The fatality attending the use of general anaesthetics led to the introduction of what are termed "local anaesthetics," some of which depend upon the therapeutic property of cold, which is properly an anaesthetic only when it freezes the part to which it is applied. The use of cold for such a purpose must necessarily be limited to small parts of the body, and its utility depends upon the ease and rapidity with which a desired spot of living flesh can be frozen, in other words, temporarily deprived of its vitality, without inflicting mechanical injury on the delicate structure of the part.

According to Dr. Richardson, the proposer of the method, the principle consists in directing on the part of the body a volatile liquid, having a boiling point at or below blood heat, in a state of fine subdivision or spray, such subdivision being produced by the action of air or other gaseous substance on the volatile oil to be dispersed. When it falls on a part of the body, it comes with force into the most minute contact with the surface upon which it strikes.

As a result, there is rapid evaporation of the volatile fluid, and so great an evolution of heat force from the surface of the part to which the spray is applied, that the blood cannot supply the equivalent loss. The part consequently dies for the moment, and is insensible; but as the power of the body is unaffected, the blood, as soon as the external reducing agency is withdrawn, quickly makes its way again through the dead parts, and restoration rapidly occurs.

The fluids used are ether, of a specific gravity not exceeding 0.723, highly rectified, and as free as possible from either alcohol or water - what is known as "absolute ether" - and rhigolene, a product of petroleum, and the lightest liquid known, ethyl or methyl chloride, and pental.

When a current of the volatile liquid, either atomized ether or rhigolene, comes in contact with the skin by the use of the spray apparatus, an intense degree of cold is produced, which deprives the nerves of the part of their power to transmit impressions to the sensorium. For the extraction of teeth, destruction of the pulps of teeth, opening abscesses, and other minor surgical operations, and neuralgia of superficial nerves, success has attended the use of such local anaesthetics. The greatest objections to such a method of inducing local anaesthesia, are the great pain which attends the first application, and the unpleasant burning sensation of the part when it is recovering from the freezing process. (See Rhigolene.) (See Aconite, for obtunding mixture.)

Liquefied chloride of methyl dissolved in ether and carefully applied by means of a hair pencil, is recommended for the painless extraction of teeth, the opening of alveolar abscesses, etc., etc. (See Cocaine, Tropacocaine, Beta-Eucaine, Vapocaine, and Chloretone, as local anaesthetics.)

A plan of local anaesthesia, known as Voltaic Narcotism, has been suggested by Dr. Richardson, of London. It consists in passing a galvanic current through a narcotic solution held in contact with the part to be operated upon. This plan appears to be successful only where the cavity of the tooth to be extracted is exposed.

Electro-magnetism has also been employed as a local anaesthetic, and it is a mooted question whether it relieves pain or complicates the sensations. It is well, however, to remember that some persons are so peculiarly constituted as to render them very susceptible to the influence of electricity. Dr. Scott describes his method of applying electricity for the extraction of teeth as follows: "Use a Kidder electro-magnetic machine, or any other giving very rapid vibrations of armature. Place the positive electrode on the gum of the tooth to be extracted, and the negative in the patient's hand, or at the back of the neck. Start with a light current and gradually increase the strength as much as can be borne without producing pain. The electrode applied to the tooth should embrace each side, the better to convey the current. For this purpose it should be bifurcated at the end of the handle, the arms of sufficient length for convenience in operating. At the end of these solder small disks about the size of a dime. The handle and arms must be insulated to prevent the current passing off at any other point than the disks. Cover the disks with thin pads of fine sponge. A cylinder electrode, also covered with fine sponge or cloth, is the proper one for the hand. Moisten each with saline water. Another method is to attach one pole of the battery to the extracting forceps, the handles of which are well insulated, the other pole being held in the hands of the patient. As soon as the forceps come in contact with the tissues about the tooth the circuit is completed, and the electrical effect produced, which is often painful. A committee of the College of Dentists, England, in a report upon the anaesthetic value of electricity, were unanimous that in no case was local anaesthesia produced by such currents, but that the effects were due to "diversion of sensation, less difficulty of extraction as compared with other extractions, syncope more or less marked, and differences in methods of operating." (See Electricity as a Therapeutic Means, etc.)

Obtunding mixtures, consisting of a combination of cocaine, eucaine, tropacocaine, chloretone, pyrethrum, aconite, chloral, veratria and alcohol, or chloroform, aconite, belladonna, opium and carbolic acid, etc., have been employed to produce local anaesthesia, and in many cases with satisfactory results. (See recipes for obtunding mixtures and local anaesthetics.) For, although entire insensibility to pain cannot in all cases be brought about, yet some diminution of it may be effected by the use of such agents. They have the merit, at least, of being less dangerous than the general anaesthetics. Such topical pain-obtunding mixtures are best applied to the parts about the neck and over the root of a tooth by means of a simple apparatus, devised by Von Bonhorst. It consists of two small metallic cups, attached to the free ends of a spring some seven inches long, and which contain sponges to hold the liquid. When used, the sponges in the cups are saturated with the obtunding mixture and applied by pressing them on the gum on each side of the tooth to be removed, where they are retained from one-half to two minutes. Previous to the application the patient should be cautioned against swallowing any portion of the mixture. (See Cocaine, hypodermic use of.)