DIGESTION 225 



The muscular movements of the large intestine are slower than 

 those of the small bowels ; possibly one reason for this may be 

 that the food has to remain a longer time in contact with the 

 absorbing surface — viz., for at least forty-eight hours, and for as 

 long as four days. The peristaltic movement of the small 

 intestines is quite distinct from that of the large ; the one ends 

 at the ileum, the other begins at the caecum. 



The muscle of the intestinal wall causes the movement known 

 as peristalsis, which normally passes in the direction stomach to 

 rectum. Relatively quick in the small intestines, it becomes 

 slower and more deliberate in the large, but the wave has always 

 the one object in view — viz., to press the ingesta onward. A 

 wave of contraction passing the reverse way — viz., in the direction 

 of rectum to stomach — is known as antiperistaltic : such a 

 movement is considered abnormal, but in the horse, according to 

 the observations of Colin, antiperistalsis of the small intestines 

 is a natural condition. Some physiologists recognise anti- 

 peristaltic movements of the large intestines as being normal 

 in certain animals, producing a to-and-fro movement of the 

 contents, but it is generally thought that in the small bowels 

 antiperistalsis is only present under abnormal circumstances. 

 If antiperistalsis be admitted for the large bowels, we see no 

 difficulty in extending it to the small, especially in view of Colin's 

 positive statement that it occurs. The peristaltic wave depends 

 upon a something peculiar to the bowel wall, for if a^piece of 

 small intestine be experimentally reversed, so that the portion 

 originally nearest the stomach is made to occupy a position 

 farthest away from it, it is found that the peristaltic wave in the 

 reversed segment is still in the original direction instead of in the 

 new direction. The actual mechanism involved in a peristaltic 

 contraction, according to Starling and Bayliss, is as follows : 

 The circular muscle on the stomach side of the bolus contracts, 

 while that on the far side is relaxed for some distance, so that the 

 advancing wave drives the bolus into a relaxed portion of bowel. 

 If a solution of cocaine or nicotine be applied to the intestinal wall 

 these movements cease, from which it is argued that they are 

 probably due to local ganglia. 



Rhythmical or Pendular movements of the small intestines 

 were first described by Starling and Bayliss. They consist of a 

 series of local contractions caused by the presence of food in the 

 canal, and occur in the dog at the rate of about twelve a minute, 

 and in the cat thirty times a minute. They have been studied 

 by means of the Rontgen rays and a bismuth diet. Pendular 

 movements are essentially connected with the division and sub- 

 division of food in the intestinal canal; by means of the rays a 

 string of material may be seen to become suddenly segmented, 



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