104 PHYSIOLOGY 



Internal in Higher Animals. — In higher animals the 

 skeleton is internal. It is entirely covered hy muscles 

 and its bones are so arranged that they move easily 

 when the muscles associated with them contract. There 

 are in man a great many bones (208) of varied shape 

 and size, usually described as long, short, flat or irregu- 

 lar. Except in the head, the hip, and the lower part of 

 the spine where the bones are fused or sutured, a joint 

 permitting more or less motion is formed wherever two 

 bones come together. The separate bones at the joint 

 are held in place by strong ligaments of fibrous, in- 

 elastic tissue. They slip over each other easily and 

 smoothly because the surfaces that come in contact are 

 covered with smooth cartilage that is constantly kept 

 moist with fluid. 



The Structure of Bones. — The bones are composed of 

 living animal matter, water, and earthy matter mostly 

 salts of lime. They are rigid, strong and light. The 

 large bones attain the necessary size without too 

 much weight by becoming hollow or by developing 

 spongy bone inside. The number, the shape of the 

 bones, the wonderful way in which they are fitted to- 

 gether, their strength and lightness insure the ease and 

 quickness with which we are able to move. 



The Spinal Column, — The fundamental part of the 

 human skeleton is the spinal column, or backbone, which 

 is so constructed that it combines the greatest strength 

 with the greatest flexibility. The separate bones of 

 which it is composed may be felt through the skin. 

 They vary in form according to their situation, and in 

 consequence they vary in the amount of motion that they 

 are able to make. In the neck, for example, there is 



