S8 THE MECHANICAL FUNCTIONS. 



of this kind are employed very extensively in the fabric of 

 insects.* 



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 re- 

 quired 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 gris- 

 tle) 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 car- 

 tilage to supply the place of bone when ridigity 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 difierent parts of the apparatus 

 of the throat and windpipe, are composed of flexible carti- 

 lage, which gives them a determinate shape and firmness. In 

 all these cases bone, which besides being three times as hea- 

 vy, is devoid of elasticity, and liable to fracture, w^ould have 

 been much less suitable. Cartilage is often employed as an 

 intermedium for connecting difierent 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 diminishes those jars which are incident to the 

 frame in all its violent actions. 



In the construction of cartilage, nature seems to have at- 

 tained the utmost degree of density which could be given 

 to an internal texture composed merely of the usual animal 



* Chabrier, Memolres du Musee, torn. vi. p. 416. 



