(From lambo, quia lambendo terpant,) a cutaneous disease called lichen, from its resemblance to the spots scattered over the tuberculated lichen. It is a papulous eruption, sometimes rising into tumours of a more considerable size; but in its milder forms rather a deformity than a disease. The term has been variously applied, and the eruption confounded with herpes, scabies, impetigo, etc. The confusion is of little consequence, since the complaint is usually trifling; and we may define it, with Dr. Wil-lan, an extensive eruption of papulae affecting adults, connected with internal disorder, usually terminating with scurf, recurrent, not contagious. This genus he divides into five species, the lichen simplex, agrius, pilaris, lividus, and tropicus. The first commences with slight feverish symptoms, which in a few days are relieved by distinct red papulae about the cheeks and chin, or on the arms; and in three or four days the same appearances take place on the neck, body, and lower extremities, accompanied with an unpleasant sensation of tingling, aggravated during the night. The eruption fades in about a week, and the whole surface is covered with large scurfy exfoliations, which continue longest in the flexure of the joints. The period of its termination is seldom the same in any two cases; and on different parts of the surface of the body there is some difference in the form of the papulae. On the face they are large, rounded, often forming small tubercles like vari; on the neck, breast, and extremities, they are most distinct and acuminated; and on the hands they resemble obscure watery pustules, which exfoliate without any discharge of fluid. This disease most commonly affects persons of a weak irritable habit, and occurs about the beginning of summer or autumn, sometimes general, at others partial, occasionally disappearing or returning without leaving any scurf, and often without any previous fever. A light cooling diet, or if necessary, some easy laxative, is only necessary. Sometimes the simple lichen is chronical, and then styled scurvy; sometimes critical, and called scabies critica. It sometimes terminates in a dry tetter.

The l. agrius, from immanis, is preceded by a fever approaching the typhus. The eruptions are of a deep dark red, with an inflamed basis, itching and tingling after any stimulating food and drink, or in the warmth of the bed. In the morning the uneasiness is inconsiderable. Straw coloured pustules are occasionally intermixed, and by the continuance of the complaint the skin is thickened, chapped, and painful. The pustules are usually confined to the upper part of the body.

Its continuance is uncertain, and it sometimes appears and disappears; but if repelled, general constitutional disorder follows. If any wound is made by scratching, it is with difficulty healed, and,the disease sometimes terminates in an ulcerated psora. This species differs from the former in the nature of the fever, the greater violence and obstinacy of the complaint. It is exasperated by mercurials, though calomel has been recommended in the beginning as a laxative; and the best medicines are the bark, with the mineral acids. We know not that a perpetual blister or an issue has been tried; but it appears a probable means of relief. The itching is best relieved by the spittle or a little rose pomatum.

The lichen pilaris is the lichen simplex, affecting chiefly, or exclusively, the roots of the hair, and from around the hair exfoliations take place. It differs in no respect from the first species.

The lichen lividus seems little different from petechia, with which they are often mixed, and the disease chiefly confined to the poor, whose diet is frequently unalimentary; it is best relieved by the bark and mineral acids. No fever precedes. Papular eruptions, resembling the lichen lividus, sometimes occur among the secondary appearances of lues; but in this last the pa-pulse are smaller, more numerous, more generally diffused; their points are, after some time, depressed; they do not disappear and return, but occasionally terminate in a foul ulcer.

The lichen tropicus is the prickly heat of tropical climates, resembling the papulae produced by sweating in the more temperate. It appears without any preceding disorders of the constitution; but the papulae, about the size of a small pin's head, are numerous, of a vivid red, and elevated so as to produce a considerable roughness in the skin; but no redness or inflammation surrounds them. The eruption is chiefly confined to those parts of the body which are usually covered, and sometimes appears on the forehead contiguous to the hair, though never on the palms of the hands, the soles of the feet, or on the hairy scalp: flannel, or warm clothing, increases the number of the papulae. Small pearly pustules, containing a limpid fluid, are often intermixed with the prickly heat, when perspiration is very copious, more especially on the breasts and about the wrists. They have no disposition to ulcerate, though violently scratched, but terminate in scales. A troublesome itching attends the prickly heat, and prevents sleeping during the night, with an acute sensation of pricking, which often also takes place suddenly after drinking any warm liquor. The eruption is sometimes stationary, appearing equally vivid in the day and night; sometimes quickly disappearing and returning, without any obvious cause; but whenever it continues for any length of time, the papulae throw off minute scales, and are succeeded by a fresh crop, without leaving any vestiges on the skin. Persons of a fair complexion, with red hair, and a soft skin, are more liable to this eruption, and have it in the greatest quantity. Those of dark complexions have it slightly, or remain free from it. As the prickly heat is considered to be a salutary eruption, no attempts should be made for its repulsion. Its sudden disappearance is rather the effect of internal disorder than a cause, and occasioned by fever, or any slight complaint of the stomach: in the latter case a stimulus applied to that organ, as spirits, or warm liquids, reproduces it. Its appearance on the skin of persons in a state of convalesence is always a favourable sign. To alleviate the itching and tingling of the prickly heat, a light and cool dress, and avoiding warm liquors, have been found most serviceable.