732 PHYSIOLOGY 



forcible contraction of the levator ani .and the other perineal muscles, and 

 this contraction also serves to restore the everted mucous membrane. 



The carrying out of this reflex act is dependent on the integrity of a certain 

 part of the lumbar spinal cord. If this ' centre ' be destroyed, the tonic con- 

 traction of the sphincter muscles disappears. This centre may be either 

 excited to increased action, or be inhibited by peripheral stimulation of 

 various nerves, or by emotion, such as fear. Application of warmth to the 

 region of the anus causes reflex relaxation of the sphincter ; application of 

 cold increases its tonic contraction. 



In man, as Hertz has shown by the skiagraphic method, the pelvic 

 colon becomes rilled with faeces from below upwards, the rectum remaining 

 empty till just before defecation. In individuals whose bowels are opened 

 regularly every morning after breakfast the entry of feces into the rectum 

 gives rise to a sensation of fulness and acts as the call to defecation. If 

 no response be made, the desire to defecate passes away, since the rectum 

 relaxes and the faecal mass no longer exercises pressure on its wall. Hertz 

 has shown that the minimal pressure required to produce the call to defecate 

 varies from 30 to 40 mm. Hg, according to the length of the gut which is the 

 seat of distension. 



