each of which is primarily responsible for one of several types of rotary
movement. In Figure 2.1, the three main axes of rotation are indicated: (1) the x-axis
which is the anteroposterior axis coincident with the line of sight; (2) the y-axis which
is horizontal and perpendicular to the line of sight; (3) the z-axis which is vertical and
perpendicular to the line of sight. The medial and lateral rectus muscles shown in
Figure 3dglobe cooperate to produce horizontal rotations (i.e., left and right) of the
eye about the z-axis. Likewise, the superior and inferior rectus muscles