752 UREA AND KREATIK [BOOK n. 



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 observation 

 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 conversion 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 thrown 

 off. Meanwhile we may state the conclusion at which we have 

 provisionally arrived, namely that the nitrogenous metabolism of 

 muscle probably gives rise to kreatin, which in some part of the 

 body other than muscle is probably split up into urea, ready for 

 excretion, and into sarcosin which also, somewhere in the body, 

 is further converted into urea. And bearing in mind the large 

 mass of the skeletal muscles, we may further conclude that a 

 large portion of the urea leaving the body by the urine is formed 

 in this way. 



485. We must not however leave this statement without 

 referring to a difficulty. Kreatinin as we have seen is so frequently 

 found in urine as to be regarded as a normal constituent, at all 

 events of human urine ; and kreatinin is as we have seen the 

 urinary form so to speak of kreatin ; the one body easily changes 

 into the other by the assumption or removal of H 2 0. This 

 suggests the question, Is not the kreatinin of urine the represen- 

 tative of the kreatin of the muscles, which is thus excreted 

 directly without undergoing the change into urea just discussed ? 

 In answer to this we may say in the first place that the quantity 

 of kreatinin in urine, though variable is small ; we may put the 

 average at about 1 grm. in 24 hours. Now muscle contains 

 from '2 to *4 p.c. of kreatin ; and this, taking the total muscle of 

 the body (to say nothing of other sources of kreatin which we 

 shall mention presently) at about 30 kilos would give 60 to 

 120 grms. kreatin as present in the muscles of the body at any 

 one moment. We can hardly suppose that the metabolism of 

 muscle is so slow as out of this stock only to provide the 1 grm. 



