OCULAR MOVEMENTS AND OCULAR MUSCLES. 867 



by two of the axes, (i) The horizontal plane of division cuts the eyeball into 

 an upper and a lower half; it is determined by the visual axis and the transverse 

 axis. In its passage through the retina, it forms the horizontal line of division 

 of this membrane, and it cuts the tunics of the eye in the horizontal meridian. 

 (2) The vertical plane of division cuts the eye into an inner and an outer half; 

 it is determined by the visual and vertical axes. It intersects the retina in its 

 vertical line of division, and the periphery of the eyeball in the vertical meridian 

 of the eyeball. (3) The equatorial plane divides the eye into an anterior and a 

 posterior half. Its position is determined by the vertical and transverse axes. 

 It cuts the sclera in the equator of the eyeball. The horizontal and vertical lines 

 of division of the retina intersect in the fovea centralis, and divide the retina 

 into four quadrants. 



v. Helmholtz has further, introduced the following terms for designating the 

 positions of the eyes: the line of fixation is the straight line connecting the center 

 of rotation with the fixed point in the external world. A plane passed through 

 the lines of fixation of both eyes is called the plane of fixation. The base line of 

 this plane is the line joining the two centers of rotation (consequently the transverse 

 axis). A sagittal plane may further be imagined as passing through the head 

 and divides it into a right and a left half. This plane will bisect the base line of 

 the plane of fixation, and if prolonged anteriorly will intersect the plane of vision 

 in its median line. The fixation-point of the eye may (i) be raised or lowered. 

 The field that it traverses is called the field of fixation. ' It is part of a hemisphere, 

 the center of which is the center of rotation of the eye. Starting from the primary 

 position of both eyes, which is characterized by the fact that the two lines of fixa- 

 tion are parallel and horizontal, the elevation of the plane of fixation may be 

 determined by the angle that it makes with the plane of the primary position. 

 This angle is called the angle of elevation of fixation. It is termed positive when 

 the plane of fixation is raised (toward the forehead) , and negative when the plane 

 is lowered (toward the chin). (2) The line of fixation may be turned also from 

 the primary position in a lateral direction in the plane of fixation, that is toward 

 the median line, or away from it. The amount of this lateral movement is 

 measured by the angle of lateral rotation, that is the angle that the line of fixation 

 makes with the median line of the plane of fixation. The angle is called positive 

 when the posterior extremity of the line of fixation moves toward the right, and 

 negative when the extremity moves toward the left. 



In accordance with the foregoing preliminary considerations 

 the eyes may assume the following positions as the result of their 

 movements : 



i. Primary position, in which both lines of fixation are parallel, and 

 the plane of fixation is horizontal. In this case the three axes of the 

 eye coincide with the three fixed axes erected in the orbital cavity. 

 2. Secondary positions result from simple movements of the eyes from 

 the primary position. There are two different kinds of secondary 

 positions, namely: (a) The lines of fixation are parallel, but are directed 

 upward or downward. The transverse axis of each eye remains the 

 same as in the primary position. The deviation of the other two axes 

 is expressed on the line of fixation by the size of the angle of elevation 

 (as has already been mentioned), (b) The second kind of secondary 

 position is produced by convergence or divergence of the lines of fixation. 

 Here the vertical axes about which the lateral rotation is effected remain 

 the same as in the primary position. The other axes form angles. The 

 amount of the deviation (as already noted) is expressed by the angle of 

 lateral rotation. The eye can be turned from the primary position out- 

 ward 42, inward 45, upward 54, and downward 57. 3. A tertiary 

 position is one assumed by the eye when the lines of fixation are con- 

 vergent, and at the same time are inclined upward or downward. None 

 of the three axes coincides now with its situation in the primary position. 

 The exact direction of the lines of fixation is determined by the size of 

 the angles of elevation and lateral rotation. In connection with tertiary 



