234 HUMAN PHYSIOLOGY 



The action of the centre for the sphincter vesicae is tonic. 

 During micturition the tonus of the sphincter is decreased 

 and the centre for the detrusor is stimulated. This process 

 is called forth reflexly by the distension of the bladder, which 

 stimulates the nerves of the bladder and thereby reflexly 

 stimulates the detrusor and inhibits the sphincter. 



(b) Defalcation centre. 



The tonus of the centre of the sphincters ani is inhibited 

 reflexly (by stimulation of centripetal nerves in the rectum 

 by the accumulated faeces) ; peristaltic movements of the 

 intestine are set up which, together with the pressure of the 

 abdominal walls, remove the faeces. 



(c) Centre for the erection of the penis, ejaculation, par- 

 turition (see third section). 



