From the primary position, the eye may make excursions in every direction so that the patient can look at a whole series of objects in succession without moving the head. This portion of space, occupied by all the objects that may thus be seen directly by moving the eye without moving the head, is called “the field of fixation.”
Binocular Movements
While either eye alone may move in all possible directions, one cannot move independently of the other eye. Under ordinary circumstances, those movements only are possible which are regularly required to subserve binocular vision, hence, binocular single vision, as well. These movements are as follows:
Parallel Movements
When one eye looks at a distant object the other is also directed to it, so that the lines of sight of the two eyes are parallel; if the distant object is moved about, the lines remain parallel, one moving as fast and as far as the other. These parallel movements of the two eyes are executed with considerable freedom in all directions, either eye being able to move readily to the right, left, up, down, or obliquely, provided the other eye moves precisely with it.
In executing any parallel movement, each eye is acted upon by at least three and sometimes by as many as five muscles. At times, but one of these muscles is required to produce any great movement of the eye, the others simply serving to steady it in its course. Thus when we look up to the right, although there are five muscles really acting upon each eye, the right eye is moved mainly by the external rectus and the left eye by the internal rectus.
Similarly, when we look up and to the right, although other muscles take part, the superior rectus is the chief muscle that moves the right eye up, and the external rectus the chief one that moves it to the right; while for the left eye the inferior oblique and the internal rectus are the efficient muscles.
A careful study of the action of the individual muscles will make it clear that these facts hold good for each of the cardinal directions of the gaze.
Furthermore, if we attentively consider the action of the twelve muscles moving the two eyes, we see that they may be divided into three groups, viz.; four lateral rotators, four elevators and four depressors.
| Lateral Rotators | ||||
| Right rotators |  | to |  | Left rotators |
| L. Internal rectus | R. Internal rectus | |||
| R. External rectus | L. External rectus | |||
| Elevators | ||||
| Right-handed elevators |  | to |  | Left-handed elevators |
| (acting mainly when the | (acting mainly when the | |||
| eyes are directed to the right) | eyes are directed to the left) | |||
| R. Superior rectus | R. Inferior oblique | |||
| L. Inferior oblique | L. Superior rectus | |||
| Depressors | ||||
| Right-handed depressors | | to | | Left-handed depressors |
| (acting mainly when the | (acting mainly when the | |||
| eyes are directed to the right) | eyes are directed to the left) | |||
| R. Inferior oblique | R. Superior oblique | |||
| L. Superior oblique | L. Inferior rectus. | |||