A good friend of mine was a boxer and now retired from the sport, but he can't remember **** from being hit too many times.
His memory is shot, as is his body, like mentioned above, his nose has been broken many times. One time that he has told me was he woke up in the emerg after having a machine attached to his face to align his cartalige(sp?) in his nose or something, anyways, he had internal bleeding and nearly bit the big one.
How much of this is possible I don't know I wasn't there, so I take his word on it.
It's a dangerous sport, if you do it, like already said, make sure it's in a controled and safe environment.