“I should, certainly.”
“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 [pg 675] 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.”
“No very unreasonable conjecture, I should think,” said Mr. Bagges.
“Well; if the air alters the blood, most likely, we should think, it gives something to the blood. So first let us see what is the difference between the air we breathe in, and the air we breathe out. You know that neither we nor animals can keep breathing the same air over and over again. You don't want me to remind you of the Black Hole of Calcutta, to convince you of that; and I dare say you will believe what I tell you, without waiting till I can catch a mouse and shut it up in an air-tight jar, and show you how soon the unlucky creature will get uncomfortable, and began to gasp, and that it will by-and-by die. But if we were to try this experiment—not having the fear of the Society for the Prevention of Cruelty to Animals, nor the fear of doing wrong, before our eyes—we should find that the poor mouse, before he died, had changed the air of his prison considerably. But it would be just as satisfactory, and much more humane, if you or I were to breathe in and out of a silk bag or a bladder till we could stand it no longer, and then collect the air which we had been breathing in and out. We should find that a jar of such air would put out a candle. If we shook some lime-water up with it, the lime-water would turn milky. In short, uncle, we should find that a great part of the air was carbonic acid, and the rest mostly nitrogen. The air we inhale is nitrogen and oxygen; the air we exhale has lost most of its oxygen, and consists of little more than nitrogen and carbonic acid. Together with this, we breathe out the vapor of water, as I said before. Therefore in breathing, we give off exactly what a candle does in burning, only not so fast, after the rate. The carbonic acid we breathe out, shows that carbon is consumed within our bodies. The watery vapor of the breath is a proof that hydrogen is so, too. We take in oxygen with the air, and the oxygen unites with carbon, and makes carbonic acid, and with hydrogen, forms water.”