One of the show's writers used to hang out on a BBS forum for Fox shows when this was still on the air. He said that if the show had survived into a fourth season, there was going to be a second set of head characters introduced, this time for the "Heddy" character. This obviously never happened, as the show was canceled after its third season. See more
[From opening credits for Series 1
I'm Herman's intellect. Without me he couldn't hold his job, pay his rent or tie his shoes.
I'm Herman's sensitivity. Without me he wouldn't feel tenderness, honesty or love - the good things in life.
Oh! I'm Herman's anxiety and I keep him out of trouble - and believe me, there's trouble *everywhere*!
I'm Herman's lust. Without me he'd miss out on all the good stuff. Y'know, fun, food - babes!