[3] It is wonderfully elastic and constantly changing in size, contracting till it will scarcely hold a quart when empty, and expanding, as food or drink is put into it, until it will easily hold two quarts, or even a gallon or more when greatly distended, as by gas. [4] If you take some pepsin which has been extracted from the stomach of a pig or a calf, melt it in water in a glass tube, then drop one or two little pieces of meat or hard-boiled white of egg into it, you can see them slowly melt away like sugar in a cup of coffee. If you add a few drops of hydrochloric acid, the melting will go on much faster; and if you warm up the tube to about the heat of the body, it will proceed faster still. So nature knew just what she was doing when she provided pepsin and acid and warmth in the stomach. [5] The liver and the bile are more fully described in chapter XVII. [6] Tiny plant cells, known also as _germs_, which cause fermentation, decay, and many diseases. CHAPTER III THE FOOD-FUEL OF THE BODY-ENGINE WHAT KIND OF FOOD SHOULD WE EAT? Generally speaking, our Appetites will Guide us. Our whole body is an ingenious machine for catching food, digesting it, and turning the energy, or fuel value, which it contains, into life, movement, and growth. Naturally, two things follow: first, that the kind and amount of food which we eat is of great importance; and second, that from the millions of years of experience that the human body has had in trying all sorts of foods, it has adapted itself to certain kinds of food and developed certain likes and dislikes which we call _appetites_. Those who happened to like unhealthy and unwholesome foods were poisoned, or grew thin and weak and died off, so that we are descended solely from people who had sound and reliable food appetites; and, in the main, what our instincts and appetites tell us about food is to be depended upon. The main questions which we have to consider are: How much of the different kinds of food it is best for us to eat, and in what proportions we should use them. Both men and animals, since the world began, have been trying to eat and digest almost everything that they could get into their mouths. And what we now like and prepare as foods are the things which have stood the test, and proved themselves able to yield strength and nourishment to the body. So practically every food that comes upon our tables has some kind of real food value, or it wouldn't appear there.