98. Mastication

The first step in the digestion of food is to reduce it to a much finer condition. This is done in the mouth, the teeth being the grinding tools. This comminution is essential for two reasons: (1) it puts the food in condition to be swallowed, and (2) fits it for the prompt and efficient action of the several digestive fluids. It is necessary for all food materials to be broken down and moistened in order that they may be swallowed. Even if they could be conveyed to the stomach in a coarse form, the process of rendering their constituents soluble would proceed very slowly. Common experience teaches us how much more quickly finely powdered sugar or salt will dissolve than will the large crystals or lumps. The more finely any solid is ground, the larger is the surface exposed to the attack of the dissolving liquid.

Prompt and rapid solution of food is essential, because, if it is too long delayed, uncomfortable and injurious fermentations are likely to set in, and, because of imperfect digestion, the final nutritive effect of a meal may be diminished, and health may be impaired. For these reasons, persons with diseased teeth, or those who have lost teeth, may not properly prepare their food for digestion.

Fig3. 1. - Glands secreting the saliva, - parotid, sublingual, submaxillary.

99. The Saliva

During mastication there is poured into the mouth a liquid called the saliva, which has two important functions: (1) it moistens the food, and (2) it causes a chemical change in certain of the constituents of the food.

The saliva has its origin in several secretory glands known as the salivary glands that are adjacent to the mouth cavity, and from these this liquid is poured into the mouth through ducts that open in the cheek and under the tongue. The chief of these glands are located in the side of the face just in front of the ear, and between the lower jaw and the floor of the mouth, and are called the parotid, the submaxillary, and the sublingual. Other glands of this character are scattered in the cheeks and at the base of the tongue. The anatomy and arrangement of these organs are not essential to our subject. We are chiefly interested in the liquid which they secrete.

100. The Saliva And Its Action

The saliva is a transparent and somewhat slimy liquid, and contains generally not less than 99 parts in 100 of water, and one part or less of solid matter. It is alkaline in reaction, because of the presence of compounds of the alkalies. The specific chemical effect exerted by this liquid on the food constituents is shown by subjecting starch to its action. When this is done, the starch gradually disappears as such and is replaced by a solution of maltose, the same sugar that we find in barley malt. The agent which is active in causing this change is a ferment, ptyalin, which is always present in the saliva of man and of some animals. It is classed among the diastatic ferments, because it has an office similar to that of a diastase in the germination of seeds; viz., the transformation of starch into a sugar. This transformation proceeds through successive stages from starch to dextrins, and from dextrins to maltose. Cooked starch is readily susceptible to the action of saliva, while raw starch is more slowly attacked by it. This does not mean that raw starch may not be finally digested by the human subject. This change begins in the mouth, and probably continues in the stomach, until the food becomes so acid that the ferment ceases to act, for ptyalin is inactive in an acid medium.

The action of saliva in the stomach does not cease suddenly, however, but proceeds until the masticated food is rendered wholly acid by mixing with the gastric juice. There is a not inconsiderable absorption of sugar from the stomach, notwithstanding the fact that the stomach secretes no agent that acts on starch. A certain proportion of the starch of foods is acted on by the saliva, partly in the stomach, but the main transformation to sugar occurs farther on in the digestive tract. The saliva also moistens the food, which is a most important office, for it is a necessary-preparation to the act of swallowing. It is estimated that an adult secretes not far from one quart of saliva in 24 hours.