[TA] three-headed muscle of posterior (extensor) compartment of arm; origin, long or scapular head: lateral border of scapula below glenoid fossa, lateral head: lateral and posterior surface of humerus below greater tubercle, medial head: posterior surface of humerus below radial groove; insertion, olecranon of ulna; action, extends elbow; nerve supply, radial. SYN: musculus triceps brachii [TA], triceps muscle of arm.