In Greek myth, Perseus used the severed snake-haired head of the Gorgon Medusa as a shield with which to turn his enemies to stone. By the sixteenth century Medusa was said to symbolize the triumph of reason over the senses; and this may have been why Cardinal Del Monte commissioned Caravaggio to paint Medusa as the figure on a ceremonial shield presented in 1601 to Ferdinand I de’ Medici, Grand Duke of Tuscany. The poet Marino claimed that it symbolized the Duke’s courage in defeating his enemies.
As a feat of perspective, the picture is remarkable, for out of the apparently concave surface of the shield —in fact convex— the Gorgon’s head seems to project into space, so that the blood round her neck appears to fall on the floor. In terms of its psychology, however, it is less successful. The boy who modelled the face (in preference to a girl) is more embarrassed than terrifying. For once Caravaggio cannot achieve an effect of horror; he was to find in the legends of the martyrs a more powerful stimulus to the dark side of his imagination than classical myth.