COMPOSITION OF BONES. 109 



curs fluid like oil. In the hollow bones of oxen, it appears 

 more like butter, and is the well known substance termed 

 marrow. 



When we trace the progress of the growth of bone, from 

 its first appearance in the foetus, unto its perfect state in the 

 adult animal, we perceive, that the cartilaginous basis and 

 the periosteum exist during every period : they are the only 

 parts which are organized, and are capable of enlargement 

 in every direction. At the commencement of the growth 

 of the bone, the cartilaginous basis is soft, flexible, and ge- 

 latinous. By degrees the earthy matter is deposited. It 

 proceeds, as it were, from centres near the surface of the 

 cartilage. In the case of flat bones, the earthy matter is 

 deposited like two plates, one above the other, which gra- 

 dually unite at their edges, leaving a space between them, 

 in which the earthy matter is more cellular than the exterior 

 crusts. This space is termed diploe. In each plate, there 

 are one or more centres of ossification, where the earthy 

 matter is first deposited, and from which it is gradually ex- 

 tended over the whole surface. In the long bones, these 

 centres of ossification are more numerous. The central 

 parts are also more cellular than the surface, and called 

 their cancelli or lattice-work. The fat, in general, resides 

 in the cancelli, and in the hollow parts, towards the centre 

 of the bone. 



The bones increase in size, not as in shells, scales, or 

 horns, by the addition of layers to the internal surface, but 

 by the expansion of the cartilaginous basis ; which, when it 

 becomes saturated with earthy matter, is incapable of far- 

 ther enlargement. This is the reason why the bones of 

 young animals are soft and flexible, while those of old ani- 

 mals are hard and brittle. 



The proportion between the cartilaginous basis, and the 

 earthy matter, differs, not only in every animal according 



