Since the muscular movements of the digestive tract, particularly of the stomach when empty, play an important part in bringing about the sensations which lead to the taking of food, it may be well to note at this point the results obtained by Cannon and Washburn in their recent investigation of hunger. Lest hunger be confused with appetite, it is essential to clearness that these terms be defined. Some consider that the two experiences differ only quantitatively, appetite being regarded as a mild state of hunger; but Cannon and Washburn hold that hunger and appetite are fundamentally different. In their view:

"Appetite is related to previous sensations of the taste and smell of food; it has therefore, as Pawlow has shown, important psychic elements. It may exist separate from hunger, as, for example, when we eat delectable dainties merely to please the palate. Sensory associations, delightful or disgusting, determine the appetite for any edible substance, and either memory or present stimulation can thus arouse desire or dislike for food."

"Hunger, on the other hand, is a dull ache or gnawing sensation referred to the lower midchest region or epigastrium. It is the organism's first strong demand for nutriment, and, not satisfied, is likely to grow into a highly uncomfortable pang, less definitely localized as it becomes more intense. It may exist separate from appetite, as, for example, when hunger forces the taking of food not only distasteful but even nauseating.

Hunger is not due merely to emptiness of the stomach. It is true that under ordinary conditions hunger is apt to appear soon after the last food has passed from the stomach to the intestine, but if the stomach be artificially emptied, the sensation of hunger may not be felt until some hours afterward. Nor is hunger due to hydrochloric acid secreted into an empty stomach, for if the empty stomach of a hungry person be washed out, but little if any acid is found.

The explanation of hunger, advanced by Cannon and Washburn, is that it is due to the muscular contractions of the walls of the empty stomach.

In order to learn whether direct proof of this might be secured experimentally in man, one of the investigators accustomed himself to swallowing a small soft rubber balloon attached to the end of a rubber tube by means of which it could be withdrawn when desired. The tube and bulb were habitually carried thus in the esophagus and stomach for two or three hours at a time until the experience ceased to have any disturbing effect. Experiments were then made in which the balloon, thus held in the stomach, was partially inflated with air and connected with a manometer and recording apparatus by means of which any pressure exerted upon the balloon was recorded automatically. In the actual experiments, the subject sat at rest with his hand on a key which he pressed whenever he experienced the sensation of hunger. This key was connected with a recording device which, like the apparatus recording the muscular contractions of the stomach upon the rubber balloon, was out of sight of the subject.

Before hunger was experienced the recording apparatus revealed no evidence of muscular activity in the stomach. The records of hunger "pangs" and of muscular contractions of the stomach were always approximately simultaneous, that is, when the subject of the experiment felt hungry, powerful contractions of the stomach were always being registered. The contractions were about 30 seconds in duration, with pauses of 30 to 90 seconds between. It was found in almost every case that the contraction reached its greatest intensity just before the record of the hunger sensation began, and that the feeling of hunger disappeared when the contraction ceased although no food or drink had been taken. Cannon considers the evidence conclusive that hunger is caused by the contractions, and not vice versa, as Boldireff had thought. Other observations in the course of Cannon's experiments showed that the lower end of the esophagus also contracts periodically in hunger, an explanation of the fact that sensations of hunger may be felt in cases where the stomach has been removed. Furthermore Cannon considers that vague sensations of hunger may also originate from muscular contractions in the intestine.

What causes the stomach contractions which give the sensation of hunger has not been determined. They do not seem to be directly related to bodily need. That they usually begin at or soon after the accustomed meal hour may be taken not only as evidence that habit plays an important role, but also as an indication of the desirability of eating at regular times; for in view of the importance of the muscular tone of the stomach walls, these observations seem to justify the view that the strong muscular contraction of the empty stomach may be regarded as an indication that the condition which causes the first sensation of hunger is that in which the stomach is in the best state of readiness to receive the food. There is also direct experimental evidence that the stomach digests more expeditiously the food which is "eaten with hunger" (Hudek and Stigler, cited by Carlson). The description of the digestive process which follows presupposes that the food is eaten under favorable conditions and received by a digestive tract which has been permitted to form good and regular habits.

The eating of food induces a flow of saliva from great numbers of minute glands in the lining membrane of the mouth and from the three pairs of large salivary glands. That saliva is secreted in response to psychic as well as chemical stimulation is shown by the fact that actual contact with the food is not necessary, since secretion may be started by the sight or odor or even the thought of food. Mixed human saliva has usually a faintly alkaline reaction and always contains ptyalin (salivary amylase), although its amylolytic power appears to vary considerably with individuals and with the same individual at different times of the day. As the food comes in contact with saliva, the digestion of starch and dextrin under the influence of the ptyalin begins at once; but as mastication is an entirely voluntary act, the thoroughness with which the food becomes mixed with saliva is subject to wide variations.