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SHAPING THE CAVITY. 221
not be perfectly adapted to them. As in the removal of decay, excava-
tors for this purpose should have curved edges, and burs should be of
a round or oval form.
If grooves are required they should neither be made deep nor too
near to the enamel, for fear of weakening the walls. At the cervical
margins of cavities grooves and starting pits should be avoided when-
ever possible, for they weaken the portion of the cavity which is sub-
jected to the greatest strain in the introduction of the filling, both
mechanically and by cutting off the nutrient supply to the cervical
margin, which tends to alter the resistive character of that portion of
the tooth structure by devitalizing it.
For the same reasons deep grooves or undercuts should not be made
near the incisal or occlusal surfaces, for the strain of mastication will be
liable to result in fracture of the wall if it is tluis unduly weakened.
In the process of shaping the cavity internally the enamel margins
will naturally be assuming their proper form, but the final part of the
preparation should consist in giving these frail portals of the cavity
very careful and minute attention.
The value and permanency of a filling will largely depend upon the
strength of the enamel icalls and their proper preparation. The enamel
cap of a tooth when intact is exceedingly strong and capable of resist-
ing great strain, but when its continuity has been broken by caries and
it is left unsupported by dentin it is very weak and brittle. This is
readily understood when we remember that enamel is composed of an
aggregation of enamel rods or prisms in close juxtaposition, slightly
joined together by a cementing substance, with their greater diameters
perpendicular to the plane of the surface of dentin upon which they
Fig. IGS.
Showing enamel structure.
rest. When continuous, these rods mutually support one another and are
thus capable of resisting great strain ; but when a lesion has occurred
they lose support on the adjoining side and hence are easilv separated
in the direction of their length. Fig. 1G
