long thoracic nerve, one of a pair of supraclavicular branches from the roots of the brachial plexus. It arises by three roots, from the fifth, the sixth, and the seventh cervical nerves. The fibers from the fifth and the sixth cervical nerves join just after they pierce the scalenus medius and are united with the fibers from the seventh cervical nerve at the level of the first rib. Compare phrenic nerve.
