598 METABOLISM, NUTRITION AND DIETETICS 



of the urine which is not formed from the creatin in the food is 

 derived' from the creatin of the muscles and other tissues, and many 

 theories have been evolved to connect the creatinin of urine with 

 the creatin of the muscles. But it is doubtful whether there is any 

 direct connection. The fact that persons who have been long ill 

 and are feeble and wasted have a low creatin content in their 

 muscles, and a low creatinin content in their urine relatively to the 

 total nitrogen (Shaffer) is, so far as it goes, in favour of some re- 

 lationship between the two. The alleged absence of creatinin from 

 muscle seemed to be opposed to the idea that the creatin store of the 

 muscular tissue was an important source of urinary creatinin; for 

 if a constant transformation of this kind was going on, traces of 

 creatinin not yet absorbed by the blood might have been expected 

 to be present in the muscles. Recently, however, it has been 

 reported that small quantities of creatinin do exist in fresh muscle 

 (4 to 8 milligrammes in 100 grammes of tissue), and that when the 

 muscle is allowed to undergo autolysis the creatinin increases at a 

 very uniform rate at the expense of the creatin. Added creatin 

 experiences the same fate as the creatin originally present, while 

 added creatinin inhibits the reaction, or even reverses it (Myers and 

 Fine). A parallelism with the conversion of glycogen into dextrose 

 in the liver easily suggests itself, and it is possible that we are here 

 in the presence of a normal reaction which may account for at 

 least a portion of the creatinin excretion. It is probable that both 

 creatin and creatinin can undergo changes in the body, especially 

 in the liver, and it is possible that the products may be further 

 utilized in metabolism. If this were so, the creatin store of the 

 muscles would acquire new significance as a reserve of useful material 

 with perhaps a long and varied metabolic career before it, and would 

 not constitute merely a temporary depot of waste material whose 

 metabolic history was ended, and which was waiting to be excreted. 

 The novel view has recently been advanced that the living muscles 

 contain little or no creatin, but that the creatin found on analysis 

 is a post-mortem product originally constituting a part of the 

 living protoplasm (Folin and Denis). 



However this may be, the constancy of the creatinin elimination 

 on a meat-free diet (j>. 482), and its complete independence of the 

 changes in the total nitrogen excretion, show that it has a. different 

 significance in protein metabolism from the urea. Evidence is accumu- 

 lating that it is especially in the metabolism of the organized or tissue 

 protein that the product eventually excreted as creatinin arises ; in other 

 words, that it represents especially the nitrogenous waste connected 

 with the wear and tear of the bodily machinery, while urea represents 

 also, and under ordinary conditions of diet chiefly, the nitrogen of the 

 surplus amino-acids which are not utilized in the building of new or the 

 repair of old tissue elements. The fact that the amount of creatinin 

 excreted by different persons seems to be related to the weight of active 

 tissue in the body, excluding fat, is in favour of this suggestion, and 



