The synovial membrane invests the inner surface of the fibrous capsule, and is reflected over any tendons passing through the joint cavity, as the tendon of the Popliteus in the knee, and the tendon of the Biceps brachii in the shoulder. It is composed of a thin, delicate, connective tissue, with branched connective-tissue corpuscles. Its secretion is thick, viscid, and glairy, like the white of an egg, and is hence termed synovia. In the fetus this membrane is said, by Toynbee, to be continued over the surfaces of the cartilages; but in the adult such a continuation is wanting, excepting at the circumference of the cartilage, upon which it encroaches for a short distance and to which it is firmly attached. In some of the joints the synovial membrane is thrown into folds which pass across the cavity; they are especially distinct in the knee. In other joints there are flattened folds, subdivided at their margins into fringe-like processes which contain convoluted vessels. These folds generally project from the synovial membrane near the margin of the cartilage, and lie flat upon its surface. They consist of connective tissue, covered with endothelium, and contain fat cells in variable quantities, and, more rarely, isolated cartilage cells; the larger folds often contain considerable quantities of fat.