i6o 



APPLIED PHYSIOLOGY 



have been born without hands have learned to use their 

 feet instead. 



318. Joints. The bones of the skull and pelvis can be 

 moved but little. Their joints permit these bones to grow 

 and also make them slightly springy. 



The bones of the spine, instep, ribs, and wrist can move 

 a little. Each bone moves scarcely enough to be notice- 

 able, but altogether they have a considerable range of 

 motion. 



The bones of the arms, fingers, legs, and toes and the 

 lower jaw can be moved freely. In them the end of one 

 bone is rounded and fits snugly into a hollow in the other 

 bone. .The two bones are bound together by tough bands 

 called ligaments, which encircle the joint like a loose col- 

 lar. Thus the bones are free to move like a door upon 

 its hinges. 



319. Cartilage. The ends of bones in the joints are 



covered with a thin layer of a very 

 tough and firm substance called carti- 

 lage. Cartilage is like bone without 

 lime. The bone of young animals is 

 cartilage at first, but as it grows it 

 takes up lime, except at the ends, which 

 remain cartilage. 



320. Synovial membrane. The in- 

 side of a movable joint is lined with a 

 bag called the synovial membrane. 

 The synovial membrane is very smooth 

 and is filled with a liquid like the white 

 of an egg. This oils the joint and 

 makes it work smoothly. Sometimes 



in old people the synovial fluid dries up. Then the joint 



is stiff and creaks when it is bent. 



i- 



Hinge joint of the elbow, 

 i humerus 2 ulna 



