"Your Cerberus, if you remember well, For that still bears his chin and gullet peeled"

(Canto 9, lines 98-99)

According to the legend of Hercules' twelve labours, when he journeyed the Underworld, Cerberus tried to stop him, but was defeated by him. Hercules tied an iron chain around Cerberus' neck, which scraped the skin off the monster's chin and neck. For some reason, it almost seems like Dante here wants to describe Hercules' (and therefore probably Theseus' as well) journey through the Underworld as willed by God, in like manner as Jesus', when he rescued certain souls from Limbo: mark how the angel says "And which has many times increased your pain" - each time the rebellious angels put themselves up against a sanctioned traveller, their punishment is increased.