Now what is it that keeps the blood whirling round and round the body in this wonderful way?
It is done by a central pump (or more correctly, a little explosive engine), with thick muscular walls, called the _heart_, which every one knows how to find by putting the hand upon the left side of the chest and feeling it beat.
The heart is really a bulb, or pouch, which has ballooned out from the central feed pipe of the blood supply system, somewhat in the same way that the stomach has ballooned out from the food tube.
The walls of this pouch, or bulb, are formed of a thick layer of very elastic and powerful muscles almost as thick as the palm of your hand.
When the great vein trunk has poured blood into this pouch until it is swollen full and tight, these muscles in its walls shut down sharply and squirt or squeeze the blood in the heart-pouch into the great artery-pipe, the aorta.

In fact, you can get a very fair, but rough, idea of the way in which the heart acts by putting your half-closed hand down into a bowl of water and then suddenly squeezing it till it is shut tight, driving the water out of the hollow of your hand in a jet, or squirt.
"But," some of you will ask at once, "what is to prevent the blood in the heart, when the muscle wall squeezes down upon it, from shooting backward into the vena cava, instead of forward into the aorta ?" Nature thought of that long ago, and ingeniously but very simply guarded against it by causing two little folds of the lining of the blood pipes to stick up both where the vena cava enters the heart and where the aorta leaves it, so as to form little flaps which act as valves.

These valves allow the blood to flow forward, but snap together and close the opening as soon as it tries to flow backward.

While largest and best developed in the heart, these valves are found at intervals of an inch or two all through the veins in most parts of the body, allowing the blood to flow freely toward the heart, but preventing it from flowing back.
As the heart has to pump all the blood in the body twice,--once around and through the lungs, and once around and through the whole of the body,--it has become divided into two halves, a right half, which pumps the blood through the lungs and is slightly the smaller and the thinner walled of the two; and a left half, which pumps the purified blood, after it has come back from the lungs, all over the rest of the body.
[Illustration: THE EXTERIOR OF THE HEART Showing the strands of muscle that compose it, the arteries and veins that feed and drain the muscle coat, and fat protecting these.] Each half, or side, of the heart has again divided itself into a receiving cavity, or pouch, known as the _auricle_; and a pumping or delivering pouch, known as the _ventricle_.

And another set of valves has grown up between the auricle and the ventricle on each side of the heart.