382 HANDBOOK OF PHYSIOLOGY. 



and of the front wall of the bladder, as expose to view its hinder wall to- 

 gether 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 fasting, two or three drops enter the 

 bladder every minute, each drop as it enters first raising up the little 

 papilla on which, in these cases, the ureter opens, and then passing 

 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 be raised, runs from them in a stream 

 till they are empty. Its flow is increased in deep inspiration, or strain- 

 ing, and in active exercise, and in fifteen or twenty minutes after a meal. 

 The urine collecting 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, 

 as before explained, 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 ab- 

 dominal muscles; and the act is completed by the accelerator urince 

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

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

 tion, 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 harmonized 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- 

 distention of the bladder, or even by a few drops of urine passing into 

 the urethra. 



