And Averroes says that the imposthume called carbuncle is most frequent in time of plague when the air is corrupt, and therefore the blood boils477 and putrefies; and an evil complexion and putrid fever composed of diverse humours come(s) thereafter. Formica is as follows: an imposthume that travels in the skin478 and is not broad: sometimes there are broad479 pustules, which move from one place to another, and form sores; they have wide roots and are of the colour of ashes (yellowish), and fiery choler is the cause thereof.