As Dante looks down from the bridge into the ninth trench, he claims that no one could hope to relate in words all the suffering he saw there. He compares the number of bleeding limbs and wounded bodies to the sum of all casualties from some of the world's greatest wars. Dante describes one body split down the middle so that its insides are completely visible. This tortured soul pulls open his own chest and cries out to Dante, identifying himself as Mohammed. He points out another soul, Ali (who instigated a great schism in Islam), whose face is split down the middle.
Dante again worries that he cannot express his story adequately through language. This worry does more to emphasize how incredible his journey was than express any real lack of self-confidence in his talent as a poet. Dante's inclusion of Mohammed allows him to incorporate another religious tradition (that of Islam) into his all-encompassing Christian epic, and to privilege Christianity above that "competitive" religion.
Mohammed tells Dante that the souls here were all sowers of scandal and discord. Since they "split" people by causing schisms and discord, they are now literally split here: a devil with a sword cuts the souls open. Their wounds heal, but then the devil splits them open again. Mohammed then asks who Dante is.
These sinners' punishment of being literally split apart corresponds very specifically to their sin of spreading social discord.
Virgil explains that Dante is not dead and is not being punished here, but is journeying through hell, guided by him. The suffering souls all stand amazed for a moment, so stunned that they forget their pain momentarily. Mohammed asks Dante to relay some advice to someone named Fra Dolcino on earth.
All the suffering souls are stunned at Dante's presence in hell as a living soul. In a place with no break from suffering, it is extremely remarkable that they are so astonished they forget their pain, if only for a brief moment.
Mohammed then walks off, and another spirit comes up to Dante with his ear and nose cut off and a wound in his throat. He asks Dante to remember him, Pier da Medicina, on earth and asks him to relay a warning to two other Italians about their impending deaths at sea.
Pier da Medicina is still concerned about those on earth. He asks Dante to remember him so that he can live on in some form through his name, preserved in Dante's poem.
Dante asks Pier da Medicina to identify another suffering soul, and he points out one whose mouth has been cut open so that he cannot speak. This now voiceless soul is Curio, who spurred on civil strife in ancient Rome. Pier then shows Dante Mosca, who sowed civil discord in Tuscany. Dante tells the man that his deeds brought death upon his family, which makes Mosca flee in misery.
Part of Curio's punishment is the fact that he cannot speak and thus cannot tell his own life story. Dante reacts to Mosca not with pity but with righteous satisfaction at his pain.
Then, Dante sees—and he cautions his reader that he would hesitate to tell this without proof, but his conscience compels him to relate the story—a headless man walking around, holding his own head in his hands. The soul walks over toward Dante and lifts his own head up to talk to Dante. He tells Dante that he is Bertrand de Born, who persuaded a young king to kill his father. Because in turning a son against his father he "sundered those that should be one," (28.139) his body is now severed in two instead of together as one.
Bertrand de Born's amazing punishment, which Dante doubts his poem can convey believably, is darkly suitable to his sin. It thus completes Bertrand's sin, bringing about divine justice.