URINE. 73 



Nervous Mechanism of Urination. When the urine has passed into 

 the bladder, it is there retained by the sphincter vesicae muscle, kept in a 

 state of tonic contraction by the action of a nerve centre in the lumbar 

 region of the spinal cord. This centre can be inhibited and the sphincter 

 relaxed, either reflexly, 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 centres 

 governing the sphincter and detrusor urintz 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 centre, with the effect of relaxing the muscle, and of 

 stimulating the centre governing the detrusor muscle, with the effect of con- 

 tracting the muscle and expelling the urine. If the act is to be sup- 

 pressed, voluntary impulses inhibit the detrusor centre and possibly stimu- 

 late the sphincter centre. 



The genito-spinal centre controlling these movements is situated in that 

 portion of the spinal cord corresponding to the origin of the 3d, 4th and 

 5th sacral nerves. 



URINE. 



Normal Urine is of a pale yellow or amber color, perfectly transparent, 

 with an aromatic odor, an acid reaction, a specific gravity of 1.020, and a 

 temperature when first discharged of 100 Fahr. 



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

 hue, due 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 is slightly acid, caused by the acid phosphate of sodium. 

 After standing for a short time, an increased acidity is observed, due to an 

 acid fermentation, from the presence of mucus. The urea is converted 

 into ammonium carbonate, giving rise to a strong ammoniacal odor. 



The specific gravity varies from l.oio to 1.025. 



The quantity of urine excreted in 24 hours is between 40 and 50 fluid 

 ounces, 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. 



