"The lamp of life" is a very old metaphor for the mysterious principle
vitalizing nerve and muscle; but no comparison could be so apt. The
full-grown adult takes in each day, through lungs and mouth, about eight
and a half pounds of dry food, water, and the air necessary for breathing
purposes. Through the pores of the skin, the lungs, kidneys, and lower
intestines, there is a corresponding waste; and both supply and waste
amount in a year to one and a half tons, or three thousand pounds.

The steadiness and clear shining of the flame of a lamp depend upon
quality, as well as amount of the oil supplied, and, too, the texture of
the wick; and so all human life and work are equally made or marred by the
food which sustains life, as well as the nature of the constitution
receiving that food.

Before the nature and quality of food can be considered, we must know the
constituents of the body to be fed, and something of the process through
which digestion and nutrition are accomplished.

I shall take for granted that you have a fairly plain idea of the stomach
and its dependences. Physiologies can always be had, and for minute
details they must be referred to. Bear in mind one or two main points:
that all food passes from the mouth to the stomach, an irregularly-shaped
pouch or bag with an opening into the duodenum, and from thence into the
larger intestine. From the mouth to the end of this intestine, the whole
may be called the alimentary canal; a tube of varying size and some
thirty-six feet in length. The mouth must be considered part of it, as it
is in the mouth that digestion actually begins; all starchy foods
depending upon the action of the saliva for genuine digestion, saliva
having some strange power by which starch is converted into sugar.