Stand up. Step forward. Bend an elbow. Button your shirt. Feed yourself. They are simple commands, carried from the brain to the legs, arms, and hands by thin white fibers, nerves no thicker than a piece of yarn, impossibly delicate yet incredibly important. Cut them, break them, shred them--a car crash or a sharp piece of broken glass will do it--and a limb hangs helpless. Don't count on most orthopedic surgeons for help--they'll fix broken bones but not broken nerves. Neurosurgeons? They like to work on the brain and spinal cord. "Peripheral nerves, the ones in the extremities, are orphans...