458 THE METABOLIC PROCESSES OF THE BODY. 



that proteid material in becoming urea passes through phases in which the 

 nitrogen exists in chemical combinations distinct from proteid material on 

 the one hand and urea on the other. 



In the second place it is extremely probable that the series of changes 

 by which proteid material becomes urea is not the same in all the tissues 

 and on all occasions. We should naturally expect to find the proteid 

 material following different lines of metabolism in different places or under 

 different circumstances, the different lines all converging to the same body, 

 urea, because for some reasons or other urea appears to be, in the main, the 

 most convenient form in which the nitrogen can leave the blood and the 

 body. 



We should accordingly expect to find, on the one hand, various nitroge- 

 nous bodies resulting from proteid metabolism in various parts of the body, 

 and, on the other hand, arrangements by means of which these various 

 bodies were reduced to the common form urea, preparatory to their dis- 

 charge from the body by the kidney. And actual observation as far as it 

 goes supports this view, though our knowledge of the whole matter is very 

 imperfect. 



398. We may turn our attention first to the metabolism of the skeletal 

 muscles, since these represent, as far as mere quantity is concerned, by far 

 the greater part of the proteid capital of the body. We may safely infer 

 that they furnish a large part of the urea of the urine; though undoubtedly 

 a small mass of tissue might by reason of its more rapid metabolism work 

 over a greater quantity of proteid material than a much larger mass with a 

 slower metabolism ; yet we have no reason to think that the proteid metab- 

 olism of skeletal muscle, obscure though it is in its nature, is so slow as to 

 neutralize the probable effect of the great bulk of muscle existing in the 

 body. 



In dealing with the chemistry of muscle ( 62) we saw that urea, save in 

 the exceptional instances of certain cartilaginous fishes, was conspicuous by 

 its absence from the extract of muscle, whereas, a very appreciable quantity 

 of kreatin was invariably present, and, indeed, was the prominent nitroge- 

 nous crystalline constituent of that extract. It seems difficult to resist the 

 conclusion that kreatin is the main normal nitrogenous product of the metab- 

 olism of skeletal muscles. If we accept this view, then, upon the fact of 

 the presence of kreatin in, and the absence of urea from, the muscle itself, 

 we may base the conclusion that while the muscle produces kreatin as an 

 antecedent of urea, the kreatin so produced is converted into urea in some 

 part of the body other than the muscle itself. Kreatin, as we have already 

 seen, may be easily split up, and we may probably with safety assume is split 

 up, somewhere in the body, into urea and sarcosin. But sarcosin does not 

 appear in the urine as such ; hence, the conversion of kreatin into (part of) 

 the urea of the urine entails as well the further conversion of sarcosin into 

 urea. Now sarcosin, as we have seen, is methyl-glycin ; we may regard it 

 for our present purposes as simple glycin, and hence the total conversion of 

 kreatin into urea entails the conversion of glycin into urea. This, however, 

 does not offer any additional difficulty, since we know from direct observa- 

 tion that glycin introduced into the alimentary canal does not reappear as 

 such in the urine, but produces a corresponding increase in the urea of the 

 urine ; from which we infer that glycin absorbed from the alimentary canal 

 is, somewhere in the body, converted into urea. We shall speak of this con- 

 version later on, and shall then see that, as far as urea is concerned, glycin 

 (amido-acetic acid) and sarcosin (methyl glycin, methyl-amido-acetic acid) 

 undergo the same change, the amide moiety in each case being converted 

 into urea, while the non-nitrogenous moiety is oxidized and thro\vn off. 



