We know of the existence of many factors which influence salivary and gastric digestion, but much less about intestinal digestion. Among these are the nature of the food, cookery, appetite, mastication, palatability, mental state, amount of fluid in the meal, the pyloric reflexes, etc. The nature of the individual plays a powerful role; some individuals can subsist almost wholly upon one or a few articles, while others quickly revolt against monotony of diet. Idiosyncrasy is another important factor; for example, some individuals cannot eat mutton, however disguised, while in others sugar is so strong a stimulant to peristalsis that it sets up a violent diarrhoea. The above being true of presumably healthy people, the factors which may influence digestion in the sick, invalid and convalescent must be still more uncertain. Hence tables which purport to give the relative digestibility of foods must be studied with allowances. We can study this problem in test tubes, or by examining the washings from the stomach after definite meals or in fistula patients, but the results can hardly be applied forthwith to all persons.

The direct stimuli to the secretion of saliva are the sight, odor or taste of food; or the mechanical irritation produced by contact of food-stuffs with the mouth. Indirectly, the thought of food may be a psychic stimulus to the flow of saliva. Excitement may so check the flow of saliva that it is impossible to swallow dry food.

The stimuli to the secretion of gastric juice are both psychic and chemical. Pawlow has demonstrated, in the case of dogs, that the sight of food or the chewing of food in the mouth may produce a very effective flow of gastric juice, to which he attributes great importance in digestion. How important this preliminary flow before food enters the stomach may be in the case of man has not been very clearly demonstrated, but it seems reasonable that the desire for eating, the act of eating and the pleasure obtained therefrom exercise a favorable influence upon gastric secretion. It has recently been shown that the products of salivary digestion act as a stimulus to the flow of gastric juice, and that once the process of gastric digestion is initiated, the products of peptic activity cause the continuance of the secretion as long as the food remains in the stomach. The mere mechanical irritation caused by the presence of food in the stomach is ineffective. Thus white of egg will cause no flow. But certain substances, such as meat extracts, bitters and condiments, promote secretion. The use of meat broths, gelatin jellies, peptones (i. e., products of gastric digestion), and toast (containing dextrins or products of salivary digestion), at the beginning of a meal, or to promote feeble gastric digestion in an invalid, is therefore a rational practice.