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 the front row and said, "Do you know what your arsehole is doing while you're having an orgasm?"

She replied, "Probably fishing with his mates."

The professor laughed so hard he could not continue with the class.