ANIMAL ORGANIZATION. 123 



not to be overlooked. This object is promoted 

 by the interposition of a layer of fat^ which is 

 another animal substance entitled to be enume- 

 rated among the elements of its structure. It 

 consists of an oily fluid, composed, according to 

 the analysis of Chevreuil, of two constituent 

 principles, which he has distinguished by the 

 terms steariue and eiaine. In warm blooded 

 animals the temperature of the body is always 

 sufficient to preserve this compound substance 

 in a fluid form : but it is prevented from'^being 

 diflused through the cellular texture by being 

 contained in separate vesicles of extreme minute- 

 ness.* Hence the whole mass of the fat, which 

 is thus formed of an aggregation of these 

 vesicles, has not the appearance of being fluid, 

 but seems to be composed of small grains united 

 by membranous investments into larger masses ; 

 a structure peculiarly adapted to the purposes of 

 a soft cushion, retaining only a small share of 

 elasticity, and yielding only in a certain limited 

 degree to pressure. 



* Dr. Monro estimated their diameter at between the 800th 

 and 600th of an inch. But their size varies in ditierent 

 animals. 



