In order to understand the actions of the superior oblique muscle, it is useful to imagine the eyeball as a sphere that is constrained—like the trackball of a computer mouse—in such a way that only certain rotational movements are possible. Allowable movements for the superior oblique are (1) rotation in a vertical plane—looking down and up (depression and elevation of the eyeball) and (2) rotation in the plane of the face (intorsion and extorsion of the eyeball).