THE BONES. 



17 



matter is such as to allow sufficient flexibility and secure 

 strength to enable man to fulfil the duties of life. 



If a bone be exposed to the action of certain acids weak 

 muriatic acid, for instance the mineral matter will be dis- 

 solved, and the bone, although retaining its normal size and 

 shape, will become flexible like a rubber 

 tube. On the other hand, if a bone be 

 placed in the fire, the animal matter will 

 disappear, leaving the bone light in weight 

 and easily crumbled into powder. 



4. Ossifying Bone. In the child at 



birth the entire framework is made of car- 

 tilage or gristle, and the gradual conversion 

 into bone is called ossification. This process 

 commences in the middle of the bone, and 

 gradually extends to the borders. Some- 

 times there are two or more points of ossifi- 

 cation, in which case the several growing 

 areas of bone approach each other and 

 coalesce, the place of the union being 

 marked by a line (see skull, page 21). 



5. The Structure of Bone. The con- 

 sistence of bone varies, according to its use 

 and according to its mode of construction. 

 In every instance there is manifested infinite 

 wisdom in securing strength without unne- 

 cessary bulk. 



In a fresh long bone, sawn lengthwise, it 

 is seen that the shaft is made up of two 

 portions, the outer or cortical portion, and length ' 

 the inner or spongy portion. The outer portion is hard and 

 firm, and is covered by a dense fibrous membrane closely 

 attached, and through which blood is supplied to the bone. 

 This is called the periosteum. 

 2 



FIG. 3. The Thigh- 

 Bone, or Femur, sawn 



