MECHANISM OF THE DISCHARGE OF URINE 335 



mucous membrane, the anatomy of which will be more fully considered 

 in connection with embryology. In the female the epithelium of the 

 urethra is like that of the bladder. In the male the epithelial cells are 

 small, pale and of the columnar variety. 



Mechanism of the Discharge of Urine. In the human subject, the 

 urine is discharged into the pelves of the kidneys and the ureters by 

 pressure due to the act of separation of liquid from the blood. Once 

 discharged into the ureters, the course of the urine is determined in 

 part by the vis a tergo, and in part, probably, by the action of the mus- 

 cular coats of these canals. 



When the urine has accumulated in certain quantity in the bladder, 

 a peculiar sensation is felt which leads to the act for its expulsion. The 

 intervals at which it is experienced are variable. The urine usually is 

 voided before retiring to rest, on rising in the morning, and two or three 

 times, in addition, during the day. The frequency of micturition, how- 

 ever, depends on habit, on the quantity of liquids ingested and on 

 the degree of activity of the skin. 



Evacuation of the bladder is accomplished by the muscular walls of 

 the organ itself, aided by contractions of the diaphragm and the abdomi- 

 nal muscles with certain muscles which operate on the urethra, and 

 it is accompanied by relaxation of the sphincter vesicse. This act is at 

 first voluntary, but once begun, it may be continued by the involuntary 

 contraction of the bladder alone. During the first part of the process, 

 the distended bladder is compressed by contraction of the diaphragm 

 and the abdominal muscles ; and this after a time excites the action of 

 the bladder itself. A certain time usually elapses then before the urine 

 begins to flow. When the bladder contracts, aided by the muscles of 

 the abdomen and the diaphragm, the resistance of the sphincter is over- 

 come and a jet of urine flows from the urethra. All voluntary action 

 may then cease for a time, and the bladder will nearly empty itself ; but 

 the force of the jet may be increased by voluntary effort. 



Toward the end of the expulsive act, when the quantity of liquid 

 remaining in the bladder is small, the diaphragm and the abdominal 

 muscles are again called into action, and there is a convulsive inter- 

 rupted discharge of the small quantity of urine that remains. At this 

 time the impulse from the bladder, and, indeed, the influence of the 

 abdominal muscles and diaphragm, are very slight, and the flow of urine 

 along the urethra is aided by the contractions of its muscular walls and 

 the action of certain of the perineal muscles, the most efficient being 

 the accelerator urinae ; but with all this muscular action, a few drops 

 of urine remain in the male urethra after the act of urination has been 

 accomplished. The process of evacuation of urine in the female is essen- 



