MOVEMENTS OF THE INTESTINES 825 



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 filled with faeces from below upwards, the 

 rectum remaining empty till just before defaecation. In individuals 

 whose bowels are opened regularly every morning after breakfast 

 the entry of faeces into the rectum gives rise to a sensation of fulness 

 and acts as the call to defaecation. If no response be made, the 

 desire to defaecate 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 

 defaecate varies from 30 to 40 mm. Hg, according to the length of the 

 gut which is the seat of distension. 



