COMPOSITION AND STRUCTURE OF BONE 57 



Composition and Structure of Bone. Bone in the 

 fresh state is pale pink in color, when dried it is grayish 

 white. The constituents of dried bone are 31 per cent, 

 organic matter and 69 per cent, inorganic matter. 

 The former is represented by bloodvessels and con- 

 nective tissue, and proteins, such as collagen, ossein, 

 elastin; the latter by mineral salts, e. g., tricalcium and 

 magnesium phosphate, calcium carbonate, and some 

 soluble salts. These ingredients may vary with the 

 age of the individual. Thus in the child there is an 

 increase in the organic matter over the mineral con- 

 stituents, while the mineral salts predominate in bones 

 of the adult. This absence of salts in the bones of 

 children accounts for the elasticity of the bones, 

 and when fractured they bend rather than break 

 after an injury. This can be seen when a fractured 

 limb, exposed to the a>rays, will appear as a twig bent, 

 but no distinct break is seen (this is termed a green- 

 stick fracture). On the other hand in extremely old 

 persons there is a tendency to fracture of the bones, 

 due to the increase of mineral salts over the organic 

 constituents, rendering them brittle, and unable to 

 withstand the slightest strain without fracture. Rick- 

 ets is another example of this decrease in lime salts 

 in the bones. It is a disease occurring in children 

 mostly, characterized by a bending of the long bones, 

 and deformity of the limbs, as a result of insufficient 

 and improper nourishment. Bone belongs to the 

 connective-tissue group, and is derived from the 

 mesoderm layer of the'tripoblast, the primitive vesicle 

 of the embryo. 



Periosteum. All bones are surrounded by a fibrous 

 sheath, called the periosteum, enclosing the bone sub- 

 stance; the latter is composed of cells and intercellular 

 substance. The periosteum consists of two layers: 

 an outer fibrous, supporting bloodvessels, an inner 

 or genetic layer, rich in cells and blood capillaries. 

 The cells in the latter become the future osteoblasts 



