Anesthesia, loss of sensation, especially the sensation of touch. It can be general (affecting the entire body and usually accompanied by loss of consciousness) or local (affecting limited areas of the body). The condition may be the result of damage to nerves or nerve centers by disease or injury, or it may be intentionally induced by the administration of drugs for the prevention or relief of pain.

History of Anesthetics
Of the few anesthetic agents known to the ancients, opium and hemp (see Cannabis) were the most important. Both were taken by ingestion or by burning the drug and inhaling the smoke. Nitrous oxide (see Nitrogen), discovered by the British chemist Sir Humphry Davy about 1800, was first used as an anesthetic in 1844 by the American dentist Horace Wells. In 1842 the American surgeon Crawford Long successfully used ethyl ether as a general anesthetic during surgery. He failed to publish his findings, however, and credit for the discovery of the anesthetic properties of ether was given to the American dentist William Morton, who in 1846 publicly demonstrated its use during a tooth extraction. In 1847 the British physician Sir James Simpson discovered the anesthetic properties of chloroform. Many other general anesthetics have since been discovered. Ether and chloroform have been largely abandoned because of their dangerous side effects and flammability. Some anesthetics act by depressing the central nervous system (barbiturates, halothane), whereas others induce amnesia and dissociation (nitrous oxide, enflurane).

General Anesthesia
Surgical anesthesia (complete general anesthesia, characterized by muscular paralysis sufficient to permit surgical manipulation) is produced by inhalation anesthetics: gases or volatile liquids such as cyclopropane, nitrous oxide, halothane, and enflurane. The anesthetic, usually mixed with oxygen, is either inhaled or administered into the windpipe through a tube.
Modern anesthesia almost always involves a combination of agents. Before administering the inhalation anesthetic, the anesthesiologist might give intravenously a short-acting barbiturate such as pentobarbital or sodium pentothal (more properly called thiopental sodium or thiopentone), or an antianxiety drug such as diazepam, to induce unconsciousness. A narcotic analgesic such as meperidine or fentanyl may be used in addition. To allow use of smaller amounts of the inhalation anesthetic, special muscle-paralyzing drugs are given. These include tubocurarine, gallamine, and succinylcholine. The combination of a narcotic, a barbiturate, a muscle-paralyzing drug, and nitrous oxide is called balanced anesthesia. Because muscular activity is prevented in all these procedures, the anesthesiologist must induce breathing in the patient mechanically.
Surgical anesthesia must continue throughout the operation, but prolonged anesthetization can kill, paralyzing first the respiratory system and then the heart. The correct level is maintained by constant monitoring of the patient's condition and increasing or decreasing the dosage as needed.
Thiopental sodium in small doses is sometimes used in psychiatry because it allows patients to talk uninhibitedly. This quality has also given the drug some value in law enforcement as a "truth serum." See also Scopolamine.

Local Anesthesia
Some surgical procedures do not require total muscular relaxation and can be performed using local anesthetics, which temporarily block nerve conduction without damaging nerve fibers. Local anesthesia is produced by injecting into the tissues to be affected a solution of a natural alkaloid such as cocaine, the oldest of all local anesthetics, or a synthetic agent such as procaine, widely known under the trade name Novocain, or lidocaine (Xylocaine).
Block anesthesia, a much more extensive local anesthesia, is produced by injecting the agent into a nerve trunk, next to a nerve, or all around the operative field, thereby deadening the entire area. The best-known block anesthesia is probably the spinal block, produced by injecting an anesthetic into the spinal canal.
A mild local anesthesia, useful in many minor dental and medical procedures, can be produced by numbing the tissues with cold, either by applying ice or by spraying with a volatile liquid such as ethyl chloride.