54 6 THE URINARY APPARATUS. [CH.XXXVI. 



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 

 \irine 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 contraction 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: (i) from the 

 lower dorsal and upper lumbar nerves ; these fibres pass to the 



