This passage, Virgil declares, was less dangerous when he last descended into Hades, for it has since been riven by the earthquake which shook this region when Christ descended into hell.
Pointing to a boiling river of blood (Phlegethon) beneath them, Virgil shows Dante sinners immersed in it at different depths, because while on earth they offered violence to their neighbors.

Although anxious to escape from these bloody waters, the wicked are kept within their appointed bounds by troops of centaurs, who, armed with bows and arrows, continually patrol the banks.

When these guards threateningly challenge Virgil, he calmly rejoins he wishes to see their leader, Chiron, and, while awaiting the arrival of this worthy, shows Dante the monster who tried to kidnap Hercules' wife.
On drawing near them, Chiron is amazed to perceive one of the intruders is alive, as is proved by the fact that he casts a shadow and that stones roll beneath his tread! Noticing his amazement, Virgil explains he has been sent here to guide his mortal companion through the Inferno, and beseeches Chiron to detail a centaur to carry Dante across the river of blood, since he cannot, spirit-like, tread air.
Selecting Nessus for this duty, Chiron bids him convey the poet safely across the bloody stream, and, while performing this office, the centaur explains that the victims more or less deeply immersed in blood are tyrants who delighted in bloodshed, such as Alexander, Dionysius, and others.

Borne by Nessus and escorted by Virgil, Dante reaches the other shore, and, taking leave of them, the centaur "alone repass'd the ford." _Canto XIII._ The travellers now enter a wild forest, which occupies the second division of the seventh circle, where Virgil declares each barren thorn-tree is inhabited by the soul of a suicide.

In the gnarly branches perch the Harpies, whose uncouth lamentations echo through the air, and who greedily devour every leaf that sprouts.