Ali's iron ribcage

i just finished watching Ali-Frazier III (the thrilla in manila) for the first time in years and first of all lemme just say that if you haven't seen this fight, you've gotta watch it - it might be the best boxing match that i've ever seen. there are lots of comments i could make about this fight, but the biggest thing i was wondering about was this: how the hell could Ali take all of those body shots from Frazier, and still have the juice left in round 14 to tko the guy?? i mean Frazier just slams him to the body over and over again, round after round, lots of good clean body blows, and Ali doesn't even seem to flinch. same thing as when he fought George Foreman, of all people - big george nailed him with some huge body punches, and Ali just took them. talk about an iron ribcage... i still think, though, that if Frazier had kept his hands up instead of coming in with his head unprotected, he would have been able to last the full 15 and possibly won the decision despite ali's incredible ability and toughness.