Squamous Inflammations

There are two important diseases of the skin with which we shall deal in this chapter, namely, pityriasis and psoriasis. In the former malady, in its typical form, the surface of the body is deeply reddened (hyperaemic), and covered by large and freely imbricated scales or flakes; hence the term applied to it - pityriasis rubra. In the disease there is no real inflammation in the form of new products. Hebra allies it to eczema, and upon the ground that "we occasionally find moist excoriated patches on other portions of the skin, especially in the flexures of the joints." But this is infinitely rare; from beginning jto end, there need be nothing but hyperaemia and scaliness present in the disease.

There is not necessarily any change in the corium tissue or the connective tissue, though the hyperaemia, if persistent, may be followed by hyperplasia and thickening of these parts, but only as accidental epiphenomena.



In psoriasis a somewhat different state of things obtains; there is hyperaemia of the papillary layer of the skin, with hyperplasia of the epithelial elements, but I believe the latter to be the more important of the two; and in this respect psoriasis contrasts with pityriasis rubra - the former being essentially a disease of cell tissue, the latter rather an hyperaemia primarily.