Peroneus tertius
The peroneus tertius muscle is generally considered by anatomists to be part of extensor digitorum longus. Peroneus tertius is often difficult to identify due to its anatomical location. Furthermore, this muscle often displays considerable anatomical variation and may be absent. When presenting in an anatomically normal manner however, peroneus tertius originates on the lower 1/3 of the medial shaft of the fibula and the crural interosseous membrane. It is indistinguishable from extensor digitorum longus at its origin. After passing beneath the extensor retinaculum, peroneus tertius inserts into the medial part of the dorsal surface of the base and shaft of the fifth metatarsal. The muscle is best identified by asking your subject to abduct, pronate and dorsiflex their foot against resistance applied to the lateral border of the foot. The tendon will appear laterally to the most lateral tendinous slip of extensor digitorum longus. It will extend toward the base of the fifth metatarsal from a dorsal direction.