TRICEPS BRACHII
A three headed, large fleshy muscle of the posterior humerus. The long head (origin-inferior margin of glenoid cavity), the lateral head (origin-posterior humerus) and the medial head (origin-distal radial groove on the posterior humerus) insert on the olecranon process of the ulna. It produces a powerful extension of the forearm which opposes the action of the forearm flexors, i.e., the brachialis and biceps brachii.