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 diploé. 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 
