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 AND SURGEONS 
 
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NOTES 
 
 ON THE 
 
 PRACTICAL COURSE 
 
 IN 
 
 Normal Histology 
 
 GIVEN IN THE 
 
 Laboratory of the Alumni Association 
 
 OF THE 
 
 COLLEGE OF PHYSICIANS AND SURGEONS 
 
 NEW YORK CITY 
 
 BY 
 
 T. MITCHELL PRUDDEN, M.D. 
 
 NEW YORK 
 TROWS PRINTING AND BOOKBINDING CO., 
 
 205-213 East 12TH Street 
 1879 
 
PREFACE. 
 
 The full course for classes in Normal Histology in this laboratory 
 comprises forty lessons, of from one and a half to two hours each. 
 Students are furnished with microscopes, reagents, and all necessary 
 apparatus, with the exception of razors, slides, cover-glasses, and boxes 
 for the preservation of specimens. So little time is usually at the dis- 
 posal of medical students for collateral reading, and so necessary is it 
 to occupy as little of the laboratory time as possible in oral descrip- 
 tions of tissues and methods, that these notes have been prepared 
 with the expectation that students will anticipate each lesson by read- 
 ing beforehand the brief section devoted to its theme, and thus be 
 ready to commence the practical work of the lesson hour without loss 
 of time. 
 
 It is not to be expected that epitomized descriptions of structures 
 as elaborate as are many of those with which we have to deal in his- 
 tology, will be in all cases perfectly clear and intelligible without the 
 aid of figures ; but the actual specimens prepared, and the sketches 
 from them which are made in the laboratory by the students them- 
 selves, will make good, it is hoped, the lack of illustration in the text. 
 
 There are many points in this as in every developing science which 
 are still unsettled — opinion in regard to them changing or being modi- 
 fied as new facts and investigations are recorded. These have been 
 treated, for the most part, very briefly in the text, it being left for the 
 supplementary oral instruction to enlarge upon and explain them as 
 the light thrown upon each by new researches may seem to require. 
 
 T. M. P. 
 
 Laboratory of the Alumni Association, College of Physicians 
 and Surgeons, New York, Sept., 1879. 
 
INTRODUCTION. 
 
 GttJNJiRAL METHODS FOR PRESERVING TISSUES AND PREPAR- 
 ING THEM FOR STUDY. 
 
 Animal tissues must conform to certain physical conditions before they 
 can be subjected to a satisfactory microscopical examination. Por- 
 tions of them subjected to study must be sufficiently thin to allow the 
 light to pass readily through them, and transparent enough to permit 
 the determination of the form, character, and relations of their struc- 
 tural elements. At the same time the refractive power of the different 
 elements should not be too nearly alike, since upon differences in this 
 respect the form and characters which microscopical objects present 
 to the eye are largely dependent ; or, in case they are so, the differ- 
 ent elements must be rendered visible by staining them with coloring 
 agents. Certain tissues naturally undergo rapid changes of structure 
 after death ; these are to be prevented by the application of preserva- 
 tive agents. Some are too soft to permit the preparation of thin sec- 
 tions and must be hardened ; others are too hard and must be softened. 
 In some specimens one, in others another structural feature, is to be 
 brought into prominence. All of these indications in the histological 
 technique are to be met in such a way as to leave the structures under 
 investigation in as natural a form as possible. Finally, specimens 
 suitably prepared for examination are, in many cases, to be rendered 
 permanent for future reference and study. 
 
 We will now consider briefly some of the methods by which these 
 indications may be fulfilled. Most histological specimens are laid 
 either with or without some enclosing fluid medium, on a glass plate, 
 and covered with a very thin slip of glass, before being brought under 
 the instrument. One of the simplest methods of studying tissues is to 
 place them, when quite fresh, and after they are reduced to a condition 
 of suitable tenuity, on a slide with some fluid which alters their physi- 
 cal condition but little or not at all, or at least very slowly, and ex- 
 amine them at once. Such fluids are called indifferent fluids ; and 
 among the best and most commonly employed are the aqueous humor, 
 blood-serum, amniotic fluid. These organic fluids, however, although 
 well suited for this purpose, are not always readily obtained, and are 
 moreover liable to undergo more or less rapid decomposition ; and 
 since for most purposes a dilute solution of common salt, one-half to 
 three-quarters per cent., answers very well, we shall generally employ 
 
6 NORMAL HISTOLOGY. 
 
 this when, in the following lessons, we have occasion to use an indif- 
 ferent fluid in the study of fresh tissues. 
 
 The examination of fresh tissues is very important, not only because 
 it enables us to follow the vital phenomena in certain 'elements, but 
 because we are able by comparison to determine the amount of change 
 which tissues undergo when prepared by more elaborate methods. 
 Still this simple mode of examination is in many respects unsatisfac- 
 tory. In the first place, it is not always easy to procure fresh tissues 
 for' every observation, and even in the indifferent fluids the tissues 
 sooner or later undergo very considerable structural alterations, so that 
 they cannot be permanently preserved. Again, fresh tissues are fre- 
 quently not sufficiently hard and firm to allow the necessary prepara- 
 tion of specimens. A still more important difficulty which this method 
 presents is the lack of clearness in the details of structure of fresh 
 tissues. A very considerable proportion of the fresh animal tissues are 
 nearly transparent, in thin pieces, and their structural elements possess 
 so nearly the same refractive power, that we see through them, but do 
 not see them : or, if we do see them indistinctly, it is not generally with 
 that definiteness which our purposes demand. Now, these difficulties 
 are usually met by the employment of agents which harden and pre- 
 serve the tissues and at the same time render the details of their struc- 
 ture visible, by changing the refractive power of one or other of their 
 elements ; or we employ, as above indicated, certain coloring agents, 
 which, being taken up with different degrees of avidity by different 
 parts, permit the recognition of details by differences in color ; or, 
 such agents are used as both harden and stain at once ; or, finally, 
 which is the most common method, we employ two or more of the 
 different classes of agents one after the other. We shall consider 
 here only some of the most commonly employed of the hardening, 
 preservative and coloring agents. 
 
 Alcohol is one of the most valuable of the preservative and harden- 
 ing agents. It causes a considerable shrinkage of the tissues by the 
 withdrawal of water from them, and, like many of the preservative 
 agents, causes a precipitation of certain of their albuminoid constitu- 
 ents, thus diminishing their transparency. 
 
 Alcohol is, in general, to be used at first diluted -with one-third 
 water, and after the bits of tissue have lain for twenty-four hours in 
 this they are transferred to commercial alcohol (eighty-five per cent.), 
 in which they may be preserved indefinitely, losing in time, how- 
 ever, somewhat of the first clearness and naturalness of structural 
 detail. 
 
 Bits of tissue to be preserved in alcohol, as in other hardening 
 agents, should be quite small — not larger, as a rule, than i or 2 cms. 
 on a side — and the quantity of fluid should be abundant. Certain 
 structures are best preserved by plunging them at once into strong 
 alcohol. 
 
 Chromic acid in solution and solutions of potassium and ammo- 
 nium bichromate are very frequently employed to preserve and harden 
 tissue, and many structures are more perfectly preserved in these fluids 
 than in alcohol. The hardening process proceeds more slowly in the 
 
INTRODUCTION. 7 
 
 chromic solutions than in alcohol, and the structures do not shrink as 
 much. The tissue seems to be hardened and preserved by a slow 
 process somewhat analogous to that of tanning. 
 
 It is a common practice to commence the hardening process with 
 one of the chromic fluids, and complete it with alcohol. 
 
 Pure chromic acid is usually employed in solution of from one- 
 sixth to one-half per cent. Potassium and ammonium bichromate are 
 usually used in 2 per cent, solutions. A very valuable and much em- 
 ployed preservative solution is the so-called Midler's Fluid, consist- 
 ing of — 
 
 Sodium Sulphate I 
 
 Potassium Bichromate 2 
 
 Water 100 
 
 The ingredients are simply dissolved in water and the solution fil- 
 tered. Chromic acid dissolves the lime salts in bone, and is often used 
 to soften them in preparation for section-cutting. Tissues which con- 
 tain fat are usually better preserved in the chromic fluids than in alco- 
 hol, since the fat is readily dissolved by the latter. 
 
 Picric Acid. — This agent hardens tissues, preserving in many 
 cases their structural features most perfectly, and at the same time 
 staining them intensely yellow. It is generally necessary to complete 
 the hardening process with alcohol. 
 
 Picric acid is one of the best agents for the decalcification of bone, 
 although it acts very slowly. It is commonly employed in cold, satu- 
 rated, aqueous solution. 
 
 Osmic Acid. — This substance has the power, in dilute solutions, 
 of fixing and hardening the tissue elements in a nearly normal form, 
 and is one of the most valuable of this class of agents. It gives tis- 
 sues a gray or brown appearance, and stains fat and certain allied 
 substances deep black. It is a very expensive substance, and hence 
 its use is at present somewhat limited. It is generally employed in a 
 one per cent, aqueous solution, and the tissues should be quite fresh 
 when immersed in it, and, as a rule, should remain for twenty-four 
 hours. Specimens hardened in osmic acid, although very perfect at 
 first, commonly become quite granular and black after a time, and 
 nearly worthless. 
 
 The preservative fluids are sometimes brought into more perfect 
 and immediate contact with the tissue elements by injecting them 
 into the blood-vessels of the part before cutting them in pieces and 
 immersing them in the fluids. This method is of special value when 
 alcohol is used for hardening small organs like the kidney. 
 
 Indications as to which of these agents are best adapted for the 
 preservation of different' tissues, and the more exact details of the 
 methods of employing them, will be given as we proceed with our 
 practical study. 
 
 Hematoxylin is one of the most generally useful of the staining 
 agents. It has the power of staining certain parts, as the nuclei of 
 cells, deeply, while other parts are much less or not at all stained. It 
 
8 NORMAL HISTOLOGY. 
 
 can be employed for staining tissues which have been hardened by 
 any of the above agents. 
 
 The following is Prof. Delafield's method of preparing the solu- 
 tion : 
 
 To make 200 c.c. of the solution, take 150 c.c. of saturated solu- 
 tion of ammonia alum ; prepare a saturated solution of haematoxylin 
 in absolute alcohol, and add 4 c.c. of it to the alum solution. This at 
 first produces a violet, or sometimes a dirty red color, which, on ex- 
 posure to the light in an open vessel, usually assumes in a few days a 
 deep violet color. If the color does not become deep enough, a few 
 drops more of the hematoxylin solution are added and the fluid exposed 
 anew to the light. After standing for at least a week, and when the 
 desired color is obtained, the solution is filtered and 25 c.c. each of 
 glycerine and wood naphtha are added. Such a solution is to be diluted 
 with several times its bulk of water before using, the exact amount 
 of dilution depending upon the rapidity with which we wish the speci- 
 men to be stained. As a rule, slow staining with a dilute solution 
 gives the best results, and is less likely to cause shrinkage of the spe- 
 cimen. In staining, bits of tissue are simply placed in a small dish of 
 the solution, so that they are bathed on all sides by it and allowed to 
 remain until sufficiently colored. The time required will depend, of 
 course, upon the strength of the solution, and also to a considerable 
 degree upon the character and previous preparation of the tissue. 
 The excess of coloring fluid is to be thoroughly washed out of the 
 specimen by water before mounting and studying. 
 
 Carmine. — This is employed in the same manner as hematoxylin, 
 and like it stains different tissue elements with different degrees of in- 
 tensity. Tissues preserved in chromic acid solutions do not stain as 
 readily in carmine as in haematoxylin. 
 
 Fre/s method for its preparation is the following : take of powdered 
 carmine 0.30 grm., and add a sufficient quantity of aqua ammonias to 
 dissolve it, mix with 30 c.c. of distilled water, filter, and add glycerine 
 30 grms., alcohol 4 grms. 
 
 Eosin. — This substance stains tissues somewhat more uniformly 
 than those just mentioned, and is especially valuable when used in 
 connection with other coloring agents, such as haematoxylin, which 
 stain the cell nuclei more deeply, since by this method of double 
 staining we have certain structural elements exhibiting one color, 
 others another. Eosin may be conveniently used either in aqueous or 
 alcoholic solutions of 1 to 100. 
 
 Picro-carminate of Ammonia or Pier o-car mine. — This substance is 
 for many purposes superior to the simple carmine. It usually stains 
 more rapidly than the latter, and we can obtain with it at once the 
 yellow color from the picric acid in certain elements, while others are 
 stained red by the carmine. 
 
 It is prepared by adding to a saturated solution of picric acid a 
 strong ammoniacal solution of carmine to saturation, evaporating the 
 mixture to one-fifth its bulk, allowing to cool, filtering from the de- 
 posit, and evaporating the filtrate to dryness over a water-bath. The 
 picro-carmine is left in the form of a crystalline ochre-red powder. 
 
INTRODUCTION. 9 
 
 This powder should be dissolved in water, for use, in the proportion 
 of t to 100. 
 
 Certain fluid tissues, such as blood, lymph, etc., are fitted for 
 study, either fresh or after suitable preservation, when they are spread 
 out in thin layers and covered. Certain tissues occur in the form of 
 membranes of sufficient thinness to admit of study without other ma- 
 nipulation than spreading them out smoothly on a slide. In other 
 cases we have recourse to the dissociation of tissues by needles, the 
 structure being carefully separated into parts sufficiently minute for 
 microscopical examination. The operation of dissociation with needles, 
 or picking, or teasing, as it is commonly called, will vary in its details, 
 depending upon the nature of the tissue and the special feature which 
 we wish to study. In many cases we wish to study the structural ele- 
 ments of a tissue in their normal relations to one another, and in parts 
 which are too thick to permit a direct observation. In such cases we 
 have recourse to thin sections cut from the tissues by a sharp knife or 
 razor — the tissue, if not sufficiently hard naturally, being hardened by 
 one or other of the above described methods. The razor employed 
 for this purpose should have a thin blade, perfectly flat on the lower 
 side, as it is held horizontally in the hand for cutting, and somewhat 
 concave on the upper side, so that a small quantity of fluid will lie 
 upon it. 
 
 It is often desirable, in studying the distribution of the blood- or 
 lymph-vessels, to fill them by injection with some colored substance 
 by means of which their ramifications may be readily recognized. One 
 of the most convenient and generally employed injecting materials is 
 a solution of gelatine colored with Prussian blue. This may be pre- 
 pared as follows : dissolve 4 grms. of gelatine in 60 c.c. of pure water 
 on a water-bath ; divide the solution into two portions : to one portion 
 add 4 c.c. of a saturated solution of sulphate of iron (green vitriol), 
 stirring constantly; to the other add first 8 c.c. saturated solution of 
 ferrocyanide of potassium, and then 8 c.c. saturated solution of oxalic 
 acid ; these two portions are now to be slowly mixed with constant 
 stirring, and then heated up to about the boiling-point of water. The 
 solution is now filtered hot through flannel and is ready for use. An 
 animal injected with this solution must of course be kept warm during 
 the injection, as must all the utensils employed, so that the gelatine 
 may not harden and stop the vessels. The details of the process of 
 injection will be given in the course of our practical study. 
 
 It often occurs that a bit of tissue from which we wish to prepare 
 a section is too small and delicate to be held in the fingers ; in such 
 cases the object may be placed between two bits of some hardened 
 tissue, such as liver, or between two bits of soft cork, and thus held 
 while the section is made. Or such a specimen may be embedded in 
 a mixture of white wax and paraffine, equal parts, melted together 
 with addition of a sufficient quantity of olive oil to give the mass the 
 proper consistency for cutting when cold. The details of the methods 
 of teasing, embedding, and section-cutting, will be learned in the 
 course of our practical studies. 
 
 Sections, bits of dissociated tissue, membranes, etc., having been 
 
10 NORMAL HISTOLOGY. 
 
 dul) T prepared, they are to be mounted on a slide for study. The 
 choice of the fluid to be employed for this purpose will depend upon 
 the nature of the specimen, the mode of preparation to which it has 
 been subjected, and the structural features which we wish especially 
 to demonstrate. One of the fluids most frequently employed is glyce- 
 rine. Many of the preservative and hardening agents, such as alcohol, 
 precipitate, as above remarked, certain albuminoid substances in the 
 tissues, in the form of small, strongly refractive particles, thus render- 
 ing them more or less opaque,* or at least translucent. Now, glycerine 
 possesses the power of penetrating many such tissues, and since, as a 
 rule, its index of refraction is much more nearly like that of the albu- 
 minous particles than is the refractive index of the substance lying 
 between them, when the tissue becomes soaked with glycerine the 
 light passes more directly through and the tissue becomes more trans- 
 parent. 
 
 Many specimens, further, preserve their structural features for a 
 long time very perfectly in glycerine. The specimens are either 
 soaked until they become transparent, in a small dish of glycerine, 
 and then transferred to a slide and covered with a drop of the same ; 
 or they may be put at once into a drop upon the slide and covered. 
 The preparations may be made permanent by painting a rim of some 
 kind of varnish — such as asphalt varnish — around the edge of the 
 cover-glass. 
 
 The strongly refractive power of glycerine, although of value in 
 rendering tissues transparent in the way just described, is, however, in 
 some cases prejudicial to our aims, because it makes them too trans- 
 parent, its refractive power being so nearly like that of the tissue- 
 elements themselves that they remain nearly, invisible, or their more 
 delicate structure is concealed. For we find within certain limits that 
 the greater the difference between the refractive power of an object 
 and that of the fluid in which it lies, the more distinct will be the out- 
 lines of the object. We have, then, in using glycerine as a mounting 
 medium which renders tissues transparent, to guard against its too 
 excessive action ; and this may be done by mixing with it, in propor- 
 tions varying with the tissue under investigation, some less refractive 
 substance such as water. For permanent preservation, however, most 
 tissues are to be put into pure glycerine. 
 
 Ca?iada balsam is for many tissues a most excellent mounting 
 medium. It possesses, to a still greater degree than glycerine, the 
 power of rendering tissues transparent, concealing proportionately, in 
 the manner above described, certain of their minute structural features. 
 This difficulty can, however, be to a considerable extent obviated with 
 balsam, as with glycerine, by the judicious use of coloring agents. 
 The most convenient way of using Canada balsam is in solution in 
 
 * A transparent object becomes more or less opaque or translucent when in any- 
 way it is made to enclose a multitude of small strongly refractive particles, because 
 the light which enters it, passing alternately from a more strongly refractive particle 
 into the less refractive substance which surrounds it, and then again into another par- 
 ticle, and so on, finally becomes to a certain extent lost, a part only reaching the eye, 
 and that after a roundabout course among the particles. 
 
INTRODUCTION. 1 1 
 
 chloroform. The pure commercial balsam is thinned with chloroform 
 until it will drop readily from the end of a glass rod. It is to be kept 
 in tightly stoppered bottles. The mode of procedure in Canada 
 balsam mounting is the following : the specimen having been suit- 
 ably prepared and stained, it is freed as completely as possible from 
 water by touching its edges with a bit of fine blotting-paper, and then 
 laid into a dish containing a few cubic centimetres of common strong 
 alcohol ; after from ten to fifteen minutes it is transferred to absolute 
 alcohol. It lies in this for from ten to fifteen minutes, when it is taken 
 out, the superfluous alcohol removed as before, and laid in oil of cloves. 
 As soon as it becomes transparent, which will usually occur in from 
 five to ten minutes, the excess of oil is removed with blotting-paper 
 and the section is transferred to a drop of balsam on a slide and cov- 
 ered. This process, which seems at first somewhat complicated, is 
 readily understood when we remember that neither the water in which 
 the specimen usually lies when the staining is completed, nor the alco- 
 hol, which is very hygroscopic and removes the water from the speci- 
 men, are miseible with balsam ; and, further, that alcohol is miscible 
 with oil of cloves, and this with balsam. Care must be taken, in these 
 manipulations, not to breathe on the specimen, nor to allow any 
 moisture to come in contact with it, since even a small amount of 
 moisture causes a precipitate in the balsam and renders the specimen 
 well-nigh worthless, unless it be remounted. Canada balsam is, as a 
 rule, best adapted for mounting those specimens in which we wish to 
 study the general structure of a tissue rather than its more minute 
 characters. Preparations in which the blood and lymphatic vessels 
 have been injected with some colored material usually show well when 
 mounted in balsam. In a very large number of cases, a double stain- 
 ing of the specimen with hematoxylin and eosin, before mounting in 
 balsam, gives excellent results. To accomplish this, we may first 
 stain in the usual way with hematoxylin, and accomplish the eosin 
 staining by adding a small amount of a solution of eosin in absolute 
 alcohol to the absolute alcohol used for the dehydration of the speci- 
 men, before laying it in balsam. 
 
 Acetic acid is an agent of great value in certain examinations of 
 tissue. It, too, possesses the power of rendering certain tissues trans- 
 parent. In accomplishing this effect, however, it acts in an entirely 
 different way from the above mentioned agents. It does not simply 
 surround the structures with a strongly refractive substance, but it 
 causes albuminoid particles to swell up and become actually more 
 transparent. Moreover, it does not act alike upon all parts of tissues ; 
 for example, while it renders cell-bodies more transparent, it does not 
 produce the same effect upon the nuclei, but rather causes them to be- 
 come more distinct, through actual shrinkage and the greater contrast 
 which they present after the change to the transparent cell-bodies. It 
 is usually employed in the study of fresh tissues, and is much less used 
 than formerly, on account of the very essential changes in form which 
 it is now known to induce in the tissue-elements. It may be used in 
 solutions of from two to three per cent. 
 
 The chemical agents which the histologist uses, and the manipula- 
 
12 NORMAL HISTOLOGY. 
 
 tive devices to which he has recourse in the study of the tissues, are 
 very numerous, and we have considered here only a few of the more 
 important and typical. We shall acquaint ourselves with others as we 
 proceed with the practical study. The preparation of each tissue 
 presents to the worker in histology a separate problem, and in few de- 
 partments of science is careful attention to technical minutiae of more 
 importance than in that which now engages us. 
 
CHAPTER I. 
 
 THE CELL IN GENERAL. 
 
 In all animal bodies are found certain tiny structural elements, 
 called cells ; and before commencing the systematic study of the tis- 
 sues, it is necessary to obtain some definite conception of the nature 
 of these elementary organisms. We may consider cells from a mor- 
 phological and from a physiological standpoint. What is the structure 
 of cells, and what do they do ? First, then, what is the structure of 
 cells ? We find a great diversity in the structure of animal cells, some 
 being extremely simple, others quite complex. The most common, 
 and usually the most prominent structural feature of the cell, that 
 portion which gives to it form and consistence, is called the cell-body. 
 This usually consists of an albuminoid material, sometimes transpar- 
 ent and apparently structureless, sometimes finely or coarsely granu- 
 lar, and not infrequently presenting, at least after death, a reticulated 
 appearance; this material is called protoplasm in its typical active 
 form, and may be then very soft and viscid, like thin jelly ; or it may 
 become so modified as to be hard, and even horny like the nail. The 
 cell-body presents a great variety of forms ; it may be spherical, cu- 
 boidal, cylindrical, fusiform, ovoid, pear-shaped, discoidal, or scale- 
 like ; it often sends off processes like branches or wings, and some- 
 times assumes the most irregular bizarre forms. We not infrequently 
 find embedded in the cell-body, pigment-granules, droplets of fat, and 
 various kinds of crystals. The form of the cell-body seems usually to 
 depend largely upon the pressure to which it is or has been subjected 
 by adjacent structures. 
 
 Within the cell-body we usually find, centrally placed or at the 
 side, one or more spherical, ovoidal, or irregular-shaped bodies, called 
 nuclei (singular, nucleus). The nucleus usually presents a sharply de- 
 fined contour, is frequently coarsely granular, and sometimes a coarse 
 or filiform network is seen suspended within or stretching from side to 
 side. The nucleus may be very small in proportion to the size of the 
 cell-body, or may make up nearly the entire bulk of the cell. In the 
 processes of degeneration and decomposition, and under the action of 
 certain chemical agents, the nucleus seems to be more resistant than 
 the cell-body, and on treatment of the cell with certain coloring 
 agents, such as carmine and hematoxylin, the nucleus is more deeply 
 stained than the cell-body. Within the nucleus, again, we frequently 
 find one or more small spherical or irregular-shaped bodies, looking 
 like vesicles or shining granules, which are called nucleoli. They 
 
14 NORMAL HISTOLOGY. 
 
 would seem, in some cases at least, to be connected with the above- 
 mentioned intra-nuclear network. Of the exact nature and signifi- 
 cance of the nucleus and nucleolus, we have, at present, little definite 
 knowledge. Finally, a small proportion of animal cells are enclosed 
 by an envelope — the cell-membrane, which may be thick or thin, now 
 presenting well-defined structural peculiarities, and again quite homo- 
 geneous and structureless. In many cases the cell-membrane would 
 seem to be simply a peripheral hardened layer of the cell-body. It 
 will thus be seen that we may have, in an animal cell, four distinct 
 structural elements : the body, the nucleus, the nucleolus, and the mem- 
 brane. It is only in a few varieties of cells, however, that all of these 
 elements are present. The cell-membrane is the least commonly pre- 
 sent of all, and there are certain cells which consist of a cell-body alone. 
 
 Regarding cells now from a physiological point of view, we find 
 the expression of their vitality in four distinct ways : they nourish 
 themselves ; they grow ; they perform certain functions for their own 
 good and for the benefit of the organism of which they form a part ; 
 and finally, under certain circumstances, they are capable of repro- 
 ducing their like. Or, in more concise language, we say : the cell ex- 
 presses its vitality in nutrition, growth, function, and reproduction. 
 Not all of these expressions of vitality, however, can be subjected to 
 direct microscopical observation. Nutrition being, as we believe, es- 
 sentially a chemical process, cannot become the subject of direct 
 microscopical study, at least until very essential, and at present ap- 
 parently impossible, improvements shall have been made in our 
 technique. The growth of cells too, is for the most part so gradual, 
 that we cannot directly follow it, but are obliged to study it in differ- 
 ent phases of its progress. 
 
 The functional activity of cells can be indirectly subjected to mi- 
 croscopical investigation when it is associated with demonstrable 
 changes in the morphological characters of the cell, or directly ob- 
 served when it expresses itself in motion ; thus, we can readily detect 
 the difference between a condition of functional activity and rest in 
 the peptic cells of the stomach ; and the observation of ciliary and 
 amoeboid movements in certain cells are among the most fascinating 
 of histological studies. Finally, regarding the reproduction of cells, 
 in a few instances the act has been directly observed under the micro- 
 scope, but in the majority of cases our knowledge is derived from the 
 study of a succession of consecutive stages in the process. Every 
 new cell which appears in the animal body, during and subsequently 
 to its development, is derived from a pre-existing cell, and all the cells 
 are derivatives of a single original cell, the ovum. New cells are pro- 
 duced by a process of division in older cells. Cell-division seems to 
 occur in a variety of ways, usually commencing in the nucleus. Of 
 the details of the process we have at present but little satisfactory 
 classified knowledge, and although interesting and valuable facts are 
 rapidly accumulating, the scope of these lessons will not permit us to 
 tarry long upon the subject. 
 
 In one of the apparently simplest modes of cell-division, one or 
 more constrictions appear around the cell-body, either before or after 
 
THE CELL IN GENERAL. 1 5 
 
 certain changes in the nucleus have led to its partial or entire division ; 
 these constrictions grow deeper and deeper, until finally the parts sepa- 
 rate and become individual cells. This variety of cell-divison is fre- 
 quently termed reproduction by fission. 
 
 If division occurs in cells which are enclosed in a distinct mem- 
 brane or envelope, so that the new organisms are not at once set free, 
 the process is called endogenous cell-reproduction. 
 
 Or finally, as frequently occurs, the cell-body may send off from 
 some part a bud-like process, which by a constriction of its pedicle is 
 at length freed ; this process is called cell-reproduction by budding 
 or gemmation. 
 
 These modes of cell-reproduction seem to be different, and yet 
 there is much reason for believing that they are really only modifica- 
 tions of one process ; but in the present condition of our knowledge 
 on the subject all general statements should be very cautiously made, 
 and such classifications as the above are to be regarded merely as for 
 convenience of study. 
 
 Cells are variously classified according to their nature and relation 
 to adjacent parts : thus, we have epithelial cells, which cover the skin 
 and mucous membranes and occur in certain parts of the glandular 
 organs; connective-tissue cells, which lie scattered through the sub- 
 stance of the structures presently to be studied as connective tissue, 
 in certain parts undergoing modification of form and relation to neigh- 
 boring parts and called endothelial cells ; gland-cells are those which, 
 possessing peculiar functional or morphological characters, make up 
 the tissue of the parenchyma of certain glands and organs. There are 
 other classes of cells which will be considered in our systematic study 
 of the tissues. 
 
 PRACTICAL STUDY. 
 
 Many of the general characters of cells may be learned from the study 
 of the epithelial cells of the bladder, the pigmented cells of the retina, 
 and the ciliated cells from the mucous membrane of the frog's mouth. 
 
 Epithelial Cells from the Rabbit's Bladder. — The bladder of a re- 
 cently killed rabbit is opened and the mucous membrane scraped with a 
 scalpel. The grayish mass which is scraped off consists almost entirely 
 of cells. It is put immediately into a mixture of alcohol (85 per cent.), 
 one part, water two parts, and allowed to remain for twenty-four 
 hours. A tiny bit of the cell-mass is then placed on a slide with a 
 drop of a dilute aqueous solution of eosin, and teased into extremely 
 fine fragments with needles. By the time the cells are sufficiently 
 separated, they will have become stained of a light rose-red color. 
 
 A small drop of glycerine is now mixed with the fluid on the slide, 
 and the whole carefully covered with a thin glass so as to exclude 
 all bubbles of air. 
 
 The specimen is now ready for study, and should be examined first 
 with a low and then with a high power. The cells will be found to 
 have a great variety of forms, some of them evidently determined by 
 the pressure of adjacent cells. They possess a finely granular cell- 
 body, with more coarsely granular nuclei, and nucleoli. If it be desired 
 
1 6 NORMAL HISTOLOGY. 
 
 to preserve the specimen, a narrow rim of asphalt varnish is painted 
 around the cover-glass, covering the edges of the latter and extending 
 for a short distance on to the slide. 
 
 Pigmented Cells of the Retina. — A fresh eye (from the ox or sheep), 
 or better, one which has lain for a few days in Miiller's fluid, is opened 
 by an equatorial section, and the vitreous body and the retina removed 
 from the posterior half. If the remaining portion, including the sclera, 
 choroid, and a portion of the pigmented epithelium of the retina, be 
 put under water in a shallow dish, and brushed gently over the surface 
 with a fine camel's-hair pencil, delicate pigmented flakes will float off 
 in the water. These are the desired cells. One or two bits should 
 be put into a drop of glycerine on a slide. Another bit is put on to 
 another slide with a drop of haematoxylon solution, and after about ten 
 minutes the coloring fluid carefully washed off with water, and the 
 stained fragments placed with the other in the glycerine on the other 
 slide. They are now covered and examined. These cells are flat, 
 hexagonal, and joined together edge to edge, giving a pavemented 
 appearance to the fragments. 
 
 Most of the cell-bodies are closely crowded with irregular-shaped 
 brown or black pigment-granules. Sometimes such are seen as have 
 but few granules within them, and occasionally they are entirely free. 
 When the cell-bodies are crowded with pigment, the nucleus appears 
 as an irregular, sharply outlined structure containing no pigment and 
 looking like a hole in the cell. When the pigment is not present in 
 considerable quantity, or not at all, the nucleus is much less clearly 
 outlined. In cells which have been stained with hematoxylin, how- 
 ever, the nuclei all present well defined outlines and are stained of a 
 violet color. 
 
 Ciliated Cells from the Frog's Month. — The mucous membrane of 
 the roof of the mouth or the gullet of a living or recently killed frog 
 is gently scraped with a scalpel. The slimy mass which is thus pro- 
 cured is transferred to a drop of one-half per cent, salt solution, on a 
 slide, and thoroughly teased apart with needles. The specimen is 
 now covered, a bit of hair being placed beforehand beside the speci- 
 men to prevent the cover-glass from pressing on the cells. The cells 
 are spheroidal or shortly columnar, and will be seen isolated or in 
 groups, the cilia springing, in the form of delicate rows of hair-like 
 processes, from the side of the cell which, when the latter is in situ, 
 lies at the free surface of the mucous membrane. A considerable 
 number of non-ciliated cells will also be observed. The form of the 
 cilia will be most readily seen in cells in which the movement is 
 becoming languid, which occurs gradually in all the cells, and finally 
 ceases altogether. The movement, when vigorous, often causes cells 
 and masses of cells to revolve and move about in the fluid and fre- 
 quently causes currents in the latter, into which floating particles are 
 drawn and then driven onward with considerable velocity. If it be 
 desired to preserve specimens of the ciliated cells, portions of the 
 mucous membrane may be scraped as before, and the mass treated 
 with dilute alcohol and stained and mounted as above directed for the 
 epithelial cells of the bladder. 
 
CHAPTER II. 
 
 CONNECTIVE TISSUE. 
 
 a. Intercellular Substance. 
 
 Our knowledge of the animal tissues is not yet extensive and exact 
 enough to enable us to make a satisfactory classification of them, but 
 for convenience of study we may regard the body as composed of 
 simple tissues and of organs. As examples of the first we have con- 
 nective tissue, muscular tissue, nerve-tissue, etc. ; of the second, the 
 liver, the lungs, the skin, mucous membranes, etc. Among the simple 
 tissues there is a large and important group, called connective tissues, 
 the members of which, though presenting many marked differences, 
 yet seem so closely allied, both in structure and life history, as to 
 justify their grouping under the above common name. The members 
 of this group of tissues may be tabulated as follows : * 
 i. Fibrillar connective tissue. 
 
 2. Embryonal and mucous tissue. 
 
 3. Fat tissue. 
 
 4. Reticular connective tissue. 
 
 5. Cartilage. 
 
 6. Bone and teeth. 
 
 Aside from many striking points of similarity in their structure, 
 which we can better appreciate after having made a practical study of 
 the group, three considerations may be briefly noticed here as of 
 weight in determining this classification. At an early period of em- 
 bryonic life, when the animal is composed almost exclusively of cells, 
 it is found that one of the first definite arrangements or groupings of 
 these cells consists in the formation of three distinct layers: an outer, 
 a middle, and an inner layer. Now it is found that the cells in these 
 different layers have an entirely distinct capacity in their power of 
 producing the tissues which are found in the adult animal. From the 
 cells in the outer layer are produced the skin with its adnexa, certain 
 organs of sense, and the central nervous system ; from the inner layer 
 are formed the intestinal epithelium and glands, and certain of the 
 large organs of the body ; while from the middle layer are developed 
 
 * In addition to these varieties we find in certain parts of the body membranous 
 layers or sheaths — sometimes structureless, sometimes having well-marked structural 
 features — which differ in many respects from the above varieties of connective tissue, 
 but whose separate consideration here would carry us beyond the scope which time 
 permits these lessons to assume. They will be briefly considered as we meet with 
 them in our systematic study of the parts of the body in which they occur. 
 2 
 
1 8 NORMAL HISTOLOGY. 
 
 the muscles, the vascular systems, and, which especially concerns us 
 here, the tissues tabulated above as members of the connective-tissue 
 group. 
 
 Besides the relationship given to them by this common origin, 
 these tissues show their close alliance by the fact that during the pro- 
 cess of development one is sometimes formed from another. Finally, 
 certain frequently observed pathological conditions seem to consist 
 chiefly in the transformation of one of these forms of tissue into 
 another of the same group. 
 
 With fibrillar connective tissue or connective tissue proper, or sim- 
 ply connective tissue, as it is often called, we commence our systematic 
 study. This tissue is most widely distributed in the human body, 
 occurring in the greatest diversity of forms, and an exact knowledge 
 of its structure and arrangement is absolutely essential to a correct 
 understanding of most other tissues and the organs, since it occurs in 
 one form or another in nearly all of them. Fibrillar connective tissue, 
 under the name of tendons and ligaments, forms bands and cords 
 which bind the muscles to the bones, and bind the bones together. It 
 is spread out in thin layers in the fasciae and aponeuroses ; it surrounds 
 the bones and cartilage as periosteum and perichondrium. It divides 
 and encloses muscular bundles and the nerves ; it supports the blood 
 and lymphatic vessels, and forms the limiting membrane of the serous 
 cavities. It forms an encasing membrane for many organs, and, ex- 
 tending into their interior, serves, under the name of interstitial tissue, 
 to support their parenchyma. Everywhere it performs these chief and 
 essential functions; it supports, it binds parts _ together, it protects. 
 Fibrillar connective tissue is composed of two distinct classes of struc- 
 tural elements : a. cells, and b. a substance lying between the cells, 
 the intercellular or basement substance. The intercellular substance 
 consists chiefly of two distinct kinds of fibres : fibrillated fibres and 
 elastic fibres. The fibrillated fibres are tiny, grayish, translucent cords, 
 which, sometimes singly, sometimes in straight or wavy bundles, lie 
 nearly parallel to one another, and again cross each other at all con- 
 ceivable angles, forming complicated net-works. When examined 
 fresh, with high magnifying powers, the individual fibres are seen to be 
 moderately refractive ; they have a delicate longitudinal striation — this 
 striation being, as we shall presently see, the expression of the fact 
 that each fibre is made up of a number of still finer fibrillar. These 
 fibrillated fibres, although frequently crossing one another, and intri- 
 cately interlacing, do not, as a rule, branch nor form anastomoses with 
 one another. On boiling for a considerable time in water they are 
 converted into gelatine, and when treated with acetic acid or dilute 
 alkalies they swell up, lose their longitudinal striations, become very 
 transparent, and finally almost invisible. 
 
 It is not easy, in studying fresh tissues, to convince one's self 
 that the longitudinal striations on the fibres are really the expression 
 of their fibrillar structure, since any attempt to pick the fibres apart 
 with needles is of little avail, because the fibrillar are bound together 
 by 3 minimal amount of cement substance. 
 
 Li, however, the tissue be placed for a few hours in some fluid 
 
CONNECTIVE TISSUE. 19 
 
 which dissolves this cement substance, as picric or osmic acid, the 
 ultimate elements may be readily separated by teasing. The second 
 variety of fibres which occur in connective tissue — the elastic fibres — 
 are much more strongly refractive than the first, hence presenting 
 more sharply marked contours ; they are not longitudinally striated, 
 and are not usually grouped together in bundles ; they often branch 
 and form anastomoses with one another, sometimes joining at frequent 
 intervals to form a narrow-meshed net, and again stretching away for 
 long distances to form a broadly-spaced reticulum. Sometimes the 
 fibres are broad and band-like, sometimes extremely fine ; on being 
 boiled in water they are not converted into gelatine, and they are un- 
 changed by acetic and dilute alkalies. These fibres, as their name 
 indicates, possess elasticity, and as a consequence of this property we 
 often find, when the fibres have been severed by teasing or other 
 modes of preparation, that the free ends curl over in the act of retrac- 
 tion, forming very characteristic curves or spirals. The elastic sub- 
 stance sometimes occurs in the form of granules instead of fibres. 
 The relative number of the fibrillar and elastic fibres varies greatly in 
 different tissues : in some we find but few elastic fibres, others contain 
 little else. The interstices of the interlacing fibres are filled with the 
 nutritive fluids of the body, or in some cases a small amount of a 
 more consistent homogeneous material binds them together. The 
 marked difference in general appearance, which is seen in different 
 parts composed of connective tissue, is due to differences in the 
 arrangement of the fibres and bundles and the relative proportion of 
 the fibrillated and elastic fibres. 
 
 PRACTICAL STUDY. 
 
 Subcutaneous connective tissue. — To study the basement substance 
 of fibrillar connective tissue, the skin should be reflected back from 
 the abdominal wall of a recently killed rabbit or dog, and, choosing a 
 part which is free from fat, a bit of the loose subcutaneous tissue is 
 seized with the forceps and snipped off with scissors. The bit of tis- 
 sue, which will -contract to a little lump around the point of the forceps, 
 is now to be carefully spread out on a slide and covered with one-half 
 per cent, salt solution. 
 
 The specimen will be seen to contain a large proportion of fibril- 
 lated fibres crossing one another in all directions, with a few narrow 
 elastic fibres. After studying in salt solution, a drop of two per cent, 
 solution of acetic acid should be allowed to run under one edge of the 
 cover-glass, the salt solution being drawn off by a bit of filter paper 
 placed at the opposite edge, and the effect carefully observed. 
 
 Tail Tendon of Mouse. — In order to demonstrate that the striated 
 fibres are composed of finer fibrillae, a bit of the tail tendon of a mouse 
 or a small tendon from any animal, should be soaked for a few days 
 in a one per cent, solution of osmic acid, washed and carefully teased 
 apart on a slide in glycerine. To make a permanent preparation the 
 cover may be surrounded by a rim of asphalt varnish. 
 
 Adventitia of the Aorta. — A shred should be stripped from the 
 
20 NORMAL HISTOLOGY. 
 
 outer layers of the aorta of any mammal, either fresh or after it has 
 been in preservative fluids, carefully teased on a slide and mounted in 
 glycerine. In this specimen the elastic fibres preponderate in the 
 basement substance. 
 
 Connective Tissue. 
 b. Cells. 
 
 We consider next the cellular elements of fibrillar connective 
 tissue. Two distinct classes of cells are found here. First : those 
 which are essential components of it, preserving a fixed and definite 
 relation to the basement substance, and being tolerably constant in 
 the different varieties of tissue, in form, size, and number, the so- 
 called fixed connective-tissue cells; and second, small spherical cells, 
 resembling in all their characters the white blood-cells, with which they 
 are believed to be identical. These latter cells escape from the blood- 
 vessels and wander through the tissues, in which they are found in 
 varying number under normal conditions, and are called wandering 
 cells. These cells will be studied in connection with the blood. 
 Among the fixed connective-tissue cells, by far the greater proportion 
 are more or less flattened, presenting to the eye, when seen from the 
 edge or in cross-section, the appearance of slender spindles. The 
 cell-bodies are for the most part quite transparent, often very thin 
 and scale-like, frequently so delicate as to be difficult of recogni- 
 tion, and sometimes the protoplasm in the vicinity of the nucleus is 
 distinctly granular. These fixed connective-tissue cells present a great 
 variety of forms, being round, ovoid, oblong, fusiform or irregularly 
 rectangular, often sending off branches which seem to be connected 
 with the branches of neighboring cells. Sometimes the thin cell-body 
 sends off one or more delicate wing-like processes at varying angles. 
 These cells may have one or more nuclei ; they lie in the interstices 
 of the fibres, which they often enwrap with their delicate bodies. The 
 form which they assume in the different varieties of connective tissue 
 would seem to be dependent upon the varying conditions of pressure 
 to which they are subjected by the adjacent fibres. These flattened 
 connective-tissue cells in many parts of the body may, and in certain 
 parts constantly do, contain pigment-granules. In certain parts of 
 the body the connective tissue presents free surfaces, lining more or 
 less well-defined closed spaces or cavities, or free surfaces which are 
 movable over one another, as is seen in the great serous cavities, in 
 the blood- and lymph-channels, tendon-sheaths, etc. In these cases 
 the flat connective-tissue cells usually undergo some modification in 
 their form, character, and relations to one another, and are called 
 endothelial cells or endothelium. Finally, in certain parts of the body, 
 usually in the vicinity of blood-vessels, are found irregular-shaped, 
 granular cells, not flat, of varying size and form, which resemble in 
 many respects cells found in the embryo, but whose nature and pur- 
 pose is little understood. These are sometimes called plasma cells. 
 
 The various forms of connective-tissue cells and their relations to 
 
CONNECTIVE TISSUE. 21 
 
 the intercellular substance may be studied by considering, somewhat 
 in detail, a few different and typical forms of connective tissue. Those 
 which we shall employ are : the subcutaneous tissue, the choroid, tendons, 
 the cornea, and for the study of endothelial cells, the serous membranes 
 of the peritoneum. 
 
 PRACTICAL STUDY. 
 
 Cells in the Subcutaneous Connective Tissue. — The fixed connective- 
 tissue cells which occur in that form of tissue in which the basement 
 substance is loosely arranged in crossing or interlacing fibres or bundles 
 — often called areolar tissue — may be studied in the subcutaneous tis- 
 sue or fasciae of such animals as the rabbit, dog, or guinea-pig, or in 
 man. The cells vary somewhat in size, shape, and number, in the 
 different animals, but those in the rabbit may be regarded as typical. 
 In the study of these cells, whose bodies are for the most part so thin 
 and transparent as to be almost invisible even with high magnifying 
 powers, we have to fulfil two important indications in our technical 
 procedure; we must treat the tissue with some agent which will 
 render the cells visible, and at the same time the form of the delicate 
 cell-body must remain as nearly as possible unchanged. These ends 
 may be attained in the following way : the skin being reflected back 
 from the abdominal walls of a freshly killed rabbit, with an ordinary 
 hypodermic syringe, inject beneath the loose adherent tissue several 
 drops of an aqueous solution of nitrate of silver, i part to 1,000 ; a 
 circumscribed cedematous area will be thus produced, from which a bit 
 may be snipped off with scissors, spread out in a thin layer on a slide, 
 covered with one-half per cent, salt solution, and studied. The silver 
 serves to fix the cells in their natural form without much shrinkage, 
 and renders the cell-body faintly visible. This method of bringing re- 
 agents into contact with the tissues, is called the method of interstitial 
 injection. In order to bring the cells still more clearly into view, the 
 specimen may be stained, first with a very dilute solution of hema- 
 toxylin, and then with a dilute aqueous solution of eosin ; the nuclei 
 will then be seen colored violet, the cell-bodies a light rose-red. The 
 cells in this tissue are for the most part extremely thin, and of various, 
 often quite irregular forms, sometimes sending off narrow processes by 
 which they join neighboring cells, and sometimes furnished with wing- 
 like projections. Nerves and capillary blood-vessels are sometimes 
 seen in the specimens, and occasionally in the vicinity of the latter are 
 found the above-mentioned plasma cells. The specimens may be per- 
 manently preserved in glycerine and enclosed by a rim of asphalt 
 varnish. 
 
 Connective-tissue Cells of the Choroid. — Pigmented connective-tis- 
 sue cells may be studied in the choroid of the eye of any of the mam- 
 malia (except of albinos, where the choroid cells are unpigmented). 
 A shred of the choroid should be torn off with forceps, and one bit 
 spread out and studied in glycerine unstained, another stained with 
 hematoxylin. Irregular-shaped, often somewhat branched and flat- 
 tened cells, are seen lying embedded in a membranous basement sub- 
 
22 NORMAL HISTOLOGY. 
 
 stance containing elastic fibres, the body being crowded, except in the 
 part occupied by the nucleus, with a multitude of brown or black 
 granules ; the nucleus looks like a clear space in the unstained speci- 
 mens, and is colored violet, as usual, by the hematoxylin. 
 
 Tendon, teased. — The tendons are composed of a varying number 
 of parallel bundles of nbrillated connective-tissue fibres, surrounded by 
 an envelope of connective tissue, whose fibres are irregularly arranged, 
 and covered, as they run in their sheaths, by a single layer of flattened 
 cells, which have the character of the cells presently to be described 
 as endothelium. The complicated forms which tendon-cells present 
 can be most easily understood when they are studied in their natural 
 relation to the parallel bundles of nbrillated fibres which form the 
 intercellular substance. Owing to their extreme tenuity and the con- 
 sequent possibility of studying them with but little manipulation, the 
 tail tendons of the mouse or rat are well suited for this purpose. If 
 the skin of the tail of a recently killed mouse or rat be torn across 
 transversely by slight twisting or pressure with the edge of the nail, 
 and gentle traction be exerted at both sides of the rupture, the caudal 
 vertebrae will readily separate, and as the traction continues a multi- 
 tude of extremely fine tendon-bundles will be seen stretching across 
 the break. When the tendons by this natural and simple dissection 
 have been freed for one or two centimetres of their length, they are 
 laid, still under slight tension, on a bit of glass of suitable size, the 
 ends cemented to the glass with a bit of soft wax or paraffine, and the 
 remainder of the tail cut off. The fastening of the ends is to prevent 
 the tendon from shrinking when immersed in the preservative fluid. 
 The bit of glass with its tendon-bundles is now immersed for twenty- 
 four hours in a one per cent, solution of osmic acid, then washed in 
 pure water. One or two of the tiny bundles is now to be carefully 
 picked into smaller fibres with needles, laid for a few hours in picro- 
 carmine, washed again, still further teased on a slide, covered with 
 glycerine, and studied. The individual bundles of nbrillated fibres 
 will be readily seen, and arranged in rows along these bundles, edge 
 to edge, with more or less irregular contours, the flattened tendon- 
 cells will be seen to lie. Their nuclei, stained red by the picro-car- 
 mine, when seen from the side appear oval or rectangular, and are 
 often arranged in couples along the fibres ; when seen edgewise they 
 look like short, thick red lines. The cell-bodies, when seen sidewise, 
 look like faintly colored, often indefinitely outlined, very thin plates, 
 stretching out from the nuclei and partially enwrapping the tendon- 
 fibres. When the cell-body is placed with its edge toward the ob- 
 server, and in close apposition with the fibre, it is not to be distin- 
 guished, owing to its extreme thinness, from the line which marks the 
 border of the fibre ; if, however, it has been separated, in part or en- 
 tirely, from the fibre, by the previous manipulations, it will appear as 
 a faint line, at some part of which is seen the elongated nucleus. 
 Examining the specimen still further, one sees in the side view of the 
 cells, sometimes along a single cell, sometimes along a row of them, 
 one or more fine lines or striae, and on looking carefully at the ceils 
 placed edgewise, he can convince himself that thin, narrow wings are 
 
CONNECTIVE TISSUE. 23 
 
 given off from the cell-body, and that it is the edges of these wings 
 which cause the striae in the cells seen on the flat. In teased speci- 
 mens of tendon one occasionally finds irregular-shaped branching 
 cells. We have then in tendon parallel bundles of fibrillated fibres, 
 arranged along and in the interstices between which lie flattened 
 cells, whose thin bodies partially enwrap certain fibres an$J send off 
 between other and neighboring fibres branches and thin narrow wings. 
 The interstices between the bundles, in which the tendon-cells lie, are 
 lymph-channels. 
 
 Transverse Sections of Tendon. — A very clear view of the rela- 
 tions between the cells and fibres of tendon, as well as the relation of 
 the wings to the cell-bodies, may be obtained by making a transverse 
 section of a hardened and stained tendon. Any small tendon from a 
 recently killed animal will answer the purpose. It should be freed 
 from surrounding tissues and immediately immersed in a small quan- 
 tity of a half per cent, solution of gold chloride. Here it remains for 
 half an hour, when it is removed, washed with pure water, and put into 
 a few cubic centimetres of the following mixture, known as the redu- 
 cing fluid : 
 
 Amyl Alcohol 1 
 
 Formic Acid 1 
 
 Water 100 
 
 After remaining for twenty-four hours in the reducing fluid, the 
 specimen will probably have assumed a rich violet color ; if not, it- 
 should be kept for twenty-four hours longer in a fresh portion of the 
 fluid. When the requisite color has been obtained the specimen is" 
 hardened for a day in alcohol, embedded in hardened liver or wax, 
 and thin cross-sections made from it. The sections are now stained 
 lightly with hasmatoxylin and mounted in glycerine. They should be 
 kept as much as possible from the light, which after a time destroys 
 gold preparations. This method of staining with gold is known as 
 Pritchard's method. The simpler method of gold-staining known as 
 Conheim's method differs from that just given, in that, after soaking in 
 the gold solution, the specimen is exposed to the light until it has as- 
 sumed the violet color. It is then mounted and preserved as before. 
 This method is less certain in its results than Pritchard's, being less apt 
 to give clear pictures. In both methods the staining action consists 
 in the precipitation of the gold in metallic form, in a state of extreme 
 comminution, in the tissue. In one case this precipitation is effected 
 by the acid mixture, in the other by the light. This deposition of 
 gold in the tissues occurs for the most part in the protoplasm of the 
 cells, bringing them into sharp contrast with surrounding parts and 
 fixing their form. There is one source of error, however, in the inter- 
 pretation of gold specimens, which should never be forgotten. The 
 gold is apparently sometimes deposited and the characteristic color 
 produced outside of the cell protoplasm, and especially in spaces in the 
 tissues which are filled with albuminoid materials, so that the utmost 
 caution is required in determining whether a given area of gold deposit 
 
24 NORMAL HISTOLOGY. 
 
 and staining really indicates the presence of cell protoplasm. And, in 
 fact, it is in some cases impossible, at present, to decide this question. 
 
 The Cornea — Transverse Sections. — We shall employ for our study 
 here the cornea of the frog, because of the ease with which it can be 
 obtained, and because, on account of its thinness, it is well suited to 
 the various manipulations to which we shall subject it. A frog's eye 
 should be enucleated directly after death, and placed entire in M tiller's 
 fluid, where it should remain for ten days. The cornea is then ex- 
 cised just within the sclero-corneal junction and laid for a few hours in 
 dilute alcohol (alcohol i, water 2), and then transferred for a day 
 to strong alcohol. Two or three short radial incisions are then made 
 at the edge, so that it will lie flat. It is embedded between two bits 
 of liver well hardened in alcohol, and thin sections made transversely 
 across it ; the sections are stained first with dilute hematoxylin, and 
 then with eosin, and mounted in glycerine. 
 
 If the sections are made so as to include the entire thickness 
 of the cornea, both the anterior and posterior edges will be seen 
 to be covered with epithelial cells which rest upon thin homoge- 
 neous membranes. On the anterior edge the cells are several layers 
 deep and have various forms ; on the posterior they are flat and ar- 
 ranged in a single row. Between these two layers of cells lies the con- 
 nective-tissue substance proper of the cornea, which alone concerns 
 us here. The basement substance of the cornea consists of fine fibril- 
 lated fibres closely bound together by a small amount of cementing 
 substance, and arranged in irregular layers or lamellae. Between these 
 lamellae are seen the cells of the cornea, which, in this view, seem 
 to have the form of slender elongated spindles, part of them being 
 closely surrounded by the intercellular substance, part lying in small 
 cavities. 
 
 La?nincB of Cornea. — In order to determine the exact form of 
 the cells, which in this mode of preparation are seen only from the 
 edge, it is necessary to study the cornea as seen from the side. For 
 this purpose a cornea should be carefully excised just within the sclero- 
 corneal junction, avoiding, as much as possible, pulling or stretching 
 the part. It is now stained with gold chloride by Pritchard's method, 
 as given above when speaking of the tendon. "When the cornea has 
 become violet it should be carefully washed and put into glycerine. 
 The epithelium on both the anterior and posterior surfaces is now 
 scraped off, and with a little care the part may be divided into several 
 thin layers by means of fine forceps. These layers are lightly stained 
 with haematoxylin and mounted in glycerine. In a specimen thus 
 prepared, the basement substance looks homogeneous or delicately 
 striated ; fine nerve-fibres are seen stretching across the specimen in 
 various directions. The corneal cells, which are the most prominent 
 objects in the specimen, are seen thickly scattered over the field, 
 stained of a reddish violet color. They have flat, irregular-shaped 
 bodies which send off a variable number of longer and shorter pro- 
 cesses ; the nuclei are large, ovoidal, or irregular-shaped, and usually 
 contain nucleoli. Fine irregular-branching, almost linear structures 
 are seen in good preparations thickly scattered over the specimen, 
 
CONNECTIVE TISSUE. 25 
 
 and very frequently lying nearly at right angles to one another. A 
 part at least of these are continuous with the cell-bodies whose 
 processes they are ; whether or not they are all cell processes is not 
 yet definitely known. It will be seen from the above studies, that 
 the connective-tissue cells of the cornea are flattened and branched 
 cells, and that certain of them, at least, lie in spaces between the fibres 
 of the intercellular substance. We have now to study more carefully 
 the nature of these spaces and their relation to the cells. 
 
 Cornea treated with Silver. — Strong solutions of nitrate of silver 
 have the power of staining the intercellular substance of connective 
 tissue light brown, while the cells are left uncolored. In order to 
 stain the cornea of the frog with silver, the following process should 
 be employed : the spinal cord of the animal having been broken up 
 with a needle, a finger is introduced into the mouth so as to press the 
 eyeball forward and bring the cornea into prominence ; the membrana 
 nictitans is drawn away with the thumb or a pair of fine forceps, so as 
 to leave the anterior surface of the cornea quite free. The eye is 
 now held for an instant over a jet of steam, when the epithelium will 
 become white or milky in appearance, and can be readily scraped off 
 by passing the blade of a scalpel lightly over the surface. It is neces- 
 sary to remove the epithelium in order that the silver solution may 
 have ready access to the connective-tissue substance of the cornea ; 
 and the advantage of steaming is, that it can be scraped off without 
 the use of much force, which would disturb the relations of the parts 
 beneath. The epithelium being thus removed, a five-per-cent. solu- 
 tion of silver nitrate is allowed to flow over the cornea and remain for 
 two or three minutes in contact with it. By this treatment the whole 
 cornea becomes opaque and stiff. The silver is now neutralized and 
 its further action prevented by washing the cornea with a one-per-cent. 
 salt solution. It is now carefully excised and placed in a dish con- 
 taining a mixture of alcohol and water, 1 to 2, and exposed to di- 
 rect sunlight or bright daylight for from a. few minutes to half an hour, 
 depending upon the intensity of the light. When it has become 
 brown it is to be laid in glycerine and stripped into thin layers, as 
 directed above, for the gold cornea ; the layers are mounted in gly- 
 cerine. If the preparation is successful, clear, branching, communi- 
 cating spaces are seen on a yellowish or brown ground. These 
 spaces, although larger, evidently correspond in position to the cells 
 in the cornea, as seen after treatment with chloride of gold ; and if a 
 specimen thus prepared be stained with haematoxylin, the corneal cells 
 will be seen lying within them. 
 
 It is difficult to determine with certainty whether or not the cells 
 send processes into all of the channels which radiate from the spaces 
 in which they lie; it is probable that some of the narrow branching 
 bodies or lines noticed above in the gold cornea are nothing more 
 than these branching and communicating spaces in which an albu- 
 minous fluid accumulates, which is stained by the gold very much as 
 cell protoplasm itself is. We thus see that the cornea is perme- 
 ated by numerous branching and intercommunicating spaces, and 
 that in these spaces the flat, branching connective-tissue cells of the 
 
26 NORMAL HISTOLOGY. 
 
 cornea lie. These spaces are called lymph-spaces, and what we have 
 been able to demonstrate in regard to the relation of cells to the 
 lymph-spaces in the cornea by these various modes of preparation 
 seems to be true, with certain modifications, of most of the varieties 
 of connective tissue. These cells lie in spaces, sometimes com- 
 pletely, sometimes only partially filling them. These spaces com- 
 municate with one another, and communicate also, on the one hand, 
 with the blood-vessels, and, on the other, with the lymphatics. 
 Through them lymph-currents pass, bathing the cells and supplying 
 them with nutritive material. The precise way in which these lymph- 
 spaces (sometimes called serous canaliculi) communicate with the 
 lymph- and blood-vessels is as yet unknown to us. It is extremely 
 probable that it is through these lymph-spaces exclusively that the 
 white blood-cells travel in their peregrinations through the tissues. 
 We study these lymph-spaces in the cornea alone, because the scope 
 of these lessons is too limited to admit of such a detailed study of all 
 the varieties of connective tissue, and because, while quite typical of 
 other tissues, the relation of the cells to the spaces is here more clearly 
 defined. 
 
 Serous Membranes — Endothelium. — The so-called serous mem- 
 branes are formed by layers of fibrillar connective-tissue fibres 
 mingled with a varying number of elastic fibres, and containing ordi- 
 nary flattened connective-tissue cells. They are more or less abun- 
 dantly furnished with blood and lymphatic vessels. On the free sur- 
 face of these membranes rests a continuous layer of flattened cells, 
 differing in many respects from the ordinary connective-tissue cells, 
 and called endothelial cells or endothelium. These cells are usually 
 transparent, irregularly polygonal in form, and frequently much elon- 
 gated ; they possess one or more ovoidal nuclei, which often project 
 above the level of the free surface of the cell-body ; they are placed 
 edge to edge, like the stones in a mosaic, and seem to be joined to- 
 gether by a minimal amount of an albuminoid cement substance. 
 
 Isolated Endothelium. — Before studying the endothelial cells in 
 situ it is advisable to see the isolated cells. This may be accom- 
 plished by macerating any serous membrane, such as the pericardium 
 or peritoneum of a mammal or the frog, for a day or two in a mixture 
 of one-third alcohol to two-thirds water, and then gently scraping the 
 free surface with a scalpel. The cells will readily separate from the 
 underlying tissue, and may be stained with hematoxylin and mounted 
 in glycerine. 
 
 Endothelium in situ — Mesentery. — The endothelial cells, in their 
 relation to adjacent parts, may be studied in the mesentery of the 
 dog, or rabbit, or frog, and in the omentum of the dog. 
 
 In order to bring the outlines of the cells clearly into view, the 
 membrane should be first treated with a solution of nitrate of silver. 
 This substance in dilute solutions forms with the cement substance 
 between the endothelium an albuminate of silver, which, on exposure 
 to light, becomes brown or black, thus clearly defining the outline of 
 the cells. 
 
 The mode of proceeding is as follows : A portion of the mesen- 
 
CONNECTIVE TISSUE. 27 
 
 tery of the recently killed dog or rabbit should be carefully removed 
 and laid over the rim of a shallow dish so that it rests loosely over the 
 opening. All stretching and pulling of the membrane should be 
 avoided, because this would destroy the natural relations of the cells 
 to one another. The membrane must be allowed to sag a little into 
 the dish, so that it may be bathed on both sides by the silver solution. 
 It is now to be carefully washed with distilled water to remove any 
 albuminous substance or blood — which would cause a granular precipi- 
 tate of silver albuminate on the surface — and the dish then filled with 
 an aqueous solution of nitrate of silver, 1 to 500. The dish should be 
 gently shaken at frequent intervals, so as to bring fresh portions of the 
 solution into contact with the membrane, and after from twenty 
 minutes to half an hour the tissue will be seen to have become cloudy 
 or milky. 
 
 The silver is now poured off and the membrane carefully washed 
 with pure water. The cells will have been fixed by the silver, so that 
 the membrane may be removed without further danger of disturbing 
 the relations of the cells, and laid in a dish containing water to which 
 one-third its bulk of alcohol has been added. It is now exposed to the 
 sunlight, and after from a few minutes to half an hour — sometimes 
 longer, depending upon the intensity of the light — the tissue will be 
 seen to have assumed a brown color. The specimens are now trans- 
 ferred to dilute hematoxylin, where they remain until sufficiently 
 stained, and finally small bits are cut off, spread smoothly on a slide, 
 and mounted in glycerine. The mesentery will be seen to consist of 
 a thin membrane of fibrillar connective tissue with a few delicate 
 elastic fibres, and here and there small blood-vessels, the whole 
 covered with the delicate mosaic of endothelial cells. The out- 
 lines of the endothelium on both the upper and under sides of the 
 membrane may be brought successively into view by careful focus- 
 sing. 
 
 Omentum. — A portion of the omentum of the dog should be treated 
 with silver in the way just directed for the mesentery, and also stained 
 with hematoxylin and mounted in glycerine. The omentum of the 
 dog, as of man, consists of an irregular meshed net, whose trabecule 
 are made up of fascicles of fibrillar connective tissue of varying thick- 
 ness, the broader containing blood- and lymph-vessels and fat-cells, 
 the narrower consisting of single bundles offibrille — all being alike 
 covered with a single layer of endothelium. Here and there are seen 
 groups of irregularly polygonal granular cells, which are connected 
 with the lymphatic apparatus. In many parts the endothelial cells 
 are seen in profile, when their nuclei appear lenticular in form, usually 
 projecting above the general level of the cell-body, which is itself in 
 most cases thicker in the vicinity of the nucleus than at the points of 
 junction with adjoining cells. The specimens of both omentum and 
 mesentery may be preserved in the usual way; but silver preparations, 
 like the gold, do not preserve their first clearness very long, unless 
 carefully kept from the light. 
 
 Stomata. — Considering the serous cavities as spaces in the con- 
 nective tissue lined with modified connective-tissue cells, it will be 
 
28 NORMAL HISTOLOGY. 
 
 seen that, although unlike in general character, they are quite analo- 
 gous with the smaller spaces, containing sometimes only one cell, 
 which are found everywhere in the connective tissue, and which were 
 studied in detail in the cornea. This analogy becomes still more evi- 
 dent when we consider their relations to the lymphatic vessels. In 
 considering the connective-tissue spaces of the cornea, it was said that 
 although serving as lymph-passages, it had never been possible as yet 
 to demonstrate their exact mode of communication with the lymphatic 
 vessels. In the large lymph-spaces, such as the peritoneal cavity, 
 however, the mode of communication has been quite definitely ascer- 
 tained. In those parts of the walls of the peritoneum which are most 
 abundantly supplied with lymphatic vessels — for example, in the cen- 
 tral tendon of the diaphragm — numerous small openings, called stoma- 
 ta, exist, leading from the peritoneal cavity directly into the ves- 
 sels, and these openings are readily recognized by a peculiar arrange- 
 ment of the endothelial cells in their vicinity. The stomata may be 
 more easily studied, however, in the frog than in the warm-blooded 
 animals. In this animal there lies directly in front of the vertebral 
 column, and extending the whole length of the trunk, a large lym- 
 phatic vessel or cistern, which is separated from the peritoneal cavity 
 by a thin connective-tissue septum covered on both sides by endo- 
 thelial cells. In this septum are found a great number of stomata. 
 They are readily demonstrated by the following procedure : the 
 spinal cord of a frog being broken up with a needle, the skin is cut 
 around just behind the front legs, and entirely stripped from the body. 
 The peritoneal cavity is opened by an incision reaching from the 
 symphysis to the sternum, and the entire visceral contents, with the 
 exception of the kidneys, removed — care being taken to cut the mes- 
 entery as far as possible from its posterior attachment, so as not to 
 injure the septum of the lymphatic sac. The hind legs are now tied 
 together by a thread ; the anterior part of the body cut off just be- 
 hind the front legs ; the remaining part washed with pure water, to 
 remove blood, and suspended in a solution of nitrate of silver, i to 
 500. Here it should be frequently agitated to bring fresh portions of 
 the solution into contact with the membrane, and after from twenty 
 minutes to half an hour, when the tissue has become milky, the whole 
 should be removed and washed, and laid in a shallow dish containing 
 pure water. A thin white membrane will now be seen floating on 
 each side of the kidneys ; this is the anterior wall of the lymph-sac. 
 After exposure to the light, until the membrane becomes brown, it 
 should be snipped out with the scissors, stained with haematoxylin, 
 and mounted in glycerine with the peritoneal surface uppermost. 
 
 The general surface is seen to be covered with polygonal endo- 
 thelium, while scattered here and there over the specimen are tiny 
 openings around which the endothelial cells are arranged in a more or 
 less radiate manner. These cells surrounding the stomata are usually 
 considerably elongated ; and whereas over the general surface the 
 nuclei are situated near the centre of the cells, here they lie near the 
 edge which borders the opening. By careful focussing one can readi- 
 ly distinguish the silver markings which outline the cells on the under 
 
CONNECTIVE TISSUE. 29 
 
 side of the specimen, those lining the lymph-sac, as well as the con- 
 nective-tissue fibres and cells above them. 
 
 The cells on the side toward the lymph-sac often extend inward 
 over the opening, so as nearly or entirely to close it : it would seem, 
 indeed, as if the stomata were rather valvular than permanently 
 patulous openings. 
 
 One sometimes sees, here and there, black irregular spots between 
 the endothelium, which should not be mistaken for stomata, as they 
 are only irregular deposits of the albuminate of silver in the cement- 
 ing substance. 
 
CHAPTER III. 
 
 EMBRYONAL AND MUCOUS TISSUE — FAT TISSUE — 
 RETICULAR TISSUE. 
 
 Embryonal and Mucous Tissue. 
 
 At a certain period of embryonic life, those parts of the body which 
 are destined finally to become fibrillar connective tissue consist almost 
 entirely of small spheroidal cells — the intercellular substance being 
 absent, or consisting only of a very small quantity of fluid lying be- 
 tween and bathing the cells. As the process of development goes on, 
 some of the cells retain their original spheroidal form, while others 
 change their character, becoming elongated and fusiform, often termi- 
 nating at their extremities in delicate single or branching processes ; 
 others become flattened and assume irregular shapes, sending off 
 branching processes by which they are joined to one another. Hand in 
 hand with this change in the cells there occurs an accumulation of 
 intercellular material, which is at first fluid, and later presents the ap- 
 pearance of a homogeneous gelatinoid substance. Then within the 
 gelatinoid intercellular substance appear fine fibrillse, which become 
 more and more abundant, arranging themselves now in bundles, and 
 again to form irregular net-works. The cells approach more and more 
 closely to the type of the adult connective-tissue cells, as development 
 goes on ; the intercellular substance loses its soft gelatinoid character, 
 and is finally replaced by the fibrillated and elastic fibres with which 
 we are already familiar. The process of development is a very grad- 
 ual one, and although the younger forms of embryonal connective 
 tissue and the adult connective tissue are distinct enough, we are yet 
 unable to separate them sharply, since they merge so gradually into 
 one another. In general, however, simply as a matter of convenience, 
 we call connective tissue, which is almost entirely made up of sphe- 
 roidal, spindle-shaped, or flattened cells, in which little accumulation 
 and little differentiation of the intercellular substance has occurred, 
 embryonal tissue ; while to that older form, which consists of variously 
 shaped, spheroidal, flat, branching and anastomosing cells, with a 
 gelatinoid, homogeneous, or partially fibrillated intercellular substance, 
 the term mucous tissue is usually applied. The name mucous tissue 
 was given to this form of young tissue, because the soft gelatinoid in- 
 tercellular substance was found to contain a certain amount oi murine, 
 which may be thrown down in the form of a whitish, often stringy 
 precipitate, by the addition of acetic acid. At present, however, tis- 
 sues presenting the above-mentioned characters are usually called 
 
EMBRYONAL AND MUCOUS TISSUE, ETC. 3 1 
 
 mucous tissues, whether the intercellular substance contains mucine 
 or not. Mucous tissue is not found in the human adult under normal 
 conditions, but frequently occurs as a pathological production. 
 
 PRACTICAL STUDY. 
 
 Subcutaneous Tissue of Embryo. — Embryonal connective tissue 
 may be studied in the subcutaneous tissue of any young mammalian 
 embryo. The animal having been placed for a few days in Muller's 
 fluid, bits of the subcutaneous tissue are torn off from the abdominal 
 wall with fine forceps, stained first with hasmatoxylin, and then with 
 eosin, carefully teased in glycerine, and covered. 
 
 Umbilical Cord. — In the umbilical cord we have a typical example 
 of mucous tissue. The cord of any young mammal, as the pig, is to 
 be put into Muller's fluid, where it remains for a week. It is then 
 washed and transferred to alcohol. After twenty-four hours it may be 
 embedded between two bits of hardened liver, and transverse sections 
 made from it. The sections are stained with hematoxylin and eosin, 
 and studied in glycerine, in which they may be preserved ; or the water 
 maybe removed and the specimens mounted in Canada balsam. The 
 surface of the cord is seen to be covered with several layers of flattened 
 epithelium, and the three large blood-vessels are seen in transverse 
 sections. The amount of fibrillation present in the intercellular sub- 
 stance depends upon the age of the embryo. 
 
 Fat Tissue. 
 
 Fat-tissue is a modified form of connective tissue, in which the 
 intercellular substance is present in proportionally small amount, a 
 large part of the protoplasm of the cells being replaced by fat, which 
 crowds the remaining part together with the nucleus to one side of the 
 cell, nearly or entirely concealing both. The fat-cells thus formed 
 are arranged in clusters or lobules, enclosed by fibrillar connective 
 tissue, which sends into the lobules and between the cells broader 
 and narrower bundles, which serve to support the cells and carry the 
 blood and lymphatic vessels, etc. Owing to the pressure to which 
 the fat-cells are subjected, they usually assume in the adult animal a 
 polyhedral form. When fresh fat is examined by teasing bits of the 
 tissue from a recently killed animal, in \ per cent, salt solution, little 
 else is seen than a congeries of more or less globular or polyhedral 
 masses of fat-cells, with larger and smaller fat droplets. The fat is char- 
 racterized by its great refractive power ; by the dark contour of its 
 globules by transmitted, and their yellowish silvery lustre by reflected 
 light ; by its solubility in alcohol and ether, and by its assumption of 
 a deep black color on treatment with osmic acid. These fat-cells are 
 enclosed by a membrane, though this is usually invisible in fresh fat, 
 and here and there the nuclei are seen crowded to one side. The 
 connective tissue which encloses the lobules is seen in picked speci- 
 mens in broken masses and shreds, scattered through the preparation. 
 Sometimes the fat is crystallized within the cells, when it appears in 
 
32 NORMAL HISTOLOGY. 
 
 the form of masses of radiating needle-like crystals. The methods by 
 which we can demonstrate the presence of cell protoplasm and nuclei 
 in adult fat-tissue, and their relations to the cell-membrane and the 
 fat, will be given below. 
 
 In order to understand clearly the nature of adult fat-tissue, it is 
 necessary to study it during the process of development. At an early 
 period of life, those parts of the body which are finally to become fat- 
 tissue possess the character of mucous tissue with a more or less fibril- 
 lated intercellular substance. The first change which we notice in 
 the cells as the transformation into fat-tissue commences, is the ap- 
 pearance in the cell-body of small shining particles. These particles, 
 of which there may be many, become gradually larger, until they pre- 
 sent the form and character of distinct droplets of fat. As these drop- 
 lets increase in size, they coalesce, forming one or more drops, which 
 presently become large enough to crowd the nucleus to one side. 
 The growing drops finally unite into one large drop, which at length 
 becomes so large as to occupy nearly the whole of the cell, leaving 
 only a thin crescent of protoplasm and a squeezed and distorted 
 nucleus crowded up against the cell-membrane. At last we can no 
 longer see, without special modes of preparation, any trace of cell 
 protoplasm, and only the deformed remnant of the nucleus. This 
 process is called fatty infiltration. In those parts of the body where 
 the fat is invariably found, this change in the cells occurs in the vicin- 
 ity of little tufts of capillary blood-vessels, so that at one period the 
 forming fat is seen lying in scattered clusters in the meshes of distinct 
 groups of blood-capillaries. It is these clusters of fat-cells, with their 
 accompanying blood-vessels, which determine, when the fat is fully 
 formed, the lobular character of the tissue. In many parts of the 
 body and under varying conditions, sometimes physiological, some- 
 times pathological, there is an accumulation of fat in the protoplasm 
 of cells ; but it is, for the most part, temporary, and the fat-cells have 
 no definite grouping in lobules and about the blood-vessels in the way 
 above described for the permanent fat. 
 
 PRACTICAL STUDY. 
 
 Fresh Fat from Rabbit. — A bit of subcutaneous fat from the 
 freshly killed rabbit should be teased on a slide in salt solution, and 
 studied without staining. 
 
 Fat Tissue treated with Nitrate of Silver. — If a bit of skin be 
 reflected back from the abdominal wall of a recently killed rabbit, 
 and an interstitial injection of a solution of nitrate of silver, i to 
 1,000, be made into one of the fat-lobules which are exposed in the 
 subcutaneous tissue, and a small bit of the oedematous fat snipped 
 off at once and carefully teased on a slide without the addition of 
 other fluid, the various parts of the fat-cells can be readily seen. 
 
 In some way which we do not yet understand, the silver causes 
 the narrow rim of cell protoplasm to swell, separating the membrane 
 and the nucleus from the fat in such a way as to make a complete 
 demonstration of the structure of the cell. 
 
EMBRYONAL AND MUCOUS TISSUE, ETC. 33 
 
 Cross-seciion of Adult Fat.— A. bit of fat-tissue from the adult 
 human subject should be hardened in alcohol, by which the fat will be 
 for the most part dissolved out of the cells, leaving them filled with 
 the alcohol. Thin sections are made and stained with hematoxylin. 
 These preparations show the relations of the cells to one another, and 
 the lobular character of the fat-tissue. If the blood-vessels of the 
 part from which the tissue is taken have been previously filled with 
 some colored injection, the relations of the lobules to the blood- 
 vessels will be well shown. 
 
 Uninjected specimens may be preserved in glycerine; injected 
 specimens may be stained with hematoxylin and eosin. and mounted 
 in Canada balsam. 
 
 Developing Fat from Young Animal— -Bits of the omentum or 
 mesentery of a nearly mature fcetal or a new-born rabbit should be snip- 
 ped off and carefully spread out on a slide with the addition of salt 
 solution. Cells, singly and in clusters, will be seen which present 
 various stages of the above described fatty infiltration. For permanent 
 preservation the specimens may be laid, for an hour, in dilute alco- 
 hol — i alcohol, & water — then stained with eosin and mounted in 
 glycerine. 
 
 Reticular Connective Tissue. 
 
 This tissue forms a large part of the supporting framework of the 
 lymphatic glands, and is found, in somewhat modified form, in other 
 parts of the body. It consists of extremely fine fibres, which, running 
 in all directions, cross and join one another at frequent intervals, 
 forming a finely meshed net-work. This net-work of fibres is not flat- 
 tened to form a membrane, but extends in all directions, like the tra- 
 becule of a sponge. Irregularly scattered over the fibres are flattened 
 nucleated cells, having the character of endothelium, which sometimes 
 lie at the points of intersection of the fibres, sometimes along their 
 sides, enwrapping them with their transparent bodies. When the cells 
 are in situ upon the fibres, the whole presents the appearance of a 
 mass of anastomosing, branched, or spindle-shaped cells ; and, as 
 such, the reticular connective tissue has until recently been regarded — 
 erroneously however, as our practical study will show. The meshes of 
 the reticular tissue are loosely filled, in the lymphatic glands, with 
 small round cells — lymph-cells — which, however, seem to have no 
 direct connection with the tissue we are studying, and may be easily 
 removed by the manipulations presently to be described. 
 
 practical study. 
 
 Lymphatic Gland of Dog treated with Osmic Acid. — From a re- 
 cently killed dog one of the mesenteric glands should be removed, 
 and a hypodermic syringe being partially filled with a one per cent, 
 solution of osmic acid, the canula is thrust well into the gland and the 
 acid slowly injected till the organ becomes somewhat tense; the 
 canula is then withdrawn and the gland dropped into strong alcohol. 
 After a few days the gland will be sufficiently hard to permit the pre- 
 '3 
 
34 NORMAL HISTOLOGY. 
 
 paration of thin sections with a razor. The sections should be carefully 
 shaken in a test-tube one-third filled with water, to remove the lymph- 
 cells, which lie in the meshes of the reticular tissue and conceal it. 
 They are then stained with hematoxylin, and studied and preserved in 
 glycerine. The object of the osmic acid injection is to harden the 
 flat cells which lie upon the fibres, and fix them firmly in place. The 
 tissue really looks, after this mode of preparation, like a mass of 
 branching anastomosing cells. 
 
 Lymphatic Gland of Dog treated with Picric Acid. — By the fol- 
 lowing mode of preparation, the cells which lie upon the fibres are 
 loosened from the latter, and after their separation, by shaking the 
 specimens in water, we see simply a mass of intersecting and anasto- 
 mosing fibres. 
 
 Another gland of the dog should be put into a saturated solution 
 of picric acid, and allowed to remain twenty-four hours (not longer), 
 then transferred to strong alcohol till it is hard enough to cut. Thin 
 sections are shaken, as before, in water and stained. The shaking 
 should be carefully and efficiently done, or many of the cells will re- 
 main clinging to the fibres. Indeed, in the best preparations, this is 
 apt to be the case in some parts of the specimen • but it is not difficult 
 to free sufficiently large areas of the section to convince ones'-self of 
 the truth of the above view of the nature of the reticular connective 
 tissue. 
 
CHAPTER IV. 
 
 CARTILAGE— BONE— TEETH. 
 
 Cartilage. 
 
 Cartilage consists, like the other members of the connective-tissue 
 group, of cells and intercellular substance. There is nothing charac- 
 teristic, however, in the form of the cartilage-cells. It is in the 
 peculiar nature of the intercellular substance and the relations which 
 the cells bear to it, that we find the distinctive features of this form of 
 connective tissue. The cartilage-cells are spheroidal, flattened, or 
 angular in form ; the cell-body is transparent, and often contains tiny 
 droplets of fat, and sometimes pigment-granules. The cells have one, 
 or sometimes two sharply defined nuclei, which are granular and often 
 seem to contain an irregular network of some more strongly refractive 
 substance. Round each cartilage-cell, in the adult animal, and closely 
 enclosing it, is a homogeneous envelope called the capsule. The sub- 
 stance forming this capsule is identical with the intercellular substance 
 of hyaline cartilage, presently to be described. The cartilage-cells 
 very readily lose their normal form and relation to the capsule by the 
 application of a great variety of substances, even by slight exposure to 
 the air. The most common change which is noticed in them is a 
 rapid shrinkage, such as occurs when cartilage is exposed to the air or 
 treated with strong salt solutions, or any substance which extracts 
 water from the tissues. Under these circumstances the cell becomes 
 more coarsely granular, it shrinks away from the capsule at certain 
 points, giving the edge of the cell a festooned appearance ; sometimes 
 large, clear spheroidal spaces, called vacuoles, appear in the cell-body, 
 and finally the cell shrinks to a shapeless mass in the centre or at one 
 side of the cavity, or retains its connection with the capsule by one or 
 more narrow irregular projections from the shrunken mass. 
 
 The basement or intercellular substance is not the same in all carti- 
 lages, and, according to the differences in its nature, cartilage is divided 
 into hyaline cartilage, fibro-cartilage, ftbro-elastic cartilage. In hya- 
 line cartilage the intercellular substance is homogeneous and transpar- 
 ent in thin layers, somewhat opalescent in thicker masses ; it is of firm 
 consistence, cutting readily with the knife, and contains at tolerably 
 regular intervals variously shaped cavities in which the cells lie, exactly 
 filling them. The layer of basement substance which immediately sur- 
 rounds the cells possesses slightly different refractive power, and it is this 
 layer which constitutes the capsule above described. The cell-spaces 
 or cavities in hyaline cartilage do not by the ordinary modes of prepa- 
 
36 NORMAL HISTOLOGY. 
 
 ration appear to be connected by lymph-channels, as are the cell-spaces 
 in other varieties of connective tissue, yet the rapid passage of fluids 
 through the tissue under certain circumstances, together with some 
 recent microscopical observations, render it extremely probable that 
 such communications do exist, though we are not at present able to 
 demonstrate them with certainty. Although by the ordinary modes of 
 preparation the basement substance of hyaline cartilage appears quite 
 homogeneous, certain changes which it undergoes under pathological 
 conditions, or by the use of certain macerating or digesting fluids, lead 
 us to believe that it really contains a groundwork of delicate fibrillae. 
 The basement substance of hyaline cartilage differs chemically from 
 the basement substance of other members of the connective-tissue 
 group, yet recent researches have thrown serious doubt upon the view 
 formerly held, i. <?., that cartilage gave, on boiling, a peculiar and 
 characteristic substance called chondrine, it having been shown that 
 the so-called chondrine is not a pure chemical substance, but a mix- 
 ture of mucine and various albuminoid materials. Hyaline cartilage is 
 found, in the adult, covering the ends of the bones, in the joints, and in 
 most of the laryngeal cartilages. The tracheal, bronchial, and costal 
 cartilages are also of this variety. Fibro-cartilage differs from hyaline 
 cartilage in having a distinctly nbrillated intercellular substance. This 
 form of cartilage is found in the intervertebral cartilages, in the menis- 
 cuses of certain joints, and at certain points where the ligaments are 
 inserted into the cartilaginous extremities of bones. In certain other 
 cartilages, such as the epiglottis, some of the small cartilages of the 
 larynx, and the cartilage of the pinna of the ear, the intercellular sub- 
 stance contains, in addition to a few fibres, such as are seen in fibro- 
 cartilage, elastic tissue, either in the form of fibres or in fine granules. 
 Such cartilage is called fibro-elastic cartilage. In both fibro- and fibro- 
 elastic cartilage the cells are identical in character with those of hya- 
 line cartilage. Except at the free surfaces, which it presents in the 
 joints, cartilage is surrounded by a layer of nbrillated connective tissue 
 of varying thickness, called the perichondrium, in which are found the 
 blood-vessels which supply nutritive material to the non-vascular car- 
 tilage within. At the surface of many cartilaginous masses the cells 
 are very much crowded together, and flattened in a plane parallel to 
 the surface. 
 
 PRACTICAL STUDY. b. 
 
 Hyaline Cartilage from Femur of Frog. — The head of the femur 
 of a recently killed animal being exposed, a thin slice of cartilage is 
 shaved off from the surface with a razor, so as to leave a flat surface 
 from which a thin section should be cut and immersed in a drop of 
 saturated solution of picric acid on a slide, and covered and surrounded 
 at once by a rim of asphalt varnish before the acid solution commences 
 to evaporate. 
 
 Most of the preservative fluids cause a rapid shrinkage of the carti- 
 lage-cells. Picric acid, however, preserves the cells in nearly normal 
 form for a long time ; but even in this they shrink somewhat after a 
 time, or become coarsely granular. 
 
CARTILAGE— BONE— TEETH. 37 
 
 In such thin sections, here and there cavities are seen from which 
 the cells have fallen out ; they may be filled with the preservative 
 fluid or with bubbles of air. 
 
 Shrinkage of Cartilage Cells by Strong Salt Solution. — Since in 
 most of the specimens containing cartilage, which we shall study in 
 subsequent lessons, the cells will have been considerably shrunken by 
 the preservative fluids, it is desirable that we should acquaint our- 
 selves with the process of shrinkage here, so that we may be able to 
 understand the varied forms which such cells present. 
 
 To do this we have simply to prepare a thin section of hyaline car- 
 tilage as above, and immerse it at once in one-half per cent, solution 
 of salt. When a good field for observation has been selected, a drop 
 of ten per cent, salt solution should be put at the edge of the cover- 
 glass, the other fluid being drawn out by a bit of blotting-paper placed 
 at the opposite side. In this way the process of shrinkage and forma- 
 tion of vacuoles may be readily observed. 
 
 Fibro-cartilage may be studied in thin sections from the inter- 
 vertebral cartilages of man or any of the domestic animals, or from 
 the head of the femur, through the insertion of the ligamentum teres, 
 parallel with the course of its fibres. The tissues should be laid for a 
 few days in alcohol before cutting. The sections are stained with 
 picro-carmine and mounted in glycerine. 
 
 Fibro-elastic Cartilage. — This is commonly studied in the epi- 
 glottis of man or the lower animals, which has been preserved in alco- 
 hol. Thin sections are stained with picro-carmine, which colors the 
 cells red, and the elastic granules and fibres yellow. They are mounted 
 and preserved in glycerine. The cells in both fibro- and fibro-elastic 
 cartilage are, of course, by the above modes of preparation, shrunken 
 and deformed. 
 
 Boxe. 
 
 In studying bone, we have to consider: 1, the hard substance, or 
 bone-tissue proper ; 2, the connective-tissue envelope which surrounds 
 the bone — the periosteum ; and 3, the marroiu contained in the central 
 cavities or spaces within. 
 
 1. The most striking feature of bone-tissue proper is its firmness 
 and hardness, which is due to the presence of certain inorganic salts 
 of lime. If a bone be soaked for some time in dilute solutions of 
 hydrochloric, or nitric, or chromic acid, or in a saturated solution of 
 picric acid, the lime salts are entirely dissolved, and there is left be- 
 hind a substance which retains, for the most part, both in general 
 form and in minute structure, all the essential features of the original 
 bone. There is a basement substance, presenting many of the optical 
 characters of hyaline cartilage, and lying in tiny, branching spaces, 
 in the basement substance, are flat, nucleated cells. 
 
 The lime salts are deposited in the intercellular substance in such an 
 extremely minute state of division as to be invisible, even with high 
 powers of the microscope. The elongated and flattened cell-spaces of 
 bone are frequently called lacunce, and the numerous fine, branching, in- 
 
38 NORMAL HISTOLOGY. 
 
 tercommunicating channels which pass off from them in all directions 
 and open into the marrow cavities, or into the passages for the blood- 
 vessels, are called canaliculi. 
 
 The bone-cells, or bone-corpuscles, as they were formerly called, 
 are, in adults, thin, flat cells, with spheroidal or oval projecting nuclei. 
 It was formerly believed that they sent fine branching processes off 
 into the ramifications of the canaliculi. Recent investigations, how- 
 ever, have thrown great doubt upon the existence of at least such 
 numerous processes as were formerly believed to exist — it having 
 been shown that some at least, if not all, of the supposed processes 
 are really portions of the intercellular substance lining the lacunae 
 and canaliculi. In young bone the cells are not flat, but spherical or 
 ovoid al. 
 
 We distinguish two kinds of bone-tissue, spongy and compact. 
 
 In spongy bone-tissue, which is found in abundance in the epiphyses 
 of the long bones, the hard substance, or bone proper, is arranged in 
 the form of thin plates, which are grouped together so as to enclose 
 tiny, irregularly-shaped cavities, filled with marrow tissue. In these 
 thin plates of spongy bone the cells lie irregularly scattered through 
 the intercellular substance, which is homogeneous. 
 
 In compact bone, such as is found in the diaphyses of the long bones, 
 the intercellular substance is arranged in layers, or lamellae, in and be- 
 tween which lie the cells. The lamellar arrangement is best seen in 
 transverse sections from the diaphyses of the long bones. If we look 
 at a thin cross-section of such a bone with a low magnifying power, we 
 notice numerous round or ovoid, or irregular-shaped openings, of 
 varying size, and around these are grouped several thin concentric 
 layers of basement substance, in and between which lie the ce]ls, flat- 
 terled in the plane of the lamellae. 
 
 These sets of concentric lamellae are called special or Haversian 
 systems of lamella. Sometimes these Haversian systems of lamellae 
 lie closely crowded together, and again they lie at varying distances 
 from one another. In the latter case the intervening space is filled 
 up by other and irregular sets of lamellae, which do not correspond 
 with the Haversian lamellae, but pass off obliquely in various direc- 
 tions. These are called, from their position relative to the Haversian 
 system, intermediate lamellae. Finally, at the surface of the bone be- 
 neath the periosteum, and sometimes at the inner surface adjacent to 
 the medullary cavity, are seen a series of lamellae which lie parallel to 
 the surfaces of the bone, which are called general or circumferential 
 lamellae. If we make a longitudinal section of a long bone we find 
 that it is traversed by a number of more or less longitudinally arranged, 
 branching and communicating canals, of varying size, in which lie the 
 blood-vessels. It is around these canals, called Haversian ca?zals, that 
 the Haversian lamellae are grouped, and the variously shaped openings 
 which are seen in the transverse sections are the transverse sections of 
 these blood-vessel, or Haversian canals. Within the Haversian canals, 
 when they are not entirely filled with the blood-vessels, we find the latter 
 enclosed in a tissue identical with that .filling the medullary cavity, and 
 presently to be described as marrow. In the flat and irregular-shaped 
 
CARTILAGE — BONE — TEETH. 39 
 
 bones essentially the same structural features are present, but the lamel- 
 lar arrangement is much less regular. 
 
 Although, for the most part, the intercellular substance of bone is, 
 by the ordinary modes of preparation, apparently quite destitute of 
 structure beyond that indicated by its lamellation, we yet find in cer- 
 tain portions a well-defined system of fibres. If, in a decalcified bone, 
 some of the external lamellae are torn off, numerous fine, fibriliated, 
 spicula-like projections are seen hanging on to the inner surface of the 
 separated fragments. These are the so-called Sharpey's fibres, which, 
 passing inward from the periosteum, pierce the bone either obliquely 
 or at right angles. As we shall see when studying the development of 
 bone, these Sharpey's fibres are the remains of fibriliated connective- 
 tissue bundles, which originally occupied the situation now filled by 
 bone. Recent investigations, moreover, have led to the belief that in 
 bone as in hyaline cartilage the basement substance is everywhere deli- 
 cately fibriliated, but we have not time in these lessons to consider 
 the methods by which this is believed to be demonstrable. 
 
 2. Periosteum. — The periosteum consists chiefly of fibriliated con- 
 nective tissue, with a few elastic fibres, and we recognize in it two 
 layers : an outer layer, composed chiefly of firm, dense connective 
 tissue, which is continuous with the muscular aponeuroses surrounding 
 the bone ; and an inner layer, which is looser in texture, more vascular, 
 and abundantly furnished with variously shaped cells. The periosteum 
 is attached to the bone by connective-tissue fibres, which pass from the 
 former into the substance of the latter, the attachment being firmer at 
 some points than at others, as, for example, near the extremities of the 
 long bones and at the points of insertion of the tendons and ligaments. 
 
 Blood-vessels pass also from the periosteum into the bone. 
 
 3. Marrow. — Marrow-tissue is found in the central or medullary 
 cavity of bones, in the tiny chambers of spongy bone, and in the Haver- 
 sian canals. Sometimes it has a yellowish color and is fatty, sometimes 
 it presents itself in the form of a reddish pulp. Red marrow is found in 
 embryos and in young animals, and in adults in certain small bones and 
 the vertebrae. 
 
 In certain animals, such as the rabbit and guinea-pig, red marrow is 
 found in most of the bones, even in adult life. In adult man, under nor- 
 mal conditions, the marrow, except in the vertebrae, ribs, and certain 
 small bones, is yellow. Yellow marrow differs from the red in that it 
 contains a large amount of fat, sometimes consisting almost exclusively 
 of fat-cells. 
 
 We find in red marrow, which is best adapted for study, blood-vessels 
 and spindle-shaped or branching cells, which constitute the supporting 
 framework of the tissue. In the interstices of the latter lie several dis- 
 tinct kinds of cells : 1. Fat-cells ; 2. smaller and larger spheroidal cells, 
 having essentially the same structure and character as the lymph-cells, 
 and called, par excellence, marrow-cells ; 3. cells somewhat larger than 
 the last mentioned, with, usually, a single very irregular-shaped and 
 sharply defined nucleus ; 4. very large granular cells, which usually have 
 several nuclei scattered through the cell-body, or grouped on one side the 
 so-called myeloplaxes or giant cells. It is not improbable that the two 
 
40 NORMAL HISTOLOGY. 
 
 last varieties are only modified forms of the same kind of cells. In the 
 marrow of developing bone are seen spheroidal, or irregularly cuboidal, 
 large granular cells, with commonly oval nuclei, usually situated at one 
 side of the cell-body. These are the so-called osteoblasts, with which 
 we shall become better acquainted when we study the process of bone- 
 development. 
 
 In addition to the above cell-forms, red blood-cells, escaped from 
 the blood-vessels, are usually seen in abundance. Not infrequently, 
 when fresh marrow is studied, cells are seen which in many respects are 
 like the true marrow-cells, but which, with a distinct nucleus, have a 
 homogeneous cell-body resembling in its color the red blood-cells. 
 These cells are the so-called nucleated red blood-cells, and are believed 
 by some observers to be destined to lose their nuclei in some way 
 or other, and assume the character of the ordinary red blood-cells. 
 Those who advocate this view regard the marrow of bones as one of 
 the blood-producing tissues of the body. The transformation of these 
 cells into red blood-cells has never been directly observed, and as the 
 peculiar appearance which they present can be accounted for on 
 other grounds, the formation of red blood-cells in the marrow, while 
 not improbable, cannot yet be regarded as definitely demonstrated. 
 
 PRACTICAL STUDY. 
 
 Decalcified Bone. — To obtain a general view of the structure of 
 bone we have recourse to transverse and longitudinal sections of one 
 of the long bones (from man or the lower animals, such as the rabbit 
 or dog), which has been freed from its lime salts by soaking in dilute 
 acids and rendered so soft as to be readily cut with a razor. 
 
 Although various acids, such as nitric and hydrochloric, effect the 
 decalcification of bone, solutions of chromic or picric acids are prefera- 
 ble, because, while very perfectly removing the lime salts, they harden 
 and preserve the soft structures in a most satisfactory manner. As 
 the salts of lime, as they exist in bone, do not undergo rapid solution 
 in these acids, the bits of bone which are to be decalcified should be 
 small, or the process will be a very protracted one. They should not 
 at most be larger than a cubic centimetre, and if they are half that 
 size the process will be much more rapidly completed. The quantity 
 of fluid should also be quite large (200 to 300 cubic centimetres to a 
 bit of bone of the above size). Picric acid is on the whole to be pre- 
 ferred, because the chromic acid often leaves the tissues in a granular 
 or cloudy condition, which interferes with subsequent study. If chromic 
 acid be employed, the bit of bone is put first into a weak aqueous solu- 
 tion (1 in 600) ; in a couple of days it is transferred to a fresh solution 
 (1 in 400), and again in a couple of days to another solution of 1 in 
 200. A stronger solution than this should never be used, but this 
 should be renewed every few days and the bottle frequently shaken. 
 In two or three weeks the process will probably be completed ; this 
 can be ascertained by passing a fine needle into the preparation. If 
 it be desirable to hasten the process, after a week or ten days' soaking 
 in chromic acid, as directed, a little nitric acid may be added to the 
 
CARTILAGE— BONE — TEETH. 4 1 
 
 solution (1 c.c. to 100 ex.). The previous action of the chromic acid 
 will prevent the swelling and partial destruction of the soft parts, 
 which nitric acid alone causes. If picric acid be employed, a saturated 
 aqueous solution should be used, the preparation frequently shaken, 
 and additional crystals of the acid occasionally added. 
 
 When the bone has become sufficiently soft by either of these 
 methods, it is allowed to soak for a day in water to remove the excess 
 of acid, and then hardened and preserved in alcohol. The picric acid 
 preparation will have to soak a day or two in alcohol before the acid 
 will be entirely removed. Sections are made transversely and longi- 
 tudinally by the usual methods, the preparation being embedded in 
 wax or a bit of hardened liver, if too small to hold in the fingers. 
 
 Bone decalcified in chromic acid is best stained in hematoxylin; 
 that softened by picric acid shows the structure very beautifully after 
 staining with picro-carmine. The specimens may be studied in gly- 
 cerine, and may be preserved in this or in Canada balsam. If the 
 periosteum has not been removed, its structure and relation to the 
 bone is well shown. 
 
 Sections of Hard Bone. — In such preparations as the above which 
 are mounted in fluids the canaliculi are for the most part invisible be- 
 cause the fluids which fill them possess very nearly the same refractive 
 power as the basement substance. The cell-spaces and canaliculi are 
 best studied in thin sections of hard bone which have been macerated 
 for some time, so as to remove the medullary fat and other soft parts, 
 and then dried. Transverse or longitudinal sections may be made, the 
 latter being most easily prepared because sections in this direction 
 are less brittle. A small piece, as thin as possible, should be sawn 
 from the bone (a diaphysis of a human long bone answers very well) 
 in the proper direction ; this is ground down very thin on a whetstone 
 or grindstone, or on a plate of glass with emery powder, the section 
 being held down with the ball of the finger or a bit of soft cork. When 
 it has become quite thin, so as to be almost transparent, it is polished 
 on a dry oil-stone free from grease, and then carefully washed and 
 brushed under water with a fine pencil to remove particles of dirt. It 
 is now allowed to dry, and may be mounted dry in a cell made by a 
 ring of asphalt varnish and covered, the cover being cemented down 
 by a rim of the same material ; or it may be mounted in Canada bal- 
 sam. In the latter case the semi-fluid Canada balsam, such as is used 
 for ordinary mounting, should not be employed, because it would run 
 into the lacunae and canaliculi, and render them invisible. A bit of 
 quite hard and solid balsam should be placed on a slide and heated 
 until it melts ; just as it begins to fairly cool, but before it gets at all 
 hard, the slip of bone is quickly immersed in the drop and covered. 
 If the proper moment has been chosen, when the balsam is neither too 
 hot nor too cold, the unevennesses upon the surface of the section, 
 which are seen when it is mounted dry, are entirely concealed, and 
 the lacunae and canaliculi are clearly defined by reason of the air with 
 which they are filled. Usually, however, in the most successful pre- 
 parations, in the very thin parts or at the edges, part of the canaliculi 
 have become filled with the balsam and rendered invisible. 
 
42 NORMAL HISTOLOGY. 
 
 Marrow. — A long bone, from a rabbit or from a child, should be 
 broken across and a little of the red marrow scooped out and laid for 
 twenty-four hours in a mixture of alcohol and water (i to 2). Fragments 
 are then teased on a slide in equal parts of glycerine and water which has 
 been colored with eosin. When the bits have been thoroughly teased 
 and are sufficiently stained, they are covered and studied. By this 
 process the red blood-cells are destroyed and the characters of the 
 so-called nucleated red blood-cells somewhat changed. For the study 
 of the latter fresh marrow should be teased on a slide in the indifferent 
 salt solution. 
 
 Development of Bone. 
 
 At a certain period of embryonic life no bone-tissue is found in the 
 body, the parts where it is finally to be being occupied either by car- 
 tilage or fibrillar connective tissue. Out of these tissues the bone is 
 developed by a process which, though presenting considerable differ- 
 ences in detail in various parts of the body, is yet, in its essential 
 nature the same in all. We recognize three ways in which bone is 
 developed: 1. In the substance of pre-existing cartilage — infra-carti- 
 laginous ; 2. beneath the periosteum — subperiosteal; 3. in the sub- 
 stance of pre-existing fibrillar connective-tissue membranes — iiitra- 
 membra?ious. 
 
 In all of these modes of bone-formation the new bone seems to be 
 deposited under the influence of certain large, granular, usually sphe- 
 roidal or cubical nucleated cells, called osteoblasts. 
 
 1. When bone is formed from cartilage, the latter bears a general 
 resemblance, in shape, to the finished bone. The first change which 
 we notice in such a cartilage which is about to undergo ossification, 
 is that at a certain point — if it be a long bone, at about the middle of 
 the diaphysis — the cartilage-cells begin to enlarge, the basement sub- 
 stance between them becoming partially absorbed, and what remains 
 of the latter becoming infiltrated with fine granules of lime salts. 
 
 Around this calcified portion we find the blood-vessels of the peri- 
 chondrium accompanied by marrow-tissue and the above-mentioned 
 osteoblasts, growing into the calcified cartilage, absorbing the latter 
 as they go, and forming irregular channels or cavities called medullary 
 spaces. These channels are at first separated from one another by 
 narrow, irregular septa of the cartilage basement substance which re- 
 mains unabsorbed, and are lined by layers of osteoblasts by whose 
 agency the septa become covered with new bone in a manner pres- 
 ently to be described. 
 
 The region in which this new bone is first deposited is called the 
 ossification zone. As the blood-vessels, accompanied by the osteo- 
 blasts and marrow-tissue, proceed farther and farther into the cartilage, 
 channelling out the medullary spaces as they go, we always find, just 
 in advance of the ends of the blood-vessels and the extremity of the 
 spaces in which they lie, a zone of calcified cartilage ; and beyond this 
 cartilage tissue which apparently prepares the way for the advancing 
 marrow-spaces and newly forming bone by very characteristic modifi- 
 cations, chiefly in the form and arrangements of its ceils. 
 
CARTILAGE — BONE — TEETH. 43 
 
 If we examine the cartilage at a considerable distance from the 
 line of ossification we find the ordinary appearance of hyaline cartilage 
 with more or less flattened cells. Approaching now the zone of ossi- 
 fication, we find that the cells are larger, are arranged in rows or 
 groups of frequently 4, 8, or 16, etc., the intercellular substance being 
 less in amount corresponding to the increase in size and number of 
 the cells. Farther inward we find the cells still more plainly arranged 
 in rows, very large, sometimes globular or flattened against one 
 another, and the basement substance reduced to quite thin septa, en- 
 closing spaces in which the rows of large cartilage-cells lie. Then 
 comes a narrow zone, in which the septa of the basement substance 
 are filled with fine granules of lime salts — calcification zone. Here 
 the cartilage- cells have assumed a peculiar granular character. Fi- 
 nally, we find that the lime salts have disappeared from the septa, and 
 that the spaces which contained the large granular cartilage-cells have 
 become continuous with the advancing, vascular, bone-walled, narrow 
 cavities above described. It is to be distinctly understood that the 
 calcification zone is not bone, but only calcified cartilage ; the true 
 bone being first formed after this lime has disappeared, on the surface 
 of the septa in which it was temporarily deposited — for what purpose, 
 we do not know. Turning our attention now to the exact way in 
 which the bone is formed under the influence of the osteoblasts, we 
 find that, just beneath these cells as they lie along the walls of the new- 
 formed medullary spaces, the basement substance of true bone begins 
 to be deposited, at first in the form of a narrow shell beneath each 
 osteoblast. These deposits, which on cross-sections have a crescentic 
 shape, become thicker and thicker, rising up around the cell, which 
 they finally enclose — the enclosed osteoblast becoming, as it would 
 seem, a bone-cell. This process going on around each osteoblast, the 
 walls of the medullary cavities soon become covered with a layer of 
 bone containing bone-cells. New osteoblasts appear on the walls, 
 and in turn become enclosed in a layer of bone, and thus the lamellar 
 arrangement of bone-tissue is produced. The remains of cartilage 
 thus covered by bone, in the septa, finally disappear in a manner un- 
 known to us. 
 
 2. Hand in hand with the formation of bone within the cartilage, 
 new bone is formed on its surface beneath the perichondrium, which 
 thus becomes periosteum. The process of sub-periosteal ossification, 
 by which the bone increases in thickness, is dependent also upon the 
 presence of osteoblasts. We find these arranging themselves along 
 the blood-vessels which enter the bone, and along the inner layer of 
 connective-tissue fibres of the periosteum, and bone is formed around 
 them in the manner above described. New bone thus formed at the 
 surface appears at first by no means in the form of smooth, contin- 
 uous layers, but as the blood-vessels and connective-tissue bundles, 
 along which the osteoblasts lie, are arranged at varying angles with 
 the surface of the bone and with each other, the effect is to produce 
 irregular branching cavities, upon whose walls the new layers of bone 
 are deposited. When these branching cavities become filled, with the 
 exception of the space occupied by the blood-vessels and marrow- 
 
44 NORMAL HISTOLOGY. 
 
 tissue, by successive lamellae of bone, they constitute the structures 
 with which we are already familiar, under the name of Haversian 
 canals and Haversian lamellae. Where the formation of bone has 
 taken place along bundles of connective tissue, these bundles some- 
 times persist for a long time, in a modified form, between the sets of 
 lamellae, and constitute the above-mentioned Sharpey's fibres. Thus, 
 by the transformation of cartilage and apposition at the surface, the 
 long bones are formed. In these bones the ossification progresses 
 toward the epiphyses, where independent centres of ossification are es- 
 tablished. The lines of ossification approach each other, and finally, 
 when the process of growth in the bone is complete, the band of carti- 
 lage which separated them disappears, and epiphysis and diaphysis 
 join to form a single bone. As the bone grows by apposition beneath 
 the periosteum, the osseous tissue which was first formed in the dia- 
 physis is absorbed, and the medullary cavity is formed in the place 
 which it originally occupied. 
 
 3. When bone is formed in membranes of fibrillar connective tissue, 
 as in the skull-cap, we notice, first, that some of the interlacing bun- 
 dles which occupy the place of the future bone become infiltrated 
 with lime salts ; along these calcareous bundles cells become very 
 abundant, osteoblasts appear and arrange themselves, and bone forms 
 around them just as in the other varieties of bone-formation. Blood- 
 vessels and marrow-tissue lie between the new-formed layers of bone, 
 so that at a certain period of embryonic life the bones of the skull-cap 
 consist of a series of bony lamellae, arranged so as to enclose branch- 
 ing and communicating cavities, which are occupied by blood-vessels 
 and marrow-tissue, and whose walls are lined with osteoblasts. A 
 well-defined periosteum is finally formed, beneath which successive 
 layers of new osseous tissue are deposited, and thus the bone increases 
 in thickness and acquires its smooth surfaces. 
 
 The mode of origin of the osteoblasts is still very obscure. Many 
 investigators believe that in the intra-cartilaginous ossification they are 
 the large cartilage-cells which we see at the calcification line, which in 
 contact with the blood-vessels become so modified in form and func- 
 tion as to assume the role of bone-formers. Others assert that the 
 large cartilage-cells disintegrate and disappear, and that the osteo- 
 blasts are produced from cells which accompany the blood-vessels. 
 Still others regard them as white blood-cells, modified and endowed 
 with new functional powers. In the intra-membranous and subperi- 
 osteal ossification, many suppose that they are formed from connective- 
 tissue cells. So little is absolutely known, however, as to their genesis, 
 that while recognizing their importance in bone-formation, we can 
 regard none of these various theories as to their origin as definitely 
 established. 
 
 PRACTICAL STUDY. 
 
 Intra-cartilaginous and Subperiosteal Ossification. — A long bone, 
 from a nearly mature foetus or a young animal, should be carefully 
 removed without injuring the periosteum, and decalcified with picric 
 
CARTILAGE— BONE— TEETH. 45 
 
 or chromic acid. Thin longitudinal sections are made with a razor 
 through the ossification zone, embracing the tissue for a considerable 
 distance on either side of it. The sections are stained, first with 
 hematoxylin and then with eosin, and mounted in glycerine or Canada 
 balsam. 
 
 The basement substance of the cartilage and the cell-nuclei will be 
 stained violet, while the cell-bodies and the new-formed bone are 
 stained rose-red. 
 
 Intra-membranous Ossification. — To study the early stages of this 
 process, a young embryo (if from the sheep or pig, 4 to 6 cms. long) 
 should be soaked for a few days in Miiller's fluid, and a bit corre- 
 sponding to the portion of one of the parietal bones cut out, and the 
 skin, muscles, and dura mater torn away with forceps under water. 
 The membrane in which ossification is occurring is now to be care- 
 fully brushed with a stiff pencil until it is thin enough to be examined 
 with tolerably high powers. It is stained with hematoxylin and eosin, 
 and mounted in glycerine or balsam. 
 
 The irregular chambers, lined with osteoblasts and filled with 
 blood-vessels and marrow, which are formed in the bones of the skull- 
 cap at a later period, are well-shown by transverse sections through 
 the decalcified skull-bones of an older foetus (human, at about 6 or 7 
 months, or from the beef or sheep, 16 to 20 cms. long). These 
 should be stained as above, and mounted in Canada balsam. 
 
 Teeth. 
 
 The teeth have many structural features in common with bone. 
 The chief bulk of the tooth is made up of a homogeneous, brittle 
 basement substance much harder than bone, called dentine. The den- 
 tine contains lime salts, and is permeated by a multitude of fine 
 branching channels which radiate from a central cavity, called the 
 pulp cavity, which the dentine encloses. 
 
 These delicate channels in the dentine are analogous with the 
 canaliculi of bone. The pulp cavity is filled with a soft vascular 
 tissue, called pulp, containing irregular-shaped, often branching cells, 
 and nerves. 
 
 Along the sides of the pulp cavity lie spheroidal or ovoid cells, 
 which send off branches into the pulp and also into the above men- 
 tioned delicate channels in the dentine. These cells are called odonto- 
 blasts, or de?itine cells, and are usually considered to be the analogues 
 of the bone-cells. The pulp cavity is open at the root of the tooth 
 for the admission of vessels and nerves. Surrounding the root of the 
 tooth is a thin layer of bone, called cement. At the crown of the 
 tooth the dentine is completely covered by a layer, of varying thick- 
 ness, of an extremely hard substance called enamel. The enamel 
 consists of a series of closely packed, small, wavy or undulating 
 prisms, placed edgewise upon the surface of the dentine, and covered, 
 over the free surface of the tooth, by a hard, tough, structureless mem- 
 brane, called the enamel cuticle. 
 
46 NORMAL HISTOLOGY. 
 
 PRACTICAL STUDY. 
 
 Hard Teeth. — To study the hard parts of teeth, thin sections of 
 a macerated and dried tooth should be ground down ' by the method 
 described when we were studying hard bone, and mounted, with the 
 same precautions, in hard Canada balsam. 
 
 Decalcified Teeth. — The soft parts of teeth may be studied in sec- 
 tions from teeth which have been decalcified with picric acid. The 
 tooth should be broken across, so as to expose the pulp cavity and 
 hasten the action of the solvent, and immersed until soft in a saturated 
 solution cf the acid, which is frequently shaken, and to which fresh 
 crystals are added at short intervals. Sections are stained with hsema- 
 toxylin and eosin. 
 
CHAPTER V. 
 
 BLOOD AND LYMPH. 
 
 Blood. 
 
 Although strikingly different in some of their physical characters 
 from most of the animal tissues, we must yet regard blood and lymph 
 as true tissues, in which, however, unlike most others, the intercellular 
 substance is fluid. Let us first consider the blood. In normal human 
 blood we find suspended in a colorless fluid — the plasma — three dis- 
 tinct classes of formed elements : r. colorless blood-cells ; 2. red 
 blood-cells ; 3. variously shaped free granules. 
 
 t. Colorless Blood-cells, or Leucocytes. — These are small spheroidal 
 cells, without a distinct membrane, the cell-body being sometimes al- 
 most transparent, or very finely granular, in other cases quite coarsely 
 granular. 
 
 They possess one or more nuclei, not usually visible in the living 
 cells, and of varying form — sometimes spherical or dumb-bell shaped, 
 sometimes having the form of a bent or twisted cylinder, and again 
 entirely irregular. 
 
 These cells possess the power, under favorable conditions, of 
 spontaneous movement. They change their form and place. While, 
 when in a state of rest, they assume in general the spheroidal form, as 
 above stated, we find that when they become active they send out 
 variously shaped processes, some fine and delicate, others broad and 
 of very irregular shape. We often see, after a process has been 
 thrown out, that it becomes gradually larger and larger, the cell-body 
 becoming correspondingly smaller, until finally the whole cell seems 
 to have passed over into the process, thus moving forward. 
 
 Sometimes processes are thrown out and again withdrawn, and not 
 infrequently the whole cell flattens out into an irregular-shaped mass, 
 so thin as to be almost invisible. Not infrequently clear rounded 
 spaces, called vacuoles, suddenly appear in the cell-body during its 
 movements, and either remain for some time, or soon disappear as 
 suddenly as they came. 
 
 These movements are called amoeboid movements ; they are always 
 very slow, and are greatly influenced by the temperature, density, 
 and oxygen-content of the fluid in which they lie. By virtue of this 
 locomotive power the white blood-cells perform certain evolutions 
 within the vessels ; they escape through their walls, and, sometimes 
 singly, sometimes in vast numbers, move through the tissues in the 
 larger and smaller lymph-spaces. This emigration of white blood- 
 
43 NORMAL HISTOLOGY. 
 
 cells occurs, apparently, to a slight extent, under normal conditions ; 
 but it is under pathological conditions that it is most active. The nu- 
 clei of these cells become visible on the application of a variety of 
 agents which determine the death of the cells, such as acetic acid, 
 dilute alcohol, and certain coloring agents. The cells vary greatly in 
 size, but on the average are, in man, larger than the red cells. 
 
 2. Red Blood-cells. — These cells having in man the form of bi- 
 concave disks, consist apparently simply of a cell-body without mem- 
 brane or nucleus — at least in the adult human blood-cells no such 
 structure is demonstrable by the ordinary modes of preparation. 
 
 Although when crowded together in great numbers the blood-cells 
 present a red color, when seen separately they have a greenish yellow 
 tint. 
 
 The cell-body is very soft and pliable, jelly-like, changing its shape 
 on the slightest pressure. It is more deeply colored at the periphery, 
 when seen from the side, because of its greater thickness at that part. 
 Owing to the peculiar shape of the cell, it acts as a lens upon the 
 light passing through it, and its central portion is either light or dark, 
 depending upon whether the objective is approached to or withdrawn 
 from it. When examined fresh in the plasma, many of the cells ar- 
 range themselves closely together side by side, in longer and shorter 
 rows. On being drawn from the vessels, if the plasma be allowed to 
 evaporate, or if certain fluids, such as strong solutions of common 
 salt or bichromate of potassium, be added, a part of the red blood- 
 cells lose their regular shape, their edges become crenulate and 
 jagged, and they sometimes seem to become smaller; finally, they 
 assume the form of irregular globular masses, beset with short, blunt 
 spines. Various other irregular forms are produced under the same 
 circumstances, which it is not necessary to describe here. The addi- 
 tion of water causes the spines to disappear, and the cells swell up 
 again and assume a globular form. The coloring matter is dissolved 
 by the water, which itself becomes colored. 
 
 These changes in form, produced by certain chemical agents and 
 by changes in the density of the fluid in which the cells lie, should not 
 be mistaken for the expression of vitality, or regarded as analogous 
 with the amoeboid movements of the white blood-cells. Not infre- 
 quently in normal blood, very often under pathological conditions, 
 red. blood-cells are seen which are much smaller than the above de- 
 scribed forms, and are spheroidal in shape. The red blood-cells are 
 much more abundant than the white, there being in normal human 
 blood about 350 to 500 of the former to one of the latter. The red 
 blood-cells owe their color as well as their power of performing certain 
 important physiological functions to the presence in them of a crys- 
 tallizable substance called haemoglobine. 
 
 The shape of haemoglobine crystals obtained from the blood of 
 different animals is not always the same ; those from the human blood 
 have in general the form of rhombic prisms. 
 
 The free granules, which are always found in greater or less num- 
 ber in human blood, are chiefly of two kinds : small spherical bodies, 
 somewhat resembling fat-droplets, and variously shaped, often angular 
 
BLOOD AND LYMPH. 49 
 
 masses, looking not unlike fragments of white blood-cells. The na- 
 ture of these granules is yet undetermined. 
 
 The above description of the blood-cells of man applies, with few 
 exceptions, to other mammalia. In other vertebrates, however, we find 
 a marked difference in the form and character, particularly of the red 
 cells. They are, for the most part, oval, and possess a distinct nucleus, 
 which often projects from both sides of the disk. 
 
 The intercellular substance, or plasma of freshly drawn blood, is 
 perfectly homogeneous ; but if we allow it to coagulate we find, on 
 subjecting it to microscopical examination, that in addition to the 
 above described elements, a multitude of delicate filaments lie among 
 the cells, stretching in all directions and joining each other at frequent 
 intervals, forming an irregular meshed net. We find, moreover, if we 
 examine a clot from which the red cells have been carefully removed 
 or rendered invisible, that in many places the filaments are grouped 
 around irregular-shaped granules, looking like those above described 
 in normal blood ; and if the process of coagulation be carefully ob- 
 served, they may be seen to actually shoot out from these granules, 
 which thus seem to form starting-points for their formation. The sub- 
 stance which thus separates from the plasma is called fibrine. 
 
 Lymph. 
 
 In lymph we find spherical cells, identical in structure with the 
 white blood-cells and variously shaped granules, composed apparently 
 of a combination of albuminoid material with fat. These granules, in 
 that variety of lymph called chyle, is so abundant as to give the fluid 
 a milky appearance. 
 
 Direct observation has shown that,. in some animals at least, the 
 white blood-cells can multiply by division. Whether the cells which 
 supply the place of those which seem to be used up in the process of 
 growth and reparation are produced in this way, and if so, whether the 
 division occurs in the blood- or lymph-vessels, or in the cell-spaces of 
 the connective tissue, or in certain special organs, or whether they are 
 produced in a manner entirely unknown to us — these are questions 
 not only of theoretical but of practical interest ; but, in spite of much 
 research and the accumulation of many observations bearing on the 
 matter, we are still unable to give them a definite answer. Still more 
 obscure, if possible, is the origin of the red blood-cells. Although in 
 the adult man they seem to possess no nucleus, yet in embryonic life 
 they certainly are furnished with that structure, we find nucleated red 
 blood-cells. Now, it has been recently shown that in certain parts of 
 the body, in adult life, cells occur which in many respects resemble 
 the nucleated red blood-cells of the embryo ; such cells are found, for 
 example, in the spleen, in the red marrow of bones, etc. 
 
 The most plausible theory in regard to the matter is that in certain 
 parts of the body — spleen, marrow, lymph-glands, and liver — white 
 blood-cells are produced, a part of which are changed into the red 
 blood-cells. The so-called nucleated red blood-cells are supposed to 
 be intermediate forms. It must be remembered, however, that this is 
 4 
 
50 NORMAL HISTOLOGY. 
 
 only theory as yet, and many observers do not ascribe to the so-called 
 nucleated red blood-cells the significance upon which the advocates 
 of this theory insist. 
 
 I PRACTICAL STUDY. 
 
 Fresh Human Blood. — This may be obtained by tying a cord tightly 
 around the finger, to cause congestion, and then pricking it sharply 
 with a needle. A clean cover-glass and slide being at hand, and the 
 microscope being in readiness, the cover-glass is touched to the drop of 
 blood so that a small droplet adheres. The cover is now carefully low- 
 ered onto the slide, no pressure being exerted beyond its own weight. 
 The film of blood beneath the cover should be very thin, or the crowd- 
 ing of the blood-cells will interfere with the observation. The speci- 
 men should be examined at once, when the various above described 
 elements will be readily recognized. As the plasma evaporates from 
 the edges the changes in form of the red blood-cells will be seen. 
 Finally, a drop of water should be added at the edge of the cover-glass 
 and its effect observed. 
 
 White Blood-cells. — In order to study more thoroughly the form of 
 the white blood-cells, and the various shapes which their nuclei pre- 
 sent, a drop of blood should be placed on a slide and mixed with a 
 few drops of dilute alcohol (alcohol i, water 3). After a few moments 
 a drop of eosin is added to the specimen, and it is covered and studied 
 at once. 
 
 Amozboid Movements. — These are most conveniently studied in the 
 blood or lymph from one of the cold-blooded animals, such as the 
 frog, for they occur here at the ordinary temperature of the air, while 
 artificial heat must be resorted to if we would maintain the blood of 
 warm-blooded animals at a proper temperature for the observation of 
 these movements. A shallow cell — about one mm. deep — having 
 been made on a slide, either by painting a narrow ring of asphalt varnish 
 upon it, or by cementing to it a ring made for the purpose, and a little 
 sweet oil having been rubbed over the rim of the cell so as to form an 
 air-tight chamber of the latter when the cover is put on, a small drop 
 of blood, obtained by snipping off the tip of one of the carefully cleansed 
 toes, is put on the cover-glass, and it is inverted over the cell. As the 
 amoeboid movements are very tardy if the blood evaporates in the least, 
 and are more vigorous if the blood is even slightly diluted, it is well, be- 
 fore inverting the cover, to breathe into the cell, so that when the cover 
 is put down and the cavity closed, the air within, in contact with the 
 blood, may be charged with moisture. 
 
 On focussing now upon the specimen, white blood-cells will readily 
 be found in contact with the cover-glass, and selecting one of them 
 which, by its irregular shape, indicates its activity, the attention must 
 be fixed on this cell, and sketches of its form made at short intervals. 
 
 If the temperature of the room be low the movements may be 
 tardy in commencing, and they can be hastened by holding the finger 
 or any warm object for a moment near the cover. It will be borne 
 in mind that in the frog's blood the red cells are oval and nucleated, 
 and that they are also larger than the colorless cells. 
 
BLOOD AND LYMPH. 5 1 
 
 Fibrine. — A small quantity of blood may be whipped and the cells 
 washed from the clot by a stream of water, and a fragment of the 
 remaining substance teased in water on a slide and studied ; but the 
 objects thus obtained are not altogether satisfactory, since the relation 
 of the fibrillse to one another is disturbed and no light is thrown on 
 the way in which they are formed. For the accomplishment of these 
 ends, the following method may be employed : a medium-sized drop 
 of blood is received on a slide and immediately covered ; after a few 
 moments, when coagulation has occurred, a mixture of alcohol and 
 water, i to 2, is allowed to run under the cover; the latter may be 
 slightly raised at the edge if the clotted blood prevents the fluid from 
 running under; this decolorizes the red blood-cells, which become 
 nearly invisible. The fibrine may now be seen with a moderately high 
 magnifying power, but it can be brought much more clearly into view 
 if a dilute solution of aniline red in equal parts of alcohol and water 
 be allowed to run under the cover; this stains the white blood-cells 
 and the fibrine. leaving the red cells uncolored. The object of adding 
 the dilute alcohol is to ged rid of the red blood-cells, whose color, when 
 they are present in large numbers, almost entirely conceals the fibrine 
 fibres. 
 
CHAPTER VI. 
 
 MUSCULAR TISSUE. 
 
 Certain muscles are under the control of the will, and are called 
 voluntary muscles; and as these have, as we shall see presently, a 
 very characteristic striation of their structural elements, they are also 
 called striated voluntary muscles. To this class belong, among others, 
 the muscles of locomotion and the voluntary muscles of the trunk 
 and head. Other muscles are not under control of the will, and are 
 hence called involuntary. A certain portion of these involuntary 
 muscles possess the same striation of their elements as the voluntary 
 muscles, and hence are called involuntary striated muscles. These 
 are found in the heart. In another kind of involuntary muscle, such 
 as is found in the intestine and bladder, the elements do not possess 
 the same kind of striations, and they are hence called smooth or non- 
 striated involuntary muscles. 
 
 Y\~e have thus to studv three kinds of muscles : 
 
 i. Ir.voiuntarv muscles - 1 f sm -°?* or ""Striated. 
 
 ( b. striated. 
 2. Voluntary muscles (striated). 
 
 i. a. — Smooth Muscular Tissue. 
 
 Smooth muscular tissue is made up of very much elongated, narrow, 
 pointed, usually fusiform nucleated cells. These cells are commonly 
 
 arranged in groups or bundles, enclosed in connective tissue and sup- 
 plied with blood-vessels and nerves. The cell-body, although usually 
 fusiform, is sometimes flattened and band-like, often divided at the 
 ends into two pointed extremities. Owing to pressure from adjacent 
 parts, the fusiform cell-bodies are often more or less flattened at the 
 sides, presenting on cross-section an irregular polygonal contour. The 
 cell-body has an indistinct longitudinal striation. is otherwise homo- 
 geneous except that occasionally, in the vicinity of the nucleus, a 
 few shining granules are seen. The nucleus is narrow and much 
 elongated, rod-like, pointed or rounded at the ends, and commonly 
 encloses one or more nucleoli. It usually lies near the middle of the 
 cell, which is often thickened or bulging at that point. 
 
 These ceils lie side by sice or lap over one another at the ends, 
 and are joined together by a small amount of an albuminoid cement 
 substance, which, like the cementing substance of endothelial and 
 other cells, is stained brown or black after exposure to light by dilute 
 
MUSCULAR TISSUE. 53 
 
 solutions of nitrate of silver. These smooth muscle-cells are variously 
 grouped in different parts of the body ; sometimes crowded together 
 in solid bundles, which are surrounded by connective tissue and ar- 
 ranged in layers as in the intestine ; sometimes arranged in narrow 
 interlacing fascicles, as in the bladder ; sometimes scattered singly 
 through certain tissues ; and again, running in various directions and 
 associated with bands of connective tissue, they form compact masses, 
 as in the uterus. 
 
 The longitudinal striation, which, under favorable circumstances, is 
 seen on the cell-body, is not a mere surface marking, but extends deep 
 into the cell, as may be seen in transverse sections of suitably pre- 
 pared cells, where fine lines are observed passing inward from the 
 periphery of the cell toward the nucleus. The blood-vessels supply- 
 ing this tissue form for the most part elongated net-works throughout 
 its substance. 
 
 PRACTICAL STUDY. 
 
 Isolated Cells. — These we obtain by teasing bits of the tissue, but 
 as they are firmly bound together by the cementing substance, this 
 must first be dissolved or softened. This can be conveniently accom- 
 plished by soaking a bit of the tissue — the wall of the intestine, for 
 example — for forty-eight hours in an aqueous solution of bichromate 
 of potassium (1 to 800 ). Shreds of the muscular layer are torn off, 
 slightly stained with hematoxylin, washed, and then carefully teased 
 in glycerine which has been colored by eosin. Since the cells are 
 apt to shrink somewhat under the action of the bichromate, it is well 
 to enclose a short segment of the intestine, distended with the solu- 
 tion between two ligatures, and immerse it in a vessel of the same 
 solution. 
 
 Transverse and Longitudinal Sections. — The intestine of the cat 
 is -well adapted for the preparation of transverse sections, since here 
 the muscle-ceils are unusually large. A segment having been dis- 
 tended as above with MuTiers fluid, it should be immersed for ten 
 days in the same, then carefully washed and put for a day or two in 
 strong alcohol. A bit is then cut out, embedded between pieces of 
 hardened liver, and thin sections made in a direction exactly at right 
 angles to the axis of the gut. The sections are stamed with hema- 
 toxylin and mounted in glycerine. In such a preparation two layers 
 of muscle-cells are seen: in one the cells are seen in transverse, in 
 the other in longitudinal section. Since the cells lap over one another 
 in the transverse sections, the forms which they present will obviously 
 differ, depending upon whether they have been cut across at die level 
 of the nucleus, or at a point nearer the extremity. In such a prepa- 
 ration, the serosa and mucous membrane may be seen on opposite 
 sides of the muscular layers. 
 
 Treatment with Nitrate of Silver. — To show the cementing sub- 
 stance between the cells, a portion of intestine — the large intestine of 
 the frog answers well — should, be distended with air, a segment en- 
 closed between two ligatures, and the surface brushed firmly under 
 
54 NORMAL HISTOLOGY. 
 
 water with a pencil, to remove the endothelium which covers the peri- 
 toneal surface. It is then immersed for two minutes in a half per cent, 
 solution of nitrate of silver, washed with pure water, and exposed to 
 the light, in a mixture of equal parts of alcohol and water, till the sur- 
 face becomes brown. Small bits are now cut out, the epithelium from 
 the mucous membrane scraped off, and the object mounted in glyce- 
 rine with the external surface uppermost. The cells will be seen to 
 be bounded by brown or black lines, which represent the cement sub- 
 stance. 
 
 Muscle-cells of Frogs Bladder. — Elegant pictures of very much 
 elongated slender muscle-cells, lying singly or arranged in narrow in- 
 terlacing fascicles, may be obtained from the frog's bladder by the fol- 
 lowing method : the spinal cord of a frog being broken up, the ab- 
 dominal cavity is largely opened by a crucial incision, and a curved 
 canula, attached to a small syringe filled with a saturated solution of 
 bichromate of potassium, is passed into the cloaca and directed for- 
 ward into the bladder. The fluid is now slowly injected, and when 
 the bladder is partially distended, a ligature is thrown around its base, 
 and the injection continued till the organ is fully distended. The 
 ligature is now drawn tight and the canula withdrawn. The bladder 
 is cut out, still distended, and put in the same bichromate solution, 
 where it remains for three days, when it is washed and transferred to 
 alcohol. After twenty-four hours it may be opened, a bit cut out, the 
 epithelium carefully brushed from the inner surface, and stained with 
 haematoxylin and eosin, and mounted in glycerine. In addition to 
 the muscle-cells, the nuclei of the endothelial cells covering the perito- 
 neal surface will be seen, as well as connective-tissue cells and fibres. 
 
 2. Striated Voluntary Muscular Tissue. 
 
 As the involuntary striated or heart muscle occupies in structure 
 an intermediate position between the smooth and the voluntary stri- 
 ated muscle, we shall find it advantageous to postpone its study until 
 we have considered the other varieties. Voluntary striated muscle, to 
 which the greater part of the muscular tissue of the body belongs, is 
 made up of narrow, cylindrical, cord-like elements, of varying length 
 and thickness, called muscular fibres. These are grouped in variously 
 shaped bundles or fascicles, surrounded by connective-tissue enve- 
 lopes or sheaths, and abundantly supplied with blood-vessels and 
 nerves. Let us first study the structure of the individual fibres. They 
 consist of three distinct elements : i, contractile substance, forming 
 the centre and making up most of the bulk of the fibre ; 2, nuclei, 
 which in man and most warm-blooded animals lie scattered upon the 
 surface of the contractile substance ; 3, the sarcolenvma, a thin homo- 
 geneous sheath or tube, which tightly encloses the other elements. 
 1. If we examine a fresh muscle-fibre, or one which has been hard- 
 ened under favorable conditions with moderately high powers, we see 
 that the contractile substance is indistinctly longitudinally striated ; 
 and if we treat muscle with certain chemical agents, such as chromic 
 acid or its salts, we find that by slightly teasing the fibres, break up 
 
MUSCULAR TISSUE. 55 
 
 along the longitudinal striae into a multitude of fine fibrillar, which 
 are called primitive muscle-ftbrillce. Again, if we examine the fresh 
 or hardened fibres still further, we find that in addition to the longitu- 
 dinal striations, they are crossed by more prominent, narrow, alter- 
 nating, dark and light bands or stripes, the relative width of the stripes 
 varying according as the muscle is seen in a state of contraction or re- 
 laxation. Still further, if we soak a fresh muscle for twenty-four hours 
 in a half per cent, solution of hydrochloric acid, and then tease it, we 
 find that the fibres, instead of breaking up longitudinally into fibrillae, 
 break across transversely into thin disks. We thus see that, by break- 
 ing up in these two directions, the whole fibre is capable of being re- 
 solved into a multitude of tiny prismatic structures, which are called 
 sarcous elements. The central portion of each prism or sarcous ele- 
 ment is occupied by a dark portion, while at each end is a lighter 
 zone. The light and dark zones of the sarcous elements, when the 
 latter are grouped together, form the alternating light and dark bands 
 of the fibres. It is believed by many observers that the sarcous ele- 
 ments are definite and independent structures, in which the dark por- 
 tion is the contractile element, and that they are joined together side 
 to side and end to end by peculiar cementing substances. 
 
 In addition to these markings on the fibres, if high magnifying 
 powers are used and the fibre is in a state of extension, a fine line is 
 seen crossing the fibre through the centre of the light transverse band ; 
 this corresponds with the dividing line between the ends of the sar- 
 cous elements, and is called Krause's line. Under favorable condi- 
 tions the dark band is also seen to be crossed by a line called Hen- 
 serfs line, whose nature is as yet but imperfectly explained. 
 
 All of the above described structural features of the muscular 
 fibres are much more distinct after treatment with chemical agents, 
 and after death of the tissue ; the longitudinal striation is not visible 
 during life, and the distinct separation of the primitive fibrillar and 
 disks can only be accomplished by chemical means. We can see the 
 various markings with sufficient clearness, on the fresh or living muscle, 
 to convince ourselves that marked optical differences, at least, exist 
 in different parts of the fibres ; but that the living fibre is made up of 
 distinct elements, having [the structure which we see in the isolated, 
 or partially isolated, sarcous elements, although probable, is by no 
 means proven, since this isolation by chemical means may signify only 
 a tendency to break up in certain directions, and not a definite, pre- 
 existing, independent structure. 
 
 2. The nuclei, which in the mammalia lie upon the surface of the 
 fibres, and directly beneath the sarcolemma, in the amphibia, fishes, 
 and certain birds, also embedded within the contractile substance, 
 are usually large, flat, and ellipsoidal in shape, contain nucleoli, and 
 lie with their long axes coincident in direction with the axis of the 
 fibres. They are irregularly scattered along the fibre, and a small 
 amount of granular matter is usually seen in their immediate vicinity. 
 
 3. The sarcolemma, a delicate, structureless, membranous sheath, 
 is so thin, and so closely encloses the contractile substance and nu- 
 clei, that we cannot usually see it, unless we separate it by artificial 
 
$6 NORMAL HISTOLOGY. 
 
 means from the underlying structures. Where the muscular fibres 
 join tendons, the sarcolemma ends in the form of a pointed or rounded 
 blind sac, to which the tendon-fibres are attached. 
 
 The muscular fibres lie closely packed together, their ends lapping 
 over onto adjacent fibres, and forming bundles which are enclosed in 
 sheaths of connective tissue. Such bundles are again grouped to form 
 larger bundles, and thus the larger and smaller muscular bellies and 
 bands are formed. Arteries enter the muscular bundles and break 
 up into capillaries, which run along the fibres forming a long and nar- 
 row-meshed net. Motor nerves also pass into the muscles, divide and 
 subdivide, and terminate at the surface of the individual fibres, in 
 structures called motor end plates. 
 
 PRACTICAL STUDY. 
 
 Fresh Muscle. — From the fresh muscle we can derive a tolerably 
 clear idea of most of the above described structural features. 
 
 From any muscle of the trunk or leg of a rabbit — which should 
 have been dead long enough to allow of the occurrence of the rigor 
 mortis, since the contraction of the fibres which occurs in perfectly 
 fresh muscle obscures the pictures — a small bit is circumscribed with 
 a sharp scalpel, and a fragment dissected off with as little stretching as 
 possible. This is carried at once to a drop of one-half per cent, solu- 
 tion of salt, on a slide, and here carefully teased, the fibres being 
 pulled apart from the ends as much as possible. It is then covered, 
 a bit of paper or hair being laid beside the specimen to prevent pres- 
 sure from the cover-glass. In such a preparation the transverse bands 
 and indistinct longitudinal striations will be seen, and here and there, 
 where the needles have pressed on the fibres, the contractile substance 
 will be seen to have been broken across and the broken ends to have 
 retracted within the sarcolemma, leaving the latter as a clear and 
 sometimes folded membrane stretching across the interval. At the 
 cut ends of the fibres, the contractile substance will often be seen 
 swelling and extruding from the sarcolemma, in the form of an ob- 
 scurely striated fungiform mass. Here and there nuclei are seen ; but 
 they may be brought much more clearly into view, by allowing a drop 
 of two per cent, acetic acid to flow under the cover-glass ; then the 
 contractile substance swells and becomes transparent, the striations 
 becoming indistinct as the somewhat shrunken nuclei become more 
 clearly defined. 
 
 Sections of Hardened Muscle. — The details of the structure of 
 muscular fibres, as well as their grouping and relation to the connec- 
 tive tissue, may be well studied in sections from hardened muscle. 
 For this purpose the tongue of some animal — such as the dog— is well 
 suited, since here we have short muscular fibres running in various 
 directions and attached to tendons, and we see in a single transverse 
 section of the organ, at once, longitudinal and transverse sections of 
 the fibres. The tongue of a recently killed animal is cut transversely 
 across the middle, and put, for ten days, in a two per cent, solution of 
 bichromate of potassium ; then soaked in water for an hour, and trans- 
 
MUSCULAR TISSUE. 57 
 
 ferred to alcohol, in which it lies till sufficiently hard. Thin transverse 
 sections are now made, including a whole or a portion of the organ, 
 and stained with hematoxylin and mounted in glycerine. The trans- 
 verse bands, with their accompanying lines, are best seen in unstained 
 sections, mounted in water, since the coloring somewhat obscures the 
 finer structure, and glycerine, owing to its high refractive power, 
 renders them less distinct. To preserve the unstained specimens 
 permanently, however, glycerine must be used. 
 
 In muscular tissue, hardened as above in bichromate of potassium, 
 the primitive fibrillar are loosened from each other, and in some parts of 
 the specimen are usually separated by the manipulation of the section. 
 
 Blood-vessels are seen in the above preparations, but they may be 
 much better demonstrated in sections of muscular tissue whose vessels 
 have been injected with some colored material, such as the blue gela- 
 tine. 
 
 i. b. — Involuntary Striated or Heart Muscle. 
 
 In mammalia, the heart-muscle differs in several important struc- 
 tural features from the voluntary muscle. 
 
 The contractile substance has essentially the same structure as the 
 latter, but, instead of being arranged in the form of elongated, un- 
 branched cylinders or fibres, without distinct cell-structure, in the 
 heart-muscle the fibres send off at frequent intervals short, narrow 
 processes, which join neighboring fibres, forming a narrow and long 
 meshed net. Further, the fibres which, owing to the numerous anas- 
 tomoses, are very irregular in form, are made up of distinct segments 
 or cells, each segment being cemented at the ends to its neighbors, 
 and furnished with a flat, elongated, ovoidal, or often rectangular nu- 
 cleus. In the vicinity of the nuclei we usually see a certain amount 
 of granular material or pigment particles. The nuclei, instead of 
 lying, as in the voluntary muscles, at the surface of the contractile sub- 
 stance, are embedded within it. We are unable, in heart-muscle, to 
 demonstrate a sarcolemma. The fibres are grouped in bundles, which 
 are enclosed in connective tissue, and supplied with blood-vessels and 
 nerves. 
 
 PRACTICAL STUDY. 
 
 Sections of Heart-Muscle. — A bit of the heart of man, or any 
 mammal, should be hardened first in Miiller's fluid, and then in alcohol. 
 Longitudinal and transverse sections are made, stained with haema- 
 toxylin, and mounted in glycerine. 
 
 Teased Heart-Muscle. — To demonstrate the segments or cells of 
 which the heart-fibres are composed, a moderately thin bit of fresh 
 heart-muscle is put for twenty-four hours into a dilute solution of 
 chromic acid (i to 5,000), then carefully washed with pure water and 
 stained with picro-carmine. It is then teased on a slide and mounted 
 in glycerine, to which a little formic acid has been added (1 to 100). 
 The formic acid renders the tissue more transparent, and brings more 
 clearly into view the narrow, jagged lines of strongly refractive sub- 
 stance which indicate the boundaries of the muscle-cells. 
 
CHAPTER VII. 
 
 NERVE-TISSUE. 
 
 The primary structural element in nerve-tissue is the nerve-cell. 
 Nerve-cells have the most diverse forms, and always possess one or 
 more branching or unbranched processes. In certain cells the un- 
 branched processes are extremely long, become associated with other 
 tissue elements, and constitute the nerve-fibres. Both nerve-cells 
 and their processes, nerve-fibres, are enclosed and supported by 
 peculiarly arranged connective tissue, and supplied with blood and 
 lymphatic vessels. The nerve-fibres form, for the most part, the 
 white matter of the nervous centres and the peripheral nerves, while 
 the cells enter largely into the composition of the gray matter. In 
 studying nerve-tissue, we have then to consider : 
 
 I. Nerve-fibres^ and the supporting connective-tissue structures, 
 with their accessories. 
 
 II. Nerve-cells. 
 
 I. Nerve-fibres, etc. 
 
 These are of two kinds : a, medullated, and b. non-medullated. 
 This distinction corresponds with the physiological and anatomical 
 classification of nerve-tissues into those of the cerebro-spinal and the 
 
 sympathetic systems : the medullated belonging to the former, the 
 non-medullated to the latter. 
 
 a. Medullated Nerve-fibres. — If we disregard, for the moment, the 
 structure of these nerves at their point of origin in the nerve-centres, 
 and at their termination in the periphery, and study their structure as 
 it is seen in the continuity of any of the larger or smaller nerves, we 
 hnd that the individual fibres present three distinct structural elements : 
 i, the axis cyli?ider ; 2, the medullary sheath ; 3, the ?ieurilemma. 
 
 1. Running through the axis of the fibre is a cylindrical, with high 
 powers, delicately longitudinally striated structure — the axis cylinder. 
 This is believed to be the essential nerve-element of the fibre — the 
 process of the nerve-cell, from which it passes without break of 
 continuity to the periphery ; and it is probable that the longitudinal 
 striations are the expression of its composition from still finer primi- 
 tive fibrils, which, as we shall see when we study the nerve-cells, seem 
 to be continued on into the cell-body itself within the nerve-centres. 
 
 2. Closely surrounding the axis cylinder is a tube or sheath of 
 varying thickness — the medullary sheath — composed of a white, semi- 
 fluid, translucent, strongly refractive substance, called myeline, which 
 
NERVE-TISSUE. 59 
 
 undergoes rapid changes after death, or on removal from the animal, 
 and swells and assumes a multitude of bizarre forms on addition of 
 water ; it is soluble in alcohol, chloroform and ether ; and, like fat, is 
 hardened and turned black under the action of osmic acid. The 
 medullary sheath does not form a continuous tube, but at tolerably 
 regular intervals is separated into segments. 
 
 3. The neurilemma or sheath of Schwann is an extremely thin, 
 structureless, membranous tube, which tightly encloses the medullary 
 sheath, and, like the latter, is broken up into segments. At the ends 
 of the segments is a constriction around the fibre, at the expense of 
 the medullary sheath, and the ends of the neurilemma segments are 
 joined together by a thin layer of cement substance, which extends 
 inward to the axis cylinder. There is reason to believe that the neu- 
 rilemma extends inward and between the medullary sheath and the 
 axis cylinder, entirely enclosing the segments of the medullary sheath. 
 
 Within each neurilemma segment, called interannular segment, 
 and about midway between the constrictions, lies a flattened, elon- 
 gated, elliptical nucleus. We may regard the neurilemma segments, 
 with their nuclei, as cylindrical cells, cemented together, end to end, 
 enclosing the segments of the medullary sheath, and surrounding the 
 axis cylinder, the latter passing uninterruptedly through the axis of 
 the segments. In addition to these structural features, we find, on 
 examining with high powers, irregularly scattered along each inter- 
 annular segment, delicate oblique lines or fissures, called the incisures 
 of Schmidt, which seem to pass from the neurilemma at the surface 
 inward to the axis cylinder, obliquely through the medullary sheath. 
 Their significance is not, as yet, definitely determined. Medullated 
 nerve-fibres vary greatly in diameter. 
 
 Connective Tissue of the Nerves. — The nerve-fibres are bound to- 
 gether by connective tissue, to form larger and smaller nerve-fascicles, 
 which, singly or in bundles, we usually call simply nerves. If we fol- 
 low the nerves outward toward their peripheral terminations, we find 
 that they divide and subdivide, becoming, as they do so, smaller and 
 smaller, until we finally come to nerves which consist of a single 
 fibre. These single nerve-fibres do not lie free in the tissues, but are 
 enclosed in a distinct sheath, called Henle's sheath, which is a tube 
 formed of a single layer of endothelial cells, placed edge to edge, and 
 cemented together. Between the sheath and the nerve-fibre is a nar- 
 row space, which, under normal conditions, is filled with lymph. 
 
 Having become acquainted with this simple structure of the single 
 terminal nerves, let us follow them backward. We find, as we do so, 
 that, as they become larger by the junction of several fibres, a small 
 amount of fibrillar connective tissue appears between the fibres within 
 Henle's sheath, and that the latter becomes attached to the surround- 
 ing structures by a layer of connective tissue. Finally, when we ar- 
 rive at the larger nerve-trunks, we find that in each the connective 
 tissue presents itself in three ways : 
 
 1. It forms a distinct sheath, which, although the analogue of 
 Henle's sheath, has a much more complicated structure, and is called 
 the lamellar sheath. This is composed of several concentric lamellae, 
 
CO NORMAL HISTOLOGY. 
 
 each of which is formed of a fenestrated membrane of fibrillar connec- 
 tive tissue containing granules of elastic tissue substance, and covered 
 with endothelial cells. The lamellae are connected by oblique fibres, 
 which pass from one to the other, binding them more or less firmly 
 together. The whole forms a compact sheath, closely investing the 
 fascicle of nerve-fibres. 
 
 2. Outside the lamellar sheath, and joining it to adjacent struc- 
 tures — with neighboring fascicles, if the nerve-trunk is composed of 
 several of these, as is the case in many large nerves — we find loose 
 fibrillar connective tissue with flattened, irregular-shaped cells — like 
 those found in the loose subcutaneous connective tissue — reinforced 
 by elastic fibres, and often containing fat-cells. The fibrillated and 
 elastic fibres, especially in the immediate vicinity of the lamellar 
 sheath, usually run in a direction approximately parallel with the axis 
 of the nerve. This tissue is called the perifascicular connective 
 tissue. 
 
 3. We find within the lamellar sheath and between the nerve-fibres 
 composing the fascicle, in the first place, prolongations inward of the 
 membranous tissue composing the lamellar sheath ; and second, fine 
 fibrillated fibres and flattened cells which lie in the interstices between 
 the nerves and fibres. This tissue is called the inlrafascictUar con- 
 nective tissue. Blood-vessels penetrate the lamellar sheath of the 
 medium-sized and larger nerves, and a very long meshed and abundant 
 capillary net-work is formed in the intrafascicular connective tissue. 
 Lymphatic channels and spaces are also abundant within the nerves, 
 so that the fibres are bathed in nutritive fluids. 
 
 Termination of Medullated Nerve -fibres. 
 
 1. In the .Nerve-centres. — We find in the nerve-centres nerves which 
 have no neurilemma, and others in which both neurilemma and med- 
 ullary sheath fail — the so-called naked axis cylinders ; we find, further, 
 extremely delicate filiform structures which are believed to be the 
 primitive nerve-fibrils. 
 
 The axis cylinders of the nerves being, as above stated, processes 
 of nerve-cells, the nerve-fibres, as we trace them back into the centres, 
 must sooner or later join cells. Their exact mode of connection with 
 the cells is not in all cases sufficiently well understood ; but it is be- 
 lieved that they either join the cells in the form of naked axis cylin- 
 ders — as we shall see when we study the nerve-cells — or, in other cases, 
 that the axis cylinders break up into their constituent primitive fibrils 
 before entering the cells. 
 
 2. In the Periphery. — The peripheral termination of nerves is a 
 subject which presents extreme difficulties to the histologist, and with 
 few exceptions the exact way in which this occurs is unknown ; and, 
 since a consideration of the subject in detail does not lie within the 
 scope of these lessons, we shall here simply describe one of the best 
 understood modes of termination — that in voluntary muscular fibre. 
 Some of the other modes will be briefly described as we study, in sub- 
 sequent lessons, the parts and organs in which they occur. 
 
NERVE-TISSUE. 6 1 
 
 Termination of Nerves in Voluntary Muscle. — If we trace a nerve as 
 it passes into a muscular bundle, we find that it divides and subdivides, 
 and finally we find a single fibre within its Henle's sheath suddenly 
 piercing the sarcolemma at the surface of a muscular fibre and going 
 out of sight. 
 
 By various modes of preparation we learn that Henle's sheath be- 
 comes continuous with the sarcolemma ; the medullary sheath disap- 
 pears, and the axis cylinder, and possibly the neurilemma, pass beneath 
 the sarcolemma. Here the axis cylinder usually divides into several 
 irregular, often knobbed branches, which lie and apparently terminate in 
 a flattened, irregular-shaped mass of granular substance, which is fur- 
 nished with variously shaped nuclei. This structure is called the 
 motor end plate. It is probable that each muscular fibre is furnished 
 with at least one of these terminal apparatuses. The above descrip- 
 tion applies to the mammalia. In certain of the lower animals, as the 
 frog, the termination is somewhat different. 
 
 b. Non-medullated Nerve-fibres. — These are also called fibres of 
 Remak. Unlike the nerve-fibres which we have just been studying, 
 they possess no medullary sheath and no neurilemma. They are 
 simply grayish translucent cords of varying diameter, are indistinctly 
 longitudinally striated, and are intimately connected with one another 
 by frequent inosculations. The fibres seem to divide and send off 
 oblique branches to join neighboring fibres. Flattened, elongated 
 nuclei lie at frequent intervals upon the surface of the fibres. These 
 fibres considerably resemble, in their general appearance, the fibril- 
 lated fibres of ordinary connective tissue, but careful examination 
 shows them to be entirely distinct structures. 
 
 They are grouped in bundles to form nerves, sometimes alone, but 
 very frequently in connection with medullated nerve-fibres. Thus, in 
 the pneumogastric, we find a considerable part of the fibres to be non- 
 medullated and intimately bound in by the intrafascicular connective 
 tissue, with medullated fibres. The non-medullated fibres originate in 
 nerve-cells of a peculiar structure, to be presently described : but of 
 their peripheral terminations we know almost nothing. 
 
 II. Nerve-cells. 
 
 Nerve-cells, or ganglion-cells, as they are frequently called, al- 
 though presenting the greatest diversity in form, have yet some quite 
 distinctive characters in common. The cell-body is finely granular 
 and delicately striated, often containing pigment-granules. The 
 nucleus is large, well-defined, vesicular in appearance, and usually 
 contains a large shining nucleolus. They all have at least one pro- 
 cess, most of them have more ; and they are hence often classified as 
 unipolar, bipolar, or multipolar ganglion-cells. The above-mentioned 
 striations in the cell-body are often seen to continue out into the pro- 
 cesses. In many nerve-cells, especially in the spinal cord, we recog- 
 nize two distinct kinds of processes : first, those which, soon after 
 leaving the cell, divide and subdivide until they become extremely 
 fine and delicate, and, in some cases, seem to join equally fine pro- 
 
62 NORMAL HISTOLOGY. 
 
 cesses of other cells — such delicate cell-processes make up a consider- 
 able portion of the gray matter of the cord, and are called branch- 
 ing processes ; second, such as pass off from the cell, and, without 
 dividing, presently are surrounded by a sheath of myeline, and become 
 medullated nerve-fibres ; the latter are called axis-cylinder processes. 
 Nerve-cells vary greatly in size, and, although the forms which they 
 present are most diverse, we yet find that a considerable proportion 
 of those found in different parts of the nerve-centres have certain 
 broadly typical forms. Thus, among the cells in the gray matter of 
 the spinal cord, we find larger and smaller fusiform or spheroidal 
 branching cells, or. which are more characteristic, large, irregular- 
 shaped cells, with several branching processes and a well-defined axis- 
 cylinder process. In the cortex of the cerebrum, while we find vari- 
 ously shaped larger and smaller cells, we find also characteristic pyra- 
 midal cells of varying size, which give off processes from both the 
 base and apex. 
 
 In the cerebellum, we find just at the inner edge of the gray corti- 
 cal matter, irregular globular or ovoidal cells, which from the side to- 
 ward the surface of the brain, send off one or two branching processes ; 
 on the opposite side we can usually demonstrate the commencement 
 of a single delicate process, which is supposed to correspond to the 
 axis-cylinder process, though since it almost invariably breaks off near 
 the cell in the attempt to isolate the latter, its nature is not yet defi- 
 nitely determined. These cells are called Purkinje's cells. 
 
 The ganglion-cells of the sympathetic are usually globular or ovoidal, 
 and are peculiar in that each cell is surrounded by a distinct capsule 
 of connective tissue, lined with flattened cells, resembling endothe- 
 lium. 
 
 They have usually but one process — sometimes two — which pierces 
 the capsule and becomes a non-medullated nerve-fibre. 
 
 PRACTICAL STUDY. 
 
 Fresh Nerve. — A bit of fresh nerve — the sciatic of the frog answers 
 very well— should be carefully and rapidly teased apart longitudinally, in 
 one-half per cent, salt solution — care being taken to pull apart the fibres 
 from the ends, so as to break them as little as possible — and covered ; 
 pressure from the cover-glass being avoided by placing a bit of paper or 
 hair beside the specimen. The nerve-fibres present if examined at once, 
 in many parts, a sharp and regular double contour, which is their nor- 
 mal appearance, and along their course the constrictions and nuclei 
 may here and there be seen. The axis cylinder and neurilemma are 
 for 'the most part invisible, the former owing to the lack of transpar- 
 ency in the medullary sheath, the latter because of its extreme thin- 
 ness and close contact with the medullary sheath. Very soon, at once 
 in some parts of the specimen, the contours of the fibres will be seen 
 to become irregular, the myeline shrinking away at some parts from 
 the neurilemma and swelling out at others. At the severed ends of 
 the fibres the myeline will be seen welling out from the neurilemma, 
 and breaking off into the fluid in irregular globular or contorted masses. 
 
NERVE-TISSUE. 6$ 
 
 After the swelling and irregular breaking up of the myeline has oc- 
 curred — this may be hastened by allowing water to run under the cover- 
 glass — the neurilemma may be seen here and there stretching across 
 between the varicosities formed by the swollen myeline, and either at 
 the broken ends of the fibres or along their course the axis cylinder 
 may occasionally be seen. 
 
 Demonstration of the Axis Cylinder by Collodion. — In order to ob- 
 tain a clear view of the axis cylinder, we should tease a fresh nerve, 
 without the addition of any fluid, and when the natural fluid of the tis- 
 sue has partially evaporated — care being taken not to allow it to be- 
 come quite dry — a drop of collodion should be added and the speci- 
 men quickly covered and examined at once. 
 
 The collodion dissolves, or renders the myeline transparent, and 
 the axis cylinder is distinctly seen within the neurilemma, either in the 
 axis of the fibre or along one side. 
 
 Nerve-fibres Treated with Osmic Acid. — The most complete dem- 
 onstration of the nerve-fibre in its entirety may be obtained by treat- 
 ment with osmic acid. This agent fixes the myeline and other con- 
 stituents of the fibre nearly in their normal form, staining the myeline 
 black. In applying this agent it is necessary to maintain the nerve in 
 a state of gentle tension, because it is otherwise somewhat contracted 
 and distorted. This is done by gently stretching the nerve — the sci- 
 atic of the rabbit will answer — along a bit of wood which has been 
 whittled away at one side, so that the nerve may lie free. It is fas- 
 tened by the ends to the wood by threads. The nerve thus prepared 
 is immersed for twenty-four hours in an aqueous solution of osmic acid 
 (i to ioo), then washed, and a small bit carefully teased apart longi- 
 tudinally in glycerine. In such a preparation nearly all the struc- 
 tures in the fibre can be readily seen : the constrictions and nuclei, the 
 medullary sheath stained black, the incisures of Schmidt, and where, 
 as will almost always occur in some parts of the specimen, the medul- 
 lary sheath has been broken across or the segments pulled asunder, or 
 the myeline has contracted at the constrictions, the neurilemma, and 
 axis-cylinder. Not infrequently, if the teasing has not been very care- 
 fully done, the segments of the medullary sheath are broken across in 
 many places and separated, giving the fibre a beaded appearance. 
 
 Transverse sections of nerves stained with osmic acid give instruc- 
 tive pictures. A nerve treated as above with osmic acid is not firm 
 enough to permit the making of thin sections, and should be hard- 
 ened by what is known as the gum embedding method, as follows : a 
 bit of the nerve, after the osmic acid treatment, is immersed for twen- 
 ty-four hours in alcohol, washed, and immersed for twenty-four hours 
 in a syrupy solution of gum-arabic, then put again for twenty-four 
 hours in alcohol. By this process the gum is deposited in solid form 
 in the interstices of the fibres, giving it sufficient consistence for cutting. 
 The sections, either stained with picro-carmine or unstained, are 
 mounted in glycerine. In such a preparation the uncolored axis 
 cylinder is seen surrounded by a black ring, the medullary sheath, and 
 here and there the nuclei of the neurilemma are seen at the edge of the 
 fibres. The connective tissue surrounding the fibres has a grayish 
 
64 NORMAL HISTOLOGY. 
 
 color. The fibres will be seen to have varying diameters and to pre- 
 sent marked differences in form, some of these depending upon artifi- 
 cial changes, others upon the difference in level at which the fibres have 
 been cut across. 
 
 Transverse Sectio?is of Nerves Preserved in Chromic Acid. — Bits 
 of the sciatic nerve from the rabbit or any other mammal should be 
 lightly stretched along a bit of wood and placed in a solution of chro- 
 mic acid (i to 500); in two weeks it is washed and transferred to alco- 
 hol. It is now embedded in gum (see above), and thin transverse sec- 
 tions made and stained with hematoxylin and eosin and mounted in 
 balsam. In such preparations the general relations of connective 
 tissue to the nerve-fibres is well seen. 
 
 Nerve-fibres Treated with Nitrate of Silver. — By this method we 
 obtain hints concerning the structure of nerves which are of no little 
 significance from a physiological point of view. A fresh nerve is 
 slightly teased apart on a slide, a large drop of a one-half per cent, 
 solution of nitrate of silver added and allowed to remain for four min- 
 utes ; this is washed off with one-half per cent, salt solution, the speci- 
 men transferred to a drop of glycerine on another slide, the fibres 
 carefully teased apart, and covered. The preparation is now exposed 
 to sunlight or diffuse daylight until it becomes brown. If now exam- 
 ined, at tolerably regular intervals along the fibre tiny brown or black 
 crosses, called Ranviefs crosses , will be seen, the transverse arm of the 
 cross being the stained cement substance between the neurilemma seg- 
 ments at the constrictions ; the longitudinal arm, which coincides with 
 the axis of the fibre and which is longer or shorter, depending upon 
 the length of time to which the fibre was exposed to the action of the 
 silver, is the axis cylinder. If a specimen be allowed to remain longer 
 than the above time in contact with the silver, the longitudinal arm of 
 the cross will be longer ; and if the contact be continued for a suffi- 
 cient time, the axis cylinder will be stained black along the whole 
 length of the fibre. It is to be observed that the axis cylinder is first 
 stained at that part which passes through the constrictions, and not 
 along the segments. Certain other soluble substances which stain the 
 axis cylinder comport themselves in the same way. We infer from 
 this that at the constrictions certain substances in solution can pass 
 into the fibre and come in contact with the axis cylinder or the nerve- 
 element proper of the fibre. This inference is significant in connec- 
 tion with the nutrition of the nerves, since we are justified in assuming 
 that nutritive substances in solution may pass also to the axis cylinder 
 in the same way. 
 
 It is not improbable that the constrictions serve yet another im- 
 portant purpose. The myeline of the medullary sheath being a semi- 
 fluid substance — perhaps serving either to isolate the axis cylinder or 
 protect it from external violence — it would inevitably tend to gravitate 
 to the lower parts of the nerves, were it not that it is held in position 
 by being enclosed, so to say, in cylindrical cases, i. e., the neurilemma- 
 cells, between the constrictions. 
 
 Naked Axis Cylinders in the Spinal Cord. — To demonstrate these, 
 a perfectly fresh bit of the white substance of the spinal cord is put for 
 
NERVE-TISSUE. 65 
 
 twenty-four hours in a one per cent, solution of osmic acid, and teased 
 in glycerine. In addition to the naked axis cylinders and medullated 
 fibres, free globules of myeline and fragments of connective tissue and 
 blood-vessels will be seen. 
 
 The non-medullated nerve-fibres may be demonstrated in connection 
 with the sympathetic ganglion- cells. — See below. 
 
 1. Nerve-cells : a. Spinal Cord. — Small bits of the gray matter from 
 the spinal cord of man, or from the ox or sheep, should be put for ten 
 days in a dilute solution of chromic acid (1 to 500), and then carefully 
 shaken in a test-tube with water colored with carmine ; when the bits 
 have become thoroughly broken up into small particles, the tube is 
 allowed to stand for a day or two, until the particles which have set- 
 tled to the bottom are sufficiently stained. The supernatant fluid is 
 then decanted, and with a glass tube a small drop of the disintegrated 
 tissue is conveyed to a drop of glycerine on a slide and covered, pres- 
 sure on the cells being avoided in the usual way. If the first prepara- 
 tion does not contain the required cells, others should be made. In 
 this way, if the shaking be carefully done, the ganglion-cells are freed 
 to a considerable degree from the surrounding parts, and such may be 
 found as present numerous long, branching processes as well as the 
 axis-cylinder process. In addition to the cells, such specimens pre- 
 sent fragments of connective tissue, bits of naked axis cylinders, and 
 medullated nerve-fibres, myeline-droplets, etc. 
 
 b. Brain. — In the way above described, cells should be prepared 
 from the gray cortical portion of the cerebrum and cerebellum. 
 
 c. Sympathetic. — For the demonstration of these cells and the 
 fibres connected with them, the frog answers very well. The animal 
 having been killed by breaking up the medulla, the abdominal cavity 
 is opened and the intestines and liver carefully removed ; the aorta 
 will then be seen lying along the vertebral column. The sympathetic 
 ganglia and nerves lie along the walls of the aorta and in the tissue 
 surrounding the origin of the spinal nerves. The head and fore-legs 
 should now be cut off close behind the latter, and the hind-legs sev- 
 ered close to the body ; the trunk is then laid in a small dish and cov- 
 ered with equal parts of one per cent, solution of osmic acid, alcohol, 
 and water. The dish is covered and set aside for twenty-four hours, 
 when the aorta, together with the tissue surrounding the commence- 
 ment of the spinal nerves, is dissected off in a single piece, spread on 
 a slide, and examined with a low power. Groups of sympathetic 
 nerve-ceils are seen here and there in the specimen, and are readily 
 distinguished by the orange color of the cells ; one or two of them are 
 to be isolated and freed as much as possible from the enclosing tissue, 
 carefully teased apart on a slide, stained lightly with hematoxylin and 
 then with eosin, and mounted in glycerine. Successful preparations 
 will show not only the nerve-cells and fibres, but also the connection 
 of the two within the capsule. 
 
 5 
 
CHAPTER VIII. 
 
 BLOOD-VESSELS— LYMPHATIC VESSELS. 
 
 Blood-vessels. 
 
 Blood-vessels are of three kinds : arteries, veins, and capillaries. 
 Although merging without sharp demarcation into one another, these 
 vessels, in their typical forms, present sufficient differences in struc- 
 ture to justify their separate consideration. The capillaries being the 
 simplest, it will be convenient to commence with them. If we exam- 
 ine a capillary vessel, either fresh or after it has been in preservative 
 fluids, it presents the appearance of a narrow tube, with very thin, 
 homogeneous walls, in which, at frequent intervals, elongated nuclei 
 are embedded, their long axis being parallel with the axis of the tube. 
 W, however, we inject the vessels with a dilute solution of nitrate of 
 silver and expose them to the light, we find that the inside of the tube 
 is divided by narrow black lines into elongated, irregular-shaped 
 spaces ; and if we then stain the specimen with hematoxylin, we find 
 that a nucleus lies in each space. The walls of the capillaries are, 
 then, not formed by a homogeneous membrane, but made up of cells 
 having the character of endothelium. The capillaries are endothelial 
 tubes. This layer of endothelial cells, which alone forms the walls of 
 the capillaries, is found lining all the other blood channels, arteries, 
 and veins, as well as the heart. In the blood-vessels it is called the 
 intima. If, now, we follow the capillaries in a direction toward the 
 arteries, we find that the connective tissue in which they lie is arranged 
 in the form of a thin layer along their walls. This layer, which is 
 also present in all arteries and veins, is called the adventitia. 
 
 Almost as soon as we find the adventitia, we notice another layer 
 between it and the intima, formed of a single row of smooth muscle- 
 cells wound transversely or obliquely around the vessel. This layer 
 is called the media or musculosa, and a vessel having these three sim- 
 ple layers in its walls is called an arteriole. In these three layers, 
 intima, media, and adventitia, we have the types of all the layers which 
 occur in the walls of the largest and most complicated blood-vessels. 
 The individual layers become, indeed, more complex in structure ; 
 but, with the exception of elastic elements, no new tissues appear. 
 Turning, now, to a larger artery — one of the digitals, for example — and 
 examining the various layers in its walls, we find that the intima is no 
 longer formed of a simple endothelial tube, but that outside of this a 
 new r layer has appeared, composed of ill-defined fibrillated and of 
 elastic fibres, among which lie large, flattened, branching cells. This 
 
BLOOD-VESSELS — LYMPHATIC VESSELS. 6 1 / 
 
 layer is called the intermediary layer of the intima, and is sharply sep- 
 arated from the media by a fenestrated, elastic membrane, called the 
 membrana elastica intimce. 
 
 The media presents here quite a thick layer of smooth muscle-cells, 
 passing transversely around the vessel, and among these we find a few 
 elastic fibres which are connected with the elastic elements in the 
 intima and adventitia. The adventitia is thicker, and consists chiefly 
 of fibrillar connective tissue with elastic fibres. In the larger arteries, 
 such as the carotids, aorta, etc., we find that the individual layers are 
 considerably less sharply defined ; in the media the elastic tissue is 
 very abundant, taking the place, to a considerable extent, of the mus- 
 cular elements ; it is arranged in irregularly distributed lamellae and 
 fibres, between which lie fibrillated fibres and connective tissue and 
 smooth muscle-cells, the latter no longer uniformly lying transversely 
 to the axis of the vessel. In the adventitia, also, of the large vessels, 
 the elastic elements are very numerous, being most abundant in the 
 vicinity of the media. In the adventitia of some of the iarge vessels, 
 smooth muscle-cells occur, arranged usually with their long axes 
 parallel with the axis of the vessel. 
 
 The walls of the veins, like those of the arteries, consist of three 
 layers, and these layers have in general an analogous structure ; but they 
 are neither as distinct, nor are their structural features as constant, as 
 those of the arteries. Moreover, we find that veins of the same cali- 
 bre present, in different parts of the body, marked differences in struc- 
 ture, and. unlike the arteries, the thickness of their walls is not uni- 
 formly proportional to the calibre of the vessel. 
 
 In general we may express the structural difference between veins 
 and arteries by saying that in the walls of the former the elastic and 
 muscular elements are much less, while the fibrillar connective-tissue 
 elements are more abundant than in the walls of the latter. Now, 
 since the muscular and elastic elements are the chief constituents of 
 the media and the fibrillar connective-tissue elements of the adven- 
 titia, we find, in general, that in the veins the media is less, while the 
 adventitia is more developed. The intermediary layer of the intima is 
 entirely absent in small veins; in many of medium size it appears, and 
 is again absent in the largest vessels. In some veins, such as those of 
 the bone, central nervous system, retina, etc., the muscular elements 
 are almost or entirely absent. In certain other veins, on the contrary, 
 such as the v. portarum, v. renalis, the adventitia contains a great 
 abundance of muscular elements arranged parallel with the axis of the 
 vessel. 
 
 The valves of the veins consist of bundles of fibrillar connective 
 tissue arranged to form a membranous projection from the walls of 
 the vessels — the bundles being arranged, in general, m a direction 
 parallel with the free edge of the valve. They contain also a net-work 
 of elastic fibres, which, at that surface of the valve which is exposed to 
 the blood-current, form a dense layer like the intermediary layer of 
 the vein-wall itself. The whole free surface is covered with endo- 
 thelium like that lining the general surface of the vessel. 
 
NORMAL HISTOLOGY. 
 
 Endocardium and Valves of the Heart. 
 
 The endocardium, which differs somewhat in thickness and structure 
 in different parts of the heart, consists, in general, of a membranous 
 expansion of fibrillar connective tissue, with elastic fibres and smooth 
 muscle-elements, which lines the cavities of the heart and is covered 
 on its free surface with a layer of endothelial cells. If we examine 
 sections made perpendicular to the surface of the endocardium, we 
 find that just beneath the endothelium is a layer of fibrillar connective 
 tissue, with flattened cells, reinforced by a net-work of elastic fibres, 
 which become coarser and more abundant in that portion of the layer 
 lying farthest from the heart-cavities. In this layer are smooth muscle- 
 cells running in various directions. Outside of this layer, and joining 
 it to the muscular tissue proper of the heart, is a layer of loose fibril- 
 lar connective tissue, in which the blood, lymphatic vessels and nerves 
 lie embedded. The valves of the heart consist of fibrillar connective 
 tissue arranged in membranes or fascicles and associated with elastic 
 tissue, the latter being most abundant at the free surfaces of the valves ; 
 and here it is present in the greatest quantity and density on the sur- 
 face which is most directly exposed to the current of blood — that is, 
 on the auricular surfaces of the tricuspid and mitral, and on the ven- 
 tricular surfaces of the aortic and pulmonary valves. The elastic ele- 
 ments are also more abundant in the valves of the left than of the 
 right side of the heart. The firmness and capacity for resistance of 
 the elastic tissue being borne in mind, the significance of its distribu- 
 tion in the valves will be readily perceived : where they are the most 
 exposed to the impact and pressure of the blood, there they are the 
 most firm and dense. 
 
 Lymphatic Vessels. 
 
 The larger lymphatic vessels have a structure quite analogous with 
 that of the veins, and like the latter they are supplied with valves. 
 They approach the arterial type, however, in that the muscular fibres 
 are quite abundant in proportion to the thickness of the walls. Fol- 
 lowing the larger lymphatics toward the periphery, we find that they 
 pass over into irregular branching and pouching channels, the walls of 
 which consist of a single layer of endothelial cells, whose edges are 
 very sinuous, dovetailing into one another like the pieces of a child's 
 puzzle-map. These channels are called lymphatic capillaries. Be- 
 tween these and certain spaces or lacunae in the tissues — those, for 
 example, in which the connective-tissue cells lie (see page 26) — there 
 is a communication, whose exact nature we do not as yet know, by 
 means of which fluids, and probably formed elements such as blood- 
 cells, pass over from the blood into the lymphatic vessels. 
 
 practical study. 
 
 Capillaries . — The general appearance of the capillaries, as well as 
 of the smaller arteries and veins, is best seen in those parts in which 
 
BLOOD-VESSELS— LYMPHATIC VESSELS. 69 
 
 the vessels are surrounded by but little tissue, as in thin membranes 
 such as the mesentery or pia mater. A small bit from that portion of 
 the pia which dips into the sulci on the surface of the hemispheres 
 should be carefully separated from the brain substance, stretched on 
 a bit of cork and fastened with pins, and laid for twenty-four hours in 
 Miiller's fluid, then for twenty-four hours in alcohol; it is then stained 
 with picro-carmine and mounted in glycerine. In such a preparation, 
 although we see the elongated nuclei in the capillary wall, we cannot, 
 as a rule, make out the outlines of the cells. To accomplish this we 
 have recourse to the use of dilute solutions of nitrate of silver. This 
 can be applied by immersing some thin membrane, such as the mesen- 
 tery, for an hour in a solution of nitrate of silver (1-500), brushing 
 off the endothelium from the surface, and exposing the specimen in 
 water to the light until it becomes brown. Or, what is better, the 
 entire vascular system of a small animal, such as a frog, may be first 
 rinsed out with water through a canula introduced into the aorta and 
 attached to a syringe, and then injected with the above silver solution. 
 Thin membranes, such as the mesentery or bladder, are removed from 
 the animal, exposed to the light for a sufficient time, and then, either 
 stained or unstained, mounted in glycerine. 
 
 Arteries and Veins. — Very small vessels can be studied entire, 
 since by careful focussing we can bring one portion after another into 
 view, obtaining thus what are called optical sections. In the larger 
 vessels, however, this simple method is no longer practicable ; hence, 
 we have to resort to actual sections of the vessels. 
 
 Small arteries and veins may be prepared for cutting by simply 
 stretching them gently, when quite fresh, over a bit of wood with pins, 
 and allowing them to dry. The dried vessels are placed between 
 two bits of soft cork, and thin sections cut in any desired direction 
 with a sharp, hard-tempered, dry razor. The sections swell out on 
 being dropped into water ; they are stained with hsematoxylin and 
 mounted in glycerine. Larger vessels, such as the aorta, vena cava, 
 etc., may be simply hardened in alcohol embedded in hardened liver 
 and cut; then stained with haematoxylin and mounted in glycerine 
 or balsam. 
 
 Heart-valves a?id Endocardium. — These may be studied in thin 
 sections made perpendicular to the surface of the valves and the inner 
 surface of pieces of the heart which have been stretched on cork and 
 hardened in alcohol. 
 
 Lymphatic Vessels. — A general view of the relation between the 
 larger lymphatic trunks and the lymphatic capillaries is best obtained 
 by studying nitrate of silver preparations of some part in which the 
 latter are abundant. Such a structure we find in the tendinous por- 
 tion of the diaphragm — so-called central tendon — of the rabbit, which 
 is very thin, covered on both sides with endothelium and very richly 
 supplied with lymphatic vessels. 
 
 To prepare the central tendon, a rabbit should be killed by break- 
 ing up the medulla with a wire, and the thoracic and abdominal cavities 
 freely opened by incisions above and below the anterior line of attach- 
 ment of the diaphragm. Two ligatures are passed around the large 
 
70 NORMAL HISTOLOGY. 
 
 vessels above the diaphragm, to prevent the escape of blood on to the 
 part, the vessels cut across between them, and the heart and lungs 
 removed from the chest. The suspensory ligament of the liver is now 
 severed, and the diaphragm is left free, stretching across between the 
 thoracic and abdominal cavities. Its upper surface is brushed with a 
 soft pencil moistened with water, so as to remove the surface endothe- 
 lium and allow the silver to penetrate readily into the lymphatics. 
 The surface is now carefully washed and a one-half per cent, solution 
 of nitrate of silver is allowed to run over it and remain for five minutes 
 in contact with it. This is now washed off with water, the diaphragm 
 carefully cut out and pinned to a cork, so as to lie flat, and exposed 
 to the light in dilute alcohol till it becomes brown. Small pieces are 
 then cut out from the tendinous portion and mounted in glycerine. In 
 such a specimen the large lymphatic channels will be readily recog- 
 nized lined with elongated endothelium like that of the blood-vessels, 
 and furnished with valves, which appear here and there as curved 
 lines crossing the vessel. The capillaries may be recognized by their 
 irregular shape and sinuous endothelium. 
 
 In the basement substance which lies between the lymph-ves- 
 sels and is stained brown, variously shaped larger and smaller irregular 
 light spaces are seen, sometimes apparently communicating with one 
 another and with the lymphatic vessels. These irregular spaces in the 
 basement substance are variously interpreted ; some regarding them 
 as the ultimate radicles of the lymphatic vessels and terming them 
 serous canaliculi ; others thinking that they are simply spaces in the 
 connective tissue bearing the same relation to the lymph-vessels that 
 the connective-tissue spaces elsewhere in the body do. 
 
CHAPTER IX. 
 
 THE GASTRO-INTESTINAL CANAL. 
 
 This canal is a tube varying greatly in its calibre in different parts, 
 and continuous at either end with the external surface of the body. 
 In certain parts of its course it is intimately connected with adjacent 
 structures ; but, for the most part, it is attached only at one side by a 
 structure which serves to convey to it its blood and lymphatic vessels 
 and nerves. The walls of the tube, although varying in structure in 
 different sections, consists in general of a muscular layer, a mucous 
 layer lining the tube, and in those parts where it is suspended in the 
 abdominal cavity a serous layer covering it. Confining our attention 
 to the stomach and intestines, we find that these layers are not simple, 
 but have each a composite structure ; thus, we find in the serosa a 
 layer consisting chiefly of dense fibrillar connective tissue, sud-serosa, 
 covered with a layer of endothelium. The muscular tunic, or muscu- 
 losa, consists of two layers of smooth muscular tissue : an external, in 
 which the cells lie longitudinally, and an internal, in which they lie 
 transversely to the axis of the canal. In certain parts of the stomach 
 an indistinct third layer is found, in which the cells have an oblique 
 course. Inside the muscularis and joining it to the mucosa, is a layer 
 of loose fibrillar connective tissue, in which the blood and lymphatic 
 vessels ramify, called the submncosa. In the mucosa, finally, we have 
 a delicate supporting framework of connective tissue varying some- 
 what in its structure and abundance in different parts of the canal ; 
 this is covered by epithelial cells and contains the glandular apparatus, 
 while at the base of the glands and adjacent to the submucosa is a 
 thin layer of smooth .muscle-cells, lying in both transverse and oblique 
 directions, called the muscularis mucosa, from which usually a few 
 muscle-cells pass up between the glands. 
 
 The chief differences in minute structure between the stomach and 
 intestines are in the mucous membrane, and since in these membranes 
 the glands are very important factors, a word should be said here about 
 the structure of glands in general. Although the term gland is popu- 
 larly applied to structures having the greatest diversity of form and 
 function, and little in common but their name, we mean by it here 
 those organs whose physiological activity expresses itself, in part at 
 least, by the elaboration of certain specific fluids, secretions, or excre- 
 tions. All such glands have a somewhat analogous structure and 
 present two distinct kinds of structural elements : i. Epithelial or 
 gland cells ; 2. A connective-tissue framework, with blood and lym- 
 
72 NORMAL HISTOLOGY. 
 
 phatic vessels and nerves. The epithelial cells, usually large, differ 
 in form in different and even in the same gland ; in the latter case 
 especially, when the gland is divided into a secreting and excretory 
 portion. They will be described when we study the glands in detail. 
 The connective tissue, varying greatly in amount in different glands, 
 is sometimes arranged in sheets and bundles so as to form variously 
 shaped cavities which are lined with the gland-epithelium ; sometimes, 
 in the form of simple or superimposed lamellae covered with flat cells 
 like endothelium, it forms thin-walled tubes or chambers on whose 
 sides the cells are placed ; in this form it is called the membrana 
 propria of the gland-cavities. 
 
 The cavities and tubes thus formed and lined with gland-epithelium 
 are variously arranged in different glands, but their different modes of 
 arrangement may be reduced to three types : 
 
 i. Tubular glands, which have the form of simple or occasionally 
 branching, straight, curved or variously contorted tubes, terminating in 
 blind extremities. Such are the glands of the stomach. 
 
 2. Racemose glands, in which the secreting portion in the form of 
 vesicular or irregular-shaped cavities — alveoli — are grouped around 
 simple or branching excretory ducts into which they open ; the whole 
 structure has been not inaptly compared to a bunch of grapes, in which 
 the fruit would correspond to the alveoli, the stem to the excretory 
 ducts ; the analogy failing at this point, however, for in the gland the 
 alveoli and ducts are bound together by connective tissue lying between 
 them — called interstitial tissue — in which the vessels and nerves ramify. 
 Such are the mammary glands and certain mucous glands of the 
 bronchi. 
 
 3. Vesicular Glands. — These consist of simple, spheroidal, or irre- 
 gular-shaped closed alveoli, surrounded by a membrana propria and 
 lmed with epithelium, the separate alveoli being embedded in inter- 
 stitial connective tissue. Such glands are the thyroid and ovary. 
 
 The Stomach. 
 
 The muscularis of the stomach differs from that of the intes- 
 tines, in that a certain number of the cells, especially in the cardiac 
 extremity, do not have the typical transverse or longitudinal arrange- 
 ment, but lie in an oblique direction. In the vicinity of the pylorus 
 again, the inner circular layers are much thickened, forming the 
 sphincter pylori. The mucosa is almost entirely made up of glands 
 supported and held together by a small amount of delicate connective 
 tissue, in which the blood and lymphatic vessels ramify. The glands 
 are tubular, usually simple, but sometimes divided at the base. They 
 have a membrana propria, and, depending upon differences in the 
 epithelium which line them, they are classified as : 1. Mucous glands ; 
 2. Peptic glands. The general surface of the stomach is covered with 
 cylindrical epithelium. In the pyloric region, and here and there in 
 other parts, the glands, or follicles as they are often called, are lined 
 throughout with cylindrical epithelium ; these are the mucous glands. 
 
 The greater proportion of the glands, however, are lined only at 
 
THE GASTROINTESTINAL CANAL. 73 
 
 their orifices with cylindrical epithelium ; deeper down in the gland 
 we find usually two kinds of cells : a, rounded or cuboidal cells with 
 transparent or very finely granular bodies ; and b, larger spheroidal or 
 somewhat flattened very granular cells, which usually lie outside the 
 others, between them and the membrana propria. These are the so- 
 called peptic cells, and these glands are called peptic glands. 
 
 The relative number of these different kinds of peptic cells varies, 
 depending upon the degree of functional activity of the glands. When 
 they are secreting rapidly, the granular cells are abundant ; when at rest 
 they are few in number, the smaller transparent cells preponderating. 
 
 The arteries pass obliquely through the serosa and musculosa, 
 divide and subdivide in the loose tissue of the submucosa, from whence 
 branches are sent in between the glands ; here, before reaching the 
 surface, they break up into a close capillary net surrounding the folli- 
 cles, and the blood is finally collected into narrow venous trunks 
 directly beneath the surface epithelium ; from these it passes back 
 into larger veins in the submucosa, where it collects in the efferent 
 veins. The lymphatic vessels lie between the glands, form anastomos- 
 ing channels in the submucosa, and pass out through the musculosa, 
 receiving larger and smaller trunks from the latter. In the connective- 
 tissue framework of the mucosa, between the follicles — sometimes at 
 their base, sometimes between them nearer the surface — are found cir- 
 cumscribed collections of small round cells, apparently analogous with 
 the lymphatic follicles presently to be described in the intestines. 
 These in man are variable in their occurrence, being sometimes abun- 
 dant, sometimes not ; they are apt to be most abundant in the pyloric 
 region, and are often visible as small grayish prominences on the 
 surface of the mucous membrane. They are frequently called the 
 lenticular glands of the stomach. 
 
 The Small Intestine. 
 
 The supporting connective-tissue frame-work of the mucosa in 
 the small intestine is more abundant than in the stomach, and is 
 richly infiltrated with small spheroidal and variously shaped cells. 
 In it lie embedded tubular glands not unlike the mucous glands of 
 the stomach, but not crowded so closely together. These glands 
 are often called the follicles of Lieberkuhn, and are lined with cylin- 
 drical epithelium. Rising from the general surface of the mucous 
 membrane, between the orifices of the glands, are very numerous 
 short, cylindrical or conical projections called villi. These are formed 
 by projections inward of the mucosa; they are covered by cylin- 
 drical epithelium, and contain an abundant vascular net-work and the 
 radicles of the lymph- or chyle-vessels. The cylindrical epithelium 
 possesses a marked peculiarity in the structure of its free border, which 
 is considerably thickened, and is crossed, in a direction corresponding 
 with the long axis of the cell, by fine parallel, closely-set lines, which 
 are usually interpreted as tiny pores or canals passing through the 
 border. Scattered here and there between the cylindrical epithelium, 
 sometimes abundant, sometimes not, their number often apparently 
 
74 NORMAL HISTOLOGY. 
 
 depending in a measure upon the preservative fluid employed, clear 
 more or less ovoidal cells are seen, with a nucleus and a small amount 
 of protoplasm in the vicinity of the narrow base ; they look as if the 
 free border of the cell had fallen off and most of the cell-contents had 
 disappeared. Not infrequently a translucent structureless substance 
 is seen protruding from the open end of the cell, as if in the act of 
 passing out of it. These cells are called, from their form, goblet-cells. 
 Their significance is not yet definitely determined in ali cases : by 
 some they are regarded as cylindrical cells changed by artificial means ; 
 others believe them to be normal structures ; by many it is supposed 
 that under certain circumstances the cell-contents undergo a mucous 
 metamorphosis, swell up, burst out of the cell, leaving little but the 
 membrane and nucleus behind, and that thus, under normal conditions, 
 a certain amount of the mucus furnished by mucous membranes is pro- 
 duced. In addition to the tubular glands which are found throughout 
 the whole extent of the small intestine — in the duodenum, especially 
 in its upper portions — racemose, probably mucous glands, are found, 
 called Brunner's glands. They lie in the submucosa, and consist of 
 variously shaped, but usually follicular alveoli surrounded by a mem- 
 brana propria and lined with cylindrical epithelium. The excretory 
 ducts are also lined with cylindrical epithelium, and open on the surface 
 of the mucous membrane. Smooth muscle-cells pass up from the 
 muscularis mucosae into the villi. The central portion of the villi is oc- 
 cupied by one, or more, usually blind canals — the chyle-vessels — which 
 pass outward to the bases of the villi, where they usually unite to form 
 a net-work around the orifices of the tubular glands of Lieberkiihn and 
 the lymph-follicles presently to be described; they then pass into 
 the submucosa, where they form larger anastomosing channels ; from 
 thence trunks pass through the musculosa, receiving vessels from its two 
 layers and from a well-developed net-work between them. 
 
 Closely connected with the lymphatic vessels, and apparently form- 
 ing a part of the lymphatic apparatus of the intestines, are found cer- 
 tain structures called lymphatic follicles, and of these it is customary to 
 distinguish two kinds : i. Solitary follicles ; and 2. Agminated folli- 
 cles, or Peyer's patches. 
 
 1. Solitary Follicles. — These are irregularly scattered through the 
 mucous membrane of both small and large intestines, in the form of 
 small grayish nodules. They lie chiefly in the mucosa, often piercing 
 the muscularis mucosce and desce?iding into the submucosa; they are 
 usually spheroidal or pear-shaped, and frequently project somewhat into 
 the intestinal cavity. Where they lie, the tubular glands are crowded 
 to one side and the villi are absent over their surfaces. They may lie 
 so near the surface as to be covered only by a single layer of cylindri- 
 cal epithelium, or they may be more deeply placed, and covered in 
 addition by a thin layer of the connective tissue of the mucosa. They 
 consist of a nodule of reticular connective tissue, whose meshes are 
 somewhat narrower at the periphery where it becomes continuous 
 with adjacent parts. The meshes are closely filled with small sphe- 
 roidal cells, having the characters of lymph-cells. In their periphery 
 the lymph-vessels of the mucous membrane form a closely anasto- 
 
THE GASTROINTESTINAL CANAL. 75 
 
 mosing net-work. The blood-vessels also interlace in their periphery, 
 and send an abundance of anastomosing capillary loops into their in- 
 terior. 
 
 2. Peyer's Patches. — These are found chiefly in the small intes- 
 tine, and here are most abundant in the lower portion of the jejunum 
 and in the ileum ; they are round, or more frequently elongated, usually 
 slightly elevated structures, and are always situated at the side oppo- 
 site the mesenteric attachment, with their long axis parallel with the 
 axis of the gut. They consist, essentially, of an aggregation of a va- 
 riable number of structures, having the characters of the solitary folli- 
 cles ; these are placed closely together, side by side, and supplied in 
 essentially the same way as the solitary follicles with blood and lym- 
 phatic vessels. 
 
 The Large Intestine. 
 
 The mucosa of the large intestine is thickly set with tubular glands 
 similar to Lieberkuhn's glands in the small intestine, but is destitute 
 of villi. Solitary lymphatic follicles are abundant and are, as a rule, 
 somewhat larger than those of the small intestine. The distribution 
 of blood and lymphatic vessels resembles in most respects that de- 
 scribed in the stomach. 
 
 PRACTICAL STUDY. 
 
 Stomach. — Bits of perfectly fresh rabbit's or dog's stomach, from 
 the fundus and the pyloric region, should be stretched on a bit of cork 
 to prevent shrinkage, and immersed in absolute alcohol. After twen- 
 ty-four hours the fluid should be changed, and in three or four days the 
 specimen will probably be hard enough to cut. Sections from both 
 regions are made perpendicular to the surface after embedding in wax. 
 They are stained with hematoxylin and eosin, and mounted in glyce- 
 rine or Canada balsam. The nuclei of all the cells are stained violet 
 by the hematoxylin ; in the peptic glands the bodies of the granular 
 peptic cells are stained a deep rose-red, while the others are but 
 slightly colored, or not at all. Sections perpendicular to the surface 
 of a stomach, whose blood-vessels are filled with the mixture of Prus- 
 sian-blue and gelatine and stained with carmine, are very instructive. 
 
 Intestine. — Bits of intestine from the upper portion of the duo- 
 denum, from the ileum, including one of Peyers patches, and from 
 the large intestine should be stretched on cork and immersed in a 
 mixture of equal parts of half per cent, chromic acid and alcohol ; 
 after four days, transferred to alcohol, in which, in a day or two, it 
 will become hard enough to cut. Perpendicular sections from the 
 various parts should be made and stained, and mounted in the same, 
 way as the stomach. Sections should be made from intestines whose 
 blood-vessels are injected with blue, stained with carmine, and mounted 
 in Canada balsam. 
 
CHAPTER X. 
 
 SUBMAXILLARY GLAND— LIVER. 
 
 Submaxillary Gland. 
 
 Connected with the digestive tract are several racemose glands, 
 which, although differing in important particulars, both in structure 
 and function, yet have many features in common. These glands are 
 the submaxillaris, the sublingual, the parotid, and the pancreas. 
 The details of structure of these glands are still insufficiently known, 
 and within the limits of these lessons we cannot consider at length even 
 what is well understood. We will simply look at some of the more 
 important features of one of the best known, the submaxillaris, con- 
 sidering this, in a general way only, as typical of the others. 
 
 The submaxillary gland differs in structure in different animals, its 
 structure in the dog being, perhaps, best known, and quite closely re- 
 sembling that in man. In the dog it consists, like other racemose 
 glands, of alveoli and excretory ducts. The alveoli are surrounded 
 by a rnembrana propria, and grouped into lobules by more or less in- 
 terstitial connective tissue, which is furnished with blood- and lymph- 
 vessels and nerves. The alveoli, depending upon the gland-epithe- 
 lium which lines them, present two distinct forms : in one form, the 
 smaller of the two, we find the cavity of the alveoli nearly filled by 
 cuboidal or polyhedral cells, whose bodies are cloudy or granular, and 
 which have spheroidal or ellipsoidal nuclei ; these cells are readily 
 stained with carmine. In the other form of alveoli — the larger — we find 
 in the first place, surrounding the cavity of the alveoli, large, irregular- 
 shaped, transparent cells, having a gelatinoid appearance, with an often 
 flattened nucleus lying at the peripheral side ; these cells are little, 
 if at all, stained by carmine. In the second place, in the periphery of 
 the alveoli, between the cells just described and the rnembrana pro- 
 pria, lie large, in cross-section, crescentic, strongly granular masses, 
 usually containing several nuclei. These are believed to be formed 
 by a number of small, angular cells, closely crowded together. They 
 are readily stained with carmine, and are, apparently, the analogues 
 of the granular peptic cells of the stomach. Like the latter they are 
 most abundant when the gland is in a condition of functional activity, 
 and are supposed to be destined to replace the inner layer of trans- 
 parent cells, as these are destroyed in furnishing the specific secretion 
 of the gland. The excretory ducts differ in their structure in differ- 
 ent parts of their course. The portion which lies outside of the gland 
 proper, conveying its secretion to the mouth, is formed by a tube of 
 
SUBMAXILLARY GLAND— LIVER. JJ 
 
 connective tissue, supported by elastic fibres, and lined by a single 
 layer of cylindrical epithelium. Inside the gland the ducts present 
 two types of structure : first, we find passing out from the alveoli, 
 broad tubes formed of a membrana propria similar to that of the alve- 
 oli, and lined by cuboidal or flattened, somewhat granular cells ; 
 second, joining the last described ducts to those lying outside the sub- 
 stance of the gland proper are tubes formed of essentially the same 
 membrana propria, but lined with cylindrical nucleated cells, whose 
 bodies, in the peripheral zone, are distinctly striated in a direction 
 parallel with the axis of the cell, and in the central zone, bordering on 
 the cavity of the tube, finely granular. 
 
 PRACTICAL STUDY. 
 
 Sections of Gland from Dog. — Small pieces of a perfectly fresh 
 gland are put into absolute alcohol, and, when sufficiently hardened, 
 thin sections are made, stained with hematoxylin and eosin, and 
 mounted in Canada balsam. 
 
 Teased Gland. — To study the minute structure of the cells, which 
 are somewhat altered by the above method, small bits of a fresh gland 
 are laid for twenty-four hours in a small quantity of one per cent, 
 osmic acid solution, washed, teased apart on a slide in glycerine, and 
 stained with carmine. 
 
 The Liver. 
 
 The liver presents three distinct elements of structure : i. The 
 cellular elements, which in form, function, and arrangement charac- 
 terize the organ, the liver-cells, or parenchyma. 2. The coiinective- 
 tissue frame-zvork, the interstitial tissue, which surrounds the organ as 
 a capsule, and in its interior supports the parenchyma and carries the 
 larger vessels. 3. The blood-, lymph-, and gall-vessels. • The liver- 
 cells are large, have the form of irregular, often somewhat elongated 
 polyhedra; they have a granular body which frequently encloses 
 granules of pigment and larger or smaller droplets of fat ; they have 
 one or more vesicular nuclei and nucleoli. When living they are very 
 soft, and the isolated cells often show depressions on their sides 
 caused by pressure from adjacent blood-vessels. If we look at the 
 cut surface of a fresh liver with the naked eye, we find that it presents, 
 more or less distinctly, small polygonal or irregular-shaped figures, 
 which are sections of certain groups of liver-cells, called acini or 
 lobules. These lobules have, in general, the form of oblong polyhe- 
 dra, and the difference in shape presented by the sections is due to 
 the fact that they lie crowded together, with their long axes lying in 
 various directions. In order to understand the structure of these 
 lobules, it is necessary to study them in connection with the blood- 
 vessels of the liver, to which they bear a very constant and character- 
 istic relation. The liver receives its blood from the portal vein and 
 the hepatic artery ; it is conveyed away by the hepatic vein. If we 
 follow the ramifications of the hepatic vein, we find that it divides and 
 
78 NORMAL HISTOLOGY. 
 
 subdivides until it finally breaks up into short terminal radicles, 
 around which as a centre the oblong liver-lobules are grouped. From 
 its smaller branches also, before it breaks up into terminal radicles, 
 small, short branchlets are given off, which form the centres of lobules. 
 Veins bearing this relation to the lobules are called central veins, or 
 venae intralobular es. If now we follow the ramifications of the portal 
 vein, on the other hand, we find that dividing, and subdividing it, too, 
 gives off small branchlets which, with the terminal branchlets into 
 which it. finally breaks up, pass to the surface of the lobules ; there 
 they pour their blood into a rich capillary net-work within the lobules, 
 whence it passes directly into the radicles of the hepatic vein at the 
 centre. The capillaries of the lobules radiate from the central vein, 
 for the most part nearly at right angles to its axis, and take more or 
 less direct, slightly divergent, courses to the periphery, being con- 
 nected with each other by frequent branches. A net-work is thus 
 formed, in whose narrow and elongated meshes the liver-cells lie, 
 sometimes in a single row, sometimes in several rows, depending upon 
 the breadth of the intercapillary spaces. The central veins do not 
 usually extend quite to the extremity of the lobules, and here the 
 capillaries are given off brush-like obliquely from its end. A liver- 
 lobule, then, is a circumscribed portion of liver-tissue, having for its 
 centre a branchlet of the hepatic vein — vena intralobular is, and at 
 its periphery the terminal branchlets of the portal vein — venae i?iter- 
 lobular es, while between these two sets of vessels, and joining them, 
 is a rich capillary net-work, in whose elongated meshes lie rows of 
 liver-cells. In certain animals, such as the pig, the lobules are very 
 distinct, being surrounded by connective tissue which is directly con- 
 tinuous with the connective tissue surrounding the larger trunks of the 
 portal vein, hepatic artery, etc., and called Glisson's capsule. In this 
 connective tissue surrounding the lobules are found, in addition to the 
 branchlets of the portal vein, certain of the terminal branchlets of the 
 hepatic artery and the capillaries connected with them, and the 
 smaller gall-ducts. In the human liver, on the contrary, there is very 
 little connective tissue between the lobules, only here and there small 
 masses are seen surrounding the branches of the portal vein and its 
 accompanying vessels, the lobules merging, for the most part, into one 
 another. The hepatic artery sends its blood into capillaries which 
 are distributed largely to the walls of the vessels and the connective 
 tissue, and it finally passes either into the terminal branchlets of the 
 portal vein at the periphery of the lobules, or directly into the intra- 
 lobular capillaries. 
 
 We have finally to consider the gall-passages. The larger gall- 
 ducts, lined with a well-developed mucous membrane, supplied with 
 tubular and racemose mucous glands, and covered with cylindrical 
 epithelium, enter the liver with the other large vessels, and dividing 
 and subdividing accompany them in the capsule of Glisson. 
 
 As they pass inward they become smaller and smaller, the mucous 
 membrane loses its glands and becomes simpler in structure — the 
 small ducts consisting of little more than a simple tube lined with low 
 cylindrical or cuboidal cells. Finally, as the ducts arrive at the 
 
SUBMAXILLARY GLAND — LIVER. 79 
 
 periphery of the lobules — intralobular ducts — they are lined with flat, 
 polygonal cells ; here they become continuous with the intralobular 
 gall-passages or gall-capillaries. The gall-capillaries are extremely 
 small and difficult of demonstration ; they form a delicate net-work 
 around the individual liver-cells, being arranged in such a way, how- 
 ever, that they never come into contact with the blood-capillaries, 
 always being separated by at least a part of the diameter of a liver- 
 cell from the latter. They do not seem to possess a distinct wall, but 
 are rather simple channels grooved in the walls of contiguous liver- 
 cells. 
 
 The connective tissue of the human -liver is chiefly found in the 
 capsule which surrounds the organ and in the capsule of Glisson ; 
 but, in addition to this, we find it in very small quantity, not only 
 in the vicinity of the hepatic vein, but also between the cells and 
 along the capillaries within the lobules ; in the latter situation it occurs 
 in the form of delicate fibres or membranes, with here and there fusi- 
 form or stellate cells. Bearing in mind that the lobules, although in 
 general of an oblong polyhedral form, lie crowded together, more or 
 less distorted by pressure, with their long axes running in various 
 directions ; and further, that where two or three terminal branchlets of 
 the hepatic vein are given off together, compound lobules may be 
 formed around them, having a common base with separate apices — it 
 will be readily conceived that sections through the liver in any direc- 
 tion will present lobules of the most diverse form. But if the charac- 
 ter of the vessels at the centre and periphery be borne in mind, the 
 determination of the relative position of any given portion will not, as 
 a rule, be difficult. The lymphatic vessels of the liver form an abun- 
 dant net-work in the capsule, and also accompany the larger vascular 
 trunks in Glisson's capsule and between the lobules. Their presence 
 within the lobules has not yet been definitely demonstrated. 
 
 PRACTICAL STUDY. 
 
 Liver-cells. — A small bit of a perfectly fresh liver is teased on a 
 slide in a drop of one-half per cent, salt solution, and covered and 
 studied in the same fluid. If it be desired to preserve the isolated 
 liver-cells, a small bit of fresh liver is put for twenty-four hours in a 
 mixture of alcohol and water (i to 2), and then teased on a slide in a 
 drop of glycerine which has been slightly colored with eosin, and 
 mounted in the same. 
 
 Sections of Pig 's Liver. — A small piece of perfectly fresh pig's liver 
 is immersed for three weeks in Muller's fluid, which should be changed 
 once or twice, then washed and transferred to alcohol. When it is 
 sufficiently hard very thin sections should be made near the surface, 
 both parallel and at right angles to it, so as to cut the lobules, which 
 are quite regularly arranged at the surface, in different directions. 
 The sections are stained with hematoxylin and mounted in Canada 
 balsam. The lobules being here surrounded by tolerably distinct lay- 
 ers of connective tissue, in which the intralobular vessels run, the 
 lobular structure is quite evident, and the different pictures obtained 
 
SO NORMAL HISTOLOGY. 
 
 by sections through the lobules in different directions are not difficult 
 of interpretation. 
 
 Sections of Human Liver. — Sections are made from a bit of human 
 liver hardened as above, and stained with hematoxylin and eosin, and 
 mounted in glycerine. Here the lobular structure is very ill-defined, 
 because of the small amount of connective tissue between the lobules, 
 and for the recognition of the different parts of the latter we are largely 
 dependent upon the determination of the different kinds of blood-ves- 
 sels, since these always bear a definite relation to the lobules. It is 
 to be remembered that the branches of the portal vein lie only in the 
 periphery of the lobules, that they are usually accompanied by other 
 vessels besides capillaries, and are in most cases surrounded by a 
 greater or less amount of connective tissue ; the central vein, on the 
 other hand, is usually unassociated with other vessels, except capilla- 
 ries, and surrounded only by a scarcely appreciable amount of connec- 
 tive tissue. 
 
 Injected Liver. — The general arrangement of the blood-vessels is 
 best studied in sections from a human or rabbit's liver, which has 
 been injected through the portal vein with a solution of gelatine 
 colored with Prussian blue. The sections are stained with carmine or 
 eosin, and mounted in Canada balsam. 
 
 Injected Gall-capillaries. — By a careful injection of the blue gela- 
 tine mixture into the hepatic duct, the gall-capillaries may be partially 
 filled. Sections are ma.de, stained with eosin, and mounted in balsam. 
 
CHAPTER XL 
 
 LYMPH-NODES— SPLEEN. 
 
 Lymphatic Glands or Lymph-Nodes. 
 
 If we follow the lymphatic vessels in their course from the periphery 
 toward the thoracic duct, we find that, sooner or later, they are inter- 
 rupted in their course by certain nodular masses, more abundant in 
 some parts of the body than in others, and of variable size, commonly 
 called lymphatic glands. They are not glands in the limited sense of 
 the word, as it has been already defined ; for, so far as we know, they 
 furnish no specific secretion, they have no excretory ducts, and seem 
 to have an entirely different structure and function from the glands 
 proper. It would be better to call them lymph-nodes, for that would 
 not be misleading, as the word gland is, in regard to their relations to 
 other structures ; but, as they are universally known as lymphatic 
 glands, we shall at present use both terms. The lymph-nodes pre- 
 sent a great diversity in form, being spherical, ovoid, discoidal, or 
 irregularly prismatic; they always present at one side a hilum-like 
 marking, and however diverse their forms, they approach more or less 
 closely in shape to the type of the kidney. If we make a section 
 through a fresh gland, at right angles to its long axis and through the 
 hilus, we find it more or less distinctly divided into two zones : an 
 outer, or cortical zone — the cortex — which is soft and grayish or red- 
 dish in color, and divided into ovoidal or irregular-shaped masses ; 
 and an inner, or medullary zone — the medulla — adjacent to the hilus, 
 which is firmer, and has a more uniform, or sometimes irregularly- 
 reticulated, grayish or brownish red surface. The nodes are sur- 
 rounded by a firm, dense capsule of connective tissue, with a few 
 elastic fibres and smooth muscle-cells. The capsule sends inward 
 numerous partitions or trabeculse, which divide the cortex into a 
 series of intercommunicating chambers, and the medulla into numer- 
 ous irregular connecting passages. These septa are formed of the 
 same elements as the capsule, and at the hilus are continuous with a 
 dense mass of connective tissue, through which the blood-vessels enter 
 the organ. If we examine the spaces left between the septa, we find 
 that in the cortex they are incompletely filled with ovoidal or globu- 
 lar bodies, which are continuous with cord-like anastomosing struc- 
 tures lying in the narrower and irregular spaces in the medulla ; the 
 bodies in the cortex are called lymph-follicles, and the cord-like 
 structures in the medulla lymph-tubes or lymph-cords. If now we 
 examine more minutely the structure of the lvmph-follicles, we find 
 6 
 
82 NORMAL HISTOLOGY. 
 
 that they consist, in the first place, of a frame-work of reticular con- 
 nective tissue, whose meshes are largest in the centre of the follicle, 
 narrower and smaller in the periphery ; indeed, so closely crowded 
 together are the trabecular here, that they give to the follicle a toler- 
 ably well defined outline. In the second place, the meshes of the 
 reticulum are closely filled with small spheroidal cells, having, in gen- 
 eral, the characters of lymph-cells ; in many of them, however, the 
 nucleus is very large, occupying the greater part of the cell. The 
 follicles — in which we recognize essentially the same structure that we 
 have found in the lymphatic follicles of the intestine — do not entirely 
 fill the cavities in which they lie, but are surrounded on all sides by a 
 narrow space. If now we examine the relation of the follicles to the 
 walls of their investing spaces, we find that, from the walls of these 
 spaces, delicate branching trabecular pass inward to the surface of the 
 follicle, where they become continuous with the reticular tissue of 
 the latter. They are, in fact, themselves reticular connective tissue, 
 similar to that of the follicle, except that the trabecular are coarser 
 and the meshes broader. Stretching across the space surrounding 
 the follicles, they suspend the latter so that they hang free in the 
 cavities. The space thus formed around the follicle is called the peri- 
 follicular space, or better, lymph-sinus, for reasons which will be 
 presently given. 
 
 If we now turn our attention to the lymph-cords of the medulla, 
 which, it will be remembered, are continuous with the follicles, we 
 find that they have an exactly similar structure, and bear the same re- 
 lation to the more irregularly arranged connective-tissue septa which 
 bound the branching spaces in which they ramify, that the follicles do 
 to the walls of their investing spaces, i. e., they are suspended in them 
 by coarse trabecular of reticular connective tissue, and surrounded 
 on all sides by lymph-sinuses. These lymph-cords, ramifying and in- 
 osculating in the medullary portion of the gland, form an intricate 
 system of intercommunications between all the follicles of the gland. 
 Injections of dilute solutions of nitrate of silver into the lymph- 
 sinuses show that the surfaces of the follicles and lymph-cords, as well 
 as the walls of their investing spaces, are covered with endothelial 
 cells. If we study the relation of the lymph-vessels to the lymph- 
 nodes, we find that the former, on arriving at the surface of the node 
 — afferent vessels — pierce the capsule and become continuous with 
 the lymph-sinuses, into which they pour their contents ; we find, fur- 
 ther, that efferent vessels, still continuous with the sinuses, leave the 
 organ at other points, frequently at the hilus. The lymph-glands, 
 then, are structures interrupting the course of the lymphatic vessels, 
 in which the lymph is forced to pass through a series of irregular 
 branching spaces or sinuses, which are crossed by a net-work of reti- 
 cular tissue, and bathing in its course certain peculiar structures — the 
 follicles and lymph-cords — whose function we do not yet understand. 
 The lymph-sinuses contain not only the fluid of the lymph, but numer- 
 ous lymph-cells, and usually a certain number of red blood-cells. 
 
 The principal blood-vessels enter the nodes at the hilus, and the 
 arteries, sending off branches to the connective tissue there and to the 
 
LYMPH-NODES— SPLEEN. 83 
 
 septa, divide and subdivide, and soon enter the lymph-cords ; they 
 pass along in the axis of these, giving off a long-meshed capillary sys- 
 tem; then entering the follicles, they break up into a loose capillary 
 net-work, from which the blood is collected into venous radicles, and 
 poured into veins which pass out through the lymph-cords and out of 
 the organ in connection with the arteries. Small blood-vessels usually 
 enter the capsule at other points than the hilus, and are largely dis- 
 tributed to the capsule and 'the connective tissue of the septa. The 
 number and arrangement of the follicles varies greatly in different 
 lymphatic glands : in some there is but a single layer in the cortex ; 
 in others, several layers are superimposed and more or less crowded 
 — thus giving to some glands a narrow, to others a broad and volumi- 
 nous cortex. In many animals, as the ox, the reticular tissue of the 
 medullary portion contains an abundance of brown pigment. 
 
 PRACTICAL STUDY. 
 
 Lymph-Sinuses Injected. — A general view of the lymph-sinuses 
 and their relations to the follicles and tubes is best obtained from 
 sections of nodes whose lymph-channels have been tilled with a col- 
 ored solution of gelatine. A gland being exposed in a recently 
 killed animal — one of the cervical glands of the dog answers well — 
 a hypodermic syringe with a sharp canula is warmed and filled with 
 a warm solution of gelatine colored with Prussian blue ; the canula is 
 thrust through the capsule at any point and the fluid injected. The 
 node will become mottled with blue, and the mass will often be seen 
 to flow into adjacent glands. If it be not desired to inject more than 
 one, a ligature should be passed around the vessels leading to the 
 others. When a sufficient quantity of the fluid has been injected to 
 render the gland firm and the capsule tense, the canula is withdrawn, 
 and the node cooled by a piece of ice laid upon it, or by a stream of 
 cold water. When the gelatine has solidified, the gland is cut out, 
 divided longitudinally, and put into strong alcohol. When it has be- 
 come sufficiently hard, sections are made through the entire gland, 
 stained with picrocarmine or eosin, and mounted in balsam. 
 
 Pencilled Section of a Lymph-node. — The meshes of the follicles 
 and cords, as well as the lymph-sinuses, are so closely crowded with 
 cells, that the structure of the supporting tissue cannot be clearly 
 made out, even in very thin sections, without removing some of the 
 cells. This may be done by shaking in water, as in the preparation 
 of reticular connective tissue (p. 34), or, better, by pencilling under 
 water with a soft camel' s-hair pencil in the way suggested by Ranvier. 
 An interstitial injection of a fresh gland with one per cent, solution of 
 osmic acid is made, in the way described on page ^^, and the gland 
 laid for an hour in water, and then immersed for three days in strong 
 alcohol. As it is not usually sufficiently hardened by the alcohol to 
 admit of very thin sections being made, it is now to be laid for a few 
 moments in water, and then in a syrupy solution of gum-arabic, in 
 which it remains for twenty-four hours, and then transferred to alco- 
 hol. This precipitates the gum in the interstices, and gives it a 
 
84 NORMAL HISTOLOGY. 
 
 proper consistence for cutting. Thin sections, made so as to em- 
 brace both cortex and medulla, are transferred to a shallow, flat-bot- 
 tomed dish of water, and as the gum begins to swell, the section 
 should be laid flat and gently pressed down with a camel' s-hair pen- 
 cil, whose end is square, so that it shall adhere at all points to the 
 bottom of the dish. When this is done, the specimen should be 
 gently tapped with the brush held perpendicularly ; as the cells are 
 freed in this way, they float off into the Water, and the specimen will 
 be seen to become more and more transparent. When the operation 
 is completed — which will be determined by the extreme delicacy 
 which the specimen will finally present when most of the cells are 
 removed — it is floated, without folds, on to a slide, stained with hema- 
 toxylin, and mounted in glycerine. 
 
 Blood-vessels. — To obtain an injection of the blood-vessels of the 
 lymph-nodes, either a whole animal, such as a rabbit or dog, may be 
 injected, through the aorta, with the blue gelatine mixture, or a single 
 gland, such as the cervical or mesenteric, through its main artery. 
 
 The glands should be hardened in alcohol, and the sections stained 
 with eosin and mounted in Canada balsam. 
 
 The Spleen. 
 
 The spleen, although differing in many important particulars from 
 the lymphatic glands, vet presents many striking analogies with them. 
 Like them, it presents on cross-section, to the naked eye, a fibrous 
 envelope — the capsule ', — from which septa and trabecular pass into the 
 organ enclosing irregular spaces. Here also we find the spaces be- 
 tween the septa filled with a soft substance presenting two distinct 
 modes of arrangement ; we find first, irregularly scattered through the 
 organ, small grayish globular structures called Malpighian bodies or 
 follicles ; and second, between these, filling up the remaining space 
 between the trabecular, a soft red tissue called the pulp. 
 
 Finally, we find blood-vessels entering the organ at the hilus. We 
 have then, in studying the spleen, to consider the connective-tissue 
 capsule and trabecules, the follicles, the pulp, and the blood-vessels. 
 
 The capside of the spleen consists of a dense envelope of inter- 
 lacing connective-tissue fibres with flattened cells, with a large number 
 of fine elastic fibres and a few smooth muscle-cells. It is covered by 
 a layer of endothelial cells similar to those of the general peritoneal 
 surface. At the hilus it passes inward, forming a sheath for the large 
 vessels, and joins a complicated system of septa and trabecules, which, 
 proceeding inward from all parts of the capsule, form a multitude of 
 irregular communicating spaces in which the follicles and splenic pulp 
 lie. These trabecular and septa are made up of the same elements as 
 the capsule, and in size and abundance vary greatly in different ani- 
 mals, being in man only moderately developed. The follicles, or 
 Malpighia7i bodies, have essentially the same structure as the follicles 
 of the lymph-nodes — that is, they are formed of a small mass of sup- 
 porting reticular connective-tissue, whose meshes are narrowest at the 
 periphery, and closely filled throughout with small spheroidal cells, 
 
LYMPH-NODES — SPLEEN. 85 
 
 and supplied with a net-work of capillaries ; we usually find here, how- 
 ever, a small artery passing either through the centre or at one side of 
 the follicle. In order to fully understand the follicles of the spleen, 
 it is necessary to study their relation to the arteries, in which respect 
 they seem entirely to differ from their analogues in the lymph-nodes. 
 The arteries and veins as they enter the hilus of the spleen are sur- 
 rounded, as above mentioned, by a connective-tissue sheath ; after 
 passing for a short distance inward, they separate, and the arteries, 
 still accompanied by a certain amount of connective tissue, rapidly 
 divide and subdivide, proceeding farther inward, until the small branches 
 finally break up into brush-like bundles of delicate twigs. If now we 
 carefully study the walls of the smaller arteries, we find that in certain 
 parts they undergo a singular modification : in the larger trunks the 
 connective-tissue sheath, in the small twigs the adventitia, become 
 very loose in texture and their meshes become filled with spheroidal 
 cells resembling lymph-cells — this is called lymphoid infiltration of the 
 arteries ; then we find that at certain points of the small twigs this in- 
 filtration suddenly becomes quite extensive, the intercellular substance 
 assuming the character of reticular connective tissue ; and distinct cir- 
 cumscribed swellings are formed either around or at one side of the 
 arteries — these are the splenic follicles or Malpighian bodies. In 
 some animals this infiltration is quite extensive and continues along 
 the arteries for considerable distances at either side of the follicles ■ in 
 others, as in man, it is not very marked except in the follicles, but 
 may frequently be seen along the arteries adjacent to them, in the form 
 of narrow cellular sheaths. The capillary net-work of the follicles is 
 connected with arterial twigs which either penetrate from without, 
 or are given off from the follicular artery as it passes through the 
 body. 
 
 Let us now turn to the pulp. This is composed, in the first place, 
 of a multitude of irregular, frequently anastomosing cords — called pulp- 
 cords — between which, and bounded closely by them, lie in the second 
 place, a series of branching channels — the cavernous veins. The pulp- 
 cords — joined on the one hand to the follicles and infiltrated arterial 
 sheaths, and on the other to the connective-tissue septa — consist of a 
 frame-work of delicate reticular connective tissue, continuous with the 
 sustaining tissue of the follicles, the meshes of which are incompletely 
 filled with various kinds of cells. Among these cells we find sphe- 
 roidal cells, like lymph-cells ; larger colorless cells with one or more 
 large nuclei ; red blood-cells ; fragments of red blood-cells ; larger 
 and smaller colorless cells containing pigment in various forms; and 
 finally, there are sometimes found in varying numbers cells which re- 
 semble the lymph and larger colorless cells in form, but whose bodies, 
 either homogeneous or granular, have a color similar to that of the 
 red blood-cells. These latter cells, like certain similar cells already 
 mentioned as occurring in the marrow of bones, are regarded by many 
 observers as intermediate forms between the colorless and the red 
 blood -cells, and are often called nucleated red blood-cells. Although 
 many recent observations would tend to confirm this idea, their nature 
 is as yet by no means absolutely determined. We have still to con- 
 
S6 NORMAL HISTOLOGY. 
 
 sider the structure of the second constituent of pulp — the cavernous 
 veins. Following the splenic veins inward from the hilus, we find 
 that they gradually lose their connective-tissue sheath, and then their 
 outer coats, and then rapidly divide and subdivide to form a multitude 
 of intercommunicating thin-walled canals of tolerably uniform calibre, 
 which occupy the irregular branching spaces between the pulp-cords. 
 These ultimate venous trunks are called cavernous veins, and their 
 walls consist of little else than a few widely separated branching circu- 
 lar and oblique fibres, upon which lie, at varying intervals, elongated, 
 curved, spindle-shaped cells, which have their long axis parallel with 
 the axis of the canal. These veins, whose walls, as will be seen, are 
 not closed but fenestrated, are in direct communication with the spaces 
 which are still left in the meshes of the pulp-cords by the cells which 
 incompletely fill them. 
 
 Still another point remains to be considered, namely, the course 
 of the blood after its exit from the above-described fine arterial twigs 
 on which the Malpighian bodies are formed, until it enters the fenes- 
 trated cavernous veins of the pulp. The opinion of different observ- 
 ers on this point differs somewhat, owing to the extreme technical 
 difficulties in the investigation; but it is believed by the majority of 
 histologists at present, that after passing out of the fine arterial twigs, 
 through the intervention of capillaries, it is poured directly into the 
 meshes of the pulp-cords, and that after circulating here around the 
 cells without distinctly walled channels, it finally finds its way through 
 their fenestrated walls into the cavernous veins, whence it passes out 
 of the organ through the large efferent veins. 
 
 PRACTICAL STUDY. 
 
 Sections of U?iinjected Spleen. — A general view of the arrangement 
 of the different structures found in the spleen may be obtained from 
 very thin sections of a spleen which has been placed for ten days in 
 two per cent, solution of bichromate of potash, and then transferred 
 to alcohol and allowed to remain until hard enough to cut. The 
 sections are stained with hematoxylin and eosin, and mounted in 
 glycerine. 
 
 Sections of Lijecied Spleen. — A spleen is injected through the 
 splenic artery with the blue gelatine mixture, hardened in alcohol, 
 and thin sections made and stained with carmine. In such speci- 
 mens, if successful, the blue fluid will be found not only in the blood- 
 vessels, but also in patches in the pulp-tissue, where it will be seen 
 lying between the cells, looking like an accidental extravasation : its 
 presence here is explained, however, by the structure of the organ. 
 
 Pencilled Sections of Spleen. — A bit of human spleen is put for a 
 week in two per cent, solution of bichromate of potash, washed and 
 transferred first to dilute, then to strong alcohol; when sufficiently 
 hard, very thin sections are made, carefully pencilled under water (see 
 page 83). stained with haematoxylin, and mounted in glycerine. 
 
 Isolated Cells. — A bit of the fresh organ is teased on a slide in half 
 per cent, salt solution, and studied in the same. Should the red blood- 
 
LYMPH-NODES— SPLEEN. 87 
 
 cells, as is apt to be the case, be so numerous as to conceal the other 
 forms, a drop or two of dilute alcohol (1 to 2) is allowed to run un- 
 der the cover-glass, and after a few moments the red blood-cells will 
 have become so transparent as not seriously to interfere with the ob- 
 servation. If the nuclei are not sufficiently well denned, a drop of 
 eosin may be added after the alcohol. In addition to the above de- 
 scribed cells of the pulp-cords in teased specimens, narrow, elongated, 
 often curved cells, with projecting nuclei, are frequently seen ; these 
 are the above-described lining cells of the cavernous veins. 
 
CHAPTER XII. 
 
 SUPRA-RENAL CAPSULES— THYROID GLAND. 
 
 Supra-renal Capsules. 
 
 In sections of the supra-renal capsules, through their thickest part, 
 and at right angles to the long axis of the organ, we see with the naked 
 eye two distinct layers : a tolerably firm, yellowish, striated cortical 
 layer of considerable thickness — the co7'tex, and a narrower central, yel- 
 low or reddish medullary portion — the medulla. Between the two, or 
 rather at the inner side of the cortical portion, a more or less distinct 
 brown zone is seen. If we examine thin sections microscopically, we 
 find that the organ is enclosed in a firm connective-tissue capsule, in 
 which are numerous elastic fibres. From this capsule delicate, con- 
 verging connective-tissue bands, or septa, pass inward, and being 
 joined together by delicate transverse bands or trabecular, divide the 
 cortex into a multitude of variously shaped chambers. In the peri- 
 phery, just beneath the capsule, the chambers are small and irregular 
 in shape ; then comes a broader zone, whose chambers are long and 
 narrow ; and again, at the inner border of the cortex, they are small 
 and irregular. In the medulla the supporting frame-work has the form 
 of a delicate reticulum with small irregular meshes. The spaces thus 
 formed are filled with cells, which differ in character in the different 
 parts ; in the cortex these parenchyma cells are, for the most part, 
 large, polyhedral, and granular ; less abundant are smaller cuboidal or 
 cylindrical forms. Those in the elongated chambers are not infre- 
 quently crowded with fat-droplets, and those lying in the inner zone 
 usually contain an abundance of brown pigment, forming the above- 
 mentioned brown zone. The meshes of the medulla are filled with 
 large, globular, or angular or branched, finely granular cells, which, in 
 marked contrast to the cortical cells, are stained intensely brown by 
 solutions of chromic acid or its salts. The blood-vessels are very 
 abundant ; many of them have extremely thin walls and lie in close 
 contact with the cells of the parenchyma. The organ is abundantly 
 supplied with nerves, which, passing along the converging trabecular, 
 form a dense plexus in the medulla, in which, as well as in the cap- 
 sule, considerable numbers cf ganglion-cells are found. 
 
 PRACTICAL STUDY. 
 
 Sections. — A human supra-renal capsule, as fresh as possible — or, 
 if this cannot be obtained, that of the guinea-pig or ox — is cut trans- 
 
SUPRA-RENAL CAPSULES— THYROID GLAND. 89 
 
 versely into two or three pieces and placed in two per cent, solution 
 of bichromate of potassium for ten days, and the hardening is then 
 completed in alcohol. Transverse sections are made, stained with 
 hsematoxylin and eosin, and mounted in glycerine or balsam. 
 
 Another section should be shaken in water in a test-tube to free 
 the parenchyma-cells, stained with hematoxylin, and mounted in 
 glycerine. 
 
 The Thyroid Gland. 
 
 The thyroid gland is composed of a congeries of small spheroidal or 
 irregular-shaped vesicles or alveoli, enclosed in connective tissue, and 
 grouped together to form lobules. Each alveolus is surrounded by a 
 delicate membrana propria, and lined with a single layer of cuboidal 
 cells. The alveoli rue entirely separate from one another, and the 
 gland has no excretory duct. The connective tissue surrounding 
 the alveoli contains numerous blood-vessels. The entire gland, which 
 consists of two lateral lobes, united at their lower extremities by a 
 transverse commissure, is enclosed in a dense connective-tissue en- 
 velope. The alveoli are filled with a clear, homogeneous fluid, which 
 in adult life is frequently transformed into or replaced by a translu- 
 cent material, commonly called colloid. Owing to the pressure which 
 the accumulating colloid substance exerts on the epithelium lining the 
 alveoli, they are often very much flattened, and not infrequently al- 
 most entirely disappear. The colloid material seems to be formed, 
 in part at least, by a transformation of the contents of the epithelial 
 cells, and its formation under pathological conditions gives rise to 
 one of the forms of goitre. The fact that the thyroid has no excre- 
 tory ducts, although otherwise having the structure of a gland, gives 
 it a unique position among those organs. 
 
 PRACTICAL STUDY. 
 
 The thyroid of a child or adult — better the former — is cut into 
 small pieces and put for a day into dilute, and then into strong alco- 
 hol. When sufficiently hard, thin sections are made, stained with 
 hematoxylin and eosin, and mounted in balsam. 
 
CHAPTER XIII. 
 
 THE RESPIRATORY APPARATUS. 
 
 The respiratory apparatus consists of a multitude of small cavities 
 or chambers, on whose walls the blood is brought into close contact 
 with the air, and of a system of branching tubes through which the 
 air is conducted to and from them. The conducting tubes are the 
 larynx, the trachea, and the bronchi. We shall limit our study of the 
 tubes to the two last. 
 
 The walls of the trachea consist of several layers ; commencing at the 
 outside, — we have, first, a layer of firm connective tissue — the fibrous 
 laye?' — in which lie embedded, incomplete cartilaginous rings, the space 
 between whose free ends, at the posterior portion of the tube, is bridged 
 over by transverse bands of smooth muscular tissue, which bind the 
 ends together. The cartilaginous rings are of the hyaline variety, and 
 their perichondrium is continuous with the looser connective tissue of 
 the layer in which they lie. This layer merges into the next, the sub- 
 ??iucosa, which consists of fibrillar connective tissue with elastic fibres, 
 the latter running chiefly in a longitudinal direction. 
 
 Still farther inward, and not distinctly separated from the submu- 
 cosa, lies the mucosa, composed of fibrillar connective tissue contain- 
 ing an abundance of variously shaped cells, and, for the most part, 
 longitudinally arranged elastic fibres. The connective tissue of the 
 mucosa, in some parts of the trachea, does not form a uniform layer, 
 but is arranged in more or less well-defined longitudinal bundles, giv- 
 ing the surface a wavy or folded appearance. Internally the mucosa 
 is bordered by a thin, homogeneous membrane, the basal membraiie, 
 upon which the epithelial cells lining the trachea rest. The epithelial 
 cells are usually arranged in about three layers. Lying upon the basal 
 membrane are irregularly spheroidal, often somewhat elongated cells ; 
 upon these lie fusiform or pear-shaped cells, while the surface is formed 
 by a layer of pyramidal or cylindrical ciliated cells. In the submucosa, 
 and sometimes extending outward along and between the cartilaginous 
 rings, lie racemose mucous glands, whose excretory ducts, lined with 
 cylindrical epithelium, pass obliquely inward and terminate on the 
 surface in expanded orifices. The alveoli of the glands are lined with 
 a single layer of slightly granular, spheroidal cells. When the glands 
 are in a condition of functional activity, however, the cells become 
 larger, their outlines indistinct, their nuclei are crowded to one side, and 
 their contents are apparently transformed into a homogeneous mucous 
 mass. Scattered here and there among the glands, and between the 
 cartilaginous rings, lie larger and smaller clusters of fat-cells. The 
 
THE RESPIRATORY APPARATUS. 9 1 
 
 blood-vessels, passing through the outer layers, furnish an abundant 
 •capillary net-work to the mucous glands, and spread out in a rich 
 capillary plexus beneath the basal membrane. 
 
 Although the larger and medium-sized bronchi have the same gen- 
 eral structure as the trachea, still we find, aside from a decrease in 
 the thickness of the walls, certain noteworthy structural modifications 
 which chiefly concern the cartilaginous and muscular elements. The 
 cartilage occurs in the form of regular, incomplete rings, only in the 
 upper portion of the larger bronchi. As the tubes become smaller 
 the cartilage occurs in the form of scattered, irregular-shaped, often 
 angular plates, which become gradually smaller and thinner. Farther, 
 a distinct layer of smooth muscular tissue, in the form of transverse 
 rings, connected with each other by interlacing cells, appears between 
 the mucosa and submucosa. We find, also, that the longitudinal 
 bundles of fibrillar and elastic connective tissue in the mucosa are 
 more strongly developed as the bronchi become smaller, so as to throw 
 the mucous membrane into pronounced longitudinal folds. Following 
 the bronchi now down toward their finer ramifications, we find that the 
 outer connective-tissue layer becomes thinner, the cartilaginous plates 
 become smaller and more infrequent, and finally altogether disappear; 
 the mucous glands too, after becoming smaller and simpler in structure, 
 disappear with the cartilages. The muscular rings assume, gradually, 
 the form of an uniform thin layer of transversely arranged muscle-cells, 
 intermingled with elastic fibres, and are finally represented only by a 
 few scattered transverse cells. The mucosa in the smaller tubes be- 
 comes gradually thinner, and finally merges into the fibrous layer with 
 the intervention only of a few scattered muscle-cells ; the lower layers 
 of epithelium gradually disappear, leaving a single row of ciliated cells 
 upon the basal membrane. Finally, we have, in the smallest tubes, 
 very thin connective-tissue walls, containing a few muscle-cells and 
 elastic fibres, and lined with cuboidal, ciliated, and finally with flattened 
 non-ciliated cells. We are thus led to the respiratory cavities of the 
 lungs — the air-chambers. If we look at the surface of a lung we see 
 that it is divided, by narrow branching lines, into irregular polygonal 
 spaces, each one of which corresponds to a pulmonary lobule. These 
 lobules have, on the surface of the lung, where they are more uniform 
 in shape than within, a pyramidal form, and are separated by narrow 
 connective-tissue septa, and each lobule is in fact a group of air-vesi- 
 cles, which are in communication with a single small bronchus and 
 its terminal twigs. As the small bronchus enters the lobule it divides 
 into several terminal twigs ; each one of these opens at its peripheral 
 extremity into two or three irregular tubular passages, called alveolar 
 passages; on all sides of these alveolar passages still other irregular, 
 sometimes branched elongated cavities, called infundibula, open out, 
 and their walls, as well as the walls of the alveolar passages, are beset 
 with tiny, irregular-shaped vesicles, called air-vesicles or alveoli, which 
 communicate with the cavities of the infundibula and alveolar passages 
 by wide, irregular openings. It is in the walls of these air-vesicles that 
 that interchange of material between air and blood occurs which is the 
 essential factor in respiration. 
 
92 NORMAL HISTOLOGY. 
 
 We have now to consider the structure of the walls of the alveolar 
 passages, infundibula, and air-vesicles. The gradual thinning which 
 we have observed in the walls of the bronchi as they approach their 
 terminations, is still more marked as we pass over into the alveolar 
 passages and infundibula. Here the walls consist of little else than a 
 thin, delicately striated, membranous, basement substance, in which 
 numerous elastic fibres ramify and a few smooth muscle-cells are em- 
 bedded, the whole being lined with flattened cells. At the openings 
 of the infundibula and air-vesicles the elastic fibres are grouped to 
 form a projecting ring, which bounds the opening. From these rings 
 of elastic fibres which surround the openings into the alveoli, other 
 elastic fibres are given off, which, dividing dichotomously, stretch over 
 the walls of the air-vesicles in the form of a wide-meshed net. the 
 spaces between the fibres being occupied by an extremely thin, struc- 
 tureless membrane, in which lies an occasional oval nucleus. The al- 
 veoli, in the adult, are lined with a single layer of flattened, polygonal, 
 epithelial cells. These are of two kinds : first, small, granular, nucle- 
 ated cells ; and second, cells which are larger, more irregular in 
 form, very thin and transparent, and usually without nuclei. The 
 relative proportion of these two kinds of cells is variable, and their 
 outlines, especially those of the larger cells, it is very difficult to see 
 without resorting to silver-staining. 
 
 In the fcetus the air-vesicles are lined at first with a distinct layer 
 of cylindrical or cuboidal epithelial cells, which become gradually 
 flattened, and when respiration is established at birth assume the form 
 of a layer of very thin polygonal cells ; these change their character 
 as the animal matures, until in adult life we have the forms above 
 described. 
 
 Just beneath the epithelial cells, and separated by them alone 
 from the air within the vesicles, lies the rich blood-capillary net-work, 
 which, in bulk as in importance, is the most essential element in the 
 walls of the vesicles. The lungs are supplied with blood through the 
 pulmonary and the bronchial arteries. The blood from the latter is 
 chiefly distributed to the walls of the bronchi and larger blood-vessels, 
 and to the connective tissue of the lungs. A large part of the blood 
 from the bronchial arteries, after passing through various sets of 
 capillaries, returns through the bronchial veins ; but a certain portion 
 of it finds its way into the pulmonary veins ; indeed, the two sets of 
 vessels seem to be in communication in various parts of the lungs. 
 The pulmonary artery, following the course of the bronchi, divides 
 and subdivides, until on reaching the lobules the small trunks break 
 up into smaller branchlets, which pass along the alveolar passages 
 and in the interlobular connective tissue to the air-vesicles, where they 
 break up into the rich capillary net- work which is spread over their 
 walls. The capillaries wind over the free edges of the alveolar walls, 
 and often project somewhat into their cavities. The net-work on the 
 alveolar walls is quite dense, and the vessels are very broad, leaving 
 only small oval or rounded spaces between them. A single net-work 
 frequently supplies the walls of adjacent vesicles, which in such cases' 
 are merged into one. The blood passes from the vesicles into the 
 
THE RESPIRATORY APPARATUS. 93 
 
 pulmonary veins in the interlobular connective tissue, and then into 
 larger trunks which, passing inward, follow the course of the other 
 large vessels. The epithelial cells of the alveoli often contain brown 
 or black pigment, and pigment deposits are of the most frequent oc- 
 currence in the adult lung, in the interlobular connective tissue, and 
 in the connective tissue and the lymph-nodes at the base of the lungs. 
 
 PRACTICAL STUDY. 
 
 Trachea. — A portion of the trachea is placed for ten days in a one- 
 sixth per cent, solution of chromic acid, washed, and the hardening 
 completed in alcohol. A small bit is cut out and embedded in wax 
 or hardened liver — care being taken not to scrape off the epithelial 
 cells in the manipulations — longitudinal and transverse sections made, 
 stained with haematoxylin and eosin, and mounted in balsam. 
 
 Uninjccted Lung. — The lungs from the human subject, or any 
 small animal, such as the dog, rabbit, or cat, having been carefully 
 removed, a canula is tied into the trachea or one of the large bronchi, 
 if one lung only is to be prepared, and the organ is distended by 
 pouring a one-sixth per cent, solution of chromic acid into the canula, 
 which, for this purpose, may be connected by a rubber tube with a 
 funnel. When the lung is filled and its surface tense, the trachea or 
 bronchus is tied, and the entire organ immersed in the same fluid with 
 which its alveoli are filled. After a couple of days the organ may be 
 cut in pieces and put into a stronger solution of the chromic acid, 
 one-fifth to one-fourth per cent. In three or four days they are 
 washed and transferred at first to dilute, and then to strong alcohol. 
 A small piece, not more than a cubic centimetre in size, in which one 
 or more small bronchi are seen, is cut out and embedded in cacoa 
 butter in the following way : the bit of tissue is transferred from the 
 alcohol to a small dish of oil of cloves, which in a short time will 
 penetrate the tissue and render it transparent. It is then placed in 
 melted cacoa butter whose temperature is kept just above its melting- 
 point on a water-bath, and allowed to remain for a couple of hours. 
 The bit of lung is then put into a little paper box, made for the pur- 
 pose from writing-paper, in such a position that transverse sections of 
 some of the small bronchi can be conveniently made, and the melted 
 cacao butter poured in around it. It is set in a cool place, and, when 
 hard, sections are made in the usual way in alcohol. By this pro- 
 cedure the air-vesicles and all the interstices of the tissue are filled 
 with a solid material of excellent consistence for cutting, and the 
 thinnest of sections can be made from the most delicate of tissues 
 when thus prepared. 
 
 In order to remove the cacao butter the sections are placed in oil 
 of cloves and gently warmed, when the fat will readily dissolve and 
 the sections again become transparent. They are then transferred to al- 
 cohol, stained with haematoxylin and eosin, and mounted in glycerine. 
 They may be permanently preserved in glycerine or mounted in 
 Canada balsam. 
 
 Epithelium of the Air-vesicles. — Ten grammes of gelatine are 
 
94 NORMAL HISTOLOGY. 
 
 added to 80 c.c. of pure water, and after soaking for half an hour, 
 melted on a water-bath ; two decigrammes of nitrate of silver are 
 dissolved in a few drops of warm water and mixed with the warm 
 gelatine. A canula is now tied into the trachea of the lungs of a 
 young animal and attached by a rubber tube to a warm glass funnel. 
 The warm gelatine-silver mixture is now poured into the funnel and 
 allowed to run into the lungs until they are well distended, the pres- 
 sure being regulated by the height at which the funnel is held above 
 the lungs. A ligature is now passed around the trachea below the 
 canula, and the latter removed. The lungs are set aside in a cool 
 place until the gelatine solidifies, when thin sections are made with a 
 razor moistened with w r ater, mounted at once in glycerine, and ex- 
 posed to the light until they become brown. They are studied with 
 high powers unstained, or are stained with dilute eosin. 
 
 Injected Lung. — From bits of lung whose blood-vessels have been 
 injected with the blue gelatine mixture through the pulmonary artery, 
 sections, which need not be very thin, are made, stained with carmine 
 or eosin, and mounted in balsam. 
 
CHAPTER XIV. 
 
 THE KIDNEY. 
 
 If a longitudinal section be made through the middle of a rabbit's 
 kidney from the convex border toward the hilus, the cut surface will 
 present a well-marked separation into an outer cortical and inner 
 medullary substance ; between these two are seen sections of large 
 blood-vessels. The medullary portion, called medullary pyramid or 
 medulla, and occupying the central part of the organ, terminates in 
 the pelvis in a single, short, rounded prolongation, called the papilla, 
 at the end of which, with a low magnifying power, several tiny open- 
 ings may be seen. The cut surface of the papilla presents a uniform 
 grayish appearance, while the segment of the medulla adjacent to the 
 cortex presents distinct bands or striae, radiating from the papilla and 
 extending, in the form of narrow, isolated, tapering rays, into the cor- 
 tex, almost to the surface of the organ ; these cortical rays are called 
 medullary rays. The cortex surrounding the medulla like a thick 
 shell, and covered by a firm, dense layer of connective tissue — the 
 capsule — consists, besides the medullary rays, of a grayish granular 
 substance lying in the form of elongated truncated pyramids between 
 the rays, and in a thin irregular layer of the same appearance directly 
 beneath the capsule. The grayish pyramidal portions of the cortex 
 are called cortical pyramids, and the entire substance of the cortex, 
 exclusive of the medullary rays, is sometimes called the labyrinth. If 
 the blood-vessels of the kidney have been injected with some colored 
 substance, or if the vessels are well filled with blood, tiny vessels can 
 be seen passing off from the large trunks which lie between the cortex 
 and medulla and extending radially toward the surface of the organ 
 through the center of the cortical pyramids, and at each side of these 
 vessels may be seen a row of minute globular structures, which are the 
 Malpighian bodies or glomeruli. 
 
 The human kidney differs from that which we have just described, 
 in that it consists of a number of just such structures crowded together 
 to form a single organ. In embryonic life its composite nature is evi- 
 dent, because each renculus — as each portion corresponding to the 
 rabbit's kidney is called — is separated from its neighbors by a certain 
 amount of connective tissue, giving the surface of the organ a lobu- 
 lated appearance. As the individual matures, the renculi usually 
 become merged into one another, so that we no longer see on the sur- 
 face any trace of its composite character. This is betrayed, however, 
 by the fact that the medullary pyramids of the primitive renculi per- 
 
g6 NORMAL HISTOLOGY. 
 
 sist as separate structures, and we thus have in the adult human kid- 
 ney just as many medullary pyramids and papillae as there were original 
 renculi. We find also, which is a striking feature in the adult human 
 kidney, that the cortical substance is not confined to the cortex, but 
 extends into the pelvis between the papillae, often as far as, and some- 
 times farther than the papillae themselves. Not very infrequently the 
 division between the primitive renculi are not entirely obliterated in 
 the process of development, and the surface of the kidney, even in the 
 adult, is distinctly lobulated. 
 
 Having thus acquainted ourselves with the general structure of the 
 kidney, we have now to study the elements of which it is composed, 
 and the way in which they are grouped to form the different parts 
 above described. The kidney is a tubular gland ; the innumerable 
 tubes of which it is mainly composed are lined with cells, differ greatly 
 in form and diameter in different parts, and run a very tortuous course 
 from their origin in the cortex to their termination in the tiny open- 
 ings, above mentioned, at the apex of the papillae. They constitute, 
 with the walls of the glomeruli, in which they originate, the paren- 
 chyma of the kidney. In addition to the parenchyma we have then to 
 study the connective tissue or interstitial tissue of the organ and the 
 blood-vessels. 
 
 The tubes of the kidney, called in general uriniferous tubules, con- 
 sist of an apparently homogeneous membrana propria lined throughout 
 with a single layer of epithelial cells, which differ greatly in character 
 and form in different parts of the tubules ; the tubules have received 
 special names in different parts of their course. Each tubule com- 
 mences within a cortical pyramid, in a glomerulus with whose capsule, 
 presently to be described, its membrana propria is continuous ; it is 
 narrow as it leaves the glomerulus, but broadens out at once into a 
 wide, convoluted canal — convoluted tubule — which winds about in the 
 pyramid, finally approaching a medullary ray; this it enters, and, sud- 
 denly becoming very narrow, descends more or less deeply into the 
 medulla; here, widening somewhat, it turns sharply on itself and 
 ascends into a medullary ray again. This portion of the tube, from 
 the point at which it becomes narrow and begins to descend in the 
 medullary ray, is called HenWs loop, and the arms of the loop are 
 called, following the course which the description has taken, the 
 descending or narrow, and the ascending or broad arms of Henle's 
 loop. The ascending arm of Henle's loop, on arriving in the cortex, 
 widens and enters a cortical pyramid, forming what is known as the 
 intercalated tubule ; this resembles the convoluted tubules, among 
 which it winds in and out in the cortical pyramids ; it is much shorter, 
 however, and passes over, entering a medullary ray into a straight 
 uriniferous tubule. This is again narrower than the convoluted tubules, 
 and passes directly downward through the medulla, joining other simi- 
 lar tubules dichotomously and becoming larger as it does so, until at 
 length it opens at the apex of a papilla. The uriniferous tubules run 
 an entirely independent course until, as straight tubules, they join one 
 another in twos to form the outlet ducts. 
 
 Vv r e have now to consider the epithelium which lines the tubules. 
 
THE KIDNEY. 97 
 
 Commencing at the straight tubules, we find that in their lower portion 
 they are lined by cylindrical cells with large nuclei and transparent 
 cell-bodies; farther up the cells become more nearly cuboidal, and 
 are often flattened in the medullary rays. The epithelium of the con- 
 voluted and intercalated tubules is nearly similar in character, and 
 consists of large, granular, nucleated cells nearly filling the tube, whose 
 outlines are not usually well defined, and which present when fresh or 
 well preserved a series of parallel striae in the protoplasm. By most 
 of the common modes of preparation, however, the cells look simply 
 like a granular mass in which nuclei are embedded, and which is often 
 divided by fissures into irregular-shaped pieces. The ascending arm 
 of Henle's loop is lined with pyramidal or low, cylindrical, granular 
 cells ; while the very narrow descending arm is lined with flat, trans- 
 parent cells, whose nuclei usually project into the lumen of the tube. 
 
 The glomeruli consist, in the first place, of a membranous, appar- 
 ently structureless capsule, similar to and continuous with the membrana 
 propria of the tubules. The epithelium of the convoluted tubules, as 
 the latter join the glomeruli, becomes flattened and continuous with a 
 layer of very thin transparent cells, which completely line the capsule. 
 At one side of the glomerulus, usually opposite to the attachment of 
 the tubule, a small artery — the vas afferens — pierces the capsule and 
 immediately divides into a number of capillary loops, which wind 
 about one another, forming a complicated vascular tuft ; the blood 
 from this tuft is collected into a vein — vas efferens — which is, as a rule, 
 somewhat smaller than the afferent artery, and leaves the glomerulus 
 near the point where the latter enters. The capillary tuft within the 
 glomerulus is covered also with a layer of flat cells like those lining the 
 capsule. 
 
 Let us now review the positions of the different parts of the tubules 
 in the kidney. In the papillae are the termini of the straight tubules ; 
 in the medulla lie the straight tubules and portions of Henle's loops ; 
 in the medullary rays are the upper portions of the straight tubules, 
 and of both arms of Henle's loops ; in the labyrinth, including the 
 cortical pyramids, are the convoluted tubules, intercalated tubes, and 
 the glomeruli. 
 
 The distribution of the blood in the kidney yet remains to be con- 
 sidered. The larger branches of the renal arteries, accompanied by 
 the veins, enter the organ at the bases of the medullary pyramids, and 
 divide into large arching trunks which pass in various directions, with 
 their convexity toward the cortex, along the irregular boundary line 
 between the cortex and medulla. From the convex side of the arte- 
 rial trunks spring numerous small branches, each of which enters at 
 once the apex of a cortical pyramid, and proceeds directly toward the 
 surface of the organ ; these arteries are called interlobular arteries* 
 
 * If we consider a circumscribed portion of the cortex of the kidney, having for its 
 centre a medullary ray, and extending on every side as far as to the nearest interlobu- 
 lar vessels, we see that in this limited area we have all of the essential structural ele- 
 ments of the cortex — a medullary ray surrounded by convoluted tubules and glome- 
 ruli. Such groups of elements, although they have by no means a separate existence, 
 and can be but indefinitely bounded, have been called lobuli ; and hence, the vessels 
 passing between them are termed interlobular vessels. 
 
 7 
 
98 NORMAL HISTOLOGY. 
 
 From these interlobular arteries lateral twigs are given off at frequent 
 intervals, which, after passing a short distance, enter the glomeruli as 
 the arteriae afferentes. On leaving the glomerulus, the efferent vein — 
 which still carries arterial blood — breaks up into a capillary net-work 
 which lies chiefly in the medullary rays, and whose meshes are narrow 
 and elongated, corresponding to the straight, narrow tubules, which it 
 supplies with blood. From this long-meshed net-work the blood 
 returns to the labyrinth again, where it is distributed in wider capilla- 
 ries through another net-work whose meshes are rounded, corresponding 
 to the convolutions of the tubules in this region. It is then collected 
 into small veins, which in turn pour it into interlobular veins, and 
 these, following the course of the interlobular arteries, finally pour it 
 into large, arched trunks at the base of the pyramids. Although, for 
 the most part, the course of the blood after leaving the glomeruli is as 
 above described, a certain portion of it in some cases enters the capil- 
 laries around the convoluted tubules at once, before or without passing 
 outward to a medullary ray. At the surface of the kidney, just beneath 
 the cortex, the smaller venous trunks may not infrequently be seen 
 centering in the commencement of an interlobular vein, thus forming 
 the well known Stellultz VerheyeniL The medulla receives its blood 
 in part from the vasa efferentes of the glomeruli, which lie near the 
 boundary, between cortex and medulla ; in greater quantity from the 
 large arterial arches. Vessels from the latter sources descend in 
 spreading tufts called vasa recta, between the tubules, and break up 
 into a long-meshed capillary net, the blood from which is collected in 
 part into a round-meshed venous net-work in the papillae, and returned 
 in straight veins along the tubules to the venous arches, and in part 
 passes directly back to the arches without going down to the papillae. 
 The capsule of the kidney receives its blood in part from the terminal 
 twiglets of the interlobular arteries, in part from terminal branches of 
 the phrenic, lumbar, and suprarenal arteries. 
 
 The connective tissue of the kidney, aside from that which forms 
 the capsule and pelvis, and is distributed along the walls of the larger 
 blood-vessels, or found in the walls of the blood-vessels themselves, 
 or in the tunicae propriae of the tubules, or the capsules of the glom- 
 eruli, is very small in quantity. It may, however, be demonstrated 
 here and there, and is most abundant in the vicinity of the papillae. 
 
 PRACTICAL STUDY. 
 
 Rabbit's Kid?iey — General View. — A fresh rabbit's kidney is cut 
 transversely across the middle, and placed in a two per cent, solution 
 of bichromate of potassium, where it remains for ten days ; it is then 
 washed and the hardening completed with alcohol. Thin sections 
 are made across the entire organ, including cortex, medulla, and 
 papilla, stained with haematoxylin and eosin, and mounted in Canada 
 balsam. 
 
 Isolated Tubules. — A small fragment of kidney, including both 
 cortical and medullary portions, is placed in a mixture of equal parts 
 of alcohol and strong hydrochloric acid ; the acid partially dissolves 
 
THE KIDNEY. 99 
 
 the intertubular tissue, so that after a time the tubules can easily be 
 pulled apart. This is usually effected in about twelve hours ; but the 
 specimen should be examined from time to time after it has been in 
 the acid for eight hours, and removed as soon as the object is accom- 
 plished. It is allowed to soak for a few hours in water to remove 
 the excess of acid, and then small bits are torn off, including cortical 
 and medullary substance, and the tubules carefully separated on a 
 slide. They are extremely brittle, and great care must be used in 
 their isolation : this may be done by needles, or by Rindfleisch's dis- 
 sociation method (see page 128) ; or better, by allowing small drops 
 of water to fall upon the specimen from a pipette. When the disso- 
 ciation is partially effected, a drop of picric acid should be added, 
 and the specimen mounted and preserved in it. 
 
 Sections of Uninfected Kidney. — A perfectly fresh kidney should 
 be cut into small pieces, and hardened in Muller's fluid and alcohol. 
 Sections are to be made in a plane vertical to the surface of the or- 
 gan, including the cortical and medullary portions, and also in a plane 
 parallel to the surface, through the cortical portion. The sections 
 are stained with hematoxylin, and mounted in glycerine ; or stained 
 with hematoxylin and eosin, and mounted in balsam. 
 
 Sections of Injected Kidney. — These should be made in the same 
 directions as the last, and mounted in balsam. 
 
CHAPTER XV. 
 
 THE GENERATIVE ORGANS. 
 
 MALE GENERATIVE ORGANS. 
 
 We shall confine our study of these organs to the testicle, prostate 
 gland, and parts of the penis. 
 
 The Testicle. 
 
 This is a tubular gland, whose chief specific secretion is the sper- 
 matozoa ; it is enclosed by a firm, dense connective-tissue capsule, 
 called the albuginea, which sends inward several incomplete septa, 
 which divide the organ into a number of communicating cavities, and 
 unite at its posterior superior part in a dense, wedge-shaped mass of 
 connective tissue, called the corpus Highmori. The more or less 
 conical cavities between the septa contain winding, anastomosing, and 
 looped tubules, called seminiferous tubules, which constitute the 
 secreting portion of the organ. As the tubules approach the corpus 
 Highmori, they become narrower and straighter — tubuli recti— and 
 are lined with cylindrical epithelium, and, on entering that body, form 
 a net-like series of intercommunicating channels lined with flattened 
 cells, called the rete testis. The channels of the rete testis are con- 
 tinuous with several tubules, which, passing upward and backward, 
 become very much convoluted, and form a number of conical masses 
 — coni vasculosi — which largely constitute the head of the epididymis. 
 The tubules of the coni vasculosi gradually unite as they descend to 
 form a single canal, which, with numerous windings and contortions, 
 constitutes the body and tail of the epididymis, and finally becomes 
 continuous with a straight, thick-walled, ascending tube — the vas 
 deferens. 
 
 The seminiferous tubules, which chiefly concern us here, lie em- 
 bedded in loose, delicate connective tissue, which is abundantly sup- 
 plied with blood-vessels, and contains many cells ; among the ordi- 
 nary connective-tissue cells of various forms, large granular, often 
 pigmented cells are not infrequently seen lying singly or in groups, 
 and sometimes in rows along the blood-vessels ; their nature and sig- 
 nificance are still doubtful. The tubules have a distinct membrana 
 propria, formed of several layers of thin, transparent cells, closely 
 packed together to form a laminated membrane ; they are lined with 
 several layers of cells piled irregularly over one another, which differ 
 in form under different circumstances, and sometimes in different 
 
THE GENERATIVE ORGANS. IOI 
 
 parts of the same gland. In a tubule which is not producing sperma- 
 tozoa, the outer row of cells — those lying upon the membrana propria 
 — are large, granular, well-defined, nucleated cells ; and upon these lie 
 two or three irregular layers of smaller nucleated cells with ill-defined 
 cell-bodies. Between the cells of the inner layers a peculiar, irregular 
 branching net-work is seen, which, by most recent observers, is be- 
 lieved to be formed, for the most part, by branches of the outer row 
 of cells ; and it is supposed that it serves as a loose frame-work, in 
 which the inner cells are supported. The cavity of these tubules may 
 be filled with a more or less granular fluid, or with detritus, or may 
 contain mature spermatozoa. 
 
 The mature spermatozoa differ in form in different animals ; in 
 man they consist of a flattened, pear-shaped portion, called the head, 
 the small end of which is directed forward ; and a delicate, tapering, 
 almost filiform portion, called the tail ; while between the head and 
 tail is a short, narrow segment, called the middle piece. When living, 
 and under favorable conditions, the spermatozoa are capable of per- 
 forming rapid movements, the whole organism being driven hither and 
 thither by wavy vibrations of the tail. 
 
 In tubules which are in full functional activity, the cavity is usually 
 filled with more or less granular material and mature and immature 
 spermatozoa, and on every side a multitude of clusters or bundles of 
 developing spermatozoa are seen, with the heads embedded among the 
 cells of the inner layers, and the tails stretching brush-like into the 
 cavity of the tube. According to the most recent observations, the 
 spermatozoa are formed from certain of the larger and smaller cells of 
 the inner layers, which are called spermatoblasts. The process of de- 
 velopment according to this view commences in the nuclei of the 
 spermatoblasts, which become converted into the body, while the tail 
 is an outgrowth from the nucleus, or is produced by a transformation 
 of a portion of the cell-protoplasm. Others believe that the sperma- 
 tozoa are formed by the growth inward, from the large outer cells, of a 
 long-stemmed bud-like process, whose dilated end divides into a num- 
 ber of longitudinal segments, each of which finally becomes a sperma- 
 tozoon. 
 
 PRACTICAL STUDY. 
 
 Living Spermatozoa. — A cut is made into the head of the epididy- 
 mis of the testicle of a recently killed buck rabbit or guinea-pig, and 
 a drop of the milky fluid which exudes is received on a slide and diluted 
 with one-half per cent, salt solution. The heads of the organisms in 
 the rabbit and guinea-pig are larger than in man, and ovoid; and not 
 infrequently in such a specimen immature forms may be seen. After 
 the movements have been sufficiently studied, water is allowed to run 
 under the cover-glass, when the movements will presently cease. 
 
 Sections of Testicle. — Small pieces of the human testicle, as fresh as 
 possible, or from the testicle of the cat or rabbit, are placed in two per 
 cent, solution of bichromate of potash, and left for ten days, the har- 
 dening being completed in alcohol. Sections from different parts are 
 stained with hematoxylin and eosin, and mounted in glycerine. 
 
102 NORMAL HISTOLOGY. 
 
 The Prostate Gland. 
 
 The prostate is a racemose gland, in which the alveoli, instead of 
 being more or less spheroidal, as is usually the case in racemose glands, 
 are often very much elongated and irregular in shape, and very fre- 
 quently present high, narrow, irregular folds in their walls. The alveoli 
 are lined for the most part with a single layer of cylindrical epithelium. 
 The excretory ducts are lined, within the gland, with cylindrical, which 
 pass over into flattened cells as they approach the urethral orifice. The 
 alveoli and ducts lie embedded in a dense mass of interlacing bundles 
 of smooth muscular tissue, intermingled with elastic fibres and a small 
 amount of fibrillar connective tissue. In the periphery of the gland 
 the muscular tissue forms a sort of capsule of varying thickness, which 
 in turn is enclosed in a fibrous tunic. 
 
 PRACTICAL STUDY. 
 
 Section of Gland. — A bit of that portion of the prostate gland 
 which lies behind the urethra is placed in two per cent, solution of 
 bichromate of potash, and after a week transferred to alcohol ; when 
 sufficiently hard, sections are made and stained with hematoxylin and 
 eosin, and mounted in balsam. 
 
 Urethra and Corpus Spongiosum. 
 
 The tissues which form the penis are so various, and present such 
 essential differences in their arrangement in different parts of the or- 
 gan, that a detailed study of its structure does not lie within the scope 
 of these lessons. Inasmuch, however, as the urethra is so frequently 
 the seat of surgical operations, and as the corpus spongiosum, which 
 surrounds a portion of it, presents us an example of a variety of tissue 
 which we shall have no opportunity to study elsewhere in these les- 
 sons, it is desirable to briefly consider their structure here. 
 
 The urethra, divided into three portions — a prostatic, a membranous, 
 and a spongy — consists of a mucous membrane which is surrounded 
 by a muscular sheath. The considerable differences in structure which 
 its different parts present are due chiefly to variations in the form of 
 the epithelium and the character and arrangement of the muscular tis- 
 sue. The mucosa is composed, in all parts alike, of fibrillar connec- 
 tive tissue containing numerous elastic fibres and richly furnished with 
 cells ; upon the free surface of this rests the epithelium, which at the 
 meatus and in the fossa navicularis is of the flat, laminated variety ; 
 this passes over into a single row of cylindrical cells which line the 
 spongy portion ; and this in turn merges into the spheroidal, pear- 
 shaped, prismatic and flattened cells of the prostatic portion, which, 
 lying in several layers, are quite similar in form and arrangement to 
 the epithelium of the bladder. Outside the mucosa, and but indis- 
 tinctly separated from it, is the submucosa, which consists of connec- 
 tive tissue with elastic fibres, and is especially characterized by a 
 dense net-work of veins which receive their blood from the rich capil- 
 
THE GENERATIVE ORGANS. IO3 
 
 lary system lying in the mucosa beneath the epithelium. This venous 
 net-work is so dense and the vessels so large as to lend to the sub- 
 mucosa, especially in the prostatic and membranous portions, the 
 character of erectile tissue (see below). Here and there, in the 
 spongy portion, larger and smaller irregular depressions, called 
 lacuna Aforgagni, are seen in the surface of the mucous membrane. 
 Racemose glands, called Littre 's glands, lie embedded in the mucous 
 membrane, sometimes extending into the muscular tunic ; their ex- 
 cretory ducts, sometimes short, sometimes long and tortuous, open on 
 the surface of the mucous membrane. In the prostatic portion the 
 prostatic and ejaculatory ducts pierce the mucous membrane, as do 
 the ducts of Cowper's glands that of the posterior segment of the 
 spongy portion. The muscular tunic of the urethra consists, in gen- 
 eral, of an inner longitudinal and an outer circular layer of smooth 
 muscle-cells, but varies greatly in structure in the different portions. 
 The outer layers of the posterior portion of the canal are formed in 
 part of striated muscle. The musculosa of the spongy portion con- 
 sists entirely of smooth muscle-cells and forms a complete circular 
 layer in the posterior region alone, while anteriorly only scattered 
 transversely and obliquely placed cells are found. 
 
 The urethra is enclosed through a part of its length in the corpus 
 spongiosum, which is largely composed of a kind of tissue called 
 erectile tissue; and before describing this bodv it will be well to con- 
 sider for a moment the nature of this kind of tissue. Erectile tissue 
 in certain cases consists simply of a somewhat circumscribed collec- 
 tion of larger and smaller veins, which under certain circumstances 
 may become distended with blood, thus causing the part in which 
 they lie to expand ; in other cases it consists of numerous larger and 
 smaller irregular communicating cavities, separated from one another 
 by broad or narrow interlacing bundles of connective tissue and 
 smooth muscular tissue, and in communication with arterial trunks 
 or capillary blood-vessels ; the cavities are lined with endothelium, 
 and, while usually containing but a small amount of blood, and in this 
 condition appearing simply as slits or narrow irregular spaces in the 
 tissue, they may, under certain conditions, become distended with 
 blood, when they assume the character of spheroidal or broad elon- 
 gated cavities, and thus cause a considerable increase in volume of the 
 part in which they are situated. The corpus spongiosum is composed 
 largely of erectile tissue of the character last described. 
 
 The corpus spongiosum is enclosed in a dense connective-tissue 
 sheath which contains elastic fibres and a few smooth muscle-cells. 
 From this a multitude of narrow trabecular or septa, composed of es- 
 sentially the same kinds of tissue, pass inward in various directions, 
 and dividing and subdividing, form a most intricate net-work, whose 
 meshes are blood-cavities and are lined with endothelium. This sys- 
 tem of trabecular and septa becomes continuous within with the sub- 
 mucosa of the urethra. This erectile tissue is most abundant below 
 the urethra ; above, the blood is collected into venous trunks, which 
 join similar trunks from the corpora cavernosa above, and convey the 
 blood away from the organ. 
 
104 NORMAL HISTOLOGY 
 
 PRACTICAL STUDY. 
 
 Transverse Sections. — The penis is to be divided transversely into 
 two or three pieces and put into two per cent, solution of bichromate 
 of potassium ; after a week it is transferred to alcohol. When the 
 hardening is complete, the corpus spongiosum, with the posterior por- 
 tion of the urethra, maybe dissected out and transverse sections made 
 from the different regions. They are stained with haematoxylin and 
 eosin, and mounted in balsam. 
 
 THE FEMALE GENERATIVE ORGANS. 
 
 The Ovary. 
 
 The ovary is a gland in which the glandular elements, in the form 
 of vesicular alveoli, have no excretory ducts, but discharge their con- 
 tents by rupture at the surface of the organ. These alveoli are called 
 Graafian follicles, and their specific secretion is the ovum. They lie 
 embedded in a connective-tissue stroma, the interstitial tissue of the 
 organ, and in the adult, are confined to the peripheral zone. We have 
 then, in studying the ovary, to consider the interstitial tissue with its 
 blood-vessels, and the glandular tissue or the Graafia?i follicles. 
 
 On looking at a thin transverse section of the ovary, we see that 
 it presents two indistinctly separated zones : an inner or central zone, 
 including the ramifications of the larger blood-vessels, and called the 
 medullary portio?i ; and an outer cortical zone, in which the follicles 
 lie. The interstitial tissue consists throughout the entire organ of 
 connective-tissue bundles, which in the medullary portion are some- 
 what loosely interwoven and associated with elastic fibres and smooth 
 muscle-cells. In the cortical zone the muscular elements fail, the 
 connective-tissue bundles are finer and cross and interlace in all 
 directions, surrounding the follicles, in whose vicinity they are modi- 
 fied so as to form a kind of capsule. Near the surface of the organ 
 the fibres arrange themselves in crossing layers to form a dense en- 
 closing sheath called the albuginea. The connective tissue of the 
 ovary, especially in the cortical zone, contains a great number of 
 spindle-shaped, flattened and irregular stellate branching cells, and 
 not infrequently larger and smaller pigmented cells. The surface of 
 the organ is covered by a single layer of cylindrical epithelium, whose 
 extreme significance we shall recognize when we study the develop- 
 ment of the Graafian follicles. The ovary has no serous covering, the 
 peritoneum being replaced by the epithelial cells. The interstitial 
 tissue of the ovary is very vascular ; the large arterial trunks, entering 
 from the broad ligament, divide and subdivide in the medullary por- 
 tion and pass off in larger and smaller twigs into the cortex, from 
 which an abundant capillary net-work is formed around the follicles. 
 The arteries frequently run a very tortuous course, twisting and turn- 
 ing upon themselves, and are characterized, moreover, by the great 
 abundance of smooth muscle-cells in their walls. Lymphatic vessels 
 
THE GENERATIVE ORGANS. 105 
 
 are abundant. Concerning the distribution of nerves, our knowledge 
 is still very meagre. 
 
 We have now to consider the structure of the Graafian follicles, to 
 which the just described structures are subservient. The Graafian 
 follicles in the ovary of the adult female, at the child-bearing age, 
 present by no means the same appearance : some are large and read- 
 ily visible to the naked eye ; others very minute, and presenting under 
 the microscope an entirely different structure. These differences in 
 structure are accounted for by the fact that certain of the follicles are 
 mature or approaching maturity, while others are still undergoing 
 development. 
 
 We will first consider the structure of those which are mature or 
 nearly so. The interstitial tissue of the ovary arranges itself in the 
 immediate vicinity of the follicle, in the form of a tolerably well-defined 
 wall, called the theca folliculi, in which it is easy to discover an external 
 denser layer and an internal layer very richly supplied with cells, and 
 containing an abundant capillary net-work. Within the theca folliculi 
 is a layer, usually several cells deep, of larger and smaller spheroidal, 
 or, in the periphery, cuboidal epithelium, called follicular epithelium, 
 and constituting the so-called membrana granulosa. At one or other 
 side the follicular epithelium is heaped up into a larger or smaller 
 mass, which projects into the cavity of the follicle and contains the 
 ovum ; this accumulation of cells is called the cumulus proligerus 
 or germ-hill. The remainder of the follicular cavity is filled with a 
 colorless fluid, in which not infrequently fine granules are sus- 
 pended. This fluid increases in quantity as the follicle approaches 
 maturity, and the pressure occasioned by its accumulation probably 
 conduces in no slight degree to the final rupture of the follicle. The 
 follicular epithelium immediately surrounding the ovum is cylindri- 
 cal and arranged radially about it. In the ovum itself, which is a cell 
 oi the most highly developed type, we recognize four structural ele- 
 ments : 1, a thick hyaline membrane, presenting, with high powers, a 
 delicate, radial striation and called the zona pellucida ; 2, within the zona 
 pellucida is the cell-body, consisting of coarsely and finely granular 
 protoplasm, and usually called the vitellus ; 3, the vitellus encloses a 
 comparatively large, vesicular, transparent, and sharply-outlined nu- 
 cleus, the germinal vesicle; 4, the germinal vesicle, which is usually 
 somewhat eccentrically placed, contains, in addition to a clear fluid, 
 a small, dark, often almost opaque nucleolus, the germinative spot. 
 
 As the Graafian follicles mature, they approach the surface of the 
 ovary, often projecting above it ; the walls become thinner and less 
 vascular at the projecting side, and finally burst at a menstrual period. 
 The ovum, with the fluid and a portion of the follicular epithelium, is 
 discharged, and through the hemorrhage which occurs from the capil- 
 laries in the follicular wall, the cavity is filled with blood. Changes 
 now occur in the walls and cavity of the follicle, which result sooner 
 or later in cicatrization and obliteration of the cavity. These changes 
 vary considerably, depending upon whether or not the ovum is im- 
 pregnated and develops ; and the difference expresses itself chiefly in 
 a difference in size and persistence of the mass of tissue called the 
 
106 NORMAL HISTOLOGY. 
 
 corpus luteum, which is produced by a growth of certain of the cells 
 which remain after the rupture of the follicle. These differences can- 
 not be considered here. The process of formation and disappearance 
 of the corpus luteum, under all circumstances, is essentially the follow- 
 ing : the blood which is poured out into the cavity passes through 
 the same retrogressive metamorphosis which extravasated blood in 
 any part of the body may undergo; it coagulates, the serum is ab- 
 sorbed, the red cells disintegrate, and the coloring matter is in part 
 taken up by surrounding tissues, in part transformed into yellowish or 
 red hematoidin crystals, which in turn may change into a dark brown 
 or black pigment, which may be taken up by surrounding tissues or 
 remain for a long time unchanged. Hand in hand with these changes 
 in the extravasated blood, go important changes in the follicular epi- 
 thelium which is left behind, and in the cells of the theca folliculi. 
 These cells proliferate and form a soft, yellowish, very vascular tissue, 
 resembling mucous tissue, which presently undergoes fatty degenera- 
 tion. This yellow mass, surrounding and enclosing the remains of the 
 extravasated blood, constitutes the corpus luteum ; and, as it disap- 
 pears, its place is occupied by firm, dense connective tissue, which 
 usually persists for a long time in the form of an irregular cicatrix, 
 whose cells not infrequently still contain yellow or brown or black pig- 
 ment. 
 
 In order to understand ail the forms which the immature Graafian 
 follicles present, it will be necessary to study the way in which these 
 structures originate. They are produced from the cylindrical epithe- 
 lium which covers the ovary, and is called germ epithelium. At an 
 early period of life the connective-tissue stroma of the ovary grows 
 rapidly, and partially encloses groups of the germinal epithelial cells, 
 which themselves proliferate and dip down into the stroma of the 
 organ, in the form of elongated, solid, or tubular masses. Presently 
 these groups of cells become separated from the germinal epithelium 
 at the surface by a still further growth of the connective tissue, and 
 appear then as irregular masses of variously shaped cells, some of 
 which are larger than the rest, enclosed on all sides by connective 
 tissue. A still further growth of the stroma separates these masses of 
 cells into smaller groups, which gradually sink farther from the surface 
 of the organ and become more widely separated from one another, 
 while new masses of germ-epithelium are being enclosed at the sur- 
 face. The cells which now lie in these separated cavities do not all 
 look alike, but we usually find that one of them is larger than the 
 others, is spheroidal, and occupies the centre of the cavity, while the 
 rest are more or less cuboidal and arranged in a single layer around 
 the wall, closely enclosing the central cell. This is the young Graafian 
 folhcle, and the central cell is the ovum ; but it presents as yet no 
 zona pellucida. Gradually the follicular epithelium increases in quan- 
 tity, forming several layers, and the ovum becomes eccentrically 
 placed. There is still no cavity, but this soon appears, at first as a 
 slit between the cells, and then grows wider and wider as fluid accu- 
 mulates "within it. Changes in the interstitial tissue lead to the for- 
 mation of the theca folliculi, the epithelium directly about the ovum 
 
THE GENERATIVE ORGANS. 107 
 
 assumes a radial arrangement, the zona pellucida is formed, and we 
 thus have the structure of the maturing follicle, with which we are 
 already acquainted. 
 
 PRACTICAL STUDY. 
 
 Sections of Adult Ovary. — The human ovary, or that of a recently 
 killed animal, should be divided transversely, great care being taken 
 not to rub the surface, since the germ-epithelium easily comes off, and 
 placed in a mixture of equal parts of one-quarter per cent, chromic 
 acid solution and alcohol. After a week it is to be transferred to 
 alcohol, in which in a few days it will become sufficiently hard. A 
 half is embedded in wax, the sections stained with hematoxylin and 
 eosin, and mounted in glycerine. 
 
 Sections of Developing Ovary. — The ovary of a foetal or new-born 
 animal is hardened and prepared as above, especial care being taken 
 not to rub off the germ-epithelium, since the chief purpose of this 
 preparation is to show the formation of the follicles. 
 
 The Uterus. 
 
 The walls of the uterus consist of crossing and interlacing bundles 
 of smooth muscle-cells, with a small amount of connective tissue en- 
 closing them and binding them together. The muscular tissue is 
 arranged in three ill-defined layers, of which the middle is the thicker. 
 It may be said, in general, that the bundles of the inner layer run 
 transversely around the organ, those of the middle layer are longitu- 
 tudinal, while those of the outer layer are quite irregular ; still in all 
 the layers there is great lack of uniformity in the direction of the cells. 
 A part of the external surface of the uterus is covered by the peritoneum, 
 while its inner surface is lined with a mucous membrane. The latter 
 consists of a frame-work formed of a delicate net-work of fibres, between 
 which lie a great number of spheroidal, fusiform, and branched cells, 
 and covered on the free surface by cylindrical ciliated cells. In the 
 mucosa the simple or branched tubular uterine glands are embedded ; 
 they are often tortuous, and, like the surface of the mucous membrane, 
 are lined with cylindrical ciliated epithelium. The surface of the mu- 
 cous membrane of the body of the uterus is smooth, but in the cervix 
 it presents regular folds, the so-called pliccz palmatce ; the connective- 
 tissue frame-work of the cervical mucous membrane is, moreover, 
 firmer in texture, contains fewer glands, and these, for the most part, 
 are more or less globular and lined with short cylindrical or cuboidal 
 epithelium. The cylindrical ciliated epithelium of the body extends 
 over on to the mucous membrane of the cervix, where it becomes 
 continuous with the laminated epithelium covering the lower portion 
 of the canal and the portio vaginalis. The uterus is a very vascular 
 organ ; the mucous membrane is supplied with a rich capillary plexus, 
 which passes inward close beneath the surface-epithelium. 
 
 During menstruation the mucous membrane becomes thickened — 
 partly owing to the engorgement of its blood-vessels, and partly to 
 the accumulation of fluid and lymph-cells in its interstices. The 
 uterine glands are enlarged, and their epithelium, as well as that of the 
 
108 NORMAL HISTOLOGY. 
 
 general surface of the cavity, is swollen. To what extent the blood, 
 which, mixed with mucous and separated epithelium, fills the cavity at 
 the menstrual period, is due to the rupture of the engorged capillaries, 
 and to what extent to diapedesis, is not yet definitely determined. 
 
 PRACTICAL STUDY. 
 
 Sections of Human Uterus. — The human uterus is cut into several 
 pieces and placed in two per cent, solution of bichromate of potassium, 
 and after ten days transferred to alcohol. The organ should be pro- 
 cured as soon as possible after death, since the cilia of the lining 
 epithelium are easily destroyed by the decomposition which commences 
 very early in the uterine cavity. Sections perpendicular to the surface 
 of the mucous membrane are made from the cervix ; and in the body, 
 from the lower portions or from the vicinity of the entrance of the 
 Fallopian tubes, since here the uterine glands are more uniformly 
 arranged at right angles to the surface. Sections are stained with 
 haematoxylin and eosin, and mounted in Canada balsam. 
 
 The Vagina. 
 
 In the walls of the vagina we recognize three layers : an outer 
 fibrous layer, a middle muscular layer, and a lining mucous membrane. 
 
 The fibrous layer, by means of which the vagina is connected with 
 adjacent parts, consists of connective tissue with elastic fibres, and in 
 texture is looser in its outer, denser in its inner portions. The mus- 
 cular layer consists of bundles of smooth muscle-cells which present 
 an indistinct grouping into an outer portion, formed of longitudinally 
 arranged cells, and an inner, in which they have in general a transverse 
 arrangement. The mucous membrane consists of delicate connective 
 tissue, loose in texture and containing coarser and finer elastic fibres ; 
 it presents numerous transverse folds and elevations and is covered by 
 laminated epithelium, the cells in the lower layers being more or less 
 spheroidal, but flat and scale-like at the surface. Numerous papillae 
 project into the epithelium. Venous plexuses are formed within the 
 deeper portions of the mucous membrane which give to some parts 
 of that structure the character of erectile tissue. 
 
 Glands have not been satisfactorily demonstrated in the vaginal 
 mucous membrane. 
 
 PRACTICAL STUDY. 
 
 Transverse Sections. — A bit of the vaginal wall should be stretched 
 on cork and placed in a mixture of equal parts of one-fourth per cent, 
 solution of chromic acid and alcohol, and after five days transferred 
 to alcohol in which the hardening is completed. Sections at right 
 angles to the surface are stained with haematoxylin, and mounted in 
 glycerine. 
 
 Isolated Surface-cells. — From a bit of vagina hardened as above, 
 or simply in alcohol, the cells from the surface of the mucous mem- 
 brane are scraped with a scalpel, and stained on a slide with haematoxylin 
 and mounted in glycerine. 
 
THE GENERATIVE ORGANS. 109 
 
 Mammary Gland. 
 
 The mammary gland is a racemose, and when fully developed a 
 lobulated gland, whose spheroidal alveoli are formed by a membrana 
 propria composed of flattened cells, and lined with cuboidal epithe- 
 lium. The excretory ducts, which in each gland are fifteen to twenty 
 in number, and open at the surface of the nipple, are lined with cylin- 
 drical epithelium. Fibrillar connective tissue with elastic fibres lies 
 between the alveoli and lobules, and is abundantly furnished with 
 blood-vessels. The gland presents, both macroscopically and micro- 
 scopically, quite marked differences in appearance at different times, 
 depending upon age, sex, and, in the adult female, upon whether or 
 not the individual is pregnant, and upon the period of pregnancy and 
 the occurrence of lactation. 
 
 In children, as in the adult male, the gland presents, in general, a 
 system of ramifying ducts, terminating in blind or in more or less 
 pouched or dilated extremities ; these lie embedded in connective tis- 
 sue, which makes up the greater part of the bulk of the gland. At 
 puberty the gland of the female undergoes considerable development : 
 well-defined alveoli are found in the periphery of the gland connected 
 with the excretory ducts, but the interstitial tissue is still more abun- 
 dant than the gland-tissue proper. Essentially in this condition the 
 gland remains until the climacteric period if pregnancy does not occur. 
 If however, the individual becomes pregnant, the number and size of 
 the alveoli rapidly increase ; the ducts, which in the virgin terminate 
 only in dilated extremities, become connected with extensive groups 
 of newly-formed alveoli; the gland assumes a lobulated character, the 
 connective tissue between the alveoli is very much less abundant in 
 proportion to the gland-tissue, is looser in texture and contains a great 
 number of larger and smaller cells, and frequently fat-cells. During 
 lactation the alveoli are very large, their epithelium contains fat-drop- 
 lets in considerable number and fat is found in greater or less quan- 
 tity in the cavities of the alveoli and in the excretory ducts. When 
 the secretory activity of the gland ceases, it undergoes involution, the 
 alveoli become smaller, fat ceases to be produced by the cells, and the 
 interstitial connective tissue becomes proportionally more abundant. 
 
 With the decline of the reproductive power the mammary gland 
 undergoes a marked and permanent involution : the terminal alveoli 
 disappear, the smaller ducts are obliterated, and finally little is left 
 of the organ excepting the deformed and collapsed larger ducts and a 
 mass of connective tissue. 
 
 PRACTICAL STUDY. 
 
 Sections of Hardened Gland. — Portions of the gland from the hu- 
 man subject or from some of the lower mammalia, preferably in a 
 condition of functional activity, should be hardened in bichromate of 
 potassium and alcohol, in the usual way, and the sections stained with 
 hematoxylin and eosin, and mounted in glycerine. 
 
CHAPTER XVI. 
 
 THE CENTRAL NERVOUS SYSTEM. 
 
 The Spinal Cord. 
 
 The spinal cord contains nerve-ceils and nerve-fibres, the former con- 
 fined to the central, the latter most abundant in the peripheral por- 
 tions. These nerve-elements lie in the meshes of peculiarly arranged 
 connective tissue in which blood-vessels ramify, and the whole is 
 surrounded by a vascular connective-tissue membrane — the pia mater 
 spinalis. On its anterior and posterior surfaces, narrow fissures, reach- 
 ing nearly to the centre, divide the cord into lateral halves ; into these 
 fissures, called the anterior and posterior longitudinal fissures, the pia 
 mater sends a membranous prolongation, the anterior fissure is com- 
 plete, the pia being found on both of its sices, which can be easily 
 separated ; while in the posterior fissure the sides are bound together 
 by the pia. In transverse sections of the cord a central gray portion 
 is seen which is surrounded by an irregular white zone. The form 
 which the gray matter assumes differs considerably in different parts 
 of the cord, but has in general the form of an unsymmetrical H ; the 
 cross-arm of the H is formed in, great part, by the nerve-substance 
 which connects the lateral halves of the cord, and is called the gray 
 commissure ; the uprights of the H lie completely embedded within 
 the white matter of the lateral halves ; the anterior and broader ends 
 being called the anterior cornua, the posterior and narrow ends the 
 posterior cornua. Within the gray commissure, and separating it into 
 an anterior and a posterior portion, a narrow canal, called the central 
 canal, runs the entire length of the cord : it is lined, in early life at 
 least, with cylindrical ciliated cells, and is surrounded by delicate con- 
 nective tissue. From the anterior and posterior cornua the spinal 
 nerves pass off, dividing the white matter into three tolerably distinct 
 portions called the anterior, lateral, and posterior columns. 
 
 In the white substance of the cord we find nerve-fibres and con- 
 nective tissue and blood- and lymph-vessels. The fibres are for the 
 most part, medullated, but have not. so far as we can determine with 
 the technical facilities at present at our disposal, any neurilemma. 
 They vary greatly in diameter, and although a large majority of them 
 run longitudinally for the greater part of their course, we find in each 
 transverse section a considerable number which run in an oblique or 
 horizontal direction. Thus, we find in front of the anterior gray com- 
 missure, at the bottom of the anterior fissure, a band of horizontally 
 arranged fibres running from one side to the other, called the white 
 
THE CENTRAL NERVOUS SYSTEM. Ill 
 
 commissure. The fibres which pass out of the gray matter to form the 
 roots of the spinal nerves, take also longitudinal and oblique courses. 
 The connective tissue of the white substance, is intimately con- 
 nected at the periphery, with the pia-mater. The inner layers of the 
 latter, consisting of delicate nbrillated and a few elastic fibres, send into 
 the cord numerous branching septa, which divide the space occupied by 
 the nerve-substance into irregular spaces, in which the fibres run, each 
 one being surrounded by a delicate net-work of fibrillae, upon which, 
 here and there, variously shaped, for the most part flattened cells, are 
 placed. This supporting connective tissue, which was formerly be- 
 lieved to be composed of branching cells and closely allied to reticu- 
 lar connective tissue, is called neuroglia ; and in addition to the 
 fibrillar elements above described there seems to be a greater or less 
 amount of a soft, homogeneous or finely granular material lying in the 
 interstices of the nerve and connective-tissue fibres ; but its nature is 
 not yet definitely determined. The amount of connective tissue in 
 the white substance of the cord relative to the nerve-fibres varies 
 considerably in the different columns ; in the posterior column for 
 example, in certain parts of the cord, the connective tissue is very 
 abundant, separating from adjacent parts the so-called cuneiform col- 
 umns or columns of Goll. 
 
 In the gray matter of the cord we have also nerve and connective- 
 tissue elements and blood- and lymph-vessels, the first consisting of 
 ganglion-cells and nerve-fibres. The nerve-fibres are in part medullated, 
 in part naked axis cylinders ; and besides these, a multitude of extremely 
 delicate gray nerve-fibrils occur, which seem partly to come from the 
 breaking up of axis cylinders, and partly to be the delicate branching 
 processes of ganglion-cells. The nerve-cells of the gray matter are, for 
 the most part, multipolar, and vary greatly in size, the largest being 
 found, as a rule, in the anterior cornua. They are arranged in irregular 
 groups in different parts of the cord. The connective-tissue elements 
 of the gray matter consist of a delicate frame-work, similar, in most re- 
 spects, to that supporting the nerve-fibres between the coarser septa of 
 the white substance. In the hinder portions of the posterior cornua 
 there is a circumscribed area, which, in the fresh cord, has a peculiar 
 gelatinous appearance, and is called the substantia gelatinosa of 
 Rolando ; in this we find comparatively few nerve-elements and much 
 connective tissue, containing larger and smaller variously shaped 
 cells. The limitations of these lessons will not permit us to consider 
 more in detail what is known of the course of the nerve-fibres through 
 the cord, and their more exact relations to the ganglion-cells. The 
 blood-vessels enter the cord from the pia, along the septa, which the 
 latter sends into the organ, and ramify in its substance, the capillary 
 plexuses being denser in the gray than in the white matter. 
 
 TRACTICAL STUDY. 
 
 Transverse Sections. — A perfectly fresh human cord should be 
 freed from its dura mater, short segments taken from different portions 
 of it, and hardened in Miiller's fluid and alcohol. Thin transverse 
 
112 NORMAL HISTOLOGY. 
 
 sections are made through the entire cord in different portions, and 
 may be stained first, lightly with hematoxylin, and then with eosin or 
 carmine, and mounted in balsam. A very excellent hardening agent 
 for the nerve-centres is the bichromate of ammonium, in which small 
 fragments should lie for a week, then be carefully washed, and the 
 hardening completed in alcohol. They may be mounted as before, 
 or they may be stained first intensely with aniline green — lying for 
 twenty-four hours in a very deeply colored aqueous solution — and 
 then passed through alcohol deeply tinged with eosin, and mounted 
 in balsam. Alcohol dissolves the green color out of the sections 
 quite rapidly, and, if allowed to remain in it too long, it will entirely 
 disappear. 
 
 The Brain. 
 
 In this organ also we have gray and white matter, but they are 
 arranged in a much more complicated manner than in the spinal cord. 
 The collections of gray matter in the brain may be arranged, in a 
 general way, in four distinct groups : first, as a direct continuation of 
 the gray matter of the cord, the gray matter investing the ventricles ; 
 second, the gray matter in the cerebral ganglia at the base of the 
 brain ; third, the gray matter in the cortex of the cerebrum ; fourth, 
 the gray matter in the cerebellum and immediately connected with it. 
 These collections of gray matter are variously associated with one 
 another by means of the nerve-fibres of the white substance. The 
 white substance of both cerebrum and cerebellum consists of coarser 
 and finer, but all very small, nerve-fibres, running in various direc- 
 tions, and supported by a delicate connective-tissue frame-work, simi- 
 lar to that of the cord. The gray matter consists here, as in the cord, 
 of ganglion-cells and fine gray fibres, supported by connective tissue. 
 In parts of the gray as of the white substance, certain cellular and 
 fibrous elements occur, of which it is at present impossible to say 
 whether they are connective or nerve tissue. Indeed, it is the diffi- 
 culty of determining the nature of certain structures in the brain, 
 together with their extreme delicacy and the difficulty of isolating 
 them, which renders the histology of the brain so difficult a theme, 
 and explains the unsatisfactory state of our knowledge concerning it. 
 
 We cannot do more in these lessons than to study briefly the 
 structure of two of the best known and at present, perhaps most inter- 
 esting parts of the brain — namely, the cortical portions of the cere- 
 brum and cerebellum. 
 
 a. Cortex of the Cerebrum. — The structure of this part of the brain 
 differs somewhat in different regions, but that of one of the frontal lobes 
 is sufficiently typical for our purpose. Here, in a section perpendicular 
 to the surface, and extending through the entire depth of the gray 
 matter, five zones or layers may be recognized, which, however, merge 
 gradually into one another. In the most superficial layer, the con- 
 nective-tissue elements preponderate, and among them, delicate 
 nerve-fibrils interlace, and a few small, scattered, globular, and elon- 
 gated nerve-cells are found; the second layer is characterized by a 
 
THE CENTRAL NERVOUS SYSTEM., 113 
 
 great number of small more or less pyramidal cells ; the third and 
 broadest layer contains a proportionately smaller number of ganglion- 
 cells than the second, but they are larger and, for the most part, 
 pyramidal, or broad, spindle-shaped, with their long axes perpendicu- 
 lar to the surface of the cortex ; in the fourth layer, which is much 
 narrower than the last, are large numbers of small, globular and 
 irregular-shaped and branching cells ; the fifth layer, finally, contains 
 medium-sized, spindle-shaped cells, with long, tapering processes, to- 
 gether with a certain number of smaller, irregular-shaped cells. In 
 the third layer, certain of the delicate nerve-fibres begin to take a 
 more regular course toward the white matter ; and in the fourth and 
 fifth layers, they are readily seen in distinct bundles passing inward 
 between the ganglion-cells. 
 
 b. Cortex of the Cerebellum. — In sections through the cortex of the 
 cerebellum perpendicular to the surface, three distinct layers are rec- 
 ognizable ; the outer, sometimes called the molecular layer, consists, 
 like the outer layer of the cerebral cortex, of a delicate connective- 
 tissue frame-work, which supports fine nerve-fibres and small spindle- 
 shaped and branching nerve-cells ; the middle, cellular layer, is char- 
 acterized by a row of large ganglion cells, Purkinje's cells, whose 
 branching processes extend into and ramify in the outer layer, while 
 the axis-cylinder process (see page 62) passes inward through the 
 inner layer ; the inner granular layer contains a great number of 
 small spheroidal cells whose nature is undetermined. The granular 
 layer merges gradually into the white substance and is thickest at the 
 summit of the convolutions, where also Purkenje's cells are most 
 abundant. The blood-vessels penetrate the cortex of both cerebrum 
 and cerebellum, in the form of small arterial twigs from the pia, and 
 form an abundant net-work in the gray substance, being somewhat 
 differently distributed in different parts of the organ. 
 
 The dura mater of the brain is a dense connective-tissue mem- 
 brane containing numerous elastic fibres and lined within by endothe- 
 lial cells. Where it is attached to the bones of the skull, to whose 
 inner surface it acts as periosteum, the tissue on the attached surface 
 is looser in texture and abundantly supplied with blood-vessels. It 
 contains the ordinary flattened connective-tissue cells and usually a 
 certain number of large granular " plasma " cells. The dura mater 
 of the cord does not form the periosteum of the bones forming the 
 spinal canal, and hence in it the looser vascular layer is for the most 
 part wanting. 
 
 The Pia Mater of the brain is a thin connective-tissue membrane, 
 covered on its outer surface with endothelium and containing an ex- 
 ceedingly abundant net-work of blood- and lymph-vessels. Over the 
 surface of the convolutions it forms a single membrane, but as it passes 
 over the sulci it divides into an inner vascular layer which dips down 
 into the sulci, and an outer layer containing but few vessels, which 
 bridges over the space between adjacent convolutions. The cavities or 
 spaces thus formed between the two layers of the pia — called subarach- 
 noidal spaces — are lined with endothelium, and are lymph-sinuses ; 
 and these, as well as the numerous lymph-channels of the pia, are in 
 8 
 
114 NORMAL HISTOLOGY. 
 
 communication with lymph-channels called perivascular lymph-chan- 
 7iels, which ensheath certain of the blood-vessels as they enter the brain 
 substance. 
 
 PRACTICAL STUDY. 
 
 Sections of Cortex of Cerebrum and Cerebellum. — These should 
 be prepared in the same way as the spinal cord ; pieces from the cor- 
 tex of the frontal lobes being taken, and from the cortex of the cere- 
 bellum at any convenient part ; they should not be larger than a cubic 
 centimetre, and the chromic fluid well washed out before immersion in 
 alcohol. 
 
 Dura Mater. — A bit of this membrane should be stretched on a 
 piece of cork with pins, hardened in alcohol, embedded in hardened 
 liver, and thin transverse sections made, stained with hematoxylin and 
 mounted in glycerine. 
 
 Pia Mater. — A small piece of the pia should be stripped carefully 
 from the brain, care being taken that none of the brain substance ad- 
 heres, stretched on a cork, hardened in alcohol, stained with carmine 
 or picro-carmine, and mounted in glycerine. For the purpose of study- 
 ing the endothelium, a small bit of the membrane is placed for forty- 
 eight hours in dilute alcohol — alcohol, i ; water, 2 ; — and the surface 
 then gently scraped with a scalpel ; a small portion of the separated 
 cellular mass is placed on a slide in a drop of glycerine which has 
 been colored distinctly red with eosin and covered and studied in the 
 same. 
 
CHAPTER XVII. 
 
 THE SKIN. 
 
 We recognize in the skin three layers of tissue : i, an cuter, epithe- 
 lial layer, the epidermis; beneath this, 2, a layer of quite firm and 
 dense connective tissue, the corium — true skin, cutis vera, or derma ; 
 3, a layer of connective tissue, the subcutaneous tissue, which, merging 
 into the corium serves to bind it to underlying parts. The skin is 
 variously modified in different parts of the body, to meet the different 
 conditions of exposure and wear to which it is subjected, and to form 
 certain supplementary structures such as the hair, nails, etc.; and con- 
 tains various sensory and secretory structures. In the epidermis we 
 recognize two tolerably distinct layers : 1, an outer or horny layer 
 consisting of very thin, flat, transparent, tough, scale-like cells, which 
 present for the most part no nuclei, and are packed closely together 
 side to side ; and 2, an inner layer, the so-called mucous or Malpighian 
 layer, consisting of larger and smaller nucleated cells of varying shape 
 and character. In the deeper portion, adjoining the corium, the cells 
 are more or less cylindrical ; above this they are spheroidal or ovoidal, 
 or considerably elongated ; still nearer the surface they become flat- 
 tened, and finally merge into the thin cells of the horny layer. In the 
 middle zone, where the cells are becoming elongated and flattened, 
 they present a peculiar jagged outline, looking as if they were bor- 
 dered by short delicate spines by which the cells seem to be dove- 
 tailed together. These spined cells are very characteristic of this 
 part of the epidermis, and are also found in certain other parts of the 
 body where laminated epithelium occurs, as in the vagina, mucous 
 membrane of the mouth, etc. The relative thickness of the horny 
 and mucous layers of the epidermis differs greatly in different parts of 
 the body ; in some parts of the palms of the hands and soles of the 
 feet, the horny layer is very thick, and here we often find that the cells 
 which lie between the horny and mucous layers form a distinct, narrow, 
 transparent zone, called the stratum lucidum. 
 
 The deeper cells of the mucous layer contain, uniformly in the ne- 
 gro, and occasionally in circumscribed regions in white men, more or 
 less brown or black pigment. The epidermis forms in but few regions 
 of the body a layer of uniform thickness, since the corium sends up 
 into it, at varying intervals, simple or branching variously shaped 
 fiapiilce, the valleys between which, as well as their summits, being 
 covered by the cells of the mucous layer. If we imagine a section 
 
I 1 6 NORMAL HISTOLOGY. 
 
 made through the skin parallel with its surface, and just deep enough 
 to touch the tops of the papillae, the cells of the mucous layer which 
 lie between the latter would appear, on looking at the cut surface, to 
 be arranged in the form of a net-work whose meshes are filled by the 
 papilla? of the corium. Hence it is that these collections of cells have 
 received the name rete Malpighi. The corium is formed of interla- 
 cing bundles of connective tissue, which are coarser in the deeper, 
 finer in the more superficial portions, where they extend into the epi- 
 dermis, forming the papilla?. Elastic fibres are present in considerable 
 number, and in the interstices between the fibres lie flattened, spin- 
 dle-shaped, branching and small spheroidal cells. In addition to these 
 elements we find muscular tissue in the corium ; striated muscular 
 fibres occur in certain parts of the skin of the face ; and smooth 
 muscular tissue, aside from that belonging to the hair-follicle, is found 
 about the sweat-glands, in the skin of the scrotum and penis, and in 
 the nipple and its areola. The subcutaneous connective tissue we 
 have already studied when considering the connective tissue in detail. 
 In some parts of the skin the texture of the subcutaneous tissue is so 
 loose that the corium and epidermis can be readily moved to and fro 
 upon the underlying parts or pinched up in folds ; in others its fibres 
 are short and tense, and bind the corium closely to the parts beneath. 
 In the subcutaneous tissue of most parts of the body, greater or 
 smaller deposits of fat occur, forming the panniculus adiposus ; but in 
 the subcutaneous tissue of the scrotum, penis, eyelids, and the pinna 
 of the ear, fat is not formed. 
 
 The Nail. 
 
 We recognize in the hard substance of the nail, which corresponds 
 to the horny layer of the epidermis, a body and a root ; the former lies 
 upon a portion of the somewhat modified corium called the nail-bed, 
 while the root is embedded in a shallow pocket of skin, the corium of 
 which constitutes the matrix of the nail. The corium of the nail does 
 not differ essentially from that of the skin in general; it is intimately 
 connected with the periosteum of the phalanx, and presents longitu- 
 dinal ridges low in the matrix, higher in the nail-bed, which are cov- 
 ered with papillae. These are bent obliquely forward, and are more 
 abundant in the nail-bed than in the matrix. Upon and between the 
 papillae several layers of variously shaped cells lie, corresponding to 
 the mucous layer of the skin. In the body these cells pass quite ab- 
 ruptly into the flat, horny, nucleated cells of the hard substance of the 
 nail ; in the matrix, however, the transition is very gradual, and it is 
 here that the growth of the nail occurs. The lunula is a portion of 
 the nail in which the mucous layer is very thick, as it is in all parts of 
 the root, and being evenly distributed over the surface of the papillae, 
 does not permit the color of the blood in the capillaries of the papil- 
 lae to be seen as it is in the rest of the nail-bed, where the longitudinal 
 ridges are higher and covered by fewer of the translucent cells of the 
 mucous layer. 
 
THE SKIN. 117 
 
 The Hair. 
 
 We distinguish in the hair: the shaft, which projects above the 
 surface of the skin ; the root, which is embedded in an oblique tubu- 
 lar depression called the follicle ; and the bulb, a dilated portion at 
 the bottom of the follicle in which the hair ends. The follicle some- 
 times extends into the subcutaneous tissue, sometimes only into the 
 corium, and its walls are formed in the first place by a sheath of con- 
 nective tissue continuous with the corium, in which numerous blood- 
 vessels ramify ; this sheath is lined by a thin transparent membrane 
 called the vitreous membrane. Within this follicular wall lies the 
 root-sheath, which consists of two layers : an outer thicker layer, 
 formed by the dipping down into the follicle of the cells of the rete 
 Malpighi, and hence consisting of cylindrical spheroidal and somewhat 
 flattened cells ; and an inner layer made up in turn of an external 
 layer called Henle's sheath, in which the cells resemble those of the 
 horny layer of the epidermis, and are closely packed together to 
 form a transparent mass ; and an internal layer called Huxley's sheath, 
 whose cells, belonging more properly to the hair itself, are irregularly 
 polygonal, somewhat flattened, and contain an elongated nucleus. 
 Both at the opening of the follicle and at its base, the layers of the 
 root- sheath become indistinct, merging on the one hand into the cells 
 of the epidermis, and on the other into those of the hair-bulb. At the 
 bottom of the follicle is a projection from the connective tissue form- 
 ing the wall of the follicle in the form of a papilla, which corresponds 
 to the papillae of the skin, and upon which the hair-bulb rests, sur- 
 rounding it at the top and sides. The hair is produced by the growth 
 of cells about the papilla ; directly covering the latter are cylindrical 
 and cuboidal cells corresponding to those of the rete Malpighi, which 
 gradually become changed in shape and more or less horny, and form 
 the substance of the hair-shaft. In the shaft we recognize three por- 
 tions : 1. A central or medullary, composed of cuboidal or more or 
 less flattened cells, not infrequently inclosing between them tiny bub- 
 bles of air which give the centre of the hair a dark appearance by 
 transmitted light. Outside of this is 2, the cortical portion, mak- 
 ing up the larger part of the bulk of the shaft, and composed of tough, 
 horny, elongated, flattened cells closely packed together, and contain- 
 ing, except in colorless hairs, granules of variously colored pigment. 
 In that portion of the hair which lies within the follicle, and between 
 the bulb and the free shaft, we find, while the hair is growing, that the 
 cells of the cortical layer are larger, less flattened and horny, and, as 
 above mentioned, merge into the large cylindrical and spheroidal cells 
 of the bulb. Finally, the shaft is covered, 3, by the so-called cuticula, 
 consisting of thin, rectangular, non-nucleated, scale-like cells, which 
 lap over one another so that the lower cells, i. e., those nearest the 
 root of the hair, cover a portion of the cells beyond, and these free 
 edges often project slightly from the surface of the hair, giving it a 
 finely serrated appearance. 
 
Il8 NORMAL HISTOLOGY. 
 
 Sebaceous Glands. 
 
 These are racemose glands whose excretory ducts are lined with 
 cuboidal or somewhat flattened cells, and whose alveoli, bounded by 
 a membrana propria, are lined with transparent, polygonal, epithelial 
 cells ; these usually contain, in addition to the nucleus, a greater or 
 less number of fat-droplets, with which, also, the cavities of the alveoli 
 are usually more or less completely filled. The sebaceous glands 
 either open into a hair-follicle near the surface of the skin, or their 
 excretory ducts are pierced near the surface by the shaft of a hair. 
 The hair-follicle, as above mentioned, is placed obliquely in the skin, 
 and at the side at which it forms an oblique angle with the surface, a 
 bundle of smooth muscle-cells is placed. It is attached to the con- 
 nective-tissue sheath of the follicle in its lower third, and, passing 
 obliquely upward, is inserted into the upper portion of the corium at 
 some distance from the opening of the follicle. A contraction of the 
 muscle thus placed will, of course, draw the hair-follicle, and with 
 it the shaft, into a position more nearly perpendicular to the surface 
 of the skin, and hence it is called the erector fiilce. As a rule, the 
 sebaceous follicle lies above the erector muscle in the angle which it 
 forms with the upper portion of the hair-follicle, and is moved with 
 the hair when the muscle contracts, and may even be pressed upon by 
 it when, as is frequently the case, it runs closely over the surface of 
 the gland. This relation of the erector pike muscle to the sebaceous 
 gland is not without significance in connection with the discharge of 
 the secretion of the latter. 
 
 Sweat-Glands. 
 
 The sweat-glands, which are found in the skin of almost all parts 
 of the body, although much more abundant in some than in others, 
 are tubular glands, the tube consisting of a membrana propria, lined 
 throughout with cuboidal epithelium. Its lower extremity, coiled into 
 a ball, and held together by loose connective tissue, lies sometimes in 
 the corium, sometimes in the subcutaneous tissue. The upper por- 
 tion of the tube, which served as the excretory duct, passes to the 
 surface of the skin, often taking a wavy course through the corium. 
 It pierces the epidermis between two papillae, and here the walls of the 
 duct cease, and it is bordered by epidermis-cells alone. If the epi- 
 dermis layer is thick, as in the palm, etc., the course of the duct 
 through it is a remarkably winding one. An abundant capillary net- 
 work lies in the loose connective tissue of the gland-coil. 
 
 Blood-vessels. 
 
 The arteries of the skin, which enter through the subcutaneous 
 tissue, give off, in general, three sets of branches, through which the 
 blood is distributed to three principal sets of capillaries : first, to those 
 which supply the fat-tissue ; second, to those which ramify in the 
 
THE SKIN. IIQ 
 
 sweat-glands ; and third, to those which supply the hair-follicles, 
 sebaceous glands, and the papillae of the coriura. Each papilla, ex- 
 cept when containing a tactile corpuscle, is furnished with a capillary 
 loop. 
 
 Nerves. 
 
 The nerves of the skin ramify in the subcutaneous tissue, and a 
 certain number of them terminate here in the so-called Pacinian 
 Bodies ; others pass into the corium, where they form plexuses, varying 
 in character in different parts of the body ; from these certain medullated 
 nerves pass into the papillae — those which contain no capillary loops 
 — and terminate in the tactile corpuscles ; still other non-medullated 
 nerves pass off from the plexuses and enter the papillae, or pass be- 
 tween the cells of the rete Malpighi, but their mode of termination is 
 not yet definitely ascertained. The Pacinian bodies are found in 
 various parts of the human body, but most abundantly along the 
 course of the nerves in the subcutaneous tissue of the palmar and 
 plantar surfaces of the fingers and toes. They are oval bodies, readily 
 visible with the naked eye, consisting of a number of concentrically- 
 arranged — apparently homogeneous — lamellae, forming a series of 
 superimposed capsules, between which lie endothelial cells. These 
 lamellae, thinner and more closely packed near the centre of the body, 
 are pierced at one end by a medullated nerve ; the neurilemma of 
 the nerve seems to become continuous with the capsular membranes, 
 while the axis cylinder, with its medullary sheath, passes inward. The 
 medullary sheath does not accompany the axis cylinder far, but gradu- 
 ally dwindles away; the naked axis cylinder passes along in a cavity 
 formed by the innermost capsule, and filled with a granular substance, 
 and terminates near the opposite end of the body in a dilated, or 
 branched or pointed extremity ; or sometimes passes entirely through 
 the body and becomes continuous, with a medullated nerve-fibre at 
 the other end. Capillary blood-vessels are also found in the Pacinian 
 bodies, for the most part in the peripheral zone. 
 
 The tactile corpuscles, called also Afeisszier's corpuscles, are most 
 abundant at the finger-tips. They are small ellipsoidal bodies, with 
 irregularly transversely striated surfaces, and lie, as above stated, in 
 the papillae of the skin. They consist of a capsule in which are granu- 
 lar material whose nature is undetermined, elongated nuclei placed 
 transversely to the long axis of the capsule, and the terminations of 
 the nerves. One or more medullated nerve-fibres pass to the lower 
 extremity, or to the side of the body, and sometimes directly, some- 
 times after winding once or twice around it, pass beneath the capsule. 
 Their course within and their mode of termination are not yet defi- 
 nitely ascertained. 
 
 PRACTICAL STUDY. 
 
 Sections of Injected Skin. — A piece of skin is removed from an in- 
 jected leg or arm, care being taken to include the subcutaneous tissue 
 to a considerable depth ; it is stretched flat on a bit of cork and placed 
 
120 NORMAL HISTOLOGY. 
 
 in a mixture of equal parts of one-half per cent, chromic acid solution 
 and alcohol ; after ten days it is transferred to strong alcohol. When 
 sufficiently hard it is embedded in wax or liver, and sections made 
 perpendicular to the surface ; these are stained lightly with hematoxy- 
 lin, then with eosin, and mounted in balsam. 
 
 Sections of Skin of Negro. — The skin is hardened in the chromic 
 acid and alcohol mixture, the sections stained with picro-carmine, 
 and mounted in glycerine. 
 
 Sections of the Nail. — A nail should be separated from a finger 
 which has been hardened in alcohol, together with as much of the 
 connective tissue which binds it to the bone as possible. In order to 
 render the soft parts more consistent and more nearly like the body 
 of the nail, so that uniformly thin sections can be made, the whole 
 should be hardened by the gum method ; see page 63. Longitudinal 
 and transverse sections are made through the entire nail, and stained 
 first very deeply with a strong solution of aniline green, and then with 
 eosin, and mounted in balsam. 
 
 Sections of Hairs from Skin of Scalp. — A piece of skin from the 
 scalp of an adult is stretched on cork and hardened in the chromic 
 acid mixture. Sections are made as nearly as possible in the direc- 
 tion of the hair-follicles, so as to include the entire root and bulb, and 
 also at right angles to them. They are stained with aniline green and 
 eosin, and mounted in balsam. 
 
 Cells of the Cuticnla of the Hair. — These are readily seen by 
 placing a hair on a slide in a drop of strong sulphuric acid and cover- 
 ing ; in a short time the ceils will have become so much loosened that 
 if the cover be lightly tapped upon, a certain number of them will float 
 off into the acid, or may be seen bristling out from all sides of the 
 shaft. 
 
 Skin from the Finger-tips. — This is prepared like the injected 
 skin, and will show the thick layer of epidermis-cells with the stratum 
 lucidum, the tortuous course of the sweat-gland ducts through the 
 epidermis, the tactile corpuscles and numerous sweat-glands, and very 
 often excellent transverse or longitudinal sections of the Pacinian 
 bodies will be obtained, if the subcutaneous tissue has been included 
 to a considerable depth in the section. The Pacinian bodies are 
 readily studied in the mesentery of the young cat, where they are 
 large and abundant ; but the description of the somewhat complicated 
 methods to which we must have recourse in order to completely 
 demonstrate their structure, would carry us beyond the limits of these 
 lessons. 
 
CHAPTER XVIII. 
 
 THE EYE. 
 
 The organ of sight is composed of the eyeball and various acces- 
 sory structures, such as the eyelids, lachrymal gland, muscles, etc. 
 The eyeball is composed, in the first place, of a dense, firm, spheroidal, 
 connective-tissue envelope, whose anterior transparent portion, the 
 cornea, is more convex than the posterior opaque segment, the scle- 
 rotic, and differs somewhat from it in structure ; the sclerotic is pierced 
 posteriorly by the optic nerve. Within the sclerotic lies a vascular 
 tunic, the choroid, formed of several layers of tissue, and thrown at 
 the surface, not far from the sclero-corneal junction, into numerous 
 longitudinal folds, called the ciliary processes. An extension from the 
 ciliary processes passes forward into the iris, which is a perforated 
 vascular curtain suspended behind the cornea, containing radial and 
 circular bundles of smooth muscle-cells, and connected peripherally 
 near the sclero-corneal junction, with a connective-tissue structure 
 called the ligamentum pectinatum. Passing backward from the liga- 
 mentum pectinatum, between the ciliary processes and the sclera, and 
 attached posteriorly to the choroid, is a muscle having the form of a 
 flattened ring, thickest in front, called the ciliary muscle. The direc- 
 tion of the muscle-cells in the ciliary muscle, which are of the smooth 
 variety, is in part meridional or oblique, in part circular. The ciliary 
 processes and muscle form together the greater part of a structure 
 known as the ciliary body. The retina, the innermost of the layers 
 forming the wall of the eyeball, spreads out from the point of entrance 
 of the optic nerve over the inner surface of the choroid. At about a 
 third of the distance back from the front of the eye, the nerve ele- 
 ments of the retina cease in a wavy line, called the ora serrata ; cer- 
 tain of the connective tissue and cellular elements continue, however, 
 somewhat modified in character, over the ciliary processes, under the 
 name of the pars ciliaris retina. The crystalline lens is suspended 
 close behind the iris by a firm, delicate, fibrillated membrane called 
 the suspensory ligament, which is attached on the one hand to a mem- 
 brane covering the ciliary processes, and on the other to the capsule 
 of the lens. The cavity of the eyeball is divided by the lens and its 
 suspensory ligament into two chambers, the anterior and smaller of 
 which is filled with a homogeneous fluid, the aqueous humor ; the pos- 
 terior with a gelatinous substance, the vitreous humor, which presents 
 an ill-defined, lamellar structure, and sometimes contains a variable 
 number of ill-defined, more or less granular cells. The vitreous is 
 
122 NORMAL HISTOLOGY. 
 
 surrounded by a delicate membrane, called the hyaloid membrane, 
 which is closely connected with the lining membrane of the retina, 
 and is hardly to be differentiated from it. 
 
 Having thus briefly described the general structure of the eye, it 
 remains for us to consider some of its parts somewhat more in detail ; 
 the limits of the course will not permit us, however, to make an ex- 
 tended study of all or even any of the structures in the eye ; we shall 
 be obliged to confine ourselves to the more marked features of the 
 cornea and sclera, the posterior portions of the choroid and retina, and 
 the crystalline lens. 
 
 Sclera. 
 
 The sclera is composed of very closely interwoven connective-tis- 
 sue fibres, with fine elastic fibres, the latter most abundant near the 
 inner surface. Between the fibres, which have little regularity in their 
 arrangement, lie flat connective-tissue cells, a certain number of which 
 frequently contain pigment-granules. On the external surface, the 
 sclera sends off delicate fibres ; anteriorly into the subconjunctival 
 tissue ; while posteriorly, behind the muscle tendons, they join to form 
 the wall of a lymph-sac, called the capsule of Tenon. On the inner 
 surface certain fibres pass directly over into the choroid, others form 
 the outer wall of a lymph-sac between the sclera and choroid, and 
 called, on account of its yellow color, the lamina f us ca ; it resembles 
 in structure the outer layers of the choroid, presently to be described 
 as the membrana supra-choroidea, of which it is indeed a part. In man 
 and many animals, the opening in the posterior segment of the sclera, 
 through which the optic nerve passes, is crossed by a net-work of con- 
 nective-tissue fibres ; these pass in from the sclera on all sides, and sur- 
 round the delicate bundles of nerve-fibres of the opticus, forming the 
 lamina cribrosa. 
 
 Cornea. 
 
 The cornea is directly continuous at the periphery with the sclera, 
 but differs from it in structure in many particulars, among the more 
 prominent of which are : the more regular lamellar arrangement of its 
 connective-tissue basement substance ; the greater abundance and 
 peculiar form of its cellular elements ; and the free anterior and pos- 
 terior surface covered with cells. In a thin section of the cornea, per- 
 pendicular to its surface, we recognize, passing from before backward, 
 five layers : i. a stratified layer of epithelial cells — the anterior corneal 
 epithelium — consisting of cells resembling in general form and arrange- 
 ment those of the epidermis ; that is, we have in the deepest layer 
 cylindrical cells, passing over into spheroidal, and these into flattened 
 cells at the surface ; 2. The anterior epithelium rests on a dense trans- 
 parent membrane, called the anterior basal membrane, or lamina elas- 
 tica anterior, which is continuous within with the connective-tissue 
 basement substance proper of the cornea, and is composed of closely 
 packed fibrillae ; 3. The body of the cornea — substantia propria comece 
 — is composed of connective tissue whose characteristics we have al- 
 ready studied, page 25 et seq.; 4. Lying closely upon the posterior 
 
THE EYE. 123 
 
 surface of the last layer, is a thin, apparently structureless membrane 
 — the membrane of Descemet, lamina elastica posterior — upon which 
 lies : 5. A single layer of flattened cells, usually called the endothelium 
 of Descemet. The cornea, except at its extreme periphery, contains 
 no blood-vessels. Nerves, on the other hand, are very abundant. 
 These, in larger and smaller trunks, enter the cornea at the periphery, 
 and dividing and subdividing, presently loose the medullary sheath 
 and break up into bundles of extremely delicate fibres • these, passing 
 toward the anterior surface, separate and form extraordinarily delicate 
 and intricate plexuses, while here and there somewhat enlarged nodal 
 points are seen along the fibres, from which the often-beaded fibrils 
 pass off in all directions. The exact mode of termination of the 
 nerve-fibrils in the cornea is not definitely ascertained ; certain of them, 
 however, seem to pass between the anterior epithelial cells, and are be- 
 lieved by some investigators to terminate in free extremities at the 
 surface. 
 
 Choroid. 
 
 In the posterior portion of the eye, that lying behind the ora ser- 
 rata, the choroid presents four layers, which although intimately con- 
 nected and presenting no sharp line of separation, may yet be more or 
 less completely separated by a careful dissection. Directly beneath 
 the lamina fusca of the sclera, and forming the inner wall of the above- 
 mentioned lymph-sac, is the outermost layer, called the lamina supra- 
 choroidea ; it is composed of a series of super-imposed connective-tissue 
 membranes, containing a net-work of delicate elastic fibres, between 
 which lie numerous flattened, irregular-shaped, often-branched, pig- 
 mented cells. Within the lamina supra-choroidea lies a layer containing 
 the larger vessels, arteries and veins of the choroid, and called the exter- 
 nal vascular layer, or the layer ofHaller. This layer is composed of a 
 ground-work similar in structure to the supra-choroidea, in which the 
 vessels are embedded, the arteries being often accompanied by smooth 
 muscle-cells. The inner vascular layer, called the chorio-capillaris, 
 follows next, and consists almost entirely of a very dense net-work of 
 broad capillary blood-vessels. Finally, the choroid is limited within 
 by an extremely delicate, finely striated membrane, called the mem- 
 brane of ' Bruch. 
 
 Retina. 
 
 Of all the animal structures the retina is one of the most delicate 
 complicated, and difficult of study, and we can do little here than indi- 
 cate briefly the general grouping of its elements. It consists of a 
 connective-tissue frame-work by which the nerve-elements are sup- 
 ported, and with which they are most intimately associated ; and in 
 some cases it is as yet impossible to say to which variety of tissue a 
 given element belongs. We distinguish, in typical parts, ten layers ; 
 commencing from within, they may be enumerated as follows : 
 
 1. Membrana limitans interna. 
 
 2. Layer of optic nerve-fibres. 
 
 3. Layer of ganglion-cells. 
 
124 NORMAL HISTOLOGY. 
 
 4. Internal molecular layer. 
 
 5. Internal granular layer. 
 
 6. External molecular layer. 
 
 7. External granular layer. 
 
 8. Membrana limitans externa. 
 
 9. Layer of rods and cones. 
 10. Pigment layer. 
 
 The limiting membranes, the outer of which is perforated by 
 numerous openings, are delicate and homogeneous, and present no 
 points of special interest for our consideration. In the layer of optic 
 nerve-fibres, the fibres run for the most part in a horizontal direction 
 and pass finally into the next layer to join the ganglion-cells; these, 
 having the general characters of branching nerve-cells, send numerous 
 processes into the internal molecular layer, where they break up into 
 a delicate fibrillar net-work, associated with the connective-tissue 
 frame-work. In the internal granular layer, the so-called granules consist 
 of small nerve-cells and large nuclei, belonging to the connective-tissue 
 frame-work of the retina. In the external molecular layer again, we 
 have a delicate net-work of nerve-fibrils mingled with connective tissue. 
 The granules of the external granular layer are nerve-elements exclu- 
 sively, and are intimately connected by processes which pass through 
 the openings in the internal limiting membrane, with the rods and 
 cones. Of the latter structures, which within the limits of these les- 
 sons we cannot even in a general way adequately describe, the rods 
 are the longer, are somewhat conical at the outer extremity where they 
 join the nerve-elements of the outer granular layer; the cones are 
 shorter, are connected also with the nerve-elements within and termi- 
 nate externally in pointed extremities. The connective-tissue elements 
 of the retina consist, in certain layers, of broad, irregular radial fibres 
 forming frequent inosculations, and in the unequally thick molecular 
 layers of a delicate reticulum within which the nerve-fibrils ramify. 
 
 The pigment epithelium, in the form of a single layer of flat poly- 
 gonal cells, which we have already studied, rests upon the outer ends 
 of the rods, and is usually left upon the choroid when the retina is 
 separated from the latter. 
 
 The Lens. 
 
 The lens is a transparent double convex body, of sufficient firm- 
 ness to maintain its form when removed from the eye, and is enclosed 
 in a homogeneous elastic capsule which is thicker on the anterior 
 than on the posterior surface. To the peripheral zone of the capsule 
 on both anterior and posterior surfaces the suspensory ligament is 
 firmly attached. The body of the lens, although perfectly transparent, 
 is by no means structureless. Behind the anterior wall of the capsule 
 Lies a single layer of flattened polygonal cells, which at the equator 
 gradually lengthen out to form very much elongated band-like nuclea- 
 ted cells, or le?is-fibres, which running meridionally make up the greater 
 part of the body of the lens. The lens-fibres have, on transverse sec- 
 tion, a flattened hexagonal form ; they are joined intimately to one 
 
THE EYE. 125 
 
 another by their narrower sides, presenting the broader sides to the 
 surfaces of the lens, so that under certain circumstances they can be 
 peeled off in layers. The course of the fibres in the lens is somewhat 
 complicated, but it may be said in general that they run meridionally 
 from one-half of the lens, in broad sweeps over into the other ; inas- 
 much, however, as the individual fibres are not long enough to reach 
 the entire distance from one pole of the lens around to the other, 
 they commence along certain definite lines at varying distances from 
 the poles ; and the farther from one pole one end of the fibre is, the 
 nearer to the other will its other end lie. In the young human lens, 
 the lines from which the fibres start may be seen on the anterior and 
 posterior surfaces, under certain circumstances, in the form of a three- 
 rayed star ; in the adult the rays usually part at the ends, giving rise 
 to complicated stellate figures. 
 
 The Eyelids. 
 
 These are formed in general by a plate of connective tissue which 
 toward the free border is very dense and firm, and called the tarsal 
 cartilage or tarsus ; they are covered on the outside by skin, on the 
 inside by the conjunctival mucous membrane ; between the tarsus and 
 the skin lie the bundles of the musculus orbicularis. The tarsus, 
 which is in no sense cartilage, is a plate formed of very dense and 
 firm fibrillar connective tissue containing ordinary flattened connective- 
 tissue cells, and is closely connected within with the palpebral con- 
 junctiva. Embedded within the tarsus lie the Meibomian glands, 
 thirty to forty in number in each lid. They consist of numerous vesi- 
 cular alveoli, lined with short cylindrical cells, arranged along and 
 opening into long excretory ducts which are lined with laminated 
 epithelium, and open on the inner border of the edge of the lids. 
 They are somewhat modified sebaceous glands, but unlike most seba- 
 ceous glands are not connected with hair-follicles. The skin of the 
 eyelids is somewhat thinner than that of the face, is beset with deli- 
 cate hairs, and supplied with sweat-glands and sebaceous follicles. It 
 passes over on to the edge of the lids, at the inner border of which it 
 becomes continuous with the mucous membrane. The eyelashes are 
 inserted obliquely into the edge of the lid in from two to four rows ; 
 the follicles, which are quite deep, are furnished with sebaceous glands. 
 The conjunctival mucous membrane of the lids consists of a basis sub- 
 stance of loose fibrillar connective tissue containing a few elastic 
 fibres and numerous small spheroidal and branching cells. The epi- 
 thelium is laminated, consisting of a deep layer of cylindrical cells, 
 then more superficially of smaller spheroidal or oblong cells, and 
 finally of flattened cells on the surface. Small racemose glands, called 
 accessory tear-glands, are often seen opening on the surface of the 
 mucous membrane. In addition to the striated muscular bundles of 
 the orbicularis, smooth muscle-cells forming a membranous layer occur 
 beneath the conjunctiva on the orbital portion of the lid. 
 
I2D NORMAL HISTOLOGY. 
 
 PRACTICAL STUDY. 
 
 General Dissection of the Eye. — For this purpose a large eye, 
 like that of the sheep or ox, is preferable ; it should be opened by a 
 short incision through the sclerotic, so that the fluid can readily come 
 into contact with the parts within, and placed in M tiller's fluid ; after 
 two weeks the dissection may be made. The eye should be divided 
 with a sharp razor into lateral halves. The cut surface shows at once 
 the general relations of the parts ; cornea, iris, lens, ciliary body, 
 vitreous, retina, choroid and sclera are seen in approximately normal 
 relations to one another. If the vitreous body be .,ow carefully re- 
 moved from one of the halves, the retina and ciliary body come 
 more fully into view ; and the suspensory ligament is readily seen. 
 As the suspensory ligament approaches the edge of the lens it divides 
 into two layers which pass, one to the anterior, the other to the poste- 
 rior surface of that body, leaving a slit-like opening between them ; 
 this is called the canal of Petit, and may be readily seen by slightly 
 pulling the edge of the lens backward and inward, when the layers 
 will separate. Now seizing the half of the lens with a pair of forceps 
 and carefully drawing it downward and backward, aw T ay from the iris, 
 the suspensory ligament, in the form of a folded fringe-like membrane, 
 may be torn away from its attachment to the hyaloid membrane ; a 
 portion of it is snipped off close to the lens, stained in hematoxylin 
 and mounted in glycerin. Bits of the lens capsule, anterior and pos- 
 terior surfaces, may be peeled off and examined in glycerine ; and a 
 fragment of the lens-tissue may be teased on a slide, stained with eosin 
 and also mounted in glycerine. After the removal of the lens, the 
 general form and attachment of the iris are readily seen. In the same 
 half of the eye the layers of the choroid may be demonstrated. The 
 retina is pulled off and the pigmented cells, which are apt to adhere 
 to the inner surface of the choroid, are brushed away under water 
 with a fine pencil. The eye is now pinned, with the flat side down, 
 on to a cork which has been attached to a bit of sheet lead and sunk 
 in a shallow dish of water. A window is cut through the sclera, in- 
 eluding the sclero-corneal junction ; the bit of sclera having been 
 removed, care being taken not to bring away the ciliary body, the 
 membranasupra-choroideawill be seen as a brown loose tissue floating 
 at the surface of the choroid. Bits of this are seized with the for- 
 ceps upon the ciliary body, and drawn carefully backward so as to 
 separate them from the parts beneath ; with care, very thin bits of 
 considerable size may be separated. They should be floated without 
 wrinkles or folds on to a slide immersed in water, then stained on the 
 slide with hematoxylin and mounted in glycerine. The floating shreds 
 of the supra-choroidea which remain after suitable specimens have 
 been obtained, should now be pulled from the eye, when the vascular 
 layer of Haller will come into view. The separation of the three 
 remaining layers is not easy under the most favorable conditions, and 
 is especially difficult in the eye of the ox and sheep, where the layers 
 are rendered more complicated and difficult of separation by the pres- 
 
THE EYE. 127 
 
 ence of a mass of interlacing fibres. Haller's layer, however, and 
 the membrana chorio-capillaris may be, with care, stripped off in 
 pieces sufficiently thin for demonstration ; sometimes a fragment will 
 be obtained, which, especially at the edges, will show both of the 
 vascular layer at once. They are stained with hematoxylin and 
 mounted in glycerine. 
 
 Cornea and Sclera. — In a fresh eye from the rabbit or dog, a 
 short incision is made through the sclera; it is placed in Muller's 
 fluid, and, after three weeks, washed, and the hardening completed in 
 alcohol. The cornea should now be excised close to the sclero-cor- 
 neal junction, embedded in wax, and thin sections made perpendicu- 
 lar to the surface, including the entire thickness, stained with hsema- 
 toxylin and eosin, and mounted in glycerine. 
 
 Sclero-comeal Junction. — The cornea and sclera, in their relations 
 to one another, and the ciliary bodies and iris may be examined in a 
 specimen prepared as follows : An eye — that of the pig is best, if a 
 fresh human eye cannot be procured — is placed, after making a short 
 incision in the sclera, in Midler's fluid, where it remains for two 
 weeks, the fluid being changed once or twice ; it is then soaked for 
 an hour or two in water, and the hardening completed in dilute and 
 strong alcohol. A bit is excised, including the sclero-comeal junc- 
 tion and adjacent parts, and embedded in cacao butter by the method 
 given on page 93. Thin sections are made perpendicular to the sur- 
 face of the eye, and after removing the cacao butter, stained in hema- 
 toxylin and eosin, and mounted in balsam. 
 
 Nerves of the Cornea. — The following is Ranvier's method for 
 their demonstration : The cornea of a recently-killed rabbit is placed 
 for five minutes in a small quantity of freshly-prepared and filtered lem- 
 on-juice ; it is then placed in a few cubic centimetres of one per cent, 
 solution of chloride of gold, in which it remains fifteen or twenty min- 
 utes ; it is then washed and placed in a small bottle, in which are 
 twenty-five to thirty cubic centimetres of distilled water, to which a 
 drop or two of acetic acid has been added. In twenty-four or forty- 
 eight hours, under the action of the light, the gold will be reduced, 
 and the cornea have a violet color. Sections are made parallel with 
 the surface ; the portions directly beneath the anterior epithelium are 
 the most desirable for study, since here the delicate nerve net-work is 
 most abundant. Sections may also be made perpendicular to the 
 surface, especial care being taken to preserve the anterior epithelium 
 undisturbed. 
 
 Sections of the Lens. — The eye of a rabbit or pig, in which the 
 sclera has been opened, should be placed in Muller's fluid ; after 
 about a fortnight the lens is removed, care being taken not to rupture 
 the capsule, kept for a few days in alcohol, and embedded in wax. 
 Thin sections are made from before backward, through the centre, 
 stained with hsematoxylin, and mounted in balsam. 
 
 Transverse Sections of the Retina. — In a fresh human or pig's 
 eye, one or two small openings should be made through the sclera, so 
 that the fluid can readily enter, and placed in Muller's fluid ; after a 
 week the eye may be cut across just behind the sclero-corneal June- 
 
123 NORMAL HISTOLOGY. 
 
 tion, and the posterior segment placed in fresh Miiller's fluid; after 
 another week the eye is transferred for twenty-four hours to dilute, 
 then to strong alcohol. After remaining for a day or two in strong 
 alcohol, a small bit is cut from the posterior portion of the retina, 
 about midway between the optic-nerve entrance and the pars-ciliaris 
 retinae, and embedded in cacao butter ; thin transverse sections are 
 then made ; the cacao butter dissolved out in the usual way : the sec- 
 tions stained with hematoxylin and eosin, and one or two mounted in 
 glycerine, one or two in balsam. By this method the general arrange- 
 ment of the layers is shown, but many of the finer structures, espe- 
 cially the rods and cones, are much altered ; and for the study of 
 these, we have recourse to osmic acid preparations. 
 
 Isolated Retiiial Elemefits. — A small fragment of the perfectly 
 fresh retina of man or the pig is placed in one per cent, solution of 
 osmic acid ; in forty-eight hours it is removed, and allowed to soak in 
 water for a few hours ; it is then placed in a mixture of equal parts of 
 glycerine, alcohol, and water, where it remains for a week or ten 
 days ; small fragments are then either teased carefully on a slide in 
 glycerine, and covered, the specimen being protected from pressure 
 by a hair ; or, what is better, a small fragment is put on a slide in a 
 drop of glycerine, and a large cover-glass, on the under side of which, 
 at the corners, tiny fragments of white wax have been placed, is in- 
 verted over it ; and the cover, just above the specimen, is gently and 
 repeatedly tapped upon with a needle ; the movement thus occa- 
 sioned in the glycerine will, after a time, cause the retina to fall apart, 
 and become separated into exceedingly small fragments. This latter 
 method is known as Rindfleisctis Dissociation Method, and may be 
 employed to advantage in the dissociation of many kinds of tissue — 
 that of the central nervous system, for example — when the use of 
 needles would be apt to cause injury to the isolated elements. 
 
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