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Paralleling the metacarpal bones, this series of twelve small muscles produce a wide array of precision movements of the fingers. They form three distinct layers and each layer contains four muscles. Moving dorsally from the palmar surface, the three layers are the lumbrical muscles, palmar interossei muscles, and dorsal interossei muscles. As with the other muscles of the hand, the ulnar and median nerves innervate these muscles. Each of these muscle layers produces a unique action at the metacarpophalangeal joint, while sharing a common action at the interphalangeal joints. After their tendons cross the metacarpophalangeal joint, they expand and wrap to the dorsal side of the phalanges where they join with the tendons of the long extensor tendons of the digits. This combined tendon complex is called the extensor expansion. This extensor expansion allows the intermetacarpal muscles to flex, abduct, and adduct the metacarpophalangeal joints while simultaneously extending the interphalangeal joints.