The peristaltic movements of the stomach begin soon after the completion of the meal, and occur every fifteen or twenty seconds. They are due to muscular contractions which pass like a wave towards the outlet, and carry the food along. They serve a double purpose. In the early stage of digestion the pylorus remains closed, and, consequently, when the food is driven against it by each peristaltic wave it causes a central return current; thus we have a churning movement by which means the food is thoroughly mixed with the gastric juice. But as the digestion proceeds the pylorus relaxes, the muscular contractions increase in strength, and the fluid or semi-fluid food is gradually ejected through it into the intestines.

The pylorus is surrounded by a strong muscular ring or sphincter, which is normally contracted to such an extent that it appears in an X-ray photograph as a fine line about 3/4 inch long. Through this narrow orifice the partially digested food is driven by strong muscular contractions or peristaltic waves. It may easily be perceived that the nature of the food influences the rate at which it passes through. When a meal is consumed consisting of lean meat, suet, and rice, so prepared that the whole mass is of an equal consistency, the X-rays show that the carbohydrate (rice) begins to pass through the pylorus in fifteen minutes, part of it being reduced to soluble starch and part into dextrin and sugar. But the protein (lean meat) and fat do not begin to leave under half an hour, and are much slower in passing through. A meal consisting of carbohydrate alone (rice or potato) leaves the stomach rapidly, no trace being left three hours after the meal. A meal of protein and fat (fat meat, ham or bacon and egg) leaves the stomach more slowly, some being present six hours after the meal. A meal of carbohydrate and protein (bread with lean meat, fish or fowl) leaves the stomach in a period which is intermediate between the other two. The addition of fat hinders the digestion of food in the stomach, and therefore delays its passage through the pylorus.

The passage of food is likewise hindered by the consistence of its particles. The X-rays show that when a particle of hard food is driven to the pylorus by the muscular contractions the aperture is immediately closed tightly. Indigestible masses are thus returned over and over again until the hard pellets are softened and reduced to a semifluid condition. At the end of gastric digestion, however, even such masses may be driven through the relaxed pylorus, for small pieces of bone, cherry stones, plum stones, and other indigestible substances manage to escape into the bowels.

The passage of liquids through the stomach is governed in the same way as other foods. Water taken on an empty stomach soon passes through the pylorus, but some of it is absorbed by the gastric mucous membrane. Fluids of a temperature higher than that of the blood are not allowed to pass through, but are retained until the temperature is the same as that of the body. Beer leaves the stomach more slowly than water; and liquids containing various salts are retained until they become of the same salinity or isotonic with the gastric juice.

Thus, by the exercise of a selective power, the stomach is able to protect the delicate mucous membrane of the bowels from the injurious effects of unsuitable materials.

The amount of digestion which is done in the stomach is great. Let us see how much has been done by the time the food has passed through the pylorus. It has been or should have been reduced to a pulp in the mouth. It is mixed with saliva. Some of the soluble substances are dissolved by that fluid; and some of the starch turned into sugar. It is now swallowed and slowly permeated by the gastric juice, which begins to be secreted during mastication. Until this permeation is complete the transformation of starch continues uninterruptedly, first into soluble starch, then into dextrin, and finally into sugar. In the stomach, however, the food undergoes further disintegration by the aid of the gastric juice. Meat, fish, or fowl is broken up into tiny fragments. The connective tissue which binds the muscular fibres together is dissolved; the chief substance, called collagen, being converted into gelatine and peptonized. The skin and gristle, also containing collagen, is converted into gelatine and peptonized. The muscular fibres, which form the chief portion of the flesh, are separated into tiny discs; the cement substance which was between the cells is peptonized; the individual cells are dissolved, the syntonin and fibrin which they contain are converted into proteoses and peptones. The covering of the muscular fibres, sarcolemma, consists of elastin, which is dissolved and peptonized. The influence of gastric juice on the coverings of animal cells varies. These coverings differ in composition; when the flesh is young and tender it consists of elastin or a similar material soluble in gastric juice; but in older animals the cell coverings become exceedingly tough, more like horn, which resist the action of gastric juice.

The fatty portion of meat undergoes similar disintegration. The connective tissue which binds it together is first gelatinized and dissolved; the covering of the fat cells is dissolved, and by this means the fat is set free and mixed in the same way as butter, lard, or oil with the other ingredients of the food.

Milk is curdled by the rennin ferment, and it should be remembered that this always occurs in the first stage of the digestion of milk. After a time the curds are broken down and their digestion by the gastric juice begun; some portion being converted to peptone. Cheese requires very careful mastication, otherwise it will not easily dissolve in the gastric juice. Nevertheless, a small amount of good ripe cheese is a stimulant to digestion, and in so far it deserves the old adage: "Cheese! thou mighty elft digesting all things but thyself! "