Superior oblique muscle of the eye innervated by the trochlear nerve is involved in intorsion (primary action), depression (secondary action) and abduction (teiary action) of the eyeball. Action of extraocular muscles: Muscle Primary action Secondary action Teiary action Medial rectus Adduction Lateral rectus Abduction Superior rectus Elevation Intorsion Adduction Inferior rectus Depression Extoion Adduction Superior oblique Intorsion Depression Abduction Inferior oblique Extoion Elevation Abduction Ref: Textbook of Ophthalmology edited by Sunita Agarwal, page 404.