Structure: Fibularis Tertius Muscle (aka Peroneus Tertius)

The peroneus tertius is a fun muscle. First of all, it is less of a distinct muscle and more of an accessory tendon of the extensor digitorum longus. When present, the peroneus tertius tendon arises from the extensor digitorum longus, and the muscle fibers it's attached to are considered its muscle belly. Secondly, not everyone has one. Third, and most important, my GTA in undergrad made us say "peroneus tertius" in a British accent, and I would advise you do the same.

Before you try memorizing the info below by rote, let's break down the name to make the structure more conceptual. "Peroneus" refers to the lateral aspect of the leg. "Fibularis" has the same meaning, and is the more preferred term since it avoids confusion with "perineum" and "peritoneum," which are completely unrelated terms. Also, "fibularis" makes more intuitive sense because that's the region where the fibula bone is. Keep in mind, though, that many clinicians and anatomists are old-school and still use "peroneus."

"Tertius" means "third." This is in reference to the fact that there are two other fibularis muscles (fibularis longus and fibularis brevis), making this the third fibularis muscle. It makes sense that this inconsistent, minor muscle would take third place to the other two fibularis muscles. The tricky part is that the fibularis tertius is not really related to the other two. By that I mean it is innervated by a different nerve and is in a different fascial compartment than the other two. The similarities are that it everts the foot and its tendon attaches near the fibularis brevis.

Basic muscle info:
Proximal attachment: Distal fibula.
Distal attachment: Base of fifth metatarsal.
Action: Assists eversion and dorsiflexion.
Innervation: Deep fibular nerve.

Advanced muscle info:
Origin: Distal third of anteromedial fibula, interosseus membrane
Insertion: Dorsomedial aspect of base of 5th metatarsal.
Action: Weak eversion of subtalar (talocalcaneal) joint. Weak dorsiflexion of ankle (talocrural) joint.
Innervation: Deep fibular nerve (L5, S1).
Arterial supply: Anterior tibial artery


References 1,2