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A Handbook of Health by Woods Hutchinson

From shooting backward into the vena cava


Though

thickest and most abundant on the inner and outer surfaces of the body, every particle of the body substance is shot through and through with a network of these tiny tubes. So close and fine is this network in the skin, for instance, that, as you can readily prove, it is impossible to thrust the point of the finest needle through the skin without piercing one of them and "drawing blood," as we say, or making it bleed. From this network of tiny, thin-walled tubes, the body-cells draw their food from the blood.

[Illustration: DIAGRAM OF ARTERY, CAPILLARIES, AND VEIN]

The Meaning of Good Color. It is the red blood in this spongy network of tiny vessels that gives a pink coloring to our lips and the flush of health to our cheeks. Whenever for any reason the blood is less richly supplied with food or oxygen, or more loaded with "smoke" and other body dirt than it should be, we lose this good color and become pale or sallow. If we will remember that our hearts, our livers, our brains, and our stomachs, are at the same time often equally "pale" and sallow--that is, badly supplied with blood--as our complexions, we can readily understand why it is that we are likely to have poor appetites, poor memories, bad tastes in our mouths, and are easily tired whenever, as we say, our "blood is out of order." The blood is the life. Starve or poison that, and you starve or poison every bit of living stuff in the body.

justify;"> THE HEART

Structure and Action of the Heart. 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.


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