This may be variously regarded either as a tumour or a hypertrophy of the skin. The frequently multiple character of the common wart and its grouping suggest an infective origin It consists of a local exaggeration of the norma] structures and occurs in numbers in the same person. A group of papilla? are elongated, and the epidermis over them thickened, so that the characters are those of a hard papilloma.

The Horn is of similar structure to the wart, but the papilla? are usually more elongated, while the horny layer of the epidermis is compacted into a more consistent mass which forms a prominent outgrowth.