Summary
Dante is deeply discouraged by seeing Virgil so upset, but Virgil recovers as soon as they reach the place where they have to climb, and turns to Dante "with that sweet look that I first saw at the foot of the mountain" (lines 20-21), back in the Dark Wood. Once again he picks Dante up and lifts him high enough that he can climb on his own, with Virgil's help. It's the hardest climb they've had yet, and Dante sits down exhausted at the top, but Virgil urges him on. Sloth won't win him fame. And Dante pulls himself together and goes on.

They go over the next high, narrow bridge, but can only see the souls down in the ditch (the Seventh Bolgia) when they come to the other side. There they see shades tormented by serpents. As they watch, one soul is bitten by a serpent, dissolves into ash, and then re-forms. It turns out to be a notorious plunderer and thief named Vanni Fucci, who is so ashamed to have Dante see him here that he foretells the coming victory of the Blacks in Florence, just to wound Dante.

When he has finished speaking, the thief lifts up both his hands in the obscene gesture called the fig (the fingers in a fist, the thumb thrust through between the index and second finger), defying God. Two serpents choke him and bind his hands, and he goes off. A centaur named Cacus, known for his thievery, pursues him. Three more spirits come in sight, and Dante and Virgil watch as they change shapes with each other, one becoming a serpent.

Analysis
Subtle thieves, who operate by deception, are worse in Dante's scheme of things that open robbers, and they are aptly tormented by creeping serpents. As they did not allow others to call their property their own, so now they cannot call their bodies their own.

The absurdity of the pride that resists and defies God, who is Reality itself, becomes more and more obvious. Farinata one can see as noble; Capaneus is, as Virgil points out, torturing himself with his defiance; and what could be more ludicrous than making an obscene gesture at God?