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PRIKCIPLES OF HISTOLOGY
A. H. TUTIXE
THHHi
THE PRINCIPLES OF HISTOLOGY
DHSCKIPTIVIC AND PRACTICAL
BOOK I.
DESCRIPTIVE HISTOLOGY.
BY
ALBERT H. TUTTLE.
PUBLISHED BY ANDERSON BROS.
UNrV'ERSlTY OF VIRGINIA.
1898.
Entered according to Act of Congress by Albert H. Tuttle, University o£ Virginia, J 898,
Press of Prout the Printer,
Charlottesville, Va.
ft
^
PREFACE.
This manual represents an effort to state the most im- ])ortant facts of descriptive histology in a manner adapt- ed to the wants of my own classes of students, both aca- demic and medical. It lays no claim to originality save in the arrangement and mode of presentation, and ac- knowledgement is here made of the extensive use of the ])est modern treatises and monographs accessible in its preparation. In the portions dealing with the nervous system I am under special obligations to the writings of Cajal and Van Gehucten among others. I am in- debted to Dr. Lyman J. Skeen for frequent and valuable aid in the preparation of the book.
This volume will be followed by a second, now in course of preparation, dealing with Practical Histology.
ALBERT H. TUTTLE. University of Virginia, Mav, 1898.
CONTENTS OF BOOK 1.
PART 1: THE CELL AND THE TISSUES.
CHAPTER L
Page
Introductory: Definitions: The Cell, - - 14— IG
CHAPTER n. The Epithelia : Endothelium: Epithelioid AND Endothelioid Structures, 17— 22
CHAPTER HL The Cartilage Group: Cartilages, - - - 23— 30
CHAPTER IV. The Fibrous Tissues: Areolar Tissue : Adi- pose Tissue: Retiform Tissue : Membranes: Tendons, 31— 4-6
CHAPTER V. The Lamellated Tissues : Corneal Tissue : Osseous Tissue : Periosteum : Marrow : Bones, 47— 58
CHAPTER VL Ossification : of Membranes ; in Cartilage, 59— 72
YI CONTENTS.
CHAPTER VII. The Blood, - 73— 82
CHAPTER VIII. The Contractile Tissues: Muscles, - - - 83 — 90
• CHAPTER IX.
The Small Vessels: Bloodvessels; Lym- phatics: Serous Cavities, - 91— 98
CHAPTER X.
The Nervous Tissues: Nerve Fibres; Cor- puscles : Terminals : Nerves ; Ganglia : Neuroglia, ._----_- 99 — ]14
CHAPTER XI.
The Structure of the Cell: Nuclear Division, --------.----- 115—123
PART II: HISTOLOGICAL ANATOMY.
CHAPTER XII.
Introductory: The Embryonic Tissue Lay- ers: Systems of Organs, ------- 127—134
CONTENTS. VII
CHAI'TBK XIII. Thk Skin and Appendages: Epidermis: Dermal Glands: Hairs: Nails, 13r)— 152
CHAPTER XIV. The Mouth and its Contents: Glands of THE M-wcous AND Serous Type;s: Salivary- Glands: the Teeth: the Tongue, - - - 153—170
CHAPTER XV. The Alimentary Canal: Component Stra- ta: The Pharynx: The Oesophagus: The Stomach : The Duodenum : The Small In- testine: The Colon: The Rectum: The Pancreas: The Liver, - - - 153—190
CHAPTER XVI. The Respiratory Apparatus: The Trachea: The Bronchi.: The Lungs: The Pulmon- ary Blood Supply, - 191—202
CHAPTER XVn.
The Urinary Organs; The Kidneys: The Bladder: The Urethra. 203 — 216
CHAPTER XVin. The Male Reproductive Organs: The Scro- tum ; The Testes: The Spermatic Ducts and Seminal Vesicles: The Male Ure- thra : The Penis : The Prostate : Cow- per's Glands, 217—232
YIII CONTENTS.
CHAPTER XIX.
The Female Reproductive Organs: The Ovaries: The Oviducts: The Uterus: The Vagina: The Vulva: The Mammary Glands: The Homologies of the Urino- Gexital Organs, __... 233—258
CHAPTER XX.
The Vascular System; The Arteries; The Heart; TheVeins: The Lymphatic Trunks; The Serous Membranes: The Synovial Membranes, .- 259—268
CHAPTER XXI.
The Ductless Bodies: Lymphatic Nodules AND Nodes: The Spleen: The Thymus: The Thyroid: The Parathyroids, Carotid Glands, AND Coccygeal Gland: The Adre- nal Bodies: The Pituitary Body: The Pineal Body, - 269—286
CHAPTER XXII.
The Nervous System : The Meninges : The Spinal Cokd: The Cerebellar Cortex: The Cerebral Cortex, 287—320
CHAPTER XXIII.
The Organs of Special Sense; The Taste Buds; The Organs of Smell; The Eye; The Ear, - - 321—360
BOOK I.
DESCRIPTIVE HISTOLOGY,
PART I.
THE CELL AND THE TLSSUES.
\-\-{^yt<YJ^v~,
CHAPTER I. INTRODUCTORY.
Histology is the science that treats of tissues; their structure, their components, their development luul modi- fication, and their arrangement to form the organs of the body. We may distinguish between Animal and Vege- table and between Normal and Pathological Histology, as well as between Human and Comparative Histology ; expressions which define themselves. As the term Histol- ogy is most commonly used in medical literature it signi- fies the normal histology of man.
In its most limited sense the term Histology is applied to the description of the tissues alone. This description is also sometimes called General Histology. The discussion of the arrangement of the tissues to form the organs of the body is distinguished as Histological Anatomy ; or, (less appropriately) Physiological Anatomy.
A Tissue may be defined as a mass of similar structural elements (cells or cell-derivatives) having similar prop- erties and functions; together with such substances as are characteristicalh'^ present between the elements (and are, as a rule, formed by their action), serving to unite them together.
14 PART I. THE TISSUES.
This definition, while true of the great majority of tis- sues, cannot be universally applied in a literal sense. There are some tissues (e. g., adenoid tissue) regularly involving more than one kind of element. Such may with propriety be called Compound Tissues.
The combination of tissues gives rise to organs of defi- nite structure and function; and, conversely, every organ is ultimately resolvable into its component tissues. There are, however, certain tissue-aggregates which, while themselves entitled to be regarded as organs, sustain a similar relation to the structure of the larger and more complex bodies to which that term is applied as do the tissues themselves. Among such are the smaller blood and lymph vessels, small glands, adenoid masses, etc. The con- sideration of these compound factors of structure may properly accompanj'- that of the tissues.
There also occur in the body substances (notably the blood) which, while they lack that coherence properly as- sociated with the idea of a tissue, contain cellular or cor- puscular elements analogous to those forming the basis of tissue-structure. While they can hardly be classed as tissues, their study is clearly within the province of His- tology.
The cells (or, more strictly, the corpuscles) to which reference has been made, although of exceedingly various form and size, are fundamentally similar in that each is a mass of (more or less modified) protoplasm provided for a
CHAPTER I. INTRODUCTORY. 15
portion if not the whole of its life with a nucleus. In some cases the outer layer of the protoplasm ^.^ives rise to or is modified to form a more or less well-defined mem- brane or w^all of varying composition, thus forming a true cell ; but the constant presence of a cell-wall of definite and approximately uniform composition characteristic of the tissue elements of plants cannot be affirmed of those ot animals, the converse being more generally true.
The study of cells as living beings, their internal struc- ture, activities and life-histories, or Cytology, is an im- portant and rapidly growing branch of Biology. We are here concerned with them as components of the tissues and with their structure and activities as related thereto. Such discussion thereof as this implies, while it logically precedes an account of the tissues, is on the whole best deferred until some practical familiarity with them and with the elements that compose them has been acquired.
It is sufficient at this time to state that the term proto- plasm is applied to what has until recently been regarded as one substance, of exceedingly complex chemical compo- sition and apparently devoid of structure; clear or slighth' granular in appearance under ordinary powers of the microscope; and viscid or semi-solid in consistency: im- proved methods and appliances of research have, how- ever, shown that it is neither homogeneous nor structure- less. It is the seat of the processes which make up physi- cal life; and it and its products compose the living body, which in its earliest stages consists of a number of appa- rentl}' similar minute nucleated masses of protoplasm
16 PART I. THE TISSUES.
(embryonic cells): the specialization in each of these masses of some one function, and the associated modification in the form of the mass and of its solid products, if any, give rise to tissue-differentiation.
The nucleus is a body usually spheroidal in form, both physically and optically denser than the protoplasm by which it is surrounded. It is jclearly seen, even with ordinary microscopes, to be more complex in structure than the latter : a nuclear membrane, a more or less defi- nite intranuclear network, and occasional granules, the largest of which, when distinctly spheroidal in form, are known as nucleoli, being in many cases readily discerni- ble. During cell-multiplication the nucleus is the seat of important changes, which will be discussed in a subse- quent chapter, together with its structure and that of the surrounding protoplasm.
While we may recognize a large number of tissue-ele- ments differing from each other in form and size, as, of course, in function, they can all be included in a very few primary groups, as follows.
Epithelial, those lining or investing a free surface.
Skeletal, those forming an investing, supporting or pro- tective framework for an organ or for the whole body.
Contractile or Muscular, those producing by their com- bined action definite movements.
Irritable or Nervous, those acting as reservoirs of energy, or as channels for its discharge, or as receivers and distributors of stimuli.
Reproductive (modified epithelial) elements.
CHAPTER II. THE EIMTHELIA. 17
CHAPTER II. THE EIMTHELIA.
Epithelium may be defined as a continuous layer of cells always (a) disposed on a free surface; (b) united by an intercellular cement substance; (c) devoid of blood-ves- sels, though not necessarily of nerve-terminals.
Epithelium of one or another form is normally present on all free surfaces, save some of those known as synovial ; it invests the skin and the mucous membranes with their various diverticula, lines the cavities of the nervous axis, occurs in the organs of special sense, in the cavities of the th\'roid and similar bodies, and (as endothelium) lines the serous surfaces and the cavities of the heart and vessels. There are also found in the structure of various solid organs (e. g., the thymus) masses of cells, which, while they no longer line free surfaces, are evidently epithelial in character, comparative and embryological studies demon- strating their epithelial origin. The same may be said of the cells which compose the greater portion of the sub- stance of the liver.
The reproductive elements have already been mentioned as epithelial in origin. They are derived from epithelial layers by processes diametrically opposed in character; those of the male being set free from the layer in which
18 PART I. THE TISSUES.
they are formed, those of the female sinking (for a time) into the subjacent tissues. Further consideration of their formation will be deferred until the discussion of the organs in which they occur.
The classification of the epitheha is based on the form, arrangement, or special modification of some or all of the constituent cells. As regards the form of the cells, epithelia may be
A. Flattened, with flattened nuclei, comprising
1. Squamous ; flattened or scale-like, with bevelled
edges ; if with vertical borders, then more properly
known as
, 2. Pavement; having the form and disposition of
!'Lvjuir~-<^'p' tiles. The terms squamous and pavement are often
"^ ' ^^T^T^ tivUv' confounded; they are here used to distinguish
lUh Ly-xH--'' clearly recognizable differences of form.
B. Isodiametric, or with nearly equal dimensions in "various directions, the nuclei central and spherical or nearly so. Known as
J-vv 3. Polyhedral; also called Cuboidal, Spheroidal,
"'* ' and, from its most frequent occurrence. Glandular;
where found on curved surfaces the cells are not un-
frequentl}' wedge-shaped or pyramidal through
compression,
C. Vertically elongated ; the nucleus undergoing a simi- lar change of form, and in some cases situated nearer the base than the free end of the cell. Termed
4. Columnar: the various modifications of form "^ ,^r, may be distinguished as cylindrical, prismatic, club-shaped, etc.
CHAI'TEK II. THE KI'lTlIELIA. 19
As regards the arrangement of cells, epithelia ma}-^ be
1 . Simple ; composed of a single layer of pavement, cuboidal, or columnar cells.
2. Transitional; composed of a layer a few cells in depth, the constituent cells varying but slightly in form. ' ^
3. Stratified: several cells deep, and more or less distinctly definable into layers . called Stratified Squamous or Stratified Columnar, according to the form of the most superficial cells.
There are numerous special modifications of epithelial cells occurring in particular localities, as in the organs of special sense : such will be described in their proper con- nections. Of more general occurrence are the following:
A. Ciliated epithelium ; usualh' columnar, occasionally cuboidal, rarely flattened ; the free surface of the cell is be- set with hair-like or lash-like prolongations of its proto- plasm (cilia), capable of vigorous flexion in one direction.
B. Goblet-cells: Columnar (rarely polyhedral) mucus- secreting cells in which the undissolved mucigen accumu- lates in the distal extremity of the cell, forming a viscid transparent mass, while the protoplasm, together with the contained nucleus, is crowded down to the base of the cell. The extremity of the cell is finally forced off b}- the escaping mucigen, leaving a chalice-shaped structure.
C. Prickle-cells: the polyhedral cells in the deeper por- tion of stratified squamous epithelium have their surfaces beset with fine immobile processes (prickles) connecting adjacent cells; the latter being separated from each other
20 PART I. THE TISSUES.
by intercellular spaces and channels. By some these pro- cesses are regarded as continuations of the protoplasm, by others of the modified surface of the cell. They will be discussed more fully in connection with the skin.
The term Endothelium is applied to the pavement epi- thelium lining the vascular, serous and to some extent the synovial surfaces. Embryological considerations led the earlier histologists to regard it as quite distinct in origin from other epithelia; more extended knowledge has shown this distinction to be doubtful, and some have urged that the use of the term endothelium should be abandoned altogether. There are reasons why it appears to be well to retain the term with the significance above indicated : as thus applied, it can also be defined as a layer of con- nective tissue cells on a free surface, a definition that will receive further consideration in another chapter.
Serous endothelium, when viewed from above, presents a mosaic of polygons whose various dimensions are nearly equal, and whose boundary lines are straight and short. Vascular endothelium is made up of cells elongated in the direction of the vessel which ihev line, and tapering at each extremity. The boundary lines are long and sinuous.
The terms epithelioid and (more commonly) endotheli- oid are applied to layers of connective tissue-corpuscles which resemble epithelium in their regularity, but are not found upon a free surface. The description of these layers does not, however, properly belong to the discussion of the epithelia.
CIIAPTKR II. THE KPITIIKLIA. 21
The statements above made concerning the form and ar- rangement of epithelial cells are purposely quite general in character: it will rejidilv be understood that within the groups indicated we may have great variety of detail, the epithelia of different localities having in almost all cases their own individual characteristics, by which they can in many instances be clearly identified, as will be pointed out from time to time in the discussion of the or- gans containing them. It should also be clearly under- stood that the differences in form described are separated by no hard and fast lines ; flattened, isodiametric, and elongated cells passing into each other by gradations at times so slight that it is often difficult if not impossible to say of a particular cell whether, for instance, it should be termed columnar or poK-hedral ; and the continuation of the same simple layer affording in some instances exam- ples of each in turn of the three fundamental forms.
As regards the arrangement of the cells, epithelia can never be said to pass into each other so gradually, since a layer of cells must always be either one cell deep or more than one cell deep. The alternative is usually between a simple epithelium and one composed of more or less dis- tinct strata, the cells on the surface differing markedly from those at the base : an arrangement transitional be- tween these is usually defined, as stated on a preceding page. Practicalh' the term transitional is by most histol- ogists applied only to the epithelium, peculiar alike in struct- ure and function, which lines the urinar}' tract, in connec- tion with which it will be discussed.
22 Part i. the TrsstJEis.
The various epithelia of the body are of exceedingly di- Terse embryonic origin, as will be pointed out in a subse- quent chapter: but however diverse their origin, and how- ever much the\' may differ in the form and arrangement of the elements of the adult structure, they in each case origin- ate as a single la^-er of isodiametric cells resting upon the subjacent membrane or its precursor. Their subsequent multiplication, if by cleavage in a vertical plane only, gives rise to a simple Xaryer of polyhedral, or, by lateral pressure, columnar cells: if cleavages occur also in a horizontal direction, or irregularh', a stratified epithelium is the re- sult.
The power of multiplication is probabh' retained throughout life b}' all epithelia : by some, however, it may be manifested at times only as a process of repair, or, if normally constant, may proceed but slowly: b}' others, however, the constant and more or less rapid production of new^ cells is a part of its chief function. This is nota- bly the case with the stratified squamous epithelia, whose outermost cells are continually being discharged from the surface, either by exfoliation or by their union to form such solid masses as the hairs and the nails.
CHAPTKR III. THE CARTILAGE GROI'P. 23
CHAPTER III. THE CARTILAGE GROUP.
The presence of an Intercellular cement-substance has been mentioned as characteristic of the structure of the epithelia : this substance, is, however, ahvavs small in quantity, and mayor may not be the product of the activ- ity of the cells themselves. Where the epithelial elements give rise to formed products of any considerable extent, thej' are either deposited in a solid form as a direct invest- ment of the cell itself (e. g., the keratin layer forming the wall of a squamous cell from the outer surface of a strati- fied epithelium), or are discharged on the free surface as secretions either in a semi-solid form (e. g., the mucigen of the goblet cells), or as a more complete solution (e. g., the products of most glands).
The tissues of the group noAv to be considered, on the other hand, have it as their chief if not as their sole com- mon characteristic that, however much the\^ may differ in appearance and consistency (from transparent, color- less, almost semi-fluid gelatinous tissue to hard, white, opaque dentine), they consist in every case of cells or cor- puscles, which, as their chief activity, give rise to a rela- tively large amount of formed products deposited as an intercellular matrix.
24 Part i, the tissues.
The matrix thus formed is at first of slight consistency, and homogeneous in structure. It may become strength- ened by subsequent direct modification in density and ten- acity; by calcification, lamination, or fibrillation; or by various combinations of these methods. The various changes in the structure of the matrix thus produced, with the concomitant changes in the form, number and arrange- ment of the corpuscles, give rise to a variety of tissues which have an essential community of function directly associated with their fundamental community of struc- ture. The former is illustrated by the facts of compara- tive anatomy, w^hich show that these tissues replace each other to a very great extent in different vertebrates ; and the latter by the substitutions and adventitious growths that occur abnormally in the human body. As will readily be supposed, they have a common embryonic origin.
The term Skeletal Tissues is here applied to the mem- bers of this group because of their chief function. Thej' are the framework tissues of the body, investing and pen- etrating every organ, and supporting and protecting every other tissue. On account of their continuity, and the part they play in binding the organs of the body to each other they are also widely known as the Connective Tissues ; a term, however, more appropriately used in its original significance, as applied to one of the principal divisions of the group.
As will be readily inferred from what has already been stated, the classification of the skeletal tissues is based in part on differences in the form and disposition of the cor-
CIIAPTKR III. CARTILAGE. 25
puscles, and in part on ditit'erences in the structure of the rnatrix. It is characteristic of the group as a whole that the corpuscles tend to branch irregularly and to remain or become united b}- the prolongation of their branches into more or less extensive protoplasmic networks whose nodes are the nucleated bodies of the corpuscles. It is within the meshes of this network that the matrix, whether sim- ple or complex in structure, is deposited.
While, however, the branching and intercommunication of the corpuscles just mentioned is shown by the evidence alike of comparative anatomy, of embryology, and of path- ology' to be characteristic of each of the skeletal tissues in some animals or in some stages and conditions, we find in the healthy adult bod}' of man and the mammals generallv differences in this and associated respects which divide the tissues in question into two principal groups quite sharply distinguished from each other as regards the primary structure of the forms included in each, although in some cases the two types are secondarih^ intermingled.
In the first, or Cartilage Group, the corpuscles, which are always of one kind only, (fixed corpuscles) are usually either spheroidal in form, or, as the result of pressure, polyhedral or flattened : in either case, however, they are simple in outline; and are isolated or at least discon- nected : in rare cases they are sparingU' branched and connected. The matrix is firm, elastic, primarily homo- geneous and finely granular, apparent^ structureless (though some facts indicate an internal structure not yet clearly demonstrated): whether more or less dense, it is
26 PART I. THE TISSUES.
alwa^'-s permeable b}' diffusion to the nutrient plasma on which the corpuscles depend for sustena ice, Ij'-mph-chan- nels being absent (or very doubtfully present), and a regu- lar blood-supply wanting, although large masses are some- times sparingly penetrated by blood-vessels: it is some- times secondarily reinforced by the intermingling of fibrous bujodles, or the deposition of lime salts.
According to the extent to which the matrix is devel- oped, and, to the character of its reinforcement, when this occurs, the various members of the cartilage groLip may be classified as follows: ■ A. Matrix simple, or not reinforced b\' fibres :
1. Cellular Cartilage: Matnx very scanty, con- sisting only of thin laj-ers deposited around the corpuscles, which are numerous and relatively large: sometimes called parenchymatous carti- lage from its resemblance to the parenchyma of plants : occurs in the embr\^os of man and many vertebrates and in the auricular cartilages of small mammals, as well as elsewhere in the permanent skeleton of some of the lower vertebrates.
2. Hyaline Cartilage: Matrix abundant, though varying in quantity, the corpuscles solitary' or gathered into small groups as the result of recent subdivisions : translucent, white or bluish-white in color, brittle, firm and elastic : the typical form of cartilage. Occurs in the encrusting cartilages of all freely movable joints (the corpuscles in them being numerous, small and near the surface flattened verticalh'); in the laryngeal cartilages, with two
CHAPTFK III. CARTII.ACK. 27
exceptions to be noted later; in the nasal, costal, tracheal and bronchial cartilages ; and in nearly all toetal cartilages.
3. Calcified Cartilage: Hyaline cartilage is fre- (luently reinforced in old age, both in man and in the mammals generally, by the regular deposition in the matrix of nodules of lime salts. This pro- cess occurs regularly in some of the lower verte- brates to such an extent as to give rise to a tissue almost bonelike in density and forming the princi- pal framework of the body. B. Matrix reinforced by the intermingling with it in smaller or larger proportions of fibrous bundles:
4-. Reticular Cartilage : Matrix continuous, pene- trated irregularly by a network of i^ellow, elastic fibres; the corpuscles relatively large and near together, approaching cellular cartilage in this re- spect. Occurs where great flexibility and toughness combined with elasticity are called for ; inthecartil- ages of the external ear, in the Eustachian tube, in the epiglottis and in the cartilages of Wrisberg and of Santorini in the larjmx. On account of its color and structure this tissue is sometimes spoken of as yellow fibro- cartilage, and on account of its phy- sical properties as elastic cartilage.
5. Fibro-Cartilage proper: Matrix largeh- replaced by bundles of white fibres ; the corpuscles small and few in number, resembling those of h^'aline cartilage in appearance. Occurs where great ten- acity combined with elasticity and moderate flexi-
28 PART I. THE TISSUES.
bilitA'' are needed ; in the intervertebral disks, in in- terarticular masses, at the margins of ball-and- socket joints, in the sacro-iliac articulations, in the sx^mph^'sls pubis. As distinguished from the pre- ceding it is sometimes termed white fibro-cartil- age.
Fibro-cartilage may^ also be described as consist- ing of masses of interwoven bundles of fibrous tis- sue with small nodules of hyaline cartilage inter- spersed sparingly in the meshes. So considered, it may be regarded as a mixture of fibrous and cartil- age tissues.
Cartilage always originates as a mass of contiguous spheroidal or polyhedral cells. As development proceeds the cells are seen to be separated by thin layers of a color- less substance, which is formed b}- the deposition about each cell of a la\'er of matrix substance known as the capsule of the cell ; cellular cartilage never proceeds be- 3'ond this stage: in the case of h\'aline cartilage the mat- rix substance accumulates between the capsules bx'^ exter- nal deposition, or else is formed b}' the gradual transfor- mation and removal of the capsules. Reticular cartilage is always pre-formed as h^^aline cartilage, the elastic fibres afterwards appearing in the matrix. In fibro-cartilage the cartilaginous substance and the fibrous tissue are said to appear simultaneousl3\
Cartilage grows by cell-division, which can without diffi- cult}^ be seen to have been in progress during life in any- good section of hj^aline cartilage, the corpuscles being
CHAPTER III. CARTILAGE. 29
tbund in «jroups of two, four or more, so related as to clearly indicate their recent origin : in some instances two cells each with a proper capsule can be found within the capsule of the cell from whose division they were devired. Such interstitial growth doubtless proceeds more rapidly in most cases near the surface than in the deeper portions of the cartilage : it may suffice merel r for the constant re- newal of the tissue, or may proceed with sufficient rapid- ity to give rise to actual increase in size. Growth in this sense is believed by some to take place chiefly by apposi- tion : that is by the deposition upon the surface of new cartilage substance.
It is customary to mention in connection with t1ie de- scription of cartilage that the matrix consists chiefly of a substance frequently called chondrogen, and said to yield chondrin on boiling; the latter is defined as a member of the gelatin group of compounds. Gelatin is itself ob- tained chiefly by boiling the fibrous tissues, which are rich in its antecedent, collagen. By some chemists the matrix of cartilage is regarded as also consisting largely of colla- gen, the so-called chondrin being regarded as only an im- pure or slightly modified gelatin. The matter is one that has no direct bearing upon the structure of tlie tissues in question (as far as our present knowledge goes) but it is well for the student to understand what is meant by the terms mentioned.
A cartilage, in the anatomical sense of the word, is an organ : that is to say, a particular part of the body hav- ing a definite form and function. As such, its description
30 PART I. THE TISSUES.
might with propriety be deferred to the second part of this book : in the case of this, however, as of some other organs consisting chiefly (though not solely) of a single tissue, it will be for various reasons desirable to discuss its struct- ure in connection with that of its prevalent tissue.
A cartilage, then, may be defined as a mass of cartilage tissue having a definite and reg'ular form. It is, especially where composed of hyaline, calcified, or reticular cartilage, usually covered by the perichondrium, a thin fibrous mem- brane moderately rich in blood vessels, which are the sole or chief source of nutriment for the mass : the elastic fibres of reticular cartilage are continuous with this membrane. A cartilage is always devoid of bloodvessels, though large cartilages are sometimes excavated by spaces of greater or less extent, through which bloodvessels pass, accom- panied by lymph-vessels and sometimes by fat, forming what is sometimes termed a "cartilage marrow." It is always devoid of nerves and insensitive to pain.
CHAI'TKK IV. FIHROl'S TISSUES. 31
CHAPTER IV. THE FIBROUS TISSUES.
In the second of the two groups of skeletal tissues above indicated, the Fibrous Tissue Group, in addition to the corpuscles primarily associated with the formation ot the tissue and permanently located in it (hence called fixed corpuscles), there ma\' be present, in some members of the group at least, characteristic accessor}' or adventitious corpuscles of various kinds. The fixed corpuscles are always irregular in form, with lamellar or filamentojus branches, the latter frequently connecting with similar processes from adjacent cells, thus forming a more or less continuous network. The matrix isalwaj's homogeneous, transparent and yielding in the embryonic state, but very early becomes penetrated by fine fibrillae running irregu- larly in various directions : the fibrillation is in most cases extensive, the matrix finally consisting chiefly of a mass of fibres variously disposed in bundles, in more or less closely felted layers, or in clearly defined laminae. In some cases the fibrillation of the matrix is regularly followed by calcifi- cation.
The number of tissues which agree in having the general structure indicated as characteristic of the Fibrous Tissue group is larger than that of all the other tissues of the
32 PART I. THE TISSUES.
body put together. Three of them, namely, corneal tis- sue, bone tissue and dentine, resemble each other and differ from all the rest in the fact that the fibres formed by the union of the fibrillae are always very minute and are closely felted together to form definite lamellae between which or exterior to which the fixed corpuscles are situated : the first of these is remarkable for its extreme transparency ; the other two are normally and extensively calcified, form- ing tissues of great density and firmness. Their further description will best be deferred to a subsequent chapter.
The remaining members of the group constitute the Fi- brous Tissues proper or the Connective Tissues in the more limited sense in which the term may best be used. Of these one is chiefly if not entirely embryonic, existing in the adult human body only in an extremely modified form. As it is an essential constituent of an important foetal structure, it merits a description as a distinct form of con- nective tissue: and since it is the precursor of most of the others, its discussion may properly precede their classifica- tion and description.
Mucous Tissue (or, as it is also called, gelatinous tis- sue) : the matrix is at first homogeneous, transparent and semi-fluid in consistency ; it is described as albuminous in composition, with the addition of mucin: fibres very early begin to appear in it, their mode of formation being not yet fully determined : the fixed corpuscles are irregular, branching, connected by their slender processes into a net- work : in addition there are to be seen here and there in thejnatrix scattered isolated corpuscles which in the fresh
CHAI'TKR IV. FIHROUS TISSUES. 33
tissue ma}^ be seen to move through the jelly-Hke sub- stance with an irregular or amoeboid motion ; these are the migratory corpuscles or leucocytes which, as we shall see, are characteristic of the connective tissues as a group. Mucous tissue constitutes an important factor of the umbilical cord, where it forms, under the name of the jelly of Wharton, the largest portion of the mass lying be- tween the epithelium upon the surface and the bloodvessels in the centre, up to the fifth month ; later the fibrillation' which has already' begun about the vessels and near the surface penetrates the w^hole mass mo^-e and more exten- siveh' : but a certain amount of sparingly fibrillated mu- cous tissue always persists. While mucous tissue is a fre- quent constituent of the skeletal framework of some of the lower animals, it is represented in the adult human body (save as a constituent of morbid growths) by the following structures, if at all.
The vitreous body (or so-called vitreous humor) of the eye may best be regarded as a modified form of mucous tissue. In the embr\'o it possesses for a time all the char- acteristics of that structure ; but in the adult the matrix undergoes watery' degeneration, and fibres are extremely rare; fixed corpuscles are altogether wanting, the only corpuscular elements present being a few leucocytes. The centre of the intervertebral djscs of fibro-cartilage con- tains a soft and yielding mass sometimes regarded as a form of mucous tissue: it lacks, however, some of the essential features of that structure and may best be re- garded as the remains of the notochord, the embryonic precursor of the vertebral column, which, while it differs
34 PART I. THE TISSUES.
in some respects from cellular cartilage, approaches more nearly to it than to mucous tissue as here defined. The pulp of the teeth also consists in part of a modified form of gelatinous tissue, which will be more fully described in connection with those organs.
Passing now to the consideration of the fibrous tissues proper, as defined by the limitations recently indicated, it is important to note at the outset that the fibres which in every case enter so largel}^ into their composition are of two kinds, both of which are present in most of the tis- sues in question, though their proportions may vary ex- ceedingly ; the characters of the tissues included in the group being in great measure based upon the proportion of the two kinds of fibres, and the modes of their disposi- tion. These are known respectively as white fibres and yellow or elastic fibres.
The former are exceedingly delicate unbranching fila- ments of collagen (which, as has been stated, is converti- ble into gelatin by boiling), rarely more than a micron in diameter, and often much less; they are almost always united into bundles of varying size ; when so united the fibres have a silky appearance, and tend to assume a char- acteristic waviness in which all the fibres of the bundle par- ticipate in such manner as to retain their almost strictly parallel arrangement : the bundles are again frequently united together in larger aggregates sometimes termed trabeculae: white fibres are very tenacious and entireh' devoid of elasticity.
The elastic fibres are coarser than the white and more variable in thickness, being from one to six micra in diam-
CHAPTER IV. FIBROUS TISSUES. 35
eter in man, and in some animals as much as fifteen micra: thev are pxismatic in form, and under some circumstances appear to be tninsverselv striated : they branch occasion- allv, and not unfrequently anastomose: when not on the stretch they tend to Jissume large sweeping curves, and the free ends, which break square across, curl up in a ehar- a£|er|stic manner : like the white fibres, they are often as- sociated in bundles. They are, as their name implies, emi- nently elastic, but are of only moderate tenacity, a bundle of them being far more easily broken across than a bundle of \yhite fibres of the same size. They are composed of a substance known as elastin, a complex nitrogenous com- pound which is not converted into gelatin on boiling. The elastic fibres are not readily affected by weak acids, as are the white fibres.
But little is as yet known of the mode of formation of either kind cf fibres. It is b\^ some held that they are in all cases formed by the transformation of a portion of the protoplasm of embryonic cells, the remainders of which, either with or without subsequent increase in size, become the fixed corjjuscles of the tissue in which they are found ; this view, however, is urged not so much on account of observations directly supporting it as of the conviction on the part of its most positive adherents that the entire liv- ing body consists and must consist onh^ of protoplasm and the immediate products of protoplasmic changes: there are, moreover, some facts very difficult of explana- tion from this standpoint. On the other hand, it is claimed that fibrillation ma}' and does result from a chemical and physical change in a part of the homogeneous ground-
36 PART L THE TISSUES,
substance of the matrix along lines not in actual contact with any mass of protoplasm ; although it is freely ad- mitted that the proximity of such masses (the corpuscles) may have a decided influence in initiating and determin- ing such a process. In the case of the elastic fibres, there is good reason for the view (first proposed by Ranvier) that the elastin is deposited in the matrix in the form of small globules, which later fuse together to form fibres : this, if true, will account for the transverse striation already mentioned.
The corpuscles of the fibrous tissues also demand pre- liminary consideration. It has already been indicated that each tissue is characterized by the presence of a spe- cific form of fixed corpuscle peculiar to it ; in the case of mucous tissue the presence of migratory corpuscles, or, as they are very frequently termed, leucocytes has been men- tioned as a feature in which this tissue may be taken as a type of the group : and reference has been made to other accessory or adventitious corpuscles. The fixed corpus- cles proper to each tissue may best be described in connec- tion with its definition: the nature and origin of leuco- cytes will be considered later in connection with the de- scription of the lymph and the blood : the accessory cor- puscles now call for discussion. They are often spoken of as modified fixed corpuscles : but while one form is cer- tainly and others are possibly derived from these bodies, there is reason to question whether those of one (if not of more than one) kind are not modified leucocytes: omit- ting further discussion of their origin (save in one instance) they may be described as follows.
CHAPTER IV. FIBROUS TISSUES. 37
What are known as fat cells arc fornied by the fatty transformation of the protoplasm of certain of the fixed corpuscles of one or more of the fibrous tissues: small droplets of oil at first appear scattered in the cell-body; these become more numerous or larger, finally fusing in one large mass, the nucleus being crowded to one side, and 'the residual protoplasm forming a thin pellicle or cell-wall.
Under the name of plasma cells arc included certain corpuscles having elongated and sometimes slightly branching bodies with central oval nuclei, the protoplasm of which contains a large number of small vacuoles (of varying size) which contain a clear fluid probabh' similar in composition to the lymph or blood-plasma, whence the name of these corpuscles.
Plasma cells were first described by Waldeyer. He in- cluded under that term not only vacuolated cells, but also what are now known as granule cells: these are usually spheroidal in form and devoid of branches ; their proto- plasm is highly granular : on account of the marked afiinity of the granules for eosin (as well as for many ani- line dyes) they are sometimes termed eosinophile cells; the same term has been otherwise applied in connection with the blood, as will be indicated later.
The name of pigment cells has been given to connective tissue corpuscles (and to some epithelial cells as well) characterized by the presence in their protoplasm of numer- ous rounded brown or black granules of a substance termed melanin. Pigment cells are of very irregular form, commonl}' branched, and often exhibiting amoeboid mo- tion when examined in a living condition : the pigment
38 PART I. THE TISSUES,
granules are often so numerous and so closely packed that the nucleus and other structural features are entirely hid- den; in some cases, however, they are much less abundant. The following classification of the fibrous tissues is based upon the general disposition of the fibres present, their abundance, the proportionate amount of the two kinds of fibres, the details of their distribution, and on the kinds and relative quantities of the associated corpuscles.
A. Fibres varying in abundance, solitary or aggregated into bundles and trabeculae running irregularly in various directions, loosely interwoven or more or less closely felted together.
1. Areolar Tissue: called by the older histologists cellular tissue: found beneath the fibrous layer of the skin as subcutaneous, beneath the mucous membranes as submucous, and the serous mem- branes as subserous, in the interspaces between adjacent organs as intermediate, upon their sur- faces as investing and forming their internal frame- work as penetrating areolar tissue, it is well nigh continuous throughout the entire body and merits in the strictest sense the name of connective tis- sue. The matrix consists in part of a semi-solid homogeneous ground-substance, penetrated in every direction by interlacing bundles and trabecu- lae of white fibres and by elastic fibres either soli- tary or in bundles : the bundles of fibres may vary greatly both in their total amount and in the pro- portion of the two varieties ; but they are never so numerous but that irregular spaces of varying
CHAPTER IV. FinROrS TISSUES, 39
A. Fibres varying in (juantity, loosely interwoven, or felted ( continued) .
size termed areolae occur in so great numbers as to be practically continuous ; a fact of great im- portance in the history of dropsical and other effu- sions. The whole structure is penetrated by blood vessels, lymphatics, and by nerves passing through it : it is colorless or whitish, filmy in texture and of but slight tenacity.
The fixed corpuscles, or areolar tissue corpuscles proper, are quite numerous : they are frequently flattened upon the surface of bundles of fibres or. situated in the angles where two or more bundles come together, in either case the processes extend- ing along the bundles and their subdivisions : leu- cocytes are of frequent occurrence : plasma cells and granule cells less so, except in particular local- ities. Pigment cells are not common in areolar tissue proper in the human body except in certain places: fat-cells are very common here and there in the areolar tissue of well nourished individuals: where they accumulate in particular localities they give rise to
2. Adipose Tissue: this, which may under favorable conditions be formed wherever areolar tissue occurs, but which is most common in connection with the subcutaneous, subserous, and interme- diate regions, is in effect little else than areolar tis- sue in which the fixed corpuscles in particular lo- calities become greatly increased in number, filling
40 PART I. THE TISSUES.
A. Fibres var\nng in quantity, loosely interwoven or felted {continued).
up the areolae, and undergo fatty transformation, giving rise to small fat lobules which are gathered together to form the fat masses visible to the naked eye: the blood supply of these localities is always greatly increased, each lobule having a capillary system of its own, while the fibres between the cor- puscles undergo no corresponding increase in num- ber, and in some cases are exceedingly scanty.
3. Retiform Tissue, or, as it is also called, reticu- lar tissue: this, as its name implies, consists chiefly of a network of fibres, or rather of fibre bun- dles and trabeculae composed chiefly of what are in all probability most nearly allied to white fibres ; true elastic fibres are very sparingly present or are wanting altogether, as is the homogeneous ground substance characteristic of areolar tissue. The fixed corpuscles are flattened, adhering closely to the surfaces of the bundles and trabeculae, and are often so numerous as to form an endothelioid in- vestment. Retiform tissue may perhaps be re- garded as a modification in the direction of greater stability of areolar tissue (with which it is often di- rectly continuous), and forms the internal frame- work of some organs, as well as the basis of the two compound tissues known respectively as ade- noid tissue and marrow. The former of these will be described in connection with the lymphatic sa's- tem, and the latter in connection with bone.
CHAPTER IV. FIMKorS TISSUES. 41
A. Fibres varying in (juantity, loosely interwoven, or felted {continued).
4. Fibrous Membrane: this differs from areolar tis- sue chiefly in the fact that the bundles and trabecu- lae of fibres, both white and elastic, are far more numerous and closely felted together, obliterating the areolae and leaving small space for the inter- fascicular ground substance : the fixed corpuscles are quite numerous, but are, as a rule, smaller than those of areolar tissue, and, with their nuclei, gen- erally flattened in the direction of the membrane in which they lie ; both plasma and granule cells may be occasionally present, and in some cases pigment cells occur in great numbers : fat cells but rarely.
The principal membranous tracts of the body {e. g., the mucous membranes) which support epi- thelial or endothelial layers are sometimes more or less clearly divisible into a stroma, which makes up by far the greatest part of the layer, and a delicate film situated just beneath the epithelium and known as the basement membrane, ormembrana propria ; this is sometimes an endothelioid layer o £ corpuscles, and in other instances a special conden- sation of the fibres, either white or elastic. Such differentiation does not occur in those investing membranes such as the perichondrium alread\' re- ferred to in connection with cartilages, the similar periosteum of bones, etc., which are not associated with a free surface. Elastic membranes consist
42 PART I. THE TISSUES.
A. Fibres varying in quantity, loosely interwoven, or felted (continued).
chiefly or entirely of elastic fibres, or, in some cases, of continuous layers of elastin. 5. Fenestrated membrane: this form of membrane is produced where, in an otherwise normally formed fibrous membrane, there are at iatervals of greater or less extent neither fibre bundles nor ground-sub- stance, thus leaving rounded openings of various size and frequency, as in the omentum of man and of numerous mammals. Fenestrated membranes may also consist chiefly, if not entirely, of elastic tissue, in which the fibrous structure at times dis- appears in great measure, as in the fenestrated elastic membranes of the blood vessels.
B. Fibres exceedingly abundant, one or the other kind predominating, aggregated into smaller and larger bun- dles, which are in a generally way disposed parallel wise : ground-substance very scanty. Fixed corpuscles few, flat- tened between the bundles of fibres.
6. Tendon tissue or white fibrous tissue : as areo- lar tissue merits in the strictest sense the name of connective tissue, so this, above all others, merits that of fibrous tissue. It consists almost exclu- sively of white fibres : these are united together in small bundles by a small quantity of ground- substance, each being covered by an endothelioid layer of flattened corpuscles : the smaller bundles are gathered together with occasional anastomo-
CHAPTKK IV. FIBROUS TISSUES. 4.'i
R. F'ibres abundant, chiefly of one kind, in parallel bun- dles {continued).
ses into larger ones of varying size, which are sep- arated by interstitial areolar tissue continuous with the investing sheath of the whole mass: elas- tic fibres are very sparingly present, chiefly in the penetrating areolar tissue. The characteristic fixed corpuscles or tendon cells are generally found in rows whiclj occupy the spaces between two or three contiguous bundles, their branches taking the form of interfascicular lamellae : leucocytes are rarely present, and other forms of corpuscles are wanting. Blood vessels and lymphatics, and, in some cases at least, nerve fibres follow the intersti- tial areolar tissue. White fibrous tissue is found in tendons and ligaments and also in tendinous aponeuroses and fasciae, which form a transition to fascicular investing membranes.
7. Elastic tissue : this, as its name implies, is com- posed chiefly of elastic or yellow fibres, which are arranged in bundles of varying size and complex- ity, the interstitial connective tissue penetrating not only the larger but also the smaller bundles, and in some cases separating individual fibres: small bundles of white fibres run in the connective tissue in various directions, and in some cases at least there are well defined bundles arranged in groups as in tendon tissue and running throughout the whole length of the structure. Elastic tissue is generally regarded as differing from all other
44 PART I. THE TISSUES.
B, Fibres abundant, chiefly of one kind, in parallel bun- dles {continued).
forms of skeletal tissue in having no characteristic fixed corpuscles, none being found in it that can with certainty be regarded as such : some authors have described in this connection flattened corpus- cles found scattered in the ground-substance be- tween the fibres and in close apposition with the latter; it is not, however, certain that these do not more properly belong with the interstitial con- nective tissue. The term elastic tissue is by some histologists applied alike to the chief constituent of elastic ligaments (in which sense it is here de- fined), and to that of elastic membranes.
We may now with profit revert to the definition of endo- thelium given in a previous chapter, which described it as a layer of connective tissue corpuscles on a free surface. We have seen that the association of corpuscles with fibrous structures is characteristic of the connective tis- sues : embryological evidence shows that the serous and vascular cavities are alike formed by cleavages or excava- tions of the mass of embryonic cells from which the skele- tal tissues are derived : the tendency of connective-tissue corpuscles to arrange themselves in layers (properly termed endothelioid) has been mentioned in connection with such structures as basement membranes, the investing layers of small tendons and tendon-bundles, etc. : and, as we shall see later, the direct continuity between endothe- lioid cells and connective tissue corpuscles can be observed
CHAPTER IV. FIUKOUS TISSUES. 45
at the free extremities of lymphatics and at the margins of synovial surfaces: it can, therefore, be readily under- stood how connective tissue corpuscles should form a con- tinuous layer under such exceptionally favorable condi- tions as those found on the free surface of a membrane. Some characteristic activities of the corpuscles found upon serous surfaces will be described in connection with the lymph and the blood.
We know at present but little of the duration of an^' of the fibrous tissues, or, indeed, of the skeletal tissues in general. Consisting largely of the constituents of the matrix, which some histologists regard as formed and in a certain sense not-living substances, it has been held that, once established, they may endure as long as the body lasts. It is possible that this is the case, to some extent at least, and that the tendon fibres, for example, of our old age are the identical tendon fibres of our childhood : on the other hand, the tendons of the child certainly in- crease both in thickness and in length toward manhood; and the mechanism by which interstitial increase takes place is certainly adequate for interstitial replacement as well ; we have, however, at present no certain evidence of any such mechanism of absorption or removal of worn- out fibres (if such there be) as a method of replacement would imply. Whether the matrix of a fibrous tissue is to be reg£irded as living or not depends entirely on what we mean by a word for which no generally accepted defini- tion has 3'et been given : it is certain, however, that under certain conditions of defective nutrition such structures
4!6 PART I. THE TISSUES.
undergo marked changes to which the name of death is certainly not inappropriate.
These changes are generally regarded as due primarily to the death of the corpuscles : but this merely shifts the problem. It is difficult to conceive that any single nucle- ated mass of protoplasm should retain its powers una- bated for half a century or more ; but if it may, what are its probable activities in the case, for example, of a tendon cell? If, on the contrary (as seems more probable), it is constantly renewed, what are the activities of the succes- sive generations ? The phenomena of tissue-repair in the case of injuries throw some light on the problem, but it is, on the whole, at present unsolved. It is desirable, how- ever for the student to know^ of its existence.
Before dismissing the discussion of the fibrous tissues it is proper briefly to mention a form of tissue at first sup- posed to be a member of this group, and described as a modification of retiform tissue: subsequent investigations have shown, however, that it differs in important re- spects from any of the true connective tissues in structure, and is of widely differing embryonic origin. It is the sus- tentacular tissue of the brain and spinal cord, and is known at present by the name of neuroglia. It is com- posed entirely of branching corpuscles and their fibrillar processes, known as glia-cells: their full description can best be given in connection with the nervous tissues with which they are associated.
CHAPTER V. I.AMKLLATKl) TISSUES. 47
CHAPTER V.
THE LAMELLATED TISSUES.
As was stated in the preceding chapter, there are three members of the skeletal tissue group (namely, corneal tis- sue, bone tissue and dentine), which, like most of the fibrous tissues described in that chapter, are character- ized bv extensive fibrillation of the matrix, the fibrillae being even more closely intermingled than are those of ordinary membranes: they differ from these structures, however, as already indicated, in the facts that the fibril- lae, which are always exceedingK^ fine and resemble most nearly those composing white fibres, are never aggregated into the bundles and trabeculae which are interwoven to form membranes, but are felted together to form d_ense layers, having in each case the characteristic fixed corpus- cles situated between them or exterior to them.
These lamellar structures have evidently much in com- mon with and are originally derived from the modification of membranes, as the facts of comparative anatomy and embryology plainly demonstrate (two of them being clearly dermal in origin in man and the lower animals alike) : the characters above stated, are, nevertheless, of such importance as to warrant their separate considera- tion. The name at the head of this chapter is, therefore,
48 PART I. THE TISSUES.
proposed for the group: but it should always be borne clearly in mind that they are more nearly related to the fibrous tissue group than are either of them to the car- tilages : this is the more important because of the fact that one of them, osseous tissue, largely replaces cartilage in the formation of most of the bones of the body, thus giving rise to the impression that these two tissues are closely allied, and that the latter is in some way trans- formable into the former, an error which leads to much unnecessary confusion.
^^^ The formation of dentine, the most peculiar member of
j^^^^^^^^jj^he group, is so intimately related to that of the other tis- io^^JaJm;:^' gygg q£ ^YiQ teeth, (of which it forms the largest part), and especially with that of the pulp contained in the tooth- cavity that it cannot well be discussed apart from these associated structures: they will be considered together when the teeth are described in connection with the other organs of the region in which they occur.
g=rr Since corneal tissue is found only in the structure from
^r which it derives its name, its consideration might in like
Ai,^^^^ manner with propriety be deferred until the description of
"^ the eye. There are, however, points of resemblance be-
I tween it and bone tissue which make its stud}' desirable
as a preliminary to that of the latter, particularly as the
absence of calcification renders far easier the recognition
of important details.
The transparent lamellae which form almost its entire bulk are quite uniform in thickness throughout the entire cornea : they are composed of white fibres running parallel
CllAPTEK V. IwVMKLLATKI) TISSUES. 49
to each other in each himella, scj disposed that those of ()ne hunella cross those of the nex^t at rij^ht angles, or nearly so, in the centre of the cornea ; toward the margins thev cross at varying degrees of oblitiuity : the lamellae near the outer surface of the cornea are traversed oblit|uely by occasional bundles of fibres, which thus unite them together. Adjacent lamellae are separated here and there by shallow lens-shaped spaces which occur frequently, but at irregular intervals; these may be designated as lacunae: those lying between the same two lamellae are connected with each other by numerous branching channels, which may with equal propriety be termed canaliculi: there is thus formed a continuous system of canals and spaces across the entire cornea between each two lamellae, the lacunae of one such system having no definite relation in position or otherwise to those of the next.
In the lacunae lie the corneal corpuscles, apparently ad- hering to the surface of one or the other of the two adja- cent lamellae. They are flattened, with flattened nuclei, and irregular in outline; they branch freely, the branches extending into the canaliculi and in many cases connecting with those from adjacent corpuscles, thus forming a pro- toplasmic network, which is possibl}' coextensive with the canal-system, but does not completely fill it. The re- mainder of the space forms a means of distribution of plasma from the blood vessels at the margin of the cor- nea; leucocytes also wander through the larger canals from lacuna to lacuna. Neither blood vessels nor lymph- atics penetrate the substance of the cornea, nor are there transverse canals which bring into communication the
50 PART I. THE TISSUES.
canal-systems separated by the lamellae. The surfaces of the cornea, both anterior and posterior, are invested by membranes in direct contact with the most superficial lamellae; but there is no genetic relationship between the membranes and the lamellae beneath. Farther description of these membranes, and of the epithelia supported by them, w^ill be deferred to a subsequent portion of this work.
It should be stated before leaving the description of cor- neal tissue that the use of the terms lacunae and canaliculi, as here applied to the corneal spaces and lymph channels, is unusual. It is warranted by the resemblance between these spaces and channels and those occurring in osseous tissue to which these names are commonly applied ; and is offered here from the conviction that a clear idea of the resemblances (and also of the differences) of the two tis- sues will aid materially in a clearer understanding of the structure of the latter.
The terms bone tissue and OSSeous tissue are applied indifferently to the chief constituent of the organs well known as bones : like all skeletal tissues, it can be defined by the structure of the matrix and the tornis and relations of the fixed corpuscles. „,jjJ^!^iiir*A The matrix consists of t^irn lamejjae composed in part of fijires (the mode of whose arrangement is not clearly demonstrable), and in part of a hjomogeneous ground- substance which is strongl}' impregnated with lime and other salts, calcium phosphate being the chief Prolonged boiling converts the fibres into a substance which has been
UJU^I'Aj)
CHAPTER Y. LAMELLATKD TISSUES. 51
called ossein, but which is probably an iinjjurc form of j^clatin : the fibres are therefore allied to if not identical with white fi lyres.
Between adjacent lamellae are found frequent lenticular spaces of exceedingly irregular outline, the lacunae ( for- merly erroneously termed the bone-cells); like the similar spaces in corneal tissue, these are connected together be- tween the lamellae by branching channels, or canaliculi: in bone, however, such canaliculi are not only present be- tween the lamellae, but also penetrate them in great num- bers, thus bringing into free communication lacunae of diflerent systems; the transverse canaliculi in some cases traversing two or more lamellae before their termination. The transverse canaliculi are both larger and more numer- ous than the interlamellar, in accordance, as will be seen, with the mode of nutrition characteristic of osseous tissue.
Within the lacunae are found the characteristic bone corpuscles: these, like the cavities which contain them, are tlattened and irregular in outline, with flattened nuclei; they can be seen to branch in some cases, but this occurs far less freely than in the case of the corneal corpuscles ; nor is there good evidenceof the connection of the branches through the canaliculi to any considerable extent, as has been supposed. Leucocytes do not traverse the Ivmph channels of bone, nor are any other corpuscles present.
The lamellated and calcified fibrous matrix, with its characteristic lacunae and canaliculi, and the enclosed cor- puscles, are the essential structural factors of osseous tis- sue wherever found : and in some of the lower vertebrates
52 PART I. THE TISSUES.
bones occur which consist simph- of a few parallel lamel- lae. In man and the higher animals, how^ever, these fac- tors are always arranged in one of two methods which are so constant as to constitute two distinct forms of osseous tissue called respectiveh' dense and spongy bone.
In the former the great majority of the lamellae are ar- ranged concentrically to narrow tubular spaces known as Haversian canals; these contain blood vessels, l^^mphatics and a small quantity of another tissue presently to be de- scribed ; they are of varying length, running in a general way parallel to the surface of the mass in which they oc- cur, and anastomosing frequently : Each is surrounded by several lamellae whose lacunae are put into communica- tion with it by means of transverse canaliculi. A Haver- sian canal and its surrounding lamellae constitute a Ha- versian system. In the irregular spaces between tbe Haversian systems of dense bone are found discontinuous lamellae also as a rule lying in a general way parallel to the surface of the mass : these are known as interstitial lamellae; and beneath the outer surface are always found more or fewer lamellae parallel therewnth and exterior to the Haversian systems : these are known as circumferen- tial lamellae.
The circumferential lamellae in all large masses of dense bone are pierced here and there by oblique canals for blood vessels, etc., which communicate with the Haver- sion canals, but differ from them in having no surround- ing lamellae: these are known as Volkmann's canals. They are also penetrated transversely by bundles of white fibres (which have undergone calcification) proceed-
\
LAMELLATED TISSUES. 53
ing from witliout in\vards, atid l)y occasional bundles of elastic fibres. The name of Sharpey's fibres is applied to both, hut more especially to the former. Dense bone con- stitutes the shafts of all long bones and forms a layer of varvingf thickness on the surface of the flat and the short bones.
In spongy bone the structure is more open, as the name implies. The solid portion consists of a meshwork of tra- beculae, or bars and plates of varying width and of great irregularity of form, each of which is several lamellae in thickness, the lamellae being in a general way parallel to the siirface and the lacunae communicating therewith by means of the transverse canaliculi. The cavities between the trabeculae, which are quite irregular in form and size, are known as Haversian spaces. Spongy bone fills the interior of the short and the flat bones and the ends of the long bones.
Just arc cartilage tissue is found in masses of definite form and function (organs), which are known as cartil- ages, so dense and spongy osseous tissues are combined in the definite masses which, with certain associated tissues, make up the organs well known as bones. As in the case of the cartilages just referred to, so in this instance it will be expedient to describe the organs in connection with the tissues of which they are chiefl\' composed.
A bone may be defined as a mass of osseous tissue sur- rounded by a proper investing and genetic membrane known as the periosteum; permeated by blood vessels and lymphatics : and containing the tissue called marrow.
/
54 PART I. THE TISSUES.
The periosteum, as found upon the surface of the bones of adult and particularly of elderly individuals, is a thin, tough, closelv felted membrane which adheres tenaciously to the structure beneath ; it can, nevertheless, be in most cases resolved into two distinct layers. In adolescence, childhood, and notably in foetal life these are clearly dis- tinguishable. The outer or fibrous layer has the structure of an ordinary membrane, the constituent bundles (chiefly of white fibres) being clo^lv^inJ:er woven, and the fixed corpuscles flattened, with flattened nuclei : here and there occasional fat^cells may be seen, with numerous blood ves- sels and lymphatics, as well as delicate nerves : the outer surface frequently exhibits extensive areas having an en- dothelioid investment.
The inner or osteogenetic layer is more loosely felted, and contains a larger proportion of elastic fibres : toward the surface of the osseous tissue it contains numerous cor- puscles which (particularly in young bone) are larger and more irregular in form than those of the outer layer ; the protoplasm is granular and the nuclei are spheroidal : these are know as osteoblasts, and in youth are directly con- cerned in the formation of new bone, as their name and that of the layer which includes them implies. In adult bone they become flattened and inactive, forming a layer just without the osseous tissue.
The complex tissue known as marrow fills the internal cavities of all bones : those in the shafts of the long bones are filled with yellow marrow ; the Haversian spaces of spongy bone, in most places where it occurs, with red
CHAPTER V. LAMELLATED TISSUES. 55
marrow : the larger Haversian eanals on the inner side of dense bone are lined with a layer of modified marrow, which is proronged in the smaller canals by a peculiar form of connective tissue cont£iining osteoblasts, and finally continuous on the outer surface with the osteogenetic layer of the periosteum ; the marrow, as will be shown later, being originally derived from an ingrowth of that layer. The two kinds of marrow are closely related in structure, the yellow being derived from the modification of the red : the latter will therefore be described first.
The basis of structure of red marrow is a delicate frame- work of retiform tissue associated with an abundant vas- cular network, which will be described more fully in an- other connection: the interstices contain corpuscles of at least three different kinds. Throughout the mass, and particularly near the surface, are the marrow cells, prop- erly so called : these are relatively large, with faintly gran- ular protoplasm and oval nuclei ; those upon the surface of the mass have the appearance and perform the function of osteoblasts in young bone. The interior of the marrow contains numerous smaller cells with granular nuclei, to which the name of erythroblasts has been given, on ac- count of the part they play in the formation of colored blood corpuscles: associated with these are great num- bers of the immature blood corpuscles themselves ; to these the color of the marrow is largely due.
Upon the surface and more rarely in the interior of the marrow are found occasional masses of protoplasm, sev- eral times larger than the ordinary marrow cells, best known as giant cells: they were called myeloplaxes by
V
56 PART L THE TISSUES.
Robin, who believed them to be peculiar to marrow ; but bodies to all appearances similar to them have since been found in other tissues. It was proposed by Kolliker to call them osteoclasts, from the belief that they were agents in the absorption of bone; where, as sometimes occurs in embryonic bone, the marrow lies in contact with tempor- ary cartilage which has begun to be absorbed, it has been proposed to call them chondroclasts for a similar reason : this view of their function is supported by the fact that they not infrequently (but by no means invariably) lie in little pits or depressions of the surface of the bone or cartilage undergoing absorption : these pits have been called How- ship's lacunae or foveolae. Nothing, however, has yet been certainly proven concerning the function of these bodies : the name first given is therefore for the present at least the most desirable.
The giant cells are of two kinds, differing chiefly as to their nuclei. Some show a large number of these bodies ; others have but a single nucleus, which is always very large, and frequently of an exceedingly irregular shape : the former are termed multinuclearand the latter uninuclear giant cells. It has been supposed that the former are derived from the latter by the fragmentation of the large irregular nucleus; but Howell has shown that this view is certainly very doubtful and probably erroneous.
From the description given it will be seen that the red marrow is a very important tissue of quite complex func- tion, having important relations not only to the surround- ing bone, but also to the elaboration of the blood. The yellow marrow, on the other hand, is, as far as our pres-
CHAPTKK V. LA.MHLLATEI) TISSUES. 57
ent knowledge u^oes, one of the most pjissive tissues ot the hodv. It differs from the retl marrow, from which it is de- rived, chietly by the facts that theerythroblastsand imma- ture blood corpuscles arealike wantinj^, and that the great majority of marrow cells in the interior ol the mass have undergone fatty transformation. Upon and near the surface both marrow cells and giant cells are found.
From the description above given it will be seen that the nutrition of dense bone is maintained by the vessels of the ])eriosteum directly for the corpuscles of circumferen- tial lamellae, and by the vessels of the Haversian canals for the concentric lamellae, and thus indirectly by the peri- osteum, from which these vessels are derived ; the perips- teum is thus seen to be ndt only a protective, but also a nutrient investing membrane: this is conspicuously shown by the fact that whenever the ])eriosteum is re- moved by accident or disease from any considerable area of bone the subjacent osseous tissue perishes. In like man- ner the corpuscles of lamellae which make up the spicules and trabeculaeof s_pongy bone depend upon the blood vessels of the ad[acent marrow for their food supply, the Haversian spaces occupying the same relation to them as the Haversian canals to the concentric lamellae of the dense bone : these spaces are in a certain sense the expan- sions of the canals, as the niiirrow which they contain is the continuation of their lining, and thus, in a roundabout way, of the inner la\'erof the periosteum. It is also to be noted that the lamellae in each case depend for their nutri- tion upon supplies drawn from a surface to which thev are
58 PART I. THE TISSUES.
parallel : the importance of the transverse canaliculi and the reason for their number and extent thus becomes evi- dent.
In speaking of tendon tissue it was stated that while a mechanism of interstitial increase was known to exist, we had no clear evidence of any continuous process of inter- stitial removal and replacement. The case of bone is dif- ferent : no one can examine a transverse section ol dense bone, and note the manner in which what appear to be re- cent Haversian systems cut into the territory of what are probably older, and how both the interstitial and the cir- cumferential lamellae are interrupted by both, without the conviction that a process of replacement is involved: and this conviction is confirmed when we compare the cross section of the femur (for example) of a child with that of an adult, into the central marrow space of which it could be thrust, each with its Haversian systems, its in- terstitial and its circumferential lamellae. Dense bone shows us, however, no tissue in process of removal, and no Haversian systems in process of formation : and the means by which both removal and replacement take place are yet to be discovered.
CIlArTKK VI. OSSIFICATION. 59
CHAPTER VI.
OSSIFICATION
Attention has already been called to the fact that the periosteum is at once an investing and nutrient, and a gen- etic membrane; and to the terms applied to its inner layer and the corpuscles contained therein. It has also been pointed out that the marrow cells upon and near the sur- face of the red marrow agree with osteoblasts in form and also in function. The function of an osteoblast may now be briefly stated by saying that it is its normal destiny to become a bone corpuscle. Bone is laid down in lamellae by the activitv, j)rol)ablv periodical in character, of an osteo- genetic layer, the osteoblasts nearest the lamella just pre- viouslv formed being enclosed by the new layer of fibril- lated and calcified matrix, and thus converted into bone cor])uscles.
It has already been stated and should always be born ill mind that the marrow, as will shortly be seen, is a de- velopment from the osteogenetic layer of the periosteum: its outer surface, with its layer of osteoblasts, is as truly an ostcojrenetic laver as that from which it is derived : attempts have, indeed, been made to define it as a distinct membrane under the name of the endosteum; but this is unnecessary: and is not advisable, if for no other reason.
60 PART I. THE TISSUES.
on account of th^ fact that it cannot bs anatomically sep- arated from the tissue below, nor its boundary on that side at all clearly defined.
The formation of osseous tissue by the enclosure of os- teoblasts in a fibrillated and calcified matrix is called ossi- fication, wherever it occurs, as, for instance, in the bones of a growing child or young animal. The term is more especialh' applied, however, to the original formation and early development of bone as it takes place in the embryo. Used in this sense, the name is applied to two processes often regarded as essential!}' different, and designated re- spectiveh-- as intramembranous ossification, or the devel- opment of bone in previously existing connective tissue, and ossification in cartilage, or the development of bone in previousl}^ existing cartilage. It is of the highest im- portance, how^ever, to keep clearl}' in mind the fact just stated, that bone is always formed in a peculiar kind of connective tissue, the osteogenetic layer already defined as in every instance derived directly or indirectly from a mem- brane. Bone is therefore alwa^'s derived from the modifi- cation of a membrane: and the facts of comparative anat- omy and of pathology, on the other hand, show that almost any membrane may under certain circumstances become osteogenetic.
The so-called ossification in cartilage, or, as it is often termed (and the expression is still more apt to mislead ), the ossification of cartilage, is in reality the replacement of cartilage by bone, the cartilage itself being absorbed and disappearing in great measure before bone forma- tion takes place. It is particularly important to avoid
lA^'
CHAI'TI;K VI. OSSIFICATION. <)1
the en oiicous impression that is sometimes caused by the exjjression iti (|uestion, to the effect that the previously ex- istinjj cartilage is in some manner transformed into hone: it" this l)c borne constantly in mind, there is no objection to the use ot the term in either form : nor, with this quali- fication, to the use of the terms "membrane bone" and ''cartilage bone," frequently employed for convenience to desi<j:nate briefly bones formed respectively in the two methods above defined.
Since bone is to be regarded as the result of a develop- ment originating in connection with a membrane, we may properlv first consider the process of intramembranous ossification : this takes place in the human body and in that of the higher vertebrates generalh' in a very small number of bones, the great majority of those entering into the structure of the adult skeleton being preceded b}' car- tilage: the tegmental bones of the skull, the squamous por- tion of the temporal bone, the bones of the face and jaws (excepting a small portion of the lower jaw), and the clav- icle are the only bones not so preceded, or, in other words, the only so called membrane bones.
A studv of the development in the embryo of any one of these bones gives substantially the following results. The place of the future bone is at first occupied by a mass of embryonic connective tissue not yet possessing the den- sity and structure of membrane, permeated by a network of blood-vessels. At one or more places, known as centres of ossification, bundles of stout fibres are formed, radia- ting outward from a point midway between the adjacent
62 PART I. THE TISSUES.
vessels : these, which are known as OSteogenetic fibres, resemble white fibres in appearance, but are less distinctly fibrillated : between them, and crowded upon the surfaces of the bundles, are large numbers of connective corpuscles, modified to form osteoblasts. The homogeneous sub- stance which forms their matrix now begins to undergo calcification, the salts being at first deposited in the form of minute globules; as these become more and more nu- merous, they fuse together forming a continuous and ap- parently homogenous mass : the osteoblasts imbedded therein becoming the bone corpuscles, and the modified, bundles of fibres becoming spicules of bone.
The formation of new fibres, Avith their associated cor- puscles, continues to go on in advance of calcification at the extremities of the spicules, the tufts of bundles diverg- ing in such manner as to anastomose with those of adja- cent spicules, their growth taking such direction as to pass between the meshes of the vascular network. As a result^ there is formed a spongy network of bone interlacing with the network of blood vessels, the interstitial embryonic connective tissue forming the basis of the primar^^ mar- row which fills the spaces and in which the blood vessels are imbedded. While this is taking place in the interior of the mass, the embryonic connective tissue upon its sur- face becomes converted into a well defined la3^er of fibrous membrane, the fibrous layer of the periosteum : from the OSteogenetic tissue immediately beneath dense periosteal bone is formed. The ossification thus set up is continued Outw^ard from the centre or centres of ossification until the whole territory involved is converted into osseous tissue,
CHAPTKK VI. OSSIFICATION. 63
with the associated periosteum, maiTow and blood ves- sels: as the botie increases in thickness new layers of dense bone are deposited by the periosteum, while that first formed is absorbed and replaced by the spongy bone of the interior.
In the replacement of cartilage by bone, or, as it has been sometimes termed, intracartilaginous ossification, blood vessels also play a conspicuous part, as we shall presently see; the centres of ossification in this, as in the preceding case, arising by the development of highly vas- cular areas of osteogenetic tissue : the formation of these areas and their subsequent extension, known as the vas- cularization of cartilage, is the immediate preliminary to the deposition of bone. The process is rendered ;nore complex by the fact that the previously existing cartilage must necessarily be removed, at least in great measure, before bone can be deposited in its stead ; and the addi- tional fact that the receding cartilage is itself the seat of noteworthy changes which invariably take place, although their direct relation to the formation of bone is by no means clear. It should also be mentioned, before begin- ning a detailed discussion of the process in question, that the replacement proper of the cartilage is also accom- panied or followed at a very early stage by its investment with bone formed by the newly developed periosteum.: and that this takes place at the outset in a manner some- what resembling the process of intramembranous ossifica- tion just described; whereas, at a later stage, the torraa- tion of dense bone takes place.
64 PART 1. THE TISSUES,
We ma}', therefore, recognize four distinct and definite stages in the process of intracartilaginous ossification, so- called (to the second of which alone, however, the term is strictl^v applicable) : these stages are in the main succes- sive for any particular point; though all may be in pro- gress at the same time at points adjacent to each other ; they are definable as follows.
The first, which may be called the transformation of cartilage, includes all those changes which take place in that tissue from the first disturbance of its normal condi- tion to its final dissolution in great measure. The second is the development of spongy bone in the spaces formed by the dissolution of the cartilage and upon its remains, or the formation of endochondral bone. The deposition of spong}^ bone beneath tbe newly developed periosteum as an 'investment of the endochondral bone and the trans- forming cartilage is the third : from the position where it occurs this is designated the formation of perichondral bone, or, as it is sometimes called, primary periosteal bone- The fourth and last stage is the formation of dense bone surrounding the perichondral bone, through the continued activity of the periosteum ; the transition from the one to the other being in some cases exceedingl}' gradual.
We may now enter upon a discussion of the changes which take place in the formation of the shaft of one of the long bones, such as the humerus, the tibia, or one of the metatarsal bones. The bone is preformed, to use a current expression, in cartilage: that is to say, its future place is occupied by a mass of hyaline cartilage having in a general way the form and relations of the bone that is
CHAPTER VI. OSSIFICATION. 65
to replace it : the mass of cartilage is covered by a single and closely adhering fibrous layer, the perichondrium, which is moderately rich in blood vessels. The transfor- mation of the cartilage involves four recognizable changes, which may be designated as the rearrangement of the corpuscles, the calcification of the matrix, the forma- tion of primary areolae, accompanied by the degenera- tion of the corpuscles, and the formation of secondary areolae by the partial dissolution of the matrix.
At a point near the centre of the mass the corpuscles be- gin to multiply and to increase in size, and (in a manner not 3'et clearly' understood) to arrange themselves in columns or rows which at first radiate from the central point, with intervening regions consisting of matrix only, thus forming a spheroidal region of transformation which continues to increase: when the surface of the mass is reached on the adjacent sides the region of transformation is of course restricted to the cartilage lying in the direc- tion of the ends : and as it advances toward them in both directions the columns of rearranged corpuscles and the intervening regions of matrix soon take on a direction parallel to the axis of the bone.
Very shortly after the rearrangement of the corpuscles just described, the deposition of lime salts in the matrix takes place, particularly in those tracts which lie between the corpuscular rows, bars and plates of calcified cartilage thus being formed. At the same time the cavities in which the cartilage corpuscles lie begin to be enlarged, while the corpuscles themselves undergo degeneration, becoming shrunken and irregular in form, and lying in the enlarged
66 PART I. THE TISSUES.
cavities, which are now known as the primary areolae above referred to. In some cases the areolae consist of two or three cavities in the same row, united by the dissolution of the thin lamella of matrix lying between them. The farther dissolution of the matrix, and the formation of the secondary areolae, in which the deposition of endochon- dral bone begins, depends upon the development of other structures presently' to be described.
About the time that the transformation of cartilage is set up in the centre of the mass important changes take place in the surrounding membrane, beginning in a zone immediately adjacent, and proceeding thence toward either extremit}' simultaneously with the advancing transfor- mation of the mass within. The fibrous layer becomes thicker and more highly vascular: its inner portion be- comes looser, and the corpuscles rapidly increase in num- ber and in size, with associated changes in form and structure: in other words, the perichondrium of the em- bryonic cartilage becomes converted into the periosteum of the future bone, with an outer fibrous and inner osteo- genetic la^^er.
By the time the central spheroidal area of transform- ing cartilage has reached the lateral surfaces of the mass, the adjacent zone of periosteum is fully formed ; and the deposition of a delicate network of spongy bone may have already taken place. As the two regions of activity come in contact, one or more loops of blood vessels grow out toward the transformed cartilage, carrying with them an investment of osteogenetic tissue: before their advance the transformed cartilage is absorbed, and they quickly reach
CHAPTHR VI. OSSIKICATION. 07
the centre of the mass. This is sometimes termed the primary vascular invasion. From the centre the forma- tion of vascular loops and the associated development of the investing osteogenetic tissue is directed toward the ends of the bone, following the advancing areas of trans- formation of cartilage ; the thin lamellae of matrix lying between the primary areolae in the longitudinal rows are aljsorbed, as well as (to some extent) those lying between adjacent rows: there is thus formed a network of elon- gated spaces, the secondary areolae, separated by inter- communicating bars and plates (trabeculae) of calcified cartilage matrix, and opening toward the centre of the bone.
These spaces are rapidly filled by the advancing blood vessels and associated osteogenetic tissue, which together form the primary marrow: the surface of the mass is in- vested with numerous osteoblasts, which are brought directly in contact with the trabeculae of calcified cartilage matrix : upon the surfaces of the latter the deposition of thin lamellae of true bone substance begins. There is thus formed a spongy mass of bone trabeculae having a small quantit}' of calcified cartilage included in their structure, and forming an area which advances towards the end of the l)ody just behind the area in which the last steps in the transformation of the cartilage are taking place: this is the endochondral bone. In the long bones with hol- low shafts it is itself a more or less temporary structure, being absorbed from behind almost as rapidly as it is formed along the area of advance, its place beingtaken by the permanent marrow. This is formed from the modifi-
68 PART I. THE TISSUES.
cation of the primary marrow, which at first consists only of the blood vessels and the associated osteogen- etic tissue, which is quite delicate and almost gelatinous in structure : later, its retiform framework becomes more fulh' developed, giant cells and erythroblasts appear, and the characters of ordinary red marrow^ are assumed; these in turn (in the region referred to) giving place to those of the yellow marrow which finally occupies the cavity.
It has already been stated that the newly developed zone of periosteum begins shortly after its formation to de- posit a layer of spongy bone upon the surface of the trans- forming cartilage in the middle of theshaft, similar to that first formed where the seat of ossification is a membrane. Along with the progress of the areas of the transforming cartilage toward the extremities of the bone, and the im- mediately successive development of endochondral bone in the interior of the mass, there is a corresponding advance in both directions of the change which converts the fibrous membrane investing the cartilage into periosteum. This, again, is immediately followed by the deposition of a layer of spongy bone, or, rather, by the extension of the layer already begun. While the layer so formed is in this man- ner steadily increasing in extent, it also undergoes increase in thickness in the region where it was first formed. There is thus formed a sheath of spongy bone, shaped like a dice box open at both ends, thickest in the middle and gradually becoming thinner towards its extremities, which contains in its middle the newly formed permanent mar- row : its ends surround the masses of endochondral bone previously described ; immediatel}' beyond lies the area of
CHAPTER VI. OSSIFICATION. 69
transforminjj^ cartila<i^c: and still beyond (and invested as yet by perichondrium) the hyalinecartilage which has thus far underg^one no chan<ye. This sheath of spongy bone is the perichondral bone, or as it is sometimes called, the primary periosteal bone.
As the periosteum extends its borders toward the ends of the future bone, its newly formed osteogenetic la3'er making the deposit of perichondral bone just described, a change, more or less gradual, takes place in the activity of its central and older portion. The meshes of the vascular network within its inner layer become more and more elongated in the direction of the long axis of the bone : the osteogenetic tissue which surrounds the vessels, instead of continuing to occupy a large portion of the intervascu- lar spaces as marrow, deposits successive lamellae of bone substance concentric to the vessels, until the spaces are re- duced to slender canals. Haversian systems are thus developed; while from the immediate surface of the perios- teum interstitial and circumferential lamellae are pro- duced, and the sheath of perichondral bone is itself invested with a layer of dense bone, or permanent peri- osteal bone. The perichondral bone is but little more en- during than the endochondral bone which it for a time surrounds : it has already been stated that as the latter advances toward the degenerating cartilage, it is itself, in the long bones, sooner or later absorbed and replaced bv the permanent marrow : the perichondral bone suffers a like fate, the mass of marrow becoming enlarged in diameter as well as in length, and the surrounding spongy bone being absorbed about it until the marrow lies directly beneath
70 PART I. THE TISSUES.
the tube of dense bone which forms the permanent shaft of the femur or tibia, as the case may be.
It will be remembered that the mass of cartilage which in the foetus represents one of the long bones under con- sideration, is very much smaller than the bone which we find in its place even in the newborn child : the increase in thickness will be readily understood. It should be borne in mind, however, that this increase in thickness is for quite a while due to the formation of bone of the peri- chondral type only : it is not until some months after birth that dense bone of the ordinary type is deposited, the shafts of the long bones remaining up to that time more or less spongy throughout.
The increase in length is due to a continuous growth of the primary cartilaginous mass itself, which up to birth and for some time after continues to increase in length and also in thickness in the part which has not as yet begun to undergo transformation. This growth continues at a constantly decreasing rate, the region of untransformed cartilage thus becoming smaller and smaller. It is en- croached upon alike by the advancing endochondral bone of the shaft, and the similarlj^ formed but more permanent mass of spongy bone in the epiphyssis, until finally the two areas of ossification meet, and the union of the epiphysis with the shaft is accomplished, the two masses of spongy bone and their investing layers of dense bone alike becom- ing confluent.
The table on the opposite page presents a summary of the various processes which have been described above as taking place in the formation of the shaft of a long bone.
REPLACEMENT OF CARTILAGE TO FORM THE SHAFT OF A LONG DONE (c. g. FEMUR.)
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72 PaUT I. THE TISSUES.
In the short bones, and in the epiphyses of the longbones, the replacement of cartilage takes place in an essentially similar manner, the vascular invasions reaching the cen- tre of the mass of transforming cartilage, and the develop- ment of spongy endochondral bone taking place on all sides. Some of the trabeculae thus formed are shortly afterwards absorbed, while others become larger, and form part of the permanent framework of the interior of the bone. It is probable even here, however, that there is in the earlier part of adult life at least more or less of ab- sorption and rebuilding continuously going on. An invest- ing layer of perichondral bone is formed, and contributes to the permanent spongy mass : and later a thin layer of dense bone is superimposed. There are some very inter- esting variations in detail as to the manner in which ossifi- cation arises in some of the bones, but their consideration would be foreign to the purpose of this work.
CHAPTER VII. THE BLOOD^ 73
CHAPTER VII THE BLOOD,
The blood consists of a fluid portion, the plasma, and numerous corpuscles found floating therein. It has some- times been described as a tissue with a fluid matrix. It is perhaps a straining of the use of these terms to apply them to the blood : it is certain, however, that a descrip- tion of the structural elements of the body would be in- complete without an account of those contained in that fluid. While they cannot be strictly defined as connective tissue elements, their origin is such as to warrant their de- scription at this time.
The plasma is during life and in health a perfect fluid as long as it is contained in the blood vessels. When allowed to escape therefrom, or more rarely under certain abnor- mal conditions while contained therein, it undergoes coag- ulation, an important process, which concerns us chiefly because of the formation of a solid substance whose ap- pearance should be familiar to the student of histology. It is known as fibrin, and when present in small quanti- ties can be plainly seen to consist of exceedingly' delicate filaments interlacing in every direction ; as the quantity increases, the meshes of the network thus formed become filled with fibres, until a practically continuous solid mass
74 PART I. THE TISSUES.
is formed : when hardened and stained such a mass pre- sents in section a granular appearance due to the cross- section of the fibres.
The corpuscular elements contained in the plasma and constituting normally a little over one-third of the vol- ume of the blood, are chiefly of two kinds, the colored (or so-called red) corpuscles, or, as they have been termed, the erythrocytes, and the colorless (or so-called white) corpuscles, which are also commonly known as leuco- cytes. There are also present in normal blood smaller or larger quantities of minute bodies known by various names, the most common of which is the blood platelets.
The colored corpuscles are by far the most numerous, about five millions being contained in a cubic millimetre of the blood of a healthy man : the number in the blood of women is about ten per cent, less : in certain forms of dis- ease their number may be very greatly reduced. When seen singly under the microscope each colored corpuscle is seen to be of a yellowish green color, the former tint pre- vailing in arterial, the latter in venous blood. The form of each is that of a biconcave circular disk, the central portion being slightly hollowed on each side and the mar- gin rounded.
The size of the colored corpuscles varies within rather wide limits, the same sample of human blood exhibiting individual corpuscles whose transverse diameter is as great as ten micra, and others that are but half as broad : by far the great majority, however, will be found to be
CHAPTER VII. THE BLOOD. 7.'
from seven to eight niicra in breadth : the average meas- urement has been variously stated by different observers after a very large number of measurements ; it is not far from the truth to say that it is about seven and three- fourths micra. Little diflerence, if any, is found in this respect in the blood of persons of different age or sex, nor is any noticeable in that of the various races of mankind. The greatest thickness of the blood corpuscle is about one-fourth its breadth. The smallest corpuscles found are sometimes distinguished as a separate form of elements under the name of microcy tes ; but this distinction is of doubtful significance.
«
The colored corpuscles of the blood of all mammals save those of the camel family resemble those of human blood in form and general appearance: they vary, however, greatly in size in different species, the differences that are found to occur having no relation to the differences in size of the animals themselves ; those of the mouse, for exam- ple, being distinctly larger than those of the horse. The largest known are those of the elephant (between nine and ten micra in diameter) and the smallest those of the musk deer (about two and one-half micra). There is, also, no necessary close resemblance in this respect between nearly allied animals ; thus, while the colored corpuscles of the blood of oxen, sheep and goats are very nearly of the same size, those of dogs and cats differ greatly.
While the colored corpuscles of the blood of many mam- mals differ in size so decidedly from those of human blood asto make their distinction a matter of absolute certainty, there are several species of mammals whose colored blood
76 PART I. THE TISSUES.
corpuscles approximate so closely in size to those of man as to render impossible a certain distinction between them. While, therefore, it might be possible under certain circum- stances to testify that a given stain was not caused by human blood, the converse is not justifiable in the present state of our knowledge.
The structure of the colored corpuscles has been and still is a matter of much debate. By some the disk is regarded as consisting of a denser colorless portion, spongy in struct- ure, the stroma, whose meshes contain the fluid colored portion, a solution of the substance known as haemoglob- in: its behavior towards certain reagents (notably to- ward water) leads others to the conviction that what we have is realh^ a closed sac (or cell in the true sense of the word) filled with the colored fluid. It is at least certain that the corpuscle is far from being a mass of protoplasm, being as greatly modified therefrom as a fat cell. It should also be noted that a nucleus is wanting in the ordinary colored blood corpuscles of all mammalia: in which re- spect they differ from those of all other vertebrates.
When blood is drawn from the vesvsels, or when, from any cause, it stagnates for a time within them, the colored corpuscles show a tendency to adhere together by their sides in masses resembling piles of coin ; this formation of rouleaux, as it is termed, was once supposed to be pecul- iar to blood corpuscles; but it has been shown that under certain circumstances the same process may be caused to take place with floating disks of cork or other substances under conditions that are purely mechanical. The rou- leaux thus formed not unfrequently arrange themselves in
CHAPTER VII. THK lU.OOD. 7<
a coarse network in whose meshes the colorless corpus- cles may be seen, and the first formed filaments of fibrin detected as coaj^ulation begins. The formation of rou- leaux does not take ])laee in the blood of vertebrates other than mammals, the thickening of the centre of the cor- puscles due to the presence of the nucleus acting as an obstacle thereto.
If sodium chloride or any other salt, sugar, glycerine, or any other reagent that tends to increase the specific grav- ity of the plasma be added to fresh blood, or if the same effect is produced by circumstances favoring rapid evapo- ration, the colored corpuscles undergo a characteristic change of form known as crenation, the corpuscle becom- ing shrivelled and the surface studded with minute projec- tions, the whole bcingin appearance not unlike the fruit of a thorn-ap])le or a horse-chestnut. This change may pos- sibly take place in the vessels themselves under circum- stances temporarily causing an appreciable difference in the proportion of water in the blood.
The colorless corpuscles, or leucocytes, are nucleated masses of protoplasm which may be regarded as tvpical animal cells. When at rest they are s])heroidal in form, but, as will be seen later, are capable of very great modi- fication in this respect. The}' are far less numerous than the colored, a cubic millimetre of healthy human blood containing on an average about ten thousand : the number varies greatly, however, not only in different individuals, but in the same individual under different conditions; thus, the number is greatly increased shortly after eating
78 PART I. THE TISSUES.
and marked!}' diminished by prolonged fasting: we may therefore find in the blood of a healthy person at one time twice as many to the cubic millimetre, at another even less than half as many as the average above given. Since the number of the colored corpuscles is not subject to such marked fluctuation, the ratio between the two varies therewith ; and the number of colorless corpuscles is often stated in terms of that ratio as ranging between one col- orless to two hundred and fifty colored and one colorless to over one thousand colored corpuscles: the average being about one colorless to five hundred colored corpus- cles. This mode of statement is, however, unsatisfactory, for the reason that, as will be readily seen, a like change of ratio would result from a marked diminution in the number of colored corpuscles or an undue increase in the number of the colorless, changes in each case of very great importance, but of quite dift'erent significance.
The colorless corpuscles vary also in size to a great ex- tent: their average diameter, when in the spheroidal state, may be stated as about ten micra. The smallest, sometimes distinguished specifically as small leucocytes, have large spheroidal nuclei, surrounded by a small amount of protoplasm : they may be regarded as newly formed elements that have only recently entered the blood stream ; they are somewhat smaller than the average col- ored corpuscles. When fully developed they may become as much as twice as large, the increase being chiefly in the amount of protoplasm : they are then sometimes termed large leucocytes; still retaining the single spherical nu- cleus.
CHAPTER YH. THK BI.OOD. 70
The smaller and lar<?cr uninuclear elements make uj) in all about tvvciity-fivc per cent, of the leucocytes of the blood. Almost all of the remainder (about seventy per cent, ot the whole number) are what are sometimes termed multinuclear leucocytes: these, which are com- paratively uniform in size and somewhat larger than the colored corpuscles, have either two or three small nuclei or one lart^^e nucleus of irregular form and apparently about to undergo fragmentation. In addition there may be found (occurring but sparsely in normal blood) a small number of leucocytes with pale nuclei and highly granular protoplasm, the contained granules staining deeply with eosin and similar reagents. These are sometimes known as granular or (from their affinity for the substance just mentioned) eosinophilous leucocytes.
When human blood or that of any manmal is examined at an ordinary temperature the colorless corpuscles are spheroidal and motionless : if, however, the stage of the microscope be so heated as to maintain the temperature of the blood at that of the living body, the spheroidal form is no longer maintained. The living corpuscle puts out one or more stout lobular processes, thus assuming an irregular form bounded by sweeping curves : these curves are vseen to change slowly but eonstantl}', the outline not remaining the same for any length of time. This shifting of form may go on about equally in all directions, the cor- puscles as a whole remaining stationary ; or there may be a flowing movement of the protoplasmic body toward one of the lobular processes, and a consequent change of posi- tion, as irregidar and indefinite as the movements by
so PART I. THE TISSUES.
which it is caused. This mode of motion, character- istic of leucocytes wherever found (and of some other elements as well) is identical in its nature with that seen to take place in the members of the genus amoeba, a group of very lowly organized unicellular animals: it is, herefore, termed amoeboid.
All the other forms of colorless corpuscles are probably derived from the modification of the small unicellular ele- ments. These, in turn, are brought into the blood stream by the Wmphatics, where, under the name of lymphocytes, they constitute the corpuscular elements of the h-mph, some, hovk^ever, attaining their full size while in that fluid.
The origin of the lymphocytes is well known. They are formed chiefl}', if not solely, in those organs which consist in great measure of w^hat is known as lymphoid or ade- noid tissue: those known as lymphatic glands, or, more properly, as lymphatic nodes, having the formation of Ivmphocytes as their principal if not their sole function. The histological anatomv of these organs will be described in a subsequent chapter: it is sufficient here to say that they consist in the main of masses of adenoid tissue well supplied with blood vessels and enclosed in each instance in a capsule, into which several small Kmphatic vessels enter and from which a single larger h'mphatic vessel leads, the current of lymph which thus passes through the mass, carrying with it the newly formed Wmph corpuscles.
What is here termed adenoid tissue is, like the marrow of bone, a compound structure of which retiform tissue is the basis. In addition to the flattened connective tissue corpuscles which form an endothelioid layer upon the reti-
CHAI'TKR VII. TMK BLOOD, 81
culuin, ami which have been described in a previous chap- ter as the fixed corpuscles proper of relitonn tissue, the trabeculae which make up the framework of tlie juass are : -MMri-r»*~4A^ 1^ in most Ccises so numerous as to entirely fill the meshes of e*r^^Ha,'jLuc the network. These cells, properly known as lympho- '*^^*' blasts, niultij)ly rapidly by cell division, the older gradu- ally falling into the lymph channels which penetrate the mass and becoming young lymph corpuscles or lympho- cytes. At this stage they are spheroidal, with kirge nuclei and a very small amount of investing protoplasm : the latter substance increases in quantity as they are carried to the heart; but, as we have already seen, many enter the blood stream before they are fully matured.
The origin of the colored corpuscles of the blood is now also well established. During the early stages of embry- onic life large nucleated colored corpuscles are formed in the newh^ forming blood vessels. Later, colored corpus- cles of the ordinary type ar^ formed first in the liver, later in the spleen, and fimilly in the red niarrow. After birth tile last named structure is the chief and probably the sole ])lace of their formation in most if not all mammals, at least during health ; it is possible that their formation may be temporarily resumed by the s])leen if not bv the liver under certain abnormal conditions.
In the description of the marrow already given it wiis stated that the interior of that structure contains numer- ous small cells with granular nuclei known as erythl'O- blasts : these are formed by cell division from larger cells not distinguishable from ordinary' marrow cells. The erythroblasts so formed multiply rapidly by the ordinarv
82 PART I. THE TISSUES.
method of cell division, the cells thus formed being sphe- roidal and nucleated and gradually becoming converted into immature blood corpuscles by the formation within them of haemoglobin. Later the nucleus is extruded from the corpuscle and the spheroidal mass becomes converted into the biconcave disk which we find in the blood stream.
Reference has been made to the blood platelets. These are minute rounded and colorless bodies (from one-third to one-fourth the diameter of a colored corpuscle) which are found in the blood either singly or adhering together in masses of considerable size: their total number in nor- mal human blood is but small. They have been described under a number of names, as the elementary particles of Zimmermann, the granules of Osier, the haematc- blasts of Hayem. The name of blood platelets was pro- posed for them b}' Bizzozero. They have been regarded by Hayem and others as concerned in the formation of red corpuscles within the blood stream, but the evidence for this view is not conclusive. Perhaps the most probable explanation of them is that they are the disintegrating fragments of broken down colorless corpuscles. Our knowledge of them is, however, exceedingly imperfect.
CJIMTKK VIM. CoNTUACni.E TISSUES. H.'i
i
CHAPTER VIII. :.«^««»..
THE COXTKACTILK TISSUES,
What is in physiological language termed contractility consists in a change of form but not of volume. Irregu lar contractility, or change of form in any direction and in an indefinite manner, is one of the powers which are inherent in living matter and may be manifested bv anv cell which is still in an embryonic condition : that speciali- zation of function and structure which con verts the embry- xm\c cell into the tissue dementis in many cases accom pan- ?*.£>. yJ^*^ ied by the disappearance of this power: it is retained, how- ever, in some cases, as in the contractility of pigment cor- puscles in many vertebrates, and notably in the amoeboid movements of leucocytes which has been described in the preceding chapter.
In one group of elements, however, this j)ower is special- ized and becomes their distinctive function. This speciali- zation is of more than one kind : there is probably an in- crease in the actual amount of contraction, and certainlv an increase in its rate; but the most important feature is its detiniteness of direction, one axis of the mass (the long- est) always becoming shorter, while the mass as a whole becomes thicker. From the fact that elements belonging to this group are the essentials of structure of the organs known as muscles, the tissues formed of them are known
84 . PART I. THE TISSUES.
as muscular tissues, although the elements of which they are composed sometimes occur singl}^ and are frequently found in masses that cannot with propriety be called mus- cles.
There are three distinct kinds of muscular elements or fibres (as they are commonly termed, from the elongation generalh' characteristic of them) the smooth, the cardiac, and the striped or striated muscular fibres. The elements of the first two kinds have in each instance a single nu- cleus, and may therefore be regarded as resulting from the direct modification of a single embrj'-onic cell. Those of the third kind are much longer as well as thicker than the others, and while they arise in each instance from a single embryonic cell, this becomes greatly elongated and the nucleus divides repeatedly; the resulting strand of proto- plasm thus becoming multmuclear.
What is variously called smooth, unstriped, plain or involuntary muscular tissue is composed of spindle shaped cells or fibres whose protoplasmic bodies show at times quite distinct evidences of longitudinal striation, but are at other times perfectly plain. The existence of a very delicate investing membrane or sheath has been dem- onstrated. The nucleus is elongated, sometimes oval but in many cases distinctly rod shaped, and is situated in the centre of the mass. Smooth muscular fibres vary some- what in size and particularly in length : the transverse diameter usually ranging between five and ten micra, while the length maybe less than tenormorc than twenty times the diameter. When so situated as to escape lateral
CHAPTER Vm. CONTRACTILE TISSl'ES. 80
[jress^ure the smoolh fibres are cjrculnr in cross section : when, however, as is frequently the case, thev are pressed together, the sides become flattened and their cross sec- tions polygonal. When seen in transverse section, there- fore, the investing membrane forms a circle or polygon, within which is seen the protoplasm of the body of the fii^rc, devoid of any clearly discernible structure, and in the centre the circular section of the nucleus.
Smooth muscular fibres are usually associated in bun- dles, the tapering extremities (which are sometimes forked ) overlapping upon the bodies of adjacent fibres and adher- ing closely thereto; a small amountof intercellular cement substance intervenes, as can be demonstrated by the use of silver nitrate. Little if any skeletal tissue pervades the bundles. The latter are in some cases more or less loosely interwoven : their most common arrangement, however, is in more or less extensive layers; as, for instance, in the muscular wall of the intestine: such la3'ers are penetrated by areolar tissue accompanying the blood vessels and nerv- ous supply of the muscle fibres. It is very rarely the case that smooth muscular fibres are aggregated together into definite masses that can with propriety be called muscles, their most common occurrence being in the blood vessels and the viscera. In no case are they under the control of the will.
The cardiac fibres are found, as their name implies, in the muscular substance of the heart, both the auricles and the ventricles beingchiefl}' composed of them : they also con- stitute an important portion of the walls of the ]ndmon-
86 PART I. THE TISSUES,
ary veins and the superior and inferior venae cavae for a short distance previous to their openings into the auricles. What are known anatomically as fibres are in this case as in others aggregates of structures not visible to the naked eve. In this instance, however, a confusion sometimes arises from the application of the term fibre to bodies which are thus compared with ordinarj' striped fibres: the}' are in realitj' rows of shorter elements more nearh' comparable to the smooth muscular fibres.
The cardiac muscular elements are short, stout, irreg- ularly prismatic bodies, intermediate in diameter between smooth and striped fibres and three or four times as long as wnde. Their ends in some cases terminate squarel}^ in others are quite jagged and irregular. They are sometimes of uniform diameter throughout, but many give off short branches which unite with those from adjacent elements, Nothing like an investing membrane or sarcolemhia has been demonstrated. The elements are faintly striated longitudinall}' and more distinctly transversely. Each element has a single nucleus, surrrounded b\' a compara- tively large amount of protoplasm showing no trace of the structure which in the superficial portion gives rise to the appearance of striation.
The c.ardiac elements are joined together by their ends to form the cardiac fibres to which reference has been made, a larger or smaller quantity of intervening cement substance being clearly demonstrable. As many of the elements branch and anastomose with those of adjacent fibres, the appearance presented is that of a network with elongated and narrow meshes.
CHAPTER VIII. CONTRACTILE TISSUES S7
The various names of striped, striated, voluntary or skeletal muscular fibres are applied to those niultinu- eleated fibres whieh form the organs ordinarily called muscles and usually attached to the bony or cartilaginous skeleton, the majority of them being under the control of the will. They are much larger than the smooth or thecar- diac muscular elements, their transverse diameter ranging from ten to seventy micra, while they are in some cases as much as three or four centimetres long: they are prismatic in form, the ends tapering more or less graduall}-. Each fibre is invested by a thin homogeneous membrane known as the sarcolemma: within this is the mass of modified protoplasm which is the seat of the function of contrac- tion : its most conspicuous feature is the transverse strip- ing or striation which gives to the fibres the name most commonlv applied. This, when seen b\' moderate powers of the microscope, presents to the eye the appearance of alternating dim and clear bands; while through the mid- dle of the clear band may be seen a narrow black line: a longitudinal striation may also be seen, but usually less distincth'.
Beneath the sarcolemma, lying between it and the con- tractile substance, may be seen here and there elongated oval nuclei: these may be shown to be surrounded with a small amount of granular protoplasm which extends as a thin disk for a short distance around the nucleus. The protoplasm and the nucleus together make up what is known as a muscle corpuscle: these, like the similar masses in the centre of the cardiac elements, may be re- garded as the residuum after the formation of the contrac-
88 CHAPTER Vlir. CONTRACTILE TISSUES.
tile substance. When a cross section of a bundle of striped muscular fibres is examined the nuclei are seen between the sarcolemmaand the contractile substance, the latter being subdivided into small polygonal areas, the fields or areas of Cohnheim.
The appearances above described are easily seen : the explanation of the structure of the contractile substance upon which most of them depend is still a matter of dis- pute. The following facts are, however, quite generally conceded, and will probably form the basis of any further positive addition to our knowledge. The contractile sub- stance may be regarded as made up of a clear viscid or semi-fluid portion, the sarcoplasm : imbedded in this are great numbers of elongated or rod-like bodies (whose ex- act form is not yet certainly demonstrated, and probably varies with different animals) ; these are know^n as the sarcous elements. They are quite uniform in length and lie in disk-like groups w^hich compose the dim bands or zones of the fibre, the clear zones being filled chiefly bv sarcoplasm : the exact cause of the dark line in the middle of the clear zone (known as Dobie's line or as the mem- brane of Krause) is not yet certainly known. The sar- cous elements are not only regularly grouped across the fibre, but also succeed each other regularly along its length, and are possibly united end to end ; the rows of sarcous elements constituting the fibrillae.
The sarcous elements are not uniformly distributed across the fibre, the fibrillae which thev compose being grouped together in strands known as muscle columns: These are separated from each other by sarcoplasm, and
CHAPTER VIII. CONTK.VCTILK TISSUES. 89
the columns themselves arc somewhat irregularly aggre- gatcd in a similar manner. This may best be seen in the cross section of a fibre, where the arcfis of Cohnheim are the cross sections of the columns, the finely granular ap- pearance of their interiors representing the ends of the fibrillae; the lines which bound the areas are composed of sarcoplasm, those which are thickest separating the groups of columns abt)ve mentioned.
The conversion of the whole of the interior of the fibre into sarcoplasm and sarcous elements and the consequent lateral position of the muscle corpuscles is characteristic of most striped fibres of adult mammals, if not of all. In the young of most mammals, however, and particularly in the embr\'o, this conversion is not entire, and the nuclei are still found in the interior of the fibre : a condition that is permanent for many of the lower vertebrates. Such fibres have also been described in certain muscles of some species of mammals when fully grown.
A muscle, in the ordinary sense of the term, is an organ consisting substantially of a mass of striped muscular fibre and its associated skeletal structures. It will there- fore be convenient in this case, as in those of the cartilages and the bones, to describe the histological anatomy of the muscles in connection with their single characteristic tis- sue.
When seen with the naked eye a muscle appears to be made up of readily distinguishable fibres of varying fine- ness : these, which are the anatomical fibres, are bundles or fasciculi of the elements which are termed fibres in the
90 PART I. THE TISSUES.
histological sense. Each fasciculus is invested b\^ a layer of areolar tissue continuous upon its outer surface with that of those adjacent and giving off from its inner surface delicate septa which lie between the individual fibres. The investing layer of the fasciculus is known as the perimys- ium; the internal skeletal tissue as the endomysium. The aggregated fasciculi which make up the body of the muscle are invested as a whole by a layer of connective tissue continuous with the outer perimysial layers, termed the epimysium. The arteries and veins proper to the mus- cle are chiefly located in the perimysium, while the capil- lary network, whose meshes are as a rule greatly elon- gated in the direction of the fasculi, are situated in the en- domysium in such a manner as to be in close proximity to every fibre.
The striped muscular fibres terminate by rounded or obliquely truncated ends, which are closely applied to the correspondingly shaped extremities of white fibre bundles, the sarcolemma of the muscular fibre being directly contin- uous therewith : these bundles are in some cases almost immediately connected with the periosteum of a bone or some similar place of attachment: in other instances they are prolonged beyond the muscle in a fibrous mass, the tendon of origin or of insertion, as the case may be.
Cn.VrTKK IX. SMALL VKSSKLS. 91
chapti:r IX.
TUB SMALL VLSSlvLS.
Mention was made in the introductorv ehapter of the iaet that eertain tissue aggregates, while themselves de- serving to rank as organs, sustain the same relation to larger and more complex organs as do the tissues them- selves. Among these compound factors of structure, as they were there termed, the most important are the small blood and lymph vessels, particularly the former. As they are built up of endothelial, skeletal and muscular tissues, their structure may now properly be described. The fol- lowing statements apply, however, only to those smaller vessels which enter the structure of other organs. The larger vascular trunks will be described in a subsequent chapter in connection with the other organs of the circu- latory system.
The blood vessels are commonly distinguished as arter- ies, which carry the blood from the heart, veins, which return it to the heart, and capillaries, which intervene between the arteries and the veins, and in which the blood is brought into the closest proximit}'^ to the tissues possi- ble in a closed system of vessels. The lymph vessels origi- nate in the interstitial spaces of the tissues (chiefly if not solely the connective tissues), these communicating directly with the open mouths of very small and thin-walled
92 PART I. THE TISSUES.
vessels known as the lymph capillaries: these unite to to form the larger vessels sometimes called lymph veins since they convey their contents toward the heart, but more commonly spoken of as lymphatics.
The interior of a small artery, such, for example, as can be just distinguished with the unaided eye, is lined with a laj^er of endothelium, whose cells are as a rule greatly elongated in the direction of the vessel ; the nuclei also being elongated. Beneath this is a layer of elastic tissue usually in the form of a membrane, either hotnogeneous or fenestrated, but occasionally composed of reticulated fibres. The endothelium and the elastic layer make up what is usually called the intima, or inner coat ; the term is, however, applied by some histologists to the elastic layer, to the exclusion of the endothelium.
Beneath the intima is the middle coat, or media; this, in the vessels under consideration, consists almostentirelv of smooth muscular fibres, arranged in a la^^er several cells deep, the long axes of the fibres crossing the vessel at right angles or nearly so. Like all layers of muscular tis- sue, this is highly elastic; during life it is always upon the stretch ; and the contraction which usually takes place in it after death throws the intima into longitudinal folds, which, when seen in cross section, give an undulating out- line to the interior of the arter}^ which is highly charac- teristic.
External to the media is the adventitia, or outer coat : this consists in the smaller arteries of a la3'er of connective tissue in most cases clearly definable on the one hand as
CIIAPTEK IX. SMALL VESSELS. 93
pertaining to the artery, on the other passing over more or less gradually into the adjacent interstitial tissue. / As the small arteries divide and subdivide, finally becom- ing lost in the ca|)illaries, there is a gradual reduction alike of the adventitia, the media and the intima. The for- mer becomes reduced to a layer of extreme thinness : the media diminishes until it is reduced to a single layer of j /transverse smooth muscular fibres and later to scattered V ^fibres not in contact and not forming a continuous layer: i the elastic layer of the intima is similarh' reduced in ex- tent, and finally disappears; the last and least of the ves- sels that may with propriety be called arterial consisting merely of the endothelial lining and an imperfectly contin- iuous layer of smooth muscular fibres, surrounded more or less definitely by a few branched connective tissue corpus-"' les.
The capillaries are the direct continuation of the arter- ies, arising either by the farther subdivision of the struct- ures just described or springing directK% as in some cases, from the sides of vessels still distinctly arterial in their structure. In either case they branch freely, forming a net- work, whose meshes have a size, form and direction in direct relation with the structure of the organ in which they occur. Thc}^ do not, like the arteries, become smaller as they branch, those of any one network being approxi- mately of the same size, though they may vary consider- ably in different parts of the same organ. Their size in life is not easily determined, but the majority of them are probably not over ten micra in diameter, though in some
94 PART I, THE TISSUES.
tissues, notably in the marrow, they may be as much as twice as large.
" In structure the capillaries are simple tubes (usually cylindricEil in form) of elongated endothelial cells of such width that from two to four may be seen in the cross sec- tion of a single capillary. As in other endothelial layers, the cells are united by an intercellular cement substance: here and there patches of this substance may be demon- strated by the silver method which may be regarded as indicating gaps between the cells ; these have been termed stigmata; and it is probable that such places offer favor- able opportunity for that migration of the leucocytes from the blood-stream into the tissues which is known to be normal to them. Capillaries are almost always situ- ated in interstitial areolar tissue or its equivalent, and are in direct relation with the connective tissue corpuscles : in some organs branched corpuscles appear to form an almost continuous layer investing the capillaries : such a layer has been termed an adventitia capiilaris.
As the capillaries originate by the subdivision of the arteries, so their union forms the origin of the veins. A small vein resembles a small artery in its general struct- ure, its wall, like that of the latter, being distinguishable into an intima, a media, and an adventitia : the chief differ- ences between them may be stated briefly as follows. The endothelium of the veins is, as a rule, composed of shorter and broader cells than that of the arteries, and the elas- tic layer of the intima is thinner: the media is very much thinner, the amount of muscular tissue being very much
CIIAPTKK IX. SMALL VESSELS. 95
reduced: the advcntiLia, which is the principal coat of tlic veins, is, if anythiiij^, thicker than that of the correspond- ing artery, and is composed largely of fibrous tissue. The veins are as a rule of a greater diameter than the arter- ies which they accompany.
Unlike tlic arteries, the veins arc not during life continu- allv on the stretch; and the recoil mentioned in connection with the former vessels does not take place after death : this reaction on the part of the arterial wall tends to drive the contained blood through the capillaries and into the veins: as a rule, therefore, the arteries are empty after death, but retain their patency on account of the thickness and elasticity of their walls. The veins, on the other hand, are usually filled with blood; or if empty become col- lapsed and flattened on account of the thinness and inelas- ticity of their walls.
A small artery and a small vein may therefore usually be distinguished when seen in cross section by the follow- ing differences : The artery is cjrcular inform : it is in most cases empty, or contains but a small amount of blood dot: the intima is thrown^ into folds, giving the elastic layer a sinuous contour, upon whicli the nuclei of the endo- thelial cells are often peculiarly conspicuous : the media is quite thick, consisting chiefly of numerous muscular fibres : the adventitia is comparatively weak, and the least con- spicuous of the three coats. The vein may be either circular or more or less irreg^ular in forni : if thejormer, it is usually filled with blood clot, readilv recognizable from the numer- ous colored corpuscles, devoid of nuclei and appearing as clear circles, and the few leucocytes with distinct nuclei:
96 PART I. THE TISSUES.
the intima is thin and siraj)le_injc:ontour : the media is also thin, containing but few smooth muscular fibres : the ad- ventitia is often the thickest and most conspicuous^ the three coats. Where the artery and vein He side_bj5?_side, the latter is usuall}^ the larger.
A similar application of the principles of structure above described w^ill enable the student to interpret the different appearances seen when a vessel is cut obliquely or longi- tudinally, and with practice to recognize an artery or a vein wherever met with in a section passing through one of the organs of the body.
The interstitial spaces of the tissues form in many cases an irregular network, in others definite lacunae or chan- nels for the lymph, a rather indefinite terra applied to the fluid originally derived from the blood and destined to be returned to it again by way of the lyraphatic vessels. The sraallest of these, the lymph capillaries, arise from the lymph-spaces of the tissues in an exceedingly irregular manner, the connective tissue corpuscles in many cases pass- ing by an insensible transition into their endothelium. When fully formed they consist of tubes which resemble blood capillaries in consisting solely of endothelial cells, but differ from those structures in the fact that they are rarely if ever cylindrical, being exceedingly irregular in form, though usually flattened, and very often quite vari' able in diameter: they anastomose freely, forming net- works of very irregular meshes. The cells of which they are composed are usually about as long as broad, but are usually characterized by a distinct sinuosity of outline.
CMAPTKK IX. SMALL VESSELS. 97
The Iviiiph c.'ipillaries unite to I'nnu the lar;.,a'r vessels known as llie lymphatics. These do not differ materially from the former exee|)t in size and in the presence of valvu- lar folds. Like the capillaries, they consist almost entirely of endothelial cells, which are, however, more elongated in the direction of the vessel than those forming the capilla- ries, but having the same sinuous outline. It should be stated here that bv some histologists it is held that the characteristic irregular outlines seen in preparations of the lymphatic endothelium by the silver method are due solely to shrinkage of the tissues after death, the outlines of the cells in the living tissues being far simpler.
A lymphatic when seen in cross section in the interior of an organ appears as an opening of irregular form, its sides usually approaching each other (due to the flatness of the vessel) ; it can in many cases hardly be distinguished from a simple tear or fissure in the connective tissue, save by the definiteness of its outline: this, except in cases where the vessel is quite collapsed, is made up of simple curves; and in it may occasionally be seen the nuclei of the endothe- lial cells of which it is composed. In some cases the open- ingis larger and more nearly equal in its various diameters, resembling the cross section of a vein : it can then be dis- tinguished from the latter b^-^the greater thinness and sim- plicity of its walls and by the fact that any clot which ma}' be present consists almost entirely of fibrin, appearing granular in the section, with here and there a few lymph- ocytes with large nuclei and a small amount of surround- ing protoplasm.
98 PART I. THE TISSUES.
The structure of the larger lymphatic vessels v/ill be de- scribed iti connection with the circulatory system.
The lymphatics have a distribution closely related to that of the blood vessels : and it is not unusual to see a small artery, its companion vein, and one or more lym- phatics in close proximity. In some cases a blood vessel may be situated within a lymphatic vessel. Such lym- phatics are termed perivascular.
The serous cavities of the body are in reality enlarged lymph cavities. They are lined by the serous membranes, which are known by distinctive names (e. g., pleura), ac- cording to their position. These consist in each case of connjective tissue which contains a more or less well de- veloped network of fine elastic fibres, surmounted by a homogeneous basement membrane. On this rests the serous endothelium, a layer of polygonal pavement cells of varying size. Here and there ma^- be found small open- ings called stomata, which put the serous cavities in com- munication with the lymph channels of the membrane and thus with the lymphatic system : they are surrounded by cells which are usually more tumid and granular than those adjacent.
In addition to the tumid cells which surround the sto- mata there may sometimes be found on the serous surfaces patches of granular cells, which may be cuboidal if not columnar in form: these give evidence of rapid cell divi- sion, and may be regarded as local centres for the forma- tion of the leucocytes which may be found in the serous cavities.
A^ ha^jik^
CHAI'TFR X. NERVOUS TISSUES. 99
CHAPTER X.
THE NERVOUS TISSUES.
The nervous tissues have for their characteristic function the reception, conduction, distribution, and discharge of impulses or stimuli. The stimuli received may be such as, when manifested in consciousness, we know as sensations ; or the\' may be such as are never reported to our con- sciousness : in either case they may be either rnechanical, physical, or chemical in their origin: the distance to which they are transmitted may vary greatl}^ in different cases, as may also the extent and nature of their distribution. The impulses which are discharged (with or without pre- vious distribution) must be regarded as liberations of energy due to changes set up in the central nervous ele- ments by the ingoing stimuli : the\' ma^' or may not re- sult in associated and more conspicuous liberations of energy in the muscular tissues.
The nervous elements, are, then, like the muscular ele- ments, reservoirs of energy : they differ from them both functionally and structurally by what has been termed their polarity, by which is expressed the fact that the im- pulses received enter in a more or less definite direction, while the ensuing discharge takes place along a definite line which is in a general way in an opposite direction to that along which the incoming stimulus enters. A nervous
100 PART I. THE TISSUES.
element of the simplest type may be conceived of as com- prising a spheroidal or spindle-shap^ed rnass of protoplasm provided with a nucleus, each of whose extremities is con- tinued into a filament of more or less length. Along one of these filaments stimiuli travel toward the central corpus- cle, and the extremity of the filament is modified for their reception: along the other, nervous impulses pass outward to their appropriate place of discharge, its extremity show- ing corresponding modification. The whole structure, from one set of terminals to the other (and including both) is the result of the modification of a single embryonic cell : on account of the delicacy of the various parts, however, and the great length in many cases of the conducting fila- ment, it is rarely (if ever) possible to isolate such a nervous element in its entirety, as we can isolate a smooth muscu- lar fibre or an epithelial cell: nor can nervous elements (with a few exceptions) be so prepared that the\^ can be seen in their entirety in a single section : in most cases our study must at any one time be chiefly directed to one or the other of the regions above indicated. Practicallv, it has been customary to distinguish, as the structural fac- tors of the nervous system, the conducting strands of greater or less length along which impulses are transmit- ted ; the central corpuscular masses ; and the terminals of reception and discharge: and although these are now known not to be independent parts, it is convenient to retain this distinction as a basis for their description: according, therefore, to the form and associated function of the vari- ous components of the nervous system, it is customary to classify them as follows.
CHArTKK X. NKRVors TISSUKS. 101
A. ComlucLiiiL,^ elements, or nerve fibres : ihese consist in every case of continuonsev linilrical ( or rounded) strands w hieh run without interruption Ironi the central cor|3us- cles to the terminal modifications ol their free extremities. The presence or absence of certain investing structures dis- tinguishes two kinds of nerve fibres, known as
1. Non-medullated fibres: also called gray or gel- atinous fibres, or (from their discoverer) the fibres of Remak: the axis consists of a grayish cylinder or a band with rounded edges, showing delicate longitudinal striations : they are invested, if at all, by a very delicate primitive sheath whose exist- ence is questioned by many histologists. Upon the surface of the fibre are seen numerous nuclei. The gray fibres not unfrequently branch and anasto- mose with adjacent fibres. ^
2. Medullated fibres: also called white fibres: the O^^^ •axis-cylinder is grayish and longitudinally striat- ed : it is surrounded by a layer of a fatty substance
termed myelin, the layer being called the medul- lary sheath or white substance of Schwann: outside of this is the clearl}^ defined primitive sheath or neurilemma. The medullary sheath is interrupted at frequent intervals, the points where this occurs being termed the nodes of Ranvier ; the intervening segments are called internodes: each exhibits one or two so-called nerve-fibre cor- puscles situated between the medullary sheath and the neurilemma and consisting of oval nuclei spar- ingly invested with protoplasm. Medullated fibres rarely branch, except near their extremities.
102 PART I. THE TISSUES.
B. Central elements, or nerve corpuscles: these are variously shaped bodies with conspicuous nuclei: they vary greatly in size also, some being among the smallest and others among the largest of the tissue elements. Their surfaces always give off one or more processes termed poles: according to the number of these processes they are commonly distinguished as unipolar, bipolar, and multipolar corpuscles. Most nerve corpuscles (in the higher vertebrates, at least) are multipolar; and in the great majority of instances one of the poles of a multipol- ar corpuscle can be distinguished from the x)thers as the process of Deiters, or, better, as its axis-cylinder process : the rest are then known as protoplasmic processes and rapidly divide into irregular branches termed dendrites : in some multipolar corpuscles no axis-cylinder process can be distinguished ; such corpuscles are termed amacrine. Other features will be discussed in a subsequent para- graph.
C. Peripheral elements, or nerve terminals, formed by the modification of the extremities of nerve fibres with or without epithelial or skeletal elements. According as they are situated at the extremities of fibres which conduct im- pulses toward the nerve centres (afferent fibres) or away from them (efferent fibres) they are distinguished as
1. Receiving terminals: of these various kinds are known, composing two groups which differ from each other alike in the form and in the arrangement of the structures included. In one group large num- bers of similar terminals are associated with modi- fied epithelial cells to form the essential structures of the three great organs of special sense, the nose.
f)
CHAPTER X. NKKVnCS TISSUI-S. 103
Fcri|)luM'al cleinenls or nerve terminals (continued). the eye, and the ear: these will be described in a subsequent chapter, together with the less highly sjjecialized organs of tiiste. In the other the term- inals are always either solitary or in groups of two or three, though they mav be more or less al)und- ant. Some of these forms of terminals have been regarded as associated with specific modes of sen- sation, but the function of most of them is alto- gether unknown : they are therefore usually dis- tinguished by peculiarities of form or of location, or by the names of their discoverers. The principal forms ma}' conveniently be described here.
a. The simplest form of receiving terminal is seen in what are known as free endings. In these the nerve fibre first looses the investing white sub- stance of Schwann, and later the neurilemma, though the nerve-fibre corpuscles are still seen for a while before the axis-cylinder becomes entireh- naked. When the latter stage is reached, the fibrils are rapidly split up into small bundles, and finally form a tuft or pencil of delicate more or less vari- cose filaments which ramify among the cells of the epithelium in the case of the skin, a mucous mem- brane, or the substance of a gland ; or among the elements of other organs. Branching terminals, essentially similar, found in tendons, are known as organs of Golgi.
b. Closely allied to the preceding are the terminals associated with what are known as tactile cells : these are spheroidal cells found in the deeper por- tion of the epidermis and probabl}' epithelial in character. The nerve fibre in relation with a group
104 PART I. THE TISSUES.
C. Peripheral elements or nerve terminals [continued). of them looses its investments and breaks up into a number of slender branches, each of which ends in a saucer-shaped disk, the tactile meniscus, which embraces the proximal surface of the tactile cell, so called. There is no positive evidence that these, rather than other cutaneous terminals, are the special organs of touch.
C. The bodies known as compound tactile cells, also sometimes called the corpuscles of Grandry, found in the skin of certain birds, and also de- scribed as occurring in mammals, may be regarded as composed of two or more layers of tactile cells, and having between adjacent layers terminal ex- pansions known as tactile disks. The neurilemma of the nerve fibre of which the tactile disks are the termination becomes continuous with a connective tissue capsule that invests the whole structure.
d. The name of tactile corpuscles has long been given to large ovoid bodies found in the papillae of the skin of the hand and foot and in other places whore the sense of touch is well developed : they are also called the corpuscles of Meissner. Each is com - posed of a mass of connective tissue about which a medullated fibre winds spirally once or twice, the sheaths of the fibre then becoming merged in the mass or continuous with its capsule: the axis- C3dinder passes as a nonmedullated fibre into the interior, where it branches more or less freely, the branches becoming varicose. Two, three, or even four medullated fibres may be connected with a single large corpuscle of Meissner.
e. What are termed end-bulbs are spheroidal or cylindroidal bodies of simple structure in which the axis cylinder of a medullated fibre enters the proxi-
CIIAPTKR X. NICKVOUS TISSUES. 105
C. Peripheral elements or nerve terminals (continued). mal end of a mass of connectivctissue which is con- tinuous with the sheaths of the fibre, and extends throughout its length, branching but little if at all. They are found in the conjunctiva of the eye, and in other modified dermal structures, as well as in- ternally. The articular corpuscles found in the vicinity of joints, as well as the genital corpuscles of the male and the female sexual organs are prob- ably to be regarded as modified end-bulbs, though both are held by some histologis ts to approach more nearly in structure to the corpuscles of Meissner.
f. The Pacinian bodies, or, as thev are sometimes called, the corpuscles of Vater, are the largest of the terminals, being easily visible to the naked eye in man\'^ cases. They are irregularly^ ovoid in form, each having for its axis the axis-cylinder of a med- ullated nerve-fibre, which may terminate near the extremity of the corpuscle by a bulbous enlarge- ment or may divide near the end into short irregu- lar branches wnth pyriform extremities. The axial structure is imbedded in a cylindrical core of doubt- ful nature: it is faintly granular and contains scat- tered nuclei, and is possibly homologous with the principal mass of an end-bulb. Surrounding this is a series of concentric tunics which maybefiftyor more in number, each consisting of a fibrous layer and an endothelial investment: the whole may be regarded as a highh^ specialized modification of the capsule of the end-bulb. Pacinian bodies are found widely distributed throughout the bodv. As is the case with the other forms of receiving termi- nals, we have as yet no certain knowledge of their specific function.
lOG PART I. THE TISSUES.
C. Peripheral elements or nerve terminals {continued). 2. Discharging terminals : Impulses sent out from the nerve centres may give rise to muscular, glan- dular, or other activities. The terminals by which discharge is made upon the elements of the tissues involved have in every case, as far as known, the form of ramifications or arborizations of the ex- tremities of efferent fibres. In the case of glands or other secretorv structures the terminal subdivi- sions are situated among the epithelial cells. In the case of smooth muscular tissue, nonmedullated fibres from an adjacent ganglion or ganglionic plexus enter the muscular layer, between whose elements the ramifications of the fibre are situated. The mechanism of discharge in the case of striped muscular tissue is somewhat more complex. Me- dullated fibres from the intramuscular plexus, fol- lowing the endomysium, divide in each instance into two or more branches, each branch passing to a single muscular fibre. The medullary sheath disappears, the neurilemma apparentU^ becomes continuous with the sarcolemma of the muscular fibre, and the axis-cylinder breaks up into a number of fine varicose branches : the latter rest upon or are imbedded in the sole-plate, a flat- tened granular mass of protoplasm containing several nuclei and lying between the sarcolemma and the body of the fibre: the whole structure is termed a motor end-plate. By some histologists the end-plates are believed to be situated altogether outside of the sarcolemma.
CHAPTKR X. NERVOI'S Tl«Sl'ES. 1(>7
Attention has l)ccn called to the delicate longitudinal striation of the axis cvlinder of the medullated fibre and of the corresponding portion of the gray fibre. This is the expression of a distinct fibrillation, the primitive fibrillae l)eing imbedded in an intervening homogeneous substance, the neuroplasm (the resemblance to the fibril- lae and sarcoplasm of striated muscular fibre is note- worthy). The terminal subdivision common to medul- lated and nonmedullated fibres consists of a breaking up of the axis into smaller bundles of these fibrillae and even- tually, in some cases, to the separation of each individual fibrilla.
The gray or nonmedullated fibres consist of little more than bundles of fibrils and neuroplasm. Those which compose the branches of the olfactory nerves have a well- defined and nucleated primitive sheath: in most gray fibres no such sheath can be demonstrated. Scattered nuclei are seen upon the surface of the gray fibres: these, like the sheath, when present must be regarded as skeletal rather than nervous in character. Gray fibres show well marked varicosities, w'hich are possibU' due to local accu- mulations of neuroplasm, the fibrillae being correspond- ingly separated at such points. Where gray fibres branch and anastomose bundles of fibrillae accompanied by neu- roplasm pass over from one fibre to another: the angles formed by the branches are frequently filled for a short distance with neuroplasm, and investing nuclei are often relatively abundant at such points.
The axis cylinders of white fibres show under certain methods of treatment transverse striation curiouslv like
108 PART I. THE TISSUES.
that of a striped muscular fibre: if a reality, and not due merely to the reagents used for its demonstration, this must be catised b\' regular variations in the size of the fibril- lae. The axis cylinder may also be shown to be suddenly thickened at the nodes of Ranvier, the spindle-shaped en- largement being apparently due to an increase in the amount of neuroplasm with an accompanying separation of the fibrillae. Some histologists maintain that the axis cylinder is invested with a delicate structureless sheath, for which Kuehne has proposed the name of the axlleaiina : whether this structure exists in the living fibre is still a matter of question.
' The medullary sheath also presents (under certain treat- ment) apparent evidence of a structure that would hardly be inferred from its semifluid character as seen in the fresh nerve. A reticular framework of a substance of a horny nature known as neurokeratin can be demonstrated, whose meshes and filaments vary greatly in size in differ- ent parts of the same fibre. That the substance in ques- tion exists as a component of myelin is probably true: but the solid framework described is quite possibly due to its coagulation by the reagents employed. Far more con- spicuous are the oblique clefts seen in the medullary sheath after treatment with certain reagents, notably osmic acid : these are evidently the view in section of conical cleavage spaces running from the primitive sheath to the axis cylinder, and dividing the medullary sheath into the me- dullary segments of Schmidt and Lantermann, a number of which may be found ineachinternode: whether these are real or artificial must, however, be regarded as still unset- tled.
,-A.I, f*^'
CHAPTKH X. NKRVOTTS TISSIKS. 109
The iiciirik'iiinia exhibits no special sLniclur.'il features worthy of remark. It should he noted that when a me- dullnted fibre joins the brain or cord, while the medullary sheath is continued within the axial structure as far as the gray matter, the neurilemma disappears: thus the columns of the cord are made up in f^^reat measure of me- dullated fibresdev^oid of neurilemma. At the distal extremi- ties of the fibres the medullary sheath is the first to disap- pear, the neurilemma bcinj^ continued for some distance toward the terminal.
Nerve fibres, both gray and meduUated, var}' considera- bly in size, their diameters ranging from two to twenty micra ; the difference appears to be associated with a cor- responding difference in the length of the fibres.
Corpuscles, fibres, and terminals are now known to be continuous structures and components of what may properly be called true tissue elements, meaning by that term in each case the result of the modification of a single embr3''onic cell. As indicated at the outset, such an element may consist of a receiving terminal, an afferent fibre (medullated or nonmedullated), a central corpuscle, an efferent fibre (of either kind) and a discharging terminal. The simplest form of terminal is in either case a tuft of fibrillae: if the subdivisions of the receiving terminal are called dendrites, and the discharging cluster an arboriza- tion, the two can readily be distinguished by these terms. A corpuscle so situated would be essentially bipolar; such corpuscles exist, though not in great numbers, in the ner- vous tissues of the higher vertebrates : more frequently the points of attachment of the two fibres become approx-
110 PARTI. THE TISSUES.
iniated and finally consolidated for a short distance, form- ing what is apparently a unipolar corpuscle with what is termed either a Y- or a T-connection according to the mode of separation of the two fibres. In certain super- ficially situated elements of a sensory character in some of the lower animals (and possibly in higher forms as well) the receiving terminal and afferent filament become so shortened and condensed as to form a mere eminence onl\' on the body of the corpuscle: such elements may be said to be in form (but even then not in function) unipolar. What have been called in the past apolar corpuscles prob- ably do not exist.
In the ganglia of the sympathetic system corpuscles are found with more than two processes, each of which be- comes an axis cylinder (or a gray fibre) : such corpuscles are in the strictest sense multipolar: whether the majority of the poles are afferent or efferent is unknown : both conditions may possibly occur.
The term multipolar has long been applied to the cor- puscles found chiefly in the brain and spinal cord in which a distinction can be made, as has been pointed out, be- tween a single axis-cylinder process and a number of so called protoplasmic processes which subdivide into a group of dendrites. It has been suggested that the latter have for their function some connection with the nutrition of the corpuscle: but a more reasonable interpretation is one which regards such a corpuscle as resulting from the disappearance of the afferent fibre, its primary subdivi- sions thus becoming processes of the corpuscle itself. The axis cylinder process may in its course give off one or more
ClIAPTKK X. NKUVOUS TISSITRS. Ill
slciidcr branches ; these leave the process at well marked angles, but soon after bend str()n<^ly to become ajjproxi- matcly parallel to it In most cases: they are known as collaterals, and like the processes from which they arise terminate in arborizations.
Axis-cylinder processes which pass from the jj^ray into the white matter of the cord become invested with a medul- lary sheath and are then true axis cylinders: elements in which this is the case are known as corpuscles of the first type: in other cases the efferent process is (piite short, the terminal arborization beinr^ situated in the gray matter: such elements are called corpuscles of the second type. The disappearance of the process altogether, making the arborization sessile, like the dendrites, gives rise to the amacrine corpuscles of Cajal.
Nerve corj)uscles alwa\^s have large and conspicuous nuclei, in the vicinity of which a patch of pigment granules is very commonly present. The iibrillae of the processes may be traced into the interior of the corpuscles, but their internal distribution is as yet unknown. The corpuscles are almost always situated in well defined lymph spaces which agree closely with them in contour. The forms of the corpuscles of the brain and cord will be described in the chapter devoted to those organs.
The nerves are definite aggregates of nerve fibres : like the blood vessels, the\' penetrate the organs of the bod\' and are consequently to be regarded among the factors of structure thereof: the same is true of manv sfanjrlia: both will therefore be described at this time.
112 PART I. THE TISSUES.
A nerve is a bundle of nerve fibres or an aggregate of such bundles. Each bundle is termed a funiculus, and is composed of a number of fibres surrounded by a cjUndrical sheath called the perineurium. The latter is lamellated in structure, the number of lamellae never being less than three save in the smallest branches of the ner v^es : they are separated by distinct lymph spaces lined with endothelioid corpuscles. The inner lamella is continued into the funi- culus by the connective tissue which lies between the fibres and supports their capillaries, called the endoneurium. In small funiculi this connective tissue is homogeneous in composition, approaching gelatinous tissue in consistency : such funiculi are termed simple: larger funiculi, called compound, show here and there in the endoneurium con- nective tissue septa which divide the funiculus irregularly.
In small nerves, consisting of but a single funiculus, the outer lamella of the perineurium is continuous with the adjacent areolartissue: where several or more bundles are associated, however, as in the larger nerves, a definite mass of connective tissue, containing more or less fat, and definitely compacted on its outer surface, invests and sup- ports the funiculi, becoming continuous with their outer lamellae: this is known as the epineurium. Within it the associated funiculi divide and anastomose from time to time, each large nerve being thus in reality a greatly elongated plexus.
As the funiculi divide into small groups of fibres and finally into single fibres in the vicinity of their destination, the perineurium becomes greatly reduced, being finally continued for a short distance on the single fibres either as
CIIAPTI^R X. Ni;UV()l'S TISSUES. 113
a single lamella or as a mere layer of etidothelioid eclls : sneli an investment is known by the name of Henle's sheath.
A ganglion is a mass of nerve corpuscles invested with a definite sheath or capsule of connective tissuecontinuous with the e|)ineurium of the nerv^cs with which it is asso- ciated; or, in the case of nerves consisting of single funi- culi, with the perineurium. These ma}^ be but two in number, the ganglion in such cases being practically seated upon a nerve trunk; or there maybe three or more, the !2fanorli<jn being situated at their intersection. In almost all the larger ganglia there may be clearly distinguished a cortical portion, consisting chiefl}'^ of nerve corpuscles, and a central portion, consisting largely of nerve fibres. Of these some pass directly through the ganglion, while others pass into or out from thecortical portion, being connected with the cori)uscles. Each corpuscle is, as a rule, con- tained in a delicate capsule continuous with the neuri- lemma of the associated fibre or fibres and enclosing, as has already been stated, a pericorpuscular lymph space.
The neuroglia, or sustentacular tissue of the brain and cord, has been referred to in the chapter devoted to the fibrous tissues. It differs from all the tissues of that group in its origin and in the absence of anythinglike the matrix characteristic of them ; consisting, as was stated, entirely of peculiar branched corpuscles known as glia-cells. These are stellate or irregularly shaped cells with large nuclei, which stain conspicuously with some reagents. Their branches, which are quite numerous, terminate in
11 4 PART I. THE TISSUES.
long slender processes : these are stated bv Ranvier to be fibrillated, the fibrillae passing through the body of the cell from one process to another: the\' are varioush^ ar- ranged upon glia-cells from different parts of the cerebro- spinal axis, those of the gray matter of the cord, for exam- ple, differing from those of the white in the number and disposition of their processes. In a general way it may be said that the latter form a dense reticulum of closely interwoven fibres: this in sections has a fineh^ granular appearance conspicuously seen in the gray matter..
The glia-cells are found in the cerebrospinal axis and are derived from the same embryonic la^^er as the nervous ele- ments themselves. While, therefore, their function is probably purely mechanical, or, in a sense, skeletal, they must be regarded as closelv related to the nervous tissues rather than to the skeletal tissues proper. These latter also penetrate the cerebrospinal axis, in the form of con- nective-tissue trabeculae which compose a proper skeletal framework : and although they gradually diminish by subdivision and become reduced to delicate fibrils inter- mingled with those of the neuroglia, nothing like a transi- tion from one tissue to the other has ever been observed.
^"^ ^ ^ II. /^ - / t
V
OLyw|-^^
en APTHR XI. STRUCTURE OF TIIH CKLL. 115
CHAPTER XI. THE STRUCTURE OF THE CELL.
\Vc have now passed briefly in review the various tissue elements, considering both their form and characters and their union to compose the tissues of the body; as also the structure of some of the simpler aggregates of tissues, or organs. The elements of the tissues were at the outset defined as cells or as derived from the modification of cells; and a cell was defined as a nucleated mass of protoplasm. It is important now for us to consider the structure of the protoplasm itself, and of the nucleus as well; and to learn something of the process by means of which new cells are formed.
It was stated in the opening chapter that the protoplasm which makes up the body of the cell is neither homogene- ous or structureless, as it was once supposed to be. The delicate granulation generally characteristic of its appear- ance as seen by ordinary' powers proves with more im- proved means of research to be the expression of a delicate reticulum or network of a somewhat denser substance which has been given the name of spongioplasm ; its meshes are filled with a less dense or semifluid substance designated as hyaloplasm: the proportion between the amounts of these two substances may var^^ greatly: as a
116 PART ]. THE TISSUES.
i^^enerjil rule the relative amount of the former increases with the age of the cell. The meshes of the spongioplasm may var^y greatly in size and in the coarseness or fineness of their constituent fibrils: the accumulations at their in- tersections are the granules most readily seen.
In cells which become surrounded b}' a cell wall com- posed of some formed product (e. g., an epidermal cell), the reticulum becomes quite close and dense near the sur- face: but in man\' cases (e. g., a leucocyte) the converse is the case, the exterior of the cell consisting almost, if not quite wholly, of hyaloplasm. Such a clear outer portion is sometimes termed ectoplasm, in distinction from the gran- ular inner portion known as endoplasm : the distinction is, however, of questionable value, since in some of the lowest animals the conditions are reversed, the same terms being applied to a denser outer and a more fluid inner portion of the cell. The terms paraplasm and deu- toplasm are also sometimes made use of in connection with the structure of the cell body to designate granules imbedded in the protoplasm, and consisting either of sub- stances taken up by the protoplasm in aj^olid form, or of formed products temporarily stored in the cell ; such, for example, as yolk granules in the ovum.
The nucleus gives evidence of a reticular structure even with ordinary powers ; and this structure is also clearly seen to vary in the different nuclei of adjacent cells or in the same cell at different times if watched while still living. Under ordinary conditions, however, the nucleus when seen in what is usually designated the "resting" condi-
CHAI'THR XI. STUrCTl'KK OF TUT CKI-L. 117
lion is ri splKToicial vt'siciilar body hounded \)y a well de- fnicd wall, and eontainiiiLi the network al)ove referred to: this is sometimes fnie and close meshed; at others com- posed of but a few coarse fibrils, which are in some cases quite irrcf^^ularly disposed ; in some cases the fibrils form a continuous filamen^which is arran^red in a tangled skein. The nodes of the network in many cases form coarse granules, which are cpiite conspicuous : in addition there are often seen in the nuclei distinct spheroidal bodies apparently different in composition from the network: such a bod}' is called a nucleolus.
The meshes of the network are filled with a clear semi- fluid substance which is not readily colored by the stain- ing fluids wdiich render the network and nucleoli conspicu- ous. This difference between the two principal substances of the nucleus led Flemming to propose for the substance composing the filaments the name of chromatin, and for the clear substance that of achromatin. More recent re- searches have made clear, however, the fact that the denser portion of the nucleus itself consists in part of a substance which does not stain any more readily than the more fluid portion: while the latter, now usually termed the nuclear matrix, is regarded as possibly similar in com- position to the hyaloplasm of the cell-body. The name of chromoplasm has therefore been proposed by Carnoy for the substance which forms the filaments, that portion which is readily stained being designated by the term chromatin, and that which resists ordinary stains by the term achromatin; a different application of these terms from that originally i)roposed by Flemming.
lis PART I. THE TISSUES.
The nuclear wall is composed of chromoplasm : by some it is regarded as consisting merely of a fine and close net- work of that substance; b\' others as a definite and con- tinuous layer. Its continuity with the network is evident from its comportment at the time of nuclear division. Whether the nucleoli consist of chromoplasm must for the present be regarded as an open question.
While the arrangement of the filaments of the nuclear network is often exceedingly irregular, especially in the resting stao^e, it can often if not alwavs be seen at the time of its greatest development to have a definite plan, whose basis is the formation of a larger or smaller num- ber of elongated loops having a meridian-like arrange- ment in relation to a definite axis. The turns of the loops are directed toward one end of the axis, termed the pole of the nucleus: the free extremities meet (and frequently interdigitate)in the region around the opposite end of the axis, the anti-pole. The sides of the loops are often ex- ceedingly irregular in their course, and may in addition branch frequently, the branches anastomosing and thus forming the irregular netw^ork commonly seen. Where the ends of adjacent loops become continuous the convo- luted filament sometimes seen is produced.
The division of older cells to form new ones is preceded in man and the higher animals generally by nuclear division. The older observers (whose imperfect micro- scopes showed them in the interior of the nucleus only the nucleoli and the coarser nodal granules) believed this pro- cess to be quite simple in its nature, consisting merely in
CHAPTKK XI. STRrCTVRE OF THK CELL. 119
the passage of a clcavaijc-planc throuijjh the nucleus in the same wav as is seen in the cell body itself. This, which is called direct division, may possibly sometimes occur; but the constant advance of our knowledge makes it yearly more and more evident that the common mode of nuclear division as it occurs in jilants and animals, and in normal and i)athological changes alike, is an exceedingly' complex process, to which the rather unfortunate name of indirect division is commonly applied : and it is quite possible, if not probable, that this is the sole method. The series of changes involved in this process has been termed karyo- kinesis. The name of mitotic division has also been ap- plied to the process, the successive stages being called mitoses: the so-called direct division being distinguished as amitotic.
The successive steps that can be recognized in what is really one continuous process have been designated by special names, based on the appearances presented b\' the nucleus from time to time. The first step is the formation of what has been variousl}' termed the spirem, or close skein : the secondary filaments are retracted into the primary filaments or loops; the nuclear membrane is also absorbed, as are the nucleoli; the latter fact indicating the possible identity of these bodies with the chromoplasm of the network. The next step is the formation of the open skein, or wreath : the j^rimary filaments contract, be- coming shorter and stouter, and having a less tortuous course: they gradually' assume the form of an equatorial wreath of loops with, as has been stated, their flexures
120 PART I. THE TISSUES.
turned toward the region of the pole. To these loops the name of chromosomes has been given.
While the-chromosomes are thus being defined, there ap- pears in the polar area a group of fibres of achromatin, known froin its form by the name of the achromatic spin- dle: this gradually moves toward thecentreof thenucleus, taking an axial position with its extremities directed toward the pole and the anti-pole respectively. The chro- mosomes having b\^ this time become quite short and stout, and V-shaped from the divergence of the limbs of the loops, attach themselves to the spindle, eventually as- suming a radial position at its equator. When the nucleus is viewed at this stage from a polar or an anti-polar direc- tion the radiating arms of the chromosomes together form a starlike figure to which the name of the aster has been given.
During the formation of the aster or immediateK^ there- after, the cleavage of the chromosomes take place, each loop being split into two similar (but of course more slender) loops by a plane passmg through them all equa- torially. This is the central process in the division of the nucleus: the changes which follow have therefore been sometimes designated by the term metakinesis: they are in a certain sense the retracing of the processes already de- scribed. In its more limited sense the term metakinesis is applied only to the cleavage of the chromosomes, and the changes immediately connected therewith.
The daughter-loops formed by the cleavage of the chro- mosomes begin to separate first at their apices, these being turned toward the extremities of the spindle: they then
CHAPTER XI. STKICTIKE OF THE CELL. 121
travel slowly along the achromatic til)rcs in each direc- tion, two sets of chromosomes thus being formed; these gradually arrange themselves about the poles of the spin- dle, which have now become the polar areas of the two daughter-nuclei. The limbs of associated loops remain for some time connected together by delicate achromatic uniting filaments: the whole figure seen from the side has a resultant barrel-shaped appearance ; at either extremity the chromosomes have a stellate arrangement, and this stage is therefore designated the dyaster.
As the dyaster is formed, a cleavage plane passes through the body of the cell, whose course in the nuclear region is sometime marked by nodal points on the uniting filaments. As their separation is completed, the free extremities of the chromosomes of the daughter-nuclei bend inward toward the new antipolar areas, which face the plane of cleavage and therefore, also, each other. Changes now ensue in the inverse order of those described as taking place at the beginning of the process, the chromosomes becoming first converted into open skeins, and later into the closed skeins whose farther modification gives rise in each to a reticulum b\' the formation of anastomosing secondary- filaments, accompanied with or followed by the formation of a nuclear membrane, the appearance in some cases of nucleoli, and the final assumption of the resting stage. The stage in which two adjacent and parallel skeins are seen is sometimes termed the double skein or dispirem : but it is evident that w'e are here dealing with structures which, though genetically associated, are now parts of Separate and distinct nuclei.
122 PART I, THE TISSUES.
The number of the chromosomes varies considerably in different plants and animals, but is probably constant for the same tissues in each species : their form may also vary greatly, particularly as regards the length of the branches of the loops. The changes above described may in many cases be followed step by step with a good microscope of ordinary powers, either as they take place in living cells, or as thc}^ may be found in adjacent cells of suitably pre- pared tissues, each of the characteristic figures mentioned being clearly recognizable : in some instances, however, owing to the irregularity in the form of the chromosomes, or to variations in the rate of their transformation, some of the phases may be so far modified as to be no longer distinguishable. The essential features of the process are, nevertheless, always to be discerned, and should be dis- tinctly borne in mind : they are, in their order, as follows : first, the collection of the chromatin of the nuclear net- work into chromosomes ; second, the equatorial arrange- ment of the latter in what has been termed the nuclear plate; third, their metakinetic cleavage ; fourth, the sepa- ration of the two sets of chromosomes thusformed ; and finally their resolution into the nuclear networks of the two resultant daughter-nuclei, whose formation b}'^ this method is accompanied by the cleavage of the protoplas- mic body of the parent cell, thus completing the forma- tion of new cells.
It has recently been made fully evident that the changes taking place in the chromatin of the nucleus during karyo- kinesis are accompanied and preceded by other equally complicated changes in structures made up wholly of ach-
CHAPTKK XI. STKICTI'KK OF THK CKLL
12vS
roniatin, the formation of the achromatic spituUc hcin<( a jiortion thereof. While these changes are doubtless as im- portant as those already described, their nature is as yet far less clearly understood. A description of them ww therefore not be necessary at this time.
END OF PART I.
' e'
Hi
9 I
PART II.
HISTOLOGICAL ANATOMY.
CHAPTER XII. INTRODT'CTORY. 127
CHAPTER XII. INTRODUCTORY.
HistolDgical Anatomy has already been defined as the study of the arrangement of the tissues to form the organs of the body : an organ has also been defined as a particu- lar part of the body having a definite form and function : and it is a familiar fact that organs having common or essentially similar functions are associated together under the name of a system, whether they are continuous, as in the case of the nervous, or discontinuous, as in that of the muscular system.
While the study of the histological anatom\' of the organs can in most cases be pursued most naturally by considering them in their relations as components of the various physiological systems, on account of the commu- nity of structure which, in most cases, characterizes asso- ciated organs, and while structure and function are with- out question closely (though not always evidenth-) re- lated, it should always be kept clearly in mind that we are here concerned with structure only ; and particularly with structure as composed of tissues : our constant endeavor should be, in the first place, to analyze the organs into the tissues of which they are composed, and to determine the relations of each to the others ; and in the second place to note any characteristic pecidiarities exhibited by any of
128 PART II. HISTOLOr.ICAL ANATOMY.
the tissues present ; this should be accompanied in each in- stance by a careful consideration of the disposition and characters of the compound factors of structure present, such as the blood, lymph, and nervous supply.
Tissues have alread}'- been defined as masses of cells or of cell-derivatives; and since tissues compose the organs, and organs make up the whole body, it follows that the body is to be regarded as a mass of more or less modified cells. The innumerable cellular elements which make up the adult human organism are in every case derived from the division of previously existing cells, and are therefore necessarily the descendents of a single ancestral cell. That cell is the fertilized, ovum or, as it is sometimes termed, the oosperm. The study of fertilization, of the segmenta- tion of the oosperm, and of the subsequent development of the tissues and organs of the body lie strictly within the province of the science of Embryology' ; but a brief state- ment of the origin of the various tissues may with advan- tage be given here, as throwing light on the structure and relations of the organs of-the body.
Repeated cell division or segmentation in a short time divides the oosperm into a spheroidal mass of apparently similar cells : these soon arrange themselves in two dis- tinct layers, from w^hich a third intermediate layer is shortly afterward derived: the outer of these layers is called the epiblast or ectoderm, the middle the mesoblast or mesoderm, and the inner the hypoblast or entoderm; the whole trilaminar structure receiving the name of the blastoderm.
CHAPTKR XII. INTRODUCTORY. 129
From the cells which compose the epiblast are derived the following structures :
The epidermis, and its appendages the hairs and the nails, and the epithelium lining the tegumentary glands (sweat glands, sebaceous glands, mammary glands).
The epithelium of the nasal passages and the asso- ciated cavities and glands.
The epithelium of the mouth and of the glands con- tinuous therewith, and of a portion of the tongue: the taste organs : the enamel of the teeth.
The epithelium of the conjunctiva and of the glands of the eyelid, and of the front of the cornea: the lens of the eye: the retina (secondarih- as an out- growth from the brain).
The epithelium of the membranous lab\'rinth of the ear.
The epithelium lining the cavities contained in the cerebrospinal axis : the nervous tissues : the neuro- glia : the pineal body : the pituitary body.
From the cells which compose the middle layer or meso- blast are derived the following structures:
The epithelium of the urinary and genital organs (with the exceptions of the epithelium of the blad- der and urethra), including the reproductive ele- ments of both sexes.
All the muscular tissues of the body, with the excep- tions of the cells (doubtfully muscular) found in the sweat glands.
The skeletal tissues of all sorts throug^hout the bod v.
130 PART II, HISTOLOGICAL ANATOMY.
The bloo(J-vascular and lymph-vascular system : the serous membranes : the spleen and other adenoid bodies: the blood and lymph corpuscles. It should be stated that by some histologists the cells which give rise to the tissues of the first two groups are regarded as having an origin somewhat different from those giving rise to the last two : the name mesoblast has been retained by them as a collective title for the former, while for the latter the name of parablast was proposed bj^ His, and later that of mesenchyma by the Hertwigs. While this is probably true of the tissues of birds, it has not yet been proven for any mammal: and there are spe- cial reasons why it might be true in one case and not in the other.
From the cells which compose the hypoblast are derived the following structures :
The epithelium of the back of the tongue, the lower part of the pharynx, the oesophagus, stomach and intestines : that of all the glandular appendages of the alimentary canal.
The epithelium of the Eustachian tube and middle ear.
The epithelium of the larynx, the trachea, the bronchi, the bronchial tubes and the air sacs of the lungs.
The epithelium lining the urinary bladder and the urethra.
The epithelium lining the vesicular alveoli of the thy- roid body.
The concentric corpuscles or epithelial nests of the thymus.
CHAPTER XII. INTRODUCTORY. 131
Even a brief study of the tabularstatementof the origin of the tissues of the body above given will make clear the facts that most organs are made up of tissues derived from more than one of the primary tissue-layers, and that in some cases at least, tissues which are structurally continu- ous and to all appearances similar arc of difterent embry- onic origin : this is notably the case with the transitional epithelium found in the ureters and the bladder; and other instances might be mentioned.
The order of study pursued in acquiring a knowledge of the histological anatom\' of the various organs and sys- tems of the body is plainly' a matter of convenience. That which will be here pursued is one shown by experience to be desirable on some accounts : but it should be understood that it may be readily varied at will. Omitting from far- ther consideration the simpler organs already described in the preceding part in connection with the tissues which chiefl\' compose them, such as the cartilages, the bones, the muscles, etc., the various regions, systems or groups of organs will be successively described as follows:
The tegumentary system will first receive attention : this includes not onh- the skin, but also those solid append- ages, the hairs and the nails, which are derived from the special modifications of its outer layer; and those in- growths of the same la\'er which constitute the sudor- iparous, sebaceous and mammary glands.
132 PART II. HISTOLOGICAL ANATOMY.
The skin upon the enter surface of the lips is continuous with the so-called mucous membrane of its inner surface : a transition which brings us naturally to the mouth and its contents: this includes the study of the lining mem- brane above referred to; the buccal and other glands which open thereon ; the teeth and the tongue.
The mouth is the antechamber of the alimentary canal,
though it is often regarded as a part of it. Beginning with the pharynx, we naturally consider in their order the oesophagus, the stomach, the small intestine in its various regions, and thelargeintestine (including the rectum ) ; and also the glandular appendages of the canal, the liver and the pancreas.
The pharynx is not only a portion of the alimentary canal, but of the respiratory tract as well : the latter being, as embryology shows, an outgrowth of the diges- tive tube ; its study includes that of the larynx, the trachea and bronchi, the bronchial tubes in their various ramifica- tions, and the terminal sacs which make up with them the proper substance of the lungs.
The bladder and the urethra are parts of an outgrowth from the posterior region of the alimentary canal, as the respiratory tract is of the anterior. With them as median structures are closely associated the paired organs which complete the urinary apparatus: and intimately related therewith are the male and female reproductive glands, and the accessory organs connected with reproduction and micturition. This group will next be studied.
CIIAPTKR XII. INTROI^rCTORY. 133
Following the groups of organs above indicated, the circulatory system may next receive attention. This in- cludes the study of the heart, the larger blood vessels, and the greater lymphatic trunks, the smaller blood vessels and lymphatics having been already considered. Since the great serous cavities, such as that of the thorax or of the abdomen, may best be regarded as lymph spaces, the special discussion of their lining membranes may appro- priateK' be considered here.
In addition to the lymph nodes, there are found in the body larger organs apparently allied to them or derived from their modification, such as the spleen and the thymus. The name of ductless glands has long been ap- plied to these and to other soft organs of uncertain func- tion, such as the thyroid, the pituitary body, and the adrenals (the so-called suprarenal capsules). The group (to which the name of the adenoid bodies is sometimes ap- plied) is a heterogeneous one, its members in some cases having little in common : but they ma}' for convenience be considered together.
The structure of the central nervous system is at pres- ent being worked out chiefly b\' histological methods: it is not as yet alwaj's easy to distinguish between what may be regarded as physiological, and what as histologi- cal anatomy in some cases. Imperfect as our knowledge is at present, the briefest statement of its details would transcend the limits of an elementarj^ course in histo- logy ; but some knowlede of itgs most salient features is
134 PART II. HISTOLOGICAL AXATOMY.
essential: the structure of the spinal cord will be discussed, together with that of the principal regions of the brain. On account of their intimate relation to the cerebrospinal axis, the membranes w^hich invest it and which form the lining of the spinal canal will be taken up in this con- nection.
Finall}'^ the study of the central nervous axis ma}^ prop- erh^ be followed by that of the complex outlj'ing struc- tures which are the essential parts of the organs of special sense, such as the eye, the ear and the nose. Beginning our course with the common investment of the body, we close it with organs w^hich are in a great measure speciali- zations thereof.
'-ci
'^"
AXA trr
CHAPTER XIII. SKIN AND APPENDAGES. 135
CHAPTER XIII.
THE SKIN AND APPENDAGES.
The skin, which is the investing and protecting mem- brane of the bod}^ is, like other membranes found upon free surfaces, composed of two primary layers, one of which is epithelial, the other skeletal : each of them being capable of division into more or less well marked secondary layers or strata. The outer or epithelial la^'^er is known variously as the epidermis, cuticle, or scarf skin : the inner or skeletal layer as the derma, corium, or cutis (or less properly the cutis vera or "true" skin).
The epidermis can under favorable circumstances be seen even with the naked eye to be made up of two distinct layers, an inner moist or mucous layer (sometimes called the rete mucosum), and an outer dry or horny layer; and each of these layers can be resolved by the microscope into strata characterized by differences in the form and arrangement of the component cells : it may therefore be regarded as the best example of a stratified squamous epi- thelium in the whole body. Beginning with the mucous layer, we find next the corium cells which are columnar in form, and which are constantl}' undergoing division ; the new cells formed at their free extremities are at first verti- cally elongated, then polyhedral in shape, and later some- what flattened vertically : they form with the basal cells
136 PART II. HISTOLOGICAL ANATOMY.
the first or lowermost stratum of the epidermis, known as the stratum Malpighii : the elements composing this stratum have the form of prickle-cells, their numerous short processes preventing the actual contact of the sur- faces of adjacent cells, thus forming channels for the circu- lation of lymph and the nutrition of the elements : all the cells of this stratum may be regarded as living. Here and there leucocytes may sometimes be found ; they have wan- dered into the epithelium from below and occupy irregular intercellular spaces. The cells of the deepest portions of the stratum Malpighii contain pjgment^ranules, the color of the skin in different races depending chiefly on the relative abundance and color of the pigment. Delicate nerve fibrils enter this stratum of the epidermis, and, as stated in a previous chapter, Merkel has described special terminals thereto under the name of tactile cells.
As new cells are constantly formed in the deeper por- tions of the stratum Malpighii, the older cells are as con- stantly pushed farther and farther from the blood vessels of the corium which constitute their basis of nutritive sup- ply through the agency of the lymph channels already mentioned. At a certain distance, which varies in differ- ent portions of the body, they begin to yield more con- spicuously to the mechanical pressure from without, be- coming more flattened in form, and at the same time to undergo degenerative changes, granules of a fat-like com- pound termed eleidin appearing in their substance in great numbers. There is thus formed a definite layer never more than a few cells deep to which the name of stratum gran- ulosum is applied: the stratum granujosum and the
CHAPTER XlII. SKIN AM) APPENDAGKS. 137
Stratum Malpigliii together make up the mucous layer, and in most parts of the body the greater portion of the epidermis.
ImmL'diately above the stratum granulosum and sharply distinguished from it is a thin layer of cells which resemble those of that stratum in b:ing compressed in form, but differ from them in greater homogeneity of substance and therefore in transluccncy : this is the stratum lucidum, the lower of the strata of the horny layer. According to Ranvier the formation of eleidin is followed by its trans- formation into keratin, the characteristic substance of horn, nails, claws, etc., which are, as we shall see, in the main developments of the stratum lucidum.
While the stratum lucidum is constantly receiving acces- sions from the cells of the stratum granulosum upon its lower or inner side, it is as constantlv undergoing mod- ifications on its upper or outer surface ; and the line which marks this transformation is equall}' well defined in either case, the stratum lucidum remaining like the stratum granulosum of nearly constant thickness and definite lim- itations. Thej:ells as they pass from its outer surface be- come somewhat swollen and more loosely disposed, form- ing a layer in which the outlines of the individual cells may be clearly discerned : the nuclei have disappeared and all traces of protoplasmic structure. This layer is some- times designated the stratum corneimi: in places where there are no hairs upon the surfacp it can be divided into two layers, the lower, or stratum epitrichium, consisting of thicker cells, more loosely disposed, and the outer or
138 PART II. HISTOLOGICAL ANATOMY.
stratum Stiuamosum of thin closely oppressed scales, which are eventually and constantly cast oflP. The stra- tum lucidum and stratum corneum (with the subdivisions of the latter) together make up the horny layer.
The epidermis varies greatly in thickness in different parts of the body: it may be no more than a tenth of a millimetre in thickness, or may be as much as a millimetre or more in places where the pressure ai»d friction upon the surface is greatest, even under ordinary circumstances : and under special conditions the cells of the stratum squa- mosum of the palms of the hand or the soles of the feet, instead of being exfoliated, may become impacted to form la^-'ers two or three millimetres in thickness. The external surface shows irregularities which in some measure corres- pond to the conformation of the corium below, but as a rule differ therefrom by their less extent : the surface of the corium is, as we shall see, covered by projections known as papillae, into the spaces between which the stratum Malpighii descends : but this stratum varies correspond- ingly in thickness to such an extent that the papillary elevations are reduced on its upper surface to mere un- dulations, to which the remaining strata conform. The deeper lines upon the surface of the epidermis corre- spond to definite folds in the corium.
Like the epidermis, the corium can be divided into strata, which are not, however, so clearly defined in the case of the latter as of the former, consisting as they do in modifica- tions of a fibrous layer. Upon the surface the fibres are
CHAPTER Xm. SKIN' AND APPENDAGES. 139
fine and very eloscly felted, forming a thin, homogeneous and almost translucent stratum, which may be distin- guished as the basement membrane of the epidermis. Its surface is closely beset with minute projections, which interlock with corresponding irregularities at the lower extremities of the columnar cells which lie at the base of the stratum Malpighii.
Beneath the basement membrane above referred to the corium consists of a felted mass of rather coarse bundles of white fibrous tissue, reinforced by elastic fibres in vary- ing quantity, and containing in some localities a greater or less amount of smooth muscular fibres, notably in con- nection with the hair-follicles. The outer portion is denser, the bundles being smaller and more closely felted and h'ing in the main parallel to the surface: it bears the papillae already- referred to in another connection, and has therefore been variously designated from its structure and conformation as the dense stratum or the stratum papil- lare: below it passes rather abruptU', but with no well- defined line of demarkation, into a region in which the bundles are coarser and less numerous, being more looseh' and irregularly disposed : from its structure this is known as the stratum reticulare. The papillae are conical or club-shaped projections upward of the dense layer: in man)' cases they are more or less subdivided at their free extremities, in which case the}' are known as compound papillae: they are most abundant in the regions of the skin where the sense of touch is most acute, and also in a modified form make up the dermal portion of the nail- bed. In some places, notably on the tips of the fingers,
140 PART 11. HISTOLOGICAL ANATOMY.
they are arranged in single or double rows along ridges of the corium, forming the familiar patterns readily seen with the naked eye: these patterns remain constant for each digit of each individual throughout life, and have therefore been used as marks of identification. Where best devel- oped the papillae are from an eighth to a fourth of a milli- metre in height.
The connective tissue corpuscles of the corium, like those of other fibrous membranes, are small, and as a rule com- pressed, their long axes lying parallel to the direction of the bundles with which they are associated : leucocytes are also present, as are pigment cells in moderate numbers.
' The mcvshes of the stratum reticulare not unfrequently contain clusters of fat-cells of varying extent : immedi- ately beneath, in most portions of the body, we come to a la^^er of larger or smaller ovoid or polj^hedral fat-lobules separated by a coarse meshwork of fibre bundles. This layer, frequently termed the panniculus adiposus, is often spoken of as subcutaneous : it is, however, in many cases no more clearly marked off from the stratum reticulare than is the latter from the stratum papillare : it may with good reason be regarded as a portion of the skin, and as such be distinguished as the stratum adiposum: whether it shall be regarded as cutaneous or subcutaneous is largely a matter of definition ; the fact of its association with the other structures of the skin as a part of the tem- perature regulating mechanism of the body is important. Below it passes into the loose layer of subcutaneous areo- lar tissue, which, save in a very few localities, intervenes
CHAPTER XIII. SKIN AND A ITKNDACES. 14-1
between the skin and the structures beneath, permitting of its more or less free movement upon them.
The skin, exclusive of the stratum adiposum, varies in thickness from h^lf a millimetre to two or three milli- metres: and may even occasionally be as much as twice the latter quantity in thickness. It is thickest upon the shoul- ders and back.
The arteries which pass to the skin branch and subdivide in the subcutaneous tissue, the small vessels thus formed proceeding toward the surface : on their way they give off twigs which supply the fat-lobes above mentioned, and the hair follicles, sweat glands, etc., presently to be described. As they approach thesurface they branch and anastomose, and finally break up into a meshwork of capillaries, situ- ated j ust below the basement membrane, suppK'ing the pap- illae together with the other portions of the dense layer. The capillaries unite to form a superficial venous network, the larger veins arising therefrom passing to the deeper por- tions of the skin in such a way as to accompany the ar- teries in large measure.
Lymphatics arise in the spaces between the bundles of fibres which make up the dense layer. These are so dis- posed as to form a lymphatic network just below the superficial capillary network just mentioned : lymphatics have also been demonstrated in some of the larger papil- lae. A second network is said to exist in the deeper por- tion of the corium, the two communicating freely with each other and with the subcutaneous lymphatics.
142 PART 11. HISTOLOGICAL ANATOMY,
The nerve supply of the skin varies greatly in different pot' tions of the bod\'. Like the blood vessels, the nerves form plexuses in the papillary region, the meshes immediately beneath the epidermis becoming ver^^ fine and close : from the fibres composing them fibrillae are sent up into the stratum Malpighii in the manner alread\^ described. As stated in a previous chapter, it is not yet certain how these fibrils terminate.
In certain (if not all) portions of the bod}', more or fewer of the papillae are supplied with nerve fibres which there terminate in the so-called tactile corpuscles of Meiss- ner: these are most abundant in the papillae of the fingers and toes. Other fibres terminate in end bulbs, while man}' others are distributed to the hair follicles and the s-weat glands. In the deeper or subcutaneous region fibres are found which end in Pacinian bodies.
The glands of the skin are chiefly of two kinds: the sudoriparous or sweat glands, distributed in Yarjmg abundance over the whole surface of the body ; and the sebaceous glands, found chiefl}' at the bases of the hairs. In certain localities specialized glands are found which maj' be regarded as modifications of one or the other of these two types.
The sudoriparous glands are situated in the reticular stratum and the outer portion of the adipose layer : they are spheroidal bodies, from one-half of a millimetre to two millimetres in diameter, each consisting of a small tube coiled into a ball : from this the tube proceeds (as the duct of the gland) with slight deviations from a direct
CHAPTER XIII. SKIN AND A PrHNDAC.ES. 143
course through the coriuni, at whose surface it becomes continuous with a closely coiled spiral opening through the epidermis ; the whole structure consisting, therefore, of a tubular depression of the surface, whose basement mem- brane is a continuation of that which forms elsewhere the outer limit of the cprium, while its lining epithelium is a direct continuation of the epidermis.
The glandular (or the greater part of thecoiled portion) is considerably larger than the rest of the tube, being sixty or seventy micra in diameter: immediately upon the basement membrane in this region is found a simple layer of elongated elements resembling smooth muscular fibres in appearance, and commonly regarded as such ; their long axes are parallel with the direction of the tube. Upon this, and surrounding the lumen of the tube, is a layer of col- umnar glandular cells about fifteen micra in diameter, which are frequently pigmented. The duct is from twenty to thirty micra in diameter: it consists of a basement membrane upon which is seated a stratified epithelium of a very simple order: immediately next the membrane is a layer usually two cells thick, composed of polvhedral cells ; while upon this is a single layer ol flattened cells often designated the cuticle. On reaching the epidermis the basement membrane is, of course, continued into that of the surrounding region : the deeper portion ot the epithe- lium becomes continuous with the stratum Malpighii : the cuticular layer lines the passage through the epidermis, which is from the first over twice the diameter of the lumen of the duct, and in the stratum corneum flares to form a trumpet-shaped opening at the surface.
144 PART II. HISTOLO '.ICAL ANATOMY.
Each sweat gland has a network of capillaries which penetrates the coil, following the interstitial connective tissue ; and is also provided with a proper nerve supply. The number of the glands varies in different portions of the bod}' from four or five hundred to the square inch (or more than a millimetre apart) upon the back of the neck and trunk to five or six time as many (or less than half a millimetre apart) on the palm of the hand or the sole of the foot. Those of the armpits are quite large, as are those at the root of the penis in the male and on the labia majores in the female. About the anus are found glands identical with the sweat glands in structure, but still larger than those just mentioned : they are sometimes dis- tinguished as the circumanal glands. In the larger sudor- iparous glands (and sometimes in the smaller) branching of the coiled portion occasionally occurs : and in some cases the duct is bifurcated, either before or after it enters the epidermis. The ceruminous glands of the external auditory meatus of the ear resemble the sweat glands in their structure, though differing from them to a marked degree in the nature of their secretion.
The sebaceous glands occur, as has been stated, at the roots of the hairs : they are also found upon the labia minores and the prepuce and occasionally in other hairless localities. Each consists of a short duct usually opening into the outer portion of a hair follicle and connecting in- ternally with the adjacent secreting portion: this commonly consists of from three or four to eighteen or twenty sac- cules, or, rarely, a single spheroidal sac. The basement membrane, which is continuous with that of the corium,
CIIAPTHR XIII. SKIN AM) APPENDAGES. 14-')
supports a conipouiul layer of polyhedral cells, wliich in the glandular portion are constantly being renewed by the division of the cells next the basement membrane. The cells thus cast off undergo a sort of fatty degeneration, the interior of. the gland thus becoming filled with a semi- Huid mass consisting of oil droplets, fragments of broken- down cells, etc., which is discharged upon the surface through the duct. The Meibomian glands of the eyelid are to be regarded as enlarged and modified sebaceous glands. The mammary glands are also to be regarded as originally derived from the modification of bodies essen- tiallv similar to sebaceous glands and are properly to be considered as tegumentary organs. On account of their size and complexity, however, their structure will be bet- ter understood after thediscussionof the general structure of glands in a subsequent chapter. They will, therefore, be discussed later in connection with the female reproduc- tive organs.
Both sudoriparous and sebaceous glands are formed by solid ingrowths of the epidermis which in the case of the former penetrate the corium, the cavity being for-med in the centre of the club-shaped mass and later communicat- ing with the surface, while the inner end becomes coiled into the shape characteristic of the adult structure. In a similar manner the mass of cells which is the precursor of the sebaceous gland becomes divided into lobules, the fatty transformation of the central cells already described be- ginning in them and advancing along the axis of the pedi- cle, which thus becomes converted into the duct.
l-iG PART II. HISTOLO ICAL ANATOMY.
While the hairs are practicalh^ outgrowths from the skin thej^ are in reality derived from the modification of ingrowths resembling in some respects those which give rise to the sebaceous and sudoriparous glands. A hair is a mass of epidermal cells which cohere together to form a cylindrical or more or less flattened rod : these cells are formed upon and around a papilla situated at the bottom of a depression of the corium known as a hair-foUicle, whose sides are lined with a specially modified layer of epi- dermal cells, continuing the Malpighian layer of the epi- dermis and the stratum' lucidum to the region around the papilla at its base. The portion of the hair which pro- jects beyond the surface is termed the shaft: that which is contained in the follicle is known as the root: the epider- mic portion of the follicle surrounding the root is com- monly called the root-sheath.
The fully formed hair, whether within or without the follicle, has on its surface a layer of imbricated scales, with their free edges directed toward the outer end of the hair ; this layer is known as the cuticle: on the form and ar- rangement of the free margin of thecuticular cells depends the pattern of the superficial markings seen on the hairs of man and of different animals. Immediately beneath the cuticle lies the cortical substance of the hair: this is com- posed of greatly elongated fusiform cells in which traces of the nucleus are still visible, though the body of the cell has largely imdergone horny transformation : these cells are so closely united that their limits are not ordinarily distinguishable, the cortex appearing to be made up of elongated fibres ; it is to the color of the cortical cells that
IokJx.IjU.^-'^-'^^^ "
ClIArTKK XIII. SKIN AND AI'I'HNDAGIiS. 14-7
the color of the hair is largely tlue. The cortical substance in some hairs extends clear to the centre: in others it sur- rounds (as its name imj)lies) an axial mass ofcuboidal or polyhedral cells, the medulla: the bodies of these cells usually contain small air-vesicles, rendering the medulla white by reflected light. Such air s])aces may also be present in the elongated cells of the cortex.
The formation of the hair at the papilla will be more clearly understood if its description is preceded b\' that of the follicle with whose epidermis it is continuous: this, like the structure of which it is a modification, consists of a fijjrous or dermal portion and an epithelial or epidermal por- tion. The dermal portion maybe resolved into three layers: of these the outerfibrouslayerconsistsof bundles of white fibrous tissue running chiefly in the direction of the follicle and containing numerous connective-tissue corpuscles ; it resembles the corium in most respects, save in the absence of elastic fibres : the middle layer, sometimes called the muscular layer, consists of connective tissue whose fibres run transversely, and of transversely-disposed elongated cells with rod-shaped nuclei which much resemble smooth muscular fibres and have been described as such, but which may perhaps be regarded as modified connective-tissue corpuscles; this layer extends from the bottom of the fol- licle to the point where the sebaceous gland opens, beyond which it is wanting: the inner or hyaline layer, or glassy membrane, as it is sometimes termed, is thin, homogene- ous and transparent ; it corresponds to the basement membrane of the corium.
148 PART II. HISTOLOGICAL ANATOMY.
The epithelial portion of the follicle, while it is structur- ally free from the hair within, is generally brought awa\'- with the latter if it be pulled out of the skin during life: it is therefore commonly termed the root-sheath of the hair. It consists of two layers, comparable in a general way to the mucous and the horny layers of the epidermis. The outer of these, or outer root-sheath, is much thicker than the inner, being a layer several cells deep ; of these the cells next the hyaline layer (or basement membrane) are columnar, like those at the base of the stratum Mal- pighii ; while those within are polyhedral prickle cells like the majority of those in the stratum just mentioned.
The inner layer of the epithelium, or inner root-sheath, may be divided into three strata: the outer of these, known as Henle's layer, is a single layer of flattened cells of a horny appearance, in which nuclei are not distinguish- able : wnthin this is Huxley's layer, composed of polyhe- dral cells with small nuclei, the layer being two or three cells deep: this is lined on the inside by the cuticle of the root-sheath a single layer of flattened cells which are im- bricated in a manner similar to that of the cells of the cuticle of the hair, but in the opposite direction, the free edges of the cells in question being directed toward the bottom of the follicle ; as a consequence, their edges inter- lock with those of the cells on the surface of the hair. The name of inner root-sheath is sometimes restricted to the part which comprises Henle's and Huxley's layers, the two becoming confluent at base of the follicle ; the cuticle of the root-sheath is in such cases described as third layer thereof; but this is entirely a question of names: no satis-
CHAPTHK XIII. SKIN AM) A I'I'K.\I)A(iKS. 14-9
factory attem])t has been made to identify either of these layers with the various strata of the skin in the way that the outer root-sheath can l)e identified with the stratum Malpighii.
As we pass to the base of the follicle the outer fibrous layer is continued around its curving extremity" to be con- tinued into the hair papilla, a large club-shaped papilla which projects upward into the lower end of the follicle: the middle layer becomes thinner and terminates near the lower end of the follicle : the hyaline layer, now resting on the fibrous layer, invests the surface of the papilla as a basement membrane. The whole follicle is slightly enlarged or bulbous at the base; the thickening being chiefl\'due to an enlargement of the epithelial portion of the follicle and of the lower end of the hair itself The outer root-sheath is continued downward with little change, the columnar cells at its base passing around the curve of the valley which surrounds the base of the papilla and over the sur- face of that body, acting here as elsewhere as the genera- tive layer of the epithelium : the poU'hedral cells become confluent with those which form the layers of Huxle>^ and of Henle, these latter having previously become merged into one, and with the cells of the cuticle of the root-sheath. The valley at the base of the papilla is thus filled with a mass of newly formed cells, which, as they are rapidlv multiplied, are pushed off as a cylindrical mass, the hair, from around the papilla. The medulla of the hair is formed from the cells developed upon the upper end of the papilla itself, and is in a certain sense the continuation of the col-
150 PART II. HISTOLOGICAL ANATOMY.
umnar layer of the outer root-sheath : the cortex of the hair represents the polyhedral layer and the la^^ers of Hux- ley and Henle ; while the cuticle of the hair corresponds to that of the root-sheath.
The outer fibrous layer of the hair follicles is richly sup- plied with blood vessels and nerves : some fibres of the lat- ter pass to the outer root-sheath, where they appear to terminate among the epithelial cells in a manner similar to that found in the stratum Malpighii of the epidermis, chiefly in the immediate vicinity of the sebaceous glands. In some of the lower animals large hairs, chiefly about the face, are provided with special forms of nerve terminals : such hairs are termed tactile hairs.
The hair follicles are rarely vertical to the surface of the skin, the degree of their obliquity varying in different local- ities and, in consequence, the position of the hair upon the surface. Many hairs have small bundles of smooth mus- cular fibres passing from a point on the papillary layer of the corium near the opening of the follicle and on the side toward which the hair is inclined, to be inserted in the outer fibrous layer of the follicle near the bulb. The con- traction of these muscles, known as the arrectores pili, tends to erect the hairs.
Hairs are formed as solid club-shaped downgrowths of the stratum Malpighii of the epidermis, which meet with specially formed papillae around which the hair-bulb is moulded : the young hair is developed as a conical mass
CHAPTKK XIII. SKIN AND APrKNDAOES. 151
above the pa])illa, the solid epithelial plug first formed undergoing sebaceous degeneration in its centre and thus permitting the escape of the hair: its lateral portions be- come the root-sheath, outgrowths therefrom giving rise to the sebaceous glands. When a hair ceases to grow, the papilla gradually disappears and the hair finally drops out of the follicle: this may or may not have been pre- ceded by the formation of a new downgrowth from the bottom of the follicle and the development of a new pa- llia, thus giving rise to a replacing hair.
The nails, like hairs, are masses of epidermal cells, con- sisting chiefly of a thickened and otherwise modified ex- tension of the stratum lucidum. Each nail can be regarded as composed of three portions : the free margin, in which growth has entireh- ceased, the nail-body, which consti- tutes its greater portion, but which receives but slight ad- ditions to its under surface, and the nail-root, which is the region of greatest increase. The body of the nail is con- tinuous below with a modification of the stratum Mal- pighii, which rests upon a modified portion of the corium, called the nail-bed: laterally this fibrous layer is folded upward to form the lateral nail-grooves, and posteriorly upward and forward to form the posterior nail-groove ; the lower portion of which is termed the nail-matrix, in- cluding the whitish curved area at the base of the nail known as the lunula.
The nail-bed and nail-matrix are continuations of the corium which has become highly vascular and is well sup-
152 PART II, HISTOLOGICAL ANATOMY.
plied with nerves : the papillae upon its surface are simple and closelv crowded tog-ether: as far as the outer marmn of the lunula thev show no definite arrangement, but throughput the nail-bed proper are arranged in longitud- inal row^s, their extremities inclining toward the free end of the nail : the^-are so closeh'set in the rows as to appear to be confluent in ridges, which are sometimes said to re- place them. Below, the nail-bed is connected with the dis- tal extremit}' of the last phalangeal bone b\' numerous strong bands of fibrous tissue: as it passes around the mar- gin of the nail to enter the walls of the nail-grooves, it assumes the structure commonh^ characteristic of it.
The stratum Malpighii is by some histologists defined as a part of the nail-bed : it is a question of names merely, but it is perhaps better on the whole to divide the two re- gions by the natural boundary between dermal and epi- dermal structures. The columnar cells of this stratum are close]}'- packed together, and multiply rapidly, partic- ularh'- in the region of the matrix: very few polyhedral cells are to be seen, the newly formed cells passing over rapidly into the substance of the nail without an inter- vening stratum granulosum. Like the stratum lucidum, the bod}^ of the nail consists of flattened horny cells, in which traces of nuclei can be seen after dissociation. Dur- ing foetal life the nail is invested by the stratum epitrich- ium, traces of which overlie its margins at birth.
CIIAPTHR XIY. MOl^TH AND CONTENTS. 153
CHAPTER XIV. THE MOUTH AND ITS CONTENTS.
The mouth is formed b}^ an ingrowth from the surface of the head ; its lining is therefore epiblastic in origin and directl\' continuous with the epidermis: the cavity of the mouth does not at first communicate with that of the phar- 3'^nx, but the two are connected later by the perforation of their common wall at the fauces. Within the cavity of the mouth are found the jaw arches, covered by the gums and bearing the teeth, and the tongue, which rises from its floor; and in the so-called mucous membrane which lines it throughout are found the labial, buccal, palatal and lin- gual glands : while other and larger glands more remoteh- situated discharge their secretion into the cavity of the mouth b}'^ means of ducts.
As we pass from the skin upon the outer surface of the lip to the mucous membrane which is found upon its inner surface marked changes are to be noted in both the epithe- lial and the fibrous layer. Near the margin of the lip the hair follicles are wanting, though sebaceous glands are present ; the derma becomes thinner and highly vascular ; while the epidermis becomes much more transparent, al- lowing the red color of the blood in the dermal capillaries to shine through. As the transition is made from a sur- face constantly dr}- from exposure to the air to one con-
154 PART 11. HISTOLOGICAL ANATOMY.
stantly moist, the stratification of the egithelium becomes less distinct: and on the inside of the mouth is found a la^^er of protoplasmic cells corresponding to the stratum Malpighii passing by insensible gradations into a layer of hornj^ cells, flattened, and with small nudei, that are con- stantly being exfoliated, corresponding to the stratum cor- neum : all trace of the inter veniiig stratum granulqsum and stratum lucidum disappearing The cells at the base of the layer are columnar, like those in the correspond- ing portion of the epidermis, and, like them, are con- stantly forming new cells to replace those lost from the outer surface.
Underneath the epithelium is the fibrous J[ayer, corres- ponding to the corium, to w^hich the name of mucosa or mucous membrane is sometime restricted : it is in most cases thinner than the corium, but bears upon its surface numerous papillae : below it breaks up into looser connec- tive tissue as a rule, the meshes being occupied by the glands of the mucosa and by fat lobules : to bhis looser subjacent tissue the names of submucosa is applied: in the region of the fauces, the soft palate, and the uvula ad- enoid tissue is present in great abundance in the raucous membrane ; a feature never found in connection with the corium of the skin.
Where the mucous membrane invests the hard palate, and where it passes over the arch of either jaw to form the gums, the fibrous layer becomes firm and dense, glands and fat lobules alike being absent or very sparingly pres- ent, and the deeper portion becomes directly continuous with the periosteum of the subjacent bone. The papillae and the investing epidermis of the dorsal surface of the
CHAPTKK XTV. MOITII AM) CONTKNTS. 155
tongue undergo special modifications best described in connection with that organ.
The term mucosa, or its equivalent, mucous membrane, above applied to the lining of the mouth, is also used to designatethe lining of all those cavities which communi- cate directly or indirectly with the outside of the body (with the exception of the abdominal cavity of the female). It thus includes the investment of the nasal passages and the associated sinuses, the respirator\' tract, the oral cav- ity and the alimentary canal, the urinogenital tract, and the middle ear with the Eustachian tube. B\' some histol- ogiststhe term is even applied to the lining of the hair folli- cles and the dermal glands. Excluding the latter, it may in general terms be described as consisting, like the skin, and as seen in the lining of the mouth, of an epithelial and a skeletal portion. The epithelium of the mouth and of the nasal passages is epiblastic in its origin : that of the kid- neys and genital glands, and of their proper ducts, meso- blastic : that of all the rest of the surfaces named hypo- blastic. It is to the regions lined with h3'poblastic epithe- lium that the term mucosa is chiefly applied.
The epithelium of a mucous surface ma\' be simple and flattened, polyhedral, or columnar; or transitional; or stratified. The skeletal layer consists usnallv of a base- ment membrane (sometimes termed a membrana propria,) which may be either a homogeneous layer of closely felted fibres, or an endothelioid layer of connective tissue cor- puscles ; beneath this is the felted fibrous layer termed va- riously the corium, the stroma, or the tunica propria, or, by some, the mucosti in a limited sense. It consists of a
156 PART II. HISTOLOGICAL ANATOMY.
thicker or thinner layer of rather loosely felted bundles of white fibres, rich in blood vessels and in some cases having its surface raised into papillae. Elastic fibres are some- times present in such great numbers as to form a definite elastic layer, and at other times are almost entirely want- ing. Adenoid tissue may be present in varying quantity, sometimes forming definite nodules or clusters of nodules. Smooth muscular fibres may also be present, forming a stratum in the deeper part of the membrane one or more layers in thickness and known as the muscularis mucosae. In exceptional cases, such as that above described in con- nection with the hard palate and the jaw^ arches, the mu- cosa is firmh' united to the subjacent structures : as a rule the deeper portion passes over into a la^^er of loose areolar tissue knowm as the submucosa, thus permitting of the free movement of the mucosa on the structures beneath.
Where the epithelium of a mucous surface is columnar a varying proportion of the elements have the form of gob- let cells, as described in a previous chapter : these are per- haps the simplest form of special secreting organs found in the human body ; and are sometimes spoken of as uni- cellular glands : the term gland being often loosely em- ployed to designate any secreting organ and, indeed, some organs that are not at all secretory in function.
Increase of secreting surface is obtained by ingrowths of the epithelium which penetrate the fibrous layer to a greater or less extent, in a manner similar to that de- scribed in connection with the skin ; like those referred to these ingrowths may be either tubular or saccular. Where
CHAPTKR XIY. MOl TH AND CONTENTS. 157
such ingrowths are not sub-divided internally they are commonly called simple tubular or simple saccular glands, as the case may be. A distinction of importance, both structural and physiological, ought, however, to be noted in this connection. Some such simple secreting or- gans are lined with cells that are similar in form and alike secretory in function throughout their whole extent : while in others the work of secretion is restricted to specialized cells in the epithelium of the deeper portions, that which is found upon the portion next the surface having lost its secretorj' activity' and become modified to form the lining of a conducting tube through which the secretion of the deeper portion is discharged. This dis- tinction can be kept in view if we always apph' to the structure which secretes throughout its whole extent the name of a follicle, and define a gland as a secreting organ provided with a duct.
Glands, as thus defined, may be either tubular or sac- cular, and either simple or, by the subdivision of the se- creting portion, multiple or compound: the secreting por- of a simple gland is called the fundus: those of a com- pound gland are called acini or alveoli. Where the final divisions of a compound gland are saccular in form, the gland is frequently designated as acinous or racemose ; where they are elongated, the gland is called compound tubular; and where both forms of alveolus are present the term acino-tubular is applied. B\' many histologists, however, the distinctions which these terms imply are re- garded as of questionable value.
Follicles, simple glands, and the smaller compound glands
158 PART II. HISTOLOGICAL ANATOMY.
rareh' penetrate deeper than the submucous or subcutane- ous tissue of the membrane Irom which they are derived. Large compound glands, on the contrar3^ are usually sit- uated at some distance from the surface where their secre- tions are discharged. Glands which are thus situated out- side of the organs to w^ich their ducts lead are called ex- trinsic, the term intrinsic being applied to those contained Avithin the organs in question.
With the exception of the tegumentary glands discussed or mentioned in the preceding chapter, all the glands of the body open upon mucous surfaces ; and with the far- ther exception of the kidne3'-s and the genital glands are derived from the modification of a mucous membrane. The secretions discharged b}^ them have always one or both of two functions : to lubricate and preserve the sur_ face in question, which is a general function, or to act as ferments upon food stuffs taken into the body, which is a special function restricted to the alimentary tract. Two principal types of glands are therefore distinguishable both structurally and functionally. Those of the first sort are known as mucous glands; those of the second (from the more water}^ character of their secretions, but less properly) as serous glands. In the case of some com- pound glands some of the alveoli are of the mucous and some of the serous t3^pe: such are designated mixed, glands.
In glands of the mucous type the acini or alveoli are lined with polj^hedral cells which do not extend quite to the centre of the cavity, thus leaving a well defined central
CHAPTER XIV. MOl'TH AND CONTEXTS. '[~>9
opening or lumen: the free extremity of the eell is trans- parent and does not stain readily with most reagents, while the ])rotoplasm and the somewhat flattened nu- cleus which it surrounds are crowded down to the base* of the cell; ijn other words, the general appearance is like that previously described as characteristic of goblet cells, which, it should be recalled, are mucigenous in function. In addition, there may in some cases be seen between the glandularcells and the basement membranecrescent shai)ed groups of granular cells which stain deeply : these were described independently by two observers, who gave them names associated with their form ; they are therefore called the crescents of Gianuzzi, or the demilunes of Heidenhain: their nature and functions are not yet fully understood: the constituent cells (sometimes solitary ) are called marginal cells.
In glands of the serous type the secreting epithelium consists of polyhedral cells which when at rest extend clear to the centre of the alveolus: a distinct lumen cannot, therefore, be recognized. The whole body of the cell is granular, the substance which is to become the character- istic secretion being thus stored up in the protoplasm : the nucleus is spherical, and situated near thecentre of the mass ; and the whole cell stains readily. No trace of any- thing like the crescentic cell-masses above mentioned or of marginal cells is to be found in any serous gland.
The appearances above described are those seen in sec- tions of glands previousK'hardenedby reagents, and taken
160 PART II. HISTOLOGICAL ANATOMY.
trom organs which had not been actively stimulated im- mediately before the preparation was made. If a piece of the fresh gland be examined in blood serum the mucigen is seen in the case of the mucigenous cells to be present in the form of very large granules : while the ferment secre- ting cells are so swollen that not only the lumen of the acinus but also the outlines of the cells are obliterated : otherwise the appearances are much as described above. As the result of prolonged stimulation the mucigenous cells discharge their secretion, and the nuclei approach a central position : while the cells of the serous type become smaller after discharge, and a distinct lumen becomes vis- ible in the acinus. The two types of alveoli, become therefore much more nearly alike, though not so much so as to prevent their distinct recognition.
It is -in the mouth that we first meet with a mucosa ; and its discussion has therefore been deferred until after a description of the lining membrane of that cavity. The glands of the mouth also furnish usw^ith examples of both mucous and serous, and both intrinsic and extrinsic glands.
The intrinsic glands of the mouth are the labial, buc- cal, palatal (including those of the uvula), and lingual. These are all racemose glands situated in the submucosa, with the exception of the lingual, which lie between the muscular bundles of the tongue. They are all of the mu- cous t3^pe save those on the posterior part of the tongue, which are serous. Their secretion contributes to form the saliva of the mouth.
CHAPTER XIV. MOUTir A.NT) CONTENTS. 101
The extrinsic glands of the mouth are those commonh' referred to under the name of the salivary glands: they are dcslpiatcd, from their positions as tlic sublingual, the submaxillary, and the parotid glands: the first- named are doubtfully to be called extrinsic, on account of their structure and position, since they consist in each in- stance of a group of small glands opening by several ducts, and are situated just beneath the mucous membrane on either side of the base of the frenum of the tongue. The nature of the salivary glands differs in different mam- mals: in man the sublingual and submaxillary are mixed glands, and the parotid is of the serous type.
The acini or alveoli of one of the larger or extrinsic salivary glands vary greatl^^ in form from flask-shaped sacs to wavy, contorted, or even branched tubules: the basement membrane is reticulated and the epithelium varies with the type of gland in question. Each alveolus leads into a tubule smaller than itself known variously as the ductule, the connecting or intermediate tubule or the intercalary duct: the basement membrane of the duc- tule is continuous with that of the alveolus; its epithelium consists of a sjngle layer of flattened cells. The ductules of a number of alveoli enter a common tube known as an intralobular duct or salivary tube of Pflueger, the ajveoli and ductules with the duct just mentioned together making up a lobule. An intralobular duct consists of a basement membrane continuous with that of the ductules, and a single la3^er of columnar epithelial cells : the latter have spherical nuclei situated near the centre of the cell ;
h^UJJrOJr di^cli r (^J ducJr
162 PART II. HISTOLOGICAL ANATOMY.
the outer extremit}^ of the cell is finely granular, while that next the basement membrane is vertically striated. The basement membrane of the alveoli, the ductules and the intralobular duct alike rest upon the interstitial con- nective tissue, which contains rich networks of capillaries surrounding the alveoli.
The intralobular ducts lead into larger conducting tubes known as interlobular ducts, around which the lobules are aggregated into lobes, their boundaries within the lobe being marked by septa of fibrous tissue. The interlobular lead into the interlobar ducts, which finally enter the chief duct of the gland. The larger ducts have beneath the basement membrane a definite fibrous layer which in some cases contains smooth niuscular fibres : the epithe- lium is columnar and simple, save in the largest ducts, in which a layer of polyhedral cells lies betw^een the colum- nar cells and the basement membrane.
The interstitial connective tissue which fills the spaces between the alveoli passes into thin laminae of fibrous tis- sue which separate the lobules : these septa are again con- tinuous with stouter structures of the same nature which lie between the lobes of the gland. The interlobar septa are continuous internally with the stroma of connective tissue which immediatelj^ surrounds the proper fibrous tunic of the chief duct and its principal subdivisions, and externalh^ with a membranous layer which surrounds the whole gland and is known as its capsule. The place where the chief duct leaves the interior of the gland is known as the hilum: the nerves, arteries, veins, and lymphatics of the gland also enter or leave at this point,
CHAPTER XIV. MOITH AND CONTENTS. 1 Orj
the capsule here becomino^ continuous with the stroma of fibrous tissue (above mentioned as surrounding the duct and its branches) in which they He, and in which occa- sional small nervous ganglia may be found.
The saliva contained in the mouth is a mixture of the secretions of the various intrinsic and extrinsic glands. With the nature and properties of the fluid itself we are not here concerned : it constantly contains, however, certain histological elements which may with propriety be men- tioned in this connection. The most abundant of these are the scpiamous cells which are constantly being exfol- iated from the surface of the stratified epithelium of the mouth : the}' occur singlj^and in patches; and when found in the latter form the overlapping of their bevelled edges can be plainU'seen: the nuclei arc small and flattened, and stain readih'. Less numerous, but quite abundant in the saliva from the back part of the mouth are the so-called salivary corpuscles, spheroidal bodies but little larger than colorless blood corpuscles, each containing one or two spheroidal nuclei and numerous minute granules which exhibit a constant dancing movement within the interior of the cell. The salivary corpuscles are really modified leucocj'tes that have escaped into the oral cavity from the tonsils or similar adjacent structures, and have become swollen by the imbibition of the water}' saliva; the gran- ules of the protoplasm being suspended in the imbibed fluid and exhibiting Brownian motion in consequence.
The mouth contains certain organs by means of which
164 PART II. HISTOLO UCAL ANATOMY.
the food is masticated and prepared for swallowing by the thorough admixture of the saliva. These are the teeth and the tongue. A tooth is a vertical]}^ elongated mass of the tissue mentioned in a previous chapter by the name of dentine, having a solid free portion, the crown, project- ing above the gum and covered with a layer of enamel: and a hollow portion, the fang, imbedded in a socket in the jaw known as an alveolus; the outer surface of the fang is covered with a layer of cementum, and the inter- nal cavit}^ occupied by the pulp. The slightly constricted region where the crown and the fang meet and the tooth pierces the gum is called the neck of the tooth.
The pulp which fills the central cavity of the tooth is a mass of connective tissue approaching in character more nearly to the gelatinous tissue of the embryo than any other structure in the adult human body, its soft and spar- ingly fibrillated matrix containing numerous branching corpuscles whose processes are connected. It contains a rich network of bloodvessels, and a small bundle of nerve fibres, entering its substance through the minute canal at the tip of the fang. Toward its surface the corpuscles rapidly increase in number and in size, forming a superfi- cial layer of crowded cells much resembling a columnar epithelium : these are the odontoblasts ; they are in all probability directly associated with the formation of den- tine.
Like osseous tissue, dentine is characterized by a lamel- lated and calcified matrix traversed by canaliculi : it differs essentially from the substance of bone, however, in the mode of calcification and the definiteness of the lamel-
CHAPTKR XIY. MOl'TH AND CONTEXTS. 165
lae, and particularly in that the characteristic corpuscles associated with its formation, the odontoblasts, do not, like the osteoblasts of bone tissue, become imbedded be- t\veen the lamellae, but remain situated upon the inner surface of the mass, their long and sparingly branched processes, the fibres of Tomes, occupying the canals, called dentinal tubules, which traverse the dentinal lamellae. Calcification takes place, at least in the outer portion of the mass, by the deposition of globular nodules of lime salts; these at first do not occupy the whole of the matrix, leaving numerous irregularly stellate spaces, the inter- globular spaces, the outer layer in which they abound being known as the granular layer. The interglobular spaces communicate outwardly with the surface of the dentine and inwardK' with the dentinal tubules : corpus- cles have been described as contained in them and commu- nicating with the extremities of the fibres of Tomes. Deeper the calcification becomes more nearh' continuous ; at certain intervals it is, however, incomplete, irregular lines in a general way parallel to the surface being seen : these, known as the incremental lines of Salter, may be regarded as marking the boundaries of the lamellae. The dentinal tubules are lined b}- a condensation of the fibres of the matrix sometimes described as a distinct membrane; they have a wavy or spiral course across the lamellae and give off branches occasionally at acute angles which pur- sue a similar course; their waviness gives rise to a striated appearance when a tooth is seen in longitudinal section with the naked eye, or a low pow-er of the microscope : the alternating dark bands seen are know^n as the lines of
166 PART II. HISTOLOGICAL ANATOMY,
Schreger. The general direction of the tubules is in the fang at right angles to the lamellae ; in the crown they pass obliquely upward.
The enamel consists of a layer of elongated calcified prisms, usually hexagonal in cross section, set in a general way vertical to the surface of the dentine of the crown of the tooth ; they are alternated lighter and darker through- out their extent, giving to the enamel as a whole a banded appearance. In addition, occasional dark brov\rn lines may be seen crossing the enamel columns parallel-wise to the surface; these, known as the stripes of Retzius, may indicate lines of growth. Here and there vertical spaces may be seen slightly separating the enamel columns near the dentine, with which the interglobular spaces of the latter possibly communicate. At the time of irruption the surface of the enamel is covered with a thin cuti- cular layer know^n as Nasmyth's membrane; it is rapidly worn away as soon as the teeth are put to active use.
The cementum, cement-substance, or crusta petrosa, as it is variously termed, investing the surface of the fang, is composed of tissue essentially similar in structure to that of dense bone : the lamellae are few and irregularly ar- ranged, the lacunae varying in size and form ; the canali- culi of the latter are said to communicate with the inter- globular spaces of the adjacent dentine: there are no clearly defined Haversian systems. The layer is formed by the so-called periodontal membrane, which is practically continuous outwardly with the periosteum lining the alveolus which contains the tooth.
CHAPTER XIV. MOITII AND CONTENTS. 107
The enamel is epithelial in its origin, being derived from the calcification of a layer of cells or formed by their secre- tion. The dentine and ccmcntum are true skeletal struc- tures. The margin of the foetal jaw early shows a thick- ening of the stratified epithelium which grov/s downward into a groove into the mesoblastic tissue beneath, the dental groove: the curved rod of epithelial cells thus formed is known as the primary enamel germ. At regu- lar intervals along the under side (jf this rod further prolif- erations of the e])ithelial cells occur, with localized down- growths, the special enamel germs, at first flask-shaped. At the same time a condensation in the mesoblastic tissue beneath each gives rise to a conical dental papilla which grows upward to meet the epithelial downgrowth. The lat- ter grows down about the papilla, thus becoming converted into a cap-like mass, the enamel organ, connected with the epithelial ridge above b}- a slender stalk: the stalk is attached to one side of the enamel organ, owing to the mode of growth of the latter. A condensation of the mesoblastic tissues surrounding the newly formed enamel organ and papilla gives rise to a membrane which soon becomes rich in bloodvessels, and is known as the dental sac.
The papilla, thus invested by the enamel organ, assumes the general shape of the crown of the future tooth : the corpuscles near the surface become more numerous and larger than those of the interior, and the layer of odonto- blasts is formed, after which the deposition of dentine is begun. Important changes have in the meantime taken place in the enamel organ : the cells upon the concave sur-
168 PART II. HISTOLOGICAL ANATOMY.
face next the papilla become greatly elongated verticall}'', and b}^ their activity begin the deposition of the enamel ; whether b}'- a process of secretion or by the transforma- tion of their own substance is still a matter of debate ; the layer of enamel-forming cells is known as the inner layer of the enamel organ : the margin of this layer is continued over the convex surface of the enamel organ b}^ the outer layer of cuboidal epithelial cells, which is in turn continuous through the stalk with the epithelium of the jaw, and which lies in contact with the dental sac: the interior of the enamel organ contains a mass of cells, the so-called middle layer, which becomes converted into branching corpuscles, their interspaces being filled with a water}' fluid. From the side of the enamel organ or from the adjacent stalk a bud arises early in the history of the tooth, and grows downward to form a second enamel organ in the case of those teeth which are succeeded by others.
As the time of irruption approaches, the middle la^^er of the enamel organ becomes greatly compressed, and the enamel forming cells of the inner layer reduced in depths to flattened scale-like bodies, forming the membrane of Nasmyth. The papilla rapidly elongates, forming the fang, thus pushing the first formed crown upward to- ward its final position. After the dentinal portion of the fang is fully formed the cementum is deposited upon its outer surface in a manner practically the same as that of the formation of bone elsewhere.
The tongue is a mass of striped muscular tissue in-
CttAl'TlvR XIV. MOITH AND CONTEMTS. 100
Vested with the niueous njembrane of tlic mouth, which upon its upper surface is specially modified. The muscu- lar fibres which make up its bulk are divided into two symmetrical masses by a median vertical partition of fibrous tissue, the lingual septum, which is better devel- oped below than above: right and left of this the}' lie in interwoven bundles some of which run kHigitudinall}^ others transversely, and a third set vertically; their arrangement to form the lingual muscles is a matter anatomical rather than of histological discussion.
The mucous membrane of the lower surface and sides of the tongue does not differ essentiall)' from that of the rest of the mouth. At the margin of the upper or dorsal sur- face, however, it suddenly becomes modified, the majority of the papillae becoming greatly enlarged to form the pro- jections of the surface visible to the naked ey^ and known as the lingual papillae. Of these there are three kinds : those most abundant, and covering the whole surface of the tongue with a velvety layer, are known as conical or filiform from their shape; they are tapering upgrowths of the corium, covered with a corresponding layer of epi- thelium, and are frequently bifid at their tips: scattered here and there among them are spheroidal elevations of the surface about the size of a small pin's head ; these are termed fungiform^ and are invested with a thin layer of stratified epithelium. Near the root of the tongue, and arranged in a V-shaped figure whose apex is directed back- ward, are eight to twelve large papillae, roughly cylindrical in form, which are sunk in depressions of the surface and surrounded by circular trenches or grooves ; these are the
170 PART II. HISTOLOGICAL ANATOMY,
cir cum vallate papillae: the stratified epithelium of their sides forms a deep la^'er in which are imbedded the taste- buds or gustatory organs. The description of these struc- tures will be given in the chapter devoted to the organs of special sense.
The papillae all contain capillary networks, the fungi- form and circumvallate papillae having a specially abun- dant blood supply. The mucous membrane of the pos- terior part of the tongue contains diffuse adenoid tissue with here and there occasional nodules of the same. The lingual glands of the anterior part of the tongue are all of the mjLicous type : some of those of the posterior part, known as the glands of Ebner, are of the serous type, their ducts opening into the grooves which surround the circumvallate papillae. The tongue is formed in part by an upgrowth from the floor of the mouth, in part by a growth forward from the ventral wall of the pharynx : its epithelium is therefore in part epiblastic and in part hypoblastic in origin ; but the two regions cannot be dis- tinguished in the adult
iJo^^^Uii^ M^^^ vhiwvt
CHAPTER XV. ALIMENTARY CANAL. 171
CHAPTER XV. THE ALIMENTARY CANAL.
As we pass from the mouth through the opening of the fauces we enter the tube lined with h vpoblastic epitheHum of which the mouth is the antechamber, and to which the name of the alimentary canal is strictly applicable. The anterior and posterior pillars of the fauces are folds of the mucous membrane rich in mucous glands and containing some diffuse adenoid tissue : between them on either side of the mouth is situated a tonsil: a rounded body of va- riable size consisting of a mass of fibrous tissue richly in- filtrated with adenoid tissue and containing a variable number (from ten to twenty) well-defined nodules of the same structure, like those found in the mucous membrane of the rest of the tongue. The surface of the adenoid mass -thus formed is covered with stratified epithelium continu- ous with that adjacent, which is in places invaded by numbers of leucocytes passing through it into the oral cavity to become the bodies described above as salivar\' corpuscles. Deep folds or depressions of the surface occur, lined with stratified epithelium : these are termed crypts, and receive the secretions of subjacent mucous glands.
The alimentary canal, p'taperWW^c^lVeiil is a tube which, beginning at the pharynx and ending with the rectum.
172 PART II. HISTOLOGICAL ANATOMY.
varies in size and form in different portions. Its structure varies correspondingly in detail, as will be stated in the descriptions of the various regions. In general plan it is a tube having essentially a double wall whose divisions are connected by a stratum of areolar tissue, and invested throughout a portion of its extent b}' the serous membrane which lines the abdominal cavity.
The inner or glandular layer, the mucosa, is a mucous membrane whose epithelium varies greatly in different parts : the fibrous portion is in all the various regions more or less infiltrated with adenoid tissue which is in places quite scanty in amount or wanting altogether, in places so abundant as to form conspicuous nodular masses. Smooth muscular fibres are also present in all the regions, usualh'- in such quantity as to form a well-defined muscu- laris mucosae, which may consist of two or even three distinct layers.
The stratum of areolar tissue into which the loose and folded mucosa passes, the submucosa, varies in extent in different regions, but always permits of free movement of the folds of the mucosa. It is characterized throughout the greater portion of its extent b}' the presence of a net- w^ork of nonmedullated nervous fibres with small ganglia at the nodes, the plexus of Meissner: and contains the larger blood and lymph vessels from w^hich those of the mucosa are derived.
Beyond the submucosa, which is continuous with its in- ternal skeletal framework, is the outer principal layer of the tube, the musculosa, or muscular layer. The anterior portion is composed of striped fibres; the remainder (and
chaptp:r XV. ali.mkntary canal. 173
greater portion) of sniooth filires. Except in the pharynx, where definite muscles are found, the bundles of fibres, whether smooth or striped, are arranp^cd in two continu- ous strata, the inner of which is composed of circular and the outer of longitudinal fibres: between them is a thin layer of connective tissue containing blood vessels and a second and larger nervous network known as the plexus of Auerbach.
In the neck atid thorax the connective tissue adjacent forms a more or less well defined membranous layer just without the musculosa sometimes described as a separate layer of the canal under the name of the fibrosa; while the term serosa is commonly applied to the peritoneal fold which rests upon the musculosa throughout the abdomi- nal cavity.
The pharnyx is the first region of the alimentary canal. Its u_pper or res^giratory portion is lined with ciliated col- umnar epithelium ; while that of the lower and larger por- tion is stratified squamous: the latter is throughout a large part of its extent more or less infiltrated with leucocytes. The fibrous membrane contains a large amount of adenoid tissue, which in the upper and posterior part of the phar- ynx is collected into a large patch containing adenoid nod- ules and crypt-like depressions, to which the name of the pharyngeal tonsil has been given : gjan^s of the rnucous type are abundant. Beneath the submucosa is found a dense layer of fibrous tissue, the pharyngeal aponeurosis, separating the mucous membrane from the muscular coat. Without are the well defined pharyngeal muscles, com-
u 0 IkAj^ ^JLCuf4- H odj^\AMM}LL{--uJ (lMy\^i.nA
174 PART II. HISTOLOGICAL ANATOMY.
posed of striped muscular fibres. The structure of the pharynx ma}^ perhaps be best understood if we regard the musculosa proper of the aHmentary tract as entirely defi- cient, the aponeurosis representing the fibrosa, and the constrictor muscles as external additions thereto.
The oesophagus, like the lower part of the pharynx, is lined with stratified squamous epithelium. The mucous membrane, which is thrown into longitudinal folds, bears numerous small papillae upon the surface, covered by a homogeneous basement membrane. Adenoid tissue is very sparingly present in the mucosa, but occasional solitary nodules may be found. The muscularis mucosae is want- ing in the uppermost part of the oesophagus : lower it at -first appears in the form of scattered longitudinal bun- dles, which near the lower end of the oesophagus become so numerous as to form a continuous layer of longitudinal fibres.
The submucosa, which is well developed, contains the intrinsic glands of the oesophagus, which are of the mucous type; their ducts traverse the mucosa : blood ves- sels and lymphatics are numerous : the plexus of Meissner is wanting or very scantily developed throughout the greater part of the oesophagus.
The musculosa consists throughout the uj22er_third of a transverse and a longitudinal layer of striped muscular fibres ; lower down the layers are made up wholly of smooth fibres. The plexus of Auerbach is present, though its ganglia are smaller and less numerous than those of the rest of the alimentarv canal. There is a more^iDi^ less
CHAPTER XV. ALIMENTARY CANAL. 175
well defined fibrosa just without the external muscular layer.
The oesophagus opens into the stomach, a saccular dila- tion of the gilimentary canal whose wall is greatly modi- fied, chiefly in the mucosa: thjs layer is much thjckened, owing to the development of long tubular glands which open upon its surface, are imbedded in it, and make up the greater part of its substance ; it is thrown into numerous folds, or rugae, and is covered with a simple la\'er of columnar epithelium containing nujnerous goblet cells, the transition between which and the stratified epithelium of the oesophagus is abrupt : its surface is dotted all over with the minute openings of the gastric glands previously mentioned.
The gastric glands are distinguished by their structure into two kinds, designated according to their position as cardiac (also called peptic), occupying the region extend- ing from the oesophageal opening to beyond the middle of the stomach, and the pyloric, which occup\^ the Ipw'er third. The former have short ducts lined with columncir epithelium similar to that of the surface of the stomach : the glandular portion is a long wavy or slightly coiled tubule, whose end is sometimes strongly bent ; from two to four such secreting tubules commonl}^ opening into a single duct : lining the tubule throughout its whole extent is a single layer of polyhedral cells, while scattered along the tubule here and there between the la\'er of cells just mentioned and the basement membrane are larger spher- oidal or ovoidal cells. The cells first mentioned are called
UvVMi:*^VAJ^^vyUL\, «,4^M0U-tUA^ J^ Jm^^UOMU^-^J,^^^^ ithM4^i
176 PART II. HISTOLOGICAL ANATOMY.
the chief or from their position the central cells, or, from their probable ferment-secreting function, the peptic cells: the others are known as the accessory or parietal cells ; they have also been called the oxyntic cells, and are sup- posed to produce the acid of the gastric juice.
The pyloric glands differ from the cardiac in the propor- tions of their parts, the ducts being much longer and the tubules shorter: in the form of the secreting tubules, which are much more contorted; and in the absence of parietal cells. Between the region of well defined cardiac and pyloric glands is a narrow intermediate zone of transition in which the ducts become longer and the tubules shorter than in the cardiac region, while the pari- etal cells become less numerous and finally are wanting altogether.
The interstices between the gastric glands are filled with interstitial areolar tissue containing a rich network of capillaries and lymphatics and a small amount of adenoid tissue, with occasional slender bundles of smooth muscu- lar fibres. About the bases of the secreting tubules the adenoid tissue is more abundant, and here and there forms small nodules with ill-defined boundaries. Below, there is a well-developed muscularis mucosae, consisting of two distinct layers, an inner transverse and an outer longitu- dinal layer : the inner is to some extent subdivided into two laminae, the fibres of one running somewhat ob- liquely to those of the other; from its inmost surface the slender bundles above mentioned as running up between the gastric glands are given off.
CHAPTKR XV. ALIMENTARY CANAL. 177
The submucosa of the stomach is a rather thick layer of areolar tissue which extends upwards into the folds of the mucosa which form the rugae. It contains the larger bloodvessels and lymphatics connected with the vascular supply of the niucosa, and a well developed plexus of Meiss- ner situated nearer to the mucosa than to the muscular coat.
The musculosa is thicker in the stomach thqji in any other portion of the alimentary canal, the increase in thickness being chiefly in the inner or transverse coat. This, in addition to being greatly increased in volume is in the cardiac portion more or less clearly divided into two layers lying obliquely to each other and to the longitudi- nal coat. The plexus of Auerbach is conspicuously seen between the two coats. On the outer surface is the serosa, a thin fibrous membrane covered by endothelium, and ad- hering closely to the longitudinal muscular coat except along a narrow strip at the attachment of the mesogaster and the omentum.
As we pass from the stomach to the duodenum the mu- cosa becomes much thinner, the glands present in the upper portion of the latter bemg situated in the submu- cosa; a narrow zone of transition is seen between the pyloric glands of the stomach and the duodenal glands, or glands of Brunner. The mucosa, which is thrown into folds, the valvulae conniventes, is covered with finger-like or leaf-like projections, the villi, between which are tubu- lar depressions, the intestinal follicles or crypts of Lie- berkuhn : the siirface of the villi and the lining of the crypts are alike covered with simple columnar epithelium con-
178 part; II. HISTOLOGICAL ANATOMV:
taining large numbers of goblet cells, the columnar cells having vertically striated cajps or borders. Beneath the epithelium of both crypts and villi is an endothelial base- ment membrane, sometimes called the subepithelial endo- thelium of De Bove.
The centre of a villus contains a lymphatic with a widely opened extremity : around it are scattered bundles of smooth muscular fibres proceeding from the muscularis mucosae below : near the surface is a network of capil- laries formed from the breaking up of a small artery at the base of the villus and uniting to form a vein near its tip : the interstices are filled with loose adenoid tissue. Between the crypts at the base of the villi is a rich network of capil- laries and Ij'mphatics and imbedded, like those of the villi, in adenoid tissue, which becomes quite dense about the bases of the crypts ; here and there well defined nodules are found.
Immediatel}'^ beneath the layer of adenoid tissue upon which the crypts abut is situated the muscularis mucosae, which is not so thick as in the stomach, but presents two distinct layers, an inner transverse and an outer longitu- dinal.
The submucosa of the upper portion of the duodenum is quite thick: in addition to the blood vessels and lymphatics and the plexus of Meissner, it contains the glands of Brunner already mentioned. These are compound tubu- lar glands, whose short alveoli, lined with polyhedral cells surrounding a distinct lumen, open into slender branching ducts which pass to the centre of the gland, there to open
CHAPTER XV. ALIMENTARY CANAI,. 179
into the central duct : this, which is lined with short col- umnar cells, penetrates the mucosa to open between the villi. The glandular epithelium resembles that of the pyloric glands, between which and the glands in question there is a distinct transition at the pylorus : the epithelial cells contain fine granules similar to those found in the cells of glands of the serous type, which these glands re- semble save in the non-essential feature of the larger size of the lumen.
The transverse layer of the musculosa, as it reaches the lower end of the stomach, becomes suddenl}^ thickened to form a muscular ring, the pylorus or gastro-duodenal valve: passing into the intestine it becomes as quickly reduced in thickness, forming a la\^er much thinner than that found in the gastric wall : a similar diminution is found in the thickness of the longitudinal layer. The mus- culosa of the small intestine does not otherwise differ from that of the stomach, like which it is invested by a serosa formed from the peritoneum.
In the lower portion of the duodenum the glands of Brunner are wanting, and thesubmucosa becomes greatly reduced, the canal assuming the structural character com- mon to the greater portion of the small intestine, whose division into the regions recognized by the anatomist is not marked by any histological characters. The only noteworthy feature in this respect is the specialization seen in the quantity and disposition of the adenoid tissue of the mucosa, which attains its highest development in the lower portion of the small intestine, the ileum.
180 PART II. HISTOLOGICAL ANATOMY.
Throughout the whole of the small intestine adenoid tis. sue is found in the villi and between the crypts, as in the upper portions ot the duodenum just described : and as in that structure, there is a continuous layer thereof between thecrypts and the muscularis mucosae. This, however, be- comes much thinner and less dense, while here and there appear well defined nodules commonly known as solitary follicles or (by the use of a term equally unfortunate) sol- itary glands: it should of course be understood that these bodies have functionally nothing in common with either glands or follicles as defined on a preceding page: they are broadly pear-shaped bodies whose bases usually extend into the submucosa, which is locally thickened in connec- tion therewith, and whose apices when large project as rounded eminences upon the surface of the mucosa covered with a single layer of columnar epithelium, both crypts and villi being wanting at that point : in structure each is a dense mass of adenoid tissue in the main uniformly dis- posed, but with a slight diminution in density toward the centre of the mass, containing a capillary network and sur- rounded by numerous lymphatics: the nodules are sur- rounded by the fibrous tissue of the mucosa, which is here quite dense, but they have no definite capsule.
The solitary nodules of adenoid tissue give place to a great extent in the lower portion of the intestine to clus- ters of similar bodies sometimes called agminated glands, but better known as Peyer's patches. Where these occur the mucosa and submucosa are both thicker than in other portions of the intestine : the nodules are closely crowded together and in the centre of the mass become confluent :
CHAPTER XV. ALIMENTARY CANAL. 181
the conical apices of most of them project upon the sur- face of the mucosa: the epithelium of the projections (as is the case to some extent with that of the solitary nodules just described) becomes infiltrated with leucocytes as in the case of that investing the tonsils, often to such an ex- tent as to be no longer clearly distinguishable, large num- bers of leucocytes thus constantly entering the intestinal cavity.
At the margin of the folds which constitute the iko- colic valve the villi cease abruptly; and as we pass the margin in question we come upon the wall of the large intestine, including its greater portion, the colon, with its anterior prolongation, the caecum (ending in the ver- miform appendix), and its posterior continuation, the rectum.
The mucosa of the colon, like that of the rest of the large intestine, is devoid of villi. It is crowded through- out its whole extent with tubular follicles resembling the crj'pts of the small intestine, but larger and showing ordi- narily a larger proportion of goblet cells. The interstitial adenoid tissue is scant)', but large solitary nodules occa- sionallv occur. The muscularis mucosae does not differ materially from that of the small intestine.
The submucosa is a layer of moderate thickness, con- taining the larger vessels and the plexus of Meissner. The musculosa is well developed : the inner or circular layer is thickened uniformly, while the outer layer is chiefly gath- ered into three longitudinal bands. The outer surface is covered for a portion of its extent with a serosa.
182 PART II. HISTOLOGICAL AMATOMY.
The caecum is essentially similar in structure to the colon. The appendix vermiformis is a rudimentary struc- ture, variable in size and development: its mucosa con- tains numerous follicles, like those of the colon, and also solitary nodules of adenoid tissue: its musculosa con- sists of two layers, the outer or longitudinal differing from that of the colon in being of uniform thickness and thicker than the inner or circular laj^er.
As the alimentary^ canal is continued downward to form the rectum, its structure undergoes important changes in each of the fundamental layers, in connection with the modification which takes place in the function of the ca- nal: these will as before, be described in connection with the layers successively.
The mucosa of the rectum, like that of the colon, is^e- void of villi : it is much thicker than that of the colon, in- creasing in thickness downwards to the anal opening. In addition to temporary foldings, chiefly longitudinal, which exist when the tube is empty, there are present three or four transverse folds, semilunar in shape, known as the valves of Houston, and near the terminus several longitu- dinal folds, the columns of Morgagni ; their relations to subjacent structures will be mentioned later. The epithe- lium of the greater portions of the rectum is columnar, like that of the small and the large intestines : just above the anus there is in man a transition from simple columnar to stratified squamous epithelium continuous with that of the surface of the body : this transition takes place upon
CHAPTER XV. AI.IMENTARY CANAL. 183
the columns of Morgagni higher up than within the de- pressions which lie between them. Follicles or crypts like those of the colon abound in the mucosa of the greater portions of the rectum : the\' disappear at the level of the commencement of the columns of Morgagni. In addition to a small amount of diffuse adenoid tissue, occasional solitary nodules arc found : they are less numerous rela- tively than in the colon.
The fibrous structure of the mucosa of the rectum is denser than that of the colon, particularly toward the lower extremity, where there is a marked increase in the proportion of elastic fibres and a diminution in the amount of adenoid tissue, the fibrous layer finally passing without abrupt transition into the corium of the skin. The mus- cularis mucosae is gradually increased in extent, and consists chiefly of longitudinal fibres: at the lower por- tion these are gathered together in several stout bundles, which, with the overlying folds of the mucosa, form the columns of Morgagni above described : these are contin- ued into tendinous bundles which are inserted into the skin in the immediate vicinity of the anal opening.
The submucosa of the rectum becomes considerably thickened toward its lower extremity', in connection with the increased mobility of the mucosa, and is in addition reinforced b\' the presence of numerous elastic fibres : it contains, moreover, a certain amount of adenoid tissue, in this respect differing markedly from the same la^-er in other portions of the canal. A distinct plexus of Meissner is present.
184 PART 11. HISTOLOGICAL ANATOMY,
The musculosa of the rectum undergoes marked in- crease in thickness from above downward in both the cir- cular and the longitudinal layer. In the former, there is in addition to the gradual increase in volume a tendency to divide into bundles of fibres of gradually increasing size : at its lowermost limit a number of these bundles are ag- gregated together to form the internal sphincter muscle which surrounds the anal opening : distinct thickenings of the circular layer also underlie the valves of Houston. The longitudinal layer, as it passes downward, also undergoes subdivision, the resultant bundles of fibres diverging more and more from the circular layer, and being roughly divisi- ble into three strata ; an inner, the bundles of which are interwoven with those that form the internal sphincter ; a middle, whose bundles terminate along the boundary of the external sphincter (an extrinsic muscular structure composed of striped fibres) ; and an outer stratum whose bundles unite with the levatores ani and the recto-coccy- geal muscles.
The extrinsic glands connected with the alimentary canal are but two in number, the pancreas and the liver, both of them opening into the duodenal portion of the small intes- tine, of which they are outgrowths. The pancreas is in most respects similar in structure to a large salivary gland of the serous type; its connective tissue framework as well as its blood and lymph vascular supply being the same, and the form of the alveoli similar, as well as their arrange-
4,
CHAPTER XV. ALIMKNTARY CANAL. 185
ment in lobules and lobes. The epithelium of the alveoli re- sembles that of a serous {.^land in its j^ranular appearance, and in the position of the nucleus; the lumen is also very small. Certain differences are, however, to be noted. The ajveoli are as a rule longer and more generally tubular in form : theepithelial cells are taller and narrow^er, as a rule, approaching to the columnar form : there have been de- scribed under the name of centroacinar cells, certain spin- dle-shaped elements occupying the lumen near the union of the alveolus with the intermediate tubule; their origin and function are alike uncertain. The intermediate tubules, with their characteristic flattened epithelium, are more extensive than in the salivary glands, replacing in great measure the intralobular ducts. The pancreas is also characterized by the presence between the alveoli of pecu- liar groups or clusters of intertubular cells which form nodular masses situated in the intralobular connective tissue or the interlobular septa; each mass having a defi- nite tuft of capillaries : these bodies whose nature is not clearly understood, are by some regarded as newly formed or embryonic alveoli.
The liver is at once the largest and the most highly modified of all the digestive glands. While it must be re- garded as formed by the modification of a primarily tubu- lar structure, the multiplication of the glandular epithe- lial cells has obliterated the lumen of the tubule in each instance, the secretion formed by the epithelium being dis- charged throughtheagency of intercellular channels which are unlike anything found in any other portion of the human body. This extreme modification in the arrange-
186 PART n. HISTOLOGICAL ANATOMY.
ment of the glandular epithelium is also accompanied by an equally great modification in the arrangement of the blood vessels, the capillary network apparently penetrat- ing the epithelial masses ; thus forming what at first sight seems to be an exception to the general rule that epithelia are devoid of blood vessels. The structure of the liver will therefore best be understood if we begin by examining the blood supply and the framework of skeletal tissue with which it is associated.
The surface of the liver is invested by a thin capsule of fibrous tissue, surmounted by the peritoneal endothelium. From this capsule thin lamellae penetrate the surface of the gland as interlobular septa : in the transverse fissure it becomes continuous at the porta (which corresponds to the hilum of an ordinary gland) with an important in- growth of connective tissue, the so-called capsule of Glis- son, which forms by far the larger part of the interlobular tissue, becoming continuous with the ingrowths from the capsule. The porta is the place of entrance for the nerve- supply of the liver, and for the large portal vein and the smaller hepatic artery ; as well as of exit for the bile duct and for the principal lymphatic trunk : these four vessels and their ramifications, together with strands of nerve-fibres, are usually found in close proximity, sur- rounded by a certain amount of connective tissue, the whole constituting what is known as a portal canal. The blood leaves the liver by means of the hepatic veins, whose branches are formed by the union of the sublobular veins, the origin of which will be presently described : the sublobular and hepatic veins are not accompanied in their
CHAPTER XV. ALIMENTARY CANAL. 187
course by branches of other vessels, a character by means of which they can be readily distinguished from the branches of the portal vein.
The lobules of the liver, sometimes (but less correctly) termed the acini or alveoli, arc masses of hepatic cells, penetrated by capillary networks, polyhedral in form as the result of pressure, and in man something over a milli- metre in diameter. Those at the surface of the liver have the axis vertical or nearly so to that surface: but there is no such regularity of position in the case of those more deeply situated. The interlobular septa are in a few mam- mals (of which the pig is one) quite stout and conspicuous : in man they are far less developed ; and the boundaries of the lobules are in consequence not always readily deter- mined. From certain points on the septa a very scanty framework of connective tissue traverses the interior of the lobule, accompanying the capillaries: its presence even in small quantity is a fact of importance in forming a clear conception of the essential structure of the liver.
The branches of the portal yein end in veinlets situated in the septa and hence known as interlobular veins, each lobule having several surrounding it : these give off capil- laries (in a manner similar to arteries elsewhere) which form a network whose meshes converge toward the axis of the lobule : this is occupied by a single vessel, the cen- tral or intralobular vein, into which the capillaries empty. The intralobular vein passes at the base of the lobule into a sublobular vein, the hepatic veins being as has been stated, formed by the confluence of the sublobu- lar veins.
188 PART II. HISTOLOGICAL ANATOMY.
The branches of the hepatic artery accompany those of the portal vein to the lobules, where they divide into inter- lobular arterioles situated in the septa, together with the interlobular veins: like the latter, they terminate in capil- laries ; these are, however, situated chiefly' in the connec- tive tissue of the septa and the walls of the veins, a por^ tion of them only penetrating the lobules for a sufficient distance to form channels of communication with the capillary network of the lobule, as a means of discharge of the blood from the arterioles. The portal vein and its final branches, the interlobular veins, are therefore to be resfarded as the channels bv means of which the blood is submitted to the glandular action of the hepatic cells ; the hepatic artery being probably in large part, if not alto- orether the channel of nutrition for the skeletal framework and vascular mechanism of the organ.
The network of capillaries with its accompanying scanty' connective tissue forming, according to some observers, perivascular lymph channels, is interwoven with another and somewhat coarser network, that of the strands of liver cells or hepatic cords. These are polj'hedral epithelial cells about twentj^ micra in average diameter, with granu- lar protoplasm and central spherical nuclei, which perform the complex secretor}^ function of the liver. The size of these cords are such that nearly every cell is brought at some point in contact with a capillary ; the surface of the cell being in many cases slightly excavated along the line of contact.
Between the hepatic cells is found a network of minute passages usually not more than a micron in diameter, the
CHAPTER XV. ALIMENTARY CANAL. 189
so-called bile capillaries or, as they are otherwise termed the bile canaliculi : these are so situated that they tra- verse the surface of the cell along a side or an angle oppo- site that in contact with the blood capillaries, never pass- ing loetw^een the latter and the cells. At the surface of the lobule the bile canaliculi communicate with small inter- lobular bile ducts whose flattened epithelium is continu- ous with the hepatic cells, and whose basement membrane is resolved into the scant}* connectiv'e tissue of the lobule. These unite as they pass to the portal canals to form the smaller bile ducts situated therein ; the epithelium becom- ing columnar and the basement membrane stouter. As the smaller bile ducts come together to form the larger trunks found in the more central region of the liver the cells of the columnar epithelium become taller and are seen to rest upon a distinct membrana propria: while the fibrous layer beneath is reinforced by smooth muscular fibres: the largest ducts have in addition srnall mucous glands in their walls. The structure of the duct leading to the intestine, and of the gall bladder as well, is essen- tially the same as that of the largest ducts within the liver.
If we recall the structure of a gland of the serous type it will be remembered that the epithelial cells which line the alveoli are so large as to reduce the lumen to a very slen- der and often imperceptible channel between their apices: the bile canaliculi may with propriety be compared to such channels. The alveoli of ordinary glands may be either spheroidal or elongated and tubular, the interstices between them containing a variable amount of connective
190 PART II. HISTOLOGICAL ANATOMY.
tissue which supports a capillary network, whose meshes conform to the structure and arrangement of the alveoli. In the lower vertebrates the liver consists of tubular alve- oli essentially similar to those of other glands. If we con- ceive of such tubules in the mammalian liver as becoming elongated and branched, and finally anastomosing, we shall understand the structure and arrangement of the hepatic cords. According to this view the anastomoses formed obliterate the boundaries of the alveoli ; but we may regard as belonging to one such division all the bile canaliculi with their surrounding epithelium which lie nearest to and presumably discharge into one of the inter- lobular bile ducts : the beginnings of the ducts, with their flattened epithelium, corresponding in position and struc- ture with intermediate tubules or intercalary ducts of other glands : the lobule is thus seen to be in fact as well as in name a lobule ; that is, a collection of acini or alveoli ; and the application of either of the latter titles to it is misleading. The most peculiar features in the structure of the liver are those connected with the blood supply.
CrtAPTfiR XVI. RESPIRATORY APPARATUS. 191
CHAPTER XVI. THE RESPIRATORY .APP.VrATUS.
In the description of the alimentary canal which has just been given, the conspicuous layers, whose modifica- tions in the various regions have been discussed, are those most readily demonstrable by the knife of the anatomist: they are those, moreover, which naturally result from the double function of the greater part of the canal, the inner layer, or mucosa, being in substance a glandular layer by means of which the nutritious portion of the food is brought into a condition suitable for the accompanying process of absorption; the outer layer, or musculosa, being in effect a mechanism by which the food mass is caused to traverse this glandular surface; while the sub- mucosa makes possible the necessary movements of the mucosa and the musculosa on each other. We should, however, bear in mind the continuity of the submucosa with the fibrous structure of the mucosa on the one hand, and on the other with the interstitial connective tissue of the musculosa, and through it with the fibrosa or serosa: considering this, and bearing in mind the embryonic origin of the tissues present, it will be quite clear that another very natural division of the wall of the alimentary canal, based on histological rather than anatomical characters, would recognize two primary la\"ers of tissues (differing
192 PART II. HISTOLOGICAL AN'ATOMY.
vastly in extent), the epithelial layer, hypoblastic in its origin; and the musculo-skeletal (and vascular) layer de- rived from the mesoblast, extending from the basement membrane, just beneath the epithelium w^ithin, to the fibrosa (or, in the intestine, to the fibrous basis of the serosa) without: the presence of this fibrous layer as one of the constituents of the alimentary canal should be clearh' borne in mind.
It is by an outgrov^'th from the phar3'ngeal region of the alimentary canal that the respiratory apparatus is formed, and its structure throughout its whole extent is de- rived from such modifications of a musculo-skeletal layer lined with epithelium as are required for the proper per- formance of the functions of the various regions. These are in the trachea and bronchial tubes such as will serve to maintain the patency of these channels for the passage of the air and in some measure to regulate the same: and in the airsacs such as will permit the freest interchange of gases between the air which they contain and the capil- laries within their walls.
The trachea is plainly seen to be composed of three well-defined layers, similarly disposed to those which ap- pear as the chief factors of the alimentary canal. The inner of these is the mucosa; it differs in some important respects from the layer of that name in the region just mentioned. It is lined with w^hat is usually termed a stratified columnar ciliated epithelium: the layer is but a few cells deep ; those at the surface are cylindrical or prismatic, with tapering bases, their free extremities in most cases beset with numerous fine cilia, whose united
ClIAPTHR XVI. RHSI'IKATOKV APPARATUS. 193
action causes constant currents toward the mouth; here and there g^ohlet cells occur; beneath the superficial cells, or intcrsjjersed between their bcises, are elongated and more or less spindle-shaped cells, which are probably des- tined to replace them ; still lower are pyriform and spher- oidal cells, many of which multiply rapidly by cell-divi- sion, replacing the older superficial cells as occasion re- quires, resembling thus in function the deepest portion of the stratified stiuamous epithelium of the oesophagus.
The epithelium of the trachea rests upon a basement membrane, a homogeneous, elastic layer of considerable thickness, perforated by occasional fine canals. This is followed abruptly by a fibrous layer, whose bundles are irregularly and somewhat loosely disposed and are inter- mintjled with a considerable number of elastic fibres: there is a well but not greatly developed network of small blood vessels, accompanied by nerves and lymphatics, and a considerable amount of adenoid infiltration. Beneath the fibrous layer, and continuous therewith, is the elastic layer, composed of a dense network of elastic fibres, most of which are longitudinally disposed, which form the inner boundary of the mucosa : it is best developed in the dorsal portion of the trachea.
The submucosa. like that of the alimentary canal, is a layer of areolar tissue serving to unite the mucosa with the denser layer beyond it. In addition to the larger blood vessels from which the blood supply of the mucosa is derived, and their associated lymphatics, it contains numerous small glands ol the mucous type, whose long ducts, lined with cuboidal epithelium, traverse the mucosa
194 PART II. HISTOLOGICAL ANATOMY.
to open on its surface. These glands are, as a rule, most abundant in those portions of the submucosa which are situated opposite the intervals between the successive fibrous rings of the trachea. Here and there in the sub- mucosa an occasional adenoid nodule may be found, fre- quently associated with a gland or its duct.
No single word will readily indicate the complex struc- ture of the outer layer of the trachea : if we for conven- ience make use of the terra fibrosa in referring to it, we shall designate it by its most constant though not most conspicuous factor. It is in effect a tube of d^nse fibrous membrane, rich in elastic fibres, reinforced at regular in- tervals by the incomplete rings of cartilage which are the most prominent components of the trachea, and bearing on the inner su_rface of its dorsal portion the rudiments of a muscular layer. The rings are composed of hyaline cartilage w^hich is far less brittle than usual: they are imbedded in the fibrous layer, which is continuous with the perichondrium of each, and situated rather toward its inner than its outer limit. The muscular layer occupies an elongated rectangular area nearly identical with that bounded b}^ the lines connecting the dorsal ends of the in- complete cartilaginous rings, but somewhat more exten- sive laterally : to the band of muscular tissue thus formed, the name of the tracheal muscle is given. It consists en- tirely of smooth muscular fibres; the great majority of these are arranged in transverse bundles disposed in groups which are separated by occasional transverse septa of fibrous tissue ; external to these are a few thin and short longitudinal bundles forming an imperfectly defined layer:
CHAPTER XVI. RESPIRATORY APPARATUS. 195
the muscular laver is invested outwardly l\v the fibrous laver which it in some measure replaces, the latter being here much thinner than in other portions of the trachea. The bundles of the trans verse la vera re inserted bet ween the rings upon the fibrous layer; those opposite the rings are in man inserted on their inner surface; in some of the lower mammals thev are inserted on the ends and in others on the outer surface of the rings. The longitudinal bundles are inserted upon the ends of the rings and upon the fib- rous layer. It is a noteworthy fact that in the dorsal por- tions of the trachea some of the mucous glands are situ- ated external to the muscular layer, their ducts penetrat- ing it as well as the other layers of tissue beneath the epi- thelium.
The bronchi, or right and left divisions of the trachea, while they differ from it anatomically, particularly in the form of the cartilages, resemble it in all essential respects as regards their histological structure. As the}- enter the lungs they branch and subdivide repeatedly, their imme- diate continuations and their subdivisions, with the ex- ception of the smallest, being known as the bronchial tubes, or, as they are sometimes termed, the intrapulmon- ary bronchi: by the time they are reduced to a diameter of one millimetre, or thereabouts, they are known from their size and structure as bronchioles; the smallest of these are never less than half a millimetre in diameter.
The largest bronchial tubes are essentially like the extra-pulmonary bronchi in structure : as we pass toward the bronchioles, their structure undergoes marked though
196 PART 11. HISTOLOGICAL ANATOMY.
gradual modification: that of a tube of medium size may be described as follows. The epithelium, like that of the trachea and larger bronchi, is stratified, columnar and ciliated, and rests on a homogeneous basement membrane. Beneath this is a fibrous layer containing a moderate amount of adenoid tissue, and numerous elastic fibres no longer forming a continuous layer, but gathered into strands, which form the bases of the folds into which the inner portion of the mucosa is frequently thrown : exter- nal to the adeno-fibrous layer is a well-defined muscularis mucosae, composed of smooth fibres transversely dis- posed, this layer, which is of variable thickness from point to point along the tube, is sometimes designated the bronchial muscle.
External to the muscular layer is the submucosa, com- posed here, as in the trachea and bronchi, of areolar tis- sue containing rnucous glands and plexuses. QJLblood and lymph vessels. The outermost layer, that which we have for convenience termed the fibrosa, is the one which first shows signs of reduction: the fibrous membrane is by no means as dense, nor as clearly defined, either from the sub- mucosa within or from the adjacent tissues without, as in the trachea; the cartilaginous rings of the latter are rep- resented by plates of cartilage of varying size and shape; and there is nothing present in this laj^er that may be re- garded as corresponding structurally to the tracheal mus- cle. Occasional lobules of fat may be seen, and the mucous glnnds not unfrequently penetrate into this layer, which thus tends to approach in its structure that of the sub- mucosa.
CHAPTER XVI. UKSIMKATOKY APPARATUS. 197
As we ])ass toward the smaller bronchial tubes the epi- thelium becomes gradually reduced to a single layer ot" columnar ciliated cells resting upon a basement membrane. The fibrous layer beneath becomes much thin^ner; the rela- tive amount of adenoid tissue becomes less and less, and elastic fibres become far less numerous, though they do not altogether disappear. The muscular layer, on the other hand, for a time at least, increases in relative thick- ness, becoming one of the niosc conspicuous features of small tubes. The submucosa and fibrosa become blended into one layer ^f loose fibrous tissue rich in lymphatics and containing blood vessels, the mucous glands djsap- pearing from the former, together with the cartilaginous plcites (and the nodules which succeed them) from the lat- ter.
Within the bronchioles (otherwise designated the term- inal bronchi) still further reductions of structure take place: the epkhelium changes from columnar to cubical, loses its cilia, and later becomes more or less flattened, forming a single layer of polyhedral granular cells upon the basement membrane. The latter rests on a thin layer of fibrous tissue with longitudinal elastic fibres: the muscu- lar layer is reduced to scattered bundles and later to iso- lated fibres, without finally disappearing altogether; while the submucosa and the fibrosa become blended with each other and with the fibrous layer of the mucosa.
The bronchioles lead into larger pyramidal or irregular shaped spaces, the infundibula, into each of which open by wide apertures a large number of the spheroidal or
198
PART II. HISTOLOGICAL ANATOMY.
polyhedral air-sacs or alveoli which are the ultimate chambers of the lung: each infundibulum with its associ- ated alveoli making up one of the component lobules of that organ. The principal change in passing from the bronchiole to the infundibulum is found in the epithelial layer: the lo\v^anuLar cells with which the distal extrem- ity of the bronchiole is lined are found in the infundibulum in patches which become smaller and less numerous as we proceed to its farther extremity : between them are found larger and thinner transparent cells w^hicli form a simple squamous epithelium to which the distinctive title of res- piratory epithelium is applied . The basement membrane, the longitudinal network of elastic fibres, and the layer of scattered muscular fibres are continued without essen- tial change. The fibrous portion of the wall of the bron- chiole is represented b}' scattered branched connective tis- sue corpuscles situated in the interstices between the elas- tic and muscular fibres.
The alveoli are in the main continuations of the walls of the infundibulum. The epithelium consists almost entirely of the large flat cells above mentioned, the smaller granu- lar cells being scattered sparingly amongthem either singly or in groups of two or three : between the cells, and in par- ticular at the angle where three or four meet, are occasional stomata, minute openings which communicate with the lymph spaces below. About the mouth of each alveolus there is an annular bundle of elastic fibres from which is given off a network, which, together with a small amount of fibrous tissue and a few connective tissue corpuscles, forms the wall of the alveolus and the support of the epi-
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theliutn and capillary network; from the torni ami dispo- sition of the alveoli it results that a single layer thus formed does duty in ureat measure for two adjacent alveoli. The capillary network conlaiucd in ilic imcraKcolar septum thus formed is cxcecdinLily dense; and its loops pass from siile to side ot the septum in a scrpeniinccourse, thus liriniiini;- the blood contained within ihcm as neat as pos- sible to the air in each ot the alveoli.
The lymph spaces '\n the alveolar walls ciMumunicate with the lymphatics situatcil \u the coi\ncctivc tissue septa which lie between the lobules Ivach K^bulc, as thus bounded, is irrcnidarly pvramitlal in form, its apex beini^ situated at the bronchiole, towartl which the linulin^ septa tend: peripherally, the interlobular septa are coii tinuous with the denser layers of tibrous tissue which fcnin the investment of the lol)es: the latter, in their ttnti. being coniitnu>us with the pleura whicii forms the scions investment o( the surface o( the whole lunu
1
The larynx is a special modilication of the proximal cwd of the trachea: it dilVers in its histological strnctuic bom the latter in the following particidars. Us epithelium is in nu)st portions stratified, columnar ami ciliatcil, as in the trachea; that ol' the true vocal cords and of the e|)i- glottis antl a portion of the intervening smface is strati lied sipiamous: in that covering the under smface of (he epiglottis immcrous taste buds nreimbeddetl. Phc mnci>sa dilVcrs from that oi' the trachea, chiellv in the giealcr amount ol adciu>id tissue, in places resembling the phar- ynx in this respect: the elastic tibres which run through out its extent ;ire <»ie.itlv increased in numbci in (lu>vtH-al
198 PART II. HISTOLOGICAL ANATOMY.
polyhedral air-sacs or alveoli which are the ultimate chambers of the lung: each infundibulum with its associ- ated alveoli making up one of the component lobules of that organ. The principal change in passing from the bronchiole to the infundibulum is found in the epithelial layer: the low granular cells with which the distal extrem- ity of the bronchiole is lined are found in the infundibulum in patches which become smaller and less numerous as we proceed to its farther extremity: between them are found larger and thinner transparent cells which form a sirnple squamous epithelium to which the distinctive title of res- piratory epithelium is applied . The basement membrane, the longitudinal network of elastic fibres, and the layer of scattered muscular fibres are continued without essen- tial change. The fibrous portion of the wall of the bron- chiole is represented by scattered branched connective tis- sue corpuscles situated in the interstices between the elas- tic and muscular fibres.
The alveoli are in the main continuations of the walls of the infundibulum. The epithelium consists almost entirely of the large flat cells above mentioned, the smaller granu- lar cells being scattered sparingly among them either singly or in groups of two or three: between the cells, and in par- ticular at the angle where three or four meet, are occasional stomata, minute openings which communicate with the lymph spaces below. About the mouth of each alveolus there is an annular bundle of elastic fibres from which is given off a network, which, together with a small amount of fibrous tissue and a few connective tissue corpuscles, forms the wall of the alveolus and the support of the epi-
CHAPTER XVI. RESPIRATORY APPARATUS, 199
theliuni and capillary network: from the form and dispo- sition of the alveoli it results that a single layer thus formed does duty in jj;^reat measure for two adjacent alveoli. The capillary network contained in the interalveolar septum thus formed isexceedin«i^ly dense; and itsloops pass from side to side of the septum in a serpentine course, thus l)rin<i^ing the blood contained within them as near as pos- sible to the air in each ot the alveoli.
The lymph spaces in the alveolar walls communicate with the lymphatics situated in the connective tissue septa which lie between the lobules. Each lobule, as thus bounded, is irregularly pyramidal in form, its apex being situated at the bronchiole, toward which the limiting septa tend : peripherally, the interlobular septa are con- tinuous with the denser laj'ers of fibrous tissue which form the investment of the lobes: the latter, in their turn, being continuous with the pleura which forms the serous investment of the surface of the whole lung.
The larynx is a special modification of the proximal end of the trachea: it differs in its histological structure from the latter in the following particulars. Its epithelium is in most portions stratified, columnar and ciljated, as in the trachea : that of the txue vocal cords and of the epi- glottis and a portion of the intervening surface is strati- fied squamous: in that covering the under surface of the epiglottis numerous taste buds are imbedded. The mucosa differs from that of the trachea, chiefly in the greater amount of adenoid tissue, in places resembling the phar- ynx in this respect : the elastic fibres which run through- out its extent are greatly increased in number in the vocal
200 PART II. HISTOLOGICAL ANATOMY.
cords, where they constitute the chief part of the mucosa. The principal cartilages are hyaline, like those of the trachea: those of Santorini, of Wrisberg and of Luschka are reticular, as is the epiglottis. The proper muscles of the larynx are composed of striped fibres, resembling in this respect the musculosa of the upper portion of the oesophagus.
The course and arrangement of the blood vessels of the lung are subjects rather for anatomical than histological discussion. It should be borne in mind, however, in study- ing sections of that organ, that the lung has a double blood supply : the branches of the pulmonary artery ac- companying the bronchial tubes to their common destina- tion in the lobules of the lung; while the smaller bronchial arteries, derived from the aorta, are distributed to the walls of the air passages themselves and to the surround- ing structures, as vessels of nutrient supply. A similar re- lation subsists between the bronchial veins, which empty into the ascending vena cava, and the branches of the pulmonary veins.
The lymphatics of the lung are divisible in a^somewhat similar manner into two groups, those associated with the bronchial tubes, and those associated with the air sacs. The former, or bronchial lymphatics, have their origin in the lymph spaces of the bronchial mucosa, forming a plexus in the submucosa which empties into larger trunks lead- ing to the root of the lung. The second group is compound of two sets, the deep or vascular lymphatics, having their origin in the connective tissue of most of the lobules of the lung, and the superficial or, as they are also termed,
CHAPTER XVI. RESPIRATORY APPARATUS. 201
the subpleural lymphatics, which arise in the vicinity of the lobules near the surface and enter into the plexus which underlies the pleura: the latter communicates with the thoracic cavity bv means of occasional stomata. The deep and superficial plexus alike empty into trunks which lead to the root of the lung, there to enter, in company with those from the bronchial lymphatics, into the bron- chial lymph nodes.
It is a common custom to speak of the lung as essentially similar in structure to a gland, the alveoli beingcompared with the similarly named divisions of the latter, theinfun- dibula and bronchioles to the ductules and intralobular ducts, and the bronchial tubes to the larger ducts. Com- parative anatom}-^ shows that the primary condition of the lung is that of a large sac-like c»utgrowth of the ali- mentary canal, its unquestionable homologue in the fish- like vertebrates being the swim-bladder. In some of these forms it undergoes more or less subdivision b\' foldings of its inner surface, and assumes something of a respiratory function; its connection with the alimentary canal remain- ing simple and entirely membranous: in the lower air- breathing vertebrates the area of respiratory surface is increased to a limited extent only by peripheral sacculation, while a rudimentary trachea and larynx appears: it is only in the higher reptiles, the birds, and the mammals, that the lung assumes the compact and spongy structure due to compound sacculation and associated with the pres- ence of a well developed system of bronchial tubes. Its origin is therefore seen to be the reverse of that of a large
202 PART II. HISTOLOGICAL ANATOMY.
gland, which is developed by an increase in the number of origin alh' small alveoli.
While the respiratory tract is to be regarded as a sub- divided saccular outgrowth of the alimentarj' canal, we must not expect to be able to recognize essential similari- ty of detailed structure in organs differing markedly in function : a general comparison may, however, aid in understanding and remembering the character and ar- rangement of the tissues present. The epithelium is, of course, continuous throughout. The mucous membrane of the alimentary canal, with its varying amount of ade- noid tissue and its subjacent muscular layer, is represented in the air-passages by the adeno-fibro-elastic layer, to which in the bronchial tubes amuscularismucosaeis added. The submucosa of one is continued into that of the other without essential modification. The outer layer of the larger air-passages may best be regarded as corresponding to the fibrosa of the pharynx and oesophagus, reinforced by the tracheal and bronchial cartilages. The musculosa of the alimentary canal is represented only by the muscu- lar area of the trachea with its well developed transverse and rudimentary longitudinal layer.
CHAPTER XVII. URINARY ORGANS. 203
CHAPTER XVII. THE URINARY ORGANS.
The urinary organs include the glandular kidneys; their ducts, the ureters; the bladder, to which the latter lead; and the urethra, through which the contents of the blad- der are discharged. The urethra of the female is strictly a urinary tract, and is throughout its entire extent a por- tion of the ventral outgrowth of the alimentary canal from a part of which the bladder is eventually formed : it will therefore be described in this connection. The pro- static portion of the male urethra includes all that part of the tract homologous with the urethra of the female; its remaining portion is a common channel for the urine and the seminal fluid, and is formed in connection with struc- tures accessory to reproduction : the description of its whole extent will therefore be deferred until it can be taken up in connection with them in a subsequent chapter.
If a kidney be cut through from the convex surface to the hilum, either longitudinally or transversely, the surface of the cut section shows to the unaided eye certain feat- ures of importance to the study of its histology. The solid portion is seen to be curved around a central cavity, the pelvis of the kidney; into this project a number (varying with the direction of the section) of conical papillae, each of which is surrounded by a cup-like extension of the pel-
204 PART II. HISTOLOGICAL ANATOMY.
vie cavit}^ termed a calyx. The solid portion itself can be readily seen to be divided with considerable sharpness into an outer region, the cortex, forming about one third of the depth, and an inner, the medulla ; the latter can be again distinctly subdivided into the boundary layer next the cortex, and forming about one fourth of the entire depth, and the papillary portion, v^hich includes the re- mainder. The papillary portion is distinctly but uni- formly striated : in the boundary layer radial tracts, termed medullary rays, similar in appearance to those of the papillary substance alternate with tracts characterized by increased transparency.
The entire mass of radiating tracts extending from the apex of a papilla to the meeting of the cortex and the boundary layer constitutes what is known as a pyramid of Malpighi; there are rarely less than ten or more than twent}^ of them in a single human kidney : in some of the lower mammals they are numerous, as in man ; in others there is a single papillary ridge which projects into the pel- vis along its length, or a single large central papilla: kid- neys such as the latter are termed umpyramidal. If the pyramids of Malpighi be conceived of as extended across the cortex, the kidney would be divided into a correspond- ing number of parts commonly called lobules : if, how- ever, we are to compare these glands with others on the basis of the arrangement of their ducts, these regions will with greater propriety be termed lobes: in the human kid- ney they are not structurally separated, but in that of some mammals, as the otter, each is invested by a capsule of its own, and is but slightly attached to its fellows : a condi"
CHAPTER XVII. I'KINARV ORGANS. 205
tion characteristic of the human kidney during foetal life. In all multipyramidal kidneys, like that of man, the corti- cal portion of each lobe extends around and beyond the base_oLthe pyramid toward the pelvis of the kidney : these interpyramidal masses, composed in part of portions of the cortical substance of the adjacent lobules, and in part of blood vessels whose origin and course will be presently described, are known as the columns of Bertin.
The medullary rays seen in the boundary layer are less readily traceable in the cortex : they are, however, contin- ued into that layer, growing smaller as they approach the outer surface, and disappearing altogether before reaching it. These prolongations of the medullary rays are known as the pyramids of Ferrein: their bases, as will be readily understood, rest on the bases of the pyramids of Mal- pighi, each of which gives rise tg a large number of them. The intervening portions of the cortex make up what is known as the labyrinth: -a definite portion thereof, not marked off bj' any visible separation, is structurally con- tinuous with each pyramid of Ferrein. Each pyramid with its associated portion of the labyrinth, and its continua- tion b}' means of the medullar}' ray to the apex of the p\'ramid of Malpighi, may, as we shall see, be with pro- priety regarded as corresponding in extent to a lobule of other glands. Scattered throughout the labyrinth are small rounded bodies, the Malpighian corpuscles, barely visible to the naked eye. Beyond the cortex maybe readily seen the tough fibrous capsule which invests the whole gland.
The kidney, like the liver, is characterized by the posses-
206 PART II. HISTOLOGICAL ANATOMY,
sion of a specially modified blood supply, the most impor- tant features of which are visible to the naked eye or with a low power of the microscope. If a section be made as above indicated through a well injected organ the follow- ing facts may be noted. The renal artery, entering at the hilum, divides into four or five branches which traverse the lining of the pelvis, their subdivisions entering the columns of Bertin. As they pass along the latter they give off twigs to the cortical substance present, and on reaching the level of the bases of the pyramids of Malpighi give rise to an arched plexus through whose meshes the pyramids of Fer- rein pass to the cortex. From the arcuate arteries of this region branches are given off which run to the surface of the kidney, known as the radiate or (from their position between the cortical areas surrounding the pyramids of Ferrein) the interlobular arteries: from these are given off twigs which supply the cortical substance in a manner presently to be described : their extremities terminate in the capillary network of the capsule of the kidney. From the arcuate arteries also arise near the point of origin of the interlobular branches slender vessels, the arteriae rectae, which supply the medullary portion, passing directly toward the apex of the papilla.
There may also be seen in the cortical portion of the kid- ney, and receiving twigs which proceed from its substance, radiate or interlobular veins situated in close proximit}^ to the arteries of that name. They arise just beneath the the capsule by small vessels having a stellate arrangement (the stellules of Verheyen), and traverse the cortex to enter into arcuate veins in the main similarly disposed to the
CHAPER XVII. IRINARY ORGANS. 207
arteries of the boundary region ; they resemble the latter in receiving vena rectae from the medullary portion, and unite to form trunks which pass by way of the columns of Bertin to traverse the lining of the pelvis and come together at or near the hilum to form the renal vein. The vasa recta pass toward the papillae in groups which alternate in the boundary layer with the medullary rays, forming the tracts of greater transparency above mentioned.
The kidney is a compound tubular gland made up of lobes and lobules which, as We have seen, are not sharph- defined from each other by fibrous septa. Its component tubules differ in a marked degree from those of any other gland in the definiteness of their course; the variation in size and in the character of the epithelium of the different regions of each ; and in the peculiar relations which they sustain to their blood supply. With the exception of the blood-vessels and their accompanying lymphatics and nerves, together with the small amount of connective tis- sue in which these are imbedded, and of a very scanty inter- stitial tissue, the whole substance of the kidney, both cor- tical and medullary, is made up of these tubules.
Each uriniferous tubule is made up throughout its whole extent of a homogeneous basement membrane lined with a single layei of epithelium. It has its origin in a Malpighian bod\', the extremity of the tubule being there expanded to form a thin walled sac whose distal portion is inverted into the proximal, forming a capsule of Bow- man: into the cavity of invagination thus formed is thrust a spheroidal mass of capillaries termed a glomerulus: its
208 PART n. HISTOLOGICAL ANATOMY.
relations will be described later. The capsule is lined throughout with a sirnple^quamous epithelium ; and with the accompanying glomerulus makes up the Malpighian bod\'. The tubule leaves the capsule at a point opposite the inversion for the glomerulus bv a constricted neck lined with small cuboidaljoells, expanding at once to form the proximal convoluted tubule : this is Hned with cuboidal cells, whose outhnes are irregular and interlock in such manner as to render it almost impossible to distinguish the boundaries of adjacent cells when seen in longitudinal section; the portion qf^thecell next the b§S£m£Dt membrane is vertically striated, and is, under the influence of certain reagents, separable into rod- like processes ; the free surface bears cilia-like processes projecting into the lumen. The tubule passes by sweep- ing curves toward the p\'ramid of Ferrein : as it enters it it turns toward the boundary layer in wavy curves as the spiral tubule of Schachowa : its structure is essentially like that of the convoluted tubule, the epithelium being distinctly "rodded."
Entering the boundary layer, the tubule suddenly be- comes much smaller and nearly straight in its course toward the papilla : it is here known as the descending tubule of Henle: this is the smallest portion of the whole tubule; its epithelium has the formof plate-like cells, whose nuclei cause a central thickening which projects into the conspicuous lumen, making its course apparently irregu- lar. Shortly after reaching the papillary portion the tubule bends upward, forming Henle's loop, the diameter in- creases slightly, and the epithelium assumes the form of
CHAPTER XVn. I'KINAKY ORGAN'S. 209
low poU'hedral cells, greatly reducin<^ the lumen. From the loop the ascending tubule of Henle passes upward through the boundary layer without essential modifica- tion, save a slight increase in diameter, as a straight or slightly wavy tubule.
The ascending tubule enters the cortex and travels for a longer or shorter distance in the pyramid of Ferrein, finally bending abiniptly to enter the labyrinth as an irregular tubule, whose size and the position of whose lumen varies greatlv in accordance with irregular variations in the height of the striated epithelium, the lumen remaining mi- nute throughout its whole extent : it pursues a zigzagcourse toward the Malpighian body whence the tubule arose. In the vicinity of the latter it resumes a nearly uniform dia- meter and exchanges its angular course for regular curves: it is now known as the distal convoluted tubule : in size, form, and structure, it is identical with the proximal. Like the latter, its course tends toward the pj'ramid of Ferrein; as it proceeds, it narrows into a junctional tubule, or, as it is also called, an arched collecting tubule, with low cuboidal epithelium and a relativeh' large lumen. It passes toward the axis of the p^'ramid to enter a straight col- lecting tubule somewhat larger, but otherwise similar in structure, which may also receive other junctional tubules from point to point as it passes toward the boundary layer.
The straight collecting tubule passes along the medul- lary ray through the boundary layer unchanged in form or size : after it enters the papillary portion it joins at an acute angle with similar tubules, the resultant tubules be-
210 PART II. HISTOLOGICAL ANATOMY.
coming larger with each union : the largest (and terminal) tubes so formed, known as the ducts of Bellini, are lined with a simple columnar epithelium: their openings, visible to the naked eye, are scattered over the apices of the pa- pillae.
The interlobular arteries, as has been previously stated, pass from the boundary to end in the capillaries of thecor- tex. Along their course through the cortex they give off on every side short arterioles which go to the neighbor- ing Malpighian bodies: these, which are known as the vasa afiferentia, on reaching these bodies give rise to the sphe- roidal masses of capillaries called glomeruli, the arteriole entering the glomerulus at a point opposite the neck of the capsule : the capillaries unite again to form small ves- sels, the vasa efferentia, which leave the glomeruli where the afferent vessels enter.
The efferent vessels, it will be seen, sustain to the glo- meruli the relation of veins : they do not, however, proceed to the interlobular veins, but divide shortly after leaving the glomeruli to give rise to the cortical network of capil- laries, which is made up throughout the labyrinth of short irregular meshes interwoven with the convoluted and irregular tubules, throughout the pyramids of Fer- rein of long meshes running in the direction of the pyra- mid. The capillary network is not distinguishable by any known means into areas corresponding to the Malpighian bodies, the individual tubules, the lobules, or even the lobes of the kidney, being apparently continuous throughout the cortex of the whole kidney.
CHAPER XYII. I RINARV ORGANS. 211
Here and there in the caj)illary network the radicles of small veins are formed, chiefly in the vicinity of the inter- lobular veins, to which they lead. The veins in question, arising from the stellate veins just beneath the capsule, gather in their course the corticid veinlets and pass to the arcuate veins of the plexus lying in the boundary. They thus form the channels of return for the blood which leaves the interlobular arteries by way of the vasa afferentia, flowing, as should be noted, through two sets of capil- laries, those of the glomeruli and those of the cortex, as well as though the intervening vasa efferentia.
The vasa recta, as has been stated, leave the arteries and veins of the boundary in close proximity to the interlobu- lar vessels, in some cases springing as branches from their bases. They pass in the transparent striae of the boundary layer to the papilla, the arteries being resolved along the way into the medullary capillaries, which form a net- work with greatly elongated meshes running in the direc- tion of the tubules between which they chiefly lie : in the tip of the papilla the meshes are much shorter, forming a denser network about the ducts of Bellini. The capillaries are gathered up into the radicles of the veins, which lie in close proximity to the arteries. Occasionally the small vessels which arise from that portion of the cortical net- work of capillaries nearest the boundary' la\er, instead of entering the interlobular veins, pass downward into the medulla: these, which are known as false vasa recta, sooner or later divide again into capillaries which enter into the medullary network. The vasa recta divide the medulla into regions corresponding to the lobules.
212 PART II. HISTOLOGICAL AXATOMY.
The capsule of the kidne}- is a thin but tough fibrous membrane, somewhat lamellated in structure, especiall}^ toward the outer surface, and having in its deep portion a scant}^ plexus of smooth muscular fibres. Atthehilumthe capsule is continuous with the fibrosa of the ureter. Its sur- face is invested with a small amount of adventitious areo- lar tissue which in some cases contains numerous fat lobules, and by which it is connected ventrally with the fibrous layer ofthe peritoneum, and dorsally with the fascia of the adjacent muscles. A plexus of lymphatics has been described in the capsule whose channels connect with spaces in the superficial portion of the cortex : it unites at the hilum with lymphatics which accompany the blood vessels along and from the medullary rays.
The lining which invests the pelvis of the kidney and its prolongations, the calyces, may be regarded as the con- tinuation of the inner coats of the ureter, and may, like the wall of the latter, be regarded as consisting of an epi- thelial and a musculo-skeletal layer. The epithelium is of the transitional type, which is found only upon urinary surfaces : its peculiarities will be discussed in connection with the bladder, in whose structure they are most readily demonstrable. Immediately beneath is a thin but dense fibrous membrane which, with the epithelium, makes up the mucosa of the pelvis. This constitutes the sole in- vestment of the papillae, the two layers ofthe mucosa be- ing continuous with the epithelium and basement mem- brane respectively of the uriniferous tubules at the mouths of the ducts of Bellini. As we pass along the surface of the calyx, however, the mucosa begins to be separated
CHAPTER XYII. URINARY ORGANS. 213
from the surface of the columns of Berlin by scattered muscular fibres, which increase in quantity as we approach the margin of the fold which surrounds the tip of the pa- pilla : here a well defined muscular ringis formed, which has been compared to the transverse layer of the muscu- losa ; it may be regarded as in a certain sense a sphincter of the papilla. The general surface of the pelvis is lined by the mucosa and the muscular layer, there being present between the two an inconspicuous submucosa of areolar tissue, which also contains a small amount of adenoid tissue and scattered mucous glands.
The ureter, as has been stated, consists of an epithelial and a musculo-skeletal layer; the components of the latter being so distributed as to form with the epithelium a mu- cosa, a submucosa, a musculosa, and a fibrosa. The mu- cosa is a continuation of that just described as lining the ;^./tv*i:^ pelvis of the kidney, with which it agrees in structure in every essential respect : it is relatively greater in quantity, being thrown, like that of the oesophagus, into longitudi- nal folds. The submucosa is correspondingly increased in quantity : as in the bladder, it contains a small amount of diffuse adenoid tissue, scattered nodules having also been described, as have occasional mucous glands. The muscu- losa shows throughout the whole length of the ureter two well defined layers, an inner longitudinal and an outer transverse ; and along the lower portion traces of a third layer, external to the circular, are found, in the form of scattered longitudinal bundles. Each of the principal layers contains a comparatively large amount of inter- stitial connective tissue as compared with the muscular
./^-
,Lm^<^fci/uiM- '• toHMltj^ 1^ J^my\hiyL
214 PART II. HISTOLOGICAL ANATOMY,
layers of the alimentary canal : outwardly this is continu- ous with a well defined but not very dense fibrosa.
The bladder resembles the ureter in the essential histo- logical structure of its wall. The epithelium which lines the mucosa resembles that of the pelvis of the kidney and of the ureter, as has been stated : the term transitional generalW applied to.it, has, as it is generally defined, little meaning; it serves, however, to connote certain character- istics pertaining to epithelium found only on urinary' surfaces, and distinguishing it from stratified squamous epithelium, with which it is often compared. The most marked pecularity of transitional epithelium is the power possessed by all of its cells (and not the deeper layers only, as is the case with stratified squamous epithelium) of changing and regaining its form in connection with the stretching or relaxation of the membrane beneath : this can be most readily seen by comparing sections of the col- lapsed bladder and of one distended by a hardening fluid: both conditions will therefore be described.
In the former case the transitional epithelium is seen to consist of three distinct forms of cells arranged in what may perhaps be termed as many layers, though, as will be seen, the boundaries which separate them are not strongly marked. The surface is invested with a single layer of cells which are almost spheroidal or cuboidal above (though usuall}' not as deep as they are wide), the upper surface being either flattish or slightly convex when the mucosa is neither stretched nor pressed together, and almost hemi- spherical when the bladder is strongly contracted: the lower surface of the superficial cells is sculptured by con-
CHAPTER XVII. IKINAKV ORirANS. 215
cavities which fit closely the surfaces of the cells of the next layer: two nuclei are sometimes seen in a single cell, and it is quite possible that the superficial cells still retain the power of cell-division. The next layer consists of large pear-shaped cells, their rounded ends fitting into the exca- vations upon the under surfaces of the superficial cells, and their smaller tapering extremities extending to the base- ment membrane beneath. The spaces between the large ends of the pyriform cells and the membrane are occupied by the constituents of the third layer, which is one or two cells deep, and consists of smallerclosely packed spheroidal or pol3'hedral cells.
If the bladder be distended, the whole epithelial layer becomes much thinner: the superficial cells become flat- tened, approaching squamous cells in form; they never, however, loose their characteristic sculpturing: the pyri- form cells become greatly shortened : and the spheroidal cells correspondingly flattened. A knowledge of the ap- pearance of the elements in both these conditions is highly important on account of the frequent appearance of bits of epithelium in morbid urine. It should be remem- bered also, that these changes doubtless take place regu- larly with the daily periodic changes in the state of the bladder. It is well, too, to note the fact that the cells which compose transitional epithelium are alwa3's closely in contact, whatever changes of form they may undergo : and also that this epithelium has a remarkable power of resisting diffusion into the blood vessels beneath of the soluble constituents of the urine.
The mucous membrane of the bladder is much thicker
216 PART II. HISTOLOGICAL ANATOMY.
and firmer than that of the ureter, and the transition to the submucosa is much more abrupt. As in the ureter, a muscularis mucosae is w^anting, while a small amount of adenoid tissue is present. The Submucosa contains numerous elastic fibres, and occasional mucous glands, particularly toward the base.
The musculosa consists throughout of smooth muscular fibres whose bundles are in a general way (and particu- larly at the equator of the bladder) arranged in three layers, a middle circular and an outer and inner longi- tudinal : the bundles are, however, quite irregularly dis- posed, and there is much interstitial connective tissue : it is, therefore, not always easy to recognize the layers. At the base of the bladder the circular laj^er is increased in quantity to form the internal sphincter. The interstitial connective tissue is continuous externally with a rather loosely woven fibrosa, which is over a part of the outer surface of the bladder invested with serous endothelium.
The female urethra continues the wall of the bladder to the mucous membrane of the vestibule. In its course the epithelium passes from transitional to stratified squa- mous, the remainder of the mucosa undergoing no impor- tant change. The submucosa contains numerous elastic fibres and near the bladder a number of mucous glands : its deeper layer is highly vascular. The musculosa con- sists of an inner circular and an outer longitudinal laj'er of smooth fibres, and there is no well defined fibrosa.
CHAPTER XVIII. REPRODICTIVE ORGANS. 217
CHAPTER ZVIII. THE MALE REPRODUCTIVE ORGANS.
The male and female reproductive bodies, or gonads, are in their orijj^in, mode of development, and primarv position essentially similar bodies. The reproductive ele- ments produced in them differ so widely, however, in their activities as to call for widely differing mechanisms for their discharge: mechanisms which are nevertheless de- rived from the modification of intimately allied structures, whose development is alwa\'s closel}-^ associated with that of the urinarv apparatus. For this reason the descrip- tion of the early stages of the latter will be deferred until the completion of this and the subsequent chapter.
The male gonads, or testes, each with its associated epididymis, are in the adult human subject suspended by the spermatic cords in saccular folds of the skin conflu- ent below the penis and forming the scrotum. The latter is formed, as will be more fully described later, by the push- ing down of the abdominal wall on either side of the base of the penis : its structure therefore conforms to that of' the body wall, subject, however, to special modifications in each of its constituent layers.
The skin of the scrotum is thin, corrugated, rich in brownish pigment and in sebaceous glands, and has scat-
218 PART II. HISTOLOGICAL ANATOMY.
tered over its surface flattened curlinghairs with conspicu- ous bulbs. The superficial fascia of the groin and adjacent parts is continued into the scrotum to form a character- istic tunic, the dartOS, which is best developed in the fore- part of the scrotum : it is quite highly vascular, and con- tains numerous smooth muscular fibres ; it is consequently of a reddish-brown color. The right and left dartos tunics unite in the mid-plane to form a partition, the septum scroti. The corrugation of the skin of the scrotum is caused by the constriction of the muscular fibres of the dartos.
Immediately internal to the dartos is a denser and firmer fibrous layer, thin and transparent, known as the sper- matic fascia: it is derived from the tendon of the external oblique muscle of the abdominal wall. Within and closeU' associated with the spermatic fascia is a layer of areolar tissue which contains numerous bundles of striped muscu- lar fibres variously disposed, constituting a more or less continuous muscular layer termed the cremaster, and formed from an extension of the external oblique muscle. Still farther within, the fascia transversalis is continued in the scrotal wall by the infundibuliform fascia, a fib- rous layer separated from the preceding by loose areolar tissue and immediately underlying the parietal portion of the tunica vaginalis, a serous membrane derived from the peritoneum, which lines the scrotum and is reflected upon the spermatic cord and testis.
The successive la^^ers characteristic of the scrotal wall are, therefore, from without inwards, the skin, the dartos tunic, the spermatic fascia, the cremaster muscle, the in-
CHAPTER XVIII. REPRODUCTIVE ORGANS. 219
fundibuliform fascia, and the tunica vaginalis. Of these the (lartos forms the basis of the median septum, while those internal to it invest the cavities which are separated thereby.
The spermatic cord of either side is composed of the ves- sels and nerves of the testis together with the duct of dis- charge, known as the vas deferens : they are imbedded in areolar tissue and surrounded by the continuations of the coverings of the testis. The structure of the duct will be described later.
The visceral portion of the tunica vaginalis invests the surface of the testis except its posterior border, along w^hich it is reflected to become continuous with the parie- tal portion : it is frequently termed the tunica adnata. Beneath it lies the proper capsule of the testis, the tunica albuginea, a dense white fibrous layer of considerable thickness: along the posterior margin this is continued into the interior for some distance as a wedge-shaped fib- rous reticulum known as the mediastinum testis, and also as the corpus Highmori: from the mediastinum ra- diate stout straight bands of fibrous tissue, the septa or trabeculae, w^hich unite with the peripheral albuginea, thus imperfectly dividing the body of the testis into a num- ber of irregularK' p} ramidal lobules. The inner stratum of the albuginea is quite vascular, and is sometimes dis- tinguished as the tunica vasculosa: from it vascular trunks pass along the septa and form the blood supply of the lobules.
220 PART II. HISTOLOGICAL ANATOMY.
Each lobule consists of a variable number of seminifer- ous tubules supported by delicate lamellae of interstitial tissue continuous with the stout fibrous septa as well as with the proper membranes of the tubules : within these lamellae peculiar epithelioid cells are found either singly or in groups, whose origin and function are still uncertain ; they are known as the interstitial cells of the testis.
Each seminiferous tubule begins near the periphery of the testis with an irregularly contorted portion extending from its rounded extremity nearly to the mediastinum and known as the convoluted tubule, or the seminifer- ous tubule in the strict sense : these occasionally branch near their free extremities, and are said to anastomose in some instances. As they approach the mediastinum they become smaller and less irregular, and unite to form the straight or conducting tubules, which lie in the apices of the pyramidal lobules. On entering the reticular medias- tinum the latter anastomose freely to form a network of tubules of variable size, forming the rete testis, whose meshes correspond with the spaces of the mediastinum.
The membrana propria of the convoluted tubules con- sists of several lamellae of endothelioid cells with flattened oval nuclei. Upon this membrane rests an epithelial layer several cells deep, from whose inner strata are derived the male reproductive elements or spermatozoa. The base- ment membrane of the straight tubules is a continuation of that of the convoluted : it is lined with a single layer of cuboidal epithelium. In the rete testis the membrane be- comes continuous with the reticular framework of the mediastinum, and can no longer be distinguished: the
CHAPTKR XVIII. REPRODUCTIVE ORGANS. 221
channels are lined with a single layer of flattened epithe- lialjcells.
The arrano^enient of the epithelial cells of the convoluted tubules, and the changes which they undergo in the pro- cess of forming the spermatozoa, which is known as sper- matogenesis, have been the subject of much controversy and cannot vet be said to be fully understood. Different methods have been described with positiveness not only in difl'erent classes of animals, but in different orders of the mammals, and even in different members of the same order. Our knowledge of the facts in the human subject are, as in most cases, less perfect than of those in the lower animals. In addition, the same terms have been applied bv different writers to what are certainly different stages. The following account is based on that given by Schaefer in the tenth edition of Quain's Anatomy.
Next to the basement membrane is found a layer of cells most of which are cubical, clear, and possessed of nuclei which are in the resting or network phase : here and there dividing nuclei are seen : these cells ma}' be termed parietal cells or spermatogonia. Scattered here and there among them are larger cells which project between the more in- ternal layers: these are known as sustentacular cells: w^hen_much elongated and joined to bundles of develop- ing spermatozoa the}- form the columns of Sertoli.
Within the parietal la\'er is seen a middle layer of some- what larger spheroidal cells whose nuclei show various phases of division, the intermediate or spermatogenic cells. The layer may be one, two or more cells in depth ;
222 PART II. HISTOLOGICAL ANATOMY.
its constituents are probably derived primarily from the cells of the parietal layer, but increase in number by lat- eral divisions. They are by some authors termed sper- matocysts.
The inner layer of distinctly cellular elements consists of smaller and more numerous cells derived from the inter- mediate layer, the spermatoblasts (or spermatids of some writers); these are probably directly transformed into the spermatozoa. When first formed they compose a layer of closely packed small granular cells: they subsequently be- come more and more elongated vertically and collected into small groups, each of which becomes connected with one of the sustentacular cells above mentioned to form a column of Sertoli: later the bundle of spermatozoa derived from each group of spermatoblasts becomes separated from its sustentacular cell; the constituent elements are set free, and accumulate in large numbers in the lumen of the tubule.
The mature spermatozoon consists of an oval and flat- tened head, about four and a half micra long, two to three broad, and one to two thick; a cylindrical rniddle part or body about six micra long and less than one in diameter; and a tapering cilia-like tail forty to fifty micra long. The head consists chiefly if not entirely of the nucleus of the spermatoblast : the origin of the body and the tail is not so certain. In the lower animals the form and struct- ure of spermatozoa vary greatly.
The p3^raraidal lobules and the body of Highmore, with their investment, the tunica albuginea, compose the whole of the gonad proper or testis in the strict sense. Closely
CHAPTER XVIII. REPRonrCTIVE ORGANS. 223
associated therewith and often regarded as a portion thereof, though in reality the beginning of the efferent appa- ratus, is the epididymis, a tubular mass attached to the testis in the region of the mediastinum. Associated with the latter are certain rudimentar\' organs known respec- tively as the hydatids of Morgagni, lying between the head of the epididymis and the upper end of the testis, the vas aberrans, attached to the lower end of the epidid- ymis, and the organ of Giraldes, found near the base of the spermatic cord.
The channels of the rete testis anastomose freely through - out the whole of the mediastinum or body of Highmore. Those of the upper or anterior portion open into the effer- ent tubules, also known as vasa efferentia, from twelve to twenty in number, which penetrate the albuginea and enter the epididymis, of which they form a part : at first straight, they soon become coiled in conical masses, the coni vasculosi, whose bases are turned away from the testis. These aggregated conical bodies, together with the upper portions of the canal of the epididymis, into which the efferent tubules open, form the globus major, or caput epididymis. Their constituent tuljules are as large as the convoluted tubules of the testis: they are lined with a cuboidal or short columnar ciliated epitheli- um, beneath which is a well defined membrana propria surrounded b}' a thin transverse layer of smooth mus- cular fibres.
The canal of the epididymis, exceedingh- flexuous from the point of its origin in the extremities of the up- permost vasa efferentia, becomes disposed in numerous
224 PART II. HISTOLOGICAL ANATOMY.
small irregular coils in the middle region and is continued at the lower portion as a densely convoluted mass, the globus minor. At its origin it is twice the diameter of the vasa efferentia: lower it becomes smaller, and is enlarged again in the globus minor, from which it is con- tinued as the beginning of the spermatic duct or vas def- erens. It is lined throughout its course with tall col- umnar cells (between whose bases smaller rounded cells occur) : these cells are provided with unusually long cilia throughout the greater portion of the tube, the cilia dis- appearing in the lower portion. Beneath the underlying membrane is a thin circular layer of smooth muscular fibres continuous with that of the vasa efferentia, which is surrounded by a thin longitudinal layer. The convolu- tions of the tube are bound together by interstitial areo- lar tissue which is here and there replaced by incomplete fibrous septa [w^hich partially divide the 'epididymis into irregular lobules.
The hydatids of Morgagni are small saccular bodies lying between the globus major and the upper end of the testis. One of these, the stalked hydatid, is provided with a short peduncle, is usually present, and has a homo- logue in the female organs of reproduction. The others, the sessile hydatids, are variable in number and some- times wanting: they are found in the male only. Both stalked and sessile hj^datids are lined with cuboidal epi- thelium upon which cilia have been described.
The vas aberrans is a long narrow blind tube, a diverti-
chaptp:r XVIII. rki»K()I)1'CTivk okcans. JJo
culum of the canal of the epididymis, which arises from the latter at or near the point where it becomes continu- ous with the vas deferens. Like the canal, it is exceedingly tortuous in its course, forming an elongated convoluted mass which extends upward among the vessels of the spenut'itic cord. It resembles the vas deferens in struct- ure. It is almost invariably present, is sometimes branch- ed, and in some cases more than one such structure oc- curs.
The organ of Giraldes, also called the paradidymis, is a small body found on the front of the spermatic cord just above the globus major. It consists of several discon- nected irregularly branched tubules lined with columnar ciliated epithelium. Their coiled masses form irregular nodules imbedded in the connective tissue of the regions between the cord and the epididj^mis.
The spermatic duct, or vas deferens, is the continua- tion of the canal of the epididymis. It is considerably larger and more complex in structure than the canal, showing like many of the tubular structures of the bod}', a definite mucosa, submucosa, and musculosa. The mu- cosa consists of a stout membrane sometimes throw^n into Ipngitudinal folds, and bearing a non-ciliated colum- nar epithelium : the submucosa is composed of areolar tis- sue somewhat laminated in arrangement : the musculosa is thick and yellowish in color and comprises an inner cir- cular and an outer longitudinal layer of smooth muscular fibres, external to which is a fibrous adventitia. At the
226 PART II. HISTOLOGICAL ANATOMY.
commencement of the duct there is also a longitudinal layer of muscular fibres internal to the circular layer.
The ampulla, or sacculated enlargement of the duct sit- uated near its junction with the seminal vesicle, resembles the rest of the duct in structure, save that the various coats are somewhat thinner. The tubular diverticula of thevasa deferentia known as the seminal vesicles are like the ampullae in structure. The spermatic duct or vas de- ferens of either side unites with its associated seminal vesi- cle to form the ejaculatory duct, or common seminal duct, which completes the passage way from the seminif- erous tubules of the testis to the urethra : in this region the walls of the tube are much thinner than in the vas deferens and the external fibrous adventitia disappears as the duct enters the substance of the prostate gland.
The male urethra, into which the common seminal ducts empty within the region surrounded by the prostate gland, is by virtue of that fact divisible into two distinct regions, the urinary and the urino-genital : it is the former alone that corresponds to the female urethra. By its anatomi- cal relations it is also divided into the prostatic, the mem- branous, and the penial urethra : the former includes the first of the two regions above mentioned and a portion of the other. Histologically the structure differs in the sev- eral regions chiefly in the character of the epithelium, in the special structures found in the mucosa of the penial region, and in the composition of the musculosa.
The prostatic urethra of the male resembles the female urethra in being essentially a continuation of the wall of
CHAPTER XVIII. REPRODUCTIVE ORGANS. 227
the bladder. It has a mucosa lined with transitional epi- thelium, which rests upon a membrane rich in elastic fibres. Beneath is a highly vascular submucosa. and beyond this a musculosa which consists of an inner longitudinal and outer circular layer of smooth muscular fibres. In front of the openings of the common seminal ducts the epjthe- lium passes by a 'gradual modification from the transi- tional to a somewhat stratified columnar form. The -^ membranous urethra continues the structure of the an- terior portion of the prostatic, and in addition receives a distinct layer of striped muscular fibres from the adjacent compressor urethrae.
In the penial urethra the epithelium is composed of a single layer of columnar cells except at the fossa navicu- ^ laris, where it passes into stratified squamous epithelium if continuous with that of the surface of the glans. Here and there the mucosa exhibits irregular depressions of variable size, known as lacunae Morgagni : it also has connected with it numerous small racemose glands, the glands of Littre, which are also sparingly found in other portions of the urethra; they are lined with cuboidal or low columnar glandular cells.
The penis consists essentially of three masses of what is commonly called erectile tissue, the right and left cor- pora cavernosa, and the median inferior corpus spongi- osum ; the latter being traversed throughout its length by the penial urethra, and having its distal portion expanded to form the glans penis : the whole, of course, invested by the somewhat modified integument. The form and rela-
228 PART II. HISTOLOGICAL ANATOMY.
tions of these bodies are matters for anatomical discus- sion : we are here concerned with their histological struct- ure onh'.
What is known as erectile tissue consists primarily "sirapl}' of a somewhat circumscribed collection of larger and smaller veins w^hich under certain circumstances may become distended with blood, thus causing the parts in which they lie to expand." Its specialization consists chiefly in the enlargement of the veins to form irregular sinuses, or cavernae, and their frequent anastomosis ; the development and modification of the circumscribing skel- etal tissues ; and accessory modifications of the arteries of supplv. That the cavernous sinuses are to be regarded as veins is showm not only b}' the striicture of their walls, but also by the fact that the blood reaches them chiefly if not entirely through capillaries. It will be seen from w^hat has just been said that erectile tissue is not constant in structure as are adenoid and other compoutjd tissues that have been previoush^ described. The form assumed in each locality where it occurs will therefore be briefly stated.
The corpora cavernosa are surrounded and united by a stout fibrous envelope, termed, like that of the testis, the tunica albuginea ; it consists of bundles of white fibres chieflv disposed in a longitudinal direction and mixed with numerous elastic fibres: within, the w^hite fibres are chiefly circularly disposed, surrounding each of the two corpora to form an individual sheath; these two sheaths are
CHAPTER XVIII. REPRODUCTIVE ORGANS. 229
confluent in the mid-plane through the greater part of the penis, tbnning a septum which is incomplete by virtue of the presence of slit-like apertures through which the erectile tissue becomes continuous from side to side: these ajiertures are most numerous in the anterior part of the penis.
From the fibrous sheath numerous stout trabeculae pass inward to form a reticulum whose meshes are the cavernous sinuses. They are composed chiefly of white fibrous tissue, with more or fewer elastic fibres inter- mingled, and contain in addition numerous bundles of smooth muscular fibres : their surfaces are lined with the vascular endothelium of the sinuses. The trabeculae are stoutest and most numerous near the surface: the sinuses are correspondingK' largest at the centre of the bod\' ; thev are also larger near the extremity of the penis than at the base, in which region their long diameter is as a rule placed transversely to the penis. The small arteries which follow the trabeculae in many cases project from their surfaces in peculiarly curled and coiled loops; they are hence known as helicine arteries: they are said in some cases to open directly into the sinuses, but such a direct communication between an arter\' and a modified vein is not easily demonstrated under conditions which exclude the possibility of error.
The corpus spongiosum differs from the corpora caver- nosa histologically in the thinness and increased elasticity of its fibrous tunic, which contains so much elastic tissue as to be yellowish in color ; in the smaller size and greater
230 PART II. HISTOLOGICAL ANATOMY.
uniformity of the trabeculae; and in the lesser amount of muscular tissue which they contain. The venous sinuses are smaller and more uniform, forming a spongy mass, from which the name is derived : their greatest dimensions are as a rule longitudinally disposed.
In the glans the meshes are quite small and uniform in size : the erectile tissue passes insensibly into the lower strata of the integument, with which its surface is invested. The derma is thin and highly vascular over the surface of the glans, and the epidermis has the form of a stratified squamous epithelium devoid of the division into layers characteristic of the cuticle and resembling in character that found on the oesophageal mucous membrane. Glands arewanting, except upon the corona and the cervix, where modified sebaceous glands, the glands of Tyson, are abundant. Special nerve terminals, the so-called genital corpuscles, are present, as are Pacinian bodies.
The skin of the penis is quite thin, highly elastic, and very movable, and contains but a very little fat : the larger portion is devoid of hair also: as it passes around the free margin of the prepuce it changes its structure and the character of its epithelium, the lining of the prepuce, like the investment of the glans, having the appearance of a mucous membrane. At the base it passes on into that of the pubes, which is quite thick, beset with coarse hairs, and provided with a dense fatty layer.
The urino-genital tract of the male has associated with it certain glandular bodies, the prostate gland, which sur-
CHAPTKR XVIII. REPRODUCTIVE ORGANS. 231
rounds the proximal region of the urethra, and the glands of Cowper, paired organs opening into it near the point where it enters the corpus spongiosum. Opening into it ventrally in close proximity to the apertures of the ejacu- latory ducts, is an interesting rudiment, the sinus pocu- laris, otherwise known as the uterus masculinus. These structures will next be described.
The prostate Ts a glandular body which differs from most organs of the kind in the fact that not only its cap- suje but also its stroma contains a ver}- considerable amount of smooth muscular tissue. This muscular tissue / is in continuity with the musculosa of the urethra and of J the ejaculatory ducts, and posteriorly with that of the » bladder. The capsule is divisible into two layers, between which is found the prostatic venous plexus: from it trabec- ulae pass inward to form the framework of the gland, con- ' sisting, in addition to the smooth muscular tissue already J mentioned, of a very small amount of white fibrous tis- sue and a larger quantity of elastic fibres. The alveoli are ) f tubular, frequently quite elongated and irregular in shape, I their walls sometimes showing conspicuous folds : the epi- thelium is columnar and simple save that frequently small and spheroidal cells are found at the base of the columnar ) cells. The ducts, which are numerous, are lined with col- umnar epithelium which changes into stratified as it ap- 1 proaches their openings upon the urethra.
Cowper's glands are small bodies of the racemose type, each consisting of several small lobes. Their capsules and
232 PART II. HISTOLOGICAL ANATOMY
supporting framework resemble those of the prostate to some extent in the presence in each of a small amount of smooth muscular fibre : a well defined longitudinal laj'-er of smooth muscular fibres is also present in the wall of the principal duct. The acini resemble those of a mucous salivary gland in form and in the general appearance of the glandular epithelium : there is a conspicuous lumen, the cells are pyramidal, and the nuclei are situated near the base. Nothing resembling the crescents or demilunes of the mucous glands has been observed. The lobar ducts are lined with cuboidal epithelium, which passes into col- umnar in the principal ducts.
The sinus pocularis is the homologue in the male of the vagina and uterus of the female. It is a diverticulum of the prostatic urethra having a well-defined muscular wall and a mucosa containing a number of short tubular glands which resemble the uterine glands in their form and structure.
chai'Ti;k XIX. reproductive organs. 23v
CHAPTER XIX. THE FEMALE REPRODUCTIVE ORGANS.
The female reproductive apparatus consists of the fe- male gonads, or ovaries, in which the reproductive ele- ments are formed, the oviducts, or Fallopian tubes, by which they are conveyed from the ovaries; the uterus, in which they are received, and in which the fertilized ovum or oosperm develops into the embryo ; the vagina, by which the uterus communicates with the exterior, and the parts composing the vulva, w^hich immediately surrounds the openingof the vagina. As in the case of the male, there are also present certain rudimentary bodies, chiefly in the vicinity of the gonad.
The ovary, like the testis, is an organ in which special- ized cells, epithelial in their origin, are matured and liber- ated : the sexual elements differ, however, in an antipodal manner as regards their size, activity, destination, mode of transportation thereto, and mode of liberation : there is a corresponding difference in the structure of the organs which produce them, the ovary having nothing of that tubular structure seen in the testis and giving to that body a close resemblance to a gland.
The framework or stroma of the ovary lacks the abund- ant white fibrous tissue found in the capsule and trabecu-
234 PART II. HISTOLOGICAL ANATOMY.
lae of the testis : it consists chiefly of a peculiar form of connective tissue characterized by the presence of elon- gated nucleated cells which are frequently spindle shaped, and b}^ the scarcity of true fibrous tissue, either white or elastic. It contains numerous smooth muscular fibres, which are most abundant in the deeper portions. Toward the surface the stroma becomes more dense, forming quite a well-defined superficial layer, to which the name of the tunica albuginea has been given ; it lacks the firmness and definiteness of the layer so designated in the testis. At the base of the ovary the stroma is especially rich in blood vessels : the region occupied by them is known as the zona vasculosa.
The region between the albuginea and the zona vascu- losa constitutes the parenchyma of the ovary. It is rather indefinitely divided into a cortical and a medullary portion by the character of the Graafian follicles contained in it. It also contains, scattered irregularly through it, groups of interstitial cells similar to those found in the testis.
The surface of the ovary is invested with a layer of cells which are structurally continuous with the serous endo- thelium of the peritoneum, but which differ therefrom in form and function. They are cuboidal or low columnar in^ shape, and constitute the germinal epithelium: they have no proper basement membrane, but rest directly up- on the tunica albuginea. Here and there may be seen, es- pecialh-^ in the embrj^o, certain cells which are larger and more rounded infoi'm : these are the primitive OVa. These, during foetal life and possibly in childhood sink into the
CIIAPTKK XIX. RHI'KODrCTIVK DKGANS. 235
Stroma, accompanied by tubular or spheroidal nests of epithelial cells : it is doubtful whether this ever takes place in the adult : it is also at present a matter of question whether the primitive ova increase in number by division after they have passed into the stroma, or whether all so situated have come from the epithelium of the surface of the ovary.
The cortical region of the parenchyma is crowded with the spheroidal masses of cells formed in the manner just described, each consisting of one or sometimes two primi- tive ova surrounded by a layer of epitheHal cells. These are the primitive Graafian follicles. In those immediately beneath the albuginea the surrounding layer is usually but a single cell deep: at first flattened, and hardly dis- tinguishable from the cells of the adjacent stroma, its cells soon become cuboidal in form. At the same time the fibres of the stroma tend to assume a disposition concen- tric to the follicle, forming the beginning of the theca folliculi.
As the follicles grow older they tend to sink deeper into the stroma of the ovary; the cells of the enveloping layer at the same time proliferating, and the layer becoming several cells thick. Shortly afterward the ovum leaves its central position for one nearer one side of the follicle, usu- uallv that farthest from the surface of the ovary, while a cleavage takes place in the cellular layer toward the other side, the space formed becoming infiltrated with a clear fluid, the liquor folliculi. The follicles now rapidly increase in size, at the same time sinking into the medul- lary region, where they are seen as large vesicles filled
236 PART II. HISTOLOGICAL ANATOMY
with fluid, invested by a well-defined theca, and lined by a layer of small isodiametric cells of irregular form, the layer being several cells deep : it is now known as the membrana granulosa. Attached to it at one side of the follicle is the rounded heap of similar cells which contains the ovum : it is termed the discus or cumulus prolig- erus.
The follicles still increasing in size, their outer wall now tends to approach the surface of the ovary, the fulh' ma- tured follicle finally projecting somewhat from the surface, b}^ whose rupture the contained ovum is eventually to be liberated. The theca is now well defined and consists of two laj'ers, an inner or vascular, and an outer or fibrous layer. The cells of the membrana granulosa next the theca and those of the cumulus next the ovum are dis- tinctly columnar in form. The most conspicuous as well as the most important body present is the mature ova- rian ovum. This is a spheroidal body now much larger than the primitive ovum from whi&hit was developed, although small as compared with the ova of many of the lower ver- tebrates. It is in the human subject about two-tenths of a millimetre in diameter. It is invested b}-- a thick cover- ing appearing when seen with microscopes like those used by the earlier observers to be quite clear : as its optical sec- tion forms a girdle or zone of considerable breadth about the ovum it was named by Von Baer the zona pellucida. Careful examination by modern instruments and meth- ods demonstrate that it contains innumerable radial striae: it is therefore now commonly called the zona striata, or still more accurately the striated membrane.
ClIAPTEK XIX. KKI'UODrCTIVi: ORCANS. 237
Several eminent and accurate observers have described a delicate membrane, which they call the vitelline mem- brane, internal to the layer just described : its presence cannot l^e readily demonstrated with certainty.
Within the envelope just described is the vitellus, or volk, wrongly so called, since it does not correspond to the bod v so termed in the eggs of many lower vertebrates: it is a homogenous protoplasmic mass, semi-fluid in con- sistencv and highly granular. It contains, usually in an eccentric position, a large spherical nucleus which was named by Purkinje the germinal vesicle: the nuclear con- tents exhibit a coarse network characteristic of the phase of complete rest : there is usually but a single nucleolus, which is quite large and rounded, and was called by Wag- ner the germinal spot.
When the ovum is discharged by the rupture of the fol- licle u])on the surface of the ovar^^ the follicular cavity is at first filled with a clot of blood. It is quickly invaded by growths from the wall of the follicle formed in part of rapidlv proliferating cells of the membrana granulosa, in part of folds and procesvses from the theca : it has been as- serted that the interstitial cells previously mentioned also enter to a large extent into the ingrowing structure. The result is the formation (about the shrunken and discolored clot as a centre) of a mass ofmingled cells and fibres known as a corpus luteum : this is at first sharply defined by the presence of the theca, but gradual!}' loses its definiteness, and becomes continuous with the mass of the ovary, the peculiar spurious tissue thus formed composing quite a
238 PART II. HISTOLOGICAL ANATOMY-
large part of that organ in age. The corpus luteum formed concurrently with pregnancy is large and well-defined and is regarded as characteristic : but corpora lutea equally large and distinct sometimes (though more rarely) occur under other conditions.
The oviducts, commonly termed the Fallopian tubes,
. »> while they vary in form in the several regions distinguish- ed by the anatomist, are quite uniform in their histologi- cal structure throughout their whole extent. Continuous at the isthmus with the uterus, they open at the fimbriated extremities into the peritoneal cavit}^ : they consequently present the only instance of direct continuitx^between a mucous and a serous surface. Like nearly all the larger tubular structures in the body, the wall is divisible into a mucosa, a submucosa and a musculosa; to which is added a serosa derived from their investment by the marginal fold of the broad ligament.
The mucosa consists of a well developed fibrous mem- brane moderately rich in elastic fibres, and well supplied with blood vessels and lymphatics, which supports a layer of simple columnar ciliated epithelium. An imperfectly developed muscularis mucosae, consisting of longitudinal bundles of smooth fibres, is also present. The mucosa throughout its extent is thrown into longitudinal folds which in the ampulla and particularly in the infundibulum are very extensive and have secondary folds upon their surfaces, giving to the cross section a peculiar arborescent appearance. As seen in such sections the bases of these folds often present the appearance of tubular glands ; but
CHAPTER XIX. F<i: PRODUCTIVE ORGANS. 230
true srlands are not present. The inner surface of the fun- briae is lined with the mucosa, while the outer surface is covered with the serosa: the two becomingconfluent along the sides.
The submucosa is a simple layer of areolar tissue of but slight depth. It is continuous with that of the uterus, like which it contains small ganglia and scattered multi- polar cells, forming the rudiments of a plexus. The mus- culosa consists of an inner circular and an outer longitu- dinal layer of smooth muscular fibres, the latter being but imperfectly developed. The serosa consists of a thin fib- rous membrane supporting the serous endothelium charac- teristic of the surface of the peritoneum.
Attached to the extremity of the tube or to one of the fimbriae is frequently found a pedunculated cyst or stalked hydatid of Margagni. It is the homologue of the body bearing the same name in the male reproductive ap- paratus and resembles it in general structure, the cavity of the sac and also of the pervious portion of the stalk being lined with cuboidal or columnar epithelium.
Situated in the broad ligament between the ovar\' and the ampulla of the oviduct is a well-defined mass of ir- regularly convoluted tubules, known as the parovarium. It is also called, from its discoverer, the organ of Rosen- mueller: Waldeyer has proposed for it the name of the epoophoron. The constituent tubules are lined with low columnar epithelium : they converge toward each other, without uniting, at the ends nearest the ovary : the other
240 PART II. HISTOLOGICAL ANATOMY.
extremities diverge somewhat, and terminate in a longi- ■ tudinal tube which runs parallel with the oviduct: in some of the lower mammals this tube is quite extensive and is known as the duct of Gartner, a term also applied to it in the human subject by many. Somewhat nearer to the uterus than the parovarium a smaller and more irregular group of rudimentary tubules occurs, similar in structure to those just described. These have been designated by Waldeyer the paroophoron. The homologies of these rudi- ments will be discussed later.
The uterus presents but two distinct regions histologi- calh%thefundusandbody agreeing in structure and differ- ing from the cervix. The most characteristic features of the upper region are found in the mucosa, the stroma of which is greatly modified, while the muscularis attains a greater development than does the structure bearing that name in any^other portion of the body. The surface is inves- ted with a single layer of columnar ciliated cells directly con- tinuous with that lining the Fallopian tube. Beneath this is a very thick mucous membrane containing but a very small quantity of fibres and composed in large part of spindle shaped cells similar to those found in the stroma of the ovary, loosely interwoven: the spongy mass so formed contains numerous lymph spaces and leucocytes. Im- bedded in it are great numbers of tubular uterine glands, wavy or convoluted in their course, not infrequently'' branched, and penetrating to the base of the stroma and quite frequently between the bundles of fibres of the mus- cular layer: they are bounded by a delicate basement mem-
CHAPTER XIX. REPRODUCTIVE ORGANS. 24-1
brane which supports columnar cells similar to those lin- ing the uterine wall : near the blind extremity of the tube the columnar cells entirely till its cavity: but throughout the greater part of its extent there is a distinct lumen.
The rnuscularis rnucosae is the chief muscular coat of the uterine wall; greatly developed at all times, it is enor- moush' hypertrophied during pregnancy, parth' by the great increase in number. It consists of bundles of fibres interwoven with a sparing amount of interstitial connect- ive tissue, and running in various directions: their dispo- sition is apparently quite irregular, and cannot be de- scribed briefly with clearness : it is, moreover, subject to considerable variations: it can perhaps be best understood bv regarding it as consisting chiefly ot circularly disposed bundles which are arranged on the fundus in two sets, one concentric to the insertion of each of the two oviducts, and which become gradually combined to form a single set as the}' approach the lower extremity of the body.
There is associated with the great development of the rnuscularis mucosae a corresponding reduction of the sub- mucosa, there being less independent movement of the mucosa and musculosa in this case than in almost any of the similar hollow structures. There is, however, a dis- tinct zone of connective tissue discernible just exterior to the muscularis mucosae characterized particularly by the presence of numerous bloodvessels and lymphatics, and by scattered nervous elements. It is of such slight extent as to be by some described as wanting.
The musculosa consists of two distinct laj^ers of smooth fibres both quite thin and varying in their relative devel-
242 PART II. HISTOLOGICAL ANATOMY,
opment in different parts of the organ. The inner or cir- cular layer is the more uniform in thickness and in the ar- rangement of its fibres : the fibres of the outer laver are in the main longitudinally disposed, but are somewhat ir- regularly arranged upon the fundus, in accordance with its irregularities of form. This laj-er also gives off bundles of muscular fibres extending out into the ligaments of the uterus. The musculosa is invested by a serosa which is a continuation of the peritoneum.
The cervix differs from the region just described chieflv in the structure of the mucosa, the stroma of which is much firmer and richer in fibres, both white and elastic, the membrane being thrown into characteristic folds, and in the lower portion beset wnth minute papillae. The up- per two-thirds is lined with columnar ciliated epithelium continuous with that of the upper regions : this passes below into the stratified squamous epithelium which in- vests the papillated lower third. There are present both tubular and saccular glands said to be lined in each case with columnar ciliated epithelium : the saccular glands contain also goblet cells and secrete the thick mucus found in the cervix. Here and there spheroidal bodies filled with a clear yellowish fluid can be seen with the naked eye: they are probably occluded and enlarged mucous glands, and are known as ovula Nabothi. The muscularis mu- cosae is well developed and consists chiefly of circular bundles : these are accumulated in greater numbers at t2 upper and lower extremities of the cervix to form the sphincters of the regions in question.
The submucosa is not conspicuous. The musculosa re-
'1/
tM/<
CHAPTER XIX. REPRODUCTIVE ORGANS. 24-3
sembles in structure the same portion of the wall of the body of the uterus: the imier circular and outer Ion git u- dinal layers are clearly defined. The region of the cervix toward the rectum is invested with a peritoneal serosa: that toward the bladder is separated from that organ b\' an adventitia of areolar tissue. The portion of the cer- vix that projects into the vagina to form the OS uteri agrees in structure on its inner aspect with the cervix : on its outer it is a continuation of the vaginal wall.
The vagina differs from the uterus in structure in accor- dance with its differing and double function, it serving at once as the channel by which the male reproductive ele- ments are brought into proximity with the female ele- ments, and as the avenue of discharge for the foetus at its maturity. Its wall is muscular, dilatable, and highly elastic, somewhat erectile, and provided with a definite reinforcement of adenoid tissue ; a feature possessed by no other portion of the reproductive tract.
The mucosa of the vagina is lined with stratified squa- mous epithelium, which rests upon a thick mucous mem- brane. The surface of the latter is beset with minute pa- pillae which project into the deeper portions of the epithe- lium but do not produce a noticeable roughness of its sur- face, the only irregularities observable being those due to the well-marked folds or rugae. The upper portion is a dense fibrous layer rich in elastic fibres : below this in the rugae are networks of large veins supported by fibrous tissue containing numerous bundles of smooth muscular fibres which may be regarded as representing the muscu-
244 PART II. HISTOLOGICAL ANATOMY.
laris mucosae, elsewhere absent : a ridge of rudimentary erectile tissue is thus formed beneath each ruga. Leuco- cytes abound in the mucosa, and scattered nodules of ade- noid tissue are found : in the anterior wall near the orifice there is a \vell-defined adenoid layer. Special nerve termi- nals, the genital corpuscles of Krause, as found in the mucosa of the vagina. It is doubtful whether glands of any sort are present.
The submucosa is quite loose in structure and contains a venous network whose meshes run chiefly in the direc- tion of the vagina. Beyond the submucosa is the mus- culosa, which is not sharply defined, as in most cases, in- to distinct strata : the inner bundles are in the main circu- larly disposed, and the outer bundles longitudinally; the two regions being, however, blended by numerous oblique bundles. A well-marked fibrosa invests the musculosa : it is composed largely of elastic tissue and is best developed on the anterior wall : it also contains an extensive plexus of large veins intermingled with bundles of smooth mus- cular fibres and forming a layer of erectile tissue best de- veloped near the lower extremity.
I^he hymen agrees in its structure with a fold of the mucosa of the vagina, and can perhaps be regarded as de- rived therefrom, though its presence in rare instances in cases of absence of the vagina has caused this mode of origin to' be questioned by those who regard it as a fold of the skin of the vestibule.
The vulva includes a number of parts or regions each characterized by certain histological features worthy of
CHAPTER XIX. REPRODrCTIYE ORGAN'S. 24."
brief mention. The surface of the area known as the ves- tibule is covered by a mucous membrane continuous with that of the vagina and of the urethra at their respective orifices. It is covered with stratified squamous epitheli- um, contains numerous elastic fibres, and is feebly erectile: it also contains numerous simple mucous glands. At the lower limit of the vestibular area are seen on either side of the vaginal orifice the openings of the ducts of the glands of Bartholin, small racemose glands of the mucous type homologous with the glands of Co wper in the male sub- ject. Beneath the mucous membrane of the vestibule and somewhat external to the proper limits of the vestibular surface are paired elongated masses of erectile tissue, the bulbi vestibuli, whose converging upper extremities are continuous with smaller plexuses whose vessels are conflu- ent above with those of the glans clitoridis. The bulbar regions may be regarded as corresponding to the bilateral bulbous portions of the corpus spongiosum of the male subject.
The clitoris consists of two small corpora cavernosa identical in structure and relations wnth those of the male, and a small glans of spongy erectile tissue, which is of course imperforate. Its surface contains numerous geni- tal corpuscles. Its preputial fold is continuous with the upper and its fraenum with the lower of the anterior di- visions of the labia minora. The latter, while resembling the surface of the vestibule in color and texture, may be regarded as folds of the skin: the}' contain numerous large sebaceous glands, but sweat glands are wanting, as are also hairs : their inner surface contains numerous genital
246 PART II. HISTOLOGICAL ANATOMY,
corpuscles. Fat is wanting in the subcutaneous connec- tive tissue, but large irregular venous channels are present, with smooth muscular fibres, composing here as elsewhere a loose form of erectile tissue. The labia majora are well defined folds of the skin whose inner surfaces resemble in appearance the outer surfaces of the labia minora, with which they are confluent, but differ from them in the pre- sence of occasional modified sweat glands and minute hairs, associated with the sebaceous glands common to both. The thick and rounded margin of the fold affords a gradual transition from the moist stratified'squamous epithelium of the mucous type found upon the greater por- tion of the vulvar surface to the epidermis of the skin, with which its outer surface agrees in general struct- ure. In the deeper portions of the integument of the labia majora is found a layer of tissue similar to that forming the dartos tunic of the male scrotum, with which the labia correspond. Like the divisions of that structure they converge above to be united upon the pubic eminence in the region known as the mons veneris, characterized, as in the male, by an abundance of coarse curling hairs, of numerous enlarged sudoriparous glands, and by a dense mass of subcutaneous fat.
The mammary glands, while essentially tegumentary organs, differ so greatly from all other dermal glands as to require consideration apart from the latter: their func- tional relations to reproduction render it appropriate to
CIIAI'TEK XIX. KlirKODlCTIVH ORGANS. 24-7
discuss them at this time. I'sualh' but not always rudi- mentary in the malesuljjcct, they are normally but not in- variably fully developed in the female. Each mammary gland so called is in reality an aggregate of fifteen or tvventy distinct glands, if we regard that term as strictly desig- nating a secretory body provided with a duct which opens independently, since each of the ducts opens by a separate orifice upon the skin of the nipple: but their union into a single anatomical structure is so intimate that it is more convenient to designate each of these regions as a lobe ot a compound racemose gland. The structure of these lobes and of the whole organ varies greatly in relation to function- al activity ; but it is characteristic of it at all times that it possesses an unusuallv large proportion of connective tis- sue and fat in its composition.
Before the gland has been called into functional activity the lactiferous ducts are present, as well as the ducts ot the lobules by whose confluence they are formed : the lat- ter have at the extremities rudimentary acini, which are, however, relatively few in number, and consist of masses of epithelial cells. Acini, lobules, and lobes are alike im- bedded in an extensive stroma of connective tissue which forms stout septa not only between the lobes but between their subdivisions as well ; while a considerable amount of subcutaneous and interstitial adipose tissue is present.
As pregnancy advances the acini become larger and more numerous, still consisting, however, of solid masses of cells. At the time of delivery the central cells undergo fatt}' degeneration and form the colostrum corpuscles of the milk of commencing lactation. In the fully active
248 PART II. HISTOLOGICAL ANATOMY.
gland the acini are sgherpidal, comparatirely large, anJ are lined b}^ a single layer of cells which when at rest are flattened, but during secretion become cuboidal or colum- nar in form, their extremities containing one or more large oil-droplets in each instance: the latter are liberated by the rupture of the cell substance, a portion of w^hich is prob- abh^ contributed to the secretion. The basement mem- brane upon which the epithelium rests consists of an en- dothelioid layer of connective tissue corpuscles : the intra- lobular stroma is greatW reduced in proportional quan- tity. The terminal branches of the ducts have a thin base- ment membrane like that of the alveoli, which is lined by a single layer of flattened cells, their appearance resemb- ling that of the ductules of the salivary glands : the larger lactiferous ducts have stouter walls, and cuboidal epithe- lium ; these empty into the still larger channels, the galac- tophorous ducts, one of w^hich leads, as has been stated, from each lobe to an independent orifice upon the skin of the nipple. These large ducts have stout walls of fibrous and elastic tissue, containing a few smooth muscular fibres : they are lined with columnar epithelium save in their outermost portions, where the epithelium becomes stratified. Each has an enlargement, the ampulla, near its termination at the nipple, whose structure does not differ from that of the rest of the duct.
In the intervals between lactation the mammary glands assume a resting condition in many respects similar to their primitive state. They always contain, however, a smaller amount of dense fibrous tissue and a larger amount of fat, and are consequently much less firm in
CHAPTER XIX. RKPRODUCTIVE ORGANS. 240
texture. At the close ot the reproductive period they be- gin to undergo a retrograde metamorphosis, the acini and smaller ducts disappearing, the larger ducts collapsing, and the shrunken organ consisting chiefly of a mass of connective tissue and fat.
The nipple is a cvlindrical projection from the surface of the gland, covered with deeply pigmented skin and composed of the extremities of the lactiferous ducts, as- sociated bloodvessels, and smooth muscular fibres ar- ranged in circular and longitudinal bundles. The der- mal papillae are rich in nerve terminals: there is no subcutaneous fat : scattered in the surface are the small racemose glands of Montgomery : the areola at its base contains sweat-glands and numerous sebaceous glands.
The intimate relations existing between the urinary and reproductive systems and the homologies between the male and female sexual organs cannot be clearly stated without a description of their formation in the embr^'o, accompanied by a statement of some at least of the facts of their comparative anatoni}'. No attempt will be made at this time to discuss systematically either the embr\'ologv' or the morphology' of these or- gans, but such account of each will be alone given as appears necessar\' to the intelligent comprehension of the relations and homologies above referred to.
The structure fundamental to the whole of the uro- genital apparatus is what is known as a nephridium.
250 PART II. HISTOLOGICAL ANATOMY.
In the great majority of the classes of the higher inver- tebrates the elimination of nitrogenous waste products takes place through the agency of organs designated by that term. A nephridium is essentially a tubular structure opening at one extremity upon the surface of the bod}' (either directly or indirectly), and at the other communicating with the bod\^ cavity, or coelom, by a more or less funnel-shaped extremity termed the ne- phrostome. Its wall is lined with an epithelium which is glandular throughout a large portion of its extent : the funnel-shaped internal opening is commonly and the larger portion of the rest of the tube frequently cili- ated, the ciliary movement invariably sweeping toward the external opening. The tube may be simple and quite direct in its course, or long, convoluted, and di- vided into specialized regions. Where a well developed vascular system is present the more or less coiled ne- phridial tube is usualh^ provided with a rich net-work of capillaries. Nephridia may be simple: a single pair opening right and left on the surface of the body, as in many mollusks; or a pair being found in each of the majority of the segments of the body, as in the annelids, where they were first observed, and designated from their disposition segmental organs: or they ma}' be compound, a number of funnel-bearing and ciliated tu- bules opening symmetrically into lateral tubes which discharge [the secretion of the tubules upon the surface of the body or into a posterior cloacal sac, as in the roti- fers. Without entering into a discussion of the homolo- gies that may exist between the nephridia of the various
CHAPTER XIX. REPRODUCTIVE ORGANS 251
classes of invertebrates, or between those of either of them and those of the vertebrates, it may be said that the urinary apparatus of the hitter conforms, in a general way at least, to the plan last described.
The gonads, in which the reproductive elements are formed, are in most classes of Metazoa developed within the body-cavity, or coelom, into which thev ])rotrude, and into which the reproductive elements, when fully mature, are in many species discharged. In some of the lower forms where this takes place, and in which the re- productive elements are produced in immense numbers, the body wall becomes distended by their presence and finalU' liberates them by rupturing, the parent organism being thereby destroyed ; a condition comparable to that of the so-called annual plants, which grow, blossom, ripen a single crop of seeds, and die. In other cases the ne- phridia serve as channels for the escape of the reproduc- tive elements : it will be noted that the small and actively moving spermatozoa would pass far more readih' through the nephridial tubules than the large and passive ova; and it is probabK" true in the case of some species that the female is destroyed by the rupture of the body wall after the ripening of the first crop of ova, while the male elements are liberated through the nephridia, the pa- rent organism surviving. In most of the higher classes of animals there are distinct channels of discharge, the gon- aducts, for the reproductive elements : these are in some cases clearly formed by the modification of nephridia.
The vertebrate urinary aparatus consists in effect of a series of nephridial tubules arranged along the dorsal wall
252 PART n. HISTOLOGICAL ANATOMY.
of the body cavity on either side of the mid-plane and opening into right and left ducts of discharge. The dis- position, form and relations of the tubules, and the origin and position of the duct of discharge of the functional kid- ney undergo important modifications in the various classes of vertebrates. Taking the group as a whole the series of tubules may be said to be divided into three reg- ions, the pronephros or head-kidney, the mesonephros or primitive kidney, and the metanephros or permanent kidney of the higher vertebrates. Each of these regions needs brief consideration.
The pronephros is best developed in the anamnia (cyclos- tomes, fish-like vertebrates, and amphibia), in all of which it is present as a well-defined structure in the embr3'o, and in some of which it is functional in the adult. The tubules are always few in number, sometimes but one on each side, open upon the body cavity by distinct nephrostomes, and discharge into ducts leading to the cloaca and known as the segmental ducts. Near the opening of the nephro- stomes there is found on each side a process of the coelo- mic wall containing a mass of capillaries and constituting a prominent glomus : the region of the coelom where this occurs is either partialK' or in some cases wholly con- stricted off from the rest of the body-cavit}- so as to form "what is practically an enormous Malpighian bod}':" thus foreshadowing the arrangement found in the perma- nent kidncA' of the mammals. The development of the pronephros is embr\'onic and rudimentarv in the sharks and their allies among the fish-like vertebrates, and in all amniota (reptiles, birds, and mammals): in the last named
CHATTER XIX REPRODUCTIVE ORGANS. 253
class it is doubtfully represented in the adult male and fe- male by rudimentary structures connected with the surprarenal capsule.
The mesonephros is the functional kidnc}- of all anam- nia except the sharks and their allies. In this class and in the various classes of the amniota it is represented by the embryonic structure termed the Wolffian body. It con- sists primarily of serially arranged tubules plainh' homol- ogous with those of the pronephros, although their mode of development is not identical, being somewhat acceler- ated ; a point of importance in connection with the origin of the metanephros. These tubules become variously coiled and convoluted, and may give rise to branches like themselves: in many anamnia they open upon the peri- toneal surface by nephrostomes : the\' have associated with them glomeruli resembling in their origin the glom- us just described in connection with the pronephros : in some cases several tubules open upon a single glomerulus. The}'^ discharge into a modification of the segmental duct known as the mesonephric or Wolffian duct: it differs from the primary segmental duct in undergoing (in some species) longitudinal cleavage to give rise to a second canal called the Muellerian duct.
The ripened gonads of many anamnia discharge the re- productive elements into the body cavity by rupture, from which the\' escape through openings posteriorly and ventrally placed and known as abdominal pores. In other cases abdominal pores are wanting: in the female the ova pass out through the Mullerian duct or oviduct ; in the male the testis lies opposite the anterior end of the
254 PART II. HISTOLOGICAL ANATOMY.
primitive kidney, or Wolffian bod}' ; diverticula grow out from the Wolffian tubules of that region and become con- nected with the testis, forming channels of escape for the spermatozoa, which thus reach the Wolffian duct ; the latter therefore functions both as a ureter and a spermi- duct.
In the selachians (the sharks and their allies), and in all amniota a diverticulum is given offfrom the posterior por- tion of the Wolffian duct. In the former class this be- comes connected with a mass of serially disposed tubules arising posterior to the Wolffian body, the metane- phros, or permanent kidney ; the duct in question being the true ureter. In the amniota the ureter grows out to- ward a mass of cells in which true uriniferous tubules with their associated Malpighian corpuscles are being formed. It is not certain that this structure is strictly homologous with the selachian metanephros: it may, per- haps, be so regarded, the difference in the mode of devel- opment being due to an intensified acceleration of growth similar to that already mentioned in the case of the Wolff- ian body. The latter structure never acts as a kidney in the higher vertebrates, its sole function being connected with the transmission of the reproductive elements of the male : the Wolffian duct becomes the spermiduct or vas deferens : in the female the whole structure is functionless and rudimentary.
This digression into the field of comparative anatomy W'ill not be without value if it aids in making clear the brief statement of the development and homologies of the male and female urogenital apparatus of the human sub-
CHAPrEK XIX KKPRODUCTIVE ORGANS. 255
ject based upon the facts nicntloncd therein, whicli is now to be entered upon.
Early in the development of the embryo there may be seen ji^rowing downward from the dorsal wall of the body cavity on eit]ier side a well-defined eminence, the Wolffian orexcretory ridge: alongitsbasearefound two ducts, the Wolffian and Mitllcrian. Just niesad of the excretor\' ridge, and separated from it by so shallow a groove at first as almost to seem a modification of its mesial surface, is a second and smaller ridge, the germinal ridge. The latter becomes the gonad : the epithelium of its surface early be- comes columnar and shows here and there numerous prim- ordial ova; the characteristiccellsbcingso termed without regard to the future sex. The changes of position which the gonads undergo in each sex will be mentioned later.
The embryonic pronephros is formed at the anterior ex- tremity of the excretory ridge. The posterior portion, as it developes backward, becomes the blastema or mass of cells in which the uriniferous tubules of the kidney are formed : it is joined by the ureter, whose extremit}' branches and forms the pelvis and calyx of the kidne\', and possibly to some extent the collecting tubules: the exact relation of the two intergrowing structures is not j-et fulh' known. At first the most posterior, the kidne}^ gradual!}^ advances in position until it occupies a location anterior to all the associated structures.
The middle region of the excretory ridge becomes the Wolffian bod\' : in both sexes it develops as a series of transverse tubules in relation with the longitudinal Wolff- ian duct. In the male the tubules of the anterior portion
256 PART II. HISTOLOGICAL ANATOMY.
become connected with the testis, forming the coni vascu- losi of the epididymis, and possibly the rete testis as welh those of the posterior portion, corresponding to the strictly renal portion of the primitive kidney of the anamnia, be- come rudimentary and form the vasa aberrantia and the paradidymis, or organ of Giraldes. The Wolffian duct be- comes the convoluted tubule of the epididymis with its continuation, the vas deferens, the latter giving off the semi- nal vesicles as it is transformed into the ejaculatory duct. In the female the whole structure, being functionless be- comes rudimentary : the anterior portion of the tubules, corresponding to the epididymis, becomes the parovarium or epoophoron ; the posterior portion becomes the paro- ophoron : the Wolffian duct becomes the rudimentary duct of Gartner.
The Miillerian duct undergoes corresponding differentia- tion. In the female, where it is functional, the upper por- tion terminates in the fimbriated extremity: the middle portion becomes the Fallopian tube, or oviduct in the strict sense. The lower portions coalesce on the mid-line, the upper region of the median structure forming the uterus and the lower the vagina: a discussion of the mode of their coalescence would take us again into the domain of comparative anatomy. In the male the Miillerian duct is, like the Wolffian duct of the female, altogether function- less. It early disappears throughout the greater part of its extent: the upper portion is perhaps represented by the sessile hydatid of the testis : the coalesced lower portions form the uterus masculinus, which sometimes showstraces of a vaginal region.
CHAPTER XIX. REPRODUCTIVK ORCANS. 2o7
In all amniota there is found very early in embryonic life a vesicular diverticulum of the ventral wall of the in- testine at a point near its posterior termination: this out- growth is called the allantois. In the mammals a consid- erable portion of it is enclosed within the body cavity: the distal portion of this becomes enlarged to form the urin- ary bladder; a narrow region corresponding to the true urethra of both sexes connects it with a proximal enlarge- ment termed the urogenital sinus: the latter opens in common with the intestine into a short cloaca formed by a depression of the ventral surface of the body. The Miil- lerian and Wolffian ducts open into the sinus: the ureters at first open into the Wolffian duct, from which they are derived ; later they open independently into the sinus : as growth advances they shift their position upward, until they reach their permanent point of discharge on the sur- face of the bladder.
The mammalian cloaca is from the first a very shallow depression : a transverse fold soon separates it into a pos- terior anal portion, and an anterior region which is the continuation ofthe urogenital sinus. Just in front of it there is found the genital eminence, whose posterior surface is grooved, the margins of the groove forming the genital folds; in front of its base the skin of the pubis is thrown into a thick fold (the future mons veneris) which is pro- longed backward right and left in the genital ridges. In the male the genital eminence elongates ; corpora caver- nosa are formed in it : the genital folds coalesce from be- hind foward, converting the groove on the posterior surface of the eminence into a canal which is the continuation of
258 PART ir. HISTOLOGICAL ANATOMY.
the narrow and elongated sinus; while the development of erectile tissue in the walls of the canal gives rise to the corpus spongiosum : the canal becomes the urethra of the penis. In the female the eminence remains small and be- comes the clitoris : the sinus becomes short and broad and is represented by the vestibule : the genital folds become the labia minores ; the genital ridges the labia majores.
Glandular diverticula of the sinus are found in both sexes. The prostate of the male may be regarded as so derived; there-is no corresponding structure in the female: the glands of Cowper and the glands of Bartholin are ho- mologous specializations of urethral structures. The change of position of the gonads has been already re- ferred to. Situated at first in the more anterior region of the abdominal portions of the body cavity, they undergo an apparent shifting backward in both sexes. The change is least in the female, where they find a permanent resting place within the pelvis. In the male they reach the brim of the pelvis, penetrate the abdominal wall, and push their way into the genital ridges : these enlarge, become saccu- lar, and coalesce below the united genital fold to form the scrotum.
CHAPTER XX. VASCULAK SYSTEM. 259
CHAPTER XX. THE VASCULAR SYSTEM.
On account of the part played by the smaller arteries, veins and lymphatics as components of the organs of the body in which they occur, the structure of such vessels was described in one of the earlier chapters of this book. The larger vessels must themselves be regarded as organs meriting separate consideration equally with the heart, which is the central organ of the vascular system.
The arteries of the body differ most conspicuously from the arterioles found in the various organs b}' the relative increase of the media : but each of the three layers under- goes both increase and modification. The intima is lined, as in the smaller vessels, by endothelial cells elongated in the direction of the tube : but this endothelium rests upon a subendothelial layer of fibrous tissue with branched corpuscles that disappears as we pass to the finer subdi- visions of the vessel : with increase in the size of the artery it becomes more fully developed. This layer is directly in contact outwardly with the elastic layer, which is well de- veloped, especially in the larger arteries, where it has the form of a fenestrated membrane, the membrana elastica intimae.
The media consists largeh^ of transversely disposed smooth muscular fibres, particularly- in the smaller ar-
260 PART II. HISTOLOGICAL ANATOMY.
teries of the limbs: in large arteries, however, there is a dis- tinct admixture of elastic tissue in the form of a network of fibres; this is connected with the elastic layer of the in- tima, and pervades the whole muscular coat: it increases in proportion with the size of the vessel. The muscular fibres of the media are quite short, and are very irregular in form, lacking the definite spindle shape characteristic of smooth fibres in most places where thej^ occur. In some of the larger arteries many of the muscular bundles of the inner part of the media are longitudinally disposed.
The adventitia is the stoutest and most resistant of the coats of the arteries. It is rich in elastic tissue, especially toward the media, the larger arteries exhibiting just ex- ternal to that coat a distinct elastic la^^er, the membrana elastica externa; this is followed by a region rich in elas- tic fibres: more outwardl}^ the adventitia consists almost wholly of closely felted bundles of white fibrous tissue w^hich at its outer surface passes over into the intersti- tial areolar tissue found between the vessel in question and the adjacent organs. In some of the larger arteries longitudinal bundles of smooth muscular fibres are found.
In connection with the discussion of the respiratory tract regarded as a diverticulum of the alimentary canal, it was pointed out that the coats of the latter were desig- nated by characters readily demonstrable with the dis- secting knife, but that the proper division based upon his- tological characters would be into an epithelial and a musculo-skeletal la3'er. The coats of an artery as above described are also examples of structures clearly disting- uishable by anatomical methods : but a consideration of
Cir.VPTF^R XX. VASCl'LAR SYSTIvM. 261
the histology of the wall of an artery will show the cor- rectness of the description that has been proposed for it as "composed of muscular and elastic tissue lined intcrnalh' by endothelium and strengthened externally by a layer of connective tissue."
The features which have been mentioned as character- istic of the largest arteries are intensified in the structure of the aorta. In it the elastic layer of the intima is not so distinctly membranous, being composed chiefly of fibres which pass into those of the media, the two coats not be- ing sharply defined. The media itself not only contains a ver}' large amount of elastic tissue in proportion to the muscular tissue present, but is also reinforced b}' a consider- able quantit\^ of white fibrous tissue, the coat being char- acterized more especially by its strength and elasticity than by its contractility. As in sojne of the larger arter- ies there are both longitudinal and transvere bundles of muscular fibres. The adventitia is not sharply defined from the media, and is relatively- thin. The pulmonary artery agrees in most respects with the aorta in struct- ure. The larger arteries and veins have, like other or- gans of the body, their own small vessels of supply ; these are called the vasa vasorum.
While the heart is quite a complex organ from the ana- tomical standpoint, its histological structure is compara- tively simple. It may be regarded as essentially a hollow mass of muscular fibres of a peculiar sort, having on its outside an investing skeletal laj-er, and a similar lin-
262 PART II. HISTOLOGICAL ANATOMY,"
ing inwardl\, each of the layers being bounded by an en- dothelium. The outer la\'er, or epicardium, is the cardiac portion of the pericardium. It is invested with serous endothelium which rests upon a membrane whose outer portion consists chiefl\^ of white fibrous tissue, but which contains in its deeper layer a considerable quantity of elastic fibres arranged in a loose and indefinite meshwork. Beneath the membrane is a stratum of areolar tissue which is rich in fat cells, and in which run the vessels and nerves of the wall of the heart ; in the auricles there are numerous small ganglia connected with the nerves. The lining membrane, or endocardium, is not unlike the epi- cardium : its investing endothelium, while continuous with that lining the arteries and veins, and agreeing in function with vascular endothelium, is composed of cells which are not elongated in form, but resemble rather in outline those found on serous surfaces. The membrane contains a few muscular fibres, and the subjacent con- nective tissue but a limited quantity of fat cells. In the auricles the elastic tissue of the membrane is quite well developed.
The myocardium or muscular layer of the heart varies greatlv in thickness, being most developed in the ventricu- lar wall and least in some portions of the auricles. The peculiar cardiac muscular elements of which it is com- posed have already been described : they are arranged in bundles which form the fibres and lamellae visible to the naked eye: the disposition of the latter is a subject of an- atomical rather than histological study. Between the bundles is a very delicate framework of connective tissue
CHAPTER XX. VASCULAR SYSTEM. 263
which supports the abundant blood and lymph capillar- ies: patches of adenoid tissue are also of occasional occur- rence. The peculiar fibres of Purkinje found just be- neath the endocardium of some mammals are of doubtful occurrence io man: they are large cellular elements (fre- quently with two nuclei) whose central portion consists of clear protoplasm but whose surface has undergone striation ; they may perhaps be regarded as imperfectly developed cardiac fibres.
The columnae carneae and the papillary muscles are alike processes of the myocardium : in them the muscular fibres are chiefly disposed in a longitudinal direction, and the endocardial membrane upon their surfaces becomes almost tendinous in structure; the latter passes over from the surface of the papillary muscles into the chordae ten- dineae, whose central strands of fibrous tissue are in the larger cords reinforced by scattered bundles of muscular fibres.
The valves of the heart are folds of the endocardium containing bundles of fibrous tissue and a variable quan- tity' of elasjtic tissue, the latter being present in greatest quantity in those places subject to the greatest pressure produced by the heart's action on the blood : the semi- lunar valves are farther reinforced by the presence in each of a nodule composed largely of elasticjtissue, the corpus arantii.
The large veins differ from the corresponding arteries in the relative thinness of the media and the thickness of the adventitia, and in the smaller amount of elastic tissue throughout the entire structure. The endothelial cells lin-
264* PART 11. HISTOLOGICAL ANATOMY.
ing the intima of the larger veins are but slightly if at all elono^ated, resembling those of the endocardium in their polygonal outline: in the smaller tributaries the form char- acteristic of vascular endothelium is assumed. The media of the two venae cavae is a continuation of the mj^ocar- dium of the right auricle, and contains cardiac muscular elements : that of the larger veins generally contains but a small quantity of muscular tissue, intermingled with bundles of white fibres, which replace the elastic tissue similarly situated in the larger arteries. The media of some veins, notably those of the nervous axis and^of the bones, is entirely devoid of muscular tissue. In a few- cases the muscular bundles situated in the inner part of the media are longitudinal in their direction. The adven- titia of the veins consists chiefly of a stout layer of felted bundles of fibrous tissue, containing a moderate amount of elastic fibres. In the portal, renal, and some other veins, the adventitia contains numerous bundles of smooth muscular fibres longitudinally disposed. The valves of the veins are folds of the intima, strengthened by bundles of fibrous tissue arranged parallel to their free borders, and containing a small amount of elastic tissue: mus- cular bundles from the media sometimes extend into the base.
The lymphatic vessels resemble the blood vessels in the fact that their walls are composed of an inner, a middle, and an outer coat : each of the three coats is, however, much thinner than in the case of the blood vessels, the middle and outer being especially so. It is customary to
CHAPTER XX. VASCULAR SYSTEM. 265
say that they resemble the veins in structure: it is more accurate to say that in the relative thickness of the coats and particularly in the proportion of muscular tissue they approach the arteries : they resemble the veins in the structure of the intima and in its development into valvu- lar folds : the adventitia is relatively to the other coats weaker and simpler than in either arteries or veins. This is not the case, however, with the most highly specialized of all the lymphatic vessels, the thoracic duct, whose ad- ventitia is quite stout and contains longitudinal bundles of smooth muscular fibres : the intima of the duct is also re- inforced with a longitudinal network of elastic fibres. In the smaller lymphatic vessels the adventitia disappears; the media contains bundles of smooth muscular fibres obliquely disposed, in addition to the ordinary transverse bundles.
The development of the heart and of the larger vascular trunks takes place in such a manner that it is difficult to discuss it without either taking for granted a knowledge of or entering into a description of the manner in which the ground plan of the embryo body is laid down. It is therefore necessary to defier it until after the subject of Embryology has been taken up by the student.
Reference mav with convenience at this time be asrain made to the great serous membranes which line the principal cavities of the body ; since these latter are best re-
266 PART II. HISTOLOGICAL ANATOMY.
garded as enormous lymph cavities. Their general struct- ure and particularlj^ that of their characteristic epithe- lium has been discussed in a previous chapter: it remains to speak briefly of some of the features characteristic of each (in so far as they have distinguishing characteristics) and to describe some allied though different structures usuall}^ associated with them.
The right and left pleurae, which invest the lungs, line the thoracic cavities and bound the mediastinum which separates the latter, while they have the general charac- ter of serous membranes, vary somewhat in structure in different localities. They are thickest over the ribs, where the subserous layer of areolar tissue permits of the ready removal of the membrane : the stomata of the endothe- lium are said to occur over the intercostal spaces onlv-. The membrane becomes thinner and more adherent as it passes onto the surface of the diaphragm. On the surface of the lungs it is quite thin and closely attached, the connective tissue becoming continuous with that of the framework of the organ: the endothelial cells of the pulmonary pleura are also taller and more granular. The pericardium, or heart sac, is in reality but little more than a double serous membrane: its outer portion being derived from the pleurae and its inner forming the proper pericardial mem- brane which is reflected upon the heart as the epicardium : between the two membranes is a layer of areolar tissue containing a small quantity of fat and numerous |blood vessels and lymphatics.
The peritoneum, which lines the cavity of the abdomen, is the largest and most complicated of the serous mem-
CHAPTER XX. VASCULAR SYSTEM 267
branes. Like the pleura, it is thicker in its parietal than its visceral portion. The subserous layer of areolar tis- sue is less developed along the ventral midline and on the under surface of the diaphragm than elsewhere in the parietal peritoneum, which is consequent!}' most closely adherent in these regions : that of the visceral portions is in every case closely connected with the skeletal structures of the organs invested. Certain of its folds, termed liga- ments, from their mechanical relations to the organs with which they are connected, for example, the spleen, the liver, and the uterus, have their applied fibrous membranes in close contact. Others, such as the omenta, have between the membranes a laj'er of areolar tissue in which a greater or less amount of fat may be developed. In the me- sentery the intervening layer contains, in addition to a variable amount of fat, an extensive system of blood and Ij'mph vessels: associated with the latter are the numer- ous nodules of adenoid tissue commonly' called the lacteal glands : their structure will be described in the following chapter in connection with other similar bodies. The tunica vaginalis of the scrotum and testis must be re- garded as an oft'set from the peritoneum : its structure and relations have been sufficiently described in a pre- vious chapter.
The meninges of the brain and spinal cord, and in par- ticular the pia mater, have been regarded as serous mem- branes : the}' differ, however, in structure and relations alike from the ordinary membranes of that name, and will best be discussed in connection with the nervous axis. The perilymphatic and endolymphatic surfaces of the internal
268 PART II. HISTOLOGICAL ANATOMY.
ear will be considered in connection with the description of that organ.
The synovial membranes, which line the capsules of joints, the sheaths of tendons, etc., may here be consid- ered, although they differ materially in structure from the serous membranes wnth which they are often associated as regards both their organization and also the charac- teristic fluid which fills the cavities bounded by them. They are in effect rather dense membranous layers of con- nective tissue, devoid of any well-defined endothelial in- vestment : here and there cells and patches of cells may be seen upon their surfaces, some of which are sufficiently close together to assume the polygonal outline character- istic of endothelium; others, how^ever, are distinctly branched, differing in no essential from ordinary connect- ive tissue corpuscles : their affinity to endothelium will perhaps be evident if we recall the definition of it previ- ousl}^ given to the effect that it is a layer of connective tissue corpuscles investing a free surface. Fringed vascu- lar folds occur upons\movial membranes which frequently bear smaller processes, the synovial villi, in which a cen- tral strand of fibrous tissue is invested with a layer of small rounded cells. Articular synovial membranes pass so gradually into the fibro-cartilage which borders the true articular cartilage of the joint that it is not possible to define the boundary between the two : even the cellular elements gradually losing their processes and presenting the appearance of cartilage corpuscles.
CHAPTER XXI. THE DUCTLESS BODIES. 269
CHAPTER XXI.
THE DUCTLESS BODIES.
The organs here included under the above title are very frequently referred to by the name of the ductless glands. If the term gland be used in its older and looser sense as a designation for any soft parench3'matous body, then a distinction between such of those bodies as are provided with and those devoid of ducts is natural and justifiable. It is better, however, to use the term in its narrower and more definite application to bodies composed essentially of acini or tubules lined with epithelial cells whose funct- tion is to secrete a specific solution or soluble substance, ordinarily removed through a definite channel of dis- charge. If the term gland be thus employed, there is but one (and a portion of another) of the bodies here consid- ered to which the expression ductless gland may with any propriety be applied : as used to designate the others it is not only erroneous, but tends to give rise to. misleading conceptions. The expression ductless bodies, while not without objections, is therefore preferable to the older title. The name of adenoid bodies has also been pro- posed, but is apt to lead to the false idea of a community of structure in all of the bodies in question.
It is quite important to note in this connection that the caption given to this chapter, unlike any other used hith-
1' ^
\
^5^70 PART n. HISTOLOGICAL ANATOMY. .
erto, does not designate any s\'stem of organs associated with the performance ofone of the great functions or groups of functions, or characterized bv anv common structure or b}' similar anatomical or morphological relations : their association under one heading is more than anything else a matter of convenience ; and the order of their consider- ation largely arbitrary, though not altogether so. The lymphatic bodies, from their close structural and func- tional relation to the Ij'mphatic vessels, might with equal propriety be described in connection with the vascular sys- tem : the spleen, differing in important respects from the bodies just named, resembles them in its close relation to the circulatory system : the thymus has much in common w^ith them in its structure: to all these bodies (and to these alone) the term adenoid bodies might with propriety be applied, owing to the prevalence in them of the tissue of that name.
The thymus, in addition to the adenoid tissue of which it is largely composed, contains definite masses of epithe- lial elements whose derivation will be discussed when that body is described : the thyroid, is largely composed of epithelial cells which line acini ; to it more than to any other of the bodies here considered may the title of duct- less gland be with propriety applied, as is shown b}^ its embryonic development. Resembling the thymus and the thyroid in the origin of some of their elements (and in little else) are the parathyroids, situated on either side of each lobe of the thyroid ; and the carotid glands ; they are closely resembled in structure, though not in origin, by the coccygeal gland; the term gland is still univer-
CHAI'TKK XXI. THE DUCTLESS BODIES. 271
sally applied to the last two; they have been called, from their associations, the arterial glands.
The suprarenal capsules, or the adrenals, as they are sometimes termed, are double structures, consisting in part of a mass of nervous tissue: they are resembled in this respect by the pituitary body, which consists in part of a mass somewhat analogous to the thyroid, and in part of an atrophied lobe of the brain: the pineal body is altogether derived from the modification of a portion of the brain, and might, in common with the pituitary body, be described in connection with that organ. This brief enumeration will suffice to show how heterogenous are the bodies here associated as regards both structure and anatomical relations: the function of most of them is at present entirely unknown. An account of the details of their structure will now be given.
The simplest lymphatic bodies are those rounded masses of adenoid tissue bounded by a more or less definite fib- brous layer which have already been mentioned in con- nection with the description of the alimentary canal. The lymphatic follicles, as they are commonly called, of the intestine, whether solitary or clustered (as in the Peyer's patches of the ileum), and the similar masses whose some- what confluent aggregations form the tonsils, are exam- ples of what may perhaps best be called lymphatic nod- ules. They consist essentiall}^ of spheroidal lumps of ade- noid tissue somewhat more dense in the outer than the inner portion, but showing no division into lobes. Thej' are pervaded by capillary networks, and may be invested
272 PART II. HISTOLOGICAL ANATOMY.
by a large lymph sinus or by a plexus of small lymphatic vessels. It is not easy to demonstrate among the latter distinctl}^ efferent or afferent trunks : and from the usual position of these bodies in mucous membranes, and from what we now know of the important bacteriophagous function of leucocytes upon mucous surfaces it may be questioned whether those formed in the lymphatic nodules do not find their chief destination there rather than in the l3''mph vStreara.
The larger and more deeply seated h^mphatic bodies commonly termed lymphatic glands, together with the altoofether similar bodies found in the mesentery and called lacteal glands, for both of which the much better name of lymphatic nodes has of late years come into use, are in reality integral parts of the lymphatic and therefore of the circulatory system. Each is a rounded body showing a distinct depression, the hilum, on one side, at which point the blood vessels of the interior enter, and from which one or more efferent lymphatics leave the organ. The surface is invested with a stout fibrous^capsule, con- taining scattered bundles of smooth muscular fibres in the larger nodes: it has a proper network of blood vessels, and into it at various points pass afferent lympahtics which traverse it quite obliquely, sometimes forming small plexuses or sinuses within it before communica- ting with the lymph channels within.
The interior is divided into a cortical and a medullary portion : from the capsule stout trabeculae of fibrous tis- sue (sometimes containing smooth muscular fibres) pass inward : they are frequently broad and lamellar in form
CHAPTER XXI. THE DUCTLESS GLANDS. 273
and divide the cortex imperfectly into suljecjual lobules sometimes called the cortical follicles and compared to the lymphatic nodules (or so-called follicles) already de- scribed, from which, however, they differ in important re- spects: the inner extremities of their trabeculae subdivide, communicating by their branches with a coarse network of fibrous tissue which forms the framework of the medul- larx' portion.
The spaces of the network just mentioned are occupied b\^ another network of rounded strands of adenoid tis- sue, the medullary cords, which become continuous at their extremities with the solid cortical lobules of the same tissue situated between the trabeculae. It will be remembered that adenoid tissue ^nsists in effect of reti- form tissue whose interstices are filled wrth lymphoblasts: the medullary cords and cortical lobules of adenoid tissue are in each case smaller than the cavities in which they are situated, the surrounding spaces containing a coarser ret- iform tissue (devoid of lymphoblasts and presenting but little resistance to the passage of fluids) which connects the cords and lobules with the medullary and trabecular framework. A network of passages throughout the whole node, known as the lymph sinuses or lymph channels, is thus formed, into which the afferent l^'mphatics open after passing through the capsule, and from which the efferent lymphatics lead.
The medullary portion of the lymphatic node extends to the surface at the hilum, the artery of supply entering it directly : the capillary network of the interior is situated almost entirely in the deeper portion of the adenoid
,>f
274 PART n. HISTOLOGICAL ANATOMY.
strands and masses. The lymph channels of the medulla converge at the hilumtororm a plexus, from which a single efferent trunk may lead, or, in the larger nodes, several smaller vessels which unite outside the node to form a single trunk. Pacinian bodies are of frequent occurrence in the interstitial connective tissue j ust without the ^ap- sjale at the hiluni.
The relation between the lymphatic nodes and the lymph stream, as regards the formation of lymphocytes and their transformation in the blood stream into leucocytes, has been discussed in a previous chapter. The minute is- lands of adenoid tissue occasionally found either on or within the walls of lymphatic vessels, and known as peri- lymphatic or endolymphatic nodules, may be regarded as rudimentary organs of the same kind.
The spleen is as closely related to the blood vascular as the lymphatic nodes to the lymphatic portion of the cir- culatory system : it may possibly be regarded as derived from the modification of one of the last named bodies, though differing from them greatly not only in its vascu- lar relations, but also in its internal structure. It is inter- esting to note that in some of the lower mammals num- erous small accessory spleen-like nodules are norrnally pre- sent in other regions of the body. Similar bodies are sometimes found in man in the vicinity of the principal organ, of which they may be regarded as diverticula.
The spleen is invested by a serosa derived from the peri- toneum which rests upon a stout capsule of fibrous tissue which, like that of the lymphatic nodes, contains occa-
CHAPTER XXI. TIIK DUCTLESS GLANDS. 275
sional sraqoth_muscular fibres: it differs from that of the bodies last mentioned in the greater predominence of elas- tic fibres, making the organ highly distensible. At the hilum the capsule is continued into the spleen to form large trabeculae. which branch and subdivide within, eventually becoming continuous with the branches of similar though small trabeculae which pass inward from the capsule at numerous points: the trabeculae, like the capsule, contain numerous elastic fibres, and some muscular fibres : the interior of the organ is pervaded by the large-meshed re- ticular framework thus produced. Continuous with this framework is a coarse retiform tissue whose fibrous net- work is invested with branched corpuscles : in many cases the fibrous element is quite scanty, and the reticulum con- sists of little more than branched corpuscles connected with each other by the tips of their branches, the so-called reticular cells of the spleen. The intervals between these cells are filled with blood which contains rather more than the usual proportion of colorless corpuscles and rather less of the colored : there are present also numerous unbranch- ed amoeboid cells somewhat larger than colorless corpus- cles, the spleen-cells: these, the reticular cells, and the ^asma of the blood itself contains disintegrating colored corpuscles, and pigment granules derived therefrom : the whole constitutes the spleen pulp, a reddish brown mass to which the characteristic color of the organ is due. The splenic arterj' divides into several branches just be- fore reaching the organ : these enter at the hilum, follow- ing the stout trabecular continuations ofthe capsule above mentioned : within the latter they branch, their branches
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276 PART n. HISTOLOGICAL ANATOMY.
in some cases followmg the subdivisions of the trabeculae. In other cases small branches leave the trabeculae and be- come divided into brush-like tufts of arterioles: on emerging, their adyentitia, heretofore continuous with the fibrous tissue of the framework of the organ, becomes replaced by a layer of adenoid tissue : here and there this sheath is suddenh' enlarged to form spheroidal masses which ma}' be as much as a millimetre in diameter, though usually less than half as large: thej^are known as the Malpighian corpuscles of the spleen, and are readily visible to the naked e^-e as whitish spots in the dark brown pulp. The adenoid tissue of the corpuscles is permeated b}' capillaries given off from the arterioles enclosed ; it is quite loose in the centre but denser at the surface, where it passes over abruptly into the retiform tissue of the pulp. The cor- puscles are found surrounding small arteries and appar- ently strung upon them, or upon their subdivisions, in which case they look like lateral outgrowths.
fOn leaving the corpuscles the arteries divide into capil- laries, which, like those arising from the smaller divisions of the arteries which follow the framework of the spleen more 1 closely, finall}^ open into the spaces of the tissue of the _y pulp, the endothelial cells of the capillaries gradually be- 1 coming looser, branching, and finally passing over into I ) the reticular cells ; thus^f|m:din^ the only instance jn the body where the blood leaves the definite vessels ,prc)ger_to \\it and circulates in the interstices of the tissues ; a condi- tion largely characteristic of the circulation of all inver- tebrates. The veinlets of the pulp originate in the same way that the arterioles terminate, or rather its converse :
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CHAPTER Xxi. THE DUCTLESS GLANDS. 277
the reticular cells passing over into branched and loosely disposed endothelial cells which later become closely united to form the lining of the commencing vessels. The latter soon enter the trabeculae, where they are gathered into larger vein&j these anastomose freely within the trabecu- lae, finalh' uniting to form the few large trunks that leave the hilum.
The thymus is a bijobed adenoid body situated just be- neath the sternum in the upper part of the thorax and ex- tending into the lower part of the neck in the embryo and the infant: it is gradually reduced to a mere vestige in the adult. The whole organ is invested by a thin capsule of fibrous tissue, beneath which it is subdivided into a num- ber of irregular lobules each but a few millimetres in di- ameter. The fibrous tissue envelope of each lobule gives off trabeculae which penetrate the interior in the same manner as the similar structures in a 13'mphatic node. The lobule in consequence exhibits a cortical and a medullarv portion. The cortex is composed of nodules of adenoid tissue not unlike those found in a tonsil : the medulla is a mass of adenoid tissue much less dense than that of the cortex, the transition from the one to the other being so rapid as to be quite conspicuous in sections. The trabec- ulae are continuous with the retiform tissue of the cortex and medulla alike, but there is no distinct raedullar\' frame- \vork of fibrous tissue and no segregation of the adenoid tissue in medullary cords: nor are there any lymph chan- nels in either cortex or medulla.
The most characteristic feature^ of the thymus is the
- — CJUa^iU^' f
278 -- PART II. HISTOLOGICAL ANATOMY.
presence in the medulla of what are known as Hassall's corpuscles, or, as that histologist termed them, concen- tric corpuscles. These are peculiar nest-like groups of epi- thelioid cells which are now" known to be derived from the breaking up of right and left tubular diverticula from the cervical hypoblast. Each corpuscle consists of a central granular mass containing one or more spheroidal cells, surrounded by two or three layers of concentric flattened cells : compound corpuscles sometimes occur, two or three ordinary corpuscles being invested by a common layer of concentric cells. The adenoid tissue of the organ contains a rich capillary network, and is the place of origin of nu- merous large lymphatics.
The thyroid resembles the thymus in being relatively large in foetal life and infancy : it differs from that organ in its persistence and evident functional importance in the adult, as indicated by the grave consequences of its com- plete extirpation. Like the tubular structures which even- tually break up into the concentric corpuscles of the thymus, its characteristic elements are derived from the cervical hypoblast. It is at first provided with a duct, whose rudi- ment becomes the foramen caecum of the dorsum of the tongue: the duct in question in rare instances persists; in the great majority of cases it aborts, converting the organ into a true ductless gland.
Unlike most glandular bodies, the thyroid is not pro- vided with a well-defined capsule. It is invested by a layer of areolar tissue considerably denser than that connecting it with adjacent organs but not passing over into a dis-
CHAPTER XXI. THE DUCTLESS GLANDS. 279
tinct fibrous ineinl)ranc. Areolar tissue of variable density ])erva(k's the interior of the organ, forming the support- ing framework of its structure, the characteristic feature of which is the jiresence of great numbers of vesicles united into imperfect lobules. The vesicles are spheroidal, polyhedral, or sometimes tubular in form, their walls con- sisting of a single laver of cubqi^al epithelium. The inte- rior of the vesicle is filled with a glairy yellowish colloid substance which frequently contains leucocytes and de- tached epithelial cells. A distinction has been] made by some observers between the colloid, cells, which are act- ivelv engaged in secreting the fluid contained in the ves- icles, and the reserve cells. A definite basement mem- brane cannot be clearly discerned, the epithelial cells ap- pearing to rest directly upon the interstitial septa of are- olar tissue already mentioned : the areolae of the septa not infrequently contain the colloid secretion of the cells: elements resembling plasma cells are found in the inter- stitial tissue. The thyroid is highly vascular, the arteries being relativel}' quite large, and anastomosing freely : the vesicles are surrounded by a rich capillar}' network : the lymphatics are also large and numerous, and the presence of colloid substance in their interior may sometimes be detected.
Imbedded in the substance of the thyroid upon both the lateral and the mesial surfaces of the lobes are small bodies a few millimeters in diameter to which the name of parathyroids has been given. They resemble the thyroid in color ^ind appearance, but differ from it in ^structure,
f';/^/
280 PART n. HISTOLOGICAL ANATOMY.
consisting of solid strands of epithelioid cells apparently anastomosing, their interspaces being occupied by numer- ous blood vessels. They have been regarded by some ob- servers as masses of embryonic thyroid tissue: this view- is denied by others, who regard their structure as more nearly approaching that of the carotid glands. There is usually associated with each a small mass of adenoid tis- sue containing concentric corpuscles and in other respects resembling the substance of the thymus.
The carotid glands, situated in the angle between the branches of the common carotid artery, are small irregu- larly shaped bodies whose envelope of connective tissue is continued inward to form a supporting framework im- bedded in which are nodular masses of epithelioid cells richly supplied with capillaries. They resemble the para- thyroids in their origin from the cervical hypoblast, and both are probably to be regarded as rudiments of larger and more important organs.
The coccygeal gland is another body of quite similar structure to those just described, and probably also rudi- mentary in its character. The epithelioid cells which con- stitute its distinguishing feature are to some extent dis- posed in columnar strands as in the parathyroids. Eberth has described among them nests of cells resembling con- centric corpuscles. Attempts have been made to show that the elements of this body are largely nervous in char- acter, but this view of their nature lacks confirmation. The mode of development is not known.
CHAPTER XXI. THE DUCTLESS GLANDS. 281
The suprarenal capsules, or, in the lan<2:uage of com- parative anatomy, the adrenal bodies, since their position is usually near but not upon the kidneys, as is the case in man,^ are in some respects the most complex in structure of all the -ductless bodies, to no other of which are they nearly allied. Each on section shows to the naked eye a distinct yellowish cortex, radialh' striated, and a dark brownish homogeneous medulla, the two being clearly de- fined from each other. The surface is invested by a thin but firm fibrous capsule whose deeper portion shows scat- tered bundles of smooth muscular fibres: from it tough fibrous septa enter the interior of the organ to form the cortical framework, which is limited internally b\' a con- tinuous layer of connective tissue which bounds the medul- la. The interior of the latter is also pervaded b}' a frame- work of fibrous tissue.
The cortex is divided by the difference in the disposition of its septa and in the consequent mode of segregation of its elements into three distinct zones which pass into each other without great abruptness : these are the thin zona glomerulosa just beneath the capsule, the zona fascicu- lata next within, which forms by far the greater portion of the cortex, and the zona reticularis, little if at all thicker than the outer zone, which lies next the medullary sheath [of connective tissue. The spaces of the fibrous framework are occupied in the outer zone by rounded no- dules, in the middle b\' columnar masses, and in the inner by a network of strands of closely packed polyhedral cells of moderate size whose protoplasm shows numerous small oil globules to which the color of the cortex is largeh^ due.
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V. / ,/
282 PART II. HISTOLOGICAL ANATOMY.
The cells of the inner zone are darker in color, frequently containing brownish pigment : those of the outer in some animals are occasionalh' columnar in form, being disposed about an ill-defined lumen in the centre of the nodule.
The medulla contains within the fibrous stroma irreg" ular cords and masses of cells larger and much more loosely arranged than those of the cortex : they are de- void of oil globules and frequentl}^ exhibit branching pro- cesses. A rich plexus of non-medullated nerve fibres is present, and connected with great number of ganglion cells either scattered or clustered in groups of varj'ing size. Numerous small ganglia are also found upon the nerves just external to the hilum.
The arteries of supply enter the surface of the capsule by numerous small branches : wathin, the vessels are distri- buted to the cortex along its framework, the capillaries not pervading the cellular masses as in the parathyroids, carotid glands, and coccygeal gland : thence they pass to the medulla, which contains a large plexus of veins whose branches unite into one at the hilum. The cortex is well supplied with lymphatics which communicate both with those of the capsule and with those of the medulla.
In the angles of the irregularly pyramidal adrenals of the human subject the cortex is folded upon itself, the me- dulla not extending into the fold ; the two layers of the zona reticularis are, however, separated b^^a continuation of the connective tissue layer which surrounds the me- dulla. The distinctness between the cortex and the medulla is associated with an important difference in their embry- onic development. The two arise independently of each
CHAPTER XXI. THE DUCTLESS ORGANS. 288
other, and, indeed, remain so throughout life in some of the fish-like vertebrates: the eortex arises as an outgrowth from the peritoneum in close proximity to the mesone- phros: the medulla is derived from an extension of the adjaeent sympathetic chain of ganglia : from its close con- nection with which and from its richness in nervous elements many are inclined to regard the adrenals as essentially portions of the nervous system.
The pituitary body is also known as the hypophysis x
cerebri. It is a double structure, consisting of an anterior j?, and a posterior portion : it would be well if the former term could be restricted to the first of these and the latter ,p(^V. to the other, since they are essentially different alike in structure and in origin. The posterior division is in real- ity a downgrowth of the brain, as the second term im- plies: it is the rudiment in man and mammals of what is a distinct and important lobe of the brain in the fish-like vertebrates. The anterior is an upgrowth from the epi- blast which lines the oral invagination, and is in sub- stance an epithelial body: it is also a rudiment of what was probabh^ in the earlier vertebrates or their inverte- brate ancestors an important glandular organ; its path- ^^-^■^■ ological relations indicate that it still has a persistent though as yet unknown function. The contact and cohe- sion of the two bodies is confined to the mammals : in all other vertebrates they remain distinct.
The anterior lobe is larger than the posterior, and is of a darker reddish color. It consists of spheroidal and sim- ple or branched[_tubularacini ; their closed cavities are lined
28-4 PART n. HISTOLOGICAL ANATOMY.
either by a mass of polyhedral ep^itheliqid cells which fill the cavitv, or by a layer of true epithelial cells which sur- round a distinct though sometimes irregular lumen fre- quently filled with colloid substance similar to that found in the thyroid : in some of the larger tubules cilia have been observed upon the cells. Between the vesicles and tubules is a framework of connective tissue which sup- ports the numerous blood vessels and lymphatics and is continuous with the fibrous capsule.
The posterior lobe is at first a hollow diverticulum of the 'twixt-brain : in man and all mammals its cavity is nearly or quite obliterated, and it becomes a small solid mass. The nervous tissues characteristic of the inferior lobe of the brain of the fish-like vertebrates fail to devel- ope in the higher forms, their place being taken by an in- growth of vessels and of bundles of fibrous tissue: within the meshes of the latter are found numerous peculiar fus- iform or stellate cells which are frequently pigmented. Where a vestige of the cavit\^ remains it is lined with col- umnar ciliated cells similar to those found lining the cavity of the third ventricle. The relation of this portion of the pituitary body to the brain is so close that the whole is frequent^ described in connection wath that organ : but the larger and apparently more important anterior por- tion resembles the thj^roid so much both in structure and in origin as to justif)^ its description among the anom- olous ductless bodies.
The pineal body, otherwise known as the epiphysis cerebri has already been spoken of in connection with the
CHAPTER XXI. THE DUCTLESS GLANDS. 285
pituitary body. Like the posterior portion of that organ, it is stricth' a portion of the brain ; and its structure should be included in a full account of that organ. Like its analogue first mentioned, however, it is in mammals altogether rudimentary and completely devoid of true nervous tissue: it has therefore no longer any histological relation with the nervous axis and may as a matter of convenience be described at this time. It consists essen- tially of a number of closed spheroidal or tubular acini lined or in most cases wholh' filled with polyhedral epithelial cells, and supported by dense interstitial con- nective tissue. Among the epithelial cells, as well as on the outer surface of the body, are numerous gritty calcare- ous particles known as brain-sand: this is also found in some other portions of the brain. The pineal body in some of the lower vertebrates contains true nervous tissue and is connected with a peculiar median sense organ sometimes called the parietal or pineal e^-e.
Attention was called In the opening paragraphs of this book to the frequent use of the term Physiological Anat- omy as a s3'nonym for Histological Anatomy. The dis- tinction between the two is very well illustrated by the facts in the case of the bodies described in this chapter. It was alike the hope and the expectation of the earlier histologists and physiologists that the investigation of the elemental structure of each organ of the body would throw great light upon its functions. This has proved to be the case in many instances, and physiology has been
286 PART II. HISTOLOGICAL ANATOMY.
under correspondingly great obligations to histology, which it has been quick to acknowledge: so ready, indeed, that the student new to the subject is to some extent liable to loose sight of the important fact that in not a few instances no such happy result has followed upon a dvances in our knowledge of minute structure. While, for example, the diiference in the form and relations of the cells lining the alveoli of serous and of mucous glands respectively can be correlated with the difference in the character of the fluids secreted by them, no one could have foretold, given the facts of structure, the associated difference in function ; there is still less evident relation be- tween structure and function in the case of the cardiac and pjdoric glands of the stomach ; and none at all that can as yet be discerned between the form and arrangement of the hepatic cells and the secretion of bile. What is true in numerous other instances is eminently so in the case of the ductless bodies : we have in them examples of quite com- plex organs whose histological anatomy has in each case been ver}^ carefully studied; but whose function is in every instance, save that of those which consist chiefly of adenoid tissue, almost if not altogether unknown.
CHAI'TKH XXII. THE NERVOUS AXIS. 287
CHAPTER XXII.
THE CBXTK.\L NERVOUS SYSTEM.
The ductless bodies afford us instances of organs exhib- iting quite complex histological structure, concerning whose functions little or nothing is known. A case not exactly the converse of this is presented b}' the nervous axis; whose ph3'siological anatomy is exceedingly com- plex, but whose histological structure is far less so. For example, experiments show that the white matter of either side of the spinal cord is resolvable into a number of more or less distinct tracts, none of which presents any single structural feature or any combination of features b}'^ which it can be demonstrated : much the same may be said of the physiological centres that have been experimentally located in the grey matter of the cord; in no case can their positions be demonstrated by histological methods.
Failure to distinguish between the provinces of ph3''si- ological and of histological anatomy has led many writ- ers upon the latter to include in their description of the nervous axis many facts (in themselves of the highest im- portance) which are demonstrable only by methods that are essentially physiological : to the confusion of the stu- dent, who is frequently led to expect that his sections will show him more than our present histological methods at least make possible; and being disappointed is apt to
288 PART n. HISTOLOGICAL ANATOMY.
doubt the validity of the statements which he has read. The present chapter will undertake no more than a brief description of such structural features as can with cer- taintv be made out by histological methods; in other words, b^' a study of the form, relations, and groupings of the structural elements present. The consideration of the different regions of the brain wnll follow that of the spinal cord : and both will be preceded by a short account of the membranes which invest the nervous axis or line the cavity in which it is contained.
The meninges, or lining and investing membranes con- nected with the nervous axis, are three in number: of these the outer, known as the dura mater, or meninx fi- brosa, lines more or less closely the spinal canal and the cavity of the skull ; the inner, called the pia mater, or meninx vasculosa, closeU^ invests the surface of the cord and the brain; while the third, the arachnoid, or meninx serosa, situated between the first and the second, is structurally connected with the latter, the two being de- rived from the differentiation of a single investing laj'er. On account of the intimate relation between these two meninges they are by some anatomists described as one, under the name of the leptomeninx, the outer membrane being termed the pachymeninx : a view which is sup- ported by the comparat ve anatomy of these structures as well as bj' their embryology ; it is also true, however, that the two meninges so designated are themselves form- ed from a single mass of indifferent tissue originally filling the space between the nervous axis and its case.
CHAPTER XXII. THE NERVOUS AXIS. 289
Tlie dura is a thick and strong mass of white fibrous tis- sue, rni.xed with a small amount of elastic fibres; the con- stituent bundles are dis]3osed chiefly in a longitudinal di- rection in the spinal portion of the membrane, those of the cranial portion being variously disposed. In the latter re- gion the dura is closely adherent to thecranial periosteum, })articularly at the base of the brain; dorsally the two fibrous layers are less intimatcU' united; it is, indeed, cus- tomary to speak of the dura as forming the intracranial periosteum, and as composed of two layers; but the inter- pretation here indicated is more in accordance with the facts of structure. The inner surface of the cranial dura is invested throughout with serous endothelium; a similar structure has been described upon the outer surface in places where it is free from the periosteum. The spinal dura is covered on both sides with a similar investment. In addition to flattened connective tissue corpuscles, both the cranial and the spinal dura contain numerous granule cells. The cranial dura is separated from the cranial periosteum here and there by the large sinuses which receive the blood from the veins of the brain, and bv the accessory or parasinoidal spaces, but is not itself highly vascular; the same is true of the spinal dura, be- tween which and the periosteal lining of the spinal canal is found the internal spinal plexus. Ossification occurs normally in the principal folds of the cranial dura (the falx and the tentorium) in some mammals.
The pia consists of two more or less clearly distinguished laN'crs, an inner or investing layer, closely applied to the subjacent nervous mass, with whose connective tissue framework it is directly continuous ; and an outer or vas- cular stratum, rich in small blood vessels whose branches enter the nervous tissue beneath. The whole membrane
290 PART II. HISTOLOGICAL ANATOMY.
is made up of interlacing bundles of white fibres, mixed with occasional elastic fibres; in the spinal pia the prevail- ing direction of the bundles is longitudinal. The outer layer of the pia is connected with the arachnoid by numerous trabeculae o^ connective tissue, which pass across the subarachnoid space; this is quite extensive along the spinal cord, and in some places around the brain ; a nearly continuous series of definitely arranged strands passes from the pia at the sides of the spinal cord across to the inner surface of the dura, traversing the arachnoid; the groups of internally converging bundles are found between the spinal nerves, with which they alternate; the whole forms the denticulate ligament of either side. Upon the free surface of the pin and along the surfaces of the trabeculae and the ligaments just de- scribed are found flattened connective tissue corpuscles, endothelioid in form and position. In the stroma of the pia plasma and granule cells occur, and in some animals great numbers of pigment cells.
The arachnoid is composed of loosely interwoven delicate bundles of fibres, the intervening meshes containingnumer- ous flattened corpuscles: the outer surface is invested with a layer of serous endothelium ; it is near to but very spar- ingly connected with the dura. The inner surface corres- ponds to the outer surface of the pia, with which it is united by the loose mesh work of trabeculae above de- scribed.
The relations of the meninges to each other and to the adjacent structures are as yet but imperfectly understood, neither their structure nor their embryonic development being as yet fully known. Such facts as are known con- cerning the latter, and such light as is afforded bj-^ com- parative anatomy, appear to justify the view which
CHAPTER XXII. THE NERVOUS AXIS. 291
regards the subdural space as the primary cleavage of the mass of connective tissue which in the embryo and in the lowest vertebrates lies between the nervous axis and the walls of the cerebrosj)inal canal ; the portion outside this cleft becoming the dura or pachymeninx, and that inter- nal to it tlie leptomeninx or arachno-pial membrane; if this view be correct, the supradural (or so-called "epi- dural") and the subarachnoid spaces must be regarded as secondary. As a modification of this view it has been suggested that the arachnoid is in reality reflected over the inner surface of the dura, thus forming a continuous serous membrane which surrounds a serous cavity for which the name of the arachnoid space (as a substitute for the more familiar name of subdural space) has been proposed ; and this interpretation has some facts in its support.
The spinal cord, surrounded by its meninges and sup- ported by them, hangs freely in the spinal canal; through- out a great part of its extent it is nearly cylindrical ; in the cervical and lumbar enlargements its transverse diameter is distinctly greater than the dorsoventral. It is divided into symmetrical halves by the so-called anterior and posterior fissures; the former of these, the ventral fissure of comparative anatomy, is a distinct furrow from a fourth to a third of the diameter of the cord in depth, containing a fold of the pia which bears an important relation to the blood supply of the cord : the latter, or dorsal fissure, while deeper than the former, is not a true cleft, the right and left masses being separated simph' bv a stout median septum given off from the inner surface of the pia. Other septa of a similar nature, but less exten- sive, are given ofif from the pia at various points along the sides of the cord, and contribute in some measure to its subdivision.
292 PART 11. HISTOLOGICAL ANATOMY,
When viewed in cross section the cord appears to the naked eye to be made up of two differently appearing sub- stances known from their color as the white and the gray mitter. The latter is in each half of the cord almost entirely surrounded by the former, but not uniformly so ; from its central portion on either side a narrow ridge pro- jects dorsally (and slightly outward) almost to the sur- face of the cord ; from its appearance in cross section it is known as the posterior or dorsal cornu; in a similar manner the gray matter projects ventrally and outward to form an anterior or ventral cornu which is much thicker than the dorsal, but does not approach the sur- face so nearly; in the thoracic portion of the cord a slightly projecting ridge extending directly outward be- tween the dorsal and ventral cornu constitutes the so- called lateral cornu. The gray matter of the two sides of the cord is continued inward in the mid-region through- out the extent of the cord to form the gray commissure, by which the two halves are connected, and in which runs the central canal; in consequence of the disposition of the gray matter of the cord its figure as seen in cross section is irregularly H-shaped.
The white matter also exhibits certain conspicuous subdivisions, due in part to the disposition of the gray matter, in part to the mode of origin of the spinal nerves. The bundles of fibres which are gathered together at regular intervals outside the cord to form the posterior or dorsal roots of the spinal nerves enter the cord close to the dorsal cornu along a well-defined line on either side, which lies in a shallow groove. Between the dorsal cornu of either side and the median septum, which extends inward to the gray commissure, lies the posterior or dorsal col- umn of the side in question ; it is limited externally by the line of entrance of the dorsal roots, between which and
CHAPTKK XXII. THE NERVOUS AXIS. 293
the dorsal midline a well-marked septal process of the pia suhdivides the column into two important tracts to be presently described. Between the dorsal and the ventral cornu of each side is situated the lateral column; anrl be- tween the ventral cornu and the ventral fissure the an- terior or ventral column. As has been already stated, the ventral cornu docs not approach very closelv to the surface, and the fibre-bundles which enter into the anterior or ventral roots of the spinal nerves are given off irregu- larly over a region of considerable width; there is there- fore no clearlv defined boundary either externally or internally between the lateral and the ventral columns ; the whole region from the dorsal cornu to the ventral fis- sure is therefore sometimes spoken of as the antero- lateral or lateroventral column. The ventral columns of the two sides are connected with each other dorsad of the ventral fissure by a narrow layer of white matter called the white commissure.
The ajjpearances thus far described can all be readily seen, most of them with the naked eye; the histological structure of which they are the expression, in some re- spects exceedingly simple, is in others of such complexity that it has hitherto taxed the resources of histological technique to the utmost. It has long been known that the gray matter contains numerous multipolar nerve corpuscles of varying size and of a distinct but not sharply defined arrangement; their branching processes; non-medullated nerve fibres; and numbers of the neuroglia cells mentioned in a previous chapter; that the central canal is lined with a layer of columnar cells which are sometimes ciliated ; that the white matter is composed chiefly of medullated nerve fibres, mingled with which are numerous neuroglia cells like those of the gray matter: that the pial septa already mentioned subdivide in the interior of the cord to
294 PART 11. HISTOLOGICAL ANATOMY,
form a framework along which are distributed the inter- nal vessels of the cord, the general disposition of which can readih'^ be determined In brief, the cord is known to consist of multipolar corpuscles and nerve fibres sup- ported largely by neuroglia cells and provided with a definite blood-supply. The complexity of the cord depends not upon the number of kinds of tissue elements present, no other active organ in the body of equal size and im- portance containing so few; but upon the arrangement of the corpuscles and fibres, and particularly upon the rela- tions existing between them ; and it is only recently that the latter has been at all satisfactorily determined by means of technical processes of great efficiency and corres- ponding delicacy. It will be profitable to consider first the more conspicuous and therefore longer known feat- ures of the histological anatomy of the cord, and subse- quently the minuter details that have more recently been discovered.
Beginning with the gray matter, the most conspicuous and in every sense the most important elements are the multipolar corpuscles scattered throughout it. While they at first seem to be quite irregularly distributed, the examination of a number of successive sections from the same region of the cord, and theircomparison with similar series from other regions will demonstrate that the corpus- cles are arranged in groups extending along the length of thecord throughout the whole or definite portions of its ex- tent with sufficient regularitv to warrant their design a- tion as distinct ganglionic columns. Of these the most readily distinguishable on account alike of its extent and the size of its constituent corpuscles is the column of the ventral cornu, or, from the know^n distribution of the axis-cylinder processes of most of its corpuscles to the ven- tral roots of the spinal nerves, the motor-corpuscle col-
CHAPTER XXII. THE NERVOUS AXIS. 295
umn; this mav a^aiii be more or less distinctly divided in some portions of the cord into a mesial tract, situated adjacent to the ventral fissure ( which probably supplies motor fibres to the nerves of the dorsal muscles ol the trunk); a ventrolateral tract, contiguous to the mesial and extending to the outer side of the cornu, (which probal)lv supplies fibres to the nerves of the lateral and ventral muscles of the trunk) ; these two tracts, which are confluent in the thoracic portion of the cord, are quite con- st'itit throughout its whole extent; and a third, the. dor- solateral tract, which is situated, as its name implies, in the outer side of the cornu and dorsad of the second above mentioned ; it is present chieflv in the cervical and lumbar enlargements ( and probablv supplies fiiires to the brachial and sacral plexuses). Still another group, more centrally located, can be distinguished in some sections; but it is much less constant than those above described The cor- puscles of the ventral cornu are the largest found in the cord, their diameters ranging in most instances from sixty-five to one hundred and thirty micra.
At the base of the ventral cornu and near the gray com- missure is a group of much smaller corpuscles, rarely ex- ceeding thirty micra in diameter, the medlocentral tract; it is well developed in the thoracic portions of the cord, but much less so in the cervical and the lumbar. At the base of the so-called lateral cornu, also in the thoracic re- gion, there is a group of corpuscles of similar size, the mediolateral tract, or, as it is called, the column of the lateral cornu.
The dorsal cornu exhibits at its juncture with the gray commissure a well-marked column which extends from the level of the seventh cervical to that of the third lumbar nerve ; throughout the thoracic region it is quite conspic- uous, being roughly cylindrical in form and quite sharply defined by the arrangement of the adjacent fibres; it was
296 PART II. HISTOLOGICAL ANATOMY.
the first column ofcorpuscles to be clearl}' recognized, and is known from its discoverer as Clarke's column. Stilling proposed for it the name of the dorsal ( thoracic j nucleus, and for similarly placed aggregations situated respectively at the level of the third and fourth cervical nerves and the second and third sacral the names of the cervical and the sacral nucleus. The corpuscles of Clarke's column are next to those of the anterior cornu in size, being from thirty to ninety micra in diameter. Small corpuscles, from twenty to thirty micra in diameter, are scattered through the rest of the cornu with little definite arrange- ment; those close to the mesial margin are in some cases quite distinctly elongated parallel thereto and are known as marginal corpuscles; near the middle of the cornu the gray matter is broken up along the mesial side and to some extent along the lateral side also by bundles of fibres; the reticular formations thus produced have associated with them irregularly defined groups of corpuscles. The posterior cornu is capped dorsally by a trani*lucent mass, rich in glia cells, known as the gelati- nous substance of Rolando ; it contains numerous small rounded elements, about fifteen micra in diameter, gener- ally regarded as nerve corpuscles, as well as other larger elements, unquestionably nervous, some of which are mar- ginal in form and position. Mention may also here be made of the fact that various observers have described outlying corpuscles scattered among the fibres of the white columns, both dorsal and lateroventral.
While the corpuscles are the most conspicuous elements of the gray matter, they form but a small portion thereof. By far the larger portion consists of a densely felted mass of fibres of various kinds ; including small medullated fibres, nonmedullated fibres, axis-cylinder processes and their branches, and the dendritic subdivisions of the protoplasmic processes of the corpuscles. These were
CHAPTER XXII. THE NERVOUS AXIS. 297
formerly suj)posc{l to be continuous .'ind to form the so- called reticulum of the gray matter: but the evidence of recent investi^^ations by methods giving results of great delicacy leads to the belief that no such continuity exists; what is now generally believed to be the true relation of the corpuscles and the fibres of the gray matter will be in- dicated later. To the interlacing fibres and fibrillae already mentioned are added the numerous delicate fila- ments proceeding from the glia cells which with the con- nective tissue elements present make up what is some- times called the spongy substance of the gray matter. About the central canal there lies a translucent layer simi- lar to that which caps the dorsal cornua, the central gelatinous substance.
The white matter of the cord consists, as has already been said, chiefly of medullated nerve fibres; these vary greatly in size, the smallest being but one or two micra in diameter, while the largest may have a diameter of over twenty-five micra. The great majority of these fibres are longitudinally disposed, the most conspicuous exception to this being found in the white commissure, through which fibres pass at greatly varying degrees of obliquit}'. Between the medullated fibres are numerous neuroglia cells so disposed as to form an interstitial framework be- tween the subdivisions of the pial septa. Along the sur- face of the septa the neuroglia cells are gathered in great numbers, forming a definite investment which is con- tinuous outwardl}'^ with a well defined layer of consider- able thickness which lines the inner surface of the pia.
While it is possible to recognize readily that certain regions of the cord, presently to be indicated, contain chiefly cither larger or smaller medullated fibres, through most portions the fibres vary so much and so irregularl}^ in size that no such division into tracts can be made as in
298 PART It. HISTOLOGICAL A.NATOMV.
the gray matter, based on the size and arrangement of the fibres; the only conspicuous visible feature on which a sub- division can be based is the presence of the important secondary septum already mentioned as situated between the median septum and the dorsal cornu of either side; this passes obliquely toward the ventral edge of the median septum, thus dividing the dorsal column into two well defined portions, long known respectively as the column of GoU (next to the median septum) and the col- umn of Burdach (next to the dorsal cornu). The remainder of the cord has been more or less accurately subdivided into tracts by other methods; while these tracts cannot be distinguished by any means now in our possession in sections of the normal adult cord, some knowledge of them is necessary in order to understand the meaning of some of the finer details of structure devel- oped by recent histological researches in both the white and gray matter: a brief account of them and of the methods by which they have been determined will there- fore be given.
We owe chiefly to Flechsig the discovery that the medul- lated nerve fibres of the different tracts of the cord attain their full development at different periods of embryonic and even of infantile life ; the difference in question being apparently correlated with similar differences in thecalling into activity of different powers and faculties of the ner- vous system : the study of the spinal cords of embryos at various stages of advancement thus revealing significant differences in structure not discernible in the cord of the adult. To Waller we are indebted for pointing out that an axis-cylinder when severed from the corpuscle of which it is a process rapidly undergoes degeneration : it follows from this that in cases of lesion of the cord, either patho- logical or experimental, not only single fibres but also tracts composed of fibres of a similar character will un-
CHAPTER XXII. TIIK \ERVOUS AXIS. 209
dergo degeneration in a manner definitely related to the place of lesion. Without entering into the physiological significance of the changes involved, it may be said that degeneration on the side of the lesion toward the brain is called ascending; and that on the opposite side of the lesion descending ; and that the same terms are applied to the tracts in which the changes in question occur. The method of Flechsig and that based on the Wallerian law of degeneration give us results which coincide to such an extent as to determine quite clearly the limit of some of the tracts about to be mentioned ; the evidence in favor of the existence of others, while generally regarded as satis- factory, is by no means as conclusive.
None of the tracts which are thus shown to exist in the white matter of the spinal cord are more clearly definable than those which pass from the pyramids of the medulla oblongata into the ventral and lateral columns of the cord. The fibres of the pyramids decussating while still in the medulla, the great majority of them enter a large com- pact bundle whose cross section is an irregularly triangular area lying (in the human spinal cord) between the dorsal cornu, from which it is separated by a thin layer of fibres, and the lateral surface of the cord, from which it is also separated by a layer of fibres save in the lumbar region, where it extends to the surface ; it is known as the lateral (or the crossed) pyramidal tract. In man the fibres of the pyramid do not, as a rule, all decussate in the medulla, a small tract passing down as a thin la\'er on the portion of the surface of the ventral column which lies within the ventral fissure on the same side as the pyramid from which it proceeds; this tract is therefore called the anterior (or the direct) pyramidal tract ; it does not extend beyond the middle of the thoracic region of the cord. It is proba- ble that decussation goes on between the right and left tracts throughout their whole course, the fibres passing
300 PART II. HISTOLOGICAL ANATOMY.
throuo^h the white commissure, instead of occurring all at once in the medulla: in some mammals the pyramidal decussation is complete, and there is in consequence no direct pyra.nidal tract: this is sometimes the case in the human subject The existence of both pyramidal tracts was demonstrated by Tuerck, though his name is usually associated with the smaller and less constant of the two, which is CO nmonly designated the column of Tuerck.
Far less clearly defined and less constant in position is another descending tract, the ventrolateral, or anterior marginal bundle of Loewenthal: it consists of a thin layer of fibres situated upon or near the surface of the ventral and a good portion of the lateral columns : its fibres proceed fro n the cerebellar cortex ot the same side.
In close contact with the ventrolateral descending cerebellar tract, the fibres of the two mingling to a greater or less extent, lies the ventrolateral ascending cerebellar tract, or anterolateral ascending^ tract of Gowers: like its companion, it is throughout the larger part of its course a thin layer of fibres, which is situated between the tract just mentioned and the surface of the cord: it is thickest in its most dorsal portion, and is limited in that direction by the crossed pyramidal tract.
Dorsad of the tract of Gowers and external to the crossed pyramidal tract is the dorsolateral ascending cerebellar tract, or the direct cerebellar tract of Flechslg. It begins in the lower part of the thoracic portion of the cord (below which level the crossed pyramidal tract comes to the surface of the cord) and extends from there up- wards, passing through the restiform body in the medulla oblongata to reach the middle lobe of the cerebellum. Like the pyramidal tracts and unlike those just described, the tract of Flechsig is very clearly defined. Between its dor- sal border and the line of entrance of the dorsal roots of the spinal nerves, limited externally by the surface of the cord, is a narrow tract, the marginal zone of Lissauer.
CHAPTER XXII. THE NERVOUS AXIS. 301
Attempts have been made to farther subdivide the white matter of the ventral and lateral columns; but the results on which these attempts arc based arc thus far so conflicting as to render theconclusions drawn therefrom cpiite doubt- ful: for the present it is best to include the whole of the territory enclosing the ventral cornu and perforated by the fibre-bundles of the ventral roots of the spinal nerves (excepting, ol course, the tracts already designated) under the title of the ventro-lateral root zone.
The columns of GoU and ol Burdach, situated in the dorsal region of the white matter, have already been de- scribed as limited structurally: they may be otherwise distinguished, according to their function, as the dorso- median and the dorso- lateral ascending tracts. Im- bedded within the latter may be detected a small bundle of fibres with descending degeneration, known Irom its outline when seen in cross section as the comma.
Wehaveseen that some of the tracts above described, both ascending and descending, are in direct relation with the brain : others are doubtless composed entirely' or in large measure of fibres that begin and end in the cord itself: others, and pcirticularly those of the dorsal tracts, are in direct relation with the spinal nerves; and these latter organs are in such close connection with the cord as to merit mention in this connection.
Each spinal nerve possesses, as is well known, a dorsal and a ventral root. The latter consists of efferent fibres chiefly if not solely motor in function, which arise from the axis-cylinder processes of the corpuscles of the ventral cornu, as has been stated, and pass almost directly out of thecord : they therefore make no important contributions to its col- umns. The dorsal root, in addition to its ganglion, which will be farther discussed in a subsequent paragraph, con- tains a few fibres probably motor in function : it consists chiefly of efferent or so called sensory fibres which are some-
302 PART 11. HISTOLOGICAL ANATOMY.
what definitely divided into two groups, a mesial and a lat- eral, in each bundle; they penetrate the surface of the cord more or less obliquely and then bifurcate, givmg rise to as- cending and descending branches, the disposition of whose terminal and collateral arborizations will be described later. Recent researches by Cajal indicate that the fibres of the lateral group, which are slender, have their bifurcation in the marginal zone of Lissauer and the adjacent part of the lateral column; their collaterals are few and delicate and end in the dorsal cornu : the fibres of the mesial group, which are stouter, reach the columns of Goll and Burdach, and there bifurcate: their collaterals form by far the larger portion of those subsequently to be described as derived from the dorsial column, and in particular those which form the channels whereby those impulses are transmitted which are involved in simple reflex movements.
The white matter of the cord, therefore, consists of med- ullated nerve fibres which may be divided according to their origin and destination into three groups, the mem- bers of which are not structurally distinguishable: those which pass from the cord to end in the brain; those, com- missural in character, which pass from one portion of the cord to end in another; and those which pass to the cord from the brain or from the spinal nerves. From the de- scription of the nervous elements given in a preceding chapter it will be evident that each of these fibres consists essentially of the axis-cylinder process of a corpuscle, and ends in an arborization, giving off along its course one or more collaterals. It is probable, but not certain, that these collaterals have their terminal arborizations in re- gions of the gray matter of the cord homologous with that in which the terminal arborization of the fibre itself is situated.
Leaving out of consideration for the present the fibres
CttAPTEk XXll. THE NERVOUS AXIS. 303
which pass from the cord to the l)rain, it may be said that the terminal arborizations of fibres, whether collateral or principal^ which enter the gray matter of the cord are in close contiguitv or actual contact with the bodies or the dendritic processes of the corpuscles of the gray matter: the latter play the part of conductors (and not a nutritive role merely), and connection is thusestablished asefficiently ns by the continuity of substance once supposed to exist. An impulse thus transmitted calls forth the activity of the corpuscle in question, resulting in a discharge along its axis-cylinder process which undergoes a similar distribu- tion.
Regarding the final arborization of a fibre as essentially similar to thoseof the collaterals, and therefore to beclassi' fied with them, we shall make the first step toward a con* ception of the physiological anatomy of the cord by an enquiry into the distribution of these structures as they leave the various regions of the white matter. The follow- ing account thereof, as well as that of the corpuscles of the gray matter to be subsequently given, is taken almost wholly from Cajal.
Collaterals of the ventral (anterior) column. These are larger than those from any other portion of the cord : springing from the large axis cylinders which compose this column they pass dorsad in irregular groups to be distrib- uted within the ventral cornu and particularly about the motor corpuscles. Some bundles pass to the mid-plane of the cord and are distributed in the ventral cornu of the opposite side, constituting the ventral (or anterior) com- missure of collaterals, situated largely dorsad of the ven- tral commissure formed of axis cylinders.
Collaterals of the lateral column. These pass inward to be distributed chiefly in the central region of the gray
304 PART II. HISTOLOGICAL ANATOMY.
matter of the same side: some, however, pass to the mid- plane, dorsad of the central canal, where thev form a por- tion of the dorsal (posterior) or gray commissure: they are divided in it into two bundles, a ventral and a ne- dian, and are distributed in the central and to some extent the dorsal region of the opposite side
Collaterals of the dorsal (posterior ) column. The tracts of Goll and of Burdach,and the marginal zone of Lissauer, from which these collaterals are chiefly derived, are formed in great part of the continuations of the fibres of the pos- terior roots of the spinal nerves. Four groups of collat- erals may be distinguished.
Sensitivo-motor (or reflexo-motor) collaterals: these arise not onh' from the continuations of the fibres of the posterior root but also from the fibres themselves before their bifurcation; passing across the gray matter, they terminate in the ventral cornu of the same side.
Dorsal cornu collaterals : these, like the preceding, are very numerous: they traverse the substance of Rolando in groups to form immediately ventrad thereof and through- out the substance of the dorsal cornu a dense network composed of the intercrossing of their terminal arboriza- tions.
Clarke's column collaterals: small bundles pass ven- . trally from the tract of Goll to terminate in the column of Clarke of the same side, there forming a thick network about the corpuscles of the column.
Commissural Collaterals: arising chiefly in the tract of Goll numerous small bundles pass to the most dorsal por- tion of the gray commissure, which they traverse, to be distributed in the dorsal cornu of the opposite side: thus forming the dorsal bundle of the commissure.
Thus, it will be seen, each white column gives off two
CIIAPTKR XXII. THE NKRVOUS AXIS. 305
kinds of coUateriils: those which furnish their terminal arborizations to the gray matter of the same side, and those, commissural in chiiracter, which are destined to ramifv in the gray matter of the opposite side. In either case their relations are eventually directly with the cor- ])uscles of the gray matter, whose disposition may now be further considered. E.Kcepting in the substance of Ro- lando, where some elements of special form ai*e found, the corpuscles of the dorsal, central, and ventral regions diflfer but little, save in size. The only important distinction to be noted in them pertains to the final disposition of their axis-cylinder processes: on this basis five groups or kinds of corpuscles may be distinguished : of these the first four send, as will be seen, their axis-cylinder processes out of the gray matter into the white, there to become medul lated : they may be therefore distinguished as corpuscles with long axis-cylinder processes (corpuscles of the first class); while the axis-cvlinder processes of the fifth end in the gray matter not far from their point of origin : they are therefore corpuscles with short axis-cjdinder processes (corpuscles of the second class, corpuscles of Golgi). The five kinds of corpuscles are as follows:
Radical corpuscles, or corpuscles directly related to the roots of the spinal nerves. These are the motor corpuscles of the ventral cornua, and comprise the largest corpuscles of the cord : their axis-cylinder processes are thick and devoid of collaterals and pass in most cases directly through the lateroventral column to enter the ventral roots of the spinal nerves. From a few^ of the corpuscles the axis-cylinder processes traverse the gray matter to leave the cord by the dorsal root : they pass through the spinal ganglia, however, without entering into relation with their corpuscles, and must be regarded as in all prob- ability motor in function.
Q'()6 PART n. HISTbLOGICAL ANATOMY?
■ The dendrites of these corpuscles are stout, lotig/aria ver}' much branched. They ma}' be distinguished as ven- tro-external, dorsal, and internal (mesial); the latter branch dichotomously in the Vicinity of the ventral (ante- rior) commissure ; some of the branches pass the niid-plane and enter the ventral cornu of the Opposite- side, intercross- ing with corresponding processes therefrom and forming the protoplasmic commissure of Caijal. The ventro-ex- ternal processes terminate in the lateroventral column, and the posterior in different regions of the ventral cornu.
Commissural Corpuscles: these are smaller than thosie just described, a:nd provided vC^ith fewer arid shorter den- drites. Golgi demonstrated their presence in all of the regions of the gray matter, and that the axis-cylinder pro- cesses pass to the mid-plane which they crOss ventrally (Iti the white commissure) to- be continued to the ventrolat^- eral column of the Opposite side. Cajal has shown that thfey there under'g'o not a sifi'gle continuation', btit a T- division: this indicates that'the conitnissural axis-cylinder process, on reaching the white inatter of the opposite side; divides into an ascending and a descending fibre of the column.
Columnar corpuscles : we may thus designate the nutn- erous medium-sized corpuscles scattered throughout the whole of the gray matter, of which the axis-cylinder pro- cesses enter into vertical 'fibres of their own side.' The greater number of the corpuscles of this kind which occur in the ventral cornii send their processes to the lateroven- tral column : those which are sittJated in the dorsal cornu direct them toward the most dorsal portion of the lateral column in manycases, though some of the corpuscles found in the substance of Rolando and the internal portion of the dorsal cornu send their processes to the dorsal column.
CHAPTKR XXII. THE NHRYODS AXIS; 307
A? rc«;ar(ls tht'columii ol Clarke, two kinds of corpuscles cati be demonslrated : conimissiiral corpuscles, whose processes enter the ventral commissure ; and columnar cor- puscles, whose processes pass to the lateral column to be- come continuous with the fibres of the cerebellar tract. This continuation takes place by two methods; by the formation of a l)end, which furnishes a sinj^le conductor, ascendiuLj or descending; and by a T-division, which forms two conductors or vertical branches, one ascending and the other descending.
Pluricolumnar corpuscles : Elements. are so termed by Cajal of which the axis-cylinder process is divided while still in the gray matter into two or more portions which enter into as many nerve fibres belonging to different col- umns : thus, for example, an element of this kind may send one fibre to the ventral column of its own side and an- other to that of the opposite side; in another the process may divide into a fibre for the dorsal column and another for the lateral or ventral, etc.
Van Gehuchten has proposed for the corpuscles here designated commissural, columnar, and pluricolumnar the names of heteromeral, tautomeral, and hecateroraeral corpuscles respectively.
Short process corpuscles: these, which are found chiefly in the dorsal portion of the gray matter, have slender and flexuous axis-cylinder processes which speedily end in arbor- izations situated in proximit\' to Other and adjacent cor- puscles.
The substance of Rolando merits special mention on ac- count of the peculiarities of some of the corpuscles con- tained therein. The latter belong chiefly to the columnar and short process types, with some pluricolumnar cor-
308 PART II. HISTOLOGICAL ANATOMY.
puscles; whilst commissural corpuscles are not known with certaint}' to occur. Those characteristic of the re- gion are of three principal forms disposed in as manv con- centric zones, passing from without inward, as follows.
The marginal corpuscles are large fu:!iform or flat- tened elements situated between the substance of Rolando and the dorsal column of white matter, thus forming a discontinuous layer. The dendritic processes line the sur- face of the dorsal column and there ramify: the axis-cylin- der process, arising sometimes from the border of the cor- puscles, sometimes from one of its processes, is directed ven- trally across the substance of Rolando; it then changes its direction to reach the posterior portion of the lateral col- umn, with one of the fibres of which it becomes continuous.
The pyriform or fusiform corpuscles are the smallest elements of the cord: their shape is quite variable, but forms indicated by the terms above are prevalent. It is characteristic of them that they are elongated dorsoven- trally and have great numbers of crooked and intermin- gled dendrites, the greater portion of which arise from a ventrally directed stalk which is prolonged almost to the head of the dorsal cornu. The axis-cylinder process gen- erally arises from the posterior portion of the corpuscle and passes either dorsally or laterally to become continu- ous with one of the fibres of the dorsal column.
The stellate corpuscles are situated nearest to the head of the dorsal cornu: they unite the substance of Rolando with that region by means of their abundant spinous den- drites. The axis-cylinder process is sometimes directed lengthwise of the cord; it then comports itself as a por- tion of a short process corpuscle, and appears to end in the substance of Rolando itself: at other times it is directed either mesially, to become continuous with a fibre of the column of Burdach, or laterally, to form a slender fibre of the marginal zone of Lissauer.
CIIAPTKK XXII. THE NERVOUS AXIS. 309
Bv a CO n():iris :)n of this (Ijscription of the corpuscles as based upon the destination of their axis-cylinder processes with that previously j^iven of their distribution in col- umnar tracts aloni^ the cord, it will be seen that these lat- ter are in nearlv everv instance composed of corpuscles of v'lrvinar relatiois. n )t evi.';i the colu n.is of the ventral cornu consistin'j^ solely of so called motor corpuscles: Each of these elon^jated clusters may therefore be regarded as consisting of corpuscles so coordinated in function as to justify the title of nuclear or ganglionic columns fre- quently applied to them.
The corpuscles of the spinal ganglia, while they are situated without the cord, should always be associated with that structure in any attempt to form a complete conception of the central nervous mechanism, since their eflferent axis-cylinder processes enter extensively into rela- tion with the dorsal columns andcornua. Their spheroidal or ])vriform contour has been described in a previouschap- ter, as has also the manner in which two medullated fibres are derived from the single pole or process borne by the bodv of the corpuscle. They are unique in the fact that one of these fibres puts them in communication with den- dritic or receptive terminals which are far more remote in most instances than those of any other nervous element.
The central canal of the cord is lined with a columnar epithelial layer of ependyma cells which are frequently but not always ciliated. These are in the embryo con- tinued b\' slender prolongations which reach to the pia, forming the primary' framework of the cord : such contin- uations persist in the lower vertebrates, but their presence in adult birds and mammals is not yet well established. In connection with the ependyma cells, and possibly de- rived therefrom, there are found in the adult cord of the
310 PART If. HISTOLOGICAL ANATOMY. ,
higher vertebrates numerous neuroglia cells which are particularly abundant in the white, matter,, immediatehr beneath the pi a and along the septa and blood vessels, in the substance of Rolando, and in the central gelatinous, substance. . , > , . • , , i
The blood supply of the cord merits special mention. A s;ingle median ventral spinal artery (the anterior spinal of human anatomy) runs along the . ventral margin of. the fold.of the pia which enter§ the ventral fissure: while a pair of dorsal spinal arteries ;,(or postejioj^ spinals). are situated just ventrad of the dorsal roots of the spinal nerves: branches of these arteries ramify. in the ]')ia tK) form an, extensive plexus. From the, ventral tr,unkv small, vessels, the central arterioles of Ross, injiumber several times as many as the iV,erte;brae follow, the .fold of, the pia into the.median fissure as far as the white commis-, sure. Here they turn altern^.tely right , and' left to reach: the central region of the gray matter of, either, side* where, they breakup into small arterioles and finally into capil; laries: the central arterioles are distributed, chiefly to the; gray matter, though some of their divisions penetrate thef white ma,tter, particularly of the lateral and ventral col- umns. ' ,
The dorsa,l vessels and the pial plexusgiyeoflT great num,- bers of peripheral arterioles which follow the dorsal me- dian septum or the other less prominent septa into the cord: their terminal divisions^ are .found not only in the w^hite matter but also in the, outer portion of the grayi matter and throughout the, dorsal coruua. Each a.irteriole, whether central or peripheral, has its own proper capil-; lary area, anastomoses between adjacent arterioles not being known to occur.
Similarly disposed central and peripheral venules o^rry the blood from the capillary networks to the irregu-
CHA<»fER XXII. TilK NERtoUS AXIS. 311
Inr plexuses of veins in the ()ia and to tlie ])rinei|)al venous trunks: these are two in number, a ventral which follows the ventral artery nmre or less eloselv, and a dorsal, \\ hieh overlies the niediiin dorsal septum and is not the compatiion of any artery. ' ' '
^'The brain is the mo'dified and specialized anterior end of the nervous axis. As we pass from the spinal cord into the medulla oblonfrata, and thence; throu<;h the region of the pons, into the crura cerebri, we find that the fibrous tracts which can be recognized in the white matter of the coi^d here become subdivided and varibusl}' modified, some of theni sooii disappearing as such, while others mav be traced almost to the most anterior portions of the brain; while new fibrous tracts variouslv related, mav be de- tected by the method of Waller and particularh'^ bv that of F'lechsig, The central canal expands here and there to form the ventricular cavities of the brain ; while its epen^ dymal lining frequently coming in contact with the pial •investment through the absence of intervening nervous tissues; is often thrown into A'^ascular folds of greater or less exrterit. The'gray matter*,' which ih the c6rd sur- rounds the ceritral canal, now lies chiefly in its floor and yides, and is penetrated and subdivided by the diversified fibre tracts already referred to. The ganglionic columns, ■v^'hich in the cord were continuous throughout the whole or large portions of its structure, are now broken up into more or less definite nuclear aggregates of corpuscles, ser- ving a«i centres for specific cranial nerves: some of these nuclei may perhaps be homologized with portions of some of the ganglionic columns ; while for others no such rela- tion is discernible.
In addition to this central prolongation and modifica- tion of the cord, other structures appear connected there
312 PART II. HISTOLOGICAL ANATOMY.
with Nvliicli. while they must unquestionably be regarded as developments of the axial region, are of such size and structural importance as to be properh' regarded as sub- stantially additions thereto. These, like the cord, and the basal portions of the brain as well, are composed of more or less definite fibre tracts and corpuscular areas, related to each other and to the more axial structures.
Within recent years great progress has been made in the process of unraveling this complex structure, and it is safe to say that the general topography of the brain is known. To attempt to sketch it. however, would take far more space than the range of the present work con- templates, and would take us largeh^ into a field where, while the method has been and must be chiefl\' that of the histologist, the results belong rather to the domain of the anatomist. Aledullated fibres, axis-cylinder processes with their collaterals, and multipolar corpuscles are much the same in appearance and in relations throughout a large portion of the brain as in the spinal cord : and we are con- cerned from a histological standpoint only with those resions of the brain in which new forms of nervous ele- m.ents appear or in w-hich some special mode of combina- tion is demonstrable. The chief of these are the cerebel- lum, the cerebral hemispheres, and the olfactory bulbs ; an account of the cerebellum and of the hemispheres will now be given : that of the olfactory bulb will be deferred until the sense organ with which it is connected has been described.
The cerebellar cortex is a superficial la^^er of gray mat- ter whose well marked folds form the laminae visible on the surface of the organ to the naked eye: the middle of each fold is occupied b}' a mass of white fibres in direct relation to the gray matter. The latter shows to the naked eye tw^o distinct strata ; an outer and paler known
CHAPTER XXII. THK NERVOUS AXIS. 313
as the molecular layer, and an inner, of a rusty brown color, called the granular layer. Between these, and par- tially imbedded in each, is a nearly continuous stratum of kir;j[e corpuscles, the most characteristic of the cerebellar cortex, the corpuscles of Purkinje. They are pyriform or flask-shaped, the large extremity being directed inward: from the latter an axis-cylinder process is given oft' which passes through thegranular layer to enter the white mat- ter as 'ci medullated nerve fibre; during its course through the granular layer it gives off^ collaterals which in many cases turn backward to enter the molecular layer.
The outer extremity of the corpuscle is prolonged for a greater or less distance, but usually soon divides into two ])rincipal branches, which rapidly and repeatedly subdi- vide to give rise to large numbers of dendritic processes, many of which are continued to the surface of the cortex: their surface is beset with .short processes which end bluntly. The ramification is in every case almost entireh' confined to a plane transverse to the lamella in which the corpuscle is situated.
The great majority of the corpuscles of the granular layer are exceedingly small, with large nuclei and very scanty surrounding protoplasm : their great numbers, and their appearance when stained with carmine or other similar stains led to the name above given for the region in which the}' occur. For a long time their nervous char- acter was doubted or denied. The more recent technical methods have demonstrated it beyond question, and the}' are now known as granule corpuscles. Their bodies con- tain a relatively large quantity of rusty brown pigment, to which the characteristic color of the layer is due, A few protoplasmic processes are given off b\' each corpus- cle: these branch sparingly to end in a sm.all number of dendrites with thickened extremities. From the bod}' of the corpuscle, or frequently from one of the processes, a
314 PART II. HISTOLOGICAT. ANATOMY.
slender axis-cylinder process is given off, which passes without collaterals into the molecular layer and there undergoes a T-division to form two slender tangential fibres whose course is alwa^'s in the direction of the lamella in which the}^ occur and therefore at right angles to the plane of the dendrites of the corpuscles of Purkinje, with which they are in close contact as the\^ pass. The tangential fibres have been shown in some of the smaller vertebrates to run the whole length of the lamellae in which they are situated.
In addition to the small granule corpuscles there are present in the inner layer, though in small numbers, other nervous elements. They are situated near the outer limit of the layer and are nearly as large as the corpuscles of Purkinje, which they somewhat resemble in form. Their outer region sends off numerous protoplasmic processes, which branch irregularh^ in every direction to form large numbers of dendrites: these project chiefly into the mole- cular layer, though many of them lie altogether within the granular layer. From the inner region there is given off a slender axis-cylinder process, which branches freelj'- almost from its origin, the whole giving rise to an exten- sive arborization-plexus which is situated entirel^within the granular layer. These corpuscles therefore opb^ng to Golgi's second type.
The finely dotted appearance to which the molecular layer owes its name, seen when the lamella is cut trans- versely, is largely due to the cut ends of the tangential fibres. There are present in this layer two kinds of cor- puscles. In the deeper portion are seen numerous corpus- cles of medium size and irregular form, whose dendritic processes branch sparingly but extend for some distance into the surrounding region : the axis-cylinder process takes a course generally parallel to the surface of the cor- tex and gives off frequent collaterals ; these pass inward
CIIAPTKK XXII. TIIK NKK VOl'S AXIS. 315
to be ji|)|)lic(l in each instance to the bodv of a cor- puscle of Puikinje, ujjon whicii, or about the base of the axis-cylinder process, tliev terminate with little if anv snb- (bvision: a»nunil)er of such fibrils surround each corjjus- cle of Purkinje, forming a nest or basket about it: the cor|)uscles Ironi which they proceed are therefore known as basket corpuscles.
Throus^hout the molecular laver, but chiefly in the outer portion thereof, are found numerous stellate COrpuscles, which though smaller in size, resemble the elements just described in their general form and in the appearance of their sparingly brdnched dendrites. The destination of their axis-cylinder processes is not known
The axis-cylinder processes of the corpuscles of Purkinje give rise to the only fibres of the central white matter of the lamella known to be sent inward from the cerebellar cortex. The fibres which pass out into it have been shown to terminate in two different methods. Some of them end in the granular layer, where their extremities branch spar- ingly, the subdivisions terminating in enlargements in such a way as to give to the whole somewhat the appear- ance of a tuft of moss: the}' have therefore been desig- nated EQQSSy fibres. The others find their way to the cor- puscles of Purkinje, traverse their surfaces, and subdivide to follow the branchings of the dendritic processes, thus forming a terminal arborization which adheres thereto like a vine to a tree: they have therefore received the name of climbing fibres.
It will be seen from the above description of the cerebel- lar cortex that each corpuscle of Purkinje is in relation with three sets of discharging terminals: the tangential fibres of the granule corpuscles, the collaterals of the bas- ket corpuscles, and the arborizations of the climbing fibres. We are aX present entirel)' ignorant of the func- tional relations which are based on this structure.
316 PART II. HISTOLOGICAL ANATOMY.
The cerebral cortex has been regarded as composed of several distinct la^-ers which differ from each other as re- gards the form and size of the contained corjDuscles. As our knowledge has increased, the boundaries between these la\'ers have been found to be less sharply defined than was at first supposed. Three can with certainty be distinguished: the outer, or so called molecular laver, the middle, or p3'ramidal laj-er, and the inner, or polymor- phous la^er.
The molecular layer, like that of the cerebellar cortex, owes its characteristic appearance largely to the cut ends of the collaterals and slender fibres which are denselv in- terwoven in it and to their terminals. Its outermost por- tion contains numerous neuroglia cells, which just beneath the pia form an almost continuous stratum, as in the spinal cord: this has been described as a distinct layer of the cortex. vScattered throughout the molecular layer are numerous nervous elements, the corpuscles of Cajal, that histologist having first demonstrated their distinguishing characteristics. Two forms have been described b}^ him, the fusiform and the stellate. The former are, as their name implies, spindle shaped, and give off from either end a polar process which runs parallel to the surface of the cortex : the two processes cannot be distinguished struc- turalh'-, and each may take on the character of an axis- cylinder process : from each collaterals are given off at right angles or nearly so, which are invariablj'' directed toward the surface of the cortex. In the second form the number of similar processes is increased to three or more. In each case the processes terminate eventually in ramifi- cations which are turned toward the cortical surface.
Cajal has also described in the molecular la^^er a third form of corpuscle under the name of polygonal; these have several protoplasmic processes which end in dendrites
CHAPTER XXII. THP: NERVOUS AXIS. 317
which nuiv extend beyond the molecular layer to enter that beneath it. The axis-cylinder process may arise either from the body of the corpuscle or from one of the processes: its terminal subdivisions are confined to the molecular layer.
The pyramidal layer is very commonly divided into two strata, the layer of small pyramids, next the molecular layer, and the layer of large pyramids, immediately sub- jacent. The elements characteristic of these two layers are, however, so nearly alike in everything but size, and the transition from the one to the other is so gradual in this respect, that they mav. at least for the present, be advMutagcouslv considered as one.
The most numerous of the elements peculiar to this layer are the pyramidal corpuscles: their form is indicated by their name. The base of the pyramid is turned away from the surface of the cortex, the apex being directed verti- cally upward and continued into a long tapering ascend- ing stem which, even from the corpuscles most deeply placed, extends nearly or quite to the molecular layer: it terminates by subdivisions into a tuft of protoplasmic processes; similar processes are given off" at right angles along its course; while others are given oft' from the bodv of the corpuscle, and particularly from the angles of its base : all these processes are probably to be regarded as dendritic : their subdivisions are distributed chiefly to the surrounding substance of the pyramidal layer, save those of the apical tuft, which are largely situated in the molecular layer. An axis-cylinder process is given off" from the base of each pj-ramidal corpuscle, usualU' from a point near the centre: it is always directed toward the white matter be- neath the cortex: collaterals are, however, given oft" while it is still in the gray matter, some of which run horizon- tally to terminal ramifications within the pyramidal
318 PART 11. HISTOLOGICAL ANATOMY.
layer, while others bend upward at a right angle to termi- nate in the molecular layer.
Within the lower portion of the pyramidal layer are cdso found, scattered here and there, certain elements confined to the cerebral cortex, the carpuscles of Martinotti : they are also found in the third or polymorphous layer. They are spindle shaped, roughly pyramidal, or irregular in form, their distinguishing characteristic being the distribu- tion of the dendritic processes chiefly outward and down- ward, and an axis-cylinder process which generally arises from the uppermost portion of the corpuscle, though some- times from one of the ascending protoplasmic processes, and ascends toward the molecular layer in which its rami- fications are usually situated : in some instances the ter- minals lie wholly or in part in the uppermost portion of the pyramidal layer.
The polymorphous layer is so called from the occur- rence therein of elements which vary greatly in form as well as in size: they may be ovoid, spindle shaped, pyramidal, or polygonal. They agree, however, in the fact that their long axes are as a rule disposed horizontally to the sur- face of the cortex, and that when a terminal or apical stalk is present it is never vertically directed as in the pyramidal layer. Some of the elements present are, as has just been indicated, ascending corpuscles of Mar- tinotti: others belong to the second type of Golgi, having short axis-cylinder processes which break upinto terminal ramifications in the immediate vicinity of the corpuscle. Still others give off long axis-cylinder processes which bend downward to enter the white matter and become medullated fibres of varying distribution.
The deeper portion of the polymorphous laj'cr contains chiefly small fusiform corpuscles, which has led to its dis- tinction en the part of some histologists as a separate
, cnAPTKK xxii. Tin-: nkkvois axis. 319
hiver: tliis is more clearly deHiied in the rej^ioii of the island of Rjil than elsevvh.'re, where the strataiii in (|ucs- tion is separated from the rest of the cortex by intervening white matter, forminf:^ the layer visible to the naked eye, known as the claustl'um. In most portions of the cortex the stratum in cjiu'stion is not clearlv definable from the rest of the ])olymorphous layer.
Brief mention may perhaps l)e made of the composition of the subjacent white fibrous layer, although the vari- ous fibres and tracts are not histologically distinguishable, save to a certain extent by the methods of Waller and of Flechsig. Fibres formed by the development of medullary sheaths about axis-cylinder processes which descend from the corpuscles of the cortical gray matter may pass on downward as projection fibres to the basal ganglia, the hindbrain, or the spinal cord itself: other fibres pass as association fibres to other ])ortions, more or less remote, of the cortex of the same hemisphere: while others still pass, chiefly by way of the corpus callosum, as commis- sural fibres to regions of the cortex of the opposite hemi- si)here; not unfrequently an axis-cylinder process may be- come a projection or an association fibre, and one or more of its collaterals an association or a commissural fibre: or the converse may occur. Still other fibres pass upward into the cortex by wa}' of projection, association, or com- missural tracts to end there, the terminal arborizations being situated either in the molecular or the upper portion of the pyramidal layer.
The neuroglia of the brain does not differ from that of the spinal cord to such an extent as to merit a detailed description in so brief an account of the organ as is here given. Much the same mav be said of the ependyma, which lines the ventricles and passage ways of the brain :
320 PART II. HISTOLOGICAL ANATOMY.
like that of the central canal of the cord, it is composed chiefly of columnar cells, whose free extremities bear for a time at least cilia-like processes not known to be vibratile. Mention should be made of the plexuses of the ventricles, formed b}^ infoldings of the pia and the ependyma, and consisting chiefly of a rich network of small bloodvessels supported by the former and invested b}' the latter. An outline of the blood supply of the cord was given above: that of the brain is far too complex and too much a mat- ter of gross anatomy to be described here. Neither will any account be attempted in this connection of the devel- opmental history of the nervous axis, beyond the state- ment that it is formed entirely from an infolding of the outer or epiblastic layer.
CHAPTER XXIII. Tlin ORGANS OF SPECIAL SENSE. 321
CHAPTER XXIir. THE ORGANS OF SPECIAL SENSE.
As was stated in a previous chapter, there are certain organs in which specially modified receiving terminals are associated with more or less highly modified forms of epi- thelium and with other special structures of a skeletal character to form in each case an apparatus for the recep- tion of a specific and clearly defined impression ; their stimulation giving rise to sensations of flavor, odor, sound, or light, commonly called special, as distinguished from the more diffused and less clearly definable sensations of temperature, contact, resistance, etc., received by the more widely scattered and possibly less specialized termi- nals described in the chapter referred to.
In each case the special receiving apparatus involved has associated therewith other special structures, chiefly skele- tal, whose function it is to render more intense or more specific the impression received: these associated struc- tures being, equally with the nervous apparatus in ques- tion, essential factors of the organ of special sense. As in the case of the brain, a full description of these structures belongs rather to the province of anatomy than to that of histolog}', and would require far more space than can with propriety be given here: an account of the histological composition of the essential apparatus will in each case be given, together with mention of any characteristic features noteworthy in the tissues of the accessory parts, some previous knowledge of the anatomy of the organs in question being presumed.
322 PART II. HISTOLOGICAL ANATOMY.
The immediate organs of taste are the taste-buds, so called from their spheroidal form, which are situated in large numbers upon both the outer and the inner sides of the valle\'S which surround the circum vallate papillae; on the fungiform papillae; and particularly upon the loose folds just in front of the anterior pillars of the fauces which in inan represent the more definiteU' circumscribed foliate papillae of some of the lower mammals: they are also found on the soft palate and the epiglottis, and arc scat- tered here and there over the surface of the tongue. They are spheroidal bodies, almost completely embedded in the stratified squamous epithelium of the surface where the\' occur: the long axis is directed vertically or nearly so to the surface, and the outer extreinitv tapers slightly, on which account their form is sometimes described as flask shaped. The mass consists of a number of elongated epi- thelial cells, of which some are spindle shaped, or flattened, and are known as sustentacular cells : a layer of these completed covers the outer surface, their grouping recall- ing somewhat the surface segmentation of acantelope: others are scattered irregularly throughout the interior. Between them lie other cells whose bodies, except just around the large nucleus, are slender and almost filamen- tous; these are the so called gustatory cells. The outer extremity of each ends in a ciliary process, the taste-hair, which projects with its fellows through a small circular opening in the squamous epithelium known as the gus- tatory pore: its inner extremity is slender, often bifur- cated, and frequently more extensively branched ; its sub- divisions, which are sometimes varicose, reach to the base of the taste-bud.
Numerous attempts have been made to demonstrate a structural relation between theelements just described and the nerve fibres which pass to the taste-buds from the sub- divisions of the glossopharyngeal nerve, but thus far
CH.VPTKK XXIII. TlIK ORCANS OF SPKCIAI, SENSE. 323
without success. This ucrvc, like the dorsal root of a spinal nerve, bears a j^anglion near its point of union with the nervous axis: examination by the chromate and silver method shows that tiie axis cylinders of the nerve fibres end in the taste-buds by branching ainoni^f the cells in the interior, forniin*,^ what are known asintrabulbar ramifica- tions: the honiolosi^y existing between the glossopharvn- jjeal and the spinal nerves would indicate that these are to be regarded as dendritic processes at the ends of long af- ferent fibres, similar to the "free endings" in the epidermis described in the chapter on the nervous tissues. Accord- ing to this view it is questionable whether the so called gustatory cells are in reality nervous in character, and some have regarded them and the sustentacular cells as alike modified epithelial elements whose form and arrange- ment favors the stimulation of the nerve terminals. There is, however, a close relationship between the senses of taste and of smell ; and the structure of the receiving ap- paratus of the latter suggests an explanation of the struc- ture of the taste-buds in which the gustatory cell would form the first member in a series of nervous elements. Reference will be made to this after the organ of smell has been described: the statements already made, however, represent the present extent of our knowledge of the facts in the case.
Mention should be made here of the fact that some of the medulla ted nerve fibres of the subdivisions of the glossopharyngeal nerve going to the taste-buds terminate bv free endings in the stratified epithelium immediately surrounding those structures, forming what are termed peribulbar ramifications : these are generally regarded as fibres of general and not of special sensibilitv. The glands of a serous type, known as the glands of Ebner, which are closely associated with the taste-buds, have been de-
324 PART ir. HISTOLOGICAL ANATOMY.
scribed in connection with the tongue. They are to be distinctly regarded as accessor}^ to the apparatus of taste perception, the fluid secreted by them aiding in the solu- tion of substances whose flavor is to be perceived by the taste-buds.
The sense of taste resembles most forms of general bodih- sensation in requiring the actual contact of the object per- ceived: the remaining special senses resemble the thermal sense in being capable of givingknowledge of objects at a distance. Of these the first and as regards its receiving mechanism the simplest is that of smell. The accessory- muscular and skeletal structures which make up the facial region known as the nose require no special description from a histological standpoint. The air passages which they enclose are lined with a mucous membrane known as the pituitary or the Schneiderian membrane : the vesti- bule, into which the nostril opens on either side, is lined with stratified squamous epithelium continuous at the margin of the nostril with the epidermis, of which it is a modification : the remainder is divided into two portions, the lower or respiratory and the upper or olfactory : the former is lined with stratified ciliated columnar epithelium, similar to that of other respiratory^ passages, beneath which is a highly vascular membrane which contains a considerable amount of adenoid tissue here and there gath- ered into distinct nodules, and numerous racemose glands, some of which are mucous and others are serous in char- acter: large numbers of goblet cells are also distributed throughout the epithelium.
The olfactory region of the nasal mucosa can be distin- guished with the naked eye b}' means of its well marked pigmentation, it being of a yellow color in man and some
CHAPTER XXIII, THE ORGANS OF SPECIAL SENSE. 325
of the lower mnmmals, nnd of a yellowish brown in others. The fibrous layer is more highly vascular than in the respiratory region, but contains less adenoid tissue: it contains numerous glands, the glands of Bowman, which diflTer from the racemose glands above mentioned in being tubular, rarely branched, and but slightly bent or convo- luted: the distal extremity is frequently the largest, the tube tapering toward the duct, which is always slender, and opens either upon the mucous surface or occasionally into a small ciliated crypt: the epithelium of the glands of Bowman is of the serous type, but the tubules resem- ble those of mucous glands in having a conspicuous lumen: ordinary racemose glands are also occasionally found in the olfactory region.
The epithelial layer of the olfactor}' region is composed chiefly of two kinds of elements. The first comprises the non-ciliated columnar supporting cells: these are chiefly prismatic in form throughout the greater portion of their extent, but with tapering inner extremities, and with oval nuclei situated at an approximateK' uniform distance from the surface: other more deeply situated epithelial cells are ])yramidal in form, their bases resting on the fibrous layer; they ma\' perhaps be regarded as immature supporting cells. Interspersed among the columnar epithelial cells are large mambers of slender elements of the second kind, whose outer extremities terminate in tufts of hair-like pro- cesses, the olfactory hairs, which project above thegeneral surface: the middle portion is suddenly thickened to con- tain large spheroidal nuclei : these are the olfactory cells: their slender varicose inner portions are now known to be continuous with the medullated fibres of the nerves of smell. They must therefore be regarded as nervous ele- ments, the thickened middle portion which contains the nucleus constituting the bod\' of the corpuscle, and the peripheral portion a greatly reduced dendritic region con'
326 Part ii. histological anatomy.
sivSting of a single protoplasmic process; while the proxi- mal portion passes over into an axis-cylinder process which shortly becomes a non-meclnllated nerve fibre.
The olfactory nerve-fibres can bo follov^^ed through the cribriform plate to their passage into the surface of the olfactory bulbs, whose structure may now be considered : as has been stated in a previous chapter, the^^ constitute a distinct region of the brain; but their histological struc- ture is so intimately associated with their relation to the sense of smell as to make their description appropriate in this connection. Each olfactory bulb is a rounded mass at the anterior extremity of the longer or shorter olfac- tory tract (or olfactory nerve, improperly so called): the whole is an outgrowth from the hemisphere and originally contains a cavity, the olfactory ventricle, which is a di- verticulum of the lateral ventricle: in many mammals this cavity persists throughout life ; in some it persists in the bulb, that of the tract being obliterated ; in man and the Primates generally it disappears altogether in the adult.
In passing across a section of the olfactory bulb from the surface in close proximity to the cribriform plate to the ependymal lining of the olfactory ventricle a more or less distinct stratification may be observed: the number of layers distinguished by difterent observers varies accord- ing to the degree of subdivision recognized: Cajal desig- nates five, distinguished by histological characteristics de- monstrable with the aid of the silver chromate method, in addition to the ependymal la\'er, which is wanting in those forms in which the bulb is solid.
The first of these is the superficial layer of nerve fibres: this is a thin stratum of slender non-medullated fibres arranged in a felted mass: it is composed exclu- sively of the constituents of the bundles which pass through the perforations of the cribriform plate, whose origin, as we have seen, is in the axis-cylinder processes of
CllAPTHK XXIll. Tin: ORGANS OF SPFXIAI. SKNSK. 327
ihc olfactory cells. The fibres leave the layer inwardly cither siniilv or in stnall *;roups to enter the second stra- lurn. or layer of olfactory glomeruli: the bodies whose presence distin«i:uishcs this layer have lon<; been known to histolosiists.as spheroidal masses present inlarji^e nnnibers near the surface of the bulb: it is only recently that their structure has been at all understood : they are composed in])arL hv the dense tufts of varicose fibrils which form the terminal arborizations of the olfactory fibres entering them from the superficial layer; in part by the similarly tufted dendritic ramifications of the extremities of pro- cesses derived from corpuscles situated in a deeper layer to be presently described: they are, therefore, the places where ingoing impulses are transmitted from the first to the second of a series of nervous elements.
The third stratum is termed by Cajal the molecular layer: as is the case with other structures similarly desig- nated in various parts of the nervous axis, the finely punctate appearance which characterizes it when seen in section is due to the cut ends of numerous fibres, and of fibre-like processes from the corpuscles of the layer next adjacent. Cajal describes in addition, as i)cculiar to this layer, certain elongated or fusiform corpuscles whose peripheral extremities are continued by slender processes which run to the glomeruli and there terminate in small dendritic tufts which are subsidiary to those of the cor puscles of the layer next within: their proximal extremi- ties give rise to axis-cylinder processes which run to the innermost layer and there bend strongly to pass toward the olfactory tracts, which they enter as medullated fibres.
The layer of mitral corpuscles is the fourth of the suc- cessive zones : it consists of large nervous elements chiefly disposed in a sitigle stratum, whose general form is indi- cated by their title. The base of each corpuscle is directed
328 PART rr. histological anatomv.
toward the outer surface of the bulb: it gives off, in mam- mals usually from a point near its centre,a stout descend- ing process which traverses the molecular la^^er to form in one of the glomeruli the important dendritic ramifica- tion already described as one of the essential constituents of each of those bodies. From the margin of the base are given off stout protoplasmic processes which diverge greatly, their ramifications interlacing to form a layer in which the corpuscles lie: their finer subdivisions extend obliquely into the molecular layer. The inwardly directed apex of the corpuscle gives rise to a stout axis-cylinder process which penetrates the layer next within and there bends abruptly to run backward in that layer, giving off collaterals whose terminal ramifications are in the mole- cular layer, and eventually to become the axis-cylinder of a medullated fibre of the olfactory tract.
The form and relations of the mitral corpuscles are sub- ject in different vertebrates to variations in detail that are of such importance, as bearing upon their functions, as to merit description here. Tn mammals generally each mitral corpuscle bears but a single descending process: this may, however, divide and send branches to more than one glom- erulus. In birds each mitral corpuscle gives off several descending processes to as many glomeruli : in either of these two ways a single corpuscle is put in relation with a number of the bipolar nervous elements of the olfactory mucosa. In some mammals, however, the glomeruli are relatively large, and each receives the dendritic ramifica- tions of several descending processes from as many mitral corpuscles: in such cases a single bipolar element may transmit a stimulus to several mitral corpuscles: this lat- ter condition obtains in the olfactory bulbs of mammals possessed of a high degree of olfactory sensibility.
Within the layer of mitral corpuscles is found the granu- lar layer or deep layer of fibres: both of these terms being
ciiMTKu xxiii Tin: ouc.ANS OF siMXiAL shnsp:. 829
ai);)lic(l to the tilth stratum as here defined. , Omitting tro 11 eoiisideration tlie ependymal liniiiij; of the eavity of the bulb, by some regarded as belonging to this layer, but which does not differ in any essential respect from the ependvma which everywhere lines the cavities of the ner- vous axis, the stratum consists almost exclusively of the two kinds of nervous elements indicated by the titles above given. The principal constituents are the nervoUS fibres, which represent the axis-cylinder processes already described as entering this layer from the subjacent corpus- cles: as these pass along the length of the bulb they give off numerous collaterals, of which some run horizontallv to end among the adjacent granules; others descend ver- tically to terminate by interlacing ramifications among the lateral protoplasmic processes of the mitral corpus- cles.
The name of granule corpuscles is used to designate the abundant cellular elements, probably nervous in char- acter, which are grouped in numerous clusters through- out the laver, the fi.bres above mentioned running in inter- lacing bundles among these clusters. They vary more or less in form, but are usually provided at their inner ex- tremities with several short, slender, rapidly branching processes: the outer extremity bears a single stouter process which runs to the inner surface of the molecular layer, there to ramify among the lateral processes of the mitral corpuscles. Other corpuscular elements, which are more doubtfully nervous in function, have been described in this layer.
The medullated fibres of this layer pass through the ol- factory tract to enter the hemisphere and there to be dis- tributed to their destinations in the cortex. In addition, Cajal describes in the tract eflferent fibres which pass into the deep fibre layer of the bulb to end in arborizations which are situated chiefly among the central processes of
33U PART II. HISTOLOGICAL ANATOMY.
the granule corpuscles. In those mammals in which the cavity of the bulb is ol)literated in the adult, the ependy- mal lining is replaced b\^ a gelatinous mass containing numerous neuroglia cells. The dorsal portion of the bulb is in mammals generalU' far simpler in structure than the ventral, the latter, by virtue of its position, being brought into far more intimate relations with the olfac- tory mucosa.
At the close of the description of the structure of the taste-buds reference u^as made to the close relationship of the senses of taste and of smell, and a possible resem- blance in structure in the two organs was intimated. That resemblance, if it exists, is chiefly between the gus- tatory and the olfactory cells : the former are strikingly like the latter as regards their general form, and particu- larly as regards the central nucleated portion and the peripheral process. As we pass inward, however, the likeness is less evident : the olfactory cell is plainly a ner- vous element, being continued by an axis-cylinder process which ends by arborizations in one of the glomeruli of the olfactory bulb: if the gustatory cell is a nervous element, its inner portion must be regarded as a corresponding arborization-region, very greatly reduced, and probably discharging the impulses which it transmits upon the intrabulbar ramifications of the fibres of the glossopharyn- geal nerve. Such an arrangement, if it exists, is without parallel as far as known ; but would find its nearest repre- sentation in the olfactory apparatus. If, on the other hand, the gustatory cells are epithelial, and not nervous, the structure of the taste-bud approaches most nearly to that of a tactile or pressure-organ, a form of sensation having little relation to the sense of taste, which resem- bles that of smell (and no other sense) in that it enables us to take cognizance of stimuli that must be regarded as
CHAPTER XXIII. 1 HI. ORGANS OF SPECIAL SENSK. 3^1
essentially chemical. It should be noted that true gusta- tory sensations are also received on portions of the tongue in which taste-buds have not been found, and that these organs have been descriljed upon surfaces other than that of the tongue which are certainly not gusta- tory. We are here undoubtedly confronted with a prob- lem whose solution depends upon discoveries yet to be made.
The apparatus of sight is far more complex in structure than that of smell, alike in its essential portions and in those which are accessory thereto. The former include the capsule, fibrous in man and in the mammals generalh' throughout the larger portion of its wall (though ]iartl\- cartilaginous or bony in some vertebrates) ; the apparatus of refraction, with its mechanisms of adjustment; and the structures directly involved in the reception of light stimuli and their conversion into nervous impulses. The accessory parts are the protecting eyelids; the investing membrane common to them and to the eyeball ; and the glands whose secretions maintain the proper condition of this membrane. It will be convenient to proceed, in des- cription, from the more external accessory parts to the more deeply seated and more complex essential structures: in each case considering anatomical characters only in so far as necessary for the elucidation of the histology of the parts in question.
The eyelids are essentially muscular folds of the skin, modified chiefly upon their inner surfaces. The outer sur- face resembles the skin of adjacent portions of the face in
332 PART II. HISTOLOGICAL ANATOMY.
the presence of diminutive hairs, accompanied by small sebaceous glands, and by occasional sweat glands : it is thrown into small irregular folds. The underlying corium is loose in texture: it also differs from that of the rest of the skin of the face in the presence of considerable numbers of branched pigment corpuscles. At the free margin ol the lid the surface curves inward ; the corium becomes more dense; and the hair follicles are suddenly enkirged for the development of the long, stout, and recurved cilia, or eye- lashes. Sebaceous glands open into the follicles of the cilia, as do also some of the ducts cf modiiied sweat glands known as the glands of Moll, others opening freely at the surface.
As the integument approaches the inner surface of the lid it bends almost at a right angle at the palpebral bor- der, and at the same time becomes modified in "structure to form the palpebral conjunctiva, which lines the sur- face of the lid in contact with the eyeball. Beneath the in- tegument is situated the orbicularis muscle, composed of striated fibres whose bundles run in a general way par- allel to the palpebral border: the group of bundles situ- ated just within the border fold, and separated from the mass of the orbicularis by the follicles of the cilia, is dis- tinguished as the ciliary or marginal muscle, or as the muscle of Riolan. Just interior to the orbicularis mus- cle lies the palpebral fascia, a layer of fibrous tissue which separates the tegumental from the conjunctival portion of the lid: in the upper lid it is blended with the tendon of the levator muscle.
The palpebral conjunctiva consists of a layer of strati- fied columnar epithelium containing scattered goblet cells, and resting upon a definite basement membrane: and a dense mass of fibrous tissue, the tarsus, or the tarsal carti- lage (erroneously so-called, as it is entirelv devoid of carti- lage corpuscles). The stratified columnar epithelium of the
CHAI'TKK XXIII. THE ORGANS OF SPKCIAL SENSE. 333
conjinictiva passes tjradually at the palpebral border into the stratified squamous epithelium ot the integument : the tarsus may be regarded as the continuation of the denser portion of the tegumentary corium. Upon the inner surface of the lid there can be seen with the naked eye. a number ol vertical rows o( apparently granular masses, of a yellowish color. These are the tarsal or Meibomian glands. com])ound structures of the sebace- ous type which are imbedded in the tarsus: each consists of a straight or somewhat curved conducting tube or duct lined with cuboidal epithelium, into the sides of which o])en numerous sebaceous saccules resembling in everv es- sential those found in connection with the hair follicles; the ducts open by minute orifices upon the margin of the lid. their mouths being lined for a short distance with stratified squamous e])ithelium.
Along the proximal margin of the tarsus, and partly imbedded therein are scattered branched tubular glands of the serous type, the accessory tear glands: they discharge their secretion upon the adjacent conjunctival surface. The conjunctival surface of this vicinity is frequentlv thrown into folds, chiefly involving the epithelium, whose appearance in cross section has led to their being de- scribed as glands. The connective tissue between the base- ment membrane of the conjunctiva and the tarsal plate contains diffuse adenoid tissue which is occasionallvgath- ered into nodules in the human subject: in some of the lower mammals these nodules are quite numerous and well-defined.
Bevond the base of the eyelids the conjunctiva passes over upon the eyeball at the fornix conjunctivae. Goblet cells are more numerous here than upon the palpebral sur- face; the fibrous portion contains a number of distinct adenoid nodules, and a few mucous glands : inwardlv.it passes over into a loose la\er of subconjunctival areolar
334 PART II. HISTOLOGICAL ANATOMY.
tissue which permits of a considerable amount of motion. The continuation of the conjunctiva upon the e\'eball will be best described in connection with that structure. The plica semilunaris, a vertical curved fold at the inner angle of the e3'-e, representing the third e\^elid of many lower vertebrates, is a mere fold of conjunctiva; it con- tains internall}' in some mammals, and sometimes in man, a thin slip of h3'aline cartilage: as well as a rudimentary racemose gland regarded as representing the Harderian gland generalh'^ present in the eyes of those vertebrates which have a functional third eyelid. The adjacent ca- runcle is a rounded fatty mass, with an investment agree- ing with the integument in structure and containing mi- nute hairs and modified sweat glands. The conjunctiva, w^hich is highH' sensitive, is richly supplied with the nerve terminals already described as end-bulbs.
The lachrymal gland, situated in the supero-lateral portion of the orbit, consists of two somewhat distinct portions, sometimes described as the superior and inferior lachrymal glands. The whole mass consists of an aggre- gation of compound racemose glands which open by in- dependent ducts upon the conjunctival surface in the region of the superior fornix. The acini, which may be either simple or branched, are lined by granular cells with large spherical nuclei, agreeing in this respect with the alveoli of serous glands ; from which the}' differ, however, b}^ the presence in each of a distinct and sometimes a large lumen. They open into ductules lined by flattened or low columnar cells: these lead into ducts whose epithelium is distinctly columnar, and in which a second layer of small cells has been described as situated near the basement membrane.
The secretion of the lachrjanal gland, after washing the surface of the eyeball, is carried away by the lachrymal
C!I.M'TI-:K XXIII. TIIK ORC/ANS OF SI'KCIAL SENSE. 335
canals, whlcli open on the ])alj)chral borders near their inner extremities. Each canal is lined with stratified squa- inons epilheliuin, which rests upon a tihrous layer rich in clastic fibres: external to this is a layer of striated mus- cular fibres which are <2^cnerally dis|)osed lon<;itudinally. The canals discharge into the lachrymal sac, which is continued to the nasal cavity by the nasal or lachrymal duct. The sac and the duct are both comj^osed of ahi elas- tic fibrous layer containing considerable adenoid tissue, and lined bv a mucous membrane which is invested by columnar ejiithelium resembling that of the nasal cavity.
The visual capsule consists of two distinct strata ; the outer, or skeletal, which is known throughout the greater ]K)rtion of the eyeball as the sclerotic, but is transformed in front to form the transparent cornea; and the inner, or musculo-vascular, which is composed of the posterior choroid, and the anterior iris: the essential nervous struc- ture of the eye lies immediately interior to the choroid. The cornea is a part of the refracting apparatus of the eve, and the iris a portion of the regulatory mechanism : but each ma\' be conveniently described in connection with the stratum of which it is a portion.
The whitish sclerotic is a dense fibrous layer resembling somewhat a greatly thickened membrane; the interlacing fibre bundles are arranged chiefly in antero-posterior and in transverse directions : elastic fibres are sparingly pres- ent: the fixed corpuscles are flattened, and lie in definite lacunae of irregular form. The inner kiyer is rich in brown- ish pigment and is known as the lamina fusca: between it and the outer layer of the choroid are extensive lymjih spaces lined with endothelium and traversed by blood- vessels and strands of connective tissue. The sclerotic is nearh' twice as thick in its posterior as in its anterior por- tion : where the optic nerve enters the eye the sclerotic be-
336 PART II. HISTOLOGICAL ANATOMY.
comes continuous with thesheath of that cylindrical bodv: the circular area enclosed is suddenly thinned, and is pierced by a number of small openings; it is therefore des- ignated the lamina cribrosa. Over the larger portion of the sclerotic its outer surface is invested by a thin layer of connective tissue loosely uniting it to the capsule of Tenon, a membranous sac lined with endothelium and enclosing the space of Tenon by means of which the e^'eball is sep- arated from the fat masses lining the orbit. In front the sclerotic, is invested, as far as the scleral sulcus bv which it is separated from the cornea, by the scleral conjunctiva: this consists chiefly of stratified squamous epithelium resting upon a thin fibrous membrane which is connected to the sclerotic by a scanty layer of looser connective tis- sue: it contains numerous end-bulbs, and, as is well known, is extremely sensitive.
The cornea is readily distinguishable from the sclerotic, not only by its transparency, but also by its greater con- vexity. Its outer surface is covered by a layer of stratified squamous epithelium continuous with that ol the scleral conjunctiva: the layer is several cells deep, the outer ele- ments being strongly flattened, but retaining their nuclei, and the inner being digitated in a manner similar to the prickle-cells of the epidermis. The deepest cells rest on a thin, dense, homogeneous layer of closely felted fibres, the membrane of Bowman, or external limiting membrane, which possibly represents the fibrous portion of the con- junctiva. Beneath this membrane, and closel}^ connected with it is the substantia propria of the cornea, a mass of the corneal tissue described in detail in a previous chapter: at its margin it passes over into the substance of the scle- rotic, of which it is presumably a modification. Internally this mass is invested with a thin homogeneous elastic in- ternal limiting membrane, otherwise known as the membrane of Descemet: its inner surface is covered
CHAPTER XXIII. THK OKOANS OF SPECIAL SENSK. 337
with a layer of endoLhcliuni and bounds the anterior chamber of the eye. Around its margin the meml)rane of Deseeniet is continued by a number of processes to form the pectinate ligament by which the cornea is attached to the iris.
The choroid is nearly coextensive with the sclerotic. It consists of a vascular layer of fibrous tissue which is ex- ceedingly rich in large pigment corpuscles, imparting to it a color which varies from brown to black: in the human eye it is dark brown in color. Its outermost portion, con- sisting entirely of pigmented connective tissue, forms a layer distinguished as the lamina suprachoroidea : it lies immediate!}' within the lamina fusca of the sclerotic, from which it is largely separated, as has been already stated, by extensive lymph spaces lined with endothelium. The body of the choroid is rich in bloodvessels, which are disposed in tw^o strata; an outer containing the arteries and veins, wdiich are arranged in a characteristic manner, and an inner, the capillary tunic, or tunic of Ruysch: the fibrous structure between the two is a layer of connective tissue rich in elastic fibres which in some mammals is so well developed as to form a distinct layer which is visible through the capillary tunic and the retina, and is known as the tapetum. Within the capillary tunic is a thin trans- jjarent layer, the membrane of Bruch, or vitreous mem- brane.
The anterior portion of the choroid is modified by the foldings of its inner surface known as the ciliary processes, and by the thickening due to the presence of the layer of bundles of smooth muscular fibres termed the ciliary muscle: the whole region, including a marginal zone, the ciliary ring, in which the capillary tunic is less well de- veloped than in the choroid generally, is sometimes desig- nated the ciliary body. The ciliary processes, upwards of seventy in number in the human eye, are meridion-
33H PART II. HISTOLOGICAL ANATOMY.
ally disposed folds which begin just anterior to the cili- ary ritig and rise gradually to the height of a half of a millimetre or so, to terminate abruptly at the margin of the iris: like the rest of the choroid, they are quite vascu- lar, the vessels being imbedded in a pigmented stroma of connective tissue, and are limited internally by the vitreous membrane. Upon their surfaces are pouch-like depressions lined by the epithelial layer, presently to be described, with which the vitreous membrane is invested in this re- gion; these have been called ciliary glands : their gland- ular function is, however, doubtful.
The ciliary muscle is b_v some regarded as a portion of the choroid, by others as interposed between it and the sclerotic. It is composed of bundles of smooth muscular fibres (of striated fibres in birds), most of which arise from the pectinate ligament at the region where the scle- rotic, the cornea, and the iris come together: of these the greater number run meridionally to be inserted into the choroid, and are therefore sometimes regarded as forming a distinct muscle, the tensor choroideae: the remainder, known as radial bundles, run obliqueh' to those just de- scribed, assuming a direction which tends toward the cen- tre of the eye, and being inserted in the ciliary processes. Other bundles, internally situated, are arranged in a more or less definite circular tract, known as the ring-muscle of Mueller- The ciliary muscle as a whole is triangular in cross section : it is thickest in hypermetropic eyes, due largely to an increase in the size of the ring-muscle.
The stroma of the choroid is continued forward from be- yond the ciliary body to form the principal portion of the iris: the latterishighly vascular, but not so much so as the rest of the musculo-vascular layer: it is also somewhat dif- ferent in texture, approaching more nearh'tothe vStructure of retiform tissue. It contains numerous pigment cor- puscles, of different colors in different persons, on whose
CIIAPTHK XXIII. Tin: ORGANS OF SIM:CIAI- SKXSK. 331)
presence the color of the iris depends: i\n exception to this occurs in cases where the iris is blue: here pigment cor- puscles are wanting in the stroma, the color depending entirely on the appearance of the post-iridal pigment (to be described-later) as seen through the body of the iris.
The anterior surface of the stroma is somewhat con- densed, and leaves a layer of endothelial corpuscles con- tinuous at the irido-corneal angle with those w'hich invest the membrane of Descemet. The posterior surface is formed by a homogenous layer continuous with the vitre- ous membrane of the choroid and the ciliary body : against it lies the layer of pigment corpuscles above referred to. Imbedded in the stroma of the iris near the pupillary mar- gin is an angular layer of smooth muscular fibres, the sphincter papillae: near the posterior surface are radi- iiting bundles forming a thin la3'er which is not continu- ous, known as the dilator pupillae: its existence is ques- tioned by some histologists, the demonstration of the scattered bundles of smooth muscular fibres, as distin- guished from the adjacent bloodvessels and bundles of connective tissue, being quite difficult.
The region lying between the outer margin of the iris and the sclero-corneal sulcus is one of great import- ance; the iris, the choroid, the ciliarx' muscle, the cornea and the sclera all coming together in this vicinit\'. Just external to the irido- corneal angle, and among the fasciculi which make up the pectinate ligament, lies a loose network of trabeculae of white and elastic fibres whose interstitial cavities, imperfectl}^ lined with endothelial cells, are known as the spaces of Fontana: they communicate freeK' with the anterior chamber of the eye and contain the same fluid. External to these and fairly within the sclerotic portion of the region is situated an annular space, irregularly flattened and in places subdivided : it is called the canal of Schlemm : whether it is a lymphatic
340 PART II. HISTOLOGICAL ANATOMY.
or a venous channel, and whether or not it communicates with the spaces of Fontana are still matters of dispute.
The parts concerned with the processes of refraction by means of which distinct images of things seen are formed on the sensitive surface within the eye are the cornea, a description of which has already been given; the aqueous humor, a water}^ fluid in which leucocytes are occasionalh^ found, but containing no other tissue elements, which fills the space between the cornea and the capsule of the lens: the crystalline lens, with its capsule, by means of which it is suspended at right angles to the eye immedi- ately behind the iris ; and the vitreous humor, which fills the cavity posterior to the lens. The regulatory mechan- isms are the iris, which, by modif\ang the size of the pupil- lary aperture, governs the amount of light which passes through the lens and indirectly (to some extent) the sharp- ness of the image formed by it ; and the ciliary muscle, whose action modifies the convexity of the lens and thus affects its definition : an account of these having already been given, there remain for description the lens and the vitreous humor, with the capsules by which they are surrounded.
The crystalline lens is composed of an epithelial layer and a fibrous mass whose components are modified epi- thelial corpuscles. The epithelial layer consists of cu- boidal elements which form nearly the whole of the ante- rior surface: as they approach the equator they become columnar, and as thej^ reach that region become greatly elongated and assume the shape of the long hexagonal ])rismatic fibres of which the greater part of the lens is composed : at the equator these retain their nuclei for some time after birth, but those of the greater portion of the mass are quickly lost.
The fibres run from the anterior to the posterior por-
CHAPTER XXIII. IHK ORGANS OF SPECIAL SKXSE. 341
tion of the mass, l)elii<.^ so disposed that their extremities eome to<2^ether in eaeh region along radifiting sutural pL'ines ])rimarily three in number; the sutures of the an- terior region alternate with those of the posterior region in position; as a consequence, the fibres pass obliquely from one region to another: the degree of obliquity is in- creased by the fact that the length of the fibres is such that those which arise nearest the centre of the anterior region terminate posteriorly nearest the equator; and vice versa. As a consequence of this arrangement of the fibres, stellate figures are formed w^hich can be easil}' seen in the artificially hardened lens, and traces of which have been discovered in the living e\^e by the aid of the ophthal- moscope. Hardening in alcohol reveals a tendency to lamination, particularly in the outer portion, the laminae peeling ofifin triangular patches which separate along the sutural planes.
The capsule of the lens is a transparent elastic sac which completely encloses that body. It is apparenth' homogenous in structure in the adult, but is regarded b}- some as composed of two laminae: an inner, cuticular in character, formed b_v the activity of the epithelial elements within while still embryonic : and an outer, composed of fibrous tissue. It is thicker in front than behind, in rela- tion with the greater change of curvature which takes place in the more highly convex posterior surface in visual adjustments. The support of the capsule and lens in place will be described later.
The vitreous humor, or vitreous body (as it also called), is a semi-fluid mass of extreme tranparenc}' derived from the modification of a quantity' of gelatinous tissue b\' the infiltration of lymph to such an extent that over ninety- eight per cent, of its substance is water. Slender trans- parent fibres are scattered through the mass, and occa- sional corpuscles are found in it : these are generally of ex-
342 PART ir. HISTOLOGICAL ANATOMY. '
qeedingly irregular shape, and are often extensively vacuo- lated ; they are probably modified leucocytes. Under cer- tain methods of hardening the vitreous body can be seen to present evidences of a laminar structure; whether this is real, or the result of the treatment employed, is not 3^et certain.
The vitreous body is invested by a thin transparent capsule, the hyaloid membrane, which is structureless throughout the greater portion of its extent. Opposite the optic nerve it is reflected forward to line the slender hyaloid, canal which perforates the vitreous body, termi- nating in front at a point opposite the centre of the pos- terior surface of the crystalline lens: here the hyaloid membrane is reflected outward from the anterior extremi- ty of the canal to line the patellary fossa, on the anterior surface of the vitreous body, which receives the convexity of the lens and its capsule, the latter being in contact with the hyaloid membrane.
At the margin of the patellary fossa the hyaloid mem brane lining it becomes continuous with that investing the outer surface of the vitreous body: here the membrane is distinctly fibrous, the fibres in some cases penetrating the gelatinous mass within. It gives off from its outer surface a fibrous layer which closely invests the ciliary processes as the zone of Zinn, or zonula ciliaris : its free portion extends beyond the ciliary processes to be inserted upon the capsule of the lens at its equator, thus forming the suspensory ligament of the lens above mentioned. The mode of insertion of the suspensory ligament is such as to leave a narrow circular space between the two layers of the hyaloid, known as the canal of Pettit.
The structures directly involved in the reception of light stimuli and their conversion into nervous impulses are contained in the retina, a highly complex organ formed by
CHAPTKR XXIII. THE OK(iANS OF SPECIAL SENSE. 34-3
the modification of a direct outgrowth from the brain. This outgrowtli, at first vcsicuhir in form, is afterwards doubled upon itself in such a manner as to form a sphe- roidal cup composed of two layers and situated chiefl\' between the vitreous humor and the choroid, its basal stalk-like portion beconiinfr the optic nerve. The fibres of the optic nerve are continued over the inner surface ot the retina, in a manner to be presently described, as far as the outer or posterior extremities of the ciliary processes; the sinuous line which marks the limit of their distribution being known as the ora serrata. In front of this line the surface of the ciliary processes is invested by a much sim- pler ciliary portion of the retina, which is in turn con- tinued into the iridal portion, or uvea, which lines the in- ner surface of the iris: the structure of these outlying por- tions will be best understood after a description of that of the principal portion of the retina.
When examined by the ordinary methods of hardening and staining the retina shows in transvervse section a num- ber of distinct strata or layers, tolerabl}^ uniform in structure throughout the greater portion of its extent. These are usually stated as eight in number: here, how- ever, as in the other sense organs, our views have in re- cent 3'ears been greatly modified by the results of the chromate and silver method. An account of the eight layers will first be given, and the relation of their compo- nents as now understood subsequently considered.
Beginning at the inner or anterior surface of the retina, there may be discerned next to the hyaloid membrane which invests the vitreous bod}'^ a delicate layer which is apparently (and onK' apparently) continuous : it has been designated the internal limiting membrane: it is in reality a mosaic formed b}^ the thin expanded ends of sup- porting structural elements, the fibres of Mueller, which
344 PART II. HISTOLOGICAL ANATOMY.
pass verticalh' toward (but not to) the outer surface of the retina.
Next to the internal limiting membrane, so called, is seen the first definite stratum of the retina, the layer of nerve fibres. These, which are non-medullated in most cases, are arranged in small bundles which form a plexi- form meshwork over the inner surface, radiating from the optic nerve, or, more exactly, converging to it from all portions of the retina. This layer, like nearly all the others, is quite transparent, and is itself insensitive to light.
The layer of nerve fibres is succeeded outwardly by the ganglionic layer, a stratum of relatively large multipolar nerve corpuscles, either spheroidal or pyriform in shape, whose axis-cylinder processes are continuous with fibres of the preceding layer, and whose other processes ramifj- in the la3^er next beyond. The ganglionic layer varies in thickness in different portions of the retina, being in some places two or three corpuscles deep, but over the greater portion of its surface consisting of a single layer of cor- puscles: toward the ora serrata these become separated from each other by considerable intervals.
Immediately beyond the ganglionic layer is situated the inner molecular layer: this, which is in most portions of the retina the thickest of the visible strata, is appar- ently composed of a granular mass, which is in reality the expression in section of the cut extremities of the rami- fying processes of the corpuscular elements of the layers next adjacent.
External to the inner molecular layer is seen the inner nuclear layer, composed chiefly of closely aggregated bipolar and multipolar nerve corpuscles, the former pre- dominating. The corpuscles var^^ greatl}^ in size, but are, on the average, decidedly smaller than those of the gan- glionic layer: the disposition of their processes will be de-
CIIVrTIvK XXIII. Tin: ORC.ANS OF SPIXIAL SKXSE. 34-0
scribed later. Xiicleatcd cMilari^ciiK'nts of the sustentacu- lar elements, or fibres of Mueller, are also found in this Layer to some extent.
The inner nuclear layer is followed by the outer molecu- lar layer, a <;ranular stratum closely resembling in its appearance the inner molecular layer, but differing from it gi'catlv in extent, being usually the thinnest of the various la vers of the retina.
Bevond the outer molecular layer, again, is seen a layer of cor j)uscular elements, the so called outer nuclear layer, which as ordinarily seen resembles the inner nuclear layer as closely as do the two molecular layers : nearly all of the constituent elements are distinctly bipolar, and, as will presently be shown, their relations are quite different from those of the elements of the inner of the two apparently similar layers. The corpuscles of this layer are, further- more, closely connected with the elements of the succeed- ing layer in a manner which differentiates them sharply from any other retinal elements.
The outer surface of the outer nuclear layer is sharply defined, the sustentacular tissue of the retina here termin- ating soabruptly as to lead to the description of a definite external limiting membrane: recent researches have shown that the application of the term membrane in this connection is even less justifiable than in the case of the inner boundary of the retina.
External to the outer nuclear layer, and apparently resting upon the so-called external limiting membrane, is the bacillary layer, or layer of rods and cones. It consists exclusively of the two kinds of elements designated by the latter of these titles. Those termed rods are b\^ far the most numerous in nearly all portions of the retina : each consists of an inner or basal portion, somewhat thicker in the middle than at the extremities, and an outer or terminal portion, slightl}' longer than the basal in man
34-6 PART II. HISTOLOGICAL ANATOMY.
and most mammals, which is of nearly uniform diameter throughout its entire length. The outer segment of each rodistransversely striated, and can be resolved into a num- ber of thin disks by the aid of certain reagents : the outer portion of the basal segment is longitudinally striated. The cones resemble the rods in consisting of two segments: the inner of these is much stouter than the basal segments of the adjacent rods, and is thicker at the base than at the outer extremity : like the corresponding regions of the rods, the outer portion of the basal segment is longitudi- nally striated. Upon its free extremity is seated the outer segment, which is shorter than that of the rods, and tapers to a point : it shows transverse striations.
The layer of rods and cones was for a long time re- garded as the limiting stratum of the retina, the adjacent layer of pigment cells being associated with the choroid, with which it is in close contact. With advancing know- ledge of the embryological development of the eye, how- ever, it has become evident that it must be regarded as retinal in nature. It consists of a single stratum ol pris- matic cells, hexagonal in form and so heavily loaded with jjigment as to hide the large central nucleus: the outer surface of the cell, in contact with the choroid, is smooth: the inner is prolonged by numerous slender processes which extend between the rods and cones of the adjacent layer to a distance which varies in relation with the in- tensity of the stimulus acting upon the retina.
The layers of the retina, as above described, are now known to be the expression as seen under the more familiar methods of preparation of a system of nervous elements ar- ranged in a manner not unlike that which is found in the other organs of special sense: their disposition may be briefly described as follows, disregarding for the present the layer of pigment cells just mentioned.
The rods and the cones of the bacillary layer are in real-
CHAPTER XXIII. THE OnOANS OF SPECIAL SENSE. 34-7
itv structurally continuous with the elements of the outer nuclear laver in such a manner that they may with propri- ety be rci^arded as their peripheral prolongations, or as their greatly morlified dendritic portions. Each rod is continued within the external limiting membrane by a slender filament of greater or less extent which terminates at the outer pole of one of the spindle sha])ed corpuscles ot the nuclear layer: the body of the corpuscle is transversely striated in a characteristic manner: from its inner pole it gives oft' a fine varicose filament, the homologue of the axis-cylinder process, which extends to the outer molecu- lar layer and there terminates in a small knob-like expan- sion, representing a greatly modified terminal arborization the rod-corpuscles are situated at various levels in the outer nuclear layer from the outer to the inner surface, their peripheral and central filamentous prolongations varying in length in a corresponding manner.
The base of each cone is continued beneath the external limiting membrane by a stout strand of protoplasm which passes almost immediately into the nucleated cor- puscle, the latter being situated just within the membrane: the corpuscle is continued inward b\' a stout smooth fibre which passes directly across the outer nuclear layer to end within the surface of the outer molecular layer by a disk-like expansion from whose margin slender filaments are given off, forming a rudimentary arborization.
The rod and cone elements, including both the bacillary and the nuclear portions, as far as the outer molecular layer, may be regarded as forming the first of the three groups of nervous elements proper to the visual appa- ratus: they are frequently distinguished as the neuro- epithelial layer, or the layer of visual cells.
The outer molecular layer, like the glomeruli of the ol- factory bulb, may be regarded as chiefly made up of the interlacement of arborizations and dendrites, the central
348 PART II. HISTOLOGICAL ANATOMY.
terminals of the rod and cone elements here coming into relation with the peripheral terminals of the corpuscles of the inner nuclear la^'-er; or, as it is now frequently desig- nated, the layer of bipolar corpuscles.
Each of these corpuscles is prolonged peripherally b\' a filament which terminates at the outer molecular layer b\' a group of dendritic processes: Cajal has shown that those of some of them pass the outer portions of that layer to form close tufts about the knob-like terminals of the rod -elements; the central processes (or axis-cylinder pro- cesses) of the same corpuscles passing to the innermost portion of the inner molecular layer: for these he has pro- posed the name of rod-bipolars. The others he has shown to ramifx' extensivel}^ in the inner portion of the outer molecular layer, in relation with the terminals of the cone-elements; while their central processes terminate in arborizations w'hich are situated at various levels in the stratified inner molecular laver: these he calls cone- bipolars.
The same investigator has demonstrated in the outer- most portion of the layer of bipolar corpuscles elements varying" in size, whose dendrites ramify in the outer mole- cular layer and whose axis-cylinder processes run for longer or shorter distances horizontally to end in arbor- izations distributed in the same layer ; for which reason he calls them the horizontal corpuscles of the retina. In the innermost portion of the same layer of corpuscles he has described pear-shaped amacrine corpuscles of vary- ing size, whose processes branch and ramify in the inner molecular layer at various levels corresponding to those indicated in connection with the terminals of the cone- bipolars. The nature and functions of the horizontal and amacrine corpuscles of this layer may perhaps stiil be re- garded as matters of question ; the bipolar corpuscles
CHAPTKK XXIII. THK OKCAXS OF SriaiAL SENSE. 349
clearly rorin ihc second members of tiie series ol nervous elements involved in visual sensation.
The inner molecular layer resembles the outer (and, in- deed, all the so-called molecular layers of the nervous sys- tem) in consistins^ chieHy of an interlacement of central and peripheral terminal filaments. It shows, as has been indicated, evidence of stratification, due to the termina- tion, at more or less definite levels, of the central pro- cesses of the cone-bipolars, and the associated horizontal distribution of their arborizations ; in relation not only with the terminals of the processes of the amacrine cells, but also with those of the dendrites of the corpuscles of the layer next within.
The layer of ganglion corpuscles, the third and in- nermost members of the visual series, is composed of ele- ments which vary much in size: according to Cajal, the smallest corpuscles send their dendrites into the inner- most stratum of the inner molecular layer: the largest to the outermost stratum : and those of intermediate size in like manner to the intervening strata ; the arborizations of the terminals from the rod-bipolars being distributed in all cases in the innermost stratum. From each of these corpuscles an axis-cylinder process is given off which eventually becomes one of the fibres of the optic nerve, its terminal arborizations being situated in the brain. Cajal has also described in the optic nerve fibres which come from the brain and enter the retina, terminatingby arbor- izations within the layer of bipolar corpuscles: he regards them as conve^-ing centrifugal impulses.
The layer of bipolar corpuscles and that of ganglionic corpuscles, taken together, have been designated the cere- bral layer, as distinguished from the neuro-epithelial layer or layer of visual cells. The cerebral and neuro- epithelial layers, taken together, are formed from the an- terior (and principal) lamina of the collapsed optic vesi-
350 PART II. HISTOLOGICAL ANATOMY.
cle, the layer of pigment cells alone representing the pos- terior lamina.
At the macula lutea the retinal layers are notably thickened, the layer of ganglionic coi'puscles in particular becoming several cells deep. Passing towards the centre of the macula, the layers become rapidl}^ thinned to form the fovea centralis, in which cone-elements only are pre- sent in the neuro-epithelial layer, and cone-bipolars in the cerebral layer, the central processes of the latter passing obliquely outward to enter the inner granular layer at the margin of the fovea. Where the optic nerve pierces the retina the retinal structures are of course wanting.
The characteristic retinal layers disappear at the ora serrata, the layer of visual cells first becoming absent. Over the ciliary portion of the retina the posterior lamina of the optic vesicle is represented by a layer of pigment cells as elsewhere: the anterior lamina bj' a layer of col- umnar cells l\nng between the pigment layer and the hya- loid membrane: they have large oval nuclei near their outer extremities. In the iridal portion of the retina both lam- inae are represented by layers of pigment cells. The masses of retinal pigment cells between the ciliary pro- cesses form the so-called ciliary glands alread}'' mentioned.
The apparatus of hearing, like that of sight, comprises a receiving and a transmitting mechanism in addition to the structure which contains the special terminal organs involved. The receiving neuro-epithelium is in some respects more complex than in any of the other organs of special sense: the accessory mechanisms are far simpler than those of the apparatus of sight.
CIIAPTHU XXIII. Tin: ORGANS OF SIM;CIAI. SKXSK. 351
The pinna cotiijists essentially of a sheet of yellow fibro- cartilage covered by integument: funncl-shiiped or vari- ously modified in mammals generidly, it is in man crum- pled and comparatively rudimentary, but retains its char- acteristic structure. In the lobe of the ear the cartilage is rejjUiced by a mass of fjit. The skin u]Jon the outer or convex surface does not in man differ materialK'from that of the adjacent portion of the head : that of the inner sur- face is thin and but slightly mobile upon the subjacent cartilage, and is devoid of sweat glands. The hairs of the integument of the pinna are in man very small, with rela- tivelv large sebaceous rrlands. The small intrinsic muscles which pass from certain folds of the cartilage to others, beneath the integument, are composed of slender striated fibres.
The external auditory meatus, in part cartilaginous and in part bony, is lined by a closely adhering tegumen- tary layer, continuous with the skin upon the inner side of the pinna, which grows thinner and simpler in struct- ure as it passes inward. The portion of the skin which invests the surface of the outer or cartilaginous portion of the tube contains fine hairs which, like those of the pinna, iire accompanied by well developed sebaceous glands : the fibrous tissue subjacent to the corium contains in addi- tion numerous convoluted tubular ceruminous glands which greatly resemble sweat glands in form and struct- ure but are larger and more closely aggregated : they are farther characterized by their brownish color and the highly refracting fatty particles seen in their secretion. The lining of the deeper bony portion of the meatus is devoid alike of hairs and of glands.
The membrana tympani, which separates the external meatus from the middle ear, is composed of a fibrous lay- er invested outwardly by a continuation of the integ-
352 PART II. HISTOLOGICAL ANATOMY.
ument from the bony wall of the meatus, and inwardly bv the mucous membrane which lines the whole of the tympanic cavity. The fibrous layer consists chiefly of ra- diating bundles diverging chiefly from the point of attach- ment of the malleus: there are in addition circularly dis- posed bundles of fibres, chiefl\^ near the margin of the membrane, which form a so-called inner layer. The tegu- mentarv la\'er resembles that of which it is a continua- tion: the mucous la\'er consists of a thin membrane rich in elastic fibres which bears a single layer of cuboidal epi- thelium whose component cells are devoid of cilia.
The auditory ossicles possess to some extent the char- acter of dense bone, their thicker portions showing dis- tinct though small Haversian systems of lamellae: defi- nite marrow^ cavities exist in the interior of the principal masses of the malleus and incus. The articular surfaces are in each case invested with hyaline cartilage. The muscles connected with the ossicles are composed of stri- ated fibres. Both bones and muscles are invested wnth the mucous membrane which lines the tympanic cavity.
The Eustachian tube, which connects the tympanic cavit}^ with the pharynx, has a bony wall in its posterior portion; in the anterior portion the wall is in part com- posed of h\^aline cartilage, sparingly reinforced by bund- les of white fibres, and in part of dense fibrous membrane. It is lined throughout its extent by a mucosa which is a continuation of that of the phar\'nx, and is in turn con- tinued by the lining of the tympanic cavity: like that of the phar3'nx, the mucosa is invested wath a layer of strat- ified columnar ciliated epithelium. In the cartilaginous ])ortion of the tube there is a submucosa which contains numerous mucous glands and a considerable, cpiantity of diffuse adenoid tissue; in this respect, again, recalling the structure of the ph^-rynx. In the bony posterior portion
CHAPTER XXIII. THE ORGANS OF SPKCMAL SENSE. 853
the mucosa is devoid of ;2;laii(ls and adheres more elosely to the wall of the tube.
The cavity of the tympanum, like that of the mastoid cells leadiuij out from it. may be re<^arded as an expan- sion of that of the Eustachian tube. It is lined with a mucous membrane which has already been frequentlv re- ferred to : between the membrane and the bony wall of the cavity is a submucosa consisting of interlaced fibrous bundles among which are seen numerous spheroidal bodies in manv wavs resem])ling Pacinian corjjuscles.
The disposition of this fibrous network, the irregulari- ties of the bony surfaces involved, and the structures pre- sent in the tympanic cavity cooperate to throw the mu- cosa into cons]3lcuous folds; their disposition is a matter for the anatomist rather than for the histologist. The epithelium of the tympanic cavity is columnar ciliated over the greater ])ortion of the surface; that of the mas- toid cells is devoid of cilia. The existence of distinct glands in the mucosa of the tympanum is a matter of question.
The inner ear is in the strict sense the organ of hearing, the middle and outer regions being merely accessory thereto. It consists of the membranous labyrinth, in which the specially modified neuro-epithelial structures in- volved in audition are situated, enclosed in the cavity within the periotic mass known as the bony labyrinth. The membranous labyrinth is formed by the ingrowth of the integument of the side of the head : this is at first a simple saccular or flask-shaped cavity lined with epithe- lium derived from the ectoderm, and communicating with the surface by a small aperture. Later, this aperture is obliterated and the sac is divided into two principal re- gions, the utricle and the saccule, which are in the adult onlvindirectlv connected. The three semicircular canals
354 PART II. HISTOLOGICAL ANATOMY.
are developments of the wall of the utricle and together with it form the labyrinth in the limited sense in which the term was formerly used. The cochlea is an extension of the saccule.
The bon}' walls of the cavity which contains the utricle and the semicircular canals are lined with a thin perios- teum invested by flattened connective tissue corpuscles which form an endothelium. The membranous structures enclosed within are adherent to the periosteum along one side of each canal and upon a portion of the surface of the utricle: throughout the rest of their extent they are sep- arated therefrom by a space filled with perihmph which is traversed by frequent trabeculae of fibrous tissue, the free surface being similarly invested. The membranous wall consists of a layer of connective tissue containing numer- ous elastic fibres, within which is a dense clear tunica propria, whose surface throughout the canals shows nu- merous low papillary eminences; lining the tunica is a layer of polygonal pavement-epithelium cells.
In the ampulla of each of the semicircular canals the tunica propria is much thickened along a projecting ridge, the transverse septum, upon whose summit is situated a crista acustica, or ampullar area of auditory neuro-epi- thelium. In passing from the general surface of the am- pulla upon the sides of the septum the pavement epithe- lium becomes first cuboidal and then columnar in form, the columnar cells being surmounted by a distinct cutic- ular layer. Within the crista the epithelium consists of cells of two sorts: fibre cells, whose elongated bodies ex- tend through the whole epithelium, their bases being larger than their free extremities, and their nuclei being variously situated within the basal half; and hair-cells, cylindrical elements which are situated in the outer half of the epithelium only, their nuclei being situated near
CIIAPTICK XXIII. TFIi: ORC.AXS OF SPKCIAL SKNSE. 'A3~}
their rounded inner extremities, find their free ends bear- ui'fl, Ion;; taperin.L!^ lilainonts, the auditory hairs.
Branches of the auditorv nerve are distributed to each crista: as the fibres enter the epithelium they loose the medullarv sheath, and quickly divide into fibrils which ramify extensively in the vicinity of the hair-cells, their free extremities being in every ease in direct contact with these ej)ithelial elements: the relation between the nerve terminals and the epithelial cells must be regarded as sim- ilar to and as specializations of that elsewhere described in connection with the free endings of nerve fibre in the epidermis.
In material hardened for section cutting the surface of each crista is found to be covered by a dome-shaped mass of a clear colorless substance of unknown composition and origin, in which the auditory hairs are imbedded : to this mass the term cupula is applied. Under suitable reagents the auditory hairs can be made to break up into numerous fine cilia-like filaments, indicating that the hairs are compound structures.
The surface of the utricle bears a large patch of neuro- epithelium, the macula acustica or macula cribrosa, as it is sometimes termed, essentially similar in structure and in the mode of nerve supply to one of the cristae of the ampullae. There is not such a marked thickening of the subjacent tunica propria, and the auditory hairs are not as long as those of the ampullar organs: the surface of the macula is invested b\' a soft gelatinous mass in which are imbedded numbers of crystals of calcium carbonate known as otoliths. A macula in every way similar to that of the utricle is found in the saccule.
The cochlea, a development of a portion of the saccule, with which it is directly connected in the lower verte- brates and in the embryo, is in the mammal in great measure constricted off from that region in the adult, be-
856 PART II. HISTOLOGICAL ANATOMY.
ing connected with it only by a slender tubular passage, the canalis reuniens. It should be regarded, however, as a tubular diverticulum of that division of the primary auditorv vesicle, differing from the rest of the membran- ous labyrinth in its spirally coiled form, its mode of at- tachment, and particularly in the complexity of its neuro- epithelium, which here attains a degree of specialization found in no other organ
Regarding, for convenience, the position of the whole structure as so far shifted from that which it occupies in the living body as to bring the base of the spiral into a horizontal plane, the apex pointing upward, the cochlear tube mav be said to be adherent outwardly for about one third of its surface to the bony wall of thecontainingcav- itv; and to be connected inwardly with the central bony spiral lamina b\' two flat membranes, the lower of which, the basilar membrane, is nearly horizontal in this posi- tion, while the upper, the membrane of Reissner, slopes at an angle of about forty-five degrees: the tube is there- fore approximately triangular in cross section.
The periosteum of the outer wall is much thickened alone: the area of adhesion of the cochlear tube to form the spiral ligament, the greatest elevation being at the point of attachment of the basilar membrane, where a fibrous ridge is found known as the crista basilaris; a short distance above this a second ridge is seen, the vas- cular prominence, containing one or more conspicuous bloodvessels : the somewhat concave surface between this and the ridge to which the membrane of Reissner is at- tached, known as the stria vascularis, exhibits a histo- logical structure without parallel in the entire body. It contains a rich network of capillaries, imbedded in ele- ments apparentl}' epithelial in character, and commonly so described: the superficial cells (which entirely overlie the capillaries) are certainly in continuity with the epithe-
CHAPTKK XXIII. THK ORGANS Ol- SI'IXIAI. SKNSK. Sf)?
Hiim linin«T the rest of the tube; those situated between the capillMi'ies may very possibly l)e epithelioid connect- ive tissue corpuscles not unlike those known to occur elsewhere.
The membrane of Reissner is an exceedingly delicate sheet of connective tissue invested on the side toward the scala vestibuli with a layer of endothelium some of whose cells are pigmented : the inner side is lined, like the greater portion of the membranous labyrinth, by a j^avement epithelium composed of polyhedral cells: the three com- ponent layers are of nearly equal thickness. The inner and lower edge of the membrane is united to the middle or inner portion of the limbus, a peculiar thickening of the periosteum of the upper surface of the bon\' spiral lamina.
The portion of the limbus situated outwardly from the attachment of the membrane of Reissner terminates ab- ruptly b}' a border excavated by the spiral groove, which is bounded by an upper and a lower lip. The upper sur- face of the upper lip is ridged and grooved and its margin developed into numerous tongue-like processes, the audi- tory teeth: the surface of the ridges and the teeth is in- vested by polyhedral pavement epithelium : that of the grooves is columnar: this is continued over the spiral groove by a layer of cuboidal cells continuous at the lower lip of the groove with the epithelial structures upon the upper surface of the basilar membrane. The lower lip extends to the margin of the bony spiral lamina.
The basilar membrane extends from this margin to the basilar crest of the spiral ligament. Its middlelayer consists of a sheet of homogeneous ground substance containing scattered nuclei, and having embedded in it an immense number of straight stout fibres running radially from the spiral lamina to the basilar crest. The surface toward the scala tympani is covered by a layer of connective tis-
358 PART II. HISTOLOGICAL ANATOMY.
sue whose elements do not take on a definite endothelial form, but are largely spindle-shaped fibres disposed at right angles to the fibres of the middle layer. The inner surface is invested with epithelium continuous with that lining the rest of the tube: that of the outer half of the membrane, or zona pectinata, is but slightly modified : that of the inner half, or zona tecta, is greatly modified to form the characteristic structure of the cochlea.
This structure, the organ of Corti, is a neuro-epithelium not unlike those found in the cristae and maculae acnsticae; being composed, like those bodies, of hair cells and sup- porting cells: its greater complexity is due chiefly to the form and arrangement of the latter elements. The central feature is a series of arches formed by the convergence above of an inner and an outer rod of Corti, the outer rods being longer and more slanting than the inner: the triangular space beneath them, which runs the whole length of the cochlea, beingknown as the tunnel of Corti. Each rod consists of a broad basal portion, or foot, a slender shaft, and an enlarged head, that of the inner rod having a concave surface upon its outer side into Vt^hich is fitted a corresponding convexity upon the inner aspect of the head of the outer rod : both the inner and the outer rods bear outwardly directed flattened phalangeal pro- cesses, those of the inner rods overlapping the inner por- tions of the processes of the outer rods. Both inner and outer rods are invested with a layer of protoplasm which is accumulated at the base on the side toward the tunnel in a mass containing an oval nucleus. The inner rods are narrower and more numerous than the outer.
On the inner side of the upper extremities of the inner rods is situated a row of inner hair cells, cylindrical in form, and, like those of the auditory structures, only extending through the upper half of the layer: the upper extremity of each Ijears a number of hair-like processes :
CHAPTER XXIII. TIIH ORGANS OF SPECIAL SKNPE. 359
the lower extremity is routuled and contains a spherical nucleus. Internal to the inner hair cells are columnar sup- portin«; cells which pass gradually over into the epithe- lium of the spiral groove.
On the outer side of the heads of the outer rods are rows, three or four in number, of outer hair cells, essentially like those of the inner row. Between them are the ujiper extremities of the cells of Deiters, elements somewhat re- sembling the rods of Corti : each has a spindle-shaped basal portion containing a spheroidal nucleus, and a slen- der rigid upper portion which terminates in an outward- ly directed phalanx. The phalangeal processes of the rods of Corti and the phalanges of the cells of Deiters are united by their angles to form a reticular membrane through whose apertures the extremities of the outer hair-cells project. Between the outer rods, the hair-cells, and the cells of Deiters are intervals, the spaces of Nuel, which communicate with each other and with the tunnel of Corti, the whole being filled with a semifluid substance.
The organ of Corti may be said to be limited by the hair cells and the cells of Deiters : the latter pass over out- wardly into tall columnar elements, the cells of Hensen, whose nuclei are situated in their large upper extremities. These pass rather abruptl}' into the shorter columnar cells of Claudius, between which and theepitheliumof the outer wall of the tube a gradual transition is seen along the surface of the zona pectinata.
The bundles of nerve fibres distributed along the cochlea pass along the under surface of the bony spiral lamina to its margin from the spiral ganglion of the mod- iolus. Here they penetrate the basilar membrane : the fibres loose their medullar}' sheath and are distributed to the epithelium in a manner quite similar to that described in the account of the cristae, some of the fibres travers- ing the tunnel of Corti to reach the vicinity of the outer
360 PART II. HISTOLOGICAL ANATOMY.
hair cells. From the margin of the upper lipofthelimbus a cuticular fold, the membrana tectoria, extends out as far as the outer cells. It probably rests upon the organ of Corti durinof life.
A comparison of the es.sential structures of the various organs of special sense shows that they agree in being modified epithelia containing more or less specialized ter- minals of nervous elements. These epithelia are in [each instance derived from the ectoderm : in the case of the re- tina indirectl}^ the organ in question being formed as a diverticulum of the nervous axis, which is itself formed from an infolding of the ectoderm : in the case of the other sense organs the derivation from the ectoderm is direct. The nerve terminals of the organs of taste and of hearing resemble each other in consisting of fibrils ramifying be- tween the specialized epithelial cells, though these senses are not at all related as regards the character of the stim- uli to which they respond. Similarly, the terminals of the organ of smell and of sight are somewhat alike, greath' as these senses differ. Farther investigation may explain these apparent resemblances and differences and exhibit a still deeper unity of structure in the mechanisms of special sense.
INDEX.
361
INDEX.
Pack
Achromatic spindle 120
Achromatin 117
Adenoid nodules 180, 183, 271
Adenoid tissue, 40, 80, 178, 183, 196
Adipose tissue 39
Adrencils 281
Adventitia 92, 94, 260, 264
Agminated glands 180
Allantois 257
Alveoli, pulmonary 188
Amacrine corpuscles 102, 111
Amitotic division 119
Amoeboid motion 33, 80
Ampulla 226
Ampullae, auditory 354
Aorta 261
Apolar c(irpuscles 110
Appendix vermilormis 182
Arborizations 109, 111
Areas of Colinheim 88
Areolae 39, 55, 66
Areolar tissue 38
Arteries 91, 92, 259
Articular corpuscles 105
Auditory meatus 551
ossicles 362
teeth 347
Aucrbach, plexus of. 173, 174
Axilemma 108
Axis-cylinder 101, 105
Axis-cylinder process 102, 110
Bacillary Layer 345
Hellini, ducts of 210
Bertin, columns of 205
Bile capillaries 189
Bipolar corpuscles 102, 109
Bladder, urinary 214,257
Blastoderm 128
Page
Blood 14, 73
corpuscles 74
platelets 74, 82
Bone corpuscles 51
nutrition of. 57
structure of. 32, 50, 53
Brain 311
Bronchi 195
Bronchial muscle 196
Bronchioles 197
Brunner, glands of. 177
Caecum 182
Calyces of kidney 204, 212
Canal of Schlemm 339
Canaliculi 49, 51
Capillaries 91 , 93
Capsule of Bowman 207
of Glisson 186
of lens 341
of Tenon 336
Cardiac fibres 85
Carotid glands 280
Cartilage 25, 29, 30
calcified 27
cellular 26
elastic 27
hyaline 26
ossification in 60
reticular 27
transformation of. 64. 65
vascularization of. 63
Cartilage bone 61
Caruncle.... 334
Cell-division 2.S, 119
Cells 14, 115
of Claudius 359
colloid 278
of Deiters 359
362
INDEX.
Cells of Hensen 359
tactile 103, 104
Cellular tissue 38
Cement um 164, 166
Central canal of cord 292, 309
Centres of ossification 61
Centroacinar cells 185
Cerebellar cortvx 312
Cerebral cortex 316
Ceruminous glands 144
Cervix uteri 242
Chondrin ^9
Chondroclasts 56
Chondrogen 29
Chordae tcndinea 263
Choroid 337
Chromatin 117
Chromoplasm 117
Chromosomes 120
Ciliary processes 337
Circumferential lamellae 52
Clitoris 245,258
Cloaca 257
Coelom 250
Coccygeal gland 280
Cochlea 355
Cohnheim, areas of. 88
Collagen 29
Collaterals Ill
of cord 303
Colloid cells ■ 278
Columnae carnae 263
Columns of Bertin 205
of Burdach 298
Clarke's 296
of Goll 298
of Morgani 182
of Sertoli 221
of spinal cord 292
of Tuerck 300.
Coni vasciilosi 223
Conjunctiva 332
Connective tissues 24, 32, 38
Contractile tissues S3
Cord, spermatic 217
spinal 291
Corium 138, 155
Cornea 336
Corneal tissue 32, 48
Corpora cavernosa 228
Corpus Arantii 263
Highmori 219, 223
luteum 237
spongiosum 229
Corpuscles 14, 23
accessory 36
adventitious 36
amacrine 102, 111
apolar 110
articular 105
basket 315
bipolar 102, 109
blood , 73
bone 51
of Cajal 316.
colored . 74
colorless 74, 77
concentric 278
. corneal 49
fixed 31, 36
genital 105, 230. 244
of Gran dry 103
, of Hassali 278
Malpighian 205. 276
. of Martinotti 318
of Melssner 104
— —migratory 33, 36
mitral 327
multipolar 102, 110
of muscle 87
nerve 102, 111
nerve-fibre 101
of Purkinje 313
pyramidal 317
red 74
of retina 348
of spinal cord 305
tactile 104
unipolar.... 102. 110
of Vater 105
wliite 74
Cowpcr, glands of 231
INDEX.
363
Crcmaster 218
Crescents of Ciianuzzi 158
Crista acustica 354
Crtista i>etrosa IfiG
Crypts of Lielirrkuhn 177
Crystalline letis 340
Cumulus proligerus 236
Cupula 355
Cytology 15
Daktos 218
DeBove. endothelium of. 178
Deiters, process of. 102
Demilunes of Heidenhain 159
Dendrites 102, 109, 110
Dense bone 52, 69
f«)rmation of. 64
Dental papilia 167
Dentinal tubules 165
Dentine ..32, 48. 164
Dcutoplasm 116
Discharging terminals 106-
Discus proligerus 236
Disks, tactile 104
Division, amitotic 119
karyokinetic 119
Dobie's line 88
Dorsal coniua 292
Duct of Gartner 240
Ductless bodies 269
Duodenum 177
Dura 289
E.\R 350
Ebncr, glands of. 170
Ectodei-m 128
Ectoplasm 116
Elastic cartilage 27
fibres 34
membrane 41
tissue 43
Elastin 35
Elementary particles 82
Enamel 164, 166
germ 167
End-bulbs of Krausc 104
Endings, free nerve 103
Endocardium 262
Endochondral bone 64, 67
Endomysium 90
Endoneurium 112
Endoplasm HG
Endosteum 59
Endothelium 20, 44
of DeBove 178
End-plates, motor 106
Entoderm 128
Eosinophile cells..... 37
leucocytes 79
Ependyma 309
Epiblast 128
Epicardium 262
Epidermis ,. 135
Epididymis 217, 223
Epineurium 112
Epiphysis cerebri 284
Epithelium 17
ciliated.... 19
columnar 8l
germinal 234
— — glandular IS
pavement 18
polyhcdrai 18
respiratory 198
simple 19
spheroidal .^. 19
squamous 18
stratified 19
transitional 19, 214
Epoophoron 239
Erectile tissue 228
Erectorcs fili 15o
Erythroblasts .'i5, 81
Erythrocytes 74
Eyelids 331
Factors of strictire 14
Fallopian tubes 238
Fasciculi, muscular 89
Fat cells .... 37
364
INDEX.
Fenestrated membrane 42
Ferrein, pyramids of. 205
Fertilized ovum 86
Fibres 34
elastic 34
gray 101. 107
medullated 101
of Mueller 344
non-meduUated 101
of Purkinjc 563
of Remak lOl
smooth muscular 84
striated 87
of Tomes 165
white 34, 101, 107
Fibrillae, muscular 88
• primitive 107
Fibrin 73
Fibro-cartilage 27
Fibrosa 173, 194, 196, 216, 244
Fibrous membrane 41
tissues 31, 32, 42, 45
Fields of Cohnheim 88
Fixed corpuscles 31, 36
Follicles 157
intestinal 177
Foveolae 56
Free nerve endings 103
Funiculus 112
Ganglia 113
Ganglionic columns 294
Gartner, duct of. 240
Gelatin 29
Gelatinous fibres 101
tissue 32
Genital corpuscles 105, 230, 244
eminence 257
ridges 257
Germinal epithelium 234
ridge 255
spot 237
vesicle 237
Gianuzzi, crescents of. 159
Giant cells 55
Giraldes, organ of 223
Glands 157
of Bartholin 245
of Brunner 177
cardiac 175
carotid 280
ceruminous 144, 351
coccygeal 280
of owper 231
of Ebner 170
gastric 175
lachrymal 334
of Littre 227
mammarj' 246
Meibomian 145, 333
- of Moll 332
of mouth 160
mucous 158
peptic 175
prostate 231
pyloric 175
salivary 161
sebaceous 144
serous 158
of skin 142
sudoriparous 152
of Tyson 230
unicellular 156
uterine 240
Glandular epithelium 18
Glans penis 230
Glia-cells 46,113
Glisson, capsule of. 186
Glomeruli, renal 207
olfactorj' 327
Goblet cells 19. 156
Golgi, organs of. 103
Gonads 217
Graafian follicles 234
Grandry, corpuscles of. 104
Granule cells 37
Gray commissure 292
fibres 101, 107
matter of cord 292
Gums 154
Haematoblasts 82
INDEX.
365
Haemoglobin 76
Hairs U6
Hassall's corpuscles 27H
Haversian canals 52
spaces ". 53
systems .- 52. 166
Head kidney 252
Heart 261
Heidenhain, demilunes of. 159
Henle's layer l+H
loop 208
sheatli 113
He|)atic cords 188
Horny layer 135
Houston, valves of. 182
Howship's lacunae 56
Huxley's layer 148
Hyaline cartilage 26
Hyaloid menibrane 342
Hyaloplasm 1 15
Hydatids of Morgagni 223. 224
stalked 239
Hymen 244
Hypoblast 128
Hypophysis cerebri 283
I.NCREMENTAL LINKS OF SALTER, 165
Infundibula, pulmonary 197
Intima 92, 259, 264
Interglobular spaces 165
Interlobular cells of pancreas.. 185
Internal sphincter 184
Internodes 101
Interstitial lamellae 52
Intestinal follicles 177
Intramembranous ossification, 60, 61
Involuntarv muscular tissue 84
Jelly of Wharto.n
33
Karvokinksis 119
Kidneys 203
bloodsupply of : 205
permanent 252
primitive 252
Krausc, end-bulbs of 104
membrane of 88
Laijia majora 246. 257
minora 245. 257
Lachrymal glands 334
Lacteal glands 267
Lacunae, of bone 49, 50
Howship's 56
of Morgagni 227
Lamellae, circumferential 52
interstitial. 52
Ivameilated tissues 47
Lamina cribrosa 335
fusca 335
- - suprachoroidea 337
Large intestine 181
Larynx 199
Lateral cornu .- 292
Leucocytes 33, 36, 74. 77
Lieberkuhn, crypts of. 177
Lines of Schreger 165
Lingual papillae 169
Littre, glands of. 227
Liver 185
Lymph 96
capillaries 92, 96
vessels 92,97, 264
Lymphatic glands 80, 272
Lymphobiasts 81
Lymphocytes 80
Lymphoid tissue 80
Macula acistica 355
Malpighian corpuscles 205, 276
Malpighi, lacunae of 227
pyramids of 204
Mammary glands 246
Marrow 40, 53, 54. 81
Matrix 23
Media 92, 259. 264
Mediastinum testis 219
Medullated fibres 101. Iu6
Medullary sheath lOl
Meibomian glands 144
Meissner, corpuscles of. 104
366
INDEX.
Meissner, plexus of 172, 174, 178, 183
Melanin 37
Membrana granulosa 236
propria 41
tectoria 360
Membrane, basilar 356
bone 61
of Bowman 336
of Descemtt 336
of Krause 88
Nasm\'tirs 166
periodontal 166
serous 98
tympanic 351
Membranous labyrinth 353
Meninges 267, 288
Menisci, tactile 104
Merkel, tactile cells of 103
Mesenchyma 130
Mesentery 267
Mesoblast 128
Mesoderm 128
Mesogaster 177
Mesonepliros 252, 254
Metanephros 252
Microcytes 75
Migratory corpuscles 33, 36
Mitosis 119
Mons veneris 246
Morgagni, columns of. 182
hydatids of 223. 224
Motor end-plate 106
Mucosa, 154, 155. 172, 175, 177,
181, 182, 192, 215,238,240,243
Mucous glands 158
layer of skin 135
membranes 153
tissue 32
Muellerian duct 253
Multipolar corpuscles 102, 110
Muscle 89
bronchial 196
corpuscles H7
papillary 263
of Kiolan 332
tracheal 194
Muscular tissues 84
cardiac 36
smooth 84
striated 37
Muscularis mucosa. ...lt>6, 172, 174,
176, 178, 183, 241. Musculosa, 172, 174. 177, llsi, 184,
216. 239, 241, 244.
Myelin 101
Myeloplaxes 55
Myocardium 262
Nails 15 1
Nasmyth's membrane 166
Nephridium 249
Nephrostome 250
Nerve corpuscles 102, 111
fibres 101, 109
fibre corpuscles 101
terminals 102
Nerves 112
Nervous tissues 99
Neurilemma 101, 109
Neuro-epithelium 222, 325, 347,
354, 308, 360.
Neuroglia 46, 113
Neurokeratin 108
Neuroplasm 107
Nodes of Ranvier 101
Nodules, adenoid IHO, 183
Non-meduUated fibres 101, 105
Nuclear division 118
Nucleoli - 117
Nucleus 16, 110
Odontoblasts 164
Oesophagus 174
Olfactory bulbs 325
cells 325
glomeruli 327
hairs 325
Omentum 177, 267
Oosperm 128
Organ 14, 127
of Corti 358
of Ciraldcs 223
INDEX.
367
Orjjf.'iii of (iolgi 103
of Roseuniucllcr 2.'}9
Os uteri 24-:{
Osier's jj;ra miles 82
Ossein 51
Osseous tissue 50
Ossification 59
centres of (il
in cartila^'c 60. 63
in membrane 60, 61
Osteo!)lasts 5+. 59
Osteoclasts 56
Osteogenetic fibres 62
- layer 54,59
tissue 66
Otoliths 355
Ova, primitive 234-
primordial 255
Ovary 233
Oviducts 238, 253
Ovula Nabotlii 242
Ovum 128, 236
I'aciman noDiKs 105, 230
Palate 154
Pancreas 184
Pannicuius adiposus 140
Pa])i!lae, foliate 322
of kidney 203
of Iin<,nial 169
Papillary muscles 263
Parablast 130
Paradidymis 225
Paraplasm 116
Paratliyroids 279
I'arietal cells 176
Paroophoron 240
Parovarium 239
Pavement epithelium 18
Pelvis of kidney 212
Penis 227, 257
Peptic cells 176
Pericardium 266
Perichondral bone 64. 69
Perichondrium 30
Periodontal membrane 166
Perimysium 90
Perineurium 112
Periosteum 53, 54. 66, 68
Peritoneum 266
Perivascular lymphatics.. 98
Permanent marrow 67
Peyer's patches 180
Pflue^er, tubes of. 161
Pharynx 173
Pia 289
Pigment cells 37
Pineal body 284
Pituitary body 283
Plain muscular tissue 84
Plasma of blood... 73
cells 37
Platelets, blood 74, 82
Pleurae 266
Plexus of Auerbach 173, 174, 177
of Meissner..l72, 174, 178, 183
Plica semilunaris 333
Poles of nerve corpuscles 102
Polyhedral epithelium 18
Portal canals 186
Prickle cells 19
Primary areolae 65, 66
marrow 67
vascular invasion 67
F'rimitivc fibrillae 107
ova 234
sheath 101
Process, axis cylinder 102
of Deiters 102
Pronephros 252
Prostate gland 231
Protoplasm 15, 115
Protoplasmic processes 102, 110
Pulmonary artery 261
Pupil, muscles of 339
Pylorus 1 79
Pyramids of Ferrein 205
of Malpighi 204
Pyriform corpuscles 101, 109
368
INDEX.
Ranvier, nodes of 101
Receiving terminals 102
Rectum 182
Red corpuscles 74
marrow 55
Remak, fibres of 100
Rete mucosum 135
testis 220
Reticular cartilage 27
Reticulum 1 15, 121
Retiform tissue 4-0
Retina 344
Retinal pigment 346
Retzius. stripes of. 106
Ring-muscle 338
Root-sheatli of hair 147
Saliva 163
Salivary glands 161
Salter, incremental lines of. 165
Sarcolemma 87
Sarcoplasm 88
Sarcous elements 88
Schachowa, spiral tubule of 208
Schmidt, medullary segments of 108
Schreger, lines of. 165
Schwann, white substance of.... 101
Sclerotic 335
Scrotum 217, 258
Sebaceous glands 144
Secondar3' areolae 65, 66
Segmental ducts 252
organs 250
Segmentation 128
Seminal vesicles 226
Seminiferous tubules 220
Serosa 173, 177, 181, 239
Serous endothelium 20, 98
glands 158, 159
membranes 99, 265
Sertoli, columns of 221
Sharpey's fibres 53
Sheath, Henle's 113
Simple epithelium 19
Sinus pocularis 231, 232
Skeletal tissues 45
Skin 135, 141, 217, 230, 331, 351
Small intestine 179
Small vessels 91
Smooth muscular fibres 84
Sole-plate 106
Solitary follicles 170
Spaces of Fontana 339
interglobular 165
Spermatic cord 217
Spermatozoa 220, 222
Spermiduct 254
Spheroidal epithelium 18
Spinal cord 291
ganglia 309
nerves 301
Spleen 274
Spirem • 119
Spongioplasm 115
Spongy bone- 52
Squamous epithelium 18
Stalked hydatid 239
Stellate corpuscles 113
Stellules of Verhuyen 206
Stigmata 94
Stomach 175
Stomata 98, 198
Stratified epithelium 19
Stratum adiposum 140
corneum 137
epitrichium 137
granulosum 136
lucidum 137, 152
Malpighii 136, 152
papillare 139
reticulare 139
squamosum I'SH
Stria vascularis 356
Striated muscular fibres 87
Striped muscular fibres ... 87
Stripes of Retzius 166
Subcutanea 140
Submucosa 154, 156, 172, 174
177, 178, 181, 183, 193, 196, 216 239, 241, 244.
INDEX.
369
Substance of Rolando Ii95. 307
Sudoriparous slj^nds 142
Su|)rarciial i.'a|)Siiles....j 2Ml
Svmpatlictic system 110
Synovial nienihrancs 268
Tactilk cklls 103, lO-l-
cor|)usclcs 104-
disks 104
hairs 150
Taste-buds 322
Teeth 16+
Tendon cells 43
tissue 42
Terminals, discharf^in^ 106
nerve 102
receiving 10 i
Testis 217, 219
Theca folliculi 235
Thoracic duct 265
Thymus 277
Thyroid 27S
Tissue 13
Tomes, fibres of. 165
Tongue 168
Tonsils 171, 173
Trabeculae of spongy bone 53
Trachea 192
Tracheal muscle 194
Tracts of cord 299
Transitional epithelium 19
Tubes of Ptlueger 161
Tunica albuginea 219, 229, 234
Tympanum 353
I'mbilical cord 33
Uniceilular glands 156
Unipolar coipuscles 102, 110
Ureter 213
Urethra, female 216
male 226
Urinary bladder 214, 257
Uriniferous tubules 207
Urogenital sinus 257
U'tcrine glands 240
Uterus 240
masculinus 231
Vagina 243
Valves ot heart 263
of Houston 182
of veins 264
Vas aberrans 223, 224
deferens 219, 224, 225
Vasa cfferentia 223
vasorum 261
Vascular endothelium .20, 90
Vascularization of cartilage 63
Vatcr, corpuscies of. 105
Veins 91, 94, 263
I Ventral cornu 292
I Verhuyen, stellules of. 206
Vermiform appendix 182
Vessels 91
Vestibule 245
Villi 177
Vitelline membrane 237
Vitellus 237
Vitreous body 33,341
Volkmann's canals 52
Vulva 244
Wharto.n's jelly 33
White commissure 293
fibres 34, lOl, lo7
fibrocartilage 28
fibrous tissue 42
matter of cord 292
substance of Schwann 101
Wolffian body ... 253
duct 253
Yellow fibres 34
fibro-cartilage 27
marrow 56
Zona pellccida 236
striata 236
vasculosa 234
Zone of Zinn 343
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