760 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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

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

 end-products of proteid metabolism. The sulphur elimination in 

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

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

 occurs in three forms: (1) In an oxidized form as inorganic sulphates. 

 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 proteids. (2) 

 The so-called conjugated or ethereal sulphates are combinations be- 

 tween sulphuric acid and indoxyl, skatoxyl, phenol, and cresol, giving 

 us phenolsulphuric acid (C 6 H 5 OSO 2 OH), cresolsulphuric acid (C 7 H 7 - 

 OSO 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, phe- 

 nol, 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 glucuronic acid 

 (C 6 H 10 O 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 proteid 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 these bodies represent 

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

 body. (3) Some of the sulphur in the urine may occur in unoxid- 

 ized form as sulphocyanid or as ethyl-sulphide (Abel) ([C 2 HJ 2 S). 

 Under certain pathological conditions (cystinuria) some sulphur may 

 be excreted in the form of cystin, but this is not a normal con- 

 stituent of the urine. For other most interesting and significant 

 changes in the composition of the urine under pathological condi- 

 tions reference must be made to special works upon the urine or 

 upon pathological chemistry. 



Water and Inorganic Salts. Water is lost from the body 

 through three main channels, namely, the lungs, the skin, and 

 the kidney, the last of these being the most important. The quan- 

 tity of water lost through the lungs probably varies within small 

 limits only. The quantity lost through the sweat varies, of course, 

 with the temperature, with exercise, etc., and it may be said that 



