60 CARTILAGE.—BONE. 
a close resemblance to animal cartilage) obtains its'nourish- 
ment from the surrounding fluid. ts 
48. The permanent Cartilages seem to undergo very little 
change from time to time. Their wear is slow; and, being 
purely mechanical, it is confined to the surface, It is replaced © 
by the materials absorbed from the blood, which are employed — 
in the development of new cells,—sometimes within the old 
ones, sometimes in the space between them. When a portion 
of cartilage has been destroyed, however, by disease or injury, — 
it is not renewed by true cartilaginous structure, but by what 
seems a condensed areolar tissue. Although cartilage does 
not usually contain vessels, yet these may be rapidly deve- 
loped in its substance, by a process which will be described 
hereafter (§ 393), when it becomes inflamed. This may be 
often seen to take place. The front of the eye is formed by 
a transparent lamina of a substance somewhat resembling 
cartilage, which bulges like a watch-glass: this, which is 
termed the cornea (§ 533), is properly nourished only by 
vessels that bring blood to its edge, where it is connected 
with the tough membrane that forms the white of the eye. 
But when the cornea becomes inflamed, minute vessels may 
be seen to spread over it, proceeding from its circular edge 
towards its centre ; and at last some of these often become of 
considerable size. Under proper treatment, however, these 
vessels gradually shrink and disappear ; and the cornea 
becomes nearly as transparent as before. | 
49, Many parts exist in the state of Cartilage in the young © 
animal, which are afterwards to become Bone; and it has 
been commonly believed that ald bone has its origin in a 
cartilaginous structure. This, however, is not the fact, as — 
will be presently shown. Before attempting to explain the 
formation of Bone, it will be desirable’ to describe its | 
structure. When we cut through a fully formed bone, such — 
as that of the thigh, we find that the shaft or elongated” 
portion is a hollow cylinder ; of which the walls are formed | 
by what appears to be solid bone ; whilst the interior is filled, | 
in the living state, by an oily substance laid up in cells, and 
termed marrow. Towards the extremities, however, the struc- 
ture of bone is very different. The outside wall becomes 
thinner ; and the interior, instead of forming one large cavity, 
is divided into a vast number of small chambers, like those 
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