The |triceps brachii| (three-headed muscle) lies at the upper portion of the inside of the arm. It is the main extensor of the arm and is made up of three teardrop shaped heads: the long head, the lateral head, and the medial head. When working with other nearby muscles it can also move the shoulder, since its upper ends are attached to the |scapula|. The long head, the largest of the three heads, is attached to the |scapula| (shoulder blade) just below the rounded socket of the |shoulder joint|, and extends almost three-fourths of the way toward the front of the arm. The lateral head lies on the back and side of the upper arm. The medial head curves around the back of the |humerus| (upper arm bone) and is mostly covered by the long head. The lower end is attached to the flattened end of the |ulna|. The |triceps brachii| extend the forearm at the |elbow joint|. It works with the |biceps brachii| to control the up and down movement of the forearm.
