KIDNEY AND SKIN AS EXCRETORY ORGANS. 777 



affected varies in different animals. In man it is estimated that 

 about one-half of the uric acid arising in the body metabolism 

 proper (endogeneous uric acid) suffers this fate. 



It is a very significant fact that the relative and absolute amount 

 of urea nitrogen in the urine varies directly with the amount of 

 protein taken as food, while other nitrogenous constituents of the 

 urine (creatinin, purin bases) are practically not affected by the 

 food, if care is taken to have the food free of these substances to 

 begin with. Folin has laid emphasis upon this fact,* and suggests 

 that most of the urea may come directly from protein of the 

 food which is hydrolyzed during digestion and absorption (action 

 of trypsin and erepsin) into simpler amino-acids. These amino- 

 bodies by further hydrolysis and oxidation may be converted, so 

 far as their nitrogen is concerned, into ammonia compounds and 

 eliminated at once as urea by the liver without entering into tissue 

 formation at all. 



Even after the removal of the liver some urea is still found in 

 the urine. It seems as though the urea-forming power of the liver 

 is shared by some of the other tissues, just as its glycogenic functions 

 are. 



Origin and Significance of the Purin Bodies (Uric Acid, 

 Xanthin, Hypoxanthin, Adenin, Guanin). These bodies are 

 related chemically, and appear also to have a common physiological 

 significance. Their chemical relations have been described by 

 Emil Fischer, to whom we owe the term purin bodies. Fischer 

 pointed out that these and other substances belonging to this 

 group have a common nucleus: 

 N C 



C C Nv which he named the purin nucleus. The 



hydrogen compound of this nucleus would be designated as purin, 



N = CH 



: HC C 



and would have the formula: HC C NH , C 5 H 4 N 4 . Addi- 



N C N^ CH 



tion of an atom of oxygen gives hypoxanthin, C 5 H 4 N 4 0: 

 HN CO 



HC C NH 



II _ II _ NX CH * Addition of two atoms of oxygen gives xan- 

 HN CO 



thin, C 5 H 4 N 4 2 : CO C NH 



H'N C NX CH - An( * addition of three atoms 

 * Folin, 'American Journal of Physiology," 13, 117, 1905. 



