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3. Tendons, Aponeuroses, and Fasciæ
FIG. 377 - Subcutaneous tissue from a young rabbit. Highly magnified. (Schäfer.) Tendons are white, glistening, fibrous cords, varying in length and thickness, sometimes round, sometimes flattened, and devoid of elasticity. They consist almost entirely of white fibrous tissue, the fibrils of which have an undulating course parallel with each other and are firmly united together. When boiled in water tendon is almost completely converted into gelatin, the white fibers being composed of the albuminoid collagen, which is often regarded as the anhydride of gelatin. They are very sparingly supplied with bloodvessels, the smaller tendons presenting in their interior no trace of them. Nerves supplying tendons have special modifications of their terminal fibers, named organs of Golgi. Aponeuroses are flattened or ribbon-shaped tendons, of a pearly white color, iridescent, glistening, and similar in structure to the tendons. They are only sparingly supplied with bloodvessels. The tendons and aponeuroses are connected, on the one hand, with the muscles, and, on the other hand, with the movable structures, as the bones, cartilages ligaments, and fibrous membranes (for instance, the sclera). Where the muscular fibers are in a direct line with those of the tendon or aponeurosis, the two are directly continuous. But where the muscular fibers join the tendon or aponeurosis at an oblique angle, they end, according to Kölliker, in rounded extremities which are received into corresponding depressions on the surface of the latter, the connective tissue between the muscular fibers being continuous with that of the tendon. The latter mode of attachment occurs in all the penniform and bipenniform muscles, and in those muscles the tendons of which commence in a membranous form, as the Gastrocnemius and Soleus. The fasciæ are fibroareolar or aponeurotic laminæ, of variable thickness and strength, found in all regions of the body, investing the softer and more delicate organs. During the process of development many of the cells of the mesoderm are differentiated into bones, muscles, vessels, etc.; the cells of the mesoderm which are not so utilized form an investment for these structures and are differentiated into the true skin and the fasciæ of the body. They have been subdivided, from the situations in which they occur, into superficial and deep. The superficial fascia is found immediately beneath the integument over almost the entire surface of the body. It connects the skin with the deep fascia, and consists of fibroareolar tissue, containing in its meshes pellicles of fat in varying quantity. Fibro-areolar tissue is composed of white fibers and yellow elastic fibers intercrossing in all directions, and united together by a homogeneous cement or ground substance, the matrix. The cells of areolar tissue are of four principal kinds: (1) Flattened lamellar cells, which may be either branched or unbranched. The branched lamellar cells are composed of clear cytoplasm, and contain oval nuclei; the processes of these cells may unite so as to form an open network, as in the cornea. The unbranched cells are joined edge to edge like the cells of an epithelium; the “tendon cells,” presently to be described, are examples of this variety. (2) Clasmatocytes, large irregular cells characterized by the presence of granules or vacuoles in their protoplasm, and containing oval nuclei. (3) Granule cells (Mastzellen), which are ovoid or spheroidal in shape. They are formed of a soft protoplasm, containing granules which are basophil in character. (4) Plasma cells of Waldeyer, usually spheroidal and distinguished by containing a vacuolated protoplasm. The vacuoles are filled with fluid, and the protoplasm between the spaces is clear, with occasionally a few scattered basophil granules. In addition to these four typical forms of connective-tissue corpuscles, areolar tissue may be seen to possess wandering cells, i.e., leucocytes which have emigrated from the neighboring vessels; in some instances, as in the choroid coat of the eye cells filled with granules of pigment (pigment cells) are found. The cells lie in spaces in the ground substance between the bundles of fibers, and these spaces may be brought into view by treating the tissue with nitrate of silver and exposing it to the light. This will color the ground substance and leave the cell-spaces unstained. Fat is entirely absent in the subcutaneous tissue of the eyelids, of the penis and scrotum, and of the labia minora. It varies in thickness in different parts of the body; in the groin it is so thick that it may be subdivided into several laminæ. Beneath the fatty layer there is generally another layer of superficial fascia, comparatively devoid of adipose tissue, in which the trunks of the subcutaneous vessels and nerves are found, as the superficial epigastric vessels in the abdominal region, the superficial veins in the forearm, the saphenous veins in the leg and thigh, and the superficial lymph glands. Certain cutaneous muscles also are situated in the superficial fascia, as the Platysma in the neck, and the Orbicularis oculi around the eyelids. This fascia is most distinct at the lower part of the abdomen, perineum, and extremities; it is very thin in those regions where muscular fibers are inserted into the integument, as on the side of the neck, the face, and around the margin of the anus. It is very dense in the scalp, in the palms of the hands, and soles of the feet, forming a fibro-fatty layer, which binds the integument firmly to the underlying structures. The superficial fascia connects the skin to the subjacent parts, facilitates the movement of the skin, serves as a soft nidus for the passage of vessels and nerves to the integument, and retains the warmth of the body, since the fat contained in its areolæ is a bad conductor of heat. The deep fascia is a dense, inelastic, fibrous membrane, forming sheaths for the muscles, and in some cases affording them broad surfaces for attachment. It consists of shining tendinous fibers, placed parallel with one another, and connected together by other fibers disposed in a rectilinear manner. It forms a strong investment which not only binds down collectively the muscles in each region, but gives a separate sheath to each, as well as to the vessels and nerves. The fasciæ are thick in unprotected situations, as on the lateral side of a limb, and thinner on the medial side. The deep fasciæ assist the muscles in their actions, by the degree of tension and pressure they make upon their surfaces; the degree of tension and pressure is regulated by the associated muscles, as, for instance, by the Tensor fasciæ latæ and Glutæus maximus in the thigh, by the Biceps in the upper and lower extremities, and Palmaris longus in the hand. In the limbs, the fasciæ not only invest the entire limb, but give off septa which separate the various muscles, and are attached to the periosteum: these prolongations of fasciæ are usually spoken of as intermuscular septa. The Fasciæ and Muscles may be arranged, according to the general division of the body, into those of the head and neck; of the trunk; of the upper extremity; and of the lower extremity.
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