ANIMAL ORGANIZATION. 95 



in which a solid basis is required for the support of 

 softer or more flexible parts, and where the mecha- 

 nical properties that are wanted are firmness, con- 

 joined 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 presents no obvious fibres ; 

 but when cut with a sharp knife, it appears as 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 em- 

 ploys cartilage to supply the place of bone, when 

 rigidity is required to be given to the fabric. In 

 an extensive order of fishes, including the Shark, 

 the Sturgeon, and the Ray, we find the whole 

 skeleton constructed of cartilage. In the fabric of 

 very young quadrupeds cartilage is substituted for 

 bone; and in the adult animal, various organs, 

 such as the external ears, the eye-lids, the nostrils, 

 and different parts of the apparatus of the throat 

 and windpipe, are composed of flexible cartilage, 

 which gives them a determinate shape and suffi- 

 cient firmness. In all these cases bone, which, 

 besides being three times as heavy, is devoid of 

 elasticity, and liable to fracture, would have been 

 much less suitable. Cartilage is often employed as 

 an intermedium for connecting different bones, as 

 for instance, between the ribs and the sternum, or 

 breast-bone ; whereby, besides the advantage of 

 greater lightness, the pliancy of the material di- 

 minishes those jars which are incident to the frame 

 in all its violent actions. 



