ANIMAL ORGANIZATION. Ill 



body is always sufficient to preserve this compound 

 substance in a fluid state : but it is prevented from 

 being diftYised through the cellular texture by being 

 contained in separate vesicles of extreme minute- 

 ness,* and through which it cannot transude. 

 Hence the whole mass of the fat, which is formed 

 of an aggregation of these vesicles, has not the 

 appearance of being fluid, but seems to be com- 

 posed of small grains united by membranous 

 investments into larger masses ; a structure pecu- 

 liarly adapted to the purposes of a soft cushion, as 

 it retains only a small share of elasticity; and, while 

 it at the same time possesses the absolute incom- 

 pressibility of fluids, it admits readily, but only to 

 a certain limited extent, of displacement by ex- 

 ternal pressure. 



These mechanical properties render it a peculiarly 

 serviceable material for filling up the spaces left 

 between adjacent organs, whose forms are not per- 

 fectly adapted to each other ; and especially those 

 whose functions require them to be in frequent or 

 continual motion. This is exemplified more par- 

 ticularly in the heart and the great vessels connected 

 with it ; between which we accordingly find a large 

 quantity of fat interposed. Diff'erent parts of the 

 alimentary tube are, in like manner, protected 

 from friction and from mechanical strains, which 

 might have occurred from mutual displacement, by 

 cushions of fat, interposed between their folds, and 



depend altogether on the proportions hi which they are united. Thus 

 a ready expedient has been provided for varying the mechanical pro- 

 perties of fat, according as circumstances required. 



* Dr. Munro estimated their diameter at between the 800th and 

 600th of an inch. But their size varies in different animals. 



