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. 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