Protein digestion begins in the stomach and is effected by the hydrochloric acid and pepsin of the gastric secretion. Like most other enzymes, pepsin has its precursor in the form of pepsinogen, which is inactive until it comes into contact with an acid, the most effective compound being pepsin-hydrochloric acid. Pavlov has demonstrated that this secretion occurs in two stages: (1) the psychic or "appetite" juice, which begins before food' is partaken of and which has always the same composition, whatever the food of the last meal may have been, and (2) the chemical juice, which is modified to suit the character of the food, bread exciting the most concentrated juice, meat the most acid juice, milk the least acid of all. Edkins has shown that the contact of certain food substances with the pyloric end of the stomach originates a certain chemical product, a gastric hormone, gastrin as he calls it, which is absorbed into the circulation and acts as a powerful stimulus to the further production of gastric secretion. The flow of chemical juice cannot be excited by all forms of nutritious substances, for white of egg and starch have no effect, and, as we have seen, fats actually diminish the flow. The most potent of all the excitants of gastrin are hardly to be classed as nutrients at all, viz., the extractives of meat; but by increasing the flow of chemical juice they contribute greatly to the digestion of the food and materially assist the appetite. Kobert has also shown that they remove the effects of muscular fatigue. When albumoses and peptones are likewise present, as in many forms of meat extract, they may be nutritious, although unfortunately, by containing "purins," they have for many people the defects of their qualities. Other substances of a similar character are dextrins, dextrose, maltose - chiefly the products of the salivary digestion of carbohydrates - and to a slight extent milk and water. Vegetable "bitters" have no influence per se on the flow of gastric juice: their mere contact with the mucous membrane of the stomach does not assist digestion, as is proved by taking them in a capsule; their sole effect is a reflex one, originating in the mouth.

Thus gastric juice begins to flow before food reaches the stomach, goes on increasing for an hour or so, and then begins to decline. Mendel has proved that the stimulation of the gastric glands may take place without direct gastric irritation, in consequence of the influence of alcohol absorbed from the intestine, a fact which goes far to explain some of the hyperacidity of the dyspeptic and the dipsomaniac. Two of Mendel's pupils have likewise demonstrated that other substances, e.g., oil of peppermint, induce a similar secretion, and have attributed this result to a reflex act, although it might as well have been occasioned by its absorption and direct stimulation of the gastric glands from the circulatory side. We are well aware that morphia and iron are thus excreted by the stomach and bowel wall, even when introduced parenterically, and there are now many observations to show that other substances, e.g., calcium and phosphorus, are excreted by the intestinal mucous membrane. This is important, as it is now recognised that "biliousness" may be due to excretion from the gastric mucous membrane, and that a diseased appendix may excite vomiting either in the same way or through reflex influence.

In the stomach proteins are converted by the pepsin-hydrochloric acid into peptones, and this takes place in various stages. Firstly, the combination with hydrochloric acid produces acid meta-protein, formerly called syntonin, or acid-albumin; then proteoses or propeptones, including albumoses, globuloses, vitelloses, elastoses, and gelatoses, are formed. All these are precipitated by nitric acid; but peptones, which are the next step in the process, are not, and although soluble in water are not coagulated by heat. If the action of digestion is sufficiently prolonged, further cleavage products arise, called polypeptides, which are probably combinations of amino-acids, and at length the individual amino-acids appear, this stage, however, not usually taking place in the stomach, but after the chyme has passed into the small intestine.

The greater the amount of protein, the longer the time will elapse before free hydrochloric acid appears. This is of importance, because until the free acid reaches a certain degree of concentration, the pylorus will remain closed, and the contents of the stomach cannot begin to pass into the duodenum. The moment the pylorus under this impulse relaxes, a certain portion of the acid gastric contents gushes out into the duodenum, and the pylorus then contracts. Contact of this acid chyme with the duodenal mucous membrane stimulates its pro-secretin-contaiuing cells to produce secretin; and Bayliss and Starling have demonstrated that this is absorbed into the blood current, and through this medium travels to the pancreas and liver directly, stimulating them to pour out pancreatic fluid and bile respectively. Doubtless, besides this direct chemical excitant or hormonic effort, reflex action plays its part in the process.