Interpretations of the unicorn myth focus on the medieval lore of beguiled lovers, whereas some religious writers interpret the unicorn and its death as the Passion of Christ. The myths refer to a beast with one horn that can only be tamed by a virgin; subsequently, some writers translated this into an allegory for Christ's relationship with the Virgin Mary. The unicorn also figured in courtly terms: for some 13th-century French authors such as Thibaut of Champagne and Richard de Fournival, the lover is attracted to his lady as the unicorn is to the virgin. With the rise of humanism, the unicorn also acquired more orthodox secular meanings, emblematic of chaste love and faithful marriage. It plays this role in Petrarch's "Triumph of Chastity", and on the reverse of Piero della Francesca's portrait of Battista Strozzi, paired with that of her husband Federico da Montefeltro (painted 1472–74), Bianca's triumphal car is drawn by a pair of unicorns. The Throne Chair of Denmark is made of "unicorn horns" – almost certainly narwhal tusks. The same material was used for ceremonial cups because the unicorn's horn continued to be believed to neutralize poison, following classical authors. The unicorn, tamable only by a virgin woman, was well established in medieval lore by the time Marco Polo described them as "scarcely smaller than elephants. They have the hair of a buffalo and feet like an elephant's. They have a single large black horn in the middle of the forehead... They have a head like a wild boar's… They spend their time by preference wallowing in mud and slime. They are very ugly brutes to look at. They are not at all such as we describe them when we relate that they let themselves be captured by virgins, but clean contrary to our notions." It is clear that Marco Polo was describing a rhinoceros. The horn itself and the substance it was made of was called alicorn, and it was believed that the horn holds magical and medicinal properties. The Danish physician Ole Worm determined in 1638 that the alleged alicorns were the tusks of narwhals. Such beliefs were examined wittily and at length in 1646 by Sir Thomas Browne in his "Pseudodoxia Epidemica". False alicorn powder, made from the tusks of narwhals or horns of various animals, has been sold in Europe for medicinal purposes as late as 1741.