you see, the joke is actually in the fact that epidermis is your skin and not your hair. So the joke isnt in Nelson saying "Ha Ha' but being incorrect and smug about his knowledge of medical terms.
I've been quoting this since I saw it the first night it aired, and have been haltingly corrected by friends every time. Sometimes, they lower their voice and look around and try to tell me on the sly that, no, your epidermis is really your skin.