KIDNEY AND SKIN AS EXCRETORY ORGANS. 855 



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, 

 giving us phenolsulphuric acid (C 6 H 5 OS0 2 OH) , cresolsulphuric acid 

 (C 7 H 7 OS0 2 OH), indoxylsulphuric acid or indican (C 8 H 6 NOSO 2 OH), 

 and skatoxylsulphuric acid (C 9 H 8 NOSO 2 OH) . The indol, skatol, 

 phenol, and cresol are formed in the large intestine as a result of bac- 

 terial putrefaction. They are eliminated in part in the feces, but 

 in part are absorbed into the blood, and after oxidation are. 

 conjugated with sulphuric acid and eliminated in the urine. The 

 process of conjugation is valuable from a physiological standpoint, 

 as it converts substances having an injurious action into harmless 

 compounds. It should be added, also, that to a small extent the 

 phenol, indoxyl, and skatoxyl may be secreted in the urine as con- 

 jugated glucuronates, that is, in combination with glycuronic acid 

 (G 6 H 10 7 ), a reducing substance closely connected with dextrose. 

 From a nutritional standpoint the amount of these substances pres- 

 ent furnishes a measure of the extent of protein putrefaction in the 

 intestine, by virtue of the indol and phenol constituents. All con- 

 ditions that increase the putrefactive processes in the intestine 

 are accompanied by a parallel increase in the ethereal sulphates. 

 By virtue of the sulphuric acid component theee bodies represent 

 also one of the forms in which sulpnur is excreted from the 



