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. Appalled by
the sighs and wailings around him, Dante questions Virgil, who directs
him to break off a twig. No sooner has he done so than he sees blood
trickle from the break and hears a voice reproach him for his cruelty.
Thus Dante learns that the inmate of this tree was once private
secretary to Frederick II, and that, having fallen into unmerited
disgrace, he basely took refuge in suicide. This victim's words have
barely died away when the blast of a horn is heard, and two naked
forms are seen fleeing madly before a huntsman and a pack of mastiffs.
The latter, pouncing upon one victim, tears him to pieces, while Dante
shudders at this sight. Meantime Virgil explains that the culprit was
a young spendthrift, and that huntsman and hounds represent the
creditors whose pursuit he tried to escape by killing himself.