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190 OPERATIVE DENTISTRY

field for operating and its proper and speedy adjustment should be
mastered. It is made in three thicknesses ; heavy, light and medium,
the medium being the weight best adapted for all purposes where
only one weight is to be kept at hand.
The Size and Shape is of little importance so long as it com-
pletely covers the mouth after it has been made to isolate the teeth
desired, as well as cover the chin and extend to either side of the
mouth sufficient for the proper engagement of the holder. This will
require a piece from five to six inches square, for all eases back of
the six anterior teeth and is most frequently the size used on the
anterior teeth. However, some economy of rubber dam may be prac-
ticed by cutting these squares in two triangular pieces, each of which
will do for a separate case. These are applied with the diagonal of
the quadrilateral (hypotenuse) uppermost.
The Holes to Receive the Teeth should be of the proper size and
smoothly cut, otherwise there is an increased liability of being torn
in adjustment. This is best done by the use of the rubber dairi
punch to be had at dental depots. However, in the absence of this,
a very good result is obtained by drawing the rubber tightly over a
tapering round handle of an instrument and touching the sharp edge
of a knife to the rubber down the side of the handle when a per-
fectly round piece will be cut out.
The Distance Between the Holes will vary according to the space
between the teeth, the height of the festoon of the gum, the weight
of the dam and the size of the teeth to be engaged. Generally speak-
'ing, the holes are cut from two to four millimeters apart in medium
dam. The lighter the dam the farther apart should be the holes.
The holes are farther spaced with extremely large gum festoons, also
when there is a considerable gum recession. If the holes are too close
together in above condition the dam may not cover the entire proxi-
mal tissues and a leakage may occur, or the gum septa may be un-
duly compressed and permanent injury result from strangulation.
If the holes are too far apart the rubber will wrinkle and bag at the
proximal spaces and seriously hinder operations in these localities.
The Location of the Holes in the piece of rubber dam depends
upon the location of the tooth to be operated upon and the teeth
to be isolated. A beginner will do well to first place the dam over
the mouth in the position desired for the outside edges, request the
patient to open the mouth and with the finger cause the dam to come
in contact with the occlusal surfaces of the teeth it is intended to
include and then punch the holes as this trial indicates. By this