When the doctor asks, "how do you feel, can you see clearly?" the man replies, "doc. i can't see a thing."
Doctor thinks for a while, then gestures a nurse to come in and undress in front of the man.
The man again replies he can't see a thing when the doctor asked him yet once again if he can see anything?
The doctor motions the nurse to spread her legs and then asks the man, "Do you see anything now?"
The man shakes his head and gives a firm reply, "No doctor, i can see nothing."
Doctor gives the man a tight bird (slap) on the head and exclaims, "If you can't see a thing, why are you getting an erection?"