technic. PART II THE CELL CHAPTER IV THE CELL In the simplest forms of animal life the entire body consists of a little albuminous structure, the essential pecuUarity of which is that it possesses properties which we recognize as characteristic of living organisms. This albuminous material basis of Hfe is known as pro- toplasm, while the structure itself is known as a cell. All plants and animals consist of cells and their derivatives, and if an attempt be made to resolve any of the more complex living structures into its 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. Xucleoplasm. 8, Attraction-sphere. 9, Centrosome. 10, Plastids. 11, Chro- matin network. 12, Nuclear membrane. 13, Nucleolus. 14, Vacuole. component elements, it is found that the smallest possible subdivi- sion still compatible with life is the cell. The cell may therefore be considered as the histological element or unit of structure. While presenting fundamental similarities which define them, cells vary greatly in size, shape and structure. Thus in the human body the cells vary in size from the small lymphocyte with a diameter of about 4M to the giant cells of bone marrow with a diameter up to loo^t. Cells also vary greatly in shape from the typical spherical of the ovum to the very irregular stellate multipolar cells of the nervous 43 44 THE CELL system. In multicellular organisms the element of pressure has frequently much to do with determining the shape of the cell. Also as most cells are soft and pliable their shapes frequently change in response to changes in function or in environment. In most cells the structure of the protoplasm is not uniform throughout the cell but is differentiated to form certain special cell structures. Thus in most cells a portion of the protoplasm is specially modified to form a body known as the nucleus. Also at the sur- face of the cell there is usually some differentiation of the protoplasm to form a more or less distinct cell wall or cell membrane. An ,; actively multiplying cell con- tains a minute structure associated with the reproduc- tive function and known as the centrosome. A typical cell thus con- sists of the following struc- tures (Fig. i): (i) The cell body; (2) the cell membrane; (3) the nucleus; (4) the cen- trosome. Of these only the cell body with its modified surface cytoplasm or cell membrane is present in all cells. A few cells contain, in their fully developed condi- tion, no nuclei. In many mature cells it is impossible Fig. 2. — Diagram Illustrating Theories of Protoplasmic Structure, a, Fibrillar theory; b, granule theory; c, "foam" theory. (The general structure of cell body and nucleus cor- responds.) to distinguish a centrosome. I. The Cell Body. — This consists of a viscid, colorless, semi-fluid substance, belonging to the general class of albumens and known as protoplasm. It is alkaline in reaction and 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 differentiated into a formed element, spongioplasm, and a homo- geneous element hyaloplasm. Distributed along the spongioplastic network are minute granules, microsomes. The exact relations which THE CELL 45 .4 these elements bear to one another and to the cell as a whole have been the su])ject of much investigation and si:)eculali()n. Tin- earlier cytologists concerned themselves with the question as to whether ])rot()i)lasin f" ~~ • — . was homogeneous {i.e., a mere solution or at most a mixture of various sub- stances) or had a definite structure. The theory of a structureless proto- plasm having been long since aban- doned, the question as to the character of the protoplasmic structure still re- mains unanswered. Altmann's granule theory considers proto- plasm 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, protoplasm is a foam or emulsion, the microscopic appearance of which can be simulated by artificial emul- sions. He ascribes the appearance of a rcti('- ulum to the fact that each little foam space forms a complete cavity filled with fluid, the cut walls of these spaces giving a reticular ap- pearance 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 feltwork of indepen- dent fibrils (filar mass or miton), and (2) a fluid or semi-fluid substance which fills in the meshes of the reticulum or separates the fibrils (interfilar mass or paramiton) (Fig. 2, o). That the question as to the ultimate struc- ture of protoplasm stiU remains unanswered is uZ'''J'~r^^^f °' emulsion struc- *^ ^ tare ot protoplasm accordmg to dependent mainly upon the extreme technical Butschli (Biitschli). A, Epidermal difficulties which have confronted the cytolo- ^?^ of ^he earthworm. B, Peri- „• , T • • * 1 I, u pneral cytoplasm of sea urchin's gist. Livmg protoplasm has a homogeneous egg. C, Artificial emulsion of oUve glossy appearance, showing even under the oil, sodium chlorid and water, 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 unde- 46 THE CELL termined. It is quite possible that the structure even of undifferentiated pro- toplasm or protoplasm proper, such as is found in early embr>'onal cells, is not entirely uniform. The protoplasm of the more highly specialized cells certainly differs markedly in structure and somewhat in chemical composition in different cells. It even differs in the same cell under different functional conditions. These dift'erences are apparently due to special development of the protoplasm for its peculiar functions. Thus, for example, in the muscle cell and the nerve cell most of what in the embryonal cell was undift'erentiated protoplasm has become differentiated into contractile or conductile fibers. The nucleus and a small amount of undifferentiated or less differentiated protoplasm remain and are probably largely active in maintaining the nutrition of the cell. Such changes in the protoplasm are of course permanent. Temporary or periodic changes in the structure of protoplasm are seen in such cells as the secreting cells of the pancreas or of the salivary glands where marked differences in the protoplasm occur, dependent upon whether the cell is in a resting, or actively secreting condition. Protoplasm is thus probably best considered as the material basis of cell acti\dty, i.e., of Hfe, rather than as a substance having fixed and definite chemical or morphological characteristics. It is convenient to use the term protoplasm 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 occur- rence in vegetable cells, and are also found in some animal cells. They are apparently to be regarded as a differentiation of the cyto- plasm, but possess a remarkable degree of independence, being capable of subdi\'ision and in some cases of existence outside of 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, deutoplasfu) granules, has been applied (Fig. i). Some of these are intimately associated with the cell acti\dties 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 endoplasm,^ the latter as exoplasm. When the exoplasm forms a THE CELL 47 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 cyto- plasm (Fig. 4). These channels may contain branching processes from other cells, forming what is known as a trophospongium. Some intracellular canaliculi are apparently secretory in character and may communicate with fine intercellular secretory channels. In this way the secretion of such cells as the serous cells forming the demilunes of mucous tubules (p. 222), or of the parietal cells of the stomach glands (Fig. 155), is carried out into the duct system of the gland. Other intracellular canaliculi, for example in ganglion cells, commu- nicate with pericellular lymph spaces, thus apparently being con- cerned with the nutrition of the cell. Closed networks of canaliculi within the l ■'• ^ X cell "apparato reticulare," having no ex- tra cellular communications have also been described. 2. The Cell Membrane (Fig. i).— In most vegetable cells the cell membrane is the most conspicuous part of the cell and ^*"-^'v^ was responsible for the name "cell" which the seventeenth-century botanists, over- , Fig 4.— Intracellular canals (trophospongium) of a ganglion looking the importance of the enclosed pro- cell (E. Holmgren), toplasm, gave to the little spaces or cavi- ties of w^hich they thought plants composed. A distinct cell mem- brane is present in but few animal cells. An exception is seen in the fat cell where a distinct membrane exists. In most animal cells no membrane has as yet been demonstrated and this has led to a persistent denial of the existence of such a membrane. It is nevertheless probable that most if not all animal cells have some m.odification, however slight, of the periphery of their protoplasm which serves to retain the protoplasm within definite bounds, to pro- tect it as it were and at the same time permit osmosis. 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 sphe- roidal, but the shape of the nucleus varies for dift'erent cells and corre- sponds somewhat to the shape of the cell body, e.g., the rod-shaped 48 THE CELL nucleus of the elongated smooth muscle cell. It may also be modified by intracellular 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 cytologists 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 earlier 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 dift'erent elements when treated with certain anihne dyes. The nuclear structures and their relations to the chemical constituents of the nucleus are as follows: (a) The nuclear membrane (amphipyrenin). Tliis 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 wdth, in some cases, the liniii, in others, the chromatin of the intranuclear network. THE CFXL 49 It may be complete, or fenestrated allowing free communication be- tween the cytoplasm and the nuclear contents. (b) The intranuclear network, or nucleoreticidum, consists of a chromatic element {nuclein or chromatin) and of an achromatic element (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 chroma- tin. These nodal points, at first thought to be nucleoh, 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-di\dsion. 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 nucleolus or plasmosome {paranuclein, pyrenin) is a small spherical body within the nucleus. Not infrequently there are sev- eral 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 regularly dis- appears during mitosis, and during functional activity in some gland cells. It stains intensely with basic dyes. Its function is unknown. {d) 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 similarity exists between the structure of nucleoplasm and cytoplasm. This similarity is empha- sized by the absence in some resting cells of any nuclear membrane, by the apparent direct continuity in some cases of nucleoreticulum and cytoreticulum, and by the continuity of karyoplasm and cyto- plasm in all cells during cell-division. 4. The centrosome (Fig. 5) is a small spheroidal body found sometimes in the nucleus, or more commonly in the cytoplasm near the nucleus. In actively dividing cells the centrosome is frequently \ \ \ 50 THE CELL double, this being apparently in preparation for the succeeding cell- division. In some cases the centrosome is triple or even multiple. It was first found in the ovum and described as peculiar to that cell. It is now believed to occur in most, if not in all, animal cells. It usually consists of (i) a minute central granule or granules — the centriole, which stains intensely with iron-haematoxylin, and out- side 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 centrosome, the term centrosome being by some appHed to the entire structure including the radiat- ing fibrils, by others to the central granule only, by still others to the central granule plus the surrounding clear area. By some -r,^ c . the radiations are believed to be composed riG. 5. — bpermatogo- '■ nium from frog (Hermann), of a different substance than the general cy to- Single centrosome at cen- , ,.,.,. , , 7,7 ^1 ter of attraction sphere or piasm, which IS designated arcJio plasm. The aster. Nucleus contains a main significance of the centrosome is in plasmosome. . . connection with cell-division, under which head it will be further considered (p. 53). 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 its own particular work in the body economy — special function. 3. To respond to stimulation — irritability. 4. To move — motion. 5. To produce other cells — reproduction. As would be expected, these properties, existing as they do in living cells, cannot always be sharply separated but frequently over- lap. Thus, e.g., in the muscle cell "motion" equals "special func- tion." 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. THE CELL 51 1. Metabolism. — This term is used to designate those cellular activities which have to do with the nutrition of the cell. A cell is able (i) to take up from without substances suitable for its nutri- tion and to transform these into its own peculiar structure, and (2) to dispose of the waste products of intracellular activities. The former is known as constructive metabolism or anabolism, tlie latter as destructive metabolism or katabolism. It is possible, for example, to watch an amoeba send out projections (pseudopodia) (see p. 52) around an adjacent bit of food material. These projections coalescing finally completely enclose the food material within the body of the amoeba where it is acted upon by the protoplasm in such a way (digested) as to completely lose its identity and to fmally become an integral part of the cytoplasm. There is normally maintained within the cell a state of equi- librium between this constructive and destructive metabolism, be- tween the intake of food on the one hand and the outflow of material products or of energy on the other. Stated as an equation, intake = outgo. Any marked -|- or — on one side of the equation without corresponding H- or — on the other side must disturb this equihbrium and must result in physical changes within the cell. Thus any marked -f in intake without corresponding + in outgo must result in growth of the cell, while any + in outgo without corre- sponding + in intake must tend toward diminution in bulk of cell and final destruction. The life-length of cells varies. Some cells, for example, some of those of the central nervous system, probably live throughout the life of the individual. At least there is good clinical evidence that once destroyed they are never replaced. Other cells, such for example as blood cells and many epithelial cells, are constantly wearing out and being replaced by new cells. 2. Special 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 pro- tection 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 accomplished by the building up within the cell of highly complex and, consequently, unstable molecular combinations. By reduction 52 THE CELL of these unstable 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 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 cheniotaxis. Some substances attract cells (positive cheniotaxis); others repel cells (negative chemotaxis). 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. ^^^. m WMi^ Fig. 6. — Amoeboid ^Movement. Successive changes in shape and position of fresh-water amceba. 4. Motion. — This is dependent wholly upon the protoplasm of the cell, and is exhibited in several somewhat different forms. (a) Amxhoid 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). While possessed by many animal cells, this property of amoeboid movement is most marked in such cells as the leucocytes or white blood cells. By their amoeboid move- ments these cells are able to pass out of the blood vessels and as so- called ''wandering cells" to circulate in the tissues. By means of their pseudopodia they can also surround tiny objects^ — granules, bacteria, etc., and sometimes digest or destroy them. Such cells are known as phagocytes and the phenomenon as phagocytosis. They play an extremely important role in the body economy both in health and disease. {h) Protoplasmic Movement. This occurs wholly within the lim- THE CELL 53 its 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 "streaming " 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 sur- faces 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 de- pendent upon the spongio- plasm, the hyaloplasm play- ing a more passive role. In muscle cells the highly de- veloped contractile powers appear to be due to the ex- cessive development and peculiar arrangement of the spongioplasm. 5 . Reproduction. — The overthrow of the long-held biological fallacy of sponta- neous 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 number 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 mitosis the division of the cell body is preceded by divi- sion of the nucleus. Direct Cell-dwision — Amitosis (Figs. 7 and 8). — In this form of cell-division the nucleus divides into two daughter nuclei without any apparent preliminary changes in its structure. The division of the nucleus may or may not be followed by division of the cell body, in the latter case resulting in the formation of polynuclear cells. This form Fig. 7. — Epithelial Cells from Ovary of Cockroach, Showing Nuclei Dividing Araitot- ically. (Wheeler.) 54 THE CELL of cell-division is uncommon in higher animals where Flemming con- siders 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-divi- sion— Mitosis (Figs, g, lo). — 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 compli- cated changes. These changes occur as a contin- uous process, but it is con- venient 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 anaphase, the formation of the two daughter nuclei; the telophase, the reconstruction 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 centro- somes. 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 derived from the linin (Fig. 9, B). These fibrils form the ce«/ra7 or achromatic spindle. Two other sets of fibrils radiate from each cen- trosome— one, known as the polar rays, passes out toward the periph- ery of the cell; the other, known as the mantle fibres, extends from the ^ -A " \ \ \ jzr Fig. 8. — Epithelial cell from bladder showing amitotic division of its nucleus. (Nemileff.) /, Cytoplasm; //, two daughter nuclei; ///, fibrils uniting daughter nuclei. \ THE CELL 55 centrosome to the chromosomes (Fig. 9, C). The two centrosomes with their fibres constitute the amphiaster. 2. During or immediately following the formation of the amphi- aster, important changes take place in the nucleus. It increases in size and loses the reticular appearance of the resting nucleus, its chromatic elements becoming arranged in a long spireme-thread or in several shorter threads, the closed skein or closed spireme. This next .1 B 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 ar- ranged around equator of nucleus; fibrils of achromatic spindle connecting centrosomes; mantle fibres passing from centrosomes to chromosomes. (E. B. Wilson, "The Cell," The Macmillan Co.) becomes thicker and more loosely arranged, thus forming the open spireme. That some chemical as well as morphological change has taken place in the transformation 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. 56 THE CELL 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). 3. The spireme next breaks up into a number of segments — chromosomes (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 equa- tor of the nucleus, their closed ends being directed centrally. The details of the transformation of the reticulum into chromo- somes vary. In some cases a single spireme-thread is formed. In others 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 difi'erent species of plants and animals, it is fixed and characteristic for a given species. 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, the most authoritative estimate is 24.^ This means that whenever mitosis occurs in Ascaris, the spireme-thread invariably segments into 4 chromosomes. Chro- mosomes and amphiaster now constitute the jnitotic figure which at this stage is known as the monaster, its formation marking the end of the prophase. (b) Metaphase (Fig. 10, E). This marks the beginning of actual division of the nucleus. Each chromosome spHts longitudinally (longitudinal cleavage) into two daughter chromosomes, each contain- ing exactly one-half the chromatin of the parent chromosome. U- and V-shaped chromosomes always begin to spHt at the apex, from which point the separation extends to the open ends. (c) Anaphase (Fig. 10, F, G). — An equal number of daughter chromosomes now travels along the fibrils of the achromatic spindle — apparently under the influence of the mantle fibres — toward each 1 Gayer (Biol. Bui. Marine Biol. Lab. Wood's Hole, Mass., Vol. XIX) describes twenty-two chromosomes in human spermatogonia, of which two are "accessory". Apparently half the resulting spermatids contain ten chromosomes, the other half twelve, two of which are accessory. In Syromastes Wilson found an identical condi- tion and it was later determined that the somatic number of chromosomes for Syro- mastes was twenty-two for the male and twenty-four for the female. Guyer concludes that the probable somatic number for the human male is twenty-two, for the femak- twenty-four. THE CELL 57 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 diastcr (Fig. lo, C). 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. Fig. io. — 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 acromatic spindle toward centrosomes; division of centrosomes; if, interzonal fibres or central spindle. G, Late anaphase. Formation of diaster; beginning division of cell body. H, Telopliase. Reappearance of nuclear membrane and nucleolus; two complete daughter cells, each containing a resting nucleus. (E.B.Wilson, "The Cell," The Macmillan Co.) (d) Telophase (Fig. lo, H). — This is marked by di\asion of the cell protoplasm and consists of a cycle of changes, by means of which each group of daughter chromosomes is transformed into the chro- matin network of a resting nucleus. These changes are the same as those described in the prophase, but occur in the reverse order, the chromosomes uniting to form the spireme, and the spireme becoming transformed into the nuclear network. The result is the formation 58 THE CELL 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 chromosomes 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. The role which each part of the cell plays in the vital activities of the cell,, and the physiological correlation of these various parts, have been but partially determined. Experiments upon some of the protozoa show that if the cell be divided into two parts, one part containing the nucleus, the other part non- nucleated, the behavior of the two parts is very different. The part contain- ing the nucleus soon again becomes a complete cell with all the properties which the cell originally possessed. The non-nucleated part responds somewhat to stimulation, is capable of some movement, makes some feeble attempts at digestion, but is incapable of secretion, of reconstructing the complete cell, of reproduction, and soon dies. Even when a small part of the nucleus remains in the cut-off piece of cytoplasm reconstruction may take place. On the other hand a nucleus completely deprived of its cytoplasm is incapable of reconstructing the cell. In Infusoria each cell has two nuclei, a macro-nucleus and a micro- nucleus, the former connected with nutrition, the latter with reproduction. These facts together with the behavior of the nucleus during the ingestion of food, during secretion, and even during motion, warrant the belief that while in all probability most of the actual work of the cell takes place in the cytoplasm, the nucleus exerts a more or less controlling influence over all cell activities. 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 nu- clear 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 THE CELL 59 fact that in the higher plants no ccntrosomc can be demonstrated during any stage of mitosis, and also that in some cases the chromosomes 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. Branca calls attention to the fact that the centrosome is an organ of no "con- stancy, permanence, or specificity," (i) that in certain cells it is impossible to demonstrate a centrosome at any time; (2) that when a centrosome has been demonstrated for a certain type of cell, it cannot always be found even in that type of cell; (3) that certain parts of the centrosome, the rays, are cytoplasmic, while another part, the centriole, reacts like nuclear material (chromatin). He concludes that the centrosome is a portion of the protoplasm temporarily differentiated for a specific function; not unlike the basal filaments which appear temporarily in gland cells when they l^ccome active, both being functional forms of protoplasm which can succeed each other as the cell changes the character of its function." 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, however, inherent in the ovum itself, but, in sexual reproduction, are acquired 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. 350) and of the ovum on the female (p. 363). The spermatozobn (Fig. 11) is developed from a cell of the sem- iniferous tubule of the testis. The nucleus of this cell so divides its chromosomes that each spermatozobn contains just one-half the num- ber of 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 centrosome, 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 undergoes mitotic GO THE CELL 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. Ji The remaining nucleus and centrosome again divide .^ M mitotically, only in this second division, instead of \y)i 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-Jialf 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 chromo- somes characteristic 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 spermatozoon enters the egg. The chromatin of the male pronucleus next becomes arranged as chromosomes. Male and /emale pronuclei now lose their limiting membranes and approach each other, their chromo- somes intermingling. As each pronucleus contained one-half the number, the monaster thus formed contains the full number of chromosomes characteristic of the 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 to the monaster of mingled male and female chromosomes, already described under mitosis. By longitudinal cleavage of these chromosomes, as in ordinary mitosis, two sets of daughter chromosomes are formed. Each set passes along the filaments of the achromatic spindle to its centro- some. Thus is formed the diaster. By continuation of the mitotic process two new nuclei are formed, each nucleus containing the number of chromosomes characteristic of the species, and each being made up equally of male and female chromosome elements. Thus .6 Fig. II.— Hu- man Spermatozoa. (After Retzius.) I, Head seen on flat; 2, head seen on edge; k, head; m, body;/, tail; e, end piece. THE CELL 61 occurs the first division of the fertilized ovum into two daughter cells. By similar mitotic processes these two cells become four, the four cells become eight, etc. This is known as segmentation of the ovum. membrane of ovum entering sper- -" matozoon protoplasm of ovum with deutoplasm granules female pronu- cleus ' -W \ head of sper- 't*-?I^i — ■ matozoon with "^^^ I centrosome ^ -- female pronucleus male pronucleus chromosome of female pronucleus chromosome of male I ' pronucleus "-— centrosome centrosome male pronu- cleus female pro- nucleus chromosome -' from female pronucleus chromosome .-"i__ from male " . ; pronucleus -• centrosome Fig. 12 — Diagram of Fertilization of the Ovum. (The somatic number of chro- mosomes being four.) (From Bohm and von Davidoff, after Boveri.) 1, Ovum surrounded by spermatozoa, only one of which is in the act of penetration. Toward the latter the protoplasm of the ovum sends out a process; 2, Head of spermato- zoon has entered ovum, its body becoming the male centrosome, its tail having disap- peared; 3, The head of spermatozoon has become the male pronucleus. Male and female pronuclei approach each other. Between them is the (male) centrosome; 4, The spiremes of male and female pronuclei have each formed two chromosomes. The centrosome has divided; 5, Male and female chromosomes have mingled and by longitu- dinal cleavage (see Mitosis, p. 54) have become eight. These become arranged in the equatorial plane of the ovum. JMantle fibres extend from centrosomes to chromosomes; 6, Division of the ovum; two daughter cells, each containing a daughter nucleus. Each daughter nucleus contains four chromosomes, two derived from each pronucleus. 62 THE CELL The earlier generations of these cells are morphologically alike and are known as Uastomeres. Soon, however, these cells become spread out and at the same time differentiated into two primary ZONA PE ULUCIDA POLAR GLOBULES A SEGMENTA- TION CAVITY. Fig. 13. — Segmentation of the Ovum. (From Gerrish, after van Beneden.) a, Two-cell stage resulting from first division of fertilized ovum; h, four-cell stage; c, d, e, later stages. A, Differentiation into inner and outer cells; B, Formation of segmentation cavity; C, Embryonic vesicle, showing two primary germ layers. Outer cells, ectoderm; inner cells, entoderm. ■jiu: (KLL 03 germ layers. The outer of these is known as the ectoderm or epihlasl, the inner as the entoderm or Jiypohlast. Between these two layers and derived from them a third layer is formed, the mesoderm or meso- blast. These three layers constitute the blastoderm. 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 cgg-centrosome disappears soon after the extrusion of the second polar body, and that the centrosome of the fer- tilized egg develops in close relation to the middle piece of the spermatozoon. These facts point to the male centrosome as the centrosome of fertilization. d 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); h, ento- derm (inner germ layer); c, mesoderm (middle germ layer); d, anlage of notochord. The ovum and spermatozoon are apparently brought together bj^ 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 be- tween ova and spermatozoa of the same species. This has been proved for lower forms by mixing ova of one species and spermatozoa from several species in an inert medium when only spermatozoa of the same species will attach themselves to the ova. That 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 powers to cause spermatozoa to enter them. 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 sper- matozoon has entered, the egg apparently loses its power of attracting sper- matozoa, 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 G4 THE CELL 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., distilled water ico 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. alcohol tinged with iodin. After from twelve to twenty-four hours the alcohol is decanted and the tube partly filled with alum-carmine solution (page 19). About twenty-four hours usually suffices for staining the nuclei. The alum-carmine is then poured oft' and the cells well shaken in water. After settling, the water is replaced by 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 MiiUer's fluid and Ranvier's alcohol (technic, 5, p. 7, and a, p. 4) for from twenty-four to forty-eight hours. The specimen is then washed in water and the cells re- moved by gently scraping the surface. These may then be stained and pre- served 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 amoebae 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 amoebae 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 move- ment 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 vitality. 6. Mitosis. The salamander tadpole and the newt are classical subjects for the study of ceU-division. The female salamander is usually full of embryo tadpoles in January and February. The embryos are removed and fixed in Flemming's fluid (technic 8, p. 8), 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. 18. 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. THE CELL 65 General References for Further Study of the Cell Branca, A.: Histologic, Paris, 1906. 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, of Aiiat., vol. iii. No. 4, 1904. Hertwig, O.: Die Zellc und die Gewebe, 1898. Hertwig, R.: Eirife, Befruchtung u. Furchungsprozess. In Hertwig's Hand- buch d. vergleich. u. experiment. Entwickelungslehre der Wirbeltiere, Bd. I, Teil I, 1903. Lillie, F. R.: A Contribution toward an Experimental Analysis of the Karyo- kinetic Figure. Science, New Series, vol. xxvii, 1908. McMurrich: The Development of the Human body. Minot: Human Embryology. A Laboratory Text-book of Embryology. Sobotta, J.: Die Befruchtung u. Furchung des Eies der Maus. Arch.f. mik. Anai., Bd. xlv, 1895. Wilson, E. B.: The Cell in Development and Inheritance, 2d ed., 1900. PART III THE TISSUES CHAPTER V HISTOGENESIS— CLASSIFICATION Ectoderm, mesoderm, and entoderm (see page 63) are known as the primary layers of the blastoderm. They differ from one an- other 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 differentiation 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 blasto- derm are as follows: Ectoderm. — (i) Epithelium of skin and its appendages — hair, nails, sweat, sebaceous and mammary glands, including smooth muscle 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, of crystalline lens, of lacrymal gland, and of lacrymal canal. (6) Epithelium of male urethra, except prostatic portion. (7) Epithelium of medulla of suprarenals, of pineal bodies, of pituitary body and of a few other small ductless glands. (8) Entire nervous system, including retina and its forward extension over iris, also muscle tissue of sphincter and dilator pupillae. Ei\jjpderm.- — (i) Epithelium of digestive tract (excepting mouth and its glands and anus,) and of glands connected with digestive tract including liver, gall bladder and pancreas. (2) Epithehum 01 respiratory tract and of its glands. (3) Epithelium of bladder except the trigonum, of female urethra, of vaginal vestibule and glands of Bartholin, of prostatic portion of male urethra, prostatic glands and glands of Cowper. 69 70 THE TISSUES (4) Epithelium of tympanum and of Eustachian tube. (5) Epithelium of thyreoid and parathyreoid, reticulum of thymus, and Hassall's corpuscles. 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. (3) Blood cells and bone-marrow. (4) Striated, cardiac and smooth muscle (with the possible excep- tion of smooth muscle of sweat glands and of iris). (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. (8) Epithelium of the suprarenal cortex. 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 con- nected 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 bound- aries are almost or wholly lost. Such a structure is known as a syncy- tium. 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. The fact that chemical changes take place in intercellular substance as well as in cells has led to the sug- gestion that the intercellular substance is endowed with the same HISTOGENESIS— CLASSIFICATIOX 71 vital properties as the cell. The consensus of opinion is, however, that the intercellular substance is derived originally from the cell, is replenished by the cell, is dependent upon the cell for its nourish- ment, and is incapable of activity or existence apart from the cell. A scientific classification of the tissues is at present impossible. The foregoing list of tissue derivations 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) nerve tissue. Of these, epithelium and connective tissue may be regarded as the more elementar}' 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. While the individuahty of the tissues classified is recognized, the physiological necessities of nutrition, innervation, etc., scarcely permit the existence of any one tissue alone by itself. Thus blood and blood vessels permeate almost all tissues except epithehum, while the latter is evervwhere traversed bv elements of nerve tissue. CHAPTER VI EPITHELIUM (INCLUDING MESOTHELIUM AND ENDOTHELIUM) General Characteristics. — Epithelium is derived from all three germ layers. In general it covers surfaces and lines cavities and diverticula from them, for example, the duct glands. Thus located, it serves for protection, takes part in the formation of neuroepithelial end organs for the reception of stimuli, and is the active agent in the elaboration of various secretions and excretions. Epithelium con- sists almost wholly of cells.- The intercellular substance is merely sufficient to attach the cells to one another and is, consequently known as cement substance. A characteristic of this cement substance is its reaction to silver nitrate (Figs. i6, 28, 29). In some instances the protoplasm of adjacent epithelial cells is seen to be even more ^Vfs* ^. ^ ^l%l'>? ^%^S'^''. Fig. 15. — Simple Cylindrical Epithelium from Uterus of Monkey — Iron-alum- hsematoxylin Stain. (Krause.) kl, cement wedges separating the free ends of the cells; ck, centrioles. 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. 20, p. 77). It seems probable that the minute spaces between the processes serve as channels for the passage of food (lymph) to the cells. Ref- erence to Figs. 159 and 191 shows that a precipitation of silver also occurs in some tubules filled with glandular secretions. It is possible that the so-called intercellular cement may be of the nature of an intercellular lymph. The surface cells of epithelium arc united by continuous cement substance in larger amount (Fig. 15), in which there are apparently no si)aces. In tliis way escape of lymph is prevented. EPITHELIUM 73 Epithelial cells vary in size and shape, the element of i)ressure being a frequent determining factor. They are often extremely elastic, allowing great changes in shape and relation. Thus the epithelium of the collapsed bladder is thick and the cells columnar but as the bladder fills, the cells become flatter until in complete distention the epithelium is thin and consists of flattened cells. Their protoplasm may be clear, finely or coarsely granular, or pig- mented. 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 basal membrane or membrana propria. Authorities differ as to whether this membrane is of connective-tissue or of epithelial origin. Surface epithelial cells frequently have thickened free borders or cuticidcB, which unite to form a continuous membrane, the ciiticular 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 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. — EpitheUa may be classified according to shape and arrangement of cells as follows: (i) Simple Epithelium. — {a) Squamous; {b) columnar. (2) Stratified EpitheUum. — {a) Squamous; {b) columnar. (3) Mesothehum and EndotheUum. Specializations of the above-mentioned types are known as: {a) Cihated epithelium; {b) pigmented epitheHum: (c) glandular epithe- lium; {d) neuro-epithelium. Pigment may occur in any type of epithelium. Ciha are found only in the simple columnar and stratified columnar forms. 74 THE TISSUES Simple Epithelium In simple epithelium the cells are arranged in a single layer, (a) Simple squamous epithelium consists of flat scale-like cells which are united by an extremely small amount of intercellular sub- FiG. i6.— From Section of Cat's Lung, stained with silver nitrate, showing out- lines of the Simple Squamous Epithelium Lining the Air Vesicle, a, Two epithelial cells; b, the wavy stained 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 center, where the nucleus , # — d Fig. 17. — Simple Columnar Epithelium from the Human Small Intestine, a, Mucous (goblet) cell; b, basement membrane; c, thickened free border (cuticula); d, leucocyte among the epithchal cells; e, replacing ceil. is situated, and thinning out toward the periphery. Simple squa- mous epithelium has but a limited distribution in man. It occurs in the lungs as non-nucleated respiratory epithelium, in Bowman's i EPITIIMLIUiM 75 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. (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 nucleus 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 cuticulse. This form of cpithehum is often ciliated. The height of the cell varies greatly, there being all gradations from Jiigh cohmmar to low cuhoidal. 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 membranous and penile portions of the male urethra, and the gall-bladder. In simple columnar epithelium, in addition to the single row of epithehal 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 epithehal cells as Fig. iS. -Diagram of Pseudostratified they are needed to replace older Epithelium, showing Nuclei situated at cells. The so-called pseudostrati- fied epithelium 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. i8). Stratified Epithelium In stratified epithelium the cells are arranged in more than one layer. (a) Stratified squamous epithelium is developed from simple epithelium by the growth of new cells between the old cells and the underlying connective tissue. It consists of several layers of cells. The number of layers varies but the shape and arrangement of the cells are quite characteristic. The cells of the deepest layer, those lying on the basement membrane which separates the epithelium 76 THE TISSUES from the underlying connective tissue are usually small regularly placed cuboi^l_cells of uniform size which in tangential sections appear as a distinct row of cells. Passing toward the surface the cells, still small, become irregular in shape and arrangement, then larger and as the surface is approached flatter, until the typical squamous surface cells are reached. The deeper cells are the softer younger cells and in the layers just above the basal layer mitotic figures arc frequent. Here also are often found \ery distinct proto- plasmic intercellular connections ("intercellular bridges," (Fig. 20), see also p. 72). Extensions of cell protoplasm similar to the inter- cellular bridges have been described as passing from the deepest cells toward or into the underlying connective tissue. The inter- Flat surface cells Polyhedral cells Large polyhedral cells \r." '- ■ Small polyhedral cells i;i Cuboidal cells ' Fig. 19. — Stratified Squamous Epithelium from Cat's Oesophagus. cellular spaces appear first as vacuoles in the exoplasm, the proto- plasmic walls of the vacuoles being the "bridges.'' The spaces are best developed in the thicker epithelia thus lending support to the view that they have to do with the nutrition of the cells. As the cells become flatter, cells without nuclei are seen and become more numeroiLS as the surface is approached, until on the surface, especially of parts exposed to air and unmoistened, hardcornified, non-nucleated cells containing keratin predominate. Such cells are in process of dying and being cast off. In the e])ithelium of mucous membranes where the surface is kept moist, there are few^r non-nucleated cells, the cells are softer and contain less keratin. In hair and nail, the surface cells though hornitied are nucleated. F.PTTITKLTUM 77 \ ^ y^^irm'^-'m It is thus seen that in stratified squamous epithelium only the surface cells are squamous. This form of epithelium rests upon a more or less distinct basement membrane, which is frequently thrown up into folds by papilLnc of the underlying connective tissue. Stratified squamous epithelium forms the surface of the skin and of mucous membranes of cavities opening upon it, mouth and oesophagus, conjunctiva, external ear, vagina and external sheath of hair follicle. {h) Stratified Columnar Epithelium. — Only the surface cells are columnar, the deeper cells being irregular in shape. The Fig. 20. — Epithelial Cells from the Stratum Spinosum of the Human Ei)idcrmis surface cells frequently send long processes show ins "Intercellular II- 11 T^u f Bridges." X7oo- (Szymo- down among the underlying cells, ihe tree nowicz.) surface is often marked by a well-developed cuticula. Some epithelia of this type are cihated. Stratified columnar epithelium is found in the larynx, nose, palpebral con- Fig. 21. — Transitional Epithelium from the Human Bladder. junctiva, largest of the gland ducts, the vas deferens, and part of the male urethra. r -K-v Wpf'-^' 9 --> ^,. ^, Fig. 22. — Stratified Columnar Epithelium from the Human Male Urethra. X4oo- Stratified epithelium composed of only from three to six layers of cells is sometimes designated '''' transitional epithelium^ This / 78 THE TISSUES type of epithelium usually rests upon a basement membrane free from papillae. 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 vrrrrTrr-rr ^ f \ Fig. 23. — Stratified Columnar Ciliated Epithelium from the Human Trachea. A mucous (goblet) cell also is present. cells are polygonal or irregularly cuboidal. This form of epithelium lines the bladder, ureter, pelvis of the kidney, and prostatic portion of male urethra. Fig. 24. — Isolated Ciliated Cells and Goblet Cells from Dog's Trachea. X700. Modified Forms of Epithelium (a) Ciliated Epithelium. — In this form of epithelium, fine hair-like f processes — cilia — extend from the surface of the cell. These cilia j vary from twelve to twenty-five for each cell and may be short as in jj J I. EPITIIKLIUM 79 the trachea or long as in the epididymis. There is usually a well- delined cuticula from which the cilia appear to spring. According to Apathy, the cilia extend through the cuticula, giving to the latter a striated appearance (Fig. 25). Just beneath the cuticula each cilium shows a swelling — the basal granule. Lenhossek considers these granules centrosomes. The intracellular extensions of the ciHa con- \.r\'/fyjt Fig. 25. Fig. 26. Fig. 25. — Ciliated Epithelial Cell from Intestine of Mollusk (Engelmann), showing a, cuticula, b, basal granules, and c, intracellular extensions of ciHa. Fig. 26. — Tangential Section through Three Cells of Skin of Aurodonta. (Kolacev.) Iron-hsmatoxylin stain, showing relation of cilia to basal granules and cytoreticulum. verge toward the nucleus, and are continuous with the reticular or fibrillar structure of the cell body (Fig. 26). The motion of cilia is wave-like, the wave always passing in the same direction. Various explanations of ciliary motion have been given. It has been sug- gested that it is due to the contractile powers of the spongioplasm, also that it is due to changes in surface tension of the film of pro- toplasm which covers each cilium. 80 THE TISSUES Cilia are confined to the surface cells of simple columnar and stratified columnar epithelium. Simple columnar ciliated epithelium occurs in the smaller bronchi, uterus, Fallopian tubes and central canal of the spinal cord. Stratified columnar ciliated epithelium occurs in large bronchi, trachea, larynx, nose, Eustachian tube, vas deferens and epididymis. (b) Pigmented epithelmm consists of cells the cytoplasm of which contains brown or black pigment (Fig. 27). It is usually present in the form of spherical or rod-like granules. Examples of it are seen in the pigmented epithelium of the ^zb^ -<-, ■>'^'?^% i-«^ ^ retina and in the pigmented cells !^!.^^;;^;..^?L>^:^'??f«^i^ ^f ^-^^ ^^^p^^ l^^^^g of ^.^g epi- dermis in colored races. (c) Glandular Epithelium. — This forms the essential or secret- ing element of glands and is mostly of the simple cylindrical variety, frequently modified to conical to conform to their concentric ar- Fig 2 7.-Pigmented Epithelial Cells ^angement with reference to the Irom the Human Retina (X350), show- » ing different degrees of pigmentation. gland lumen. The general fea- The clear spots in the centres of the cells , ^ , , , • . i 1 • represent the unstained nuclei. tures of glandular epithelium are described on p. 217. The dift"erent kinds of glands and their epithelia are described among the organs. (d) N euro-epithelium. — This is a highly specialized form of epithe- Hum which occurs in connection with the end organs of nerves, under which heading it will be described. Mesothelium and Endothelitim 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 iudice 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 mesenchyme, the term endothelium is retained. The term mesothe- lium is used for those tissues which resemble epithelium but are de- EPITHELIUM 81 rived from a subdivision of the mesoderm which he designates the mesothehal layer. Mesothehum and endothehum are similar in structure. Each consists of thin flattened cells with clear or slightly granular proto- FiG. 28. — Mesothelium from Omentum of Dog Treated according to Technic 7, p. S2. 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. plasm and bulging oval or spherical nuclei. The edges of the cells are usually wavy 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 Fig. 29. — The Endothelium of a Small Blood-vessel. Silver nitrate stain. X350. 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. Recent researches make it seem probable that the surface cells 82 THE TISSUES of serous membranes are modified connective-tissue cells rather than epithelium. TECHNIC 1. Simple Squamous Epithelium. — That of the lung may be demonstrated by injecting with silver solution (technic i, p. 28) through a bronchus and then immersing the tissue in the same solution. The lungs of young kittens furnish especially satisfactory material. 2. Simple Columnar Epithelium.— A piece of small intestine, human or animal, is pinned out flat on cork and fixed in formalin-Miiller's fluid (technic 6, p. 7). Sections are cut perpendicular to the surface, stained with hsematoxy- lin and eosin (technic i, p. 20) and mounted in glycerin tinged with cosin (p. 22). Little processes known as villi project from the inner surface of the intes- tine. These are covered by a single layer of columnar epithelial cells. The cuticulae 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 stud- ied in their various stages of secretion, from the ceU in which only a small amount of mucus is present near the outer margin, to the cell whose protoplasm is almost wholly replaced 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 pre- ceding. The cells are laid down in from six to eight layers. The oesophagus may be used instead of the cornea, its mucous membrane being lined by a some- what thicker epithelium. 4. Transitional Epithelium. — -This is conveniently studied in the mucous membrane 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 are ciliated so that this specimen also serves to demonstrate that type of modified epithelium. Isolated cells or clumps of cells may be obtained from the trachea in the manner described in technic 3, p. 6^. 6. Pigmented Epithelium. — Fix a freshly removed eye in formalin- ]Muller's fluid (p. 7). After hardening, cut transversely and remove the vitreous and 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 liandling. 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 KIMTUELIUM 83 stained with haematoxylin (stain 5, p. iq). 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 connec- tive-tissue cells a paler blue. It must be borne in mind in studying this specimen that the strands or trabecular 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 VII THE CONNECTIVE TISSUES Under this head are classn.jd Connective Tissue Proper, Carti- lage, and Bone. General Characteristics. — The most prominent characteristic of the connective tissues is the predominance of the intercellular sub- stance. In this respect the connf^r' '^-e tissues differ markedly from epithelium. IMoreover it is the 'I'-'^'^'srcellular substance and not the cells which determines the physical character of the tissue. Thus, for example, the hardness of bone and teeth, the firmness and elasticity of cartilage, the toughness of tendon, the softness of sub- cutaneous connective tissue, are all due to the character and arrange- ment of the intercellular elements. In most forms of connective tissue the cellular elements are very similar and in no way determine the physical character of the tissue. The role of the connective tissues is mainly passive, and the cells, instead of playing the most active part, as in epithelial, muscle and nerve tissues, serve mainly for the maintenance of the nutrition of the more important intercellular substance. The latter thus predominates in function as well as in quantity, its character accord- ing with the specific function of the tissue. Thus, where rigidity and preservation of form are essential, are found connective tissues with such intercellular substance as occurs in bone, where toughness and elasticity, such types of intercellular substance as occur in tendon and cartilage, where softness and flexibility, such loosely arranged intercellular elements as are present in areolar tissue. Most of the connective tissues are extremely vascular, differing also in this respect from epithehum. An exception to this rule is hyalin cartilage. v A verj^ close relation exists between the different forms of connective tissue as evidenced by a marked tendency and ability of one form to be closelj' united to, or to be transformed into, or to be replaced by, another form. This may be progressive in line of developmenl or retrogressive in line of degeneration. For example the close physical union which exists between tendon and bone or between cartilage and bone or tendon: or the manner in which the forms of bone are first laid down in connecti\-e tissue, which is rei^laced by bone, or by cartilage 84 THE CONNECTIVE TISSUES 85 followed by bone. Again in the callus which occurs in the uniting of fractures, there is first fibrous tissue, then bone, or there may be an intervening cartilagin- ous stage, or repair may fail of completion, leaving permanent connective tissue or cartilage. The correlation of the differeiu forms of connective tissue is also shown by the fact that ill different species the same anatomical structure is sometimes formed by different members of the group. Thus in some birds the leg tendons are formed of bone, while in certain fish the skeletal system is cartilaginous. In Mammals the sclera of the eye is fibrous ' acctive tissue. In Batrachians the same structure is cartilage, in Birds, bone. Development. — All of the connective tissues have a common origin in the more loosely arranged portion of the mesoderm which is known as the mesenchyme. This consists at first wholly of small spheroidal or ovoid cells. These cells unite to form a networkior, syncytium with closely packed nuclei. As the cytoplasm increases more r ~ ' ^ly than the nuclei, the latter become more widely separated. There next takes place a differentiation wliich by some has been described as a differentiation of the cytoplasm into a central portion or endo- plasm surrounding the nucleus, and a peripheral portion or exoplasm, by others as a differentiation into cells and a primitive intercellular or ground substance. Such a tissue is known as mucous or embryonal connective tissue (Fig. 30) and is widely distributed throughout the embryo. It represents a stage in the development of the more specialized forms of connective tissue. Portions of this embryonal connective tissue develop fibres. The first formed fibres are known as white or fibrillated fibres or, because of their chemical constitution, as collagenous fibres. Somewhat later other fibres appear which are known as yellow or elastic fibres and consist of elastin. This differentiation of fibres gives rise to connective-tissue proper. At other points in the mesen- chyme, cartilaginous material appears. These points are numerous and widely separated and each represents a chondrification center or point of development of cartilage. At some points in the connective tissue or in the cartilage there are depositions of lime salts — calcification. This is followed by the formation of true bone — ossification. Regarding the development of the connective-tissue fibrils, there arc two theories: (i) According to one, they are developed directly from the protop'iasm of the connective-tissue cells. The cells increase in length, and each she ,vs a differentiation into an endoplasmic and an ectoplasmic portion. Fine granules appear in the ectoplasm, become arranged in rows and unite to form fibrils. Such cells are known as fibroblasts, and their fibrils are the fore-runners of the inter- cellular fibrils of connective tissue. (2) According to the other theory the fibrils are developed from the matrix, minute granules first becoming arranged in rows and later uniting to form fibrils. Regarded as opposing theories, there is in reality but little antagonism be- tween them, it being probable that what has been described by some observers as ectoplasm has been considered by others intercellular substance. Whichever theory is accepted, the entire intercellular substance, fibres and ground substance are ultimate derivatives of the cell. Recent studies, especially those of Mall, are decidedly in favor of the first of the theories given above, that is of the intra- cellular or ectoplasmic origin of the connective-tissue fibrils. 86 THE TISSUES Two similar theories exist as to ihe development of elastic fibres, a cellular theory and an extracellular theory. As in the case of the white fibrils the weight of evidence points to their development in the ectoplasm, apparently in much the same manner as the white fibrils. Classificalion:- Tissue Cartilage Fibrous [ Elastic Bone r (i) Embryonal Connective Tissue • Proper Connective J | Ilyalin (2) Fibrillar (3) Elastic (4) Reticular f (a) loose or areolar (fat tissue, pigment- ed tissue) {b) formed (tendon and aponeurosis) CONNECTIVE TISSUE PROPER EMBRYONAL CONNECTIVE TISSUE This is the least differentiated of the connective tissues and has been already partly described (p. 85). It is not found in the normal human adult but is described on account of its important develop- mental relations to the mature forms. It is also of interest from the standpoint of pathology as it is the type of connective tissue found in certain new growths, and also occurs during connective-tissue repair after injuries. Its structure varies considerably for different developmental stages. The younger forms are little more than mesenchyme with irregular stellate branching and anastomosing cells scattered through an apparently structureless or slightly fibril- lated semifluid ground substance. Older forms contain more dis- tinct fibres of both white and elastic varieties. To a somewhat more differentiated embryonal tissue in which the ground substance is rich in mucin, the name mucous tissue has been given (Fig. 30). It consists of a mucin-containing ground substance in which are small bundles of fine white fibrils and irregular branching cells. It occurs as Wharton's jelly in the umbilical cord. By some the vitreous of the eye is classed as mucous tissue. It contains only a few cells and only slight traces of a reticular fibrillation. FIBRILLAR CONNECTIVE TISSUE Fibrillar connective tissue, also known as white fibrous tissue, and itself sometimes called connective tissue proper, consists of cells and fibres lying in a basement or ground substance. The elements of fibrillar tissue mav be classified as follows: Cells f I. Fixed THE CONNECTIVE TISSUES 87 [ (a) Ordinary connective-tissues cells. (b) Plasma cells. (c) Mast cells. (d) Clasmatocytes 2. Wandering. Intercellular substance [ . V „., I white or fibrillated, (a) Fibres i ,, , ,. I [ yellow or elastic. (b) Ground or basement substance. r m- r. Fig. 30. — Mucous Connective Tissue from Umbilical Cord of Eight-inch Foetal Pig. X600. At this stage the ground substance shows some fibrillation. Connective -tissue Cells. — (a) The ordinary fixed connective- tissue cell is often the only connective-tissue cell seen in ordinary sections. It is an irregular shaped cell, often quite flattened with clear or slightly granular cytoplasm and an oval nucleus. (Figs. 31 and 32,) In loosely arranged tissue where the cells are well separated, the cell is usually stellate with many branches, which anastomose with branches of neighboring cells. In a dense tissue such as the cornea, these cells apparently lie in little cell spaces or lacunae from which minute channels (canaliculi) extend in all directions to unite with canaliculi from adjoining spaces (^Fig. 33). Delicate cell processes extend into the canaliculi and there anastomose with processes from other cells thus forming a sort of syncytium (Fig. 34). Owing to the extreme sensitiveness of the protoplasm of the connective-tissue cell to most fixatives, its usual 88 THE TISSUES b ^•^.) Fig. 31. — Areolar Connective Tissue (Rauber-Kopsch). a, White fibre, b, Elastic fibre, c, Fixed connective tissue cell, d, Clasmatocyte. e, Leucocyte (wandering cell), f, ]Mast cell. d -./^ % Fig. 32. — Fibrillar Connective Tissue (Areolar Type) from Subcutaneous Tissue of Rabbit (technic 2,-p. 100). Xsoo. a, Fixed connective-tissue cell; h, fibrillated fibres; c, elastic fibre with curled broken end; d, elastic fibres sho^\•ing Y-shaped branching. THE COXXF.CTTVE TISSUES 89 appearance is that of a minute amount of cytoplasm shrunken down around a nucleus. Somewhat similar cells with more coarsely- granular or vacuolated cytoplasm have been designated ''clas- mocytes" (Ran\'ier). By some these are believed to be of leucocyte- origin, by others to be an earlier stage in the development of the fixed connective-tissue cell. {b) Plasfua 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. Their nuclei are small, spherical and usually excentric. Small plasma cells i5x?«^'";^'?^ ' ■ .•:'.'"-*w-^»»^ia.-.w^-6^.' - ^^: ^ji.: < .■.-, '"^ If- ' . •■..'- - .- ■-■■:j^<^ k-y -•-Si, ^ >--■»=■ -c-t Fig. ^t,. — Section of Human Cornea cut Tangential to Surface. X350. (Technic 9, p. loi.) Connective-tissue Cell Spaces (Lacunae) and Anastomosing Canaliculi, white; Intercellular Substance (Ground Substance and Fibres), dark. 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- plasm contains coarse granules which stain intensely with basic aniline dyes (Fig. 31). They are believed by some investigators to be connected \^ith the formation of fat; by others to represent a stage in the development of the fixed connective-tissue cell. (d) Clasmatocytes . — These are large, mostly spindle shaped cells with granular protoplasm. Connective- tissue cells may be pigmented (Fig. 35). In suck cells the cytoplasm is more or less filled with brown or black pigment 90 THE TISSUES granules. In man pigmented connective-tissue cells occur in the skin, chorioid and iris. The so-called wandering cells (Fig. 31) are not properly a part of connective tissue, being merely amoeboid white blood cells (see page * - .»wr'.": _.-— J • ■-'-••M... ■:< : .' - / /..0% /■■'/ //i"\\s/./ ^-^^^^ \ lArn .^-"-f ■■y^%^:mw\ \ \ 1. / , / .- , r~-*:'V<-v% — '(~< » / V/ --T' , ' \ \ ::;*^:xi. ,...- .- ,- -._ . -...,.. . _= ; ', Fig. 34. — Section of Human Cornea cut Tangential to Surface. X350. (Technic 8, p. 10 1.) Connective-tissue Cells with Anastomosing Processes, stained; Intercellular Substance. (Ground Substance and Fibres), unstained. Ill) which have passed out from the vessels into the tissues. They are not pecuhar to connective tissue, being found in other tissues, e.g., in epithelium. The Intercellular Substance.— (a) Fibres. White or fihrillated fibres are bundles of ex- tremely fine fibrillae (0.5/x in diameter) (Fig. 32). The fibrillae lie parallel to one another giving the fibre a striated appearance and are '>^^% .€&^^i^'''^^ ^^^^^- •« , M%W^' united by a small amount '^■"' s^S:^^!^ i^^^^ ^Sff of cement substance. The , ^ , . ^ „ fibrillae do not branch. The Fig. 35. — Pigmented Connective-tissue Cells from Chorioid Coat of Human Eve. X3S0. fibre bundles, on the other (Technic 7, p. 100.) " ^^^^^ ^^^^^^^ dichotO- mously and anastomose. White fibres are soft. and flexible but very slightly elastic. Chemically they consist of an albuminous material known as collagen and, on boiling, yield gelatin. Yellow or elastic fibres are apparently homogeneous, highly re- fractive fibres, varying in diameter from i to loju (Fig. 32). They branch and anastomose, forming networks. The smaller fibres are TITF. rOXXECTIVE TISSUES 91 round on cross section, the larger flattened or hexagonal (Figs. 45 and 46). Their elasticity is easily demonstrated in teased specimens by curUng of the broken ends of the fibres (Fig. 32). On boiUng 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 fibres and elastic fibres above described, so-called ''reticular" fibres are frequently present in fibrillated connective tissue. (See p. 99.) {b) Basement or ground substance constitutes the matrix in which the connective-tissue cells and fibres lie. It also occurs in extremely minute amounts between the Individual fibrill.T of the white fibres, where it acts as a cement substance (Fig. 31). Difiiculty in seeing this ground substance is due to its transparency. It may be demon- strated by staining with silver nitrate. (See technic 9, p. loi.) 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 hard and is known as dense fibrous tissue. The terms fine connective tissue and coarse connective tissue desig- nate the character of the fibres. When many cells are present, the tissue is softer and is known as cellular connective tissue. According to the arrangement of the white fibres, fibrous connec- tive tissue is subdivided into areolar or loose connective tissue and formed connective tissue. Areolar or Loose Connective Tissue In this the fibres are irregular, running in all directions and in- terlacing, leaving between them meshes or areolce (Fig. 32). Subcutaneous connective tissue is a typical example of areolar tissue. Both white and elastic fibres are present, although the former predominate. Areolar tissue varies greatly as regards the relative number of cells and fibres and the closeness with which the different elements are packed. It thus varies greatly in density. Fat Tissue, — Adipose tissue or fat tissue is a form of areolar tissue in which some of the cells have become changed into fat cells. Fat tissue is peculiar among the connective tissues in that the cells and not the intercellular substance make up the bulk and determine the 92 THE TISSUES character of the tissue. The adult fat cell is surrounded by a distinct cell membrane, and almost the entire cell is occupied by a single droplet of fat (Figs. 37 and 38). The nucleus, flattened and sur- rounded by a small amount of cytoplasm, is usually found pressed against the cell wall (Fig. 38). This appearance of a distinct cell membrane enclosing the spherical fat droplet, with the nucleus and cytoplasm pressed into a cresent-shaped mass at one side, has given rise to the term "signet-ring cell." Fat cells which occur singly, Fig. 36.- — Fat Tissue from Human Subcutaneous Tissue (Child) to show Lobulation. X25. (Technic i, p. 99.) or in small groups, or in the developing fat of young animals are spherical (Fig. 37). In large masses of adult fat, the closely packed cells are subjected to pressure and are polyhedral (Fig. 38). Fat cells are usually arranged in groups or lobules, each lobule being separated from its neighbors by fibrillar connective tissue (Fig. 36). The appearance which adult fat presents can be understood only by reference to its histogenesis. Fat cells are developed directly from embryonic connective-tissue cells. In the human embryo they are first distinguishable as fat cells about the thirteenth week. The connective-tissue cells which are to become fat cells gather in groups in the meshes of the capillary network which marks the ending of a small artery. Each group is destined to become an adult fat lobule (Fig. 39). TIIF, COXXF.rTIVK TISSUES a b 93 -> c Fig. 37.— Young Fat from Human Subcutaneous Tissue. (Child.) Xi75- (Technic II, p. loi.) a, Interlobular connective tissue; b, fixed connective-tissue cell; c, fat cells d, artery; e, nucleus of fat cell and remains of cytoplasm ("signet ring"). -— b d c Fig. 38.— Adult Fat Tissue from Human Subcutaneous Tissue. Xi75- (Technic II, p. loi.) a, Fat cells; b, interlobular connective tissue; c, nucleus of fat cell and remains of cytoplasm ("signet ring"); d, artery. 94 THE TISSUES Fat first appears as minute droplets in the cytoplasm of the em- bryonic connective-tissue cell (Fig. 40). These small droplets in- I PIT .:'f £- ->^ *- Fig. 39. — Developing Fat Tissue from Subcutaneous Tissue of Five-inch Foeta Pig- X75. (Technic 12, p. loi.) a, Arteriole breaking up into capillary network; b embryonal connective tissue; c, embryonal fat lobule developing around blood-vessels • • • ■■»■■•■•- Fig. 40. — Developing Fat Tissue from Subcutaneous Tissue of Five-inch Foetal Pig. (Technic 12, p. loi.) a, Arteriole breaking up into capillary network; b, embryonal connective tissue, embrj-onal cells from which fat cells are developing; c, capillaries. Fat droplets stained black. At the right are five individual cells showing stages of development from an embryonal cell to an adult fat cell. crease in number and finally coalesce to form a single larger droplet. This increases in size and ultimately almost wholly replaces the cyto- THE CONXECTIVF. TISSUES 95 plasm. In this way the nucleus and remaining cytoplasm are pressed to one side and come to occupy the inconspicuous position which they have in adult fat. The blood supply of fat is rich and the adult lobule maintains its embryonic vascular relations, in that the vascular supply of each lobule is complete and independent. One artery runs to each lobule, where it breaks up into an intralobular capillary network, which in turn gives rise to the intralobular veins, usually two in number. Fat is thus seen to be a connective tissue in which some of the cells have undergone specialization. There still remain, however, embry- onal connective-tissue cells which arc not destined to become fat cells, but which develop into cells and fibres of ordinary fibrous connective tissue. A few of these remain among the fat cells to become the delicate intralobular connective tissue seen in adult fat. The majority are, however, pushed to one side by the developing lobules, where they form the inter- lobular septa. t:.i= ■.SO ..*,,j- Fig. 41. Fig. 42, Fig. 41. — Longitudinal Section of Tendon from Frog's Gastrocnemius. X 250. The nuclei of the flattened cells are seen lying in rows between the connective- tissue fibres. Fig. 42. — Teased tendon fibres with cells lying on their surface. X400. (Ranvier.) Formed Connective Tissue In formed connective tissue the tissue elements instead of being disposed irregularly as in areolar tissue, are arranged with some regularity or order, thus giving the tissue more or less definite form. Tendons and ligaments are examples of formed connective tissue in which the fibres all run in approximately the same direction (Fig. 41). Elastic fibres are absent or present in very small numbers. Tbe Fit-' ' linance of the white fibres and their parallel arrange- in great strength with almost no extensibihty. While 96 THE TISSUES the individual librils do not branch, groups of fibrils pass from one bundle to another. There is little ground substance and cells are comparatively few. The latter are, however, so characteristic as to have received the name of tendon cells (Fig. 43). They are irregu- larly rectangular cells, have rather more breadth than length and are arranged in characteristic rows between the fibre bundles. The cell margins are contiguous and the usually excentric nuclei tend to lie in adjacent sides of two cells, thus giving the cells the appearance of being arranged in pairs (Fig. 42). Thin plate-like extensions of the — A — B Fig. 43. Fu;. 44. Fig. 43. — Tendon Cells from the Tail of a Rat. Stained by methylene-blue {intra vitam). (Bohm-Davidoff.) Fig. 44. — Pavement Endothelium of Tendon of Rat. A , intercellular substance im- pregnated with silver nitrate; 5, tendon fibres, Xirs- (Branca.) cell (Fig. 43) pass into the ground substance between the fibre bundles and when the cell is seen on flat, the greater thickness of the cell through the extensions gives the optical effect of dark lines in the cell body. Aponeuroses. — In thin aponeurotic tissue the fibres are disposed in two planes, the fibres of one plane running approximately at right angles to those of the other plane. The cells resemble tendon cells and he. upon the fibre bundles. In thicker aponeuroses the fibres are arranged in planes but their disposition is more irregular. ELASTIC TISSUE Elastic fibres occurring in fibrous connective tissue have been described. When the elastic fibres predominate the tissue is known as elastic tissue. Almost pure elastic tissue is found in the Uga- mentum nuchai of quadrupeds. In man it occurs mainly in the ligamenta subflava, in some of the laryngeal ligaments, in the walls THE conm:cti\'f. tissues 97 of the trachea and bronchi and of arteries. In the ligamentum nuchae the fibres are coarse (lo to 15^) and arranged in bundles separated from one another by white fibrous tissue containing con- FiG. 45. — Coarse Elastic Fibres from Ligamentum Nuchae. (Technic 10, p. 101.) X500. Teased specimen. nective-tissue cells. This white fibrous tissue also penetrates the bundles and separates the individual elastic fibres. A few white fibres and connective-tissue cells are also present (Figs. 45 and 46). Fig. 46. — Cross Section of Coarse Elastic Fibres from Ligamentum Nuch:e. X500. (Technic 10, p. loi.) a, Elastic fibres; b, white fibrous tissue and cement substance. The nuclei are the nuclei of fixed connective-tissue cells. Elastic tissue may be arranged as thin membranes, as e.g., in the walls of blood-vessels. These membranes are usually described as composed of a dense mass of flat, ribbon-like elastic fibres, which 7 98 THE TISSUES interlace in such a manner as to leave openings in the membrane. Hence the term ''fenestrated membrane." They have been re- cently described as consisting of a central layer composed of elastin, staining with magenta, and on either side a thin, transparent sheath unstained by magenta. This is seen to correspond to Mall's de- scription of the structure of the elastic fibre. Only the middle of these layers is fenestrated. RETICULAR TISSUE Reticular connective tissue is a form of fibrillar connective tissue. It consists of extremely delicate fibrils with no ground Fig. 47. — Reticular Tissue from a Human Lj^mph Node. (Teciiuic, 13, p. 101.) a, Reticular connective tissue, in the meshes of which are suspended b, leucocytes, and c, ( lymphocytes. The reticular connectiv^e tissue is present also in the more dense lymphatic tissue seen in the lower part of the figure, but is not visible on account of the closely packed cells. ! substance. The fibrils are grouped in larger or smaller bundles I which form a network or feltwork enclosing spaces, thus constitut- j THE CONNECTIVE TISSUES 99 ing a reticulum. The fibrils present much the same microscopic appearance as the white fibres of areolar tissue. Also in certain organs, e.g., the lymph nodes, the direct continuity of the fibres of the coarser fibrous tissue of the trabeculoe with the fibrils of the reticular tissue can be easily demonstrated. In some locations, e.g., in the lymph nodes (Fig. 47) the cells of the reticular tissue lie upon the surface of the fibrils and so completely invest them that the fibrillar character of the tissue cannot be seen until the overlying cells have been removed. This has led to two views regarding the relations between the cells and fibres of reticular tissue, one that the relation is the same as in other forms of connective tissue, the cells lying on the fibres but being entirely separate from them, the other that the fibres run through the protoplasm of the cells. This view considers reticular tissue a less dift'erentiated type than fibrous tissue, develop- ment having stopped at a point where the cells (fibroblasts) still have fibrils running through their protoplasm. Reticular tissue has been described as yielding on boiling, a sub- stance called by some elastin by others reticulin instead of gelatin which results from boiling fibrous tissue. Other recent studies upon the chemistry of reticular tissue are not however in accord with this view, reticular tissue being found to yield gelatin on boiling. Both tissues resist pancreatic digestion, and the question as to the structural and chemical relations of the two tissues remains at present unsettled. Reticular connective tissue forms the framework of adenoid tissue and of bone-marrow. It is also present in large amounts in the spleen and in the mucous membrane of the gastro-intestinal tract, lung, liver, kidney, and other organs where it forms the finer part of the framework, supporting the capillaries, and often apparently serving as a basement membrane for the gland cells. TECHNIC I. Areolar Tissue, to show White and Elastic Fibres. — Remove a bit of the subcutaneous tissue, as free from fat as possible, from a recently killed animal. Place it upon a mounting slide and with teasing needles quickly spread it out in a thin layer. During this manipulation the specimen should be kept moist by breathing on it. Put a drop of sodium chlorid solution upon the specimen and cover. As the specimen is unstained, a small diaphragm should be used for the micro- scopic examination. The white fibres are straight or wa\y, are crossed in all directions, and are longitudinally striated. The elastic fibres have been stretched and show as sharp lines with curled ends where the fibres are broken. j^r 100 v' THE TISSUES Place a drop of hycftic acelatc, i-per-cent. aqueous solution, at one side of the cover and a bit of filter paper at the other side. The filter paper absorbs the salt solution, which is replaced by the hydric acetate. The latter causes the white fibres to swell and become indistinct while the elastic fibres show more plainly. 2. Areolar Tissue, to show Cells and Elastic Fibres. — Prepare second speci- men of areolar tissue in the same manner as the preceding. Instead of mounting in salt solution, allow it to become perfectly dry, then stain in the following solution : Gentian violet, saturated aqueous solution, 40 c.c. Water, 60 c.c. Wash thoroughl}-, dry, and mount in balsam. The nuclei of the fixed connective-tissue cells are stained violet. Their deli- cate cell bodies show as an irregular haze around the nuclei. Both nuclei and cell bodies appear cut in all directions by the stretched elastic fibres. Wander- ing cells (leucocytes) may usually be seen. Plasma cells are frequently not demonstrable, and mast cells are only occasionally present. The elastic fibres are stained violet. The white fibres are almost unstained. While these methods are most satisfactory for bringing out the different con- nective-tissue elements, they are misleading to the student in that they show a picture of connective tissue after special preparation, rather than as it usually appears in sections. For contrast the student should study carefully the con- nective tissue as it appears in sections through the skin, the mucous membranes and other organs. 3. Formed Connective Tissue. — Fibrous tissue arranged in the form of a net- work may be seen in the specimen of omentum (technic 7, p. 82). 4. Densely formed connective tissue may be studied in tendon. Cut through the skin of the tail of a recently killed mouse about half an inch from the tip and break the tail at this point. By pulling on the end of the tail this portion may now be separated from the rest of the tail, carrying with it long delicate tendon fibrils, which have been pulled out of their sheaths. These should be immedi- ately examined in salt solution, using the high power and a small diaphragm. The fibrils are seen arranged in parallel bundles. 5. Place a drop of hydric acetate (2-per-cent. aqueous solution) at one side of the cover-glass, absorbing the salt solution from the opposite side by means of filter paper. The fibres swell and become almost invisible, while rows of connective-tissue cells (tendon cells) can now be seen. The cells may be stained by allowing a drop of haematoxylin or of carmine solution to run under the cover. After the cells are sufiiciently stained, the excess of stain is removed by washing, and the specimen mounted in glycerin. 6. Fix a small piece of any good-sized tendon in formalin-Miiller's fluid (page 7). After a week, harden in alcohol, embed in celloidin, and make longitudinal and transverse sections. Stain strongly with haematoxylin, followed by picro- acid-fuchsin (page 21). Mount in balsam. 7. Pigmented connective-tissue cells are most conveniently obtained from the chorioid coat of the eye. Fix an eye in formalin-Miiller's fluid (see page 7), cut in half, remove chorioid and retina and pick ofif the dark shreds which cling to the outer surface of the chorioid and inner surface of the sclera. These may TUV. rOXXKCTIVE TISSUES 101 be transferred directly to glycerin, in which tlicy arc mounted, or the bits of tissue may be first stained with haematoxylin (page i8). In addition to the pig- mented cells should be noted the ordinary fixed connective-tissue cells which lie among them. Only the nuclei of these cells can be seen. 8. ConneiiLhigr tissue cells to show anastomosing processes. — Stain a cornea with gold(chIori(j/ i^see page 28). Sections are made tangential to the convex surface and are mounted in glycerin. 9. Connective-tissue cell spaces (lacunae) and their anastomosing canaliculi may be demonstrated by staining a cornea with silver nitrate (see page 28). The silver stains the ground substance of the cornea, leaving the lacuna) and canaliculi unstained. The relation which this picture bears to the preceding should be borne in mind (see Figs. 31 and 32). 10. Coarse elastic fibres may be obtained from the ligamentum nuchae, which consists almost wholly of elastic tissue. A piece of the ligament is fixed in satu- rated aqueous solution of picric acid and hardened in alcohol. A bit of this tissue is teased apart on a glass slide in a drop of pure glycerin, in which it is also mounted. Before putting into glycerin, the specimen may be stained with picro- acid-fuchsin. This intensifies the yellow of the elastic fibres and brings out in red the fibrillar connective tissue. Pieces of the ligament fixed and hardened in the same manner may be embedded in celloidin and cut into longitudinal and transverse sections. These stained with picro-acid-fuchsin show well the rela- tion of the coarse elastic fibres (yellow) to the more delicate fibrous tissues (red). 11. Fat Tissue. — Human subcutaneous fat as fresh as possible is fixed in formalin-Miiller's fluid (technic 6, p. 7), hardened in alcohol and embedded in celloidin. Sections are stained with haematoxylin and picro-acid-fuchsin (technic 3, p. 21). The alcohol and ether of the celloidin remove the fat from the fat cells, leaving only the cell membranes. The fat gives the celloidin a milky appearance. Such celloidin does not cut well. The celloidin should, there- fore, be changed until it ceases to turn white. The sections are cleared in oil of origanum or carbol-xylol, and mounted in balsam. The fibrillar tissue is stained red by the fuchsin, and the protoplasm of the fat cell yellow by the picric .icid. Fat tissue may also be satisfactorily stained by Sudan III or Scharlack R. F details see p. 31. 12. Developing Fat Tissue. — Remove bits of tissue from the axilla or groin of a five-inch foetal pig, or other foetus of about the same development. Fix twenty-four hours in a i-per-cent. aqueous solution of osmic acid (technic 10, p. 31), wash thoroughly and mount in glycerin. A part of the tissue mounted should be thoroughly teased, the rest gently pulled apart. The teased portion will show the fat cells in various stages of development. The unteased part will usually show brownish blood-vessels and the grouping of fat cells around them, to form embryonic fat lobules. Note the developing connective tissue between the groups of fat cells. It is from this that the areolar tissue, which envelops and separates the lobules of adult fat, is developed. 13. Reticular Tissue. — Fix a lymph node in formalin-Miiller's fluid (technic 6, p. 7), and stain very thin sections with haematoxylin and picro-acid-fuchsin (technic 3, p. 21). In the lymph sinuses of the medulla the reticulum can usually be plainly seen. 102 THE TISSUES CARTILAGE Cartilage is a form of connective tissue in which the ground sub- stance is firm and dense and determines the physical character of the tissue. On boiling it yields chondrin. Cartilage cells are differen- tiated connective-tissue cells. While varying greatly in shape they are most frequently spherical or oval. Each cell lies in a cell space or lacuna, which it completely fills. The intercellular substance im- mediately surrounding a lacuna is frequently arranged concentrically, forming a sort of capsule. Fine canaliculi connecting the lacuna} are present in some of the lower animals and have been described in human cartilage. They can be demonstrated, however, in human cartilage, only by special methods, and probably represent artefacts. Cartilage contains no blood-vessels, and in human cartilage no lymph channels have been positively demonstrated. .4-^ m% %' M ■fp!m iM^}ii \^5 -^^^S^'iy "7<®^'' IM^.m ■ :> '. ^^m ooie ^ m- ■' 'B m^-^ ^m Fig. 48. — Hyaline Cartilage from Head of Frog's Femur. X350. (Technic i, p. 104.) (Jroups of cartilage cells in apparently homogeneous matrix. Cartilage is subdivided according to the character of its intercellu- lar substance into three varieties: (i) Hyaline, (2) elastic, (3) fibrous. I. Hya ine Cartilage (Fig. 48). — 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 i.s centrally placed, and has a distinct intranuclear network and TIIK CONNECTIVE TISSUES 103 ■Mm -' /'V^'^-'i , .^: •l"^ ,1 :£■ '■. "i+V.^JJte 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 intercellular matrix, when subjected to the usual technic, appears homogeneous. By the use of special methods, such, e.g., as artificial digestion, this ajiparently structureless matrix has been shown to be made up of bundles of fibres, quite similar to those found in fibrous connective tissue. Hyaline cartilage forms the articular cartilages of joints, the costal cartilages, and the cartilages of the nose, trachea, and bronchi. In the embryo a young type of hyaline cartilage, known as em- bryonal cartilage, forms the matrix in which most of the bones are developed. 2. Elastic cartilage (Fig. 49) resembles hyaline, but dift'ers from the latter in that its hya- line matrix contains a large number of elastic fibres. These vary in size, many being ex- tremely fine. The elastic fibres branch and run in all directions, forming a dense network of inter- lacing and anastomosing fibres. Elastic cartilage occurs in the external ear, the Eustachian tube, the epiglottis, and in some of the laryngeal cartilages. ' 3. Fibrous cartilage (Fig. 50) is composed mainly of fibrillar con- nective 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 sternoclavic- ular articulations, in the symphysis pubis, and in the intervertebral discs. Cartilage, ej^cept 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 superficial layers of the cartilage. Like the other connective tissues, cartilage develops from meso- L- Fig. 49. — Elastic Cartilage from Dog's Ear. X350. (Technic 2, p. 104.) Groups of cartilage cells in fibro-elastic matrix. 104 THE TISSUES derm. 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. Fig. 50. — Fibrous Cartilage from Dog's Intervertebral Disc. X350. (Technic 3, below.) Groups of cartilage cells in matrix of fibrillar connective 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 solution, cementing the cover-glass immediately. (2) Elastic Cartilage. — Remove a piece of cartilage from the ear and fix in formalin-Muller's fluid (technic 6, p. yj. Stain sections strongly with hema- toxylin, followed by picro-acid-fuchsin (technic 3, p. 21). Clear in carbol-xylol and mount in balsam. The capsules around the cartilage cells are thick and, as they usually retain some haematoxylin, 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-Muller's fluid. Sections are stained either with haematoxylin-eosin or with hasmato.xy- lin-picro-acid-fuchsin and mounted in balsam. BONE 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 THE CONXFXTIVE TISSUES 105 Fig. 51. — Bone Tissue showing Lacunae and Canaliculi. X700. (Technic i, p. 106.) phosphate and the carbonate of calcium. These salts are not merely 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 com- pose two-thirds of bone by weight, it is impossible to distinguish them by the highest magnification. Fur- thermore, if either the lime salts are dissolved out by means of acids (decalcifica- tion) or the organic matter removed by heating (calcina- tion), the histological struc- ture of the bone still remains. Like the other connective tissues, bone consists morphologically of cells and intercellular substance. Bone cells or bone corpuscles lie in distinct cell spaces or lacunce. From the lacunae pass off in all directions minute canals — canaliculi — which anastomose with canaliculi of neighboring lacunae (Fig. 51). At the surface of bone these canaliculi open into the periosteal lym- phatics. A complete system of canals is thus formed, which traverse the bone and serve for the passage of nutritive fluids. The bone cells themselves (Fig. 52) are flat, ovoid, nucleated cells, with numerous fine processes, which extend in all directions into the can- aliculi. In young developing bones the Fig. 52.— Bone Cell and processes of adjacent cells anastomose. In ^hTcS-bod^hlf shrunken, ^dult bone the processes extend but a short allowing the outhne of the distance into the canalicuH, and probably do lacuna to be seen. 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 lameUce. Less commonly the fibre bundles are coarser and have an irregular arrangement. 106 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 lacuns 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. 202.) CHAPTER Vni THE BLOOD I* WiJi^ 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 substance as in other tissues. There are about 5 litres of blood in the adult body, blood thus constituting about one-thirteenth of the entire body weight. The fluid intercellular substance or plasma is slightly alkahne in reaction. It consists of serum albumen, globulin, fibrinogen and inorganic salts, chiefly the chlorid, carbonate, bicarbonate and phos- -lii?, phate of soda. The reaction of blood is distinctly alkaline, due mainly to the phosphate of soda. Its specific gravity is about 1.030, while that of the whole blood is about 1.060. The bulk of the plasma is about equal to that of the red and the white cells. The formed elements of the blood are: (i) Red blood cefls (red blood corpuscles, erythrocytes); (2) white blood cefls (color- less corpuscles — leucocytes); (3) blood platelets (thrombocytes); (4) blood dust (haematokoniaV I. Red blood cells (erythrocytes) (Fig. 53, non-nucleated circular discs. ^ Their average diameter is about 7.5,", their thickness 2,« at the thin centre.- A few red blood cells '~>f a diameter of 8/« to8.5/< (macrocytes) and about the same number ;f red cells only about one-half the usual diameter (microcytes) ^ Some observers describe the red blood cell as bell- or cup-shaped. (Lewis: Jour. -led. Research, N. S., vol. v, 1904; Radasch: Anat. Anz. xxviii, 1906. Weidenreich: .>gebn. d. Anat., 1903, 1904, 1909; Arch. f. mikr. Anat., Ixi, 1903, Ixix, 1906.) - As this is a quite uniform average diameter and as red blood cells are to be seen in Imost every microscopic field, the diameter of the red blood cells is frequently used as unit of microscopic measurement. 107 ;« Fig. 53. — Cells from Human Blood. X 600. (Technic 2, p. 114.) I, Red blood cell seen on flat; 2, red blood cell seen on edge; 3, red blood cells forming rouleaux; 4, 4, small and large lymphocytes; 5, mononuclear leucocj'te; 6, transitional leucocyte; 7, poh'morphonuclear leucocyte, containing neutrophile gran- ules; 8, poly nuclear leucocyte, containing eosinophile gran- ules; 9, mononuclear leuco- cyte, containing basophile granules. I, 2, 3) are in man 108 THE TISSUES are usually present. Red blood cells are biconcave, with rounded edges. Seen on the flat, the difference in thickness between centre and pe- riphery is evidenced by the difference in refraction (Fig. 53, i). Seen on edge, the shape resembles that of a dumb-bell (Fig. 53, 2). Singly or in small numbers, red blood cells have a pale straw color, due to the presence of haemoglobin. Redness is apparent only when the cells are seen in large numbers. If fresh blood be allowed to stand for a moment, the red cells are seen to adhere to one another by their flat surfaces, forming row^s or rouleaux (Fig. 53, 3). Subjected to the usual technic, the red blood cell appears homo- geneous. By the use of special methods, this apparently homogene- ous substance can be separated into (a) a color-bearing proteid — hcEmoglohin, and {h) a stroma, the latter representing the protoplasm of the cell. Peripherally the stroma probably forms an extremely delicate cell membrane, although the presence of any membrane w^hatever is denied by some. It is the haemoglobin which gives color to the corpuscles. Haemoglobin is a complex proteid, which can be resolved into a globulin and a pigment, h(B?natin. It is held in solu- tion or in suspension in the stroma. Red blood cells are soft and elastic, and are easily twisted to accommodate themselves to the smallest capillaries. This elasticity results in their assuming many shapes, the most common next to their disc shape being that of a cup or cap. The red blood cell is extremely susceptible to changes in the plasma. Thus even shght 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 eft'ect, resulting in swelling of the cell. It also causes solution of the haemoglobin, w^hich leaves the cell, the latter then appearing colorless, with a faint circular outhne — the membrane of the cell. This separation of the haemoglobin from the corpuscle is also caused by freezing and thawing, by heat (6o°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 hcemolysis-, and may also be cfl'ected by mixing the blood of one species with that of another. Dilute aceti9 acid causes swelling and / THE BLOOD TOO 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 millimeter of blood. 2. White blood cells (leucocytes) (Fig. 53, 4 to 9 inclusive) are colorless nucleated structures which have a generally si)herical 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 lo/i, and are much less numerous than the red cells, the proportion being about one white cell to live hundred red cells, or about 10,000 to the cubic millimeter. This proportion is, however, subject to wide variation. Leucocytes may be classified as follows: (a) Lymphocytes; (b) mononuclear leucocytes; (c) transitional leucocytes; (d) polymor- phonuclear or polynuclear leucocytes. (a) Lymphocytes (Fig. 53, 4). — These vary in diameter from 5 to 8/x, and are sometimes subdivided into small lymphocytes and large lymphocytes. The nucleus is spherical, stains deeply, and al- most 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. (6) Mononuclear leucocytes (Fig. 53, 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. 53, 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 transitional stage between the mononuclear and the polymorphonuclear and polynuclear varieties. (d) Polymorphonuclear and polynuclear leucocytes (Fig. 53, 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 difficulty. The no THE TISSUES polynuclear form is derived from the polymorphonuclear by breaking down of the connecting cords, leaving several separate nuclei or nuclear segments. Granules in small numbers may be present in the protoplasm of any of the leucocytes, but the protoplasm of about 70 per cent, of all leucocytes is so distinctly and regularly granular that by some authors a primary division into granular leucocytes and non-granular leuco- cytes is made. Under this classification, lymphocytes and some mononuclear leucocytes are placed in the non-granular group, while transitional leucocytes, polymorphonuclear leucocytes and some mononuclear leucocytes, are placed in the granular group. Aniline dyes may be di\dded into acid, basic and neutral, according to whether the coloring matter is an acid, a base, or a combination of an acid and a base, and the granules of the granular leucocytes react in a definite manner to these dyes, thus allowing the following classificjttion : [ I neutrophile. Granular leucocytes] 2 acidophile (eosinophile). [ 3 basophile. As the neutrophile granules are fine and the eosinophile granules coarse, a classification of leucocytes into finely granular and coarsely granular is sometimes made. 1. Neutrophile Leucocytes. — These are the most numerous of all leucocytes, making up about 68 per cent. Their protoplasm is thickly studded with very fine granules which stain violet with a mixture of eosin (acid) and toluidin blue (basic). Most neutrophiles are polymorphonuclear, a few are transitional. They have a wide distribution, being found not only in the blood itself, but in the spleen and lymph nodes and as wandering cells in various tissues and organs. 2. Acidophile Leucocytes or, because the most common acid dye used is eosin, eosinophile. The granules in these cells are coarse and sharply defined. They stain strongly with acid dyes. Eosino- philes are mainly polymorphonuclear, more rarely they are transi- tional. They make up from i per cent, to 4 per cent, of all leuco- cytes. In certain pathological conditions their number is greatly increased. 3. Basophile Leucocytes.— ThQ granules in these cells are rather coarse and irregular in shape and are distributed unevenly through THE HLOUD 111 the cytoplasm. They stain strongly with basic dyes. They are present in small numbers (Ehrlich, 0.2 per cent, to 0.5 per cent.) in normal blood, or they may not be demonstrable. Ehrlich identifies them with the "mast cells" which are found in various tissues and organs, especiallx' in areolar connective tissue, but this identity has been questioned. Upon the basis of the foregoing description the following classification of leucocytes on the basis of granulation may be made. Leucocytes Xon-granular Granular Lymphocytes 22-25 per cent. Mononuclear leucocytes 1-4 per cent. Neutrophile 65-72 per cent, (mainly polymorphonuclear, few transitional and mononuclear) Acidophile 1-4 per cent, (mainly polymorphonuclear, few transi- tional and mononuclear) ^ Basophile 0. 2-0.5 per cent, (mononuclear and transitional) The function of the red cells is primarily the carrying of oxygen from the lungs to the tissues and of carbonic acid from the tissues to the lungs. This oxygen-carrying ability is dependent upon the haemoglobin and is directly proportionate to the number of red cells and to the richness of the individual cells in haemoglobin. In the capillaries of the tissues and again in the capillaries of the lung the haemoglobin is undergoing constant change. The haemoglobin of arterial blood is known as oxy haemoglobin, of venous blood as re- duced haemoglobin. The difference is readily demonstrable by the spectroscope.^ Amcehoid Movement. — That leucocytes possess in a marked degree the power of amoeboid movement has been noted (p. 52). On ac- count of this motility leucocytes are able (i) to leave the blood-vessels (diapedesis) and move about freely in the tissues (wandering cells), (2) to surround and take up substances from without (phagocytosis). (See also p. 52.) Diapedesis. — This power is directly dependent upon motility and while possessed by all leucocytes is most markedly character- istic of the polymorphonuclear forms. Phagocytosis. — Phagocytic powers are not possessed equally by all leucocytes, but are confined largely to the mononuclear ^Of interest in this connection is the fact that in poisoning by illuminating gas, a very definite and stable combination of the carbon dioxid with the haemoglobin is formed (carboxyhaemoglobin). It is this substance which determines the darker red color of the blood in this condition and is apparently the cause of death. 1 1 2 THE TISSUES and polymorphonuclear forms. Such cells can take up foreign sub- stances, bacteria., degeneration products, etc., carry them to other parts of the body or entirely outside the body (salivary corpuscles), or apparently absorb or digest them. (See also p. 52.)^ 3. Blood platelets (thrombocytes) are minute round or oval bodies from 2« to 4/« in diameter. They are colorless and vary in shape. In fixed preparations they often appear stellate. Their num- ber has been variously estimated. The average is probably about 200,000 to 300,000 per cubic millimeter of blood. In the blood stream they are separate, but show a marked tendency to aggluti- nation directly the blood is drawn. Some comparatively recent observations tend toward considering the platelets as true cells. Thus they have been described as amoeboid, as having the same chemical composition as cells, and as containing either granules of chromatin or distinct nuclear structures. The so-called throm- bocytes of Ovipara are larger than those of man and are unques- tionably nucleated cells. Their appearance is, however, wholly un- like human thrombocytes and the identity of the two forms is doubtful. Various functions have been ascribed to the thrombocytes. Under certain conditions — exposure to air passing over a roughened vessel wall — the fibrinogen is precipitated as fine crystals of fibrin. Thrombin, or prothrombin, probably a blood platelet derivative, is apparently the active element in determining the precipitation of the fibrinogen. The formed elements of the blood are carried down in the precipitation and there is thus formed a solid mass, the blood clot or thrombus, leaving a clear defibrinated fluid almost free from cells, the blood serum. The time required for the clotting of human blood outside the body is usually from two to eight minutes ac- cording to conditions. In certain individuals otherwise apparently normal, also in certain diseases, the blood clots more slowly, a matter of some surgical importance. 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 ^It is to be noted that phagocytosis is not confined to leucocytes but has been ob- served in other cells, e.g., fixed connective-tissue cells including endothelium. It is also to be noted that phagocytic cells are not equally phagocytic to all substances. In other words phagocytes apparently have some powers of selection. Thus if two kinds of bacteria be presented to them, they may take up only one kind, or one kind much more readily than the other. TTTF. BT.nOD 1 13 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 blood islands. It is from these "islands" that both blood and blood-vessels develop. The peripheral cells arrange themselves as the primitive vessel walls, within which the central cells soon become free as the first blood corpuscles. In this way vascular channels are formed, inside of which are developing blood cells. This division of the mesoblastic cells of the blood islands into (i) endothe- lial cells of the vessel walls and (2) progenitors of the blood cells or primitive blood cells, is quite generally accepted. From this point, however, opinions diverge, the two main theories of blood-cell formation being known as the poly- phyletic and the monophyletic theories. According to the polyphyletic theory, after the original division of the meso- blast cells of the blood islands into vessel wall cells (endothelium) and blood cells (primary blood cells), the latter go on to the development of erythroblasts and these to the development of erythrocytes. Leucocytes develop later in con- nective tissue, in the liver, spleen and bone marrow, while lymphocytes have their origin in the germ centers of the lymphoid organs. There are thus three separate sources of blood cells: (i) Red blood cells originally from erythroblasts of the blood islands; in adult life from the erythroblasts of the bone marrow and possibly of spleen; (2) leucocytes, first in connective tissue, liver, spleen and mar- row; in adult Hfe in the marrow; (3) lymphocytes, in the lymphoid organs. Ac- cording to the polyphyletic theory these three types are genetically independent and remain so throughout life, the cells of each type undergoing mitotic division to produce new cells of the same type only. According to the monophyletic theory the blood-cell-forming elements of the blood islands fall into two groups as regards their future development: (i) cells which give rise to primary erythroblasts, these to secondary erythroblasts and these again to erythrocytes, thus completing the line of red blood cells, and (2) cells which remain undifferentiated, not only in the embryo but throughout life, and retain the capabiUty of differentiating into erythroblasts or into white blood cells, either leucocytes or lymphocytes. According to Weidenreich^ and other supporters of this theory, red and white blood cells stand in very close genetic relation. He claims that an undifferentiated mother cell exists which is capable of differentiating either in the direction of the red cell or of the white cell; that this mother cell exists not only in the embryo but throughout life and is con- stantly giving birth to cells which are to develop into red blood cells or white blood cells to replace those being constantly used up and cast oft". Morpho- logically the mother cell is apparently identical with the lymphocyte. In post- embryonic life, this mother ceU is found in marrow in the germ centers of lymphoid organs and possibly in connective tissue. On the other hand a once differentiated cell remains a differentiated cell, and while able to divide mitotically and thus ^ Ant. Rec, vol. iv., Sept., 1910, s 114 THE TISSUES give rise to cells of its own kind, it is never capable of producing ceUs of any other kind. 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. According to one the nucleus is absorbed within the cell and gradually secreted; 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 considering of the liver as a blood-forming organ. The liver cells themselves, however, take no actual part in the formation of blood cells, the blind pouch-like venous capillaries of the liver, with their slow-moving blood currents, merely furnishing a peculiarly suitable place for cellular proliferation. Before birth the splenic pulp and bone marrow become blood-forming organs. In the adult the bone marrow is prob- ably, under normal conditions, the main if not the sole seat of red-cell formation. During foetal 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 origin of the blood platelets is not known. They have been described as originating in the extruded nuclei of the red cells, as disintegrating leucocyte products, as red cells in process of development, as red cells in process of disin- tegration, as albuminoid precipitates, as a specific blood cell. According to Wright,^ they are derived from the megakaryocytes of bone marrow and other blood-forming organs. 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 pre- vented 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 draw 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 11, p. 32. By this method the acidophile granules are stained red, basophile 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 paits alcohol and ether and staining first in a strong alcoholic solution of eosin, then in a rather weak aqueous solution of methylene blue. ^ Jour. Morpli., xxi, igio. CHAPTER IX 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. IMoreover, in muscle this contractiUty is along definite Fig. 54.— Isolated Smooth Muscle Cells from Human Small Intestine. X4oo- (Technic i, p. i20-) Rod-shaped nucleus surrounded by area of finely granular pro- toplasm; 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. Involuntary Smooth Muscle. — This is the simplest form of muscle tissue and consists of long spindle-shaped cells (Fig. a b '^^^^^ A B Fig. 55. — .\pparent Intercellular Bridges of Smooth Muscle. A, From longitudinal section of intestine of guinea-pig ; B, from transverse section of intestine of rabbit. X 420. a, Nerve cell; b, end of muscle cell. (Stohr.) 54) which are prismatic on transverse section (Fig. 55). The length of the smooth muscle cell varies usually from 30 to 2ooijl, its width from 3 to 8m, ^ except in the pregnant uterus where the cells fre- quently attain a much greater size. Less commonly the smooth ^ In the walls of very small blood-vessels smooth muscle cells 15 to 20yu are found. (Apathy). 115 llf) THE TISSUES \'¥m^ muscle cell is flattened, and, in tissues rich in elastic fibres, e.g., the media of some arteries, especially the aorta, is quite irregular in shape. The central thickest part of each cell contains an elliptical or long rod-shaped nucleus. The nucleus has a rather coarse intra- nuclear network and one or more nucleoli. | In some cells a centrosome has been demonstrated. It lies out- side and usually just to one side of the nucleus. )j The irregular, wavy and twisted nuclei often seen are probably due to contractions of the cytoplasm. The nucleus is sur- rounded by an area of finely granu- lar cytoplasm, most abundant at the poles of the nucleus where it frequently forms a little pointed cap. The rest of the cytoplasm shows delicate longitudinal striations, which probably represent a longitudinal arrangement of the spongioplasm. These fibrils are extremely fine, are frequently present in small numbers, are not arranged in bundles, are ap- parently homogeneous > and anisotro- pic, and are often very difficult of demonstration. \jThe fibrils lie in a less differentiated cytoplasm. The smooth muscle cell has no such dis- tinct envelop as the sarcolemma of striated muscle. The outer cyto- plasm is, however, modified to form a delicate cell membrane or at least a modified surface layer. The cells are united by a small amount of intercellular "cement" substance '/which reacts to silver i' nitrate, f Intercellular "bridges" similar to those connecting epithelial cells have been described (Fig. 55), but are regarded by many observers as artefacts. By others fine Fig. 56. — Preparation of Smooth Muscle Cells to show Fibrillar Struc- ture. From intestine of Triton. X 2300. (Heidenhain.) MUSCLE TISSUE ii: intercellular fibrils are described as continuous from cell to cell, thus forming a syncytium. Between and surrounding the smooth muscle cells is a network of delicate connective-tissue fibrils' (Fig. 57). ^^"^■> T^in r-^i-. '^^*»i?>, n- Fig. 57. — Longitudinal Section of the Musculature of Cat's Intestine to show Intercellular Connective Tissue. (Bohemann.) These are not apparent in sections which have been subjected to the usual technic, but can be demonstrated either by digesting the smooth muscle cells by means of trypsin, thus bringing out the undigestible collagenous fibrils, or by special staining methods. {Xr-'<1VJAA' V'^\i^\ hill ir-- Kinr7^r-v^--':"^iH^ j^ IViL. yfJL:.../ ' ■ Fig. 58. — Elastic Fibres in the Smooth Muscle of Intestine of Cat. (Holmgren.) if 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 (Figs. 58 and 59). In such masses 118 THE TISSUE 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 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 and in the capsules and trabeculce of some glands. -' b Fig. 59. — Smooth ]\Iuscle 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. 129.) a, Transversely cut cells of inner circular layer; in comparatively few has the plane of section passed through the nucleus; d, longitudinally cut cells of outer longitudinal layer. In many of the cells the plane of section has not passed through the nucleus; c, intermuscular septum (connective tissue); d, small artery. Voluntary Striated Muscle. — This consists of cylindrical fibres from 50 to 130 mm. in length and from 10 to ioo,« in diameter. Generally speaking, the fibres of greater diameter are found in the larger animals. There is apparently no relation between the size of the muscle and either the length or diameter of its component fibres. There is also no relation of length to diameter, within the same muscle, fibres of many different sizes intermingling. 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, very elastic membrane, which adheres so closely to the underlying muscle sub- stance as to be indistinguishable in most preparations. i| In teased specimens it may frequently be seen at the torn ends of the fibres (Fig. 60). 1/ In staining reaction it differs from both white and elastic tissue and is apparently to be considered a distinct cell membrane. The muscle substance consists of ergastoplasm (fibrillce) and MUSCLE TISSUE 119 sar CO plasm (interfibrillar substance) and shows (Fig. 6i), longitudinal striations and cross striations. 'V\\v longitudinal striations are due to i^arallel running ulliniatc t'lbrillic (Fig. Oi, a), which lie in and are more or less sep- arated from one another by the sarcoplasm. Each iibrilla when examined, unstained, by reflected light is seen to be comj)osed of alter- nating light and dark segments. As like segments lie in the same transverse plane, the whole muscle fibre appears composed of alternate light and dark bands (Figs. 6i and 62), and this distinction is maintained even in stained specimens, as the light bands stain little if any, with most staining agents, while the dark bands react strongly to stains. The light band is composed of a singly re- fracting (isotropic) substance, the dark band of a doubly refracting (anisotropic) sub- stance. Through the middle of the light band runs a tine line {Krauses line, Fig. 61, r, 62, r), while an even finer line (Hensen's line, Fig. 61, (7, 62, a) can sometimes be seen run- ning through the middle lighter portion of the dark band. Both Krause's and Hensen's lines cross the intervening sarcoplasm as well as the fibrillae and extend to the sarcolemma, thus completely crossing the fibre. As all of these structures run through the entire thick- ness of the fibre, they in reality constitute discs of muscle substance (Fig. 62). By means of certain chemicals these discs may be separated, the separation taking place along the lines of Krause. Each "muscle disc" thus consists of that portion of a fibre in- cluded 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 ull Fig. 6 1 , a) , two sets of striations m Fig. 60. — Semidiagram- matic Drawing of Parts of two Muscle Fibres which have been broken, showing the relations between Mus- cle Substance Proper and Sarcolemma (Ranvier.) m, a, Retracted ends of muscle substance, between which is seen the sarcolem- ma with several adherent muscle nuclei; B, thin layer of muscle substance which has adhered to the sarco- lemma; n, muscle nucleus; s, sarcolemma; p, space between sarcolemma and muscle substance. transversely into mus- 120 THE TISSUES cle discs (Fig. 62, /). What is known as the sarcous element of Bow- man is that portion of a single fibrilla which is included in a single disc, i.e.. between two adjacent lines of Krause (Fig. 62, b). The sarcoplasm is not evenly distributed throughout the fibre. On cross section irregular trabeculae of sarcoplasm are seen extend- ing in from the sarcolemma (Fig. 63). These separate the fibrillas into bundles, the muscle columns of KolUkcr. A transverse section Fu, 01. Fig. 62. X350- (Technic 4, Fig. 61. — Portion of Striated Voluntary Muscle Fibre. X350. VJ-Ccnnic 4, p. 129.) The fibre is seen to be marked transversely by alternate light and dark bands. Through the centre of the light band is a deUcate dark line (Krause's line); through the centre of the dark band a fine fight line indicates Hensen's line. The black line outlining the fibre represents the sarcolemma. a, Fibrillse; b, muscle nucleus; c, Krause's line; d, Hensen's line. Fig. 62. — Diagram of Structure of a Muscle Column of KoUiker. The appearance presented by the cross-cut muscle column = Cohnheim's field, a, Muscle fibrillae; b, sarcous element; c, Krause's fine; d, Hensen's fine; e, Cohnheim's field;/, muscle disc. of one of these columns presents the appearance of a- network of sarcoplasm and of interfibrillar cement substance enclosing the fibrillae. This appearance is known as CoJniheim^s field (Figs. 62 and 63).. Many of the details of structure of striated muscle have been determined by studies upon the muscles of lower animals. These details are extremely com- plicated and the numerous terms used to designate the same structure very con- MUSCLE TISSUE 121 fusing. The following is the scheme of structure and the nomenclature accord- ing to Heidenhain (Fig. 64). The older terms muscle cell for the smooth muscle cell, and muscle fibre for the analogous multinuclcar element of striated muscle are retained, also Apathy's myofibril for the ultimate fibrilla, Kolliker's muscle column for the smallest bun- dles of fibrils, and sarcoplasm for interfibrillar substance. The muscle fibre is subdivided into a series of segments by transverse discs which completely cross the fibre, involving both fibrilla; and sarcoplasm and are attached to the sarcolemma. This disc (ground membrane — membrane of Krause,) Heidenhain designates the telophragma (Z). That part of a fibre which hes between two telophragmata is an inokomma. The middle of each inokom- ma is crossed by a disc (Hensen's membrane) which also involves both fibrillae Fig. 6,3. — Semidiagrammatic Drawing of Transv^erse Section of a Voluntary Muscle Fibre, showing Sarcolemma; sarcoplasm separating fibrils into bundles, each bundle constituting a muscle column of Kolliker and the appearance of its cross-cut end being Cohnheim's field, a, Sarcoplasm; b, Cohnheim's fields; c, sarcolemma. and sarcoplasm and is attached to the sarcolemma. This membrane having apparently the same structure as the telophragma is designated the mesophragma (M). The mid-portion of the inokomma consists of anisotropic substance (Q), the ends of isotropic (J). That portion of Q which lies on either side of M is lighter than the rest of Q and is designated Qh (h = hell = clear) . That portion of J which lies close on either side of Z is darker than the rest of J and is designated Jd (d = dicht = thick). In the sarcoplasm between the fibrillae are granules; those in J are arranged in a regular row and are known as J-granules, those in Q are more irregularly placed and are known as Q-granules. In Fig. 64 are also shown the "cross-fibre-nets" which are brought out by metallic impregnation and which possibly represent intracellular canals. I Two varieties of striated voluntary muscle fibres are distinguished, white fibres and red fibres. The difference betv^een 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 122 THE TISSUES and mastication. In some of the lower animals are found muscles made up wholly of red fibres. INIuscle fibres ending within the substance of a muscle have pointed extremities. Where muscle fibres join tendon, the fibre ends f z — ^s ]i Jd ■H""iiBI» i"!4" fHiii" # 9# # • I Qh } Qh I P 1 i|" " I H H g g g g Telophragma J-granules Q -granules Mesophragma Q-granules J-granules J-granules Cross-fibre nets dililK Cross-fibre nest L 7, — iittiiiiMJ wiife^itiW^'^ Fig. 64. — Scheme of Structure of Striated Voluntary Muscle Fibre with Nomenclature of Heidenhain. (llcidcnhain.) in a rounded or blunt extremity, the sarcolemma being continuous with the tendon fibres (Figs. 65 and 66). Muscle fibres are usually unbranched. In some muscles — e.g., those of the tongue and of the eye — anastomosing branches occur. MUSCLE TISSUE 123 5 • >-v/ — "- 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 hundreds or even thou- sands of nuclei.^ In the white fibres the nuclei are situated at the periphery just beneath the sarcolemma. In red fibres they are centrally placed. Involuntary Striated Muscle (Heart Mus- cle).— -This, as its name implies, is a striated muscle not under control of the will. It occurs only in the cardiac musculature. Like voluntary muscle, heart muscle is striated both longitudinally and transversely. Like smooth muscle, on the other hand, it is com- posed of units which at least resemble cells and have long been called heart muscle cells. Heart muscle also resembles smooth muscle in that there is usually but one nucleus to a cell and this nucleus is centrally placed instead of lying at the periphery as in voluntary muscle. Also the sarcolemma if present at all is extremely delicate and is probably, as in smooth muscle, only a modification of the surface sarcoplasm. The amount of sarcoplasm throughout the cell is large. Around the nucleus is an area of sar- coplasm free from fibrillae. This area often ex- tends some distance toward the ends of the cell. Heart muscle fibres are not the parallel- running unbranched fibres of voluntary mus- cle, but, while having a generally parallel arrangement, give off side branches which anastomose, making it impossible to trace a single fibre for any great distance. These side branches have the same structure as the main fibre except that they are for the most part of smaller diameter and are nonnucleated. ^ Stohr calls attention to the fact that such a structure may be considered either a multinuclear cell or a syncytium and that there is really no true distinction between the two. Fig. 65. — Semidia- grammatic Illustration of Endings of Muscle Fibres within a Muscle and in Tendon. (Gage.) a, Tapering end of fibre terminating within the muscle; the lower end of the central fibre shows the same method of termination; c, c, each fibre termi- nates above in pointed intramuscular ending, below in blunt ending connected with tendon. 124 THE TISSUES The striations of heart muscle are less distinct than are those of voluntary muscle, but apparently represent very similar structures. The longitudinal striations indicate fihrillcB lying in the sarcoplasm. From the central mass of sarcoplasm which surrounds the nucleus, strands radiate toward the periphery. These strands, anastomosing, separate the fibrillas into columns, the muscle columns of Kolliker. In cross section these present the appearance described under volun- tary muscle as Cohnlieim^s fields. The disposition of the sarcoplasm, ■ ' 7 f?-" t STi t Fig. 66. Fig. 67. Fig. 66. — Two Muscle Fibres from Upper End of Human Sartorius, to show con- nection of muscle and tendon. X350. (Gage.) m, Muscle fibres; /, tendon fibres. Fig. 67. — Muscle Cells from the Human Heart (technic 6, p. 129), showing 4ateral branches and lines of union between cells. X500. extending outward from the region of the nucleus like the spokes of a wheel, gives to the cross section a characteristic radiate appearance (Fig. 68), not seen in cross sections of voluntary muscle. The transverse markings represent, as in voluntary muscle, alternate light and dark discs. Through the middle of the light disc can be seen the membrane of Krause. Mc Galium describes Krause's membrane as crossing not only the librillae. but also the sarco- plasm, as in the voluntary muscle fibre. The sarcoplasm he describes as further subdivided by membranes, which are transversely con- tinuous with Krause's membranes, into minute discs. The centre :\IUSCLE TISSUE 125 of the cell around the nucleus is wholly composed of these little discs of sarcoplasm. //Peculiar to heart muscle are what appear in longitudinal sections to' be dark lines which cross transversely both main fibres and side' branches. These arc known as itilcrcallated discs and dixidr the muscle fibre into irregular, short, thick cylindrical segments (Fig. 69). A disc may pass straight across a fibre, or it may cross it in a series of steps, or it may extend only part way across the fibre. It always touches at some point one of the ground membranes^ and sometimes r' - ■ . ■ ■ ^ -■•■ , , !-l4;n-:fe?'; ^!v>^': ■1 Fig. 68. — Section of Heart Muscle. X350. (Technic 7, p _ 129.) a, Cells cut longitudinally; b, cells cut transversely (only three nuclei have been included in the plane of section); c, cells cut obliquely; d, connective-tissue septum. entirely fills the space between two ground membranes which lie unusually close together. Special technic has demonstrated the fact that the fibrillae do not stop at the disc but are continuous through it, although they show in passage some modification of structure. 1/ The one question which has been most discussed in regard to the structure of heart muscle and which remains still unanswered is whether heart muscle is cellular or a syncytium. The solution of the problem is of course dependent upon the determination of the significance of the intercallated discs — whether or not they represent true cell boundaries. The segments into which the heart muscle fibre is divided by the discs have long been described as cells. The fact that the nuclei are usually placed about midway between two discs; that the discs show the same staining reaction as intercelltilar substance when subjected to the action of silver nitrate, and the ease with which heart muscle may be 1 Some authorities deny this. 126 THE TISSUES separated into "cells," especially in young animals and in lower vertebrates, by use of those chemicals usually used to break down intercellular cement; have aU been used as arguments in favor of a cellular structure. On the other hand, the fact that it has not been proved that the segments bounded by the^discs corre- spond to the original myoblasts; that in the later stages of development of heart muscle the type of nuclear division is usually amitotic, a type frequently unac- companied by division of the cytoplasm; that the fibrillae pass uninterrupted through the discs; that some discs only partially cross a fibre; that some segments contain more than one nucleus while others are non-nucleated; all favor a syncytial interpretation. How open the question still remains is shown by the fact that Heidcnhain regards the "cells" as "growth segqients"; that Marceau considers them non-contractile "supports" for the fibrillae; that Jordan from M * .r'-i'ft "^ v'l i . f--^JSjii» Fig. 69. — Longitudinal Section of Heart Muscle of Rabbit (Werner) showing cross and longitudinal striations and intercallated discs. In several "areas" two nuclei are seen. Fig. 70. — Longitudinal Sec- tion of Heart Muscle (Przen- oski), showing continuity of fi- brils through intercallated disc. his work on humming-birds concludes that they are a coarser development of the anisotropic bands; while Schaffer looks upon them as post-mortem contrac- tion artefacts. Development of Muscle Tissue Smooth Muscle. — In the higher animals, muscle tissue with the exception of that connected with the sweat, lacrymal and mammary glands, which is of ectodermic origin, is derived wholly from mesoderm. The cells (myoblasts) which are to become smooth muscle cells develop in the general mesenchymal tissue among cells which are to become connective-tissue cells and with which they are at first apparently identical. In becoming a smooth muscle cell the myoblast chariges its shape, becoming greatly elongated, its nucleus at the same time becoming oval or rod shaped. Such cells anastomose freely. During these MUSCLE TISSUE 127 changes in shape, delicate longitudinal fibrils appear in the cytoplasm. Just how these fibrils originate is not known, but they probably represent a speciali- zation and rearrangement of the spongioplasm. t:.:rr>i^> ii?S Fig. 71. — Myoblasts in Different Stages of Development. (Godlewski.) The upper cell represents a myoblast with granular cytoplasm (from sheep embryo of 13 mm.); the middle, a myoblast with fibrils in process of formation (from guinea-pig embryo of 10 mm.); the lower, a myoblast with still further differentiated, segmented fibrils (from a rabbit embryo of 8.5 mm-). Striated Voluntary Muscle. — This develops from the mesoblastic somites. Each somite early divides into an outer part, sclerotome or cutis plate, and an inner part, the myotome or muscle plate. Cells of the myotome soon show changes Fig. 72. — From a Section of Developing Heart Muscle from a Rabbit Embryo of 9 mm. (Godlewski.) a, Cell body with granules arranged in series; b, cell body with centrosome and attrac- tion sphere; c, branching fibril; d, fibrils extending through several cells. which distinguish them as myoblasts or muscle-forming cells. These cells, which are at first spherical, become elongated and spindle shaped. The nucleus is at this stage centrally placed and the spongioplasm is in the form of a reticulum. 128 THE TISSUES Fibrillar arrangement of the spongioplasm first appears around the periphery where granules form in the cytoplasm and become arranged in rows lengthwise of the cell. The central portion of the cell is at this stage still occupied by retic- ular spongioplasm and the nucleus. These granules next unite to form deli- cate fibrils. New fibrils form both by longitudinal arrangement of more granules and by longitudinal splitting of fibrils already formed, until they finally fill the entire cell. After the union of the granules to form fibrils the latter are apparently homogeneous, but later differentiate into the alternate light and dark substances which determine the cross striations characteristic of striated muscle. Just how this differentiation takes place is not known. During this process of fibrillation the nucleus has been undergoing mitotic division without corresponding division of the cytoplasm. In white fibres these nuclei migrate to the surface and come to lie just beneath the sarcolemma. The sarcoplasm in which the fibrils lie probably represents the remains of the undifferentiated pro- toplasm (hyaloplasm). i«iS=5^t^'**l-'-,'>"'':v-=-==?^^-i=1?^^§^?^^ Fig. 73. — From a Section of Developing Heart ^luscle in a Rabbit Embrj-o of 10 mm. (Godlewski.) The fibrils are segmented, indicating the beginning of the cros5 striation characteris- tic of heart muscle. Some authorities deny the origin of the muscle fibre from a single cell, describ- ing it as derived from a number of myoblasts which unite to form a fibre. Heart muscle develops from mesenchyme. The myoblasts are at first small, spheroidal, and closely arranged. With the appearance of intercellular substance the cells become separated and irregular in shape, and anastomose to form a syn- cytium. A little later the cells become arranged in parallel columns and cross markings, the " intercallated discs" appear, dividing the syncytium into the so-called "heart-muscle cells." Whether these coincide with the original myo- blasts is not known. As in voluntary muscle, fibrils first develop in the periphery of the cell apparently by union of granules which arrange themselves in lines lengthwise of the cell. The fibrils increase in number and invade the entire cell except the small area of undifferentiated protoplasm which remains around the nucleus. Later the fibrils show the alternate light and dark markings or cross striations. As with voluntary muscle the way in which these cross markings develop is not known. Attention has already been called (p. 5,0 to the spongioplasm as the con- tractile element of protoplasm. It is to be noted that in the development of MUSCLE TISSUE 129 muscle no new element appears, the contractile fibrillcE representing nothing more than a specialization of the already contractile spongioplasm. TECHNIC (i) Isolated Smooth Muscle Cells. — Place small pieces of the muscular coat of the intestine in o.i-per-ccnt. 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 (p. 19). This is poured off, the tissue again washed in water and preserved in eosin-glyc- erin, 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 intes- tine 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 (p. 19). 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 6, p. 7) or in Zenker's fluid (technic 10, p. 8). Thin transverse or longitudinal sections are stained with haematoxylin-eosin (technic i, p. 20), 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 Voluntary Muscle Fibres.— One of the long muscles removed from 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 formalin-Miilier's fluid or in Zenker's fluid (p. 8). Thin sections are stained with haematoxylin-picro-acid-fuchsin (technics, p. 21) and mounted in balsam. As the muscle fibres of the tongue run in all directions, fibres cut transversely, longitudinally, and obUquely may be studied in the same section. The sarco- lemma, the pointed endings of the fibres, and the relation of the fibres to the con- nective tissue can also be seen. (6) Isolated heart-muscle cells may be obtained in the same manner as smooth muscle cells. (See technic i, above.) (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 haematoxyhn-eosin (technic i, p. 20), or haematoxylin-picro-acid-fuchsin (technic 3, p. 21). CHAPTER X 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-cyUnder 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 off from this cell body two kinds of processes, the axis-cylinder process and the dendritic processes (Fig. 74). An important exception to this rule is presented by the cells of the cerebro- spinal ganglia (Fig. 300). Here the typical neurone has two processes, a periph- eral process, and a central process which enters the central nervous system. Both processes have a typical axis-cylinder structure. Some authorities have interpreted the peripheral process as a modified dendrite. I. The Cell Body.- — ^Like most other cells, the nerve cell body consists of a mass of protoplasm surrounding a nucleus (Fig, 75). 130 NERVE TISSUE 131 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 ventral horns of the cord, which are among the largest in the body. There is as much variation in shape 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 MA Fig. 74. Fig. 75. Fig. 74.^ — Scheme of Peripheral 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 iieiirone. c. Cytoplasm of cell' body containing chromophilic 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 (probably not present in central nervous system); m' , striated muscle fibre; tel, motor end plate. Fig. 75. — 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. of the spinal ganglia are spheroidal; of most of the cells of the cortex cerebri, pyramidal; of the cells of Purkinje, pyriform; of the cells of the ventral horns of the cord, irregularly stellate. According to the number of processes given ofT, nerve cells are often referred to as unipolar, bipolar, or multipolar. The NUCLEUS of the nerve cell (Fig. 75) differs in no essential 132 THE TISSUES 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 at least two distinct elements: (i) Neurofibrils, and (2) perifibrillar substance. In most nerve cells a third element is present, (3) chromophilic bodies. P *'= ^-^ii; u'# Fig. 76. — Ganglion Cells, Stained by Bethe's Method, showng 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.) In B the chromophilic bodies are shown. In this picture the netirofibrils are shown as not anastomosing. (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. 76 and 77). (2) The perifibrillar substance (Fig. 76) is a fluid or semi-fluid substance which both in the cell body and in the processes surrounds NERVE TISSUE 133 and separates the neurolibrils. It is believed by some to be like the fibrils, continuous throughout the cell body and processes, by others to be interrupted at certain points in the axone (see p. 142). (3) The chromophilic bodies (Fig. 75) are granules or groups of 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 39). Fig. 77. — Body of Large Pyramidal Cell from Cortex of Cat. Silver Method of Cajal. Shows nucleus pale and arrjingement of neurofibrils within the cell; a, axone; h, main or apical dendrite. (Cajal). In this picture the neurofibrils are shown as anasttmioslng. When subjected to this technic, nerve cells present two very dif- ferent types of reaction. In certain small cells the amount of cytoplasm is extremely small and only the nuclei stain with the Nissl method. Such cells aire found in the granule layers of the cerebellum, olfactory lobe, and retina. They are known as caryo- chrome cells, and apparently consist wholly of neurofibrils and peri- fibrillar substance. Other cells react, both as to their nuclei and as to their cell bodies, to the Nissl stain. These cells are known as somatochrome cells. Taking as an example of this latter type of cell one of the motor cells of the ventral horn of the cord and subjecting 134 THE TISSUES it to the Nissl technic, we note that the cytoplasm is composed of two distinct elements: (a) a clear, unstained ground substance, and, scattered through this, (b) deep-blue-staining masses, the chromo- philic bodies (Fig. 75), These bodies are granular in character and differ in shape, size, and arrangement. They may be large or small, regular 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 particular type of cell remains constant, and has thus been used by Nissl as a basis of classification.^ 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 chromophiHc 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 structureless, and it is this part of the cytoplasm that corresponds to the neuro- fibrils and a part at least of the perifibril- lar substance. Fig. 78.— Pyramidal CeU from Human Cerebral Cortex. (Golgi bichlorid method. See 2, p. 36.) Golgi cell type I. fl, Cell body; b, main or apical dendrite showing gemmules; c, lateral dendrites showing gemmules; d, axone with col- laterals. Only part of axone is included in drawing. 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 pre- ^ For this classi&cation, the significance of which is somewhat doubtful, the reader is referred to Barker, "The Nervous System and Its Constituent Neurones," p. 121. NERVE TISSUE 135 sents the same appearance ("equivalent picture"), and that this appearance furnishes a norm for comparison with cells showing pathological changes, and which have been subjected to the same technic. Many nerve cells contain more or less brownish or yellowish pigment (Fig. 75). This pigment, which is a lipochrome, is not present in the cells of the new-born, but appears in increasing amounts with age. Its significance is not known. Another black pigment known as melanin is present in certain cells in the central nervous system {e.g., the substantia nigra). This pigment is said to increase in amount until adolescence. Fig. 79. — 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. In addition to its characteristic structure, the nerve cell may contain many elements found in other cells (p. 46). Golgi, Holm- gren, Cajal, and others have also demonstrated a network of canals within the nerve cell similar to that found in other cells (p. 47, Fig. 4). n. 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. 75 and 76). Dendrites branch dichotomously, become 136 THE TISSUES rapidly smaller, and usually end at no great distance from the cell body (Figs. 78 and 79). m. The Axone. — This differs from the cell body and dendrites in that it contains no chromophiHc bodies (Fig. 75), 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. 75), which is also 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 axones have been described. In Golgi preparations the axone is distinguished by its straighter course, more uniform diameter, and smoother outline (Fig. 78). 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 extends a long distance from the cell body. Such cells are known as Golgi cell type I or long axone neurones (Fig. 78). In others the axone branches and ends in the gray matter in the vicinity of its cell of origin — Golgi cell type II or short axone neurones (Fig. 79). 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 of Schwann. Still others are surrounded by a sheath of considerable thickness known as the medullary or ?nyelin sheath. Depending upon the presence or absence of a medullary sheath, axones may thus be divided into two main groups — ■meduUated axones and non-mednllated axones. I. NoN-MEDULLATED Axones (non-medullated nerve fibres) (Fig. 80). 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 J^ NERVE TISSUE 137 short distance after leaving the nerve cell body, and also just before reaching their terminations. (b) N on-medidlated axones with a neurilemma — fibres of Remak (Fig. 80). In these the axone is surrounded by a delicate homo- geneous, nucleated sheath, the neurilemma or sheath of Schwann (see p. 139). These axones are described by some writers as having r ^"1 ABC Fig. 80. Fig. 81. Fig. 80. — Non-meduUated Nerve Fibres with Neurilemma, only the nuclei of which can be seen. X300. Fig. 81. — 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 myehn. a, Node of Ranvier; b, incisures of Schmidt; c, nucleus of neurilemma. no true neurilemma, but merely a discontinuous covering of flat connective-tissue cells, which wrap around the axone and corre- spond to the endoneurium of the nerve trunk (see page 426). The majority of the axones of the cells of the sympathetic ganglia fall under this category. 2. Medullated Axones (medullated or myelinated nerve fibres). — These, like the non-medullated, are subdivided according to the 138 THE TISSUES presence or absence of a neurilemma into mcdullated axones with a neurilemma and medullated axones without a neurilemma, (a) Medullated axones ivith a neurilemma constitute the bulk of the fibres of the cerebro-spinal nerves. Each fibre consists of (i) an axone or axis-cylinder, (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 neuroplasm (Fig. 86). In the fresh condition the axone is broad. Fig. 82. — I, Fibres of Remak; 2, nerve fibre from central nervous system (medul- lated but without sheath of Schwann); 3 and 4, nerve fibres from the sciatic of frog, showing nodes of Ranvier, and in 4 the incisures of Schmidt. and shows faint longitudinal striations corresponding to the neuro* fibrils, or appears homogeneous (Fig. 81, A). Fixatives usually cause the axone to shrink down to a thin axial thread, w^hence its older name of axis-cylinder (Fig. 81, C). A delicate membrane has been described by some as enveloping the axone. It is known as the axolemma or periaxial sheath (Fig, 84). (2) The medullary sheath (Figs. 82 and 84) is a thick sheath largely composed of a semi-fluid substance resembling fat and known as myelin. In the fresh state the myelin has a glistening homo- geneous appearance. It is not continuous, but is divided at intervals of from 80 to 6coyu by constrictions, the nodes or constrictions of Ranvier. That portion of a fibre included between two nodes is known as an internode (Fig. 84), The length of the internode is XERVE TISSUE 139 usually proportionate to the size of the fibre, the smaller fibres having the shorter internodes. In fresh specimens the medullary sheath of an inter- node appears continuous (Fig. Si, A), but in fixed specimens it is broken up into irregular segments, Schmidt-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. 8i, C). On boiling medullated nerve fibres in alcohol and ether a fine network is brought out in the medullary sheath, the neurokeratin network. Owing to the resistance of neurokeratin to the action of trypsin, it has been considered as possibly similar in composition to horn. (3) The neurilemma or sheatJi of Schwann (Figs. 86, B, and 85) is a delicate structureless membrane which encloses the myelin. At the nodes of Ranvier the neurilemma dips into the constriction and comes in contact with the axone or axolemma. Silver nitrate staining shows a black transverse line in the neurilemma at the node of Ranvier. This line represents the bound- ary between two neurilemma cells. Against the inner surface of the neurilemma, usually about midway between two nodes, is an oval-shaped nucleus, the nucleus of the neurilemma (Figs. 80, C, and 85). Each nucleus is surrounded by an area of granular protoplasm, and makes a little depression in the myelin and a slight bulging of the neurilemma (Fig. 81, C). According to most observers no neurilemma is present in the central nervous system. An important exception is Cajal, who describes the medullated fibre of the central nervous system as having a neurilemma. In addition to the above-described sheaths, most medullated fibres of peripheral nerves have, Fig. 83. — Dia- grammatic Repre- sentation of Portions of Two Medullated Nerve Fibres, as seen in Longitudinal Sec- tion, stained with OsmicAcid. (Length of internode is pro- portionately short- ened.) i?,7?, Nodes of Ranvier, with axis cylinder passing through; a, neurilemma; c, nucleus surrounded by protoplasm, King at about the middle of the internode between the neurilemma and the medullary sheath. (From drawing by J. E. Neale in Quain's Anatomy.) 140 THE TISSUES h \ ii / i^ / outside the neurilemma, a nucleated sheath of connective-tissue origin, known as the sheath of Henle (Fig. 85). n]P||fl| Two views as to the relation of the ./''M axolemma to the neurilemma are illus- ' trated in Fig. 84. According to one the neurilemma is continuous, merely dipping into the nodes of Ranvicr, where it touches the axolemma or the axone. According to the second both neurilemma and axolem- ma are interrupted at the node, but unite with each other there to enclose completely the medullary substance of the internode. According to the views illustrated in Fig. 86, that part of the axone which lies between two nodes is enveloped by a cell or by several cells forming a syncytium. The outer homogeneous membrane there pictured would thus be of the nature of a cell membrane or cuticle, and would cor- respond to the neurilemma. The trabeculae (protoplasmic strands and neurokeratin network, of which some of the larger strands would represent the incisures of Schmidt) would correspond to the spongio- plasm, and just along the outer side of the axone would constitute the axolemma. The myelin would thus be enclosed within the cylindrical neurilemma cell which sur- rounds the axone. Recent experiments of Bethe and others tend to prove an interruption of the peri- fibrillar substance at the node of Ranvier. They consider the axone at the node as probably crossed by a sieve-like plate, through the holes of which the fibrils pass, but which completely interrupts the peri- fibrillar substance. The accuracy of these observations has been disputed. Medullated nerve fibres vary- greatly in size. The finer fibres have a diameter of from 2 to 4/i, those of medium size from 4 to lo/x, the largest from 10 to 20^1. They have few branches, and these are always given off at the nodes of Ranvier- {b) Medullated axones without a neurilemma are the medullated Fig. 84. Fig. 85. Fig. 84. — -Diagram of Structure of a Medullated Nerve Fibre of a Peri- pheral Nerve, showing two different views (one on each side) as to rela- tions of neurilemma and axolemma and their behavior at the nodes of Ranvier. (Szymonowicz.) a, Neuro- fibrils; b, cement substance; c, axone; d, incisure of Schmidt; c, nucleus of neurilemma; /, medullary sheath; g, sheath of Schwann; h, axone; i, axo- lemma; j, sheath of Schwann; k, node of Ranv'ier. Fig. 85. — Piece of Medullated Nerve Fibre from Human Radial Nerve. X400. Osmic-acid fixation and stain. (Szymonowicz.) a, Me- dullary sheath; b, axone; c, sheath of Henle; d, nuclei of Henle's sheath; e, nucleus of neurilemma. NERVE TISSUE 141 B nerve fibres of the central nervous system as described by most observers. Cajal, as already mentioned (p. 139), describes these fibres as having a neurilemma. Their structure is similar to the above-described structure of a medullated nerve fibre with a neuri- lemma, except for the absence of the latter sheath. As to the physiological significance of the structural elements of the neurone, we have little absolute knowledge but certain fairly well- sz grounded theories. That portion of the neurone which surrounds the nucleus — the cell body — is, as already stated. " the genetic centre of the neurone, the nucleus as in other cells being probably concerned in the sp general cell metabolism. From the behavior of the processes when cut off from the cell body it /^, is evident that the latter is the trophic or nutritive centre of the neurone. It is probable that in most or perhaps all p^ neurones the usual direction of conduction along the axone is cellulifugal, i.e., from cell body to gs terminal arborization. The dendrites and cell body would receive and probably integrate the various nerve impulses received by the neurone. In other words, the dendrites and cell body receive, the axone transmits and the axone branches and terminal arborizations distribute. This general direction of conduction indicates a certain polarity of the neurone. WhUe the cerebro-spinal ganglion cell is obviously polar- 0 — sg le - ss Fig. 86. — Scheme of Structure of Medullated Per- ipheral Nerve Fibre of a Fish (Nemileff). A, Cross section; B, longitudinal section; on left, fibre is shown as stained intra vitam with methylene blue; on right, myelin is shown black as in osmic acid staining, with the incisures of Schmidt indicated; sz, cells of sheath of Schwann; n, their nuclei; .J.r, sheath of Schwann; sp, processes of the cells of sheath of Schwann or the myelin sheath network; le, larger trabeculje of proto- plasmic framework of medullary sheath arranged obliquely to a.xis-cylinder and forming the so-called "funnels"; leo, clear streaks in libres treated with osmic acid, corresponding to le, incisures of Schmidt; mo, myelin blackened with osmic acid; ax, axis- cylinder; pa, periaxial space around axis-cyhnder; gs, "coagulum sheath," granules probably representing coagulated fluid in periaxial space; pf, peripheral, non-fibrillar, part of axis-cylinder; /, neurofibrils of axis-cylinder; r, ring-like thickening of Schwann's sheath at node of Ranvier: 0, cavity in r. n sz ax ss sz sp Pi g^ f 142 THE TISSUES ized, yet if its peripheral process be regarded as an axone, the latter would evidently constitute an exception to conduction along the axone being cellu- lifugal. There are cases {e.g., unipolar cerebro-spinal ganglion cell) where the nerv^ous impulse may apparently pass from one process to another without traversing the body of the cell. Regarding the chromophilic substance, certain facts, such for example as the entire absence of chromophilic bodies in many nerve cells, which nevertheless undoubtedly functionate; the absence of these bodies in all axones; the diminu- tion of the chromatic substance during functional activity; its much greater diminution if activity be carried to the point of exhaustion; these together with its behavior under certain pathological conditions, all favor the theory that the stainable substance of Nissl is not the active nerve element of the cell, but is rather of the nature of a nutritive element. There thus remains to be considered as possible factors in the transmission of the nervous impulse the neurofibrils and the perifibrillar substance. While a few investigators are inclined to magnify the importance of the latter, the ma- jority agree in considering the neurofibrils as the principal conducting mechanism of the neurone. The already referred to observations of Bethe regarding the interruption of the perifibrillar substance at the constricted portion of the axone and at the nodes of Ranvier, would, if true, be obviously in favor of this view. It is also obvious that if the neurofibrils are the sole conducting substance and if they do not anastomose within the body of the vertebrate neurone, we would be driven to conclude there must be an extracellidar anastomosis. The proba- bility, however, is rather against both of these premises. 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 the- ories, the contact theory and the continuity theory. 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 prominent neurologists as Apathy, Bethe, Held, and Nissl, on the ground that in some cases the neurofibrils are continuous throughout a series of neurones. The point of contact between two neurones is called a synapsis (Fig. 83), and the conception that there is some kind of interruption or discontinuity in neural circuits involving more than one neurone has proved useful to physiologists. It affords an explanation of certain differences between conduction through a cir- cuit of two or more neurones and conduction through a nerve fibre alone. 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. Based upon the contact theory is the so-called "retraction hypothesis," which held that a neurone being asso- ciated 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. NERVE TISSUE 143 According to the continuity theory, while the perifibrillar substance is inter- rupted as above described, the neurofibrils, are continuous. According to this theory the neurofibrils, which form a plexus or network, within the cell body and dendrites, are connected with a pericellular network — the Golgi net — which closely invests the cell body and its dendrites. Externally the Golgi net is further connected wiih the neurofibrils of the axoncs 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, accord- FiG. 87. — .4, B, C, Three cells of the Ventral Cochlear Nucleus of Rabbit, showing terminals of fibres of the 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; h, c, terminal rings. The points of contact between the terminals of the axone of one neurone and the cell body and dendrites of the other neurone constitute a " synapsis." ing to this theor>', continuous and form two or possibly three continuous net- works: {a) an intracellular network, {b) a pericellular network (Golgi), and (c) a more diffuse extracellular network, lying between the cells. The existence of (c) is extremely doubtful and it seems probable that the Golgi network is either non-nervous or an artefact. Thus the main point at issue is whether the neurofibrils, in such, pericellular terminals as are illustrated in Fig. 87, are con- tinuous 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 144 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. 35) stain. Later, when the degenerated fibres have been largely replaced by connective tissue, the Wcigert method is most satisfactory, especially in the central nervous system. In this case, of course, the degeneration is indicated by an absence of stain, while in the Marchi method a positive picture of the degenerating myelin sheaths is A B C ' Fig. 88. — A, Normal Nerve Fibres from Sciatic Nerve of Rabbit, osmic acid fixa- tion 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 only traces of the axones remain. seen. When the nerve is cut the first changes affect the cut ends and seem to be of a traumatic character such as formation of bulbous enlargements of the ends of the axones, increase and separation of neurofibrils and formation of sprouts. This phase is soon passed over in the distal stump, while in the central it may pass over into a beginning regeneration of the nerve fibres. In the distal portion a degeneration of the nerve fibres throughout their length and including their termi- nal arborization now takes place. All parts of the nerve .fibre are affected. In the medullary sheath the changes consist in segmentation of the sheath, break- ing up of the segments into granules and finally complete absorption (Fig. 88). While undergoing these physical changes chemical changes arc also taking place in the myelin which result in its breaking down into simpler fatty substances which give the fat reaction to the Marchi stain. At the same time the neuro- fibrils become irregular and granular and the axis-cylinders finally disappear. NERVE TISSUE 145 The neurilemma cells of the peripheral nerve fibres are peculiar in that they do not degenerate; instead of this their protoplasm increases, and their nuclei pro- liferate. These cells are apparently concerned in the disintegration and absorp- tion of the myelin. They also form protoplasmic bands which play an important part in the regeneration of the nerve (see below). The same rule holds good for dendrites as for axones as far as it has been possible to determine, 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 JNIarchi (for nerve fibres) and Fig. 89. — 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 chromatoly- sis, 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 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 three factors (i) the type of neurone — some neurones being apparently more resistant than others to injury; (2) the character of the injury — e.g., tearing the nerve causing the greatest reaction; cutting the nerve, a reaction of less intensity; pinching the nerve, the least degree of reaction; and (3), the location of the injur>', an injury near the cell body caus- ing more effect centrally than one at a distance. In other words, the effect de- pends upon the percentage of the neurone cut off. If the injury is very near the cell body the latter may ultimately disappear, the proximal portion of the nerve fibre which remains attached to it undergoing a final degeneration similar to that of the distal severed portion. In the cell body there is an initial turgescence, 10 146 THE TISSUES followed by disintegralion and disappearance of the chromophilic bodies begin- ning near the centre 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 sufficiently characteristic to have led to the designation "axonal degeneration" (Fig. 89). If the neurone body survive the injury regeneration takes place. This con- sists in a reappearance of the chromophOic substance, beginning near the nucleus, a return of the nucleus to its normal position, and a slow subsidence of the tur- gescence. While this is going on the axones of the central stump grow across the scar to the protoplasmic bands beyond formed by the neurilemma cells of the peripheral stump, and find their way along or in them to their former termina- tions. According to some authorities the neurilemma cells can do more than this and form, in young animals at least, new nerve fibres which, however, do not persist unless they form connections with the central stump. The outgrowth (centrogenetic) theory seems more probable than the latter (autogenetic) theory. A B Fig. 90. — A, Neuroglia Cell — Spider Type — Human Cerebrum. — Mossy Type — Human Cerebrum. B, Neuroglia Cell The rapidity of the above degenerative and regenerative changes varies according to the metabolic activity of the animal, i.e., they are much more rapid in warm-blooded than in cold-blooded animals, and in summer than during hibernation. They are also influenced by the age of the- animal. The importance of these degenerations from the standpoint of anatomy lies in the fact that, by using such methods as Nissl, Weigert, and Marchi, one is enabled to trace the connections between cell bodies and nerve fibres throughout the nervous system. Neuroglia This is a peculiar form of connective tissue found only in the nervous system. Unlike the other connective tissues, neuroglia is of ectodermic origin, being developed from the ectodermic cells which line the embryonic neural canal. These cells, at first mor- phologically identical, soon differentiate into neuroblasts or future NERVE TISSUE 147 neurones, and spongioblasts or future neuroglia cells, the latter most probably being in the form of a syncytium. Later this syncytium differentiates fibres, the neuroglia fibres, which, according to Weigert and others, may be entirely separate from the cells (Fig. 91), but more probal)ly lie wholly or at least partly within them. In the syncytium there are also to be distinguished an endoplasm of granular protoplasm and a clear ectoplasm which may perhaps be regarded as a matrix. (See also Fig. 321.) The structure of neuroglia would thus be analogous to that of fibrous connective tissue, i.e., composed Fig. 91.— Neuroglia Cells and Fibres from the White Matter of the Human Cere- bellum stained by Weigert's neuroglia stain. A, NeurogUa cell; B, blood-vessel cut longitudinally, and C, blood-vessel cut transversely, showing enveloping neuroglia fibres; a, neurogUa fibres; b, cytoplasm of neurogUa cell. (Cajal.) of cells, the neuroglia cells, a fibrillar substance, the neuroglia fibres, and a ground substance. The Golgi method apparently reveals a great variety of neuroglia cells which may be divided into cells with straight radiating unbranched processes, 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 por- tions of the cells or syncytium, this method not differentiating between cells and fibres. An increased activity of the neuroglia, due it may be to some pathological cause, is indicated by increase in the endoplasm, by proliferation of the nuclei, and by some cells detaching themselves 148 THE TISSUES from the syncytium and becoming amoeboid. Such amoeboid cells may convey various products of degeneration to the vicinity of the perivascular lymph spaces. There may also result a temporary in- crease in the glial fibres (gliosis). According to good authorities a continuous glial membrane forms the outer boundary of the ectodermal elements of the central nervous system, i.e., the nerve tissue and glia. This glial membrane is of course in apposition with the mesodermic connective tissue which forms the pia or inner covering of the brain and cord and also with the connective tissue of the adventitia of the blood-vessels which penetrate the central nervous system. The glial membrane is formed by superficial flattened cells and processes of other glial cells which come into contact with the connective tissues. The lining of the cavity of the neural tube is formed by the ependymal cells which are a form of neuroglia cells. These cells are columnar epi- thelial cells each with a process penetrating a variable distance into the wall of the neural tube and joining the general neuroglia syncy- tium. Their nuclei lie near the central cavity of the tube and they contain glia fibres. The inner fining of the chorioid plexuses of the brain is a single layer of cuboidal epithelial cells which are believed to have a secretory function and contribute to the production of the cerebro-spinal fluid. It is probable that the ependyma cells also have a secretory activity. The neurilemma cells also probably originate from the neural tube. If this is the case it is evident that they may be regarded as a special form of neuroglia cell. It has already been seen that in pathological changes they behave similarly to the neuroglia cells in the central nervous system. 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 bichlorid method is used, the sec- tions may be covered. Sections are cut from 75 to loo/^ in thickness, cleared in carbol-xylol or oil of origanum and mounted in balsam. This section shows only the external morphology of the neurone. It is also to be used for studying the different varieties of neurogha cells as demonstrated by Golgi's method (see page 147). (2) Thin transverse slices from one of the enlargements of the spinal cord are fixed in absolute alcohol. Thin sections (5 to lo/t) are stained by Nissl's method (page 39) and mounted in balsam. This section is for the purpose of NERVE TISSUE 149 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 lon- gitudinally. 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 Ran- vier. 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 formalin- IMiiller's lluid (tcchnic 5, p. 7), and hardened in alcohol. Small strands are stained twenty minutes in strong picro-acid-fuchsin solution (technic 2, p. 20), washed thoroughly in strong alcohol, cleared in oil of origanum, thor- oughly teased longitudinally and mounted in balsam. General References for Further Study of Tissues Barker: The Nervous System. Bethe: Allgemeine Anatomic und Physiologic des Nervensystem. Cabot: A Guide to the Clinical Examination of the Blood for Diagnostic Purposes. 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 A tissue such as any one of those described in the preceding chap- ters scarcely exists in the pure state, that is alone by itself, in the adult body. Two or more tissues are always associated and such an associa- tion of two or more tissues for the purpose of performing a definite function constitutes an organ. In practically all cases one of the tissues is connective tissue, the main function of which is to form a supportive framework for the more active specific tissue of the organ. The connective tissue is often spoken of as the interstitial tissue, the specific tissue as the parenchyma. In many cases the connective tissue forms a definite covering or capsule. From the capsule strands of connective tissue frequently extend down into the organ where they divide and sub- divide to form its connective tissue framework. Sometimes the subdivision of the organ by connective tissue is quite regular, macro- scopic subdivisions being marked off by coarse connective tissue septa, and these again being subdivided by finer septa. In such case the former are known as lobes, the latter as lobules. But while an organ has been defined as consisting of two or more tissues and while in general one of these tissues is the connective tissue framework and the other the specific functioning tissue of the organ, such a simple combination of two tissue does not actually exist, for all organs are supplied with blood and lymph which are distributed to them and through them by the blood and lymph vessels. There are thus carried into the organs not only the blood and lymph themselves, but also the tissues which compose the walls of the blood and lymph vessels. Also every organ has its nerve supply; thus nerve tissue is distributed through all organs. Many organs are hollow tubes and their study is the study of the structure of the walls of the tube, such, e.g., are the stomach and intestines, the heart and the blood-vessels, the trachea and bronchi. Other organs not so apparently tubular still show their tubular struc- ture on closer analysis, such, e.g., are the lungs and the duct glands. As an organ has a specific function and as this function is performed mainly by the specific cells of the organ, these cells show variations in structure dependent upon whether the organ is at work or at rest. Many adult organs are so complex that it is possible to understand 153 c 154 THE ORGANS them only by reference to their development from more simple structures. From the foregoing it follows that in studying an organ there are to be considered primarily: (i) The specific tissue of the organ. (Parenchyma.) (2) The connective tissue framework. (Interstitial tissue.) (3) The blood supply. (4) The nerve supply, (5) Function. (6) Development. f CHAPTER XI 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 propelKng organ, the heart; (b) a series of efferent tubules — the arteries — which by branching constantly increase in number and decrease in cahbre, 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 hning, 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 hning for walls of muscle and connective tissue. Capillaries It is convenient to describe these first on account of their simpUcity of structure. A capillary is a small vessel from 4.5 to i6fjL in diameter. Its waU consists of a single layer of endothelial cells. The cells are somewhat elongated in the long axis of the vessel. The smaller the calibre of the capillary the more elongated are the cells. Two endo- thelial cells suffice to complete the circumference of small capillaries, while larger require three or four. The protoplasm of the cells is clear or finely granular. The nuclei are oval, with their long axes of the vessel. In fixed material the nuclei bulge into the lumen. In 155 156 THE ORGANS the living condition the lining of the capillary is probably nearly smooth. According to some investigators a delicate cuticle limits the cytoplasm on the side toward the lumen. The edges of the cells are serrated and are united by a small amount of intercellular substance (p. 8i), which can be demonstrated by the silver- nitrate stain. ^ In certain 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. 70). Capillaries branch without diminution in calibre, and these branches anasto- I Fig. 92. — ^Large and Small Capillaries. Silver-nitrate and hsematoxylin stain (technic 7, p. 79), to show outlines of endothelial cells and their nuclei. mose to form capillary networks, the meshes of which differ in size and shape in different tissues and organs (Figs. 92, 93, 94). The largest meshed capillary networks are found in the serous membranes and in the muscles, while the smallest are found in the glands, as, e.g., the liver. The largest capillaries are found in the liver, the smallest in muscles. 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 centre of the vessel than at its periphery. It is slower in the veins and slowest in the capillaries. In the case of the frog's bladder, the mere expo- sure to the air acts as a sufficient irritant to cause slight inflammatory changes, and the leucocytes may be seen adhering to the waUs 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. ^ Some authors describe delicate protoplasmic anastomoses between the cells and an irregular precipitation of the silver nitrate corresponding to the spaces between the anastomosing threads- These spaces are interpreted as intercellular channels through which the leucocytes and plasma pass. Intracellular spaces reacting to silver nitrate and supposed to have a similar function have also been described. THE CTRCn.ATORY SYSTEM 157 Arteries The wall of an artery consists of three coats: (i) An inner coat, the intima. (2) The middle coat, the media. (3) An outer coat, the adventitia. The intima consists of a single layer of endothelial cells, continu- ous with and similar to that forming the walls of the capillaries, or, >c fed Fig. 93. — Diagram of Capillaries and Small Artery showing their structure and relations, a, Capillaries; h, nuclei of capillary endothelium; c, precapillary arteries; d, arteriole; e, large capillary; /, small artery. in arteries of considerable size, of this layer plus more or less connec- tive 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, (&) medium sized arteries, and (c) large arteries. Small Arteries. — Passing from a capillary to an artery, the first change is the addition of a thin sheath of connective tissue, the fibres 158 THE ORGANS of which are disposed longitudinally, around the outside of the endo- thelial 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. 93, d\ Fig. 94, h). Mediimi-sized Arteries. — This group comprises all the named arteries of the body with the exception of the aorta and the pulmo- FiG. 94. — Capillary Network from Human Pia Mater, showing also an arteriole in " optical section" and a small vein. X3S0. (Technic i, p. 164.) a, Vein; h, arteriole; c, large capillary; d, small capillaries. nary. Their walls are 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. 95). (a) An inner endothelial layer already described. {h) A middle layer, the intermediary layer of the intima. This is composed of delicate white and elastic fibrils which run longi- tudinally, and connective-tissue cells. (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 CIRCUI.ATORY SYSTEM 159 the medium-sized arteries the intermediary layer is often wanting, the endotheUal cells resting directly upon the elastic membrane. Owing to the extensive amount of elastic tissue in their walls, there is a postmortem contraction of arteries which results in the intima being thrown up into folds. For tliis reason the elastic membrane presents, in transverse sections of an artery, the appearance of a wavy band (Fig. 95). Fig. 95. — From Cross-section through Walls of Medium-sized Artery and its Accompanying Vein. XJS- (Technic3, p. 164.) .4, Intima of artery; d, its endothe- lial layer; b, 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. 2. The MEDIA is a thick coat of circularly disposed smooth muscle cells (Fig. 95, ^). Its thickness depends largely upon the size of the vessel, though varying somewhat for different vessels of the same size, A small amount of fibrillar connective tissue supports the muscle cells. Elastic tissue is present in the media, the amount being usually pro- portionate to the size of the vessel. In the smaller of the medium- sized arteries, the elastic tissue is disposed as dehcate fibrils among the muscle cells. In larger arteries many coarse fibres are intermin- gled 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 IGO THE ORGANS occurs not only as fibrils but also as circularly disposed plates or fenestrated membranes. 3. The ADVENTiTiA (Fig. 95, C) is composed of loose fibrous connective 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 f i. I "I (J /-^ *-- ., t. ^ 'A ■J S^is >" v 3 Fig. 96. — From Transverse Section of Dog's Aorta. X60. (Technic 4, p. 164.) Intima; Z;, media; c, adventitia; d, vasa vasorum; e, elastic tissue;/, endothelium. 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 adventitia forms an especially well-defined layer at the outer margin 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. 96) have the same three coats as small and medium-sized arteries. The layers arc not, however, so distinct. Tin's is due mainh' to the excessive amount of elastic THE CIRCULATORY SYSTEM 161 tissue in the media (Fig. 97), which makes indistinct the boundaries between intima and media, and between media and adventitia. 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 Fig. 97. — From Transverse Section of Dog's Aorta, to show Elastic Tissue. X60. (Technic 7, p. 165.) Elastic tissue stained black, a, Intima; h, media; c, adventitia. 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 advejititia 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 11 162 THE ORGANS 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 mesenteric and dorsalis penis. The radial, femoral and coeliac arteries have comparatively little clastic 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. 95). Venous walls are not, however, so thick as those of arteries of the same calibre, and the coats are not so distinctly differentiated from one another. The transition from capillary through the precapillary vein to the small vein is similar to that described under arteries (page 157). Unlike the artery, the thickness of the wall of a vein and its structure a,re not directly proportionate to the size of the vessel, but depend also upon other factors such as the position of the vein and the support given to its walls by surrounding 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. 95). 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- ing at intervals from the inner surfaces of the walls of some veins are valves. These are derived entirely from intima and consist of loose connective tissue covered by a single layer of endothelium. Beneath the endothelium of the convex surface of the valve (the surface directed peripherally or against the blood current) is a rich network of elastic fibres continuous with the elastic tissue of the intima of the adjacent vein wall. The connective tissue along the opposite margin of the valve is entirely free from or contains very few elastic fibres. 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 visceral 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 circularly THE CIRCUT.ATORV SYSTEM 163 disposed smooth mustlc cells. In most veins the amount of muscle is comparative!}- small and the cells are distributed in ^n-oui).-- through the connective tissue. The media is thickest in the veins of the lower extremities, especially the ])opliteal. 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 ca\a and in the veins of bones, of the pia mater, dura mater, and retina, there is an almost entire absence of mecha. The ADVENTITIA is well developed in i)roportion to the media and forms the bulk of the vessel wall. It consists of mixed fibrous and elastic tissue and usually contains along its inner margin small bundles of longitudinally disposed smooth muscle cells. 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 hepadc 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 mem- branes, the retina, the placenta and the bones, no muscle is present. Vasa Vasorum. — Medium and large arteries and veins are sup- pUed with small nutrient vessels — vasa vasorum. These vessels run in the adventitia, small branches penetrating the media (Figs. 95 and 96). Lymph channels are found on the outer surface of many blood- vessels. Some of the smaller vessels are surrounded by spaces Uned by endothelium — perivascular lymph spaces. These communicate with the general lymphatic system. Nerves. — The walls of the blood-vessels are suppHed with both medulla ted and non-medullated fibres. The latter are axones of sympathetic neurones. As these nerves control the caHbre 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 gangHon cells. 164 THE ORGANS 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-Miiller's fluid for twenty- four hours (tcchnic 6, 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 haematoxylin (lightly) and eosin (strongly) (technic I, p. 20), 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 haemoglobin-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 living tissues may be obtained by curarizing a frog, distending the bladder with normal saline introduced through a small catheter or cannula, opening the abdo- men 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 network and into the veins is beautifully demonstrated. ; (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 ves- ' sels, with a small weight attached, in formalin-Miiller's fluid (technic 6, p. 7). [ Sections should be cut transversely, stained with haematoxylin-eosin (technic I, p. 20), or with haematoxylin-picro-acid fuchsin (technic 3, p. 21), and mounted in balsam. The vessels of the adventitia — vasa vasorum — are con- venient 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 haema- toxylin-eosin or with haematoxylin-picro-acid fuchsin and mount in balsam. (5) The outlines of the lining endothelial cells may be demonstrated as fol- lows: 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 glycerin, and expose THE CIRCULATORY SYSTIQI 165 to the direct sunlight. After the specimen has turned brown examine with the low power. The outlines of the ceils 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. 8^. (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. 28. 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 123). This constitutes the myocardium. On its inner and outer sides the myo- cardium is covered by connective- tissue membranes Uned, respect- ively, with endothelium and mesotheUum and known as the erido- cardium and epicardium. The MYOCARDiUii 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 aurictdo-ventricular ring, com- pletely separates the muscle of the auricles from that of the ventri- cles, except in the median septum where the auricular and ventricu- lar muscle is continuous. The auricular muscle consists of an outer coat common to both auricles, the fibres of which have a transverse or somewhat oblique direction, and of an inner coat, independent for each auricle, the fibres of which are longitudinally disposed. Bun- dles of these fibres stand out as ridges along the inside of the auricles. They are more strongly developed in the right than in the left auricle. 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 more compHcated. It is usually described as composed of several layers, the fibres of which run in dift'erent directions. The meaning of these fibre layers becomes apparent when we study the arrangement 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 approximately right angles to each other. Both sets of fibres begin at the auriculo-ventricular rings. The superficial fibres wdnd around both ventricles in a spiral 166 THE ORGANS manner, becoming constantly deeper, to terminate in the papillary muscles of the opposite ventricle. The deeper fibres pass from the auriculo-ventricular ring around the ventricle of the same side, through the interventricular septum and terminate in the papillary muscles of the opposite ventricle. The ENDOCARDIUM covers 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 similar in structure to the intima of the vessels. It consists of two layers: (a) an inner composed of a single layer of endothelial cells, corresponding to the endotheHal Hning of the blood-vessels; and (b) 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 {annuli fibrosi), composed of mixed fibrous and elastic tissue, surround the openings between auricles and ventricles. 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 structure. It consists of a layer of mixed fibrous and elastic tissue covered over by a single layer of mesothelial cells. Beneath the epicardium 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 branches pass in among the individual fibres, where 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 endocardium and enter the valves. Within the myocardium minute lymph vessels have been demonstrated between the muscle fibres and accompanying the blood-vessels. THE CIRCULATORY SYSTEM 1(17 Nerves. — These are derived from both cerebro-spinal (vagus) and sympathetic systems (cervical ganglia) and consist of both medullated and non-medullated fibres. Sympathetic ganglion cells are distributed in groups throughout the myocardium, the largest, lying in the epicardium near the base of the heart, being known as the cardiac ganglion or ganglion of Wrisberg. Among these cells the nerve fibres form plexuses from which both motor and sensory terminals are given oft' to the muscle. (For nerve endings in heart muscle see page 446.) TECHNIC (i) The Heart. — Cut pieces through the entire thickness of the wall of one of the ventricle's, care being taken not to touch either the serous surface or the lining endothelium. Fi.x in formalin-^Iiiller's fluid (tcchnic 6, p. 7). Cut trans- verse and longitudinal sections; stain with haimatoxylin-eosin (technic i, p. 20) 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 condi- tion by filling with formalin- IMiiller'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. De\elopment of the Circulatory System The blood-vessels and the heart begin their development separately and afterward become united. Both are derived from mesoderm, but while the heart develops within the embryo, the earliest blood-vessels and blood are developed from extra-embrj-onic mesoderm. The earliest vessels to be formed are the capillaries. These make their appearance in the mesoderraic tissue near the periphery of the area vasculosa which surrounds the developing embryo. Here groups of ceUs known as "blood islands" differentiate from the rest of the mesodermic cells, appearing in the chick by the end of the first day of incubation. The superficial cells of these islands become flattened to form the endothelium, which is at this stage apparently a syncj'tium through which the nuclei are scattered and showing no e\ddences of cell boundaries. This endotheHal syncytium surrounds the remaining more central cells, from which blood cells are developed. These represent the earliest blood-vessels. The channels, which are at first unconnected, anastomose and give rise to a network of channels which are the earliest capillaries. These develop rapidly in the area vasculosa and some of them increase in size to become arteries and veins, the smooth muscle and connective tissue of their walls being dift'erentiated from the surrounding mesoderm. These grow toward, and finally into, the embryo where they unite with the heart. In granulation tissue, and in new growths in general, both normal and pathological, new blood-vessels apparently develop as oft"-shoots from other vessels. These are at first solid extensions 168 THE ORGANS of endothelium which become hollowed out. In regard to the origin of the later vessels in the extraembryonic area, and those within the embr^'o, 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 vascular system is at first represented by a network of capillaries. As development proceeds some of the channels enlarge to form the arteries and veins. The heart first appears as an endothelial tube, the primitive endocardium, at a very early age of embryonic life, in the chick embryo during the first day of incubation. It apparently begins its rhythmic contraction before any Coelom Parietal mesoderm Ectoderm Visceral mesoderm Blood islands Fig. 98. — Section of Blastoderm of Chick of 42 Hours Incubation. Photograph. The cells of the blood islands are differentiated into nucleated red blood cells (crythro- blasts) and the endothelium of the vessels. contractile fibrils can be distinguished in its walls and before any connection with blood-vessels has been established. 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. The foregoing description applies to the chick and to those mammals thus far studied. In the earliest human embryos (2 to 3 mm.) the heart is already a single, slightly coiled tube connected 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 sepa- rate origin, being, constricted off from that portion of the mesoderm which lines the primary body cavity. THE CIRCULATORY SYSTEM 169 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. These are folds of the intima, and always occur in pairs. They are arranged with considerable regu- larity and in small vessels where the intima forms the entire wall there is a distinct bulging just above the valve which gives the vessel a quite characteristic appearance. 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 which occur in groups separated by connective tissue containing a few elastic fibres and an adventitia of longitudinally running connective tissue contain- ing elastic fibres and strengthened by bundles of longitudinal smooth muscle. 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 caHbre within short distances. In cer- tain tissues dense networks of these lymph capillaries are found. Cleft-like lymph spaces — perivascular lymph spaces — partially sur- round 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 canaliculi of the cornea and of bone (pages 87 and 105). Similar in character to lymph spaces are the body cavities, peri- toneal, pleural, and pericardial, with their finings of serous membranes. These cavities first appear in the embryo as a cleft in the mesoderm — the ccelom, body cavity, or pleuro peritoneal cleft. This cleft is lined with mesothelium beneath W'hich 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. Thev consist of a stroma of mixed fibrous and elastic tissue, covered on its inner side by a layer 170 THE ORGANS of mesotheliuni , the two being separated by a homogeneous basement membrane. The stroma contains numerous lymphatics. These have been described as communicating with the free surfaces by means of openings — stoma tu. 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 origin of the lymphatic vessels is not clear. According to some investi- gators they originate as evaginations from the vascular system, starting as four buds, two from the veins of the neck, and two from the veins in the inguinal region. The two anterior buds appear first (pig embryo of 14.5 mm.) and are in communication with the anterior cardinal vein. A little later the posterior buds connected with the posterior cardinals appear. From these buds the entire lymphatic plexus develops by a process of evagination. According to other investigators, the lymphatic vessels first appear as min- ute spaces in the mesenchyme of the axUla and groin (during second month in human embryo). These spaces enlarge and by sending off branches give rise to the network of lymphatic vessels. According to this veiw, the connection between lymphatic vessels and veins is secondary. 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. 82. 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, fix in forma- lin-Miiller's fluid (technic 6, p. 7), and stain sections with hsematoxylin-eosin (technic i, p. 20). General References for Further Study of the Circulatory System KoUiker: Handbuch der Gewebelehre des Menschen, vol. iii. Stohr: Text-book of Histology. Schiifer: Histology and Microscopic Anatomy, in Quain's Elements of Anat- omy, tenth edition. CHAPTER XII LYMPHATIC ORGANS Lymphatic Tissue ' )0-CALLED lymphatic tissue consists of reticular connective tissue and a special type of cells, lymphoid cells, which fill in the meshes of the reticulum. Lymphoid cells are small spheroidal cells, each hav- ing a single nucleus which almost completely fills the cell. Lym- phatic tissue may be difi'use or circumscribed. In diffuse lymphatic tissue the cells are not closely packed and there is no distinct de- marcation between the lymphatic and the surrounding tissues. An example of difi'use lymphatic tissue is seen in the stroma of the mucous membrane of the gastro-intestinal canal. In circumscribed lymphatic tissue the cells are very closely packed, often completely obscuring the reticulum. There is also a quite distinct demarca- tion between the lymphatic and the surrounding tissues. Such a circumscribed mass of lymphatic tissue is known as a lymph nodule. 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 they are arranged in series forming "chains" of lymph node^as, e.g., the axillary and inguinal. 'liEach lymph node is surrounded by a capsule of connective tissue which sends trabeculaor sepja into the organ. The capsule and septa constitute the connective-tissue^[ramewqrk_ of the node, and serve as a support for the lymphatic tissue (Fig. 99). The capsule is composed of fibrous connective tissue. Toward the surface of the capsule the fibres are loosely arranged and there are comparatively few elastic fibres. This outer layer of the capsule ^ In the preceding editions " lymphatic tissue" was placed according to name rather than structure among the tissues. As it consists of a connective tissue framework sup- porting a special type of cell which has a specific function, it is more properly classi- fied as an organ. On account of the peculiar structure and wide distribution of the "tissue" and on account of convenience and long usage, the term " lymphatic tissue" is still retained. 171 172 THE ORGANS blends with the surrounding tissuesk3,nd serves, hke the fibres of the arterial adventitia, to attach the organ to them. ^The inner layer of the capsule consists of a more dense connective tissue, is richer in elastic fibres, and contains some smooth muscle cells. At one point, known as the // Hum (Fig. 99), there is a depression where the con- nective tissue of the capsule extends deep into the substance of the node. This serves as the point of entrance for the main arteries and nerves, and of exit jp_r_the veiiis and efferent lymph vessels.:; It g f Fig. 99. — Section through Entire Human Lymph Node, including Hilum. X15 (Technic i,p. 176.) Dark zone, cortex; light central area, medulla, a, Lymph nodule of cortex; b, germinal centres; c, trabeculae containing blood-vessels; d, capsule; e, hilum; /, lymph sinus of medulla; g, lymph cords of medulla; It, lymph sinuses of medulla and cortex. The connective-tissue septa, which extend from the capsule into the interior of the node incompletely divide 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. Within the compartments formed by the capsule and the septa is the lymphatic tissue. Near the capsule where the compartments are large and spheroidal or pear-shaped, the lymphatic tissue is arranged in masses which correspond in shape to the compartments. LV.Ml'IIATIC ORf;AXS 173 These are known as lymph nodules (Fig. 99). 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 lymphoid cells takes place, this area is known as the germinal centre (Figs. 99 and 100). Immediately surrounding the germinal centre is a zone in which the lymphoid cells are more closely packed than elsewhere in the nodule (Fig. 100). This is apparently due to the active production of new cells at the germinal centre and the consequent pushing outward of tlic surrounding cells. In stained sections the centre of the nodule is thus lightly stained, while immediately surrounding this light area is the darkest 0735 Fig. 100. — Section through Cortex and Portion of Medulla of Human Lymph Node (Technic 2, p. 176.) a, Capsule; b, lymph sinus; c, trabecula; d, closely packed cells at outer border of lymph nodule; e, germinal centre; /, lymph cords in medulla. portion of the nodule (Fig. 100). From the inner sides of the nodule, strands of lymphoid tissue extend into the center of the node. These are known as lym^i cords, and anastomose freely. The regular arrangement of tFe lymph nodules and trabecul^e in the peripheral portion of the node contrasts strongly wdth their irregu- lar arrangement in the centre. This determines a division of the nodule into two zones, an outer peripheral zone or cortex and a cen- tral zone or medulla. In both cortex and medulla the lymphoid tissue is always separated from the capsule or from the septa by a distinct space — the lympji sinus — which is bridged over by reticular tissue containing comparatively few lymphoid cells (Fig. 100). These sinuses form a continuous system of anastomosing incompletely walled channels throughout the node. 174 THE ORGANS The regular arrangement of trabecular, and lymph nodules with sinuses betv;een, which is characteristic of the cortex, makes this part of the organ easily understood. To appreciate the structure of the medulla it must be borne in mind that all of these cortical struc- tures extend down into the medulla, the trabeculse as anastomosing networks of connective tissue, the lymph nodules as cord-like struc- tures which divide and anastomose, the sinuses as more or less clear channels which always separate the connective tissue from the lymph cords. These parts — trabecula, sinus, lymph cord— all anas- tomosing freely and most irregularly in the medulla, always main- tain the same relation to one another as in the cortex, namely, that sinus is always interposed between lymph tissue and trabecula. The reticular connective tissue (page 98), 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 ob- scured.''a Crossing the sinuses, especially those of the medulla, and in specimens in which the cells have been largely washed out or removed by maceration, 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- ulas. As both nodules and cords are everywhere separated from cap- sule and trabecular 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.! 1 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- 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 net- \ LYMPHATIC ORGANS 175 works. Some of the arteries, instead of passing directly to the lym- phatic 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 Afferent „.,i,;V;%/ /r^% {. .'.^i.^, /, yrmph. ves. '^(p:^.. ^l i . ;..V^" Fig. lor.- — ^From a Section through the Axilla of a Human Embryo of 125 mm. (4-5 months), showing an Early Stage of a Lymph Gland. (Kling.) 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 know^n. !i Lymph.^ — This is a colorless fluid containing rather few formed ements in the w^ay of w^hite blood cells, blood platelets, granules and fat. Most of the cells are of the small lymphocyte type although some large lymphocytes and polymorphonuclear leucocytes are found. In the lymph of the intestinal mucosa, especially during digestion, fat droplets are present in large numbers giving the fluid a milky appearance. This is known as chyleh 176 THE ORGANS Development. — The first indications of lymph node formation are found in the axilla and groin (pig embryo of about 30 mm.; human toward end of third month) in the connective tissue in which the lymphatic vessels are best developed (p. 170). Here groups of more closel}^ packed cells appear. As the tissue is richly vascular it seems impossible to determine whether the closely packed cells originate within the blood-vessels or develop from fixed connective tissue cells. Each group of cells is the anlage of a lymph node (Fig. loi). The immediately surrounding connective tissue forms the capsule, while the lymph channels just beneath form the subcapsular or marginal sinus (Fig. 102). The point of main connection with outside l)lood-vessels becomes the hilum. As the lymph node Afferent lymphatic vessels Marginal sinus Capsule Dense lymph tissue Marginal sinus (plexus) Capsule Trabecula Reticular tissue Intermediary plexus Efferent lymph, vessel Blood vessels Fig. 102. — Diagram Illustrating a Stage (Later Than Fig. 97J in the Development of a Lymph Gland. (Stohr.) grows outward, parts of the capsule remain within to form trabeculse while the lymph channels within the nodule apparently develop as ingrowths from the marginal sinus (Figs. loi and 102). TECHNIC (i) Remove several lymph nodes from one of the lower animals (ox, cat, dog, rabbit), fix in formalin-]\Iiiller"s fluid (technic 6, p. 7), and harden in alcohol. Cut thin sections through the hilum, stain with hsematoxylin-eosin (technic 1, p. 20), or with hsematoxylin-picro-acid-fuchsin (technic 3, p. 21), and mount in balsam. (2) Expose a chain of lymph nodules {e.g., the cervical or inguinal of a re- cently killed dog or cat). Insert a small cannula or needle into the uppermost node and inject formalin-MUller's fluid until the node becomes tense. By now slightly increasirtg 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 lvjMphatic organs 177 and placed for twenty-four hours in fornialin-MuUcr's fluid, llieii hardened in alcohol. Sections are cut through thr hikun, stained with haematoxylin-eosin or with hsematoxylin-picro-acid-fuchsin and mounted in balsam. Near the centre of the chain are usually found nodes in which the lymph sinuses arc properly dis- tended. The most proximal nodes are apt to be overdistendcd, 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 may be treated by either of the above methods. Owing to the coalescence of their cortical nodules their structure is not so easily demonstrated as that of the lymph nodes of lower animals. Haemolymph Nodes These are lymphoid structures which closely resemble ordinary lymph nodes, but with the essential difference that their sinuses are hlood sinuses instead of lymph sinuses. Fig. 103. — Section through Human Haemolymph Node, including Hilum, showing cap- sule, trabeculse, sinuses filled with blood, and Ij^mph nodules. (VVarthin.) I Each node is surrounded by a capsule of varying thickness, com- posed of fibro-elastic' tissue and smooth muscle cells. From the cap- sule traheculoe of the same structure pass down into the node, forming its framework (Fig. 103) . Beneath the capsule is a hlood sinus, which may be' broad or narrow, and usually completely surrounds the node. Less commonly the sinus is interrupted by lymphoid tissue extending 12 178 THE ORGANS 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 Jiilum. 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 lymph node. Reticular connective tissue crosses the sinuses and supports the cells of the lymph nodules and cords (Fig. 104). G^^^L^.^-...^^&-^.^^^^^X2^^^''^-^:i-.,'' /C2: Fig. 104. — Section through Superficial Portion of Human Hasmolymph Node (Marrowlymph Node). (Warthin.) Capsule, trabeculse, 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. 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 no). Polynuclear leucocytes and large leucocytes with a single lobulated nucleus are less numerous. The very large multinuclear cells of red marrow are also found, but usually in small numbers. LYJMPHATIC ORGANS 179 Large pJiagocytes 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 fdl the sinuses. Further classification of haemolymph nodes has been attempted, but is unsatisfactory, owing to the large number of transitional forms. Thus many nodes arc transitional in structure betwen the hcemo- lymph node and the ordinary lymph node, between the splenolymph node and the marrowlymph node, and between the splenolymph node and the spleen. Under normal conditions the haemolymph 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. The development of the haemolymph nodes is probably much the same as that of the lymph nodes, except for the relation of the latter to the lymphatic vessels, the sinuses of the haemolymph nodes being developed from blood-vessels. TECHNIC Same as for lymph nodes (technic i, p. 176). The nodes are found in greatest numbers in the prevertebral tissue, and are often difficult to recognize. Fixing the tissues in s-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 foetal 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 180 THE ORGANS and connective tissue, until by the twentieth year scarcely a vestige of glandular tissue remains. The fully developed thymus presents the following structure. The entire gland is surrounded by a rather delicate and loose connective-tissue capsule which attaches it to the surrounding tissues. From the capsule septa extend down into the organ. These branch and subdivide the gland into lobes and lobules. From the perilobular connective tissue, septa extend into the lobule, incompletely separating it into a number of chambers. Each lobule consists of a cortical portion and a medullary portion. The cortex Fig. 105. — From Section of Human Thymus, showing parts of five lobules and interlobular septa. X20. (Technic, page 182.) a, Cortex; b, medulla; c, interlobular septum. consists of nodules of compact lymphatic tissue, composed of reticular tissue and lymphoid cells, similar to those found in the lymph node. These occupy the chambers formed by the connective-tissue septa. The medulla consists of the same elements only more loosely arranged, the cells being much less closely packed, thus forming a more diffuse lymphatic tissue. There are also in the medulla no connective-tissue septa. In some lobules the more dense cortical substance completely encloses the medulla. It is common, however, for the medullary tissue to extend to the surface of a lobule at one or more points and to be there continuous with the medullary substance of an adjacent lobule. These interlobular connecting strands of medullary substance LVMl'llATIC URUAXS 181 are known as medullary cords. In the medulla are found a number of spherical or oval bodies composed of concentrically arranged epi- thelial cells. These are known as IlassaWs corpuscles (Fig. io6), and represent the only remains of the original glandular epithelium. They are characteristic of the thymus. The central cells of the cor- puscles 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-hasmatoxylin, the corpuscles stand out sharply from the surrounding bluish or pinkish lymphatic tissue. With low magnihcations they are apt to be mistaken for blood- vessels. Unlike ' the other lymphatic (t; organs, the lymph nodules of the thymus contain no germinal centres. Mitosis can, however, ■ usually be seen in the lymphoid '■< . cells. No definite lymph sinuses have been demonstrated. Nu- ^ , ^ Cleated red blood cells occur m j,^^_ io6.-Hassall's"Corpuscle and Small the thymus, which must there- Portion of Surrounding Tissue. X6oo . ' . , , ^ ,1 (Technic page 182.) tore be considered one 01 the sources of red blood cells as well as of lymphoid 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 networks in the cortex and medulla. The capillaries pass over into veins. These converge to form larger veins, which accompany the arteries. Of the lymphatics of the 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. The thymus originates in the entoderm in the region of the third bran- chial groove, first as two hollow evaginations of the endothelium of the pharyngeal cavity, which later become solid cords, and then separate entirely from the pharynx. The thymus thus begins its fcetal existence as a typical epithehal gland. Into this epithelial structure mesodermic cells grow and differentiate into lymphatic tissue. This almost completely replaces the epi- thelial tissue, only rudiments of which remain as Hassall's corpuscles. 182 THE ORGANS Stohr denies the mesodermic invasion of the thymus and consequently the lymphatic character of the gland. He considers the specific cells of the thymus as modified epithelial cells which have become "deceptively like lymphoid cells." This explanation has not, however, been generally accepted. TECHNIC Fix the thymus of a new-born infant in formalin-Miiller's fluid (technic 6, p. 7), and harden in alcohol. Stain sections with hsematoxylin-eosin (technic i, p. 20), or with haematoxylin-picro-acid-fuchsin (technic 3, p. 21), and mount in balsam. The Tonsils The Palatine Tonsils or True Tonsils. — These are lymphatic organs, essentially similar in structure to those already described. V V Fig. 107. — Vertical Section of Dog's Tonsil through Crypt. X15. (Szymonowicz.) a. Lymph nodule; b, epithelium of crypt; c, blood-vessel; d, crypt; e, connective-tissue capsule; /, mucous glands; g, epithelium of pharynx. The usual librotis capsule is present only over the attached sur- face, where it is firmly adherent on the one side to the tonsillar tissue and on the other to the surrounding structures from which it LYMPHATIC ORGANS 183 separates the tonsils. From the capsule, connective-tissue trabeculce extend into the substance of the organ and branch to form its frame- work. The free surface of the tonsil is covered by a reflection of the stratified squamous epithelium of the pharynx (Fig. 107). This epithe- lium presents the same structure as elsewhere in the pharynx, flat surface cells, beneath which are irregular cells, while the deepest cells are more or less distinctly columnar. The latter rest upon a papillated stroma from which they are separated by thehasement membrane. At several places on the surface of the tonsil deep indentations or pockets occur. These are from ten to twenty in number, are known as the crypts of the tonsil (Fig. 107), and are lined throughout by a continua- fv '^^^^ ^:-^ sSrJ ve Fig. ioS. — Vertical Section through Wall of Crypt in Dog's Tonsil, showing lymphoid infiltration of epithelium. Xiso. (Bohm and von DavidofT.) a, Leucocytes in epithe- lium; b, space in epithelium filled with leucocytes and changed epithelial cells; c, blood- vessel; d, epithelium be\'ond area of infiltration; e, basal layer of cells. tion 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. The stroma 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 germ centre, where active mitosis is going on, and a surrounding zone of more densely packed cells. The nod- ules may have a fairly definite arrangement, forming a single layer beneath the epithelium of the crypts or may be arranged quite irregu- larly, several nodules uniting to form masses of dense lymphatic tissue. At various points on the surface of the tonsil, and especially in the crypts, occurs what is known as lymphoid infiltration of the epithelium (Fig. 108). This consists in an invasion of the epithelium by the underlying lymphoid cells. It varies from the presence of 184 THE ORGANS only a few lymphoid cells scattered among the epithelium, to an almost complete replacement of epithelial by lymphoid tissue. In this way the latter reaches the surface and lymphoid cells are discharged 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. They are similar in structure to the true tonsils. Each Ungual tonsil has usually one rather wide-mouthed deep crypt (the forafuen ccBcum lingiics) which may be branched and which is Hned with a continuation of the surface stratified squamous epithelium. Arranged around the crypt are the lymph nodules, each usually having a distinct germinal center and more or less sharply separated from the surrounding tissue by a fibrous capsule. In most crypts there is marked lymphoid infiltration of the epithelium with free "salivary corpuscles." Into these crypts frequently 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, with consequent obstruction of the nasal openings, is common especially in children, constituting what are known as adenoids. The blood-vessels 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. The nerves wliich are branches of the glosso-pharyngeal and of the spheno-palatine ganglion also enter the organ along its attached side. The palatine tonsils make their first appearance during the third month of intra-uterine life as hollow evaginations of entoderm which grow down into the underlying mesenchyme, in the region of the second inner branchial groove. The earliest of the tonsillar lymphoid cells are apparently white blood cells which have migrated from the vessels of the stroma of the mucosa and have infiltrated the surrounding connective tissue. Further development of the tonsil is by proliferation of these cells. The crypts are at first solid ingrowths of surface epithelium. These later become hollowed out. LYMPHATIC ORGANS 185 The lingual and pharyngeal tonsils begin their development during the later months of intra-uterine life. In the pharyngeal tonsils, definite nodules appear about the time of birth or during the first or second year. In the lingual tonsils the nodules are not fully formed until the fifth or sixth year. 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. 176, care being taken that sections pass longitudinally through one of the crypts. The Spleen The spleen is a lympliatic organ and might be quite properly classed as a hasmolymph node. Its peculiar structure appears to depend largely upon the arrangement of its blood-vessels. e Fig. ioq. — Section through Portion of Cat's Spleen, to show general topography. X15. (Technic i, p. i^i.) a, Capsule; b, septa containing blood-vessels; c, center of lymph nodules; d, septa; e, germinal centres. Except where attached, the spleen is covered by a serous mem- brane, the peritoneum (page 272) . Beneath this is a capsule of fibrous tissue containing numerous elastic fibres and some smooth muscle cells. Along the broad attached surface this capsular connective tissue blends with the connective tissue of the posterior abdominal wall. From the capsule strong connective-tissue septa, similar to the capsule in structure, extend into the interior of the organ. These branch and unite with one another to form very incomplete anas- tomosing chambers. At one point on the surface of the spleen known as the hilum a deep indentation occurs. This marks the entrance 186 THE ORGANS and exit of the large splenic vessels. Accompanying the vessels broad strands of capsular tissue extend deep into the organ where they radiate and subdivide, to form with smaller trabecular which extend in from other parts of the capsule, the connective-tissue frame- work of the organ (Fig. 109). 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. 188. This tissue constitutes the substantia propria of the organ and is everywhere traversed by thin-walled vascular channels, the tissue and vascular channels together con- o /a~ ' 0 o o "" 0 Fig. 1 10. — Section of Human Spleen, including portion of Malpighian body with its artery and adjacent splenic pulp. X300. (Technic 2, p. 191.) o, INIalpighian body; h, pulp cords, c, cavernous veins; h and c together constituting the splenic pulp. stituting the splenic pulp (Fig. no). Compact lymphatic tissue occurs in the spleen as spherical, oval, or cyHndrical aggregations of closely packed lymphoid cells. These are known as Malpighian bodies or splenic corpuscles (Figs. log and no) 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 a structure to the 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 LY.MrHATIC ORGANS 187 centre (see page 173)- In the adult human spleen germ centres are rarely seen. The blood-vessels of the spleen have a very character- istic arrangement, which must be described before considering further the minute structure of the organ. The arteries enter the spleen at the hilum 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 Spleen sinus Sheath artery Pulp artery- Pulp vein Beginning of in- terlobular vein Capillary net- work of nodule Central arterv Interlobular vein Interlobular artery Trabeculae Penicillus Lobule Hilus Reticulum Spleen nodule Capsule Fig. hi. — Scheme of Human Spleen, x, Opening 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 too far from margin of nodule. (Stohr.) course through the splenic pulp. Here the adventitia of the smaller arteries assumes the character of reticular tissue and becomes in- filtrated with lymphoid cells. In certain animals, as e.g., the guinea- pig, this infiltration is continuous, forming long cord-hke 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, 188 THE ORGANS B these arteries are known as central arteries. They give rise to a few capillaries in the spindles, to a larger number in the nodules. Be- yond the latter the arteries divide into thick sheathed terminal artenes— -pulp arteries — wliich do not anastomose, but lie close to- gether like the bristles of a brush or penicillus. The pulp arteries break up into unusually thick walled arterial capillaries which still retain an adventitia, the so-called sheathed arteries. These are of re- markably uniform diameter — 6-8 cc — and empty into broader spaces from lo to 40yu in diameter — the spleen sinuses or ampiillcB — which in turn empty in to the cavernous veins of the splenic pulp (Fig. no). 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. no), and with the caver- nous veins constitute, as already mentioned, the splenic pulp. The pulp cords consist of a delicate framework of reticular connective tissue, in the meshes of which are found, in addition to lymphoid cells, the following (Fig. 112): (i) Red blood cells, including nucleated red blood cells and fragments of red cells in process of disintegration. (2) White blood cells. (3) Mononuclear cells, the so-called spleen cells. These are rather large, granular, spher- ical, 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 beheved to be concerned in the destruction of red blood cells. (4) MuUinuclear 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- J? H a \ F (■;■. di Fig. 112. — Isolated Spleen Cells. X700. (KoUiker.) A, Cell containing red blood cells; b, blood cell; k, nucleus; B, leucocyte with polymorphous nucleus; C, "spleen" cell with pigment granules; D, lymphocyte; E, large cell with lobulated nucleus (megalo- cyte); F, nucleated red blood cells; G, red blood cell; //, multinuclear leucocyte; /, cell containing eosinophile granules. L^'^IPTTATTC ORGANS 189 shaped cells with bulging nuclei. These come from the walls of the cavernous veins. Two views have been held regarding the vascular channels of the pulp. According to one, these channels have complete walls, the blood-vessel system of the splenic pulp being a closed system as in other organs. According to the other, the cavernous veins or spleen sinuses into which the arterial capillaries open, have fenestrated w^^' 3^ m \ £i \fci ,&^ ( r ,?■•'•' , Dark band :l t^ '1 li ill »••• Transition zone ... - ■'■ Tendon fibrils Nucleus Fig. I2g. — Longitudinal Section through Muscle-tendon Junction. Human Intercostal. X750. Only part of a fibre is shown, without sarcolemma. (Stohr.) 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 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 break? 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 com- plete disappearance of the fibre. From the free sarcoplasm new THE ^MUSCULAR SYSTEM 215 muscle fibres are formed. In Ihe 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. 126). 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 oft' at right angles. These in ;^ t y Sarcoplasm Muscle nucleus Transition zone Sarcolemma ' ,'/' Tendon fibre bundles Tendon nucleus Fig. 130. — Junction of Muscle and Tendon showing Continuity of Fibrils and Ex- tension of Sarcolemma beyond the Limits of Cross Striations. Rectus abdominis of frog. X750- (Stohr.) turn give rise to an anastomosing capillary network with elongated^ meshes, which surrounds the individual 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 con- nective 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. 216 THE ORGANS Nerves. — The terminations of nerves in muscle and tendon are described under nerve endings (page 440). 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 formalin-Miiller's fluid (tech- nic 6, p. 7), and harden in alcohol. Stain transverse sections with ha^matoxylin- picro-acid-fuchsin (technic 3, p. 21) and mount in balsam. (2) Junction of Muscle and Tendon. — Any muscle-tendon junction may be selected. Fix in formalin-Miiller's fluid, keeping stretched by means of a weight attached to the lower end. Cut longitudinal sections through the muscle- tendon junction, stain with hsematoxylin-picro-acid-fuchsin, and mount in bal- sam. 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 prepa- ration in the fixative removing the muscle-tendon from the bone after fixation. 4 9v V I CHAPTER XV GLANDS General Structure and Classification Attention was called in describing the functional activities of cells (page 51) 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 delinite 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 which result in the production within itself of a substance which is to be used outside the cell. The appearance which this cell presents depends upon the stage of secretion. It is thus possible to differen- tiate 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 epithelium 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 substance which does not stain Hke cytoplasm, but reacts to hasmatoxylin. 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 sur- rounds 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 difi'er as to the further behavior of this cell. According to some, its life history is now ended, and its place is taken by other cells which pass through the same process. Others 217 218 Tin: ORGANS believe that in most cases the cell is reconstructed from the nucleus and unchanged cytoplasm, and again passes through the process of secretion. In stratified epithelium secretion may begin while the cell is still deeply situated, but is completed only as the cell reaches the surface, where its mucus is to be discharged. In describing the vital properties of cells (p. 50) attention was called to the fact that all cells take up from the surrounding blood and lymph substances required for their own nutrition, and give off waste products (excretions). In such sense all cells secrete and excrete. What distinguishes the gland cell is that in addition to carrying on its own metabolism (p. 51) it manufactures a specific substance not for its own use ^- — -Diagram of Section through an Incisor, showing Blood-vessels and Peridental Membrane. (Noyes.) THE DIGESTIVE SYSTEM 235 Organic matter, 28.01 Calcium phosphate and fluoric!, 66.72 Calcium carbonate, 3-36 Magnesium phosphate, i . 18 Other salts, 0.73 Dentine constitutes the bulk of the tooth and is peculiar in that it contains canaliculi, dentinal canals (Figs. 139 and 140, Dk), but no lacunae or bone cells. The latter are represented by the odonto- id ♦ L**- ^'-i- '.■'•. .'i » j^;-^*? ^* • •'.. ^- *# « '♦.** ♦r*^ ^7^^ 1*** -fee •■ x: ^% '■ ip* st If ,■/■■■/* * ■■ -> Fig. i39.^Cro33 Section through Root of Human Canine Tooth (X25) (Sobotta), showing relations of pulp cavity, dentine, and cementum. P, Pulp cavity; D, dentine; C, cementum; K, Tomes' granular layer. blasts of the pulp, which, as already noted, lie at the inner side of the dentine, into the canaliculi of which they send the dentinal jihres. Dentine is non-vascular. The dentinal canals begin at the dental pulp, into which they open and where they have a calibre of 2 to 5/*. They pass outward radially, to the limit of the dentine, and, while taking different directions in different parts of the dentine, are essen- 236 THE ORGANS tially parallel. In their passage through the dentine the dentinal canals describe two series of curves, known as primary and secondary curves. The primary curves take the form of an elongated S from pulp to enamel or cementum, the secondary are twistings of the canals (drawn out corkscrew). These twistings are very fine as many as 200 turns having been described in the length of a canal. In their passage through the dentine the main canals gradually grow smaller until their diameter is from 0.5 to in. They give off minute side branches from 0.3 to o.6/( in diameter, which leave the main tubules KH Dk Fig. 140. — From Longitudinal Section through Root of Human Molar Tooth (X200) (Sobotta), showing junction of dentine and cementum. C, Cementum; D, dentine; A', Tomes' granular layer; Dk, dentinal canals; A'//, lacunse of cementum. at almost right angles, but soon turn slightly outward. They anasto- mose 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 neigh- boring tubules. Some of the tubules have quite extensive terminal branchings, other tubules have only two or three end branches. A few tubules run sHghtly beyond the limits of the dentine into the enamel. The arrangement of the dentinal canals and their branches differs in different parts of the tooth. In the crown there arc few large branches and the main canals show distinct primary and second- ary curves, most of them ending blindly in brush-like branchings THE DIGESTIVE SYSTEM 237 just under the enamel, but some continuing over a short distance into the enamel where they lie in the cement between the prisms. In the root the canals have an almost straight direction (without primary curves). Large branches are more numerous and the main canals have a somewhat more irregular arrangement. They probably do not pass over into the cementum, but end at the granular layer. The dentine immediately around a dentinal canal is harder and more dense than elsewhere and forms a sort of sheath for the canal — Neumann^ s dental sheath. Between the dentinal canals is a calcified ground substance, in which are connective-tissue fibres running in a direction parallel to the surface of the pulp, this corresponding, as in bone, to the deposition of the dentine in successive layers. In a longi- tudinal section of the dentine of the crown, lines are seen running parallel to the surface of the pulp. They are known as the lines of Schreger and are probably due to irregularities in deposition of the dentine. 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. 141, Jg). They are filled with a substance resembling uncalcified dentine. The inter- globular spaces do not interrupt the dentinal 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. 140, A'). In the root of the tooth this layer is quite thick, separating the cementum from the dentine. Its spaces or lacunas 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. 140) 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 dentinal tubules extending to, and some of them entering slightly, (see above) the enamel.-^ ^Distinction is sometimes made between primary and secondary dentine. Primary dentine is that dentine normally formed during the development of a tooth and corre- sponds to the description given. Later under some stimulus, e.g., excessive wear, for- mation of dentine again takes place, in which case the canals are often much more ir- regularly arranged; this is known as secondary dentine. 238 THE ORGANS The ENAMEL covers the exposed part or crown of the tooth and is the hardest substance in the body. It is thickest over the crown and gradually decreases in thickness along the sides of the tooth until it reaches the neck where it stops. 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 prisons (Fig. 141, Sp) — which take a sHghtly wavy course through the entire thickness of the enamel. The prisms are attached 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 em- bryo they show a longitudinal fibrillation. Rather indistinct parallel lines (the Imes of Retzins) cross the enamel prisms. They probably represent the deposition in layers of the lime salts, although they are considered by some as artefacts. The enamel is covered by a very thin apparently structureless membrane, the cuticula dentis. The CEiviENTUM (Fig. 140, C) covers the dentine of the root in a manner similar to that in which the enamel covers the dentine of the crown (Fig. 137, i and 3). It forms a thin layer at the neck, but in- creases in thickness as the deeper part of the root is reached. Cemen- tumis hone tissue. It contains /acz/;z«and hone cells. These vary much more in size and shape than do those of bone, are very irregularly distributed and may be absent from considerable areas. From the lacunae radiate canalicuH, but there is no distinct lamellation and no Haversian systems excepting in the large teeth of the larger mammaUa, and in the teeth of the aged, where they may be present. Channels, similar to Volkmann's canals in bone, not surrounded by concentric lamellae, but serving for the passage of blood-vessels, are quite fre- quent in the thicker portions of the cementum. The ground sub- stance of the cementum is continuous with that of the dentine and many canahcuK of the former open into the interglobular spaces of the latter. Many uncalcilied Sharpey's fibres penetrate the cementum. THR DKIKSTIVE SYSTEM 239 The Peridental Membrane. — SurrouiKling the root of the tooth and filling in the space between it and the wall of I lie alveolus is a layer of connective tissue which is known as the peridental membrane (Fig. 13S). It attaches the tooth to the alveolus, attaches the teeth to each other, supports the free margin of the gum and holds it to the tooth, and serves for the transmission of vessels and nerves. It consists of dense white fibrous tissue with few or no elastic fibres. In general it resembles periosteum and has been described as a reflec- tion of the alveolar periosteum upon the root of the tooth. The fibres fall into two classes, long fibres which pass from the ccmcntum to the coarser connective .^A Bk ■k s Fig. 141. — From Longitudinal Section of Crown of Human Premolar (X200) (Sobotta), showing junction of enamel and dentine. S, Enamel; D, dentine; Sp, 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. tissue of the gum, to the alveolar wall or to the cementum of an adjacent tooth, and short fibres which fill in the interstices between the long fibres. IMany of the long fibres are continued as calcified fibres into the cementum on the one hand and into the bone of the alveolus on the other. They are analogous to Sharpey's fibres of bone (p. 197). As they enter the cementum or bone, the fibres are grouped in bundles but in the central portion of the membrane these bundles break up and their fibres interlace in all directions. The long fibres differ in direction in different parts of the membrane. Those which spring from the cementum near its junction with enamel, pass out at right angles and then turn sharply toward the free margin of the gum which they support. Passing toward the apex, the next fibres instead of bending toward the free margin of the gum pass out at right angles and blend with the gum connective tissue. xAll fibres 240 THE ORGANS to the gums are more developed on the lingual than on the labial side. Between the teeth, fibres of this level pass from the cementum of one tooth to that of the next adjacent, here also supporting the free margin of the gum. Still further apically the fibres are grouped in coarser bundles to form the so-called dental ligament, and pass over or into the edge of the alveolus; between the teeth into the edge of the septum or into the cementum of the adjacent tooth. From the dental ligament for about one-third the depth of the root, rather coarse bundles of fibres pass from cementum almost straight to alveolar wall. For most of the remaining two-thirds, the direction of the fibres is away from the apex, the bundles tending to break up and to radiate to a more extended inser- tion into the alveolar bone. The apical fibres also radiate to their insertion into bone. Among the connective-tissue fibres are found fixed connective-tissue cells, groups or cords of cells which are apparently epithelial, and, during development and sometimes sparingly later, osteoblasts, osteoclasts, and cementoblasts. The fixed connective-tissue cells are described on p. 87. The epithelioid cells are mostly arranged in cords which anastomose to form a network. The cells themselves are granular and have oval or round nuclei rich in chromatin. In some places their arrangement resembles tubules, but whether they have a glandular character is not known. Osteoblasts and osteoclasts are the same as in developing bone (p. 202). The latter are present wherever absorption is taking place. They are especially numerous during the absorption of the roots of the milk teeth to make way for the permanent set. Cementoblasts are cells which lie against the cementum and are analogous to the osteoblasts, from which however they differ morphologically. They are flattened and their protoplasm fills in the spaces between the ends of the fibres so that the cell bodies show the indentations of the fibres lying against them. From the cementoblasts processes extend into the cementum in much the same manner as processes of bone cells extend into their canaliculi. Blood-vessels. — The arteries which supply the tooth and peri- dental membrane, enter the apical portion of the latter from the ad- jacent bone of the alveolus (Fig. 138). On entering the membrane the vessels divide into two main sets one of which passes through the foramina of the apex to supply the dental pulp, while the other passes along the outside of the tooth in the peridental membrane which it suppHes (Fig. 138). The vessels in the membrane anastomose with vessels from the bone of the alveolar wall and at the margin of the alveolar cavity with vessels from the gums. From the capillary net- work veins follow the arteries back to the apical portion of the mem- brane and into the bone at the bottom of the alveolus. The arteries to the pulp run mainly through its centre giving off branches which form a capillary network which is especially rich at the periphery of the pulp (Fig. 142). From these capillaries arise veins which pass THE DIfiKSTIVE SYSTEM' 241 back through the apical foramina into the peridental membrane where they unite with veins from the membrane and pass into the bone of the alveolus. Enamel, cementum, and dentine are non-vascular. A marked feature of the pulp vessels is the thinness of their walls, ves- sels of much greater calibre than are usually classed as capillaries having walls of capillary thinness. Lymph Vessels. — Most author- ities have denied the existence of definite lymph channels in the pulp. Schweitzer on the other hand describes an arborization of small lymph vessels in the pulp of the crown, converging to a few larger lymph vessels in the root pulp and accompanying the blood- vessels through the foramina of the apex. Nerves. — The distribution of nerves to the tooth follows quite closely that of the blood-vessels. Bundles of medullated fibres from the bone at the bottom of the alveolus enter the peridental mem- brane just beneath the apex where they divide into two main sets, one of which follows the vessels of the membrane while the other passes with the vessels through the apical foramina to the dental pulp. The membrane surrounding the tooth also receives nerves from the bone of the side wall of the alveolus. The branches to the pulp pass up through its center, giving off branches which are mostly non-medullated and which radiate toward the periphery where they form a plexus in the layer of Weil just 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.^ ^Noyes calls attention to the extreme sensitiveness of dentine in spite of the fact that no nerv-e fibres have been demonstrated in its canals, and believes that sensation is carried to the nerve fibres through the processes (dentinal fibres) and cell bodies of the odontoblasts. 16 Fig, 142.- •Diagram of Blood-vessels of Pulp. (Stowell). 242 THE ORGANS -^l^iJ.- • Fig. 143- Fig. 144. L ' '■^•— " '"*'^J vrs.: ■•^r- 6 5// . V^>^ i^ ■■£-:yW\ '.■:}■ - • -/-i- \l^^-^:i/ ]•..,- -. f:S-^ ;7> - .•■/ irr: ;i WC^''r'\ ^\-_ ^■-■~'\\ i-'"- A,- •,>*.■ '^^^/i .- i' o Fig. 145. Fig.' 146. Figs. 143, 144, 145, 146. — Four Stages in the Development of a Tooth (from lower jaw of sheep embryo). (Bohm-Davidofif.) Fig. 143, Beginning of enamel organ show- ing connection with epithelium of mouth; Fig. 144, Later stage showing same with first trace of papilla; Fig. 145, Later stage showing papilla well formed, the ditTerentiation of the enamel pulp and of the inner and outer enamel cells can be seen; odontoblast appearing along periphery of the papilla; Fig. 146, shows also beginning enamel organ of permanent tooth; Figs. 143, 144, 145, X no, Fig. 146, X40. a, Epithelium of mouth; h, its basal layer; c, superficial cells of enamel organ; d, enamel pulp; p, dental papilla; s, enamel cells; 0, odontoblasts; S, enamel organ of permanent tooth just beginning to differentiate; v, remains of enamel ledge of milk tooth: u. surrounding connective tissue. THE DIGESTIVE SYSTEM 243 Development. — The enamel of the teeth is of ectodermic origin, the re- mainder of mesodermic. The earliest indication of tooth formation occurs about the seventh week of intra-uterine hfe (embryos 12 to 15 mm.). It consists in a dipping down of the epithelium covering the edge of the jaw into the underlying connective tissue (mesoderm) where it forms the dental shelf, or common denial 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 epithelium of the dental shelf is at first of uniform thickness. Soon, however, at intervals along the outer side of the dental shelf, the cells of the shelf undergo proliferation and form thickenings, ten in .-«>; V Fig. 147. — Developing Tooth from Three-and-one-half-months' Human Embryo. X65. (Szymonowicz.) a, Epithelium of gums; b, neck of enamel organ; c, dental germ of permanent tooth; d, bone of lower jaw; e, dental papilla;/, inner enamel cells; g, enamel pulp; h, outer enamel cells. 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 (embn,-os of about 40 mm.) an invagination of the underlying con- nective 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. 145), over which the tissue of the 244 THE ORGANS special dental germ forms a sort of cap, the latter being known from its sub- sequent 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 of the original invagination. The next step is the almost complete separation of the special dental germs and ridge from the surface epithelium (Fig. 146), and the formation around each special dental germ of a vascular membrane, the denial sac. The attenuated strand of epithelial cells, which Epithelium of mouth } Enamel cells Uental sac Bone of jaw- Blood-vessel Remnant of enamel pulp Papilla Fig. 148. — Longitudinal Section of a Developing Tooth of a New-born Puppy. (Bonnett.) Late Stage- still maintains a connection between the dental germs and the epithelium of the gums, is known as the neck of the emanel organ and it is from this that an extension soon occurs to the inner side of the dental gerrris of the milk teeth, to form the dental germs of the permanent teeth (Fig. 147, 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 fa?tal existence the germs of allmilk and of some permanent teeth. The ENAMEL is formed by the enamel organ. At the stage represented in Fig. 149, it consists of three layers: (i) The outer enamel cells, somewhat flat- THE DIGESTIVE SYSTEM 245 tened; (2) the inner enamel cells, high columnar epithelium; (3) a layer of enamel pulp, situated between the other layers, and consisting of stellate anastomosing cells with considerable intercellular substance (Figs. 149 and 150)- A mem- brane, 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 cement substance, beginning in the ends nearest Enamel Dentine | Enamel prisms Outer Ij^^^gl __ Inner J cells Enamel pulp Cuticle 1 of Basal memb. J enamel cells Fig. 149. — Section through Border of a Developing Tooth of a New-born Puppy. (Bennett.) 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 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 grows. It has been suggested that it may furnish nutrition or serve as an avenue through which nutrition reaches the non-vascular enamel organ. It may serve as an area of least resistance through which the tooth grows. 246 THE ORGANS b '■ The DENTINE is the first of the dental tissues to become hard. Both dentine and pulp develop, as noted on p. 243, from the mesoderm of the dental papilla. When the latter is first formed it is of the same structure as the surrounding mesoderm with which it is continuous, 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 peripheral cells of the pulp, those lying nearest the enamel organ, differentiate from the rest of the pulp to form a single layer of columnar or 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 anastomoses with other cell processes. These cells are analogous to the osteoblasts of de- veloping bone and like them appear to determine the deposition of lime salts. The lime salts are first laid down in a membrane-like structure — the mem- brana preformativa — which the odonto- blasts apparently form between them- selves 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 internal 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 in- terglobular spaces. As calcification pro- ceeds the odontoblasts do not become fe^£Si&>iS&isiai:^rsi* V a'.'Ji Fig. 150. — From Cross-section through a Developing Tooth. X 720. (Bohm and von Davidoif.) Note close relationship between odontoblasts and tissue of dental pulp, a, Dental pulp; b, odontoblasts; c, dentine; d, inner enamel cells; e, enamel pulp. enclosed within the dentine as do the osteoblasts within bone. They leave merely long slender processes, the dentinal fibres, lying in minute channels through the dentine, dentinal 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, persist, not only throughout embr>'onic but through adult life. While the tooth lies within the gum, the somewhat condensed connective tissue which surrounds it constitutes the dental sac. As the germs of the milk teeth develop, the dental shelf broadens by extend- ing inward toward the tongue. Along this inner margin appear the germs of the permanent teeth, the development of the various teeth structures from the germs being identical with the process described in the case of the milk teeth. THE DIGESTIVE SYSTEM 247 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 permanent 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 (w-isdom teeth) molars do not appear 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. 207), without the formation of Haversian systems. TECHNIC (i) Teeth are extremely difficult organs from which to obtain satisfactory material for study. Sections of hard (undecalcified) and of decalcified teeth may be prepared in the same manner as sections of bone — technics i, and 2 p. 202. 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-jNIiiller's fluid (technic 6, p. 7), hardened in alcohol, and decalcified (page 10). Subsequent treatment is the same as for developing bone (technic i, p. 208). The Pharynx The wall of the pharynx consists of three coats — mucous, muscu- lar, 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 cihated, 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 184). Beneath the stratified squamous epithelium the stroma is thrown up into numerous low papillce. These are absent in regions 248 THE ORGANS 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 Hmit, 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. 228. 11. THE FOREGUT The (Esophagus viThe walls of the oesophagus are continuous with those of the phar- ynx and closely resemble the latter in structure. They consist of four layers, which from within outward are mucous, submucous, muscular, and fibrous (Figs. 151 and 153). 1. The mucous membrane resembles that of the pharynx except that beneath the stroma is a well-developed muscularis miicosce com- posed of smooth muscle cells arranged longitudinally. The muscu- laris mucosae forms a complete coat only in the lower part of the oesophagus. The epithehum 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 the upper portion of the oesophagus this coat is composed of striated muscle fibres; in the middle portion, of mixed striated and smooth muscle. In the lower portion there are THE DIGF.STI\'K SYSTEM 249 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. 151. — Transverse Section through Wall of Dog's (Esophagus. X18. (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 obHquely 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 250 THE ORGANS profusely. Some contain both chief and acid cells, others only chief cells (see stomach, page 254). 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 nerves in the oesophagus is similar to their distribution in the mouth (p. 227). 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. 4 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 6, 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 different levels should be taken: one near the upper end, one at about the middle, and the other in the lower third. Stain with haematoxyUn-eosin or haematoxylin-picro-acid-fuchsin (technic i, p. 20; 3, p. 21) 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. 152). These from the lumen outward are mucous, submucous, muscular, and serous. I. The mucous membrane (Fig. 152) consists of surface epithe- lium, glands, stroma, and muscularis mucosas. The surface epithe- lium 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 diffuse 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 mucosce consists of smooth muscle cells, which have a generally longitudinal arrange- ment. Where, however, the muscularis mucosae is thick there are usually two distinct layers — an inner circular and an outer longitudi- THE DIGESTIVE SYSTEM 251 nal. Folds of considerable extent occur in the mucous membrane. Those of the 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. ffi^-}}* Fig. 152. — Diagram of Structure of Wall of Gastro-intestinal Canal (longitudinal section). A, Mucous membrane; a, glands; b, surface epithelium; c, goblet cells; rf, stroma; e, inner circular,/, outer longitudinal layers of g, muscularis mucosae. B, Sub- mucosa. C, Muscular coat; h, ;its inner circular layer; j, its outer longitudinal layer; i, intermuscular connective- tissue septum. D, serous coat; k, its connective-tissue laj-er; /, its endothelial layer. 2. The submucosa (Fig. 152) is a loose connective- tissue structure. It contains the larger blood-vessels, lymphatics, and nerves. 3. The muscular coat (Fig. 152) consists of two layers of smooth muscle, which in the intestine are sharply differentiated into an inner circular and an outer lo?igitudinal. In the stomach the direction of the layers of the muscular coat is less definite. A thin layer of 252 THE ORGANS connective tissue separates the two layers of muscle. From this septa extend into the muscle tissue, separating it into bundles. 4. The serous coat (Fig. 152) is the visceral layer of the perito- neum. It consists of a thin layer of connective tissue covered by a single layer of mesothehum. Along the attachment of the mesentery the serous coat is wanting. .^ 0.C .;^CS^ W0!^':>?9rp ■M ■^^^■.■<^ Fig. 153. — Section through Junction of (Esophagus and Stomach of Man. X121. (Schafer.) Oe, (Esophagus; M, stomach; cd, cardiac glands; wd, dilated ducts of cardiac glands; 5, stroma; E, stratilied squamous epithehum of oesophagus; mm, muscularis mucosae; cd, irregularly cut tubules of cardiac glands; dd, cardiac glands in lower end of the oesophagus; u, limit of stratified oesophageal epithelium. 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. The Stomach At the junction of oesophagus and stomach there is an abrupt transition from the stratified squamous epithehum of the former with its smooth surface to the simple columnar epithelium of the latter THE DKiESTIVE SYSTEM 253 with its elevations and depressions. In the deeper structures the Hne of demarcation is not so abrupt, the muscularis mucosae of the oesoph- agus being continuous with that of the stomach, and glands of the stomach type extending up under the stratified epithelium of the oesophagus (Fig. 153). I . The mucous membrane of the stomach is folded into ridges or rugos, the height and number of which depend, as already noted, upon the degree of distention of the organ. The rugae are most prominent Fig. 1 54. — Outline Diagram of Stomach to showLocation of Different Kinds of Glands. ■c, cardia; p, pylorus; vvvvvv, cardiac glands; , fundus glands; + + + + + , pyloric glands; oooooo, intestinal type glands (of Lieberkuhn). (Jouvenal.) in the collapsed organ, almost absent when the organ is fully dis- tended. In addition to the rugae the entire mucous membrane is studded with minute depressions barely visible to the naked eye, the so-called gastric pits (Fig. 156, Mg). These mark the openings of the gastric glands. In the fundus they are comparatively shallow, ex- tending through about one-fifth the thickness of the mucosa; in the pylorus the pits are much deeper, extending through half or more of the thickness of the mucous membrane (compare Figs. 156 and 160). The Epithelium. — This is of the simple columnar type, covers the entire surface of the gastric mucosa and extends down into the pits (Fig. 156). The cells are of the high, clear, mucous type (Fig. 157, M and M'). The end of the cell toward the lumen is clear, usually consists mostly of mucus, and consequently stains lightly. There 254 THE ORGANS is no such distinct cuticle as in the intestine. The basal end of the cell contains the spheroidal, oval, or sometimes flattened nucleus, is granular, and takes a darker stain. The amount of mucus in the cell depends upon its functional condition. The cells rest upon a distinct basement membrane. The Gastric Glands. — Extending from the bottoms of the pits, their epithelium continuous with that of the pits themselves, are the gastric glands. These are of two main kinds, fundus or peptic glands, distributed through the greater part of the gastric mucosa, and py- loric glands, confined to the immediate region of the pylorus. Gastric pits 9^^^^HKr%^iS^HH^L^i^^Bi ^^^'"^ ^ ' ;FiG. 155. — Surface View of Mucous Membrane of Stomach showing gastric pits (Spalteholz). The fundus glands (Fig. 156) are simple, sometimes branched, tubular glands, of which from three to seven open into each gastric pit. They extend through the entire thickness of the stroma, to the muscularis mucosae. Each gland consists of (i) a mouth opening into the pit; (2) a constricted portion, the neck; (3) the body or main portion of the tubule; and (4) a slightly dilated and bent bhnd extremity, the fundus (Fig. 156). The mouth marks the transition from the higher epithelium of the pit to the low cuboidal of the neck (Fig. 157, h). In the body and fundus of the gland two types of cells are found: (a) chief cells (central, peptic, or adelomorphous), and (b) parietal cells (acid, oxyntic, or delomorphous). The chief cells (Figs. 157, 158) are the more numerous. They are of the low columnar type, often pyramidal with apices directed toward the lumen. Their bases rest either on the basement membrane or against the parietal cells. The appearance which these cells present THE DIGESTIVE SYSTEM 255 depends upon their lunclional cundiLion (p. 277). They usually appear clear and granular and take a light stain. The parietal cells (Fig. 157), are oval or polygonal in shape, and ^^U lie here and there against the base- ment membrane. The nucleus is spherical, somewhat larger than that of the chief cell, and is usually situ- ated at the center of the cell. There may be two nuclei. The protoplasm is finely granular^ and in the fresh unstained condition appears clearer than that of the chief cells. The parietal cells stain intensely with the aniline dyes with the result that in Fig. 156. Fig. 157. FiG 156. — ^Vertical Section through the Mucous Membrane of the Fundus of the Stomach. X85. (Kolliker.) Mg, Gastric pits; /?, neck; k, body; g, fundus of peptic glands; h, chief cells; h, parietal cells; m, muscularis mucosae. Fig. 157. — Cross-sections at Various Levels of Peptic Glands of Stomach. X400. (Kolliker.) M, Section through gastric pit near surface; M', section through gastric pit near bottom; //, mouth of gland; k, neck; g, body near fundus; a, chief cells; b, parietal cells. stained specimens the two kinds of cells are in marked contrast, the parietal cells being much darker than the chief cells. Although lying against the basement membrane and frequently pushing it out so as to form little protuberances beyond the even line of the gland tubule. 256 THE ORGANS the parietal cells always maintain a connection with the lumen. This is accomplished by means of little clefts between the chief cells, intercellular secretory tubules, which extend down to the parietal cells, ly means of the method of Golgi may be demonstrated not only the intercellular secretory tubules, but also the fact that upon reaching the cells these are continuous with a network of minute spaces within the cell — the intracellular secretory tubicles (Figs. 158 and 159). Parietal ^/-^ iiu^^^€ © c/i.m c . t^c.ft.m Fig. 158. — Sections through Different Parts of a Fundus Gland of Human Stomach. A, mouth of gland; cells resemble surface cells except that they are shorter. B, neck of gland; cp, principal cells; cpm, mucous cells; ch, parietal cells, one contain- ing two nuclei. C, middle portion of gland; cp, chief cells; ch, parietal cells. D, deeper part of gland; cp, chief cells containing secretorj- granules, and at their bases ergastoplasm filaments {erg) : at c are shown intracellular secretor}^ canals pene- trating base of cell; cb, parietal cells, one of which shows canaliculus leading to lumen of gland (i), the latter being cut twice in section owing to its irregulaf course. E, blind end of gland; cp, chief cells with secretion granules and ergastoplasm fila- ments (erg); cb, parietal cells, one containing three nuclei; cpm, mucous cells; /, lumen of gland. X250. (Prenant.) cells are not distributed uniformly throughout the gland, but are most numerous in the body, where they frequently almost obscure the chief cells. In the fundus of the gland parietal cells are less nu- merous. For this reason and because of the wider lumen of the fundus, transverse and longitudinal sections of this part of the tubule are most satisfactory for the study of the relations of the two kinds of cells (Figs. 156 and 157). Mitosis is most active at the junction of the neck and body of the tubule which has consequently been desig- nated the "growing point" of the tubule. THE DIGESTIVE SYSTEM 257 - d Lying near the basement membrane among the bases of the colum- nar epithelial cells are small spherical or irregular cells with dark nuclei. These are young epithelial cells which from their function are known as "replacing cells" (see page 75). The PYLORIC GLANDS (Figs. 160 and 16 j) are simple branched tubular glands, several of which open into each of the deep pyloric pits. These pits occupy a much greater pro- portion of the thickness of the mucous membrane than do the ])its of the fun- dus glands (compare Figs. 156 and 160) and the proportionate depth occupied by the glands themselves is correspond- ingly less. The pyloric glands, though short, are quite tortuous, so that in sec- tions the tubules are seen cut mainly transversely or obliquely. In most of the pyloric glands but one type of cell is found. These resemble the chief cells of the fundus, but present a more uni- Fig. isg.-Longitudinal Section ' J^ \ of fundus of Gland from rj-lonc form appearance, probablyMue to the End of Dog's Stomach. (Golgi , c • . 1 11 A • ^1 method. See <;, p. 20.) a, Lumen absence of parietal cells. As m the of gland; i, intracellular canals in fundus, "replacing cells" lie between parietal cells; c cut-off portion of . . parietal cell; a, chief cells; e, inter- the bases of the columnar epitheHal cellular canals leading from lumen cells. Parietal cells are not always en- °^ ^land to canals in parietal ceUs. tirely absent, but occur here and there in the pyloric tubules, espe- cially near the fundus. The transition from fundus to pylorus is not abrupt, but is marked by a "transitional border zone," in which fundus and pyloric glands are interiTiingled, and in which are also found single glands which resemble in structure both fundus and pyloric. In the transition zone between oesophagus and stomach are found glands which resemble those found in the lower end of the oesophagus (p. 250). Their cells are clear resembling those of the pyloric glands. On account of their location they have been designated cardiac glands. Those nearest the cjesophagus are lined with clear cells and re- semble closely the pyloric glands. As one passes further from the oesophageal junction, cells with coarse granules begin to make their appearance among the clear cells and, assuming the character of parietal cells, become more and more numerous, the glands thus passing over by gradual transition into typical fundus glands. There 258 THE ORGANS ^a b are also found in the stomach glands which are apparently identical in structure with those of the intestine (p. 265). ]\Iost of these resemble the glands of Lieberkiihn and are distributed in small groups mainly in the pylorus and lesser curvature. Glands resembling B runner's glands have also been described. The STROMA (Figs. 156 and 160) or tunica propria, in which the glands are embedded, consists of mixed fibrillar and reticular connective '.n^\:ihX- f Fig. 160. Fig. 161. Fig. 160. — Vertical Section through Mucous ^Membrane of Pyloric End of Stomach. X85. (KoUiker.) Mg, Gastric pit; b, blood-vessel in stroma; d, longitudinal section of body of gland; m, muscularis mucosae. Fig. 161. — Pyloric Gland from Vertical Section through Wall of Dog's Stomach. (Ebstein.) m, Gastric pit in which are seen some transversely cut cells; n, neck of gland; /, fundus cut transversely. tissue infiltrated with lymphoid cells. In the fundus of the stomach the glands are so closely packed that the stroma is reduced to thin strands, which pass up between the glands and also separate them from the muscularis mucosae. In the pylorus the glands are more widely separated and the stroma is correspondingly greater in amount. THE DIGESTIVE SYSTEM 259 In both fundus and pylorus thicker strands of stroma surround a num- ber of gland tubules, thus separating them into more or less well- defined groups. In addition to the diffuse lymphatic tissue of the stroma, closely packed aggregations of lymphoid cells are found in the shape of distinct nodules, known as ^^ solitary follicles." These occur throughout the entire gastric mucosa, but are most numerous in the pylorus. The nodules are usually egg-shaped, their apices lying just beneath the epithelium, their bases resting upon the muscu- laris mucosae. Less commonly they lie partly in the submucosa. Over the nodules the epithelium is more or less infiltrated with migra- tory leucocytes. Most of the nodules contain germinal centres around which the lymphoid cells are more closely packed than else- where (see page 173). The MUSCULARis MUCOSAE (Figs. 156 and 160, m) may consist of a single layer of smooth muscle with cells arranged longitudinally or obliquely, or there may be two distinct layers, an inner circular and an outer longitudinal. From the muscularis mucosae single cells and groups of cells extend into the stroma between the gland tubules. 2. The submucosa consists of connective tissue, loosely arranged, many elastic fibres, and some fat. It contains the larger blood- vessels and nerves, including the plexus of Meissner (p. 276). 3. The muscular coat is usually described as consisting of three layers, an inner oblique, a middle circular ,^nd an outer longitudinal. In the fundus the muscle bundles run in various directions, so that the division of the muscular coat into layers having definite directions can be made out only in the pylorus. 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 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 connec- tive 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 remove 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, mucous membrane side up, on pieces of cork, fix in formalin- IMiiller's fluid (technic 6, p. 7) or in Zenker's fluid (technic 10, p. 8), and harden in alcohol. Sections are cut as thin as possible, care being taken that the plane is such that 260 THE O the glands are cut longitudinally, stained with hcEmatoxylin-eosin (technic i, p. 20), 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 stomach may be filled with the fixative, the ends being tied, and then placed in a large cjuantity 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 preceding 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, p. 259. Examine unstained sections and sections stained with hajmatoxylin-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, p. 259. (5) For the study of the distribution of the blood-vessels sections of an injected stomach should be made. This is best accomplished by selecting a small animal, such as a rat or guinea-pig, and injecting in toto through the as- cending aorta, or by injecting only the hind part of the animal through the ab- dominal aorta. Technic, p. 25. in. THE MIDGUT The Small Intestine On passing from stomach to small intestine the rugaj of the former disappear, but are replaced by much more definite foldings of the mucosa, the vahula conniventes (Fig. 163). 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 obUque 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. 164 and 165). 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 valvulae conniventes and the vilh are characteristic of the small intestine. It is important to note that while the pits of the stomach are depressions in the mucous membrane, the intestinal vilU are definite projections above its general surface (Fig. 162). The wall of the intestine consists of the same four coats described rilK DinKSTIVK SYSTEM 2(il as constituting the wall of the stomach, mucosa, submucosa, mnscii- laris, and serosa. I. The mucosa, as in the stomach, is composed of a lining epithelium, stroma, glands, and Diuscularis mucosce. Of these, the epithehum, the stroma, and cells from the inuscularis mucosx-, are concerned in the formation of the villi. The VILLUS consists of a central core — a fold of the stroma — of mixed fibrous and reticular tissue infiltrated with lymphoid cells, and of a covering epithelium. The epithelium is disposed in a single layer, separated from the underlying connective tissue by a basement membrane. It consists of ■in^^m ^^j^^"^ _^&^ ^TS. Fig. 162. — 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 Lieberkuhn; s, secreting tubules of Brunner'js glands; g, pyloric glands; t, tubules of Brunner's glands in submucosa of stomach; m, muscularis mucosae. two apparently quite different kinds of cells, columnar cells Sind goblet or mucous cells. The former are the more numerous and are pecuhar to the intestine, while the mucous cells are identical with those found in other mucous membranes. The columnar cells are quite elastic and while .generally long and narrow, vary as to length and breadth as they adapt themselves to movements of the intestine. Their pro- toplasm is finely granular, its appearance depending somewhat on the activity of secretion and absorption. It frequently contains fat droplets. The nucleus is void and is usually situated in the basal third 262 THE ORGANS of the cell. A peculiarity of these cells is the striated free border, con- tiguous free borders uniting to form the so-called cuticidar membrafie (Fig. 167, c). By means of special technic these striations can be re- solved into delicate parallel lying rods or fibrils. They have been inter- preted by some as marking channels through the cuticle, by others as a fibrillar arrangement of the protoplasm. Scattered among the col- umnar cells are mucous or goblet cells (Figs. 166 and 167, b). These vary in appearance according to the amount of secretion which they contain. A cell at the beginning of secretion contains only a small amount of mucus near its free border. As secretion increases the % V«/. X X '*A WW X ^^M / — . 8- Fig. 163. — Vertical Longitudinal Section of Human Jejunum (X16) (Stohr), includ- ing two valvulae conniventes. a, Villi, in many of which the stroma has shrunken away from the epithelium leaving a clear space, XX. Lying free in the lumen of the gut are seen sections of villi cut in various directions, b, Epithelium; c, stroma; d, cr\'pts of Lieberkiihn; X, soHtary lymph nodule with germinal centre; e, tissue of sub mucosa forming centre of one of the valvule conniventes;/, submucosa; g, inner circular layer of muscle; k, outer longitudinal layer of muscle; i, Auerbach's plexus;/, serous coat. 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 sides by the cuticulae of the adjacent columnar cells. Opinions THE DIGESTIVE SYSTEM 203 differ as to the relation of the mucous to the cokimnar cell. Some authorities consider them two different and independent cells. The majority, however, look upon the mucous cell as a speciahzation of the Mouths of crypts Lymph nodules Villi Fig. 164. — Surface View of Small Intestine near upper end, showing villi and one solitary lymph nodule. X12. (Spalteholz.) / ~ I ff^.f Ik fit fx^ ., r ^¥ Fig. 165. — Vertical Section through Mucous Membrane of Human Jejunum. X80. (Stohr.) a and h, Artifacts due to shrinkage; c, intestinal crypts (Lieberkiihn); d, obique and transverse sections of crypts; e, stroma;/, epithelium; g, tangentially cut villi; h, muscularis mucosae; i, submucosa. 264 THE ORGANS columnar cell, the younger cells being indifferent as it were and able to become columnar cells or goblet cells as conditions may re- quire. Again opinions differ as to whether the mucous cell, having discharged its secretion, dies or re-secretes, or returns to the condi- tion of the columnar cell. Small spherical cells with deeply staining nuclei are found in varying numbers among and sometimes within the epithehal cells. ^ These are so-called wandering cells, migratory Srrz. Li©(jR^fe^c^- always occur on the side of the gut opposite 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 he 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 are not confined to the stroma, but usually spread out in the submucosa. The relation of the patch to the stroma and submucosa can be best appreciated by following the course of the muscularis mucosae. This is seen to stop abruptly at the circumference of the patch, appearing through- out the patch as isolated groups of smooth muscle cells. The nodules rarely remain distinct, but are confluent with the ex- ception of their apices and bases. It should be noted that both solitary nodules and Peyer's patches are structures of the mucosa, and that their presence in the submucosa is secondary. The muscularis mucosae (Figs. 165 and 171) consists of an inner circular and an outer longitudinal layer of smooth muscle. 2. The submucosa (Figs. 163, 165, 171) consists, as in the stomach, of loosely arranged connective tissue and contains the larger blood-vessels. It is ' " ' "' '■•••■•■••••••••• -••••••■•••■—■-^•_^_ Fig. 171 . — From Vertical Longitudi nal Sec- tion of Cat's Duodenum to show Brunner's Glands. (Larrabee.) a, Villus; b, epithe- lium; c, stroma; d, glands; e, muscularis mucosas; /, Brunner's glands; g, submucosa; free .from glands except in the /'- circular muscle layer. duodenum, where it contains the glands of Brunner (Fig. 171). These are branched tubular glands hned with a granular columnar epitheHum similar to that of the pyloric glands. The ducts are also lined with simple columnar epithehnin. They pass through the 268 THE ORGANS muscularis mucosae and empty either into a crypt of Lieberkiihn or on the surface between the vilK. Brunner's glands frequently occur in the pylorus, and it is not uncommon 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 sub-mucosa (see page 276). 3. The muscular coat (Figs. 163 and 171) consists of two well- defined layers of smooth muscle, an inner circular and an outer longitudinal. Connective-tissue septa divide the muscle cells into groups or bundles, while between the two layers of muscle is a con- nective-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 276). 4. The serous coat consists as in the stomach of loose connective tissue covered by a single la}'er of mesothelium. 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. 1 . The mucous membrane has a comparatively smooth surface, there being neither pits as in the stomach nor villi as in the small intestine (Fig. 172). The glands are of the simple tubular variety, are considerably longer than those of the small intestine, are almost straight, and extend through the entire thickness of the stroma. Owing to the closeness with which the gland tubules are packed, the amount of stroma is usually small. The surface cells (Fig. 172, 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. 173, 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 MUCOSA (Fig. 173, c) consists of an inner circular and an outer longitudinal layer of smooth muscle. 2. The submucosa (Fig. 172, e) consists of loosely arranged con- nective tissue. It contains large blood-vessels and the nerve plexus TTTK DrOKSTIVE SYSTEM 269 of Meissner (see page 276). Solitary lymph follicles occur throughout the mucous membrane of the large intestini'. W'liilc 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 171.) r ^rmm^'^^ ;;••.•^:.^■^^^U.■^^.V^■?r:•"■■";C•■^•^•■:^•^■.;'.:• ^'^- ;•••■•' ;v . •. .v;t-.-. *(-:..: ■' ■'"■(••■.' ■ rR3 Fig. 172. Fig. 173. Fig. 172. — From Vertical Longitudinal Section of Cat's Large Intestine. (Larra- bee.) a, Epithelium; b, stroma; c, fundus of gland; d, muscularis mucosae; c, submucosa; /, circular muscle layer; g, longitudinal muscle layer; h, sefous coat; i, Auerbach's plexus. Fig. 173. — From Vertical Longitudinal Section of the Mucous Membrane of the Human Large Intestine. (Technic i, p. 280.) a, Mucous (goblet) cells; b, fundus of a gland cut obhquely; c, muscularis mucosae; d, lumen of a gland cut longitudinally; e, stroma between the glands;/, leucocytes in the epithelium; g, stroma between fundi of glands and muscularis mucosae. 3 C comple arrange coh. I aris (Fig. 172) the inner circular layer only is ) tissue of the external longitudinal coat being hree strong, flat, longitudinal bands, the lineae bands the longitudinal muscular coat is either 270 THE ORGANS 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. J^For details see page 276.) 4. The serous coat consists, as in the stomach and small intestine, of loose connective tissue covered by a single layer of mesothelium. The Vermiform Appendix *^"' The vermiform appendix is a diverticulum from the large intes- tine. Its walls are continuous with those of the latter, and closely Oc>tl< il*^?' Fig. 1 74. — Transverse Section of Human Vermiform Appendix. (Technic 2, p. 275.) a, Mesoappendix; &, serous membrane (serosa); c, 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; i, stroma; j, glands opening into lumen and cut in various planes; musculans mucosae not i)rcsent. resemble them in general structure. There are the same four coats, mucous, submucous, muscular, and serous. I. The mucous membrane (Fig. 174) consists of epithelium, glands, stroma, and muscularis mucosae. The epithelit^m resembles THE DIGESTIVE SYSTEM 271 that of the large intestine. The glands vary in number, but are usu- ally much less closely packed than in the large intestine. They are most numerous in the appendices of infants and children. The glajid tubules (Fig. 174, j) 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 miiscularis mucosce 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 obHteration of the lumen. 2. The submucosa (Fig. 174, e) is similar to that of the intestine. 3. The muscular coat varies greatly, both as to thickness and as to the amount of admixture of fibrous tissue. The inneru;ircular layer (Fig. 174, d) is usually thick and well developed.- The outer longitudinal layer (Fig. 174, 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, be- tween which the outer coat is thin or wanting, is observed. 4. The serosa has the usual structure of peritoneum. The lymph nodules (Fig. 174, h) constitute the most conspicuous feature of the appendix. They He mainly in the submucosa. In children and young adults the nodules are oval or spherical; in later Ufe 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 Rectum I. The mucous membrane of the rectum has a structure similar to that of the large intestine (Fig. 175). 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 272 THE ORGANS so-called columncB rectales. A change in the character of the mucous membrane begins at the upper end of the columnae rectales. Here the simple columnar epithelium of the gut passes over into a stratified squamous epithehum, beneath which is a papillated stroma. The glands continue 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 225). gl Fig. 175. — ^lucous Membrane of Human Rectum, a, Superficial epithelium, com- posed almost wholly of mucous cells, the columnar cells lying between them being so compressed as to appear as thin dark lines; gl, lumen of gland of Lieberkuhn. X60. (Prenant.) 2. The submucosa is similar in structure to that of the large intestine. The muscularis of the rectum differs from that of the large intes- tine in that the longitudinal layer is continuous and thick. The serous coat is absent in the lower part of the rectum, being replaced by a fibrous connective-tissue layer, which connects the rec- tum with the surrounding structures. The Peritoneum, Mesentery, and Omentum The peritoneum (see also p. 169) is a serous membrane which lines the walls of the abdomen (parietal peritoneum) and is reflected over the contained viscera (visceral peritoneum). It consists of two THE DIGESTIVE SYSTEM 273 layers, a connective-tissue stroma and mcsollu'liuin. The stroma consists of loosely arranged connective-tissue bundles, which inter- lace in a plane parallel to the surface. There are numerous elastic fibres, especially in the deeper .'layer of the parietal peritoneum. There are comparativel\- frw connective-tissue cells. The mesothe- lium consists of a single layer of flat polygonal cells with bulging nuclei. The cells have in^gular wavy outlines, which are easily dem- onstrated with silver nwfrate (Fig. 28). The shape of the cells varies considerably according 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 i)u^toneum is freely movable, a considerable amount of loose conn(^i\e li^|Mc, rich in elastic fibres and containing varying numbers df fat ^P^ connects the peritoneum with the underlying tissue. This i^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 covered with peritoneum. It is reflected from the p6st-abdominal wall to the viscera, and serves to carry to these organs their blood- vessels, lymphatics, and nerves. In the case of the stomach, duo- denum, and large intestine, the mesentery is comparatively short, and the organs are therefore quite firmly fixed to the abdominal wall. In the case of the small intestine the mesentery is long and the intestine, 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. 28) 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 bundles are arranged in networks, the strands and meshes of which vary greatly in size and shape. The strands are covered by a single layer of mesothelium. Blood-vessels of the Stomach and Intestines The arteries reach the gastro-intestinal canal through the mesen- tery, give off small branches to the serosa, and pass through the muscular coats to the submucosa, where they form an extensive 18 274 THE ORGANS plexus of large vessels (Heller's plexus) (Fig. 176, 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 muscular coats, the other inward to supply the mucous membrane (Fig. 176). Of the former, the larger vessels pass directly to the intermuscular septum, where they form a plexus from which branches are given off to the .■i B C Fig. 176. — Scheme of Blood-vessels and Lymphatics of Stomach. X70. (Szy- monowicz, after Mall.) a, Mucous membrane; b, muscularis mucosae; c, submucosa; d, inner circular muscle layer; e, outer longitudinal muscle layer; A, blood-vessels; B, structure of coats; C, lymphatics. two muscular tunics. A few small branches from the larger re- current vessels also supply the inner muscular layer. Of the branches of the submucosa plexus which pass to the mucous membrane, the shorter supply the muscularis mucosae, while the longer branches* pierce the latter to form a capillary plexus among the glands of the stroma. These capillaries have a comparatively small diameter and are most numerous around the bodies and necks of the glands. * Stohr describes these small arterial branches to the stroma of the gastric mucosa as terminal or end-arteries each supplying a segment of mucosa from i to 2.5 m.m. in diameter. As the capillaries of one end-artery, do not anastomose with those of other end-arteries, any obstruction or destruction of one of these arteries results in death of the dependent area of mucosa. THE DIGKSTIVE SYSTEM 275 Thcv pass over into much larger capillaries which form a dense network just beneath the surface ci)ilheliuni. From the capillaries small veins take origin which pierce the muscularis mucosae and form a close-meshed venous plexus in the submucosa (Fig. 176). 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 villi (Fig. 178). Each villus receives one small artery, or, in the case of the larger villi, two or three small arteries. The artery passes through the long axis of the villus close under the epithelium to its summit, giving off a network of fine capil- laries, which for the most part lie just beneath the epithelium. From these, one or two small veins arise which lie on the opposite side of the villus from the artery. Lymphatics of the Stomach and Intestine Small lymph or chyle capillaries begin as blind canals in the stroma of the mucous membrane among the tubular glands (Fig. 176). 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. 178). These vessels unite to form a narrow- meshed plexus of lymph capillaries in the deeper part of the stroma, lying parallel to the muscularis mucosae. Vessels from this plexus pass through the muscularis mucosas and form a wider meshed plexus of larger lymph vessels in the submucosa. A third lymphatic plexus lies in the con- FiG. 177. — Diagram of wall of Stomach to show General Dis- position of Lymphatics a, Term- inal lymph channel in tissues separating glands; b, superficial lymphatic plexus in connective tissue surrounding bases of glands; c, submucous lymphatic plexus; d, intermuscular lym- phatic plexus; e, subperitoneal lymphatic plexus. (Cuneo.) nective 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 intermus- cular plexus, and then pierce the outer muscular layer to pass into the mesentery in company with the arteries and veins. 276 THE ORGANS 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 sym- pathetic gangUon cells to form the plexus myentericus or plexus oj Aiierhach. The dendrites of the ganglion cells interlace, forming a large part of the plexus. The axones are grouped together in small Fig. 178. — Scheme of Blood-vessels and Lymphatics of Human Small Intestine. (From Bohm and von Davidoff, after Mall.) a, Central lacteal of villus; h, lacteal; c, stroma; d, muscularis mucosae; e, submucosa;/, plexus of lymph vessels; g, circular mus- cle layer; h, plexus of lymph vessels; i, longitudinal muscle layer; j, serous coat; k, vein; I, arter}'; m, base of villus; n, crypt; 0, artery of villus; p, vein of villus; q, epithelium. bundles of non-medullated fibres, which pass into the muscular coats, where they form intricate plexuses, from which are given off club- shaped terminals to tne smooth muscle cells. From Auerbach's plexus fibres pass to the submucosa, where they form a similar but finer-meshed, more dehcate plexus, also associated with groups of sympathetic ganglion cells, the plexus of Meissner. Both fibres and cells are smaller than those of Auerbach's plexus. From Meissner's TITE DTCKSTIVE SYSTEM 277 plexus delicate fibrils pass to their terminations in submucosa, mus- cularis mucosee, and mucous membrane. Secretion and Absorption The secretory activities of ci)ithchal cells have already been men- tioned (page 217). The epitheHum of the gastro-intestinal tract must be considered as having two main functions: (i) The secretion of substances necessary to digestion; and (2) the absorption of the products of digestion. (i) Secretion. — The production of mucus takes place in the mucous or goblet cell, which is present throughout the gastro- enteric mucosa and, as already mentioned, probably represents a differentiation of the ordinary columnar epithelial cell. The chief cells, "peptic cells," of the stomach glands are compar- atively clear during fasting, with development of ergas- toplasm in their basal ends. With the onset of digestion these cells increase in size and become generally cloudy from development of gran- ules with reduction of the ergastoplasm. These gran- ules represent a pre-ferment or pepsinogen and with their discharge into the lumen of the gland, the cells again become smaller, clearer, and assume the resting condition. Bensley and Theohara have recently very clearly demonstrated the secretory chain in the chief cells from ergastoplasm or basal filaments through pepsinogen granules to pepsin. While some authorities still contend that a minor pepsin-forming role is played by the pyloric glands, the above facts together with the fact that activity of the chief cells (Fig. 180) is coincident with an increase in the pepsin found in the stomach, and that the amount of pepsinogen in the gastric mucosa is proportionate to the number of granules in the chief cells (Langley and Sewell) , may be accepted as proving these cells the main if not the sole producers of pepsin, and the granules as some stage in the elaboration of the ferment. As their name of "acid cells" Fig. 1 70. — Section through Glands of Fundus of Human Stomach in Condition of Hunger. X500. (Bohm and von Davidoff.) <7, Stroma; b, parietal cell; c, lumen; d, chief cell. 278 THE ORGANS would indicate, the parietal cells have been considered the source of the hydrochloric acid of the stomach. While doubt still exists as to the function of these cells, recent investigations make it probable that they secrete substances which as chlorides are transformed into hydrochloric acid by the action of the carbonic acid of the blood. According to some authorities other cells assist in the production of the stomach acid. The cells of Brunner's glands undergo changes during digestion, which are somewhat similar to those described as occurring in the chief cells of the stomach glands. By some they are beUeved to be concerned in the production of pepsin, by others to play a special role in the secretion of one or more of the intestinal ^MY \ffy^^A <»,. (V %\«^'s ■^i^i Fig. i8o. — Section through Glands of Fundus of Human Stomach during Diges- tion. X500. (Bohm and von Davidoff.) a, Lumen; b, stroma; c, chief cell; d, parietal cell. ferments. The only function of the surface epithelium and of that of the intestinal crypts which has yet been determined is the secretion of mucus. The cells of Paneth are typical gland cells containing secretion granules and probably produce a specific secretion. (See also page 265.) Whether this secretion is the so-called "inverse ferment" or "invertin^' which changes cane sugar into glucose and levulose or one of the other intestinal ferments has not been deter- mined. It has been known that pancreatic fluid is inactive toward albuminoids unless mixed with intestinal secretions. This is due according to Pavlow to a special ferment " enter okinasey According to some this is a secretion of the epithelium, to others it is elaborated THE DIGESTIVE SYSTEM 279 by eosinophile leucocytes which pass through the intestinal wall, and become a part of the intestinal secretion. (2) Absorption of Fat. — While various other products of diges- tion are absorbed by the intestine, the absorption of fat is the one most easily observed and the one of most interest from the histological © >'^- m^ dm® ®J %'--c!5i- -■■ © ^ ® o. ..;;«:/'^r^ . © '--MP uh © ^^fVltmirf^^^^-: b f©^ ■^^^-'^^^s >•'■• .- Fig. 188. — Section of Human Pancreas. X112. (KoUiker.) az), Alveoli; a, inter- lobular duct surrounded by interlobular connective tissue; L, islands of Langerhans; v, small vein. size of these zones depends upon whether the cell is in the active or resting state (compare Fig. 190, 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 are increased in number to such an extent that they almost fill the cell, while after prolonged secretion they are again almost absent. The cell now returns to the resting state in which the two zones are about equal. The in- crease and disappearance of the granules are marked by the appearance of the fluid secretion of the gland in the lumen. It would thus seem probable that the zymogen granules are the intracellular representatives of the secre- tion of the gland. ^t^ :^- — — a /^^l^" Fig. iSq. — From Section of Human Pancreas. Xyoo. (Kolliker.) a, Gland cell; 6, basement membrane; 5, intermediate tubule; c, centro- acinar cells; sk, intracellular secretory tubule. THE DIGESTIVE SYSTEM 289 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 •a £. FiG. igo. — Sections of Alveoli from Rabbit's Pancreas. (Foster, after Kiihne and Lea.) A, Resting alveolus, the inner zone (a), containing zj'mogen granules, occupying a little more of the cell than the outer clear zone (b); c, indistinct lumen. B, Active alveolus, granules coarser, fewer, and confined to inner ends of the cell (a), the outer clear zone (b) being much larger; outlines of cells and of- lumen much more distinct. tubule. They are known as the centra- acinar {centro-tuhular) cells of Langerhans (Fig. 189, c). Their significance is not definitely known. Langerhans beheved that they were derived from the intermediate tubule, the epithelium of which, instead of directly joining that of Fig. iqi. — Sections through Alveoli of Human Pancreas — Golgi Method — (Dogiel), to show intracellular secretory tubules, a, Intermediate tubule giving off several terminal tubules, from which pass off minute intracellular secretory tubules; b, gland cells hning terminal tubules. the terminal tubule as in the submaxillary gland, was continued over into the lumen of the terminal tubule (Fig. 189). This interpretation has been quite generally accepted. Cells which differ from the secreting cells are frequently found 19 290 THE ORGANS wedged in between the latter. They extend from the lumen to the basement membrane and are probably siistentacular . Passing from the lumen of the terminal tubule, sometimes between the centro-tubular cells, directly into the cytoplasm of the secreting cells are minute intracellular secretory tubules. These are demonstra- ble only by special methods (Golgi) (Fig. 191). The pancreas also contains peculiar groups of cells, the cell-islands of Lajigerhans, having a diameter from 200 to 300/^ (Figs. 188, 192, and 193). The "island" cells differ quite markedly both in arrange- ment and structure from those which line the terminal tubules (Fig. a .1 w. © ^"^■1 'ft©® © '^^- ■f-v ' -^^ ?5' Fig. 192. — Island of Langerhans and few surrounding Pancreatic Tubules. (Bohm and von Davidofif.) a, Capillary; b, tubule. 192). They contain no zymogen granules. They are arranged in anastomosing cords or strands which are separated from one another by capillaries. There are no ducts and the method of Golgi shows no inter- or intra-cellular secretory tubules. Their protoplasm is un- stained by basic dyes, but stains homogeneously with acid dyes. Their nuclei vary greatly in size, some, especially where the cells are closely packed, being small, others being large and vesicular. Some of the islands are quite sharply outlined by delicate fibrils of connec- tive tissue containing a few elastic fibres (Fig. 192). Others blend with the surrounding tissues. THE DIGESTIVE SYSTEM 291 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 surrounding the secret- ing tubules. The close relation of cells and capillaries and the absence of any ducts have led to the hypothesis that these cells furnish a secretion — internal 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 cell-islands of Langerhans are definite structures "formed in embryological Fig. 193. — From Section 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. 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. 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 Httle 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 292 THE ORGANS 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 tcchnic for the pancreas is the same as for the salivary glands (page 286). (2) Zymogen granules may be demonstrated by fixation in formalin-Miil- ler's fluid (technic 6, p. 7), and staining with picro-acid-fuchsin (technic 2, p. 21), or with Heidenhain's iron haematoxylin (technic 3, p. 18). (3) The arrangement of the blood-vessels in the islands of Langerhans may be studied in specimens in which the vascular system has been injected (page 25). The Liver The liver is a compound tubular gland, the secreting tubules of which anastomose. There are thus, strictly speaking, no "terminal Fig. 194. — Section of Lobule of Pig's Liver X60 (technic i, p. 301), 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; //, hepatic vein. tubules" in the liver, the lumina and walls of neighboring tubules anastomosing without any distinct line of demarcation. THE DIGESTIVE SYSTEM 293 The liver is surrounded b}- a connective-tissue capsule, the capsule ojGlisson. At the Jiilum this capsule extends deep into the substance of the liver, giving off broad connective-tissue septa, which divide the organ into lobes. From the capsule and from these interlobar septa, trabeculae pass into the lobes, subdividing them into lobules. In some animals, as for example the pig, each lobule is completely invested by connective tissue (Fig. 194). In man, only islands of connective B y H FiG.^ 195. — Section of Human Liver. X80. (Hendrickson.) P, Portal vein; H, hepatic'artery; B, bile duct. P, H, B constitute the portal canal and.lie in the connec- tive tissue between the lobules. tissue are found, usually at points where three or more lobules meet (Fig. 195). The lobules are cylindrical or irregularly polyhedral in shape, about i mm. in breadth and 2 mm. in length. Excepting just beneath the capsule, where they are frequently arranged with their apices toward the surface, the liver lobules have an irregular arrangement. The lobule (Fig. 194) w^hich may be considered the anatomic unit of structure of the liver, consists of secreting tubules arranged in a 294 THE ORGANS delinite manner relatively to the blood-vessels. The blood-vessels of the liver must therefore be first considered. 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 bloody the latter venous blood from the intestine. Both 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 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 con- verge toward the center of the lobule, where they empty into the central vein (Fig. 194). The central veins 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 capillaries 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 sublobular vein which is a bfarseli of the hepatic (Fig. 205). The hepatic artery accompanies the portal vein, following the branchings of the latter through the interlobar and interlobular con- nective tissue, where its fmer 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 MAIN EXCRETORY DUCT- — hepatic duct — leaves the liver at the Fig. 196. — Portal Canal. X315 (Klein and Smith.) a, Hepatic artery; F, portal vein; i, bile duct. THE Dir.F.STIVK S^-STEM 29; hilum near the entrance of the portal vein and hepatic arter}-. Within the liver the duct divides 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 — thejiepatic artery, the portal vein, and the bile duct, which always occur together in the con- necdve tissue which marks the point of separation of three or more lobules — together constitute the portal canal (Fig. 196). From the interlobular ducts short branches pass to the surfaces of the lobules. 4 mm^-Ji V.:.-,.. , ^ ^^ - -^ /,■■. ■ _' ■ , > -•: . J^.\-: ^^tV'--'' - ' ■ '■"■-:.'■"' ■'"■ " ■'"'■'.' ' ■ ' .-'i^y- ■-, ,'.\".V,'-;f.''.-* / ■^te,- ""'>"<,/■■ 7 //iv^>y Cartilage ^'' \ \ .|4'!f7,\V?>h ^^5^' .;, Fibrous coat Gla I ■.4\ VJ A''\M Excretory duct Fig. 209. — Cross Section of Human Bronchus (of a child) of 2 mm. diameter. X30. (Stohr.) are frequently fibrocartilage rather than hyaline. These plates de- crease 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 belong to the duct system down to a diameter of about THE RESPIRATORY SYSTEM 313 0.5 mm. From the small interlobular bronchi are given off the ter- minal bronchi. These are respiratory in character and are described with the lungs. "?•„ Fig. 210. — Transverse Section of Small Bronchus from Human Lung. X115. (Tech- nic 2, p. 32^.) a, Stroma; b, epithelium; c, muscularis mucosae; d, fibrous coat. --jr::-* 5i*\ ^5!*--i- >r^ <«-'^. • •\'\ ^ t-^^ ^^7 Fig. 211 . — 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 bronchi it is convenient to arbitrarily divide them into large, medium-sized, and small bronchi. Large bronchi have essentially the same structure as the trachea except for somewhat thinner walls. Medium-sized bronchi (Fig. 2c8) have an epithelium about three layers deep, disconnected plates of cartilage, a continuous layer of smooth muscle disposed circularly as a muscularis mucosae, and tubular glands 314 THE ORGANS Small bronclii have a single layer of ciliated epithelium, a thinner muscular coat, no glands, and no cartilage. (Figs. 210, 211.) 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 alveoH. 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 lung, or hilum, is reflected upon the inner surface of the chest - 1 Fig. 212. — From Lung of an Ape. The bronchi and their dependent ducts and alveoH have been filled with quicksilver. Xi5. (Kolhker, after Schulze.) 6, Terminal bronchus; a, alveolar duct; i, alveoli. wall as the parietal pleura. It consists of fibrillar connective tissue containing fine elastic fibres which are more numerous in the visceral than in the parietal layer. From the capsule broad connective- tissue septa pass into the organ, dividing it into lohes. From the capsule and interlobar septa are given off smaller septa, which sub- divide the lobes into lobules. The human pulmonary lobule (Figs. 213 and 215) is the anatomic unit of lung structure. Each lobule is complete in itself, having its own bronchial system, its own vascular system, and being more or less completely separated from its neighbors by connective tissue. In the young and in some lower animals the lobule is quite plainly outlined by connective tissue, but in the human adult the amount of connective tissue is extremely small, and the lobules, especially the more central, difficult of definition. The lobules are best observed at the surface of the lung where their bases which lie against the pleura can be seen with the naked eye. This is especially true in the aged THE RESPIRATORY SYSTEM 315 where carbon deposits in the interlobular connective tissue assist in outlining the lobules. Thebase of the peripheral lobule is four- to eight- sided and from i to 1.5 cm. in diameter. It is pyramidal in shape, narrowing to an apex about i to 1.5 cm. from the base. In the inte- rior of the lung the lobules are not pyramidal but irregularly poly- hedral, the apex being, however, distinguishable by the entrance of the lobular bronchus. The apex of each lobule is the point of entrance of the lobular bronchus (branch of inter- or sublobular) (Figs. 212, 213 and 215, bl) which is about 0.5 nini. in diameter, and of the lobular branch of the Bronchial artery Pulmonary vein — A^ Pulmonary artery Terminal bronchus Alveolar passage Pleural capillaries Fig. 213. — 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. pulmonary artery. Accompanied by the artery (Fig. 213) the bron- chus passes through the central axis of the lobule, giving off collateral branches (Fig. 212, a, b), to about the middle of the lobule, where it divides into two branches (Fig. 213). These branches and also the collaterals branch dichotomously giving rise to from 50 to 100 ter- minal or alveolar bronchi ("terminal", as being the last subdivision of the bronchial tree which preserves its identity as a bronchus; "alveolar", as it gives off, especially toward its distal end, some alveoK) . From each terminal bronchus (Fig. 215. ba) open from three 316 THE ORGANS to six narrow passages — alveolar passages or alveolar ducts (Fig. 215, ^^) — from which are given off the alveoli — air vesicles or air cells* (Figs. 214 and 215, d). The somewhat dilated distal end of the alveolar passage is sometimes designated the alveolar sac or infundibulum (Fig. 215, /). Laguesse divides the lobule schematically into three parts a proximal part containing the intralobular bronchus and Alveolar sacs Blood-vessel Terminal bronchus Alveoli ssT Small bronchus.. Fig. 214. — Section of Cat's Lung (Szymonowicz), surface lobule; terminal bronchus opening into alveolar duct from which are given off two alveolar sacs. collaterals, a middle part in which the main division of the lobular bronchus occurs, and a third part containing the alveolar bronchi and alveoli. The terminal bronchus (Figs. 213, 214, 215, ha). The proximal portion of the terminal bronchus is hned by a simple columnar * The dilatation of the terminal bronchus from which are given olY the alveolar passages is sometimes called the vestibule (Fig. 215, v). THE RESPIRATORY SYSTEM 317 ciliated epithelium, resting upon a basement membrane. Beneath this is a richly elastic stroma containing bundles of circularly dis- posed 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. h.l. K-rc / r.A .'-Xrx "7-^,"- \*•'^-<'^ Fig. 215. — Section of Lung of Rat to show Arrangement of Bronchial Ramifications and of Alveoli within a Single Lobule. W, Lobular or sublobular bronchus; &zY, intralobular bronchus; ba, terminal bronchus; ;•, dilatation sometimes called vestibule; ca, alveolar passage; i, portion sometimes called infundibulum; d, alveoU, some of which are so cut as to show their openings into the infundibulum and alveolar canals, etc., while others appear closed. X60. (Prenant.) The alveolar passage (Fig. 215, ca). Here the cuboidal epithelium is almost completely replaced by the respiratory. Beneath the epithelium the walls 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. 318 THE ORGANS The alveolus (Figs. 215, (/, 216). The epitheUum of the alveolus consists of two kinds of cells, respiratory cells and so-called "festal" cells (see Development, page 321). The respiratory cells (Fig. 216) 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. 82). Fig. 216. — From Section of Cat's Lung Stained with Silver Nitrate. (Klein.) (Technic i, p. 82.) Small bronchus surrounded by alv^eoli, in which are seen both flat cells (respiratory epithehum) and cuboidal cells (foetal cells). The "fcetal" cells are granular, nucleated cells which are scattered among the respiratory cells (Fig. 216). Their position appears to be less superficial than that of the respiratory cells, the foetal cells lying in the meshes 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. Delicate elastic fibrils support the respiratory and foetal cells. Around the opening of the alveolus the elastic fibres are more numerous, forming a more or less definite ring. The disposition of elastic tissue in the wall of the TTTK RT:SPTRATf)RV SYSTEM 319 alveoli is undoubtedly of importance in determining the contraction and expansion of the 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 passage, or alveolar bronchus, by means of a broad opening, but alveoli arc connected with one another by minute openings in their walls. The interaheolar connective tissue, while extremely small in amount, serves to separate the alveoli from one another. Somewhat thicker connective tissue separates the alveoli of one alveolar passage from Fig. 217. — Section Through Three AlveoU of Human Lung. _X235. Weigert's elastic-tissue stain (technic 3, p. 28) to show arrangement of elastic tissue, a, Alveolus cut through side walls only; b, alveolus cut through side walls and portion of laottom or top; c, alveolus in which either the bottom or top is included in section. those of another. Still stronger connective-tissue bands as already noted separate adjacent lobules. Blood-vessels. — Two systems of vessels distribute blood to the lungs. One, the bronchial system, carries blood for the nutrition of the lung tissue. The other, the much larger pulmonary system^ carries blood for the respiratory function (Fig. 213). The bronchial 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. 213). The pulmonary vessels are much the larger and run in the connective tissue outside the . bronchial walls. The bronchial vessels lie within the fibrous coat of the bronchus. A section of a bronchus thus usually shows 320 THE ORGANS the large pulmonary vessels, one on either side of the bronchus, and two or more small bronchial vessels in the walls of the bronchus (Fig. 208). 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. 213). From these are given off minute terminal arterioles which pass to the ^ Fig. 218. — Parts of Four Alveoli from Section of Injected Human Lung. X200. (Technic 5, p. 323.) a, Wall of ah^eolus 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. 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. 218). 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 capilaries 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 THE RESPIRATORY SYSTEM 321 respiratory epithelial plates; and (3) the delicate basement membrane upon which the respiratory epithelium rests together with an extremely small amount of fibrous and elastic tissues (see diagram, Fig. 219.) The veins begin as small radicles, one from the base of each alve- olus (Fig. 2 13). 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 broncJilal 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. The bronchial arteries supply the walls of the bronchi, 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. b Blood ■ C Fig. 21Q. — Diagram of Tissues Interposed Between Blood and Air in Alveolus, a, Respiratory epithelium; b, fibro-elastic tissue; c, endothelium of capillary. As b does not form a continuous membrane, the capillary wall is in many places in direct apposi- tion with the respiratory epithelium, so that only two layers, a and b, are interposed between blood and air. 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 pul- monary 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. — ^Bundlesof 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 Respiratory System The epithelium of the respiratory system develops from entoderm, the con- nective-tissue elements from mesoderm. The first differentiation of respiratory 322 THE ORGANS system appears as a dipping down of the entoderm of the floor of the primitive pharynx (some investigators describe two original evaginations, one for each lung). 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 lung, (Fig. 220, b, b, b) 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. Up to this point (about six months in human foetus) the development Fig. 220. — Scheme of Development of Lung (Right), b, b, b, the three primary bron- chial buds; b' , b', b', collateral branches and secondary buds, terminating in vpp, the primary vesicles; vpd, pulmonary vesicles proper or alveoli; ca, alveolar canals. The broken line limits the stage of the three primary lung buds; between this line and the line only the three primary buds, their collaterals and secondary buds; between the line and line , the stage of dichotomous division and of termination in primary vesicles. Up to this point the development is that of a com- pound alveolar gland. From this line to the surface represents the final period of development, which is peculiar to the lung and results in the formation of the pulmonary alveoli. (Prenant.) is that of a compound alveolar gland, (Fig. 220). The last to develop are the respiratory divisions of the bronchi with their alveolar passages and alveoli. The appearance of the alveoli is wholly characteristic of lung (Fig. 220, vpd.) The epithelium of the alvuoli is at first entirely of the fcEtal-cell type, the large flat respiratory plates appearing only late in foetal life. Just how and when the flattening of the epithelial cells takes place is not definitely known. The accepted theory has been that the cells become flattened rather suddenly at birth as a result of the first inspiration. Some authors describe a gradual thinning of the cells from the sixth foetal month on. Bikfalir describes a gradual thinning THE RESPIRATORY SYSTEM 323 which is completed rather rapidly on inspiration. The foetal and respiratory cells of the adult lung have the sanae 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. 304). 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-Miiller'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 ha;ma- toxylin-eosin (technic i, p. 20), 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 pulmonary lobules is best seen in sections near and horizontal to the sur- face. Sections perpendicular to and including the surface show the pulmonary pleura. (3) Respiratory Epithelium (technic i, p. 78). (4) Elastic Tissue of the Lung (technic 3, p. 28). (5) Blood-vessels. — For the study of the blood-vessels, especially of the cap- illary networks of the alveoU, sections of injected lung should be made. A fresh lung is injected (page 25) 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. 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. CHAPTER XVII 1 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 single /o5e (Fig. 221). In the adult human kidney the division into lobes is not complete, the peripheral parts of the differ- ent lobes blending. Rarely the foetal division into lobes persists in adult life, such a kidney being known as a "lobulated kidney." On the mesially directed side of the kidney is a depression known as the hilum (Fig. 221). This serves as the point of entrance for the renal artery and of exit for the Fig. 221. —Longitudinal Section renal vein SiUd Ureter . Through Kidney of Guinea-pig, includ- ^ cprtinn t Hi'vi'm'nn af fhp ing hilum and beginning of ureter. X5. ^^ section, a QU ISlon 01 tne (Technic I, p. 338.) a, Pelvis; 6, papilla; organ into two zones is apparent c, wall of pelvis; d, ureter; e, ducts of , . , BeUini;/, cortical pyramids; g, medullary tO the naked eye (rigS. 221 and rays; h, cortex; /, medulla; j, renal cor- 322). The OUter ZOne Or COrtCX puscles. ^ 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 they run in parallel radial lines alternating with straight blood-vessels. The medullary portion of 324 THE URINARY SYSTEM 325 the kidney projects into the pelvis, or upper expanded beginning of the ureter (Figs. 221, a and 222, g) in the form of papilla; (Figs. 221, h and 222, 0). The number of papilLne varies from ten to fifteen, Fig. 222. Fig. 223. Fig. 222. — Longitudinal Section of Kidney Through Hilum. a, Cortical pyramid; h, medullary ray; c, medulla; d, cortex; e, renal calyx;/, hilum; g, ureter; h, renal artery; i, obliquely cut tubules of medulla; j and k, renal arches; I, coluinn of Bertini; m, connective tissue and fat surrounding renal vessels; n, medulla cut obhqWy; 0, papilla; p, medullary pyramid. (Merkel-Henle.) » Fig. 223.— Scheme of Uriniferous Tubule and of the Blood-vessels of the Kidney showing their relation to each other and to the different parts of the kidney. G, Glorner- ulus; BC, Bowman's capsule; N, neck; PC, proximal convoluted tubule; S, spiral tubule; D, descending arm of Henle's loop; L, Henle's loop; A, ascending arm of Henle's loop; I, DC, distal convoluted tubule; AC, arched tubule; SC, straight collecting tubule; ED, duct of BeUini; ^, arcuate artery, and V, arcuate vein, giving oS interlobular vessels to cortex and vasa recta to medulla; a, afferent vessel of glomerulus; c, efferent vessel of glomerulus; c, capillary network in cortical labyrinth; s, stellate veins; vr, vasa recta and capillary network of medulla. (Pearsol.) corresponding to the number of lobes in the foetal kidney. The pyramidal segment of medulla, the apex of which is a papilla — in other words, the medullary portion of a foetal lobe — is known as a 326 THE ORGANS medullary or Malpighian pyramid (Fig. 223). The extensions down- ward of cortical substance between the Malpighian pyramids con- stitute the columns of Bertini or septa renis (Fig. 222, /). Radiating lines — medullary rays or pyramids of Ferrein — extend outward from the base of each Malpighian j)yramid into the cortex (Fig. 222.) As the rays extend outward in groups they outline pyramidal cortical areas. These are known as the cortical pyramids or cortical labyrinths (Fig. 223). The secreting portion of the kidney is composed of a large number of long tortuous tubules, the uriniferous tubules. Each URINIFEROUS TUBULE begins in an expansion known as Bowman's capsule (Figs. 223, BC, and 224, 3, 4, 5). This encloses a Fig. 224. — Diagrams Illustrating Successive Stages in Development of the Renal Corpuscle, i and 2, Approach of blood-vessel and bhnd end of tubule; 3, invagination of tubule by blood-vessels; 4 and 5, later stages, showing development of glomerulus and of the two-laj'ered capsule of the renal corpuscle, the outer layer being the capsule of Bowman continuous with the epithelium of the first convoluted tubule. tuft of blood capillaries, the glomerulus. Bowman's capsule and the glomerulus together constitute the Malpighian body or renal corpuscle (Fig. 225). As it leaves the Malpighian body the uriniferous tubule becomes constricted to form the neck (Figs. 223, N, 224, and 225, h). It next broadens out into a greatly convoluted portion, the first con- voluted tubule (Fig. 223, PC, and Fig. 226). The Malpighian body, the neck, and the first convoluted tubule are situated in the corti T pyramid (Fig. 223). The tubule next takes a quite straight courst" downward into the medulla — descending arm of Henle's loop (Fig. 223, D) — turns sharply upon itself — Henle's loop (Fig. 223. L)— and passes again toward the surface — ascending arm of Henle's loop (Fig. 223, A) — through the medulla and medullary ray. Leaving the medullary ray, it enters the same cortical pyramid from which it took origin to become the second convoluted tubule (Fig. 223, DC). This tubule is in THE URINARY SYSTEM 327 close proximity to the jMalpighian body from which it started, lying, however, on the side of the afferent and efferent blood-vessels, i.e., on the side opposite its point of origin. The second convoluted tubule passes into the arched tubule (AC) which enters a medullary ray and continues straight down through the medullary ray and medulla as the straight or collecting tubule (SC). During its course the col- lecting tubule receives other arched tubules. As it descends it be- comes broader, enters the papilla, where it is known as the duct of Bellini (ED), and opens on the surface of the papilla into the kid- ney pelvis. About twenty ducts of Bellini open upon the surface of each papilla, their openings being known as the foramina papillaria. Fig. 225. — Malpighian Body from Human Kidney. X280. (Technic 2, p. 331.) a, Bowman's capsule; b, neck'; c, first convoluted tubule; d, afferent and efferent vessels. Each tubule consists of a delicate homogeneous membrana propria upon which rests a single layer of epithelial cells. The shape and structure of the epithelium differ in different portions of the tubule. I. The Malpighian body is spheroidal, and has a diameter of from 120 to 2oo/«. The structure of the Malpighian body can be best understood by reference to its development (Fig. 224). During the ^' velopment of the uriniferous tubules and of the blood-vessels of the .dney, the growing end of a vessel meets the growing end of a tubule m such a way that there is an invagination of the tubule by the blood- vessel (see Fig. 224). The result is that the end of the vessel which develops a tuft-like network of capillaries — -the glomerulus — comes to lie within the expanded end of the tubule, which thus forms a two- layered capsule for the glomerulus. One layer of the capsule closely invests the tuft of capillaries, dipping down into it and separating the 328 THE ORGANS groups of capillaries (see p. 333). This layer by modification of the original epithelium of the tubule is finally composed of a single layer of flat epithelial cells with projecting nuclei. The outer layer of the capsule lies against the delicate connective tissue which surrounds the Malpighian body. This layer consists of a similar though slightly higher epithelium and is known as Bowman's capsule. Between the glomerular layer of the capsule and Bowman's capsule proper is a | space which represents the beginning of the lumen of the uriniferous tubule (Fig. 225), the epithelium of Bowman's capsule being directly continuous with that of the neck of the tubule. B . ^^ ,-t3 ■ "^^ V '^';:....- • '-:,-•- -&= i"- ft- A Fig. 226. — Proximal Convoluted Tubules of Human Kidney. X35°- (Technic 2, P- 33^-) A, Cross-section; B, oblique section. 2. The Neck. — This is short and narrow, and is lined by a few cuboidal epithelial cells. Toward its glomerular end the epithelium is transitional between the flat epithelium of Bowman's capsule and the cuboidal epithelium of the neck proper. At its other end the epitheHum of the neck becomes larger and more irregular as it passes over into that lining the next division of the tubule (Fig. 225. c). 3. The first convoluted tubule (Fig. 226) measures from 40 to 70," in diameter. It is lined by irregularly cuboidal or pyramidal epithe- lium, with very indistinct demarcation between the cells. The cyto- plasm is granular, and the granules are arranged in rows, giving the cell a striated appearance. This is especially marked at the basal end of the cell where the nucleus is situated. A zone of fine striations along the free surface fiequently presents somewhat the appearance of cilia. 4. The descending arm of Henle's loop is narrow (Fig. 227, i), 10 to i5/< in diameter. It is lined by a simple flat epithelium. The THE URINARY SYSTEM 329 part of the cell which contains the nucleus bulges into the lumen, and as the nuclei of opposite sides of the tubule usually alternate, the lumen is apt to present a wavy appearance in longitudinal sections. 5. Ilcnlcs Loop. — The epithelium here changes from the flat of the descending arm to the cuboidal of the ascending arm. The exact point where the transition occurs varies. It may take place during the turn of the loop, or in either the ascending or descending arm. I S. ■1 7- "^^ .1 ■Si, m< m Fig. 227. — Tubules of Human Kidney. X560. From longitudinal section. (Technic 2, p. 338.) i. Descending arm of Henle's loop; 2, ascending arm of Henle's loop; 3, collecting tubule; 4, duct of Bellini. Beneath the longitudinal sections are seen cross sections of the same tubules. 6. The ascending arm of Henle's loop (Fig. 227, 2) is broader than the descending, measuring from 20 to 30,". in diameter. Its epithelium is cuboidal with granular striated protoplasm. The cells thus resemble those of the convoluted tubule, but are smaller, more regular, and less granular. 7. The second convoluted tubule has a diameter of 40 to 5o/-«. It is much less tortuous than the first convoluted tubule. Its epithelium is similar to that lining the first convoluted tubule except that it is shghtly lower and less distinctly striated. 8. The arched tubule has a somewhat narrower lumen (about 25") than the second convoluted. It is lined with a low cuboidal epithe- lium with only slightly granular cytoplasm. 330 THE ORGANS 9. The straight or collecting tubule (Fig. 227,3) has at its commence- ment at the apex of a medullary ray a diameter of from 40 to 50,". As it descends it receives other arched tubules, and increases in diameter until in the ducts of Bellini (Fig. 227, 4) of the papilla it has a diameter of from 200 to 300/^ and a widely open lumen. The epithelium is at first low and gradually increases in height. In the ^v ^^ V'.-'VV:r:-'-:' "V.;-^; Fig. 228. — Cross Section Through Cortex of Human Kidney. X60. (Technic 2, p. 338.) a, Convoluted tubules of cortical pyramid; b, interlobular arterj-; c, medullary rays; d, ISIalpighian bodies. ducts of Bellini it is 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 kidney Cortical labyrinth Portion of tubule Epithelium Bowman's capsule. . Flat with bulging nuclei. Neck Cuboidal, granular. First convoluted Pyramidal, granular; large cells with granules in rows, 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 URIXARY SYSTEM 331 Medullary ray in cor tex Location in kidney Portion of tubule Epithelium Arched (passing from Rather clear cuboidal cells, labyrinth to ray) Part of ascending Cuboidal, granular, regular, arm of Henle's loop Collecting tubule... Cuboidal or columnar, clear; varying in height with diameter of tubule. Descending arni of Clear flat cells with bulging Henle's loop nuclei. Henle's loop Usually like descending, rarely like ascending arm. \ Part of ascending Cuboidal, granular, arm of Henle's loop Collecting tubule. . . Cuboidal or columnar, clear; varying in height with di- ameter 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. Rlihle describes the basement membrane as consisting of delicate longitudinal and circular connec- tive-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 ]\Ialpighian 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 tubules and of the ascending arm of Henle's loop. Blood-vessels (diagram, Fig. 230). — 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 332 THE ORGANS immediately splits up into a number of branches — the interlobar arteries (Fig. 230, g). These give off twigs to the calyces and to the capsule, then without further branching pass between the papillse 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. 230, d). From the arcuate arteries two sets of vessels arise, one supplying the cortex, the other the me- dulla (Figs. 223 and 230). d < <^ "^ ^: I b c Fig. 229 — Cross Section through Medulla of Human Kidney. X465. (Technic 2, P- 33^-) a, Capillaries; b, collecting tubule; c, ascending arms of Henle's loops; d,ide- scending arms of Henle's loops. The arteries to the cortex spring from the outer (Fig. 230, b) sides of the arterial arches, and as interlobular arteries pursue a quite straight 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 THE LRIXARV SYSTEM 333 glomerulus. Each group of glomerular capillaries arising from a single arteriole is separated from its neighbors by a rather larger amount of connective 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 some- A B C Fig. 230. — Diagram to Illustrate (left) the Course of the Uriniferous Tubule; (right) the Course of the Renal Vessels. (Szymonowicz.) A, B, C, D, each represents a kidney lobule limited laterally by the interlobular vessels; 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 convoluted tubule; 6 and 7, collecting tubules; 8, duct of BeUini; b, interlobular artery; c, interlobular vein; d, renal arch (arcuate artery above and arcuate vein below); in their transverse course these vessels he along the boundary line between cortex and medulla; /, interlobar vein; g, interlobar artery; h, medulla; i, medullary ray;y, cortex. what larger capillaries, which unite to form the efferent vessel of the glomerulus. As afferent and efferent vessels lie side by side, the glomerulus has the appearance of being suspended from this point (Figs. 223,225). The entire vascular system of the glomerulus is arterial. 334 THE ORGANS After leaving the glomerulus, the efferent vessel breaks up into a second system of capillaries, which form a dense network among the 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 interlohular veins (Fig. 230, 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. 230, 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 arteri^ rectae) and (2) by medullary 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. 230, d). In addition to the distribution just described, some of the inter- lobular arteries extend to the surface of the kidnev, where thev 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 estabhshed bv 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 oj 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 S3'stems. The medullated fibres appear to pass mainly to the walls of the blood-vessels which supply the kidney capsule. THE URIXARY SVSTT^^r 335 Plexuses of fine non-medullated libres (sympathetic) accompany the arteries to the glomeruli. Delicate terminals have been described as passing from these plexuses, piercing the basemrnt luenibrane and ending freely between the epithelial cells of the tubules. The Kidney-Pelvis and Ureter The kidney-pelvis, with its subdivisions 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. . cm c / ,. • /|. ;*;#%# if ^iS^'l^^s/ .■■::'^^^ • •:. . ^ .', 1 1 ^' #^^ #-^? %,;.|:^^ -.. ; I ; ■ 0 . ^^ ,K \ ■ ; .■•>V^.^'v . .•; IPS' m p;-.^,^;->; Fig. 231. — Transverse Section of Human Ureter. (Prenant.) Z, lumen; c/), epi- thelium; ch, stroma; cml, inner longitudinal muscular layer; cmc, outer circular muscular layer; ad, fibrosa. X25. The mucosa is lined by epithelium of the transitional type (Fig. 231, cp). There are from four to eight layers of cells, the cell outlines are usually well defined, and the surface cells instead of being dis- tinctly squamous are only slightly flattened. Less commonly large flat plate-like cells, each containing several nuclei, are present. The. cells rest upon a basement membrane, beneath which is a stroma of 336 THE ORGANS delicate fibrous tissue containing few elastic fibrils and rich in cells (Fig. 231, cli). Diffuse 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 (Fig. 231, cml and cmc). 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 (Fig. 231, ad). 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 w^here they lose their medullary sheaths. Terminals of these fibres have been traced to the lining epithelium. 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 compara- tively smooth, according to the degree of distention of the organ. The epithelium is of the same general type — transitional epithelium (seepage 77) — as that of the ureter. The number of layers of cells and the shapes of the cells depend largely upon whether the bladder is 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 Tin: URINARY SYSTE^r 337 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. 232). In the distended organ there is still further flattening of the superficial cells and thinning of the entire epithelium. The stroma consists of fme loosely arranged connective tissue containing many lymphoid cells and sometimes small lymph nodules. It merges without distinct demarcation into the less cellular submucosa (Fig. 232, c). '^■=^^-^ ■ ■ ' • • V --. / Fig. 232. — Vertical Section through Wall of moderately distended Human Bladder. X60. (Technic 5, p. 331.) a, EpitheHum, b, stroma, of mucous membrane; c, sub- mucosa; d, inner muscle layer; e, middle muscle layer;/, outer muscle layer. 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. 232, 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. 22 338 THE ORGANS Nerves. — Sensory medullated iibres pierce the muscularis, branch repeatedly in the stroma, lose their medullary sheaths, and terminate among the cells of the lining epithelium. Sympathetic hbres form plexuses in the fibrous coat, where they are interspersed with numerous small groups of ganglion cells. Axones of these syrApathetic 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 382. TECHNIC (i) Fix the simple kidney of a rabbit or guinea-pig in formalin-^Miiller's fluid (technic 6, p. 7). Make sections through the entire organ including the papilla and pelvis, stain with hsematoxylin-eosin (technic i, p. 20), 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 27). (4) Ureter. — Cut transversely into short segments, fix in formalin- ^Miiller's fluid (technic 6, p. 7), and stain transverse sections with haematoxylin-eosin (technic i, p. 20), or with haematoxylin-picro-acid-fuchsin (technic 3, p. 21). Mount in balsam. (5) Bladder (technic i, p. 250, or technic 2, p. 260). By the latter method any desired degree of distention may be obtained. General References for Further Study KoUiker: 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. BuL, vol. xi., 1900. Miiller: Ueber die Ausscheidung des Methylenblau durch die Nieren. Deutsches Archiv f. klin. Med., Bd. 63, 1899. CHAPTER XIX 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 alhuginea (Fig. 233, 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 vasciilosa. Posteriorly the tunica albu- ginea is greatly thickened to form the corpus Highmori, or mediastinum testis, from which strong connective-tissue gj^ea; b, connective-tissue septum septa radiate (Figs. 2^7,, m and 2^4,b). between lobules; m, mediastinum; t, ^ \ o vjvj7 1 convoluted portion of semmiterous These septa pass completely through tubule; s, straight tubule; r, rete -1 ^ ~ J ui J „ vu 4-U„ i-.-^:^^ testis; e, vasa efferentia; c, tubules the organ and blend with the tumca ^^ ^^'^^'^^ epididymis; te,v^s epi- albuginea at various points. In this didymis; vd, vas deferens; va, vas . , . . , ,. aberrans; *, paradidymis. way the mterior 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. '.S3 and 234). Behind the testis and outside of its tunica albuginea is an elongated )ody^the epididymis (Figs. 233, c and 234, c), consisting of convoluted 339 Fig. 233.- — Diagram illustrating the Course and Relations of the Semi- niferous Tubules and their Excretory Ducts. (Piersol.) a, Tunica albu- 340 THE ORGANS \} Ac, ■on 0 ~< '•^ s 7 tubules continuous with those of the mediastinum. The epididymis is divided into three parts: an expanded upper extremity, the head or globus major (Figs. 2,^3 and 234, c) ; a middle piece, the body (Fig. 234, d)\ 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. 233, vd). All of the tubules of the epididymis are continuous on the one e a 1 ---— r^ hand with the tubules of : .^'"rv 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. 237, a). As they approach the apex of a lobule several of these convoluted tubules unite to form (2) the straight tubule (Fig. 233, s, 237). 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 retejestis (Fig. 237, 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,". 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. 237, a). The wall of the convoluted tubule (Fig. 235) consists of three Fig. 234. — Longitudinal Section through Human Testis and Epididymis. X2. (Bohm and von Davidoff.) The light strands are connective- tissue septa, a, Tunica albuginea; b, mediastinum and rete testis; c, head of epididymis; d, body of epididymis; c, lobule; s, straight tubules; t, vas epididymis. THE REPRODUCTIVE SYSTEM 341 layers: (a) An outer layer composed of several rows ol llaUened con- nective-tissue cells which closely invest the tubule; (b) a thin base- ment membrane; and (c) a lining epithelium. The epithelium con- sists of two kinds of cells, the so-called supporting or susientacular cells and the gandular cells proper, the s per mato genie cells. The suskntaciilar cells, or columns of Sertoli, are irregular, high, epithehal structures, whose bases rest upon the basement membrane, sf 5 Fig. 235.— Cross Section of Convoluted Portion of Human Seminiferous Tubule X480. (Kolliker.) M, Basement membrane; i, its inner homogeneous layer; fs, its outer fibrous layer; s, nucleus of Sertoli cell; sp, spermatogone; sc, spermatocyte; sc' , spermatocyte showing mitosis; sf, nearly mature spermatozoon; sf , spermatozoon free in lumen of tubule; d, degenerating nucleus in lumen; /, fat droplets stained by osmic acid. and wliich extend through or nearly through the entire epithelium (Fig. 236,5). Their sides show marked irregularities and depressions, due to the pressure of surrounding spermatogenic cells. Their nuclei are clear, being poor in chromatin and their protoplasm contains brownish fat droplets. The cells of Sertoli have long been considered as sustentacular in character. It has recently been suggested that these cells are derived from the spermatogenic cells, but that, in- stead of developing into spermatozoa, they undergo retrograde changes, r 342 THE ORGANS their protoplasm mingling with the intercellular substance, their nuclei becoming lost and the cells finally disappearing. According to this theory the tuft-hke arrangement of the spermatozoa about the ends of the Sertoli cells is due to pressure by surrounding spermato- genic cells (Figs. 236, h and 238, /). ^v-f:-\;^ ^^^- ^^^^'^3^ — SI sp sc sp m •sp' si ■i^^mm'] in sp L^ ^ y Fig. 236. — Parts of Transverse Section of three Seminiferous Tubules from Testis of White Mouse. X600. (Szj-monowicz.) 5, Sertoh cell with nucleus; 5/), spermatogone, resting state; sp' , spermatogone in mitosis; sc, spermatocyte; si, spermatid; sj, spermatid developing into spermatozoon; h, head of spermatozoon; t, tails of developing sper- matozoa; h, blood-vessel; c, interstitial cell; m, basal membrane;/, fat droplets. The appearance which the spermatogenic cells present depends upon the functional condition of the tubule. In the resting state the epithelium consists of several layers of spherical cells containing nuclei which stain with varying degrees of intensity. In the active state several distinct layers of spermatogenic cells can be dift'eren- tiated. These from without inward are as follows:. (i) S^i^XMa-logau^s (Figs. 235 and 236, sp). — These are small cu- boidal cells which lie against the basement membrane. Their nuclei are spherical andrich in chromatin. By mitotic division of the spermat- ogones are formed the cells of the second layer, the spermatocytes. THE REl'RCMJUCTIVE SYSTEM 343 (2) Spermatocytes (Figs. 235 and 236, sc). — These are larger spher- ical cells with abundant cytoplasm and large vesicular nuclei showing various stages of mitosis. They form from two to four layers to the inner side of the spermatogones, and are sometimes differentiated into spermat- ocytes of the first order and spermato- cytes of the second order. By mitotic division of the innermost spermatocytes are formed the spermatids. (3) The sperjiialids (Figs. 235 and 236, st) are small round cells which line the Fig. 237. , Fig. 237. — Passage of Convoluted Part of Seminiferous Tubules into Straight Tu- bules and of these into the Rete Testis (Milhalkowicz.) a, Convoluted part of tubule; b, fibrous stroma continued from the mediastinum testis; c, rete testis. Fig. 238. — Spermatoblast with some Adjacent Sperm Cells, from Testis of Sparrow. (From Kolliker, after Etzold.) M, Basement membrane; s, nucleus of SertoH cell; sp, spermatogones; sc, spermatocyte; sti and st-2, spermatids lying along the surface of the Sertoli cell, s' and sh; at 5/3 are seen the nearly mature spermatozoa; /, tuft-like arrangement of bodies of spermatids around free end of Sertoli cell, with two mature spermatozoa. lumen of the seminiferous tubule. They are the direct progenitors of the spermatozoa. (For details of spermatogenesis see page 350.) In the actively secreting testicle spermatozoa are frequently found either free in the lumen of the tubule or with their heads among the superficial cells and their tails extending out into the lumen (Figs. 235, sf^ and 238). There are also found in the lumen many small 344 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 >^^<^' 4^ ; 'V? X--'-'-:"^'^-^^ iv ;-• ; . . #»v />/ V? //'« — /VSJ SfC Fig. 239. — From Section through Human ^lediastinum and Rete Testis. X96. (Kolliker.) A, Artery; V, vein; L, lymph space; C, canals of rete testis; s, cords of tis- sue projecting into the lumina of the tubules and so cut transversely or obliquely; Sk, section of convoluted portion of seminiferous tubule. and nerves. Among the usual connective-tissue elements are found groups of rather large spherical cells w^ith large nuclei — interstitial * Tn contrast with the actively functionating seminiferous tubules described above, are the tubules of non-active testes. Typical examples of non-active tubules can be studied in animals having definite breeding' seasons with long intermissions. Here between seasons one finds no spermatozoa in tlu' tubules, no spermatogenesis, only cells of the spermatogone type and Sertoli cells. .\ similar condition is sometimes found in the human testis in senility or after prolonged sickness. In senility there is a general increase in' fibrous tissue at the expense of elastic fibres and cells, while in the tubules sperm cells may entirely disappear, the Sertoli cells alone remaining. THE REPRODUCTIVE SYSTEM 345 lf cells (Fig. 236, c). They are believed to represent remains of the Wolffian body. 2. The Straight Tubule. — With the termination of the con- voluted portion, the spermatogenic tissue of the gland ends, the remainder of the tubule con- stituting a complex system of , ,r-}^' ^ excretory ducts. The straight tubule is much narrower than the convoluted, having a dia- meter of from 20 to ^o/'.. It is lined with a single layer of cuboidal cells resting upon a thin basement membrane. At the apex of the lobule the straight tubules become con- tinuous with the tubules of the rete testis. 3. The Tubules of the Rete Testis.— These are irregular canals which vary greatly in shape and size. They are lined with a single layer of low cuboidal or fiat epithelial cells (Fig. 239, C). Fig. 240. — Part of a Cross Section through a Vas Efferens of the Human Epididymis. X140. (KoUiker.) F, High columnar ciliated epithelium; d, lower non-ciliatcd epithelium, presenting appearance of a gland; d', the same cut obliquely. x » 'm Fig. 241. — From Cross Section through Head of Epididymis. Ap5. (KoUiker.) b, Interstitial connective tissue; c, sections through tubules of epididymis, showing two- laj-ered columnar epithelium; g, blood-vessel. 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 jiroper, which serve to 346 THE ORGANS transmit the secretion of the testis, and are known as the seminal ducts. On leaving the testis these ducts form the epididymis, after which they converge to form the main excretory duct of the testis, the vas deferens. The Epididymis. — From the tubules of the rete testis arise from eight to fifteen tubules, the vasa ejferentia, or efferent ducts of the testis (Fig. 233, 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 epithehum of the vasa efferentia consists of two kinds of cells, high columnar ciliated cells (Fig. 240, F), and, interspersed among these, low cuboidal non- ciliated cells (Fig. 240, d). Occasionally some of the high cells are free from cilia and some of the cuboidal cells may bear cilia. The cuboidal cells He in groups between\groups of the higher cells, often giving the appearance of crypt- like depressions. These have been referred to as intraepithelial glands. They do not, however, invaginate the underlying tissues. The epithelium rests upon a base- ment membrane, beneath which are several layers of circularly disposed smooth muscle cells. The vasa efferentia converge to form the vas epididymis (Fig. 241) . Here the epithelium is of the stratified variety, there being two or three rows of cells. The sur- face cells are narrow, liigh, and ciliated, and their nuclei are placed at dift'erent levels (Fig. 242). The ciha are long and each cell has only a few ciha. The deeper cells are irregular in shape. The basement membrane and muscular layer'- a'-e the same as in the vasa efferentia. As the vas deferens is approached the muscular coat becomes thickened, and is sometimes strengthened by the addition of scattered bundles of longitudinally disposed cells. The Vas Deperens. — The walls of the vas deferens consist of four coats — Jnucosa, submucosa, muscularis, and fibrosa (Fig. 243). The mucosc is folded longitudinally, and is composed of a stroma Fig. 242. — Vertical Section through Wall of Tubule of Epididymis. X700. (Kolhker.) (Fig. 241 more highly magni- fied.) b, Connective-tissue and smooth muscle cells; e, basal layer of epithelial cells; /, high columnar epithelial cells; p, pigment granules in columnar cells; c, cuticula; h, cilia. THE REPRODUCTIVE SYSTEM 347 and a lining epilhclium. The epithelium 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 epithelium 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 epi- didymis. The epithelium rests upon a basement membrane beneath which is a fibro-elastic cellular stroma. The stroma merges without distinct demarcation into the more vascular suhmucosa. ^*» o \ -6 C ■ d ^^^. e Fig. 243. — Cross Section of Human Vas Deferens. X37. (Szymonowicz.) a, Epithelium; h, stroma; c, submucosa; d, inner circular muscle layer; c, outer longitudinal muscle layer;/, fibrous layer; g, blood-vessels. The miiscularis consists of two strongly developed layers of smooth muscle, an inner circular and an outer longitudinal (Fig. 243), 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 composed of longitudinal bundles, and situated between the inner' circular layer and the submucosa. The fibrosa 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 vas. The lining epithelium is, however, frequently markedly pig- mented and the mucosa contains branched tubular glands. 348 THE ORGANS 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 epithelium is pseudo-stratified 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 giving the appearance of an ordinary simple columnar epithelium. Beneath the epithelium is a thin stroma, out- 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 ejaculatory 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 Giraldcs, 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. In- stead of a single ductus aberrans, several ducts may be present. (3) The appendix testis (stalked hydatid or hydatid of INIorgagni), in the upper part of the globus major. It consists of a vascular connective tissue surrounding a cavity lined with simple columnar ciliated 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 epididy- midis are believed by some to be derived from the primitive kidney, by others from the embryonal duct of Miiller. 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 capillaries arise veins which accompany the arteries. Lymph capillaries begin as clefts in the tunica albuginca and in the connective tissue surrounding the seminiferous tubules. These THE REPRODUCTIVE SYSTEM 349 connect with the more dehnite lymph vessels of the mediastinum and of the spermatic cord. 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 epidid- ymis are found small sympathetic Head < ganglia. The walls of the vasa Galea capitis Neck Body - End ring ■ I Main segment of tail Anterior end knob Posterior end knob Spiral fibers Sheath of ' axial thread Axial thread Capsule efferentia, vas epididymis, and vas deferens contain plexuses of non- medullated nerve fibres, which give off terminals to the smooth muscle cells and to the mucosa. The Spermatozoa. — The spermatozoa are the specific secre- tion of the testicle. Human spermatozoa are long, slender flagellate bodies, from 50 to 70/1 in length, and are suspended in the semen, which is a secretion of the accessory sexual glands. The general shape is that of a tadpole; and by means of an undulatory motion of the tail, the spermato- zoon is capable of swimming about freely in a suitable medium. It has been estimated 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. 244). The head, from 3 to 5,« long and about half that in breadth, is oval in shape when seen on flat, pearshaped when seen on edge. It con- sists mainly of chromatin drived from the nucleus of the parent cell. Terminal filament Fig. 244. — Diagram of a Human Sper- matozoon. (Meves, Bonnet.) 350 THE ORGANS Enveloping the nuclear material of the head is a thin layer or delicate membrane of cytoplasm, the galea capitis. The front of the head 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 acrosome 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 iibrillated 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 fits 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 disc. 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 objectives. Development or the Spermatozoa. — As already noted in describing the testicle, the spermatozoa are developed from the epithelial cells of the semi- niferous tubules. The most peripheral of the tubule cells, the spermatogones (Fig. 235, sp and Fig. 236, sp) are small round cells with nuclei rich in chromatin. By mitosis the spermatogone 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. 236, sc and Fig. 238, sc). The latter are rather large spherical cells, whose nuclei show verj' distinct chromatin networks. By mitotic division of the spermatocytes of the innermost row are formed the spermatids (Fig. 236, st and Fig. 238, si). These are small spherical cells, which THE REPRODUCTIVE SYSTEiM 351 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 59). The transformation of the spermatid into the spermatozoon differs somewhat in different animals and the details of the processmust be regarded as not yet definitely determined. The nucleus of the spermatid first becomes oval in shape, and its chromosomes be- come condensed into a small homogeneous mass, which forms the head of the spermatozoon. During their transformation into the heads of the spermatozoa, Head Anterior end knob Posterior end knob Anterior end knob Posterior end knob End ring Nucleus Cytoplasm Proximal centrosome Distal centroEcne Fig. 245. — Transformation of a Spermatid into a Spermatozoon (human). Schematic. (Meves, Bonnet.) the nuclei of the spermatids arrange themselves in tufts against the inner ends of the cells of SertoH. This compound structure, consisting of a Sertoli cell and of a group of developing spermatozoa attached to its central end, is known as a spermatoblast (Fig. 238). 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. 245. The centrosome either divides completely, forming two 352 THE ORGANS 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 disc-shaped. The one lying nearer the centre 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 there 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 ofT 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. Atypical spermatozoa are not infrec}uent. Thus are found very large, or very small types, or spermatozoa with several tails attached to a single head or less commonly several heads with only one tail. 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 chromosomes. The head and body, containing the chromatin and the centro- somes, are the parts of the spermatozoon essential to fertilization. The acro- some is an accessory which in some forms at least aids the spermatozoon in attaching itself to and in entering the ovum. The tail is an accessory structure upon which depends the motility of the spermatozoon. In the lumen of the seminiferous tubules very little motion is observable, only after they have become a part of the semen (p. 349) does their motion become active. This activity is retained in the fluids of the female generative tract. Considering their minuteness, their speed is considerable, 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 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 chUd, 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 6, p. 7). Antero-posterior longitudinal sections through the entire organ and in- cluding the epididymis should be stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 21) or with haematoxylin-eosin (technic i, p. 20) and mounted in balsam. (2) The testis of a young adult is removed as soon after death as possible, is cut into thin transverse sUces, which include the epididymis, and is fixed in forma- lin-M tiller's or in Zenker's fluid (technic 10, p. 8). Select a slice which includes the head of the epididymis, cut away the anterior half or two-thirds of the testis THE REPRODUCTIVE SYSTEM 353 proper in order to reduce ihe 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 haematoxyUn-eosin (technic i, p. 20) and mount in balsam. (3) For the study of spermatogenesis fix a mouse's testis in chrome-acetic- osmic mixture (technic 8, p. 8). Harden in alcohol and mount thin unstained sections in balsam or in glycerin. (4) Spermatozoa. — ^Human spermatozoa may be examined fresh in warm normal saHne 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 haema- toxj-lin-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 (V .'^f '. -■;..' It V • '' .»* Ml.;.- :?' Fig. 246. — Section of Human Prostate. X150. (Technic i, p. 349.) a, Epithelium of tubule; b, interstitial connective tissue; c, corpora amjdacea. 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 twenty main ducts, which open into the urethra. The gland is'surrounded by a capsule of fibro-elastic tissue and smooth 23 354 THE ORGANS muscle cells, the muscle cells predominating. From the capsule broad trabeculcB 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 prostate 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. 246). Less commonly the epithelium is pseudo-strati- fied. The ducts are hned with simple columnar epithehum until near their terminations where they are lined with transitional epithe- lium similar to that lining the urethra. Peculiar concentrically laminated bodies, crescentic corpuscles, or corpora amylacea, are fre- quently present in the terminal tubules (Fig. 246, c). They are more numerous after middle life. Through the prostate runs the prostatic portion of the urethra. Within the prostate is found the vesicula prostatica {utricidiis prostaticus — uterus masculinus). It represents the remains of a foetal structure, the Mullerian 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 secre- tion is serous. The blood-vessels of the prostate ramify in the capsule and tra- beculse. 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. The lymphatics begin as blind clefts in the trabeculae and fol- low 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 trabeculae and of the walls of the blood-vessels. Their mode of termination is not known. Timofeew describes afferent medullated 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 small varicose fibrils which form a network around the terminals of the large fibre. TPIE REPRODUCTIVE SYSTEM 355 Cowper's Glands The bulbo-urcthral glands, or glands of Cowper, are small, com- pound tubular glands. Both tubules and ducts are irregular in diam- eter 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 6, p. 7). Stain sections with haematoxylin-eosin (technic i, p. 20J 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 cylindrical 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. 247). All three are enclosed in a common connective-tissue capsule which is loosely attached to the overlying skin. Iruaddi- tion each corpus has its own special capsule or tunica albu- ginea, about a millimetre in thickness, and composed of dense connective tissue contain- ^ ^ c .- ..u i. Fig. 247. — Transverse Section through ing many elastic fibres. • . Human Penis. a, Skin; b, subcutaneous Tbp rnrhu^ ^bnnpiomm a,nd ^''^''^' c. fibrous tunic; d, dorsal vein; e, ine corpus spongiosum d,na j-o^pora cavernosa; /, corpus spongiosum; cor pora cavernosahsLve essentially g, urethra, the same structure, being com- posed of so-called erectile tissue (Fig. 248). This consists of thick trabecute of intermingled fibro-elastic tissue and bundles of smooth 356 THE ORGANS muscle cells, which anastomose to form a coarse meshed network, the spaces of which are lined with endothelium. 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 broader venous channels in the periphery of the erectile tissue, and :4.- -V ■ • , ■'..^ 'A'- •_:''- ,-> b ,' §s'--- ■■ ■■■■,"■ |)-' ■ ■ a, ■ ■'•-..,■ >. -; Fig. 248. — Erectile Tissue of Corpus Spongiosum of Human Penis. X60. a Trabeculse of connective tissue and smooth muscle; h, cavernous sinuses; c, groups of leucocytes in sinus. these in turn communicate with the cavernous sinuses. The usual direct anastomosis between arterial and venous capillaries also occurs. 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. 247). In the corpus spongiosum there is probably no direct opening of arteries into sinuses. Both trabecular and sinuses are also smaller. Of the lymphatics of the penis little definite is known. The nerve endings, according to Dogiel, consist of: {a) free sensory endings, (i) deeply situated genital corpuscles, (c) Pacinian corpus- THE REPROnrc^TTVE SYSTKM 357 r cles and Krause's end-bulbs in the more supcrlicial connective tissue, and {d} Meissner's corpuscles in the papillae. (For details see pages 438 and 439.) The ghms 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 attached to the fibrous sheath of the underlying erectile tissue. A few small sebaceous glands, unconnected 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. .1 I B • >'-,:'^^ f e::: ■^^..rn y - d -■.^■riSi Fig. 249. Fig. 250. Fig. 249. — From Transverse Section of Urethra and Corpus Spongiosum, including Mucous Membrane and part of Submucosa. X15. The dark spots represent the cavernous veins. Fig. 250. — Vertical Section through Portion of Wall of Human Male Urethra. X3S0. A, Mucous membrane; B, submucosa; a, epithelium; b, stroma; c, cavernous veins; d, connective 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 essen- tially the same structure as the male urethra. The epithehum 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 Li re pre?, t, but are fewer than in the male. 358 THE ORGANS mucous, submucous, and muscular. The structure of the wall dilters in the different parts of the urethra. The mucous membrane (Fig. 250) consists of ejnthelium and stroma. The epithelium of the prostatic part is stratified squamous (transitional), 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 base- ment membrane, beneath which is a thin stroma rich in elastic fibres and "having 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. 250). 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 (i) For the study of the general topography of the penis, remove the skin from the organ and cut into transverse slices about 0.5 cm. in thickness. Fix in formalin-Miiller's fluid (technic 6, p. 7), cut rather thick sections across the entire penis, stain with hasmatoxylin-picro-acid-fuchsin (technic 3, p. 21) or with hjematoxylin-eosin (technic i, p. 20) 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, leaving only the corpus spongiosum and contained urethra, and treat as above. Sections should be thin and stained with hajmatoxylin-eosin. (3) The same technic is to be used for the membranous and prostatic portions of the urethra. 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 TITF KKPRODUCTTVE SySTF,>r 359 directly continuous with its structure. In place of this it is provided 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 hilum, to the broad ligament. Elsewhere the surface of the ovary is covered bv a modified peritoneum. At llu- hilum the tissues of the broad ligament pass into the ovary and spread out there to form the ovarian Fig. 251. — Ovary opened l)y Longitudinal Incision. Ovum has Escaped through Tear in Surface. Cavity of follicle filled with blood clot (corpus hfemorrhagicum) and irregular projections composed of lutein cells. (Kollmann's Atlas.) stroma. This consists of fibrous connective tissue rich in elastic fibres and containing many smooth muscle cells. In the deeper cen- tral portion of the organ, stroma alone is found. Here it contains many large blood-vessels, and constitutes the medulla or zona vas- culosa of the ovary (Fig. 252, 2). From the medulla the stroma radi- ates toward the surface of the ovary and becomes interspersed with glandular elements forming the ovarian cortex (Fig. 252, 3, 3'). At the surface of the ovary, just beneath the peritoneum, the stroma forms a rather dense layer of fibrous tissue, the tunica alhuginea. At the margin of the peritoneal surface of the ovary the connective tissue of the peritoneum becomes continuous with the stroma of the ovary, while the llai mesothelium of the general peritoneum is replaced by a single layer of cuboidal cells, which covers the surface of the 360 THE ORGANS ovary and is known from its function as the germinal epithelium (Fig. 253, he). The parenchyma or secreting portion of the ovary consists of peculiar glandular elements, the Graafian follicles. The structure of the Graafian follicle can be best appreciated by studying its development. The follicles originate from the ger- minal epithelium during foetal life. At this time the germinal epithe- lium is proliferating, and certain of its cells differentiate into larger 'MfQ''% <<^-•''•.".■■■',''-■ Fig. 256. — Section through Graafian FolUcle of Ape's Ovary. X90. (Szymono- wicz.) Later stage of development than Fig. 255. a, Germ hill; &, ovum with clear zona pellucida, germinal vesicle, and germinal spot; d, follicular epithelium (membrana granulosa); e, foUicular cavity;/, theca folliculi; g, blood-vessel. The youngest of the Graafian follicles are found just under the tunica albuginea near the germinal epithelium, from which they orig- inate (Fig. 252, 5). As the follicle matures it passes deeper into the cortex. With complete maturity the follicle usually assumes macro- scopic 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 (Fig. 259), while at the same time an increase in the liquor folliculi determines increased intrafollicular pressure. This results in rupture of the Graafian follicle and the discharge of its ovum, together with the liquor folliculi and some of the follicular cells. THE REPRODUCTIVE SYSTEM 365 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 organized by the ingrowth of vessels from the theca, to form the corpus hcemorrhagiciim (Fig. 260), which represents the earliest stage in the development of the corpus luteum. Fig. 257. — Graafian Follicle and Contained Ovum of Cat; directly reproduced from a photograph of a preparation by Dahlgren. X 235. (From "The Cell in Devel- opment 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. EncircHng the follicle is the connective tissue of the theca folliculi. The corpus luteum (Fig. 261), which replaces the corpus haemor- rhagicum, consists of large yellow cells — lutein cells — and of connec- tive 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 by some as derived from the connective-tissue cells of the theca; by others as the result of proliferation 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 366 THE ORGANS these granules that the characteristic yellow color of the corpus luteum is due. A dehnite cellular structure with a supporting con- nective-tissue framework thus replaces the corpus htcmorrhagicum, remains of which are usually present in the shape of orange-colored crystals of haematoidin. By degeneration and subsequent absorption of its tissues the corpus luteum becomes gradually reduced in size, Corona radiata Volk granules. Zona pellucida Fig. 258. — From a Section of a Human Ovum. Section 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.) 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 lutea (corpora lutea vera or corpora lutea of pregnancy) and false corpora lutea (corpora lutea spuria); The former replace follicles whose ova have under- gone fertilization, the latter, follicles whose ova have not been fer- THE RKPROnUCTIVK SYSTEM 307 Germinal cpjlhelium I ■ I ".+1 ' m fa- 4 Tunica albuginea Theca folliculi (vascular layer) Theca folliculi (fibrous layer) Stratum granulosum ^ .^;^ 'Mhi^f^ ■^;t --=>s:gvjSir Fig. 259. — From Section of Human Ovary, showing mature Graafian follicle ready to rupture. (Kollmann's Atlas.) f^^ii'^' ■••,?. tit. Point of rupture Lutein cells Corpus haemorrhagicum < Blood vessel of theca Cavity of follicle Theca folliculi Fig. 260. — From or( lior. -^i Human Ovary, showing early stage in formation of Corpus Ltiteum. ;Kollmann's Atlas.) 368 THE ORGANS tilized. 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 corpus luteum is not known, the recent experiments of Fraenkel seem to be confirmatory of the theory advanced by Born, that the corpus luteum is a gland having an inter- Point of rupture Blood vessel of theca Connective tissue Remnant of corpus haemorrhagicum Lutein cells Connective tissue from theca Theca folliculi Blood vessels of theca Fig. 261. — From Section of Human Ov^ary, showing later stage of Corpus Luteum than Fig. 260. (KoUmann's Atlas.) nal secretion, which appears to have some influence upon the attach- ment of the fecundated ovum to the uterus and upon its nutrition during the first few weeks of its development. According to Fraenkel the corpus luteum is a periodically rejuvenated ovarian gland, which gives to the uterus a cyclic nutritional impn t t . which prepares it for the implantation of the ovum or favors nc-nstruation whenever the ovum is not fertilized. THE REPRODUCTIVE SYSTEM 369 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, to- gether with their follicles, retrogressive changes known as atresia of the follicle. The nucleus of the ovum, as well as the nuclei of the follicular cells, passes through a series of chromatolytic changes, or in some cases apparently simply atrophies. The cell bodies un- dergo fatty or albuminous degeneration and the cells become reduced to a homogeneous mass, which is fmally 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 capillary 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. — INIeduUated 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 foUicuH. Some describe fibres as passing through the theca and ending in the follicular epithelium. Others claim that nerve fibres do not enter the follicle proper. Groups 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 parobpJwron consists of a number of cords or tubules of epi- theHal cells, sometimes ciliated, sometimes non-ciliated. It is found in the medulla, or, more commonly, in the connective tissue of the hilum. The epobpJioron is a similar structure found in the folds of the broad Kgament. Its tubules open into a duct known as Gartner's duct. In man this duct ends blindly. In some of the lower animals it opens into the vagina. Both paroophoron and epoophoron are 24 370 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 r-c 111 ^'-c^'jjiv, —a n '=? — •"•'■? ^^"^ ' Fig. 262. — 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.) length of 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. 262 and 263). The mucous mernbrane- presents numerous longitudinal foldings. In the embryo four of these folds can usually be distinguished, and these are known as prii\iary 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. 263). The epithelium lining the tube is of the simple columnar ciliated type, "and completely covers the foldings of the mucous mem- THE REPRODUCTIVE SYSTEM 371 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 isth- mus, consists of discontinuous groups of muscle cells in the ampulla, and in the fimbriated extremity is frequently absent. *'- -^^i^. "^■t! :'iy iM ^iS^%< mmsm&Sw Fig. 263. — Cross Section of Oviduct near Fimbriated Extremity, showing complicated foldings 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 epithelial lining. TECHNIC (i) Child's Ovary. — Remove the ovarj^ of a new-born child, being careful not to touch the surface epithelium, fix in Zenker's fluid (technic 10, p. 8), and harden in alcohol. Cut sections of the entire organ through the hilum. Stain with haematoxylin-eosin (technic i, p. 20) and mount in balsam. (2) For the purpose of studying the Graafian follicle in the different stages of 372 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 corpus 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 possi- ble a normal human ovary should be obtained from a 3'oung subject for purposes of comparison with the above. Technic (i). (4) For studying the egg cords of Wliiger 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 ovar>\ For the study of other parts of the tube, cut out thin pieces from different regions, fix in formahn-MuIler's fluid, stain transverse sec- tions with haematoxyhn-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 mtiscularis 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 suhmucosum — is mainly longitudinal, although some obliquely running bundles are usually present. The middle layer — called from the large venous 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 se'rosa. Fig. 264. — Muscle cells from (a) non- pregnant uterus; h, pregnant uterus; drawn to same scale. (Sellheim.) THE REPRODUCTIVE SYSTEM 373 The muscle cells of the uterus are long spindle-shaped elements, some having pointed, others blunt, branched, or frayed ends. In the virgin uterus they have a length of from 40 to 60/'.. During preg- nancy 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 have a length of from 250 to 6oo/«. (Fig. 264.) The mucous membrane. As the mucosa presents marked variation in structure, dependent upon the functional condition of the organ, it is necessary to describe: 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 v/- • ,i • .- ■■■:'■ •■■..••■: ■■:::<: '^•■- ••;•..■. ■■■■ V'Wy V-'- ■ >- a :&=; This is from i to 2 mm. thick, and consists of a stroma, glands, and a lining epithelium (Fig. 265). The stroma resembles embrvonal 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 cihated 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 frequently turned at right angles to the rest of the tubule. In the cervix the stroma is firmer and less cellular, and the mu- Fig. 265. — From Uterus of Young Woman. (Bohm andvonDavidoff; preparation by Dr. J. Amann.) X34. a, Mucous membrane; h, surface epithelium; c, gland; e, muscle. 374 THE ORGANS cous membrane is thicker and presents numerous folds — the plicce palmatcB. The epitheUum 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. .^ l--r-»' ^ occurs, leading to the formation . _^ of retention cysts, the so-called ovula Nabothi. At about the ^ .. .^. . " ^ '' junction of middle and lower 5 .. .- d thirds of the cervical canal a I change takes place in the epi- ^ '' , thelium. Here the simple .. - columnar ciliated epithelium / ; " ii of the upper part of the cervix Fig. 266.-From Section of Dog's Cervix, gradually paSSeS OVer into a X4- (Technic 2, p. 385.) a, Cervical canal; stratified SquamoUS epithe- b, mucosa; c, folds of mucosa: (plicas pal- . "XT +>i f 1 matae); d, muscle layers of cervix; e, epithe- lium. JNI ear tne external OS Hum of vagina and vaginal surface of cervix; p^piH^ appear, the vaginal /, vagmal epithelium; g, vaginal mucosa; h, ^ ^ ^ ^ ^ ° _ sub mucosa and muscularis of vagina; I, blood- surface of the cervix being ''®^^^^^- 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. {h) The stage of menstruation proper. {c) The stage of reparation. (a) The Stage of Preparation. — This begins several days before the actual flow of blood, and is marked by an intense hyperaemia determining a swelling and growth of the entire mucosa. Tlir: REPRODUCTIVE SYSTEM 375 b -tf The 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 menstrualis (Fig. 267). {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 rup- ture 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 cavity 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 re- mains almost intact. Complete destruction of the epithelium is hardly compatible 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 con- siderable depth, the deeper portions of the glands always remain to take part in the succeeding regenerative phenomena. (c) The Stage of Reparation. — x\fter from three to five days the bleeding from the uterine mucosa ceases and the return to the resting condition begins. This is marked by disappearance of the Fig. 267. — Section through Mucous Membrane of Virgin Uterus during First Da}' of Menstruation. X30. (Schaper.) a, Surface epithelium; b, disintegrating surface; c, pit-Hke depression in mucous membrane; d, excretory duct; e, blood- vessels; ,(j, gland tubule; //, dilated gland tubule; ;«, muscularis. m 376 THE ORGANS congestion, by decrease in thickness of the mucosa and in the size of the glands, and by restoration of the surface epithelium. 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. {b) The decidua reflexa or decidua capsularis — that part of the mucosa which surrounds the ovum. {c) 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 between the mouths of the glands becomes quite dense and firm, forming the compact layer. The deeper part of the stroma contains numerous 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 connective tissue. They are of large size (30 to ioo«), vary greatly in shape, and in the later months of pregnancy have a rather charac- teristic brown color, which they impart to the superficial layers of the decidua vera. They are mostly mononuclear, although poly- nuclear forms occur. During 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 vera. 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 REPRODUCTIVE SYSTEM 377 The decidua serotina undergoes changes connected with the development of the placenta. The Placenta^ The placenta consists of two parts, one of which is of maternal origin — placenta uterina- — the other of foetal origin — the placenta Jcetalis. 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 hnger-like projections — villi — which penetrate the mater- nal 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 fcetus the chorion after the third month is covered by a delicate foetal membrane, the placen- tal 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 (&) an inner villous layer, which gives rise to finger-like projections which extend down from 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. 268) . The secondary villi are given off laterally from the primary villi, end freely in the spaces between the latter — intervillous spaces (Fig. 268) — and are known as free, terminal or floating villi (Fig. 268). The chorionic villus thus consists of a central core of connective ^ 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 fifth edition of Stohr's "Textbook of Histolosv." 378 THE ORGANS c i- 3 Kl Co o n o o G s a to s 00 6 O O :^ p. >- 0 a)-- _3 3 5 S JS5 a o ■ 3 'II IK REPRODUCTIVE SYSTEM 379 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 villi of early pregnancy the epithelium consists of an inner layer of distinctly outlined cells and an outer layer of fused cell bodies — a syncytium (Fig. 270, 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 homogeneous membrane, probably the remains of the syncytium. At various points on the surface of the villus are groups of nuclei. These stain "Giant" cell .^.r. ^'v.'."*^:; •.-. :■ f •;:;:^F>'''-«^ -> A' Syncytium 3^ ''iit-V-.. Ikt^J Trophoder:;! mass Stroma of villus Fig. 269. — Section of Chorion of Human Embryo of one month (9 mm.). (Grosser.) intensely, are surrounded by a homogeneous protoplasm, and form knob-like projections above the general surface of the villus. They are known as cell patches, or more properly as nuclear groups (Fig. 270, c), and represent remains of the nuclei of the epithelium of the younger villus. Between the nuclear groups the villus is covered only by a thin homogeneous membrane. Small villi usually resemble more closely in structure the younger villus, being frequently covered by a nucleated syncytium. Portions of the syncytium, especially of older villi, sometimes become changed into a peculiar hyaline sub- stance containing numerous channels. This is known as canalized 380 THE ORGANS c — d — "C fibrin, and may form dense layers upon the surface of the chorion. (Fig. 269.) The Placenta Uterina.^ — This develops from the decidua sero- tina. The latter becomes much thinner than the rest of the decidua (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 pla- centa, as described above, separat- ing its villi into cotyledons. Near the margins of the placenta these septa pass to the chorionic mem- brane and form beneath it a thin membrane, the subcJiorionic pla- cental decidua. At the edge of the placenta, 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 espe- cial 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. 268) their walls lose their muscular and connective- tissue ele- ments and become 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. It is to be noted that the blood-vessel systems of the mother and of the foetus are both closed systems, and that consequently there is no direct admixture of maternal and foetal blood. Inter- change of niaterials must therefore always take place through the capillary walls and through the walls of the chorionic vilh. (Fig. 268.) B Fig. 270. — Cross Sections of Human Chorionic Villi at End of Pregnancy. X250. (Schaper.) A, Small villus; B, larger villus, a, Protoplasmic coat (sj'ncytium); b, epithelial nucleus; c, nuclear groups; d, small artery; e, small vein; /, capillaries. TUE RKI'RODUCTIVE SYSTEM 381 Blood-vessels.— The arteries enter the uterus from the broad ligament and pass to the stratum vasculare of the muscularis, where they undergo extensive ramification. From the arteries of the stra- tum vasculare branches pass to the mucosa and give rise to ca])ilhiry networks, 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 vas- culare. 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, submucous, 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 submucosa 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 diffuse 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, 382 THE ORGANS but is folded transverseh', forming the so-called rugce. Most au- thorities agree that glands are wanting in the vagina, the mucus found there being derived from the glands of the cervix. Blood-vessels. — The larger blood-vessels run in the submucosa, giving off branches which break up into capillary networks in the sub- 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. IMedullated (sensory) fibres, the dendrites of spinal ganglion cells, form plexuses in the mucosa, from which are given off delicate non-medullated terminals to the epithe- lial 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 — glandulce vestib- ulares minores — especially numerous around the clitoris and opening of the urethra. Larger mucous glands — glandulce vestibulares ma- jores, 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 papillae, and is richly supplied with nerves having highly specialized terminations. ' Develop AiEiXT of the Urinary and Reproducti\'e Systems The development of the genitourinaty system is complicated by the appear- ance, and disappearance for the most part, of two sets of urinary organs, and the linal 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 present 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 intimately 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 pf reptiles and in all birds and mammals. The pronephroi are represented in the human embryo of 3 to 5 mm. by one or two small, condensed masses of mesoderm just lateral to the primitive segments THE REPRODUCTIVE SYSTEM 383 on each side in the cervical region. These masses, which are probably derived from the mesothelial lining of the body cavity, may or may not become hollow, but do not connect with the pronephric duct, and soon disappear. The pro- nephric 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 pronephroi and just caudal to them. Condensations appear in the mesodermal tissue lateral to the primitive segments and become more or less tortuous. 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 struc- tures 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 develop- ment 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 glomeruli but also by means of the epithehum 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 meta- nephroi 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 efferentia, while a few in the caudal portion remain as the paradidymis and vasa aberrantia; the duct persists as the vas epididy- midis, vas deferens, and ejaculatory duct. In the female the mesonephric tu- bules disappear for the most part, only a few remaining to form the epoopho- ron 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 the mesonephros. The proximal portion remains more slender as the ureter, while the distal end becomes 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 convoluted tubule. The portion of each tubule derived from the mesodermal 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). 384 THE ORGANS During foetal life ihe kidneys are disLincUy lobulated, but after birth the surface becomes quite smooth. The genital gland on each side appears on the mesial surface of the meso- nephros 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 deeper layer which is des- tined 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 superficial 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 about 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 disappear. The surface epithelium again sends plugs of cells (Pfl tiger'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 oocyte (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 com- pleted before birth. (See also p. 360.) 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 flat cells. The sex cords which first grew into the underlying tissue and which contain the sex cells, are destined to give rise to the convoluted 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, Plliiger'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. 341.) THE REPRODUCTIVE SYSTEM 385 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 lo, p. 8) or in formahn-Mullcr's fluid (technic 6, 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 haematoxjdin-picro-acid-fuchsin (technic 3, p. 21) 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 hajmatoxylin-eosin (technic i, p. 20), 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 the same technic (i). (4) If a human or animal uterus with the placenta m 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 utcro-placental junc- tion. They may be stained with haematoxylin-eosin or with ha^matoxylin-picro- acid-fuchsin (see above). (5) Treat pieces of the human vagina according to technic i, p. 259. 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 Wirbelthiere, Jena, 1896. Kolliker: Handbuch der Gewebelehre des Menschen. Xagel: 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, 1S96. — Ueber die Bildung des Corpus luteum beim Kaninchen. Anat. Hefte, I. Abth., Bd. viii, 1897. 2J CHAPTER XX 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 epithehal derivative of the ectoderm. ---- - 6 ig|p?a >>«;«1 Fig. 271. — Vertical Section of Thin Skin, Human. X60. (Technic 2, p. 391.) a Epidermis; b, pars papillaris of derma; c, papilla; d, pars reticularis of derma; e, duct of sweat gland;/, sweat gland; g, subcutaneous fat. The Derma. — This is divided into two layers which blend without distinct demarcation. The deeper is known as the pars reticularis, the more superficial as the pars papillaris (Fig. 263). The pars ^''cularis is made up of rather coarse, loosely arrai -red 386 TITK SKTX AXD FTS APPEXDAGES 387 white and elastic fibres with connective-tissue cells in \arying num- bers. The fibres run lor the most part parallel to the surface of the skin. ^w The pars papillaris is similar in structure to the precedingT^ut 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 tlic thick skin of the palms and soles these furrows arc close together and parallel, while between them are long corresponding ridges. In addition to the furrows and A\ Bl C [ Fig. 272. — 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- pillm — others in several points — compound papillcE; some containing blood-vessels — vascular papillcs; others containing special nerve terminations — nerve papilla (Fig. 273). 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 penetrate the corium. "■ eneath the corium is the subcutaneous tissue. ^^ '"; consists of 388 THE ORGANS 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 pannicidus adiposiis. The Epideemis.- — This is composed of stratilied squamous epithelium. In the comparatively thin skin of the general body sur- face the epidermis is divided into two sublayers: (i) One lying just above the papillary layer of the derma, and known as the stratum '^.Ki:- -i^ '^'m ' -:;d«'- if ?'-5"»'S:i. \? O' '* ..... t..i - -p;.y^ //• «>« V \ > - ^ Fig. 273. — From Vertical Section through Skin of Human Finger Tip. X200. (Schafer.) a, Stratum corneum; b, stratum hicidum; c, stratum granulosum; d, stratum germinativum. To the left a vascular papilla; to the right a nerve papilla containing tactile corpuscle. germinativum (stratum mucosum — stratum Malpighii).; (2) the other constituting the superficial layer of the skin — the horny layer or stratum corneum. In the thick skin of the palms and soles two ad- ditional layers are developed; (3) the stratum granulosum; and (4) the stratum lucidum (Fig. 272). (i) The stratum germinativum consists of several layers of cells, The deepest cells are columnar and form a single layer (stratum cylindricum), which rests upon a basement membrane separating it THE SKTX AND ITS APPENDAGES 389 ■'t-p/'v ,-V.' -I' — f. V r I* V ® <^ ■<© f ^ t fc& I 1 0t> x/ <§> «?P ^ # 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 ^>==:f^ 3 "prickle" cells, and the layer, the - | ''stratum spinosum." The spines * _ ' S cross minute spaces between the cells, which are believed to communicate with the lymph spaces of the derma (Fig. 274, c) . The cells of the stratum germinativum are usually in a state of active mitosis. (2) The stratum granidosum 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. 274, b). (3) The stratum lucidum is also best developed where the skin is thickest. It consists of two or three layers of fiat 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. 274, a). (4) The stratum corneum varies greatly in thickness, reaching its greatest development in the skin of the palms and soles. The cells ^. <^ t a r^. Fig. 283. — Five stages in the development of a human hair. (Stohr.) a, Papilla; b, arrector pili muscle; c, beginning of hair shaft; d, point where hair shaft grows through epidermis; e, anlage of sebaceous gland; /, hair germ; g, hair shaft; h, Henle's layer; i, Huxley's layer; k, cuticle of root sheath; /, inner root sheat; m, outer root sheath in tangential section; 11, outer root sheath; 0, connective-tissue follicle. 26 402 THE ORGANS an outer, the future stratum corneum, and an inner, the future stratum germ- inativum. The stratum granulosum and stratum lucidum are special develop- ments of the stratum germinativum. 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 stra- tum 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 disappears, 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 thicken- ing 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 wall, 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 dift'erentiate 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. 283). 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 germina- tivum into the underlying corium. The lower end of the ingrowth becomes thickened and convoluted to form the coiled portion of the gland, and somewhat later the central portion becomes channelled out to form the lumen. The muscle tissue of the sweat glands, which lies between the epithelium and the basement membrane, is the only muscle of the body derived from the ectoderm. The Mammary Gland The mammary gland is a compound alveolar gland. It consists of from fifteen to twenty lobes, each of which is subdivided into lobules. The gland is surrounded by a layer of connective tissue containing more or less fat. From this periglandular connective tissue broad septa extend into the gland, separating the lobes (inter- lobar septa). From the latter finer connective-tissue bands pass in between the lobules (interlobular septa). From the interlobular septa strands of connective tissue extend into the lobule where they act as support for the glandular structures proper. An excretory duct passes to each lobe where it divides into a number of smaller ducts THE SKIN AND TTS APPEXDAGES 4().S (lobular ducts), one of which runs to each lobule. Within the latter the lobular duct breaks up into a number of terminal ducts, which in turn open into groups of alveoli. The lifteen to twenty main excre- tory ducts pass through the nipple and open on its surface. At the base of the nipple each main duct presents a sac-like dilatation, the ampulla, which appears to act as a reservoir for the storage of the milk. m '&■■ Q-O ■f J (m _^i Fig. 284. — From Section of Human Inactive Mammary Gland. X25. (Technic I, p. 407.) Gland composed almost wholly of connective tissue; few scattered groups of tubules. Until puberty the gland continues to develop alike in both sexes, but after about the twelfth year the male gland undergoes retrogres- sive changes, while the female gland continues its development. The inactive mammary gland, by which is meant the female gland up to the advent of the first pregnancy and between periods of lactation, consists mainly of connective tissue and a few scattered groups of excretory ducts (Fig. 284). Around the ends of some of the ducts are small groups of collapsed alveoli. Both ducts and alve- oli are lined with a low columnar, often rather flat epithelium. In some cases the flat cells are two or three layers thick, forming a thin stratified squamous epithelium. The relative amount of fat and 404 THE ORGANS connective tissue varies greatly, some inactive mammae consisting almost wholly of fat tissue. The Active Mammary Gland. — Throughout pregnancy the gland undergoes extensive developmental changes and becomes func- tional at about the time of birth of the child. The microscopic appearance of the active gland differs greatly from that of the inactive (Fig. 285). There is a marked reduction in the connective tissue of the gland, its place being taken by newly developed ducts and alveoli. The alveoli are spheroidal, oval, or irregular in shape, and vary con- -/) Fig. 285. — From Section of Human Mammary Gland during Lactation. Xso. (Stohr.) a, Branch of excretory duct; b, interlobular connective tissue; c, alveoli. siderably in size. The alveoli are lined by a single layer of low col- umnar or cuboidal epithelial cells which rest upon a homogeneous basement membrane. The appearance of the cells differs according to their secretory conditions. The resting cell is cuboidal and its protoplasm granular. With the onset of secretion the cell elongates, and a number of minute fat droplets appear. These unite to form one or two large globules of fat in the free end of the cell. The fat is next discharged into the lumen of the alveolus, and regeneration of the cell takes place from the unchanged basal portion (Fig. 287). As to the number of times a cell is able to go through this process of secretion and repair before it must be replaced by a new cell. THE SKIN AND ITS APPENDAGES 405 nothing dcrmite is known. Active secretion does not as a rule take place in all the alveoli of a lobule at the same time. Each lobule thus contains both active and inactive alveoli. The smallest ducts are lined with a low columnar or cuboidal epithelium. This in- creases in height with increase in the diameter of the duct until in the largest ducts the epithelium is of the high columnar type. The secretion of the gland is milk. This consists microscopic- ally of a clear fluid or plasma in which are suspended the milk Fig. 2S6. — From Section of Mammary Gland of Guinea-pig during Lactation, X500. (Osmic acid.) (Szymonowicz.) a, Basement membrane; 6, lumen oi alveolus; c, tangential section of alveolus; d, fat globules. globules. The latter are droplets of fat from 3 to 5," in diameter, each enclosed in a thin albuminous membrane which prevents the droplets from coalescing. Cells, probably leucocytes, containing fat droplets may also be present. In the secretion of the gland during the later months of pregnancy, and also for a few days following the birth of the child, a relatively large number of large fat-containing leucocytes — colostrum corpuscles — are found. Blood-vessels. — These enter the gland, branch and ramify in the interlobar and interlobular connective tissue, and finally terminate in capillary networks among the alveoli and ducts. From the capillaries arise veins which accompany the arteries. 406 THE ORGANS Lymphatics.— Lymph capillaries form networks among the alveoli and terminal ducts. The lymph capillaries empty into larger lymph- atics in the connective tissue. These in turn communicate with several lymph vessels which convey the lymph to the axillary glands. Nerves.— Both cerebro-spinal and sympathetic nerves supply the gland, the larger trunks following the interlobar and interlobula ST.. n n A / X » -\ ^^^^^<^^,, rr\ n B Q) /" ^ C Fig. 287. — -^j Mammary gland cells — secreting stage; B, same — excreting stage; the secretion having separated from the cells; C, resting stage; ?i = nucleus. e = ergasto- plasm filaments. ^ = droplets of secretion. (Simon.) connective-tissue septa. The nerve terminals break up into ple.xuse which surround the alveoli just outside their basement membranes. From these plexuses delicate fibrils have been described passing through the basement membrane and ending between the secreting cells. THE SKTX AXD ITS APPENDAGES 407 Development. — The development of ihe mammary gland is quite similar to the development of the sebaceous glands. The gland first appears as a dipping down of solid cord-like masses of cells from the stratum mucosum. The alveoli remain rudimentary until the advent of pregnane}'. After lacta- tion the alveoli atrophy, being replaced by connective tissue, and the gland returns to the resting state. After the menopause a permanent atrophy of the gland begins, fat and connective tissue ultimately almost wholly replacing the glandular elements. TECHNIC (i) Fix thin slices of an inactive mammary gland in formalin-Miiller's fluid (technic 6, p. 7). Stain sections with haimatoxylin-cosin (technic i, p. 20), and mount in balsam. (2) Prepare sections of an active mammary gland, as in preceding technic (i). (3) Fix very thin small pieces of an active gland in one per cent, aqueous solution of osmic acid. After twenty-four hours wash in water and harden in graded alcohols. Thin sections may be mounted unstained, or after slight eosin stain, in glycerin. General References for Further Study Kolliker; Handbuch der Gewebelehre des Menschen. ]\Ic;Murrick: Development of the Human body. Ranvier: Traite Technique d' Histologic. Schafer: Essentials of Histology. Spalteholz: Die Vertheilung der Blutgefasse in der Haut. Arch. Anat. u. Phys., Anat. Abth., 1893. CHAPTER XXI THE THYREOID AND PARATHYREOID, THE PITUITARY BODY, THE PARAGANGLIA AND THE ADRENAL. The Thyreoid The thyreoid (Fig. 288) is a ductless structure built upon the general principle of a compound alveolar gland. There are usually two 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 Fig. 28S. — Section of Human Thyreoid. Most of the alveoli contain colloid. 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 i2o«) 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 alveoli are lined with a single 40s THE THYREOID AND PARATHYREOID 409 or double layer of cuboidal c{)ithclial cells. Two types of cells are recognized. One of these is actively secretin.^ colloid and is known as a secreting 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 rest- ing cells. In certain alveoli which are much distended with colloid the lining epithelium is flattened. That the thyreoid exerts a decided influence upon general body metabolism is shown by the symptoms resulting from congenital absence of the gland (con- genital myxcedema 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. L5miphatics 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 against the bases of, 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 ento- derm 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 lies transversely across the neck. It is composed of solid cords of cells which become hollow 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. In- growths of connective tissue divide the solid cords of cells of which the gland at this stage consists into groups or lobules, and at the same time break up the long cords into short segments. Dilatation of the alveoli occurs with the formation of colloid. 410 Till-: ORGANS The Parathyreoids These are small ductless glands which usually lie upon the posterior surface of the lateral lobes of the thyreoid. There are commonly two 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 ])readth, and 2 mm. thick. Small groups of cells having the structure of the parathyreoids have been found below the thyreoid and within the thyreoid and thymus. Fig. 289. — Section of Human Parathyreoid; showing mainly "clear," "principal,' or "chief" cells. (Pool.) 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 tis- sue and is very vascular. The number and arrangement of the cells vary. The gland may be almost wholly cellular with very little con- nective tissue, the groups of cells may be widely separated by inter- stitial tissue, or there may be any intermediate condition. The cells are arranged in irregular groups or cords (Figs. 289, 290) sometimes around tubules, sometimes having a distinctly alveolar structure (Fig. THE THYREOID AXI) PARATHYREOID 411 291); in the latter case colloid niay 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) CJiieJ or clear cells {Fig. 2Sg). These are the more numerous. Their bodies are small and clear, and the cytoplasm does not stain readily. The nuclei Fig. 290. — Section of Human Parathyreoid showing groups of oxyphile cells. (Pool.) are large in proportion to the cell and are clear and vesicular with loosely arranged pale chromatin network. (2) Oxyphile cells (Fig. 290). 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 de- 412 THE ORGANS scribed. It is probable that the dear cells represent the resting, the granular cells the active secreting condition of the same cell. The parathyreoids originate as epithelial cv aginations 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.' Fig. 291. — Section of Human Parathyreoid, showing lumina indicating tubular or tubulo-alveolar structure. (Pool.) TECHNIC The thyreoid and parathyreoid glands are best fixed in formalin-MuUer's fluid. Sections may be stained with haematoxylin-eosin or hajmatoxylin-picro-acid- fuchsin and mounted in balsam. General Reference for Further Study 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. PITUITARY liODV 413 The Pituitary Body The PitiiiUiiy Eod} or Hypophysis Cerebri consists of two main lobes which are totally different both in structure and in origin. They are separated by a cleft, the interglandular cleft, and a narrow strip of the posterior lobe bordering the cleft presents a different struc- ture from other parts of the gland. This has been designated the pars intermedia (Fig. 292). The Anterior Lobe {pars anterior). — This is the larger of the two lobes and is distinctly 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 diverticulum ultimately atrophies and disappears, leaving the gland entirely unconnected with the sur- face. 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 clear or finely granular cytoplasm {chief cells). Others, somewhat less numerous than the preceding, are larger and polygonal with centrally placed nuclei, and cytoplasm containing coarse basophile granules {chromophile cells). Cells with distinctly eosinophile granules may also be present. There has been much controversy as to whether these cells are fundamentally different or merely represent different secretory conditions or stages. The large variation in relative number of the different forms, and the occurrence of cells which apparently represent intermediate stages, render it probable that all should be considered as merely different functional conditions of the one type of cell. As in all ductless glands, the blood supply is rich, the vessels being sinusoidal in character, and the relations of capillaries to gland cells is extremely intimate, dense networks of capillaries surrounding the alveoli on all sides. In some places the cells are so placed around a capillary as to resemble the relation of gland cells to the lumen. The Posterior Lobe {pars posterior, pars nervosa). — This like the anterior lobe is surrounded by a connective- tissue capsule which sends trabeculae into its substance. In the human adult the lobe consists mainly of neuroglia with a few scattered" cells, which probably 414 THE ORGANS represent rudimentary ganglion cells and a few nerve fibres. 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 e f Fig. 2Q2. — Mesial Sagittal Section through Pituitar\' Body of Five Months Human foetus. (Herring.) a, Optic chiasma; b, anterior extension of pituitary body; c, third ventricle (infundibulum) ; d, pars anterior; e, neck or isthmus of pars nervosa connecting it with bottom of infundibulum; /, epithelium continuous with pars intermedia sur- rounding neck; g, intcrglandular cleft; h, pars nervosa or posterior. 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. (3) An inner layer of nerve cells and neuroglia cells. These react to the Golgi stain, the nerve cells hav- ing 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 THE PARAGAXCWJA 415 diverticulum from the floor of the third ventricle. The remains of the diverticulum constitute the infundibulum. The so-called middle lobe or pars intermedia is a thin layer of tissue which covers the anterior aspect of the posterior lobe thus lying between it and the interglandular cleft. It develops with the anterior lobe and like it consists of a connective-tissue framework and epithe- lial cells. It is less vascular, however, and its cells are smaller and less distinctly granular. The cells lining the cleft are columnar, thus contrasting with the flat cells of the opposite wall. Character- istic of the pars intermedia are small cyst-like structures which con- tain colloid, presenting an appearance not wholly unlike thyreoid, although chemical!}- the colloid cf the two glands is not identical. Lining these tiny cysts is a cuboidal epithelium. The nature of ihe secretion, its function, and its relation to the various kinds of cells have been subjects of much controversy. The granules within the cells undoubtedly represent an intracellular stage of the secretion. According to some investigators each type of cell contributes its own special secretion to the general secretion of the gland. According to most authorities the different types of cells represent merely different stages in the elaboration of the secretion. Some consider the colloid a special secretion, others derive it from the cell granules and consider it as possibly representing a final stage, or "normal de- generative condition of the secretion." Pregnancy apparently induces increased activity of the gland. This is of interest in connection with the clinical use of the extract of pituitary body for the purpose of inducing contractions in an atonic uterus during parturition or for the control of post-partum bleeding. Pathological conditions of the pitu- itary, more particularly tumors or hyperplasia of the anterior lobe, are apparently associated with a clinical condition known as acromegal}^ in which there is marked connective-tissue hypertrophy, especially of certain bones. The con- nection between acromegaly and lesions of the pituitary body would seem to be somewhat similar to that between myxcedema and lesions of the thyreoid. Re- moval of the thyreoid results in enlargement of the pituitary body, especially in increase of colloid. Increase of colloid has also been reported after removal of the pancreas. The Paraganglia Under the head of ParagangHa are grouped certain small ductless glands and small groups of cells which are closely associated both anatomically and embryologically with the sympathetic system. They include the carotid, the coccygeal, and tympanic glands, the para- sympathetic organ of Zuckerkandl, and the medulla of the adrenals. The most marked characteristic of these organs aside from their 'O" 416 THE ORGANS close relation to the sympathetic, is that their cells take a yellowish brown stain when placed in solutions of chromic acid or chrome salts, retaining the color even after prolonged washing in water. For this reason the cells are sometimes referred to as chromaffin cells and the organs as chromaffin organs} While each organ -has its own peculiar structure, all agree in certain general features, (i) The cells are polyhedral and have a general arrangement into cord-like structures rather than into lobules; (2) they are all ductless glands; (3) the cells lie in very close relation I *E^^gi;,^ Septum.- Trabecula of cells in cross- section. Distended blood capil- laries. Sci.^^- Efferent vein. Fig. 293. — Section of human carotid gland. X 160. (Schaper.) to rich capillary networks of large vessels, some of which resemble sinusoids and have been described as having incomplete walls, the gland cells thus being in direct contact with the blood stream; (4) unstained the cells show a clear, highly refractive cytoplasm, but con- tain secretion granules which stain yellowish brown with chromic acid and chrome salts, red with safranine, and black with osmic acid; (5) all, as far as has been determined, produce an internal secretion which passes directly into the blood and which acts as a regulator of vascular tone. The Carotid Glands. — These are two small ductless glands, each about the size of a rice grain, which lie one on either side of the ^ Cells containing chromal^in granules have also been described as occurring in sympathetic ganglia, and by Rose as present in many different organs, e.g., the ovary, testicle, blood-vessel walls, etc. THE COCCYGEAL GLAXD 417 bifurcation of the carotid artery. They are composed of a vascular connective tissue supporting spheroidal groups of large polyhedral epithelial ceils with poorly marked boundaries and closely associated with tufts of capillaries. These capillaries are of large diameter and thin walled, and have been described as sinusoidal in character. The amount of connective tissue and the blood-vessels increase with age at the expense of the epithelial elements. This probably led to ft ^...^-^ 2 2 Ttt*i '^\ J / 2 Fig, 294, — Section through coccygeal gland. (Walker.) i. Blood space; 2. epithelium; 3. connective tissue. the earlier description of the gland as a vascular or glomerular struc- ture. The gland cells themselves contain chromaffin granules (p. 416) and secrete a blood-pressure-raising substance apparently identical with, or similar in nature to, the secretion of the adrenal (p. 418). The Coccygeal Gland. — This is a small ductless gland which lies just in front of the apex of the coccyx. It is similar in structure to the preceding but has its cell groups more irregularly arranged (Fig. 294). There is the same general arrangement of gland cells, the same relation of gland cells to the connective-tissue framework and to large sinusoidal blood-vessels, the same vascular and con- 27. 418 THE ORGANS Kr-i i-^ ■••^.'- ■:?'^'' •'•'';-v:'-^.i^|/' nective tissue changes with age, the same reaction of its cells to chrome salts, and the same or a similar secretion. The t3mipanic gland and the organ of Zuckerkandl are small col- lections of chromaffin cells, the former lying on Jacobson's nerve in the tympanic canal, the latter lying I ^ in the retroperitoneal tissue at about J the level of the bifurcation of the abdominal aorta. The Adrenal The adrenal is a ductless gland situated on the upper and anterior surface of the kidney. It is sur- rounded by a capsule and consists of an outer zone or cortex and a central portion or medtilla. The cortex and medulla are sharply differentiated both in general appearance and in histological structure. The former is of rather lirm consistency, its cells are arranged in rows wdth 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, vas- cular, has a dark reddish appearance, and its cells contain granules known as chromaffin or phceochrome granules. The CAPSULE (Fig. 295, A) is composed of fibrous connective tissue and smooth muscle. In the outer part of the capsule the connec- tive 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 organ forming its framework and outlining compartments, (.-.•■.: :2 m ■■M ■y9^ :'■ '{jS ■■•■.■■ ■.::.: -.'''fj;!'- t'i^.- '-—■ -■'-*^-' J of A. Fig. 2Q5. — Vertical Section Adrenal. (Merkel-Henle.) Capsule; B, cortex; C, medulla; (7, glomerular zone; b, fascicular zone; c. reticular zone; v, vein in medulla. THE ADRENAL 110 which CDiitain the glancluhir ci)ithelmm. This coiuicctivc tissue is reticular in character. The CORTEX (Fig 295, 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. 295, a) the high, irregularly columnar epithelium is arranged in spherical or oval groups. The protoplasm of the cells is granular, contains fat droplets, and their nuclei arc rich in chromatin. In the fascicular zone (Fig. 295, b) polyhedral cells are arranged in long columns or fascicles. The cytoplasm is granular and usually contains many large fat droplets. The nuclei contains less chromatin than those in the glomerular zone. The appearance which the protoplasm of the cells richest in fat presents has led to their being called "spongi- oblasts." In the reticular zone (Fig. 295, c) similar though somewhat more darkly staining cells, containing small fat droplets or sometimes no fat droplets, form a coarse reticulum of irregular anastomosing cords. This division of the cortex into zones is more distinct in some of the lower animals. It does not indicate any fundamental structural or functional differ- ences. The cells in the different zones show only minor variations in structure, the characteristic appearance of the zone depending upon the arrangement of the cells and the shape of the connective-tissue compartments. According to Gottschau the glomerular zone is the zone in which active formation of new cells takes place, these cells gradually passing toward the medulla, according to some, finally completing their history as medullary cells. Others terminate the life history of these cells with the deepest layer of the cortex. The pigment formation in the cells of the reticular zone is considered by some, a degenerative, by others a secretory process. Still other investigators look upon the deeper cor- tical layers as the site of most active cell proliferation and an indifferent cell here situated as giving rise on the one hand to pigment cells and on the other to fat-producing cells. If the deeper cells are the older, the deeper zone would contain the more mature and probably the more functionally active cells, and it is here that are found cells crowded with fat droplets or with pigment granules. The former begin, according to some investigators, in the superficial cell as droplets of ordinary fat, which becomes changed into lecithin — an easily broken- down, acid, phosphorus-containing fat with which many of the cells of the deeper layers are filled. As already noted, this pigment is regarded by some as the last stage in fat formation, by others as an entirely independent secretion. This interrelation of cortical and medullary cells is not, however, in accord with the findings of embryology or of comparative anatomy (p. 421). 420 THE ORGANS The MEDULLA (Fig. 295, C) consists of spherical and oval groups and cords of polygonal cells. After alcohol or formalin fixa- tion these cells take a paler stain than those of the cortex. After fixation in solutions containing 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 chromafhn granules which they contain (p. 411). The chromaffin content varies in different animals and with age. Thus while in the adult human the chromaffin reaction is strong, little or no re- action is present in the foetal adrenal. The secretion of the med- ullary part of the adrenal is known as adrenalin, apparently the ma- ture condition of the intra-cellular chromaffin granules. As already noted (p. 417) it is probably an active agent in the regulation of arterial tension. Scattered in irregular groups among the chromaffin cells are many sympathetic ganglion cells. Blood-vessels. — The arteries supplying the adrenal first form a 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 v^essels to the me- dulla pass directly through the cortex without branching and form dense capillary networks among the groups of medullary cells. The re- lations of the capillaries to these glands cells are extremely intimate, especially in the reticular zone and medulla, where the cells in many cases immediately 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 center of the medulla. Lymphatics. — These follow in general the course of the blood- vessels. The exact distribution of the adrenal lymph system has not been as yet satisfactorily determined. Nerves. — The nerve supply of the adrenal 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 THE ADRENAL .421 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 appar- ently do net 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 surround the individual cells. Associated 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 different develop- mental 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 mamnials 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 mesone- phros the mesothelium sends outgrowths into the mesenchyme. These out- growths soon lose their connection with the main mass of mesothelium and con- stitute the anlage of the adrenal cortex. The medulla has an entirely inde- pendent origin, being derived from ectoderm, as part of the peripheral sym- pathetic nervous system. The cells of some of the sympathetic ganglia differ- entiate into sympathoblasts and phceochromohlasts , which give rise to the sym- pathetic 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. General References for Further Study Flint: The Blood-vessels, Angiogenesis, Organogenesis, Reticulum and Histology of the Adrenal. Contributions to the Science of Medicine, Johns Hopkins Press, 1900. Pfaundler: zur Anatomie der Nebenniere. Anzeiger Akad. Wien, 29, 1892. Nagel: Ueber die Entwickelung des Urogenitalsystem des Menschen. Arch. f. Mik. Anat., Bd. xxxiv. Stohr: Lehrbuch der Histologie B. 15th Ed. Prenant: Traite d'Histologie. CHAPTER XXII THE NERVOUS SYSTEM The nervous mechanism in man consists of two distinct though associated systems, the cerebrospinal nervous system and the sympa- thetic nervous system. Each of these systems is composed of a central portion which is its center of nervous activity, and of a peripheral portion which serves to place the center in connection with the organs which it controls. In the cerebro-spinal system the central portion consists of the cerebro-spinal axis, or brain and spinal cord. The peripheral portion is formed by the cranial and spinal nerves. The central portion of the sympathetic system consists of a series of ganglia from which the sympathetic nerves take origin. These latter constitute its peripheral portion. The cerebro-spinal axis, or brain and spinal cord, constitutes the central nervous system. The cranial and spinal nerves and the sympathetic system constitute the peripheral nervous system. , HISTOLOGICAL DEVELOPMENT AND GENERAL STRUCTURE The beginning differentiation of the nervous system appears very early in embryonic life. It is first indicated by a longitudinal median thickening of the outer embryonic layer or ectoderm, to' form the neural plate. The sides of the plate become elevated to form the neural folds, leaving between them the neural groove. By the dorsal union of these folds the neural groove is converted into the neural tube. The lumen of the neural tube corresponds to the central canal of the cord and the ventricles of the brain in the adult, and from the ec- todermic cells which form the walls of this tube practically the entire nervous system is developed. The caudal portion of the tube is of nearly uniform diameter^ — the spinal cord. At the cephalic end, the neural plate, even before its closure, is wider and forms, when closed, an expanded portion of the tube, the brain. In their further develop- ment, the walls of the brain form three expansions — the three brain ''vesicles" known as the forebrain {prosencephalon), midbrain {mes-^ 422 THE NERVOUS SYSTEM 423 enceplialon), and hindbrain {rhombencephalon) . In the forebrain two main divisions are usually distinguished, the endbrain {telencephalon) and the interbrain {diencepJialon). The basal part of the endbrain forms the corpora striata and rhinencephalon, while the dorsal part ex- pands into the pallium {cerebral hemispheres) . In the interbrain may be distinguished a dorsal part, the epithalamus; a middle (largest) part, the thalamus; and a ventral expansion, the hypothalamus. In the midbrain the basal part becomes the tegmentum, and the dor- sal part expands into the corpora qiiadrigemina or inferior and superior collicuU. The narrower part, connecting midbrain and hindbrain, is the isthmus. The basal part of the hindbrain forms the medulla ob- longata and part of the dorsal wall expands into the cerebellum. In the later development of the brain, the pallium, and with it parts of the cerebellum, becomes enormously enlarged and structures are formed constituting connections between the pallium and the rest of the brain. The most massive of these are the pes pedunculi, added ventrally to the thalamus and midbrain; the pons Varolii, added ventrally to part of the midbrain, isthmus and part of the hind- brain; and the pyramids, added ventrally to the medulla. The basal part of the mid- and hindbrain, thus covered ventrally by the pes and pons, is the tegmentum. Other portions of the dorsal walls of the forebrain and hindbrain form thin-walled expansions which, together with vascular mesodermic coverings, are the chorioid plexuses of the lateral, third, and fourth ventricles. The cavities of the cere- bral hemispheres are the lateral ventricles; the cavity of the interbrain is the third ventricle; that of the midbrain is the iter or aquaductus Sylvii; that of the hindbrain is the fourth ventricle. The wall of the neural tube is at first composed of a single layer of epithelial cells. By proliferation of these cells the epithelium soon becomes many-layered, and forms a syncytium — the myelospongium of His — although some of the original epithelial cells appear to extend through the entire thickness of the wall. Some of the syncytial cells which extend through the entire thick- ness of the wall of the neural tube {spongioblasts of His) increase in length as the wall increases in thickness. The inner ends of these cells form the lining of the tube, while other parts of the cells between the lumen and the surface tend to collapse, forming cord-like struc- tures. The outer ends of the cells, on the other hand, become per- forated and unite to form a thick network^ — the marginal veil of His. Of these cells, some retain this position, with nuclei near the lumen, 424 THE ORGANS in the adult and are known as ependymal cells; others move away from the central canal and become neuroglia cells. Still other of the cells of the neural tube are destined to become neurones, and as such are known as neuroblasts. From the neuro- blast a neurofibrillated process grows out — the future axone. Den- drites which at this stage are absent develop later in a similar manner, i.e., by extensions of the cell protoplasm.^ The neuroblasts soon leave their original position near the lumen and pass outward along the spaces between the elongated ependymal cells, but their bodies do not usually penetrate the marginal veil. Some may, however, pass through and even leave the neural tube along with the efferent roots. The axones of many neuroblasts located in the ventral part of the neural tube pierce the marginal veil and leave the neural tube as the efferent root fibres. These nerve cells together with many of the sympathetic neurones (see below) are the efferent peripheral neu- rones. Their axones pass to sympathetic ganglia or to various struc- tures (muscles, glandular epithelia) the activities of which they affect. Such structures may be collectively termed effectors. The axones of other neuroblasts do not completely pierce the marginal veil, but are directed upward and downward within it, thus forming fibres con- necting various parts of the central nervous system. Axones of still other neuroblasts, especially in suprasegmental structures (see p. 426) , are directed toward the lumen. All such neurones whose axones do not leave the neural tube may be termed intermediate or central neurones, as distinguished from the peripheral neurones: inter- mediate, because they serve as intermediate links connecting, within the central nervous system, the terminations of the afferent per- ipheral neurones (see below) with the bodies of the efferent peripheral neurones; central, because they are confined to the central nervous system. Later becoming medullated, their axones constitute the great majority of the fibres of the white matter of the central nervous system. During the closure of the neural groove, groups of cells from the crest of each neural fold become separated from the rest of the de- veloping nervous system. Some of these cells form the cerebrospinal ganglia, while, according to most authorities, others migrate still further from the neural tube and form the various sympathetic ganglia. Some cells (neuroblasts) develop into the nerve cells of the cerebro- axone ^According to other views, other cells may participate in the formation of the ne, and the dendrites may anastomose with other neuroblasts (see Chap. VI). THE NERVOUS SYSTEM 425 spinal and sympathetic ganglia. Others lurin, according to some authorities, enveloping cells, such as the capsule cells around the ganglion cell bodies and the neurilemma cells around the peripheral nerve fibres. These latter would thus correspond to the neuroglia cells of the central nervous system. The majority of the neuroblasts of the cerebro-spinal ganglia develop two processes: peripheral processes {aferent nerve fibres) to various structures {re- ceptors) which receive stimuli; and central processes, forming the afferent roots and passing into the central nervous system where they usually bifurcate and form longitudinal ascending and descending arms. These cells are at first bipolar, later the cell body withdraws from the two processes and thus assumes the adult unipolar con- dition (see page 433). Other processes may also appear. Many of the neuroblasts of the sympathetic ganglia develop dendrites and axones while others form branching cells in which the two kinds of processes cannot be easily distinguished. Sympathetic cells are also derived from cells which migrate from the neural tube along the efferent root (p. 424). The cerebro-spinal ganglionic neurones, together with some sym- pathetic neurones, certain neurones in the olfactory mucous mem- brane, retina, and possibly midbrain roof, constitute the aferent per- ipheral neurones of the entire nervous system. Most of the sympa- thetic neurones are efferent. The peripheral processes of the afferent peripheral neurones, the axones of the efferent peripheral neurones after they have emerged from the central nervous system, and the axones of the sympathetic ganglion cells, together with all their sheaths and connective-tissue investments, form the peripheral nerves. The bodies of the afferent peripheral neurones and sym- pathetic neurones form groups known as ganglia. The peripheral neurones are arranged segmentally, as shown by the series of ganglia and nerves. The sympathetic neurones and those cerebro-spinal neurones which innervate sympathetic ganglia, viscera, glands, blood-vessels, and smooth and heart muscle are peripheral visceral or splanchnic neurones. Those cranial nerve neurones which innervate the striated voluntary muscles of jaw, ear, face, pharynx, and larynx (branchio- motor) are usually also classed as splanchnic. The remainder of the cerebro-spinal peripheral neurones are peripheral 5c wa/zc neurones. From the foregoing it will be seen that all the neurones of the nervous system fall into two categories: I. Peripheral neurones, a 42G THE ORGANS part of whose processes, at least, lie outside the central nervous sys- tem, forming the peripheral nerves. II. Central or intermediate neurones. These lie entirely within the central nervous system. The peripheral neurones may be classified as follows: A. Afferent peripheral (all the neurone bodies located outside the central nervous system, except possibly some in midbrain roof). The afferent peripheral neurones are (i) the cerebro-spinal ganglion cells, which may be (a) somatic, (b) splanchnic; (2) cells in olfactory membrane, retina, and possibly midbrain; (3) some sympathetic neurones (splanchnic). B. Efferent peripheral nenrones. These are (i) cerebro- spinal (neurone bodies located in ventral part of central nervous system), (a) somatic (to voluntary striated muscles except (c)), (b) splanchnic (sending axones to sympathetic ganglia) , (c) splanchnic to voluntary striated branchiomotor muscles (p. 425); (2) sympathetic (splanchnic, bodies in sympathetic ganglia, sending axones to smooth muscle, heart muscle and glands). In the central nervous system the bodies and dendrites of the neurones are usually aggregated within certain localities which, on account of their appearance when examined in fresh condition, are collectively termed the gray matter (substantia grisea). The gray matter also contains, of course, the beginnings and endings of the nerve fibres. The parts of the nervous system where the bodies and dendrites are absent and which are composed (exclusive of the neuroglia, blood-vessels, etc.) of the medullated nerve fibers are collectively termed the white matter {substantia alba). The central nervous system may be divided into a segtnental part and certain suprasegmental parts. The former is in more immediate relation with the peripheral (segmental) nerves. It comprises the spinal cord and basal part of the brain [segmental brain). It con- tains all the bodies of the efferent cerebro-spinal neurones ami practically all the terminations of the central processes (afferent root fibres) of the afferent peripheral neurones. In it the gray matter is internal, as a rule, and the white matter external. The supraseg- mental parts comprise the expanded portions of the dorsal wall of the neural tube already mentioned, namely the pallium, corpora quad- rigemina, and cerebellum. These constitute the highest coordi- nating centers of the nervous system. Here the gray matter is exter- nal, constituting a cortex, and the white matter is internal. The grouping of peripheral neurones into ganglia and nerves has already been mentioned. In the central nervous system more or TTIE XF.RVOUS SVSTKM 427 less definite groups or systems of neurones having certain definite con- nections may also be distinguished. The axones of such a system constitute a tract or, when aggregated into a definite bundle, c\ fascicu- lus. The groups of neurone bodies are termed nuclei. The group or collection of bodies whose axones form a certain tract is the nucleus of origin of that tract. The same nucleus may receive the termina- tions of some other tract and is then the terminal nucleus of that tract. A given neurone system serves as a path for the conduction of some particular kind of nerve impulse. A conduction path, however, is often composed of several neurone systems linked together, thus forming a series of relays. Three -neitrone a/Yerenf sufwciseyme^iiz^/toiy/i j^/fie^re/t/rieri/iJi era/ neu- FiG. 296. — Diagram illustrating an arc transversing only the segmental, and an arc transversing the suprasegmental part of the nervous system. All reactions performed by the nervous system must ultimately take effect upon some part of the body or effector and are usually ini- tiated by changes in some receptor. Such a circuit from receptor to effector may be termed a neural arc (Fig. 288), and will involve affer- ent peripheral neurones, efferent peripheral neurones and usually imermediate neurones. The complexity of such arcs depends largely upon the number and character of intermediate neurones intercalated in the arc between the aff'erent and efferent peripheral neurones. Such arcs may obviously traverse centrally only the cord or segmental brain or may also traverse one or more of the suprasegmental parts of the nervous system. Intermediate neurones which link together dift'erent parts of the same segment may be termed intrasegmental neurones. Intermediate neurones which link together different segments of the cord and segmental brain may be termed interseg- mental neurones. Other intermediate neurones form conduction paths to and from suprasegmental parts {afferent and efferent supra- 428 THE ORGANS segmental neurones) and still others are suprasegmental associative neurones, confined to suprasegmental structures (Fig. 296). MEMBRANES OF THE BRAIN AND CORD The brain and cord are enclosed by two connective-tissue mem- branes, the dura mater and the pia mater, a part of the latter being often referred to as a separate membrane, the arachnoid (Fig. 297). The dura mater is the outer of the two membranes and consists of dense fibrous tissue. The cerebral dura serves both as an investing membrane for the brain and as periosteum for the inner surfaces of the cranial bones. It consists of two layers: (a) An inner layer of closely packed fibro-elastic tissue containing many connective- tissue cells, and lined on its brain surface with a single layer of fiat cells; and {h) an outer layer, which forms the periosteum and is similar in structure to the inner layer, but much richer in blood-vessels and nerves. Between the two layers are large venous sinuses. The spinal dura corresponds to the inner layer of the cerebral dura, which it resembles in structure, the vertebras having their own separate peri- osteum. The outer surface of the spinal dura is covered with a single layer of fiat cells, and is separated from the periosteum by the epidural space, which contains anastomosing venous channels lying in an areolar tissue rich in fat. The spinal dura is said to contain lymphatics which open on both of its surfaces. Beneath the spinal dura, between it and the arachnoid, is the subdural space, a narrow cleft containing a fluid probably of the nature of lymph. It is stated that this space communicates by lymph clefts with the lymph spaces ■ in the sheaths of nerves and also communicates with the deep lymph- atic vessels and glands of the neck and groin. It has no direct com- munication with the subarachnoid space (see below). 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 lamellae, forming a spongy tissue, the cavities of which contain more or less fluid. The outer lamellae are the most compact, and are covered, on the dural surface by a single layer of fiat cells. It is this external layer of the pia which is frequently described as a separate membrane, the arachnoid. The space beneath the arachnoid is the subarachnoid space and contains cerebrospinal fluid. There is direct communication in the roof of the fourth ventricle between the subarachnoid space and the brain cavity. THK XKRVOUS SYSTEM 429 ventricle. The subarachnoid space communicates with the lymphatic spaces within the nerve sheaths. It is also stated that it communi- cates with the lymph spaces contained within the vascular connective tissue (media and adventitia) of the blood-vessels which penetrate the central nervous system. Whether there is normally a peri- 3 •c o (0 o o o -*-> c (A xt o o Q. 2 ►< rt "5 J2 rt ei vascular lymph space between adventitia and ghal membrane is doubtful and the existence has also been denied, by good authorities, of lymph spaces within the glia, the gUal membrane forming in normal conditions a continuous structure. The inner lamellae of the pia are more loosely arranged, are more cellular and more vascular. Espe- 430 THE ORGANS cially conspicuous arc 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 fre- quently 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 membranes attached, in formaUn-Miiller's fluid (technic 6, p. 7) and stain sec- tions with haematoxylin-eosin (technic i, p. 20). 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 afterent part. Of the cranial nerves some are purely efferent, others purely afferent, w^hile still others consist, like the spinal nerves, of both efferent and afferent fibres. The efferent fibres of the spinal nerves are axones of cell bodies situated in the anterior horns of the cord (see p. 457, and Figs. 316 and 326) and axones of sympathetic ganglion cells. The former leave the cord in several bundles, which join to form the motor or efferent root. The afferent fibres are peripheral processes of cell bodies situated in the spinal ganglia (p. 435 and Figs. 316, 300). These leave the ganglion and join with the fibres of the motor root to form the mixed spinal nerve (Fig. 316, /). The connection of the ganglion with the cord is by means of the central processes of the spinal ganghon cells, which enter the cord as the posterior root. Among the aff'erent fibres of the posterior root are also found, in some animals, a few efferent fibres (Fig. 316, c), processes of cells in the cord. Some fibres from the spinal ganglion and from the efferent roots form the white ramus THE NERVOUS SYSTEM 431 communicans to the sympathetic ganglia. Fibres from the sympa- thetic ganglia form the gray ramus communicans to the mixed spinal nerve (Figs. 297 and 310). For further details regarding cranial nerves see pp. 480, 481. The peripheral nerve consists of nerve fibres supported by con- nective tissue (Fig. 298). Enclosing the entire nerve is a sheath of dense connective tissue, the epineiirmm. This sends septa into the nerve which divide the fibres into a number of bundles or fascicles. Surrounding each fascicle the tissue forms a fairly distinct sheath, the Fig. 298. — From Transverse Section of Human Nerve Trunk. (Osmic acid fixa- tion.) (Quain.) e/), Nerve sheath or epineurium surrounding the entire nerve and con- taining blood-vessels iv) and small groups of fat cells (/); per, perifascicular sheath or perineurium surrounding each bundle or fascicle of nerve fibres; end, interior of fascicle showing supporting connective tissue, the endoneurium. perifascicular sheath or perineurium, which is covered with a layer of flat cells. From the perineurium, delicate strands of connective tissue pass into the fascicle, separating the individual nerve fibres. This con- stitutes the intrafascicular connective tissue or endoneurium. In the connective-tissue layers of the perineurium are blood-vessels, and lymph spaces lined with endothelium, which communicate with lymph channels within the fascicle. When nerves branch, the con- nective tissue sheaths follow the branchings. When the nerve becomes reduced to a single fibre, the perineural arid endoneural 432 THE ORGANS connective tissue still remaining constitutes the sheath of Henle. The space between the neurilemma and sheath of Henle has been described as a lymph space communicating with the rest of the lymphatic system of the nerve. On emerging from the central nervous system, the root libres receive an investment of connec- tive tissue (endoneurium and perineurium) as they pass through the pia mater. This is reinforced by additional connective tissue epineurium,) as the nerve passes through the dura mater. For description of medullated and non-medullated nerve fibres see Chap. VI. 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 6, p. 7). Stain transverse sections in haematoxylin-picro-acid-fuchsin (technic 3, p. 21), or haematoxylin-eosin (technic i, p. 20), 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 tested and examined without staining. THE AFFERENT PERIPHERAL NEURONES These comprise, as already stated, the bodies and processes of the cerebro-spinal ganghon cells or cerebro-spinal ganghonic neurones, 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 A cerebro-spinal ganghon contains the bodies of the cerebro-spinal ganglionic neurones whose processes form the afferent root of the cerebro-spinal nerve. It lies in the dorsal root, a dorsal root being that part of the nerve which lies between the exit of the dorsal root fibres from the cord and their junction with the ventral root. Each ganglion is surrounded by a connective-tissue capsule which is continuous with the perineurium and epineurium of the peripheral nerves. (Fig. 297.) 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 and by bundles of nerve fibres. Each ganglion cell contains a centrally located nucleus and a dis- THE NERVOUS SYSTEM 433 tinct nucleolus, and is surrounded by a capsule of flat, concentrically arranged cells which are probably derived from the neural plate (p. 425) and are often caWed am pJdcytes or satellite cells (Fig. 301). 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 vicinity of the point of origin of the main process of the cell. The majority of the Fig. 299. — Longitudinal Section through a Spinal Ganglion. X20. (Stohr.) a, Ventral ner\-e root; b, dorsal nerve root; c, mixed spinal nerve; d, groups of ganglion cells; e, nerve fibres;/, perineurium; g, fat; h, blood-vessel. cells of the spinal ganglia have one principal process which, at some distance from the cell body, divides into a peripheral branch which becomes an afferent fibre of the peripheral nerv^e and a central branch which enters the cord as a dorsal root fibre (p. 426). These cells are usually called unipolar cells. The principal process usually becomes medullated soon after emerging from the cell cap- sule (Fig. 300). 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. 300, i and Fig. 301, A). 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 collaterals. These lie within the capsule or are given ofif outside the capsule and 28 434 THE ORGANS terminate in other parts of the ganglion and its covering, either in terminal arborizations 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. 300, 2 and 3, Fig. 301, 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 intra- capsular network, finally uniting to form the single main process (Fig. 300, 4 and 5 and Fig. 301 , C and D) . (d) Cells with a number of dendrite-like processes Fig. 300. — 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. 433-435). which divide, forming an intracapsular network which finally fuses into the main process (Fig. 300, 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. 300, 7). (f) Bipolar cells (Fig. 300, 8). (g) Multipolar cells with a number of intracapsular dendrites and a main process (Fig. 300, 10; Fig. 301, E and F). (h) Cells with a principal process which probably enters the dorsal root and a number of proc- esses which may be dendrite-like in character, but also become meduUated in places, and which branch and terminate in arborizations or bulbs in various parts of the ganglion. These latter apparently collectively represent the per- ipheral process which here ends in the ganglion (Fig. 300, g) The various endings in the ganglion of collaterals and other processes of ganglion cells often have capsules and resemble the terminations in receptors in THE NERVOUS SYSTEM 435 other parts of the body (corpuscles of Pacini, end bulbs, etc.) and, together with their envelopes, 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. .1 B C a.f. s.p. '^c^ p p p c D E F Fig. 301. — 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 bulbous dendrites and enveloped with pericellular arborizations (p.a.) of fibres (a.f.) terminating around cell; c, collateral; d, dendrite; p, principal process; s.p., short process. (Cajal's silver stain.) The peripheral processes of the cerebro-spinal ganglion CELLS are the afferent fibres of the cerebro-spinal nerves (p. 430). The modes of termination of these peripheral processes in receptors 436 THE ORGANS (p. 425) are extremely varied and complicated. These peripheral terminations are always free, in the sense that, while possibly some- times penetrating cells, they probably never become directly con- tinuous 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, the axis-cylinder splits up into minute fibrils (terminal ar- borization) which pass in between the cells and terminate there, often in little knob-like swellings (Fig. 302). Such terminal Fig. 302. — Free Endings of Afferent Nerve Fibres in Epithelium of Rabbit's Bladder. (Retzius.) 0, Surface epithelium of bladder; b g, subepithelial connective tissue; n, nerve fibre entering epithelium where it breaks up into numerous terminals among the epithe- lial cells. fibrils do not penetrate among the squamous cells. Similar ^^ dif- fuse^' endings are found in serous membranes and in simple epitheha, also in connective tissue. In the case of glandular epithelia such endings may, in part at least, be terminations of efferent (secretory) fibres from sympathetic ganglion cells (p. 445). Diffuse endings have also boen described on endothelial surfaces such as endocardium and in smooth muscle. The latter are to be distin- guished from the regular motor endings described below. An impor- tant form of diffuse ending is that found encircUng and ending in the outer root sheaths of hair follicles (Fig. 303). Nerve endings are abundant in the pulp of teeth. There has been some dispute THE NERVOUS SYSTEM 437 as to whether they penetrate the dentine. In addition to such com- paratively simple nerve endings, there are also found in the skin 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. Fig. 303. — Nerves and Nerve Endings in the Skin and Hair Follicles. (After G. Retzius.) As, Outer root sheath; c, most superficial nerve-fibre plexus in the cutis: dr, sebaceous glands; //, the hair itself; list, stratum corneum; is, inner root sheath of hair; n, cutaneous nerve; rm, stratum germinativum Malpighii. 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 438 THE ORGANS 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. Of the tactile corpuscles (Fig. 304) those of Meissner, which occur ■;fr Fig. 304. Fig. 305. Fig. 304. — Tactile Corpuscles of Meissner, tactile cell and free nerve ending. (Mer- kel-Henle.) a, Corpuscle proper, outside of which is seen in the connective-tissue cap- sule, b, fibre ending on tactile cell; c, fibre ending freely among epithelial cells. Fig. 305. — Taste Bud from Circumvallate Papilla of Tongue. (^Merkel-Henle.) a, Taste pore; b, nerve fibres entering taste bud and ending upon neuro-epithelial cells. On either side fibres ending freely among epithelial cells. 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 cells. From one to four medullated nerve fibres are distributed to Fig. 306. — End Bulb from Conjunctiva. (Dogiel.) a, Medullated nerve fibre, axone of which passes ov'er into dense end skein. each corpuscle. As a fibre approaches a corpuscle, its connective- tissue sheath becomes continuous with the fibrous capsule, the medullary sheath disappears, and the fibrillar of the terminal abori- zation pass in a" spiral manner in and out among the epithelial cells. TIIR NERVOUS SYSTEM 439 These terminal fibrils usually end in a flattened expansion consisting of neurofibrils and perifibrillar substance. Tactile corpuscles are also found on the volar surface of the forearm, eyelids, lips and tip of the tongue. Of the so-called end bulbs, the simplest, which are found in the mucous membrane of the mouth and conjunctiva, consist of a central core formed by the usually more or less expanded end of the usually branched axis cylinder, surrounded by a mass of finely granular, nucleated protoplasm — the inner bulb — the whole enclosed in a capsule of flattened connective-tissue cells continuous with the sheath of Henle. Other end bulbs may be compound. End bulbs are found also in the mucous membrane of the tongue, epiglottis, nasal cavities, lower end of rectum, peritoneum, serous mem- branes, tendons, Hgaments, connective tissue of nerve trunks, synovial mem- branes of certain joints and external geni- tals, especially the glans penis and cHtoris.^ The Pacinian bodies (Fig. ^^07) are laminated, elliptical structures which differ ^^^ aoy.-Pacinian Body from the more simple end bulbs already from Mesentery of Cat. (Ran- ... i J 1 vier.) c, Lamina of capsule; described, mainly in the greater develop- (/, epithelioid cells lying between ment of the perineural capsule. The cap- ifmin^ of capsule; n, nerve tr IT 1 fibre, consistmg of axis cyimder sule is formed by a large number of con- surrounded by Henle's sheath, , • 1 11 11 11 • 4-* ™ ^c enterins; Pacinian body; /, centric lamelte, each lamella consisting of perineural sheath; m, inner connective-tissue fibres Hned bv a single bulb; n, terminal fibre which , breaks up at a mto an irregular layer of flat connective- tissue cells. ihe bulbous terminal arborization. lamellaj are separated from one another by a clear fluid or semifluid substance. As in the simpler end bulbs, there is a cyHndrical 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. Arteries in the vicinity of the cor- puscle break up into capillary networks surrounding the corpuscle. 1 This description of the distribution of the various receptors is principally taken from the excellent account in Schafer's "Text-Book of Microscopic Anatomy." An admirable and still more detailed account is to be found in "The Nervous System," by Barker. 440 THE ORGANS From this network capillaries enter the corpuscle, usually near the nerve fibre, and penetrate among the lamellae, even reaching the distal end of the corpuscle. They do not enter the inner bulb. The Pacinian bodies are found in the subcutaneous fat, especially of the hand and foot, in the parietal peritoneum, mesentery, penis, clitoris, urethra, nipple, mammary gland, in the vicinity of tendons, ligaments and joints. In voluntary muscle afferent nerves terminate in Pacinian cor- puscles, in end bulbs, and in complicated end organs called muscle spindles, or neuromuscular bundles. The muscle spindle (Fig. 308) 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 various points by nerve fibres. A single spindle contains several muscle fibres and nerve fibres. The muscle fibres differ from ordi- nary fibres, being fine and containing more nuclei and sarcoplasm in the middle. The muscle spindles are more numerous in the limb muscles than in those of the trunk, and in the distal than the proximal part of the limb. There are few in eye muscles and in some muscles they have not been detected. According to Ruffini, there are three modes of ultimate terminations of the nerve fibres within the spindles: one in which the end fibrils form a series of rings which encircle the individual muscle fibre, termed annular termina- tions; a second in which the nerve fibrils wrap around the muscle fibres in a spiral manner — spiral terminations; a third in which the terminations take the form of delicate expansions on the muscle fibre — arborescent terminations. At the junction of muscle and tendon are found the elaborate afferent terminal structures, known as the muscle-tendon organs of Golgi (Fig. 309). This is a spindle-shaped body composed of several tendon bundles. Into this there enter one or several nerve fibres which break up into complicated terminal arborizations upon the tendon bundles. Other forms of nerve endings enclosed within connective-tissue sheaths are corpuscles of Ruffini found in the subcutaneous tissue of the finger and corpuscles of Golgi-Mazzoni found on the surfaces of tendons and also in the subcutaneous tissue of the fingers and in other parts of the skin. It is evident from the above that the nerve terminations are onlj' stimulated through the intermediation of surrounding cells which may form quite elab- THE NERVOUS SYSTEM 441 Fig. 308. — Middle Third of Muscle Spindle from Striated Voluntary Muscles of Cat. (From Barker, after Ruftini.) A, annular terminations; S, spiral termina- tions; F, arborescent terminations. Fig. 309. — Tendon of Muscle of Eye of Ox. (Ciaccio.) Two muscle-tendon organs of Golgi, each showing ring-like and brush-like endings, gh, sheath of Henle; sr, node of Ranvier. 442 THE ORGANS orate structures. These cells constitute the receptors and probably render the various nerve terminations they envelop, more or less inaccessible to all but one particular kind of stimulus. The receptors described above are scattered through- out head and body {general or common senses) as distinguished from those which are concentrated into the organs of the special senses (smell, sight, hearing, taste) present only in the head (pp. 480, 481 and Chap. XXIII). The various stimuli received by them may give rise to sensations of light pressure or touch (tactile cells and tactile corpuscles, terminations in hair sheaths), temperature and pain (dififuse terminations in epithelium and connective 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 sensation (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 {iutcro-ccptors, including taste) and those giving infor- mation 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 consist- ing of all the central processes of the corresponding ganglion. Hav- ing entered the central nervous system, the central processes divide into ascending and descending arms, as already mentioned (p. 425). In the spinal cord the ascending arms are longer than the descending arms. In the brain the descending is usually the longer. These arms give off collaterals. Both collaterals and also the terminals of the arms enter the gray matter of the cord and segmental brain and terminate around various cell bodies and dendrites (terminal nuclei). THE SYMPATHETIC GANGLIA The sympathetic system of the neck and trunk consists of (i) a series of vertebral or chain ganglia, lying ventro-lateral to the vertebrae, connected with the cord by white rami comniunicantes and connected with each other by longitudinal cords, (2) of gangliated prevertebral plexuses connected with the vertebral ganglia and also connected with ((3) ill-defined peripheral or terminal ganglia in the walls of the viscera {e.g., plexuses of Auerbach and Meissner). The sympa- thetic ganglia of the head are the ciliary, sphenopalatine, otic, and submaxillary.. The peripheral processes of certain spinal ganglion cells pass via THE NERVOUS SYSTEM 44o the white rami communicantes to the vertebral ganglia and thence to visceral receptors. Axones of splanchnic efferent spinal neurones (preganglionic fibres) pass from the spinal cord via the ventral roots and white rami communicantes to terminate in various sympathetic ganglia. The axones of the cells of these ganglia (postganglionic fibres) complete the path by passing to the splanchnic effectors (heart muscle, smooth muscle or gland). Thus while the somatic efferent peripheral path consists of only one neurone, whose body is in the ventral gray of the cord or brain, and whose axone passes without interruption to the striated voluntary muscle, the splanchnic efferent peripheral path (except the branchiomotor) consists of two neurones. The first neurone body is in the gray of the cord or brain, the second neurone body is in a sympathetic ganglion. The preganglionic fibres emerging in the seventh cervical to the third lumbar spinal roots end either in the vertebral or prevertebral (cocliac, superior mesenteric, inferior mesenteric) ganglia. The vertebral ganglia send part of their axones to the peripheral nerves (gray rami communicantes) , thence passing to superficial splanchnic eft'ectors (smooth muscle of hairs and of superficial blood-vessels and glands of skin). Other axones may pass to the head. The axones of the prevertebral ganglia pass to the glands and smooth muscle of the viscera (Fig. 310). The other segments of the cord do not send out white rami communicantes except the second, third and fourth sacral. These fibres do not communicate with vertebral ganglia but terminate in prevertebral (pelvic) ganglia, which in turn send axones to the lower colon, rectum, bladder and genitals. In the head, preganglionic fibres emerge with the third nerve and pass to the cihary ganglion, thence as postganglionic fibres to the ciliary and sphincter iridis muscles of the eye; others emerge with the seventh and pass to the sphenopalatine and submaxillary ganglia; others with the ninth and pass to the otic ganglion. Many of the axones of these ganglia pass to the salivary gland, others may be vasomotor. The vagus nerve sends preganglionic fibres to most of the viscera. These fibres prob- ably terminate in the peripheral sympathetic gangha. There is thus a cranial (III, VII, IX and X nerves), thoraco-lumbar and sacral outflow of preganglionic fibres. Most visceral structures appear to receive a double innervation, one from the thoraco-lumbar and one from either the cranial or sacral system. By some writers the term sympathetic has been restricted to the thoraco-lumbar, the two others being termed autonomic. 444 THE ORGANS Smooth muscle Type II ? Somatic efferent Somatic afferent Splanchnic cerebrospinal afferent Splanchnic cerebrospinal efferent Sj'mpathetic Dorsal root Spinal ganglion Dorsal ramus Ventral ramus Blood vessel Smooth muscle Prevertebral ganglion White r. com. — — Gray r. com. -- — Vertebral or chain gang. Afferent sym path, neurone — Gland "^^ Periph. gang. Blood vessel Sens, ending Pacinian corpuscle Fig. 310. — Diagram of the Sympathetic System and the Arrangement of its Neu- rones 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 gray rami, threj ganglia of the chain, one prevertebral ganghon 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 body. THE NERVOUS SYSTEM 445 Some of the sympathetic cells are probably afferent but the nia- jority are efferent. The fibres of the white rami communicantes are mostly fine and medullated. The axones of the sympathetic cells are fine and non-mcdullated or thinly medullated. For further details see Fig. 310. The larger ganglia resemble the spinal ganglia in having a connec- tive-tissue capsule and framework. The cells are smaller and often densely pigmented. Each cell is surrounded by a capsule of cells Fig. 311. — Sympathetic Xerve Cells (woman of 36 years). (Cajal.) A and B, cells whose dendrites {b) form a pericellular plexus. C, cell with long dendrites, a, axone; c and d, terminal part of a dendrite. The capsular cells are fainth' indicated. (Cajal's silver stain.) similar to those surrounding the spinal ganghon cells. Often two or three cells and their interlocked dendrites are enclosed within a com- mon capsule (Fig. ^12, 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. Sometimes a dendrite will pass some distance from the cell, arborize, and interlock with a similar dendritic arborization of another cell. Another form of sympathetic cell has long slender dendrites often indistinguishable from axones. These cells are more frequent in the 446 THE ORGANS peripheral ganglia. Some of these cells have been considered to be afferent sympathetic cells. (Figs. 311 and 312.) te a.j. a.f. Fig. 312. — Sympathetic Nerve-cells and their Capsules. (Cajal.) .4 , Two-celled glomerulus; B, cell surrounded with the pericellular terminal arborizations of two fibres {a.f.) passing to the cell; a, axones; d, fibre, probably dendritic, with bulbous termina- tion. (Cajal's silver stain.) The sympathetic cells receive fibres which form arborizations around and within their capsules and also around the long dendrites. Many of these are terminations of the visceral cerebro-spinal efferent neurones, ie., the pre- ganglionic libers (Figs 310 and 312, B). As already noted, the axones of the efferent sympathetic cells terminate in heart muscle (cardio-motor), in smooth muscle of viscera (viscero-motor), of blood-vessels (vaso-motor) and of hairs (pilomotor), and in glandular epithelia (secretory). In heart muscle (Fig. 313) and in smooth muscle (Fig. 314) the nerves of the sympathetic system end in fine Jeltworks of fibres, which are in relation with ■^ Fig. 313. — Nerve End- ings on Heart Muscle Cells. (From Barker, after Huber and De Witt.) THE NERVOUS SYSTEM 447 the muscle cells. Satisfactory differentiation between efferent terminals and afferent terminals in heart and in smooth muscle has not yet been made. In organs whose parenchyma is made up of glandular epithelium, the sympathetic 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. A. b ■ „ b Fig. 314. — Nerve Endings on Smooth Muscle Cells. (From Barker, after Huber and De Witt.) a, Axis cylinder; h, its termination; n, nucleus of muscle cell. TECHNIC (i) Fix spinal and sympathetic ganglia in formalin-Miiller's fluid (technic 6, p. 7). Stain sections with ha^matoxylin-eosin (technic i, p. 20), or with hiema- toxylin-picro-acid-fuchsin (technic 3, p. 21). (2) Fix spinal and sympathetic gangUa in absolute alcohol or in lo-per-cent. formaUn, and stain sections by Nissl's method (technic, p. 39). (3) See also technic i, p. 452. (4) Ganglia should also be prepared by Cajal's silver method, using the al- cohol fixation (p. 38). 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 coms municans, in spinal nerves) to various sympathetic gangUa to ter- minate there, or proceed as the efferent fibres of the peripheral nerve- to terminate in the striated voluntary muscles of the body and head. In the spinal nerves these fibres pass beyond the spinal ganglia and then join the afferent fibres; 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 448 THE ORGANS more than one muscle fibre. In the perimysium the nerve fibres undergo further branching, after which the fibres lose their medul- lary sheaths and pass to the individual muscle fibres. Here each fibre breaks up into several club-like terminals. These terminals are imbedded in a nucleated protoplasmic mass on the outside of the muscle fibre and probably composed of sarcoplasm. It is stated that the neurilemma of the nerve fibre is continous with the sarco- lemma and that a continuation of the sheath of Henle covers'the ■^' Fig. 315. — Motor nerve-endings in abdominal muscles of a rat. X170. (Szymonowicz.) Gold preparation. whole structure which is known as the motor end plate. As a rule each muscle fibre is supplied with a single end plate, though in large fibres there may be several. (Fig. 315.) 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 vertebra 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 THE NERVOUS SYSTEM 449 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 medullated 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. The bodies of these cells, the medullated axones of which form the white matter of the cord are situated as follows: (i) Cells outside the spinal cord. (Extrinsic cells.) B. Cells situated in the gray matter of the cord. (In- trinsic cells.) (3) (5) Cells outside the central nervous system (spinal ganglion cells). ] (2) Cells in other parts of the central nervous system (the brain). Root cells, such as those of the anterior horn, whose axones form the ventral root (efferent peripheral neurones) . These pass out directly, and thus do not become longi- tudinal column fibres. (4) Intermediate neurones, whose axones enter into formation of the fibre columns of the cord (column cells.) 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.) (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 spHt into ascending and de- scending longitudinal arms composing the greater part of the dorsal funiculus and the zone of Lissauer. They are described more in detail later (pp. 467 and 468). (2) Cells Situated in Other Parts of the Central Nervous System which Contrlbute Axones to the White Columns OF the Cord. These cells are situated in the motor areas of the cortex of the 29 450 THE ORGANS pallium, in the midbrain, cerebellum(?), and various parts of the segmental brain. The axones of these cells pass down the cord, forming the descending fibre tracts of the cord (p. 470). Collaterals and terminals of these fibres enter the gray matter of the cord to ter- minate there. / Fig. 316. — Transverse Section through Spinal Cord and Posterior Root Ganglia of an Embryo Chick. (Van 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 (b) ; within the pos- terior 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 gang- lion 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.) I (3) Root Cells — Motor Cells of the Anterior Horn The course of the axones of these cells has been described (p. 447). The bodies are described on pp. 457 and 458. (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. 456) 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 side of the cord. These are known as tautomeric cells (Fig. 317, 3). Others send their axones as fibres of the anterior commissure to the white matter of the opposite side of the cord — THE NERVOUS SYSTEIM 451 lieteromeric cells (Fig. 317,1 and 2). 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. 317, 4). The axones of many of these cells are short, constituting the short fibre tracts (fundamental columns — ground bundles) of the cord (see VeniraX Fig. 317. — Transverse Section through Spinal Cord of Embryo Chick of Seven Days Incubation (Golgi Preparation). The Ijoundary between the future gray and white matter is indicated by the dotted line, i and 2, Heteromeric column cells whose axones {ax) pass through the anterior commissure to the opposite ventral white columns; 3, tautomeric column cells whose axone (ax) passes to the ventro-lateral white column on the same side; 4, hecateromeric column cell whose axone (ax) bifurcates, one branch entering the ventral white column of the same side and the other branch passing through the anterior commissure to the opposite side. A number of longitudinal fibres in the white columns are shown, cut transversely or obliquely. page 474; others form long ascending fibre tracts, passing up through the cord to the brain (see page 467.) Terminals and collateral branches of these longitudinal axones, especially of the short ones, are constantly re-entering the gray matter to end in arborizations around the nerve cells (Fig. 318). 452 THE ORGANS (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 relations 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. 36). Rather thick (75/i) transverse and longitudinal sections are made and mounted in balsam without a cover-glass. Owing to the uncertainty 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 joining the peripheral processes of the spinal ganglion cells to form the spinal nerves, can usually be seen in the trans- verse 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 men- tioned uncertainty of the Golgi reaction, it is usually necessary to make a large num- ber of 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, some both, while in very few will the hecateromeric type be found. Sections containing fewest impregnated cells frequently show collaterals to best fringe of fine transverse fibres crossing the and white matter. Fig. 318. — From Longitudinal Section of Spinal Cord of Embryo Chick. (Cajal.) A, White column of cord; B, gray matter. The cells of the gray matter (column cells) are seen sending their axones 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 de- scending arms becoming fibres of the white column. From the laiter are seen fibres (collaterals and terminals) passing into the gray matter and end- ing in arborizations (Golgi Method). advantage. These are seen as a boundary line between gray matter THE NERVOUS SYSTEM 453 (4) The silver method of Cajal, using the alcohol fixation (technic 2, p. 30), may also be advantageously used to display many of the above details of struc- ture in embr>'o chicks. It is also capricious. Whole neurones with long axoncs 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. 452). — Using a lower-power objective, first locate the cord and determine the outlines of gray matter and white matter. Observe the spinal ganglia lying one on either side of the cord (Fig. 316, a). At least one of the ganglia will probably show one or more bipolar cells, sending 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. 316, 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. 316,/). Observe the central processes of the spinal ganglion cells entering the dorsal column of the cord and bifurcating (Fig. 316, 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. The fact that they are finer than the longitudi- nal fibres in the white matter shows that they are branches of the latter. Even in transverse section the point at which they leave the latter may occa- sionally be made out. 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 begin- ning of the sympathetic system with its chain of ganglia. Note the relation 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. Axones 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. 317). Study also the collaterals if they are stained. Remember that only a few of the elements present are stained in Golgi preparations and that there are apt to be present 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 representation of the ependyma cells of the adult cord, except the cell does not usually in the latter extend entirely through the wall of the neural tube. Longitudinal Section of Six-day Chick Embryo (Technic i, p. 452). — Using a low-power objective locate gray matter and white matter and identify plane 454 THE ORGANS 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. 318). The longitudinal fibres of the posterior columns are the as- cending and descending branches of the central processes of the spinal ganglion cells, and the branches passing into the gray matter are their collaterals and terminals. If the section happens to include the entering fibres of a posterior root, these can be seen branching in the posterior columns into ascending and descending arms (Fig. 318). The longitudinal fibres of the lateral and anterior columns are axones of column cells and of cells situated in higher centres (see pages 450 and 451). 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. 319 and 320). — The general features of the sec- tions can be best seen with the naked eye or with a low-power dissecting lens. Note the shape and size of the cord, and that it is surrounded by a thin mem- brane, the pia malcr 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 somewhat 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 cormia). The ventral 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 white 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 funicu- lus {ventro -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 funiculus {ventral white column). In the concavity between the an- terior and posterior horns some processes of the gray matter extend out into the white matter where they interlace with the longitudinally running-fibres of the latter to form the reticular process (not well marked in the lumbar cord). For the study of further rletails the low-power objective should be used. THE NERVOUS SYSTEM 455 Gray Matter. — 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 as ihe 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 surrounding the central canal, the gelatinous substance of Rolando. The head is connected with the base of the dorsal horn by a narrower neck or cervix. External to the gelatinous substance of Rolando is a thin zone containing a plexus of fine medullated fibres (Weigert stain) known as the marginal zone or zona spongiosa, and external to this, occupying the space 456 THE ORGANS between it and the periphery, is a zone composed of fine longitudinal meduUated fibres rather sparsely arranged and therefore staining more lightly with the Wei- gert method. This is the zona terminalis or zone of Lissauer. It belongs ob- viously to the white matter of the cord (see page 458). 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. 319 and 320.) White jVIatter. — Note the general appearance of the white matter and the disposition of the supporting strands of neuroglia tissue (light in the Weigert, usually darker in other stains). The neuroglia is seen to form a fairly thick layer just beneath 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 ventral white commissure. (Fig. 320.) 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 transversely 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. 319 and 320.) (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 stellate 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 pro- cessus 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 from the dorsal spino-cerebellar tract (see page 469). 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 Intermediate Gray. — (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) intermedio-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 as preganglionic fibres into the sympathetic system. This nucleus T}IE XERXUUS SYSTEM 457 is more conspicuous in the thoracic cord, but extends from the seventh or eighth cervical to about the third lumbar segment and is also present in the sacral cord (especially the third segment), (pp. 462, 463 and 464.) From the above it is seen that all the cells of the dorsal and intermediate gray, except the intermedio-lateral group, are column cells, either tautomeric, heteromeric or hecateromeric. c s to (U m a B t-c 3 O 43 a a. O .si tn u ^^ rt > 1-1 r^ 'o'o J-CJ bO O o CO 6 I— ( (C) Cells of the Ventral Horx. — These fall into two categories: (i) Column Cells. These may be tautomeric, sending axones to the adjoining white matter or heteromeric, the axones of which cross in the ventral commissure to the white matter of the opposite side. Among the latter may be a well-marked group in the dorso-mesial part of the horn (commissural nucleus). Hecateromeric cells may be present. (2) Root cells. Two main divisions may be distinguished: (a) Mesial group, present throughout the cord above the fifth sacral segment 458 THE ORGANS (Bruce). This group probably innervates the striated voluntary (somatic) muscles of the trunk. The mesial 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 re- gions. This group innervates the muscles of the extremities and exhibits the following subdivisions: An antcro-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 Xhc phrenic group (C4), cen- trally located, cilio-spinal and other cells (C8 to Th2, to sympathetic ganglia which send fibres to dilator pupillae 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. In the lumbo-sacral cord below the fourth lumbar there is also a medio-ventral splanchnic group which together with the lateral horn group of the sacral cord furnishes the preganglionic fibres emerging from the sacral cord. 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, see Chapter X) is used. Arrangement of Fibres (Fig. 320). — With the low- and high-power objectives the course of the transverse (i.e., longitudinally cut) nerve fibres should be care- fully 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 fibers by turning or splitting, or they are the col- laterals 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 pass along the dorsal and mesial side of the dorsal horn, forming the zone of entry of the dorsal roots. By bifurcating (not visible in the preparation) they become the majority of 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 ter- minate 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. Col- laterals 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 NERVOUS SYSTEM 459 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 fibres in the ventral horn separate the cell groups, but between individual cells are seen numerous fine mcduUated 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 Structures.' — Study with the high power the general histological struc- ture of the gray and white matter. In the gray matter note (Cajal, Nissl, H.-E.), besides the nerve cells and their processes, the neuroglia nuclei. Note also the structure and size of the medullatcd nerve fibres (Weigert). In the white matter Fig. 321. — From Transverse Section of Elephant's Cord. (Hardesty.) Benda's Neuroglia Stain, b, c, d and i. Four types of neuroglia cells; k, neuroglia fibre passing through several neuroglia cells; /, leucocyte. note the 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. 321.) Note also the pia mater and the connective-tissue septa (usually perivascular) entering the cord from the pia accompanied usually by a denser aggregation of glia fibres. 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 and glia stains). Study the internal structure of the nerve-cells of various sizes present, espe- cially the amount and arrangement of the chromophilic substance (Nissl). The smallest nerve cells of the cord have a limited amount of chromophilic substance, often either in the form of perinuclear caps or small bodies near the periphery ' It may sometimes be advantageous to have the high power study of the general histological structure of the gray and white matter of the cord precede the study of the architectural arrangements of the cord here placed first. 460 THE ORGANS 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 chromo- philic substance (for further details see Chap. X). In general it seems that the intermediate neurones belonging to efferent paths {i.e., acting on periph- eral motor neurones) have the definite, clear-cut, coarse chromophilic granules characteristic of the peripheral motor neurones; while the neurones forming parts of afferent paths have the fine, indefinitely grouped granules character- istic of the ccrcbro-spinal ganglionic neurones (Jacobsohn and Malone). Blood-vessels (Fig. 322. — ) Study the arrangement and structure of the blood- vessels of the cord and pia. There are three principal longitudinal arteries, the anterior spinal artery given ofT from the vertebral arteries near their union Posterior spinal artery Region sup- plied by sulco- commissurae artery Column cells Anterior spinal artery (giving off a sulco-commissural artery) Root cells Fig. 322. — Schematic transverse section of Cord, Showing General Distribution of Blood-vessels (leftj 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 broken black lines on the surface of the cord are portions of the vascular network in the pia mater. into the basilar artery, and two posterior spinal arteries, given off also from the vertebral 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.e., non-anastomosing) branches enter the cord, supplying all parts except the ventral horn and column of Clarke. The latter are supplied by branches from the anterior 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 into a rich capillar}- 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 ati terior median veins can be distinguished. Portions 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. THK NERVOUS SVSTKM 4G1 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 white matter, vary in different parts of the cord, which must therefore be separately considered. These variations are due to: (i) Variations 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. PRACTICAL STUDY Section through the Twelfth Thoracic Segment (Fig. 324). — Note that the cord is smaller than in the lumbar enlargement and somewhat flattened 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 comparatively 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. 456). 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. 469). Section through the Mid-thoracic Region (Fig. 324). — Compare with the lumbar sections. Note 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 stUl fewer cells. Clarke's column is present, but not so large. Section through the Cervical Enlargement (Fig. 323). — Note the marked in- crease 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 reticular 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 462 THE ORGANS C.II cm ^im^^m M. ^--•■•:i-:- ^^^5r3-^ ' '.': ;«-^-"/.: I*'. '' ,-\~}f^ii-'' CIV Oi' /^<^.:^'~ \s^ V y?« C VI -^ /''■ / % C. VII C. VIII Fig. 323. — Transverse Sections through the Cervical (II-VIII) Segments of the Cord. Weigert preparations. (Rauber-Kopsch.) THE NERVOUS SYSTEM 465 Th.I Th. VI Th. IX Th. II Th. VII .:;•** ^.o --^ Th. VIII Th.X Th.XI Th. XII Fig. 324. — Transverse Sections through the Thoracic (I-XII) Segments of the Cord. Weigert preparations. (Rauber-Kopsch.) 464 THE ORGANS y L.I L.II L.IV ':c^'\^k ^ L. V S.II %< ^ rs- 5. IV S. V S. Ill Fig. 325. — Transverse Sections through the Lumbar (I-V) and Sacral (I-V) Segments ot" the Cord. Weigert preparations. Rauber-Kopsch.) THE NERVOUS SYSTEM 465 definite septum divides the posterior column into an inner part, the column oJGoll, and an outer part, the column of Burdach. For further variations and differences between the segments of the cord, compare Figs. 323, 324 and 325. Fibre Tracts of the Cord The determination of the fibre tracts of the cord has been accom- pHshed principally by two methods: (i) The myelogenetic method, which is based upon the fact that the fibres of different systems ac- quire their myelin sheaths at different periods of embryonic develop- ment. Thus by examining cords from embryos of various ages and young specimens it is possible, using a myelin stain {e.g., Weigert), to distinguish different tracts by the presence or absence of myeliniza- tion of their fibres. (2) The method of secondary or axonal degenera- tion, based upon the fact that a fibre separated from its cell under- goes degenerative changes and ultimately disappears and that the cell body also usually shows certain changes (see page 143). The fibres distal to the injury can be distinguished during active degenera- tion by applying the Marchi stain (page 35). After their disappear- ance, however, a negative picture is obtained by staining the sur- rounding normal fibres (Weigert). The changes in the cell bodies whose axones are injured are distinguished by applying the Nissl stain (p. 39). 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 degenera- tion changes. On the other hand, at any level below the cut, fibres which originate from cells above the cut will show degenera- tion (''descending" degeneration), while the cell bodies of these fibres, located above the cut, will exhibit axonal degeneration. The indication this gives as to the direction of conduction is evident when it is remembered that the impulse passes from neu- rone body along the axone to its termination. (3) Atrophy (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 pre- ceding, 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 30 466 THE ORGANS r — ■ ni M. W C THE XKRX'OUS SYSTIO.M -107 systems of lower forms and the correlated development or absence of related parts of the nervous system, and the method of pliysiology, i.e., study of the physiological effects of stimulation or extirpation of various portions of the nervous system thereby indirectly demon- strating anatomical pathways. Ascending Tracts. It will be recalled that various groups or systems of neurones (nuclei and tracts) are linked together to form conduction paths (p. 427). In general an afferent conduction path consists of (i) a primary system (afferent ganglionic) whose central processes (aff'erent root fibres) end in its terminal nucleus; (2) a secondary system whose bodies constitute the terminal nucleus of (i) and whose axones form a second ary tract and end in a secondary terminal nucleus; (3) the conduction path may continue through tertiary nuclei and tracts, etc. A . Tracts form ill i^ parts of afferent pallia! paths. I. Long Ascending Arms of Dorsal Root Fibres (Posterior Funiculi). — The origin of these tracts — central processes of the cells of the spinal gangha — has been described (page 442). The distribu- tion of the posterior root fibres to the gray matter of the cord was noted in connection with the study of the lumbar enlargement sec- tion (page 458). The general arrangement of these fibres in the dorsal funiculi remains to be noted. Each successive dorsal root sends its fibres into the cord next to the dorsal horn and therefore lateral to the ascending fibres from the next root below. Thus the fibres of the lower roots as they ascend the cord are gradually pushed toward the median line until they finally occupy that part of the posterior column lying near the posterior sep- tum. The separation of the posterior column by a connective-tissue septum into the column of Goll and the column of Burdach occurs Fig. 326. — 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 first 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). (3) The third zone borders the gray matter and includes the ground or fundamental bundles of the cord (chiefly spinal intersegmental fibres). In the figure, for nu, Darlischewitschi read neucleus of medial longitudinal fasciculus. 468 THE ORGAXS only in the cervical cord (Fig. 319 and 323). Here the most median fibres, i.e., those lying in the column of Goll, are the longest fibres of the posterior columns, having come from the lower spinal ganglia (lower thoracic, lumbar and sacral), while the column of Burdach (Fig. 319) consists of short and medium length fibres (upper thoracic and cervical dorsal root fibres). The fibres of Goll's column end in the nucleus Juniculi gracilis or nucleus of the column of Goll in the medulla (see p. 491 and Fig. 334). Those fibres of Burdach's which do not terminate in the spinal cord terminate in the medulla in the nucleus funiculi cuneati or nucleus of the column of Burdach (p. 491 and Fig. 334). The nucleus gracilis and nucleus cuneatus — which will be seen in sections of the medulla (Fig. 334) — thus serve as terminal nuclei for the afferent root fibres in the columns of Goll and those of the columns 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 con- stitute 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, (probably principally to the post-central area). This path is, in brief, as fol- lows: spinal ganglionic (long ascending arms of dorsal roots) + fillet + thalamo-cortical path, decussating in the medulla (Fig. 330). II. The Spine -thalamic Tract.- — This arises from heteromeric cells lying probably principally in the dorsal horn (groups c and d, p. 456). 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 ganglion (short arms and collaterals of dorsal roots) + spinothalamic -j- thalamo-cortical neurones, three systems of neurones. Its decussation takes place in the cord at about the level of entry of the dorsal roots involved. Associated with this system may be some fibres to the superior colliculus (spino-collicular tract). (Figs. 326, 327 and 331). B. Tracts forming part of paths to the cerebellum. THE NERVOUS SYSTEM 469 III. The Dorsal Spino-cerebellar Tract {Tract of Flechsig, Direct or Uncrossed Cerebellar Tract). — This tract lies along the dorsal-lateral periphery of the cord, being bounded internally by the crossed pyramidal tract (Fig. 319 and Fig. 326). The fibres of the direct cerebellar tract are the axones of the cells of Clarke's column (tautomeric column cells) (Figs. 326, 327 and 331). These axones cross the intervening gray matter and white matter of the same side and turn upward as the direct cerebellar tract. In the medulla they form part of the restiform body or inferior cere- bellar peduncle and pass to the cerebellum. • Here they enter the gray matter of the vermis of the same or opposite side, ending in ramifications 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 456). As already noted above, 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 and that some of the fibres from the nucleus lateralis cross). (Fig. 345.) rv. The Ventral Spino-cerebellar Tract. — This tract lies along the periphery of the cord, extending from the anterior Hmit of the direct cerebellar to about the exit of the ventral roots (Fig. 319 and Fig. 326). It is probably formed by axones whose cell bodies are scattered Fig. 327. — Diagram showing Beginnings of Principal Long x\scending 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. 470 THE ORGANS through the intermediate gray matter, possibly group a (p. 456 and Fig. 319). Some fibres come from tautomeric, others from hetero- meric cells, the axones of the latter crossing in the ventral commis- sure. The tract first appears in the upper lumbar cord and natur- ally increases in size as it passes upward. The fibres of this tract also end in the vermis of the cerebellum. They reach their desti- nation in the cerebellum by a dift'erent 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 ver- mis. This path is thus also a two-neurone path (spinal ganglion cells and spino-cerebellar neurones) and is partly crossed and partly uncrossed. The ventral spino-cerebellar and spino-thalamic tracts are sometimes referred to as Gower's tract. (Figs. 331 and 345.) 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 (thus 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 stimuli from muscle- tendon receptors. Ascending paths may also be formed by successive relays of shorter tracts in the ground bundles of the cord and the reticular formation of the brain. This has been especially claimed for the pain pathway. The afferent visceral path to the pallium is not known. Descending Tracts I. The Pyramidal Tracts {Tradus Cortico-spinalis, Cerebro- 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. 365). Their axones converge in the corona radiata and pass downward through the internal capsule, pes pedunculi, pons, and medulla, sending off fibres to the motor 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 (a) most of the fibres of each tract cross to the opposite dorso-lateral region of the cord and continue downward as the crossed or lateral pyramidal tract. This Hes in the dorsal part of the lateral column (Figs. 319 and 326). It extends to the lower- most part of the cord. In the cervical and dorsal region it is sepa- rated from the surface of the cord by the direct cerebellar tract. In THE NERVOUS SYSTEM 471 the lumbar region the latter tract is no longer present and the crossed pyramidal tract comes to the surface, (b) The minority of the fibres of the anterior pyramids, instead of decussating, remain on the same side to pass down the cord along the anterior median fissure as the direct or anterior pyramidal tract, occupying a small oval area adjacent to the anterior sulcus (Fig. 326). It does not usually extend below the middle or lower dorsal region of the cord. As the pyramidal 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 probably cross through the anterior 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 homolateral (uncrossed) fibres in the lateral pyram- idal 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 decus- sate 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-peripJieral efferent conduction path. According to some authorities the pyramidal fibres terminate around cells in the intermediate gray matter whose axones, which form a part of the ground bundles, in turn terminate around the efferent root cells. Short axone (Golgi type II) cells might also of course be intercalated in this connection. (Figs. 330 and 331.) 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.). n. The Colliculo -spinal Tract {Tectospinal Tract) orginates in the colliculi of the midbrain roof, decussates and descends to the cord, where it lies near the ventral sulcus. Its presence in the cord has been disputed. (Fig. 358.) in. The Tract from the Nucleus of the Posterior or Medial Longitudinal Fasciculus. — This nucleus is located in the reticular formation of the tegmentum of the midbrain cephalad to the nucleus of nerve III.^ The tract originating from it forms in the brain a part ^ 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. By the nucleus of the medial longitudinal fasciculus is here meant the interstitial nucleus of Cajal. 472 THE ORGANS of the posterior or medial longitudinal fasciculus. It is uncrossed and lies near the floor of the brain cavity and next the median line. In the cord it occupies a similar position near the ventral sulcus- Its fibres terminate in the ventral horn. Some fibres have been traced into the lumbar cord. (Figs. 326 and 358.) IV. The Rubro-spinal Tract {von Monakow's Tract). — This con- sists of axones of the red nucleus (nucleus ruber) located in the teg- mentum of the midbrain. These axones cross and descend to the cord, being joined by axones of the other cells in the reticular forma 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 cerebellum to cord composed as follows: (a) Axones of cells in cerebellar cortex to nucleus dentatus in cerebel- lum; (b) axones of cells in nucleus dentatus via superior cerebellar peduncle to the nucleus ruber; (c) axones of nucleus ruber as the rubro-spinal tract to (d) efferent peripheral neurones of cord. (Figs. 326, 331 and 345.) V. The Deitero-spinal Tract {Vestihulo-spinal Tract).- — This tract originates from Deiters' nucleus which lies in the medulla and receives fibres from the vestibular division of the acoustic nerve and also from the cerebellum (see below). It occupies the ven- tral 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 terminal nuclei, and of other nuclei in the gray re- ticular 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 path- way: (a) Axones of cells in cerebellar cortex to nucleus fastigii in cerebellum; {b) Axones of cells of nucleus fastigii as the fastigio- bulbar tract to Deiters' nucleus; (c) axones of cells of Deiters' nucleus as the Deitero-spinal tract to (d) efferent peripheral neurones of cord. According to some authorities some fibres proceed from cerebellum to cord without interruption in Deiters' nucleus. (Figs. 326 and 331.) All the fibres of V are sometimes 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 NERVOUS SYSTEM 473 the descending fibres of an important bundle in the segmental brain known as the medial longitudinal fasciculus. VT. The Fasciculus of Thomas. — Besides the reticulo-spinal fibres already mentioned are hljres in llie lateral column which originate in the reticular forma- tion of the medulla and terminate in the gray of the cervical cord. These are known as the tract of Thomas. (Fig. 326.) 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. 326). The origin and destination of its fibres are not definitely known. Some of its fibres appear to originate in the cord and terminate in the inferior olivary nucleus in the medulla (spino-olivary fibres). VIII. The Septo-marginal Tract. — This is a small bundle of fibres lying next the posterior 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 Flcchsig) at the middle of the septum and a superficial bundle, in the thoracic and cervical cord its fibres are more scattered. It is probably composed of descending axones of ceils in the cord. (Fig. 326.) 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. 326). 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 ceUs situated in the gray matter of the cord (column cells). In general the descending tracts fall into two categories: (i) Tracts which are descending or efferent suprasegmental paths, or are parts of such paths. These comprise (a) the efferent pallial path (tract I), {h) the efferent midbrain paths (tracts II and III) and (c) the efferent cerebellar paths (tracts IV and V) . (2) Descending inter- segmental tracts. Of the latter some originate in nuclei lying in the segmental brain (nucleus of the medial longitudinal fasciculus, nucleus ruber, nucleus of Deiters) which receive efferent supraseg- mental 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 descending fibres in the ground bundles (see below). Tracts VIII and possibly IX are also in this category. The descending path from the pallium which terminates around the bodies of the splanchnic efferent neurones in the cord is stated to consist of two neurones, a pallio-bulbar neurone to the medulla and a bulbo-spinal neurone to the efferent peripheral neurone in the cord. The axones of the latter pass out of the cord as preganglionic fibers to the sympathetic ganglion cells. In the cord the fibres of this path 474 THE ORGANS probably lie in the ventro-lateral columns, though some authorities place them in the dorsal columns. This pallio-bulbo-spino-sympathetic path is the path by which various psychic states (emotions, perceptions, memories) affect the splanchnic effectors, as seen in blushing, perspiring, erection, pupillary changes, etc. Many tracts contain fibres proceeding in a direction opposite to that of most of the fibres. Fundamental Columns or Ground Bundles (Shorter Inter- segmental Tracts of Cord or Spino-spinal Tracts) 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 Receptor Fig. 328. — -Diagram illustrating a Two-neurone Spinal Reflex Arc. Groups of neu- rones are represented by one neurone, gg, Spinal ganglion. (Van Gehuchten.) the lateral side of the posterior horn between the latter and the crossed pyramidal tract (Figs. 319 and 326). 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 THE NERVOUS SYSTEM 475 tibres serve as longitudinal commissural fibres to bring the different segments of the cord into communication (Fig. 329). 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. 450 and 451). The fact, alluded to above, that the shorter fibres lie nearest, or mingled with the gray is, in a general way, true through- out the central nervous system. A result of this in the cord is the superficial posi- tion 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 427). It must be kept in mind that there are prob- ably no isolated neural arcs and that every neural reaction involving any given arc always influences and is influenced by other parts of the nervous system. Fig. 329. — Diagram iUustrat- ing Three-neurone Spinal Reflex Arcs of one segment and more than one segment. Groups of neurones are represented by one neurone. g, Spinal ganglion cells; h.c.c, heteromeric column cell; t.c.c, tautomeric column cell; v.r., ventral root. From the neurones thus far studied and the tracts which their axones form, the following neural arcs may be con- structed: (i) A Two-neurone Spinal Reflex Arc (Fig. 328). — (a) Peripheral afferent neu- rones; their peripheral processes and receptors, the spinal ganglion cells, their central processes with collaterals terminating around motor cells of anterior horn; (6) peripheral efferent neurones, i.e., motor cells of anterior horn with axones passing to effectors. Such a two-neurone reflex arc is chiefly uncrossed and in most cases involves only one segment or closely adjacent segments. As it involves only one synapsis (see chapter X) (in the ventral gray) it is sometimes termed a monosynaptic arc. (2) A Three-neurone S pinal Reflex Arc (Fig. 329). — {a) Peripheral afferent neiirones as in (i), but terminating around column cells of the cord, (b) Cord neurones (column cells) — axones in the fundamental 476 THE ORGANS columns with collaterals and terminals to anterior horn cells of different levels, (c) Peripheral ejferent neurones as in the two-neu- rone 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) A Cerebellar Arc may be constituted as follows: (a) Peripheral afferent neurones to {b) column cells in cord {e.g., Clarke's column) via spino-cerebellar tracts to cerebellar cortex; (c) various associative cortical cerebellar 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 peripheral neurones in cord. Another arc would consist of {a), (b) and (c) the same, (d) cerebellar cortex to nucleus fastigii in cerebellum to (e) nucleus of Deiters to (/) efferent peripheral neurones to eft'ectors (Figs. 331 and 345)- Fig. 330. — Diagram showing the Most Important Direct Paths which an Impulse follows in passing from a Receptor (S) 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 cere- bral 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 fillet or sensory decussation ; F, level of pyramidal or motor decussation; G, spinal cord. From Periphery to Cortex. Neurone 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 ganghon cell as fibre of dorsal root to column of GoU or of Burdach, thence to nucleus of one of these columns in the medulla. Vi, Cranial (example, fifth cranial nerve, trigeminus; cell bodies in Gasserian ganghon); receptor; peripheral arm of Gasserian ganglion cell; central arm of Gasserian ganglion cell to medulla as afferent root of fifth nerve, thence to terminal nuclei in medulla. Neurone No. 2. — 2, Spinal connection — Cell body in nucleus of GoU or of Burdach; axone passing as fibre of fillet to thalamus. V 2, 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, axone 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 cap- sule and pes to {a) motor nuclei of cranial nerves {h) by means of pyramidal tracts to ventral gray of spinal cord. Neurone No. 5. — 5, Spiml, cell body in ventral gray of cord; axone as motor fibre of ventral root through mixed spinal nerve to effector (muscle). Neurone No. 5. — Cranial — Vb, Cell body in motor nucleus of trigeminus; axone passing to muscle as motor fibre of fifth nerve. Illh, Peripheral efferent neurone of third nerve — oculomotor. F/5, Peripheral efferent neurone of sixth nerve — abducens. F//5, Peripheral efferent neurone of seventh nerve — facial. Xlli, Peripheral efferent neurone of twelfth nerve — hypo- glossal. THE m:r\ous system 477 7? (4) A 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 neu- rones via pyramid to (/") efferent peripheral neurones to effectors. Another arc would in- volve the spino-thala- mic tract instead of the lemniscus. (Figs. 330 and 331.) Similar arcs ma}' in- clude efferent sympa- thetic neurones. (See pp. 471 and 442.) TECHNIC (i) 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 dura to the cork. Care must be taken to leave the dura very loose, otherwise it will flat- ten 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 i5.« in thickness. 478 THE ORGANS (2) For the study of the general internal structure of the cord, stain a section through the lumbar enlargement of a cord prepared according to the preceding technic (i) in haematoxylin-picro-acid-fuchsin (technic 3, p. 21) and another section through the same level in Weigert's ha^matoxylin (technic p. 33). Mount both in balsam. For Weigert staining, material fixed in formalin or in Orlh'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. 30). The silver method of Cajal (alcohol-fixation) should also be used (technic, p. 38) and that of Nissl. (3) From a cord prepared according to technic i, remove 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 2o;£ thick, stained by Weigert's method (page TiS), and mounted in balsam. McduUated 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 :i^). The picture in the cord is dependent upon the fact that axones cut off from their cells of origin degenerate and disappear. After a com- plete transverse lesion of the cord, therefore, all ascending tracts are found de- generated above the lesion, all descending tracts below the lesion. The method of Marchi gives a positive picture of osmic-acid -stained degenerated myelin in the affected tracts. The method of Weigert gives a negative picture, the neu- roglia tissue which has replaced the degenerated tracts being unstained in con- trast with the normal tracts, the myelin sheaths of whose fibres stain, as usual, dark blue or black. (5) Human cords from cases which have lived some time after the destruc- tion 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 complications being much less than after complete section. (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 pyramidal tracts are at this age non-meduUated and are consequently unstained in Weigert preparations. The cord of an infant of about one month is also ex- cellent. Here the majority of the pallio-spinal fibres are medullated but very thinly so that they are easily distinguishable by their lighter stain. The Wei- gert-Pal method gives the best results. (8) For the study of the course of the posterior root fibres within the cord, cut any desired number of posterior roots between the ganglia and the cord THE XER\ OUS SYSTEM 479 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 re- ceptors 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. 442), there are in the head the highly specialized receptors of smell, sight, hearing and posi- tion (semicircular 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 con- tinuations. It has already been stated that all the afferent periph- eral neurones which innervate these receptors (except the mesen- cephalic V) follow the general law of having their bodies located out- side the neural tube. The central processes (root fibres) usually split on entering the tube, but the descending arms are the longer. Nerves I and II present certain special peculiarities. The splanchnic effectors of the head include the usual splanchnic effectors — smooth muscle and glands — and also the branchial stri- ated voluntary muscles. The somatic effectors are the remain- ing (myotomic) striated voluntary muscles. The striated voluntary muscles of the head fall into three groups; those of the eye (somatic); of the jaw, face, pharynx and larynx (splanchnic, modified branchial musculature); and of the tongue (somatic). The peripheral path to the smooth muscles and glands follows the same general law as in the body, i.e., neurone bodies in the central nervous system send pre- ganglionic root fibres to sympathetic ganglion cells, -which in turn send their axones to the eft'ectors (p. 445). The peripheral path to 480 THE ORGANS the splanchnic (branchial) striated voluntary muscles, on the other hand, follows the same law as obtains for the somatic striated volun- tary muscles, i.e., neurone bodies in the central nervous system send their efferent root fibres uninterrupted to the muscle. These dis- tinctions are shown centrally by differences in grouping of the neu- rone bodies which supply respectively the somatic muscles, the splanchnic voluntary muscles, and the smooth muscles and glands via the sympathetic ganglia The higher coordinating apparatus or suprasegmental structures (p. 426) of the brain are essentially expansions of the dorsal walls of parts of the brain, each expansion having manifold afferent and efferent connections with the rest of the nervous system and having the endings and beginnings in it of its afferent and efferent paths complexly interrelated by enormous numbers of association neurones. The presence of these latter has probably 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. 426), 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) Splanchnic 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 or general sensibihty). — Semilunar ganglion of V. The peripheral arms of the semilunar ganglion cells pass to the skin of the anterior part of the head, to the mouth and meninges. The central arms of the ganglion cells form a descending tract in the medulla, the radix spinalis V. The terminal nucleus of this tract is the continuation in the medulla of the dorsal THE NERVOUS SYSTEM 481 horn. The axones of the terminal nucleus form a secondary tract to the thalamus and thence a tertiary thalamo-cortical tract passes to the central cortex cerebri. Axones of the mesencephalic nucleus of nerve V innervate the muscle-tendon receptors of the jaw muscles and nerves III, I\' and VI probably contain fibres to the muscle- tendon receptors of the eye muscles. (3) V esUbulo-Semicircular Canal Group. — Ganglion of Scarpa of VIII. The peripheral arms pass to vestibule and semicircular canals. The central arms constitute the vestibular portion of VIII, forming descending tracts in medulla and terminating in several vestibular terminal nuclei (including Deiters' nucleus). (4) Acoustic Group. — Ganglion spirale of VIII. The peripheral arms terminate in the organ of Corti of the cochlea. The central arms form the cochlear part of VIII and terminate in the medulla in various nuclei which originate the secondary tract (lateral fillet) to the midbrain, and medial geniculate body in thalamus. A third (or fourth?) system of thalamo-cortical neurones passes to the temporal cortex cerebri (also p. 588). (5) Visual Group. — Ganghon in retina. The second neurone system begins in the retina and forms the secondary tract (optic "nerve") to the lateral geniculate body in thalamus. The third thalamo-cortical neurone system passes to the calcarine cortex cerebri (also p. 567). (6) Olfactory Group. — " Ganglion " cells in olfactory mucous mem- brane form the olfactory nerve (lila olfactoria) which terminates in the olfactory bulb. Secondary tracts from the olfactory bulb (and tertiary tracts) proceed to diencephalon and hippocampus (also p. 591). Efferent Peripheral (Segmental) Neurones.^(i) Splanch- nic, (a) Lateral nuclei in the gray matter of the hindbrain. Their axones pass to the striated voluntary muscles of the jaw (V), face (VII), pharynx and larynx (IX and X). (b) Nuclei more deeply placed in the gray of mid- and hindbrain. Their axones pass as preganglionic fibres to sympathetic gangKa of head and body, (c) S>Tiipathetic. Their bodies compose the sympathetic ganglia of the head (ciliary, sphenopalatine, submaxillary and otic). These ganglia receive the above preganglionic fibres and are thus connected with cranial nerves III, V, VTI, IX, and X. (2) Somatic. — Medial nuclei located near the median line in the gray matter of hindbrain (XII and VI), and midbrain (IV and III), to 31 482 THE ORGANS muscles of tongue (XII) and eye (VI, IV and III). Nerve III also contains splanchnic neurones whose axones pass to sympathetic ganglia (ciliary). Nerves III, IV and VI probably also contain afferent nerve fibres (p. 480, (2)). Intrasegmental and 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 intermingled. Among the former are certain well differentiated nuclei (e.g., nucleus ruber, nucleus of Deiters, and nucleus of the medial longitudinal fasciculus) the axones of which form long, principally descending, intersegmental tracts external to the gray reticular formation. These may also be links in efferent suprasegmental paths. (See also pp. 483-4, XIV, XVI, XVII and XVIII.) Other cells in the gray reticular formation form the shorter tracts within it. The reticular formation may also con- tain motor nuclei of the cranial nerves and is traversed by various fibres passing to tracts and by terminals from tracts. Afferent and Efferent Suprasegmental Paths These paths are paths from receptors of body and head to cere- bellum, midbrain roof (colliculi) and pallium and paths from cere- bellum, midbrain roof and pallium to effectors of head and body. There are also paths connecting pallium, mid-brain roof and cerebellum. Afferent Suprasegmental Paths A. Afferent Pallial I. General Somatic Sensation: Spinal and trigeminal ganglionic -|- spino- thalamic and bulbo-thalamic (medial fillet) crossed + thalamo-pallial (to post- central cortex) neurones. (Fig. 330.) II. Splanchnic Sensation, including Taste. This important path is not well known. The gustatory path enters by ganglionic neurones of nerves V (?), IX and X (fasciculus solitarius). Its secondary tract may lie partly in the medial fillet. III. Hearing (cochlear) : Spiral ganglionic -}- lateral fillet (crossed) and brachium of inferior colliculus -|- geniculo-pallial (to temporal cortex) neurones (Fig. 33S). IV. Vestibular: A somewhat doubtful path except perhaps to the pallium through the cerebellum (paths X -|- VII). THE NERVOUS SYSTEM 483 V. Sight: Retinal bipolar ganglionic + rctino-gcniculate (optic nerve and tract, crossed and uncrossed in chiasma) + geniculo-pallial (to calcarine cortex) neurones. (Fig. 358.) VI. Smell: Olfactory ganglionic + bulbo-rhincnccphalic-pallial (crossed and uncrossed to hippocampal cortex) neurones. (Fig. 359.) \'II. Cerebello-pallial: Cerebellar cortical + dcntato-rubral and dentato- thalamic (superior cerebellar peduncle, crossed) + rubro-thalamo-pallial neurones. (Fig. 345.) B. Afferent Mesencephalic \1II, A comparatively unimportant path composed of spinal ganglionic + spino-coUicular (crossed) and l)ulho-coUicular (? medial fillet, crossed) neurones. C. Afferent Cerebellar IX. The spinal ganglionic (innervating proprioceptors of body) + spino- cerebellar path to vermis of cerebellum. The dorsal spino-cerebellar and ventral spino-cerebellar tracts of this path pursue somewhat different routes. (Figs. 331 and 345.) This path may receive accessions from the columns of GoU and Burdach and their nuclei in the bulb. X. The vestibular ganglionic (Scarpa) + Deitero-cerebellar path. The vestibular gangUonic neurones may send axones to the cerebellum without inter- ruption. (Figs. 331, 339 and 345.) Paths via other cranial nerves, especially nerves V and II, may also pass to the cerebellum. OUvo-cerebellar neurones form part of another, not well known, afferent cerebellar path. Efferent pallial path XIII is obviously also an afferent cerebellar path. Efferent Suprasegmental Paths A . Efferent Pallial XI. Voluntary Motor: Pallio (precentral cortex) — spinal (crossed) + spinal peripheral somatic motor neurones, also pallio-tegmental and pallio-bulbar (crossed and uncrossed) + cranial peripheral somatic motor and branchiomotor neurones. A short intersegmental system probably is intercalated between the pallial and peripheral neurone systems. (Figs. 330, 331.) XII. Splanchnic Efferent (except branchiomotor) : This is the pallio-bulbo- spino-sympathetic path mentioned on page 473 and comprises also descending pallial fibres which act upon neurones in the brain whose axones pass out as cranial preganglionic fibres. XIII. Pallio-cerebeUar: Pallio-pontile + ponto-cerebellar (middle cere- bellar peduncle, crossed) neurones. (Figs. 331, 345.) XIV. PaUio-rubral + rubro-spinal (crossed) -f- peripheral motor neurones (p. 518). This path may partly supplement, physiologically, path XI. Other less important paths are mentioned on p. 529. 484 THE ORGANS B. Efferent Mesencephalic XV. CoUicuIo-bulbar and coUiculo-spinal (crossed) + cranial (especially VII for squint reflex) and spinal peripheral motor neurones. (Fig. 35S.) XVI. CoUiculo-medial longitudinal fasciculus: This path probably consists of coUicular neurones which pass to the nucleus of the medial longitudinal fascic- ulus + the latter nucleus and its descending axones in the medial longitudinal fasciculus + peripheral motor neurones of brain (especially oculomotor) and cord. (Fig. 358.) Inasmuch as the superior coUiculus receives optic fibres, paths XV and XVI are probably largely concerned in optic reflexes. C. Efferent Cerebellar XVII. Cerebellar cortico (cortex of cerebellar hemispheres) — dentate -f dentato-rubral (superior cerebellar peduncle, crossed) + rubro-spinal (crossed) + peripheral motor neurones. (Figs. 331, 345.) XVIII. Cerebellar cortico (cortex of vermis) — fastigial + fastigio — Deiters (mesial part of inferior cerebellar peduncle) + Deitero-bulbar and Deitero-spinal (crossed and uncrossed) -j- peripheral oculomotor and spinal motor neurones. (Figs. 331, 339.) Inasmuch as Deiters' nucleus also receives directly vestibular nerve fibres, there exists the important vestibulo-Deiters + Deitero-bulbar and Deitero-spinal -f peripheral motor neurones reflex path whereby the vestibulo-semicircular canal receptors directly influence the position of eyes and body. Aft'erent pallial path \TI is obviously also an efferent cerebellar path. (Figs. 331, 345-) The large efferent pallial paths XI and XIII markedly affect the configura- tion of the brain. These two paths are added ventraUy to the segmental and intersegmental apparatus and form the pes pedunculi or crusta (added ventraUy to the tegmentum of the midbrain), the pons \'arolii (added ventraUy to the teg- mentum of midbrain, isthmus and hindbrain) and the pyramids (added ventraUy to the hindbrain). Arising from the neopaUium (p. 538), they are to be regarded as largely more recent acquisitions by the vertebrate nervous system. SUPRASEGMENTAL STRUCTURES • These are the pallium or cerebral hemispheres, the inferior and superior colliculi or corpora quadrigemina and the cerebellum. They consist essentially of the endings and beginnings of their re- spective 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 accorda.nce with the above there are usually to be distinguished in transverse sections of the brain at various levels the following: THE NERVOUS SYSTEM 485 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, {h) 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. I ntrasegmental and Intersegmental nuclei and tracts of the seg- mental brain, consisting principally of the gray reticular formation and long descending tracts (arrangement much modified in forebrain). D. Nuclei and tracts forming parts of afferent and efferent supra- segmental paths. 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 efferent suprasegmental tracts to them. E. Suprasegmental structures (not present in many transections). The general histology of the brain is similar to that of the cord. Cells of the motor cranial nuclei have an arrangement of chromophihc substance similar to their analogs in the cord, while cells of aft'erent cranial gangHa present a chromophihc picture similar to their cord analogs. Certain cells whose axones act directly upon motor cells in cord and brain {e.g., cells in motor cortex and certain cells in reticular formation) resemble motor cells, in arrangement of chromo- phihc substance, while certain cells in close connection with afferent peripheral neurones resemble the latter. The neurogha 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 tegmentum and pons. Its peripheral nerves are the V, VI, VII, VIII, IX, X, and XII. 1 The jMedulla Oblongata or Bulb is the continuation upward of the spinal cord and extends from the lower Hmit of the pyramidal decussation below to the lower margin of the pons above. ^ ^ It is better probably to reckon the so-called meduUarv or bulbar 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. 486 THE ORGANS «; P H. N.V N.C SP. QANC. CELL Fig. 331. THE NERVOUS SYSTEM 487 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 bulgin<^ 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 VI and XII (somatic motor) emerge near the mid- ventral line, the others, including the splanchnomotor portions of the V, VII, IX and X, emerge more laterally. The posterior surface shows two prominences on either side. The more median of these, known as the clava, is caused by the nucleus gracilis, or nucleus of the column of Goll; the other, the cuneus, lying just to the outer side of the clava, is due to the nucleus cuneatus or nucleus of the column of Burdach. Lateral to this is a third eminence, the tuber culum cmere/fw, due in part to the descending root of the V and its underlying terminal nucleus, the continuation of the dorsal gray column of the cord. This eminence merges anteriorly with the eminence of the restiform body. The central canal of the cord continues into the me- dulla, 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 hypo- glossi (trigonum hypoglossi). Lateral to this is a triangular area, the ala cinerea {trigonmn vagi), surrounded by furrows. This is partly occupied by nuclei of the vagus. Cephalad and laterally a broader Fig. 331. — Principal afferent and efferent suprasegmental pathways (excepting the rhinopallial connections, the efferent 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 geniculate body to temporal lobe; br. conj, brachium conjunctivum (superior cerebellar peduncle); br. pontis, brachium pontis, from pons to cerebellum; b.q.i, brachium quadrigeminuminferius; e.g./, lateral or external geniculate body; c.g.m, medial or internal geniculate body; c.quad, corpora quadrigemina;/.cor/.-5/>,pallio-spinal fascic- ulus (pyramidal tract); f.c.-p.f, frontal pallio-pontile fasciculus (from frontal lobe); f.c.-p.t, temporal pallio-pontile fasciculus (from temporal lobe); f.c.-p.o, occipital pallio-pontile fasciculus (from occipital lobe);/.c«w, fasciculus cuneatus (column of Burdach); t.f.-b, fastigio-bulbar tract;/, grac, fasciculus gracilis (column of Goll); f.s.-t, spino-thalamic fasciculus; /.^/'.-c.f/, dorsal spino-cerebellar fasciculus (tract of Flechsig); f.sp.-c.v, ventral spino-cerebellar fasciculus; lem. lat, lateral lemniscus or lateral fillet; Icm. vied, medial lemniscus or fillet; n. coch, cochlear nerve; n.cmi, (terminal) nucleus of the column of Burdach; 7i.d, nucleus of Deiters; 7i.dent, nucleus dentatus; n.grac, nucleus of the column of Goll; n.opt, optic nerve; n.r, nucleus ruber; 7i.t, nucleus tecti (or fastigii) ; n.trig, trigeminal nerve; n.vesl, vestibular nerve; pes.ped, pes pedunculi (crusta); ptilv thai, pulvinar thalami; pyr, pyramid; rad. ant, ventral spinal root; rad. post, dorsal spinal root; rad. opt, optic radiation (from lateral geniculate bod\^ to calcarine region) ; 50?ki-- — -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. 357 is not quite accurately indicated. 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 continua- tion forward of the medulla, may be included in the term tegmentum of the hindbrain. The Cerebellum is described on p. 513. r TECHNIC The technic of the medulla (and the rest of the segmental brain) is the same as that of the cord (page 477). Transverse sections should be cut through the following typical levels, stained by Weigert's method (page 2)i), and mounted in balsam: THE NERVOUS SYSTEM 489 1. Through the pyramidal decussation. 2. Through the sensory decussation. 3. Through the lower part of the olivary nucleus. 4. Through the middle of the oliwary 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 roots of the fifth cranial nerve. The methods of Nissl, Cajal and glia stains should also be used when practic- able. PRACTICAL STUDY I. Transverse Section of the Medulla through the Decussation of the F^yram- idal Tracts (Motor Decussation) (Figs. 332 and 333) The most conspicuous features of this section are the decussation of the pyra- 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 fibers passing out to emerge on ventral aspect. Nuclei of XI, in mesial position in central gray, or in ventral gray. Axones pass out laterally from latter and emerge on the lateral surface. The mesial or deep nuclei are best reckoned with nerve X. 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 fibres of the V-cranial nerve or traclus 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 medullatcd 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. 512 and Fig. 342). 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 colliculo-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 colliculo-spinal; i.e., (a) rubro-spinal (in lateral arm of U), (b) Deitero-spinal (lateral and mesial), 490 THE ORGAXS THE NERVOUS SYSTEM 491 (c) tract from nucleus of medial longiLutlinal fasciculus (mesial), (d) colliculo- spinal (mesial), (e) shorter descending and ascending tracts which may be re- garded as the equivalent of the ground bundles of the cord comprising shorter reticulo-spinal and spino-ret'cular fibres. The shortest of these fibers, which in the cord were next the lateral gray, arc now mingled with the gray, the combina- tion constituting the gray reticular formation. Other short intra- and interseg- mental tracts lie in and adjoining the dorsal horn, as in the cord. Descending suprasegmental paths include certain of the above long descend- ing intersegmental tracts as previously explained. Besides these there are the efferent suprasegmental neurones known as the pallio-spinal or pyramidal tracts and the colliculo-spinal tracts. Bundles of fibres are seen crossing {pyramidal decussation) from the anterior 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 pyram- idal decussation most of these fibres cross to the opposite dorso-lateral region to pass down the cord as the crossed pyramidal tract (p. 470, and Fig. 326; Fig. 330, F). The remaining fibres stay in their original anterior position and continue down the cord as the direct pyramidal tract (p. 471, and Fig. 326; Fig. 330, F). A few pass to the ventral tract in the same side, thus becoming 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 transverse 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. 332). 2. Transverse Section of the Medulla through the Decussation of the Fillet or Lemniscus (Sensory Decussation) (Figs. 332 and 334) The most conspicuous features are the appearance of the nuclei cuneatus and gracilis, the decussation and formation of the medial lemniscus or fillet, 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 cinerecB) . In the ventral part of the central gray is the nucleus hypo gloss i 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 vagoglossopharyngeal and trigeminal tracts (three-neurone reflex, and from various parts of the reticular formation. Whether pyramidal fibres reach the nucleus directly or via intercalated neurones is uncertain. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — Entering afferent root fibres are usually not present. The funiculi or fascicuH cuneatus and gracilis have diminished, and internal to them have appeared large masses of gray. These are the nuclei of the columns, 492 THE ORGANS 3 a jj c o m.« o ■M :2S W3 .2-a faii y rt-2 ~ — ' Eg 4.3 M Ifci 11 "3 fc> ( K **-* o o ^l i ibto 4-1 / '■ .^ a"" l^^vUB a^ H -< :**j b^jgi^ \ K>.JBBt^^ \ .28 THE NERVOUS SYSTEM 493 and are known, respectively, as the nucleus of the column of Goll or the nucleus gracilis, and the nucleus of the cohimn 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 completion 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 corresponding 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. 467, 468.) Study the plexus of fine fibres in these nuclei, formed by the terminals of the column fibres, also the coarser fibres (a.xoncs of the cells of the nuclei) gathered in the ventral part of the nuclei, whence they emerge and curve^ around the cen- tral 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 medial 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 Nevirones. — The rubro-spinal and Deitero-spinal tracts and the tract from the nucleus of the medial longitudinal fasciculus occupy about the same positions. The 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 longitudinal 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 parts of the anterior pyramidal tracts which form two large masses of fibres one on each side of the ventral sulcus. The coUiculo- spinal tract occupies the same position. 3. Transverse Section of the Medulla through the Lower Part of the Inferior Olivary Nucleus (Figs. 332 and 335) The central canal has opened into the fourth ventricle, the central gray (in- cluding the central gelatinous substance) now being spread out on its floor. The roof of the ventricle is formed by its chorioid plexus. The most conspicuous new feature is the olive. Efferent Peripheral Neurones — The nucleus of the XII is large and occupies ^ Fibres having a transverse curved or arched course are in general termed arcuate fibres. If they are deeply located, they are internal arcttate fibres, if near the periphery, they are superficial or external arcuate fibres. Obviously the same fibre may be, in different parts of its course, internal arcuate, external arcuate, and longitudinal. 494 THE ORGANS 6 u u E • en o o O (J s .2 a u 3 o « d •M > > > c O a ^__, o o o 3 '_a) , 2; o "o o o o / 3 3 3 3 3 IL. ;5 Z 2; 2; 2 ^i'^^^^^'-^'^j^ii^ f^ d •a c a! P4 o o Pi o .M 'S , a tn _a) "o ■4-* 3 M iz; 3 O >» <4-i M Ut a O > rt o -^ O 43 1— 1 s o CU^ r-t -.-> (D ^4-< ^ o -W t<>-l l-> o t:i V) <=;•- J ^1) s"^ ' "c. '^ r- d ^ c ; « O D 7) o 3 r-' ^ H •So 1 o 5 S 373 1 * eu ^ o i) r- SH o 43^ ^ ■^^ r-« p^-ri rt O c/1 *-» o u c o" .2^ d -*~i 1 '-' h d o u 3 C/2^ u 3 w o cS d o g^ d o ^^ £ > .Q c/2 s fiG. 335.— Tran preparation. THE NERVOUS SYSTEM 495 tne swelling in the floor of the ventricle each side of the median line known as the eminentia or trigonum 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 sympathetic neurones) some, at least, of the smooth muscles, heart (and glands ?), innervated by the vagus (X). The dorsal nucleus appears to be relatively deficient in termi- nals. What it does receive ap- pears to come from the secondary vago-glossopharyngeal and tri- geminal tracts. The bodies of another group of peripheral efl'erent neurones form the nucleus ambiguiis, often difficult to distinguish, in the reticular formation. Their ax- ones pass obliquely dorsally and mesially, join the other root fibres of the X, and then bending ab- ruptly, pass with them to the lateral surface of the medulla. Some pass across the median line and leave by the root of the op- posite side. They probably in- nervate the striated muscles of the pharynx, larynx (and oesoph- agus ?). The nucleus receives various terminals, some at least appearing to come from the secondary trigeminal tracts and from the lateral part of the retic- ular formation. (Fig. 336.) Afferent Peripheral Roots, their Terminal Nuclei and Secondary Tracts. — Other root fibres are the afferent fibres of the X w^hich form a common root with the preceding. Sometimes they can be seen joining the fasciculus solitarius of which they form a part. Some fibres or collaterals may enter the adjacent gray {terminal nucleus of the X) (see Fig. 336). 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 oUvo-cerebellar fibres (see below) . Its terminal nucleus is less conspicuous. Two new bundles of descending root fibres have appeared; one is the fasciculus solitarius composed of the afferent root fibres of the X, IX (including gustatory Fig. 336. — Diagram of Origin of Cranial Nerves X and XII. (Schafer.) pyr, Pyramid; 0, olivary nucleus; r, restiform body; d.V, spinal root of fifth nerve; n.XII, nucleus of hypo- glossal; XII, h>=poglossal nerve; d.n.X.XI, dorsal nucleus of vagus; n.amb, nucleus am- biguus;/.^., solitary fasciculus (descending root of vagus and glosso-pharyngeal) ; f.s.n, nucleus of solitary fasciculus; X, 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 glossophar^Tigeal tract.) This course of the secondary tract is doubtful. 496 THE ORGANS 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 vestibular 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 emerging from these regions ma}?- represent secondary tracts (probably reflex) from these nuclei, (p. 499; Fig. 339.) The nucleus gracilis has disappeared. The nucleus cuneatus may be pre- sent, much diminished, and give rise to some internal arcuate fibres to the medial lemniscus. 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 decussating 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. 513; Fig. 345). 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 the bulge of 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 body, thus formed by these spino-cere- bellar 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. According to many authorities fibres from the columns and nuclei of Goll and Burdach of the same side (dorsal external arcuate fibres) and opposite side (ventral external arcuate fibres) may join the restiform body. (Comp, P- 513-) , . Fibres appearing on the external surface of the olivary nucleus are the term- ination of a large tract descending to the olivary nucleus, the central tegmental tract. Its origin in higher levels is not accurately known. Intersegmental Neurones. — The reticular formation is now still more extensive. The original U-shaped mass of intersegmental tracts (and the coUiculo-spinal tract) has now become widely separated into two parts. The lateral part, con- THE NERVOUS SYSTEM 497 sisting principally of the rubro-spinal tract and uncrossed Deitero-spinal fibres, lies mesial to, or partly mingled with, the spino-thalamic and ventral spino-cere- bellar tracts. The mesial part of the U, consisting principally of crossed and uncrossed Deitero-spinal fibres and fibres from other nuclei in the reticular formation, and of fibres from the nucleus of the medial longitudinal fasciculus, now forms the medial longitudinal fasciculus dorsal to the fillet. Near this bundle, or united with it, is the coUiculo-spinal tract (predorsal tract). When these tracts have passed down to below the formation of the fillet and the olives, they assume I he positions noted in the lower levels of the medulla. Efferent Suprasegmental Neurones. — The pyramids are the same. Small bundles of more lightly stained fibres present in the fillet here and in higher levels (Weigert stain, not indicated in the figures) are efferent pallial fibres detached from pes or pyramids. They are aberrant fibres which rejoin the pyra- mids or are fibres innervating motor cranial nuclei. The colliculo-spinal tract (see above). 4. Transverse Section of the Medulla through the Middle of the Ohvary Nucleus Such a section is so similar to 3 and 5 that its detailed description may be omit- ted. 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. 332 and 337) 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 afferent fibres (see below). The nucleus XII has disappeared 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 solitarius or its terminal nucleus. The fasciculus solitarius is smaller, and just above the entrance of the IX consists of only a comparatively few descending 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 externall}^ the restiform body and known as the ventral (or accessory) and dorsal (or lateral) cochlear nuclei. ]Most of the axones of the dorsal nucleus pass across in the floor of the ventricle {strice medullares) 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, (trapezius), and also form a part of the lateral lemniscus. This latter decussation takes place at a higher level (see next section). 498 THE ORGANS o O.X =' <" M Rl c r-* .2 ^ en Hi CO • ii . . o ^ f-( '^ f-' n: o THE NERVOUS SYSTEM 499 The auditory nerve is divided into two parts: the cochlear nerve (ganglion spirale) and the vestibular nerve (ganglion of Scarpa). The fibres of the coch- lear 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 author- ities, 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 and send collaterals to, or are partially interrupted in, the nucleus olivaris superior, trapezoideus, nucleus of lateral fillet and inferior coUiculus (posterior corpus quadrigeminum). According to some authorities all the fibres of the lateral lemniscus terminate in the inferior coUiculus. From the inferior colliculus 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 (decussation) lateral lemniscus, (3) posterior corpus quadrigeminum and its brachium, (4) medial geniculate body of the thal- amus and geniculo-cortical fibres. If the lateral lemniscus fibres be regarded as simply passing by the posterior corpus quadrigeminum, giving collaterals to it (Cajal), the path would in part consist of three neurone systems analogous to those of the paths from the cord, trigeminus and eye. (Figs. 331, 338.) The fibres of the vestibular root enter higher and mesial to those of the coch- lear 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; (o) Deiters' nucleus (lateral vestibular nucleus) situated at the end of the main bundle of root fibres, just internal to the restiform body; (5) von Bechterew's nucleus (supe- rior 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 descending 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 vestibular tracts, some axones going to (a) the cerebellum (?),(b) the midbrain, especially 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 various motor nuclei, via the medial longitudinal fasciculus, lateral tract from Deiters' nucleus and other tracts in the reticular formation. (Figs. 331, 339 and 345.) 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. Secondary trigeminal tracts cannot be distinguished — such fibres probably either join the medial lemniscus or form an independent ascending tract in the reticular formation. The fillet is about the same. The ventral spino-cerebellar and spino- thalamic tracts are in the same positions. Other Afferent Cerebellar Neurones. — The olives are still larger and send many bundles of olivo-cerebellar fibres to the opposite restiform body which has 500 THE ORGANS 'lio.tera.li3. '!' B 'Lemniscus lateralis Nu. Corf) trabe^Otdei Oliva. 5ubtri.or Ifu ace e$ tort Fig. 338 THE NERVOUS SYSTEM 501 EXPLANATION OF FIG. 338 Fig. 338. — Diagram showing Connections of the Cochlear Portion of the Auditory (VIII) Nerve. A, Section at level of superior colliculi (Attt. corp. quad.) and red nucleus; B, through level of inferior colliculi {Corpus, quad. ^05^); C, through level of nucleus of lateral lemniscus {Nu. lent, lal.); D, through pons at level of VIII nerve. Spacing between the ditYerent levels is not proportionate. In B and C the basis pedunculi is omitted. Each neurone group is indicated by one or several individual neurones. Neurone No. i. — Cell bodies in spiral ganglion (gang, spiralis); peripheral processes end in organ of Corti; central processes terminate principally in ventral or accessory nucleus {N'u. accessorius) and lateral nucleus {A^u. lateralis) or tubcrculum acusticum; some also terminate in superior olives {Oliva superior), and nuclei of trapezoid body {Nu. Corp. trapezoidei) of same and opposite sides. Neurone jYo. 2. (and 3 ?).• — rAxones of cells in accessory nucleus, in superior olives, and in nuclei of trapezoid body, constitute a ventral path in the lower border of the tegmen- tum, and form the lateral part of the lateral lemniscus {Lemniscus laleralis) or lateral fillet on the opposite side. Axones of cells in the lateral nucleus trav'erse the floor of the fourth ventricle as the striae meduUares, forming a dorsal pathway, decussate and then turn ventrally to a point dorsal to the superior olive and join the lateral lemniscus as its mesial part. Some axones also of cells in the accessory and lateral nuclei pass dorsally, looping around the restiform bod}', and then proceed ventrally (bundle of Held) to join the opposite lemniscus. The lateral lemniscus passes upward to the inferior colliculus, some of the axones terminating en route in the nucleus of the lateral lemniscus. From cells in this nucleus some axones again join the lateral lemniscus, and a few decussate and then pass upward to the inferior coUiculius. The axones of the lateral lemniscus ter- minate in the inferior colliculus, or pass on to terminate in the internal geniculate body, merely giving off collaterals to the superior colliculus. Some fibres of the lateral lemniscus probably go to the superior colliculus. Neurone No. 3 (and 4?). — Axones of cells in the gray matter of the inferior col- liculus form its brachium {Brachium corp. quad, post.) and ascend to terminate in the internal or medial geniculate body {Corp. genie, inter.). Neurone iVo. 4(and 5?). — Axones of cells in the internal geniculate body pass as a part of the thalamic radiation 'cia the posterior part of the internal capsule to the cortex of the temporal lobe of the cerebrum. The axones which constitute the ventral path (Neurones i and 2) form a bundle of fibres known as the trapezoid bod}' {Corpus trapezoideuin) or trapezius. The decussa- tion of these is peculiar in that the dorsal axones of the bundle on one side become the ventral ones on the opposite side; this accounts for the convergence of the axones at the median raphe. Axones of cells in the superior olive pass to the nucleus of VI nerve (reflex). There is possibly also a descending path from the lateral nucleus to the spinal cord (not indicated). 502 THE ORGANS fiu.nvestd^sc C-a.no. Sca.rha.e Ved.il 1^. lyj+en cerebelli Fig. 339. THE NERVOUS SYSTEM 503 EXPLANATION OF FIG. 339 Fig. 339. — Principal Connections of the Vestibular Portion of the Auditory (VIII) Nerve. A, Section at level of oculomotor (III) nerve; B, section through pons and cere- bellum; C, through inferior olives; D, through spinal cord. Each neurone group is indicated by one or several individual neurones. Neurone No. i. — -Cell bodies in ganglion of Scarpa; peripheral processes end in semi- circular canals; central processes bifurcate, and ascending arms go to Deiters' nucleus (Nu. lal. 11. veslib.) (i a), to von Bechterew's nucleus {Nu. sup. n. veslib.) (i h), and to nuclei fastigii and cortex of vermis of cerebellum (i c); descending arms go to nucleus of descending root {Nu. n. vestib. desc.) (1 d) and (collaterals?) to principal or median nucleus {Nu. med. n. vestib.) (i e). Neurone No. 2. — Axones of some cells in Deiters' nucleus descend {Tr. desc. nu. Deilersi) uncrossed to antero-lateral column of the cord, axones of other cells enter the posterior longitudinal fasciculus {Fasc. long, post., 2 b) of same side and descend to anterior column of the cord, others pass to the medial longitudinal fasciculus of opposite side whence some (2 c) descend to anterior column of the cord, occupying a position near the anterior median fissure, while some (2 d) ascend in the medial longitudinal fasciculus and terminate principally in the nuclei of VI, IV, and III nerves. Axones of cells in von Bechterew's nucleus ascend (2 e), joining lateral part of medial longitudinal fasciculus of same side, and terminate in nuclei of IV and III nerves. Axones of cells in the nucleus of the descending root probably pass in part to lateral part of reticular formation of same and opposite sides, ascending and descending (to other motor nuclei?). Axones of cells in the median nucleus probably pass largely into the reticular formation, possibly also to the medial longitudinal fasciculus (not indicated). Axones of cells in the nuclei fastigii of the cerebellum pass to von Bechterew's nucleus (2/) and to Deiters' nucleus (2 g). The cerebellar associations intercalated between these (2/, 2 g) and the vestibular fibres to the cerebellum (i c) are not known. (It is evident that impulses other than vestibular ones entering the cerebellum may also by 2 /and 2g act indirectly upon the motor nuclei innervated by axones of the cells in Deiters' and von Bechterew's nuclei. Compare Figs. 331 and 345.) THE ORGANS Vvm Fig. .■^40, — Section through the Hindbrain at the Level of the Junction of Pons and Cerebellum and the Entrance of the Vestibular Part of the Eighth Nerve. Weigert preparation. (Marburg.) Va, Radix spinalis trigemini (spinal root of the fifth); VI, nervus abducens (external eye muscle nerve); Vila, pars nuclearis nervi facialis (pars prima, crus of origin or ascending facial root); VIII, nervus acusticus (eighth, vestibular part); 5Po, brachium pontis (middle cerebellar peduncle); i?rr;, brachium conjunctivum (superior cerebellar peduncle) ;ciJ, fasciculus tegmenti centralis; C/'ft, corpus pontobulbare (Essick) ; Crj/I, corpus restiforme; Dca, decussatio cerebelli anterior; iPec/, declive; Enib, embolus (nucleus emboliformis);/c Po, fibrje pontocerebellares; Ffb, fasciculus fastigio- bulbaris (uncinate bundle of Russell); //,?/, pedunculus flocculi; glob, nucleus globosus; Lm, Lemniscus medialis (mesial fillet tract); XVII, nucleus facialis (motor nucleus of facial nerve); NaB, nucleus angularis, or superior, vestibularis (Bechterew); ?\'dt, nucleus dentatus cerebelli; Nod, nodulus cerebelli; Nos, nucleus olivaris superior; Nrl, nucleus reticularis lateralis (nucleus of the lateral column); Nrtg, nucleus reticularis tegmenti; Nt, nucleus tecti (nucleus fastigii); Nvm, nucleus vestibularis magnocellulaiis or lateralis (Deiters) (large-celled nucleus of vestibular nerve); Plchl, plexus chorioideus lateralis; Fo, pons;Pv, pyramid;5/n", stratum intermedium (pedunculi); Tr, corpus trapezoides; vIV, ventriculus quartus; vNdt, vellus nuclei dentati cerebelli (fleece of the cerebellar olive)- THE NERVOUS SYSTEM 505 still further increased in size. External arcuate fibres may be present and prob- ably contain fibres to the restiform body and possibly fibres from the cerebellum, which end in the reticular formation. The arcuate nuclei are present. The central tegmental tract is larger. Intersegmental Neurones. — The reticular formation is very extended. Its composition (longitudinal and transverse fibres, and cells) should be examined carefully. The rubro-spinal tract is in the same position, but the lateral Deitero- spinal tract is now more internally located. Its fibres cannot usually be dis- tinguished, but are bending inward and toward its nucleus of origin (lo- cated somewhat higher). The medial longitudinal fasciculus may be partially separated from the medial lemniscus. It is a complex bundle and contains at va- rious levels (c) descending and ascending fibres from Deiters' nucleus and other cells scattered in the reticular formation, (/)) descending fibres from the nucleus of the medial longitudinal fasciculus in the tegmentum of the m.idbrain. The fibres of this fasciculus probably terminate in many nuclei, especially those of eye-muscle nerves (III, IV and VI) (comp. Figs. 339, 345 and 358). Efferent Suprasegmental Neurones. — The pyramids and colliculo-spinal tracts are in the same positions. The aberrant efferent pallial fibres already noted (p. 497) may be seen in the lemniscus. 6. Section through the Hindbrain at Level of Junction of Pons and Cerebellum and Entrance of Vestibular Nerve (Figs. 332 and 340) The most conspicuous features are the nuclei and fibres of the pons, added ventrally to the preceding structures, which are now collectively known as the tegmentum; the cerebellum, enclosing dorsally the fourth ventricle; and the connec- tions {inferior and middle peduncles, arms, or brachia) of the cerebellum with the rest of the brain. The greater part (restiform body) of the inferior peduncle represents ascending cerebellar connections from all parts below this level, the middle peduncle (from the pons) is the second link in the descending connection from the pallium to the cerebellum (pallio-cerebellar path). The superior ped- uncle (efferent cerebellar) is not fully formed at this level. (Comp. p. 514.) Efferent Peripheral Nevirones. — In this level and that of the next section are present the nuclei and root fibres of nerves VII and VI. The nucleus of nerve VII, or nucleus facialis is seen occupying a lateral posi- tion in the reticular formation similar to that of the nucleus ambiguus. In it may be made out the usual plexus of fine terminals and the coarser root fibres which proceed dorso-mesially to the floor of the ventricle, where they partially envelop the nucleus of the VI (usually not present in this level). They then turn cephalad, forming a compact longitudinal bundle (next section) and finally turn ventro-laterally and caudally to emerge on the lateral aspect at the caudal border of the pons. This latter part is the second part as distinguished from the first part of the connection. The bend is known as the genu facialis. Ac- cording to some authorities, some of the fibres cross and pass out in the root of the opposite side. Four groups of cells composing the nucleus facialis have been distinguished: three ventral groups which, passing from the most mesial to the most lateral group, innervate respectively the muscles of tympanum, of pinna and of mouth 506 THE ORGANS ventr /y m!i and face. The dorsal group (to superior branch of facial) innervates the frontalis, corrugator supercilii and orbicularis palpebrarum Among the terminals in the nucleus have been distinguished collaterals (reflex) from the lateral part of the reticular formation, from the secondary acoustic and trigeminal tracts and other adjacent fibres; also terminals of the coUiculo-spinal tract. Whether the nucleus receives direct terminals from the pyramids or whether fibres of the latter are only connected with it via intercalated neurones is uncertain. Some of the root fibres of the VI are usually seen in the ventral border of the tegmentum. For nucleus see next section. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The afferent vestibular root fibres enter at the caudal border of the pons and pass lateral to the spinal V, mesial to the ventral cochlear nucleus and restiform body, and enter the field previously occupied by the descending vestibular root, the fibres of which are a continua- tion of the root. Scattered large cells in this region form the nucleus of Deiters. Dorso-mesial to this is still the medial vestibular terminal nucleus and dorsal to Deiters' at the external angle of the fourth ventricle is the superior vestibular terminal nucleus (von Bechterew). Fibres seen passing from the vestibular region to the cerebellum and lying near the ventricle are partly vestibular root fibres to the cerebellum (especially to the nucleus tecti, or fastigii, see below), and partly descend- ing fibres from cerebellar nuclei (especially from the nuclei fastigii, forming jastigio-bulbar fibres) to Deiters' nucleus, other vestibular nuclei, and other cells in the reticular formation. It is thus evident that such nuclei as Deiters' may act as parts of vestibular - bulbo-spinal reflex arcs and also as parts of eft'erent and possibly afferent cerebellar paths. Internal arcuate fibres from the vestibular area are probably principally fibres (secondary tracts) from the various vestibular nuclei to the medial longitudinal fasciculus and other tracts in the reticular formation. (Comp. pp. 499, 507, 513, Figs. 339, 345.) The nucleus olivaris superior lies ventral to the nucleus facialis and lateral to the central tegmental tract. This nucleus together with several other small nuclei in its immediate vicinit}^ (preolivary nucleus, semilunar nucleus, trapezoid nucleus) is one of the nuclei intercalated in the cochlear path (Fig. 338) which provides reflex connections {e.g., with the \1 and \TI motor nuclei). Lateral /'/•'■ Fig. 341. — Diagram of Origin of Sixth and Seventh Cranial Nerves. (Schafer.) pyr, Pyramid; or, restiform body; dV, spinal root of fifth nerve; Vcntr. IV, fourth ventricle; VIII .v, vestibular root of eighth nerve; n.VI, chief nucleus of sixth nerve; n'VI , accessory nucleus of sixth nerve; VI, sixth nerve; n.VII, nucleus of seventh nerve, from which the axones pass dorso- mesially to the floor of the ventricle, where the}- turn brainward, appearing as a bundle of transversely cut fibres, a VII, and ascend to the "genu," g, where they turn and pass ventro-laterally and somewhat caudally to the surface as the seventh nerve, VII. THE NERVOUS SYSTEM 507 to it is seen a mass of fibres which pass by it toward the median line through the medial lemniscus, and decussate, finally turning longitudinally dorso-lateral to the opposite superior olive. These are fibres of the trapezius, and together with the more dorsal secondary cochlear fibres (p. 499) form the lateral lemniscus (See Fig. 338 and page 499). The lateral lemniscus is thus one of the links in the cochlear or auditory pathway. Fibres pass from superior olive to nucleus of nerve VI (reflex). In some cases the slender afferent root of VII {portio inter- media or nerve of Wrisberg) from the ganglion geniculi may be seen entering between the vestibular and main facial roots. Its fibres proceed to a gray mass which may be regarded as a continuation of the nucleus fasciculi solitarii and there probably partly terminate and partly send descending arms to join the fasciculus solitarius. The spinal V occupies the same position though separated from the surface by the pontile fibres; internal to it is its terminal nucleus. Note the change in the shape of the lemniscus. The ventral spino-cerebellar and spino-thalamic tracts are in the same position though her-j separated from the surface by the added pontile fibres. Other Afferent Cerebellar Neurones.— The inferior olives are not present, and the olivo-cerebellar fibres are here entering the cerebellum as a part of the restiform body. The central tegmental tract (to the olives) occupies the ventral part of the reticular formation. The restiform body is entering the white matter of the cerebellum. It has been seen to be composed of the dorsal spino-cerebellar tract, olivo-cerebellar fibres and fibres from the lateral and possibly other nuclei in the reticular formation. The dorsal spino-cerebellar tract terminates in the cortex of the vermis or middle lobe of the cerebellum, the olivo-cerebellar fibres terminate in all parts of the cerebellar cortex. The fibres mesial to the restiform body, consisting of ascending vestibular fibres to the cerebellum and descending fibres to vestibular and other nuclei (see cerebellum), are sometimes called the in- ternal or juxta-restiform body. This and the restiform body proper constitute the inferior cerebellar peduncle. The pons consists of gray matter— the pontile nuclei — and of transverse a.nd longitudinal fibres. The longitudinal fibres include the pyramids which pass through to the medulla and cord, and other fibres from the pallium (pallio-pontile or cerebro-pontile) which terminate in the pontile nuclei. The axones of the latter form the transverse pontile fibres (ponto-cerebellar fibres) which cross and pass to the cortex of the opposite cerebellar hemisphere. They constitute the middle cerebellar peduncle or brachium pontis. (Comp. "P- 514-) The pallio-pontile and ponto-cerebellar neurones constitute the pallio-ponto- cerebellar path connecting one cerebral with the opposite cerebellar hemisphere (p. 483, path XIII). There are probably also transverse fibres in the pons connecting cerebellum and reticular formation. Fibres passing vertically in the raphe from pons to reticular formation (perpendicular fibres of pons) may be in part continuations of these and in part efferent pallial fibres from pes to teg- mentum. The latter are either aberrant fibres or fibres innervating directly or indirectly motor cranial nuclei. Intersegmental Neurones. — The reticular formation is extensive. In it there may be distinguished, besides the nuclei already mentioned, various more V. 508 THE ORGANS or less well-defined reticular nuclei (see Fig. 340). The fibres of the lateral Deitero-spinal tract (not distinguishable) are here emerging from Deiters' nucleus. The medial longitudinal fasciculus occupies the same position, but is here well separated from the fillet. Some of the internal arcuate fibres in the dorsal part of the reticular formation may be fibers from Deiters' nucleus to the medial longitudinal fasciculus. They may be crossed or uncrossed, and may descend in it as already mentioned (pp. 472 and 505, or ascend (see Fig. 339). '{ Other internal arcuate fibres here, as elsewhere, pass from the various terminal nuclei to form secondary tracts. Other transverse fibres are axones of cells of 7 reticular nuclei or collaterals and terminals ending in them. 7 Efferent Suprasegmental Neurones. — The coUiculo-spinal tract lies ventral to the medial longitudinal fasciculus. The pyramids are in the same position, but are partly surrounded by pontile fibres and nuclei. (See also pons, above.) t The Cerebellum. — The gray matter consists of the external gray or cortex, 1 and inlernal nuclei forming interruptions or relays in paths from the cortex. The white matter consists of the fibres of various afferent and efferent cerebellar paths and possibly association fibres of the cerebellum. The cortex is studied elsewhere. The internal nuclei can usually be distinguished. They are the nucleus dentatus cerebelli {corpus dentatum), a convoluted mass of gray resembling the inferior olives (and sometimes called the cerebellar olives), and mesial to this the nucleus globosus, nucleus enibolijormis and the nucleus tecti or fastigii. The nucleus fastigii receives fibres from various parts of the cerebellar cortex and also vestibular root fibres (p. 503). Its axones, in part at least, pass to Deiters' nucleus and other nuclei in the reticular formation {fastigio-bulbar tract). This forms a link of the cortico-fastigio-Deitero-spinal path (p. 484, XVIII). The nucleus dentatus, nucleus globosus, and nucleus emboliformis also receive fibres from the cerebellar cortex. Their axones form the superior cerebellar peduncle {brachium conjunctivum) , cross and pass to the red nucleus, reticular formation, nucleus of nerve III, and thalamus. At the level of the section the superior pe- duncle is not yet fully formed. This forms links in both the cortico-dentato-rubro- spinal and the cerebello-pallial paths (p. 484, XVH and \TI. See also p. 513.) Note where possible the structure of the plexus chorioideus of the fourth ventricle. It consists of a layer of cuboidal epithelial cells next the ventricle which are ectodermic, and an outer mesodermic part consisting of connective tissue and blood-vessels. 7. Transverse Section of the Hindbrain through the Roots of Nerves VI (Ab- ducens and VII (Facial) (Figs. 332 and 342) Efferent Peripheral Neurones. — The nucleus facialis is usually not present, but various portions of the root fibres may be present (see preceding section), especially the longitudinal part. The nucleus oj the VI or nucleus abduceniis is present in about the middle of the floor of the ventricle and just beneath the central gray or partly within it. Its fibres, the root fibres of the abducens, are seen passing ventrally. The nucleus receives collaterals from the axones of Deiters' nucleus (secondary vestibular) on the same and opposite sides and collaterals or terminals from the superior THE NERVOUS SYSTEM 509 E « .:-v:. ••v..-.# . (U I. • '{I •H _: ! ..(,;. .V....;v-: c> w 'V ';•/,■■,-- /■ £ ^--. i ^' - w o .4>-vii^M^ s^^aipm mm4mmk " LiV? J:iV»^ ■^SPSM r'fr.'t TV" I ^y /..ssA* ■'*ii*?^^ ■'"'•- o i; c r c S S o p '5.5'"' . m 4^ . £^5 o o o o M BMEf-i n OJ ^^^L J > ll ^^Hflj ■ u ^H 1 »«j :z; ^^^H ^ u-t H^^^D . a 0 1 'x w 0 ijfU^U^ Nj/Tji -Q uMSr c« Hv N m m r^ -*-» ^ C 0 c 'cS ,,,'f^v A w 0 V. ^^.2 J5 s -*-• 0 c J3 0 •d M a c ^ c 0 1) r^ c -i-> t/; (U f-* > 2 _o 0 a 2 ^ E 0 .!-> .M CA) 'So U 1 c 2 '. ^ £ 0 E rO w 6 tn 512 THE ORGANS tracts of V (three-neurone arc). The nature of its connections with efferent paUial fibres is not known. Many collaterals are also received from the mesencephalic root. (Fig. 3-14.) Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The afferent fibres of the \' pass through the pons and enter the tegmentum where they divide into short ascending and long descending arms. The former, together with collaterals, terminate in the cephalic end of the terminal nucleus of the V. This is broken up into groups of cells which lie dorso-lateral to the entering fibres and is sometimes known as the "principal sensory" nucleus of the V. The long descending arms pass down to the cord as the spinal V, giving off collat- erals and terminals to the nucleus en route. (Fig. 344.) A third source of C'/f /• elf rn'n. I'. Fig. 344. — ^Diagram of Origin of Fifth Cranial Nerve. (Schafer.) G, Gasserian ganglion; a, h, c, the three divisions of the nerve; m.n.V, principal motor nucleus; p.s.n.V, principal terminal "sensory" nucleus; d.s.n.V, terminal nucleus of spinal root; d.s.V, descending or spinal root; c.V and c'.V, secondary trigeminal tracts (axones of cells in terminal nuclei); r, median raphe; 7>i'.n.V, mesencephalic nucleus. fibres of the V is a series of cells extending upward into the roof of the mesen- cephalon. The axones of these cells form the mesencephalic root of the V. There is reason to suppose, from their peculiar location and for other reasons, that these are afferent peripheral neurones which have remained within the neural tube. From the region of the terminal nucleus of the V, a transverse bundle passes to the opposite side in the floor of the fourth ventricle. This is considered a secondary decussating trigeminal tract which forms an ascending tract in the dorsal part of the reticular formation. Fibres of secondary tracts give off collaterals to v.arious efferent nuclei and probably axones of some cells of the terminal nuclei become intersegmental fibres in the reticular formation. Second- / THE NERVOUS SYSTEM 513 ary tracts to the thalamus {via fillet and also in reticular formation?) form part of the trigeminal afferent pallial path. The superior terminal vestibular nucleus (of von Bechterew) may still be present. The superior olivary and trapezoid nuclei may be present. The secondary (and tertiary (?), see Fig. 338) cochlear tract or lateral lemniscus is now well formed and may be seen lying dorso-lateral to the superior olivary and trapezoid nuclei. The medial lemniscus is still more llattened. The spino-thalamic and ventral spino-cerebellar tracts occupy the same positions. Other Afferent Cerebellar Neurones. — The transverse pons fibres are the same, but the longitudinal fibres have increased owing to the presence of more pallio-pontile fibres. The perpendicular pontile fibres are seen passing dorsal ly in the raphe into the tegmentum. The central tegmental tract is in nearly the same position. Intersegmental Neurones. — The reticular formation is somewhat diminished. In it is the nucleus reticularis tegmenti. The rubro-spinal tract is in the same position, mingled v.-ith the spino-thalamic and ventral spino-cerebellar tracts. The medial longitudinal fasciculus is unchanged. Efferent Suprasegmental Neurones. — The pyramids now occupy the cen- tral part of the pons, and are broken up into a number of bundles. In the dorsal part of the pons fibres pass obliquely dorsally. These are probably efferent pallial fibres which act directly or indirectly on some of the efferent nuclei of cranial nerves (motor path to cranial nerves). The pallio-pontile fibres have been mentioned. The superior peduncle is now a large bundle of fibres flattened against the ventricular surface of the dorso-lateral brain wall. The colliculo-spina' tract is in the same position. Cerebellum The cerebellum, connected with the rest of the brain by its three peduncles, consists of two lateral lobes or hemispheres connected by a median lobe, the vermis. These are divided into various lobules, the surfaces of which are marked by parallel transverse folds or laminae. When these are cut across it is seen that they give off secondary or tertiary laminae, the whole producing the appearance known as the arbor vitas. The surface of the cerebellum is composed of gray matter, the cortex, enveloping the white matter. Besides this there are masses of gray within, the internal nuclei of the cerebellum (dentatus, globosus, emboliformis, and fastigii), embedded in its white matter. Fibres entering the cortex are the terminations of the fibres of the restif orm body (dorsal spino-cerebellar tract to the cortex of the vermis, olivo-cerebellar fibres to the whole cortex, fibres from the ateral nucleus and possibly other nuclei in the reticular formation, also per- 33 514 THE ORGANS haps some fibres from the columns and nuclei of Goll and Burdach of the same and opposite sides), vestibular root fibres to the vermis, the ventral spino-cerebellar tract to the vermis, and the pontile fibres to the cortex of the hemispheres. The cortical cells do not send axones outside the cerebellum, all efferent fibres being interrupted in the internal nuclei. The dentate nucleus receives fibres from the cortex of the hemispheres; the globose and emboHform nuclei receive fibres from the cortex of the vermis; and the nucleus fastigii receives fibres from various parts. The axones of the first three form the superior i a A '-4-Mu ■■mmm& '^■■•^ !'■ ■■■''■■ sm W Fig. 346. — Part of a Vertical Section through the Adult Human Cerebellar Cortex. Nissl Method. (Cajal.) A, Inner portion of the molecular layer; B, granular layer; C, body of a Purkinje cell; a, stellate cell of the molecular layer; b, nuclei of the epithelial- like neuroglia cells (cells of the fibres of Bergmann); c, stellate cell with marginal chromo- philic substance; d, fibrillar mass corresponding to the baskets; e, nuclei of the granule cells;/, islands or glomeruH in the granular layer; g,h, Golgi cells in the granular layer; i, nuclei of neuroglia cells. peduncle; the axones of the nucleus fastigii are fastigio-bulbar fibres, principally crossed, to vestibular nuclei and possibly other reticular formation nuclei. There may be some efferent fibres in the middle peduncle to reticular formation nuclei, but the major part, at least, of this peduncle consists of the ponto-cerebellar fibres already de- scribed. The inferior and middle peduncles are thus largely afferent and the superior peduncle is eft'erent, to red nucleus, thal- amus and nucleus of nerve III. (Fig. 331. p. 552 and Fig. 345.) See also pp. 481 and 482. THE NERXOUS SYSTEM ilo In the cortex can be distinguished an outer or molecidar layer with few cells and few mcdullated fibres, an inner, granular or nuclear layer, and between the two a single row of large flask-shaped cells, the cells of Piirkinje (Figs. 346, 347). These latter give off several main dendrites, which enter the molecular layer and form a remarkably rich arborization extending to the surface. The Golgi method shows the larger and medium branches smooth, but the terminal branches thickly beset with "gemmules." The dendritic arborization is flattened, extending at right angles to the laminae. The axone is Fig. 347. — Purkinje Cell of Adult Human Cerebellum. Golgi preparation. (Cajal.) a, Axone; b, recurrent collateral; d, spaces occupied by basket cells; c, spaces occupied by blood-vessels. given off from the end opposite to the dendrites and passes through the granular layer into* the white matter, either to one of the internal cerebellar nuclei where it terminates, or to some other part of the cortex. The Purkinje cells are almost the only cells the axones of which enter the white matter. It is evident that all intracortical connections must ultimately converge on these cells to reach the eft'erent cerebellar paths. The axones of the Purkinje cells give off collaterals not far from their origin, which pass into the molecular layer and appear to terminate there in end "buttons" upon the 516 THE ORGANS bodies of adjacent Purkinje cells. The medullated axones form the coarser fibres traversing the granular layer. The cell body is fairly well filled with small chromophilic bodies of uniform size, often show- ing a sHghtly concentric arrangement (Fig. 346). The cells in the molecular layer (stellate cells) are either superficial stellate cells with irregular branching dendrites and a short axone or deep stellate (basket) cells. These latter are cells the axones of which (apparently non-medullated) have a narrow neck and unusual thicken- ing beyond the neck. They extend at right angles to the laminae i Fig. 348. — Section of Adult Human Cerebellum. Silver Method of Cajal. (Cajal.) A, B, Cells in the granular layer enveloped by basket fibres; C, cell of Purkinje. The axone of one of the cells of Purkinje is shown. for a distance of several Purkinje cells, giving off to each Purkinje cell one or more collaterals which pass toward the granular layer and envelop, with their terminal arborizations, the body and proximal, non-medullated portion of the axone of the Purkinje cell. Collaterals of other basket cell axones may terminate around the same Purkinje cell, forming the "basket" (Fig. 348). The dendrites of the basket cells ramify throughout the molecular layer. Besides the cells con- tained in it, and the dendritic arborizations of the Purkinje cells, the molecular layer contains the axones of the granule cells and the ter- minations of the cKmbing fibres (see below) . In ordinary stains it pre- sents a general punctate appearance, with the scattered nuclei of the THE NERVOUS SYSTEM 517 Fig. 349. — Diagram of Longitudinal Section of Cerebellar Lamina. Golgi method. (KoUiker.) gr, Cell of the granular layer; n, axone of granule cell; n', the same in molec- ular layer where it branches and runs in long axis of lamina; p, Purkinje cell showing how much less extensively its dendrites (/>') branch in long axis of lamina. (Compare Fig. 35I-) B b Fig. 350. — Granule Cells and Mossy Fibres in the Cerebellum of Adult Cat. Silver method of Cajal. (Cajal.) A, Granule cell; B, Golgi cell; a, dendritic arborization of granule cell; b, mossy fibres passing by Golgi cell; c, mossy fibre; d, termination of a mossy fibre; e, terminal processes given off from a thickening in a mossy fibre. 518 THE ORGANS short axone and basket cells, and the coarser dendrites of the Purkinje cells distinguishable (Fig. 346). The granular layer with ordinary stains presents the appearance of closely packed nuclei with clear spaces here and there {^'islands'' or "glomeruli) and also a few larger cells (Fig. 346). Most of these nuclei belong to the granule cells, which are caryochrome cells. The granule cells are small and possess three to six dendrites which are Fig. 351. — Semi-diagrammatic transverse Section of a Cerebellar Lamina of a Mam- mal, as shown by the Golgi jMethod. (Cajal.) A, IMolecular layer; B, granular layer; C, white matter; a, Purkinje ceU, seen flat; b, basket cells of the molecular layer; d, their terminal arborizations which envelop the bodies of the Purkinje cells; e, superficial stel- late cells;/, Golgi cell; g, granule cells with their axis-cylinders ascending and bifurcating at i; h, mossy fibres; j, neuroglia cell; m, neuroglia cell in granular layer; n, climbing fibres. comparatively short and terminate in the glomeruli with a compact arborization, each branch of which ends in a small varicosity. The axones of the granule cells, which are non-medullated, ascend into the molecular layer where each divides into two branches running longi- tudinally along the laminae and terminating in varicosities (Figs. 349, 351). These are the parallel fibres of the molecular layer. They thus run at right angles to and through the dendritic expansions of THE NERVOUS SYSTEM 519 the Purkinje cells and their cross sections together with the terminal dendritic arborizations of the Purkinje cells give the molecular layer its punctate appearance. The scattered larger cells in the granular layer are principally short axone or Golgi cells, whose main dendrites usually penetrate and branch within the molecular layer. Their axones often form very extensive and complicated arborizations in the granular layer, the terminations of which are concentrated in the glomeruli (Fig. 351,7). Dislocated cells of this type may have their cell bodies in the molecular laver. Fig. 352. — Cross Section of a Cerebellar Convolution Stained by Weigert's Method (Kolliker.) m, Molecular layer; K, granular layer; w, white matter; q, fine fibres passing from white matter into the molecular layer; tr, dots represent longitudinal fibres of molec- ular layer among bodies of Purkinje cells. In the cortex there are also the terminations of the afferent cerebellar fibres already mentioned (p. 513). These are of two types, mossy fibres and climbing fibres. The mossy fibres, so called from the appearance of their terminations in embryos, are the coarsest fibres of the white matter. While in the latter they bifurcate, branches going to different laminae. These main branches give off secondary branches which enter the granular layer and there arborize. During their course, and also at their terminations, these branches are thickened in places and there give off short, thick, terminal 520 THE ORGANS branches which end in varicosities. These terminal branches are located within the glomeruli. The glomeruli thus contain the dendritic terminations of the granule cells, the axonal terminations of the Golgi cells, and the terminations of the mossy fibres. (Figs, 351 and 350.) The climbing fibres pass from the white matter, through the granular layer to the cells of Purkinje. Passing by the bodies of the latter they arborize into terminals which envelop the smooth dendritic branches of the Purkinje cells; i.e., all but the terminal dendritic arborizations. (Fig. 351, n). Three kinds of fibres thus terminate around the Purkinje cells; the granule axones, probably in contact with its terminal dendritic arborizations; the climbing fibres around its coarser dendritic branches; and the basket fibres around its body. The respective sources of the mossy and climbing fibres are unknown. There is some evidence that the climbing fibres are from the pons. It is evident from the above that all of the cells of the cerebellar cortex except the Purkinje cells are association cells of the cortex. The medullated fibres of the cerebellum (Fig. 352) pass from the white matter into the granular layer and ramify throughout the latter, forming quite a dense plexus separating groups of granule cells. Sometimes straight fibres can be seen passing through toward the molecular layer which are probably either the climbing fibres or axones of the Purkinje cells. Beneath and between the bodies of the Purkinje cells is a plexus of fibres extending into the deeper part of the molecular layer, the remainder of this layer containing few or no medullated fibres. These fibres in the vicinity of the Purkinje cells are probably principally formed by the recurrent collaterals of the Purkinje cells already mentioned. The remaining fibres of the granular plexus would apparently consist of the arborizations of mossy fibres and of the Golgi cells. Whether the former are medul- lated is, however, somewhat uncertain. Most of the neuroglia cells in the cerebellum are of the same general type as seen elsewhere, but in the Purkinje cell layer are apparently epithelial-like cells which send vertical processes to the periphery. Some of these processes, as seen in the Golgi method, are rough and branched, others are smooth. In ordinary stains these processes are sometimes visible and are known as the fibres of Bergmann. , (Figs. 346, 351.) THE NERVOUS SYSTEM 521 Isthmus PRACTICAL STUDY 9. Transverse Section through the Isthmus at the Exit of Nerve IV ( Trochlearis) (Figs. 332 and 353) In this there are to be distinguished three parts, the thin roof (superior medul- lary velum), the tegmentum and, ventral to the latter, the pons. The tegmentum consists essentially of the reticular formation, efferent cerebellar and midbrain connections, and externally the afferent pallial connections. The pons contains the efferent pallial paths to the cerebellum and to parts of the nervous system caudal to it. The cavity is the iter or aquaductus Sylvii. Next to this is the central gray of the brain wall. Efferent Peripheral Neurones. — The root fibres of the IV are seen in the roof. They originate from nuclei lying further forward in the ventral part of the central gray. The fibres pass from the nuclei dorsally and caudally in the outer part of the central gray and finally decussate in the roof and emerge. It is only the latter part of this course which is seen in this level. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The mesen- cephalic root of the \' lies in the lateral part of the central gray. Mingled with its fibres may be seen the rounded cells, the axones of which form these fibres. The lateral lemniscus occupies part of the lateral swelling on the surface of the tegmentum, forming the major part of the external structure known as the trigo- num lemnisci. Groups of cells among its fibres constitute the dorsal nucleus of the lateral lemniscus. The medial lemniscus is now still more flattened. The spino-thalamic tract is in about the same position, between the two lemnisci. Thus at this level, the principal afferent suprasegmental paths form an L-shaped mass, enveloping the rest of the tegmentum and representing general bodily sensation and hearing. There are also cranial nerve ascending paths lying probably within the reticular formation and fillet (secondary vago-glossopharyn- geal and trigeminal tracts, representing visceral, taste, and general head sen- sation). These cannot be distinguished in the section. The ventral spino-cerebellar tract is on the surface, and now comes to lie external to the superior cerebellar peduncle. At about this point it turns caudally, and passes back into the cerebellum, accompanying the superior peduncle. Otl er Afferent Cerebellar Connections. — The central tegmental tract oc- cupies the same position. (For the pallio-cerebellar connection see ''Efi'erent Suprasegmental Neurones" below.) Intersegmental Neurones. — The reticular formation is diminished in extent. One of its nuclei, the nucleus centralis superior, lies near the raphe. The rubro- spinal tract has moved somewhat mesially. It is dorsal to the medial lemniscus. The medial longitudinal fasciculus is in the same position, and is a well-marked bundle lying at the boundary between the ventral part of the central gray and the reticular formation. Efferent Suprasegmental Neurones. — The pyramids are now broken up into bundles which may show a tendency to gather in the ventral part of the pons 522 THE ORGANS THE NERVOUS SYSTEM 523 (distinguishable in one-month infant, where they are medullated while the pallio- pontile system is not). Bundles apparently forming lateral and mesial portions of the medial lemniscus (not indicated in the figure) are aberrant efTerent i)allial fibres. Such bundles have been seen passing from pons to tegmentum and also imbedded in the medial lemniscus in lower levels (pp. 507, 497). Some of these fibres are possibly fibres acting directly or indirectly on the efferent periph- eral neurones or motor nuclei of the cranial nerves (see p. 527). The pallio-pontile fibres are still more numerous. The gray matter in the pons (nuclei pontis) is very extensive. 'J'he transverse fibres of the pons no longer pass at this level into the cerebellum, but are collected at the sides of the pons to pass backward to the cerebellum (compare with an external view of the brain). The superior cerebellar peduncles or brachia conjunctiva are two large crcscentric bundles of fibres in the lateral part of the reticular formation. Some of their fibres have begun to decussate in the ventral part of the reticular formation. The colliculo-spinal tract or predorsal fasciculus lies ventral lo the medial longitudinal fasciculus. Midbrain or Mesencephalon The dorsal surface of the midbrain presents four rounded promi- nences, the two inferior and two superior colliculi (posterior and anterior corpora quadrigemina) . Ventrally are seen two diverging masses of longitudinal fibres, the pes pedunculi, separated by a deep groove or sulcus. In the midbrain are to be distinguished, (a) the expanded roof, the colliculi or corpora quadrigemina, (b) the tegmentum containing the segmental (cranial nerves IV and III) and interseg- mental apparatus and the afferent suprasegmental paths, and {c) the basis pedunculi, ventral to the tegmentum and comprising the principal efferent pallial paths {pes pedunculi) and the substantia nigra. The cavity of the midbrain is the aquediictus Sylvii or iter. PRACTICAL STUDY 10. Transverse Section through Midbrain at Level of Superior Colliculi ("An- terior Corpora Quadrigemina) and Exit of Nerve III (Oculomotorj (Figs. 332 and 355) Compared with the preceding section, the following arc the most conspicuous changes: The roof has now enlarged into the superior colliculi; the tegmentum now contains the nuclei and roots of nerve III and the red nucleus; instead of the pons, the ventral part of the brain is now composed of the basis pedunculi, consisting of a mass of efferent pallial fibres and the substantia nigra. The 524 THE ORGANS terra crura cerebri or cerebral peduncles is loosely used lo include all except the roof of the brain at this level, i.e., tegmentum and basis pedunculi. Efferent Peripheral Neurones. — The nucleus of nerve III or oculomotor nucleus is located in the ventral part of the central gray in a V-shaped trough formed by the fibres of the medial longitudinal fasciculus. The nucleus is divided into large and small-celled groups. The large-celled groups are two lateral groups subdivided ^issura ^(fr^.. Fig. 354. — The region of the aqua;ductus Sylvii seen from above. Schema showing the position of the nuclei of nerves III and IV and their subdivisions. (Edinger.) I. The small-celled nucleus (here represented as one on each side) a, its ciliary, ^, its pupillary portion. 2, The portion of the large-celled nucleus sending uncrossed fibres to M. levator palpebras; 3, portion sending uncrossed fibres to M. rectus superior; 4 and 5, portions sending crossed and uncrossed fibres to Mm. rectus internus and obliquus inferior; 6, portion sending crossed fibres to M. rectus inferior. The nucleus trochlearis sends crossed fibres to M. obliquus superior. into anterior and posterior dorso-lateral and anterior and posterior ventro-mesial, and a central or median group — nine in all. Between the cephalic or anterior groups are on each side a small-celled group known as the Edinger-Westphal nucleus, and still further forward are two small-celled anterior median nuclei. The connections of these groups with the extrinsic muscles of the eye innervated by nerve III (internal, superior and inferior recti, and inferior oblique) and the levator palpebrae superioris and intrinsic eye muscles (ciliary and sphincter THE NERVOUS SYSTEM 525 c 0) W CJ 3 o 2 3 rt c u E 52G THE ORGANS pupillae, via ciliary sympathetic ganglion) are uncertain. From a priori grounds, the innervation of the intrinsic muscles by the small-celled groups and the other muscles by the large-celled groups would seem probable. Some of the fibres, usually stated to be from the posterior dorsal-lateral group, decussate. Recent observations (Cajal) would indicate that the decussating fibres come from the ventro-mesial lateral groups. What is perhaps the prevailing view as to these relations is shown in Fig. 354. The various fibres pass ventrally in a number of bundles, some passing mesial to, some traversing, and some passing lateral to the superior cerebellar peduncle and red nucleus. Ventral to these the root fibres come together and emerge on the ventral aspect of the midbrain (Figs. 355 and 356.) The nucleus of nerve III receives terminals and collaterals from the medial longitudinal fasciculus, i.e., from the ascending axones from Deiters' nucleus and the descending axones from the nucleus of the medial longitudinal fasciculus, possibly collaterals from the colliculo-spinal tract, collaterals or terminals from the superior cerebellar peduncle and from the reticular formation. Its exact relation to efferent' pallial fibres is not known. Fibres in the medial longitudinal fasciculus connect the nuclei of III, IV and VI (synergic movements of eye muscles). Immediately caudal to the nucleus of nerve III (not in the plane of the section) is the nucleus of the nerve IV occupying a position similar to the lateral groups of the nerve III nucleus. The course of the root fibres of the IV has been mentioned (preceding section). It receives terminals similar to those received by III. Afferent Roots, their Terminal Nuclei and Secondary Tracts. — The mesen- cephalic trigeminal root is sometimes distinguishable on the lateral border of the central gray. The lateral lemniscus has partly or wholly terminated in the inferior coUiculus at a lower level. Fibres from the latter form its arm or brachium, representing another link of the cochlear path. In the present section the fibres of the brachium of the inferior coUiculus are seen entering the medial geniculate body (which is a part of the thalamus). Axones of the cells of this body constitute the last relay of the cochlear path (III, p. 4S2) to the temporal cortex cerebri (not present in this section). As already stated, there is doubt as to how far this path is interrupted in various nuclei along its course such as the superior olives, trapezoid, lateral lemniscus nucleus and inferior coUiculus. It may be reducible to three principal neurone groups: (i) the spinal ganglion and its cochlear nerve, (2)the lateral lemniscus and (3) the geniculo-temporal system. The medial lemniscus is now a laterally placed, curved bundle, displaced laterally by the red nucleus. According to some authorities, some of its fibres terminate in the superior coUiculus. The spino-thalamic tract is plainly distin- 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 superior coUiculus are terminations of the optic tract (the continuation past the optic chiasma of the 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- THE NERVOUS SYSTEM 527 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 formation. It consists of a large-celled part which gives rise to the rubro-spinal tract and to rubro-bulbar fibres, and a smaller-celled portion. The latter sends fibres to the thalamus and thence to the pallium. The nucleus ruber, probably the first named part, also receives fibres from the pallium. According to this it will be seen that the nucleus ruber is a part of the descend- ing cerebello-rubro-bulbar and spinal path (XVII, p. 484), of the cerebello- pallial path (VII, p. 483), and of the pallio-rubro-bulbar and spinal path (XIV, p. 483). The red nucleus probably also receives collaterals from the colliculo- spinal tract. 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 pedunculi. 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 pallial 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 VII and XII. Besides the above pallio-pontile and pallio-spinal fibres in the pes there are two other aberrant peduncular bundles (p. 523) which probably contain efferent pallial fibres which pass to the motor nuclei of nerves V, VII and XII. These two bundles, which may be termed the mesial and lateral pedunculo-tegmental bundles ("medial accessory fillet" and "lateral peduncular fillet") detach them- selves from the pes higher up, and below this level come to lie in the vicinity of the medial lemniscus. 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. The substantia nigra receives collaterals from the ad- joining pes fibres. These are probably fibres from the motor cortex but according to some may be from the corpus striatum or corpus subthalamicum. The axones of the cells of the substantia nigra enter the tegmentum. Their des- tination is unknown. The superior cerebellar peduncle has completed its decussation below this level and its fibers are seen surrounding or within the nucleus ruber which is one of their terminal nuclei. Other fibres of the superior cerebellar peduncle terminate in the thalamus and some are stated to terminate in the nucleus of nerve III. 528 THE ORGANS Internal arcuate fibres from the gray matter of the superior colliculus pass through the reticular formation, and form an oblique decussation. This decussation is the dorsal, or fountain-like decussation of Meynert. The fibres originate from cells in the superior colliculus (tectum opticum), and after decussation form the descending colliciilo-btdbar and spinal tract (tecto-spinal or predorsal tract) (see also below). The Anterior Corpus Quadrigeminum or Superior Colliculus. — In this four principal layers may be distinguished besides the usual covering of neuroglia cells and fibres: (i) An outer white laj'cr, stratum zonale. This consists of fine nerve fibres coming from the superior brachium, possibly fibres from the optic tract and cerebral cortex. Among them are small nerve cells, mostly horizontal and with tangential or centrally directed axones. (2) A gray layer, the stratum cinereum. This consists of radially arranged nerve cells with their larger den- drites proceeding outward, and their axones inward. The largest cells lie deep- est. In this layer the optic fibers principally terminate. (3) The stratum opticum consists principally of optic fibres which send their terminals mostly into the preceding 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 lemnisci, 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 superficial layers, either pass across to the opposite colliculus or sweep ventrally around the central gray, decussate in the raphe and proceed caudally as the colHculo-bulbar and 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 (pallio- collicular fibres). 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 coUicular cells (in turn receiving optic fibres), decussate and terminate in the nucleus of the medial longitudinal fasciculus 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/&) 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 distin- guished ; the thalamus, epithalanius, and hypothalamus. The epithala- mus consists principally of the pineal body, the habenulas, and striae thalami. The hypothalamus consists mainly of the structures in THE NERVOUS SYSTEM 52!) the ventral expansion of the interbrain, such as the corpora mamil- laria, tuber cinereum, infundibulum and posterior lobe of the hy- pophysis. The epithalamus and hypothalamus are principally con- nected with olfactory paths (see p. 536 and Fig. 359). Certain extensions forward of the tegmentum are also termed subthalamic {e.g., corpus subthalamicum or corpus Luysii). The thalamus comprises the great bulk of the interbrain. It con- sists of a number of nuclei forming links in afferent and efferent pallial 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 their connections. According to some authorities the thalamus may be regarded as divided into internal and external segments (usually separated by the lamina meduUaris medialis). The in- ternal segment consists of an anterior nucleus, median nucleus, the "median center" or nucleus of Luys, and a nucleus arcuatus. The external segment consists 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 (metathalamus) . The various nuclei of this external segment receive the fibres of the aferent 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 geniculate body and thence to the occipital pallial cortex; (4) part of the superior cere- bellar 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. (See also p. 481.) 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 de- scending fibres from the cortex palHi. 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- 34 530 THE ORGANS ventrally, so that the libres 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 palHi. The anterior nucleus receives the bundle of Vicq d' Azyr (mamillo-thalamic tract) and probably sends libres to the nucleus caudatus (see p. 539). It thus belongs to the olfactory apparatus. The median nucleus is also probably connected with the nucleus caudatus. 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 afhrm that 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. PRACTICAL STUDY II. Transverse Section through the Junction of Midbrain and Thalamus. (Figs. 332 and 356.) The most conspicuous change from the last section is the appearance, or increase, 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 colliculus {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 completing 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 I Till'. XKRVOUS SVSTK.M 531 • •^i I '■* E/1 cn cn r^ c/1 C ^"^ O 'ZV'^ 3 3 o C 3 .S £ - ^J S| a.2-1 rt C '^— !J 3 :3 <^ t-.j-j fc o ^ •- .2 3 ^ p r:=- •«- •SPlS • - -cCS:! 3 .^ p: 3 3 oj £_:;-^?f-u— c P u r- i: o ^3 c o 5 F! O rt P — ^ ^ 0 - CO _ - . 3 ,- p t y ,i^ "^ 1^ ■- c -o S .^--^ E^ «,nH ""3 ^1 — '—< :" •/; M c ^ t/] c C 3 ui 3 CT' Is I 3 > ^ .2 ►-H v^l ^ .'^ C1<:ji: ^ .22 t^ tb^ c o 2 rt c:^ 3i!.S 3 „ , radiatio optica; S, fissura Sylvii (posterior branch); Sgc, substantia grisea centralis; sM, sulcus Monroi; Sge, substantia grisea subependymalis; ssc, stratum subcallosum: Strz, stratum zonale thalami; Tbc, tuber cinereum; Th, thalamus opticus; ti, sulcus temporalis in- ferior; Ti, gjTus temporalis inferior; tin, sulcus temporalis medius; Tm, gyrus temporahs medius; ts, sulcus temporahs superior; Ts, gyrus temporahs superior; Tte, taenia tecta; U, uncus; VI, ventriculus lateralis; Vli, ventriculus lateraHs (cornu inferius); vst, pedunculus anterior thalami; A', pedunculus putaminis; CM, Commissure of Meynert. 542 THE ORGANS 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 paUial connections are hardly distin- guishable among the fibres connecting thalamus and pallium. Efferent Suprasegmental Neurones. — A great part of the pes has now entered the corona radiala. The part now about to enter the corona is the anterior limb of the internal capsule (Fig. 360). 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. They are con- sidered by some as composed of cortico-thalamic fibres. The anterior pillars of 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 termination in the mammillary body. For other structures of thalamus, epithalamus, and hypothalamus see Fig. 361. 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, ajid fasciculus arcuatus) (seep. 541). Note the nucleus amygdaliformis , the anterior perforated space and the anterior commissure, belonging to the rhinencephalon. Other details shown in Fig. 361 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 dendrites. By the Golgi and EhrHch 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 ofl several collaterals on its way to the white matter. (2) Stellate cells. These have dendrites passing in various directions. Many, especially the smaller (granules), may have short, axones (Golgi's second type). (3) PolymorpJious 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) Hori- zontal cells (of Cajal), found in the outer layer, with long horizontal dendrites and axones confined to the outer layer. (5) Inverted pyramidal cells (of Alartinotti) with axones directed toward the surface. (Fig. 363.) The largest cells of the cortex (giant cells of Betz) are very rich in chromophilic substance arranged similarly to that in the efferent THE XKRVOUS SYSTEM 543 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. '■'J "i^^NrAi^t^ l: '•''.••'■^Vi ',' •'■■.. ''I'.i'' . I'" ■■.'')■ '■ '■',!:■■"'". '.'■. '.',••''•:■' ". V r- >'■; .-: ^^'- ^-i^'-t^/.; M:•^•. ..,:•;/ ^•^'i v\;' m'-'' i V ^ /^ Fig. 362. — Vertical Sections of Calcarine Area of Adult Human Cortex. Left, Weigert preparation showing fibre arrangement. Right, Arrangement of Cells. (Campbell) G, Line of Gennari; R, radiary layer; S, 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), (nternal granular layer; 6, ganglionic layer (containing solitary cells of Meynert); 7, imultiform layer. 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 544 THE ORGANS such as suppression, diminu- tion, enlargement, or subdi- vision 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 Martinotti cells. Besides this, it contains the terminal branches of the apical den- II Fig. 363. Fig. 363. — Vertical section of Calcarine Area of Cortex of an In- fant 15-20 days old. (Cajal, com- bined from three, /, // and ///, somewhat overlapping figures. The multiform layer is not included). Golgi's method. /. — A, Molecular layer; B, ex- ternal granular layer (of small pyra- mids); C, pyramidal layer (of me- dium pyramids); a, descending ax- ones; b, ascending collaterals; c, apical dendrites of large pyramidal cells in ganglionic layer. //. — A , Sublayer of large stellate cells; B, sublayer of small stellate cells; C, outer part of ganghonic layer; a, crescentic stellate cells; b, and /, horizontal, spindle-shaped stellate cells; c, medium-sized pyram- idal cells; e, stellate cells with arched axones; g, triangular stellate cells with stout, arched collaterals; //, pyramidal cells with arched axones. ///. — A, Part of internal granu- lar layer; B, sublayer of small pyram- idal cells with arched ascending axones; C, sublayer of large pyra- mids; a, large pyramidal cell; b, medium-sized pyramidal cell with long descending axone; c, small pyr- B "3) amidal 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 as- cending axones which branch in B and in the sublayer of small stel- late cells; i,j,k, pyramidal cells with arched ascending axones which send branches into the ganglionic layer. c O nil-; \i;r\ous system 545 drites 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 com- posed 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 Marti- notti. (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 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 ]\Iartinotti cells. (6) Multiform layer or layer of polymorphous cells. These usually send their axones into the white matter. Mingled with them are short axone and Martinotti cells. (Figs. 362, 363 and 364.) The cells of the cortex obviously fall into two classes : efferent pro- jection cells and association cells. Which cells are projection cells is not definitely known, except in the case of the precentral motor cor- tex where it has been established that these cells are the cells of Betz, the axones of which form the pyramidal tract and the fibres to motor cranial nerve nuclei. An examination of this area shows that the association cells must enormously outnumber the efferent pro- jection cells in the cortex. The association cells comprise the short axone cells and cells the fibres of which enter the white matter, but terininate in some other part of the cortex, forming the association fibres of the white matter. (Compare p. 539.) It is thus evident that every part of the cortex contains termina- tions of association fibres. The areas containing the terminations of afferent projection fibres are those which receive the thalamocorti- cal continuations of the afferent pallial paths and the continuations of the olfactory paths. From observations made with the Golgi method it seems probable that the afferent projection fibres are 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 af- ferent projection fibres are the hippocampal area (olfactory), the cal- 35 546 THE ORGANS 4*^ 'i .{.' I i •»• 'Vi,. ^ i .■ 4 , •W I * . i t ■ ■ * ► i-v /. •i'>*' i . >' V A ' 1 V 1-- ' * • : . ■■ V > ^ V i 4- .'> Fig. 364. — -Vertical Sections of Precentral or Motor Area of Adult Human Cortex. Lefi, Weigert preparation showing fibre arrangement. Right, Arrangement of cells. (Campbell.) B, Position of line of Baillarger, its position obscured by surrounding wealth of fibres; R, radiary layer; S, supraradiary lajer; Z, layer of superficial tangential fibres in molecular layer, dense and well defmed; i, molecular layer; 2, external granular lajx'r (small pyramids), 3 (medium-sized) and 4 (large), pyramid layer; 5, internal granuar layer, indistinct and with scattered granule or stellate cells; 6, ganglionic layer (large deep pyramids); 7, multiform layer. THE NERVOUS SYSTEM 547 carine area (visual, fibres from lateral geniculate body), the trans- verse temporal gyri of Heschl (auditory, fibres from medial geniculate body), and the pre- and postcentral areas (postcentral only, according to some authorities, area of general sensation from body and head, fibres from ventrolateral thalamic nuclei). (Fig. 365.) 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 caUbre, 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 supraradiary plexus. A denser aggregation of irregular fibres constitute 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 represent- ing the terminals of fibres from the white matter the oblique fibres in general are also composed of medullated collaterals of axones of pyramids and possibly arborizations 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 convolutions and in the sulci, are considered short association fibres belonging to the fibrie 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. 362 and 364.) 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), {b) by the number and arrangement of the medullated 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 (cytoarchitecture). 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 548 THE ORGANS ura^ Prcmb-nl po'" V.iuo-pjyrAi* Visuo-seiiiorjl ^udtto-sensoru 1l1T>.V .,N* ot?" V I, Tnterme-ir„te )«l < II Fig. 365. — Diagram (orthogonal) showing Cortical Areas as deLcrmincd by 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-scn- sory, within the calcarine fissure) precentral (motor) and postcentral (within the fissure of Rolando) and especially the acoustic {aiidito-sensory) which is almost completely hidden within the Sylvian fissure. A , B and C, parts of the limbic lobe. THE NERVOUS SYSTEM 549 which tR'ere is perhaps the most general agreement are the vari- ous sensory (afferent projection) areas already enumerated (p. 545) and the motor (efferent projection) precentral area (Fig. 365). These areas myeHnate 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 myehnating areas are suppHed with projection fibres. According 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 362, 363 and 364. 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 fibres of the optic path (geniculo-calcarine fibres). TECHNIC (i) The general structure of the cerebellum is well brought out by staining sections of formalin-MuIIer's fluid-fixed material with haematoxylin-picro-acid- fuchsin (technic 3, p. 21), 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 staihed by the method of Nissl (technic, P- 39)- (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 Weigert or Weigert-Pal method (technic, p. 34). (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. 36 and 37). Especially in the case of the cerebellum, sections should be made both at right angles, and longitudinal 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- ^ 550 THE ORGANS tion (technic p. 39, Xo. 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 {See page 413.) 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, being connected with the former. The pineal body is appar- ently of the nature of a rudimentary sense organ, being sometimes referred to as the median or pineal eye. In man it is surrounded by a firm connective-tissue capsule, w^hich is a continuation of the pia mater. This sends trabeculae 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 pineal body can be studied by fixing material in formalin-lMuller's fluid (technic 6, p. 7) and staining sections with haematoxylin-eosin (technic i, p. 20). General References for Further Study Bailey and Miller: A Text-book of Embryology, New York, 1909. Chaps XVII and XVIII. Barker: The N^ervous System and its Constituent X'eurones, X^ew York, 1899. Dejerine: Anatomic des centres nerveux, Paris, 1895. Edinger L.: Vorlesungen iiber den Bau der nervosen Zentralorgane des Menschen und der Tiere, Leipsig, 1908 and 191 1. \'an Gehuchten: Anatomie du systeme nerveux de I'homme, Louvaine, 1906. Golgi: Untersuchungen iiber den feineren Bau des centralen und peripher- ischen X'ervensystems, Jena, 1894. Johnston, J. B.: The Xervous System of Veterbrates, 1906. Kolliker: Handbuch der Gewebelehre des Menschen, Leipsic, 1896. Von Lenhossek: Der feinere Bau des Nervensystems im Lichte neuester Forschungen, Berlin, 1895. -2 go V) ^ a! j: C ■!-> C -w o CO V > u •—I •S 'S. en •o c a £ OS JD ai H a OS «-c u 0)^ w w ol c» o «) M ca O V ^ 0) O t- -*-> t^ — ja jj -u i CU H c g c O 0 (ij Ov c .2 +3 8 :3 , 1 ^ nl J3 a O o w U C 3 P. o c o 5 rt "- Ih o o 0) rt X ?; iH (U c M « ^ t.^ t 4-> t« o ca.2 t; J3 OJ3 ■" g .S -a O Mic^ c aj 4) ^ S-tv ■ d"" « i" « CTI -^ i> r c o > ^-S^ <-> Co, O I fj I |^° o ° " e o 5 1) o o CO g.2 CO c-o * ?i «! e o u u t. C o. H H O a 3 O u o X a> ■♦-> u O u •a a O c. 3 £ 3 o (A 'c E o o ta m jj g c"^ g ca~ o gj: u u o c <-■ t, o I to • 3 — -a C 0! c "d.S ") c Co (3 Wpjj C o p. o 2 ■o e '5 ti« 55'-' " ^"d o .-o w *? c oj'o E ^> oo C Co d >'0 O M U C O 4)^ U 4) " w o w > ^ « 3 « I* O p O O I- u o o 4*« *> ^ o M E & TJ > «; C 4> c 'S o 0] •a u O o a; •4-> ■«-• » 4< Q 3 C o > o H ■O ■(3 o u o 4> i. 4* ^-' « i3 4)T)tc u E i, o s! Ov-i 4J ^ Q ca I o c3 « - 3 rt ^ o 1) o Q ca 3 P Ul 4> ca O 3-2 M 4) 3 g-o^ CO 3 2 ^^ o -!-> 3— 3 • 3^ . „• 4) ca o r^ 4) o o-^ o.&'o s; o «j 3 ca 3 o 3 4) 3ffl 2: Z 2 2 E _3 13 o (-< lU O ca ;3 ca o-S 4> ?« ft -3.1, „ w > M 4) 3 n,j2 X o lu^ " ft ■^ -^ 4) 3 & d ca -w —, o O'O E ^ii >^ E 41 4J <" ft-2'm ° ~~ O, I-, o 3 c ■^^"^ ca ceo ca ca-c; o . a; ca 3 o O C > ca ft o . o o •— K O o « C 3 P 3 o ca ^ 3" . o m en 'O 3 3' SI" O r ^ «j E-og o : o ca S 2 2 m" w _: "r o P L= ca "J ? ';" M ftc 3 o ^+j ft o c^ Jfto v: G . ca ^ ft C^ ca 3 o o . ca o to o w "E| O '^ M." 3 en E c, < o — cfl ca o c . o ca •^ c C o ca iH O > c c^ 3'S _o E O r- o . > c o ca «i en 'O _o E 3.5 2 u 4) tn tn ca O c _o "5) c ca O c > o tn u: 3 ^ ^ ft 3 S .c « 3 S:; 4^ S o tn o 4J 073 O S 2 2 «! 3:^ OTS 3 4> 2E >-i 3 > .H i£ Se§ 2 O ca ft c ca O _3 o o ca O ft O o £ .2 en- 3 3 ■n 3 . ca c : o 7^ *^ 'S « c •c 1, o " so >^ o .« o c 4) 4J 3 •a tn ■U c rt (]> ca r fe 4> 3.^ O 3 u «■. it! ca < > 3 a o to > o § O) a ■ ^ o *^ 2 s ^• Is ^S8 ni.ii — .2- ■a c o -*-> (U 3 C X ^ fe ClJ +» t4-l (0 O < m I I CTJ u C u> — • (U (U -^ td . SO O 3"c O M C h I* ^f " •a > nj • O O V H o o 9i cd H S "Jo +^ +-> jj c 9 c o o 53 O 0) S o O I-, '^ Oi •J1 I-. •^ I-. p. .S 'n o o O > is 3 . "u O. O 3 CA (A CA a) j/ <-> z. o — rt o o o 3 3 22 o O c a! bo bC C O u o u •So N C U (U S « •*^ a, 3 iS o o o^^ 03 he-— 1^ o c o c o 2 "o V- 3 tr ^ •" -.: 1) ^ 3 2 3 2 o 3 2- ni fa o 3 - ftr ni fa Oft"" 0. M - •^ ft 'J3 a-; ca +j re y; -M n! a> ■J3.2 rE~ I, I3 00 lU o 5 gao w c 0! y. c >> 03 ft (U c M m n! 3 c"n r (U O ft 12 "J M 3 . I" 3 M C O •-• ^ — • rt Cxi »o *-■ ^ cj C C CO ca k! c ■5?., 2 o c ^ ^-s 3 E C3 O o o oj'-:-^ o ■se; t, oJ rt 00" .S.S s ft ft tA w w 3 w o) y 3 3.2 o o p: rt n! b U U Qt ■HHJ 'S x:t3 oj Q_td o — 4) O " Shu ni C 'ft oO -o C^ ni 0 C w E u aj 3 X : 5 MftO 3 .^ 60 C '^ C ? w n! 60 O O ca to '^ O o> •a u . a a |'"3 ^-^JS e o "3.2 ^ 3 ca __. 00^ "33 3 c-d c 5 4) ^-5E -C) X '2'S _ S jC to c S O 1-, c o e o a) ft ft to " C 3? E" +j ta to a 1' O '-■ ca i 2^ bci-t .2 2^-^. « — to ca O X ft o u It) ft to .^j 3 C Oj 3 >. to .— . to r3 <:E i-H'a . o ft ft o o 2.2 ft.E.2 to £ ^ ^ to to 60 '^^ O . *:> 4J +J ft « C ft C >, «J 4) 4) 0) W7a "- '- •- " 4) 4) 4) .Stetatc !_, ft 4) 41 ca t3 .0 X I s THE NERVOUS SYSTEM 553 Marburg: Atlas des mcnschlichen Centralnervensystems, Leipzig and W'ein, 1910. Meyer, Adolf: Critical Review of the Data and General Methods and Deductions of Modern Neurology. Journ. of Comp. Neurol., Vol. VIII, Nos. 3 and 4, 1898. Obcrsteiner: Anleitung beim Studieren des Baues der nervosen Central- organe, Leipsic. Quain's Elements of Anatomy, \'ol. Ill, Neurology, Parts i and 2, 1908. Ramon y Cajal: 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. — Histologic du systeme nerveux de 1' homme et des vertebreS; trad, par L. Azoulay, Tomes I and II, Paris. 1909. Studien iiber die Hirnrinde des Menschen, Leipsig, 1900. Spalteholz, W.: Handatlas of Human Anatomy (trans, by L. F. Barker) Vol. Ill, 1903. CHAPTER XXIII 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 OcuU.- — 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. 366). The Sclera (Figs. 366 and 367).- — 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 oblique muscles with the equatorial fibres. The few cells of the sclera lie in 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. 375). 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 endothehum. Anteriorly a loose connective tissue attaches the sclera to the scleral conjunctiva. The Cornea (Figs. 368 and 371). — 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. 368) : (i) Anterior epithelium. (2) Anterior elastic membrane or membrane of Bowman. (3) Substantia propria corneae. 554 THE ORGANS OF SPECIAL SENSE 555 (4) Posterior elastic membrane or membrane of Descemet. (5) Posterior endothelium or endothelium of Descemet. (i) The anterior epithelium (Fig. 368, 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 Fig 366. — Diagram of Eyeball showing Coats. (Merkel-Henle.) a, Sclera; b, chorioid; c, retina; d, cornea; e, lens; /, iris; g, conjunctiva; h, ciliary body; i, sclero-cor- neal junction and canal of Schlemm; j, fovea centralis; k, optic nerve. intercellular bridges. The surface cells are flat. Along the margin of the cornea the epithelium is continuous with that of the conjunctiva (Fig. 371). (2) The anterior elastic membrane (Fig. 368, 2) is a highly de- veloped basement membrane, its anterior surface being pitted to receive the bases of the deepest epithelial cells. It is apparently homogeneous, and while called an elastic membrane, does not con- form chemically to either fibrous or elastic tissue. By means of special technic, a fibrillar structure has been demonstrated. 556 THE ORGANS (3) The substantia propria (Fig. 368, 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 lamelhe are parallel to one another and to the surface 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 lamelhx from posterior to anterior elastic membranes hold the lamellae tirmly together. They are known as perforating or arcuate fibres. ■i-'^^^^-?^^^^ \A \B c Fig. 367. — 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 canaliculi. 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 flat cells corresponding in shape to the spaces and sending out processes into the canaliculi (Figs. 369 and 370). (4) The posterior elastic membrane or membrane of Descemel (Fig. 368, 4) resembles the anterior, but is much thinner. Like the anterior, it does not give the chemical reaction of elastic tissue. (5) The posterior endothelium or endothelium of Descemet (Fig. 368, 5) 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. 367): THE ORGANS OF SPECIAL SENSE 55"; of (i) The lamina suprachorioidea. (2) The layer of straight vessels — Mailer's layer. (3) The capillary layer — choriocapillaris. (4) The vitreous membrane — lamina citrea — membrane Brach. (i) The lamina suprachorioidea (Fig. 367, 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 pericliorioidal lymph spaces (Fig. 371). (2) The layer of straight vessels (Fig. 367, e) consists of fibro-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, as they are straight and parallel, give to the layer a striated appearance. The arteries lie to the inner side. The veins — vence vorticosce — are larger than the arteries and converge toward four points 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 tapetum fibrosum, while in the carnivora its structure — several layers of flat cells — gives it the name of the tapetum cellulosum. (3) The choriocapillaris (Fig. 367,/) consists of connective tissue supporting a dense network of capillaries, which is most dense in the region of the macula lutea. This layer is usually described as free from pigment, although it not infrequently contains some pigmented cells. Fig. 368. — Vertical Section of Cornea. (Merkel-Henle.) i, An- terior epithelium; 2, anterior elastic membrane; 3, substantia propria corneae; 4, posterior elastic mem- brane; 5, posterior endothelium. 558 THE ORGANS (4) The litreous memhrane (Fig. 367, g) is a clear, apparently structureless 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. ' fl.n.n- Fig. 369. — Section of Human Cornea cut Tangential to Surface — X350 (technic 9, p. 97) — showing corneal cell spaces (lacunae) and anastomosing canaliculi. The Ciliary Body. — This is the anterior extension of the chorioid and consists of the ciHary processes and the ciliary muscle (Fig. 371). It extends from the ora serrata (a wavy edge which marks the anterior ":k^%?-; / / .." ( .,=.-■•">, -^tr. X :■. i_ ■ ^. i.-V. -'-pi' .,-' — ' >.« / ■; N r /' .'■ ""^•••r~- -.-. ' .■•-4 ,>--^i;{iip.''" "~': / '' / / ■■-■■/•-... I \ " — *A?t"\ ' 7 -v / ■■— ■ 4 ""■•- — 4 J : ■ Fig. 370. — Section of Human Cornea cut Tangential to Surface — X3S0 (technics, p. 97) — showing ccrneal cells 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. 371), from seventy to eighty in number, THE ORGANS OF SPECIAL SENSE 559 are meridionally-running folds of the chorioid from wliich 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 Cornea Anterio? chamber Iris Pars iridica retinn CaDal of Sclilemm- Spaces of Fontaua- Conjunctiva^ Ciliary process Ligaini-ntum pectinatuni iridis Circular fibres of ciliary muscle P Pars ciliaris retinae Sclera — \ Perichorioidal lymph space- Zonule of Zinn Retina Fig. 371. — 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 reiince (Fig. 371). This consists of two layers of columnar epithelial cells, the outer layer being pigmented, the inner non- pigmented. The ciliary muscle (Fig. 371) 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 Miiller; {h) an 560 THE ORGANS ^W^ outer meridional group lying next to the sclera and known as the tensor choricidese, 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 era serrata, the latter radiating fan-like to a broad insertion in the ciliary body and processes. The cihary body is closely attached to the sclero-corneal junction by the ligamentum pectinatum (Fig. 371), a continuation of the posterior elastic lamina of the cornea. Within the ligament are spaces {spaces of Fontana) hned with endothelium. These are ap- parently lymph spaces, and com- municate with each other, with similar spaces around the canal of Schlemm, and with the anterior chamber. The canal of Sclilemm (Fig. 371) 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. 372). — This represents a further continuation forward of the chorioid. Its base is attached to the cihary body and ligamentum pectinatum. From tliis point it extends forward as a diaphragm in front of the lens, its centre being perforated to form the pupillary opening. It is deeply pigmented, and to its pigment the color of the eye is due. Four layers may be distinguished, which from before backward are as follows: (i) The anterior endothelium. (2) The stroma. (3) The vitreous membrane. (4) The pigmented epithelium. (i) The anterior endothelium is a single layer of pigmented cells continuous with the posterior endothelium of the cornea (Fig, 364, a). Fig. 372. — Vertical Section through Iris. (Merkel-Henle.) a, Anterior en- dothelium; 6, stroma or substantia pro- pria; c. vitreous membrane; d, pigment layer; v, blood-vessel. THE ORGANS OF SPECIAL SENSE 5()1 (2) The stroma is divisible into two layers: an anterior reticular layer, containing many cells, seme of which are pigmented, and a vas- cular layer, the vessels of which are peculiar in that their walls con- 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. 372, d) consists of several layers of cells and is continuous with the pars ciharis 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 ora serrata, which marks the outer limit of the ciliary body (Fig. 371). The nervous part of the retina is known as the pars optica retincB, the non-nervous extension over the ciliary processes as the pars ciliaris retince, its further continuation over 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. TJie Pars Optica Retince. — 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. 373): (i) Layer of pigmented epitheUum. (2) Layer of rods and cones. (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. Layer of neuro-epithelium. Ganglionic layer. The layer oj pigmented epithelium (Fig. 373, B, i) consists of a single layer of regular hexagonal cells (Fig. 27, p. 80 J. The nuclei 36 562 THE ORGANS lie in the outer part of the cell, while from the inner side thread-Uke projections extend down between the rods and cones of the layer next internal. The pigment has the form of rod-shaped granules. Its distribution seems to depend upon the amount of hght being admitted to the retina. When little or no light is being admitted, the pigment is found in the body of the cell, the processes being wholly or almost A B 1 ^^^.g!5.f ^|j^Sjf;?^r-v:^r?r r ,C^ ''tM Fig. 373. — A, Scheme of retina as shown by the Golgi method. B, Vertical section of retina to show layers as demonstrated by the hasmatoxylin-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 gang- lion cells of nerve-cell layer; 10, fibres of optic nerve forming layer of nerve fibres; 11 and 12, t>-pes of horizontal cells; 13, 14, 15, and 16, types of cells the bodies of which He in the inner nuclear layer; 17, efferent optic-nerve fibre ending around cell of inner nuclear layer; 18, neuroglia cells; 19, Miiller's fibre; 20, rod-bipolar cell of inner nuclear layer. m 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. The layer of rods and cones and the outer nuclear layer (Fig. 373, S, 2, 4) are best considered as subdivisions of a single layer, the neuro- epithelial layer. This consists essentially of two forms of neuro- THE ORGANS OF SPECIAI. SENSE 563 epithelial elements, rod visual cells and cone visual cells. These, with sup])orting connective tissue, constitute the layer of rods and cones and the outer nuclear layer, the separation into sub-layers being due to the sharp demarcation between the nucleated and non-nucleated parts of the cells, and the separation of the two parts by the per- forated outer limiting membrane. The rod visual cell (Fig. 373, A, 4) consists of rod, rud-librc, and nucleus. The rod (Fig. 373, yl, 3) is a cylinder from 30 to 40," 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 line 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. 373, B, 4). The cone visual cell (Fig. 373, A, 5,6) consists of cone, cone-fibre, and nucleus. The cone (Fig. 373, ^, 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. 2>73, A, 6) is much broader than the rod fibre, passes completely through the outer nuclear layer and ends in an expansion at the mar- gin 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. 373, B, 6) and the layer of nerve cells (Fig. 373, B, 8) are composed largely of nerve-cell bodies, while the two molecular layers (Fig. 373, B, 5, 7) are formed mainly of the ramifications 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. 373, yl, 20) processes {dendrites) pass outward to ramify in the outer molecular layer around the terminations of the rod fibres. From the cone bipolars (Fig. 373, A, 7) similar processes {dendrites) extend into the outer molecular layer where they ramify around the terminations of the 564 THE ORGANS cone cells. Two other forms of nerve cells occur in the inner nu- clear layer. One is known as the horizontal cell (Fig. 373, A, 12). Its processes ramify almost wholly in the outer molecular layer. The other lies along the inner margin of the inner nuclear layer and sends its dendrites into the inner molecular layer (Fig. 373, 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 horizontal 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. 373, 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 565). 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. 373, 5, 9) consists mainly of the ax- ones of the just-described ganglion cells, although a few centrifugal axones of brain cells (Fig. 373, A, 17) are probably intermingled. • The cuter and inner limiting layers or membranes (Fig. 3 73, 5, 3,1c) are parts of the sustentacular apparatus of the retina, being connected with the cells or fibres of Midler (Fig. 373, ^4, 19 and Fig. 374). 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 the cells, which are conical and fibrillated, unite to form the inner limiting membrane (Fig. 374, 10). Through the inner molecular layer the cell takes the form of a narrow stalk with nu- merous fringe-hke side fibrils (Fig. 374, 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. 374, b). This wide portion of the cell in the inner nuclear layer con- tains the nucleus (Fig. 374, a). In the outer molecular layer the THE ORGANS OF SPECI.\L SENSE 5G5 cell again becomes narrow (Fig. 374, 5) and in the outer nuclear layer broadens out into a sponge- like reticulum (Fig. 374, 4), which sup- ports the rod and cone bipolars. At the inner margin of the layer of rods and cones the protoplasm of the IMiiller's cells spreads out and unites to form the so-called cuter hmiting membrane (Fig. 374,3), from which delicate fibrils (fibre baskets) pass outward between the rods and cones. In addition to the Miil- ler's cells, which are neurogha elements, spider cells also occur in the retina (Fig. 373,^4, 18). The retina of the macula 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 mole- cular 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 re- duction in the number of rods, while the number of cones is correspondingly increased. In the centre of the macula is a depres- sion, the 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 ret- inal extension over the ciliary body (pars ciliaris retina) and over the iris {pars iridica retince) have been described in con- nection with the ciHary body and iris. The Optic Nerve. — The optic nerve (Fig. 375, d) is enclosed by two connective-tissue sheaths, both of which are extensions of the brain membranes. The outer or diiral sheath, (Fig. 375, a) is continu- ous with the dura mater of the brain posteriorly, while anteriorly it blends with the sclera. The inner or pial sheath, (Fig. 375, &) is an ex- tension of the pia mater and is separated from the outer sheath by Fig. 374. — Two M tiller's Fibres from Retina of Ox showing Relation to Layers of Retina. (Ramon y Cajal.) 3, Outer limiting layer; 4, outer nuclear layer; 5, outer molecular layer; 6, inner nu- clear layer; 7, inner molecular layer; 8, layer of nerve cells; 9, layer of nerve iibres; 10, in- ner limiting layer; a, nucleus; b, cup-like depression caused bv pressure from surrounding cells. 566 THE ORGANS Wfg^g the subdural space (Fig. 375, 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 subarachnoid space. The optic nerve fibres, in passing through the sclera and chorioid, separate the connective-tissue bundles so that they form a lattice-work, the already mentioned lamina crihrosa (Fig. 375, }i)- The optic nerve fibres are medullated, but have no neurilemma. As they pass through the lamina cribrosa the medul- lary 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-epithelial beginnings of the visual tract (Fig. 373, A, 3, 4, 5, and 6). By their expanded bases in the outer molecular layer, the rod and cone cells communicate with the neu- rone system No. I. of the optic tract. This comprises (fl) rod neurones, {h) cone Fig. 375. — Section through Entrance of Optic Nerve into Eyeball. (Merkel-Henle.) a, Dural sheath; b, pial sheath, inner and outer layers; c, space between inner and outer layers of pia mater; d, optic nerve; c, central artery of retina; a' , sclera; /, chorioid; , retina; h, lamina cribrosa. neurones, (c) horizontal neurones. Neurone System No. I. — {a) Rod neurones. The cell bodies of these neurones (Fig. 373, A, 20) lie in the inner nuclear layer. Their dendrites enter the outer molecular layer where they form net- works 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. II.). {h) Cone neurones (Fig. 373, ^, 7). These have their cell bodies in the inner nuclear layer. Their dendrites pass to the outer molecular layer where they form networks around the expanded bases THE ORGANS OF SPFXIAL SENSE 567 of the cone cells. Their axones pass only into the inner molecular layer where they end in arborizations around the dendrites of neu- rones whose cell bodies are in the layer of nerve cells (neurone system No. II.). (c) Horizontal neurones (Fig. 373, A, 11 and 12). These serve as association neurones between the visual cells and may be divided into rod association neu- rones and cone association neu- rones. The cone association neurones are the smaller and more superficial, and both den- drites and axones end in the outer molecular layer around the terminal expansions of the cone visual cells (Fig. 373, A, 11). The rod association neurones are larger, more deeply seated, and behave in a similar manner to- ward the rod visual cells (Fig. 373, A, 12). Some of these cells send processes to the inner mole- cular layer. Xeuroxe 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. 373, A, 8, 9) are in the layer of nerve cells and are, as above noted, associated either directly or by means of their dendrites \^dth 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. 373, A, 10). The optic nerves (Fig. 376, No) unite at the base of the brain to form the optic decussation or chiasma (Fig. 376, CM). Here the axones from the mesial part of the retina cross to the optic tract of the opposite side, while those from the lateral part of the retina remain in the optic tract of the same side. The axones of the optic tract Fig. 376. — Diagram showing Main Relations of Optic Tract. (Testut.) R, Retina; No, optic nerve; CM, optic decussation or chiasma; Tro, optic tract; Tlio, thalamus; Cgl, lateral geniculate body; Qa, anterior corpus quadrigemi- num; Rd, fibre of optic tract passing directly to cortex; Sm, third neurone sys- tem of optic tract (excepting Rd) connect- ing thalamus, lateral geniculate body, and anterior corpus quadrigemnium with the cortex, Co. 568 THE ORGANS Corfe. Fig. 37; THE NERVOUS SYSTEM 569 EXI'LAXATIUX OF FIG. 377. Fig. ,577. — Diagram of the Optic (II) Nerve and some ol 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 No. 2.- — 2a, Axones of ganglion cells in temporal part of retina pass to pulvi- nar of thalamus of same side; 2 b, axones of ganglion cells in temporal retina pass to superior coUiculus 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 superior coUiculus 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. — 3a, 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 ^b constitute the primary optic radiation; 3 c, 3 , spiral ganglion; q, blood-vessel. lymph spaces of the veins of the cochlear duct. The scala vestibuli and the scala tympani communicate with each other in the cupola by means of a minute canal, the helicotrema. 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 spiral turns from the vestibule, where it is connected with the saccule through the canalis reuniens. THE ORGANS OF SPECIAL SENSE 585 to its blind 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. 389, Dc). The upper or vestibular wall is formed by the thin membrane of Reissner (Fig. 389, h) which separates the cochlear duct from the scala vestibuli. The membrane consists of a thin central lamina of connective tissue covered on its vestibular side by the vestibular endothelium, 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 more loosely arranged tissue. From it, two folds project slightly into the duct. One, the crista basilar is (Fig. 389, r), serves for the attachment of the membranous spiral lamina; the other, the spiral ■prominence (Fig. 389, /"), contains several small veins. Between the two projections is a depression, the external spiral sulcus (Fig. 389, v). That part of the spiral ligament between the spiral prominence and the attachment of Reissner's membrane is known as the stria vascu- laris (Fig. 389, 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 epithelial cells, giving the unusual appearance of a vascular epithehum. 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 583). The membranous spiral lamina consists of a substantia propria or basilar membrane, its tympanic covering, and its cochlear covering. The basilar membrane (Fig. 389) is a connective-tissue membrane 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 586 THE ORGANS membrane is divided into an outer portion, the zona pectinata (Fig. 389, i) and an inner portion, the zona tecta (Fig. 389, s). The epithe- lium 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. 389, g, and Fig. 390) 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 amina, and consists of a complex arrangement of four different kinds limlrns mcmiraTia tectorict outer hair-cells •iierve fibres inner rod vas hasUar outer cells of Beitera spiialc Membrane rod Fig. 3QO. — 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 spiral ganglion) are seen passing 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. of epithelial cells. These are known as: (i) pillar cells, (2) hair cells, (3) Deiter's cells, and (4) Hensen's cells (Fig. 390). (i) The pillar cells are divided into outer pillar cells and ijiner 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 "articulating" with each other. From their articulation the pillars diverge, so that their bases which rest upon the basilar membrane are THE ORGANS OF SPECI.VL SENSE 587 widely separated. There are thus formed by the pillars a series of arches known as Corti's arches, enclosing a triangular canal, CorlVs tunnel. This canal is filled with a gelatinous substance and crossed by delicate nerve fibrils. As the outer pillar cells are the larger, they are fewer in number, the estimated number in the human coch- lea being forty-five hundred of the cuter cells and about six thousand of the inner cells. (2) The hair cells or auditory cells lie on either side of the arches of Corli, 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 a point, while from its free surface are given off a number of fine stift" hairs. 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. 390). — 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 cuter pillar is a space known as NueVs space (Fig. 390, x). (4) Hensen's Cells (Fig. 390, a). — These are sustentacular cells, which form about eight rows to the outer side of the outermost Deiter 's ceils. These cells form the outer crest of Corti's organ and consequently 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. 390, b), the name given to the coch- lear 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- 588 THE ORGANS brane also extends out as a cuticula over the cells of Hensen and of Claudius. The Memhrana Tcctorla. — This is a peculiar membranous struc- ture attached to a projection of the bony spiral lamina known as the spiral lifubus (Fig. 390), the concavity beneath its attachment being the internal spiral sulcus (Fig. 390, c). The membrane is non-nucle- ated 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 auditory nerve to the utricle, saccule, and semicircular canals. It supplies these parts, giving rise to a capillary network, w^hich is coarse meshed except in the cristse and maculae, where the meshes are fine. The cochlear artery also starts out in company with the audi- tory nerve, but accompanies it only to the first tarn of the cochlea. Here it enters the modiolus where it gives off several much coiled branches, the glomerular 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 ac- company 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 en- dolymphatic duct, the endolymphatic sac, and minute lymph chan- nels 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 581. The ganglion of the vestibular branch is situated in the internal auditory 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. THE ORGANS OF SPECIAL SENSE 589 numerous fibres radiate to the bony spiral laminae, in the bases of which they enter the spiral ganglia (Fig. 389, p). The cells of the spiral ganglia are peculiar, in that while of the same general type as the spinal ganghon cell they maintain their embryonic bipolar condition (see page 425) 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 499). Their dendrites become medul- lated like the dendrites of the spinal ganglion cells and pass outward in bundles in the bony spiral lamina (Fig. 389, 0, and Fig. 390). From these are given off branches which enter the tympanic portion of the lamina, where they lose their medullary sheaths and pass 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 tun- nel. One bundle lies just inside the inner pillar beneath the inner row of hair cells (Fig. 390). A second bundle runs in the tunnel to the outer side of the inner pillar (Fig. 390). The third bundle crosses the tunnel (tunnel-fibres) and turns at right angles to run between the cells of Deiter beneath the outer hair cells (Fig. 390). From all of these bundles of fibres are given off delicate terminals which end on the hair cells. (For details of acoustic tract see pp. 481, 482, and 499.) Development of the Ear The essential auditory part of the organ of hearing, the membranous laby- rinth, 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. Diver- ticula soon appear passing off from the otic vesicle. These are three in number and correspond respectively to the future endolymphatic duct, the cochlear duct and the membranous semicircular canals. Within the saccule, utricle, and ampulla special differentiations of the lining epithelium give rise to the maculae and cristae acusticas. 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 under- going an elaborate specialization to form the organ 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 connective tissue at first com- 590 THE ORGANS pletely fills in the space between the cochlear duct and the bony canal. Ab- sorption 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 vesicle. 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 branchial cleft and adjacent branchial arches. The tympanic mem]:)rane is formed from the mesoderm of the first branchial arch, its outer covering 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 fixed in formalin-Miiller's fluid (technic 6, p. 7) and sections stained with haematoxylin-eosin (technic i, p. 20). In sections of the wall ot the cartilaginous meatus the ceruminous glands may be studied, material from children and from new^-born infants furnishing the best demon- strations 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 8, p. 8). 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 g). The bone is next decalcified, either by one of the methods mentioned on page 10 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 paraffin, cut sections through the long axis of the modiolus, through the utricle and saccule, and through the semicircular canals. Stain with haematoxylin-eosin and mount in balsam. (3) The neurone relations of the cristte, maculcC, 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 36). Sections should be thick and must of course be cut through undecalcified bone. Good results are difiicult 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 305). The THE ORGANS OF SPECIAL SENSE 591 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 peripherally 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. 391, elhm) to its terminal nucleus in the olfac- tory bulb (Fig. 391). The Olfactory Bulb. — This is a somewhat rudimentary structure analogous to the much more prominent olfactory brain lobe of some Fig. 39 1 . — Diagram of Structure of Olfactory Mucosa and Olfactory Bulb. (Ram6n y Cajal.) be, Bipolar cells of olfactory raucosa; sm, submucosa; ethm, cribriform plate of ethmoid; a, layer of olfactory fibres; og, olfactory glomeruli; mc, mitral cells; ep, epithelium of olfactory ventricle. 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 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: 592 THE ORGANS The layer of olfactory fibres (Fig. 391, a) consists of a dense plexi- form arrangement of the axones of the above-described olfactory cells. 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. 391, og) . The latter mark the ending of neurone system No. I. of the olfac- tory conduction path. (For olfactory tract see pp. 481, 483,) The molecular layer contains both small nerve cells and large nerve cells. These send their dendrites into the olfactory glomeruli. The smaller cells belong to Golgi Type II. (seepage 135) and appear to be association neurones between adjacent glomeruli. The axones of the larger cells, the so-called brush cells, become fibres of the olfac- tory tract. Of the mitral cells (Fig. 391, 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 centrally directed axones of the mitral and brush cells. These fibres run in distinct bundles separated by neurogHa. 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 consti- tute 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 distincdy brown color. Fix in Flemming's fluid (technic 8, p. 8), or in Zenker's (technic 10, p. 8). Stain thin vertical sections with hajmat- oxylin-eosin (technic i, p. 20) 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 36). THE ORGANS OF SPECIAL SENSE 593 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 231) and under sensory end-organs (p. 438). The taste buds are found in the side walls of the circumvallate papillai (page 231), of some few of the fungiform papilkc, in the mucosa of the posterior surface of the epiglottis, and especially in folds (fohate papillae) which occur along the postero-lateral margin of the tongue. The taste bud (Fig. 392) is an ovoid epithehal structure embedded in the epithehum and connected with the surface by means of a min- ute canal, the gustatory canal (Fig. 392, 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 or gustatory cells and sustentacular cells (Fig. 392). The gustatory cells are long, delicate, spindle-shaped cells which occupy the centre of the taste bud, each ending externallv 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. 392, 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 between the taste buds. These are finer and smoother than the intrageminal fibres and are known as inter- geminal fibres (Fig. 392). Fig. 392. — Taste-bud from Side Wall of Circumvallate Pa- pilla. (Merkel-Henle.) a, Taste- pore; h, nerve fibres, some of which enter the taste-bud — in- trageminal fibres; while others end freely in the surrounding epithelium — intergeminal fibres. TECHNIC (i) The general structure of the taste buds is shown in the sections of tongue (technic, p. 232). (2) For the studj' of the nerve terminals the method of Golgi should be used (page 36). 38 594 THE ORGANS General References for Further Study Kolliker: Handbuch der Gewebelehre des Menschen. IMcMurrich: The Development of the Human Body. Ramon y Cajal: La retine des vertebres. La Cellule, ix., 1S93. Schwalbe: Lehrbuch der Anatomie der Sinnesorgane, 1887. INDEX AunucENS (nerve \'l', 4S1, 505, 508; see also Nerves, crania! Absorption, 277 of fat, 279 Accessorius (nerve XI), 489 Accessory nasal sinuses, 305 olivary nucleus, 493, 496 Achromatic element of intranuclear net- work, 49 spindle, 54 Acid aniline dj'es, 20 cells, 254 Acidophile granules, no Acini, 222 Acoustic group of segmental neurones, 481 Acromegal)', 415 Acrosome, 350 Acusticus (nerve MIT); see Xcrvcs, auditory Adelomorphous cells, 254 Adenoids, 184 Adipose tissue. 91 Adrenal gland, 418 blood-vessels of, 420 chromaffin granules of, 418, 420 development of, 421 lipoid granules of, 418 lymphatics of, 420 nerves of, 420 pha^ochrome granules, 418 phseochromoblasts, 421 secretion of, 419 sympathoblasts, 421 Adrenalin, 420 Adventitia of arteries, 160 of lymph vessels, 169 of veins, 162 Afferent cerebellar neurones, 483, 499, 507, 510, 513 paths, 482 pallial, 467, 468, 470, 480. 482, 529 peripheral nerves, 425 neurones, 425, 432, 480 Afferent peripheral segmental neurones, 485 roots, their terminal nuclei and sec- ondary tracts, 489, 491, 495, 497, 506, 510, 512, 521, 526, 530, 532, 542 suprasegmental neurones, 427 Agminated follicles, 265 Air cells, 316 passages, 316 sacs, 316 vesicles, 316 Ala cinerea, 487, 495 Alcohol, as a fixative, 6 dilute, as a fixative, 7 for hardening, 9 Ranvier's, 4 strong, as a fixative, 6 Alcohol-ether celloidin, 11 Alimentary canal, primitive, 302 tract, 224 development of, 302 endgut, 268 foregut, 248 headgut, 225 midgut, 26c Altmann's granule theory of protoplasmic structure, 45 Alum-carmine, 19 for staining in bulk, 22 Alveolar ducts, 316* glands, 219, 222 passages, 317 sacs, 316 Alveoli, of glands, 222 of lung, 318 Amacrine cells, 564, 592 Amitosis, 53 Amoeboid movement, 52, in technic for, 64 Amphiaster. 55 Amphicytes, 433 Amphip3"renin. 48 Ampulla, 580 595 596 INDEX Am3-gdaliforni nucleus, 542 Anabolism, 51 Anaphase, 56 Aniline dyes, acid, 20 basic, 20 Anistropic line, 119 substance, 119 Annular terminations, 440 Annuli fibrosi, 166 Ansa lenticularis, 542 Anterior corpus quadrigemini, 523, 528 cerebral commissure, 538, 542 horns, 441, 450, 454, 457 root or motor cells of, 450 perforated space, 538, 542 pyramids, 470 white commissure, 542 Antero-lateral funiculus, 454 white column, 454 Anus, 272 Aorta, 160 Apathy, concerning cilia, 79 Apical body, 350 foramina, 233 Aponeuroses, 96 Apparatus reticulare. 47 Appendix epidid3'midis, 348 testis, 348 vermiformis, 270 Aqueductus Sylvii, 423, 521, 523 Arachnoid membrane, 428, 566 Arbor vitae, 513 Arborescent terminations, 440 Arborizations, terminal, 136, 436 Arc cerebellar, 476 cerebral, 477 pallial, 477 spinal reflex, 475 three-neurone, 475, 491 two-neurone, 475 Archipallium, 538 Archoplasm, 50 Arciform nucleus, 496, 505, 536 Arcuate fasciculus, 541 fibres, 493 external, 493, 496, 505 internal, 493, 506, 508. 528 Area, acustica, 488 hippocampal, 545 tegmenti, 536 Areolar (loose) connective tissue, 91 Arrector pili muscle, 397 332 Artcria; arciformes, rectae, 334 Arteries, 157 advenlitia of, 160 anterior spinal, 460 aorta and other large, 160 arcuate, 332 arteriole, 158 bronchial, 319 ciliary, 571 coats of, 157 coronary, 166 development of, 167 greater arterial circle of iris, 572 hepatic, 294 interlobar, 332 interlobular, 332 intima of, 158 large, like the aorta, 160 lesser arterial circle of the iris, 572 lymph channels of, 163 media of, 159 medium-sized, 158 nerves of, 163 posterior spinal, 460 precapillar}', 158 pulmonary, 319 pulp, 188 renal, 324, 331 sheathed, 188 small, 157 splenic, 188 structural peculiarities of some, 161 sulco-commissural, 460 technic of, 164 vasa vasorum, 163 Arteriole, 158 Articular cartilages, 209 Articulations, 209; see Joints diarthrosis. 210 synarthrosis, 209 sj-nchondrosis, 209 syndesmosis, 209 technic of, 211 Arytenoid cartilages, 307 Ascending degeneration. 465 fibre tracts of spinal cord, 467 direct cerebellar, 469 Gowers', 469 long arms of dorsal root fibres, 467 posterior columns, 467 funiculi, 454, 467 INDEX 597 Ascending fibre, spino-tcctal, 471 -thalamic, 468 tract of Flechsig, 469 tracts forming parts of afferent pal- lial paths, 467 to cerebellum, 468 tractus spino-cerebellaris dorsalis, 469 ventralis, 469 uncrossed cerebellar, 469 Asters, 58 Atresia of follicle, 369 Atrophy method of determining fibre tracts of the cord, 465 Attraction sphere, 50 Auditory canal, 578 cells, 587 hairs, 581 nerve, 497, 499 cochlear branch of, 497, 499 vestibular branch of, 481, 497, 499 path, 481, 497, 499, 530 pit, 589 Auerbach, end buttons of, 515 end foot of, 39 plexus of, 268, 276 Auricle, 165, 573 muscle of, 165 Auriculo-ventricular ring, 165 Autogenetic theory of neurones, 146 Axis cylinder, 130, 138; see also Axone Axolemma, 138 and neurilemma, relation of, 140 Axonal degeneration, 146, 465 method of determining fibre tracts by means of, 465 Axone, the, 130, 136, 424 Bethe's views of, 140 Cajal's views of, 141 collaterals of, 136 degenerative changes in, 144 development of, 424 fibres of Remak, 137 medullary sheath of, 137 meduUated, 137 naked axone, 136 non-meduUated, 136 terminal arborizations of, 136 Axone-hill, 136 Baillarger, line of, 547 Balsam, Canada, for mounting, 23 Barker, concerning the neurone, 131, 134 Bartholin, duct of, 283 glands of, 69, 382 Basal filament, 218 granule, 79 Basic aniline dyes, 20 Basket cells, 227, 516 Basophile granules, no Bellini, duct of, 327, 330 Bcnsley and Theobara, concerning secre- tion, 277 Bergman, nerve fibres of, 520 Berkley, concerning pituitary body, 414 Bertini, columns of, 32() Bathe, concerning continuity of axo- lemma and neurilemma, 140 Betz, cells of, 539, 542, 545 Bielschowsky's stain, ^Maresh's modifica- tion of, 31 Bile duct, 295 Bioblasts, 45 Bipolar nerve cells, 131, 434 Bladder, urinary, 336 Blastoderm, 63 Blastomeres, 62 Blocking, 12 Blood, 107 corpuscles, 107 crenation of red cells, 108 development of, 113 diapedesis, in dust, 112 erythrocytes of, 107 granules, elementary, 113 granules of Ehrlich, ni haematin, 108 hsematokonia, 112 haemoglobin of, 108 haemolysis, 108 Jenner's stain for, 32 leucocytes of, 109 macrocytes, 107 microcytes, 107 phagocytosis, in plasma of, 107 platelets, 112 red cells of, 107 crenation of, 108 technic for, 63 smears, technic of, 32, 114 specific gravity of, 107 stroma of, 108 technic of, 114 598 INDEX Blood, thrombocytes, 112 vascular unit, 320 white cells of, ioq Blood-islands, 113, 167 Blood-sinuses of haemolymph nodes, 177 Blood-smear, 6 Blood-vascular unit. 320 Blood-vessel system, 155 arteries, 157 capillaries, 'D3 development of, 167 heart, 165 lymph channels of, 163 nerves of, 163 technic of, 164, 167 vasa vasorum, 163 veins, 162 Blood-vessels, 155 lymph channels of, 163 nerves of, 163 technic of, 164 Body cavities, 169 Bone, breakers, 204 cells, IDS, 195, 20s, 238 decalcification of, 10 development of, 202 formers, 204 tissue, 104 calcination of, 105 cells of, 105 cementum, 238 corpuscles of, 105 decalcification of, 10, 106, 202 intercellular substance of, 105 lacunae and canaliculi of, 105 lamellse of, 105 technic of, 106, 202 Bone-marrow, 197 blood-vessels of, 201 cells of, 197 development of, 206, 208 endosteum, 197, 201 erythroblasts of, 198 fat cells of, 199 f^elatinous, 201 giant cells, 198 leucocytes, 197 marrow cells, 197 spaces, 197, 206 mast cells of, 199 megakaryocytes, 198 myelocytes of, 197 Jionc- marrow, myeloplaxes of, 198 non-nucleated red blood cells of, 107, 197 normoblasts. 198 nucleated red blood cells of, 198 osteoclasts, 198 plasma cells of, 199 polykaryocytes of, 19S red, 199 technic of, 202 yellow, 201 Bones, 193 blood-vessels of, 201 canaliculi of, 196, 202 cancellous or spongy, 193 cells of, 195 origin of, 204 circumferential lamellae of, 195 development of, 202 intracartilaginous, 205 intramembranous, 203 subperichondrial development of, 207 subperiosteal, 207 growth of, 208 hard or compact, 193 Haversian canals of, 194 lamella;, 195 spaces, 207 systems, 196 Howship's lacunae, 204 intermediate lamella?, 195 interstitial lamellae, 195 lacunae, 195, 202, 204 lymphatics of, 202 marrow, 197 red, 199 yellow, 201 nerves of, 202 nutrient cana\, 201 foramen, 201 vessels, 201 osteoblasts, 203 osteoclasts, 188, 204 osteogenetic tissue, 203 perforating fibres, 197 perichondrium, 205 pericranium, 204 periosteal buds, 205 periosteum of, 196, 204 Sharpey's fibres, 197 spongy, 194, 207 INDEX 599 Bones, tcchnic of, 202 developing bone, 208 Volkmann's canals, 196, 201, 202 Bony cochlea, 582 spiral lamina, 583 Borax-carmine, alcoholic solution, 22 Born's theory of corpus luteum, 368 Bowman, capsule of, 326, 328 glands of, 306 membrane of, 554 sarcous elements of. 120 Brachia conjunctiva, 508, 523 Brachium of posterior corpus quadrigemi- num, 526 pontis, 507 Brain, the, 479 cerebral cortex of, 542 contrasted with spinal cord, 479 development o(, 422 efTectors, 479 endbrain (telencephalon), 423, 538 corpus striatum, 538 pallium, 539, 542 rliinenccphalon, 538 forebrain (prosencephalon), 422, 528 diencephalon (ihalamencephalon), epithalamus, 528 hj'pothalamus, 529 thalamus, 529, 536 general histology of, 485 structure of, 479 hemispheres of, 484 higher coordinating apparatus of, 479 hindbrain (rhombencephalon), 423, 485 cerebellum, 513 isthmus, 521 medulla oblongata (bulb), 485 nerves of, 487 pons, 423, 488, 505 tegmentum, 485, 505 interbrain, 528 membranes of, 428 arachnoid, 428 blood-vessels of, 430 cerebral dura, 428 dura mater, 428 pia mater, 428 cerebralis, 430 Pacchionian bodies of, 430 relation of optic nerve to, 566 Brain membranes, technic of, 430 midbrain (mesencephalon), 422, 523 aqueductus Sylvii, 523 basis pedunruli, 523 corpora quadrigemina, 484, 528 iter, 523 pes pcdunculi, 527 posterior commissure, 528 substantia nigra, 527 superior collie ulus, 528 tegmentum, 523, 505 pallium, 423, 539, 542 pineal eye, 550 pituitary body, 413 receptors, 479 relation of, to optic nerve, 566 sand, 553 segmental brain and nerves, 426, 480 afferent peripheral neurones, 4S0 efferent peripheral neurones, 481 semicircular canals, 481 suprasegmental structures, 484 technic of, 488, 549 ventricles, 422, 423 vesicles, 422 Branca, concerning the centrosome, 59 Bridges, intercellular, 70, 72, }6 Bronchi, 310 alveolar, 315 blood-vessels of, 31Q cartilages of, 312 development of, 322 lobular, 315 lymphatics, 321 nerves of, 321 primary, 310 respiratory, 313, 316 structure of walls of, 310 technic of, 323 terminal, 316 Bruch, membrane of, 557 Brunner's glands, 258, 267, 278 Brush cells, 592 Bulb, 485; see Medulla Bulbo-urethral gland, 355 Bulbus oculi, 554; see Eyeball Bundle of Lowenthal, 472 of Vicq d'Azyr, 530,^538 Burdach, column of, 455, 465, 467, 468 nucleus of. 468, 487, 493 Bursse, 213 Busch-Marchi staining method, 35 600 INDEX Biitschli's theory of protoplasm struc- ture, 45 Cachkxia strumipriva, 409 Coelom, 169 Cajal, cells of, 542 concerning the neurone, 135, 139, 141 interstitial nucleus of, 471, 473, 491, 497, 499, 506, 510, 513, 521, 524 methods for staining neurofibrils in nerve cells, 38 plexiform layer of, 544 Cajeput oil for clearing sections, 23 Calcarine area, 543 Calcification, 85 center, 203, 205 zone, 207 Calcination, 105 Calyces of kidney-pelvis, 335 Canada balsam, 23 Canal, gastro-intestinal, 250 gustatory, 593 Haversian, 194 lacrymal, 573 of Cloquet, 571 of Petit, 571 of Schlemm, 560 portal, 295 Volkmann's, 196, 202 Canaliculi of bone, 105, 196, 202 of connective tissue, 87 Canalis communis, 581 Canalized fibrin, 379 Cancellous bone, 193, 204 Capillaries, blood, 155 chyle, 265, 27s development of, 167 lymph, 169, 275 technic of, 164 Capillary endothelium, 155 network, 156 Capsule of Bowman, 326, 328 -cells, 425 internal, 536 of ganglion cells, 425 of Glisson, 293 of Tenon, 572 Carbol-xylol for clearing specimens, 23 Carboxyhicmoglobin, in Cardiac glands, 257 Carmine, alum, 19, 22 borax, 22 Carmine, gelatin, 25 neutral, 20 picro-, 21 Carotid gland, 416 Cartilage, 102, 202 arytenoid, 307 articular, 210 cells, 102 chondrin. 102 classification of, 102 cricoid, 307 development of, 84, 103 elastic, 103, 307 embryonal, 103 epiphyseal, 208 fibrous, 103, 209 hyaline, 102, 205, 307 intercellular matrix of, 104 intermediate, 208 laryngeal, 307 of developing bone, 202 perichondrium of, 103 Santorini's, 307 technic of, 104 thyreoid, 307 tracheal, 308 of Wrisburg's, 307 Cartilages, articular, 209 costal, 209 interarticular, 210 skeletal, 209 Caryochromes, 133, 518 Caudate nucleus, 532, 538, 541 Cavernous sinuses, 356 Cavity of embrxonic vesicle, 62 Cedarwood oil as solvent, 14 Cell, the, 43 amitosis, 53 body of, 44, 130 centrosome of, 49 crusta of, 47 cuticula of, 47 cytoplasm of, 46 cytoreticulum of, 46 deutoplasm granules of, 46 division, direct, 53 indirect, 54 endoplasm of, 46 exoplasm of, 46 function of, 51 hyaloplasm of, 44 intranuclear network of, 49 INDEX 601 Cell, irritability of, 52 karyo{)lasm of, 46, 49 linin of, 49 membrane of, 47 metabolism of, 51 metaplasm granules of, 46 microsomes, 44 mitosis, 54 motion of, 52 nuclear membrane of, 48 nuclein of, 49 nucleolus of, 49 nucleoreticulum, 49 nucleus, 47 origin of word, 47 paraplasm granules of, 46 pellicula, 47 plastids, 46 plastin. 44 protoplasm of, 43 Altmann's granule theory of, 45 Biitschli's foam or emulsion theory of, 45 fibrillar theory of, 45 reproduction of, 53 space (lacuna), 105 spongioplasm of, 44 technic for study of, 63 trophospongium, 47 typical, 44 diagram of, 43 structure of, 43 vital properties of, 58, 218 Cell-islands of Langerhans, 29c function of, Opie's theorj' of, 291 origin of, 291 structure of, 291 technic of, 292 Cell-patches, 290 Celloidin, alcohol-ether, 11 clove-oil, 13 embedding, 11 sections, 15 clearing of, 23 mounting of, 23 Cells, acid, 250 active, 217 adelomorphous, 254 air, 316 amacrine, 564. 592 amoeboid, 148 auditory, 587 Cells, basal, 306 basket, 227, 516 blood, 107 bone. 105, 195, 204, 238 brush, 592 capsule, 424 cartilage, 102, 205 cementoblasts, 240 central, 254 centro-acini, of Langerhans, 289 centro-tubular, 289 chief, 250, 254, 278, 411, 413 chromaffm, 416 chromophile, 413 ciliated, 79 clasmatocytes, 87, 89 clasmocytes, 89 colloid, 408, 411 columnar, 75, 261 compound tactile, 438 connective-tissue, 87 crescents of Gianuzzi, 227 cuboidal, 75 daughter, 54, 58 decidual, 376 Baiter's, 587 delomorphous, 254 demilunes of Heidenhain, 227 empty, 217 endothelial, 155 eosinophile, no, 178 epithelial, 73 erythroblasts, 198 erythrocytes, 107 extrinsic, 449 fat, 92, 178, 199 fibroblasts, 85, 99 fixed, 87 fcetal, 318, 322 fusiform, 233 ganglia, 424, 432, 449 giant, 198 gland, 217 goblet, 78, 82, 217, 261, 277, 307 Golgi, Type I., 134, 136 Type II., 136, 452 granule, 518 gustatory, 593 hair, 581, 587 hecatomeric, 551 Hensen's, 587 heteromeric, 451 602 INDEX Cells, interstitial of seminiferous tubule, 344 intrinsic of cord. 449 inverted pyramidal, 542 KuptTcr's, 299 Langerhans', 289 leucocytes, 109, 197 Leydig's, 514 liver, 297 loaded, 217 lutein, 365 lymphoid, 171, 187, 307 macrocytes, 107 marrow, 197 mast, 87, 1/8, 199 megakaryocj'te, 198 megalocyte, 188 microcyte, 107 migrators- leucocytes, 264 mitral, 592 mononuclear, 188 moss}', 147 mucous, 21/, 226, 261 multinuclear, 178, 188 muscle, 115 myelocytes, 197 nn-eloplaxes, 198 nerve, 130; for classification see Nerve cells neurilemma, 148, 425 neuroblasts, 130, 146, 424 neuro-epithelial, 8c neuroglia, 147, 424, 459, 520 normoblasts, 198 nucleated red blood, 113, 198 odontoblasts, 233, 246 of Claudius, 587 of oral glands, 227 olfactory, 306 osteoblasts, 203, 240 osteoclasts, 198, 204, 240 ovum, S3,, 360, 361 oxyntic, 254 oxyphile, 41 1 I'ancth's, 265, 278 parietal. 254 peptic, 254, 277 I)ha;ochromoblasts, 418, 421 phagocytes, 1 1 1 , 178 I)igmented, 46, 75, So, 135, 390, 394, 560 pillar ) 0"^ Cells, plasma, 87, 89, 199 polykaryocytes, 198 prickle, 389, 393 primitive ova, 360, 384 Purkinje, 515 pyramidal, 542 red blood, 107, 18S replacing, 75, 257, 264 respiratory, 318 resting, 217 satellite, 433 secreting, 218, 289 serous, 226 Sertoli's, 341, 384 sex, 384 signet-ring, 02 simple tactile, 437 single primitive, 53 smooth muscle, 3, 115 spermatids, 343, 350 spermatoblasts, 351 spermatocytes, 343, 350 spermatogenic, 342 spermatogones, 342, 350 spider, 147 spinal ganglion, 434 spleen, 188 squamous, 76 stellate, 233 supporting, 341 sustentacular, 290, 306, 341 sympathoblasts, 421 tactile, 437 tautomeric, 551 tendon, 96 thrombocytes, 112 wandering, 52, 87, 90, in, 264 white blood, 109, 188 Cementing glj-cerin mounts, 23 Cementoblasts, 240 Cementum, 238 development of, 247 Central canal, 422, 455 cells, 254 chromatolysis, 146 gelatinous substance, 455 gray, 489, 491, 493, 508 nervous system, 422 neurones, 424 spindle, 54 tegmental tract, 496, 507. 513, 521, 524, 532 INDKX 603 Central vein, 2g4 Centriolc, 50 Centro-acinar cells of Langerhans, 289 Centrogenetic theory, 146 Centrosome, 49, 54, 60 archoplasm, 50 attraction sphere, 50 centriole, 50 daughter, 54 of fertilization, 63 Centro-tubular cells of Langerhans, 289 Cerebellar arc, 476 connections, 469, 470, 505, 507, 508, 513 cortex, 513 peduncles, see Peduncle, cerebellar Cerebcllo-olivary fibres, 496, 497, 505 Cerebellum, 423, 505, 508, 513 arbor vitae, 513 ascending paths to the, 468 association cells of, 520 basket cells of, 516' cells of, 515, 516, 517 climbing fibres of, 519 cortex of, 508, 515, 520 dentate nucleus of, 508, 513 descending tracts from, 473 development of, 423 fibres of, 516 climbing, 519 mossy, 519 of Bergmann, 520 parallel, 51S general histology, 513 glomeruli of (islands), 518 granular layer, 515 granule cells, 518 gray matter of, 513 hemispheres of, 508, 513 internal nuclei of, 508, 513 laminae, 513 molecular layer, 515 neuroglia of, 520 nuclear layer, 515 nuclei of, 508, 513 nucleus dentatus, 508 emboliformis, 508, 513 globosus, 508, 513 tecti or fastigii, 508, 513 peduncles of, 505, 514 Purkinje cells of, 515 stellate cells, 516 Cerebellum, tcchnic of, 549 vermis, 513 wliitf matter of, 513 Cerebral arc, 476 areas, 547 cortex, 539, 541 dura. 428 hemisj)heres, 423 (K'xclopment of, 423 nuinbranes, 428 peduncles, 524 vesicle, 422 Cerebro-spinal ganglia, 130, 424, 432 central processes of, 442 Dogiel's classification of, 433 peripheral processes of, 435 technic of, 447 nervous system, 422; see also Ner- vous system neurones, efferent peripheral, 447 Ceruminous glands, 578 Cervical enlargement of cord, 448 segments of cord, 44S Cervix, 372 epithelium of, 374 external os, 374 ovula Nabothi, 374 plicae palmatae, 374 technic of, 385 Chain ganglia, 442 Cheeks, mucous membrane of, 225 Chemo taxis, 52 Chiasma, optic, 532, 567 Chief cells, 254 Chloride of gold for staining connective- tissue cells, 28 Chloroform as solvent, 14 Choledochus ductus, 303 Chondrin, 102 Choriocapillaris, 557 Chorioid, the, 556 choriocapillaris of, 557 fissure, 576 Mailer's layer of, 557 lamina citrea, 557 suprachorioidea, 557 perichoriodal lymph spaces of, 557 plexus, 423, 486, 493, 508 tapetum cellulosum of, 557 fibrosum of, 557 venae vorticosae, 557 vitreous membrane of, 558 604 INDEX Chorion, 377 Chorionic villi, 377 Chromaffin cells, 416 granules, 418 organs, 416 Rose, concerning chromaffin cells, 416 Chromatic element of intranuclear net- work, 49 Chromatin, 49 Chromatolysis, 146 Chrome-silver method of Oolgi, 29 Chromophile cells, 413 Chromophilic bodies, 133 significance of, 142 Nissl technic for, 39, 133, 148 Chromosomes, 56 daughter, 56 probable number for species, 56 Chyle, 17s vessels, 265, 275 Cilia, vessels, 53, 73, 78 Ciliary artery, 571 body, 558 blood-vessels of, 571 canal of Schlemm, 560 ligamentum pectinatum, 560 muscles of, 558 processes of, 558 spaces of Fontana, 560 vitreous membrane of, 559 ganglion, 442 movement, 53 technic for, 64 muscle, 559 plexus, 572 processes, 558 Cingulum, 541 Circulatory system, 155 blood-vessel system, 155 development of, 167 lymph-vessel system, 169 Circumferential lamellae, 196 Circumvallate papillae, 230 Cirl, concerning fibres of internal capsule, 532 Clarke's columns, 456, 461, 469 Clasmatocytes, 87, 89 Clasmocytes, 89 Claude, concerning development of pan- creas, 303 Claudius, cells of, 587 Clava, the, 487 Clearing specimens before mounting, 2^ Clefts of Schmidt-Lantermann, 139 Climbing fibres, 520 Clitoris. 382 Cloquet's canal, 571 Closed skein (spireme), 55 Clove-oil celloidin, 13 Coccygeal glands, 417 segments of spinal cord, 449 Cochlea, 582 bony spiral lamina of, 583 cupola of, 582 cupula of, 582 hamulus of, 583 helicotrema, 584 membranous spiral ligament of, 583 modiolus of, 582 scala tympani, 584 vestibuli, 584 spiral ligament of, 583 Cochlear duct, 584 basilar membrane of, 585 crista basilaris, 585 external spiral sulcus, 585 membrane of Reissner, 585 nuclei, 497 organ of Corti, 586 spiral prominence of, 585 stria vascularis, 585 zona pectinata, 586 tecta, 586 nerve, 481, 495, 497, 499 tracts, 497, 506, 510 Coelom, 169 Cohnheim's field, 120 Collagenous fibres, 85 Collaterals, 136, 453 CollicuH, 423, 499, 523; 528 Colliculo-spinal tract, 471, 484, 491, 496, 505, 510, 513, 523 Colloid, 408, 411, 414 Colostrum corpuscles, 405 Column cells, 450 hecateromeric, 451 heteromeric, 451 tautomeric, 450 technic of, 452 of Burdach, 455, 465, 467, 468 of GoU, 455, 465, 468, 491 Columnaj rectales, 272 Columns of Bertini, 326 of Sertoli, 341 INDEX 605 Comma tract of Schullzc, 473 Commercial formalin, 5 Commissura habenularis, 537 Commissure, dorsal gray, 455 neutral gray, 455 Common dental germ, 243 senses, 442 Compact bone, 103 Compountl tactile cells, 438 Conduction path, 427, 467 afferent and efferent suprasegmental, 482, 483 afferent-cerebellar, 482 -mesencephalic, 482 pallial, 467, 468, 470, 480. 482, 529 auditory, 499 efferent cerebellar, 484, 505 mesencephalic, 483 pallial, 483 pallio-cerebellar, 505 pallio-spino-peripheral efferent, 471 to cerebellum, 468 trigeminal afferent pallial, 513 Cone association neurones, 567 -bipolar, 563 fibres, 563 neurones, 566 -visual cell, 563 Cones, layer of rods and. 562 Conjunctiva. 574 end bulbs of, 439, 575 Connective tissue, 84, 86 adipose or fat, 91 aponeurotic, 96 areolar, 91 bone, 104 cartilage, 102 cells of, 87 cellular. 91 characteristics of, 84 chlorid-of-gold method for demon- strating cells of, 28 classification of, 86 dense fibrous, 91 development of, 85 elastic, 96 elastin of, 91, 99 embryonal, 85, 86, 202, 208 fat, 91 fibres of, 90 elastic, 90 fibrillated, 90 Connective tissue, fibres of, reticular, 91, 99 white, 90 yellow, 85, 90 fibrillar, 86 fibroblasts, 99 fixed cells of, 87 formed, 95 gelatin of, 90 histogenesis of, 84 intralobular, 95, 220 interalveolar, 319 intercellular substance of, 87, 90 interlobar, 220 interlobular, 93, 220 intrafascicular, 213, 431 ligaments, 95 loose, 91 Mallory's stain for, 29, 30 mast cells of, 89 mucous, 86 neuroglia, 146, 424, 459, 520 pigmented cells of, 89 plasma cells of, 89 reticular, 98 retinaculse cutis, 388 staining of, 28 technic for, 99, 104, 106 theories of development of fibres of, 85 wandering cells of, 90 white fibrous, 86 Constrictions of Ranvier, 138 Contact theory of neurones, 142 Continuity theory of neurones, 143 Convoluted tubules of kidney, 326, 328, of testis, 340 Cord, spinal, 422; see Spinal Cord Corium, 386 Cornea, the, 554 anterior elastic membrane of, 555 epithelium of, 555 corpuscles of, 556 endothelium of Descemet of, 556 membrane of Bowman of, 554 of Descemet of, 556 posterior elastic membrane of, 556 substantia propria of, 556 Corneal corpuscles, 556 Cornu ammonis, 538 Cornua of cord, 454 606 INDEX Corona radiala of ovum, 362 of pallium. 443, 539, 542 Coronary arteries, 166 Corpora amylacea, 354 cavernosa, 355 lutea of pregnancy, 366 spuria, 366 vera, 366 mammillaria, 537 quadrigemina, 423, 49Q, 523, 528 anterior, 523, 528 development of, 423 posterior, 499, 523 striata, 423 Corpus albicans, 366 callosum, 542 dentatum, 508 haemorrhagicum, 365 Highmori, or mediastinum testis, 339 luteum, 365 theory of, 368 Luysii, 530 quadrigeminum, anterior, 423, 523, 528 posterior, 499 spongiosum, 355 striatum, 527, 538, 541 caudate nucleus, 538 lenticular nucleus, 53S putamen, 541 subthalamicum. 527, 536 trapezoideum, _sof>, 510 Corpuscles, blood, 107 bone, 105 colostrum, 405 corneal, 556 crescentic, of prostate, 354 genital, 356 Golgi-Mazzoni, 400, 440 Grandry's, 437 Hassall's, 181 Meissner's, 357, 400, 438 Merkel's, 437 Pacinian, 202, 356, 439 renal, 326 Ruilini's, 400, 440 salivary, 184 splenic, 186 tactile, 438 Vater-Pacinian, 356, 400 '\\'agner, 400 Cortex cerebelli, 508, 515, 520; see also Cerebellum cerebri, 539, 541 areas of, 547 association fibres of, 549 cells of, 541, 549 of Betz, 539, 542, 545 of Cajal, 542 of C.olgi, Type II. 135, 136, 452 of Martinotti, .S42, 545 pyramidal, 542 commissural fibres, 539 corona radiata of, 536, 539, 542 deep tangential fibres of, 547 external granular layer, 545 ganglionic layer, 545 horizontal cells of Cajal, 542 internal granular layer, 545 intcrradiary plexus, 547 in\-erted pyramidal cells of Marti- notti, 542 layer of polymorphous cells, 542 of p\'ramidal cells of, 542, 545 line of Baillarger, 547 of Gennari, 549 molecular layer, 544 multiform layer, 545 plexiform laj^er of Cajal, 544 jjolymorphous cells of, 542 projection fibres, 549 radiations of Meynert, 547 stellate cells of, 542 superficial, tangential fibres of, 547 supraradiary plexus of, 547 Cortical labyrinths, 326 pyramids, 326 Corti's arches, 587 organ, 586; see also Organ of Corli tunnel, 586, 589 Cotyledons, 377 Cowper's glands, 69, 355 technic of, 355 Cox-Golgi method of staining ner\-e tis- sue, 37 Cranial nerves, see Xerves. cranial Crcnation of red blood cells, 108 Crescentic corpuscles, of prostate, 354 Crescents of Gianuzzi, 227, 283, 308 Cretinism, 409 Cricoid cartilage, 307 Crista acustica, 582 basilaris, 585 INDEX 607 Crossed pyramidal tracts, 470, 491 Crura cerebri, 524 Crusta (exoplasm), 47 Crypt of Licljcrkuhn, 265 of tonsils, 183 Cumulus ovigerus, 362 Cuncus, 487 Cupola, 582 Cupula, 582 * Cutaneous sensation, 442 Cuticle, 3S8 Cuticula, 47, 73 dentis, 238 Cuticular membrane, 73, 245, 262 Cystic duct, 300, 303 Cytoarchitecture, 547 Cytoplasm, 46 of nerve cells, 132 Cytoreticulum, 45 Cytosomes, 218 Darkschewitsch, nucleus of, 471 Daughter cells, 58 ccntrosomes, 54 chromosomes, 56 stars, 57 Davidoff, concerning cells of stomach, 264 Decalcitication, 3, 10 Decalcifying, 10 fluids, 10 Decidua basalis, 376 capsularis, 376 gravaditatis, 376 menstrualis, 375 placentalis subchorialias, 380 reflexa, 376 serotina, 376 subchorionic-placental, 380 vera, 376 Decidual cells, 376 Decolorizing fluid for Weigert's ha;ma- toxylin, 34 Decussation of fillet, 491, 493 motor, 489 of Forel, 528 of Meynert, 528 of pyramids, 470, 491, 493 optic, 567 sensory, 491, 493 Deep sensation, 442 Degenerating nerves, Marchi's method for staining, 35, 144 Degeneration of neurones, 144 Wallerian law of, 145 Dehiscent glands, 223 Deilcr's cells, 587 nucleus, 472, 482, 489, 496, 506, 508, 510 descending tract from, 472, 489, 493. 497, 505 > 508 Deitcro-spinal tract, 472, 473, 484, 489, 493, 497, 508 Dclalield's h;cmatoxylin, 18 Delomorphous cells, 254 Demilunes of Ileidenliain, 227 Dendrites, the, 135, 418 Dental germ, 243 groove, 243 papilla, 243 periosteum, 239 pulp, 233 layer of Weil of, 233 odontoblasts, 233, 246 ridge, 243 sac, 244 sheath, Neumann's, 237 shelf, 243 Dentate nucleus, 508, 513, 514 Dentinal canals, 235 development of, 246 fibres, 233, 235 interglobular spaces of. 237 Hnes of Schreger, 237 nerves of, 241 Neumann's dental sheath, 237 Tomes' granular layer, 237 Dentine, 2:^7, chemical composition of, 235 secondary, 237 Derma, or corium, 386 corpuscles of Meissner, 356, 400, 438 muscle cells of, 387 papillae, compound, 387 nerve, 387 simple, 387 vascular, 387 pars papillaris, 387 reticularis, 386 pigmentation of, 390 subcutaneous tissue of, 387 Descemet, endothelium of, 556 membrane of, 556 Descending degeneration, 465 608 INDEX Descending degeneration fibre tracts of the spinal cord, 470; see Fibre tracts of spinal cord (descending) Dcutoplasm, 46, 363 Diapedesis, iii Diaphysis of bone, 208 Diarthrosis, 210 articular cartilages, 210 glenoid ligaments, 210 interarticular cartilages, 210 joint capsule, 210 Diaster, 57, 60 Diencephalon, 423, 528 Digestive system, 224 absorption, 277, 279 alimentary tract of, 224 development of, 243, 302 endgut, 268 foregut, 248 gall-bladder, 301 gastro-intestinal canal, 250 headgut, 225 large intestine, 268 larger glands of, 281 liver, 292 mesentery, 272 midgut, 260 mouth, 225 muscular tissue of, 224 oesophagus, 248 omentum, 272 pancreas, 286 peritoneum, 272 pharynx, 247 rectum, 271 salivary glands, 281 secretion, 277 small intestine, 260 stomach, 252 teeth, 232 tongue, 228 vermiform appendix, 270 Direct cerebellar tract, 469 pyramidal tract, 470, 471 Discus proligerus, 362 Dissociation of tissue elements, 4 Disynaptic arc, 476 Dogiel's end plates, 573 theorj' of structure of spinal gan- glion, 433 Dorsal accessory oli\"ar\' nucleus, 496 decussation of'Meynert, 528 Dorsal gray commissure, 455 root fibres of white matter, 456 spino-cerebellar tract, 469 white commissure, 456 Duct glands, 221 systems of glands, 219, 222 Ductless glands, 222 Ducts, aberrans Halleri, 348 alveolar, 316 Bartholini's, 283 Bellini's, 330 bile, 295 choledochus, 303 cochlear, 578 common, 300 cystic, 300, 303 ejaculatory, 348 endolymphatic, 581 excretory, 219, 282 Fallopian tube, 370 Gartner's, 369 hepatic, 294, 300 kidney-pelvis and ureter, 335 mesonephric, 383 Miillerian, 348, 354 nasal, 573 of Muller (embryonal), 348, 354 of sweat glands, 390 oviduct, 370 pancreatic, 287 pronephric, 383 reuniens, 581 Santorini's, 287 secondary pancreatic, 287 seminal, 348 Stenoni's, 282 thoracic, 169 thyreo-glossal, 409 utriculo-saccular, 581 vas deferens, 346 vas efferentia, 346 vas epididymis, 346 \\"harton's, 284 Wirsung's, 287 Wolffian, 383 Ductus aberrans Halleri, 348 reuniens, 581 Dura mater, 428 blood-vessels of, 430 cerebral, 428 spinal, 428 technic of, 430 INDEX G09 Dyes, acid aniline, 20 basic aniline, 20 nuclear, 17 plasma, 20 Dynamic centre of cell, 58 Ear, 578; see also Organ of Hearing blood-vessels of, 588 development of, 589 drum, 579 external, 578 auditory canal, 578 auricle, 578 blood-vessels of, 579 ceruminous glands of, 578 ear drum, 579 Ij-mphatics of, 579 nerves of, 579 pinna, 578 tympanic membrane, 579 internal, 580 ampulla, 580 blood-vessels of, 587 canalis communis, 581 cochlea, 582 ducts of, 584 ductus reuniens, 581 endolymph of, 580 endolymphatic duct, 58S sac, 588 fenestra ovalis, 580 rotunda, 580 h'mphatics of, 588 membrana tectoria, 588 membranous labyrinth, 580 nerves of, 588 organ of Corti, 586 osseous labyrinth of, 580 perih'mph of, 588 saccule, 581 scala media, 584 semicircular canals, 581 utricle, 581 utriculo-saccular duct, 581 vestibule, 580 middle, or tj'mpanum, 579 fenestra rotunda of, 580 ossicles of, 5S0 stapes, 580 wax, 579 Ebner's glands, 231 hydrochloric salt solution, 10 39 Ectoderm, 63 derivations from, 69, 130, 146, 243, 401 Ectoplasm, 85 Edinger-Wcstphal nucleus, 524 Effectors, 424, 430, 432, 479 Efferent-cerebellar paths, 484, 505 Efferent pallial paths, 467, 468, 470, 481, 483 peripheral neurones, 424, 447, 481, 491, 493, 497, 505, 508, 510, 521, 524, 530 root fibres, 424 suprasegmental neurones, 427, 491, 493, 505, 510, 513, 521, 527, 532, 536, 542 Egg cords, Pfliiger's, 360 technic of, 372 nests, 360 Ehrlich, granules of, iii Ejaculatory ducts, 348 Elastic cartilage, 103 fibres, 85, 97 tissue, 96 Verhoeff's differential stain for, 28 Weigert's stain for, 28 Elastin, 85, 91, 99 Eleidin, 389 Ellipsoid of Krause, 563 Embedding, 11 celloidin, 11 paraffin, 13 Embolifprm nucleus, 508, 513, 514 Embryonal tissue, 86 fat tissue, 91 Eminentia hypoglossi, 487, 495 medialis, 487 Emulsion theory of protoplasmic struc- ture, 45 Enamel, 238 cells, 242, 244 chemical composition of, 238 cuticula dentis of, 238 membrane of, 245 development of, 243 fibres, 238 organ, 244 prisms, 238, 245 lines of Retzius of, 238 Tomes' process, 245 Endbrain (telencephalon), 423, 538 corpus striatum, 538 610 INDEX Endbrain, olfactory {lallium, 538 pallium, 539 neopallium, 538 rhincnccphalon, 538 anterior perforated space, 538 gyrus hippocampi, 538 nerve, 538 olfactory bulb, 538, 591 pyriform lobe, 538 trigonum olfactorium, 538 tuberculum olfactorium, 538 End-buttons, 515 -bulbs, 439 of Krause, 232, 357, 400 Endgut, 268 large intestine, 268 mesentery, 273 omentum, 273 peritoneum, 272 rectum, 271 vermiform appendix, 270 Endocardium, 165 primitive, 168 Endochondral ossification, 203, 205 Endolymph, 580 Endolymphatic duct, 581 sac, 581 Endomysium, 213 Endoneurium, 137, 431 Endoplasm, 46, 85 Endosteum, 197 Endothelial tube, 168 Endothelium, 72, 80 of Descemet, 556 Engelmann, showing ciliated epithelial cell, 79 Enterokinase, 278 Entoderm, 63 tissue derivations from, 09, 302, 321 Eosin, 20 -glycerin, 23 -hematoxylin stain, 20 Eosinophile granules, no, 178 Ependymal cells, 424 Epiblast, 63 Epicardium, 165 Epicranium, 204 Epidermis (or cuticle), 388 eleidin, 389 keratin, 389 kcratohyaline granules, 389 mitosis of cells of, 389 Epidermis, pareleidin, 390 pigmentation of, 390 prickle cells of, 389 stratum corneum of, 389 c^lindricum of, 388 germinativum of, 388 granulosum of, 389 lucidum of, 389 Malpighii of, 388 mucosum of, 388 spinosum of, 389 Epididymis, 339, 346 body of, 340 cells of, 346 globus major, 340 minor, 340 vas deferens, 340, 346 vas epididymis of, 340, 346 vasa efferentia of, 346 Epidural space, 428 Epiglottis, 307 Epimysium, 213 Epineurium, 431 Epiphyseal cartilage, 208 Epiphysis of bone, 208 Epithalamus, 423, 528, 536 Epithelium, 72 basal membrane of, 73 cells" of, 73 ciliated, 78 classification of, 73 cuticular membrane of, 73 derivation of, 69 endothelium, 80 follicular, 361 general characteristics of, 72 germinal, 360, 384 glandular, 80, 217 histogenesis of, 69 intercellular bridges of, 72 lens, 570 membrana propria of, 73 mesenchj'mal, 210 mesothelium, 80 neuro, 80 pigmented, 80 pseudo-stratitied, 75 replacing cells of, 75 respiratory, 317 simple, 74 columnar, 75 pseudo-stratified, 75 INDEX (ill Epithelium, simple squamous, 74 stratified, 75 columnar, 77 squamous, 75, 183 transitional, 77 surface, of mucous membranes, 223 sync\-tium, 379 tactile cells of, 437 technic'of, 82 transitional, 77 Eponychium, ;^q^ Epoophoron, 369 Erectile tissue, 355, 382 Ergastoplasm, 218, 277 Erythroblasts, iqS Erythrocytes, 107 Erythrosin, 20 Eustachian tube, 580 Excretion, 218, 248 Excretory ducts, 219, 282 substances in cells, 46, 218 Exoplasm, 46, 76, 85 External arcuate fibres, 493, 496, 505 ear, 578; see Ear, external geniculate bodies, 499, 526, 530 OS, 374 spiral sulcus, 585 Extero-ceptors, 442 Extracellular network, 143 Eye, the, 554; see Organ of Vision blood-vessels of, 571 development of, 575 eyeball or bulbous oculi, 554 eyelid, 573 lacrymal apparatus, 573 lens, 570 lymphatics, 572 nerves of, 572 neurone systems of, 566 optic nerve, 565 technic of, 576 vitreous body of, 571 Eyeball (or bulbus oculi;, 554 blood-vessels of, 571 chorioid of, 556 ciliary body of, 558 cornea of, 554 development of, 575 iris of, 560 lens, 570 lymphatics of, 572 nerves of, 561, 565, 571 Eyeball, retina of, 561 sclera of, 554 technic of, 576 P^yelashes, 574 Eyelid, the, 573 blood-vessels of, 575 conjunctiva of, 574 epidermis of, 573 glands of, 574 of .Mall, 574 lymphatics of, 575 Meibomian glands of, 574 muscles of, 574 nerves of, 575 tarsus of, 574 technic of, 575 Facialis (nerve VII), 479, 480, 481 Fallopian tube, 370 ampulla of, 370 blood-vessels of, 371 coats of, 370 development of, 382 fimbriated extremity of, 370 isthmus of, 370 lymphatics of, 371 nerves of, 371 ovarian extremity, 370 technic of, 372 False corpora lutea, 366 Fascicles of muscles, 212 of nerves, 431 Fasciculus arcuatus, 541, 542 gracilis, 489 of Thomas, 473 inferior longitudinal, 541, 542 medial longitudinal, 473, 491, 497, 506, 510, 513, 521, 524 perpendicular of Wernicke, 541 predorsal, 510 retroflexus of Meynert, 537 solitarius, 493, 495, 497 superior longitudinal, 541 uncinate, 541, 542 Fastigio-bulbar tract, 506, 508 Fat, absorption of, 279 technic of, 280 blood supply of, 95 development of, 92, 94 technic of, loi osmic-acid stain for, 31 subcutaneous, 388 612 INDEX Fat, Sudan III and Scharlack R stain for, tissue, 9 1 histogenesis of, 92 technic of, loi Fat-droplets in cells, 46, 94, 261 Fat-lobules, 92 Fauces, mucous membrane of, 225 Female genital organs, 358 pronucleus, 60 Fenestra ovalis, 580 rotuntla, 580 Fenestrated membrane, 98 Ferrein, pyramids of, 326 Fertilization of the ovum, 59, 364 theory of, 63 Fibrae propria; of Meynert, 541, 547 Fibre baskets, 565 systems, 427 efferent, 430 short, 451, 474 proprio-spinal, 474 spino-spinal, 474 tracts of cord, 465 ascending, 467 Burdach's, 467 diagram of, 466 direct cerebellar, 469 dorsal columns, 467 dorsal-spino-cerebellar, 469 Flechsig's, 469 fundamental, 474 Goll's, 467 Gower's, 469 long ascending arms of dorsal root fibres, 467 posterior columns, 467 short, 451, 474 spino-collicular, 468 spino-tectal, 471 spino-thalamic, 468 uncrossed cerebellar, 469 ventral-spino-cerebellar, 469 descending, 470 antero-lateral, 472 cerebro-spinalis, 470 colliculo-spinal, 471 comma tract of Schultze, 473 cortico-spinalis, 470 crossed pyramidal, 470 Deitero-spinal, 472, 473, 484, 4^59, 493> 497, 508 Fibre tracts descending, direct pyra- midal, 471 fasciculus of Thomas, 473 from nucleus of posterior longi- tudinal fasciculus, 471 fundamental, 451, 474 Hclweg's, 473 marginal bundles of Lowen- thal, 472 origin of tracts, 449 oval bundle of Flechsig, 473 pallio-spinalis, 470, 539 pyramidal, 470, 539 rubro-spinal, 472, 483, 487, 489 493, 497, 5^o, 5i9, 521, 526 septo-marginal, 473 short, 451, 474 tecto-spinal tract, 471 tractus cortico-spinalis, 470 vestibulo-spinal, 472 Von Monakow's tract, 472 methods of determining, 465; see Methods Fibres, afferent nerve, 425; see also Nerve fibres arcuate, 493, 496 association of pallium, 539, 541 calcified, 203 cartilage, 103 collagenous, 85 commissural of cerebral cortex, 539 cone, 563 connective-tissue, 90 development of, 85 cortical of hair, 394 of cerebellum, see Cerebellar cortex of cerebrum, see Cerebral cortex dentinal, 233, 235 efferent root, 424 elastic, 85, 96 enamel, 238 external arcuate, 493, 496, 505 genioglossal, 229 heart muscle, 123 hyoglossal, 229 intergeminal, 593 internal arcuate, 493, 506, 508, 528 interzonal, 57 intragcminal, 593 in\olunlary striated (heart) muscle 123 lens, 570 INDEX 013 Fibres, Mallory's method of staining con- nective-tissue, 29, 30 mantle, 54 Miiller's, 564 nerve, 136; see also Nerve fibres medullated, 137 non-medullated, 136 neuroglia, 147 of areolar tissue, 91 of bone, 105 of developing muscle, 12S of formed connective tissue, 95 of Remak, 137 of Sharpej^ 197, 238 olfactory, \&\tx of, 590 perforating or arcuate, of cornea, 556 projection, 539, 541, 545, 549 radiate of liver, 298 reticular, 91, 99 reticulo-spinal, 473 rod, 563 stjdoglossal, 229 superficial arcuate, see External arcuate fibres tendon, 95, 213 tunnel of Corti's organ, 589 voluntary muscle, 118, 121 Weigert's method for staining elastic, 28 method for staining nerve, 33 white or fibrillated, 90, 213 yellow or elastic, 90, 213 Fibrillar connective tissue, 86 theory of protoplasmic structure, 44 Fibrinogen, 112 Fibroblasts, 85, 99 Fibrous cartilage, 103 Field of Forel, 536 Fila olfactoria, 481 Filar mass, 45 Filiform papillae, 230 Fillet (or medial lemniscus), 468, 491, 493, 497, 499, 510, 532 lateral peduncular, 427 medial accessory, 427 Filum terminale, 448 Fimbria, 538 Fissure, anterior median, 454 chorioid, 576 Fixation, 5 b}- injection, 6 in tola, 6 Fixatives, 6, 7, 8 Fixed connective tissue cells, 87 Flechsig, oval bundle of, 473 myclogenctic method of, 465, 547 tract of, 469 P'lemming concerning cell-division, 54 Flemming's fluid, 8 Foam theory of protoplasm structure, 45 Foetal cells, 318, 322 structures, appendix epididymidis, 348 of genital system, 348, 369 testis, 348 ductus, aberrans Halleri, 348 organ of Giraldes, 348 paradidymis, 348, 383 Foliate papilhc, 593 Follicle, Graafian, 360; see also Graafian follicle hair, 394 Follicles, agminated, 265 solitary, 259 Follicular cavity or antrum, 361 Folliculi linguales, 184; see Tonsils Fontana, spaces of, 560 Foramen caecum lingua;, 184 Forebrain (prosencephalon). 422, 528 diencephalon (thalamcncc phalon) , 423 528 epithalamus, 528, 529, 536 hypothalamus, 529 thalamus, 423, 529, 532, 536 interbrain, 528 section through junction of mid- brain and thalamus, 530 Foregut, the, 248 general structure of walls of the gas- tro-intestinal canal, 250 oesophagus, 248 stomach, 252 Forel, decussation of, 527 field of, 536 Formaldehyde, for macerating, 5 -bichromate method, 37 Formalin, commercial, 5 Formalin-alcohol (Schaffer), 7 Formalin-^Miiller's fluid (Orth's), 7 Formed connective tissue, 95 FornLx, 537 anterior pillars of, 542 commissure, 539 Fossa navicularis, 358 614 INDEX Fountain-like decussation of ^leynert, $26 Fourth ventricle, 487, 493, 505 Fovea centralis, 565 Fracnkel's theory of corpus luteum, 368 Free endings of sympathetic nerves, 447 Frozen sections, 16 Fuchsin, 20 Function of cells, 51 Fundamental columns of spinal cord, 451, 474 Fundus, 253 glands, 254 Fungiform papillte, 230 Funiculus cuneatus, 468, 491 gracilis, 468, 491 posterior, 467 Gage, showing muscle fibres, 123 Gage's hematoxylin, 1 7 Galea capitis, 349 Gall-bladder, 301 Galvanotaxis, 52 Ganglia, 424, 425 amphicytes, 433 cerebral, 432 cerebro-spinal, 424, 430 chain, 442 ciliarj-, 442 Gasserian, 512 geniculate, 480 habenularis, 536 of Scarpa of VIII, 481, 499 otic, 442 peripheral, 442 satellite cells, 433 sphenopalatine, 442 spinal, 432, 434 spiral, 589 spirale of VIII., 481, 499 structure of, 432 submaxillary, 442 sympathetic, 424, 442 technic for, 447 terminal, 442 vertebral, 442 Ganglion cells, 425 capsule of, 425 cardiac, 167 development of. 424 Gartner's canal, 3S3 duct, 369 Gasserian ganglion, 512 Gastric cr^'pts, 253 glands, 253, 254 pits, 253 Gastro-hepatic omentum, 273 Gastro-intestinal canal, general structure of the walls of, 250 Gelatin, 90, 99 carmine for injecting, 25 Prussian blue, for injecting, 25 Gelatinous marrow, 201 substance of Rolando, 455 Gemmules, 515 Geniculate body, 499, 526, 530 ganglion of, VII., 480 Genio-glossal fibres, 229 Genital gland, 384 organs, female, 358 male, 339 ridge, 384 system, 339; see also Reproductive system development of, 382 rudimentary structures connected with development of, 348, 369 Genito-urinary system; see Urinary sys- tem, 324 and Reproductive sys- tem, 339 Gennari, line of, 549 Gentian violet, 20 Genu-facialis, 505 Germ hill, 362 layers, 63 tissues derived from, 69 primary, 63, 69 Germinal epithelium, 360, 384 Germinal spot, 363 vesicle, 59, 363 Giant cells of Betz, 539, 542 Gianuzzi, crescents of, 227, 283, 308 Giraldes, organ of, 348, 383 Gland cells, 217 Glands, 217 accessory thyreoid, 409 acini of, 222 adrenal, 41S alveolar, saccular, 219, 222 compound, 222 simple, 222 alveoli of, 222 Barthohn's, 382 Bowman's, 306 Brunner's, 258, 267 I NT) EX 615 Glands, bulbourethral, 355 cardiac, 257 carotid, 415 cells of, 217 ccruminous, 578 classification of, 217, 220 coccygeal, 417 compound, 219 corpus lutcum, 365 Cowper's, 355 dehiscent, 223 development of, 220 duct, 219 ductless, 219, 222 Ebner's, 23 1 epithelium of, 217 ergastoplasm, 218 excretoty ducts of, 219 fundus, 254 gall-bladder, 301 gastric, 254 general structure of, 217 genital, 384 haemoK'mph, 177 internal secreting, 219. 222 interstitial tissue of, 220 intraepithelial, 346 kidney, 324 lacrymal, 573 large, of digestive system, 281 Lieberkiihn's, 258, 265 lingual, 227 Littre's, 357, 358 liver, 292 lobes of, 220 lobules, 220 lymph, 176 IMall's, 574 mammary, 402 Meibomian, 222, 574 mixed, 226 mucous, 226, 249 membranes of, 223 of internal secretion, 223 of the oral mucosa, 225 ovary, 358 pancreas, 286 paraganglia, 415 parathyreoids, 410 parenchyma of, 220, 282 parotid, 282 peptic, 254 Glands, pineal, 550 prehyoid, 409 prostate, 353 pyloric, 254, 257 racemose, 220 reticular, 222 saccular, 219 salivary, 281 sebaceous, 357, 390, 397, 574, 576 secreting portions of, 219 serous, 226 smiple, 219 spleen, 185 sublingual, 283 submaxillarj', 284 sudoriferous, 390 suprahyoid, 409 sweat, 390 tarsal, 574 thymus, 179 thyreoid, 408 accessory, 409 tonsils, 182 tubular, 219, 221 compound, 222, 324 simple, 221, branched, 249 tubulo-alveolar, 219, 287 tympanic, 415 Tyson's, 357 uterine, 373 Glandulae sudoriparse, 390 vestibulares majores, 382 minores, 382 Glandular epithelium, 80, 217 Glans penis, 357 Glenoid, ligaments, 210 Glia, 148 Gliosis 148 Glisson, capsule of, 293 Globus major, 340 minor, 340 pallidus, 541 Glomerulus of kidney, 327 blood-vessels of, 331 Bowman's capsule of, 326, 328 olfactory, 591 Glosso-pharyngeal (IX nerve) 480, 497 Glycerin for mounting specimens, 22 jelly, 23 Glycogen granules, 295 Goblet cells, 78, 82, 217, 261, 277, 307 616 INDEX Gold chloric! for staining connective- tissue cells, 28 Gold-size for glycerin mounts, 23 Golgi cell, Type I., 134, 136 cell, Type II., 135, 136, 452, 471 method, bichlorid, 37 chrome silver, 29 Cox modification, 37 formalin bichromate, 37 mixed, 36 rapid, 36 silver, for nerve tissue, 36 slow, for nerve tissue, 36 muscle-tendon organs of, 440 net, 143 organs of, 440 Golgi-Mazzoni corpuscles, 400, 440 GoU, column of, 455, 468 nucleus of, 468, 487, 493 Gowers' tract, 469 Graafian follicles, 360 antrum of, 361 corona radiata, 362 cumulus ovigerus, 362 development of, 362, 382 discus proligerus, 362 egg nest, 360 epithelium of, 360 follicular cavity of, 361 germ hill of, 362 liquor folliculi, 361 nerves of, 369 ovum of, 362 Pfliiger's egg cords, 360 primitive, 361 ova, 360 rupture of, 364 stratum granulosum, 361 technic of, 371 theca folliculi, 362 tunica fibrosa, 362 vasculosa, 362 Graded alcohols, 7 Grandry, corpuscles of, 437 Granule cells of cerebellum, 518 theory of protoplasmic structure, 45 Gray matter of spinal cord, 426, 455 rami communicantes, 429, 443 reticular formation, 482, 491 Greater omentum, 273 Ground bundles of spinal cord, 451, 474 Griibler's methylene blue, 31, 39 Griibler's water-soluble eosin, 31 Gums, mucous membrane of, 225 Gustatory canal, 593 Gyrus dentatus, 538 hippocampi, 538 of Heschl, 547 Habenul^, 536 Haemalum, Mayer's, 18 Hffimatcin, 17, 108 Hsmatoidin, crystals of, 366 Hasmatokonia, 112 Haematoxylin, 17 and eosin, for staining double, 20 and picro-acid fuchsin, 21 Delafield's, 18 Gage's, 17 Heidenhain's, 18 Mallory's stain, 29, 30 Weigert's, 19, S3 Haemoglobin, 108 Haemolymph nodes, 177 blood sinuses of, 177 blood-vessels of, 177 cells of, 178 eosinophiles, 178 mast cells, 178 phagocytes, 179 development of, 179 function of, 179 hilum of, 178 marrow-lymph, 178 relation of, to lymphatic system, 179 spleno-lymph, 178 technic of, 179 Haemolysis, 108 Hair, 393 arrector pili muscle of the, 397 blood-vessels of, 399 bulb, 393 cells of the, 394 connective-tissue follicle of, 395 cortex of, 394 cortical fibres of, 394 cuticle of, 394 development of the, 401 eyelashes, 574 follicle, 394 germ, 398, 402 growth of the, 398 hyaline membrane, 395 inner root sheath, 394 INDEX 017 Hair, inner root, cuticle of, 3g5 Henle's layer of, 395 Huxley's layer of, 395 lanugo, the, 394 lymphatics, 400 medulla of, 393 nerves of, 400, 436, 437, 446 outer root sheath, 305 papilla of, 393 pigment of, 394 prickle cells, 395 root of the, 393 root sheath, 394 sebaceous glands of the, 397 sebum of the, 398 shaft of, 393 shedding of the, 398 stratum cylindricum, 395 technic of the, 399 vitreous membrane, 395 Hair cells, 581 Halleri, ductus aberrans, 348 Haller's layer, 557 Hamulus, 583 Hardening, 9 Hassal's corpuscles, 70, iSi Haversian canals, 194 development of, 208 fringes, 210 lamellae, 195 spaces, 207 systems, 196 development of, 208 Hayem's fluid, 64 Headgut, 225 mouth, 225 pharynx, 247 teeth, 232 tongue, 228 Hearing, organ of, 578; see Ear Heart, 165 annuli fibrosi, 166 auricular muscle, 163 auriculo-ventricular ring, 165 blood-vessels of, 166 coronary arteries of, 166 development of, 167 endocardium of, 165 epicardium of, 165 lymphatics of, 166 -muscle, 123, 165; see also Involuntary striated muscle Heart, muscles of, 165 myocardium of, 165 nerves of, 167, 446 technic of, 167 valves of, 166 Hecateromeres, 451 Heidenhain, concerning vohmtary striated muscle, 121 demilunes of, 227 Heidenhain's hematoxylin, 18 Heisterian valve, 301 Helicotrema, 584 Heller's hlexus, 274 Helweg, tract of, 473 Hemispheres of cerebellum, 508, 513 of cerebrum, 423 Hendrickson, concerning coats of liver ducts, 301 Henle, concerning ovum, 369 Henle's laj'cr of hair follicle, 395 loop of uriniferous tubule, 326, 328, 329 sheath of nerve fibre, 140, 431 Hensen's cells, 587 line, 119 Hepatic artery, 294 cells, 298 cords, 297 cylinders, 303 duct, 294, 300 Heschl, transverse temporal gyri of. 547 Heteromeres, 451, 453 Hilum of liver, 293 of kidney, 324 Hindbrain, 423, 485 {rhombencephalon) , bulb, 485 cerebellum, 424, 508 medulla oblongata, 424, 485 section of, through, at level of junc- tion of pons and cerebellum, and entrance VIII, vestibular, 505 through roots of VI, abducens, and VII facial nerves, 505 through roots of V, trigeminus nerve, 508 tegmentum, 488 Hippocampus, 481 His, marginal veil of, 423 myelospongium of, 423 spongioblasts of, 423 Histogenesis, 69 Holmgren, showing trophospongium, 47 618 INDEX Horizontal cells, 544, 564 Howship's lacunie, 204 Huxley's layer, 395 Hyaline cartilage, 102 Hjaloid canal, 571 membrane, 571 Hyaloplasm, 44 Hydatid of Morgagni, 34 507, 510. 513 roots, 489, 491, 495 secondary tracts of, 489, 491. 495 terminal nuclei of, 489, 491, 493, 505 ala cinerea, 487, 495 anterior fissure, 487 pyramid, 487, 505 arciform (arcuate) nucleus, 496, 505 arcuate fibres, 493 area acustica, 488 auditory nerve, 499 central canal, 455 gelatinous substance, 493 gray matter, 489 tegmental tract, 496. 505 cerebellar peduncles, 505 cerebello-olivary fibres, 495, 496 chorioid plexus, 493 clava, 487 cochlear nerve, 479, 481, 499 nuclei, 499 colliculo-spinal tract. 471, 484, 491, 496, 505. 510. 515. 523 column of Burdach, 487, 491, 493 of Goll, 487, 491, 493 compared with spinal cord, 487 corpus restiforme, 487 cranial nerves of, 485, 487 crossed pj-ramidal tract, 491 cuneus, 487 decussation of fillet, 491 Deiter's nucleus, 499, 506 tract, 493 descending root of fifth nerve. 487, 489 Medulla descending or spinal root of ves- tibular portion of eighth nerve, 499 suprasegmental paths, 491 tract from Deiter's nucleus, 489, 497, 506 from the vestibular nuclei, 496 development of, 422 direct cerebellar tract, 489 pyramidal tract, 491 funiculus, 489 horns, dorsal, 493 nucleus of ninth cranial nerve, 497 of tenth nerve, 495 spino-ccrebellar tract, 489, 493, 496 efferent peripheral neurones, 489, 491. 495, 505, 508 suprasegmental neurones, 491, 497, 505 eminentia hypoglossi, 495 external arcuate fibres, 493, 496, 505,. 508 fasciculus cuneatus, 489 gracilis, 489 solitarius, 495, 497 fillet or medial lemniscus, 493 formatio reticularis, 491, 493, 495 fourth ventricle, 487 funiculus cuneatus, 491 gracilis, 491 gelatinous substance of Rolando, 489. general structure of, 485 genu facialis, 505 Gowers' tract, 489 gray reticular formation, 491, 493,. 496 internal arcuate fibres of, 493, 506, 508 intersegmental neurones, 489, 493,, 496 lateral fillet, 497, 499 lemniscus, 497, 499 longitudinal fasciculus, 497, 506 median lemniscus, 491, 493, 495, 497,. 499 longitudinal fasciculus, 491, 493 raphe, 494, 496 motor decussation, 489 nuclei arcuati, 496, 505 laterales, 496 of posterior columns, 493 of the floor of the ventricle, 487 624 INDEX jMeduUa, nucleus, abducentis, 488 accessory cuneate, 493 als cinereae, 491 ambiguus, 495 arcuatus, 496 commissuralis, 493 cuneatus, 487, 491, 493, 496 gracilis, 487, 491, 493 hypoglossi, 487, 491 of acoustic nerve, 488, 499 of the column of Burdach, 487, 493 of the column of Goll, 487, 493 of the fifth spinal nerve, 493, 510, 512 of origin of eleventh cranial {spinal- accessory) nerve, 489 of origin of twelfth cranial {hypo- glossal) nerve, 487, 491, 493 of vagus nerve, 480, 495 olivary nucleus, 493, 496, 497 olives, 487, 493 olivo-cerebellar fibres, 496, 499 pallio-spinal tract, 491 peduncles of, 505 peripheral neurones of, 487 plexus chorioideus, 493 pons Varolii, 488, 505 posterior columns of, 493 longitudinal fasciculus, 473 septum, 487 predorsal tract, 497 pyramidal decussation, 491, 493 tracts, 491, 493 raphe, 494, 496, 513 restiform body, 487, 495, 496, 505, 507, 510 root fibres of spinal V., 489, 495 and nucleus of origin of sixth {ahducens) cranial nerve, '506, S08 of seventh {facial) cranial nerve, 497 and nuclei of eighth {andilory) cranial nerve, 499 of eleventh {spinal-accessory) cran- ial nerve, 489 of ninth nerve {glosso- pharyngeal), 480, 495, 497 of tenth nerve {vagus), 480, 491, 495 of twelfth {hypoglossal) cranial nerve, 491" Medulla, rubro-spinal tract, 489, 493, 497 secondary cochlear tract, 499 sensory tract of fifth nerve, 489, 493 vestibular tract, 499 section through decussation of fillet, 491 entrance of cochlear branch of eighth, 497 lower part of inferior olivary nucleus, 493 mitldle of olivary nucleus, 497 pyramidal decussation, 489 sensory decussation, 491 solitary fasciculus, 493, 495, 497 spinal (descending) root of fifth cranial nerve, 489 V, 481, 483, 489, 493, 496, 499, 506, 507, 510, 512 spino-cerebellar tract, 469, 489, 493 -thalamic tract, 493 striffi meduUares, 497, 536 technic of, 488 tecto-spinal tract, 471, 489, 484, 491, 496, 505. 510, SI3j 523 tegmentum, see Tegmentum terminal nucleus of the descending (sensory) root fibres of the fifth nerve, 4 tract of Gowers, see Gowcrs' tract from interstitial nucleus of Cajal, see Cajal of Helweg, see Helweg of Lowenthal, see Lowenthal tractus spinalis trigemini, 489 trapezius, 506 trigonum hypoglossi, 487, 495 vagi, 487 tuberculum cinereum, 487 vestibular nerve, see Auditory nerve vestibulo-spinal tract, 489 Medullary cords, 181 lamina, 536 pyramid (Malpighian), 326 rays, 326 sheath, 136, 138 MeduUated axones, 137 nerve fibres, Weigerts' stain for, 33 ^Slegalocytes, 188 Megakaryocytes, 198 Meibomian glands, 222, 574 Meissner, corpuscles of, 356, 400, 438 INDEX 625 Meissner, plexus of, 259, 268, 269, 276,442 Melanin, 135 Membrana chorii, 377 elastica externa, 160 interna, 158 limitans olfactoria, 306 prcformativa, 246 propria, 73 tectoria, 588 Membrane, basal, 73 cuticular, 73, 245 glial, 148 mucous, 223 of Bowman, 554 of Descemet, 556 of Krausc, 121, 124 of Reissner, 585 peridental, 239 serous, 169 synovial, 210 Membranes of brain and cord, 428 Membranous cochlea, 584 labyrinth, 580 spiral lamina, 583 ligament, 583 Meninges, 428 Meniscus, tactile, 437 Menopause, 407 Menstrualis, decidua, 375 Menstruating uterus, 374 Menstruation, 375 Mercuric chlorid as a fixative, 8 Merkel's corpuscles, 437 Mesencephalic root of fifth (trigeminus) cranial nerve, see Xcrves, cranial V Mesencephalon, 423, 523 Mesenchymal epithelium, 210 Mesenchj-me, 85 Mesentery, 273 Mesoappendix, 270 Mesoblast, 63 Mesoderm, 63 tissue derivations from, 70, 126, 243, 302, 321 Mesonephros, 383 derivations from, 348, 369 Mesophragma, 121 Mesothelium, 72, 80 Metabolism of cells, 51, 218 Metanephroi, 383 Metaphase, 56 40 Metaplasm, 46 Metathalamus, 529 Methods for studying fibre tracts of the cord, 4O5 atrophy, 465 axonal degeneration, 465 comparative anatomy, 465 myclogenetic, 465 physiology, 467 secondary degeneration, 465 von Ciuddcns, 465 Methyl blue, 20 green, 20 violet, 20 Methylene blue for staining nerve cells, 39 Meynert, decussation of, 528 fasciculus retroflexus of, 537 fibnc propriai of, 541 radiations of, 547 Microcytes, 107 Micron (micromillimeter), 15 Micro-nucleus, 58 ]\Iicrosomes, 44 Microtome, 14 Midbrain, 422, 523 aqueductus Sylvii, 523 basis pedunculi, 523 brachia conjunctiva, 508 cerebral peduncles, 523, 524 coUiculi, 423, 523 corpora quadrigemina, 423, 499, 523, cranial nerves III and IV, 523, 526; also see Nerves, cranial crura cerebri, 524 decussation of Forel, 527 of Meynert, 528 Edinger-Westphal nucleus, 524 geniculate bodies of, 499, 526, 530 inferior brachium quadrigeminum, 526 coUiculi, 523 internal arcuate fibres, see Internal arcuate fibres iter, 523 lateral peduncular fillet, 527 lemniscus, see Lemniscus medial accessory fillet, 527 mesencephalic root of fifth nerve, see Nerves, cranial optic nerve, see Nerves, II, opticus pes pedunculi, 523, 527 626 INDEX Midbrain, posterior commissure, 528 longitudinal fasciculus, 527 red nucleus of, 523, 526, 527 reticular formation, 526 root fibres and nucleus of origin of third {oculomotor) cranial nerve, section through exit of third (oculo- motor) cranial nerve, 523 spino-tectal tract, 528 substantia nigra, 523, 527 superior cerebellar peduncles of, 527 colliculi, 523, 528 tegmentum, 424, 523 ^liddle ear, 579; see Ear, middle Midgut, 260 small intestine, 260 Migrator}- leucocytes, 264 Milk, 405 cells of, 405 colostrum corpuscles of, 405 teeth, 244, 245, 246 Minot, concerning endothelium and meso- thelium, 80 concerning the pregnant uterus, 376 Mitochondria, 218 Miton, 45 Mitosis, 54 anaphase, 56 metaphase, 56 method of demonstrating by Flem- ming's fluid, 64 prophase, 53 technic for, 64 telophase, 57 ^Mitotic figure, 56 Mitral cells, 590 Mixed glands, 226 spinal nerve, 430 Modiolus, 582 MoUier, concerning splenic pulp, 189 Monaster, 56 Mononuclear cells, 188 leucocytes, 109 Monophyletic theory of blood cells, 113 Monosynaptic arc, 475 Mordanting, 33 Morgagni, hydatid of, 348 flossy cells, 147 fibres, 519 Motion of cells, 52 Motor cells of antf^rior horns, 450 Motor cells, decussation, 489 end plate, 448 nuclei, 447 peripheral nerves, 447 precentral area, 549 Mounting, 22 celloidin specimens, 23 in balsam, 23 in glycerin, 22 paraffm sections, 24 Mouth, the, 225 blood-vessel of, 227 end bulbs in mucous membrane, 439 glands of, 225 lymphatics of, 227 mucous membrane of, 225 nerves of, 227, 232, 438, 593 technic of, 228 Mucin, 86, 226 Mucous glands, 226 membranes, 223 end bulbs in, 439 general structure of, 223 of alimentary tract, 224 tactile cells of, 437 corpuscles of, 438 tissue, 86 ]\Iucus, 217, 226, 253, 262, 277 ^liiller, cells of, 564 circular muscle of, 559 fibres of, 564 Miiller's fluid, 7 Miillerian ducts, 348, 354 Multinuclear cells, 188 Multipolar nerve cells, 131, 434, 450 Muscle, arrector pili, 397 auricular, 165 cells, 115 ciliary, 559 circular, of ^Miiller, 559 columns of Kolliker, 120 discs, 119 fibrillje, n8 nuclei, 116, 123 of sweat glands, 402 spindles, or neuro-muscular bundles, 440 tendon junction, 213 organs of Golgi in, 440 peripheral nerve terminations in, 440 tissue, 115 IXDEX 027 Muscle tissue, classification of, 115 development of, 1 26 histogenesis, 1 26 intercallated discs of, 125 heart, 123 involuntary smooth, 115 intercellular bridges of, 116 striated, 123 technic of, 129 voluntary striated, 118 anisotropic substance, 119 Cohnheim's fields, 120, 124 cross fibre nets of, 121 end bulbs of, 440 ergastoplasm, 118 Heidenheim's scheme of struc- ture of, 121 Hensen's line, 119 inokomma, 121 isotropic substance, 119 Krause's line, 119 membrane, 121, 124 mesophragma, 121 muscle columns of KoUiker, 1 20, 124 discs, 119 spindles, 440 myofibrils, 121 nerves, terminations in, 440. 446 Pacinian corpuscles of, 440 Rufiini's theory of nerve termi- nations in, 440 sarcolemma, 118 sarcoplasm, 119 sarcous element of Bowman, 120 technic of, 129 telophragma, 121 ultimate fibrillae, 119 white and red fibres, 121 Muscles, voluntary, 212 capsule of, 212 endomysium, 213 epimysium, 212 fascicles of, 212 growth of, 214 intrafascicular connective tissue of, 213 perifascicular sheath, 213 perimysium, 213 Muscular system, 212 blood-vessels of, 215 bursae, 213 Muscular system, lymphatics of, 215 nerves of, 215 technic of, 215 tendons of, 95, 213 voluntary muscle, 212 Muscularis mucosa; of mucous mem- branes, 223 Musculature of intestine, 117, 267, 271 Myelin, 138 sheath, 136 Myeloarchitecture, 547 Mj-elocytes, 197 Myelogenetic method for determining fibre tracts of cord, 465, 547 Myeloplaxes, 198 Myelospongium of His, 423 Myentericus, plexus, 276 ^Myoblast, 126, 215 Myocardium, 165 primitive, 168 Myofibril, 121 Myotome, 127 ^Myxoedema, 409 Nabothi, ovula, 374 Nail-bed, 391 root, 391 groove, 391 wall, 391 Xails, the, 391 cells of the, 392 development of, 407 eponychium of, 393 growth of, 393 hyponychium, 393 keratohyalin of, 393 lunula of, 393 matrix of, 391 prickle cells of, 393 structure of, 391 technic of, 393 Xares, 305 accessory nasal sinuses, 305 cells of, 306 basal, 306 olfactory, 306 sustentacular, 306 development of, 321 glands of Bowman, 306 membrana limitans olfactoria, 306 structure of, 305 olfactory region, 305 628 INDEX Nares, struclure of, respirator}- region, 305 vestibular region, 305 technic of, 310 zone of oval nuclei, 306 of round nuclei, 306 Nasal duct, 573 NemilelY, showing amitosis, 54 scheme of medullated nerve fibre, 141 Neopallium, 538, 539, 541 association fibres of, 541 fibrie of Mejnert, 541 Nerve cells, 130; see also Neurone amacrine, 564, 592 amphicytes, 433 anterior horn, 450 association, 520, 545 basket, 516, 527, 542 Betz', 539, 542, 545 bipolar, 131, 434 brush, 592 Cajal's, 542 capsule, 425 caryochromes, 133, 512, 518 cerebro-spinal ganglia, 424, 432, 449 column, 450, 457, 469, 475 cone-bipolar, 563 cone-visual, 563 efferent projection, 545 ependymal, 148, 424 extrinsic, 449 ganglion, 424, 432, 449, 545 giant, of Betz, 539, 542, 545 glia, 148 Golgi, Type I, 134, 136 Type II, 135, 136, 452, 542 granule cells of cerebellum, 518 hecateromeric, 451 heteromeric, 451 horizontal, 544, 564 in gray matter of cord, 450 intrinsic, 449 inverted pyramidal, 542 marginal, 456 !Martinotti's, 542, 545 mitral, 590 mossy, 147 motor, of the anterior horn, 131, 450 Miiller's, 564 multipolar, 131, 434, 450 Nerve cells, neurilemma, 148, 425 neuroblasts. 130, 146, 424 neuroglia, 147. 424, 459, 520 nucleus of, 131 of motor area of cerebral cortex, 545 outside the spinal cord, 449 peripheral motor, 447 sensory, 450 pigment in, 135 polymorphous, 542, 545 Purkinje, 515 pyramidal, 542 inverted, 542 rod-bipolar, 563 rod-visual, 563 root, 447, 450, 457 satellite, 433 somatochromes, 133 spider, 147 spinal ganglion, 424, 432, 449 spongioblasts, 147, 423 stellate of cerebellum, 516 of cerebral cortex, 542 sympathetic ganglion, 442, 445 tautomeric, 450, 469 unipolar, 131, 433 endings, 435; see Peripheral nerve terminations fibres, 135 afferent, 425 root, 426 arcuate, 493, 496, 505 association, 427, 539, 541 cerebro-pontile, 507 cHmbing, 516, 519 commissural, 539 cone, 563 deep tangential, 547 dorsal root, 456 efferent root, 424 fastigio-bulbar, 506, 510 felt works of, 446 glial, 148 laj'er of, of retina, 564 lens, 570 medullated, 137 mossy, 519 motor end plates of, 448 myelinated, 137 nerve, 136 neuroglia. 147 TXDEX 629 Nerve fibres, non-mcdullated, 136 of Bergmann, 520 of Remak. 137 of trapezius, 507 origin of, of white mailer of cord, 449 pallia!, 53Q pallio-pontile, 507, 510 pallio-'tectal, 541 pallio-lhalamic, 541 parallel, of the cerebellum, 518 perpendicular pontile, 505, 507, 510. 513 postganglionic, 443 preganglionic, 443 projection, 539 rod and cone, 563 superficial tangential, 541 roots, dorsal, 456 terminations, 436, 440, 44O motor, 446 sensory, 436 tissue, 130 Golgi methods of staining, 36 neuroglia, 146 neurone, 130 axone, 136 cell body, 130 dendrites, 135 protoplasmic processes, 135 technic for, 33, 148 Nerves, cranial, table of, 551, 552 I (olfactorius), 479, 481, 538 II (opticus), 479, 526, 530, 565 optic decussation of, 532, 567 chiasma, 5^2, 567 tract, 526, 530, 532, 542 motor and sensory nuclei of, 427 III (oculomotor), 481, 523, 524, 526 oculomotor nucleus, 523, 524 root fibres and nucleus of origin of third, 508, 523. 524 IV (trochlearis), 481, 521, 524, 526 root fibres and nucleus of origin of fourth, 521, 524, 526 V (trigeminus), 489, 493, 495, 496, 499, 506, 507, 510, 512 mesencephalic root of fifth, 481, 512, 521, 526 motor nucleus of fifth, 510 "principal sensory" nucleus, 512 semilunar ganglion of fifth, 480 Nerves, sensory nucleus of fifth, 512 and motor root fibres of fifth, 512 spinal root of fifth, 489, 493, 506 terminal nucleus of, 489 VI (abducens), 481, 505, 508 nucleus of origin of sixth, 505, 506, 508, 510 abducenlis, 505, 508 root fibres of sixth, 505, 506, 507, 50S \'II (facial), 479, 480, 481 ganglion geniculate, 480 nucleus facialis, 505 of origin of seventh, 505 rool fibres of seventh, 497, 508 \'III (auditory), 479, 481, 499 cochlear branch of eighth, 481,497 ganglion of Scarpa, 481, 499 spirale, 481, 499 nuclei of eighth, 497, 510 Deiter's, 587 von Bechtcrew's, 499, 513 root fibres of eighth, 499 vestibular branch of eighth, 481, 496, 497, 499, 506 IX (glosso-pharyngeal), 480, 497 descending or sensory root fibres of the ninth, 495 dorsal nucleus of ninth, 496 ganglia of, 480 motor nucleus of ninth, 496 root fibres of ninth, 497 X (vagus), 480, 495 descending or sensory root fibres of tenth, 495 dorsal nucleus of tenth, 491, 495. 497 ganglion jugular, 480 nodose, 480 motor nucleus of tenth, 495 nucleus ake cinereae, 491 root fibres of tenth, 495 XI (spinal accessory), 489 nucleus of origin of eleventh, 489 root fibres of eleventh, 489 XII (hypoglossal), 481, 485, 487, 491, 493 nucleus of origin of twelfth, 487, 491. 493 root fibres of twelfth, 491, 495 mixed spinal, 430 olfactory. 538 630 INDEX Nerves, peripheral, 425, 430 spinal, anterior, motor or eflierent roots of, 491 sensory, or afferent portions, 432 Nervous system, the, 422 cerebro-spinal, 422 connective tissue of, 146 development of, 69, 422 general structure of, 422 sympathetic, 422 cerebro-spinal, 422 afferent peripheral neurones, 430 brain, 479 cerebro-spinal ganglia, 430 cranial nerves, 479, 551, 552 development of, 422 efferent peripheral cerebro-spinal neurones, 447 general structure of, 422 histological development of, 422 membranes of brain and cord, 428 segmental part, 426, 480 spinal cord, 448 nerves, 430, 551 suprasegmental, 426, 484 sympathetic, 422 development of, 424 ganglia, 422, 442 Neumann's dental sheath, 237 Neural arc, 427, 475 cerebellar, 476 cerebral, 476 disynaptic, 476 mono-synaptic, 475 paUial, 476 canal, 130 fold, 422 groove, 422 plate, 422 tube, 422 Neuraxone, 130 Neurilemma, 136, 139 and axolemma, relation of, 140 -cells, 425 Neurite, 130 Neuroblasts, 130, 146, 424 Neuro-epithelium, 80 cone bipolar cells, 563 visual cells, 563 rod bipolar cells, 563 visual cells, 563 Neurofibrils, 132 Neurofibrils, Cajal's method of staining, 38 importance of, in neurone, 142 Neuroglia, 146 cells, 459 fibres, 459 mossy cells of, 147 Miillcr's cells of, 564 neuroblasts, 130, 146, 424 of cerebellum, 520 spider cells, 147 spongioblasts of, 147, 423 technic for, 148 Neurokeratin network, 139 Neurological staining methods, ^^ Neuromuscular bundles, 440 Neurone, the, 130 arc, 475 three-neurone, spinal, 475, 491, 512 two-neurone, spinal, 475, 510 axone of, 136 Barker's classification, 134 canals in, 135 carj'ochromes, 133 cell body, 130 chromophilic bodies, 133 clefts or incisures of Schmidt-Lanter- mann, 139 contact theory of, 142 continuity theory of, 143 cytoplasm of, 132 degenerative changes in, 143 dendrites of, 135 development of, 130 extracellular network of, 143 functional centre of, 141 genetic centre of, 141 Golgi net, 143 long axone neurone, 136 neurofibrils of, 132 Nissl, special method of technic for, 39 nucleolus, 132 nucleus, 131 nutritive centre of, 141 pericellular network of, 143 perifibrillar substance, 132 physiological significance of, 141 pigment in, 135 protoplasmic processes, 135 retraction theory of, 142 Schmidt-Lantermann segments, 139 TXDEX 031 Neurone, sheath of Schwann, 139 short axone neurone, 136 somatochromcs, 133 substantia nigra, 135 synapsis of, 142 technic for, ^2> 148 theory, 141 trophic centre of, 141 Neurones, afferent peripheral, 426, 432, 480 segmental, 427, 480 suprasegmental, 427 associative, 476 central, 424, 426 cone association, 567 cord, 475 cortical prccentral, 475 efferent peripheral, segmental, 426, 447, 480, 481 peripheral segmental, 427, 480, 481 suprasegmental, 427, 491, 493, 505, 510- 513. 521, 527, 532, 536, 542 intermediate, 424, 426 intersegmental, 427, 482, 493, 496 intrasegmental, 427, 482 pallio-pontile, 507 afferent, 426, 432, 475, 480 efferent, 424, 426, 474, 481 peripheral, 425 ponto-cerebellar, 507, 510 rod-association. 567 somatic (peripheral), 425, 480 splanchnic (peripheral), 425, 480 suprasegmental associative, 428 thalamo-cortical, 468, 481 visceral (peripheral), 425 Neuroplasm, 138 Neutral carmine, 20 Neutrophile granules, 1 10 Nipple, 403 Nissl method for staining nerve cells, 39 concerning chromophilic bodies, 134 pathological value of, 134 Nitric acid for decalcifying, 11 for dissociating muscle tissue, 5 Nodes, Ij-mph, 171 of Ranvier, 138 Nodose ganglion of X nerve, 480 Non-meduUated axones, 136 Normoblasts, 198 Notochord, anlage of, 63, 209 Noyes concerning nerves of teeth, 241 Nuclear dyes, 1 7 alum carmine, 19 basic anilin, 20 carmine, 1 7 combination of Gage's and Mayer's formulas, 18 Delafield's ha^matoxylin, iS Gage's ha^matoxylin, 17 haimatoxylin, 17 Heidenhain's haematoxylin, 18 ^Mayer's h;cmalum, iS Weigert's h;ematoxylin, 19 eccentricit}- in nerve cell, 446 fluid, 49 groups of placenta, 379 membrane, 48 sap, 49 structures, method of demonstrating by Flemming's tluid, 8 X'uclein, 48 Nucleolus of typical cell, 49 false, 49 X'ucleoplasm, 49 Nucleoreticulum, 49 Nucleus, the, 47 abducentis, 508 accessory olivary, 496 achromatic element of, 49 alse cinereae, 491 ambiguous, 495, 505 amphipyrenin of, 48 amygdaliformis, 542 arciform, see Nucleus arcuatus arcuatus, 496, 505, 536 caudatus, 532, 538, 541 centralis superior, 521 chromatin of, 49 commissuralis, 457, 493 cuneatus, 468, 493 Deiter's, see Deite/s nucleus dentate, 472, 508 dorsal, 456 accessory olivary, 496 cochlear, 499 Edinger-Westphal, 524 emboliformis, 508, 513 facialis, 505 fastigii, 472, 506, 508, 513 function of, 48 funiculi cuneati, 468, 491 gracilis, 468, 493 globosus, 508, 513, 514 I 632 INDEX Nucleus, gracilis, 493 hypoglossi, 491 inferior olivary, 496 interstitial, of Cajal, see Cajal, iulcr- slitial, nucleus of kar>'oplasm of, 49 karj'osomes, 49 lateralis, 496 lenticularis, 53S, 541 linin of, 48 medial accessory olivary, 496 medialis, 496 membrane of, 48 network of, 48 nuclein of, 48 nucleoreticulum of, 49 nucleolus of, 49 oculomotor, 524 of acoustic tubercle, 499 of a tyjiical cell, 47 of column of Burdach, 468, 487, 493 of Goll, 468, 487, 493 of Darkschewitsch, 471 of Luys, 529 of origin, 427 olivarius superior, 506 olivary, 493, 496 plasmosome of, 49 pontis, 510 preolivary, 506 pulposus, 209 red, see Nucleus ruber resting, 57 reticularis tegmenti, 510 ruber, 472, 482, 487, 526, 530, 536, 541 semilunar, 506 superior olivary, 506, 510 tecti, 472, 506, 508 terminal, 427, 442 trapezoid, 497, 506. 510, 513 triangular, 496 ventro-lateral, 536 vestibular, 496, 499, 506 von Bechterew's, 499, 506, 513 Xuel's space, 587 Nutrient canal, 201 foramen, 201 vessels, of bone, 201 Nutritive center of neurone, 141 Oculomotor III nerve, 481, 523, 524, 526 nucleus, 523, 524 Odontoblasts, 233, 246 CEsophagus, the, 248 glands of, 248, 249 technic of, 250 Oil of origanum Cretici for clearing specimens, 23 Oifactorius (I nerve), 479, 481, 538 Olfactory bulb, 538, 591 granule layer, 592 of longitudinal fibre bundles, 592 of mitral cells, 592 of olfactory fibres, 592 molecular layer, 592 olfactory glomeruli of, 592 path, 481, 538, 545 pallial commissure, 539 glomeruli, 592 group of segmental neurones, 481 nerve, 481, 538 organ, 580 pallium, 538 tract, 481, 545, 592 Olivary nucleus, 487, 493, 499, 506, 513 Olives, 493, 496, 499 dorsal accessory, 496 medial accessor}-, 496 Olivo-cerebellar fibres, 496, 499, 507 Omentum, 273 gastro-hepatic, 273 greater, 273 Opie, concerning the cell-islands of Lan- gerhans, 291 concerning the pancreas, 291 Oppel's method of staining intralobular connective tissue of liver, 302 Optic chiasma, 532, 567 cup, 576 decussation, 567 depressions, 575 nerve, 479, 526, 530, 565 arachnoid of, 566 dural sheath of, 565 pial sheath, 565 relation to retina and brain, 566 subarachnoid space, 566 subdural space, 566 technic of, 576 stalk, 575 tract, 526, 530, 532, 542 vesicle, 5 7 5.- 576 Opticus (II nerve), 479 Ora serrata, 558, 561 TXDEX 633 Oral glands, cells of, 226 crescents of Gianuzzi, 227 demilunes of Hcidenhain, 227 mixed glands, 226 mucous glands, 226 serous glands, 226 technic of, 228 Orange G, 20 Organ an, 153 of Corti, 586, 589 acoustic neurones to, 481 cells of Claudius of, 587 Corti's arches, 587 tunnel, 587 Deiter's cells, 587 hair or auditory cells, 587 Hensen's cells, 587 lamina reticularis of, 587 Nuel's space of, 587 phalangeal processes, 587 pillar cells, 586 of Giraldcs (paradidymis), 348, 383 of hearing, 578; see also Ear blood-vessels of, 588 development of, 589 ear, external, 578 internal, 580 middle, 579 lymphatics, 588 nerves, 588 technic of, 590 of smell, 590 olfactory bulb, 591 mucosa, 590 tract, 592 technic of, 592 of taste, 593 cells of, 593 foliate papillffi, 593 gustatory canal, 593 intergeminal fibres of, 593 intrageminal fibres of, 593 taste buds, 231, 232, 438, 439, 593 technic of, 593 of vision, 554 blood-vessels of, 571 development of, 575 eyeball or bubus oculi, 554 eyelid, 573 .lacrymal apparatus, 573 lens, 570 lymphatics of, 572 Organ of vision, nerves of, 572 neurone systems of, 566 optic nerve, 479, 526, 530, 565 technic of, 576 of Zuckerkandl, 418 Organs, the, 153 of Golgi, peripheral nerve termina- tions in, 440 Orth's lluid (formalin-Muller's), 7 Osmic acid as a fixative, 8 action on fat, 8 on myelin, 8 stain for fat, 31 Osmosis, 47 Osseous labyrinth, 580 Ossicles of middle ear, 580 Ossification, 85 centres, J03 endochondral, 203, 205 intracartilaginous, 205 intramembranous, 203 subpcrichondral, 207 subperiosteal, 207 Osteoblasts, 203, 240 Osteoclasts, 198, 204, 240 Osteogenetic tissue, 203 Otic ganglion, 442 vesicle, 589 Otocyst, 589 Otolithic membrane, 581 Otoliths, 581 Oval bundle of Flechsig, 467 Ovary, the, 358 blood-vessels of, 369 corpora lutea, of pregnancy, 366- spuria, 366 vera, 366 corpus albicans, 366 haemorrhagicum, 365 luteum, 365 cortex of, 359 development of, 382 egg nest, 360 epoophoron, 369 Fallopian tube, 359, 370 germinal epithelium of, 360- Graafian follicles, 360 hsematoidin crystals, 366 hilum of, 359 lutein cells, 365 lymphatics of, 369 medulla of, 359 634 INDEX Ovary, nerves of, 369 ovarian stroma, 359 oviduct, 359, 370 ovum, 358, 362 paroophoron, 369 I'fluger's egg tubes or cords, 360 primitive Graafian follicle, 361 ova, 360, 384 rudimentary structures connected with, 369 secretion of, 358 structure of, 359 technic of, 371 tunica albuginea, 359 zona vasculosa, 359 Oviduct, the, 370; see Fallopian tubes Ovula Nabothi, 374 Ovum, the, 53, 59, 362 atresia of follicle. 369 deutoplasm granules, 363 development of, 362 fertilization of, 59, 364 germinal spot, 363 maturation of, 59, 363 perivitelline space, 363 primitive, 360, 384 segmentation of, 62 yolk granules of, 363 zona pellucida of, 363 Oxyhaemoglobin, iii Oxyntic cells, 254 Oxyphile cells, 411 Pacchionian bodies, 430 Pacinian bodies, 202, 439 corpuscles, 211, 356, 440 Palate, mucous membrane of, 225 Palatine tonsils, 182; see Tonsils Pallial connections, 467, 468, 470, 476, 483. 505, 507, 510. 523> 52S, 529, 541 Pallio-pontile fibres, 507, 510, 523 Pallio-ponto-cerebellar path, 505, 507 Pallio-spino-peripheral efferent conduc- tion path, 571 Pallio-thalamic fibres, 541 Pallio-tectal fibres, 541 Pallium, 423, 539, 542 cortical areas of, 542 fibres of, 539, 541 Pancreas, the, 286 blood-vessels of, 291 Pancreas, cell-islands of Langerhans, 290 centro-acinar cells of Langerhans, 289 development of, 303 duct of Santorini, 287 of Wirsung, 287 ductus choledochus, 303 excretory ducts, 287 intracellular secretory tubules of, 290 lobules of, 286 lymphatics of, 291 nerves of, 291 Opie, concerning cell-islands, 291 secondary excretory duct of, 287 secretion of, 288 sustentacular cells of, 290 technic of, 292 terminal tubules of, 287 zymogen granules of, 287 Paneth, cells of, 265, 278 Panniculus adiposus, 388 Papillae, circumvallate, 230, 593 compound, 387 filiform, 230 foliate, 593 fungiform, 230, 593 nerve, 387 of hair, 396 of mouth, 225 of skin, 387 pharynx, 247 simple, 386 vascular, 387 Paradidj-mis, or organ of Giraldes, 34S, 383 Paraffin embedding, 13 apparatus for, 14 oven, 13 section-cutting, 15 sections, staining and mounting of, 24 Paraganglia, 415 carotid gland, 416 chromatTm organs, 416 coccygeal gland, 417 organ of Zuckerkandl, 418 tympanic gland, 418 Paralinin, 48 Paramiton, 45 Paranuclein, 49 Paranucleus, 218 Paraplasm, 46 Parathyreoids, 410 chief i)T clear cells of, 411 I INDEX 635 Parathyreoids, development of, 412 function of, 412 oxyphilc cells, 411 Pool's theory of, 412 technic of, 412 Pareleidin, 390 Parenchyma, 153 of glands, 220, 282 Parietal cells," 254, 255 peritoneum, 272, 440 pleura, 314 Paroophoron, 369 Parotid gland, 282 development of, 303 nerves of, 285 Stenoni's duct of, 282 technic of, 286 tubules of, 282 Pars ciliaris retinae, 561, 565 iridica retinae, 561, 565 optica retina;, 561 papillaris, 387 reticularis, 386 Pavlow, concerning secretion, 278 Peduncle, cerebellar, inferior, 505, 506, 507, 514 middle, 505, 514 superior, 505, 506, 508, 510, 513, 514, 523 cerebral, 523, 527, 532 Pellicula, 47 Penicillus, 188 Penis, 355 arteries of, 356 cavernous sinuses, 356 corpora cavernosa of, 355 corpus spongiosum of, 355 erectile tissue, 355 glans, 357 glands of Tyson of, 357 lymphatics, 356 nerve endings of, 356, 440 prepuce of, 357 sebaceous glands of, 357 technic of, 358 tunica albuginea of, 355 Pepsinogen granules, 277 Peptic cells, 254, 272 glands, 254 Perforated space, anterior, 542 Perforating fibres, 197 of cornea, 556 Perforating fibres, of Sharpey, 197 Perforatorium, 350 Periaxial sheath, 138 Pericardial cavity, 169 Pericellular network, 143 Perichondria! ossification, 203 Perichondrium of bone, 205 of cartilage, 103 Perichorioidal lymph spaces, 557 Pericranium, 204 Peridental membrane, 239 Perifascicular sheath, 213, 431 Perifibrillar substance, 132 Perilymph, 580 Perimysium, 213 Perineurium, 431 Periosteal buds, 205 Periosteum, 196, 204, 205 Peripheral afferent neurones, 425, 432, 480 cerebro-spinal ganglia, 432 sympathetic ganglia, 442 efferent neurones, 426, 447, 481, 491 motor neurone system, 447 nerve terminations, 435 annular, 440 diffuse, 436 end-bulbs, 437, 439 free endings, 436 in penis, 356, 440 golgi jNIazzoni corpuscles, 400, 440 Grandry, corpuscles, 437 in mucous membrane of mouth and conjunctiva, 439 in muscle-tendon junctions, 440 in skin, 400, 436 in smooth muscle, 436 in voluntary muscle, 440 Krause's end-bulbs in penis, 357 Aleissner's corpuscles in papilke of penis, 357, 438 muscle spindles, 440 muscle-tendon organs of Golgi, 440 nerve terminations, Ruffini's the- ory of, 440 neuromuscular bundles, 440 Pacinian bodies, 202, 439 corpuscles of penis, 357 spiral terminations, 440 tactile cells, 437 corpuscles, 438 meniscus, 437 taste buds, 438 636 INDEX Peripheral nerves, 425, 430 afferent part of, 430 cranial, 430 efferent part of, 430 endoneurium of, 431 epineurium of, 431 fascicles of, 431 intrafascicular connective tissue of, 431 meduUated fibres of, 137 motor or cfTerent, 430 motor nerve terminations, 447 non-medullated fibres of, 136 perifascicular sheath of, 431 perineurium of, 431 sensory or afferent, 430 sensory nerve terminations, 435 sheath of Henle, 432 spinal, 430 structure of, 430 technic of, 432 nervous sj'stem, 422 spinal nerves, 430 Peritoneal cavity, 169 Peritoneum, 272 parietal, 272 subserous tissue of, 273 visceral, 272 Perivitelline space, 363 Perpendicular fasciculus of Wernicke, 541 Pes pedunculi, 423, 523, 527, 532, 536, 542 Petit, canal of, 571 Petrosal ganglion of IX, 480 Peyer's patches, 174, 265 Pfliiger's egg tubes or cords, 360 Phaeochrome granules, 418 Pha^ochromoblasts, 421 Phagocytes, 52, 178 Phagocytosis, 52, iii Phalangeal processes, 587 Pharyngeal tonsils, 184; see Tonsils Pharynx, the, 247 blood-vessels of, 248 lymph nodules, 184, 247 lymphatics of, 248 nerves of, 248 . structure of, 247 technic of, 248 Phloroglucin, n Pia mater, 428 arachnoid, 428 blood-vessels of, 430 Pia mater, cerebralis, 428 Pacchionian bodies, 430 spinalis, 428 technic of, 430 Picric acid as a fixative, 9 as plasma dye, 20 Picro-acid-fuchsin, 21 Picro-carminc, 21 Pigment granules in cells, 46, 390 in connective tissue, 89 in epithelium, 73, 80 of hair, 394 in iris, 560 in nerve cells, 135 Pillar cells, 587 Pineal body, 550 brain sand of, 553 technic of, 553 eye, 550 Pinna, 578 Pituitary body, 413 anterior lobe, 413 Berkle}-, concerning posterior lobe,. 414 chief cells of, 413 chromophile cells of, 413 function of, 415 middle lobe, 415 posterior lobe, 413 relation of, to pregnancy, 415 Placenta, 377 blood-vessels of, 380, 381 canalized fibrin, 379 cell patches, 379 chorionic villi, 377 cotyledons, 377 fastening villi, 377 foetalis, 377 free or floating villi of, 377 lymphatics of, 381 membrana chorii of, 377 nerves of, 381 nuclear groups, 379 septa of, 380 subchorionic placental decidua, 380 syncytium of, 379 technic of, 385 uterina, 379 villi of, 377 Plasma cells, 89, 199 dyes, 17, 20 acid aniline, 20 i IXDEX 637 Plasma dyes, eosin, 20 neutral carmine, 20 picric acid, 20 Plasmosome, 49 Plastids, 46 Plastin, 44 Platelets, blood, 112 Pleura, parietal, 314 pulmonar*-, 314 Pleural cavity, 169 Pleuroperitoneal cleft, 160 Plexiform layer of Cajal, 544 Plexus annularis, 573 Auerbach's, 268, 276, 442 ciliary, 572 chorioideus, 423, 488, 493, 508 Heller's, 274 interradiary, 547 Meissner's, 259, 268, 269, 276, 442 myentericus, 276 prevertebral, 442 supraradiary, 547 PliciE palmate, 374 Pneumogastricus (vagus nerve), 480, 495 Polar bodies, 60, 364 globules, 62 rays, 54 Polykaryocytes, 198 Polymorphonuclear leucocytes, 109 Polynuclear leucocytes, 109 Pons Varolii, 423, 488, 505, 507, 521 longitudinal fibres of, 505, 507 perpendicular fibres, 505. 507, 513 pontile nuclei of. 507 pyramid of, 507 transverse fibres of, 507 Pool, E. H., concerning parathyreoid gland, 412 Poroophoron, 369 Portal canal, 295 vein, 294 Posterior column of Burdach, 455, 465, 467, 468, 491 of Goll, 455, 465, 468, 491 columns of spinal cord, 449, 454, 467 origin of fibres of, 449 commissure, 455, 528. 532 corpus quadrigemium, 484, 499 distribution of fibres of, 454 funiculus, 454, 467 horns, 454 Posterior longitudinal fasciculus, sec Lo)i- giludinal fasciculus, medial median septum, 454 nerve root, 456 root fibres, 456 nucleus funiculi cuneati, 468, 491 funiculi gracilis, 468, 491 origin of, 449 tract, or terminal zone of Lissauer, 449, 456 zone of Lissauer, 449 Potassium hydrate, as a macerating fluid, 5 Precapillary artery, 158 Prcdorsal fasciculus, 497, 510 Prcolivary nucleus, 506 Preparation of sections, 5 Prepuce, 357 Preserving, 10 Prevertebral plexuses, 442 Prickle cells, 389, 393 Principal sensory nucleus of nerve V, 512 Processus reticularis, 455 Projection fibres, 539, 541, 545, 549 Pronephric or Wolfiian ducts, 383 Pronephroi, 382 Pronucleus, female, 60 male, 60 Prophase, 54 Proprio-ceptors, 442 Proprio-spinal tract of cord, 474 Prosencephalon, 422, 528 Prostate gland, 353 blood-vessels of, 354 corpora amylacea of, 354 crescentic corpuscles of, 354 lymphatics of, 354 Miillerian duct, 354 nerves of, 354 senile, 354 technic of, 355 uterus masculinus, 354 utriculus prostaticus, 354 vesicula prostatica, 354 Protargol, for staining intercellular sub- stance, 28 Prothrombin, 112 Protoplasm, 44 streaming of, 53 theories of structure of, 45 Protoplasma superieur, 218 Protoplasmic movement, 52 638 INDEX Protoplasmic processes, 135 Proximal convoluted tubule, 326 Prussian blue gelatin as an injecting fluid, -3 Pseudo-chromosomes, 218 Pseudopodia, 51 Pseudo-stratificd epithelium, 75 Pulvinar radiations, 530 thalami, 530 Pulmonary artery, 319 lobule, 314, 320 pleura, 314 system, 319 Pulp cavity, 232 cord, 188 splenic, 188 Purkinje cells, 515 Putamen, 541 Pyloric glands, 257 Pylorus, 257 Pyramid, cortical, 326 of Ferrein, 326 Malpighian, 326 Pyramidal cells, 542 decussation, 470, 491 tracts, 470 Pyramids, 423, 505, 508, 510 Pyrenin, 49 Pyriform lobe, 538 P^'rogallol for neurofibrils, 38 Racemose glands, 220 Radiations of Meynert, 547 Radix spinalis, V, 489, 493, 506 Rami communicantes, gray, 429, 443 white, 430, 442 Ranvier's alcohol as a macerating fluid, 4 nodes or constrictions of, 138 showing muscle fibres, 119 Raphe, of semicircular canals, 582 median, 494, 496, 513 Receptors, 425, 430, 431, 435, 442, 479 extero-ceptors, 442 intero-ceptors, 442 of special senses, 479 proprio-ceptors, 442 Sherrington's classification of, 442 somatic, 479 visceral, 479 Rectum, 271 anus, 272 columnar rectales, 272 Rectum, tcchnic of, 280 Red blood cells, 107; see also Blood nucleated, 113, 188 bone marrow, 199 nucleus, see Nucleus ruber Reduction of chromosomes, 59, 351 Reflex arc, disynaptic, 476 monosj-naptic, 475 three-neurone spinal, 475 two-neurone spinal, 475 Reissner, membrane of, 585 Remak, fibres of, 137 Renal artery, 324, 331 corpuscle, 326 development of, 383 vein, 324 Renculus, 324 Replacing cells, 75, 257 Reproduction of cells, 53 Reproductive system, 339 development of, 382 rudimentary structures connected with the, 348, 369 female organs, 358 clitoris, 382 Fallopian tube, 359, 370 ovary, 358 oviduct, 359, 370 placenta, 377 uterus, 372 vagina, 381 vestibule, 382 ■ male organs, 339 Cowper's glands, 355 ejaculatory ducts, 348 epididymis, 339 penis, 355 prostate gland, 353 seminal ducts, 345 vesicle, 348 seminiferous tubule, 340 spermatozoa, 343, 349 testis, 339 Respiratory bronchus, 313, 316 cells, 318 epithelium, 317 system, 305 bronchi, 310 development of, 321 larynx, 307 lungs, 314 nares, 305 I INDEX 639 Respiratory system, technic of, 310, 323 trachea, 307 Restiform body, 487, 405, 496, 497, 505, 507, 510 Rete testis, tubules of, 340, 345 vasa efiferentia of, 345 Reticular formation, 482, 491, 493, 495, 496, 505, 507 glands, 2,22 process, 454, 461 tissue, 98, 174 Reticulin, 99 Retina, 561 blood-vessels of, 571 ellipsoid of Krause, 563 fibre-basket of, 565 fovea centralis, 565 ijanglionic layer of, 561 horizontal cells of, 564 inner limiting membrane of, 564 layer of nerve cells, 564 of nerve fibres, 564 of neuro-epithelium, 561, 562 of pigmented epithelium, 561 of rods and cones, 562 macula lutca, 565 molecular layer, 563, 564 Miiller's cells and fibres, 564 nuclear layer, 563 ora serrata, 561 outer limiting membrane, 564 molecular layer, 564 nuclear layer, 562 pars ciliaris retinas, 561, 565 iridica retina?, 561, 565 optica retinas, 561 relation to optic nerve, 566 rod and cone cells of, 563 visual purple of, 563 Retinaculae cutis, 388 Retraction hypothesis, 142 Retrolenticular portion of internal cap- sule (Cirl), 532 Retzius, lines of, 238 Rhinencephalon, 423, 538 Rhinopallium, 538 Rhombencephalon, 423, 485 Ribboning paraffin sections, 15 Rod association neurones, 567 fibres, 563 neurones, 566 Rod-visual cells, 563 Rods, layer of rods and coaes, 563 Rolando, gelatinous substance of, 455 Root canal, 232 cells, 447, 450, 457 Roots, afferent, 425 Rose concerning chromafiin cells, 416 Rubro-spinal tract, 472, 487, 489, 493, 497, 505. 510, 519, 521, 526 Ruffini, corpuscles of, 400, 440 theory of nerve terminations, 440 Ruga;, 251, 253 Riihle, concerning the uriniferous tubule, 331 Saccular glands, 219 Saccule, and utricle, 581 • auditory hairs of, 581 macula acustica, 581 neuro-epithelial cells of, 581 otolithic membrane of, 581 otoliths of, 581 sustentacular cells of, 581 Sachs, E., concerning thalamus, 530 Sacral segments of spinal cord, 464 Safranin, 20 Salivary corpuscles, 184 glands, 281 blood-vessels of, 285 development of, 303 general structure, 282 interstitial tissue, 282 lymphatics of, 285 minute structure of, 226 nerves of, 285 parenchyma of, 282 parotid, the, 282 sublingual, the, 283 submaxillary, the, 284 technic of, 286 tubules, 282 Santorini, cartilage of, 307 duct of, 287 Sarcolemma, 118 Sarcoplasm, 119 Sarcostyles, 214 Sarcous elements of Bowman, 120 Satellite cells, 433 Scala media, 584 tympani, 584 vestibuli, 584 Scarpa's ganglion, 481, 499 Schaffer's formalin-alcohol for fixation, 7 <)40 INDEX Schaper, concerning placenta, 377 Scharlack R method for staining fat, 31 Schlemm, canal of, 560 Schmidt-Lantermann, clefts of, 139 incisures of, 139 segments of, 139 Schreger, lines of, 237 Schultze, comma tract of, 473 Schwalbe, lymph paths of, 572 Schwann, sheath of, 137, 139 Schweitzer, concerning lymph-vessels of teeth, 241 Sclera, the, 554 Sebaceous glands, 357, 390, 397 development of, 220 of glans penis, 357, 390 of labia minora, 390 of margin of lips, 390 of prepuce, 390 of skin, 397 Sebum, 398 Secondary cochlear tract, 499, 510, 513 dentine, 237 trigeminal tract, 489 vestibular tract, 499 Secretion, 46, 218, 262, 277 Langley and Sewell concerning, 277 Theohara and Bensley concerning, 277 Secretory tubules, 282 Golgi method of demonstrating, 29 of parietal cells of stomach, 256 Section cutting, 14 celloidin specimens, 15 frozen sections, 16 paraffin specimens, 15 ' staining, 17, 20 Segmental brain, 426, 480 nerves, 426, 480 paths, 482 Segmentation cavity, 62 of ovum, 61 Segments of Schmidt-Lantermann, 139 of spinal cord, 449, 461 Semen, 349 Semicircular canals, 481, 582 crista acustica of, 582 cupula of, 582 raphe of, 582 semilunar, fold of, 582 Semilunar fold, 582 ganglion of V, 480 Semilunar nucleus, 506 Seminal ducts, 345 vas epididymis, 346 vas deferens, 346 vasa efferentia, 346 vesicles, 348 Seminiferous tubules, 340 cells of, 341 columns of Sertoli, 341 convoluted tubule of, 340 development of, 350 glandular cells of, 341 interstitial cells, 344 rete testis, 340, 345 spermatids, 343 spermatocytes, 343 spermatogenic cells, 342 spermatogones, 342 spermatozoa, 340 straight tubule, 345 supporting cells of, 341 sustentacular cells of, 341 Sensation, cutaneous, 442 deep, 442 superficial, 442 Senses, common, 422 general, 442 special, 442 Sensory decussation, 491, 493 path, general, see Efferent paths peripheral nerves, 430 Septa renis, 326 Septo-marginal tract, 473 Septum lingua?, 229 Serial sections, 15 Serous cells, 226 glands, 226 membranes, 169 Sertoli, cells of, 341, 384 columns of, 341 Sewell and Langley concerning secretion, 277 Sex cells, 384 Sharpey, showing bone lamellae, 196 Sharpey's fibres, 197, 238 Sheath of Henle, 140, 431 medullary, 136, 138 myelin, 136 Neumann's dental sheath, 237 of Schwann, 136 perifascicular, 213, 431 Sherrington concerning receptors, 442 ■A INDEX 641 Short fibre IracLs, 451 Signet-ring cell, 92 Silver nitrate method, Golgi, 36 of slainin^' intercellular substance, 28 Sinusoidal circulation of liver, 304 Skein, closed, 55 Skeletal system, 193 articulations, 209 bone-marrow, 197 bones, 193 cartilages, 209 Skin and its appendages, 386 blood-vessels of, 405 color of, 390 corium, 386 corpuscles of Grandry, 437 of Meissner, 400, 438 of Merkel, 437 of Ruflini, 400, 440 of Wagner, 400 cuticle, 388 derma, 386 development of, 401 eleidin of, 389 end-bulbs in, 436 epidermis of, 388 glands of, 390 glandulse sudoriparae, 390 Golgi-jNIazzoni corpuscles of, 400, 440 hair follicles of, 393 junction of, with mucous membrane of mouth, 225, 390 keratohjalin granules, 389, 393 Krause's end-bulbs, 400 lymphatics of, 400 mammary gland, 402 mitosis of cells of, 389 nails, 391 nerves of, 400, 436 of nipple, 387 of scrotum, 387 Pacinian bodies of, 440 panniculus adiposus of, 388 papillae of, 387 pareleidin, 390 pars papillaris, 387 pars reticularis, 386 peripheral nerve terminations in, 400, • 430 prickle cells of, 389, 393 retinaculae cutis, 388 41 Skin, sebaceous glands of, 390, 397 smooth muscle of, 387 subcutaneous tissue of, 387 sweat glands (glandular suboriparae), 390 pores of, 390 tactile cells of, 437 corpuscles, 439 technic of, 390 for blood-vessels of, 401 Vatcr-Pacinian corpuscles of, 400 Small intestines, 260 agminated follicles, 265 Auerbach's plexus, 268 blood-vessels of, 273 Brunner's glands of, 267, 278 cells of, 261 columnar, 261 connective tissue, 264 goblet, 262 lymphoid, 265 mucous, 262 of Paneth, 265, 278 replacing, 264 secreting, 262, 264 smooth muscle, 264 wandering, 264 chyle capillaries of, 275 coats of, 261, 267, 268 crypts of Lieberkiihn, 265 Davidoff, concerning cells of stomach, 264 development of, 303 lacteals of, 280 lymphatics, 275 Meissner's plexus, 268 migratory leucocytes, 264 muscle of, 267 nerves of, 276 Paneth's cells, 265, 278 Peyer's patches of, 265 plexus myentericus, 276 solitary follicles, 265 technic of, 82, 280 valvulae conniventes of, 251, 260 villi of, 260, 261 Smell, organ of, 584 Smooth muscle, 115; see Involuntary muscle Sodium hydrate as a macerating fluid, 5 Solitary fasciculus, 493, 495, 497 follicles, 259 642 INDEX Somatic receptors, 479 (peripheral) neurones, 426, 479, 481 Somatochromes, 133 Spaces of Fontana, 560 Special denial germs, 243 senses, 442 Spermatids, 343 Spermatoblast, 351 Spermatocytes, 343, 350 Spermatogenesis, 350 technic of, 353 Spermatogones, 342 Spermatozoa, 59, 343, 349 acrosome, 350 anterior end-knob, 350 apical body, 350 axial thread, 350 development of, 59, 350 diagram of, 349 galea capitis, 350 perforatorium, 350 posterior end-knob, 350 structure of, 59, 350 technic of, 352 Sphenopalatine ganglion, 442 Sphincter pylori, 259 Spider cells, 147 Spinal accessory nerve, see Nerves, cranial Spinal cord, 422, 448 anterior columns of, 454 funiculus, 454 horns of, 450, 454 marginal bundle of Lowenthal, 472 median fissure, 454 pyramids, 471 white commissure of, 456 antero-lateral columns of, 454 funiculus, 454 white column, 454 arachnoid membrane of, 428 arrangement of fibres of, 458 arteries of, 460 ascending tracts of, 467 axonal method of determining fibres tracts, 465 blood-vessels of, 460 cell-groupings of, 456 cells of dorsal horn, 456 of Golgi, T^'pe II., 452 of the intermediate gray mutter, 456 of ventral iiorn, 457 Spinal cord, cell-groupings of outside cord, with axones to white matter of cord, 440 central canal of, 422, 455 gelatinous substance, 455 cervical enlargement of, 448, 461 segments of, 449 Clarke's column of, 456, 461, 469 coccygeal segments of, 449 collaterals of, 453 column of Burdach, 455, 465, 467, 468. of GoU, 455, 468 cells, 449, 450, 457 hecateromeric, 451 heteromeric, 451 tautomeric, 450 conduction paths of, 427, 467 cornua of, 454 descending paths from higher centres, 470 diagram showing tracts of, 466 direct ascending paths to higher centres, 469 cerebellar tracts, 469 pyramidal tract, 470 reflex collaterals. 458 dorsal gray columns, 454 commissure, 455 spino-cerebellar tract, 469 white columns, 454 commissure, 456 dura mater of, 428 ependyma of, 459 fasciculus, medial longitudinal, 473 of Thomas, 473 fibre tracts of, 465 methods of determining. 465 filum terminale of, 44S Flechsig's tract, 469 fundamental columns of, 451, 474 ganglion cells of, 449 gelatinous substance of Rolando, 455 general topography of, 454 Gowers' tract, 470 gray matter of, 426, 455 ground bundles of, 451, 474 Helweg's tract, 473 interchange of fibres, 458 intermediate gray matter, 456 lateral horn of, 456 ligamentum denliculatum, 455 INDEX 643 Spinal cord, long ascending arms of dorsal root fibres, 467 long ascending fibre tracts, 451 longitudinal section of six days' chick embryo, 453 lumbar enlargement of, 449, 454 segments of, 44Q main motor fibre systems of, 465 marginal' bundle of Lowenthal, 472 zone, 455 medial fillet (lemniscus), 468 membranes of, 428 arachnoid, 428 blood-vessels of, 460 dura mater, 428 pia mater, 428 spinal dura, 428 technic of, 430 mixed spinal ner\e, 430 motor cells of anterior horn, 450 multipolar ganglion cells of, 131, 445, 450 myelogenetic method of determining fibres, 465 neuroglia cells, 459 fibres, 459 neurone systems of, 427 nucleus, Darkschewitsch's, 471 Deiter's, 472 funiculi cuneati, 468 gracilis, 468 origin of fibres of white matter, 449 of posterior columns of. 449 oval bundle of Flechsig, 473 peripheral motor or efferent neurone system, 424, 447 sensory or afferent neurone system, 425, 432 pia mater, spinalis, 428, 454 posterior columns, 449, 454 funiculus, 454, 467 horns. 454 median septum, 454 nerve roots, 456 root fibres, 456 postero-lateral grooves, 454 sulci, 454 processus reticularis, 455 pyramidal decussation, 470 tracts, 470 reflex arcs, 474 relation to spinal ganglion, 449 Spinal cord, reticular process, 454, 461 root cells. 447, 450, 457 rubro-spinal tract, 472 sacral segments of, 464 schematic section of, 455 scheme of neurone relations of, 466 section through cervical enlarge- ment of, 461 through lumbar enlargement, 454 through mid-thoracic region, 461 through six-da_\' chick embryo, 453 through twelfth thoracic segment, 461 septo-marginal tract, 473 shape of, 448 short fibre systems of, 451, 468, 474 shorter intersegmental tracts, 474 size of, 448 spino tectal tract, 471 thalamic tract, 468 technic of, 451, 471 thoracic segments of, 449 tract from interstitial nucleus of Cajal, 471 tracts of, 449, 465; see Tracts variations in structure at different levels, 461 veins of, 460 ventral gray columns, 454 commissure, 455 white, 456 white columns, 454 white commissure, 456 matter, 426, 449, 456 zona spongiosa, 455 terminalis, 456 zone of Lissauer, 449, 456 Spinal ganglia, 432, 434 amphicj'tes, 433 capsule of, 432 development of, 424 technic of, 447, 452 ganglion cells, 435 442, 449 capsules of, 435 central processes of, 442 classification of, 433 collaterals from, 451, 453 descending arms from central pro- cesses of, 442 development of, 424 Dogiel's classification, 433 ectodermic origin, 422 644 INDEX Spinal ganglion cells, modes of termina- tion of peripheral processes of, 436 peripheral processes of, 435 Ruffini's classification of termi- nations in muscle spindles, 440 satellite cells, 433 sructure of, 432 technic of, '447, 452 nerves, 430 Spindle, achromatic, 54 Spino-cerebellar tract (dorsal), 469, 489 (ventral), 469, 489, 496, 507, 510, 521 Spino-collicular tract, 468 Spino-peripheral motor neurone system, 448 Spino-spinal tract, 474 Spino-tectal tract, 471, 489, 493, 496 Spino-thalamic tract, 468, 489, 493, 496, 499, 507, 5io> 521, 532 Spiral ganglion, 481, 499 lamina, 583 ligament, 583 limbus, 588 organ, 585 prominence, 585 sulcus, external, 585 terminations, 440 Spireme, closed, 55 open, 55 Spireme-thread, 56 Splanchnic effectors, 479 (peripheral) neurones, 425, 4S0, 481 Spleen, 185 ampullae, 188 blood-vessels, 187 cavernous veins, 188 cells of, 188 central arteries of, 188 connective- tissue framework, 186 cords of, 188 corpuscles of, 186 development of, 189, 190 function of, 190 germinal centres of, 187 lymphatics of, 191 IMalpighian bodies, 186 Mollier, concerning splenic pulp, 189 nerves of, 191 penicillus, 188 pulp of, 186, 188 Spleen, pulp of, cords of, 188 spindles of, 187 technic of, 191 Spleen-sinus, 188 Splenic corpuscles, 186 pulp, 188 Spheno-lymph nodes, 178 Spongioblasts, 147, 423 of His, 423 Spongioplasm, 44 Spongy bone (cancellous), 193 Staining, 17 differential, 4 double with h;umatoxylin-eosin, 20 in bulk, 22 methods, special, 28 chlorid of gold, 28 Golgi's chrome silver for secretory tubules, 29 Jenner's, for blood, 32 Mallory's aniline blue for connect- ive tissue, 30 phosphomolybdic acid hema- toxylin stain for connective tissue, 29 phosphotungstic acid ha^matoxj'- lin stain for connective tis- sue, 30 Maresh's modification of Bielsch- owsky's stain for fine connective- tissue fibrils, 2i^ osmic acid, foe fat, 31 siKer nitrate, for intercellular sub- stance, 28 Scharlack R, for fat, 31 Sudan III, for fat, 31 Verhoeff's " differential stain for elastic tissue, 28 Weigert's elastic-tissue stain, 28 paraffin sections, 24 sections, 20 double with ha^matoxyhn-eosin, 20 triple with ha^matox_\lin-picro-acid- fuchsin, 21 with picro-acid-fuchsin. 20 with picro-carmine, 21 selective, 4 special neurological methods, 33 Cajal's methods for neurofibrils in nerve cells, 38 Cox-golgi method, 37 Golgi bichlorid method, 37 INDEX ()45 Staining, special (iolgi silver method, 36 Marchi's, for degenerating nerves, 35 Nissl's method, 39 Weigert's, for medullatcd nerve fibres, SS Weigert-Pal method, 34 Stains, nuclear, dyes, 17 plasma dyes, 20 Stalked hydatid, 348 Stapes, 580 Stellate cells, 233, 516, 542 Stenoni, duct of, 282 Stohr, concerning muscle fibres, 123, 214 concerning th\mus, 182 scheme of spleen, 187 Stomach, 252 absorption of, 277 acid cells of, 255, 277 adelomorphous cells of, 254 Auerbach's plexus, 276 blood-vessels of, 273 cardiac glands of, 257 chief cells of, 254, 277 delomorphous cells of, 254 development of, 253 epithelium of, 253 fundus glands of, 254 gastric glands of, 253 pits of, 253 Heller's plexus, 274 Ij'mphatics of, 275 Meissner's plexus, 276 mucous membrane of, 253 muscular coat of, 259 nerves of, 276 oxyntic cells, 255 parietal cells of, 255 peptic cells of, 254, 277 glands of, 254 plexus myentericus, 276 pyloric glands of, 257 replacing cells, 257 rugae of, 251, 253 secretion of, 277 solitary follicles of, 259 stroma of, 258 submucosa, 259 technic of, 259 tubules of, 256 tunica propria of, 258 Slomata, 170 Straight tubules of testis, 245 Stratum cinereum, 528 corncum, 223, 389 cj'lindricum, 388 fibrosum, 210 germinativum, 388 granulosum, 223, 361, 389 Icmnisci, 528 luciilum, 223, 389 Maipighii, 388 mucosum, 388 opticum, 528 spinosum, 389 submuscosum, 372 supravasculare, 372 synoviale, 210 vasculare, 372 zonalc, 528 Streaming of protoplasm, 53 Stria cornea, 537 meduUaris, 497, 536 of Baillarger, 547 terminalis, 537 vascularis, 585 Stria; thalami, 528 Styloglossal fibres, 229 Subarachnoid space, 428 Subchorionic placental decidua, 380 Subcutaneous tissue, 388 Subdural space, 428 Sublingual gland, 283 crescents of Gianuzzi of, 283 development of, 303 duct of Barthohn of, 283 nerves of, 285 technic of, 286 Sublingualis minor, 284 Submaxillary ganglion, 442 gland, 284 development of, 303 duct of Wharton of, 284 nerves of, 285 technic of, 186 Submucosa, 223 Subperichondrial ossification, 207 Subperiosteal ossification, 207 Subserous tissue, 273 Substantia alba, 426 grisea, 426 nigra, 135, 523, 532 propria corneae, 556 646 INDEX Sudan 111 and Scharlack R method for staining fat, 31 Sudoriferous glands, 390 Sulcus, external spiral, 585 Superficial sensation, 441 Superior cerebellar peduncles, 505, 506, 508, 510, 513, 514, 523 colliculus, 423, 523, 528 ganglion of IX, 480 longitudinal fasciculus, 491, 500, 504, 515, 526 olive. 499, 506, 510, 513 Suprachorioidea, 557 Supraradiar\' plexus, 547 Suprasegmental arc, 427 brain, 484 cerebral hemispheres, 426, 484 connections (afferent and efferent), 482 corpora quadrigemina, 426, 484 intersegmental nuclei and tracts of segmental brain, 485 intrasegmental nuclei and tracts of segmental brain, 485 nuclei and tracts forming supraseg- mental paths, 485 pallium, 426, 484 paths, 482 ■• afferent, 482 efferent, 483, 491 peripheral (segmental) neurones, 485 structures, 484 terminal nuclei, 485 neurones, 427 afferent, 427 associative, 427 efferent, 427, 493, 497, 505 Suspensory ligament, 571 Sustentacular cells, 290, 306, 341 Sweat glands, 390 development of, 401 ducts of, 390 muscle tissue of, 402 pore, 390 Sympathetic ganglia, 424. 442 cells of, 445 chain ganglia, 442 development of, 424 in Aucrbach's plexus, 442 in Meissner's plexus, 442 pigmentation of cells of, 445 Sympathetic ganglia, prevertebral plex- uses, 442 structure of, 442 technic of, 452 termination of nerves, 446 vertebral ganglia, 442 nervous system, 422 diagram of, 444 Synapsis of neurones, 142 Synarthrosis, 209 Synchondrosis, 209 Syncytial cells, 423 Syncytium, 70 of placental villi, 379 Syndesmosis, 209 Synovial membrane, 210 villi, 210 System, a, 70 Szymonowizc, showing intercellular bridges, 77 showing meduUated nerve fibre, 140 Tactile cells, 437 compound, 438 simple, 437 corpuscles, 228, 438 of ^leissner, 400, 438 of \\'agner, 400 meniscus, 437 Taenia thalami, 536 Tapetum cellulosum, 557 . fibrosum, 557 Tarsal glands, 574 Tarsus, 574 Taste buds, 231, 232, 438, 439, 593 organ of, 593; see Organ of Taste Tautomeres, 450, 452, 463 Teasing, 4 Technic, general, 3 Tecto-spinal tract, see Tract, tecto- spinal Teeth, 232 apical foramina, 233 blood-vessels of, 240 cementum of, 232, 238 crown of, 232 culicula dentis, 238 dental germ, 243 groove, 243 papilla, 242, 243 periosteum, 239 pulp, 233 INDEX G47 Teeth, denial sac, 244 shelf. 243 dentinal canals, 235, 246 fibres, 233, 235, 246 dentine of, 232, 233 development of, 243 common dental germ, 243 cuticular mvmbranc, 245 dental papilla, 243 enamel organ, 244 mumhrana preformativa, 246 special dental germ, 243 technic of, 247 Tomes' process, 245 enamel of, 232, 238 cells, 242 fibres, 238 organ, 244 prisms, 238, 245 fang of, 232 interglobular spaces, 237, 246 layer of Weil, 233 lines of Retzius, 238 of Schreger, 237 lymphatics of, 241 milk, 244, 245, 246 neck of, 232 nerves of, 241, 436 Neumann's dental sheath, 237 odontoblasts of, 233, 246 peridental membrane, 239 permanent, 244, 247 pulp cavity, 232, 246 root of, 232 root-canal of, 232 special dental germs, 243 technic of, 247 Tomes' granular layer, 237 process, 245 true molars, 247 wisdom, 247 Tegmental tract, central, see Tract, central tegmental Tegmentum, 423, 485, 488, 521, 523 Telencephalon, 423, 538 Telophragma, 121 Telophase, 57 Tendon, structure of, 95 sheaths, 213 Tendon-muscle junction, 214 organs of Golgi in, 440 peripheral-nerve terminations in, 440 Tenon, capsule of, 572 Tensor chorioideic, 560 Terminal arborizations, 136, 436 bronchus, 316 ganglia, 442 nucleus, 427, 442 Terminations, nerve, 435 annular, 440 arborescent, 440 Ruffini's theory of, 440 spiral, 440 Testis, 339 blood-vessels of, 348 convoluted tubules of, 340 corpus Highmori, 339 development of, 382 ducts of, 340, 345, 348 ejaculatorj', 347 epididymis of, 346 h'mphatics of, 348 mediastinum, 339 nerves, 349 rete, 345 secretion of, 349 semen, 349 seminal ducts of, 345 vesicles, 348 seminiferous tubules of, 340 spermatozoa, 343, 349 technic of, 352 tubules of non-active, 344 tunica albuginea of, 339 vaginalis, 339 vasculosa, 339 vas deferens, 346 Thalamencephalon, 528 Thalamic radiations, 529, 532 Thalamo-cortical neurones, 468, 529 Thalamus, 423, 529, 532, 536 anterior peduncle of, 542 bundle of Vicq d'Azyr, 530 external segment of, 529 geniculate bodies, 526 internal segment of, 529 mamillo-thalamic tract, 530 metathalamus, 529 nuclei of, 529 nucleus of Luys, 529 pulvinar, 529 Sachs, E., concerning the, 529 thalamic radiations, 529, 532 Theca folliculi, 362 648 INDEX Theoharra and Bcnsle}- concerning secre- tion, 277 Thermostat, 13 Thermotaxis, 52 Thionin, 20 Third ventricle, 423 Thomas, fasciculus of, 473 Thoracic duct, 169 Three-neurone afferent suprasegmental conduction path, 427 spinal reflex arc, 475, 512 Thrombin, 112 Thrombocytes, 112 Thymus, 179 blood-vessels of, 181 development of, 181 Hassall's corpuscles, 181 lymphatics of, 181 nerves of, 181 technic, 182 Thyreoid, 408 absence of, 409 blood-vessels of, 409 cartilage, 307 cells of, 409 colloid of, 408 development of, 409 isthmus of, 408 lymphatics of, 409 nerves of, 409 Timofeew, concerning nerve fibres in prostate gland, 454 Tissue, 70 -elements, dissociation of, 4 Tissues, 71 adipose, 91 aponeurotic, 96 areolar, 91 blood, 71, 107 bone, 104 cartilage, 102 classification of, 71 connective, 84, 86 derivatives from ectoderm entoderm, mesoderm, 69, 70 dissociation of, 4 elastic, 96 embryonal connective, 86, 202 endothelium, 80 epithelial, 70 erectile, 355, 382 examination of fresh, 4 . . Tissues, histogenesis of, 69 interstitial, 153, 220 lymphatic 171 lymphoid, 174 mesothtiium, 80 muscle, 71, 115, 213 nerve, 71, 130 osteogenetic, 203 reticular, 98, 174 subcutaneous, 388 subserous, 273 white fibrous, 90 Toluidin blue, 20 Toluol, as solvent, 14 Tomes' granular layer, 237 process, 245 Tongue, 228 blood-vessels of, 231 circumvallate papilla?, 230 connective tissue of, 229 Ebner's glands, 231 end-bulbs of Krause, 232 fibres of, 229 filiform papillae, 230 fungiform papillae, 230 glands of, 227, 231 longitudinal fibres of, 229 lymph follicles of, 184, 231 spaces of, 232 mucous membrane of, 229 muscles of, 228 nerves of, 232 papillae of, 229 septum linguae, 229 taste buds, 231, 232, 438, 593 technic of, 232 transverse fibres of, 229 vertical fibres of, 229 Tonsils, 182 blood-vessels of, 184 crj'pts of, 183 development of, 184 germ centre of, 183 lingual; folliculse linguales, 184 foramen caecum linguae of, 184 lymphatics of, 184 lymphoid infiltration of epithelium, 183 nerves of, 184 nodule of, 183 palatine or true, 182 pharyngeal, 184 INDEX 649 Tonsils, pharyngeal, adenoids of, 184 salivary corpuscles of, 184 technic of, 185 Trachea, 307 blood-vessels oi, 309 cartilages of, 308 crescents of Gianuzzi, 30S glands of, 308 h'mphatics of, 310 nerves of, 310 technic of, 310 Tract, a, 427 antero-lateral ascending-v c n t r a 1 spino-cerebellar, 469 antero-lateral descending, 472 Burdach's, 465, 467 central tegmental, 496, 505, 507, 513 cerebro-spinalis, 470 cochlear, 497, 506, 510 colliculo-spinal, 471, 484, 491, 496, 505, 510, 513, 523 comma, of Schultze, 473 cortico-spinalis, 470 crossed or lateral, pj^ramidal, 470, 491 Deitero-spinal tract, 472, 473, 484, 489, 493, 497, 505, 508 descending from pallium to motor nuclei, 539 direct cerebellar, 469 pyramidal. 471, 491 dorsal spino-cerebcllar, 469, 489, 493, 496, 507 dorso-lateral ascending — dorsal spino- cerebellar, 469, 489 fasciculus of Thomas, 473 fastigio-bulbar, 506, 508 Flechsig's, 469, 473 from interstitial nucleus of Cajal, 471,491, 493 from nucleus of posterior longi- tudinal fasciculus, 471 from vestibular nuclei, 472 fundamental or ground bundles, 451, 474 Goll's, 465, 467 Gower's, 469, 470 Hehveg's, 473 intersegmental (shorter), 451, 474 Lissauer's, 449, 456 long ascending arms of dorsal root fibres, 467 mamillo-thalamic, 530, 538 Tract, marginal bundle of Ltiwenthal, 472 medial lemniscus, 468, 491, 49?, 495, 497> 499. 503. 523 longitudinal fasciculus, 471, 473, 482, 484, 491, 497, 505, 506, 508, 510, 513, 521, 524, 527, 528, 532, 536, 542 olfactory, 481 optic, 526, 530 oval bundle of Flechsig, 473 pallio-spinalis, 470, 491, 539 -tectal, 541 posterior columns, 467 funiculi, 454, 467 posterior longitudinal fasciculus, see Tract, medial longitudinal fas- ciculus predorsal, 497 fasciculus, 510, 523 pyramid, 470, 539, 591 anterior, 471 rubro-spinal, 472, 483, 487, 489, 493. 497, 505, 510, 519, 521, 526 secondary cochlear, 499, 510, 513 trigeminal, 491, 495 vago-glossopharyngeal and trigem- inal, 491, 521, 532 vestibular, 499 septo-marginal, 473 short fibre, 451, 474 spinalis trigemini, 489, 506 spino-cerebellar, ventral, 469, 489, 493. 496, 499. 507. 510 -colHculo, 468, 489, 491, 493, 496, 505. 508, 510, 523 -spinal, 474 -tectal, 471, 489, 493, 496 -thalamic, 468, 489, 493, 496, 499, 507, 510, 521, 532 tecto-spinal, 471, 484, 491, 496, 505, 510, 513, 523 uncrossed cerebellar, 469 ventral spino-cerebellar, 469, 489, 496, 507, 510. 521 vestibulo-spinal, 472 Von ^Monakow's, 472 Tractus, see Tract Transitional leucocytes, 109 Transverse temporal gyri of Heschl, 547 Trapezium, 499 Trapezius, 506 Trapezoid nucleus, 499, 506, 510 650 INDEX Trigeminus (V nerve), 481, 493, 495, 496, 499» 506, 510 Trigonum hypoglossi, 487, 495 olfactorium, 538 vagi, 487 Triple staining, 21 Trochlearis (IV nerve), 481, 521, 524, 526 Trophic centre of neurone, 141 Trophospongium, 47 True corpora lutea, 366 tonsils, 182 vocal cords, 301 Tuber cinereum, 537 Tuberculum cinereum, 487, 537 olfactorium, 538 Tubular glands, 219, 221, 324 Tubules arched, 329 collecting, 330 convoluted, 328 distal, 329 lirst or proximal, 328 intercalated, 282 intercellular secretory, 256 intermediate, 282, 284 intracellular secretory, 256 junctional, 377 mucous, 227 salivary, 282 second or distal, 320 secreting. 282 seminiferous, 340 serous, 226 straight, 330 terminal, 283 uriniferous, 326 Tubulo-alveolar gland, 287 Tunica albuginea, of ovary, 359 of penis, 355 of testis, 339 dartos, 387 fibrosa, 362 propria, 22^, vaginalis, 339 vasculosa, 339, 348, 362 Two-neurone spinal reflex arc, 475, 510 Tympanic gland, 415 membrane, 579 Tympanum, 579; see also Ear, middle Tyson, glands of, 357 Ultimate fibrilhc, 121 Uncinate fasciculus, 441 Unipolar nerve cells, 131, 433 Ureter, 335 development of, 382 technic of, 338 Urethra, female, 357 glands of Littre of, 358 male, 357 fossa navicularis, 358 glands of I^ittre of, 358 technic of, 358 Urinary bladder, 336 development of, 382 system, 324 developement of, 382 kidney, 324 -pelvis, 335 technic of, 338 ureter, 335 urinary bladder, 336 Uriniferous tubules, 326 arched tubule of Henle's loop, 326, 329 blood-vessels of, 331 Bowman's capsule, 326, 328 cortical labyrinth, 330 descending arm of Henle's loop, 326, 328 development of, 383 duct of Bellini, 327, 330 epithelium of, 331 lirst or proximal convoluted, 326, 328 foramina papillaria, 327 glomerulus, 326 Henle's loop, 326, 329 Malpighian body, 326 membrana propria of, 327 neck of, 328 renal corpuscle, 326 Riihle, concerning epithelium of, 331 second or distal convoluted, 326, 329 straight or collecting, 327, 330 Utericulo-saccular duct, 581 Uterus, 372 blood-vessels of, 381 cervix, 372 coats of, 372 . decidua basalis, 376 capsularis, 376 graviditatus, 376 menstrualis, 375 reflexa, 376 serotina, 376 - INDEX 051 Uterus, deciduii suhchorionic placental, 380 vera, 376 decidual cells of, 376, development of, 382 lymphatics of, 381 mucosa of menstruating, 374 of pregnant, 376 of resting, 373 nerves of, 381 placenta, 377 plicai palmatse, 374 pregnant, 376 theories concerning, 375 stage of menstruation proper, 375 of preparation, 374 of reparation, 375 stratum submucosum, 372 supravasculare, 372 vasculare, 372 technic of, 385 willa placenta in sihi, technic of, 385 Uterus-masculinus, 354 Utricle. 581; see Saccule and utricle Utriculus prostaticus, 354 Uvula, mucous membrane of, 225 V'agina, 381 blood-vessels of, 382 coats of, 381 lymphatics of, 382 nerves of, 382 rugae of, 382 technic of, 382 V'agus (X) nerve, 480, 495 \'alve, Heisterian, 301 Valves of heart, 166 of veins, 162 Valvulae conniventes, 251, 260 Van Gieson's picric-acid-fuchsin stain, 21 Vas deferens, 346 technic of, 352 epididymis, 346 Vasa efferentia, 346 vasorum, 163 Vascular papillae, 387 system, 155; see also Circulatory system \'ater-Pacinian corpuscles, 400 Veins, 162 adventitia of, 162 anterior median, 460 X'eins, arcuate, 334 cavernous, 188 central, 294 coats of, 162 development of, 167 intima of, 162 lymph channels of, 163 media of, 162 musculature of, 163 nerves of, 163 perivascular lymph spaces of, 163 portal, 29s posterior median, 460 renal, 324 splenic, 187 stellate, of Verheyn, 334 sublobular, 294 technic of, 164 valves of, 162 vasa vasorum, 163 venae vorticosae, 557 Venae vorticosae, 557 Ventral horns, 441, 450, 457; see Trad, ventral spino-cerchcllar Ventricles of brain, 422, 423, 488, 495, 505 of heart, 165 Verhe\-n, stellate veins of, 334 Verhorff's differential elastic-tissue stain, 28 Velum, superior medullary, 521 \'crmiform appendix, 270 lymph nodules of, 271 mesoappendix, 270 technic of, 280 \'ermis of cerebellum, 513 Vertebral ganglia, 442 Vesicle, air, 316 brain, 422 germinal, 59 optic, 575 otic, 589 seminal, 348 Vesicula prostatica, 354 Vestibular ganglion, 481, 499 nerve, 43 1. 44 , 506 nuclei, 4 48 descending tract from, 447 Vestibule, 581 ductus reuniens of, 581 endolymphatic duct, 581 sac, 581 saccule of, 581 652 INDEX Vestibule, utricle of, 581 utriculo-saccular duct of, 581 Vestibule-semicircular canal group of seg- mental neurones, 481 -spinal tract, 472 Vicq d'Azyr bundle of, 530, 538 \'illi, 261 chorionic, 377 fastening, 377 floating, 377 free, 377 of small intestine, 261 synovial, 210 terminal, 377 Visceral neurones, 425, 481 peritoneum, 224, 272 Vision, organ of, 554; see Organ of Vision Visual area, 494 group of segmental neurones, 481 path, 481, 524 purple, 563 Vital properties of cells, 50, 218 function, 51 irritability, 52 metabolism, 51, 218 motion, 52 reproduction, 53 Vitreous body of the eye, 571 Cloquet's canal, 571 hyaloid canal of, 571 membrane of chorioid, 559 of iris, 559 Vocal cords, 307 Volkmann's canal, 196, 202 Voluntary striated muscle, 118; see Mus- cle, striated, voluntary Von Bechterew's nucleus, 499, 506, 513 Von Bibra, concerning chemical composi- tion of dentine, 233 of enamel, 238 Von Gudden, concerning method of deter- mining fibre tracts of cord, 465 Von Monakow's bundle, 472 Wagner, corpuscles of, 400 Wallerian degeneration, law of, 144 Wandering cells, 52, 87, 90, iii, 264 Washing after fixation, 9 Wax, ear, 579 Weigert's elastic-tissue stain, 28 hematoxylin, 19 method of staining meduUated nerve fibres, 33 Weigert-Pal method, 34 Weil, layer of, 233 Wernicke, perpendicular fasciculus of, 541 Wharton's duct, 284 jelly, 86 White blood cells (leucocytes), 109, 188 fibrous tissue, 86 or fibrillated fibres, 90 rami communicantes, 431, 443 Wilson, E. B., diagrams showdng mitosis, 55, 57 Wirsung, duct of, 287 Wolfliian body, 345, 383 Wrisburg, cartilage of, 307 nerve of, 507 Xylol and cajeput oil for clearing, 23 -balsam, 23 damar, 30 Xylol-paraffin for embedding, 14 Yellow or elastic fibres, 85, 97 Yellow bone marrow, 201 Yolk granules, 363 Zenker's fluid for decalcifying, 11 for fixation, 8 Zinn, zonule of, 571 Zona incerta, 536 pectinata, 586 pellucida, 62, 363 spongiosa, 455 tecta, 586 Zone of Lissauer, 449, 456 of oval nuclei, 306 of round nuclei, 306 Zonula ciliaris, 571 Zonule of Zinn, 571 Zuckerkandl, organ of, 418 Zymogen granules, 287 technic of, 292 I i PLEASE DO NOT REMOVE CARDS OR SLIPS FROM THIS POCKET UNIVERSITY OF TORONTO LIBRARY