_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_).
=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.
=M. peroneus brevis= (Fig. 90, _s_) lies beneath the other peronei.
_Origin_ by fleshy fibres from the distal half of the surface of the fibula, which it embraces. At the lateral malleolus the muscle ends in a thick tendon which passes through the canal on the ventral surface of the malleolus. In the canal it is enlarged and surrounded by a synovial bursa. It then turns onto the foot and passes over the dorsal surface of the calcaneus on its lateral side and has its
_Insertion_ into the tubercle on the lateral side of the base of the fifth metatarsal.
_Relations._--Outer surface with the peronei longus (_q_) and tertius (_r_). Inner surface with the bone.