William Blake, The Circle of the Thieves; Agnolo Brunelleschi Attacked by a Six-Footed Serpent, Canto XXV, 1827

We’re recapping the Inferno. Read along! This week: Virgil shuts up and men become reptiles.

Canto 25 is known for having the least dialogue of any canto in the Inferno. It seems like a minor feat, but when you remember how many questions Dante likes to ask, and how long Virgil will typically spend explaining things, and how sinners really like to chat it up with the living, canto 25 begins to seem remarkable. In fact, Virgil hardly has the chance to explain anything at all here.

It begins with Vanni, the sinner from canto 24 who, in a fit of shame and spiteful anger, revealed to Dante the sad fate of the White Guelph party in Florence. Vanni makes an obscene gesture into the air, and curses God. And although we do not know exactly what “Making the figs with both his thumbs” means, we can guess that it is the centuries old Italian way of flipping God the bird. Dante wants to get away, and the snakes from the previous canto attack Vanni, hog-tying him and wrapping themselves around his neck to silence him.

As Dante leaves Vanni, a centaur—with snakes on his back and a dragon on his shoulder—starts screaming. The centaur’s voice was the mysterious sound Dante heard in canto 24, before he lost interest. This is Virgil’s only didactic moment in the canto: he tells Dante that the centaur is Cacus, who was clubbed to death by Hercules. (Accounts of Cacus’s death vary depending on the poet’s preference—strangely, the version Virgil tells Dante is not the version Virgil himself wrote about. Virgil’s Cacus was strangled to death.) Dante uses canto 25 to quarrel with other poets. Later, he describes a scene in which two sinners change their physical appearances, and he essentially tells the reader that this scene is far more striking than any of the shape-changing moments written by Ovid or Lucan.

The rest of canto 25 features a lineup of sinners, all of them Florentines—just in case Dante’s thoughts on the state of Florence weren’t clear enough. One has to wonder: With all the terrible sinners in Florence and the disastrous state of politics in the city, what’s left there for Dante to like at all?

First, Dante watches as a reptile bites a sinner; as the beast wraps itself around the sinner’s body, clinging to him like ivy, Dante says, “Slowly, the two become one,” becoming some frightening, unknown beast with one head, one pair of limbs, and “members never seen before.” Unlike many of the other punishments, which are designed primarily to inflict pain on the sinners, this one is psychological—it’s demeaning.

And it seems rather effective. Our pilgrim is bewildered by the scene before him. But he’s about to witness something far more harrowing: as the first transformation finishes, another reptile scuttles out and bites another sinner in the stomach. As the two become transfixed, staring at one another, a thick smoke pours from the reptile’s mouth and from the sinner’s wound. Soon, the two plumes of smoke merge, and slowly the reptile takes on the shape of a man. Its tail becomes legs, its limbs become arms, it grows human skin and walks upright, and its animal features become a human face. As the reptilian hiss slowly becomes a human voice, the wounded sinner and the reptile exchange bodies—the sinner makes the same transformation, but in reverse. Dante believes he may be able to recognize the sinner who had just taken a human form—Francesco de’ Cavalcanti.

There is something gruesome and poetic in the absolute precision of Dante’s descriptions of the metamorphoses. Canto 25 offers a rare, scientific glimpse into the nature of punishment in hell, and it stands out, along with the forest of suicides and Pier delle Vigne, as one of the more definitive examples of how punishments in hell actually work.

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Alexander Aciman is the author of Twitterature. He has written for the New York Times, Tablet, the Wall Street Journal, and TIME. Follow him on Twitter at @acimania.