Origin Dorsal 1 - 4: Adjacent sides of two metacarpals (bipennate muscles) Insertion Dorsal 1 - 4: Extensor expansions and bases of proximal phalanges of digits 2 - 4 Action Dorsal 1 - 4: Abduct digits from axial line and act with lumbricals to flex metacarpophalangeal joints and extend interphalangeal joints Innervation Deep branch of ulnar nerve (C8 and T1) (C8, T1) Arterial Supply Dorsal 1 - 4: Dorsal and palmar metacarpal arteries