428 FAT. [BOOK it 



quantity of urea the amount of carbon is far less than that found 

 in the quantity of proteid containing the same amount of nitrogen. 

 Thus the percentage composition of the two being respectively, 



Carbon. Hydrogen. Oxygen. Nitrogen. Sulpliur. 



Urea 20'00 6'66 26'67 46*67 



Proteid 53 7'30 23'04 15'53 113 



100 grms. of urea contain about as much nitrogen as 300 grms. of 

 proteid; but the 300 grms. of proteid contain 139 grms. (159 20) 

 more carbon than do the 100 grms. urea. Hence the 300 grms. of 

 proteid in passing through the body and giving rise to 100 grms. 

 of urea, would leave behind 139 grms. of carbon, in some combi- 

 nation or other ; and this surplus of carbon, if the needs of the 

 economy did not demand that it should be immediately converted 

 into carbonic acid and thrown off from the body, might be deposited 

 somewhere in the form of fat. We have already seen, in treating 

 of the action of the pancreatic juice (p. 252), that there is evidence 

 of a fatty element (viz. leucin, which is amido-caproic acid, and 

 so belongs to the fatty acid series) being thrown off from the 

 complex proteid compound in the very process of digestion. 



It is clear that a construction of fat does occur in the body 

 somewhere. What limits can we place on the degree to which 

 this construction is carried? In reference to this point it is worthy 

 of notice that the composition of fat varies in different animals. 

 The fat of a man differs from the fat of a dog, even if both feed on 

 exactly the same food, fatty or otherwise. Were the fat which is 

 taken as food stored up as adipose tissue directly and without 

 change, recourse being had to other sources of food for the con- 

 struction of fat only in cases where the fat in the food was de- 

 ficient, we should expect to find that the constitution of the fat of 

 the body would vary greatly with the food. So far from this being 

 the case, direct experiment shews that the fat of the dog is, as far 

 as composition is concerned, almost entirely independent of the 

 food, that the normal constituents of fat make their appearance as 

 usual, though some of them, such as stearin or olein, may wholly be 

 absent in the food, and that abnormal fats such as spermaceti 

 presented as food are not to be found in the fat which is stored up 

 in the body as a consequence of a large supply of that food. 



Of course it is quite possible that in such cases as these, though 

 the stearin, or the olein, when absent from the food, was in some 

 way or other constructed anew, yet at the same time those con- 

 stituents which were present were simply stored up; but it is also 

 open for us to suppose that all the fat taken as food was in some 

 way or other disposed of, and that all the new fat which made its 

 appearance was constructed anew. And the latter view is supported 

 by the histological facts just mentioned, as well as by other con- 

 siderations, which we shall presently have to urge. At the pre~ 



