KIDNEY AND SKIN AS EXCRETORY ORGANS. 781 



not made by the body and hence muscular creatin is probably not 

 an antecedent substance to the urinary creatinin.* As is described 

 in the section on Nutrition, it is known that increased muscular 

 work may or may not increase the nitrogen output in the urine 

 according to the diet used. Several observers have claimed that 

 muscular activity increases the amount of creatinin in the urine, f 

 but the increase is not so distinct nor so invariable that one may 

 conclude satisfactorily that it is due to actual increase in production 

 in the muscle. Others state that the increase is observable only 

 after excessive muscular activity. 



Hippuric Acid. This substance has the formula C 9 H 9 NO 3 . Its 

 molecular structure is known, since upon decomposition it yields 

 benzoic acid and glycocoll, and, moreover, it may be produced syn- 

 thetically by the union of these two substances. Hippuric acid 

 may be described, therefore, as a benzoyl-amino-acetic acid (CH 2 - 

 NH[C 6 H 5 CO]COOH). It is found in considerable quantities in the 

 urine of herbivorous animals (1.5 to 2.5 per cent.), and in much 

 smaller amounts in the urine of man and of the carnivora. In 

 human urine, on an average diet, about 0.7 gm. are excreted in 

 twenty-four hours. If the diet is largely vegetable, this amount may 

 be much increased. This last fact is readily explained, for it has been 

 found that if benzoic acid or substances containing this grouping 

 are fed to animals they appear in the urine as hippuric acid. Evi- 

 dently a synthesis occurs in the body, and Bunge and Schmie- 

 deberg proved conclusively that in dogs the union of benzoic acid 

 and glycocoll to form hippuric acid takes place in the kidney 

 itself. Later it was discovered:}: that the same synthesis may be 

 effected by ground-up kidney tissue, mixed with blood and kept 

 under oxygen pressure. It seems possible, therefore, that the 

 synthesis is due to some specific constituent of the kidney cells, 

 possibly an enzyme. Vegetable foods contain benzoic acid com- 

 pounds, and we can understand, therefore, why when fed they in- 

 crease the hippuric acid output of the urine. Since, however, in 

 starving animals or animals fed upon meat hippuric acid is still 

 present in the urine, although reduced in amount, it is evident that 

 it arises in part as a result of the body metabolism. It should be 

 added finally that some of the hippuric acid may be derived from 

 the process of protein putrefaction that occurs in the large intestine. 



The Conjugated Sulphates and the Sulphur Excretion. 

 The sulphur excretion of the urine possesses an importance similar 



* Folin, "Festschrift fur Olof Hammarsten," Upsala, 1906. 



t Gregor, "Zeitschrift f. physiol. Chemie," 31, 98, 1900. 



j Bashford and Cramer, 'Zeitschrift f. physiolog. Chemie," 35, 324, 1902. 



