THE METABOLISM OF THE PURIN BODIES 393 



between birds and mammals. Why should the chief end product of 

 proteid metabolism be uric acid in birds and urea in mammals ? 



In birds, uric acid is, almost entirely, produced by the synthesis 

 of two urea molecules with a tri-carbon chain. If this synthesis be, 

 from any cause, prevented, urea, and not uric acid, is excreted, 

 and the bird behaves, so far as its proteid metabolism is con- 

 cerned, like a mammal. The difference between the two groups of 

 animals is therefore more apparent than real ; in both cases urea is 

 the end product of proteid metabolism, but in mammals it is excreted 

 in the urine as such, whereas in birds it is excreted as uric acid. 1 



Our next question, therefore, is, whether or not the small amount 

 of uric acid which mammals do excrete arises also by a synthetic 

 process ? Is it, as was at one time suggested, the remnant of an 

 evolutionary process which betrays the development of birds and 

 mammals from some common stock in which the synthetic process 

 alone obtained ? Or is the source of uric acid in mammals an entirely 

 different one from that in birds ? To answer this question is to 

 anticipate much of what will follow in the succeeding pages, but 

 it is necessary to do so briefly in order to understand what more 

 immediately concerns us. The trace of uric acid in mammals is 

 not, to any appreciable extent at least, produced by a synthetic 

 process, but arises by the oxidation of other purin bodies in the 

 organism ; about one-half of it from purins given in the food and 

 the other half from the purins which are set free in the tissues. 



All the purins given in the food, or all that are liberated in the 

 tissues, do not, however, appear in the urine. A certain portion (in 



1 This analogy between uric acid in birds and urea in mammals has been 

 strikingly demonstrated in some experiments by T. H. Milroy ( 37 ). By making 

 an artificial anus in large birds (geese, ducks, and turkeys), Milroy was able to 

 collect urine unmixed with fasces, aud so to estimate accurately in the urine 

 how much total nitrogen, uric acid, purin bases, and ammonia were excreted. 

 When nitrogenous equilibrium had become established and the excretion of 

 purins constant in amount, he administered to the birds sufficient acid 

 (hydrochloric or lactic) to produce mild symptoms of acid toxaarnia, and found 

 besides a distinct diuresis that the amount of uric acid in the urine became 

 greatly diminished (^th its previous amount), but that of ammonia greatly 

 increased. In mammals, acid intoxication greatly diminishes the excretion of 

 urea, but causes the ammonia excretion to increase, and, no doubt, this is what 

 had primarily occurred in Milroy's experiment ; the acid had by combining with 

 ammonia prevented urea formation, and consequently also that of uric acid. 



The acid administration did not, in the above experiments, influence the 

 excretion of the purin bases, and it is possible that these may be produced by 

 some process akin to that obtaining in mammals, viz., oxidation. No certain 

 evidence of this could, however, be obtained by Milroy, 



