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Insertion: Into the extensor expansion of the respective digit, with possible attachment to base of proximal phalanx as follows: First: Ulnar side of thumb. Second: Ulnar side of index finger. Third: Radial side of ring finger. Fourth:  Radial side of little finger.

Innervation: Ulnar, C8, T1

Action: Adduct the thumb, index, ring, and little finger toward the axial line through the third digit.  Assist in flexion of MCP joints, and extension of IP joints of the three fingers.