Eye Associates
The muscles forming any one pair are called associates. Any two associates acting together will move their respective eyes in precisely the same direction and to the same extent. Thus the right superior rectus moves the eye up to the left and rotates its vertical meridian to the left; and its associate, the left inferior oblique, moves its eye up to the left and rotates its vertical meridian to the left. This likewise applies to each of the other five groups of associates.
If one eye fails to keep pace with the other in executing parallel movements, diplopia ensues. If the eyes are moved in all directions and the point noted where the patient just begins to see double, we delimit the field of binocular single vision.
Normally, however, the two eyes maintain parallelism to the very limit of their excursion, so that diplopia occurs only at the extreme periphery of the field of vision, if at all. In fact, the field of binocular single vision usually extends not less than 40 degrees from the primary position in every direction.
Each of the various parallel movements of the eye appear to be governed by a distinct nerve mechanism, there being one centre for movements to the right, one for movements to the left, one for movements up, etc.