xin BONES AND JOINTS 175 



adult the mineral matter makes up about two thirds of 

 the entire bone. The other portion is animal matter or 

 connective tissue. By placing a small bone like a 

 sheep's rib in a mixture of one part muriatic acid 

 and six parts water for two or three days, the mineral 

 matter can be dissolved, leaving the bone so soft that 



FIG. 90. A sheep's rib tied into a knot after the mineral matter was removed 

 (Johnstone). 



it may be tied into a small knot. By placing a bone 

 in a fire for a short time, the animal matter can be 

 burned away, leaving a very brittle bone. 



In children the bones are soft and may be easily bent 

 out of shape because they have a large amount of ani- 

 mal matter and as yet very little mineral matter. This 

 is why children's bones are not easily broken. Slowly 

 the bones of the growing person become hard and 

 firm because more and more lime displaces the animal 

 matter and they grow strong but become brittle. In 

 old age bones break easily and heal slowly when 

 broken. 



127. Structure of Bone. Most of the long bones 

 are hollow through the center. The cavities are filled 

 with soft yellow marrow. This form of structure makes 

 them stronger and very much lighter than they would 



