ALLEN'S HIM AN ANATOMY SECTION I. HISTOLOGY. ^WiJ^.llt^Sfc* tS^**$EFSk <{Cff^l S;*<3^' . *r^SL5L^-Aw . \i ^!V^! FROM THE LIBRARY OF FRANK BRANSON PETRIE, M.D v, :,: A SYSTEM OF HUMAN ANATOMY. INCLUDING ITS MEDICAL AND SURGICAL RELATIONS p.v HARRISON ALLEN, M. D., I'K'il ].>~MK OF PHYSIOLOGY IN THE TSlVERSITY OF PENNSYLVANIA, ETC., ETC. ILLUSTRATED WITH THREE HUNDRED AM) KIGHTV FIGURES OX OXE HUXDREI) AND NINE PLATES, MANY OF WHICH ARE BEAUTIFULLY COLORED. THE DRAWINGS BY HERMANN EABER, FROM DISSECTIONS BY THE AUTHOR. ALSO, UPWARDS OF TWO HUNDRED AXD FIFTY WOODCUTS IN THE TEXT. SECTION I. H ISTO LOGY. E. O. SHAKESPEARE, M.D., OPIITIIALMOJ/KHST TO THK PHILADELPHIA HOSPITAL. PHILADELPHIA: HENRY C. LEA'S SON & CO. 1882. DUNGLISON, ROBLEY, M. D. MEDICAL LEXICON; A DICTIONARY OF MEDICAL SCIENCE. Containing a concise explanation of the various subjects and terms of Anatomy, Physiology, Pathology, Hygiene, Therapeutics, Pharmacology, Pharmacy, Surgery, Obstetrics, Medical Jurisprudence and Dentistry; Notices of Climate and of Mineral Waters; Formula; for Officinal, Empirical and Dietetic Preparations; with the accentuation and etymology of the terms, and the French and other synonymes, so as to constitute a French as well as an English Medical Lexicon. A new edition. Thoroughly revised and very greatly modified and augmented. By RICHARD J. DUNGLISON, M. D. In one very large and handsome royal octavo volume of 1139 pages. Cloth, $6.50; leather, raised bands, $7.50; very handsome half Russia, raised bands, $8. STILLE, ALFRED, M. D., LL. D., and MAISCH, JOHN M., Phar. D. THE NATIONAL DISPENSATORY, CONTAINING THE NATURAL HISTORY, CHEMISTRY, PHAR- MACY, ACTIONS AND USES OF MEDICINES, including those recognized in the Pharmacopoeias of the United States, Great Britain and Germany, with numerous references to the French Codex. Second edition, thoroughly revised, with numerous additions. In one very handsome octavo volume of 1692 pages, with 239 illustrations." Extra cloth, $6.75; leather, raised bands, $7.50; very handsome half Russia, raised bands and open back, $8.25. FLINT, AUSTIN, M. D. A TREATISE ON THE PRINCIPLES AND PRACTICE OF MEDICINE. Designed for the use of Students and Practitioners. Fifth edition, thoroughly revised and much improved. In one large and closely- printed octavo volume of 1153 pages. Cloth, $5.50; leather, $6.50; very handsome half Russia, raised bands, $7. THOMAS, T. GAILLARD, M. D. A PRACTICAL TREATISE ON THE DISEASES OF WOMEN. Fifth edition, thoroughly revised and rewritten. In one large and handsome octavo volume of Sio pages, with 266 illustrations. Cloth, $5 ; leather, $6; very handsome half Russia, raised bands, $6.50. PLAYFAIR, W. S., M. D., F. R. C. P. A TREATISE ON THE SCIENCE AND PRACTICE OF MIDWIFERY. Third American edition, specially revised by the Author. Edited with additions by ROBERT P. HARRIS,^!. D. In one handsome octavo volume of 655 pages, with 183 illustrations. Cloth, $4; leather, $5 ; half Russia, raised bands, $5.50. SMITH, J. LEWIS, M. D. A COMPLETE PRACTICAL TREATISE ON THE DISEASES OF CHILDREN. Fifth edition, thor- oughly revised and rewritten. In one handsome octavo volume of 836 pages, with illustrations. Cloth, $4.50; leather, $5.50; very handsome half Russia, raised bands, $6. CORNIL, V., and RANVIER, L. A MANUAL OF PATHOLOGICAL HISTOLOGY. Translated with Notes and Additions, by E. O. SHAKESPEARE, M. D., Pathologist and Ophthalmic Surgeon to the Philadelphia Hospital, and J. HENRY C. SIMES, M. D., Demonstrator of Pathological Histology in the University of Pennsylvania. In one very hand- some octavo volume of 800 pages, with 360 illustrations. Cloth, $5.50; leather, $6.50; very handsome half Russia, raised bands, s;. DALTON, JOHN C., M.D. A TREATISE ON HUMAN PHYSIOLOGY. Designed for the use of Students and Practitioners of Medi- cine. Seventh edition, thoroughly revised and rewritten. In one very beautiful octavo volume of 722 pages, with 252 elaborate illustrations. Cloth, $5; leather, $6; very handsome half Russia, raised bands, $6.50. GRAY, HENRY, F. R. S. ANATOMY, DESCRIPTIVE AND SURGICAL. The Drawings by H. V. CARTER, M. D., and Dr. WEST- MACOTT. The Dissections jointly by the ATTIIOR and Dr. CARTER. With an Introduction on General Anatomy and Development by T. HOLMES, M. A., Surgeon to St. George's Hospital. A new American from the eighth enlarged and improved London edition. To which is added LANDMARKS, MEDICAL AND SURGICAL. By LUTHER HOLDEN, F. R. C. S. In one magnificent imperial octavo volume of 993 pages, with 523 large and elaborate engravings on wood. Cloth, $6; leather, $7; half Russia, raised bands, $7.50. ALLEN'S HUMAN ANATOMY HISTOLOGY. Entered according to the Act of Congress, in the year 1882, by HENRY C. LEA'S SON & CO., in the Office of the Librarian of Congress. All rights reserved. CONTENTS. HUMAN ANATOMY. INTRODUCTION . DEFINITIONS ANATOMICAL NOMENCLATURE HISTOLOGY. LYMPH ....... CHYLE BLOOD ENDOTHELIUM ' CONNECTIVE-TISSUE CELLS EPITHELIUM .... The Epithelium of the Skin The Epithelium of Mucous Membranes Glandular Epithelium THE CONNECTIVE-TISSUE SYSTEM . Mucous- or Gelatinous-Tissue White Fibrous-Tissue Yellow Elastic-Tissue Adipose Tissue PAGE 17 19 20 23 28 29 34 35 37 37 38 40 45 45 46 49 50 CARTILAGE Hyaline Cartilage Yellow Elastic or Eeticular Cartilage White Fibrous Cartilage BONE OR OSSEOUS TISSUE Ossification .... DEVELOPMENT OF BONE . Endochondral Bone Intermembranous Bone TEETH ...... Structure of the Teeth MUSCLE Smooth or Unstriped Muscle Striated Muscle .... Development of Muscle BLOODVESSELS .... NERVOUS TISSUE .... Cerebro-Spinal Nerves Peripheral Terminations of Nerves Nerve-Centres . Spinal Nerve Ganglia Sympathetic Nerve Ganglia THE LYMPHATIC SYSTEM . Lymphatic Glands . PAGE 51 52 54 54 54 57 58 59 60 60 61 63 G3 64 69 69 75 75 79 84 88 89 90 94 (iii) HUMAN ANATOMY, INCLUDING ITS MEDICAL AND SURGICAL RELATIONS. INTRODUCTION. IT is the design of this book to present the facts of human anatomy in the manner best suited to the requirements of the student and the practitioner of medicine. The author believes that such a book is needed, inasmuch as no treatise, as far as he knows, contains, in addition to the text descriptive of the subject, a systematic presentation of such anatomical facts as can be applied to practice. Works on anatomy may be placed in two groups : those written by scientists which have no special ap- plication of any kind, and those written by surgeons which have a decided leaning to surgical application. The model for the latter group originated in Europe, where the line is sharply drawn between surgical and medical practice. It requires but little discernment to detect the faulty plan upon which both these va- rieties of books are constructed. The scientist neces- sarily lacks clinical knowledge and sympathy ; the surgeon lacks interest in all but one class of subjects. A book which will be at once accurate in statement and concise in terms; which will be an acceptable ex- pression of the present state of the science of anat- omy; which will exclude nothing that can be made applicable to the medical art, and which will thus embrace all of surgical importance, while omitting nothing of value to clinical medicine would appear to have an excuse for existence in a country where most surgeons are general practitioners, and where there are few general practitioners who have no in- terest in surgery. The author may be allowed to say that, in the performance of his self-imposed task, nothing lias been hastily or inconsiderately undertaken. He has been actuated throughout by a sincere desire to produce a useful book. He has subordinated all other ten- dencies and notions to this end. In occasionally attempting a method of treatment of a subject some- what different from the one usually accepted, he has not departed from established ways of teaching for the sake of appearing to be original, but for good reasons, as he trusts will appear when the reader compares the text with that of other books on anatomy. The plan adopted is one necessarily encyclopedic. The author lias gleaned his materials from every source accessible to him, and, so far from fearing a charge of plagiarism, he will be glad to have the instances noted in which he has had the good taste to appropriate an occasional apt phrase or striking adjective. There is doubtless a greater degree of indebtedness due English works than the author is aware of, since the powerful impressions they have made on his mind must remain unconsciously to in- fluence his style. By way of introduction to the essential features of the volume, the attention of the reader is invited to the kinds of knowledge of the human body the physician demands. In the first place, the physician demands an exact acquaintance with the form and construction of the organs of the body. But, inasmuch as an anatomical fact is of little use unless the range of the application of the fact is known, the due connection between the normal condition of an organ and the variation in the condition of that organ within the limits of health will ( 17 ) 18 INTRODUCTION. receive proper attention. Accordingly, the typical description of each organ will be followed by a brief statement of " variations." In the second place, the physician demands a knowledge of the relations of the parts. This information it is necessary to possess in performing operations and in explaining signs or symptoms. Anatomical relations may be interpreted to be the mutual disposition of those parts which occupy the same neighborhood. The local value of relation has been enforced by the surgeon whose accurate know- ledge of each special region is held by him to be of great importance; but the general practitioner cannot appreciate the necessity of keeping up ever refresh- ened impressions of these regions. The anatomical relations he needs are determined at the examination of the sick, or at the autopsy. While parts in a given region may hold both surgical and medical rela- tions, this need not of necessi'ty be the case. In many instances the medical relations involve parts remote from one another and separated by one or more topographical regions. The former will receive the name of the topographical or the direct relation ; and the latter the clinical or the indirect relation. In the third place, the physician needs some account of the uses of the organs. This subject overlaps physiological anatomy. That much only will be succinctly given as may be said properly to illustrate the subject from an anatomical point of view, and at the same time to be free from controversy. In the fourth place, the physician must have a true conception of the nature and general behavior of morbid processes, and of the manner in which such processes are modified by locality. His comprehension of the changes due to diseased action in a given place must be fairly proportional to his knowledge of the normal anatomy of that place. This subject, which will receive the name of localization of diseased action, will be illustrated for the most part by concise state- ments of recorded cases, in which the essential feature of each case will be emphasized and the bearing it has on the subject treated of clearly shown. The material in these sections is capable of being used by the student in two ways : first, in bringing forcibly to his mind the value of the facts themselves, since cases similar to those quoted may occur to himself after graduation ; and, secondly, in lightening the task of remembering important though otherwise un- inviting details. In a word, anatomy may thus be made what unfortunately it rarely is an interesting study. In presenting anatomical features in explanation of given lesions, or of signs, or symptoms, care has been taken to give the sources of the statements made. It is hoped that the original papers or volumes con- taining such statements will be consulted whenever this is practicable. May not a yet more important use be made of these cases? May not a series of such abridgments be available in assisting the practitioner in detect- ing the significance of obscure conditions in relation to which the underlying facts are anatomical ? Should these questions be answered affirmatively, this book, it is hoped, will take a place among the physician's volumes of daily reference. In order to assist in the attainment of this object a copious index of diseases and injuries, in addition to the index of subjects, will be appended. Among other matters, the book will be found to contain an elaborate description of the tissues ; an ac- count of the normal development of the body ; a section on the nature and varieties of monstrosities; a section on the method of conducting post-mortem examina- tions; and a section on the study of the superficies of the body taken as a guide to the position of the deeper structures. These will appear in their appropriate places, duly subordinated to the design of presenting a text essentially anatomical. DEFINITIONS. 19 DEFINITIONS. ANATOMY is the science that treats of the enume- ration of organized bodies, and the description of their structure. HUMAN ANATOMY treats of the anatomy of man. COMPARATIVE ANATOMY, as usually understood, treats of the anatomy of all animals excepting man. In a better sense, Comparative Anatomy includes the form and structure of animals as related among them- selves rather than as related to man. The term may be also used in speaking of the anatomy of the differ- ent races of mankind as compared with one another. DESCRIPTIVE OR SYSTEMIC ANATOMY treats of the body as classified by its tissues or organs. Thus, the bones, the muscles, the bloodvessels, the viscera, etc., are severally distinct from one another. Descriptive Anatomy is opposed to Topographical or Eegional Anatomy. In this subdivision the body is divided by the relations of its parts to one another into a .number of more or less arbitrarily defined regions. Surgical Anatomy is a term often used to designate that branch of topographical anatomy which treats of regions of special importance in the study of surgical operations and of the effects of injury. Medical Anatomy is of similar import to the foregoing, but refers chiefly to the relations of parts as specially considered by the student of clinical conditions as distinguished from surgical. GENERAL ANATOMY treats of the composition and general relations of the tissues and organs. Thus, by the general anatomy of bone are understood, first, the composition of the fibrous tissue, the cartilage, and the salts contained in the bone; secondly, the relations that these hold one to another; and, thirdly, the consideration of similar structures or ingredients in allied tissues. The study of the ultimate elements of structure, as resolvable by the microscope, has led authors of late years to speak of this branch of gene- ral anatomy as Histology or Microscopical Anatomy. In like manner, the chemical analysis of tissue is treated of under the head of Zoo-chemistry, or Phy- siological Chemistry. General Anatomy is opposed to Special Anatomy, , which deals with the elucidation of a single part. Comparative anatomists apply this term to the struc- ture of a single animal when no comparison is en- tered upon. In this sense human anatomy itself is a Special Anatomy. MORPHOLOGICAL ANATOMY, or Morphology, is the science of organic form, and treats of homologies and the comprehensive relations of parts, especially those relations indicating zoological affinity. It is often in- exactly spoken of as Philosophical or Transcendental Anatomy. It is opposed to Teleological Anatomy, or Teleology, which treats of the adaptations of parts or organs to certain final specific uses. PHYSIOLOGICAL ANATOMY includes the considera- tion of the functions of organs, but in a more general sense than teleology. There is no sharply-defined line separating physiological anatomy from physiology or physics. The physiological anatomy of the eye is at once its physiology, which, in turn, can be explained only by reference to the principles of physics as ap- plied to vision. MORBID ANATOMY is the science which treats of the variations in the normal anatomy as determined by diseased action. It is conventionally held to in- clude congenital defects, or gross variations in struc- ture: but these subjects are best included under the head of TERATOLOGY. PRACTICAL ANATOMY is a term much in use to embrace the special kinds of printed directions best suited to those engaged in dissecting, together with the methods of making anatomical preparations, etc. It may also be said to include the study of human anatomy by dissection in contradistinction to the 20 INTKODUGTION. study of human anatomy as a branch of general knowledge. FCETAL ANATOMY, or EMBRYOLOGY, treats of the origin and formation of the organs in the embryo. As it is naturally considered in connection with the physiological anatomy of the organs of generation, it is often included under the head of Physiology. DESCRIPTIVE ANATOMY. Its divisions are as fol- lows : The Bones, or Osteology. " Joints, or Arthrology. " Muscles, or Myology. " Bloodvessels, or Angciology. " Viscera, or Splanchnology. " Nerves, or Neurology. " Special Senses. ANATOMICAL NOMENCLATURE. In this book the word distal (following Barclay) will be understood to refer to a point away from the centre of the body ; and the word proximal to a point toward the centre. For example, the trochlea of the humerus is at the distal extremity of the bone, while the head is at the proximal extremity. The word central also refers to a portion of a nerve or vessel which is connected with the centre of the system to which the part treated of belongs. The remaining portion, as opposed to the central, is called peripheral. In the case of the bloodvessels the central end is often spoken of as cardiac. When the axis of the body or a limb is understood, the words median and lateral are often used in de- scribing parts. Median means near or related to the axis (median line). Lateral means near or related to the surface or periphery, as distinct from median. "Inner" and "outer," "internal" and "external," are words very generally employed in the same sense as median and lateral. They are less exact, however, since they are also used to denote a central in opposi- tion to a peripheral part, as in the contents of a section. Median and lateral are synonymous with visceral (splanchnic) and parietal, in describing surfaces of the pleura, pericardium, peritoneum, etc. A longitudinal section is a section cut parallel to the longitudinal axis of the body or limb. It may be made from before backward, when it is called the sagittal section (vertico-longitudinal), because it is parallel to the sagittal suture of the cranium ; or it may be made from side to side, when it is called frontal (vertico- transverse), because it is parallel to the frontal suture of the cranium. 1 The frontal section is, of 1 The terms sagittal and frontal are in general use among Ger- man writers. That they relate to the disposition of the cranial sutures is an assumption of the writer. He cannot give the au- thority for their first employment. course, perpendicular to the sagittal section. Some writers restrict the term longitudinal to the sagittal section, in which case the frontal becomes to it a dex- tro-sinistral transverse. It is in this sense that Charcot uses the latter term. The frontal section of the cra- nium and contents becomes a true transverse section of the brain, owing to the angulation of the brain with the axis of the trunk. But a frontal section of the spinal cord is a longitudinal section, since it is parallel to the axis of the trunk. With the exercise of a little care in the use of these terms, no confusion need occur. A transverse section is a section cut perpendicular to the longitudinal axis. Thus a transverse section of a limb is perpendicular to the axis of the limb. A vertical section can be opposed only to a trans- verse as the author defines the word, and may include both the frontal and the sagittal. The term should be restricted to sections made with direct reference to the study in which the vertical position of the part is of importance. The term horizontal is sometimes used to express a section made parallel to the plane on which the organ or approximate parts rest. Thus, one can speak of a horizontal section of the brain and of a horizontal semicircular canal, because these are parallel (or ap- proximately so) to the plane of the base of the skull. In a transverse section of the parts confined in that portion of the trunk, neck, or head which contains the large vessels Owen has named the structures in relation to the position of the central nervous system and aorta. Let it be supposed that it is desired to describe the parts in a transverse section of the thorax; then the structures above the body of the vertebra be- come neural, and those below the body become hemal, since the former are near the central nervous system as expressed here in the section, and the latter are near the aorta. In the same way the section of the ANATOMICAL NOMENCLATURE. 21 vertebral canal becomes the neural space, and the cavity of the thorax the hemal space. Anything toward the neural space becomes neurad; and any- thing toward the hemal space becomes hemad, etc. Ventral and dorsal are terms nearly equal in value to those just given. They more commonly relate to surfaces. Huxley proposes the terms epi-axial and hypo-axial to designate the relation of parts to any given axis, either of the trunk or of the limbs. According to this method, the longissimus dorsi muscle is epi-axial to the axis of the spine, while the psoas muscle is hypo-axial to it. The biceps cubiti muscle is epi-axial, the triceps is hypo-axial, etc. In making these dis- tinctions the body is assumed to be prone or supine. The terms pre-axial and post-axial may be substi- tuted for the foregoing in studying a body, like that of man, in the erect position. In the naming of organs, it must be acknowledged that little order exists in the employment of terms. The terms are often inappropriate, cumbersome in form, and vague in meaning. They are as likely as not to be applied in a manner at variance with their legitimate use. Authors have multiplied terms to such a degree that there are but few structures which are designated by a single name ; and since no cus- tom has fixed the choice to be made in such sy- nonymy, clinical writers are perhaps excusable in consulting their own convenience. Whenever prac- ticable, the terminology used by clinical writers will be preferred throughout this treatise. Femurs will have " heads" and " necks," and convolutions will continue to " ascend" or " descend," as long as prac- tical physicians employ these words in recording their cases. With a view of preventing confusion, the more common of the synonymous terms will be placed in brackets after those adopted by the author. The significance of anatomical terms not in general use is fully exhibited in works readily accessible to the student. HISTOLOGY. LYMPH. OF all the tissues of the human frame, perhaps the lymph is the most important; it is certainly one of the most extensive. Possessing a volume nearly one-third that of the entire body, it surrounds every constituent of the connective framework, and is in close contact with the elementary parts of all organs. It is the ever-present medium of transportation from the highways of the blood to the cell-elements of the body, of the pabulum necessary to their life and func- tion ; it is the common carrier of the products both of elaboration and of waste of the great connective-tissue system ; and it is the perennial stream, through whose agency the depuration of the blood, during its course in the capillaries, is balanced by a complementary accession. The morphology of the lymph is all that concerns us in this place. Viewed from this standpoint, the lymph is one of the simplest tissues studied under the microscope ; and it is for this reason that we have chosen to begin with it. Under a high magnifying power, lymph is seen to consist, when freshly examined, of numbers of form- elements, imbedded usually in a clear, colorless, trans- parent, structureless substance of a fluid consistence (the lymph-plasma). These form-elements may readily change their relative positions in the surrounding medium, by means of currents in the latter, or by means of an individual power of locomotion which seems to be inherent in some. In structure, shape, and dimensions, these forms differ much among them- selves, particularly in the warm-blooded animals; and their number in a given volume of the fluid medium, in which they are loosely suspended, varies greatly in different parts of the lymphatic system, according to the many circumstances which influence the density and chemical constitution of the lymph-plasma, as well as the activity of the form-elements themselves. The great majority of these elements do not differ so much in the general plan of their construction, as in the proportions of their constituent parts. Before speaking particularly of this, however, it should be well understood, that in every collection of lymph there are present in the plasma forms in widely vary- ing numbers representing three general classes of elements : (a) minute granules ; (6) cells consisting of one or more nuclei, and a protoplasmic body ; (c) forms more or less closely resembling red blood- corpuscles. a. Minute granules. There are always present in every 0.03937 cubic inch of the lymph numbers of particles, which, under a magnifying power of 500 or 600 diameters, present the form of very minute gran- ules; they are somewhat spherical (sometimes angu- lar), have a gray, opalescent appearance, and are in a state of constant agitation thus exhibiting the so- called Brownian movement. It is the presence of these elementary particles in vast numbers which gives rise to the opalescence of chyle. A more detailed de- scription of them will be given when the constitution of chyle is discussed. In the lymph their number varies greatly in different parts of the lymphatic system ; it varies also from one time to another in collections made at the same point. b. Lymph-corpuscles. The characteristic form-ele- ment of the lymph is the so-called lymph-corpuscle, variously termed leucocyte, white-corpuscle, or wan- dering-cell. Size. The lymph-corpuscles vary much in dimen- sions. In warm-blooded animals their diameter ranges, in the thoracic duct, from -5-^5-^ to -%-fa-Q of an inch, while, in the lymph of the peritoneum, the size of (23) 24 HISTOLOGY. many cells may even reach J^Q of an inch. Their mean diameter is generally less in the efferent than in the afferent vessels of the lymph-glands. Number. Their number in a given volume of the plasma has been found to vary quite as widely as their dimensions. Owing to several causes, particularly the viscosity (adhesiveness) of the corpuscles, the enumera- tion of these elements has always been accomplished with great difficulty, and consequently with much irre- gularity as to results. Notwithstanding this, however, the data obtained tend to establish, with considerable certainty, the following dicta : 1st. The number of lymph-corpuscles in any given volume of the plasma varies widely in different parts of the lymphatic system. 2d. In the efferent lymph-vessels of lymphatic glands and follicles they are much more numerous than in the afferent vessels of the same glands. 3d. They are usual- ly much less numerous in the smaller than in the larger lymph- vessels of the same course. Indeed, in many locations, the small radicles of the peripheral lymph- capillaries are almost entirely free of lymph-corpuscles. 4th. The small lymph-spaces and lymph-capillaries of the tendons and aponeuroses contain almost none in health, while in the loose connective tissues lymph- corpuscles are much more abundantly present. In the dog, lymph from the thoracic duct, at one observation, was found to contain 4800 globules per 0.03937 c. i. ; at another time the number reached 7500 per 0.03937 c. i., while the number of white corpuscles in the blood was 25000 per 0.03937 c. i. In the rabbit, the same observer (Ranvier) found in the thoracic duct 11300 per 0.03937 c. i., whilst in the blood of the aorta only 7500 were enumerated. Minute constitution. Both nucleus and cell-body consist of a fine network of colorless albumenoid ma- terial, which incloses in its meshes a semi-fluid sub- stance, usually also colorless. This network is visible only under very favorable conditions, generally after the action of certain reagents, yet it has been seen in other cells of the economy in situ natura during the life of the animal. The nuclear portion of this network has been termed the intra-nuclear, while that of the surrounding cell-body has been named the intra-cellu- lar network; the fibres of the two intercommunicate through the limiting membrane of the nucleus. The opalescent or finely-granular appearance of this and of the preceding class of white cells or lymph -corpuscles is entirely due to the optical effect of the fine fibres forming the network. Seen in optical transverse sec- tion these minute fibres appear as fine grayish granules, and at the nodal or crossing points, resemble dots of similar aspect, the minute intermediate spaces seem- Fig. l. a. White blood-corpuscle, showing an intra-collular and an intra-nuclear reticulum. b. Elliptical colored blood- corpuscle, showing similar reticula. High power. (Klein.) ing more brilliant. It is this finely- mottled appearance which has suggested the use of the term "finely granu- lar" universally employed in describing some cells, for in many heal thy living cells there are really no granules! to be found, a, Fig. 1, represents very fairly the net- works already referred to. The drawing also very well shows the difference in the closeness of the two reticula. By reference to the figure it will be readily observed that the meshes of the intra-cellular net- work are much wider than those of the intra-nuclear reticulum. It can now be readily understood that the semblance of a granule or pseudo-nucleus in the cell-body, or of a spot or pseudo-nucleolus within the nucleus, may be produced by means of a conden- sation or contraction of this reticulum at any point. Varieties. For convenience of description lymph- corpuscles may be divided into three classes the ex- treme forms of each class, however, gradually shading off' into those 'of the others as follows: 1. In every specimen of lymph there are to be found small colorless corpuscles, more or less spheroid in shape, composed of a single roundish nucleus, sur- rounded by an exceedingly small protoplasmic body. In their construction these small cells do not visibly differ from those of the next succeeding class, except in the relative proportion of nucleus to cell-body. They are present in numbers varying according to the location from which the lymph may be obtained. In the thoracic duct their number is about equal to the elements of the second class, while in the efferent lymph-vessels of lymphatic follicles or glands they are much more numerous, and in the afferent vessels of the same glands much less numerous than the larger lymph-corpuscles. In the lymph-glands them- selves these small colorless corpuscles preponderate in the medullary portion, while, on the contrary, the larger cells far outnumber the smaller in the cortical portions. Because of the very small protoplasmic body of these cells they have frequently been de- scribed as free nuclei. Their diameter is often not more than -5^-$ of an inch. 2. A larger finely granular cell, with one nucleus about the size of that of the preceding variety, or with two or more smaller ones, and with a surround- ing cell-body of much greater extent, more or less spherical in outline when at rest, and composed of a LYMPH. minute structure apparently identical with that of the preceding forms, may be considered to represent the second class of lymph -corpuscles. In the thoracic duct, and in the afferent vessels of lymph-glands, their diameter often reaches 2 * a of an inch, while in serous cavities it not infrequently measures 20 6 f an inch. The cell consists essentially of two parts, nucleus and cell-body. The nucleus, whether there be one or more contained within the body of the cell, is usually spheroid, vesicular, and possesses a limiting membrane of double contour. When single, the nucleus is about TTjVff f an i Qca i n diameter. During life, the nucleus is, as a rule, invisible, being masked by the natural slight opalescence of the cellular body surrounding it. 3. The intra-cellular reticulum of the lymph-cor- puscle may contain, in its meshes, besides a colorless hyaline semi-fluid substance, real collections of colored or highly refracting material genuine granules. The cell is then called a granular cell. The size of these granular cells may equal or exceed the dimensions of the finely granular corpuscle, but they are present in the Ivmph in much fewer numbers than the latter. The granules are not distributed evenly throughout the cell, but may be more or less grouped in various portions. Their predilection is for the body of the cell. This is so strong, indeed, that when seen during life, light, areas are often observed of considerable size wherein scarcely any granules are visible. These lighter areas generally correspond to the position of the nuclei when the cells are motionless. The lymph-corpuscle is destitute of an enveloping membrane. When living, its substance is soft and gelatinous, and extremely free to assume any shape which extraneous or inherent forces may direct. Liv- ing lymph-corpuscles, when removed from the animal which they inhabit, and observed under conditions of heat, surrounding fluids, gases, etc., which are as nearly as possible natural to them, evince their vi- tality in numerous ways, and for longer or shorter periods. Movements. Many of these are contractile, and when watched sufficiently long and close, exhibit various phenomena of an individual motion, which, when energetic and protracted, may ultimately result in cell-multiplication or locomotion. This contrac- tility seems to reside in the fibres which constitute the reticulum, the fluid and the granules which may be suspended in it having only a secondary or passive motion. It appears also that the intra-cellular reticu- lum is usually much more powerfully active in the various movements of the cell than is the intra- nuclear network. Even a movement which accom- 4 plishes the division of the nucleus, but stops short of complete cell-division, may be effected almost entirely through the agency of the intra-cellular network. Yet the intra-nuclear network is by no means en- tirely passive. It possesses and exercises a measure of moving power, for certain reliable observers have seen spots or nucleoli move within the nucleus when the cell presented no other movements. During the contractions and expansions of the reticulum the fluid and the suspended particles contained in the meshes are set in motion, and currents more or less limited are thus produced. In this way suspended particles, whether elaborated in the cell or imported thither, may move from place to place, while the invisible contraction or expansion of the reticulum may have a location in the cell quite different from that of the movement of the visible particles. The lymph-corpuscles of the second and third class are, par excellence, those cells of the lymph which exhibit active movements. The small colorless cor- puscles, having only a very thin cell-body around the nucleus, and consequently a very small amount of intra-cellular network, as a rule, show very feeble movements, or none at all. When the fresh lymph of a batrachian or mammalian animal is, immediately after extraction, placed in a moist chamber and prop- erly prepared for examination under a high power of the microscope, at first, the colorless corpuscles are, more or less, perfectly globular, and so opalescent that the nucleus cannot be seen. Presently, if the temperature of the lymph be kept sufficiently near that normal to the animal from which it has been taken, some of the finely granular, as well as many of the granular corpuscles are seen to put forth from one or more portions of their surface hyaline masses, which may persist indefinitely, or be at length with- drawn. These masses, at first hyaline, or, in the case of the granular corpuscles, free from granules, soon, in a greater or lesser part of their area, present the same optical appearance as does the body of the cell. They may then increase in size until perhaps half the volume of the cell has, so to speak, flowed into them. After or before this stage of alteration of form has been reached, one of two things may happen. The substance of the projection or bud may sink back again into the original body of the cell, which may then present its primary form, to be perchance subse- quently altered again by similar manoeuvres ; or the remainder of the cell -substance may continue to pass into the projection, until the latter has completely absorbed the former. In the latter case, it is evident that the location of the cell has been changed. In 26 HISTOLOGY. this manner the lymph-cells may slowly move from place to place. After a considerable portion of the substance of the cell has passed into a bud or projec- tion, the two main masses may increase the distance between their centres by a lengthening out and thin- ning of the pedicle which unites them (see Fig. 2). Subsequently the newly-moulded mass may still flow back into the original cell, as before suggested, or it may draw the original or mother mass into it, or the two masses may continue to separate until the uniting bond becomes so fine that it breaks, when the two become independent individuals, each endowed with the characteristics of the original cell. This is one of the modes of multiplication which the student can readily follow. Instead of one such budding mass, two or more may sprout from the original cell-body, and experience the various changes already men- tioned. Figs. 2 and 3 very fairly represent the out- Fig. 2. Fig. 3. WHITE CORPUSCLES (OR LYMPH-CELLS) undergoing division, and active move- ments. High power. (Car- penter.) A GRANULAR CORPUSCLE OF NEWT, showing changes undergone in fifteen minutes. High power. (Klein.) lines which the finely granular and the coarsely gran- ular corpuscles present at various stages of their alterations, although they are drawn from white blood-cells. The structure of the cells, however, is not portrayed with equal truthfulness, the granular aspect of the granular corpuscles being exaggerated while the nuclei of the finely granular cells are much too prominent. The changes in form already men- tioned are essentially those of budding or gemmation. At the same time that they are transpiring the original nucleus, if there be but one, may suffer division either by fission or gemmation. Usually each large bud draws into it a nucleus, if the budding is to result in cell-multiplication. The lymph-corpuscle, instead of putting out buds, may present a constriction in the cell-bodv, which may progress until both the nucleus and cell are cut into two. Thus, two new cells may be the result of division by scission. The portion of the cell cut off' from the original mass in the two ways above indicated may vary muoh in size. Unless the new mass contain a nucleus it cannot be regarded as a complete cell. Whether such a nucleated mass be capable of development into a perfectly formed corpuscle endowed with powers of reproduction and of ulterior usefulness is a question which remains open. In every specimen of lymph there are such masses to be found, varying greatly in size, but never presenting large dimensions. The movements above considered, because of their resemblance to those of the uni-cellular animals called amoebae, were termed by Max Schultze amoeboid movements, and the jelly-like substance of which the cells are composed, on account of its general resem- blance to the material of certain vegetable cells, re- ceived the name of protoplasm. Degrees of vitality. The various colorless elements of the lymph appear to be endowed with different de- grees of vitality and of activity. In every collection of fresh lymph properly prepared for microscopic ex- amination, there are some colorless corpuscles which, as long as the examination is continued, remain un- changed, and, of those which show signs of life, some present much more sluggish movements than others. Of the immobile cells, those which show a sharply de- fined nucleus, or two or more of the same kind, or which contain great numbers of fatty molecules, may be con- sidered moribund or perfectly inert, and perhaps already advanced in the process of disintegration. Eanvier re- gards these as identical with varieties of pus- corpuscles (see Fig. 4). Those which still preserve their opales- PUS-CORPUSCLES. 1, a, 6, in water ; c, d, e, after the action of acetic acid. 2, division of nuclei a, 6, division progressing ; c, d, more or less complete. ( Virchow.) cent finely granular appearance, and their nucleus partly or not at all visible, may, for periods, remain in a dormant state, and be capable of being awakened therefrom by the action of a sufficient stimulant. Chemical and physical influences. Of the agents which variously affect the lymph-corpuscles, some of the chief are heat, moisture, oxygen, acids, and electricity. Whilst a certain degree of heat is essen- tial, not only for the manifestations of amoeboid LYMPH. 27 movements, but also, even for the existence of life in the corpuscles, yet, on the other hand, the tem- perature cannot pass above a certain elevation with- out endangering the life of the cell. The two ex- tremes of heat within which the amoeboid cells are active, seem to be for the warm-blooded animals about 70 and 106^ F. The temperature of the lymph can be brought much lower than 70 F. without neces- sarily destroying vitality. Indeed, these cells have been seen to show every phenomenon of life after fivc/dng and thawing. Protracted lowering of the tem- perature below the extreme, however, is certain to induce the death of the cellule. The nearer the heat approaches the highest extreme the more rapid become the amoeboid .movements until the limit is reached, when they suddenly cease in the destruction of the life of the element, the crisis being manifested by the retraction of the amoeboid prolongations, the resump- tion of the spherical form of the corpuscle, and the dis- tinct appearance of the nucleus, and of the granules contained in the body of the cell. The highest ex- treme of temperature cannot be surpassed without resulting in death. Not only is warmth necessary for the life and ac- tivitv of the lymph-corpuscles of warm-blooded ani- mals, but, according to Ranvier, oxygen seems also to be essential. When the cells are deprived of it, they become sluggish, asphyxiated, and finally die. Amoe- boid movements of the cells are excited by it, and sluggish corpuscles in which the vital activities lie dormant or in suspense are resuscitated and put into vigorous motion. As will be seen below, the lymph- plasma, especially in the larger trunks, contains only a minimum of oxygen, a circumstance in harmony with the known comparative inactivity of the lymph-cor- puscles in the passage-ways of the lymphatic system. When floating in a medium nearly devoid of oxygen their activities being, as a rule, dormant, their form is generally spherical, and their movements are mostly only passive. When it is remembered that it is by means of their amoeboid movements that they apply themselves to surfaces, creep along, and pass through them, and that it is upon these movements their stickiness or so-called viscosity, depends, it will be readily understood how important for the economy is the fact that in the great lymphatic system, where the fluids move so slowly, and where, consequently, sticky elements could readily crowd together and obstruct the channels, one of the essential stimulators of the amoeboid movements, oxygen, should be almost en- tirely wanting. A sufficient fluidity of the elements themselves and of their surrounding medium is also necessary for the lively movements of the lymph-corpuscles. An in- creased specific gravity of the plasma retards, while, vice versa, within certain limits, a lowered specific gravity accelerates the amoeboid movements. By the addition of water, in sufficient quantity, to the medium in which the lymph-corpuscles float, the cellular protoplasm is swollen and made more transparent, the nucleus becomes distinct, finally the granules which may be contained within the cell assume the Brownian movement and the cell dies. These va- rious phenomena of the death of the cell become more marked and follow each other in more rapid succes- sion if an acid be added to the water. Viability. The viability of lymph-corpuscles is great. Under favorable conditions of heat and oxy- gen and other surroundings, they readily live a long time without the organism. When properly pre- served they have given evidence of life hours, days, and even weeks after extraction from the animal. Lymph-corpuscles exposed in the moist chamber to the action of vapor of iodine become fixed and killed the instant the vapor reaches them. Most of the cells assume a yellow tint, while the nucleus and granules become distinct. According to Ranvier, a few cells are stained mahogany-brown, the color cha- racteristic of glycogeuic matter. This author states that, in these cells the glycogenic matter is diffused throughout the whole of the element. It may be ex- truded from the cell in the form of drops, and if the action of the iodine be sufficiently prolonged these drops may fuse together and form a halo or atmo- sphere around the cell of a brown-mahogany color. The presence in the lymph-corpuscles of this matter explains why chemical analyses have revealed the presence of sugar in the lymph. Absorption. By means of their amceboid movements, lymph-corpuscles possess the faculty of drawing into themselves minute particles with which they come in contact. They have been seen also to expel such parti- cles from within their interior. Thus they may absorb minute particles at one location, carry them for awhile in their meanderings, and subsequently discharge their cargo at a distant point, to be absorbed perhaps again by another living cell. This circumstance has an important bearing upon the positiveness of deduc- tions, respecting the nature of inflammation, which have been drawn concerning the identity, in the tis- sues outside of the bloodvessels, of lymphoid and other cells containing in their interior minute foreign particles which at some point have been injected into the blood-stream. 28 HISTOLOGY. Lymph-corpuscles may become loaded with ab- sorbed innocuous minute foreign particles without having their activity seriously impaired. Many lymph-corpuscles are found which inclose in their interior various substances naturally met with in the course of their circulation, as for example fat drops, granules of blood -pigment, fragments of red blood- globules, and sometimes even small red blood-disks. Those cells which contain small red blood-disks have given rise to much dispute among physiologists, some believing that the blood-disks are formed within the cell, others claiming that the red disks reach their singular location in the same manner that other ex- traneous particles become absorbed. From their sup- posed power of elaboration of certain products, which may be discharged either as waste material or as matter to be used by other elements of the economy, Eanvier has suggested that the lymph-corpuscle is a urn-cellular wandering gland. c. Red corpuscles. Another element constantly found in the lymph is the red blood-corpuscle. It is usually met with only in small numbers when the lymph has been very carefully extracted. Its source may be traced to diapedesis from the blood-capillaries, which doubtless is to some extent a never-ceasing means ot supply. But it is also possible that many may be the outcome of a new formation somewhere within the lymphatic system. Plasma. The plasma of the lymph, as has already been stated, is, when fresh and normal, a colorless transparent substance of. fluid consistence. Accord- ing toChevreul, a thousand parts of the lymph of a dog yielded, of water, 926.4; of albumen, 61; of fibrin, 4.2 ; of salts, 8.4. In estimating the gases in the lymph, Hammarsten found in 3.527 ounces of fluid 1.664 cubic inches of gas, of which .054 c. i. were nitrogen, .016 c. i. oxygen, and 1.594 c. i. carbonic acid. In consequence of the fibrin-elements which it con- tains, the plasma of the lymph soon coagulates into a gelatinous mass when collected. If set aside for a while, this mass separates, like the blood-plasma, into two portions, the serum and the clot, which latter, in its elements, does not differ microscopically from that of the blood. CHYLE. The fluid collected by the lacteals, transported through the mesenteric lymphatic vessels and emptied into the thoracic duct with the lymphatic fluids from other locations, is lymph which contains elements al- ready described. In the intervals of digestion, it cou- Fig- 5. tains many more minute granules (a, Fig. 6) in a given volume of fluid than the lymph usually car- ries, but during digestion the number of minute par- ticles is so enormous that, whenviewed by the naked eye, the chyle presents an opalescent appearance, which is entirely due to their increased numbers. The most minute of them usually exhibit the Brown- ian movement. These granules do not consist solely of fat, as some have contended, but they pos- sess a protoplasmic body which forms an envelope or frame for the load of fat which each of them holds. Under a high power of the microscope, after treat- ment with the proper re- agents, each minute parti- cle is seen to consist of a small fat-drop imbedded within a mass of proto- plasm, which envelops it as a thin shell. Acetic acid added to the fluid dissolves these delicate protoplasmic envelopes and sets free the fat-glo- bules, many of which then run together to form larger fat-drops (ft, c, Fig. 6). When a collection of chyle is placed in a flask, and Fig. G. MOLECULAR BASE AND CORPUSCLES OF CHYLE. At a, from a lacteal ou the intestine ; 6, from a mesenteric gland ; c, from the rcceptaculum chyli. From Man. (Carpenter.) CONTENTS OF CHYLE. At a, primary molecules of chyle; 6, secondary mole- cules of chyle ; c, fatty globules ; d, chyle-corpuscles ; , pale cells ; /, red corpuscles. (Carpenter.) BLOOD. 29 after the addition of ether is set aside for a day or two, there is found at the bottom of the vessel a deposit which consists of lyrnph-corpuscles and minute gran- ules. The latter have retained their original globular or angular shape, have become more translucent, and have been entirely deprived of their fat. BLOOD. Like the lymph, the blood, when naturally flowing, consists of a colorless transparent fluid plasma, in which freely float numerous and varied minute ani- mal forms. The proportion of the volume of these form-elements to that of the plasma is about as fol- lows: In 1000 parts of blood there are Water Globules . Proteid substances . Fatty matter and salts 790. 127. 73. 10. Besides the fluids and solids above enumerated, the blood is always more or less charged with gases, of which carbonic acid, oxygen, and nitrogen are the chief. The capacity of the blood for the absorption of oxygen is peculiarly great, being more than eight times that of water. Whilst the oxygen of the blood is almost, if not quite, exclusively held in the red corpuscles, the carbonic acid is united with the plasma. Number and size of red corpusc'es. By far the most numerous and most important form-element of the blood of vertebrate animals is the red blood-cor- puscle, or red blood-disk. We learn from the enume- ration of Malassez, that in cartilaginous fishes the number of these elements ranges from 140,000 to 230,000 per .03937 cubic inch (a cubic millimetre); in the osseous fishes the number varies between 700,000 and 2,000,000 per cubic millimetre. The same in- vestigator places the number of these corpuscles per c. m. in birds at 1,600,000 to 4,000,000, while the ex- treme numbers per c. m. in the rnammiferseare recorded at 3,500,000 and 18,000,000. We have the authority of the same writer for the statement that the mean volume of the corpuscles is almost always in inverse ratio to their numbers. This proposition is not abso- lute, however, for a small number of the colored corpuscles may not be entirely compensated by an increase in the volume of the corpuscle. By consulting Fig. 7 and the table subjoined, the enormous difference in size, as well as in shape, of the colored corpuscles of the blood of different vertebrate animals can be readily appreciated. It will be noticed that the long axis of the red corpuscle of the proteus is recorded in the table as T ^ of an inch, a magni- tude sufficiently large to be appreciated by the naked eye under favorable circumstances. Yet the colored blood-disk of the Amphiuma (Congo eel) is quite one- third larger still. Fig. 7. Ofl O < '0 All the corpuscles here shown are drawn to the uniform scale, at the bottom of the wood-cut, of l-4000th of an English inch, and the measurements are expressed in vulgar fractions of that inch. T. D, signifies transverse diameter; L. D. long diameter; a. D. short diameter. (Gulliver.) MAMMALIA. T. D. L. D. 8. D. 1. Ked Corpuscle of Man, seen on the flat surface and also on the edge ; thickness 1-12400 .... 1-3200 2. of Elephant .... 1-2745 3. of Mask Deer .... 1-1232.5 4. of Dromedary, thickness 1-15337 1-3254 1-5921 AVE8. 5. of Ostrich . 1-1649 1-3000 Nucleus of Ostrich 1-3200 1-9166 6. of Pigeon 1-2314 1-3429 7. of Humming Bird 1-2666 1-4000 RHPTILIA. 8. of Crocodile 1-1231 1-22S6 9. of Python 1-1440 1-2400 10. of Proteus 1-400 1-727 PISCES. 11. of Perch 1-2461 3-3000 12. of Pike 1-2000 1-3555 13. of Shark 2-1143 1-1684 According to the statements of various observers, the mean number of the colored corpuscles in the blood of man may be regarded as varying in health between four and five millions per cubic millimetre. The counts of the corpuscles made upon the blood of the same individual appear to vary considerably, according to the location whence the blood is obtained. It seems, however, to be pretty well established that in any portion of the economy where there is a con- densation of the plasma of the blood by loss of fluid, 30 HISTOLOGY. cither from evaporation, or from excretion, or the like, there is usually to be found in the blood of the part an increased number of corpuscular elements. On the contrary, under opposite conditions, the number of red globules has usually been found to be below the mean. In the left heart, and in the large arteries of the limbs, the number of globules is identical, but in the arterioles it is increased. The venous blood of the skin, where evaporation is rapidly going on, and the venous blood of the kidneys, where excretion is taking place, is much more rich in red blood- disks per c. m. than is the blood of the corresponding arteries. In the interim of digestion, the mesenteric venous blood is richer in red cells than is the arterial blood, yet less rich in this respect than the blood of the cutaneous veins. During digestion, however, there are fewer red corpuscles per c. m. in the mesen- teric veins than in the supplying arteries. There is, of course, in the latter case an accession of fluid in the veins by absorption of the intestinal juices. The emptying of the lymph into the blood-stream at the mouth of the thoracic duct, also causes a lowering of the number per c. m. of the red corpuscles in the blood- current on the proximal side of that point. The splenic vein is much richer in red disks than is the artery. The subhepatic veins have fewer red cells per c. m. than have either the portal veins or the ascending cava. There is in the liver a probable de- struction of red globules. Besides these, so to speak, local variations in the number of colored corpuscles in a given volume of the blood, there is, perhaps, a physiological mean varia- tion for each individual, as well as a difference in indi- viduals dependent upon the conditions of sex. Accord- ing to some investigators, there would also appear to be slight fluctuations in the mean volume of the colored cells without a marked disturbance of health, ts well as differences in the intensity of the coloring matter of the corpuscle. The same authors claim that certain abnormal states of the economy show these fluctuations in a more or less marked degree. Thus Hayem declares that in chronic ansemia the mean dimensions of the red 'globules are always lessened. 100 globules of anasmic blood may correspond in volume to less than 80 healthy globules. At the same time, the intensity of the coloring matter of the corpuscles may be lessened one-quarter or one-half. The fresh fluid blood of vertebrates, as is well known, presents to the naked eye, when seen by reflected light, and in considerable quantity, a homo- geneous opaque aspect and an intense red color. It is opaque to the naked eye for the same reason that an emulsion of oil is opaque. When spread out in an extremely thin layer, however, and examined under a very high magnifying power by transmitted light, the fluid or plasma of the blood is seen to be per- fectly transparent, colorless, and structureless. The form-elements which float in it in enormous numbers are of three general classes : colored cells, colorless cells, and minute free granules. The colored cells or corpuscles, instead of being red, are, when examined by transmitted light, of a slight orange-green tint if seen single, and only approach a reddish tinge when cell is imposed upon cell several layers deep. Form of colored corpuscles. The general form of the colored corpuscle has been found to vary greatly in different orders of animals. In nearly all mammals the outline has been found circular, whilst in nearly all of the lower vertebrates it has been seen to be more or less elliptical. There are a few fishes (the lamprey eel and allied forms), however, in which round colored corpuscles have been met with ; while, on the other hand, two species of mammalian animals (the camel and the llama) possess red cells of oval out- line. A more rigid dividing line between the mam- malian and the lower vertebrate animals with respect to their colored blood-corpuscles is afforded in the presence or absence of a nucleus. In the lower ver- tebrates, whether the red cell be round or elliptical, there is a central nucleus present. In the adult healthy mammalian corpuscle, the nucleus is gener- ally admitted to be absent. Nevertheless, in oppo- sition to this latter statement, Bcetcher and a few others have sought to prove the existence of the re- mains of a nucleus in the red corpuscle of mammalians also (man, among others). Varieties of colored corpuscles. In the healthy blood of adult man, the red corpuscles present two general varieties of form and dimension the red blood-disk or corpuscle proper and the microphyte. Much the more numerous of these is the red blood-disk. The red blood-disk or corpuscle. As the word indi- cates, instead of being a spherical body, the red blood- cell is flattened into the form of a thin disk. Its outline is naturally circular, and in fresh arterial blood the central area of the disk presents a concavity on each side. In other words, besides being circular, the disk is normally bi-concave. The edge of the disk instead of being sharp or acute is rounded. In con- sequence of its peculiar shape, this form of the colored corpuscle, when seen in surface by trans- mitted light and properly focussed, should be most intense in color in the peripheral ring corresponding to the thicker portion. The color appears still more BLOOD. 31 HUMAN BI.OOD-OI.OBFI,ES. rt, seen from the surface ; &, from tho side; c, united iu rouleaux ; d, rendered spheri- cal by water ; e, decolorized by the same ; /, blood-glo- bulos shrunk by evaporation. (Gray.) intense when the disk is seen in profile. The colored corpuscle consists of three principal parts: the stroma, the fluid contents, and the coloring matter. Accord- ing to some late investigators, the body of the colored blood-corpuscle is composed of a fine felt-work of minute fibrillas (!>, Fig. 1) of an albuminoid material, holding in its meshes a soft semifluid substance. The latter holds more or less closely united with it the coloring matter of the corpuscle, a substance which has been named hemoglobin. The weight of authority seems to incline to the opinion that the corpuscle is membraneless. The red disk is naturally elas- tic, capable of assuming any form which a moderate pres- sure may require, and of re- turning again to its original shape when the pressure is removed. Many experienced and accurate observers declare that they have seen it manifest limited con- tractile power. But most writers assert that it is incapable of any other than passive movements, re- garding it as a more or less inert body, the altered remains of a once complete cell. The red disks of human blood, when the latter is freshly drawn and spread out upon a glass slide for microscopic examination, frequently show a marked tendency to apply themselves one against the other by their broad surfaces to form rows similar to rou- leaux of coins -a phenomenon the cause of which is unknown. Alterations of the red corpuscles. The colored cor- puscle of man may experience various alterations of form and composition, which are purely physical or chemical, in contradistinction to amoeboid or vital movements. Water causes them to swell, to lose their bi-convex discoid shape and become more or less spherical, and to discharge their coloring matter into the surround- ing plasma -which latter is now uniformly stained, while the corpuscle is apparently structureless, color- less, and sharply outlined. Many solutions of salts of less density than that of the blood have much the same action upon the red corpuscles. On the contrary, when the density of the blood- plasma or of the artificial serum in which the colored corpuscles are examined is increased either by slow evaporation or by other means, the surface of the cor- puscles, which is notably smooth, becomes wrinkled; subsequently, little prickles or spines appear all over the surface of the elements; finally, the corpuscles be- come more or less globular in form and diminished in diameter, retaining the while their prickly appear- ance. They are then known as crenated corpuscles. A few of this kind of cells are met with in nearly every specimen of blood. The addition of water causes them to swell and their spines to disappear, but they retain the spherical form. Carbonic acid causes the bi-concave disks to swell. When acting upon the crenate corpuscles, it causes the spines to disappear and the corpuscles to partly resume their former con- cave appearance. The action of this gas is generally not sufficient to effect a return of the concave surface upon both sides, but the corpuscle is made to assume a saucer shape. A strong electric current first causes the red disks to become crenate and spherical ; afterwards the cor- puscles swell and lose their color. Bile causes the red globules at first to become pale; after that they suddenly disappear, leaving no trace. Urea causes the red disks to become globular, but does not effect a decoloration. Upon the surface of the corpuscle there are formed little drops of matter apparently entirely similar to the body of the cell, which are united with each other and with the body of the corpuscle by fine filaments. The addition of tannic acid in sufficient amount to a portion of blood causes the hemoglobin to sepa- rate more or less completely from the body of the corpuscle and to form globular projections upon the surface, of the characteristic color. Eapid desiccation of a thin film of blood, spread out on a glass slide, very perfectly preserves the form and dimensions of colored corpuscles. When blood is raised to a temperature near 134 F., the globules begin to lose their discoid form, and to assume a spherical shape. At the same time little buds make their appearance upon the surface of the corpuscle, united to it and to each other by fine fila- ments. When the temperature reaches 158 F. the globules become decolored and broken up into small transparent spheres of very unequal size. Cold causes a dissolution of the hemoglobin, and an effect similar to that of water. Blood-crystals. When extra vasated into the tis- sues, colored corpuscles disintegrate by breaking into minute portions, the coloring matter of which finally is transformed into brown pigment-granules, known as hematin. Besides these granules of pigment, such extravasations may contain crystalline needles of a reddish-brown color hematoidin-crystals. But the characteristic crystals of the blood are those of hema- 32 HISTOLOGY. globin, usually deposited from a considerable quan- tity of blood. The form of the hemaglobin-crystals varies in different animals, and is often quite peculiar to certain species. These characteristic forms often Fig. 9. ' BLOOD-CRYSTALS. A, trihedral crystals from blood of Guinea-pig. B, pen- tagonal crystals from blood of Squirrel. C, octahedral crystals from blood of Rat and Mouse. D, heinatin-crystals from Human blood. E, hematoidin-crys- tals from an old apoplectic clot. F, hemin-crystala from blood treated with acetic acid. (Gray.) yield valuable suggestions as to the kind of animal from whence they came. Crystalline forms, artificially derived from hemo- globin, and valuable in medico- legal examinations of supposed blood-stains, are those obtained from dry blood by the action of glacial acetic acid. They consist of nut-brown, rhombic, crystal- line plates, called Aemz'w-crystals, or hydrochlorate of hematin. The red blood-disk of man varies somewhat in dimensions. Its mean diameter, as is seen by reference to the table on a previous page, is about -5^5-5- of an inch. This size may vary somewhat among different individuals, and in the same person at different times. The microphyte. In every specimen of human blood there are among the red bi-concave disks, already dc- HEMIN - CRYSTALS, by Teichmaun, in hydrochlo- rate of hematin. scribed, a small number of colored globular corpuscles of much less diameter, and of a darker tint. Their size may be not greater than -^-^ or -g-J-^ of an inch. They appear to possess the same intimate structure, and to give the same chemical reactions as do the red bi-concave disks. They have been called microcytes. By some histologists they are regarded as young, not yet fully developed, red blood-corpuscles (hemato- blasts of Hayem), since they seem to be more numer- ous at times when the blood is undergoing repair. By others they are thought to be the old bi-concave disks undergoing retrograde changes. They are especially abundant in progressive pernicious anasmia. The white or colorless corpuscles. Of the colorless corpuscles of human blood, cells similar to those of the three general classes of colorless cells described as present in the lymph are found, and it is not necessary here to review their characteristics. Their number, however, is usually far below that of the colored elements. It varies much even in health, and in the same individual, from one time to an- other. In comparing the white corpuscles with the red, Welcker has the average as 1 white to 335 red, Moleschott 1 white to 357 red, Klein has placed the highest number in health at 1 white to 650 red. We have the authority of Moleschott for the statement that boys have 1 colorless to 226 colored corpuscles, old men 1 to 381, girls 1 to 389, young women men- struating 1 to 247, the same women when not men- struating 1 to 405, pregnant women 1 to 281. Ac- cording to Hirt the proportion of white corpuscles to the red is low immediately after eating, while two or three hours later, the proportion is again near the normal. The same author states that the proportion in the blood of the splenic vein is 1-60 ; in the splenic artery 1-2260 ; in the hepatic veins 1-170 ; in the portal vein 1-740. It would seem from these figures that there may be at times a physiological leucocy- tosis, although this has been denied by some later investigators. Groucher declares that the proportion of the white to the red corpuscles varies little in the course of the day. In disease the relative number of the white corpuscles has even surpassed the propor- tion of 1 white to 3 red cells. Red granular corpuscles. Besides the ordinary white corpuscles, Semrner has observed in the blood of mammals certain numbers of more or less granu- lar, nucleated, colorless cells, of somewhat larger size than those above considered, which contain red granules of considerable size, and has named them red granular corpuscles. According to this author, as well as A. Schmidt, they are probably intermediary BLOOD. 33 forms between the white cell and the red corpuscle of the blood. Ovoid colorless cells. Von Eecklinghausen has met with a few colorless, ovoid, granular, nucleated cells in the blood of frogs. In these animals they seem to bo more numerous in certain seasons of the year, being much more frequent in the spring and summer. Many accurate observers have seen similar color- less elements in the blood of man. They have been met with chiefly in the blood of patients suffering with certain febrile disorders. In the blood of re- lapsing fever subjects they are not infrequent ; they have also sometimes been encountered in typhoid fever. In the two latter cases these cells are often loaded with fine fatty granules. The writer has met with such cells, in some abundance, in inflammations aftbeting the bloodvessels, and believes with Sernmer that they may be regarded as normal to the blood of certain, if not all, mammals. During the same study of inflammation of bloodvessels we had the opportu- nity once or twice to observe under the microscope endothelial cells, lining the vessels, become detached and carried off in the blood-current. These observa- tions leave us no doubt that at least some of the ovoid, colorless cells, above mentioned, are to be assigned to a similar origin, namely, a limited desquamation of the endothclia lining the vessel walls. Free ijrannles. The plasma of the blood also car- ries in suspension numbers of minute particles appa- rently identical with the free granules described as present in the lyrnph ; their number as well as their nature varies in much the same manner. Of course, in the blood some of these minute particles arise from a destruction by disintegration and fragmentation of the superannuated red blood-disks. Regeneration. In the blood, as in the other tissues of the organism, the various stages of generation, de- velopment, vigorous activity, decay, and disintegra- tion are constantly presented by the different elements. After hemorrhage, whether natural or accidental, it would seem that the investigator has offered to him very ample opportunity of studying the methods of regeneration by which immense losses of the red blood-globules are rapidly repaired. To replace the ordinary periodic destruction of red blood-disks which women suffer at the time of their catamenia, about one hundred and seventy-five million of red corpus- cles, it has been calculated, must be produced every minute during the intermenstrual period. Yet, the question of the renewed supply of colored elements of the blood, notwithstanding the many exhaustive investigations of hematologists, has not up to the 5 present moment been satisfactorily answered. It seems to be generally admitted, however, that in some way the red disks are elaborated from the colorless cells, either of the blood or of the lymph, but the diversity of opinion as to the precise method of for- mation during adult life, is great among some leading modern histologists. Movements of the corpuscles. Heat and the other reagents which were mentioned when considering the elements of the lymph, have identical effects upon the colorless cells of the blood. Wherever the cir- culation of the blood is sluggish, and heat and oxygen are present in sufficient quantities, large numbers of the white corpuscles put forth active movements. These movements are much more energetic in the small venules than in any other portion of the circu- latory system. It is here and in the capillary vessels that the viscosity or stickiness of the colored ele- ments especially shows itself, and it is mainly here also that the amoeboid corpuscles stick to the walls of the vessels and ultimately pass through them. The fact that they do traverse the walls of these vessels is not now questioned ; but writers are not yet agreed as to the manner in which the act is accomplished, some believing that it is effected by the active creeping of the corpuscles through minute pores, which ordi- narily are filled with a soft permeable subs-tance, others claiming that the extravasation of the cor- puscle is passive, entirely effected by the outward pressure of the blood against the walls of the vessel ; still other opinions have been advanced. It is ad- mitted, however, that the more active the amoeboid movements from any cause, the more rapid and ex- tensive is the emigration. It has long been known that the red elements also escape through the vessel walls. The diapedesis of these elements, however, is generally believed to be in the main passive ; perhaps it takes place through passage-ways already opened by the previous emigration of a white amoeboid cell. In these emigrations of the elements of the blood through the walls of the vessels, the cells are often fragmented, and the fragments are according to cir- cumstances carried off by the lymph current without the vessel or by the blood current within it. Like the lymphatic fluid, the, plasma of the blood contains a certain proportion of fibrin which sooner or later deposits itself when the blood is left to stand. The nature and structure of the fibrin of the blood are very similar to those of the fibrin of the lymph. It is supposed to be the result of the action of a so-called fibrinogenous upon a fibrinoplastic substance contained in the plasma. HISTOLOGY. Derivation of blood and lymph. All the foregoing elements in the lymph and in the blood are to be regarded as derivatives immediately or indirectly of the connective-tissues of the organism descendants from, the middle or connective-tissue layer of the blastoderm, or mesoblast. ENDOTHELIUM. We now pass to the consideration, in the endothelia, of another variety of the connective-tissue elements which are widely distributed. They are flat cells, which usually in a single layer arc fouud lining the surface of serous cavities, the inner surface of blood- and lymph-vessels, and the synovial membranes. They consist of a more or less flat cell-plate, of an albumi- nous somewhat elastic substance, and one or more nuclei possessingadouble-contoured membrane, amore or less oval outline, and an excentric location in the body of the cell. The cell-plate consists of an elastic network of fibrils (the intra-cellular reticulum) in- closing in its meshes a semifluid homogeneous color- less substance. The nucleus also consists of a similar but denser network (the intra-nuclear), holding in its spaces a similar semifluid substance (see c, Fig. 11). Fig. 11. 'I CELLS SHOWIXO THE RETIODI.CM ix THE PROTOPLASM AXI> NUCLEUS. a. Co- lumnar epithelial cell provided with cilia, the latter being prolongations of the iutra-ccllnlar network. 6. Nucleus of a glandular epithelial cell from the stomach of a Newt, showing the intra-uuclear network, c. Endothelial cell of the mesentery of a Newt, containing in a hyaline ground-substance a plexus of fine fibre-bundles intra-cellular network in connection with the intra- nuclear network, d. Connective- tissue corpuscle from mesentery of X'\vt, showing very clearly the intra-cellular network of fibrils and the hyaline ground-substance ; the former extends into the branched processes, and is also connected with the more delicate intra-nuclear reticulnm. e. Goblet-cell from the stomach of a Newt, showing the intra-cellular network in connection with fibrils of the intra-nuclcar network ; the upper part of the eel! is greatly swollen by mucus. (Klein.) The cell appears to be without a membrane. The nucleus is much flattened when the cell-plate is thin, and causes a prominence corresponding with its loca- tion. Normally the cell-plate is so thin and transparent that its outlines are indistinguishable, tire nucleus alone being visible. Neighboring cells are placed nearly in contact by their edges, being separated only by a very small amount of structureless transparent viscid sub- stance which holds them together. This material has been called cement -substance (intercellular cement). When these cells are seen in profile they appear to be more or less linear or spindle-form, the thickness of the spindle corresponding to the nucleus of the cell. After staining with a weak solution of nitrate of sil- ver, the intercellular cement is so darkened that the outline of the cell is made very distinct. It is thus found that endothelial cells have more or less irregular polygonal outlines (consult Fig. 12), and that the Fiji. 12. silver-treated NORMAL EXDOTHBLIBM OF VISCERAL PKRir.inntrM or A 1 and highly magnified. (('Jiapnuin.) extent of the cell-plate is extremely variable. In the arteries and larger lymph-vessels the cells are more or less lozenge shape, with the border lines only slightly wavy. In the veins and lymph-capillaries. the cells are much broader as a rule, and their periph- ery is represented by extremely irregular indented lines (consult figs. 1, 3, Plate XII.). Upon the surface of the serous cavities the outlines of the elastic cell- plates present still another picture, as will be seen by reference to fig. 1, Plate I. Besides variations in the outlines of endothelial cells, there are other dif- ferences which are shown mainly upon serous surfaces In the adult animal the greater part of these surfaces is covered by a single layer of extremely thin, nearly hyaline, and somewhat broad cell-plates. There are, however, limited areas more or less numerous scat- tered here and there upon which the cells offer a very different aspect. They are much narrower, their edges are much less irregular, they are much thicker when seen in profile (approaching a cubical form), are quite granular, arid possess two or more nuclei. In fact, they appear to be in a decidedly active state, while, on CONNECTIVE-TISSUE CELLS. 35 the other hand, the vital proclivities of the first men- tioned very thin hyaline cell-plates seem to be entirely dormant. These granular cells are said by Klein, and some others to be germinating, and have been called by them germinating endothelia (see e, fig. 1, Plate I.). They are generally arranged around the mouth of a vertical canal connecting a lymph-channel of the sub jacent connective-tissue with the serous cavity. The activity of this germinating endotheliurn may be such that the granular cells proliferate and form new cells, some of which may be set free in the serous cavity. These new cells are similar in every respect to the other lymph-corpuscles which float in the lymph- plasma, and cannot, after separation from their place of generation, be distinguished from them. The endothelia covering some portions of the abdominal cavities and the fenestrated septa of the mediastinum are thus a very fruitful source of supply of the color- less corpuscles of the lymph. The endothelia are, as already stated, somewhat elastic. When the membrane which they cover is stretched, they become thinner, and spread out in order to cover a wider extent of surface, and their border becomes straighter. On the contrary, when the membrane retracts, their broad diameter lessens, their borders become more sinuous, and the cell be comes thicker. Thus during full inspiration the endo- thelia covering the pleural surface of the lungs are much thinner and broader than at the end of expira- tion. At the latter instant the pulmonary endothelia are quite cubical in shape. As has already been indicated, the endothelia not only of the serous cavities, and of the lymph-vessels, but also of the bloodvessels as well, may be regarded as some of the sources of the colorless elements to the lymph and blood. CONNECTIVE-TISSUE CELLS. Still another group of the elements derived from the rnesoblast is formed by the cells of the connective- tissue. They may be considered under two general heads the wandering cells and the fixed cells. Wandering cells. The wunderiny cells of the connec- tive-tissue are not different from the colorless cells already described when considering the lymph. They are in reality lymph-corpuscles, existing in the radi- cles of the lymphatic system, and do not call for any particular description here. One form may be con- sidered for a moment, however, before dismissing them. There are found in the interstices of the loose connective-tissues which are especially vascular, small numbers of large granular corpuscles slightly pig- mented (plasmatic corpuscles of Waldeyer). They are more frequent along the walls of vessels, and do not usually exhibit marked amoeboid movements. Connective-tissue corjmscles. The fixed cells of the connective-tissue vary much in shape according to the arrangement of the fibrous tissue in which they are imbedded. The cellular elements of the great connective-tissue system are met with only within or upon the sides of the spaces which permeate that system, whether they be in the form of fine channels or in the nature of lymph-cavities, and whether those spaces be large or small. The large spaces, as, for example, the perito- neal cavity, the pleural cavity, the arachnoid cavity, the heart and bloodvessels, the larger Ivmph-vessels, are lined throughout by a complete covering of endo- thelial cells; the minute spaces which exist between the bundles of connective-tissue fibrils have as a rule only a more or less incomplete lining. But whether the space be of one kind or of the other, it is lined by cells of an essentially identical nature; only their forms and the arrangement of some of their structures, vary according to the circumstances which surround them. Although the fixed cells of the connective- tissue are here considered under a special section dif- ferent from that in which the endothelia were dis- cussed, it is not because these two species of cells differ in anything more than form. The so-called fixed cell of the connective tissue tends to assume a form which is a more or less perfect mould of the space in which it is found. It presents two general forms, the one plate-like, the other stel- late. The plate-like forms more nearly resemble the cell-plates of the endothelia than do the stellate forms, and will therefore be considered first (see c, fig. 3, PI. IX.). Flat tendon-cells. In white fibrous tissue such as tendon, etc., the spaces formed by the apposition of several bundles are linear, and are limited laterally by the convex surfaces of the fibrous bundles. The surface of the space thus formed, is usually partially lined only. On one side is a longitudinal row of flat elastic cell- plates, applied more or less tightly to the surface of the fibrous bundles forming that side. The opposite side of this minute lymph-space is void of cellular lining. The cells constituting this row are placed edge to edge, and at the line of junction the border of the cell is quite straight, and generally transverse to the direction of the bundle. These cells thus have two long straight parallel sides. The lateral borders of the cell are more or less notched, HISTOLOGY. and sometimes processes of considerable length, and more or less branched, spring from them. A single cell will generally spread across two or more bundles. When this is the case, the indentation of the space caused by the convex surface of two bundles corning together, is filled out by the substance of the cell- plate. In consequence of this circumstance, when the cell- plate is detached from its position and examined at once, a ridge extends from one parallel side of the cell to the other (see e, B, fig. 2, Plate III.). This ridge appears like a band running in the body of the cell when the latter is viewed in surface. Sometimes it may be developed into a secondary cell-plate of some width, springing from the first, thus complicating the form of the cell. These cells are known as the flat tendon-cells of Ranvier. They may very justly be re- garded as a special form of endothelium. The cell- plate is elastic, and contains an ovoid nucleus near one straight side. It consists of an intercellular network which corresponds Avith the extent of the elastic plate, and at the notched sides extends beyond the plate into the processes; and the nucleus contains an inter- nuclear reticulum. Seen edgewise, these flat tendon- cells appear spindle-form, and when seen in optical or real transverse section they present a more or less branched or stellate aspect. Stellate cells. In loose connective tissue, or areolar tissue, where the bundles of fibrils intercross in every conceivable direction, the most irregularly formed minute lymph-spaces are found. These spaces are more or less stellate, and they often contain one or more fixed stellate cells. These are the connective- tissue corpuscles, par excellence. They lie in the small lymph-cavitv, loosely attached to one of the sides of the space. The simplest form of this cell in adult tissue is that of a thin cell-plate of more or less irregu- lar outline, and containing an ovoid nucleus. The nucleus has an enveloping membrane of double con- tour, and contains an intra-nuclear, very dense net- work, the nucleus seeming to be implanted nearer to EXPLANATION OF PLATE I. Fig. 1. Peritoneal surface of the centrum tendineum of a Rabbit, silver-stained and highly magnified. (After Klein.) b, Smaller endothelial plates, situated over the straight lymphatic vessels which lie between (a) the tendon-bundles; e, true stomata, some widely open, some collapsed, they form a means of communication between the serous cavity of the peritoneum and those straight lymphatics last men- tioned, and are lined with endothelial cells of a germinating character; the dotted line starting from c represents the outline of a lymph-sinus below the surface, and in communi- cation with the before-mentioned straight lymph-vessels. Fig. 2. Highly magnified view of the spine-covered epi- thelial cells of the rete mticosum, or deep layer of the cutaneous epithelium. The section is parallel with the surface, and includes some underlying connective-tis- sue. (After Ranvier.) a, Bloodvessels with surrounding lymph-spaces ; /, 'con- nective-tissue bundles cut transversely; b, union of the epi- thelium with the tissue of the cutis ; e, polyhedral epithelial cells of the rete mucosum, containing oval nuclei and minute brilliant nucleoli. These cells are united together by means of their spines, c ; an intercellular cement fills the interspaces formed by the spines. Fig. 3. Various forms of epithelium, fresh and much mag- nified. (After Ranvier.) a, Thin, broad, epithelial scale from the inside of the cheek, showing a very small nucleus, and finely-granular contents, the imprint of adjoining cells is observable in the innermost line ; b, smaller epithelia, from the deeper epithelial layers of the bladder, and c, a larger epithelial cell from a more superficial layer ; d, an isolated spinous epithelial cell from the rete mucosum of the skin ; n, the nucleus of a ciliated columnar epithelial cell, whose vibratile cilia are shown at g ; k, the elastic striated plate from which project the cilia of a smaller ciliated columnar epithelium; p, q, the deep extremities of columnar cells. Fig. 4. Three isolated columnar epithelia from the intestine. h, The striated elastic plate which limits their free ex- tremity ; n, the oval, double-contoured nucleus of the cell. Fig. 5. A. Profile view of epithelia of the bronchus of a Rabbit, showing, between the regular ciliated columnar epi- thelium, d, the existence of branched cells, e, of a different nature, whose processes form a more or less complete network with each other, as well as a communication with the branched connective-tissue corpuscles, a. B. A surface view of the same epithelia, the letters hav- ing a similar indication. C. Shows a longitudinal section of a minute bronchus, d, Columnar epithelia. of which, c are the cilia ; v is the muscular coat, the smooth muscle-fibres being cut trans- versely ; a is a lymph-vessel of the adventitious coat of the bronchus. (After Klein.) PLATE Fig.l Fig. 2 4 EPITHELIUM. 37 one surface of the cell than to the other. This intra- nuclear network is in connection with an intra-ccllular network, which is in part imbedded in the elastic cell- plate, but it at some points extends beyond the edge of this plate to form more or less branched processes (see d, Fig. 11), which attach themselves to the adja- cent fibrous bundles, or to the processes of other branches or of other cells. Instead of presenting this simple form, secondary cell-plates may spring from the primary body and send off processes, when the connective-tissue corpuscle becomes a very complex affair. Yet it essentially consists of a thin elastic plate similar to that of the flat tendon-cell already described, and can be classed in the same category. These fixed cells of the connective-tissues are believed by some accomplished histologists to be capable of limited vital movements during health. It is undeniable that under the influence of irrita- tion they can, and often do, return to an active state, when they may put forth all the powers of the amoe- boid lymph-corpuscle. EPITHELIUM. With this subject we come to the study of a class of elements possessing an ancestry and functions totally different from those of the minute forms previously considered. The cutaneous epithelia are the representatives of the epiblast, or the upper layer of the blastoderm. On the other hand, the epithelial cells of the intestinal canal spring from the lower layer, or the hypoblast of the blastoderm of the embryo. THE EPITHELIUM OF THE SKIN. Sjiinous cells. -The cellular elements covering the skin afford several varieties of investing epithelium. In the rete mucosum the epithelial cell is soft, appa- rently granular, membraneless, and polyhedral from mutual pressure. It is constituted by a cell-body, and generally only one spherical vesicular nucleus. The cell-body consists of a network of fibrils holding in its meshes a semifluid, hyaline substance. The nucleus also is formed of a network, and an inter- fibrillar substance, and is limited by a double-con- toured membrane. There are also in the nucleus often one or more small granules (nucleoli), the remains of a hyaline, albuminoid material, from which the nucleus was originally developed. The periphery of these soft membraneless cells of the rete mucosum is furnished with small prickles or spines (see c, fig. 2, Plate L), which are frequently wanting on the peri- phery of neighboring cells. These spinous cells have been called by various names, such as the dentate-cell, the prickle-cell, and the riff-cell. Adjacent cells are effectually held together by the union of their spines, and by a viscid cement substance (intercellular cement) similar to that above mentioned for the endotheliurn. During irritation this cement substance increases in amount but lessens in viscidity. The lines of separation between the cells then become quite dis- tinct. Fig. 2, Plate I., presents a very truthful picture of the relations which these cells bear to each other. The deepest of the rete-cells, namely those in con- tact with the fibrous tissue of the cutis, are more or less columnar, or rather conical in form. The large ends abut against the polyhedral rete-cells previously mentioned, while the small ends fit accurately into indentations of the papillary layer of the derm. In the dark-skinned races, these columnar cells are more or less pigmented. Stratum yranulosum, and stratum, lucidum. The outermost layer of the rete-cells is covered with a thin lamina of flattened, intensely granular cells, forming a layer about two cells deep, the granular layer of Langerhans (see Fig. 13). Immediately external to the thin granular layer is a second, which has received the name of the stratum lucidum of Schron. This layer is of slight thickness, the cells are transparent and hyaline, and occasionally exhibit nuclei. Corneous layer. Placed upon the stratum lucidum are the most superficial cells of the cutaneous epi- thelium. They are all converted into dense, corneous plates, with no trace of their reticular or nuclear structure remaining. They form a layer of variable thickness in different locations the stratum corneum, or corneous layer. The outermost cells of the rete mucosum, or rete Malpighii, have lost their regularly polyhedral form, and have become somewhat flattened, so that in a vertical section of the skin they are seen in profile, and appear more or less spindle-shaped. The cells of the granular layer are still more flat. Those of the stratum lucidum have been flattened into mere scales, which, seen in profile, look like interrupted lines. In the cutaneous epithelium we have, there- fore, five varieties of epithelial cells, named from below upwards as follows: the columnar, granular, spinous, lucida, and corneus. Among the soft, mem- braneless, polyhedral cells of the rete mucosum are, according to some late investigators, a few spindle- form, or stellate nucleated cells, whose branching pro- cesses run in the intercellular cement-substance, and 38 HISTOLOGY. ~0j0le columnar epithelium. Simple columnar epithelium. The columnar epithelial cell, when it presents one of its ends upon a free sur- face, usually is limited at that end by a thin plate of considerable stiffness, which seen edge-wise, as when the cell is viewed in profile, appears like a brilliant band. In the simple columnar cells of the intestinal canal, under favorable conditions, this brilliant band scums to be vertically striated (see Fig. 15). By violence the thin limiting plate is often partially or completely detached from the end of the cell. Frequently the cell possesses a very delicate limiting mem- brane, which, however, is probably only a con- densed film of the body of the cell. The colum- nar cell is somewhat soft and pliable, and may, therefore, readily assume shapes imposed upon it by pressure. It is rarely absolutely co- lumnar; it is frequently more nearly conical. Often its deep end divides into two or more short thick branches (see p, fig. 3, Plate I.). It is provided with Fig. 15. SECTION OF A VILLCTS OF A RABBIT, fur- nishing examples of non-ciliated colum- nar epithelium. High power. (Strieker.) a nucleus which is oval or more or less rod-shaped possesses a thin membrane of double contour, and generally one or more distinct nucleoli. The nucleus is usually located near the deep extremity of the cell. The body of the cell is composed of a network of fibrils, whose meshes have a decided linear shape, mainly parallel with a long axis of the cell. The nucleus also contains a network which, like the nuclei of other cells, is denser than that of the cell-body. The finely granular appearance of the cells is due to the existence of this reticulum. The semifluid mate- rial contained in the meshes of the reticulum may vary in amount, and cause the interfibrillar spaces to increase more or less in extent, thus increasing or lessening thereby the granular aspect of the whole or a part of the cell. Ciliated columnar epithelium. The ciliated colum- nar epithelium is essentially similar in structure to the simple columnar cell. It differs from the latter only by the presence of a number of fine cilia or hairs vertically attached to the thin plate at the free ex- tremity of the cell (see g, fig. 3, Plate I.). These cilia pass through the thin limiting plate and are directly attached to the fibres of the intra-cellular reticulum (see a, Fig. 11). During the life and the activity of the cell the cilia are in more or less vigor- ous vibratile motion, and those agents which excite or retard movements in the amoeboid corpuscles have the same action upon the movements of the cilia! In stratified ciliated columnar epithelial coverings, the ciliated columnar cells constitute the superficial layer. They are usually present in a single row. Be- tween and below the deep extremities of these ciliated cells are a greater or lesser number of spindle-form and more or less irregular polyhedral epithelial cells. These are usually membraneless, and contain one or more spherical nuclei, the minute structure of the cell being reticular. Scattered here and there among the columnar and other cells are a few soft granular membraneless bodies, sometimes fusiform, sometimes branched, apparently similar to the analogous cells mentioned under the cutaneous epithelium (see e, fig. 5, Plate I.). Goblet-cells. Some of the columnar cells, whether ciliated or not, may become distended and distorted with a collection of mucus near the free end. Such cells are known as goblet-cells (see e, Fig. 11). The nucleus is generally pressed aside and crowded into the deep end of the cell. If the accumulation of the mucus continue, the thin limiting plate is broken or detached, an occurrence which results in the dis- charge of mucous drops upon the free surface of the 40 HISTOLOGY. epithelium, and the probable ultimate destruction and desquamation of the cell. During the progress of gastric digestion, these goblet-shaped columnar cells are to be found in great numbers upon the mucous coat of the stomach. The epithelial cells of the mucous membranes, like those of the skin, are slightly separated, yet closely and firmly held together by an intercellular cement. According to the investigations of some authors (Klein among others), foreign particles placed upon the surfaces of mucous membranes are absorbed mainly, if not exclusively, by means of the intercel- lular cement-substance, which, at the surface of the fibrous tissue, is in direct communication with the superficial lymph-spaces of the connective -tissue. Even in the intestines the minute particles of the chyle, according to Klein, are not taken up by the columnar cells, and thence passed into the radicles of the lacteals, but they enter the intercellular cement, which reaches the free surface, and pass along this until they reach the lacteal capillaries. GLANDULAR EPITHELIUM. Under this caption we consider cells of widely- varying function and location. As a rule, they have an embryonal derivation similar to that of the invest- ing epithelium, upon which the ducts of their glands empty. In fact, they commonly begin to develop at an early period of intra-uterine life by epithelial buds, which project from the deepest layers of this investing epithelium into the subepithelial connective- tissue. Glands of mucous membranes. The epithelial glands, which open upon the surface of mucous membranes, seem to be constructed upon two general plans, if we are to judge from the nature of their lining epithelium. The various forms of these glands seern to be only modifications of two models. 1. Simple tubular mucous ijlands. The simplest kind of epithelial gland is, perhaps, the crypt or follicle of Lieberkuhn. It usually presents the form of a small tube, closed at the deep extremity by a caecal end, and opening at the other upon the surface of the intestinal canal. The cells, lining the tube in a single layer,, are apparently identical in structure and form to the columnar epithelium of the surface. They are short, cubical, or columnar cells, which possess a spherical or oval nucleus, with a thin, double-contoured limiting membrane. The gland- tube is usually single, but sometimes may be branched in its deep extremity. The deeper por- tion of the gland is frequently slightly larger in diameter than is the upper portion or neck of the gland. In the wider portion the cells are longer. The columnar cells consist of a network, which is denser in the nucleus than in the cell-body. The fibres of the reticulum have a general direction parallel with the long axis of the columnar cell, namely, perpendicular to the wall of the tube. They are separated from the latter by a thin, flat, sometimes branched, endothelial cell, and separated from their neighbor-cells by a small amount of cement-substance, into which sometimes branches of the endothelial cells above mentioned may project. The free end of the columnar cell may contain a drop of mucus, when it constitutes what has already been described as the goblet-cell. The chief, if not, indeed, the only function of these cells is the secretion of mucus, and the gland itself may be regarded as the simplest form of a mucous gland. The cells in the quiescent stiite are smaller, and apparently more granular than when the cell is actively forming and excreting mucus. At no time, however, during health, is the cell coarsely or densely granular. It is to be remarked also that the longitudinal striation of the cell an appearance in itself due to the prevalent longitudi- nal direction of the meshes of the intra-cellular and intra-nuclear reticula is somewhat more marked in the neck, and at the orifice of the gland, than in its fundus. Besides the foregoing characteristics, these purely mucous tubular glands possess a distinct lumen throughout their length. 2. Compound tubular mucous glands. In the mucous glands of the mouth we have an example of a com- pound tubular gland, whose chief function is the ex- cretion of mucus. Here we have not only a modifi- cation of the form of the simple tubular gland, but there is also a complication of the lining of the tubes. Some of the largest of these glands may be taken as a type for description. Each gland consists of a large duct, funnel-shaped at its mouth. The duct passes more or less obliquely through the mucosa. Reaching the submucous tissue it divides into a number of smaller tubular branches, each of which soon slightly enlarges to form an infundibulurn. Very soon the infundi- bulum again narrows, and passes into the secreting portion of the tube, which, after turning irregularly in numerous convolutions, ends in a csecal extremity. In man, the funnel-shaped mouth of the duct has a lining of stratified pavement epithelium. At a slight depth, however, the epithelial lining of the duct is seen to consist of a single layer of long, narrow columnar cells, with intra-cellular and intra-nuclear EPITHELIUM. 41 networks, producing a distinct longitudinal striation, and a slightly granular appearance of the cell. The lumen of the duct has considerable width. In the narrow tubes, into which the duct branches, the epi- thelium is still in a single layer only, but the cells are now much shorter and wider. They may be cubical, or even more flattened. The cells still con- tain networks, but these are neither dense nor arranged so as to produce an apparent striation. In the infundibulum the lining epithelium still pre- serves this more or less flattened form. The lumen of the infundibulum is consequently comparatively wide. As the deep portion of the infundibulum is reached it again slightly narrows to pass into the convoluted tubes. At the same time the character of the lining epithelium again changes. Their cells now become columnar and very slightly granular. They contain a round or oval double-contoured nucleus, which is located near the outer end of the cell, both cell-body and nucleus consisting of a network of fibrils forming comparatively large meshes when the gland is fully developed. When the latter is active, these cells con- tain drops of mucin in their inner portion, and present every characteristic of goblet-cells. When exhausted, they shorten very considerably and become very granular, and consequently somewhat opaque. The lumen of the convoluted tube is characteristically large. The epithelia of these glands rest upon a layer of endothclial or connective-tissue cells which are more or less branched, some of the branches penetrat- ing as septa between the intercellular cement which holds the cells together. Sometimes in the convoluted portion of the glands two, three, or more embryonal cells are found at rare intervals along the course of the tube massed together in thin clumps, and they are always located between the previously described columnar epithelium and the layer of endothelial cells which form the basement- membrane of the tubes.. The sublingual gland of man and the submaxillary gland of the dog are mucous glands still larger than those last considered, but they are constructed upon the same general plan with, however, an epithelial lining a little more complicated. In the convoluted portion of the tubes, protoplasmic collections between the mucus-cells and the basement-membrane of stel- late endothelia, somewhat similar to the embryonal cells mentioned as occasionally present in the last- described mucous glands of the mouth, are in these glands very numerous. These protoplasmic masses are more or less crescentic, and have received the name of the crescents of Gianuzzi, after the anatomist G who first accurately described them. They are densely granular uni- or poly-nucleated masses without a de- fined membrane, but with projections which fit into the spaces between the epithelial cells with which Fig. 16. a, StTBMAXIl,l,ARY GLAND op Don. a. Mucns-colls. b. Protoplasm cells, a. Crescents of Gianuzzi. d. Traunverse section of excretory duct with its pecu- liar columnar cells. High power. (Strieker.) they are in contact. In other respects the contents of these compound tubular glands are apparently identical with those of the smaller mucous glands of the mouth. 3. Compound tubular salivary glands. The parotid gland of man is built upon the same plan as is the sublingual so far as the arrangement of the tubes is concerned, but its function is quite different, and so also are the characters of the lining epithelia of the convoluted secreting tubes. The epithelial cells of this portion of the tubes are more or less cubical, and are densely granular and proportionately opaque. They contain a round nucleus located near the base- ment-membrane, and limited by a thin envelope of double contour. The nucleus arid cell-body consist of a dense network of fibrils with irregular and small meshes, containing, in the quiescent state, a minimum of fluid substance. During activity, this fluid in- creases very considerably, and distends the meshes, thus causing the cell to become considerably larger arid more transparent. Crescents of Gianuzzi are found in these tubes also, but in fewer numbers than in the purely mucous glands. Another distinction between these purely salivary glands and the purely mucous glands is in the diameter and the size of the lumen of the convoluted tube. In the mucous gland the lumen is distinctly recognizable, of considerable size, and is generally patulous, while, on the contrary, in the purely salivary glands it is often doubtful if the 42 HISTOLOGY. convoluted tube has a real lumen. It is at all times so small and generally so plugged with a substance much like the intercellular cement, that some authors have denied the existence of a true lumen in this portion of the tube, and have claimed that the small intercellular spaces act the part of a duct for the secretions. In the submaxillary gland of man we have presented a large compound tubular gland still more compli- cated than either the large purely salivary, as the parotid, or the large purely mucous, like the sublin- gual, for in this gland the functions and the structure of the two latter seem to be united in one. It is a compound tubular gland of mixed function the secretion of a salivary fluid and the excretion of mucus, and it combines more or less intimately and perfectly the anatomy of each. Some of the lobules of the gland are composed of convoluted tubules, which possess an entirely salivary character, while others present a purely mucous structure and func- tion. Further, even in the same lobule, some of the convoluted tubes may possess the one character, while others represent the other variety. 4. Gastric glands. In the stomach are two varieties of compound tubular glands, which possess an epithe- lial lining presenting peculiarities somewhat different from the preceding. In looking at the mucous surface of the stomach with a good magnifying hand-lens, immense numbers of minute holes can be seen. They are generally collected together in groups of three to five or more. These small holes are orifices of the ducts of glands occupying the thickness of the mucosa. They have been called peptic (/lands. They are constructed upon the model of the compound tubular gland. They consist of a common duct with a wide lumen. The surface of the mucous membrane of the stomach is covered by a single layer of columnar epithelia. During digestion many of these cells elaborate and discharge mucus, and in doing so assume the form of goblet-cells. Columnar cells, identical in appearance with those of the mucous surface, line in a single layer the walls of these ducts. Near the middle of the thickness of the rete mucosa the ducts suddenly narrow and divide into two or more smaller branches, which immediately become constricted, to form the nock or intermediary portion of the secreting tube. The neck of the secreting tube is lined by a continuation of the epithelium in the duct, but the cells are much shorter. Immediately outside of the latter, and between them and the basement-membrane, are scattered here and there single cells of a very granular aspect containing a round or oval nucleus, both cell and nucleus being more or less flattened. They consist of an intra-cellu- lar and an intra-nuclear network, and have been called by some authors peptic cells (Figs. 17, 18). They do Fig. 17. PEPTIC GASTRIC GLANDS. a. Com- mon duct. 6, b. Its chief branches. PORTION OF ONE OF THE C^CJE OF A PEPTIC GASTRIC GI.AND MOKE HIGHLY MAGNIFIED: seen longitudinally at A ; transversely at B. a. Basement-mem- brane. &. Large glandular or peptic c. Terminal Cffica with spheroidal cell. c. Small epithelial cells sur- gland or peptic cells. (Carpenter.) rounding the lumen. (Carpenter.) not constitute a continuous layer. They are more closely aggregated in the neck and upper portion of the secreting tube than towards the fundus, although they are also present in the deepest end of the tube. The secreting tubes into which the large duct divides are somewhat wavy in their course. They are often quite curved at their csecal extremities. After the constriction of the neck they gradually increase in diameter until the deep end of the tube is reached. As the tube widens the lining epithelium lengthens, still, however, leaving a distinct lumen for the passage of the mucous secretion. The cells of the fundus contain a spherical nucleus in the outer end of the cell, and an intra-cellular and intra-nuclcar network whose meshes do not, however, present a linear ar- rangement such as is seen in the cells which line the neck of the secreting tubes and the common duct. Between the epithelia upon the surface of the gastric mucous membrane is a scant reticulurn of branched connective-tissue or endothelial cells. Similar cell$, and their processes also penetrate the intercellular cement of the whole epithelial lining of the peptic glands. EPITHELIUM. 43 Fig. 19. ISOLATED HEPATIC CELLS. a, &, Normal, but b more highly mag- nified, showing the nucleus and distinct oil-particles ; c, cells in various stages of fatty degeneration. (Carpenter.) Liver- and kidney -cells. The gland cells of the liver possess one, sometimes two spherical vesicular nuclei with an investing membrane of double contour and an intra-cellular and intra- nuclear network. The net- work appears to be ar- ranged somewhat as a honeycomb. In the spaces of this honeycomb are drops of fat, with biliary granules, and occasionally pigment. The cells are polyhedral from mutual pressure, their outlines generally presenting five or six sides. They are soft, rnembraneless, and ap- parently granular. The cells lining the tubules of the kidneys pre- sent diverse aspects accord- ing to the relative portion of the tubules in which they are located. Those in the convoluted por- tions of the tubes are more or less granular and opaque. Here they are mernbraneless, and each con- tains a round or slightly oval nucleus a little towards the deep part of the cell. The cells are more or less cubical in form with a tendency to a columnar shape. They are longitudinally striated an appearance due to the presence of large numbers of minute rod-like fibres with a general direction perpendicular to the axis of the tube. These minute rods or fibres are in truth united into a network, the meshes of which are extremely long and narrow. The nucleus also contains a network. Glands of the skin. 1. Sweat-glands. The sudori- parous glands of the cutaneous surface are simple tubes which at their deep portions are very much convo- luted. In the straight portion of each tube, as it passes through the derma, there is an epithelial lining con- sisting of a single layer of more or less cubical or short columnar cells. These cells contain a nucleus, and are more or less longitudinally striated an appearance due to the presence of an intra-cellular and intra- nuclear network, the prevalent direction of whose fibrillae is longitudinal. In the convoluted portion the epithelial cells are much less striated, because the meshes of the reticulum of which they are in part composed are much more irregularly arranged. 2. Sebaceous (/lands. The sebaceous glands consist of two or more lobules attached to an excretory duct. The lobules are as a rule regular, ampullar dilata- tions, lined with two or more layers of epithelial cells. The peripheral layer of cells rests upon the basement- membrane, and consists of cubical epithelia having spherical or flattened nuclei containing one or more nucleoli. These cells are more or less granular an appearance due in part to the presence of a fine reti- culum in the cell-body and nucleus, and also in part to the presence in the meshes of this reticulum of small fatty particles. The second layer of elements is composed in the main of cells similar to those of the outermost layer, which have undergone a metamor- phosis into fat- vesicles. The molecules of fat, which are at first scattered through the meshes of the reti- culum, accumulate so rapidly that thev distend its meshes inordinately, and finally cause the atrophy and destruction of the nucleus and the breaking- down of the fibres of the reticulum. The small fatty particles then run together and form a large fat-drop, which ultimately bursts its protoplasmic envelope and becomes free in the lumen of the gland. Hair and hair-follicles. Epithelium enters into the construction of the shaft and the sheath of hair structures met with very extensively on the surface of the body. The hair-follicle begins its development in the skin as early as the third or fourth month of the existence of the foetus. The first stages of de- velopment are exactly similar to those of the sweat- gland. Both these organs arise by an ingrowth of the cutaneous epithelium, in the shape of buds, which rapidly extend in length, burrowing their way into the depth of the connnective-tissue; for some time after development has begun, it is impossible to say whether a certain epithelial bud, growing into the connective-tissue, will ultimately form a sweat-gland or a hair-follicle. After the epithelial bud has thus entered some distance, the mass presents the first characteristic indication of the formation of a hair, by a more or less definite appearance of two layers: viz., an axial and a peripheral layer, or the true hair and its investing sheath. By further evolution these two portions, into which the mass of cells filling the depression in the derma has separated, subsequently develop, the axial portion into the shaft, the peripheral portion into the sheath of the hair. After this stage of growth has been reached, the space thus channelled out of the connective- tissue, and filled with the sheath and shaft of the hair, receives definitely the name of hair-follicle. The adult hair follicle normally contains epithelial elements very similar to those of the com- mon epidermis, but presenting some characteristics of form and arrangement which are peculiar to the hairs. Since the hair-follicle, sheath, and shaft are formed by 44 HISTOLOGY. an infolding of cutaneous surface, the arrangement of the various elements is essentially the same as upon the surface of the skin. The sheath of the hair is composed of an inner and an outer portion. The outer portion is a continuation of the rete inucosum of the skin, and consists of epithelial cells identical in struc- ture and arrangement with those of the normal rete mucosurn (b, fig. 1, Plate II.). Interspersed among these epithelia is a small number of branched proto- plasmic cells, in every way similar to those upon the surface of the derma. Internal to the cells correspond- ing to the rete mucosum of the skin is the inner sheath (c, fig. 2, Plate II.). It is composed of two strata of transparent cells, the innermost stratum containing elements with a visible nucleus. Both cell and nucleus are somewhat ovoid in shape, while the cells of the outermost stratum are destitute of nuclei. The latter are, perhaps, a continuation of the cells of the stratum granulosum of the surface, although they contain no granules. Within the innermost sheath, is the axial shaft or hair proper, itself covered with a very thin cuticle (e, fig. 2, Plate II.). The cuticle consists of a single imbricated layer of thin epithelial scales without visible nuclei (c, fig. 1, Plate II.). The hair-shaft comprises two portions when seen trans- versely, viz., a cortical cylinder and a medullary axis. With its investing cuticle, the shaft, except in and near the hair-bulb at the root of the hair, consists entirely of extremely thin corneous epithelial scales without a vestige of nuclei. They are so densely packed together, that it is impossible, even after mechanical dissociation, to distinguish the outlines of the cells. After the action of sulphuric or nitric acid upon them, the intercellular cement-substance is dis- solved, when the thin scales can be readily separated by means of needles, and their characters demon- strated (fig. <, Plate II.). The corneous cells of the shaft represent, therefore, the stratum corneum ot the epidermis. Some of the minute interstices between the corneous scales may contain particles of fatty matter or pigment-granules. The greater or lesser abundance of the latter determine the light or dark color of the hair. Some of these minute capillary spaces may contain air. When many of them are filled with air, and at the same time the pigment par- ticles, previously mentioned, are present only in small numbers or are entirely absent, the hair presents a glistening silvery 'aspect. The medullary or axial portion of the shaft consists of an irregular aggrega- tion of shrunken epithelia of an original form more or less polygonal. These shrunken angular cells some- times contain pigment-granules, sometimes minute vacuoles filled with air, and the same gaseous sub- stance generally fills the interstices between the shrunken cells. The medullary axis of the hair-shaft, no matter ^vhat the color of the cortical cylinder may be, shines through the latter, presenting a lighter and more glistening appearance. The foregoing characters of the hair are commonly observed in those portions above the bulb or root of adult hair. As the bottom of the follicle is reached, the distinctions between the inner and outer sheath become lost in the general embryonal character of the epithelium. The hair-shaft is now also changed both in shape and in the character of the constituent ele- ments. It expands into a bulb-shaped extremity a little above the bottom of the follicle. The cuticle covering this lulb is composed of slightly granular epi- thelia containing distinct nuclei. At the bottom of the follicle these cells are more or less spherical, but they flatten and their size and the distinctness of their nuclei lessen gradually from below upwards, as can be seen by reference to the figure already indicated, until, above the bulb, the nucleus is lost entirely, and the cell is converted into the thin transparent cuticu- lar plates or scales already mentioned. The two portions of the hair-shaft also present in the root an aspect entirely different from that of the outermost end. For a slight distance above the bulb the cells of the cortical cylinder are less corneous and they contain a trace of a nucleus. As the bulb is approached, the epithelia of this cylinder gradually increase in thickness, while their nuclei also become larger and more defined. In the bulb itself, the epi- thelial cells present the characteristics of the most superficial cells of the rete mucosum. A vascular connective-tissue papilla springs from the bottom of the hair-follicle and penetrates the centre of the hair-bulb. On the sides, it is ensheathed by the cellular accumulation which is continuous above with the corneous cortical cylinder of the hair- shaft. Above this papilla is a mass of epithelial cells, similar to those of the cortical part of the bulb, which is continuous with the shrunken irregular corneous cells of the medulla or axis of the shaft. These medullary cells of the bulb pass by slow gradations from below upwards into the irregular corneous cells already described as existing in the axis of the hair, but they still present distinct nuclei at a level much higher than that of the bulb. The papilla seems to be the main source of the vitality and growth of the hair. Destruction and regeneration of epithelium. The various investing epithelia, of the cutaneous surfaces THE CONNECTIVE-TISSUE SYSTEM. 45 and of the mucous surfaces as well, seem to undergo a constant destruction and regeneration. Desquarna- tion is constant upon the external surfaces, and it seems also to be a common although less rapidly occurring phenomenon upon the mucous surfaces. This constant loss of cells must be fully compen- sated by an active new formation of elements. It is generally conceded that the new supply of cells must come from the layer corresponding to the rete muco- suin in the stratified epithelial coverings. Whether these new cells are produced by a proliferation of the soft finely granular polyhedral epithelial cells of the rete mucosum, or whether they arise from the wandering cells which are always present in greater or lesser numbers in the epithelial coverings, or whether they are in part derived in both ways, in- vestigators have not entirely determined. Many up- hold one of ih.3 extreme opinions, while some defend the middle ground. It is quite certain that under the influence of irri- tation not only do the rete cells proliferate, but even those which have begun to approach the corneous condition return to the embryonal state, or exhibit other evidences of an awakened formative power. Under such a stimulus, some of the cells may contain an endogenous progeny. Such a condition is fre- quently met with in and around tumors, and upon epithelial surfaces secreting pus. The regeneration upon granulating surfaces of epi- thelium, or its new formation over abrasions, seems in some way to be more or less closely related to the action of previously existing epithelial cells. The newly formed epithelium is almost always connected directly with the old epithelium at the edges of the wound. If a large abraded surface be sprinkled freely with epithelial scales, isolated islands of newly formed epithelia, which exhibit no direct connection with the epithelium at the edges, may after some time make their appearance. Skin-yraftinrj. One means of healing extensive granulating surfaces is, by the employment of skin- grafts. This method is in some respects essentially identical with that of dusting the surface with epithe- lial scales. In both instances, the presence of epithe- lium seems to affect the granulation-cells in a peculiar manner, and cause the latter to develop into epithelia. There seems to be an infection of the granular cells by the epithelia, or, so to speak, an "action of presence" of the epithelial scales, which is shown in the tendency of the embryonal cells acted upon to form epithelium rather than connective-tissue. THE CONNECTIVE-TISSUE SYSTEM. Having reviewed some of the numerous cellular elements which in part constitute the human organ- ism, we now enter upon the consideration of the histological relation which they bear to other consti- tuents. We have already studied the lymph and the blood tissues which from their derivation may be classed as congeners of connective-tissue. The connective-tissue, in one form or another, be- sides special offices which it may perform in the human economy, acts as the framework upon which and within which the various elements and organs are supported. Upon it rests the investing epithe- lium. In it are imbedded the glands, the muscles, the vessels, and the nerves. The great connective-tissue system comprises the following groups of tissue, which will be examined in the order in which they are here enumerated : Mucous- Tissue; Varieties of Connective-Tissue, properly so called Cartilage, Bone, Dentine, and Cementurn; Muscle, Nerve, and the Connective-Tissue exhibited in the Bloodvessels and Lymphatics. MUCOUS- OR GELATINOUS-TISSUE. In the human adult this form of tissue is of ex- tremely limited distribution. It is found in the vitreous humor of the eye, and perhaps in the enamel- organ of the teeth. In the embryo it is very exten- sive. It presents two general varieties, which are distinguished by the character of the cellular ele- ments, and the intercellular substance which sur- rounds them. The simplest form of mucous- or gelatinous-tissue is that represented by the vitreous humor of the eye. It consists of a transparent, colorless, gelatinous semi- fluid substance, which contains a more or less con- siderable quantity of mucin, or of a substance which has a very similar reaction. It is precipitated by weak acids in the form of minute granules, which then give to this semifluid ground-substance under the microscope a finely granular aspect and a slight opacity when viewed by the naked eye. Imbedded in this gelatinous semifluid ground-substance, cellular elements are present in more or less considerable numbers, according to the age of the subject and the quiescent state of the tissue. In the adult they are scattered at rare intervals. These cells are lymphoid elements of more or less spherical or oval outline. HISTOLOGY. They are capable of limited movements, which, however, are perhaps not usually sufficient for loco- motion, although some of them are doubtless wander- ing cells. They have no limiting membrane; pos- sess one, sometimes two or more nuclei, and consist of an intra-cellular and an intra-nuolear network, which give to the element a more or less granular aspect. The ground-substance seems to be void of bloodvessels in the adult. Another variety of mucous-tissue, a type of which is met with in the Whartonian jelly of the umbilical cord, has a more complicated structure, and is a grade higher in the scale of development. In a gelatinous mucin-containing ground-substance similar to that of the simplest variety, is to be found a network, or rather felt-work of soft, delicate, slightly refracting fibres. These are collected into bundles or bands, sometimes of considerable width. Upon and near these bundles, cells more or less stellate, and in other respects similar to the flat-branched cells of loose connective-tissue which have already been particu- larly described, are to be seen, these branches form- ing by their communications a more or less complete network of stellate cells. Some of the bundles con- tain, in their interior, bloodvessels with distinct and somewhat thick walls. The inter-fibrillar areas are also sometimes permeated by capillary bloodvessels with large loose meshes, and with strong, distinct walls. These capillary vessels are ensheathed by a network of branched cells. The intcrfibrillar spaces also contain, in small numbers, the lymphoid cells common to the simplest variety. Sometimes net- works, composed of fusiform or of branched connec- tive-tissue cells (fig. 5, Plate II.), are seen occupying the inter-fibrillar spaces. In some pathological for- mations of mucous-tissue there is, in addition to the foregoing structure, a sparse network of fine single elastic fibres. WHITE FIBROUS-TISSUE. Fibrous bundles. White fibrous connective-tissue is most extensive in its distribution. It consists of extremely fine fibrils collected together into bundles varying widely in thickness and form. These bundles of fibrils may be cylindrical or band like, the s'des of the cylinders or bands being usually parallel. The bundles may be branched, but the individual fibres never are so. The course of the bundles may be straight, or more or less wavy, according to whether they are tense or loose. The minute fibrils which constitute the bundles are united and held together by a transparent viscid cement, which may be dis- solved by weak acids, lime-water, baryta water, 10 per cent, solution of salt, and by other means known to the histologist. In some locations, the bundles are EXPLANATION OF PLATE II. Fig. 1. A profile view of a Human hair-follicle, containing a hair under a high power. (After Frey.) a, The fibrous sheath ; b, the external root sheath ; c, the cuticle of the hair-shaft ; p, the papilla of the hair ; h, e, the medullary axis of the hair-shaft ; h, the cortical cylinder of the hair-shaft ; g, the transition of the cor- neous epithelial scales of the cortical cylinder of the hair- shaft above, with the soft, nucleated, and membrane- less epithelium of the bulbar portion of the hair ; e, point of transition of the soft nucleated epithelium of the bulbar portion of the medulla of the hair-shaft into the shrunkened, deformed, and dry corneous granules which fill the medulla of the upper portion of the shaft. Fig. 2. Transverse section of half of a Human hair, with its root sheaths, still higher power. (After Frey.) h, Hair-shaft ; e, hair-cuticle ; c, two layers of the internal root-sheath ; b, external root-sheath, showing an outer layer of columnar cells ; /, basement membrane ; a, the external fibrous sheath. Fig. 3. a, b, c, Stellate mucous-tissue cells, whose processes form a more or less complex network. (Moderate en- largement.) Fig. 4. Represents isolated cells of various parts of the hair. Highly magnified. (After Frey.) e, Nucleated epithelial cells of the bulbar portion of the hair ; b, h, cells from the cuticle of the hair ; h', corneous epithelial cells of the cortical portion of the shaft, treated with sulphuric acid, the same resolved into separate plates at A." Fig. 5. Mucous tissue (gelatinous tissue) from the umbilical cord of a Lamb, very highly magnified. (After Kan- vie r.) c, Branched cells ; n, embryonal cells, or lymph corpuscles ; F, connective fibres; B, intercellular amorphous fluid-sub- stance, containing mucin. PLATE Fig.S. Fig. 4. THE CONNECTIVE. TISSUE SYSTEM. 47 more or less completely enveloped in a thin elastic sheath. Such ensheathed bundles are to be found in the subcutaneous connective-tissue, in the subarach- noid of the brain, in aponeuroses, in tendons, and in some other locations. Structure and arrangement of fibrous bundles. The bundles of fibrils present a distinct longitudinal fibrillation when seen under a sufficient magnifying power. When they are loose and wavy, they also appear to have an indistinct, transverse striation. Tins is an optical illusion caused by short and very frequent waves in the course of the fibrils. This wavy appearance of the fibrils, and consequent appa- rent transverse striation, may be seen sometimes even when the sides of the bundle seem to be perfectly straight. Water and weak acids cause the interfibrillar ce- ment to swell, and give the bundle a hyaline or homogeneous aspect. These agents have no such effect upon the elastic sheath which envelops some of the bundles. Ensheathed bundles appear swollen in some places, and constricted in others, thus present- ing a beaded outline. The bundles of fibrous tissue may be variously ar- ranged with respect to each other. The simplest arrangement is that of a collection into secondary bundles or bands, which run parallel with each other and are spread out, side by side, to form lamellar membranes, as in aponeuroses, or are collected to- gether to form rounded cords, as in tendons. They may, on the other hand, cross each other in various directions, forming a loose felt-work with wide meshes, as in the loose connective- tissues ; for example, the sub- cutaneous- or mucous-tissue, the inter-muscular con- nective-tissue, the loose interstitial connective-tissue of glands, etc. Or they may be closely packed together to form a very dense felt-work, such as the cutis, the dura mater, etc. On the other hand, the bundles may form a loose network spread out in the form of a membrane with more or less wide meshes, like the mesentery, the ligamentum dentatum of the spinal cord, etc. Interfibrous spaces, and their cellular contents. The spaces left between the fibrous bundles are conse- quently of forms varying according to the direction of the bundles. They are filled with lymph, and con- stitute the radicals of the great lymphatic system. In tendons, the spaces left between the bundles are more or less linear when viewed longitudinally, and when seen in transverse section appear more or less stellate. When speaking of cndothelia and connective-tissue corpuscles, we described the flat elastic cells which form a partial lining of the lymph-space formed between adjacent parallel bundles of tendons, and stated their relation to the bundles of fibrils upon which they were more or less closely applied. It was then stated that these cells (the flat tendon-plates of Eanvier) spread across two or more bundles. Fig. 2, Plate III., shows very beautifully this peculiar arrangement of the tendon-cells. The drawing re- presents a number of (primary) bundles united to- gether into a (secondary) larger bundle forming a small tendinous cord in the tail of a young rat. In larger tendons we have a number of such secondary bundles collected together to form the tendon. Ten- dons of this kind have their external surface covered by a complete layer of large flat endothelial plates such as line serous cavities, which rest upon an ex- tremely thin elastic apparently structureless mem- brane. Such a tendon, after proper treatment by ni- trate of silver, when placed under the microscope and seen longitudinally, shows this superficial covering of endothelial cells. By lowering the focus the flat tendon-cells of Eanvier next come into view. The lat- ter if seen in surface present the appearances already described Apropos of the cells of the connective-tissue. If their position upon the secondary bundles is such that they are seen in profile, they then appear as lines or spindles with slightly projecting bellies correspond- ing to the location of the nucleus. When a tendon composed of secondary bundles is cut transversely, and properly stained, it presents a number of more or less markedly stellate bodies (Fig. 20) connected the Fig. 20. TRANSVERSE SECTION op TENDON: showing so-called branched corpuscles, inclosing spaces which, loft bluuk, are naturally filled with tendinous fasci- culi, nigh power. (Carpenter.) one with the other by anastomosing branches (C, fig. 2, Plate III.). These bodies were formerly regarded as stellate connective corpuscles. They are, in normal tendon, generally nothing more than the cross cuts of the more or less flat tendon plates of Ranvier. These stellate forms may consist of two or more of such plates in apposition. Sometimes even one or more lymphoid 43 HISTOLOGY. cells may be mingled with them. They may thus be formed of aggregations of cells, in which case several nuclei may be visible within them. The anastomosing branches appear to be sections of the primary or secondary cell-plates and their lateral ramifications. The clear spaces constituting the meshes formed by the previously mentioned anastomoses correspond to transverse sections of bundles of fibrils which have not taken the staining. Besides the fixed cells above described, leucocytes or wandering cells are found in small numbers in the lymph spaces of the connective- tissue. Elastic fibres. When tendons are thoroughly boiled the white fibrous tissue which constitutes them is dis- solved, and the other elements previously mentioned disappear. The more complete this effect of boiling, the more visible becomes a network formed of single highly refracting elastic fibres. These elastic fibres are very small in diameter, are more or less perfectly cylindrical, are branched at intervals, and are appa- rently homogeneous. Their branches unite to form a network, whose meshes are long and narrow, the long diameter of the meshes being parallel with the axis of the tendon bundles which they accompany. These elastic fibres are in contact with the sheath which envelops the pri- mary bundles more or less completely. They are so difficult to be seen that unless they are specially pre- pared they are generally invisible. In aponeuroses, where the fibrous bundles are arranged so as to form thin lamellae of considerable width, the fibres of a given lamella run parallel with each other, and are crossed at an angle by those of the lamella next above or below. In many such lamellar fibrous-tissues, the fibres in adjoining lamellae cross at right angles. The tendon-cells are then found to be much more irregularly formed than in the simplest variety of tendon. In the adult these cells are not so frequently arranged in rows as in the simple tendons, and the secondary plates which spring from the body of the cell may then have a direction corresponding to that of the different fibrous bundles with which they are in contact. Loose connective-tissue. In the loose connective-tis- sue, for example the subcutaneous cellular-tissue so called, the bundles of fibrils run in every conceivable direction, sometimes branching and forming frequent anastomoses. The spaces left between the fibrous bundles are generally large, and they freely inter- communicate. Fluid or air injected into them readily passes from one to another. After having been dis- tended by air and dried, they present the appearance to the naked eye of large cells or vesicles, hence the name of cellular- tissue. Some of the bundles of fibres are ensheathed as already mentioned, many of them are not. The fixed cells of the connective-tissue are not very numerous in adult tissue of this kind. They are loosely applied to the surface of one or more bundles, frequently at their points of crossing (see fig. 3, PI. IX). The wandering lymphoid cells are much more numerous in this form of connective-tissue than in the tendons or aponeuroses. Yellow elastic fibres are also to be met with. Here they are frequently much larger than in tendon, but they bear much the same relations to the fibrous bundles in the one case as in the other. The larger elastic fibres have been found by some investigators to be enveloped by a thin, dense membrane, which in most locations is extremely difficult to demonstrate. In the loose subarachnoid connective-tissues of the brain, and in some other locations, this investing sheath is quite distinct. This variety of fibrous tissue may exist in the form of a thin, fenestrated membrane. Some portions of the omentum furnish an example of such, the fenestrag in this case representing the dilated lymph-spaces exist- ing between the bundles of fibrous tissue. It is thought by some authors (Axel Key and Retzius) that many of the loose connective-tissues are composed of scant felt-works of fibrous bundles spread out in many superimposed Iamel]a3, which are more or less sepa- rated from each other by a single layer of flat endo- thelial cells resting upon one side of an extremely thin, structureless elastic membrane. Dense connective-tissue. The dense fults of -white fibrous tissue, as, for example, those of the true skin, of the dura mater, etc., are essentially identical in structure to the loose felts of the subcutaneous con- nective-tissue, of the submucous-tissue, and of the interstitial connective-tissue of muscles, and of glands. The fibrous bundles being now much more closely packed together, the lymph-spaces are consequently smaller. The cellular elements are also present in much smaller numbers. Bloodvessels. The bloodvessels supplying this form of connective-tissue have much the same character as in the fibrous variety of mucous or gelatinous- tissue already described. There are two other varieties of white fibrous con- nective-tissue, namely, the neuroglia and the reticular, or lymphoid tissue, which will be briefly considered when we study respectively the nervous system and the lymphatic system. TIIE CONNECTIVE-TISSUE SYSTEM. 49 YELLOW ELASTIC-TISSUE. Another form of connective-tissue presents itself in the yellow elastic-tissue. The fibres or bands com- posing it differ in chemical constitution from those of white fibrous tissue. They are not influenced by the action of water, weak acid, or strong alkalies. Unlike the white fibrous tissue, the individual elastic fibres frequently branch, and by the anastomoses of those branches form a genuine network with meshes, varying in size and shape according to the location and the variety of elastic-tissue. Fine elastic fibres. Ks already indicated, every form of white fibrous tissue contains a variable amount of elastic tissue in one form or another. Perhaps the simplest and most widely distributed variety of yellow elastic-tissue is that which is present among the bundles of white fibrous tissue in the form of a loose sparse network of fine cylindrical apparently homogeneous and highly refracting fibres. These fine elastic fibres lie in close proximity to the (pri- mary) white fibrous bundles, never within them. After nearly every method of preparation they seem homogeneous, presenting no trace of fibrillation or other structure. But when freshly submitted to the action of osmic acid, and examined under a mode- rately high magnifying power, they appear trans- versely striated. If a power of eight or niiie hundred diameters with a good lens is employed, the transverse strias are resolved into highly refracting lenticular or somewhat spherical forms, imbedded in a less dense, hyaline, transparent substance. The fibres of elastic-tissue are straight or curved according to the tense or loose condition of the tissue in which they are found. When broken they curl up at the extremity. This form of elastic fibres is met with also in the matrix of yellow elastic cartilage. Coarse elastic fibres. Besides the network formed of such delicate elastic fibres just described, many white fibrous tissues contain an elastic plexus, composed of much coarser fibres. These coarser, elastic fibres are cylindrical or band-like. Schwalbe proved them to be covered by a delicate elastic sheath, which is ordi- narily invisible. After the action of osmic acid upon perfectly fresh specimens, the coarse fibres are also seen to possess the peculiarity of structure already de- scribed for the finer fibres. This variety of yellow elastic fibres is also met with in the skin and other felt- works of white fibrous tissues. In the superficial portion of the trachea and bronchi they are closely packed together to form, with but a sparse amount of interstitial white fibrous tissue, a dense lon<-i- o 7 tudinal layer. The most marked development of this form of yellow elastic-tissue is found in the ligamentum nuchaj and the ligamenta subflava, which may be regarded as, par excellence, types of yellow elastic tissue. Even in these two locations there is always a certain quantity of white fibrous bundles Fig. 21. YELLOW ELASTIC-TISSUE. High power. (Gray.) scattered through the meshes formed by the large yellow fibres, and there are also a small number of the formative connective-tissue cells present. The latter bear their customary relations to the lymph- spaces and the fibres. In this tissue the elastic fibres pursue mainly a longitudinal course, and are closely applied to each other. But if thin sections are made, and pulled out laterally by needles, it is easily seen that they really constitute a network. This form of yellow elastic-tissue is also found well developed in the walls of large arteries and veins. The elastic networks heretofore considered are more or less sponge-like in their arrangement. There are elastic fibres which are band-like and in some locations form only lateral anastomoses. In this case we have an elastic fenestrated membrane, such as is present in the inner and middle coats of arteries and veins (see fig. 3, Plate III.). Furthermore, instead of a fenestrated membrane, we may have a continuous layer of yellow elastic- 50 HISTOLOGY. tissue such as is found upon the posterior surface of the cornea, and upon the surface of serous cavities immediately beueath the endothelia. The former membrane (membrane of Descemef), however, is known to be somewhat fibrillar in structure, although ordi- narily it does not appear so. The more or less distinct basement- membrane of gland-tubes, as has been already hinted, is an elastic membrane formed sometimes entirely of flat, poly- gonal or branched, elastic endothelial plates. ADIPOSE TISSUE. This is another member of the group of connective- tissues. It may be present in a diffuse manner in the subcutaneous and submucous tissues of the economy. In the well-nourished it is frequently present in the interstitial connective-tissue of muscles and of some glands, and it may be found in greater or lesser quan- tity in subserous connective-tissues. Fatty or adipose tissue, however, is usually found aggregated into lobules, each one of which generally has its own spe- cial vascular apparatus, being supplied by an afferent artery, one or more afferent veins, and an interme- diary capillary network of small meshes, in which one or two adipose cells lay (Fig. 22). The aggregation Fig. 22. BLOODVESSELS OF FAT. 1. Minute flattened fat-lobule, in which the vessels only are represented. 3. Terminal artery. 4. Primitive vein. 5. Fat-cells of one border of the globule separately represented. (Magnified 100 diameters.) 2. Plan of arrangement of capillaries on exterior of fat-cells, more highly magnified. (Gray.) of the cells into lobules, the possession by each one of these lobules of its own vascular supply, and a mode of development which is similar to that of some lymph- glands, have suggested to some histologists a com- parison of this tissue with that of secreting glands. Klein, Creighton, and others consider the lobules of adipose tissue essentially acini of ductless glands. However true or false this assumed analogy may be in fact, there is undoubtedly, up to a certain point, a wonderful parallelism in a common method of de- velopment for the adipose lobule and the lymph- follicle. Development and growth of adipose tit sue. Accord- ing to Klein, and some other accomplished investi- gators, the origin and growth of a lobule of fatty tissue in the mesentery may be briefly sketched as follows : In the vicinity of an artery of somewhat consider- able size a small spot can be seen with the microscope, in which the following conditions are present. In the lymph-spaces the flat, fixed cells of the connective- tissue are undergoing proliferation ; considerable num- bers of lymphoid cells are being formed by division and budding. The connective-tissue septa between neighboring lymph-spaces, in which the cells are proliferating, become gradually thinned until there remains only a delicate reticulum, in the meshes of which are to be found newly-formed lymphoid cells. In other words, a genuine lymphatic reticular-tissue is in process of formation. The spot of reticular- tissue at first forms at some distance from a blood- capillary. After the cellular activity has somewhat progressed, the nearest capillary begins to send out one or more loops towards the little lymphoid spot or nodule. One arm of this loop subsequently grows into an arteriole, the other develops into a venule ; the middle portion sends out protoplasmic branches to ramify in the minute nodule of lymphoid tissue. Tn"ese protoplasmic branches become hollowed out, finally constituting a capillary network, which per- meates the newly-formed cellular mass. Often stel- late protoplasmic cells in the depth of the nodule have no direct connection with the walls of previ- ously formed or developing capillaries, but, by a pro- cess of vacuolation, become hollowed out and finally united with the capillaries to form new bloodvessels. In this manner the newly-formed nodules, after the commencement of their growth, are supplied with newly-formed bloodvessels. Adjacent spots or no- dules, by spreading, may unite together to form greater or lesser areas of similar newly-formed cellular tissue of a lymphoid character. At this stage of de- velopment we have, as far as the eye can tell, genuine lymphatic or reticular-tissue, such as is seen especially well-defined in the mucosa of the intestines. These spots in the mesentery are always covered, at least on one surface, with a layer of germinating endothelia. It is after this that the adipose tissue, as such, makes its first appearance. In some of these nodules CARTILAGE. 51 or lobules, to use a better term, a larger or smaller number of the cells begin to charge themselves with minute molecules of oil a circumstance which causes them to become extremely granular and somewhat opaque. These minute fatty granules subsequently coalesce to form fewer and larger fat-drops which now load the cell in varying numbers. These fat- drops in their turn increase in size and finally run together. At this stage of development of the fat-cell we find the element consisting of a cell-body which is spread out over the oil-globule as a thin shell of protoplasm. The nucleus, which has been pushed to one side and much flattened, has an enveloping membrane of double contour, and it contains, as does also the cell- body, a network of fine fibres holding in its close meshes a small amount of semifluid, transparent, struc- tureless substance. The fat continues to accumulate in the cell until the enveloping protoplasmic shell is so stretched and distended that the only visible trace of it to be seen, under the microscope, is in the form of an extremely thin limiting membrane surrounding the oil-globule when the latter is viewed in optical sec- tion. This membrane is a little thicker at one side a single indication of the location and existence of the nucleus. The cell is now known as the adipose vesicle (Fig. 23). In a growing lobule of adipose Fig. 23. ADIPOSB TISSUE. - z, Star-like bodies, from crystallization of fatty acids. High power. (Gray.) tissue the different fat-cells may present various stages of the elaboration of fat. In a fully-developed one, however, all, or nearly all, the cells present the aspect of a fully-developed vesicle. The vesicles rest in the meshes of the rich capillary plexus which sur- rounds and permeates the lobule. At the periphery of a full-grown lobule a small amount of loose fibrous bundles may be seen between the cells, accompanied by a small number of fixed and mobile connective - tissue corpuscles. In the depth of the lobule this fibrous framework has apparently disappeared, at any rate it is here generally invisible, since only the cap- illary vessels are to be seen. Whether the adipose lobules are developed in the mesentery or in the subcutaneous, submucous, or interstitial connective tissues, it is usually after the method above sketched : First, the formation of a lymphoid nodule ; then, the supply of this nodule by a special newly-formed vascular system; last, the gradual transformation of the vascularized lymphoid nodule into an adipose lobule. Diffuse formation of fat. There is also a not infre- quent formation of diffused adipose tissue in the sub- cutaneous, submucous, and intermuscular loose con- nective tissue by the direct infiltration and transform- ation into adipose vesicles, of the fixed and other cells already existing in these tissues. Such a direct trans- formation of the connective-tissue cells into fat-vesi^es may involve wide areas of tissue, and result in the formation of more or less thick and extensive strata of adipose tissue. The large granular plasmatic cells of Waldeyer, which are usually found in greatest numbers in the neighborhood of vessels, very fre- quently experience this direct transformation or infil- tration. In this latter method of formation, the blood vessels actively increase, for adipose tissue is pre-emi- nently a vascular tissue. The fat once formed, it and the cell contain- ing it may suffer various changes. By an inter- ference with the vascular supply the fat may ultimately decompose, and fat-crystals form in the interior of the vesicles. Or, on the other hand, under the influence of an increased activity of the cell-irritation in inflammation for exam- ple or of a demand for nutritive material, sucli as may be occasioned by inanition or fasting, the fat may gradually disappear, leaving in its place at first a serous fluid which still distends the cell and preserves its vesicular form. If the irritation or the demand for nutritive material still continue, this serous fluid gradually disappears and the cell finally returns to the original form it possessed before the fat first made its appearance. CARTILAGE. In cartilage we have a structure which has been classed with the various forms of connective tissue, but there seems to be a wide variance between it and other groups of the great connective- tissue system, 52 HISTOLOGY. both as to its chemical constitution and its intimate structure. While the other groups of the connective- tissues yield gelatine on boiling, the cartilages, on the contrary, yield chondrin. Cartilage consists, like the other tissues, of cells and intercellular substance; the latter is .now known as the ground substance or matrix. There are three principal varieties of carti- lago which arc distinguished from each other by variations in the nature of the matrix rather than in the cells. These principal varieties are hyaline carti- lage, reticular or yellow elastic cartilage, and white fibrous cartilage. HYALINE CARTILAGE. The simplicity of the structure of hyaline cartilage is nearly equal to that of the simplest form of mucus or gelatinous tissue. Cartila'jn cells. The cells are generally more or less spherical or ovoid, when free to assume shapes which are not the result of unequal pressure. They contain one, occasionally two, large spherical vesicu- lar nuclei with a thin limiting membrane of double contour. The nucleus may contain one or more com- paratively large and distinct nucleoli. The cell-body often incloses a few small fat drops and fatty granules. The nucleus consists of a network of fibrils similar to the intra-nuclear network of other cells, but the fibrils which constitute, with a semifluid inter-fibrillar sub- stance, the body of the cell, instead of forming a true network as in other cells, are arranged in a dense felt-work with the fibres crossing each other, and interlacing in all directions. The cartilage-cell pos- sesses one characteristic which unmistakably distin- guishes it from other cells, and marks the tissue in which it is formed as cartilaginous. It has the faculty of causing a halo to be formed around it of substance similar to the matrix of hyaline cartilage a more or less thin film known as the capsule. This peculiarity of the cartilage cell, it should be remarked here once for all, is a characteristic of all cartilage-cells, whether of one variety of cartilage or of another. The carti- lage cell is capable of limited movement. It can swallow minute particles brought directly in contact with its body, and under the stimulus of physiological growth, or of pathological irritation, can multiply by division. Under the influence of such a stimulus the cell first enlarges ; then the nucleus divides by fission or by gemmation; finally, the cell-body suffers a constriction and an ultimate division into new cells, each containing a micleus. If the stimulus is ener- getic, each new cell may again divide. Thus two, four, eight, or even sixteen or thirty-two new cells may be formed as the progeny of the original cell. The original cell has been termed the mother cull, EXPLANATION OF PLATE III. Fig. 1. White fibrous connective-tissue, highly magnified. a, The fibrous bundles ; 5, nuclei of endothelial cells ap- plied to the surface of the bundles, or of stellate connective- tissue corpuscles resting in the interfibrous lymph spaces ; c, a lymphatic lacuna. Fig. 2. A. Highly magnified view of an isolated cord of primary bundles of white fibrous tissue from the tail of a yo'ung Rat. Fresh specimen, silver-treated. High power. n, Nucleus of flat endothelial cell-plate applied to the sur- face of the cord, the cell is seen to spread across two or more bundles ; similar endothelial cells are arranged in a row they do not completely envelop the cord ; f, fibrous bundles composing the cord. B. A view, under the same power, of two isolated endo- thelial cell-plates similar to those which in part cover the fibrous cord in A. n, The nucleus ; e, a ridge on the surface of the cell plate, which sinks into the crevice or groove formed by the apposition of two or more bundles. C. A transverse view, under a much lower power, of several juxtaposed fibrous cords, such as mentioned above, f, The fibrous bundles cut across ; c, stellate bodies, having processes which sometimes communicate with those of neighboring bodies they have been regarded as stellate corpuscles or cells, but are nothing more than the cross cuts of the above- mentioned endothelial cell-plates. (All after Ranvier.) Fig. 3. Represents, under a moderate power, an elastic lamina of the middle tunic of the Human aorta. (Ran- vier.) b, Elastic membrane ; a, elastic fibres arising from it. Fig. 4. Hyaline cartilage of Man, .medium amplification. rf, Structureless hyaline matrix, in which are imbedded cells enveloped within a capsule, a, which latter is indicated by the light halo; b, c, cells in process of multiplication. Fig. 5. Section of cartilage from the head of the femur of a Frog, examined perfectly fresh and without any mount- ing fluid. Very high power. (After Ranvier.) s, Fundamental substance, or hyaline matrix ; c, capsule ; n, double-contoured nucleus ; n', small brilliant nucleolus. PLATE III Fig 1. . 3. Fig. 2. c FiS 5. Fig 4. T- iirclur *Sn MK CARTILAGE. 53 Fig. 24. while the descendants have received the name of dauijhler cells. If the process of division be slow each new cell may form around it a new capsule (see Fig. 24). In this case the mother cap- sule contains a series of daughter capsules. If, how- ever, the process of multipli- cation is rapid the newly born cells will not have time to secure the formation of a capsule before they give birth to new cells. Hence, the orginal mother capsule must now contain two or a greater number of cells of a more or less embryonal nature. The healthy carti- lage-cell lives, then, within an envelope or shell which it builds for itself as a habitation. The inner surface of this envelope, the cartllaye capsule, forms the boundary of a lymph-space, more or less spherical, which is normally quite filled by the body of the cartilage-cell (see figs. 4, 5, Plate III.). Pre- parations of cartilage usually show the cartilage-cell to be more or less angular, and occupying only a small Fiff. 25. PROLIFERATING CARTILAOE- (':;:,!.,-. c. Protoplasm of the coll ; , nucleolus ; &, nucleus ; d, pri- mary and secondary cartilage cap- sules ; c, ground substance. In one of the card lag c-cclls are seen two nuclei. a. r CARTILAGE OF A CUTTLE-FISH. a, d. Body of cell. &. Anastomosing branches of the cells, c. Fundamental substance. X 400. (Kanvie.r.) proportion of the space within the capsule. This is a distortion, due to the death and shrinking of the cell. In the depth of cartilage the capsules usually have a tendency to assume the spheroid form, while to- wards the surface they become more or less flattened or lenticular in outline. In the superficial layer, at the articular surfaces, the cells are quite flat, and sometimes branched. In the articular cartilages, near the periphery, where the fibrous tissue of the syno- vial membrane is attached, the cartilage-cells are often branched, and at the line of attachment the processes of the branched cells sometimes communicate with those of the connective-tissue corpuscles of the fibrous tissue. Branched cells, like those seen, in Fig. 25, are not uncommon in hyaline cartilage of some lower animals. Matrix. The intercellular substance, the ground- substance, or, as it is technically called, the matrix of hyaline cartilage, is usually hard, somewhat elastic, homogeneous, and transparent. In adult cartilage, after staining, the matrix faintly shows, by slight dif- erences in shade, very indistinct and shadowy mark- ing for some distance around the capsule (fig. 5, Plate III.). The cartilage-cell seems to have been a centre around which on all sides the matrix has been de- posited in successive layers from without inwards, the existing capsule being last deposited. Under* some circumstances the shadowy areas around the capsule seem to be composed of a number of concentric shells. The whole ground-substance appears to consist of an aggregation of these shadowy areas surrounding the cells. fibrillation of matrix. Sometimes in adult hyaline cartilage the matrix is seen to be very finely fibril- lated. This condition is met with most frequently in the costal cartilages of the aged, and in the calcifying layer of ossification of cartilage (see fig. 1, Plate IV.). Embryonal cartilage. The cells of hyaline carti- lages may be closely aggregated, or they may be sparsely distributed through the matrix. When the intercellular substance is so small in amount that it is scarcely appreciable, the cartilage is known as parenchymatous or embryonal cartilage. In the deep portion of articular cartilages, in the neighborhood of lines of ossification, the cartilage cor- puscles assume an arrangement in parallel rows more or less vertical to the articular surface. Perichondrium. Each variety of cartilage is en- veloped in a fibrous membrane, the perichondrium, which covers it everywhere except upon the articular surfaces and in the lines of ossification. Calcification of cartilage. Cartilage may be infil- trated with calcareous particles. When this is the case lime granules, minute and more or less angular, are first deposited in the ground-substance, immediately around the cartilage capsules. From these points the 54 HISTOLOGY. infiltration may gradually spread until the whole matrix may be incrusted, and the cells transformed into calcified elements. By transmitted light the particles of lime are dark and opaque, large collec- tions of them consequently appear more or less black. By reflected light they are brilliant and shining. This deposit of lirne does not appear to permanently change the constitution of the cells or matrix. Weak acids readily dissolve it, leaving both apparently in their original condition. YELLOW ELASTIC OR RETICULAR CARTILAGE. This variety of cartilage is met with in man, in the Eustachian tube, in the epiglottis, and a few other places. It differs from hyaline cartilage only by the presence in the matrix* of fine, yellow elastic fibres in greater or lesser numbers. These fibres are gen- erally collected together so as to form a trabecular network in the matrix. In the meshes thus formed the cells are found enveloped in capsules, which are surrounded by a variable amount of hyaline ground- substance (see fig. 3, Plate IV.). Near the perichondriurn the elastic fibres gradually disappear. WHITE FIBROUS CARTILAGE. This form of cartilage appears to occupy, histo- logically, an intermediate place between white fibrous connective-tissue and hyaline cartilage. Its distribution in man is somewhat more exten- sive than the yellow elastic cartilages. It is chiefly found in the intervertebral disks and in the bursse of tendons. The intercellular substance or the matrix consists of white fibrous tissue arranged in parallel bundles. Frequently the fibres are arranged in lamellae, and sometimes, as in the intervertebral disks and the symphysis pubis, the bundles constituting the lamellae have a concentric arrangement. The cells of this form of cartilage have the same relation to the bun- dles as the flat cells of Kanvier do to the parallel bundles of tendons or aponeuroses. The cells them- selves are not quite so flat or so much branched as the endothelial cell-plates of tendons. They are en- veloped by the characteristic capsule of the cartilage- cell (fig. 2, Plate IV.). In some places the capsules are surrounded by a very small amount of hyaline substance. In the transition of tendon into fibro-car- tilage the tendon-cells gradually thicken and become invested with a capsule, the only visible change noticeable. In the transition of fibre-cartilage into hyaline cartilage, as is Observed at the edges of the intervertebral cartilages for example, the fibrous intercellular substance slowly disappears and merges in the Iryaline matrix, while the cartilage-cells gradu- ally swell and approach the spherical form. Lymphatics of cartilage. Cartilage, like all the other connective-tissues, is permeated by lymph spaces. The cartilage-capsules are perforated by almost num- berless minute canals. The openings of these minute capillary tubes or pores communicate at one end with the interior of the capsule, which is a lymph-space containing the cartilage -cell, and at the other end open into large canaliculi which ramify in the cartilage matrix, and which, in their turn, communicate witli the lymphatics of the perichondrium. BONE OR OSSEOUS TISSUE. Bone is the greatest in weight of all the solid tis- sues of the organism, and is the most extensive of the groups of the great connective system. It presents for study a ground-substance, inclosing cellular elements and vessels, both lymph and capil- EXPLANATION OF PLATE IV. Fig. 1. Section of calcified and fibrillated hyaline cartilage, from the costal cartilage of an old Man. High power. a, Intercellular matrix infiltrated with minute calcareous granules, they are found more densely aggregated at c, around the cartilage capsules ; b, fine fibrillae, imbedded in the in- tercellular hyaline matrix. Fig. 2. Fibro-cartilage from the intervertebral disk of Man. High power, a, Fibrous matrix or intercellular substance ; c, b, cells contained within a cartilage capsule, and exhibiting various stages of multiplication. Fig. 3. Reticular or elastic cartilage, from the epiglottis of Man, showing the so-called cartilage cells in various phases of division within their enveloping capsules. High power. b, Elastic fibres imbedded in the intercellular matrix. PLATE IV "- Ny" FiiX *& 'M $f ?<*, ^,,' rl ' a. ^ T SijcHrt i San lit]-. BONE OR OSSEOUS TISSUE. 57 by their size and structure, cannot when isolated be distinguished from colorless lymph-corpuscles. The (Smallest of the latter have been called medulla cells (d,f, Fig. 26). Multinuclear giant cells (myeloplaxes, , 6, Fig. 26) are also present in small numbers. The latter elements consist of a large, soft, membraneless, irregularly-formed, 'sometimes branched and flattened cell-body, of a fine granular appearance, and contain a fibrillar network like that of other cells. They inclose a large number of nuclei. The vessels are similar to those of the other varieties of marrow. Red marrow of bone is met with in the Haversian spaces of spongy bone. To the naked eye it differs from the yellow marrow mainly by the entire ab- sence of fat vesicles, or by their presence only in very small numbers, and by the extreme vascularity of the tissue. This form of marrow consists of reticulated fibres similar to those of lymph-glands or lymphoid tissues to be described later. In the meshes of this reticulated tissue, cells of the following forms are closely crowded : 1st, perhaps an extremely few fat- cells; 2d, large numbers of medulla-cells ; 3d, con- siderable numbers of multinuclear giant cells; 4th, polynucleated cells, which have a diameter slightly larger than that of the red blood-corpuscles, and which possess a smooth, apparently homogeneous body presenting a yellowish-green tinge, these cells, according to Neumann and Bizzozero, subse- quently become converted into red blood-corpuscles, by losing their nucleus and becoming bi-concave ; 5th, along the osseous trabeculas are a single row of cells, which are larger than the ordinary colorless elements of the marrow, and are more or less pris- matic or flattened by mutual pressure. They have been called by Gegenbauer osteoblasts, since they seem to take an active part in the formation of the bony substance. All of the foregoing cellular elements possess, in a more or less active degree, the power of movement. Besides the cellular forms already described there are always present in the bony marrow some colored cells, which are not distinguishable from red blood- corpuscles. Red bone-marrow is very richly supplied with bloodvessels, which have extremely thin and delicate walls. Periosteum. The external surface of bone is covered by a fibrous membrane the periosteum. It consists of two layers, an outer and an inner. The outer layer is composed of one or more lamellaa of dense white fibrous tissue, the direction of whose bundles is parallel to the surface of the bone. Among these white fibrous 8 bundles is a limited quantity of fine yellow elastic fibres, and in the lymph-spaces formed by the apposi- tion of the bundles are cellular elements similar to those of dense, white fibrous tissue. In this external, or fibrous layer of the periosteum, blood and lymph- vessels ramify and form networks. The inner or osteogenetic layer of the periosteum consists of an extremely loose fibrous tissue, the meshes of which are filled by cells very similar to the osteoblasts described as existing upon the trabeculse of spongy bone. Among these are numbers of ele- ments which present characters similar to those of lymph-corpuscles. The osteogenetic layer of the peri- osteum is richly supplied by bloodvessels, which run among the cells occupying the meshes. Beneath the periosteum the surface of growing bone is covered by a bony network, the meshes of which are crowded by cells which are contiguous with those which fill the inter-fibrillar spaces of the osteogenetic layer. Here and there the most super- ficial portion of this bony network sends a pointed and somewhat curved spicule of bone into the depth of the osteogenetic layer of the periosteum. The points of these somewhat conical spicules are usually continuous with fibrous bundles of the osteogenetic layer. The surface of the trabecula3 forming this osseous network, as well as of the bony spicules which project into the osteogenetic layer of the periosteum, are covered by the so-called osteoblasts of Gegenbauer, through the agency of which not only the bony tra- beculse and spicules springing therefrom increase in size, but also the fibrous bundles at the end of the spicules are converted into bone. In this manner the bone grows beneath the periosteum. OSSIFICATION. Bones increase in length by a process which is known as ossification. In young growing bone, the epiphyses are united to the diaphyses by the inter- vention of a plate of cartilage the intermediary cartilage. The successive conversion into spongy bone of the layers of this cartilage which are in immediate apposition with the diaphysis, furnishes an excellent opportunity to examine the process of ossification of cartilage. Line of ossification. A longitudinal section passing through the diaphysis and intermediary cartilage of a long bone shows to the naked eye a straight line of union between the spongy bone and the cartilage. This line is called the line of ossification. The carti- lage above it seems to be divided more or less dis- 58 HISTOLOGY. tinctly into three layers the lowest layer or the one forming the line of ossification is more or less opaque, the middle layer is unusually transparent, the upper layer is quite normal in appearance. If a thin piece of the spongy bone and cartilage is shaved off', and after being properly prepared for examination, is placed under the microscope, the following appear- ances may be noted (see fig. 4, Plate V.). In the upper layer or zone of cartilage, the capsules and in- tercellular ground-substance are normal. In the middle layer, the cells have begun to enlarge and to multiply. Some of them show signs of disintegration, and the spaces which contain the cells have also con- siderably increased in dimensions. The increase is mainly in a direction perpendicular to the line of ossification, and on account of this tendency the cells are applied against each other so as to form linear rows. The intercellular substance is consequently encroached upon and somewhat softened. In the lower zone, the cartilage matrix which re- mains between the now greatly elongated capsules, in consequence of the further enlargement of the latter, is reduced to narrow trabeculce which, are infiltrated with calcareous deposits. The enlarged elongated capsules frequently fuse together. They are now in part filled by an embryonal marrow. The cartilage- cells which occupy their upper ends approach a wedge- shape, and are piled upon one another in such a manner that the bases alternate with the spaces as seen at a, fig. 4, Plate V. In a yet lower zone, the linear spaces between the narrow trabeculae of cartilage matrix are completely filled with a mass of cells similar in every respect to that of the bony marrow in the Haversian spaces, and they are now permeated by loops of capillary blood- vessels, which are in communication with the vessels of the marrow of the spongy bone below. The thin trabeeulae of cartilaginous matrix, separating these spaces, are still infiltrated with calcareous salts. They are now more or less completely encrusted with a thin layer of osseous substance of variable thickness (see g, fig. 4, Plate V.). This thin osseous incrustation is itself covered by a layer of osteoblasts of a prismatic, ovoid, or some- times slightly-branched outline. Here and there in the thickest points of this incrusting layer of osseous substance an oval-shaped, incompletely-branched bone- corpuscle is visible. By carefully searching along these incrustations pouch-shaped notches are occasionally met with opening upon the surface. These are filled by osteoblastic cells which may be connected with other cells in the osteoblastic layer by broad or narrow" branches (see/, fig. 4, Plate V.). It is thus seen that the bone-cells are osteoblasts which have become in- cluded and completely imbedded in 'the osseous sub- stance which they have formed around them. In a still lower zone, we recognize the same general features presented by that last described. But the lamellae of bone incrusting the cartilaginous trabeculse are much thicker, and the cartilage matrix has entirely disappeared in many places. Below this zone, there is no trace of cartilage matrix, and we have only the ordinary structure of spongy bone. The spongy substance thus gradually en- croaches upon the intermediary cartilages until they finally disappear. The line of ossification presents essentially the same features whether the cartilages be intermediary plates between diaphyses and epiphyses, or cartilages which cover articular surfaces. It is then noticeable, that in the growth of spongy bone at the expense of hyaline cartilage, the latter does not really become bone, is not really ossified, but is gradually substituted and replaced by bony substance which is the direct product of some of the cells belonging to the marrow contained in the Haversian spaces of the spongy bone. This latter tissue is more or less directly an outgrowth from the osteogenetic layer of the periosteum. It may be stated in general terms that there is no ossification which does not come either directly or indirectly from the periosteum or from similar mem- branes which represent it. The perichondrium which envelops cartilage as the periosteum does the bone, may be regarded as such a representative, for the structure of the one is similar to that of the other, and their functions are parallel. DEVELOPMENT OF BONE. The original development of bone almost always is accomplished through the intermediation of cartilage. The bones of the face, however, and some of those of the cranium, are built directly upon fibrous mem- branes as a foundation. But whether or not cartilage be employed in the process of development, the for- mation of bone always starts from the periosteum. In the embryonal cartilage which nearly always precedes the formation of bone, we have areas where- in certain changes take place preliminary to the de- velopment of bone. These areas have been called points of ossification. When these points of ossifica- tion appear in cartilage we have a development of what has been termed cndochondral bone. DEVELOPMENT OF BONE. 59 ENDOCHONDRAL BONE. In an embryonal cartilage which is preparing to be replaced by bone, the following succession of altera- tions may be observed. Formation of embryonal spongy bone. At a certain point in the inner or chondrogenetic (osteogenetic) layer of the perichondrium, a loop or two of capillary bloodvessels project into the cartilage-substance. In advance of these loops the cartilage-cells are enlarg- ing and multiplying within their capsules, which also are enlarged. These cells finally absorb the car tilage-matrix between them and the mass of cellular elements which surrounds the projecting capillary loops; they then communicate with the cells of the osteogenetic layer. By a continuation of this pro- cess of absorption of the cartilage in front of the advancing capillary loops, and by the addition of the contents of the capsules to the mass of cells surround- ing the vessels, the embryonal cartilage becomes channelled by vascularizcd cellular trabecula? or granulations springing from the inner layer of the perichondrium. The next step everywhere in the close vicinity of the vascular granulations, is the enlargement of the cartilage-capsules, and the multiplication of their cell- contents. This enlargement continues, and results in the intercommunication of many adjacent capsules and the connection of their contents with the vascu- lar granulations. By the enlargement of the spaces containing the cartilage -cells, the intervening ground- substance is eaten away until nothing is left of it but an irregular network of anastomozing trabeculas, the surfaces of which are notched at the place where the capsules have communicated with the general cellu- lar mass. At the same time the cartilage-matrix in this riddled area is infiltrated with a deposit of the insoluble salts of lime. We have now an area more or less irregular of calcified, spongy cartilage, the meshes of this spongy formation being filled with, a vascularized marrow, similar in constitution to that already described when discussing the encroachment of spongy bone upon the intermediary cartilage. The next step is the incrustation of the calcified cartilage-trabeculas with a thin envelope of osseous tissue. This first takes place near the central portion of the spongy area, and the bone is formed through the agency of osteoblasts which cover the trabecula. This process advances until the whole cartilage is converted into spongy bone. (Fig. 27). Formation of bony marrow. Sooner or later, in the central portions of this embryonal spongy bone, the osseous trabeoulte soften and disappear (osteoporosis). In this manner a central marrow cavity results. The spongy bone continues to be slowly absorbed, and converted into marrow until finally the whole of the embryonal spongy bone may be thus absorbed. Fig. 27. TRANSVERSE SECTION FROM THE FEMUR OF A HUMAN EMBRYO ABOUT ELEVEN WEEKS OLD. a. A medullary sinus cut transversely, b. Another, cut longitu- dinally, c. Osteoblasts. d. Newly formed osseous substance of a lighter color. e. That of greater age, /. Lacunse, or blood-corpuscles, with their cells, g. A cell etill united to an osteoblast. (After Gegenbanr.) At the same time that the embryonal spongy bone is being absorbed at the centre, there is a continual new formation of spongy bone by the osteogenetic layer of the perichondrium (which latter may now be regarded as a periosteum), practically identical with that already described for the periosteal growth of bone. In the older portions of periosteal spongy bone, Ha- versian systems and canals begin to form through the successive development of concentric lamella within the Haversian spaces, by means of the osteoblasts which cover their surfaces. Layer within layer is thus formed in concentric deposits until the Haver- sian space is converted into a Haversian system with its narrow central canal. The original osseous tra- beculas, which formed the meshes of the periosteal spongy bone, persist as interstitial bands between the Haversian systems. Absorption of bone (Osteoporosis'). Around the cen- tral marrow-cavity, which has been formed asindicated above, a process of absorption is at work. The com- pact tissue is disappearing, and being reconverted into spongy tissue through the absorption of the Haversian systems and their substitution by bone-marrow. The 00 HISTOLOGY. process of absorption does not stop at the accomplish- ment of this result. The interstitial osseous trabeculse of the spongy bone next succumb, and the softened area is added to the central cavity. Thus it is that while the bones are growing at the periphery they are being eroded at the centre. According to Kolliker, Eindfleisch, Klein, and others, solution of bone is effected by the agency of the multinuclear giant-cells. These cells have con- sequently received the name of osteoclasts. They are believed to elaborate an acid by whose action the bony substance with which they are in contact is first softened and then dissolved. In the development of bone, the various stages in the process of ossification may succeed each other with varying rapidity in different bones, and in dif- ferent parts of the same bone. In long bones, for example, the middle portion of the diaphyses often entirely consists of periosteal bone, while at the ex- tremities the embryonal spongy bone has scarcely begun to disappear. INTERMEMBRANOUS BONE. The intermembranous formation of bone is analo- gous to the development of bone from the periosteum. For instance, the bones of the cranium have their origin in a fibrous membrane which soon presents a division_into two layers similar both in structure and function to the outer and inner layers of the perios- Fig. 28. OsTEOBT.ASTB FROM THE PAHIETAL BONE OF A TlfMAN EMBRYO THIRTEEN WEEKS OLD. a. Bony septa, with the cells of the lacuna, or bone-corpuscles. 6. Layers of osteoblasts. c. The latter in transition to bone-corpuscles. Very high power. (Gegenbaur.) teum. Spongy bone is formed by the inner or osteo- genetic layer precisely in the same manner as it is formed beneath the periosteum of other bones. This spongy bone is converted into compact substance by the same method, and finally osteoporosis progresses in a manner already familiar. TEETH. Although the first rudiments of the teeth are off- shoots from the epithelium covering the surface of the gums, and although the enamel which covers the exposed surface of the fully-developed tooth is of epi- thelial derivation, yet the greater portions of the teeth, namely, the dentine, the cement, and the pulp, are of connective-tissue origin, and may properly be classed with the connective-tissues. Because of the entrance of bone into their structure, as well as for other rea- sons, an examination of their histology would seem to have an appropriate place after the study of bone. Development of the teeth. The first rudiment of the tooth is met with early in embryonic life, at a period when the connective-tissue of the gum has scarcely advanced in development beyond the state of fibrous mucous-tissue. It is observed in the form of a club- shaped duplicature of the stratified epithelium of the gum. (See Fig. 29.) This epithelial infolding is con- Fig. 29. VERTICAL SECTION OF THE UPPER JAW OK A POSTAL SHEEP, about 1% inches long, showing the enamel-germ, with the semilunar rudiments of the dentine- germ and dental sac in transverse section. 1. Dental groove ; 2. Palatal pro- cess. Magnified 50 diameters. stituted externally by a single layer of columnar epi- thelia, identical with those which form the deepest layer of the covering of the gum, and internally by a collection of polyhedral epithelial-cells. These cells, like those of various other epithelial coverings, are held together by an intercellular -cement-sub- stance. This club-shaped mass of epithelial cells is the pri- mary enamel- or (jan. Later this club-shaped mass pene- trates deeper into the connective-tissue and increases greatly in thickness, at the same time changing its outline. The club-shaped extremity has now spread out and become indented by a slight elevation of the connective-tissue which has begun to advance into it. (See /, Fig. 30.) This elevation is the first appearance of what will subsequently constitute the dentine and the pulp of the tooth. The milk, or first teeth, are now TEETH. 61 00< in full process of development. A preparation for the growth of the permanent teeth is, even at this stage, often to be met with in the shape of an offshoot from Fig. 30. I ce SAME, ATA LATER PERIODOF DEVBLUPMENT. a. Epithelium. 6. Younger layer of epithelium, c. luferior layer of the epithelium, e. Enamel-organ. /. I)en- tiue-gcrm or papilla, g, h. Inner and outer layers of the sacculua that is about to form. (Carpenter.) the epithelial mass already spoken of as the primary enamel-organ. 4, Fig. 31, represents such an epithelial Fig. 31. VERTICAL SECTION OF THE LOWER JAW OF A HUMAN FOETUS, measuring about four inches in length, magnified 25 diam. 1. Dental-groove. 2. Remains of the enamel-germ. 3. Enamel-organ of a deciduous tooth, presenting epithelium on both its outer and inner surface, i. e. t where it lines the saccnlus and where it covers the papilla. 4. Enamel-germ of the permanent tooth. 5. Dentine-germ. 6. Section of inferior maxilla. 7. Meckel's cartilage. The dental sacculus will boobserved to present a number of fine papilla; opposite the dental papiliie. (Carpenter.) offshoot after the changes in the growth of the milk- tooth have so far progressed that the connection with the enamel-organ of the latter has been severed. Fig. 31 represents a much later stage in the forma- tion of the tooth. The connective-tissue papilla has grown into the enamel-organ until the latter has been completely invaginated. The enamel-organ now covers the apex and sides of the tooth -papilla like a cap, and it has been cut off from its former connec- tion with the epithelial covering of the gum. This cap-shaped epithelial mass is termed the secondary enamel-organ. Through the invagination of the epi- thelial mass constituting the enamel-organ, it results that the cap covering the papilla, when seen in section longitudinal to the long axis of the tooth, seems to be formed by three principal layers. The uppermost is composed of a single row of cubical epithelium ; the lowest layer consists of a single row of long cylindri- cal cells; the middle layer is formed of more or less compressed and branched epithelial cells, with a large amount of intercellular cement between them. They contain small oval or spherical nuclei, and, according to some authors, afford with the intercellular substance an example of mucous-tissue. Klein denies to this layer any other than an epithelial constitution. It is the lowest layer of long columnar or pris- matic epithelial cells which ultimately furnishes the enamel of the tooth. The two upper layers gradu- ally diminish in thickness, and finally form a thin epithelial covering, which is found upon the surface of the enamel when the tooth first makes its appear- ance above the gurn (membrane of Nasmytli). At first the lower layer of enamel-cells is separated from the papilla by a thin elastic membrane, the re- mains of the basement-membrane upon which the epithelium of mucous surfaces is implanted. Later this disappears, when the enamel rests directly upon the dentine, which, as we shall see below, is formed bv the papilla. It is still a mooted question whether the enamel of the tooth is secreted by the lower layer of columnar cells, or whether it is the product of a direct transformation of the cells themselves. The dentine of the tooth is formed by the media- tion of a double row of branched fusiform and colum- nar cells, which cover the pulp or papilla. The dental cement, or bony incrustation of the den- tine in the root and neck of the tooth, is developed from the fibrous tissue of the dental processes or alveoli. This tissue here has the structure and func- tions of the periosteum of bone. STRUCTURE OF THE TEETH. The fully-developed tooth consists of a crown, neck, and one or more roots, which like the long bones have a central marrow cavity. In this central cavity run the nerves and walls of the vessels. Dentine. Nearly the whole solid portion of the tooth is formed of a hard, compact, brittle substance called dentine. In the crown, the dentine is covered by a coating of enamel, the hardest substance met with in the human frame. In the neck and roots the den- tine is incrusted by a shell of true bone of varying thickness. This incrustation of bone, technically 62 HISTOLOGY. termed cement, is thickest at the deep end of the root. It gradually thins off toward the neck, until at this location it is lost^in the enamel. Dentinal pulp. The dentinal pulp consists of an in- tricate reticulum of delicate branched connective-tis- sue corpuscles, with proportionally large and distinct round or oval double- contoured nucleus, and a small amount of cell-body, except that which constitutes the branched processes. Among these processes is a small number of lymphoid corpuscles and delicate connective-tissue fibres. The capillary bloodvessels have an investing cellular sheath just as in the fibrous form of mucous-tissue. Upon the external surface of the pulp is placed a double layer of branched columnar and fusiform cells, already alluded to. The cells nearest the pulp are more or less fusiform or stellate, with processes running into the pulp, and a few short lateral branches also, which connect one cell of the row with another. The outer ends of the cells of this row taper off into a fine long extremity, which passes between the cells of the outer row, and perhaps beyond them into the deutinal canals. Odontoblasts, and dentinal fibres. The outer row of columnar cells (so-called odontoblasts) is in contact with the dentine. These cells are more or less club-shaped, with the thick end of the club toward the pulp. The nucleus,which is usually somewhat oval in shape, is in this portion of the cell. The outer end of the club- shaped cell tapers off into a fine, long, somewhat tough, and apparently elastic process (dentinal fibre of Tomes). This long process of the odontoblast passes outward through the dentine, and* in doing so frequently branches. The general course of the main fibres is straight or slightly wavy, and is perpendicular to the external surface of the dentine. By means of the lateral branches anastomoses are frequently formed with the processes of neighboring odontoblasts. Ac- cording to most recent investigators, the dentinal fibre is surrounded by a thin structureless membrane, the dentinal sheath of Neumann. The interstitial substance between these dentinal sheaths, which latter constitute the dentinal canals, is a dense reticular substance made extremely hard and brittle by infil- tration with the carbonates and phosphates of lime. When a thin plate of dentine is mounted dry the dentinal canals arc filled with air, and when examined by transmitted light appear dark, like the canaliculi of bone-corpuscles similarly prepared. According to Klein, the dentinal fibres which lie in the dentinal canals are the processes of the inner row of the double layer of cells covering the pulp, and have no distinct connection with the outer row of odontoblasts. The same author thinks that those processes of the latter which enter the dentine, become calcified to form the interstitial substance between the dentinal canals. Nerve-fibres have been traced between the odonto- blasts. Inter globular spaces. At the outer surface of the dentine the dentinal canals open into what are known as the interglobular spaces (b, Fig. 32). These spaces are bounded on the side of the dentine by more or less globular projections of the ground-substance. They contain branched cor- puscles, apparently similar in every respect to Fig. 32. SECTION THROUGH THE ROOT OF A MOLAR TOOTH. a. Dentine traversed by its tubuli. ft. Nodular layer, n. Cementum. (Carpenter.) branched bone-cells. Their branches are continuous with the dentinal fibres, and with branches of cells in neighboring interglobular spaces. Where the dentine is covered with cement, these spaces communicate with adjacent bone-corpuscles, by means of the fine canaliculi of the latter. Beneath the enamel the inter- globular spaces send a few short and irregular blind tubes among the deep ends of the enamel prisms. Cement (cementum). The cement, or bony crust around the root of the tooth, presents the laminated ground-substance and the branching corpuscles of bone. Sharpey's fibres arc present in small num- bers. In the thickest portions, even Haversian sys- tems with their small central canals are sometimes met with. Except in these rare instances of Haver- sian systems, the lamellae are mainly found parallel to the surface of the root. The bone-corpuscles are sometimes unusually large ; they possess numerous branched canaliculi, having the ordinary relations and structure of bone-corpuscles elsewhere, and they in- close typical bone-cells. Enamel. The enamel of the fully formed and healthy tooth is densely calcified. Seen under 'a high power, in a thin section vertical to the sur- face, the enamel appears to be formed of closely- MUSCLE. 63 packed strias, whose direction in general radiates from the pulp-cavity as a centre. In certain spots these radiating striae are crossed by less distinct lines running mainly parallel to the surface of the enamel (transverse striae). Besides these fine striations, there are often two, three, or more narrow, faint, darkish stripes (parallel stripes of Re.tzius) of considerable length ; they exhibit a slightly undulating course, and, as a rule, run parallel to the enamel-surface. The significance of these stripes is not fully under- stood. They may, perhaps, represent the division be- tween successive deposits of enamel. The places at which transverse strise are seen crossing the radiating stria3, contain enamel-fibres running in opposite direc- tions. A minute description of the radiating stride will suffice for both. They consist of long prisms or cylinders of calcified enamel-cells. When seen in transverse section these enamel-prisms appear more or less regularly hexagonal in outline (A, Fig. 33). In longitudinal section after maceration in hydrochloric acid, the enamel-prisms present the appearance rep- resented in B, Fig, 33. At regular intervals the transparent hyaline-substance of the prism is seen to be crossed by extremely minute lines. A further maceration in the acid results in the breaking up of Fig. 33. DIAGRAMMATIC REPRESENTATION OF THE STRUCTURE OF ENAMEL. A. Trans- Verse section of enamel, showing the hexagonal form of its prisms. B. Sepa- rated prisms seen lengthwise. (Carpenter.) the prism into as many somewhat cubical sections as there are areas occupied by these minute cross-lines. These prisms are separated from each other by a small amount of cement-substance. When the enamel begins to form, the enamel-cells lengthen towards the dentine. The lengthened portion soon becomes cal- cified, the calcareous deposit first appearing at the sides of the prism, or in the intercellular cement. After the calcification of the newly-formed end of the columnar enamel-cell has measurably progressed, the dentinal end of the cell again lengthens, and this new portion of the cell is in its turn calcified. This process repeats itself, until the original columnar enamel-cell is much lengthened, and the whole is gradually converted into successive sections of the enamel-prism. It is this successive periodic trans- formation of the enamel-cell into calcified prisms which gives to the latter the peculiar appearance shown in B, Fig. 33. MUSCLE. Muscular tissue comprises two general varieties, unstriped or smooth, and striated. The elements of this tissue are derived from the mesoblast, and are well supplied with capillary vessels and nerves. SMOOTH OR UNSTRIPED MUSCLE. This variety of muscular tissue is composed of spindle or fusiform cells, whose transverse diameter is usually small in proportion to the length of the long axis of the cell. These cells are soft, and are often more or less prismatic from mutual pressure. They may occur more or less isolated, but are usually collected into bun- dles. The cells constituting the bundles are closely packed together, the spindle extremity of one cell fitting between the bellies of two or more. These closely - packed muscle -cells are slightly separated from each other by a small amount of intercellular cement, appa- rently similar to that which unites the cells of epithelial surfaces. In this albuminous intercellular cement are often found a small number of flat, connective-tissue cells, more or less branched, and some- times a very few scattered, de Fig. 34. SMOOTH OR UNSTRIPEDMUSCU- LAR-FlBRE-CELLS FROMAHTERIES [ HUM AS]. 1. From the popliteal artery. A, without, B, with acetic acid. 2. From a branch of the anterior tibial ; a? rod- shaped nuclei of the fibres. Mag- nified 350 diamters. (Gray.) licate connective fibres. This intercellular material corresponds to the endomysium of striped muscles. Arrangement and distribution of smooth muscle- fibres. The muscular bundles thus composed are sep- arated by a variable quantity of ordinary loose con- nective-tissue an analogue of the perimysium sur- rounding bundles of striped muscles. The bundles of smooth muscles may anastomose with each other so as to form a regular muscular network. Such a muscular network may be spread out with a fenes- trated layer. Frequently the bundles are placed side II I S T O L G Y. by side to form continuous muscular membranes, or they may be collected into large bundles or cords. Besides the familiar locations in which smooth muscle- fibres are present, such as the muscular coat of the intestine, the walls of arteries, veins, and large lym- phatic trunks, the iris and ciliary body, etc., they are also found around the ducts of glands and sometimes in the loose connective-tissue between their acini, in the infundibula of the lungs, in the fibrous trabeculas of the spleen, sometimes in the trabecula? of lymph- glands of the lower animals, and in many other places. Form and minute structure of smooth muscle- fibres. The spindle-cell of smooth muscle is sometimes branched at the extremity. The largest cells are found in the pregnant uterus near term, the smallest occur around the ducts of sweat-glands. The minute structure of the smooth, fully-devel- oped muscle-cell consists externally of a very delicate, scarcely visible, enveloping membrane, which seems to be of an elastic hyaline construction. Within this delicate membrane is inclosed the cell-body and nucleus. The body of the cell consists of a soft, gela- tinous substance, in which are imbedded a number of fine fibrils. These fine fibrils run in the direction of the long axis of the cell, and when the element is seen longitudinally give it a delicate, longitudinal striation. The nucleus is generally more or less rod- shaped, and is located near the middle of the cell. It is limited by a thin envelope of double contour, and contains one or more small nucleoli. Within the nucleus is a close reticulum of fine fibrils, which, at the two extremities of the nucleus, are in connection with the fibrils of the cell-body. Seen in transverse section, if the cut passes through at the level of the nucleus, the latter offers a circular outline near the centre of the cross-cut of the cell, which itself now appears more or less circular or polyhedral. In transverse sectioa the cell-body ap- pears dotted with fine points the cross-cuts of the longitudinal fibrils above mentioned. Smooth muscle- fibres, when isolated and examined fresh, sometimes present little thickenings in their course, causing a slight moniliform outline of the element when seen longitudinally. These swellings are supposed to be due to irregular contractions of the substance of the fibre. Over the position of these slight swellings the thin enveloping membrane is often thrown into minute transverse folds or ridges. These occasional ridges of the membranous envelope of the smooth muscle-cell frequently give to the latter an apparent interrupted transverse striation. STRIATED MUSCLE. The voluntary muscles, namely, those which move the skeleton, the tongue, the pharynx, the upper part of the oesophagus, and some involuntary muscles, as for example the heart, and the diaphragm, are com- posed of muscular fibres, which, when fully formed, are striated, or striped. This form of muscular tissue is usually red or flesh color, while, on the contrary, the smooth muscular tissue is ordinarily quite pale. Muscular bundles. Striated muscles are composed of aggregations of very fine striated fibres, imbedded in a connective-tissue framework, which carries the necessary vessels and nerves. The muscle-fibres have certain relations to this connective-tissue framework, as well as to each other. The smallest striped mus- cles consist of a greater or lesser number of striated muscle-fibres (primitive fibres), running parallel to each other, and closely packed together to form a primary bundle. The primitive fibres during life are soft, and capable of assuming outlines due to mutual pressure. Between these more or less closely- packed primitive muscle-fibres, and separating as well as holding them together, is a certain amount of semi- fluid, albuminous cement-substance, in which are to be found a few delicate connective-tissue fibrils, and here and there a flat, perhaps branched, connective- tissue corpuscle (figs. 2, 3, Plate VI.). Occasionally in this interfibrous tissue we see also some large, granular plasrnatic cells of Waldcyer, flattened be- tween the primitive fibres. In this scant framework ramify small nerves and capillary vessels. Endomysium and perimysium. This delicate inter- fibrous tissue is known as the endomysium. It is in connection with the perimysium, a name reserved for the connective-tissue which surrounds the primary bundles. When several primary muscular bundles are bound together to form a larger muscle, we have a secondary bundle formed. In the perimysium or tis- sue surrounding the secondary bundles we have all the characteristics of loose connective-tissue. Even the presence of adipose vesicles may be noted, and arteri- oles and venules are quite frequent, as well as the cor- responding lymphatics. In the larger muscles of the bodjr, we meet with tertiary muscle-bundles, to form which a number of secondary bundles are bound to- gether within a common sheath. Muscle-fibres. The primitive muscle-fibre, when not influenced by lateral pressure, is usually some- what cylindrical in form. Its length is exceedingly great in comparison to the width of its cross-sec- tion. In small, short muscles, it is possible that MUSCLE. 65 each primitive muscle-fibre may extend the whole length of the muscle. The longer and larger muscles are probably composed by many lengths of fibres whose ends overlap. It is believed by some authors that in such muscles no primitive fibre ever ex- ceeds a length of one and a half or two inches. Near their extremities the muscle-fibres taper off' to a more or less sharp conical point. The fibre is, therefore, somewhat spindle-shaped, and has a variable diameter according to the part selected. As in the fusiform smooth muscle-cells, the tapering extremity of one fibre is inserted between the thicker portions of two or more adjacent fibres. A cross-section of a muscu- lar bundle composed of such an arrangement of primitive fibres will necessarily show fibres appa- rently of a widely varying diameter. While there are some actual variations in the size of the several primitive fibres which constitute a given muscular bundle, yet in a muscle which is not rapidly changing, the muscle-fibres have a very uniform diameter. The average diameter of striped muscle-fibres, how- ever, not only differs enormously in different species of animals, but it also varies considerably between different muscles in the same animal. Form and structure of muscle-fibres. The striped muscle-fibre is nearly always unbranched, but in human striated muscles we find some examples of branched fibres, as in the tongue, in some of the facial muscles, and in the heart. In the latter organ the branched fibres unite to form a genuine muscular retioulum. The peculiarities of the cardiac muscle will be considered later. When a striped muscle-fibre is examined under the microscope during life, it is at first a gray and appa- rently nearly homogeneous cylinder. Soon after re- moval from the relations in which it naturally exists, it begins to show a faint segmentation into alternate dark and light cross-bands of unequal thickness. During a state of contraction of the fibre the dark bands are narrowest ; in elongation of the fibre the dark bands reach their greatest width. The reverse is true of the light bands. Contractile disks. The action of hydrochloric acid upon the fibres causes them to be readily broken up into a countless number of thin transverse disks piled one upon another like rouleaux of coins. (See 3, fig. 1, Plate VI.) When the transverse marking into alternate bands of light and dark substance makes its appearance in fresh muscle, the fibres present the as- pect shown in A, Fig. 35, when examined under a magnifying power of 700 or 800 diameters. The dark bands have been called contractile disks, the light bands interstitial disks. Sarcous elements. Sooner or later after the removal of the fibre from its normal surroundings the dark con- tractile disks begin to show extremely narrow light lines which run vertically and divide the tissue into a Fig. 35. B STRIATED MCSCOLAR FIBRE [HUMAN]. A. Portion of a medium-sized muscu- lar fibre, magnified nearly 800 diameters. B. Separated bundles of fibrils, equally magnified, a a, larger, and b b, smaller collections; c, still smaller ; d d, the smallest which could be detached. (Gray.) large number of short rod-like elements of a dark ma- terial. Under these conditions each disk appears to be constituted by a single row of alternating light and dark lines extending parallel to the axis of the mus- cle-fibre. The minute dark rods of which each con- tractile disk is composed are regarded as the ultimate contractile units, and are known as the sarcous elements of Bowman. They lengthen during elongation of the fibre, and shorten and increase in thickness during contraction. When the successive contractile disks of a muscle-fibre are differentiated into these alternate light and dark lines parallel to the axis of the fibre, the optical effect produced is that of a longitudinal striation, for the ends of the dark rods of one con- tractile disk are placed vertically above or below those of adjacent disks. Very frequently, indeed, the longitudinal and the transverse striations are simulta- neously visible in the same fibre. Certain reagents have the effect of bringing out the one form of striation more distinctly than the other. Thus alcohol usually makes the longitudinal striation quite prominent, while on the contrary hydrochloric acid accentuates the transverse strias. 66 HISTOLOGY. &', Fig. 36, gives a vertical view of a transverse disk after the latter has been differentiated into its ulti- Fig. 36. FRAHMESTB OF STRIPED EI.BMBXTARY FIBRES, showing a cleavage in opposite directions. High power. A. Longitudinal cleavage. The longi- tudinal and transverse lines are both seen. Some longitudinal lines are darker and wider than the rest, and are not continuous from end to end. This results from partial separation of the fibrillx. c. Fibrillaj separated from one another by violence at the broken end of the fibre, and marked by transverse lines, c', c". Two appearances commonly presented by the separated single fibrills more highly magnified. At c' the borders and transverse lines are all perfectly rectilinear, and the included spaces perfectly rectangular. At c" the borders are scalloped and the spaces bead-like. When most distinct and definite, the fibrilla presents the former of those appearances. B. Transverse cleavage. The longitudinal lines are scarcely visible, a. Incomplete fracture following the op- posite surfaces of a disk, which latter stretches across the interval, and retains the two fragments in connection. The edge and surfaces of this disk are seen to be minutely granular, the granules corresponding in size to the thickness of tho disk, and to the distance between the faint longitudinal lines, b. Another disk nearly detached, b'. Detached disk (more highly magnified), showing the sarcous elements. (Gray.) mate sarcous elements, by the action of some reagent which has softened or dissolved the interstitial sub- stance between them. Fields of Cohnheim. If a perfectly fresh and living muscle is frozen, and cut into thin transverse sections, and immediately examined under a high magnifying power, the following appearances are noted : At first the surface of the section is uniformly gray. Very soon the field begins to be mottled. The surface is now everywhere marked by fine brilliant lines which cross each other in such a manner as to form an irregular bright network inclosing darkish-gray areas. The dark-gray areas are the ends of the ultimate sarcous elements or the dark rods seen as above stated when the longitudinal fibrillation becomes apparent. The light lines correspond to sections of the bright substance between the dark rods. The width of the bright lines mottling the gray field gradually increases a little during the obser- vation, while at the same time the dark areas limited by the meshes correspondingly lessen in extent. The dark areas are the so-called fields of Cohnheim. The central object in Fig. 37 very well represents the ap- pearance above described, although it was intended merely to show the manner of termination of a nerve. The dark areas or fields of Cohnheim are dotted with extremely fine points. These are the ends of fine fibrils which can be observed in the sarcous elements when the latter are seen longitudinally under favor- able conditions. Various reagents which effect the 7. TRANSVERSE SECTION OF ONE OF THE MUSCLES OF Tin; THIOH op THE LACEETA AGILIS (A COMMON EUROPEAN LIZARD), made whilst frozen, and magnified 400 diameters. N. Nerve of a muscle-fibre which is surrounded by portions of six others, a. Nucleus of tho nerve-sheath, b. Nucleus of the sarcolemma. c. Sec- tion of nucleus of terminal plate of nerve, d. Transverse section of terminal plate, surrounded by granular material, e. Transverse section of muscle-nuclei. /. Fine fat-drops. The angular dark particles are sections of groups of sarcous elements. ( Carpenter.) death of the constituents of the muscle-fibre cause the sarcous elements to shrink from each other and leave proportionately large spaces between them, which are filled with an interstitial cement-substance the light network seen in transverse sections, and the.light lines observed between the rod-shaped sar- cous elements when the fibre is viewed lengthwise. Sarcolemma, intermediate disks, etc. The muscle- fibre is more or less closely enveloped in a thin, tough, elastic, apparently hyaline sheath the sarco- lemma. This elastic tube is partitioned across at short, regular intervals by thin plates which are offshoots from the inner wall of the sarcolemma. Such plates consist of substance somewhat similar to that of the sarcolemma, but are not so tough and resistant. They cross the tube through the middle of the light or interstitial disk, and are known as the membranes or intermediate disks of Krause (z, Fig. 38). The intermediate disks of Krause, therefore, divide the tube formed by the sarcolemma into cylindrical sections, placed end to end. Each of these cylindrical sections contains a dark contractile disk, with a half of an interstitial disk above and below, separating it from the intermediate disks of Krause. In each half of the light interstitial disk is frequently found a thin transverse layer of somewhat dark sub- stance which separates into granules when the dark contractile disk shows a division into sarcous ele- ments. This thin layer of dark substance has been MUSCLE. 67 called the secondary or collateral disk (n, Fig. 38). It requires strong and well-defining lenses for the demon- stration of these details of the structure of muscle- Fig. 38. n - x, STRUCTURE OF STRIATED MUSCULAR FIBRE, AFTER ENRELMANX, FROM TEI.E- PIIORl'3 MELANITRU3 (A COMMON EUROPEAN BEETLE). CUTANEOl'3 MUSCLE FROM THK ABDOMEN. ni. Median disk. n. Secondary disk. z. Intermediate ilisk. Magnified 1000 diameters. The sarcolemma U seen ou the left side. fibres. Engelmann and Henscnn have each recorded the presence, in the middle of the transverse or con- tractile disk, of a transverse, lighter band which they have called the median disk (diagrammaticallv shown at TO, Fig. 38). The above-described dark elements of the striped muscle have been found to be doubly refracting; they consequently polarize light, while the light substance which separates them has not a similar effect upon the luminous rays. The shortening and transverse thickening of the dark rods or sarcous elements of the contractile disk, which takes place during contraction of the fibre, causes the interstitial substance to apparently increase in amount. Since the thickened rods are then more closely pressed together, the fluid portion of the in- terstitial substance must necessarily be squeezed out above and below, and increase the volume of the half of the light or interstitial disk above and below the contractile disk. Frequently a part of this expressed interstitial substance finds its way between the sarco- lemma and the edge of the contractile disk, and sepa- rates the two, thus producing a convexity or bulging of the sarcolemma at these points. This condition is shown, somewhat diagram matically on the left in Fig. 38. Sometimes the attachment of the intermediate disk of Krausc to the sarcolemma breaks, either from a destructive process at work in the fibre itself, or from the action of reagents which soften the intermediate disk and cause the substance of the light interstitial disk to swell greatly. The sarcolemma may then be separated from the surface of the muscle-fibre for a considerable part or the whole of its length. Occa- sionally the muscle-fibre may suffer a transverse frac- ture within the sarcolemma. A retraction of each fragment then takes place, resulting in their separa- tion. Such a condition is shown in i, fig. 1, Plate VI. Theensheathing sarcolemma remaining unbroken be- comes wrinkled, and perhaps twisted, as is seen at n, 1, in the figure last mentioned. These fractures in- variably pass through the light or interstitial disk. The interstitial substance between the sarcous ele- ments often contains, especially in the lower animals, minute molecules, sometimes of pigment, sometimes of fat the interstitial granules of Kolliker. Muscle-corpuscles. Besides the constituents above enumerated, striped muscle-fibres contain, at more or less scattered intervals, cellular elements which ap- pear to be closely analogous to connective-tissue cor- puscles. In nearly all the muscles of most of the higher animals, these elements rest upon the surface of the muscle-fibre, and form a part of it. They are not connected with the sarcolemma. In some of the Amphibia, these cells are scattered through the sub- stance of the fibre (see e, Fig. 37). In birds they are found in both locations. In the muscle of the heart of man they exist near the centre of the fibre. When in the depth of the fibre, they are im- bedded in the interstitial substance, and they consist of a flattened oval nucleus, containing one or more nucleoli, the long axis running parallel with the length of the fibre. In the adult, fully-matured, normal muscle-fibre, the nucleus is surrounded by a very small amount of protoplasm, which is more ex- tensive at the ends of the oval nucleus, where it is frequently lengthened out into a tapering extremity. The broad surface of the cell then presents a some- what fusiform outline. When seen in profile the out- line appears more linear. The protoplasm usually contains a certain number of dark, or, sometimes, shining granules, mostly aggregated at the -poles of the cell. These cells are known as muscle-corpuscles. Their office has been variously interpreted. Some have thought them to be the terminal organs of the nerves, which supply the fibre. Others have regarded them as simple connective-tissue corpuscles. The weight of opinion, however, seems to support the view that they are the builders of the muscle-fibres. The rela- tion which they bear to the growth of muscle will be considered when we speak of its development. Cardiac muscle -fibres. In the heart of man the muscle-fibres possess certain peculiarities. In the first place they seem to be destitute of an investing sarcolemma. In the second place they are very short, and are branched in the manner shown in Fig. 39. Each fibre possesses one or more nuclei, which are imbedded in the depth of the fibre. The end of a branch of one fibre abuts against the end of that HISTOLOGY. of another. In this way an anastomosis of the branched muscle-fibres is formed. When heart- Fig. 39. ASASTOMOSINQ Mcscr.E-FlBREs op THE HEART, seen in a longitudinal section. On the right tho limits of the separate cells with their nuclei ure exhibited somewhat diagrammatical ly. (Gray.) muscle is cut transverse to the direction of the muscle-fibres, instead of the cross-section of the fibre presenting a circular outline, as in ordinary striped muscle, it is often elongated. This is the case when the cut has passed through the body of the fibre near the point where it branches. Because of the extreme shortness of the branched muscle-fibres almost every one of them in the cross-section is seen to contain near its centre a muscle-corpuscle. Termination of muscle fibres. As has already been mentioned, each individual muscle-fibre termi- nates in a tapering conical extremity. Whether these extremities end in tendon, or in the depth or body of the muscle, in the endoinysium between the muscle-fibres of tho primary bundles, they are at- tached to connective-tissue fibres (t, 2, fig. 1, Plate VI.). It seems to be still unsettled exactly how this connection is established. Unless the tissue is especially prepared for exami- nation, the longitudinal tibrillas appear to be directly continuous at their extremity with the connective- fibres, and the one seems to pass insensibly into the other. Perhaps such a simple connection may really exist in many instances; but it is probable that, in most cases, where muscle is inserted into tendon, there is a different mode of communication. The sarco- lemma probably continues without interruption around the conical end of the muscle-fibre, and at this point is attached by its external surface to the bundles of fibrous tissue which form the tendon. Whether the EXPLANATION OF PLATE VI. Fig. 1. The appearances of striated muscle-fibres after varied treatment. Moderate power. (Frey.) 1. A muscle-fibre (m) ruptured at n, showing the sheath or sarcolemma partly emptied and twisted. 2. The end of a muscle-fibre from the biceps of Man, m ; a fibrous bundle, t, of the interstitial connective-tissue is attached to its pointed extremity. 3. A Human muscle-fibre after prolonged treatment with hydrochloric acid. It can now be readily split up into transverse disks, n, nucleus of muscle-corpuscles. 4. A muscle-fibre from the leg of a Frog after protracted treatment with dilute hydrochloric acid. From the cut end, t, very fine fibres, upon which minute granules are distrib- uted, are seen projecting. Fig. 2. Transverse section of Human biceps. High power. (After Frey.) m, Muscle-fibres, cut across ; v, a section of a bloodvessel ; e, a fat vesicle in the interstitial connective-tissue. Fig. 3. Fragment of sartorius muscle of a Frog. High power. (After Ranvier.) n, Nuclei of muscle-corpuscles, seen partly in face. Fig. 4. Muscle-bundle of the dorsal fin of the Hippocampus (a fish commonly known as the sea-horse), showing method of attachment of the fibrous bundle of the tendon. High power. (After Ranvier.) m, Muscle-fibres ; at t they are attached to the tendon- fibres through the intermediation of a double-contoured membrane, which is continuous with the elastic sheath (n) of the bundle. Fig. 5. Highly-magnified capillary bloodvessel, after injec- tion with a weak solution of nitrate of silver, and sub- sequent staining with picrocarminate of ammonia. The dark lines mark out the boundaries of the endothelial cells forming the wall of the vessel; the nuclei are also distinctly seen. (Ranvier.) Fig. C. Shows a capillary bloodvessel, surrounded by and attached to the fibres of the reticular tissue. (Ran- vier.) c, The capillary ; r, the reticular fibres ; n, nuclei of flat endothelial cells upon the reticular fibres. PLATE VI n- ni Fig .4. Fig 5. Fig 6 Fig. 3. BLOODVESSELS. 69 fibres of the tendon are inserted into the sarcolemma at this point, or are simply glued upon its exterior surface by the intervention of a tough, stieky mate- rial, has not yet been determined. Fig. 4, Plate VI., represents such a mode of insertion of a muscle-fibre into tendon. The preparation from which the draw- ing was made is from the large dorsal fin of the Hip- pocampus, or sea-horse. But the artist has failed to show distinctly the most important feature of the preparation. At t, where the muscle- fibre ends in the tendon, the line between the two should have a double contour, and be continuous with the double-contoured line representing the sarcolemma ensheathing the muscle-fibre. DEVELOPMENT OF MUSCLE. Muscular tissue is developed from the cells of the middle or connective-tissue layer of the blastoderm. The fibres of striped muscles originate in the follow- ing manner: A fusiform cell in the connective- or gelatinous tissue of the embryo suffers a division of its nucleus. The two new nuclei divide again, and this process continues until the original uni-nucleated spindle-cell has become more or less completely filled with a number of nuclei arranged in a linear series from one end of the cell to the other. Proceeding equally with this increase of nuclei, the cell thickens somewhat, and greatly elongates. Here and there the cell-body soon begins to show cross-markings - the earliest appearance of the transverse disks. This transformation of the cellular protoplasm into the con- tractile elements of the full-grown muscle continues to spread throughout the fusiform cell-body, until nearly the whole of the latter becomes transversely striated. The protoplasm immediately surrounding the nucleus is the last to experience this metamor- phosis. Even in the full-grown and adult fibre a small por- tion of the original protoplasm remains around the nucleus, and constitutes, with the latter, the muscle- corpuscle above described. It is not yet known whether the sarcolemma is the product of an excre- tion by the protoplasm of the rnuscle-ccll, or whether it is a formation from the surrounding connective-tissue. Reproduction of muscle fibres. In various muscles of the human organism are many striped fibres contain- ing a very large number of muscle-corpuscles, sur- rounded by a proportionately large quantity of proto- plasm, and presenting other evidences of growth. It is probable that in health there is a continual destruc- tion and reproduction of muscle-fibres. It is certainly so in many diseases. BLOODVESSELS. The blood of man flows throughout the body in a system of channels, which, according to their size, construction, and the character of the blood passing through them, are denominated arteries, veins, capil- laries, sinuses. Each of these species of bloodvessels has a char- acteristic structure which generally differentiates it from all the others. But while there is a general plan of construction common to the members of each species, there are slight differences which constitute varieties. Some of these variations will be incident- ally noted. Capillaries. The simplest form of bloodvessel met with in man is the blood -capillary an ex- tremely minute tube. The most primitive form of the capillary is that of a simple cylindrical channel hollowed out in the connective-tissue, with no other definite wall than that of a delicate, elastic, limiting membrane, consisting of a single complete layer of flat, thin, elastic, endothelial plates, such as have already been described as covering serous surfaces. Each of these endothelial plates contains one, some- times two, flattened ovoid nuclei. Under favorable conditions intra-nuclcar and intra-cellular networks can be demonstrated in these cells. Staining by nitrate of silver and exposure to light blackens the intercellular cement between their edges, which are normally in apposition with edges of the adjacent endothclia, and if the preparation is subsequently stained with carmine the nuclei show a brilliant red. The capillary, of which fig. 5, Plate VI., is a very faithful drawing, has been treated in this manner. The outlines of the cells are shown deep black upon the top of the capillary cylinder, and by dotted lines on the bottom portion. The cells are seen to be more or less lozenge shaped, with the long axis of the cell running with the length of the capillary. The edges of cells are observed to be sinuous. The sinuosity of the edges of the cells is much lessened when the capillaries are distended. Lender the influence of irritation or inflammation the endothelia swell up. Their edges then become separated at points, thus forming openings in the capillary wall. The smallest of these are called stigmata, the largest sto- mata. Probably it is through these openings that the elements of the blood escape during inflammation. According to some authors, they are in direct com- munication with the lymph-spaces of the surrounding connective-tissue. 70 HISTOLOGY. A little more complex form of the blood-capillary consists of an addition to the vessel of an incomplete sheath of fusiform or stellate connective-tissue cells, whose branches form a network. These cells float in a thin lymph-space, which in part surrounds the ves- sel, and the two together constitute a delicate outer or adventitious coat (tunica advenlitia). In some loca- tions the capillaries are completely invested by a cylin- drical lymph-channel, in which case the walls of the channel are lined with endothelium, and the exterior surfaces of the capillaries are also covered by similar cells. This is usually the character of the capillaries of the brain and spinal cord. When capillaries run through reticular tissue their walls are connected with the branches of the reticu- lum (fig. 6, Plate VI.). The capillaries vary greatly in size in different locations. In the cerebro-spinal nervous system and in the lungs they are smallest, in the marrow of bone they are largest. Moreover, in some locations, they are quite irregu- lar in calibre. In the inner layer of the dura mater, and in the interstitial connective-tissue of many mus- cles, they sometimes present varicosities or even diverticula of odd forms. Many authors have claimed for the capillaries a moderate contractile power, through the agency of which their calibre may be more or less modified. Arteries. The simplest structure of the artery is found in the smallest vessels of this class. Such ves- sels are known as arterioles. The capillaries pass so gradually into the arterioles that it is often difficult to say exactly where the one begins and the other ends. Tue wall of each arteriole has a much greater thick- ness than that of the capillary in which it ends, and is more complex in its structure. Next to the blood- stream lies the same endothelial layer as is found in the capillaries. It has also an adventitious coat ex- ternally, which is a further development of the deli- cate tunica adventitia surrounding many capillaries. Between these two inner and outer coats of the arte- riole is a third or middle muscular coat. The dis- tinctive feature of the arteriole is the existence of this muscular tissue in the middle coat of the vessel. The muscle-fibres are short, smooth, and spindle-form, and run around the inner coat in a transverse direc- tion. In that part of the arteriole nearest the capillary, these transversely arranged smooth muscle-fibres do not form a continuous layer. The cells are usually a little too short to completely encircle the vessel. At intervals, two or three are grouped together on one side of the arteriole, while the opposite side remains uncovered. By means of an alternation of these groups of cells around opposite sides of the vessel, the arteriole is practically supplied with the means of narrowing and widening its lumen. A little nearer the heart the layer of muscle-fibres Fig. 40. CEREBRAL ARTERIOLES (HUMAN). 1. Smallest artery. 2. Transition vessel. 3. Coarser capillaries. 4. Finer capillaries, tt. Structureless membrane still with some nuclei, representative of the adventitious coat. 6. Nuclei of the muscle-fibre-cells, c. Nuclei within the small artery, perhaps appertaining to the eudothelium. '/. Nuclei in the transition vessels. Highly magnified. (dray.) forms a continuous uninterrupted membrane. The wall of the smallest artery may then be regarded as composed of three coats or tunics the external coat or tunica adventitia, the middle (muscular) coat or tunica media, and the inner coat or tunica intima. The tunica adventitia of the larger arterioles still represents the tunica adventitia or lymph-sheath of the capillaries above described. There is a network of branched corpuscles which lie in lymph-spaces formed by a loose reticulum and felt-work of white fibrous tissue. Between the tunica adventitia and the tunica media in the larger vessels of this class there are fre- quently found a few fine elastic fibres collected into a network. Between the tunica media and the tunica intima in these vessels is also a small number of elas- tic fibres, the elastic layer of the tunica intima. Be- tween such a delicate elastic layer and the endothelia lining the tunica intima exist, even in arteries of this size, a small number of branched connective-tissue cells connected together into a membranous network. In arteries of a larger calibre, the various elements BLOODVESSELS. 71 entering into the construction of the different arterial tunics are more fully developed, while a few other characteristics are added. The tunica adventitia has gained in thickness by a more complete development of a connective felt- work, whose fibres now have a prevalent longitudinal course. Scattered among these bundles of white fibrous tissue appear a few fine elastic fibres. In the loose meshes formed by the inter- crossing of the fibrous bundles are more or less numerous connective-tissue corpuscles and lymphoid cells. These loose meshes thus formed are lyrnph- spaces, which very freely intercommunicate. The elastic fibres become more abundant and larger as the tunica media is approached. At the line of union between the outer and middle coats, the elastic fibres form a dense network, and spread out more or less into a fenestrated membrane which constitutes the line of division between these two tunics. This dense collection of elastic fibres has been termed the external elastic membrane. The tunica media is now constituted by 'a much more numerous collection of smooth muscle-fibres; but, instead of forming as before a continuous mus- cular membrane composed of a single layer of cells, the latter are arranged in several more or less con- tinuous layers, the cells of each layer, however, still running transversely around the axis of the vessel. The several muscular layers of which the tunica media is now composed are separated from each other by plates of elastic tissue in the form of fenes- trated membranes. These elastic plates run mainly longitudinally and at the same time parallel to the curved surface of the vessel. They are connected j with those internal and external to them by means ; of networks of fine elastic fibres (fig. 3, Plate III.) which run among the cells of the muscle-layers. Be- tween the muscular membranes which the last-named cells constitute, is also to be found now for the first time a very small amount of fibrous connective-tissue with elements which usually accompany it. At the external boundary of the tunica intima, and forming a sharp, distinct line of division between it ; and the media tunica, is another dense accumulation j of elastic tissue called the internal elastic membrane. It consists, in small arteries, of two or more fenestrated elastic layers so closely packed against each other as to present in section an appearance of a simple struc- tureless elastic membrane. Internal to this elastic layer of the tunica intima is a slight accumulation of delicate white fibrous tissue. The direction of these fibres is mainly longitudinal. They intercross, how- ever, at acute angles, and form between them lymph- spaces elongated with the axis of the vessel. These spaces contain fusiform and branched connective-tissue corpuscles, as well as an occasional lyrnphoid cell. This connective layer is covered internally by the endothelia lining the lumen of the vessel. The out- line of these cell plates is that of a sharp-pointed lozenge, and their edges are somewhat sinuous. When the artery is cut transversely, the internal elastic layer of the tunica intima is shown beautifully fes- tooned an appearance which gives the inner surface of the arterial wall an extremely wavy outline. Seen in face, the inner surface of the artery appears cov- ered with longitudinal folds or ridges. In the large arterial trunks, the tunica media and intima become much thicker. In the tunica media, the number of muscular layers is much increased, as well as are the thickness and size of the elastic plates between them. The elastic fibres which form a net- work among the muscle-cells are also much stouter than before. Instead of nearly all the muscle-fibres running transversely, as in the smaller vessels, there are to be found in some arteries longitudinal and ob- lique bundles, especially in the inner portion of the tunica media. These are occasionally met with in the tunica adventitia, but rarely in the longitudinal fibrous layer of the tunica intima. The elastic layer of the tunica intima is much thicker, and is also laminated. Between the laminae is to be found a small amount of connective-tissue. The layer of longitudinal fibrous bundles beneath the endothelial lining of the lumen of the vessel is now quite distinct. As a rule, the larger the artery the thicker becomes the muscular tunic and the more numerous the muscle- fibres. In the aorta, however, we have a partial exception to this rule (fig. 1, Plate VII.). Of this great vessel the following characteristics may be enumerated. The tunica adventitia is here compara- tively thin. The tunica media is thick, but the lavers of muscular tissue are thin, and the muscle-fibres which constitute them are scattering. In the inner portion of the tunica media the elastic plates which separate the muscular layers are also thickened and laminated. The fibres of the elastic network which unite the plates and which anastomose among the muscle-fibres are thick, tough, elastic cylinders, whose main direction is longitudinal to the axis of the vessel. In the outer portion of the tunica media the elastic plates are less laminated and are not so thick as in smaller arteries; neither are the elastic fibres so large. An appreciable amount of connective-tissue is scat- tered through the middle coat. The tunica intima is 72 HISTOLOGY. thicker, but does not otherwise differ from that of other large arteries. The muscle-fibres of the arteries are in the main simple, smooth, fusiform cells with rod-shaped nuclei. In the larger trunks the ends may be more or less bifurcated or even branched. In the aorta, flattened, stellate muscle-cells are often met with. The walls of the arteries are relatively thick, and, owing to the large amount of elastic tissue compos- ing them, the lumen is usually patulous. The thick- ness of the arterial wall varies considerably, according to whether the vessel is distended or not. In the large vessels the outer and middle coats are supplied with bloodvessels the vasa vasorum. In a few instances, capillary vessels even enter the tunica intima. Veins. The veius differ considerably from the arteries. In the first place, the lumen of the vein is usually considerably larger than in the artery of the same grade, yet the wall of the vein is much the thinner. There are differences also in the minute structure. The smallest vessels which collect the blood from the capillaries are known as venules. Their walls are extremely thin, yet they can be readily differentiated, as with the arteries, into three coats the external or tunica adventitia, the middle or tunica m.edia, and the internal or tunica intima. The tunica adventitia consists of a simple network, of fusiform or stellate cells floating in a narrow lymph-channel. The tunica media is composed of simple connective-tissue scarcely differentiated. It contains as yet no muscle- elements. The tunica intima is very thin, and is separated into thin layers which closely correspond to those of the tunica intima of arterioles. The outer layer consists of a delicate fibrous membrane with the fibres running longitudinally. The middle layer is represented by a few stellate cells with their branches anastomosing. Resting upon these stel- late cells is the inner layer composed of thin endo- thelial cell-plates similar to those of the arteries in all except outline. They are much broader and shorter than are the same cells of the arteries, and have rather more sinuous outlines. Sti/jmata and ttomata are formed here as in the capillaries, and there may be consequently out-wandering of the blood-cells. In veins of a little larger calibre, the tunica adven- titia increases in thickness and strength by an acces- sion of fibrous bundles, which have a prevalent longitudinal direction, but which branch and inter- lace in such manner as to form a loose meshwork in which lie branched connective-tissue corpuscles, and a few lymphoid elements. At the junction of the tunica adventitia with the tunica media the connec- tive-tissue bundles are aggregated so as to form a more or less distinct fibrous membrane, corresponding in position to the external elastic membrane of the arteries. In most veins of this size the tunica media contains, in addition to the elements of loose connec- tive-tissue, a few scattered, smooth muscle-fibres ar- EXPLANATION OF PLATE VII. Fig. 1. Longitudinal section of thoracic aorta of Man. High power. (After Ranvier.) The central part of the middle coat is not drawn. 1. Internal layer of the internal coat, or tunica intima. 2. External layer of the internal coat. 3. Elastic lamina dividing the internal and middle tunics of the artery. 4. 5. Smooth muscular fibres of the middle coat, cut transversely ; among them are elastic fibres and elastic plates. 6. External coat. Fig. 2. Showing development of capillary bloodvessels in the normal omentum of a Rabbit. High power. (After Klein.) a, Capillary bloodvessels ; 5, connection of the capillaries witli branched cells of the extravascular tissue. It is by metamorphosis of these branched cells that new vessels are formed. Fi": 3. Shows an artificial injection of the blood and lymph- vessels of the parietal peritoneum. Medium enlarge- ment. I, Network of lymph-trunks ; v, venules and arterioles ; c, capillary bloodvessels. Fig. 4. A. Shows, under a moderate power, an injection of the capillary network in the walls of the alveoli of au inflated lung. The vessels were filled from the pulmonary artery. B. An injection of the pulmonary blood-capillaries (I) in the lung of a human foetus the air-vesicles (a) never having been inflated. Fig. 5. Shows a silver injection of the bloodvessels of the lung of a Frog. High power. v, Larger vessel ; v', small arteriole which distributes its blood to capillaries (c) in the walls of the air- vesicles ; a, inter-capillary areas. p L A T V ; ! FigT BLOODVESSELS. 73 ranged transversely. The tunica intima is extremely thin, and presents the same structure as in the venules. In the larger venous trunks the structure of the vessel is essentially the same as that of the vessels last described. The tunica adventitia is strengthened by the presence of a greater quantity of connective- tissue fibres. The tunica media in most cases contains a number of muscle-layers with the cells, for the most part running transversely ; elastic tissue is, however, entirely absent. The layers of muscle-cells are sepa- rated by lamellas of fibrous tissue whose individual bundles pursue a general longitudinal course. The outer layer of the tunica intima consists of a dense fibrous membrane, sometimes laminated. The sub- endothelial layer contains longitudinal fibrous bun- dles, in the interspaces of which are stellate and lymphoid cells. The endothelial lining is not dif- ferent from that of the smaller veins. In many veins, the tunica media possesses no mus- cle-fibres. The veins of bone, of muscle, of the retina, of the membranes of the brain and spinal cord, the cardiac ends of venous trunks emptying into the superior vena cava have no muscles. Some veins possess only a longitudinal muscular coat, as the veins of the pregnant uterus. Others possess an outer lon- gitudinal and inner circular layer of muscle-fibres. In some veins, the distribution of the muscle-fibres is not limited to the middle coat. They are not infre- quently found in the tunica adventitia, and are occa- sionally present even in the tunica intima. The foregoing division of the walls of veins into three distinct coats as in the arteries is not accepted by all investigators. Eanvier thinks that the walls of these vessels should be regarded as consisting of ' only two tunics, an inner and an outer. According to him, it is in the innermost portion of the latter that muscle-fibres are usually located, but they may at times be found throughout the greater part of its thickness. Nearly all the veins are furnished with valves for the purpose of preventing a backward flow of the 'blood. At the location of the valves, there is a slight ampullar enlargement of the calibre of the vein, a provision which prevents a serious encroachment upon the diameter of the blood-channel when the valves I are open and their leaflets flattened against the walls of the vessel. The following peculiarities in the structure of the valves maybe adverted to here: Each surface is covered with a single layer of endothelial plates. Upon the inner surface the endothelia are entirely similar to those lining the vein, i. e., more or less 10 lozenge-shaped, with the long axis parallel to the axis of the vein. Upon the outer surface, however, the long axis of the endothelial plates is, in the main, transverse to the axis of the bloodvessel. Imme- diately beneath the endothelium is a subendothelial layer of connective-tissue fibres interspersed with a few elastic fibres. This layer is thicker and much more abundant in elastic fibres upon the inner than upon the outer surface of the valve. Between and upon these fibres connective-tissue cells, both fixed and wandering, are present in variable number. The subendothelial layers of the two surfaces of the valve are separated from each other by a thin, tough, fibrous membrane composed of interlacing white fibrous bundles whose general direction is parallel to the edge of the valve. Some elastic fibres are also scat- tered among the white fibrous bundles, and according to the statements of some authors a few muscle-fibres may be found near the base of the valve. When muscle-fibres are present, the direction of their long axis is usually transverse to the axis of the vessel. The endothelium and the subendothelial layer, lining the ampullar enlargement of the vein at the location of the valves, are similar to those which cover the outer surface of leaflets of the valve. Sinuses. Custom among anatomists has fixed upon the term sinus a double and somewhat indefinite sig- nificance. Many vessels of the human economy which have received this name possess no circumstance which distinguishes them from veins, other than the simple fact that they constitute various channels in fibrous membranes, e. in an extremely fine terminal net- work. Besides a nerve-fibre, each Pacinian corpuscle is supplied with a minute afferent and efferent blood- vessel and an intermediato capillary plexus. The bloodvessels usually enter and emerge with the nerve- fibre, but sometimes they pass into the capsule at the opposite extremity. The capillaries are distributed between the lamellae of the fibrous capsule, and never reach the central axial space. Occasionally, instead of one Pacinian body being connected with a nerve- fibre, as is usual, the fibre may divide at a node of Eanvier, and each branch end in a Pacinian corpuscle. The group of corpuscles is then often connected to- gether by longer or shorter bands of fibrous tissue. Hair-bulls. The hair-bulbs receive the termina- tions of medullary nerve-fibres, and are sometimes exquisitely sensitive. Nerve-endings in gland-cells. Pfliiger, among others, has carefully studied the relations of nerves to some of the secreting glands, and has found nerve- fibres in direct connection with the nuclei of cells in the acini, and even in the ducts of glands. Fig. 53 shows four modes of direct termination of nerve- fibres in gland-cells. b. Termination of motor nerves. The motor nerves have their peripheral endings among muscle-fibres. Nerve-endings in smooth muscle. In smooth muscles their character, distribution, and termination are not the same as in striped muscle. Non-medullated nerve bundles run and branch in the loose connective-tissue between the bundles of muscular fibres. They are enveloped in a cellular sheath, which is a representa- tion of the perineurium, and they are composed of NERVOUS TISSUE. 83 interlacing fibrils which have no medullary sheath, or sheath of Schwann, but are covered at intervals by flat cells, which are probably the remains of the nerve-corpuscles belonging to the medullary sheath. MOPES OF TERMINATION OF THE NERVES IN THE SALIVARY GLANDS. 1 and 2. Brandling of the nerves between the salivary cells. 3. Termination of the nerve in the nucleus. 4. Union of a ganglion-cell with a salivary cell. 5. Varicose nerve-fibres entering the cylindrical cells of the excretory ducts. (PJHiger.) Such branches communicate with their neighbors to form a plexus the ground-plexus of Arnold. From this ground-plexus come oft' smaller branches com- posed of small groups of individual fibrils. These smaller branches are still covered by a cellular-sheath. They also unite into a plexus, which envelops the perimysium of primary bundles of smooth fibres the intermediary plexus of Arnold. The intermediary plexus immediately enveloping the muscular bundles gives off" branches composed of single beaded fibrils, which enter the endomysiurn and unite to form a deli- cate network surrounding the individual muscle-fibres. Some authors claim that this last network again gives off extremely minute fibrils, which penetrate the muscle-cell and end in the nucleus; others declare the network to be the peripheral termination of these nerve-fibres. Occasionally a few pear-shaped enlarge- ments are met with upon the fine fibrils of this last network. Fig. 4, Plate X., represents, according to Arnold, the nerve-supply of the smooth muscle-fibres in the walls of a small artery. Non-medullated bundles of fibrils, similar to those of the ground-plexus described above for smooth muscles, unite to form a ground-plexus in the tunica adventitia. (It should be mentioned here, in passing, that in the nodes of these ground- plexuses of nerves in smooth muscle there are often located one or more cells, which many regard as gan- glion nerve-cells.) This ground plexus gives origin to minute but still compound branches, which them- selves again unite into a more delicate plexus the intermediary plexus. This last plexus gives off' single fibrils which enter the tunica media, pass between the individual rnuscle-cells, and unite there to form a fine network. The same difference of opinion exists as in the former case concerning the direct connection of the nerve -fibrils with the nuclei of the muscle-cells. In many places capillary bloodvessels are surrounded by networks of nerve-fibrils, as shown in fig. 5, Plate X. Nerve-endings in striped muscle. The termination of motor nerves in striped muscle is peculiar. The nerve-fibres preserve their medullary cylinder and sheath of Schwann until they enter the muscle- fibres. According to the investigations of the most recent authors, each individual rnuscle-fibre receives one or more rnedullated nerve-fibres. The nerve- fibre passes to the muscle-fibre obliquely, and enters the sarcolemma, the neurilemma, or sheath of Fig. 54. a. !> MUSCULAR FIBRES OF LACERTA, WITH THE TERMINATIONS OF NERVES IN END-PLATES OF KBHSE. a. Seen in profile. P. The nerve end-plates. s. The base of the plate, consisting of a protoplasmic mass with nuclei, b. The same seen in face, when a perfectly fresh fibre is examined, the nerve-ends probably being still excitable, c. The same as seen two hours after death from poisoning by curare. Highly magnified. (Kuhne.) HISTOLOGY. Scliwann, ending in and being continuous with the elastic envelope of the muscle-fibre (Figs. 87 and 54). The nerve-fibre, composed of a medullary sheath and axis-cylinder, penetrates the sarcolemma and passes into and is lost in a large flat body, located upon the substance of the muscle-fibre, called the end- plate of Kuhne. Before disappearing in the substance of this end-plate the still medullated fibre may branch and the branches ramify upon the end-plate, each yet retaining its medullated sheath. Where the medul- lary sheath ends, the axis-cylinder spreads out into the surface of the end-plate, and is soon lost to view. The end-plate is usually more or less granular, and sometimes contains a considerable number of oval nuclei. The granular appearance is generally due to the presence of a minute reticulum, which may, per- haps, be continuous with the fibrils of the axis-cylin- der. The nerve-fibrils are not known to pass into j the contractile substance of the muscle-fibre. -Some j authors believe they have seen the branches of the j nerve-fibre extend upon the surface of the sarcous substance beyond the end-plate. Beale denies entirely this mode of ending of the nerves in striped muscles, and substitutes for the end plate of Krause within the sarcolemma a simple net- work of medullated nerve fibres with a number of nuclei in the meshes, claiming that the network rests upon the outside of the sarcolemma. NERVE-CENTRES. I The nerve-centres are constituted by gray or vesicular nerve-substance, and white fibrous substance. The latter consists of nerve-fibres in most respects similar to those which have already been described when considering the nerves, but they have no defi- nite neurilemma. The nerve-fibres will, therefore, not occupy particular attention in this place. The former, the gray matter, contains special cells of peculiar form and structure which have been called nerve- or ganglion -cells. Neuroglia. In the cerebro-spinal nerve-centres both nerve-fibres and nerve-cells are found imbedded in a soft finely-granular variety of connective-tissue which has received from Virchow the special name of neuro'jlia. In the gray substance of the cerebro- spinal centres it is possible, as we shall see below, that the neuroglia may possess peculiarities of func- tion which are probably not common to the neuroglia of the white substance. The neuroglia of the latter consists of a semifluid, -homogeneous substance con- taining a network of extremely minute fibrils which are probably of an elastic nature. At occasional in- tervals, among this minute elastic network, are to be seen branched connective-tissue corpuscles whose fine ramifications are in communication with the fine net- work already described. These branched connective- tissue corpuscles are known as neuroylia-cells. Be- tween the medullated nerve-fibres of the white sub- stance the minute fibrils of the neuroglia-network have mainly a longitudinal direction. In the gray substance, the meshes formed by the neuroglia-fibres are extremely minute and are quite irregular and sponge-like. In addition to the neu- roglia-network above mentioned, there is superadded a minute network of fine fibrils which are derived from the branching processes of the ganglion nerve- cells present in large numbers in the gray matter a network difficult, if not impossible, to distinguish from that composed of simple neuroglia-fibres. This has been termed the nervous reticulum of Gerlach. Some investigators add to the neuroglia of the gray substance of the spinal marrow still another nervous reticulum, w'hich derives its minute fibres from a re- peated division, in the posterior portions of the cord, of nerve-fibres from the sensory roots. In this complex neuroglia of the gray substance the vessels, the ganglion-cells with their branches, and a limited number of medullated fibres, are imbedded. Ganglion-cells. The ganglion-cell is contained with- in a small lyrnph-space in the neuroglia. It is of an irregular outline more or less closely corresponding to the shape of the cell. The size of the latter is comparatively large, but different ganglion-cells vary widely in their dimensions. Some are relatively of enormous volume. The largest are usually found in the anterior horns of the spinal cord, and in the motor centres of the cerebrum. They also vary greatly in form. Some Fig. 55. are oval without processes (apolar) ; some fusiform with one or more processes at either end (bipolar); but the majority of them have many branches which may come off at any portion of their surface (rmtltipolar). The body of the ganglion - cell STELLATE NERVE-CELL, from the contains an intra - cellular nucleus cervicis cornu (posterior . vesicular column) of a Foetus of network of fine fibrils. six months, showing the minute Nearly all the prOGBSSBS of reticulum formed by the dichoto- in mons branching of the processes these cells repeatedly branch (nervous retioulum of Gerlach). dicllOtomOUslv Until the Miuniifled 420 diameters. (Car- penter.) resulting fibres become NERVOUS TISSUE. 85 extremely fine, when they form a minute network in the neuroglia the nervous network of Gerlach above mentioned. Each one of these branching pro- cesses contains a continuation of the intra-cellular Fig. 56. A MEDIUM-SIZED GANOLTON-CEM., from the anterior horn of the gray matter of tho spinal cord of a Calf, isolated after a short maceration in serum containing a little iodine in solution. Magnified 600 diameters. Some of the processes (as at b) are abruptly broken off; a is tho axis-cylinder process of Deiters. (Strieker.) network, the meshes of which are narrow and greatly elongated in the direction of the length of the process. The fibres of the reticula of the processes spread out in the cell-body, and cause the latter to appear to be crossed in various directions throughout its substance by fine fibrillae (Fig. 56, and refer also to fig. 2, Plate XL). In the motor areas of the cerebro-spinal system are found large multipolar ganglion nerve-cells, of which one of the processes differs from the previously described branching processes in several respects. In the first place, the process is smaller at its connection with the body of the cell than are the others (a, Fig. 56). Secondly, as the distance from the cell increases, so also the process enlarges, until finally it becomes surrounded by a medullary sheath (a, Fig. 43). This process has been called the axis-cylinder process of Deiters. It has a distinct longitudinal fibrillation. Those branched ganglion nerve-corpuscles which possess an axis-cylinder process of Deiters on the one hand communicate with the motor nerve-fibres through their axis-cylinder process, whilst on the other hand they are united intimately with the nervous reticulum of Gerlach by means of their finely branching processes. Each ganglion-cell con- tains near its central portion a large spherical nucleus limited by a double - contoured membrane, and inclosing sometimes one or more distinct brilliant nucleoli. The nucleus, like the cell-body, is com- posed of an intra-nuclear network in connection with the intercellular reticulum. In the meshes of these reticula is inclosed a soft semifluid substance some- times holding in suspension brownish-yellow pig- ment-granules. As has been already indicated, these ganglion-cells are suspended in lymph-spaces the per'cellular lymph- spaces which sometimes even in health may contain a small number of lymph-cells. The ganglion-cells of the gray matter of the cerebro- spinal centres vary greatly in size, shape, and distri- bution. Gray matter of the spinal cord. In the spinal cord they are smallest in the posterior portion of the pos- terior gray horns, and are largest in the lateral portion of the anterior horns. Instead of being scattered evenly or irregularly throughout the gray matter of the cord, they are mostly collected into certain well-known groups which extend up and down the cord and form columns of cells. In Fig. 57, three such groups are represented in the anterior horn the so-called internal, anterior, and lateral groups. It is thought by some authorities that these are the only ganglion-cells of the spinal cord which possess axis- cylinder processes, and a direct connection with medul- lated nerve-fibres. In front of and in the anterior \ 86 HISTOLOGY. portion of tbe gelatinous substance in the posterior horn is another column. At the root of the posterior horns in the dorsal region there is still another column of ganglion-cells located near the posterior white col- umns (Clarke's column). Around the central canal or its remains is another aggregation of ganglion-cells. The finely-branched ganglion nerve-cells of the poste- rior horns possess no axis-cylinder processes, and are iu communication with the nervous network of Gerlach only by means of their minute branches. Fig. 87. TRAN3VE T. > jlttin :RSE SECTION OF THE GRAY SUBSTANCE op THE SPINAL CORD THROUOH THE MIDDLE OF THE LUMBAR ENLARGEMENT. On the left side of the figure groups of large cells are seen ; on the right side, the course of the fibres is shown without the cells. Magnified 13 diameters. (After J. L. Clarke.) In the cervical region of the cord a few ganglion-cells are found in the white columns adjoining the lateral group of multipolar cells in the anterior horns. Ependyma of the cord. Around the central canal of the cord the neuroglia of the gray substance be- comes a little more dense than elsewhere, and the fibres of the reticulum have an arrangement peculiar to this location. They follow three main directions. Some are longitudinal, parallel with the axis of the canal; others are concentric, and a few radiate per- pendicular to the surface of the canal. The radial fibres are continuous with fine processes of ciliated columnar epithelial cells which line the surface of the central canal in a single layer. In the human adult the central canal is rudimentary below the cervical region of the cord, and is not patulous. This condensation of the neuroglia at the surface of the central canal constitutes the ependyma of the cord. The canals and various ventricles of the brain are lined by a similar tissue which is there also known as the ependyma, and is invested by a single layer of ciliated columnar cells whose deep ends di- vide into processes which communicate with the neu- roglia-fibres. The particular arrangement of the nerve-fibres and the nerve-cells of the cord will be described in the subsequent chapters of this work. Cortical or yray matter of the train. In the cortex of the brain the gray substance presents general microscopic appearances which are peculiar to it. It seems to be arranged in several illy-defined layers, one passing almost insensibly, and by small grada- tions into those adjoining. Fig. 3, Plate XI., very well represents a view of the cortex of the human cerebrum as it appears when ordinarily prepared for examination, but Fig. 58 gives a more intelligible dia- grammatic sketch of the minute anatomy of the gray matter covering a cerebral convolution. According to Meynert, " speaking generally, the cortex presents five Iamina3(see Fig. 58). The first orsuperficial lamina (J) is principally composed of an evenly punctated non-nervous matrix, with a few small stellate cells, and near its surface numerous fine varicose nerve-fibres decussating in all directions. The second (2) is a layer of close-set, small pyramidal corpuscles. The third (3) is a layer of large pyramidal corpuscles. The fourth (i) is a layer of small, close-set, irregular-shaped cor- NERVOUS TISSUE. 87 puscles; and the fifth (5) is a layer of fusiform corpuscles. " The different parts of the same hemisphere are connected, first, by the numerous inter- communicating processes of the cells, and second, by a system of arcuate-fibres (in, Fig. 58) of different lengths lying imme- diately inside the cortex." Lockhart Clarke differs some- what from Meynert in his de- scription of the general cortex of the cerebrum. He recognizes seven layers, and describes them as follows: - " Most of the convolutions, when properly examined, may be seen to consist of at least seven distinct and concentric layers of nervous substance, which are alternately paler and darker from the circumference to the centre. The laminated structure is most strongly marked at the extremity of the posterior lobe. In this situa- tion all the nerve-cells are small, but differ considerably in shape, and are much more abundant in some layers thau in others. In the superficial layer, which is pale, they are round, oval, fusiform, and angular, but not numerous. The second and darker layer is densely crowded with cells of a similar kind, in company with others that are pyriform and pyra- midal, and lie with their tapering ends either to- wards the surface or parallel with it, in connection with fibres which run in corresponding directions. The broader ends of the pyramidal cells give off two, three, four, or more processes, which run partly through the white axis of the convolution, and in part horizontally along the plane of the layer, to be continuous, like those at the opposite ends of the cells, with nerve-fibres running in different directions. The third layer is of a much paler color. It is crossed, however, at right angles by narrow and elongated groups of small cells and nuclei of the same general appearance as those of the preceding layer. These groups are separated from each other by bundles of fibres, radiating towards the surface from the central white axis of the convolution, and together with thorn TRANSPARENT SECTION op A FCRROW OP THE THIRD CEN- TRAL CONVOLUTION op MAX. Moderately magnified. 1. Lay- er of scattered small cortical corpuscles. 2. Layer of close- set, small, pyramidal, cortical corpuscles. 3. Layer of largo pyramidal corpuscles. 4. Layer of small, close-set, irregularly shaped, cortical corpuscles (granule-like for- mations). 5. Layer of fusiform cortical corpuscles, m. Me- dullary layer. (Meynert.) form a beautiful fanlike structure. The fourth layer also contains elongated groups of small cells and nuclei, radiating at right angles to its plane ; but the groups are broader, more regular, and, together with the bundles of fibres between them, present a more distinctly fanlike structure. The fifth layer is again paler and somewhat white. It contains, however, cells and nuclei which have a general resemblance to those of the preceding layers, but they exhibit only a faintly radiating arrangement. The sixth and most internal layer is reddish gray. It not only abounds in cells like those already described, but contains others that are rather larger. It is only here and there the cells are collected into elongated groups, which give the appearance of radiations. On its under side it gradually blends with the central white axis of the convolution, into which its cells are scat- tered for some distance. "The seventh layer is the central white stem or axis of the convolution. On every side it gives off bundles of fibres, which diverge in all directions, and in a fanlike manner towards the surface, through the several gray layers. As they pass between the elon- gated and radiating groups of cells in the inner gray layers, some of them become continuous with the processes of the cells in the same section or plane, but others bend round and run horizontally, both in a transverse and longitudinal direction (in reference to the course of the entire convolution), and with various degrees of obliquity. While the bundles themselves are by this means reduced in size, their component fibres become finer in proportion as they traverse the layers towards the surface, in conse- quence, apparently, of branches which they give off to be connected with cells in their course. Those which reach the outer gray layer are reduced to the finest dimensions, and form a close network with which the nuclei and cells are in connection. " Besides these fibres which diverge from the central white axis of the convolution, another set, springing from the same source, converge or rather curve inwards from opposite sides, to form arches along some of the gray layers. These arciform fibres run in different planes transversely, obliquely, and longitudinally and appear to be partly continuous with those of the diverging set which bend round, as already stated, to follow a similar course. All these fibres establish an infinite number of communications in every direction, between different parts of each convolution, between different convolutions, and be- tween these and the central white substance." The cerebro-spinal nerve-centres are enveloped by 88 HISTOLOGY. membranous structures, which will be especially con- sidered hereafter. They carry the blood and lymph to and from the nervous tissue. In the white and gray substance of the cerebro-spinal centres, the bloodvessels of all sizes run in lymph-channels. The blood-capillaries of the brain are some of the smallest in the whole organism. The capillary vessels are much more numerous in the gray than in the white substance. SPINAL NERVE-GANGLIA. As is well known, the sensory roots of the spinal nerves pass through a collection of ganglion nerve-cells before uniting with the anterior roots to form a nerve of double function. In leaving the spinal marrow, and passing out of the spinal canal, the nerves per- forate the frail envelope of the cord without receiving any part of it as a covering. In penetrating the arachnoidal and the dural investment of the cord, however, the roots of the spinal nerves receive fibres from eacli of these membranes, and are consequently surrounded by an inner or arachnoidal and an outer or dural sheath, which form respectively a subarach- noid and a subdural lymph-space around the nerve- root, each entirely separated from the other ; but communicating freely with the corresponding space of the spinal cord. As the nerve-roots pass on and unite to form the spinal nerve, the dural sheath be- comes continuous with, and is represented by, the epineurium, while the arachnoid is represented by the perineurium. The spinal ganglia are enveloped in fibrous tissue arranged in a manner quite analogous to that of the nerves. Groups of ganglion-cells are surrounded by a laminated connective-tissue identical in structure and continuous with the perineurium of the nerve- bundles. The ganglion is composed of a larger or smaller number of such groups separated and held together by a loose, tough connective-tissue in which small arteries and veins ramify. This represents the epineurium of the nerves, and is continuous with it. The ganglion-cells constituting a group are separated from each other by a variable amount of delicate, loose connective-tissue very similar to the endoneu- rium separating the individual fibres of the nerve- bundles which penetrate between the ganglion-cells of a group. In this endoneurium capillary blood- vessels course, and single nerve-fibres or groups of medullated and non-mcdullated fibres run between the ganglion-cells. Ganglion-cells. The yanylion nerve-cells are com- paratively large, and of divers sizes and forms. The prevalent form is that ef an oval or somewhat pear- shaped body, with one, rarely two or more processes. This large nerve-cell contains one large spherical, generally excentric, nucleus, containing one or more refractile nucleoli. The nucleus is enveloped in a EXPLANATION OF PLATE XI. Fig. 1. Represents Iialf of a transverse section of the spinal cord of Man in the lumbar region. Very slight enlarge- ment. /, The anterior median fissure ; f, the posterior median fissure ; d, the remains of the central canal lined with colum- nar epithelium, and located in the gray commissure. In front of the latter is seen the decussation of the white fibres of the commissure ; c, the anterior horn of the gray matter ; a, bands of nerve-fibres issuing from the anterior horn to form the anterior root of a spinal nerve ; b, posterior root issuing in a single band from the gray matter of the pos- terior horn ; m, the lateral tract ; t, the posterior radical tract; g, the column of Goll ; the fibres of the two latter together form the posterior column. Fig. 2. Transverse section of a portion of the external an- terior border of the gray matter of an anterior horn of the spinal cord of Man. Highly magnified. c, Cross-sections of medullated nerve-fibres of the adjoin- ing white substance ; a, nuclei of the neuroglia of the gray matter; n, n, nerve-fibres running in the gray matter; a multipolar ganglion-cell, finely striated, is seen imbedded in the gray substance ; within the ganglion- cell a large vesicular nucleus containing a nucleolus is distinctly visible. Fi<*. 3. Vertical section of the cortex of a cerebral convolu- O tion of Man. Medium enlargement. a, Cortical layer in which the prevalent direction of the connective-tissue or neuroglia-fibres is parallel to the sur- face ; b, layer of small club-shaped nerve-cells ; g, layer of larger club-shaped nerve-cells ; c, capillary bloodvessels. Fi, A venule filled with blood-corpuscles ; the venule givss off capillary branches ; I, lymphatic capillary, .invaginating the bloodvessels, and lined with endothelium, the outlines of which are mapped out by the serrated dark lines ; s', branches of the lymph-vessels in some places communicating with the neighboring lymph-spaces. Fig. 2. Internal tunic of the Human aorta, treated with sil- ver. High power. (After Langhans.) a, Superficial lymph-spaces ; b, deeper lymph-spaces ; both freely intercommunicate, and are separated from one anollier by the ground-substance, g. Fig. 3. A. From a section of a Guinea-pig's lung, which had been injected with silver nitrate, showing the relations of blood and lymph-vessels. High power, a, Branch of pulmonary artery ; I, lymph-vessel lined with endothelial plates, and in connection with the lymph-canali- cular system or inter-alveolar lymph-spaces, c, of the walls of the alveoli. B. Same lung, showing the inter-alveolar lymph-spaces, in surface. (After Klein.) PLATE XII . 2 T. S,nclMTlS OI - lilh. PROSPECTUS A SYSTEM OF HUMAN ANATOMY, INCLUDING ITS MEDICAL AND SURGICAL RELATIONS. HARRISON ALLEN, M. IX, PROFESSOR OF PHYSIOLOGY IN THE UNIVERSITY OF PEXXSYI.YA.VIA, ETC., ETC. WITH A CHAPTER ON HISTOLOGY, E. O. SHAKESPEARE, M. D., OPHTHALMOLOGIST TO THE PHILADELPHIA HOSPITAL. To be completed in Six Sections, containing about jjo pages of letter-press, illustrated with jSo figures on 109 plates, many of which are beautifully colored. The drawings by Hermann Fabcr. from dissections by the Author. Also, 250 woodcuts in the text. PRICE PER SECTION, $3.50. Section I. HISTOLOGY. Section IV. ARTERIES, VEINS 'AND LYMPHATICS. II. BONES AND JOINTS. V. NERVOUS SYSTEM. III. MUSCLES AND FASCIAE. " VI. ORGANS OF SENSE, OF DIGESTION AND GENITO-URINARY ORGANS. The plan and scope of the work may be gathered from the following brief extract from the introduction. " It is the design of this book to present the facts of human anatomy in the manner best suited to the requirements of the student and practitioner of medicine. The author believes that such a book is needed, inasmuch as no treatise, as far as he knows, contains, in addition to the text descriptive of the subject, a systematic presentation of such anatomical facts as can be applied to practice. A book which will be at once accurate in statement and concise in terms; which will be an acceptable expression of the present state of the science of anatomy; which will exclude nothing that can be made applicable to the medical art, and which will thus embrace all of surgical importance, while omitting nothing of value to clinical medicine, would appear to have an excuse for existence in a country where most surgeons are general practitioners, and where there are few general practitioners who have no interest in surgery." As a brief introduction to the essential features of the volume, attention is invited to the kinds of knowledge of the human body which the physician demands. First. An exact acquaintance with the form and construction of the organs of the body. But, inasmuch as an anatomical fact is of little use unless the range of application of the fact is known, the due connection between the normal condition of the organs and their variations within the limits of health will receive proper attention, accordingly the typical description of each organ will be followed by a brief statement of such variations. Second. The physician demands a knowledge of the relations of the parts. This information it is necessary to possess in performing operations and in explaining signs and symptoms. Third. The physician needs some account of the uses of the organs. This subject overlaps physiological anatomy. That much only will be succinctly given as may be said properly to illustrate the subject from an anatomical point of view, and at the same time be free from controversy. Fourth. The physician must have a true conception of the nature and general behavior of morbid processes, and of the manner in which such processes are modified by locality. His comprehension of the changes due to diseased action in a given place must be fairly proportional to his knowledge of the normal anatomy of that place. This subject, which will receive the name of localization of diseased action, will be illustrated for the most part by concise statements of recorded cases, in which the essential feature of each case will be emphasized, and the bearing it has on the subject treated of clearly shown. In presenting anatomical features in explanation of given lesions, or of signs or symptoms, care has been taken to give the sources of the statements made. ' "Among other matters, the book will be found to contain an elaborate description of the tissues; an account of the normal development of the body; a section on the nature and varieties of monstrosities; a section on the method of conducting post-mortem examinations; and a section on the study of the superficies of the body taken as a guide to the position of the deeper structures. These will appear in their appropriate places, duly subordinated to the design of presenting a text essentially anatomical." In the preparation of this elaborate work no pains have been spared. The illustrations of normal anatomy, with a few exceptions, are from original dissections, engraved on the stone, with the name of every part clearly drawn upon the figure after the manner of "Holden" and "Gray," and in every typographical detail it has been the effort of the publishers to render the volume worthy of the distinguished position anticipated for it. Each section will be enclosed in an individual portfolio, thus preserving all in a perfect condition in case it is subsequently desired to bind them as a volume. FOR SALE BY SUBSCRIPTION ONLY. REYNOLDS' SYSTEM OF MEDICINE. Revised Edition. Now Ready. V SYSTEM OF MEDICINE. Edited by J. RUSSELL REYNOLDS, M.I)., Professor of the Principles and Practice of Medicine in University College, London. With notes and additions by HENRY HARTSHORNE, M. D., late Professor of Hygiene in the University of Pennsylvania. In three large and handsome royal octavo volumes, containing 3056 double-columned pages, with 317 illustrations. Per volume, cloth, $5 ; leather, $6; half Russia, 6.50. Per set, cloth, $15 ; leather, $18; half Russia, $19.50. The labors of the American editor, Dr. Hartsborne, have been very conscientiously : any other language, for that matter, wliirli equals, mneh less excels, .KeynoMV System. performed, and his judicious notes distributed tbroui:boul, afford abundant evidence of the Each volume contains a complete index, a feature which tbns.- \vho may have encyclopaedic thoroughness of the revision at his hands. In conclusion, we take pleasure ID commending works of medicine not containing this index can fully appre 'Kite. .!//<.'/ I>IH Metlirnl this work to our readers, feeling confident that it will not only become t'aniliinl, but from Xtin, June 10, 1880. its containing just th:it information which the busy practitioner t'requently finds himself in \Ve regard this the tinest work on the practice of medicine iu the Kn-lisb language. In need of, from its completeness, its fulness of detail, .and its excellence, it will >> justly up- fact, we do not think it lias its superior in any lannna^e in the world. Combining, as it predated by the entire profession on this continent. Kml/tern J'rnrllliuner, April, Issu. does, a c miplete history ofdla aaes, a thorough account of their patliolo-y. a lull di'*eripiin For conciseness- and comprehensiveness in the treatment of all the subjects embraced of therapeutics, and a minute detail of treatment, etc., it embodies all that a practitioner under the head of "Practice of Medicine," there is no work in the English language, or in can wish. Ciix-iunni! .V>v//,w/ 7V//;<,<, August 1880. HOLMES' SYSTEM OF SURGERY. Americanized. Just Ready. A SYSTEM OF SURGERY, THEORETICAL AND PRACTICAL. In Treatises by Various Authors. Edited by TIMOTHY HOLMES, M.A., Surgeon and Lecturer on Surgery at St. George's Hospital, London. American edition, thoroughly revised and much enlarged, by JOHN H. PACKARD, M. D., Surgeon to the Episcopal and St. Joseph's Hospitals, Philadelphia, assisted by a corps of thirty-three of the most eminent surgeons of America. In three large and very handsome royal octavo volumes containing 3137 double-columned pages, with 979 illustrations on wood, and 13 lithographic plates, beautifully colored. Per volume, cloth, $6; leather, $7; half Russia, raised bands, $7.50. Per set, cloth, $18; leather, $21 ; half Russia, raised bands. 522.50. Representing originally the most advanced school of British surgery, it has been supple- sufficient guarantee that the work has not only been brought fully up to date, but aKo iliai meuted, through the labors of its editor, by what is latest and best in the surgery of America. it lias been accomplished in this large, thorough and scientific spirit, which cbaracteri/es It may therefore be regarded as embodying whatever is of established value in this depart- the contributions to the original edition. Canada .Imntnl of .Mnlii-nl .sv/?nc<<, Nov. ISM. ment of our art. No surgeon who proposes to keep abreast with bis rivals can afford to be The elegant American revision of this justly celebrated work, w r ill give to the American without it. Ainerii-on Praetilioner, April, 1882. practitioners, a corresponding collaboration of surgical lore, to that they already have in It is a subject for congratulation that the idea of an American edition, incorporating possessing the volumes of Reynold.-.' System of Medicine, and with the two works, the all recently acquired knowledge and experience, should have been conceived and its execution entrusted to such able hands as Packard's. The names of coadjutors atlbrcl a fortunate possessor has a most complete library on Practical Medicine and Surgerv. Brai(htrtiil<'K I!<",/>,-;petl and Quai't> .'""<'' ;'" J'f'/*'fe j\/ wHT" sPiSr^ y^su? -JStf- &%JsfeTW miS^SBK 5&&uP ?32*^ 3? ^ ^& % lH ^%MM igKf^-sra**" TUCr ^?s ^^yErOT ^ft m : $j? * j% '~T^^J^*^ 4^yp^ ^^^S3B; ^s^^^ H i^a?. > rlfife ^-^^ mtf ^'^^i ^^JfZSBMS ^ tR'faSy^.^Fj^'aei. yffiM&Eim