SURFACE VEINS AND DEEP-LYING ARTERIES OF INNER SIDE OF RIGHT ARM AND HAND
The deep-lying veins that run parallel to the arteries have been omitted; so have the veins of three of the fingers.
One of the first of these branches to be given off by the aorta is a large blood pipe, or artery, to supply the shoulder and arm; this artery runs across the chest, thence across the armpit, and down the arm to the elbow. Here it divides into two branches, one to supply the right, and the other the left, side of the forearm and hand. These branches have by this time got down to about the size of a wheat straw; the one supplying the right side is the artery which we feel throbbing in the wrist, and which we use in counting the pulse. From it run off smaller branches to supply the thumb and fingers. These branches break up again into still smaller branches, and they into a multitude of tiny capillaries, which run in every direction among all the muscle cells, delivering the food and oxygen at their very doors, as it were. The muscle cells eagerly suck out the food-stuffs, and breathe in the oxygen of the blood; at the same time, they pour into it their waste stuffs of all sorts, including carbon dioxid. These rob the blood of its bright red oxygen color and turn it a dirty purplish, or bluish, tint.
The loops of capillaries again run together, as they did in the liver and in the lung, to form tiny veins; and these run together at the base of the thumb and in the wrist, to form larger ones through which the now poor and dirty blood is carried back up the arm over much the same course as it took in coming down it. Indeed, the veins usually run parallel with, and often directly alongside of, the arteries. The blood passes through the armpit, across the chest, into the great main pipe for impure blood, the vena cava, and through this into the right side of the heart, where it again meets the rich, but waste-laden blood from the food tube and liver, and starts on its circuit through the lungs and around the body again.
The blood reaches every portion of our body in precisely this same manner, only taking a different branch of the great pure-blood delivery pipe, the aorta, according to the part of the body which it is to reach, and coming back by a different vein-pipe.
Why the Arteries are more deeply Placed than the Veins. In the limbs and over the surface of the body generally, the arteries are more deeply placed than the veins, so as to protect them from injury, because the blood in the arteries is driven at much higher pressure than in the veins and spurts out with dangerous rapidity, if they are cut. Some of the veins, indeed, run quite a little distance away from any artery and quite close to the surface of the body, so that you can see them as bluish streaks showing through the skin, particularly upon the front and inner side of the arms.
The Capillaries. Of course, the blood pipes into which the food is sucked through the walls of the food tube, and those in the lung, through which the oxygen is breathed, as well as those in the thumb through which food is taken to the muscle-cells, have the tiniest and thinnest walls imaginable. For once, the name given them by the wise men—capillaries (from the Latin capilla, a little hair)—fits them beautifully, except that the hairs in this case are hollow, and about one-twentieth of the size of the finest hair you can see with the naked eye. So tiny are they that they compare with the big veins near the heart into which they finally empty much as the smallest and slenderest twigs of an elm do with its trunk. What they lack in size, however, they more than make up in numbers; and a network of them as fine and close as the most delicate gauze goes completely around the food tube between its mucous lining and muscular coat.
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.
DIAGRAM OF ARTERY, CAPILLARIES, AND VEIN 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.
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
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