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EXCAVATION OF CAVITIES BY CLASSES. 163

sary, as a ])rotectiou against the effects of thermal changes.
Notice particularly in this series of di'awiugs the supposed
approach of the mesial horn of the pulp is diagrammatically
shown by a darkening of the recessional line of the mesial crest
of the pulp. Cutting deeper than this supposed point has been
strictly avoided except in the removal of decayed dentin in the cen-
tral portion where the form of the pulp gives more room. Also
the cutting of the step has been rounded about the recessional line
of the mesio-buccal horn of the pulp in order to avoid cutting
that line at a point even nearer the occlusal enamel. In the prep-
aration of cavities for young people, this precaution should never
be omitted unless rendered necessary in the actual removal of
decay. Often the point of the mesio-buccal horn of the pulp
approaches much closer to the enamel than is anticipated and
an accidental exposure results. The split teeth, Figures 107,
109, Volume I, which are not very young teeth, show what one
may sometimes find in the persistence of this horn of the pulp.
In one of these it is not only very long, but it is also very large.
CROSS SECTIONS OF PEOXIMAL CAVITIES AND FILLINGS.
ILLUSTRATIONS: FIGURES 197-205.
In order to further illustrate the form that may be given the
mesial or distal portion of proximo-occlusal cavities to the gingi-
val of the step, several photomicrographs with low powers from
cross sections are introduced here. Figure 197 is from a cross sec-
tion of a bicuspid, enlarging it about twelve diameters, showing
extensive injury by caries in the mesial surface and a beginning
decay in the enamel in the distal surface. With the low power
of the lens with which this is made, the direction of the enamel
rods could not be well shown, but there is enough checking from
drying to give a general idea of the direction in which the enamel
splits most readily and from this the direction of the enamel
rods may be fairly well made out. This section shows how irreg-
ular this direction is occasionally found to be and the necessity
for determining this in any given locality by watching during
the chipping of the enamel. The buccal horn of the pulp pene-
trated this section, showing its position perfectly. The lingual
horn did not penetrate the section, but its recessional line is cut
across, showing imperfectly the position it occupied when the
person was younger. The decayed area in the upper part of
the picture, or mesial portion, is an ordinary one for bicuspids.
Figure 198 shows the form of the prepared cavity, omitting
the convenience points for starting the filling, and Figure 199