STRUCTURE AND COMPOSITION OF BONE. JOINTS. 93 



lage which has an extremely smooth surface. The bony cup 

 is also deepened a little by a cartilaginous rim. The proximal 

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

 borne on a somewhat narrower neck, and fitting into the ace- 

 tabulum. This head also is covered with articular cartilage; 

 and it rolls in the acetabulum like a ball in a socket. To 

 keep the bones together and limit the amount of movement, 

 ligaments pass from one to the other. These are composed 

 of white fibrous connective tissue (Chap. VIII) and are ex- 

 tremely pliable, but quite inextensible and very strong and 



FIG. 43. Section through the hip-joint. 



tough. One is the capsular ligament, which forms a sort of 

 loose bag all round the joint, and another is the round liga- 

 ment, which passes from the acetabulum to the head of the 

 femur. Should the latter rotate above a certain extent in 

 i'ts socket, the round ligament and one side of the capsular 

 ligament are put on the stretch, and any further movement 

 which might dislocate the femur (that is, remove the head 

 from its socket) is checked. Covering the inside of the cap- 

 sular ligament and the outside of the round ligament is a 

 layer of flat cells, which are continued in a modified form 

 over the articular cartilages and form the synovial membrane. 

 This, which thus forms the lining of the joint, is always 



