M. Femoritibialis Internus, [Figs. 13], [15]
T. pallidicinctus
General Description and Relations.—Elongate; on posteromedial surface of femur; bounded anteriorly by M. vastus medialis and posteriorly by M. adductor profundus (overlapping anterior edge of latter); anteroproximal part lateral to M. ambiens; anterodistal corner deep to distal end of M. extensor iliotibialis anticus; distal part of muscle split into superficial and deep layers; superficial layer thin, narrow, and tendinous except for proximal end; deep layer wider, much thicker, and fleshy except for distal end taking form of flat tendon; anterior edge of latter somewhat fused to medial edge of tendon of M. vastus medialis; deep layer widest near distal end of fleshy part; posterior edge of superficial layer fused to underlying deep layer, and anterior edge fused to (continuous with) posterior edge of tendon of M. extensor iliotibialis anticus.
Origin.—The origin is mostly fleshy from the posteromedial surface of the femur between the origin of M. vastus medialis and the posterior intermuscular line, terminating immediately proximal to the internal condyle.
Insertion.—The tendons of both superficial and deep layers attach to the medial part of the rotular crest, forming the medialmost part of the patellar tendon.
Innervation.—The posteriormost branch of the middle division of the femoral nerve penetrates the medial surface of the muscle near the proximal end.
Individual Variation.—None of significance in any of the three species studied.
M. Extensor Iliofibularis (M. biceps femoris), [Figs. 12], [14], [16], [17]
The term extensor in the name of this muscle does not refer to its function. Howell (1938) used the term extensor to indicate derivation of the muscle from the primitive dorsal extensor muscle mass. (Likewise he used the term flexor to indicate derivation from the primitive ventral flexor muscle mass.)