CHAP, v.] NUTRITION. 411 



respiration. The final causes of the divergence are to be sought 

 rather in the fact that urea is the form adapted to a fluid, and uric 

 acid to a more solid excrement. 



Hippnric Acid. In the urine of herbivora uric acid is for 

 the most part absent, being replaced by hippuric acid. In the 

 urine of omnivorous man, both acids may be present together. 

 The history of the hippuric acid of urine is very instructive ; for 

 though at first sight its presence might appear to indicate that the 

 metabolism of the herbivora is in some points fundamentally 

 different from that of carnivora, there can be little doubt that the 

 hippuric acid which appears in the urine of herbivora comes 

 directly from the ingested food. Hippuric acid is a compound of, 

 or rather a result of, the union or conjugation of benzoic acid and 

 glycin ; and when benzoic acid is introduced into the stomach of 

 an animal, whether herbivorous or not, it reappears not as benzoic 

 but as hippuric acid. It evidently meets, somewhere in the body, 

 with glycin ; and uniting with this becomes hippuric acid, in which 

 form it passes out by the urine. Nitrobenzoic acid in a similar 

 way becomes nitrohippuric acid; and many other bodies of the 

 aromatic class, by a like assumption of glycin, become conjugated 

 in their passage through the body. 



The knowledge of the fact that benzoic acid is thus converted 

 into hippuric acid naturally suggested the idea that the food of 

 herbivora might contain either benzoic acid, or some allied body, 

 and that the presence of hippuric acid as a normal constituent of 

 urine might be thus accounted for. And it would appear 

 that all the hippuric acid of herbivorous urine is in reality 

 due to the presence in ordinary fodder (hay) of a particular 

 constituent containing a benzoic residue ; when this constituent is 

 withdrawn, the hippuric acid disappears from the urine. 



The transformation or conjugation appears to take place chiefly 

 in the kidneys, for when blood containing benzoic acid is driven 

 through the vessels of a fresh, that is of a living, kidney, it is found 

 after the transit to contain hippuric acid. And indeed the same 

 change may be effected by simply mixing benzoic acid with 

 portions of fresh and still warm kidney, broken to pieces and as it 

 were mashed up, the mixture being exposed to the temperature of 

 the body. If the kidney be kept some time before the benzoic 

 acid is submitted to its action, the transformation fails, indicating 

 that the change is effected by certain elements, probably by the 

 renal epithelium cells, which retain this power so long only as they 

 remain alive after removal from the body. 



A similar transformation of benzoic into hippuric acid is said 

 to take place in some animals at least in the liver, benzoic acid 

 injected into the portal vein reappearing as hippuric acid in the 

 blood of the hepatic vein. In both these cases it is worthy of note, 

 and the observation bears on the formation of urea just discussed, 

 that the change takes place even if no glycin be added with the 



