Full Muscles of the Eye Description
[Continued from above] . . . the constant, opposing muscle tension. Tremor causes an almost unseen trembling of a point image (like a spotlight in a dark room), and drift makes the image move slowly off-center. Before the movement becomes really noticeable, there is a quick flick to bring the image to the center. These movements may seem like they would be distracting, but they make sure that the image constantly moves over unused parts of the retina and, as a result, the receptors at any spot do not get overloaded with images, and thereby effective vision is maintained. Smooth pursuit movements are used to follow objects at a high speed; for example, from word to word and line to line when reading. The sixth eye movement, called vergence, helps the eyes turn inward to direct the images directly onto small, rodless areas of the retina. During these movements, the brain registers the amount of tension and uses it to estimate the distance of the object.
Simultaneously, the intrinsic muscles of the eye serve to focus the eye and control the amount of light entering it.
Each eye is held in place by three pairs of taut, elastic muscles (extrinsic muscles) that constantly balance the pull of the others. The superior rectus acts to roll the eyeball back and up, but it is opposed by the inferior rectus. In the same way, the lateral rectus pulls to the side, while the medial rectus pulls toward the nose, and the two oblique muscles roll the eye clockwise or counterclockwise. The muscles of each eye work together to move the eyes in unison. Because of the constant tension in the muscles, they can move the eye very quickly, much faster than any other body movement.