CHAPTER XLI.
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.