CH. XXXVI. ] MICTURITION 549 



the walls of the abdomen, and of the front wall of the bladder, as 

 expose to view its hinder wall together with the orifices of the ureters. 

 The urine does not enter the bladder at any regular rate, nor is there 

 a synchronism in its movement through the two ureters. During fast- 

 ing, two or three drops enter the bladder every minute ; each drop as 

 it enters first raises up the little papilla through which the ureter 

 opens, and then passes slowly through its orifice, which at once again 

 closes like a sphincter. In 'the recumbent posture, the urine collects 

 .for a little time in the ureters, then flows gently, and, if the body is 

 raised, runs from them in a stream till they are empty. Its flow is 

 aided by the peristaltic contractions of the ureters, and is increased 

 in deep inspiration, or by straining, and in active exercise, and in 

 fifteen or twenty minutes after a meal. The urine is prevented from 

 regurgitation into the ureters by the mode in which these pass 

 through the walls of the bladder, namely, by their lying for between 

 half and three-quarters of an inch between the muscular and mucous 

 coats before they turn rather abruptly forwards, and open through 

 the latter into the interior of the bladder. 



Micturition. 



The contraction of the muscular walls of the bladder may by 

 itself expel the urine with little or no help from other muscles. In 

 so far, however, as it is a voluntary act, it is performed by means of 

 the abdominal and other expiratory muscles, which in their contrac- 

 tion press on the abdominal viscera, the diaphragm being fixed, and 

 cause the expulsion of the contents of those whose sphincter muscles 

 are at the same time relaxed. The muscular coat of the bladder 

 co-operates, in micturition, by reflex involuntary action, with the 

 abdominal muscles ; and the act is completed by the accelerator urince, 

 which, as its name implies, quickens the stream, and expels the last 

 drop of urine from the urethra. The act, so far as it is not directed 

 by volition, is under the control of a nervous centre in the lumbar 

 spinal cord, through which, as in the case of the similar centre for 

 defsecation, the various muscles concerned are harmonised in their 

 action. It is well known that the act may be reflexly induced, e.g. 

 in children who suffer from intestinal worms, or other such irritation. 

 Generally the afferent impulse which calls into action the desire to 

 micturate is excited by over-distension of the bladder, or even by a 

 few drops of urine passing into the urethra. The impulse passes up 

 to the lumbar centre, and produces, on the one hand, inhibition of the 

 sphincter, and on the other hand contraction of the necessary muscles 

 for the expulsion of the contents of the bladder. The tonic action of 

 the lumbar centre can also be inhibited by the will. 



The bladder receives nerves from two sources: (1) from the 



