This muscle lies on the outer surface of the scapula or blade-bone, the anterior half of which it covers.

Origin. - From the whole of the outer surface of the scapula in front of its spine, and from the spine itself.

Insertion. - Below this muscle ends in two short tendons, which become inserted into the inner and outer tubercle on the head of the humerus respectively. The large tendon of the biceps passes between them.

Action. - To extend the humerus on the scapula.