CHAP, iv.] METABOLIC PKOCESSES OF THE BODY. 591 



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 con- 

 version 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 conver- 

 sion 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, so 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. 



383. We must not however leave this statement without 

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

 quently found in urine as to be regarded as a normal constit- 

 uent, 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 O. This suggests the question, Is not the kreatinin of urine 

 the representative 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 



