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