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Peroneus tertius is one of the muscles of the anterior crural compartment of the leg. It is sometimes considered as an accessory part of extensor digitorum longus.
It originates from the inferior third of the anterior surface of the fibula and the adjoining interosseous membrane. It passes distally in the company of extensor digitorum longus under the superior and then the inferior extensor retinaculum. It inserts into the base and shaft of the dorsal fifth metatarsal.
It is innervated by the deep peroneal nerve (L5,S1).
Peroneus tertius acts to dorsiflex and evert the foot.