Dante again invokes the Muses to help him write what he sees, just as he had done at the beginning of his journey. This time the invocation is longer and even more passionate. He knows that this portion of the journey is going to be harsh and horrible, and he hopes that he has the words for it.
Dante the Poet is fully aware that the noble art of poetry is not designed to describe the horrors of this dreadful abode. Poetry is not usually devoted to harsh and grating and vulgar sounds. Thus, he invokes the Muses of Poetry to help describe horrors in poetic terms.
The geography of the final pit of Hell is explained in these two cantos. There are four rounds in this circle of traitors. The first, Caina, reserved for those who were traitors to their kin, is named for the Biblical Cain who slew his brother, Abel. Remember also that Francesca (Canto V), in her story, says that Caina awaits hers and Paolo's murderer, Paolo's brother. The sinners that Dante finds here are two brothers who killed one another in a squabble over their inheritance, hence they must spend all of eternity locked together, bickering and butting heads.
