ORIGIN.--_Posterior head_: This arises fleshily from the posterior surface of the fibula beginning almost at the proximal end and from the medial surface of the fibula beginning deep to the distal part of M. popliteus. _Lateral head_: This arises fleshily (sometimes partly tendinously) from the lateral surface of the fibula proximal to the fibular tubercle. Some fibers arise from the distal edge of the tendon of insertion of M. extensor iliofibularis. _Medial head_: This arises fleshily from the posterior surface of the tibiotarsus just medial to the distal part of the posterior head, distal to M. popliteus, and either lateral or distolateral to the origin of M. plantaris. Distal to the junction of the three heads, the muscle arises fleshily from the posterior surface of the tibiotarsus (except the distal part) and from the medial and posterior surfaces of the fibula. INSERTION.--The slender ossified tendon becomes flexible and passes through the canal in the tibial cartilage that lies anterolateral to the canal for M. flexor perforans et perforatus digiti II and anteromedial to the canal for M. flexor perforatus digiti II, then passes through the bony canal of the hypotarsus that is deep to all the other flexor tendons; the tendon ossifies again and lies adjacent (lateral) to the posterior metatarsal crest; the vinculum from the tendon of M. flexor hallucis longus fuses extensively to the superficial surface of the present tendon a short distance below the midpoint of the tarsometatarsus; the tendon is considerably broader below this point than above it. At the level of the first metatarsal, the tendon divides into three branches (unossified) that diverge, each passing through a groove on the ventral surface of the subarticular cartilages ventral to the trochleae, then pass onto the ventral surfaces of digits II, III, and IV. On _digit IV_ the tendon gives off two dorsal fibro-elastic slips before attaching to the ventral surface of the ungual phalanx; one slip attaches to the subarticular cartilage ventral to the third interphalangeal joint, the other to the subarticular cartilage of the fourth joint and may also attach in part to the distal end of the fourth phalanx. On _digit III_ the tendon gives off two dorsal slips before attaching to the ventral surface of the ungual phalanx; one slip attaches to the subarticular cartilage of the second interphalangeal joint, the other to the subarticular cartilage of the third joint and may also attach in part to the distal end of the third phalanx. On _digit II_ the tendon gives off one dorsal slip before attaching to the ventral surface of the ungual phalanx; the slip attaches to the subarticular cartilage of the second interphalangeal joint and may also attach in part to the distal end of the second phalanx. INNERVATION.--A branch of the medial division of the tibial nerve penetrates the medial surface of the posterior head.