THE MECHANICAL PHENOMENA OF DIGESTION 331 



of the ileum through the wall of the colon (so-called ileo-caecal 

 valve), but essentially by the tonic contraction of the ileo-colic 

 sphincter. The sphincter usually permits the passage of material 

 only in the direction from small to large intestine. But as an 

 occasional phenomenon, a reverse movement may occur. Thus 

 food may actually pass back through the ileo-colic sphincter into 

 the small intestine under the action of a long-continued and vigorous 

 antiperistalsis, and in this way a considerable portion of a bulky 

 enema may be eventually disposed of (Cannon). This so-called 

 antiperistalsis is not precisely the same kind of movement, leaving 

 out of account its direction, as the peristalsis already described in the 

 small intestine, since it is not preceded by a wave of inhibition. 

 True peristaltic contractions preceded by relaxation of the gut may 

 also be observed to start in the caecum, and to travel down the large 

 intestine. They are not very frequent in comparison with those 

 of the small intestine, and they die away before reaching the end 

 of the colon, allowing the food to be driven back again towards 

 the caecum by the antiperistalsis. A true downward peristalsis 

 is more commonly seen in the descending colon, and is here asso- 

 ciated with the propulsion and collection of the faeces, which are 

 mainly stored in the sigmoid flexure. These peristaltic contractions 

 do not normally reach the rectum, which, except during defaecation, 

 remains at rest. 



Influence of the Central Nervous System on the Gastro- Intestinal 

 Movements. As we have already said, these movements are much 

 less closely dependent on the central nervous system than are those 

 of the oesophagus. They can not only go on, but are in general 

 better marked when the extrinsic nervous connections are cut; they 

 cannot spread when the continuity of the tube is destroyed, and 

 the mere presence of food will excite them when other than local 

 reflex action has been excluded by section of the nerves. Never- 

 theless, the central nervous system does exercise some influence 

 in the way of regulation and control, if not in the way of direct 

 initiation of the movements, and the swallowing or even the smell 

 of food has been observed to strengthen the contractions of a loop 

 of intestine severed from the rest, but with its nerves still intact. 

 The vagus is the efferent channel of this reflex action : stimulation 

 of its peripheral end may cause movements of all parts of the 

 alimentary canal from cesophagus to large intestine, and may 

 strengthen movements already going on; but section of it does not 

 stop them, nor hinder the food from causing peristalsis wherever 

 it comes. The vagus also contains inhibitory fibres for the lower 

 end of the cesophagus and the whole of the stomach. Stimulation 

 of it is followed first by inhibition, and then, after an interval, by 

 an increase of tone and augmentation of the contraction of the 

 whole stomach, including the cardiac and pyloric sphincters. The 



