The Long Thoracic Nerve (n. thoracalis longus; external respiratory nerve of Bell; posterior thoracic nerve) supplies the Serratus anterior. It usually arises by three roots from the fifth, sixth, and seventh cervical nerves; but the root from the seventh nerve may be absent.
The roots from the fifth and sixth nerves pierce the Scalenus medius, while that from the seventh passes in front of the muscle.
The nerve descends behind the brachial plexus and the axillary vessels, resting on the outer surface of the Serratus anterior. It extends along the side of the thorax to the lower border of that muscle, supplying filaments to each of its digitations.