Actually, this matter is far simpler than all of you are making it out to be. It's this: do you want to screw over the player, or the monster? Are you on the side of the players or the monster? Answer that, and stick to your guns.
Of course in the case of house rules, you can also ask the question "Is it cooler or better for the game that the rogue be able to slice and dice the monster from the inside, or is it cooler and better for the game if he dies slowly screaming?"
Really, you guys are overthinking things, with your discussion of anatomy and such. If you want a truly generic ruling, use the truly generic principles.