EXCRETION. 423 



though it assumes a variety of forms. Uric acid was long regarded 

 as a product of general proteid metabolism and for chemic reasons 

 an antecedent of urea. This view has been abandoned. /) At present 

 it is believed that itis a cleavageproduct of nuclein, a 

 rriliti mLUbuluuTO! liudeilffPBleid 



from the latter of which uric acid is derived. Nu- 

 cleic acid when decomposed yields a series of bases, such as xanthin, 

 hypoxanthin, adenin, guanin, etc., known collectively as the alloxur 

 bases, including uric acid, a name which indicates their relation, 

 on the one hand, to alloxan and on the other to urea. Though there 

 is a close relationship between uric acid and the alloxur bases, it has 

 been impossible to experimentally derive one from -the other. When 

 hypoxanthin, however, is given internally it is oxidized and converted 

 into uric acid. It is extremely probable, therefore, that uric acid is 

 an oxidation product of one or more of the alloxur bases. 



Xanthin, hypoxanthin, guanin, etc., are also found in urine 

 in small but variable amounts. They are nitrogenized compounds 

 derived mainly from the metabolism of the nuclein bodies. 



Kreatinin is a crystalline nitrogenous compound closely resem- 

 bling kreatin, one of the constituents of muscular tissue. The amount 

 excreted daily is about i gram. Though kreatinin may arise in conse- 

 quence of proteid metabolism, it is probable that it is largely derived 

 from a transformation of the kreatin contained in the meat consumed 

 as food. 



Hippuric acid in combination with sodium and potassium is 

 very generally present in urine, though in small amounts. It is more 

 abundant in the urine of the herbivora than the carnivora. In man 

 the amount excreted daily is about 0.7 gram, though the amount may 

 be raised by a diet of asparagus, plums, cranberries, etc., and by the 

 administration of benzoic and cinnamic acids. There is evidence that 

 hippuric acid is formed in the kidney from benzoic acid, its pre- 

 cursors, or related bodies. Various compounds of this class are found 

 in vegetable foods, a fact which may account for the increase in the 

 excretion of uric acid on a vegetable diet. 



Leucin, tyrosin, phenol, cystin, indoxyl, skatoxyl, are found 

 in small amounts even under normal conditions. They arise from 

 putrefactive change in the intestine. 



Inorganic Salts. Sodium and potassium phosphates, known 

 as the alkaline phosphates, are found in both blood and urine. J 

 The total quantity excreted daily is about 4 grams. Calcium y|j^ 

 and magnesium phosphates, known as the earthy phosphates, are 

 ]5resent to the extent of i gram. Though insoluble in water, they 

 are held in solution in the urine by its acid constituents. If the 

 urine be rendered alkaline, they are at once precipitated. Sodium 

 and potassium sulphates are also present to the extent of about 



