This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
Local sedatives are substances which diminish, and local anaesthetics are substances which destroy, the sensibility of the skin for the time being.
Their effect in some degree is due to a paralysing action upon the terminal branches of the cutaneous nerves. It is probably, to some extent, also due to an effect upon the vessels and tissues analogous to that which is produced by rubbing or scratching, which, as everyone knows, gives temporary relief to itching. Sweating also relieves the itching, which is sometimes felt just before it begins.
Local sedatives are employed to relieve itching and to lessen pain, whether it be due to neuralgia or inflammation. Local anaesthetics are employed temporarily to abolish the sensibility of the skin, and allow slight incisions or operations to be made painlessly.
Stimulating: Action of Drugs on the Peripheral Ends of Sensory Nerves. - The peripheral terminations of sensory nerves appear to become more sensitive when the supply of blood to the part is increased. This is markedly seen, not only in inflammation, where the part becomes exceedingly tender, but in cases where turgescence of the vessels occurs under physiological conditions. Besides the class of irritants which act on the peripheral terminations of sensory nerves so as to cause pain when locally applied, there are several drugs which appear to have a special irritant action on the ends of sensory nerves when introduced into the circulation: these are aconite and aconitine, which give rise to a peculiar tingling and numbness in the tongue, lips, cheeks, and indeed in all parts supplied by the fifth nerve. Vera-trine also causes peculiar sensations in the sensory nerves when taken internally, but these are felt more in the lingers and toes, and in the joints, than in the tongue.1
1 Lewin, Ueber Piper methysticum (Kaica). Berlin, 1886.
1 Von Schroff, Pharmacologie, 4th ed. p. 584.