60 CAETILAGE. BONE. 



a close resemblance to animal cartilage) obtains its nourish- 

 ment from the surrounding fluid. 



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 all 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 



