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THE ENAMEL. 57

edge is reached. This very strong inclination of the enamel
rods toward the incisal upon this surface renders it exceedingly-
difficult to make good margins if the marginal lines of the cavity
approach nearer the incisal edge than the junction of the middle
and incisal thirds of the crown. If the cavo-surface angle of
the enamel is beveled enough to remove all short ends of enamel
rods the filling material becomes too thin to stand well. This is
the reason we so frequently see imperfect margins when the
marginal lines of fillings have been laid inthis position.
On the labial surface of the incisors and cuspids, the
direction of the enamel rods is generally about perpendicular to
the surface of the junction of the middle and gingival third, and
as the incisal margin of the surface is approached there is a
gradual inclination toward the incisal. This change is slow in
the middle third, so that it is rarely more than six centigrades
at the junction of the middle and incisal thirds. In the incisal
third the increased inclination to the incisal is much more rapid,
so that upon the incisal edge the direction of the enamel rods is
parallel to the long axis of the tooth. These directions of the
rods must be taken into account in any case in which the mar-
ginal lines of labial cavities" approach the incisal margin of the
surface. Fortunately, such cavities are rare.
On the mesial and distal surfaces the direction of the
enamel rods is generally perpendicular to the surface through-
out the middle third of the length of the crown. In the incisal
third an inclination toward the angle begins. The extent of
this is quite variable. The rule is that the more acute the angle
the closer the angle is approached before the inclination of the
rods toward the angle becomes pronounced. Therefore, the
distal angle being more rounded the inclination of the rods
toward it begins further from the angle upon the distal surface,
but often approaches the angle quite closely upon the mesial
before the inclination becomes great. For this reason cavity
lines in mesial surfaces may approach safely quite close to the
incisal angle if the enamel is well supported by sound dentin.
In the distal surfaces much more care must be exercised in lay-
ing cavity lines close to the incisal angle, for the reason that if
the enamel wall is inclined enough to give a good cavo-surface
angle the edge of the filling is likely to become too thin to stand
well.