Each group, it will be seen, comprises two pairs of muscles; one pair acting solely when the eyes are directed to the right, the other when they are directed to the left. It will further be noted that of the two muscles constituting any one pair, one is situated in the right eye, the other in the left.
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.
Movements of Convergence
In order to see an object at a nearby point, the eyes have to converge—a movement affected by a simultaneous and equal contraction of both internal recti. This movement may be combined with a vertical, lateral or oblique parallel movement. Thus, when we wish to look at a near object situated twenty degrees to our right, we first turn both eyes twenty degrees to the right, then converge both equally, turning the left a little more to the right and the right a little back toward the left.
Convergence is governed by a distinct mechanism of the nerves, the source of which has not been determined.