336 THE ANIMALS AND MAN 



rounded head of the femur (F) fitting exactly into the 

 socket and attached to its base by the capsular lig- 

 aments. 



The surface of the ball and of the socket are each covered 

 with a smooth elastic cartilage to prevent friction. Cover- 

 ing- the inside of the ligaments there is a thin membrane, 

 the synovial membrane, which secretes a thick viscid fluid 



Fig. 171. Articulation of pelvis and hip-joint seen from before. (The 

 anterior half of the capsular ligament of the left hip-joint has been 

 removed and the femur rotated outvi'ards.) F, femur; H L, capsular 

 ligament of hip-joint; S, sacrum; L, vertebral ligament inserted on 

 sacrum; L I, ligaments inserted on vertebrae. (After Allen Thomson.) 



called synovial fluid. This acts as a lubricating fluid for 

 joints. 



In a hinge joint, as at the elbow, the bones are so connected 

 as to admit of motion in only two directions, like a door 

 upon hinges. 



Dislocation, fractures and sprains. — A dislocation occurs 

 when the bones of a joint are forced out of place. The bones 

 must be replaced at once by a physician. In the meantime 

 the pain may be relieved by hot or ice cold applications. 



