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100 PATHOLOGY OF THE HARD TISSUES OF THE TEETH.
following directly the length of the enamel rods. This represents
a new decay of enamel in the form of an extension, but begin-
ning upon the surface. It is not a lateral extension within the
tissue, but marks the spreading of microorganisms on its surface.
It is well to note particularly the direction of the enamel rods
along the occlusal side of the flamelike tongue of decay shooting
down from z, with reference to the inclination of the enamel
wall that would be required if this were prepared as a simple
proximal cavity. It will be seen that this inclination of the
enamel rods is too great to fill against safely, for it is in such
a position that the thinness of the margin of the filling material
would be insufficient to give it the necessary strength.
MISPLACEMENT OF BEGINNING PROXIMAL DECAYS.
ILLUSTRATION'S: FIGURES 119, 120.
It has been noted, in considering Figure 92, that the proxi-
mal decay upon the mesial of the bicuspid is out of the ordinary
position to the lingual, passing partially around the lingual
angle of the tooth. Such displacements from the normal position
occur frequently on account of irregularity of the teeth bringing
their surfaces together in unusual relations to each other, or
some such accidental condition.
In Figure 119 a peculiar shaped cavity is presented in the
mesial surface of the central incisor, having a prolongation
running labio-incisally. When the tooth is seen standing alone,
such form of beginning decay might seem difficult to explain,
but examination of Figure 120 explains the reason for this unus-
ual form. The relative position of the two teeth is such as to
bring very near contact directly along the line which this decay
has taken in the enamel, and is the local influence which has
caused this peculiarity. In any case, if a tooth, a bicuspid for
instance, is turned one quarter around upon its axis, so that the
buccal surface, proper, becomes the mesial surface, and the
lingual surface, proper, becomes the distal surface, decay, if it
occurs, will start in the portion of the enamel that is in near
contact with the neighboring tooth.
Therefore, we see again in this that the nature or perfection
of the enamel is in no wise an element in the localization of decay
on the smooth surfaces of the teeth. These unusual forms, and
apparently unusual positions, of beginning caries are always
traceable to some condition that serves to invite lodgment and