410 MANUAL OF PHYSIOLOGY. 



When the bladder is full, the elastic forces tending to expel its 

 contents increase, and, as we have seen, the resistance is propor- 

 tionately augmented. Under ordinary circumstances, then, there 

 is a combat going on between the expelling and retaining powers 

 (neither the muscle in the wall of the bladder nor voluntary 

 effort, however, coming into action), in which the retaining forces 

 are just able to overcome the expelling elastic pressure. If the 

 urine be retained for a considerable time, a moment arrives when 

 the reflex stimulation of the sphincter no longer suffices to keep 

 back the fluid, and the voluntary contraction of the neighboring 

 muscles has to be called to the aid of the sphincter. Under these 

 circumstances, if a drop of urine make its way into the sensitive 

 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 urinse; 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 

 sphincter is more readily fatigued. 



Under ordinary favorable circumstances, however, we mictu- 

 rate voluntarily, and the bladder is never allowed to become so 

 over-distended that the reflex contraction of the sphincter is in- 

 sufficient to retain the urine. Almost at any time we can call 

 forth the reflex act just described by increasing the pressure on 



