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the upper jaw falls partly upon the two bicuspides in the
lower jaw. The second bicuspis shuts upon the second
bicuspis and the first molaris. The first upper molaris cov-
ers two thirds of the first and part of the second molaris of
the under jaw. The second upper molaris shuts upon the
remainder of the second and part of the third ; and the
third molaris of the upper jaw, being smaller than that in
the under jaw, shuts even upon it. From this mechanism of
the teeth, their power in mastication is increased, and if one
tooth be extracted, the antagonist tooth does not become
useless, since it can in part act upon another.
A tooth is composed of two substances, one of which,
called the enamel, is spread over that part which is not cov-
ered by the gums. The other substance is bone ; it con-
sists of the fang and all the body of the tooth situated within
the enamel. The bone of the tooth is formed from the pulp
;
and the enamel, from the investing membrane. The bony
part of the tooth is begun to be formed before the enamel.
When the ossification of a tooth is commencing, bone is de-
posited from the vessels of the pulp upon its extreme points.
In the incisores it begins upon the edges, and in the molares
upon the points of their grinding surfaces. The ossification
usually begins in the incisores in three spots ; these increase,
soon unite, and produce the cutting edge of the tooth. In
the molares it begins in as many spots as there are grinding
points, which in the lower jaw are commonly four, and in the
upper, five. These soon unite and form one thin layer of
bone over the upper surface of the pulp. The ossification
soon extends to the sides of the pulp, and a thin shell of bone
is spread over its whole surface. If this shell be removed,
the pulp, when uncovered, will be found very vascular.
This is extremely well seen in the teeth of large animals,
when in a state of formation.