=The Story of a Meal.=—The chemistry of digestion will be understood most readily if the constituents of a meal are traced from their entrance into the mouth to their absorption through the wall of the alimentary canal, or abandonment as indigestible.
We may describe as a typical meal one consisting of bread, vegetables, cane-sugar, meat, milk, fat, and cheese. In the mouth the various foods are crushed and mixed with the alkaline secretions of the salivary glands. A certain amount of the cooked starch contained in the bread is changed into maltose. In the stomach the digestion of starch is continued for a time, but a large part even of the cooked starch awaits the action of pancreatic juice. A certain amount of cane-sugar is converted into dextrose and levulose, which are rapidly absorbed into the blood; but this action is due to hydrochloric acid, and probably affects a comparatively small part of the cane-sugar swallowed. Fat is quite unaltered in the stomach. All proteins are attacked by pepsin, but some yield to digestion more readily than others. Gluten of bread, like all vegetable proteins, is comparatively resistant; but since it is presented to the action of pepsin in small quantities and in a spongy form—very suitable for digestion—it is probable that most of it is peptonized in the stomach. Chemists experimenting with gastric juice taken from the stomach, and reproducing the conditions as to temperature, removal of products of action, etc., as closely as it is possible to reproduce them in the laboratory, find that the various foods take different times to digest. The proteins of meat are more quickly peptonized when raw than after coagulation by heat. The same is true of white of egg. Amongst different varieties of cooked flesh, beef is more quickly peptonized than fish. The casein of milk is more quickly peptonized than any other protein; and it also is no exception to the rule that digestibility is diminished by cooking. Similar data may be obtained for all foods. They are no doubt useful indications of the course of action which we may expect to occur within the stomach, but we can never be sure that my lord will obey the ruling of the chemist. Practice with a captive golf-ball is a useful preparation for the game; but there are conditions on the links which cannot be reproduced on the lawn. In an artificial stomach the clean fibre of raw fish digests more slowly than raw beef. Even when the beef is roasted and the fish fried or boiled in the ordinary way, the beef disappears through the dialyser (the bag of membrane suspended in a vessel of warm water in which experimental digestion is carried out) more quickly than the fish. Nevertheless, the living stomach is better disposed towards a mixed meal containing a certain weight of fish than towards a meal in which, the other constituents remaining the same, beef takes the place of fish. Important conclusions may, no doubt, be drawn from observations of the time occupied in the peptonization of pure food—_i.e._, fibrin, white of egg, clean meat, etc.—under conditions simulating those which are present in the stomach; but they must be accepted with many reservations. In the stomach it is not pure substances, but mixtures, that the gastric juice has to deal with. And here a most important factor comes into play, to which further reference will be made later on. The amount and quality of the secretion of the gastric glands depends upon the nature of the food. Hence a food, or a combination of foods, which digest readily in the laboratory may take a long time to disappear from the stomach, and _vice versâ_. Digestibility depends upon the nature of the food. It depends also upon its physical state. To take simple illustrations: Cheese contains coagulated casein, one of the most easily digestible of proteins, but the casein is intimately mixed with fat, upon which gastric juice can make no impression. Even when finely divided, the particles of casein are protected from the action of the juice by fat. In the same way the meat of pork is as digestible as mutton, but the fat of pork is quickly melted and very liquid. In the process of cooking the muscle-fibres become saturated with fat.