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 itself. 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 The brain sends back word to take the hand away, and the hand is withdrawn. But all this is done so quickly that the hand seems to be withdrawn the very instant that it comes in contact with the fire.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 parts 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. The food which a person eats is acted upon by the digestive fluids in the body and is turned over and dissolved until it becomes fluid itself. It is then taken up by the blood and carried to different parts of the body, so that each organ and muscle gets what it needs. We shall learn, a little later, just how the food gets into the blood. We have seen that the heart sends the blood out through vessels, which are called arteries. This blood starts from the heart bright red, full of fresh air and food for the body. As it goes on its journey, each tissue takes from it what is needed to keep that part healthy, and at last the blood has given out all the good things with which it was loaded. It no longer carries food and fresh air, but is full of the impurities taken in on its journey. If something were not done to cleanse it, the man would die. [Illustration: THE CIRCULATION OF THE BLOOD. The blood is forced by the heart through the arteries (black lines), and returns to it through the veins (dotted lines). ]