agent which produces a local or general loss of sensation, including pain. General anesthesia involves loss of consciousness, usually for the purpose of relieving the pain of surgery. Local anesthesia involves loss of sensation in one area of the body by the blockage of conduction in nerves.

Drugs (drug) of various kinds have been used for many centuries to reduce the distress of surgical operations. Homer wrote of nepenthe, which was probably cannabis or opium. Arabian physicians used opium and henbane. More recently powerful rum was administered freely to British sailors before emergency amputations were carried out on board ship in the aftermath of battle.

In 1799 Sir Humphry Davy (Davy, Sir Humphry, Baronet), British chemist and inventor, tried inhaling nitrous oxide (laughing gas) and discovered its anesthetic properties, but the implications of his findings for surgery were ignored. By the early 1840s parties had become fashionable in Britain and the United States at which nitrous oxide, contained in bladders, was passed around and inhaled for its soporific effect. It was soon found that ether, which could be carried much more conveniently in small bottles, was equally potent. In the United States several young dentists and doctors experimented independently with the use of nitrous oxide or ether to dull the pain of tooth extractions and other minor operations. In 1845, Horace Wells (Wells, Horace), an American dentist, attempted to publicly demonstrate the use of nitrous oxide anesthesia for dental extractions. Unfortunately, the demonstration was unsuccessful.

Historians argue about who should be credited with the first use of true surgical anesthesia, but it fell to William Morton, an American dentist, to convince the medical world that general anesthesia was a practical proposition. He administered ether to a patient having a neck tumour removed at the Massachusetts General Hospital, Boston, in October 1846. Crawford Long (Long, Crawford Williamson), an American surgeon, had used ether in his practice since 1842 but did not make his findings public until 1849.

A few weeks after Morton's demonstration, ether was used during a leg amputation performed by Robert Liston at University College Hospital, London. In Britain, official royal sanction was given to anesthetics by Queen Victoria, who accepted chloroform from her physician, John Snow, when giving birth to her eighth child, Prince Leopold, in 1853.

Early anesthetics had unpleasant side effects (often causing vomiting on recovery) and were somewhat hazardous, since the dose needed to produce unconsciousness and full muscle relaxation (so that the surgeon could work unimpeded) was not far short of that which would paralyze the breathing centre of the brain. In addition, the early anesthetics were administered by simple devices consisting of glass or metal containers for sponges soaked in ether or chloroform (which was introduced as an anesthetic in 1847) and allowed no control of dosage.

Modern inhalation anesthetics such as trichlorethylene and halothane have a much wider safety margin and are administered, mixed with oxygen and nitrous oxide, from an anesthetic machine. The anesthetist can control the flow and composition of the gas mixture precisely and, using a tube placed down the trachea (windpipe) after the patient is unconscious, can, if necessary, maintain respiration by mechanical means. Delivering the gas mixture to the lungs through a close-fitting endotracheal tube also prevents accidental inhalation of mucus, saliva, and vomit.

With respiration artificially maintained it is possible to paralyze the muscles with drugs like curare, a neuromuscular blocking agent, so that procedures requiring full muscle relaxation, such as chest and abdominal surgery, can be carried out under light anesthesia.

Many short operations can be carried out under anesthesia produced by injecting an agent such as the barbiturate sodium thiopental (Pentothal) into a vein, either as a single dose or intermittently through a saline drip. Patients are also commonly put to sleep by this method before the administration of an inhalation anesthetic is begun, since it is a distress-free procedure, and unconsciousness occurs smoothly within 10 or 15 seconds of starting the injection.

Local anesthetics work by blocking the passage of impulses along nerves. cocaine was thus used for eye operations in 1884 by a Viennese surgeon, Carl Koller (Koller, Carl), acting on the suggestion of Sigmund Freud. A solution of the drug was applied directly to the part to be operated on. Soon it was being injected under the skin to facilitate small, local operations, and it was later successfully used for larger procedures by injecting it into the trunks of nerves supplying a part. Now, synthetic cocaine substitutes are widely used.

Major operations on the lower half of the body can be carried out after injecting a suitable local anesthetic into the fluid-filled space between the spinal cord and its outer membrane coverings (spinal anesthesia). Painless childbirth (parturition) can be achieved by an epidural block, which involves injecting the anesthetic agent through a fine tube threaded into the space surrounding the tough membrane covering the lower end of the spinal cord, thus dulling the emerging nerves that supply the pelvic organs.

In recent years much interest has been shown in acupuncture anesthesia, whereby apparently painless major operations are carried out after the insertion of acupuncture needles into specified points on the skin. Often an electric current is passed through the needle used. The results of some research into the efficacy of acupuncture have suggested that the stimulation of the peripheral nerves by the needles triggers the release of endorphins, a group of neurochemicals that have pain-killing effects.