466 PHYSIOLOGY. 



Micturition. When the act of micturition takes place the spinal 

 detrusor center is excited into activity by the pressure of the urine; 

 the sphincter reflex center is also independently excited by the pres- 

 sure of the urine, and opens to expel the secretion. The spinal 

 detrusor and spinal sphincter are under the control of a cerebral 

 detrusor center which I have shown to be seated in the locus niger, 

 and which is set in activity by the cerebral hemisphere in voluntary 

 micturition. The detrusor center sends its impulses down the lateral 

 columns of the spinal cord to the vesico-sphial center. 



Voluntary micturition is materially aided by the action of the 

 abdominal and respiratory muscles. 



