ANIMAL ORGANIZATION. 109 



The animal substance which comes next in 

 the order of density is Cartilage. The purposes 

 for which this kind of structure is employed are 

 those in which a solid basis is required for the 

 support of softer or more flexible parts, and 

 where the mechanical properties that are wanted 

 are firmness, conjoined with some degree of 

 elasticity. Cartilage (or gristle) is composed of 

 a finer and more uniform material than any of 

 the preceding structures. It consists almost 

 wholly of albumen, with a slight proportion of 

 calcareous matter. Unlike membrane in any 

 of its forms, it contains no fibres, but, on being 

 cut with a sharp knife, presents the appearances 

 of a dense homogeneous substance of a white 

 pearly hue. Its surface is smooth, and where 

 it is exposed to friction, as in the joints, is often 

 highly polished. 



In all the inferior tribes of animals Nature 

 employs cartilage to supply the place of bone 

 when rigidity is required to be given to the 

 fabric. In an extensive order of fishes, in- 

 cluding the shark, the sturgeon, and the ray, 

 we find the whole skeleton constructed of car- 

 tilage. In the fabric of very young quadrupeds 

 cartilage is substituted for bone ; and in the 

 adult animal, various organs, such as the exter- 

 nal ears, the eye-lids, the nostrils, and different 

 parts of the apparatus of the throat and wind- 

 pipe, are composed of flexible cartilage, which 



