DEFECATION 319 



of the rectum by the faeces. When the pressure of the faeces in 

 the rectum reaches a value of about 30-40 mm. Hg, there is a 

 call to defaecate. If no response be made, the call is not repeated 

 immediately, as the rectal walls relax and so lose their irritability 

 to pressure. While the initiation is voluntary the act itself is 

 purely reflex like the other movements of the intestine. The 

 reflex contractions and relaxations are generally aided by voluntary 

 contraction of all the muscles, which will increase abdominal 

 pressure. 



From a physico-chemical standpoint practically nothing can 

 be said of the mechanism of alimentary transport. While the 

 movements, etc., are apparent the underlying causes are com- 

 pletely hidden. No help so far is given by attempting to trace 

 the development of the highly complex system of the vertebrate 

 from the apparently simple physico-chemical response of the 

 amoeba to contact with food. 



