The movements of the intestines are of two kinds. At all times they exhibit swaying movements, in the production of which the longitudinal fibres play the chief part, although the circular fibres also contract. The object of this undulation is to thoroughly mix the contents of the gut with its secretions. If pills of subnitrate of bismuth are administered, and their progress observed by the aid of Röntgen rays, they are seen to oscillate backwards and forwards on their way down the canal. The slower vermicular movement which squeezes the contents forwards is called “peristalsis.” It resembles the progressive contraction of an elastic tube which may be effected by drawing it through a ring, but is rather more complicated. At the point at which it is occurring the circular coat is sharply contracted. Above this it is also somewhat contracted; below it is relaxed. The longitudinal fibres, using the constricted portion as a _point d’appui_, pull up the segment of the intestine which lies immediately below it, drawing it off the contents of the tube as a glove from a finger. When food is swallowed, it falls down the œsophagus, aided by slight peristalsis. As soon as sufficient has accumulated on the upper surface of the cardiac valve of the stomach, the valve relaxes; at the same time a stronger peristalsis of the lower portion of the œsophagus squeezes its contents into the stomach. Food remains in the stomach until it has reached a certain stage of digestion, the chief object of which is its subdivision into small particles. Until this stage is reached, the pyloric valve is firmly closed. The contractions of the wall of the stomach drive its contents round and round—down the greater and up the lesser curvature—mixing them thoroughly with the gastric juice (_cf._ p. 124). As the acidity of the mixture increases, the peristaltic contractions of the stomach become more vigorous, until, the pyloric valve relaxing, the food is little by little driven into the duodenum.