Medical Definition of plantar interosseus
: any of three small muscles of the plantar aspect of the foot each of which lies along the plantar side of one of the third, fourth, and fifth toes facing the second toe, arises from the metatarsal bone of the toe along which it lies, inserts into its proximal phalanx, and acts to flex the proximal phalanx and extend the distal phalanges of its toe and to adduct its toe toward the second toe—called also interosseus plantaris, plantar interosseus muscle
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