THE MECHANICAL PHENOMENA OF DIGESTION 329 



to side. Under abnormal conditions, as in the exposed ' surviving ' 

 intestines of the rabbit, contractions, probably similar to the 

 pendulum movements, but running indifferently in both direc- 

 tions, can be set up by local stimulation. The function of these 

 pendulum movements seems to be the thorough mixing of the food 

 with the digestive juices in the intestine. 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 rhyth- 

 mically 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 divid- 

 ing again, without sensibly shifting its position. In addition to the 

 relatively 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 pre- 

 cedes 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-csecal 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 disturb- 

 ance of nutrition. The most probable explanation is that the peri- 

 stalsis 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 stimula- 

 tion, 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 



