This oxygen is picked up by, and combines with, the red coloring matter of the millions of little oxygen sponges, or baskets--the red corpuscles--and turns them a light red color, causing the blood to become bright red, such as runs in the arteries and is known as _arterial blood_. The loops of tiny capillaries around the air cells of the lungs run together again to form larger pipes; and these unite, at the point of each lung nearest the heart, to form two large blood pipes--one from each lung--which pour the rich, pure blood, loaded with both food and oxygen into the left side of the heart. The left side of the heart pumps this blood out into the great main delivery-pipe for pure blood, known as the _aorta_, and this begins to give off branches to the different parts of the body, within a few inches of where it leaves the heart. [Illustration: 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.