Fig. 17. Effect of pulleys on the rotational axis of horizontal rectus extraocular muscles (EOMs). The axis for the EOM is perpendicular to the line connecting the pulley to the scleral insertion, and is thus vertical in straight-ahead gaze. During visually guided eye movements with the eye in the secondary position of elevation of angle α, the distance D1 from the pulley (ring) to globe center is equal to distance D2 from the insertion to globe center. This causes the axis of the EOM to tilt posteriorly by approximately angle α/2, satisfying the half-angle rule to implement Listing's law.