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96 THE TECHNICAL PROCEDURES IN FILLING TEETH.

enamel wall relates especially to its deviation from the direction
of the enamel rods.
The direction of the rods may be known, first, by oljtainiug
a good general knowledge of their course by the stiKty 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, which, again, is along the length
of the enamel rods. Therefore, for onr purpose, the terms
"cleavage" and "splitting" are practically sjTionjTiious.

STATEMENTS OF THE DIRECTIONS OF THE ENAMEL RODS IN DIFFERENT
PARTS OF THE CROWNS OF THE TEETH.
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 toward
the surface, or better, they are most commonly at right angles
to the surface. Everywhere on the crown of the tooth the ends
of the enamel rods present to the surface. Over the greater por-
tion of the crown, the direction of the enamel rods is perpen-
dicular to the surface, but in every tooth there are certain por-
tions in which the enamel rods approach the surface at a more or
less considerable inclination.
(1.) In a central perpendicular section labio-lingually
through an incisor tooth, the enamel rods will be cut parallel
with their length in all of its parts, but they will not all be at
right angles to the surface of the tooth. The deviations in this
respect are shown in diagram, Figure 104.
(2.) In a central perpendicular section bucco-lingnally
through a bicuspid tooth, the enamel rods will be cut parallel
with their length in all of its parts, but the enamel rods will not
all stand at right angles to the surface of the tooth. The devia-
tions in this respect are shown in diagram in Figure 105.
(3.) In a central section mesio-distally through a lower
molar tooth, the enamel rods will be cut parallel with their length
on the mesial and distal surfaces and over the marginal ridges
onto the occlusal surface, but will be cut irregularly diagonally
across in parts of the occlusal surface, depending upon tlie rela-
tion of the section to the developmental grooves and [)its. This
is illustrated in diagram. Figure 10().
(4.) In a perpendicular section from a ]ioiiit in the occlusal
surface of a molar tooth in a line passing dii-cctly over the point
of any cusp and along the axial line angle to the gingival line, the