The Skeletal System 113 



the two bones may be formed by connective tissue, thin sheets of 

 cartilage, or direct bone-to-bone contact. Often the suture is ob- 

 literated in the adult. 



The second type of joint, the diarthroses (Fig. 27), is that in which 

 the adjacent elements are freely moving. In these the adjoining surfaces 

 are usually protected by cartilage. Between the cartilaginous portion 

 is a liquid-filled joint cavity or bursa. The liquid filling this sac is 

 the lubricating synovial fluid. Connecting the bones are heavy fibers 

 of connective tissue, the ligaments. These form a protective capsule 

 around the joint. 



Diarthroses may be of several dififerent types. The articulation 

 of the humerus into the glenoid cavity is a ball and socket joint; the 

 elbow is a hinge joint. The rotation of the head by means of the odon- 

 toid process of the axis fitting into the atlas is an example of the 

 action of a pivot joint. The wrist is an example of a gliding joint. 



