Erythema nodosum, a painful disorder of the subcutaneous fat, is the most common type of panniculitis. It was originally described in 1798 by English dermatologist Robert Willan (1757-1812) in his classic monograph on erythemas:
In the erythema nodosum, many of the red patches are large and rounded. The central parts of them are very gradually elevated, and on the sixth or seventh day, form hard and painful protuberances, which are often taken for imposthumes, but from the seventh to the tenth, they constantly soften and subside, without ulceration. On the eighth or ninth day, the red colour changes to bluish or livid, and the affected limb appears as if it had been severly bruised. The appearance remains for a week or 10 days, when the cuticle begins to separate in scurf.1(p483)