Extensor indicis

Origin: distal third of the dorsal part of the body of ulna and interosseous membrane. 

Insertion: index finger (extensor hood)

Nerve: posterior interosseous nerve

Action:  extends the index finger (and wrist)

Description:
The Extensor indicis is a narrow, elongated muscle, placed medial to, and parallel with, the extensor pollicis longus. 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.
Variations.—Doubling; the ulnar part may pass beneath the dorsal carpal ligament with the Extensor digitorum communis; a slip from the tendon may pass to the index finger.