As to the _solubilities_ of uric acid and urates in gouty blood he points out that the suspension capability of the blood-stream for uric acid much transcends the highest amount of uric acid as yet found in the gouty subject. Accordingly, to him, therefore, it seems that “neither chemical nor physico-chemical processes suffice to explain the problem. There must be something more, something vital, biological.” Having ascertained as far as possible the measure of our knowledge in regard to the foregoing points, we shall, in the succeeding chapter, proceed to discuss the _sources_ of _uric acid_, whether of intrinsic or extrinsic origin. CHAPTER VII SOURCES OF URIC ACID Uric acid, like the “purin bodies” (xanthine, hypoxanthine, guanine, and adenine), is derived from _nucleins_, _i.e._, from the breaking down of tissues rich in cells. The end-product of purin or nuclein katabolism _uric acid_ represents but a further stage in the oxidation of the _purin_ bodies. To the serial enzymatic transformations that mark its derivation from _nucleic acid_ we shall allude later, but at this juncture we are concerned not with the _mode_ of _formation_ of uric acid, but with the sources thereof. In this sphere we are greatly indebted to the pioneer researches of Burian and Schur. These observers noted that on a diet rich in _nucleins_ (sweetbreads, liver, kidneys) the total daily excretion of uric acid was considerably higher than on a milk or purin-free diet. This difference in response to varying dietaries, in respect of the excretion of uric acid, led Burian and Schur to the conclusion that the purins excreted must be partly of _exogenous_ and partly of _endogenous_ origin; in other words, the exogenous purins are derived from the nucleins ingested in the food, whereas the endogenous are the outcome of the breaking down of the cellular tissue of the organism itself. Here it may be noted that all the ingested purins are not excreted in the urine as uric acid, for some pass away as _purins_. Moreover, the amount excreted will vary with the kind of purin ingested, and also with the _species_ of the animal that consumes it. Thus, in man “only one half of the hypoxanthine administered as such appears as uric acid in the urine, and but one fourth of the purin in nuclein when that is fed. In the dog, compared with man, about ten times as much purin disappears in its passage through the organism; in the rabbit, about three times” (Flack and Hill).[11] In amount about 0·4-0·7 gramme of uric acid is excreted in _human_ urine daily, and the purin bodies, hypoxanthine, xanthine, and adenine, in small quantities.