BONES 157 



308. The skull. The frame of the head is called the 

 skull. It is made of twenty-two flat plates of bone. Those 

 that cover the brain and the other parts of the head are 

 thick and strong, for they are likely to be hurt by blows. 

 The bones which make the nose and the inside of the 

 skull are thin, for they have little work to do, and blows 

 cannot reach them. 



The bones of the skull are joined together by rough 

 edges, which fit exactly into each other. In a man some 

 of these bones grow together into a single bone. These 

 joints can move only enough to prevent a little of the 

 jarring when we jump or strike the head against something 

 hard, but in a young baby the bones can be moved and 

 the head can be pressed into any shape. 



309. The spine. The skull is balanced upon a stiff 

 string of bones called the spine or backbone. The spine 

 runs the whole length of the back, and is made up of 

 thirty-three rings of bone; but in a man the lower ones 

 grow together so that there are only twenty-six separate 

 rings. The spinal cord is hung in the middle of these 

 rings. Between the rings are thick, strong pads of tough 

 flesh or gristle which make strong, close joints, and also 

 act as springs to keep the body from being jarred when 

 we run or jump. By means of these joints the backbone 

 can be bent and twisted, but the motion is small. In the 

 circus are men whose backbones are so loose jointed that 

 they can twist themselves into a knot. 



310. Ribs. From the sides of the backbone slender 

 bones, called ribs, extend around the sides of the body, 

 and are joined in front to the sides of a flat bone called 

 the breast bone. There are twelve ribs on each side. 

 They have a little motion up and down, and out and in, 

 as in breathing. They form a box called the chest. 



