SYNO'VIAL MEMBRANES AND FLUID. In every joint in which a considerable range of motion is required, the osseous segments (or contiguous extremities of bones) are sep arated by a space, which is called the cavity of the joint. The end of each of the bones entering into the composition of the joint is incrusted by a layer of articular cartilage adapted to its form, and the entire cavity of the joint is lined by a delicate membrane, which is termed the synovial membrane, which secretes a peculiar viscid matter, termed synovia, or synovial fluid, for the purpose of lubricating the inner surface. In its micro scopical characters, a synovial membrane so closely resembles a serous membrane, that we shall content ourselves with referring the reader to the article on the latter structures._ There are, however, certain points of difference, which are fully described in the article " Serous and Synovial Membranes" in The Cyclopadia of Anatomy and Physiology. Like a serous membrane, a synovial membrane is always a closed bag, like the pleurre, for -example, with an attached and a free surface, the latter being smooth and moist. A
very simple form of synovial membrane—anatomically known as a bursa--is employed to facilitate the gliding of a tendon of a muscle or of the integument over a projection of bone. It consists of a bag connected by areolar tissue with the neighboring parts, and secreting a fluid in its interior. These bags are sometimes prolonged into synovial *heaths, which surround long tendons, such as those of the flexor and extensor muscles of the fingers and toes. In deep-seated whitlow (q.v.), when inflammation extends to one of the sheaths, and gives rise to the formation of adhesions, the motion of the inclosed tendon is destroyed, and a permanently stiff finger is the result.
The synodal fluid, or synovia derives its name from its resemblance to the white of an egg (Gr. syn, with, and oYn, an egg). It consists of water holding iu solution mucin, albumen, extractive matters, fat, and inorganic salts. The analysis of Frerichs show that the composition and quality of the synovia vary essentially according as an animal is at rest or leads a wandering life.