_Origin_ by a thin flat tendon from the lateral surface of the lateral epicondyle of the femur just dorsad of the origin of the popliteus. The tendon becomes narrower and thicker and passes through the capsule of the knee-joint and over a slight groove on the tibia just dorsad of the head of the fibula. The belly of the muscle extends the entire length of the tibia, passing beneath the transverse ligament (5) along with the tendon of the tibialis anterior (_n_). At the ankle it runs through a fibrous loop which is attached to the dorsal surface of the calcaneus (the annular ligament of the calcaneus); it then turns and passes onto the dorsum of the foot. The muscle ends in four slender tendons, the two middle of which begin at the transverse ligament, while the others begin at the fibrous loop. The tendons diverge to the four digits. Each tendon is connected with the fibrous sheath surrounding the base of the first phalanx, and each has a synovial bursa beneath it at this point. Near the distal end of the first phalanx the tendon is united on the lateral side to the conjoined tendon of the extensor brevis digitorum and an interosseus muscle, and on the medial side to the tendon of the interosseus. The lateral side of the most lateral tendon is not thus united, but is joined on the outer side by the tendon of the peroneus tertius. _Insertion._--The tendon thus formed on the dorsum of each phalanx by the junction of the two or three tendons above mentioned is firmly attached to the base of the second phalanx at its proximal end in the middle of its dorsal surface. In crossing the joint between the first and second phalanges the tendon forms a fibrous pad which protects the dorsum of the joint and glides over it. It then continues to be inserted into the base of the terminal phalanx. _Relations._--Outer surface with the tibialis anterior (_n_) and the superficial fascia. Inner surface with a part of the tibialis anterior, with the bones, the interosseous membrane, and the peroneus brevis (_s_). _Action._--Extensor of the phalanges. =M. tibialis anterior= (Fig. 90, _n_).--This is the superficial muscle covering the lateral side of the tibia. _Origin_ by fleshy fibres from the proximal one-sixth of the lateral surface of the shaft of the tibia, from the proximal third of the medial border of the shaft and head of the fibula, and from the intervening interosseous ligament. The triangular body of the muscle covers the lateral surface of the tibia and the outer surface of the extensor longus digitorum (_p_). Near the malleolus it ends in a strong tendon which passes beneath the transverse ligament along with the tendon of the extensor longus (_p_). The tendon crosses the dorsal surface of the foot obliquely toward its medial side. _Insertion_ into the outer surface of the first metatarsal.