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.”