374 A YEAR IX SCIENCE 



stance now left is mineral matter, chiefly calcium car- 

 bonate and calcium phosphate. 



In an adult about two-thirds of the bone is mineral 

 matter. In a child there is much less. As a result, the 

 bones of a child are much more flexible than those of 

 an adult. 



Growth of bone. Bone is not a solid compact mass, 

 as one might be led to suppose from its appearance. 



Canal 



Canal 



Fig-. 172. Sections through bone highly magnified ; A, longitudinal 

 section ; B, cross section. 



If a section of a bone is examined with a microscope, 

 it will be found to be composed of a series of canals 

 and irregular cavities. These canals contain blood 

 which passes through smaller canals to the bone cells 

 which lie in the irregular cavities. These bone cells 

 absorb the food and transform it into bone material. 



Covering the bone everywhere except at the joints 

 is a tough membrane called the periosteum. This mem- 

 brane is of great importance in the growth of the bone, 

 because the new bone is formed by the cells of the 

 periosteum. If a bone is broken, or if a piece is taken 



