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A Handbook of Health by Woods Hutchinson

The loops of capillaries again run together


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.


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