Lichen

Lichen, the name of a group of skin diseases, very variously employed by different writers, but now generally restricted to cases characterised by 'the development of solid persistent papules which undergo but little change till they gradually disappear.' The commoner skin-eruptions formerly called lichen, L. simplex and L. tropicus (or 'prickly-heat'), are abortive forms of eczema. None of those retained under this name is common. L. circumscriptus occurs in bright red patches on the back or front of the chest in adults, apparently from the irritation of thick woollen garments worn next the skin. L. scrofulosorum is a pale, very chronic papular eruption, sometimes seen in delicate children. L. planus or ruber is the most characteristic and important of the group: it manifests itself in raised flat patches, of a dull-red colour, usually very chronic in their course, often somewhat itchy. It does not usually interfere much with the health: occurs generally in adults, never in young children; and commonly yields to treatment by arsenic.