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THE RUBBER DAM. ' 71

faces, or iu using the mouth mirror. To serve this purpose, the
holes should be cut slightly farther apart than the whole width
of the teeth over which the rubber is placed. The holes should be
of a size to hug the necks of the teeth tightly, Init not so small as
to endanger the rubber by too severe stretching in getting it
over the teeth. Further, wlien the rublier is in phice, the septum
of the rubber between the holes will be stretched much more
than any other part, and, unless there is much more width of it
than at first glance would seem necessary, it may be too narrow
to hug closely around the necks of the teeth and leakage will
result. This will occur whenever the holes are cut too close
together, which is a very common error. In case of the lower
incisors, it is often necessary to cut the holes so far apart as to
cause just a little wrinkling of the rubber in order to have the
septum of rubber between the teeth broad enough to make it
tight. The necks of these teeth are much the broader from labial
to lingual, and their mesial and distal surfaces are often very
nearly flat. It is therefore often difficult to make the dam water
tight. It will not be water tight unless the space Ijetween the
holes is broader than the teeth. The bicuspids often offer a
similar difficulty. This is to be remedied by cutting the holes
a little farther apart than the mesio-distal breadth of the teeth.
Therefore, the rule should be to cut the holes as much farther
apart than the mesio-distal breadth of the teeth as can be done
without causing the rubber to wrinkle. If in any case the proxi-
mal surfaces are very flat, it is better to overstep this a little,
a slight wrinkling of the dam being better than leakage.
In case there are special conditions, the position of the
holes in the rubber dam must be varied to meet them. For
instance. Figure 63 illustrates the position of the holes for a
case of some recession of gum with decay far to the gingival on
the buccal surface of an upper first bicusi^id, on which it is
intended to set a Hatch clamp. Notice that the hole intended
for the first bicuspid is a little out of the line of the arch. If this
were not so, by stretching the rubber to the gingival enough to
expose the cavity it is likely to be pulled away from the cuspid
and second bicuspid and cause leakage. (See Figures 97, 98,
99.) In other cases, the positions of the holes must be varied
to meet the conditions ; as when teeth are very irregular in the
arch or the rubber is to be stretched this way or that, a tooth
missing, etc. Forms for other parts of the mouth are shown in
Figure 64 for the lower incisors and Figure 65 for the bicuspids
and molars, upper or lower. Notice particularly that in Figure