Cutting off the Arrow Shaft

Long, long ago, in a battle a soldier was shot in the leg, and suffered from constant pain. An officer in the troop sent for a surgeon versed in external medicine to treat the soldier's wound.
The surgeon came to have a look, then sain, "This is easy!" He cut off the arrow shaft at the leg with a big pair of scissors, and immediately asked for fees for the surgical operation.
"Anyone can do that," the soldier, getting upset, cried, "The arrow head is still in the leg, why haven't you taken it out?"
"My surgical operation is finished. The arrowhead in your leg should be cured by a physician who practices internal medicine."