KIDNEY AND SKIN AS EXCRETORY ORGANS. 759 



the food is eliminated as 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 in- 

 creases the amount of creatinin in the urine,* 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. Koch| has suggested, on account of the methyl groups 

 present, that a part of the creatinin may arise from a metabolism 

 of the lecithin. 



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-amido-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. is 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 proteid putrefaction that occurs in the large intestine. 



The Conjugated Sulphates and the Sulphur Excretion. 

 The sulphur excretion of the urine possesses an importance similar 



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

 f " American Journal of Physiology, " 13, xix, 1905. 

 t Bashford and Cramer, "Zeitschrift f. physiolog. Chemie," 35, 324, 

 1902. 



