728 PHYSIOLOGY 



wards over the reversed length of gut. The intestine above this point there- 

 fore becomes dilated. If however the reactions of the local nervous system 

 be paralysed or inhibited, a reflux of intestinal contents is quite possible, since 

 the contractions excited at any spot by local stimulation of the muscle have 

 the effect of driving the food either upwards or downwards ; the direction of 

 movement of the food will be that of least resistance. 



The movements of the small intestine are also subject to the central 

 nervous system. Stimulation of the vagus has the effect of producing an 

 initial inhibition of the whole small intestine, followed by increased irrita- 

 bility and increased contractions (Fig. 346). On the other hand, stimulation 

 of the splanchnic nerves causes complete relaxation of both coats of the small 

 gut (Fig. 347). It seems that the splanchnics normally exercise a tonic 



FIG. 345. Passage of bolus. Contractions of longitudinal coat (enterograph). The 

 bolus (of soap and cotton- wool) was inserted into the intestine four inches above the 

 recorded spot at A. The figures below the tracing indicate the distance of the middle 

 of the bolus from the recording levers. As the bolus arrived two inches above the 

 levers there is cessation of the rhythmic contractions and inhibition of the tone of 

 the muscle. This is followed, as the bolus is forced past, by a strong contraction in 

 the rear of the bolus. 



inhibitory influence on the intestinal movements, which can be increased by 

 all manner of peripheral stimuli. On this account it is often impossible to 

 obtain any movements in the exposed intestine so long as these remain in 

 connection with the central nervous system through the splanchnic nerves. 

 The relaxed condition of the gut which obtains in many abdominal affections 

 is probably also reflex in origin, and is due to reflex inhibition through the 

 splanchnic nerves. 



As a result of the two sets of movements described above, the food is 

 thoroughly mixed with the digestive juices, and the greater part of the 

 products of digestion are brought into contact with the intestinal wall and 

 absorbed. What is left a proportion varying in different animals according 

 to the nature of the food is passed on by occasional peristaltic contractions 

 through the lower end of the ileum into the colon, or large intestine. The 

 lowest two centimetres of the ileum present a distinct thickening of the cir- 

 cular muscular coat, forming the ileocolic sphincter. This sphincter relaxes 

 in front of a peristaltic wave and so allows the passage of food into the colon. 

 On the other hand, it contracts as a rule against any regurgitation which 

 might be caused by contractions in the colon. Although thus falling into 



