I have already said that air reaches the lungs through the larynx. The larynx (of which we shall speak further when I have explained another curious thing very valuable to little girls—the voice), the larynx is a tube composed of five pieces of cartilage (you know now what cartilage or gristle is), the firm resisting texture of which keeps it always open. After these five pieces of cartilage, come others, and the tube is continued; but it then takes the name of the trachea; the larynx and trachea constituting the windpipe. At its entrance into the chest, the trachea divides into two branches, which are called bronchial tubes, and which run, one into the right lung, the other into the left. You sometimes hear people talking about bronchitis. It is an inflammation of these bronchial tubes, which are within an inch or two of the lungs. It is necessary, therefore, to be very careful in such circumstances, and do exactly what the doctor prescribes, because—one step further, and the inflammation extends from the bronchial tubes into the lungs themselves, with which it is not safe to play tricks.
Having reached the lungs, the bronchial tubes subdivide into branches, which ramify again in their turn like the boughs of a tree, and the whole ramification terminates in imperceptible little tubes, each of which comes out in one of those little chambers I was talking about just now. And this is the way in which air gets there at all.
The venous blood which leaves the heart, arrives on its side by one large canal, which passes out from the right ventricle, and which is called the pulmonary artery. And, to tell you the truth, while there is no learned man present to be angry with us, it is a very ill-chosen name, because it is venous blood which flows in this so-called artery. But the doctors have decided that all the vessels which run from the heart should be called arteries, and all those which go back to it veins, whatever may be the nature of the blood which they contain. We cannot help it, because they manage all these matters in their own way; but in that case it was scarcely worth their while to talk about arterial and venous blood. It would have been better to have said simply, red blood and black blood.
Be this as it may, venous blood arrives from the right ventricle through the pulmonary artery. This divides itself, like the bronchial tubes, into thousands of little pipes, whose extremities come creeping along the partitions of the little chambers in question.
And here, then, takes place, between the air and the blood, that mysterious intercourse for the account of which I have kept you waiting so long; and at the end of which the black blood becomes red, or, in other words, from venous becomes arterial. I have called it "intercourse," and this is really the proper phrase; for this transformation of the blood is accomplished by means of an exchange. The air gives something to the blood, and the blood gives something to the air—each giving, in exchange, like two people over a bargain in the marketplace.
With your permission, my dear child, we will stop here to-day. We have now got to the charcoal market, and it is a little black.
LETTER XX.
CARBON AND OXYGEN.
Here, then, my dear child, we have arrived at the explanation of that great mystery, WHY we breathe. Keep on the alert, for we are now entering into a region where everything will be new to you.
Here we are at the charcoal market, I said to you just now, and no doubt you concluded that I was beginning another comparison.