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 assure
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 assisted 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 assist
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 _albumen_ 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 albumen 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 pass 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.