_Insertion._—(1) The internal tuberosity of the humerus; (2) the external lip of the bicipital groove; (3) the tendon of origin of the coraco-brachialis. _Action._—To adduct and retract the limb; if the limb is advanced and fixed, to draw the trunk forward. _Structure._—This muscle is also almost entirely fleshy. Its posterior part is wide and thin, but as the muscle is traced forward, it becomes narrower and much thicker. It passes forward and slightly upward in a gentle curve to its insertion. The humeral insertion is just below that of the inner division of the supraspinatus. Part of the fibers are inserted by means of a tendinous band which binds down the tendon of the biceps and is attached to the external lip of the bicipital groove, and a small part is attached to the tendon of origin of the coraco-brachialis. _Relations._—Superficially, the skin, panniculus, and superficial pectoral; deeply, the abdominal tunic, the external oblique, the rectus abdominis et thoracis, the brachial vessels, and branches of the brachial plexus of nerves. The external thoracic vein lies along the outer or upper border. _Blood-supply._—Internal and external thoracic, inferior cervical, anterior circumflex, and intercostal arteries. _Nerve-supply._—Pectoral (or thoracic) nerves, from the brachial plexus. 4. =Serratus magnus= (M. serratus ventralis).—This is a large, fan-shaped muscle, situated on the lateral surface of the neck and thorax. It derives its name from the serrated ventral edge of its thoracic portion. It consists of a cervical and a thoracic portion. (=a=) =Cervical part= (M. serratus cervicis; levator scapulæ hominis). _Origin._—The transverse processes of the last four or five cervical vertebræ. _Insertion._—The anterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage. (=b=) =Thoracic part= (M. serratus thoracis; serratus anterior hominis). _Origin._—The external surfaces of the first eight or nine ribs. _Insertion._—The posterior triangular area on the costal surface of the scapula and the adjacent part of the cartilage. _Action._—The two muscles form a sort of sling in which the trunk is suspended. Contracting together, they raise the thorax; contracting singly, the weight is shifted to the limb on the side of the muscle acting. The two parts can contract separately and are antagonistic in their action on the scapula. The cervical part draws the base of the scapula toward the neck, while the thoracic part has the opposite action; these effects concur in the backward and forward swing of the limb respectively. With the limb fixed, the cervical part extends (raises) the neck or inclines it laterally. The thoracic part may act as a muscle of forced inspiration.