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The clinical discipline concerned with the reversible production of insensibility to pain is known as ‘anaesthesia’, a term coined by Oliver Wendell Holmes in 1846 to describe a new phenomenon in a single word. It is essentially a practical subject and although becoming increasingly based on science it still retains some of the attributes of an art. In veterinary practice anaesthesia has to satisfy two requirements: (i) the humane handling of animals and (ii) technical efficiency. Humanitarian considerations dictate that gentle handling and restraint should always be employed; these minimize apprehension and protect the struggling animal from possible injury. Technical efficiency is not restricted to facilitation of the procedure to be carried out on the animal, it must also take into account the protection of personnel from bites, scratches or kicks as well as the risks of accidental or deliberate self-injection with dangerous or addictive drugs. Moreover, today it is considered that personnel need protection from the possible harmful effects of breathing low concentrations of inhalation anaesthetic agents. While anaesthesia has precisely the same meaning as when it was first coined, i.e. the state in which an animal is insensible to pain resulting from the trauma of surgery, it is now used much more widely and can be compared to terms such as ‘illness’ and ‘shock’ which are too non-specific to be of real value. Starting with the premise that ‘pain is the conscious perception of a noxious stimulus’ two conditions may be envisaged: general anaesthesia where the animal is unconscious and apparently unaware of its surroundings, and analgesia orlocal anaesthesia where the animal, although seemingly aware of its surroundings, shows diminished or no perception of pain. General anaesthesia is a reversible, controlled drug-induced intoxication of the central nervous system in which the patient neither perceives nor recalls noxious or painful stimuli. Analgesia may be produced by centrally acting drugs such as morphine given in doses insufficient to produce unconsciousness or by substances having a local, transient, selective paralytic action on sensory nerves and nerve endings (local anaesthetics). The analgesia produced by these latter substances may be classified as local or regional, applied as they are by topical application, subdermal or submucous infiltration and by peripheral, paravertebral or spinal perineural injection.

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