These are certain drugs which so blunt the senses that little or no pain results from causes which in their absence induce it. They have a local or general effect according to the mode of application. Some, as cocaine, veratrium, and aconite, when applied to the skin, deprive the nerves of the part of sensation, so that they may be cut or even burned without causing pain at the time. This action is called local anaesthesia, and the agent employed, an anaesthetic.
Ether spray or other applications producing intense cold by rapid evaporation have also the effect of producing insensibility, but experience has proved that this method of producing anaesthesia is sometimes attended with after consequences of a very undesirable character. Evaporating lotions, composed largely of the cheaper forms of spirit, are frequently prescribed to reduce pain in inflamed limbs, tendons, and joints, their effect being the result of the cold induced in the part. The application of ice and ice-water is attended with similar results.
General Anaesthesia is produced by the inhalation of drugs such as chloroform and ether, and the same end may be attained by introducing them into the blood either through the stomach or by subcutaneous injection. These agents, by acting on the brain and spinal cord, induce sleep and insensibility to outward impressions and inward pain, and in the latter connection rank with that class of agents termed anodynes. Given in certain doses, they soothe the whole body and reduce the activity of different organs; then they are classed as sedatives. Since their effect is also to relieve spasm, they are known as anti-spasmodics.