228 URINE 



end as soon as the intravesical pressure falls below that 

 which is required to keep the sphincter open. The bladder 

 is therefore never completely emptied — a fact of great chnical 

 importance. 



When the connections between the bladder and cord are 

 intact but the cord transected in the thoracic region, 

 stretching of the bladder wall not only causes rhythmic 

 contraction but gives rise to impulses which travel to 

 the cord when they reflexly produce impulses motor 

 to the body of the bladder and inhibitory to the 

 sphincter. By this means the bladder is emptied com- 

 pletely, and the urethra is emptied by reflex contraction 

 of the muscles surrounding it. Purely reflex micturition 

 of this sort can be brought about by stimulation of any 

 sensory nerve, particularly those arising in the pelvis. 



In the intact organism the mechanism is to a great 

 extent under the control of the will. The sudden rise in 

 intravesical pressure is recognised subjectively. The 

 evacuation of the bladder is aided by contraction of the 

 abdominal muscles. There is evidence, too, that the 

 sphincter trigoni and even the musculature of the bladder 

 wall are under voluntary control. 



