DIGESTION 289 



relaxed and motionless, so that we may say that a wave of 

 inhibition precedes the wave of contraction. The peristaltic 

 movements of the small intestine, the most typical of their 

 kind, are most easily excited by mechanical stimulation of 

 the mucous membrane, as by the contact of a morsel of food 

 or an artificial bolus of cotton-wool. Travelling, under 

 normal conditions, always downwards, the constriction 

 squeezes the contents of the tube before it, and the wave 

 usually ends at the ileo-caecal valve, which separates the 

 small intestine from the large. The cause of the definite 

 direction of the peristaltic wave is grounded in the anatomical 

 relations of the intestinal wall. For when a portion of the 

 intestine is resected, turned round in its place and sutured, 

 so that what was before its upper is now its lower end, the 

 contraction wave appears to be unable to pass, and the 

 obstruction to the onward flow of the intestinal contents 

 causes marked dilatation of the gut, and sometimes serious 

 disturbance of nutrition. The most probable explanation is 

 that the peristalsis is governed by a local reflex nervous 

 mechanism (Auerbach's plexus), tthe stimulation of which by 

 the contact of the food with the mucous membrane or by 

 the distension of the gut causes excitation of the circular 

 muscular fibres above the point of stimulation and inhibition 

 of them below it./ On the other hand, the pendulum move- 

 ments appear to be myogenic in origin and independent of 

 the nervous system (Bayliss and Starling). This distinction 

 is borne out by the difference in the effect of poisons like 

 -cocaine and nicotine, which paralyze the local nervous 

 mechanism, on the two kinds of contractions. The pendulum 

 movements are, if anything, increased in intensity and made 

 more regular. But the peristaltic waves, although they can 

 be locally excited by direct stimulation of the muscular 

 fibres, are no longer propagated, and a bolus introduced into 

 the intestine remains at rest where it is placed. Under 

 certain conditions it appears that a reverse or anti-peristalsis 

 can be set up in the intact body, e.g., in the case of obstruc- 

 tion of the intestine leading to vomiting of its contents. But 

 such a reversal of the normal direction is not easy to realize 

 iby artificial stimulation, and even when an antiperistaltic 



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