904 METABOLISM. 



metabolism, even at a time when the tissues are temporarily supplied with 

 circulating proteid from the blood of a well-nourished animal. 



That a continuance of liberal proteid diet does produce an increased growth 

 of muscular tissue, may also be looked upon as extremely probable, from the 

 daily experience of athletes. As is well known, the diet upon which training 

 is chiefly carried out consists very largely of proteid matter, the proteids of the 

 food being in much larger proportion to the fats and carbohydrates than in the 

 normal diet of untrained persons. It would appear likely that this, which is 

 the result of the experience of many generations of trainers, must have a 

 physiological basis, and that the effect of such excess of proteid in the diet 

 must in itself not only cause an increase of the proteid metabolism, but also 

 lead to the formation of actual tissue proteid. Under ordinary circumstances, 

 however, whether the proteid which passers to the muscles is actually built up 

 into their tissue, or whether it is simply included in the interstices of the 

 living substance, it is not stored there for long ; for it is found, after a meal 

 containing much proteid, that within a few hours practically the whole of the 

 proteid which has been absorbed is removed in the form of urea. 



That the change in proteid which results in the formation of urea 

 must primarily occur within the muscles, within which, as we have 

 seen, the greater part of the oxidations of the body occur, there can be very 

 little doubt. But there has always been this difficulty in connection 

 with the question, that although urea is the ultimate product of proteid 

 metabolism, the muscles practically contain either no urea or only a 

 very small amount. An exception is, it is true, found in certain animals, 

 e.g. the Elasmobranch f fishes, the muscles of which contain a considerable 

 amount of urea. But this is not the case with most animals, and it 

 cannot be supposed that urea is formed to any appreciable amount in 

 the muscles, especially since we know that by far the greatest amount 

 is actually formed in the liver. 



What precursor, therefore, of urea is formed in the muscles from the 

 proteid which is metabolised within them ? The nitrogenous substance 

 which could best be supposed to be produced from the metabolism of pro- 

 teids, is creatine, since this is the one found in largest amount within 

 the muscles ; and it is natural to suppose that creatine, which is capable 

 of being converted in the laboratory without any great difficulty into 

 urea and sarcosine, might be the immediate precursor of urea. It is, 

 however, found that if creatine is injected into the blood or subcutane- 

 ously, or if it is taken with food, and thus absorbed into the blood, it 

 does not become converted into urea, but is found in the urine as 

 creatinine ; and we cannot therefore suppose that the creatine of the 

 muscles is absorbed by the blood, and carried by that fluid to the liver, 

 and there converted into urea, since we find that creatine added to the 

 blood does not become so converted. 



Without ignoring the possibility that the creatine which is found in 

 muscle may still be a preliminary stage in the transformation of 

 proteid into urea, we must look for other products of nitrogenous 

 metabolism passing from the muscles which, whether derived immedi- 

 ately from the proteid or indirectly from it through creatine, may be 

 supposed to be the real precursors of urea. As a matter of fact, such 

 products are found in the form of ammonia salts. It was noticed by 

 Schondorff, in the experiments already quoted, that in cases in which 

 the blood of a fasting animal was sent through the limbs only of a well- 

 1 Stiideler and Frerichs, Journ. f. prakt. Chem., Leipzig, 1858, Bd. Ixxiii. S. 48. 



