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100 THE TECHXIC.iL PROCEDTJEES IN FILLING TEETH.

noted in Figure 97, Volume I, in a photomicrograph showing a
segment of a cross section of carious enamel. Such examples
show that the normal directions shown by the diagrams can not
be too implicitly depended upon in preparing cavity walls. These
must be verified by the chisel and corrected.
HOW TO STUDY THE DIEECTIONS AND INCLINATIONS OF THE
ENAMEL RODS.
The first studies of the enamel rods should, of course, be
made by grinding sections, etching them before mounting, so
that the rods may be seen well, and studying these with the aid
of the microscope. But when a fairly good idea of the structure
has been obtained in this way, one should continue the study by
using the chisel in splitting the enamel oft", which is the easiest
way of removing frail margins, and noting carefully the cleavage
of the enamel while operating at the chair. In preparing cavi-
ties, the direction of the cleavage should be carefully noted, and,
knowing that this cleavage follows the length of the enamel rods,
one may be continuously studying their direction in different
positions upon any and all of the teeth. This study, when pur-
sued for some time, enables an operator to so place his instru-
ments (hand instruments only should be used) as to split off the
enamel easily in opening ca%dties, which is of immense advan-
tage, as he will do it easier and quicker than by any other method.
Irregularities in the course of the rods, which occur frequently,
will be detected and the enamel wall may be inclined more or
less to avoid leaving short ends of rods at any point. This
knowledge can be obtained only by the use of hand-cutting instru-
ments. If there was no other reason for using them instead of
burs this would be sufficient. The operations will also be greatly
improved because of more perfect preparation of the enamel
walls and cavo-surface angles. One soon learns to feel for the
direction of the enamel rods with the cutting instruments and
to take advantage of the irregularities of the cleavage in cutting
the enamel, and then readily feels this in the formation of the
enamel wall and its cavo-surface angle and knows when the
enamel margin is firm and strong. One who will carry on the
study at the chair will become a rapid and strong operator.
The shaving or planing of the enamel in finishing the
enamel wall should be looked to with gi"eat care. Those points
at which the enamel rods are more or less broken apart will gen-
erally appear a little whitish, and, in pushing a sharp edge
lightly along them, the loosened rods are easily dislodged in what