The medial head of the triceps brachii muscle is one head of the triceps muscle and originates from the back of the humerus. The triceps brachii has three such heads (connective immovable muscle) and is the only muscle on the back of the upper arm. It connects the humerus (upper arm bone) and the scapula (shoulder blade) to the ulna (longest of the forearm bones) and is the primary extensor of the elbow. The three heads are the lateral, the medial, and the long head.