6 ELEMENTARY PHYSIOLOGY. 



are nearly round, like these bones of the wrist. 

 Some are quite flat, like this large, spreading bone 

 at the shoulder (16), or these broad bones in the 

 lower part of the main body (3). 

 structure ^~^ 6 bones are very hard and strong. 

 "Hard as a bone" is a familiar compar- 

 ison. We shall not be surprised at their hardness 

 aod strength when we shall have learned what im- 

 portant uses they serve in the body. There are two 

 kinds of material in the structure of a bone. One 

 part is called animal matter and the other is called 

 mineral matter. If the bone were composed of 

 animal matter alone, it would bear no pressure and 

 keep no permanent shape. If it were made entirely 

 of mineral matter it would be too brittle, and conse- 

 quently would break very easily. So these two kinds 

 of material 'are united together in such a way as to 

 secure strength without too great brittleness. In 

 childhood the bones are not easily broken. This is 

 because in early life they contain about twice as 

 much animal matter as mineral matter. What a wise 

 protection against the "bumps" and "tumbles" of 

 the little ones. In middle life the two kinds of 

 material are more nearly equally divided. In old 

 age, however, the bones are very brittle, because, 

 then, there is about twice as much mineral matter as 

 animal matter in their structure. 



It is an easy and interesting experiment to sepa- 

 rate these two kinds of bone material . Throw a flat 

 bone, or piece of bone, into the fire. After a while 



