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240 EXCLUSION OF MOISTURE.
view and the exclusion of blood, the presence of which is inse]):irable
from thor()ii
material generally becomes impaired by the action of the instruments
in their free use at the cervix ; but the economy of time and the essen-
tials of thorough performance of this class of operations warrant the
application during this portion of the treatment.
When the case is ready for the filling process a new piece of the dam
should be prepared, and adjusted with great care to prevent the ingress
of the least moisture. Without this appliance the greatest skill is pow-
erless to secure sound results in large, difficult, or complicated cases.
The introduction of this invention has made it possible to execute
with gold, operations which previously were impossible ; not the least
advantage resulting from its use is that the operator has free use of the
left hand to assist the right.
Quality of the Rubber.—The quality of the rubber greatly modi-
fies the facility of its application. It should be of medium thickness
and of light color, as it then absorbs less light. It should be freely
extensible and so elastic that when the thumb is forcibly pressed into it
it returns to its normal form on the removal of the force. If it re-
sponds to this test it will not tear if fairly applied.
The size and form of the piece should be such as to avoid encum-
bering the face of the patient and to permit the lateral extension to be
folded out of the way in such manner as to prevent obstruction of the
view. The form generally best suited is a triaufjle, which form also
permits of its most economical use.
For the front teeth the piece should be moderately small ; for the
bicuspids and molars the size should be ample and is best adapted when
cut from strips about seven and a half inches in width.
The selected piece should have holes cut in it of such size as to
correspond with the dimensions of the teeth over which it is to pass.
When more than one hole is required the holes should be at such dis-
tances apart as will present a sufficient amount of material to allow for
the take-up in the application, so that the strait which passes between
the teeth shall be sufficient to allow the edge to be carried upward to
form a valve at the cervices of both teeth and not be under such strain
as to interfere with the valvular action of the edges of the rubber. At
the same time there should be no excess to hamper the view or inter-
fere with the placement of the filling material.
Attention to the valvular arrangement of the dam at the cervix wall
avoid subsequent difficulty and will prevent in many instances the
infliction of pain in using ligatures except upon the tooth under treat-
ment and the adjacent one. The appearance of this valve is shown in
