RACTICAL 
 
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 A.Ml'^... 
 
Cornell University 
 Library 
 
 The original of tliis book is in 
 tine Cornell University Library. 
 
 There are no known copyright restrictions in 
 the United States on the use of the text. 
 
 http://www.archive.org/details/cu31924031267663 
 
A MANUAL 
 
 OP 
 
 PRACTICAL NORMAL 
 HISTOLOGY 
 
 T. MITCHELL ERUDDEN, M. D. 
 
 l*IRKCTOR OF TKE PHYSIOLOGICAL AND PATHOLOGICAL LABORATORY OF THE ALUMNI 
 
 ASSOCIATION OF THK COLLEGE OF PHYSICIANS AND SURGEONS, W. Y, ; 
 
 LECTURER ON rJORMAL HISTOLOGY IN YALE COLLEGE ; PATHOLOGIST 
 
 TO THE MANHATTAN EYE AND EAR HOSPITAL, 
 
 SECOND EDITION. 
 Revised^ 'with an Appendix. 
 
 NEW YORK & LONDON 
 
 P. PUTNAM'S SONS 
 %\t luachnboctui %u%* 
 
 1887 N 
 
COPYKIGHT 
 
 G. P. PUTNAM'S SONS 
 
 Press of 
 P. Putnam's Sons 
 New York 
 
PREFACE TO THE SECOND EDITION. 
 
 The advances in Normal Histology since the first 
 edition of this book was written have been largely in the 
 direction of improved technical procedures. These im- 
 provements have been embodied in this edition, for the 
 most part in an appendix, to which references are made 
 in the text by foot-notes. 
 
 T. M. P. 
 
 New York, July, 1884. 
 
PREFACE. 
 
 This book has been prepared for the use of those 
 students and practitioners of medicine, who with a 
 limited amount of time at their disposal, wish to 
 acquaint themselves in a practical way with Normal 
 Histology. It is especially designed for those who 
 study the science in classes, with an instructor 
 in a laboratory ; but the technical procedures are 
 described with sufficient fulness for the needs of 
 those who are obliged to pursue the study by them- 
 selves. 
 
 The method adopted is to give a brief description 
 of the tissues and organs in appropriate sequence, 
 following each description with an account of the 
 way in which the structures described may be dem- 
 onstrated. The descriptions were written for the 
 most part at the microscrope table, with the prepar- 
 ations made by the methods recommended, under 
 the eye of the writer, so that it is believed that the 
 student will have no difficulty in verifying them. 
 
 Too much stress cannot be laid upon the neces- 
 sity of each student making outline sketches of all 
 
VI PREFACE. 
 
 but the more complicated structures examined, and 
 for this provision has been made in the book. 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 Human Histology, will be 
 in all cases perfectly clear and intelligible without 
 the aid of plates ; but the specimens which the stu- 
 dent prepares, and the sketches from them which 
 he makes, will make good, it is hoped, the lack of 
 illustration in the text. Indeed, the more critical 
 examination which accurate sketching requires, as 
 well as the facility which this exercise cultivates, 
 will enlarge the achievements of such a course of 
 study beyond the acquirement of a knowledge of 
 this theme alone, so as to embrace a valuable train- 
 ing of the eye and hand. 
 
 This book is not designed to take the place of 
 more elaborate treatises on this subject ; nor is it 
 written with the design of fostering the deplorably 
 wide-spread tendency among medical students, to 
 be content with the barest smattering of those 
 branches which are not in the most evident manner 
 " practical." On the contrary, where time permits, 
 collateral reading and additional practical work are 
 most urgently recommended. But the necessity for 
 improvement in medical education, which is expres- 
 sing itself in the medical colleges of this country, 
 especially in the establishment of laboratories and 
 practical courses of instruction, is, unfortunately, 
 
PREFACE. vii 
 
 not yet sufficiently deeply felt as to have led to the 
 general lengthening of the period of undergraduate 
 study ; so that very little time is usually at the dis- 
 posal of medical students for collateral reading, or 
 for the pursuit of elaborate practical investigations. 
 It is desirable, moreover, since the laboratory time 
 itself is usually limited, to occupy as little of it as 
 may be, in oral descriptions of tissues and methods. 
 
 It is these considerations which seem to justify the 
 addition of another to the long list of elementary 
 text-books. 
 
 There are many points in this, as in every develop- 
 ing science, which are still unsettled — opinion in re- 
 gard to them changing or being modified 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 instruc- 
 tion to enlarge upon and explain them, as the light 
 thrown upon each by new researches may seem to 
 require. 
 
 In the simpler form of " Notes on the Practical 
 Course in Normal Histology " the substance of this 
 book has been in use for two years in the laboratory 
 of the Alumni Association of the College of Phy- 
 sicians and Surgeons, and it has been found that 
 with some preliminary preparation of tissues by the 
 instructor, the subject essentially as presented here, 
 can be embraced in a course of forty lessons of about 
 two hours each. 
 
Vlll PREFACE. 
 
 The writer wishes, in conclusion, to express his 
 sincere thanks to Prof. Francis Delafield, to whose 
 wise counsel and unwearied assistance in many mat- 
 ters requiring a wider experience than his own, he 
 is greatly indebted. T. M. P. 
 
 Laboratory of the Alumni Associa- 
 tion OF THE College of Physicians 
 AND Surgeons. 
 
 New York, May^ x88x. 
 
CONTENTS. 
 
 INTRODUCTION - - ... j 
 
 I THE CELL IN GENERAL . . 17 
 
 II — CONNECTIVE TISSUE • . -27 
 III — EMBRYONAL AND MUCOUS TISSUE — FAT TIS- 
 SUE RETICULAR CONNECTIVE TISSUE 48 
 
 IV — CARTILAGE — BONE — TEETH • - 56 
 
 V — BLOOD AND LYMPH - . .80 
 
 VI — MUSCULAR TISSUE • - • 91 
 
 VII NERVE-TISSUE .... 103 
 
 VIII BLOOD-VESSELS LYMPHATIC VESSELS - II 9 
 
 IX — LYMPH-NODES — SPLEEN . - - 129 
 
 X THE GASTRO-INTESTINAL CANAL - 143 
 
 XI SUBMAXILLARY GLAND LIVER . 155 
 
 XII SUPRA-RENAL CAPSULES —THYROID GLAND 164 
 
 XIII THE RESPIRATORY APPARATUS - 168 
 
 XIV THE KIDNEY ... j^g 
 
 XV THE GENERATIVE ORGANS ... i88 
 
 XVI THE CENTRAL NERVOUS SYSTEM . 211 
 
 XVII THE SKIN AND ITS ADNEXA • - 220 
 
 XVIII THE EYE .... 232 
 
 APPENDIX . - - - - 251 
 
 ix 
 
INTRODUCTION. 
 
 GENERAL METHODS FOR PRESERVING TISSUES A*IO ppp- 
 PARING THEM FOR STUDY. 
 
 Animal tissues must conform to certain physical 
 conditions before they can be subjected to a satis- 
 factory microscopical examination. Portions ol 
 them subjected to study must be sufficiently thin 
 to allow the light to pass readily through them, and 
 transparent enough to permit the determination ol 
 the form, character, and relations of their structural 
 elements. At the same time the refractive powei 
 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 different 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 use of preservative agents. Some are too 
 soft to permit the preparation of thin sections, and 
 must be hardened ; others are too hard and must be 
 softened. In some specimens one, in others another, 
 
2 NORMAL HISTOLOGY. 
 
 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 struc- 
 tures under investigation in as natural a form as 
 possible. Finally, specimens suitably prepared for 
 examination, are in many cases, to be rendered per- 
 manent 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 in some enclosing 
 fluid medium, on a glass plate and covered with a 
 thin slip of glass, before being brought upon the 
 microscope. One of the simplest methods of study- 
 ing 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 
 physical condition but little or not at all, or at least 
 very slowly, and examine them at once. Such a 
 fluid is called an indifferent fluid, and for most pur- 
 poses a dilute solution of common salt, one-half to 
 three-quarters per cent., answers very well. 
 
 The examination of fresh tissues is very important, 
 not only because it enables us to study the vital 
 phenomena in certain elements, but because we are 
 thus enabled by comparison to determine the 
 amount of change which tissues undergo when pre- 
 pared by more elaborate methods. Still it is in 
 many respects unsatisfactory. In the first place, it 
 is not always easy to procure fresh tissues for every 
 
INTRODUCTION, 3 
 
 observation, and even in an indifferent fluid, tissues 
 sooner or later undergo considerable alterations, so 
 that they cannot be permanently preserved. A 
 still more important deficiency in this method is the 
 lack of distinctness in structural details which it in- 
 volves. Most of the fresh animal tissues are nearly 
 transparent in thin pieces, and their structural ele- 
 ments have so nearly the same refractive power, 
 that we see through them, but do not see them ; or, 
 if we do see them, it is not with that definiteness 
 which our purposes demand. Now these difficulties 
 are usually met by the use of agents which harden 
 and preserve the tissues and at the same time ren- 
 der the details of their structure visible, by changing 
 the refractive power of one or other of their ele- 
 ments ; or, we employ certain coloring agents, 
 which, being taken up with different degrees of 
 avidity by different parts, assist in 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 common and useful of these agents. 
 
 HARDENING AND PRESERVATIVE AGENTS. 
 
 Alcohol \s one of the most valuable of these. It 
 causes a considerable shrinkage of most tissues, 
 partly by the withdrawal of water from them, and. 
 
4 NORMAL HISTOLOGY. 
 
 like many of these fluids, precipitates certain of 
 their albuminoid constituents, thus diminishing 
 their transparency. It is in general to be used at 
 first diluted with one-third water, and after the tis- 
 sues have lain for 24 hours in this, they are trans- 
 ferred to strong alcohol in which they may be pre- 
 served indefinitely. 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 abun- 
 dant. Certain structures are best preserved by 
 plunging them at once into strong alcohol. 
 
 Aqueous solutions of chromic acid and of potas- 
 sium and ammonium bichromate and of the neutral 
 chromate of ammonium are much used to preserve 
 and harden tissues, and many structures are more 
 perfectly preserved in these fluids than in alcohol. 
 The hardening proceeds more slowly than in alco- 
 hol, and the structures do not shrink as much. It 
 is a common practice to commence the hardening 
 with one of the chromic fluids and complete it with 
 alcohol, the former being thoroughly washed out 
 before immersion in the latter. Specimens should 
 not remain too long in chromic fluids since they are 
 apt to become brittle. An immersion of from 2-3 
 weeks usually suffices, especially if alcohol is to be 
 used afterward.* Chromic acid may be used in solu- 
 tions of from ^ to ^ per cent., potassium and am- 
 monium bichromate in 2 per cent., and the neutral 
 ammonium chromate in J-per-cent. solution. 
 
 * See Appendix, p. 2 
 
INTRODUCTION. S 
 
 A very useful preservative solution is Mailer's 
 Fluid, consisting of 
 
 Sodium sulphate, i 
 
 Potassium bichromate, 2 
 Water, 100 
 
 Chromic acid renders the lime salts in bone soluble, 
 and is often used to soften them in preparation for 
 section-cutting. 
 
 Picric Acid. — This agent while hardening, pre- 
 serves the structur-e of many tissues very perfectly, 
 and at the same time stains them yellow. It is usu- 
 ally necessary to complete the hardening with alco- 
 hol. For the decalcification of bone it is one of the 
 best of agents, although it acts very slowly. It is 
 used in saturated aqueous solution. 
 
 Osmic Acid. — This substance has the power 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 tissues a gray or brown ap- 
 pearance, and stains fat and certain allied substances 
 deep black. It is used in one-per-cent. aqueous 
 solution. The tissue should be quite fresh when 
 immersed in it, and, as a rule, should remain for 
 twenty-four hours. Specimens hardened in osmic 
 acid commonly become quite granular and dark 
 after a time. 
 
 Preservative fluids are sometimes brought into 
 more direct contact with the tissues, by injecting 
 
6 NORMAL HISTOLOGY. 
 
 them into the blood-vessels of the part befofe cut- 
 ting it in pieces ; or they may be dfkrert directly 
 into the interstices of the tissiae, by means of a 
 small syringe with a sharp-pointecf canula ; this is 
 called interstitial injection. 
 
 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 empkiying 
 them, will be given as we proceed with our practical 
 study. 
 
 STAINING AGENTS. 
 
 Hamatoxylin is one of the most gene^tBy useful 
 of the staining agents. It has the powef of coloriWg 
 certain parts, as the nuclei of cells, deeply; while 
 other parts are stained much leSs, of not at all. 
 The following is Prof. Delafield's method of prepar- 
 ing the solution : To make six hurtdfed c. c. of the 
 solution, take four hundred c. c. of satufafed solu- 
 tion of ammonia alum, with an excess of the alum 
 crystals, and add to this four grms. of haemaCco^lM 
 (Merck's is preferable, the crude extract will not 
 answer), dissolved in twenty-five c. c. strong com- 
 mercial alcohol. This, at first, produces a light 
 violet, or sometimes a difty-red color, but on ex- 
 posure to the light, in an unstoppered bottle, the 
 color deepens ; after standing for three or four days 
 exposed to the air and light, the solution is filtered, 
 and one hundred c. c. each, of glycerin and Hast* 
 
INTRODUCTION. 7 
 
 ings' wood naphtha (pyroxylic spirit) are added. 
 The solution is now allowed to stand for a day or 
 two, and is then filtered again, and this filtration is 
 repeated, after standing, until a dark sediment is 
 no longer fornied. The color is now usually very 
 deep, and the solution should be kept in a tightly- 
 stoppered bottle. 
 
 Such a solution is usually to be diluted with water 
 before using, the exact degree of dilution depending 
 upon the rapidity with which we wish the specimen 
 to be stained. As a rule, slow staining with a dilute 
 solution gives the best result, and is less likely to 
 cause shrinkage of the specimen. In staining, bits 
 of tissue are placed in a small dish of the solution, 
 so that they are bathed on all sides by it', and al- 
 lowed to remain until sufificiently colored. The 
 time required will depend on 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 further 
 manipulation. 
 
 Carmine. — This is a red stain, and is employed in 
 , the same manner as haematoxylin, and is useful 
 where we do not vwsh as great body of color in the 
 specimen as the haematoxylin imparts. It may be 
 prepared by dissolving two grms. of commercial 
 carmine in a few drops of strong ammonia, and 
 adding one hundred c. g. of water. It should be 
 
8 NORMAL HISTOLOGY. 
 
 allowed to stand in an open vessel until the excess 
 of ammonia evaporates. As a rule, old carmine so- 
 lutions stain better than those recently prepared, 
 
 Picro-carminate of Ammonia, or Pkro-carmine. — 
 With this substance we can obtain at once a yellow 
 color from the picric acid in certain elements, while 
 others are stained red by the carmine. It is pre- 
 pared 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 deposit, 
 and evaporating the filtrate to dryness over a water 
 bath. The picro-carmine is left in the form of an 
 ochre-red powder, which, for use, may be dissolved 
 in water in the proportion of one to one hundred. 
 
 Eosin. — This substance stains tissues more uni- 
 formly than many other dyes, and is especially 
 valuable when used in connection with other color- 
 ing agents, such as haematoxylin, which stains the 
 cell nuclei more deeply, since by this method of 
 double staining we have certain elements exhibiting 
 one color, others another. Eosin may be conveni- 
 ently used either in aqueous or alcoholic solutions 
 of one to one hundred. 
 
 METHODS OF PREPARING SPECIMENS FOR STUDY. 
 
 Certain fluid tissues, such as blood, lymph, etc., 
 are fitted for study, either fresh, or after suitable 
 preservation, when a drop is placed on a slide, and 
 
INTRODUCTION. 9 
 
 covered. Certain tissues occur in the form of mem. 
 branes, of sufficient thinness to admit of study with- 
 out other manipulation than spreading them out 
 smoothly on a slide. In other cases we have re^ 
 course to the dissociation of tissues by needles, 
 called teasing. 
 
 Section Cutting. — In many cases we wish to study 
 the structural elements 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 
 tissue 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, and somewhat con- 
 cave on the upper Side, so that a small quantity of 
 fluid will lie upon it. A flat, shallow dish is partly 
 filled with alcohol, with which the surface of the 
 specimen to be cut, as well as th^ razor blade, 
 should be constantly covered, the blade being 
 dipped into the alcohol, and as much taken up as 
 will lie upon it. The bit of tissue being held ^rmly 
 in one hand, and the razor firmly but lightly in the 
 other, the sections are made by long, slow, diagonal 
 sweeps of the razor, the blade being drawn from 
 heel to tip along the tissue, and not crowded di- 
 rectly forward. Much practice is required for mak- 
 ing large, thin, and even sections, and the endeavor 
 
lO NORMAL HISTOLOGY. 
 
 at first should be to get thin and even sections, no 
 matter how small they may be. The razor should 
 never be allowed to get dull, its edge being fre- 
 quently freshened by a few light passes along a 
 leathern strop.* 
 
 INJECTIONS. 
 
 It is often desirable in studying the distribution 
 of the blood- or lymph*vessels to fill them by injec- 
 tion with some colored substance by means of 
 which their ramifications may be readily recognized. 
 One of the most commonly employed injecting 
 materials is a solution of gelatin colored with 
 Prussiati blue. This may be prepared as follows : 
 Dissolve 4 grms. of gelatin in 60 c. c. of water on a 
 water bath ; divide the solution into two portions ; 
 to one portion add 4 c^p. of a saturated solution of 
 sulphate of iron (green vitriol), stirring constantly ;t 
 to the other add first 8 c. c. saturated soj. of fer- 
 rocyanide of potassium, and then 8 c. c. saturated 
 sol. 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 or organ injected 
 with this mixture should be kept warm during the 
 injection, as should all the u tensils employed, so 
 
 * See Appendix, p. 251. 
 
 + 6bp»W the iron cause a pasty precipitate ip the geUtin, this 
 portion should be aUowed to cool, when on warming again with 
 stirring, it wUl dissolve. ^ ^ ' ^^ 
 
IN TROD UCTION. 1 1 
 
 that the gelatin may not harden prematurely and 
 stop the vessels. The injection may be made 
 with a syringe, or better with some form of appa- 
 ratus furnishing a constant pressure of variable de- 
 gree. 
 
 EMBEDDING. 
 
 It often occurs that a bit of tissue from which we 
 wish to prepare a section is too small or delicate to 
 be held in the fingers ; in such cases ^he object may 
 be placed between two bits of hardened tissue, 
 such as liver, tied round with a thread, and thus held 
 while the sections are made. Or, such .a specimen 
 may be embedded in a mixture of equal parts of 
 white wax and paraffin melted together, with ad 
 dition of a sufficient quantity of" olive oiJ to giv 
 the mass the proper consistence for cutting wh? i 
 cold. 
 
 Certain tissues are very friable, so that thin s»;c- 
 tiojis fall apart as soon as they are made ; or they 
 may contain cavities so that they do not afford suf- 
 ficient resistance to the razor. In such cases the 
 interstices may be filled with some fluid material 
 which can afterward be rendered solid and resis- 
 tant, so that the whole forms a firm mass ; after the 
 sections are cut the impregnating material may be 
 dissolved out, so as not to interfere with study. 
 The details of such impregnating processes will be 
 found on p. 1 15.* 
 
 * See also the celloidin method. Appendix, p. 252. 
 
li NORMAL HISTOLOGY. 
 
 MOUNTIMG. 
 
 Sections, bits of dissociated tissue, membranes, 
 etc., having been duly prepared, they are to be 
 mounted on a slide for study. The choice of a 
 a fluid 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 for this pur- 
 pose is glycerin. Many of the hardening agents, 
 such as alcohol, precipitates, as above remarked, 
 certain albuminoid substances in the tissues, in the 
 form of minute strongly refractive particles, thus 
 rendering them more or less opaque, or at least 
 translucent. Now glycerin has the power of pen- 
 etrating many such tissues ; and since, as a rule, its 
 index of refraction is much more nearly like that 
 of the albuminous particles, than is the refractive 
 index of the substance lying between them, when 
 the tissue becomes soaked with glycerin the light 
 passes more directly through, and the tissue is 
 more transparent. Many specimens, furthermore, 
 preserve their structural features very perfectly for 
 a long time in glycerin. 
 
 The stained specimens are either soaked until 
 they become transparent in a small dish of glycer- 
 in, and then transferred to a slide and mounted in 
 the same ; or they may be mounted at once, with- 
 
INTRODJJCTION. 13 
 
 out the preliminary soaking. Eosin is soluble in 
 glycerin, so that if a tissue stained in it is mounted 
 in pure glycerin it gradually fades. To prevent 
 this the glycerin used for mounting should be 
 slightly tinged beforehand with eosin. 
 
 The strong refractive power of glycerin, although 
 of value in rendering tissues transparent, is, how- 
 ever, in some cases prejudicial to our aims, because 
 it makes them too transparent ; its refractive power 
 being so nearly like that of the tissue elements 
 themselves that their more minute structure is con- 
 cealed. 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 outlines of the object. 
 
 We have then in using glycerin 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 varying proportions, 
 some less refractive substance, such as water. For 
 permanent preservation, however, most tissues are 
 to be put into pure or nearly pure glycerin. 
 
 Canada balsam is for many tissues a most excel- 
 lent mounting medium. It possesses to a still 
 greater degree than glycerin the power of render- 
 ing them transparent, obscuring proportionately, in 
 the manner above described, certain of their min- 
 ute structural features. This difficulty can, how- 
 ever, be to a considerable extent obviated with bal- 
 
,14 NORMAL JirSTO^^ 
 
 sam as with glycerin, by the judicious use oi col- 
 oring agents. Thus, for example, suppose we have an 
 object containing cells, the exact outlines of which 
 we wish to_ bring clearly into view ; if we stain with 
 haematoxylin alone, and then mount the object in 
 balsam, the nuclei will be distinctly seen, because 
 they have a violet color ; but the cell body will in 
 many cases be well nigh invisible, because it ha3 al- 
 most the same refractive power as the bals^W 
 which surrounds it. If, however, before mounting 
 in balsam we stain with eosin, which colors the cell 
 body, this will be distinctly visible under any cir- 
 cumstances. We shall stain most specimens which 
 are to be mounted in balsam first with haematoxy- 
 lin and then with eosin, and throughout this man- 
 , ual, when the direction is given to " stain a speci- 
 men double," this use of haematoxylin ,and eosin is 
 to be understood. 
 
 The. most convenient way of using Canada bal- 
 sam is in solution in chloroform. The commercial 
 balsam is thinned with chloroform until it drops 
 readily from the end of a glass rod. The mode pf 
 procedure in Canada balsam mounting is the follow- 
 ing : The specimen having been suitably prepared 
 and stained, it is freed as completely as possible 
 from water by touching its edges with a .bit of fil- 
 ter paper, and then placed in a small wide-mouthed 
 bottle containing a few cubic centimetres of common 
 strong alcofiol ; a^fter from ten Hq fifteen minutes it is 
 
:::trod uction. i 5 
 
 transterred to absolute alcohol, where it remains fif- 
 teen minutes. It is then taken out, the superEuous al- 
 cohol removed by filter paper, and laid in oil of cloves. 
 As soon as it becomes transparent, when it usually 
 sinks to the bottom of the dish, it is spread on a 
 slide, the excess of oil removed, a drop of balsam 
 put upon it, and covered by thin glass. This pro- 
 cess, which seems at first somewhat complicated, is 
 readily understood, if we remember that neither 
 the water in which the specimen usually lies when 
 the staining is completed, nor the alcohol, which, be- 
 ing hygroscopic, removes the water, are miscible 
 with balsam ; and further, that alcohol is micsible 
 with oil of cloves, and oil of cloves with balsam. 
 C3.re 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 produces a precipitate in the 
 balsam which greatly diminishes the clearness of 
 the preparation. 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. Prepara- 
 tions in which the blood or lymph vessels are in- 
 jected with some colored material, usually show 
 best when mounted in balsam. 
 
 To make the double staining with hEematoxylin 
 and eosin, as mentioned above, we may first stain in 
 the usual way with an aqueous solution of haema- 
 
l6 NORMAL HISTOLOGY. 
 
 toxylin, and then accomplish the eosin staining by 
 adding a few drops of a saturated solution of eosin 
 in absolute alcohol to the absolute alcohol used for 
 the final dehydration of the specimen, before laying 
 it in oil of cloves. We thus complete the dehydra- 
 tion and stain with eosin at one operation. 
 
 Acetic acid is sometimes used to render tissues 
 transparent. This it does by causing the albumi- 
 noid materials and particles within them to swell and 
 become actually more transparent, differing in this 
 from glycerin and balsam. It produces, however, 
 very considerable changes in the form and character 
 of many tissue elements, so that although very val- 
 uable for special purposes, as will presently be seen, 
 it is now much less frequently employed than for- 
 merly. 
 
 The chemical agents which the histologist uses, 
 and the manipulative devices to which he has re- 
 course in the study of the tissues, are very numer- 
 ous, and we have considered here only a few of the 
 more important and typical. Th6 preparation of 
 each tissue presents to the worker in histology a 
 separate problem, and.in tew departments of science 
 is careful attention to technical minutiae of more im- 
 portance than in that which is now to engage us. 
 
CHAPTER I. 
 THE CELL IN GENERAL. 
 
 In all animal bodies are found certain tiny struc- 
 tural elements called cells. These are parts which 
 at one time or another are alive ; and all the varied 
 activities of the body are the result of the single or 
 combined activities of the cells which are within it. 
 It is very desirable, owing to their great significance, 
 that before commencing the systematic study of the 
 tissues we should acquire a definite conception of 
 the nature of these elementary organisms. We may 
 consider cells from a morphological and from ,i 
 physiological standpoint, asking first, what is their 
 structure ? and second, what do they do ? 
 
 First, then, what is the structure of cells ? We 
 find in this a great diversity, 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 con- 
 sists of an albuminoid material, sometimes transpar- 
 ent and apparently structureless, sometimes finely 
 or coarsely granular, and not infrequently present- 
 ly 
 
1 8 - NORMAL HISTOLOGY. 
 
 ing, at least after death, a reticulated appearance ; 
 this material is z'aAe.A protoplasms 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 pre- 
 sents a great varietj^ of forms ; it may be spherical, 
 cuboidal, cylindrical, fusiform, ovoid, pear-shaped, 
 discoidal, or scale-like ; it often sends off processes 
 like branches or wings, and sometimes assumes the 
 most irregular bizarre^ forms. We not infrequently 
 find imbedded 'in the cell-body, pigment granules, 
 droplets of fat, and various kinds of crystals. The 
 form of the cell-body seems usually to depend large- 
 ly upon the pressure to which it is or has been 
 subjected by adjacent structures. 
 
 Within the cell-body and making up a part of th'e 
 protoplasm, we usually find one or more spherical, 
 ovoidal, or irregular-shaped bodies, called nuclei 
 (singular, nucleus). The nucleus usually presents a 
 sharply defined contour, is frequently coarsely gran- 
 ular, 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 de- 
 generation and decomposition, and under the action 
 of certain chemical agents, the nucleus is more re- 
 sistent than the cell-body, and on treatment of the 
 cell with certain coloring agents, such as haematoxy' 
 
TtiE CELL IN GENERAL. I9 
 
 lin, the nucleus is more deeply stained than the cell- 
 body. Within the nucleus, again, we frequently 
 find one or more small spherical or irregulaf-shaped 
 bodies, looking like vesicles or shining granules, 
 which are called nucleoli. They would seem, in 
 some cases at least, to be connected with the above- 
 mentioned intra-nuclear network. Of the exact 
 nature and significance of the nucleus and nucleolus, 
 we have at present little definite knowledge, but the 
 nucleus, at least, seems to be a very important part 
 of the cell. 
 
 . 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 membrane. It is only 
 in a few varieties of cells, however, that all of these 
 elements are present. The cell-membrane is the 
 least commonly present 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 
 
20 tfORMAL HISTOLOGY. 
 
 for the benefit of the organism of which they form a 
 part; and, finally, under certain circumstances, they 
 are capable of reproducing their like. Or, in more 
 concise language, we say the cell expresses its 
 vitality in nutrition, growth, function, and reproduo 
 tion. Not all of these expressions of vitality, how- 
 ever, can be subjected to direct microscopical 
 observation. Nutrition, being as we believe, essen 
 tially a chemical process, cannot, with our present 
 facilities, become to any considerable extent the ob- 
 ject of direct microscopical study. 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 different phases of its progress. 
 
 The functional activity of cells can be indirectly 
 subjected to microscopical investigation when it i? 
 associated with demonstrable changes in the mor- 
 phological characters of the cell, or directly observed 
 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 is 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 microscope, but in the majority of cases 
 our knowledge is derived from the study of a suc- 
 cession of consecutive stages in the process. Every 
 
THE CELL IN GENERAL. 21 
 
 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 deriva- 
 tives of a single original cell, the ovum. New cells 
 are produced 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 satisfac- 
 tory classified knowledge, and although interesting 
 and valuable facts are rapidly accumulating, we can- 
 not tarry long upon the subject here. 
 
 In one of the apparently simplest modes of cell- 
 division, one or more constrictions appear around 
 the cell-body, after certain changes in the nucleus * 
 have led to its partial or entire division ; these con- 
 strictions grow deeper and deeper, until finally the 
 parts separate and become individual cells, each fur- 
 nished with a nucleus. This form of cell-division is 
 called reproduction by fission. Or, the cell-body 
 may send off from some part a bud-like process, 
 which, becoming nucleated, is, by a constriction of 
 its, pedicle, at length set free as an independent 
 cell ; this is called cell-reproduction by budding or 
 
 * These preliminary changes in the nuclei of proliferating cells, 
 which consist in a readjustment and perhaps new formation of fibres 
 of the intra-nuclear network, and the development of bizarre and 
 often symmetrical figures within the nucleus, are at present engaging 
 the attention of many eminent histologists. The results of their re- 
 searches, while now affording much, and promising far deeper, insight 
 into the nature of cell-life, are not yet sufficiently classified, and are 
 far too intricate to come within the scope of this manual. 
 
22 NORMAL HISTOLOGY. 
 
 gemmation. A mode of division, called endogenous 
 cell-reproduction, is described, in which the division 
 is said to occur entirely within the membrane of a 
 cell, called the parent cell, so that the new organ- 
 isms — the daughter cells — are not at once set free. 
 This mode of cell-division is, however, doubtful, or 
 at least extremely infrequent. These iriodes of cell- 
 reproduction seem to be different, and yet there is 
 much reason for believing that they are really only 
 modifications 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 re- 
 garded merely as for convenience of study, and not 
 as expressing any absolute and fully established 
 truth. 
 
 Cells are variously classified according to their 
 nature and relations to adjacent parts : thus we have 
 epithelial cells, which cover the skin and mucous 
 membranes, and occur in certain parts of the glan- 
 dular organs ; connective-tissue cells, which lie scat- 
 tered throughout the substance of the structures, 
 presently to be described as connective tissue, and 
 in certain parts undergoing modification of form 
 and relation to neighboring parts, and called endo- 
 thelial cells ; gland cells are those which, possessing 
 peculiar functional or morphological characters, 
 make up the parenchyma of certain glands and or- 
 gans. The special characters of these classes of 
 
THE CELL IN GENERAL. 23 
 
 cells, together with those of other classes not here 
 mentioned, will be considered in our systematic 
 study of the tissues. 
 
 PRACTICAL STUDY. 
 
 Many of the above-described general characters of 
 cells may be seen by studying the epithelial cells of the 
 bladder, the pigmented cells of the retina, and the living 
 ciliated cells from the mucous membrane of the frog's 
 mouth. 
 
 Epithelial Cells from the Rabbit's Bladder. — A small piece, 
 including the mucous membrane, is cut from the wall of 
 the bladder of a recently-killed rabbit and immersed for 
 24 hours in a s-per-cent. solution of neutral chromate of 
 ammonium. It is then soaked for an hour in water to 
 remove the chromate solution, and the now loosened cells 
 are scraped from the surface of the mucous membrane 
 and placed in a few cubic centimetres of the picro-car- 
 mine solution, where they remain until they have acquired 
 a distinct red color. A small fragment of the cell mass is 
 now transferred to a slide, covered with a drop of gly- 
 cerin, teased apart with needles, and carefully covered 
 with thin glass so as to exclude all bubbles of air. The 
 cover-glass should be allowed to close down upon the 
 specimen by its own weight alone, since much pressure 
 upon these delicate cells distorts and breaks them. This, 
 like nearly all specimens, 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 have 
 
24 NORMAL HISTOLOGY. 
 
 a finely granular body, with more coarsely granular nuclei, 
 and nucleoli. In many cases, instead of appearing 
 coarsely granular, the nucleus is seen to contain a distinct 
 reticulum (intra-nuclear net-work) of strongly refractive 
 material, the thickened nodal points of which may be re- 
 garded as nucleoli. To preserve this, and all specimens 
 mounted in glycerin, a narrow rim of asphalt varnish is 
 painted around the cover-glass, lapping over the edges of 
 the latter and extending for a short distance on to the 
 slide. 
 
 Pigmented Cells ef the Retina. — An eye from the ox or 
 sheep, 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 pig- 
 mented flakes will float off in the water. These are the 
 desired cells. One or two bits should be put into a drop 
 of glycerin on a slide. Another bit is put on to another 
 slide with a large drop of hsematoxylin solution, and after 
 about ten minutes the coloring fluid carefully washed off 
 with water, and the stained fragments placed with the 
 other in the glycerin on the other slide. They are now 
 covered and examined. 
 
 These cells appear hexagonal* and are joined together 
 
 * These cells have really a very complicated structure, see page 241, 
 but it is not necessary to study this in detail here, since they are only 
 studied now for the purpose of demonstrating the occurrence of pig. 
 ment in the bodies of certain cells. 
 
THE CELL IN GENERAL. 25 
 
 edge to edge, giving a pavemented appearance to the frag- 
 ments. Most of the cell bodies are closely crowded with 
 elongated 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 appear as a rounded, 
 indefinitely outlined structure, containing no pigment and 
 looking like a hole in the cell. When the pigment is 
 present in small quantity or entirely absent, the nucleus 
 is much less sharply outlined. In the cells which have 
 been stained with haematoxylin, however, the nuclei all 
 present well-defined outlines, and are stained of a violet 
 color. 
 
 Ciliary Movement in Living Ciliated Cells from the Frog's 
 Mouth. — The mucous membrane of the roof of the mouth 
 of a living frog is gently scraped with a scalpel, and the 
 slimy mass thus procured is transferred to a large drop 
 of one-half-per-cent. salt solution on a slide, and thor- 
 oughly teased apart. The specimen is now covered, a bit 
 of hair being placed beforehand beside it to prevent pres- 
 sure from the cover-glass. The cells are mostly sphe- 
 roidal, and will be seen isolated or in clusters the cilia, 
 in the form of rows of delicate hair-like processes, spring- 
 ing from one side of the cells. 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, and finally ceases altogether. The movement, when 
 vigorous, often causes cells and masses of cells to revolve 
 and move about in the fluid, and frequently produces 
 
26 NORMAL HISTOLOGY. 
 
 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 ciliated cells, 
 a fragment of the palate of the frog, or, better, of the 
 trachea of the rabbit — since the cells here are much more 
 like those in the air-passages of man, — should be excised 
 and treated in the manner above described for the prepa- 
 ration of the epithelial cells of the bladder. 
 
CHAPTER IL 
 CONNECTIVE TISSUE. 
 
 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 conven- 
 ience of study we may regard the body as composed 
 of simple tissues and of organs As examples of 
 the first we have connective 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 im- 
 portant group, called connective tissues, the members 
 of which, though presenting many marked differ- 
 ences, 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. 
 
 * 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 re- 
 spects from the above-mentioned varieties of connective tissue, but 
 which cannot be separately described here. They will be briefly 
 considered as we meet with them in our systematic study of the parts 
 of the body in which they occur. 
 
 27 
 
28 NORMAL HISTOLOGY. 
 
 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 embryonic life, when the animal is 
 composed almost exclusively of cells, it is found 
 that one of the first definite arrangements or group- 
 ings 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 dif- 
 ferent 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 epi- 
 thelium and glands, and certain of the large organs 
 of the body ; while from the middle layer are de- 
 veloped the muscles, the vascular systems, and, 
 which especially concerns us here, the tissues tabu- 
 lated above as members of the connective-tissue 
 group. Besides the relationship given to them by 
 this common origin, these tissues show their close 
 
CONNECTIVE TISSUE. 29 
 
 alliance by the fact that during the process of devel- 
 opment one is sometimes formed from another. 
 Finally, certain frequently observed pathological 
 conditions seem to consist chiefly in the transforma- 
 tion of one of these forms of tissue into another of 
 the same group. 
 
 With fibrillar connective tissue or connective tissue 
 proper, or simply 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 sup- 
 ports the blood and lymphatic vessels, and forms 
 the limiting membrane of the serous cavities. It 
 forms an encasing membrane for many organs, and, 
 extending into their interior, serves, under the 
 name of interstitial tissue, to support their paren- 
 chyma. 
 
 Fibrillar connective tissue is composed of two dis- 
 
30 NORMAL HISTOLOGY. 
 
 tinct classes of structures : a. cells, and b. a substance 
 lying between the cells, the intercellular or basement 
 substance. We shall consider the latter, first. 
 
 b. — INTERCELLULAR SUBSTANCE. 
 
 The intercellular substance consists chiefly of two 
 distinct kinds of fibres : fibrillated fibres and elastic 
 fibres. 
 
 'YVt. 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 conceivable 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 fibrillae. 
 
 On boiling for a considerable time in water they 
 are converted into gelatin, and when treated with 
 acetic acid or dilute alkalies, they swell up, lose 
 their longitudinal striations, become very trans- 
 parent, and finally almost invisible. 
 
 The second variety of fibres which occurs in con- 
 nective tissue — the elastic fibres — are much more 
 strongly refractive than the first, hence presenting 
 more sharply-marked contours ; they are not longi- 
 tudinally striated, and often branch and form anas- 
 
CONNECTIVE TISSUE. 31 
 
 tomoses 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 gelatin, and they are unchanged by acetic acid 
 and dilute alkalies. These fibres, as their name in- 
 dicates, possess elasticity, and as a consequence of 
 this property we often find, when the fibres have 
 been severed by teasing or other modes of pfepara- 
 tion, that the free ends curl over in the act of re- 
 traction, forming very characteristic curves or 
 spirals. The elastic substance sometimes occurs in 
 the form of granules instead of fibres, or as nearly 
 homogeneous membranes. 
 
 The relative number of the fibrillar and elastic 
 fibres in connective tissue varies greatly in different 
 parts of the body ; in some we find but few elastic 
 fibres, others contain little else. The interstices of 
 the interlacing fibres — both fibrillated and elastic — 
 are filled with the nutritive fluids of the body ; or 
 in some cases, a small amount of a more consistent 
 homogeneous material cements them together. 
 The marked difference in general appearance 
 which is seen in different parts composed of con- 
 nective tissue, is largely due to differences in the 
 arrangement of the fibres and bundles, and in the 
 relative proportion of the fibrillated and elastic 
 fibres. 
 
32 NORMAL HISTOLOGY. 
 
 «.— CELLS. 
 
 We consider next the cellular elements of fibril- 
 lar connective tissue. These are of two distinct 
 classes. First, those which are essential components 
 of it, preserving a fixed and definite relation to the 
 basement substance, and which are quite constant 
 in the different varieties of tissue, in form, size, 
 and number ; these are called fixed connective-tissue 
 cells. Second, small spheroidal cells, the white 
 blood-cells, which, escaping in varying number 
 from the blood-vessels, move about through the in- 
 terstices of the tissues, and are called wandering 
 cells. These will be studied 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 trans- 
 parent, often very thin and scale-like, frequently so 
 delicate as to be difficult of recognition, and some- 
 times 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 rec- 
 tangular, 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 ; 
 
CONNECTIVE TISSUE. 33 
 
 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 con- 
 nective tissue would seem to be largely 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-de- 
 fined closed spaces or cavities, or free surfaces 
 which are movable over one another, as in the great 
 serous cavities, in the blood- and lymph-channels, 
 tendon-sheaths, etc. In these cases the flat con. 
 nective-tissue cells usually undergo some modifica. 
 tion in their form, character, and relations to one 
 another, and are called endothelial cells or endothe- 
 lium. 
 
 Finally, in certain parts of the body, usually in 
 the vicinity of blood-vessels, are found irregular- 
 shaped granular cells, not usually flat, and of vary- 
 ing size and form, called plasma cells, which resem- 
 ble in many respects cells found in the embryo. 
 
 PRACTICAL STUDY. 
 i. — INTERCELLULAR SUBSTANCE. 
 
 Fibres of Subcutaneous Connective Tissue. — To study 
 these, the skin should be reflected back trom the abdomi- 
 
34 JVOSMAL HISTOLOGY. 
 
 nal wall of a recently killed animal (mammal), and choos- 
 ing a part whicH is free from fat, a bit of the loose, so- 
 called areolar tissue is seized with the forceps and 
 snipped off with scissors. The bit of tissue, which will 
 contract to a little lump around the point of the forceps, 
 is to be spread out very thin on a slide and covered with 
 a ^-per-cent. salt solution. The specimen will be seen to 
 consist largely of fibrillated fibres crossing one another 
 in all directions, with a few delicate elastic fibres. After 
 studying in salt solution, a drop of 2-per-cent. sol. 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. This preparation is not to be pre- 
 served. 
 
 Fibrilla in Tail Tendon of Mouse. — It is not easy in 
 studying fresh tissues to convince one's self that the longi- 
 tudinal 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 
 fibrillae are bound together by a small amount of cement 
 substance. If, however, the tissue be placed for a few 
 hours in some fluid which dissolves this cement substance, 
 as osmic acid, the ultimate elements, the fibrillse, may be 
 readily separated. A bit of the tail tendon of a mouse, 
 or a small tendon from any mammal, should be soaked 
 for a few days in a i-per-cent. sol. of osmic acid, washed 
 and teased apart on a slide and mounted in glycerin. 
 
 Elastic Fibres from the Ligamentum Nuchce, — A small 
 fragment of this structure — conveniently obtained from 
 the ox — is preserved in strong alcohol. A tiny bit is 
 
CONNECTIVE TISSUE. 3$ 
 
 teased thoroughly and mounted in glycerin. It will be 
 seen to consist of a dense net-work of broad, closely-an- 
 astamosing elastic fibres which curl over at the free 
 ends. 
 
 «.— CELLS. 
 
 Cells in the Subcutaneous Connective Tissue. — The 
 fixed connective-tissue cells, which occur in this form 
 of tissue, may be studied in any mammal They 
 vary somewhat in size, shape, and number in differ- 
 ent animals, but those in the rabbit are sufficiently 
 typical. In the study of these cells, whose bodies are, 
 for the most part, so thin and transparent as to be 
 almost invisible when fresh, and very liable to shrink and 
 become distorted by contact with the usual hardening 
 agents, 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, not greatly alter the form of the delicate cell- 
 body. 
 
 These ends are best attained by the method suggested 
 by Dr. W. H. Welch. The skin being reflected back 
 from the abdominal wall of a freshly-killed rabbit, with 
 a hypodermic syringe, inject beneath the loose adherent 
 tissue a few cubic centimetres of a solution of analine 
 red (Fuchsine), prepared by adding one part of saturated 
 alcoholic sol. of the powder to forty parts of water. After 
 the red fluid has remained for two or three hours in con- 
 tact with the tissues, a small fragment is snipped off from 
 the colored oedematous portion, spread very thin on a slide, 
 covered with the same fluid, and enclosed at once With 
 
36 NORMAL HISTOLOGY. 
 
 asphalt varnish. The cells in this tissue are, for the 
 most part, extremely thin, and of various, often quite 
 irregular forms, sometimes sending off narrow branching 
 processes, by which they join neighboring cells, and 
 sometimes furnished with wing-like projections. In ad- 
 dition to these cells, and the intercellular fibres, nerves 
 and capillary blood-vessels are sometimes seen in the 
 specimens, and, occasionally, in the vicinity of the ves- 
 sels are found the above-mentioned plasma cells. The 
 cells prepared by this method preserve their form for a 
 considerable time, but finally shrink, and lose their 
 color. 
 
 Pigmented Connective-tissue Cells of the Choroid. — These 
 may be taken from the eye of any mammal (except al- 
 binos) which has been hardened in Miiller's fluid and 
 alcohol. A shred of the outer layers of the choroid 
 should be torn off, stained with haematoxylin, and 
 mounted in glycerin. Irregular- shaped, often branched 
 and flattened cells are seen lying embedded in a mem- 
 branous nucleated basement substance, containing deli- 
 cate elastic fibrils, the cells being more or less crowded, 
 except in the part occupied by the nucleus, with a multi- 
 tude of minute brown or black granules. 
 
 Cells in Teased Tendon. — Tendons are composed essen- 
 tially of parallel bundles of fibrillated connective-tissue 
 fibres, with thin irregular-shaped cells between them. 
 The complicated forms which tendon cells present can 
 be most easily understood when they are studied in their 
 natural relation to the fibres. 
 
 Owing to their extreme tenuity, and the consequent 
 possibility of studying them with but little manipulation. 
 
CONNECTIVE TISSUE. IJ 
 
 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 twist- 
 ing, or pressure with the edge of the nail, and gentle 
 traction be exerted at both sides of the rupture, the 
 caudal vertebrse will readily separate, and, as the trac- 
 tion continues, a multitude 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 parafifin, 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 water. 
 
 One or two of the tiny bundles is now to be partially 
 teased apart, lightly stained with hsematoxylin, and then 
 with picro-carmine ; then still further very finely teased 
 on a slide, and mounted in glycerin. The individual 
 fibrillated fibres will now be readily seen, and arranged in 
 rows along them, edge to edge, with more or less irregu- 
 lar contours, the flattened tendon cells will be seen to 
 lie. Their nuclei, stained violet by the hsematoxylin, 
 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 lines. The 
 cell-bodies, when seen sidewise, look like faintly-colored, 
 often indefinitely-outlined, very thin plates, stretching 
 
38 NORMAL HISTOLOGY. 
 
 out from the nuclei, and partially enwrapping the tendon- 
 fibres. When the cell-body is placed with its edge toward 
 the observer and in close apposition with the fibre, it is 
 not to be distinguished, owing to its extreme thinness, 
 from the line which marks the border of the fibre ; if, 
 however, it has been separated, in part or entirely, 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 cells placed 
 edgewise, he can convince himself that thin, narrow 
 wings are given off from the cell-body, and that it is the 
 edges of these wings which cause the strias in the cells 
 seen on the flat. In teased specimens of tendon one oc- 
 casionally 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 en- 
 wrap certain fibres, and 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. 
 
 Cells in Transverse Sections of Tendon. — A very clear 
 view of the relations 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 imme- 
 
CONNECTIVE TISSUE, 39 
 
 diately immersed in a small quantity 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 centimetre of the following mixture, 
 known as the reducing fluid : 
 
 Amyl Alcohol i 
 
 Formic Acid, . . i 
 
 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 hard- 
 ened for a day in alcohol, embedded in hardened liver 
 or wax, and thin cross-sections made from it. The sec- 
 tions are now stained lightly with hasmatoxylin, and 
 mounted in glycerin. 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 older method of gold staining, known as Conheim's method, 
 differs from that just given, in that, after soaking in the gold solu- 
 tion, the specimen is exposed to the light until it has assumed 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 shaip contrast with surrounding parts, and 
 "■ fixing their form. There is one source of error, however, in the in- 
 terpretation of gold specimens, which should never be forgotten. 
 The gold is apparently sometimes deposited, and the characteristic 
 
40 NORMAL HISTOLOGY. 
 
 Transverse Sections of the Cornea. — Cells seen from the 
 edge. — We shall employ for our study here the cornea of 
 the frog, because of the ease with which it can be ob- 
 tained, 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 Miiller's fluid, where it should 
 remain for ten days. It is then washed, and put, for a 
 few hours, into dilute alcohol (alcohol one, water two), 
 then transferred to, and left, for forty-eight hours, in 
 strong alcohol. The cornea is now excised, just within 
 the sclero-corneal junction, and, two or three short 
 radial incisions having been made at the edge, so that 
 it will lie flat, it is embedded between two bits of hard- 
 ened liver,* and thin transverse sections cut from it. 
 These are stained double, and mounted in glycerin, 
 slightly tinged red with eosin. 
 
 If the sections are made so as to include the entire 
 thickness of the cornea, both the anterior and posterior 
 edges of the section will be seen to be covered with epi- 
 thelial cells. Between these two layers of cells lies the 
 connective tissue su1)stance of the cornea, which alone 
 concerns us here. This consists of delicate fibrillated 
 fibres, closely bound together by a small amount of ce- 
 menting substance, and arranged in lamellae. Between 
 these lamellae are seen the corneal cells, which, in this 
 
 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 and staining really indicates the presence of cell proto- 
 plasm. And, in fact, it is in some cases impossible, at present, to 
 decide this question. 
 
 * Better in celloidin.' See Appendix, p. 252. 
 
CONNECTIVE TISSUE. 4I 
 
 View, seem to have the form of slender elongated spin- 
 dles, part of them being closely surrounded by the in- 
 ter-cellular substance, part lying in small elongated cavi- 
 ties. 
 
 Lamince of Cornea. — the Cells seen on the Flat. — In order 
 to determine the exact shape of the cells, which in the 
 former prepd,ration are seen only from the edge, it is 
 necessary to look at the cornea from the side. For this 
 purpose a fresh cornea should be very carefully excised, 
 all pulling or stretching of the part being avoided, since 
 this distorts the cells. It is now stained with gold 
 chloride by Pritchard's method, and then laid on a slide 
 in glycerin. 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 hsematoxylin and mounted in glycerin. 
 
 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 promi- 
 nent 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 vari- 
 able number of longer and shorter processes ; the nuclei 
 are large, ovoidal, or irregular-shaped, and usually con- 
 tain nucleoli. Fine irregular-branching, almost linear 
 structures are seen in good preparations thickly scattered 
 over the specimen, 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 arej 
 
42 NORMAL HISTOLOGY. 
 
 whether or not they are all cell processes is not yet defi- 
 nitely known. 
 
 It will be seen from the above studies, that the connec- 
 tive-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. — Relation of the Cells to 
 the Basement Substance. — Strong solutions of nitrate of 
 silver have the power of staining the intercellular sub- 
 stance 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 an- 
 terior surface of the cornea quite free. The eye is now 
 held for an instant over a jet of steam, when the epithel- 
 ium 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 necessary 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 epithe- 
 lium being thus removed, a five-per-cent. solution of 
 silver nitrate is allowed to flow over the cornea and re- 
 
CONNECTIVE TISSUE. 43 
 
 main 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 containing a mixture of alcohol and water, one to 
 two, and exposed to direct 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 glycerin and stripped into thin layers, as 
 directed above, for the gold cornea ; the layers are 
 mounted in glycerin. 
 
 If the preparation is successful, clear, branching, com- 
 municating spaces are seen on a yellowish or brown 
 ground. These spaces, although larger, evidently corre- 
 spond in position and in a general way, in shape, 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 permeated by numerous 
 branching and intercommunicating spaces, and that in 
 these spaces the flat, branching connective-tissue cells lie. 
 
44 NORMAL HISTOLOGY. 
 
 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 modifica- 
 tions, of the cells in most of the varieties of connective 
 tissue. These cells lie in spaces, sometimes completely, 
 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 ma- 
 terial. 
 
 It is extremely probable that it is through these lymph 
 spaces exclusively that the white blood-cells travel, in 
 their perigrinations through the tissues. We study them 
 in the cornea alone, because the scope of this manual is 
 too limited to admit of such a detailed study of all vari- 
 eties of connective tissue, and because here the relation 
 of the cells to the spaces is clearly defined. 
 
 Endothelium of the Serous Membranes. — The serous 
 membranes are formed by layers of fibrillar connective 
 tissue fibres mingled with a varying number of elastic 
 fibres, and containing ordinary flattened connective-tissue 
 cells. They are more or less abundantly furnished with 
 blood and lymphatic vessels. On the free surface of 
 these membranes rests a continuous layer of flattened 
 cells, differing in many respects from the ordinary con- 
 nective-tissue cells, and called endothelial cells or endothel- 
 ium. These ce^s are usually transparent, irregularly 
 polygonal in form, and frequently much elongated ; they 
 possess one or more ovoidal nuclei, which often project 
 
 • 
 
CON^NECTIVE TISSUE. 45 
 
 above the general 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 together by a minimal 
 amount of an albuminoid cement substance.* 
 
 Endothelium covering the Mesentery. — 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 sil- 
 ver, 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 mesentery 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 stretch- 
 ing and pulling of the membrane should be avoided, be- 
 cause 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 water to remove any albuminous substance 
 or blood — which would cause a granular precipitate of 
 silver albuminate on the surface — and the dish then filled 
 with an aqueous solution of nitrate of silver, i to 500. 
 The dish should be gently shaken at frequent intervals, 
 so as to bring fresh portions of the solution into con- 
 tact with the membrane, and after from twenty minutes 
 
 * There is reason for believing that the cement substance between 
 the endothelium, as well as between many kinds of epithelial cells is 
 permeable to fluids, and, under certain conditions, to solid particles 
 also, and thus forms an avenue of communication between adjacent 
 but separated cavities or lymph spaces, 
 
46 NORMAL HISTOLOGY. 
 
 to half an hour the tissue will be seen to have become 
 cloudy or milky. 
 
 The silver is now poured off, and the membrane care- 
 fully washed with water. The cells will have been fixed 
 by the silver, so that the membrane may be removed 
 without further danger of disturbing the relation of the 
 cells, and laid in a dish containing water to which one- 
 third its bulk of alcohol has been added. It is now ex- 
 posed to the sunlight, and after from a few minutes to 
 half an hour — sometimes longer, depending upon the in- 
 tensity of the light — the tissue will be seen to have as- 
 sumed a brown color. 
 
 A small piece from the thinner portion should now be 
 lightly stained with hsematoxylin and mounted in glyc- 
 erin. 
 
 These portions of the mesentery consist of a thin mem- 
 brane of fibrillar connective tissue containing a delicate 
 network of elastic fibres and a few small blood-vessels ; 
 the whole being covered on both sides by the delicate 
 mosaic of endothelial cells. The outlines of these cells 
 on both sides, may be brought successively into view by 
 careful focussing. 
 
 Endothelium of the Omentum. — A portion of the omen- 
 tum of the dog should be treated with silver in the way 
 just directed for the mesentery, and also stained with hse- 
 matoxylin and mounted in glycerin. The omentum of 
 the dog, as of man, consists of an irregular meshed net, 
 whose trabeculae are made up of fascicles of fibrillar 
 connective tissue of varying thickness, the broader con- 
 taining blood- and lymph- vessels and fat-cells, the nar- 
 rower consisting of single bundles of fibrillse — all being 
 
CONNECTIVE TISSUE. 47 
 
 alike covered with a single layer of endothelium. 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 speci- 
 mens 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 care- 
 fully kept from the light. 
 
CHAPTER III. 
 
 EMBRYONAL AND MUCOUS TISSUE.— FAT TISSUE.— 
 RETICULAR CONNECTIVE 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 between 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 elon^ 
 gated and fusiform, often terminating at their ex- 
 tremities 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 accumula- 
 tion of interceillular material, which is at first fluid, 
 and later presents the appearance of a homogene- 
 ous gelatinoid substance. Then within the gelati- 
 noid intercellular substance appear fine fibrillse, 
 which become more and more abundant, arranging 
 themselves now in bundles, and again to form irregu- 
 
 48.' 
 
EMBRYONAL AND MUCOUS TISSUE. 49 
 
 lar networks. The cells approach more and niore 
 closely to the type of the adult connective-tissue 
 cells, as development goes on ; the intercellular sub- 
 stance 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 gradual 
 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 mat- 
 ter of convenience, we call connective tissue which 
 is almost entirely made up of spheroidal, spindle- 
 shaped, or flattened cells, in which little accumula- 
 tion 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 fibri- 
 lated 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 intercellular substance was found to con- 
 tain a certain amount of mucin, which may be 
 thrown down in the form of a whitish, often stringy 
 precipitate, by the addition of acetic acid. At 
 present, however, tissues presenting the above 
 mentioned morphological characters are usually 
 
so NORMAL HISTOLOGY. 
 
 called murous tissues, whether the intercellular sub- 
 stance contains mucin or not. Mucous tissue is not 
 found in the healthy human adult, but frequently 
 occurs under pathological conditions. 
 
 PRACTICAL STUDY. 
 
 Subcutaneous Tissue of Emh-yo. — Embryonal connec- 
 tive tissue may be studied in any young mammalian em- 
 bryo, preserved in Miiller's fluid and alcohol. Bits of the 
 subcutaneous tissue are torn off from the abdominal wall, 
 stained double, finely teased, and mounted in glycerin. 
 
 Mucous Tissue from the Umbilical Cord. — In the um- 
 bilical cord of a nearly mature foetus we find typical mu- 
 cous tissue. A bit of the cord of any mammalian foetus, 
 as the pig, is hardened in Miiller's fluid and alcohol, then 
 embedded in liver and transverse sections made. These 
 are stained double and mounted in glycerin or balsam. 
 The surface of the cord is seen to be covered with lami- 
 nated epithelium, and the three large blood-vessels are 
 seen to be cut across. The amount of fibrillation present in 
 the intercellular substance depends upon the age of the 
 foetus. 
 
 FAT TISSUE. 
 
 Fat-tissue is a modified form of connective tissue, 
 in which the intercellular substance is present in 
 proportionally small amount, and a large part of the 
 protoplasm of the cells is replaced by fat, which 
 crowds the remaining part, together with the nu- 
 cleus, to the side of the cell, nearly or entirely con- 
 
FAT TISSUE. 51 
 
 cealingboth. 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 lym- 
 phatic vessels, etc. Owing to the pressure to which 
 the fat-cells are subjected, they usually assume, in 
 the adult animal, a polyhedral form. 
 
 Sometimes the fat appears within the cell in the 
 form of clusters of radiating needle-like crystals. 
 
 In order to understand clearly the nature of adult 
 fat-tissue, it is necessary to study it during the pro- 
 cess 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 fibrillated 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, be- 
 come gradually larger, until they present the form 
 and character of distinct droplets of fat. As these 
 droplets 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 pro- 
 toplasm and a squeezed and distorted nucleus 
 
52 NORMAL HISTOLOGY. 
 
 crowded up against the cell-membrane. At last we 
 can no longer see, withput special modes of prepara- 
 tion, any trace of cell protoplasm — although a small 
 amount of this really persists as a thin shell within 
 the membrane — and only the deformed remnant of 
 the nucleus. This process is called fatty infiltra- 
 tion. 
 
 In those parts of the body where the fat is invari- 
 ably found, this change in the cells occurs in the 
 vicinity 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 deter- 
 mine, when the fat is fully formed, the lobular char- 
 acter of this tissue. 
 
 In many parts of the body and under varying 
 conditions — sometimes physiological, sometimes 
 pathological — there is an accumulation of fat in the 
 protoplasm of cells; but it is, under normal condi- 
 tions 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, teased. — A bit of fresh subcutaneous fat 
 should be teased and covered in salt solution, without 
 staining. In such a specimen little else is seen than a 
 
FAT TISSUE. 53 
 
 congeries of more or less globular or polyhedral masses 
 of fat-cells, with larger and smaller free fat droplets. The 
 fat is characterized by its great refractive power ; by the 
 dark contour of its globules by transmitted, and their yel- 
 lowish silvery lustre by reflected, light ; by its solubility in 
 alcohol and ether, and by the deep black color which it 
 acquires on treatment with osmic acid. The connective 
 tissue which encloses the lobules is seen in broken masses 
 and shreds, scattered through the preparation. 
 
 Section of Adult Fat IHssue. — A small piece of subcuta- 
 neous fat from man should be hardened in alcohol, by 
 which the fat will be for the most part dissolved out of 
 the cells. Thin sections are made, stained double, and 
 mounted in balsam. This preparation shows the relation 
 of the cells to one another, and the lobular structure of 
 the tissue. If the blood-vessels of the part from which 
 the tissue is taken have been previously injected with the 
 blue gelatin mixture, the relations of the vessels to the 
 lobules and cells will be well shown. 
 
 Developing Fat from Young Animals. — To study this in 
 its early stages, some of the fresh mucous tissue from the 
 axilla or groin of a foetal animal, such as a pig, 5 inches 
 long, should be immersed for 24 hours in i-per-cent. osmic 
 acid ; then washed and teased, and mounted in glycerin 
 slightly tinged with eosin. The larger and smaller fat 
 droplets within the cells will be black, and numerous cells 
 will be seen in which the fatty infiltration has not com- 
 menced. 
 
 To study the fat cells at a later period of development, 
 thin sections may be made from the subcutaneous fat of 
 the human fcetus, at from 6 to 8 months, or from any 
 
54 NORMAL HISTOLOGY. 
 
 mammal of corresponding age. These are stained double 
 and mounted in balsam. At this period, while the fat 
 droplet in many parts occupies the greater part of the 
 cell, a distinct crescentic mass of protoolasm is still seen 
 at one side, enclosing the nucleus. 
 
 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 delicate fibres, of varying diameter, 
 which cross and join one another at -frequent inter- 
 vals, forming a fine meshed network. This net- 
 work of fibres is not flattened to form a membrane, 
 but extends in all directions, like the trabeculae 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 these cells are in j^Vwupon the fibres, 
 the whole presents the appearance of a mass of an- 
 astomosing, branched, or spindle-shaped cells ; and, 
 as such, the reticular connective tissue has until re- 
 cently been regarded,-^erroneously, however, as is 
 shown by the fact that by appropriate manipulation 
 the flat cells can be entirely freed from the underly- 
 ing fibre-net, leaving the latter intact. The meshes 
 of the reticular tissue are loosely filled, in the lym- 
 
RETICULAR CONNECTIVE TISSUE. 55 
 
 phatic glands, with small spheroidal cells — lymph 
 cells — which, however, seem to have no direct con- 
 nection with the tissue we are studying, and may be 
 easily removed. 
 
 PRACTICAL STUDY. 
 
 Section of the Lymphatic Gland of Dog, Treated with Osmic 
 Acid. — One of the mesenteric or cervical glands is re- 
 moved from a recently-killed dog, and a hypodermic 
 syringe being partially filled with i-per-cent. solution of 
 osmic acid, the canula is thrust well into the gland, and 
 the acid slowly injected until the organ becomes quite 
 tense ; the canula is then withdrawn, and the gland placed 
 in strong alcohol. After a few days it will be hard 
 enough to make sections from.* The sections, which must 
 be very thin, 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. As these cells are shaken out, the sections look thin- 
 ner, and when the operation is completed they are stained 
 with hsematoxylin and mounted in glycerin. 
 * 1 1 should be embedded in celloidin. 
 
CHAPTER IV. 
 
 CARTILAGE— BONE— TEETH. 
 
 CARTILAGE. 
 
 Cartilage consists, like other members of the 
 connective-tissue group, of cells and intercellular 
 substance. There is nothing characteristic, how- 
 ever, 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 finely granular 
 and often contains tiny droplets of fat, and some- 
 times pigment-granules. The cells have one, or 
 sometimes two, sharply defined nuclei, which are 
 coarsely granular and often contain an irregular 
 network of a more strongly refractive substance. 
 Around each cartilage-cell, in the adult animal, and 
 closely enclosing it, is a homogeneous envelope 
 called the capsule. The substance forming this cap- 
 sule 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 
 
 56 
 
CARTILAGE. 57 
 
 of a great variety of substances, and 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 ap- 
 pearance ; 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 cartilages, and, according to the dif- 
 ferences in its nature, cartilage is divided into hya- 
 aline cartilage, fibro-cartilage, fibro-elastic cartilage. 
 
 In hyaline cartilage the intercellular substance is 
 homogeneous and transparent in thin layers, some- 
 what opalescent in thicker masses ; it is of firm 
 consistence, and contains at tolerably regular inter- 
 vals variously shaped cavities in which the cells lie, 
 exactly filling them. The layer of basement sub- 
 stance which immediately surrounds the cells pos- 
 sesses slightly different refractive power, and it is 
 this layer which constitutes the capsule above men- 
 mentioned. 
 
S8 NORMAL HISTOLOGY. 
 
 The cell-spaces or cavities in hyaline cartilage do 
 not by the ordinary modes of preparation 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, renders it ex- 
 tremely probable that such communications do 
 exist, though we are not at present able to demon- 
 strate them with certainty. Although by the ordi- 
 nary modes of preparation the basement substance 
 of hyahne cartilage appears quite homogeneous, 
 certain changes which it undergoes under patholog- 
 ical 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 dif- 
 fers 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. e., that cartilage gave, on 
 boiling, a peculiar and characteristic substance 
 called chondrin, it having been shown that the so- 
 Cjilled chondrin is not a pure chemical substance, 
 but a mixture of gelatin, mucin, and certain salts. 
 
 Hyaline cartilage is found, in the adult, covering 
 the ends of the bones in the joints ; and most of the 
 laryngeal and the tracheal, bronchial, costal and nasal 
 cartilages are of this variety. 
 
CARTILAGE. 59 
 
 Fibro-cartilage differs from hyaline cartilage in 
 having a distinctly fibrillated intercellular substance. 
 This form of cartilage is found in the intervertebral 
 cartilages, in the meniscuses 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 
 substance contains, in addition to a few fibrillated 
 fibres, 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 
 hyaline cartilage. 
 
 Except at the free surfaces, which it presents in 
 the joints, cartilage is surrounded by a layer of 
 fibrillated connective tissue of varying ' thickness, 
 called the perichondrium, in which are found the 
 blood-vessels which supply nutritive material to the 
 non-vascular cartilage 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. 
 
 Hyaline Cartilage from Femur of Frog. — The head of 
 a femur of a recently killed animal being exposed, a thin 
 slice of cartilage is shaved off with a razor so as to leave 
 
6o NORMAL HISTOLOGY. 
 
 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 com- 
 mences to evaporate. 
 
 In such thin sections, cavities are seen here and there 
 from which the cells have fallen out ; these may be filled 
 with the preservative fluid, or with bubbles of air. Picric 
 acid is one of the best agents for preserving the normal 
 characters of cartilage cells, but even in this they shrink 
 somewhat, and after a time become coarsely granular. 
 
 Shrinkage of Cartilage Cells by Strong Salt Solution. — 
 Since in most of the specimens containing cartilage, 
 which we shall study later, the cells will have been con- 
 siderably shrunken by the preservative fluids, it is desira- 
 ble for us to acquaint ourselves here with the process of 
 shrinkage, so that we may understand the various dis- 
 torted forms which such cells present. To do this, we 
 have simply to prepare a thin section of hyaline cartilage, 
 as above, and mount it at once in half-per-cent. salt solu- 
 tion. When a good field for observation has been se- 
 lected, a drop of ten-per-cent. salt solution (alcohol will 
 do as well) is run in at one edge of the cover glass, the 
 agent being drawn through by filter paper at the other. 
 In this way the process of shrinkage and formation of 
 vacuoles may be readily observed. 
 
 Fibro-cartilage may be studied in thin sections from 
 the intervertebral cartilages of man or any of the domes- 
 tic 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 
 
BONE. 6l 
 
 alcohol before cutting. The sections are stained with 
 picro-carmine, and mounted in glycerin. 
 
 Fibro-elastic Cartilage. — This is best studied in the 
 epiglottis of man or the lower animals, which has been 
 preserved in alcohol. Thin sections are stained with 
 picro-carmine, which colors the cells red, and the elastic 
 granules and fibres yellow. They are mounted and pre- 
 served in glycerin. The cells in both fibro- and fibro- 
 elastic cartilage are, by the above modes of preparation, 
 more or less shrunken and deformed. 
 
 BONE. 
 
 In studying bone we have to consider: i, the 
 hard substance, or bone-tissue proper ; 2, the connec- 
 tive tissue envelope which surrounds the bone — the 
 periosteum ; and, 3, the marrow contained in the 
 central cavities or spaces within. 
 
 I. 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. Various 
 acids dissolve these lime salts, and when they are 
 removed a substance 'is left behind which retains, 
 for the most part, both in general form and 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 
 
62 NORMAL HISTOLOGY. 
 
 an extremely minute state of division as to be in- 
 visible, even with high powers of the microscope. 
 The elongated and flattened cell-spaces of bone are 
 frequently called lacuncB, and the numerous fine, 
 branching, intercommunicating 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-curpuscles, as they were for- 
 merly called, are, in adults, thin, flat cells, with sphe- 
 roidal or oval projecting nuclei. It was formerly be- 
 lieved that they sent fine branching processes ofi into 
 the ramifications of the canaliculi. Recent investiga- 
 tions, however, 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 sup- 
 posed processes are really portions of the intercel- 
 lular substance lining the lacunse and canaliculi. 
 In young bone the cells are not flat, but spherical 
 or ovoidal. 
 
 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 sub- 
 stance, or bone proper, is arranged in the form of 
 thin plates, which are grouped together so as to 
 enclose tiny, irregular-shaped cavities, filled with 
 marrow-tissue. In these thin plates of spongy 
 
BONB. 63 
 
 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 ar- 
 ranged in layers, or lamellae, in and between 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 nu- 
 merous round or ovoid, or irregular-shaped open- 
 ings, of varying size, and around these are grouped 
 several thin concentric layers of basement substance, 
 in and between which lie the cells, flattened in the 
 plane of the lamellae. 
 
 These sets of concentric lamellae are called special 
 or Haversian systems of lamellce. 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 interven- 
 ing 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 directions. 
 These are called, from their position relative to the 
 Haversian system, intermediate lamellcB. Finally, at 
 the surface of the bone beneath the periosteum, and 
 sometimes at the inner surface adjacent to the me- 
 dullary cavity, are seen a series of lamellae which lie 
 parallel to the surfaces of the bone, and are called 
 general or circumferential lamellce. If we make a 
 
64 NORMAL HISTOLOGY. 
 
 longitudinal section of a long bone, we find that it 
 is traversed by a number of more or less longi- 
 tudinally arranged, branching and communicating 
 canals, of varying size, in which lie the blood- and 
 lymph-vessels. It is around these canals, called 
 Haversian canals, that the Haversian lamellae are 
 grouped, and the variously shaped openings which 
 are seen in the transverse sections, are transverse 
 sections of these vascular or Haversian canals. 
 Within the Haversian canals, when they are not en- 
 tirely 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 bones 
 essentially thesame structural features are present, 
 but the lamellar arrangement is much less regular. 
 
 Although, for the most part, the intercellular sub- 
 stance of bone is, by the ordinary modes of prepa- 
 ration, apparently quite destitute of structure be- 
 yond that indicated by its lamellation, we yet find 
 in certain portions a well-defined system of fibres. 
 If, in a decalcified bone, some of the external la- 
 mellae are torn off, numerous fine, fibrillated, 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 
 
BONE. 65 
 
 remains of fibrillated 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 delicately 
 fibrillated, but we have not space in this manual to 
 consider the methods by which this may be demon- 
 strated. 
 
 2, Periosteum. — The periosteum consists chiefly 
 of fibrillated connective 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 apo- 
 neuroses surrounding the bone ; and an inner layer, 
 which is looser in texture, more vascular, and abun- 
 dantly 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 cen- 
 tral or medullary cavity of bones, in the tiny cham- 
 bers of spongy bone, and in the Haversian canals. 
 Sometimes it has a yellowish color and is fatty, 
 sometimes it presents itself in the form of a red- 
 
66 NORMAL HISTOLOGY. 
 
 dish pulp. Red marrow is found in embryos and 
 in young animals, and in adults in certain small 
 bones and in 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 normal conditions, the marrow — except 
 in the vertebrae, ribs, and certain small bones — is yel- 
 low. Yellow marrow differs from the red in that it 
 contains a large amount of fat, sometimes consist- 
 ing 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 sev- 
 eral distinct kinds of cells: i. 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 some- 
 what larger than the last mentioned, with, usually, a 
 single very irregular-shaped and sharply defined nu- 
 cleus ; 4. very large granular cells, which usually have 
 several nuclei scattered through the cell-body, or 
 grouped on one side, the so-called myeloplax'es or 
 giant cells. It is not improbable that the two 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 
 
BONE. 67 
 
 osteoblasts, with which we shall become better ac- 
 quainted when we study the process of bone-de- 
 velopment. 
 
 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, 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 trans- 
 formation 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 struc- 
 ture of bone, we have recourse to transverse and longitudi- 
 nal 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. 
 
68 NORMAL HISTOLOGY^ 
 
 Although various acids, such as nitric and hydro- 
 chloric, effect the decalcification of bone, solutions of 
 chromic or picric acids are preferable, 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. The quantity of fluid should 
 also be quite large (two hundred to three hundred cubic 
 centimetres to a bit of bone of the above size). Picric 
 acid, although slow in its action, is, on the whole, to be 
 preferred, because the chromic acid often leaves the tis- 
 sues 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 solution of 
 one in six hundred ; in a couple of days it is transferred 
 to a fresh solution of one in four hundred, and again in a 
 couple of days to another solution of one in two hun- 
 dred. A stronger solution than this should not 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 
 jjassing a fine needle into the preparation. If it be de- 
 sirable 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 solution (one c.c. to one hundred 
 CO.). The previous action of the chromic acid will pre- 
 vent the swelling and partial destruction of the soft 
 parts, which nitric acid alone causes. If picric acid be 
 
BONE. 69 
 
 employed, a saturated aqueous solution should be used, 
 the preparation frequently shaken, and additional crys- 
 tals 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. Longitudinal and transverse 
 sections should be made, and, if decalcified by chro- 
 mic acid, are best stained double with hsematoxylin 
 and eosin ; if by picric acid, the structure shows very 
 well after staining with picro-carmine. Both may be 
 mounted in glycerin. If the periosteum has not been 
 removed, its structure and relation to the bone are 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, because 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 (the 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 
 
70 NORMAL HISTOLOGY. 
 
 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 w^er with a 
 fine pencil to remove particles of dirt. It is now al- 
 lowed to dry, and is mounted in balsam. For this pur- 
 pose 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 
 lacunae and canaliculi are clearly defined by reason of 
 the air with which they are filled. Usually, however, in 
 the most successful preparations, in the very thin parts or 
 at the edges, part of the canaliculi have become filled 
 with the balsam and rendered invisible. 
 
 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 (one to two). Fragments are then 
 teased on a slide in glycerin which has been colored 
 with eosin, and mounted in the same. By this process 
 the red blood-cells are destroyed, and the characters of 
 the so-called nucleated red blood-cells changed. For 
 the study of the latter, fresh ma»rtow should be teased on 
 a slide in the indifferent salt solution. 
 
DEVELOPMENT OF BONE. 7I 
 
 DEVELOPMENT OF BONE. 
 
 At a certain period of embryonic life no bone-tis- 
 sue is found in the body, the parts where it is finally 
 to be, being occupied either by cartilage or fibrillar 
 connective tissue. Out of these tissues the bone is 
 developed by a process which, though presenting 
 considerable differences 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 de- 
 veloped : I. In the substance of preexisting car- 
 tilage — intra-cartilaginous ; 2. beneath the perios- 
 teum — sub-periosteal ; 3. in the substance of preex- 
 isting fibrillar connective-tissue membranes — intra- 
 membranous. In all of these modes of bone-forma- 
 tion the new bone seems to be deposited under the 
 influence of certain large, granular, usually spheroidal 
 or cuboidal cells, called osteoblasts. 
 
 I. 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 substance between 
 them becoming partially* absorbed, and what re- 
 mains of the latter becoming infiltrated with fine 
 granules of lime salts. Around this calcified por- 
 tion we find the blood-vessels of the perichondrium 
 
72 NORMAL HISTOLOGY. 
 
 accompanied by marrow-tisssue and the above- 
 mentioned osteoblasts, growing into the calcified 
 cartilage, absorbing the latter as they go, and form- 
 ing irregular channels or cavities called medullary 
 spaces. These channels are first separated from 
 one another by narrow, irregular septa of the cartil- 
 age basement substance which remains unabsorbed, 
 and are lined by layers of osteoblasts, by whose 
 agency the septa become covered with new bone, 
 in a manner presently to be described. 
 
 The region in which this new bone is first depos- 
 ited is called the ossification zone. As the blood- 
 vessels, accompanied by the osteoblasts and mar- 
 row-tissue, proceed further and further into the car- 
 tilage, 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 cartil- 
 age ; and beyond this, cartilage-tissue which appar- 
 ently prepares the way for the advancing marrow- 
 spaces and newly forming bone, by very character- 
 istic modifications, chiefly in the form and arrange- 
 ment of its cells. 
 
 If we examine the cartilage at a considerable 
 distance from the line of ossification, we find the or- 
 dinary appearance of hyaKne cartilage with more or 
 less flattened cells. Approaching now the zone of 
 ossification, we find that the cells are larger, are 
 arranged in rows or groups of frequently four, eight. 
 
DEVELOPMENT OF BONB. 7$ 
 
 or sixteen, etc., the intercellular substance being less 
 in amount, corresponding to the increase in size and 
 number of the cells. Further 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, enclosing 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. Finally, still nearer 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 marrow-cavities, 
 above described. It is to be distinctly understood 
 that the calcification zone is not bone, but only cal- 
 cified 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 medul- 
 lary spaces, the basement substance of true bone 
 begins to be deposited, at first in the form of a nar- 
 row shell beneath each osteoblast. These deposits, 
 
74 NORMAL HISTOLOGY. 
 
 which on cross-sections have a crescentic shape, be- 
 come 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 osteo- 
 blasts appear on the walls, and in turn become en- 
 closed in a layer of bone, and thus the lamellar 
 arrangement of bone-tissue is produced. The re- 
 mains of cartilage basement substance between the 
 medullary space thus covered by bone finally dis- 
 appear in a manner unknown to us. 
 
 2. Hand in hand with the formation of bone 
 within the cartilage, new bone is formed on its sur- 
 face beneath the perichondrium, which thus becomes 
 periosteum. The process of sub-periosteal ossifica- 
 tion, 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 sur- 
 face appears at first by no means in the form of 
 smooth, continuous layers, for as the blood-vessels 
 and connective-tissue bundles, along which the oste- 
 oblasts lie, are arranged at varying angles with the 
 surface of the bone and with each other, the effect 
 
DEVELOPMENT OF BONE. ?$ 
 
 is to produce irregular-branching cavities, oipon 
 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-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 forma- 
 tion of bone has taken place along the bundles of 
 connective tissue, these bundles sometimes persist 
 for a long time, in a modified form, among the 
 lamellae, and constitute the above-mentioned SAar- 
 pej/'s fibres. 
 
 Thus, by the transformation of cartilage and ap- 
 position at the surface, the long bones are formed. 
 In these bones the ossification progresses toward the 
 epiphyses, where independent centres of ossification 
 are established. The lines of ossification approach 
 each other, and finally, when the process of growth 
 in the bone is complete, the band of cartilage which 
 separated them disappears, and epiphysis and diai- 
 physis join to form a single bone. As the bone 
 grows by apposition beneath the periosteum, the 
 osseous tissue which was first formed in the diaphy- 
 sis 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 bundles which occupy 
 
76 NORMAL HISTOLOGY. 
 
 the place 6f the future bone become infiltrated with 
 lime salts ; along these calcareous bundles cells be- 
 come 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 branching 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 sur- 
 faces. 
 
 The mode of origin of the osteoblasts is still very 
 obscure. Many investigators believe that in the 
 intra-cartilaginous ossification they are the large car- 
 tilage-cells which we see at the calcification line, 
 which in contact with the blood-vessels become so 
 modified in form and function as to assume the role 
 of bone-formers. Others assert that the large carti- 
 lage-cells disintegrate and disappear, and that the 
 osteoblasts are produced from cells which accom- 
 pany the blood-vessels. Still others regard them as 
 white blood-cells, modified and endowed with new 
 functional powers. In the intra-membranous and 
 sub-periosteal ossification, many suppose that they 
 
DEVELOPMENT OF BONE. 77 
 
 are formed from connective-tissue cells. So little is 
 absolutely known, however, as to their genesis, that 
 while recognizing their importance in bone-forma- 
 tion, we can regard none of these various theories as 
 to their origin as definitely established. 
 
 PRACTICAL STUDY. 
 
 Inira-cartilaginous and Sub-periosteal Ossification. — A 
 long bone, from a nearly mature foetus or a young ani- 
 mal, should be carefully removed without injuring the 
 periosteum, and decalcified.* Thin longitudinal sections 
 are made with a razor through the ossification zone, em- 
 bracing the tissue for a considerable distance on either 
 side of it. The sections are stained double and mounted 
 in balsam. 
 
 Jntra-membranous Ossification. — To study the early 
 stages of this process, a young embryo (if from the sheep 
 or pig, four to six cms. long) should be soaked for a few 
 days in Muller's fluid, and a bit corresponding to the por- 
 tion 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 carefully brushed with a stiff pencil until 
 it is thin enough to be examined with tolerably high 
 powers. It is stained double and mounted in 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 six or 
 
 * Embed in celloidin. 
 
78 NORMAL HISTOLOGY. 
 
 seven months, or from the beef or sheep, sixteen to 
 twenty cms. long). These should be stained and mounted 
 as above. 
 
 TEETH. 
 
 The teeth have many structural features in com- 
 mon with bone. The chief bulk of the tooth is 
 made up of homogeneous, brittle basement sub- 
 stance much harder than bone, called dentine. The 
 dentine 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 /M^, containing irregular-shaped, often 
 branching cells, and nerves. Along the sides of the 
 pulp cavity lie spheroidal or ovoid cells, which send 
 oil branches into the pulp and also into the above- 
 mentioned delicate channels in the dentine. These 
 cells are called odontoblasts or dentine 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. Sur- 
 rounding 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 vary- 
 ing .thickness, of an extremely hard substance called 
 
TEETH, 79 
 
 enamel. The enamel consists of a series of closely 
 packed, small, wavy or undulating prisms, placed 
 edgewise upon the surface of the dentine, and cov- 
 ered, over the free surface of the tooth, by a hard, 
 tough, structureless membrane, called the enamel 
 cuticle. 
 
 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 sections from teeth which have been decalci- 
 fied with picric acid. The tooth should be broken 
 across, so as to expose the pulp cavity and hasten the 
 action of the solvent. Sections are stained with haema- 
 toxylin and eosin, and mounted in glycerin. 
 
CHAPTER V. 
 BLOOD AND LYMPH. 
 
 BLOOD. 
 
 Although strikingly different in physical char- 
 acter from most animal tissues, we must yet regard 
 blood and lymph as true tissues — tissues with a fluid 
 intercellular substance. Let us first consider the 
 blood. 
 
 In normal human blood we find suspended in a 
 colorless fluid, the plasma, three distinct kinds of 
 formed elements : i. Colorless blood-cells ; 2. red blood- 
 cells ; 3. vd^rionsly shdi^ed free granules* 
 
 I. Colorless or White Blood-cells or Leucocytes. — 
 These are small, usually spheroidal nucleated cells, 
 without a membrane, the cell-body being finely, or 
 sometimes coarsely, granular. The nuclei, of which 
 there may be one or more, are not usually visible in 
 the living cells on account of the granular character 
 of the body which conceals them^ and are of vary- 
 ing form, — sometimes spheroidals or dumb-bell- 
 shaped, sometimes having the forni\ of a bent or 
 twisted cylinder, and again entirely irregular. 
 
 These cells possess the power, under favorable 
 conditions, of spontaneous movement. They change 
 • See Appendix, p. 253. _^ 
 
BLOOD. 8 1 
 
 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 with- 
 drawn, 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 loco- 
 motive 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-cells occurs apparently, to a slight extent, 
 under normal conditions ; but it is under patho- 
 logical conditions that it is most active. 
 
82 NORMAL HISTOLOGY. 
 
 The nuclei 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 discs, consist apparently, simply 
 of a cell-body without membrane or nucleus. Al- 
 though when crowded together in great numbers 
 they give the blood a distinct red color, when seen 
 singly 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 with- 
 drawn from it. When examined fresh in the plasma, 
 many of the cells arrange themselves closely to- 
 gether side by side, in longer and shorter rows. 
 
 If water is mixed with blood, the red cells soon 
 begin to swell and lose their color. Sometimes 
 one side swells faster than the other and the cells 
 become cup-shaped ; finally they become globular, 
 are considerably larger than at first, and colorless. 
 On being drawn from the vessels, if the plasma 
 
BLOOD. 83 
 
 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 crenu- 
 late and jagged, and they sometimes seem to be- 
 come smaller ; finally, they assume the form of ir- 
 regular globular masses, beset with short, blunt 
 spines. Various other irregular forms are produced 
 under the same circumstances which it is not neces- 
 sary to describe here. The addition of water causes 
 the spines to disappear, and the cells swell up and 
 assume a globular form. These changes in form, 
 produced by chemical agents and by change in the 
 density of the fluid in which the cells lie, should not 
 be mistaken for an expression of vitality, or re- 
 garded as analogous with the amoeboid movements 
 of the white blood-cells. 
 
 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. It is 
 estimated that in man there are between four and 
 five million red blood-cells in one cubic millimetre 
 of blood. The diameter of the average cell ii 
 about lir^^ °^ ^ millimetre, or about 7.9 jx.^ Not 
 infrequently in normal blood — very often under pa- 
 thological conditions — red blood-cells are seen which 
 
 ^ The Greek letter /^ is frequently employed to represent the 
 micro-millimetre, or the one-thousandth part of a millimetre, this 
 having been widely adopted as the unit for microscopic measurement. 
 In English measurement the average red blood-cell has a diameter of 
 about TsViF^ °' ^'^ 'va(^ 
 
84 NORMAL HISTOLOGY. 
 
 are much smaller than the above-described forms, 
 and are often spheroidal in shape. 
 
 The red blood-cells owe their color, as well as 
 their capacity for performing certain important 
 physiological functions, to the presence in them of 
 a crystallizable substance called hmmoglobin. The 
 exact relation existing between the haemoglobin and 
 the substance of the cell is but little understood. 
 They are but loosely combined, for the haemoglobin 
 is readily dissolved out by water, which itself be- 
 comes colored, while the colorless and swollen cell 
 or stroma is left behind. The shape of haemoglobin 
 crystals obtained from the blood of different ani- 
 mals is not always the same ; those from human 
 blood have, in general, the form of rhombic prisms. 
 
 3. Free Granules. — These are always found in 
 greater or less number in human blood when drawn 
 from the vessels. They are chiefly of two kinds; 
 either small spherical bodies, somewhat resembling 
 fat-droplets, or variously shaped, often angular 
 masses, looking not unlike fragments of white blood- 
 cells. They are sometimes seen singly, sometimes 
 in clusters. Their nature is still undetermined. 
 
 The above general description of the blood-cells 
 of man applies, with few exceptions, to other mam- 
 malia. The differences in size, however, which ex- 
 ist between the red blood-cells of man and those of 
 certain of the mammalia, are so considerable that 
 they can be distinguished with certainty from one 
 
BLOOD. 85 
 
 another by microscopic measurements. It should, 
 on the other hand, never be forgotten that the red 
 blood-cells of certain other mammalia, e. g., the dog, 
 have so nearly the same average diameter as those 
 of man that they cannot be distinguished with ab- 
 solute certainty by measurements. In other verte- 
 brates the form and character of the red blood-cells 
 differ greatly from those above described ; being for 
 the most part oval, and having a distinct nucleus. 
 
 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 direc- 
 tions, and joining each other at frequent intervals, 
 forming an irregular-meshed net. We find, more- 
 over, 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 observed, they may be seen 
 to actually shoot out from these granules, which 
 thus seem to form starting-points for their forma- 
 tion. The substance which thus separates from the 
 plasma is called fibrin. 
 
86 NORMAL HISTOLOGY. 
 
 LYMPH. 
 
 In lymph, in addition to the plasma from which 
 fibrin is formed on separation from the body, we 
 find spheroidal cells identical in structure with the 
 white blood-cells; sometimes a few red blood-cells; 
 and variously shaped granules or minute globules, 
 composed apparently of a combination of albu- 
 minoid material, with fat. These globules, in that 
 variety of lymph called chyle, are so abundant as to 
 give the fluid a milky appearance. 
 
 Origin of Blood-cells — Direct observation has 
 shown that, in some ainimals 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 repara- 
 tion 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 pro- 
 duced in a manner entirely unknown to us — these 
 are questions not only of theoretical but of practi- 
 cal 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 
 
LYMPH. 87 
 
 find nucleated red blood-cells. Now, it has been re- 
 cently shown that in certain parts of the body, in 
 adult life, cells occur which in many respects resem- 
 ble 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, mar- 
 row, 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 view is not estab- 
 lished as yet, and many observers do not ascribe to 
 the so-called nucleated red blood-cells the signifi- 
 cance upon which the advocates of this theory 
 insist. 
 
 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 at the side of the nail with a 
 bright, clean sewing needle. A small drop is received on 
 a slide and covered at once, care being taken not to press 
 upon the cover-glass. The film of blood should be 
 very thin or the crowding of the cells will interfere with 
 the observation. As the plasma ^vaporates, the changes 
 in form due to shrinkage of the red blood-cells may be ob- 
 served near the edges of the preparation. Finally, a 
 drop of water should be allowed to run under the cover- 
 
88 NORMAL HISTOLOGY. 
 
 glass and its action observed, first, in causing the dis- 
 appearance of the crenulations on the shrunken cells and 
 the swelling of all the red blood-cells, and second, in dis- 
 solving the haemoglobin out of them. 
 
 Crystals of Hcemoglobin from Rat's Blood. — Although 
 readily separated by water from its combination with the 
 stroma of the red blood-cells of man, the hasmoglobin 
 does not readily crystallize. But when separated from 
 the red cells of certain animals, the rat for example, it 
 commences to crystallize, under favorable conditions, al- 
 most immediately. A small drop of rat's blood is mixed 
 on a slide with an equal quantity of water, covered and 
 examined at once. The color begins to be discharged 
 from the red cells very soon, and within a few moments, 
 near the edges of the cover- glass, small crystals may be 
 found in abundance. If the specimen is set aside and 
 examined after a few hours, many very large and beauti- 
 ful prismatic crystals may be seen. Haemoglobin crys- 
 tals do not keep long enough for permanent preserva- 
 tion. 
 
 Demonstration of the Nuclei of the White Blood-cells. 
 — In order to see their outlines distinctly, the nuclei 
 must be stained and the cell-body rendered transparent. 
 This may be accomplished by mixing on a slide a small 
 drop of blood from the finger with an equal quantity of 
 the following fluid : 
 
 Saturated alcoholic solution of Analine Red I part, 
 Alcohol ....... 5 parts, 
 
 Water lo parts. 
 
 After thoroughly mixing the blood with the fluid, and 
 
L YMPH. 89 
 
 covering, it will be found that while the red cells are par- 
 tially decolorized and inconspicuous, the bodies of the 
 white blood-cells have become transparent and their 
 nuclei are stained deep red. 
 
 Ammboid 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 temperatures 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 their 
 occurrence. 
 
 The leg and toes of a frog having been carefully 
 cleansed, the tip of one of* the toes is snipped off, and on 
 stripping the leg downward with the thumb and finger, 
 a drop of mixed blood and lymph will presently exude 
 from the toe. This is received on a slide, protected from 
 pressure by a bit of hair, and covered. To prevent evap- 
 oration of the plasma, a rim of oil is painted around the 
 edge of the cover. On focussing now upon the specimen, 
 white blood-cells will readily be found, and selecting one 
 which, by its irregular shape, indicates its activity, the at- 
 tention must be fixed upon this cell and sketches of its 
 form made at short intervals — every two minutes. Al- 
 though the movement is usually too slow to be actually 
 detected by the eye, if the cell is fairly active, it will be 
 sufficiently evident, after a few sketches, that it has 
 changed its shape and perhaps its place also. 
 
 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 should be borne in mind that in the 
 
90 NORMAL HISTOLOGY. 
 
 frog's blood the red cells are oval and nucleated, and that 
 they are also larger than the colorless cells. 
 
 Fibrin. — 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 fibrillae 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 coagu- 
 lation has occurred, the cover-glass is gently raised with 
 the forceps, and the blood-cells washed out of the clot by 
 allowing drops of water to fall upon the inverted cover- 
 glass from a pipette held a few inches above the prepara- 
 tion. (If, in removing the cover-glass, the clot adheres 
 to the slide, the water is, of course, to be applied here.) 
 When no more color is seen in the clot, a drop of the above 
 solution of Analine Red is placed upon it, and it is again 
 covered. The fibrin will be seen in the form of minute 
 inosculating filaments which often radiate from what ap- 
 pear to be the above-described free granules. 
 
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 
 transverse striation of their structural elements, 
 they are also called striated voluntary muscles. To 
 this class belong, among others, the muscles of lo- 
 comotion and the voluntary muscles of the trunk 
 and head. Other muscles are not under control of 
 the will, and are hence called involuntary. A cer- 
 tain portion of these involuntary muscles possess 
 the same striation of their elements as the volun- 
 tary muscles, and hence are called involuntary 
 striated muscles. These are found in the heart. In 
 another kind of involuntary muscles, 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 mus- 
 cles. 
 
 We have thus to study three kinds of muscles : 
 
 T , . , \ a, smooth or non-striated, 
 
 1. Involuntary muscles | ^; ^^^j^^^j_ 
 
 2. Voluntary muscles (striated). 
 
 91 
 
92 NORMAL HISTOLOGY. 
 
 1. a. — SMOOTH MUSCULAR TISSUE. 
 
 Smooth muscular tissue is made up of very much 
 elongated, narrow, pointed, usually fusiform cells. 
 These cells are commonly arranged in groups or bun- 
 dles, enclosed in connective tissue and supplied 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 ex- 
 tremities. Owing to pressure from adjacent parts, 
 the fusiform cell-bodies are often more or less flat- 
 tened at the sides, presenting on cross-section an 
 irregular polygonal contour. The cell-body has an 
 indistinct longitudinal striation, and frequently in the 
 vicinity of the nucleus a few shining granules are 
 seen. The nucleus is usually narrow, and much 
 elongated, rod-like, 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 cells lie side by side or lap over one 
 another at the ends, and are joined together by a 
 small amount of an albuminoid cement substance. 
 These smooth muscle-cells are variously grouped in 
 different parts of the body ; sometimes crowded 
 together in solid bundles, which are arranged in 
 layers and surrounded by connective tissue, as in 
 the intestines ; sometimes arranged in narrow inter- 
 lacing fascicles, as in the bladder, or scattered 
 
MUSCULAR TISSUE. 93 
 
 singly through certain tissues ; sometimes wound 
 in single or double layers around the blood-vessels ; 
 and again running in various directions and associ- 
 ated with bands of connective tissue, they form 
 large 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 
 prepared cells where fine lines are observed passing 
 inward from the periphery of the cell toward the 
 nucleus. The blood-vessels supplying this tissue 
 form for the most part elongated net-works through- 
 out 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 accomplished 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 (i to 800). Shreds of the 
 muscular layer are torn off, slightly stained with haema- 
 toxylin, washed, and then carefully teased in glycerin 
 which has been colored by eosin. To prevent the shrink- 
 age of the cells under the action of the bichromate, it is 
 well to enclose a short segment of the intestine, dis- 
 tended with the solution, between two ligatures, and im- 
 merse it in a vessel of the same solution. 
 
94 NORMAL HISTOLOGY. 
 
 The cells may be more perfectly separated by soaking 
 a bit of the fresh tissue for a few minutes in forty-per- 
 cent, solution of caustic potash and then teasing, but 
 they cannot be stained or preserved. 
 
 Transverse and Longitudinal Sections of the Cells. — 
 The intestine of the cat is well adapted for this prepara- 
 tion, since here the muscle-cells are unusually large. A seg- 
 ment having been distended as above with Muller's fluid., 
 it should be immersed for ten days in the same, then 
 carefully washed and put for a day or two in strong al- 
 cohol. 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 sec- 
 tions are stained double and mounted in balsam or glyc- 
 erin. 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 the level of the 
 nucleus, or at a point nearer the extremity. In such a 
 preparation, the cementing substance between these cells 
 may be seen in the transverse sections ; and the serosa 
 and mucous membrane are seen on opposite sides of the 
 muscular layers. 
 
 Muscle-cells of Frog's Bladder. — Elegant pictures of 
 very much elongated, slender muscle-cells, lying singly or 
 arranged in narrow interlacing fascicles, may be obtained 
 from the frog's bladder by the following method : the 
 spinal cord of a frog being broken up, the abdominal 
 cavity is largely opened by a crucial incision, and a 
 
 * Better in celloidin. 
 
MUSCULAR TISSUE. 9$ 
 
 curved canula, attached to a small syringe filled with a 
 saturated solution of bichromate of potassium, is passed 
 into the cloaca and directed forward 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 dis- 
 tended, and put in the same bichromate solution, where 
 it remains for three days, when it is washed and trans- 
 ferred to alcohol. After twenty-four hours it may be 
 opened, and a bit cut out, the epithelium carefully 
 brushed from the inner surface, and stained double and 
 mounted in glycerin. In addition to the muscle-cells, 
 the nuclei of the endothelial cells covering the peritoneal 
 surface will be seen, as well as connective- tissue cells and 
 fibres in the wall of the bladder. 
 
 2. STRIATED VOLUNTARY MUSCULAR TISSUE. 
 
 ■ As the involuntary striated or heart muscle oc- 
 cupies, in structure, an intermediate position between 
 the smooth and the voluntary striated 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- 
 
96 NORMAL HISTOLOGY. 
 
 tissue envelopes or sheaths, and abundantly sup' 
 plied 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 
 sarcolemma, a thin homogeneous sheath or tube, 
 which tightly encloses the other elements. 
 
 I. If we examine a fresh muscle-fibre, or one 
 which has been hardened under favorable condi- 
 tions, with moderately high powers, we see that the 
 contractile substance is indistinctly longitudinally 
 striated ; and if we treat muscle with certain chem- 
 ical agents, such as chromic acid or its salts, we 
 find that by slightly teasing, the fibres break up 
 along the longitudinal striae into a multitude of fine 
 fibrillse, which are called primitive ' muscle-fibrillce. 
 Again, if we examine the fresh or hardened fibres 
 still further, we find that in addition to the longi- 
 tudinal striations, they are crossed by more promi- 
 nent, narrow, alternating, dark and light bands or 
 stripes, the relative width of the stripes varying ac- 
 cording as the muscle is seen in a state of contrac- 
 tion or relaxation. 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 longi- 
 
MUSCULAR TISSUE. 97 
 
 tudinally into fibrillse, break across transversely into 
 thin discs. We thus see that, by breaking up in 
 these two directions, we may conceive of the fibre 
 as being resolvable into a multitude of tiny pris- 
 matic 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 elements are definite and independ- 
 ent structures, in which the dark portion is the 
 contractile element, and that they are joined 
 together side by 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 sarcous elements, and is 
 called Krauses line. Under favorable conditions 
 the dark band is also seen to be crossed by a line 
 called Hensen's line, whose nature is as yet but 
 imperfectly understood. 
 
 All of the above-described structural features of 
 the muscular fibres are much more distinct after 
 treatment with chemical agents, and after the 
 
98 NORMAL HISTOLOGY. 
 
 death of the tissue; the longitudinal striation is 
 not visible during life, and the distinct separation 
 of the primitive fibrillae and discs can only be 
 accomplished by chemical means. We can see the 
 various markings with sufificient 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, preexisting separate 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, con- 
 tain nucleoli, and lie with their long axes coinci- 
 dent 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 en- 
 closes the contractile substance and nuclei, that we 
 cannot usually see it, unless we separate it by 
 artificial means from the underlying structures. 
 
MUSCULAR TISSUE. 99 
 
 Where the muscular fibres join tendons, the sarco- 
 lemma 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 on to 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 narrow-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. — A very small bit is dissected off, with 
 as little stretching as possible, from one of the voluntary 
 muscles of a recently-killed mammal, and carefully teased 
 longitudinally in salt solution. 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 may 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 ex- 
 truding from the sarcolemma in the form of an obscurely 
 
lOO NORMAL HISTOLOGY. 
 
 Striated fungiform mass. Here and there nuclei are seen ; 
 but they maybe 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. 
 
 Muscular Fibres Hardened with Osmic Acid. — The 
 above-described finer structural details of the muscle 
 fibres are much more evident when they are in a state of 
 extension than when contracted. We may render this 
 condition permanent for study, by the following method : 
 the skin is quickly removed from the leg of a freshly-killed 
 animal (rabbit or dog), and one of the large muscles of 
 the tkigh is forcibly extended with the fingers ; the canula 
 of a hypodermic syringe is then thrust into the muscle, 
 and an interstitial injection is made of a mixture of equal 
 parts of i-per-cent. solution of osmic acid and strong 
 alcohol. This fluid, in three or four minutes, fixes the 
 fibres in the extended condition. A small bit of that por- 
 tion which has become brown is now snipped off and 
 carefully teased and mounted in a mixture of equal parts 
 of glycerin and water. In such a preparation some of the 
 fibres frequently escape extension, in which case, the 
 marked difference may be observed between the extended 
 and non-extended condition of the fibres. 
 
 Sections of Hardened Muscle. — The details of the struc- 
 ture of muscular fibres, as well as their grouping and 
 relation to the connective 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, 
 
MUSCULAR TISSUE. lOI 
 
 since here we have short muscular fibres running in vari- 
 ous directions and attached to tendons, and we see in a 
 single transverse section of the organ, at once, longitudi- 
 nal and transverse sections of the fibres. The tongue of 
 a dog is hardened in bichromate of potassium and 
 alcohol, and transverse sections through the anterior half 
 of the organ are stained double and mounted in glycerin. 
 
 In muscular tissue thus hardened, the primitive fibrillae 
 are loosened from one another, and in some parts of the 
 specimen are usually more or less separated. The con- 
 tractile substance, morever, usually shrinks away some- 
 what from the sarcolemma, which then appears in the 
 transverse section as a delicate ring around the fibre. 
 
 Blood-vessels are seen in the above preparation, but 
 they may be much better demonstrated in longitudinal 
 sections of a muscle whose vessels have been injected. 
 
 I. b. INVOLUNTARY STRIATED OR HEART-MUSCLE. 
 
 In mammalia, the heart-muscle differs in several 
 important structural 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, unbranched cylin- 
 ders or iibres, 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 
 anastomoses, are very irregular in form, are made up 
 
I02 NORMAL HISTOLOGY. 
 
 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 
 nucleus. 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 
 substance, 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 in Miiller's fluid and 
 alcohol. Longitudinal and transverse sections are stained 
 double and mounted in glycerin. 
 
 Teased Heart-muscle. — To demonstrate the segments 
 or cells of which the heart-fibres are composed, a mod- 
 erately thin bit of fresh heart-muscle is put for twenty- 
 four hours into a dilute solution of chromic acid (i to 
 S,ooo), then carefully washed with pure water and stained 
 with picro-carmine. It is then teased on a slide and 
 mounted in glycerin, to which a little formic acid has 
 been added (i to loo). The formic acid renders the tis- 
 sue more transparent, and brings more clearly into view 
 the narrow, jagged lines of strongly refractive substance 
 which indicate the longitudinal 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 as- 
 sociated with other tissue elements, and constitute 
 the nerve-fibres. Both nerve-cells and their processes, 
 nerve-fibres, are enclosed and supported by pecu- 
 liarly arranged connective tissue, and supplied with 
 blood and lymphatic vessels. The nerve-fibres form, 
 for the most part, the white matter of the nerve- 
 centres and the peripheral nerves, while the cells 
 enter largely into the composition of the gray mat- 
 ter. In studying nerve-tissues, 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 
 
 103 
 
I04 NORMAL HISTOLOGY. 
 
 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-meduUated 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 con- 
 tinuity of any of the larger or smaller nerves, we 
 find that the individual fibres present three distinct 
 structural elements: i, the axis cylinder; 2, the 
 medullary sheath ; 3, the neurilemma. 
 
 I. Running through the axis of the fibre is a 
 cylindrical, with high powers, delicately longitudi- 
 nally 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 periph- 
 ery ; and it is probable that the longitudinal stri- 
 ations are the expression of its composition from 
 still finer primitive 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-cen- 
 tres. 
 
 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 myelin, which 
 
NER VE- TISSUE. IDS 
 
 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 con- 
 tinuous 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 rea- 
 son to believe that the neurilemma extends inward 
 and between the medullary sheath and the axis 
 cylinder, entirely enclosing the segments of the me- 
 dullary sheath. 
 
 Within each neurilemma segment, called interannu- 
 lar segment, and about midway between the constric- 
 tions, lies a flattened, elongated, 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 seg- 
 
•tmiitJMt rrt>K ^r fr-jL'^ri caLcr:: tie mdmres »f 
 it* i.-, ■^vrju'jt i' vfc t::> tie zais -zfsncisz- ciiEq-zisiy 
 
 C^^«^r*?^> 7',tt-t^ if the I^srses. — ^^^ serv&ffivES 
 
 a'* '■/■.', :A xrx^j^^cjtz br.- cscssrSfrs ts::e. ti form 
 
 v,r>cC.*rt, we -iv-,^v call sjaap-h- r^rres. I: -we filltT 
 d»« ji«ryet <>',f»'<irc t >-a-ird tben- pei^Jtml tertnina- 
 iMiia%, V; find tr^a^i di^ f h-ide £r,c sobdivide, be- 
 'yyfr/ir.g, as they do so, sntElIer and 5-^-=" er. ^nnl we 
 f,r.;ij;/ com^ to r.er-.es whicii ct-~"5t :f a sin^ 
 fibre. 'Ihc't^ sm^e. r.er-.e-Sbres don^tlie tee in 
 the ti'^r^ues, but are enclosed in a distinct sheath, 
 CAlled Henl^s sluath, which is a tube formed of a 
 »inglc layer of endothelial ceUs, placed edge to edge, 
 and cemented together. Between the sheath and 
 tht nerve-fibre is a narrow space, which, under nor- 
 mal conditions, is filled with lymph. 
 
 Having become acquainted with this simple struc- 
 ture of the single terminal' nerves, let us follow them 
 backward. We find, as we do so, that as they be- 
 ( oine larger by the junction of several fibres a small 
 iiinount of fibrillar connective tissue appears be- 
 
NERVE-TISSUE. I07 
 
 tween the fibres within Henle's sheath, and that the 
 latter becomes attached to the surrounding struc- 
 tures by a layer of connective tissue. Finally, when 
 we arrive 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 com- 
 plicated structure, and is called the lamellar sheath. 
 This is composed of several concentric lamellae, each 
 of which is formed of a fenestrated membrane of 
 fibrillar connective tissue containing granules of 
 elastic-tissue substance, and covered with endothe- 
 lial 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 of the lamellar sheath, and joining it 
 to adjacent structures — to neighboring nerve-fasci- 
 cles, 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, ir- 
 regular-shaped cells — like those found in the loose 
 subcutaneous connective tissue — reinforced by elas- 
 tic fibres, and often containing fat-cells. The fibril 
 lated and elastic fibres, especially in the immediate 
 vicinity of the lamellar sheath, usually run in a di- 
 rection approximately parallel with the axis of the 
 
I08 NORMAL HISTOLOGY. 
 
 nerve. This tissue is called the peri-fascicuhr connec- 
 tive tissue. 
 
 3. We find within the lamellar sheath and be- 
 tween the nerve-fibres composing the fascicle, in the 
 first place, prolongations inward of the tissue com- 
 posing the lamellar sheath ; and, second, fine fibril- 
 lated fibres and flattened cells which lie in the 
 interstices between the nerves and fibres. This tis- 
 sue is called the intra-fascicular connective 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 
 
 I. In the Nerve-centres. — We find in the nerve- 
 centres, nerves which have no neurilemma, and 
 others in which both neurilemma and medullary 
 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 un- 
 
NERVE-TISSUE. IO9 
 
 derstood ; but it is believed that they either join 
 the cells in the form of naked axis-cylinders, 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 dififi- 
 culties to the histologist, and with few exceptions 
 the exact way in which this occurs is unknown. The 
 motor nerves, which are distributed in the voluntary 
 striated muscles, terminate in distinct, nucleated, 
 finely granular structures on the surface of the 
 fibres, called end plates ; those which go to the 
 smooth muscle tissue break up into fine plexuses, 
 from which fibrils seem to pass either to the individ- 
 ual muscle-cells or to the surface of cell-bundles. In 
 the case of some of the nerves of special sense, we 
 have elaborate nerve-structures such as the retina, 
 auditory apparatus, etc. Again, we find the nerves 
 ending in small, complex, isolated bodies, such as the 
 so-called tactile corpuscles, etc. In some cases as 
 the nerves approach their peripheral terminations, 
 they lose the medullary sheath and neurilemma, and 
 the axis-cylinder breaks up into very numerous, 
 exceedingly delicate fibrils which sometimes form in- 
 tricate plexuses ; some of the fibrils appear to 
 terminate by free extremities ; others, it is probable, 
 end in single cells of various kinds ; but the whole 
 subject of peripheral nerve-endings is far too intrL 
 
no NORMAL HISTOLOGY. 
 
 cate and too little understood, to demand more than 
 a passing mention in a course of study as elemen- 
 tary as that which now engages us. 
 
 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 neuri- 
 lemma. They are simply grayish translucent cords 
 of varying diameter ; they are indistinctly longitu- 
 dinally 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 fibrillated fibres of ordinary connec- 
 tive 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 pneumo- 
 gastric, 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. 
 
NERVE-TISSUE. Ill 
 
 II. NERVE- CELLS. 
 
 Nerve-cells, or ganglion-cells, as they are frequently 
 called, although 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-gran- 
 ules. The nucleus is large, well-defined, vesicular in 
 appearance, and usually contains a large shining 
 nucleolus. They all have at least one process, most 
 of them have more ; and they are hence often 
 classified as unipolar, bipolar, multipolar ganglion- 
 cells. The above-mentioned striations in the cell- 
 body are often seen to continue out into the 
 processes, and are apparently continuous with the 
 striations or fibrils of the axis-cylinder of the 
 nerves. 
 
 In many nerve-cells, especially in the spinal cord, 
 we recognize 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 processes of other cells — such delicate cell- 
 processes make up a considerable portion of the 
 gray matter of the cord, and are called branching 
 processes ; second, such as pass off from the cell, and, 
 without dividing, presently are surrounded by a 
 sheath of myelin, and become medullated nerve- 
 fibres ; the latter are called axis-cylinder processes. 
 
112 NORMAL HISTOLOGY. 
 
 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, 
 and, 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 variously shaped 
 larger and smaller cells, we find also characteristic 
 pyramidal 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 cortical matter, irregular globular or ovoidal 
 cells, which, from the side toward 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 definitely determined. These 
 cells are called Purkinjes 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 endc 
 
NER VE- TISSUE. 1 1 3 
 
 thelium. They have one or more processes which 
 pierce the capsule and become non-medullated 
 nerve-fibres. 
 
 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 so- 
 lution — care being taken to pull apart the fibres from the 
 ends, so as to break them as little as possible, — and cov- 
 ered ; 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 normal 
 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 for- 
 mer owing to the lack of transparency in the medullary 
 sheath, the latter because of its extreme thinness 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 myelin shrink- 
 ing away at some parts from the neurilemma, and 
 swelling out at others. At the severed ends of the fibres 
 the myelin will be seen welling out from the neuri- 
 lemma, and breaking off into the fluid in irregular glob- 
 ular or contorted masses. 
 
 After the swelling and irregular breaking up of the 
 myelin has occurred — 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 va^ 
 
114 NORMAL HISTOLOCr. 
 
 ricorities fonned by the swollen myelin, and either at the 
 broken ends of the fibres or along their course, the axis- 
 cylinder may occasionally be seen. 
 
 Nerve-fibres Treated with OsTtiic Acid. — The most 
 complete demonstration of the nerve-fibre may be ob- 
 tained by treatment with osmic acid. This agent fixes 
 the myelin and other constituents of the fibre nearly in 
 their normal form, staining the myelin black. In apply- 
 ing 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 stretch- 
 ing the nerve — the sciatic 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 fastened by the ends 
 to the wood by threads. The nerve thus prepared is im- 
 mersed for twenty-four hours in an aqueous solution of 
 osmic acid (i to loo), then washed, and a small bit care- 
 fully teased apart longitudinally in glycerin. In such a 
 preparation nearly all the structures 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 medullary sheath has been broken across, or the seg- 
 ments pulled asunder, or the myelin has contracted at 
 the constrictions, the neurilemma and axis-cylinder. Not 
 infrequently, if the teasing has not been very carefully 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. 
 — A nerve treated as above with osmic acid is not firm 
 
NER VE- TISSUE. 1 1 5 
 
 enough to permit the making of thin sections, and should 
 be hardened by what is known as the gum embeddifig 
 method, as follows : a bit of the nerve, after the osmic 
 acid treatment, is immersed for twenty-four hours in al- 
 cohol, soaked for a few moments in water, to remove the 
 alcohol, 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 precipitated 
 by the alcohol in solid form in the interstices of the fibres, 
 giving it sufficient consistence for cutting. The sections 
 (which must be very thin), either stained with picro-car- 
 mine or unstained, are mounted in glycerin. In such a 
 preparation the uncolored axis-cylinder is seen sur- 
 rounded 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 color. The nerve-fibres will be 
 seen to have varying diameters and to present marked 
 differences in form, some of them depending upon artifi- 
 cial changes, others upon the difference in level at which 
 the fibres have been cut across. 
 
 Transverse Sections 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 chromic acid (i to 500); 
 in two weeks it is washed and transferred to alcohol. It 
 is now embedded in gum^ (see above),* and thin transverse 
 sections made and stained double and mounted in balsam. 
 In such preparations the general relation of connective 
 tissue to the nerve-fibres is well seen. 
 
 Nerve-fibres Treated with Nitrate of Silver. — By this 
 
 * Better in celloidin. 
 
Il6 NORMAL HISTOLOGY. 
 
 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 
 minutes ; this is washed off with one-half-per-cent. salt 
 solution, the specimen transferred to a drop of glycerin 
 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 ex- 
 amined, at tolerably regular intervals along the fibre, tiny 
 brown or black crosses, called Ranviers crosses, will bd 
 seen ; the transverse arm of the cross being the stained 
 cement substance between the neurilemma segments 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 the specimen be allowed to remain longer than the 
 above time in contact with the silver, the longitudinal arm 
 of the cross will be longer. 
 
 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 connection with the nutrition of the nerves, since we 
 are justified in assuming that nutritive substances in 
 
NERVE-TISSUE. 11/ 
 
 solution may pass also to the axis-cylinder in the same 
 way. 
 
 It is not improbable that the constrictions serve yet an- 
 other important purpose. The myelin of the medullary 
 sheath being a semi-fluid substance — perhaps serving 
 either to isolate the axis-cylinder or protect it from ex- 
 ternal 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, 
 /. e., the neurilemma-cells, between the constrictions. 
 
 The non-meduUated nerve-fibres may be demonstrated 
 in connection with the sympathetic ganglion-cells ; see 
 below. 
 
 I. 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 (one to five hundred), and then carefully 
 shaken in a test-tube with water colored lightly with car- 
 mine ; 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 settled to the bot- 
 tom 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 glycerin on 
 a slide and covered, pressure on the cells being avoided 
 in the usual way. If the first preparation does not con- 
 tain 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. 
 
Il8 NORMAL HISTOLOGY. 
 
 In addition to the cells, such specimens present frag- 
 ments of connective-tissue, bits of naked axis-cylinders, 
 and medullated nerve-fibres, myelin-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 forelegs should now be 
 cut off close behind the latter, and the hind legs severed 
 close to the body ; the trunk is then laid in a small dish, 
 and covered 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 commencement of the 
 spinal nerves, is dissected off in a single piece, spread on 
 a slide, and examined with a low power. Groups of sym- 
 pathetic nerve-cells are seen here and there in the speci- 
 men, 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, care- 
 fully leased apart on a slide, stained lightly with hsema- 
 toxylin and then with eosin, and mounted in glycerin. 
 Successful preparations will show not only the nerve- 
 cells and non-medullated nerve-fibres, but also the con- 
 nection of the two within the capsule. 
 
 ♦Neuroglia or " spider cells " may also be seen. See Appendix, 
 p. 254. 
 
CHAPTER Vin. 
 
 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 distinct differences in struc- 
 ture. The capillaries being the simplest, it will be 
 convenient to commence with them. 
 
 If we examine 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 axes 
 being parallel with the axis of the tube. If, how- 
 ever, 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 haematoxy- 
 lin, 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 
 
 "9 
 
I20 NORMAL HISTOLOGY. 
 
 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 adven- 
 titia, we notice another layer between it and the 
 intima, formed of a single row of smooth muscle- 
 cells, or of scattered cells, wound transversely or 
 obliquely around the vessel. Tkis layer is called 
 the media or musculosa, and a vessel having these 
 three simple 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 — the radial, 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 layer has appeared, composed of ill- 
 defined fibrillated and of elastic fibres, among which 
 
BLOOD- VESSELS. 121 
 
 lie large, flattened branching cells. This layer is 
 called the intermediary layer of the intima, and is 
 sharply separated from the media by a fenestrated, 
 elastic membrane, called the membrana elastica in- 
 timce, which in contracted vessels is usually folded. 
 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 consid- 
 erably less sharply defined. The three layers of the 
 intima are much less distinct ; in the media the 
 elastic tissue is very abundant, taking the place, to 
 a considerable extent, of the muscular elements ; it 
 is arranged in irregular lamellae and fibres, between 
 which lie fibrillated fibres, and connective-tissue and 
 smooth muscle-cells, the latter no longer all lying 
 uniformly transversely to the axis of the vessel. In 
 <he adventitia also of the large vessels the elastic 
 Tilements are very numerous, being most abundant 
 fn the vicinity of the media. In the adventitia of 
 ^ome of the large vessels, smooth muscle-cells occur, 
 tirranged usually with their long axes parallel with 
 U\e axis of the vessel. 
 
 "fire walls of the veins, like those of the arteries, 
 V 
 
122 NORMAL HISTOLOGY. 
 
 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 calibre present, in dif- 
 ferent parts of the body, marked differences in 
 structure, and, unlike the arteries, the thickness of 
 their walls is not uniformly 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 
 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 ele- 
 ments, of the adventitia, 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 mus- 
 cular 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 walls of the larger arteries and veins contain 
 blood and lymph-vessels, called vasa vasorum. 
 
BLOOD-VESSELS. I23 
 
 The valves of the veins consist of bundles of fibril- 
 lar connective tissue arranged to form a membran- 
 ous projection from the walls of the vessels — the 
 bundles being arranged, in general, in a direction 
 parallel with the free edge of the valve. They con- 
 tain 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 endothelium like that lining the 
 general surface of the vessel. 
 
 ENDOCARDIUM AND VALVES OF THE HEART. 
 
 The endocardium, which differs somewhat in thick- 
 ness 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 con- 
 nective 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 far- 
 thest 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 
 
124 NORMAL HISTOLOGY. 
 
 proper of the heart, is a layer of loose fibrillar con- 
 nective tissue, in which the blood, lymphatic ves- 
 sels, and nerves lie embedded. 
 
 The valves of the heart consist of fibrillar connec- 
 tive tissue, arranged in membranes or fasdcles, 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 surface which is most directly ex- 
 posed to the current of blood — that is, on the auric- 
 ular surfaces of the tricuspid and mitral, and on 
 the ventricular surfaces of the aortic and pulmonary 
 valves. The elastic elements are also more abun- 
 dant in the valves of the left than of the right side 
 of the heart. The firmness and capacity for re- 
 sistance of the elastic tissue being borne in mind, 
 the significance of its distribution in the valves will 
 be readily perceived ; where they are the most ex- 
 posed 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 similar to 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, although the walls themselves are very 
 thin in proportion to the size of the lumen. Fol- 
 
LYMPHATIC VESSELS. i2$ 
 
 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. Between these and 
 certain spaces or lacunae in the tissues — those, for 
 example, in which the connective-tissue cells lie (see 
 page 44) — there seems to be a direct communica- 
 tion, 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 the vessels are surrounded by but 
 little tissue, as in thin membranes such as the mesentery 
 or pia mater. A slice about an inch thick should be 
 made from the surface of the cerebrum and laid for 
 twenty- four hours in Mtiller's fluid.* A small fragment 
 from that part of the pia which dips into the sulci should 
 now be carefully separated from the brain substance, 
 stretched on a bit of thin cork, and fastened with pins. It 
 is then laid for twenty-fours each, in dilute and strong 
 alcohol, and finally stained in picro-carmine and mounted 
 in glycerin ; or stained double and mounted in balsam. 
 
 ' The normal relations of the parts are much better preserved by 
 a preliminary hardening in this way before the pia is removed from 
 Che brain, than if it is removed while quite fresh. 
 
126 NORMAL HISTOLOGY. 
 
 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 
 lecourse to the use of dilute solutions of nitrate of silver. 
 This can be applied by immersing some thin membrane, 
 such as the mesentery, 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 glycerin.* 
 
 Arteries and Veins. — Very small vessels can be studied 
 entire, since by careful focussing we can bring one por- 
 tion after another into view, obtaining thus what are 
 called optical sections. 
 
 The arterioles may be prepared by pulling out some of 
 the arteries which enter the brain substance ; at the ends 
 of these the arterioles into which they divide are seen. 
 Some of the finer twigs are cut off, stained double, 
 mounted in balsam, and studied entire. 
 
 In larger vessels this simple method is no longer prac- 
 ticable, and we have to resort to actual sections. Small- 
 er medium-sized arteries and veins may be prepared by 
 stretching them, when fresh, along a bit of wood, with 
 pins, and allowing them to dry. The dried vessels are 
 placed between two bits of soft cork or elder-pith, and 
 
 * See Appendix, p. 254 
 
LYMPHATIC VESSELS. 127 
 
 thin sections shaved of, in any desired direction, with a 
 hard-tempered dry razor. The sections swell out on be- 
 ing dropped into water ; they are then stained double 
 and mounted in balsam. 
 
 Larger vessels, such as the aorta, vena cava, etc., may 
 be hardened in alcohol, embedded in liver, and the sec- 
 tions stained and mounted as before* 
 
 Heart-valves and 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 be- 
 tween the larger lymphatic trunks and the lymphatic 
 capillaries, is best obtained by studying nitrate of silver 
 prepartions of some part in which the latter are abundant. 
 Such a structure we find in the tendinous portion 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 he 
 killed by breaking up the medulla, and the thoracic and 
 abdominal cavities freely opened by incisions above and 
 below the anterior line of attachment of the diaphragm. 
 Two ligatures are passed around the large 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 lig- 
 ament of the liver is now severed, and the diaphragm is 
 left free, stretching across between the thoracic and ab- 
 dominal cavities. Its upper surface is brushed with a 
 soft pencil moistened with water, so as to remove the sur- 
 
 * These structures may all be embedded in celloidin. 
 
128 NORMAL HISTOLOGY. 
 
 face endothelium 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 al- 
 lowed to run over it and remain for five minutes in con- 
 tact with it. This is now washed off with water, the dia- 
 phragm 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 glycerin. In such a 
 specimen the large lymphatic channels will be readily 
 recognized, 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. 
 
CHAPTER IX. 
 
 LYMPH-NODES— SPLEEN. 
 tYMPHATIC 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 interrupted 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, 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 present 
 a great diversity in form, being spherical, ovoid, 
 discoidal, or irregularly prismatic ; they always 
 present at one side a hilus, at which the larger 
 blood-vessels enter. 
 
 If we make a section through a fresh gland, at 
 129 
 
130 NORMAL HISTOLOGY. 
 
 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 reddish 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 surrounded by a firm, dense capsule 
 of connective tissue, with a few elastic fibres and 
 smooth muscle-cells. The capsule sends inward 
 numerous partitions or trabecules, which divide the 
 cortex into a series of intercommunicating cham- 
 bers, and the medulla into numerous irregular con- 
 necting 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 globular bodies, which are continu- 
 ous with cord-like anastomosing structures 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- 
 cords. 
 
 If, now, we examine more minutely the structure 
 of the lymph-follicles, we find that they consist, in 
 
LYMPH-NODES. 13 1 
 
 the first place, of a framework 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 trabeculae here, that they give to the follicle a 
 tolerably well-defined outline. In the second place, 
 the meshes of the reticulum are closely filled with 
 small spheroidal cells, having, in general, the char- 
 acters of lymph-cells ; in many of them, however, 
 the nucleus is very large, occupying the greater 
 part of the cell. The follicles do not entirely fill 
 the cavities in which they lie, but are surrounded 
 on all sides by a narrow space. 
 
 If 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 trabec- 
 ulae 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 trabeculae 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 follicles 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 
 
132 NORMAL HISTOLOGY. 
 
 continuous with the follicles, we find that they have 
 an exactly similar structure, and bear the same 
 relation 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 trabeculse of reticular connective 
 tissue, and surrounded on all sides by lymph- 
 sinuses. These lymph-cords, ramifying and inoscu- 
 lating 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 endothelium. 
 
 If we study the relation of the lymph-vessels to 
 the lymph-nodes, we find that the former, on arriv- 
 ing 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, further, 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 lymph- 
 atic vessels, in which the lymph is forced to pass 
 through a series of irregular-branching spaces or 
 sinuses, bathing in its course certain peculiar struc- 
 tures — the follicles and lymph-cords — whose function 
 
L YMPH-NODES. 133 
 
 we do not yet understand. The lymph-sinuses con- 
 tain not only the fluid of the lymph, but numerous 
 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 septa, divide and 
 subdivide, and soon enter the lymph-cords ; they 
 pass along in the axis of these, giving off a long- 
 meshed capillary system ; 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 distributed to the capsule and the connective 
 tissue of the septa. 
 
 The number and arrangement of the follicles 
 vary 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 voluminous cortex. In many 
 animals, as the ox, the reticular tissue of the me- 
 dullary portion contains an abundance of brown 
 pigment. 
 
 There are, in many parts of the body, small dense 
 masses of tissue, some of them sharply circumscribed, 
 
134 NORMAL HISTOLOGY. 
 
 Others diffuse and merging into adjacent tissues, 
 which seem to be somewhat analogous to the lymph- 
 nodes, although not, as a rule, forming well-defined 
 organs. Thus we find in the intestines and stomach, 
 either single or in clusters, circumscribed nodules of 
 reticular connective tissue, whose meshes are filled 
 with small spheroidal cells, and resembling in most 
 respects the follicles in the cortex of the lymph- 
 nodes. These, which will be more fully considered 
 when we study the gastro-intestinal canal, are called 
 the solitary follicles of the stomach and intestines, 
 and Peyer's patches. Less well defined than these, 
 we find scattered in various parts of the body, larger 
 and smaller diffuse collections of small spheroidal 
 cells, lying in a reticular stroma, and forming the so- 
 called lymphoid tissue, which recent investigations 
 have shown to be of no little importance under cer- 
 tain pathological conditions, although of their rela- 
 tions to the lymph-vessels or their physiological sig- 
 nificance we know very little. These diffuse collec- 
 tions of lymphoid tissue are found in the mucous 
 membrane of the bronchi, beneath certain serous 
 membranes, in the liver, kidneys, and elsewhere, and 
 may be seen in the preparations of these parts pres- 
 ently to be studied. 
 
 PRACTICAL STUDY. 
 
 Lymph-Sinuses Injected. — A general view of the lymph- 
 sinuses and their relations to the follicles and cords is 
 
L YMPH-NODES. 1 3 5 
 
 best obtained from sections of nodes whose lymph-channels 
 have been filled with a colored solution of gelatin. A 
 gland being exposed in a recently killed animal — one of 
 the cervical glands of the dog answers well — a hypodermic 
 syringe is warmed and filled with the warmed blue 
 gelatin mixture ; the canula is now 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 nodes. 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 ice or cold water. When the gelatin 
 has solidified, the gland is cut out, divided longitudinally, 
 and put into strong alcohol. When it has become suffi- 
 ciently hard, sections are made through the entire gland, 
 stained with picro-carmine or eosin, and mounted in 
 balsam. 
 
 Shaken 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 sup- 
 porting 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. 5 5). When the operation 
 is completed — which will be determined by the extreme 
 thinness which the section presents when most of the loose 
 cells are removed — it is floated, without folds, on to a 
 slide, stained double and mounted in glycerin. 
 
 Blood-vessels. — To obtain an injection of the blood- 
 
136 NORMAL HISTOLOGY. 
 
 vessels of the lymph-nodes, either a whole animal, such 
 as the rabbit or dog, may be injected through the aorta, 
 with the blue gelatin mixture ; or a single gland, such as 
 the cervical or mesenteric, may be injected through its 
 main artery. The glands should be hardened in alcohol, 
 and the sections stained deeply with eosin and mounted 
 in balsam. 
 
 THE SPLEEN. 
 
 The spleen, although differing in many important 
 and probably most essential particulars from the 
 lymphatic glands, yet presents many striking analo- 
 gies with them. Like them, it presents, on cross- 
 section, to the naked eye, a fibrous envelope — the 
 capsule, — from which septa and trabeculae pass into 
 the organ, enclosing irregular spaces. Here also we 
 find the spaces between the septa filled with a soft 
 substance presenting two distinct modes of arrange- 
 ment ; we find first, irregularly scattered through the 
 organ, small grayish globular or elongated struc- 
 tures, called Malpighian bodies, or follicles ; and, sec- 
 ond, between these, filling up the remaining space 
 between the trabeculae, a soft red tissue called the 
 
 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 
 trabeculce, the follicles, the pulp, and the blood 
 vessels. 
 
 The capsule of the spleen consists of a dense en- 
 
THE SPLEEN. 137 
 
 velope of interlacing 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 cap- 
 sule, form a multitude of irregular communicating 
 spaces in which the follicles and splenic pulp lie. 
 These trabeculae and septa are made up of the same 
 elements as the capsule, and in size and abundance 
 vary greatly in different animals, being in man only 
 moderately developed. TYiq follicles, ox Malpighian 
 bodies, have essentially the same structure as the fol- 
 licles of the lymph-nodes — that is, they are formed 
 by a small mass of supporting reticular connective- 
 tissue, whose meshes are narrowest at the periphery, 
 and closely filled throughout with small spheroidal 
 cells, and supplied with a net-work of capillaries ; 
 we usually find here, however, a small artery passing 
 either through the centre or at one side of the fol- 
 licle. 
 
 But 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 surrounded, as above mentioned, by a 
 
138 NORMAL HISTOLOGY. 
 
 connective-tissue sheath ; after passing for a short 
 distance inward, they separate, and the arteries, still 
 accompanied by a certain amount of connective 
 tissue, divide and subdivide, proceeding farther in- 
 ward, 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 : at first the connective-tissue sheath 
 and the adventitia become very loose in texture, and 
 their meshes become filled with spheroidal cells re- 
 sembling lymph-cells — this is called lymphoid infil- 
 tration of the walls of the arteries ; then we find that 
 at certain points this infiltration becomes quite ex- 
 tensive, the intercellular substance assuming the 
 character of reticular connective tissue ; and thus 
 distinct spheroidal or much elongated 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 a considerable 
 distance 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 ad- 
 jacent 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. 
 
TH£ SPLEEN. 139 
 
 Let us now turn to the pulp. This is composed, 
 in the first place, of a multitude of irregular, fre- 
 quently 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 framework of delicate reticular 
 connective tissue, continuous with the sustaining 
 tissue of the follicles, the meshes of which are in- 
 completely filled with various kinds of cells. Among 
 these cells we find spheroidal 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. Finally, there are sometimes found 
 in varying number, cells which resemble the lymph- 
 and larger colorless cells in form, but whose bodies, 
 either homogeneous or granular, have a color sim- 
 ilar to that of the red blood-cells. These latter 
 cells, like certain similar cells already mentioned as 
 occurring in the marrow of bones, are called nu- 
 cleated red blood-cells, and are regarded by many 
 observers as intermediate forms between the color- 
 less and the red blood-cells. Although many re- 
 cent observations would tend to confirm this idea, 
 their nature is as yet by no means absolutely de- 
 termined. 
 
140 NORMAL HISTOLOGY. 
 
 We have still to consider the structure of 
 the second constituent of the 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 sub- 
 divide to form a multitude of intercommunica- 
 ting thin-walled canals of tolerably uniform calibre, 
 which occupy the irregular-branching spaces be- 
 tween the pulp-cords. These ultimate venous trunks 
 are called cavernous veins, and their walls consist of 
 little else than a few widely separated, branching, 
 circular and oblique fibres, upon which lie, at vary- 
 ing intervals, elongated, curved, spindle-shaped, and 
 flattened endothelial cells, which have their long 
 axes parallel with the axis of the canal. These veins, 
 whose walls, as will be seen, are not closed but fenes- 
 trated, 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 
 fenestrated cavernous veins of the pulp. The 
 opinion of different observers on this point differs 
 somewhat, owing to the extreme technical difficul- 
 ties in the investigation ; but it seems probable that 
 after passing out of the fine arterial twigs, through 
 
THE SPLEEN. I4I 
 
 the intervention of the capillaries it is poured direct- 
 ly into the meshes of the pulp-cords, and that after 
 circulating here around the cells, without distinctly 
 walled channels, it iinally finds its way through their 
 fenestrated walls into the cavernous veins, whence 
 it passes out of the organ through the large efferent 
 veins at the hilus. 
 
 PRACTICAL STUDY. 
 
 Sections of Uninjected Spleen, a. Cat. — A general 
 view of the arrangement of the different structures of the 
 spleen may be obtained from very thin sections of a cat's 
 spleen hardened in bichromate of potassium and alco- 
 hol. They are stained double and mounted in bal- 
 sam, b. Human. — Structural details may be studied in a 
 section from a human spleen hardened as above. Before 
 mounting, the sections should be placed in a shallow, flat-" 
 bottomed dish, and just covered with water ; with a fine 
 camel's hair pencil, held perpendicularly, the section is 
 gently tapped until it appears thinner and more transpa- 
 rent from the brushing out of the loose cells. It may 
 then be stained double, and mounted in glycerin or bal- 
 sam. 
 
 Section of Spleen with Injected Cavernous Veins. — A 
 spleen is injected, under low pressure, through the vein, 
 with the blue gelatin mixture, hardened in alcohol, stained 
 deeply with eosin, and mounted in balsam. 
 
 Isolated Cells of the Spleen. — A bit of the fresh organ is 
 teased on a slide in a half-per-cent. salt solution, and 
 studied in the same. Should the red blood-celk, as is 
 
142 NORMAL HISTOLOGY. 
 
 apt to be the case, be so numerous as to conceal the 
 other forms, a drop or two of dilute alcohol (one to 
 two) is allowed to run under the cover-glass, and after a 
 few moments the red blood-cells will have become so 
 transparent as not seriously to interfere with the observa- 
 tion. If the nuclei are not sufiSciently well defined, a 
 drop of eosin may be added after the alcohol. In addi- 
 tion to the above-described 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 X. 
 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 adja- 
 cent structures ; but, for the most part, it is attached 
 only at one side by a structure — the mesentery — 
 which serves to convey to it its blood- and lym- 
 phatic-vessels and nerves. The walls of the tube, 
 although varying in structure in different sections, 
 consist 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 in- 
 testines, 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, subserosa, covered with a 
 layer of endothelium. The muscular tunic, or mus- 
 culosa, consists of two layers of smooth muscular 
 tissue: an external, in which the cells lie longitiidi- 
 
144 NORMAL HISTOLOGY. 
 
 nally, 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 of the 
 muscularis and joining it to the mucosa, is a layer of 
 loose fibrillar connective tissue, called the submucosa, 
 in which the blood and lymphatic vessels ramify. 
 In the mucosa, finally, we have a delicate support- 
 ing 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 submu- 
 cosa is a thin layer of smooth muscle-cells, lying in 
 both transverse and oblique directions, called the 
 muscularis mucosae, 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 mem- 
 brane, and since in this the glands are very impor- 
 tant factors, a word should be said here about the 
 structure of glands in general. 
 
 Although the term gland is popularly 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 
 excretions. All such glands have a somewhat analo- 
 
THE GASTRO-INTESriNAL CANAL. I4S 
 
 gous structure, and present two distinct kinds of 
 structural elements : I. epithelial or gland-cells; 2. 
 a connective-tissue framework, with blood- and 
 jymphatic-vessels and nerves. The epithelial cells, 
 usually large, differ in form in different and even in 
 the same glands : 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: 
 
 1. Tubular glands, which have the form of simple 
 or occasionally branching, straight, curved, or vari- 
 ously contorted tubes, terminating in blind extremi- 
 ties. 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 
 
146 NORMAL HISTOLOGY. 
 
 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 inter- 
 stitial tissue — in which the vessels and nerves ramify. 
 The alveoli which open into the same excretory 
 duct are usually joined more closely to one another 
 than to those opening into different ducts, and these 
 clusters of alveoli are called the lobules or acini of 
 the gland. Such are the mammary glands and cer- 
 tain mucous glands of the bronchi. 
 
 3. Vesicular Glands. — These consist of simple, 
 spheroidal, or irregular-shaped closed alveoli, sur- 
 rounded by a membrana propria, and lined with 
 epithelium, the separate alveoli being embedded in 
 interstitial connective tissue. Such glands are the 
 thyroid and ovary. 
 
 THE STOMACH. 
 
 The muscularis of the stomach differs from that 
 of the intestines, 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 entirely made up of glands supported 
 
THE GASTRO-INTESTINAL CANAL. 147 
 
 and held together by a small amount of delicate 
 connective tissue, in which the blood and lymphatic 
 vessels ramify. The glands are tubular, sometimes 
 simple, sometimes divided, and often tortuous at 
 the base. They have a membrana propria, and, de- 
 pending upon differences in the epithelium which 
 line them, they are classified as: i. 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 through- 
 out with cylindrical epithelium ; these are the mu- 
 cous glands. 
 
 The greater proportion of the glands, however, 
 are lined only at their orifices with cylindrical epi- 
 thelium ; deeper down in the gland we find usually 
 two kinds of cells : a, spheroidal or polyhedral 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 
 caWed 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. 
 
148 NORMAL HISTOLOGY. 
 
 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 follicles, and the blood is finally 
 collected into narrow venous trunks directly be- 
 neath the surface epithelium ; from these it passes 
 back into larger veins in the submucosa, where it 
 collects in the efferent veins. The lymphatic ves- 
 sels lie between the glands, form anastomosing 
 channels in the submucosa, and pass out through 
 the musculosa, receiving larger and smaller trunks 
 from the latter. 
 
 The nerve-trunks from the sympathetic and pneu- 
 mogastric form a plexus, associated with minute 
 ganglia, called A uerback's plexus, between the layers 
 of the musculosa ; from this branches pass into the 
 submucosa and form another similar plexus, called 
 Meissner's plexus. 
 
 Nodules of lymphoid tissue, varying greatly in 
 size and number, are found in the mucosa of the 
 stomach, at the base of the follicles, and sometimes 
 extending up between them. These are sometimes 
 visible to the naked eye as small grayish promi- 
 nences on the surface of the mucous membrane, and 
 have been called the lenticular glands or follicles of 
 the stomach. 
 
THE GASTIiO-INTESTINAL CANAL. I49 
 
 THE SMALL INTESTINE. 
 
 The supporting connective-tissue framework 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 folli- 
 cles of Lieberkuhn, and are lined with cylindrical 
 epithelium. Rising from the general surface of the 
 mucous membrane, between the orifices of the 
 glands, are very numerous short cylindrical or coni- 
 cal projections called villi. These are formed by 
 projections inward of the mucosa ; they are covered 
 by cylindrical epithelium, and contain an abundant 
 vascular net-work, and the radicles of the lymph- or 
 chyle-vessels. 
 
 The cylindrical epithelial cells are joined to- 
 gether, side by side, by cement substance, and pos- 
 sess a marked peculiarity in the structure of the 
 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. It was formerly taught 
 that through these pores the chyle passed to enter 
 the epithelium on its way to the lymph-vessels. 
 More recently, however, the view has been ad- 
 
ISO NORMAL HISTOLOGY. 
 
 vanced that substances absorbed into the lymph- 
 vessels from the intestines pass, not through the 
 epithelial cells, but through the cement substance 
 between them. 
 
 Scattered here and there between the cylindrical 
 epithelium, sometimes abundant, sometimes not, 
 are transparent more or less ovoidal cells 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 infre- 
 quently 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 all cases : by some, 
 they are regarded as cylindrical cells changed by 
 artificial means ; but most observers believe them 
 to be normal structures, and suppose 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 produced. 
 
 In addition to the tubular glands which are found 
 throughout the whole extent of the small intestine 
 — in the duodenum, especially in its upper por- 
 tions — racemose, probably mucous glands, are 
 
THE GASTRO-INTESTINAL CANAL. IJI 
 
 found, called Brunner's glands. They lie in the 
 submucosa, and consist of variously shaped, but 
 usually elongated 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 occupied 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 pres- 
 ently 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. 
 The distribution of the nerves is essentially similar 
 to that in the stomach. 
 
 Closely connected with the lymphatic vessels, and 
 apparently forming a part of the lymphatic appa- 
 ratus of the intestines, are found certain structures 
 called lymphatic follicles, and of these it is custom- 
 ary to distinguish two kinds: i. Solitary follicles ; 
 and 2. Agminated follicles, or Peyer's patches. 
 
 I. Solitary Follicles. — These are irregularly scat 
 tered through the mucous membrane of both small 
 
152 NORMAL HISTOLOGY. 
 
 and large intestines, in the form of small gra/ish 
 nodules. They lie chiefly in the mucosa, often 
 piercing the muscularis mucosce and descending 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 cylindrical 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 ad- 
 jacent parts. The meshes are closely filled with 
 small spheroidal cells, having the characters of 
 lymph-cells. In their periphery the lymph-vessels 
 of the mucous membrane form a closely anasto- 
 mosing net-work. The blood-vessels also interlace 
 in their periphery, and send an abundance of anas- 
 tomosing capillary loops into their interior. 
 
 2. Peyer's Patches. — These are found chiefly in the 
 small intestine, 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 opposite the mesenteric attach- 
 ment, with their long axes parallel with the axis of 
 
THE GASTRO-lNTESTtNAL CANAL. I53 
 
 the gut. They consist, essentially, of an aggrega- 
 tion of a variable number of structures, having the 
 characters of the solitary follicles ; these are placed 
 closely together, side by side, and supplied in essen- 
 tially the same way as the solitary follicles with 
 blood- and lymphatic-vessels. 
 
 THE LARGE INTESTINE. 
 
 The mucosa of the large intestine is thickly set 
 with tubular glands similar to Lieberkiihn's glands 
 in the small intestine, but is destitute of villi. 
 Solitary lymphatic nodules 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 described in 
 the stomach and small intestine. 
 
 PRACTICAL STUDY. 
 
 Sections of 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 shrink- 
 age, and immersed in absolute alcohol. After twenty- 
 four hours the fluid should be changed, and in three or 
 four days the specimen will probably be hard enough to 
 cut. After embedding in wax,* sections from both 
 regions are made perpendicular to the surface, stained 
 double and mounted in balsam. The nuclei of all the 
 cells are stained violet by the haematoxylin ; in the pep- 
 tic glands the bodies of the granular peptic cells are 
 stained a deep rose-red, while the others are but slightly 
 
 * Better in celloidin. 
 
154 NORMAL HISTOLOGY. 
 
 colored, or not at all. Sections perpendicular to the 
 surface of a stomach, whose blood-vessels are filled with 
 the mixture of Prussian-blue and gelatin, and stained 
 with carmine, are very instructive. 
 
 Sections of Intestine. — Bits of intestine from the upper 
 portion of the duodenum, from the ileum, including one 
 of Peyer's 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 sol. and alcohol ; 
 after four days they are transferred to alcohol, in which, 
 in a day or two, they will become hard enough to cut. 
 Perpendicular sections from the various parts should be 
 stained and mounted in the same way as the stomach. 
 Sections from intestines, whose blood-vessels are injected 
 with blue, are stained with eosin and mounted in balsam. 
 
CHAPTER XL 
 
 SUBMAXILLARY GLAND—LIVER. 
 
 SUBMAXILLARY GLAND. 
 
 Connected with the digestive tract are several 
 racemose glands, which, although differing in impor- 
 tant particulars, both in structure and function, yet 
 have many features in common. These glands are 
 the submaxillaris, the sublingual, the parotid, and 
 \ht pancreas. Their details of structure are still in- 
 sufificiently known, and within the limits of this 
 manual 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, considering this, in a gen- 
 eral way only, as typical of the others. 
 
 The submaxillary gland differs in structure in 'dif- 
 ferent animals, its structure in the dog being per- 
 haps best known, and quite closely resembling that 
 in man. In the dog it consists, like other racemose 
 glands, of alveoli and excretory ducts. The elon- 
 gated alveoli are surrounded by a membrana propria, 
 and grouped into lobules by more or less interstitial 
 connective tissue, which is furnished with blood- and 
 lymph-vessels and nerves. The alveoli, depending 
 
 155 
 
IS6 NORMAL HISTOLOGY. 
 
 upon the gland-epithelium 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. 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 periphe- 
 ral side ; these cells are not readily stained by eosin, 
 and are called mucous cells. In the second place, in 
 the periphery of the alveoli, between the cells just 
 described* and the membrana propria, lie large, in 
 cross-section, crescentic, strongly granular masses, 
 usually containing several nuclei ; these are called 
 the crescents of Gianuzzi. They are believed to be 
 formed by a number of small angular cells closely 
 crowded together; they are readily stained with 
 eosin, and are apparently 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 believed to be des- 
 tined to replace the inner layer of transparent cells 
 as these are destroyed or changed in 'urnishing the 
 specific secretion of the gland. 
 
 The excretory ducts differ in structure and in the 
 diameter of the lumen, in different parts of the 
 gland ; the difference depending chiefly upon differ- 
 
THE LIVER. 157 
 
 ences in the structure of the epithelial layer which 
 lines them ; this, in the larger ducts, is columnar, 
 and in the smaller, flattened. 
 
 PRACTICAL STUDY. 
 
 Section of Gland of Dog. — Small pieces of a perfectly 
 fresh gland are hardened in absolute alcohol. Very thin 
 sections are to be stained with haematoxylin and eosin, 
 and mounted in glycerin or balsam. One frequently 
 finds in a gland taken from a dog shortly after eating, 
 certain groups of alveoli with the cells in an active, others 
 with the cells in a resting, condition. 
 
 THE LIVER. 
 
 The liver presents three distinct elements of 
 structure : i. The cellular elements, which in form, 
 function, and arrangement, characterize the organ — 
 the liver-cells or parenchyma. 2. The connective- 
 tissue framework — the interstitial tissue, which sur- 
 rounds the organ as a capsule, and in its interior 
 supports the parenchyma and carries the larger 
 vessels. 3. The blood-, lympk-, and gall-vessels. 
 
 The liver-cells are large, have the form of irregu 
 lar, 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 the pressure from adjacent blood-vessels. 
 
158 NORMAL HISTOLOGY. 
 
 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 polyhedra, and the dif- 
 ference 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 neces- 
 sary to study them in connection with the blood- 
 vessels of the liver, to which they bear a very 
 constant and characteristic 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 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 vencz intralobulares. 
 
 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 
 
THE LIVER. 159 
 
 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 connected 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 vein does 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 intralobu- 
 laris, — and at its periphery the terminal branchlets of 
 the portal vein — vencz interlobulares, — 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 continuous v/ith the connective 
 tissue surrounding the larger trunks of the portal 
 vein, hepatic artery, etc., and called Glissons capsule. 
 In this connective tissue between the lobules are 
 found, in addition to the branchlets of the portal 
 
l6o NORMAL HISTOLOGY. 
 
 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, insensibly 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, directly or indirectly, 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 epithe- 
 lium, 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 periphery 
 of the lobules — interlobular gall-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 
 
THE LIVER. l6l 
 
 narrow, and form a delicate net-work around the 
 individual liver-cells ; being arranged in such a way, 
 however, 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 GHsson, which accompanies the 
 larger vessels ; 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 situ- 
 ation it occurs in the form of delicate fibres or 
 membranes, with here and there fusiform or stellate 
 cells. 
 
 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, and are connected with minute 
 intralobular lymph-spaces. Here and there in the 
 interstitial tissue of the liver are found small irregu- 
 lar nodules of lymphoid tissue, see page 134. 
 
 PRACTICAL STUDY. 
 
 Liver-cells. — A small fragment of fresh liver is teased 
 in bait solution and studied in the same. It need not be 
 
l62 NORMAL HISTOLOGY. 
 
 preserved, as hardened cells will be seen in the following 
 preparations. 
 
 Sections of Pig's Liver. — A small piece of pig's liver 
 should be hardened in Miiller's fluid and alcohol, and 
 very thin sections 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 double and mounted 
 in balsam. The lobules being here surrounded by toler- 
 ably distinct layers of connective tissue, in which the in- 
 terlobular vessels run, the lobular structure is quite evi- 
 dent, and the pictures obtained by sections in different 
 directions are easy of interpretation. 
 
 Sections of Human Liver. — Sections are made from a 
 bit of human liver hardened as above, stained double and 
 mounted in balsam. 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- 
 vessels, 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 be- 
 sides 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 dissimilar vessels, except capillaries, and sur- 
 rounded only by a scarcely appreciable amount of con- 
 nective tissue. 
 
 Injected Liver. — The general arrangement of the blood- 
 
THE LIVER. 163 
 
 vessels is best studied in sections from a human or rab- 
 bit's liver, which has been injected through the portal 
 vein with the blue gelatin mixture. The sections are 
 stained with eosin and mounted in balsam. 
 
 Injected Gall-capillaries. — By a careful injection of the 
 blue gelatin mixture into the hepatic duct, the gall-capil' 
 laries may be partially filled. Sections are stained with 
 eosin and mounted in balsam. 
 
 To show the relation between the blood- and gall-capil- 
 laries, both may be simultaneously injected with gelatin 
 of different colors, one forced into the hepatic duct, the 
 other into the portal vein. 
 
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 cortex, 
 — and a narrower central, yellow or reddish portion 
 — the medulla. Between the two, or rather at the 
 inner side of the cortical portion, a more or less dis- 
 tinct 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 
 and smooth muscle-cells. From this capsule, deli- 
 cate converging connective-tissue bands or septa, 
 similar in structure to the capsule, pass inward, and 
 being joined together by delicate transverse bands 
 or trabeculae, divide the cortex into a multitude of 
 variously shaped chambers. In the periphery. 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 
 
 164 
 
SUPRA-RENAL CAPSULES. 1 65 
 
 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 — -parenchyma cells, — which differ in char- 
 acter in the different parts ; in the cortex they are, 
 for the most part, large, polyhedral, and granular ; 
 less abundant are smaller cuboidal or cylindrical 
 forms. Those in the elongated chambers are usually 
 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 globu- 
 lar 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 very 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 trabeculse, form a dense plexus 
 in the medulla, in which, as well as in the capsule, 
 considerable numbers of ganglion-cells are found. 
 Lymphatic vessels and sinuses are also abundant. 
 
 PRACTICAL STUDY. 
 
 Sections. — A human supra-renal capsule, as fresh as 
 possible — or, if this cannot be obtained, that of the 
 
1 66 NORMAL HISTOLOGY. 
 
 guinea-pig or ox, — is cut transversely into two or three 
 pieces and hardened in bichromate of potassium and 
 alcohol. Transverse sections are stained double and 
 mounted in glycerin or balsam. 
 
 THE THYROID GLAND. 
 
 The thyroid gland is composed of a congeries of 
 larger and smaller spheroidal or irregular-shaped 
 alveoli, enclosed in connective tissue and grouped 
 together to form lobules. The alveoli are entirely 
 separate from one another, and they have no excre- 
 tory ducts. Each alveolus has a delicate membrana 
 propria, and is lined with a single layer of cylindri- 
 cal or cuboidal cells. The connective tissue between 
 the alveoli contains numerous blood- and lymphatic- 
 vessels. 
 
 The entire gland, which consists of two lateral 
 lobes, united at their lower extremities by a trans- 
 verse commissure, is enclosed in a dense connective- 
 tissue envelope. The alveoli are filled with a clear, 
 homogeneous albuminous fluid, which in adult life 
 is frequently transformed into or replaced by a 
 translucent material, 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 
 almost entirely disappear. The colloid material 
 seems to be formed, in part at least, by a transforma- 
 tion of the contents of the epithelial cells, and its 
 
THE THYROID GLAND. 1 67 
 
 formation under pathologicjil conditions gives rise 
 to one of the forms of goitre. 
 
 PRACTICAL STUDY. 
 
 The thyroid of a child or adult — better the former — is 
 cut into small pieces and hardened in alcohol. Sections 
 are stained double and mounted in glycerin or balsam. 
 
CHAPTER XIIL 
 THE RESPIRATORY APPARATUS. 
 
 The respiratory apparatus consists of a multi- 
 tude 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 con- 
 ducting 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 — t\ie fibrous layer — in which 
 lie embedded, incomplete cartilaginous rings, the 
 space between whose free ends, at the posterior por- 
 tion of the tube, is bridged over by transverse bands 
 of smooth muscular tissue, which bind the ends to- 
 gether. 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- 
 mucosa, which consists of loose, fibrillar connective 
 tissue with elastic fibres, the latter running chiefly 
 in a longitudinal direction. 
 
 i68 
 
THE RESPIRA TOR Y APPARA TUS. I69 
 
 Still farther inward, and not distinctly separated 
 from the submucosa, lies the mucosa, composed of 
 fibrillar connective tissue containing an abundance of 
 variously shaped cells, and, for the most part, longi- 
 tudinally arranged elastic fibres. In some parts of 
 the mucosa the tissue resembles diffuse lymphoid 
 tissue. The connective tissue of the mucosa, in 
 some parts of the trachea, does not form an uniform 
 layer, but is arranged in more or less well-defined 
 longitudinal bundles, giving the surface a wavy or 
 folded appearance. 
 
 Internally, the mucosa is bordered by a thin, 
 homogeneous membrane, the basal membrane, 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 epi- 
 thelium, pass obliquely inward and terminate on the 
 surface in expanded orifices. The alveoli of the 
 glands are lined with a single layer of slightly granu- 
 lar, polyhedral cells. When the glands are in a con- 
 dition of functional activity, however, the cells 
 become larger, the outlines indistinct, their nuclei 
 
170 NORMAL HISTOLOGY. 
 
 are crowded to one side, and their contents are 
 apparently transformed into a homogeneous mu- 
 cous mass. Scattered here and there among the 
 glands and between the cartilaginous rings, lie larger 
 and smaller clusters of fat-cells. 
 
 The blood-vessels, passing through the outer lay- 
 ers, 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 general structure as the trachea, still 
 we find, aside from a decrease in the thickness of 
 the walls, certain noteworthy structural modifica- 
 tions 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. Furthermore, a dis- 
 tinct layer of smooth muscular tissue, in the form of 
 transverse rings, connected with each other by in- 
 terlacing cells, appears between the mucosa and the 
 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 thei^ 
 
THE RESPIRA TOR V APPARA TVS. 1 7 1 
 
 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, disap- 
 pear with the cartilages. The muscular rings as- 
 sume, 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 becomes gradually thinner, and 
 finally merges into the fibrous layer, with 'the inter- 
 vention only of a few scattered muscle-cells ; the 
 lower layers of epithelium gradually disappear, leav- 
 a single row of ciliated cells upon the basal mem- 
 brane. Finally, we have, in the smallest tubes, very 
 thin connective-tissue walls, containing a few mus- 
 cle-cells and elastic fibres, and lined with cuboidal, 
 ciliated, and at last 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 more or less distinctly 
 divided, by narrow branching lines, into irregular 
 polygonal spaces, each one of which corresponds to 
 ^pulmonary lobule. These lobules have, on the sur- 
 face of the lung, where they are more uniform in 
 shape than within, a pyramidal form, and are sepa- 
 rated by narrow connective-tissue septa, and each 
 lobule is, in fact, a group of air-vesicles, which are 
 
1/2 NORMAL HISTOLOGY. 
 
 in communication with a single small bronchus and 
 its terminal twigs. As the small bronchus enters 
 the lobule, it divides into several terminal twigs ; 
 these open at the peripheral extremity into irregular 
 tubular passages, called alveolar passages ; on the 
 sides of these alveolar passages still other irregular, 
 sometimes branched elongated cavities, called in- 
 fundibula, 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 infun- 
 dibula and alveolar passages by wide, irregular open- 
 ings. All of these cavities are closely crowded to- 
 gether, and their walls intimately joined. It is in the 
 walls of the air-vesicles that that interchange of 
 material between air and blood occurs, which is the 
 essential factor in respiration. 
 
 We have now to consider the structure of the 
 walls of the alveolar passages, infundibula, and air- 
 vesicles. The gradual thinning which we have ob- 
 served in the walls of the bronchi as they approach 
 their termination, 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 
 connective-tissue and smooth muscle-cells are em- 
 bedded, the whole being lined with flattened epithe- 
 lium. At the opening of the infundibula and air- 
 
THE RESPJRA TOR Y APPARA TVS, 173 
 
 vesicles the elastic fibres are grouped to form pro- 
 jecting rings, which bound the opening. From these 
 rings of elastic fibres which surround the openings 
 into the alveoli, other elastic fibres are given ofif, 
 which, dividing and subdividing, stretch over the 
 walls of the air-vessels in the form of a wide-meshed 
 net, the spaces between the fibres being occupied 
 by an extremely thin, structureless membrane, in 
 which lies an occasional oval nucleus. 
 
 The alveoli, in the adult, are lined with a single 
 layer of flattened, polygonal, epithelial cells. These 
 are of two kinds: first, small granular, nucleated 
 cells ; and, second, cells which are larger, more irreg- 
 ular 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 difficult to 
 see distinctly without resorting to silver-staining.* 
 
 In the foetus 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 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 
 
 * See Appendix, p. 254. 
 
174 NORMAL HISTOLOGY. 
 
 walls of the vesicles. The lungs are supplied with 
 blood through the pulmonary and the bronchial 
 arteries. The blood from the latter is chiefly dis- 
 tributed 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 bron- 
 chial arteries, after passing through various sets of 
 capillaries, returns through the bronchial veins ; but 
 a certain portion of it finds its way into the pulmon- 
 ary 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 reach- 
 ing 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 al- 
 veolar walls, the vessels often projecting 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 vesi- 
 cles into the pulmonary veins in the interlobular 
 connective tissue, and then into larger trunks which, 
 passing inward, follow the course of the other large 
 vessels. 
 
THE RESPIRA TOR Y APPARA TCS. 1 7 5 
 
 The surface of the lungs is invested with a thin 
 layer of connective tissue, — 'C!\& pulmonary pleura, — 
 which contains numerous blood- and lymph-vessels, 
 and is covered with endothelium. Here and there, 
 beneath the pleura, as well as elsewhere in the 
 lungs, between the lobules and around the alveolar 
 passages and small bronchi, are small irregular 
 nodules of lymphoid tissue. 
 
 The epithelial cells of the alveoli often contain 
 brown or black pigment, and pigment deposits are 
 of the most frequent occurrence, in the adult lung, 
 in the interlobular connective tissue, and in the con- 
 nective tissue and lymph nodes at the base of the 
 lungs, as well as in the above-mentioned lymphoid 
 nodules. The greater part of this pigment is prob- 
 ably taken up from the respired air, and transported 
 through the cells or lymph-channels to the parts in 
 which it is found. 
 
 PRACTICAL STUDY. 
 
 Trachea. — A portion of the human trachea is placed 
 for ten days in |-per-cent. solution of chromic acid, and 
 the hardening completed with alcohol. A small bit is 
 embedded in wax or hardened liver,* care being taken not 
 to rub off the ciliated cells in the manipulation, and the 
 longitudinal and transverse sections are stained double 
 and mounted in balsam. 
 
 Uninjected Lung. — The lungs from the human subject, 
 or any small animal, such as the dog, rabbit, or cat, hav- 
 ing been carefully removed, a canula is tied into the 
 * Much better in celloidin. 
 
176 NORMAL HISTOLOGY. 
 
 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 
 short 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. Sections* may 
 be stained double and mounted in glycerin. 
 
 To demonstrate the elastic fibres in the walls of the air- 
 vesicles, thin sections of an unstained human lung are 
 mounted in glycerin to which strong acetic acid has been 
 added in the proportion of i-ioo. 
 
 Epithelium of the Air-vesicles. — The lung of a freshly- 
 killed animal (a young cat is best) is filled with a solution 
 of silver nitrate 1—500, in the manner just described, and 
 after remaining for half an hour, the tube between the 
 funnel and the lungs should be lengthened to about six 
 or eight inches, so as to increase the pressure, and the 
 funnel filled with a mixture of equal parts of alcohol and 
 water. Under this increased pressure, the silver solution 
 will be partially driven out through the pleura and re- 
 placed by the dilute alcohol. When this replacement is 
 partially accomplished, the lung is placed entire in alcohol 
 of Jhe same strength and exposed to the light. After a 
 few hours it may be cut into pieces and preserved in 
 strong alcohol. Sections are made* from the surfaces 
 
 * After embedding in celloidin. 
 
THE RESPIRA TOR Y APPARA TUS. 1 77 
 
 which have become brown, and mounted in glycerin 
 tinged with eosin. 
 
 Injected Lung. — From bits of human lung whose blood- 
 vessels have been injected with the blue gelatin mixture 
 through the pulmonary artery, sections, which need not 
 be very thin, are made, stained with eosin and mounted 
 in balsam. 
 
CHAPTER XIV. 
 
 THE KIDNEY. 
 
 If a longitudinal section be made through the 
 middle of a rabbit's kidney, the cut surface will pre- 
 sent a well-marked separation into an outer cortical 
 and an inner medullary substance; between these 
 two are seen sections of large blood-vessels. The 
 medullary portion, called medullary pyramid or me- 
 dulla, 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 openings 
 may be seen. The cut surface of the papilla pre- 
 sents an uniform grayish appearance, while the seg- 
 ment 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 cortex, 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 connec- 
 tive tissue — the capsule — consists, besides the medul- 
 
 178 
 
THE KIDNEY. 179 
 
 / 
 lary rays, of a grayish 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, pyra- 
 midal 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 beween the cortex and medulla ; and extending 
 radially toward the surface of the organ through the 
 centre of the cortical pyramids, and at each side of 
 these vessels, may be seen a row of minute globular 
 structures, which are the Malpighian bodi's 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 evident, 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 
 lobulated appearance. As the individual matures, 
 the renculi usually become merged into one another, 
 so that we no longer see on the surface any trace of 
 its composite character. This is betrayed, however. 
 
l80 NORMAL HISTOLOGY. 
 
 by the fact that the medullary pyramids of the 
 primitive renculi persist as separate structures, and 
 we thus have in the adult human kidney 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 sometimes farther than the papillae themselves. 
 Not very infrequently the divisions between the 
 primitive renculi are not entirely obliterated in the 
 process of development, and the surface of the kid- 
 ney, 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 com- 
 posed, are lined with epithelial cells, and run a very 
 tortuous course from their origin in the cortex to 
 their termination in the tiny openings, above men- 
 tioned, at the apex of the papillae. They constitute, 
 with the cells lining the walls of the glomeruli, in 
 which they originate, the parenchyma 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 urin- 
 
THE KIDNEY. l8l 
 
 i/erous tubules, consist of an apparently homogeneous 
 membrana propria lined throughout with a single 
 layer of epithelial cells, which differ greatly in char- 
 acter and form in different parts of the tubules; the 
 tubules have received special names in different 
 parts of their course. 
 
 Each tubule commences within a cortical pyramid, 
 in a dilatation called the 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 medul- 
 lary ray ; this it enters, and, suddenly 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 Henle's 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, form- 
 ing what is known as the intercalated tubule ; this 
 resembles the convoluted tubules, among which it 
 winds in and out, and then passes over, entering a 
 medullary ray, into a straight uriniferous tubule. 
 
1 82 NORMAL HISTOLOGY. 
 
 This is again narrower than the convoluted tubules, 
 and passes directly downward through the medulla, 
 joining other s"imilar 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 by twos, to 
 form the outlet ducts. 
 
 We have now to consider the epithelium which 
 lines the tubules. Commencing at the straight 
 tubules in the papillae, we find that in their lower 
 portion they are lined by cylindrical cells with large 
 nuclei and transparent bodies ; further 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 similar in charac- 
 ter, and consists of large granular striated cells, 
 whose outlines are not well defined, and which 
 nearly fill the lumen of the tube. 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, transparent cells, 
 whose nuclei usually project into the lumen of the 
 tube. 
 
 The glomeruli consist, in the first place, of a mem- 
 branous, apparently 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 
 
THE KIDNEY. 1 83 
 
 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 at- 
 tachment of the tubule, a small artery — the vas 
 afferens — pierces the capsule and immediately di- 
 vides 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 cov- 
 ered 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, inter- 
 calated tubes, and the glomeruli. 
 
 The distribution of the blood in the kidney yet 
 remains to be considered. 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 
 
1 84 NORMAL HISTOLOGY. 
 
 cortex, along the irregular boundary line between 
 the cortex and medulla. From the convex side of 
 the arterial 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.^ From these interlobular arteries lateral 
 twigs are given off at frequent intervals, which, after 
 passing a short distance, enter the glomeruli as the 
 arterise afferentes. On leaving the glomerulus, the 
 efferent vein — which still carries arterial blood — 
 breaks up into a capillary net-work, which lies 
 among the adjacent convoluted tubules and in the 
 neighboring medullary rays, the meshes in the for- 
 mer region being rounded, in the latter, elongated, 
 corresponding to the character of the tubules among 
 which they lie. From these the blood is collected 
 into small veins, which in turn pour it into interlob- 
 ular veins, and these, following the course of the 
 interlobular arteries, finally pour it into the large 
 arched trunks between the cortex and medulla. In 
 the superficial portions of the cortex there are no 
 glomeruli, and here small venous trunks centre in 
 
 ' If we consider a circumscribed portion of the cortex of the kid- 
 ney, having for its centre a medullary ray, and extending on every 
 side as far as to the nearest interlobular vessels, we see that in this 
 limited area we have all of the essential structural elements of the 
 cortex — a medullary ray surrounded by convoluted tubules and glomer- 
 uli. 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. 
 
THE KIDNEY. 185 
 
 the commencement of an interlobular vein, forming 
 the well-known stellulce Verheyenii. 
 
 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 ar- 
 teries, in part from terminal branches of the phrenic, 
 lumbar, and supra-renal arteries, and it passes into 
 the stellate veins. 
 
 The connective tissue of the kidney, aside from 
 that which forms the capsule, lines the pelvis, and 
 is distributed along the walls of the larger blood- 
 vessels and around the glomeruli, is very small in 
 amount. It may, however, be demonstrated here 
 and there, and is most abundant in the vicinity of 
 the papilla. 
 
 PRACTICAL STUDY. 
 
 Rabbit's Kidney — General View. — A rabbit's kidney is 
 cut transversely across the middle and hardened in two- 
 
1 86 NORMAL HISTOLOGY. 
 
 per-cent. solution of bichromate of potassium and alco- 
 cohol. Thin sections are made across the entire organ, 
 including cortex, medulla, and papilla, stained double 
 and mounted in balsam. 
 
 Isolated Tubules. — A small fragment of fresh kidney, 
 including both cortical and medullary portions, is placed 
 in a mixture of equal parts of alcohol and strong hydro- 
 chloric acid ; the acid partially dissolves 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 accomplished. It is allowed to 
 soak for a few hours in water to remove the acid, and 
 then small bits are torn off, including cortical and medul- 
 lary 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 better by allowing small drops of water to fall upon 
 the specimen on a slide, from a pipette. When the dis- 
 sociation is partially effected, the excess of water should 
 be removed with filter paper, and a mixture of equal parts 
 of saturated solution of picric acid and glycerin added, 
 in which it is mounted and preserved. 
 
 Sections of Uninfected Human Kidney. — A perfectly 
 fresh human kidney is cut into small pieces and hardened 
 in strong alcohol. Sections are made in a plane vertical 
 to the surface of the organ, including both cortical and 
 medullary portions, and also parallel to the surface, 
 through the cortex. They are stained double and 
 mounted in glycerin. 
 
THE KIDNEY. 1 87 
 
 Sections of Injected Kidney. — The kidney is injected, 
 through both the renal artery and vein, with the blue 
 gelatin mixture, and sections in different directions are 
 stained with %osin 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, zx^di parts of the penis. 
 
 THE TESTICLE — SPERMATOZOA. 
 
 This is a tubular gland, whose chief specific secre- 
 tion is the spermatozoa ; it is enclosed by a firm, 
 dense connective-tissue capsule, called the albu- 
 ginea, which sends inward several incomplete septa, 
 which divide the organ into a number of communi- 
 cating 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 wind- 
 ing, anastomosing, and looped tubules, called semi- 
 niferous tubules, which constitute the secreting por- 
 tion of the organ. As the tubules approach the 
 corpus Highmori, they become narrower and 
 straighter — tubuli recti — and are lined with cylindri- 
 cal epithelium, and, on entering that body, form a 
 net-like series of intercommunicating channels lined 
 
 l88 
 
THE GENERATIVE ORGANS. 1 89 
 
 with flattened cells, called the rete testis. The chan- 
 nels of the rete testis are continuous with several 
 tubules, which, passing upward and backward, be- 
 come very much convoluted, and form a number of 
 conical masses — coni vasculosi-r-vfhich largely consti- 
 tute the head of the epididymis. The tubules of 
 the coni vasculosi gradually unite as they descend 
 to form a single canal, which, with numerous wind- 
 ings 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 embedded in loose, delicate, lamellated 
 connective tissue, which is abundantly supplied with 
 blood-vessels, and contains many cells ; among the 
 ordinary connective-tissue cells of various forms, 
 large granular, often pigmented cells are not infre- 
 quently seen lying singly or in groups, and some- 
 times in rows along the blood-vessels ; their nature 
 and significance are still doubtful. The tubules 
 have a distinct membrana propria, and are lined 
 with several layers of cells piled irregularly over 
 one another, which differ in form under different 
 circumstances, and sometimes in different parts of 
 the same gland. 
 
 In a tubule which is not producing spermatozoa, 
 the outer row of cells — those lying upon the mem- 
 brana propria — are large, granular, well-defined, nu- 
 
1 9° NORMAL HISTOLOGY. 
 
 cleated cells ; and upon these lie two or three irreg- 
 ular 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 some recent observers, is believed 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 framework in which the inner cells are sup- 
 ported. Others believe it to be only intercellular 
 cement-substance. The lumen of these tubules 
 may be filled with granular material or may con- 
 tain spermatozoa. 
 
 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, which 
 are similar to the lining cells above described, and 
 the tails stretching brush-like into the cavity of the 
 tube. 
 
 According to some observers, the spermatozoa are 
 formed from certain of the cells of the inner layers, 
 which are called spermatoblasts. The process of de- 
 velopment, according'to this view, commences in the 
 muclei of the spermatoblasts, which become con- 
 verted into the head, while the tail is an outgrowth 
 from the nucleus, or is produced by a transformation 
 of a portion of the cell-protoplasm. Others believe 
 
THE GENERATIVE ORGANS. I9I 
 
 that the spermatozoa are formed by the growth in- 
 ward, from the large outer cells, or from cells lying 
 among these, of a long-stemmed bud-like process, 
 whose dilated end divides into a number of longitud- 
 inal segments, each of which finally becomes a sper- 
 matozoon. That the latter is the mode of development 
 in certain animals, e. g. the rat, would seem to be 
 unquestionable?. 
 
 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 be- 
 tween the head and tail is a short, narrow segment, 
 called the middle piece. When living, and under 
 favorable conditions, the spermatozoa are capable 
 of performing rapid movements, the whole organism 
 being driven hither and thither by wavy vibrations 
 of the tail. 
 
 PRACTICAL STUDY. 
 
 Spermatozoa. — These may be obtained from the sem- 
 inal vessels of man, or from the sediment of urine in 
 which they occur either normally, or under pathological 
 conditions. They are well preserved by a mixture of 
 equal parts of saturated sol. of picric acid, glycerin, and 
 water, in which they may be mounted. 
 
 Spermatozoa for comparative study may be readily ob- 
 tained by making an incision into the head of the epi- 
 didymis of a dog, rabbit, or guinea-pig, and receiving the 
 milky fluid which exudes in the above picric acid fluid. 
 
192 NORMAL HISTOLOGY. 
 
 The movements of the living spermatozoa may be 
 studied by mixing some of the milky fluid from the epi- 
 didymis of a freshly killed animal, on a warm slide, with 
 J-per-cent. salt solution, and protecting from pressure 
 by a hair. 
 
 The temperature of the slide should be kept at about 
 the same elevation as that of the animal from which they 
 are taken. The movement may be stopped by the ad- 
 dition of water. 
 
 Sections of Testicle. — Small pieces of the human tes- 
 ticle, obtained as soon as possible after death, or, if this 
 cannot be obtained, of the testicle of the cat, rabbit, dog, 
 or rat, are hardened in 2-per-cent. sol. of potassium bichro- 
 mate and alcohol. Sections from the different parts are 
 stained double, and mounted in glycerin or balsam. The 
 differences between the structure of the testicle in the 
 lower animals and man cannot be described here. 
 
 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 frequently 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 
 
THE GENERATIVE ORGANS. I93 
 
 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 vary- 
 ing thickness, which, in turn, is enclosed in a fibrous 
 tunic. 
 
 PRACTICAL STUDY. 
 
 Section of Gland. — A bit of that portion of the pros- 
 tate gland of man which lies behind the urethra, is 
 hardened in potassium bichromate and alcohol, and the 
 sections are stained double and mounted in balsam. 
 
 URETHRA AND CORPUS SPONGIOSUM. 
 
 The tissues which form the penis are so various, 
 and present such essential differences in their ar- 
 rangement in different parts of the organ, that a de- 
 tailed study of its structure does not lie within the 
 scope of this manual. Inasmuch, however, as the 
 urethra is so frequently the seat of surgical opera- 
 tions, and as the corpus spongiosum, which sur- 
 rounds a portion of it, presents an example of a 
 variety of tissue which we shall have no opportunity 
 to study elsewhere, it is desirable to briefly consider 
 their structure here. 
 
 The urethra, divided into three portions — a pros- 
 tatic, a membranous, and a spongy, — consists of a 
 mucous membrane which is surrounded by a muscu- 
 lar sheath. The considerable differences in struc- 
 ture which its different parts present are due chiefly 
 
194 NORMAL HISTOLOGY. 
 
 to variations in the form of the epithelium and the 
 character and arrangement of the muscular tissue 
 The mucosa is composed, in all parts alike, ■ of 
 fibrillar connective tissue containing numerous elas- 
 tic 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 por- 
 tion ; 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 epithe- 
 lium of the bladder. 
 
 Outside of* the mucosa, and but indistinctly sep- 
 arated from it, is the submucosa, which consists of 
 connective tissue with elastic fibres, and is especially 
 characterized by a dense net-work of veins which re- 
 ceive their blood from the rich capillary 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 submucosa, 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 Morgagni, 
 are seen in the surface of the mucous membrane. 
 Racemose glands, called Littres glands, lie embed- 
 ded in the mucous membrane, sometimes extending 
 
THE GENERATIVE ORGANS. IQS 
 
 into the muscular tunic ; their excretory 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 
 general, of an inner longitudinal and an outer circu- 
 lar 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 consists 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. 
 
 When the urethra is closed the mucous membrane 
 is thrown into irregular longitudinal folds. 
 
 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 body it will be well to 
 consider for a moment the nature of this kind of 
 tissue. 
 
 Erectile tissue, in certain cases, consists simply of 
 a somewhat circumscribed collection of larger and 
 smaller veins, which under, certain circumstances, 
 
196 NORMAL HISTOLOGY. 
 
 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 communi- 
 cation 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 cer- 
 tain conditions, become distended with blood, when 
 they assume the character of spheroidal, or broad 
 elongated cavities, and thus cause a considerable 
 increasa in volume of the part in which they are 
 situated. The corpus spongiosum is composed 
 largely of erectile tissue of the character last de- 
 scribed. 
 
 The corpus spongiosum is enclosed in a dense con- 
 nective-tissue sheath which contains elastic fibres 
 and a few smooth muscle-cells. From this a multi- 
 tude of narrow trabeculse or septa, composed largely 
 of smooth muscle-tissue, pass inward in various 
 directions, and dividing and subdividing, form ai\ 
 intricate series of spaces, which are the above-men- 
 tioned blood-cavities. This system of trabeculae and 
 septa is continuous, within, with the submucosa of 
 the urethra. 
 
 The erectile tissue is most abundant below the 
 
THE GENERATIVE ORGANS. I97 
 
 urethra ; above, the blood is collected into venous 
 trunks, which, joining similar trunks from the cor- 
 pora cavernosa above, convey the blood away from 
 the part. The corpora cavernosa are similar in 
 structure to the corpus spongiosum. 
 
 PRACTICAL STUDY. 
 
 Transverse Sections. — The corpus spongiosum and the 
 posterior portions of the urethra are dissected from the 
 remainder of the penis and hardened in potassium bichro- 
 mate and alcohol. Transverse sections from the different 
 regions are stained double and mounted in balsam. 
 
 FEMALE GENERATIVE ORGANS. 
 THE OVARY. 
 
 The ovary is a gland whose vesicular alveoli have 
 no excretory ducts, but discharge their contents by 
 periodic rupture at the surface of the organ. These 
 alveoli are called Graafian follicles, and the ovum 
 may be regarded as their specific secretion. They 
 lie embedded in a peculiar connective-tissue stroma, 
 the interstitial tissue, 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 Graafian 
 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 
 
198 ' NORMAL HISTOLOGY. 
 
 the ramifications of the larger blood-vessels, and called 
 the medullary portion ; and an outer, denser 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 
 somewhat loosely interwoven and associated with 
 elastic fibres and smooth muscle-cells. In the corti- 
 cal 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 modified so as to form a kind of 
 capsule. Near the surface of the organ the connec- 
 tive-tissue fibres arrange themselves in crossing lay- 
 ers, to form a dense but not distinct sheath, called 
 the albuginea. 
 
 The connective tissue of the ovary, especially in 
 the cortical zone, contains a great number, and in- 
 deed in some parts seems almost entirely to consist 
 of spindle-shaped, flattened, and irregular branch- 
 ing cells, among which are not infrequently larger 
 and smaller pigmented cells and polyhedral cells 
 resembling epithelium. 
 
 The surface of the organ is covered by a single 
 layer of cylindrical epithelium, whose extreme sig- 
 nificance we shall recognize when we study the de- 
 velopment 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 vascu- 
 
THE GENERATIVE ORGANS. I99 
 
 lar; the large arterial trunks, entering from the 
 broad ligament, divide and subdivide in the medul- 
 lary portion, and pass off in larger and smaller twigs 
 into the cortex, from which an abundant capillary 
 net-work is formed around the follicles. The ar- 
 teries frequently run a very tortuous course, twist- 
 ing and turning upon themselves, and are character- 
 ized, moreover, by the great abundance of smooth 
 muscle-cells in their walls. Lymphatic vessels 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 parts just described 
 are subservient. The Graafian follicles in the ovary 
 of the adult female, at the ohild-bearing age, present 
 by no means the same appearance ; some are large 
 and readily visible to the naked eye ; others very 
 minute, and presenting under the microscope an en- 
 tirely 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, consisting of an ex- 
 ternal denser layer and an internal layer very richly 
 supplied with cells, and containing an abundant 
 
200 NORMAL HISTOLOGY. 
 
 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 consti- 
 tuting 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 cumulus proligerus 
 or germ-hill. The remainder of the follicular cav- 
 ity is filled with a colorless fluid, in which not 
 infrequently fine granules are suspended. This fluid 
 increases in quantity as the follicle approaches ma- 
 turity, and the pressure occasioned by its accumu- 
 lation probably conduces in no slight degree to 
 the final rupture of the follicle. The follicular 
 epithelium immediately surrounding the ovum is 
 cylindrical and arranged radially about it. 
 
 In the ovum itself, which is a cell of the most 
 highly developed type, we recognize four structural 
 elements: i, a thick hyaline membrane, presenting, 
 with high powers, a delicate radial striation, and 
 called the zona pellucida; 2, within the zona pellu- 
 cida 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 
 nucleus, the germinal vesicle ; 4, the germinal vesicle, 
 which is usually somewhat eccentrically placed, con- 
 
THE GENERATIVE ORGANS. 20I 
 
 tains, in addition to a nearly transparent fluid, a 
 small dark, often almost opaque nucleolus, the ger- 
 minative 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 pro- 
 jecting side, and finally burst at a menstrual period. 
 The ovum, with the fluid and a portion of the follic- 
 ular epithelium, is discharged, and through the 
 heinorrhage which occurs from the capillaries in the 
 follicular wall, the cavity becomes 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 impregnated and develops ; and the differ- 
 ence expresses itself chiefly in a difference in size 
 and persistence of the mass of tissue called the cor- 
 pus lufeum, which is produced by a growth of certain 
 of the cells which remain after the rupture of the 
 follicle. These differences cannot be considered 
 here. The process of formation and disappearance 
 of the corpus luteum, under all circumstances, is 
 essentially the following : The blood which is poured 
 out into the cavity passes through the same retro- 
 gressive metamorphosis which extravasated blood 
 in any part of the body may undergo : it coagulates, 
 the serum is absorbed, the red cells disintegrate, and 
 the coloring matter is in part taken up by surround- 
 
202 NORMAL HISTOLOGY. 
 
 ing tissues, in part transformed into yellowish or red 
 hematoidin (bilirubin) crystals, which in turn may 
 change into a dark brown or black pigment, and 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 epithelium which is left behind, and in 
 the cells of the theca folliculi. These cells prolifer- 
 ate and form a soft, yellowish, very vascular tissue, 
 resembling mucous tissue, which presently under- 
 goes fatty degeneration. This yellow mass, sur- 
 rounding and enclosing the remains of the extrava- 
 sated blood, constitutes the corpus luteum ; and, as 
 it disappears, 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 pigment. 
 
 In order to understand all the forms which the 
 immature Graafian follicles present, it will be neces- 
 sary to study the way in which these structures 
 originate. They are produced from the cylindrical 
 epithelium which covers the ovary and is called 
 germ-epithelium. At an early period of life the con- 
 nective-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 elon- 
 sated, solid or tubular masses. Presently these 
 
THE GENERATIVE ORGANS. 20^ 
 
 groups of cells become separated from, the gernynal 
 epithelium at the surface, and appear then as irregu- 
 lar masses of polyhedral or spheroidal cells, 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 
 surface. The cells which now lie in these separated 
 cavities may at first all look alike, but later 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 follicle, and the central cell is the ovum ; 
 but it presents as yet no zona pellucida. Gradually 
 the follicular epithelium increases in quantity, form- 
 ing several layers, and the ovum becomes eccentri- 
 cally 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 accumulates within 
 it. Changes in the interstitial tissue lead to the for- 
 mation of the theca folliculi, the epithelium directly 
 about the ovum 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. 
 
204 NORMAL HISTOLOGY. 
 
 PRACTICAL STUDY. 
 
 Section of Adult Ovary. — The human ovary, or that of 
 a recently killed dog or cat, 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 so- 
 lution and alcohol. After a week it is to be transferred 
 to alcohol, in which in a few days it will become suffi-- 
 ciently hard. A half is embedded in wax,* the sections 
 stained double and mounted in balsam. 
 
 Section of Developing Ovary. — The ovary of a foetal or 
 new-born animal is hardened and prepared as above. 
 
 THE UTERUS. 
 
 The walls of the uterus consist of crossing and 
 interlacing bundles of smooth muscle-Gells, with a 
 small amount of connective tissue enclosing 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 
 longitudinal, 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 
 
 * Better in celloidin. 
 
THE GENERATIVE ORGANS. 205 
 
 which lie a great number of spheroidal, fusiform, and 
 branched cells, and is 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 mucous membrane of the body 
 of the uterus is smooth, but in the cervix it presents 
 regular folds, the so-called plicce palmatce ; the con- 
 nective-tissue framework 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 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 sup- 
 plied with a rich capillary plexus, which passes inward 
 close beneath the surface-epithelium. 
 
 During menstruation the mucous membrane be- 
 comes 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 general surface of the cavity, 
 is swollen. To what extent the blood, which, mixed 
 
2o6 NORMAL HISTOLOGY. 
 
 with mucus and separated epithelium, is present 
 in 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. 
 
 Section of Human Uterus. — The uterus is cut into sev- 
 eral pieces, and hardened in potassium bichromate and 
 alcohol. The organ should be procured as soon as pos- 
 sible after death, since the cilia of the lining epithelium 
 are easily destroyed by the decomposition which com- 
 mences very early in the uterine cavity. Sections per- 
 pendicular 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 Fal- 
 lopian tubes, since here the uterine glands are more uni- 
 formly arranged at right angles to the surface. They are 
 stained double and mounted in balsam. 
 
 THE VAGINA. 
 
 In the walls of the vagina we recognize three lay- 
 ers : an outer fibrous layer, a middle muscular layer, 
 and z. lining mucous metnbrane. 
 
 The fibrous layer, by means of which the vagina 
 is connected with adjacent parts, consists of connec- 
 tive 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 
 
THE GENERATIVE ORGANS. 207 
 
 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 connec- 
 tive tissue, loose in texture and containing coarser 
 and finer elastic fibres ; it presents numerous trans- 
 verse folds and elevations, and is covered by lami- 
 nated 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 epi- 
 thelium. 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. 
 
 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 an- 
 gles to the surface are stained double and mounted in 
 glycerin. 
 
 Isolated Surface-cells.- — From a bit of vagina hardened 
 as above, or simply in alcohol, the cells from the surface 
 of the mucous membrane are scraped with a scalpel, 
 stained on a. slide with hsematoxylin, and mounted in glyc- 
 erin. Familiarity with the appearance of these surface 
 islls is of practical importance, since they are often found 
 'jx\ urine. 
 
208 NORMAL HISTOLOGY. 
 
 MAMMARY GLAND. 
 
 The mammary gland is a racemose, and when 
 fully developed a lobulated, gland, whose spheroidal 
 and elongated alveoli are formed by a membrana 
 propria composed of flattened cells, and lined with 
 cuboidal epithelium. The excretory ducts, which 
 in each gland are fifteen- to twenty in number, and 
 open at the surface of the nipple, are lined with 
 cylindrical 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 mi- 
 croscopically, quite marked differences in appearance 
 at different times, depending upon age, sex, and, in 
 the adult female, upon whether or not the individ- 
 ual is pregnant, and upon the period of pregnancy 
 and the occurrence of lactation. 
 
 In children, as in the adult male, the gland pre- 
 sents, in general, a system of ramifying ducts, ter- 
 minating in blind or in more or less pouched or 
 dilated extremities ; these lie embedded in connec- 
 tive tissue, which makes up the greater part of the 
 bulk of the gland. At puberty the gland of the fe- 
 male 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 abundant than the 
 
THE GENERATIVE ORGANS. 209 
 
 gland-tissue proper. Essentially in this condition 
 the gland usually 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 lobu- 
 lated 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 fre- 
 quently fat-cells. During lactation the alveoli are 
 very large, their epithelium contains fat-droplets in 
 considerable number, and fat is found in greater or 
 less quantity in the dilated 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 perma- 
 nent 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. 
 
2IO NORMAL JUSTOLOGY. 
 
 PRACTICAL STODT. 
 
 Sections of Hardened Gland. — Portions of the gland 
 from the human subject or from some of the lower mam- 
 malia — if possible, both in a condition of rest and of func- 
 tional activity — should be hardened in potassium bichro- 
 mate and alcohol, and the sections stained double and 
 mounted in glycerin or balsam. 
 
CHAPTER XVI. 
 
 THE CENTRAL NERVOUS SYSTEM. 
 
 THE SPINAL CORD. 
 
 The spinal cord contains nerve-cells and nerve' 
 fibres ; the former confined to the dentral, the latter 
 most abundant in the peripheral, portions. 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 fis- 
 sures, reaching nearly to the centre, divide the cord 
 into lateral halves ; into these fissures, called the 
 anterior and posterior longitudinal fissures, the pia 
 mater sends membranous prolongations. The 
 anterior fissure is complete, the pia being^ found on 
 both of its sides, 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 as- 
 sumes differs considerably in different parts of the 
 cord, but has, in general, the form of an unsyn\met- 
 
212 NORMAL HISTOLOGY. 
 
 rical 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 s 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 connective tissue. From 
 the anterior and posterior cornua the spinal nerves 
 pass off, dividing the white matter into three toler- 
 ably distinct portions called the anterior, lateral, 
 ■axiA posterior columns. 
 
 In the white substance of the cord we find nerve- 
 fibres, connective tissue, and blood- and lymph-ves- 
 sels. The fibres are for the most part meduUated, 
 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 longitudi- 
 nally 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 commis- 
 sure, at the bottom of the anterior fissure, a band 
 
THE CENTRAL NERVOUS SYSTEM. 213 
 
 of Horizontally arranged fibres running from one 
 side to the other, called the white commissure. The 
 fibres which pass out of the gray matter to form 
 the roots of the spinal nerves, take also longitudi- 
 nal and oblique courses. 
 
 The connective tissue of the white substance is 
 intimately connected at the periphery with the pia 
 mater. The inner layers of the latter, consisting 
 of delicate fibrillated and a few elastic fibres, send 
 into the cord numerous branching septa, which 
 divide the space occupied by the nerve-substance 
 into smaller irregular spaces, in which the nerve- 
 fibres run, these being surrounded by a delicate 
 net-work of fibrillae, among which, here and there, 
 variously shaped, for the most part flattened, some- 
 times branching, connective-tissue cells are found.* 
 This delicate supporting connective tissue is called 
 neuroglia. In addition to the fibrillar and cellular 
 elements above described, there seems to be a 
 greater or less amount of soft homogeneous or 
 finely granular material lying in the interstices of 
 nerve- and connective-tissue fibres. 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 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 
 
 * Neuroglia cells. See Appendix, p. 254. 
 
214 NORMAL HISTOLOGY. 
 
 meduUated, in part naked axis cylinders ; and be- 
 sides 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 mat- 
 ter consist of a delicate neuroglia framework, simi- 
 lar, in most respects, 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. The limitations of 
 this manual will not permit us to consider more in 
 detail what is known of the course of the nerve- . 
 fibres through the cord, and the more exact rela- 
 tions 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. 
 
THE CENTRAL NERVOUS SYSTEM. 215 
 
 THE BRAIN. 
 
 In this organ also we have gray and white mat- 
 ter; but they are arranged in a much more compli- 
 cated manner than in the spinal cord, the grouping 
 being far too intricate for consideration here. The 
 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 cere- 
 bellum consists of coarser and finer, but all very 
 small, nerve-fibres, running in various directions, and 
 supported by a delicate connective-tissue frame- 
 work, similar to the neuroglia of the cord. The 
 gray matter consists here, as in the cord, of gang- 
 lion-cells, and fine gray fibres supported by connec- 
 tive tissue. In parts of the gray as of the white 
 substance, certain cellular and fibrous elements oc- 
 cur, of which it is at present impossible to say 
 whether they are connective or nerve-tissue. In- 
 deed, it is the difficulty 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 interesting parts of the 
 
2l6 NORMAL HISTOLOGY. 
 
 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 re- 
 gions, but that of one of the frontal lobes is suffi- 
 ciently 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 
 into one another. 
 
 In the most superficial layer, the connective-tissue 
 elements preponderate, and among them, delicate 
 nerve-fibrils interlace, and a few small, scattered 
 globular and elongated branching nerve-cells are 
 found ; the second layer is characterized by a 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, pyra- 
 midal, or broad spindle-shaped, and multipolar, with 
 their long axes perpendicular to the surface of the 
 cortex ; in the fourth layer, which is much narrower 
 than the la.st, 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, together 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 
 
THE CENTRAL NERVOUS SYSTEM. 2\J 
 
 white matter; and in the fourth and fifth layers, 
 they are readily seen in distinct bundles passing in- 
 ward 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 recognizable : the 
 outer, sometimes called the molecular layer, consists, 
 like the outer layer of the cerebral cortex, of a deli- 
 cate connective-tissue framework, which supports 
 fine nerve-fibres and small spindle-shaped and 
 branching nerve-cells ; the middle, cellular layer, \s 
 formed by an irregular row of large ganglion cells, 
 Purkinj'e's cells, whose branching processes extend 
 into and ramify in the outer layer, while the axis- 
 cylinder process passes inward through the inner 
 layer; the inner, granular layer, contains a great 
 number of small spheroidal cells whose nature is un- 
 determined. The granular layer merges gradually 
 into the white substance and is thickest at the sum- 
 mit of the convolutions, where also Purkinje's cells 
 are most abundant. 
 
 The blood-vessels penetrate the cortex of both 
 cerebrum and cerebellum, in the form of small arte- 
 rial twigs from the pia, and form an abundant net- 
 work in the gray substance, being somewhat differ- 
 ently distributed in different parts of the organ, and 
 less abundant in the white vhan in the gray matter. 
 
 The Dura Mater of the brain is a dense connective- 
 tissue membrane containing numerous elastic fibres 
 and lined within by endothelial cells. Where it is 
 
2l8 J>iORMAL HISTOLOGY. 
 
 attached to the bones of the skull, to whose inner 
 surface it acts as periosteum, the tissue on the at- 
 tached surface is looser in texture and abundantly- 
 supplied with blood-vessels. It contains the ordi- 
 nary flattened connective-tissue cells, and usually a 
 certain number of " 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 exceedingly abun- 
 dant net-work of blood- and lymph-vessels. Over 
 the surface of the convolutions it forms a single mem- 
 brane containing numerous small lymph-sinuses ; 
 but as it approaches the sulci it is partially separated 
 into two indistinct layers, the outer bridging over 
 the sulci, while the inner and more vascular layer 
 dips down to the bottom of them. The space 
 within the sulci, between the two layers, is occupied 
 by numerous larger and smaller lymph-sinuses, 
 which under normal conditions are little more than 
 slits in the connective tissue, lined with endothelium ; 
 but they are capable of considerable dilatation when, 
 for any reason, fluid accumulates in the meshes of 
 the pia. These spaces are called sub-arachnoidal 
 lymph-spaces ; the outer layer of the pia having been 
 formerly regarded as a distinct membrane, and called 
 the arachnoid. These sub-arachnoidal lymph spacesy 
 as well as the other numerous lymph-channels of the 
 
THE CENTRAL NERVOVS SVSTEM. 21$ 
 
 pia, are in communication with lymph-channels, 
 called perivascular lymph-channels, which ensheath 
 certain of the blood-vessels as they enter the brain- 
 substance. 
 
 PRACTICAL STUDY. 
 
 Transverse Sections of Spinal Cord. — A perfectly fresh 
 human cord should be freed from its dura mater, and short 
 segments from different portions of it hardened in Mtiller's 
 fluid and alcohol.* Thin transverse sections are made 
 through the entire cord in different pordons, and may be 
 stained first lightly with haematoxylin and then deeply 
 with eosin and mounted in balsam. A very excellent 
 hardening agent for the nerve-centres is the bichromate 
 of ammonium, 2-per-cent. solution, in which small frag- 
 ments should lie for a week, and then, after careful wash- 
 ing, the hardening completed with dilute and strong al- 
 cohol. They may be stained and mounted as above.^ 
 
 Sections of Cortex of Cerebrum and Cerebellum. — These 
 should be prepared in the same way as the spinal cord, 
 pieces from the cortex of the frontal lobes being taken, 
 and from the cortex of the cerebellum 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 alco- 
 hol, embedded in hardened liver, and thin transverse 
 sections made, stained double and mounted in glycerin 
 or balsam. 
 
 Pia Mater. — This may be prepared by the method 
 given on page 125, 
 
 * Embed in celloidin. 
 
 i "?<"(> new stainins melhnd. Appendix, p. 254. 
 
CHAPTER XVII. - 
 THE SKIN AND ITS ADNEXA. 
 THE SKIN. 
 
 We recognize in the skin three layers of tissue : 
 I, an outer, epithelial layer, the epidermis ; beneath 
 this, 2, a layer of quite firm and dense connective 
 tissue, the coriuin — true skin, cutis vera, or derma ; 
 3, a layer of looser connective tissue, the subcu- 
 taneous tissue, which, merging into the corium, 
 serves to bind it to the underlying parts. The skin 
 is variously modified in structure in different parts 
 of the body, corresponding to the different condi- 
 tions of exposure and wear to which it is subjected, 
 and to form certain supplementary structures, such 
 as the hair, nails, etc. ; and contains various sensory 
 and secretory structures. 
 
 In the epidermis we recognize two tolerably dis- 
 tinct layers of cells: i, an outer or horny layer, con- 
 sisting of very thin, transparent, tough, scale-like 
 cells, which present, for the most part, no nuclei, 
 and are packed closely together ; 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, ad- 
 
 220 
 
THE SKIN AND ITS ADNEXA. 22 1 
 
 joining the coriam, the cells are more or less cyl- 
 indrical; above this they are spheroidal or polyhe- 
 dral or elongated ; still nearer the surface they be- 
 come flattened, and finally merge into the thin cells 
 of the horny layer. In the middle zone the cells 
 present a peculiar jagged outline, looking as if they 
 were bordered by short delicate spines, by which 
 the cells appear dove-tailed together. These spined 
 cells — called prickle cells — are very characteristic of 
 this part of the epidermis, and are also found in 
 certain other parts of the body where stratified 
 epithelium occurs, as in the vagina, mucous mem- 
 brane of the mouth, etc. 
 
 The relative thickness of the horny and Mal- 
 pighian 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 Malpighian 
 layers form a distinct narrow, transparent zone, 
 called the stratum lucidum. The deeper cells of the 
 Malpighian layer contain, uniformly in the negro, 
 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 papillce, the valleys be- 
 tween which, as well as their summits, being cov- 
 
222 NORMAL HISTOLOGY. 
 
 ered by the cells of the Malpighian layer. If we 
 imagine a section made through the skin, parallel 
 with its surface, and just deep enough to cut off the 
 tops of the papillae, the cells of the Malpighian 
 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 
 papillae of the corium. Hence it is that these col- 
 lections of cells have received the name rete Mal- 
 pighi. 
 
 The corium is formed of interlacing bundles of 
 connective tissue, which are coarser in the deeper, 
 finer in the more superficial portions, where they 
 extend into the epidermis, forming the papillae. 
 Embedded in the papillae are capillary blood-vessels, 
 nerves, and special terminal nerve-apparatuses. Elas- 
 tic fibres are present in considerable number, and in 
 the interstices of the fibres lie flattened, spindle- 
 shaped, branching, and small spheroidal cells. In 
 addition to these elements we sometimes find mus- 
 cular tissue in the corium ; thus striated muscular 
 fibres occur in certain parts of the skin of the face ; 
 and smooth muscular tissue, aside from that belong- 
 ing 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 al- 
 ready studied when considering the connective tissue 
 in detail. In some parts of the skin its texture is 
 
THE SKIN AND ITS ADNEXA. 223 
 
 SO loose that the cerium 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 s"ubcu- 
 taneous tissue of the scrotum, penis, eyelids, and the 
 pinna of the ear, fat is not formed. 
 
 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 capil- 
 laries: first, to those which supply the fat-tissue; 
 second, to those which ramify in the sweat-glands ; 
 third, to those which supply the hair-follicles, se- 
 baceous glands, and the papillae of the corium. 
 Each papilla is furnished with a capillary loop, ex- 
 cept when it contains a tactile corpuscle, when the 
 former may be absent. 
 
 THK NAIL. 
 
 We recognize in the hard substance of the nail, 
 which corresponds to the horny layer of the epi- 
 dermis, a body and a root ; the former lies upon a por- 
 tion 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 
 
224" NORMAL HISTOLOGY. 
 
 not differ essentially from that of the skin in gen-^ 
 eral ; it is intimately connected with the periosteum 
 of the phalanx, and presents longitudinal ridges, 
 low in the matrix, higher in the nail-bed, which are 
 covered with papillae. The latter are bent obliquely 
 forward, and are more abundant in the nail-bed 
 than in the matrix. Upon and between the pa- 
 pillae several layers of variously shaped cells lie,' 
 corresponding to the Malpighian layer of the skin.' 
 In the body these cells pass quite abruptly 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 Malpighian 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 papillae 
 to be seen, as it is in the rest of the nail-bed, where 
 the longitudinal ridges are higher, and covered by 
 fewer cells. 
 
 THE HAIR. 
 
 We distinguish in the hair : the shaft, which pro- 
 jects above the surface of the skin ; the root, which 
 is embedded in an oblique tubular depression, called 
 the follicle ; and the bulb, a dilated portion at the 
 bottom of the follicle in which the hair ends. The 
 follicle sometitnes extends into the subcutaneous' 
 
THE SKIN AND ITS ADNEXA. 225 
 
 tissue, sometimes only into the cbrium, and its walls' 
 are fornied in the first place by a sheath of connec- 
 tive 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 prop- 
 erly to the hair itself, are irregularly polygonal, 
 somewhat flattened, and contain an elongated nu- 
 cleus. 
 
 Both at the opening of the follicle, and at its, 
 base, the layers of the root-sheath become indis- 
 tinct, 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 forming the wall of the 
 follicle, in the form of a papilla, which corresponds 
 to the papilla; of the skin, and upon which the hair- 
 bulb rests, surrounding it at the top and sides. 
 The hair is produced by the growth of cells about 
 
226 NORMAL HISTOLOGY. 
 
 the papillae; directly covering the latter are cylin- 
 drical and cuboidal cells, corresponding to those of 
 the rete Malpighi, which gradually become changed 
 in shape, and more or less horny, and form the sub- 
 stance of the hair-shaft. 
 
 In the shaft we recognize three portions: i. A 
 central or medullary portion, composed of cuboidal 
 or more or less flattened cells, not infrequently en- 
 closing between them tiny bubbles of air, which 
 give the centre of the hair a dark appearance by 
 transmitted light. Outside of this is 2, the cortical 
 portion, making up the larger part of the bulk of 
 the shaft, and composed of tough, horny, elongated, 
 flattened cells closely packed together, and having 
 within and between them, 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 cor- 
 tical 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. 
 
THE SKIN AND ITS ADNEXA. 22^ 
 
 SEBACEOUS GLANDS. 
 
 These are racemose glands, whose excretory ducts 
 are lined with polyhedral and somewhat flattened 
 cells. The alveoli, bounded by a membrana propria, 
 are lined with granular polygonal epithelium, and 
 the cavity is more or less filled with larger polyhed- 
 ral cells, crowded with fat-droplets. The sebaceous 
 glands, as a rule, 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. This is attached 
 to the connective-tissue sheath of the follicle in its 
 lower third, and, passing obliquely upward, is in- 
 serted into the upper portion of the corium at some 
 distance from the opening of the follicle. A con- 
 traction 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 pilce. 
 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. 
 
228 NORMAL HISTOLOGY. 
 
 This relation of the erector pilae muscle to the se- 
 baceous gland is probably 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 polyhedral and cuboidal epithelium. 
 Its lower extremity, coiled into a ball, and held to- 
 gether by loose connective tissue, lies sometimes in 
 the corium, sometimes in the subcutaneous tissue. 
 The upper portion of the tube, which serves 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 epidermis-layer is 
 thick, as in the palm, etc., the course of the . duct 
 through it is a remarkably winding one. An abun- 
 dant capillary net-work lies in the loose connective 
 tissue of the gland-coil. 
 
 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 StCiN AND ITS ADNEXA. 229 
 
 the corium, where they form plexuses, varying in 
 character in different parts of the body. From 
 these, certain medullated nerves pass into the papillae 
 and terminate in the tactile corpuscles (called Meiss- 
 ner's corpuscles) ; others pass to the hair-follicles 
 and sebaceous glands; still other, non-medullated 
 nerves enter the papillae or pass between the c»lls 
 of the rete Malpighi, but their mode of termination 
 is not yet definitely ascertained. The structure of 
 the Pacinian bodies and Meissner's corpuscles is too 
 intricate, and the methods required for their com- 
 plete demonstration too elaborate, to justify their 
 further consideration here. 
 
 PRACTICAL STUDY, 
 
 Sections of Injected Skin. — A piece of skin is removed 
 from an injected 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 in a mixture of 
 equal parts of one-half-per-cent. chromic acid solution 
 and alcohol ; after ten days it is transferred to strong al- 
 cohol. When sufficiently hard it is embedded in wax or 
 liver,* and sections made perpendicular to the surface ; 
 these are stained lightly with haematoxylin, 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 glycerin. 
 
 Sections of the Nail. — A nail should be separated from 
 a finger which has been hardened in alcohol, together 
 
 * Better in celloidin. 
 
230 NORMAL HISTOLOGY. 
 
 with as much as possible of the connective tissue which 
 binds it to the bone. In order to render the soft parts 
 more consistent and more nearly like the body of the iiail, 
 so that uniformly thin sections can be made, the whole 
 should be hardened by the gum method; see page 115. 
 Longitudinal and transverse sections are made through 
 thg entire nail, and stained double 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 hard- 
 ened in the above chromic acid mixture. Sections* are 
 made as nearly as possible in the direction of the hair- 
 follicles, so as to include the entire root and bulb, and 
 also at right angles to them. They may be stained double 
 and mounted in balsam ; or instead of first staining with 
 haematoxynn they may be laid for twenty-four hours in a 
 strong aqueous solution of analin green and then dehy- 
 drated in the usual way in the eosin alcohol. By the 
 latter method a very brilliant differentiation in color in 
 the layers of the inner root-sheath may be obtained. 
 
 Cells of the Cuticula of the Hair. — These are readily 
 seen by placing a hair on a slide in a drop of strong sul- 
 phuric acid, and covering ; in a short time the cells 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. 
 
 Section of 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. 
 
 * After embedding, in celloidin. 
 
THE SKIN AND ITS ADNEXA. 23 1 
 
 In this preparation the ovoidal tactile corpuscles may be 
 seen lying in some of the papillae, and if the subcutaneous 
 tissue has been included in the section to a considerable 
 depth, transverse or longitudinal sections of a Pacinian 
 body may be found. 
 
CHAPTER XVIII. 
 THE EYE. 
 
 The organ of sight is composed of the eyeball 
 and various accessory structures, such as the eye- 
 lids, lachrymal gland, muscles, etc. The eyeball is 
 composed, in the first place, of a dense, firm, sphe- 
 roidal connective-tissue envelope, whose anterior 
 transparent portion, the cornea, is more convex than 
 the posterior opaque segment, the sclerotic, 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 an- 
 teriorly, just behind the sclero-corneal junction, 
 into numerous longitudinal folds, called the ciliary 
 processes. An extension from the ciliary processes 
 passes forward, constituting the iris, which is a 
 perforated vascular connective-tissue and muscular 
 curtain, suspended behind the cornea, and con- 
 nected peripherally, near the sclero-corneal junction, 
 with a connective-tissue structure called the liga- 
 mentuin pectinatum. 
 
 Passing backward from the ligamentum pectina- 
 232 
 
THE EYE. 233 
 
 turn, 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 direction 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-elements of the retina cease in a 
 wavy line, called the ora serrata ; certain cellular 
 elements continue, however, over the ciliary pro- 
 cesses, under the name oi pars ciliaris retince. 
 
 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 membrane covering the ciliary pro- 
 cesses, 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 \^& aqueous, fluid ; the posterior, 
 
 * By the anterior and posterior chambers of the eye, ophthalmolo- 
 gists at present mean the cavities in front of the lens, separated by 
 the iris, and formerly regarded as constituting the anterior chamber 
 alone, While thalf containing the vitreous was called the posterior 
 
234 NORMAL HISTOLOGY. 
 
 with a gelatinous substance, the vitreous body, which 
 presents an ill-defined lamellar structure, and some- 
 times contains a variable number of ill-defined 
 more or less granular cells. The vitreous is sur 
 rounded by a delicate membrane, called the hyaloid 
 membrane, which is closely connected posteriorly 
 with the lining rnembrane of the retina, and is 
 hardly to be differentiated from it. The hyaloid 
 membrane is thickened and fibrillated over the 
 ciliary processes, where it is called the zonula ciliaris, 
 and a prolongation forward from this constitutes 
 the suspensory ligament of the lens. 
 
 Having thus briefly described the general struc- 
 ture of the eye, it remains for us to consider some 
 of its parts somewhat more in detail ; the scope of 
 this manual will not permit us, however, to make 
 an extended study of all or even any of the struc- 
 tures in the eye ; we shall be obliged to confine 
 ourselves to the more marked structural features of 
 the cornea and sclera, the posterior portions of the 
 choroid and retina, the iris and crystalline lens. 
 
 THE SCLERA. 
 
 The sclera is composed of very closely interwoven 
 connective-tissue fibres, with fine elastic fibres, the 
 latter most abundant near the inner surface. Be- 
 tween the fibres, which have little regularity in their 
 arrangement, lie flat connective-tissue cells, a certain 
 number of which frequently contain pignient-gran- 
 
THE EYE. 235 
 
 ijles. 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 fusca; it resembles in 
 structure the outer layers of the choroid, presently 
 to be described as the membrana supra-cJioroidea, 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 connective-tissue fibres ; these pass 
 in from the sclera on all sides, and surround the 
 delicate bundles of nerve-fibres of the opticus, form- 
 ing the lamina cribrosa. 
 
 THE CORNEA. 
 
 The cornea is directly continuous at its periph- 
 ery with the sclera, but differs from it in struc- 
 ture in many particulars, among the more promi- 
 nent of which are, the more regular lamellar 
 arrangement of its connective-tissue basement-sub- 
 stance, the greater transparency of the latter, the 
 peculiar form of its cellular elements, and the free 
 surfaces covered with cells. 
 
 In a thin section of the cornea, perpendicular to 
 
236 NORMAL HISTOLOGY. 
 
 its surface, we recognize, passing from before back- 
 ward, five layers : i. A stratified layer of epithelial 
 cells — the anterior corneal epithelium — consisting of 
 cells resembling in general form and arrangement 
 those of the epidermis; that is, we have in the 
 deepest layer, cylindrical cells, passing over into 
 polyhedral, and these into flattened cells at the 
 surface ; 2. The anterior epithelium rests on a 
 dense transparent membrane, called the anterior 
 basal membrane, or lamina elastica anterior, which 
 is composed of closely packed fibrillae ; 3. The body 
 of the cornea — substantia propria cornece — is com- 
 posed of connective tissue whose characteristics we 
 have already studied ; 4. Lying closely upon the 
 posterior surface of the last layer, is a thin, appar- 
 ently structureless membrane — the metnbrane of 
 Descemet or lamina elastica posterior — -upon which 
 lies : 5- A single layer of flattened polyhedral cells, 
 called the endothelium of Descemet. 
 
 Except at its extreme periphery, the cornea 
 contains no blood-vessels. Nerves, on the other 
 hand, are very abundant. These, in larger and 
 smaller trunks, enter the cornea at the periph- 
 ery, and dividing and subdividing, break up into 
 bundles of extremely delicate fibrils, some of 
 which are distributed to the superficial, others to 
 the deep, layers of the cornea. These fibrils form 
 extraordinarily delicate and intricate plexuses, and 
 are finally resolved into the ultimate nerve-fibrils 
 
THE EYE. 237 
 
 which, often finely beaded, pass off to their termi- 
 nations. The exact mode of termination of the 
 nerve-fibrils in the cornea is not yet sufficiently 
 definitely known ; certain of them, however, seem 
 to pass between the anterior epithelial cells, and are 
 believed by some investigators to end in free ex- 
 tremities at the surface. 
 
 THE CHOROID. 
 
 In the posterior portion of the eye, the choroid 
 presents four layers, which, although intimately 
 connected, and presenting no sharp line of division, 
 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 com- 
 posed of a series of superimposed connective-tissue 
 membranes containing delicate elastic fibres and 
 numerous flattened, irregular-shaped, often branch- 
 ing pigmented cells. The layers are covered with 
 endothelium, and the spaces between them are 
 lymph-spaces or sinuses. 
 
 Within the lamina suprachoroidea lies a layer, 
 containing the larger arteries and veins of the 
 choroid, called the external vascular layer, or the 
 layer of Haller. This layer is composed of a 
 groundwork similar to the suprachoroidea, in which 
 the vessels are embedded, the arteries being often 
 
238 NORMAL HISTOLOGY. 
 
 closely surrounded by dense masses of pigmented 
 cells. 
 
 The inner vascular layer, called the chorio-capil- 
 laris, follows next, and consists almost entirely of a 
 very dense net-work of broad capillary blood-ves- 
 sels. Finally, the choroid is limited within, by an 
 extremely delicate, finely striated membrane, called 
 the lamina vitrea, or membrane of Bruch. 
 
 THE IRIS. 
 
 The iris is a thin connective-tissue membrane, 
 pierced near the centre by an opening, the pupil, 
 and joined at the periphery to the ligamentuni 
 pectinatum and the ciliary body. The bulk of the 
 iris, the substantia propria, consists of delicate inter- 
 lacing connective-tissue fibres, among which are 
 numerous variously shaped, often branching pig- 
 mented and unpigmented cells. 
 
 It contains numerous blood-vessels which are 
 characterized by an extraordinary thickness of the 
 walls. Near the pupillary margin lies a circular 
 band of smooth muscle-cells — sphincter pupillm — 
 while radiating bands of similar cells passing from 
 the periphery toward the pupil — dilator pupillce — 
 are found in certain animals, but not in man. The 
 anterior surface is covered by a layer of endothe- 
 lial cells, while the posterior surface is covered by 
 an irregular thicker layer of polyhedral cells, which 
 are densely crowded with pigment, and constitute 
 the so-called uvea 
 
THE EYE. 239 
 
 THE RETINA. 
 
 Of all the animal structures the retina is one of 
 
 the most delicate, complicated, and difficult of 
 study, and we can do little more here than indicate 
 briefly the general grouping of its elements. It 
 consists of a connective-tissue framework by which 
 the nerve-elements are supported, 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 nerve-fibres. 
 
 3. Layer of ganglion-cells. 
 
 4. Internal molecular layer. 
 
 5. Internal nuclear layer, 
 
 6. External molecular layer. 
 
 7. External nuclear 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 very delicate 
 ^nd homogeneous, and belong to the connective- 
 tissue framework. In the layer of nerve-fibres, 
 which is thickest around the entrance of the optic 
 nerve, — thus forming the papilla, — the fibres spread 
 
246 NORMAL HISTOLOGY. 
 
 out, intricately interlacing, into a thir. sheet, and 
 then pass outward into the next layer to join the 
 ganglion-cells. These, which have the general 
 characters of branching nerve-cells, send numerous 
 processes into the internal molecular layer, where 
 they break up into an extremely delicate fibrillar 
 net-work associated with the connective-tissue 
 framework. Most of the nuclei in the internal nu- 
 clear layer are believed to belong to small nerve- 
 cells, while the larger ones belong to the connective- 
 tissue framework. In the external molecular layer, 
 again, we have a delicate net-work of nerve-fibrils 
 intermingled with connective tissue. The nuclei of 
 the external nuclear layer seem to belong exclusively 
 to nerve-elements, and are directly connected, by 
 processes which pass through the openings in the 
 external limiting membrane, with the rods and 
 cones. 
 
 Of the rods and cones, which within the limits of 
 this book cannot even in a general. way be ade- 
 quately described, the rods are the longer, are usu- 
 ally somewhat pointed at the inner extremity where 
 they join the nerve-elements of the outer nuclear 
 layer ; the cones are shorter, are connected also with 
 nerve-elements within, and terminate externally in 
 pointed or rounded extremities. ^ 
 
 The connective-tissue elements of the retina con- 
 sist, in certain layers, of broad, irregular radial fibres 
 forming frequent inosculations, and, in the molecu- 
 
THE EYE. 241 
 
 lar layers, of a delicate reticulum, within which the 
 nerve fibrils ramify. 
 
 The pigmented epithelium of the retina consists 
 of large polyhedral cells set together side by side, 
 and forming a continuous layer over the distal ends 
 of the rjds. As seen from the side, as we have al- 
 ready studied them, p. 24, they appear like flat 
 pentagonal or hexagonal plates, but when seen in 
 profile while in situ, it is evident that they send 
 down long, slender pigmented processes between 
 the rods. 
 
 The outer portion of these cells, that next to the 
 choroid, usually contains the nucleus and but little 
 pigment, while the amount of pigment in the pro- 
 cesses seems to depend upon whether the eye had 
 been exposed to light or not immediately before 
 death. For it has been recently shown that in some 
 animals the pigment particles under the influence of 
 light can move within the narrow cell-processes so 
 as to be now collected within the inner portion of 
 the cell-body, and again grouped in larger and 
 smaller masses betwen the rods. In this movement 
 the pigment particles are themselves passive, the 
 change in position being due to active movements 
 in the protoplasm, induced by light. 
 
 The larger arteries and veins ramify beneath the 
 internal limiting membrane in the layer of nerve- 
 fibres, and from these blood is distributed outward 
 to all the layers, as far as to the external nuclear 
 layer, beyond which no blood-vessels are found. 
 
242 NORMAL HISTOLOGY. 
 
 THE LENS. 
 
 The lens is a transparent double convex body, of 
 suflficient firmness to maintain its form when re- 
 moved from the eye, and is enclosed in a homoge- 
 neous elastic capsule which is thicker on the ante- 
 rior than on the posterior surface. To the periphe- 
 ral 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 nucleated cells, or lens-fibres, which run- 
 ning meridionally, and joined by interfibrillar ce- 
 ment-substance make up the greater part of the 
 body of the lens. The lens-fibres are slightly ridged 
 upon the surface, and have, on transverse section, a 
 flattened hexagonal form ; they are so intimately 
 joined to one another at their sides by the cement- 
 substance, that under certain circumstances they 
 may 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 ; inasmuch, however, as 
 the individual fibres are not long enough to reach 
 the entire distance from one pole of the lens around 
 
THE EYE. 243 
 
 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 sur- 
 faces, under certain circumstances, in the form of a 
 three-rayed star ; in the adult, the rays usually part 
 at the ends, giving rise to secondary rays. 
 
 THE EYELIDS. 
 
 These are formed in general by a plate of connec- 
 tive tissue, which toward the free border is very 
 dense and firm, and called the tarsal cartilage, or tar- 
 sus ; they are covered on the outside by skin, on the 
 inside by the conjunctival mucous membrane; 
 between the tarsus and the skin lie the bun- 
 dles of the musculus orbicularis. The tarsus, 
 which is, in no sense, cartilage, is a plate 
 of very dense and firm fibrillar connective tissue, 
 containing ordinary flattened connective-tissue cells, 
 and is closely connected within with the palpebral 
 conjunctiva. Embedded within the tarsus He the 
 Meibomian glands, thirty to forty in number in each 
 lid. They consist of numerous vesicular alveoli, 
 lined with short cylindrical cells, arranged along and 
 opening into long excretory ducts, which are lined 
 with laininated epithelium, and open on the inner 
 border of the edge of the lids. They are some- 
 
244 NORMAL HISTOLOGY 
 
 what modified sebaceous glands, but, unlike most 
 sebaceous glands are not connected with hair-fol- 
 licles. 
 
 The skin of the eyelids is somewhat thinner than 
 that of the face, is beset with delicate hairs, and 
 supplied with sweat-glands and sebaceous follicles. 
 It passes over on to the edge of the lids, at the in- 
 ner 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 ; and the follicles, which are quite deep, 
 are furnished with sebaceous glands. 
 
 The conjunctival mucous membrane of the lids 
 consists of a basis substance of loose fibrillar connec- 
 tive tissue containing a few elastic fibres and numer- 
 ous small spheroidal and branching cells. The 
 epithelium is laminated, consisting of a deep layer of 
 polyhedral cells, then more superficially of more or 
 less columnar cells. 
 
 The epithelium of the bulbar conjunctiva ap- 
 proaches more and more closely in structure that of 
 the cornea, as we pass from the lid over toward the 
 sclero-corneal junction. 
 
 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 con- 
 junctiva on the orbital portion of the lids. 
 
THE EYE. 245 
 
 PRACTICAL STUDT. 
 
 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 scle- 
 rotic, so that the fluid can readily come into contact with 
 the parts within, and placed in MuUer's fluid ; after two 
 weeks it is carefully washed and placed in alcohol for a 
 week, when the dissection may be made. 
 
 The eye should be divided with a sharp razor into 
 lateral halves, the section passing through the optic 
 nerve. The cut surface shows clearly the general rela- 
 tions of the parts ; cornea, iris, lens, ciliary body, vitreous, 
 retina, choroid, and sclera are seen in approximately nor- 
 mal relations to one another. 
 
 If the vitreous be now removed from one of the halves, 
 the retina and ciliary body come more fully into view. 
 As the zonula ciliaris approaches the edge of the lens, 
 it divides into two layers, which pass, one to the anterior, 
 the other to the posterior surface of the body, forming 
 the suspensory ligament ; the slit-like opening between 
 the layers is called the canal of Petit, which may be seen 
 by pulling the lens slightly backward, when the layers will 
 separate. 
 
 Now seizing the half of the lens with forceps and care- 
 fully drawing it downward and backward away from the 
 iris, the zonula ciliaris, in the form of a folded fringe- 
 like membrane, will be separated from the surface of the 
 ciliary body. A portion of this, in connection with a 
 fragment of the lens-capsule, is detached from the lens, 
 stained deeply in eosin, and mounted in glycerin. After 
 
246 NORMAL HISTOLOGY. 
 
 the removal of the lens, the form and attachment of 
 the iris are readily seen. 
 
 In the same half of the eye,- the layers of the choroid 
 may be demonstrated. For this purpose the retina is 
 pulled off, and the pigmented cells, which are apt to ad- 
 here to the inner surface of the choroid, are brushed or 
 scraped off. The eye is now firmly pinned with the flat 
 side down to a flat bit of cork which has been attached 
 to a piece of sheet-lead and sunk in a dish of water deep 
 enough to cover the eye. A window about a centimetre 
 square is now cut through the sclera, including the sclero- 
 corneal junction, and the bit of sclera having been re- 
 moved, care being taken not to disturb the ciliary body, 
 the membrana suprachoroidea will be seen as a brown 
 loose tissue floating at the surface of the choroid. Bits 
 of this are pulled off with the forceps, stained with he- 
 matoxylin, floated smoothly on to a slide immersed in 
 water, and mounted in glycerin. The floating shreds 
 of the suprachoroidea which remain after suitable speci- 
 mens 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 hot 
 easy under the most favorable conditions, and is espe- 
 cially difficult in the eye of the ox and sheep, where the 
 layers are rendered more complicated and difficult of 
 separation by the presence 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 frag- 
 ment will be obtained, which, especially at the edges, 
 
THE EYE. 247 
 
 will show both of the vascular layers at once. They are 
 stained double and mounted in glycerin or balsam. 
 
 Cornea and Sclera. — A fresh eye from the rabbit or dog 
 is hardened in Muller's fluid and alcohol. The cornea 
 should now be excised close to the sclero-corneal junc- 
 tion, embedded in wax or liver, and thin sections made 
 perpendicular to the surface, including the entire thick- 
 ness. They may be stained double and mounted in 
 glycerin. 
 
 A transverse section may be made from a bit of the 
 sclera from the same eye ; stained double and mounted 
 in balsam. 
 
 Sclero-corneal Junction, and Iris. — The cornea and 
 sclera, in their relations to one another, and the ciliary 
 bodies, and iris, may be examined in a specimen pre- 
 pared as follows : An eye — that of the pig is best, if a 
 fresh huma:n eye cannot be procured — is placed, after 
 making a short incision in the sclera, in Muller'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-corneal 
 junction and adjacent parts, and embedded in cacao 
 butter by the following method.* 
 
 This cacao-butter method of embedding is of great 
 use in making sections of delicate parts where the tis- 
 sues enclose spaces, or are not close enough together or 
 firm enough to afford a sufficient resistance to the razor. 
 The piece of tissue is transferred from alcohol to a small 
 dish of oil of cloves, which in a short time will pene- 
 trate the tissue and render it transparent. It is then 
 
 * It is better to use celloidin. See Appendix, p. 252. 
 
248 NORMAL HISTOLOGY. 
 
 placed in melted cacao butter, whose temperature is 
 kept just above its melting point, on a water-bath, for a 
 couple of hours. It is now placed in a little box made 
 from writing paper, in such a position that sections can be 
 made in a suitable direction at the proper time, and the 
 box filled with the melted cacao butter or with the wax 
 mixture described on page II When the wax is cold 
 the whole forms a solid mass of suitable consistence for 
 cutting, and the wax serves to hold tlie delicate parts 
 together while the section is being made. Sections are 
 made at right angles to the ciliary body. 
 
 The wax and the cacao butter must now be dissolved 
 out by oil of cloves, the latter replaced by alcohol, and 
 this by water ; the sections are then stained double and 
 mounted in balsam. This method seems long and com- 
 plicated, but the excellence of the sections of delicate 
 parts obtained by its use will well repay the worker for 
 his pains. 
 
 Lens Fibres by Teasing. — A short incision is made 
 through the sclera of a fresh eye which is soaked for 
 three or four days in a mixture of alcohol and water, i 
 to 2. The lens will be found on removal to be white 
 and soft, and readily breaks up into layers ; a bit of one 
 of these is teased on a slide in eosin -glycerin and 
 mounted in the same. 
 
 Sections of the Lens. — The eye of a rabbit or pig 
 should be kept for a fortnight in Mailer's fluid ; the 
 lens is then removed, care being taken not to rupture 
 the capsule, and placed for a day or two in dilute and 
 then in strong alcohol. It is embedded in wax,* and thin 
 sections, made in an antero-posterior direction through 
 
 * Or celloidin. 
 
THE EYE. 249 
 
 the centre, are stained double and mounted in glycerin 
 or balsam. 
 
 Transverse Sections of the Retina. — In a perfectly fresh 
 human or pig's eye, one or two small openings are made 
 through the sclera and it is placed in Miiller's fluid ; 
 after a week the eye may be cut across just behind the 
 sclero-corneal junction, and the posterior segment placed 
 in fresh Miiller's fluid. After another week it is trans- 
 ferred for twenty-four hours to dilute, and then put for 
 two or three days in strong alcohol. A small piece is 
 now cut from the retina at a little distance from the 
 optic-nerve entrance and embedded in cacao butter (see 
 above).* Thin transverse sections are made, the cacao 
 butter dissolved out in the usual way, and stained double 
 and mounted in balsam. By this method the general 
 arrangement of the layers is well shown, but many of 
 the finer details of structure are obscure. 
 
 * Celloidin is much better for embedding the retina. 
 
APPENDIX. 
 
 Page io. Section-Cutting with the Microtome. — Although it is 
 very important for every worker in normal Histology to be able to 
 cut thin sections with the free hand, it is desirable in many cases to 
 make use of an instrument called the Microtome for this purpose ; 
 since, when many sections are to be cut, much time is thereby saved, 
 and sections can be made much thinner and smoother. One of the 
 best instruments is that devised by Prof. R. Thoma, of Heidelberg, 
 and known as Thoma's Microtome. This instrument is made of 
 three sizes, and the intermediate or largest size is most useful. This 
 can be imported from the maker, Rudolph Jung, of Heidelberg, 
 Germany ; or it can be obtained from the Prang Educational Co. , 7 
 Park Street, Boston, Mass., or of Meyrowitz Bros., corner of Fourth 
 Ave. and Twenty-third St., N. Y. The method of using the instru- 
 ment need not be described here, since with the instrument at hand 
 any worker will readily make out for himself the necessary pro- 
 cedure. 
 
 The instrument and mode of using are described in Jour. Royal 
 Microscopical Soc, vol. iii., p. 298, 1883. 
 
 The Freezing Microtome. — For many purposes it is desirable to 
 study thin sections of fresh tissues which have not been subjected to 
 the action of hardening agents. Such sections are best prepared 
 with a so-called freezing microtome, by means of which, by the action 
 usually of a spray of ether or rhigolene, smiall pieces of tissue may in a 
 few seconds be made hard and easily cut off in thin slices. The 
 
 251 
 
252 APPENDIX. 
 
 fre'ezing microtome of Thoma is one of the simplest and cheapest, 
 and can be obtained as above. A thin bit of the fresh tissue, not 
 more than 2-3 mm. thick, is placed on the metal plate, and aspia.yof 
 ether from an ordinary two-bulbed atomizer being directed against the 
 lower side of the plate, the tissue will soon become solid, and sections 
 may be shaved off by passing the knife — ^held a little obliquely on the 
 edge, like the knife of a plane — over the glass plate. These sections 
 may be studied unstained, or be stained with a one-per-cent. aqueous 
 solution of analin green. This method is very useful when it is de- 
 sirable to determine the nature of a tissue without waiting for the 
 action of hardening agents, as well as for seeing it in a nearly natural 
 condition. 
 
 Page ii. Embedding in Celloidin. — Although the methods of em- 
 bedding given on pages 11, 115, and 247, in wax and paraffin, gum 
 and cacao butter, are useful for some purposes and were formerly uni- 
 versally employed, celloidin, which has been recently suggested for 
 this purpose, is much better in almost all cases, and has largely super- 
 seded them. 
 
 Celloidin is a purified form of collodion, which is non-explosive, 
 and comes in the market in the form of dense glue-like sheets or 
 shavings. It may be obtained in a pure form from Bachrach and 
 Brother, corner Eutaw and Lexington streets, Baltimore, Md. It is 
 prepared for use by making a saturated solution in equal parts of com- 
 mon sulphuric ether and alcohol. The solution should be of about 
 the consistence of moderately thick mucilage. The specimen to be 
 embedded should have been previously soaked in alcohol. It is placed 
 for a few hours in a mixture of equal parts of alcohol and ether, and 
 then transferred to the celloidin solution and left until it is thoroughly 
 penetrated by the latter. If the specimen is small this will usually 
 be accomplished in from twelve to twenty-four hours. A small square 
 box is now made of filter paper, and the specimen being placed in a 
 proper position in this, the celloidin solution is poured around it so 
 as to fill up the box. The box should be make considerably larger 
 than the specimen, as the celloidin shrinks considerably in hardening. 
 If the microtome is to be used for cutting the section, or if the bits of 
 tissue are small, it is well to make the box by rolling the paper around 
 
APPENDIX. 253 
 
 one end of a straight cork of the proper size, so that it will project 
 from the end, and t)ring it with a thread. The cork is now set upon 
 end with the cylindrical box uppermost, and the specimen placed in 
 this, and the celloidin poured in as before. In this way the celloidin 
 with the specimen will harden on to the cork which can be readily 
 clamped in the microtome. In many cases, if the specimen is small, it 
 will suffice, after it is permeated with the celloidin, to place it directly 
 on the end of a cork, and pour a little celloidin — as much as will lie 
 on the end of the cork — directly over it. 
 
 After enclosing the specimen in any of these ways in the semifluid 
 celloidin, the latter is hardened by allowing the preparation to stand 
 in the air or under a bell jar until some of the ether has evaporated, and 
 the celloidin becomes partially hardened on the outside, and then 
 placing it in a mixture of equal parts of alcohol and water. In this, 
 after a longer or shorter time, the celloidin will acquire the proper 
 consistency for cutting, shrinking considerably as it does so. By a 
 little practice, the worker will soon learn just how thick to make the 
 celloidin solution, and how long to leave it exposed to the air, and in 
 the dilute alcohol for hardening. 
 
 When the sections are cut they may be stained in the usual way, 
 and mounted in glycerin or balsam. It is not necessary to remove 
 the celloidin from the section, since it is transparent, and for most pur- 
 poses interferes but little with the subsequent study, and it greatly 
 facilitates the handling of delicate specimens, holding as it does very 
 fragile or friable tissues perfectly in place In mounting in Canada 
 balsam, if oil of cloves he used, the celloidin is dissolved by it, leav- 
 ing the section free. The celloidin may be kept on the section by 
 using for clearing up the specimen the white oil of thyme instead of 
 oil of cloves. The uncut portion of the specimen may be preserved 
 indefinitely, ready for cutting, in eighty per cent, alcohol. 
 
 The So-called Third Corpuscle of the Blood. — Several observers have 
 described the occurrence in the blood of numerous pale discoidal 
 bodies considerably smaller than the red blood cells. They are apt, 
 when the blood is withdrawn from the vessels, to become clustered in 
 irregular masses. These are often called blood-plates. It is probable 
 that many of the " free granules " described in the text are the struc- 
 
254 APPENDIX. 
 
 tures described under this name. Too little is as yet known about 
 their structure and function, or the conditions under which they occur, 
 to make it necessary to describe them further here. 
 
 Neuroglia. — The nerve elements of the central nervous system are 
 supported and held in place in part by connective-tissue septa and 
 prolongations which pass inward from the pia mater ; in part by a deli- 
 cate net-work of a peculiar form of connective tissue called neu- 
 roglia. The latter consists for the most part of fine fibrils, and of 
 irregular-shaped flat-bodied cells, which frequently send off numerous 
 exceedingly delicate branching processes. These cells are called 
 neuroglia cells, and from their numerous and delicate process are 
 very commonly known as ' ' spider cells. " 
 
 Silver Staining of Bladder. — As the injection of an entire frog is 
 somewhat difficult without considerable practice, the following pro- 
 cedure may be substituted for it : The bladder is exposed in a freshly- 
 killed frog, and a canula being passed into it, the organ is moderately 
 distended with air and ligated in this condition. It is then cut out, 
 rinsed in water, and laid for twenty minutes in one-half-per-cent. 
 solution of silver nitrate. It is then rinsed and exposed to the light, 
 and treated as on page 126. The pictures are more distinct if the 
 epithelium be scraped from the inner surface before mounting. 
 
 Respiratory Epithelium. — These thin transparent cells which 
 partially line the air-vesicles are sometimes called respiratory 
 epithelium, and it was formerly supposed that such cells were con- 
 fined to the terminal air-spaces. It has been recently shown, how- 
 ever, chiefly by the researches of Kalliker, that the respiratory 
 epithelium is abundant in the smaller bronchi as well, and these he 
 accordingly calls respiratory bronchioles. The peculiar character and 
 distribution of this epithelium, which seems so well fitted to facili- 
 tate the interchange of material between air and blood in the lungs, 
 would seem to indicate, therefore, that the actual respiratory surface 
 in the lungs is greater than we have been wont to believe. 
 
 Weigert's Acid-Fuchsin Method for Staining Nerve- Tissue. — While 
 for a standard method in studying the central nervous system, the 
 double staining with hematoxylin and eosin answers a very good pur- 
 pose, it fails to show the nerve-elements with sufficient distinctness. 
 
APPENDIX. 255 
 
 .being best adapted to reveal the connective tissue. A method recently 
 described by Weigert, on the contrary, when carefully followed, brings 
 out the nerve fibres, even in their finest ramifications, with great dis- 
 tinctness. It is called the acid-fuchsin method, because the stain em- 
 ployed is one of the so-called acid anilines. It is rosanilin in which 
 one atom of H is replaced by the radicle S O'H. It is a sulphate of 
 rosanilin. It does not stain the nuclei of cells, but stains the tissues 
 diffusely. It is found, however, that on treating the stained sections 
 with caustic potash, the color is largely removed from every thing but 
 the nerve fibres, which remain of a deep red color. The ganglion 
 cells are not much colored by it. It is necessary to use a particular 
 preparation of acid-fuchsin for this purpose ; namely, one known as 
 acid-fuchsin S. of the Baden anilin factory, which may be obtained 
 in small quantities from Dr. Grllbler, 17 Dufour Strasse, Leipzig, 
 Germany, or from Meyrowitz Bros., Fourth Ave. and Twenty-third 
 Street, New York. 
 
 The details of this method are as follows : The nerve tissues 
 (brain or spinal cord) must be carefully hardened in MuUer's fluid. 
 Alcohol may be used to complete the hardening, but should be used 
 only at the end of the process. Sections are stained from one to 
 twenty-four hours in a saturated aqueous solution of the fuchsin. 
 The differentiation of the elements is accomplished by a saturated 
 alcoholic solution of caustic potash, diluted with alcohol in the pro- 
 portion of i-io. The sections are taken from the stain, ringed in 
 water, and placed in the alkaline bath. The exposure in the latter 
 should be very brief, usually not more than five or six seconds, the 
 section being moved to and fro. If left in the potash too long the 
 color will be removed altogether, and this is consequently a delicate 
 part of the operation. A little practice will enable the operator to 
 determine how long an alkaline exposure to give the section. The 
 latter is now transferred immediately to pure water, which should be 
 changed once or twice to insure the complete removal of the potash. 
 The section may now be dehydrated with alcohol in the usual way, 
 cleared up with oil of cloves, and mounted in balsam. The nuclei 
 of the cells may be brought out by staining lightly with hematoxylin 
 before dehydrating. The color of ihe preparations is somewhat 
 
256 APPENDIX. 
 
 deepened by dipping the section for an instant, after the removal of 
 the alkali, into a solution of hydrochloric acid and water 1-5. 
 
 In sections stained in this way, the connective-tissue elements have 
 a gray or light violet color, the nuclei alone, if hematoxylin be used, 
 being distinctly stained. Although not especially stained, the gangli- 
 on cells may usually be seen with great distinctness. The nerve 
 fibres on the other hand stand out, in successful preparations, with 
 great distinctness, in light or deep red, and we are able to trace their 
 courses and ramifications in a highly satisfactory manner. 
 
 The hardening in chromic fluids — especially Muller's fluid — may 
 be greatly hastened by keeping the fluid in which the specimens are 
 immersed at a slightly elevated temperature — 35° C. In this way the 
 hardening may be accomplished in about one third the time required 
 at ordinary temperatures. The fluid should be frequently changed. 
 A little gum camphor may be put into the fluid to retard the de- 
 velopment of bacteria. 
 
INDEX 
 
 Acetic Acid, action of, on tissues . . , 
 
 Air-vesicles of lungs, demonstration of epithelium of 
 
 structure of 
 Albuginea, of ovary . / . 
 
 of testicle .... 
 
 Alcohol, as a hardening and preservative agent 
 Alveolar passages of lungs, structure of 
 Ammonium bichr,, as a hardening agent 
 Ammonium chromate (neutral), as a hardening agent 
 Amceboid movement, in white blood-cells, character of 
 
 method of studying 
 Aniline-red, as a staining agent . 
 Arachnoid, of brain 
 Areolar tissue 
 Arteries, structure of 
 
 method of studying 
 Arterioles .... 
 Asphalt varnish, for enclosing specimens 
 Axis cylinder of nerve-fibres 
 
 Balsam, method of mounting in . 
 
 Blood 
 
 coagulation of 
 free granules in 
 method of studying fresh , 
 plasma of . . . 
 
 Blood-cells, red 
 
 action of water on 
 
 change of form in 
 
 coloring matter of 
 
 di£Ference in, in different animals 
 
 nucleated, in marrow of bone 
 
 in spleen 
 number of 
 
 «57 
 
 PAGE 
 
 i6 
 176 
 
 173 
 
 193 
 
 188 
 
 3 
 
 I7« 
 
 4 
 
 4 
 
 81 
 
 89 
 
 3S 
 ai8 
 3+ 
 
 ISO, ISI 
 
 X26 
 
 ISO 
 
 "4 
 104 
 
 IS 
 
 80 
 85 
 84 
 87 
 8S 
 83 
 8a 
 83 
 84 
 84 
 67 
 139 
 83 
 
258 
 
 INDEX. 
 
 Blood-cells, red 
 
 
 
 
 PAGE 
 
 number of, relative to white blood-cells . 
 
 83 
 
 origin of . 
 
 
 
 86 
 
 size of 
 
 . 
 
 
 
 83 
 
 stroma of . 
 
 • • 
 
 
 
 84 
 
 varieties in 
 
 
 
 
 84 
 
 white . 
 
 
 
 
 80 
 
 amoeboid movements in 
 
 
 
 81,89 
 
 demonstration of nuclei in 
 
 
 
 83 
 
 origin of . 
 
 
 
 86 
 
 Blood-crystals ...... 
 
 
 
 84,88 
 
 Blood-vessels, adventitia of . . . 
 
 
 
 IZO 
 
 arteries .... 
 
 
 
 Z20, 121 
 
 capillaries .... 
 
 
 
 119 
 
 classification of , 
 
 
 
 rig 
 
 intima of . 
 
 
 
 I20 
 
 musculosa of . . . 
 
 
 
 120 
 
 method of preparing, for study 
 
 
 
 125, 126 
 
 veins .... 
 
 
 
 121 
 
 Blue gelatine mixture for injecting, formula for prep. 
 
 of 
 
 
 10 
 
 Bone 
 
 
 
 61 
 
 canaliculi of . 
 
 
 
 62 
 
 cells 
 
 
 
 «2 
 
 decalcified, method of preparing . 
 
 
 
 67 
 
 developing, preparation of . 
 
 
 
 77 
 
 development of, intra-cartilaginous 
 
 
 
 7« 
 
 sub-periosteal 
 
 
 
 74 
 
 intra-membranous 
 
 
 
 75 
 
 hard, method of preparing sections of 
 
 
 
 «9 
 
 Haversian canals of . 
 
 
 
 64 
 
 marrow of 
 
 
 
 
 6S 
 
 periosteum of 
 
 
 
 
 6s 
 
 Bone-tissue, spongy 
 
 
 
 
 62 
 
 compact 
 
 
 
 
 63 
 
 Sharpey's fibres in 
 
 
 
 
 64,7s 
 
 Brain, arachnoid of . 
 
 
 
 
 Z18 
 
 blood-vessels of 
 
 
 
 
 217 
 
 dura mater 
 
 
 
 
 ai7 
 
 general structure of 
 
 
 
 
 21S 
 
 pia mater 
 
 
 
 
 318 
 
 preparation of sections of 
 
 
 
 
 ai9 
 
 structure of cortex of cerebellum 
 
 
 
 217 
 
 structure of cortex of cerebrum . , 
 
 
 
 S16 
 
 structure of white matter 
 
 of . 
 
 
 
 SIS 
 
INDEX. 
 
 259 
 
 Bronchi, lai^e, structure of , 
 
 small, structure of 
 Bruch, membrane of, in choroid . 
 Brunner, glands of, in duodenum 
 Budding, cell multiplication by . 
 
 Cacao Butter, method of embedding In 
 Canada balsam, hard, for mounting bone 
 
 solution in chloroform for mountingf 
 Canaliculi of t>one ...... 
 
 Capillaries, structure of . . . . 
 
 method of studying ... 
 Capsule, of cartilage cells .... 
 
 supra-renal . . . . . 
 
 Carmine, preparation and use of, as a staining agent 
 Cartilage ...... 
 
 cells of . 
 
 artificial shrinkage of . • 
 
 classtfication of . . . . 
 
 fibro- . • . 
 
 preparation of . . 
 
 fibro-elastic . . t . 
 
 preparation of , . . 
 
 hyaline, basement substance of . 
 
 preparation of . . . 
 
 Cells, body of . . . 
 
 bone ...... 
 
 ciliated, from frog^s mouth . . 
 
 classification of ... . 
 
 connective-tissue . • . 
 
 corneal, seen on edge 
 
 seen on flat . . . 
 
 relation of, to basement substance 
 division of . 
 endothelial 
 
 epithelial, of small intestine ■ ■ * 
 
 ganglion . . . • • 
 
 general structure of . • • 
 
 goblet, in small intestine . . • 
 
 liver ...... 
 
 niarrow . . * • • 
 
 membrane of . . • • 
 
 nuclei of . ... 
 
26o 
 
 INDEX, 
 
 Cells, nucleoli of . 
 
 peptic . . . 
 
 pig^mented, in choroid 
 physiology of 
 plasma 
 
 practical study of . 
 reproduction of 
 tendon 
 Cell-spaces of connective tissue 
 Cement, in roots of teeth . 
 Central nervous system 
 Central tendon, in diaphragm of rabbit 
 Chondrin ..... 
 Chromic acid as a hardening agent 
 Chyle vessels, in small intestine 
 CUiary motion, in cells from frog's mouth 
 Coloring tissues (see staining) 
 Conheim*s method of gold staining 
 Coni vasculosi of testicle . 
 Connective tissue, classification of 
 
 distribution of, in body 
 
 fibrillar 
 
 of nerves . , 
 
 origin of, in embryo 
 
 practical study of 
 
 reticular 
 
 method of preparing 
 Corium of skin .... 
 Corpus Highmori of testicle 
 
 luteum in ovary ... 
 spongiosum of penis 
 
 preparation of 
 
 Decalcification of bone . 
 Delafield's method of preparing haematoxylin 
 Dentine ..... 
 
 Double -stfuning .... 
 
 in Canada balsam mounting 
 Dura mater, structure of . 
 preparation of 
 
 Elastic Fibres in connective tissue 
 
 in ligamentum nuchae . 
 
 PAGE 
 
 19 
 
 "47 
 
 36 
 '9 
 33 
 23 
 30 
 
 36,38 
 
 SIX 
 
 T27 
 
 4 
 
 151 
 
 35 
 
 6 
 
 39 
 189 
 
 27 
 27 
 =9 
 106 
 98 
 33 
 54 
 55 
 
 232 
 
 188 
 SOI 
 196 
 197 
 
 67 
 
 e 
 
 78 
 
 8 
 
 14 
 
 217 
 
 2x9 
 
 30 
 34 
 
JNDKX. 
 
 261 
 
 Elastic granules in connective tissue 
 Bmbeddiog in cacao butter 
 in gum 
 in liver 
 in wax 
 Emigration of white blood-cells 
 Enamel cuticle 
 of teeth 
 Endocardium, structure of 
 
 method of studying 
 Endogenous cell reproduction 
 Endothelial cells 
 Endothelium, of mesentery 
 of omentum 
 Eosin, mounting specimens stained with, 
 
 preparation and use of 
 Epidermis 
 
 Epithelial cells from rabbit^s bladder 
 Epithelium, renal 
 Erectile tissue, nature of 
 Eye . 
 
 aqueous fluid of 
 canal of Petit . 
 capsule of Tenon 
 chambers of 
 choroid . 
 ciliary body 
 cornea, structure of 
 
 preparadon of 
 conjunctiva 
 dissection of 
 general structure of 
 hyaloid membrane 
 iris, structure of 
 
 preparation of 
 lamina fusca 
 lens, structure of 
 
 preparation of 
 ora serrata 
 retina, structure of 
 
 preparation of 
 sclera, structure of 
 Bclero-corneal junction, preparaUon of 
 
 glycerin 
 
262 
 
 INDEX. 
 
 Eye, suspensory ligament 
 
 uvea. 
 
 vitreous body 
 
 zonula ciliaris 
 Byelids . . • 
 
 Fat Cells, structure of . 
 
 development of 
 Fat tissue 
 
 fresh, preparation of 
 method of studying, in adult 
 method of studying, in young animal 
 Fatty iD61tration . . . • 
 
 Fibres, elastic, in connective tissue 
 
 fibrillated, in connective tissue . 
 
 nerve .... 
 
 muscle . , . • 
 
 Fibrillse in tail tendon of mouse . • 
 
 primitive muscle . 
 Fibrillar cmnective tissue 
 Fibrin f romn^lood, characters of 
 
 method of preparing 
 Fibro-cartilage, structure of 
 Fibro-elastic cartilage 
 Fission, cell multiplication by 
 Fresh tissues, method of examining 
 Fuchsine (see aniline red) 
 
 Ganglion Cells . . • ■ 
 
 Gastro-intestinal canal 
 Gelatin, for injecting 
 Gemmation, cell multiplication by 
 Generative organs, female 
 male . 
 Germ epithelium of ovary 
 Giant cells, in marrow of bone . 
 Gianuzzi, crescents of, in submaxillary gland 
 Glands, Brunner's .... 
 
 general characters of 
 
 lymphatic .... 
 
 mammary .... 
 
 Meibomian . . . 
 
 peptic 
 
INDEX. 
 
 263 
 
 
 
 
 
 
 
 PAGB 
 
 Gluids,]mMla» 
 
 
 
 10» 
 
 racemose . 
 
 • < 
 
 
 
 
 MS 
 
 sebaceous . 
 
 • 
 
 
 
 
 227 
 
 submaxillaiy 
 
 
 
 
 
 
 «55 
 
 sweat . ■ 
 
 
 
 
 
 
 S28 
 
 thyroid 
 
 
 
 
 
 
 166 
 
 tubular . 
 
 
 
 
 
 
 >4S 
 
 vesicular . 
 
 
 
 
 
 
 146 
 
 Gltsson's capsule of liver 
 
 
 
 
 
 
 159 
 
 Glomeruli of kidney 
 
 
 
 
 
 
 179, 182 
 
 Glycerin as a mounUng medium 
 
 
 
 
 
 12 
 
 Goblet cellR . 
 
 
 
 
 
 15° 
 
 Gold chloride as a staining agent 
 
 
 
 
 39 
 
 Graafian follicles, structure of 
 
 
 
 
 199 
 
 development of 
 
 
 
 S02 
 
 HjBMATOXYLiN, preparation and use of 
 
 
 
 6 
 
 Haemoglobin, characters of 
 
 
 
 
 84 
 
 preparation of crystals of 
 
 
 
 
 88 
 
 Hat, cells in cuticula of . 
 
 
 
 
 230 
 
 follicle, structure of . 
 
 
 
 
 S25 
 
 general structure and relations of 
 
 
 
 
 224 
 
 papillae of . . . 
 
 
 
 
 225 
 
 preparation qf sections ov . 
 
 
 
 
 230 
 
 abaft, structure of . 
 
 
 
 
 226 
 
 Haaer's layer of choroid . 
 
 
 
 
 "37 
 
 Hardening agents , . 
 
 
 
 
 3 
 
 Haversian lamellae . . 
 
 
 
 
 63 
 
 canals • • 
 
 
 
 
 64 
 
 Heart, endocardium 
 
 
 
 
 123 
 
 muscle of . . 
 
 
 
 
 JOI 
 
 valves of 
 
 
 
 
 124 
 
 Henle, sheath of 
 
 
 
 
 106 
 
 Hensen's line In muscle-fibre 
 
 
 
 
 97 
 
 Highmori, corpus, in testicle 
 
 
 
 
 188 
 
 
 
 
 
 57 
 
 method of studyhig 
 
 
 
 
 59 
 
 Indifiibrent Fluids, use of 
 
 
 
 
 ■ 
 
 Infundibula of lungs, structure of 
 
 
 
 
 17s 
 
 InJecUon, interstitial 
 
 
 
 
 6 
 
 of blood-vessels . 
 
 
 Y 10 
 
 Intestine, luge, structure 
 
 9f 
 
 
 1 
 
 
 
 >53 
 
264 
 
 INDEX. 
 
 
 
 
 PAGE 
 
 Intestine, large, method of studying .... I54 
 
 small, .... 
 
 
 149 
 
 agmina ted glands of 
 
 
 
 152 
 
 Brunner's glands . 
 
 
 
 150 
 
 chyle vessels of . 
 
 
 
 IS' 
 
 epithelial cells of 
 
 
 
 149 
 
 follicles of Lieberkufan 
 
 
 
 149 
 
 goblet cells of 
 
 
 
 150 
 
 lymph-vessels of 
 
 
 
 •SI 
 
 muscularis mucosSB of 
 
 
 
 IS' 
 
 method of study of 
 
 
 
 >54 
 
 Peyer's patches 
 
 
 
 >52 
 
 solitary follicles of 
 
 
 
 551 
 
 villi of . 
 
 
 
 149 
 
 Kidney, capsule of ... 
 
 
 
 178 
 
 connective tissue of 
 
 
 
 »8s 
 
 convoluted tubules of . 
 
 
 
 181 
 
 cortex of . . • 
 
 
 
 178 
 
 cortical pyramids of . . 
 
 
 
 179 
 
 epithelium of . . . 
 
 
 
 182 
 
 general structure of . . 
 
 
 
 178 
 
 glomeruli 6f . . . 
 
 
 
 »79. 182 
 
 Henle's loops of 
 
 
 
 181 
 
 injected, preparation of • 
 
 
 
 .87 
 
 intercalated tubules of 
 
 
 
 181 
 
 labyrinth of 
 
 
 
 179 
 
 medulla of .... 
 
 
 
 178 
 
 medullary lays of 
 
 
 
 178 
 
 method of examining , • 
 
 
 
 i8s 
 
 papilla of ... 
 
 
 
 178 
 
 parenchyma of . 
 
 
 
 l8a 
 
 position of tubules in . 
 
 
 
 .83 
 
 preparation of isolated tubules of 
 
 
 
 l86 
 
 rabbit's, preparation of 
 
 
 
 iSs 
 
 straight tubules of . . . 
 
 
 
 181 
 
 uninjected, preparation of 
 
 
 
 186 
 
 uriniferous tubules of 
 
 
 ^ « 
 
 180 
 
 Krause, line of, in muscl&-fibres 
 
 
 
 97 
 
 Lactation, changes of mammary gland during . 
 
 
 S09 
 
 Lacunae, of bone ...... 
 
 
 6a 
 
 Morgagnl, in urethrs. . 
 
 
 
 >9« 
 
INDEX. 
 
 265 
 
 Lamellse, of bona 
 Leucocytes . 
 Lieberkuhn, follicles of, in small intesUoe 
 Ligamentum nuchae . 
 
 littre, glands of, in urethia 
 Liver .... 
 
 blood-vessels of 
 cells of ... 
 
 preparation of . 
 connective tissue of 
 gall-vessels of 
 genera! structure of 
 Glisson's capsule in 
 injection of blood-vessels of 
 
 gail-vessels of 
 lobules of 
 lympliatic vessels of 
 lymphoid tissue in . 
 pig's, preparation of 
 human, preparation of 
 Lungs, air vesicles of 
 
 alveolar passages cf 
 alveoli of . • 
 
 blood-vessels of . 
 infundibula of . 
 Injected blood-vessels of 
 hibules of . 
 method of examining 
 pigment in . 
 uninjected, preparation of 
 Lymph 
 
 follicles 
 Lymphatic capillailes 
 
 glands . . 
 
 spaces . . 
 
 Lymphoid tissue 
 
 Malfighiah Bodies, of kidnejr 
 of spleen 
 Malpigliian layer, of skin 
 Mammary gland, structure of 
 preparatioo of 
 
 PAGE 
 
 63 
 80 
 
 149 
 
 34 
 
 194 
 
 IS7 
 
 158 
 
 '57 
 161 
 161 
 160 
 158 
 »S9 
 162 
 163 
 •59 
 161 
 161 
 162 
 Ifo 
 17a 
 17a 
 17a 
 •74 
 172 
 177 
 171 
 «7S 
 «7S 
 «7S 
 
 86 
 
 »34 
 I«5 
 
 139 
 44 
 134 
 »34 
 
 179 
 «37 
 aao 
 aoS 
 
 ma 
 
266 
 
 INDEX. 
 
 Marrow, of bone, structure of 
 
 preparation of 
 Medullary sheath, of nerve-fibres 
 Meibomian glands 
 Meissner^s corpuscles 
 
 Menstruation, changes of uterine mucous membrane 
 Mesentery, endothelium of 
 Motor-end plates . 
 Mounting', in glycerin 
 
 in Canada balsam 
 Mucin, in embryonal tissue 
 Mucous tissue 
 
 in umbilical cord, preparation of 
 Mtiller^s fluid, formula for 
 Muscle-cells, smooth 
 
 isolation of 
 preparation of, from frog's bladder 
 sections of, from intestine 
 Muscle, erector pilse. 
 Muscle-fibres, general structure of 
 Hensen's line in 
 Krause's line in , 
 nuclei of . 
 preparation of fresh 
 
 by osmic acid 
 sections from tongue 
 sarcolemma of 
 sarcous elements of . 
 
 Muscle-fibrillse, primitive 
 Muscle-tissue, classification of . 
 of heart 
 
 preparation of . 
 smooth 
 striated voluntary 
 
 Nail, structure of . 
 
 preparation of sections of 
 Nerves, connective tissue of 
 
 Henle's sheath of 
 
 Intra-fascicular connective tissue of 
 
 lamellar sheath of 
 
 peri-fascicular connective tissue of 
 
 preparation of sections of 
 
tNt)EX. 
 
 267 
 
 
 PAGB 
 
 Nerve>cells, classification of . . . . . 
 
 ZII 
 
 preparacion of, from brain . . , 
 
 118 
 
 from spinal cord . 
 
 117 
 
 processes of . . . . , , 
 
 XII 
 
 sympathetic, structure of . 
 
 112 
 
 preparation of 
 
 118 
 
 Neive-fibres, axis-cylinder of . 
 
 104 
 
 classification of .... . 
 
 103 
 
 fresh, study of .... . 
 
 "3 
 
 incisures of Schmidt in . 
 
 106 
 
 inter-annular segments of 
 
 los 
 
 medullary sheath of ... . 
 
 104 
 
 meduUated ..... 
 
 104 
 
 neurilemma of .... 
 
 105 
 
 non-medullated ..... 
 
 no 
 
 physiological significance of structure of 
 
 116 
 
 preparation of, with osmic acid 
 
 "4 
 
 with silver 
 
 "S 
 
 transverse sections of 
 
 "M 
 
 Ranvier's crosses on ... . 
 
 "5 
 
 ofRemak 
 
 no 
 
 sympathetic, preparation of . . . 
 
 118 
 
 termination of, in nerve-centres 
 
 loS 
 
 in the periphery 
 
 lo» 
 
 Kerve-tissue, classification of . 
 
 103 
 
 Neuroglia, character of, in spinal cord .... 
 
 «I3 
 
 Nodes, lymph ....... 
 
 "9 
 
 Nucleated red blood-cells, in marrow .... 
 
 67 
 
 in spleen .... 
 
 139 
 
 Odontoblasts ....... 
 
 78 
 
 Oil of cloves, use of, in Canada balsam mounting • • 
 
 «5 
 
 Omentum, endothelium of ..... 
 
 46 
 
 Optical sections ....... 
 
 ia6 
 
 Osmic acid, action of, on tissues .... 
 
 S 
 
 Osteoblasts ........ 
 
 67.73 
 
 origin of ...... 
 
 76 
 
 Ovary, connective tissue of .... . 
 
 198 
 
 g^eneral structure of .... • 
 
 197 
 
 Graafian follicles of 
 
 «99 
 
 preparation of . • • • • • 
 
 »04 
 
 Ovum, structure of . . . . . . . 
 
 900 
 
 Pacinian Bodies, in sidn ...... 
 
 MB 
 
268 
 
 INDEX. 
 
 Panniculus adiposuB ■ ■ 
 
 Papillae, of skin . , , 
 
 of hair 
 Periosteum .... 
 Payer's patches 
 Pia-mater, of biain 
 
 preparation of 
 of cord . 
 Pigmented cells, of retina 
 of choroid 
 Pleura, pulmonary, structure of . 
 Potassium bichromate 
 Preservative agents 
 Prickle-cells, in epidermis 
 Pritchard's method for gold staining 
 Prostate gland, structure of 
 
 preparation of . 
 Protoplasm .... 
 Pulp, of spleen 
 
 of tooth . . , 
 
 cords, in spleen 
 Purkinje's cells, structure of 
 
 position of, in cerebellum 
 
 Ranvibr, crosses of, on nerve-fibres 
 Remak, nerve-fibres of 
 Respiratory apparatus 
 Rolando, substantia gelatiaosa of 
 
 Sarcolemma 
 
 Saicous elements 
 
 Schmidt, incisures of, In nerve-fibres 
 
 Schwann, sheath of 
 
 Sebaceous glands . 
 
 Section cutting 
 
 Seminiferous tubules 
 
 Serous membranes . 
 
 Shaking, method of, in preparing tissues 
 
 Sharpey, fibres of, in bone 
 
 Silver nitrate, methods of staining with 
 
 Skin, blood-vessels of 
 
 coriumof 
 
 epidermis of 
 
 VAGB 
 
 as 
 
 "4 
 
 36 
 
 »7S 
 
 4 
 
 3 
 
 921 
 
 39 
 192 
 193 
 
 18 
 
 »39 
 78 
 «39 
 
 >I2 
 
 «7 
 
 no 
 i£8 
 ■14 
 
 g6 
 
 97 
 
 lo« 
 
 10s 
 
 «27 
 
 9 
 
 188,189 
 
 44 
 
 55 
 
 64.75 
 
 49.45 
 
 223 
 
 923 
 99a 
 
INDEX, 
 
 269 
 
 Skin, from finger tip, preparation of 
 general structure of 
 horny layer of 
 injected, preparation of 
 mucous layer of 
 negro*s 
 nerves of 
 papillae of 
 prickle-cells of 
 rete Malpighi 
 stratum lucidum 
 Spermatoblasts 
 Spermatozoa, development of 
 movements of 
 preparation of 
 structure of 
 Spinal cord, arrangement of gray matter in 
 blood-vessels of 
 central canal of 
 columns of 
 connective tissue of , 
 cornua of . . 
 
 general structure of 
 gray commissures of 
 gray matter of, structure of 
 neuroglia 
 
 sections, preparation of 
 substantia gelatinosa of Rolando 
 white commissure . 
 white matter, structure of 
 Spleen, cavernous venis of 
 
 distribution of blood in 
 follicles of 
 general structure of 
 preparation of 
 pulp of 
 Staining agents 
 Stellulae Verheyenll 
 Stomach, blood-vessels of 
 
 general structure of 
 lenticular glands of 
 mucous glands of 
 nerves of 
 
270 INDEX. 
 
 
 
 PAGE 
 
 Stomach, peptic glands of ... 
 
 ■ 147 
 
 preparation of sections of 
 
 "53 
 
 Subcutaneous connective tissue, ceils of 
 
 35 
 
 fibres of 
 
 33 
 
 Submaxillary g:land, preparation of . , , 
 
 157 
 
 relations and structure of 
 
 155 
 
 Suprarenal capsule, preparation of 
 
 164 
 
 structure of . . , 
 
 .65 
 
 Sweat-glands ..... 
 
 228 
 
 Sympathetic, cells of ... . 
 
 IIZ 
 
 fibres of .... 
 
 HO 
 
 Teeth. 
 
 78 
 
 Tendon, cells of, demonstration of . . . 
 
 36.38 
 
 fibrillEe of, demonstration of . 
 
 34 
 
 Testicle, preparation of . 
 
 192 
 
 structure of . , , . , 
 
 18S 
 
 Thyroid gland, formation of colloid in 
 
 166 
 
 general structure of . , 
 
 t66 
 
 preparation of . . 
 
 167 
 
 Trachea, blood-vessels of ... 
 
 , 170 
 
 mucous glands of ... . 
 
 169 
 
 preparation of . 
 
 175 
 
 structure of ... , 
 
 168 
 
 Umbilical Cord, mucous tissue in , , , 
 
 SO 
 
 Urethra, general structure of . . , 
 
 193 
 
 lacunae Morgagni of , 
 
 194 
 
 Littre's glands of . . , . 
 
 194 
 
 preparation of . 
 
 197 
 
 Uriniferous tubules . . . . . 
 
 180 
 
 Utsrus, general structure of ... , 
 
 204 
 
 mucous membrane of . . . . 
 
 205 
 
 preparation of .... . 
 
 • . S06 
 
 Vagina, general structure of ... . 
 
 • • 206 
 
 mucous membrane of . , 
 
 . . 207 
 
 preparation of ' . 
 
 . . 207 
 
 Vas deferens ...... 
 
 189 
 
 Veins, preparation of .... . 
 
 126 
 
 structure of . . . , , 
 
 
 valves of . . . . , 
 
 123 
 
INDEX. 
 
 271 
 
 Wax-mass, for embedding ...... 
 
 Welch's method of demonstrating subcutaneous connective-tissue 
 cells 
 
 Yellow elastic tissue .... 
 
 Zonula ciliaris ..... 
 
 35 
 30 
 
 U4 
 
 INDEX TO THE APPENDIX. 
 
 Acid, chromic, mode of hastening action of , . . . 256 
 
 hydrochloric, use of in fuchsin staining* of nerves . 356 
 
 Bladder, silver staining of blood-vessels of . . . 254 
 
 Btood>plates ......... 253 
 
 so-called third corpuscle of .... . 253 
 
 Bronchioles, respiratory of lungs . . . 254 
 
 Camphor, use of in preventing the formation of bacteria in chromic 
 
 fluids .... . . 256 
 
 Celloidin, use of as an embedding medium . . 252 
 
 Cells, neuroglia . . . ... 254 
 
 spider ....... 254 
 
 Chromic acid, method of hastening action of solutions of 256 
 
 Epithelium, respiratory in lungs ... 254 
 
 Freezing Microtome, cutting fresh tissues with ... 251 
 
 Fuchsin, acid, use of in staining nerve tissues . . . 254 
 
 Green, anilin, use of in staining fresh tissues . . . 252 
 
 Microtome, freezing, for cutting fresh tissues . . . 251 
 
 section cutting with ...... 251 
 
 MUller's fluid, mode of hastening action of . . . 356 
 
 Nerve Tissue, Weigert's method of staining with fuchsin . . 254 
 
 Neuroglia cells ........ 254 
 
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