n America, where they make the best locomotive engines in the world, they say that the life of an engine is about twenty years. That is, when they build an engine, they know about how much work it will have to do and what usage it is likely to have. They know that the engine is strong enough to do such work and stand such usage for twenty years. So they say that the length of the engine's life is twenty years.Now, a man's body is, in its way, a machine. It is made to do certain work, and if it has the right sort of care, it ought to be healthy and do the work required of it, to the end of the man's life. 8It is estimated that the natural life of a man is seventy years. This little book is intended to tell us how to live and something about caring for our bodies so that they shall last as long as possible, and be ready and able to do their work in the world.The Skull, Chest, and Abdomen.In a general way, we may compare the human body to three closed boxes, one above another. These boxes are the skull, the chest, and the abdomen. Each one has its own special contents, formed to do a special work for the body. The skull is a hard, bony case made to contain the brain. This is where the mind lives, and it is part of the work of the mind to take care of the body and direct its movements. The brain maintains a sort of telegraph station within 9itself. Wires, which we call nerves, branch out from it to all parts of the body, and the brain is constantly receiving messages over these wires and sending others telling the muscles what to do. For instance, if the hand comes in contact with something hot, a message instantly goes to the brain, telling this fact. The brain sends back word to take the hand away, and the hand is withdrawn. But all this is done so quickly that the hand seems to be withdrawn the very instant that it comes in contact with the fire. The skull is supported by the backbone, which connects it with the second closed box.This second cavity is the chest, which is really a sort of cage formed by the ribs, the backbone, and the breastbone. In the chest are the heart and the lungs. The heart is an engine. Put your hand over it and you can feel the steady throb of its beat, day and night. It is working all the time, whether you are awake or asleep. The business of the heart is to send blood to all 10parts of the body. It does this by driving the blood through tubes, called arteries and veins, that go all over the body. The arteries are deep down among the muscles, but some of the veins are close to the surface. We can see blue veins at the temples and on the backs of our hands. All the blood goes to every part of the body once in two minutes.