he patches
develop to the size of coins--_psoriasis nummularis_. In some cases
there is a strong tendency for the central part of the lesions to
disappear, and the process then remain stationary, the patches being
ring-shaped--_psoriasis circinata_; and occasionally several such rings
coalesce, the coalescing portions disappearing and the eruption be more
or less serpentine--_psoriasis gyrata_. Or, in other instances, several
large contiguous lesions may coalesce and a diffused, infiltrated patch
covering considerable surface results--_psoriasis diffusa, psoriasis
inveterata_.

[Illustration: Fig. 17. Psoriasis.]

#Is the eruption of psoriasis always dry?#

Yes.

#What course does psoriasis pursue?#

As a rule, eminently chronic. Patches may remain almost indefinitely, or
may gradually disappear and new lesions appear elsewhere, and so the
disease may continue for months and, sometimes, for years; or, after
continuing for a longer or shorter period, may subside and the skin
remain free for several months or one or two years, and, in rare
instances, may never return.

[Illustration: Fig. 18. Psoriasis.]

#Is the course of psoriasis influenced by the seasons?#

As a rule, yes; there is a natural tendency for the disease to become
less active or to disappear altogether during the warm months.

#What is known in regard to the etiology of psoriasis?#

The causes of the disease are always more or less obscure. There is
often a hereditary tendency, and the gouty and rheumatic diathesis must
occasionally be considered potential. In some instances it is apparently
influenced by the state of the general health. It is a rather common
disease and is met with in all walks of life.

#Is psoriasis contagious?#

No. In recent years the fact of its exhibiting a family tendency has
been thought as much suggestive of contagiousness as of heredity.

#What is the pathology?#

According to modern investigations, it is an inflammation induced by
hyperplasia of the rete mucosum; and it is beginning to be believed that
this hyperplasia may have a parasitic factor as the starting-cause.

#With what diseases are you likely to confound psoriasis?#

Chiefly with squamous eczema and the papulo-squamous syphiloderm; and on
the scalp, also with seborrh[oe]a. It can scarcely be confounded with
ringworm.

#How is psoriasis to be distinguished from squamous eczema?#

By the sharply-defined, circumscribed, scattered,