VI] THE EXCRETORY ORGANS 81 



various organs and tissues (endogenous formation). The former 

 mode of production is largely responsiy)le for those disorders 

 (e.g. gout) which are due to excess of uric acid, and such dis- 

 orders may be palliated or cured by restricting the diet (as by 

 avoiding meat which is especially rich in the purine base hypo- 

 xanthine, C5H4N4O). 



Hippuric acid, (C^HjCO.NH.CHgCOOH benzoyl glycocoll), is 

 especially characteristic of the urine of Herbivora in which it is 

 present in the form of hippiurate salts. It may be crystallised out 

 from horse's urine by evaporating to a syrup and saturating with 

 hydrochloric acid. As its formula suggests, it orginates through 

 the combination of benzoic acid (CgH^COOH) and glycocoll 

 (CH^NHaCOOH) with the elimination of water. This synthesis 

 occurs in the kidnej^ itself, and is possibl}' due to the action 

 of a ferment. This is rendered probable hy the fact that 

 Ijenzoic acid and glycocoll can be made to unite to form hippuric 

 acid in the presence of ground up kidney extract mixed with 

 blood. There is a hippiuric acid forming substance in hay, 

 •arass and grains. 



Benzoic acid, (('gHjCOOH), is said to be present in the urine 

 of working horses, and as has been stated it is a precursor of 

 hippuric acid. Benzoic acid is apparently derived from various 

 aromatic substances which are present in plants and are absorbed 

 in the food. 



Creatine, (C4H9N3O2 methyl guanidine acetic acid), is a con- 

 stituent of muscle and is not found in urine excepting diiring 

 starvation, and in certain febrile and other pathological conditions. 



Creatinine, (C4H7N3O), which is the anhydride of creatine is 

 present in normal urine and in generally constant amount. There 

 is evidence that the hver is the chief seat of its formation. It 

 is a product of protein metabolism, and according to Mellanby 

 l)robably gives rise to the creatine of the muscles, any excess 

 ( )f creatinine, \\hich is not required for this purpose, being excreted 

 by the kidneys. 



Indican. Of the ethereal sulphates occurring in the urine 

 ])hen\d sulphate of potassium and indoxyl sulphate of potassium 

 are chief. The latter is formed from indol potassium hydrogen 

 sulphate. Indol is a product of intestinal disintegration pro- 

 duced from tryptophane by bacterial putrefaction. It is not 

 normal in human urine, though it very frequently occurs, but it 



