THE DIGESTIVE SYSTEM. 327 



membrane. This is the only known local reflex, with the possible 

 exception of the opening and closing of the pyloric sphincter. 



The Segmental Contractions. The segmental contractions differ 

 from the peristaltic waves in occurring regularly and rhythmically. 

 When they are recorded by means of a balloon in the intestinal lumen, 

 they are found to be most marked at the middle of the balloon where 

 the tension is greatest. The result of such contractions is to sub- 

 divide any bolus over which the contraction takes place, and thus 

 to bring about a thorough admixture of the intestinal contents. Both 

 layers of the muscular coat take part in these contractions, as well as 



FIG. 124. Tracing showing relaxation of intestinal wall on stimulation of splanchnic 

 nerve (balloon method). (Bayliss and Starling. ) 



in the peristaltic waves, and as a result the bowel exhibits pendular or 

 swaying movements. The segmental contractions have no influence in 

 promoting the onward movement of the intestinal contents. 



It has not been definitely determined whether the segmental move- 

 ments are myogenic or neurogenic in origin, though the latter seems 

 the more probable. They are not abolished by painting the intestine 

 with nicotine or cocaine, and this would point to their being of 

 myogenic origin ; on the other hand, they occur in isolated strips of 

 muscular coat to which the myenteric plexus is attached, and do not 

 occur if the plexus is absent, so that the presence of the plexus 

 would seem to be necessarj^ for them to take place 



The Nerves of the Small Intestine. The small intestine is supplied 

 by the vagus and the splanchnic nerves, and although, as has been 



