Third dorsal interosseous muscle

Origin

The third dorsal interosseous muscle occupies the third metacarpal space, situated between the middle and ring finger. It arises by two heads from the adjacent sides of the third and fourth metacarpal bones.

• The lateral head originates from the ulnar border of the third metacarpal bone.

• The medial head arises from the radial border of the fourth metacarpal bone.

Insertion

The muscle's tendon inserts into the ulnar side of the base of the proximal phalanx of the middle finger and its extensor hood.

Action

Functionally, the primary role of the third dorsal interosseous muscle is to abduct (spread) the middle finger medially (ulnar deviation), moving it away from the hand's central axis. Additionally, via its connection to the dorsal aponeurosis (extensor hood), it assists in flexion at the metacarpophalangeal (MCP) joint and extension at the interphalangeal (IP) joints of the middle finger.

Innervation

Like all dorsal interossei muscles of the hand, it is innervated by the deep branch of the ulnar nerve (derived from spinal segments C8 and T1).