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 cen tral organs of the nervous system, and of the retina, which supports the nerve cells, nerve fibres, and blood-vessels of those parts. Microscopically it consists of small round or ovoid corpuscles, imbedded in a soft Glionia, is sometimes produced by the excessive growth in the brain or retina of this variety of connective tissue.
b. leetiform 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 network or reticulum. In the 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 t.,-,,,,, ft,rInc:torinv11 f o 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 Gelatinous or mucous connective tissue (Schleimgewebc), forms the connective tissue of the embryo, the vitreous humour of the eye-ball, and the jelly of Wharton, which invests the blood-vessels of the umbilical cord. It is soft and jelly-like in consistency. Microscopically it consists of rounded, or spindle-like, or stellate cells, imbedded in a soft gelatinous 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.
/connect together the two structures between which the tendon or ligament passes.
In the fibrous form of connective tissue, both cells and intercellular substance, the latter of which is differentiated into fibres, may be recognised. The cells are, as a rule, either elongated, or fusiforna, or caudate, or stellate branched cells, and are familiarly known as the connective tissue coipuscles. In these cells the nucleus is round or oval, and usually well marked. It is surrounded by granular protoplasm, but it is very doubtful if the protoplasm is invested by a cell wall. Not unfrequently, more especially where the cells arc stellate, the delicate branched protoplasm processes of adjacent cells appear to blend at their extremities with each other, and form an anastomosing network. In tendons the cells are arranged in linear rows, which lie parallel to the long axis of the tendon itself. In adults these cells are flattened, but in younger tendons they are more polygonal in form. There seems reason to think, indeed, as Thin has shown, that the bundles of connective tissue are invested by a layer of flattened cells. The wide diffusion of the connective tissue throughout the body, and the great importance of its cellular elements, have been especially dwelt on by Virchow as sources of origin of the new cell forms which arise in various pathological processes.
The intercellular substance consists of fibres, which are not uniform in shape, and are divided into the two groups of white and yellow fibres.
The white ,fibres of connective tissue constitute the most common form, and make up the great bulk of most ligaments, tendons, and fibrous membranes. They consist of excessively delicate filaments, varying from Tasub-th to -5A-0--0-th inch in thickness, which are united together in bundles or fasciculi of variable size. The bundles, as well as the filaments of which they are composed, have a wavy course, and the filaments in each bundle lie almost parallel to each other. The bundles also in some cases are parallel, though in others they cross at various angles. Not only the filaments in each bundle, but the bundles themselves, are cemented together ; the firmness of the adhesion varies in the different modifications of the fibrous connective tissue, being much more decided in the tendons, ligaments, and fasciae, than in the lax areolar tissue.
The yellow fibres of connective tissue, named elastic fibres, from their elasticity, make up the mass of the ligamentum nuchm, the ligaments sub-flava, and the yellow elastic coat of the arteries. They are also found, mingled with the white fibres, in the fibrous membranes, the skin, mucous and serous membranes, the areolar tissue, in ten dons, and some ligaments. In the lin; menta sub-flava and nuehm the yellow fibres are arranged in bundles, the individual fibres of which are comparatively broad, with a distinct dark outline. They branch, and their branches readily break across, and the broken end then curls upon itself. Their diameter is about to-1„th inch. In the coats of the arteries the elastic fibres form an anastomosing network. When mingled with 'the white fibres they are much finer, and sometimes do not exceed Tri--5-6th inch in diameter. They possess, however, a distinct and definite outline; they branch and occasion ally anastomose; and the individual fibres, possessing a ringlike, spiral, or twisted course, are wound around the bundles of the white fibres. The white fibres yield gelatine on boil ing, but the elastic fibres do not. The white fibres swell up and become so transparent under the action of acetic acid as to be no longer recognisable. The yellow fibres, again are not affected by that reagent. Quekett pointed out that the elastic fibres of the ligamentmn nuche of the giraffe were marked by transverse striae, and M. Watson has seen a similar appearance in the elastic pericardiae ligament of the elephant. These transverse stria are apparently cracks in the fibre; and, as Beale has shown, are not 'infrequently seen in the elastic fibres in beef and mutton which have passed through the alimentary canal.
Bearing on the mode of nutrition of the tendons, and other fibrous forms of connective tissues, the existence of plasma, or juice, canals has been described, along which, not blood, but the liquor sanguinis is supposed to flow. Virchow conceived that the connective tissue corpuscles formed an anastomosing network for this purpose. Briicke believed that delicate channels or lacunae existed between the bundles of connective tissue, whilst Recklinghausen maintained that serous canalicnli were situated in the homogeneous substance which connects the fibrous fasciculi and lamellm of the connective tissue with each other. These lacunae or caualiculi are, in all probability, the rootlets of origin of the lymphatic system of vessels. There can indeed be no doubt, as the recent injections of Ludwig and Schweigger-Seidel have shown, that tendons and fasciae are well provided with lymph vessels. for they have in , jected in them a minute network, consisting in part of polygonal meshes, and in part of vessels running longitudinally and parallel to the connective tissue bundles, and the walls of these vessels were formed of endothelial cells. Recklinghausen and othet-- have recently described corpuscles in the connective tissue which resemble in size and appearance the white corpuscles of the blood and lymph. These corpuscles are believed to move about in the juice canals already referred to, and it is possible that they may have migrated into the tissue through the walls of its nutrient bleed-vessels.
The vascularity of the connective tissue varies in different localities. The periosteum and perichondrium are very vascular; but their numerous vessels are concerned in the nutrition not merely of these fibrous membranes, but of the bone and cartilage which they invest. The sheath of connective tissue which invests a tendon is more vascular than the substance of the tendon itself. As a rule, it may be stated that the fibrous connective tissues are not highly vascular, and that the nutritive changes which take place in them after their growth is completed are not very active.
The mode of development of the connective tissue has been much discussed by anatomists, and various views have been advanced as to the changes which lead to its production. It is now, however, generally admitted that it arises from the embryonic cells by a special morphological and chemical differentiation of their protoplasm, but the degree to which this differentiation may proceed varies with the particular form of the texture. In the neuroglia the tissue is apparently a simple nucleated protoplasm. In the retiform connective tissue the cells have assumed a stellate shape, and their branches anastomose. In the gelatinous and fibrous forms an intercellular matrix is extensively pre-' duced, and exhibits a differentiation into fibres. In these' last-named forms, which are the most characteristic varies tics of the tissue, the cells of the embryo change their form, and assume a fusiform, caudate, or stellate shape ; and, subsequently a delicate fibrillated structure appears between them, which assumes the characters of the bundles of white fibrous tissue, and by separating the cells from each other forms the fibrous intercellular matrix. It has been much disputed whether these white fibres take their rise immediately from the peripheral portion of the cells by a direct differentiation of their protoplasm, or whether this protoplasm is not in the first instance converted into a homogeneous matrix in which the fibrous differentiation then occurs. There can be no doubt that the fibres are formed by a metamorphosis of the protoplasm of the cells; whether the metamorphosis takes place directly, or through the intermediate stage of a homogeneous matrix, is a secondary question, and in all probability both modes of conversion take place at different times and in different localities. As the differentiation into fibres progresses, the tissue becomes firmer and tougher, and the proportion of the cellular to the fibrous element diminishes. Hence, say in a young tendon, the rows of connective tissue cells are not only closer together, but are much more readily seen than in an adult tendon, in which the increased production of fibres obscures the cellular element.
The mode of origin of the yellow elastic fibres has also been much discussed. At one time it was believed that they were derived from nuclei, and on this supposition they were named nuclear fibres. But from more recent observations there is reason to believe that they are produced, like the white fibres, by a special differentiation of the protoplasm of the embryonic cells, or of a homogeneous matrix derived from that protoplasm. In such localities as the ligamentum nuchm, where the fibres are both large and numerous, the whole of the cell protoplasm appears to become converted into elastic tissue. In tendons, and those parts where these fibres are slender and scanty, and coil round the bundles of white fibrous tissue, they apparently arise from a differentiation of the protoplasm on the surface only of the formative embryonic cells.