BONES l6l 



321. Hinge joints. In the fingers, toes, wrists, ankles, 

 elbows, and knees the surfaces of the joints are long and 

 round like a hinge. So they can open and shut in one 

 direction only like a penknife. In the other direction they 

 can only straighten the limb. If you bend the limb in the 

 wrong direction, you will break the bones or else put them 

 out of joint. 



322. Ball and socket joints. The shoulders and hip 

 joints can be moved in any direction. In each the upper 

 end of the bone of the limb is round like half of a ball. 

 This fits into a cup in the other bone. Thus the limb can 

 be turned in any direction. In fourfooted animals the 

 fore and hind legs cannot be moved nearly so freely as a 

 man's arms and legs. So these animals could not do a 

 man's work even if they had hands. 



323. Broken bones. The bones of children are more 

 springy and are softer than those of old people. A child 

 may fall very hard without danger, while an old person's 

 bones will break from a slight fall. But a grown person 

 weighs many times as much as a child, and so, when* he 

 falls, there is more strain on the bones. 



When a bone is broken, its ends must be put in place, 

 or " set," and kept there by splints and bandages. Then 

 new cells grow in place of the injured ones. Lime is 

 mixed with the new cells, and the bone repairs itself in 

 about a month. It will then be as strong as it was before 

 it was broken, or even stronger. If the ends of the bones 

 are not put in their proper place and kept there until heal- 

 ing begins, the bone will grow crooked. 



324. Sprains. When a joint is bent too far, or in the 

 wrong direction, its ligaments are stretched and partly 

 torn. This makes a sprain. A sprained joint is very 

 tender and painful, and gets well slowly. 



ov. PHYSIOL. (INTER.) 1 1 



