JOINTS 



341 



353. Joints. A joint is a place where bones come to- 

 gether. Some joints are intended to be bent freely; 

 these are called flexible, or movable, joints. Some joints 

 are not arranged for bending; these are inflexible, or im- 

 movable, joints. Some of the bones of the skull are held 

 together in inflexible joints. 



Between inflexible joints there is usually a layer of 

 cartilage or of material much like it. Its use is shown 

 admirably in the backbone, where it forms pads between 

 the vertebrae. These pads permit the backbone to be 

 bent somewhat, and 

 act as cushions to 

 relieve the skull as 

 much as possible 

 from the jar caused 

 by walking and 

 running. 



At movable joints 

 the bones are held 

 together by tough 

 bands, called liga- 

 ments, and the ends 



of the bones are smooth and round. When muscles 

 pass over joints, they also assist in holding the bones 

 together. We distinguish three principal kinds of mov- 

 able joints (Fig. 274): 



(1) Those in which the movement is in two opposite direc- 

 tions only, as in an ordinary hinge; these are called hinge 

 joints. The knees, elbows, and fingers have hinge joints. 



(2) Those that can be moved in any direction, as the 

 arm can at the shoulder. Such joints have the rounded 



Tibia 



FIG. 274. 

 A Hinge Joint and a Ball-and-Socket Joint. 



