THE ABDOMEN. 157 



fore, is not purely a process of secretion but requires 

 some action on the part of the kidney, though no new 

 substances are secreted in the kidney. 



The passage of the urine down through the ureters 

 is assisted by a kind of peristaltic action in the walls 

 of the ureters and it is expelled from the body by the 

 act of micturition, which is mostly voluntary, though 

 a certain amount of nervous mechanism controls it. 

 The seat of this nervous mechanism is in the lum- 

 bar enlargement of the spinal cord. In some nervous 

 conditions, especially where there is injury to the spinal 

 cord, there is involuntary micturition. 



The urine is a watery solution containing many waste 

 products, especially urea. It is generally amber in 

 color, varying in shade with circumstances, with an 

 aromatic, characteristic odor when fresh. It is acid 

 in reaction and has a specific gravity of about 1020, 

 though this too varies with circumstances. Besides 

 water, which is its chief constituent, it contains urea, 

 uric acid, organic acids, urates, inorganic salts, includ- 

 ing sodium chloride and phosphates of calcium and 

 magnesium, a certain amount of ammonia, and certain 

 pigments. Its acidity is due to acid sodium phosphate 

 in solution but varies with the food, and in disease the 

 urine may become alkaline when passed. After stand- 

 ing a few hours in a warm place it decomposes and be- 

 comes alkaline. 



The quantity, which is normally three pints or fif- 

 teen hundred cubic centimeters in twenty-four hours, 

 varies with the amount of fluid drunk, the amount of 

 perspiration, etc. The amount secreted depends chiefly, 

 however, upon the flow of the blood through the kidneys; 

 the greater the flow of blood, the larger the amount of 

 urine formed; and the blood flow is determined by blood 

 pressure and by vasomotor action. Secretion also seems 

 to be increased by the presence of urea, which apparently 

 serves as a stimulant to the kidney cells. 



The excretion of waste materials takes place by three 



