CLASSES OF JOINTS. 125 



between structures whicli 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 

 a lubricating fluid, called synovia or joint-oil ; the foundation of 

 these membranes 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 vasculosus, 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 capsular, 

 which line the capsular ligaments of all true joints; bursal mem- 

 branes, found where one structure moves upon another, as when 

 a tendon plays over a bone, aod known as synovial hiirsce ; 

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

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

 as hursce mucosce. The third form, %'aginal nnenihranes 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 ceils 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. 



CLASSES OF JOINTS. 

 Joints may be divided into three classes — Immovable or 

 Synarthrodial, Movable or Diarthrodial, and Mixed or Amphi- 

 arthrodial. 



