DR. mm n^sLii G^fvl^''6'i PbK »^1<^ Columbia S!nit)er£?ftp tntlifCttpofiilmgork COLLEGE OF PHYSICIANS AND SURGEONS LIBRARY Digitized by tlie Internet Arcliive in 2010 witli funding from Open Knowledge Commons (for the Medical Heritage Library project) http://www.archive.org/details/textbookofnorma1899pier TEXT-BOOK OF Normal Histology: INCLUDING AN ACCOUNT OF THE DEVELOPMENT OF THE TISSUES AND OF THE ORGANS. BY GEORGE A. PIERSOL, M.D., PROFESSOR OF ANATOMY IN THE UNIVERSITY OF PENNSYLVANIA. IVITH FOUR HUNDRED AND NINE ILLUSTRATIONS, OF WHICH THREE HUNDRED AND FIFTY-EIGHT ARE FROM ORIGINAL DRA WINGS B Y THE A UTHOR. SIXTH EDITION. PHILADELPHIA: J. B. LIPPINCOTT COMPANY. Copyright, 1893, by J. B. LlPPINCOTT COMPANY. Copyright, 1899, by J. B. LlPPINCOTT Company. Electhotyped and Printed bv J. B. Lippincott Company, Philadelphia, U.&A. PREFACE. In the preparation of these pages the aim of the author has been to present _descriptions which should include the salient features of the various structures with sufficient fulness to impress important details without wearying minutiae : many years of teaching have con- vincingly shown that too great conciseness of statement, on the one hand, and too great elaboration of detail, on the other, are alike unsatisfactory to the student in his efforts to gain an adequate and lasting knowledge of minute anatomy. The recognition of the underlying morphological relations of the tissues alone can bring the appreciation of the broad principles requisite for the elevation of histology from a maze of barren details to a study full of interest and suggestion. In order that these wider bearings may become apparent, a brief account of the embryological processes and the histological differentiation concerned in the de- velopment of the tissues and the organs has been added to the descriptions of the adult structures. The desirability of keeping the size and scope of the volume within the limits adapted to its primary purpose of text-book has forbidden the systematic consideration of embryological data, and much of interest relating to the earlier stages of development has been necessarily omitted. In adopting the character of the illustrations choice has been influenced by the reflection that the mission of such drawings is instruction, and that the illustrations best accomplishing that end are of most value for the object at hand. With the exception of those taken from other, duly acknowledged sources, the drawings have been made by the author in nearly all cases with the aid of the camera lucida or from photo-micrographs. While sufficiently dia- grammatic to be efficient aids in the comprehension of the text, the drawings are faithful likenesses of the original preparations ; the latter as far as possible have been taken from human tissues. IV PREFACE. For manifest reasons, references and bibliography have been omitted, except in connection with statements where mention of the name of the authority has seemed desirable. The author wishes to express his obligation to the writings of KoUiker, Ranvier, Schwalbe, Waldeyer, Retzius, Stohr, Flemming, O. Hertwig, Schaefer, Golgi, Ram6n y Cajal, and others, as well as to many papers found in the Archiv fiir mikroskopische Anatomie and other journals. G. A. P. University of Pennsylvania, Philadelphia, September 30, 1893. CONTENTS. CHAPTER I. PAGES The Cell and the Tissues 11-25 The elementary tissues ; The typical cell ; Protoplasm — arrangement and structure ; The nucleus and the nucleolus ; The paranucleus ; The centrosome and the attraction-spheres ; Vital manifestations of the cell ; Direct cell-division ; Indirect cell-division — karyokinesis ; Maturation and fecundation of the ovum ; Segmentation of the ovum ; The tissues — cellular and intercellular constituents ; The blastodermic layers and their derivatives. CHAPTER II. The Epithelial Tissues 26-34 Varieties of epithelium ; Squamous epithelium ; Columnar epithelium ; Modified epithelium ; Glandular epithelium ; Neuro-epithelium ; Endothelium ; Development of epithelium ; De"velopment of endothelium. CHAPTER III. The Connective Tissues 35-57 Forms of connective tissue ; Cellular elements ; Intercellular constituents — white fibrous and yellow elastic tissue ; Mucoid tissue ; Tendon ; Elastic tissue ; Development of fibrous and elastic tissue ; Adipose tissue ; Hyaline cartilage ; Elastic cartilage ; Fibro-cartilage ; Development of cartilage ; Bone — spongy and compact ; Structure of compact bone ; CONTENTS. The periosteum ; The marrow of bone ; Development of bone ; Endochondral bone ; Periosteal bone ; Summary of bone-development. CHAPTER IV. The Muscular Tissues 58-68 Non-striated muscle— distribution and structure ; Striated muscle-structure ; Arrangement of muscle-fibres ; Cardiac muscle ; Development of muscular tissue. CHAPTER V. The Nervous Tissues 69-82 Nerve-cells ; Nerve-fibres — structure ; Medullated nerve-fibres ; Non-medullated nerve-fibres ; Nerve-trunks — structure ; Supporting tissues of nerve-centres ; Ganglia — structure ; Development of nervous tissues. CHAPTER VI. The Peripheral Nerve-Endings 83-93 Terminations of sensory nerves ; Special sensory nerve-endings ; Tactile cells ; Tactile corpuscles ; End-bulbs ; Nerve-endings in non-striated muscle ; Nerve-endings in striated muscle ; Nerve-endings in tendon ; Nerve-endings in blood-vessels ; Nerve-endings in glands ; Neuro-epithelium of the sftnse-organs. CHAPTER VII. The Circulatory System 94-ii4 The arteries ; The veins ; The capillary blood-vessels ; The heart ; Development of the blood-vessels ; Development of the heart ; The blood ; The colorless blood-cells ; The colored blood-cells ; Effect of reagents on human blood ; Blood-crystals ; Development of the blood-corpuscles. CONTENTS. Vil CHAPTER VIII. PAGES The Lymphatic System 115-135 The lymphatic spaces ; The lymphatic vessels ; The lymphatic tissues ; Simple lymph-follicles ; Compound lymph-glands ; The spleen ; The thymus body ; The serous membranes ; The synovial membranes ; Development of the lymphatic system ; Development of the spleen ; Development of the thymus body. CHAPTER IX. Mucous Membranes and Glands 13^-^43 Mucous membranes — structure ; Glands — varieties ; Tubular glands ; Saccular glands ; Glandular epithelium ; Glandular ducts ; Mucous glands ; Serous glands ; Changes due to functional activity ; Development of glands. CHAPTER X. The Digestive Tract = . . 144-190 The oral cavity ; The teeth ; Development of the teeth ; The tongue ; The papillae of the tongue ; The salivary corpuscles ; The tonsils ; The pharynx ; The oesophagus ; The stomach ; The glands of the stomach ; The intestines ; The intestinal villi ; The intestinal glands ; The glands of Lieberkiihn ; The glands of Brunner ; The solitary glands ; The agminated glands, or Peyer's patches ; The liver ; The gall-bladder ; The accessory digestive glands ; The parotid gland ; viii CONTENTS. PAGES The submaxillary gland ; The sublingual gland ; The pancreas ; Development of the digestive tract ; Development of the accessory digestive glands. CHAPTER XI. The Urinary Organs 191-206 The kidney ; The renal sinus and the ureter ; The urinary bladder ; The urethra ; Development of the urinary organs. CHAPTER XII. The Male Reproductive Organs 207-223 The testicle ; Spermatogenesis ; The epididymis ; The semen ; The penis ; The prostate gland ; The glands of Cowper. CHAPTER XIII. The Female Reproductive Organs 224-245 The ovary ; The ovum ; The escape of the ovum ; The parovarium ; The paroophoron ; The oviduct ; The uterus ; The vagina ; The genitalia ; The glands of Bartholin ; The mammary glands ; Milk ; Development of the reproductive organs. CHAPTER XIV. The Respiratory Organs 246-260 The larj'nx ; The trachea ; The bronchi ; The lungs ; The pleura ; The thyroid bod\' ; Development of the respiratory organs ; Development of the thyroid body. CONTENTS. ix CHAPTER XV. PAGES The Skin and its Appendages 261-281 The skin ; The nails ; The hair ; The sebaceous glands ; The sweat-glands ; Development of the skin and its appendages. CHAPTER XVI. The Central Nervous System 282-335 The membranes of brain and cord ; The spinal cord ; The medulla ; The pons ; The crura cerebri ; The cerebellum ; The cerebral cortex ; The hippocampus major ; Tne fascia dentata ; The fimbria ; The septum lucidum ; The corpus striatum ; The optic thalamus ; The corpora quadrigemina ; The olfactory lobe ; The white matter of the cerebrum ; The pituitary body ; The pineal body ; The suprarenal body ; Development of the nervous tissues. CHAPTER XVII. The Eye and its Appendages 3Z^2>7^ The cornea ; The sclera ; The choroid ; The ciliary body ; The iris ; The irido-corneal angle ; The retina ; The optic nerve and entrance ; The crystalline lens ; The vitreous body ; The blood-vessels of the eye ; The lymphatics of the eye ; The nerves of the eye ; The eyelids ; The Meibomian glands ; The conjunctiva ; The lachrymal apparatus ; The capsule of Tenon ; Development of the eye. CONTENTS. CHAPTER XVIII. PACKS The Organ of Hearing 377-401 The external ear ; The tympanic membrane ; The middle ear; The ear-ossicles ; The Eustachian tube ; * The internal ear ; The saccule and the utricle ; The semicircular canals ; The cochlea ; The ductus cochlearis ; The scala vestibuli ; The scala tympani ; The ductus endolymphaticus ; The development of the ear. CHAPTER XIX. The Nasal Mucous Membrane 402-406 The respiratory region ; The olfactory region ; Development of the nasal fossae. APPENDIX. The Most Useful Histological Methods 407-429 Fixation of the tissue ; Fixation reagents ; Preservation of the tissue ; Staining ; Staining solutions ; Embedding ; Interstitial embedding; Paraffin method ; Celloidin method ; Section-cutting ; Cutting ribbon-series ; Fixing sections to the slide ; Mounting sections ; Finishing, labelling, and storing slides ; Outline of standard method ; Weigert's staining method ; Golgi's silver method ; Golgi's gold method ; Silver staining ; Staining chromatin filaments ; Injecting capillary blood-vessels. Index 431 LIST OF ILLUSTRATIONS. FIG. PAGE 1. Colorless blood-cell ii 2. Typical cell — ovum of cat 12 3. Structure of the cell 13 4. Cells exhibiting the paranucleus {Plat7ier) 14 5. Segmenting ova of ascaris ynegalocephala {Boveri) 14 6. Direct cell-division of colorless blood-corpuscle 15 7. Karyokinesis — diagram of close skein [Rabl- Schiefferdecker) 16 8. Karyokinesis — diagram of loose skein (Rabl- Schiefferdecker) 16 9. Karyokinesis — diagram of polar field {Rabl- Schiefferdecker) 17 10. Karyokinesis — diagram of migration of segments (i?aW) 18 11. Karyokinesis — epidermal cells from larva of newt 19 12. Segmenting ova, showing centrosomes and attraction-spheres {Boveri) . 20 13. Large marrow-cell with multiple nuclei 20 14. Maturation and fecundation of ovum ( O. Hertwig) 22 15. Blastodermic layers of rabbit embryo 24 16. Squamous epithelium 28 17. Stratified squamous epithelium in section 28 18. Isolated cells of stratified squamous epithelium 28 19. Prickle-cells from epidermis 29 20. Simple columnar epithelium 29 21. Stratified columnar epithelium 29 22. Ciliated epithelium 30 23. Isolated elements of ciliated epithelium 30 24. Goblet-cells ? 31 25. Pigmented epithelium 31 26. Glandular epithelium 32 27. Rod-epithelium {Heidenhain and Schiefferdecker) 32 28. Isolated neuro-epithelium 32 29. Endothelium T)?) 30. Endothelium showing stomata 34 31. Young connective-tissue cell 36 32. Embryonal connective tissue 36 2)'i. Subcutaneous areolar tissue -yj 34. Special connective-tissue elements 37 35. Pigmented connective-tissue cells jH 36. Pigment-cell 38 2)"]. Plate-like connective-tissue cells 38 38. Cell-spaces of dense connective tissue 38 39. Branched connective-tissue cells 38 40. White fibrous tissue 39 41. Elastic fibres isolated {Schiefferdecker) 39 42. Young connective-tissue cells 40 43. Tendon in transverse section 41 xii LIST OF ILLUSTRATIONS. "G- PACK 44. Primary bundles of tendon 41 45. Primary tendon-bundles in section 42 46. Elastic fibres in transverse section 42 47. Elastic fibres forming fenestrated membrane 42 48. Subcutaneous tissue with fat-cells 43 49. Hyaline (costal) cartilage 44 50. Hyaline cartilage with perichondrium 45 51. Elastic cartilage 46 52. Fibro-cartilage 46 53. Transverse section of dried bone 47 54. Longitudinal section of dried bone 48 55. Lacunae and canaliculi of dried bone 48 56. Bone-cell within lacuna 49 57. Fragments of bone, showing Sharpey's fibres 49 58. Cells of bone-marrow 50 59. Primary embryonal cartilage 51 60. Developing bone — centre of ossification 52 61. Developing bone — zone of calcification 53 62. Developing bone — trabeculoe of endochondral bone ... 54 63. Developing bone — conversion of osteoblasts into bone-cells 54 64. Developing bone — periosteal and endochondral bone 55 65. Developing bone — longitudinal section of embryonal phalanx 55 66. Developing bone — showing Howship's lacuna 56 67. Isolated involuntary-muscle cells 59 68. Involuntary-muscle cells 59 69. Involuntary muscle in transverse section 60 70. Involuntary muscle in longitudinal section 60 71. Voluntary-muscle fibres 61 72. Voluntary-muscle fibres in section 62 •JT). Voluntary-muscle fibres 62 74. Diagram of arrangement of contractile substance 64 75. Muscle-fibres, showing Cohnheim's fields 65 76. Voluntary muscle in transverse section 65 Tj. Branched fibres of voluntary muscle 65 78. Heart-muscle, showing branched fibres 66 79. Heart-muscle fibres in section 67 80. Injected voluntary muscle 67 81. Developing voluntary muscle 68 82. Nerve-cell from cerebral cortex . . 70 83. Nerve-cell isolated from spinal cord 71 84. Nerve-cell of first type 72 85. Nerve-cell of second type 72 86. Basket-work around Purkinje's cell 72 87. Nerve-cell from sympathetic (Retzius) 'jt, 88. Medullated nerve-fibres 74 89. Ultimate fibrillae of axis-cylinder 74 90. Medullated nerve-fibres treated with osmic acid 75 91. Silvered nerve-fibres 75 92. Non-medullated nerve-fibres 76 93. Section of nerve-trunk 77 94. Section of single funiculus of nerve 78 95. Supporting tissues of nerve-centres 79 96. Longitudinal section of spinal ganglion 80 97. Section of portion of spinal ganglion 80 98. Ganglion nerve-cell, showing spiral fibre ( .SifAzV^ifrafif^^tfr) 81 LIST OF ILLUSTRATIONS. xiii FIG. PAGE 99. Termination of sensory nerve fibres 83 100. Termination of sensory nerve fibres within the epidermis 84 loi. Special nerve-endings within the epidermis (Ranvier) 84 102. Tactile corpuscles — simple and compound 85 103. Tactile corpuscle of Meissner ( 5cAz^.f/w/'-/?z<27a/) 297 332. Diagram of sensory decussation of medulla ( TesUit-Duval ) 298 2)2,i- Diagram of medulla through olivary bodies ( 7"if.y/z?/!ya/ ) 298 334. Section of medulla of child 299 335. Section through human pons ( 75/m/) 393 Section through auditory pit of rabbit embryo 398 Section through otic vesicle of rabbit embryo 398 406. Section through developing ear of rabbit embryo 399 407. Section through developing cochlea of rabbit embryo 400 408. Section of human respiratory nasal mucous membrane 402 409. Section of olfactory mucous membrane of child 403 399- 400. 404. 405- NORMAL HISTOLOGY. CHAPTER I. THE CELL AND THE TISSUES. Histology, literally, the science of tissues, represents that part of general morphology which treats of the structural elements of organisms, by the various arrangement of which the textures and organs of the body are formed. The term is, evidently, equally applicable to the structural components of plants as well as to those of animals; "histology," however, is usually accepted as relating especially to animal tissues, "vegetal histology" expressing the extension of the study to the tissues of plants. At first sight apparently complex and numerous, the structures composing the animal economy are really made up of but few elementary tissues ; these latter may be divided into four funda- mental groups: Epithelial tissues; Connective tissues; Muscular tissues; Nervous tissues. Each of these tissues may be further resolved into the compo- nent morphological constituents, the cells and the intercellular substances. ~ All animal cells are the descendants of the embryonal elements derived from the division of the primary parent cell — the ovum; the intercellular substances, on the other hand, are formed through the more or less direct agency of the cells. The animal cell may exist in either the embryonal, matured, or metamorphosed condition. The embryonal cell, as represented by the early generations of the direct offspring of the .ovum, or by the lymphoid or colorless blood-cells of the adult, is a pj^ j_ small irregularly round or /r^x:>^ /rc^ y<^ ^ oval mass of finely granular ^f|^tii ^-'^M^ \^W\ ^"S^'^^) gelatinous substance — the "'^^^^ c^^'S^w cilij^ cui^:^^ ___.„_l 11 , . Colorless blood-cell exhibiting amoeboid movement. . protoplasm or cell-contents ^ — in some part of which a smaller and often indistinct spherical body — the nucleus — lies embedded. In the embryonal condition, 12 NORMAL HISTOLOGY. Fig. 2. when the coll is without a hmitin_o; membrane and composed al- most entirely of active living substance, the outlines are frequently changing, these variations in shape being ' known as amoeboid movements, from their similarity to the changes observed in the outline of an active amoeba, one of the simplest forms of animal life. As the embryonal cell advances in its life-history, the surrounding conditions to which it is subjected induce, with few exceptions, further specialization. Among the earliest of such effects is the condensation of the peripheral zone of the cell, whereby the reten- tion of a definite form is greatly favored; such peripheral condensa- tion may progress to the production of a distinct limiting membrane — the cell- wall. This structure is very frequently wanting; when present, however, it is usually so thin that its optical expression is a single delicate line. The cell-wall is to be regarded as a product of the specialization of a portion of the protoplasm, rather than as an essential part of the cell. The adult cell consists of the protoplasm, or cell-contents, possibly limited by a cell-wall, en- closing a nucleus, which latter, in turn, often con- tains one or more minute spherical bodies, the nu- cleoli. The more or less definite and characteristic forms which the elements of the various tissues possess on reaching their full development, depend largely upon the changes effected by growth and dif- ferentiation in the proto- plasm during the younger condition of the cells. The protoplasm of Typical cell,— ovum of cat: a. protoplasm; i, nucleus: <:, which the greater part of nuclear membrane; d, nucleolus; e, true cell-wall, closely rells is COmOOScd Usino" applied to the surrounding secondary envelope, the zona i . . , ' , "^ peiiucida. the term m its broadest application and as sy- nonymous with cell-contents, usually appears as a finely granular semi-fluid or gelatinous substance, in \vhich darker and coarser granules or other particles of extraneous matters are often embedded. The structure of protoplasm is now recognized as far more com- plicated than was formerly supposed, comprising a highly elastic and extensible portion — the spongioplasm — and an interstitial. mmmPm ^ms^r!^,^:. '^.^■'■•'. TW^rC THE CELL AND THE TISSUES. 1 3 seemingly less active substance — the hyaloplasm. The active contractility which has_ been generally credited to the spongioplasm has been recently questioned (Schaefer), since the characteristic amoeboid movements of living cells are by some attributed to the changes taking place within the hyaloplasm. The arrangement of these constituents of the protoplasm is vari- able. When they exist closely and uniformly intermingled, the customary finely granular appearance of the cell-contents is produced; not infrequently, however, the spongioplasm is disposed as a more or less well-defined reticulum. In living cells this reticulation is transient, and, to a certain degree, acci- dental, since it often depends upon an ' ■^' unequal distribution of the hyaloplasm induced by the presence of vacuoles or of particles of foreign substance, as se- cretion within glandular epithelium. Chemically, protoplasm consists of various albuminous substances in com- bination with a special nitrogenous pro- teid, plastin, together with water and salts. It is probable that in the albu- structure of the cell : a, sponglo- 1 , 1 .1 _ 4- „r plasm, arranged as reticulum, hyalo- mmous substances alone the property of J,,,^'!;,, J^hin the latter; ^,ceii- COntraCtility resides; the plastin, on the wall ; ^ /SJir^ /55^ simplest organisms, \vjj?j5>^^^^ \_^£^- — k.'^'J , daughter-stars. (After iPaW.) (8) The daughter-Stars, or diaster, each of these corresponding to a new nucleus. About this time the cell-protoplasm, which until now has been almost passive, begins to exhibit a constriction of its body, w'hich impression now steadily progresses until the protoplasm of the cell completely separates into the portions destined to become the bodies of the cells, enveloping the new nuclei. The karyokinetic cycle is completed by each (9) Daughter-wreath or star in turn assuming (10) The stage of the daughter-skeins, at first loose and afterwards close; on obtaining nuclear membranes and the nucleoli reappearing, the new nuclei finally pass into the stage of rest. * The term "equatorial plate" has been employed by some authors to indicate the later phases of the aster stage. THE CELL AND THE TISSUES. 19 In recapitulation, the above changes may be tabulated as follows : Resting Mother-Nucleus. Prophases : The inauguration of division is marked by migration and division of centrosome, increase of chromatin, resulting in the formation of The Mother- Skein {^Spirent) : a. Close skein, — Disappearance of nucleoli. Disappearance of nuclear membrane. Fig. II. G Cells from the epidermis of very young larva of newt : A , resting nucleus ; B, close skein ; C, loose skein ; Z> and £, mother-stars, seen from the polar field and appearing as the wreath stage ; J^, mother- star from the side ; G, migration of segments ; //, daughter-stars ; / and y, segments grouped about new polar fields (in y the protoplasm exhibits constriction) ; A", daughter-skeins, — division of nucleus complete with slight constriction of cell-body; L, completed division of nucleus and protoplasm. d. Loose skein, — Separation of skein into segments or chromosomes. Appearance of polar field around the centrosomes. Rearrangement of chromosomes around polar field. The Mother- "Wreath, or Aster: Appearance of nuclear spindle. Metaphase : Longitudinal cleavage of chromatin segments. 20 NORMAL HISTOLOGY. Anaphases : Migration of segments i^Metakinesis). Seg^ments pass towards the poles of the new nuclei. Equatorial plate produced by massing of migrating segments. Separation of segments into polar groups. Appearance of connecting filaments. Daughter- Wreaths, or Asters. Telophases : Beginning division of cell-protoplasm. Daughter-Skeins : a. Loose skein. b. Close skein. Completion of new nuclei. Acquisition of nuclear membranes. Reappearance of nucleoli. Completed separation of cell-protoplasm. Resting Daughter-Nuclei. Fission of the nucleus is ordinarily followed by cleavage of the protoplasm, the resulting new cells being entirely distinct elements. A deviation from this usual . ^^' ^^' procedure is, however, some- times encountered where the division of the nucleus has not been followed by cleavage Fig. 13. Segmenting oy^L oi ascaris megalocephala : /I , cell Large marrow-cell: the nu- contains nucleus, two centrosomes (c), surrounded by cleus has undergone repealed af.raction-spheres, and adherent polar body (/); B, division without cleavage of the beginning polar striation around the centrosomes and protoplasm, attraction spheres ; C, cell viewed from polar field, tVie striation proceeding from the centrosome ; D, cell seen from the side, apices of nuclear spind'e correspond with centrosomes. (.A.f.er Bcrfci.) of the cell-protoplasm, the latter remaining undivided even after the repeated division of the nuclei. Examples of such ' ' endogenous' ' formation are seen in the multinucleated giant marrow-cells. Not infrequently under such conditions the nucleus undergoes repeated abortive division, resulting in the production of irregular lobulated or cleft forms embedded within the common protoplasmic mass. THE CF.LL AND THE TISSUES. 21 The foregoing vital manifestations, being chiefly concerned in the mere existence and perpetuation of the cell, are appropriately termed vegetative ; irritability and motion, on the contrary, are the ex- pressions of a higher and more individual existence, and hence are called animal. It is to be remarked that the term "animal," as here employed, must not be regarded as indicating distinctions be- tween plants and animals; for this purpose such manifestations are inadequate, since the elements of certain plants (Mimoseae, Dionaea) possess irritability, and the protoplasm of others (Myxomycetes, Volvocineae) exhibits motion in a marked degree. Irritability is that property of living matter by virtue of which external influences are responded to by changes within the cell; these changes may, in turn, induce secondary phenomena. Instances of such impressions are frequent among the lower forms, where surface elements, or, as among the still simpler unicellular protozoa, the pe- ripheral zone of the protoplasm common to the entire animal, exhibit susceptibility to external stimuli. Among the higher animals irri- tability is manifested by nerve-cells, which, through their processes, influence other tissues. Concerning the exact nature of the intimate changes taking place within the cell, the sum of which we call nervous phenomena, little is known; it is probable, however, that the al- buminous constituents of the protoplasm are the particular seat of these obscure molecular changes. Motion, more or less pronounced, is a characteristic of all ani- mal cells — and, likewise, of very many vegetal ones — during some portion of their existence. The development and specialization of the adult cell usually result in limitation of the activity of the protoplasm, by reason both of its decrease and of its intimate relations with the surrounding tissues; the cells exhibiting motion in the adult condition are those which retain, to a certain degree at least, their embryonic type: such are the lymphoid and connective-tissue cells. Motion may be exhibited by elements devoid of, as well as by those provided with, special appendages. The lowest degree of this vital manifestation is encountered in the streaming of the protoplasm within cells, as in plants, enclosed within limiting membranes which do not permit such motion to affect the exterior of the cells. Con- spicuous examples of the more marked effects of protoplasmic streaming are familiar in the changes readily observed in amoebae or in the colorless blood-cells of higher types. In these latter elements, however, the motion is manifested rather in change of form than by marked variation in position. The highest expression of motion is displayed by those cells whose protoplasm has undergone specialization, resulting either in the pro- duction of a peculiar tissue, as that of the voluntary muscle fibre, or 22 NORMAL HISTOLOGY. of external appendages, as the cilia of many unicellular organisms or of the epithelial elements of the higher animals. Since every cell is derived from a pre-existing cell, it follows that all the cells of the organism are the descendants of the parent ele- ment— the ovum. The ripe mammalian egg, while small in com- parison with many other ova, is among the largest histological elements, measuring about .2 millimetre in diameter, and, further, possessing all parts of the typical cell. Before the ovum is capable of uniting with the male sexual element to carry out the changes attendant upon fecundation, it passes through a cycle of preparatory stages collectively known as maturation. These changes consist in the repeated very unequal division of the ovum, resulting in the expulsion of minute portions of its proto- plasm, the polar bodies ; of these latter, usually two are extruded. Maturation and fecundation in ova o( ascaris megalocephala : I, k, nucleus of ovum before matura- tion ; s, entering spermatozoon ; //, nucleus («) has passed to periphery of cell preparatory to di- viding ; s, spermatozoon now within the ovum ; ///, nucleus dividing into first polar body (/) ; >«, male pronucleus resulting from spermatozoon ; IV,p,p' , first and second polar bodies, the last still in process of formation ; m, male pronucleus ; V, p, p' , polar bodies ; / and rn, respectively female and male pronuclei, in contact but not yet fused ; c. centrosomes, indicating poles of nuclear spindle ; VI, pronuclei now fused ; striation proceeds from centrosomes preparatory to division of ovum. (After O. Hert-Mig.) The nucleus which appears within the ovum after the formation of the polar bodies is the female pronucleus. Upon the completion of these phenomena, maturation has taken place and the ovum is prepared for the reception of the male sexual element. Under THE CELL AND THE TISSUES. 2^ favorable conditions the spermatozoa reach the ovum, when a single element penetrates the envelopes of the egg and is received within the protoplasm of the female cell. The entrance of the spermatozoon causes a new disturbance within the ovum, resulting in the formation of the male pronucleus. Subsequently the latter joins with the female pronucleus, the fusion of the two pronuclei being followed by a temporary disappearance of all nucleus within the ovum. Shortly afterwards the new nucleus of segmentation appears, so called from the fact that within this body cleavage of the ovum is first established. The process of segmentation following the fertilization of the ovum is essentially one of indirect cell-division, in which the stages, although modified in certain details, are essentially the same as those already described. The mammalian ovum undergoes a total segmentation; although the resulting segments are, strictly regarded, not quite equal in size, yet, as a matter of simplicity, they may be regarded as such, and the division characterized as total equal segmentation. The repeated cleavage of the segmentation-spheres into which the ovum is divided soon produces a mass of innumerable cells con- stituting the blastoderm ; the latter, by continued division and further diiferentiation, subsequently gives place to a cell-area, in which at first two layers, an outer and an inner, and later a third middle stratum, of cells appear. These more or less imperfectly defined tracts constitute the important primary blastodermic layers, the ectoderm, mesoderm, and entoderm, from which are derived all the tissues of the body. The reader must be referred to' the various text-books of embryology for a detailed account of the complicated and often obscure processes of maturation, fertilization, segmentation^ and blastulation, of which only the most salient points have been indicated above. THE TISSUES. Every tissue is composed of two parts, — the cellular elements and the intercellular substance. Upon the first of these depends the vitality of the tissue, while its physical properties are determined by the character of the second. The physical condition of the inter- cellular substances includes a wide latitude, varying from that of a fluid, as blood or lymph, through all degrees of density, until, by the additional impregnation of calcareous matters, the well-known hardness of bone or dentine is attained. The proportion between the cellular elements and the intercellular substance of mesodermic tissues varies with age and development, the intercellular substance in the early stages being scanty and very o^ NORISIAL HISTOLOGY. yielding, while with adolescence they may become tough and re- sistant. Accompanying the growth of the tissue, an increase of the intercellular substance usually takes place through the direct or indirect participation of the cells, these latter, in consequence, suf- fering marked reduction in number and size. The younger the mesodermic tissue, the richer is it in cells and the poorer in intercellu- lar substances; conversely, the older the tissue, the more prominent the intercellular substance and less conspicuous the cellular elements. A marked example of this law is presented by tendon, where, in the embryonic condition, the cells constitute the greater bulk of the tissue, while in the adult the intercellular fibrous tissue so overwhelms the cellular elements that reagents are frequently necessary to satis- factorily demonstrate their existence. While increase of the intercellular substance usually accompanies the growth of the mesodermic tissues, those derived from the ecto- and entoderm present a marked contrast. In these latter tissues the intercellular constituent is represented by the very scanty cement substance, increase in which occurs only as necessitated by the growth of the surrounding cells, the proportion between the two elements being practically constant throughout life. Instances of this constant relation are seen in the varieties and modifications of the epithelial tissues. The primary blastodermic layers — ectoderm, mesoderm, and entoderm — early exhibit histological differences which suffice to distinguish the one from the other, and especially to indicate, at least in a general manner, the tendency of the outer and inner layers to Fig. 15. Blastodermic layers of rabbit embrj-o : a, ectoderm ; 3, entoderm ; f, entodermal cells destined to fijrm notochord ; m, mesoderm. form epithelial structures in contrast to the less compact and more reticular formations of the mesoderm. The epithelia of the genito- urinary tract, however, are marked exceptions in their origin, being derived, as well as the connective and muscular tissues, from the mesoderm, in this respect constituting conspicuous specializations. THE CELL AND THE TISSUES. 25 Derivatives of the Primary Blastodermic Layers. From the ectoderm are derived — The epitheHum of the outer surface of the body, including that of the conjunctiva and anterior surface of the cornea, the external auditory canal, together with the epithelial append- ages of the skin, as hair, nails, sebaceous and sweat glands (including the involuntary muscle of the latter). ■ The epithelium of the nasal tract, with its glands, as well as of the cavities communicating therewith. The epithelium of the mouth and of the salivary and other glands opening into the oral cavity. The enamel of the teeth. The tissues of the nervous system. The retina; the crystalline lens. The epithelium of the membranous labyrinth. The epithelium of the pituitary and pineal bodies. * From the mesoderm are derived — The connective tissues, including areolar tissue, tendon, cartilage, bone, dentine of the teeth. The muscular tissues, with the exception of the muscle of the sweat-glands. The tissues of the vascular and lymphatic systems, including their endothelium and circulating cells. The sexual glands and their excretory passages, as far as the termination of the ejaculatory ducts and vagina. The kidney and ureter (but not the bladder). From the entoderm are derived — The epithelium of the digestive tract, with that of all glandular appendages except those portions derived from ectodermic origin at the beginning (oral cavity) and termination of the tube. The epithelium of the respiratory tract. The epithelium of the urinary bladder and urethra. The epithelium of the thyroid and thymus bodies, the atrophic primary epithelium of the latter being represented by Hassall' s corpuscles. 26 NORMAL HISTOLOGY. CHAPTER II. THE EPITHELL\L TISSUES. The free surface of the skin and of the various mucous membranes is covered by epithelium, which affords protection to the more dehcate parts lying beneath. In this tissue the intercellular con- stituent is reduced to a minimum, being represented alone by the scanty cement-substance between the cells; the latter, in consequence of this relation, form practically an unbroken sheet. The epithelia are best grouped under two chief heads — squamous and columnar. The designation as tessellated or pavement is not distinctive, since either variety may present a mosaic when viewed from the free surface. These tissues may be classified in several divisions as below indicated. VARIETIES OF EPITHELIUM. / Squamous. II. Columnar. a. Simple — consisting of a single layer — a. Simple. b. Stratified — consisting of several layers — b. Stratified. /// Modified, a. Ciliated; b. Goblet; c, Pigmented. /K Specialized, a. Glandular epithelium; b. Neuro-epithelium. The epithelium contains no blood-vessels, the nutrition of the tissue being maintained by the absorption of the nutritive juices conveyed by means of the intercellular clefts within the cement- substance. The nervous supply of epithelium is likewise ordinarily very scanty, the existence of nerve-fibrils within the epithelium in many localities being doubtful; in certain regions possessed of high sensibility, as the corneal or tactile surfaces, the termination of nerve- fibres among the epithelial elements may be regarded as definitely established. The epithelial cells usually rest upon a basement- membrane, or membrana propria, a modification of the subjacent connective tissue of which it is part. The principal distributions of the various forms of epithelium follow. THE EPITHELIAL TISSUES. 27 Simple squamous epitheliumi occurs in but few places : Partially lining- the tympanic cavity, including the mastoid cells ; parts of the membranous labyrinth ; the infun- dibula and alveoli of the lungs ; the posterior surface of the anterior capsule of the crystalline lens ; parts of ducts of glands ; the capsule of the Malpighian body and the descending limb of Henle's loop in the kidney; choroid plexuses and parts of brain-ventricles. Stratified squamous epithelium occurs widely distributed, cover- ing— The skin and its extensions, as the external auditory canal, conjunctival sac, and cornea ; the mouth, lower part of pharynx, and oesophagus ; the epiglottis and upper part of larynx, together with the false and true vocal cords ; the pelvis of kidney, ureter, bladder, beginning and end of male and entire female urethra ; the vagina. Simple columnar epithelium occurs: a. Non-ciliated, in — The digestive tract, from the oesophageal opening of stomach to anus, as well as in the larger ducts of the glands com- municating with this tube.; ducts of mammary glands; seminal vesicles and ejaculatory duets ; membranous and penile portions of urethra. b. Ciliated, in — Oviduct, uterus, and part of canal of cervix ; greater part of brain-ventricles and canal of spinal cord. Stratified columnar epithelium occurs : a. Non-ciliated, in- Terminal part of the vas deferens ; olfactory part of nasal fossae. b. Ciliated, in— The Eustachian tube and parts of tympanic cavity ; lachry- mal passages ; respiratory part of nasal fossae, with com- municating sinuses ; ventricle of larynx, trachea, and bronchiae ; epididymis and first part of vas deferens. Squamous Epithelium. When occurring as a simple layer, the flattened, polyhedral, nucleated plates form a regular mosaic; such epithelium is found but seldom in the human body, the lining of the air-sacs of the lung, the posterior surface of the anterior capsule of the crystalline lens, the membranous labyrinth, and a few other localities being its principal seats. 28 NORMAL HISTOLOGY. Fig. i6. A far more usual arrangement is as several layers, constituting the stratified squamous variety. The isolated cells of such epi- thelium differ greatly in form, size, and appearance according to the layer from which they are taken. The cells com- posing the deepest stratum are not scaly, but irregularly columnar, resting, with slightly expanded bases, upon the sub- jacent membrana propria. The irregular borders of these cells join with neighboring elements in such a manner that minute intercellular clefts are formed ; these are occupied by the yielding cement-sub- stance, and allow the passage of the nutrient juices, as well as of the migratory leucocytes, or wandering cells. The nuclei of the columnar elements are oval, and often situated nearer the outer ends of the cells. Passing from the basement-membrane towards the free surface, the form of the cells undergoes a radical change. The pronounced columnar type belongs to the deepest layer alone ; the cells next become irregularly polyhedral. Fig. 17. Squamous epithelium from frog's skin, viewed from the free surface. Fig. 18. Stratified squamous epithelium in section, from the cornea : the deepest cells are columnar ; the superficial are scaly plates. Isolated cells of stratified squamous epithelium : a, surface-cell ; i and c, cells from middle layers ; th. cells he embedded "^Xt and dificdng but w.th.n ihe l-cuna, cihcr ^..n^Iy, m pairs, ur .n gruups ; little from tCndoU. SinCe matrix exhibits diflerentiation around the cell-spaces as more u " rarfilacrp" thp t\'-n\ deeply staining areas. ^ ^ ■'I cal hyaline variety is usually understood, that form first claims attention. Hyaline cartilage, so named from the transparent, apparently homogeneous character of the intercellular matrix, enjoys a very THE CONNECTIVE TISSUES. 45 wide distribution, occurring as the articular cartilage of bones, costal cartilages, the larger cartilages of the larynx, trachea, or bronchi, nose. Eustachian tube, etc. ; in the embryo the entire skeleton, with the exception of the vault of the cranium, the bones of the face, and the greater part of the lower jaw, is mapped out by primary hyaline cartilage. The homogeneity of the hyaline matrix is only apparent, since, as long ago pointed out by Leidy, the intercellular substance may be resolved into bundles of fibrous connective tissue, which, however, are so closely united and intimately blended by the cementing ground- substance that the presence of the fibres is, ordinarily, not evident. After prolonged boiling, cartilage matrix yields chondrin. Embedded within the hyaline matrix lie the cartilage-cells ; these are irregularly oval or angular nucleated protoplasmic bodies, which, during life, almost fill the spaces, or lacunae, which they occupy. In adult tissue usually two or more cells share the same compartment, Fig. 50. the original occupant of the space having undergone division, so that two, four, or even more daughter-cells form a single group. The matrix immediately sur- rounding the lacuna is specialized as a layer of different density, thereby as- suming the appearance of a distinct limit- ing membrane, described as the capsule. A further differentiation of the ground- substance is seen in the greater intensity with which the more recently formed matrix enveloping the cells stains; such re- sulting figures constitute the cell-areas. It is to be remembered that the cartilage- cells are but connective-tissue cells, and that the lacunae correspond to the lymph- or cell-spaces found in other dense connective tissues. Since it is usual to find these cell-spaces in communication through minute channels, or canaliculi, their absence and the apparent isolation of the lacunae in cartilage are to be regarded as deviations from the typical arrangement ; among some lower forms, however, such a communication exists, the minute canaliculi passing between the neighboring lacunae. The free surface of the cartilage is covered by an envelope of dense connective tissue, the perichondrium ; this consists of an external Hyaline cartilage with perichon- drium (p) attached : y, zone of youngest cartilage-cells ; ?k, hyaline matrix enclosing the lacunae contain- ing the cartilage-cells ; /, space from which the cell has been lost. 46 NORMAL HISTOLOGY. or fibrous layer of dense fibro-elastic tissue and an inner, much looser stratum, between the fibres of which are numerous connective- tissue cells. This inner portion is intimately concerned in the pro- duction of new cartilage, and is known as the chondrogenetic layer. The cells of the latter arrange themselves in rows parallel to the surface, and gradually assume the characteristics of the cartilage corpuscles, being at first spindle-shaped, but gradually assuming the more spherical form. The new cells soon become surrounded by the recently-formed matrix, which, at first small in amount, soon in- creases so that the groups of cartilage-cells become separated, by more extensive tracts of intercellular substance ; as the nests of cells formed by the division of the original single occupant of the lacuna recede from the perichondrial surface they lose their primary parallel disposition and become irregularly arranged and further separated. Sometimes in those portions most removed from the perichondrium the ground-substance appears granular ; this feature is intensified when a deposition of calcareous matter takes place, which not infre- quently happens in old subjects. Elastic cartilage is distinguished by the presence of elastic fibres within the intercellular substance. The typical hyaline matrix is confined to areas of F^'G. 51. Fig. 52. limited extent immedi- ately surrounding the cell-nests, while the in- tervening matrix is penetrated by net- works of elastic fibres extending in all directions. The cells within the lacunae, in the midst of the hy- aline areas, resemble closely the usual ele- ments of hyaline car- tilage. Elastic cartilage has a much less general distribution than the hyaline variety, occur- Elastic cartilage from the epi- glottis : c, cartilage-cells sur- rounded by a very limited area of hyaline matrix (A) ; theremaming part of the intercellular substance is penetrated by net-works of elastic fibres {e), cross-sections of which appear as minute points. Fibro-cartilage from the knee-joint : c, cartilage- cells surrounded by very limited areas of hyaline ^\^cr principally in the matrix [h) ; the space be- ., ' r 1 tween these areas is occu- CartllagCS Of the CX- pied by the fibrous tissue, tcmal Car, part of the Eustachian tube, epi- glottis, arytenoid cartilages, cartilages of Wrisberg and of Santorini. This tissue presents an opaque, yellowish tinge in contrast to the THE CONNECTIVE TISSUES. 47 opalescent, bluish tint of the hyaline variety. It is covered by a perichondrium of the usual description. Fibro- cartilage, as implied by its name, is largely composed of interlacing bundles of fibrous connective tissue, embedded in which the round or oval cartilage-cells lie, singly or in groups, immediately surrounded by a narrow zone of hyaline matrix. The number of the cells and the proportion of fibrous tissue present differ in various specimens. Fibro-cartilage is found in comparatively few localities : around the margin of articular surfaces and within certain joints, the sym- physes and the intervertebral disks, constitute its chief distribution. The tissue is closely akin to tendon, presenting a white, tough, re- sistant but pliable tissue. A proper perichondrium is wanting. The development of cartilage proceeds directly from the ele- ments of the mesoderm. The primary close aggregation of the embryonal cells, which early indicates the position of the future cartilage, subsequently gives way to a looser disposition of the cells, resulting from the appearance of the young matrix. After the formation of the perichondrium, the cartilage grows by the addition of new layers beneath the membrane. Fig BONE. Bone is a dense form of connective tissue impregnated with lime salts. Composed of the same histological elements as other compact connective tissues, bone differs from these in having a deposit of calcareous matter within the interfascicular cement-substance, to which peculiarity the well-known hardness of the tissue is due. The microscopical appearance of bone varies with the character of the prepara- tion, especially as to whether the earthy matter has been removed before sectioning, or whether thin plates of dried bone are examined ; it is in sections of dried bone that the classical pictures of this tissue are seen. Dependent upon the arrangement of the matrix, two varieties of bone are recog- nized— spongy and compact. Although the spongy bone is, as we shall see, the fundamental form, yet the compact variety alone presents all the structural peculiarities of the tissue. A transverse section of the compact osseous tissue constituting the shaft of one of the long bones presents a number Transverse section of dried bone : /;, one of the Haversian canals, about which the lamellae are con- centrically disposed, constituting^ the Haversian systems ; g, the ground or interstitial lamellae. Fig .g NORMAL HISTOLOGY. of round or oval openings — the Haversian canals — each sur- rounded by a broad band or zone composed of concentrically- disposed lamellae ; the canal and tlie surnjunding lamcllai form an Haversian system. Seen in longitudinal sections, the Haversian canals appear as extended channels, some closely corresponding in their course with the general axis of the bone, while others run obliquely and es- tablish free communication between the adjacent canals. The concentric bone lamellae in such sections appear as parallel bands bordering the large channels. The Haversian canals communicate with the central marrow-cavity, of which they are really continuations ; variable in width and length, each canal contains an extension of the bone-marrow, comprising a delicate connective-tissue reticulum, rich in cells, blood-vessels, and lymphatics. The areas between the Haversian systems are filled out by osseous lamellae, disposed without regard to the concentric systems ; these are the interstitial or ground lamellae, and represent the older parts of the bone, being the remains of the primary spongy net-work of periosteal bone. The concentric lamellae constituting the Haversian systems are secondarily deposited within the enlarged spaces of the bony reticu- lum. In addition to the lamellae already mentioned, superficial os- seous strata encircle the bone on Fig. 55. both its outer and inner (medul- lary) free surfaces ; these are the outer and inner circum- ferential or fundamental la- mellae. Between the bundles of the ground - matrix spindle - shaped spaces — the lacunae — are seen, from which minute channels — the canaliculi — radiate in all direc- tions ; these dark, stellate figures with their minute lateral canals form a system of intercommunicating lymph-spaces within the bone ; the canaliculi belonging to the same space or to the adjoining lacunae of the same Haversian system anastomose with one another, but not with the canals of different systems. Longitudinal section of dried bone : /(, Haversian canals opened lengthwise and bordered by the longitudinally-cut lamella:. ^r The lacunae and canaliculi of dried bone under high ainptification. THE CONNECTIVE TISSUES. 49 A bone-cell lying within the lacuna of the osseous matrix : decalcified and stained. In dried bone the spaces are filled with air, the lacunae and cana- liculi consequently appearing dark and sharply defined when viewed by transmitted light. The lacunae, sometimes improperly called "bone-cells," in dried preparations are empty, or, at most, contain the remains of the soft, protoplasmic bodies, the true bone-cells, which during life partially fill the spaces ; these, like the cells of other dense connective tissues, lie within the lymph-spaces of the ground- matrix. In sections of young, well-stained, decalci- fied fresh bone, after the usual manipulations, the bone-corpuscles are seen as nucleated, stellate, protoplasmic bodies, whose processes extend into the canaliculi ; in adult and old bones, however, the cells become reduced in size and very inconspicuous. The lacunae being lenticular, they present different figures according to the direction in which they are sectioned : cut transversely, they appear as short, narrow ovals ; opened longitudinally, but not parallel to the lamellae, they are seen as long, narrow, elliptical figures ; while when cut longitudinally, and at the same time parallel to the lamellae, they present a broad, oval surface, sometimes almost circular ; the canaliculi, extending in all planes, appear much the same in all sections. The periosteum, an envelope of vascular connective tissue, closely invests the outer surface of all bones except the articular facets. This important structure is composed of t'WO portions — an outer, dense, protective, fibrous layer, and an inner, much looser stratum, rich in cells and blood-vessels, which, from its intimate relations to the formation of bone, is known as the osteogenetic layer. This latter contains within its meshes numerous round or spindle cells, many of which later be- come bone-forming ele- ments— the osteoblasts. If a decalcified bone be sectioned parallel to the superficial lamellae, especially if these be of a spongy bone, or if the outer lamellae be forcibly torn off, a number of transverse or perpendicular fibres of 4 Fig Fragments torn from the surface of a decalcified bone : A, surface ; B, oblique view ; s, Sharpey's perforating fibres ; /, the lacunse. CO NORMAL HISTOLOGY. more or less delicacy will be exposed ; these are the perforating fibres of Sharpey, and represent periosteal fibres which have failed to underfjo calcification ; of these Kolliker recognizes two kinds — those entirely soft and uncalcified, the most numerous and, at the same time, the smallest; and those partly calcified and of larii;^er size, which, in fact, are bundles of fibrous tissue. Sharpey' s fibres are most numerous in the superficial lamellae of spongy bones, although found in the interstitial lamellae of other bones, pinning together the lamellae which they transfix. The jjcrforating fibres, being derived from the periosteum, never occur in the lamellae of the Haversian systems, since the latter, it will be found, are not directly produced by the periosteum, but as secondary deposits. Additional elements of the bone-matrix are the elastic fibres, which are found in the outer fundamental lamellae, as well as occasion- ally in the deeper interstitial lamellae; these elastic fibres are generally associated with the uncalcified Sharpey' s fibres ; not infrequently the elastic fibres are contained within the uncalcified bundles of fibrous tissue composing the large perforating fibres. tained within the shafts of the Fig. 58. Marrow of Bone. The cavities within bones, as well as the elaborate intercommunicating nutrient channels extending through- out the osseous tissue, are filled with the highly vascular marrow, which genetically is an extension of the osteogenetic layer of the periosteum, since the primary marrow is a direct ingrowth and ex- tension of this latter tissue. The marrow of all bones in very young animals is red in color ; after a certain time, however, that con- tubular and the spaces of some other bones assumes a lighter tint, finally becoming of a straw color, owing to the ;--, accumulation of fat within ■ . ■■ ■ the marrow-cells. Depend- ing upon this difference, two varieties — the red marrow and the yellow marrow — are recognized : it is to be remembered that the red marrow is genetically the older and represents the primary condition. The elements of the red marrow comprise a delicate connective- tissue reticulum supporting a rich vascular distribution, composed of arterioles breaking up into numerous capillaries, which, in turn, give place to venous radicles of large size and extremely thin walls. The Elements of red marrow ; stained and highly magni- fied, a, var.ous forms of marrow-cells : 6, eosinophilic ceil ; c, mast-cell ; d giant-cell ; e, nucleated red blood- cells ; r, red blood-cell. THE CONNECTIVE TISSUES. 51 meshes of the tissue contain great numbers of soft, plastic connective- tissue elements, the marrow-cells ; many of these, in actively- growing bone, become the osteoblasts. In yellow marrow the majority of the marrow-cells have undergone transformation into fat-cells. Additional huge, irregular, multinucleated, protoplasmic masses are occasionally encountered ; these are the giant-cells, or myeloplaxes (Robin), and are of interest as being elements es- pecially concerned in the absorption of osseous tissue, being iden- tical with the osteoclasts (KoUiker). These cells, with their nuclei, offer an example of what formerly was described as the endogenous mode of cell-formation. Dentine is analogous to bone, although differing in details of arrangement, since it is derived from embryonal connective tissue. The matrix becomes calcified, and contains, embedded within the ground-substance, numerous long, parallel, partly-branched tubes, the dentinal tubules. These correspond with the lacunae of bone, enclosing in some places delicate processes, the dentinal fibres. A more extended account of the structure and development of den- tine will be found in connection with the structure of the teeth. Fig. 59. Development of Bone. With the exception of the bones of the vault of the cranium, of the face, and of part of the lower jaw, the skeleton is mapped out, in its foetal condition, by solid cartilages which correspond in form more or less closely with the future bones. The primary embryonal cartilage is of the hyaline variety, being extremely rich in cells, many of which are engaged in division; the cell-groups are separated by a relatively small amount of inter- cellular substance, and the outer surface of these solid cartilages is closely in- vested by an important membrane, the primary periosteum. When bone is formed at the centres of ossification within the cartilage, it is termed endochondral bone ; when formed directly from and beneath the periosteum, periosteal bone. While quite complicated in its sequence of changes, it must be remembered that endochondral development results in the formation of structures which are largely temporary, and which finally, for the most part, suffer absorption. The permanent bones of the skeleton are, chiefly, the products of Primary embryonal cartilage repre- senting one of the carpal bones : /, perichondrium, or primary periosteum ; «, nutrient canals extending from the periphery. C2 NORMAL HISTOLOGY. the periosteum ; where bone is developed directly from the periosteum, and without being mapped out by primary cartilage, the process is spoken ol as intermembranous bone-formation, although differ- ing in no important respect from that pnxlucing the periosteal bone. Endochondral Bone. The first indications of the future pro- found changes within the solid cartilage correspond in position to the so-called centres of ossification, 60. and consist in an increase in the size g,%9^i^H of the embryonal cartilage-cells, as well ^ '"»'?-«*f as in the amount of intercellular sub- stance separating the cell-nests, followed by a characteristic rearrangement of the enlarged cells into vertical rows or columns ; in the matrix between and around these columnar groups a cal- careous deposit subsequently takes - :'®®®^J'»f^j^^./ place. These enlarged cartilage-cells, «■ - •>i,^-<*e-^'* surrounded by the calcified matrix, are Developing bone — centre of ossifica- . 1 ^ „_:.„„_.. „_„^1 r Cl .. . I . r 1 J the primary areolae 01 bharpev. tion in a carpal bone : 2, area of enlarged ^ f J ^ <~^ iw|yv.^. cartilage- lacunae and calcified matrix; «) ; at / an osteoblast just being isolated. places forming an outer envelope closely embracing the central endo- chondral bone. The details of the process by which the osteoblasts are converted into the bone-cells are the same in both the intracartilaginous and the periosteal formation. The bone-matrix, deposited through the agency of the cells, gradually accumulates around the osteoblast, until this lies completely surrounded by the young matrix, when, after its isolation from the marrow-cavity, it becomes the bone-cor-puscle. At first the canaliculi are wanting, as are, also, calcareous matters ; these later appear. The conversion of the original spongy into compact bone depends upon the development of additional lamellae within the meshes of the primary osseous net-work. As an initial step, a local absorption takes place, resulting in the enlargement of the pri- mary medullary spaces contained between the trabeculae of the perio.steal net-work; these osseous bands are thus reduced to thin bony partitions between large oval cavities, the Haversian spaces. A new growth of bone subsequently takes place within these spaces, THE CONNECTIVE TISSUES. 55 the osteoblasts depositing new bone upon the walls of these cylin- drical cavities, layer upon layer, until only a small central channel — the Haversian canal — remains as the representative of the large Haversian space. The outer boundary of the Haversian system, therefore, corresponds to the limits of the Haversian space, while the remains of the primary bone-trabeculae constitute the older interstitial lamellae of the adult tissue. Osseous tissue, wherever developed, is formed through the agency of the osteoblasts, the deriva- tives and descendants of the special- ized mesoblastic cells of the embryo ; whether in endochondral or periosteal formation, the bone-producing elements Fig. 64. Developing bone — both periosteal and endochondral : /, outer fibrous, o, inner osteogenetic layer of perios- teum ; p, trabecule of periosteal bone covered by the osteoblasts ; e, endochondral bone ; m, primary marrow- cavities. Developing bone — longitudinal section of embryonal phalanx : e, the primary cartilage of the extremities of the bone ; a, zone of enlarged and vertically-dis- posed cartilage-lacunae ; c, zone of calcifi- cation ; t, trabeculae of calcified cartilage covered with new bone ; in, marrow- cavity ; b, periosteal bone formed directly beneath the overlying periosteum,/. arrange themselves over the surfaces of the cartilage-trabeculae or the periosteal fibres respectively, and soon are surrounded by osseous 56 NORMAL HISTOLOGY. Fig. 66. Developing bone — portion of trabecula undergoing absorp- tion : b, bone-cells ; c, osteo- blasts ; w, bone-matrix ; o, multi- nucleated osleoclast lying within the absorption-pit, or Howship's lacuna. matrix ; this gradually thickens and encloses the osteoblasts, which now lie within minute bays or recesses, the entrances to which become gradually contracted, until the opposed edges join and the cells lie within lacunae completely surrounded by the bone-matrix : the osteoblasts have now become the bone- cells. The matrix is deposited as lamellae, especially marked in the bone formed in the later stages of foetal life ; between these are included the lacunae. The matrix is at first soft and possessed of a distinct ftbrillated structure in which the subsequent deposit of lime salts — principally the phosphate and carbonate — takes place. When, on the contrary, bone or carti- lage is absorbed, it is through the agency of the giant-cells, the osteoclasts, or chon- droclasts (Klein); these large multinucleated elements usually lie upon the surface of the bone-trabeculae within larger or smaller pits which have been excavated by them ; these are Howship's lacunae. In recapitulation, the following summary of the phases of de- velopment during the growth of a tubular long bone may be noted : 1. Solid embryonal cartilage. 2. Enlargement and rearrangeivrnt of cartilage-cells and lacunae and calcification of matri.x at centre of ossification. 3. Penetration of periosteal tissue to the focus of calcification ; vascularization of the cartilage. 4. Formation of medullary spaces by the breaking down of lacunae surrounded by the zone of calcifying cartilage. 5. Covering of the surface of calcified cartilage trabeculae by the layer of osteoblasts and the productio7i of a?i enveloping sheath of true bone. 6. Resulting cefitral net-work of eyidochondral bone, with gradual absorption of encased cartilage trabeculae. 7. Absorption of central sp07igy bone in shaft and formation of central marrow-cavity. 8. Formation, mean-ivhile, of peripheral periosteal net-work of spongy bone. 9. Co7iversion into compact bojie by partial absorption of tra- beculae to form Haversian spaces ; secondary deposit of concentric lamellae within these spaces forming Haversian systems of compact bone. THE CONNECTIVE TISSUES. 5; 10. Absorption of inner lamellcz of cotnpact bone as the shaft increases in diameter by the deposition beneath the periosteum ; production of enlarged medullary cavity. 11. Continued absorption of endochoyidral central bone until the latter is found alone in the epiphyses, where it continues to be pro- duced at the expense of the intermediate cartilage during the entire future growth of the bone. 58 NORMAL HISTOLOGY. CHAPTER IV. THE MUSCULAR TISSUES Contractility is possessed, to a certain degree, in common by all cells rich in active protoplasm ; the distinguishing characteristic of muscular tissue, however, is that this property is so conspicuously developed in highly specialized structures, and that the contractions take place along definite lines in limited directions alone. Con- tractile tissue or muscle occurs in two principal forms: (i) as the non-striated, smooth, or vegetative muscle, usually beyond the control of the will, and hence called involuntary , and (2) as the striated, striped, or animal muscle, which, being influenced by volition, is known as voluntary . The sharp differences separating the two groups of muscle in man and the higher animals cannot be regarded as fundamental, since in the embryonal condition of these higher forms temporarily, and in the adult form of the lower types permanently, the striped and non- striated varieties of muscle depend upon the degree of specialization rather than upon inherent differences. It is a suggestive fact that long before the cells forming the embryonal heart show indications of differentiation into muscle-tissue the contractions of the organ have commenced. The association of the striped fibres with response to the will and, on the contrary, of the plain tissue with involuntary action must be, likewise, only provisionally accepted, since in some animals the development of marked striae never takes place in the voluntary fibres. Standing between and connecting the extremes of these groups is the cardiac muscle of the higher vertebrates, in which the fibres are striated, although beyond the control of the will. non-striated or involuntary muscle. Non- striated, smooth, or involuntary muscle, while never occurring in large individual masses, enjoys a wide distribution ; its principal localities are — 1. The Digestive Tract: the muscularis mucosae from oesophagus to anus and the delicate bundles of mucosa and villi ; muscular tunic from the lower half of oesophagus to anus. 2. The Accessory Digestive Glands : in the large excretory ducts of liver, pancreas, and some salivary glands ; also in the gall-bladder. 3. The Uritiary Tract : in the capsule and the pelvis of kidney, ureter, bladder, and urethra. THE MUSCULAR TISSUES. eg 4. The Male Generative Organs : in epididymis, vas deferens, i^esiculae seminales, prostate body, Cowper's glands, cavernous and spongy bodies of penis. 5. The pemale Generative Orga7is : in oviducts, uterus, and vagina ; in the erectile tissue of external genitals ; in broad and round ligaments, and in erectile tissue of nipple. 6. ^ The Respiratory Tract : in the posterior part of trachea ; encircling bands in bronchial tubes, and bundles within pleura. 7. The Vascular System : in the coats of arteries, veins, and larger lymphatics. 8. The Lymphatic Glands : in the capsule and the trabeculae of spleen ; sometimes in the trabeculae of lymphatic glands. 9. The Eye : in iris and ciliary body, and in eyelids. 10. The Integument : as the arrectores pili connected with the hair-follicles ; in sweat and some sebaceous glands ; in skin covering the scrotum and parts of the external genitals. Involuntary muscle is composed of delicate spindle, often rib- bon-like, fibre-cells ; these vary greatly in size, measuring 75-225 Fig. 68. M* long and 4-8 ii wide. The cells found in arteries are short Fig. 67. Isolated involuntary-muscle cells from intestine of man. Involuntary-muscle cells from mesentery of newt : n, nuclei ; _/", axial fibre ; m, transverse markings on surface of cell ; B, muscle-cell with forked extremity. and flat, being but 25-45 !'■ ^on^ and 9-12 ,a wide; the largest ele- ments are found in the gravid uterus, where they reach a length * I m {micron) = th.e 1000th part of a millimetre. 6o NORMAL HISTOLOGY. of over 500 jj. and a breadth of 20 11. Occasional cells with bi- furcated ends are encountered, especially among the lower verte- brates. The spindle muscle-cell is invested with a very delicate, homo- geneous, hyaline sheath, closely resembling elastic tissue, and corresponding to the sarcolemma of the striated fibre ; within this envelope lies the soft, semi-fluid, contractile protoplasm, embedded in which, near the centre of the cell, lies a characteristic, narrow, rod-shaped nucleus. Delicate longitudinal fibrillae sometimes can be made out extending the entire length of the cell ; these are re- garded by many histologists as representing the actively contractile parts of the cell, the surrounding protoplasm being largely passive. Transverse markings are also often seen ; these correspond in posi- tion to local variations in the diameter of the cell, and are probably due to corrugations in the enveloping mem- brane. The individual spindle-cells are closely fitted together and united by an albu- minous cement-substance ; they are dis- posed in groups or bundles, which, on cross-section, are made up of rounded polygonal areas of varying size, the larger possessing round nuclei, while the smaller have hone. Since these areas are the sections of nucleated spindle- cells, the large nucleated fields corre- spond to sections passing through the nucleus of the cell, while the small ones are sections of the cell fall- ing near the pointed ends. The bundles of muscle-cells are arranged to form layers or sheets, as in the digestive tract, or net-works, as in the eye, pleura, etc. ^'*^- 70- Examined in longitudinal sec- tion, or in considerable masses, it is difficult to distinguish the individual component fibre- cells, the involuntary muscle in such cases closely resembling fibrous connective tissue ; how- ever, the numerous more or less regularly disposed rod- shaped nuclei, and the absence of the delicate wavy fibres, together with the impression of greater density, usually suffice to establish the identity of the muscle. Involuntary muscle in transverse section : portions of three bundles are represented, separated by areolar tissue (a): the nucleated areas are sections of the muscle-cells through their nuclei ; the smaller figures repre- sent sections of the cells cut nearer the ends. Involuntary muscle in longitudinal section : the muscle-cells are often cut obliquely, and hence appear shorter than when isolated. THE MUSCULAR TISSUES. 6i The connective tissue uniting the larger bundles of muscle-cells supports the blood-vessels and nerves. The larger blood-vessels break up into capillary net-works, which pass between the muscle- cells. The nerves, derived principally from the sympathetic system, likewise penetrate the intercellular spaces and terminate between the cells in the manner more fully described in the chapter devoted to nerve-endings. Lymphatics occur, as in parts of the digestive tract, closely associated with the muscular tissue. STRIATED OR VOLUNTARY MUSCLE. Striated or voluntary muscle, in addition to the extensive system attached to the skeleton, supplies the special muscles connected with many organs, including the tongue, pharynx, middle ear, larynx, upper half of the oesophagus, diaphragm, generative organs, etc. This form of muscle is composed of long, irregularly cylindrical fibres, each of which represents the high specialization resulting from the development of the single original embryonal cell ; the fibre is, therefore, the structural unit of the striated muscular tissue, and corresponds to the spindle fibre-cell of the involuntary variety. The fibre of striped muscle comprises (a) the sarcolemma, (d) the micscle- nuclei, and (c) the muscle-stibstance. Each fibre is closely invested by a clear, homogeneous, elastic sheath — the sarcolemma — which, ordinarily, so tightly adheres to the enclosed musclcTSubstance that the two are optically blended together; in favorable positions, Fig. 71. as where breaks in the sarcous substance occur, or after the action of water, the sarcolemma is separated from the muscle-substance, and is then seen in profile as a delicate line spanning the break in the continuity of the fibre. The sar- colemma forms a closed sac completely envel- oping the contractile substance of the fibre. Immediately beneath the sarcolemma, lying within minute depressions on the surface of the muscle-substance, are the muscle-nuclei. These are oval or fusiform, usually placed parallel to the long axis of the fibre, and sur- rounded, especially at their ends, by a small amount of granular protoplasm. These accumu- lations represent the meagre remains of the indifferent protoplasm which has not undergone conversion into the highly specialized muscle-substance of the fibre. In mammalian muscle the nuclei lie always upon the surface of the sarcous substance of the fibre and immediately beneath the sarco- Voluntary-muscle fibres, somewhat broken after treatment with water, showing the sarcolemma (s) in several places. NORMAL HISTOLOGY. lemma ; in the majority of other vertebrates, however, the nuclei are distributed irregularly throughout all parts of the contractile substance. These differences are well shown in the accompanying figures. The muscle -fibres present alternate light and dark transverse markings, or striae, to which the tissue owes its characteristic appear- ance. The highly specialized contents of the sarcolemma are composed of two substances pos- sessing different refractive prop- erties, that forming the dark bands being doubly refracting, or anisotropic, while that of Fig. 72. B Fibres of voluntary muscle in section : A , human fibres, with nuclei upon the surface and beneath the sarcolemma ; B, fibres from frog, with nuclei embedded within the muscle-substance. Fig. 73. Voluntary muscle, portions of two fibres show- ing the characteristic transverse markings ; the lighter band is divided by the row of minute beads constituting the intermediate disk : a, termination of muscular substance and attachment of adjoin- ing fibrous tissue ; n, nuclei of muscle-fibres. the light striae is singly refracting, or isotropic. When fresh or well-preserved mammalian muscle is examined under high am- plification it is seen that the dark striae, or transverse disks, are not unbroken homogeneous bands, but that each is composed of a number of minute prismatic elements placed side by side and sep- arated from one another by a thin layer of a substance corresponding to and continuous with that forming the light zone. This latter, in addition, is divided transversely by a delicate interrupted line or row of dark dots — the intermediate disk, or membrane of Krause. That part of the light zone between the dim intermediate and trans- verse disks constitutes the lateral disk. The explanation of these appearances has caused many and pro- longed discussions, and even at present, notwithstanding the careful study bestowed upon the subject, the exact structure of voluntary muscle must be regarded as still unsettled. Heretofore two promi- nent and opposed views have prevailed : the one regards the fibre as composed of parallel longitudinal rows of minute prisms forming fibrillae (as rows of bricks placed end to end) ; the other considers the fibre as built up by the apposition of their disks, whose diameter corresponds to that of the entire fibre (as cheese-boxes piled one THE MUSCULAR TISSUES. 6^ Upon the other). After treatment with alcohol, the fibres of striped muscle readily split up lengthwise into delicate bundles, which, with care, may be subdivided to such an extent that the resulting threads embrace in their width only a single row of alternating light and dark elements. These ultimate fibrillae were formerly considered by KoUiker as the normal elements of the fibre ; the dark prisms of these , fibrillae correspond to the sarcous elements, which were regarded by Bowman as the component units of muscular tissue. The transverse cleavage of the fibre following the action of diluted mineral acids, on the other hand, has been upheld as representing the natural division. According to Krause, the fibre is divided through the light bands by a number of transverse partitions con- tinuous with the sarcolemma ; these assumed septa appear as delicate broken lines — the membranes of Krause — and are identical with the intermediate disks already mentioned. Adopting this view, the fibre is composed of numerous thin zones or contractile disks, each of which embraces the dark dim band in its centre and half of the light stripe at either end. Each contractile disk is further sub- divided by vertical partitions extending between the neighboring membranes of Krause, thus forming in every disk a row of com- partments or muscle -caskets. The portion of the dim band con- tained within each muscle-casket has been regarded as itself being composed of a series of thin prisms of contractile substance — the muscle-rods. After renewed critical study of the subject, Rollett has presented a view regarding the structure of voluntary muscle which not only offers the most plausible solution of this difficult problem, but is, likewise, in harmony with the history of the development of the tissue. According to this theory, the muscular tissue is composed of the highly specialized, darker, anisotropic contractile substance, and the relatively passive, lighter, semi-fluid, isotropic sarcoplasm. The contractile substance is arranged as delicate spindles, the appo- sition of whose thicker parts produces the dhn transverse disk seen under medium amplification ; at either end the spindle is prolonged as an extremely thin thread, which terminates in a minute sphere or bead ; the apposition of these beads in the transverse row gives rise to the appearance of the interrupted line constituting the inter- mediate disk, or Krazise' s 7ne?nbrane. The darker anisotropic sub- stance forms, therefore, numbers of continuous contractile fibrillae, which extend in parallel bundles the entire length of the fibre ; all the remaining interfibrillar space within the sarcolemma is filled with the lighter sarcoplasm, which appears faintly granular in preserved tissue, but is, probably, almost fluid during life. On comparing this description with the usual appearances presented by striated muscle, 64 NORMAL HISTOLOGY. Fig. it will be seen that the lateral apposition of the thicker parts of the contractile fibrilla produces the dark band, or trayisverse disk, while the row of minute spherical masses appears as the interrupted dark line bisecting the light zone, or intermediate disk. The threads bridging between these beads and the chief mass of the fibrillae are too delicate to be appreciated under ordinary powers, and that portion of the fibre corresponding to the lateral disk consequently appears as if made up of the lighter sarcoplasvi alone. In certain forms of invertebrate muscle a more complicated arrangement exi^s, since on either side of the intermediate disk a row of dark granules crosses the light lateral disk, forming a dim secondary disk ; these gran- ules are connected with the intermediate and transverse disks by delicate bridges of con- tractile substance, along which they occur as local thickenings. The dim transverse disk sometimes contains a central lighter band, the median disk of Hansen, which is due, probably, to diminished thickness of the con- tractile fibrils. The contractile fibrillae, however, are not uniformly distributed throughout the fibre, but are aggregated into bundles — the muscle- columns — each of which is enveloped in a thicker layer of the sarcoplasm than the partitions separating the individual fibrillae. In transverse section each muscle-fibre presents a number of small, polygonal, dark areas, enclosed by lighter lines, which areas, under high amplification, exhibit minute punctations. These areas are sections of the viuscle-columns and correspond to Cohnheim's fields, the dots being sections of the individual constituent fibrillae; the lighter intervening and surrounding substance is the sarcoplasm, thicker layers of which surround and separate the larger groups into which the muscle-columns are further collected. The individual muscle-fibres, which usually are not circular in cross-section, but rather irregularly polygonal with rounded angles, are held together by a small amount of areolar tissue, the endo- mysium. They are grouped into primary bundles, which latter are enveloped and separated from other primary bundles by the thicker bands of connective tissue constituting the perimysium. The primary bundles are united to form larger secondary groups or fasciculi, upon the width and arrangement of which the coarseness A, diagram of arrangement of the contractile substance according to the view of Rol- lett : the granular figures rep- resent the contractile element*, the intervening light areas the sarcoplasm ; B, small muscle- fibre of man ; the correspond- ing parts in the two figures are indicated : t, i, I, respectively the transverse, intermediate, and lateral disks ; n, muscle- nuclei. THE MUSCULAR TISSUES. 5e or the fineness, macroscopically appreciable, of the muscle largely depends. The entire muscle is invested in a fibrous sheath, the epimysium, derived from the denser layers of the interfascicular connective tissue. When contraction takes place, the entire muscle becomes shorter and, at the same time, broader ; the striae also participate in the changes, becoming narrower. Fig. 75. These phenomena, however, affect only sl A B limited part of the fibre at one time, consecu- tive portions being influenced in regular se- quence, so that the changes pass along the fibre as a contraction wave ; after the passage of the wave the muscle resumes its previous condition. In short muscles the individual fibres quite frequently extend the entire length ; in long ones, on the contrary, the fibres are shorter than the muscle, being generally some 30-45 mm. long ; sometimes, however, the fibres reach a length of 120 mm. by 10-50 mm. in width (Felix). The fibres, as a whole, are gen- erally somewhat spindle-shaped, being slightly larger in the middle ; the ends of the fibres are more or less pointed, although blunted or club-shaped, and, more rarely, Fig. 76. branched, extremities are not un- vk common. Branched and anas- tomosing fibres frequently occur (Gage), especially in the tongue Fig. 77. Muscle-fibres in transverse section, highly magnified : A , portion of human muscle : the small, irregular areas are the fields of Cohnheim (c) ; B, semi-diagrammatic view show- ing the groups of muscle-col- umns composing Cohnheim's fields ; n, nucleus ; m, groups of muscle-columns. Voluntary muscle in transverse section : the irregular polyhedral areas (y") are the individual muscle-fibres in section, held together by the en- domysium («) ; the primary bundles of the fibres are enclosed by the denser perimysium (/). Branched voluntary-muscle fibres from the tongue. and ocular muscles. When the individual fibres do not extend the length of the entire muscle, the sarcous substance terminates in 5 66 NORMAL HISTOLOGY. Fig. 78. pointed or rounded extremities, while the sarcolemma is united with the endomysium of the surrounding fibres. The muscle-substance is never directly continuous with adjacent tissues, but is always enclosed within the sac of the sarcolemma ; the union between the fibres and other structures is effected by the blending of the endo- mysium of the muscle-fibres with the connective tissue of the attach- ments, whether these be tendon, periosteum, perichondrium, or subcutaneous tissue ; the sarcolemma closely invests the sarcous contents, being simply received into the connective tissue without becoming directly continuous. CARDIAC MUSCLE. The muscular tissue of the heart, as well as of the cardiac ends of the large veins, forms an intermediate group of contractile tissue, standing in its development between the simple spindle non-striated cell on the one hand and the highly differentiated striped fibre orf the other. Among the lower vertebrates (fishes, amphibians) the cardiac muscle is composed of nucleated spindle-cells possessing distinct transverse striations and often branched ends ; in man and the higher vertebrates these spindle- cells give place to short, striated, cylindrical fibres, provided with lateral processes. By the apposition of these richly-branched cells a close, narrow-meshed net-work is formed, the juncture between the individual elements being indicated by transverse lines of cement-sub- stance. The peculiarities of heart-muscle are — of the sarcolemma, the transversely striated longitudinally marked muscular tissue being Heart - muscle, showing several joined branched fibres : around the poles of the nuclei are aggregations of pigment-granules. 1. The absence and more faintly naked. 2. The situation of muscle-nuclei within the sarcous substance, usually near the centre of the cell. 3. The characteristic arrangement of the contractile fibrillae, since these are so placed that the peripheral fibrillae are grouped into flat, ribbon-like muscle-columns, somewhat radially disposed about the circumference of the fibre ; the remaining central portion is occupied by prismatic bundles of fibrillae, together with the nuclei and the associated protoplasm (Ranvier, Kolliker). The small masses of pro- toplasm which surround the muscle-nuclei usually contain minute fat-drops and pigment-granules. The amount of pigment normally present varies with age, increasing from the tenth year (Maass). Fig Heart-muscle fibres in sec- tion : the peripheral zone is composed of radially-ar- ranged groups (/) of muscle- columns ; a zone {c) of less differentiated sarcoplasm surrounds the nucleus. Fig. So. THE MUSCULAR TISSUES. 57 Sometimes, in preserved tissue, the position of the nucleus is occu- pied by a clear vacuole. Ranvier has called attention to certain differences in the muscles of the rabbit, describing two varieties — the red or especially dark (semitendinosus, soleus) and the white or pale (adductor magnus). The red muscles are char- acterized by slow response to electrical stimulus, less regular transverse striation, greater dis- tinctness of longitudinal markings, and great number of round nuclei. The blood-vessels of striated muscle are very numerous. The larger vessels, together with the nerve-trunks and, less frequently, the lymphatics, are contained within the perimy- sium, where they give off numerous smaller branches ; these, in turn, extend between the primitive bundles and break up into extremely thin capillaries, which form a characteristic rectangular-meshed net-work around the indi- vidual muscle-fibres. The longer sides of the meshes correspond with the axis of the fibre. At various points along the course of these vessels peculiar dilatations, or ampullae, occur, the object of which is, probably, the relief of sud- den temporary interference with the circulation during muscular contractions. The relation be- tween the capillary blood-vessels and the muscu- lar fibres of the heart is very intimate ; in many places the vessels lie embedded within or even en- tirely surrounded by the muscular tissue (Meigs). Lrymphatic vessels occur in striated muscle in small numbers, but are entirely wanting in many small muscles ; when distinct lymphatic vessels do occur, they are confined to the larger and looser masses of the perimysium (KoUiker). The nerves supplying the striated muscle include the principal trunks which run within the perimysium, where they subdivide into smaller groups of medullated fibres, in order to reach the individual muscle-fibres ; these latter receive their nervous supply at certain points only, the nerves passing to the muscle to end in the special end-plates in the manner described more fully in connection with the peripheral nerve-endings. The development of all varieties of muscular tissue is closely related to the mesoderm, of which they are the direct descendants. The plain or non-striated muscle is formed by the differentiation, Injected voluntary mus- cle : the capillaries form rectangular-meshed net- works enclosing the indi- vidual fibres. 68 NORMAL HISTOLOGY. within certain areas, oi the irregular mesodermic elements into the elongated fusiform fibre-cells. In suitable preparations all gradations between the ordinary embryonal connective-tissue cells and the muscular elements may be observed, emphasizing the common ancestry of the two forms of tissue. Voluntary muscle, representing a higher specialization, is de- rived from definite areas constituting the imter layer of the muscle- plates, which are referable to the early stages of the primary segmenta- tion into sotnites. The cells of the muscle-plate soon elongate, wi^h pro- liferation of the nuclei, to become the primitive muscle-fibres. These at first consist of greatly extended ele- ments, possessed of numerous nuclei and composed of granular indifterent protoplasm. After a time the fibre exhibits a difterentiation into longi- tudinal striae, which, later, are supple- mented by the transverse markings characteristic of voluntary muscle. The sarcolemma appears about the time the longitudinal markings are seen. The striations are limited, at first, to one side of the fibre, then extend over the entire periphery, but still for some time do not reach the centre of the fibre, an inner zone of undiffer- entiated sarcoplasm occupying the middle. Later, this area also becomes converted into striated tissue, while the once numerous nuclei are reduced to the few collected beneath the sarcolemma. Cardiac muscle, likewise, develops from the mesoderm immediately surrounding the primary heart-tubes, the contractions of the cells being displayed even before the histo- logical differentiation becomes apparent. In its development it represents an intermediate stage, since the original spindle-cells be- come converted into protoplasmic fibres containing a central area which always remains less differentiated and nearer its primary con- dition of indifferent sarcoplasm than the peripheral portions of the fibre. The fibres of Purkinje, found in the hearts of certain rumi- nants, represent muscular fibres in which the sarcoplasm remains in part still undifferentiated. Developing voluntary muscle: A, young muscle-cells ; a, very young spindle-cell ; b, older element, exhibiting indications of future striation on one side ; the remaining part of the cell is composed of the undifferentiated sarco- plasm ; B, embryonal muscle-fibres pos- sessing many nuclei and traces of striae ; C, developing muscle-fibres in section ; in the larger fibres a differentiated peripheral zone of striae [d) is seen in section ; an area of still indifferent sar- coplasm occupies the centre of the fibre and surrounds the nucleus («). THE NERVOUS TISSUES. 6g CHAPTER V. THE NERVOUS TISSUES. The nervous system is composed of two principal parts — the tissues originating and transmitting nervous impulse, the nerve- cells and the nerve-fibres, and the tissues uniting and supporting the nervous elements, the neuroglia and connective-tissue frame- work. While convenience warrants the consideration of the nerve- cells and the nerve-fibres as distinct elements, it must be appreciated that the nerve-fibres are only the extended processes of the nerve- cells, and that both cells and fibres together constitute the morpho- logical units, the neurons, of which, in conjunction with the sup- porting tissues, the entire nervous system is composed. In the simplest type of nervous system the perception of external stimuli and the generation and transmission of the responsive impulse are performed by the same cell, the more centrally lying parts of the cell serving to convey and expend the force originating within the protoplasm. Such simplicity, however, is unusual, the nerve-cell soon becoming specialized and removed from the peripheral area with which it maintains connection by means of its process — the nerve-fibre. NERVE-CELLS. Nerve- or ganglion-cells of man and other vertebrates differ greatly in form and size, since they may be either spherical (Gasserian, spinal, or other ganglia), ellipsoidal (spinal cord), pyriform (cere- bellum), pyramidal (cerebrum), or stellate (spinal cord), and vary from ID to ICO /i in size. The ganglion-cells are composed of granular or striated proto- plasm, containing a large round or oval vesicular nucleus within which lies a prominent nucleolus ; after certain stains the protoplasm, nucleus, and nucleolus present distinct tints. Many nerve-cells are deeply colored, owing to the presence of considerable quantities of pigment-granules around the nucleus ; a certain amount of pigment within the protoplasm is almost constant. The protoplasm of every nerve-cell is prolonged into at least two and frequently several processes, dependent upon the number of which it is customary to speak of nerve-cells as bipolar, or mtilti- polar. Since an apolar nerve-cell is, evidently, functionally use- less, it is doubtful whether such cells ever normally exist ; apolar cells are frequently seen in preparations, but the absence of the jQ NORMAL IllSTOLOCiY. processes is only apparent, being- due either to mutilation or to the process lying without the plane of the section ; where processes are really wanting, an immature or pathological condition must be suspected. Tlic processes of nerve-cells arc of two principal kinds — the protoplasmic {dtmdn'ts) and the axis-cylinder {iieurits) processes. The protoplasmic pro(fesses Fig. 82. or dendrits rapidly undergo division, splitting up and sub- dividing until the resulting branches form rich terminal arborizations of slender threads, which frequently interlace, but probably never actually join, with similar fibrils of adjacent cells. Nerve-cells, in one sense, are but nucleated local accumula- tions of the interfibrillar pro- toplasm, which latter may be termed neuroplasm (K61- liker) ; the large striated multipolar ganglion-cells may be regarded as switch-boards for the redistribution of the numerous ultimate fibrillae continued into the axis-cylin- ders. The fibrillae pass off in divergent paths, along the several processes of the cell, to form new combinations and relations. The peculiarities formerly supposed to constitute the distinguishing characteristics of the axis-cylinder processes are no longer sufficient in the light of recent advances in our knowledge regarding the structure of the nervous system. The investigations of Golgi and others have shown that, in addition to greater delicacy and a straighter course, the axis-cylinder processes present variations which separate gan- glion-cells into two groups— cells of the first and cells of the second type. Nerve-cells of the first type include elements, as those of the motor areas, possessing the characteristic axis-cylinder processes directly continuous with the axis cylinder of the nerve-fibre. While Nerve-cell from the cerebral cortex, exhibiting the striations of the protoplasm and the conspicuous char- acter of the nucleus and the nucleolus: /, pigment- granules ; a, axis-cylinder process ; b, I, apical and lateral protoplasmic processes. THE NERVOUS TISSUES. n these processes, when compared with the richly-divided protoplasmic, may be regarded as unbranched, the existence of delicate lateral off- shoots, or collateral fibrils, has been established ; these delicate branches pass backward towards the gray matter, within which they end, — often at considerable distances, in relation with other cells. Fig. 83. Nerve-cell from the spinal cord, isolated by maceration and teasing : the numerous branched pro- toplasmic processes arc somewhat displaced and distorted, owing to manipulation : a, axis-cylinder process. Nerve-cells of the second type are distinguished by the be- havior of the axis-cylinder process ; this, instead of passing into the white matter to become the centre of a nerve-fibre, never leaves the gray tnatter in which the ganglion-cell lies, but, after a longer or shorter course, rapidly undergoes division and subdivision in the production of a terminal arborization of delicate fibrillae ; these ramifications are limited entirely to the gray matter, their exact manner of ending and their relations to other cells varying in dif- ferent parts. The free division of the axis-cylinder process does not curtail the branching of the protoplasmic extensions, which are often very conspicuous, notwithstanding the numerous bifurcations of the former. In some instances the axis-cylinder processes of cells of this type split up into fibrils which enclose the bodies of other nerve-cells within basket-like net-works ; a notable ex- ample of this arrangement exists in the cerebellum around the cells of Purkinje. 72 NORMAL HISTOLOGY. Fig. 84. The a.xis-cylinder processes usually are directed towards the nearest mass of white matter, since the a.xis-cyliiider of the nerve- fibre is directly continuous with that of the cell. Exceptional arrangements are sometimes encountered, as where one process of a bipolar cell becomes wound about the remainini^ straighter fibre, con- stituting a spiral process ; such cells are comparatively frequent in the sym- pathetic ganglia of the frog. Ganglion-cells lie within peri-cellu- lar lymph-spaces, which appear with greater or less Fig- 85. distinctness ac- p iiM'f't/ cording to the condition of the Nerve-cell of first type — from cere- bral corte.x : /', /, protoplasmic pro- cesses directed respectively towards the free surface and laterally ; a, axis-cylin- der or nerve-process giving off collateral branches, c, c. Golgi staining. Nerve-cell of second type — from cerebellum : /, branched protoplasmic processes ; c, cell- body ; a, axis-cylinder process breaking up into arborization (h), but entirely confined to gray matter. Golgi staining. Basket - work, formed by the extensions of the branched axis -cylinder process of a nerve-cell, surrounding the body of one of the ganglion-cells of Purkinje : /, base of branched process of Pur- kinje's cell ; n. fibrils con- stituting basket-work. protoplasm of the enclosed cell ; when this is contracted and shrunken the space is, obviously, more conspicuous than when almost entirely filled by the cell. These lymph-spaces are limited by a delicate, elastic, hyaline membrane, and lined with nucleated endothelial plates. NERVE FIBRES. Depending upon the character of the investing coats, nerve-fibres appear as two kinds— the meduUated, or white, and the non- medullated, or gray. These do not, however, constitute two sharply defined and distinct classes, but depend upon variations in the condition of fibres, which often represent both varieties at dif- ferent portions of their course. Every medullated nerve-fibre, start- ing from the nerve-cell as the axis-cylinder process, at first lacks the THE NERVOUS TISSUES. n medullary substance ; on obtaining the latter, it then courses as the typical meduUated fibre until near its termination, where again the white substance of Schwann is wanting, the end arborizations consisting of the ramifications of the naked axis-cylinder. The majority of nerve-fibres constituting the great cerebro-spinal tract may be classed as medullated, although numbers of gray fibres like- wise o^ccur here ; the non-meduUated fibres are especially numerous in the sympathetic system, where they predominate, as well as in certain of the cranial nerves, as the olfactory. In other cases, as conspicuously in the pneumogastric nerve of the dog, distinct bundles of both medullated and non-meduUated fibres are associated in the for- mation of a common nerve-trunk. A typical medullated nerve-fibre consists of the following parts : 1. The axis-cylinder, surrounded, possibly, by its sheath, or axilemma (Kiihne). 2. The medullary sicbstance, or white matter of Schwann. 3. The neurilem^na, or sheath of Schwann, with the nerve-corpuscles. Perfectly fresh, uninjured, medullated nerve -fibres, when examined by transmitted light, appear as homogeneous, hyaline cylinders, with dark contours and no appreciable structure ; seen by reflected light, the fatty character of the medullary substance is indicated by the glisten- ing appearance of the fibres, and their dull white color when viewed in masses. Shortly after death the fibres exhibit characteristic double contours, enclosing an apparently structureless centre; later, the fibres become mottled by irregular spherical masses, derived from the dis- torted medullary substance. The axis-cylinder appears, in fresh nerves or in those fixed with osmic acid and teased, as an inconspicuous, clear, delicate rod extending along the central part of the fibre, or, perhaps, projecting beyond the outer sheaths ;it the broken end. The longitudinal striations occasionally seen, under high amplification, in carefully fixed preparations, are indi- cations of the ultimate fibrillae of which the axis-cylinder is com- posed ; these fibrilte are cemented together by a finely granular, interstitial substance, or neuroplasm (Kolliker). According to Kiihne, the axis-cylinder is enveloped by a special, delicate, elastic Nerve-cell from a sym- pathetic ganglion of frog, showing the tortuous course and terminal net-work of the spiral fibre: n, neuri- lemma continued as a deli- cate sheath. (After Ket- zius.) 74 NORMAL HISTOLOGY. Medullated nerve-fibres : A , teased in salt solution, x, shortly after death ; y, a node of Ranvier ; z, post-mortem distortions of med- ullary substance. />', an isolated stained fibre; a, axis-cylinder ; r, node of Ranvier; tii, medullary substance ; n, neurilemma, beneath which a nerve-corpuscle is seen in the lower .segment. sheath — the axilemma (Kiihnc) ; other authorities reejard this appearance as an artificial production. Since every axis-cyHnder is connected with the corresponding process of a nerve-cell, axis-cylinders may be regarded as direct continuations of the ganglion-cells, their component fibrillae forming uninterrupted paths which con- nect the periphery with the presiding nerve-centres. On approaching its ter- mination, the axis-cylinder splits into smaller bundles of component fibrillae ; these groups subsequently divide, until, finally, the naked ner- FiG. 89. vous threads reach their ultimate destination as the terminal arborizations. The nerve-librillae not in- frequently exhibit nu- merous minute fusiform enlargements or vari- cosities along their course, giving to the fibrils a characteristic beaded appearance, es- pecially after gold-stain- ing. The medullary sub- stance, or white matter of Schwann, surrounds the axis-cylinder, and forms the most con- spicuous investment of the fibre. The existence of a narrow lym- l^hatic cleft described as lying between the medullary substance and the axilemma is still uncertain. The medullary substance consists of two parts: one of these occurs as a delicate reticulated frame- work, composed of a resistant material probably resembling neuro- keratin (Kiihne and Ewald) ; the other fills the interstices of the reticulum and appears as a semi-fiuid, highly refracting, fatty sub- stance— the myelin — which affords protection to the enclosed axis- cylinder. Other authorities regard the reticulated framework as the effect of reagents, citing the variability in the appearances of the net-work as opposed to its presence as a normal constituent of the coat. At regular interv^als along the medullated nerve-fibres well-marked annular constrictions occur ; these are the nodes of Ranvier, and mark the interruption of the white substance of Schwann at certain points. Gold-stained axis- cylinder («) and bear the motor end-plates ; s, some of the sensory nerve-fibres sup- plying the muscle. primary bundles of the muscle-cells, and probai)ly terminate in finely pointed or slightly thickened free ends ; the direct connection be- tween the nerve-fibrillae and the nuclei of the muscle-cells is, at best, extremely doubtful. Striated Muscle is supplied with both motor and sensory nerves ; the latter are distributed as a loose net- work, the fibrillse of which appar- ently terminate between the individual muscle- fibres. The meduUated nerve- fibres composing the motor supply of a vol- untary muscle unite to form an intramuscular plexus, from which small bundles of nerve-fibres spring, and subsequently divide in such manner that a single meduUated axis-cylinder passes to each muscle-fibre. At the point where the nerve pierces the sarcolemma the medullary substance abruptly ends, while the neurilemma, blended with the sarcolemma, joins the peri- neuria! (Henle's) sheath in forming the telolemma, or the sheath investing the end-organ. The axis-cylinder, now beneath the muscle-sheath, continues upon the surface of the sarcous sub- stance, and, later, breaks up into a number of somewhat tortuous ultimate fibrillae, which irregularly unite and end in thickened bulbous extremities. The terminations of the nerve are embedded in a flattened nucleated mass — the sole-plate — composed of soft faintly granular protoplasm, which resembles sarcoplasm and is closely applied to the surface of the muscular substance ; this mass, together with the embedded nerve-fibrillae, constitutes the motor disk, or end-plate. Each muscle-fibre possesses usually but a single end-plate ; in exceptional cases, however, there may be two or more ; likewise, Motor end-plate of voluntary muscle from rabbit : n, meduUated nerve-fibre passing to muscle (>«), on the surface of which the axis-cylinder ends in the dark arborescent figure ; the latter lies em- bedded within the nucleated sole-plate (j) composed of granular protoplasm. THE PERIPHERAL NERVE-ENDINGS. 91 several nerve-fibres instead of a single one may supply the end-plate. The nerve-endings in the voluntary muscle of amphibians and bony fishes differ from the foregoing in the absence of the granular protoplasmic disk, and in the more diffuse disposi- tion of the terminal nerve- fibres. The axis-cyhnders, in these cases, branch into fibrillse which extend for some distance parallel to the axis of the muscle- fibre and end in slight bulbous expansions ; gran- ular pyriform nuclei also occur along the course of these fibrillce. The muscle-spindles described by Kiihne, and considered by some (Kerschner) as special sensory nerve endings, appear to be transient developmental structures connected with the cleav- age of the muscle-fibres (KoUiker). Tendon. In addition to the sensory end- plates of tendon, studied by Kolliker, Rollett, Sachs, Golgi, and others, which consist of an intri- cate net-work of pale non - medullated fibres, Golgi has described pe- culiar nerve-endings in tendon to be found in the immediate vicinity of the union with the muscle. Golgi's corpuscle or tendon-spindle from the human tendo Achillis ; gold preparation : N, nerve-fibres surrounded by the perineurial sheath {Fs) spreading out into the reticular ramifications {Ev) of the axis-cylinder; A, the tendon- bundles, one of which is separated at b ; Mf, the muscle- fibres; R, node of Ranvier. (After Ciaccio.) 92 NORMAL HISTOLOGY. Fig. 113. These tendon-spindles appear as sharply-defined, greatly-elon- gated, elliptical masses (in the rabbit .25-75 mm. long and .02-. 01 mm. broad), one end of which extends upon the tendon, while the muscular pole is usually, although not always, continuous with the adjoining muscle-fibres. The tendon-spindle is composed of a distinct connective-tissue capsule, which, embracing two or more of the primary bundles of the tendon, becomes united with the sheath of the latter ; the inner surface of the spindle is covered with endo- thelial plates, MeduUated nerve-fibres to the number of two, three, or four join the organ near its widest part, sometimes, however, at one end ; after repeated division as medullated fibres, the nerves spread out on the surface of the tendon as pale, non-medullated fibres, whose axis-cylinders unite to form a richly but irregularly meshed arborescent figure ; the ultimate fibrillee, in addition to the net-work, present numerous knobbed free ends. Blood- Vessels. The blood-vessels are accompanied by nerve- fibres derived from the sympathetic system ; in addition to the pale fibres, a few medullated ones usually take part in the production of the irregular net-work surrounding the larger vessels. From this plexus fine branches are given off, which ultimately end between the muscu- lar bundles of the media and within the fibro-elastic tissue of the adven- titia. The capillaries are accom- panied and partly surrounded by delicate non-medullated nerve- fibres. The muscular tunics of the large lymphatic trunks are supplied with nerves in a manner similar to the blood-vessels ; the delicate, thin-walled lymphatics are probably without nerves. Glands. A detailed account of the nervous supply of the larger glands will be given in connection with the consideration of the several organs ; it may be mentioned here, in general, that the more important glands are provided, in addition to the medullated nerves often found passing through the substance of the gland in their course to the contiguous skin or mucous membrane, with nervous bundles in which non-medullated fibres predominate, but in which some medullated ones also occur. These bundles form an interlobular plexus, rich in ganglion-cells, which accompanies the larger excretory ducts and blood-vessels, and gives off a few branches to be distributed to the muscular coats of these tubes. Thin bundles of pale fibres bear the smaller ducts company as far as the primary groups of acini, and there break up into minute bundles of free axis-cylinders passing Nerve-fibres accompanying a small artery (f), from ihe mesentery of rabbit; gold prep- aration. THE PERIPHERAL NERVE-ENDINGS. 93 Fig. 114. between the acini. The nerve-fibrillse may be traced readily to the membrana propria of the acini, around which a net-work is spun ; regarding their ultimate distribution and relation to the secreting cells much uncertainty still exists, notwithstanding many elaborate investigations and positive statements. The exact mode in which the nerves terminate within the acini is still doubtful ; it is probable, however, that the fibrillse end between, or in apposition with, the ends of the secreting cells directed towards the basement-membrane ; proof of direct con- nection between the nerve-fibrillae and the se- creting cells, as often described, is wanting. Likewise the mode of termination of the med- ullated fibres, which, as already stated, con- tribute to form the interlobular net-work, is uncertain ; in some glands, as in the pancreas of the oat and the buccal glands of the hedge- hog, they terminate in special nerve-endings resembling the corpuscles of Vater. The perceptive apparatus connected with the termination of the nerves of special sense include the highly specialized epithelial struct- ures made up of the neuro-epithelium ; the rod- and cone-cells of the retina, the hair-cells of the internal ear, the olfactory cells of the nasal fossae, and the gus- tatory cells of the taste-buds are important examples of such tissue. In these structures the specialized epithelium forms the apparatus for the reception of the external stimuli, while the nerve-fibres provide for the further transmission of the impressions so appreciated. The relation between the receptive cells and the conducting nerve-fibres must be, evidently, very intimate; a direct anatomical continuity between the two, however, must be regarded as extremely doubtful in the light of recent research. Nerves ending in glands, from the parotid of dog; gold preparation : s, group of secreting cells of single acinus; k, nerve-fibre lying outside the membrana pro- pria and giving off twigs which enclose the acinus within a net-work of ter- minal nerve-fibrillae. 94 NORMAL HISTOLOGY. CHAPTER VII. THE CIRCULATORY SYSTEM. The circulatory apparatus comprises the channels for the con- veyance of the blood-stream, the vessels, and the dilated and special- ized portion of the vascular tube, constituting the heart, for the pro- pulsion of the current. In development and structure the several parts of the vascular system possess much in common, although variations in the details of the walls of the blood-channels suffice to distinguish the different portions. THE BLOOD-VESSELS. The blood-vessels occur in three forms, as arteries, veins, and capillaries, the latter constituting an expanded system of thin- walled tubules, intimately related to the organs, and especially de- signed to facilitate the interchanges be- FiG. 115. tween the nutritive current which they gZ carry and the tissues through which they pass. The arteries possess three coats — the inner, or intima, the middle, or media, and the external, or adventitia. Since these coats vary in relative thickness and 'Mt%^Wt:i^00^ y\ m Fig. 116. '■ Section of human arterj' of medium size : /, the intima, consisting of the endothelium (e), the sub-endothelial tissue (f), and the internal elastic membrane (j-) ; M, the media, com- posed of the involuntary muscle and the bundles of elastic tissue (y) ; A , the adventitia, containing irregular elastic trabeculae iz). Endothelium of artery of frog : the vessel has been treated with silver, hence the boundaries of the endothelial plates are indicated by the dark lines of stained cement-substance. Sev- eral pseudo-stomata appear as minute dark areas between the cells. in details of structure with the size of the vessel, it is usual to classify THE CIRCULATORY SYSTEM. gC arteries as small, medium, and large. The first group includes the terminal branches near transformation into capillaries, the second, all the named arteries of the body, except those which, as the aorta or the pulmonary artery, are recognized as belonging to the third group of large arterial vessels. The inner coat, or intima, as seen in a typical artery of medium size, comprises three layers : (a) an endothelial lining, made up of long, lanceolate, nucleated plates, united by a sinuous Hne of cement-substance and placed parallel to the axis of the vessel ; (d) a sub-endothelial layer of delicate fibrous connective tissue, with branched corpuscles ; (c) a band of elastic tissue — the internal elastic membrane — which forms the most prom- inent part of the intima, appearing in sections of medium-sized ar- FlG. Fig. ii8. Portion of the intima of the human aorta, silver stained : the larger stellate figures are the cell-spaces in the ground-substance be- tween the elastic bundles and contain the connective-tissue corpuscles. Portion of the elastic tissue of the intima of the human aorta; the fibres are so broad and so closely grouped that they constitute an elastic sheet — the ' fenestrated membrane of Henle. teries as a clear, glistening, and usually corrugated band separating the tissue of the inner coat from that of the media. The sub-endothe- lial tissue, which separates the en- dothelium from the internal elastic membrane, is wanting in the smaller arterioles, but appears in vessels of medium size as a longitudinally disposed layer; becoming more conspicuous with the increased calibre of the artery. In tubes of large diameter, as in the aorta, the sub-endothelial tissue appears as a stratum composed of layers made up of fibrous tissue, elastic net- works, and flattened connective-tissue cells. Likewise, the elastic tissue of the intima increases in amount and in complexity, in the large arteries the broad elastic fibres becoming fused together to form an almost continuous sheet — the fenestrated membrane of Henle. The middle coat, or media, is the muscular tunic, and consists principally of circularly disposed bundles of non-striated muscle-cells ; these elements, when isolated, appear as broad, nucleated, irregular spindle-cells, presenting ragged outlines. In many arteries, con- spicuously the subclavian, the inner portion of the media con- tains additional muscle-cells longitudinally disposed. In the smaller arteries the muscular tissue constitutes almost the entire media, but an insignificant amount of intermuscular fibrous connective tissue Fig q5 normal histology. being present ; with the increase in the size of the vessel, however, the quantity of such tissue becomes greater, in addition to which bands of elastic tissue also make their appearance between the muscle-bundles. In the large vessels the fibro-elastic tissue forms a considerable portion of the media ; in the aorta the elastic tissue occurs as robust circularly arranged bands, supplemented by oblique and longitudinal tra- beculae of similar nature ; these elastic fibres, together with the accompanying fibrous tissue, constitute the predominating structure, the muscle being less conspicuous in places than the intermuscular fibro-elastic strata. Owing to this generous admixture of fibrous tissue, the large arteries, while possessed abso- lutely of a greater amount of elastic tissue, have walls relatively less contractile than those of the smaller arteries, whose media is composed of almost pure muscular tissue. The external coat, or adventitia, is the most resistant tunic of the vessel, its characteristic strength being due to the generous amount of component fibro-elastic tissue. The fibrous tissue is arranged as closely- felted bundles, irregularly placed and intermingled with longitudinal bands of elastic tissue ; numerous flattened con- nective-tissue cells lie between the bundles applied to the fibrous trabeculae. The mesh-work is closer and the amount of elastic tissue greater next the media than towards the outer surrounding con- nective tissue into which the adventitia insensibly blends. In the larger arteries the middle and outer coats are separated by a band of condensed elastic tissue — the external elastic membrane. Cer- tain arteries present peculiarities in their coats ; as examples of such varia- tions may be noted the slight develop- ment of the sub-endothelial tissue of the intima of the external iliac, renal, mesen- teric, and coeliac arteries, the appear- ance of longitudinal muscle-cells within and the presence of longitudinally dis- Muscle-cells isolated from the media of human artery. Fig. 120. irJ/ Section of aorta of child: /, M, and A, respectively intima, media, and adventitia. The thick stratum of sub-endothelial tissue and the layer of longitudinally disposed bundles of muscle {b) are peculiarities of the inner coat. the intima of the aorta, THE CIRCULATORY SYSTEM. 97 posed muscular tissue within the adventitia of other vessels (su- perior mesenteric, splenic, renal, and iliac arteries). In passing from medium-sized ar- teries towards smaller vessels, the coats become reduced in thickness, the media being earliest affected. The intima of the smallest ar- terioles consists of an endothelial layer alone, the middle coat in- cludes but a single layer of muscle- cells, while the external tunic is re- duced to a few longitudinal bundles. The vessels intermediate between small arteries and true capillaries no longer possess a complete layer of muscle-cells, the media being represented in such arterioles by scattered groups of circularly placed spindle-cells, forming an imperfect muscular sheet, which partially en- circles the vessel. The nuclei of these circular muscle-cells are trans- versely placed, while those of the endothelial plates are usually longi- tudinal or parallel with the axis of the vessel A, small human artery, in which the coats are reduced each to a single layer of cells ; the media here consists of only one layer of muscle-cells («/), which are seen in optical section : /, intima ; a, adventitia ; e, nuclei of the endothelial plates. B, an arteriole just before becoming a capillary ; the vessel still possesses muscle-cells (in), but these are now arranged as irregular groups. C, true capillary vessel, consisting of only an endothelial coat, the other tunics having disappeared ; the nuclei are those of the endothelial plates. THE VEINS. The veins possess the same tunics as the arteries, but, in general, are characterized by thinner walls and a preponderance of connective over the muscular and elastic tissues. There is, further, less regu- larity and constancy in the structure of the coats. The inner layer of the intima consists of a single layer of endo- thelial cells, rather broader and more polyhedral in form than those lining the arteries, the spindle shape being best marked in the smaller veins. The subendothelial tissue contains numerous con- nective-tissue corpuscles, and, in the larger veins, is arranged in distinct lamellae. An inner elastic membrane is generally present, in some cases taking the form of a fenestrated layer. The media consists of circular bundles of muscle-cells, associated with lamellae of fibro-elastic tissue in the larger veins. This coat is best developed in the veins of the inferior extremities, less so in those of the upper limbs. The muscle-tissue of the veins is sub- ject to many variations, both in amount and in arrangement, that 7 Fig. 122. gg NORMAL HISTOLOGY. of the media is very scant or altogether wanting in a number of veins, including the thoracic part of the vena cava, the internal and external jugular veins, the veins of the pia and dura, of the retina, of bone, and of the corpora cavernosa. Certain veins possess longi- tudinal muscular bundles in the inner part of the media ; such are the mesenteric, umbilical, iliac, and femoral. The adventitia, often the thickest coat of the vein, consists of stout net-works composed of bands of fibro-elastic tissue ; in some veins additional bundles of plain muscle occur within this tunic. Among the venous trunks possessing well- marked, longitudinally arranged muscu- lar tissue in the external coat are the abdominal cava, azygos, hepatic, portal, ^3 splenic, axillary, superior mesenteric, ^ V^; renal, spermatic, and external iliac veins. .J-;^y The veins of the gravid uterus contain Tz^^z muscular tissue in all the coats, the prin- " ~', cipal bundles running longitudinally. The valves with which many veins are provided consist of crescentic folds of the inner tunic of the vessel, strengthened by additional fibro-elastic tissue ; in some instances the muscular bundles extend for a short distance into the valve. The base or the attached margin of the valve is often its thinnest part, the free edges being somewhat thickened. The striated car- diac muscular tissue is continued for a short distance in the walls of those parts of the venae cavae and of the pulmonary veins immediately adjoining the heart ; the explanation of this fact is found in the derivation of these portions of the vessels from the tissues of the primitive heart- tube. THE CAPILLARIES. The capillaries establish the only communication, with few excep- tions, between the arteries and the veins, and, further, provide the intimate anatomical relation between the nutritive current and the tissues of the body necessary for the maintenance of the integrity and functional activity of the various organs. As exceptions to the usual inter^'ention of the capillaries between the arterial and venous radicles, the direct communication between these vessels existing in the erectile tissue of the genital organs, in the spleen, and in some parts of the peripheral circulation, as in the tips of the fingers and toes and of the nose, may be mentioned. Section of human vein of medium size : /, At, and A, respectively intima, media, and adventitia. THE CIRCULATORY SYSTEM. 99 Fig. 123. Capillary blood-vessels from mesentery of young dog : n, the capillaries, with the nuclei of the endothelial plates, lying within the connective tissue [^). The capillaries form rich net-works in almost all tissues and organs, the principal localities where these vessels are wanting being epi- thelium, the hairs, the nails, teeth, cartilage, the cornea, the crys- talline lens, and certain parts of the nervous system. The capillary net-works vary in the size both of the meshes and of the constituent vessels. The average diameter of the capillaries is 7-fo !J. ; the smallest are found in the brain, retina, and muscle ; the largest in bone-marrow, dentinal pulp, and the liver. The closest meshes are found in the air-vesicles of the lungs, the choroid, the liver, and other glands ; the widest in the serous membranes, tendon, etc. Young tissues are more richly supplied than old ones. The capillaries consist of a single layer of endothelial cells, united by intercellular cement-substance ; they are, consequently, protoplasmic tubes of high vitality, admirably designed to facilitate the interchanges constituting nutrition. After staining with silver the endothelial plates are seen as extended spindle-cells, united by irregular lines of darkened cement-substance ; at the points where the vessels branch, irregular triangular cells are not infrequently seen. In such preparations, likewise, along the lines of union or at the juncture of several plates, irregular darkened areas — the stigmata — may be observed ; these are probably minute spaces occupied by stained albuminous sub- stances ; these areas are supposed to aid the diapedesis or trans- migration of the blood-cells. Some capillaries are invested by an imperfect adventitious coat, formed by a net-work of surrounding branched connective-tissue cells, and resembling the reticulum present in lymphoid tissue. The intimate relation existing between the endothelium of the vessels and the surrounding connective-tissue corpuscles is well exhibited in young growing tissues, as the omentum. The peculiarities distinguishing the capillaries from the small ' ' capillary' ' arteries or veins consist not so much in the size of the vessels — for the capillaries may have absolutely the greater calibre — as in the character of their walls. The true capillary possesses no muscle-cells, these first appearing in irregular groups beyond the limits of the capillary vessel ; in those cases where, as in certain veins, muscular tissue is wanting, the character of the adventitia of the vein will aid in determining the character of the vessel. Small blood-vessels — the vasa vasortim — provide for the nutri- lOO NORMAL HISTOLOGY. tion of the walls of the medium- antl large-sized arteries and veins. These vessels arise some distance from the area which they supply, frequently coming from a different branch or, as in the case of the veins, from a neighboring arterial stem. The nerves of blood-vessels are mainly derived from the sym- pathetic system, and hence are principally of the non-medullated kind ; a few medullated fibres, however, are usually present. The nerves accompanying the blood-vessel give off branches, which form surrounding plexuses ; from these minute bundles pass, whose com- ponent fibrilte are distributed to the media and the adventitia. The capillaries are accompanied by correspondingly delicate fibres. Lymphatic clefts and vessels are found in the external coat of the larger vessels. In many places, as in the nerve-centres, including the organs of special sense, in the peritoneum, etc., the lymphatic clefts of the adventitia unite to form a large ensheathing circular sinus — the perivascular lymph-space — which separates a portion of the adventitia from the remainder of the vessel ; as a result of this arrangement, the blood-vessel seemingly lies within the lymph-space. Perivascular lymphatics may be readily observed in the peritoneum of the frog. THE HEART. The heart-walls consist of three layers — the endocardium, the muscular layer, and the pericardium. The endocardium forms the serous lining of all parts of the organ, becoming continuous with the inner tunic of the blood-vessels at the several cardiac orifices. The inner free sur- face of the heart is covered with a single layer of polyhedral nucleated endothelial cells. These latter rest upon the substance proper of the endocardium, a stratum composed of fibrous connective tissue mingled with a felt- work of elastic fibres ; the elastic net-works are especially well developed in the auricles, in certain parts of which the broad fibres join to form fenestrated membranes. The outer connective-tissue layer of the endocardium is continuous with the perimysium of the muscular tissue. The heart-valves are formed by duplica- tures of the endocardium strengthened by bands of fibrous tissue enclosing numerous elastic fibres. The endocardial layer of the auricular side of the Fig ft''' ■■ " t> Section of human heart show- ing endocardium : a, endothe- lium ; b, subendothelial con- nective-tissue stroma in outer layer (c), containing net-work of elastic fibres (e) ; d, trans- versely-cut bundles; /, mus- cular tissue. THE CIRCULATORY SYSTEM. lOI auriculo-ventricular valves is thicker than that of the ventricular surface. The roots or attached portions of these valves possess thickenings — the annuli fibrosi — composed of supplementary- masses of fibro-elastic tissue. The auricular muscle is continued into the valves for about one-third of their width, following closely the general contours of the fold. Within the larger chordae tendineae the papillary muscles extend for some distance, in addition to which isolated muscle-bundles are also sometimes present. The semilunar valves possess a thin elastic layer on the arterial surface, aug- mented by a thick stratum of connective tissue, the bundles ex- tending parallel with the margin of the valve; increased strength is secured by a fibro-elastic nodule, or corpus Arantii, which occupies the middle of each leaflet. Beneath the ventric- ular endocardium, in many animals (deer, sheep, calf, pig, horse, goat, dog, certain birds, etc.), but not in man, peculiar bands — the fibres of Purkinje — occur; these are muscular fibres whose transverse striations are limited to the pe- ripheral zone, while their centre is occupied by a large continuous mass of nucleated pro- toplasm. The fibres of Purkinje represent an embryonal condition of the muscular tissue, since the peripheral part of the fibre alone has undergone differen- tiation, while the central portion has remained indifferent protoplasm. Among some lower vertebrates, as fishes, a similar condition of the muscle-fibres is constant. Section of the heart, including a leaflet of the semilunar valve of the pulmonary artery of child : a, a, cardiac, i, b, arterial, surface ; c, recess behind the valve (y), constituting part of a sinus of Valsalva ; d, free border of valve ; e, thickening near edge of valve corresponding to a corpus Arantii ; g, endothe- lium, h, intima, i, media, k, adventitia, of the pulmonary artery ; the adventitia is continuous with the principal fibrous layer of the endocardium ; m, cardiac muscle ; n, areolar tissue. I02 NORMAL HISTOLOGY, Fig. 126. The muscular tissue of the heart j)ossesses the pecuHarities already described in Chapter IV. : it is coiniposed of short, branched, nucleated fibre-cells, devoid of a sarcolemma, which unite to form an intricate net-work. The naked muscle-fibres are enveloped within a perimysium and are grouped into i)rimary and secondary bundles, which are associated to form lamellce disposed in a very irregular and complex manner. The muscular tissue of the auricles is arranged in general as an outer transverse and an inner longitudinal layer, many small ad- ditional bundles deviating from the principal disposition to pursue independent courses in various directions. The muscle-bundles of the ventricles have a very intricate arrangement, the majority extending in an irregular oblique or spiral direction, some, in fact, describing a figure- of-eight in their course. The pericardium, which invests the exterior of the heart, and by reflection forms the pericardial sac, resembles the endocardium in possessing a single layer of endothelial plates covering its free surface, and a stratum of fibro-elastic con- nective tissue beneath. The parietal pericardium is distinctly thicker than the visceral, all the constituent layers being better developed. The subpericardial tissue covering the heart is continuous with the intermuscular connective tissue of the outer muscular layer ; in this posi- tion numerous fat-cells lie between the bundles of the fibrous and the muscular tissue. The blood-vessels supplying the muscle of the heart are derived as branches of The principal trunks are situated in the larger interlamellar masses of connective tissue, within which they divide into numerous twigs giving origin to the capillaries ; the latter penetrate the primary muscle-bundles, among and parallel to which they run. The relation between the individual muscle-fibres and the capillaries is more intimate than usually supposed, since, as shown by Meigs, the blood-vessels deeply impress the fibres, and in many places are surrounded completely by the muscular tissue. The extraordinary demands made upon the nutrition of the heart- tissue as the result of its remarkable functional activity explain the Section of human heart including pericardium a endothehum of pericardial surface ; i, subendo- thelial fibrous tissue ; c, net-works of elastic fibres ; d, subpericardial areolar tissue containing fat-cells embedded between pericardium and muscle (c) ; z>, blood-vessel. the coronary arteries. THE CIRCULATORY SYSTEM. 103 necessity for such close arrangement. The deeper fibrous layers of the pericardium and of the endocardium receive numerous capil- laries, a few being also found within the chordae tendinese and the valves. The lymphatics of the heart are very numerous. They form a comprehensive system, embracing the lymph-spaces occupying the clefts between the muscle-fibres and the rich net-works of more definite channels extending within the pericardium and endo- cardium, including the valves. These two sets of lymph-radicles communicate but sparingly and pursue largely independent courses. Lymphatic vessels also accompany the branches of the coronary arteries. The rich nervous supply of the heart is derived from the coro- nary plexuses, and includes numerous medullated fibres coming from the pneumogastric, as well as the non-meduUated sympathetic fibres proceeding from the cervical ganglia. Numerous microscopic gan- glia are found along the course of the larger nerve-trunks accom- panying the branches of the coronary arteries, especially in the longitudinal interventricular and in the auriculo-ventricular furrows. Many additional small groups of ganglion-cells occur within the muscular tissue associated with the fibres supplying the intimate structure. The nerves and the blood-vessels are covered by the visceral pericardium. The development of all parts of the circulatory apparatus takes place within the mesoderm; while possessing a common origin, the blood-vessels and the heart, however, develop in- dependently, and, for a time, are distinct and dis- connected. The earliest blood-vessels appear near the periphery of the vascu- lar area, outside the limits of the proper body of the embryo ; later and second- arily they extend centrally and unite with the primitive heart and those parts of the large trunks which have been formed coincidently within the embryo. The mesodermic elements within certain tracts near the periphery of the vascular area undergo proliferation, which results in the pro- duction of deeply staining densely nucleated areas known as the blood-islands of Pander ; these are the direct progenitors of the Developing capillary blood-vessels within the omentum of young rabbit : a, a, elongated protoplasmic processes connecting the walls of the newly-formed capillary {c) with the angioblastic connective-tissue corpuscles (b). 104 NORMAL HISTOLOGY. earliest blood-vessels and the first blood-cells. The blood-channels appear within the nucleated ' ' islands' ' as spaces which follow the partial breaking down of the inner portions of the areas. The peripheral zone of the nucleated cell-mass becomes the endothelium of the future blood-vessel, while, probably, certain of the enclosed mesodermic elements persist as the primary blood- cells. After a time the mesoderm surrounding the newly-formed endothelial tube differentiates into the muscular and other tissue of the remaining coats. The endothelium is, therefore, genetically the oldest part of the vessel, although its characteristic appearance, as seen in silvered adult tissue, is not visible until further dift'erentiation has taken place. The blood-channels are further extended by the fusion of elongated mesoblastic cells with those of the walls of the primary vessels, the lumina of the latter gradually entering the solid processes, which are thus converted into tubes. After the development of the earliest vessels in the manner indicated, the formation of all new vessels subsequently, in pathological processes as well as in normal ones, is associated closely with the connective-tissue cells, since solid protoplasmic processes of the united cells become later the walls of the young vessel. The development of the heart resembles that of the extra- embryonic vessels in so far that the jjart first formed— the primary endothelial tube — originates by the differentiation of the mesodermic cells and the hollowing out of the tissue lying enclosed. In its very early stage the ^si'T&si®^^^-. mammalian heart exists as two distinct and /l^l'/^i oX^*'^*^*) Widely-separated tubes, which later unite to |l.fs*^/r ^^f^i! form a single sac. Outside the primary ^®^^S^«^^^^ endothelial heart the mesoderm differ- <^^<^'M^il'' entiates into the muscular tissue of the Section of a part of the develop- cardiac Wall, but for some time the endo- ing heart of a rabbit embr\-o of ^, ,. , . . i eleven days: ., the endothelial thclial and muscular laycrs coutinuc as tube, within which lie several of independent tubcs, the inner endothelial the primary nucleated red blood- i- • • i_ i ceus w; \n, the slightly differ- ^"^^^S appearing as a shrunken cast rcpro- entiated mesoblastic cells, which ducing the contours of the larger muscular later become the muscular tissue. ' T-v . . ■ .. j i organ. 1 he two tunics are connected by numerous bridging bands, which increase in number and size with the progress of the development of the organ ; these primary tra- beculae are represented in the adult organ by the columnae carneae and musculi pectinati. The pericardium originates as the special- ized layer of mesoderm — the mesothelium — forming the immediate boundary of the general primary body-cavity, of which the peri- cardial sac is only a constricted portion. THE CIRCULATORY SYSTEM. IOC THE BLOOD. While, when physiologically considered, the blood is regarded, with Bernard, best as an internal medium of exchange, histologically it may be classed as a mesodermic tissue possessing a fluid inter- cellular substance, the liquor sanguinis ; in the latter float the cellular elements — the blood-corpuscles. The morphological constituents of the blood are of two kinds, the colorless or white corpuscles and the colored or red cells ; to these must be added a third variety, the blood-platelets or blood-plaques, which are probably constant and independent ele- ments. THE COLORLESS CELLS OF THE BLOOD. The colorless blood-cells, or leucocytes, are not peculiar to the blood, since they originate in lymphoid tissues and are carried by the lymphatic trunks into the blood-current, in which fluid they usually are observed. Critical examination of suitably stained preparations of human blood shows that the colorless cells in- clude several varieties which are characterized by differences of size, nucleus, and granularity. These varieties are : 1. Small mononuclear leucocytes or lymphocytes, which are about the same size as the red blood- cells, or some- what smaller, and contain a nucleus relatively so large that almost the entire cell is occupied by this body. The protoplasm is confined to a thin inconspicuous envelope and is usually devoid of granules. This variety of leucocyte may be regarded as the youngest form. 2. Large mononuclear leucocytes, two or three times larger than the red cells, are frequently of an oval form and contain a nu- cleus which stains less intensely than in the younger cells, owing to its comparative poverty in chromatin. The protoplasm of these leu- cocytes contains but few, if any, pale granules. 3. Transitional leucocytes, which represent more advanced stages of development, are characterized by a nucleus exhibiting vary- ing degrees of indentation, so that the nucleus often appears kidne}^- e^' 'c Leucocytes in normal human blood, stamed and highly magnified, a, small mononuclear; i, large mononuclear ; c, transition form ; ^, polymorphonu- clear cells; e, eosinophilic leucocyte ; r, red corpus- cles. I06 NORMAL HISTOLOGY. shaped or of a horseshoe form. The protoplasm resembles that of the large mononuclear cells, presenting ordinarily no granularity, a few eosinophile granules, however, sometimes being distinguishable. 4. Polymorphonuclear or polynuclear leucocytes constitute the form most frequently encountered, and probably represent the fully developed condition of the white blood-cell. These elements are distinguished by the variously distorted nucleus, which, being fairly rich in chromatin, presents a striking figure in stained prepa- rations. The nuclei appear imperfectly separated into variously dis- posed segments, so that they recall the letters O, S, U, V, Z, etc. The segments usually retain connection by delicate threads of chromatin ; exceptionally these bridges become broken, in which case the term "polynuclear" is appropriate. Occasionally cells may be observed containing granules which stain deeply with eosin. Such "eosino- philic" leucocytes probably represent the final phase of development. Ehrlich has classified the granules occurring within the leucocytes into the following four groups, according to their afiinity for various stains : 1. a-granules, which are coarse and highly refracting, appearing in fresh blood as minute fat particles. These granules possess a strong affinity for eosin and acid stains ; hence they are known as eosinophile and oxyphile. 2. y-gramdes, which are coarse, intensely basophilic, and occur in the mononuclear "mast" cells. 3. 8-granules, which are fine, basophilic, and are seen in the large mononuclear cells. 4. t-granules, which exhibit an affinity for neutral staining mix- tures and are neutrophilic. These constitute the most abundant and important variety of granules, producing the fine granularity encoun- tered in the polymorphonuclear cells. THE COLORED CELLS OF THE BLOOD. The adult mammalian red blood-cell represents a condition of retrogression, since in its development it has suffered the loss of its nucleus and a profound metamorphosis of its protoplasm, changes of such importance that some authorities dispute the propriety of regarding the mammalian red blood-corpuscles as true cells. The presence or absence of the nucleus within the colored corpuscle, together with its general form, furnishes a basis for a division of all vertebrate bloods into — A. Those having nucleated, oval red corpuscles : including fishes (except cyclostomata, which have round, discoidal cells, as the lamprey), amphibians, reptiles, and birds. B. Those having non-nucleated, round, discoidal red cor- THE CIRCULATORY SYSTEM. IO7 puscles : including man and other mammals, except the camel family, which have oval, non-7itccleated red blood-cells. Since an oval corpuscle on being subjected to certain reagents may present a circular outline, the presence or absence of a nucleus offers the most reliable means of differential diagnosis between mammalian and other bloods. The human colored blood-cell is a small round disk, measuring about 8 p. in diameter, and exhibiting individually a faint greenish- yellow tinge. The well-known color of the blood appears only when great numbers of ^"^- -^^o- these corpuscles are massed ; the term ' ' red' ' conventionally applied to these elements is, strictly regarded, incorrect and less appro- priate than "colored." The two surfaces of the blood-disk are not perfectly flat, the centre of the corpuscle being slightly biconcave, while its edges are rounded, biconvex, and Human biood-ceiis:^^, coior- somewhat thickened : in consequence of this ''^^^ corpuscle, surrounded by ,. ,ii • • )> r 11 1 r ^ red cells ; those at r exhibit a peCUhar "biscuit form, all planes of the partially-formed rouleau. corpuscle are not seen accurately focused at one time, the centre usually appearing either darker or lighter than the marginal parts of the cell, depending upon the focal adjustment. The structure of the colored blood-corpuscles is still a subject of discussion. According to the generally accepted view, the cor- puscles consist of two parts : (a) the transparent, colorless, apparently homogeneous, and plastic stroma, extensible and pliable to a high degree, and (J>) the coloring matter, or haemoglobin, which is held within, and uniformly distributed throughout, the former. This conception of the corpuscle assumes the presence of a uniform though highly flexible stroma-mass of definite form, colored by the imbibition of the soluble haemoglobin. On the other hand, the behavior of these elements when treated with water, upon the addition of which the corpuscles swell, lose the discoidal form, and become globular, as well as the suggestive appearances following the staining with anihne of such bleached corpuscles, the outlines of the cells then showing as distinct rosy rings, offers strong arguments, in the opinion of not a few, for the belief that the red corpuscles are minute sacs, consisting of a limiting membrane and the colored fluid contents. The nuclei of the red cells, when present, lie embedded within the colored stroma ; in perfectly fresh or circulating corpuscles they are made out with great difficulty, since they possess a refractive index almost identical with that of the other parts of the cell. After reagents, or after the expiration of some minutes, the nuclei become I08 NORMAL HISTOLOGY. very evident, and correspond in appearance and structure with those of other cells, one or more nucleoli often being visible. In fresh blood the red corpuscles within a few minutes arrange themselves in rows or piles by the apposition of their broader sur- faces, thus forming figures which, from their resemblance to rolls of coin, are termed rouleaux. The cause of this phenomenon is still uncertain, although it is not improbable that it is to be attributed to the presence, in the fresh corpuscles, of a film of a nature repelling the liquor sanguinis and favoring the adhesion of the disks ; the rouleaux are only temporary, the corpuscles later spontaneously separating and remaining apart. It is of interest to note. that only discoidal corpuscles of mammalian bloods (including, however, the discoidal cells of the lamprey) run together to form these figures, the projecting nuclei and the slight biconvexity of the oval nucleated cells affording surfaces evidently unfavorable for adhesion. The average diameter of the red corpuscles in the various races of mankind is identical, being between 7 and 8 m, or about i -3200th of an inch. The size of the animal bears no relation to that of its red blood- cells, as shown by the following measurements of some mammalian bloods, based on the observations of Gulliver : Millimetre. Millimetre. Millimetre. Elephant . . . .0092 Guinea-pig . . .0071 Pig 0060 Sloth 0086 Dog 0071 Horse 0059 Whale 0080 Rabbit 0070 Cat 0058 Man 0079 Bear 0070 Sheep 0048 Beaver 0076 Mouse 0067 Goat 0040 Monkey . . . .0074 Ox 0048 Muskdeer . . .0024 The largest corpuscles are those of the amphibians, the red cells of the frog measuring .0016 mm. in breadth by .022 mm. in length, those of the triton, .019 by .029, and those of the proteus, .035 by .058. The maximum size is reached in the huge red cells of the amphiuma, which are no less than .046 mm. wide by .075 mm. long, and are readily distinguishable by the unaided eye. The number of colored cells normally present in one cubic millimetre of human blood, as determined by the haemacytometer, is about five millions ; these figures are modified by sex, the male subject usually having more corpuscles than the female. The number of red corpuscles varies in different animals : the carnivora possess a greater number of cells in a given quantity of blood than do the herbivora ; in birds the proportion is still larger ; while in the sluggish amphibians the number of the huge red cells is reduced to thousands. Effect of Reagents applied to Human Blood. No elements Fig. Red blood-cells of man and of am- phiuma, magnified to the same extent to show the size of the human cor- puscles in comparison with that of the largest known blood-cell. THE CIRCULATORY SYSTEM. ' lOQ are more sensitive to changes in environment or to the effects of reagents than are the cells of the blood. An appreciation of the alterations referable to external causes is important as guarding against unwarranted conclusions as to the existence of pathological conditions, since not infrequently ap- pearances which lead the tyro to infer disease may be ascribed to influences acting on the corpuscles outside the body. If fresh blood be exposed to a current of air, subjected to undue pressure or to other disturbing influences, alterations of the corpuscles at once take place. One of the most common distortions affects the exterior of the red corpuscles, and results in the formation of a number of minute projections, or spines, pro- ducing a condition known as crenation. Saline Solutions. The application of a weak saline solution or of urine is attended with similar effect ; if the strength of the reagent be gradually increased, a corresponding progressive degree in the distortion is observed, until, finally, upon the addition of a concentrated brine, a shrivelled, shapeless mass replaces the former discoidal red corpuscle. The reaction is less marked upon the colorless cells, weak sahnes pro- ducing no perceptible change, while a slight shrinkage is noticeable after the stronger solutions. ^A^ater. Upon the application of water the colored cells swell up, lose the discoidal form, and become spherical, and at the same time part with their coloring matter, the haemoglobin ; the latter, being dissolved, leaves the bleached and colorless stroma to form the "ghost." That the red corpuscles are not destroyed by the water, as sometimes stated, may be demonstrated by the addition of a suitable aniline dye, when the presence of the bleached corpuscles is made evident by the colored rings which mark their outlines. The action of water upon the living color- less blood-cells is somewhat different. These corpuscles cease their amoeboid movements, retract their Fig. 132. Reactions of human blood- cells with various reagents : A, effect of treatment with water upon the white (w) and the colored cell (r) ; 3, red cells after the addition of saline solutions, crenation following the application of weak solu- tions, great shrinking and dis- tortion (s) succeeding the action of the concentrated reagent ; C, action of dilute acetic acid on the colorless cell {■w) and on the red cor- puscle (r) ; D, red blood-cell after the addition of one-per- cent, solution of tannic acid. no NORMAL HISTOLOGY. processes, become round, and swell up into larger spheres ; mean- while, the protoplasm resolves itself into a number of sharply-cut granules, which, owing to their suspension within a fluid of less density than the blood-plasma, exhibit the active dancing or oscil- latory movements which constitute the " Brownian motion," a phenomenon entirely physical in nature. The nuclei of the colorless cells after treatment with water appear as clear or slightly granular areas among the vibrating particles. After a time the distention of the corpuscle becomes too great, and rupture takes place, followed by the escape of the particles of disintegrated protoplasm. Acids. Upon the addition of weak acetic acid the red cells become rapidly decolorized, at the same time losing tlie discoidal form and approaching the spherical. The protoplasm of the color- less corpuscles clears up entirely, the nuclei coming very con- spicuously into view. Upon subsequent treatment of acid prepara- tions with aniline, the nuclei of the white cells appear deeply stained, while the red cells are outlined by faintly-colored rings. Tannic acid, when applied to the red corpuscles in weak (one- half to one per cent.) solutions, produces a peculiar effect: the coloring matter of the corpuscle is coagulated as it escapes from the cell and becomes conspicuous as a minute accumulation adhering to one edge of the corpuscle. Where strong solutions of tannic acid are employed, the haemoglobin is coagulated within the corpuscle before it has had an opportunity to escape, producing appearances which have been mistaken for nuclei and other details of cell-structure. The Blood-Platelets. If human blood be drawn directly into a drop of osmic acid solution (one per cent.) or of a three-fourths per cent, solution of sodium chloride, covered at once, and examined with a high power, numbers of small, colorless, circular disks will be seen on careful observation ; these are the blood-platelets of Bizzozero, sometimes called the third corpuscular ele- ments of the blood. They are very unstable, f' ^ prone to disintegration, and are variable in A, human red blood-cells and sizc, posscssiug an average diameter of about ?a:;'",t;'a^^cilfwh1;htr one-third of that of the red cells; they isolated or in masses: B, fibrin occur Singly, but show a marked dispositiou Sr/t-inteTafer^t;;:::: to mn together in groups preparatory to lets. breaking up into the minute particles long known as the granules of Max Schultze. Unless great precaution is taken to insure the immediate action of the preserving fluids, the blood-platelets will not be seen in their normal form. THE CIRCULATORY SYSTEM. HI These bodies may be recognized in the circulating blood, as ob- served by Osier and others, and are constant, although numerically variable, elements of mammalian bloods. The peculiar elongated elliptical ' ' blood-spindles' ' found in the blood of other vertebrates are probably to be regarded as the homologues of the blood-plaques of mammals. While the presence of the blood-platelets as distinct, constant, and normal constituents of the human blood is now gener- ally recognized, authorities are far from accord as to their significance. The evidence at present seems to point to a close relation between these bodies and the process of coagulation, in view of their probable active role in the production of the factors in the formation of fibrin. Fibrin filaments are to be observed in a drop of blood mounted in the usual manner for microscopical examination and allowed to stand for some time in a moist chamber ; they appear as very delicate straight interlacing threads which occupy the interspaces between the corpuscles and frequently radiate from a common centre, con- taining a group of partially broken-down blood-platelets. Additional minute particles are to be seen in human blood, regarding the nature, source, and significance of which much has been surmised and but little definitely established. These include the small colored disks, the microcytes or the hsematoblasts of Hayem, according to whose authority they constitute an important source of the red corpuscles ; by others they are regarded as sep- arated portions of the ordinary red cells. Other minute, colorless, often highly refracting, granules are encountered floating in the liquor sanguinis ; such are the elementary particles of Zimmer- mann and the granules of Max Schultze. These particles differ in nature as well as in source ; some probably are derived from the disintegration of the white corpuscles and of the blood-platelets, others from that of the Fig. 134. endothelial plates of the vascular channels, ■while many represent fatty granules absorbed during digestion or taken up, possibly, in the course of pathological processes. Blood-Crystals. The coloring matter of the H^mm crystals from dried i_i 1 1 1 II. -i-T ■,,. human blood. blood— the haemoglobm — readily crystalhzes in man and most mammals as elongated, rhombic prisms ; the haemo- globin crystals of the squirrel and of the guinea-pig, however, are respectively hexagonal plates and rhombic tetrahedra. These blood- crystals, of a deeper or lighter red color according to their size, often form in preparations of blood which have been sealed and allowed to stand after the addition of a few drops of water ; the blood of the rat is especially favorable for their production. If dried blood be treated and thoroughly mixed with glacial acetic acid (the addition 112 NORMAL HISTOLOGY. of a few granules of common salt being advantageous in the case of old clots), on slightly heating until bubbles appear, numbers of dark-brown irregular rhombic prisms form. These are the haemin crystals of Teichmann, which are positive indications of the presence of blood, but have no value in the determination of its source. They vary greatly in size and considerably in form, the peculiar unequally-notched ends presented by the larger crystals being quite characteristic. DEVELOPMENT OF THE BLOOD-CORPUSCLES. The origin of the colorless blood-cells must be referred to the lymphoid tissues, since these elements are identicarwith those occurring within the lymph with which they are poured into the blood-current. The colorless corpuscles appear later than the red cells, the first ones probably entering the circulation as migratory mesodermic elements. The lymphatic or adenoid tissues, however, undoubtedly constitute the principal sources of the colorless blood- corpuscles, which are produced by the division of the numberless masses of active protoplasm contained within the various aggrega- tions of lymphoid tissue throughout the body. The multiplication of existing colorless cells which takes place normally, but which is especially active under the stimulus of patho- logical conditions, accounts for the origin of a certain number of white corpuscles ; the division of the cellular elements of connective tissue is regarded by some as an additional source of these blood- cells. The efferent lymph-streams passing from the lymphatic tissue, as well as the blood contained in the splenic vein, are richer in color- less cells than are the corresponding afferent currents, showing that the augmentation is due to the new elements contributed by the lymphoid tissues through which the currents pass. The origin of the colored blood-cells is usually considered as taking place during two epochs — before and after birth. It must be remembered, however, that such division is conventional and largely arbitrary, since the period at which the primary embryonic processes of such formation cease and are replaced by those maintained throughout life is uncertain and variable ; in man and mammals born in a condition of advanced development the production of blood-corpuscles within the marrow is instituted before the termina- tion of intra-uterine life. Before Birth. The first blood-cells originate outside the body of the embryo, within the angioblastic cells of the mesodermic tract of the vascular area. Certain cells of this layer increase in size and undergo proliferation of their nuclei, forming multinucleated areas known as the blood-islands of Pander. These subsequently THE CIRCULATORY SYSTEM. 113 unite into an irregular net-work, the nodal points of which are dis- tinguished by an active production of new nuclei. Some of these acquire protoplasm and later become the endothelium of the blood- vessel, while others, more centrally situated, are converted into the primary blood-corpuscles, the intervening tissue undergoing liquefaction to constitute the blood-plasma. These earliest blood- cells, although destined to become the red corpuscles, are at first colorless masses of active protoplasm, provided with nuclei and exhibiting amoeboid movements. After a time the protoplasm gradually acquires the characteristic tinge and assumes a discoidal form, the elements then constituting the nucleated red blood-disks of the embryo. The earliest red cells unquestionably increase by the division of the primary corpuscles, the reproduction being attended by the changes of karyokinesis. This multiplication of the early red corpuscles probably ceases in man long before the end of gestation, the embryonal colored corpuscles meanwhile becoming smaller and losing their nuclei, so that at birth all the nucleated red cells have disappeared. The exact details of the metamorphosis from the embryonal to the adult form are still uncertain. There is no evidence at present to establish the descent of the red corpuscles from the colorless cells, the two being distinct elements having independent origins. The liver must probably be reckoned among the situations in which the formation of blood-cells takes place during embryonal life ; in this same category is included the spleen by some authorities, indeed, Bizzozero regards it as a post-natal source. After Birth. Of the many suggested sources for the post-natal production of the red blood-cells, of which great numbers must be formed constantly to replace those continually undergoing destruction, the red marrow of bones is undoubtedly the most important. Among the more common elements of the red marrow, cells usu- ally are to be observed which strongly resemble the embryonal red blood-corpuscles, being distinguished from the ordinary marrow- cells by their haemoglobin-colored protoplasm, smaller size, and unstable nuclei. These cells, often called the erythroblasts, are undoubtedly transitional forms of red blood-corpuscles, the nuclei disappearing and the protoplasm assuming the usual appearance of such elements. As to the source of the erythroblasts, however, whether they are transformed colorless marrow-cells or distinct elements, the descend- ants of the red corpuscles of the embryo, much uncertainty still exists. There are strong reasons for regarding the latter supposition the true indication of their nature and origin, the production of the red corpuscles both before and after birth being thus closely related. Direct transformation of the colorless cells, production within the jj. NORMAL HISTOLOGY. spleen, and growth from the blood-platelets, or haematoblasts of Hay em, have been advanced from time to time as additional sources of origin of the red blood-corpuscles. Without entering upon a detailed critical consideration of the evidence supporting these views, it may be stated that, at present at least, they all lack the conclusive proof of unimpeachable direct observation. Concerning the rela- tions of the "haematoblasts" much confusion exists in consequence of the application of the term to different objects by various writers. The exceptionally small red corpuscles, or " microcytes," together with those of unusually large diameter, may be regarded as ex- pressing the extremes of variation in size to which all morphological elements are subject. The formative processes within the* red bone- marrow may be regarded, in the light of our present knowledge, aS' the most important source, if, indeed, not the sole authentic one, of the new red blood-corpuscles produced throughout life. Mention may be made in this place of the problematic organs the so-called arterial glands, which include the coccygeal and carotid glands. The first of these, the glandula coccygea, or Luschka's gland, occurs near the tip, in front of the apex, of the coccyx, associated with the middle sacral artery, which contributes the blood-vessels largely forming its pea-sized mass. The carotid gland lies at the bifurcation of the common carotid artery, frequently between the resulting branches, and appears as a somewhat flattened ovoid nodule. These peculiar bodies are identical in structure, both consisting of dense arterial net-works surrounded by irregular groups of granular polyhedral cells, whose presence suggested the once supposed glandular nature of the organs. The entire plexiform mass is invested by connective tissue, from which fibrous septa pene- trate between the vascular structures. Numerous non-medullated nerve-fibres are also present. The true nature and function of these rudimentary organs are en- tirely unknown, and probably will remain so until the embryology of these bodies is better understood. THE LYMPHATIC SYSTEM. 115 CHAPTER VIII. THE LYMPHATIC SYSTEM. The lymphatic system consists of two parts — the lymph-channels and their contents the lymph, and the lymphatic tissue. The former may be represented by irregular interfascicular clefts between the bundles of fibrous tissue or by vessels with well-defined walls, while the latter may exist as diffuse adenoid tissue, the simple lym- phatic nodule, or the complicated compound lymph-gland. THE LYMPH-CHANNELS. The lymphatic spaces, the radicles of the more distinct vessels, are almost universally present, since they exist in almost every locality where connective tissue abounds, forming intercommuni- cating systems of greater or less perfection throughout the Fig. 135. various organs. The relation between the connective tissue and the lymph-radicles is very intimate, and it may be assumed that all mterfascicidar clefts are directly or indirectly con- nected with the lymphatics. In loose areolar tissues, as the sub- cutaneous, the lymph-spaces are ill-defined clefts, irregular in form and size, which are bounded by the neighboring bundles of fibrous tissue and hned by an imperfect layer of endo- thelioid connective-tissue cells. In the denser forms of fibrous tissue, as the central tendon of the diaphragm, cornea, etc., the lymph-spaces are more limited and form well-defined intercommuni- cating systems of canals, or "juice-channels ;" of such the corneal spaces and the bone-lacunae are familiar examples. These spaces are filled incompletely by the connective-tissue cor- puscles, which usually are applied to one wall of the cavity to form a partial lining. The number of cells occupying a single space varies : sometimes several lie side by side (kitten's cornea) united by lines of cement-substance ; in such cases, after silvering, the cells present the appearance of endothelial plates. The large serous Lymph-spaces between bundles of fibrous tissue seen in profile, from tlie human cornea : b, b, bundles of fibrous tissue ; s, lymph-spaces containing flattened connective-tissue cells. II6 NORMAL HISTOLOGY. cavities, as the peritoneal or pleural sacs, are, in principle, but greatly-dilated lymph-spaces, lined by P^^ modified connective-tissue cells, the en- dothelial plates, which by mutual press- ure become polygonal in outline ; in- FiG. 136. Lymph-spaces of cornea, surface view ; a, the spaces within the ground-substance (c) con- nected by the minute canals (6), or canaliculi. Lymph - capillary from silver-stained mesentery of frog': a number of lymph- corpuscles occupy the deli- cate endothelial tube which constitutes the vessel. Stead of a few cells sufficing for the formation of a lining membrane, as in the case of the minute Fig. 138. lymph-space, innumerable ele- ments are required to clothe the large serous cavity. The lymphatic spaces within the connective tissue join to form definite channels at the margins of the fibrous tissue, the lymph being carried by the lymphatic vessels from the organs to the adjacent masses of adenoid tissue, the lym- phatic glands. The lymph- vessels immediately succeed- ing the spaces may be regarded as the lymphatic capil- laries, being protoplasmic tubes of great delicacy, com- posed of a single layer of en- dothelial plates. The contours of the lym- phatic vessels are not uniform, but present numerous dilatations and constrictions, which indicate the positions of the imperfect valves : Lymphatics of silvered diaphragm of rabbit : s, s, lymph-spaces lying within the deeply-stained ground- substance ; /, /, lymphatic vessels lined with endo- thelium and possessing valves (z/) and corresponding dilatations. Fig THE LYMPHATIC SYSTEM. JI7 these latter consist of a fold of endothelium, strengthened often by a minute quantity of elastic tissue. The relation of the lymph-spaces to the capillary blood-vessels on the one hand and to the lymphatic vessels on the other is very inti- mate ; in certain localities, as in the omentum, indirect communication between the blood-vessels and lyrriphatics is established by means of the spaces of the groundwork of the dense connec- tive tissue (Klein). Many nerve-trunks are en- closed by perineurial lymphatic channels, into which the lymph-spaces of the surrounding tissue open. The blood-vessels of the central nervous system, especially of the retina, likewise are surrounded by distinct perivascular lymph- sheaths, formed by the enlargement and con- fluence of the clefts within the adventitia of the vessels. In some membranous structures, notably the amphibian mesentery, the vessels lie encased within distinct endothelial tubes. Lymphatic vessels of large size have walls of considerable thickness, resembling those of the veins. In such vessels three coats are recog- nizable— the inner, or endothelial, the middle, or muscular, and the outer, or connective tissue. The thoracic duct possesses a well-developed intima, composed of a considerable layer of subendothelial connective tissue con- taining a net-work of longitudinally dis- posed elastic fibres. The muscular tissue of the media is supplemented by bundles of involuntary muscle extending length- wise within the outer coat, which in the vessel under consideration is particularly robust. The lymph contained within the lym- phatic vessels, like the blood, consists of two parts — the clear, straw-colored plasma, or liquor lymphse, and the cellular elements, the lymph-cor- puscles. The cells of the lymph are small nucleated masses of active proto- plasm, when at rest presenting a spherical form and measuring about .01 mm. in diameter ; in their usual condition of activity, however, their outlines Perivascular lymphatic (i) enclosing a small ar- tery {a), from the silvered mesentery of frog : c, branching lymphatic cap- illary. Fig. 140. Transverse section of human tho- racic duct : 1, m, and o, respectively the inner, middle, and outer tunics ; jt:, endothelial lining, beneath which lies the fibrous stratum containing net-work of longitudinal elastic fibres (jj/) ; z, longitudinally disposed bun- dles of muscular tissue within adven- titia ; v, capillary blood-vessels. ii8 NORMAL HISTOLOGY. are continually undergoing the changes effected by amoeboid move- ment. These elements, in short, possess all the peculiarities of the colorless blood-corpuscles with which, in fact, they are identical. In addition to the lymph-corpuscles, numerous fatty granules are usually present within the plasma ; in the lymphatic vessels of the intestinal tract the absorption of fatty matters is made conspicuous by the presence of the chyle, an emulsion occupying the so-called lacteals, or chyle-vessels ; these latter are not distinct tubes, but only those portions of the lymphatic net-work which convey the milky-looking chyle during certain stages of digestion. The sources of the lymph-corpuscles are those already con- sidered in connection with the colorless cells of the blood, the lym- phoid or adenoid tissues of the body being unquestionably the most ' important and prolific se'ats for the production of these elements. The presence of a few cells within the lymph-radicles, between their commencement and the first masses of adenoid tissue occurring on their course, is due to the entrance within the vessels of migratory cells from the surrounding connective tissue ; only after the lymph- stream has passed through considerable masses of lymphoid tissue do the corpuscles appear with profusion. Fig THE LYMPHATIC TISSUES. Lymphatic, lymphoid, or adenoid tissue usually occurs as circumscribed masses known as lymphatic nodules or "glands;" in certain localities, however, as in parts of the mucous membranes of the larynx, the pharynx, the stomach, the intestines, etc., ill-defined masses of diffuse lymphatic tissue occur. These are recognized as aggregations of small round cells, fading away among the surround- ing structures. Lymphatic tissue, wherever found, is com- posed structurally of two elements — the deli- cate connective-tissue reticulum, on the surface of the fibres of which plate-like, often stellate, connective-tissue corpuscles are applied, and the small round cells contained within the reticulum. These elements — the lymphoid or adenoid cells — become the lymph-cor- puscles and the colorless blood-cells on their escape from the denser reticulum into the lymph-current and their subsequent entrance into the blood. The variations in the compactness with which the cells are lodged Elements of adenoid tissue from partially brushed sec- tion of lymphatic gland of child : a, fibres of reticulum ; b, lymphoid cells ; c, ex- panded connective - tissue plate. THE LYMPHATIC SYSTEM. 119 within the net-work constitute the denser or looser forms of adenoid tissue found in the lymphatic nodules ; ordinarily the cells are so closely placed that the reticulum is greatly masked, satisfactory views of the latter being obtained only in sections of great thinness or after the cells have been removed by brushing or by violent agitation. The reticulum of lymphoid tissue consists of intertwining and anastomosing bundles of connective tissue ; along the fibrous trabeculae, especially Fig. 142. Fig. 143. at the nodal points, flattened plate-like or stellate connective-tis- sue cells are applied after the manner of an imperfect endothelial investment. In parts of many adenoid struct- ures the delicate re- ticulum seems to be formed by the union of the protoplasmic pro- cesses of the branching connective-tissue cells themselves ; this ar- rangement, however, Diffuse lymphoid tissue occu- pying deeper layers of mucosa of human stomach : the lym- phoid cells infiltrate the fibrous tissue between the glands with- out being definitely limited. Simple lymph -follicle from conjunctiva of dog : a, lym- phoid tissue, limited by the fibrous capsule {6) ; c, sur- rounding connective tissue. is usually only seem- ing, the cells really being applied to the surface of the fibres and not constituting an integral part of the reticulum. It is probable that in the splenic pulp and in a few other localities the processes of the stellate cells do unite to form protoplasmic net-works. Diffuse adenoid tissue represents the least specialized form of the lymphoid structures ; the mucosae of the digestive and of the respiratory tracts afford good illustrations of the presence of such tissue. Simple lymphatic nodules, or solitary follicles, stand next in differentiation ; these are found in almost all mucous membranes (those of the bladder and of the sexual organs excepted), while they occur in great numbers in the respiratory and digestive tracts, the solitary glands of the latter being important examples of these structures. The simple nodules consist of oval masses of adenoid tissue, limited by a delicate connective-tissue wall or capsule, com- posed of fibrous lamellae. The adenoid tissue of such simple follicles presents no considerable variations in its arrangement, that occupy- ing the more central portions of the nodule, however, being fre- I20 NORMAL HISTOLOGY. quently somewhat less closely packed than the tissue at the periphery. The afferent lymph-vessels conveying the lymph to the simple follicles break up at the periphery of the nodule into branches, which distribute the lymph to the adenoid tissue ; corresponding efferent vessels carry off the fluid returned from the lymphoid tissue and unite to form larger lymphatic trunks. Compound lymphatic follicles, the lymphatic glands of gross anatomy, are formed by the aggregation and partial fusion of a Fig. 144. \ e-^' ..A , J.^\^ *1«4^ , i>^- d Section of lymph-gland from child, showing general arrangement of lymphoid tissue and lymph- sinuses: a, capsule from which trabeculae {6, b) extend ; c, masses of dense adenoid tissue composing the cortical follicles ; d, the same, of the medullary cords ; e, lymph-sinuses. Fig. 145. number of simple nodules. These structures enjoy a wide distri- bution, and are represented by the numerous chains of deep and superficial lymph-glands, of which the axillary and inguinal glands are familiar instances. The periphery of these lymph-glands is occupied by a firm capsule composed of fibrous connective tissue, inter- mingled with which, in the largest glands, bundles of involuntary muscle are sometimes present. At the position of entrance and exit of the larger blood-vessels and the efferent lymphatic trunks, usually opposite the most convex surface of the organ, the capsule dips deeply into the interior of the gland and forms the hilum. The space included within the capsule is subdivided into a peripheral zone, the cortex, and a centrally situated part, the medulla, which at the hilum reaches fegcal^g^^^"???^ Section of lymphatic gland of child, including portion of cortex at periphery : c, capsule ; s, loose tissue of the lymph- sinus ; /.denser lymph-tissue of the cortical follicle. THE LYMPHATIC SYSTEM. 121 the exterior. The details of arrangement distinguishing these portions of the gland depend primarily upon the distribution of the trabeculae which continue the tissue of the capsule into all parts of the organ. The trabeculae, composed of stout bundles of fibrous tissue, ex- tend from the inner surface of the capsule towards the hilum and divide the cortex into a number of imperfect spherical compartments which enclose masses of adenoid tissue, the cortical follicles, which correspond to simple lymph-follicles. The continuations of the tra- beculae towards the centre of the gland unite at much more frequent intervals and form throughout the medulla a series of incomplete par- titions which separate imperfect compartments occupied by elongated masses of adenoid tissue, the med- ullary cords. These latter and Fig. 147. the cortical follicles constitute one ^^.^ - ^ "^^1- continuous mass of dense lymphoid .,c4fafeMgfe?^^v>V^ Fig. 146. Section of lymphatic gland of child, in- cluding portion of medulla : i, part of tra- becula, on either side of which narrow lymph-sinuses are seen, bounded by denser structure of medullary cords (/). Portion of human lymph-gland, showing de- tails of structure : a, lymph-sinus ; 6, adenoid tissue ; c, trabeculae ; rf, coarser reticulum of lymph-sinus ; e, expanded connective-tissue plate applied to fibres ; _/, lymphoid cells. tissue, which follows the contours of the spaces occupied, but does not completely fill the compartments formed by the fibrous trabecute. The spaces included between the fibrous trabeculae and the masses of dense adenoid tissue are occupied by a very loose reticulum and sparingly distributed lymphoid cells ; these channels are the lymph- sinuses, into which the lymph brought by the peripherally-situ- ated afferent vessels is poured and through which it finds its sluggish course, thus securing the opportunity of taking up numerous new cells in its journey through the organ. The lymph-sinuses form a freely intercommunicating system of canals throughout the gland, beginning at the periphery, where they receive the afferent lymph- vessels, and ending in the hilum, where the lymph is collected and carried off" by the efferent trunks. 122 NORMAL HISTOLOGY. The trabeculae all along their course give off numerous ramifi- cations ; each of these breaks up into still finer bands, until the final divisions of the fibrous tissue terminate in the delicate reticulum con- stituting the supporting framework in whose meshes the lymphoid cells are held. In the areas of denser tissue the cells are so closely placed that the supporting reticulum is almost completely masked. The surfaces of the fibrous bundles and partitions, especially those directed towards the. lymph-sinuses, support numerous plate-like connective-tissue cells, in places these elements constituting almost an endothelial covering. The blood-vessels supplying the lymphatic glands are arranged as two groups : the one set gains entrance at the periph'ery and is distributed principally to the capsule and larger trabeculae ; the other group enters at the hilum, the majority of the arterial branches pass- ing directly into the lymphoid tissue, while a few follow the course of the larger septa ; these, following the latter course, give off numerous twigs to the surrounding adenoid tissue, the terminal branches con- tinuing to the capsule, where they finally are distributed. The cap- illaries derived from the breaking up of the arterial twigs entering at the hilum especially ramify through the denser adenoid tissue, avoid- ing the loosely reticulated lymph-sinuses. The distribution of the nerves passing to the compound lymphatic glands is uncertain, the supply including bundles of both the meduUated and the pale fibres. In addition to the numerous well-developed compound lymphatic follicles, many of which, as the mesenteric and the bronchial glands, reach conspicuous dimensions, certain organs present special modifi- cations of adenoid tissue ; such are the spleen and the fully-developed thymus body, which therefore may be included with propriety in the account of the lymphatic structures. THE SPLEEN. The spleen may be regarded as a specialized compound lymphatic gland, modified by the arrangement of its blood-supply. The organ is invested by a firm capsule, composed of a dense felt-work of bundles of fibrous tissue, with which are mixed numerous elastic fibres. The outer surface of the capsule, with the exception of a limited area, is covered by the serous coat of the peritoneum, the union between the two being very intimate. On the inner surface the capsule is continuous with numerous prolongations, the trabeculae. These penetrate deeply into the interior from all sides, and by the free union of their processes form a spongy connective-tissue framework throughout the organ, enclosing an elaborate system of intercommunicating spaces occupied by the lymphoid tissue. THE LYMPHATIC SYSTEM. 123 In certain animals (dog, cat, hog) the capsule contains bundles of involuntary muscle ; these are only exceptionally present in man. Fig. 149. Fig. 150. Section of spleen of dog, showing general structure : a, capsule, from which trabeculse extend ; sec- tions of these latter are seen in several places, as at d ; b, tissue of splenic pulp ; c, c, Malpighian corpuscles ; e, sections of blood-vessels. Likewise, bundles of muscular tissue are constituents of the trabfeculse in many mammals, including man to a limited degree ; the muscle- cells are distin- guishable from the surrounding connective tissue by their rod- shaped nuclei. The stoutest tra- beculae are found at the hilum, which c o r r e- sponds to the position at which the larger blood- vessels enter and leave the organ. The lymphoid tissue filling the intertrabecular spaces exists in two forms — as the loose adenoid tissue which, together with the Section of human spleen, showing trabeculse (a) and fibrous reticulum (b) continued into the surrounding splenic pulp ; c, lymphoid cells Transverse section of large trabecula of human spleen : a, fibrous tissue, containing a few groups of plane muscle-cells {b) ; c, extension of trabecula into fibrous reticulum; d, lymph- corpuscles. 124 NORMAL HISTOLOGY. intimately related vascular channels, forms the splenic pulp, and as the cylindrical or spherical masses of dense adenoid tissue ensheath- ing the arteries, constituting the Malpighian corpuscles. The largest trabecute support the branches of the splenic artery ; on entering at the hilum, these twigs receive a strong fibrous invest- ment, or adventitious Fig. 151. sheath, which accom- panies the vessel and becomes gradually reduced as the arte- ries diminish in size ; finally, this sheath blends with the con- nective-tissue frame- work of the paren- chyma. Many of the smaller branches of the splenic ar- tery are deflected from the trabec- ulse and enter the surrounding tissue, where they become ensheathed at irregu- lar intervals by cy-» lindrical or spherical masses of dense adenoid tissue and constitute the Malpighian corpuscles. The artery usually pierces the mass somewhat eccentrically, sometimes, however, passing near the centre. Numerous small twigs are distributed to the tissue composing the corpuscles ; after forming a net-work they eventually open into the channels of the pulp ; the main artery of the Malpighian corpuscle has a similar destination. The form of these ensheathing masses of adenoid tissue varies in different animals ; in some (guinea-pigs) the arteries are accompanied throughout their entire course by a layer of lymph-cells, while in others (man, cat) the investment is limited to irregularly spherical masses ; between these extremes numerous intermediate forms exist. The peripheral zone of the Malpighian corpuscle is usually denser than the central part, an arrangement favoring the sharp demarca- tion of the body from the surrounding looser parenchyma ; in man the corpuscles are less clearly defined than in many lower animals. The splenic pulp, which makes up the larger part of the bulk of the organ, consists of a loose net-work of slender bands and imperfect septa, composed of delicate fibres and broad plate-like connective- Section of human spleen cutting transversely a Malpighian cor- puscle : a, section of the somewhat eccentrically situated artery ; 6, capillaries distributed to the tissue of the corpuscle ; /, the sur- rounding lymphoid tissue of the splenic pulp. Portion of channel within splenic pulp from human spleen : a, endothelioid connective-tis- sue plates of the imperfect wall of the space ; b, red blood-corpuscles ; c, lymphoid cells ; d, larger amoeboid elements, containing pigment- granules; e, large multinucleated cell. THE LYMPHATIC SYSTEM. I2q tissue cells. The processes of the latter unite with one another to form imperfect partitions ; in young animals multinucleated plates are frequently encountered. Ad- hering to the delicate reticulum, Fig- 152. partially occluding the channels throughout the pulp, are numerous lymphoid cells or leucocytes, which are largely the offspring of the ele- ments forming the adenoid tissue. The spaces of the splenic pulp are additionally occupied by num- berless colored blood-cells, brought by the arteries which open directly into the channels within the pulp ; the dark-red appearance of the organ is thus explained. As a re- sult of the breaking down of numer- ous worn-out red blood-cells, — in which process of destruction the leucocytes may take an active part, — pigment - granules, both free and within the lymph-cells, are con- stantly encountered. The splenic pulp, in addition to giving origin to numerous leucocytes, in common with other lymphoid tissues, is regarded by many histologists as the birthplace, as well as the "graveyard," of a certain number of colored blood-cells ; the pres- ence of young nucleated red cells supports this view. The blood-vessels of the spleen form an important part of the organ. After entering at the hilum, the splenic artery gives off tra- becular branches which rapidly diminish in size by repeated division. As already described, many of the smaller arteries leave the septa and become ensheathed by the Malpighian corpuscles, to which they contribute with capillary net-works. A certain number of the arteries extend the entire length of the trabeculae, and hence never become encased within the masses of adenoid tissue ; both these latter and those bearing the corpuscles eventually open into the spaces of the pulp, pouring their streams of blood into the parenchyma. The pulp-spaces communicate, on the other hand, with a wide-meshed net-work of venous channels ; the latter unite to form a number of large veins, which pass out at the hilum in company with the principal arteries. All the blood conveyed by the smaller arteries finally reaches the spaces of the splenic pulp, whether directly or indirectly afte;- having first passed through the tissue composing the Malpighian corpuscle ; 126 NORMAL HISTOLOGY. the blood then slowly traverses the partially obstructed channels within the pulp and is collected by the venous spaces and passed on to the larger veins, by which it escapes from F'«- 153- the orc^an. The retarded current within the splenic pulp is favorable to the removal and destruction of the worn-out red cells and to the acquisition of additional leucocytes. Within the pulp, while passinj^ from the arteries to the veins, the blood is probably not confined to channels provided with defi- nite walls, but comes into direct relation with the lymphoid tissue. The lymphatics of the spleen are limited to the connective-tissue framework of the organ, in which they form a superficial plexus in the deeper layers of the capsule, and a deeper plexus within the trabeculae. The lymphatic clefts within the adventitia of the arteries communicate with the deeper lymphatics of the trabeculae ; regarding the definite relations of the deeper lymphatics our knowledge is incomplete. The nerves of the spleen are composed mostly of non-meduUated fibres, although a few of the meduUated variety are present ; they are distributed to the walls of the blood- vessels ; also ganglion-cells have been ob- served along the nerve-trunks. Diagram of the relations of splenic vessels to the tissue of the pulp : a, V, small arterial and venous branches of splenic vessels within trabecula (/, /) ; one twig of artery is diverted and becomes ensheathed by tissue of the Mal- pighian corpuscle, M; the remain- ing part of the artery follows the trabecula and passes directly into the spaces of the pulp — in either case the arterial branches termi- nate in the spaces (/, /) within the pulp surrounded by the lym- phoid tissue (/, /) ; the venous radicles take up the blood and carry it from the spaces of the pulp into the larger venous trunks. THE THYMUS BODY. The thymus body is included among the lymphatic tissues on ac- count of the histological characteristics of the fully-developed organ ; in its early stages, however, the bulk of the organ is epithelial in nature, being derived from the endodermic cells and closely resem- bling many glands in its earliest growth. The rapid invasion of mesodermic tissues, at a later period, so changes the character of the organ that tissues of a lymphoid type predominate, while the original epithelial structures are reduced to mere rudimentary remains. The entire organ usually consists of two lateral lobes, more or less intimately united, composed of numbers of lobules, held together by the interlobular areolar tissue and enveloped within the general fibrous capsule of the organ. The irregularly ovoid lobules, 5-10 mm. in diameter, are further divided by connective-tissue septa into compartments, each of which includes several smaller secondary THE LYMPHATIC -SYSTEM. 127 lobules ; these, in turn, are made up of groups of the primary alveoli or follicles. The latter closely resemble lymph-follicles in structure, being limited by a fibrous envelope giv- Fig. 154. ing off slender tra- beculae, which are soon lost in the deli- cate reticulum of connective tissue pervading all parts of the follicles. The meshes of the re- ticulum are occupied by numerous lym- phoid cells, among which many capillary blood - vessels run. The adenoid tissue of the peripheral zone, or cortex, of the follicles is more Section of human thymus body, showing general arrangement of follicles : a, fibrous tissue enveloping lymphoid tissue and sending septa (of) between the follicles (b) ; d, interfoUicular tissue, contain- ing blood-vessels (<:). closely packed with cells than that occupying the centre, or medulla, in consequence of which variation the medulla appears lighter than the denser cortex. Scattered throughout the follicles Fig. 156. Portion of the periphery of one of the folli- cles of the foregoing section, more highly magnified : a, fibrous tissue ; b, lymphoid tissue, containing numerous capillaries (c). Portion of the same follicle, showing corpuscles of Hassall (a), which represent the original epithelial constituents of the organ. round or oval bodies are seen, which vary greatly in number and size (20-175 At), usually stain but faintly, and present an irregularly concen- J 28 NORMAL HISTOLOGY. trie striation, with occasional nuclei ; these bodies are the corpuscles of Hassall, or the concentric corpuscles. They represent the re- mains of the epithelial structures which, as already stated, in the early stages of the thymus constitute the principal tissue of the organ. The larger blood-vessels of the thymus run within the inter- lobular connective tissue, giving off branches which penetrate the follicles and break up into a rich capillary net-work supplying the adenoid tissue of cortex and medulla. As may be inferred from the character of the organ, the lym- phatics occur in large numbers. The radicles coming directly from the follicles are received by the interlobular vessels, which, in turn, communicate with the superficial net-work occupying the surface of the organ. Bundles of nerve-fibres accompany the ramifications of the arteries and veins, to the coats of which they seem principally to be distributed. The thymus body reaches its highest development about the second year, after which time it gradually diminishes, undergoing retrogressive changes and absorption, until, by the eighteenth to the twenty-first year, the characteristic tissues have disappeared or have been replaced by fibrous connective tissue and fat. THE SEROUS MEMBRANES. The serous membranes are intimately related to the lymphatic system, since the cavities which they enclose form parts of the gen- eral lymph-tract of the body ; when considered in their widest sig- nificance they include the lining of all cavities clothed with endothe- lial cells and cut off from atmosphere. Regarded in a more limited and critical sense, such cavities may be separated into certain groups, following which the connective-tissue linings may be divided into : a. The serous membranes proper, as the peritoneum, the pleura, and the pericardium. b. The synovial membrajies, including the synovial capsules of the joints, the synovial sheaths of tendon, and the synovial bursse placed between opposed movable surfaces to reduce friction. c. The endothelial lining of the vascular system, comprising that of the heart, of the blood-vessels, and of the lymphatics. d. The lining of various spaces developed within the connective tissues; such spaces are usually small and provided with very rudi- mentary linings ; they may be, however, of considerable size, as in the case of the perilymphatic spaces of the internal ear. The serous membranes proper, represented by the peritoneum, the pleura, the pericardium, and the tunica vaginalis, are all derived as constrictions from the originally single pleuro-peritoneal cavity THE LYMPHATIC SYSTEM. I2q first formed. In the closed sacs constituted by the serous mem- branes a parietal and a visceral layer are always distinguishable ; the connection of these with the subjacent structures is slight or intimate according to the character and amount of the subserous tissue. Every organ which projects beyond the wall of the serous pouch into its cavity must be enveloped by the serous membrane to a greater or less degree. When the organ remains closely attached to the wall of the body-cavity, as does the kidney, it obtains only a partial serous investment ; where, on the other hand, the organ leaves the parietes and encroaches upon the cavity, the serous invest- ment becomes almost complete, as in the case of the small intestines. In all cases the viscera lie outside the serous sac, the membrane which constitutes the lining of the space being pushed before the encroaching organ to form a serous covering more or less complete. The serous cavity of greatest extent — that of the peritoneum — in the female presents an exceptional arrangement in possessing outlets at the orifices of the oviducts ; in this connection, however, it must be remembered that the oviduct is the persistent Miillerian duct, which is only one of a number of tubes formed during early foetal life by evagination of the primary serous membrane, thus establishing communication with surfaces exterior to the serous cavity. While such tubes in the Fig. higher animals are only transient, in the lower types they may remain as permanent structures. The serous membranes are sufficiently thin and transparent to permit the color of the underlying parts to be seen readily through them ; moderate strength, extensi- bility, and elasticity are among their physical properties. These membranes consist of the endothelium covering their free surface and resting upon the connective-tissue stroma, which constitutes the chief substance of the Peritoneal endothelium of dog, membrane ; external to this layer a variable tZr::^;^^:'^^^ amount of subserous tissue usually is pres- among the ceiis. ent. The endothehum comprises a single layer of the large, thin, irregularly - polyhedral connective - tissue plates already described and figured in Chapter II. In addition to the minute deeply-stained intercellular areas, or pseudo-stomata, true openings, or stomata, also exist in the sev- eral serous membranes. These orifices are especially well seen in silver preparations from the posterior wall of the frog's peritoneal 9 I .Q NORMAL HISTOLOGY. cavity (Fig. 30); but they may be demonstrated also in the tissues of man and of the higher animals : the central tendon of the dia- phragm, on which they were first discovered in the peritoneum by von Recklinghausen, offers a favorable place for their study. In addition to the stomata occurring in the peritoneum covering the diaphragm, similar apertures have been observed in the omentum, the j^leura, and the pericardium. The stomata, either directly '^' '^"' or through minute canals, lead into the sub- jacent lymphatic vessels and are surrounded by cuboidal or spherical guard-cells. The stroma of the serous membranes con- sists of interlacing bundles of white fibrous tissue, mingled with elastic fibres, which afe especially numerous in the more superficial Peritoneum in section from p^rts, wlicre they frequently form a reticular dog: /, peritoneum proper, . ^i. ru consisting of endothelium of layer. The uitersticcs between the fibrous free surface and subendothelial buudlcS are OCCUpicd by the gfOUnd-SUb- fibrous stroma containing net- , , ^ . . work of elastic fibres; s, sub- stancc ; the latter after a time m some cases, peritoneal vascular connective ^s in the omcntum, suficrs local absorption, interfascicular orifices then partially taking its place. The serous membrane, which in its earlier condition forms a continuous sheet, may become riddled with apertures, and is said to be fenestrated. Where the ground-substance and stroma are well developed and of considerable thickness, particularly in the vicinity of folds, adipose and sometimes lymphoid tissue occur in addition to the blood-vessels and lymphatics. The ground-substance in places where dense is penetrated by an intercommunicating system of lymph-spaces opening into the lymphatic vessels of the serous membrane. Branched connective-tissue cells are also frequently seen with processes extending between the endothelial plates of the free surface ; such processes when stained with silver probably form the pseudo-stomata already mentioned ; other protoplasmic exten- sions of the cells may come into relation with the walls of the blood- vessels or of the larger lymphatics. The subserous layer, where well developed, is composed of loosely-arranged bundles of fibro-elastic tissue, between which blood- vessels and lymphatics, with migratory leucocytes, are situated. The blood-vessels of serous membranes contribute wide-meshed net-works both to the layer of proper stroma and to the subserous tissue ; in positions where tracts of adipose or of lymphoid tissue exist, the capillaries form net-works enclosing the fat-sacs or the lymphoid masses. The lymphatics of serous membranes are very numerous, and are represented by the definite lymphatic vessels and the lymph-spaces THE LYMPHATIC SYSTEM. j^j within the ground-substance ; by means of the stomata and the minute passages leading from them the lymphatics communicate with the serous cavities, while, on the other hand, they join with the wide, irregular lymph-channels within the subserous tissue. The nerves supplying these membranes are limited, those which are present being largely derived from the sympathetic system, com- posed of pale, non-medullated fibres destined chiefly for the blood- vessels. The few fibres passing into the substance of the membrane form a loose reticulum throughout its deeper layers, from which finer fibrillae extend beneath the surface. The synovial membranes, which constitute a second group of serous membranes, include the lining of the clefts developed within the connective tissue (mesoderm) surrounding opposed movable surfaces, embracing the capsules enclosing the articulating surfaces of the various joints, the synovial sheaths in which the tendons glide, and the bursal sacs interposed between surfaces ; these varieties of synovial membranes are known respectively as the articular, the vaginal, and the vesicular. Synovial membranes differ from the serous in the character of their secretion ; that of the former — the synovia — is a glairy, viscid fluid, resembling the white of egg, well adapted for the lubrication of the opposed parts, and contains fat particles, lymphoid cells, and degenerated endothelial plates. Fig. 159. Section of synovial membrane at edge of articular surface : s, s, tissue of synovial membrane bearing villous projections (v, v) ; x, position at which tissues of membrane become continuous with those of periphery of cartilage ; f, group of fat-cells ; /, fibrous tissue constituting peripheral zone of cartilage {c). The secretion moistening serous membranes is thinner, watery, and less suited to the reduction of friction. The articular synovial membranes surround the joints, tightly embracing the bones and enclosing them within their sacs, but do not extend over the articulating surfaces, which are composed of 1^2 NORMAL HISTOLOGY. naked cartilage, over whose surfaces of contact not even the imper- fect endotheUal covering is continued ; tendons or other structures traversing the joint-cavity receive an investment of the synovial membrane. The marginal zone, embracing the attachment of the membrane to the cartilage, is marked by the gradual alteration of the tissues of the synovial membrane to assume the characters first of fibro-cartilage, and finally of the typical articular cartilage of which the membrane then seems a part. The synovial sacs, originating as clefts within the mesoderm surrounding the extremities of the young bones, exhibit a structure corresponding to slightly condensed connective tissue. The mem- brane is composed chiefly of closely-felted bundles of fibfous tissue, mingled with elastic fibres, containing the usual connective-tissue elements ; the free surface of the membrane possesses an imperfect covering of connective-tissue cells, which, when closely placed, as in the younger tissue, present the characters of an endothelium ; when less densely arranged, they retain their processes and appear as branched elements, resembling those of other dense fibrous tissues ; in the vaginal membranes the cells are often elongated to correspond with the axis of the sheath. Cleft folds of the synovial membrane project into the serous cavity as the Haversian fringes ; they are free processes of the membrane containing vascular loops and, in the larger ones, fat ; the smaller secondary fringes, or villi, often present as finger-like processes attached to the edges of the larger folds, contain no blood-vessels, but consist principally of small, irregularly-round cells, separated by a scanty intercellular substance. In some cases these villi enclose a denser core, which consists of fibrous bundles ; occasionally the entire villus is formed of fibro-cartilage, the superficial round cells being wanting. Blood-vessels are quite numerous within the synovial mem- branes, as well as in the subjacent tissues, nearly all parts of the joints being generously supplied. Many of the Haversian fringes contain vascular tufts, while the termination of the blood-vessels around the margin of the cartilages is marked by vascular loops possessing greatly dilated terminal arches. The nerves of the synovial membranes, by no means numerous, form a loose plexus beneath the free surface ; in connection with the joints, peculiar special nerve-endings, the articular end-bulbs of Krause, have been found attached to the ner\'e-fibres ; Pacinian corpuscles have likewise been observed in relation with the synovial membranes. ^ The serous surfaces lining the blood-vessels and the lymphatic THE LYMPHATIC SYSTEM. j^^ channels and spaces have been considered in connection with their respective systems. The development of the lymphatic system in all its parts involves the mesoderm alone. Very early in the life-history of the embryo, shortly after the appearance of the three blastodermic layers, the mesoderm undergoes cleavage into two leaves, the separation affecting the mesodermic layer on either side as far as the lateral margin of the uncleft axial band. The resulting sheets of meso- dermic tissue become the parietal layer {somatopleuric) and vis- ceral layer {splanchnopleuric) ; the former clings to the ectoderm to become the future wall of the body-cavity, while the latter adheres to the entoderm to form the wall of the digestive tube. The space included between these leaves is the primitive body- cavity, or coelom, and the mesodermic tissue forming its imme- diate wall becomes dif- ferentiated into a special Fig. i6o. lining — the mesothe- lium — whose elements are the ancestors of the later endothelium. The fully-formed se- rous membranes, represented by the peri- toneum, the pleura, the pericardium, and the tunica vaginalis, are all derived as constric- tions from the common pleuro - peritoneal sac, or body - cavity, first formed, the subdivision of which into the above- mentioned special serous compartments occurs secondartly and at a much later period. Bearing in mind the origin of the primary lining of the serous membranes, the claims of endothelium to near kinship with con- nective tissue must be admitted ; likewise, the reasons for regarding endothelium as distinct in nature from epithelium will be appreciated. Inasmuch as the epithelium of the genito-urinary tract is derived indirectly from the mesoderm, it is related genetically to the endo- thelium of the abdominal serous membrane. In the course of the differentiation and growth of the fibrous con- nective tissue, clefts appear within the ground-substance between the bundles of young tissue, which become the lymph-spaces of the Transverse section of ten-day rabbit embryo, showing the cleavage of the mesoderm and the formation of the primary body-cavity : E, ectoderm ; M, M, the letters occupy the body-cavity and have the parietal (/) and visceral (z/) layers of the cleft mesoderm respectively above and below them ; the immediate lining of the cavity constitutes the mesothelium ; En, entoderm; TV, neural canal; c, notochord; s,s, cavities within the somites — really parts of the body-cavity; a, one of the paired primitive aortae. 124 NORMAL HISTOLOGY. maturer stages. The formation of the lymphatic vessels takes place in a manner very similar to that by which the blood-channels are produced. The protoplasmic net-works established by the united processes of the connective-tissue corpuscles are at first solid ; sub- sequently they acquire a lumen and become converted into a series of protoplasmic tubes, the nuclei of whose endothelial plates are de- rived from the jjroliferated nucleus of the original elements. The earliest lymph-corpuscles are, probably, migrated mesoblastic cells which have entered the young vessels. The additional coats of the larger lymphatic trunks are derived from the condensation and differentiation of the surrounding young connective tissue^. The development of the lymphoid tissue occurs at a rela- tively late period. The position of the future lymph-gland is indi-' cated by a cleft or fissure which appears within the mesoderm and completely isolates the gland-area on all sides except that destined to become the future hilum, where the tissue devoted to the produc- tion of the gland and the surrounding mesoderm are continuous. The development of the lymphoid tissue is marked by increased numbers and greater compactness of the mesodermic elements ; the supporting reticulum, the capsule, and other details of the adenoid tissue appear later. The development of the spleen begins about the commence- ment of the third month, some time after the pancreas has become defined ; a condensation of the mesodermic cells, lymphoid in character, within the primitive omentum, or the mesogastrium, in the near vicinity of the pancreas, is the earliest indication of the future organ. The lymphoid aggregation first established is sup- plemented by the elements lying beneath the peritoneum, which differentiate into elongated spindle-cells especially devoted to the formation of the trabeculse and connective-tissue framework. Numerous blood-vessels soon grow into the splenic tissue, the sub- sequent accumulations of lymphoid cells within the tissue around some branches of the arteries giving rise to the Malpighian cor- puscles. The history of the development of the thymus body demon- strates an origin markedly at variance with the character of the fully- formed organ, since, notwithstanding the pronounced lymphatic type of the tissue constituting almost the entire body when most complete, its structure in the earliest stages corresponds entirely to embryonal epithelium which is derived as the direct outgrowth of the ento- derm. The first trace of the thymus body appears as a cylindrical bud of entodermic tissue springing on either side from the third pharyngeal pouch, or inner visceral furrow. The epithelial nature of the early thymus is for some time very evident, the original cell- THE LYMPHATIC SYSTEM. j,,- mass appearing also similar to the earliest stage of a glandular area ; repeated division rapidly converts the at first simple cylindrical aggre- gation into a complex figure, in which an elongated main part bears numerous lateral branches. At a later period the surrounding meso- derm becomes richer in cells and more compact and grows into the original epithelial structure, the result of which invasion is the final complete atrophy and disappearance of the epithelial constituents, with the exception of the inconspicuous but constant corpuscles of Hassall, which alone bear witness to the primary epithelial nature of the organ. 136 NORMAL HISTOLOGY. CHAPTER IX. MUCOUS MEMBRANES AND GLANDS. Fig. 161. All passages and cavities directly or indirectly communicating with the exterior of the body and the atmosphere are lined by mucous membranes. These structures consist of two parts : the connective-tissue stroma, or tunica propria, and the epithelial covering ; the outer surface of the connective-tissue layer is (juite usually special- ized to form an extremely delicate basement-membrane, or mem- brana propria, which thus separates the epithelium from the under- lying tissue and forms a third constituent of the mucous membrane. The basement-membrane is often scarcely demonstrable as a distinct layer, while in certain organs, as many glands or the hair-follicles, it is highly developed. The epithelium of mucous surfaces varies both in character and in arrange- ment, as already described in Chajjter II. The proper substance or stroma of the mucous membrane consists of a felt-work of bands of fibrous connective tissue together with net-works of elastic fibres ; these latter may be so plentiful that an especial elastic layer is formed, as in parts of the respiratory tract. Numerous con- nective-tissue cells lie between or upon the fibrous bundles, the flattened plate- like cells forming in many places par- tial linings for the interfascicular lymph- spaces found throughout this layer. Not infrequently the surface of the connective-tissue stroma is beset with numerous elevations or papillae, over which the epithelium extends. Such irregularities, when slight, may be present without impressing the free surface of the mucous membrane, since the epithelial layer completely fills the depres- sions between the elevations : when very pronounced, the papillae or folds of the connective tissue produce such conspicuous sculpt- urings of the surface as the papillae of the tongue or the rug« of the vagina. Diagram of a typical mucous mem- brane : e, epithelium of free surface con- tinuing into the glandular depression to become the secreting cells ; h, base- ment-membrane separating epithelium and connective-tissue stroma; s,s, fibro-elastic tissue of tunica propria ; V, blood vessels forming net-works beneath epithelium and around gland. MUCOUS MEMBRANES AND GLANDS. 137 Fig. 162. Mucous membranes may be invaded to a greater or less degree by lymphoid cells, as in many localities in the digestive tract ; sometimes, as in the villi of the small intestine, the tissue assumes still more closely the lymphoid type, a delicate connective-tissue reticulum supporting the lymphoid cells. The membrana propria, or basement-membrane, usually appears as a delicate homogeneous line beneath the epithelium. It must be regarded as a modification of the connective tissue, and when well developed, after suitable staining with silver, appears as a more or less complete covering of flattened, endothelioid cell-plates. The deeper layers of the mucous mem- brane fade away into the surrounding areolar tissue or into the adjacent submucosa ; sometimes, how- ever, the mucosa is limited by a delicate zone of involuntary muscle, the muscularis mucosae, consisting often of two distinct, although delicate, layers of muscle-cells. Mucous membranes are usually provided with glands, which in their simplest type are depressed portions of the general mucous surface, Hned with modified epithelium — the secreting cells. A single cell may constitute an entire gland, instances of such arrangement being found in the unicellular glands of the lower forms ; the familiar goblet-cells are, in fact, such structures ; it is, however, the more developed forms of secreting apparatus which the term ' ' gland' ' usually represents. Glands are of two chief varieties, tubular and saccular, each of these occurring as simple and compound. Simple tubular Plate-like endothe- lioid connective-tissue cells constituting base- ment-membrane. Diagram illustrating the forms of glands : A , simple tubular ; B, compound tubular ; C, modified (coiled) tubular; £>, simple saccular; £, compound saccular, or racemose. glands are frequent, the peptic glands and the mucous follicles of the intestines being well-known examples. Compound tubular glands vary in complexity, from a simple bifurcation of the fundus, 1 28 NORMAL HISTOLOGY. as in many pyloric or uterine glands, to the intricate arrangement of the tubules of the kidney or the testicle. Simple saccular glands do not occur in the higher animals, but are conspicuous in the lower types, as in the integument of am- phibians. Compound saccular, or racemose, glands, on the other hand, are represented in man and mammals by such important organs as the pancreas and the salivary glands. In the least complex type of gland, the simple tubular, the t>vo fundamental parts of all glands are distinguishable in their primi- tive form : these are the deeper actively secreting portion, the fundus, and the superficial division, or duct, through which the products of the secreting cells escape. Dilatation of the fundus of tlie primitive type produces the simple saccular gland ; division of the fundus and of part of the duct originates the compound tubular variety ; repeated cleavage and subdivision of the duct, with accompanying expansion of the associated terminal tracts, lead to the production of the com- pound saccular, or racemose, type. The tubular glands may exist as perfectly straight cylindrical depressions ; more usually, however, the tubes are somewhat wavy or tortuous : when the torsion of the fundus Fig. 164. reaches its highest expression, such modi- fications as the coiled sweat-glands result. Glandular epithelium is the direct de- rivative of the cells covering the adjacent mucous membrane, so modified and special- ized as to adapt it to the requirements of the several parts of the gland. In simple tubular follicles the cells of the adjacent free surface pass into those lining the neck of the gland with little change ; cells of the increased size and spherical form become more pronounced towards the fundus, where the elements assume the characters of se- creting epithelium. The cells lining the ui)]:)er part of the duct of such glands not infrequently exhibit a distinctly imbricated arrangement ; this is well seen in the peptic glands. The greater complexity of the racemose glands resulting from the system of freely branching excretory tubes renders the recognition of several parts desirable. These are, towards the ducts, proceeding from the ter- minal compartments, the alveoli or acini, the intercalated or intermediate tubules, the intralobular tubes, the interlobular Tubular glands : /4, simple tubu- lar crypt from human small intes- tine ; B, compound tubular gland from pyloric end of human stomach. MUCOUS MEMBRANES AND GLANDS. 139 ducts, and the excretory ducts, which latter usually unite to form a single common duct of large size. At the open end of the acinus the lining cells of the latter become flattened or cuboidal, and, together with the basement-membrane, are directly continuous with the similar structures forming the walls of the narrow intermediate tubule ; the latter succeeds the acinus as the continuation of the narrow intercellular clefts of several adjacent acini, and, after a longer or shorter course as a delicate narrow- lumened canal, passes into the intralobular tube. The distinctive characters of the latter are its larger lumen and the columnar epithe- lium, many cells of which exhibit a distinct vertical striation through- out the peripheral zone next the basement-membrane. The branch- . ing intralobular tubes, on emerging from the lobular tissue, join to form the interlobular duct which occupies the connective tissue lying between and holding together the divisions of the glandular sub- FiG. 165. Fig. 166. Section of racemose gland showing relation of glandular tissue to origin of duct : x, acini lined with secreting cells which are directly- continuous with those of the intermediate tubule {z) ; y, interlobular connective tissue. Section of the human parotid gland shiiwing the interlobular tissue : s, s, se- creting cells of surrounding acini ; d, inter- lobular duct ; V, blood-vessels within the fibrous tissue ; g;, group of ganglion-cells. stance. The interlobular ducts are clothed with simple columnar cells, which form "a passive lining to the canal for the conveyance of the secretions of the more active parts of the gland. Towards the free surface of the mucous membrane the interlobular ducts unite to form the chief, often single, excretory duct of large lumen, whose walls for a variable distance from the point of discharge are covered with epithelium similar to that covering the adjoining mucous surface ; this is soon replaced, however, by the columnar cells which then continue into the smaller tubes. In the large ducts the subepithelial tissue is strengthened by net-works of elastic fibres. The saccules or alveoli are limited by a basement-membrane j.Q NORMAL HISTOLO(;Y. upon which rests a single layer of irreg-ularly spherical or polygonal secreting cells ; these latter do not entirely till the acinus, but leave an intra-cellular cleft, in which the system of tubes for the conveyance of the secretions commences. Glands are often divided into serous and mucous, a differentia- tion depending upon the peculiarities of the cells lining the acini as well as upon the character of their secretion. The cells of the serous glands are distinguished by being distinctly granular, generally spherical in form, readily and deeply stained with carmine, and by having conspicuous nuclei situated near the centre of the cells ; the elements of the mucous glands, on the contrary, are distended, Fig. i68. Fig. 167. Serous acini of human pa- rotid gland ; the deeply-stain- ing granular cells are sur- rounded by the basement- membrane. Mucous acini of human lingual gland : the secreting cells (a), being loaded with the slightly-staining secretion, appear clear and transparent ; c, c, crescentic masses of granular cells — the demi-lunes of Heiden- hain ; b, interacinous connective tissue. very clear and transparent, slightly stained with carmine, and have the nuclei displaced to the outer edge of the cells, not infrequently immediately beneath the basement-membrane. In the embryonal pre-functionating condition these two kinds of glands are identical, both as to mode of origin and histological characteristics ; the varia- tions and the conspicuous differences subsequently appearing depend on differences of physiological function and character of secretions, and not on structural differences in the original cells. Fluids elaborated by the serous glands are thin and watery, appear- ing within the protoplasm of the secreting cells as minute dark gran- ules ; the general appearance of the cells depends upon the number of these granules stored up within their protoplasm. When a serous gland is in a condition of rest, the cells are loaded with granules, and when stained they appear larger, clearer, and coarsely granular ; after active secretion the cells are exhausted and contain fewer granules, appearing, therefore, srnaller. darker, and finely granular. MUCOUS MEMBRANES AND GLANDS. 141 The mucous glands secrete a clear, viscid, homogeneous sub- stance, or mucine, having little affinity for carmine, but staining deeply with haematoxylin. During rest the cells of such glands become loaded and distended with the mucoid secretion, while the nuclei are crowded to the periphery of the cells ; under these condi- tions the cells lining the acini appear clear with well-defined outlines, and, on the sides next the basement-mem- brane, present a thin zone containing the displaced nuclei and granular protoplasm. After prolonged secretion the exhausted cells contain relatively little mucoid sub- stance ; hence the threads of the protoplasm are no longer widely separated, but are more closely placed ; in consequence of these changes the cells assume appearances resembHng those of the elements of the serous glands, being smaller, darker, and more granular than the cells of the quies- cent mucous gland. In the acini of mucous glands small crescentic groups of granular, deeply-staining cells are often seen lying between the clearer elements and the basement-membrane ; these are the crescents of Gian- nuzzi, or the demi-lunes of Heidenhain, the significance of which has caused extended discussion. The recent researches of Lingual glands from tongue of cat : a, b, the serous and the mucous acini containing respec- tively the granular and the clear cells. Fig. 170. B A and B, serous and mucous acini in different stages of functional activity : r, condition of rest, the cells being gorged with secretion ; a, condiiion of exhaustion after great activity : following the discharge of the secretijn the elements of the protoplasm become more closely placed, producing an appearance of increased granularity. Kuchenmeister, Solger, R. Krause, and others, have shown that these crescents are composed of cells identical with those filHng the acini of the serous glands. The crescents, therefore, are groups of serous cells, compressed and displaced by the preponderating mucous elements, which add their albuminous secretion to that of the mucous gland. Glands containing the crescents are to be classed as mixed, or muco-serous. The vascular supply of glands is always rich. The larger blood- vessels, conveyed by the submucosa, send off branches into the 142 NORMAL HISTOLOGY. Fig. i-i. mucosa to break up into capillaries which enclo.sc the tubules and acini in close net-works, lying outside but in intimate relation with the basement-membrane, an arrangement favoring the passage of substances from the blood into the protoplasm of the secreting cells, which are thus placed between the blood-current on the one hand and the lumen of the gland on the other. Numerous lymphatic spaces are contained within the connective tissue surrounding the acini and the tubules, some of the clefts being immediately beneath the membrana propria and in close relation with the gland. The nerve-supply of glandular structures is often very rich. The nerve-trunks accompany the larger blood-vessels in the .Submucous tissue and give off numerous small bundles which follow the smaller arteries in their distribution to the mucosa, where they form delicate plexuses about the acini and the tubules immediately outside the basement-membrane. The exact mode of the final termination of the nerves and their relation to the individual secreting cells are still matters for investigation ; whether the fibres pierce the basement- membrane to terminate among the glandular epi- thelium, while probable, must be regarded as still unproved. The development of glands proceeds from the epithelial tissue of the young mucous mem- ,)^VT)>^i^;^?S)t v brane, the penetrating cylinder of epithelium rep- "'^ 0>^!^^rS^')^>t' resenting ectodermic or entodermic tissue, except sS'i^'l''^^^^}^ ^" those cases where the glands are formed in -rCV. connection with the parts of the genito-urinary tract derived entirely from the mesoderm. The first trace of the glands consists of a cylindrical ingrowth of the epithelium into the subjacent mesodermic tissue, both the tubular and the saccular glands alike starting as simple epithelial processes. Where, however, the struct- ure is destined to become a gland of the racemose type, the branching cords of epithelial elements early indicate the nature of the future gland as distinguished from one of the compound tubular variety ; since, in this case, the terminations of the epithelial masses soon become markedly ex- panded and club-shaped, from which dilatations the ultimate divisions or primary alveoli of the racemose glands are extended secondarily. The epithelial cords, at first solid, later acquire a lumen which extends as far as the terminal compartments of the gland. Sometimes, as conspicuously instanced by the liver, V^; v:^^ Peveloping sa'ivary gland Irom fifteen-day rabbit embryo. J'he ectodermic ingrowth has divided into secondary branches which termi- nate in slightly expanded club-shaped ends; e, epithelium of oral sur- face ; nt, young connec- tive tissue of future tunica propria into which the epithelium grows. MUCOUS MEMBRANES AND GLANDS. I^^ the primary arrangement of the gland is modified by subsequent changes to such a degree that the original plan of its structure is recognized with difficulty. The sexual glands are so highly special- ized that in their development they deviate materially from the mode of the formation of the typical secretory organs. Ordinarily the elaborating glandular cells are ectodermic and entodermic in origin, while the basement-membranes and supporting tissues are meso- dermic. 144 NORMAL HISTOLOGY. CHAPTER X. THE DIGESTIVE TRACT. THE MOUTH. The mucous membrane of the oral cavity consists of the epi- thehal covering and the connective-tissue stroma or tunica propria ; the deeper layers of the latter fade insensibly into the subjacent tissues which unite the mucous membrane with the surrounding deeper parts. The epithelium lining the entire oral cavity is of' the stratified squamous variety, continuous with the epidermis on the one hand and with the covering of the pharynx on the other. The tunica propria is composed of interlacing bundles of fibrous connective tissue containing elastic net- ^"^- ^~~- works, and possesses numerous simple ^.- '. - -.' -' ;f5 papillae which encroach on the epithelial 1^; -V:■.^^0• -'•'-'-■• '■-■•■''JI l^y^^> but do not appear on the free surface of the mucous membrane. The latter is broken in many places by the openings of the ducts of the numerous glands which occupy the submucosa and deeper parts of the mucosa. In the transition of the skin on the lips, where the skin passes into the ^ . ^ , ^ ^ mucous membrane, the epithelium is Section of oral mucous membrane ot _ _ ' child; the surface of the fibrous tunica greatly thickcncd, whilc the connectivc- propria is broken by minute papilte, ^^^^^^ J^ dcCrCaSCS in thlcknCSS ; the •which contain the endings of the blood- ... vessels and the nerves. The papillae Subepithelial papillae llCrC bcCOniC VCry are covered by the stratified squamous prominent. The hair-folliclcs disappear, epithelium. \ i i -n but the sebaceous glands still are present, especially near the angles of the mouth and in the upper lip. The mucous membrane covering the cheeks adheres tightly to the bucci- nator muscle, and possesses small papillae ; that covering the gums is dense, and contains numerous well-marked papillae beneath the epithelium, the submucous tissue being closely united with the peri- osteum. The portion covering the hard palate is thin and firmly united to the periosteum, while that investing the soft palate, the uvula, and the fauces is much thicker, less dense, possesses numerous mucous glands, and, in many places, is so densely crowded with lym- phoid cells that the entire mucous membrane assumes the appearance of adenoid tissue. THE DIGESTIVE TRACT. 145 The oral mucous membrane is thickly beset with small mucous racemose glands in nearly all parts. These are especially well marked on the lips, the cheeks, the under surface of the tongue, and the soft palate, constituting, respectively, the labial, the buccal, the lingual, and the palatine glands ; on the gums and the hard palate such structures are absent or present in very limited numbers. The acini are situated within the deeper layers of the mucosa, while the ducts pierce the superficial layers to open on the free surface. The squamous epithelium of the latter is continued within the duct usually as far as its first division. Small lateral isolated groups of acini, constituting accessory mucous glands, sometimes open into the long narrow excretory duct of the main glandular mass during its journey to the free surface. The larger blood-vessels supplying the oral mucous membrane he within the submucous tissue and give off branches which extend through the deeper layers of the mucosa to the superficial portions of the connective-tissue stratum ; on reaching the outer boundary of the latter the arteries break up into rich subepithelial capillary net- works, or, where papillae are present, enter the minute elevations to supply their apices with terminal capillary loops. The capillaries likewise enclose the acini of the oral glands. The lymphatics begin in the irregular net-work of interfascicular spaces between the connective-tissue bundles of the tunica propria ; these spaces unite to form definite lymphatics in the deepest layers of the mucosa, which in turn are taken up by the larger lymph- vessels of the submucous tissues. " Nerve-fibres, largely of the meduUated variety, accompany the blood-vessels, and form a subepithelial plexus ; special terminations — the end-bulbs — are found in the apices of some of the papillae, while additional numerous tactile corpuscles occur on the lips. THE TEETH. In principle, and among many of the lower animals in fact as v/ell, the teeth may be regarded as hardened papillae of the oral mucous membrane. The teeth are firmly retained within their appropriate sockets by the close attachment afforded by the alveolar periosteum which holds together the alveolus and the root of the tooth. The perios- teum lining the alveolus is composed of dense fibrous tissue, whose fibres have a general transverse disposition : elastic tissue is almost wanting, nerves and blood-vessels being, however, numerous. At its neck the tooth is especially embraced by the thickened perios- teum, which then becomes continuous with the periosteum covering the alveolar process of the jaw and with the gum. 146 NORMAL HISTOLOGY. The tooth comprises the dentine, the enamel, and the cemen- tum. The dentine, or ivory, principally contributes the bulk and the characteristic form of the tooth, completely enclosint^ a central pulp- cavity, except where the narrow nutrient canal, admitting the blood- vessels and nerves to the pulp, pierces the aj)cx of the fang. The dentine is composed of a matrix or ground-substance, which, as that of bone, must be regarded as modified connective tissue, formed of bundles of Fig- 173- fibrous tissue intimately united and subsequently impregnated with calca- reous salts. Piercing the ground- substance and appear- ;^) ing under low amplifica- FiG. 174. / X 1i>v Ax ■y-^'->i^"t^' ^-^ ' c -^ Longitudinal section of molar too{h of kitten : a, pulp-cavity, continued by canals {/) to apices of roots; d, dentine; e, en- amel ; c, cementum ; /, alveolar periosteum ; n, neck of tooth ; b, osseous tissue of jaw. Section of dried human tooth showing portions of enamel and dentine : a, ground-substance of dentine ; b, branching dentinal tu- bules ; c, terminal zone of tubules within the enamel {d). tion as a radial striation, the dentinal tubules extend the entire thickness of the dentine as minute channels ; they are seen espe- cially well in sections of the dried tooth in which the canals are filled with air. Starting from the pulp-surface with a diameter of 20-26 /i, the dentinal tubules pass in a slightly wavy and spiral course through the dentine, to terminate in irregular clefts, the interglobular spaces, situated at the juncture of the dentine with the enamel or the cementum. THE DIGESTIVE TRACT. H7 The tubules give off numerous secondary canals along their course, by which means the adjacent tubules communicate ; on approaching the enamel or the cement the tubules undergo repeated division, the resulting smaller secondary channels corresponding in their general direction with the larger canals. The marked parallel curves described by the dentinal tubules pro- duce optical effects which are appreciated as a coarse striation con- centric with the outline of the pulp-cavity ; these appearances, known as Schrager's lines, may be seen in sections with the unaided eye. That part of the dentinal matrix immediately surrounding the tubules is especially dense and resistant, and constitutes the so-called den- tinal sheaths which may be isolated by acids. Within the tubules lie the deHcate dentinal fibres, which are the modified processes of the connective-tissue cells forming the peripheral layer of pulp- cells. When cut across the tubules appear circular or slightly oval, and contain a minute dot, the dentinal fibre in transverse section. Want of uniformity in the calcification of the outer zone of dentine gives rise to the incremental lines of Salter. The interglobular spaces are irregular stellate intercommuni- cating clefts situated at the margin of the dentine, into which open on the one hand a number of dentinal tubules and on the other hand the Fig. 176. Interglobular spaces of dentine from dried human tooth : t, i, spaces into which certain dentinal tubules {d) open. Section of enamel from dried human tooth : a, i, longitudinal and trans- verse views of enamel rods. spaces or the lacunae of the cementum. Each space contains a pro- toplasmic body, the connective-tissue cell, the processes of which unite with the dentinal fibres. The enamel covers the exposed parts of the softer underlying dentine, and is composed of irregular 4-6-sided columns, the enamel prisms, closely placed and generally vertical to the surface of the dentine. After suitable isolation the enamel prisms appear slightly varicose in outline, the minute concavities producing the irregular dark bands often seen traversing the prisms. The prisms are held together by a delicate layer of cement-substance and grouped into Fig 1^3 NORMAL HISTOLOGY. bundles which cross one another, producing the alternate dark and lijrht radial bands seen in the enamel. The additional dark lines extending more or less parallel to the free surface of the tooth — the stripes of Retzius — are probably due to inequalities in growth and density. At birth, and for a variable time thereafter, the outer sur- face of the enamel is covered by a delicate but resistant cuticle, the membrane of Nasmyth, composed of keratose epithelial plates, the remains of the enamel organ. This cuticle is soon worn away after the teeth are actively used. Next the dentine numerous clefts exist for a short distance between the enamel prisms ; they com- municate with the interglobular spaces and thus indirectly with the dentinal tubules. The cementum, or crusta petrosa, invests the fang of the tooth and closely resembles in structure ordinary bone ; the lamellae extend parallel to the dentine, as do likewise the long axes of the bone lacunae. Where the cementum reaches a considerable thickness, as at the apex of the root of the tooth. Ha- versian canals may exist, although usually these are wanting ; the outer layers of the cement contain fewer and smaller lacunae. The lacunae communicate with the dentinal tubules, while the protoplasmic processes of their contained bone-cells may come in contact with the filaments of the odonto- blasts lying within the dentinal tubules. The pulp consists of a matrix of soft embryonal connective tissue, in which nu- merous stellate and spindle cells form pro- toplasmic net-works JDy their anastomosing processes. At the periphery the connective- tissue elements are arranged as layers of elongated cylindrical cells perpendicular to the inner surface of the dentine, in contact with which they lie ; these cells are the odontoblasts, being the representatives of the cells which were actively engaged in producing the dentinal matrix. The protoplasm of many of these cells is prolonged peripherally as delicate threads into the dentinal tubules, the processes becoming modified to form the stiff elastic dentinal fibres ; centrally, the odontoblasts frequently are connected with the stellate connective-tissue cells. The pulp is richly supplied with blood-vessels and nerves. The arteries run in the long axis of the tooth, breaking up into capillary net-works which are closest in the periphery. The nerves accom- Section of human tooih junction of the dentine and the cementum : D, dentine with its tubules, which communicate with interglobular spaces {B i and with lacunae of cementum (C). THE DIGESTIVE TRACT. 149 Fig. 178. pany the larger blood-vessels as meduUated fibres ; these give off filaments which pass to the layers of odontoblasts, among which they extend as pale fibres. The ulti- mate distribution of these latter is still unsettled ; the assertion that fine fibrillae accompany the dentinal fibres into the tubules lacks confirmation. Distinct lymphatic vessels have not been demonstrated within the pulp, although the clefts within the matrix between the connective-tissue fibres represent the lymph-spaces and are in close relation with the adjacent lym- phatic channels. DEVELOPMENT OF THE TEETH. Section of young tooth of child, show- ing peripheral portion of pulp and ad- joining dentine : b, pulp-cells, some of which send processes {a) within dentinal tubules ; c, stroma of delicate connective tissue ; d, blood-vessels. Fig The teeth of man and the higher animals are really exaggerated papillae, the peripheral parts of which have become specialized and have under- gone calcification. The ectoderm con- tributes the enamel, while the dentine, cementum, and pulp are derived from the mesoderm. A linear thickening of the primitive oral epithelium marks the earliest indication of the formation of the teeth ; in man this band appears before the end of the sixth week (Rose), and is adherent to the under surface of the epithelial layer. Following the ex- pansion of this ectodermic thickening a con- tinuous lateral projection, the dental ridge, grows obliquely into the mesodermic tissue. The dental ridge continues to grow back- ward towards the mandibular articulation, forming an unbroken arch of ectodermic tissue connected with the under side of the oral epithelium. The line of this attach- ment is later marked on the oral surface by a longitudinal furrow, the dental groove, which has been long known, and which was formerly regarded as the initial step in the dental development. While the dental ridge constitutes a shelf-like common epithelial invagination, the position and further development of the individual teeth are marked by local thickenings along the under surface of c • Section of jaw of rabbit embryo, showing thickening of ectodermic epithelium {ec) from which dental ridge (e) begins its growth into mesoderm (;«). I50 NORMAL HISTOLOGY. the ridge. These secondary aggregations are the first indications of the enamel organs of the temporary teeth. After the estabUsh- FlG. iSo. Fig. i8i. Model of jaw of human embrj'o of 40 mm. : r, r, arch of increased epiihelium constituting dental ridge; /. local thick- enings corresponding to positions of future enamel sacs. (After Rose.) Section of jaw of rabbit embryo, showing dental ridge cut across : ec, oral ectoderm : e, epithelial outgrowth corre- sponding to future enamel organ ; in, mesodermic tissue. Fig. 182. ment of these structures the ectodermic tissue composing the dental ridge atrophies and eventually disappears in the intervals between the individual teeth. The enamel sacs of the permanent teeth are formed at a later date from the remains of the dental ridge, those for the three permanent molars being derived from a special extension of the dental ridge which grows independently of ectodermic attachments. The primitive enamel organ which grows from the dental ridge at first con- sists of a solid cylindrical process of epi- thelial tissue ; soon, however, the ex- tremity becomes club-shaped and slightly tortuous, and later distinctly expanded and flask-shaped. Coincident with these changes the surrounding mesoderm be- gins to exhibit proliferation and conden- sation of its elements, this dificrentiation marking the earliest stage in the forma- tion of the important mesodermic dental papilla, which very soon becomes a conical mass of closely-aggregated meso- dermic elements. Along with the growth of the latter the now expanded end of the ectodermic plug becomes indented Section of jaw of rabbit embryo, showing later stage of enamel organ, which now exhibits diflfercntiation into outer (b) and inner (e) cells : m, mesodermic tissue which at a has undergone already some condensa- tion ; ec, oral ectoderm. THE DIGESTIVE TRACT. jCj or invaginated to form an epithelial cap, which embraces the meso- dermic dental papilla, and, from its future important function, is known as the enamel organ. The impression of the dental papilla upon the overlying enamel organ is probably not to be attributed ^'°- ^^•5- to mechanical obstruction op- posed to the advancing ecto- dermic tissue, but has its cause in more deeply lying laws of ex- pansion along lines of unequal growth. As the invagination of the enamel organ progresses, more and more of the dental papilla becomes covered, until about two-thirds of the meso- dermic cone are embraced within the sides of the ectodermic cap. The enamel organ itself under- goes a differentiation into three distinct layers : the outer layer, directly continuous for a long time with the ectodermic cells of the oral cavity, is composed of one or two layers of low columnar or polyhedral cells ; at the point where they are reflected to form the inner, invaginated part of the original epithelial sac, the cells become elongated and 0 Section of jaw of cat embryo ; the dental papilla is seen as a projecting conical mass (/) of con- densed mesoderm, whose summit is enveloped by the invaginated enamel organ (e) ; ec, oral epi- thelium, still attached by the atrophic isthmus (a) with the -enamel organ, whose outer {b), middle (c), and inner (d) layers are differentiated ; e' , beginning of enamel organ for permanent tooth. Fig. 184. Section of jaw of cat embryo with four developing teeth slightly farther advanced than in the pre- ceding stage : ec, oral epithelium ; a, dental groove ; e, enamel organ ; /, /, dental papillse ; m, mesodermic tissue ; b, b, bone. distinctly columnar, constituting the inner layer of the enamel organ containing the beautiful enamel cells. The outer and inner lavers ■c^/^' IC2 NORMAL HISTOLOGY. of the enamel organ are separated at first by the narrow zone of epithelial elements of the middle layer ; the cells of the latter soon undergo characteristic changes, owing to an accumulation of fluid, resulting in the complete transformation of the cells, which become pressed together and reduced to thin plates, the tissue appearing as if composed of irregularly anastomosing connective-tissue fibres rather than of epithelial elements. The enamel organ retains for a considerable time its connection with the epithelium of the oral cavity, a thin atrophic cord of cells indicating the position of the former robust stalk. At the side ^■J ^' of this attachment a lateral cylin- drical projection early marks the beginning of the development of the second enamel organ for the permanent tooth. The columnar cells of the inner layer alone are concerned in the production of the enamel. This process consists essentially of a gradual deposition on the inner side of the enamel cells — that is, next the new dentine — of homo- geneous prisms arranged verti- cally to the surface of the inner layer of the enamel organ. The layer of enamel increases by the addition of increments deposited from within out, the latest-formed enamel always lying immediately internal to the inner layer of the enamel organ. During the later stages the inner and outer layers are approximated at the expense of the intervening middle layer, which finally becomes reduced to an attenuated stratum, the other coats of the enamel sac coming almost in actual contact. During the changes described in the enamel organ the central dental papilla is actively engaged in producing the dentine. The top and sides of the papilla are covered by a layer of elongated, columnar or pyriform connective-tissue cells, the odontoblasts, which are the immediate agents in causing the deposition of the dentinal matrix, the formative process being similar to that producing bone. The dentine is first formed at the apex of the papilla, and appears as a thin lamina of homogeneous matrix into which the delicate processes of the odontoblasts extend, becoming the dentinal Section of developing tooth from cat embryo : m, mesodermic tissue condensed in dental pa- pilla (/), at whose summit osteoblasts {d) are forming young dentine (c) ; inner layer {a) of enamel organ is engaged in producing layer of young enamel (b) ; e, middle, h, outer layer of enamel organ. THE DIGESTIVE TRACT. 153 Fig. I fibres ; the canals left within the matrix to maintain the nutrition of the tissue constitute the dentinal tubules, the homologues of the lacunae and canaliculi of bone. With the continued growth the sides of the papill^ as well as the apex be- come covered by the layer of newly- formed dentine ; the central part of the dental papilla remains, after all the dentine has been formed, as the pulp-tissue, into which the blood- vessels and nerves grow at a later period. At first both dentine and enamel are soft, the impregnation with lime salts occurring subsequently ; the layer of the soft, most recently formed matrix is readily distin- guished in stained sections from the older calcified tissue. The cemen- tum, wanting during foetal life, is produced by the alveolar periosteum. THE TONGUE. Section of developing tooth from cat em- • bryo, portion of preceding figure more highly magnified : fn, mesodermic elements consti- tuting pulp-tissue ; /, layer of odontoblasts en- gaged in producing dentine (/i) ; a and b, cells of middle layer, c and d, cells of inner layer of enamel organ ; e, zone of young enamel. The bulk of the tongue is com- posed of variously-disposed bundles of striated fibres of the lingualis, together with those of the accessory muscles, over the unattached surfaces of which the oral mucous membrane is reflected. The muscular tissue of the organ is arranged in bundles extending in three planes : (i) vertically and slightly radially (genio-hyoglossus, vertical fibres of lingualis and hyoglossus) ; (2) transversely (trans- verse fibres of lingualis) ; (3) longitudinally (lingualis superior and inferior, and styloglossus), A vertical median partition, the septum lingualse, divides the muscular tissue into two halves ; the inter- fascicular spaces are filled by delicate connective tissue and fat, in which lie embedded numerous small lingual glands. Many of the muscle-fibres find insertion in the deeper layer of the mucosa, into which their sarcolemma fades. Branched striped muscle-fibres are of common occurrence in the tongue. The mucous membrane forms the most conspicuous part of the organ. That covering the sides and inferior surfaces of the tongue is thin, containing small papillae and numerous mucous glands : on reaching the superior surface the mucous membrane greatly increases in thickness, and presents additional conspicuous irregularities, the 154 NORMAL HISTOLOGY. the filiform or conical, Fig. W'M. .r,-^S^r papillae. The papillae are of three kinds the fungiform, and the circumvallate. The conical papillae are widely distributed, occurring on all parts of the upoer surface of the tongue. They consist of a conical or cylindrical elevation of the connective tissue of the mucosa, .5-2.5 mm. in height, covered with a thick layer of epithelium, the cells of which, as the most exposed part of the papillae, are partially removed by abrasion, the remaining epi- thelium presenting a ragged sur- face. The fungiform papillae are likewise found on all parts of the tongue, but they are fewer in number, lower, and broader than the conical, appearing as isolated but distinct red points. The connective-tissue stalks of these papillae are composed of a dense felt-work of fibrous tissue, and bear secondary papillae on their upper surface, the epithelium completely enveloping the entire connective-tissue core. The circumvallate papillae, usually eight to ten in number, are placed in 'two rows forming a A at the posterior part of the dorsum of the tongue. Each consists of a large flattened fungiform papilla surrounded by a deep furrow and a secondary encircling ridge or S wall — an arransrement which has ^A^ suggested the name. The upper surface of the mucosa is beset with minute secondary elevations, which, however, are not apparent on the free surface, being hidden by the thick stratum of covering epithelium. Lying altogether within the epithelium lining the sides of the deep circular furrow, the taste-buds appear as inconspicuous oval bodies Section of human tongue showing conical papillae: a, connective tissue of mucosa, which forms core of papillae; i, b, partially abraded epithelium; c, masses of epithelial cells filling interpapillary recesses. Section of tongue of child, showing a fungiform papilla; the connective-tissue stroma is covered by the epithelium. THE DIGESTIVE TRACT. occupying almost the entire thickness of the epithelium. Additional taste-buds are found in the folds in the vicinity of the circumvallate papillae, as likewise on some fungiform papillae. At the sides of the Fig Section of circumvallate papilla from tongue of child : a, main central elevation, surrounded by the annular ridge {b) and the intervening deep furrow; c, taste-buds within the epithelium; d, ducts of neighboring glands (g, g' } ; e, blood-vessels. tongue, just in front of the anterior pillars of the fauces, are groups of parallel folds containing a number of taste-buds ; these folds con- stitute the papillae foliatae, which are highly developed in some of the lower animals, as in the rabbit. The taste-buds are oval, flask-shaped bodies, embedded within the epithelium, occu- pying usually the entire thickness of the latter, with their long axes placed in general vertically to the free surface of the epithelium. Each taste-bud consists of an enveloping layer of greatly-elongated epithelial cells, the cortical or tegmental cells, which form a complete covering, except over a small area correspond- ing to the superficial pole of the bud ; at this point a minute canal, the taste-pore, connects the interior of the bud with the surface of the mucous membrane. Within the epithelial capsule lies a group of highly-specialized elements, the gustatory cells. These neuro-epithelial elements appear as spindle, rod-like, or forked cells, each being possessed of an oval nucleus situated about the centre of the elon- gated body. The peripheral or outer ends of these cells are usually prolonged with fine pointed extremities, some of which terminate in stiff" hair-like processes projecting within the taste-pore almost as Taste-bud from circumval- late papilla of child. The oval structure is limited to the epithelium {e) lining the furrow, encroaching slightly upon the adjacent connec- tive tissue (/\; o, taste-pore through which the taste-cells communicate with the mucous surface. 156 NORMAL HISTOLOGY. far as the free surface. The inner or central ends of the gustatory cells are prolonged as slender, sometimes forked, processes ; the minute swellings or varicosities which these extensions often exhibit are supposed to indicate the direct connection of the neuro-epithelial cells with the fibres of the nerve of the special sense of taste. It must be remembered, however, that no such continuity has been or is likely to be demonstrated. The submucous and interfascicular tissue of the tongue contains numerous glands, both of the mucous and of the serous type. The mucous glands resemble those of other parts of the oral cavity, being small racemose clusters of acini more or less filled with clear mucoid secretion. They are situated in the deeper layers of the submucous tissue, as well as between the bundles of the muscle-fibres, principally in the posterior part of the tongue, although a group of small mucous glands (Nuhn's) is found near the tip. The ducts of those at the root of the tongue are some- times lined by ciliated epithelium. The serous glands are limited to the immediate neighborhood of the circumvallate and of the foliate papillae. The acini appear darkly granular and pour out a thin watery secretion well adapted to aid in producing gustatory impressions. The mucous membrane covering the root of the tongue contains also much adenoid tissue, which occurs either as diffuse masses or as circumscribed irregularly spherical lymph-follicles, 1-5 mm. in diameter. The position of these follicles is fre- ■ '^^' quently indicated by slight elevations of the mucosa, in the centre of which a minute pit leads into the interior of the lymphatic crypt. The ^ epithelium lining such recesses is completely in- \^^ *^)y filtrated with lymphoid cells, while the surround- „ ,. " , ing diffuse adenoid tissue contains several minute salivary corpuscles " from human saliva: x, sphcHcal masscs of denser structure. group of corpuscles near Amoug the formed elements observed in the epithelial cells ; y, cor- , ■=> puscie which has burst, saliva the so-callcd salivary corpuscles are allowing granules to es- couspicuous. Thcsc are sphcrical bodies, some- cape ; r, salivary cor- '^ 111 puscie highly magnified, what larger than the leucocytes, and possess a showing granules and (jistiuct nuclcus and minute granules within the nucleus. -i 1 • 1 i-r ■ 1 cell-contents ; under high amplification these gran- ules exhibit the agitation characteristic of the molecular or Brown- ian motion. The salivary corpuscles are derived from the adenoid tissue of the mouth, and are really escaped lymphoid cells, which, in consequence of the action of the saliva, become swollen by the imbibition of a fluid less dense than the tissue-juices ; they thereupon exhibit a reaction similar to that seen when the colorless blood-cell is treated with water. THE DIGESTIVE TRACT. jc- The blood-supply of the tongue is very rich, the vessels forming a superficial net-work in the mucosa, from which minute twigs as- cend within the papillae to terminate at the summit in close capillary plexuses. The acini of the various glands are surrounded by capillaries, as are also the lymph-follicles through the adenoid tissue of which many minute vessels extend. The capillary net-works supplying the mus- cular tissue follow the general arrangement and direction of the mus- cular fibres, surrounding the latter by the characteristic rectangular- meshed net-works. The lymphatics of the tongue are numerous ; they are arranged as a superficial plexus within the submucous tissue, which re- ceives the lymphatics from the bases of the papillae ; the Fig. 193- latter vessels, in turn, take up /-/f^T^^ the smaller trunks having their Fig. 192 Section of tonsil of dog : a, epithelium of mucous membrane passing into central recess {6), where it becomes infiltrated with lymphoid cells (e) ; c, lymph-nodules embedded within diffuse adenoid tissue ; d, neighboring mucous glands. Section of tonsil of child ; the epithelium of adja- cent surface passes into the deep pits which extend into the adenoid tissue. origin in the numerous interfascicular lymph-spaces within the cen- tral papillary connective tissue. The lymph-follicles at the root of the tongue are well provided with lymphatics, which surround the follicles and give ofif radicles to the adenoid tissue. Fig. 194. 158 NORMAL HISTOLOGY. The nerves supplying the mucous membrane — the glosso-pharyn- geal and the hngual branch of the trifacial — end either beneath the epithelium in the usual manner, or in close relation with the organs of special sense — the taste-buds. Numerous microscopic ganglia also occur along their course, especially in connection with the fibres of the glosso-pharyngeal nerve. THE TONSILS. The tonsils represent compound lymphatic glands, while con- siderable variation exists as to form and size, each organ consisting of an aggregation of from ten to eighteen lymph-follicles, closely resembling those found at the root of the tongue embedded within the sur-' rounding diffuse adenoid tissue. The entire mass is separated from the adjacent structures on the attached borders by a fibrous capsule, and is covered with a reflection of the oral epithelium on the mucous surface, in- cluding the deep central pit on which the lymph-follicles abut. The epi- thelium covering the folds and de- pressions of these surfaces is com- Wi^^iWf>^^^^i^''<^%^^ P^e^^^o"-^ o^ ^"^^e suriaces is com- ^^^e^i^'^^^Mk^ pletely infiltrated with lymphoid cells, '^^?'^S^^y'^f^i