The evident purpose of the saliva when food is not present is to keep the lining of the mouth moist.
Salivary digestion is carried on in the stomach until the food becomes thoroughly mixed with the gastric juice, which, being acid, inhibits the action of the ptyalin.
The thorough and regular cleansing of the teeth is an important aid to digestion. Food products allowed to remain about the teeth ferment, rendering the mouth acid. When the mouth is acid, the alkaline saliva does not secrete in sufficient amount and the mouth is more or less dry.
The mouth is acid in rheumatism and allied conditions and the saliva may be thick and ropy so that it does not moisten the food properly. On the other hand the flow of saliva may be too free, the ptyalin is then too much diluted to promptly act on the food. This may result from overstimulation of the salivary glands occasioned by the excessive chewing of gum, or tobacco. These excesses also carry too much air into the stomach, resulting in flatulence.
The flow of saliva is controlled, largely, by nerves centering in the medulla oblongata. The sight of food, pleasingly served, or even the thought of food which one likes, will increase the flow. This is one instance of the control of thought over digestion, and the importance of forming the habit of cultivating a taste for all kinds of food is apparent. The stronger the relish for the food, and the more thoroughly it is masticated and mixed with the saliva, the more perfectly will the first step in digestion be accomplished.
Thorough mastication is important, not only because the chemical action on the starch molecules is facilitated by the softening and mixing with the saliva, but also because thorough mastication tends to prevent overeating—the appetite is more quickly satisfied when the food is well masticated.
Cool water encourages the flow of saliva and for this reason should be drunk before meals, particularly when digestion is weak. It may be taken at rest periods during the meal. (See page [31].)
The Mouth and Nasal Passages
The relation of the mouth and nasal passages to the digestive processes is seldom considered by the average individual. Their importance to the growing child is being recognized by the examination of school children which is now being made a part of the health program in many of our cities. Their importance to the adult is no less.