It is evident from what has been stated that the gastric digestion of proteid foods is a comparatively slow process, involving several hours of time; and further, that food material in general remains in the stomach for varying periods, dependent upon its chemical composition. It would appear further, that relaxation of the pyloric sphincter, allowing passage of chyme into the intestine, must depend somewhat upon chemical stimulation, as this offers the most plausible explanation of the diversity of action seen with the different foodstuffs. As has been pointed out, gastric digestion is primarily a process for the conversion of proteid food into soluble products. It would be a mistake, however, to assume that the digestion of proteid foods is complete in the stomach. Stomach digestion is to be considered more as a preliminary step, paving the way for further changes to be carried forward by the combined action of intestinal and pancreatic juice in the small intestine. The importance of gastric digestion is frequently overrated. It is unquestionably an important process, but not absolutely essential for the maintenance of life. Dogs have lived and flourished with their stomachs removed, the intestine being joined to the oesophagus. The intestine is a much more important part of the alimentary tract; it is likewise far more sensitive to changing conditions than the stomach, and undoubtedly one function of the latter organ is to protect the intestine and preserve it from insult.

The stomach may be compared to a vestibule or reservoir, capable of receiving without detriment moderately large amounts of food, together with fluid, in different forms and combinations, with the power to hold them there, until by action of the gastric juice they are so transformed that their onward passage into the intestine can be permitted with perfect safety. Then, small portions of the properly prepared material may be discharged from time to time through the pylorus without danger of overloading the intestine, and in a form capable of undergoing rapid and complete digestion. Further, the stomach as a reservoir is very useful in bringing everything to a proper and constant temperature before allowing its entry into the intestine. Another fact of some importance is that, contrary to the general view, absorption from the stomach of the products of digestion is not very rapid under ordinary conditions. Even water and soluble salts pass very slowly into the circulation from the stomach. Like the partially digested food material, they are carried forward through the pyloric sphincter into the intestine, where absorption of all classes of material is most marked. It is in the small intestine that both digestion and absorption are seen at their best. It is here that all three classes of foodstuffs are acted upon simultaneously through the agency of the pancreatic juice, intestinal juice, and bile. Here, too, are witnessed some of the most complicated and interesting reactions and changes occurring in the whole range of digestive functions. Especially noteworthy is the peculiar mechanism by which the secretion of pancreatic juice is set up and maintained. On demand, pancreatic juice is manufactured in the pancreas and poured into the intestine just beyond the pylorus through a small duct - the duct of Wirsung. Secretion is started by contact of the acid contents of the stomach with the mucous membrane of the small intestine, so that as soon as the acid chyme passes through the pyloric sphincter there commences an outflow of pancreatic juice into the intestine. While acid is plainly the inciting agent in this secretory process, its action is indirect. It does not cause secretion through reflex action on nerve fibres, but it acts upon a substance formed in the mucous membrane of the intestine, transforming it into secretin, which is absorbed by the blood and carried to the pancreas, where it excites secretory activity. As would be expected from the foregoing statements, the secretion of pancreatic juice commences very soon after food finds its way into the stomach, and naturally increases in amount with the onward passage of acid chyme into the intestine, the maximum flow being obtained in the neighborhood of the third or fourth hour, after which the secretion gradually decreases. In man, it is estimated on the basis of one or two observations that the amount secreted during 24 hours is about 700 cc, or a pint and a half. Careful experiments, however, tend to show that the quantity of secretion depends in some measure at least upon the character of the food, and also that the composition of the secretion varies with the character of the food. Thus, on a diet composed mainly of meat, the proteid-digesting enzyme is especially conspicuous, while on a bread diet, with its large content of starch, the starch-digesting enzyme is increased in amount. In other words, there is suggested the possibility of an adaptation in the composition of the secretion to the character of the food to be digested.

Pancreatic juice is an alkaline fluid, rather strongly alkaline in fact, from its content of sodium carbonate, and is especially characterized by the presence of at least three distinct enzymes; viz., trypsin, a proteid-digesting ferment; lipase, a fat-splitting enzyme; and amylopsin, a starch-digesting enzyme. It has already been pointed out how dependent the secretion of pancreatic juice is upon the cooperation of the intestinal mucous membrane. A similar dependence is found when the digestive activity of the secretion is studied. As just stated, pancreatic juice contains a proteid-digesting enzyme. This statement, however, is not strictly correct, for if the secretion is collected through a cannula so that it does not come in contact with the mucous membrane of the intestine, it is found free from any digestive action on proteids. The secretion is activated, however, by contact with the duodenal membrane. Expressed in different language, pancreatic juice as it is secreted by the gland does not contain ready-formed trypsin; it does contain, however, an inactive pro-enzyme, which, under the influence of a specific substance contained in the intestinal mucous membrane, known as enterokinase, is transformed into the active enzyme trypsin. There is thus seen another suggestive example of the close physiological relationship between the small intestine and the activity of the pancreatic gland, or its secretion.