This particular troll brain, courtesy of Harrie Skarie, is from the rare species of Morian Cave Troll. Even though the Morian caves are geographically situated only two parallel dimensions away from the Mines of Moria it's considered a much nicer neighborhood owing to the fact that the Morian Cave Council has staunchly refused to allow Starbucks to set up commerce, and they don't have any of those nasty Orcs.
The majority of my troll knowledge comes from Terry Pratchett's Discworld. Being made of stone one could also assume they'd have rock brains, and since they aren't known for their vast intellect, except for under extremely frigid conditions, one might also assume that they have teeny tiny brains.
The rock pictured is only an 1.5 long, but one could claim it came from a cranium the size of a compact electric car.
A little Spanish moss, a small rock, a jar and you've got yourself a nice little specimen.