Joelle's teeth and the dentist dream — "as her spread mouth spreads farther…"
as her spread mouth spreads farther to cry out in fear all she can see in the little round mirror are endless red-stained rows of teeth leading back and away down a pitch-black pipe, and the image of all these rows of teeth in the disk blots out the big dentist’s good face as he probes with a hook and says he assures her that these can be saved.
Context
The reflected image usurps the person: the disk's picture "blots out the big dentist's good face" — a small screen overwriting the face behind it, terror by pure optics. And the regress runs "down a pitch-black pipe": the scene opened with 'base cocaine eating teeth, and the mirror routes the damage back down the glass pipe it came from (pipe-reading inferred, but it sits directly on the scene's own setup).