Lastly, Walker Hall, discussing endogenous uric acid excretion, emphasises the necessity of discriminating between the _uric acid_ output and the _total purin_ output. He reminds us that the actual cell nucleins belong chiefly to the group of amino-purins, _i.e._, guanine and adenine, and that the oxypurines, xanthine and hypoxanthine, are intermediate products on their way to excretion, another and more advanced intermediate product being uric acid. Now, only a proportion of these intermediary products appears in the urine, this commonly cited to be approximately 50 per cent. But this, as Walker Hall states, must be taken only as a very broad estimate, for in the same individual the output varies with the number of conditions, not as yet fully determined. But the point most emphasised by him is, that though “the _uric acid_ output varies considerably, the _total purin_ output does not show similar variations; for when the uric acid excretion wanes that of the purin bases usually rises. As a consequence, the total purin output is more constant, less influenced by circumstances, than the output of _uric acid_.” This being so, we shall now pass on to consider other conditions influencing endogenous uric acid excretion. FACTORS INFLUENCING ENDOGENOUS URIC ACID EXCRETION The output of endogenous uric acid excretion is influenced by (1) Physiological conditions, (2) Pathological states, and (3) The ingestion of certain drugs. PHYSIOLOGICAL CONDITIONS It is now recognised that the purin bases of the body exist not only in the bound form (nucleic acid), but also _free_, especially in _muscular_ tissue. Also, that from such free purin bases uric acid can be readily formed as from those liberated by the disruption of nucleic acid. Thus, _inosinic_ acid, a nucleotid first isolated from meat extract, yields phosphoric acid and the purin base, _hypoxanthine_. In possession of these facts, we shall be better able to appreciate the significance of the researches of Burian and others. (_a_) _Muscular Exercise._—According to Burian a large increase in the excretion of uric acid was found to follow _muscular exercise_. The same observer also noted the presence of _hypoxanthine_ in defibrinated blood after its perfusion through the hind legs of a dog whose muscles had been thrown into tetanus. Moreover, subsequent to contraction, the muscles themselves contained an increased amount of oxypurine. From these findings Burian concluded that hypoxanthine was a product of _muscular action_, and that this substance or its precursor, _inosinic acid_, was an important source of _endogenous uric acid_. The uric acid thus formed by oxidation was then partly destroyed in the liver and partly excreted by the kidneys. But Burian noted also during activity of the muscles that a certain amount of the _purin bases_ failed of oxidation, and consequently a larger amount of the same, as compared with uric acid, passed into the circulation.