Origin Lateral third of clavicle, acromion, and spine of scapula Insertion Deltoid tuberosity of humerus Action Anterior part: flexes and medially rotates arm; Middle part: abducts arm; Posterior part: extends and laterally rotates arm Innervation Axillary nerve (C5 and C6) (C5, C6) Arterial Supply Deltoid branch of thoracoacromial artery