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DENTINAL ANESTHESIA BY CHEMICAL AGENTS. 209

blast should be gently a])plied at first at intervals of a couple of sec-
onds ; when the pain induced by the abstraction of the water some-
what diminishes, the force should l^e increased and made continuous,
Avhen in most cases the excavation may be continued. The air may
also and preferably be heated by an electric warm-air syringe (Fig.
158), which has tlie advantage of maintaining an even degree of heat.
As stated before, this means is of less use with soft teeth, and fre-
quently fails when the teeth have a high grade of sensitivity which
appears to be due to constitutional conditions,—where the sensitivity is
not confined to the surface of the tissues immediately beneath the caries
but pertains to the whole of the dentin.
Preparatory to the use of heated air, the application to the cavity
of absolute alcohol is serviceable, on account of its high affinitv for
water.
Carbolic Acid.—This substance, \\hile of little efficiencv in con-
trolling acute sensitivity, is of benefit in moderating that condition.
Its efficacy is increased by adding to it a proportion of one-third of oil
of cloves, which latter has some anesthetic intluence. When other
more active means are not admissible and the effect is not immediately
satisfactory, a better result is produced by placing this combination in
the cavity and sealing it in with zinc phosphate until a subsequent
visit, as before described. On account of the feeble affinity of carbolic
acid for water, the obtundent effect is facilitated by the previous partial
desiccation of the surface of the cavity by warm-air blasts. Carbolic
acid in combination with caustic potassa, equal parts of each (Robin-
son's Remedy), is often of much service in subacute sensitivity. The
preparation should be laid in the cavity in contact with the denuded
dentin and should be allowed to remain until it deliquesces.
Carbolic acid in combination with tannic acid is also serviceable Avhen
sealed in the cavity l)y an impermeable temporary stopping.
Zinc Chlorid.—Of all substances, when not interdicted by proximity
of the dental pulp, zinc chlorid is the most efficient of the topical
remedies for the condition under consideration. Its action is explained
by the double power of its affinity for water and its extreme coagulating
effect upon albumin. It is evident that if the tissue be deprived of two
of its elements the function of sensitivity must be impaired or destroyed.
In the degree to which this action takes place the tissue loses its capacity
for irritation.
As zinc chlorid in concentrated solution is an active escharotic to
organic tissue, it must be employed with caution. After paralyzing the
vital resistance of the part its action is by combining in definite ])ropor-
tions with the albuminous elements of the structure. It has the fur-
ther property of an excessive affinity for M^ater, which enables one to
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