SYNOVIAL MEMBRANES. 125 



SYNOVIAL MEMBRANES. 



These are thin membranes lining the capsular or binding liga- 

 ments of true joints, or they are interposed elsewhere between 

 structures which move one upon another, and which would 

 otherwise be injured by the friction. They resemble what are 

 termed serous membranes, in lining closed cavities, and in 

 containing a layer of endothelial cells which secrete a lubricating 

 fluid, called synovia or joint-oil ; the foundation of these mem- 

 branes is a layer of connective tissue, which varies considerably 

 both in density and thickness. 



The synovial membrane terminates at the border of the articu- 

 lar cartilage, which in the adult it never covers ; in the foetus it 

 is said by some authorities that it covers the cartilage also. 

 Near the borders of articular cartilages the membrane is generally 

 found as a projecting fold, the projection being due to a small 

 pad of fat, interposed between the membrane and the capsular 

 ligament. These projections were once erroneously termed 

 synovial glands, but their use is probably to assist in forcing the 

 synovia between the opposing surfaces of cartilage. 



Under the cellular layer blood-vessels are found, and near the 

 articular cartilage there is formed a vascular zone, the circulus 

 articuli vasculosiis, in which the capillaries terminate by dilated 

 loops ; articular cartilage derives its nourishment partly from this 

 source (see page 121). 



There are three forms of these membranes : — the capsulm% 

 which line the capsular or other ligaments of true joints ; bursal 

 membranes, found where one structure moves upon another, as 

 when a tendon plays over a bone, and known as synovial bursce ; 

 or, when they exist in the subcutaneous tissue, between the 

 skin and certain prominent parts of the skeleton, they are known 

 as bursal mucosa?. The third form, vaginal membranes or sheaths, 

 exist where one tendon forms a sheath for another, or in other 

 canals through which tendons glide. 



Synovia or joint oil, is a viscid, transparent fluid, colourless, 

 or pale yellow, physically resembling oil, but it contains very 

 little fatty material consisting chiefly of albumen, salts, and 

 water ; it is secreted by the cells on the inner surface of the 

 membrane. When an animal is in active exertion, there is a 

 greater demand for joint oil than when at rest, consequently there 

 is an increased secretion of it. 



