62 -THE HUMAN BODY. 



on the head of the thigh-bone lies close against that lining 

 the acetabulum, so that practically there is no cavity left 

 in the joint. This close contact is not maintained by the 

 ligaments (which, are much too loose, and serve mainly to 

 prevent such excessive movement as might roll the femur 

 quite out of its socket), but by the many strong muscles 

 which pass between pelvis and thigh-bone and hold 

 both firmly together. In addition, the pressure of the 

 atmosphere is transmitted by the skin and muscles to 

 the exterior of the air-tight joint, and helps to keep its 

 surfaces together. If all the muscles be cut away from 

 around the hip-joint of a dead body, it is found that the 

 head of the femur is still held in its place by the pressure 

 of the air ; and so firmly that the weight of the whole 

 limb will not draw it out ; but if a hole be pierced into the 

 bottom of the acetabulum, and air be thus let into the 

 joint, then the thigh-bone falls out of place as far as the 

 ligaments will let it. 



In all joints we find the same essential parts ; bones, 

 articular cartilages, synovial membrane, synovial liquid, 

 and ligaments.* 



Ball and socket joints. Such a joint as that at the hip 



Is there in health any definite space between the bones of the hip- 

 joint? What is the chief use of the ligaments? How are the bones 

 held together? What in addition to muscles helps to keep the bones 

 of the joint in contact? Describe an experiment illustrating the 

 effect of atmospheric pressure in keeping the bones together? 



What essential parts are round in all joints? 



What is such a joint as the hip- joint called? 



* The structure of joints can be readily seen in those of a fresh calf's or 

 sheep's foot. The synovial membrane is so thin and so closely adherent to the 

 parts it lines that a microscope is needed for its demonstration ; but all the 

 other parts are readily made out. 



