The moisture afforded by the salivary secretion is also necessary to enable us to swallow the food.
"And the Lord said unto him, Who hath made man's mouth? or who maketh the dumb, or the seeing, or the blind? have not I the Lord?"—Exodus iv.
873. Why does the salivary juice enter the mouth just at the moment that we are eating?
Because the glands, which are buried in the muscles of the mouth, and which in their form are much like bunches of currants, are always full of salivary secretion. There are nerves which are distributed from the brain to these glands, and when other nerves which belong to the senses of taste, of sight, or of feeling, are excited by the presence of food, a stimulus is imparted to the salivary glands, through the nerves that surround them, their cells collapse, and the juice which they contain is poured out through their stems, or ducts, into the mouth.
874. How do we know that impressions imparted to one set of nerves, may be imparted to another set, so as to put any particular organ in action.
Because very frequently the mere sight of rich fruit, or acid substances, will cause the saliva to flow freely. In this case it is evident that the salivary glands could not see or know that such substances were present. An impression must, therefore, be made upon the brain, through the organ of vision, and the desire to taste the substances being awakened, a nervous stimulus is imparted to the glands of the mouth, and they at once commence their action, as if food were present.