CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 609 



cess of digestion ; and though, as we have said, we have no proof 

 that this action of pancreatic juice takes place largely in the 

 normal body, its value as an example is none the less important. 

 . Some observers have pushed this view* of the production of 

 fat out of proteids so far as to insist that all the fat formed in 

 the body arises in this way out of proteid material, and that 

 when carbohydrate food gives rise to the formation of fat it 

 does so by shielding from oxidation the carbon moiety of the 

 proteid food taken at the same time and thus permitting it to 

 be stored up as fat. The carbohydrate itself, they argue, never 

 becomes fat but its presence allows fat to be formed out of pro- 

 teid material. This view has obviously a very important eco- 

 nomical bearing, since, if it were true, it would be useless to 

 increase the carbohydrate material of food for the purpose of fat- 

 tening, unless a sufficient proportion of proteid material be given 

 at the same time. It has however been proved to be untenable. 

 402. It is clear then that a construction of fat does occur 

 in the body somewhere. What limits can we place on the de- 

 gree to which this construction is carried? When the food 

 contains sufficient actual fat to account for the fat stored up in 

 the body, does any construction of fat take place? In the first 

 place we find that when the food contains abnormal fats such 

 as are not present in the body, spermaceti for instance, or eru- 

 cin (from rape-seed oil), these fats are not to be found, or are 

 found in very small quantity only, in the fat which is stored 

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

 In the second place we may call to mind the statement previ- 

 ously made, that the x;omposition 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 construction of fat only in cases where the fat 

 in the food was deficient, we should expect to find that the 

 nature 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, very 

 largely independent of the food, that the normal constituents 

 of fat make their appearance very much as usual and in very 

 much their appropriate proportion, though their proportion in 

 the food may largely vary, and though some of them may be 

 wholly absent. Thus in one experiment the fat of the body 

 contained considerable quantities of stearin after a diet free 

 from stearin, and in another preserved the normal amount of 

 olein after a diet free from olein. This shews that the con- 

 structive power of the economy is, as regards fat, very great ; 

 indeed it is even possible that all the fat stored up in the body 

 is fat formed anew. 



39 



