The eye is likewise carried straight down by the inferior rectus and the superior oblique; down and out by the inferior and external recti, and down and in by the superior oblique and internal recti.
Field of Action of Muscles
As will be seen, each muscle acts most energetically in some special direction of the gaze, termed field of action of that particular muscle; thus the external rectus acts most powerfully when the eye is directed outward, and acts little or not at all when the eye is directed inward, except by purely passive traction. Likewise the superior rectus acts mainly when the eye is directed down. Furthermore, its action is limited to the upper and outer field; for in the upper and inner field elevation is performed chiefly by the inferior oblique.
This is also true of all the other muscles.
Direction of the Gaze
There are six cardinal directions of the gaze, each corresponding to the field of action of one of the six ocular muscles as follows:
| Cardinal Direction: | Muscles Specially Active: |
|---|---|
| Straight out | External rectus |
| Straight in | Internal rectus |
| Up and out | Superior rectus (as an elevator) |
| Up and in | Inferior oblique (as an elevator) |
| Down and out | Inferior rectus (as a depressor) |
| Down and in | Superior oblique (as a depressor) |
It is to be noted that the action of each muscle does not absolutely stop at the middle line, but extends somewhat beyond it. Thus the action of the right externus extends not only throughout the whole right half of the field of vision, but also some fifteen to twenty degrees to the left of the median line; and that of the superior rectus extends not only above the horizontal plane but also somewhat below.
Primary Position—Field of Fixation
Under normal conditions, when the head is erect and the eye is directed straight forward—that is, when its line of sight is perpendicular to the line joining the centres of rotation of the two eyes in the horizontal plane—the muscles are all balanced. This is called “the position of equilibrium” or the primary position. It is this position which must be assumed by the patient in conducting tests for balance of the muscles.