DIGESTION 333 



relaxed, and its site becomes the centre of another com- 

 partment. This process goes on repeating itself, and thus 

 the contents of the gut are thoroughly mixed and churned. 

 This may be seen by feeding with food mixed with bismuth 

 and employing X-rays. These movements occur when all 

 the nerves have been divided. How far they are dependent 

 upon the action of the myenteric plexus is not definitely 

 established. 



2. The peristaltic movements are much more complex 

 and powerful. They consist of a constriction of the muscles, 

 which seems to be excited by the distension caused by the 

 contents, and they may be caused by inserting a bolus of 



J / 





Fir;. 156. — Skiagrams to show Segmentation of tlie Small Intestine. (Hertz.) 



cotton-wool covered with vaseUne. Starting at some point 

 of the intestine, the wave passes slowly downwards and 

 gradually dies away. In front of the contraction, the 

 muscular fibres are relaxed, and thus the contracting part 

 drives its contents into the relaxed part below. This move- 

 ment of the contents of the intestine, when they are 

 mixed with gases produced by fermentation, frequently 

 produces gurgling noises. 



The peristaltic movements go on after the nerves to the 

 gut are cut, but they are stopped when the ganglia in the 

 wall of the intestine are poisoned by nicotine. The 

 myenteric plexus undoubtedly forms a local reflex mechanism 

 which is stimulated by the presence of the residue of the 

 food in the intestine and which brings about the co- 



