872. _Why does saliva enter the mouth when we are eating?_
Because, in addition to the _mechanical_ grinding of the food by the action of the teeth, it is necessary that it should undergo certain chemical modifications to adapt it to our use. There are placed, therefore, in various parts of the body, _glands_, which secrete peculiar fluids, that have a chemical influence upon the food.
The first of these glands are the _salivary glands of the mouth_, which pour out a clear watery fluid upon the food we eat, and which fluid has been found to possess a property which contributes to the digestion of food.
The moisture afforded by the salivary secretion is also necessary to enable us to swallow the food.
[Verse: "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_.
875. _Why does food descend into the stomach?_
Because, after the teeth, the tongue, and the muscles of the mouth generally, have rolled the food into a soft bolus, it is conveyed to the back of the mouth, where it is set upon the opening of the throat (_oesophagus_). It does not then descend through the throat by its own gravity, because the throat is generally in a compressed or collapsed state, like an empty tube; and we know that persons can eat or drink when with their heads downwards. The oesophagus is formed of a number of muscular threads, or rings, and _each little thread is like a hand ready to grasp at the morsel that is coming_. As soon as the bolus is presented at the top of the throat, these little muscular hands lay hold of it, and transmit it downward, passing it from one to another, until it is conveyed through the long passage, to the door of the stomach, which it enters.