_Action._--Flexor of the phalanges. =M. tibialis posterior= (Fig. 91, _o_).--A slender flat fusiform muscle beneath the flexor longus digitorum and between it and the flexor longus hallucis. _Origin_ by fleshy fibres from nearly the whole medial surface of the head of the fibula, from the inner surface of the aponeurosis between it and the flexor longus digitorum (_n_), and by a few fleshy fibres from the ventral surface of the tibia between the oblique ridges. Some fibres may also arise from the outer surface of the aponeurosis covering the flexor longus hallucis (_m_). The muscle ends at about the middle of the tibia in a slender flat tendon which passes parallel to the tendon of the flexor longus digitorum (_n_) through the dorsal groove on the medial surface of the distal end of the tibia. This groove is converted into a canal by a transverse ligament. Beyond the canal the tendon turns onto the plantar surface of the foot and passes through a groove on the ventral surface of the scaphoid bone. It then divides. _Insertion_ into the outer tuberosity on the surface of the scaphoid, and onto the proximal end of the ventral surface of the medial cuneiform. _Relations._--Outer and medial surface with the flexor longus digitorum (_n_). Lateral surface with the flexor longus hallucis (_m_). _Action._--Extensor of the foot. B. MUSCLES ON THE DORSAL AND LATERAL SURFACES OF THE LOWER LEG.--=M. peroneus longus= (Fig. 90, _q_).--A slender, fusiform muscle lying superficially on the lateral side of the leg, dorsad of the soleus. _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_).