Surgical Anatomy.—The function of a joint is to permit of the movement of one bone upon another. The articular surfaces are covered with a thin layer of hyaline cartilage, and are retained in apposition by the tension of ligaments and of the muscles surrounding the joint. The articular capsule (capsular ligament) is directly continuous with the periosteum, and is lined by a synovial layer, which at the line of attachment of the capsule is reflected on to the bone as far as the articular cartilage. The synovial layer invests intra-articular ligaments, and is projected into the interior of the joint in the form of loose folds wherever the articulating surfaces are not in immediate contact. The surface of the synovial layer is covered with minute processes or villi, which in diseased conditions may become hypertrophied. The synovia owes its lubricating property to mucin, derived from the solution of the endothelial cells on the free surface of the synovial layer. The opposing surfaces of a joint being always in accurate contact, the so-called cavity is only a potential one. If fluid is poured out into the joint, the synovial layer and the capsule are put upon the stretch, causing discomfort or actual pain, which is partly relieved by slightly flexing the joint. If the distension persists, the ligaments become elongated and the joint unstable.
The common origin of bone, cartilage, periosteum, and synovial layer from one parent tissue of the embryo, accords with the readiness with which any one of these tissues may be converted into another under traumatic or pathological influences; and how in ligaments and in synovial membrane foci of hyaline cartilage may form and, after increasing in size, undergo ossification.
Joints derive an abundant blood supply through the articular arteries. The lymphatics, which take origin in the synovial layer, pass to efferent vessels which run in the intermuscular and other connective-tissue planes of the limb. The nerve supply is derived chiefly from the nerves distributed to the muscles acting on the joint and to the skin over it.
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