The History of a Mouthful of Bread Part 4

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 fit to take charge of her teeth." No sooner said than done: other masons set to work in other cells, placed under the first set, and as the permanent teeth keep growing and growing, they gradually push out the milk-teeth, which were only keeping their places ready for them till they came.

This is just your case at present, and you now understand your responsibility, and how necessary it is to preserve those good teeth which have placed so generous a confidence in your care of them, and which, once gone, can never be replaced.

You have no loss by the exchange; you had twenty-four at first, you will now have twenty-eight. Twenty-eight, did I say? nay, you will have thirty-two; but the last four will come later still. The last _molars_ on each side, above and below, in both jaws, will not make their appearance till you are grown up. They are a fastidious and timid set, and will not run any risks; and they are called _wisdom-teeth_, because they do not appear till we are supposed to have arrived at years of discretion. Some people do not cut them before they are thirty, and you will agree that, if they have not become wise by that time, they have but a very poor chance of ever being so!

There is much more still to be said about the teeth; but I think I have told you quite enough to teach you the importance of these little bony possessions of yours, which children do not always value as they deserve, and whose safety they endanger as carelessly as if they had fresh supplies of them ready in their pockets. If so many skilful contrivances have been devised for enabling us to masticate our food properly, it is clear that this process is not an unimportant one.

Those, therefore, who swallow a mouthful after two or three turns, forget that they are thereby forcing the stomach to do the work the teeth have neglected to do, and this is very bad economy, I can a.s.sure you. You will see hereafter, when we speak about animals, that by a marvellous compensation of nature, the power of the stomach is always great in proportion to the _in_efficiency of the teeth, and that by the same rule, it is weakest when the jaws are best furnished. Now, no jaw is more completely furnished than the human one; it is clear, then, that it should do its own work and not leave it to be done by those who are less able: and the little girl who, in order to finish her dinner more quickly, shirks the use of her teeth, and sends food, half chewed, into her stomach, is like a man who, having two servants, the one strong and vigorous, the other feeble and delicate, allows the first to dawdle at his ease, and puts all the hard work on the other.

He would be very unjust in so doing, would he not? And as injustice always meets with its reward, his work is sure to be badly done.

Now, the work in question consists in reducing what we eat into a sort of pulp or liquid paste, from which the blood extracts at last whatever it requires. But the teeth may bite and tear the materials as they please, they can make nothing of them but a powder, which would never turn into a pulp, if during their labors they were not a.s.sisted by an indispensable auxiliary. To make pap for infants what do we add to the bread after it is cut in little bits? Without being a very clever cook, you will know that it is water which is wanted. And thus, to a.s.sist us in making pap for the blood, Providence has furnished us with a number of small spongy organs within the mouth, which are always filled with water. These are called _salivary glands_. This water oozes out from them of itself, on the least movement of the jaw, which presses upon the sponges as it goes up and down. The name of this water, as I need scarcely tell you, is _saliva_.

When I call it water, it is not merely from its resemblance; _saliva_ is really pure water with a little _alb.u.men_ added. Do not be afraid of that word--it is not so alarming as it appears to be. It means simply the substance you know as the _white of egg_. There is also a little soda in the water, which you know is one of the ingredients of which soap is made. And this explains why the saliva becomes frothy, when the cheeks and tongue set it in motion in the mouth while we are talking; just as the whites of egg, or soapy water, become frothy when whipped up or beaten in a basin.

But the alb.u.men and the soda have not been added to the saliva, in our case, merely to make it frothy; that would have been of very little use. They give to the water a greater power to dissolve the food into paste, and thus to begin that series of transformations by which it gradually becomes the fine red blood which shows itself in little drops at the tip of your finger when you have been using your needle awkwardly.

When once minced up by the teeth and moistened by the saliva, the food is reduced to a state of pulp, and having nothing further to do in the mouth, is ready to pa.s.s forward. But getting out of the mouth on its journey downwards is not so simple an affair as getting into it by the _front door_, as it did at first. Swallowing is in fact a complicated action, and not to be explained in half a dozen words, and I think we have already chatted enough for to-day. I only wish I may not have tired you out with these interminable teeth! But you may expect something quite new when I begin again.

LETTER VII.

THE THROAT.

You remember a certain door-keeper, or porter, of whom we have already spoken a good deal, who resides in the mouth--the sense of taste, I mean?

Well, it is a porter's business to sweep out the entrance to a house, and you may always recognize him in the courtyard by his broom.

And accordingly our porter too has a broom specially placed at his service, namely, the tongue; and an unrivalled broom it is--for it is self-acting, never wears out, and makes no dust--qualities we cannot succeed in obtaining in any brooms of our own manufacture.

When the time has come for the pounded mouthful (described in the last chapter) to travel forward (the teeth having properly prepared it), the broom begins its work; scouring all along the gums, twisting and turning right and left, backwards and forwards, up and down; picking up the least grains of the pulp which have been manufactured in the mouth; and as the heap increases, it makes itself into a shovel--another accomplishment one would scarcely have expected it to possess. What it gathers together thus, rolls by degrees on its surface into a ball, which at last finds itself fixed between the palate and the tongue in such a manner that it cannot escape; at which moment the tongue presses its tip against the upper front teeth, forms of itself an inclined plane, and--but stop! we are getting on too fast.

At the back of the mouth, (which is the antechamber, as we said before,) is a sort of lobby, separated from the mouth by a little fleshy tongue_let_, suspended to the palate, exactly like those tapestry curtains which are sometimes hung between two rooms, under which one is enabled to pa.s.s, by just lifting them up.

If this lobby led only from the mouth to the stomach, the act of swallowing would be the simplest thing in the world; the tongue would be raised, the pounded ball would glide on, would pa.s.s under the curtain, and then good-bye to it. Unfortunately, however, the architect of the house seems to have economized his construction-apparatus here.

The lobby serves two purposes; it is the pa.s.sage from the mouth to the stomach, as well as from the nose to the lungs.

The air we breathe has its two separate doors there--one opening towards the nose, the other towards the lungs; through neither of which is any sort of food allowed to pa.s.s. But, as you may imagine, the food itself knows nothing of such spiteful restraints, and it is a matter of perfect indifference to it through which of the doors it pa.s.ses.

Not unlike a good many children who, though they are reasonable creatures, will push their way into places where they have been forbidden to go; and who can expect a pulpy food-ball to be more reasonable than a child? It was necessary, therefore, so to arrange matters that there should be no choice on the subject; that when the food-ball got into the lobby it should find no door open but its own, namely, that which led to the stomach. And that is exactly what is done.

You have not, perhaps, remarked that in the act of swallowing, something rises and contracts itself at the same moment in your throat, producing a kind of internal convulsion which jerks whatever is inside. People do not think about it when they are eating, because it is an involuntary action, and their attention is otherwise engaged.

But try to swallow when there is nothing in your mouth, and you will perceive what I mean at once.

Now, imagine our lobby at the back of the throat as a small closet, with a doorway in its wall, half-way up, the doorway being closed by a curtain. In the ceiling is a hole, which leads to the nose; in the floor two large tubes open out; the front one leading to the lungs, the one behind, to the stomach.

Now swallow, and I will tell you what happens. The curtain rises up and clings to the ceiling, and thus the pa.s.sage to the nose is stopped up. The lung-tube rises along the wall, and hides itself under the door, contracting itself, and making itself quite small, as if it wished to leave plenty of room for the mouthful of food which is about to pa.s.s over it; and, for still greater security, at the very moment it rises, it pushes against a small trap-door which shuts up its mouth.

No other road remains, therefore, but through the tube which leads to the stomach; the pulpy mouthful drops straight therein, without risk of mistake, and when it is once there, everything readjusts itself as before.

These are very ingenious contrivances, and I will venture to say that if we would but study the wonders of the marvellous and varied machinery which is constantly at work in our behalf within us, we should be much better employed than in learning things from which no practical good can be derived. Moreover, we should be ashamed to trust, like the lower animals, only to our instinct, (which, after all, is much less developed in us than in them,) for blindly escaping the thousand chances of destruction that beset a structure so fragile and delicate in its contrivances as the human body. Besides, it is not only our own machinery that is entrusted to us, we are liable to be responsible for that of others, whose development it is our duty to guard and watch; and how can we do this with a safe conscience, if we are ignorant of the construction, the action, the laws of all sorts which the great Artificer has, so to speak, made use of in forming our bodies?

When you, in your turn, are a mother, you dear little rogue, who sit there opening wide your bright eyes, and not comprehending a word of what I am saying, you will be glad that you were taught when you were little, how your own little girl ought to be managed. You will find a hundred opportunities of making good use, in her behalf, of what you and I are learning together, and in the meantime there is no reason why you should not yourself profit by the knowledge you have gained.

I am quite sure, for instance, that in repeating to your child the simple rule of politeness, with which everybody is acquainted, "_Never talk when you are eating_," you will be very careful to add, "_and especially when you are swallowing_," for reasons I am about to detail.

When we want to speak we have to drive the air from the lungs into the mouth, and our words are sounds produced by this air as it pa.s.ses through. This is the reason why I advise you to go on gently, and make the proper stops in reading aloud: to _take breath_, in fact, as it is called; otherwise, breath would all at once fail you, and you would be obliged to stop short in the middle of a sentence and wait like a simpleton till you had refilled the lungs with air by breathing.

It was for this purpose, also, and not for mere economy's sake, as you may have thought, that the little cross-road of four doors has been placed at the back of the mouth, enabling it to communicate at pleasure with either the lungs or the stomach. It is a dangerous pa.s.sage for food-parcels making their way to the stomach; but if you could subst.i.tute for it, as it may have occurred to you to do lately, a simple tube going directly to the stomach,--behold! you would find yourself dumb;--a serious misfortune, eh? for a little girl! But come, I am quizzing too much, so console yourself. I know many grown-up people who would be at least as sorry as yourself.

To return to our subject. We have said that, in order to guard against accidents, the lung-tube is closed at the moment we are about to swallow. But if by any unlucky chance the air is coming up from the lungs at the same moment, it must have a free pa.s.sage. Its tube cannot help returning to its place; the little trap-door which shuts up the opening opens whether or no, and then adieu to all the precautions of good Mother Nature! The mouthful when it drops, falls outside of its proper tube--that is to say, into the other, which is exactly in front of it, and we find that we have _swallowed the wrong way_.

You know what happens in such a case. You cough and cough till you are torn to pieces, till you grow scarlet, or even blue in the face; till you lose your breath; till your body trembles; till your eyes start out of their sockets. Let who will be there, there is no resource but to hide your face in your handkerchief. The tube, which was only made for the pa.s.sage of air, on finding an intruder forcing an entrance, does its utmost to drive it back through the door. Then the lungs, which would be destroyed by its getting to them, come to the a.s.sistance of the faithful servant who is struggling for their protection: they agitate themselves violently, and send forth gusts of air which drive all before them. Thence arises the cough, and by this means at last the enemy is thrust out of the mouth, like dust before the wind. And it is only when the pa.s.sages are cleared that the storm subsides. But the commotion is no laughing matter, I a.s.sure you; for if one had swallowed a little _too far_ the wrong way, or if the substance swallowed had been too heavy for the air-tube, aided by the lungs, to eject within a certain time, death would have ensued: instances of which are by no means unknown. Nature does nothing in vain; this is no case of a man frightened by a mouse. When you find your whole being concentrating its efforts to one point, and betraying such distress, at an accident apparently so trifling, you may be sure there is danger, and real danger too; and if you doubt it, that makes no difference--happily for you.

Now you have learned why little girls should not attempt to talk and swallow at the same time, and, I may add, still less laugh; for laughingis a kind of somersault, performed by the lungs, and is always accompanied by the ejectment of a great deal more breath than is necessary in speaking, so that the jerks it occasions derange still more the wise provisions made to protect life whenever we swallow anything, and therefore we are more apt to swallow the wrong way while laughing than while speaking.

Need I say that we ought equally to guard against making others laugh or talk; or exciting, or frightening them, while they are swallowing; in short, avoid doing anything to create a sudden shock which might suddenly force the air out of their lungs, and cause them in the same manner to swallow the wrong way? Politeness requires this from us, and what I have now said will fix the lesson still more strongly on your mind. What would become of you if you were to see a person die in your presence in consequence of some foolish joke, however apparently innocent?

Not to conclude with so painful a picture, I will, before we part, give you the right names of the _curtain_, the _lobby_ or _closet_, and the _tubes_ of which we have been speaking.