HIPPURIG ACID 2O5 



111 some animals uric acid takes the place of urea, none of 

 the latter being formed. In these cases it is manufactured 

 by the liver from ammonia compounds. This does not, how- 

 ever, seem to be the origin of uric acid in human urine. It 

 has been looked upon as unconverted urea, i. e., as a product 

 antecedent to urea ; but at present such does not seem to be 

 the case. A theory that it is the end product of the destruc- 

 tion of certain materials in the nuclei of cells has consider- 

 able support. 



Hippuric acid exists in the urine as hippurates. It differs 

 from most of the other urinary constituents in being formed 

 in the kidney ; it does not preexist in the blood. The daily 

 output of this substance is about 10 grains, though the 

 amount may be considerably increased on a vegetable diet. 

 The benzoic acid of vegetables seems to be synthesized into 

 hippuric. In proteid dissimilation some benzoic acid may be 

 produced and eliminated in this shape. 



The various lactates are not formed by the kidney, but 

 pass unchanged into it from the blood. The lactic acid from 

 which they are formed probably results from the transfor- 

 mation of dextrose. 



Creatinin is normally present in the urine. It is a nitro- 

 genous body differing from creatin only by a molecule of 

 water. It is eliminated to the extent of about 15 grains per 

 day. A part comes from proteid destruction in the body, and 

 another part is said to come directly, without metabolism, 

 from creatin which is a constituent of ordinary meat. It is 

 not formed in the kidney. 



Xanthin, hypoxanthin, etc., are to be regarded as nitro- 

 genous excreta allied to uric acid and resulting in some way 

 from proteid metabolism. They are regarded by some as 

 having the same probable origin as uric acid, viz., the disin- 

 tegration of cell nuclei. 



The non-nitrogenous constituents scarcely deserve separ- 

 ate mention. It is through the kidney that the largest variety 

 of these materials are discharged. Certain of these are con- 



