486 DISEASES OF THE SKIN. 



causing the papillae to produce an unhealthy epidermis, which, mingled 

 with the exudation, scales off in large flakes. The cause of psoriasis 

 is quite unknown. This disease, which is of very frequent occurrence 

 among all classes of society, cannot be regarded as the exponent of a 

 dyscrasia, for it attacks perfectly healthy subjects, and, in fact, seems 

 to prefer them, while invalids and decrepit persons often remain ex- 

 empt. In some families psoriasis is hereditary. Men and women seem 

 to be about equally liable to it, while it rarely attacks either children 

 in their first year or very aged persons. 



SYMPTOMS AND COTJESE. The disease always commences in small 

 round spots upon the skin. These are reddened; project slightly 

 above the surrounding level, and immediately after their appearance 

 are smooth, looking as if the covering of epidermis had been raised by 

 a serous effusion, and had sunk back again after the liquid had been 

 reabsorbed. This small, round, red, infiltrated spot soon becomes cov- 

 ered with dry, white scales. According to Hebrews apt and simple 

 description, it is from this form of psoriasis psoriasis guttata that 

 all the other varieties of psoriasis arise, developing sometimes through 

 extension of the morbid process, sometimes through retrocession of the 

 disease at the point first attacked. An enlargement of the affected 

 spot converts a psoriasis guttata into a p. nurmdaris ; this latter, 

 when the process begins to decline in the central and older portion of 

 the area of disease, so that the scales grow thinner and commence to 

 fall off, becomes a p. scutellata, and this again becomes a p. annulata, 

 when the redness disappears from the centre, and the skin at that 

 point resumes its healthy appearance (lepra vulgaris, Willari). When 

 the circles thus formed touch one another, interruptions occur at the 

 points of contact, so that at last there merely remain segments of cir- 

 cles, producing a new form of the disease, called p. gyrata. Finally, 

 p. conferta s. diffusa is the result of the confluence of numerous spots 

 of the eruption. The favorite seat of psoriasis is upon the dorsal or 

 extensor surfaces of the extremities, and, above all, upon the knees 

 and elbows. The disease often remains confined to these regions, no 

 trace of it being discoverable elsewhere upon the body. In cases of 

 this kind, and indeed in cases where the eruption is quite extensive, it 

 is often arranged upon each half of the body with remarkable sym- 

 metry. This, however, occurs in other cutaneous diseases also ; for 

 instance, in eczema. In very rare instances, psoriasis is confined to the 

 eyelids, or to the lips, the prepuce, the scrotum, or to the labia majora r 

 and there is no reason why these varieties should not be called p. pal 

 pebrarum, labiarum,prceputii, scroti, or pudendorum, etc. But neither 

 the diffuse nor the circumscribed form of psoriasis palmaris and plan- 

 taris is included in this series of varieties. In the diffuse form, the 



