Bartolomeo Corsi was a Florentine monk from the 12th Century, one of many individuals who wound up exchanging bodies with one of the Great Race of Yith. He was met (in his temporary Yith body) by Nathaniel Wingate Peaslee (also in one of the Yith).
Corsi went mad for a time after being returned to his body, and was imprisoned by local authorities on suspicion of supporting the Holy Roman Emperors. Pope Innocent IV, upon learning of this, commuted Corsi's sentence to exile and sent him to the island of Stromboli. There, he wrote the Harmaticon. It has been suspected that Corsi met a young Dante Alighieri, since portions of Dante's Inferno are alleged to bear a resemblance to parts of the Harmaticon.