The Framework of the Body 



The principal structures which 

 enter into the formation of a joint 

 are : bone, cartilage, synovial mem- 

 brane, and ligaments. 



Bones make the chief element 

 of all the joints, and their -adjoin- 

 ing surfaces are shaped to meet 

 the special demands of each 

 joint. The joint-end of 

 bones is coated with a thin 

 layer of tough, elastic carti- 

 lage. This is also used at 

 the edge of joint-cavities, 

 forming a ring to deepen 

 them. The rounded heads 

 of bones which move in 

 them are thus more securely 

 held in their sockets. 



Besides these structures, 

 the muscles al^o help to 

 maintain the joint-surfaces 

 in their proper relation. 

 Another essential to the 

 action of the joints is the 

 pressure of the outside 

 air. This may be suffi- 

 cient to keep the articular 

 surfaces in contact even 

 after all the muscles are 

 removed. Thus the hip 

 joint is so completely 

 surrounded by ligaments as 

 to be air-tight. 



FIG. 26. Bones of Right Fooc. 

 (Dorsal surface.) 



