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THE ENAMEL, 51

screw made tight. Then the rubber may be allowed to draw
tightly around it, and is fairly certain to exclude moisture per-
• fectly. For this purpose the hole in the rubber should be cut a
little larger than usual.
With the most skilful, some impossible cases occur with any
and all of these instruments. Then resort must be had to hold-
ing the dam in position with an instrument while performing
the operation with the other hand. This is difficult, but prac-
ticable. The best instrument for this purpose is a straight shaft
with a broad flat point cut in the form of a fork, or a V-shaped
notch.
Often in proximate cavities, where the gingival wall is very
difficult to reach, the rubber may be forced into position and
held with a matrix, or the matrix may be placed first and
securely tied and the rubber applied over it. Sometimes a
similar device will accomplish the same upon buccal surfaces.
When the operation is completed, great care should be had
to remove all ligatures before removing the rubber, for if the
rubber is pulled away with a ligature on, a ring of rubber will
sometimes be torn away and remain around the neck of the
tooth unobserved, and do great damage before the cause is dis-
covered. When the contacts between the teeth are close, or
when gutta-percha fillings have been placed to seal in treat-
ments, the rubber should be drawn to the buccal or labial, and
the septum passing between the teeth cut before removing the
rubber. This will avoid the danger of leaving bits of rubber
dam hanging between the teeth or of disturbing a soft filling.
Finally, when the rubber has been removed, the gums should
be well kneaded with the fingers, while being flooded with warm
water from the syringe. The gums have been compressed and
the circulation interfered with, and this will clean the parts and
start the blood into full activity and prevent the severe soreness
that is so apt to follow.
The Enamel.
The structure of the enamel is of such importance in its rela-
tion to the preparation of cavities for filling that it requires spe-
cial study. It is difiicult to so prepare specimens of the enamel
that they show its structure well, and when the specimens are
well prepared, it requires a large amount of study to gain that