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54 THE TECHNICAL PROCEDURES IN FILLING TEETH.

The enamel rods are almost always parallel AA/^ith
each other in the outer half or near the surface of the
enamel. This fact you should observe carefully, for it is of the
utmost importance in the preparation of the enamel margins of
cavities. All that we have said of the splitting apart of the
enamel rods applies with all of its force to this outer portion of
the enamel, no matter how much the rods may be twisted in the
deeper portions, and for this reason the enamel margins of cavi-
ties should be so prepared that no portion of the cavo-surface
angle will present short ends or rods unsupported; that is, the
enamel wall should be parallel with the length of the rods, and
the bevel of the cavo-surface angle should cut off the ends so
that there may be no loose ends of rods upon the surface to fall
away while packing gold or after finishing the filling. This may
readily be accomplished if we know the direction of the enamel
rods.
The direction of the rods may be known, first, by obtaining
a good general knowledge of their course by the study of pre-
pared sections of enamel; second, by observing the direction of
cleavage during the preparation of cavities. By the term cleav-
age we express the tendency of a substance to split or separate
in given directions. The direction of cleavage is the direction in
which it splits most readily. Therefore, for our purposes the
terms cleavage and splitting are practically synonymous.
It may be stated as a general rule that the direction of the
enamel rods is from the center of the crown of the tooth
tow^ard the surface. Everywhere on the crown of the tooth
the ends of the enamel rods present to the surface. Over the
greater portion of the crown the direction of the enamel rods is
perpendicular to the surface, but in every tooth there are certain
portions in which the enamel rods approach the surface at a more
or less considerable inclination.
If we draw a line around the crown of a molar or bicuspid
tooth in the middle of its length and examine the enamel rods
in that section we will find them everywhere very nearly perpen-
dicular to the plane of the surface. If we move, now, gradu-
ally, toward the cusps of the tooth we will find the direction of
the enamel rods progressively leaving the perpendicular and
inclining toward the cusps. This inclination becomes continually
greater, until when the point of the cusp is reached the direc-