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. 

 Miiller 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. 



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

 tion is felt which leads to the act for its expulsion. The intervals at which it is experi- 

 enced 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 ac- 

 tivity of the skin. 



Evacuation of the bladder is accomplished 

 by the muscular walls of the organ itself, aid- 

 ed by contractions of the diaphragm and the 

 abdominal muscles with certain muscles which 

 operate upon the urethra, and it is accompa- 

 nied by relaxation of the sphincter vesica3. 

 This act is at first voluntary, but, once begun, 

 it may be continued by the involuntary con- 

 traction 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 abdo- 

 men and the diaphragm, the resistance of the 

 sphincter is overcome, 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 considerably 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, 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- 



FIG. 116*. Diagram shoicinfl the mechanism of mic- 

 turition. (Kiiss.) 



5; but the walls cannot approach nearer the base 

 without the aid of the abdominal muscles, which, by 

 a voluntary effort, bring the summit to the position 

 indicated by the line 6. 



