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23 Natural History of the Teeth.
having spots on its surface. These spots* general are found
in rows near the cutting edges of the incisnres and grinding
surfaces of the molares, and are caused by irregular or defective
action of the membrane which secretes the enamel ; and this
morbid action is generally caused by some peculiarity of con-
stitution or derangement of the system, at the time of its
deposition. Thus, we sometimes find the enamel of the teeth
of a whole family defective ; and again, the first teeth a
child gets, will often be defective, while those that come in
three or four years after will have the enamel perfect : we
have seen many such examples. The teeth are more highly
organized in some individuals than in others ; and with this
organization, the coustitution and state of the general system
at the time of their formation, have much to do.
PHYSIOLOGICAL OBSERVATIONS ON THE FOOD OF MAN,
DEDUCED FROM THE CHARACTERS OF HIS TEETH.
Physiologists have had much discussion, whether man be a
carnivorous, omnivorous, herbivorous, or frugivorous animal,
and have deduced their conclusions from the structure and
shape of his teeth, by comparing them with those of the dif-
ferent classes of animals, whose teeth are found to be adapted
to the particular kind of food on which they live. Thus the
carnivora have very long canine teeth, fitted for tearing the
food in pieces ; the incisores and molares are also well adapted
for the same purpose ; they have no lateral motion to the jaw
and the enamel is confined to the external surface of the teeth.
In the graminivora, especially the ruminantia, the incisores
are fitted for cropping the grass, and the molares for grinding
it, and to render them better adapted for this purpose, their bony
structure is intermixed with perpendicular layers of enamel,
which, as the bone wears away, make the grinding surfaces
very rou»h. All gramiuivorous animals have lateral motion
to the lower jaw; the reason for this is plain.
* Dental authors give us to understand, that as soon as the internal
membrane has formed a shell of the crown of a tooth, that the external
takes on a new action and secretes the enamel simultaneously from its
whole inner surface. The phenomena occasionally displayed in the
formation of the enamel, make it probable almost to a certainty, that it
first begins to form, like the bone of the teeth on the points of the grinding
surfaces of them, and then proceeds gradually towards the gum.