MATERIAL OF THE FRAME. 43 



All the bones in the body are very hard on 

 the outside. Perhaps the teeth are most so. 

 The inside of the teeth is not much harder 

 than other bones; but the outside is coated 

 with a substance called enamel, which is very 

 hard indeed. 



PARTICULAR DESCRIPTION. You have al- 

 ready been told that the long round bones, 

 such as the humerus and the femur, are hollow, 

 and have marrow in them. This marrow nearly 

 or quite fills up the hollow.* There is a very 

 thin membrane that lines the hollow, and also 

 runs among the marrow. The same sort of 

 membrane lines also the little cells in the 

 spongy bones. These cells have a small quan- 

 tity of liquid in them. None of them appear 

 to be entirely empty. 



Most of the bones are pierced through their 

 outside with one or more holes of considerable 

 size, through each of which goes an artery to 

 convey blood to the bones ; and a vein comes 



* This is true of the bones of most other animals 

 besides man. Some of the bones of birds, however, 

 are said to be entirely hollow and empty. 



