KIDNEY AND SKIN AS SECRETORY ORGANS. 853 



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

 upon pathological chemistr}'. 



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 tlirough the skin, the less will be secreted by the 

 kidneys. Through these tlii'ee 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 eliminated through 

 the kidneys. 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 

 intervals finally ejected from the bladder through the urethra by 

 the act of micturition. 



Movements of the Ureters. — The ureters possess a muscular coat 

 consisting of an internal longitudinal and external circular layer. 

 The contractions of this muscular coat form the means by which 

 the urine is driven from the pehds of the kidney into the bladder. 

 The movements of the ureter have been carefully studied by Engel- 

 maim. * According to his description, the musculature of the ureter 



* "Pflliger's Archiv. f. die gesammte Physiologie," 2. 243, 1869, and 4, 33; 

 see also Lucas, "American Journal of Physiology," 17, 392, 1906. 



