Origin by muscle- and tendon-fibres from the ventral surface of the tibia over its proximal half between the oblique ridges; by fleshy fibres from an aponeurosis between it and the tibialis posterior ([Fig. 91], o), and by tendon from the medial surface of the head of the fibula. At the junction of the middle and distal thirds of the bone the muscle ends in a slender tendon which passes through the ventral groove on the medial surface of the tibia and, curving onto the sole of the foot, becomes attached to the medial border of the common tendon [described] under the last.
Relations.—Outer surface with the medial head of the gastrocnemius ([Fig. 91], k) and with the superficial fascia. Inner surface with the tibialis posterior ([Fig. 91], o). Medial border at the proximal end with the popliteus ([Fig. 92], k), lateral border with the tibialis posterior ([Fig. 91], o), and flexor longus hallucis ([Fig. 91], m).
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.