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flexor accessorius; quadratus plantae third-layer intrinsic foot muscle
origin from medial and lateral margins of plantar aspect of calcaneum
insertion into medial aspect of flexor digitorum longus tendon
nerve supply lateral plantar nerve
action aligns direction of pull of flexor digitorum longus along sagittal plane of the foot; indirect flexion of lesser toes (via its insertion into flexor digitorum longus)