Day 21 - Stuck in the pitch

The canto begins with an interesting sentence, stating that Dante and Virgil traverse from one bridge to another, "talking of things my Comedy is not concerned to sing." What an interesting way to start things out.  I really wonder what they were discussing, and why Dante felt compelled to tell his readers they were speaking of things that don't concern his work.  Why even bring it up? My old professor, Dr. Antonio Rutigliano, who inspired a love of Dante would often talk about the myriad of ways in which Dante wrote this poem and it could be looked at.  You could look poetically, metaphorically, spiritually, philosophically, and his favorite, mystically.  It is in the mysticism that Dante reveals the many things he hides (thought various acrostics and alliterations and with the sounds of  certain words mimicking phenomenae).  But i we never discussed this particular phrase.  When Dante hides the action from the reader but also tells the reader he is hiding it, it creates a certain intrigue.  Were they discussing poetic techniques, food differences in italy in the centuries gap between them? It makes the imagination run.
Beyond this curious first sentence, Canto 21 begins with oozing sludge and pitch. They are in some sort of ship bay, an inferno version os the venetian port.  Ships are permanently stuck in the goop that, instead of water, is tar and pitch, rendering them all immovable.  The souls aboard work tirelessly replacing keels, hammering at the prow and the stern, and bulding sails, all for ships that certainly couldn't cast off in this muck.
A demon almost runs Dante over, barking orders about another sinner he needs to cast down.  It's almost humorous in description.  The demon proclaims to another demon he's got another one for him and tosses him into the pitch, like a shipbuilder would toss a hammer to another working on repairs. Or maybe a sous chef going into the wlk in fridge, as the sinner is tossed into a gigantic cauldron; just another ingredient in the hell-stew. The demons shoot Dante and Virgil nast looks, and Virgil urges Dante to not make too big of a scene in witnessing this busy-body scene. But for the most part, they are left unnoticed, just a bit light ribbing to terrify Dante. One of the demons actually gives our protagonists some advice, saying the bridge ahead is out and there best method forward is to walk along the mountain ridge. It's just a weird scene that tempts comedy until then last line, when one of the few laugh out loud moments occur in this tale.  As Dante and Virgil are exiting this damned shipyard along the ridge, a Demon,the leader named Barbariccia (whose name means curling beard), bids them adeiu in the most memorable fashion.  He makes a trumpet of his ass.  Let me just repeat that line.  He makes a trumpet of his ass, giving our heroes the salute of breaking wind. 
One wonders what the next conversation, one not concerned to sing about in this Comedy, was like.