THE DIFFERENT SHAPES OF TEETH
“Man has thirty-two teeth, sixteen to each jaw,” Uncle Paul continued.
Emile already had his finger in his mouth, passing it from one tooth to
another, to count them. His uncle paused until he had finished the
count.
“But I have only twenty, all told,” declared the boy; “twenty, and not
thirty-two.”
“The other twelve will come some day, my boy; at present you have the
right number of teeth for a child of your age. They do not all come at
one time, but one after another. We begin with twenty, and no more.
They are called milk teeth, or first teeth. When we are about seven
years old they begin to fall out and are replaced by others stronger
and set in more firmly. In addition to this second score of teeth there
appear later twelve others, bringing the total number up to thirty-two.
Those farthest back, in the inmost cavity of the mouth, come late, when
we are eighteen or twenty years old, or even older, for which reason
they are called wisdom teeth to signify that they appear at an age when
the reason is well developed. These thirty-two last teeth constitute
the second cutting. I call them last because they are never replaced by
any others; if we lose them, that is the end of our teeth; no more will
come.”
“I have two now that are loose,” said Emile.
“They must come out soon to leave room for the new teeth that are to
take their place. The others will get loose, too, and the twenty that
you have now will be succeeded by twenty others, to which, sooner or
later, will be added twelve more which come only once. These last
occupy the back part of the jaws, three on each side, top and bottom.
Thus the final number will be thirty-two.
“These thirty-two teeth are divided into three classes according to
their shape and the work they must do. The same names being repeated
top and bottom and right and left, I show you merely the eight teeth of
half a jaw. In every tooth there are two parts to be distinguished, the
crown and the root. The root is the part that is embedded in the
jaw-bone like a nail hammered into wood; the crown is the part that
comes into view, and it may be likened to the head of a nail. The root
holds the tooth in place, fixes it firmly; the crown cuts, tears, and
grinds the food.
“In the two front teeth in each half-jaw the crown grows thinner toward
the top. The edge is straight and sharp, fitted for cutting food,
dividing it into small mouthfuls. Therefore these teeth are called
incisors, from the Latin incidere, meaning to cut. Their root is a
simple pivot. The next tooth is called canine. Its root is a little
longer than those of the preceding teeth, and its crown is slightly
pointed. The dog, the cat, the wolf, and carnivorous animals in general
have this tooth shaped like a powerful fang, which serves to catch and
hold prey, but above all acts as a weapon in attack and defense. It is
the canine teeth that you see crossing one another, long and pointed,
two on each side, when you raise the upper lip of a cat or a dog.
Because of these remarkable fangs of carnivorous animals, especially
the dog, which in Latin is canis, the name canine has been given to the
teeth that in man are like them, if not in form and use, at least in
the position they occupy.
“The next five teeth are the most useful of all. They are called
molars, from the Latin mola, a millstone, because they play the part of
millstones in grinding the food. For this purpose their crowns are
blunt and broad and slightly irregular, not flat like the horse’s
molars or with sharp cutting edges like the wolf’s, because man’s food
is not composed exclusively of either vegetables or flesh, but of both
at the same time. For food as varied as man’s there is need of molars
fit for all sorts of service: they must grind like those of the
herbivorous animals and cut like those of the carnivorous; in short,
they must be like those of both. And, indeed, their wide crowns are
suited to vegetable food, and their rather sharp irregularities are
adapted to animal food.
“The first two are called little molars or, in more learned language,
bicuspids, because they have each two cusps or points. They are the
least strong of the five and have only one root each. The two little
molars, the canine tooth, and the two incisors (of each half-jaw) are
the only teeth that are renewed. Multiply them by four and you will
have the twenty teeth of the first cutting, teeth that begin to fall
out toward the age of seven and are gradually replaced by others. That
is the state of Emile’s teeth at present, there being but twenty of
them.
“The other three teeth, in each half-jaw, come only once. They are the
large molars, of which the very end one is also called the wisdom
tooth. As in the act of mastication the large molars have to bear
strong pressure, the root is composed of several pivots or prongs
reaching down each into a special cavity or socket. This makes them
strong and firm, so that they can stand pressure both downward and
sideways.
“To sum up, the grown man has thirty-two teeth in all, sixteen to each
jaw; namely, four incisors, two canines, and ten molars. These last are
divided into four bicuspids or little molars and six large molars; the
milk teeth do not include these last six.”
Here Jules had a question to ask. “Ivory and enamel,” said he, “those
two substances of different degrees of hardness that you told us were
arranged in such a wonderful way in horses’ and wolves’ teeth—are they
in our teeth, too?”
“Yes, they are there. Ivory forms the entire root, which must serve as
a firm support, and it also fills the crown, while enamel merely covers
the outside as a hard protecting layer.”
“I am going to get the cat and look at her teeth,” said Emile. “Has she
twenty, like me, or has she thirty-two?”
“Neither twenty nor thirty-two, but thirty when full-grown. Dogs and
wolves have forty-two; horses and donkeys forty-four. In fact, the
number varies with different animals as much as the shape. Perhaps a
few words on this subject will not be out of place.
“First, here is the picture of a wolf’s mouth. If one did not already
know, one could easily guess the animal’s diet by merely looking at its
teeth. Those deeply indented molars, those strong, curved
canines—surely they call for wild prey and show great strength. The
whole set indicates clearly enough a carnivorous appetite. At i are the
incisors, six in number. They are small and of slight use, for the
animal does not cut its prey into little mouthfuls, but swallows it
gluttonously in great strips. At c are the canines, veritable daggers
which the bandit plunges into the sheep’s neck. The little molars are
at m. The large molars come next. The first, marked r, is the
strongest, and it is with this that the wolf and the dog crack the
hardest bones. Finally, the picture shows the salivary glands; that is,
the organs that prepare the saliva and let it ooze into the mouth
through the canal s as the animal eats. Without dwelling on this point,
which would take me too far from my subject, I will merely say that
saliva serves to soak the food and make a soft mouthful that can be
easily swallowed, and it also plays an important part in the stomach in
reducing to a fluid pap the food taken in; that is to say, it helps to
digest the food.
“Let us pass on to the cat, another typical flesh-eater. Six small
incisors are ranged in the front of the jaw like a row of elegant but
useless pearls. They are ornamental rather than useful to the animal. A
mouse-hunter needs very long and pointed canines for piercing the prey
seized by the claws. In this respect the cat is armed in a very
formidable manner. What do you think of it, Louis?”
“I think,” he replied, “a rat must be very uncomfortable between those
curved canines the picture shows us.”
“One day,” said Emile, “when I was pulling the cat’s mustache, she gave
me a bite that felt like the sharp prick of a needle. It was done so
quickly I had no time to draw my hand back.”
“The cat brought her canines into play and wounded you with one of them
as quickly as a steel point could have done.
“Now look at the molars. There are four above, the last one very small,
and three below. Their cusps or points are still sharper than the
wolf’s; so, too, the cat’s appetite—like that of its kindred, the
tiger, the panther, the jaguar, and others—demands more flesh than that
of the wolf and animals like it, such as the fox, the jackal, and
especially the dog. Have you ever noticed how disdainful the cat is
when you throw her only a piece of bread? Scarcely has she smelt it
when she makes a movement of superb scorn, tail in the air, back
raised, and looks at you as if to say: ‘Are you making fun of me? I
want something else.’ Or, if very hungry, she reluctantly bites the
bread, chews it awkwardly, and swallows it with distaste. The dog, on
the contrary, our good Azor for example, catches the bread joyfully in
his mouth without letting it touch the ground, and if he finds any
fault with the piece it is for being too small. You call the cat a
glutton. I take her part and maintain that it is not the vice of
gluttony she shows, but that her teeth must have meat. What could you
expect her pointed canines and keen-edged molars to do with a crust of
bread? They demand, above all, a prey that bleeds, a quivering bit of
flesh.
“What a difference between the teeth of the hunter and those of the
peaceful chewer of grass! Let us examine this picture of a horse’s
head. Here the incisors, six in number, are powerful; they seize the
forage and cut it, a mouthful at a time. The canines, of no use here,
show only as little knobs on the jaw-bone. Next beyond comes a long
vacant space called the bar; that is where the bit is held in the
horse’s mouth. Back of the bar you see the real grinding mechanism,
composed of twelve pairs of strong molars with square, flat crowns
furnished with slightly projecting folds whose usefulness I have
already pointed out to you. If I am not much mistaken, here we have a
mill capable of grinding tough straw and fibrous hay.
“Finally, here is a rabbit’s head. Each jaw is furnished with two
enormous incisors set deep into the bone, bent backward above, and
ending each in a sharp-edged crown. What are such incisors as those
made for?”
“I know,” Jules quickly replied. “The rabbit is always nibbling. For
want of better food it will gnaw the bark of a tree and even the wood.
It uses its incisors to cut its food very fine, to gnaw it.”
“To gnaw it—that is the right word; hence we give the name of rodents
or gnawers to the various animals having incisors of that kind. Such
are the squirrel, the hare, the rabbit, the rat, and the mouse, those
poor creatures which must gnaw the toughest vegetable substances and
fill their bellies with wood, paper, rags even, when there is nothing
better for supplying the mill that is kept always going. But it is not
merely to satisfy their hunger that these animals are almost
incessantly gnawing; there is another reason for their doing it. Their
incisors grow all their lives and tend to lengthen indefinitely;
consequently, the animal must wear them away by continual friction, as
otherwise their crowns would at last so far overlap that they could not
be made to meet. Then the poor beast would be unable to seize its food
and would perish. In order to be able to eat when hungry, the rat and
the rabbit must eat when not hungry, so as to sharpen their incisors
and keep them the right length. It is true that they often turn their
attention to very poor fodder. A splinter of wood, a straw, a mere
nothing suffices to maintain the play of their indefatigable incisors.
Remember, children, the expressive term rodents (which means gnawers),
applied to a whole class of animals akin to the rabbit and the rat;
remember their curious incisors, for we shall have occasion to speak of
them again hereafter. For the present let us finish our examination of
the rabbit’s teeth.
“The canines are lacking; in their place the jaw shows a bar or, in
other words, a large open space. At the extreme back of the mouth are
the molars, few in number but strong, with flat crowns and several
folds of enamel. In fact, they make an excellent grinding machine.
“In giving you these details concerning the different shapes of teeth
in different species of animals, I wished particularly to point out the
following truth: Each species eats a particular kind of food for which
the teeth are especially formed, so that one might say of any animal,
‘Show me its teeth and I will tell you what it eats.’ In many instances
where we cannot examine the teeth we do not know what such and such a
creature feeds on, and in our hasty judgment we mistake a friend for an
enemy, a helper for a destroyer. If the animal is ugly we condemn it on
the spot and hate it, accusing it of any number of misdeeds. We declare
war against it, and never suspect, in our foolishness, that it is a war
at our own expense. But there is a very simple precaution by which we
can avoid these regrettable mistakes: let us yield to no prejudice,
however wide-spread, and before condemning an animal as harmful let us
find out what sort of teeth it has. They will tell us the animal’s way
of living, as you shall soon see for yourselves.”