Involuntary Muscular Contractions:

A Professor was giving a lecture on ' Involuntary Muscular
Contractions' to
his first year medical students.

Realizing that this was not the most riveting subject, the Professor
decided to lighten the mood slightly. He pointed to a young woman in
front row and said, 'Do you know what your ass hole is doing while
having an orgasm?'

She replied, "Probably deer hunting with his buddies."

He laughed so hard he couldn't finish teaching the class.