There was no special reason for the six fingers. Hannibal was just born that way. There is a name for the condition, it mentions it in the book, but I can't remember it off the top of my head. In the book Hannibal also has very unusual eyes.

It is hard to avoid attention when you have six fingers so there does come a point where hannibal has one of them removed. He also performs surgery on his own face. They really are very good and The Silence of the Lambs has one of the best closing paragraphs I have ever read. It only works though if you have read the full story. I wrote a review about it, which can be found HERE and make some of the same points there.

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A Passion For Horror