144 DIGESTION 



Movements of the Large Intestine 



Food begins to enter the large intestine within three 

 hours of ingestion. As a peristaltic wave approaches the 

 ileocaecal valve the colon in the neighbourhood of the 

 valve first contracts, then relaxes as the wave disgorges 

 the food into it. The ileocsecal valve is a true sphincter, 

 having a nervous mechanism of its own. It appears both 

 from X-ray observations and from the direct observation 

 of the intestine exposed in warm sahne solution that the 

 principal movement in animals consists of antiperistaltic 

 waves. These begin at about the middle of the transverse 

 colon, and at the rate of about five per minute (in the 

 cat), sweep towards the caecum. Prevented by the closing 

 of the ileocsecal valve from regurgitating into the ileum, 

 the contents escape distally through the peristaltic ring. 

 By this means is ensured the maximum exposure to the 

 absorbing surface. From the fact that enemata introduced 

 at the rectum appear in csecal fistulse, the same process is 

 beheved to occur in man, though it has not actually been 

 observed. The contents fill up the ascending colon, and 

 as they proceed gradually attain the faecal consistency. 

 In the transverse colon the advancing column is split up 

 by waves of contraction, which travel slowly towards the 

 pelvis. 



Normally the contents take about two hours to traverse 

 the ascending colon, and another two hours to reach the 

 splenic flexure. The part of the large intestine which hes 

 between the middle of the transverse colon and the rectum 

 is in a state of constant tonic contraction, interrupted only 

 by slow peristaltic waves. These have the effect of filling 

 this part of the intestine from below upwards. As they 

 pass along, the faeces become gradually harder by absorption 

 of water. 



Defaecation 



Defsecation consists of a train of events partly involuntary 

 and partly voluntary. The faeces accumulate from the 



