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FILLING TEETH 157

The operator, finding nothing but amalgam is visible
on completion of the filling, thinks everything is
right ; but in a short time he finds that the force
of mastication has broken the thin border of amal-
gam, and a broad line of cement has become
exposed. This necessitates cutting out the cement
at the border sufficiently deep to admit of proper
repair with amalgam. Lining cavities is excellent
practice, and efficiency is only attained by care and
experience.
In certain cases the amalgam will be the better
retained for the cement lining ; shocks to the pulp
also, from thermal change, will be to a great extent
prevented, and should the amalgam shrink or curl
up at the edges, the tooth will be better protected
than if no lining were used. When, however, an
amalgam is used that has the tooth-hardening,
or preservative properties of copper, it is well for
it to be in direct contacrt with the dentine. The
use of a cement lining also prevents an amalgam
from showing through a thin buccal wall, and con-
sequently distinctly helps to preserve the appearance
of the tooth ; for this reason it may be useful in
certain cases to place the cement only against the
buccal wall.
Amalgam and cement are also combined by
mixing together. Some advise the incorporation of