An unique variation of the peroneus tertius muscle.

Peroneus tertius (fibularis tertius) is a muscle unique to humans. It often appears to be a part of extensor digitorum longus, and might be described as its "fifth tendon". Although its insertion variation has been reported by many authors, variations of its origin points are not common. A variation of the peroneus tertius muscle was found during routine dissection of a 75-year-old male cadaver. The muscle originated from the extensor hallucis longus. The muscle belly of the extensor hallucis longus arose from the middle two-fourths of the medial surface of the fibula, medial to the extensor digitorum longus, and anterior surface of the interosseous membrane. It lay under the extensor digitorum longus, and lateral to the tibialis anterior muscle. The muscle belly of the extensor hallucis longus divided into medial and lateral parts 17 cm below its origin point. The lateral part, named as peroneus tertius, continued downward to reach the medial part of the dorsal surface of the base of the fifth metatarsal bone. The medial part ran also downward and divided into two tendons reaching the dorsal surface of the base of the distal phalanx of the great toe. This kind of variation may be important during foot or leg surgery.