NUTRITION. 461 



Kreatin, too, exists in the brain, and probably there and else- 

 where in the nervous system is produced by chemical degra- 

 dation of protoplasm; the spleen also contains a good deal 

 of kreatin, and so do many glands. This substance would 

 therefore seem to be constantly produced in considerable 

 quantities by the protoplasmic tissues generally; and since 

 it belongs to a group of nitrogenous compounds which the 

 Body is unable to utilize for reconstruction into proteids, it 

 must be carried off somehow. The urine, however, contains 

 no kreatin and but little of its immediate derivative, krea- 

 tinin, and what kreatinin it does contain depends mainly on 

 the feeding, since its amount varies with the diet and it 

 disappears during starvation. Kreatin can readily be chem- 

 ically broken up with hyd ration, yielding urea and sarkosin; 

 and sarkosin in turn can be decomposed so as to yield its 

 nitrogen in the form of urea. Hence there are no great 

 chemical difficulties in regarding kreatin as the main im- 

 mediate source of the urea of normal urine. There are some 

 reasons for thinking that kreatin is not the form of the 

 actual nitrogen waste in living muscle but a post-mortem 

 product from that waste; but that is not of importance in- 

 the present connection. Whatever the original form of the' 

 waste substance be, if it be not kreatin it is certainly very 

 easily converted into it. The formation of the final product,, 

 urea, does not occur in the muscles. They never contain 

 urea; and very little of it, if any, can be extracted from the; 

 brain. 



Where the kreatin is finally changed into urea is doubt- 

 ful. We have seen (Chap. XXVIII) that it is not formed in 

 the kidneys but merely separated by them from the blood. 

 A good deal of urea is found in the liver, which suggests some 

 part played by that organ in urea formation. Further, in 

 certain cases of hepatic disease (acute yellow atrophy) in 

 which the liver cells are profoundly changed, the urea of the 

 urine is greatly diminished and a quite different substance, 

 leucin, takes its place; and this favors the view that the liver 

 has much to do with the final elaboration of urea. It may 

 also be noted in this connection that, quite apart from kreatin 

 as a source of urea, there may be another in leucin, for leucin 

 is very widely distributed through the Body, and when proteids 

 are decomposed by various chemical methods leucin is very 

 constant among the products. It is therefore a possible form 



