Serratus anterior. The serratus anterior a muscle that originates on the top surface of the eight or nine upper ribs. The serratus anterior muscle inserts exactly at the front border of the scapula, or shoulder blade. The muscle has three sections: the superior, intermediate or medial, and the inferior. The function of the serratus anterior muscle is to allow the forward rotation of the arm and to pull the scapula forward and around the rib cage.