Serratus anterior(boxer’s muscle) | SHOULDER MOVERS ORIGIN: by series of muscle slips from ribs 1-9. INSERTION: the anterior surface of the vertebral border of the scapula. ACTION: an agonist to protract & hold scapula against the rib cage rotates scapula (inferior angle laterally & upward) abduct & raise arm & horizontal arm movements. NERVE SUPPLY: The long thoracic nerve.