~Motor Processes in the Stomach.~--There are no peristaltic waves in the fundus of the stomach, and the movement of the food mass is accomplished through the stretching and contraction of the muscular walls of the organ which tends to churn and further mix it with the salivary juices as it is gently pushed out into the middle region. In this region the peristaltic waves begin and travel toward the pylorus and increase in force as digestion progresses, ceasing only with the emptying of the organ. When the first stratum of food reaches the middle of the stomach it is caught by these oscillating peristaltic waves and forced forward through the pyloric region and against the pylorus, from whence it is returned back through rings of constriction. This forward and backward movement continues as long as there is food in the stomach, thus thoroughly mixing the mass with the gastric juice and allowing the enzymes existing in the juices to have an opportunity for action (chemical digestion). ~Passage of Food from Stomach.~--The material prepared in the stomach, known as ~chyme~, is passed into the duodenum through the pylorus. The opening of this sphincter is controlled, according to Cannon, to a certain extent by the liquefication of the chyme, but more especially by the presence of free acid in the stomach side of the pyloric orifice. ~Behavior of Food in the Intestines.~--The food does not pass at once along the canal, but waits in the duodenum until several portions have passed through. As the food mass is made alkaline in the presence of the intestinal juices, the pyloric valve closes, opening again as the contents nearest it on the stomach side are acidified. ~Intestinal Movements.~--The peristaltic waves in the small intestines begin in the upper part and start a course ever downward. These waves in the intestines are two-fold in character; the quick shallow wave which forces the food string forward, breaking it up into segments, and backward joining the segments together again, and the strong deep wave which carries the entire mass forward after each segmentation. This method of movement in the small intestines is the best one possible under the conditions which prevail in this region of the digestional apparatus, since it not only mixes the food material with the juices necessary for its digestion, but likewise spreads it out over a wide space, insuring a greater contact with the absorbing walls of the small intestines. ~The Effect of Muscular Constrictions.~--The muscular constrictions occurring in the intestines producing segmentation of the food string have, according to Sherman, the effect of "(1) further mixing of the food and digestive juices, (2) bringing the digested food into contact with the absorbing membrane, (3) emptying the venous and lymphatic radicles in the membrane, the material which they have absorbed being forced into the veins and lymph vessels by the compression of the intestinal walls."[53]