Object lesson

A group of medical students are having their first encounter with a cadaver. The instructor tells them the first lesson they have to learn is to overcome their squeamishness. As an object lesson, he inserts his finger into the cadaver's rectum. He invites each student in turn to do the same. They do so, with varying degrees of reluctance and disgust.
"Now, watch!" the instructor says. And he puts his finger in his mouth and sucks it. The students gasp.
"Now, you do the same!" he instructs.
One by one they do so, gagging and heaving.
"The second lesson you have to learn is to be VERY observant. I put my middle finger into the cadaver's rectum, but I put my forefinger in my mouth."