DIARTHROSES. 201 



possession of synovial cavities and the movement of the ends 

 of the bones on one another. 



The articular ends of the bones entering into the formation 

 of the joint are covered with a layer of hyaline cartilage, which 

 takes the place of a periosteum. The articular surface of this 

 cartilage is smooth, polished, and naked, not being covered by 

 perichondrium, synovial membrane, or endothelium ; the 

 superficial cartilage-cells are flattened, the deeper ones often 

 arranged in rows perpendicular to the surface; the deepest 

 stratum of the cartilage is calcified. 



At the margins of the joint the articular cartilages are con- 

 tinuous with synovial membrane, which closes the sides of the 

 joint, and together with the cartilages completely incloses and 

 lines the joint-cavity. 



Beneath the synovial membrane is areolar and fatty tissue, 

 filling in the spaces between the bony parts. 



The synovial membrane in places projects into the cavity as 

 vascular, fatty, or fibrous ridges, fringes, or villi. 



The exterior of the joint, outside the subsynovial tissue, is 

 firmly bound together by ligaments. In some joints the mar- 

 gins of the articulating surfaces are deepened by rims ofjibro- 

 cartilage, as in the shoulder (glenoid ligament), hip (cotyloid 

 ligament), and knee (semilunar cartilages). 



