DIGESTION 307 



When an animal is fed with food containing bismuth subnitrate 

 and observed with the Rontgen rays, it is seen that the food 

 in a coil is often divided into small segments, which then join 

 together to form longer masses, these being in turn again divided. 

 This segmentation is rhythmically repeated (in the cat at the 

 rate of thirty times a minute). Although of itself it insures only 

 the mixing of the contents of the gut, and not their onward 

 progress, it is usually accompanied by peristalsis, so that while 

 the food is undergoing segmentation, it is also slowly passing 

 down the intestine. Often, however, a column of food remains 

 for a considerable time, dividing, uniting, and dividing again, 

 without sensibly shifting its position. In addition to the rela- 

 tively rapid pendulum movements, much slower periodic varia- 

 tions of tone of the whole musculature may be normally observed. 

 (2) True peristaltic movements, in which a ring of constriction, 

 obliterating the lumen, moves slowly down the tube, with a speed, 

 it may be, no greater than i mm. per second. The portion of 

 the intestine immediately below the advancing constriction is 

 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 is 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), the 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. The 

 automatic pendulum movements and also the slow, rhythmical 

 variations of tone, have a different relation to the local nervous 

 mechanism, for they behave differently to poisons like cocaine 

 and nicotine, which act on that mechanism. The pendulum 

 movements are, if anything, increased in intensity and made 

 more regular. But the peristaltic waves, although they can 



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