878 PHYSIOLOGY 



constituents. It has been shown that digestion of the primary 

 purine bases of most nucleic acids, namely, guanine and adenine, with 

 extracts of tissues such as the liver, converts these bodies into hypo- 

 xanthine and xanthine, the change involved being one of deamination. 

 as is evident from comparing the formula) of these bodies. 



N=C NH 2 NH-CO 



HC C NH NH 2 .C C NH 



Adenine Guanine 



HN-CO HN-CO 



II II 



HC C-NH CO C-NH 



Hypoxanthine Xanthine 



Under the action of other oxidising ferments also contained in 

 tissues xanthine may be converted into uric acid. If spleen pulp be 

 digested with blood for some time it is possible to extract a consider- 

 able amount of xanthine from the mixture. If, however, oxygen be 

 bubbled through the mixture the xanthine disappears, its place being 

 taken by uric acid. It is evident that these changes, resulting 

 ultimately in the formation of uric acid, might affect either the 

 nucleins derived from the food or any nucleins set free by disintegra- 

 tion of the tissues of the body itself. Uric acid being a constituent of 

 urine, we might therefore assign to it a twofold origin, namely, 

 (a) exogenous from the nucleins of the food, (6) endogenous from the 

 nucleins of the tissues. This would not, however, exhaust' all the 

 possibilities. We have seen already that in the bird the greater part 

 of the uric acid, which represents the chief nitrogenous excreta, is 

 formed not from purines at all, but by a process of synthesis from 

 lactic acid and ammonia. This synthesis occurs in the liver, so that 

 after extirpation of this organ the place of the uric acid in the urine 

 is taken by lactic acid and ammonia. Though we have no evidence 

 of a similar change taking place in the mammal, we cannot exclude 

 the possibility of some formation of uric acid by a process of 

 synthesis. 



Even after uric acid is formed in any of the ways mentioned above, 

 it does not necessarily follow that it will be excreted as such. Uric 

 acid may be further broken down in the body, as it is outside the 

 body., under the action of oxidising agents. Thus if nitric acid be 



