THE PHYSIOLOGY OF MICTURITION 1213 



of the pelvic organs in children may give rise to reflex involuntary 

 micturition. 



In the adult the processes of retention and evacuation of urine are 

 modified and controlled by voluntary effort. The normal action of the 

 sphincter mechanism may be aided by the contraction of the perineal 

 muscles which keep the urethra closed. The reflex process of evacuation 

 may be set in motion by voluntary contraction of the abdominal muscles, 

 by which the pressure in the bladder is increased and the normal sphincter 

 action overcome. It is probable too that the individual has a certain degree 

 of voluntary power over the unstriated muscles of the bladder, and that the 

 contraction of the muscular wall may be directly augmented by impulses 

 proceeding from the cortex to the upper part of the lumbar cord. This view 

 is favoured by the fact that stimulation of the eras cerebri has been observed 

 to cause contraction of the detrusor urinae. In this experiment the abdo- 

 men was opened, so there could be no question of the contraction of the 

 abdominal muscles. 



