Summary Pectoralis Major Origin Clavicular head anterior surface of medial half of clavicle Sternocostal head anterior surface of sternum, superior six costal cartilages, and aponeurosis of external oblique muscle Insertion Lateral lip of intertubercular groove of humerus Action Adducts and medially rotates humerus Draws scapula anteriorly and inferiorly Acting alone: clavicular head flexes humerus and sternocostal head extends it Innervation Lateral and medial pectoral nerves (C5-T1) Arterial Supply Pectoral branch of the thoracoacromial trunk