In conclusion, it would appear that some _drugs_ influence more or less markedly the excretion of endogenous uric acid, notably, atophan; but discussion of these will, we think, be best postponed to the section dealing with the medicinal treatment of gout. Meanwhile we shall proceed to consider the vexed question of the formation within the organism of uric acid by synthesis. SYNTHETIC FORMATION OF URIC ACID Birds eliminate most of their nitrogen in the form of uric acid, and, undoubtedly, in their instance synthetic formation of uric acid in the liver takes place on a large scale. Thus, when blood containing ammonium lactate is perfused through the liver of the goose, an increase in the uric acid content of the blood occurs. Also the ingestion of lactic, pyruvic and other organic acids leads to augmented output of uric acid; in short, it is generally agreed that in birds _synthesis_ is the chief mode of formation of uric acid, homologous with the formation of _urea_ in the liver of mammals. If this be true of birds, on the other hand, _splitting_ and _oxidation_ of _nucleins_ is in _mammals_ the most important source of uric acid, but there is evidence that it cannot all be accounted for in this way. As before remarked, the old belief that purin excretion remains almost constant on a _purin-free_ diet, despite great variations in the amount of the ingests, is not strictly true. Thus, using swifter and more reliable methods for the estimation of nitrogenous metabolites, Folin noted, on an absolutely purin-free diet, that an increase in purin excretion ensued, given _marked_ variations in the intake of food. Again, the Dalmatian dog, as we have seen, excretes uric acid in his urine. S. R. Benedict was therefore able to demonstrate that a very distinct increase in his uric acid output ensued in sequence to increase in the amount of his _non-purin_ food; moreover, that even when such non-purin foods were continued for a year, “the total amount of uric acid excreted was at least ten times greater than could have come from the traces unavoidably included in the food” (MacLeod). Also Ascoli and Izar, experimenting with dog livers, noted on incubation thereof and passage through the same of oxygen that the uric acid disappeared; but on the substitution of carbon dioxide an accumulation thereof ensued. Wells, however, was unable to confirm this re-synthesis of uric acid by dog livers, and Spiers also failed to corroborate their findings. On the other hand, there is evidence pointing to the fact that a certain small percentage of synthetic formation does take place in the organism. Thus certain chemical substances, and these not purin, do cause an appreciable though slight increase in the purin excretion of mammals, and a very marked augmentation of the same in birds, viz., _lactic_, _tartronic_ and _B-oxybutyric acids_.