Cruel Professor

An Autopsy professor was talking to his class over a human corpse one day. "To have a successful medical career you need two things; the first is no fear." With that, he took his finger, stuck it up the corpse's anus, and licked it. "Now I want all of you to do the same." After much hesitation the class did the same. After that the professor said, "The second thing you will need is a keen sense of observation; for example, how many of you noticed that I stuck my middle finger up the corpse's anus but that I licked my pointer finger?"