The first palmar interosseous is a small unipennate muscle that pulls the index finger toward the middle finger. It arises from the ulnar side of the second metacarpal and lies in the space between the second and third metacarpals, palmar to the second dorsal interosseous. Some older texts count a small slip to the thumb as the first palmar interosseous, but in standard modern numbering that slip is excluded and the first palmar interosseous belongs to the index finger.
| Origin | Ulnar (medial) side of the second metacarpal (unipennate) |
|---|---|
| Insertion | Ulnar side of the base of the proximal phalanx of the index finger and its extensor hood |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | Palmar metacarpal artery of the second intermetacarpal space (from the deep palmar arch) |
| Actions |
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|---|
Like all palmar interossei, it is supplied by the deep branch of the ulnar nerve, so ulnar nerve lesions at the elbow or in Guyon's canal weaken index finger adduction. Weakness shows up in the card test, where the patient cannot hold a card between the index and middle fingers against a pull. Together with first dorsal interosseous and adductor pollicis weakness, it contributes to poor pinch and grip in ulnar neuropathy.
Not reliably palpable on its own because it lies deep in the palm between the second and third metacarpals; its action is tested by resisted adduction of the index finger toward the middle finger.
Ulnar nerve injury denervates the first palmar interosseous, so the index finger cannot be held firmly against the middle finger. It is detected with the card test and is often accompanied by wasting of the first dorsal interosseous.
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