f fifteen and thirty. It is rarely seen
before the tenth year, and a first attack is uncommon after the age of
forty.

#Has psoriasis any special parts of predilection?#

The extensor surfaces of the limbs, especially the elbows and knees, are
favorite localities, and even when the eruption is more or less general,
these regions are usually most conspicuously involved. The face often
escapes, and the palms and soles, likewise the nails, are rarely
involved. In exceptional instances, the eruption is limited almost
exclusively to the scalp.

#Are there any constitutional or subjective symptoms in psoriasis?#

There is no systemic disturbance; but a variable amount of itching may
be present, although, as a rule, it is not a troublesome symptom.

#Describe the clinical appearances of a typical, well developed case.#

Twenty or a hundred or more lesions, varying in size from a pin-head to
a silver dollar, are usually present. They are sharply defined against
the sound skin, are reddish, slightly elevated and infiltrated, and more
or less abundantly covered with whitish, grayish or mother-of-pearl
colored scales. The patches are usually scattered over the general
surface, but are frequently more numerous on the extensor surfaces of
the arms and legs, especially about the elbows and knees. Several
closely-lying lesions may coalesce and a large, irregular patch be
formed; some of the patches, also, may be more or less circinate, the
central portion having, in a measure or completely, disappeared.

#Give the development and history of a single lesion.#

Every single patch of psoriasis begins as a pin-point or pin-head-sized,
hyperaemic, scaly, slightly-elevated lesion; it increases gradually, and
in the course of several days or weeks usually reaches the size of a
dime or larger, and then may remain stationary; or involution begins to
take place, usually by a disappearance, partially or completely, of the
central portion, and finally of the whole patch.

#Describe the so-called clinical varieties of psoriasis.#

As clinically met with, the patches present are, as a rule, in all
stages of development. In some instances, however, the lesions, or the
most of them, progress no further than pin-head in size, and then remain
stationary, constituting _psoriasis punctata_; in other cases, they
may stop short after having reached the size of drops--_psoriasis
guttata_; in others (and this is the usual clinical type) t