Saints Cosmas and Damian were early Christian martyrs who practised medicine and surgery without payment according to their legend, and were therefore represented to the lay public as medical ideals. In this Spanish altarpiece they appear in a vision, dressed in the full finery of academic doctors as they perform the miracle of transplanting a leg. The vision is described in a medieval book called 'The Golden Legend', written by Jacobus de Voragine. The vision was received by a verger in the church of Saints Cosmas and Damian in Rome. The verger had a disease of the thigh which was eating away his flesh like cancer or gangrene. In his sleep he dreamt that the two saints came and cut off his bad limb and transplanted in its place the leg of a dead African who had just been buried in a nearby churchyard.