CORNELL UNIVERSITY LIBRARY GIFT OF The Estate of S. Simpson Cornell University Library QM 551.S53 1907 The essentials of histology, descriptive 3 1924 003 131 327 The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924003131327 HISTOLOGY By the same Juthor. DIRECTIONS FOR CLASS WORK IN PRACTICAL PHYSIOLOGY: Elementary Physiology of Muscle and Nerve and of the Vascular and Nervous Systems. With 48 Diagrams to show arrangement of Apparatus. 8vo. 3s. net. LONDON : LONGMANS, GREEN & CO. A COURSE OF PRACTICAL HISTOLOGY Containing plain directions for individual work in Histology. 8vo. 7s. 6d. LONDON : SMITH, ELDER & CO. THE ESSENTIALS OF HISTOLOGY DESCRIPTIVE AND PEACTICAL FOR TEE USE OF STUDENTS E. A. SCHAFER, LL.D., si.D., F.E.S. PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF EDINBURGH FORMERLY JODRELL PROFESSOR OF PHYSIOLOGY IN UNIVERSITY COLLEGE, LONDON SEVENTH EDITION LONQMANS, GREEN AND CO. 39 PATEENOSTER ROW, LONDON NEW YORK, BOMBAY AND CALCUTTA 1907 BIBLlOGRAPHtCAL NOTE. First Edition, May 1885 ; Second Edition, January 1887 ; Third Edition, May 1892 ; Fourth Edition, April 1894; Fifth Edition, June i8g8 ; Sixth Edition, May 1902 ; Seventh Edition, January 1907. PREFACE TO THE SEVENTH EDITION. This Book is written with the object of supplying the student with directions for the microscopical examination of the tissues. At the same time it is intended to serve as an Elementary Text-hook of Histology; comprising the essential facts of the science, but omitting less important details. For conveniently accompanying the work of a class of medical students, the book is divided into fifty lessons. Each of these may be supposed to occupy from one to three hours, according to the relative extent to which the preparations are made beforehand by the teacher, or during the lesson by the students. A few of the preparations cannot well be made by a class, but it has been thought advisable not to injure the ' completeness of the work by omitting mention of them. Only those methods are recommended upon which experience has proved that full dependence can be placed, but the directions given are for the most pa;rt capable of easy verbal modification in accord- ance with the ideas or experience of different teachers. The present edition has been considerably enlarged, partly by additions to the text — especially that descriptive of the structure vi PEEFACE. of the central nervous system, a proper knowledge of which is essential to students of medicine — partly by the provision of new illustrations derived from many sources. The author desires to express his recognition of the readiness with which other authors have placed illustrations at his disposal. This recognition is especially, due to Professor Sobotta and to Professor Eam6n Cajal, the latter of whom was good enough to lend many of his original drawings for reproduction in this work. A new feature is the printing of many of the illustrations in colour, which it is believed will give a better idea of the appear- ance of the stained preparations. To Dr. P. T. Herring,, who has read and corrected the final proofs; and to Professor Sherrington, who has looked through the chapters on the central nervous system, the author begs to offer his grateful acknowledgments. CONTENTS. INTRODUCTORY. PAGE Enumeration of the Tissues — General Structure of Animal Cells, 1 LESSON I. Use of the Microscope — Examination of Common Objects, . 24 LESSONS II. AND III. Human Blood-Corpuscles — Development of Blood-Corpuscles — Bone-Marrow, . . .... 28 LESSON IV. Action of Reagents upon the Human Blood-Corpuscles, . 41 LESSON V. Blood-Corpuscles of Amphibia, 45 LESSON VI. Amceboid Phenomena of the Colourless Blood-Corpuscles, . 48 LESSON VII. Epithelium, . . . . ■ ■ ■ 52 viii CONTENTS. LESSON VIII. PAGE Columnar and Ciliated Epithelium, . . 60 LESSON IX. Connective Tissues : Areolar and Adipose Tissue — Retiform Tissue, . . .67 LESSON X. Connective Tissues {continued) : Elastic Tissue — Fibrous Tissue- Development OP Connective Tissue, . . . .78 LESSON XL Connective Tissues {continued) : Articular Cartilage — Synovial Membranes, .86 LESSON XII. Connective Tissues {continued) : Costal Cartilage — Fibro- Cartilaqe, ... ... 92 LESSON XIII. ■ Connective Tissues {continued) : Structure and Development OF Bone, ... . . . .96 LESSON XIV. Striated Muscle, . .... . . 110 LESSON XV. Connection of Muscle with Tendon — Blood- Vessels of Muscle — Cardiac Mdsolb — Development of Muscle — Plain Muscle, 120 LESSON XVI. Nerve-Fibres, . . .128 CONTENTS. ix LESSONS XVII. AND XVIII. PAOE Nbrve-Cells^Nedroqlia — Development of Nerve-Fibres and Nerve-Cells — Degeneration and Eegeneration, . . .137 LESSON XIX. Modes of Termination of Nerve-Fibres, .... 166 LESSON XX. The Larger Blood-Vessels, 184 LESSON XXL Smaller Blood-Vessels — Lymph-Vessels — Serous Membranes. — Microscopic Study of the Circulation — Development of Blood- and Lymph- Vessels, 191 LESSON XXII. Lymph-Glands — Tonsil— Thymus, 203 LESSON XXIII. Spleen— Suprarenal Capsules— Thyroid Body — Pituitary Body, 213 LESSONS XXIV. AND XXV. Skin, Nails, Haies, etc. — Mammary Gland^, 226 LESSON XXVI. Heart, . 2.50 LESSON XXVII. Trachea and Lungs, .... . • 254 X CONTENTS. LESSON XXVIII. FAOS Structure and Development op the 'Teeth, . . . . 263 LESSON XXIX. Tongue and Mucous Membrane of the Mouth — Taste-Buds — Pharynx and {Esophagus, . 275 LESSON XXX. Salivary Glands, . 281 LESSON XXXI. Stomach, . . 287 LESSONS XXXII. and XXXIII. Small and Large Intestine, . ... . 295 LESSONS XXXIV. and XXXV. Liver and Pancreas, .... 309 LESSON XXXVI. Kidney, ... . . 320 LESSON XXXVII. Ureter, Bladder, and Male Generative Organs, . . 328 LESSON XXXVIII. Generative Organs of the Female, . . . 344 LESSON XXXIX. and XL. Spinal Coed, ... 355 CONTENTS. xi LESSON XLI. PAGE Medulla Oblongata, ... . . . . 374 LESSONS XLII. AND XLIII. Pons Varolii, Mesencephalon, and Thalambncbphalon, . . 390 LESSONS XLIV. and XLV. Cerebellum and Cerebrum, . 417 LESSONS XLVI., XLVII. and XLVIII. Eye, ... 443 LESSON XLIX. OLrACTORT Mucous Membrane— External and Middle Ear, . 467 LESSON L. Internal Ear, . . 472 APPENDIX. Methods, ... . . ... 484 INDEX, 501 THE ESSENTIALS OF HISTOLOGY INTRODUCTORY. ENUMERATION OF THE TISSUES AND THE GENERAL STRUCTURE OF ANIMAL CELLS. Animal Histology ^ is the science whicli treats of the minute struc- ture of the tissues and organs of the animal body ; it is studied with the aid of the microscope, and is therefore also termed Microscopic Anatomy. Every part or organ of the body, when separated into minute fragments, or when examined in thin sections, is found to consist of certain textures or tissues, which differ in their arrangement in different organs, but each of which exhibits characteristic structural features. The following is a list of the principal tissues which compose the body : — 1. Epithelial. 2. Connective : Areolar, Fibrous, Elastic, Adipose, Lymphoid, Cartilage, Bone. 3. Muscular : Voluntary, Involuntary or plain. Cardiac. 4. Nervous. Some organs are formed of several of the above tissues, others contain only one or two. It is convenient to include such fluids as the blood and lymph amongst the tissues, because they are studied in the same manner and contain cellular elements similar to those met with in some of the other tissues. All the tissues are, prior to differentiation, masses of cells (embryonic cells). In some tissues other tissue-elements become developed which take the form oi fibres. Thus the epithelial tissues are com- posed throughout life entirely of cells, only slightly modified in 1 From larbs, a web or texture. A 2 THE ESSENTIALS OF HISTOLOGY. structure, and the nervous and muscular tissues are formed of cells which are greatly modified to form the chai'acteristic fibres of those tissues. On the other hand, in the connective tissues an amorphous material becomes formed between the cells which is termed intercellular substance or ground substance, and in this substance fibres make their appearance, sometimes, as in the fibrous connective tissue, in so large an amount as to occupy the whole of the intercellular substance, and greatly to preponderate over the cells. This ground substance, by virtue of its containing a certain amount of inorganic chlorides, has the property of becoming stained brown or black by nitrate of silver and subsequent exposure to light, in which case the cells, which remain unstained, look like white spaces (cell-spaces) in the ground substance. When an epithelial tissue is similarly treated, the narrow interstices between the cells are also stained, from which it may be concluded that a similar substance exists in small amount between the cells of this tissue. It has here been termed cement-substance, but it is better to apply to it the general term intercellular substance. The cells of a tissue are not always separate from one another, but are in many cases connected by bridges of the cell-substance, which pass across the intercellular spaces. This is especially the case with the cells of the higher plants, but it has also been found to occur in animal tissues, as in some varieties of epithelium and in cardiac and plain muscular tissue. Occasionally the connexion of the cells of a tissue is even closer, and lines of separation between them are almost or entirely absent. The term syncytium is given to any such united mass of cells. Fig. 1.— Diagbam of a cell, highly magnified. p, protoplasm, consisting of hyaloplasm and a network of spongioplasm ; ex, oxoplasm ; end, endoplasm, with distinct granules and vacuoles ; c, double centrosome ; n, nucleus ; n', nucleolus. Cells. — A cell is a minute portion of living substance (cytoplasm), which is sometimes inclosed by a cell-membrane and always contains a specially differentiated part which is known as the nucleus. The cytoplasm of a cell (fig. 1, ^) is composed oi protoplami, which consists chemically of proteid or nucleoproteid substances, with which STRUCTURE OF THE CELL. a combination of fatty acid with glycerophosphoric acid, and 'clwlesterin, a monatomie alcohol, having many of the physical characters of fats, appear always to be associated. The protoplasm 1 2 3 4 5 6 7 8 Fio. 2. — Successive changes exhibited by an am(eba. (Verworn.) tends during life to exhibit movements which are apparently spon- taneous, and when the cell is uninclosed by a membrane a change in the shape, or even in the position of the cell, may be thereby produced. This is characteristically shown in the movements of the unicellular organism known as the amoeba (fig. 2) ; hence the name Fig. 3.- -Protoplasmic structure in a psbudopodium op a fobaminifek (miliola). (Verworn, after Butschli.) amoeboid movement, by which it is generally designated.^ The proto- plasm often, but not always, contains a fine spongework, which takes under high powers of the microscope the appearance of a network (figs. 1, 3), the remainder of the protoplasm being a clear ^The araceboid phenomena of cells will be studied later (in the colourless corpuscles of blood). . ' THE ESSENTIALS OF HISTOLOGY. substance which occupies the interstices of the sponge, and may also cover the surface or project beyond the rest of the cell. A granular appearance is often produced by the knots in the network when imper- fectly observed looking like separate granules. The material which forms the reticulum is termed spongioplasm ; the clearer material which occupies its meshes is hyaloplasm. The protoplasm of some cells shows a considerable degree of differentiation into fibrils which may be unbranched or may form a network Fig. 4.-TEopHospoNGmM (CANAL- ^itliin *e cell. Some cells exhibit a isATioN) WITHIN A GANGLION fine caualisation of their protoplasm CELL. (E. Holmgren.) ,. tt i ^u (fig. 4), and according to Holmgren the canaliculi are in many cases occupied by branching processes of other (nutrient) cells, which form what he terms a " trophospongium." Protoplasm often, if not always, includes actual granules of a Fig. 5.— Cells peom thb testiolk of the mouse in pbocess op tbanspoe- MATioN into spermatozoa. (Beuda.) The "mitochondria" are darkly stained and are seen in the sucoessive staees fa to M to be arranging themselves so as to constitute the spiral flUment of the spermatozoon proteid nature. Some of these granules may be essential con- stituents of the protoplasm (Altmann); others are materials which STEUCTUEE OF THE CELL. 5 have been formed by the protoplasm, and which are in a sense accidental inclusions. That the former are of importance appears to be evident from the fact that many of the chemical changes of cells occur in them. Moreover they are closely associated with the most active part of the protoplasm, the part, namely, in the neighbourhood of the nucleus, and appear to become formed in this part, and from it to extend through the cell. When fibrils are formed in the protoplasm, they are believed to be produced from the granules in question, to which the name mitochondria has been given (Benda), (fig. 5). The mitochondria are sometimes collected into a spherical mass near the nucleus which stains more deeply than the rest of the cytoplasm (fig. 6). To this body the term pa/ra- nvdeus has been applied. The granules referred to may be regarded as actual constituents of the cytoplasm, and formed Fig- 6.— Pancreas cells of fkog, J. ,, J. .. , , k ■ T ^ SHOWING PAKANDCLBUS AND directly irom its protoplasm. As indicat- ohondbomitombmbkilsfokmed ing this close connexion with protoplasm ™^^ after*Matthews ) '^"'" they may conveniently be termed deuto- plasm. This name has also been used to include materials which are merely included in the cytoplasm and not factors in its constitu- tion, such as pigment granules, fat globules, and vacuoles containing watery fluid, with or without glycogen or other substances in solution. Materials which are thus included in the protoplasm of a cell are either stored up for the nutrition of the cell itself, or are converted into substances which are eventually extruded from the cell in order to serve some purpose useful to the whole organism, or to be got rid of from the body. The term paraplasm may be employed to denote any such materials within a cell. Paraplasm is often present in sufiScient quantity to reduce the cytoplasm to a relatively small amount, the bulk of the cell being occupied by other material, as when starch becomes collected within vegetable cells or fat within the cells of adipose tissue. It is frequently the case that the para- plasm and deutoplasm are confined mainly to the middle of the cell in the neighbourhood of the nucleus, an external zone of the protoplasm being left clear. The two portions of protoplasm which are thus somewhat imperfectly differentiated off from one another are termed respectively the endoplasm and the exoplasm (fig. 1). They are exhibited in the amoeba (fig. 2), and also in the white blood-corpuscle (fig. 8). 6 THE ESSENTIALS OF HISTOLOGY According to the view advocated by Biitschli the apparent reticulum' or spongioplasm of a cell is the optical eflfect of a soft honeycomb or froth-like structure : in other words, the meshes of the reticulum do not communicate with one another as in a sponge, but are closed cavities as in a honeycomb. Biitschli finds indications of the same alveolar structure: in all cells, including nerve-fibres and muscle-fibres, and has devised experiments with drops of froth made up of a mixture of oil and alkaline carbonate or sugar solution, which, when examined in water under the microscope, imitate very closely not only the structural appearance (fig. 7) but even the so-called spontaneous Fig. 7.— Comparison of protoplasm with oil and water emulsion. A, Protoplasm of ThalasBicola: B, Froth-like appearance of a mixture of oil and cane sugar. (Verworn, after BUtscbli.) or amoeboid movements of actual protoplasm. It may be stated, however, that although it is a matter of difficulty to determine whether a microscopic reticulum is a sponge-work or a honeycomb, it is probable that neither structure is essential to living substance, for the outermost layer of the ceU protoplasm, which is usually the most active in exhibiting movements, often shows no indication of such structure. And further, it has been shown by Hardy that a colloid solution such as that which exists in protoplasm may, under some circumstances, appear homogeneous and under others may separate out into two parts, one more solid the other more fluid, and after such separation may exhibit either a granular, a reticular, or a honeycomb structure, according to circumstances. Nor is a "froth" necessary for the imitation of amoeboid movements, for similar movements, due to changes in surface tension, are brought about in a simple oil drop or in a drop of oil-clad albumen when brought in contact with solution of soap or of any alkali (Berthold, Quincke). A drop of any colloid solution containing electrolytes is also subject to such changes of surface tension when exposed to varying electrical influences, so that these amoeboid movements, which are certainly " vital," are capable of being explained by well-known physical laws There are grounds for believing that a very fine pellicle covers the exterior of the protoplasm of all free cells, and that this pellicle is composed of a material which, although not soluble in water, is permeable to watery fluids, and may also allow the passage of solids without rupture. Such a material might be furnished by the lecithin and cholesterin (Overton), which are, as we have seen, constant constituents of cell-protoplasm. It must, however, be stated that it has not been. proved that these substances are collected at the surface of protoplasm. Properties of living matter.— Living cells exhibit (l) irritability or the property of responding to stimuli ; (2) metabolic or chemical changes which result in assimilation or the taking in of nutrient matter and converting it into living substance (anabolism), and disassimilation, the property of break- STRUCTURE OP THE CELL. 7 ing down or getting rid of such substance (kataJDoIism) ; (3) reproduction resulting in the multiplication of cells. Of these properties (2) and (3) are •certainly governed or influenced by the cell-nucleus, and (3) appears to be usually initiated by the centrosome (see below). The irritability of the cell depends, however, mainly upon the cytoplasm itself. It is in consequence of this property that protoplasm reacts, sometimes by contraction sometimes by relaxation, to mechanical, chemical, thermal, and electrical stimuli, and in the case of some cells {e.g. the pigment-cells and cones of the retina) to the stimulus of light. 'The amoeboid movements of cells are a manifestation of irritability, being produced and influenced by various external conditions and stimuli. Sometimes the result of a stimulus is to cause a cell or organism to move towards the source of excitation (attraction) ; in other cases the movement is in the reverse direction (repulsion). The terms positive and negative chemotaxis, phototaxis, thermotaxis, and the like, are used to indi- cate the nature of the effects produced by various forms of stimulation. Attraction-sphere and centrosome. — In some cells, as already indi- ■cated, there are fine but distinct striae or fibrils {cytomitome) running in definite directions. These are very commonly met with in fixed Fig. 8. — An am(eboid cell (white coBPUscLE or newt) veet highly MAGNIFIED. fihowiag a double nucleus witli recticulum of chromoplasiu, and the protoplasm com- posed of two portions, a clearer exoplasm, , and a granular-looking endoplasm. Fig. 9.— a cell (white bloodcorpusole) showing its attkaction-spheke. CWUson, after M. Heidenhain. ) In this, as in most cases, the attraction-sphere, w, lies near the nucleus, n. cells, such as various kinds of epithelium-cells, nerve-cells, and muscle- cells. But besides these special differentiations, which appear to be related to special functions, there are other fibril-like structures in the cell-protoplasm, associated with what is known as the centrosome (figs. 1, 9). This is a minute particle (centriole), usually situated near the nucleus, and staining darkly with iron-hsematoxylin, surrounded by a clear area (attraciion-sphere), and from it radiate into the surround- ing protoplasm a number of fine fibrils with dot-like enlargements at intervals. The attraction-sphere, with its central particle, was ■first noticed in the ovum and was at first supposed to be peculiar 8 THE ESSENTIALS OF HISTOLOGY. to the egg-cell, but it has now been recognised in very many kinds of cells, and, in animal cells, is of nearly universal occurrence. The structures in question are frequently double (fig. 1), the twin, spheres being connected by a spindle-shaped system of delicate fibrils {achromatic spindle) : this duplication invariably precedes the division, of a cell into two. In some cells the centrioles are multiple ; this is frequently the case with leucocytes and always with the giant-cells of bone marrow. The material which immediately surrounds the centrosome, and of which the radiating fibres and the fibres of the spindle are composed, is considered by some to be distinct in nature from the general protoplasm: it has been termed the archoplasm. It appears clear that in some cells the centrosome and archoplasm may have an existence independent of one another; thus no centrosome has been found in the cells of the higher plants, although the archoplasmic fibres are very well marked in them during cell-division. A cell-membrane is rarely distinct in animal cells. When, present,. it is usually formed by transformation of the external layer of the protoplasm, but its chemical nature has not been sufficiently investi- gated. In plant cells a cellulose membrane is of common occurrence. The nucleus of the cell (fig. 1, n) is a small vesicle, spherical, ovoid, elongated, annular, or irregularly lobulated (figs. 8, 9, 10), embedded in the protoplasm. Cells have sometimes two or more nuclei. The nucleus is bounded by a membrane which incloses a clear sub- stance {nuclear hyaloplasm, haryoplasm), and the whole of this substance is generally pervaded by an irregular network of fibres, some coarser, others finer {tmclear reticulum, Jmryomitome). The membrane is the outermost layer of the nuclear reticulum, and may itself have meshes, like a basket-work, thus allowing direct communication between the hyaloplasm of the cell and that of the nucleus. The knots of the nuclear reticulum are sometimes very distinct and give an appearance of conspicuous granules within the nucleus {pseudomtcleoli). The nucleus- usually contains a single distinct highly refracting spherical particle known as the nucleolus. This is sometimes multiple, and occasionally has a vacuole-like globule in its interior. The material of the nucleolus differs in its chemical and staining reactions from the nuclear reticulum, but prior to cell-division it becomes indistinguishable from the substance of the nuclear fibres. Whfether it blends with them or becomes absorbed and removed is at present uncertain. The nuclear membrane, intranuclear fibres, and nucleoli all stain deeply with hsematoxylin and with basic dyes generally ; this property distin- guishes them from the nuclear matrix, and they arte accordingly spoken of as chromatic (containing chromatin, which in the nucleus appears to^ be chemically identical with nuclein), the hyaloplasm being achromatic. Sometimes instead of being united into a network the intranuclear STRUCTURE OF THE CELL. 9 fibres take the form of convoluted filaments (chromosomes), having a skein-like arrangement (fig. 11). This is always found when a nucleus is about to divide, but it may also occur in the resting Fig. 10.— Cell prom bone-makbow. Fig. 11. — Gland-cell of Chieono- (Carnoy.) Mus. (Flemming.) p, protoplasm with fine reticulum ; n, nucleus, long and folded, -with intra- nuclear network. condition. These filaments may sometimes be seen with high magni- fication to be made up of fine juxtaposed particles (chromomeres) arranged either, in single or double rows (fig. 12), which impart a crosg-striated appearance to the filament. The chromomeres are united to form the chromosomes by a non-staining material to which X T-'M Fig. 12.— Spermatocyte of PKOTEna, showing chkomosombs of nucleus FORMED OF PARTICLES OF CHROMATIS UNITED BY ACKOMATIO FILAMENTS. (Gurwitsoh, after Hermann. ) The nucleolus is distinct from the chromosomes. In the cytoplasm an archoplasmic mass containing mitochondria is seen on the right. the term linin has been applied. The nuclear fibres are sometimes clumped together into a solid mass which comprehends the nucleolus when present, and has the appearance of an enlarged nucleolus. The 10 THE ESSENTIALS OF HISTOLOGY. fibres within the nucleus have been observed to undergo spontaneous changes of form and arrangement, but these become much more evident during its division. The division of the protoplasm is always preceded by that of the nucleus, and the nuclear iibres undergo during division a series of remarkable transformations which are known collectively by the term haryoUnesis (Schleicher) or mitosis (Flemming). These changes may easily be studied in the division of epithelium cells, but exactly similar phenomena have been shown to occur in cells belonging to other tissues. Fig. 13.— Celi or bladder epithelidm, showino amitotio division ott NUCLEUS. (Nemileff.) I. cytoplasm ; //. daughter nuclei ; ///. strand of fibrils uniting daughter nuclei. The simple division of a nucleus by a process of fission without karyokinetic changes is termed amitotic division : it occurs in com- paratively rare instances, and is not usually followed by the division of the cell, so that it is apt to result in the formation of bi-nucleated and multi-nucleated cells, as in the superficial layer of the epithelium of the urinary bladder and in some of the giant cells of bone-marrow. The nucleus of the eell is not only concerned with its division aijd multiplication in the manner shown below, but takes an active part in the chemical (metabolic) processes which occur in the protoplasm. Hence cells deprived artificially of their nuclei do not assimilate nourishment, and lose any power of secretion they may have possessed, although the protoplasm may continue for a time, to live and exhibit amoeboid movements. DIVISION OF CELLS. 11 Division of cells. — The division of a cell is preceded by the division of its attraction-sphere, and this again appears to determine the division of the nucleus. The latter, in dividing, passes through a series of remarkable changes, which may thus be briefly summarised Fig. 14.— Epithelium-oblls op salamandha larva in different phases op DIVISION BT kartokinesis OR MITOSIS. (Flemming.) as they occur in typical animal cells such as the epithelium cells of Salamandra : — 1. The network of chromoplasm-filaments of the resting nucleus b^colnes transformed into a sort of skein, formed apparently of one long convoluted filament, but -in reality consisting of a number of filaments ' (spirem) ; the nuclear membrane and the nucleoli disappear or are merged into the skein (fig. 14, a to (f). 2. The filament breaks 'into a number of sejjarate portions, often 12 THE ESSENTIALS OF HISTOLOGY. V-shaped, the chromosmies. The number of chromosomes varies with the species of animal or plant ; in some animals the dividing nuclei may contain at this stage only four chromosomes ; in man there are said to be twenty -four in the ordinary or somatic cells; a like number occurs in many animals and plants; others have more or fewer. As soon as the chromosomes become distinct they are often arranged radially around the equator of the nucleus like a star {aster, fig. 14, e, f, g). Fig. 15. — The pkinoipai. phases of the nuoleak chromatin filaments in the PROCESS OF OEDINART MITOSIS OF THE SOMATIC CELL. (Flemming.) A, skein or Bpirem ;.B, aster with splitting of chromosomes; C, separation of the split chromosomes (metakineais) ; J), continuation of this process ; E, diaster ; F, di- spirem. The cell protoplasm is represented in outline in F: it has itself undergone division at this sl^ge. In this figure the (somatic) cells represented are supposed to have eight chromosomes. 3. Each of the chromosomes splits longitudinally into two, so that they are now twice as numerous as before {stage of cleavage, fig. 14, h, i). This longitudinal cleavage may occur at an earlier stage. 4. The fibres separate into two groups, the ends being for a' time interlocked (fig: 14, y, h). 5. The two groups pass to the opposite poles of the now elongated nucleus and form a star-shaped figure at either 'pole {diaster, fig. 14, I). Each of the stars represents a daughter nucleus. 6. 7, 8. Each star of the diaster goes through the same changes as the original nucleus, but in the reverse order — viz. a skein, at first more open and rosette-like , (fig. 14, m), then a closer skein (fig. 14, n), DIVISION OF CELLS. 13 Fig. 16.— Heteeo-typical and homo-ttpioal mitosis^ of the GENERATIVE CELL. (Flemming.) The asterisk marks the middle, the cross the end of a chromosome. The changes are to be compared with those shown in fig. 15. In the hetero-typical form the chromosomes are ahready arranged in pairs hefore division commences ; this condition (geminal condition, a) having been produced in the synaptic prophase. A longitudinal split is seen in the diaster stage and the daughter nuclei have the somatic number of chromosomes (8 in this instance). In the homo-typical form there is no longitudinal split of the V-shaped chromosomes and the separation to form the daughter nuclei results in a reduction to one-half the somatic number. 14 THE ESSENTIALS OF HISTOLOGY. then a network (fig. 14, o, p, q) ; passing finally into the typical| reticular condition of a resting nucleus. Fig. 17.— Spkrmocttbs op salamandka showing v-shaped chromosomes AT THE EQTJATOK OF THE SPINDLE. CWilson, after Drtiner.) A, seen in profile ; four chromosomes only arc represented. B, seen end-on. All twenty-four chromosomes are represented ; the fibrils of the spindle are seen in optical section. The splitting and separation of the chromosomes is often spoken of as the metaphase {metaJcinesis)-; the stages leading up to this beiug termed the anaphase and those by which the process is terminated the kataphase or telophase. The mode of division of the nuclear chromatin above described is frequently spoken of as somatic or ordinary mitosis (fig. 15) to distinguish it from two modes of division which are only seen normally at certain stages of multiplication of the generative cells, and which are known as Jietero- typical and homotypical mitosis (fig. 16). In the latter the chromosomes do not undergo the usual longitudinal splitting, but one half of the total number passes into each daughter nucleus, so that the number of chromosomes io each of these is only one half the usual somatic number. This is termed the reduction-division.^ Heterotypical mitosis (which immediately precedes the homotypical) is characterised by a peculiar arrangement of the chromosomes, the split halves of which, before separating to pass to the daughter nuclei, tend to adhere together in the form of loops or rings, or in the case of short straight chromosomes into small quadrangular masses (tetrads), all of which are observable in various instances of heterotypical mitosis, (see fig. 19). It is further noteworthy that the generative cells which later undergo the reduction-division above described exhibit either immediately (sperm-cells)- or a long while (germ-cells) before the maiotic divisions a remarkable series of changes in their nuclear chromatin ; the chromosomes becoming first distinct in place of forming a network, then entangled together at one side of the nucleus (synaptic condition), and finally becoming again distinct, but now arranged in pairs (gejoini) which later take various forms, such as double rods, loops, or rings as in heterotypical mitosis, but without forth- with proceeding to nuclear' division. The protoplasm of the cell divides soon after the formation of the diaster (fig. 14, n). During division fine lines are seen in the proto- ' "Maiotic division" or "maiosis" of Farmer, Moore and Walker. DIVISION OF CELLS. 15 plasm, radiating from the centrosomes at the poles of the nucleus, whilst other lines form a spindle-shaped system of achromatic] fibres II. ---,. M vir. , VIII. .- w Fig. 18.— Diaoeam showing thb changes which occur in the obnteo- somes and ndolkus or a cell in the pbocess oe mitotic division. The nucleus is supposed to have four chromosomes. within the nucleus, diverging from the poles towards the equator (fig. 18). These are usually less easily seen than the chromatic fibres 16 THE ESSENTIALS OF HISTOLOGY. or chromosomes already described, but are not less important, for they are derived from the attraction-spheres. These, with their centrosomes, as we have seen, always initiate the division of the cell; indeed they are often found divided in the apparently resting nucleus, the two particles being united by a small system of fibres forming a minute spindle at one side of the nucleus (fig. 1). When mitosis is about to take place this spindle enlarges, and as the changes in the chromatin of the nucleus which have been above Fig. 19. — Thbee stages of hbtekotype mitosis in spermatocyte op TBITON. (Moore.) It, geminal condition of chromosomes ; h, gemini arranged in quadrate loops or tetrads ; c, separation of tetrads into the duplex chromosomes of the daughter nuclei. described occur-^which changes involve the disappearance of the nuclear membrane — the spindle gradually passes into the middle of the mitotic nucleus, with the two poles of the spindle at the poles of the nucleus, and with the fibres of the spindle therefore completely traversing the nucleus (fig. 18). The spindle-fibres appear to form directing lines, along which the chromosomes pass, after the cleavage, towards the nuclear poles to form the daughter nuclei. In some cells, especially in plants, the line of division of the proto- plasm of the cell becomes marked out by thickenings upon the fibres of the spindle which occur just in the plane of subsequent division, and have been termed collectively the cdUplate (fig. 20). But in most animal cells no cell-plate is formed, the protoplasm simply becoming constricted into two parts midway between the two DIVISION OF CELLS. 17 •daughter nuclei. Each daughter cell so formed retains one of the two attraction-particles of the spindle as its centrosome, and when the daughter cells are in their turn again about to divide this centro- some divides first and forms a new spindle, and the whole process goes on as before. riG. 20. — Cell-plate in dividinq SPOBB-CELL OF LILT. (Gurwitach, after Zimmermann.) Fig. 21. — Dividing cell constbioted to form two daughter cells each WITH CENTROSOME. (Geberg.) Tke particle at the junction of the daughter cells represents a rudimentary cell-plate. Earely the division of a nucleus is into three or more parts instead of two. In such cases the centrosome becomes correspondingly multiplied and the achromatic system of fibres takes a more complex form than the simple spindle. Division of the Ovum. — Usually the two daughter cells are of equal size ; but there is a notable exception in the case of the ovum, which, prior to fertilisation, divides twice (by hetero- and homotypical mitosis respectively) into two very unequal parts, the larger of which retains the designation of ovum, while the two small parts which become detached from it are known as the ^olar bodies (fig. 22). Further, in the formation of the second polar body a reduction-division occurs, and the nucleus of the ovum, after the polar bodies are extruded, contains only one half the number of chromosomes that it had previously {e.g. twelve in place of the normal twenty-four in man, and two instead of four in Ascaris megalocephala (var. bivalens), fig. 22, (7). Should fertilisation supervene the chromosomes which are lacking are supplied by the male element (sperm-cell), the nucleus of which has also undergone, in the final cell-division by which it was produced, the process of reduction in the number of chromosomes to one half the normal number. The two reduced nuclei — which are formed respectively from the remainder of the nucleus of the oocyte 18 THE ESSENTIALS OF HISTOLOGY. (ovum) after extrusion of the polar bodies, and from the head of the spermatozoon, which contains the nucleus of the sperm-cell — are known (within the ovum) as the sperm and germ nuclei or the male and Fig. 22.— Formation of the polak glob- ules AND KEDUOTION OF THE NnMBER OP CHROMOSOMES IN THE OVUM OF ASOAKIS MEGALOOEFHALA. A, By ovum showing division of nucleus to form first polarglobule (Van Gehuchten). m, gelatin- ous envelope of ovum ; m', membrane dividing tUe polar globule from the ovum ; cb (in A), head of spermatozoon. C, formation of second polar globule (Camoy); g\ first ; ^, second polar globule ; n, nucleus of ovum, now containing only two chromo- somes ; ne, nucleus formed from head of sper- matozoon. Fig. 23.— OyuM of bat with polar BomBS and germ- and SPERM-NUOLBi. (Van der Strioht.) The development of the sperm-nucleus from the- head of the spermatozoon Is very evident in this case, because the rest of the spermatozoon happens not to have been thrown off. DIVISION OF THE OVUM. 19 female pronuclei When these blend, the ovum again contains a nucleus with the number of chromosomes normal to the species (fig. 24, j^). Fig. 24,— Fertilisation and first division of ovum of ascaris mhgalo- CEPHALA (slightly modified from Boveri and Wilson). A, second polar globule just formed ; the head of the spermatozoon is becoming changed into a reticular nucleus (^), which, however, shows distinctly two chromosomes ; just above it, its archoplasm is shown : the egg-nucleus ($) also shows two chromo- somes. B, both pro-nyclei are now reticular and enlarged ; a double centrosome (a) is visible in the archoplasm which lies between them- C, the chromatin in each nucleus is now converted into two filamentous chromosomes ; the centrosomes are separating from one another. J), tho chromosomes are more distinct and shortened ; tho nuclear membranes have dis- appeared ; the attraction-spheres are distinct. Ef mingling and splitting of the four chromosomes (c) ; the achromatic spindle is fully formed. J^, separation (towards the poles of the spindle) of the halves of the split chromosomeSj and commencing division of the cytoplasm. Bach of the daughter cells now has four chromosomes ; two of these have been derived from the ovum nucleus, two from the spermatozoon nucleus. 20 THE ESSENTIALS OP HISTOLOGY. When it divides after fertilisation each daughter cell is found to .contain the normal or somatic number of chromosomes, derived from the splitting of both male and female elements, half the number from the one and half from the other. Fig. 25.— Human ovum {oocyte) feom gbaafian folliolk : examined fresh IN LIQUOR FOLLICDLI WITH VEKT HIGH MAONIFTING POWER. (Waldeyer.) The cells which are attached to the outside, and which appear to be joined into a ayn- cytium around the ovum, are follicular colls belonging to the dUeua proligerus. Wittiin them is the clear membrane of the ovum (zona pellucida). The cytoplasm of the ovum shows a distinction between clear ectoplasm and granular endoplasm ; the large granules are yolk particles. The nticleus (germinal vesicle) is clear. The nucleolus (germinal spot) is distinct. Formation of the tissues. — It appears to be established beyond doubt that new cells can only be formed from pre-existing cells. rOEMATION or THE TISSUES. ^ , B c 21 ^ Fig. 26.— Formation of blastoderm in babbit by division of ovum into A number of cells. {Allen Thomson, after E. v. Beneden.) A to E, diviflion of ovum and formation of " mulberry mass " ; p gl, polar globules ; s, e, cells of primary division which already show a difference of appearance. This early differentiation is not, however, accepted by most authorities. F to /, sections of the ovum in subsequent stages, zp, membrane of ovum (zona pellucida) ; sz, .subzonal layer, by means of which the ovum becomes, attached to the uterine mucous membrane ; i, inner cell-mass, which gives rise to the blastodermic layers. The accumulation of fluid between ectoderm and entoderm in G, H, and / has swollen the ovum out to form the so-called blastodermic vesicle. The subzonal layer of clear cells should be shown in F extending all round the inner cell-mass. 22 THE ESSENTIALS OF HISTOLOGY. In the early embryo the whole body is an agglomeration of cells. These have all been formed from the ovum or egg-cell (fig. 25), which, after fertilisation, divides first into two cells, these again into two, and so on until a large number of cells (embryonic cells) are pro- duced. These form at first an outer clear stratum lying at the surface (fig. 26 sz) and a darker granular mass attached to this layer at one part, but elsewhere separated from it by clear fluid. Eventually the cells of the inner mass arrange themselves in the form of a membrane (blastoderm) which is composed of three layers. These layers are known respectively as the ectoderm, mesoderm, and entoderm. The ectoderm gives rise to most of the epithelial tissues and to the tissues of the nervous system; the entoderm to the epithelium of the alimentary canal (except the mouth), and the glands in connection with it; and the mesoderm to the connective and muscular tissues. Fio. 27. — Section of blastodekm showiitg the commencing formation of THE MESODERM. (Kolliker.) ep, ectoderm ; hy, entodevm ; vie, mesoderm ; ax, axial part of ectoderm with cells under- going division (*). The mesoderm is growing from this part. The tissues are formed either by changes which occur in the inter- cellular substance!, or by changes in the cells themselves ; frequently' by both these processes combined amongst the cells which are least altered from their embryonic condition are the white corpuscles of the blood, and these may be regarded therefore as typical animal cells. The histogenetical relation between the three layers of the blasto- derm and the several tissues and organs of the body is exhibited in the following' table : — .The epithelium of the skin or epidermis, and its appendages, viz., the hairs, nails, sebaceous and sweat glands, and mammary glands. The epithelium of the mouth, and the epithelium of the anus and anal canal. The salivary and other glands which open into the mouth. The enamel of the teeth. The gustatory organs. The epithelium of the lower part of the urethra and vagina. The epithelium of the nasal passages, and of the cavities and glands which open into them. The epithelium covering the front of the eye. The crystalline lens. The retina. The epithelium lining the membranous labyrinth of the ear. The epithelium lining the external auditory meatus, The epithelium lining the central canal of the spinal cord, the aqueduct of Sylvius and the fourth, third, and lateral ventricles of the brain. The tissues of the nervous system. ^The pituitary body. The pineal gland. The medulla of ttie suprarenal capsules. Ectoderm - ORIGIN OF THE TISSUES. 23 Mesoderm. 'The connective tissues. The blood- and lymph-corpuscles. The spleen and other vascular and lymphatic glands. The epithelial lining of the heart, blood-vessela, lymphatics, and serous mern- branoB (endothelium). The epithelium of the uriniferous tubules. The epithelium of the internal generative organs, and the generative products in both sexes. ^The muscular tissues, voluntary, involuntary, and cardiac. 'The epithelium of the alimentary canal (from the pharynx to the lower end of the rectum) and of all the glands which open into it (including the liver and pancreas). The epithelium of the Eustachian tube and cavity of the tympanum. The epithelium of the larynx, trachea, and bronchi, and of all their ramifications. The epithelium of the pulmonary alveoli. The thyroid'body. The thymus gland. The epithelium of the urinary bladder and ureters, and of part of the urethra. lAU the connective tissues, the endothelium (mesothelium) of the vascular system, and the vascular and lymphatic glands are formed from, a special part of the mesoderm termed parablast or mesenchyme, which consists of a syncytium of branched cells with a homogeneous intercellular matrix. Hain muscular tissue is for. the most part also formed from mesenchyme, but in certain situations, as in the sweat glands and muscular tissue of the iris, it is said to bo ecto- dermal in oris^iu. Entoderm. 24 THE ESSENTIALS OF HISTOLOGY. LESSON I. USE OF THE MICROSCOPE. EXAMINATION OF CERTAIN COMMON OBJECTS. Fig. 28. — Diagram of microscope. The requisites -for practical histology- are a good compound microscope ; slips of glass teclinically known as 'slides,' upon which the preparations are made; pieces of thin glass used as covers for the preparations ; a few instruments, such as a microtome, a scalpel, scissors, forceps, and needles mounted in wooden handles ; and a set of fluid re-agents for mounting and staining microscopic preparations? A sketch-book and pencil are also necessary, and must be constantly employed. The microscope (fig. 28) consists of a tube (t f) 160 millimeters (6'4 inches) long having two systems of lenses, one at the upper end termed the 'eye-piece' or ' ocular ' (fic), the other at the lower end termed the 'objective' (ohf). For ordi- nary work there should be at least two objectives — a low power working at about 8 millimeters (J inch) from the object, and a high power, having a focal dis- tance of about 3 millimeters (^ inch) ; it is useful also to have a lower power (commanding a larger field of view) for finding objects readily, and two or more oculars of different power. The focus is obtained by cautiously bringing the tube and lenses down towards the object by the coarse adjustment, which is usually a rack-and-pinion movement (fldj), and focussing exactly by the tine adjustment, which is always a finely cut screw (adf). The stage (si) upon which the prepar- tions are placed for examination," the mirror (m) which serves to reflect light up through the central aperture in the stage and along the tube of the instrument, and the diaphragm (d) below the stage 1 The directions for making the principal fluids used in histological work will be found in the Appendix. MICROSCOPICAL EXAMINATION OF COMMON OBJECTS. 25 ■which is used to regulate the amount of light thus thrown up, are all parts the employment of which is readily understood. A substage con- denser (not shown in the diagram), which serves to concentrate the light thrown up by the mirror to the centre of the object, is valuable when high powers and stained preparations are employed. The combinations of objectives and oculars above referred to will generally give a magnifying power of from 50 to 400 diameters, and this ia sufficient for most purposes of histology. But to bring out minute points of detail in the structure of cells and of certain tissues examination with much higher magnifying powers may be necessary. Objectives of high power are usually made as immersion-lenses ; i.e. they are constructed to form a proper image of the object when the lowermost lens of the system is immersed in a layer of liquid which lies on the cover-glass of the object and has a refractive index not far removed from that of the glass itself. For this purpose either water or an essential oil (oil of cedar- wood) is used. The advantages obtained by the employment of these lenses, especially those for oil-immersion, are : — increased working distance from the object, increased angle of aperture with sharper definition of the object, and increased amount of light traversing the microscope. The best lenses for histological work are made of the so-called ' apochro- matic' glass ; specially constructed 'compensating' eye-pieces are used with these. A scale for measuring objects should be constructed for each microscope. To do this, put a stage-micrometer (which is a glass slide ruled in the centre with lines ^ and y^ millimeter apart) under the microscope in such a manner that the lines run from left to right (the microscope must not be inclined). Focus them exactly. Put a piece of white card on the table at the right of the microscope. Look through the instrument with the left eye, keeping the right eye open. The lines of the micrometer will appear projected upon the paper. Mark their apparent distance with pencil upon the card, and afterwards make a scale of lines in ink, of the same interval apart. A magnified representation is thus obtained of the micrometer scale. Mark upon it the number of the eye-piece and of the objective, and the length of the microscope-tube. This scale-card will serve for the measurement of any object without the further use of the , micrometer. To measure an object, place the scale-card upon the table to the right of the microscope and view the object with the left eye, keeping the right eye open. The object appears projected upon the scale, and its size in ^ or j^ of a millimeter can be read off. It is essential that the same objective and eye-piece should be employed as were used in making the scale, and that the microscope tube should be of the same length. The lines on English stage-micrometers are often ruled j-jhr ^^^ T^tns ""'^ apart.' Before beginning the study of histology the student should endeavour to familiarise himself with the use of the microscope, and at the same time learn to recognise some of the chief objects which are liable to occur accidentally in microscopic specimens. On this account it has been con- sidered desirable to introduce directions for the examination and recogni- tion of starch-granules, moulds and torulse, air-bubbles, linen, cotton, and woollen fibres, and the usual constituents of the dust of a room, into the first practical lesson (fig. 29). * 1. Examination of starch-granules. Gently scrape the cut surface of a potato with the point of a knife ; shake the starch-granules so obtained into a drop of water upon a clean slide and apply a cover-glass. With the low power the starch granules look like dark specks differing ' For the method of measuripg with an ocular micrometer, and for determining the magnifying power of a microscope, the reader is referred to the author's Course of Practical Histology. 26 THE ESSENTIALS OF HISTOLOGY. Fig. 29. — Objects frequently present in microscopic preparations. 1 starch granules ; 2, a small air bubble and part of a large one ; 3, yeast torulie ; 4, a mould (Aspergillus glaucus) ; 5, linen fibres ; ij, cotton fibres ; 7, -wool ; 8, hair, human; 9, epithelium scales; 10, mierocoeei ; 11, bacilli and spores (B. subtilis). Magnified about 260 diameters. MICROSCOPICAL EXAMINATION OF COMMON OBJECTS. 27 considerably in size ; under the high power they are clear, flat, ovoid particles (fig. 29, 1), with a sharp outline when exactly focussed. Notice the change in appearance of the outline as the microscope is focussed up and down. On close examination fine concentric lines are to be seen in the granules arranged around a minute spot which is generally placed eccen- trically near the smaller end of the granule. Sketch two or three starch granules. Notice the appearance of air-bubbles in the water. If comparatively large they are clear in the middle, with a broad dark border due to refraction of the light ; if small they may look entirely dark. 2. Examine some yeast which has been grown in solution of sugar. Observe the yeast-particles or torulse, some of them budding. Each torula contains a clear vacuole, and has a well-defined outline, due to a membrane. Sketch two or three torulae. 3. Examine some mould in water. Notice the long branching filaments (hyphse), and also the torula-like particles (spores) from which byphas may in some instances be seen sprouting. Sketch part of a hypha. 4. Examine fibres of linen and of cotton in water, using a high power. Compare the well-defined, rounded, relatively straight or but slightly twisted linen, with the longer, broader but thinner, and more twisted cotton fibres. Sketch one of each kind. 5. Mount one or two hairs from the head in water and look at them first with the low, then with the high power. Examine also fibres from any woollen material and compare them with the hairs. They have the same structure, although the wool is finer and is curled ; its structure may be obscured by the dye. Draw one or two of each. 6. Examine a drop of hay infusion, which has been standing a day or two, for bacteria and other putrefactive organisms. The active movements which these exhibit are due to minute cilia or flagella, which can only be made visible by special staining methods and a very high magnifying power. Any minute inactive particles, organic or inorganic, which occur in this or other fluids may be seen to exhibit a peculiar tremulous dancing movement which is known as the 'Brownian ' movement. 7. Examine some dust of the room in water with a high power. In addition to groups of black particles of carbon (soot) there will probably be seen fibres or linen, cotton, or wool, and shed epithelium-cells derived from the epidermis. 28 THE ESSENTIALS OF HISTOLOGY. LESSONS II. AND III. STUDY OF THE HUMAN BLOOD-CORPUSCLES. 1. Having cleaned a slide and cover-glass, prick the finger above the nail and mount a small drop of blood quickly, so that it has time neither to dry nor to coagulate. Examine it at once with the high power. Note (as) the coloured corpuscles mostly in rouleaux and clumps, but some lying apart seen flat or in profile ; (6) the colourless corpuscles, easily made out if the cover-glass is touched by a needle, on account of their tendency, to stick to the glass, whilst the coloured coi-puscles are driven past by the currents set up ; (c) in the clear spaces, fibrin-filaments and elementary particles or blood-platelets. Sketch a roll of coloured corpuscles and one or two colourless corpuscles. Count the number of colourless corpuscles in a field of the microscope, 2. To be made as in § 1 , but the drop of blood is to be mixed upon the slide with an equal amount of isotonic saline solution, so that the red corpuscles tend to be less massed together, and their peculiar shape is better displayed. Sketch a red corpuscle seen on the flat and another in profile or (optical section). Also a crenated corpuscle. Measure ten red corpuscles, and from the results ascertain the average diameter of a corpuscle. Measure also the largest and the smallest you can find, 3. Make a preparation of blood as in § 1 and put it aside to coagulate. Keep the edges from drying by placing it in a moist chamber or by occasion- ally breathing upon it. After a few minutes place a drop of 1 p.c. methyl violet at one edge of the cover and allow this to pass in and mix with the blood : it may be drawn through the preparation by applying a small piece of blotting paper to the opposite edge. The dye stains 'the nuclei of the white corpuscles, the blood-platelets, the network of fibrin-filaments, and the membranes of the red blood-corpuscles. The three preparations just described cannot be kept, but the two follow- ing will serve as permanent preparations of blood : — 4. To fix and stain the coloured corpuscles : — Place upon a slide a drop of 1 p.c. osmic acid mixed with an equal amount of saturated aqueous solution of eosin. Prick the finger, and mix the blood directly with the coloured fluid, stirring them together with a needle. Cover the mixture and put aside for an hour, protected from evaporation ; then place a drop of glycerine and water at the edge of the cover-glass. When this has passed under fix the cover-glass with gold size. 5. To study the granules of the colourless corpuscles and their different reactions to staining reagents, a film of blood is inclosed between two cover- glasses, which are at once separated and the film on each quickly dried in the air. A slide may be used instead of a cover-glass ; the drop of blood is placed close to the ground edge of one slide and this is drawn evenly over the middle of another. The films are fixed by immersion for one hour or more in a mixture of alcohol and ether, equal parts of each. They are then stained by (1) a saturated solution of eosin in 75 p.c. alcohol (three minutes), after which they are rinsed with water, and are then treated with (2) a 1 p.c. STUDY OF THE HUMAN BLOOD-CORPUSCLES. 29 Fig. 30. — ^HiEMAOTTOMEXKK SLIDE, RULED IN SQUARES FOR THE ENUMERATION OF BLOOD-COBPUSOLES. Fig. 31.— Oliver's apparatus for estimating the number of corpusolks IN blood by means of the opacity method. a, pipette for measuring blood ; 6, dropper for adding mixing solution ; c, graduated tube ; d, mode of observing. a, &, e, natural size. 30 THE ESSENTIALS OF HISTOLOGY. :; solution of methylene blue (one minute). The film is again rinsed with i water, rapidly dried, and mounted in xylol balsam or dammar.^ 6. Mount in xylol balsam or dammar sections of marrow from a long bone of a rabbit fixed with mercuric chloride or formol and stained with eosin and methylene blue. Observe the fat-cells, the supporting reticular tissue, the proper marrow-cells in this tissue, the myeloplaxes and the erythroblasts. 7. Tease in salt solution or serum some of the red marrow from the rib of a recently killed animal. Observe and sketch the proper marrow cells and look for myeloplaxes and nucleated coloured blood-corpuscles (erythroblasts). 8. Make a film preparation of red marrow by smearing a little upon- a cover-glass or slide, allowing it to dry quickly, and placing it in a mixture of equal parts of ether and alcohol. After an hour or more in this, the prepara- tion may be stained with eosin and methylene blue in exactly the same way as a film preparation of blood (see § 5), and mounted in xylol balsam or dammar. 9. Enumeration of the blood-corpuscles. This is done by some form of blood-counter such as the hsemacytometer of Gowers, or the similar apparatus of Thoma. This instrument consists of a glass slide (fig. 30), the centre of which is ruled into ^ millimeter squares and surrounded by a glass ring ^ mm. thick (in Gowers' instrument, the ruling is into \ mm. squares with a ring | mm. thick). There must also be provided a pipette (fig, 31, a) for measuring the blood, constructed to hold about 5 cubic millimeters of fluid ; a dropper (fig. 31, 6) to deliver the diluting solution; a small cylindrical mixing glass, not shown in the figure, with a mark indicating 100 times the capacity of the blood pipette ; a small glass stirrer, and a guarded needle. The diluting solution may either be that of Hayem, viz. distilled water 200 cc, sulphate of soda 5 grms., common salt 1 grm., corrosive sublimate 0'5 grm., or that of Marcano, viz. 97 cc. of a solution of sulphate of soda in distilled water of sp. gr. 1020, to which is added chloride of sodium 1 grra., and formol 3 cc. A little of the diluting solution is put in the mixing vessel, the finger is pricked, and the pipette filled exactly with blood (by capillarity). The blood is then washed out of it with diluting solution, by aid of the dropper, into the mixing vessel, which is now filled up to the 100 mark with diluting solution, and the blood and this are thoroughly mixed. A drop of the mixture is next placed in the centre of the cell, the cover-glass is gently laid on (so as to touch the drop, which thus forms a layer ^j^jy mm. thick between the slide and cover-glass), and pressed down by two brass springs. In a few minutes the corpuscles have sunk to the bottom of the layer of fluid and rest on the squares. The number in ten squares is then counted, and this, multiplied by 100, gives the number in a cubic millimeter of the mixture, or if again multiplied by 100 (the amount of dilution) the number in a cubic millimeter of blood. For the enumeration of the white corpuscles the blood is diluted only 10 times instead of 100 times. It is also convenient to use one half per cent, solution of acetic acid just coloured with methyl violet as a diluent (Thoma). This destroys the coloured corpuscles and stains the nuclei of the white. A rapid method of estimating the number of coloured corpuscles is that devised by G. Oliver. The blood is taken up as before in a capillary pipette (fig. 31, a), and is washed out of this with Hayem's fiuid by the dropper, b, into a flattened graduated glass mixer, c, the diluent being added until the flame of a small wax candle in a dark room will just show sharply through the mixture, when the vessel is held close to the eye and about ten feet from the candle and so that the light traverses the greater thickness of fluid. The ^ Other stains, such as Ehrlioh's tri-acid and the Ehrlich-Biondi, may also be employed for films. STUDY OF THE HUMAN BLOOD-CORPUSCLES. 31 graduations are so arranged that for normal blood (5,000,000 corpuscles per cub. rum.), the mixture will now stand exactly at the 100 mark : if the blood contain more or fewer corpuscles than normal, it will require a greater or less dilution to attain the requisite tianslucency, and the mark at which the mixture then stands will indicate the percentage, of corpuscles as compared with the normal. Another rapid method of estimating the relative number of blood-cor- puscles is to determine the corpuscular volume in a known amount of blood. This is done by the use of the htematocrit, in which the blood, suitably diluted, is centrifugalised and the volume of corpuscles read off on a scale. The coloured blood-corpuscles. — The coloured corpuscles are com- posed of a delicate colourless highly elastic (? protoplasmic) envelope, and coloured fluid contents, consisting mainly of a solution of hajmoglobin. >>'" r ^ V '•■ ';}-■ ■■'■■ 'i. >' ^■^-:^ Fig. 32. — Human bed blood-coepuscles : Photograph magnified 650 diameters. The existence of such an envelope is shown by the osmotic effect of water upon the corpuscle, which passing in through the envelope, distends, and eventually bursts the corpuscle and sets free the con- tents. The description which is current in many text-books that the red corpuscles consist of a porous solid stroma, permeated with dissolved hasmoglobin, is incompatible with this and similar reactions. Moreover, the envelope can be distinctly seen with the microscope, especially in the amphibian corpuscle, and can be stained by reagents. The envelope contains lecithin and cholesterin in considerable amount, and these substances impart a certain greasiness to the surface of the corpuscle. It is in all probability due to such greasiness that 32 THE ESSENTIALS OF HISTOLOGY. the corpuscles run together into rouleaux when the blood comes to rest (see p. 43). Under the microscope blood is seen to consist of a clear fluid {plasma), in which are suspended the blood-corpuscles (fig. 32). The latter are of two kinds : the red or coloured (erythrocytes), which are by far the most numerous, and the white, pale, or colourless (leucocytes). In addition to these more obvious corpuscles, blood contains a variable number of minute particles which were termed by Zimmer- mann the elementary particles of the blood, but which are now more usually known as the blood-platelets on account of their flattened form. Erythrocytes. — When seen singly the coloured corpuscles are not distinctly red, but appear of a reddish-yellow tinge. In the blood of man and of all other mammals, exc'ept the Camelidae, they are biconcave circular disks. Their central part usually has a lightly shaded aspect under a moderately high power, but this is due to their biconcave shape, not to the presence of a nucleus. They have, as just stated, a strong tendency to become aggregated into rouleaux and clumps when the blood is at rest, but if it is disturbed they readily become separated. If the density of the plasma is increased in any way, as by evapora- tion, many of the red corpuscles become shrunken and crenated by the passage of water out of the corpuscle. On the other hand, a diminution in the density of the plasma tends to cause the red corpuscles to become cup-shaped, but it is erroneous to describe this as the normal form of the corpuscle. The average diameter of the human red corpuscle is 0'0075 milli- meteri (about ^-^tj inch), but a few will always be found somewhat larger (0'0085) and a few somewhat smaller (0'0065 mm.).^ There are from four to five millions of coloured corpuscles in a cubic millimeter of blood. Leucocytes. — The colourless corpuscles of human blood are proto- plasmic cells, averaging 0-01 mm. (^-^ inch) in diameter when spheroidal, but they vary much in size. They are far fewer than the coloured corpuscles, usually numbering not more than eight to ten thousand in a cubic millimeter (about 1 to 600 red corpuscles). Moreover, they are specifically lighter, and tend to come to the surface of the preparation. If examined immediately the blood is drawn, they are spherical in shape, but soon become flattened and ^Also expressed as 7 '5 m or mioromillimeters ; a mioromillimeter being xiFinr millimeter. ,2 The following list gives the diameter in parts of a millimeter of the red blood- corpuscles of some of the common domestic animals : — Doe, 0"0073 ■ rabbit 0-0069 ; oat, 0-0065 ; goat, 0-0041. LEUCOCYTES. 33 then irregular in form (fig. 33), and their outline continually alters, owing to the amoeba-like changes to which they are subject. In some kinds (phagocytes) the protoplasm tends to take in foreign particles with which the cells come in contact ; in others there seems to be little or no such tendency. Some of the colourless corpuscles are very pale and filled with fine granules, others contain coarser and more distinct granules in their protoplasm ; others again have a hyaline protoplasm without any apparent granules. In some corpuscles (lymphocytes) the protoplasm forms only a relatively thin coating to the nucleus. The corpuscles are classified according to the character and appearance of the nucleus and the nature and staining qualities of the granules in Fig. 33. — Three .\m(eboid white coepuscles of the newt, killed by instantaneods application of steam. d, eosinophil cell ; &, c, polymorphous cells. The nuclei appear multiple, hut are seen to be connected with fine filaments of nuclear substance ti-aversing the protoplasm. the protoplasm. Thus some granules are readily stained by basic dyes such as methylene blue, and such granules are accordingly termed basophil. Distinct coarse basophil granules are, however, rare in normal blood, although cells with these granules are normally present in the marrow and in some connective tissues, and make their appear- ance in the blood in leucocythsemia. On the other hand, some granules more readily take up colour from acid dyes, such as eosin, and these have been termed oxyphil or eosinophil. Other cells possess granules (amphophil) which are stained by both acid and basic dyes ; and others chiefly by neutral dyes (neutrophil). In some cells more than one kind of granule is met with. The protoplasm may also contain clear spaces or vacuoles. Each leucocyte has at least one nucleus, which is difl&cult to see in a fresh preparation, but is easily seen after the action of most reagents and after staining. There is also a centrosome with attraction-sphere, but special methods of staining are necessary to exhibit these. (See fig. 9, p. 7.) The following are the chief varieties of leucocytes : — 1. Polymorphs. Cells with lobed or multipartite nuclei and a relatively large amount of protoplasm, which is highly amoeboid (fig. 33, b and c). These are often termed multi-(poly-)nuclear, but the nucleus is rarely if ever multiple, C 34 THE ESSENTIALS OF HISTOLOGY. its several parts being nearly always joine4 by threads of nuclear substance. . The cells in question vary in size, but when spherical are usually not quite 0-01 mm. in diameter. Their protoplasm stains with eosin, this being due to the presence of fine oxyphil granules (Kanthack and Hardy). They are highly amoeboid and phagocytixj,. and constitute from sixty to seventy per cent, of all the leucocytes of the blood (fig. 34, a). Fig. 34.— VAjaious kinds of coloueless oobpuscles, showino the dikfebent . OHAKACTEBS OF THE GEANULES. (From a, film prepfiration of normal human blood. ) Two of each kind are represented. 2. Lymphocytes. — These are small cells, with a very limited amount of clear protoplasm around the nucleus, which is simple, not lobed or divided (fig. 34, b). The amoeboid phenomena are less marked in them than in the other varieties of leucocytes. The protoplasm stains with methylene blue. They are about 0-0065 mm. in diameter, but some are larger and appear to be transitionaJ between this and the next variety. They constitute from fifteen to thirty per cent, of the total number of leucocytes in the blood. They are relatively more numerous in infancy. 3. Macrocytes. — Large uninucleated cells similar to the last, but larger,, and containing much more protoplasm (fig. 34, e). Some, however, are smaller and are regarded as transitional forms from the last variety. The nucleus may be spherical, oval, or kidney-shaped. The protoplasm is hyaline; it stains slightly with methylene blue, perhaps owing to very fine basophil granules. These cells are highly amoeboid and phagocytic. Including the transitional forms, they constitute about five per cent, of all the leucocytes in blood. 4. Eosinophils. — These are characterised by their coarse granules which stain deeply with acid dyes, such as eosin. Their average LEUC0C3fTES. 35 diameter in the spherical condition is O'Ol mm. The nucleus may be simple or lobed (fig. 34, d; fig. 33, a). They are amoeboid, but less actively so than the finely granular cells. They are more variable in number than the other varieties, constitutingsometimes not more than one per cent., and at other times as much as ten per cent, of the total leucocytes of blood. 5. Basophils. — These are rarely if ever found in normal blood (adult), but occur in children and in certain pathological conditions affecting the bone marrow. Blood-platelets. — In the clear fluid in which the blood-corpuscles are suspended, a network of fine straight intercrossing filaments (fibrin) Fig. 35. — Netwokk op fibein, shown aftkk washing away the cokpusclbs prom a pbepakation op blood that has been allowed to clot ; many op the i'llaments eadiatk prom small clumps of blood-platelets. Fig. 36. — Blood-corpuscles and elemen- tary PARTICLES OB BLOOD-PLATELETS, WITHIN A SMALL VEIN. (From Oaler:) Fig. 38.— Blood-platblets, highly . magnified, showing the amceboid FORMS which they ASSUME WHEN EXAMINED UNDER SUITABLE CONDI- TIONS, AND ALSO EXHIBITING THE CHROMATIC PARTICLE WHICH EACH PLATELET CONTAINS, AND WHICH HAS BEEN RBGAEDED AS A NUCLEUS. (After Fig. 37.— a mass of blood-pl,vtelets, Kopsch.) FROM HUMA.V BLOOD. (Osler. ) A few at the edge are detacbed from the rest. The preparation had been kept in salt solu- tion on the warm stage for some time, thus causing a partial breaking up of the mass of platelets.- These will be observed to have filaments attached to them. soon makes its appearance (fig. 35). These often seem to radiate from minute round colourless discoid particles less than one-third the diameter of a red corpuscle, either separate or collected into groups or masses, of variable, sometimes of considerable, size. These are the elementary particles, blood-platelets, or thrombocytes. In the blood-vessels they are 36 THE ESSENTIALS OF HISTOLOGY. discrete but immediately clump together in drawn blood (fig. 37). If, however, the blood is examined on agar jelly containing certain salts in definite proportions, the platelets can be kept separate, and may then be submitted to very high powers of the microscope. The result of such examination seems to show that the blood-platelets are not mere inert particles, as has generally been supposed, but that they are protoplasmic and amoeboid, and that each one contains a nucleus (fig. 38), that they are in fact minute cells (Deetjen). Blood platelets vary greatly in number : they are estimated by Brodie and Russell to amount to from 5 millions to 45 millions in the cubic centimeter of blood. Fatty particles, derived from the chyle, may also occur in the plasma. Fig. 39. — DKVBLDPMENT of blood-vessels and ELOOD-OOEPnSOLES IN THE VASCDLAK AREA OE THE GUINEA-PIG. bl, blood-corpuscles becoming free in the interior of a nucleated protoplaBmlc mass. Development of red blood-corpuscles. — In the embryo, the first-formed coloured blood-corpuscles are amoeboid nucleated cells, the protoplasm of which contains haemoglobin. These embryonic blood-corpuscles are developed within cells of the mesoderm (mesenchyme), which are united with one another to form a syncytium (fig. 39). The nuclei- of the cells multiply, and around some of them there occurs an aggregation of coloured protoplasm. Finally the network becomes hollowed out by an accumulation of fluid in the syncytial protoplasm, and thus are produced a number of capillary blood-vessels, within which the coloured nucleated portions of protoplasm are set free as embryonic blood-corpuscles (erythroblasts, fig. 39, bl). Within the circulation these multiply by mitotic division, and thus becpme. rapidly more numerous. In later embryonic life, nucleated coloured corpuscles disappear from mammalian blood, and are replaced by the usual discoid corpuscles. Many of these are doubtless derived from the nucleated embryonic DEVELOPMENT OF BLOOD-COEPUSCLES. 37 blood-cells, the absence of the nucleus being accounted for either by its atrophy or extrusion from the cell or by the separation of a part of the coloured cell-substance. The foetal liver has been supposed to be one of the places of formation of red blood-corpuscles. Erythrocytes are also formed at a somewhat later stage of development within certain cells of the connective tissue {vasoformative cells), a portion of the substance of the cell becoming coloured by hsemoglobin, and separated into globular particles (fig. 40, a, h, c), which are gradually moulded into disk-shaped red corpuscles. In the meantime the cells Fig 40.— Blood-corpuscles developing within conneotive-tissue cells. a, a coll containing diffused hsemoglobin ; 6, a cell filled with coloured globules ; c, a cell containing coloured globules in the protoplasm, within which also are numerous vacuoles ; d, an elongated cell with a cavity in its protoplasm occupied by fluid and blood-corpuscles mostly globular ; e, a hollow cell, the nucleus of which has multi- plied. The new nuclei are arranged around the wall of the cavity, the corpuscles in which have now become discoid ; /, shows the mode of union of a ' hEemapoietic 'cell, which in this instance contains only one corpuscle, with the prolongation (bl) of a previously existing vessel. become hollowed out, and join with similar neighbouring cells to form blood-vessels (fig. 40, d, e, /). The process is therefore the same as in the early embryo, except that cell-nuclei are not included in the ha&moglobin-holding protoplasm.^ ^It has been suggested by some writers that the vasoformative cells con- taining coloured corpuscles in various stages of formation are in reality portions of an already formed vascular network which is undergoing atrophy ; and that the cor|)uacIes within such cells are not in process of formation but of disappearance. But since the appearances in question are seen in parts in which vascular tissues (such as fat) are undergoing not atrophy but formation ; and since, moreover, the hEematoidiu crystals and pigment granules which are character- istic of the disintegration of erythrocytes within cells are not present, it seems more reasonable to interpret the appearances as indicative of intracellular de- velopment of blood-corpuscles by differentiation of part of the protoplasm of the vasoformative mesenchyme cell, rather than a degeneration of already formed blood-vessels and blood-corpuscles. 38 THE ESSENTIALS OF HISTOLOGY. Formation in bone-marrow.— Although no nucleated coloured cor- puscles (erythroblasts) are as a rule to be seen in the blood in post- embryonic life, they are found in the marrow of the bones, and in some animals also found in the spleen. They vary in size, most measuring about -007 mm. (normoblasts), but some being considerably larger (megaloblasts), and others considerably smaller (microblasts). It is. probable that the red disks are formed from these nucleated red corpuscles of the marrow by the nucleus disappeaiing and the coloured - # * b m me' m' r" Fig 41. — Ebd mabbow of young babbit. Magnified 450 diameters. e, erythrocytes ; el, erythroblasts ; e", an erythroblast undergoing mitotic division ; I, a polymorph leucocyte ; m, ordinary myelocytes ; m', myelocytes undergoing mitotic division ; eo, an eosinophil myolooyte ; 6, a basophil myelocyte ; meg, a giant-cell or megakaryocyte. protoplasm becoming moulded into a discoid shape. At what time this formation of blood-corpuscles in the bone-marrow begins has not been ascertained, but after it has commenced it continues throughout the whole of life — the red marrow, especially that of the ribs, being especially active in this respect. In mammals the formation' of nucleated coloured corpuscles appears to take place within the tissue of the marrow external to the blood-vessels. It is uncertain, to what extent the capillary vessels of the marrow are limited by a complete endothelium (see p. 40), but in any case, the formed erythroblasts seem to readily pass into the blood stream.^ ■ . ^In Ijirds the erythroblasts are confined to. the large blood-chaimels of the marrow, and the transformation into erythrocytes occurs within theseHlhalinels. MARROW OF BONE. 39 The marrow of bone is of a yellow colour in the shafts of the long bones of most animals, and is there largely composed of adipose tissue, but in the shafts of the long bones of some animals, and in the cancellated tissue of most, it is usually red, the colour being partly due to the large amount of blood in its vessels. This red marrow (fig. 41) is chiefly composed of spherical cells — the myelocytes or marrow-cells — which resemble rather large blood-leucocytes, and, like these, are amoeboid. They also exhibit the same kind of dififerences as to the character of the granules which they contain, some being oxyphil and others amphophil or neutrophil. But while the blood-leucocytes rarely contain any coarse basophil granules, some p q r s t m n op Fig. 42.— Cells of the bed markow oi? the gdinea-pis. (Highly magnified.) a, a large cell, the nucleus of which appears to be partly divided into three by constric- tions ; h, a cell, the enlarged nucleus of which shows an appearance o£ being con- stricted into a number of smaller nuclei ; c, a ^o-oalled giant-cell or inyeloplaxe with many nuclei ; d, a smaller myeloplaxe with three nuclei ; e-i, proper cells of the marrow; j-t, various forms of coloured nucleated cells (erythroblasts), some in prooesB of division ; in others the nucleus appears to be undergoing atrophy. of the marrow-cells contain these in considerable numbers. There are also to be seen mingled with the marrow-leucocytes a number of corpuscles somewhat smaller in size, nucleated, and at least some of them amcEboid, but of a reddish tint (fig. 41, e'). These cells, which are termed erythroblasts, resemble the nucleated coloured blood-corpuscles of the embryo, and are believed to be cells from which the coloured blood-disks become developed. Many of them are in process of mitotic division. Others are seen, with the nucleus in a more or less atrophied condition (fig. 42, h) ; from this it may perhaps be inferred that the transformation into a discoid blood-corpuscle is accompanied by, the disappearance of the nucleus (Bizzozero). Lastly, the marrow 40 THE ESSENTIALS OF HISTOLOGY. contains a number of very large cells, the gicmt-cells or myeloplaxes of Robin (fig. 41, meg: fig. 42, a.-d). These are especially numerous wherever bone is becoming absorbed, but are not confined to such situations, being indeed normal constituents of marrow. Sometimes they possess several nuclei, but most — the so-called megakwryocytes^ contain but one large nucleus, which has usually an annular form. They are also characterised by possessing a number of centrioles grouped together near the nucleus. Lastly, the existence of cells within the marrow containing blood-corpuscles in various stages of transformation into pigment, similar to those which occur in the spleen-pulp, has been noted (Osier). The marrow is very vascular, the capillaries and veins being large and thin-walled; indeed, according to some authorities, the walls of the capillaries are imperfect, so that there is an open communication between them and the interstices of the tissue, and in this way it is supposed that the coloured blood-disks, which are, it is believed, produced from the coloured nucleated cells (erythroblasts) of the marrow, may get into the circulation. There is not, however, an interstitial circulation of blood in the marrow such as is found in the spleen, nor does injection material such as carmine gelatine pass into the interspaces of the tissue, but remains confined to the vessels, so that the existence of an open communication is doubtful. Development of white corpuscles. — The white blood- and lymph- corpuscles occur originally as free cells, which are believed to find their way into the vessels from the circumjacent mesoderm. They do not occur within the first-formed blood-vessels of the embryo nor within the vasoformative cells. In later stages of foetal life and during the whole of post-embryonic life they become formed in the bone-marrow as well as in lymph-glands and other organs composed of lymphoid tissue, and pass from these directly into the lymphatics and into the blood. It is probable, but has not been ascertained with certainty, that the lymphocytes are all produced in lymph-glands and other lymphoid tissues, and that the macrocytes are formed by enlargement of the lymphocytes. On the other hand, the polymorphs and the coarsely granular oxyphil cells are believed to be formed within the bone marrow, which contains cells of similar character. Cells with well- marked basophil granules are also met with in bone marrow, and sometimes, in abnormal conditions, pass in large numbers into the blood. HOMAN BLOOD-COEPUSCLES. 41 LESSON IV. ACTION OF REAGENTS UPON THE HUMAN BLOOD- CORPUSCLES. 1. Make a preparation of human blood, and apply a drop of water, at one edge of the cover-glass. Examine at a place where the two fluids are becoming mixed. Notice particularly the first effect of water upon both red and white corpuscles, as well as the ultimate action. Sketch both kinds of corpuscles under the action of water. 2. Eepeat on another preparation, using very dilute alkali (0'2 per cent, caustic potash) instead of water. Notice the complete solution first of the white and then of the coloured corpuscles as the alkali reaches them. 3. Eepeat on another preparation, using dilute acetic acid (1 per cent.). Observe that the effect of the acid upon the coloured corpuscles is similar to that of water, but that it has a different action upon the colourless corpuscles. Sketch two or three of the latter after the action is completed. 4. Make a preparation of blood mixed with salt solution, as in Lesson II. 2, and investigate the action of tannic acid (1 part tannic acid to 100 of distilled water) in the same way. Sketch two or three coloured corpuscles after the action is complete. 5. Examine blood-crystals of rat, guinea-pig, and squirrel. Preparations of hsemoglobin crystals cannot be kept permanently. 6. Prepare haemin by heating a dry smear of blood on a slide with glacial acetic acid. The crystals of hseniin are permanent. Structure of erythrocytes. — The action of reagents upon the human red blood-corpuscles shows that, although to all appearance homogeneous, they in reality consist of an external envelope of colourless material a i c d e which forms a thin film inclosing the dis- f| p-) W '^ji ' * solved colouring matter or hmmoglobin. Thus, when water reaches the corpuscles, it passes > i,-|^ ^_^ through the film and swells the corpuscle, •' \^ ^V_J^ pausing it to become globular ; eventually Fig. 43. the film is burst through, and the colouring "-\^","e1'rA.f-it°/refl:cro°£ matter escapes into the serum. The addition Snn"o " cid. °''" ' "' ^"°''' °* of hyperisptonic solution of salt, on the other hand, by increasing the density of the fluid in which the corpuscles float, causes diff'usion of water out of the corpuscle, and consequent shrinking and corrugation of the surface, the crenated form 42 THE ESSENTIALS OF HISTOLOGY. (fig. 43,/) being thereby produced. The same change is brought about by evaporation of water, if the blood is exposed to air. The separation of hsemoglobin from the corpuscle can be effected not only by water (fig. 43, Ore), but also by dilute acids, by the action of heat (60° C), the freezing and thawing of blood, the action of ether or chloroform, and the passage of electric shocks. Bile and dilute alkalies rapidly cause the red corpuscles to become spherical and then almost instantly eflfect their complete solution (haimolysis). The mixing of blood from one species of animal with the blood or serum of animals of other species frequently also has a similar eflFect. In this case the haemolytic Fig. 44. — Blood crystals, magnified. 1, from human blood ; 2, from tlie guiaca-pig ; 3, squirrel ; 4, hamster. action is exerted by some constituent (heemolysin) of the foreign blood, which is special for each species and against which the " host " can render itself immune if, prior to any large quantity of the foreign blood or serum being injected, successive small injections be made; an " antihsemolysin " being gradually produced. This fact is not only of interest as bearing upon the general doctrine of immunity, but also serves to detect the source of a given sample of blood. Tannic acid produces a peculiar effect upon the red corpuscles (fig. iS, g); the haemoglobin is discharged from the corpuscle, but is im- mediately altered and precipitated, remaining adherent to the envelope in the form of a round or irregular globule of a brownish tinge (hsematin ?). Some of these reactions occur by a process of osmosis as in the case of water, but in others a solution of the envelope of the corpuscle is produced HUMAN BLOOD-COEPUSCLES. 43 by the reagent, and the hsemoglobin js thus allowed to escape. The film or envelope is probably composed of protoplasm containing, besides nueleo- proteids, lecithin and cholesterin (myelin), and these are substances which possess many of the physical properties of fats, although of a different chemical composition. If we assume that such fatty substances form an external film to the corpuscle, the running of the red disks into rouleaux can readily be explained, since it has been shown by Norris that disks of any material, e.g. cork, suspended in a fluid, tend in the same way to adhere in rouleaux, provided their surfaces are covered with a layer which is not wetted by the fluid. We may also explain on the same hypothesis the fact that no rent is ever seen in the envelopes of the red corpuscles even when they appear to have burst after imbibition of water, for, if the film which represents an envelope is myelinic in nature, any rent in it would tend immediately to close up again when the opposed edges come in contact. It was also shown by Norris that droplets of fluid encompassed by myelin have a tendency to assume a flattened shape. -^f J [ x*-? Fig. 45.— Hjbmin crystals, magnified. Fig. 46. — H^ematoidin crystals. (Preyer.) (Frey.)' The more solid part of the red corpuscle is often termed the stroma,, but this name rests upon an entirely false conception of the structure of the corpuscle. In adopting the name, it was supposed that the corpuscle is formed of a homogeneous porous material (stroma — EoUett), in the pores of which the hsemoglobin is contained, but there is no reasonable foundation for this belief, which fails to explain, except on the assumption of a still more complex hypothesis, the well-known osmotic phenomena of the corpuscle ; whereas the supposition that there exists a delicate external film or envelope inclosing a coloured fluid is in accordance with all the known facts regarding the action of reagents upon these bodies. It is true that in the fresh mammalian corpuscle the envelope is too delicate to be actually observed in the optical section of the corpuscle, but in the blood-corpuscles of amphibia it can be quite distinctly seen, and with any slight increase in density of the plasma it tends to become wrinkled and the creases in it are plainly visible. In these corpuscles also the nucleus becomes readily displaced in drawn blood from its position in the centre of the corpuscle and may lie quite at the side ; this is a clear indication of the fluid nature of the contents of the corpuscle, and by analogy we may fairly assume a similar constitution for the mammalian corpuscle. Lastly, it is possible to stain the envelope of the red corpuscles of a diiferent colour from the remainder of the corpuscle. Blood-crystals — Hsemoglobin. — In the blood of many animals (fig. 44), crystals of hsemoglobin readily form after its separation from the red 44 , THE ESSENTIALS OF HISTOLOGY. corpuscles. These crystals are rhombic prisms in man and most animals, e.g. the rat, but tetrahedra in the guinea-pig, and hexagonal plates in the s(^uirrel. In these animals they at once appear on shaking up the blood with chloroform or ether, or even on the addition of water, with or without subsequent evaporation. Haemin. — This name has been applied to the minute dark-brown rhombic crystals of hydrochlorate of hsematin (fig. 45), which are formed when dried blood from any source whatever is heated with glacial acetic acid. HsBmatoidin. — This occurs in the form of brownish yellow crystals (fig. 46). It is found in old blood extravasations and in other places where blood- corpuscles are undergoing disintegration within the tissues. Action of reagents on leucocytes. — The structure of the colourless corpuscles is also brought out by the action of some of the reagents above noticed. As the water reaches them their amoeboid movements cease; they become swollen out into a globular form by imbibition Fis. 47. 1, first effect of the action of water upon a white blood-corpuscle ; 2, 3, white corpuscles treated with dilute acetic acid ; n, nucleus. of fluid (fig. 47, 1), and the granules within the protoplasm can be seen to be in active Brownian motion. Their nuclei also become clear and globular, and are more - conspicuous than before. With the further action of the water, the corpuscle bursts and the granules are set free. Acids have an entirely different action upon the white corpuscles. Their nuclei become somewhat shrunken and very distinct (fig. 47, 2 and -3), and a granular precipitate is formed in the protoplasm around the nucleus. At the same time, a part of the protoplasni generally swells out so as to form a clear bleb-like expansion (ah appearance which also often accompanies the death of the corpuscle from other causes). Dilute caustic alkalies rapidly cause the complete destruction of the white corpuscles. BLOOD-CORPUSCLES OF AMPHIBIA. 45 LESSON V. THE BLOOD-CORPUSCLES OF AMPHIBIA. 1. Obtain a drop of frog's, toad's or newt's blood, and mix it with a very- small quantity of salt solution upon a slide. Examine with the high power. Notice the shape of the coloured corpuscles both when seen flat and edge- ways, and the nucleus within each. Measure ten corpuscles (long and short diameters), and from the results obtain the average dimensions of a corpuscle. Notice also the colourless corpuscles, smaller than the red, but larger than the pale corpuscles of human blood, although otherwise generally resembling these. Sketch two or three red corpuscles and as many white. Be careful not to mistake the rounded liberated nuclei of crushed red corpuscles for pale corpuscles. Enormous cells and nuclei belonging to the cutaneous glands as well as the granular secretion of those glands may be present in this preparation if it is obtained from the newt's tail. 2. Apply a drop of water to the edge of the cover-glass of the same preparation and notice its action upon the corpuscles. Sketch two or three corpuscles altered by the action of the water. 3. Mount another drop of blood, and apply dilute acetic acid (1 per cent.) instead of water at the edge of the cover-glass. Make sketches showing the effect of the acid upon both red and white corpuscles. 4. Examine the corpuscles of newt's blood which has been allowed to flow into boric acid solution (2 per cent.). Notice the effect produced upon the coloured corpuscles. Sketch one or two. 5. Mount drops of glycerine-jelly containing (a) frog's blood and (6) bird's blood, previously flxed by Flemming's solution and stained with piero- carmine. 6. Make a film preparation of. amphibian or avian blood as described on P; 28, § 5. The coloured blood-corpuscles of amphibia (fig. 48), as well as of nearly all vertebrates below mammals, are biconvex elliptical disks, considerably larger than the biconcave circular disks of mammals. ^ In addition to the coloured body of the corpuscle, which consists, as in mammals, of hsemoglobin inclosed within an envelope, there ' The following are the dimensions in parts of a millimeter of the coloured oorpuscles of some oviparous vertebrates : — Long diameter. Short diameter Pigeon, 00147 00065 Frog, 00223 0-0157 Newt, 0-0293 0-0195 Proteus, 0-0580 0-0350 Amphiuma, 00770 0460 46 THE ESSENTIALS OF HISTOLOGY. is a colourless nucleus, also of an elliptical shape, but easily becoming globular, especially if liberated by any means from the corpuscle. The nucleus resembles that of other cells in structure, being bounded by a membrane and having a network of chromatin. It is not very I w y Fig. 48. — Amphibian erythkocytes. (From photographs.) Magnified 4.50 diameters. A, from the frog. B, from the toad. distinct in the unaltered corpuscle, but is brought clearly into view by the action of reagents, especially acids. The action of reagents upon the rod corpuscle of amphibia is otherwise similar to that upon the mammalian corpuscle, water and acetic acid causing it to swell into a globular form and then to become decolorised ; solution of salt causing wrinkling of the envelope, and so on. As a first effect, water and certain other fluids may cause the haemoglobin to retire from the envelope at the points where the fluid is passing through the membrane : a stellate appearance is thereby often pro- duced (Hiinefeldt, Hensen). Boric acid causes the hfemoglobin of the newt's corpuscle to become partially or wholly collected around the nucleus, which may then be extruded from the corpuscle (Briicke). Immediately within the envelope, at the periphery of the amphibian erythrocyte, is a band of fine fibrils which are stained by gentian violet (Meves) and can also be seen cut across in sections of the corpuscles (Bryce). The colourless corpuscles of amphibia, although larger, are very similar to those of mammals. Like them, they are either wholly pale and finely granular or inclose a number of very BLOOD-COEPUSCLES OF AMPHIBIA. 47 distinct granules of similar nature to those met with in mammals. These corpuscles vary much in size and in the activity of their amoeboid movements : those which have a multilobular nucleus (fig. 33, b, c) are usually the most active. Reagents have the same effect upon them as on those of mammals. The presence of glycogen may be demonstrated in them by its reaction with iodine (port-wine colour). The 'blood-platelets (thrombocytes) in the frog are fewer in number than in mammals. Many are of a spindle shape. They contain a nucleus-like body and like the blood-platelets of mammals they show amoeboid changes and tend rapidly to clump together in drawn blood. 48 THE ESSENTIALS OF HISTOLOGY. LESSON VI. THE AMBOEBOID PHENOMENA OF THE COLOURLESS BLOOD-CORPUSCLES. 1. Make a preparation of blood from the finger in the usual way. Draw a brush just moistened with perfectly neutral oil around the edge of the cover- glass to check evaporation. Place the preparation upon a ' warm stage,' and heat this to about the temperature of the body (38° C). Bring a white corpuscle under observation with the high power, and watch the changes of shape which it undergoes. To become convinced of these alterations in form, make a series of outline sketches of the same corpuscle at intervals of a minute. Fig. 49.— Simple wabming apparatdb, complete, shown in operation. The simplest form of warm stage is a copper plate of about the size of an ordinary slide, perforated in the centre and with a long tongue of the same metal projecting from the middle of one edge (fig. 49). The copper plate rests upon the stage of the microscope, with a piece of cloth or other non- conducting material between. The preparation is made upon an ordinary slide or on a large cover-glass, which is placed upon the warm stage and AMOEBOID PHENOMENA OE COLOUELESS CORPUSCLES. 49 pressed into contact with it by the brass clips. Heat is applied to the copper tongue by a small spirit-lamp flame, and a greater or less amount is con- ducted to the warm stage and the superjacent preparation according to the point to which the flame is applied. To ascertain that the right temperature is got and maintained, put two pieces of paraffin, one melting at 35° C. {95° F.) and another at 38° C. (100° F.), on either side of the preparation. The temperature must be such that the first piece is melted and remains so whilst the second remains uumelted.' 2. Mount a drop of frog's or newt's blood diluted with an equal amount of salt solution, and examine it in the same manner upon the copper stage, at first cold, afterwards warm ; the temperature must, however, be kept below 30° C. Observe the effect of beat in acolerating the amoeboid movements of the pale corpuscles. Sketch one at intervals of a minute (a) in the cold, (6) whilst warmed. 3. Take some yeast which has been mixed with salt solution, and mix a very little of the yeast and salt solution with a fresh drop of newt's blood, slightly oiling the edge of the cover-glass as before. Endeavour to observe the inception of the yeast-torulse by the white corpuscles. Sketch one or two corpuscles containing torulse. Milk-globules or particles of carbon or of vermilion may also be used for this experiment, but the process of inception or 'feeding' is most readily observed with the yeast particles. 4. At the beginning of the lesson collect a drop of newt's or frog's blood into a fine capillary tube, seal the ends of the tube, and mount it in a drop of oil of cedar-wood or dammar varnish (or the clot may be blown out into a drop of salt solution on a slide and mounted in this solution). Towards the end of the lesson examine it to see white corpuscles emigrating from the shrunken clot (see fi.g. 50). 5. To obtain a specimen with the white corpuscles fixed in amoeboid con- dition, make a preparation of newt's blood, mixed with salt solution, and set it aside for ten minutes. By this time the corpuscles will be freely amoeboid, and will probably show well-marked pseudopodia. To fix them in this condition let a jet of steam from a flask or kettle play for two or three seconds upon the cover-glass. The heat instantly kills the corpuscles, and they are fixed in the form they presented at the moment the steam was applied. They may now be stained by passing dilute hsemalum^ under the cover-glass, or by removing the latter and staining with eosin and methylene blue in the manner recommended on p. 28, § 5. If hsemalum is used, the stain is followed by dilute glycerine, after which the cover may be cemented and the preparation kept. The amoeboid phenomena which are exhibited . by the protoplasm of the colourless blood-corpuscles consist of spontaneous changes of form, produced by the throwing out of processes or pseudopodia in various directions. When first thrown out the pseudopodia are quite clear; they appear to be produced by a flowing of the hyaloplasm (see p, 4). If the corpuscle is stimulated, either mechanically, as by ■"For exact work, an apparatus somewhat more complex than the above is required. For description of such, see A Course of Practical Histology. "Delafield's orEhrlich'shjematoxylin can be substituted for hsemalum wherever the latter is mentioned. The water used for the dilution of haematoxylin solu- tions must always be distilled. D 50 THE ESSENTIALS OF HISTOLOGY. tapping the cover-glass, or electrically, the pseudopodia are retracted, the corpuscle becoming spherical. A change of form caused by the protrusion of the pseudopodia may, when active, be followed by changes in place or actual locomotion (migration) of the corpuscle. When a pseudopodium, or the external surface of the corpuscle, comes in contact with any foreign particle, the protoplasm tends to flow round and enwrap the particle, which is then drawn into the corpuscle ; particles thus incepted may be conveyed by the corpuscle in its movements from one place to another (fig. 51). This property plays an important part in many physiological and pathological pro- cesses; thus cells in the spleen resembling large leucocytes— the Fig. 50. — 'White coepusclbs of frog's blood migrating prom. shrunken CLOT WITHIN A CAPILLARY TIIBE. (From Sanderson's Handbook for the Physiological Laboratory.) so-called sfplenie cells — incept blood-corpuscles, which become broken down within them; and pathogenic bacteria become taken into the protoplasm of certain leucocytes (on this account termed phagocytes)', there to be destroyed (Metchnikofi). The phagocytic properties of the leucocytes become especially developed as the result of the action upon the bacteria of certain chemicEjl substances which are present to a greater or less extent in blood and which are termed opsonins (Wright). It is probable that particles of organic matter which are taken up by the pale corpuscles may undergo some slow process of intracellular digestion within their protoplasm., AMCEBOID PHENOMENA OF COLOUELESS CORPUSCLES. 51 The processes of the granular corpuscles are quite clear at first; the granules afterwards flow into them. The migration of the colourless corpuscles from the blood-vessels into the surrounding tissues (which especially occurs in inflamed parts), or from a blood-clot into the surrounding serum (fig. 50), is due to these amoeboid properties. The conditions which are favourable to this amoeboid activity of the white corpuscles are (1) the natural slightly alkaline medium, such as plasma, serum, or lymph, or faintly alkaline normal saline solution. Any increase of density of the medium produces a diminu- tion of amoeboid activity, whilst, on the other hand, a slight decrease in its density has the opposite efi'ect; (2) a certain temperature. In Fig. 51. — Cbanges of toem of a white blood-ookpuscle sketched at intekvals op a few minutes, showing the inception of two small gbanclk8 and the changes of position these hndebwent within the corpuscle. warm-blooded animals the phenomena cease below about 10° C. When gradually warmed the white corpuscles become more and more active up to a certain point, the maximum being a few degrees above the natural temperature of the blood. Above this point they become spheroidal and at a somewhat higher temperature their protoplasm is coagulated and killed. Acids at once kill the corpuscles and stop the movements. Narcotic gases and vapours, such as carbonic acid gas or chloroform vapour, also arrest the movement, but it recom- mences after a, time if their action is not too prolonged. 52 THE ESSENTIALS OF HISTOLOGY. LESSON VII. EPITHELIUM AND SECRETING GLANDS. 1. Mount a drop of saliva and examine first with a low, afterwards with a high power. Observe the nucleated epithelium-cells, some single, and others still adhering together by overlapping edges. Measure three or four, and also their nuclei. Sketch one or two on the fiat and one edgeways. Notice the salivary corpuscles, which are migrated white blood-corpuscles, swollen out by imbibition of water. The preparation may be stained with diluted hsemalum and preserved with glycerine. 2. Put a small shred of human epidermis into a drop of strong caustic potash solution (35 p.c.) for five minutes. Then break it up in water with needles, cover and examine. Observe the now isolated swollen cells. 3. Study the arrangement of the cells in a section through some stratified ^epithelium, such as that of the mouth, skin, or cornea.^ Notice the changes in .shape of the cells as they are traced towards the free surface. Measure the thickness of the epithelium. Count the number of layers of cells. 4. Make a preparation of the epithelium of the urinary bladder, which may be moderately distended with bichromate of potash solution (1 part to 800 of salt solution), and after an hour or two cut open and placed in more of the same solution. Take a small scraping of the lining epithelium on the point of a scalpel, and break it up by tapping it in a drop of very dilvte hsBmatoxylin on a slide. Put a small hair in the drop and cover. Add a small d/rop of glycerine at one edge : allow this to diffuse under. Cement next day. Observe the large flat superficial cells, and the pear-shaped cells of the second layer. Sketch one of each kind. The cells will vary greatly in appearance according to the amount of distension of the organ. 5. Study the minute structure of epithelium-cells and their nuclei, both at rest and dividing, in sections of the skin of the newt's tail, or in shreds of peritoneum or of epidermis or in sections of the salamander-tadpole. The preparation may, for this purpose, be stained either with hsematoxylin or iron-hsematoxylin, or with some aniline dye such as saffranin.^ Sketch an epithelium-cell with resting nucleus, and others with nuclei in different phases of mitosis. The simple saccular skin-glands of Amphibia may also be studied in these preparations. An epithelium is a tissue composed entirely of cells separated by a very small amount of intercellular substance (cement-substance) and generally arranged so as to form a membrane covering either an external or internal free surface. The structure of epithelium-cells, and the changes which they undergo in cell-division, are best seen in the epidermis of the newt ^ The methods of preparing and staining sections are given in the Appendix. EPITHELIUM. 53 or of the salainander-tadpole (fig.- 52); in the latter especially, the 6eils and nuclei are much larger than in mammals. Structiire of the cells. — Each epithelium-cell consists of protoplasm containing a nucleits. The protoplasm may either look granular, or it may have a reticulated appearance, or may exhibit fibrils. The nucleus is spherical or ovoid. Usually there is only one, but there may be two or more. The cell-substance is often modified in its chemical nature; its external layer may become hardened to form a [ jOiI Fig. 52. — Bpidbkmis cells of- a labval salamandbb. Magnified 400 diameters. CWilson. ) Three -of the cells are- undergoing division. The intercellular channels are bridged across by fine fibres.^ At one -place a, branched pigment; cell is. lying' between the epithelium cells. ■ , ., . - '' .'. sOrt of membrane, or the whole cell may become horny (keratinised) ; or there may be a separation of materials (granules) within the cell which are ultimately used by the organism, as in some secreting glands. Classification of epithelia. — Epithelia are somewhat illogically classified partly according to the shape and arrangement of the cells, partly according to their function. Thus we speak of scaZy or pavement, mihkal, columnar-, glwndular, and ciUated epithelium. Most of these are simple epithelia, with the cells only one layer deep. If forming several superposed layers, the epithelium is said to be stratified, and then the shape of the cells diifers in the difi'erent layers. Where there are only three or four layers in an epithelium, it is termed transitional. ..■--, ,.■ 54 THE ESSENTIALS OF HISTOLOGY. Stratified epithelium covers the anterior surface of the cornea, lines the mouth, pharnyx (lower part), gullet, anal canal and part of the urethra, and forms the epidermis which covers the skin. The vocal cords are also covered by stratified epithelium. In the female it also lines the vagina and covers the os uteri. The cells nearest the Fig. 53. — Section oif the stratified epithelium oovbrikg the front OF the cornea of the eye (man). c, lowermost columnar cells ; p, polygonal cells above these ; ^, flattened cells near the surface. Between the cells are seen intercellular channels bridged over by processes which pass from cell to cell. surface are always flattened and scale-like (fig. 6S,fl; fig. 54), whereas the deeper cells are polyhedral, and those of the deepest layer some- what columnar in shape (fig. 53, c). Moreover, the deep cells are soft and protoplasmic, and are separated from one another by a system of intercellular channels, which are bridged across by numerous fibres passing from cell to cell; giving the cells, when separated, the appearance of being beset with short spines {prickle-cells of Max Schultze). These ' bridging fibres ' are not peculiar to stratified epithelium, but occur in many if not in all kinds of epithelia. The deeper cells multiply by mitotic division, the nuclei first dividing in the manner already described. The ^fl™60 dfa^JeterB^™' ^^^^' ^^^^^ formed cells tend as they enlarge to push those external to them nearer to the surface, from which they are eventually thrown off. As they approach the surface they become hard and horny, and in the case of the epidermis entirely lose their cellular appearance, which can, however, be in a measure restored by the action of alkalies (§ 2). The cast-off superficial cells of the stratified epithelium of the mouth, which are seen in abundance in the saliva (§ 1), are less altered, and the remain? of a nucleus is still visible in them (fig. 54). The stratified epithelium of the human skin (epidermis) shows many peculiarities : these will be considered when the skin itself is treated of. Fig. 54. — Epithelium-scales from TRANSITIONAL EPITHELIUM. 55 Transitional epithelium is a stratified epithelium consisting of only- three or four layers of cells. It occurs in the urinary bladder, the ureter, and the pelvis of the kidney. The superficial cells (fig. 55, a) are large and flattened ; they often have two nuclei. Their free sur- face is covered with a cuticular stratum (Eggeling), and on their under surface they exhibit depressions, into which fit the larger ends of pyriform cells, which form the next layer (fig. 55, h). Between the tapered ends of the pyriform cells one or two layers of smaller polyhedral cells are found. The epithelium seems to be renewed by mitotic division of these deeper cells ; it is probable that the superficial cells also multiply, but in this case by amitosis. (Klein.) Fig, 55. — Epithelial cells from the bladder ov the babbit. (Magnified 500 diameters.) a, largo flattened ceU from the superficial layer, with two nuclei and with strongly marked ridges and intervening depressions on its under surface ; &, pear-shaped cell of the second layer adapted to a depression on one of the superficial cells. Simple scaly or pavement epithelium is found in the saccules of the lungs, in the ducts of the mammary glands, in the kidney (in the tubes of Henle, lining the capsules of the Malpighian bodies, and covering the glomeruli), and also lining the cavities of serous membranes (fig. 56), and the interior of the heart, blood-vessels, and lymphatics. When occurring on internal surfaces, such as those of the serous membranes, blood-vessels, and lymphatics, it is often spoken of as endothelium or mesotheUum. According to v. Brunn the cells of a serous epithelium may be provided with a striated border on their free surface, somewhat like that which is found on columnar cells. Columnar epithelium and ciliated epithelium are for the most part found covering the inner surface of mucous membranes; which are membranes moistened by mucus and lining passages in communication with the exterior, such as the alimentary canal and the respiratory and generative passages. The cells of a columnar epithelium form a single 56 THE ESSENTIALS OF HISTOLOGY. layer, varying in thickness according to the length of the constituent cells, and when the cells of a columnar epithelium are short, the epithelium is spoken of as cubical, such as that which lines the vesicles of the thyroid gland. Fig. 56. — Pavsmbnt epithelium or esdothelidm of a serous membrane. Nitrate op. silver preparation. Carmine staining m- nuclel Ciliated epithelium is found. in man throughout the whole extent of the air-passages and their prolongations, but not in the uppermost part of the nostrils which is supplied by the olfactory nerves, nor in the lower part of the pharynx, nor in the terminal bronchioles and pulmonary alveoli. It also occurs in the Fallopian tubes and the greater part of the uterus ; in the efferent tubes of the testicle ; and in the ventricles of the brain, and the central canal of the spinal cord. OtANDULAE EPITHELIUM AND SECRETING GLANDS. Glandular epithelium is the essential tissue of all the organs which are known as secreting- glands. Glands are of two chief kinds. Those which are best known and which are termed secreUng glands proper are furnished with a duct which ramifies in all parts of the gland and by means of which the products of the secretory activity of the gland-cells are brought to a free surface. ' Such glands have been developed- as involutions of the surface updn which they open, and their epithelium is continuous with that of this surface, and is in some oases, especially where the surface upon which the gland opens is covered with columnar GLANDULAR EPITHELIUM. 57 Fig. 57.— Various kinds of glands. I. Simple sacclilar 'gland from amphibian Skin (Flemming). 11^ Sim^ple tubular gland from intestine {Fl^mroing), III. A small racemose gland with a single duct into - which a number of irregularly tubular acini open (Flemming). IV. Part of a tubulo- . racemose gland with the acini unravelled (Flemming). V. Wax model of a small - tubulo-^raeemose gland' from the epiglottis (Maziarski). 58 THE ESSENTIALS OF HISTOLOGY. epithelium, of a similar character to the epithelium of the surface ; in others diflferent in character. In most glands the epithelium alters as we trace the duct back into the recesses or alveoli of the gland, and it is in these that the characteristic glandular cells, which are generally poly- hedral in shape, are found. Every such involution or ingrowth of epithelium to form a gland is, when first formed, of a simple character, Fig. 58." -Simple tubular glands sekn in a section of the mucous membra.ne or THE stomach of the kangaroo. e, epithelium of general surface ; bm, baBement membrane ; n, neck or duct of gland ; 6, base or fundus ; gl, glandular epithelium ; It^ lymphoid tissue ; mm, muscular tissue of the mucous membrane. shaped like a test-tube or flask and filled with a solid mass of cells, but it presently becomes hollowed out and the cells, are left as a lining to the connective tissue membrane which bounds the involution. The gland may remain simple and unbranched (simple sacculwr and simple tubular glands, fig. 57, I. and II.), or it may branch again and again until a complicated structure, in some cases small, in others of considerable size, is produced (compound tubular and compound saccular (or racemose) glands (fig, 57, III., IV., v.), instances of which are furnished by the kidneys and salivary glands respectively). The cells which furnish the secretion of the gland and which line the secreting parts of the tubules of a tubular SECRETING GLANDS. 59 gland, or the alveolar enlargements (acini) at the ends of the ducts of a racemose gland, are frequently partly or wholly filled with granules in the intervals of secretory activity, and these granules become discharged or dissolved and pass into t'he secretion during activity. Secreting glands are always abundantly supplied with blood-vessels and nerves. The former are distributed in the connective tissue which holds together the acini and groups of acini (lobules) of the gland ; the latter are supplied partly to the blood-vessels and partly ramify amongst the secretory epithelium cells. The liver differs from all other secreting glands in being composed of solid masses of cells (hepatic lobules) instead of tubular acini lined by epithelium. It exhibits also other important difiFerences in the nature of its blood-supply and the relation between the blood and the liver- cells. The other kind of secreting glands, known as the internally secreting glands, are not furnished with ducts and are usually described (along with the spleen and the lymphoid structures) as ductless gla/nds. The internally secreting glands are, like the externally secreting organs, composed of epithelial cells, sometimes grouped in solid masses {e.g. suprarenal gland), in other cases disposed around hollow vesicles {e.g. thyroid) which become filled with the material of the secretion. But as in these glands there is no duct the secretion is carried into the blood either directly by the blood-vessels of the gland or indirectly through the lymphatics. The detailed study of the glands and of other epithelial structures may be reserved until the organs in which they occur are described, but columnar and ciliated epithelia will be dealt with in the next Lesson. The hairs and nails and the enamel of the teeth are modified epithelial tissues. They will be described along with the skin and structures connected with the mouth respectively. 60 THE ESSENTIALS OF HISTOLOGY. LESSON VIII. COLUMNAR AND CILIATED EPITHELIUM: ACTION OF CILIA. 1. Break up in dilute glycerine a shred of epithelium from a minute piece of the mucous membrane of intestine (frog) that has been treated with 1 per cent, osmic acid for some hours, and has subsequently macerated in water for a few days. The cells easily separate on tapping the coverrglass. Measure and sketch one or two cells. The cover-glass may be at once fixed by gold size. 2 Prepare ciliated epithelium from a trachea that has been in chromic acid solution (1 to 2000 normal saline) for a few days, in the same way as with transitional epithelium (§ 4, p. 52). Measure in one or two of the cells (a) the length of the cells, (6) the length of the cilia, (c) the size of the nucleus. Sketch two or three cells. 3. Mount in sea-water one or two bars of the gill of the marine mussel (fig. 59). Study the action of the large cilia. Now place the preparation upon the copper warrai stage (see Lesson VI.) and observe the effect of raising the temperature. Fig. 59. — Valve ob' mussel (mttilus bdulis) showing br, br, the expanded GILLS OB BKANOHI^, WHICH, OWING TO THE LITTLE BAKS OF WHICH THEY AKE COMPOSED, PEKSENT A STRIATED ASPECT. ml, mantle ; m, cut adductor muscle ; i, mass of Yiscera ; the dark projection just above is the foot. Keep this preparation until the end of the lesson, by which time many of the cilia will have become languid. When this is the case pass a drop of dilute potash solution (1 part KHO to 1000 of sea-water) under the cover- glass and observe the effect. 4. Cement with sealing-wax a piece of small glass tubing to a slide so that one end of the tube comes nearly to the centre of the slide. To do this effectually the slide must be heated and some sealing-wax melted on to it and allowed to cool. The glass tube is then made hot and applied to the slide, embedding itself as it does so in the sealing-wax. Apply a ring COLUMNAR AND CILIATED EPITHELIUM. 61 of putty or modelling wax (half an inch in diameter and rising above the glass tube) so as to include the end of the tube. Make a deep notch in the ring opposite the tube for the exit of the gas. Place a drop of water within the ring (fig. 60). Fig. 60.- -MOIST CHAMBEK ADAPTED FOR PASSING A GAS OR VAPOUR TO A PREPARATION UNDER THE MICROSCOPE. Put a bar from the gill upon a eover-glaas in the least possible quantity of sea-water ; invert the cover-glass over the putty ring, and press it gently and evenly down. The preparation hangs in a moist chamber within which it can be studied through the cover-glass, and into which gases or vapours can he passed and their eflFects observed. Fig. 61. — Method oe subjecting a preparation to a stream oe carbon DIOXIDE. 6, bottle containing marble and Iiydrochlorie acid ', 6', wash-bottle, connected by india- rubber tube, t, with the moist chamber, «. Pass CO2 through the chamber, and after observing the effect replace it by air (see fig. 61). Bepeat with ether and with chloroform vapour. Colnnmar epithelium. — The cells of a columnar epithelium (fig. 62) are prismatic columns, which are set closely side by side, so that when seen from the surface a mosaic appearance is produced. They often 62 THE ESSENTIALS OF HISTOLOGY. taper somewhat towards their attached end, which is generally truncated, and set upon a basement membrane. Their free surface is Fig. 62. Fig. 63. Fig. 62. — A Eow of oolumnak cells from the intestine oe the rabbit. Smaller cells are seen between the epithelium-cells ; these are leucocytes. Fig. 63.— Columnar epithelium-cells of the rabbit's intestine. The cells have been isolated after maceration in very weak chromic acid. The cells are much vacuolated, and one of them has a fat -globule adhering to it near its attached end ; the striated border (str) is well seen, and the bright disk separating it from the cell-protoplasm ; n, nucleus, with intranuclear network ; a, a thinned-out wing- like projection of the cell which probably fitted between two adjacent cells. Fig. ea. Fig. 64. Fig. 66. Fig. 64.— a columnar epithelium-cell, showing mass of fibrils (cttomitome) within the cytoplasm. (M. Heldenhain.) Fig. 65. — ^A goblet or muoos-seoreting cell in columnar epithelium. (M. Heidenhain.) The oentrosoma is in the mucigon-mass. An ordinary columnar cell is also shown. Fig. 66. — Ciliated columnar epithelium, from the trachea of a rabbit. ml, vfi, mS, mucus-secreting cells in various stages of mucigon formation. The prepara- tion was treated with dilute chromic acid. covered by a thick striated border (fig. 63, str.) which may sometimes become detached in teased preparations. The protoplasm of the cell is highly vacuolated and reticular, and fine longitudinal strise may be COLUMNAR EPITHELIUM. 63 seen in it, which appear continuous with the striae of the free border. Between the striated border and the protoplasm of the cell is a highly refracting disk which contains iine dumb-bell shaped particles set vertically, connected below with the fibrils or strise which run through the cell protoplasm (fig. 64j 65). It has been suggested that these particles are formed by multiplication of the centrosome, but the fact cannot be regarded as established. The nucleus is ovoid and reticular. The lateral borders of the cells are often somewhat irregular or jagged, the result of the presence of amoeboid cells, which are generally found between the columnar cells, at least in the intes- tine. After a meal containing fat the epithelium-cells of the small intestine con- tain fat globules, which become stained black in osmic preparations. Columnar epithelium-cells are found lining the whole of the interior of the stomach and intestines : they are also present in the ducts of most glands, and sometimes also in their secreting tubes and saccules. The epithelium which covers the ovary is also of a modified columnar shape, but cells having all the structural peculiarities indicated above are found only in the alimentary canal and in its diverticula. Goblet-cells. — Some of the cells of a columnar epithelium, and also cells of glandular, ciliated, and transitional epi- thelia, contain mucigen, which is laid down within the cell in the form of granules (fig. 65, fig. 66, to\ m^ ; fig. 67) swell up to form globular masses which may run together and greatly distend the part of the cell nearest the free border. When the mucigen is extruded as mucus the cell takes the form of an open cup or chalice (fig. 66, m^), hence the name. These goblet-cells, or, as they are more appropriately termed, mucus- secreting cells, are probably not mere temporary modifications of the ordinary columnar and ciliated cells amongst which they are found, but permanently differentiated cells, which, after having got rid of their mucus by extrusion, again form a fresh supply in the same way as Fig. 67. — Theeb MUons-SEORBT- INO CELLS FBOM THE STOMACH, FILLED WriH MUCIGEN GEAN- ULES, SOME OF WHICH ARE IN PROCESS OF BXTRDSIOK. (M. Heidenhain.) These granules eventually €4 THE ESSENTIALS OF HISTOLOGY. before. In the gastric mucous membrane all the surface epithelium' is composed of mucus-secreting cells, and they extend also into the mouths of the glands. In the large intestine also most of the cells both of the surface and in the glands are goblet-cells. According to the observationa J of Carlier those of the gastric mucous men»brane are connected together ; laterally by protoplasmic fibres. Ciliated epithelium. — The cells of a ciliated epithelium are usually columnar in shape (figs. 66, 68), but in place of the striated border of the ordinary columnar cell the free surface is surmounted by a bunch of fine tapering filaments {vibratile ciUa), which, during life, move spon- taneously to and fro, and serve to produce a current in the fluid which covers them. The border upon which the cilia are set is bright in the living condition: after fixation it appears formed of little juxtaposed knobs or basal particles, to each of which a cilium is attached. In" the large ciliated cells which line the alimentary canal of some molluscs (figs. 68, 70), and with leas distiuctness in the ciliated cells of vertebrates, the knob, may be ob- served to be prolonged into the protoplasm of , the cell .as a fine varicose filament, termed the rootlet of the cilium. Since the axial fibril in the tail of the spermatozoon (which is commonly regarded as a ciliuiJi^ is developed in connection with the centrosome, it has been supposed that the cilia of an ordinary ciliated cell may also be outgrowths from the (multiplied) centrosome. But although it may be the case that the basal particles are formed by the division of the centrosome of the cell, in which case the rootlets may represent the fibrils of archoplasm which radiate from the centrosome of such a cell as the white corpuscle (fig. 9), it appears not to be true that the cilia are developed from these basal par- ticles, for the cilia sometimes appear before the basal particles. In plant spores, which have no centrosomes, the cilia are developed from amoeboid processes of the ectoplasm of the cell (Strassburger). Similar basal particles and longitudinal fibrils are found in columnar cell^ (pp. 62, 63), and these are probably homologous with the knobs and rootlets of the ciliated cell, while the bunch of cilia of the latter is represented by the striated borde* of the columnar cell. Fig. 68.— Focb ciliated cells. (Unhossek.) CILIATED EPITHELIUM. 65 , Sphuberg lias described in the cilia of certain infusoria an end-j)iece which stains differently from the rest of the cilium (fig 71). The action of cilia,.^When in motion a cilium is bent quickly over in one direction -with a lashing whip-like movement, immediately recovering itself. When vigorous the action is so rapid, and the rhythm so frequent (ten or more times in a second), that it is im- FiG. 69.— Columnar ciliated EPITHELIUM-OELLS IfEOM THE LOWEB PART OP THE HUMAN NASAL PASSAGES. EXAMINED PKESH IN SEKUM. (Sharpey.) Fig. 71.— Cilia or peontonia leuoas. (A. Sohuberg.) Lnffler's Bagellum stain, x 2250. Fig. 70.— Ciliated cell, from THE intestine 01' A MOLLUSC (Engelmann.) lie possible to follow the motion with the eye. All the cilia upon a ciliated surface are not in action at the same instant, but the move- ment travels in waves over the surface. If a cell is detached from the general surface, its cilia continue to act for a while, but their movement at once ceases if they are detached from the cell. If, however, a portion of the cell protoplasm is detached with them, they will continue to move for a time. The rhythm is slowed by cold, quickened by warmth ; but heat 66 THE ESSENTIALS OF HISTOLOGY. beyond a certain point kills the cells. The movement will continue for some time in water deprived of oxygen. Both CO^ gas and ether and chloroform vapour arrest the action, but it recommences on restoring air, if their action is not too prolonged. Dilute alkaline solutions quicken the activity of cilia, or may even restore it shortly after it has ceased. Theories of ciliary action. — Various attempts have been made to explain the manner in which cilia act. One hypothesis supposes that one side only of each cilium is contractile, the other side being elastic, or that there is a more rigid but elastic axis and a contractile covering. This supposition is negatived by the fact that in heat rigor the cilia are not bent over as they would be by the contraction which always accompanies rigor, but stand up straight. It is moreover impossible to suppose that a soft structure like a cilium could be bent over in a uniform gentle curve by contraction along one side ; such contraction could only produce shortening and wrinkling of the cilium, effects which are never observed. Another hypothesis assumes that the projecting cilia are set in action by rhythmic lateral contractions in the protoplasm ; which, by moving the rootlets, cause the cilia to bend over as a whip is bent by movements of the wrist applied to its handle. But this again implies an amount of rigidity which cilia do not possess, for it must be borne in mind that they have to overcome the resistance of fluid, and of fluid which is in many oases highly viscous. If in our ignorance of the structure of the individual cilia we are to form an idea as to the cause of the rhythmical bending over of these minute cell processes, the simplest hypothesis appears to be to assume that they are curved flattened hollow filaments, the interior communicating with the cell- protoplasm.^ If this is the case, then rhythmipal variations of pressure within the cell-protoplasm, which might, as in the case of amoeboid move- ments, be caused by alterations in surface tension, would be transmitted to the cilia and would cause the curve to open out, and again to assert itself, according to the degree of tension within the tubular filament. Such action can be imitated with a fine curved and flattened indiarubber tube attached to a pressure bag. Any increase of pressure within the tube causes it to straighten out; on again decreasing the pressure the tube bends over exactly in the manner of a cilium. This hypothesis has the advantage over the others which have been off'ered that it explains the movements of cilia on a theory -which is precisely similar to that which gives the most probable explanation of amoeboid movements of protoplasm, viz., that they are due to variations in surface tension, and it thus brings these two forms of protoplasmic activity into line with one another. It will presently be shown that the changes which occur in muscle in contraction are suscep- tible of a similar explanation. 'All cilia and oilium-like structures (flagella) which are sufficiently large to show any structural differentiation, exhibit an external membranous covering and a clear and usually structureless contents, but the minute size of ordinary cilia prevents one from determining whether this is also the case with them. THE CONNECTIVE TISSUES. 67 LESSON IX. THE CONNECTIVE TISSUES. AKBOLAR AND ADIPOSE TISSUE. RBTIFORM TISSUE. 1. Take a little of the subcutaneous tissue or of the intermuscular connective tis.sue of a rabbit or guinea-pig and spread it out with needles on a dry slide into a large thin film. Eeep the centre moist by occasionally breathing on it, but allow the edges to dry to the slide. Before commencing put a drop of salt solution on a cover-glass, and now invert this over the film. Examine with a high power. Sketch one or two bundles of white fibres and also one or two elastic fibres, distinguishable from the former by their sharp outline, isolated course, and by their branching. Sketch also one or more connective- tissue corpuscles, if any such are visible in the clear interspaces. Look also for migratory cells (leucocytes). Next carefully remove the cover-glass and replace the salt solution by dilute acetic acid (1 per cent.). Watch its efl'ect in swelling the white fibres and bringing more clearly into view the elastic fibres and corpuscles. Look for constricted bundles of white fibres. 2. Make another very thin film in the same way, but allow to dry com- pletely. Pour over the film a 1 per cent, solution of magenta in 50 per cent, alcohol, to which 1 drop per cubic centimeter of a 1 per cent, solution of gentian violet in alcohol has jtist been added. After one minute drain this otf, wipe round the specimen and allow^ the remainder of the staining solution to dry on the film. When completely dry mount in dammar. The elastic fibres are deeply stained ; the cells are also well shown. 3. Prepare another film of the subcutaneous tissue, including a little adipose tissue. Fix by pouring over it formol (10 p.c.) and leave this in contact with the film for 20 minutes. Wash with water and stain with saturated solution of Sudan III. or Scharlach R. in 75 p.c. alcohol ; wash with 75 p.c. alcohol to remove stain from everything except fat, then wash with water and stain with dilute hsematoxylin. Mount in glycerine and water. Examine first with a low and afterwards with a high power. The fat is well brought out by the Sudan III. or Scharlach B. stain ; if the preparation is from a young animal, fat-cells will be found in process of formation. Measure and sketch two or three of the cells. The fat may also be stained, without prior fixation, by treatment with 1 p.c. osmic acid solution. 4. Spread out another large film of connective tissue, letting its edges dry to the slide, but keeping the centre moist by the breath. Place on its centre a large drop of nitrate of silver solution (1 per cent.). After five minutes wash this away with distilled water, and expose to direct sunlight until stained brown. Now allow the film to dry completely, and cover it in dammar varnish or Canada balsam dissolved iu xylol. Sketch the outlines of two or three of the cell-spaces. 5.. To display retiform tissue the following method is recommended (Spalteholz). Place a piece of the organ (e.g. lymphatic gland) for twenty- 68 THE ESSENTIALS OF HISTOLOGY. four hours or more in alcohol, then overnight at 38° C. in a 1 per cent, solution of carbonate of soda to which a few drops of a solution containing trypsin have been added. Cautiously transfer the semi-digested structure to alcohol again, and leave it for a few hours. Embed in paraffin in the usual way and stain the sections with iron hsematoxylin. The fibrils of connective and retiform tissue are the only structures which have remained undigested and they are deeply coloured by the hsematoxylin. The connective tissues include areolar tissue, adipose tissue, elaslk. tissue, fibrous tissue, reticular and lymphoid tissue, cartilage and bone. All these tissues agree in certain microscopical and chemical characters. They, for the most part, have a large amouTit of intercellular substance in which fibres are developed, and these fibres are of two kinds — white and yelloui or elastic. Moreover, there are many points of similarity between the cells which occur in these tissues ; they are all developed from the same embryonic formation, and they tend to pass imper- ceptibly the one into the other. Besides this, the use of these several tissues is similar; they mostly serve to connect and support the other tissues, performing thus a passive mechanical function. They may therefore be grouped together, although differing considerably in extel-nal and even in microscopic characters. Of the connective tissues, however, there are three which are so intimately allied as to be .naturally considered together, being composed of exactly the same elements, although differing in the relative development of those elements : these are the areolar, elastic, apd fibrous tissues. Adipose tissue and reticular tissue may both be looked upon as special modi- fications of areolar tissue. Areolar tissue being the commonest and, in a sense, the most typical, its structure may be considered first. Areolar tissue. — The areolar tissue presents to the naked eye an appearance of fine transparent threads and laminse which intercross in every direction with one another, leaving intercommunicating meshes, or areolae, between them. When examined with the microscope, these threads and fibres are seen to be principally made up of wavy bundles of exquisitely fine transparent fibres (white fibres, &g. 73, A). The bundles run in different directions, and may braiich and intercommunicate with one another (fig. 75); but the individual fibres, although they pass from one bundle to another, never branch or join other fibres. The fibres are cemented together into the bundles- by a clear substance containing mucin, and the same clear nlaterial forms also the basis or ground- substance of the tissue, in which the bundles themselves course, and in which also the corpuscles of the tissue lie embedded. This ground- suisstance betweeii the bundles can with difficulty be seen in the fresh tisgiie on account.of its extreme transparency ; but it can be brought to view by staining with nitrate of silver, as in §4. The whole of the . AREOLAR TISSUE. tissue is thereby stained of a yellowish brown colour, with the excep^ tion of the spaces which are occupied by the corpuscles {celUspaces), Fig. 72. Gbound substance of connective tissue stained bt silver. The cell-spaces are unstained. Prom a photograph. Magnified 260 diameters, B A i? Fig. 73. — White and elastic fibbes of areolae tissue. A, bundles of white fibres partly unravelled. B, elastic fibres. As Macallum has shown, this reaction is due to the presence of chlorides in the intercellular substance, whereas the cell-protoplasm contains none. Besides the white iibres of connective tissue here described, fibres of a different kind (fig. 73, B) may be made out in the preparations; these are the elastic fibres. They are especially well seen after treatment 70 THE ESSENTIALS OF HISTOLOGY. with acetic acid, and after staining with magenta, or, in sections, with orcein ; but they can be detected also in the fresh preparation. They are characterised by their distinct outline, their straight course, the fact that they never run in bundles, but singly, and that they branch or join neighbouring fibres. If broken by the needles in making the preparation, the elastic recoil causes them to curl up, especially near the broken ends. Besides these histological differ ences, the two kinds of fibres differ also in their chemical characters. Thus the white fibres are formed of a material {collagen) which is dis- solved by boiling in water yielding gelatin, and by peptic digestion, but is not dissolved by tryptic digestion ; whereas the substance of which the elastic fibres are composed (elastin) resists for a long time the action of boiling water and peptic digestion, although it is dis- solved by tryptic digestion. Moreover, the white fibres swell and Fig. 74. — A white bundle swollen by acetic acid. From the subaeaohnoid TISSUE AT THE BASK OF THE BKAIN. (Toldt. ) become indistinct under the action of acetic acid ; the elastic fibres are unaltered by this reagent. Elastic fibres appear to have a sheath which is more resistant to reagents than the rest of the fibre. The bundles of white fibres which have been swollen out by acid sometimes exhibit constrictions at irregular intervals (fig. 74). These are in many instances due to elastic fibres coiling round 'the white bundles. The cells of areolar tissue. — Several varieties of connective tissue cells are distinguished, viz. : (1) Lamellar cells, which are flattened and often branched (fig. 75, c, c') and may be united one to the other by their branches, as in the cornea. Sometimes they are unbranched and may lie along the fibril-bundles and even themselves show a fibrillar appearance. Some authors have inferred from this that these cells are transformed into white fibril-bundles and have termed them " fibroplasts " ; but the fibrillation which they exhibit is not of the same character as that of the white fibres, and is pro- bably a form of cytomitome, such as is seen in many protoplasmic cells. In certain situations the lamellar connective-tissue cells THE CELLS OF AREOLAR TISSUE. 71 Fig. 75.— Sdboutaneoub tissue feom a touns kabbit, prepared as dieboted IN § 1. Highly magnified. The white fibres are in wavy bundles ; the elastic fibres form an open network, p, p, plasma-cells ; g, granule-cell ; c, c', lamellar-cells ; /, fibriUated-cell. Fig. 76. — Epithelioid oells op oonneotive tissue pbom the surface of AN APONBUKOSIS. (Nitrate of silver preparation.) 72 THE ESSENTIALS OP HISTOLOGY are greatly flattened out, especially when they lie upon the surface of aponeuroses and they are there joined edge to edge like the cells of an endothelium (fig. 76. The apparent cell-spaces in silver prapatatiomi have of course in all cases a similar arrangement to that of the cells). (2) Plaskn cells (fig. 75, p), which are composed of a soft much- vacuolated ^protoplasm, rarely flattened, but otherwise varying greatly in sh^e and size. (3) Ch-anular cells (g) {Mast-zeUen of Ehrlich, clasrmtocytes of Ranvier), usually spheroidal or ovoidal in shape, and formed, like the plasma-cells, of soft protoplasm, but thickly occupied with albuminous granules, which are deeply stained by gentian violet Fig. 77. — A few cells from the margin of a fat lobule. Highly magnified. From a photograph. f.g, fat-globule distending a fat-cell ; n, nucleus ; m, membranous envelope of the f*t- cell ; c. r. buncb of crystals within a fat-cell ; c, capillary vessel ; v, venule ; c.t. con- nective-tissue cell ; g, granular cell ; the connective-tissue fibres are not represented. and by other basic aniljne dyes. (4) Migratory leucocytes may also be seen here and there in the areolar tissue (wander-cells). (5) In the middle coat of the eye in mammals, and in some parts of the skin, some of the connective-tissue cells are filled with granules of pigment [pigment-cells). These are much more extensively present in lower vertebrates, especially in amphibia and fishes, where they exhibit amoeboid changes which result im the pigment being at one time diflfused over a considerable area and at another time restricted to the immediate neighbourhood of the nucleus. The changes thus produced cause alteration in the general colour and shade of the integument, where such pigment cells are very numerous, and serve' the purpose of protective adaptation of the animals to their environment. The cells lie in spaces in the ground-substance, between the bundles of white fibres. In some parts of the connective tissue the white THE CELLS OF AREOLAE TISSUE. 73 bundles are developed to such an extent as to pervade almost the whole of the ground-substance, and then the connective-tissue corpuscles become' aqueezed into the interstices, flattened lamellar expansions of the cells extending between the bundles, as in tendon (see next Lesson)., A" %\ Fie. 78. — Deposition of fat ijj oonneotivb-tisshk cells. /, a cell with a few isolated fat-droplets in its protoplasm ; f, a cell with a single large and several minute drops ; /", fusion of two large drops ; g, granular cell, not yet exhibiting any fat-deposition ; c.t,, flat connective-tissue corpuscle ; c, c, network of capillaries. The cells and cell-spaces of areolar tissue come into intimate relation with the cells lining the lymphatic vessels' and small blood-vessels. This connection can best be seen in silvered preparations ; it will be again referred to in speaking of the origin of the lymphatics. Adipose tissue consists of vesicles filled with fat (figs. 77, 79) and collected into lobules, or into tracts which accompany the small blood- vessels. The vesicles are round or oval in shape, except where closely packed, when they become polyhedral from mutual compres- sion. The fat-drop is contained with- in a delicate protoplasmic envelope (fig. 77, m) which is thickened at one part, and here includes an oval flattened nucleus. The fat is stained black by osmic acid; a deep yellow colour by Sudan III. ; and an intense red by Scharlach E. t Fig. 79.— Fat-oells from young animal (Ranvier.) OSMIO AOID PREPARATION. The drops of fat are stained of an intense black, n, nucleus ; g, small gloluules of fat. 74 THE ESSENTIALS OF HISTOLOGY. The vesicles are supported partly by filaments of areolar tissue, but chiefly by a fine network of capillary blood-vessels. The fat when first formed in the embryo is deposited within large granular cells of areolar tissue (fig. 78) similar in general appearance to the " Mast "-cells of Ehrlich; some authorities regard them as of a specific nature, for they are in certain situations collected into large gland- 4^ tf V V.i» XU ^'« Fig. -Two STAGES OF FORMATION OF ADIPOSE TISSUE. (H. Batty Shaw.) In A the tissue is formed of a gland-like mass of cells, in some of which the cytoplasm is occupied by fat globules (looking white in the sections). In B the fat fills many of the cells. like masses (fig. 80) abundantly supplied with blood-vessels, which gradually become transformed into fat-cells by the deposition of fat in the cell-protoplasm. Fat is, however, also laid down in ordinary Fig. 81.— Retifobm tissue from a lymph-gland. Moderately magnified. tr, a trabeculum of connective tissue ; r, r', retitorm tissue, with more open meshes at r and denser at /. branched cells of connective-tissue. The fat appears to be produced by a transformation of albuminous granules which the cells con- tain into droplets of fat. As the droplets increase in size they run ADIPOSE TISSUE. 75 together into a larger drop, which gradually fills the cell more and more, swelling it out so that the cell-protoplasm eventually appears merely as the envelope of the fat-vesicle. Fig. 82. — Pobtion of the above, more highly , magnified, showing the continuity of the rfetiform tissue r, r, with the connective tissue of a trabeculum, tr. Fat is found most abundantly in' subcutaneous areolar tissue, and under the serous membranes ; especially in some parts, as at the back of the peritoneum around the kidneys, under the epicardium, Fig. 83.— EetIodldm of bone-mabbow. (Enderlen.) and in the mesentery and omentum. The yellow marrow of the bones is also principally composed of fat. There is no adipose tissue within the cavity of the cranium. Retiform or reticular tissue (figs. 81, 82, 83) is a variety of con- nective tissue in which the intercellular or ground-substance has largely 76 THE ESSENTIALS OF HISTOLOGY. disappeared or is replaced by fluid. There are very few or no elastic fibres in it, but a dense network of white fibres, the meshes of which vary in size, being very small and close in some parts ; more open and like areolar tissue in other parts. In some places where the tissue occurs the fibres are enwrapped by flattened branched con- nective-tissue cells, and until these are removed it is not easy to see the fibres. Chemical diff'erences between the fibres of retiform tissue and those of ordinary areolar tissue have been described by Mall, but microscopically the fibres of the two tissues are indistinguishable and are found in continuity with one another (see figs. 82, 84). This Fig. 84.— Lymphoid tissuk of a lymph-gland. tissue forms a fine framework in many organs, supporting the proper elements and extending into all the interstices between the coarser connective tissue bundles. It can best be shown by dissolving the cells of the tissue by tryptic digestion and subsequently staining the fibres forming the reticulum (p. 67, g 5). In this way it may be demon- strated in lymph-glands, in the spleen, liver, bone-marrow (fig. 83), mucous membranes, and many other parts. Lymphoid or adenoid tissue is reticular tissue in which the meshes of the network are largely occupied by lymph-corpuscles (fig. 84). This is by far the most common condition of a retiform tissue, and BASEMENT-MEMBEANES. 77 is met with in the lymph-glands and allied structures, and also in parts of the alimentary mucous membrane, and in some other situations. Basement-membranes (membrance proprice) are homogeneous-looking membranes, which are found forming the surface layers of connective- tissue expansions in many parts, especially where there is a covering of epithelium, as on mucous membranes, in secreting glands, and else- where. They are generally formed of flattened connective-tissue cells joined together to form a membrane; but in some cases they are evidently formed not of cells, but of condensed ground-substance, and in yet other cases they are composed of elastic substance ; the name basement-membrane is therefore used to denote structures of an entirely different nature. Jelly-like connective tissue, although occurring largely in the embryo, is found only in one situation in the adult — viz. forming the vitreous humour of the eye. It is composed mainly of soft, fluid, ground-substance, with cells scattered here and there through it, and with a few fibres which interlace throughout the tissue and confine the fluid of the ground-substance within their meshes ; thus conferring upon the tissue its jelly-like character. All embryonic connective-tissue is in the first instance of this jelly-like nature (see p. 82). 78 THE ESSENTIALS OF HISTOLOGY. LESSON X. TEE CONNECTIVE TISSUES {continued). ELASTIC TISSUE. FIBROUS TISSUE. DEVELOPMENT OF CONNECTIVE TISSUE. 1. Tease out as finely as possible a small shred of elastic tissue (ligamentum nuchae of the ox or ligamentum subflavum of man) in glycerine and water, slightly coloured by nlagenta. Cover and cement the preparation. Note the large well-defined fibres constantly branching and uniting with one another. Sketch a small part of the network. Note the existence of bundles of white fibres amongst the elastic fibres. 2. Examine a thin transverse section of ligamentum nnchse which has been hardened in 2 per cent, solution of bichromate of potash. The section is to be stained with haemalum and mounted in Canada balsam or dammar by the usual process, or simply in glycerine and water. Observe the grouping of the fibres and their angular shape. 3. Pinch off the end of the tail of a dead mouse or rat, draw out the long silk-like tendons and put them into salt solution. Take one of the threads, which should be nearly three inches long, and stretch it along a slide, letting the ends dry firmly to the glass but keeping the middle part wet. Put a piece of hair on either side and cover in salt solution. Observe with a high power the fine wavy fibrillation of the tendon. Draw. Now run dilute acetic acid (0'75 per cent.) under the cover-glass, watch the tendon where it is becoming swollen by the acetic acid. Notice the oblpng nucleated cells coniiug into view between the tendon-bundles. Sketch three or four cells in a row. Lastly, lift the cover-glass, wash away the acid with distilled water, place a drop of Ehrlich's haematoxylin or carmalum solution on the tendons, and leave the preparation until it is deeply stained ; then wash away the stain and mount the preparation in faintly acidulated glycerine. 4. Take another long piece of tendon, and after washing it in distilled water, stretch it upon a slide as before, fixing the ends by allowing them to dry on to the slide. Put a drop of nitrate of silver solution (1 per cent.) on the middle of the tendon, and leave it on for five minutes. Then wash oflf the silver nitrate with distilled water, and expose the slide to direct sunlight. In a very few minutes the silvered part of the tendon will be brown. As soon as this is the case, dehydrate the tendon with absolute alcohol in situ upon the slide, run off the alcohol, and at once put a drop of clove oil on the preparation. In a minute or two the clove oil can be replaced by xylol balsam or dammar and the preparation covered. 5. Stain, with magenta solution, a thin section of a tendon which has been hardened in 70 per cent, alcohol. Mount in dilute glycerine aud cement at once. 6. For developing connective tissue study sections of the umbilical cord at different periods. Fix with formol. Stain with acid fuchsine and hsematoxylin. ELASTIC TISSUE. 79 Elastic tissue is a variety of connective tissue in which the elastic fibres preponderate. It is found most characteristically in the liga- mentum nuchae of quadrupeds and the ligamenta subflava of the vertebrae, but the connective tissue of other parts may also have a Fig. 85. — El.^stic fibkes from the ligamentum nuoh.e of the ox, showing transverse markings on the fibres. considerable development of elastic fibres. It occurs in an almost pure form in the walls of the air-tubes, and uniting the cartilages of the larynx. It also enters largely into the formation of the lungs and of the walls of the blood-vessels, especially the arteries. Fig. -CeOSS-SKCTION op elastic FIBRES FROM THE LIGAMENTDM NU0H*1 OP THE OX. In the ligamentum nuchas most of the fibres are very large (figs. 85, 86). They often exhibit cross markings or even transverse clefts. When dragged asunder, they break sharply across. They constantly branch and unite, so as to form a close network. In transverse section they are seen to be separated into small groups or bundles (fig. 86) by intervening septa of areolar tissue. 80 THE ESSENTIALS OF HISTOLOGY. Elastic tissue does not always take the form of fibres, but may Occur as membranes {e.g. in the blood-vessels). Sometimes the fibres are very small, but tbeir microscopic and chemical characters are always very marked (see p. 70).. Fig. 87. — Section of tendon, human. (Sobotta.) x 32. i, tendon-bundles ; s, septa of areolar tissue ; v, vessels. Fibrous tissue is almost wholly made up of bundles of White fibres running in a determinate direction. These again are collected into larger bundles, which give the fibrous appearance to the tissue. The bundles are constantly uniting with one another in their course, although their component fibres remain perfectly distinct. The interspaces between the larger bundles are occupied by areolar FIBROUS TISSUE. 81 tissue (fig. 87, s ; fig. 88, c, d, .e) in which the blood-vessels and lymphatics of the fibrous tissue are conveyed. The interstices between the smallest bundles are occupied by rows of lamellar connective-tissue Fig. 88.— Part of a large tendon in transverse section. More highly magnified. a, areolar sheath of the tendon, with, the fibres for the most part running transversely ; hut with- two or three longitudinal bundles, ,b; I,- lymphatic cleft in the sheath'; iittmediately. over it a blo.od-vessel is aeon cut across, and on the other side of the figure a small artery is shown cut longitudinally ; c, large septum of areolar tissue ; d, smaller septum ; e, still smaller septum. The irregularly stellate bodies are the tendon-cells in section. Fig, 89,— Tendon op mouse's tail (175 diameters) ; showing chains of CELLS between THE TENDON-BUNDL^B. A, stained with hasmatoxylin. B, stained with silver nitrate, showing the cell-spaces. P 82 THE ESSENTIALS OF HISTOLOGY. corpuscles (tendon-cells), which, from, being squeezed up between three or more bundles, become flattened out in two or three directions. In transverse section the cells appear somewhat stellate (fig. 88), but when seen on the flat they appear lamellar (fig. 89), and from this aspect their general shape is square or oblong. They lie, as before said, in rows between the tendon-bundles, and the nuclei of adjacent cells are placed opposite one another in pairs (fig. 90). The cell-spaces correspond in general figure and arrangement to the cells which occupy them. Fibrous tissue forms the tendons and ligaments, and also certain membranes, such as the dura mater, the fibrous pericardium, the fasciae of the limbs, the fibrous covering of certain organs, etc. It is found wherever great strength, combined with flexibility, is concerned. It receives a few blood-vessels, disposed longitudinally for the most part, and contains many lymphatics. Both blood-vessels and lymphatics run in the areolar tissue which separates and surrounds the tendon- bundles. Tendons and ligaments also receive nerve-fibres, which, in Fig. 90. — Eight oblls feom the same tendon as eepkesented in fig. 89. (425 diameters.) The dark lines on the surface of the cells are the optical sections of lamellar extensions directed towards or away from the observer. some cases, end in localised ramifications within fusiform enlargements of the tendon-bundles (organs of G-olgi), while others terminate in end-bulbs or in simple Pacinian corpuscles. These will be described along with the modes of ending of nerve-fibres. Development of connective tissue. — Connective tissue is developed in and from the cells of the mesoderm (mesenchyme) of the embryo. In those parts which are to form connective tissue, there may frequently be seen a clear space separating the cell-layerj which are already formed, this clear space being permeated with fibres which appear to be produced from the cells bounding the space. Presently branching mesenchyme cells, which are derived from the bounding cell- layers, are found forming a syncytium within the clear space (fig. 91, m). In the meshes of the reticular syncytium is a muco-albuminous semi- fluid intercellular substance (ground-substance). The connective-tissue fibres, both white and elastic, are deposited in this ground-substance, the elastic substance appearing in the form of granules (fig. 95, g), which subsequently become connected together into elastic fibres or DEVELOPMENT OF CONNECTIVE TISSUE. 83 Fia. 91. — Developing oonnbotive tissue in heart op ohiok-embryo of 48 hours. (Szili.) my, cells forming myocardium ; j, jelly formed of reticulum with enclosed fluid ; e, endo- thelium (mesothelium) of heart ; m, mesenchyme cells in jelly; .bl, blood-corpuscles. Fig. 92. — Developing connective tissde from the umbilical.oohd of A HUMAN embryo 21 MM. long. (Minot.) x540. ■84 THE ESSENTIALS OF HISTOLOGY. Y rf} ) ^ J55^ 17 ,> ^ ^ / ,-"A:'i }v ^^1 "6' _.-#=^ ».:.-A^: ''^-^:- / ' ^ "'-'-' ^1 ^^^'/ '-^' -" ^ o i ^ %'^ Fig. 93. — Developing co.n'nective tissue from the umbilical cokd op a THBEE months' HUMAN EMBRYO. (Minot.) X 511. y \ , ;^%. f I 1 1 'f ./ Fig. 94. — Jelly op wharton prom umbilical cokd op new-bokn child (Sobotta.) x280. /, connective-tissue fibres ; c, cells. DEVELOPMENT OF CONNECTIVE TISSUE. 85 laminae, as the case may be, the white fibres appearing at first in the form of very fine bundles, which afterwards become gradually larger (fig. 93); so that in fibrous tissue the whole ground-substance is eventually pervaded by the bundles, and the cells of the tissue become squeezed up into the intervals between them. Before any considerable development of fibres has taken place, the embryonic connective tissue has a jelly-like appearance ; in this form it occurs in the umbilical cord, where it is known as the jelly of Wharton (fig. 94). There has been always a considerable difference of opinion as to the origin of the fibres of connective tissue, some histologists holding that they are formed within the protoplasm of cells, which gradually lose Fig. 95. ^Development of elastic tissue by deposition op fine gkandles. (Kanvier.) g, fibres being formed of rows of ' elastin ' granules ; p, flat plate-like expansion of elastic substance formed by the fusion of ' elastin ' granules. their cell-characters as the fibres become developed within them ; others taking the view that the fibres, both white and elastic, are extracellular formations. While there is no doubt that they are produced under the influence of the cells, for they first appear in close proximity to those structures, it seems on the whole probable that the fibres are deposited in the ground-substance and not actually in the cell-protoplasm, so that they are rather to be looked upon, like the ground-substance itself, as formed by a process of secretion than ■ by one of direct cell-transformation. THE ESSENTIALS OF HISTOLOGY. LESSON XI. TEE CONNECTIVE TISSUES (continued). ARTICULAR CARTILAGE. SYNOVIAL MEMBRANES. 1. Cut two or three very thin tangential slices of the fresh cartilage of a joint, mount them in salt solution, and examine with the high power. Observe the form and grouping of the cells. Look at the thin edge of the section for spaces from which the cells have dropped out. Measure two or three cells and their nuclei, and sketch one or two groups. Now replace . the salt solution by water and set the preparation aside for a little while. On again examining it, many of the cartilage cells will be found to have retracted away from their containing capsules. 2. Make other sections of the cartilage (1) from near the middle, (2) from near the edge. Place the sections for two or three minutes in acetic acid (1 per cent.), wash them with water, and stain with dilute hsemalum or carmalum solution. When stained mount in dilute glycerine and cement the. cover-glass. In (2) look for branched cartilage cells. 3. Study vertical sections of articular cartilage from an end of bone which has been fixed and decalcified, and mount the sections in glycerine and water, or, after staining with hsemalum, in dammar or xylol balsam. Sketch the arrangement of the cells in the difi'erent layers. 4. Brush a fresh joint with distilled water ; drop 1 per cent, nitrate of silver solution over it ; after five minutes wash away the nitrate of silver and expose in water to direct sunlight. When browned, place in spirit for half an hour or more, and then with a razor wetted with spirit cut thin sections from the surface and mount in xylol balsam or dammar after passing through clove oil. The cells and cell-spaces show white in the brown ground-substance. 5. To study the structure of the synovial membrane mount other slices from the silvered preparation of the joint (§ 4) just beyond the limits of the articular cartilage, and also look for small fringed projections of the membrane. Snip them off with scissors and mount as before. 6." The superficial flexor tendons of the foot of the ox or sheep run in grooves formed by the deep flexors, and these grooves are lined, and the tendons which pass through them are covered by vaginal synovial mem- branes. To show the structure of these treat one of the superficial flexor tendons with silver nitrate in the manner recommended for the joint, § 4, and after hardening in spirit out sections from the surface and mount them in balsam or dammar varnish. Cartilage or gristle is a translucent bluish-white tissue, firm, and at the same time elastic, and for the most part found in connection with bones of the skeleton, most of which are in the embryo at first repre- sented entirely by cartilage. Three chief varieties of cartilage are CARTILAGE. 87 distinguished. In one, which is termed hyaline, the matrix or ground- substance is almost clear, and free from obvious fibres; in the other two, which are termed fibro-cartUage, the matrix is everywhere pervaded by connective-tissue fibres. When these are of the white variety, the tissue is white fihro-eartilage ; when they are elastic fibres, it is yellow or ■elastic fibro-cartilage. Hyaline cartilage occurs principally in two situations — namely (1) ■covering the ends of the bones in the joints, where it is known as articular cartilage ; and (2) forming the rib-cartilages, where it is known » 9 -. « Pig. 96i— Articular cartilage from hbad of metatarsal bonk of ampu- tated FOOT, HUMAN (OSMIO ACID PREPARATION). THE CELL-BODIES ENTIRELY PILL THE SPACES IN THE MATRIX. (340 diameters.) a, group of two cells; b, group of four cells; h, protoplasm of cell, with g, fatty granules ; n, nucleus. as costal cartilage. It also forms the cartilages of the nose, of the external auditory meatus (but not the pinna), most of those of the larynx, and the cartilages of the windpipe; in these places it serves to maintain the shape and patency of the orifices and tubes. Articular cartilage. — The cells of articular cartilage are generally scattered in groups of two or four throughout the matrix (fig. 96). The lajiter is free from obvious fibres, except at the extreme edge of the cartilage, where the connective-tissue fibres from the synovial 88 THE ESSENTIALS OE HISTOLOGY. membrane extend into it ; and here also the cartilage-cells are often branched, and offer transitions to the branched comiective-tisgue corpuscles of that membrane (transitional cartilage, fig. 97). By long maceration in brine, however, evideince of a fibrous structure may be •s^ Fig. 97.— Border of AKTionLAB oaetilage showing tkansition of cabtil- AGB CELLS INTO CONNEOTIVE-TISSnE 0OKPUSCLE8 OF SYNOVIAL MEMBBANE. Feom head of metatabsal bone, human. (About 340 diameters.) a, ordinary cartilage-cells ; &, b, with branching processes. obtained, even in the matrix of true hyaline cartilage. Some his- tologists also describe fine communications in the matrix uniting the cartilage-cells with one another, but these are of doubtful occurrence in vertebrate cartilage, although they unquestionably exist in the cartilage of cephalopods. The matrix immediately around the cartilage-cells is often marked ofi^ from the rest by a concentric line or lines, this part of the matrix, which is the latest formed, being known as the cwpside of the- cell (fig. 98). The cells are bluntly angular in form, the sides opposite to one another in the groups being generally flattened. The proto- plasm is very clear, but it may contain droplets of fat; and with a high power fine interlacing filaments and granules have been observed in it. During life the protoplasm entirely fills the cavity or cell-space which it occupies in the matrix ; but after death, and in consequence of the action of water and other agents, it tends to shrink away from the capsule. The nucleus is round, and shows the usual intranuclear network. Fig. 98. — A group of oabtilage-cellb SHOWING THE OAPSULAR OUTLINES IN THE MATRIX 8URB00NDING THE GROUP. (Ranvier- ) a, nucleolus ; b, nucleus ; c, cytoplasm of a coU ; d, capsular lines in pericellular matrix ; e, fibrils in cartilage matrix. ARTICULAR CARTILAGE. 89- In vertical section (fig. 99) the deeper cell groups (c) are seen to be arranged vertically to the surface, the more superficial ones (a) parallel to the surface ; whilst in an intermediate zone the groups are irregu- riG. 99.— Vbbtioal section m' abticulae cartilage ooveking the lower END oi' THE TIBIA, HUMAN. (Magnified about 30 diameters.) a, cells and cell-groups flattened conformably with the surface ; &, cell-groups irregu- larly arranged ; e, cell-groups disposed perpendicularly to the surnice ; d, layer of calcified cartilage ; e, bone. ABODE U !!ii'iilVl!l'!!li!]i|lllSI'i!ll!|il'!lil!l!1i;fi|l'ii'l!i|ilii!l1if;l|if'||^^^^ '!' Fig. 100. — Plan of the multiplication of cells of cartilage. (Sharpey.) A, cell in its ' capsule ; b, divided into two, each with a capsule ; c, primary capsule disappeared, secondary capsules coherent with matrix ; n, tertiary division ; e, secondary capsules disappeared, tertiary coherent with matrix. larly disposed (6). In the deepest part of the cartilage, next to the bone, there is often a deposit of calcareous salts in the matrix (calcified cartilage, d). The disposition of the cells of cartilage in groups of two, four, eight, etc., is apparently due to the fact that these groups have originated 90 THE ESSENTIALS OE HISTOLOGY. from the division of a single cell first into two, and these again into two, and so on (fig. 100). The division of the cartilage-cell, like that of most other cells, is mitotic. _ i It would seem that the matrix is formed of successive portions, which are deposited around each cartilage-cell as the so-called ' capsules,' each newly formed portion soon blending in its turn with the previously formed matrix, whilst a new capsule is formed within it. The most newly formed portions of matrix stain with hsematoxylin more deeply than the rest, and in some cartilages this gives the appearance of rounded balls of darkly stained matter surrounding each cell or cell-group (chondrin-balls, Morner). Embryonic cartilage is characterised by the cells being usually more sharply angular and irregular ; they are even in some cases markedly branched, like those which occur at the junction of cartilage and synovial membrane in the adult. The cells are also more closely packed, the matrix being in relatively less amount than in later life. Development. — Cartilage is formed in the embryo from mesenchyme similar to that which gives origin- to other forme of connective-tissue. Each cell forms a capsule around itself, and the blended capsules compose the first matrix. Cartil- age sometimes remains in this condition throughout life ; it is then termed parerir chymatous cartilage. This can be seen in the mouse's ear ; where also the cartilage cells become filled with fat. Cartilage grows at first partly by interstitial expan- sion (accompanied by cell multiplication and by formation around and between the cells of intercellular sub- stance), partly by apposition at the perichondrium, the connective tissue becoming here transformed into cartilage. At a later period of growth the increase in size and change in shape of cartilages are due almost entirely to the agency of the perichondrium. FlQ. 101. — ViLLnS OF SYNOVIAL MEMBRANE. (Hammar.) Synovial Membkanes. The synovial membranes are often compared with the serous membranes. They are indeed, like the latter, connective-tissue SYNOVIAL MEMBEANES. 91 membranes which bound closed cavities moistened with fluid, but they are not connected with the lymphatic system, nor is the fluid (synovia) which moistens them of the nature of lymph. Moreover, there is either no endothelial lining, or it occurs only in patches, in place of the continuous lining which we find in the serous membranes. Long villus-like projections occur in many parts; they are often covered by small rounded cells, and probably serve to extend the surface for the secretion of synovia. The blood-vessels of synovial membranes are numerous, and approach close to the inner surface of the membrane. They are well seen in preparations from an injected limb. 92 THE ESSENTIALS OF HISTOLOGY. LESSON XII. THE CONNECTIVE TISSUES (eontin-ued). COSTAL CARTILAGE. FIBRO-CAETILAGE. 1. Make transverse and tangential sections of a rib-eartilage, which may either be fresh, or may have been preserved in spirit or formol. Stain them with hsemalum or carmalum (if fresh, after treatment with acetic acid as in Lesson XL § 2, or they may be placed for an hour or two in '5 p.c. osmie acid), and mount in glycerine. Sketch a part of a transverse section under a low power and a cell-group from one of the tangential sections under a high power. Notice especially the arrangement of the cells, somewhat concentric near the surface but radial near the centre. The costal cartilages tend to become ossified near the middle in most animals, but in man when ossification occurs it is the superficial layer which is invaded. 2. Make sections of the cartilage of the external ear (pinna), either fresh or after hardening in alcohol. Mount in dilute glycerine faintly coloured with magenta, or stain with orcein and mount in balsam. If from the ox, notice the very large reticulating elastic fibres in the matrix. Notice also the isolated granules of elastin, and around the cartilage-cells an area of clear ground-substance. If from the mouse or rat there is very little matrix and no elastic fibres, and the cells are almost in contact (parenchymatous cartilage) ; they also contain fat (staining with osmic acid). 3. Mount a section of the epiglottis in the same way. Notice the closer network of much finer fibres in its cartilage. 4. Cut sections of white fibro-cartilage (intervertebral disk or semilunar cartilage of knee), which has been hardened in picric acid, followed by spirit, or in spirit only. Stain the sections with dilute hsemalum or carmalum. Mount in dilute glycerine. Observe the wavy fibres in the matrix and the cartilage-cells lying in clear areas often concentrically striated. Look for branched cartilage-cells. Sketch three or four cells and the adjoining fibrous matrix. Costal cartilage. — In the costal cartilages the matrix is not always so clear as in the cartilages of the joints, for it more often happens that fibres become developed in it. The cells are generally larger and more angular than those of articular cartilage, and collected into larger groups (fig. 102). Near the circumference, and under the perichondrium or fibrous covering of the cartilage, they are flattened and parallel to the surface, but in the deeper parts they have a more irregular or a radiated arrangement. They frequently contain fat. The cartilages of the larynx and windpipe and of the nose resemble on the whole the costal cartilages, but the COSTAL CARTILAGE. 93 study of them may be deferred until the organs where they occur are dealt with. Elastic or yellow flbro-cartilage occurs in only a few situations. These are, the cartilage of the external ear and that of the Eustachian tube, and the epiglottis and cartilages of Santorini of the larynx. The matrix is everywhere pervaded with well-defined branching fibres, which unite with one another to form a close network (figs. 103, 104). These fibres resist the action of acetic acid, and are stained deeply Fig. 102. — Section of bib-oartilage, showing cells and oell-geoups in AN indistinctly EIBKOUS MATRIX. Two or three empty cell-Bpaces are seen from which the cells have dropped out in preparing the section. by magenta; they are evidently elastic fibres. In the ox they are very large, but smaller in man, especially in the cartilage of the epiglottis. They appear to be developed, as with elastic tissue else- where (see p. 82), by the deposition of granules of elastin in the matrix, which at first lie singly, but afterwards become joined to form bhe fibres. WMte fibre-cartilage is found wherever great strength combined with a certain amount of rigidity is required : thus we frequently Bnd fibro-cartilage joining bones together, as in the intervertebral Jisks and other symphyses. But in these cases the part in contact with the bone is always hyaline cartilage, which passes gradually into [the fibro-cartilage forming the bulk of the symphysis. Fibro- jartilage is often found lining grooves in which tendons run, 94 THE ESSENTIALS OP HISTOLOGY. -> m f Fig. 103.— Section op elastic caetilage of bab, human. (Sobotta.) x280. i oartilaKe cells; cap, their capsules; m, clear matrix around cells and eeU-groups; ' /, elastic fibres. Fio. 104.— Section op the elastic cartilage op the eak. (R. Hertwig.) Highly magnified. Fig. 105. — White pibbo-oaktilaqe prom an intervertebral disk, hdman. Highly magnified. The concentric lines around the cells indicate the limits of deposit of successive capsules. OnJe of the colls has a forked process which extends beytond the hyaline area surrounding the cell, amongst the fibres of the general mati-ix. FIBRO-CARTILAGE. 95 and it may be found in the tendons themselves. It is also em- ployed to deepen cup-shaped articular surfaces; and in the case of the interarticular cartilages, such as those of the knee and lower jaw, to allow greater freedom of movement whilst diminishing the liability to dislocation. Under the microscope white fibro -cartilage looks very like fibrous tissue, but its cells are cartilage-cells, not tendon-cells (fig. 105). They are rounded or bluntly angular and surrounded by a concentrically striated area of clear cartilage-matrix. In some parts of the intervertebral disk many of the cells are branched, and may be looked upon as transitional forms to connective-tissue corpuscles. S6 THE ESSENTIALS OF HISTOLOGY. LESSON XIII. THE CONNECTIVE TISSUES (continued). BONE; STBUCTURE AND DEVELOPMENT. 1. In thin sections of hard bone made by grinding,' observe the Haversian ■canals, lamellae, lacunae, canaJiculi, etc. Make a sketch first under a low and afterwards under a high power. 2. With fine forceps strip ofi' a thin shred from the superficial layers of a bone which has been decalcified in 5 p.c. commercial sulphurous acid and afterwards washed with water for 24 hours. It may be kept in dilute alcohol. Mount the shred in water. Observe the fibrous structure of the lamellae. Look for perforating fibres or the holes from which they have been dragged out. Sketch a small piece of the thin edge of a lamella. 3. Stain with dilute magenta and hsemalum solution, or with methyl-blue and eosin, very thin sections of compact bone which has been fixed with 10 p.c. formol (1 to 3 days) and then decalcified in sulphurous acid as above. Mount in dilute glycerine, cementing at once. Look for fibres of Sharpey piercing the circumferential lamellae. The elastic perforating fibres are more darkly stained than the others. Notice the stained nuclei of the bone- corpuscles in the lacunae. In the thinnest parts of the sections try to make out the blood-vessels and other structures in the Haversian canals. 4. Mount in xylol balsam or dammar a section of a foetal lower jaw which has been stained in bulk and embedded in paraflBn. Find the part where the lower jaw-bone is becoming ossified, and carefully study the appearance which it presents. The bone is prolonged in the form of osteogenic fibres which are covered with osteoblasts. 5. Intramembranous ossification may also be studied in the parietal bone of a foetus which has been preserved in Miiller's fluid. A piece of the growing edge is scraped or brushed free from its investing membranes, and from most of the cells which cover and conceal it, and is mounted in glycerine with or without previous staining with carmalum. 6. Mount in balsam or dammar sections of a foetal limb (which may have been stained 4n bulk). The bones will be found in different stages of ossification, those of the digits being least developed. Make sketches illus- trating the three chief stages of endochondral ossification. Notice the peculiar terminal ossification of the third phalanx. Bone is a connective tissue in which the ground-substance is impregnated with salts of lime, chiefly phosphate, these salts consti- tuting about two-thirds of the weight of the bon6. When bones are macerated this earthy matter prevents the putrefaction of the animal matter. When bones are calcined they lose one- third of their weight, owing to the destruction of the animal matter ; when steeped in acid ' It is best to purchase these. BONE. 97 the earthy salts are dissolved and only the animal matter is left. This, like areolar and fibrous tissue, is converted into gelatine by boiling. .> P'V Fig. 106. — Section op a decalciwed homan eadius. (Sobotta.) x48. p, periosteum ; pi, periosteal bony lamellse ; pfl\ deeply seated lamellae parallel with periosteal surface ; H, Haversian systems ; tr, tr, trabeculas of spongy substance ; ml, lamellae bounding medullary spaces. Bony tissue is either compact or cancellated. Compact bone is dense, like ivory ; cancellated ii spongy with obvious interstices. The outer layers of all bones are compact, and the inner part is generally 98 THE ESSENTIALS OF HISTOLOGY. cancellated, but the shaft of a long bone is almost entirely made up of compact substance, except along the centre, which is hollow and filled with marrow. The interstices of cancellated bone are also occupied by marrow. Externally bones are covered except at the joints by & vascular fibrous membrane, the periosteum. True bone is always made up of lamellm, and these again are com- posed of fine fibres lying in a calcified ground-substance. , Between the lamellae are branched cells, the lone-corpuscles, which lie in cell-spaces or lacuncB. The ramified passages which contain the cell-processes are termed canaliculi. Fig. 107.— Tkaksvkrse section op compact tissue (of nnMEBUs). (Sharpey.) Magnified about 150 diameters. Three of the Haversian canals are seen, with their concentric rings ; also the lacunse, with the canaliculi extending from them across the direction of the lamellse. The Haversian apertures had become filled with air and debris in grinding down the section, and therefore appear black in the figure, which represents the object as viewed by transmitted light. In cancellated bone the blood-vessels run in the interstices supported by the marrow. In compact bone they are contained in little canals — the Haversian canals — which everywhere pervade the bone. These canals are about 0-05 mm. {-^-^ inch) in diameter, but some are smaller, others larger than this. Their general direction is longi- tudinal, i.e. parallel to the long axis of the bone, but they are constantly united by transversely and obliquely running passages. In a section across the shaft of a long bone they are seen as small rounded or irregular holes (fig. 106). When the section has been made by grinding, the holes get filled up with air and debris, and they then look black by transmitted light, as do also the lacunse BONE. 99 and canaliculi (fig. 107). Most of the lamellae in compact bone are disposed concentrically around the Haversian canals ; they are known as the Haversian lamellae, and with the included canal form what is known as a Haversian system. The lacunae of a Haversian system communicate with one another and with the Haversian canal, but not as a rule with the lacunae of adjacent Haversian systems. The angular interstices between the Haversian systems are generally Fig. 108. — Tkahsverse section of decalcified human tibia, fkom near the scbfacb of the shaft. H, H, Haversian canals, with their systems of concentric lamellfe ; in all the rest of the figure the laniellxi are circumferential ; s, ordinary perforating fibres of Sharpey ; e, e, elastic perforating fibres. Drawn under a power of about 150 diameters. occupied by bony substance which is fibrous but not lamellar. Besides the lamellae of the Haversian systems there is a certain thickness of bone at the surface, immediately underneath the periosteum, which is composed of lamellae arranged parallel with the surface; these are the circumferential or periosteal lamellce (fig. 106, pi). They are pierced here and there by simple canals for blood-vessels, the so-called Folkmann's canals, which are proceeding from the periosteum to join the system of Haversian canals, and also by calcified bundles of white fibres and by elastic fibres which may also be prolonged from the periosteum. These are the per f orating fibres of Shwrpey (fig. 108). 100 THE ESSENTIAI^j^ MBBE, SHOWING ABEAS OF COHN- '™ HEIM. Three nuclei are seen lying close to the Fig. 124.— Small portion of a saroolemma. MDSOLE FIBKE OF CRAB SPLIT- TING UP INTO FIBRILS. (From a photograph. ) Magnified 600 diameters. in the muscles of insects. They indicate the divisions between the longitudinal elements {fibrils or sarcostyles) which compose the fibre, and in preparations treated with dilute acid they appear to form part of a fine network, which pervades that substance, and serves to unite the granules both transversely and longitudinally. This network, which is sometimes very distinct in preparations of muscle treated with chloride of gold, is, however, a network in appearance only : in reality it is the optical expression of the interstitial substance which lies between the fibrils. This substance is termed sarcoplasm. On examining the transverse section of a fibre with a high power, it is seen to be subdivided everywhere into small angular fields, Coknheim's areas (fig. 12.5), which are themselves again divided up. The smallest divisions represent sections of the fibrils of w;hich the fibres are composed, and into which they may be split after death, especially after being hardened in certain reagents, e.g. chromic acid VOLUNTARY MUSCLE. 113 or osmic acid. The larger areas represent groups of fibrils. These areas of Cohnheim are usually polyhedral, but they may be elongated, and disposed either radially, or concentrically with the circumference of the section. The interstitial substance or sarcoplasm lies between them and can be made visible by treatment with dilute acid or by staining with chloride of gold (figs. 127, 128, and 129). It is some- times in relatively large amount, but in most muscular fibres is reduced to a very fine interstitium. An ill-defined clear line is sometimes seen running trans verselj' across the fibre in the middle of each dark band. This is termed Hensen's line. If instead of focussing the surface of the fibre it be observed in its depth, an appearance different from that shown in fig. 122 is frequently visible, namely, a fine dotted line (Dobie's Ime), bisecting each clear stripe (fig. 123) ; this appearance is often considered to represent a membrane [Krause's membrane), which subdivides the fibrils at regular intervals (see p. 116). But the membrane of the individual fibrils or sarcostyles is rarely, if ever, visible in an intact mammalian fibre, and it is certain that the appearance of such a line in the middle of the clear stripe of an intact fibre is in most cases due to interference, caused by the light being transmitted between disks of different refrangibility. Hayoraft has suggested that the cross-striation of voluntary muscle is due to refractive effects produced by a varicosity of the component fibrils, basing his view upon the fact that in impressions of the fibres made in soft collodion all the cross-striations which are observed in the fibre itself are reproduced. There is no doubt that a well-marked cross-striated appearance can be produced in homogeneous fibrils by regularly-occurring varicosities, and many of the appearances observed in muscle may, as Haycraft contends, be referred to this cause. But even when a fibre or fibril is stretched so that it exhibits no varicosities, the cross-striations are still perfectly distinct. Moreover, in view of the entirely different manner in which the substance of the dark and clear stripes behave to many staining reagents, and especially to chloride of gold when applied as directed in § 7, the fact being that very definite structural appearances can under these circumstances be made out, the homogeneity of the muscle-fibril cannot be admitted. This inference is strongly confirmed by the microchemical work of A. B. Macallum, who has shown that the potassium salts of the muscle are mainly accumulated in the sarcous elements. Nuclei. — Besides the sarcolemma and striated substance, a muscular fibre also exhibits a number of oval nuclei which have the usual structure of cell-nuclei : their chromatin often has a spiral arrangement. Sometimes there is a little granular substance (protoplasm) at each pole of the nucleus ; each nucleus with the adjacent protoplasm has then been spoken of as a muscle-corpuscle. But the protoplasm which is adjacent to the nuclei is in all probability continuous with the H 114 THE ESSENTIALS OF HISTOLOGY. sarcoplasm between the fibrils ; both being the remains of the original undiflFerentiated protoplasm of the cells from which the muscular fibres are developed. In mammalian muscle the nuclei usually lie imme- d Fig. 126.— Living muscle of water-beetle (dttiscus maeginalis.) (Highly magnified.) s, sarcolcmma ; o, dim stripe ; 6, bright stripe ; c, row of dots in bright stripe, which seem to be the enlarged ends of rod-shaped particles, d, but are really expansions of the interstitial sarcoplasm which appear in the living muscles as fine dark lines with dot-like enlargements upon them. Fig. 127. — Portion of leg-muscle of insect treated with dilute ACID. S, sarcolemma ; B, dot-like enlargement of sarcoplasm ; Kj Krause's membrane. The sarcous elements are dissolved or at least rendered invisible by the acid. iiiiii ■"ii'lill fiiillUIli Fig. 128. Fig. 129. Fig 128. — Transverse section of leg-muscle fibre of an insect, STAINED with GOLD CHLORIDE The sarcoplasm is here stained, and appears in the form of a network, in the meshes of which lie the sections of the fibrils. Notice the mottled appearance of the sections of the sarcostylos or fibrils, indicating a porous structure, as in the wing fibrils (see fig. 132). The central protoplasm (with a nucleus) is also evident. (From a photograph.) Fig 129.— Leg-musole fibre of insect treated with dilute acid, showing a tendency to break across into disks. The sarcoplasm is in the form of fine lines. The ordinary dark stripes of the fibre have disappeared m the acid. A, a disk seen partly in section and exhibiting the reticular arrangement of the sarcoplasm ; B, longitudinal view of fibre. VOLUNTARY MUSCLE. 115 ■iiP^ -S.K diately under the sarcolemma (figs. 121, 122, 125), in frog's muscle they are scattered throughout the substance of the fibre; in insect muscle they occupy the middle of the fibre, embedded in granular firotoplasm (fig. 128). Some animals, such as the rabbit, have, besides muscles of the ordinary type of structure, which in this animal are pale in colour, others of a deep red colour. These rti muscles were found by Eanvier to exhibit certain differences both in structure and function. One difference of structure is that the nuclei, which are numerous, are not confined to the surface, but are scattered throughout the substance of the fibres. The fibres in question also contain more sarcoplasm than the ordinary fibres, and their blood- vessels have a peculiarity of structure which will be afterwards noticed. Here and there, in all mammals, amongst the ordinary fibres are some in which the nuclei are distributed through the thick- ness of the fibres ; this is the case also, as just remarked, with all the muscular fibres of the frog. In muscles which are in constant activity, such as the dia- phragm and the dorsal fin muscles of Hippocampus, the protoplasm (sarco- plasm) of the fibres is present in rela- tively large proportion, and this is also the case with the wing .muscles of insects. The transverse section of a muscle shows the fibres to be nearly cylindrical in figure. Between the fibres there is a certain amount of areolar tissue, which serves to support the blood-vessels and also unites the fibres into fasciculi ; the fasciculi are again united together by a larger amount of this intramuscular connective tissue {endo- mysium.) Ordinary or leg-muscles of insects. — In the muscles of insects the stripes are relatively broad, and their structure can be more readily seen than in mammals. In the living fibres from the muscles which move the legs, the sarcoplasm presents a striking appearance of fine longitudinal lines traversing the muscle, and enlarging within the Kght stripes into rows of dots (fig. 126). This is still better seen in fibres and portions of fibres which have been treated with dilute acid (fig. 127). In separated disks produced by. the breaking across of muscle-fibres, the surfaces of the disks show a network with poly hedral meshes in some insects (fig. 129, a), one formed of lines radiating YL Fig. 130.~Leg-musclk fibre of insect, stained with gold CHLORIDE BY ROLLETT'S METHOD. K, line formed by membranes of Kiause; S.E., dark stripe formed by sarcous elements. The sarco- plasm has the appearance of longi- tudinal lines. 116 THE ESSENTIALS OP HISTOLOGY. from the centre of the fibre in others. The nuclei, with some inclosing protoplasm, lie in the middle of the fibre. Wing-muscles of insects.— The wing-muscles of insects are easily broken up into fibrils (sarcostyles), which also show alternate dark and light striae (fig. 131). The sarcostyles are subdivided at regular intervals by thin transverse disks (membranes of Krause) into successive portions, which may be Fig. 131. — Fibrils of the wing-muscles of a wasp, prepared by rollbt's METHOD. Highly magnified. (From photographs.) A, a. contracted fibril. B, a stretched fibril, with its sarcous elements separated at the line of Hensen. C, an uncontracted fibril, showing the porous structure of the sarcous elements. termed sarcomeres. Each sarcomere is occupied by a portion of the dark stria of the wholfe fibre (sarcous element) : the sarcous element is really double, and in the stretched fibre separates into two at the line of Hensen (fig. 131, b). At either end of the sarcous element is a clear substance (probably fluid or semi-fluid) separating it from the membrane of Krause : this clear substance is more evident the more the fibril is extended, but diminishes, even to complete disappearance, in the contracted muscle (fig. 131, A). The cause of this change is WING-MUSCLES OF INSECTS. 117 explained when we study more minutely the structure of the sarcous element. For we find that each sarcous element is pervaded with longitudinal canals or pores, which are open in the direction of Krause's membranes, but close.d at the middle of the sarcous element (fig. 132). In the contracted muscle, the clear part of the muscle-substance has disappeared from view, but the sarcous element is swollen and the sarcomere is thus shortened : in the uncontracted muscle, on the other hand, the clear part occupies a considerable interval between the sarcous element and the membrane of Krause, the sarcomere being Fig. 132. — Isolated sabcous elements of a ■wino-musclb, showins the tubulak or poeous steuotuke. (Magnified 2300 diameters,) Some are seen in profile ; others on the flat. Fio. 133. — Diagram of a saroomeee in a MODERATELY EXTENDED CONDITION, A, AND IN A CONTRACTED CONDITION, B. K, K, membranes of Krause ; H, line or plane of Hensen ; S.E., poriferous sarcous element. lengthened and narrowed. The sarcous element does not lie free in the middle of the sarcomere, but is attached at either end to Krause's membrane by very fine lines, which may represent fine septa, running through the clear substance (fig. 133); on the other hand, Krause's membrane appears to be attached laterally to a fine membrane which limits the fibril externally. The pla,nes of sarcous elements set side by side in a muscle-fibre form the dark stripe (the so-called principal disk) of the muscle- substance of ordinary muscle-fibres (fig. 130). But in the wing-muscles of- insects the sarcous elements of the fibrils less constantly lie in continuous planes, and the whole fibre is therefore very indistinctly and irregularly cross-striated, although each individual fibril is markedly so (fig. 131). As already stated, the sarcous elements are remarkable for containing a large proportion of potassium salts (Macallum). Sometimes in the ordinary (leg) muscles of arthropods what look like detached dot-like portions of the sarcous element are seen within the clear stripes, lying usually near Krause's membrane. The rows of such dots have been termed accessory dish. Most muscles show no accessory disks, but the safcoplasmic enlargements between the fibrils (fig. 127, d) are often mistaken for them. 118 THE ESSENTIALS OF HISTOLOGY. Muscle in polarised light.— When muscle-fibres are examined with polarised light between crossed Nichol's prisms, the sarcous elements (which form the dark stripe) are seen to be doubly refracting (anisotropous), while the clear substance (forming the light stripe) is singly refracting (isotropous). In contracted parts of the muscle the (anisotropous) sarcous elements are seen to have increased in bulk, while the isotropous 'substance of the clear stripe has correspondingly diminished in amount (fig. 134, b). Fig 134— Leg-mdscle fibre or chktsomela coeedlea with (fixed) con- TBAOTION WAVE PHOTOGRAPHED UNDER POLARISING MICEOSCOPB.' A, with uucrossed Nichols ; B, with crossed Nichols. E. Merkel described a reversal of the stripes during contraction, i.e. a transference of the anisotropous substance of the dark stripe from Hensen's line to Krause's membrane, the place of the dark stripes thus becoming occupied by clear material, that of the light stripes by dark. He further described this condition as being preceded by an intermediate stage in which the fibril shows homogeneity of shading. No doubt in the ordinary muscle- fibres of arthropods, when we observe the so-called 'fixed' waves of con- traction, there is an apparent blurring of the cross-striation of the fibre just where the muscle is passing from extension to contraction, but this appear- ance is explicable by the unequal pull of the contracted parts of the fibrils upon those which are not yet contracted. The contraction in each fibre starts from the nerve-ending, which is at one side of the fibre, and spreads first across the fibre and then tends to pass as a wave towards either end. But the one side always has a start in the progress of this wave, and the fibrils must thus receive an unequal pull, so that they are shifted along one another and the line of cross-striping is broken up. That no transference of anisotropous substance really occurs is at once clear from the appearance ^ I am indebted to Professor Engelmann for these two photographs. MUSCLE IN POLARISED LIGHT. 119 of the contracting fibre under polarised light (fig. 134, b), and the study of the isolated fibrils of wing-muscle gives no support to the theory of reversal, although it is widely held by German authors. That the apparent reversal is not real is also illustrated by fig. 135, which represents a leg muscle fibre of an insect in process of contraction. The dark bands of the contraction- wave are seen to be really due to accumulations of sarcoplasm. These accumulations appear as dark lines which obscure the continuity of the fibrils, and by contrast cause the whole of the sarcomeres between them to appear light. Mechanism of contraction.— Comparing the structure of the sar- comere with that of the protoplasm of an amoeboid cell we fiud in both a framework (spongioplasm, sub- stance of sarcous element) which incloses in its meshes or pores a clear, probably fluid substance (hyaloplasm, clear substance of sarcomere). In both instances also the clear substance or hyaloplasm, when the tissue is subjected to stimulation, passes into the pores of the porous substance or spongio- plasm (contraction), whilst in the absence of such stimulation it tends to pass out from the spongioplasm (formation of pseudopodia, resling condition of muscle). The effect of stimulation appears in both structures to be the production of a change in surface tension (perhaps between the hyaloplasm and spon- gioplasm) ; this change being de- monstrably accompanied in muscle by a difference in electric potential. In all prbbability such an electric change occurs in all protoplasm. Thus both the movements of cell-protoplasm and those of muscle seem brought about by like means, although at first sight the structure of muscle is quite dissimilar from that of protoplasm. We have already noticed that the movements of cilia are susceptible of a somewhat similar explanation. Fig. 135. — "Wave of contraction passing ovek a leg-muscle tibre of dttisous. Highly magnified. 120 THE ESSENTIALS OP HISTOLOGY. LESSON XV. CONNEXION OF MUSCLE WITH TENDON; BLOOD- VESSELS OF MUSCLE; CARDIAC MUSCULAR TISSUE; DEVELOPMENT OF MUSCLE; PLAIN MUSCULAR TISSUE. 1. To study the connexion of muscle with tendon, a frog is killed by destruc- tion of the brain and spinal cord, and placed in about a litre of water raised to a temperature of 55° C. It is left in this for 15 minutes, the water gradually cooling. It is then easy to dissociate the muscular fibres m lar^e numbers. To observe their attachment to the tendon-bundles a fine longi- tudinal shred must be snipped oflF with scissors at the tendinous attachment, and dissociated upon a slide in a drop of water. It will usually be found that the muscular substance is retracted from the end of the sarcolemma tube, which is firmly cemented to the tendon-bundle. The structure may be brought more distinctly into view by adding to the dissociated fibres a drop of a weak solution of iodine in salt solution or in serum (iodised serum).' 2. The blood-vessels of muscle. These are studied in longitudinal and transverse sections or in flattened-out pieces of injected muscle. It will be noticed that the capillaries are very numerous, and form a network with oblong meshes. In the red muscles of the rabbit, small dilatations are seen on the transverse cords of the network. 3. The muscular tissue of the heart is studied in sections of that or^an (see Lesson XXVII.) and also in teased preparations. To prepare the latter,, place a small piece of heart-muscle in 33 per cent, alcohol for a few days ; stain in picro-carmine solution for some hours or days ; and tease in dilute glycerine. 4. Tear off a small shred of the muscular coat of a piece of cat's intestine which has been for 48 hours or more in Fper cent, bichromate of potash solution or in 33 per cent, alcohol. Hold the shred with forceps in a drop of water and fray it out with a needle. In this process many cells will be set free and can be found with a low power. The preparation may then be covered and examined with a high power. Sketch one of the cells. Then allow dilute hsematoxylin solution to pass under the cover-glass and lastly a drop of glycerine. Sketch another cell after staining. Measure two or three cells and their nuclei. Ending of muscle in tendon. — A small tendon-bundle passes to- each muscular fibre and becomes firmly united with the sarcolemma, which extends over the end of the fibre (fig. 136). Besides this- immediate attachment, a further connexion is established by the 'This method is the one given by Ranvier {Traiti Technique, p. 395). The mascle-endings may also sometimes be well seen at the extremities of the tendons which are removed from the mouse's tail in the manner described iu Lesson X. BLOOD-VESSELS OF MUSCLE. 121 fact that the areolar tissue between the tendon-bundles is continuous with that which lies between the muscular fibres. Blood-vessels of muscle. — The capillaries of muscular tissue are very numerous. They run, for the most part, longitudinally, with transverse branches, so as to form long oblong meshes (fig. 137). No blood-vessels ever penetrate the sarcolemma. In the red muscles of the rabbit, the transverse capillaries have small dilatations upon mil. i,ii,.„i,|;,,|,..| S3 Fig. 136.— Tbemination of a mcs- CULAK FIBKE IN TENDON. (Ran- ™''-) Fig. 137.— Capillakt vessels of 7)1, sarcolemma ; s, the same membrane MUSCLE, passing over the end of the fihre ; p, extremity of muscular substance, c, retracted from the lower end of the sarcolemma-tube ; t, a tendon-bundle passing to be fixed to the sarcolemma. them (fig. 138). Associated with this and other peculiarities of structure (see p. 115), it is found that the red muscles have a much slower rate of contraction, and a much longer period of latency than the ordinary muscles. Lymph-vessels, although present in the connective-tissue sheath (perimysium) of a muscle, do not penetrate between the component fibres. The motor nerves of voluntary muscles pierce the sarcolemma 122 THE ESSENTIALS OF HISTOLOGY. and terminate in ramified expansions known as end^lates or moiar end-m-gans ; the sejisory nerves end in groups of specially modified muscle fibres known as muscle-spindles (see Lesson XIX.). Development.— Voluntary muscular fibres are developed from em- bryonic cells of the mesoderm (muscle-plate), which become elongated, and the nuclei of which become multiplied, so as to produce long slender multi-nucleated fusiform or cylindrical embryonic fibres. According to most recent authorities the Embryonic fibres are not formed by the growth of a single cell, but by the joining together Fig. 138. — Vascclae netwokk or a bed muscle (sbmi-tendinosus) of the KABBIT. (Ranvier.) a, arteriole ; v, v, venules ; n, dilatation on transverse brancli of capillaries. end to end of a number of cells of the muscle-plate (or even of more than one muscle-plate), so as to produce a syncytium, within which the striated fibrils make their appearance. These appear at first along one side of the fibre, the change gradually extending around the circumference and also penetrating towards the centre; but the protoplasm at the middle of the fibre, to which the nuclei are presently confined, and at the side opposite to that at which the differentiation began, remains for some time unaltered in character (fig. 139). Eventually the change in structure extends to these parts also, and the nuclei pass gradually to occupy their ordinary position under the sarcolemma, which has by this time become formed. The sarcolemma is believed to be produced, not by the muscle-fibre itself. BLOOD-VESSELS OF MUSCLE. 123 but by the mesenchyme or connective-tissue cells betwfeen the fibres, since it is directly continuous with the connective-tissue bundles of the tendon and of the interstitial tissue. Fig. 139. — Developing muscular fibres. A, elongated cell with two nuclei. A striation is beginning in the protoplasm along one side of the cell ; from foetal aheep. (Wilson Fox.) B, from human foetus of two months. (Eanvier.) p, central protoplasm with several nuclei, n, scattered In it; s, commencing sarcolemma, with striated muscular sub- stance developing immediately beneath it. C, from human foetus of three months, (Banvier.) The contractile substance, s, /, now almost incloses the unaltered protoplasm, g ; only one nucleus, n, is represented. CARDIAC MUSCLE. The muscular substance of the heart is composed of transversely striated muscular fibres, which differ from those of voluntary muscle in the following particulars, viz. : — their striations are less distinct ; they have no sarcolemma, although there is a thin superficial layer of non-fibrillated substance; they branch and unite by their branches and also at the side with neighbouring fibres, and their 124 THE ESSENTIALS OF HISTOLOGY. nuclei lie in the substance and often near the centre of the fibres. In man and many mammals the fibres are marked off into a series of short cylindrical cells (figs. 140, 141) joined together end to end and side to side, each corresponding to one of the nuclei. The junctions of these cells may be seen in longitudinal sections appropriately stained ; they come also distinctly into view in sections of the fresh tissue stained with nitrate of silver. They appear to be bridged across by fine fibrils, continued into the cells above and below the lines of junction (fig. 143). These lines have usually been regarded as intercellular spaces separating the constituent cells of the tissue from one another Fig. 140. — Musculak fibres from the heart, magnified, showing their oross-8tri2e, divisions, and junc- TIONS. (Schweigger-Seidel. ) The nuclei and cell-junctions are only repre- sented on the right-hand side of the figure. Fig. 141.— Six muscular fibre-cells FROM THE HEART. (Magnified 425 diameters.) a, line of junction between two cells ; b, c, branching of cells. (From a drawing by J. E. Neale.) (Schweigger-Seidel). But recent authorities (Przewosky, v. Ebner, M. Heidenhain) are inclined to regard the cardiac muscular tissue as forming a syncytium, the cells being all continuous both laterally and longitudinally, and the apparent intercellular lines being special differentiations. These, according to v. Ebner, are due to localised con- tractions, but, according to Heidenhain, represent portions of the fibres at which growth in length occurs (analogous to the suture-lines between the flat bones of the cranium). As against this view of the structure of the heart-muscle, and in favour of that of Schweigger- Seidel, must be set the silver-staining of the supposed cell-junctions, and the fact that it is easily possible in some animals to separate the fibres after maceration into short uninucleated fragments as in fig. 141. CARDIAC MUSCLE. 125 The short non-nucleated lengths of fibres (fig. 142), which Heidenhain regards as fatal to the cellular theory, may be parts of cells lying in other planes of the myocardium, which are inserted between those belonging to the plane included in the longitudinal section. On the other hand, the continuity of the muscular fibrils within the masses of Purkinje's fibres under the endocardium in the sheep, the fibrils around one cell being freely continued around the neighbouring cells (see fig. 304, p. 252), is in favour of the syncytial theory. Further, Fig. 142. -Diageam oi' segmentation Fig. 143.— Portion of cardiac muscle OF HEART MUSCLE. (M. Heideiihaiu. ) exhibiting continuity of fibrils Parts of the segments contain one across .iunctional line. (Przewoskv.) OE TWO nuclei, but SOME ARE QUITE Highly magnified. SMALL AND NON-NUCLEATED. in many vertebrates, including some mammals, no cell-territories can be made out in the myocardium, whilst in others, and especially in man and some mammals, although definite cell-territories can be shown to exist in the adult condition, they are absent in young animals. We must therefore conclude that both conditions may occur. The explanation of these differences appears to lie in the fact that in all heart-muscle at a certain period of development the cells form a syncytium within which the contractile fibrils are developed, and only in mammals is a differentiation of the syncytium into cells produced; the lines of junction being even here bridged across by the muscle-fibrils. 126 THE ESSENTIALS OF HISTOLOGY, NON-STRTATED, SMOOTH OR PLAIN MUSCLE. Involuntary or plain muscular tissue is composed of long, somewhat ilattened, fusiform cells (fig. 144), which vary much in length. Each cell has an oval or rod-shaped nucleus, which shows the usual intra- nuclear network and commonly one or two nucleoli. The cell-substance Fig. 144.— Muscclae fibre-cells fkosi the muscdlak coat or THE SMALL INTESTINE, HIGHLY MAGNIFIED. A, a complete cell, showing the nucleus with intra-nuclear network, and the longitudinal fibrillation of the cell-substance, with finely vacuolated protoplasm between the fibrils ; B, a cell broken in the process of isolation ; a delicate external layer projects at the broken end a little beyond the striated substance of the cell. Fig. 144. ^i*''i Fig. 145. — IVIuscle-cells of intew- TINE. (Szymonowicz.) Magnified 530 diameters. The fibres are represented in longitudinal section ; and the interstices between them are seen to be bridged across by fine fibrils, i, interstice ; )', nucleus. Fig. 146. — Plain muscle fibre, .showing nd- CLECS, CENTRIOLE, AND CYTOPLASM WITH FIB- RILS. (Lenhossek. ) is finely fibrillated, but does not exhibit cross-stria like those of voluntary muscle. There appears, as in cardiac muscle, to be a delicate non-striated external layer, probably a stratum of undifferen- tiated protoplasm, certainly not a true sarcolemma. Next to this, in some smooth muscle, is a layer containing coarser fibrils (boundary fibrils of M. Heidenhain). There is a little intercellular substance PLAIN MUSCLE. 127 which can be stained by nitrate of silver, and which is bridged across by filaments passing from cell to cell (fig. 145). Some authorities, however, deny that the involuntary cells are thus connected, and hold that the appearance of bridging fibres is due to intercellular connective tissue. It is however difficult to understand how the contractions are propagated from cell to cell if there is no sort of continuity between the cells. Plain muscular tissue is found chiefly in the walls of hollow viscera ; thus it forms the muscular coat of the stomach and intestines, and occurs abundantly in the muscular coat of the gullet, although it is here intermixed with cross-striated muscle ; it is found also in the mucous membrane of the whole alimentary canal from the oesophagus downwards ; in the trachea and its ramifications ; in the urinary bladder and ureters ; in the uterus and Fallopian tubes ; in the prostate ; the spleen and lymphatic glands ; the muscle of Miiller in the orbit, and in the ciliary muscle and iris. The walls of gland-ducts also contain it ; and the middle coat of the arteries, veins and. lymphatics isilargely composed of this tissue. It occurs in the skin, both in the secreting part of the sweat glands, and in small bundles attached to the hair- follicles ; in the scrotum it is found abundantly in the subcutaneous tissue (dartos), and it also occurs in the areola of the nipple. Development. — According to the observations of C. M'Grill, the smooth muscle of the alimentary canal (pig) is developed from the syncytium of mesenchyme cells which surrounds the entoderm. Some of these cells become elongated and spindle-shaped while retaining their inter-connexion. Myofibrils are developed in their 'protoplasm. These are not confined to the limits of a single cell, but extend over two or even a large number of cells. The myofibrils are of two kinds, coarse and fine, varying in relative number in different parts. The distinction is seen even in the fully formed muscle, which retains its syncytial character, and is not formed of completely separated cells. In certain situations smootb muscle is formed from epithelium, as with th* miiiSBnlar tissue of the sweat glands (Ranvier) and that of the iris (Nussbaum, Szili). 128 THE ESSENTIALS OP HISTOLOGY. LESSON XVI. STRUCTURE OF NERVE-FIBRES. 1. Tbasb a .piece of fresh nerve rapidly in salt solution (or by the method of semidesiccation, afterwards mounting in salt solution), injuring the fibres as little and obtaining them as long and straight as possible. Study the medul- lated fibres, carefully noticing all the structures that are visible — viz., nodes of Eanvier, nucleus of primitive sheath, double contour of medullary sheath, medullary segments, etc. Measure the diameter of half a dozen fibres. Draw a short length of a fibre very exactly. 2. Prepare a piece of sympathetic nerve in the same way. The nerves passing to the spleen are well adapted for the study of non-meduUated fibres. They may also be found amongst the medullated fibres of the ordinarjfc nerves. The nuclei may be stained by gentian violet. 3. Separate (in dilute glycerine) into its fibres a small piece of nerve or nerve-root that has been twenty -four hours in 1 per cent, osmic acid. The nerve should have been moderately stretched on a piece of cork by means of glass pins before being placed in the acid. Keep the fibres as straight as possible and only touch them near their ends with the needles. Sketch two portions of a fibre under a high power, one showing a node of Banvier and the other a nucleus of the primitive sheath. Look for fibres of Bemak. Measure the length of the nerve-segments between the nodes of Eanvier. 4. Mount in xylol balsam or dammar sections of a nerve which has been hardened in picric acid and alcohol, or fixed with osmic acid and hardened in alcohol. The sections may be stained with picjro-carmine or hssmatoxylin. The nerve should be pinned out straight upon a cork with glass pins before being placed in the hardening solutions. Examine the sections first with a low and afterwards with a high power. Notice the lamellar structure of the perineurium, the varying size of the nerve-fibres, the axis cylinder in the centre of each fibre, etc. Measure the diameter of five or six fibres, and sketch a small portion of one of the sections. 5. Study sections of splenic nerve placed as soon as possible after death in Plemming's solution. 6. Teased preparations and sections from nerves which, some days pre- viously, have been cut nearer the spinal cord. The nerves should have been prepared with osmic acid, as in § 3. "Notice the breaking up of the myelin of the medullary sheath, varying in degree according to the length of time the section has been made previously. In preparations from the central cut end of the nerve prepared by Cajal's reduced silver method i new fibres may be seen budding from near the extremities of the undegenerated fibres of the stump. Nerve-fibres are of two kinds, medullated and novr-rmdullated. The cerebro-spinal nerves and the white matter of the nerve-centres are ^ See Appendix. STRUCTURE OF NERVE-FIBRES. 129 composed of medullated fibres ; the sympathetic nerves near their peripheral distribution are largely made up of non-medullated fibres. Fig. 147. — White or medullated nerve-fibres, showing the sinuous outline and double contours. Fig. 148. — Portions of two nervk- fibbes stained with osmio acid, FROM A YOUNG ANIMAL. (Diagram- matic. ) i?, R, constrictions of Ranvier, with axis- cylinder passing througli. a, neurolemma of the nerve ; c, opposite the middle of the segment, indicates the nucleus and proto- plasm lying between the primitive sheath and the medullary sheath. In A the nodes are wider, and the intersegmental sub- stance more apparent than in B. Fig. 148. The medullated or white fibres are characterised, as their name implies, by the presence of the so-called medullar// sheath or whiic substance. This is a layer of soft substance, physically of a fatty 130 THE ESSENTIALS OF HISTOLOGY. nature, which encircles the essential part of a nerve-fibre, viz., the axis-cyliiider. Outside the medullary sheath is a delicate but tough homogeneous membrane, the primitive sheath or nucleated sheath of Schimnn, but this is not present in all medullated fibres, being absent in those which are within the nerve-centres. The primitive sheath is known as the neurolemma.'^ ijlill!!;: ,;$£;;( Fig. 14!).— a small pakt of a medullated FIBRE. (Highly magnified. ) The fibre looks in optical section like a tube- hence the tenn tubular, formerlj' applied to these fibres. Two partial breaches of continuity (medullary clefts) are seen in the medullary sheath, which at these places exhibitsa tendency to split into laminse. The primitive sheath is here and there apparent outside the medullary sheath, and the delicate strise which are visible in the middle of the fibre indicate the fibrilla- tions of the axis cylinder. Fig. IfjO.— Two portions op siedol- lated neevefibees, after treat- ment WITH OSMIC ACID, SHOWING THE AXIS-CYLINDEE AND THE MEDULLARY AND PRIMITIVE SHEATHS. (Key and Retzius. ) A, node of Ranvier. B, middle of an inter- node with nucleus, c, axis-cylinder pro- jecting; p, primitive sheath, within which the medullary sheath, which is stained dark by the osmic acid, is broken away for a short distance. The medullary sheath is composed of a highly refracting fatty material (myelin), which gives a characteristic dark contour and tubular appearance to the nerve-fibres (fig. 147). It affords a continuous investment to the axis-cylinder, except that, as was shown by Eanvier, in the peripheral nerve-fibres it is interrupted at regular intervals. At these places the neurolemma appears to produce a constriction in the nerve-fibre, and the interruptions of the medullary sheath are accordingly known as the constrictions (Eanvier) or nodes (figs. 148, 151), the latter term being applied from the resemblance which they bear to the nodes of a bamboo. It is, however, uncertain whether the constriction is 1 Often termed "neurilemma," a name formerly applied also to the sheath of Henle (see p. 136). NERVE-FIBRES. 131 entirely occupied by the neurolemma itself or partly by a special band {constricting band of Ranvier) of a material which resembles intercellular substance in its reaction to nitrate of silver (fig. 162). The length of nerve between two successive nodes is termed an inter- node ; in the middle of each internode is one of the nuclei of the Fig. 151. — Nerve-fibke pkepaked with osmic acid. (Szymonowicz.) b, constriction of Ranvier. The intervals between the medullary scgmontB appear as clear oblique lines, a, a. neurolemma. Besides these interruptions the medullary sheath shows a variable number of oblique clefts (Lantermann) (figs. 149, 151), sub- dividing it into irregular portions, which have been termed medullary segments; but there is some reason to believe that the clefts are artificially produced. At the clefts there is an appearance of spiral fibres in the medullary sheath, especially after treatment of the nerve Fig. 152.- -Spiral and ketioulab fibkii.s in the sheath op a nbeve- PIBKE. (GoJgi. ) with certain reagents (Golgi) (fig. 152); it is, however, possible that this appearance does not represent any pre-existing structure. A reticular appearance has also been described in the medullary sheath {neurokeratin network of Kiihne), and can be readily seen in nerve fibres fixed in alcohol and treated with ether, but it varies greatly in aspect, and is perhaps produced by the action of the reagents Fig. 153.— Eecticulab appkaeanoe in the medullaky sheath of a NERVE-PIBKE. (Gedoelst.) (From the guinea-pig. ) employed to show it (figs. 152, 153). By other modes of fixation {e.g. picric acid) the medullary sheath seems to have a rod-like structure (fig. 155) ; this again may be due to the manner in which certain of its constituents are coagulated by the reagent. Osmic acid stains the medullary sheath black (figs. 151, 154, 156). 132 THE ESSENTIALS OF HISTOLOGY. The axis-cylinder, which runs along the middle of the nerve-fibre, is a soft transparent thread which is cantinuous from end to end of the nerve. On account of the peculiar refractive nature of the medullary- sheath it is difficult to see the axis-cylinder in the fresh nerve except at the nodes, where it may be observed stretching across the interruptions Fig. 154. — Lonsitddinal and transverse section of medullated nerve- riBRE OF FKOG (osMic ACID AND ACID fhchsine). (After Biedermann. ) The longitudinal section shows one node of Ranvier and two of Lantermann's clefts. The fibrillar structure of the axis-cylinder is shown in both longitudinal and transverse section. in the medullary sheath ; it may also sometimes be seen projecting from a broken end of a nerve-fibre. It is longitudinally striated, being made up of exceedingly fine fibrils (neuro-fibrils, fig. 154). They are readily seen at the terminations of nerves as in the cornea and are also visible in the section of a nerve- fibre as fine dots (fig. 154), which sometimes appear to have a clear centre (fig. 155), as if the fibrils were tubular. Staining with nitrate of silver produces a curious trans- versely-striated appearance in the axis-cylinder (Fromann) (fig. 162, c), but this is due to the precipita- tion of chlorides, and does not indicate a pre-existing structure (Macallum). Medullated nerve - fibres vary greatly in size (figs. 155, 156), but may be classified as large, inter- mediate, and small. The largest are those which are passing to the skin and to the voluntary muscles ; the smallest are those which are distributed to the viscera and blood- vessels by way of the autonomic nerves, i As shown by Gaskell, the •This term has been introduced by Laugley to include both the nerves of the sympathetic system and also the analogous nerves which proceed from the cranial and sacral regions for the innervation of certain involuntarv muscles and seoretmg glands. Fig. 105.— Section across five nerve- FiBEES. (Magnified 1000 diameters.) The nerve was hardened in picric acid and stained with picro-carmine. The radial striation of the medullary sheath is very apparent. In one fibre the rays are broken by shrinkage of the axis-cylinder. The fibrils of the axis-cylinder appear tubular. (From a photograph.) NERVE-FIBRES. 133 anterior roots of the last one or two cervical nerves, of all the thoracic, of the first and second lumbar, and of the second and third sacral nerves contain besides the ordinary large meduUated fibres a bundle of very small medullated fibres which are destined for the ar Fis. 136. — Section or the sciatic neeve of a cat, showing the vabiations IN SIZE 01' its constituent fibees. (Magnified 300 diameters.) The nerve was fixed with osmic acid. viscera and blood-vessels, and which for the most part pass to the sympathetic system. The roots of some of the cranial nerves (the spinal accessory, vagus, glossopharyngeal, and facial) contain similar fine medullated fibres. Non-medullated fibres. — Intermingled with the medullated fibres there may always, even in the cerebro-spinal nerves, be found a certain Fig. 157. — NoN-MEDULLATED NEBVE-FIBEES. (Magnified 400 diameters.) number of pale fibres devoid of the dark double contour which is characteristic of the presence of a nledullary sheath. These are the grey avirum-medullated fibres, also called, after their discoverer, fibres of Bemak (fig. 157).' They frequently branch, which the medullated 134 THE ESSENTIALS OF HISTOLOGY. fibres rarely do except near their termination, and they are beset with numerous nuclei which perhaps belong to a delicate sheath, but this is not certain, and undoubtedly both in longitudinal view and in cross section the nuclei seem to lie in the substance of the fibres. The sympathetic nerves, as they approach their peripheral dis- tribution, are largely made up of fibres of this nature, but many of the fibres contained in the sympathetic nerves possess a thin medul- lary sheath, and have the usual structure of medullated fibres. Structure of the nerve-trunks. — In their course through the body the nerve-fibres are gathered up into bundles or funieuU, and the funiculi are again united together to form the nerves which we meet with in dissection. The connective tissue which unites, the funiculi and invests the whole nerve, connecting it to neighbouring Fig. 158.— Section across non-meddllated fibres from the splenic nerve OP THE OX. (Tuokett.) Fig. 1.59.— Sbotiok of part of a nerve-trunk fixed with osmio acid. (From a, photograph.) Magnified 40 diameters. Three email funiculi and a small part of a larger funiculus are shown. The fat-cells in the epineurium are stained black by the osmic acid.' parts and conveying to it blood-vessels, lymphatics, and even nerve- fibres destined for its coats, is termed the epineurivm; it frequently contains fat-cells. That which ensheaths the funiculi is known as the perinmrium (figs. 159 to 161). It has a distinctly lamellar structure (fig. 160), the lamellae being composed of connective tissue covered by flattened epithelioid cells (fig. 162, a). Between STRUCTURE OF NERVE-TRUNKS. 135 the lamellae are clefts for the conveyance of lymph to the lymphatics of the epineurium. The delicate connective tissue which lies between the nerve-fibres of the funiculus is the endoneurium. It assists in Fib. 160.— Section of part op a funiculus of the sciatic nerve of a cat FIXED WITH Flemming's SOLUTION. Magnififed 400 diameters. ep, epineurium with blood-vessels ; e, section of an end-bulb ; p, perineurium ; m, medul- lated fibre cut at the level of a nucleus ; n, n, bundles of non-meduUated fibres. Fig. 161. — Section of the thoeaoio sympathetic cobd of the cat. (Fischer.) Osmio preparation. The nerve is composed in almost equal parts of fine medullated fibres (3, 4) derived from the thoracic anterior roots, and grey fibres (5) derived from the sympathetic ganglion-cells. The dark bodies in the epineurium (1) are fat-cells ; 2, perineurium. supporting the longitudinally arranged meshwork of blood-capillaries, and its interstices communicate with the lymph-clefts of the perineurium. All the branches of a nerve, and even single nerve-fibres which 136 THE ESSENTIALS OF HISTOLOGY. are passing to their distribution, are invested with a prolongation of the perineural sheath, which is then known as the sheath of Henle. goo 600 1 cy Fig. 162. — Nerves stained with silver nitrate. (Eanvier.) In A, the epitheliaMike layer of flattened cells Ijelongang to the sheath of Heul^ is stained. In B, the cross-like markings at the nodes are exhibited. In G, a single fibre is shown more highly magnified, with Fromann's transverse markings of the axis- . cylinder, a, constricting band ; m, medullary sheath ; <^, axis-cylinder. The nerve-trunks themselves receive nerve-iibres {nervi nervorum) which ramify chiefly in the epineurium and terminate within this in end-bulbs (Horsley) (fig. 160, e). The degenerative processes which occur in cut nerve-fibres as well as the subsequent reparative processes will be dealt with after the structure of nerve-cells ' has been studied (see p. 154). N-EEVE-CELLS. 137 LESSONS XVII. AND XVIII. NERVE-CELLS. 1. Pot » small piece of spinal ganglion into 1 per cent, osmic acid for a few hours. Place in water containing a fragment of thymol for two days or more. Tease in dilute glycerine. Notice the spheroidal ganglion-cells ; their large nuclei and distinct nucleoli. Many of the cells may still be seen within their nucleated membranous sheath. Look for cells which still retain the axis-cylinder process and for T-shaped junctions of nerve-fibres with this. Fat-cells may be present in the periganglionic connective tissue. These will appear intensely black in osmic preparations. 2. Prepare in the same way a spinal ganglion or the Gasserian ganglion of the skate or cod. Notice the bipolar character of most of the cells. 3. Prepare a piece of sympathetic ganglion as in §§ 1 and 2. If from a rabbit observe that many of the cells are bi-nucleated. Measure two or three cells in each of the above preparations. 4. Mount stained sections of ganglia, both spinal and sympathetic. These will serve to show the arrangement of the cells and fibres in the ganglion and the nucleated sheaths around the nerve cells. The ganglia may be fixed and hardened in saturated solution of corrosive sublimate or of picric acid or in 10 per cent, formol. They may either be stained in bulk or sections cut from paraffin and stained on the slide by Nissl's method. Ehrlich's methylene blue method, Golgi's silver chromate method, or Cajal's silver reduction method, especially the Kist named, are all useful for showing the cells and their connections with nerve-fibres. These methods are described in the Appendix. 5. Place a portion of the grey matter from a piece of Spinal cord in 33 per cent, alcohol. After macerating for two days or longer in this fluid, a little of the grey matter may be shaken up in a test-tube with water so as to bieak it up into fine fragments. Allow these to subside, decant off the water and substitute a dilute solution (1 to 500) of methylene blue or solution of picrocarmine. When it appears sufficiently stained some of the debris is pipetted off and examined under a low power of the microscope ; at first without a cover-glass so that the cells may, if necessary, be separated from the rest of the tissue. Mount in water with a thick hair under the cover- glass. Notice the large branching cells, some with a mass of pigment near the nucleus. Observe the fibrillation of the cell-processes. Many axis- cylinders will be seen in this preparation deprived wholly or partially of their medullary sheath, and their fibrillar structure can then also be well seen. Carefully sketch these appearances. To keep the methylene blue preparation the stain must be fixed with picrate of ammonia, after which a mixture of glycerine and picrate of ammonia may be used for mounting. If picrocarmine is used the specimen is simply preserved in dilute glycerine. Similar preparations may be made from the grey matter of the cerebral cortex and cerebellar cortex. 138 THE ESSENTIALS OF HISTOLOGY. 6. Examine sections of spinal cord, medulla oblongata and brain stained by methylene blue (Nissl's method), to exhibit the angular particles withm the nerve-cells. 7. Examine sections of parts of brain, spinal cord and ganglia prepared by Cajal's method, to exhibit the neuro-fibrils in the cells and cell-processes. 8. Examine the nerve-cells and neuroglia-celk in sections from the spinal cord, cerebrum, or cerebellum of a small animal, e.g. young rat or kitten, prepared by Golgi's method. The sections must be mounted in thick xylol balsam or dammar varnish, without a cover-glass, and dried rapidly on a warm plate. 9. Examine sections of spinal cord (lumbar enlargement) and correspond- ing spinal ganglia from an animal in which the sciatic nerve had been cut about three weeks before it was killed. The sections are to be stained by Nissl's method. Many of the anterior horn nerve-cells and of the ganglion- cells on the side of the lesion will exhibit the chromatolysis or breaking down of the Nissl granules, which is characteristic of cells the axons of which have been severed. They may be compared with the normal cells on the intact side. Nerve-cells, neurocytes or neurones. — Nerve-cells occur in the grey matter of the nerve centres, and in little groups on the course of certain of the peripheral nerves, these groups often causing nodular enlargements of the nerves,, which are known as ganglia. The most conspicuous ganglia are those which are found upon the posterior roots of the spinal nerves, upon the roots of some of the cranial nerves, and upon the trunk and principal branches of the sympathetic nerve. Minute ganglia are also found very numerously in connection with the nerves which are supplied to glands and involuntary muscular tissue, as in the salivary glands, heart, alimentary canal, bladder, uterus, etc. Nerve-cells vary much in size and shape ; many are large, some being amongst the largest cells met with in the body, but others are quite small. All nerve-cells possess at least one process, the axon, which becomes either a non-medullated fibrie or the axis- cylinder of a medullated fibre. If other processes are present they are always branched almost from their commencement at the cell-body, and they are therefore termed dendrons (dendrites). The nucleus is generally large, clear, and spherical, with a single large and distinct nucleolus; there may also be a network of chromatin, but this is not always to be seen. The cytoplasm is fibrillated, the fibrils passing into the processes; they are known as neuro-fibrils (p. 132), and are believed to be the actual conductors of nerve-impulses. It also contains peculiar angular particles (Nissl granules) staining deeply with methylene blue, but the size, number, and arrangement of these in different cells vary greatly (fig. 163). The granules also vary in number and size with the physiological condition of the cells ; thus it is found NERVE-CELLS 139 Fig. 163.— Multipolar and unipolar types of nervb-oell. A, iarge pyramidal cell of cerebral cortex, tumaij. . Nissl method... (Cajal.) a, axon ; b, cell-body ; c, apical dendron ; d, placed between two of the basal dendrons points to the nucleus of a neuroglia cell. B, Unipolar cell from spinal ganglion of rabbit. Nissl method., (Cajal.) a, axon ; 6, circumnuclear zone, poor iu granules ; c, capsule ; d, network within nucleus ; e, nucleolus. 140 THE ESSENTIALS OF HISTOLOGY. that nerve-cells which have been fatigued by prolonged activity (fig. 164), and also those the axis-cylinder process of which has been cut| (fig. 165), show the Nissl granules, becoming disintegrated ; they may even disappear for a time from the cell. A similar result is found to occur after the action of poisons which especially affect the nervous system. The Nissl granules of the nerve-cell appear to consist chemically mainly of nucleoproteid; they contain organically combined iron (Macallum). Many nerve-cells have also a clump of pigment- granules, containing lecithin, at one side of the nucleus. This is especially marked in certain locali- ties (locus coeruleus, locus niger), and is more frequent in man than Fio. 164.— Two MOTOB NEBVE-oELLs in the lowcr animals. The pigment FROM THE DOQ. ^jg^ tcnds to incrcasc in amount as a, normal ; i, after a period of prolonged activity. (Photographed from preparations age advances, by Dr. Gustav Mann.) ° . , , , i ^i r j e As already stated, the body ot every nerve-cell is traversed by fine fibrils {newo-fihils) continuous with those in the axis-cylinder of the issuing nerve and with similar '^^% Fig. 165.— Ohbomatoltsis of neeve-cblls, produced, by sbvbbanoe of axon. . (Diagrammatic.) A, Kissl granules normal ; B, commencing chromatolysis, the cell &nd nudeas swollen and the granules beginning to disintegrate ([the nucleus is usually close to the ' periphery at this stage); 0, advanced condition of chromatolysis, the cell and nucleus shrunken.. "^^' NERVE-CELLS. 141 fibrils in their dendrons. They were noticed by Max Schultze, but their course and connections were first accurately described by Apathy in the nerve-cells of certain annelids. They can be seen without any diffictilty in the nerve-cells of vertebrates (fig. 166) by the employ- FiG. 166. — Nerve-cells of kitten (from the anterior corpora quadrigemina) SHOWING NEUKO-FIBRILS. (Cajal.) a, axon ; b, c, d, various parts of the intracellular plexus of fibrils. ment of the silver reduction method of Cajal. The neuro-fibrils are said to present variations in thickness according to the condition of activity of the animal at the time of death. Most, if not all, nerve-cells show a delicate superficial reticulum (fig. 167), described by Golgi, which is generally regarded as composed of neuro-fibrils, but, according to J. Turner, may be an investment derived from neuroglia-cells. Golgi has also described another network 142 THE ESSENTIALS OF HISTOLOGY. of fibrils with somewhat larger meshes {deep reticnihm of Golgi) (fig. 168) in the deeper parts of the cell. According to some authorities both Fig. 167.— Superficial nbtwork of golgi sdrrodnding two oblls from the cerebral cortex op the cat; BHRLIOH's METHOD. (Cajal.) A, large cell ; B, small cell, a, a, folds in the network ; b, a ring- like condensation of tiie network at the poles of the larger cell ; c, spinous projections from the surface. Fig. 168.— Nerve-cell prom spinal ganglion, showing NETWORK AROOND THE NUOLEDS. (Golgi.) Ill Fig. 169. — Axis-cylindeb process op a nerve- cell FROM the SPINAl CORD. (M. Sohultze.) X X , portion of the cell-body, out of which the fibrils of the axis-cylinder proceasj a, are seen to emerge. At ^a', this process acquires ' a medullary sheath. (Highly magnified.) NERVE-CELLS. 143 superficial and deep networks are in continuity throughout the cell, and receive and are prolonged from the neuro-fibrils of an entering axon on the one hand, and with those of the axis-cylinder process of the nerve-cell, and also of the dendrons, on the other hand. Other authorities regard these networks as distinct from the neuro-fibrils, which they suppose to run independently through the nerve-cell body, entering it by way of the dendrons and emerging in the axon. "fROPHOSPONGiUMiOF NEKVE-CELLS. — Entirely distinct from the fibrils is a system of fine canaliculi, which has been described by E. Holmgren, permeating the cytoplasm of the nerve-cell body for the purpose of subserving its nutrition by conveying plasma into its substance (see fig. 4, p. 4). These channels are stated by Holmgren to be occupied by branching processes of other (connective-tissue or neuroglia) cells. In the very large nerve-cells from which the nerves of the electric organs of Malapterurus arise blood-vessels penetrate into the cytoplasm. Fig. 170.— Two bipolar ganglion cells (fish). (Holmgren.) In B the medullary sheath is continued as a thin layer over the cell-body. Processes of nerve-cells. — As already intimated the processes are of two kinds. The first is that known as the axis-cylinder process (Deiters) or nerve-fibre process, so called because it becomes the axis-cylinder of a nerve-fibre ' (fig. 169 a, a'); in the case of the non-medullated fibres, it becomes the nerve-fibre itself. It is also termed the neuraxon or simply the axon. Probably no nerve-cell is without this process. The place where it arises from the body of the nerve-cell (cone of origin) is marked ofi' from the rest of the cell-substance by absence of Nissl granules (see fig. 163). The other processes of the nerve-cell are those which were termed by Deiters the protoplasmic processes ; they are now usually termed the 144 THE ESSENTIALS OF HISTOLOGY. dendrons or dendrites and are generally multiple, whereas the axon is generally single. The dendrons are characterised by the fact that as soon as they leave the cell they begin to branch dendritically, whereas the axis-cylinder process does not branch until near its Fig. 171.— VaBIOUS forms OI' PEKIOELLUtAB ENDING OP ENTERING NEHVE FIBRES IN THE TEAPEZOID NUCLEUS OF THE CAT. (Edlnger,, after Veratti.) termination, with the exception of a few fine lateral offshoots, which are sometimes given off in its course. Dendrons may be absent; the cell is then adendric. Most nerve-cells have only one nerve-fibre process (unipoh/r), but some have two or more (bipolar, multipolw(). The dendrons contain Mssl's granules, but the axons do not. , PROCESSES OF NERVE-CELLS. 145 The shape of the cell depends largely on the number of processes, and the manner in which they come off from the cell. If there is but one chief process the cell is generally nearly spherical. This is the <:ase with most of the cells of the spinal ganglia (fig. 163, B) ; in these the single process, after a short course, divides into two fibres, which pass the one centrally the other peripherally (fig. 178). When there are two main processes from a nerve-cell they often go off in opposite directions from the cell, which is thus rendered somewhat spindle-shaped (fig. 170), but occasionally they emerge at the same part. ' When there are three or more processes, the cell becomes irregularly angular, as in the motor-cells of the spinal cord and the pyramidal cells of the cerebral cortex. In some cases where there appear to be two fibres connected with a cell, one of them is derived from another nerve-cell elsewhere; and is passing to end in a ramification which envelops the cell-body. In certain situations the ramification is coarse and forms a calyx-like investment to the cell-body : this investment may be so intimately united to the body of the second cell that it appears to be rooted into the external layer (fig. 171); in other places the pericellular fibrils are very fine and form a felt-work over the cell-body (fig. 172), the fibrils coming in contact with the surface of the cell and sometimes ending in small button-like enlargements or varicosities. In preparations made by Golgi's chromate of silver method the nerve-cells and their processes are coloured black by a deposit of reduced silver, so that the processes can be traced for a considerable distance from the body of the cell, in fact in many instances as far as their remotest ramifications. It is found by the employment of this method that the axis-cylinder process is not always an unbranched process, as was formerly supposed, but that it usually, if not invariably, 'I am indebted to Dr. J. Turner for the drawing here reproduced. K Fis. 172. — Pericellular neoeo-fibbils AROUND A LARGE PTKAMIDAL CELL OF THE HUMAN CORTEX CEREBRI. 1 Methylene blue preparation. 146 THE ESSENTIALS OF HISTOLOGY- io FIG. 173.-A PYBAMIDAL OEH OF THE OOETBX OEBBBEI OF IHK BABBIT. (CajM.>. haaal dendrons ; p, apioftl dendron ramifying near surface ; e, axon ; c, its „, basal d^^^^'ii^^^r^gTi,, fibres of white matter of brain. PROCESSES OF NERVE CELLS. 147 W Fig. 174. — Cell ow type II. op GtOlgi, with short axon kamipting in the ADJACENT GBET MATTER. SSXT^ I'iG. 175.— Synaptic connections op sympathetic cells pbom the SUPERIOR cervical GANGLION OP MAN. (Cajal.) The cells (A, B) show well-marked intracapsular deodrons. C, X>, synapses between dendrons outside the cell-capsules ; E, a fibre, which is itself surrounded by afinc spirally wound fibril, passing to a cell and forming a synapse with the cell-dendrons within i^e ' capsule, a, a, axons; b, c, d, e, /, extra- capsular dendrons. •* 148 THj; ESSENTIALS OF HISTOLOGY. gives off fine lateral branches (collaterals), which themselves tend to ramify in the adjacent nerve-substance (fig. 173). And although the main part of the axis- cylinder process usually passes on and becomes part of a long meduUated nerve-fibre (cell of type I. of Golgi, fig. 173), this is not always the case, for in another type of nerve-cell within the nerve-centres (cell of type II. of Golgi, fig. 174) the axis-cylinder pro- cess breaks up almost immediately into an arborescence. Moreover, the long process of type I. (which becomes the axis-cylinder of a long nerve-fibre) ulti- mately ends in a similar manner, that is to say, in a terminal ramification or arborescence, as will be seen in study- ing the endings of nerve-fibres, and the structure of the central nervous system. Neurone theory.— Each nerve-cell is generally regarded as an anatomically independent element (nerve-^nii, neurone), and the connection of one nerve-cell with another is believed to be effected through the medium of the terminal arborisations of the dendrons or axons. Such arborisations from different cells may interlace with one another (as in the olfactory glomeruli, in the retina, and in the sympathetic ganglia) (fig. 175), or a terminal arborisation from one cell may embrace the body or the cell-processes of another cell ; as with the cells of the spinal cord (fig. 176) and the cells of the trapezoid pucleus of the pons Varolii (fig. 171) and in many other places. The term neuro-synapse may be applied to these modes of junction. By them nerve-cells are linked together into long chains of neurones, the physiological path being uninterrupted, although the anatomical path is, as above indi- cated, believed to be interrupted at the synapses. Fig. 176.— Arbokisation of col- laterals FROM THE POSTBRIOB ROOT-FIBEES ABOUNU CELLS IN THE POSTERIOR HORN OF GBET MATTER. (Cajal.) A, fibres of posterior column derived from posterior root ; B, collaterals ; C, D, nerve-cells In grey matter sur- rounded by the arborisations of the collaterals ; E, an arborisation shown separately. The doctrine of the anatomical independence of the nerve-cell is known as the "neurone-theory" (Waldeyer). It is supported by the appearances of chromate of silver preparations of nerve-cells, i In these the reduction of the silver is strictly confined to single cells, which become stained with all their NEUEONE THEORY. 149 processes ; and these processes, when demonstrated by this method, are never found in continuity either with the processes or with the bodies of other nerve-cells. Moreover many of the facts relating to nerve-degeneration can be more readily interpreted by this theory than by one which assumes the existence of direct continuity between the nerve-units. But it has been shown by Apdthy that in annelids (the nervous system of which was formerly supposed to offer a typical example of isolated, li'nked "neurones"), the fibrils are in fact continuous from cell to cell and are not interrupted at the synapses ; it is therefore possible that the same may prove true for vertebrates also, in which case the doctrine of independent units would require modification. We may at any rate assume the truth of the hypothesis so far as the nutrition of all the processes of the nerve-cell to their remotest termination is concerned, independently of the question whether there is or is not anatomical continuity of nerve-fibrils from one unit to the other ; for there are many examples in both animal and plant cells of such interdependence by means of fibrils, combined with trophic independence. STRUCTURE OF GANGLIA. In the ganglia (fig. 177) each nerve-cell has a nucleated sheath which is continuous with the neurolemma of the nerve-fibre with which the '- *S!sr ' V.-'. Fig. 177.— liQNGITCDINAL SECTION THBODGH THE MIDDLE OF A GANGLION ON THE P08TEEI0E HOOT OF ONE OF THE SACRAL NERVES OF THE DOG, AS SEEN DNDEK A LOW MAGNIFYING POWER. a, nerve-root entering the 'ganglion ; 6, fibres leaving the ganglion to join the mixed spinal nerve ; c, connective-tissue coat of the ganglion ; d, principal group of nerve- cells, with fibres passing down from amongst the cells, to unite with the longitu- dinally coursing nerve-fibres by T-shaped iunctions. cell is connected. In the spinal ganglia, and in many of the corre- sponding ganglia on the roots of the cranial nerves of mammals and of most other vertebrates, the cells have only one issuing process, the axis-cylinder process, which soon acquires a medullary sheath and then passes with a somewhat convoluted course to some little distance from the cell-body, where, still within the ganglion, it divides into two, one fibre passing to the nerve-centre, and the other towards the periphery^ 150 THE ESSENTIALS OF HISTOLOGY. The branching is T-shaped or Y-shaped, and always occurs at a node of Kanvier (figs. 178, 179). The neuro-fibrils of the central and peripheral branches retain their individuality in the common trunk and are traceable into a neuro-fibril network within the cell-body. These spinal ganglion-cells have, as a rule, no dendrons, but some show Fig. 178. — Two spinal ganguon-oells, showing bifurcation of thbik NKRVE-MBKE PROCBSSBS. (Ranvier.) TC, nucleus of one of the cells ; ti', nuclei of capsules; n", nuclei of Schwann's sheath; c, c, c', c*, constrictions of Banvler. short dendrons terminating in bulbous enlargements (fig. 182) either within the cell-capsule or immediately outside it (Huber, Cajal). The origin of the axon is not always simple, but may be multiple, the several parts forming at first a plexus close to the cell, eventually joining to produce a single axon. This multiple condition tends to become accentuated with age (fig. 183). The intracapsular dendrons also occur in sympathetic ganglia (Cajal) (figs. 175, 185). Two chief types of cells occur in the spinal ganglia, one large and clear, the other small and staining almost uniformly dark (fig. 179). As was first shown by Dogiel, the cell-body of the spinal gangliouTcell is partially invested by the convoluted ramifications of a fine afferent uerve-fibre, derived either from one of the other cells of the same ganglion or from a cell in a neighbouring sympathetic ganglion (fig. 180). Similar afferent fibres forming pericellular plexuses also occur in the sympathetic ganglia- (fig. 186). -^ *^ . ^ ^ In the sympathetic ganglia the nerve-cells usually have several •dendrons and one axon ; this usually becomes a non-meduUated nerve- STRUCTURE GF GANGLIA. 151 Tig. 179. — Ttpss of oeeebeo-spinal ganglio^-oblis, fbom vagus ganglion or OAT. (Ehrlioli'a metliod.) (Cajal.) A, B, large cells with much convoluted commencement of OKon ; 0, D, smallercells j E, F, smallest cells, staining darkly and without convolutions. -Tig. 180.— Pekioellulab arbobisations in spinal ganglion-oells. (Cajal.) In A the arborisation extends over the cell-body ; in B it is limited to the axon. 152 THE ESSENTIALS OF HISTOLOGY. Fig. 181. — Diagram showing some of the cells or a beinal ganglion- AND THEIK CONNECTION WITH NEBVE-FIBEES. (Dogiel.) .'I u, p, anterior and posterior root of spinal nerve ; n, an issuing nerve bundle ; ay, fibres ■ from sympathetic ; x, a cell, the axon of which ends in ramifications around the cell-bodies of the ordinary ganglion-cells. Fig. 182.— Oerebbo-spinal ganglion-cells, man. (Caj'al.; It, 6, intracapsular dendrons, with knobbed extremities. SYMPATHETIC GANGLIA. 153- fibre, but is occasionally finely medullated. In certain animals (rabbit, hare, guinea-pig) the sympathetic cells have each two nuclei (fig. 184). In the frog they are unipolar, but sometimes with a second spiral fibre ■winding round the issuing axon. Fig. 183.— Senile type op ceebbro-spinal ganglion-cell. (Cajal.) a, issuing axon ; &, part of pericellular plexus ; c, pericellular loops. Fig. 184. — A sympathetic nerve-cell. (Eanvier.) nn, nuclei of cell; /,/, pale fibres issuing from cell ; n', n", nuclei on fibres. The cells of ganglia are disposed in aggregations of different size, seiparated by the bundles of nerve-fibres which are traversing the 154 THE ESSENTIALS OF HISTOLOGY. ganglion (fig. 177). The ganglion if large is inclosed by an investing capsule of connective tissue which is continuous with the epineurium and perineurium of the entering and issuing nerve-trunks. Fig. 185. — Two sympathetic cells, man. (Cajal.) a, tty axon ; b, c, intracapsular dendrons ; d, knob-like ending of au intracapsular dendron. DEGENERATION AND REGENERATION OF NERVE-FIBRES AND NERVE-CELLS. Since each nerve-fibre is the process of a nerve-cell, when a nerve is cut, the separated part degenerates. Its axis-cylinder becomes broken up and disappears, the nuclei of the neurolemma multiply, and the medullary sheath undergoes a process of disintegration into droplets of fatty substance which stain intensely like fat itself in a mixture of bichro- mate of potash and osmic acid which does not stain the medullary sheath of normal fibres. The change which results in the fibres was described by A. Waller in 1850, and is known as Wallerian degeneration, {&g 187, A to c). In man and mammals these changes begin 24 to 48 hours after section of the nerve, and proceed rapidly, so" that by the DEGENERATION OF NERVE-FIBRES. 155 third day the nerve-fibres cease to conduct impulses. When a peri- pheral nerve is cut, all the nerve-fibres distal to the point of section must degenerate, because all have grown from and are processes of nerve-cells in or near the nerve-centre — the aff'erent fibres from the cells of the ganglion on the posterior root, the eff'erent fibres from the cells of the anterior horn of the spinal cord. FlS. 186. — Two CELLS FROM A SYMPATHETIC GANGLION OF MAN SHOWING THE TEEMINATIOK OF AFFERENT FIBRES WITHIN THE CELL-CAPSULE. (Cajal.) A, large ; B, small cell, a, b, afferent fibres surroundlug a dendron aud passing into capsule. Waller supposed that no changes are produced centrally to the injury when a nerve is cut, nor indeed is there any obvious immediate alteration in the nerve-fibre itself between the injury and the cell- body, although it is stated that the fibrils of the axis-cylinder dis- appear for a time. But it was found by Nissl that degenerative changes occur in the cell-body of every cell, whether motor or sensory, the axis-cylinder of which has been severed. ^ These changes become ' But section of the posterior root-fibres central to the ganglia does not entail A, C, and D are from osmic preparations; B, from an alcohol and carmine pr,e^'V';V paration. nucleus become shrunken in volume. This process of disintegration and disappearance of chromatin may be termed Nissl degeneration : it DEGENERATION OF NERVE-FIBRES. 157 is also known as chromatolysis. It is brought about not only by section of the axon, but also as the result of excessive fatigue of the intact cell (fig. 164), and of the action of a large number of drugs and poisons. , The chromatolysis may be persistent or may be recovered from. Sometimes it is . followed by almost complete atrophy of the cell- hody, and when this is marked there may be a secondary Wallerian degeneration of the part of the nerve-fibre still attached to the cell. The chromatolysis is accompanied by changes iii the neurofibrils of the cells, which stain differently and become granular (Marinesco). Begeueration.'^ — After a certain lapse of time, especially if the cut «nds of the nerve are in apposition, continuity between them may become re-established. But when such regeneration takes place in the cut nerve, it is effected not by a re-establishment of connection between the degenerated fibres and the fibres of the central stump, "but by an outgrowth of new fibres from the stump (figs. 187, D; 188), -which endeavour to find their way to the periphery along the course of the degenerated fibres. If they succeed in doing so, the continuity and conducting pbwer of the nerve become ultimately restored. This may not happen for three months or more, according to the length •of nerve cut oif and the nature of the severance, although the process begins within a few days of the injury in man. Some investigators lave attempted to show that regeneration may take place independently in the peripheral part of the cut nerve, but the evidence offered is not conclusive, although changes occur in the peripheral part pre- paratory to the down-growth of new fibres into it (Mott, Halliburton And Edmunds). There appears, however, to be no union of the down- growing fibres with regenerated fibres in the peripheral part. The recent investigations of Cajal have shown conclusively that whenever con- tinuity is re-established it is invariably due to the growth of fibres from the central stump of the cut nerve. These down-growing fibres are usually terminated by a button-like swelling similar to that which •characterises the growing fibres of the embryonic nerves (incremental ■cone), and they may also exhibit numerous lateral ramifications (figs. 188, 189). Even when the cut central stump is turned backwards and fixed amongst the muscles or under the skin a certain number of newly-budded fibres may find their way from it into the degenerated peripheral part of the nerve. If regeneration fail to establish itself, the central end of the cut fibre and the cell-body from which it takes origin undergo slow atrophic changes resulting from disuse. These atrophic changes may oiltimately extend to other links in the cell-chain, so that even 158 THE ESSENTIALS OF HISTOLOaY. remote cells in the same physiological path may eventually become atrophied (Gudden's atrophy). Fig. 188.— Fibres from the central cut end of sciatic nerve (of YOUNG rabbit) CUT 10 DAYS BEFORE DEATH. (Cajal.) A, fibres showing down-growth of axis-cylin- ders (6) which are invested Mj flattened nucleated cells ; a, intact part still my- elinated. B, a fibre, the axis-cylinder of which has not grown down with the rest, but which shows various degenerative appearances, such as buds from the axis- cylinder and at d, a separation of the fibrils. Fig. 189.— From the peripheral end of a nerve out 78 days before death. (Cajal.) a, c, enlarged growing ends of axis-cylinder sprouts which have grown down from the central cut end into the old shesiths of th& cut nerve-fibres (myelin dr6ps are stilL visible within the sheaths). The middle fibres (h) are interstitial (not in old she^th§),i they show a new formation of a nucleftited* sheath. The fibre d has an enlarged^ end^. ' a, with sheath &; e, very fine fibres '^thitf an old sheath ; to the left of it, em oltL sheath without nerve-fibres. * REGENERATION. 15& No regeneration of cut nerve-fibres ever occurs in the brain or spinal cord, although the process of degeneration of fibres which are cut off from their cell-bodies occurs in the same manner as at the periphery, and the Nissl degeneration also takes place in the cell-bodies. Both in the nerve-centres and in the peripheral nerves (if regeneration fail to occur), the place of the degenerated nerve-fibres becomes eventually occupied by strands of fine fibres, somewhat similar to the fibres of cicatricial tissue. These strands stain deeply with carmine and remain unstained by osmic acid and by the Weigert-Pal method, and are thus differentiated from the surrounding normal meduUated nerves. NEUROGLIA. In the brain and spinal cord the nerve-cells and nerve-fibres are supported by a peculiar tissue which has been termed the neuroglia. It is composed of cells and fibres, the latter being prolonged from and Fio. 190. — Section of spinal cord of embryo chick, showing neuroglia FIBRES PROLONGED FROM THE EPITHELinM OF THE CENTRAL CANAL. (Cajal.) d, dorsal ; v, ventral surface ; c, central canal from which the neuroglia cells and fibres are seen to radiate to the periphery of the cord. Some detached neuroglia cells are also represented. through the cells. Of the fibres some are radially disposed. These start partly from the lining layer of the central canal of the spinal cord and the, ventricles of the brain, where they are originally if not permanently continuous with the ciliated epithelium cells lining 160 THE ESSENTIALS OF HISTOLOGY Pig. 191.— Neuroglia cells of the cerebellum. Golgi method. (6. Eetzius.)! , a cells with long parallel processes extending to surface ; !), arborescent cells ; ' c, "spider" ceUs. NEUROGLIA. 161 these cavities. They course in a radial direction, slightly diverging as they proceed, and constantly branching, towards the surface of the organ, where they end in enlargements attached to the pia mater (iig. 191, a). The radial neuroglia cells and fibres are best seen in the embryo before the nervous elements are fully developed (fig. 190); when first distinct they are termed spongioblasts (His). Other neuroglia-fibres are prolonga- tions or cell-processes of branching neuroglia-cells (glia-cells). The cells are stellate in shape (fig. 192), and their fine processes pass as neuroglia- fibres between the nerve-cells and nerve-fibres, which they aid in sup- porting. There appear to be two kinds of these neuroglia-cells difi'ering from one another in the character of their processes (Andriezen). In the one kind the processes branch re- peatedly (arborescent cells) (fig. 191, 6) ; in the other kind they remain unbranched from their origin in the cell-body to their termination (spider-cells) (fig. 191, c). Some authorities (e.g. Weigert) have thought that the fibres of the neuroglia are inter- not intra-cellular, although it is admitted by all that they are formed originally by the neuroglia-cells. Fig. 192.— Neurosiia cell from SPINAL COBD. (Eanvier.) Isolated after maceration in 33 p.c. alcohol. DEVELOPMENT OF NERVE-CELLS AND NERVE-FIBRES. All nerve-cells in the body are developed from the cells of the neural _ groove and neural crest of the early embryo ; the neural groove closing to form the neural canal (fig. 193), the cells of which form the spinal cord and brain, and the neural crest giving off' at intervals sprouts which become the germs of the spinal ganglia. The cells which line the neural canal are at first all long columnar cells, biit amongst these, and probably produced by cell-division from some of these (fig. 194, g), rounded cells (neuroblasts) make their appearance, the remaining elongated cells forming the spongioblasts. Soon from each neuroblast a process begins to grow out (fig. 194, n, and fig. 195). This is the axon, and it is soon characterised by an enlarged extremity (incremental cone) (fig. 196, h, h; fig. 197, B, c). As it grows, it may emerge from the antero-lateral region of the canal and become the axis-cylinder of a motor nerve or anterior root-fibre. The dendrons of L 162 THE ESSENTIALS OP HISTOLOGY. the cell appear somewhat later than the axon. The axis-cylindel? processes of some of the neuroblasts remain within the nerve-centre^ and are developed into commissural, association, and intercentral fibres. Fig. 193. — Closure oi' nedral canal of human embkto, showing the cells of the neukal crest becoming separated to form the germs of the SPINAL GANGLIA. (L^nhossek.) A, canal still open ; B, canal closed. Fig. 194. Fig. 195. Fig. 194.— Section of neural epithelium of early embryo. (His.) Higbl; magnlBed 7iew of part of a section, at the time of the first differentiation of the neuroblasts, showing, s', apongework formed of the outer ends of columnar epithelium cells, s ; g, rounded " germinal cells " in process of division (probably to form neuroblasts) ; n, a neuroblast. Fig. 195.— Neuroblasts from a pig-embryo, showing three stages of development. (Gurwitsoh, after Scott.) (Highly magnified.) The sprouts from the neural crest contain the neuroblasts from whicli the posterior root-fibres are developed. Axons grow out from these neuroblasts in two directions, so that the cells become bipolar (fig. 198). DEVELOPMENT OF NERVE-CELLS. 168 One set of processes, forming the posterior root-fibres, grow into the postero-lateral portion of the spinal cord and ramify in the developing grey matter ; the other set, containing the afferent fibres^ of the spinal nerves, grow towards the developing anterior roots, and eventually mingle with them to form the mixed nerves. As development pro- ceeds, the bipolar ganglion cells become gradually transformed in most Fig. 196. FiQ. 197. Fig. 196. — Section of spinal ookd of ohiok ov third day of incubation. (Cajal.) A, anterior root-fibres formed by outgrowttis of motor neuroblasts, c, e ; B, posterior root-fibres formed by ingrowths of bipolar sensory neuroblasts, o, in ganglion rudi- ment ; a, early neuroblasts ; h, neuroblast giving rise to a commissural nerve-fibre, d; h,i, enlarged ends of growing axons J e, e, neuroblasts of wbich the dendrons are beginning to appear. Fig. 197. — Neckoblasts feom the spinal ookd or A third-day chick EMBKYO. (Cajal. ) A, three neuroblasts, stained by Cajal's reduced silver method, showing a network of neuro-fibrils in the cell-body ; o, a bipolar cell. B, a neuroblast stained by the method of Golgi, sliowlng the incremental cone, c. vertebrates, by a shifting of the two axons, into unipolar cells (fig. 198, h, i, j ; fig. 199) ; but in some fishes the cells remain permanently bipolar (fig. 170). This is also the case with the ganglion-cells of the eighth cranial nerve (ganglion of Scarpa and ganglion of the cochlea). The ganglia on the sympathetic and on other peripheral nerves are developed from small masses of neuroblast-cells which separate off from the germs of the spinal ganglia and give origin to axons and dendrons much in the same way as do the neuroblasts within* the central nervous system. 164 .THE ESSENTIALS OF HISTOLOGY. The manner in which the medullary sheath and neurolemma of the nerve-fibres are formed is not well understood. It is usually assumed that they are also ectodermie in origin and are developed Fig 198.— Spinal and sympathetic ganglia and part op spinal cokd op OHIOK OF SEVENTEENTH DAY OP INODBATION. (Cajal.) A, antero-lateml part of spinal cord with d, a motor nerve-cell ; the fibres of the anterior root are seen emerging and passing to B (the connection appears interrupted in the section) ; C, posterior root formed of fibres which have grown from the ganglion-cells in D, spinal ganglion; B, mixed spinal nerve; P, sympathetic ganglion ; a, a, axons of .sympathetic cells, passing to join the spinal nerve ; b, den- drons of these cells; e, axons passing to the sympathetic cord; h, cells of spinal ganglion still bipolar ; i, i, bipolar cells becoming transformed into unipolar ; 7, unipolar cell with T-junction ; /, section of an artery ; g^ body of vertebra. Fig. 199.- -SpINAL ganglion-cells showing TRANSITIONS FROM BIPOLAR TO UNIPOLAR CONDITION. (Holmgren.) from ectoderm cells which grow out from the embryonic central nervous system along the axis-cylinder processes of the neuroblasts. But this is by no means clear. It is more probable that the medullary PBVELOPMENT OF NEEVE-FIBEES. 165 substance is formed by the axis-cylinder itself, and that the neurolemma with its nuclei is derived from extrinsic cells, perhaps of mesodermic origin. The neuroglia-cells appear to be developed from ectoderm cells (spongioblasts) of the wall of the neural canal, which, in place of giving off axon and dendrons like the neuroblasts, send out a number of iine processes in all directions from the cell to form the fibres of the neuroglia. It is held by some authorities that the neuroglia has a double origin, some of the cells being developed from ectoderm and others from mesoderm. , Some neurologists are of opinion that the nerve-fibres do not grow out from single nervercells in the manner above described, but are formed of chains of cells which emerge from the neural ectoderm or from the ganglion- rudiments, and join end to end into a syncytium, which gradually lengthens out into the nerve-fibre, the nuclei of the syncytium becoming the nuclei of the sheath of Schwann, and the protoplasm of the syncytium becoming differentiated into axis-cylinder, medullary sheath, and neurolemma as development advances. Others, whilst agreeing ' that the axis-cylinders grow out as cell processes from the neuroblasts of the. neural . canal and ganglia, describe those outgrowing processes as surrounded by other neural ectoderm cells — lemmal cells- — which accompany them in their progress through the, tissues, multiplying as they proceed, and forming eventually the nucleated sheath of Schwann of the medullated nerve. 166 THE ESSENTIALS OV HISTOLOGY. LESSON XIX. MODES OF TERMINATION OF NERVE-FIBRES. 1. Shell out a Pacinian corpuscle from a piece of cat's mesentery either fresh or after having been kept for two or three days in ^ per cent, chromic acid or in 5 per cent, formol. Clear it as much as possible of adhering fat, but be careful not to prick or otherwise injure the corpuscle itself. Mount in water or saline with a thick hair to prevent crushing with the cover-glass. Sketch the corpuscle under a low power, and afterwards draw under a high power the part of the core where the nerve enters and the part where it temiiinates. Notice the fibrous structure of the lamellar tunics of the corpuscle and the oval nuclei belonging to flattened epithelioid cells which cover the tunics. The distinct lines, which when seen in the fresh corpuscles are generally taken, for the tunics, are really the optical sections of these flattened cells. Pacinian corpuscles may be observed in sections of skin ; tactile corpuscles and end-bulbs may also be seen in certain parts of the integument. 2. Study the corpuscles of Grandry and of Herbst in sections of the akin covering the duck's bill. 3. Mount in dilute glycerine sections of a rabbit's cornea which has been stained with chloride of gold by Klein's method. Notice the arrangement in plexuses of the darkly-stained nerve-fibres and fibrils, (1) in the connective- tissue substance, (2) under the epithelium, and (3) between the epithelial cells. Make one or two sketches showing the arrangement of the fibrils. 4. Spread out a small piece of muscle which has been stained with chloride of gold by Lowit's method, or with hsematoxylin by Sihler's method, and examine it with a low power to find the nerve-fibres crossing the muscular fibres and distributed to them. The pieces of muscle may advantageously be thinned out for observation by pressure upon the cover-glass. Search thoroughly for the close terminal ramifications (end-plates) of the axis- cylinders immediately within the sar- colemma. These nerve-endings as well as others elsewhere can also be displayed in preparations made by Ehrlich's, Golgi's or Cajal's methods (see Appendix). Modes of ending of sensory nerve-fibres. — Nerve-fibres which are distributed to sensory parts end either in special organs or in free terminal ramifications, these last being usually in epithelia. Within the special organs the actual nerve-ending is also generally ramified. Nerve-endings in special connective-tissue organs. — Three chief kinds of these special organs are usually described, represented in man by Pacinian corpuscles, tactile corpuscles, and end-bulbs. The typ6 is the same in all : a lamellated connective-tissue capsule enclosing a core of a soft material which appears to be composed of nucleated proto- plasmic cells ; the capsule being an expansion of the perineurium, and the core of the endoneurium of the nerve. Within the core the axis- ENDING OF NEEVE-FIBRES. 167 cylinder terminates either simply or by a more or less complex arborescence. The variations which occur are chiefly due to the complexity of the capsule, which is simplest in the end-bulbs and most complex in the Pacinian corpuscles. In the tactile corpuscles Fig. 200. Fig. 201. Fig. 200.— Tactile coepusole within a papilla oif the skin oi' the hand, STAINED WITH CHLORIDE OP GOLD. (Eanvier.) n, two nerve-fibres passing to the corpuscle ; a, a, varicose ramifications of tlie axis- cylinders within the corpuscle. Fig. 201. — End-bulbs at the terminations of nehvbs in the human con- JONOTIVA, AS SEEN WITH A LENS. (LoUgWOrth.) Fig, 202.— a medulla^'bd fibre terminating in several bnd-bulbs in the HUMAN PERITONEUM. (Dogiel.) Methylene blue preparation. Low power. 168 THE ESSENTIALS OF HISTOLOGY. and end-bulbs the connective-tissue sheath of the meduUated fibre expands to form a bulbous enlargement, which is cylindrical or spheroidal in the end-bulbs and ellipsoidal in the tactile corpuscles. In both kinds of end-organ as the nerve-fibre enters (which in the tactile corpuscle only happens when it has reached the distal part, Fig. 203. — End-bulbs feom the human peritoneum. (Dogiel.) More highly magnified. Methylene blue preparation. a, medullated fibre ; b, nuclea.ted lajuellated capsule of end-bulb ; c, non-medullated fibres, probably destined for the capillaries which surround the end-bulbs. Fie. 204. — Bnd-bulb peom the central tendon of the diaphbaom op the dog. (Dogiel.) Showing besides the main medullated fibre terminating by an arborescence within the core, a second very fine medullated fibre, forming a more delicate arborescence around the ending of tlie main fibre in the outer part of the core. Methylene blue preparation. after having wound spirally once or twice round the corpuscle) it loses its sheaths and is prolonged as an axis-cylinder only ; this gene- rally ramifies and its branches terminate after either a straight or a convoluted course within the organ; but it sometimes remains almost unbranched (see figs. 200 to 205). Tactile corpuscles occur in some of the papillae of the skin of the hand and foot, in sections of which they can be studied (see fig. 277). End-bulbs are found in the conjunctiva of the eye, where in most animals they have a cylifadrical or oblong shape, but in man they are spheroidal (fig. 201). ENDING 0| NERVE-FIBEES. 169 They have also been found in papillae of the lips and tongue, in serous membranes, in tendons and aponeuroses, and in the epineurium of the nerve-trunks; and somewhat similar sensory end-organs {genital corpuscles) also occur in the integument of the external genital organs of both sexes (fig. 205). Similar bodies of larger size are also met with in the neighbourhood of the joints (articular corpuscles). In the skin covering the bills of certain birds (e.g. duck), a simple form of end-organ (corpuscle of Grandry, fig. 206) occurs, consisting of two or more cells arranged in rows within a capsule, with the axis-cylinder terminating in flattened expansions (tactile disks) between the cells. Fig. 206.^ — Tactile cokpdsoles fbom the duck's tongue. (Izquierdo.) A, composed of three cells, with two inter- posed disks, into which the axis-cylinder of the nerve, n, is observed to pass ; in B there is but one tactile disk inclosed between two Fig. 205.— End-bulb fbom the glans tactile ceUs. PENIS, SHOWING ENDING OF AXIS-CTLIN- DER. Methylene blue preparation. (Dogiel.) a, medullated nerve-fibre; 6, sheath of end -bulb. The Pacinian corpuscles are larger, and have a more complex structure, than the tactile corpuscles and end-bulbs (fig. 207). They are composed of a number of concentric coats arranged like the layers of an onion, and inclosing the prolonged end of a nerve-fibre. A single medullated nerve-fibre goes to each Pacinian corpuscle, encircled by a prolongation of the perineurium (sheath of Henle), and within this by endoneurium ; when it reaches the corpuscle, of which it appears to form the stalk, the lamellse of the perineurium expand into the tunics of the capsule. The nerve passes on, piercing the tunics, sur- rounded by endoneurium, and still provided with medullary sheath, to reach the central part of the corpuscle. Here the endoneurium is prolonged to form a core of cylindrical shape, along the middle of which the nerve-fibre, now deprived of its medullary and primitive sheaths, passes in a straight course as a simple axis-cylinder (tigs. 170 THE ESSENTIALS OF HISTOLOGY. 207, n' ; 208, c.f) to terminate at the farther end of the core, either in an arborisation or in a bulbous enlargement. In its course through the core it may give off lateral ramifications, which penetrate to all parts of the core, and themselves end in fine branches. Fig. 207.— Magnified view op a pacinian body from the oat's mbsenteey. (Ranvier. ) ■A, stalk of corpuscle with nerve-fibre, Inclosecl in sheath of Henle, passing to the qorpusple; n', its continuation through the core, m, as axie-oylinder only; o, its terminal arborisation ; c, d, sections of epithelioid cells of tunics, often mistaken for the tunics themselves ; /, channel through the tunics whlcli ejfpanclB into the core of the corpuscle. PACINIAN CORPUSCLES. 171 Besides the medullated fibre, which is always very conspicuous, it has been shown that both the Pacinian and Herbst corpuscles receive in addition a fine non-medullated nerve-fibre, which arborizes over the outer surface of the core. A similar arrangement also obtains in Grandry's corpuscles, where the tactile cells are surrounded with such an arborization (Dogiel and others). The tunics of the capsule are composed of connective tissue, the fibres of which for the most part run circularly. They are covered Fig. 209. Fig. 208.— Pabt of pacihian body, showing the nbbve-fibbe entering THE COEE. FBOM AN OSMIO ACID PBEPABATION. ms, entering nerve-fibre, the medullary sheath of which is stained darkly, and ends abruptly at the core, c; ps, prolongation' of primitive sheath or neurolemma passing towards the outer part of the core ; c^, axis-cylinder passing through the core as the central fibre ; e, some of the inner tunics of the corpuscle, en- larged where they abut against the canal through which the nerve-fibre passes — t^e dots within them are sections of the fibres of which they are composed ; 71, nuclei of the tunics ; n', nuclei of the endoneurium-cells, continued by others in the outer part of the core. Fig. 209.- -PACINIAN COEPDSOLE FHOM the OAT, STAINED WITH SILVER NITEATE. on both surfaces with a layer of flattened epithelioid cells (fig. 209), and here and there cleft-like lymph-spaces can be seen between them like those between the layers of the perineurium. Pacinian corpuscles occur in many parts, e.g. in the deeper layers of the skin of the hands and feet, in the periosteum of some bones, in lie neighbourhood of tendons and liga,ments, in the connective tissue 172 THE .ESSENTIALS OF HISTOLOGY. Fig. 210.— Section of pacinian cokpuscie. (Szymonowioz.) , one of the layers of epithelioid colls ; n, nucleus of epithelioid cell. It Is seen that the tunics are very closely packed around the core, in the middle of which the axial-fibre is cut across'. X Fig. 211. — Herest cdkpusoli of duck, (Sobotta.) x38b. n, jnedullated nerve-flbre ; a, its axis-cylinder, terminating in an enlargement at end ■of -core ; c, nuclei of cells of core ; «, nuclei 'Oi ceils of outer tunics ; f, inner tunics; PACINIAN CORPUSCLES. 173 at the back of the abdomen, and (in the cat) very numerously in the mesentery, where they are most easily got for observation. A simple form of Pacinian corpuscle with fewer tunics and a core formed of regularly arranged cells occurs in birds (corpuscles of Herbst, fig. 211). Although most of the nerve endings in connective-tissue structures are enclosed within lamellated capsules, nerves are found to end in some situations in arborisations between the bundles of connective- tissue fibres. This has been shown by Dogiel to occur in intermuscular Fig. 212.— TEEMINAL arborisation from the INTBRMnSOULAR CONNEOTIVB TISSDE OF THE EBCTDS ABnOMINIS OF THE RABBIT. METHYLENE BLUE PREPARATION. (Dogiel.) " . f ■ --»-^. Fig. 213.— Terminal arborisation from the superficial later of the PERITONEUM OF THE BABBIT. METHYLENE BLUE PREPARATION. (Dogiel.) a, meduUated fibre ; b, fibre connecting the arborisation with another one not here represented. connective-tissue septa (fig. 212); and in serous membranes (fig. 213); in the latter such arborisations may be quite superficial and placed just below the endothellurii. Organs of Euffini..— These, which resemble long cylindrical end-bulbs, are composed of connective-tissue bundles, within which the axis- cylinders of the nerves ramify, and end in flattened expansions. They 174 • THE ESSENTIALS OF HISTOLOGY. occur commonly in the subcutaneous tissue of the human finger (fig. 214). Other end-bulb-like organs, spheroidal, oval, or cylifldricaLin form, have been described by Rulfini under, the name of G;olgi-Mazzoni corpuscles; they appear to be varieties of the ordinary end-bulb of W. Krause. They occur in tendons and in the subcutaneous tissue of the pulp of the finger. Fig. 214. — A nerve fibre is shown dividing into sbven secondary fibres TO WHICH ABE ATTACHED FIVE ORGANS OF RDFFINI. (Barker, after Euffini.) Organs of Golgi. — A special mode of nerve-ending is met with in many tendons, near the points of attachment of the muscular fibres. The tendon-bundles become somevi^hat enlarged and split into a number of smaller fasciculi, and the nerve-fibres — one, tvro, or even more in number — pass to the enlarged part, and penetrating between the fasciculi of the tendon lose their medullary sheaths, while the axis- cylinders end in a terminal arborisation, beset with irregular vari- cosities. The structure (fig. 215) is enclosed within a fibrous capsule continuous with the areolar tissue covering the bundles of the tendon ; and between the capsule and the organ proper is a lymph-space, similar to that which is found in the muscle-spindle (see p. 179). Free nerve-endings. — When sensory nerve-fibres terminate in epi- thelium, they generally branch once or twice in the subepithelial connective tissue on nearing their termination. The sheaths of the fibres then successively become lost, first the connective tissue or perineural sheath, then the medullary sheath, and lastly the neuro- lemma, the axis-cylinder being alone continued as a bundle of primitive fibrils (fig. 216). This branches, and with the ramifications of the axis-cylinders of neighbouring nerve-fibres forms a primary plexus. FEEE NERVE-ENDINGS. 1V5 Fig. 215.— Oroan of golgi from thk human tendo aohillis. Chloride OF GOLD PREPARATION. (Ciaooio.) m, muscular fibres ; t, tendon-bundles ; 0, Golgi's organ ; re, two nerve-flbres passing to It. Fig. 216.— Plexus of nerve fibres in the rabbit's cornea: Methylene blub. (Cajal.) A, trabeoula of primary plexus ; B, secondary plexus ; C, intraepithelial fibrils. 176 THE ESSENTIALS OF HISTOLOGY. From the primary plexus smaller branches come off, and these form a secondary plexus nearer the surface, generally immediately under the epithelium if the ending is in a membrane covered by that tissue. Finally, from the secondary plexus nerve-fibrils proceed and form Fig. 217. — Vbktioal section of cornea stained with ohlokide op gold. (Ranvier. ) n, r, primary plexus in connective tissue of cornea ; a, branch passing to subepithelial plexus, s ; p, intraepithelial plexus ; b, terminations of fibrils. terminal ramifications amongst the tissue cells (fig. 217, ^, b), the actual ending being generally in free varicose fibrils (b). This mode of ending is characteristically seen in the cornea of the eye, but can also be rendered evident in other epithelia (fig. 218). The fibrillar Fig. 218.— Intra-epithelial nebvs-tbrminations in the l.akynx: GoLGi method. (G. Retzius.) On the left the epithelium is stratified and on the right ciliated columnar. n, nerve-fibres in cerium. Structure of the ramifications of the axis-cylinders is very apparent in some of the preparations figured. In some situations the nerve-fibrils within a stratified epithelium terminate in flattened or crescentic expansions which lie in the inter- stices of the deeper epithelium cells, to some of which they are applied. FREE NERVE-ENDINGS. 177 These expansions are known as tactile disks ; they are character- istically developed in the skin of the pig's snout (fig. 219), and are also found in the outer root sheath of hairs and in the deeper parts of the epidermis in various parts. "With appropriate treatment it may be shown that they consist of a fine network of neuro-fibrils (fig. 220). .0/:?7^ Fig. 219.— Ending os' nerve in tactile disks in the pig's snout. (Ranvier.) •It, meduUated fibre ; m, terminal disks or menisci; e, cells of the Malpighian layer of the epidermis ; a, somewhat modified cell to which a tactile disk is applied. Fig. 220.— a nerve-fibre ending in a number op tactile menisci FROM A tactile haib, BABBIT. (Cajal and Telle. ) A, point of ramification of axis-cylinder of nerve-fibre ; B, isthmus between two menisci ; 0, terminal meniscus ; D, large meniscus at branching of several divisions of the nerve-ending. Sensory nerves of muscles. — The sensory nerves of muscles end in peculiar organs which were termed by Kiihne muscle-spindles. Their structure has recently been specially investigated by Kuffini, M 178 THE ESSENTIALS OP HISTOLOGY. Huber, and Dogiel ; and also ' by Shierrington, who has showii that the large meduUated nerves which they receive (about three i :S¥ M « ,- -b Fig. 221. Fig. 222. Fig. 221.— Nkbve-endinos dpon the intrafusal mdsom- rlbres of a muscle-spindle of the rabbit; modebatsli magnified. Methylene blue preparation. (Dogiel.) a, large meduUated fibre coming off from * ' spindle" nerve and passing to end in an annulo-spiral termination on and' between the intra- fusal fibres ; b, a fine meduUated fibre coming off from the same stem and dividing. Its branches, c, pass towards the ends °^W ' muscle-fibres and terminate in a number of small localised arb£>^ tions, like end-plates. Fig. 222. — An annulo-spiral ending op intrafusal pibbe; highly magnified. methylene blue prbpabatios. (DogieL) MUSCLE-SPINDLES. 179 or four such fibres entering each spindle not far from its equator)^ are derived from the posterior root-ganglia. j The muscle-spindle is a fusiform body, from 0-75 to 4 mm. long, and from 0-08 to 0'2 mm. in diameter; it lies parallel with the general direction of the fibres of a muscle. It consists of a lamellated connective-tissue sheath externally, within which is a bundle (intrafusal bundle) of from two to twelve peculiar muscle-fibres. These form an axial mass with some connective tissue and the nerve-fibres ; between this axial bundle and the sheath is a lymphatic periaxial space, bridged Fig. 223. — Sensory nerve terminating in arborisations around the ENDS OF MUSOLE-HBKES. (Cecoherelli.) < , across by filaments of connective tissue. The intrafusal muscle-fibres are somewhat like embryonic fibres in appearance, being smaller than the ordinary fibres of the muscle and having a relatively large number of nuclei with surrounding protoplasm, as in the red variety of muscle. At the proximal end of the spindle they are usually only two or three in number, but they become cleft as they pass through it ; at the distal end they may terminate in tendon-bundles. The nerve-fibres which pass to the spindle are mostly of large size ,■■ they divide after reaching the intrafusal bundle, but retain their medullary sheath for a time, although eventually terminating as axis-cylinders merely, which wind in a spiral manner around the intrafusal muscle-fibres (figs. 221, 222)> 180 THE ESSENTIALS OF HISTOLOGY. wkich they clasp by flattened encircling branches (mnulo-spiral endings). Other, much finer, medullated fibres also pass to the spindle and termi- nate in neighbouring parts, of the intrafusal bundles in flower-like or plate-like expansions (fig:'22i). Accordihg to some observers these fine fibres are prolonged from the.annulo-spiral endings of the coarser fibres ; but DogieWstates that they may run independently to the intrafusal bundle. No motor nerve-fibres appear to pass into the. spindles, unless the fine fibres above mentioned are to be so regarded, nor do the muscle-fibres of the spindle undergo atrophy on section of the motor nerve-roots, as is the case eventually with the ordinary muscle-fibres. It is not uncommon to find two or three spindles close together or even Fig. 224. — NsEVK-ENDiNe in musoulae nsKE of a lizard (Laoerta viridis). (Kuhne.) A, end-plate seen edgeways ; B, from the surface ; s, s, sarcoleionia ; p, p, expansion of axis-cylinder. In B the expansion of the axis-cylinder appears as a clear network branching from the divisions of the medullated fibres. inclosed in a common sheath. Muscle-spindles are few in number in the eye-muscles, and have not yet been found in the muscles of the tongue, but 'Otherwise their occurrence is general. In the frog both motor and sensory nerves may terminate in and between the same inuscle- fibres, but at different parts of the fibre. It is not known' whether the muscle-fibres of the spindles also receive motor nerves in mammals. Another kind of ending of sensory fibres in muscle has been described by Ceccherelli, in the form of an arborisation of nerve-fibrils around the ends of the muscle-fibres which are inserted into tendon (fig. 223). Ending of motor nerves. — The motor nerves to muscles terminate in fine: ramifications of the axis-cylinder; in striated (voluntary) muscles the ramification is localised in special organs termed moior end-organs, or, less correctly, end-plates. ENDING OF MOTOR NERVES. 181 Fig. 225.— Motok neeve-endings in the abdominal muscles of a eat. Gold preparation. Magnified 170 diameters. (Szymonowicz.) -»2 t'iG. 226.— Motor end-obgan of a lizard, gold preparation. , (Kiibne.)' rt, nerve-fibre dividing as it approaches the end-organ ; r, ramification of axis-cylinder upon, A, granular bed or sole of the end-organ ,m, clear substance surrounding the ramifications of the axis-cylinder. 182 THE ESSENTIALS OF HISTOLOGY. In voluntary muscle, the nerves, which are always meduUated, terminate, as just stated, in special end-organs (figs. 224 to 226). A meduUated fibre will branch two or three times before ending, and then each branch passes straight to a muscular fibre. Having reached this, the neurolemma of the nerve-fibre is continued into the sarcolemma of the muscle, the medullary sheath stops short, and the axis-cylinder ends in a close terminal ramification with varicose expansions upon its branches. This ramification is embedded in a layer of granular nucleated protoplasm (sole) (fig. 226, b), probably a development of the sarcoplasm of the muscle. In some cases the ramification is restricted to a small portion of the muscular fibre, and forms with the granular bed a slight prominence (eminence of Boylre). This is the case in insects. Fie. 227.— Ending of motok nerves in babbit's muscle. Ekduoed SILVER METHOD. (Cajal.) a,, axis-cylinder of entering ilerve ; h, c, d, parts o£ terminal ramification showing network of neuro-fibrils.; and mammals. In the lizard the ramification is rather more extended than in mammals, whilst in the frog it is spread over a considerable length of the fibre. The ramification shows a fibrillar structure (fig. 227), which is especially evident at the enlargements. In mammals there appears to be only one end-plate to each fibre, while in reptiles there may be several. The end-plate is covered, externally . . INVOLUNTARY MUSCLE. 183 to the sarcolemma, by an expansion of the sheath of Henle of the nerve-fibre (telolemma). In involuntary muscle, both plain and cardiac (fig. 228), the nerve- fibres, which near their termination are entirely non-medullated, end in plexuses. The primary plexuses are generally furnished with ganglion- cells in abundance. Such gangliated plexuses are best developed in Fig. 228. — Ending op nbevb-wbkks in oakdiao muscle. (Smirnow.) connection with the intestine. From the cells of these plexuses other nerve-fibres pass which form secondary plexuses and terminal ramifica- tions amongst the contractile fibre-cells, to the surface of which the endings of the branches, often slightly enlarged, are applied (Huber and de Witt). 184 THE ESSENTIALS OF HISTOLQOY. LESSON XX. STRUCTURE OF THE LARGER BLOOD-VESSELS. 1. Sections of a medium-sized peripheral artery aud vein, e.g. popliteal or radial. In this preparation the limits of the vascular coats can be well seen and also the differences which they present in the arteries and veins respec- tively. The sections may be stained with hsemalum and eosin or with orcein, and mounted' in dammar or xylol balsam. 2. Mount in xylol balsam or dammar a thin slice cut from the inner surface of a large artery which, after having been cut open longitudinally and washed with distilled water, has been rinsed with nitrate of silver solution and then with distilled water and exposed to the sunlight. The vessel should then be hardened in alcohol, or it may be exposed in this to the light. This preparation will show the outlines of the epithelium-cells which line the vessel. ' A similar preparation may be made from a large vein. 3. A piece of an artery which has been macerated for some days in 33 per cent, alcohol is to be teased so as to isolate some of the muscular cells of thfe middle coat and portions of the elastic layers (networks and fenestrated membranes) of the inner and middle coats. The tissue may be stained, cautiously with diluted hsemalum, and glycerine afterwards added. The muscular cells are recognisable by their irregular outline and long rod- shaped nuclei. Sketch one or two aud also a piece of the elastic network or of fenestrated membrane. The fenestrated membrane is best obtained from one of the arteries of the base of the brain ; it is also well seen in the arteries within the kidney. 4. Transverse sections of aorta and carotid. Notice the differences in structure between these and the section of the smaller artery. 5. Transverse section of vena cava inferior. Notice the comparatively thin layer of circular muscle, and outside this the thick layer of longitudinal muscular bundles in the adventitia. Make sketches from 1, 4, and 5 under a low power, from 2 and 3 under a high power. An artery is usually described as being composed of three coats, an iwner or elastic, a middle or muscular, and an external or areolar (fig. 229, 6, c, d). It is, however, more correct to describe the wall of an artery as being mainly composed of muscular and elastic tissue, lined internally by a pavement epithelium {endothelium), and strengthened externally by a layer of connective tissue {adventitia). The mnefr coat {tunica intima) is lined by a thin layer .of pavement epithelium {endothelium), the cells of which are somewhat elongated in the direction of the axis of the vessel (fig. 230), and form a smooth lining to the tube. After death they become easily detached. ■STRUCTURE OF ARTERIES. 185 The endothelium is the essential layer in all blood-vessels. It is always ■the first part to be developed, and in some it remains as the only layer of the vessel. This is the case with all true capillaries and with certain veins JK- FiG. 229.— Transverse section of part of the wall of the posterior TIBIAL ARTERY. (75 diameters.) a, epithelial and subepithelial layers of Inner coat ; b, elastic layer (fenestrated mom- brane) of inner coat, appearing as a bright line in section ; c, muscular layer (middle coat) ; d, outer coat, consisting of connective-tissue bundles. In the interstices of the bundles are some connective-tissue nuclei, and, especially near the muscular coat, a number of elastic fibres cut across. ^ ■> . ^ a and also with the lacunar spaces or sinusoids, which, as Minot has pointed out, take the place of capillaries in certain parts (e.g. in the liver, the medulla of the suprarenal capsules and the Wolffian body of the embryo) • Fig. 230. — Epithelial later lining THE posterior TIBIAL ARTERY. (250 diameters.) Fig. 231.— Portion of fenestrated membrane of henle from an ARTERY. (Toldt.) it is also true of the sinuses of erectile tissue, as well as the sinus-like blood-vessels which are met with in invertebrates. In some structures the endothelial layer of the blood-vessels is imperfect, viz. : in the capillaries and 186 THE ESSENTIALS OF HISTOLOGy. blood-sinuses of the spleen, the placental, mucous membrane of the pregnant uterus, and probably the sinusoids (capillaries) of the liver ; in these places Fig. 232; — Elastic netwoek of ARTERY. (Toldfr.) Fig. 233. — Muscular fibre-cells from SDPERIOE THYROID ARTERY. (340 dia- meters.) intima media adventUia Fig. 234.— Section of the lingual artery. (Griinstein.) «,( epithelium and subepithelial layer of inner eddit ; ■ !>, its elastic layer ; a c d inner; most and outermost layers of middle coat, -with, elastic fibres passing obliquely to join the elastic layers which bound that'coat; e, innermost part of outer coat or adventitia, showing many elastic fibres out aoroea ; /, outer-part of adventitia STRUCTURE OF ARTERIES. 187 •the blood finds its way into the interstices of the organ and comes in direct contact with the .tissue-cells. • Next to the endotheliuin cpmes an elastic layer in the form either of elastic networks (fig. 232) or of a, fenestrated membrane (fig. 231).,. In some arteries there is a layer of fine connective tissue intervening between the epithelium and the fenestrated membrane (subepithelial layer). The middle coat (tunica media) consists mainly of circularly disposed plain muscular fibres, but it is also pervaded in most arteries by a network of elastic fibres which are connected with the fenestrated Fig. 235. — Section of thoracic aorta as seen under a low power. (Toldt.) a, the inner coat consisting of three layers, viz. ; 1. Epithelium seen as a fine line. 2. Subepithelial layer, 3. Elastic layers. In the outer part of the inner coat, at its junction with the middle, a layer of longitudinal muscular fibres is represented as cut across, b, middle coat with alternating layers of muscle and elastic mem- branes ; c, outer coat with two vasa vasorum. membrane of the inner coat and are sometimes almost as much developed as the muscular tissue itself. This is especially the case with the larger arteries, such as the aorta and the carotid and its immediate branches, but in the smaller arteries of the limbs the middle coat is composed almost purely of muscular tissue. The muscular fibres are comparatively short, with long rod-shaped nuclei, and are often' irregular in shape (as in fig. 233), especially if the middle coat contains much elastic tissue. The outer coat is formed of connective tissue with a good many elastic fibres, especially next to the middle coat. The strength of an artery depends largely upon this coat ; it is far less easily cut or torn 188 THE ESSENTIALS OF HISTOLOGY. than the other coats, and it serves to resist undue expansion of the vessel. Its outer limit is not sharply marked, for it tends to blend with the surrounding connective tissue; hence it has been termed tunica odveMtia. intimai 5 inedUiJ advtntitia^ Fig. 236.— Sbotion op aobta more magnified. (Griinstein.) a, epithelial and subepithelial layers of inner coat; 6, c, outer layers of inner coat containing many fine elastic fibres ; d, e, parts of outer coat. ., STRUCTURE OF ARTERIES. 189 Variations in structiyre.— The aorta (figs. 235, 236) differs Id some respects in structure from an ordinary artery. Its inner coat contains a considerable thickness of subepithelial connective tissue, but the elastic layers of this coat are chiefly composed of fine fibres, and are not especially marked off from those of the middle coat, so that the inner and middle coats appear blended with one another. On the other hand, there is a very great develop- ment of elastic tissue in the middle coat, forming membranous layers which alternate with layers of the muscular tissue. A good deal of connective tissue also takes part in the formation of the middle coat, making this coat unusually strong. The inner and middle coats constitute almost the entire thickness of the wall, the outer coat being relatively thin. The other variations which occur in the arterial system have reference chiefly to the development and arrangement of the muscular tissue. Thus in many of the larger arteries there are a few longitudinal muscular fibres at the inner boundary of the middle coat, and in some arteries amongst the circular fibres of the middle coat. This is the case in the aorta. In the part of the umbilical arteries within the umbilical cord there is a complete layer of longitudinal fibres internal to the circular fibres and another external to them, whilst the amount of elastic tissue is very small. Longi- tudinal fibres are also present in some other arteries (iliac, superior mesenteric, splenic, renal, etc.), external to the circular fibres, and therefore in the outer coat of the artery. Fig. 237.— Transverse section of part of the wall of one of the POSTERIOR TIBIAL VEINS (man). About 200 diameters. a, epithelial, and h, aubepithsUal layers of innor. coat ; c, middle coaf consisting of irregular layers of muscular tissue, alternating -with connective tissue, and passmg ■ '■' somewhat gradually into the outer connective tissue and Elastic coat, d. The veins (fig. 237) on the whole resemble the arteries in structure, but they present certain diflFerences. In the internal coat the same layers may be present, but the elastic tissue is less developed, and may be quite inconspicuous ; it seldom takes the form of a complete membrane. The epithelium cells are less elongated than those of the arteries. The middle coat (c) contains less elastic tissue and also much less muscular tissue, being partly occupied by bundles of white 190 THE ESSENTIALS OF HISTOLOGY. connective-tissue fibres. These are continuous with those of the external coat, which is relatively better developed in the veins than in the arteries, so that, although thinner, their walls are often stronger. Many of the veins are provided with valves, which are crescentic folds of the internal coat strengthened by a little fibrous tissue : a few muscular fibres may be found in the valve near its attach- ment. The layer of the inner coat is rather thicker and the epithelium-cells are more elon- gated on the side which is subject to friction from the current of blood than on that which is turned towards the wall of the vessel. Variations in different veins. -^'^ The veins vary in structure more than do the arteries. In many veins longitudinal muscular fibres are found in. the inner part of the middle coat, as in the iliac, femoral, umbilical ; the umbilical vein within the umbilical cord having three muscular layers like the correspond- ing arteries ; it has a well-developed internal elastic layer. In other veins, longitudinal fibres occur ex- ternal to the circularly disposed fibres, and may be described aa belonging to the outer coat. This is the case with the abdominal and especially the hepatic portions of the inferior vena cava (fig. 238), and to a less extent with the hepatic veins and the portal vein and its tribu- taries. In the superior vena cava, in the upper part of the inferior vena cava and in the jugular, subclavian and innominate veins muscular fibres are almost entirely absent in the middle coat, and there are but few in the adventitia. The veins of the pia mater, brain and spinal cord, retina, and bones, and the venous sinuses of the dura mater and placenta have no muscular tissue. It is only the larger veins, especially those of the limbs, that possess^ valves. They are wanting in moat of the veins of the viscera (although occurring abundantly in some of the tributaries of the portal vein), in those within the cranium and vertebral canal, in the veins of the bones, and in the umbilical vein. Fig. 238.— Transvekse section op thb infbbioe vbna cava of the dog. (Szymonowicz.) Magnified 150 dia- meters. a, intima ; b, thin layer of circular muscle ; c, thick adventitia with longitudinal muscu- lar bundles ; d, a vas vasis. SMALLER BLOOD-VESSELS. 191 LESSON XXI. SMALLER BLOOD-VESSELS AND LYMPH- VESSELS. SEROUS MEMBRANES. MICROSGOPIG STUDT OF THE GIRGULA- TION. DEVELOPMENT OF BLOOD-VESSELS. 1. Take a piece of pia mater which has been fixed with 2 per cent, bichromate of potassium and stained with hsematoxylin, and separate from it some of the small blood-vessels of which it is chiefly composed. Mount the shreds in dilute glycerine, or after dehydrating with alcohol and passing through clove oil they can be mounted in dammar or xylol balsam. The structure of the small arteries can be studied in this preparation, the nuclei of the epithelium and of the muscular coat being brought distinctly into view by the stain. The veins of the pia mater possess no muscular tissue. Capillary vessels which have been dragged out from the brain in removing the pia mater may also be seen in this preparation. Sketch two small arteries of different sizes, giving also their measurements. 2. Mount in dammar or xylol balsam a piece of the omentum of the rabbit, stained with silver nitrate. The membrane should be stretched over a cork or a ring of wood or vulcanite, rinsed with distilled water, treated for five minutes with 1 per cent, nitrate of silver solution, again washed and exposed to sunlight in spirit. When stained brown, the preparation is removed from the light and placed in oil of cloves. Pieces may now be cut off from the membrane and mounted in balsam or dammar ; they should include one or more blood-vessels. This preparation is intended to show the epithelium of the smaller blood- vessels and accompanying lymphatics, and also the epithelium of the serous membrane. Sketch a small piece showing the epithelium of the vessels. 3. Mount in balsam or dammar a piece of the central tendon of the rabbit's diaphragm which has been prepared with silver nitrate, the pleural surface having been first brushed to remove the superficial epithelium and thus enable the nitrate of silver more readily to penetrate to the network of underlyi^ lymphatic vessels. Observe the lymphatic plexus under a low power ; sketch a portion of the network. If the peritoneal surface is focussed, the epithelium which covers that surface will be seen, and oppo- site the clefts between the radially disposed tendon-bundles stomata may be looked for in this epithelium. 4. Examine sections of the thoracic duct. These may be made in the same way as sections of the blood-vessels. 5. Open the abdomen of a freshly killed frog, preferably a male, and remove the abdominal viscera, taking care not to injure the membrane or septum at the back of the abdomen, which lies over and between the kidneys and separates the peritoneal cavity from the cisterna lynvp/iatica magna, a large lymph-space in which the aorta and vena cava are contained. Cut out the kidneys along with as much as possible of the above septum ; rinse with distilled water ; anil place in a watch-glass of I per cent, silver nitrate for 5 minutes. Einse again in distilled water and expose in tap water to the light. When slightly browned snip off a portion of the mem- branous septum, float it flat on a slide, drain off the superfluous water and 192 THE ESSENTIALS OF HISTOLOGY. allow it to dry ; then add a drop of xylol balsam or dammar and cover the preparation. 6. Kill a frog by destroying the brain, and study the circulation of the blood in the mesentery. It can also be studied in the web of the frog's foot, and in the lung and tongue of the frog or toad, or in the tail of the tadpole or of any small fish. But for observing the phenomena attending com- mencing inflammation and the emigration of leucocytes from the vessels, the mesentery is the most convenient object. The frog can 'be immobilised •with curari or by placing it in water in which chloroform or ether has been shaken up: a laterar incision is made in the abdominal wall, a loop of intestine drawn out, and laid over a ring of cork whichis fixed 'to a glass plate and covered with athin piece of glass. The membrane must be kept wet with salt solution.' The coats of the small arteries and veins are much simpler in structure than those of the larger vessels, but they contain at first all Fig. 239. — Small artekt, A, with cokeesponding vein, B, tkbated with AOKTIO ACID. (Kolliker.) (Magnified 350 diameters.) it, external coat with elongated nuclei ; 6, nuclei of the transTerse muscular tissue of the middle coat (when seen endwise, as at the sides of the vessel, their outline is circular) ; c, nuclei of the epithelium-cells ; d, elastic layer of the inner coat. the same elements. Thus there is a lining endothelium and an elastic layer, the two together forming an inner a at; a, middle coat of circularly disposed plain muscular tissue; and a thin adventitia. The same differences are found between the smaller arteries and veins as with the larger, the walls of the veins being thinner and containing far less muscular tissue (fig. 239), and the lining epithelium-cells, much 1 For details of the methods of studying the circulation and also of iniectimg the blood-vessels, see A Course of Practical Histology. SMALLER BLOOD-VESSELS. 193 elongated in both vessels, are far longer and narrower in the small arteries than in the corresponding veins (fig. 241). In the smallest vessels it will be found that the elastic layer has entirely disappeared in the veins, and the muscular tissue is consider- ably reduced in thickness in both kinds of vessel. Indeed, it is soon represented by but a single layer of contractile cells, and even these no longer form a complete layer. By this time also, the outer coat as well as the elastic layer of the inner coat have disappeared both from arteries and veins. The vessels are reduced, therefore, to the condition of a tube formed of pavement-epithelium cells, with a partial covering, of circularly disposed muscular cells. Fig. 240.— Tbansvekbe section op a small abtert and vein. Magnified 250 diameters. Even in the smallest vessels, which are not capillaries, the differences between arteries and veins are still manifested. These differences may be enumerated as follows : — The veins are larger than the correspond- ing arteries ; they branch at less acute angles ; their muscular cells are fewer, and their epithelium- cells less elongated; the elastic layer of the inner coat is always less marked, and sooner disappears as the vessels become smaller. Capillary vessels. — When traced to their smallest branches the arteries and veins eventually are seen to be continued into a network of the smallest blood-vessels or capillaries. The walls of these are composed only of flattened epithelium-cells (fig. 242) continuous with those that line the arteries and veins ; these cells can be exhibited by staining a tissue with nitrate of silver. The cell-outlines are not shown in developing capillaries; in these, silver nitrate stains the whole 194 THE ESSENTIALS OF HISTOLOGY. wall, i This is the case also with the capillaries of the villi, those of. the choroid coat of thp eye (Eberth), and those of the kidney-gjomeruli , (Eanvier); in all these places the walls are formed of a syncytium. <:, Fig. 241. — A small artkky, A, and vein, V, from the subodtanbous oon- NEOTIVB TIBSDE QF THE BAT, TKBATBD WITH'NITEATE OF 8ILVEE, WITH SUBSEQUENT STAINING OF NUCLEL 175 diameters. ■ a, a, epithelial cells with b, b their nuclei ; m, m, transvei^se markings due to staining of substance between the muscular fibre-cells ; c, c, nuclei of connective-tissue corpuscles attached to exterior of vessel. The capillaries vary somewhat in size and in the closeness of their meshes; their arrangement in different parts, which is mainly deter- mined by the disposition of the tissue-elements,^may best be studied in injected preparations, and will be described when the structure of the several organs is considered. In the transparent parts of animals, the blood may be seen flowing . CAPILLARY VESSELS. 195 through the capillary network from the arteries into the veins. The current is very rapid in the small arteries, somewhat less rapid in the veins, and slow in the capillaries. The current is fastest in the centre of the vessels, slowest near the wall (inert layer). In this layer the leucocytes are carried along by the stream and may be observed— especially where there is commencing inflammation of the part,, as in Fig. 242.— Capillaet vessels from the bladdee of the oat, mag- NIFIED. The outlines of the cells are stained by nitrate of silver. Fig. 243. — Blood flowing THRonoH a SMALL VEIN OF THE FKOG's MESENTERY. The mesentery had been exposed for a short time, so that there was commencing inflammation and many of the white corpuscles are observed sticking to and even passing through the vas- cular wall, a, central rapid layer containing the coloured corpuscles ; &, outer slower layer (inert layer) containing the white corpuscles. Fig. 244. — Ending of seksokt nerve-fibres in arborescenoes in the WALL OF A SMALL AETEET. (Dogiel.) The endothelium-cells of the vessel are outlined by dotted lines and the outlines of the muscular fibres are faintly indicated. the mesentery in consequence of exposure — to adhere to the inner surface of the blood-vessels, and here and there to pass through the coats of the small vessels, and appear as migratory cells in the surround- ing connective tissue (fig. 243). The blood-platelets are also to be seen in the inert layer, and show a tendency to adhere to the wall and to one another in commencing inflammation. , 196 THE ESSENTIALS OF HISTOLOGY. Vessels and nerves of the blood-vessels.— The larger arteries and veins possess blood-vessels (vasa vasorum) and lymphatics, both of which ramify chiefly in the external coat. Nerves are distributed to the muscular tissue of the middle coat, after forming a plexus in the outer coat. Most of the nerves are non-medullated. But there are a certain number of medullated fibres intermingled with the non-medullated and passing to end in localised arborescences (fig. 244) partly in the adventitia, partly in the intima. These medullated fibres are doubtless afierent ; the majority of the non-medullated are probably efierent (vaso-motors). In the aorta of man and in some of the larger trunks Pacinian corpuscles are here and there met with. The capillary vessels also receive nerve-fibres, which form a fine plexus of fibrils in close contact with the endothelium-cells of which the walls of these vessels are composed. Small cells are found at intervals in connection with these plexuses, but whether they are of the nature of nerve-cells or not is uncertain. Development of the blood-vessels. — The blood-vessels are developed in the connective tissue or in the mesenchyme which precedes it, the first vessels being formed in the vascular area which surrounds the early embryo. Their development may be studied in the embryo Fig. 245. — Isolated capillaby network fobmbd bt the junction op a hollowbd-ont stnottidm, containing coloubed blood-cobpuscles in a clear fluid. c, a hollow cell the cavity of which does not yet communicate with the network; p, Pf pointed cell processes, extending in different directions for union with neigh- bouring capillaries. chick or mammal, in the omentum of the new-born rabbit, or in the serous membranes and subcutaneous connective tissue of foetal animals. They are originally developed from cells (vaso-formatwe cells or angioUasts) which become hollowed out by vacuolation : coloured blood-corpuscles may be formed within them (see Development of Blood-corpuscles, Lesson II.). The cells branch and unite with one another to form a syncytium, and their cavities extend into the branches. In the meantime their nuclei multiply and become dis- tributed along the branches, cell-areas being at a later stage marked out around the nuclei. In this way intercommunicat- ing vessels — capillaries containing blood — are produced (fig. 245). DEVELOPMENT OF BLOOD-VESSELS. 197 These presently become connected with previously formed vessels, which extend themselves by sending out sprouts, at first solid, and afterwards hollowed out.i Even the larger blood-vessels appear first to be developed in the same way as the capillaries, in so far that the epithelium is first formed and the muscular and other tissues are subsequently added; but whether they are formed as clefts in the mesoblastic tissue, which become bounded by flattened cells, or whether as a hoUowed-out syncytium has not been definitely ascer- tained. VCZ .Int V Ar Fig. 246. — DrAGBAM To illustbate the development of blood-oapihabies (eight side), and sinusoids (left side) kespeotivelt. (F. T. Lewis.) Int, tntestinal entoderm with outgrowth on the left to form the liver and gall-bladder, and on the right to form the pancreas. V.C.I. , vena cava inferior: V.P., vena porta; v., vein and Ar, artery supplying pancreas. It is seen that the sinusoids or apparent capillaries of the liver are formed by the breaking up of a large blood-space into channels by the growth into it of cell-columns derived from the hepatic outgrowth of the entoderm. Sinusoids. — These are sinus-like blood-spaces between the cells of a tissue, which may when fully developed bear a superficial resemblance to blood-capillaries, but which difiier essentially from them both in their mode of development and in their relationship to the connective tissue, as well as to the tissue-elements of the organs in which they occur. Whereas capillary blood-vessels are developed amongst and between the tissue-elements and are connected with and grow from neighbouring capillaries which are themselves surrounded by areolar tissue, sinusoids make their first appearance in the form of comparatively large blood- spaces connected with the venous (or arterial) system. Into these, the walls of which are formed only of a single layer of endothelial cells, the tissue-elements of the developing organ (Wolffian body, liver, suprarenals, blood-glands, etc.) grow, invaginating the thin wall and forming cell-trabeculse within the sinus (figs. 246, 247), so that ' Many authorities consider that new blood-vessels are exclusively formed by sprouts from pre-existing vessels, and regard the appearances above described as being due to retrogressive development of an already formed vascular net- work (see footnote, p. 37). 198 THE ESSENTIALS OF HISTOLOGY. the. cells of the organ are directly in contact with the invaginated endothelium, and are only separated by this from the blood contained within the sinus. But the •connection may become yet closer than this, for, as happens in the liver, the invaginated endothelium may X300 ^"-'-^ ^' -< k; , > Fig. 247 — Livbk of embryo chick of eleven days (C S Minot.) h.e., hepatic cylinders , &, sinusoids. become defective, so that the blood within the sinus comes into actual contact with the cells of the organ, and runs into the interstices between them. As development proceeds these interstices may come to resemble blood-capillaries in general arrangement and shape; but the resemblance is only superficial, and the intimate relationship between the blood and the tissue-elements, which are both enclosed within the original sinus, is usually maintained.' The distinctive character of sinusoids was first recognised by Minot. LYMPHATICS OR LYMPH-VESSELS. To the lymphatic system belong not only the lymphatic vessels and lymphatic glands, but also the cavities of the serous memh'anes, which are moistened with lymph and are in |open communication with lymphatic vessels which run in their parietes. The larger lymph-vessels somewhat resemble the veins in structure, except that their coats are much thinner and valves much more numerous. In lymphatics of smaller size, the wall of the vessel is formed, first, by a lining of pavement-epithelium cells (lymphatic endothelium), which are elongated in the direction of the axis of the vessel; and, secondly, by a layer of circularly and obliquely disposed muscular fibres. In the smallest vessels (so-called lymph-capillaries, which are generally considerably larger than the blood-capillaries). LYMPH-VESSELS. 199 Fig. 248. — A small pabt of the lymphatic plexus oe t^b plbueal later OE THE DIAPHRAGM. Magnified 110 diameters. (Eanvier.) I, lymphatics with characteristic epithelium : c, cell-spaces of the connective tissue here and there abutting against the lymphatic. vvi^ I'm. 249. — Nebyes of a lymphatic vessel, shown by methylene blue, (Dogiel.) he membrane is prepared by the nitrate of silver method the stomata and the cells which surround them on either side of the membrane are well shown. The pavement-epithelium of the serous membrane rests upon a homogeneous basement-membrane, which is especially well marked Fig. 251. 1. Epithelium from the postekioe part of the frog's peritoneum, showing THREE stomata LEADING INTO THE CISTERNA LTMPHATIOA MAGNA. (v. Bbner, after Sohweigger Seidel and Dogiel.) 2. A PORTION OF EPITHELIUM FROM THE PERITONEAL SURFACE OF THE RABBIT'S DIAPHRAGM. THREE PORES ARE VISIBLE BETWEEN THE EPITHELIUM CELLS. (v. Ebner, after Ludwig and Schweigger Seidel. ) in the serous membranes of man. The rest of the thickness of the membrane is composed of connective tissue, with a network of fine elastic fibres near the inner surface. The cavities of the serous membranes are originally formed in the embryo as a cleft in the mesoderm (pleuro-peritoneal split, coelom) which becomes lined with epithelium, outside which the coelomic wall eventually becomes diflFerentiated into the serous membrane. LYMPH-GLANDS. 203 LESSON XXII. LTMPE-GLANDS. TONSILS. THYMUS. 1. Sections of a lymph-gland which has been hardened either in formol or potassium bichromate, or in chromic acid or picric acid followed by alcohol, stained in bulk, and embedded in paraffin. Or the sections may be stained with hsematoxylin and eosin. Notice (1) the fibrous and muscular capsule, with trabeculse extending inwards from it through the cortex and anastomosing with one another in the medulla, (2) the dense lymphoid tissue (adenoid tissue of some authors) forming large masses in the cortex (cortical nodules) and rounded cords in the medulla (medullary cords). Notice also the clearer channel or lymph-sinus which everywhere intervenes between the fibrous tissue and the lymphoid tissue. Observe the fine fibres and branched cells which bridge across this channel. Make a general sketch under a low power of a portion of the cortex together with the adjoining part of the medulla, and under a high power drawings of small portions of cortex and medulla. The retiform tissue of the lymph-glands has already been studied (p. 75). 2. Sections of a hsemal lymph-gland. These may be readily found in the neck of the ox, in the neighbourhood of the large blood-vessels. Stain vtith eosin and hsematoxylin or with eosin and methylene blue. Notice that the channels around the lymphoid nodules (or some of them) contain blood instead of lymph. 3. In sections of tonsil prepared similarly to those of the lymphatic gland, notice the large amount of lymphoid tissue, partly collected into nodules. Observe also that the stratified epithelium, which covers the mucous membrane here as elsewhere in the mouth, is infiltrated with lymph- corpuscles. The tonsil is beset with pit-like recesses, with mucus-secreting glands opening into the pits. 4. Lymphoid nodules of mucous membranes. In other mucous membranes besides that of the back of the mouth and pharynx, collections of lymphoid tissue, occur which resemble those of the tonsils ; such nodules form the solitary glands of the stomach and intestines and the agminated glands of the small intestine, and are also found in the trachea and bronchial tubes and in the cesophagus. They may be studied later in sections of those parts. 5. Sections of the thymus gland of an infant or young animal. Notice that the masses of lymphoid (?) tissue which form the lobules of the gland are separated by septa of connective tissue, and that the lobules show a distinc- tion into two parts, cortical and medullary. There are no lymph-paths. Observe the differences of structure of the cortex and medulla, and especially notice the concentric corpuscles in the medullary part. Make a sketch of one of the lobules under a low power and of a small part of the medulla under a high power, including one or two concentric corpuscles. Measure the latter. Lymph-Glands. Structure of a lymph-gland. — A lymph^gland (lymphatic gland) is composed of a framework of fibrous and plain muscular tissue, which. 204 THE ESSENTIALS OF HISTOLOGY. incloses and supports the proper glandular substance, but is everywhere separated from it by a narrow channel, bridged across by cells and fibres, which is known as the lymphrcharmel. The frameworh consists of an envelope or capsule (fig. 252, c), and of trabeculce (ir), which pass at intervals inwards from the capsule, and after traversing the cortex of the gland, divide and reunite with one another to form a network of fibrous bands. At one part of the gland there is usually a depression (hilus), and at the bottom of this the medulla comes to the surface and its fibrous bands are directly continuous with the capsule. Fig. 252. — Diaobammatio section or ltmph-oland. (Sharpey.) a.l. afferent, e.L efferent lympbatica ; C, cortical substance ; M, reticulating cords of medullary substance ; l.h. lympboid tissue ; I.e. lympb-sinus ; c, capsule sending trabeculse, tr, into tbe substance of tbe gland. The proper glcmdidar substance (l.h.) is composed of lymphoid tissue, i.e. a fine reticulum with the meshes thickly occupied by lymph- corpuscles. It occupies all the interstices of the gland, forming com- paratively large rounded masses in the cortex (lymphoid nodules, C), which may be two or three deep, and smaller reticulating cord-like masses (lymphoid cords, M) in the medulla. The cells which bridge across the lymph-channel in the medulla (fig. 254, c) are branching nucleated cells which often contain pigmen^ so that this part of the gland has a dark colour. Some may contain disintegrating erythrocytes. The lymph-channel is bridged across not only by these branched cells, but also by fibres derived from the LYMPH-GLANDS. 205 capsule and trabeeulse, which pass to the lymphoid tissue and become lost in its reticulum. But the fibres are often completely concealed by the cells. Afferent lymph-vessels (fig. 252, a.l.) enter the lymph-channels after ramifying in the capsule, and the lymph is conveyed slowly along the channels of the cortical and medullary part towards the hilus, taking up many lymph-corpuscles in its passage. At the hilus it is gathered up by an efferent vessel or vessels («.Z.) taking origin in the lymph-sinuses of the medulla. Fig, 258. — Section of a lymph-gland rnoM the neck of an eight teae OLD child, (v. Ebner.) xl3. c, capsule; c.%, cortical nodules, some with germ-centres; l.c, lymptLOid cords of medulla (dark); l.p, lymph-path (light); s, cortical sinus; t, trabeeulse; v, vein; I, efferent lymph-vessels, accompanying and partly surrounding blood-vessels, bl. The efferent lymphatics always contain many more lymph-corpuscles than those which enter the gland, for lymph-corpuscles are constantly being formed by mitotic division of the pre-existing cells in the glandular substance, especially in the clearer centre of each cortical nodule (germ-centre of Flemming) ; they gradually find their way into the lymph-channel. The leucocytes of the germ-centres frequently show in sections peculiar darkly-coloured bodies — the stainahle-hodies of Flemming — the origin of which has not been determined. 206 THE ESSENTIALS OF HISTOLOGY. An artery passes into each gland at the hilus; its branches are conveyed at first along the fibrous cords, but soon become surrounded by the lymphoid cords, where they break up into capillaries (fig. 254, 4).. The blood is returned by small veins, which are conducted along the fibrous trabeculse to the hilus. Fig. 254. — Section of the mbdullakt substance of a lymph-gland. 300 diameters. (Becklinghauaen. ) . u, u, a, lymphoid cords ; c, lymph-sinus ; 6, b, trabeculse ; d, d, capillary blood-vessels. In some lymph-glands the fibrous trabeculse are very slightly de- veloped, so that the gland seems in section to be almost uniformly a mass of lymphoid tissue, pervaded by lymph-channels and with clearer rounded nodules (germ-centres) scattered about, especially in the cortex (fig. 253). This is the case with most of the lymph-glands of man and some other animals. In other animals, such as the dog and ox, the trabeculse are very well developed and contain much muscular tissue. Nerve-fibres pass to lymph-glands and appear to be distributed chiefly as non-meduUated fibres to the plain muscular tissue of the blood-vessels and trabeculse. Ordinary lymph-glands are confined to mammals, but Vincent and Harrison have found hsemal lymph-glands in birds. Hsemal lymph-glands. — In many animals a certain number of lymph- glands are observable which have a red colour. Some of these on H^MAL LYMPH-GLANDS. 207- section show that what corresponds to the peripheral lymph-channel in ordinary lymph-glands is in them occupied by blood. Others have the gr6ater part of the interior occupied by large sinuses filled with blood ; but some parts have the ordinary structure of a lymph- gland. The names hmmal glands and hcemal lymph-glands (Robertson) have been given to these organs. The blood passes into the sinuses from the arterial capillaries, which probably, as in the spleen, become incomplete, and open into the tissue interstices, from which at other Fis. 255. — Sbction through one op the cripts of the tonsil. (Stohr.) e, e, stratified epitholium of surface of mucous membrane, continued into crypt ; f, f, follicles or nodules of the lymphoid tissue, which is elsewhere diffuse ; some show clear -"germ-centres"; opposite each nodule numbers of lymph-cells are passing through the epithelium ; «, masses of cells which have thus escaped from the organ to mix with the saliva as salivary corpuscles. parts the small veins in like manner arise. Like the spleen these haemal glands show cells (phagocytes) which contain red blood- corpuscles in various stages of transformation into pigment. Some haemal glands are said by Weidenreich to have no lymph- channels, but this statement requires confirmation. The Tonsils. The tonsils are two masses of lymphoid tissue placed one on each side of the pharynx, into which they project. They are covered on the free surface with the stratified epithelium of the mucous membrane. THE ESSENTIALS OF HISTOLOGY. Fig. 256.— Lymphatics op a peyee's patch, injected with silver nitrate; OAT. (Kolliker.) MagDi&ed 85 diameters. /, a lymphoid nodule or follicle ; /', its base, resting upon the muscular coat, m ; s.m.^ submucosa ; I, lymph- vessels ; «, sinus-like enlargement of lymph-vessel surrounding follicle. Fig. 257. —Developing lymphoid nodules from the guinea-pig's omentum. (Klein.) a, perilymphatic nodule ; a, lymphatic ; c, its endothelium ; e, lymph-corpuscles ; h, accumulation of lymphoid tissue on one side of it ; d, blood-capillaries within this. B, endolymphatic nodule consisting of an enlarged lymphatic vessel, <2, within which. is a capillary network c, c, an artery; b, and a vein, a ; e, lymphoid tissue within the lymphatic, its branched cells being, joined to and derived from the lymphatic endothelium f. THE TONSILS. 209 and this surface is pitted with apertures which lead into recesses or crypts in the substance of the organ (fig. 255). These recesses are all lined by a prolongation of the stratified epithelium, and into them the ducts of numerous small mucous glands open. The tonsils are composed almost entirely of lymphoid tissue,, which, besides being dififused over the whole organ, is at intervals aggregated into nodules, in which the lymph-cells are more closely arranged than elsewhere. In the clear centre {germ-centre) of some of these nodules active multiplication of the lymph-cells by mitosis is constantly proceeding, and is, in fact, the cause of the formation of nodules in the tissue, as in other organs in which lymphoid tissue occurs. Even the epithelium which covers the tonsils is infiltrated with lymph-corpuscles (Stohr), and these may also wander out on to the free surface, and become mingled with the saliva as salivary corpuscles. The lymphoid tissue is highly vascular, and contains many lymphatics. The mucous membrane of the neighbouring part of the pharynx and of the back of the tongue and that of the upper part of the pharynx near the orifices of the Eustachian tubes shows crypts and masses of lymphoid tissue similar in structure to those of the tonsils. Other Lymphoid Structures, Lymphoid tissue occurs in many other parts of the body in addition to the lymphatic glands and tonsils, although it may not, as in these structures, constitute the bulk of the organ. Thus it is found in many mucous membranes, such as those of the intestine and of the respiratory tract, both in a difiiise form and also collected into nodular masses which are like the cortical nodules of a lymphatic gland, and may, like these, be partially surrounded by a lymph-sinus. In the intestine such nodules constitute the so-called solitary glands and Peyer's patches. The lymphatics form plexuses of large sinus-like vessels which to a large extent enclose the nodules (fig. 256). In the spleen a large amount of lymphoid tissue is found eiisheathing the smaller arteries, and also expanded into nodular masses (Malpighian corpuscles of the spleen). All these structures will be studied subse- quently. Lymphoid tissue also occurs in considerable amount in the serous membranes, especially in young animals ; in the adult it is here mostly replaced by adipose tissue. Development of lymphoid tissue.— Lymph-glands are developed in connection with plexuses of lymph-vessels, an accumulation of retiform tissue and lymph-cells taking place either external to and around the lymphatics {perilymphdtic formation) ; or some of the lymphatics are dilated 210 THE ESSENTIALS OF HISTOLOGY. into a sinus or sinuses and the formation of lymphoid tissue occurs within it {endolymphatic formation) (fig. 257, a and b). When there is a develop- ment of lymphoid tissue outside the lymphatic vessels this may form a considerable accumulation before the formation of lymph-paths within the tissue. Blood-vessels are early developed amongst the lymphatic plexuses, and by these, according to GuUand, the first lymph-corpuscles of the lymphoid tissue are brought to the gland. The marginal sinus is produced by the fusion of a number of lymph- vessels which surround the accumulation of lymphoid tissue, while in the situation of the future hilus other lymph-vessels grow into the glandular substance and form channels which subdivide it up into cords and nodules (Kling). The branched cells of the lymph-path are said to be derived from the lymphatic endothelium. The axillary glands were found by Stiles to increase in number and size during lactation, diminishing again after lactation has ceased. In the developing tonsils Gulland occasionally found nests of epithelial cells detached from the surface epithelium, somewhat like those found per- manently in the thymus. Thymus. The thymus gland is an organ which in man is found only in the embryo and during infancy. It is composed of a number of lobules (fig. 258) varying in size, which are separated from one another by septa of connective tissue, along which the blood-vessels and lymphatics pass to and from the lobules. Each lobule shows Fig. 258. — A LOBDLE of the thymus of a child, as seen under a low-powe^ u, cortex ; c, oonceutrio oorpusoles within medulla ; 6, blood-vessels ; tr, trabeoulse. plainly, when examined with a low power, a distinction into an outer cortical and an inner medullary portion. The cortical part of the lobule is imperfectly divided into nodules by trabeculse of connective tissue. It is highly vascular, and is superficially similar in structure to the lymphoid tissue of the lymph-glands and tonsils, with which it also agrees in exhibiting numerous indications of indirect cell- THYMUS. 211 division, but without definite germ-centres. Besides leucocytes it contains a number of peculiar granular cells. The medulla is more open in its texture, and its reticulum is formed by large transparent, branched cells (fig. 259), which are sometimes massed together and then resemble epithelium-cells. The medulla contains fewer lymph- corpuscles than the cortex and has a clearer aspect. Connective tissue fibres are not wholly absent from it. Within the medulla, but not f IG. 259. — Section of medulla of thymus, showing branched (epithelial) CELLS OF KETICULnM AND A CERTAIN NUJIBEK OP LYMPHOID CELLS IN THE MESHES. (Hammar. ) in the cortex, are found peculiar concentrically striated bodies (the concentric corpuscles of Hassal, figs. 258, 260), which are " nests " of flattened epithelial cells arranged concentrically around one or more central cells, which have often undergone a- degenerative process. Sometimes these corpuscles are compound, two or three being grouped together and similarly enclosed by flattened cells. They represent part of the remains of an epithelial tube, which forms the thymus rudiment of the early embryo and is derived from certain of the branchial clefts. According to the observations of Hammar the reticulum of the gland is also derived from this epithelium, and Stohr believes that the apparent lymphoid cells of the gland have a similar origin. The inference drawn by J. Beard from his observ- ations in Elasmobranchs that the thymus is the original seat of 212 THE ESSENTIALS OF HISTOLOGY, formation of leucocytes in the embryo, appears from more extended investigations to be incorrect. Nucleated red blood- corpuscles (erythroblasts), similar to those found in red marrow, have also been described in the thymus (J. SchaflFer), and occasionally cysts lined by ciliated epithelium are found. In some animals islands of striated muscular cells are seen in the medulla. Multinucleated giant-cells are also found (Watney). The lobules, the cortex especially, are abundantly supplied with capillary blood- vessels.' In man the arteries penetrate to the junction of cortex and medulla, and give off most of their capillaries radially into the cortical nodules. Veins pass away both from the surface of the lobules and to a less extent directly from the medulla. The mode of distribution of the lymphatics has not been definitely ascertained, but none are seen within the lobules. Nevertheless, large lymphatic vessels, containing many lymphocytes, issue from the interstitial connective tissue of the thymus, but in what way they are connected with the lobules has not been ascertained. In the human subject the thymus gland undergoes after childhood a process of retrogression, its lobules ceasing to grow and becoming surrounded and concealed by a quantity of adipose tissue which develops in the interstitial connective tissue of the gland. Eventually the lobules atrophy. Fig. 260.— Elements of the THYMUS. 300 diameters. (Cadiat.) u, lymph-corpuscles ; b, con- centric corpuscle. STEUCTUTIE OF THE SPLEEN. 213 LESSON XXIII. STRUCTURE OP THE SPLEEN, SUPRARENAL CAPSULES, THYROID BODY, AND PITUITARY BODY. 1. Sections of the spleen hardened in Miiller's fluid or formol and stained with hsematoxylin and eosin. Notice the trabeculse extending into the substance of the organ from the capsule. Notice also that the glandular substance is of two kinds, (1) lymphoid tissue accumulated around the small arteries and here and there massed to form lymphoid nodule's — the Malpighian corpuscles — and (2) a tissue — the red pulp — consisting of a reticulum of fibrils and branching cells : this tissue contains blood in its interstices. Sketch part of a section under a low power and a small portion of the pulp under a high power. 2. Sections across a suprarenal capsule hardened in 2 per cent, bichromate of potassium. In sections not otherwise stained, notice the deep brown coloration of the medulla (action of the chromic salt). Stain other sections with eosin and hsematoxylin. Examine first with a low power, noticing the general arrangement and extent of the cortical and medullary parts of the organ, and making a general sketch which shall include both. After- wards sketch carefully under the high power a group of cells from each part of the organ. 3. Sections of the thyroid body stained with eosin and hsematoxylin. Notice the vesicles lined with cubical epithelium and filled with a " colloid " substance which becoines stained with hsematoxylin. Sketch one or two vesicles. Measure several vesicles. The sections will probably also include one or more parathyroids. 4. Sections (antero-posterior) through the pituitary body. Notice the (epithelial) anterior lobe separated by a cleft from the (nervous) posterior lobe. (The anterior part of the posterior lobe is also covered by an epithelial layer.) 5. Injected preparations of these organs may also be studied : the spleen is usually naturally injected with blood. The Spleen. The spleen is the largest of the so-called ductless glands. It appears to be functionally connected with the blood, white blood-corpuscles being formed and coloured blood-corpuscles being submitted to destruc- tion within it. Like the lymph-glands, the spleen is invested with a fibrous and muscular capsule (fig. 261), which is however stronger and has far more plain muscular tissue ; outside the capsule is a covering derived from the serous membrane. The capsule sends bands of trabeculse 214 THE ESSENTIALS OF HISTOLOGY. into the organ, and these join with a network of similar trabeculae which pass into the gland at the hilus along with the blood-vessels. In the interstices of the framework thus constituted lies a soft pulpy substance containing a large amount of blood, and therefore of a deep red colour, dotted within which are here and there to be seen small round bodies, whiter than the pulp in the fresh organ but darker in Via. 261.— Vertical section of a poktion of the monkey's spleen, as SEEN WITH A LOW POWEE, stained sections, the Malpighian cmpuscles of the spleen. These are composed of lymphoid tissue which is gathered up into masses which envelop the smaller arteries, whilst the red pulp which everywhere surrounds them and which forms the bulk of the organ is composed (Carlier) of a close network of connective tissue fibrils (fig. 262), partly covered by flattened and branched cells (fig. 263). Passing into the pulp and communicating with its interstices are capillary blood- vessels which are connected with the terminations of the arteries ; whilst in other parts venous channels — characterised in the human spleen by an encirclement of reticulum-fibres, possibly of an elastic nature (fig. 264), and by the presence of a layer of highly characteristic. STEUCTURE OF THE SPLEEN. 215 Fig. 262. — Reticulum of sflbbn, golgi method. (Oppel.) a, Malplghian corpuaclo ; b, part of its reticulum ; c, condensed reticulum at its margin ; d, more open tissue >next' to this; e, wall of arteriole; /, capillaries of Malpighian corpuscle ; g, reiiiculum of arteriole expanding into that of the Malpighian corpuscle. Fio. 263. — Small veins of spleen pulp with retioulab tissue, (Hoyer.) The veins, which are invested by encircling fibres, show gaps in their walls whereby they communicate with the interstices of the pulp. 216 THE ESSENTIALS OF HISTOLOGY. comparatively thick and prominent endothelium-cells, which exhibit a longitudinally striated structure— course through the pulp and bring the blood which has passed into its interstices from the arterial capillaries towards the larger veins of the organ, which run in the trabeculse, and are by them conducted to the hilus. The arteries, which are also at first conducted from the hilus along the trabeculse into the interior of the organ, presently leave the trabeculse, and their Fio. 264. — Venous spaoks of splbbn pui-p, showing the enoirolinb pibbes IN THEIR WALLS. MAN. (v. Ebner.) CD, capillary veins ; 33, pulp (the tissue elements are not represented). external coat becomes gradually converted into a thick sheath of lymphoid tissue which invests them in the remainder of their course, and in places becomes swollen into the Malpighian corpuscles already mentioned. The small arteries distribute a few capillaries to the Malpighian corpuscles, and then break up into pencils of capillary vessels which open into the interstices of the pulp.^ The Malpighian corpuscles frequently but not always show a clearer central nodule or germ-centre, characterised by the presence of numerous mitoses ; and the stainable bodies of Flemming (see p. 205) are also seen in them. « ^It is right to state that many authorities hold that the arterial capillaries open into the venous sinuses and that the blood-system of the spleen is there- fore a closed one, the blood- corpuscles passing into the pulp-interstices by diapedesis. STRUCTURE OF THE SPLEEN. 217 ; ^V4^^'^-^^ '11 Fig. 26.5. — Thin .section of spleen-pulp of child, highly magnified, SHOWING the' APPAKKNT MODE OF ORIGIN OF A SMALL VEIN IN THE INTEBSTIOES OF THE PULP. Magnified 400 diameters, a, blood in pulp ; a', blood in vein ; 6, phagocyte in vein ; c, 1>rancliQd cell of pulp ; d, splenic cell. it"- -tBaamtext^ Fig. 266.— a giant cell pbom the SPLEEN OF A KITTEN. Magnified 400 diameters. Fig. 267. — A vertical section of the suprarenal body OP A FCBTUS, twice THE NATURAL SIZE, SHOWING THE DISTINCTION BETWEEN THE MEDULLARY AND COR- TICAL SUBSTANCE. (A. Thomson. ) V, issuing vein ; r, summit of kidney. 218 THE ESSENTIALS OF HISTOLOGY. The special cellular elements of the spleen-pulp, are of three kinds, viz. (1) peculiar, large, amoeboid phagocytic cells, (2) megakaryocytes or giami cells, and (3) bramched and generally flattened cells which assist in forming the spongework. The pulp also contains all the corpuscular elements of blood. The phagocytic cells are frequently found to contain coloured blood-corpuscles in their interior in various stages of transformation into pigment. They occur both in the interstices of the pulp and in the venous sinuses and veins, where they are often filled with erythro- cytes (fig. 265). The giant cells are most frequent in young animals (fig. 266) : their function has not been ascertained. The branched cells of the spongework are probably of the same nature as the endothelium cells of the terminal capillaries . and veins of the pulp. They are connected with one another and with the endothelial cells of the small vessels by branches. The ph&,gocytie spleen cells are perhaps derived from them. Nucleated coloured corpuscles are found in the embryo, and occasionally after birth, in the spleen-pulp. The blood of the splenic vein is very rich in leucocytes. The lymphatics of the spleen run partly in the trabeculse and capsule, and partly in the lymphoid tissue ensheathing the arteries. They join to form larger vessels which emerge together at the hiliis. There are no lymphatics in the spleen pulp. The nerves, which are numerous and mostly non-meduUated, are distributed to the muscular tissue of the arteries and to that in the capsule and trabeculse. Mall states that the distribution of the trabeculse and of the blood-vesselfl within the spleen is such as to indicate a differentiation of the pulp into divisions which he terms "spleen lobules," each of which has its own arteriole and venule, and in which the pulp is arranged in columns or cords surrounded by venous spaces. It must, however, be understood that there is nothing of the nature of partitions separating such lobules : to all appearance the pulp is in continuity throughout the organ. The Suprarenal Capsules. The suprarenal capsules (adrenals) belong to the class of bodies known as ductless glands, but they are entirely different in structure and function from the spleen and lymphatic glands. A section through the fresh organ (fig. 267) shows a cortex which is striated verti- cally to the surface, and of a yellowish colour, and a medulla which is soft and highly vascular, and of a dark-red colour. The whole organ is invested by a fibrous capsule which sends fibrous septa inwards through the cortical substance (fig. 268, a), subdividing this for the most part THE SUPRARENAL CAPSULES. 219 into columnar groups of cells {zma fascieulata, c). Immediately under- neath the capsule, however, the groups are more rounded, and the cells tend to assume a columnar form {zona glomeruhsa, b), whilst next to the medulla they have a reticular arrangement {zona reticularis, d). Fie. 268. — Vertical section op ooktex of supraebnal of dog. (Bohm and V. Davidoff). Magnified about 150 diameters. u, capsule ; h, zona glomerulosa ; c, zona fascieulata ; d, zuua reticularis. The cells which form the cortical substance are, for the most part, polyhedral in form ; each contains a clear round nucleus, and there are often yellowish oil-globules in their protoplasm. No arteries and veins penetrate between these cells, both these and the lymphatics of the cortex running in the fibrous septa between the columns of cells, which they surround with a capillary network. In the zona reticularis the capillaries widen out and occupy the spaces between the cell- columns (fig. 268, d). The lymphatics communicate with fine canals between the cells of the cortex. 220 THE ESSENTIALS OF HISTOLOGY. The cells of the mfiduUa (fig. 269) are more irregularly disposedi They are supported by a network of elastic fibres. They lie in very close relation to the large capillary blood-spaces (sinusoids) which Fig. 269. — Section showing zona RBTionLAEis of oortbx, r, and medulla, m, OF SUPEARENAL OF DOG. (Szy monowicz. ) Magnified 384 diameters, pervade the medulla and they probably pour a secretion directly into the blood. Their protoplasm is granular; in some animals it containf a brownish pigment, but in man the dark red colour of the medulla is due to the blood contained in the large sinusoid spaces by which it is pervaded, and which receive the blood after it has traversed the capillaries of the cortex. A few arterioles pass straight, to the medulla through the cortex. One large vein usually passes out at THE THYROID BODY. 221 the hilus in the anterior surface of the gland. Investing the larger veins are longitudinal bundles of plain muscular fibres ; but most of the veins have only an endothelium. Numerous nerves, after traversing the cortical substance, are distributed throughout the medulla, where they form a close plexus provided here and there with ganglion-cells. The cells of the medulla are characterised by staining brown by chromic acid and its salts, provided the organ is fresh (chromophil or chromaffin cells). The medulla of the suprarenal capsule is developed from cells which become detached from the rudiments of the sympathetic ganglia, and are therefore of ectodermal origin. The cortex is developed from mesoderm. The Thyroid Body. The thyroid body consists of a framework of connective tissue inclosing numerous spherical or oval vesicles (fig. 270) which are lined Fig. 270. — Seotion of human thyroid. (Szymonowioz.) Magnified about 180 diameters, a Vesicle occupied by colloid, whicb has partly shrunk away from the epithelium ; 6, epithelium of a large vesicle ; c, c, epithelium of vesicles which are cut tangen- tially ; d, interstitial connective tissue. with cubical epithelium-cells ; these often contain granules of a fatty elaraeter. The cavities of the vesicles are usually occupied by a peculiar viscid liquid (colloid) which is coagulated by alcohol and which then becomes stained with haematoxylin. A similar material 222 THE ESSENTIALS OF HISTOLOGY. has been found in the lymphatics of the gland, and may often be detected also in the interstices of the connective tissue. The blood-vessels of the thyroid are numerous and give a deep red colour to the organ. The capillaries form close plexuses round the vesicles (fig. 271), and even extend between the .lining epithelium cells. Parathyroids. — In close proximity to or embedded in the substance of the thyroid are always to be found four small glandular organs Fig. 271.— Thyroid of dog injected. Fig. 272.— Paeathtroid of monkbt. (Vincent and Jolly.) Ut parathyroid tissue ; &, blood-vessels ; c, connective tissue ; d, junction of para. thyroid with thyroid ; e, thyroid vesicles ; e'. colloid. of different structure from the thyroid proper, although somewhat resembling its embryonic condition (fig. 272). These bodies, one of which usually lies on the lateral and one on the mesial surface- of each lateral lobe, a,re formed of columns of granular epithelium-cells, with a very vascular connective tissue between the columns. If left after removal of the thyroid, they are stated to undergo hypertrophy, and to develop a vesicular structure (Vincent and Jolly). Besides these bodies, there is also frequently to be found in connexion with the thyroid a small mass of lymphoid tissue which resembles the thymus tissue in structure, and, like it, contains concentric corpuscles. CAROTID AND COCCYGEAL GLANDS. These are minute glandular organs without ducts, lying respectively at the bifurcation of the carotid artery and in front of the apex of the coccyx. They are composed of polyhedral cells, with numerous blood- capillaries between them. In the carotid gland the cells are collected CAROTID AND COCCYGEAL GLANDS. 223 d Fig. 273. ^A clump oe oell-bali, from the oakotid gland, injected. (Sohaper. ) a, arteriole ; v, venules ; c, sinus-like capillary within nodule ; gl, group of gland cells ; c, boundary of nodule surrounded by lymph space ; d, inter-nodular connective tissue of gland. 3 Tf-rn VT± — .SimnTTriK OP COCCYGEAL GLAND. (Walker.) 224 THE ESSENTIALS OF HISTOLOGY. into spheroidal clumps, in the coccygeal gland into irregular nodules. The blood-vessels, at least in the coccygeal gland, have a sinusoidal character (Walker). Amongst the cells are some which stain dark brown with chromic acid like those of the medulla of the suprarenal capsules (chromophil cells). A certain number of such cells occur also, according to Kohn, in sympathetic ganglia. The Pituitary Body. The pituitary body or gland (hypophysis cerebri) is a small reddish mass which lies in the sella turcica, and is connected with the third ventricle by the infundibulum. It consists of two lobes, a larger anterior and a smaller posterior (fig. 275). The anterior lobe is InfimdUnOum. Cranium Fig. 276.— Section thkough hypophysis. (Edinger.) Fig. 275 a.— Sbotion of antebioe lobe of hypophysis of ox. (Doetoiewsky.) U, blood-sinuses ; c, oell-stranda containirg clear cells ; d, strands o£ darker granular cells. Other strands contain botli kinds of cell. THE PITUITARY BODY. 225 originally developed as a hollow protrusion of the buccal epithelium. It consists of a number of tubules, which are lined by epithelium and united by connective tissue. In some of the tubes the epithelium is ciliated, and occasionally a colloid substance is found in them, but for the most part the lumen of the tubules has become obliterated in the adult, and they present the appearance of solid cell-masses between which are numerous large venous capillaries, perhaps sinusoids. Some of the cells are clear, others darkly granular in appearance (fig. 275 A). The posterior lobe of the pituitary body, which is developed from the infundibulum of the third ventricle, consists chiefly of vascular connec- tive tissue and neuroglia, but it also includes masses of cells of an epithelial character (pars intermedia), which are continuous with those of the anterior lobe. It is partly separated from the anterior lobe by a cleft-like space containing glairy fluid. In pian the posterior lobe is stated to contain no cells in the adult of distinctly nervous character, but it receives many nerve-fibres which arise from large cells in the grey matter just behind the optic chiasma, some of which penetrate into the glandular substance. 226 THE ESSENTIALS OF HISTOLOGY. LESSONS XXIV. AND XXV. TEE SKIN. 1. Sections of' skin from the palmar surface of the fingers. The skin is hardened in picric acid or formol, followed by alcohol. The sections are made vertical to the surface, and should extend down as far as the sub- cutaneous tissue. Notice the layers of the epidermis and their different behaviour to staining fluids. Notice also the papillae projecting from the corium into the epidermis and look for tactile corpuscles within them. In very thin parts of the sections the fine intercellular channels in the deeper parts of the epithelium (see Lesson VII.) may be seen with a high power. The convoluted tubes of the sweat-glands are visible here and there in the deeper parts of the corium, and in thick sections the corkscrew-like channels by which the sweat is conducted through the epidermis may also be observed. Make a sketch showing the general structure under a low power, and other sketches to exhibit the most important details under a high power. Measure the thickness of the epidewpis and the length of the papillae. 2. Sections of the skin of ^e scalp, vertical to the surface and parallel to the slope of the hair-follicles, and others parallel to the surface, and therefore across the hair-follicles. Stain and mount in the same way as in the last preparation. Examine also the structure of the hairs. In these preparations the details of structure of the hairs and hair-follicles, together with the sebaceous glands and the little muscles of the hair-follicles, are to be made out. 3. Vertical sections of the nail and nail-bed. To cut such hard structures as the nail it is best, after fixing with picric acid or formol followed by 75 p.c. alcohol, to soak the tissue in strong gum arable for a few days, then place it in an appropriate position upon a cork or upon the object- carrier of a microtome, and plunge the whole into 70 per cent, alcohol. This renders the gum hard, and enables sections to be out of sufficient fineness. A plane iron should be used with the microtome, since the hardness of the nail will turn the edge of a razor. To remove the gum the sections are placed in water for a few hours ; they may then be stained and mounted. Notice the ridges (not papillse) of the corium, projecting into the epidermis. Observe also the distinction of the epidermis into Malpighian layer and nail proper. 4. Mount a section from a portion of skin in which the blood-vessels have been injected, and notice the distribution of the capillaries to the sweat- glands, to the hair-follicles, and to the papillary surface of the corium. 5. The cells which compose the nails and hairs can be isolated by warming a small piece of nail or hair in strong sulphuric acid ; after this treatment they are readily separated from one another by pressure upon the cover- glass. 6. Sections of mammary gland during lactation. The gland may be fixed in Zenker's fluid (see Appendix) and the sections stained with hsematoxylin and eosin. The skin is composed of two parts, epidermis and cutis vera (fig. 276). The epidermis, or scarf skin, is a stratified epithelium (fig. 277). It is composed of a number of layers of cells, the deeper of which are THE SKIN. 227 soft and protoplasmic, and form the rete mucosvm of Malpighi, whilst the superficial layers are hard and horny, this horny portion sometimes \ \stratum corneuin rete mucosum -cutis vera sweat glands adipose tissue Fig. 276. — Vkktioal section throdgh the skin op the solu of the ffooT. Magnified about 25 diameters. constituting the greater part of the thickness of the epidermis. The deepest cells of the Tete mucosum, ■which are" set on the surfade of the Cutis vera, are columnar in shape. In the coloured races of mankind 228 THE ESSENTIALS OF HISTOLOGY. these cells contain pigment-granules. In the layers immediately above them the cells are polyhedral. Between all these cells of the rete mucosum there are fine intercellular clefts which separate the cells from one another, but are bridged across by fibres which pass from cell to cell, and also through the. substance of the cells (Ranvier, Del6pine). The intercellular, channels serve for the passage of lymph, stratum corneum stratum lucidum stratum granuloBum >rete mucosum •cutis vera ^ Fig. 277. — Vertical section thkough the skin of the palmar side of the pingek, showing two papill/e (one of which contains a tactile COBPDSOLE) and the deeper L.1TER OF THE EPIDERMIS. Magnified about 200 diameters. and within them occasionally lymph-oorpuscles may be found, often having a stellate figure from becoming shaped to the interstices. The superficial layer of the rete mucosum is formed of somewhat fiattened cells filled with granules or droplets of a material (ekidin) which stains deeply with carmine and hsematoxylin (stratum grari/aiosim, fig. 277; fig. 278). This is not sharply marked o£f from the cells of the rete mucosum which lie next to it, for many of these, show similar granules, although they less completely fill the cell. THE SKIN. 229 Superficial to the stratum granulosum is a layer in which the cell- outlines are indistinct and the cells contain flakes or larger droplets of a hyaline material (kerato-hyalin), which stain less intensely than the granules in the last layer, and which tend to run together. This layer has a clear appearance in section, and is known as the stratum lucidum. Immediately superficial to the stratum lucidum is the horni/ part (stratum commm) of the epidermis. It is composed of a number of layers of epithelium cells, the nuclei of which are no longer visible. These cells near the surface take the form of thin horny scales ^*^ a^ h Fig. 279. — Sectiok of epidbemis. (Rauvier.) a, superficial horny scales; s-w, swoUeu homy cells; s.l., stratum lucidum ; p, prickle- cells, several rows deep ; c, elongated cells forming a single stratum near the corium ; s.gr^ stratum grauulbsum o£ Langerhans, just below the stratum lucidum. ~ Fart of a plexus of nerve-fihres is seeii in the superficial layer of the cutis vera. From this plexus fine varicose nerve-fibrils may be traced passing up between the epithelium-, '; cells of the Malpighian layer. the deeper epidermis cells. Such terminations are seen in the skin over the pig's snout (fig. 219) and in the root-sheaths of hairs. They also occur in the skin in the neighbourhood of the entrance of the sweat-ducts into the epidermis (Eanvier) (fig. 280). The cutis vera or corium is composed of dense connective tissue, which becomes more open and reticular in its texture in its deeper part, where it merges into the subcutaneous tissue. It is thickest over the posterior aspect of the trunk, whereas the epidermis is thickest on the palms of the hsnds and soles of the feet. The superficial or vascular layer of the' corium bears microscopic ^i»pi^te, which project up THE SKIN. 231 into the epidermis, which is moulded over them. These papillae for the most part contain looped capillary vessels, but some, especially those of the palmar surface of the hand and fingers, and the corresponding part of the foot, contain tactile corpuscles, to which medullated nerve-fibres pass (fig. 277). In some parts of the body (scrotum, penis, nipple, and its areola), involuntary muscular tissue occurs in the deeper portions of the cutis vera, and, in addition, wherever hairs occur, small bundles of this tissue are attached to the hair-follicles. A.KMMhNSKi Fig. 280. — Section of the skin of the pulp op the fingbk of a child, stained with gold chloride, showing nerves terminating in an ivt-like arbobesoence at the sukpacb 01? the cutis vera and in the DEEPEST PART OF THE EPIDERMIS. (Ranvier.) p, p, outlines of papillae ; it, n', nerve-fibres in cutis vera ; in, terminal menisci ; », duct of a sweat-gland. The blood-vessels of the skin are distributed almost entirely to the surface, where they form a close capillary network, sending up loops into the papillae (fig. 281). Special branches are also distributed to the various appendages of the skin, viz. the sweat-glands and hair-follicles, with their sebaceous glands and little muscles, as well as to the masses of adipose tissue which may be found in the deeper parts of the cutis. The lymphatics originate near the surface in a network of vessels, which is placed a little deeper than the blood-capillary network. They receive branches from the papillae, and pass into larger vessels, which are valved, and which run in the deeper or reticular part of the corium. From these the lymph is carried away by still larger vessels, which course in the subcutaneous tissue. The appendages of the skin are the nails, the hairs, with their 232 THE ESSENTIALS OF HISTOLOGY. sebaceous glands, and the sweat-glands. They are all developed as thick- enings and downgrowths of the Malpighian layer of the epidermis. The Nails. The nails are thickenings of the deeper part of the stratum corneuro developed over a specially modified portion of the skin (fig. 282), which r.m. Fig. 281. — Duct of a sweat-gland passing theough the epidermis. Magnified 200 diameters. (HeitzmaDn. ) p, papillse with blood-Tossels injected ; r.m., rate mucosum between the papillae ; c, e, stratum corneum ; s.g,^ stratum granulosum ; d, d, sweat-duct passing through epidermis. is known as the bed of the nail, the depression at the posterior part of the nail-bed from which the root of the nail grows being known as the nail-groove. The part of the bed which occupies the inner or central portion of the groove is termed the nail-matrix, since it is from this part that the growth of the nail proceeds. The distal part of the nail forms the/«e boi-der, and is the thickest part of the body of the nail. The substance of the nail (fig. 283, N) is composed of clear horny cells, each containing the remains of a nucleus ; it rests immediately upon a Malpighian layer (B) similar to that which is found in the epidermis generally, but destitute of a defined stratum granulosum. Never- theless, in the more superficial cells both of the bed and matrix there are a large number of granules to be seen, which appear to represent those of the stratum granulosum of the epidermis. These granules are, however, not composed of eleidin, but of a material {onychogenic substance, Ranvier) which stains brown instead of red with carmine; a similar material occurs in the cells which form the fibrous substance and cuticula of the hairs. The corium of the nail-bed THp NAILS. e id 233 ':iM '■X' ?TG. 382.— Longitudinal section thbough the hoot of the nail and its MATKix. Magnified about 10 diameters. a, root of nail ; &, Malpighian layer of matrix ; c, ridges in dermis of nail-bed ; d, epitrlchial layer of epidermis ; «, oponychium ; f, bone (terminal phalanx) of finger. Fig. 283.— Section aceoss the nail and nail-bed. Magnified 100 diameters. (Heitzmann.) P, ridges with blood-vessels ; S, rote mucosum ; N, nail. 234 THE ESSENTIALS OF HISTOLOGY. is beset with longitudinal ridges instead of the papillae which are present over the rest of the skin ; these, like the rest of the superficial part of the corium, are extremely vascular. The nail-bed also receives many nerve-fibres, some of which end in Pacinian corpuscles whilst others ramify in the ridges of the corium, and others again penetrate amongst the deeper epithelium cells. The nails are developed in the foetus at about the third month, the groove being formed at this time in the corium, and the nail rudiment appearing in it as a thickening of the stratum lucidum, which lies over the bed. It becomes free in the sixth month, its free end being at first thin, but as it grows forward over the bed it receives additions on its under surface — at least in the posterior part of the bed — so that after a time the distal end becomes thicker. The epitrichial layer of the cuticle which originally covered the developing nail becomes detached after the fifth month, and, after birth, only remains as the narrow border of cuticle (eponychium) which overlies the lunula at the root. being Hairs. The hairs are growths of the epidermis, developed in little pits — the hair-follicles — which extend downwards into the deeper part of the corium, or even into the subcutaneous tissue. The ' hair grows from the bottom of the follicle, the part which thus lies within the follicle known as the root (fig. 285). The substance of a hair is mainly composed of a pigmented, horny, fibrous material (fig.284,/), which can be separated by the action of sul- phuric acid into long tapering fibril- lated cells, the nuclei of which are still visible. The fibrous substance of the hair is covered by a layer of delicate imbricated scales, termed the hair-cuticle (c). In many hairs, but not in all, the centre is occupied by an axial substance {medulla, m), formed of angular cells which contain granules of eleidin, and frequently have a dark appearance from the presence of minute air-bubbles. The latter may also occur in interstices in the fibrous substance. When they are present, the hair looks white by reflected light. The root has the same structure as the body of the hair, except at its extremity, which is enlarged (fig. 285) ; this enlargement is com- FiG. 284.— Piece of human haik. Magnified. A, seen from the surface ; B, in optical section, c, cuticle ; /, fibrous sub- stance ; m, medulla, the air having been expelled by Canada balsam. THE HAIRS. 235 posed mainly of soft, growing cells, and fits over a vascular papilla, which projects up into the bottom of the follicle (fig. 287). Structure of hair-follicle (figs. 285 to 288).— The follicle, like the skin itself, of which it is a recess, is composed of two parts : one epithelial, and the other connective-tissue. The epithelial or epidermic part of the follicle closely invests the hair-root, and is often in great part dragged out with it ; hence it is known as the root-sheath. It consists of an outer layer of soft columnar and polyhedral cells, like the ^^^^^^ epidermis sebaceous gland outer root-sheafch inner root-sheath junction of inner and outer root-sheaths ■ cuticle of hair medulla fibrous substance papilla blood-vessels Fig. 285.— Diagkam to explain the formation op a hair. (Maurer.) Malpighian layer of the epidermis, but without stratum granulosum — the outer root-sheath ; and of- an inner, thinner, horny stratum next to the hair— the inner root-sheath. The inner root-sheath itself consists of three layers, the outermost being composed of horny, fibrous, oblong cells the nuclei of which are obscure and difficult to make out (Henle's layer), the next of polyhedral nucleated cells containing eleidin (Huxley's layer), and the third— the cuticle of the root-sheath— a. layer of down- wardly imbricated scales, which fit over the upwardly imbricated scales of the hair itself. In the more superficial part of the hair-follicle the layers of Huxley and Henle are indistinguishable, the cells of both 236 THE ESSENTIALS OF HISTOLOGY. being clear and keratinised ; even lower down where distinguishable they show a tendency to dovetail into one another. At the bottom of the follicle no differentiation into layers can be made out in the root-sheath, which is here formed by a- uniform mass of soft cells surrounding the papilla. Fig. 286. — Sections across hair-polliolbs tkom the scalp of an infant. I. Through papilla. II. Just above papilla. Ill, About middle of follicle. IV. Near outer part of follicle. In I. : — p, papilla ; e, epithelium surrounding papilla, with pigment in cells ; Ay, hyaline layer of dermic coat with thin outer root-l^eath just within it. In II., III., IV. : — o, outer root-sheath; i', layer of Henle and i", layer of Huxley of the inner root-sheath ; c, cuticle of root-sheath ; A, hair. In the greater extent of the follicle the outer root-sheath is several layers deep, but as the bottom of the follicle is approached it becomesi thinner and is finally reduced to a single stratum of cells which become^ flattened out into a very thin layer in the papillary part (fig. 286, I.). The connective tissue or dermic part of the hair-follicle is compose^ internally of a vascular layer, which is separated from the root-sheath THE HAIES. 237 by a basement-membrane termed the hyaline layer of the follicle. This inner vascular layer corresponds to the superficial layer of the cutis vera. Its fibres and cells have a regular circular arrangement around Tig. 287.— Longitudinal seotion of a haik-folliole. Magnified 200 diameters. 0, outer; i, inner root-sheath; h, hair ; x, part shown magnified in fig. 288. the follicle, the cells being flattened against the hyaline layer. Exter- nally the dermic coat of the follicle has a more open texture, correspond- ing to the deeper part of the cutis, and contains the larger branches of the arteries and veins. In the large tactile hairs of animals, the 238 / THE ESSENTIALS OF HISTOLOGY. d hy. i' i" c' c" Fig. 288. — A small poetion of the section shown in fig. 287 enlarged TO EXHIBIT THE STBUCTDRB OF THE SEVERAL LAYERS. h, hair ; c", its cuticle ; c', cuticle of root-sheath ; i", Huxley's layer ; i', Henle's layer ; 0, duter root-sheath ; h^, hyaline layer ; d, dermic coat ; /, fat-cells. — Ast Fig. 289.— Nerves anu nebvb-bndings in the skin and hair-foliioles. (G. Ketzius.) -' hit, horny stratum; rm, rote Malpighii; c, superficial nerve-flbre plexus in the cutis •' ! '' m, cutaneous nerve ; is, inner root-sheath of hair ; as, outer root-sheath : h, hair : drj .. sebaceous glands. . , .., "..i , u.^^ .(^^ THE HAIRS. 239 veins near the bottom of the follicle are dilated into sinuses, so as to produce a kind of erectile structure. The hair-follicle receives nerve-fibres which pass into the papilla, and others which enter the root-sheath. These last are derived from the superficial nerves of the corium and form ring-like arborisations in the upper part of the hair follicle. They are especially well developed in the large tactile hairs (whiskers) of animals (figs. 289, 290, 291). Fig. 290. — Fkom a section of skin pkepaeed by the cheomate of silver method, showing the uppeii part of two haies and the terminal arborisations of nervb-fibees in their eoot-sheaths. (van Gehuchten.) The hair grows from the bottom of the follicle by multiplication of the soft cells which cover the papilla, these cells becoming elongated and pigmented to form the fibres of the fibrous substance, and other- wise modified to produce the medulla and cuticle of the hair and the several layers of the root-sheath. The cells which form the medulla of the hair and the inner root-sheath are filled with granules of eleidin, but those which form the fibrous substance and cuticula of the hair have granules which stain brown with carmine, and appear similar to those -which are met with in the corresponding cells of the nail-matrix , (Eanvier) (see p. 232). On the side to which the hair slopes a small patch of richly innervated thickened epidermis is usually to be found, developed over an enlarged papilla of tlie cutis vera : while on the opposite side of the hair is a fiat area of skin with thickened scale-like epidermis, which may represent a vestige of the reptilian scale (Pinkus). 240 THE ESSENTIALS OF HISTOLOGY. The hair germs when they first appear (as at a, fig. 293) are singularly like certain tactile patche.s which are found in the skin of amphibia and some reptiles, and it is possible that hairs have become developed phylogenetically from these patches. It is well known that the tactile sensibility of many parts of the skin is intimately associated with the hairs, where these occur, although parts devoid of hairs may also have a highly developed sense of touch. Besides the hair-follicles already described, which are provided with a papilla, from the cells on the surface of which the hair and its inner root-sheath grow {growing hairs, papillated hairs, hairs mth hollow bulb). hy iiip Fio. 291. — Nerve ending in outer root-sheath op tactile hair of rabbit. (Ranvier. ) w, nerve-fibre ; m, tactile meniscus ; o, outer root-sheath ; i, inner root-sheath ; A, hair ; hy, hyaline membrane. there are niany hairs which are unprovided with a papilla and the follicle of which ceases at the level of attachment of the arrectot pili muscle (cMi-hairs, non-papillated hairs, hairs mth solid bulb). These are hairs which have lost their papilla and have ceased to grow ; they are more easily eradicated than the growing hairs, and tend to fall out spontaneously after a time. In their follicles the whole of the lower part, including the original papilla and the soft growing cells which cover it, have entirely disappeared, the hair being now attached at its. sides and below to the root-sheath. A hair which has thus ceased to grow eventually becomes lost, but its place is presently supplied by a new hair, which becomes developed in a down-growth from the bottom of the follicle, a new papilla becoming formed at the extremity of the THE HAIRS. 241 down-growth (fig. 292). If not previously detached, the old hair is pushed out from the follicle by the one which replaces it. The detachment of the non-papillated hairs is preceded by an absorp- tion of the root of the hair and of the investing inner root-sheath. Fig. 292.— LONOtTDDINAL SECTION THROUGH THE POLLICLE OF A HAIE WHICH HAS CEASED TO GROW AND THE BOOT OF WHICH IS UNDEBGOING ABSOEP- TION. Magnified 200 diameters. This absorption appears to be effected by the cells of the outer sheath, which multiply at the expense of tlje keratinised' parts of the hair root and thus undermine its attachment to the follicle (fig. 292). The hairs are originally developed in the embryo in the form of small solid down-growths from the Malpighian layer of the epidermis (fig. 293). The hair-germ, as it is called (although it gives rise not only to the hair proper but to the epithelium-cells of the hair-follicle Q 242 THE ESSENTIALS OF HISTOLOGY. also), is at first composed entirely of soft growing cells, the outermost and deepest having a columnar shape; but presently those in the centre become differentiated, so as to produce a minute hair invested by inner root-sheath, its base resting upon a papilla which has become inclosed by the extremity of the hair-germ and which is continuous . with the connective tissue of the corium (figs. 294, 295). As the minute hair grows, it pushes its way through the lajrers of the epidermis, which it finally perforates, the epitrichial layer being thrown off (p. 229).. At the same time the follicle grows more deeply into the cutis vera, carrying the papilla down with it. Fig. 293.— Haie-germs in a section of the scalp of a human fcetos. (Szjmonowicz.) Magnified 230 diameters. u, commencing down-growth of epidermis,; 6, further stage of down-growth ; c, eonnoc- tive-tissue cells beginning to accumulate to produce the dermic coat of the follicle ; d, hair-follicle more advanced in development ; e, section of a blood-vessel. The hair-rudiments begin to appear at the third or fourth month of foetal life ; their growth is completed about the fifth or sixth month, and the fine hairs which they form constitute a complete hairy cover- ing termed the lanugo. This is entirely shed within a few months of birth, the new hairs being formed in down-growths from the old hair- follicles in the manner already mentioned. Hairs grow at the rate of half an inch per month. They are found all over the surface of the body except on the palms of the hands and the soles of the feet, and on the distal phalanges of the fingers and toes. They usually slant, and in the negro the hair-follicles are even considerably curved. On the scalp they are set in groups, as is well seen in a horizontal section. The hairs of animals are often curiously marked by the arrangement, of their medvilla, the markings being characteristic of particular species. Ill some animals, e.ff. the mole, the hairs have a varicose form with alter- nate enlargements and constrictions. In human hair the disappearance of GROWTH OF THE HAIRS. 243 the papilla is preceded by its gradual diminution in size, and during this period the root of the hair is becoming gradually more slender (Ranvier)^ so that when such a hair is pulled out it appears to be of least diameter near the bulb, instead of being largest there, as is the case under ordinary circumstances. Muscles of the hairs. — A small muscle composed of bundles of plain muscular tissue is attached to each hair-follicle {arredw pili) ; it passes from the superficial part of the corium, on the side to which the hair slopes, obliquely downwards, to be attached near the bottom of the follicle to a projection formed by a localised hypertrophy of the outer root-sheath. When the muscle con- tracts, the hair becomes more erect, and the follicle is dragged upwards so as to cause a prominence on the general surface of the skin, whilst the part of the corium from which the little muscle arises is correspond- ingly depressed; the roughened con- dition known as 'goose skin' being in this way produced. There is always a sebaceous gland in the triangle formed between the arrector pili, the mouth of the hair-follicle, and the epidermis, so that the con- traction of the arrector generally causes the secretion of the gland to be extruded. Glands of the Skin. Fig. 294.— Djsveloping hair from human embryo of 44 months. (Ranvier. ) The sebaceous glands (fig. 285) are ^' ^^t^\i tn^r^ZZlLktV^Z- small saccular glands, the ducts from which open into the mouths of the hair-follicles, but they are also found in a few situations which are devoid of hairs (margin of lips, labia minora, glans, and prepuce). The Meibomian glands of the eyelid may also be regarded as modified sebaceous glands. Both the duct and the saccules are lined by epithelium, which becomes charged with fatty matter. This sebaceous matter is ing formed ; k, keratiniaed part of inner root-sheath, uncoloured by carmine ; o, outer root-sheath ; 6, epithelial projec- tion for insertion of arrector pili ; s, sebaceous gland ; t, sebaceous degenera- tion of cells in the part which will become the neck of the follicle. This forma a channel for the passage of the hair-point through the Malpighian layer. 244 THE ESSENTIALS OF HISTOLOGY. discharged into the cavity of the saccule, probably owing to the disintegration of the cells within which it is formed. There may be more than one sebaceous gland attached to each hair-follicle. The sebaceous glands are developed as outgrowths from the outer root-sheath (figs. 294, 295, s). Fig. 29.5. — Lonoitudinal section of a hair with its polliolb from a six-months' H0MAN EMBRYO. (Szymonowicz.) Magnified about 150 diameters. ^, papilla ; h^ young hair ; i, inner root-sheath ; d, dermic coat o£ follicle ; o, outer root- sheath ; s, sebaceous gland rudiment ; 6, projection for insertion of arrector pili. The sweat-glands are abundant over the whole skin, but they are most numerous on the palm of the hand and on the sole of the foot. .They are composed of coiled tubes, which lie in the deeper part of the integument and send their ducts up through the cutis SWEAT-GLANDS. 245 to open on the surface by corkscrew-like channels in the epidermis (figs. 276, 281). The glandular or secreting tube is a convoluted tube composed of a basement-membrane lined by a single layer of cubical or columnar epi- thelium-cells, and with a layer of longitudinally or obliquely disposed fibres between the epithelium and basement-membrane (fig. 296). These fibres are usually regarded as muscular, but the evidence on this point is not conclusive. The secreting tube is considerably larger than the efferent tube or duct, which begins within the gland and usually makes several convolution^ before leaving the gland to traverse the cutis vera. Fig. 296. — Sbotion of a sweat-siand in the skik of man. a, a, secreting tube in section ; b, a coil seen from above ; e, c, efferent tube ; d, intertubular connective tissue with blood-vessels. 1, basement-membrane ; 2, muscular fibres cut across ; 3, secreting epithelium of tubule. The efferent tube has an epithelium consisting of two or three layers of cells, within which is a well-marked cuticular lining; but there is no muscular layer. The passage through the epidermis has no proper wall, but is merely a channel excavated between the epithelium-cells. Very large sweat-glands occur in the axilla. The eeruminous glamds of the ear (fig. 297) are modified sweat-glands. The secretion is of a sebaceous nature, instead of being watery like that of the ordinary sweat-glands. The sweat-glands are developed, like the hairs, from down-growths of the Malpighian layer of the epidermis into the corium. They are distinguishable from the hair-germs by the fact that the cells of the outermost layer are not columnar in shape, but spheroidal or poly- 246 THE ESSENTIALS OF HISTOLOGY. hedral. The sweat-gland germs which are thus formed become eventually coiled up at their extremities and converted into hollow tubes. The muscular fibres of the 'tubes as well as the secreting epithelium- cells are ectodermic structures. The sweat-glands receive nerve-fibres, and each gland has a special cluster of capillary blood-vessels. Root-sheath of "^ follicle. eath of\ e. / Boot of hair. Hair. Sebaceous glands. Hair-follicle. Gerumincua gland. Fig. 297.— Section of skin of auditory meatus, including two hair- follicles WITH THEIR SEBACEOUS GLANDS AND TWO OBRUMINOUS GLANDS. (Griiber. ) Thk Mammary Glands. The mammary glands are compou,n4; racemose glaiids which open by numerous ducts upon the apex of the nipple. The ducts are dilated into small reservoirs just before reaching the nipple. If traced backwards, they are found to commence in groups of saccular alveoli (fig. 298). The walls of the ducts and alveoli are formed of THE MAMMARY GLANDS. 247 r Fig. 298.— Section of mammaet gland of woman during lactation. (Testut, after de Sin^ty.) a, lobule of gland ; 6, acini lined by cubical epithelium ; c, duct ; t, connective-tissue stroma. Fig. 299.— Section of mammaey gland, human, in full activity. (v. Ebner.) xllO. a, a', a", alveoli variously cut, and distended by secretion ; g, g', commencing ducts ; i, connective tissue. 248 THE ESSENTIALS OF HISTOLOGY. a basement-membrane lined by a simple layer of flattened epithelium (fig. 299). Milk globules may be seen within the alveoli and duets, and at the commencement of lactation amoeboid cells containing fat-particles appear in the secretion (colostrum corpuscles). These are Fig. 300.— An alveolus with pat-deops in cells, (v. Bbner.) x360. , sub-ondocardial tissue ; c, fibrous tissue of the valve, thickened at d near the free edge ; d^ section of the lunula ; e, section of the fibi'ous ring ; /, muscular fibres of the ventricle attached to it ; ^, loose areolar tissue at the base of the ventricle; «.F. sinus of Valsalva; i, ;2, S, inner, middle, and outer coats of the aorta. 252 THE ESSENTIALS OF HISTOLOGY. The myocardium is covered externally by a layer of serous mem- brane — the epkardium (cardiac pericardium fig. 303, A) — composed, like other serous membranes, of connective tissue and elastic fibres, the latter being most numerous in its deeper parts. Underneath the epicardium run the blood-vessels, nerves, and lymphatic vessels of the heart embedded in areolar and adipose tissue, this tissue being con- tinuous with that which lies between the muscular bundles ; the free surface of the membrane is covered by serous epithelium. The endocardium (fig. 303, B) has a structure not very unlike the pericardium. It is lined by a pavement-epithelium (endothelium), like that of a serous membrane, and consists of connective tissue with Fig. 304.— Fbagmekt oi' the network op Pukkinje's pibkes fbom the VENTBIOULAB ENDOCARDIUM OP THE SHEEP. (Ranvier.) c, clear cell body; n, nuclei; /, striated fibrils. elastic fibres in its deeper part, between which there may, in some parts, be found a few plain muscular fibres. Fat is sometimes met with under the endocardium. In some animals, e.g. the sheep and ox, large beaded fibres are found under the endocardium. These are formed of clear cells joined both end to end and laterally, and generally containing in their centre two nuclei, whilst the peripheral part of the cell is formed of cross- striated muscular tissue ; the chains of cells form the fibres of Purhmje (fig. 304). They appear to be cardiac cells which have undergone differentiation into striated muscle substance only at their periphery, the non-differentiated part of the cell having continued to grow until it has attained a considerable size. In man distinct fibres of Purkinje are not seen, but the innermost muscular fibres of the ventricles are larger than those which lie more externally : they also undergo development somewhat later (J. B. MacCallum).- A muscular bundle which sliows less differentiation than the rest of the cardiac muscle has been described by Stanley Kent, His and others, running STEUCTUEE OF THE HEAET. 253 in the septum and affording a bridging connexion between the muscle of the auricles and that of "the ventricles. This bundle is commonly believed to serve to propagate the contractions of the auricles to the ventricles and thus to maintain their regularity of rhythm ; and it is stated that when the bundle in question is severed experimentally or by disease this propagation is no longer possible, and the ventricles in consequence beat with a much slower rhythm than the auricles. The accuracy of this statement is, however, denied by Kronecker, who regards the regularity of the cardiac contractions as a function, not of the muscular substance of the heart, but of the nerve- fibres, which are distributed to every part of the myocardium. The valves of the heart are formed of folds of endocardium strengthened by fibrous tissue (fig. 303, c). This tissue forms a thicken- ing near the free edge of the valve (c'). At the base of the auriculo- ventricular valves the muscular tissue of the auricle may be found passing a short distance into the valve. In the foetus these valves are at first entirely muscular. The nerves of the heart are seen in sections underneath the epi- cardium of both auricles and ventricles ; in the former situation they are connected at intervals with small ganglia (fig. 303, A, g). Their branches pass to the muscular substance, and after dividing into fine fibrils, these end in enlarged extremities, which are applied directly to the muscular fibres (Ranvier). Other nerve-fibres, which are probably afferent, terminate in complex ramifications in the endocardium in ■connection with small masses of nucleated cells, forming a kind of end- plate (Smirnow). The blood-vessels of the heart are very numerous, and the veins thin- walled, retaining the capillary structure (endothelium only) in vessels of as much as 0-25 mm. in diameter. They are accompanied by numerous lymphatic vessels, which also form plexuses under the cardiac pericardium and endocardium. The lymphatics appear to be in free ■communication with the spaces of the interstitial connective tissue between the muscle-fibres. 254 THE ESSENTIALS OF HISTOLOGY LESSON XXVII. THE TRACHEA AND LUNGS. 1. In sections of the trachea and larynx, notice the epithelinm, the basement- membraiie (of some thickness in the human trachea), the lymphoid tissue of the mucous membrane, the elastic tissue external to this,, and, lastly, the iibrous membrane containing the cartilages; In the mucous membrane and submucous areolar tissue look for sections of mucous glands, ducts of which may be seen opening on the surface. At the back of the trachea notice the plain muscular fibres transversely arranged ; there may be larger mucous glands external to these. « , 2. In sections of lung notice the sections of the alveoli collected into groups (air-sacs). Find sections of bronchial tubes, some cut longitudinally and passing at their extremities into thealveolar passages, others cut across. In each tube notice the ciliated epithelium internally. Next to this the mucous membrane containing numerous elastic fibres and often thrown into folds ; then the layer of circular muscular fibres, and, outside this, loose fibrous tissue in which in larger bronchial tubes pieces of cartilage may be seen embedded. Small mucous glands may also be observed in the fibrous tissue sending their ducts through the other layers to open on the inner surface. Notice that the section of a branch of the pulmonary artery always accom- panies a section of a bronchial tube. In the sections of the alveoli observe the capillary vessels passing from one side to the other of the intervening, septa ; and in places where the thin wall of an alveolus is to be seen in the section, the network of blood-capillaries upon it. Notice within the alveoli nucleated corpuscles whidh frequently contain dark particles in their protoplasm. They are amoeboid cells which have migrated from the blood-vessels and lymphatics, and have taken in inhaled particles of carbon. They may pass back into the lung tissue, for similar cells are seen in this. Make a sketch of part of the wall of one or more bronchial tubes and of one or two of the alveoli. 3. In sections of a fresh lung the air-cells of which have been filled with a mixture of gelatine and nitrate of silver solution, the epithelium of the alveoli may be studied. The sections can be made with the freezing microtome, and mounted in glycerine, which should be warmed after the cover-glass is applied in order to melt the gelatine. 4. Mount a section of lung in which the pulmonary vessels have been injected. Study the general arrangement of the vessels with a low power, and the network of capillaries of the alveoli with a high power. Observe that the veins run apart from the arteries. Sketch the capillary network of one or two adjoining alveoli. The Trachea. The trachea or windpipe is a fibrous and muscular tube, the wall of which is rendered somewhat rigid by C-shaped hoops of cartilage THE TRACHEA AND LUNGS. 255 which are embedded in the fibrous tissue. The muscular tissue, which is of the plain variety, forms a flat band, the fibres of which run trans- versely at the back of the tube. The trachea is lined by a mucous membrane (fig. 305, a to d), which has ciliated epithelium upon its inner surface. The epithelium-cells, which have been already described (Lesson VIIL), rest upon a thick basement-membrane. The corium of Fig. 305. — Longitudinal section of the human trachea, including portions OP two cartilaginous kings. (Klein.) Moderately magnified. a, ciliated epithelium ; 6, basement-membrane ; e, supei-ficial part of the mucous mem- brane, containing the sections of numerous capillary blood-vessels and much lymphoid tissue; d, deeper part of the mucous membrane, consisting mainly of elastic fibres ; e, submucous areolar tissue, containing the larger blood-vessels, small mucous glands (their ducts and alveoli are seen in section), fat, etc. ; /, fibrous tissue investing and uniting the cartilages ; g, a small mass of adipose tissue in the fibrous layer ; h, cartilage. the mucous membrane consists of areolar and lymphoid tissue, and contains numerous blood-vessels and lymphatics. In its deepest part is a well-marked layer of longitudinal elastic fibres (d). Many small glands — mucous and mixed mucous and serous — are found in the wall of the trachea. They may lie either within the mucous membrane or in the submucous areolar tissue (e) or, lastly, at the back of the trachea, outside the transverse muscular fibres. The two divisions of the trachea, the bronchi, are precisely similar in structure to the main tube. 256 THE ESSENTIAXS OF HISTOLOGY. Tlie larjmx is also very like the trachea so far as the structure of the mucous membrane is concerned. It is lined by ciliated epithelium, but over the true vocal cords and upon the epiglottis, as well as here and there in the part above the glottis, stratified epithelium is found ; and taste-buds may occur in this epithelium, except over the vocal cords. The nerve-endings in the epithelium are shown in fig. 218, p. 176. The lymphoid tissue is especially abundant in the mucous membrane of the ventricle of Morgagni (fig. 306, d), and a large number of mucous glands open into this cavity and into that of the sacculus. — ffi Fig. 306. — Longitddinal section through the vektricle op the larynx OF A CHILD. (Klein.). a, true vocal cord ; &, false vocal cord ; c,' nodule of cartilage ; d, ventricle of Morgagni ; If lymphoid tissue ; m, thyro-arytenoid muscle. The true vocal cords are composed of fine elastic fibres. The cartilages of the trachea and the thyroid, cricoid and arytenoid cartilages of the larynx are hyaline ; all these are liable to ossify as age advances. The epiglottis and the cartilages of Santorini and of Wrisberg are composed of elastic fibro-cartilage. This is also the case with the uppermost part of the arytenoid and the tip of the vocal process. The Lungs. The lungs are formed by the ramifications of the bronchial tubes and their terminal expansions, which form groups or lobules of sacculated THE LUNGS. 257 dilatations (air-sacs, infundibula), beset everywhere with small irregu- larly hemispherical or cubical bulgings, known as the air-cells or fidmorwLry alveoli. The broncMal tubes (figs. 307, 308, 309) are lined (except the Fig. 307. — Portion of a transverse section of a bronchial tube, human, 6 MM. in diameter. (F. E. Schultze.) Magnified 30 diameters. tt, cartilage and fibrous layer with mucous glands, and, in the outer part, a little fat ; in the middle, the duct of a gland opens on the inner surface of the tube ; &, annular layer of involuntary muscular fibres ; c, elastic layer, the elastic fibres in bundles which are seen cut across ; d, columnar ciliated epithelium. Fig. 308. — Section op part of a bronchial tube. Magnified 200 diameters. ■\- mm s. •**^ ^5s:-v*^^. R flpWW*-^',' -:'* ^■- ii V-''''''"''^ Fig. 317.— Okoss-seotion op root op canine tooth, human. (Sobotta.) x26. D, dentine ; G, its granular layer ; 0, cement ; P, pulp cavity. fig. 315, shown magnified in -fig. 322). After decalcification the dentine can be separated into lamellse along these incremental lines. The animal matter of dentine resembles bone and the connective tissues generally in having its ground-substance pervaded by fibres which yield gelatine on boiling. These fibres, which have been THE TEETH. 267 especially investigated by v. Ebner and by Mummery, are difficult of demonstration in the fully calcified dentine; but in developing dentine and in dentine which is attacked by caries they are more easily shown. Fig. 318. — Section thkough the enamel of a tooth. Magnified 200 diameters. (Eauber.) a projection of dentine, showing some of its tubules, b, penetrating into the ' enamel; c, c, enamel fibres out longitudinally; d, d, prisms out transversely; c, cuticle of the enamel. Fig. 319.— Enamel pbisms. Magnified 350 diameters. (Kolljker.) A, Fragments and single fibres of the enamel, isolated by the action o£ hydrochloric aoid. B, Surface of a small fragment of enamel, showing the hexagonal ends of the fibres. 268 THE ESSENTIALS OF HISTOLOGY. The pulp (fig. 323) consists of a soft, somewhat jelly-like, connective tissue, containing many branched cells, a network of blood-vessels, and many nerve-fibres which pass into the pulp-cavity along with the blood- vessels by a minute canal at the apex of the fang. The superficial Fig. 320. Fig. 322. Fig. 320.— Section of fang, paballel to the dentinal tubulbb. Magnified 300 diameters. (Waldeyer.) 1, cement, with large bone lacunae and indications of lamellae; 2, granular layer of Purkinje (interglobular spaces) ; S, dentinal tubules. Fig. 321. — Sections oe dentinal tubules. Magnified about 300 diameters. (Fraenolcel.) a, cut across ; h, cut obliquely. Fig. 322. — A small poetion of dentine with inteeglobulak spaces. Magnified 350 diameters. (KoUiker.) c, portion of incremental line formed by the interglobular spaces, wbich are here filled up by a transparent material. cells of the pulp form an almost continuous layer, like an epithelium (fig. 323, Od, Od'). They are known as odontoblasts, from having been concerned in the formation of the dentine. The nerve-fibres are said to pass eventually between the odontoblasts and to end in arborisa- THE TEETH. 269 tions close to the dentine, but they have not been followed into the dentinal tubules. The crusta petrosa (fig. 317, 320) is a layer of lamellated bone, including lacunae and canaliculi, but without Haversian canals, at least normally, in the human teeth. It is covered with periosteum {dental periosteum), which also lines the socket, and serves to fix the tooth securely. Fig. 323.— Section aoeoss the boot of a young tooth showing the pulp IN SITU. (Rose.) P, pulp ; y, y, Teins ; A, A, A, arterioles ; H, nerve bundles ; Od, columnar odontoblasts still depositing dentine ; Od', flattened odontoblasts which have ceased to form dentine. Formation of the teeth. — The teeth are developed somewhat similarly to the hairs. A continuous thickening of the epithelium occurs along the line of the gums, and grows into the corium of the mucous membrane {common dental germ or dental lamina, fig. 324, a). At regular intervals there is yet a further thickening and growth from the common germ into the tissue of the mucous membrane, each of these special rudi- ments, which are ten in number, swelling out below into a flask-shaped mass of cells, the special dental germ (fig. 324, b) of a milk tooth. The intermediate parts of the dental lamina long remain, forming a common epithelial strand uniting the several special dental germs to one another and to the epithelium covering the gum (fig. 324, c, D,/). A vascular 'papilla is continued from the corium into the bottom of each special 270 THE ESSENTIALS OF HISTOLOGY. germ (fig. 324, C, D, p) ; this papilla has the shape of the crown of the future tooth. Each special dental germ, with its included papilla, B t^^ff^W Fig. 324. A. Section across the upper jaw or a f(etal sheep, 3 centimeters long. (Waldeyer.) 1, common dental lamina dipping down into the mucous membrane where it is half sur- rounded by a horseshoe-ahaped more dense-looking tissue, the germ of the dentine and dental sac ; Z, palatine process of the maxilla. B. Section from fcetal calf similar to that shown in A, but passing through ONE OF THE SPECIAL DENTAL GERMS HERE BECOMING FLASK-SHAPED. (Bose,) a, epithelium of mouth, thickened at 6, above special dental germ ; c, papilla ; d, special dental germ ; e, enamel epithelium ; /, dental sac. C AND D. Sections at later stages than A and B, the papilla having BECOME FORMED AND HAVING BECOME PARTLY SURROUNDED BY THE EPITHELIAL GERM. (KoUiker.) c, epithelium of gum, sketched in outline ; /, neck of dental germ ; /, enamel organ ; e, its deeper columnar cells ; e', projections into the corium ; p, papilla ; s, dental sac forming. In D, the dental germ (Jp) of the corresponding permanent tooth is seen. FORMATION OF THE TEETH. 271 presently becomes almost entirely cut ofi' from the epithelium of the mouth, and surrounded by a vascular membrane— the de-ntal sac. The papilla becomes transformed into the dentine and pulp of the future tooth, and the enamel is deposited upon its surface by the epithelial cells of the dental germ. The root of the tooth, with its covering of cement, is formed at a later period, when the tooth is beginning to grow up through the gum, by a gradual elongation of the base of the papilla. The shaping of this into the form of the root is Fig. 325.— Section of a developing inoisok tooth op a human embeto. (Rose.) The section also includes the qekm of the adjacent tooth. J)Kt dental papilla ; od, odontoblasts ; &, bone of jaw ; e, e', outer and inner layers of enamel-organ; S.P., enamel pulp; d.f. dental furrow ; c, remains of common dental germ or lamina ; n, neck or bridge of cells connecting this with the enamel-organ ; m.«., mouth-epithelium; e", enamel organ of adjacent tooth-germ; r, reserve germ of permanent tooth. determined by a growth of the epithelium of the edge of the enamel germ, which extends in the form of a fold (the ejpithelial sheath of V. Brunn) towards the future apex of each fang. Previously to the deposition of the enamel, the dental germ under- goes a peculiar transformation of its previously polyhedral epithelium- cells, into three layers of modified cells. One of these is a layer of columnar cells {ameloblasts, fig. 326, a), immediately covering the surface of the dentine. The enamel-prisms are produced by a fibrous forma- tion (fig. 327, /) followed by a deposition of calcareous salts ; these changes taking place altogether external to the cells (or, as some hold, by a direct calcification of their protoplasm). The cells next to the dental sac form a single layer of cubical epithelium (i3g. 325, e), and 272 THE ESSENTIALS OF HISTOLOGY. nearly all the other cells of the dental germ become transformed into branching corpuscles (fig. 325, SP ; fig. 326, p) communicating by their processes, and thus forming a continuous network. The dental germ, after it is thus modified, is known as the enamel organ. The dentine of the tooth is formed by calcification of the surface of the papilla. At this surface there is a well-marked layer of odonto- blasts (fig. 328, od, fig. 329, c), and these produce a layer of dentinal matrix which forms a sort of cap to the papilla, and which soon str. int. Fig. 327. Fie. 326. Fig. 326.— Section showing the strdotuee of the part op the enamel OKGAN which LIES NEXT TO THE DENTINE. (Rose.) d, dentine ; e, newly formed enamel stained black by osmic acid ; T, Tomes' processes from the ameloblasts, a; str. int., stratum intermedium of enamel-organ; p, branched cells of enamel pulp. • FiG. 327. — Developing enamel showing ameloblasts and the fibbods sub- stance PBODUOED BY THESE CELLS, WHICH FORMS THE BASIS OF THE ENAMEL PRISMS. (From a photograph by Leon Williams. ) a, portions of the ameloblasts ; /, fibrous basis of enamel prisms ; e, calcified part of enamel. becomes calcified by the deposition of globules of calcareous matter. Processes of the odontoblasts remain in the dentine as it is forming, and thus the dentinal tubules are produced. Subsequently other layers of dentine are formed within the first by a repetition of -the same process, and in this way the papilla gradually becomes calcified. A part, however, remains unaltered in the centre of the tooth, and with its covering of odontoblasts forms the pulp. The ten milk-teeth are formed in each jaw in the manner described. These, however, become lost within a few years after birth, and are replaced by permanent teeth in much the same way that a new succes- sion of hair occurs. A small outgrowth takes place at an early period FOEMATION OF THE TEETH. 273 from the dental germ close to each of the milk-teeth (fig. 324, D, fp), and this eventually becomes the germ of the corresponding permanent tooth. It gradually enlarges, acquires a papilla, forms an enamel Pig. 328. — Section op pabt of a developing tooth. (From a photograph by Leon Williams.) ■^ ^".ifc-f ^ '"•r sis* Ji muscular JOyres m muiCvXar Jlbres Fig. 365.— Section op the small intestine (jejunum) of oat. (Magnided about.40 diameters.) the general surface becomes regenerated from them (Bizzozero). The mucous membrane between these glands is mainly composed of reticular tissue, which contains here and there nodules of lymphoid tissue. These nodules constitute when they occur singly the so-called solntwry glands of the intestine (fig. 368), and when aggregated together 300 THE ESSENTIALS OE HISTOLOGY. form the agminaied glmds oi patches of Feyer (fig. 374). The latter occur chiefly in the ileum. Fig. 367.— Cboss-skction of a small fkagment of thb mucx)us membrane of the intestine, inoludino one entibe OBTPT OF LIEBEKkBhn AND PARTS OF THREE OTHERS. (Magnified 400 dia- meters. ) (Frey . ) u,, cavity of the tubular glanda or crypts ; b, one of the lining epithelium-cells ; c, the inter- glandular retiform tibue ; d, lymph-cells. Fie. 366.— A crypt of Lie- BBRKUHN FROM THE HUMAN intestIIje. (Flemming. ) I Fig. 368.— Section of the ileum theough a lymphoid nodule. (Cadiat.) ■ a, middle of the nodule'with the, lymphoid tissue partly fallen away from the soction ; 6, epithelium of the intestine ; c, villi : their epithelium is partly broken away ; d, " ■ crypts of LieberkUhn; e, /, muscularie m-ucosse. THE SMALL INTESTINE. 301 The glamds of Brurmet; which have been already noticed (p. 294), occur in the duodenum. They are small tubulo-raceniose glands of W^'^^f^' ■ i'3i W''- VvV circular tj^ muscular layer serous coat Fig. 369.— Section of ddooenum of oat, showing Bbunnbr's glands. (Magnified about 60 diameters.) serous character, situated in the submucosa (fig. 369) ; they send their ducts to the inner surface of the mucous membrane either between the crypts of Lieberkiihn or into them. 302 THE ESSENTIALS OF HISTOLOGY. The villi with which the whole of the inner surface of the small intestine is closely beset are clavate or finger-shaped projections of the mucous membrane, and are composed, like that, of retiform tissue covered with columnar epithelium (figs. 370 to 372). The characters of this epithelium have already been described (Lesson VIIL). Between and at the base of the epithelium-cells many lymph- corpuscles occur, as well as in the meshes of the retiform tissue. Fig. 370.— Lokgitudikal section Of a villus from a rat killed thkbe houbs AFTEB PEBDING WITH BREAD AND WATBB. The columnar epithelium shows numerous lymph-corpuscles between the cells ; i, lacteal, containing lymph-corpuscles, c, some partly disintegrated. ' The epithelium rests upon a basement-membrane. In the middle of the villus is a lymphatic or lacteal vessel which may be somewhat enlarged near its commencement, but the enlargement is replaced in some animals by a network of lacteals. Surrounding this vessel are small bundles of plain muscular tissue prolonged from the muscularis mucogse. The network of blood-capillaries (figs. 370, 373) lies for the most part near the surface within the basement-membrane; it is supplied with blood by a small artery which joins the capillary network at the base of the villus ; the corresponding vein, generalljr arises near the extremity. THE SMALL INTESTINE. 303 Fig. 371.— Tkansvekse section of a villds, man. (v. Ebner.) Magnified 530 diameters, a, basement-membrane, here somewhat shrunken away from epithelium ; 6, goblet- cells ; c, cutioula ; d, lacteal ; e, columnar epithelium ; /, leucocytes in epithehum ; f, leucocytes below epithelium ; /", large leucocytes ; g, blood-vessels. Fig. 372.— Paet of a seotion thbough a villus of the dog, highly magnified. (R. Heidenhain.) m, plain muscle ; I, V, I", leucocytes, which are seen in large numbers in the interstices of the reticular tissue ; U, vessels ; e, connective-tissue cells, covering the nnriis oi the reticulum. The epithelium of the villus is not represented. 304 THE ESSENTIALS OF HISTOLOGY. The lyimphatics (lacteals) of the mucous membrane (fig. 374), after receiving the central lacteals of the villi, pour their contents into a plexus of large valved lymphatics which lie in the submucous tissue and form sinuses around the bases of the lymphoid nodules (fig. 256, Fig. 373.— Small intestine (vbbtioal transverse section), with the BLOOD-VESSELS INJECTED. (Heitzmann. ) r, a villuB ; G, glands of Lieberktihn ; M, nausoularis mucosas ; A^ areolar coat ; R, circular muscular coat ; Z, longitudinal muscular coat ; P, peritoneal coat. p. 208). From the submucous tissue efferent vessels pass through the muscular coat, receiving the lymph from an intramuscular plexus of lymphatics, and are conveyed away between the layers of the mesentery. Absorption of fat. — In order to study the process of fat transference in the intestine, it is convenient to stain the fat with osmic acid, which ABSORPTION OP FAT. 305 Fig. 374. — Vertical section of a portion of a Peybr's patch, with the LACTEAL VKSSELS INJECTED. Magnified 32 diameters. (Frey.) The specimen is from the lower part of the ileum ; a, villi, with their lacteals left white ; &, some of the tubular glands; c, the muscular layer of the mucous membrane; d, cupola or projecting part of the nodule ; e, central part ; /, the reticulated lacteal vessels occupying the lymphoid tissue between the nodules, joined above by the lacteals from the villi and mucous surface, and passing below into g, the sinus-like lacteals under the nodules, which again pass into the large efferent lacteals, g' ; i, part of the muscular coat. Fio. 375.— Section of the villus of a eat killed during fat-absobption. ep, epithelium ; «(r, striated border ; c, leucocytes ; c", leucocytes in .the epithelium ; I, central lacteal containing chyle and disintegrating leucocytes. U 306 THE ESSENTIALS OF HISTOLOGY. colours it black. It can then be observed that in animals which have been fed with food containing fat, particles of fat are present (1) in comparatively large globules within the columnar epithelium-cells ; (2) in fine granules in the interstitial tissue of the villus, but often confined to the amoeboid leucocytes, which abound in this tissue ; (3) in fine granules within the central lacteal of the villus. The leucocytes are present not only in the reticular tissue of the villus, but also in con- siderable numbers between and at the base of the epithelium-cells ; and they can also be seen in thin sections from bichromate-osmic prepara- tions within the commencing lacteal ; in the last situation they are undergoing disintegration (figs. 375, 376). These observations are easily made in the frog. Fig. 376.— Muoocs membrane of frog's intestine dubing fat- absorption. ep, epithelium ; sir, striated border ; c, leucocytes ; I, lacteal. Since the leucocytes are amceboid, it is probable from these facts that the mechanism of fat-absorption consists of the following processes — viz. (1) absorption or formation of fat in the columnar epithelium- cells of the surface ; (2) ejection of fat-granules from the epithelium into the tissue of the villus ; (3) inception of fat by leucocytes, these taking it up after it has passed out of the epithelium cells ; (4) migration of leucocytes carrying fat particles through the tissue of the villus and into the central lacteal; (5) disintegration and solution of the immigrated leucocytes, and setting free their contents. Since fat-particles are never seen in the striated border of the columnar cell it is probable that the fat first becomes saponified by the action of the digestive juices, and reaches the epithelium-cell in the form of dissolved soap ; the fat which is seen and stained by osmic acid within the cells having become re-formed by a process of synthesis. In some young animals (puppy, kitten) the fat which is undergoing absorption is seen not only in the epithelium-cells and leucocytes, but also in the form of streaks of liquid, stained black by osmic acid, in the interstices of the reticular tissue of the villi. It has probably passed THE, LARGE INTESTINE. 307 Fie. 377.— Glands of the labgb :ntk3tine of child. (300 diameters.) A, in longitudinal section ; B, in transverse section. 308 THE ESSENTIALS OF HISTOLOGY. out from the epithelium in a dissolved condition by a kind of reversed secretion. The migration of leucocytes into the lacteals of the villi is not a special feature of absorption of fat, but occurs also when absorp- tion of other matters is proceeding (fig. 370); the transference of fat-particles is merely a part of a more general phenomenon of migration of leucopytes which accompanies the process of absorption. The large intestine has the usual four coats, except near its termina- tion, where the serous coat is absent. In man the muscular coat is peculiar in the fact that along the csecum and colon the longitudinal muscular fibres are gathered up into three thickened bands which produce puckerings in the wall of the gut. The mucous membrane of the large intestine is beset with simple tubular glands somewhat resembling the crypts of Lieberkuhn of the small intestine, and lined by columnar epithelium similar to that of the inner surface of the gut, but containing many more mucus-secreting or goblet-cells (fig. 377). The blind extremity of each gland is usually slightly dilated. These glands of the large intestine are not strictly homologous with the crypts of the small intestine, for whereas the latter are developed as depressions in the general surface between the villi, the glands of the large intestine are formed by the growing together of villus-like projections of the surface. The interglandular tissue is a reticular tissue and is beset here and there with solitary glands, especially in the csecum. The mucous membrane of the vermiform appendix is in great part of its extent packed full of lymphoid nodules. The arrangement of the blood-vessels and lymphatics in the large intestine resembles that in the stomach. The nerves of the large intestine also resemble those of the stomach and small intestine in their arrangement. At the lower end of the rectum the circular muscular fibres of the gut become thickened a little above the anus to form the internal sphincter muscle. In the anal region there are a number of compound racemose mucous glands opening on the surface of the mucous mem- brane {anal glands). The anus has a lining of stratified epithelium continuous with that of the skin. THE LIVER. 309 ■ LESSONS XXXIV. AND XXXV. THE LIVER AND PANCREAS. 1. Sections of liver are to be studied cfirefully. They may be stained with eosin and hsematoxylin ; or by eosin and methylene-blue after Muir's method (see Appendix). Sketch the general arrangement of the cells in a lobule under the low power ; and under the high power make very' careful drawings of some of the hepatic cells and also of a portal canal. If from the pig, the outlines of the lobules are observed to be very well marked. Notice that the hepatic cells are in intimate contact with the blood-capil- laries or sinusoids. Some cells may be found to contain red blood-corpuscles ; many are filled with eosinophil granules. Notice in the sinusoid capillaries the large partly detached endothelial cells (cells of Kupffer). These also frequently contain erythrocytes, which appear to be in process of destruction. 2. To observe the glyc6gen within the liver-cells, kill a rabbit or rat (pre- ferably about six hours after a full meal of carrot), and at once throw a thin piece of the liver into 96 per cent, alcohol. When well hardened the piece may be embedded in paraffin in the usual way, or sections may be cut with the free hand without embedding. Some of the sections so obtained are to be treated with a 1 per cent, solution of iodine in potassium iodide for five minutes ; they may then be mounted in a nearly saturated solution of potas- sium acetate, the cover-glass being cemented with gold size. 3. Presence of iron. Other sections of alcohol-hardened liver are to be treated first with potassium ferrocyanide solution and then with hydrochloric acid and alcohol (1 to 10), passed through absolute alcohol into xylol, and mounted in dammar ; in these many of" the pigment granules wiU be stained blue (Prussian blue). Or the sections may simply be placed in an aqueous solution of hsematoxylin (1 to 300), with or without previous treat- ment with alcohol containing 10 parts per cent, hydrochloric acid (to set free organically combined iron), after which they are mounted in the ordinary way (Macallum's method). 4. Study, first with the low power and afterwards with the high power, a section of the liver in which the blood-vessels have been' injected. Almost invariably the injection will be found to have penetrated into canaliculi within the liver-cells themselves. Make a general sketch of a lobule under the low power and draw a small part of the network of blood-vessels and intracellular canaliculi under the high power. 5. Take a small piece of liver which has been several weeks in 2 per cent, bichromate of potassium solution or Mlillftr's fluid and plunge it in 1 per cent, nitrate of silver solution, changing the fluid after half an hour. Leave the piece of liver in the silver solution overnight. It may then be trans- ferred to alcohol, and after complete dehydration embedded and cut in paraffin in the usual way and the sections mounted in dammar. In many parts of such sections the bile-canaliculi' are stained. They can also be brought to view (at the periphery of the lobules) by injection with solution of Berlin blue from the hepatic duct ; or, throughout the whole of the lobule, by injecting about 60 c.c. of saturated sulphindi- gotate of soda solution in three successive portions, at intervaVs of half an 310 THE ESSENTIALS OF HISTOLOGY. hour, into the blood-vessels of an anaesthetized cat or rabbit. Two hours after the last injection the animal is killed, and the blood-vessels washed out with saturated solution of potassium chloride. The organ is then fixed with absolute alcohol. But the chromate of silver method is easier and surer than the injection methods. 6. Tease a piece of fresh liver in serum or salt solution for the study of the appearance of the hepatic cells in the recent or living condition. 7. Stained sections of pancreas from a gland which has been hardened in alcohol, or in formol followed by alcohol. The sections may be double stained with eosin .and hsematoxylin or with eosin and methylene blue. Notice the islets of Langerhans betjyeen the alveoli ; largest and most evident in animals which have been long -fasting and also very well marked after the gland has been stimulated by secretin. Make sketches under both low and high power. R. Tease a small piece of fresh pancreas in serum or salt solution. Notice the granules in the alveolar cells, chiefly accumulated in the half of the cell which is nearest the lumen of the alveolus, leaving the outer zone of the cell clear. Sketch a small port;ion of an alveolus under a high power. 9. The ducts of the pancreas, and the termination of nerve-fibres in the alveoli may be seen in jpreparations made by the Golgi method. THE LIVER. The liver is a solid glandular organ, made up of the hepatic These are polyhedral masses (fig. 378) about 1 mm. {-^-g inch) in Fig. 378. — Diaqeammatio kepresentation oi? two hepatic lobulhs. The left-hand lobule is represented with the intralobular vein out across ; in the right- • ' hand one the section takes the course, of the intralobular vein, p, interlobular branches of the portal vein ; h, intralobular branches of the hepatic veins ; s, su&l iSt':;! lobular vein ; c, capillaries of the lobules. The arrows indicate the direction of iHl%j^,J^'i^J course of the blood. The liver-cells are only represented in one part of each lobule.^ ^ a^/J diameter, composed, of cells, and separated from one another by connective tissue. Iti some animals, as in the pig, this separation is complete, and each lobule is isolated, but in man apd most a,nimals it is incomplete. There is also a layer of eonnectivei tissue underneath THE LIVEE. 311 the serous covering of the liver, forming the so-called capsule of the organ. Each lobule is penetrated by a fine network of reticular tissue which helps to support the columns of cells within the lobule . The afferent blood-vessels of the liver (portal vein and hepatic artery) «nter it on its under surface, where also the bile-duct passes away from the gland. The branches of these three vessels accompany one another in their course through the organ, and are inclosed by loose connective Fig. 379- — Reticdlcm of a liver-lobule. (Oppel.) V.C, central vein ; i, interlobular interval. tissue (capsule of Glisson), in which are lymphatic vessels,. the whole being termed a portal canal (fig. 380). The smaller branches of the vessels penetrate to the intervals between the hepatic lobules, and are known as the interlobular branches. The blood leaves the liver at the back of the organ by the. hepatic veins; the branches of these run through the gland unaccompanied by other vessels (except lymphatics) and can also be traced to the lobules, from each of which they receive a minute branch (central or intralobular vein) which passes from the centre of the lobule, and opens directly into the (sublobular) branch of the hepatic vein. ;:■: Each lobule is a mass of hepatic cells pierced everywhere with a 312 THE ESSENTIALS OF HISTOLOGY. network of sinusoid blood-vessels, the so-called hepatic capillaries (fig. 378), which at the periphery of the lobule receive blood from the interlobular branches of the portal vein (p), and converge to the centre of the lobule, where they unite to form the intralobular branch of the hepatic vein. The interlobular branches of the hepatic arteries join this network a short distance from the periphery of the lobule. The blood-capillaries are in direct contact with the liver-cells ; indeed, it would appear as if the endothelium is deficient, for artificial injections are seen to be in contact with the cells and even pass between their interstices and run into canaliculi within their protoplasm. The endothelium of the blood-vessels (or sinusoids) is in part at least Fig. 380. — Section of a poktal canal. a, branch of hepatic artery ; v, branch of portal vein ; d, bile-duct ; I, i, lymphatics in the areolar tissue of Glisaon's capsule which incloses the vessels, represented by certain conspicuous cells which occur at intervals on the wall of the sinusoids, and lie in contact with the liver cells. These cells were described by Kupffer. They are phagocytic, like the endothelial cells of the blood^sinuses of the spleen, and ingest erythrocytes, which can be seen within them. The hepatic cells, which everywhere lie between and surround the capillaries, are polyhedral, somewhat granular-looking cells, each containing a spherical nucleus. The protoplasm of each cell is pervaded by an irregular network of fine canaliculi (fig. 383), which in preparations of injected liver become filled with the injection material, which has passed into them from the blood-vessels (Herring and Simpson). They thus form a system of intracellular canals which probably receive the blood-plasma directly from the vessels. Such canals were conjectured to exist by Browicz, who showed that under THE LIVER. 313 certain circumstances not only hsemoglobin but whole red blood- corpuscles, and even groups of blood-corpuscles, which are in process of breaking down, are to be found in the interior of the hepatic cells. In the dog's liver both hsemoglobin and bilirubin may be found in the form of crystals within the nuclei of the liver-cells (Browicz). It is easy to inject these minute canals from the blood- vessels, and they are clearly shown filled with the injection mass in the preparation of injected liver of rabbit shown in fig. 384. Besides these plasma-channels, the liver cells may show fine, short Fig. 381. — Section of babbit's liver with the intbeceilulab network oi' BILE-CANALIODLI INJECTED. Highly magnified. (Hering.) Two or three layers of cells are represented ; 6, blood-capillaries. canals which communicate with the intercellular bile-ducts (see below) and generally commence within the cell by a dilatation (secretion- vacuoles). After a meal many of the liver cells may contain fat, and masses of glycogen can also be- seen within them (fig. 385) if the liver be hardened in alcohol and treated in the manner described in section 2. The cells also contain pigment-granules, many of which are stained by potassium ferrocyanide and hydrochloric acid, or by pure hsematoxylin (presence of iron^). The ducts commence between the hepatic cells in the form of inter- celMar Mle-eanalvmli, which lie between the adjacent sides of the ' The iron which is in organic combination can be set free by treatment for a short time with alcohol to which 10 p.c. hydrochloric acid has been added. 314 THE ESSENTIALS OF HISTOLOGY. cells, and receive the contents of the secretion-vacuoles above mentioned. They form a network, the meshes of which correspond in size to the cells (fig. 381), and at the periphery of the lobule they pass into the interlobular bile-ducts (fig. 382). In many animals the network of bile-canaliculi is incomplete (G. Eetzius). Fig. 382. — Lobule of babbit's livek: vessels and bile-ducts injected. (Cadiat.) a, central vein ; &, 6, peripheral or interlobular veins ; c, interlobular bile-duct. The liver-cells are not represented. The bile-ducts are lined by clear columnar epithelium (fig. 380, d). Outside this is a basement-membrane, and in the larger ducts some fibrous and plain muscular tissue. Many of the large ducts are beset with small blind diverticula. The gall-bladder is in its general structure similar to the larger bile-ducts. It is lined by columnar epithelium, and its wfilL is formed of fibrous and muscular tissue. THE LIVER. 315 I The lymphatics of thfe liver have been described as commencing as perivascular lymphatic spaces inclosing the capillaries of the lobules (MacGrillavry). But this cannot be so, since -there is no space between Fic. 383. Fig. 384. Fig. 383. — A cell FBOM the human LIVEE, showing INTBAOELLnLAB OANALIOULI. (Bcowioz. ) Fig. 384. — Fkom a section op babbit's livkb injected fbom the foetal vein, showing inteacelltllab oanalicdli communicating with the inteecellulab blood-sinusoids. Fig. 385 Livee cells containing OLirooGEN. (Dunham, from Barfurth.) the liver-cells and the sinusoid capillaries with which they come into Immediate relationship (Herring and Simpson). All that can be posi- tively asserted is that there are numerous lymphatics accompanying the branches of the portal vein, and others, less numerous, accompanying 316 THE ESSENTIALS OF HISTOLOGY. the tributaries of the hepatic veins, but so far as can be ascertained no direct communication exists between the two sets of lymphatics within the lobules, although they communicate freely near their exit from the liver. Most of the lymph passes out by the portal lymphatics. Nerves are described as distributed both to the blood-vessels and to the liver-cells. THE PANCREAS. The pancreas is a tubulo-racemose gland, resembling the salivary glands so far as its general structure is concerned, but differing from Fig. 386. — Section of human pancreas. (Bohm and v. Davidoff.) if". a, group of cells in interstitial tissue (islet of Langerhans) ; b, connective tissue ; c, larger duct ; d, d, alveoli with centro-acinar cells ; e, small duct passing into alveoli ; /, inner granular zone of alveolus. them in the fact that the alveoli are longer and more tubular in character. Moreover, the connective tissue of the gland is somewhat looser, and there occur in the glandular substance here and there small groups of epithelium-like cells unfurnished with ducts {islets of Langerhans) (fig. 386 a ; fig. 387), which are supplied with a close network of large convoluted capillary vessels (fig. 388). Their function is unknown, but their presence is very characteristic of the pancreas. They increase in size during starvation and also as the result of increasing the activity of the gland by injection of secretin (Dale), apparently at the expense of the proper glandular alveplU The degeneration which they sometimes undergo in cases of diabetes THE PANCREAS. 317 mellitus seems to suggest that they are concerned with the influence exerted by the pancreas on the metabolism of carbohydrates. , Fig. 387. — Section of pancreas of akmadillo showing several alveoli AND A LAKGE INTEEALVEOLAK CELL-ISLET. (V. D. Harris.) The cells of the alveoli are shrunken, but they show markedly tho two zones, the outer or non-granular stained deeply by heematoxylin. The cells which line the alveoli are columnar or polyhedral in shape. When examined in the fresh condition, or in sections stained by certain methods, their protoplasm is seen to be filled in the inner two-thirds with granules, but the outer third is left clear or is Fig. 388. — Injection of blood-vessels of an "islet" of the pancreas. (Kiihne and Lea.) striated (fig. 390, A ; fig. 386). After a period of activity the clear part of the cell becomes larger, and the granular part smaller (fig. 390, B). In hsematoxylin-stained sections the outer part is coloured more deeply than the inner (fig. 387). 318 THE ESSENTIALS OF HISTOLOGY. Pancreas cells frequently exhibit a rounded mass of granules or fibrils (mitpchondria) near the nucleus, which is known as the para,^ nucleus (Nebenkern) : this is probably related to the secretory activity of the cells (see p. 5). Fig. 389. -Fkom a section op human panckbas. {v. Ebner.) 530 diameters. Magnified li, a, outer zones of alveolar cells with striated appearance ; b, inner granular zones ; m, membrana propria ; c, centro-acinar cells, here occuiTing in unusually large amount ; d, junctional part of duct ; its epithelium is continuous with the centro- acinar cells. ' In the centre of each acinus there may generally be seen some spindle-shaped cells {centro-acinar cells of Langerhans — fig. 386, d), the nature of which has not been definitely determined ; but they appear to be continued from the cells which lirie the smallest ducts (fig. 386, e) Fig. 390.— Part op an alveoius op the babbit's pakckkab. A, at rest ; B, AFTER ACTIVE SECRETION. (From Foster, after Kiihne and Lea.) a, the inner granular zone, which in A is larger and more closely studded with fine ' granules than in B, ia which the granules are fewer a!nd coarser ; b, the outer trans- parent zone, small in Ay larger in B, arid in the latter marked with faint strife ; c, the lumen, very obvious in B, but indistinct in A; d, an indentation at the junction of two cells, only distinct in B. Sometimes they are much more conspicuous and fill the parts of the alveoli which are nearest to the duct (fig. 389) ; in these cases the mass of cells which they form is liable to be mistaken for a Langerhans' islet. THE PANCREAS. 319, Diverticula from the lumen of the alveolus penetrate between the alveolar cells (fig. 391), as in the salivary glands (p. 285). The pancreas has many nerves, with numerous small nerve-cells distributed upon Fig. 391.— a duct of the panoeeas with lateral diverticula into the alveoli; golgi method. (E. Miiller.) ■ In A the duot is shown cut longitudinally and giving off ductules, m, to the alveoli, where they extend between the cells {L). In B the details of their termination are shown more highly magnified. their course ; the nerve-fibrils end by ramifying amongst the cells of the alveoli, as in the salivary glands. In the cat, which has Pacinian bodies in its mesentery, these terminal organs are also found numer- ously in the substance of the pancreas (V. D. Harris). 320 THE ESSENTIALS OF HISTOLOGY. LESSON XXXVI. STRUCTURE OF THE KIDNEY. 1. Sections passing through the whole kidney of a small mammal, such as a mouse or rat. These sections will show the general arrangement of the organ and the disposition of the tubules and of the Malpighian corpuscles. 2. Thin sections of the kidney of a larger mammal, such as the dog or cat, may next be studied. In some the direction of the section should be parallel with the rays of the medulla, and in others across their direction. The characters of the epithelium of the several parts of the uriniferous tubules and the structure of the glomeruli are to be made out in these sections. .3. Separate portions of the uriniferous tubules may be studied in teased preparations from a, kidney which has been macerated in diluted hydro- chloric acid (1 to 5 water). This renders it possible to unravel the uriniferous tubules for some distance. 4. Thick sections of a kidney in which the blood-vessels have been injected. Exaniine these with a low power of the microscope. Follow the course of the arteries — those to the cortex sending their branches to the glomeruli, those to the medulla rapidly dividing into pencils of fine vessels which run between the straight uriniferous tubules of the boundary zone. Notice also the efferent vessels from the glomeruli breaking up into the capillaries which are distributed to the tubules of the cortical substance. The kidney is a compound tubular gland. To the naked eye it appears formed of two portions — a cortical and a medullary. The latter is subdivided into a number of pyramidal portions (pyramids of Malpighi), the base (boundary zone) of each being surrounded by cortical substance, while the apex projects in the form of a papilla into the dilated commencement of the ureter (pelvis of the kidney).'^ Both cortex and medulla are composed entirely of tubules — the wriniferous tubules — which have a straight direction in the medulla and a contorted arrangement in the cortex; but groups of straight tubules also pass from the medulla through the thickness of the cortex (medullary rays). The uriniferous tubules begin in the cortical part of the organ in dilatations, each inclosing a tuft or glomerulus of convoluted capillary blood-vessels (corpuscles of Malpighi), the dilated commencement of the tubule being known as the capsule (fig. 396, 1). The glomerulus is ^In many animals (e.g. dog, oat, rabbit, most monkeyB) the whole kidney is formed of only a single pyramid, but in man there are about twelve. UErNIPEEOUS TUfeULES. 321 lobulated (figs. 394, 395); the lobules being united by the afferent and efferent vessels and covered by a syncytium reflected from the Fig. 392.— Diageam of the course or the tubdI/BS in a unipykamidal KIDNEY, SUCH AS THAT OF THE RABBIT. (Toldt.) a, Malpighian bodies ; 6, first convoluted tubule ; c, d, looped tube of Henle ; e, second convoluted ; /, collecting tube ; g, ducts of Bellini. Fig. 393.— Section through part of a dog's kidkey. (Ludwig.) p, papillary, and g, boundary zones of the medulla ; r, cortical layer ; h, bundles of tubules in the boundary layer, separated by spaces, b, containing bunches of vessels (not here represented), and prolonged into the cortex as the medullary rays, m ; c, intervals of cortex,, composed chiefly of con- voluted tubules, with irregular rows of glomeruli, between the medullary rays. epithelium lining the capsule. The glomeruli near the medulla are larger than the rest and have more lobules. The capillary-wall in all the glomeruli is a syncytium, showing no cell-outlines in silver pre- '!■ parations (Drasch), 322 THE ESSENTIALS OF HISTOLOGY. The tubule leaves the capsule by a neck (2), which is rarely narrower than the rest of the tubule in mammals, but in some animals (e.g. frog) Fig. 394.— a malpighian oobpusole from the kidney op the monkey. (Szymonowicz.) Magnified 350 diameters. a, a, sectionB of convoluted tubules ; a', commencement of convoluted tube from capsule ; b, ca23sule ; c, afferent and efferent vessels of glomerulus. Fig. 395.— Model of a gi.ome!rdlus. (Johnson.) a, afferent : e, efferent blood-vessel. UEINIFEROUS TUBULES. 323 is long, and has ciliated epithelium ; the tubule is at first convoluted {first convoluted tubule, 3), but soon becomes nearly straight or slightly spiral only {spiral tubule, 4), and then, rapidly narrowing, passes down Fig. 396.— Diagbam of the conE.SE of two CRiNiFEBOtrs tubules. (Klein.) A, cortex; b, boundary- zone ; c, papillary zoiie of the medulla ; a, a', superficial and deep layers of cortex, free from glomeruli. For the explanation of the numerals, see the text. ■ into the medulla towards the dilated commencement of the ureter as the descending tubule of Henle (5). It does not at once, however, open into the pelvis of the kidney, but before reaching the end of the 324 THE. ESSENTIALS OF HISTOLOGY. papilla it turns round in the form of a loop (hop of Eenle, 6) and passes upwards again towards the cortex, parallel to its former course, and at first somewhat larger than before, but afterwards diminishing in size (ascmding tubule of Eenle, 7, 8, 9). Arrived at the cortex it approaches close to the capsule from which the tubule took origin, but at a point opposite to the origin, viz. near the afferent and efferent vessels of the glomerulus (Golgi). It then becomes larger and irregularly zigzag (zigzag or irregular iubvle, 10), and may again be somewhat convoluted (second convoluted tubule, 11), eventually, however, narrowing into a small vessel (junctional tubule, 12), which joins a straight or collecting tubule (13). The last-named unites with others to form large collecting tubes which pass through the medul- lary substance of the kidney (14) to open at the apex of the papilla as the ducts of Bellini (15). The tubules are throughout bounded by a basement-membrane, which is lined by epithelium, but the characters of the epithelium-cells vary in the diflferent parts of a tubule. In the capsule the epithelium is flattened and is reflected over the glomerulus. In some animals (e.g. mouse) the granular epithelium of the convoluted tube is proloiiged a little way into the capsule. In the first convoluted and spiral tubules the epithelium is thick, and the cells are markedly granular, with a tendency for the granules to be arranged in lines perpendicular to the basement-membrane (rodded or fibrillar appearance, fig. 397). The granules of the cells are particularly well displayed in sections stained by Muir's method ; they are eosinophil, like the granules of secreting cells generally. They often exhibit a brush of eilium-like processes projecting into the lumen (figs. 397, 400), but these are not vibratile in mammals. In the narrow descending limb of the looped tubules, and in the loop itself, the cells are clear and flattened and leave a relatively large lumen ; in the ascending limb they again acquire a granular structure and may nearly fill the lumen. The arrangement of the cell-granules in lines perpendicular to the basement-membrane IS still more marked in the zigzag tubules, and a similar structure is present also in the second convoluted tubules, into which these pass. On the other hand, the junctional tubule has a large lumen and is lined by clear flattened cells, and the collecting tubes have also a very Fig. 397. — Section of a convoluted TUBULE OF THE KABBIT'S KIDNEY, SHOWING THE STEUCTURE OF THE EPITHELIUM. (Szymonowioz.) (Mag- nified 1100 diameters.) UEINIFEROUS TUBULES. 325 distinct lumen and are lined by a clear cubical or columnar epithelium (tig. 398, a). Fig. 398. — Section across a papilla of the kidney. (Cadiat.) a, large collectiug tubes (ducts of Bellini) ; &, c, c2, tubules of Henle ; e, /, blood-capillarles. The following gives a tabular view of the parts which compose a uriniferous tubule, and the nature of the epithelium in each part : — PoKTioN OF Tubule. Nature of Epithelium. Position of Tubule. Capsule First convoluted tube . Spiral tube . Small or descending tube of Henle . Loop of Henle Larger or ascending tube of Henle . Zigzag tube . Second convoluted tube Junctional tube . Straight or collecting tube .... Duct of Bellini . Flattened, reflected over glomerulus, where its cells form a syncytium. Cubical, granular, with appearance of fibrillation ("rodded"), the cells interlocking . ... Like the last Clear flattened cells . Like the last Cubical, granular: the cells some- times imbricated .... Cells strongly "rodded": varying height, lumen small Similar to first convoluted tube, but cells are longer, with larger nuclei, and they have a more refractive aspect Clear flattened and cubical cells Clear cubical and columnar cells Clear columnar cells .... Labyrinth of cortex, i Labyrinth of cortex. Medullary ray of cortex. Boundary zone and partly papillary zone of medulla. Papillary zone of medulla. Medulla, and medul- lary ray of cortex. Labyrinth of cortex. Labyrinth of cortex. Labyrinth passing to medullary ray. , Medullary ray and medulla. Opens at apex of papilla. 1 The part of the cortex between and surrounding the medullary rays is so named. 326 THE .ESSENTIALS OF HISTOLOGY. Blood-vessels.— The renal artery divides into branches on entering the organ, and these branches pass towards the cortex, forming incomplete arches between the cortex and the medulla (fig. 399, a). The branches of the renal vein form similar but more complete Fig. 399. — Vasoulak stjpplt op kidney. (Oadiat.) Diagrammatic. aj part of arterial arch ; 6, ihterldbular artery ; c, glomerulus ; d, efferent vessel passing to medulla as false arteria recta ; e, capillaries of cortex : /, capillaries of medulla ; g, venous arch ; h, straight veins of medulla ; ^', vena stellula ; i, interlobular vein. arches (g). — From the arterial arches vessels pass through the cortex {cortical or mterlobular arteries, h), and give off at intervals small arteri- oles {afferent vessels of the glomeruli), each of which enters the dilated commencement of a uriniferous tubule, within which its capillaries form a glomerulus.-- From the glomerulus a somewhat smaller efferent vessd passes out, and this at once again breaks up into capillaries, which are BLOOD-VESSELS OF KIDNEY. 327 distributed amongst the tubules of the cortex (e) ; their blood is collected by veins which run parallel with the cortical arteries but not in juxtaposition with them ; these veins join the venous arches between the cortex and the medulla ; they receive blood from certain other veins which arise by radicles having a somewhat stellate arrange- ment near the capsule {verm stellulm, j). The medulla derives its blood-supply from special offsets of the arterial arches, which almost .immediately break up into pencils of fine straight arterioles running in groups between the straight tubules of the medulla. These arterioles supply a capillary network with Fig. '400. — Nerve i'iekils ending ovek capillary blood-vessels and AMONGST THE EPITHELIUM CELLS OF A OONVOHITED TUBE OP THE PROG'S KIDNEY. (Smirnow. ) elongated meshes which pervades the medulla (fig. 399,/), and which terminates in a plexus of somewhat larger venous capillaries in the papillse. From these, and from the other capillaries the venules of the medulla collect the blood, and pass, accompanying the straight arterioles, into the venous arches between the cortex and medulla. The groups of small arteries and veins {vasa recta) in the part of the medulla nearest to the cortex alternate with groups of the uriniferous tubules, and this arrangement confers a striated aspect upon this portion of the medulla {bomidary zone, fig. 393, g). The efferent vessels of those glomeruli which are situated nearest to the medulla also break up into pencils of fine vessels (false vasa r«cte) which join the capillary network of the medulla (fig. 399, d). ■ Between the uriniferous tubules, and supporting the blood-vessels, is a certain amount of connective tissue (fig. 400), within which are cleft-like lymphatics. Nerve-fibrils are described as ramifying amongst the epithelium-cells of the tubules (fig. 400), but most of the nerves to the kidneys are •distributed to the blood-vessels. 328 THE ESSENTIALS OF HISTOLOGY. LESSON XXXVII. STRUCTURE OF THE URETER, BLADDER, AND MALE GENERATIVE ORGANS. 1. Section across the lower part of the ureter. Another section may be taken across the upper part near the pelvis of the kidney. 2. Section of the urinary bladder vertical to the surface. In the sections of the ureter and of the urinary bladder, notice the tran- sitional epithelium resting on a mucous membrane, which is composed of areolar tissue without glands (in most animals), and the muscular coat outside this. In the ureter there is a layer of connective tissue outside the muscular coat, and at the upper part of the bladder a layer of serous membrane covering the muscular tissue. 3. Section across the penis (child or monkey). The blood-vessels of the organ should be injected with the hardening fluid so as the better to exhibit the arrangement of the venous spaces which constitute the erectile tissue. Notice the large venous sinuses of the corpora cavernosa and the smaller spaces of the corpus spongiosum, in the middle of which is seen the (flattened) tube of the urethra. 4. Section across urethra and prostate gland (child or monkey). Notice the glandular tubes and the plain muscular tissue of the prostate, and the character of the urethral epithelium. 5. Section of testis and epididymis. The sections may be made from a rat's testis which has been hardened in alcohol ; they can be stained with iron-hsematoxylin. In these sections notice the strong capsule sur- rounding the gland, the substance of which consists of tubules which are variously cut ; and the epithelium of the tubules, which is in different phases of development in diflferent tubules. .Observe the strands of poly- hedral interstitial cells, much more numerous in some animals, lying in the loose tissue between the tubules ; also the lymphatic clefts in that tissue. Notice'in sections throvigh the epididymis the epithelium of that tube. Sketch carefully under a high power the contents of some of the semini- ferous tubules to illustrate the mode of formation of the spermatozoa. 6. Examination of spermatozoa. Spermatozoa may be obtained fi'esh from the testicle or seminal vesicles of a recently killed mammal and examined in saline solution. Their movements may be studied on the warm stage ; to display their structure a very high power of the microscope is necessary. They may be preserved and stained as " film " preparations, as with marrow (P- 30). The ureter (fig. 401) is a muscular tube lined by mucous membrane. The musmla/r coat consists of two layers of plain muscular tissue, an outer circular, and an inner longitudinal. In the lower part there are some longitudinal bundles external to the circular. Outside the muscular coat is a layer of connective tissue in which the blood-vessels and nerves ramify before entering the muscular layer. THE BLADDER. 329 The mucous membrane is composed of areolar tissue and is lined by transitional epithelium (fig. 402). The urinary bladder has a muscular wall lined by a strong mucous membrane and covered in part by a serous coat. FiQ. 401. — Section aoboss the upper part of the dreter. (v. Bbner.] Magnified 14 diameters. i!, epithelium ; s, mucous membrane ; I, longitudinal muscle ; r, circular muscle. Fie. 402. —Section oe the Mucons membrane op the bladder to show its epithelium. (Szymonowicz. ) a, 6, superficial epithelium-cells ; e, leucocyte ; d, areolar tissue of mucous membrane. The muscular coat consists of three layers, but the innermost is incomplete. The principal fibres run longitudinally and circularly, and the circular fibres are collected into a layer of some thickness which immediately surrounds the commencement of the urethra. The mucous membrane is lined by a transitional stratified epithelium like that of 330 THE ESSENTIALS OF HISTOLOGY. the ureter. The shape and structure of the cells have already been studied (p. 55). Many of the superficial cells have two nuclei. . The nerves to the bladder form gangliated plexuses, and are dis- tributed to the muscular tissue. and blood-vessels, but some are said to enter the epithelium. The penis is mainly composed of cavernous tissue which is collected into two principal tracts — the corpmn cavernosa, one on each side, and the corpus spongiosum in the middle line inferiorly. All these are bounded by a strong capsule of fibrous and plain muscular tissue, con- taining also many elastic fibres and sending in strong septa or trabeculee Fig. 403.— Section of ekbotile tissue. (Cadiat.) a, trabeculae of connective tissue, with elastic fibres, and bundles of plain muscular tissue, some cut across (c) ; b, venous spaces. of the same tissues, which form the boundaries of the cavernous spaces of the erectile tissue (fig. 403). The arteries of the tissue run in these trabeculae, and their capillaries open into the cavernous spaces. On the other hand, the spaces are connected with efferent veins. The arteries of the cavernous tissue may sometimes in injected specimens be observed to form looped or twisted projections into the cavernous spaces {helicine arteries of Muller), into which they may open directly. The integument of the penis and clitoris, especially that of the glans, contains numerous special nerve end organs of the nature of end- bulbs (see p. 169), and Pacinian bodies are also found upon the nerves. Lymphatic vessels are numerous in the integument of the organ and allso'in' the submucous tissue of the urethra. THE URETHRA. 331 Urethra.— The cross-section of the urethra appears in the middle of the corpus spongiosum in the form of a transverse cleft. It is lined in the prostatic part by transitional, but elsewhere by columnar epithelium, except near its orifice, where the epithelium is stratified. In the female urethra it is stratified throughout. The epithelium rests upon a vascular mucous membrane, which contains longitudinally disposed plain muscular fibres, and in the membranous urethra, cir- cularly disposed cross-striated fibres. Outside the mucous membrane is a coating of submucous tissue, with two layers of plain muscular fibre— an inner longitudinal and an outer circular. Outside this again Fig. 404. — Section ok prostate. (Heitzmann.) M, muscular tissue ; E, epithelium ; C, concretions. is a close- plexus of small veins which is connected with, and may be said to form part of, the corpus spongiosum. The mucous membrane of the urethra is beset with small mucous glands, simple and compound {glands of LitM). There are also a number of oblique recesses termed lacunce. Besides these small glands and glandular recesses, two compound racemose glands open into the bulbous portion of the urethra {Cowper's glands). Their acini are lined by clear columnar cells which yield a mucus-like secretion. The prostate, which surrounds the commencement of the urethra, is a muscular and glandular mass, the glands of which are composed of tubular alveoli, lined by columnar epithelium, with smaller cells lying between them and the basement-membrane (fig. 404). Their ducts 332 THE ESSENTIALS OF HISTOLOGY. open upon the floor of the urethra. In old subjects the tubules often contain colloid or calcareous concretions. The muscular tissue is of the plain variety. Blood-vessels and nerves are numerous. The nerves are provided , with small ganglia and are distributed partly to the muscular tissue, partly to the glands, and others (sensory) to the capsule, and to the wall of the urethra. The sensory nerves end in plexuses and in peculiar terminal corpuscles like simple Pacinian bodies (Timofeew). i ^■"' -cW'^-'^r '. -d Fig. 40.5.— Sectio.v ok human testis and epididymis, somewhat magnified. (Bohm and v. Davidoff.) a, glandular substance divided into lobules by septa of connective tissue ; 6, tunica albu- ginea ; c, bead of epididymis ; d, rete testis ; e, middle part or body of epididymis ; /, mediastinum giving origin to -the septa ; g, sections of the commencing vas deferens. The testicle is inclosed by a strong fibrous capsule, the tunica albuginea (fig. 405, b). This is covered externally with a layer of serous epithelium reflected from the tunica vaginalis. From its inner surface there proceed fibrous processes or trabecwlce, which imperfectly subdivide the organ into lobules, and posteriorly the capsule is prolonged into the interior of the gland in the form of a mass of fibrous tissue, which is known as the mediastinum testis (fig. 405, /). Attached to the posterior margin of the body of the gland is a mass (epididymis) which when investigated is found to consist of a single convoluted tube, receiving at THE TESTICLE. 333 its upper end the efferent ducts of the testis and prolonged at its lower end into a thick-walled muscular tube, the vas deferens, which conducts the secretion to the urethra. The glandular substance of the testicle is wholly made up of ■convoluted tubules, which when unravelled are of very considerable length. Each commences near the tunica albugitiea, and after many windings terminates, usually after joining one or two others, in a Fig. 406.— Passage of cONVOLnTED seminifbkous tubules into steaight TUBULES AND OF THESE INTO THE EETE TESTIS. (Mihalkowioz. ) a, seminiferous tubules ; b, fibrous stroma continued from the mediastinum testis ; c, rete testis. straight tubule, which passes into the mediastinum, and there forms, by uniting with the other straight tubules, a network of intercom- municating vessels of varying size, which is known as the rete testis (fig. 406). From the rete a certain number of efferent tubules arise, and after a few convolutions pass into the tube of the epididymis. The straight tubules which lead from the convoluted seminiferous tubes into the rete testis are lined only by a single layer of clear flattened or cubical epithelium. The tubules of the rete also. have a 334 THE ESSENTIALS OF HISTOLOGY. simple epithelial lining; both in these and in the straight tubules the basement-membrane is absent, the epithelium being supported directly by the connective tissue of the mediastinum. The efferent tubules which pass from the rete to the epididymis are lined by columnar ciliated epithelium. In man their lumen is irregular in section, and the inner surface pitted with depressions (intra-epithelial glands) lined by short clear ' non-ciliated cells (J. Schaifer). The tttbe of the epididymis is lined by long columnar cells having at their bases Fig. 407.~Seotion of thb tube op the epididymis. (Szymonowlcz.) (Magnified 300 diameters.) a, bloqd-veasel ; b, circular muacular fibres ; c, epithelium. smaller cubical cells with spherical nuclei (fig. 407). The columnar cells are provided with what appear to be bunches of cilium-like fibrils pro- jecting into the lumen of the tube. These apparent cilia are, however, not vibratile as was formerly supposed, and are therefore not true cilia (Neumann, Myers-Ward). They appear to vary in development in different cells, and are probably connected in some way with the formation of the secretion of the epididymis and its extrusion into the lumen of the tube. The epididymis cells exhibit can.aliculi in their cytoplasm, which according to Holmgren, communicate with the exterior at the attached border of the cell (fig. 408). The tube of THE TESTICLE. 335 t;he epididymis has a considerable amount of plain muscular tissue in its wall (fig. 407). The VOLS deferens (fig. 409) is a thick-walled tube, formed of an outer Fig. 408.— Cells of epididymis, showing canalization ov the cytoplasm. (B. Holmgren.) Fig. 409.— Section across the commencement ov the vas defeeens. (Klein.) a, epithelium; b, mucous membrane; c, d, e, inner middle, and outer layers of the muscular ioat ; /, bundles of the internal cremaster muscle ; g, section of a blood- layer of longitudinal bundles of plain muscular tissue ; within this an equally thick layer of circular bundles of the same tissue, and within this again a thinner layer of longitudinal muscle. There is a good deal of connective and elastic tissue between the muscular bundles. The 336 THE ESSENTIALS OF HISTOLOGY. tube is lined by a mucous membrane, the inner surface of which is covered by columnar non-ciliated epithelium. The ampullm of the vasa deferentia, and the vesiculce seminales, are in structure similar to the vas deferens, but their corrugated walls are much thinner and less muscular. The connective tissue between the tubules of the testis is of very loose texture, and contains numerous lymphatic clefts, which form an intercommunicating system of commencing lymphatic vessels. Lying in this intertubular tissue are strands of polyhedral epithelium-like Fig. 410. Fig. 411. Fig. 410. — Section op pakts of three SEMiuiB'ERons tubcles or the bat. a, with the spermatozoa least advanced in development ; 6, more advanced ; c, containing fully developed epermatozoa. Between the tubules are seen strands of interstitial cells with blood-vessels and lymph-spaces. Fig. 411. — Human spermatozoa. :Li^, (G. Retzius.) 1, in profile ; 2, viewed on the flat ; b, head ; c, middle piece ; d, tail ; «, end-piece of the tail, which is described as a distinct part by Retzius. cells (interstitial cells, see fig. 410) of a yellowish colour; they are much more abundant in some species of animals (cat, boar) than in others. They accompany the blood-vessels before these break up to form fhe capillary networks which cover the walls of the seminiferous tubules. The interstitial cells contain in many animals yellowish-brown fat- globules (staining with osmic acid), and also sometimes needle-shaped crystals (proteid). Similar fatty globules may occur in the Sertoli cells of the seminiferous tubules (see above), and have been thought to be derived from those of the interstitial tissue. THE SEMINIFEROUS TUBULES. 337 Structure of the tubules.— The seminiferous Mules are formed of a thick basement-membrane, and contain several layers of epithelium- cells. Of these layers, the one next to the basement-membrane is a Fig. 412. — Hdman spekmatozoa on the flat and in pbopilb. (Bramman.) Those on the right still show protoplasm adhering to them. Only the commencement of the tail is represented in the two which are shown in profile. Magnified 2500 diameters. Tstratum of clear cubical cells {spermatogofda or spermogonSj figs. 410, 414, a), the nuclei of which for the most part exhibit the irregular network wljich is characteristic of the resting condition, but in certain tubules show indications of division. Here and there between the spermatogonia Y 338 THE ESSENTIALS OF HISTOLOGY. some of the lining epithelium-cells are enlarged, and project between the more internal layers, being connected with groups of developing spermatozoa. These enlarged cells are the cells of Sertolii&g. 414, a',, a" ; fig. 417). Next to this lining epithelium is a zone of larger cells (spermatocytes or spermocytes, fig. 414, b), the nuclei of which are usually in some stage of hetero- or homo-typical mitotic division; these cells may be two or three deep (as in a, fig. 410). Next to them, and most internal, are to be seen in some tubules (fig. 410, 6 and c) a large a d / 9 h Fia. 413.— Different forms of spermatozoa. (From Verworn.) u, o£ bat ; h, c, of frog ; d, of finch ; e, of ram ; /, g, of boar ; %, of a jelly-fish ; i, of a monkey ; Z, of crab ; A, of round-worm. number of small protoplasmic cells with simple spherical nuclei (spermatids or spermids, fig. 414, c). In other tubules the spermatids are elongated, and the nucleus is at one end, and in others again these elongated cells are converted into evident spermatozoa, which lie in groups : their heads projecting between the deeper cells and connected with one of the Sertoli cells of the lining epithelium, and their tails emerging into the lumen of the tubule (fig. 410, b). As they become matured they gradually shift altogether towards the lumen, where they eventually become free (c). During the time that this crop of spermatozoa has been forming, another set of spermocytes has been produced by the division of the sperm ogonia, and on the discharge of the spermatozoa the process is repeated as before (see diagram, fig. 414). The sperfiiatozoa. — Each spermatozoon or sperm consists of three parts, a head, a middle part or body, and a long tapering and vibra- tile tail (figs. 411, 412). In man the head is of a flattened oval shape, somewhat more flattened anteriorly ; in some animals it bears a small barb-like projection at its extremity, but, this appears to be THE SPERMATOZOA. 339 flo. 414. — dlaokam bxhibitlig the ctole of phases of spermogenpsis (rat). a, lining epithelium-cells or spermatogonia, seen dividing in 6 ; a', a", Sertoli cells ; b^ spermatocytes, witli skein-like nuclear filaments. These cells are seen actively dividing in 5, c, spermatids, forming an irregular column or clump in 6, 7, 8, and 1, and connected to an enlarged Sertoli cell, a', of the lining epithelium in 2, 3, 4, and 5. In 6, 7, and 8 advanced spermatozoa of one crop are seen between columns of spermatids of the next crop. «', parts of the spermatids which disappear when the spermatozoa are fully formed ; s, seminal granules. Fig. 415.— Spermatozoa fhom the eat in different stages of develop- ment. (H. H. Brown.) 1-6, developing spermatozoa from the testicle; 7, a mature spermatozoon f*-om the yas deferens. The remains of the protoplasm of the cell, which is seen in 6 still adhering to the middle piece of the spermatozoon and containing a number of chromatin granules .appears to he thrown off as the spermatozoon matures. , 340 THE ESSENTIALS OE HISTOLOGY. absent in the human spermatozoon. The apical part is covered by a cap of a somewhat different appearance from the rest — the head-cap. The middle-piece is in man short and cylindrical, and has a spiral fibre passing round it. An axid fibre, itself fibrillated, passes from a knob close to the head right through the body and tail. The tail is the longest part of the spermatozoon, and when examined with the micro- scope in the fresh condition is seen to be in continual vibratile motion, the action resembling that of a cilium. The extremity of the tail {end-piece) forms a distinct part of the spermatozoon, and in some Fig. 416. — Changes in the spermatids in the conRSE of foemation of THE SPERMATOZOA. (Niessing.) The tail filament is seen (in a and e) to extend from the centrosome, which Ilea close to the ' nucleus. The head-cap (shown in c) is produced by a tiransformation of part of the archoplasm which becomes vacuolated (ii, c, d). animals may split into two or three fibrils ; these can also sometimes be traced along the whole length of the tail. Human spermatozoa are about 0-05 mm. (-^^ inch) long, the head and middle-piece each measuring about J^th of this amount. In different animals the shape of the head and the extent of middle- piece and tail vary greatly (fig. 413). In the rat (fig. 415, 7) the head is long, and is recurved anteriorly ; it is set obliquely on the middle- piece, which is also of considerable extent, and which has. a closely wound spiral filament encircling it (H. H. Brown). In the newt the bead is long and tapering, and the tail has a membranous, expansion, attached in a spiral- manner along its whole length. This has also been SPERMOGENESIS. 341 described in the human spermatozoon, but its existence here is doubt- ful. In decapods, which possess no cilia, the spermatozoa are stellate and motionless (fig. 413, I) ; in nematoid worms they are amoeboid (fig. 413, k). Sometimes two distinct kinds of spermatozoa are met with in the same species of animal, one kind being far the larger in size (giant spermatozoa) but much less numerous. Such giant spermatozoa have been observed in man. Although the tail of the spermatozoon is usually classed with cilia, it is obvious that it exhibits far greater complexity and is a much more highly differentiated structure. Spermatozoa also differ from cilia in being highly resistant to putrefaction and to chemical reagents, even including the strongest acids and alkalies. Spermogenesis. — The spermatozoa are developed from the small cells (spermatids) which form the innermost stratum of the seminal epithelium, and these are themselves produced by the division of the large spermocytes of the second layer. It is probable that fresh spermocytes are formed by division of some of the lining epithelium- cells or spermpgons. The cycle of changes therefore which takes place is as follows: — 1. Division of a lining epithelium-cell or spermogon into two, one of which grows larger (" growing cells " of H. H. Brown), becomes a spermocyte, and passes into the second layer, while the other remains in the first layer. 2. Division of the spermocyte. 3. Further division of the daughter-spermocytes thus produced. The four cells (spermatids) which result from this double division possess only one-half the somatic number of chromosomes in their nuclei, " reduc- tion" having been effected in the final cell-divisions by which the spermatids are produced (see p. 14). 4. Elongation of the spermatids and their gradual conversion into spermatozoa. As they undergo this conversion their grouping becomes more evident, and each group is found to be connected with a cell of Sertoli (figs. 414, a', 417), which probably ministers to their nutrition. This cell undergoes a gradual process of elongation so that the spermatozoa by the time they are fully developed are brought to the lumen of the tube, in which they then become free. In the meantime other alternate groups of spermatids from which the next crop of spermatozoa will be derived are being formed in the same manner, passing through the same cycle- of changes. So that in a longitudinal section even of the same tubule, different phases of development may be observed, and in, different tubules of the same testicle every phase may be traced, The accompanying diagram (fig. 414), which is constructed from drawings by H. H. Brown, illustrates the cycle of changes above described : it is divided into eight parts, each of which shows 342 THE ESSENTIALS OF HISTOLOGY. "the condition of the epithelium of a semiiliiferOus tubule at a particular stage. Each spermatid becomes converted into a spermatozoon in - the following manner (figs. 415, 416, 418): The nuteleus forms the chief part of the head, while the tail develops as an outgrowth of the centrosome and cytoplasm. The tail-filament appears within the protoplasm, growing out from the centriole of the cell which lies close Fig. 41S. '<■' ■Tj"(i.'417.— A CElil OF Sertoii with which thk spermatids (three of which . ./^^.n shown) are beginning to be connected : HUMAN. (Bramman.) The cell contains globules (of nutritive substance) staining witb osmic .acid, and similar .i buf.stnaller globules are also seen in the'Spermatids. The "ring " forniea around the - i tail'iil^ment by one of the particles of the centrosome (see text) is shown in each of . ' -^ ' thesespermatidsclosd to the "head." . Fig. ;^'8:— Stages of spermogbnesis; with trans formation of the granules • '■ 1 ;qs,mito0hondria of the spermatid into the spiral fibre of the middle . ■. , piece: MOUSE. (Benpla.) to the nucleus (fig. 416). The centriole is double, and one of its two particles forms an annular expansion or ring which, as development proceeds, moves down the tail-filament until it reaches the place where this leaves the cytoplasm: here it ultimately forms the limit of the body or middle piece of the spermato'zo6n. The archoplasm (see p^ 8) assists in forming the head of the spermatozoon ; a portion (the idiozome of Moves) at an early stage separates from the rest, lying iapically to the nucleus. Within this portion vacuoles form (fig. 416, J, Cj (i) which presently run together into a clear non-stainable SPERMOGENESIS. 343 globule which flattens out over the nucleus and forms (fig. 416, «) the head-cap of the spermatozoon ; as development proceeds, this may become indistinguishable from the rest of the head. The spiral fibre of the middle piece is developed from mitochondria (see p. 5) in the spermatid (Benda) (fig. 418). A portion of the protoplasm of each spermatid containing a number of chromatin-particles (seminal granules) becomes detached and disin- tegrated before the spermatozoon is fully matured (fig. 414, s, s'). A few spermocytes undergo incomplete division, and the resulting spermatids are large (giant spermatids) and contain either one large nucleus or two or more nuclei which ultimately blend to form the head of the spermatozoon. In these cases there are a corresponding number of centrosomes, from each of which a tail-filament may become developed. 344 THE ESSENTIALS OF HISTOLOGY. LESSON XXXVIII. GENERATIVE ORGANS OF THE FEMALE. 1. Sections of the ovary of the non-pregnant rabbit or cat. (If from a pregnant animal the organ may be largely occupied by luteal tissue.) Study the sections with a low power, observing the small and large Graafian follicles, each inclosing an ovum, scattered through the stroma. Measure some Graafian follicles of difierent sizes ; make a general sketch of a section under the low power. Then sketch carefully two or more of the follicles with their contents under a high power. 2. Sections across the Fallopian tube. Sketch a section under the low power. 3. Section across the body of the uterus, or across a cornu of a bicorned uterus. Observe with the naked eye the thickness of the muscular and mucous coats respectively. Notice the ciliated columnar epithelium lining the organ and extending into the glands of the mucous membrane. Draw a part of the section under the low power. 4. Section of the mucous membrane of the vdgina. Notice the stratified scaly epithelium which lines it. and which is continued over the projecting part of the os uteri. 5. Take the fresh ovary of a recently killed animal and with a needle or fine scalpel-point prick one of the largest and most prominent of the Graafian follicles. The organ must be held just over a slide so that on pricking the follicle the fluid contents may spurt out on to the glass. Examine the drop of liquor folliculi with a low power for the escaped ovum, which will be surrounded by follicular cells. "When found place a piece of hair in the drop, cover with cover-glass and examine with high power. THE OVARY. The ovary is a small solid organ, composed of a stroma of fibrous tissue, with many spindle-shaped cells, and also containing, near its attachment to the broad ligament, a large number of plain muscular fibres. It is covered by a layer of small columnar epithelium-cells '(germinal epithelium, fig. 420, a), between which may here and there be seen a few larger spheroidal cells, with large round nuclei. In the young subject the epithelium occasionally dips down into the subjacent stroma. The stroma is beset with vesicles of different sizes, the smallest being near the surface of the organ, the larger ones placed more deeply in the stroma, although, as they increase in size, they extend towards the surface. THE OVARY. 345 These vesicles are the Graafian follides. Each Graafian follicle has a proper wall {theca follkuli) formed of a layer derived from the stroma, and a special inner layer containing large cells : both are highly vascular. Each follicle contains an ovum and epithelium. In the smallest follicles the ovum is small, and the epithelium of the follicle is formed of a single layer of cells, which may be flattened against the ovum (fig. 421). In somewhat larger follicles the epithelium-cells are in two layers, and these are columnar in shape (fig. 423, E). In still larger ones, each of these two layers is formed of several strata of cells, and fluid has begun to collect between the layers at one part. Of the two layers, the one which lines the cavity of the follicle is termed the membrana granulosa, while the mass of cells which more immediately surrounds the ovum is known as the cumulus or discus proligerus. Fig. 419. — Section oi' the ovakt or the oat. J. (Sohron.) 1, outer covering and free border of the ovary ; 1\ ajitached border ; S, the central ovarian stroiha, showing a fibrous and vascular structure ; * S, peripheral stroma ; It, blood- vessels ; 5, Graafian follicles in their earliest stages lying near the surface ; fi, 7, 8, more advanced follicles whibh are embedded more deeply in the stroma ; 9, an almost mature follicle containing the ovum in its deepest part ; &, a follicle from which the ovum has fallen out in preparing the section ; 10, corpus luteum. In the largest follicles the fluid has much increased in amount, so that the follicle has become gradually larger and more tense. Finally it reaches the surface of the ovary, and projects from that surface, where it eventually bursts, and the liquor folliculi, with its contained ovum, is set free. This event is believed to occur usually at some time during menstruation. Some of the Graafian follicles do not burst, but, after attaining a certain stage of maturity, undergo a process of retrograde metamor- phosis and eventually disappear. The ovarian ova or ovocytes are large spherical cells (fig. 424), about 0-2 mm. ( j^-j- inch), in diameter. When fully formed, as in the 346 THE ESSENTIALS OF HISTOLOGY. largest Graafian follicles, each ovum is surrounded by a thick trans- parent membrane {zma pelhicida). Within this is the protoplasm of the ovocyte (mtellus), filled with fatty and albuminous granules (yolk granules). Lying in the vitellus, generally eccentrically, is the large Fig. 420.— Section of the ovabt of an adult bitch. (Waldeyer.) a, germ-epithelium : 6, remaias of egg-tubes ; c, small follicles ; d, more advanced follicle ; e, discus proligerus and ovum ; /, second ovum in the same follicle (this occurs but rarely) ; g, outer tunic of the follicle ; h, inner tunic ; i, membrana granulosa ; fc, collapsed retrograded foUiclo; J, • blood-vessels ; m, nij longitudinal and transverse' ■ sections of tubes of the parovarium ; y, involuted portion of the germ -epithelium of the surface ; z, place of the transition from peritoneal to germinal or ovarian epithelium. ' clear round nucleus {germinal vesicle), which may show an intranuclear network, and invariably has a well-marked nucleolus {germinal spot), sometimes more than one.' ' - ' . Oogenesis. — ^Both the ova and the epithelium of the Graafian follicles THE OVARY. 347 originate from the germinal epithelium of the embryo. This forms at first a simple layer covering the stroma, but later becomes thickened and multiple. After a time rounded cords of epithelium-cells {egg- tubes of Pfliiger ; fig. 423, a), grow down into the stroma, whilst this Fig. 421. — Section of pakt of human ovart showinr small Graafian FOLLICLES IMBEDDED IN A FIBBO-CELLULAK STROMA. (Sellheim.) Fib. 422.— a moderately large Graafian follicle from the human ovARYi SHOWING OVUM SURROUNDED BY " DISCUS PBOLIGEEUS " AND WALL OP FOLIjICl-E LIJJED BY "mEm'bBANA GRANULOSA." BETWEEN THEM IS AN AOCUMCLATION OF LIQUOR FOLLlCULIl' (Sellheim.) 348 THE ESSENTIALS OF HISTOLOGY. at the same time grows into the epithelium. The cords presently- become broken up by ingrowths of stroma into small isolated nests of Fig. 423. — Figuees showing vabious stages in the development or the Graafian follicles of the babbit. A, from ovary of young rabbit, showing " egg-tubes " of PflUger growing in from germinal epithelium ; some of the tubes contain primitive ova ; b, primitive Graafian follicles formed from the breaking up of an egg-tube ; c, a young Graafian follicle, with a single layer of follicle-epithelium ; d, a somewhat older follicle, with the second layer forming within the first ; b, a more advanced follicle, showing two complete layers of columnar epithelium surrounding the ovum within the follicle. epithelium-cells, each of which may represent a Graafian follicle. To form the ova, some of the cells become enlarged (primitive ova), and usually there is one such enlarged cell in each of the isolated THE OVARY. 349 nests.i The remaining cells form the epithelium of the follicle (see fig. 423, B, c). It is stated that the protoplasm of the ovum remains connected with the cells of the discus proligerus by fine processes Fig. 424.— Human ovum; highly magnified. (Waldeyer.) The zona pellucida is surrounded by cells of the discus proligerus, which are adherent to it. which pass through pores in the zona pellucida, and on the other hand, the epithelium-cells of the follicle are themselves inter-connected by protoplasmic bridges, so that the vi'hole forms a syncytium. ' The nuclei of the primitive ova pass through the pre-maiotio changes mentioned on p. 14. 350 THE ESSENTIALS OE HISTOLOGY. The stroma of the ovary contains, besides the spindle-shaped con- nective-tissue cells and plain muscular fibres already mentioned, a _ number of epithelium-like interstitial cells. Some of these are derived from the germinal epithelium, and appear capable of developing into ova and follicle epithelium-cells (Lane-Claypon); others have originated from cells of corpora lutea. These last are large yellow Fis. 425.— Three sta'gbs in the for- mation or THE CORPUS LDTEnM IN ■ THE MOUSE. (Sobotta.) A. The follicular epithelium, fe, is hyper- trophied, and vascular processes, a, of the theca, th, or wall of the follicle are growing into ft. B. The epithelial mass is now subdivided into lobule-like masses, I, of luteal cells by the thecal ingrowths; e, epithelium of surface of ovary. G. There are now very numerous thecal septa or ivabecu^B, and the columns of £-- .■•T-.-,-»»-«iM^t' 4\ "*.asw luteal cells- are much narrower. A central '^^:!&\/l)'\, - ^li IW^ cavity is stiU seen. c nodules which are developed out of the Graafian follicles after the ova have been extruded (figs. 425, 426). They consist of columns of large yellowish cells {luteal cells), with intervening trabecules of vascular fibrous tissue, which converge to a central strand of connective tissue occupying the axis of the nodule (fig. 426). The columns of cells are not unlike those of the cortex of the suprarenal capsule. The corpus luteum is derived from the wall — probably in the main from the epithelium — of the follicle, which becomes thickened and folded by multiplication and hypertrophy of its cells ; between the folds THE OVAEY. 351 connective tissue and blood-vessels grow in from the theca towards the centre of the follicle; in this way the columnar arrangement above mentioned is produced. After persisting for a time the corpus luteum gradually disappears, its tissue becoming merged in the surrounding stroma. Corpora lutea grow much larger and remain much longer persistent in the event of pregnancy supervening. Fig. 426. — CoKPns luteum of mouse. (Sobotta.) This figure shows a more advanced stage of development, the luteal tissue being now vascularized and the central -cavity .-ohliterated. The use of the corpus luteum is not known certainly, but it has recently been suggested that it may yield an internal secretion, the effect of which is to produce the fixation of the fertilized ovum in the uterine mucous membrane (Born). In confirmation of this, experiments seem to iftdicate that gestation does not supervene in animals whose corpora lutea have been destroyed (Fraenkel and Cohn), or from which the ovaries have been removed during the first stages of pregnancy (Marshall and Jolly). The blood-vessels of the ovary are very large and numerous, and are especially distributed to the walls of the Graafian follicles, over which they form a close network. THE FALLOPIAN TUBES AND UTEKUS, The Fallopian tubes are lined by a very vascular mucous membrane which is covered with ciliated epithelium, and has numerous longi- tudinal folds (fig. 427). Externally they are covered by a serous coat, within which is a thin longitudinal stratum of plain muscular 352 THE ESSENTIALS OF HISTOLOGY. fibres overlying circular fibres of the same tissue, but these layers are not distinctly marked off from one another. The human uterus is composed of two parts, the body and cervix. The body of the uterus is formed of the following layers : 1. A sermis layer, derived from the peritoneum, which covers the greater part of the fundus. 2. A muscular layer, which is of considerable thickness and is formed of plain muscular fibres disposed in three, more or less blended, strata. Of these the outer has its fibres arranged partly longitudinally, partly circularly. The middle muscular layer, on the other hand, is thick; Fis. 427.— Section aoboss the fallopian tobe. (Diagrammatic, its fibres run in different directions, and it contains the ramifications of the larger blood-vessels. The inner layer, again, is thinner and has both longitudinal and circular fibres, many of the latter being pro- longed internally into the deeper part of the mucous membrane ; the extremities of the uterine glands extend between and amongst its fibres. 3. A mucous mem.brane, which is very thick and is composed of soft connective tissue containing a large number of spindle-shaped cells. It is lined by ciliated epithelium and contains long, simple, tubular glands, which take a curved or convoluted course in passing through the membrane (fig. 429). Their (ciliated) epithelium is continuous with that which covers the inner surface of the mucous membrane. In the cervix the mucous membrane is marked by longitudinal and oblique ridges, and the glands are shorter but more complex than those of the THE FALLOPIAN TUBES. 353 Fig. 428.^Seqtioii of mucous membbanb of human uterus duking mbn- 3tkuati0k, showing masses of blood which have escaped fkom ruptuebd capillaries into the interglandular tissue, and have at one place (#) BROKEN THROUGH THE SURFACE EPITHELIUM. (Sellheitn.) Fio. 429.— Section of a coenu of the rabbit's uterus. «, serous layer; l.m., longitudinal muscular fibres; cm., circular muscular fibres of the muscular coat ; a, areolar tissue with large blood-vessels ; m.m., muscularis muoosie ; m, mucous membrane. Z 354 THE. ESSENTIALS OP HISTOLOGY. body of the uterus, and are lined by columnar mucus-secretin cells. Near; the os uteri the epithelium becomes'Tnon-ciliated columnar, and at the margin I of| [^the os uteri this passes into a stratified epithelium which overlies vascular papillae of the corium. The mucous mem- brane is very vascular, and it also contains a large number of lymph-vessels. In many animals the uterus is composed of two long tubes (obrnua uteri) : the arrange- ment of the muscular tissue in these is simpler than in the human uterus, which has been formed by the fusion of two such tubes. Fig. 429 exhibits the structure of a cornu of the uterus of the rabbit. At each menstrual period the mucous membrane of the uterus undergoes a partial process of disintegration accompanied by an escape of blood from the capillaries of the membrane (fig. 428). This is succeeded by a rapid renewal of the disintegrated part. Should gestation supervene, the process of renewal results in the formation over certain parts of a greatly thickened mucous mem- brane, with long convoluted glands, which is then known as the decidua. The muscular layer also becomes enormously hypertrophied, this hypertrophy being produced by the Fig. 430. — McacuLAK enlargement of the individual muscle cells FIBRES (a) FROM NON- j^n\ PRBGNANT, (6) FROM PRBG- (fig. 430). NANT CTBRITS, DRAWN TO THE SAUE SCALE. (Sell- heim.) THE SPINAL CORD. 355 LESSONS XXXIX. AND XL. STRUCTURE OF TEE SPINAL CORD. 1. Sections of the spinal cord from the cervical, dorsal, and lumbar regions. If the human spinal cord cannot be obtained suificieutly fresh, that of a dog, cat, or monkey may be used. It is to be hardened by suspending it immediately after removal from the body in a tall jar of formol (10 per cent, solution). After a few days it may be transferred to alcohol. Sections are to be made either by the paraffin or celloidin method : the former is prefer- able for small cords.' The sections may be stained by Nissl's method, \irhich brings to view the nerve-cells and also stains the axis-cylinders of the nerve- fibres. If it is desired to stain by the Weigert-Pal method, which colours the medullary sheaths of the nerve-fibres, the pieces of cord should be placed in 2 per cent, bichromate of potassium solution or Miiller's fluid (either at once or after formol) and should be left for about a month, after which they are cut by a freezing microtome. (For the details of these methods see Appendix.) Carminate of ammonia or thionin may also be employed to stain the nerve-cells and axis-cylinders. Notice the relative extent of the grey as compared with the white matter in the different regions of the cord. Sketch a section from each region under a low power. Sketch also a small portion of the white substance, two or three nerve-cells, and the central canal with its lining epithelium and surrounding neuroglia under the high power. Measure the diameter of some of the nerve-fibres in the anterior columns, in the lateral columns, and in the posterior columns. 2. Tracts in ^the spinal cord. The conducting tracts of the spinal cord may be studied in two ways, viz. : (1) by preparing sections of embryonic cordUi (from the 5th to the 9th mouth), the sections being stained by the Weigert-Pal process (Flechsig's method) ; (2) by preparing sections from the cord of an animal in which either a complete section or a hemi-section has been performed about 15 days before the animal is killed, and staining thin pieces of the cord from below and from above the section by placing them in a solution consisting of two parts of Miiller's fluid and 1 part of 1 per cent, osraic acid (Marehi's method). The cord must first be partly hardened by placing it for a few days in Miiller's fluid. The spinal cord is composed of grey matter in the centre and of white matter externally. It is closely invested by a layer of connective- tissue containing numerous blood-vessels {pia mater), and less closely by two other membranes (fig. 431). One of these is an areolar mem- brane, resembling a serous membrane in general structure, but non- vascular and more delicate in texture {arachnoid). The other, which lines the vertebral canal, is a strong fibrous membrane known as the dura mater. At the middle of the anterior and posterior (ventral and 356 THE ESSENTIALS OF HISTOLOGY. dorsal) surfaces the pia mater dips into the substance of the cord in the anterior and posteriw median fisswres, so as to divide it almost completely into two lateral halves. These are, however, united by an isthmus or bridge, which is composed anteriorly of transversely crossing white fibres {white w anterior commissure), posteriorly of grey matter {grey commissure), in the middle of which is a minute canal lined by ciliated epithelium {central canal). Each lateral half of the spina! cord contains a crescent of grey matter, which is joined to the corresponding crescent of the opposite side by the grey commissure. Of the two horns of the crescent the posterior or dorsal is the narrower and comes near the surface of the « d Fig. 431. — Section op the spinal oobd within its MEMBRANES. (Key and Retzius.)^ a, dura tnatcr ; b, arachnoid ; c, septum of aiachuoid ; {2, e, trabeculse of arachnoid ; ^, llgamentum denticulatum ; /, bundles of poaterior roof; h, bundles of anterior root ; k, I, subarachnoid space. cord; close to it the bundles of the posterior nerve-roots enter the cord. The bundles of the anterior nerve-roots emerge from the anterior horn. According to Ingbert about 1,300,000 nerve-fibres enter the cord by the posterior roots, aud about one-third that number leave it by the anterior roots. The posterior l-oot-fibrea are derived from the cells of the spinal ganglia, which lie outside the cord ; the anterior root-fibres from cells within the grey matter, chiefly from cells in the anterior horn, but also from some cells in the middle and posterior parts of the grej' matter and (especially in the thoracic region) from cells in the intermedio-lateral tract (lateral horn). The latter probably furnish the autonomic (sympathetic) fibres of the anterior roots, while the cells of the anterior horn fiirnisli the fibres which are distributed to the voluntary muscles. The uhite matter of each half of the cord is subdivided by the approach of the posterior horn to the surface into two unequal columns — antero-lateral and posterior. A distinction is sometimes drawn between anterior and lateral portions of the antero-lateral THE SPINAL CORD. 357 postero-lateral fissure poatero-mesial column postero-naediau fissure posterior root-bundle posterior column- subst. gfelat. of post, horn tractof Flechsig lat. pyram. tr. form, retic, lateral horn central canal {Lut. commissure anterior horn — ant. median fissure Fig. 432.— Section of human spinal cord i<'rom upper cervical region. (Photograph.) Magnified about 8 diameters. .,-4 ') Fig. 433. —A small pobtion of a transvebse section of the human spinal CORU IN the region OF THE LATEBAL COLUMN, TO SHOW THE SUPERFICIAL NEUKOGLIA. a, a, superficial neuroglia ; b, b, traiievetse section of part ot the lateral column of the cord, in which the dark points are the axia-cyhnders, and the clear areas the . medullary substance of the nerve-flbres. The superficial neuroglia is seen to exhibit the appearance of a fine feltwork in which numerous nuclei and one or two rorpora omytoffia, c.a., are embedded, and to extend inwards (c, c) among the nerve-fibres. 358 THE ESSENTIALS OF HISTOLOGY. column, although there is no line of demarcation between them. In the upper part of the cord the posterior column is subdivided by a septum of connective tissue into two — the poster o-mesial column or funiculiis gracilis, and the posterolateral colurrm or fwniculus cvmeatus. The white matter is composed of longitudinally coursing meduUated nerve-fibres, which in sections stained with carmine or thionin appear as clear circular areas with a stained dot, the axis-cylinder, near the middle (fig. 433) ; while in sections stained by the Weigert-Pal method they appear as black circles with a clear centre. The nerve-fibres vary in size in different parts ; on the whole those which are nearest to the surface of the cord are larger than those nearest to the grey matter, but there is a bundle of very small fibres (at M, fig. 434) opposite the tip of the posterior horn. The medullated fibres are supported by neuroglia, which is com- posed of fibrillated neuroglia-cells (fig. 192, p. 161). The neuroglia is accumulated in greater amount at the surface of the cord, underneath the pia mater (particularly in the human cord, near the entrance of the posterior roots (fig. 433)), and it extends into the grey matter, in which it is especially accumulated in the substantia gelatinosa at the apex (caput) of the posterior horn and around the bentral canal. The grey matter, besides neuroglia, contains an interlacement of nerve-fibres and the arborisations of the nerve-cells which are embedded in it. Characters of the. spinal cord in the several regions (figs. 434, 439). — In the cervical region the white matter, especially that of the lateral columns, occurs in largest proportion. The grey matter in the cervical enlargement is also in considerable amount, and it encroaches, especi- ally in the upper part of the region, in the form of a network {formatio reticularis) upon the adjacent part of the lateral white column (fig. 432). The anterior horns are thick and the posterior slender. The postero- mesial column is distinctly marked off. In the dorsal region the grey matter is small in amount, and both horns are slender. The whole cord is smaller in diameter than either in the cervical or lumbar region. The columns of nerve-cells known as Clarke's column and the intermedio -lateral tract are well marked. In the Iwnibar region the crescents of grey matter are very thick, and the white substance, especially the lateral columns, relatively small in amount. The isthmus lies nearly in the centre of the corrl, whereas in the cervical and dorsal regions it is" nearer the anterior surface. In the part of the spinal cord from which the sacral and coccygeal nerve-roots take origin the grey matter largely preponderates, the THE SPINAL COED. 359 / -> Fig. 434.— Sections of human spinal cord from the lower cervical (a), mid-dorsal (b), and mid-lumbar (c) regions, showing the principal groups op nerve-cells, and on the right side of each section the conducting tracts as thet occur in the several regions. a, b, c, groups of cells of the interior horn ; d, cells of the lateral horn ; e, middle group of cells ; /, cells of Clarke's column ; g, cells of posterior horn; c, e, central canal ; a.c. anterior commissure ; m, marginal bundle of Liasauer ; p-?n, septomarginal tract. POSTERIOR ROOT BUNDLES r 360 THE ESSENTIALS OF HISTOLOGY. crescents form thick irregular masses, and the grey isthmus is also of considerable thickness. TRACTS OF NEKVE-FIBRES IN THE WHITE COLUMNS. The course of the nerve-tracts in the spinal cord, and in other parts of the central nervous system, can be made out by the method of Flechsig, which involves the study of sections of the developing cord; for it is found that the formation of medullary substance occurs sooner in some tracts than in others, so that it is easy to make out the distinction between them. Thus, the peripheral nerves and nerve- roots become myelinated in the first half of the fifth month of foetal life. Of the tracts of the spinal cord, those of Burdach and Goll (see below) are the first to be myelinated, then the tracts of Flechsig and Gowers, all of these being sensory or centfipetally conducting, while the pyramidal tracts, which are motor or centrifugally conducting, do not receive their myelin sheath until after birth. ^ Another method (that of A. Waller) consists of investigating the course which is pursued by degeneration of the nerve-fibres in consequence of lesions produced accidentally or purposely. Those tracts in which degeneration of fibres occurs below the lesion are termed " descending " tracts ; those in which it occurs above the lesion are termed "ascending." The cells whence the fibres of any tract arise can be identified after a lesion of the tract by the chromatolysis or degeneration of Nissl which nerve-cells undergo after section of their axons (see pp. 154 to 159). Tracts of the posterior column. — 1. Tract of GoW. - The fibres of the postero-mesial column belong to a tract which is known as the tract of Goll (fig. 435, 6). This consists of fibres derived from the posterior nerve-roots of the sacral, lumbar, and lower dorsal nerves, which, after having entered the postero-lateral columns, pass, as they ascend, towards the posterior median fissure and form a distinct tract, which is marked off from the rest of the posterior column in the cervical region by a slight furrow and a septum of pia mater (fig. 432). This tract ends amongst the cells of the nucleus gracilis of the medulla oblongata. 2. Tract of Burdach. — The postero-lateral column (tract of Burdach) is also composed of fibres of the posterior nerve-roots, which all run for a certain distance in it before entering the grey matter of the cord or of the medulla oblongata. As each mass of posterior root-bundles 'Flechsig finds that the fibres of the posterior roots are myelinated in at least three stages, and that the postero-lateral tract shows a corresponding differentia- tion into three chief parts: the ventral, middle and dorsal root-zones. He suggests that this difierentiation corresponds with functional dififerences of the fibres. THE SBINAL CORD. 361 enters the column close to the apex of the posterior horn it, so to speak, pushes the root-fibres which have already entered nearer to the median fissure ; hence those which are derived from the lowest nerve-roots are nearest that fissure (in the tract of Goll), while those . which are derived from the highest remain near the posterior horn (in the tract of Burdach). Many of the fibres of both tracts pass into the grey matter either immediately on entering the cord or in their course upwards ; the rest are continued into the medulla oblongata and those of the tract of Burdach end by arborising amongst the cells of the nucleus- cuneatus. 3. Comma tract. — Besides the tracts of Burdach and Goll, which are wholly composed of long " ascending " fibres having their cells of origin Fig. 435.— Diagbam showing THE ASCENDING (bIGHT SIDE) AND DESCENDING (LEFT SIDE) TBACTS IN THE SPINAL CORD. 1, Crossed pyramidal; 2, direct pyramidal ; 3, antero-lateral de- scending ; 3a, bundle of Helweg ; 4, prepyramidal ; 5, comma ; 6, postero-mesial; 7, postero-lateral; 8, tract of Liasauer ; 9, dorsal cere- bellar ; 10, antero-lateral ascend- ing or ventral cerebellar ; s-m, septo-marginal ;s.p.l.y superficial postero-lateral fibres (dorsal root zone of Flechsig) ; a to tiS, groups of cells in the anterior horn ; i, intermedio-lateral group or cell- column in the lateral part of the grey matter ; p, cells of posterior horn ; d, dorsal nucleus of Stilling or cell-column of Clarke. The fine dots indicate the situation of "endogenous" fibres (arising in grey matter of cord) having for tile most part a short course in the ganglia on the posterior roots, there are a few fibres which have a shorter "descending'' course in the posterior column. These are believed by some authorities to arise from descending branches of the posterior root-fibres, by others to arise from cells in the grey matter of the cord. They form the so-called comma tract (fig. 435, 5). Proprio-spinal or endogenous fibres of the posterior column. — There are a few fibres (septomarginal), chiefly accumulated near the median fissure (oval bundle) and near the posterior surface (median triamgle bundle), but also scattered in other parts of the column, which are derived from cells in the grey matter of the cord itself. These take a "descending" course in the posterior column; while others which arise in the grey matter and have an "ascending" course are especially numerous in the ventral part of the column. Descending tracts of the antero-lateral column.— 1. Pyramidal or corticospinal tract.— At the posterior part of the lateral column there is a tract of moderately large "descending" fibres (intermingled with 362 THE ESSENTIALS OF HISTOLOGY. CEREBELLAR HEMISPHERE Fig. 43fi.— Diagram showing the course, origin, and termination op the FIBRES of the PRINCIPAL TRACTS OF THE WHITE MATTER OF THE SPINAL CORD. (The numliers in this diagram refer to fibres of the tracts shown with corresponding numbers in fig. 435.) "Doscending" tracts,:— ia, a fibre of the crossed pyramidal tract ; lb, an uncrossed fibre of the pyramidal tract passing to the lateral column of the same side ; SB,, a 0bro of the direct pyramidal tract ; 3, a fibre of the antero-lateral descending tract ; 4, a fibre of the ijrepyramidal tract; S, fibres of the comma tract. "Ascending" tracts:— fi. a fibre of the postero-mesial tract ; 7, fibres of the postero-lateral tract ; 9, one belonging to the dorsal cerebellar; to. a fibre of ihp. ftHfiendiniT?iTitevn.in,tj>ra.i nr v^Mfttii ?*or'*bellar THE SPINAL CORD. 363 smaller fibres) which are found to run in the lateral column of the spinal cord from the opposite side of the brain, after having for the most part crossed at the decussation of the pyramids of the medulla oblongata (crossed lateral pyramidal tract, fig. 435, 1 ; fig. 436, la)- Intermingled' with the fibres of the crossed pyramidal tract in the lateral column are a few fibres of the pyramid which have not crossed in the medulla oblongata, and which are therefore derived from the cerebral cortex of the same side (uncrossed lateral pyramidal fibres, fig. 436, lb). The large fibres which lie in the anterior columns next to the anterior median fissure, which are especially numerous in the upper part of the human cord, also belong to a portion of the same tract which has not undergone decussation (direct pyramidal tract, figs. 435, 436, S). The direct pyramidal tract is only found in man and the anthropoid apes; in some individuals it is absent, and it varies considerably in extent. The pyramidal tracts are composed of "descending" fibres, which have their cells of origin in the cerebral cortex (ascending frontal and paracentral gyri) and end by arborisations in the grey matter at the base of the posterior cornua of the spinal cord. In some mammals (rat, mouse, guinea-pig, sheep, kangaroo, squirrel, etc.), the pyramidal tracts are situated in the posterior columns of the cord, in others, including the monkey, dog, cat, and rabbit, they run. in the lateral columns The pyramidal tracts are very small in the lower mammals, and are not found at all in vertebrates below mammals. It has been calculated that there are about 80,000 fibres of the pyramidal tract in each half of the human cord. The pyramidal tracts are generally regarded as the paths along which volitional impulses are conveyed from the cerebral cortex to the spinal cord. But experiments have shown that they are not the only cortico-spinal paths nor even the most important in many animals, for the paralysis which results from their section is soon recovered from in most animals, whereas that resulting from section of the anterior column and adjacent part of the lateral column may be more marked and permanent. In man it appears to be the finer and more delicate movements which are permanently lost when the pyramidal tract is affected by disease. 2. Tract of Loewenthal. — Besides the pyramidal tracts there are four other " descending " tracts of fibres in the antero-lateral column. One of these (the antero-lateral descending tract or tract of Loewenthal, figs. 435, 436, 3) lies on the side of the anterior median fissure, and extends along the margin of the cord in the "root" zone, even reaching the anterior part of the lateral column. These fibres are continued down, chiefly from the posterior longitudinal bundle (testibulo- 364 THE ESSENTIALS OF HISTOLOGY. Fig. 437.— Diagram showing the course op the tracts of Fleohsig and op gowers in the spinal cord and their oontinnations to the cere- bellum, corpora quadrioemina, thalamus and cortex cerebri. a, posteyior root-fibres ; h, tract of Flechsig, passing at 6', by the restiform body to the > cerebellar vermis ; c, tract of Gowors ; d, passage of most of its fibres alc^lig the superior peduncle to the cerebellum ; e, fibres to the corpora quadrigemina, «' ; /, others to the thalamus ; g, fibres from thalamus to cerebral cortex. THE SPJNAL CORD. 365 ^inal fibres) of the medulla oblongata and pons Varolii, partly from other sources which will be afterwards referred to. They end by arborisations in the anterior horn. Similar arborisations pass from the posterior longitudinal bundle to the nuclei of the motor cranial nerves. This tract is mainly uncrossed. 3. Rubrospinal tract. — Another "descending" tract in the antero- lateral column lies just in front of the crossed pyramidal tract. This is the prepyramidal or rubrospinal tract (figs. 435, 436, .4); its fibres end by arborising in the grey matter of the middle of the crescent; the situation of its cells of origin is the red nucleus of the tegmentum in the mid-brain. This tract is also known as Monakow's trad. Some of its fibres are stated to be derived from cells in the reticular formation of the pons and medulla oblongata. 4. Tectospinal fibres. — Intermingled with the fibres of the rubro- spinal tract (but far fewer in number in man) are fibres derived from the quadrigeminal bodies of the opposite side. These fibres form a part of the tectospinal tract. Another part of this tract passes into the anterior column of the cord in the tract of Loewenthal above mentioned. 5. Olivospinal tract. — Lastly a small triangular group of "descend- ing" f.bres traceable from the neighbourhood of the olive in the medulla oblongata, and passing down the cervical cord in the anterior part of the lateral column (fig. 435, Sa), (the exact origin and destination of the fibres is unknown) is termed the bundle of Helweg or olivospinal tract. Ascending tracts of the antero-lateral column. — 1. Tract of Flechsig. — This is a well-marked tract, which is however only distinct in the cervical and dorsal regions, where it lies external to the crossed pyramidal tract. It consists of large fibres which are derived from the. cells of Clarke's column (fig. 434,/) and which pass up into the cerebellar vermis by way of the inferior peduncle of the same side {dorsal spino-cerebellar bundle or direct cerebellar tract of Flechsig, fig. 434; figs. 435, 436, 9 ; 437, b, V). 2. Tract of Gowers. — This is situated more anteriorly, lying in front of the crossed pyramidal and direct cerebellar tracts in the lumbar region ; while in the dorsal and cervical regions it forms a narrow band of fibres curving round close to the external surface of the cord, and extending even into the anterior column. It was termed the antero-lateral ascending tract by Gowers (figs. 435, 436, 10), Its fibres are intermingled with those of the antero-lateral descend- ing tract. Most of the fibres of the tract of Gowers are con- nected with the vermis of the cerebellum, constituting the ventral 366 THE ESSENTIALS OF HISTOLOUY. spimo-cerebellaf bundle, which passes to that organ over and parallel with the superior cerebellar peduncle (fig. 437). According to Van Gehuchten, confirmed by Collier and Buzzard, the tract of Gowers gives oiF a few fibres to enter the opposite cerebellar hemisphere by the middle peduncle. Some of the fibres of the antero-lateral ascending tract (spino-teetd fihres) are continued up to the corpora quadrigemina. Others pass into the tegmentum of the crus cerebri, where they can be traced as far as the lower part of the thalamus (spino-thalamic jihres). The cells from which the fibres of Gowers' tract take origin are not certainly known, but it is probable that they are cells situated in the middle and posterior parts of the grey crescent, partly on the same but chiefly on the opposite side of the cord. The latter is almost certainly the case with the cells from which the spino-thalamic fibres arise. 3. Tract of Lissauer. — Lastly, there is another small tract of fibres which undergoes degeneration above the point of section. This is the marginal bundle of Lissauer (marked M in fig. 434). It is formed by fine fibres from the posterior roots. Other portions of the antero-lateral columns near the grey matter which are differentiated by the method of Flechsig are probably short tracts uniting adjacent portions of the grey matter of the cord. Proprio-spinal or endogenous fibres of the antero-lateral column. — Sherrington has shown that in the dog the lateral column in the dorsal region of the cord contains a certain number of long fibres which take origin in the cervical, dorsal and upper lumbar segments and are traceable down to the lumbo-sacral enlargement. These must serve to convey excito-reflex impulses from the upper to the lower parts of the body. Probably similar fibres arise all along the cord from the cells of the lateral column and pass upwards as well as downwards. A tract of endogenous fibres has been observed in man close to the anterior median fissure lying amongst the fibres of the direct pyramidal tract. This is known as the anterior sulco-marginal tract of Marie. The antero-lateral column contains also many endogenous fibres, both ascending and descending, derived from cells in the grey matter of the cord, which have only a short course, serving to connect adjacent segments. GEEY MATTER OF COED. ^^ The nerve-cells which are scattered through the grey matter are in part disposed in definite groups. Thus there are several groups of large multipolar nerve-cells in the anterior horn in the cervical and p' THE SPINAL CORD. 367 VII. VI. VIII. Fio. 4.S8. — Diagram op sections of the spinal coed of the monkey show- ing THE position OF DEGENERATED TRACTS OF NERVE-FIBRES AFTER SPECIFIC LESIONS OP THE CORD ITSEIP, THE EPPBEENT NERVE-ROOTS AND OP THE MOTOR BBGION OP THE CEREBRAL CORTEX. (The degenerations are shown by the method of Marohi. ) The left side of the cord is at the reader's left hand. I. Degenerations resulting from extirpation o£ the motor area of the cortex of the left cerebral hemisphere. II. Degenerations produced by section of the posterior longitudinal bundles in the upper part of the medulla oblongata. III. and IV. Result of section of posterior roots of the first, second, and third lumbar nerves on the right side. Section III. is from the segment of cord between the last thoracic and first lumbar roots ; section IV. from the same cord in the cervical region. V. to VIII. Degenerations resulting from (right) semi-section of the cord in the upper thoracic region. V. is taken a short distance above the level of section ; VI., higher up the cord (cervical region) ; VII., alittle below the level of section ; VIII., lumbar region. 368 THE ESSENTIALS OF HISTOLOGY Sao: I Fig. 439.— Diagram op sections of human spinal coed at dipfebbkt LEVELS. (Edinger.) The names refer to the origin of the oorrespondlng nerve roots. The relative shape and size of the'oord and grey matter, and the relative amounts of -grey and white matter, and the- principal cell-groups are shown. THE SPINAL COED lumbar enlargements (fig. 435), although in other regions of the cord the number of groups in this situation is reduced to two, a mesial and a lateraJ. The larger groups in the enlargements correspond with segments of the limb (Van Gehuchten) ; thus there . appear to be groups associated with foot, leg, and thigh, and with hand, arm, and shoulder movements respectively. The groups from which the motor nerves to the shoulder and arm muscles arise appear in somewhat b a u t ■ ■ I Fig. 440. — Diageam showing the pbobaele relations op some of tiie CELLS OF THE CORD TO THE "WHITE COLUMNS. On the left side the col- laterals from the fibres of the white columns are shown passing into the grey matter. (Cajal. ) a, 6, fibres of posterior column sending collaterals into the grey matter ; c, d, fibres of posterior root entering posterior column ; e, /, collaterals passing from lateral and anterior columns into grey matter ; g, h, i, fibres of white commissure ; 3, anterior root-fibre springing from k, cell of anterior horn ; I, m, n, other cells of greycrescent sending their axons into the white matter ; 0, axon of cell of Clatke's column passing into the dorsal cerebellar tract ; p, axon of cell of substantia gclatinosa ; g, fibre of dorsal cerebellar tract ; r, fibre of posterior root passing to tract of Lissauer ; s, t, cells of substantia gelatinosa ; w, cell of Clarke's column. higher segments of the cervical cord than those belonging to the hand muscles. The same holds good, mutatis mutandis, for the lumbar cord in relation to the leg and foot. Further, the larger groups show subdivisions which may be related to particular movements, i.e. to particular groups of muscles. In the case of the diaphragm there is a special cell-group or cell-column in the cervical cord (anterior horn) from which the fibres of the phrenic nerve arise, so that in this case a cell-group is set apart for a special muscle. The axis-cylinder processes of the anterior horn cells mostly pass out into the corresponding anterior nerve-roots (fig. 436, m ; fig. 440, j), but a few send their axons to the anterior column of the opposite side through the white commissure (fig. 440, m) or to the anterior or lateral column of the same side (I, n). It is noteworthy that in birds a few 2a 370 THE ESSENTIALS OF HISTOLOGY. cells of the anterior horn send their axons into the posterior roots. A. well-marked group of large rounded nerve-cells, best marked in the thoracic region, lies at the base of the posterior horn {nucl&us of Stilling, Clarke's column, fig. 434, /; fig. 435, d; fig. 440, m). The cells of Clarke's column send their axis-cylinder . processes into the dorsal cerebellar tract (Mott), and if this tract be cut experimentally, the Fig. 441. — From a longitudinal sec- tion OF spinal cobd, showing the ENTRANCE OE POSTERIOR BOOT-PIBEBS. (Cajal.) A, A, fibres entering the postero-lateral . column, and bifurcating into an ascending and descending division ; B, C, collaterals passing from them into the grey matter ; B, other fibres of the posterior white columns also giving off collaterals. Xk Fig. 442. — Arborisation of col- laterals FROM THE POSTERIOR ROOT- FIBRES AROUND CELLS IN THE POS- TERIOR HORN OP GEST MATTER. (Cajal.) A, fibres of posterior column derived from posterior root ; B, collaterals ; C, D, nerve- cells in grey matter surrounded by the arborisations of the collaterals; E, an arborisation shown separately. large cells of Clarke's column on the same side below the section undergo Nissl degeneration and eventually atrophy. There are, how- ever, a few small cells with short axons in Clarke's column which da not undergo this change. THE SPINAL COEi). 37 i Another group is seen on the outer side of the grey matter lying in a projection which is sometimes known as the lateral horn {lateral cell-column, intermedia-lateral column, figs. 434, d ; 435, i). This is most distinct in the dorsal region (as far up as the second thoracic segment). The axons from its cells for the most part leave the cord along with the anterior roots, and probably furnish the outgoing visceral and vascular fibres. Another group {middle cell-column) lies in the middle of the crescent (fig. 434, e). The cells of the posterior horn (g) are very numerous but are not collected into definite" gr.oups. Those of the substantia gelatinosa of Rolando send their nerve-fibre processes partly into the lateral, partly into the adjacent posterior columns (fig. 440, s, t). The cells which send their axons into the adjacent parts of the white columns but not into any special tract are sometimes termed the " cells of the white columns." Connection of nerve-roots with spinal cord. — The anterior- roots leave the anterior horn in a number of bundles. Most of their fibres are directly continued from the nerve-cells in the anterior and lateral horns, and according to Golgi in part also from cells in the posterior horn. These cells, from which the anterior root-fibres arise, are surrounded by an interlacement of ramified nerve-endings, which are derived from various sources, especially the axons of cells of the posterior horn, from collaterals of the posterior root-fibres (see below), and from those of the fibres of the adjacent white columns. The fibres of the posterior roots originate in the cells of the posterior root ganglia and pass into the posterolateral column (see diagram, fig. 436), but the smallest fibres enter the marginal bundle of Lissauer, and some pass directly into the posterior horn of grey matter. On entering the spinal cord the fibres bifurcate (fig. 441), one branch passing upwards, the other down- wards. Both from the main fibre and from its branches collateral fibres pass at "Sequent intervals into the grey matter, and end in arborisations of fibrils which ^''^i^-'^^Z the™ Zl envelop the nerve-cells both of the cord of a child, showing 5 1 /. 1 • 1 in "S CILIATED EPITHELIUM AND posterior and of the anterior horn (fag. the subkounding central 442) and in the dorsal region the cells of S°f "^" ^^'"^''^'"^^ "''^^ Clarke's column and those of the inter- medio-lateral tract. Many of the main fibres also ultimately end,, in a similar manner in the grey matter, some after a short course only, but 372 THE ESSENTIALS OF HISTOLOGY. Fig. 444. — Pakt of epithelium of central canal of new-bohn child, STAINED BT GOLQi'S METHOD. (Sobotta.) X 120. ep, epithelium ; ng, neuroglia cells in adjacent grey matter. Fig. 445.— Section op coed of embbto, showing some of the ependtma cells DETACHED AND BECOMING CONVERTED INTO NEUHOGLIA-CELLS. (L^nhosaek. ) THE SPINAL COED. 373 others after a longer course. But a considerable number of fibres pass upwards in the postero-lateral and postero-mesial columns (in the latter especially those of the lower spinal nerves), until they arrive at the medulla oblongata, where they end in terminal arborisations around the cells of the nucleus gracilis and nucleus cuneatus. The central canal of the spinal cord is lined by columnar ciliated epithelium-cells (ependyma), which are surrounded by a quantity of neuroglia. The cells are best seen in the spinal cord of animals and in the child (figs. 443, 444); in the human adult they have frequently become proliferated, and their cilia are no longer visible. In the early embryo their fixed extremities extend through the whole thickness of the cord to reach the pia mater. This condition is permanent in the cord of many of the lower vertebrata. Blood-vessels of the spinal cord.— The blood-supply of the grey matter is derived mainly from a series of arterioles, which come off from the mesially- situated anterior spinal arterj', pass into the anterior median fissure, and at the bottom of this divide each into two branches, one for the grey matter of each lateral half of the cord. In the grey matter is a very close capillary plexus which is supplied not alone by the vessels just mentioned, but also by small arterioles, which converge from the small arteries of the pia mater, passiiig through the white matter, and supplying this as they pass through it. These arterioles are branches of the .above- mentioned anterior spinal artery and of the posterior spinal arteries (which run on each side along the line of the posterior roots). The capillary plexus (if the white matter is far less dense than that of the grey matter. It forms longitudinal meshes. The veins of the spinal cord accompany the arteries. Two longitudinal venous vessels, accompanying corresponding anastomotic arterioles, are seen, one on either side of the central canal, in most transverse sections of the cord. 374 THE ESSENTIALS OF HISTOLOGY. LESSON XLI. THE MEDULLA OBLONGATA. Sections of the medulla oblongata (made in the same way as with the spinal eord) : {a) at the level of the decussation of the pyramids, (5) just above the decussation, (c) opposite the middle of the olivary body, and, {d) either through the uppermost part of the olivary body, or just above it. The brain consists of three great morphological divisions associated with the three primary cerebral vesicles of the embryo ; they are termed respectively the hind-brain, mid-brain, and fore-brain. The hind-brain is formed of the parts around the fourth ventricle, viz., the medulla oblongata or spinal bulb (myelencephalon), and above this the pons Varolii with the cerebellum (metencephalon) ; the region of the corpora quadrigemina forms the mid-brain (mesencephalon); the parts immediately above that region, and centring around the third ventricle, including the optic thalami, form the thalamencephalon ; and the corpora striata and cerebral hemispheres constitute the telen- cephalon. The structure of the medulla oblongata or spinal bulb can best be made out by the study of a series of sections taken from below upwards, and by tracing in these the changes which occur in the constituent parts of the spinal cord, taking note at the same time of any parts which may be superadded. A section through the region of the decussation of the pyramids (fig. 446) has much the same form as a section through the upper part of the spinal cord, and most of the structures of the cord can be easily recognised. A considerable alteration of the grey matter is, however, produced by the passage of the large bundles of the crossed pyramidal tract from the lateral column of the spinal cord on each side through the root of the anterior horn and across the anterior median fissure to the opposite anterior column of the medulla oblongata, where, together with the fibres of the direct pyramidal tract, they constitute the prominent mass of white fibres which is seen on the front of the bulb, on each side of the middle line, and which is known as the pyramid. By this passage of fibres through the grey matter the tip of the anterior horn is cut ofi" from THE MEDULLA OBLONGATA. 375 the rest and becomes pushed as it were to the side ; part of it appears as an isolated mass or masses of grey matter, one of which becomes known as the lateral nuclms. In sections just above the decussation of the pyramids a wavy mass of grey matter makes its funiculus gracilis post, mediau fissure fuuiculus cuneatus nucleus gracilis desc. Vtli l)undle from fun, cun. subst. gelat. Rol. tract of Flechsig. pyramidal tract bundles decussation of pyramids anterior horn- ant. median fissure Fig. 446.— Section across the lower pabt op the mbdulIa oblongata in THE REGION Oi' THE DECUSSATION OF THE PYRAMIDS. (Magnified 6J diameters.) appearance on the lateral aspect of 'each pyramid, corresponding with a prominence on the surface which is known as the olive. The wavy or plicated grey matter is termed the olivary micleus (figs. 447 to 449). The pyramids (anterior pyramids) of the medulla oblongata are formed of fibres which originate in the motor region of the cerebral cortex, and which can be traced from the axons of large cells in the grey matter of that cortex through the white matter of the hemisphere, through the middle third or more of the internal capsule and crusta, through the pyramid bundles of the pons Varolii and into these Structures (pyramids) of the bulb. As we have just seen, they pass at the lower limit of the bulb chiefly to the opposite or crossed, 376 THE ESSENTIALS OF HISTOLOGY. lateral column of the cord, but partly to the lateral column of the same side, and, in man and anthropoid apes, partly to the anterior column. They collectively constitute the. pyramidal tract, which is smaller in the medulla oblongata than in the pons Varolii, since many of its fibres have left the main tract whilst within the pons and have passed across the middle line towards the grey matter on post, median fissure nucleus gracilis' funiculus cuneatusi nucleus cuneatus*- deac. root of 5th' central canal, substantia Kolandi' central fibres of Vthi int. arcuate fibresi tract of Flechsig" tract of Gowersi raphe) accessory oliv. nucl. ailiqua olivaji olivary nucleus' pyramid- arcuate nucleus- Fig. 447.— Section taken immediately above the ueodbsation of the FYEAMIDS. (Magnified 6| diameters.) ant. long. {^ vy bundle . ■']■' the dorsal aspect of the pons and meduUa oblongata. Sometimes such a bundle of fibres, after passing towards the sensory nuclei in the lateral part of the medulla oblongata, does not end in them, but again comes ventral-wards and joins the main or central part of the tract near its decussation {bwndle of Pick). It is not a little remarkable that although the fibres of the pyramidal tract give oflF numerous collaterals to the grey matter of the cerebral cortex, the basal ganglia of the cerebrum, the substantia nigra of the mid-brain, the nuclei pontis of the pons Varolii, and the base of the posterior horn of the spinal cord, no collaterals are seen to leave them in their course through the medulla' oblongata, except a very few to the olivary nuclei. Various observers have described collaterals and terminations of the pyramidal fibres THE MEDULLA OBLONGATA. 317 as passing to the motor nuclei of the cranial nerves as well as to the anterior horns of the spinal cord, but statements to this effect must be received with caution for although current in most text-books, they have not been suId- stantiated by accurate observations. It is certain that most if not all of the fibres of the pyramidal tract end not in the ventral but in the dorsal part of the grey matter of the cord. A change also occurs in the posterior horn in consequence of the increased development of the posterior column of white matter. This causes the posterior horns to be pushed towards the side, the V which they form with one a,nother being thus opened out ; at the same time the tip of the horn swells out and causes a prominence upon the surface of the medulla oblongata, which is known as the tubercle of Bolando. , Its grey matter forms the prolongation of the sensory nucleus of the fifth nerve. On its outer side and partly embracing it is a bundle of fibres seen in every section of the medulla oblongata, and traceable up to the pons Varolii. This is the inferior or descending root of the fifth nerve — formerly known as the "ascending" root. Its fibres extend down as far as the upper cervical region of the spinal cord. Grey matter also soon becomes formed within the upward prolongations of the gracile funiculus (postero-mesial column), and of the cuneate funiculus (postero-lateral column) appearing at first as thin strands in the middle of the columns, but rapidly increasing in thickness so as eventually to occupy almost the whole of them, and forming the nucleus gracilis and the nucleus cuneatus respectively. It is in these nuclei that the fibres of Goll's and Burdach's tracts, which are continued up from the posterior columns of the spinal cord, find their ultimate ending in complicated arborisations amongst the cells of the nuclei. These nuclei do not, however, receive all the ascending branches of the posterior root fibres, for a considerable number of these have already disappeared by entering the grey matter of the cord, in which they also end by arborisation amongst its cells. The cells of the nucleus gracilis and nucleus cuneatus are small or of moderate size with long dendrons. Their axons pass as internal arcuate fibres through the reticular formation into the inter-olivary layer, cross the median raphe dorsal to the pyramids (fig. 447), and then turn upwards, constituting the tract of the fillet. This tract, which in its lowest part is thus formed by the nerve-fibres which belong to the second relay (or second neurones) of one of the sensory spinal paths, is reinforced in the higher regions of the medulla oblongata and in the pons by fibres derived from cells of the sensory nuclei of the cranial nerves. The majority of its fibres end in the lateral nucleus of the thalamus, but some pass to both the anterior and posterior corpora quadrigemina. 378 THE ESSENTIALS OF HISTOLOGY. According to Van Gehuchten the fibres of the fillet which are derived from the nucleus cuneatus lie dorsally to those which are derived from the nucleus gracilis. The continuation of the central canal of the spinal cord is still seen in the lower medulla oblongata (figs. 446, 447), but it comes nearer to the posterior surface and eventually opens out at the point of the calamus scriptorius of the 4th ventricle (fig. 448). The grey matter which nucleus gracilis, funiculus cuneatus' nucleus cuneatus^ fasciculus Bolitarius, dorsal nucleus of Xth desC. root of Vtk.. nucleus of Xllth. subst. gelat, Kolandl tract of Flechsig. int. arcuate fibres. rubro-spinal tract^ \ issuing fibres of Xllth, tract of Gowers slliqua olivse, olivary nucleus, ext. arcuate fibres. pyramid arcuate nucleus Fig. 448.— Section aceoss the mkdulla oblongata at the point op the CALAMUS SCRIPTORIUS OF THE 4th VENTRICLE. (Magnified 6^ diameters.) surrounds it contains two well-marked groups of nerve-cells; the anterior (ventral) of these is the lower part of the nucleus of the hypoglossal or twelfth nerve, the posterior (dorsal), with smaller cells, that of the vago-accessory or tenth and eleventh. But most of the grey matter of the crescent becomes broken up, by the passage of bundles of nerve-fibres through it, into a reticular formation the production of which is already foreshadowed in the upper part of the spinal cord. Instead of the comparatively narrow isthmus which joins the two halves of the spinal cord, a broad raphe now makes its appearance j THE MEDULLA OBLONGATA. 379 this is formed of obliquely and antero-posteriorly coursing fibres, together with some grey matter containing nerve-cells. In the section at about the middle of the olive (fig. 449), it will be seen that a marked change has been produced in the form of the medulla oblongata and the arrangement of its grey matter, by the opening out of the central canal into the fourth ventricle. This causes the grey vestibular nucleus ^ / I NUCLEUS ^fe- f- 4Bt ) acuLO-\ -^ /'v Tf .-■' M0T0RIU3--.J.-- --V ■■■ 1 i / \ 1 s f auAORICEMlhUM T y conpus' GENICULATUM EXTEHMUfl > OPTIC -^^^v^ ^Cy^ TRACT ^S^v y^V^ h y v\ K I'IG. 471. — DiAGKAM TO SHOW THE PKOBABLE COURSE AND BBLATIONS OP THE OPTIC FIBRES.l composed of white matter externally and grey matter internally. It receives fibres from the anterior pillar of the fornix of the same side j these fibres arise from cells in the hippocampus and end in the mammil- lary body. According to Edinger some fibres from the olfactory tract pass directly to it. The axons of its cells bifurcate, one branch, the coarser, passing into the anterior and upper part of the thalamus in ^ Only single fibres are shown emerging from the anterior quadrigeminal and external geniculate bodies, conijnuing the course of th6 two fibres from correspond- ing points in the retinae. This is merely tt/ simplify the diagram and is not intended to imply that- the retinal impressions are fused in those situations. THE THALAMENCEPHALON. 415 the bundle of Vicq d'Azyr, and the other into the tegmentum of the mid-brain in v. Gudden's bundle. The corpora mammillaria form part of the central olfactory apparatus (fig. 496). Subthalamic region. — The tegmentum of the crus cerebri is pro- longed below the thalamus opticus, and between it and the internal Fig. 472.— Section taken obliquely thkough the optic thalamds and internal capsule showins some of the steands of fibres op the SCBTHALAMUS. (Magnified 2^ diameters.) Til., thalamus; v.m., third ventricle; t,, taenia, or attachment of epithelial roof of ventricle; str., stria meduUaris or habenula ; g', ganglion of the babeniila ; n.t., mesial nucleus of thalamus ; opt., optic fibres passing into pulvinnr of thalamus ; zL, zona incerta, from which fibres are seen emerging and sweeping as the ansa lentieularis, a.l., round the internal capsule, ifi^,, to pasft^,towaifdB the 4gDticuIar nucleus; cs., corpus subthalamicum ; /., anterior ]pillar of fornix passing ba'CEwards to corpus mammillare ; V.A., bundle of Vicq d'Azyr, passing itpwards and forwards from corpus mammillare into thalymus ; g, group of nerve cells, probably belonging to the nucleus of the corpus mammillare ; x, fasciculus relrofiexus. capsule, into a mass of grey substance, with longitudinally and obliquely crossing white bundles, which is known under the name of subthalamus (fig. 472). Its deepest part contains a lens shaped mass of grey matter prolonged forwards from the substantia nigra, known as the cmpus subthalamicum (Luys). A mass of fibres sweeps round this 416 THE ESSENTIALS OF HISTOLOGY. and round the internal capsule passing between the thalamus and the nucleus lenticularis, this is known as the ansa lenticularis. The pineal gland or epiphysis cerebri (fig. 461), which is developed in the roof of the third ventricle, but passes backward between the anterior corpora quadrigemina, is composed of a number of tubes and saccules lined and sometimes almost filled with epithelium, and con- taining deposits of earthy salts (brain sand). (Similar deposits may also occur in other parts of the brain, especially in the pia^-mater.) The follicles are separated from one another by vascular connective tissue derived from the pia-tnater, and along with the vessels are numerous nerve-fibres of sympathetic type (Cajal). No true nerve- cells can be seen, although there are a number of cells similar in general appearance to the "granules" of the cerebellum, but apparently without axons. In some animals (ox) striated muscular fibres have been met with. In the chameleon and some other reptiles, the pineal is better developed, and is connected by nerve-fibres with a rudimentary median eye of inverte- brate type, placed upon the upper sarfaoe of the head. The pituitary body or gland (hypophysis cerebri) is connected with the third ventricle by the infundibulum. It consists of two lobes, a large anterior and a smaller posterior. The structure of the pituitary body has already been described (p. 224). THE CEREBELLUM. 417 LESSONS XLIV. and XLV. STRUCTURE OF THE CEREBELLUM AND CEREBRUM. 1. Sections of the cerebellum vertical to the surface, {a) across the direction of the laminse, (6) parallel with the laminae. 2. Sections across the whole of one hemisphere of the cerebrum of a monkey passing through the third ventricle. 3. Vertical sections of the cerebral cortex : — one across the ascending frontal and ascending parietal gyri, another from the occipital lobe (calcarine region), another across the superior temporal gyrus and island of Reil, and one across the hippocampal gyrus and hippocampus. 4. Transverse sections of the olfactory tract and bulb. In all these preparations make sketches under a low power of the general arrangement of the grey and white matter, and also of the nerve-cells in the grey matter. Sketch some of the details under a high power. The preparations are made in the same way as those of the spinal cord. Other preparations should be made by the Golgi method to exhibit the relation of the cells to one another. Such preparations have been already partly studied (Lessons XVII. and XVIII). The Cerebellum. The cerebellum is composed of a white centre, and of a grey cortex. Both extend into all the folds or laminse, so that when the laminse are cut across, an appearance is presented of a white arborescence covered superficially by grey matter. The white matter is in largest amount in the middle of each cerebellar hemisphere. There is here present a peculiar wavy lamina of grey matter, similar to that in the olivary body, and known as the nucleus dentatus (fig. 473, n.d.). This receives numerous nei-ve-fibres from the cells of Purkinje of the cortex, which end by arborising around its cells. The latter give off axons which become the fibres of the superior cerebellar peduncles, and for the most part end in the opposite red nucleus, but some pass beyond this into the subthalamic region. The dentate nucleus also receives collaterals from fibres of the inferior peduncle (Cajal). Other isolated grey nuclei lie in the white matter of the middle lobe over the roof of the 4th ventricle and constitute collectively the nuclei of Stilling. The most important of these appears to be the nucleus tecti (s.fastigii) (fig. 473). This receives many of the ascending fibres of the vestibular nerve (p. 385) and collaterals from the spino-cerebellar tracts, and gives origin to a bundle of fibres which crosses to the opposite side 2d 418 THE ESSENTIALS OF HISTOLOGY. and descends in the mesial part of the restiform body to the reticular formation of the medulla oblongata (Risien Russell). The grey matter of the cerebellum appears essentially of similar structure throughout the whole extent of the cortex. It consists of two layers. The inner one (that next to the white centre) is composed of a large number of very small nerve-cells intermingled with a few larger ones and some neuroglia-cells {granule layer, fig. 474, d). The outer one is thicker, and is formed chiefly of fine nerve-fibres (fig. 476, A) Tic. emboLvRrhm^s n-.aZobpsus Choroid' jjZex-us Fig. 473. — Section across the ceeebbllum and medulla oblongata SHOWING the position Or THE NDOLEI IN THE WHITE CBNTEB OP THE CEREBELLUM. (Stilling.) n.d.j nucleus dentatus cerebelli ; s.c.-p., fibres of superior peduncle; com^ c, ,j amacrine cell with diffuse arborisation of its processes in inner molecular layer ;^ i, j, )tt,,nerve-fibrilspa8Sing respectively to outer molecular, inner nuclear, and inner molecular layers ; n, ganglionic cells, with axons passing into nerve-fibre layer. THE EETINA. 457 a fine axis-cylinder process prolonged into a fibre of the layer just noticed, and a thick branching process, the ramifications of which terminate in the next layer in flattened arborisations at different levels (fig. 515, A, B, c). The inner synapse layer or inner molecular layer is comparatively thick, and has an appearance very like parts of the grey matter of the nerve- FiG. 514. — Section thboogh the inner latees of the eetina op a bikd, PREPAKBD BT GOLGl'S METHOD. (Cajal.) A, nerve-fibres of optic nerve layer ; B, sonae of these fibres passing through the inner molecular layer to end in an arborisation at the junction of the inner molecular and inner nuclear layers. The layers in this and in the two succeeding cuts are numbered in correspondence vrith the layers in fig. 511. centres. A few nuclei are scattered through it, and it is occupied by the processes of the nerve-ceslls and of the inner granules (bipolars and amacrine cells) which form synapses in it ; it is also traversed by the centrifugal fibres from the optic nerve layer, as well as by the fibres of Miiller. Fig. 515. — Section across the molecular and ganglionic layers of bird's RETINA, PREPARED BT GOLGl's METHOD. (Cajal.) Three or four ganglionic cells, A, B, C, and the terminal arborisations of their dSndrons, a, 6, c, in the molecular layer, are shown. The inner granule layer (also termed irmer nuclear layer) is mainly composed of bipolar nerve-cells containing large nuclei. A process (the axon) of each of these cells (fig. 513) extends inwards into the inner molecular layer where it spreads out into a terminal arbori- sation. These arborisations occur at diflferent levels in the layer, forming synapses with the optic nerve-cells. Another process (dendron) is directed outwards, and arborises in the outer molecular 458 THE ESSENTIALS OF HISTOLOGY. layer, where it forms synapses with the terminations of the rod- and cone-fibres. It has been shown by Eam6n y Cajal that there are two kinds of bipolars, one kind (rod-Upolars, fig. 513, c.d) being connected externally with the rods of the retina, and passing inwards to ramify over the bodies of the nerve-cells, whereas those of the other kind (cone-bipolars, e) are connected with the cone-fibres, and ramify in the middle of the inner molecular layer. The outwardly - directed pro- cesses of these cone-bipolars are, in some animals, but not in mammals. Fig. 516. — Section of bird's bbtina, prepared by golgi's method. (Cajal.) A, large nerve-^sell of inner nuclear layer ; B, C, amacrine cells ; I), small bipolar nerve- cells with one process, ramifying in tlie inner molecular layer and the other one. ramifying in the outer molecular layer, and extending (E) as far as the rods and cones as a fibre of Landolt ; F, G, rod- and cone-nuclei respectively; H, I, cells with dendrons ramifying in outer molecular layer ; J, fibre of Mtiller. continued on as far as the external limiting membrane, where each ends in a free extremity (fibre of Landolt, fig. 516, e). Besides these bipolar nerve-cells, there are other larger inner granules (spongioblasts of some authors) which are different in character, having ramified processes which extend into the inner molecular layer (figs. 513, h; 516, A, B, c), in which the bodies of these cells are often partly embedded. The cells in question have been regarded as of the nature of neuroglia-cells, but according to Cajal they are probably all nerve- cells. He has termed them amacrine-c'ells, from the fact that they are destitute of a long process ; but some have been noticed to give off, besides the branching processes or dendrons, which ramify in the molecular layer, an axis-cylinder process which may extend into the THE RETINA. 459 nerve-fibre layer. There are also some cells in the outer part of the granule layer which send their processes entirely into the outer molecular layer (fig. 516, h). These are the horizontal-cells of Eam6n y Cajal (termed spongioblasts of outer molecular layer by some authors). The fibres of Muller have nucleated enlargements (fig. 516, j) in the inner nuclear layer. The outer molecular layer is thin, and is composed mainly of the arborisations of the inner granules, of the rod and cone-fibres, and of the horizontal cells (figs. 513, 516), which all form synapses in this layer. The outer mtclear layer and the layer of rods and cones are composed of elements which are continuous through the two layers, and they should pro- perly, therefore, be described as one. It has been termed the sensory epi- thelium of the retina (fig. 517, 6 and 7). The elements of which this nerve- epithelium consists are elongated nerve-cells of two kinds. The most numerous, which may be termed the rod-dements, consist of peculiar rod-like structures (retinal rods) set closely side by side, each of which is prolonged internally into a fine varicose fibre (rod-fibre) which swells out at one part of its course into a nucleated enlarge- ment, and ultimately ends (in mam- mals) in a minute knob within the outer molecular layer, where it is em- bedded in the ramifications of the dendrons of the rod-bipolars. The rod consists of two segments, an outer cylindrical and trans- versely striated segment, which during life has a purplish-red colour if the eye has not been recently exposed to light, and an inner slightly bulged segment which in part of its , length is longitudinally striated. The nucleus of the rod-element in some animals, but accord- ing to Flemming not in man, has a transversely shaded aspect in the fresh condition (fig. 517). The cone-elements are formed of a conical tapering external part, the retinal cone, which is directly prolonged into a nucleated enlargement, from the farther side of which the cmhe-fihre, considerably thicker (in mammals) than the rod-fibre, passes inwards, to terminate by an expanded arborisation in the outer mole- FlG. 517. — DiAGKAMMATIO EKPBE- 8ENTATI0N OF THE BOD A^D CONE ELEMENTS OF THE RETINA. (After Sohwalbe. ) The designation of the numbers is the same as in fig. 511. 460 THE ESSENTIALS OF HISTOLOGY. cular layer ; here it comes into relation with a similar arborisation ■of dendrons of a cone-bipolar. The cone, like the rod, is formed of two segments, the outer of which, much the smaller, is transversely striated ; the inner, bulged segment being longitudinally striated. Fig. 519. — Pigmented epitheliom op the HUMAN RETINA. (M. Sohultze.) (Highly magnified.) u., cells seen from the outer surface with clear lines of intercellular substance between; 6, two cells seen in profile with fine offsets extending inwards ; c, a cell still in connection with the outer ends of the rods. Fig. 518. — Diagram of the conneotions of the retinal elements with ONE another and WITH THE CENTRAL NERVOUS SYSTEM. (Cajal.) a to g, layers of retina ; a, rods and cones ; 6, outer nuclear layer ; c, outer molecular , . layer ; d. Inner nuclear layer ; e, inner molecular layer ; /, nerve-cells giving origin to fibres of optic nerve ; g, A, i, a centrifugally conducting fibre, with a terminal .' arborescence in the retina ; j, grey matter of corpus geniculatum or corpus quadri- ■ gcminum. The inner ends of the rod- and cone-fibres, as already stated, form synapses with the peripheral arborisations of the bipolars, and through the latter elements and their synapses in the inner molecular layer a connection is brought about with the nerve-cells and nerve-fibres of the innermost layers. The connection of the retinal elements with one another and through the optic fibres with the central nervous THE RETINA. 461 'W system (anterior corpora quadrigemina and lateral geniculate bodies) is shown. diagrammatically in fig. 518. In birds, reptiles, and amphibia, a small oil-globule, often brightly coloured red, yellow, or green, is found in the inner segment of each cone. Other variations of structure are met with in different animals. The cones are most numerous at the back of the retina; they are fewer in number, and the rods are proportionally more numerous towards the anterior part. A B Fig. 520. — A. Part of a section of the betina pkom the ete op a pbog WHICH HAD BEEN KEPT IN THE DAEK FOE SOME HOUES BEFOEE DEATH, (v. Genderen-Stort. ) The pigment is collected towards the outer ends of the rods, which were red, except the outer detached rod, which was green. The cones, which in the frog are much smaller than the rods, are mostly elongated. B. A SIMILAR SECTION PBOM A FROG WHICH HAD BEEN EXPOSED TO LIGHT. The pigment is extended between the rdds, and is accumulated near their bases. The ' rods were colourless. All the cones are contracted. The pigmentary layer forms the most external part of the retina. It consists of hexagonal epithelium-cells (fig. 519), which are smooth externally where they rest against the choroid, but are prolonged internally into fine filaments which extend between the rods. The pigment-granules, many of which are in the form of minute crystals, lie in the inner part of the cell, and after prolonged exposure to light they are found extending along the cell-processes between the rods (Kiihne), their function being probably connected with the restoration of the purple colouring matter which has been bleached by the light. This extension of the pigment is accompanied by a shortening of the cones (Engelmann) (fig. 520). 462 THE ESSENTIALS OF HISTOLOGY. Fibres of Mm&r.—Th.e fibres of Miiller (fig. 516, J, and -fig. 521) are long stiff cells which pass through several of the retinal layers. Commencing at the inner surface of the retina by expanded bases which unite with one another to form the so-called internal limiting membrane (fig. 522), they pass through all the layers in succession, until they reach the outer granule layer. Here they branch and m.l.e. m.l.i. Fig. 521. Fig. 562. Fig. 521. — A fibre of muller froji the dog's RETINA, GOLGI METHOD. (Cajal.) 1, nerve-fibre layer ; 2, nerve-cell layer ; 3, inner molecular layer ; 4, inner granule lay er ; 5, outer molecular layer ; 6^ outer granule layer ; 6, nucleus of the fibre ; a, a proceed extending into inner molecular layer; ni.Z.i., membrana limitans interna; tn.l.e., membrana limltana externa. Fig. 522. — Internal limiting membrane of retina treated with silver nitrate, showing the odt- lines op the bases of the fibres of muller. (G. Retzius.) expand into a sort of honeycomb tissue which serves to support the fibres and nuclei of the rod- and cone-elements. At the bases of the rods and cones, this sustentacular tissue ceases, being here bounded by a distinct margin which has been called the external limiting membrane (fig. 521, m.l.e.), but delicate sheaths pass from it around the bases of the rods and cones. Each Miillerian fibre, as it passes through the inner granule layer, has a nucleated enlargement (S), indicating the cell-nature of the fibre. The fibres of Muller represent ependyma THE RETINA. 463 cells or periaps long neuroglia-cells such as are found in some parts of the nerve-centres, e.g. the cerebellum (see fig. 479, gV). There are two parts of the retina which call for special description. The macula lutea (yellow spot), with its central fovea, is the part of the retina which is immediately concerned in direct vision. It is characterised firstly by its greater thickness (except at the middle of the fovea), secondly by the large number of its ganglion-cells, which are rounded or conical, and thirdly by the large number of cones ^^..- ..,.:>U\\\\\\U\\! 1 liHlUilWli* Fig. 523. — Section theoush the central paet of the fovea oentkalis. J'^. (From a preparation by C. H. Gelding-Bird.) Mf bases of Mtillerian fibres ; c.b., nuclei of Inner granules (bipolars) ; c.«., cone-fibre nuclei ; c, cones. it contains as compared with the rods. In the central fovea itself (fig. 523) there are no rods, and the cones are very long and slender, measuring not more than 2/i in diameter ; all the other layers become gradually thinned down almost to complete disappearance, so that the middle of the central fovea is the thinnest part of the retina. Since there are few rods, the outer granule layer loses in great measure its appearance of being composed of closely packed nuclei, and the cone-fibres are very distinct, forming the so-called fibrous layer. The direction of these fibres is for the most part very oblique in this part of the retina. 464 THE ESSENTIALS OF HISTOLOGY. , Fig. 524. — A small poktion of the ciliary PART OF THE RETINA. (Kdlliker.) 350 diameters. 1, pigment-cells ; 2, columnar-cella. Fig. 525. — Section through the mar- gin OP THE rabbit's LENS, SHOWING THE TRANSITION OF THE EPITHELIUM OF THE CAPSULE INTO THE LENS- FIBRES. (Babucliin. ) FiG. .526. — Fibres of the ortstallinb LENS. (350 diameters,) A, longitudinal view of the fibres of the lens from the ox, showing the serrated edges. B, trans- verse section of the fibres of the lens from the human eye. C, longitudinal view of a few of the fibres from the equatorial region of the human lens. Most of the fibres in G are seen edgewise, and, towards 1, present the swell- ings and nuclei of the 'nuclear zone' ; at 2, the flattened sides of two fibres are seen. A and Tt fvnm Kflllilfftr ; c from Henle.1 THE LENS. 465 The pigmentary layer is thickened over the fovea, and there is also a thickening in the choroid coat here, due to a large accumulation of capillary vessels. The pars ciliaris retinse, which commences at the ora s&rrata, where the retina proper abruptly ends, is composed of two epithelial layers (fig. 524), and has no nervous structures. Of the two layers, the external is a thick stratum of pigmented epithelium formed of rounded cells and continuous with the pigmentary layer of the retina on the one hand, and with the uvea of the iris on the other ; the inner is a layer of columnar cells, each containing an oval nucleus. They probably represent the Mullerian fibres of the retina. Fig. 527.— Cells ob' vitebous. (Sohwalbe.) a, d, without vacuoles ; b, e, e, /, g, witli vacuoles. The retina contains but few blood-vessels. The central artery enters and the vein leaves it in the middle of the optic nerve. The larger vessels ramify in the nerve-fibre layer, and there are capillary net- works in this layer and in the inner nuclear layer. There are peri- vascular (lymph) spaces around the veins and capillaries. The sensory epithelium receives no blood-vessels, but is nourished from the vessels of the choroid. The lens. — The lens is a laminated fibrous body inclosed by a trans- parent elastic capsule to which, around the circumference, the fibres of the suspensory ligament are attached (fig. 508). Immediately within the capsule, in front and at the sides, there is a layer of cubical epithelium termed the epithelium of the capsule, but at the margin of the lens the cells become longer and pass by a gradual transition into the lens-fibres (fig. 525). The fibres which compose the lens are long and riband-shaped, with finely serrated edges (fig. 526, A); in transverse section they appear prismatic (b). Many of the superficial fibres are 2g 466 THE ESSENTIALS OF HISTOLOGY, nucleated (c), the lens-fibres having originally been developed by the elongation of epithelium-cells. The vitreous humour. — This is composed of soft gelatinous tissue, apparently structureless when examined in the fresh condition, but containing fibres and a few scattered cells, the "processes of which are often long and varicose, and the cell-bodies distended by large vacuoles (fig. 527). The hyaloid memhrane, which invests the vitreous humour, is homogeneous and structureless except in the region of the ciliary processes, where it is fibrous in structure, forming the zonule of Zinn and spreading out into the suspensory ligament of the lens (fig. 508). This part of the hyaloid membrane is connected with a circular fibrous portion of the vitreous humour which serves to give addi- tional firmness to the attachment of the fibres of the suspensory ligament of the lens (Anderson Stuart). OLFACTORY MEMBRANE. 467 LESSON XLIX. STRUCTURE OF THE OLFACTORY MUCOUS MEMBRANE AND OF THE EXTERNAL AND MIDDLE EAR. 1. Vektical sections of the nasal mucous membrane. The sections may be carried either across the upper turbinate bone, after decalcification or across the upper part of the nasal septum. Make a sketch under the low power. Notice the difference in the character of the epithelium in the olfactory and respiratory- parts of the membrane. 2. Teased preparation of the epithelium of the olfactory mucous membrane. A piece of the membrane is placed quite fresh in osmic acid (1 per cent.) for a few hours, and is then macerated for two days or more in water. The epithelium is broken up in dilute glycerine ; the cells easily separate from one another on tapping the cover-glass. Notice the two kinds of cells. Sketch some of the cells under a high power.' 3. Sections of the external ear (these have been already studied for the cartilage, Lesson XII.). 4. Sections across the cartilaginous part of the Eustachian tube. Sketch under the low power. 5. Preparation of the membrana tympani. A piece of the membrane, stained with magenta and gentian violet (see Lesson IX., § 2), is mounted flat in xylol balsam or dammar. Determine the composition of the membrane — i.e. the several layers com- posing it — by focussing carefully with the high power. THE OLFACTORY MUOOUS MEMBRANE. The olfactory region of the nasal fossae includes the upper and middle turbinate processes and the upper third of the septum. It is covered by a soft vascular mucous membrane of a yellow colour in man. The epithelivm of the olfactory mucous membrane (figs. 528, 529) is very thick and is composed of long cells, set closely side by side and bounded superficially by a cuticular lamina, through which the free ends of the cells project. The cells are of two kinds : 1. Long narrow spindle-shaped or bipolar nerve-cells consisting of a larger part or body (6), containing the nucleus, and of two processes or poles, one (c) straight and cylindrical and extending to the free surface, the other (d) very delicate and varicose, looking not unlike a nerve-fibril and ' The connection of the olfactory cells with the olfactory nerve-fibres is displayed in embryos, the method of Golgi being employed. 468 THE ESSENTIALS OP HISTOLOGY. extending down towards the corium. The position of the nuclear enlargement varies, and with it the relative length of the two processes. The distal or free process terminates in a small clear projection, which passes beyond the euticular membrane; in amphibia, reptiles, and birds, and perhaps also in mammals, it bears fine stiff hairlike fila- ments. The proximal or varicose process becomes lost Amongst the plexus of olfactory nerve-fibres at the base of the epithelium ; it is connected with one of these fibres, and ultimately passes through the Fio. 528. — Cells and terminal nerve-fibres of the olfactory region. (Highly magnified.) 1, from the frog ; ^ and 5, from man. In 1 and S :—a, epithelial cell, extending deeply into a ramified process ; &, olfactory cells ; c, their peripheral rods ; e, the extremi- ties of these, seen in I to be prolonged into fine hairs ; (i, their central filaments. In S : — h, hairlets.; c, free border of cell ; p, peripheral process ; &, body of cell ; n, nerve-fibre. 1 and 2 from M. Sohultze ; S from v. Brunn. cribriform plate of the ethmoid to end in an arborisation within one of the olfactory glomeruli (see diagram, fig. 495, p. 438). These cells have been termed the olfactory cells. 2. Long columnar epithelium-cells (a), with comparatively broad cylindrical nucleated cell-bodies placed next to the free surface, and long, forked, and branching tail-like pro- cesses extending down to the corium. These are regarded not as sensory epithelium-cells, but merely as serving to support the proper olfactory cells. They are the columnar or sustentacula/r cells. 3. Taper- ing cells are present, at least in some animals, in the deeper part of the epithelium. They rest by their bases upon the corium, and project between the other cells, which they assist to support. OLFACTORY MEMBRANE. 469 The corium of the olfactory mucous membrane is also very thick (fig. 529). It contains numerous blood-vessels, bundles of the olfactory nerve-fibres (which are non-meduUated), and a large number of serous glands known as Bowman's glands (b), which open upon the surface by ducts which pass between the epithelium-cells. Fig. 529. — ^Sbotion of OLFACTOEr mucous membrane. (Cadiat.) a, epithelium ; 6, glands of Bowman ; c, nerve-bundles. THE EXTERNAL AND MIDDLE EAR. The external ear proper (pinna) is composed of elastic fibro-cartilage, invested by a .thin closely adherent skin. The skin is covered by small hairs, and connected with these are the usual sebaceous follicles. In the lobule there is a considerable amount of adipose tissue ; and voluntary muscular fibres are in places attached to the cartilage of the pinna, and are seen in sections. The external auditory meatus is a canal formed partly of cartilage continuous with that of the pinna, partly of bone. It is lined by a prolongation of the skin and is closed by the membrana tympani, over which the skin is prolonged as a very thin layer. Near the orifice the skin has hairs and sebaceous glands, and the meatus is also provided throughout the cartilaginous part with small convoluted tubular glands of a brownish-yellow colour, which yield a waxy secretion (ceruminous glands). They appear to represent modified sweat-glands. They are represented in fig. 530. The tympanum is lined by a mucou* membrane which jis continuous through the Eustachian tube with the mucous membrane of the pharynx ; it is also prolonged into the mastoid cells. The epithelium 470 THE ESSENTIALS OF HISTOLOGY. is columnar and ciliated in some parts, but in others — e.g. roof, promontory, ossicles, and membrana tympani — it is a pavement- epithelium. The membrana tympani is a thin membrane formed of fibrous bundles which radiate from a central depression (umbo). Within the radial fibres are a few annular bundles. Covering the fibrous Hair. Boot-sheath of \ follicle. ) Root of hair. Sebaceous glands. Hair-follicle. Geruminous gland. Fig. 530. — CERntiiNous glands and haibs of the external eab. (Griiber.) membrane externally is a thin layer continuous with the skin of the meatus ; covering it internally is another thin layer, derived from the mucous membrane of the tympanic cavity. Blood-vessels and lym- phatics are distributed to the membrane chiefly in the cutaneous and mucous layers. , The Eustachian tube is the canal leading from the tympanum to the pharynx. It is formed of bone near the tympanum, but below, THE EUSTACHIAN TUBE. 471 near the pharynx, it is bounded partly by a bent piece of cartilage (fig. 531, 1, 2), partly by fibrous tissue. The latter contains numerous Fig. 531.— Section aokoss the oartilaginous part op the eustachian TUBE. (Riidinger.) 1, S, bent cartilaginous plate ; S, muse, dilatator tubse ; to the left of U, part of the attachment of the levator palati muscle ; 5, tissue uniting the tube to the base of the skull : 6 and 7, mucous glands ; 8, 10, fat ; 9 to 11, lumen of the tube ; IS, connective tissue on the lateral aspect of the tube. mucous glands (6, 7), which open into the tube, and on the outer side a band of muscular tissue (3) which joins the tensor palati. The epithelium is ciliated. 472 THE ESSENTIALS OF HISTOLOGY. LESSON L. THE INTERNAL EAR. 1. Sections across one of the membranous semicircular canals of , a fish (skate). 2. Longitudinal sections through the ampulla of a semicircular caual (skate). 1 and 2 may be hardened in chromic and osmic acid (see below under 5) and embedded in celloidin. The semicircular canals and their ampuUse may also be seen cut across in sections of the petrosal of the guinea-pig or other mammal. 3. Golgi preparations of the macula of the utricle from the skate. 4. Teased preparations of the auditory epithelium of an ampulla or of the macula of the utricle, from the skate. 5. Vertical sections through the middle of the cochlea of a mammal (guinea-pig). The cochlea is put quite fresh into 0'2 per cent, chromic acid containing one-fifth its volume of 1 per cent, osmic acid, or into Flemming's solution, or 10 per cent, formol. The decalcification can be effected by the use of the phloroglucin-nitric acid fluid, or by sulphurous acid.' When decalcified, the preparation is well washed, and then transferred to alcohols of gradually increasing strength. In preparing sections of the above three preparations it is advisable, in order that the epithelium should be kept in position, to embed in celloidin. If the parafiSn method of embedding be used, the sections are fixed to the slide by an adhesive process. The organ should preferably be stained in bulk. 6. Teased preparations of the epithelium of the organ of Corti from the guinea-pig. Both 4 and 6 are made from osmic preparations. Make sketches from all these preparations under the high power.^ The labyrinth, which is the essential part of the auditory organ, consists of a complex membranous tube lined by epithelium and filled with endolymph, contained within a bony tube — the osseous labyrinth — of corresponding complexity of shape (figs. 532, 533). The mem- branous labyrinth does not wholly fill the bony cavity ; the rest of the space is occupied by perilymph. The membranous labyrinth (fig. 532) is composed of the utricle (u), and the three semicircular canals (each ' See Appendix. '' For details of the methods of obtaining the various parts of the labyrinth for microscopical examination, the student ia referred to the author's Cowse of Practical Histology. THE tABYEINTH. 473; with an enlargement or ampulla which opens into it), the saccule (s), and the canal of the cochlea (ex.). The branches of the auditory nerve pass to certain parts only of the membranous labyrinth, viz. the maculse of the utricle and saccule, the cristse of the ampullae, and along the whole length of the canal of the cochlea (the shaded parts in fig. 532). At these places the lining epithelium is specially modified to form a sensory or nerve-epithelium ; elsewhere it is a simple pavement- epithelium. The membranous semicircular canals and the utricle and saccule^ are composed of fibrous tissue, which is adherent along one side to the lpj:ff. Fig. .')32.— Plan of the eight, mem- bkanous labtkinth viewed feo.w ?i. 1 ■Uy utricle, with its macula and s.s,c., p.s.c, and e.8.c., the three semicircular canals with their ampullse; s, saccule; aq.v.j aquaeductus vestibuli ; s.e,, saccus endo- lymphaticus ; c.r., canalis reuniens ; c.c, canal of the cochlea. Fig. 533. — View of the intekiok or THE LEFT OSSEOUS LABTKINTH. The bony wall of the labyrinth is removed superiorly and externally. 1, fovea hemi- elliptica ; 2, fovea hemisphaerica ; 3, com- mon opening of the superior and posterior- semicircular canals ; 4, opening of the- aqueduct of the vestibule ; 6, the superior, G, the posterior,' and, 7, the external semi- circular canals ; 8, spiral tube of the coch- lea ; 9, scala tympani ; 10, scala vestibuli. endosteum of the bony canal ; from the opposite side bands of fibrous tissue pass across the perilymph (fig. 534). Within the fibrous mem- brane is a thick clear tunica propria, which, in the semicircular canals,, may form papilliform elevations in the interior of the tube (fig. 535). The places of entrance of the nerve-fibres are marked in each ampulla by a transverse, inwardly projecting ridge (crista), in the saccule and utricle by a thickening of the tunica propria (macula). The epithelium at these places is formed of columnar cells (fig. 536), which are surmounted by long, stiff, tapering hairs (auditory hairs, fig. 536, h). Around these hair-cells the axis-cylinders of the nerve- fibres ramify (fig. 538) ; they are therefore— like the gustatory cells of the taste-buds — sensory epithelium cells. Between them are a number ■474 THE ESSENTIALS OF HISTOLOGY. of thin and somewhat rigid nucleated cells (fibre-cells of Retzius), which rest upon the basement-membrane, and are connected at their free extremity with a cuticular membrane, through which the auditory hairs project. The auditory hairs do not jut freely into the endolymph, but into a soft mucus-like substance, of a dome-like form in the ampullae (cupula end Pig. 534.— Section of a semicikculab oanal, new-born cHitD. (Sobotta.) x55. €.£., connective tissue strands, between membranous canal and endosteum of bony canal; yti, membraBous canal ; b, wall of bony canal ; c, remains of fcetal cartilage ; end, endosteum ; v, blood-vessels. is, fig. 536), and which in the saccule and utricle has a mass of calcareous particles (otoliths) embedded in it. The cochlea consists of a bony tube coiled spirally around an axis THE COCHLEA. 475 Fig. 535. — Section of mbmbeanotjs semicieoclab canal. (Riidinger.) (More magnified.) 1, outer fibrous layer ; 2, tunica propria ; 3, 6, papillifonn projections witli epithelial covering ; 5, fixed side of the canal, with very thin tunica propria without papillse ;. 7, fibrous bands passing to periosteum. Fig. 536.— Longitudinal section of an ampulla through the okista A0U8TICA (dIAGIIAMMATIO). rnim., cavity of the ampulla; sec, semicircular canal opening out of it ; c, connective tissue attached to the wall of the membranous ampulla and traversing the penlymph , e, e, flattened epithelium of ampulla; *, auditory hairs projectmg from the columnar cells of the auditory epithelium into the cupula, cup.term ; v, blood-vessels ; n, nerve- fibres entering the base of the crista and passing iuto the columnar cells. 476 THE ESSENTIALS OF HISTOLOGY. which is known as the columella (fig. 539, 540). The tube is divided longitudinally by a partition which is formed partly by a projecting lamina of bone (spiral lamina), partly by a flat membrane (basilar ep Fig. 537. — Sbction of macula acostioa, cat. (Sobotta.) x 120. ep, epithelium ; n, n, fibres of vestibular nerve. Fig. 538. — Nerve Teeminations in macula, goloi method. (Barker, from Unhossek. ) membrane), into two parts or scalce ; the upper (supposing the cochlea resting base downwards) being termed the scala vestibuli, the lower scala tympani ; the latter is closed near its larger end by the membrane of the fenestra rotunda. The scalse are lined- by endosteum, and are filled with perilymph, continuous with that of the rest of the osseous THE COCHLEA. 47V labyrinth at the commencement of the scala vestibuli ; they communi- oate at the apex by an opening, the helkotrema. The scala vestibuli. does not occupy the whole of that part of the tony tube of the cochlea which is above the partition. Its outer and str.v. Fie. 539. — Section thbough the cochlea of the oat. (Sobotta.) x 25. dc, duct of cochlea ; scv, scala vestibuli ; set, scala tympani ; w, bony wall of cocblea ; C, organ of Oorti on membrana basilaris ; mR, membrane of Beissner ; ji, nerve fibres of cochlear nerve ; ^gp, ganglion spirale ; str.v., stria vascularis. lower third is cut off by a delicate connective-tissue membrane {memi)rane of Beissner, fig. 541, B), which springs from near the end of the spiral lamina, and passes upwards and outwards to the outer wall, thus separating a canal (d.c.) triangular in section, which is lined by epithelium, and represents the membranous labyrinth of the cochlea (duet or canal of the cochlea). 478 THE ESSENTIALS OF HISTOLOGY. canal of scala urnml^rane of the cochlea vestibuli Reissner \ion scala basilar tympani membraffbe Fig. 540. — Vbktical section through the middle of thb human cochlea. (Oiagrammatic. ) basilar Tnembrane Fig. 541.— Veetioal section of the fiest turn of the human cochlea. (G. Retzius.) J. II, aoala vestibuli ; s.t, aoala tympani ; d.c, canal or duet of the cochlea ; sp.l, spiral lamina ; n, nerve-fibres ; i.sp, spiral ligament ; s«r.?>, stria vascularis; s.sj), spiral sulcus; R, section of Reissner's membrane ; I, limbus laminae spiralis ; m.t, membrana tectoria; to, tunnel of Corti ; h.m, basilar membrane ; h.i, h.e, internal and external hair-cells. THE COCHLEA. 47» Canal of the cochlea. — The floor of the canal of the cochlea is formed (1) of the extremity of the spiral lamina, which is thickened above by a peculiar kind of connective tissue, forming an overhanging projection known as the limbus (fig. 541, 1) ; and (2) of the basilar membrane (b.m..), which stretches across from the end of the bony lamina to the outer wall, and is attached to this by a projection of reticular connective tissue termed the spiral ligament (l.sp). Fig. 542^ — A paik of bods of oorti, feom the rabbit's Cochlea, in side VIEW. (Highly magoified.) 6, 6, basilar memblrane; i.r., inner rod ; e.r., outer rod. The nucleated protoplasmic masses at the feet are also shown. LimbUB. Nerve- fibres. Intier Stood- Basilar Outer ' — , — ' rod. vessel, membrane. rod. Celts of Deiters. Fig. .543. — Sectiox thbough the oegan of coeti of the human cochlea. (G. Eetzius.) (Highly magnified. ) The basilwr membrane is composed of stiff straight fibres, which extend from within out, and are embedded in a homogeneous .substance. The membrane is covered below by a layer of connective tissue continuous with the endosteum of the scala tympani; above, the modified epi- thelium which forms the organ of Gorti rests upon it. It becomes gradually broader in the upper turns of the cochlea (rather more than twice as broad in the uppermost as in the lowermost turn), and its constituent fibres become therefore gradually longer. 480 THE ESSENTIALS OE HISTOLOGY. The organ of Oorti consists of the following structures : 1. The rods of Corti, two series (inner and outer) of stiff, striated ^structures, of a peculiar shape, the inner somewhat like a human ulna, the outer like a swan's head and neck (fig. 542). They rest by one •extremity (the foot) on the basilar membrane a short distance apart, and are inclined towards one another, their larger ends (heads) being jointed together ; the series of rods thus enclose a sort of tunnel, the floor of which is formed by a part of the basilar membrane (fig. 544). Close to their feet may usually be seen the remains of the cells from which they have been formed. The inner rods are narrower and Or. I _■ >f Fig. 544. — Semi-diageammatic view of part op the basilab membrane and tunnel of cohti op the babbit, fkom above and the side. (Much magnified. ) I, limbuB ; CV., extremity or crest of limbus with tooth-lilce projections ; 6, basilar mem- brane ; gp.Lf spiral lamina with, p, perforations for transmission of nerve-fibres ; i.r., fifteen of the inner rods of Corti; h.i., their filattened heads seen from above; e.r., nine outer rods of Corti ; h.e., their heads, with the phalangeal processes extend- ing outward from them and forming, with the two rows of phalanges, the lamina reticularis, l,r. Tather more numerous than the outer. The head of each outer rod has a process which extends outwards and is known as the phalangeal process. This forms part of — 2. A reticular lamina (fig. 544, I.r.), which is a cuticular structure -extending like a wire-net over the outer epithelium-cells of the organ of Corti, and is composed of two or three series of stiff fiddle-shaped rings (phakmges) cemented together in such a manner as to leave square or oblong apertures through which the hair-cells (see below)' project. 3. The outer hair-cells placed external to the rods of Corti. These are epithelium-cells of columnar shape, arranged in three or four series THE COCHLEA. 481 (fig. 543). The free extremity of the cell is surmounted by a bundle of short auditory hairs, and projects through one of the apertures in the reticular lamina; the fixed extremity is prolonged into a stiff cuticular process, which is attached to the basilar membrane. Between them are other supporting cells which are tapered in the same manner, but rest by their larger end upon the basilar membrane, and are prolonged above into a cuticular process which is attached to the reticular lamina {cells of Deiters, figs. 543, 545). 4. The inner hair-cells (fig. 543), placed internal to the rods of Corti. They form a single series of columnar cells surmounted by auditory hairs, lying in close apposition to the inner rods. Fig. 545. Fig. 546. Fig. 545.— Foue cells of deiteks fbom the babbit. (After G. Eetzius.) (Highly magnified. ) The varicose lines are nerve-flbrils. The phalangeal processes are attached above to a portion of the lamina reticularis. Fig. 546.— Gbnekal view of the mode of distkibution of the coohleab NEBVE, ALL THE OTHEB PAETS HAVING BEEN EBMOVBD. (Arnoia.) The remaining epithelium-cells have no important characteristics. They are long and columnar next to the outer hair-cells, but soon diminish in size, becoming cubical, and in this form they are con- tinued over the outer wall of the cochlear canal. Here they cover a very vascular membrane {stria vascularis, fig. 541, str.v.), which is frequently pigmented; its capillary blood-vessels penetrate between the epithelium-cells. Internal to the inner hair-cells the epithelium also soon becomes cubical ; it is prolonged in this form over the limbus of the spiral lamina. The epithelium of Eeissner's membrane is of the pavement variety. 2 H 482 THE ESSENTIALS OF HISTOLOGY. The membrana tectmia (figs. 541, 543) is a soft, fibrillated structure, wMch is attached along the upper surface of the limbus, and lies like a pad over the organ of Corti. It thins out towards the distal margin, here becoming somewhat reticular, and, according to Retzius, Fig. 547.— Ending of some op the fibres op the cochlear nerve amongst THE HAIK-CELLS, (G. Eetzius.) This preparation is made by Golgi's method, and is viewed from above, g, a cell belonging to the spiral ganglion. it is attached to the lamina reticularis. In sections it usually appears raised a short distance above the auditory hairs, but it is probable that it always rests upon them during life. The fibres of the cochlear branch of the auditory nerve enter the base of the columella, and run in canals through its substance (figs. 539, 540), being gradually deflected outwards as they pass upwards.?; into the spiral lamina, at the base of which they swell out into a ganglionic cord {spiral ganglion). The fibres take origin from the cells of this ganglion. THE COCHLEA. 483 After traversing the spiral lamina they emerge in bundles, and the fibres then, having lost their medullary sheath, pass into the epithelium of the inner hair-cell region. Here some of them course at right angles and are directly applied to the inner hair-cells, whilst others cross the tunnel of Corti, to become applied in like manner to the outer hair-cells and the cells of Deiters (figs. 543, 545). They apparently lie between and in close contact with these cells, but there does not appear to be any direct continuity between the nerve-fibrils and the cell-substance. 484 THE ESSENTIALS OF HISTOLOGY. APPENDIX. Mounting solutions :— 1. Normal saline solution.— A 0'6 to 0"9 per cent, solution of common salt is used in place of serum for mounting fresh tissues for immediate examination. The lower percentage is used for frog's tissues, the higher for mammals'. Preparations mounted in salt solution cannot be preserved permanently. 2. Olycerine, diluted with an equal quantity of water. The cover-glass may be fixed by gold size. 3. Canada balsam, from which the volatile oils have been driven off by heat, dissolved in xylol. 4. Dammar varnish, made by dissolving dammar resin in xylol. The solution is filtered through paper wetted with chloroform. This is used for the same purposes as xylol balsam and has the advantage of remaining colourless, whereas Canada balsam becomes yellow after long keeping. 5. Acetate of potassium, a. nearly saturated solution. This is the best mediuiB for osmic preparations and for iodine-stained liver, to show glycogen within the cells. The cover-glass may be fixed by soluble glass or by gold size. General methods of preserving and hardening tissues and organs.'— The following fluids may be used : — Alcohol (75 per cent, to absolute) ; acetone ; Carnoy's fluid (absolute alcohol 60 o.c, chloroform 30 c.c, glacial acetic acid 10 c.c.) ; formol (diluted with from 9 to 99 parts of water) ; corrosive sublimate (saturated solution in water or spirit) ; chromic acid solution (1 in 200 to 1 in 500, to which glacial acetic acid may advantageously be added in the proportion of 2 parts acetic acid to 1000 chromic solution); picric acid solution (saturated, either alone or containing 2 parts of nitric or sulphuric acid to lOOO) ; Mann's fluid (a mixture of equal parts of saturated aqueous solutions of mercuric chloride and picric acid) ; osmic acid solution (1 per cent.) ; bichromate of potassium solution (3 per cent.), to which for more rapid hardening glacial acetic acid may be added to the extent of 5 per cent, or lees ; MUller's fluid (bichromate of potash 2^ parts, sulphate of soda 1 part, water 100,, parts); Zenker's fluid (which is MUller's fluid containing 5 parts per cent, of mercuric chloride, to which 5 c.c. of acetic acid is added at the time of using) ; and mixtures of MUller's fluid and osmic acid 1 per cent, in varying proportions. 1 Details of the methods of preparing fixing and staining solutions as well as a discussion of the theory of their action will be found in Mann's Physiological Histology, Oxford, 1902. A:PPENDIX. 485 ■It i3 best, if possible, to inject the fluid used for hardening into the blood- vessels-after washing them out with warm normal saline; if this is not possible, very small pieces of tissue should be ta,ken, and always a consider- able amount of the hardening fluid. The fluid of most universal application is forraol. This is a 40 per cent, solution of formaldehyde. Mixed in the proportion of 10 parts formol to 90 water, it penetrates readily and hardens quickly. The tissue may remain in formol a few days and should then be transferred to alcohol. For rapid fixation a very small piece of the tissue is placed in 10 per cent, formol and warmed to a temperature of about 40° or 50° ; it will be suflnlciently hardened in half an hour, and may then be transferred first to weak and then gradually to absolute alcohol, so that it is ready for the preparation of sections in about an hour. Pure acetone is also of utility for rapid fixation and hardening. Small pieces of the tissue are dropped into a large amount of acetone, which not only fixes and hardens but also dehydrates, so that the tissue can be trans- ferred in an hour or so direct to molten paraffin for embedding. But much better results are got by placing in 10 per cent, formol for thirty minutes before transferring to acetone. For preserving the structure of cells and nuclei, one of the best fixing fluids is that recommended by Flemming. This consists of 15 vols, of 1 per cent, chromic acid, 4 vols, of 2 per cent, osmic acid, and 1 vol. glacial acetic acid. It must be freshly prepared. It is sometimes diluted with from two .to five times its bulk of water before use. One or two days is sufficient for fixation and hardening. The tissue should be washed for several hours in running water after removal from the mixture, and then placed in dilute alcohol. Carnoy's fluid is in many cases excellent for cell-structure and mitotic changes, and very rapid in its action. Tissues to be hardened in alcohol are usually placed at once in absolute alcohol, but for some tissues it is best to begin with 50 per cent, alcohol, and pass the pieces through successive grades of 75 per cent., 95 per centy into absolute alcohol, leaving them a few hours in each. They are ready for cutting as soon as they are dehydrated, but as a rule they may be left a long time in alcohol without deterioration. Organs which contain much fibrous tissue, such as the skin and tendons, should not go into stronger alcohol than about 80 or 90 per cent. ; otherwise they become too hard to cut. Alcohol is generally used after the other fixing reagents, partly to complete the hardening, partly on account of its dehydrating property, since previous to embedding in paraffin all trace of water must be eliminated from the tissue. If mercuric chloride be used for hardening, tincture of iodine must be added to the alcohols subsequently used (except the final alcohol), to get rid of the excess of sublimate. Mercuric chloride in alcohol (saturated solution) is one of the most rapid fixing and hardening reagents, and may be used if sections are desired within a very short time. It can also be used in place of alcohol and ether mixture for fixing blood films (Lesson II., § 5), in which case it may be saturated with eosin, and used for fixing and staining at the same time. An immersion of 5 minutes is sufficient. 486 THE ESSENTIALS OF HISTOLOGY. Many tissues can be instantly hardened by being plunged for a minute nto boiling water and then placed in alcohol : this is not, however, a good method for glandular organs. For tissues that are to be hardened in chromic acid an immersion of from seven to fourteen days is generally necessary ; they may then, after washing for some hours or days in tap-water, be placed in alcohol for preservation and to complete the process of hardening. The alcohol should be changed once or twice. Organs placed in bichromate of potassium or Miiller's fluid are ready for section in a fortnight or three weeks ; they may, however, be left for a much longer time in those fluids without deterioration. With picric acid the hardening process is generally complete in two or three days ; the organs may then be transferred to alcohol, which ought to be frequently changed. The hardening of the brain and spinal cord in Muller's fluid takes from three weeks to as many months. It can be hastened by warmth, and by the addition of acetic acid, or by placing small pieces in Marchi's solution (see below), after they have been a week or ten days in Muller's fluid. Tissues containing calcareous matter, e.g. bone and tooth, may be rapidly decalcified in a solution made by dissolving, with the aid of heat, 1 grm. phloroglucin in 10 c.c. nitric acid, and filling up to 100 c.c. with water, to which more nitric acid may be added if desired. Another rapid decalci- fying fluid is commercial sulphurous acid solution. If it is desired to preserve the soft parts within the bone, it should first be placed for a- few hours in 10 per cent, formol. For decalcifying more slowly a 1 to 5 per cent, solution of nitric acid in water or alcohol, a saturated solution of picric acid containing a, superabundance of crystals, or a 1 per cent, solution of. chromic acid are employed. Embedding of hardened tissues, and preparation of sections.— Sections are most advantageously made with some form of microtome. It is generally needful to support the hardened tissue whilst it is being cut, and with this object it is embedded in some substance which is applied to it in the fluid condition and becomes solid on standing. The embedding substance can either simply inclose the tissue, or the tissue may be soaked in it ; the latter method is the one commonly employed. The embedding substance chiefly used is paraffin of about 50' C. melting point. Embedding in paraffin. — Before being soaked in melted paraffin, the piece of tissue^ may be stained in bulk ; it is then dehydrated by a series of alcohols (50 per cent., 75 per cent., 95 per cent.), finishing up with absolute alcohol ; after which it is soaked in cedar-wood oil, xylol, or chloroform. It is now transferred to molten paraffin, which should not be too hot, and is soaked in this for one or several hours, according to thickness. Very delicate objects- are sometimes passed through » solution of paraffin in chloroform. When thoroughly impregnated with the paraffin the object is placed in a mould or glass which has been smeared with glycerine, and is covered with molten paraffin which is APPENDIX. 487 allowed to cool quickly. A square block of the paraffin containing the tissue is then fixed in the desired position on the microtome, thin sections are cut and fixed to a slide (see below), the paraffin dissolved out by turpentine or xylol, and -the sections mounted in Canada balsam or dammar. If it be desired to cut a riband of successive sections, and the paraffin used prove too hard for them to stick to one another at the edges, a parafflnf of lower melting point (40° C.) is smeared over the opposite sides of the block ; the sections then adhere together as they are cut. Preparation of frozen sections. — The bichromate solutions and formol are the best fluids to use for preserving tissues which are to be frozen in place of being embedded. The tissue requires to be soaked in gum-water before being placed upon the freezing microtome. A thin syrup of either gum arabic or dextrin may be used. Emheddin^ in celloidin — The piece to be embedded is dehydrated by alcohol, and is then placed over night in a solution of celloidin in alcohol and ether similar to ordinary collodion, and afterwards in collodion of double strength. After twenty-four hours more it is removed from the celloidin (collodion) and placed upon a wood or metal holder. When tlie celloidin is set by evaporation of its ether the holder is plunged in alcohol (85 per cent), and after a few hours sections may be cut with a knife wetted with spirit of the same strength. The sections are placed in 95 per cent, alcohol ; then passed through cedar- wood oil or bergamot oil into xylol balsam. They must not go into clove-oil, nor into absolute alcohol. The advantage of the method is that the celloidin, which is quite transparent, need not be got rid of in mounting the sections, and serves to keep the parts of a section together ; it is thus very useful for friable tissues or for large sections. The tissue may either be stained in bulk before embedding, or the sections may be stained. Microtomes. — A section-cutting apparatus or microtome is essential for histological work. Useful instruments for students are the Cathcart micro- tome for freezing and the tripod microtome for objects which have been embedded in paraffin. The tripod microtome is a simple and efficient little instrument, and has the advantage of being inexpensive. It consists of a metal frame (fig. 548) in which the razor is fixed, provided with a micrometer screw by which the height of the razor-edge is adjusted. The paraffin block containing the tissue is fixed by the aid of heat on a flat piece of glass over which the tripod slides. The razor-edge is lowered after each successive section. In the Cathcart freezing microtome (fig. 549) the tissue, after being soaked in gum-water, is placed on a metal plate and frozen by playing an ether- spray on the under surface of the plate. The plate is moved upwards by a finely-cut screw, and the knife or plane used to cut the sections is guided over the plate by passing over glass slips. In the Williams microtome the freezing agent is ice and salt mixture. In using any freezing microtome, especially for the nervous system, it is important not to freeze the tissue too hard, or the section will roll up. 488 THE ESSENTIALS OF HISTOLOGY. Sorae-what more expensive and complicated, bnt also more efficient, instru- ments are the rocking microtome of the Cambridge Scientific Instrument Company and the microtomes designed by C. S. Minot and by Del6piue. The :Fig. 548.— Tripod miokotome. (Bircli's pattern. Fia. 549.— CATfiOAKT FREEZme MIOROTOME. AI^iPENDIX. 489 action of all of these is automatic. Tor example, with the rocker microtome every to-and-fro movement of the handle, h, not only cuts a section of the tissue of definite thickness, but also moves the paraffin block forwards in readiness for the next section. And by employing a rectangular, block of Fig. 550.— Rocking microtome. Fig. 551.— Minot's automatic kotakt microtome. 490 THE ESSENTIALS OE HISTOLOGY. paraffin of the proper consistency, a long series of sections of the _ same object, of equal thickness, can be obtained and made to adhere together m a riband (as shown in iig. 550). The sections can be kept in series upon Fis. 552.— Mtnot's pkeoision miokotome. This is especially adapted for large sections. Fig. 553.— Inolinbd plane mickotome. APPENDIX. 491 the slide by the employment of some adhesive method of mounting the riband. For celloidin-embedded preparations it is necessary to cut the sections with a knife kept wetted with spirit. For this purpose a sliding micro- tome, in which the knife or razor is moved horizontally over the tissue, with the edge obliquely inclined to the direction of movement, is the most useful. The best instrument for this purpose, especially for large sections of brain, is one in which the celloidin-soaked object is immersed in spirit during the actual process of making the sections. It is all-important for every kind of micirotome that the knife should be in perfect order. Methods of mounting in xylol balsam or dammar.— Individual paraflfin- cut sections or ribands of sections, such as are cut with the rocking and other microtomes, are fixed to a slide or cover-glass — preliminary to being treated with stains and other fluids— in the following way ; — The slide (or cover-glass), after having been carefully cleaned, is smeared very thinly with fresh white of egg : this can be done with the finger or with a clean rag, and the slides may be put aside to dry, protected from dust. It is con- venient to prepare a large number of slides [at a time in this way, and to keep them at hand in) a suitable receptacle. When required for use a little water is poured onjjto the slide and the riband of sections is placed on the water, which is then warmed on a hot plate or over a small flame until the paraffin becomes flattened out, without actually melting. The water is then ^drained oflF, the slide put in [a warm place for-the remainder of the water to evaporate (this will take from half an hour to an hour acoordijig to the size of the section and the temperature at which it is kept), and then heated sufficiently to melt the paraffin. It is next immersed in xylol to remove the paraffin, after which the sections may, if already stained, be mounted at once in xylol balsam or dammar ; if not stained, treat, after xylol, first with absolute and then with gradually lower grades of alcohol, then water, and then stain, and finally pass through water, alcohol (in grades), clove-oil, and xylol, into xylol balsam or dammar. For many sections some of the grades of alcohol can be omitted, but it is always best to place in 50 per cent, between water and absolute alcohol. A simpler method, but one which, in most [cases, answers the purpose perfectly well, is to place the riband or the individual sections cut from paraffin on the surface of water in a basin, just sufficiently warm to flatten out the paraffin, but not to melt it, then pass a perfectly clean slide under the surface of the water and float the sections on to it ; remove, drain oflf the water, and put the slide and sections aside for one or more hours until all the water haS evaporated. The sections are found to have adhered firmly to the slide (they may, if desired, be yet more firmly fixed by drawing a brush moistened with solution of celloidin in oil of cloves over them). The paraffin can now be removed by washing the slide with xylol or immersing it in xylol. If not previously stained they can then be passed through alcohols and stained and mounted as just described. It is con- venient to keep the several solutions which are required in cylindrical tubes or grooved glaSs receptacles in a regular row upon the working table. 492 THE ESSENTIALS OF HISTOLOGY. and transfer the slide from one to the other in succession. Thus such a series would be (1) xylol ; (2) absolute alcohol ; (3) 75 per cent, alcohol ; (4) 50 per cent, alcohol ; (5) distilled water ; (6) staining solution ; (7) tap water ; (8) distilled water ; (9) 50 per cent, alcohol ; (10) 75 per cent, alcohol ; (11) absolute alcohol ; (12) clove oil or xylol. The changes can also be effected by pouring the solutions over the sections and draining oflf, but this is less satisfactory. The following table shows the methods which may be adopted for the treatment of parafiQn-cut sections or ribands of sections : 1. Place on a slide or cover-glass in a drop of tap- water : the glass may previously have been smeared with egg-white : warm gently. 2. Drain off water and allow to dry completely. I 3. Warm until paraffin is just melted. 4. Dissolve paraffin away with xylol. If tissue is already stained in bulk. If tissue is not already stained. I 1 Mount in xylol balsam or dammar. 5. Absolute alcohol. 6. Descending grades of alcohol. 7. Stain. For sections cut by the freezing | or celloidin methods, if the 8. Water. tissue has already been stained | in bulk, the sections need only 9. Ascending grades of alcohol. be put through the ascending | grades of alcohol and bergamot 10. Xylol or bergamot oil or oil, and then mounted in creosote or clove-oil. dammar. If the tissue has | not already been stained, begin Mount in xylol balsam or at No. 7. dammar. Staining of sections. — The fluids most commonly employed for the staining of sections are ; — (1) Solutions of hsematoxylin and alum ; (2) solutions of carmine with or without alum ; (3) certain aniline dyes. The time of immersion in the staining fluid varies according to the strength of the fluid and the mode by which the tissue has been hardened. The necessity of staining sections may be avoided if the tissue is stained in bulk before embedding. For this purpose a piece of tissue is left to stain for twenty-four hours or more in a moderately diluted hsematoxylin solution or in carmalum or borax carmine, and is then embedded and cut into sections by the paraffin or freezing methods. If the latter be employed the sections are thoroughly washed with tap-water, dehydrated by alcohol, and passed through clove-oil or xylol into xylol balsam or dammar. For some purposes (e.ff. the study of ossifying cartilage) an alcoholic solution of magenta is useful for staining in bulk ; from this the tissue goes direct into a small quaijtity of oil of cloves, and after being soaked with this it is passed into molten paraffin. Sections may also be stained whilst still infiltrated with paraffin by floating them on to the surface of the staining solution, which APPENDIX. 493 may be gently warmed (but not enough to melt the paraffin). They generally require far longer exposure to the stain. The subsequent treat- ment is quite simple, for they need only be transferred to warm water, floated on to a slide and allowed to dry. The paraffin is then melted, dissolved away with xylol, and the sections are mounted in dammar. The following are some of the principal staining solutions and methods of staining for special purposes : 1. Delafield's hoematoxylin. — To 150 cubic centimetres of a saturated solution of potash alum in water add 4 cubic centimetres of a, saturated solution of hsematoxylin in alcohol. Let the mixture stand eight days, then decant, and add 25 cubic centimetres of glycerine, arid 25 cubic centimetres of methylic alcohol. The solution must stand a few days before it is ready for use. To stain sections add a few drops of this solution to a watch-glassful of distilled water. If overstained the excess of colour can be removed by alcohol containing 1 per cent, nitric or hydrochloric acid. With long keeping this solution becomes red instead of blue ; a trace of ammonia will restore the blue colour. 2. EhrlicKs hematoxylin. — Dissolve 2 grammes hsematoxylin (or, better, haematein) in 100 cubic centimetres alcohol ; add 100 cubic centimetres water, 100 cubic centimetres glycerine, and 10 cubic centimetres glacial acetic acid : add potash alum to saturation. This solution will keep almost indefinitely : it is valuable for staining in bulk, as it does not easily overstain. TTor staining sections it is best to dilute the solution either with distilled water or with 30 per cent, alcohol. After the sections have been stained they must be thoroughly washed with tap-water. This develops the blue colour of the hsematoxylin. 3. Kultschitzkjf s hcematoscylin. — Dissolve 1 gramme hsematoxylin in a little alcohol, and add- to it 100 cubic centimetres of a 2 per cent, solution of acetic acid. This solution is valuable for staining sections of the nervous system (see Weigert-Pal process). 4. HcBmalwm. — Hsematoxylin-alum solutions acquire their colouring properties only as the hsematoxylin on keeping becomes converted into hseraatein. The latter substance may, therefore, as recommended by Mayer, be used advantageously in place of Hsematoxylin if the stain is required immediately. The following mode of preparing the solution may be adopted : — Dissolve 50 grammes of ammonia alum in 1 litre of water, and ) gramme of hsematein in 100 c.c. of rectified spirit. Add the h^mateln solution gradually to the alum. The mixture is ready for staining at once, either as it is or diluted with distilled water. A small piece of thymol or a little carbolic acid should be added to prevent the growth of moulds. 5. R. Heidenhain's method. — After hardening in alcohol, or in saturated solution of picric acid and then in alcohol, place the tissue from twelve to fourteen hours in a ^ per cent, watery solution of hsematoxylin, and then from twelve to twenty-four hours more in a | per cent, solution of yellow ehromate of potash, which may be changed more than once. Then wash in water, place in alcohol, pass through xylol, and embed in paraffin. 494 THE ESSENTIALS OP HISTOLOGY. 6. M. HeidenhaiiCs method. — Harden in sublimate, followed by alcohol ; fix sections to slide by water method ; treat with iodised alcohol, transfer to 2'5 per cent, iron-alum (solution of sulphate (or tartrate) of iron and ammonia) and leave a quarter of an hour or longer ; rinse with distilled water; place in 1 to 0"5 per cent, pure hsematoxylin in water containing 10 per cent, alcohol for a few minutes ; wash with water ; differentiate in the iron and ammonia solution until nearly [decolorised ; wash for fifteen minutes in tap-water.; dehydrate jand 'mount jlin the [usual way. This method is especially adapted for exhibiting the oentrosomes of cells. It is also useful for retiform tissue and glands. Both the process of mordanting with iron-alum and the subsequent staining with hsematoxylin may be considerably prolonged with advantage for some tissues. 7. Carmcdum (Mayer). — Useful either for sections or bulk-staining. If the sections are subsequently passed through alcohol containing picric acid in solution a double stain is produced. Garminio acid, 1 gramme. Ammonia alum, - 10 grammes. Distilled water, 200 c.e. Boil together, allow to cool and filter. Add thymol or a little carbolic acid to prevent the growth of moulds. 8. Carminate of ammonia. — Prepared by dissolving carmine in ammonia, and allowing the excess of ammonia to escape by slow evaporation. The salt should be allowed to dry and be dissolved in water as required. 9. Picric acid. — A saturated solution of picric acid in spirit may be used as a second stain after hsematoxylin or carmine. Any excess of picric acid is dissolved out by rinsing with strong spirit. This form of double stain is valuable for exhibiting keratinised tissues and muscle-fibres. 10. Picro-carminate of ammonia, a double stain, u,. Ranvier's picro- carmine. — To a saturated solution of picric acid add a strong solution of carmine in ammonia, until a precipitate begins to form. Evaporate on the water bath (or, better, allow it to evaporate spontaneously) to one half its bulk ; add a little carbolic acid to prevent the growth of moulds ; filter from the sediment. ;8. Bournes picro-carmine. — " Add 5 c.e. of ammonia to 2 grammes carmine in a bottle capable of containing about 250 c.e. Stopper, shake, and put aside till next day. Add slowly, shaking the while, 200 c.e. of a saturated solution of picric acid in distilled water. Put aside till next day. Add slowly, constantly stirring, 1 1 c.e. of 5 per cent, acetic acid. Put aside till next day. Filter ; to the filtrate add four drops of ammonia, put back in the stoppered bottle " (Langley). 11. Borax-carmine. — Dissolve 4 grammes borax and 3 grammes carmine in 100 cubic centimetres of warm water. After three days add 100 cubic centimetres of 70 per cent, alcohol, let stand two days and filter. This solution improves on keeping. It is useful for staining in bulk. After staining with borax-carmine, the tissue should be placed in 70 per APPENDIX. 495 cent, alcohol containing 5 drops of hydrochloric acid to 100 cubic centi- metres. 12. Aniline rf^^es.— These are used either in aqueous solution (which may contain O'Ol per cent, of caustic potash) or in water shaken up with aniline oil, and it is usual to overstain a tissue with them, and subsequently to decolorise with absolute alcohol containing i its bulk of aniline oil (from which the sections can pass directly into xylol) or with acid-alcohol (1 to 10 per lOOO hydrochloric acid) followed by absolute alcohol and this by xylol. Those most employed are the " basic " dyes— methyl-blue, methylene- blue, gentian-violet, toluidin-blue, thionin, saffranin, and vesuvin ; and the " acid " dyes — eosin, erythrosin, acid magenta or acid f uchsin, and orange G. A double stain is obtained by combining eosin with methylene blue or toluidin blue, the sections being first stained for ten minutes in 1 per cent, aqueous eosin and then, after rinsing with water, for twenty minutes in 1 per cent, of the blue solution, after which they are decolorised by absolute alcohol or absolute alcohol and aniline oil. The decolorisation is arrested by xylol. Other good double stains are the eosin-methyl blue mixture devised by G. Mann' and Jenner's stain, which is made by dissolving in pure methyl alcohol the precipitate which is produced when eosin solution is added to methylene blue solution. Jenner's stain is valuable for blood films. For the same purpose Ehrlich's triple stain is also used. This is formed by mixing together aqueous solution of orange G., aoid- fuchsin, and methyl-green in certain proportions.^ 13. Eosin. — A 1 per cent, solution in water may be used. The sections are first stained deeply with hsematoxylin and rinsed with water. They are then stained with the eosin solution, passed through 75 per cent, alcohol, and then through strong spirit — which is allowed to dissolve out some but not all of the eosin stain — into clove-oil : they are finally mounted in xylol balsam or dammar. Eosin stains haemoglobin of an orange red colour ; so that the blood corpuscles are well shown by it when fixing fluids have been employed which do not remove the hsemoglobin from them (such as mercuric chloride, bichromate of potassium, and formol. 14. Midr's m,ethod of double-staining with eosin and methylene blue. — For staining hsemoglobin and oxyphil granules in cells the method devised by Richard Muir will be found valuable. It consists in staining the sections of formol-hardened tissue (which are fixed on a slide) with saturated solution of alcohol-soluble eosin crystals dissolved in rectified spirit. This solution is poured over the sections, and evaporated over a flame until the alcohol is driven oflf, leaving only a watery solution. Rinse with water, place in saturated solution of potash alum for three minutes, and wash again. Decolorise with alcohol rendered very faintly alkaline with ammonia. Wash. Stain with saturated water-solution of methylene blue ; wash with water ; dehydrate and mount in usual way. 15. Acid fuchsin.—A. 1 per cent, solution in 50 per cent, alcohol (to which iSee Methods of Physiological Histology, p. 216, ''It is best to purchase this solution ready-made. 496 THE ESSENTIALS OF HISTOLOGY. 1 drop of 1 per cent, alcohol-solution of gentian-violet may be added per cubic centimetre), is an excellent stain for connective tissue (see p. 67). It may also advantageously be used for developing bone and tooth and for lymph-glands. The piece of tissue is left for several days in a 1 per cent, solution in 95 per cent, alcohol and is then placed direct in a small quantity of clove-oil for a few hours, after which it is transferred to molten paraffin. 16. Orcein. — Dissolve 1 gramme orcein in 100 c.c. absolute alcohol con- taining 1 c.c. hydrochloric acid. Place the sections in some of this solution in a watch-glass and warm slightly, allowing the fluid to nearly evaporate to dryne.9S. Dehydrate in alcohol, which removes the excess of stain ; pass through xylol into dammar. Orcein stains especially the elastic fibres. 17. Flemming's method for haryokvmtic nuclei. — This is especially valuable for staining cell-nuclei in mitosis. The tissue having been appropriately fixed, small shreds or thin sections are placed for two days in saturated alcoholic solution of saffranin, mixed with an equal amount of aniline- water. They are then washed with distilled water and decolorised in aniline-alcohol or in alcohol containing 1 per 1000 hydrochloric acid until the colour is washed out from everything except the nuclei. They are then again rinsed in water and placed in saturated aqueous solution of gentian-violet for two hours, washed again in distilled water, decolorised with aniline-alcohol until only the nuclei are left stained, then transferred to bergamot oil or xylol, and from this are mounted in xylol balsam or dammar. Gentian-violet and several other aniline colours may be employed in place of saffranin from the first. Delafield's hsematoxylin (followed by acid), or Ehrlioh's hsematoxylin also stain the mitotic figures well. 18. Staining with nitrate of silver (Recklinghausen). — "Wash the fresh tissue with distilled water ; immerse in ^ to 1 per cent, nitrate of silver solution for from one to five minutes ; rinse with distilled water and expose to bright sunlight either in water, 70 per cent, alcohol, or glycerine. The tissue, which is generally a thin membrane, may either be mounted in glycerine, or it may be spread out flat in water on a slide, the water drained off, the tissue allowed to dry completely, and then dammar added. This method is used to exhibit endothelium, and generally to stain intei'cellular substance. It depends upon the fact that the chlorides of the tissues are almost exclusively confined to the intercellular substance. The following methods are especially useful in investigations relating to the nervous system : 19. MarcMs solution. — This is a mixture of Mliller's fluid (2 parts) with 1 per cent, osmic acid (1 part). It is of value for staining nerve-fibres in the earlier stages of degeneration, before sclerosis sets in (especially a few days after the establishment of a lesion). All the degenerated medullated fibres are stained black, whilst the rest of the section remains almost unstained. It is best to put thin pieces of the brain or cord to be investi- gated singly into a large quantity of the solution (after previously hardening for ten days in Miiller's fluid), and to leave them in it for a week or more ; but if necessary sections can be stained ; in this case the process is more APPENDIX. 497 complicated.1 In either case they are fixed on a slide and mounted by the usual process in xylol balsam or dammar. 20. Weigert-Pal method.— 1}i\s method is chiefly used for the central nervous system. By it all meduUated nerve-fibres are stained dark, while the grey matter and any sclerosed traets of white matter are left uncoloured. The following modification of the original method can be recommended : Pieces which have been hardened in Muller's fluid and afterwards kept a short time in alcohol (without washing in water) are embedded in cel- loidin, and sections are cut as thin as possible. Or sections may be made by the freezing method direct from Muller's fluid, if the tissue is first soaked in gum-water for a few hours. In either case they are placed in water, and from this are transfereed to Marchi's fluid (see above, § 19), in which they are left for a few hours. They are then again washed in water and transferred to Kulschitzky's hsematoxylin (see above, § 3). In this they are left overnight, by which time they will be completely black. After again washing in water they are ready to be bleached. This is accomplished by Pal's method as follows : Place the overstained sections, first in J per cent, solution of potassium permanganate for five minutes (or for a longer time in a weaker solution) ; rinse with water and transfer to Pal's solution (sulphite of soda 1 gramme, oxalic acid 1 gramme, distilled water 200 cubic centimetres), in which the actual bleaching takes place,^ They are usually sufficiently difi'erentiated in a few minutes : if not, they can be left longer in the solution without detriment. If after half an hour they are not difi'erentiated enough, they must be put again (after washing) into the permanganate for some minutes, and then again into Pal's solution. After differeutiation they are passed through water, alcohol (with or without eosin), and oil of bergamot (or xylol), to be mounted in xylol balsam or dammar. The advantages which this modification has over the original method are (1) even the finest raedullated fibres are brought to view with great surety ; (2) the staining of the fibres is jet black, and ofifers a strong contrast to the colourless grey matter ; (3) the sections are easily seen and lifted out of the acid hsematoxylin, which has very little colour ; (4) it is difficult to overbleach the sections ; (5) the stain is remarkably per- manent. Aa a modification of the above, Bolton recommends to harden with formol, place the sections for a few minutes in 1 per cent, osmic acid, stain for two hours in Kulschitzky's hsematoxylin at 40° C, and then proceed with the bleaching process. 21. Staining with chloride of gold. — a. Cohnheim's method.— £\a,ce the fresh tissue for from thirty to sixty minutes in a ^ per cent, solution of chloride of gold ; then wash and transfer to a large quantity of water faintly acidu- lated with acetic acid. Keep for two or three daya in the light in a warm place. This answers very well for the cornea. If it be principally desired to stain the nerve-fibrils within the epithelium, the cornea may be trans- ferred after twenty-four hours (the outlines of the larger nerves should be 1 See Hamilton, Brain, 1897, p. 180. 2 Diluted sulphurous acid solution may be employed instead of Pal's solution. 2i 498 THE ESSENTIALS OF HISTOLOGY. just apparent to the naked eye) to a mixture of glycerine (1 part) and water (2 parts), and left in this for twenty-foul- hours more (Klein). p. LomCs method. — Place small pieces of the fresh tissue in a mixture of 1 part of formic acid to 2 to 4 parts of water for one-half to one minute ; then in 1 per cent, chloride of gold solution for ten to fifteen minutes ; then back again into the formic acid mixture for twenty-four hours, and into pure formic acid for twenty -four hours more. After removal from the gold, and whilst in the acid, the tissue must be kept in the dark. This method is especially good for motor nerve endings in skeletal muscle. y. Ranvier's method. — Immerse in lemon-juice for five to ten minutes, then wash with water and place in 1 per cent, gold-chloride solution for twenty minutes. Then treat either as in Cohnheim's or as in LSwit's method. 22. Oolgi's chromate of silver methods. — These are chiefly employed for investigating the relations of cells and fibres in the central nervous system. Two methods are mostly used, as follows : IX.. Very small pieces of the tissue which has been hardened for some weeks in 3 per cent, bichromate of potassium or Miiller's fluid are placed for half an hour in the dark in 075 per cent, nitrate of silver solution, and are then transferred for twenty-four hours or more to a fresh quantity of the same solution (to which a trace of formic acid may be added). They may then be placed in 96 per cent, alcohol (half an hour), and sections, which need not be thin, are cut either from celloidin with a microtome or with the free hand after embedding (but not soaking) with paraffin. The sections are mounted in xylol balsam, which is allowed to dry on the slide : they must not be covered with a cover-glass, but the balsam must remain exposed to the air. |8. Instead of being slowly hardened in bichromate, the tissue is placed at once in very small pieces in a mixture of bichromate and osmic (3 parts of 3 per cent, bichromate of potash or of Miiller's fluid to 1 of osmic acid). In this it remains from one to eight days, a piece being transferred each day to 0'75 per cent, silver nitrate. The subsequent procedure is the same as described under a. For some organs it is found advantageous to repeat the process, replacing them for a day or two in the osmic-biohromate mixture after silver nitrate and then putting them back into silver nitrate (Cajal's double method). This method is not only more rapid than that in which bichromate of potassium alone is used, but is more sure in its results. 23. Ehrlich's methylene-hlue method. — This method is one of great value for exhibiting nerve-terminations, and in some cases the relations of nerve- cells and fibres in the central nervous system. For its application the tissue must be living : it is therefore best applied by injecting a solution of methylene-blue (1 part to 100 of warm saline solution) into a vein in an ansesthetised mammal, until the whole blood is of a bluish colour ; or the injection may be made through the vessels of the part to be investi- gated, immediately after killing an animal. But fairly good results can also be obtained by immersing small pieces of freshly-excised living tissue in a less concentrated solution (O'l per cent.), or, in the case of the central APPENDIX. 499 nervous system, by dusting the methylene-blue powder over a freshly-cut surface, allowing some time for it to penetrate, and then treating it with picrate of ammonia and Bethe's solution. In either case the tissue should be freely exposed to air; the blue colour then appears in the nerve- cells and axis-cylinders, even to their finest ramifications. It does not however remain, but after a time fades from them while other tissues become coloured. To fix the stain the tissue is taken at the moment that the nerve-fibres are most distinctly seen and is placed for an hour or two in saturated solution of picrate of ammonia, after which the preparation can be mounted in glycerine containing picrate of ammonia. But to allow of sections being made from it for mounting in balsam or dammar, it must, subsequently to the treatment with picrate of ammonia, be placed for some hours in Bethe's fluid, viz. : Molybdate of ammonia, 1 gramme. Chromic acid 2 per cent, solution, 10 o.c. Distilled water,- 10 c.o. Hydrochloric acid, 1 drop. This renders the colour insoluble in alcohol. 24. Sihler's method of staining nerve-endings in muscle and blood-vessels.— Macerate the tissue for eighteen hours in the following solution : Ordinary acetic acid, 1 part. Glycerine, 1 pa^^t. 1 per cent, chloral hydrate solution, 6 parts. From this transfer to glycerine for from one to two hours ; then unravel somewhat with needles and place for from three to ten days in the following : Ehrlich's hsematoxylin, . 1 P="^*- Glycerine, 1 P^""'- 1 per cent, chloral hydrate solution, 6 parts. It may then be kept for any desired time in glycerine, which should be changed several times. . - preparations are made by careful dissociation with needles. If over- stained they may be differentiated by acetic acid until the dark-blue colour is changed to violet. The muscle spindles and the end-plates are well shown by this method. 25. Nissl's method of staining the chromatic granules in nerve-cells.— Th\a is a method of overstaining with methylene blue and subsequent differ- entiation with alcohol (see § 12). Nissl recommended 90 per cent, alcohol as the hardening agent, but both formol and corrosive sublimate followed by alcohol may be employed also. Toluidin-blue (Mann) may be used m place of methylene-blue. The sections may first be stained with 1 per cent, aqueous solution of eosin, and then, after rinsing in water, with 1 per cent, methylene-blue solution : they are best differentiated in an.lme-alcohol. The effect of heating the solutions to about 70° C. is to accelerate and accentuate the staining, which will then take only a few minutes. 2~l2 500 THE ESSENTIALS OF HISTOLOGY. A Nissl stain may also be obtained by placing thin pieces of the fixed and hardened nervous tissue in 1 per cent, solution of thionin for several days ; after which the tissue is dehydrated and embedded in parafi&n. 26. Cajal's reduced silver method for exhibiting neurofibrils within nerve-cells and -fibres. — A small piece of the tissue (brain, spinal cord, ganglion, etc.), not more than 4 mm. thick, and preferably from' a young animal, is placed in 50 c.c. of rectified spirit to which 5 drops of ammonia are added. After twenty-four hours in this, rinse with distilled water and place in a large quantity of 1 per cent, solution of silver nitrate, which is maintained at a temperature of about 30° C. After being five or six days in this solution, the piece is removed, mixed for a few seconds in distilled water, and transferred for twenty-four hours to the following solution : Hydrokinone (or pyrogallio acid), 1 to 1 '5 grammes. Distilled water, 100 cub. cent. Formol, 5 to 10 cub. cent. Rectified spirit, 10 to 15 cub. cent. The addition of alcohol to the above is not indispensable, but favours penetration. The piece is then washed in water for some minutes, trans- ferred to alcohol, embedded in celloidiu, and sections are prepared and mounted in the ordinary way. INBBX. 501 INDEX. ACH Achromatic spindle, 7, 31. Achromatic substance, 8. Adenoid tissue, 76. Adipose tissue, 73. Adrenals. See suprarenal capsules. Air-bubbles, 27. Ameloblasts, 271. Amoeba, 3. Angioblasts, 196. Ansa lenticularis, 415. Aorta, structure of, 189. Appendix, 484. {See also vermiform.) Archoplasm, 8. Areolar tissue, 68. cells of, 70. — — fibres of, 68. Arrector pili, 243. Arteries, nerves of, 196. — structure of, 184. — variation in structure of, 189. — and veins, smaller, structure of, 192. Articular cartilage, 87. — corpuscles, 169. Attraction sphere, 7. Auerbach, plexus of, 296. Autonomic nerves, 132. Axon, 138, 143. Bacteria, 27. Baillarger, lines of, 428. Basilar membrane, 479. Basement membranes, 77. Bechterew, nucleus of. See nucleus. Bellini, ducts of, 324. Bile-ducts, 313. Bladder, 329. Blastoderm, 22. Blood-corpuscles, action of reagents upon, 41, 44. • — of amphibia, 45. — coloured, 31, 32, 41. — colourless, 32. amoeboid phenomena of, 48. granules of, 33. migration from blood-vessels, 51, 72. varieties of, 33. — development of, 36, 40. — enumeration of, 30. — structure of, 31. CEL Blood-crystals, 43. ^ Blood-film, 28. Blood-platelets, 35, 47. Blood-vessels, development of, 36, 196. — structure of, 184, 192. Bone, 96. — development of, 101. — lacunae and canaliculi of, 98. — lamellsB of, 98. — marrow of, 38. Bowman, glands of. See glands. — membrane of, 447. Bronchi, 255. Bronchial tubes, 257. Brain. See cerebrum, cerebellum, medulla oblongata, mesenceph- alon, pons Varolii. — divisions of, 374. — membranes of, 441. Brunner, glands of. See glands. Burdach, tract of. See tracts. Bundle. See tracts. Calleja, islands of, 396. Capillaries, 193. — circulation in, 194. Carotid gland, 222. Cartilage, 86. — articular, 87. — costal, 87, 92. — development of, 90. — embryonic, 90. — hyaline, 87. — ossification of, 101. — parenchymatous, 90. — transitional, 88. — varieties of, 86. Cartilage-cells, 87. — capsules of, 88. Cajal's method of staining neuro- fibrils, 499. Celloidin for embedding, 487. Cell-plate, 16. Cells, division of, 10. — — amitotic, 10. reduction, 14. — embryonic, 1. — membrane of, 8. — nucleus of, 8. — structure of, 2. SOS INDE±. OEM Cement. See crusta petrosa. Central fovea of retina, 463. — tendon of diaphragm, 191. Centriole, 7. Centrosome, 7. Cerebellum, 417. — peduncles of, 398, 402, 424. superior, 398, 411, 424. inferior. See restiform body. middle, 390, 424. Cerebrum, 424. — jiasal ganglia of, 440. — ■ cortex of, 424. structure of different parts, 432. — peduncle of, 405. Chondrin-balls, 90. Choroid coat of eye, 451. Chromatic substance, 8. Chromatolysis, 140. [224. Chromaffin or chroraophil cells, 221, Chromosomes, 9, 12. Cilia, 64. — action of, 65. theories regarding, 66. Ciliary muscle, 452. Clarke, column of, 370. Claustrum, 426. Coccygeal gland. See glands. Cochlea, 474. Cohnheim, areas of, 112. — method of staining nerve-endings, 497. Collaterals, 148. Colostrum-corpuscles, 248. Comma tract. See tracts. Commissures of cerebrum, anterior, 439. posterior, 410. — of spinal cord, 356. Conjunctiva, 444. Connective tissue, cells of, 70. development of, 82. fibres of, 68. jelly-like, 77. — tissues, 68. Cornea, 447. — nerve endings in, 176, 449. Corpora albicantia (mammillaria), 414. — geniculata, 412. — quadrigemina, 406. Corpus luteum, 350. — striatum, 440. — subthalamioum, 415. Corti, organ of, 479. Cotton fibres, 27. Cowper, glands of. See glands. Crusta, 405. — petrosa, 269. Cutis vera, 230. Cytomitome, 7. Cytoplasm, 2. FIB Deitbks, cells of, 481. — nucleus of. See nucleus. Dendrons, 138. Dentine, 263. — formation of, 272. Descemet, membrane of, 449. Deutoplasm, 5. Dilatator pupilla;, 454. Dobie, line of, 113. Doy^re, eminence of, 182. Dust, 27. Ear, 469. Ebner, glands of. See glands. Ehrlich's methylene-blue method, 498. Elastic tissue, 79. Eleiden, 228. Embedding, methods of, 486. Enamel, 263. — formation of, 271. — organ, 272. End-bulbs, 168. Endocardium, 252. Endomysium, 115. Endoneurium, 1.35. Endoplasm, 5. Endothelium, 55. End-plates, ISO. Ependyma, 373, 389. Epicardium, 252. Epidermis, 226. Epididymis, 334. Epineurium, 134. Epiphysis cerebri. See pineal gland. Epithelium, 52. — ciliated, 55, 64. — classification of, 53. — columnar, 55, 61. — glandular, 66. — nerve endings in, 174. — pavement, 55. — stratified, 54. — transitional, 55. Epitrichial layer, 229. Erectile tissue, 330. Erythroblasts, 36. Erythrocytes. See blood-corpuscles, coloured. Eustachian tube, 470. Exoplasm, 5. Eye, 443. Eyelids, 444. Eye-piece, 24. Fallopian tubes, 351. Fat. See adipose tissue. — absorption of, 304. — in cartilage cells, 90, 92. Fenestrated membrane, 187. Fibres. See connective tissue, muscle, nerve, etc. Fibrin, 35. INDEX. 503 FIB Fibro-oartilage, elastic, 93. — white, 93. Fibrous tissue, 80. Fillet. See tract of fillet. Fimbria, 434. Flechsig, method of, 360. — tract of. See tracts. b'lemming,gerin-centreof, 205, 209,216. — stainable bodies of, '205, 216. — method of staining nuclei, 480. Forel, decussation of, 403 (footnote). Freezing method for preparation of sections, 487. Gall-bladdbb, 314. Ganglia, 138. — cells of, 149, — development of, 162, Ganglion of cochlea, 385. of glossopharyngeal, 384. Scarpa, 385. of vagus, 383. — Gasserian, 395. — geniculate, 393. — of habeuula, 405, 414. — interpeduncular, 405, Gas-chamber, 60. Genital corpuscles, 169. Gennari, line of, 428, Germ-ceutre, 205, 209, 216, Germ-nuclei, 18, Gianuzzi, crescents of, 283. Gland or glands. — agminated, 209, 300. — anal, 308. — of Bowman, 469. — of Brnnner, 294, 301. , — carotid, 222. — ceruminous, 245, 469, — classification of, 56. — coccygeal, 222. — of Cowper, 331. — ductless, 59. — of Ebner, 276. — gastric, 288. — hsemal, 207, — internally secreting, 59. — laorymal, 446, — of Lieberkuhn, 279. — of Littrfe, 331. — lymph, 203. — mammary, 246. — Meibomian, 445. — Pacchionian, 442. — pineal, 415. — pituitary, 224 — racemose, 58. — saccular, 58. — salivary, 281. — sebaceous, 243. — secreting, 56. HEN Gland or glands, secreting, varieties of, 58. ~ serous, 276. -* solitary, 209, 299. — sweat, 244. — thymus, 210. — tubular, 58. Glisson, capsule of, 311. Glomeruli of kidney, 320. — olfactory, 439. Glycogen in colourless blood-cor- puscles, 47, — in liver cells, 313. Goblet-cells, 63. Gold-methods o? staining nerve- endings, 497, 498. Golgi, organs of, 174, — cells of, 418. — methods of preparing the nervous system, 498. — reticulum of, 141, 142. — types of nerve-cells, 148. . GoU, tract of. See tracts. Gowers, tract of. See tracts. Graafian follicles, 345. Grandry, corpuscles of, 169. Granules of protoplasm, 4. — of colourless blood-corpuscles, 33. Ground-substance of connective tissue, 2, 68. Gudden, atrophy of, 158. — bundle of. See tracts. — commissure of, 410. Gullet. See oesophagus. Gustatory cells, 278. — pore, 278. H^MAL glands. See glands. Hsematoidin, 44. Hsemin, 44. Heemoglobin, 43. Haemolysis, 42. Hair-cells of internal ear, 473, 480, 481. Hair-follicle, structure of, 235. Hairs, 27, 234. — development of, 241. — muscles of, 243. Hassal, concentric corpuscles of, 211. Haversian canals, 98, — systems, 99. Haycraft, views of, on muscle struc- ture, 113. Heart, 250. — muscle of, 123. — nerves of, 253. — valves of, 253. Helweg, bundle of. See tracts. Henle, fenestrated membrane of, 187. — looped tubules of, 324. — sheath of, 136. Hensen, line of, 113. S04 INDEX. HEP Hepatic lobules, 310. — cells, 312. Herbst, corpuscles of, 173. Hippocampus major, 434. * His, bundle of, 252. Histogenesis, 20. Histology, meaning of term, 1. Hyaloplasm, 4. Hypophysis cerebri. See pituitary body. Idiozomb, 342. Internal capsule, 440. Intestine, large, 308. — small, 296. • Iris, 453. Jelly of Wharton, 85. Kartokinbsis, 10. Kerato-hyaline, 229. Kidney, 320. — blood-vessels of, 326. Krause, membrane of, 113, 116. Labyrinth of ear, 472. — of kidney, 325. Lacteals, 304. Langerhans, islets of, 316. — centro-acinar cells of, 318. Lanugo, 242. Larynx, 256. Lens, 465. Leucocytes. {See blood-corpuscles, colourless). Lieberkiihn, crypts of, 297. Linen fibres, 27. Lissauer, bundle of. See tracts. Littr^, glands of. See glands. Liver, 310. — blood-vessels of, 198, 311. — cells of, 257. — ducts of, 313. — lobules of, 310. — lymphatics of, 315. Loewenthal, tract of. See tracts. Lung, 256. — alveoli of, 261. — blood-vessels of, 261. — lymph-vessels of, 262. Lymph-glands or lymphatic glands, 203. haimal, 206. Lymph- vessels or lymphatics, 198. — connection with cells of connective tissue, 73, 201. — development of, 201. — nerves of, 200. Lymph-corpuscles. See blood-cor- puscles, colourless. Lymphocytes, 34. MUS Lymphoid tissue, 76, 209. development of, 209. Maoula lutea of retina, 463. Malpighi, rete muoosum of, 227. — pyramids of, 320. Malpighian corpuscles of kidney, 320. of spleen, 209, 214. Mammary glands, 246. Marchi's method of staining degen- erated nerve-fibres, 496. Marrowf, 38. Measuring objects, 25. Medulla oblongata, 374. Megakaryocytes, 40, 218. Meissner, plexus of, 297. Membrana tectoria, 482. — tympani, 469. Mesencephalon, 401. Mesothelium, 55. Methods of embedding, 486. — of measuring microscopic objects, 25, — of mounting sections, 491. — of preparing sections, 486. — of preserving and hardening, 484. — of staining, 492. Meynert, bundle of. See tracts. — decussation of, 403, 409. Micrometer, 25. Microscope, 24. Microscopic work, requisites for, 24. Microtomes, 487. Migration of colourless blood-corpus- cles, 51, 195. Mitochondria, 5. Mitosis, 10. Moist chamber, 61. Monakow, bundle of. See tracts. Mould, 27. Mounting solutions, 484. Mucus-secreting cells, 63. Miiller, fibres of, 462. — helicine arteries of, 330. — muscle of, 453. Muscle, accessory disks of, 117. — blood-vessels and lymphatics of, 121. — cardiac, 123. — changes in contraction, 116, 119. — corpuscles, 113. — development of, 122. — ending of, in tendon, 120. — involuntary or plain, 126. — — development of, 127. of arteries, 187. — nerves of, 121, 177, 180. — nuclei of, 113. — of heart, 123. — of insects, 115, 116. — in polarized light, 118. INDEX. 505 MUS Muscle, prinoipEll disk of, 117. — red, 115. — spindles, 122, 177. — structure of, compared with proto- plasm, 119. — voluntary or cross-striated, HI. Myeloplaxes, 40. Myocardium, 250. Nails, 232. — development of, 234. Nerve-cells, 138. — development of, 161. — processes of, 138, 143. — reticulum of, 141. — trophospongium of, 143. — types of, 148. Nerve-fibres, axis cylinder of, 1.32. — degeneration of, 154. — development of, 161. — meduUated, 129. — medullary segments of, 131. — motor, terminations of, 180. — non-meduUated, 133. — regeneration of, 157. — sensory, modes of termination of, 166, 177. — sheaths of, 130. — varieties of, 128. Nerve-trunks, structure of, 134. Nervi nervorum, 136. Neuroblasts, 161. Neurofibrils, 132, 140, 177. Neuroglia, 159. Neurokeratin, 131, Neurolemma, 130. Neurone, 138. — theory, 148. Neuro-synapse, 148. Nissl, granules of, in nerve-cells, 138. Nissl's degeneration of nerve-cells, 140, 155. — method of staining nerve-cells, 499. Nucleolus, 8. Nucleus of cell. Nucleus or nuclei. — of abducens, 394. — of accessory, 378, 381. — of Bechterew, 389, 397. — oaudatus, 440. — of cochlear nerve, 385. — cuneatus, 377. — of Deiters, 388, 392, 397, 402. — dentatus cerebelli, 417. — of facial, 392. — of glossopharyngeal, 379, 384. — gracilis, 377. — of hypoglossal, 378, 381. — lenticularis, 440. — oculomotor, 401. — of olive, 380, PIT Nucleus or nuclei. — of pons, 390. — of posterior longitudinal bundle, 403. — preolivary, 392. — red, of tegumentum, 402. — semilunar, 392. — of Stilling, 417. - superior olivary, 392. — tecti (s. fastigii), 417. — of thalamus, 411. — of trapezium, 391. — of trigeminal, 394. — of vagus, 379, 383. — of vestibular nerve, 387. Objective, 24. Ocular, 24. Odontoblasts, 268. CEsophagus, 280. Olfactory bulb, 436. — cells, 439, 468. — mucous membrane, 467. — path, 439. — tract, 436. Olive, 375. — superior, 392. Omentum, 191. , Onychogenic substance, 232. Opsonins, 50. Optic chiasma, 409. — nerves, 409. — thalamus, 410. — tract, 409. Ossification in cartilage, 102. — in membrane, 108. Osteoblasts, 100, 103. Osteoclasts, 105. Osteogenic fibres, 108. Ovary, 344. Ovum, 22, 343. — division of, 17. Pacinian corpuscles, 169. Pancreas, 316. Papillae of tongue, 275. — of skin, 230. Paranucleus, 5, 318. Paraplasm, 5. Parathyroids, 222. Penis, 330. Pericardium, 252. Perineurium, 134. Periosteum, 100. Peyer, patches of, 209, 300. Phagocytes, 33, 50, 218. Pharynx, 280. Pick, bundle of, 376. Pigment-cells, 72. Pineal gland, 415. Pituitary body, 224. 506 INDEX. PLE Pleura, 262. Polar bodies, 17. Pons Varolii, 390. Portal canal, 311. Posterior longitudinal bundle. See • tracts. — commissure. See commissures. Prickle-cells, 54. Pronuclei, 18. Proprio-spinal fibres of cord, 361, 366. Prostate, 331. Protoplasm, 2. Purkinje, cells of, 418. — fibres of, 125, 252. Pyramids of medulla oblongata, 375. Ranvibb, constrictions of, 130. Recklinghausen, method of staining with silver nitrate, 496. Rell, fillet of, 398. — island of, 426. Reissner, membrane of, 477. Remak, fibres of, 133. Restiform body, 380, 389, 424. Reticular or retiform tissue, 75. Retina, 454. — macula lutea of, 463. — pars ciliaris of, 465. Rhinencephalon, 433. Rolando, tubercle of, 377. RoUett's method of staining muscle, no. Rouleaux (of blood-corpuscles), causa- tion of, 43. Ruffini, organs of, 173. Saccule, 473. Salivary corpuscles, 52. — glands, 281. Sarcolemma, 111. Sarcomeres, 116. Sarcoplasm, 112. Saroostyles, 112. Sarcous element^ 116. Schwann, sheath of, 130. Sclerotic coat of eye, 446. Sebaceous glands. See glands. Sections, preparation of, 486. Semicircular canals, 473. Seminiferous tubules, 337. Serous membranes, 201. Sertoli, cells of, 341. Sharpey, fibres of, 99, 100. Sihler's method of staining, nerve- endings, 499. Silver-methods, 496, 499. Sinusoids, 185, 197. Skin, 226. Spermatogenesis, 341, Spermatozoa, 338, TRA Sphincter ani, internal, 308. Sphincter pupillse, 453. Spinal bulb. See medulla oblongata. Spinal cord, 355. blood-vessels of, 373. central canal of, 356, 373, 378. — — characters in different parts, 358. — — connection of nerve roots with, 371. grey matter of, 366. membranes of, 355. nerve-cells of, 366. tracts in, 360. Spinal ganglia, 149. Spleen, 213. Spongioblasts, 161. Spongioplasm, 4. Staining of sections, 493. Stanley-Kent, bundle of, 252. Starch granules, 25. Stilling, nucleus of, in cord, 370. Stomach, 287. — blood-vessels of, 294. — glands of, 288. — lymphatics of, 294. Stomata, 201. Stroma (of blood corpuscle), 43. Substantia nigra, 405. Subthalamus, 415. Suprarenal capsules, 218. Sweat-glands, 244. Sylvian aqueduct, 401. Sympathetic ganglia, 150. — nerves, 133. Synapse, 148. Syncytium, 2, 125, 194. Synovial membranes, 90. Tactile corpuscles, 167. — disks, 169, 177. Taste-buds, 277. Teeth, structure of, 263. — formation of, 269. — pulp of, 268. Tegmentum, 402. Tendon, 82. — nerve-endings in, 174. Testicle, 332. Thalamenuephalon, 410. Thalamus, 410. Thrombocytes, 35. Thymus gland, 210. Thyroid body, 221. Tissues, enumeration of, 1. — formation from blastodermic layers, 22. Tongue, 275. Tonsils, 207. Tooth. See teeth. Trachea, 254, INDEX. 507 TEA Tract or tracts or bundles. — anterior longitudinal. See tecto- spinal tract. — anterolateral ascending. See tract of Gowers. descending. See tract of Loew- enthal. — bulbothalamio. See tract of fillet. — of Burdach, 360, 377. — central, of cranial nerves, 398. — central, of tegmentum, 381, 397. — cerebello-bulbar, 384. — comma, 361. — of cord, 360. — eortioo-bulbar, 396, 398. — cortico-spinal. jSee pyramidal tract. — crossed pyramidal. See pyramidal tract. — descending cerebellar, 399. — direct pyramidal. See pyramidal tract. — of GoU, 360, 377. — myelination of, 360. — of fillet, 377, 387, 397, 404, 411. — of Fleohsig, 365, 381. — of Gowers, 365, 381, 399. — of Gudden, 403 (footnote), 414. — of Helweg, 365. — of Lissauer, 366, 371. — of Loewenthal, 363. — of Marie, 366. — of Meynert, 405. — of Monakow, 365, 396, 403. — of Miinzer, 403. — of Pick, 376. — olfactory, 436. — olivo-spinal. See tract of Helweg. — optic, 409. — ponto-spinal, 397. — posterior longitudinal, 363, 388, 392, 396, 402. — prepyramidal. See tract of Mona- kow. — pyramidal, 361, 376, 390, 396, 405, 409. — of Risien-Russell, 418. — rubro-spinal. See tract of Mona- kow. YEA Tract or tracts or bundles. — spino-cerebellar, 365, 370. — spino-tectal, 366. — spino-thalamic, 366. — tecto-spinal, 365, 397, 403. — thalamo-bulbar, 398. — thalamo-olivary, 397. — transverse peduncular, 410. — ventral longitudinal. See tecto- spinal tract. — vestibulo-motor. See posterior longitudinal tract. — vestibulo-spinal, 363, 397. — of Vicq d'Azyr, 414. Trapezium, 391. Trophospongium, 4, 143. Tympanum, 469. Ueeter, 328. Urethra, 231. Urinary bladder, 329. Uriniferous tubules, course of, 322. Uterus, 352. Utricle, 473. Vas deferens, 335. Vasa vasorum, 196. Vasoformative cells, 36, 196. Veins, structure of, 189. — valves of, 190. — variations in, 190. Vermiform appendix, 308. Vesiculse seminales, 336. Villi, arachnoidal, 442. — of intestine, 302. — of synovial membrane, 91. Vitreous humour, 466. Volkmann, canals of, 99. Walleriah degeneration, 154, 360. Warming apparatus, 48. Weigert-Pal method for staining sec- tions of the nervous system, 496. Woollen fibres, 27. Yeast, 27. GLA9Q0W : PRINTED AT THE UNIVBR8ITT PKKSS BY ROBERT MACLEHOSE AND CO. LTD.