First Dorsal Interosseus
| Pronuncation | in-ter-OSS-ee-us |
| Derivation | Between (inter) the bones (osseus) on the dorsal side of the hand. |
| Origin | The facing surfaces of the first and second metacarpals. |
| Insertion | Base of the proximal phalanx of the index finger. |
| Action | Pulls the first and second metacarpals together, abducting the index finger. |
There is a network of muscles between all of the metacarpals that forms the bulk of the hand. Of these muscles the first dorsal interosseus is the most distinct. This muscle forms the thumb-side wall of the hand.
It becomes prominent on the dorsal surface when the thumb is squeezed against the side of the hand.