Not only are the bodies of all except the very smallest animals and plants at the same time gas engines and battlegrounds; they are also living apothecary shops. Indeed, it is a pretty well stocked drug store that has more different chemicals in it than a man and his horse, and his dog, and the garden that he works in, all together, manufacture every day of their lives.

Think to begin with, of all the different perfumes of all the different flowers, and all the various tastes of all the different vegetables which we eat, and of all the various spices and herbs which we use for flavorings in our food. Think of all the different coloring matters in flowers and leaves. Think of the tar, rosin, pitch, maple syrup, turpentine, gum, varnish, shelac, india rubber, tan bark, and the rest, that we get from a few trees alone. The chemicals made by even the ordinary plants could fit out the shelves of a fair-sized drug store.

We animals have vastly more different things inside us than any plant. Every time we move a muscle, we manufacture soda water. Every time we think, we turn a half dozen different chemicals into the blood. We take a mouthful of cracker, which is mostly starch; and straightway a ferment, or “enzyme,” in the mouth begins to turn the starch into sugar. That is why a cracker or a potato tastes sweeter and sweeter the longer one chews it. When the sugar gets into the blood, another enzyme (the “y” is like “i,” and the word sounds like en-zime) in the liver turns it back to starch again, so that it stays there in the liver, and can’t get out. Then as you know, this liver-starch slowly turns back to sugar again, and leaks out into the blood to feed the muscles. In the muscles, still another enzyme helps it to explode into carbon dioxid and water, only sometimes it forms lactic acid instead, which is the acid of sour milk. Because of these enzymes in the muscles, the gas engines which they are, can run when only moderately warm, instead of needing to be too hot to touch, like other engines when they are at work.

But if instead of making flour into crackers, or having the baker do it, and then eating them, we had made the flour into bread with yeast: then the yeast, which is a living and growing plant, would have formed another enzyme, which in turn would have turned the sugar in the bread into carbon dioxid and alcohol, and the carbon dioxid would have puffed up the bread and made it light. That is what yeast is for. The yeast of beer also, turns the sugar of the grain into carbon dioxid and alcohol. The first of these gives us the fizz, the second we use to poison ourselves with.

Even the color of our hair and eyes depends on an enzyme which manufactures the coloring matter. People who have the enzyme, have dark hair and brown eyes. People who haven’t it, have light hair and blue eyes.

In fact, almost anything that any living part of the body has to do, whether to take its food out of the blood stream and build it into its own substance, or to do its work, or merely to grow, it generally has to employ one or more of these enzymes to do it with. These are, in fact, the tools of the living jelly, or protoplasm, which makes our bodies and does our living for us. Without them we could not live a single hour.

I suppose there must be hundreds, or perhaps even thousands, of different chemicals and drugs and medicines and poisons and antitoxins and enzymes forming all the time in different parts of the body. The various organs put them to all sorts of various uses. Among others, they employ certain of them as messengers, to carry signals from one part of the body to another.

I have already explained that the head is one of the first organs in the body to be formed. It gets a start, therefore, over the rest, and a little baby’s head is something like four times as big as it need be to fit its body. But the baby’s legs and arms, which started late, are not nearly large enough to fit the rest of him. So the limbs have to grow fast, and the head to grow slowly, in order to come out right in the end. How do they know enough?

How fast little girls’ feet sometimes grow! At twelve they can wear their mamma’s shoes. Then the feet stop growing and the rest of the body catches up. Or when it is time for a boy’s voice to change, all of a sudden, his Adam’s apple, where the voice comes from, starts growing. In a few months it has increased from boy’s size to man’s, and the voice has dropped to a deep, if uncertain, growl. Everywhere throughout the body, the different parts start growing, and stop, and keep along together, or get ahead of each other in the most complicated fashion, but always right.

How they manage it, we do not altogether know. We do, however, know that the different parts of the body do signal to one another by means of these substances which they form within their cells, and turn loose in the blood stream. In this way, each organ of the body is able to send messages to the rest—“Start growing,” “Steady now,” “Slow down,” “Stop”—as the case may be. You already know how a working muscle signals to the heart and lungs, and how the blood cells get wind of an invasion, by the altered smell or taste of the blood, and rush to the point where the enemy has broken through the defense, and the fight is on.

So in general, when a message has to go quickly from one part of the body to another, it goes by way of the nerves. But when there is no special hurry, or when the signal must go to some tissue or organ which, like the blood corpuscles, has no nerve connection, then the message has to go by way of the blood.

If then, our legs want to say: “We’re sitting on something hard and sharp, please may we move,” they call us up over the nerves. If they want to say: “We have been growing too fast, and it’s high time we stopped and gave something else a chance,” they do it by turning something into the blood. But if they want to say: “We’re so dead tired that we simply can’t walk another step,” then as you know, they use both methods—fatigue toxins and very dreadful aches.