In its natural position it bulges upwards in the middle, like a cloth swollen out by the wind, and thus occupies a portion of the chest at the expense of the lungs. When air has to be admitted, its fibres tighten and bring it flat again, as you and your brother brought the handkerchief flat just now by tightening it.
The whole space previously occupied by the arch of the diaphragm is thus given up to the lungs, which, being elastic, instantly stretch themselves out to it; while air, running in through the nose and mouth, fills up in proportion the empty place (vacuum) created by the extension of the lungs, exactly as in the case of the bellows.
But soon the fibres of the diaphragm relax. It rises up again into its old position, driving back the lungs as it does so; and the air finding there is now no room for it, goes out by the same way the other came in. I say the other, observe, because the air that goes out is no longer the same as when it came in; and this is the secret of why we breathe; while the up and down movement of the diaphragm is the explanation of how we breathe.
As you perceive, then, the mechanism of these bellows of ours, is of the most simple, and consequently of the most ingenious character, and leaves far behind it anything we have ever imagined.
Are you disappointed? Do you feel inclined to exclaim, "Is this all?" to ask where are the wonders I promised you? to protest that I may talk as I please about the inflating and flattening of a pocket-handkerchief? you can see nothing so marvellous in the matter; nothing worth making your mouth water for.
A little patience, Mademoiselle! Hitherto we have talked only of the machine; but there is a goblin inside it, and our fairy tale is going to begin again.
There are in some families certain old servants who belong to the house, more, it may be said, than their masters, in some ways. They educate the children, and they serve them till death; they live for them alone, and know so well what they have to do, both by day and night, that there is no need to give them any orders. Nay, not only is it unnecessary to give them directions—it is for the most part labor in vain. They are so completely at home in their business, that they will go nobody's way but their own. If you wish them to alter their habits they may obey you for an instant, but it is only to return into the old groove directly after; for they know better than you do what you want.
I was very little when I first read in the story-books of my day, some bitter complaints of the disappearance of this race of old-fashioned servants of the good old times. And you very likely may have seen it said that they are no longer to be met with. Yet there will always be some, depend upon it, in families, who know how to make and to keep them. Good old times or not, they have never been found in any other but these cases.
Still, I have just such a one as I have described—even I who am talking to you—and so has your mamma; and what is more, you have one yourself; and what is more still, everybody else has one. This servant of the good old times, who will never disappear (and this is more than one can promise of any other) is the Diaphragm! When you came into the world, my dear child, and were merely a poor little lump of flesh, without strength, intelligence, or will; incapable of giving any orders whatever to those organs of yours, of whose existence you were not even aware, your diaphragm quietly began his duties, without leave or inquiry from you, and with your first breath your life began. Since which he has always gone on, whether you attended to him or not, and his last effort will be your last sigh.
When you go to sleep, careless of all that is to happen, until you awake again, that servant of yours, indefatigable at his post, labors for you still, and the light breath which half opens your rosy little lips as it passes through them; that light breath which your happy mother watches with such pleasure, is his work. Midnight strikes—one o'clock—two; all around you are buried in sleep—but he is awake still. Were it otherwise—were he to go to sleep when you do, you would never awake again!