The movement of each eyeball is controlled by the six extraocular muscles (four rectus and two oblique) innervated by CNN III, IV, and VI. The extraocular muscles are named according to their position on the eyeball (Figure 18-3B and C).
All of the muscles except the inferior oblique muscle arise from a common tendinous ring
in the posterior part of the orbit. To determine the action of each muscle, it should be noted that the apex of the orbit
(where the superior and inferior rectus and oblique muscles originate) is not parallel with the optical axis
(when looking directly forward) (Figure 18-3D
). Therefore, when the superior and inferior rectus muscles and the oblique muscles contract, there are secondary actions on the eyeball movement, as follows (Figure 18-3E