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98 THE TECHNICAL PROCEDXJKES IN FILLING TEETH.

which must be found by noting the direction of cleavage, or feel-
ing for it, as hereafter described.
If the attempt be made to prepare a cavity upon an axial
surface of one of these teeth for filling, and this cavity approaches
a cusp or a marginal ridge, the enamel wall must be inclined out-
ward (or toward the occlusal) very greatly, or else short ends
of enamel rods will be left at the surface. And if inclined enough
to prevent this, the filling material will be so thin at its margin
that it will not have sufficient strength. These facts forbid us
laying enamel margins near the occlusal margins of the axial
surfaces of the teeth. Figures 104, 105, 106.
On the occlusal surfaces of the bicuspids and molars, the
matter would seem more complicated because of the greater
variations of the disturbing causes affecting the directions of
rods. In the larger sense this is true, but, from the standpoint
of the preparation of cavities, it is, in fact, simpler. If we exam-
ine carefully the diagrams in Figures 105, 106, we will discover
that the direction of the enamel rods in the occlusal surfaces is
such that perpendicular and parallel cavity walls about cavities
occurring in pits, will, except in very large cavities, be particu-
larly strong enamel walls, because the inclinations of the enamel
rods are all toward the center of the area in which fillings are
made. In this, variations from this normal are rarely so wide
as to turn the inclination in the opposite direction. It is only
in the approach to the marginal ridges and apices of the cusps
around the occlusal surfaces that especial care must be taken
regarding the directions of the rods. In very large occlusal
fillings this is always to be reckoned with, and the enamel walls
sloped outward to agree with the outward inclination of the rods
as shown in the diagrams. This inclination is found by examina-
tion of the cleavage, as will be described later.
In the incisors and cusi^ids, the general directions of the
enamel rods are as shown in the diagrams. On the distal sur-
faces, where the distal angle is much rounded, the deviation of
the enamel rods from the right angle to the surface begins far-
ther from the incisal angle than upon the mesial surface and is
often much more considerable before reaching close to the angle.
The enamel wall at this point requires special care. The great-
est difference occurs in the distal surface of the ui)])er lateral
incisor, which has the most rounded incisal angle. On the mesial
surface, where the angle is acute, the change of direction occurs
very suddcTily at the incisal angle, and tlie considerable inclina-
tion that would seriouslv interfere with the formation of the