Unlike its proximal predecessors, the pectoral girdle muscles and shoulder joint muscles, the brachial muscle group is confined completely within the superior limb proper. These muscles range in size from the small articularis cubiti muscle to the large triceps brachii muscle and form the fleshy mass that surrounds the humerus. The humerus separates the muscles into two distinct masses, which each arise from different developmental primordia. Anterior to the humerus and enclosed within a distinct fascial envelope is the anterior muscle compartment of the brachium, which consists of three muscles. Posterior to the humerus and enclosed within a distinct fascial envelope is the posterior muscle compartment of the brachium, which consists of two muscles. The muscles of the brachium are prime movers of the elbow joint.