412 MANUAL OF PHYSIOLOGY. 



the muscle coat of the bladder, called the detrusor urince, as we 

 shall see presently, is the chief agent in actively expelling the 

 urine. When these muscles are in good working order, much 

 more urine can be conveniently retained than the elasticity of 

 the tissues about the urethra would permit of. If the spinal cord 

 be destroyed, the bladder can only retain about one-third the 

 quantity of urine it conveniently holds when the cord is intact. 

 We must, then, suppose that the sphincter muscle acts more pow- 

 erfully when the elastic forces are equalized. The accumulation 

 of urine after a certain time gives the sensation known as a full 

 bladder, but this feeling is not necessarily accompanied by any 

 immediate call to make water, though it soon produces a desire 

 in that direction. We suppose, then, that the stimulus given to 

 the sensory nerves by filling the bladder causes reflexly a con- 

 striction of the sphincter muscle, so that, in proportion as the 

 pressure within the bladder increases, the resistance to its outflow 

 is also augmented. This does not imply any automatic action 

 of the sphincter vesicse, but merely a constant reflex excitation 

 of that muscle, which secures its contraction and the retention 

 of a considerable amount of urine without the intervention of 

 voluntary influences or attention. 



Micturition, or the expulsion of the urine, does not normally 

 ever depend on elastic forces alone, as in the case mentioned of 

 paralytic incontinence, when the urine commences to dribble 

 away as soon as a certain pressure is attained within the bladder. 



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 



