DIGESTION 309 



eventually disposed of (Cannon). This so-called antiperistalsis 

 is not precisely the same kind of movement, except for its direc- 

 tion, 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 associated 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 defalcation, remains at rest. 



Influence of the Central Nervous System on the Gastro- 

 intestinal Movements. As we have already said, these move- 

 ments 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. Nevertheless, 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 

 oesophagus 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 oesophagus 

 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 splanchnic 

 nerves contain fibres by which the intestinal movements can be 

 inhibited, and they appear to be always in action, for after 

 section of these nerves the movements are strengthened. On the 

 other hand, stimulation of the peripheral end of the cut splanchnic 

 causes arrest of the movements. Occasionally, however, it has 

 the opposite effect. Contractions of the small intestine are more 

 easily caused by excitation of the vagus after the inhibitory 

 splanchnic nerves have been cut. The splanchnics also contain 



