THE URINE 



3*7 



Fig. 99.— Crystals of Purified Hippuric 

 Acid (Funke). 



acid in vegetable feeders has not, as far as we know, been worked 

 at, though its physiological interest is considerable. 



Hippuric Acid. — This acid is characteristic of the urine of the 

 herbivora ; traces may also be found in the urine of man, but 

 none in birds. Hippuric acid cannot be detected in the blood, 

 nor does it exist free in the urine, but as hippurates of lime or 

 potash, probably the former 

 (Figs. 99, 100). Chemically 

 it is a conjugated acid formed 

 by the union of glycine with 

 benzoic acid. The synthesis 

 takes place in the kidney, 

 and hippuric acid is one of 

 the few substances actually 

 formed by that gland. In 

 the rabbit the synthesis may 

 occur elsewhere, as the ani- 

 mal can form hippuric acid 

 after the removal of the 

 kidneys. The benzoic acid 

 is derived from vegetable 

 food. It is known that hay, 

 grass, and grain contain in 

 their cuticular covering a 

 substance which yields hip- 

 puric acid in the body. If 

 the foods be treated with 

 caustic potash, the hippuric- 

 acid-forming substance is 

 removed, and animals fed 

 on this forage form no hip- 

 puric acid. Even, it is said, 

 if the husk be removed from 

 grain, the latter is incapable 

 of giving rise to hippuric 

 acid. The benzoic acid de- 

 rived from the various aro- 

 matic bodies contained in 

 plants, is combined with 

 glycine, the latter being de- 

 rived either from the decomposition of protein substances or 

 from the bile acid. There is a doubt about its source, as no 

 free glycine has been detected either in the blood or tissues. 

 The synthesis occurs in the kidney, and is brought about in the 

 cells of the tubules in conjunction with the oxygen of the red 

 corpuscles, the kidney providing the glycine. 



Fig. 100.- 



-Crystals of Impure Hippuric 

 Acid. 



