Encyclopaedia Britannica > Volume 3, Anatomy-Astronomy

106
ANATOMY.
Compara¬
tive
Anatomy.
Simple
teeth.
Compound
teeth.
Semiconi'
pound
t-eth.
The ce¬
ment or
crusta
petrosa.
the tooth. The hardness of this substance may be in¬
ferred from the fact that it strikes fire with steel. These
component fibres, however, are not always rectilineal.
Most frequently they describe curves with the convexity
of incurvation towards the crown and the concavity to¬
wards the root. This arrangement at least is observed
in the ruminants. The distinction between the enamel
and bony matter is recognised by a gray line, and another
whiter which belongs to the latter substance..
The enamel varies chiefly in thickness in different ani¬
mals. The tusks which project from the mouth are ge¬
nerally observed to be less white, less hard, and more
similar to bone or ivory than the other teeth ; and on this
account, probably, the existence of enamel has been de¬
nied in the tusks of the elephant. It is nevertheless cer¬
tain that the external layer of these tusks presents radiat¬
ing fibres, though it is by no means so hard, or possesses
the same grain, as the enamel of the other teeth. Enamel
is more apparent, though thinner, in the tusks of the
morse, dugong, and boar; and it is quite as distinct in
those of the hippopotamus as in the other teeth of that
animal. Lastly, the enamel of the teeth of the cachalot,
which is very thick, shows in its section only strice paiallel
to the surface of the osseous substance.
Teeth may be distinguished according to the mode in
which their component tissues are arranged into three
sorts. ls£, When the enamel invests the axis all round,
and does not penetrate the latter, the tooth is said to
be simple (dens simplex). Such is the character of the
human teeth, and those of the Quadrumana and Zoo-
phaga, and several other animals, and all the reptiles.
2d, When the enamel is folded as it were round the bony
part, but without inclosing it, so that the latter forms a
continuous band several times folded on itself, and sec¬
tions of the tooth in every direction divide repeatedly the
component substances, the tooth is said to be compound
or complex (dens multiplex vel compositus). A good exam¬
ple of this structure is seen in the grinders of the ele¬
phant. 3c?, When the base or root of the tooth is simple,
and the folds of the enamel and bone penetrate only to a
certain depth, they are said to be semicompound. Ex¬
amples of this modification of arrangement are seen in
the grinders of the ruminating animals.
In the compound, and part of the semicompound teeth,
the enamel is covered by a third substance; and as the
latter is arranged, especially in the former sort, so as to
leave intervals between it and the next layer of enamel,
this substance serves to fill all these intervals, and conso¬
lidates the component lobes of the tooth even before their
osseous parts are united below. This substance, which is
denominated by Cuvier cement, by Tenon cortex osseus,
and by Blake crusta petrosa, though less firm than either
bone or enamel, is dissolved by acids more slowly than
the former, and sooner becomes black in the fire. In the
teeth of the elephant and cabiai it forms half their mass
at least. In most genera it presents no apparent organiza¬
tion, and resembles a sort of crystalline tartar incrusted
on the tooth. In the cabiai, however, it presents nume¬
rous pores very regularly arranged. Tenon was of opi¬
nion that it arose from ossification of the membrane which
enveloped the tooth ; blake ascribed it to deposition from
the opposite surface of the enamel membrane; but Cuvier
ascertained that it is deposited by the same membrane
and the same surface as the enamel. This accurate
observer found, on inspecting the germs ot the teeth
of the elephant, that when the internal membrane of the
dental capsule has deposited the enamel, it undergoes a
change of structure, and becomes thick, spongy, opaque,
and reddish, to furnish the cement, which is then deposit¬
ed, not in regular crystalline fibres, but in random drops.
The teeth of the Reptiles consist of hard, compact, os- Compara.
seous matter, invested by a thin covering of enamel, and
without cement. .
The teeth of Fishes vary much m structure. They are Teeth of
either simple or compound. The simple teeth are those gshe&
which consist of bone invested by enamel. They may be
distinguished into two species, according to their mode of
attachment. The first are the simple teeth, which are not
implanted in alveoli, but merely attached to the gum, or
fixed by articulation to the jaw, as those of the shark
tribe ; the second are simple teeth growing m alveolar
cavities, as is observed in the majority of fishes, the pike,
dory, &c. The simple teeth attached to the gum are
chiefly distinguished by their fibres intersecting in the
manner of the cancellated tissue of bones, and being
therefore at first light, porous, and spongy, and becoming
afterwards uniformly hard and compact like ivory.
The compound teeth, which consist of an infinite num¬
ber of minute tubes mutually aggregated and invested by
a common covering of enamel, form plates of different
sizes, adhering to the bones of the jaws or palate by an
intermediate membrane only. In some they affect the
disposition of the quincunx; in others they occupy the
whole breadth of the upper jaw at least, as in the ray as
seen on the small scale, and in the same manner in larger
fishes ; others are in straight transverse bars ; others as¬
sume the shape of a circular segment, or the figure in
heraldry denominated the chevron.
In the wolf-fish the jaws are provided with eminences
composed of fibres or tubes proceeding from the base to
the circumference, and which are connected to the jaw
by a substance more spongy than the rest of the bone.
After their first formation the teeth retain nearly their Mecham.
original shape in the Zoophaga, man, and the Quad-cadetri
rumana. In the two latter only their crowns begin to be
worn, rendering the incisor and canine less pointed by the
use of food partly vegetable; but in the zoophagous tribes
they undergo no detrition whatever. In the herbivorous
animals, however, the crown begins to undergo detrition
more or less rapidly; and in no long time the superior
layer of enamel is entirely worn off, and the surface of
the tooth exposes the succession of bone or ivory, enamel,
and cement. These substances are well seen in the teeth
of the Rodentia, for instance the hare; those of the
Pachydermata, as the elephant; the Ruminants, as the
stag, sheep, and ox ; and the Solidungula, as the horse.
In all these animals the enamel, which is hardest, forms
prominent lines or ridges; while the bone and cement are
indicated by depressions.
This detrition, which is purely mechanical, might pro¬
ceed to such an extent in the herbivorous quadrupeds
as to destroy the whole of the crown of the tooth, and
leave the process of mastication to be performed by the
jaws only. It appears to be chiefly to obviate this incon-
venience that the dentition of the elephant, the Ethiopian uc^.e.*n
boar, and perhaps all the Pachydermata, is conducted
in a successive manner through a series of six or eight ejephani
sets of teeth at least. In the former animal, in which
this process has been best observed, and was ably explain¬
ed many years ago by Mr John Corse Scott (Phil. Trans.
1799), each half-jaw, whatever it contains, exhibits at
one time only one complete grinder and part of another
behind it, the prominent parts of which are placed ob¬
liquely to the horizon, forming an inclined plane, so that
the anterior parts are worn before the posterior. The
anterior complete one, which is employed in mastication,
undergoes progressive detrition till its anterior portion is
worn down to the level of the jaw. In this state the fangs
of the anterior part of the tooth begin to diminish, render¬
ing the tooth narrow before ; while the crown of the poste-