Involuntary Muscular Contractions.

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

Realising that this was not the most riveting subject, the Professor decided to lighten the mood slightly.

He pointed to a young woman in the front row and said, "Do you know what your arse hole is doing while you're having an orgasm?"

She replied, "Probably watching football & drinking beer with his mates."

The professor laughed so much he could not continue with the lecture.