Extensor indicis proprius

The extensor indicis proprius (Extensor indicis) is a narrow, elongated muscle, placed medial to, and parallel with, the extensor pollicis longus.

Origin and insertion
It arises, from the dorsal surface of the body of the ulna below the origin of the Extensor pollicis longus, and from the interosseous membrane.

Its tendon passes under the dorsal carpal ligament in the same compartment as that which transmits the tendons of the Extensor digitorum communis, and opposite the head of the second metacarpal bone, joins the ulnar side of the tendon of the Extensor digitorum communis which belongs to the index finger.

Action
The Extensor indicis proprius extends the index finger, and by its continued action assists in extending the wrist.