Beyond exogenous and endogenous purins there is yet one other possible source of uric acid, viz., its _synthetic formation_ within the organism. This supposition took origin in Horbaczewski’s discovery that in the laboratory he was able to produce uric acid by the interaction of _urea_ and _glycocine_, a finding afterwards confirmed by Latham. The theory was then advanced that a similar synthesis might be effected by the _kidneys_; but it was found that glycocine and urea, even when given in excess to mammals, caused no change in the uric acid excretion. So much by way of preface to our detailed discussion _seriatim_ of the various sources of uric acid, and to which we now pass on. EXOGENOUS PURINS The foodstuffs that cause an increase in purin excretion are divisible into three groups:— (_a_) Amino-purins. (_b_) Oxy-purins. (_c_) Methyl-purins. _Amino-purins._—In man the taking of food rich in nucleated cells and therefore in nucleo-protein and nucleins, increases the quantity of uric acid in the urine. Thymus gland, pig’s pancreas, and herring roe, containing the characteristic conjugated proteins of nuclei, or Liebig’s meat extract, rich in purin bases, when ingested, lead to a distinct increase in purin excretion. The researches of Kossel and Horbaczewski showed that such augmentation was mainly due to the production of uric acid from the _nuclein_ substances of the food; in other words, it was due to the katabolism of _nuclein_, the cleavage products of which comprise _adenine_ derived from thymus, and _guanine_ from the pancreas, both of these bodies being amino-purins. According to Burian and Schur, of the amino-purins ingested, a fourth is excreted as purin. _Oxy-purins._—To this group belong xanthine and hypoxanthine. These substances occur in muscle, and in great abundance in meat extract, and Minkowski noted that the ingestion of xanthine bases markedly augmented the amount of uric acid excreted. In man, given ingestion of hypoxanthine as such, only one half thereof appears as uric acid in the urine. It may here be mentioned that not all the purin bases ingested exist bound up in the nuclein substances. An appreciable amount is present in the tissues in a _free_ state, _e.g._, hypoxanthine in the muscles; consequently, a moiety of the intake of purin bodies, especially in the animal constituents of the food, is to hand ready formed, and does not require the disruption of _nucleic acid_ for its liberation.