I originally wanted to do the write-up on Phineas Gage, but it looks like it has been done very well. However, after thinking about the case some more, I do have something to add.
When I thought about the entire story some more, I see that the usual explanation, that Phineas Gage had a wound that only affected an area of his frontal lobe concerned with personality and emotion while leaving everything else untouched, is not quite a straight conclusion from the facts. It is quite possible that the physical brain damage was quite unrelated to his personality change.
It could be that the changes in Mr. Gage's personality were due to nothing more then the extreme shock and pain of having a metal object explode through his skull. Most people who have been in a violent or tramautic experience usually do incur some (at least temporary) personality changes. Having a metal rod shoot through your head would probably unnerve most of us enough to change our way of looking at the world. Briefly, it could be possible that Phineas Gage had nothing more then a case of PTSD.
Another explanation is that the wound only worked through the power of suggestion. Perhaps the sober, polite persona that Mr. Gage presented to the world and himself was only an act. When he was wounded, and people were expecting him to act differently, he used it as a chance to slip into a personality that was closer to his true self.