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82 PATHOLOGY OF THE HAED TISSUES OF THE TEETH.
ing both the spreading buccally and lingually and at the same
time the penetration of the enamel along this line in its rela-
tions to the individual teeth as a whole. In three of the four
figures, the decay has spread across the proximal surfaces to
the rounding of the angle which opens the embrasures, both
to the buccal and to the lingual, practically before decay of the
dentin has begun. Figure 92 is from a mouth with thick necked,
squarely built teeth, in which the lingual surfaces of the second
bicuspids are equally broad with the buccal, and with very fiat
proximal surfaces. On the distal, the decay reaches fully from
angle to angle. It is easily seen in this that there were two
nearly equal beginning points which have run together on the
surface, but are still divided in the deeper part. On the mesial
surface, it is noted that the original beginning was much to the
lingual at the point where whitening completely penetrates the
enamel for a little space. The spreading is toward the buccal
and toward the lingual from that point. It spreads a little
around the mesio-lingual angle but does not reach the mesio-
buccal angle. It is easy to determine by this that the position of
the first bicuspid was abnormally to the lingual of the line of
the arch. This gave the opportunity for decay in this unusual
position. It was prevented from reaching the mesio-buccal
angle by the cleaning of this part by excursions of food crushed
along this portion of the surface in mastication.
In Figure 93 there has been an unusual number of very
small beginning points forming a line which had just fused
together by the superficial spreading. It is particularly inter-
esting from that fact. The tooth seems out of form for the
reason that the decayed side was cut farther from the occlusal
surface than the sound side. Figure 94 shows an area of decay
on the distal surface of a molar that is of the more regular
type, showing a solid advance of decay of the enamel, except
as it thins out at the angles of the tooth. This photograph gives
in relief, to speak figuratively, the reason for many cases of
recurrence of decay after filling. The cutting had been carried
to hard enamel, but not far enough to remove the thin portion
of the decay spreading farther around toward the angles. If
the filling were finished without sufficient separation of the teeth
having been made, so that a little of the mesio-distal breadth
of the tooth was lost, the area of near contact was increased
by that much, giving better opportunity for decay to rebegin.
Therefore, the filling is quickly undermined.
Many of the beginning decays observed in the proximal sur-