SOURCES OF UREA. 433 



nitrogen of its food reappears in its urine, and that when 

 it is laying up fat; so that then we get a state of things in 

 which proteids are broken up more easily than fats. This 

 indicates that proteid in the Body may exist under two 

 -conditions; one, when it forms part of a living tissue and is 

 protected to a great extent from oxidation, and another, in 

 which it is oxidized with readiness and is presumably in a 

 different condition from the first, being not yet built up into 

 part of a living cell. The use of proteids for direct oxidation 

 is known as luxus consumption; how far it occurs under 

 ordinary circumstances will be considered presently. The 

 main point now to be borne in mind is that while all organic 

 non-nitrogenous foods cannot be called respiratory, neither 

 can proteids under all circumstances be called plastic, in 

 Liebig's sense. 



The Antecedents of Urea. In the long run the pro- 

 genitors of the urea excreted from the Body are the proteids 

 taken in the food; but it remains still to be considered 

 what intermediate steps, these take before excretion in the 

 urine; and whether urea itself is finally- formed in the kid- 

 neys or merely separated by them from the blood. 



In seeking antecedents of urea one naturally turns first 

 to the muscles, which form by far the largest mass of pro- 

 teid tissues in the Body. Analysis shows that they always 

 >oontain kreatin, a body intermediate chemically between 

 proteids and urea. The quantity of this in muscles is prac- 

 tically unaffected by work, and is from 0.2 to 0.4 per cent. 

 .Since it is readily soluble and dialyzable, and therefore fit- 

 ted to pass rapidly out of the muscles into the blood stream, 

 it is a fair conclusion that a good deal of it is formed in 

 the muscles daily and carried off from them. Kreatin, 

 : too, exists in the brain, and probably there and elsewhere 

 in the nervous system, is produced by chemical degrada- 

 tion 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, 



