The History of a Mouthful of Bread. Jean Macé
close up to their hinge; particularly if you have no scruple about spoiling the scissors, by the way!
If you were a grown-up lad, and I were teaching you natural philosophy, I should have here a fine opportunity for explaining what is called the theory of the lever. But I think the theory of the lever would frighten you; so we must get out of the difficulty in some other way.
I find, however, that I have been joking so much as I went along, that I have but little space left, and feel quite ashamed of myself. We seem quite unlucky over these teeth.
I have already been scolded by people who are not altogether wrong in accusing me of losing my time in chattering, first of one thing and then of another. They complain that by thus nibbling at every blade of grass on the way-side we shall never get to the end of our journey; and there is some truth in what they say. Still, I will whisper to you in excuse that I thought we might play truant a little bit while we were on familiar ground, where naturally you were sure to feel a particular interest in everything. The hand, the tongue, the teeth—these are all old friends of yours—and I thought you would like to hear all about them. By-and-bye we shall be in the little black hole, and then we shall get on much more rapidly.
LETTER VI.
THE TEETH (continued).
I left off at the molars, which are the teeth one selects to crack nuts with; and if I remember rightly, we talked about different ways of cutting with scissors.
Let us look at the subject from a distance, that we may understand it more clearly. Let us imagine a horse drawing a heavy cart slowly along. Ask it to gallop, and it will answer, "With all my heart! but you must give me a lighter carriage to draw." And now fancy another flying over the ground with a gig behind it. Ask it to exchange the gig for the cart, and it will say, "Yes; but then I shall have to go slowly."
Whereby you see that with the same amount of strength to work with, one has the choice of two things: either of conquering a great resistance slowly, or a slight one quickly.
And it is partly on this account, dear child, that I teach you so gradually; for young heads, fresh to the work, are less easily drawn along than others, and have but a certain amount of strength.
Hitherto all has been clear as the day. Now take your scissors in your left hand; hold the lower ring of the handle firmly between your thumb and closed hand, so that the blade shall remain straight and immovable: then with your other hand cause the upper ring to go up and down, and watch the blade as it moves. The whole of it moves at once, and is put in motion by the same power—viz., your right hand. But the point makes a long circuit in the air, while the hinge end makes only a very little one—indeed, moves almost imperceptibly: and, as you may imagine, a different sort of effort is required from the motive power (your hand) according as resistance is made at the point or at the hinge. The point goes full gallop: it is the horse in the gig; the light work is for him. The hinge moves slowly; it is the cart-horse, and takes the heavy labor.
I hope I have made you understand this, for it explains the cracking of our nut, though you may not suspect it. Move your scissors once more in the same way. Now, you have before you the pattern of the two jaws on one side of your face, from the ear to the nose; the upper one, which never moves (as you may convince yourself by placing a finger on your upper lip when you either speak or eat), and the lower one which goes up and down. Two pairs of scissors set points to points give you the whole jaw. The incisors are at the points, they gallop up and down, and are worthless for doing hard work; the molars are at the hinges, and move slowly; and if anything tough has to be dealt with, it comes to them as a matter of course; hence they are the nutcrackers. You must own that it is pleasant to reflect thus upon what we are doing every day, and the next time you see a stonemason moving stones of twenty times his own weight with his iron bar, ask your papa to explain to you the principle of the lever. After what I have told you, you will understand it very readily, or at least enough of it to satisfy your mind.
But, besides this power of moving up and down, the lower jaw possesses another less obvious one, by means of which it goes from right to left. This is precisely what naughty children make use of when they grind their teeth: not that I mean this remark for you, for I have a better opinion of you than to suppose you do such things. Those who make such bad use of their jaws deserve to lose the power of ever moving them thus, and then they would find themselves sadly at a loss how to chew their bread—for their molars would be of but little service to them in such a case; as it is chiefly by this second action of the jaw that the food is pounded. Try to chew a bit of bread by only moving your jaw up and down, and you will soon tire of the attempt.
One word more to complete my description of the teeth: that portion of them which is in the jaw is called the root; and the incisors, which cannot work hard because, like the gig-horses, they have but little resisting power, possess only small and short roots; whereas the canines, whose duty it is to tear the food sideways, would run the risk of being dragged out and left sticking in the substances they are at work upon, if they were not well secured; these, therefore, have roots which go much deeper into the jaw, and in consequence of this they give us more pain than the others when the dentist extracts them: those famous eye-teeth, which so terrify people on such occasions, are the canines of the upper jaw, and lie, in fact, just below the eye.
The molars meanwhile would be in danger of being shaken in the sideway movement, while chewing: so they do as you would do if you were pushed aside. Now you would throw out your feet right and left in order to steady yourself, and thus the molars, which have always two roots, throw them out right and left for the same purpose. Some have three, some four, and they require no less for the business they have to do.
Above the root comes what is called the crown; that is the part of the tooth which is exposed to the air; the part which does the work, and which bears the brunt of all the rubbing. Now, however hard it may be, it would soon end in being worn out by all this fun if it were not covered by a still harder substance, which is called enamel. The enamel which forms the coating of china plates, and which you can easily distinguish by examining a broken plate, will give you a very exact idea of it. It is this enamel which gives the teeth the polish and brilliancy we so much admire, and it is desirable to be very careful of it, not out of vanity, though there is no objection to a little vanity on the subject, but because the enamel is the protector of the teeth, and when that is destroyed, you may say good-bye to the teeth themselves. All acids eat into the enamel, as vinegar or lemon-juice does into marble; and one of the best means of preserving this protecting armor of the teeth is never to eat the unripe windfalls of fruit, which I have seen unreasonable children pick up in orchards and devour so recklessly. They give sufficient warning, by their acidity, that they are not fit for food, and when this warning is neglected, they take their revenge by corroding the enamel of the teeth; not to speak of the disturbance which they afterwards cause in the poor stomach.
I said that without this coating of enamel, the teeth would be prematurely worn out, the reason of which is, that the teeth have not the property of growing again, as the nails and hair have. When those little germs of which I spoke when we began to describe the teeth, have finished their work, they perish and fall out, like masons who, when they have built the house, take their departure forever.
But the "forever" wants explanation. For such stern conditions would fall hard on very little children, who, not having come to their reason, cannot be expected to understand the great value of their teeth, and take all the care they need of them. So to them a second chance is given.
Your first teeth, the milk-teeth, as they are called, count for nothing: they are a kind of specimen, just to serve while you are very young.
When you are approaching what is called the age of reason, (and this word implies a great deal, my dear child,) the real teeth, the teeth which are to serve you for life, begin to whisper among themselves, "Now, here is a little girl who is becoming reasonable, and who will soon, or else never, be