In the Simpsons episode where he becomes a boxer, the doctor tells Homer he has a thicker skull than normal and it is why he can take so many blows to the head without getting hurt. I was watching a documentary recently about Marvin Hagler and it was said that Hagler had a thicker than normal skull or a muscle surrounding his skull that was about an inch thicker than usual. This gave him an uncanny ability to take punishment without showing much hurt.