THE SKELETON 



31 



Chemical Composition of Bone. — Experiments (2 and 3) 

 show that the bones contain a mineral or earthy substance, 

 which makes them hard and stiff, and ~ 

 a certain amount of animal matter, 

 called gelatine, which binds the min- 

 eral matter together and makes the 

 bones tough and somewhat elastic. 

 The fire burned out the animal matter rj, 

 of the first bone, and the acid dissolved 

 out the mineral matter of the second 

 bone. The mineral matter is chiefly 

 lime, and makes up about two thirds of 

 the weight of the bone. (Why is more 

 mineral than animal matter needed ?) 

 The animal gelatine is a gristly sub- 

 stance. As the body grows old, the 

 animal matter of the bones decreases, 

 and they become lighter. They are [_. 

 more easily broken and do not heal so 

 readily as the bones of young persons. 



The skeleton is subdivided into the 

 bones of the head, trunk, and limbs. 

 The bones of the trunk are those of 

 the spine, the chest, the shoulder blades, 

 collar bone, and hip bones. 



The spinal or vertebral column is 

 made up of twenty-six bones (Fig. 31). 

 It is the axis of the human skeleton, 

 to which all other bones are directly 

 or indirectly attached. Animals with 

 inside skeletons have this column, and 

 are called vertebrates. Fish, reptiles, birds, beasts, apes, 

 and man are vertebrates. The spine, as this column is some- 



G-..5 



Fig. 31. — Vertebral 

 Column. Side view. 



