The class is grouped around the body fully covered with a sheet and the professor says:
"Class, a good doctor must always take care of two things. One, he must respect every part of the human body and not be ashamed of it".
To demonstrate, he lifts the sheet, inserts a finger into the posterior of the body and then puts finger in the mouth. The students look at him aghast. He says, "Ok, now it is your turn". The students look disgusted but since they have seen the Prof. do it, one by one they come forward, put a finger inside the posterior of the dead body and put it in their mouth (Yuck and double Yuck).
After everybody is through, the prof. says, "And now for the second important thing that a doctor should know...
YOU MUST BE MORE OBSERVANT.
If you all noted, I put in my middle finger inside the posterior of the body, but sucked on my index finger!"