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THE INCISORS. 23

The mechanical stnicture of the crown is a matter of importance. It
will be observed that it consists of several elements : first, a broad cut-
ting blade (Fig. 7, a) supported by two strong lateral columns (6) on
each side, and that these columns
Fig.
are upheld by two strong marginal
Fig. 8.

The mechanical design of the crown of
the upper central incisor : a, the blade ; 6,
the two columns supporting the blade ; c,
d f
the marginal ridges acting as guys, brac-
ing the columns ; d, the basal ridge as the Diagram of the labial face of the upper central
base of attachment for the guys. incisor.
ridges (e) leading up from the loAvcr ridge (d). These ridges are but-
tresses, which gu}' and uphold the columns which contain and carry the
blade. Hence, when these ridges are destroyed by caries or in operating,
the support of the column is lost and the blade readily breaks away.
The form of the crown is spade-like, or a compressed-wedge shape,
the edge being quite thin and the thickness increasing rapidly to the
base. It is slightly bent toAvard the lingual side, or much curled over in
some cases.
The labial face is imperfectly square or oblong, the cervical margin
being rounded (Fig. 8, a). It is convex from side to side, but only
slightly so from cervix to edge. Two shallow depressions or furrows
extend the length of the face perpendicularly (6) dividing it into
thirds, called lobes,—the mesial, (e), median (d)
aud distal lobes (e). These furrows and lobes are
quite conspicuous when the tooth is erupted, but
are abraded by age and the wear of use and denti-
frices, until the face becomes smooth. The mesial
margin is a little longer than the distal so that
the cutting edge slopes upw^ard toward the distal
side (/).
The lingiud face is smaller than the labial, Diasram of the lingual face
of the upper central in-
being on the inner and smaller curve of the
cisor.
crown, and is narrower from side to side (Fig. 9).
It is triangular in outline, being wide at the r-dge and narrow and
rounded at the base or cervix. The marginal ridges {a) are high
and conspicuous, and extend from the basal ridge to the edge on the