Anæsthe'sia (Gr., 'lack of sensation') is a term used to express a loss of sensibility to external impressions, which may involve a part or the whole surface of the body. It may occur naturally as the result of disease, or may be produced artificially by the administration of anæsthetics. In some diseased conditions of the nervous centres, a part of the body may become totally insensible to pain, while in another part sensation may be unnaturally acute, constituting a state of hyperæsthesia. When a nerve is divided, there is no feeling of touch or pain referred to the parts which it supplies, because these are cut off from communication with the brain; and in some diseases, as the elephantiasis græcorum, a loss of sensation in patches of the skin is an early and characteristic symptom. Insensibility to external impressions may be either general—i.e. affecting the whole body, or local, where only that part is affected to which the anæsthetic agent is applied.
In ancient writers, we read of insensibility or indifference to pain being obtained by means of Indian hemp (Cannabis indica), either smoked or taken into the stomach. The Chinese, more than 1500 years ago, used a preparation of hemp, or ma-yo, to annul pain. The Greeks and Romans used mandragora for a similar purpose (poiein anæsthê'sian); and as late as the 13th century, the vapour from a sponge filled with mandragora, opium, and other sedatives was used. The mandragora, however, occasionally induced convulsions, with other alarming symptoms; and though
Bullein, an English physician (died 1579), mentions the possibility of putting patients who were to be operated upon into 'a trance or a deepe terrible dreame' by its use, it gradually became obsolete, and was banished from the pharmacopœia. John Baptista Porta, of Naples, in his work on Natural Magic (1597), speaks of a quintessence extracted from medicines by somniferous menstrua. This was kept in leaden vessels, hermetically closed, lest the aura should escape. 'When it is used, the cover being removed, it is applied to the nostrils of the sleeper, who draws in the most subtle power of the vapour by smelling, and so blocks up the fortress of the senses, that he is plunged into the most profound sleep, and cannot be roused without the greatest effort. . . . These things are plain to the skilful physician, but unintelligible to the wicked.' In 1784, Dr Moore, of London, used compression on the nerves of a limb requiring amputation, but this method was in itself productive of much pain. In 1800, Sir Humphry Davy, experimenting with the nitrous oxide or laughing-gas, suggested its usefulness as an anæsthetic; and in 1828, Dr Hickman suggested carbonic acid gas.
As early as 1795, Dr Pearson had used the vapour of sulphuric ether for the relief of spasmodic affections of the respiration. The fact that sulphuric ether could produce insensibility was shown by the American physicians, Godwin (1822), Mitchell (1832), Jackson (1833), Wood and Bache (1834); but it was first used to prevent the pain of an operation in 1846, by Dr Morton, a dentist of Boston. The news of his success reached England on 17th December 1846; on the 22d, Mr Robinson, a dentist, and Mr Liston, the eminent surgeon, operated on patients rendered insensible by the inhalation of sulphuric ether. This material was extensively used for a year, when Sir J. Y. Simpson, of Edinburgh, discovered the anæsthetic powers of Chloroform (q.v.), and introduced the use of it into his special department, midwifery. Since that time, chloroform has been the anæsthetic in general use in Europe, but ether is preferred in America. It is now the opinion of most medical men that chloroform should not be given where there is weak action of the heart from disease. Other substances have been used by inhalation, such as nitrous oxide gas, which is the best and safest anæsthetic for operations that last only one or two minutes, as in the extraction of teeth; bichloride of methylene and tetrachloride of carbon have also been employed, but are not so reliable as those above mentioned.
The employment of general anæsthetics in surgery has greatly increased the scope of the surgeon's usefulness, and has been a great boon to suffering humanity. It is, however, fraught with a certain amount of danger. However much care may be taken in its administration, an occasional fatal accident occurs from the action of the anæsthetics employed. In these cases, there is generally disease of the heart, or a hypersensitive nervous system, predisposing to sudden sinking, or to shock.
Local anæsthesia, artificially produced, is of great value in minor operations, and in painful affections of limited areas of the body. It depends upon a paralysis of the sensory nerves of the part, and may be induced by the application of cold, or of medicinal agents. An ether spray thrown on the part, produces such intense cold by its evaporation, that the part is completely numbed, and a layer of ice forms on its surface. The after effects, however, when reaction sets in, are very painful, and there is danger that in weak constitutions sloughing and ulceration may follow. Of medicinal agents, the best is cocaine, prepared from the coca shrub of Peru (Erythroxylon coca). In the form of a five to ten per cent. watery solution, this drug is introduced into the tissues by a hypodermic needle, and produces complete anæsthesia of the part thus treated in from three to fifteen minutes. Rarely it produces giddiness, but has no unpleasant local after effects. Thymol, menthol, aconite, belladonna, chloroform (the last three as the well-known A B C liniment), phenol, chloral, and Indian hemp, have also a local anæsthetic action if rubbed on the skin, or applied to abraded surfaces.