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82 THE TREATMENT OF TEETH

but not immediately comiected with the margin.
The margin should be made as smooth as possible
with chisels, spoon excavators, fine-cut engine-burs,
fine files, emery or sandpaper tapes or discs, &c.,
whichever may be most suitable to the case in hand.
The introduction of the small stones known as
"Gem Cavity Trimmers " renders the proper prepara-

tion of margins in many cases easy and certain. Care
must be taken in using these stones, for if they
catch or become jammed between the teeth ever so
slightly the stone will be broken ; otherwise they
are fairly durable.
Extension for Preveyition.—What is known as
extension for prevention has provoked much dis-
cussion. This method of preparing cavities con-
sists in cutting the cervical margins of approximal
cavities well below the gum, and cutting away the
lateral walls, until the junction of the filling and the
margins of the cavity is sufficiently exposed to be
kept clean, and free from food deposits, by the action
of the tongue, lips, and cheeks, and the toothbrush.
The cervical margin is protected by the gum, and it
may be conceded that if the margins of a well-made
and well-extended gold filling are attacked by decay,
it is due to carelessness on the part of the patient in
keeping the teeth clean. It is very nice to be able
to say to a patient, " Your teeth are now restored in