If you will think back over what you have already learned in this book, you will see that we began by finding out something about how we men, the animals, and the plants come to have any such things as bodies at all. We learned how the little chick forms inside the egg, and the little plant inside the seed. We learned, too, about the wonderful life-jelly or protoplasm of which all living things are made; how it shapes itself into cells; how it builds these cells into our various members, eyes and bones and hair and muscles; and how the body changes, as we grow from youth to maturity, and from maturity to old age.

Then, after we had learned something about this body of ours, we turned to consider how we use it. We found about something of what animals cannot do, and what they can do, and how they do it. We learned how animals of various sorts, and plants as well, see and feel and act; and we learned also something about how we ourselves do our thinking, which is so very different, and so very much better done, than that of any animal or plant.

Now we turn to a different matter. We have taken up being, and doing, and thinking. Now we shall consider living. We shall learn about how the body of the plant or animal feeds itself and keeps alive, and how the different parts of it, the bones and skin and leaves and bark, manage to get on with one another, and work together like a well-made machine.

For, of course, the body is a machine. It is a vastly complex machine, many, many times more complicated than any machine ever made by hands; but still after all a machine. It has been likened to a steam engine. But that was before we knew as much about the way it works as we know now. It really is a gas engine; like the engine of an automobile, a motorboat, or an airplane.

I don’t suppose that any boy, at least, needs to be told the difference between a gas engine and a steam engine. In the one, we build a fire under the boiler, and turn water to steam. Then the steam goes thru a pipe to the cylinder, where it pushes the piston back and forth, first on one side, then on the other, and so turns the wheels.

In the gas engine, on the other hand, there is no boiler, no steam, and no fire. A mixture of air and gasolene vapor flows into the cylinder, cold. There it explodes, set off by an electric spark, and the push of that explosion moves the piston and makes the wheels go round.

We, I say, are not steam engines. We have neither boiler nor steam nor fire. But each little working cell is like a little cylinder, which takes up from the blood air and food, mixes them together inside itself, waits with everything ready to go off, gets the proper signal thru a nerve, then explodes and does something.

That’s the way a muscle does its work. It is a many-thousand-cylindered engine. Each little fiber of the muscle is a cylinder; and each time you lift your hand or move your foot there is a perfect battery of minute explosions. You cannot hear them, for there is no pop—the muffling is vastly better than any engine-builder ever devised. But you do feel the heat; and if you move fast and hard enough, you have to stop to cool off and get a drink.

The plants also are many-cylindered gas engines. They do not do so much work as animals do, not so much running round and moving things. But they do move, and certainly grow and lift themselves high in the air. This much work they do by exploding their cells, just as animals or automobiles do theirs. The growing plants take their food out of the air thru their leaves; and they take also the air itself in the same way. They mix these together inside their cells; and when there is work to be done, growing, moving, or any other sort, they explode a little of the mixture and do it.

Don’t think then that animals and plants and human beings are merely like automobiles. They are automobiles. Their fuel is their food. They mix it with air. They explode the mixture, and move. Anything that does that is an automobile, and runs with a gas engine.