The third dorsal interosseous abducts the middle finger toward the ring finger, opposing the second dorsal interosseous. Together with the second DI, it allows the middle finger to spread bilaterally — the only finger with two dorsal interossei. The middle finger cannot be moved by a palmar interosseous as the reference axis of hand abduction runs through it.
| Origin | Adjacent sides of the third and fourth metacarpals (bipennate) |
|---|---|
| Insertion | Ulnar base of the proximal phalanx of the middle finger and extensor hood |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Third dorsal metacarpal artery |
| Actions | Abducts the middle finger ulnarly (toward the ring finger); Flexes the MCP and extends the IP joints of the middle finger |
|---|
The third dorsal interosseous atrophy is visible between the third and fourth metacarpals on the dorsal hand in ulnar nerve palsy. The middle finger has no palmar interosseous because it lies on the reference axis of hand abduction — it can only be abducted, never adducted, in the strict anatomical sense. In Dupuytren's disease the third web space cord can tether the third DI.
Palpated between the third and fourth metacarpals on the dorsal hand during resisted middle finger ulnar abduction.
Dupuytren's cord in the third interspace incorporating the third dorsal interosseous fasciae, producing limited ring finger extension and web space contracture.
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