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) the 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, scaly patches, and by the history and course of the individual lesions. #In what respects does the papulo-squamous syphiloderm differ from psoriasis?#