MECHANISM OF THE DISCHARGE OF URINE. 409 



brane is tolerably vascular. Lymphatics have been described as existing in the walls of 

 the bladder, but Sappey, whose researches in the lymphatic system have been very ex- 

 tended and successful, has failed to demonstrate them in this situation. The nerves of 

 the bladder are derived from the hypogastric plexus. 



The urethra is provided with muscular fibres, and it is lined by a mucous membrane, 

 the anatomy of which will be more fully considered in connection with the function of 

 generation. 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 some of the lower orders of animals in 

 which the urine is of a semisolid consistence, the movement of vibratile cilia in the uri- 

 niferous tubes probably aids in the discharge of the excretion ; but, in the human subject, 

 the existence, even, of cilia is doubtful, and the urine is discharged into the pelves of the 

 kidneys and the ureters by pressure due to the act of separation of the fluid 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 muscular coats of these canals. 

 Mtiller has found that the ureters can be made to undergo a powerful local contraction 

 upon the application of a galvanic current ; and Bernard has shown that this may be 

 produced by galvanization of the anterior root of the eleventh dorsal nerve. Notwith- 

 standing these facts, it is difficult to estimate the amount of influence ordinarily exerted 

 by peristaltic contractions of the ureters ; but, when there is excessive accumulation of 

 urine in the bladder, or when there is obstruction from any cause, such as the presence 

 of a renal calculus, these contractions are probably quite energetic. 



When the urine has accumulated to a certain extent in the bladder, a peculiar sensa- 

 tion is experienced which leads to the act for its expulsion. This desire to discharge the 

 urine is probably due to the impression produced by the distention of the bladder. The 

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

 before retiring to rest and upon rising in the morning, and generally two or three times, 

 in addition, during the day. The frequency of micturition, however, depends very much 

 upon habit, upon the quantity of liquids ingested, and upon the degree of activity of the 

 skin ; the latter conditions modifying the quantity of urine. 



Evacuation of the bladder is accomplished by the muscular walls of the organ itself, 

 aided by contractions of the diaphragm and the abdominal muscles with certain muscles 

 which operate upon the urethra, and it is accompanied by relaxation of the sphincter 

 vesicse. This act is at first voluntary, but, once commenced, 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 period 

 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 with considerable force 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 at any time be considerably increased by voluntary effort. 



It is a question whether the bladder be capable of entirely emptying itself by the ac- 

 tion of its muscular walls. That almost all the urine may be expelled in this way in the 

 human subject, there can be no doubt ; and it has been shown by experiments upon some 

 of the inferior animals that the bladder may be completely evacuated when it has been 

 drawn out of the abdominal cavity. In vivisections, we have frequently observed the 

 bladder so firmly contracted that it could contain hardly more than a few drops of liquid. 



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

 bladder is slight, the diaphragm and the abdominal muscles are again called into action, 

 and there is a convulsive, interrupted discharge of the small quantity of urine that re- 

 mains. At this time, the impulse from the bladder, and, indeed, the influence of the ab- 



