KIDNEY AND SKIN AS EXCRETORY ORGANS. 861 



molecular structure is known, since upon decomposition it yields 

 benzole acid and giycocoll, and, moreover, it may be produced 

 synthetically 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 giycocoll 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 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. The physio- 

 logical significance of this nitrogenous product is different from 

 those previously considered in that it does not represent the 

 result of a special metabolism of protein food or protein tissue, 

 but is rather a provision by which benzoic acid when present 

 in the food or formed in the body is conjugated with glycin and 

 excreted. 



The Conjugated Sulphates and the Sulphur Excretion. 

 The sulphur excretion of the urine possesses an importance similar 

 to that of nitrogen. Sulphur constitutes an element in most of the 

 proteins, and in some form, therefore, it will be represented in the 

 end-products of protein metabolism. The sulphur elimination in 

 the urine, like the nitrogen elimination, has been taken as a measure 

 of the amount of protein destruction. In the urine the sulphur 

 occurs in three forms: (1) In an oxidized form as inorganic sul- 

 phates. Some of the sulphates are undoubtedly derived or may be 

 derived from the mineral sulphates ingested with the food, but the 

 larger part arises from the oxidation of the sulphur of the proteins. 

 (2) The so-called conjugated or ethereal sulphates are combinations 

 between sulphuric acid and indoxyl, skatoxyl, phenol, and cresol, 



* Bashford and Cramer, "Zeitschrift f. physiol. Chemie," 35, 324, 1902. 



