Psoriasis on the hand. Artwork from Robert Willan's 1808 medical textbook, On Cutaneous Diseases, showing what he calls psoriasis palmaria, or psorasis affecting the palm of the hand. Psoriasis is a condition caused by skin cells replicting more rapidly than normal, leading to inflamed patches, or plaques. Willan (1757- 1812) founded the science of dermatology.