TEETH
“Is it not true,” resumed Uncle Paul, “that each kind of work demands
its own special tool? The plowman must have the plow, the blacksmith
the anvil, the mason the trowel, the weaver the shuttle, the carpenter
the plane; and these different tools, all excellent for the work to
which they are applied, would be of no use in any other. Could the
mason rough-cast his wall with a shuttle? Could the weaver weave his
cloth with a trowel? Evidently not. Is it not true, then, that from the
tool one may easily guess the kind of work it does?”
“Nothing could be easier, it seems to me,” replied Jules. “If I see
planes and saws hanging on the wall, I know that I am in a carpenter’s
shop.”
“And I should know,” said Emile, “from seeing an anvil, a hammer, and a
pair of tongs, that I was in a blacksmith’s shop. But if I saw a
mortar-board and a trowel, I should look around for the mason.”
“Well,” Uncle Paul went on, “every creature has its special task in
creation’s great workshop, where all take part, all work, according to
the design of Divine Wisdom. Each species has its mission—I might say
its trade to follow—a trade that requires special tools just as does
any work done by man. Now, among the innumerable trades of animals
there is one that is common to all without exception, the most
important trade of all, as without it life itself would be impossible:
it is the business of eating.
“But all animals do not take the same kind of food. Some need prey, raw
flesh, others fodder; some eat roots, others seeds and fruit. In every
instance teeth are the tools used in the work of eating; so they must
have the shape appropriate to the kind of food eaten, whether that be
tough or tender, hard or easy to chew. Therefore, just as from his tool
the artisan’s work may be inferred, so from the shape of its teeth one
can usually tell the kind of food eaten by any animal.
“Herbivorous animals are those that live on grass, fodder, hay; and
carnivorous animals are those that eat flesh. The horse, the donkey,
the ox, and the sheep are herbivorous; the dog, the cat, and the wolf,
carnivorous. The food of the herbivorous animal is tough, hard,
fibrous, and must be ground for a long time by the teeth in order to be
reduced to a paste-like mass suitable for swallowing and, after that,
for easy digestion. In this case the teeth in both upper and lower jaw
must have broad and almost flat surfaces that will come together and
grind the food as millstones grind grain. On the other hand, the flesh
eaten by the carnivorous animal is soft, easy to swallow, and easy to
digest. All that the animal has to do is to tear it apart and cut it
into shreds. So the teeth here must have sharp edges that come together
and operate like the blades of a pair of scissors.
“I think I have said enough on that subject. Now, which of you will
tell me what kind of food goes with each of the teeth I show you here?”
And Uncle Paul laid before his hearers the two teeth pictured on these
pages, with others to follow.
“The first tooth,” said Emile, “is flattened and very wide at the top;
it must crush and grind by rubbing against a tooth of the same kind in
the opposite jaw. So it is the tooth of an animal that eats fodder.”
“It is indeed,” Uncle Paul replied, “the tooth of an herbivorous
animal, a horse.”
“The second,” continued Emile, “is composed of several broad points
with edges almost as sharp as knife blades. It must be meant for
cutting flesh.”
“Those winding folds that you see in the horse’s tooth—what are they
for?” asked Jules. “There is nothing like them in the wolf’s tooth.”
“I was going to tell you about them,” his uncle replied. “If the
horse’s teeth had perfectly smooth surfaces, without any roughness to
act as a grater, is it not true that in pressing and rubbing, each
against the opposite tooth, they would simply crush the fodder or hay
as you would crush it between two smooth stones without changing it
into fine powder? Millstones, if they were polished like marble tables,
would flatten the grain without making flour of it; they must be rough
on the surface in order to seize the wheat during the grinding of the
upper stone on the stationary lower one and to make it into powder.
When by long use the surface is worn smooth, the stones are of no
service until they are dented again with the hammer. Well, the folds of
a horse’s teeth may be likened to the roughness of a millstone: they
project a little above the general surface of the tooth, making a sort
of coarse file that tears to pieces blades of grass or hay when rubbed
by the opposite tooth.”
“I think I see a danger threatening the herbivorous animal,” put in
Jules at this point. “Those projecting folds must soon be worn down by
rubbing against one another, just the same as the roughness on the
millstone. If smooth millstones can’t make flour without being
roughened again, no more can the herbivorous animal’s worn teeth go on
grinding.”
“That is provided for, admirably provided for, my boy. Everything in
the world is arranged so that it can do its work: a wisdom that nothing
escapes watches over the smallest details; everything, even to a
donkey’s jaw, shows this to be so. Listen, and judge for yourselves.
“There are two different substances in a tooth: one very hard, a little
like glass and called enamel; the other quicker to wear out, but very
difficult to break, and known as ivory. These two substances are
combined in different ways, according to the animal’s diet. In the
horse, the sheep, the ox, the donkey, and many other herbivorous
animals the ivory makes up the main part of the tooth, while the harder
substance, the enamel, extends in winding sheets throughout the former,
projecting a little above its surface in a fold which varies in form in
the different kinds of animals. So, then, it is the enamel, a substance
as hard as a pebble, that composes the folds in the herbivorous
animal’s teeth. From the rubbing of the lower teeth against the upper
the ivory wears away faster than the enamel, so that the folds of the
latter embedded in the mass of the tooth have their cutting edges
brought above the general level as fast as required, and thus the
grinding surfaces are kept in constant repair. You see how it is: in
the donkey’s food-mill, for instance, the millstones re-roughen
themselves as fast as necessary for the chewing of a thistle; the
machinery is self-repairing even while at work.”
“What you tell us, Uncle, is wonderful,” commented Jules. “I never
should have guessed that such an arrangement was necessary for chewing
a thistle.”
“And only the other day,” put in Louis, “I kicked out of my way a
jaw-bone that was lying in the road. How gladly should I have looked at
it closely if I had known all these things!”
“Ignorance always kicks things aside like that, my boy, but science is
interested in everything, knowing that it can always learn something.
But let us return to the teeth of the carnivorous animals and examine
those of the wolf.
“Here the irregularities of the nutmeg-grater, the parallel ridges of
the file, and the roughness of the millstone would be of no use, since
the animal’s food is to be torn into shreds and not chewed into paste.
For the wolf’s food cutting blades are needed—sharp scissors which are
hard enough not to become blunt. Hence the working edges of the wolf’s
teeth are not flat like millstones, but shaped rather like pointed
chisels. The ivory forms the central body of the tooth, making it tough
and strong, while the enamel, harder but more brittle, is spread as a
continuous layer over the tooth and furnishes the requisite cutting
edges. In like manner a skilful cutler, when he wishes to make an edged
tool that will cut well and at the same time withstand violent blows,
makes its central mass of iron, a tough material that bears
considerable violence without injury, but is not hard enough to furnish
a keen cutting edge. He then overlays it, to obtain such an edge, with
fine steel, which combines excessive hardness with the fragility of
glass. The best that man can contrive in the making of edged tools is
met with in perfection in the teeth of carnivorous animals.”
“If I understand you, then,” said Jules, “ivory, which is not so hard
as enamel, but less brittle, forms the interior of the teeth of
carnivorous animals, and enamel, which is harder and more brittle,
forms the outside layer. Ivory makes the teeth strong; enamel makes
them cut.”
“Yes, that is it.”
“Now, I don’t know which is the more wonderful, the donkey’s or the
wolf’s set of teeth.”
“Both are wonderful, as both are admirably adapted to the kind of work
they have to do.”
“What surprises me most,” Emile interposed, “is that a lot of things we
should never pay any attention to turn out to be very interesting when
Uncle Paul explains them to us. I never should have thought that the
time would come when I should listen with pleasure to the history of a
tooth.”
“Since that interests you,” said Uncle Paul, “I will continue the
subject a little further and will tell you about human teeth, about
yours, my boy, so white and so well arranged, and so admirably adapted
for biting a slice of bread and butter.”