f opened, they exude a small quantity
of blood and serum. This disease more frequently occurs between the ages
of twelve and fifty, but occasionally appears during dentition, when it
is called "tooth rash." The lichen pimples are sometimes dispersed
singly over the skin and gradually subside, forming a minute scale,
corresponding in position with the summit of the pimple. When the
pimples appear in clusters, there is a diffused redness in the affected
part, and, if they are irritated, minute scabs will be formed. Lichen
generally appears on the upper portion of the body, as on the face,
arms, hands, back, and chest.

The various forms of lichen are designated according to their causes,
signs, location, manner of distribution, and the form of the pimples.

_Lichen Simplex_ is the simplest form of this disorder, and is indicated
by the appearance of minute pimples, which, when the distribution is
general, are arranged like the blotches of measles. Sometimes the
eruption is local and bounded by the limits of an article of clothing,
as at the waist. In eight or ten days, the cuticle separates into minute
scales, which are detached and thrown off; but a new crop of pimples
soon appears and runs the same course, only to be succeeded by another,
and thus the affection continues for months and even years.

_Lichen circumscriptus_ is an aggravated form of _lichen simplex_, and
is characterized by a circular arrangement of the pimples. The
circumference which marks the limit of the patch is sharply defined.
This form of lichen usually appears on the chest, hips, or limbs, and is
not unfrequently mistaken for ringworm.

_Lichen strophulosus_ is a variety peculiar to infants. Dermatologists
recognize several subdivisions of this species, but the general
characteristics are the same in all. The pimples are much larger than in
the other forms of lichen, of a vivid red color and the duration of the
eruption is limited to two or three weeks.

_Lichen urticatus_ is also an infantile affection and begins with
inflammation, which is soon succeeded by the eruption. In a few days the
pimples shrink, the redness disappears, and the skin has a peculiar
bleached appearance. The eruption is attended by an intense itching
sensation and, if the skin is ruptured, a small quantity of blood is
discharged and a black scab formed. This variety of lichen is very
obstinate and of long duration.

_Lichen tropicus_, popularly known as _prickly heat_