any muscle with three heads, or points of origin, particularly the large extensor along the back of the upper arm in humans. It originates just below the socket of the scapula (shoulder blade) and at two distinct areas of the humerus, the bone of the upper arm. It extends downward and inserts on (attaches to) the upper part of the ulna, in the forearm. Its major action is extension of the forearm upon the elbow joint, in opposition to the biceps brachii.