This small muscle arises from the ulna just distal to the former muscle. Its tendon remains deep and passes across the lower end of the radius covered by the bunched-up tendons of extensor digitorum with which it shares a common synovial sheath. From here it passes over the dorsal surface of the metacarpal bone of the index finger lying to the ulnar side of the digitorum tendon. It joins the dorsal expansion of the index finger. Thus, like the dorsal surface of the little finger, the index finger has two tendons passing to it but they belong to different muscles: the indicis tendon is on the ulnar side of the digitorum tendon.
Nerve supply of extensor indicis:
By the posterior interosseous nerve (C7, 8).
Action of extensor indicis:
It extends the index finger, as in pointing.