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LOCAL ANVESTHESL\. 269

are retained from one-half to two minutes. Previous to the ap-
plication the patient should be cautioned against swallowing any
portion of the mixture. (See Cocaine, hypodermic use of.)
Pressure Anasthesia by Compressed Air.—Compressed air is
recommended as essential to the dentist for relieving the pain of
many operations, and also for securing better results in the action
of certain remedial agents by bringing them in closer contact
with the tissues to be anaesthetized. It has been successfully
employed by a method which consists in forcing, by compressed
air with an even and constant force, an obtunding agent into the
tubuli of the dentine, in cases of hypersensitiveness. It is a
more simple method of obtunding than that of cataphoresis,
electricity being the pressure in the cataphoric method.
Some twenty-hve to thirty or forty pounds of air-pressure are
sufficient, which can be secured by an apparatus designed for the
purpose, which will produce double the amount of pressure in air
than in water. A solution of cocaine in sulphuric ether —
vapocaine, for example, has the advantage of easy access to the
tubuli of the dentine, on account of the ether holding the cocaine
and finding its way into the tubuli, when it evaporates leaving the
cocaine, which then combines with the fluid present.
Besides its use as an obtundent of sensitive dentine, com-
pressed air may also be used to diffuse medicines in root-canals,
and into the pus-pockets of alveolar pyorrhcea, in bleaching
teeth, in forcing out blood and pus in pyorrhcea, in forcing back
the gum and contents of such pockets, as a spray, and as a dessi-
cant of pulp-canals, in drying cavities in teeth, and also drying
the mouth in rapidly hardening cement fillings with warm air, in
cooling impressions of modeling compound, or wax, in keeping
dry the banding of a root by forcing back the blood and gum, in
setting crowns and bridge-work, etc. In the apparatus em-
ployed, the outlet of air is under control so as to secure an even
and constant pressure, and the supply is controlled by a cut-off.
Liquid Air.—The temperature of liquid air is 312° F. below
zero. It is applied in the form of spray. Liquid air has re-
ceived attention from a surgical standpoint in regard to its ef-
ficacy as a local anaesthetic, in the treatment of ulcers, opening