“Well, then; first you ought to have some idea of the breathing apparatus. The laboratory that contains this is the chest, you know. The chest, you also know, has in it the heart and lungs, which, with other things in it, fill it quite out, so as to leave no hollow space between themselves and it. The lungs are a sort of air-sponges, and when you enlarge your chest to draw breath, they swell out with it, and suck the air in. On the other hand, you narrow your chest, and squeeze the lungs, and press the air from them;—that is breathing out. The lungs are made up of a lot of little cells. A small pipe—a little branch of the windpipe—opens into each cell. Two blood-vessels, a little tiny artery, and a vein to match, run into it also. The arteries bring into the little cells dark-colored blood, which _has been_ all over the body. The veins carry out of the little cells bright scarlet-colored blood, which _is to go_ all over the body. So all the blood passes through the lungs, and in so doing, is changed from dark to bright scarlet.” “Black blood, didn’t you say, in the arteries, and scarlet in the veins? I thought it was just the reverse,” interrupted Mr. Bagges. “So it is,” replied Harry, “with all the other arteries and veins, except those that circulate the blood through the lung-cells. The heart has two sides, with a partition between them that keeps the blood on the right side separate from the blood on the left; both sides being hollow, mind. The blood on the right side of the heart comes there from all over the body, by a couple of large veins, dark, before it goes to the lungs. From the right side of the heart, it goes on to the lungs, dark still, through an artery. It comes back to the left side of the heart from the lungs, bright scarlet, through four veins. Then it goes all over the rest of the body from the left side of the heart, through an artery that branches into smaller arteries, all carrying bright scarlet blood. So the arteries and veins of the lungs on one hand, and of the rest of the body on the other, do exactly opposite work, you understand.” “I hope so.” “Now,” continued Harry, “it requires a strong magnifying glass to see the lung-cells plainly, they are so small. But you can fancy them as big as you please. Picture any one of them to yourself of the size of an orange, say, for convenience in thinking about it; that one cell, with whatever takes place in it, will be a specimen of the rest. Then you have to imagine an artery carrying blood of one color into it, and a vein taking away blood of another color from it, and the blood changing its color in the cell.” “Ay, but what makes the blood change its color?” “Recollect, uncle, you have a little branch from the windpipe opening into the cell which lets in the air. Then the blood and the air are brought together, and the blood alters in color. The reason, I suppose you would guess, is that it is somehow altered by the air.”