OUTFLOW OF URINE. 409 



tents. Thus where no active muscular forces can possibly come 

 into play, as in the case of the dead subject, or in complete paral- 

 ysis following destruction of the spinal cord, a considerable 

 amount of urine is retained. But when a certain pressure is 

 attained by the gradual accumulation of urine within the blad- 

 der, the elasticity of the sphincter and the other tissues around 

 the outlet is overcome by the elasticity of the bladder wall, and 

 the urine slowly dribbles away. 



In the normal condition there are two sets of muscular mech- 

 anisms which aid the elastic forces just named. 



They may be regarded as antagonistic the one, the sphincter 

 muscle, by contracting, strengthens the elastic power of the tissues 

 around the urethra which retains the urine; the other formed by 

 the muscle-coat of the bladder, called the detrusor urines, 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. 



