372 



EXCRETION BY THE SKIN AND KIDNEYS. 



is discharged into the pelves of the kidneys and the ureters by pressure due 

 to the act of separation of 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. M tiller 

 has found that the ureters can be made to undergo a powerful local contrac- 

 tion by the application of a Faradic current ; and Bernard has shown that 

 this may be produced by stimulation of the anterior roots of the eleventh 

 dorsal nerves. 



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

 culiar sensation is felt which leads to the act for its expulsion. The intervals 

 at which it is experienced are very 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, how- 

 ever, depends very much upon habit, upon the quantity of liquids ingested 

 and upon 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 abdominal mus- 

 cles with certain muscles which oper- 

 ate upon the urethra, and it is accom- 

 panied by relaxation of the sphincter 

 vesicae. 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 di- 

 aphraghm and the abdominal mus- 

 cles ; and this after a time excites the 

 action of the bladder itself. A cer- 

 tain time usually elapses then before 

 the urine begins to flow. When the 

 bladder contracts, aided by the mus- 

 cles of the abdomen and the dia- 

 phragm, the resistance of the sphinc- 

 ter is overcome, and a jet of urine 

 flows from the urethra. All voluntary 



ripfi nr , tYiav tVipn ppasp for a limp anrl 

 ^CtlOn may tnen Ctase I H6, ail 



nearer the base without the aid of the abdom- +V, P "hlarlrlpv will nparlv pmntv it<?plf 

 inal muscles, which, by a voluntary effort, bring tlie Dlaaaei wm nearly empty 1IS61I , 



the summit to the position indicated by the but the f orce of the jet may be con- 

 siderably increased by voluntary effort. 



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

 ing 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 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 con- 



FIG. 118. Diagram showing the mechanism of mic- 

 turition (Ktiss.). 



1, bladder distended with liquid ; by the contrac- 

 tion of its walls it assumes successively the po- 

 sitions 2, 3, 4, 5 ; but the walls can not approach 



