Med students (adult)

A professor is giving the first year medical students their first lecture on autopsies, and decides to give them a few basics before starting.
"You must be capable of two things to do an autopsy. The first thing is that you must have no sense of fear."
At this point, the lecturer sticks his finger into the dead man's anus, pulls it out, and then licks it. He asks all the students to do the same thing with the corpses in front of them. After a couple of minutes silence, they follow through with his disgusting command.
"The second thing is that you must have an acute sense of observation: How many of you noticed that I stuck my middle finger into the corpse's anus, but I licked my index finger ?"