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EROSION OF THE TEETH. 43

particularly in the central incisors and cuspids above and below,
and sometimes in other teeth.
In examining the illustration it will be noted that in this
case the lower centrals present an offshoot which has hollowed
out the labial surface to the cutting edge. The same has been
done in the upper central incisors, but the eroded area has
become so large in these as to almost obscure this feature. It
occurs in no other teeth in this case.
Figure 49 also presents the dish shape of the eroded areas,
but in this case it has two complications. There is recession
of the gums and the dish-shaped portion of the erosion is largely
in the cementum, but cutting the cementum, enamel and dentin
without distinction. This complication of recession of the gums
with erosion is not uncommon. The erosion never passes under
the free margin of the gum, and while the gum may be tumified
or swollen, it is rare that it ever laps over into the eroded area,
differing markedly from caries in this respect. Some of the cases
look as though the recession of the gum may have been caused
by the erosion, but we see many cases of recession of the gima
of similar character without erosion. We also see many cases of
erosion cutting away the teeth very close to the gum without
recession of the gum. It would therefore seem that there is
no causal relation between the two. Figure 49 also has a second
complication not so frequently seen, in the squared-out projec-
tions toward the incisal from the dish-shaped portion which
were noted in Figure 48. In this case the progress, however,
has been much less and these curious projections are cut squarely
into the tooth but little more than through the enamel, and with
square angles toward the incisal. They do not appear in the
lower jaw, but appear in every tooth eroded in the upper jaw.
This ease was bilateral, being much alike on the two sides.
Wedge-shaped aeeas. These are the "keilformige defecte"
of German writers. This form usually has its place of beginning
near the free margin of the gum, and if seen at its beginning
looks like a little groove cut across the crown of the tooth from
mesial to distal. These gradually deepen, and soon the case
looks as if a wedge-shaped piece had been cut out of the labial
surface of the tooth, presenting a flat side reaching toward the
gingival and a square shoulder toward the occlusal, as if filed
away with a square file. In many of the cases the cut is as square
and the angles as sharp as they could be made with such an
instrument. In others there is more inclination to rounding
of the angle in the deeper part of the cut.