That form of psoriasis which approximates most nearly the common, non-syphilitic psoriasis, is psoriasis diffusa; and more particularly the variety designated as psoriasis guttata. Psoriasis diffusa consists of rather large, round or irregular, yellowish, pale-red, or copper-colored spots, that become covered with hard, brittle, faint-white scales, the centre of which frequently ulcerates and forms a blackish scurf; the scaly spot frequently shows slight cracks from which a clear serum oozes out, which may be succeeded by ulcerations and condylomatous growths. This form is most frequently seen in the face, though it may likewise break out on the hands and around the ankles, and, according to some, even around the anus and on the scrotum.
Psoriasis guttata, which occurs much more frequently, is not, as some assert, confined exclusively to the hairy scalp; on the contrary, it may spread over the whole surface of the body. On the body it is seen in the shape of small, prominent spots, of the size of a three-cent piece, generally round or oval, not pitted in the centre, generally isolated and scattered in tolerably large numbers. These small spots, which are of a marked copper red, and retain this color for a long time before it changes to gray, soon become covered with a gray, rather firmly-adhering scale, which, after it once drops off, is not apt to form again, and leaves a small, red, somewhat raised spot, surrounded by a narrow white border. These spots often remain a long time before they flatten down and grow paler; even after they have sunk to a level with the skin, the redness often preserves for a long time and imparts to the spots an appearance of being elevated above the skin. After a time, their color changes to brown and then gray, and the eruption disappears without leaving a trace behind.
Psoriasis cornuta, also designated as palmaris and plantaris, because it breaks out almost exclusively in the palms of the hands and on the soles of the feet, may break out in two different forms. In one form it may assume the shape of small risings, of the size of a pea, somewhat wart-shaped, and, seated upon a red, not always distinctly perceptible base, with a hard, white, horny centre, that sometimes can be picked off with the nails, but in other cases seems to be sunk into the integuments, like a wedge This horny elevation is generally surrounded by a ring, of one or two lines in width, having characteristic color but not always distinctly visible, if the hands are not clean, and becoming more prominent after the hands have been washed with cold water. I have seen this form break out, sometimes even after the lapse of a fortnight, in individuals in whom a chancre had been suppressed by external means, without the use of Mercury; whereas the second variety has only been seen by me on persons who had taken a good deal of Mercury for chancre, and even after chronic mercurial poisoning, in cases where no syphilitic taint was present. This variety consists of a more or less broad, scaly spot, or of several of them, on which the epidermis seems to become detached in large scarlet flaps, whose scales are sometimes so thick that it seems as though several layers might be torn away. These scales sometimes crack, forming painful rhagades having a very clean appearance, and being entirely devoid of any of the signs of similar rhagades at the anus, or between the toes. These scales are very often surrounded by a tolerably broad border of an unmistakably syphilitic color; in such cases a so-called mercurial syphilis, or a coalescing of syphilis and mercurial disease most likely prevails.