Blood. Red corpuscles. It is not an easy matter to devise a classification of the tissues, based on their structural characters, which shall be in all respects logically perfect; but a convenient basis of arrangement for descriptive purposes may be found by dividing them into those which consist-1st, of cells suspended in fluids; 2d, of cells placed on free surfaces; 3d, of cells imbedded in solid tissues. 1st Group.-Cells Suspended in Fluids. The fluids of the body which have cells or other minute solid particles suspended in them are the blood, the lymph, and the chyle. Sometimes cells are found floating in the secretions of glands. THE BLOOD.-The blood is the well-known red fluid which circulates throughout the blood-vascular system. As its composition and general properties will be described in the article PHYSIOLOGY, the solid particles only, which are suspended in the liquor sanguinis, will be considered here. If a drop of human blood be examined under the microscope, crowds of minute bodies, the blood corpuscles, or blood globules, may be seen in it. These present two different appearances, and are distinguished by the names of red and white blood corpuscles. The red corpuscles, which are by far the more numerous, are minute circular discs, slightly concave on both surfaces. Their average diameter is about 6th of an inch, and their thickness about 4th of that measurement; hence they are not spheres, as the old name blood globules would imply. They are non-nucleated. Single corpuscles have a faint fawn-coloured hue, but collectively they give to the blood its characteristic red colour. This colour is due to the presence in the corpuscles of the substance termed hæmoglobin. It has been estimated by Vierordt and Welcker that 5,000,000 red corpuscles are present in every cubic millimetre of healthy human blood. The red corpuscles in the blood of all mammals, except the tribe of camels, are circular bi-concave discs; but in these exceptional mammals they have an elliptical outline. In all mammals the red corpuscles are non-nucleated, though appearances of nucleation have been seen in exceptional individual cases; for Rolleston saw a nucleated appearance in a small proportion of the dried red blood corpuscles of a two-toed sloth; and Turner observed in a proportion of the red blood discs of a Hoffmann's sloth an appearance of a central nucleus. O 1 2 3 In all birds, reptiles, and amphibia the red corpuscles are oval or elliptical, and in each corpuscle an oval or elliptical nucleus is situated. In all fishes they are nucleated and also elliptical in form, except in some of the Cyclostomata, which possess circular discs. In the elliptical nucleated corpuscles the surfaces are not biconcave, but have central projections, which correspond in position to the nucleus (2, 4, 5, Fig. 28). The red corpuscles vary materially in size in different vertebrata, and these variations have been especially studied by Gulliver. He has found them to vary in mammals from an average diameter of th of an inch in the elephant, and 7th in Orycteropus capensis, toth in Tragulus javanicus, and he concludes that the smallest blood discs 6 FIG. 28.-1, red corpuscles of human blood; 2, red corpuscles of blood of common fowl, seen on the surface and edgeways; 3, red corpuscles of frog; 4, of Squalus squatina; 5, of Lophius piscatorius; 6, corpuscles of the blood of a scor pion. In occur in the small species of an order or family, the largest in the large species. In birds they are larger than in mammals, and vary in length from an average of inch in Casuarius javanicus to th in Linaria minor. reptiles they are still larger, and vary in length from an average of 8th in Anguis fragilis toth in Lacerta viridis. In amphibia the largest corpuscles, according to Gulliver, are about inch in length in Proteus and Siren, though Riddell states that in Amphiuma tridactylum they are 3d larger; whilst the smallest, as in the common frog, average in length inch. In cartilaginous fish the corpuscles are larger than in osseous. In Lamna cornubica Gulliver found their long diameter to be inch; while in the Salmonida, which have the largest blood discs among osseous fish, the long diameter in the salmon and common trout is only about Too inch. The white or colourless corpuscles are comparatively few White in number in the healthy human blood. Welcker has corpuscles. estimated the normal relative number as one white to 335 red; in pregnant and menstruating women the proportion is increased to about 1 to 280. In some forms of disease the proportion is so very materially increased that they appear to be almost as numerous as the red. They are rounded in form, finely granulated or mulberry-like in appearance, and nucleated-the nucleus becoming more distinct after the addition of acetic acid; moreover, they are larger than the red corpuscles, their average diameter being from th to th of an inch. Corpuscles of a similar form are found in the blood of all vertebrata. They do not vary so much in size in different animals as do the red corpuscles. In Triton, according to Gulliver, their average diameter is th, whilst in Herpestes griseus they are not more than 37 inch. The white blood corpuscles are minute nucleated clumps of protoplasm; they are therefore minute cells. It is very doubtful if they possess a cell wall, the evidence being against rather than in favour of its presence. If, The red blood corpuscles in all vertebrata, except the mammalia, are nucleated clumps of protoplasm; they are therefore minute cells. In mammals, owing to the absence of a nucleus, they do not accord with the definition of a cell adopted in this article, and they are not therefore morphologically identical with the red corpuscles in other vertebrates. What their precise homology may be is somewhat difficult to say, owing to the obscurity which prevails as to their exact origin. If they are merely clumps of specially modified protoplasm, budded off from the white corpuscles, then they are cytodes. If, as some have supposed, they are the nuclei of the white corpuscles, specially modified in composition, then they are free nuclei. again, they are the white corpuscles, the cell substance of which has undergone a special differentiation, and the nucleus has disappeared, then they are potentially cells, though no nucleus is visible. Whatever may be their exact homology, there can be no doubt that the non-nucleated mammalian red corpuscle, and that part of the nucleated red corpuscle which lies outside the nucleus, are functionally identical with each other; the protoplasm having undergone a special chemical differentiation into hæmoglobin, a proximate principle characterised by containing iron as its essential constituent. The action of water, spirit, acids, alkalies, various gases, heat, cold, and electrical currents, on the red corpuscles has been studied by several observers, and the conclusion has been reached that the corpuscles consist of a stroma," with which the colouring matter is blended, but from which it may be separated without the stroma affording any evidence of the presence of an investing envelope or membrane. Wher blood is drawn from the vessels the red corpuscles, in about half a minute, run together into piles, like rouleaux of coins (Fig. 29), which arrange themselves into irregular meshes. | blood corpuscles are nucleated and larger than the future In inflammatory diseases, and in the blood of pregnant women, the piles of corpuscles form more readily, and at the same time sink rapidly below the surface of the fluid, red discs, but as development goes on, non-nucleated red corpuscles appear, and as their number increases, both absolutely and relatively with the progress of the fœtus, in course of time all the nucleated red corpuscles have disappeared, and are replaced by the non-nucleated discs. In adults the red corpuscles are believed to be derived from the white corpuscles, though the exact process of metamorphosis has not been satisfactorily ascertained. It is also believed that red corpuscles may be new-formed in the spleen, and Neumann has recently stated that the red marrow of the bones may serve as a centre of origin for the red blood corpuscles. In the foetus the liver apparently serves as a centre of origin for the white corpuscles, but its blood corpuscle forming function ceases at the time of birth. Throughout extra-uterine life the spleen and the lymphatic glands are without doubt organs of formation of the colourless corpuscles, those produced in the lymphatic glands, under the name of lymph corpuscles, being FIG. 29.-1, red corpuscles of healthy human blood; 2, red corpuscles beginning mingled with the blood-stream where the fluid lymph flows to form rouleaux; 3, mesh-like arrangement in healthy blood; 4, mesh-like arrangement in buffy blood, where the meshes are larger than in healthy blood. so as to cause the "buffy coat" seen in the blood coagulum. In the healthy blood of horses a buffy coat is formed as a natural condition of the coagulation. One of the most curious properties possessed by the living white blood corpuscle is that of protruding delicate processes from its circumference, which processes may change their shape, or be again withdrawn into the substance of the corpuscle, which then resumes its former circular outline. These processes resemble the sarcode prolongations which Amoeba and other Rhizopods can project from various parts of their circumference; and as a white blood corpuscle, like an Amoeba, can by the movements of the processes change its position, the term "amoeboid movements" has been applied to the phenomena in question. Like an Amoeba, also, a white corpuscle can by these movements include within its substance minute particles of solid matter which it may come in contact with in its path. Thirty years ago W. Addison stated that the white blood corpuscles could pass through the walls of the blood-vessels into the surrounding tissue, where they formed mucus corpuscles, and, under certain pathological conditions, the corpuscles of pus or inflammatory lymph. The passage of white blood corpuscles through the wall of the capillaries was seen in 1846 by A. Waller; and though for many years his observations were ignored, yet the more recent inquiries of Cohnheim and others into the subject have anew directed attention to them. It is now generally admitted that the migration of these corpuscles from the blood through the wall of the capillaries into the tissues does take place, and that they may then "wander" to and fro, owing to the mobility of their contractile protoplasm. These migrated corpuscles are also believed to play an important part in many physiological and pathological processes. But the blood contains, in addition to the red and white corpuscles, still more minute particles, which are, however, inconstant in number. Minute globules have been described by Beale and Max Schultze, which are probably detached fragments of protoplasm budded off from the white corpuscles; and Zimmermann has described, as elementary corpuscles, minute particles, which are apparently derived from broken-up red corpuscles. In the very young embryo the blood corpuscles, like the capillary blood-vessels themselves, are formed by special differentiation of certain of the cells of the embryo, and these young corpuscles seem to have the power of multiplying by fission. At first they are colourless, but afterwards assume a red colour. Even in mammals the earliest red into the venous system. When mixed with the blood, the lymph corpuscles become the white blood corpuscles. Corpuscles are also found in the blood of the invertebrata. They are as a rule colourless, but R. Wagner pointed out that in the Cephalopods they are coloured. They are sometimes round, at others oval or fusiform, and in worms and insects have even branched processes. They are always nucleated. THE LYMPH AND CHYLE.-The lymph is the fluid found Lymph. in a subdivision of the vascular system named the lymph vascular system. It is transparent and colourless, and contains numerous corpuscles floating in it, which correspond, in appearance, structure, and the possession of the property of amoeboid movements, to the white corpuscles of the blood. The lymph corpuscles are formed in the glands situated in the course of the lymph vessels, and are carried away from the glands by the stream of lymph which flows through them. The chyle is a milky fluid found during the period of Chyle. digestion in the delicate lacteal vessels which pass from the walls of the intestine. The lacteals join the lymphatics at the back of the abdomen to form the thoracic duct in which the lymph and chyle become mingled together. The chyle contains corpuscles similar to the lymph corpuscles, which are apparently derived from the lymph glands in the mesentery, through which the chyle flows on its way to the thoracic duct. The fluid of the lymph, the chyle, and the blood, in which the corpuscles are suspended, is sometimes described as a fluid intercellular substance. Corpuscles possessing the type of structure of the lymph corpuscles, are named lymphoid cells or leucocytes. Cells are also met with floating free in the secretions formed in the interior of some of the glands. They are more particularly found in the secretion of mucus from the mucous glands, and of saliva from the salivary glands. They are round, colourless, nucleated corpuscles, not unlike the white corpuscles of the blood, and have been detached from their original position in the gland follicles. 2d Group.-Cells placed on Free Surfaces. By the term free surface is meant a surface which is not blended with or attached to adjacent structures, but is free or separable from them without dissection. Every free surface is covered by one or more layers of cells. Sometimes these cells are named an Epithelium, at others an Endothelium. By the term Epithelium is meant the cells situated on free surfaces which are exposed either directly or indirectly to the air. By the term Endothelium is meant the cells situated on free surfaces which are not exposed either directly or indirectly to the air. Epithe lium. Mucous Membranes. 18 mucous membrane of the mouth. EPITHELIUM.—The free surfaces oovered by an epithe- | in diameter from th tooth inch. Those in the same lium are the skin and the membranes, named, from the layer, being in contact by their character of their secretion, mucous membranes. The edges, form a tessellated, paveMucous Membranes line internal passages and canals, and ment-like arrangement, whilst are continuous at certain orifices with the skin,-e.g., the the cells in adjacent layers have mucous membrane of the alimentary canal opens on the their flattened surfaces in consurface at the mouth and anus; the respiratory mucous tact with each other. Sometimes membrane opens on the surface at the nostrils, and is the cells have jagged, serrated continuous in the pharynx with the alimentary mucous edges, or fluted surfaces, and membrane-it is also prolonged through the Eustachian usually they contain scattered tube into the tympanum, and is continuous through the granular particles. In the forma- FIG. 30.-Scaly epithelium from the nasal duct with the conjunctiva; the genito-urinary mucous tion of this epithelium a morphomembrane opens on the surface at the orifice of the urethra logical differentiation of the protoplasm of the rounded and vagina. Mucous membranes also line the ducts of the cells of the deeper strata into flattened scales, and at the various glands which open on the surface either of the skin same time a chemical differentiation of their soft contents or the several mucous membranes. The epithelial cells are into a horny material, have occurred. as a rule arranged in layers or strata, and the shape of the cells is by no means uniform in the different layers. The cells of the deeper strata are usually smaller, softer, more rounded, and more recently formed than those of the superficial strata, though sometimes, as in the bladder, conjunctiva, and some other mucous surfaces, they may be irregular in form and size, or even elongated into short columns. The cells next the free surface have a tendency to be shed, and their place is then taken by the cells of the deeper layers, which become modified in form as they approach the surface. The form of the cells of the superficial layer varies in different localities, which has led to a division of epithelium into groups bearing appropriate names. Epithelium is distinguished further by being devoid of bloodvessels, i.e., it is non-vascular; and also, with some exceptions, devoid of nerves, i.e., non-sensitive. | B A Columnar or cylindrical epithelium is situated on the free surface of the mucous lining of the alimentary canal from the œsophageal orifice of the stomach to the anus, it is prolonged into the ducts of various glands which open on the alimentary mucous membrane; it covers the mucous lining of the urethra and the mucous membrane of the gall bladder. Its cells are elongated, cylindrical columns, about th inch long, placed side by side like a row of palisades, and with their long axes perpendicular to the surface on which the cells rest. Sometimes the cells are uniformly cylindrical; at other times they are compressed at the sides; at others they vary in circumference, the broader end, lying next the surface, being rounded or polygonal; the deeper extremity being narrower and more pointed. The nuclei are distinct, and the cell contents are finely granular. Usually this epithelium forms only a single layer of cells. The columnar cells which cover the intestinal villi have a clear space at their broad free ends, which is often streaked with fine parallel lines. Intermingled with the cells of the columnar epithelium of the alimentary canal are small goblet-shaped cells. FIG. 31.-Columnar epithelium. A, side view of a group of cells; B, larger free end of a group of cells; C, a striated columnar cell from intestinal villus. The epithelial cells, whether arranged in one or several strata, rest upon a subjacent tissue, which, from its relation to the cells, may be called sub-epithelial. The subepithelial tissue is a delicate modification of the fibrous form of connective tissue, to be subsequently described, and in it the nerves and the blood and lymph vessels of the skin and mucous membranes ramify; hence it is sometimes described as a fibro-vascular tissue or corium. It was for a long time believed that between the deeper surface of the epithelium and the corium a homogeneous continuous membrane, named by Bowman a basement membrane, intervened. Bowman, however, himself admitted that in some of the localities where this membrane was theoretically supposed to exist it could not satisfactorily be demonstrated; and the general opinion of anatomists now is, that a distinct separable membrane does not intervene between the epithelium and the fibro-vascular corium, but that the cells of the former rest directly upon the surface of the latter. The corium is also the seat of the numerous glands, with the blood and lymph vessels and the nerves belonging to them, found in connection with both the skin and the mucous membranes; and the epithelial lining of the g'ands is continuous at their orifices with the epithelial investment of the corium. The surface both of the skin and mucous membranes is usually more or less undulated-sometimes it is thrown into strong folds or ruga, at others it is elevated into minute, frequently conical, processes, named in some localities papillæ, in others villi; but in all these cases the epithelium is prolonged as a continuous covering over the undulat-phery possesses the firmer consistence FIG. 32.-Ciliated epithelium ing free surface. The free surface of all mucous membranes is kept moist by the secretion or mucus which lubricates it. Tessellated, pavement, scaly, or squamous epithelium is situated on the free surface of the mucous lining of the mouth, pharynx, œsophagus, vestibular entrance to the nose, ocular conjunctiva, and entrance to the urethra and vagina. It forms, under the special name of the horny layer of the cuticle or epidermis, the superficial investment of the skin. Its cells are nucleated flattened scales, varying Ciliated epithelium is situated on the free surface of the nasal mucous membrane, which extends into the air-sinuses within the cranial bones, into the nasal duct and lachrymal sac, into the Eustachian tube and tympanum; on the free surface of the mucous membrane of the windpipe as far as the terminal branches of the bronchial tubes; on the mucous surface of the uterus and Fallopian tubes; on the mucous lining of the commencement of the vas deferens, and on the lining membrane of the ventricles of the brain and central canal of the spinal cord. It generally consists of columnar cells, which have at their free ends extremely slender, soft, pellucid, hair-like processes, or cilia. These cilia are specially differentiated at the free ends of the epithelium cells from which they project. Beale states that the soft bioplasm (protoplasm) of the body of the cell is prolonged along the axis of each cilium, whilst the peri cells. of formed or differentiated material. Secreting glands. Endothelium are either spheroidal or cylindrical; but as the cavities lined | the surfaces or parts which move on each other, and the they move, and play an important part in the economy of many animals; in some of the invertebrata they serve as organs of locomotion, in others they propel currents over respiratory surfaces, and in others aid in bringing food within the animal's reach. Spheroidal or glandular epithelium is situated on the free surface of the follicles or ultimate secreting apparatus of glands, and the commencement of gland ducts. The cells are often spheroidal in form, though not unfrequently they are polyhedral. Their contents are specially differentiated into the secretion of the particular gland in which they are situated. The epithelial cells of a Secreting Gland rest upon a subepithelial tissue. Not unfrequently this tissue has the appearance of a membrane; it represents, indeed, the base ment membrane of Bowman, and is called membrana propria. Deeper than this apparent membrane is a delicate connective tissue in which the blood and lymph vessels and the nerves of the gland ramify. The anatomical structures necessary for secretion are cells, blood-vessels, and nervesThe blood-vessels convey the blood from which the secretion has to be derived: the cells, as Goodsir showed by a variety of proofs, are B the liver; B, spheroidal gland cells from the saliva the active agents in separat- FIG. 33.-A, polyhedral gland cells from ing the secretion from the blood; the nerves regulate the size of the blood-vessels, and therefore the amount of blood which circulates through the gland, and perhaps also exercise some direct influence on the activity of the cells. The connective tissue and the membrana propria are merely supporting structures for the cells, vessels, and nerves. All secreting glands have the same general type of structure, though they differ from each other, as will be pointed out when the individual glands are described, in the degree of complexity in which their constituent parts are arranged. Transitional epithelium is the name applied to epithelial cells, situated on some free surfaces, which possess transitional forms either between the columnar and tessellated epithelia, or the columnar and spheroidal. The epithelium of the mucous lining of the bladder is transitional between the columnar and scaly varieties; and in many glands the continuity of the epithelial layer from the spheroidal epithelium of the gland follicles to the columnar epithelium of the ducts is preserved by the interposition of intermediate transitional forms of cells. The epithelial surfaces of the upper part of the mucous lining of the nose and of the back of the tongue are specially modified in connection with the senses of smell and taste localised in those regions, as will afterwards be considered when their anatomy is described. ENDOTHELIUM.-The free surfaces covered by an endothelium are the serous membranes, the inner surface of the walls of the lymph and blood vessels and of the heart, the synovial membranes of the joints and of synovial bursæ, the free surface of the osseous and membranous labyrinth of the internal ear, and the free surface of the ventricular cavities of the brain and central canal of the spinal cord. The tubes, canals, and cavities lined by an endothelium are shut off from all communication with the external atmosphere. The cells of the endothelium are arranged so as to give perfect smoothness to the surface which they cover. In the blood and lymph vessels this smoothness of surface is in order to facilitate the flow of the blood and lymph in the course of the circulation. The serons and synovial membranes are found covering branes. Each Serous Membrane consists of a portion which invests Serous the viscus or organ, named the visceral layer, and a portion memwhich lines the walls of the cavity in which the organ is situated, named the parietal layer. Between these two layers is the so-called serous cavity, the wall of which is formed by the smooth surfaces of both the parietal and the visceral layers. The serous membranes are as follows:-The two pleuræ situated in the cavity of the chest, one investing each lung, and lining the interior of that part of the thoracic cavity in which the lung is situated; the pericar dium, which invests the heart, and lines the bag in which the heart is contained; the peritoneum, which invests the abdominal viscera, and lines the abdominal cavity; and the arachnoid membrane, which invests the brain and spinal cord, and is regarded by many as lining the dura mater, which encloses these important organs. The smooth free surfaces of the serous membranes are moistened by a limpid fluid, or serum, which facilitates their movement on each other, just as the free smooth surfaces of the synovial membranes are lubricated by the viscid synovia which they secrete. Endothelial cells form usually only a single layer, and are, as a rule, flattened scale-like cells, arranged after the manner of a tessellated epithelium. Endothelium, like epithelium, is non-vascular, and, so far as is known, non-nervous. The endothelial cells rest upon a sub-endothelial tissue, consisting of a delicate modification of the fibrous form of connective tissue. Here, as in the surfaces covered by epithelium, a basement membrane was at one time supposed to intervene between the cells and the connective tissue; but it is now believed that the cells are in direct contact, by their deeper surface, with the connective tissue itself. In the serous membranes and in the coats of the larger blood-vessels elastic fibres are present in considerable numbers in the sub-endothelial tissue, which serves as the framework of support for the blood and lymph vessels and the nerves of the part. In the serous membranes the lymphvessels are very abundant in the sub-epithelial tissue, where they form a layer parallel to the free surface of the membrane, from which short vessels pass vertically to open by minute orifices into the serous cavity. The serous membranes are attached by the sub-endothelial connective tissuз to the organs which they invest. The endothelium of the Serous Membranes consists of irregular and squamous cells, the edges of which may be smooth or slightly serrated. The cells are closely adapted to each other by their edges, so as to form a continuous smooth layer, which forms the free surface of the serous membrane. Scattered irregularly over this surface are the minute orifices, or stomata, which open into lymphatic vessels. The cells which surround the stomata differ in form and appearance from the ordinary endothelium; FIG. 34. Endothelial cells from the they are smaller, and are polyhedral, their contents are granular, and the nucleus is more distinct. peritoneal serous membrane. Three stomata may be seen surrounded by polyhedral nucleated cells; the one to the left is closed. The light band marks the position of a vertical lymphatic vessel. (After Klein.) The endothelium lining the Lymphatic Vessels consists of flattened cells, which, instead of having an irregular shape, are elongated spindles, slightly sinuous in outline. The endothelium of the lymphatics is continuous with that of the serous membranes through the stomata, so that the cavities of the serous membranes are now regarded as great lymph-sacs. Connective tissue. The endothelial lining of the Blood Vessels corresponds | in general characters with that of the lymphatics. In the small blood capillaries the cells are fusiform; in those of larger size, more irregular: in the veins they are broader, more irregular, and less distinctly fusiform than in the arteries. The endothelial covering of the endocardial lining of the heart consists of a layer of flattened cells with irregular outlines. The endothelial lining of the blood-vascular system is continuous with that of the lymph-vascular system, where the thoracic duct and other large lymph-vessels open into the great veins, and thus a continuity of surface is established between the serous membranes and the lining membrane of the blood-vascular system through the lymphatics. The endothelium of the Synovial Membranes is formed of roundish, or polygonal, or tessellated cells, arranged after the manner of a stratified epithelium. Not unfrequently processes of the sub-endothelial vascular connective tissue covered by the endothelium project into the cavities of joints and synovial bursæ. They have been called synovial fringes, and contribute to the formation of the synovia which lubricates the surfaces of a synovial membrane. The endothelium of the Cerebral Ventricles and Central Canal of the spinal cord is, as already stated, formed of spheroidal or cylindrical cells, possessing cilia on the free surface. The endothelial lining of the osseous labyrinth consists of flattened scales, whilst the membranous laby rinth possesses a layer of polygonal cells. 3d Group.-Cells imbedded in Solid Tissues. The cells which are imbedded in the solid tissues are either grouped together in considerable masses, or, as not unfrequently happens, are more or less separated by an intermediate matrix or intercellular substance. The matrix substance varies in its character in different tissues, and sometimes is so abundant as to obscure the cells. The textures which are constructed on this plan are of great importance, and constitute by far the larger proportion of the tissues not only of the human body, but of the bodies of animals generally. Sometimes these tissues are elongated into delicate threads or fibres, at other times they are expanded into thin membranes, at others they form solid masses of considerable thickness. CONNECTIVE TISSUE.-By the term connective tissue is meant a group of tissues which, though the members of the group differ in various respects from each other, both in naked eye and microscopic characters, yet agree in the property of binding or connecting together other tissues or parts of the body, and in serving as a supporting framework for more delicate tissues. This group of tissues is the most extensively diffused of all the textures, for there is no organ in the body which does not contain one or other of its forms. The following varieties, based on modifications in their appearance and structure, may be recognised. a. Neuroglia. This name, which means nerve glue, has been applied by Virchow to the delicate tissue in the central organs of the Glioma, is sometimes produced by the excessive growth in the brain or retina of this variety of connective tissue. b. Retiform connective tissue constitutes the stroma or supporting framework of the lymphatic and other glands which possess the adenoid type of tissue. It also forms the middle subdivision of the enamel organ of the teeth. It consists of stellate branching cells, the branches of which blend with each other, and form a delicate anastomosing FIG. 36.-Retiform connective network or reticulum. In the tissue from a lymphatic gland. lymph glands, the colourless lymph corpuscles are set in the meshes of this network. In the solitary and Peyer's glands of the alimentary canal, in the tonsils, the back of the tongue, the posterior wall of the nasal part of the pharynx, the palpebral conjunctiva, the thymus gland, the pulp and Malpighian bodies of the spleen, colourless lymph-like corpuscles are also included in the meshes of a reticulum. The name adenoid or lymphoid tissue is sometimes employed in describing this type of structure, and in some forms of disease the tissue increases in a soft gela- FIG. 88.-Gelatinous connective tissue. The fusiform and tinous intercellular substance. Sometimes the intercellular substance is in part differentiated into short delicate fibres. Under some pathological conditions, this form of tissue increases largely in quantity in some parts of the body, and forms a kind of tumour named Myxoma. stellate cells, and the partial differentiation into fibres of the intercellular substance, are shown. d. Fibrous connective tissue presents four modifications in appearance. It may be soft and delicate, with the fibres short and but faintly marked, as in the sub-epithelial tissue of the skin and mucous membranes. It may be loose, flocculent, and filamentous, and may contain small spaces or areolae (when it is called areolar tissue), which is well seen in the subcutaneous tissue of the adult, and in the omenta. It may be expanded in the form of a fibrous membrane, as in the fascia or aponeuroses, and the threads or fibres, strong and well marked, sometimes run parallel, sometimes cross each other at various angles. It may be collected into rounded or flattened bands, as in tendons and ligaments, where it forms the tendinous and ligamentous tissues. Here also the threads or fibres may be distinctly recognised and seen to run in parallel bundles, so as to I. 107 |