NERVOUS MECHANISM OF MICTURITION. 413 



urethra, matters are greatly altered. Now, even voluntary effort 

 does not suffice to keep back the stream, and an imperative call 

 is made upon the local mechanisms to empty the bladder. This 

 is accomplished by the contraction of the muscular coat of the 

 bladder, which is excited reflexly by the stimulus starting from 

 the mucous membrane lining the urethra. The evacuation of 

 the bladder is then accomplished quite independently of the will 

 by a reflex act, which may even be unconscious. 



When the urine once commences to flow, it continues until the 

 bladder is quite empty, the last drops of urine being expelled 

 from the urethra by rhythmical spasms of the muscles around 

 the bulbous portion of that canal. The sequence of events will 

 then be stimulation of the mucous membrane of the urethra 

 by escape of urine ; contraction of the detrusor urinae ; relaxation 

 of the sphincter ; rhythmical contraction of the ejaculator urinse ; 

 and, finally, a voluntary twitch of the levator ani and neighboring 

 muscles. 



This sequence of events may go on in sleep, as a result of 

 slight local excitations, frequently in children, when probably 

 the retention mechanisms are not yet well educated. 



At an early age we learn, under ordinary favorable circum- 

 stances, to micturate voluntarily, and the bladder is never allowed 

 to become so over-distended that the reflex contraction of the 

 sphincter is insufficient to retain the urine. Almost at any time 

 we can call forth the reflex act just described by increasing the 

 pressure on the bladder by voluntary contraction of the abdominal 

 muscles ; the diaphragm being depressed and fixed, the muscles 

 of expiration are put into action, and the contraction of the 

 sphincter muscle being at the same time probably checked by 

 the will, the power of retention is overcome. 



The moment the balance of power is thus turned in favor of 

 the expelling agencies and a drop of urine reaches the urethra, 

 the excitation thus produced brings about the complete evacua- 

 tion of the bladder without further voluntary effort. 



The nervous mechanism that controls the act of micturition 

 consists essentially of ganglionic centres which are situated in 

 the lumbar enlargement of the spinal cord, and of two sets of 



