[Verse: "Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God."--CORINTH. X.] CHAPTER XLI. 869. _Why do we eat food?_ Because the atoms of which our bodies are composed are _continually changing_. Those atoms that have fulfilled the purposes of nature are removed from the system, and, therefore, new matter must be introduced to supply their place. 870. _Why do we eat animal and vegetable food?_ Because their substances are composed of _oxygen_, _hydrogen_, _carbon_, and _nitrogen_--the four chemical elements of which the human system is formed. They are, therefore, capable of nourishing the body, after undergoing digestion. 871. _Why do we masticate our food?_ Because mastication is _the first process towards the digestion of food_. Before animal or vegetable substances can nourish us, their condition must be entirely changed, their _organic_ states must be dissolved, and they must become simple matter, in a homogeneous mass, consisting of the four chemical elements necessary to nutrition, and they must again be restored to an organic condition. 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._