KIDNEY AND SKIN AS EXCRETORY ORGANS. 761 



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. The amount of water secreted may be in- 

 creased 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, which salts are eliminated from the blood by the kid- 

 ney in the water secretion, and in part they are formed in the de- 

 structive metabolism that takes place in the body, particularly 

 that involving the proteids 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 are the means by which 

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

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

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

 contracts spontaneously at intervals of ten to twenty seconds (rab- 

 bit), the contraction beginning at the kidney and progressing 

 toward the bladder in the form of a peristaltic wave and with a 

 velocity of about 20 to 30 mms. per second. The result of this 

 movement should be the forcing of the urine into the bladder in a 

 series of gentle, rhythmical spurts, and this method of filling the 



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



