CHAPTER VII.

THE BODY AND ITS COMPOSITION.

“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.  Swallowed whole, or placed directly in the stomach, such food passes through the body unchanged.  Each division of the alimentary canal has its own distinct digestive juice, and I give them in the order in which they occur.

First, The saliva; secreted from the glands of the mouth:—­alkaline, glairy, adhesive.

Second, The gastric juice; secreted in the inner or third lining of the stomach,—­an acid, and powerful enough to dissolve all the fiber and albumen of flesh food.

Third, The pancreatic juice; secreted by the pancreas, which you know in animals as sweetbreads.  This juice has a peculiar influence upon fats, which remain unchanged by saliva and gastric juice; and not until dissolved by pancreatic juice, and made into what chemists call an emulsion, can they be absorbed into the system.

Fourth, The bile; which no physiologist as yet thoroughly understands.  We know its action, but hardly why it acts.  It is a necessity, however; for if by disease the supply be cut off, an animal emaciates and soon dies.