The rotation of each eye is controlled by six muscles that are each attached to the sclera (outer eyeball surface) by a tendon. Figures 5.17 and 5.18 show their names and arrangement. The tendons pull on the eye in opposite pairs. For example, to perform a yaw (side-to-side) rotation, the tensions on the medial rectus and lateral rectus are varied while the other muscles are largely unaffected. To cause a pitch motion, four muscles per eye become involved. All six are involved to perform both a pitch and yaw, for example, looking upward and to the right. A small amount of roll can be generated; however, our eyes are generally not designed for much roll motion. Imagine if you could turn your eyeballs upside-down inside of their sockets! Thus, it is reasonable in most cases to approximate eye rotations as a 2D set that includes only yaw and pitch, rather than the full 3 DOFs obtained for rigid body rotations in Section 3.2.
Steven M LaValle 2016-12-31