[TA] muscle of posterior (extensor) compartment of forearm; origin, dorsal surface of distal ulna and adjacent interosseous membrane; insertion, extensor expansion of index finger; action, independently extends the index finger and assists in extending hand at wrist joint; nerve supply, radial (posterior interosseous). SYN: musculus extensor indicis [TA], index extensor (muscle), musculus extensor indicis proprius.