CHAPTER XXIV.
THE AIR AND THE LUNGS.
"In order to live," continued Monsieur Roger, "you must breathe. Youdon't doubt that?"
"No," said Miss Miette, seriously.
"Now, respiration consists in the absorption by the blood of some of theoxygen of the air and in breathing out carbonic acid. The oxygen, incombining with the carbon and hydrogen of the blood, excites a realcombustion in the lungs, which results in the production of heat and inthe exhalation of vapor and carbonic acid."
Monsieur Roger was going to continue in the same scientific tone, whenMonsieur Dalize remarked to him that his explanation did not seem to beat all understood by the children.
The latter, a little embarrassed, held their tongues.
"You are right," replied Monsieur Roger, addressing Monsieur Dalize;"that is a silence which is certainly not very flattering. I intend toprofit by this lesson by beginning once more at the beginning."
"You are right," said Miette.
"Well, then, respiration is the very important function whose object isto introduce air into our lungs.
"What are the lungs, and why is it necessary to introduce air into them?And, in the first place, how is this air introduced? Through the mouthand through the nose. Then it passes through the larynx and arrives at alarge tube, which is called the trachea, or wind-pipe. It is this tubewhich, as I shall show you, forms the two lungs. As it enters the chest,this tube branches out into two smaller tubes, which are called theprimary bronches. One of these bronches goes to the right, to make theright lung; the other to the left, to make the left lung. Each primarybronche is soon divided into a number of little tubes, called secondarybronches. The secondary bronches divide up into a number of other tubes,which are still smaller; and so on, and so on. Imagine a tree with twobranches, one spreading towards the right, and the other towards theleft. Upon these two branches grow other branches; upon these otherbranches still others, and so on. The branches become smaller andsmaller until they become mere twigs. Now, imagine these twigs ending inleaves, and you will have some idea of that which is sometimes calledthe pulmonary tree, with its thousand branches."
"No," said Miette: "bronches."
"Bronches,--you are right," said Monsieur Roger, who could not helpsmiling at Miss Miette. "The tree which I have taken as a comparisonfinishes by dividing itself into twigs, which, as I have said, end inleaves. But you know, of course, that the twigs of the pulmonary tree inour breast do not end in leaves. They end in a sort of very small cells,surrounded by very thin walls. These cells are so small that they need amicroscope to detect them, and their walls are very, very thin; thecells are all stuck fast together, and together they constitute a spongymass, which is the lung. Now let us pass to the second question: Why isit necessary to introduce air into the lungs?"
"Yes," said Miette; "let us pass to that."
"The blood, in going out of the heart, circulates to all the parts ofthe body in order to make necessary repairs; at the same time it chargesitself with all the old matter which has been used up and is no longerany good and carries it along. Now, what is it going to do with this oldmatter? It will burn it. Where will it burn it? In the lungs. Now, therecan be no combustion when there is no air. The blood, wishing to burnits waste matter, and wishing also to purify the alimentary principleswhich the veins have drawn from the stomach, has need of air. Where willit find it? In the lungs. And that is why it is necessary to introduceair into our lungs, or, in other words, that is why we breathe. Thelungs are a simple intermediary between the air and the blood. Among thecells of the lungs veins finer than hair wind and turn. These veinsgather up the blood filled with waste matter. It is blood of a blackcolor, which is called venous blood. The walls of the veins whichtransport the blood are so thin that air, under the atmosphericpressure,--this pressure which I have told you all about,--passesthrough them and into the blood. Then the venous blood charges itselfwith the oxygen contained in the air, and frees itself from what I havecalled its waste material, and which is nothing less than carbon.Immediately its aspect changes. This venous blood becomes what is calledarterial blood; this black blood becomes rich vermilion,--it isregenerated. It goes out again to carry life to all our organs. Now,this time," asked Monsieur Roger, pausing, "have I made myselfunderstood?"
"Yes," said Miette, speaking both for Paul and for herself; "yes, wehave understood,--except when you speak of oxygen, of carbon, and ofcombustion."
"Oh, I was wrong to speak of them," answered Monsieur Roger, pretendingto be vexed.
"That may be," answered Miss Miette, very calmly; "but as you did speakof them, you must tell us what they are."
"Yes, you must, my friend," remarked Monsieur Dalize, taking sides withhis little girl.
"Mustn't he, papa? mustn't Monsieur Roger explain?" asked Miette.
"Come, now," said Monsieur Roger, in a resigned tone. "You must know,then, that air is composed of two gases,--oxygen and nitrogen;therefore, when we breathe, we send into our lungs oxygen and nitrogen.You might think, when we throw out this air, when we exhale,--you mightthink, I say, that this air coming out of our lungs is still composed ofoxygen and of nitrogen in the same proportions. Now, it is not so atall. The quantity of nitrogen has not varied, but, in the first place,there is less oxygen, and there is another gas,--carbonic acid gas;where, then, is the oxygen which we have not exhaled, and whence comesthis carbonic acid which we did not inhale? Then, besides, in the airexhaled there is vapor. Where does that come from? These phenomenaresult from the combustion of which I speak; but, in order that youshould understand how this combustion occurs, I must explain to you whatis oxygen and what is nitrogen. And as it is a long story, you must letme put it off till this evening; then I will talk until you are weary,my dear little Miette."
Miette looked at Albert and Paul, and answered for them with remarkablefrankness:
"It will be only right if you do weary us. It is we who asked you, and,besides, we have so often wearied you that it is only right you shouldhave your revenge on us. Still----"
"Still, what?"
"Still, we can trust you," added Miette, laughing, and throwing her armsaround Roger's neck.