In studying the passage of food from the stomach into the intestine, Cannon found that the pylorus does not open at the approach of each wave of constriction which passes over this part of the stomach, but only at irregular intervals. When the observations made by means of the Roentgen rays were supplemented by chemical examinations of stomach and intestinal contents removed at different stages, it appeared that the presence of free acid in the pyloric part of the stomach causes the pylorus to open, and its presence in the small intestine causes the pylorus to close. Thus it would appear that under normal conditions it is only when the protein of the food has become more or less completely saturated with hydrochloric acid and some of the latter remains in the free state, that the food is allowed to pass into the intestine.

Ordinarily, when each is fed separately, protein food stays longer in the stomach than carbohydrate, fat longer than protein, and mixtures of fat and protein leave the stomach more slowly than either alone. This is probably because fat tends to retard both the motility of the stomach and the secretion of the acid gastric juice. In general the softer or more fluid the fat the more rapidly it will leave the stomach; also emulsified fats tend to pass on more promptly than fat of the same kind taken in larger masses.

The difference noted between protein and carbohydrate is doubtless due to the fact that combination of the acid of the gastric juice with the protein of the food delays the appearance of free acid at the pylorus; for when protein food was acidulated before feeding and carbohydrate food was made alkaline, the protein was found to leave the stomach more rapidly than the carbohydrate. That the passage of food from stomach to intestine is governed mainly by the degree of acidity reached in the pyloric part of the stomach is of interest in view of the importance to the organism of the action of the acidity of the gastric juice in effecting a partial disinfection of the food. It has been found that when through any cause the hydrochloric acid of the gastric juice is abnormally decreased, the numbers of bacteria in the stomach contents may increase greatly. It will be seen also that the acidity of the chyme as it passes the pylorus has an important influence upon the secretion of the pancreatic juice.

The most important characteristics of gastric juice are the presence of free hydrochloric acid and of pepsin. While other acids may be found in stomach contents, the acidity of gastric juice appears to be due entirely to hydrochloric acid. Normal human gastric juice has been found by different observers to contain about 0.2 to 0.4 per cent of free hydrochloric acid.*

*According to Carlson, "hunger juice" and "appetite juice" in man contain respectively 0.25 per cent and 0.40 per cent of free hydrochloric acid - averages of hundreds of observations upon a healthy man having a gastric fistula.

The stimuli which bring about secretion of gastric juice are both psychical and chemical.

Psychical stimulation results from the sensations of eating and may also be due to the sight and odor of food. The psychical secretion is studied chiefly by means of the "fictitious feeding" ("sham feeding") experiments in which food is given to dogs which have been prepared with esophageal openings through which the swallowed food escapes without entering the stomach. When such a dog is fed with meat, for example, there is a considerable secretion of gastric juice in spite of the fact that no food reaches the stomach. Such a flow of gastric juice is due to impulses received through the nervous system and specifically through the vagus nerve, for fictitious feeding has been found to cause a flow of gastric juice when the vagi are intact, but not after they have been cut. Secretion produced in this way reflexly as the result of the sensation of taste, odor, etc., is called by Pawlow a "psychic secretion" or "appetite juice." When the secretion is once started, even if no food enters the stomach, the flow of juice may continue for some time after the stimulus has ceased.

On the other hand, the normal secretion of gastric juice may be checked by unpleasant feelings such as fear, anger, or pain. This has been repeatedly observed with frightened or angry animals. Hornborg reports a similar observation upon a small boy. Food was shown but withheld, and the child became vexed and distressed, whereupon no gastric juice was secreted. After he was calmed, and given the food, it was some time before secretion began. Cannon infers, furthermore, that there is a "psychic tone" or "psychic contraction" of the gastro-intestinal muscles, analogous to the psychic secretion. In the same fashion that secretion may be checked, so also the movements of the stomach, bringing about the mixing of food with gastric juice and insuring its passage on into the duodenum, may be stopped during excitement or pain. This fact has been observed many times in experiments with various animals, as well as in the case of human beings.

If psychic secretion is normally excited, it insures the prompt beginning of gastric digestion. Stimulations arising within the stomach itself supplement the psychic influences and provide for the continued secretion of the gastric juice long after the mental effects of a meal have disappeared. This second stimulation is chemical and depends upon the production in the pyloric mucous membrane of a specific substance, or hormone, which acts as a chemical messenger to all parts of the stomach, being absorbed into the blood and thence exciting the activity of the various secreting cells of the gastric glands (Starling). Meat extracts, soups, etc., are particularly active in exciting the secretion which depends upon chemical stimulation; milk causes less secretion; white of egg is said to have no effect.

Under normal conditions, the amount of nutritive material absorbed from the stomach is insignificant as compared with the amount absorbed from the intestine. Nearly all the food eaten is passed from the stomach into the intestine in the form of chyme, having been more or less perfectly liquefied and acidulated by its thorough mixing with the gastric juice in the middle and pyloric regions of the stomach.

The stomach therefore has several functions. It serves (1) as a storage reservoir receiving food in relatively large quantities, say three times a day, and passing it on to the intestine in small portions at frequent intervals, (2) as a place for the continuation of the salivary digestion of starch, and (3) for the beginning of the digestion of proteins and perhaps fats, and finally (4) as a disinfecting station by virtue of the germicidal action of the hydrochloric acid of the gastric juice.