Having now considered the nature and constitution of the various foods which sustain and build up the body, I will proceed to describe the methods and processes by which they are digested and assimilated.

Digestion Of Food In The Mouth

When solid food is placed in the mouth, it is masticated or ground by the molar teeth. It is at the same time thoroughly mixed with the saliva, which is poured out in abundance at the moment required, by the salivary glands, the ducts of which open into the mouth on the inner side of the cheeks and under the tongue. The period of time that the food remains in the mouth, subject to mastication and to the influence of the saliva, varies with different individuals; but it is well that this period should be as long as possible, in order that the food may be completely broken up, and the tougher and harder portions rendered fit for digestion in the stomach. Besides mastication, the first step in the digestion of starch takes place in the mouth.

Starch is contained in a great number of the vegetable foods which are common articles of diet - namely, potatoes, flour, peas, beans, etc. In its uncooked condition, it is incapable of digestion by man. It exists in the form of small granules, composed of concentric layers of material.

These granules are insoluble in cold water, but on being boiled or placed in hot water their outer envelope bursts, and the contents swell up, the whole forming an opalescent gelatinous mass. In order that starch may be made perfectly soluble, so as to pass through the coats of the minute blood vessels of the intestines, it is necessary for it to be converted into sugar, and, therefore, one of the most important acts of digestion is the conversion of starch into sugar. This is brought about by the action of a ferment or diastase. Such a ferment or diastase is present in the saliva, and is called ptyalin. It acts on the starch contained in the food, and partially converts it into sugar while mastication is going on.

The mouthful of food, having been thoroughly ground by the action of the molar teeth or grinders, and well mixed with the saliva, is rolled into a ball or bolus by the tongue, and passed, by the act of swallowing, to the back of the mouth. It is here seized by the self-acting or involuntary muscles which form the pillars of the throat, and it is passed by their action, and by the rolling upwards and backwards of the root of the tongue over the epiglottis, or trap-door which closes the opening into the windpipe, into the gullet or oesophagus, a long tube which conducts it to the stomach.