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58 THE TECHNICAL PROCEDURES IN FILLING TEETH.

In following the inclination of the enamel rods around the
incisors and cuspids in the circumferential direction, we find
them generally standing perpendicular to the surface. A notable
exception to this is the approach to the mesial and distal marginal
ridges on the lingual surface and over the lingual marginal ridges.
Here the enamel rods incline somewhat toward the marginal
ridges, but in passing over these ridges their direction or inclina-
tion changes rather suddenly. For this reason this becomes
rather a dangerous point in the preparation of proximate cavities
in the incisors. When the marginal lines of these proximate
cavities reach to the lingual marginal ridge, it is rarely safe to
leave any of the ridge remaining, because of the very uncertain
direction of the enamel rods. Especially is this true of lateral
incisors, in which the curve of the ridge is often very abrupt.
While this ridge is very strong in the perfect tooth, it becomes
very frail when its support on either side has been destroyed,
and the only safe course seems to be to cut it away sufficiently to
be certain of the direction of the enamel rods upon the margin
formed. The rounding of the labio-mesial or labio-distal angles
is not so abrupt, and the enamel rods usually hold closely to a
direction perpendicular to the surface, so that good margins can
be made at any point by observing carefully the form of the
tooth and the enamel cleavage.
In passing toward the gingival line from the junction of the
middle and gingival third of the length of the crown, the inclina-
tion of the enamel rods changes more and more toward the
gingival. This varies considerably in different specimens ; gen-
erally it is not much more than six or eight centigrades at the
gingival margin of the enamel, but in some specimens it is ten
or twelve centigrades. This calls for extreme care in beveling
gingival cavo-surface angles in cavities that approach close to
the gingival line.
How to Study the Direction and Inclination of the Enamel
Rods.
The first studies of the enamel rods should, of course, be
made by grinding sections and studying these with the aid of the
microscope, and in lantern illustrations. But when a fairly good
idea of the structure has been obtained in this way it is better to
continue the study by noting carefully the cleavage of the enamel