MUSCULAR POWER. 97 



pulp begins to shrink, and the diameter of the tube dimi- 

 nishes; so that it exhibits a tapering form at both ends. Thus, 

 mere variations in the bulk and the action of the pulp, ac- 

 companied with changes in that of the capsule, are sufficient 

 to account for every diversity in the form and condition of 

 the resulting structures. 



Among the mechanical uses of the integument, that of 

 serving as a cushion for relieving the more prominent parts 

 of the frame, and especially of the bones, from unequal pres- 

 sure, ought not to be overlooked. This object is promoted 

 by the interposition of a layer oi fat, which is another ani- 

 mal substance entitled to be enumerated among the elements 

 of its structure. It consists of an oily fluid, composed, ac- 

 cording to the analysis of Chevreuil, of two constituent prin- 

 ciples, which he has distinguished by the terms .s/eflfr2?ie and 

 elaine. In warm-blooded animals the temperature of the 

 body is always sufficient to preserve this compound sub- 

 stance in a fluid form: but it is prevented from being dif- 

 fused through the cellular texture by being contained in 

 separate vesicles of extreme minuteness.* lience, 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. 



§ 5. Muscular Power, 



In Machines contrived by human skill the chief art con- 

 sists in devising expedients for regulating and directing the 

 giving moving power, so that it may bear, in the proper 

 degree, and in the proper order, upon some particular ob- 

 jects, and produce some particular effect. The whole of the 

 apparatus employed with this intention, however numerous 



• Dr. Monro estimated their diameter at between the 800th and 600th of 

 an inch. But their size varies in different animals. 



Vol. I. 13 



