168 HUMAN PHYSIOLOGY. 



in the lumbar region of the spinal cord. This center can be in- 

 hibited and the sphincte'r relaxed, either reftexly, by impressions 

 coming through sensory nerves from the mucous membrane of the 

 bladder, or directly, by a voluntary impulse descending the spinal 

 cord. When the desire to urinate is experienced, impressions made 

 upon the vesical sensory nerves are carried to the centers governing 

 the sphincter and detrusor urince muscles and to the brain. If now 

 the act of urination is to take place, a voluntary impulse originating 

 in the brain passes down the spinal cord and still further inhibits 

 the sphincter vesicse center, with the effect of relaxing the muscle 

 and of stimulating the center governing the detrusor muscle, with 

 the effect of contracting the muscle and expelling the urine. If 

 the act is to be suppressed, voluntary impulses inhibit the detrusor 

 center and possibly stimulate the sphincter center. 



The genitospinal center controlling these movements is situated 

 in that portion of the spinal cord corresponding to the origin of the 

 third, fourth, and fifth sacral nerves. 



URINE. 



Normal urine is of a pale yellow or amber color, perfectly trans- 

 parent, with an aromatic odor, an acid reaction, a specific gravity of 

 1020, and a temperature when first discharged of 100 F. 



The color varies considerably in health, from a pale yellow to a 

 brown hue, owing to the presence of the coloring-matter, urobilin or 

 urochrome. 



The transparency is diminished by the presence of mucus, the 

 calcium and magnesium phosphates, and the mixed urates. 



The reaction of the urine is acid, owing to the presence of acid 

 phosphate of sodium. The degree of acidity, however, varies at 

 different periods of the day. Urine passed in the morning is strongly 

 acid, while that passed during and after digestion, especially if the 

 food is largely vegetable in character, is either neutral or alkaline. 



The specific gravity varies from 1015 to 1025. 



The quantity of urine excreted in twenty-four hours is between 

 forty and fifty fluidounces, but ranges above and below this standard. 



The odor is characteristic, and caused by the presence of taurylic 

 and phenylic acids, but is influenced by vegetable foods and other 

 substances eliminated by the kidneys. 



