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DENTINAL ANESTHESIA BY ELECTRICAL OSMOSIS. 205

proven to be the case from the profound effect in cavities upon buccal
and labial surfaces and in shallow cavities of occlusal surfaces. Besides
the less diifusion of the cocain when the carious matter is removed,
a degree of electrical force which in the former case is easily tolerated
becomes painful. These facts make conclusive the importance of retain-
ing some of the carious contents of the cavity.
An explanation of the influence of the current is found in the prin-
ciples and examples given on page 194. As the anode is put in con-
nection with the lint saturated with the cocain solution the fluids of the
tooth advance toward the pulp through the canaliculi, their place being
taken by the solution of cocain. At the same time it is observed
that loss of fluid from the lint occurs, necessitating additions to
maintain the proper wetness, some loss of water taking place by
evaporation.
Conditions Infliioicing Tolerance of the Current.—As already stated,
when the electrical force is brought into connection with the carious
matter the irritation caused bv the current is of triflino^ deg-ree and
soon subsides, indicating that the anesthetic eifect has been produced
;
but when the cavity is denuded of caries the above-stated degree of
current force is not so tolerable, the irritation continues longer and
does not subside in the same manner, but the effect upon the tissue is
nearly if not quite as marked. The nearer the bottom of the cavity
is to the pulp, the greater the irritation. Hence in this condition it
becomes necessary to begin with a low degree of voltage. While in
the one case fifteen cells may be selected, in the other ten cells are
more satisfactory.
To avoid the removal of caries the condition of the dentin as regards
sensitivity should be tested at the line of its connection with the
enamel.
Some stress has been laid upon the necessity for rendering the solu-
tion of cocain more highly conductive. This claim is probably more
theoretical than practical in its character, since experience with the
solutions given indicates that the conductivity is sufficient, and that the
resistance is more to be looked for in the dentin than in the solution,
and that when the tooth has become tolerant of the current at a com-
paratively low voltage an increase of pressure of the current is suf-
ficient to complete the anesthesia.
The form of the platinum anode should be such as to permit its
easy entrance into the cavity when its point is covered with a layer of
absorbent lint. For all cavities in the approximal surfaces and in most
occlusal positions an excellent form of anode is made by curling the
end of a fine platinum wire (No. 25) into a flat knot, or forming it in a
loop. On the loop a properly sized piece of lint may be gathered.