GENERAL DESCRIPTION AND RELATIONS.--Thick; on anterior aspect of thigh deep to M. peroneus longus; bounded posteriorly by M. extensor digitorum longus and posterolaterally by Mm. flexor digitorum longus and peroneus brevis; divided into two heads--tibial and femoral; small femoral head adjacent to posterolateral surface of much larger tibial head; two heads joined near midpoint of fleshy part of muscle, forming bipinnate belly (pinnate structure most evident on deep surface); proximal part of femoral head situated between outer cnemial crest and head of fibula; proximal part of anterior surface of tibial head somewhat fused to overlying M. peroneus longus; medial surface fused to aponeurosis of latter. ORIGIN.--_Tibial head_: This arises by fleshy and tendinous fibers from the edge of the inner cnemial crest, from the rotular crest between the inner and outer cnemial crests, and from the anterior surface, distal edge, and posterior surface of the outer cnemial crest; the attachment may or may not extend onto the superficial surface of the distal part of the patellar tendon; the attachment is adjacent to the origin of the underlying M. extensor digitorum longus. _Femoral head_: This arises by a slender tendon from the notch in the distal end of the external condyle of the femur. INSERTION.--The slender ossified tendon extends along the anterior surface of the distal end of the tibiotarsus and passes through a large, strong, oblique retinaculum (superficial to the supratendinal bridge); the lateral end of the retinaculum attaches to the lateral end of the supratendinal bridge; the medial end attaches immediately proximal to the medial end of the bridge. The tendon widens and becomes flexible as it passes across the anterior surface of the intratarsal joint, then narrows and attaches to the tubercle on the anterior surface of the proximal part of the tarsometatarsus between Mm. extensor hallucis longus and extensor brevis digiti IV. The distalmost bundle of tendinous fibers does not attach to the tubercle, but extends distally along the anterior surface of the tarsometatarsus and attaches to the latter a few mm. distal to the tubercle, forming an accessory insertion. A part of the peroneal nerve passes between the main and accessory insertions. INNERVATION.--A variable number of branches of the peroneal nerve penetrate the lateral surface of the femoral head; a variable number of branches of the same division pass deep to the femoral head and enter the posterior edge of the tibial head. INDIVIDUAL VARIATION.--- In one leg, the accessory insertion is absent. _T. cupido_ INDIVIDUAL VARIATION.--None of significance. _P. p. jamesi_ DIFFERENCES FROM TYPICAL _T. pallidicinctus_.--The origin of the tibial head does not extend onto the patellar tendon. INDIVIDUAL VARIATION.--The accessory insertion is absent in one leg. =_M. Extensor Digitorum Longus_=, Figs. 15, 17 _T. pallidicinctus_