60 THE HUMAN BODY. 



Joints. Articulations which permit of movement by 

 the gliding of one bone over another are called 'joints ; all 

 are constructed on the same general plan, though the 

 range and direction of movement permitted are different in 

 different joints. As an example we may take the hip- 

 joint, a section through which is represented in Fig. 26. 



FIG. 26. Section through the hip-joint. 



On the outer side of the os innominatum (s, Fig. 8) 

 is a deep hollow, the acetabulum, which receives the upper 

 end of the thigh-bone. The acetabulum is lined by a thin 

 layer of cartilage, with an extremely smooth surface, and 

 its cavity is also deepened by a cartilaginous rim. The 

 upper end of the femur consists of a nearly spherical head, 

 borne on a narrower neck ; this head is covered by car- 

 tilage, and rolls smoothly in the acetabulum like a ball in 



What is a joint? How do joints differ? Describe the hip-joint. 



