Origin by tendon-fibres from the lateral surface of the head of the fibula and from the proximal half of the lateral surface of its shaft.
At the junction of the middle and distal thirds of the lower leg the muscle ends in a slender tendon which passes through the groove on the lateral surface of the lateral malleolus. This groove is converted into a canal by a transverse ligament. The tendon passes through the groove on the peroneal tubercle of the calcaneus, then turns onto the sole of the foot and passes through the peroneal groove on the cuboid bone. It then turns mediad and passes through the groove between the ventral processes of the lateral cuneiform and the metatarsal until it reaches the first metatarsal. The entire groove is converted into a canal by the overlying ligaments.
Insertion into (1) the base of the fifth metatarsal, and (2) the outer side of the base of the first metatarsal, and (3) by slender branches into the bases of the other metatarsals.
Relations.—Outer (lateral) surface with the superficial fascia and the tendon of the biceps femoris. Inner surface with the peroneus tertius (r) and peroneus brevis (s), and with the extensor longus digitorum (p). At the proximal end the dorsal border touches the tibialis anterior (n).
Action.—Flexor of the foot.
M. peroneus tertius ([Fig. 90], r).—A slender fusiform muscle beneath the peroneus longus (q).
Origin by fleshy fibres from about the second quarter of the lateral surface of the fibula. The muscle ends in a slender tendon (r′), which passes with that of the peroneus brevis (s) through the groove on the ventral border of the lateral malleolus. The groove is converted into a canal by a transverse ligament. Emerging from it, the tendon turns and passes along the outer margin of the foot. In passing the sesamoid at the base of the first phalanx of the fifth digit it is united to it by a band which passes from the sesamoid to the tendon.
The insertion is finally into the lateral border of the extensor tendon of the fifth digit as it passes from the first to the second phalanx.
Relations.—Outer surface with the peroneus longus (q), the soleus (o), and the superficial fascia. Ventral border with the soleus (o) and flexor longus hallucis ([Fig. 91], m). Inner surface with the peroneus brevis ([Fig. 90], s).
Action.—Extensor and abductor of the fifth digit and flexor of the foot.