This is a broad, flat muscle of considerable length. It extends from the under surface of the abdomen along the side of the chest to the inner aspect of the arm.

Origin. - From the fascia covering the under surface of the abdomen and the posterior part of the sternum.

Insertion. - Into the internal tubercle of the humerus, the tendon of origin of the biceps, and the fascia which binds it down to the bicipital groove.

Action.- To pull the scapula, and with it the limb as a whole, in a backward direction.