For Testing and Correcting Muscular Imbalance—
Providing a Comfortable Form of Trial-Frame.
Chapter XV
MOVEMENTS OF THE EYEBALLS
AND THEIR ANOMALIES
After a careful study of the foregoing chapters, the refractionist may desire further knowledge concerning muscular imbalance—a matter in which the Ski-optometer plays an exceptionally important part.
It should be remembered that it is only the general utility of the instrument, plus one’s knowledge of refraction and individual diagnosis that enables the refractionist to attain maximum efficiency in every examination, a fact which largely accounts for the following chapter.
Monocular Fixation
When we view an object directly, so that it appears to be more distinct than surrounding objects, we are said to “fix” or “fixate” it.
As the fovea is normally the most sensitive part of the retina, affording by far the most distinct vision, “fixation,” in the great majority of cases, is so performed that the image of the object that is “fixated” falls upon the fovea of the eye that is “fixing.” This is known as central or muscular “fixation.”
When central vision is absent, however, the patient is compelled to see with a portion of the retina outside of the fovea. The eye must then be so directed as to cause the image of the object to fall on this outlying portion of the retina. This is termed “eccentric fixation,” and usually denotes that vision is exceptionally poor.
The ability to “fix” is apparently acquired in early infancy by constant practice in looking at objects. Any marked interference with vision, particularly with central vision—present at birth or soon thereafter—will tend to prevent the acquisition of this ability, and in extreme cases the eye does not learn to “fix” at all, but aimlessly wanders in all directions.