METABOLISM, NUTRITION AND DIETETICS 443 



muscle containing from 0*2 to 0-4 per cent, of it; and the total 

 quantity of nitrogen present at any given time as kreatin is not 

 only greater than that of the nitrogen present in urea, but greater 

 than the whole excretion of nitrogen in twenty-four hours. To 

 kreatin, then, we should naturally look first, among all these nitro- 

 genous metabolites, in our search for a forerunner of urea. But 

 there is a difficulty in accepting it as such, for although in the 

 laboratory kreatin can be changed into kreatinin, and kreatinin into 

 urea, there is no proof that in the body anything more than the first 

 step in this process is accomplished. When kreatin is introduced 

 into the intestine or injected into the blood, it appears in the urine, 

 not as urea, but as kreatinin. Uric acid is, indeed, very closely 

 related to urea, and can be made to yield it by oxidation outside the 

 body. Not only so, but it is excreted as urea when given to a 

 mammal by the mouth, and it replaces urea as the great end-product 

 of nitrogenous metabolism almost wholly in the urine of birds and 

 reptiles, and partially in the human subject in leukaemia. But none 

 of these things can be admitted as evidence that in the normal 

 metabolism of mammals uric acid lies on the direct line from pro- 

 teid to urea. 



Then, again, the amido-acids, leucin, glycin and asparaginic acid, 

 when given by the mouth, increase the output of urea, so that the 

 leucin formed in the intestine during digestion is doubtless a pre- 

 cursor of a small portion of the urea. And since leucin and tyrosin 

 are very widely spread in the solids and liquids of the body, it has 

 been asserted that the amido-acids are the form in which nitrogen 

 leaves the tissues to be converted into urea in the liver. But it is 

 against this view that there is not enough carbon in proteids to 

 convert all their nitrogen into amido-acids (Bunge). It may be, 

 however, that a portion of the proteid is changed into amido-acids 

 relatively rich in carbon, and the remainder into simple ammonia 

 compounds relatively rich in nitrogen, the amido-acids being then 

 further split up into simpler bodies, which combine with each other 

 to form urea. Lea has suggested that the amido-acids have quite 

 another significance than that of intermediate steps in the downward 

 metabolism of proteids that they are destined, in fact, to take part 

 in synthetic processes within the liver that they are on the up, and 

 not on the down, grade. And he points out, in support of this view, 

 that even when the urea in the urine is increased by the administra- 

 tion of these substances, the increase does not correspond to the 

 whole of their nitrogen ; a part of it is therefore devoted to other 

 purposes in the body. Outside of the body proteids can be decom- 

 posed so as to yield a large number of products, ammonium salts, 

 leucin, tyrosin, aspartic and glutaminic acids, lysine and arginine. 

 These can also be all obtained by tryptic digestion, and from the 

 arginine urea can be artificially derived. But we must not conclude 

 without special proof that in the body the course of proteid decom- 

 position is the same. 



The conclusion of the whole matter is that, if anyone 



