856 PHYSIOLOGY OF DIGESTION AND SECRETION. 



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 

 the amounts of water secreted through kidney and skin stand in 

 something of an inverse proportion to each other; that is, the greater 

 the quantity lost through the skin, the less will be secreted by the 

 kidneys. Through these three organs, but mainly through the 

 kidneys, the blood is being continually depleted of water, and the 

 loss must be made up by the ingestion of new water. When water 

 is swallowed in excess the superfluous amount is rapidly eliminated 

 through the kidneys, and there is some evidence (p. 843) that in 

 this case the excretion of the excess of water is controlled by a 

 special hormone found in the small intestine. The amount of 

 water secreted may be increased by the action of diuretics, such as 

 potassium nitrate and caffein. 



The inorganic salts of urine consist chiefly of the chlorids, phos- 

 phates, and sulphates of the alkalies and the alkaline earths. It 

 may be said, in general, that they arise partly from the salts ingested 

 with the food, and are eliminated from the blood by the kidney 

 in the water secretion; and in part they are formed in the destruc- 

 tive metabolism that takes place in the body, particularly that 

 involving the proteins and related bodies. Sodium chlorid occurs 

 in the largest quantities, averaging about 15 gms. per day, of 

 which the larger part, doubtless, is derived directly from the salt 

 taken in the food. The phosphates occur in combination with cal- 

 cium and magnesium, but chiefly as the acid phosphates of sodium 

 or potassium. The acid reaction of the urine is usually attributed 

 to these latter substances. The phosphates result in part from the 

 destruction of phosphorus-containing tissues in the body, but 

 chiefly from the phosphates of the food. The sulphates of urine 

 are found partly in an oxidized form as simple sulphates or con- 

 jugated with organic compounds, as described above. 



Micturition. The urine is secreted continuously by the kid- 

 neys, is carried to the bladder through the ureters, and is then at 



