Anodynes are remedies which relieve pain by lessening the excitability of nerves or of nerve-centres. They are divided into local or general: Local Anodynes.

Cold-Cold water. Ice-bags.

Warmth - Poultices. Fomentations.

Aconite.

Acupuncture.

Atropine.

Belladonna.

Blood-letting - Leeches. Cupping.

Carbolic acid.

Carbonic acid.

Cocaine.

Conium.

Creasote.

Gelsemium.

Hydrocyanic acid.

Morphine.

Opium.

Veratrine.

General Anodynes.

Anaesthetics in small doses.

Atropine.

Belladonna.

Butyl-chloral.

Chloral.

Conium.

Conime.

Gelsemium.

Hyoscyamus.

Hyoscyamine.

Lupulus.

Lupulin.

Morphine.

Opium.

Stramonium.

Action

The sensation of pain is due to a change in some part of the cerebrum, and is usually excited by injury to some part of the body.

According to Ferrier the hippocampal region is the seat of sensation. Pain may be of central origin; for if these convolutions should from any cause undergo changes similar to what usually take place in them on the application of a painful stimulus to a nerve, pain will be felt, even although no injury whatever has been done to the body. Something of this sort appears to occur in certain cases of hysteria.

Conversely, if the changes which ordinarily occur in these convolutions on severe irritation of a sensory nerve are prevented from taking place, pain will not be felt, however great the stimulus to the nerve may be.

The sensory nerves of the head pass directly to the brain, but all other sensory nerves have to pass for a greater or less distance along the spinal cord before they reach the brain.

The transmission of painful impressions along the spinal cord occurs in the grey matter, and the effect of anaesthetics in preventing the transmission of painful impressions while tactile stimuli are still conducted has been already discussed.

Pain may be occasioned by irritation applied to nerves anywhere between the brain and the periphery; and whatever its point of application may be, it is usually referred to the peripheral distribution of the nerve. Sometimes irritation of a nerve, instead of being referred by the brain to the proper spot, is referred to a branch of the same nerve going to a different point.

Pain may be caused by violent stimulation of the peripheral distribution of a nerve, of its trunk, of the spinal cord through which the fibres pass to the brain, or of the encephalic centres themselves.

Pain may be relieved by (a) removing the source of irritation, (b) by preventing the irritation from affecting the cerebrum. Thus, if necrosis of the jaw should give rise to intense pain, the pain will at once cease on dividing the sensory nerve by which the impulses are transmitted to the brain. It may be relieved, also, while the source of irritation still remains, by lessening the excitability of the peripheral terminations of the sensory nerves which receive the painful impression; or of the nerve-trunks; or of the spinal cord along which the impression travels; or of the cerebral centres in which it is perceived.

Opium probably acts on them all, diminishing the excitability of the cerebral centre, of the spinal cord, and of the sensory nerves; and bromide of potassium is also supposed to affect all these structures, though to a much less degree than opium.

Chloral, butyl-chloral, lupulin, gelsemium, and cannabis indica probably act on the cerebral centres.

Belladonna and atropine lessen the excitability of the sensory nerves, and probably this is effected also by hyoscyamus, stramonium, aconite, aconitine, and veratrine.

Uses

It is evident that if the nerve-centre by which pain is perceived is deadened, the pain will cease wherever its seat may be; and therefore opium and morphine are used to relieve pain whatever may be its cause. Cannabis indica and bromide of potassium, having likewise a central action, may also be employed, but they are very much less efficient than opium. Chloral and butyl-chloral have an anaesthetic action when given in very large doses, but in moderate doses their power to relieve pain is not so marked as their hypnotic action. Butyl-chloral, however, seems to have a special sedative action on the fifth nerve, and so has gelsemium: consequently both of them are used in the treatment of facial neuralgia.

As cocaine, belladonna, aconite, and veratrine have a local action on the peripheral ends of the sensory nerves, they are usually applied directly to the painful part in the form of lotion, ointment, liniment, or plaster. Local injections of cocaine, morphine, atropine, or ether, in the neighbourhood of the painful part, are often of the greatest service.

Adjuncts To Anodynes

As pain depends on the condition of the cerebral centre by which it is perceived, as well as on irritation of sensory nerves, it is obvious that it may vary with the condition of these centres, although the irritation remains. Thus a decayed tooth does not always cause toothache, and when the toothache comes on, it may frequently be removed by means of a brisk purgative, even although the tooth be not extracted. It is possible that the purgative may act partly by lessening congestion around the tooth, but partly also by altering the condition of the cerebral centres. When the attention is fixed upon other things, also, the pain may be to a great extent, or even completely, abolished, as in mesmerism or hypnotism. The sensory stimuli, also, which would usually produce pain may be diverted voluntarily or involuntarily into motor channels. Thus, during the heat of action, the pain of a wound is not felt; and the pain felt during the extraction of a tooth is lessened by the employment of violent muscular effort, as in grasping the arms of the dentist's chair. Other most powerful adjuncts are electricity applied along the course of the nerves, and counter-irritation, especially by means of the actual cautery to the painful part, and, when other means fail, stretching the nerve may succeed.

Cold also, applied to the surface over a painful part, will relieve pain, and so may dry heat, applied by a sand-bag or hot cloth, or moist heat in the form of a poultice; for the mode of action of these vide 'Action of Irritants.'

Pain has been ascribed by Mortimer Granville to vibrations of nerves or of the sheaths; and, in order to lessen it, he proposes to produce vibrations of a different nature: this he does by percussing over the painful nerve with a small hammer, worked either by clockwork or electricity. For a dull heavy pain he uses quick and short vibrations of the hammer, and for a sharp lancinating pain he uses large and slow vibrations.