868 OCULAR MOVEMENTS AND OCULAR MUSCLES. 



positions another important point comes into consideration, namely 

 that the eyeball rotates at the same time about its line of fixation and its 

 axis. As the iris rotates about the line of fixation as a wheel around its 

 axle, these movements of rotation, which are always associated with 

 the tertiary positions, are termed wheel-movements. Every oblique 

 movement can be considered as composed of a rotation (i) about the 

 vertical axis, and (2) about the transverse axis; or it may be conceived 

 as a rotation about a single, constant axis, lying between these two 

 axes, and passing through the center of rotation of the eye, perpendicu- 

 lar to the primary and the secondary direction of the visual axis (line of 

 fixation). The amount of the wheel-movement (circular rotation) 

 is measured by the angle that the horizontal line of division of the 

 retina forms with the horizontal line of division of the retina when the 

 eyes are in the primary position. This angle is positive when the eye 

 has turned in the same direction as the hand of a clock that it observes, 

 that is, when the upper end of the vertical line of division of the retina 

 deviates toward the right. 



According to Bonders the angle of rotation increases with the angles of ele- 

 vation and lateral rotation; it may increase to more than 10. With equally 

 great elevation or depression of the plane of fixation, the rotation is stronger the 

 greater the elevation or depression of the line of fixation. 



In looking upward in the tertiary position, the upper ends of the vertical 

 lines of division of the retina diverge; in looking downward they converge. If 

 the plane of fixation is raised, the circular rotation is toward the left when the 

 eye turns laterally toward the right; and, conversely, the circular rotation is 

 toward the right when the eye turns laterally toward the left. If the plane of 

 fixation is lowered, however, the eye rotates in the same direction, to the right 

 or the left, when it turns laterally respectively toward the right or the left. Ex- 

 pressed otherwise : if the angles of elevation and of lateral movement have the 

 same signs (+ or ), the rotation of the eyeball is negative, but if they have dis- 

 similar signs, the rotation is positive. In order to make the wheel-movement 

 visible in one's own eye, a surface divided by vertical and horizontal lines is fixed 

 with one eye, a positive after-image is excited, and then the eye is rapidly placed 

 in a tertiary position. The lines of the after-image then form angles with the lines 

 of the background. As the position of the vertical meridian of the eye is important 

 from a medical point of view, it may be again particularly pointed out that in 

 the primary and secondary positions of the eyes the vertical meridian retains 

 its vertical position. In looking upward and to the left, and downward and to 

 the right, the vertical meridians of both eyes are inclined to the left; conversely 

 they are inclined to the right in looking downward and to the left, or upward 

 and to the right. 



In the secondary positions of the eye rotations never take place. Exceedingly 

 slight rotation of the eyes occurs, however, when the head is inclined toward 

 the shoulder, and in the opposite direction from the inclination. It amounts to i 

 for every 10 of inclination of the head. 



Ocular Muscles. The movements of the eyeball are effected by the 

 four straight and the two oblique ocular muscles. In order to determine 

 the action of each of these muscles, a knowledge of the plane of traction 

 of the muscle and of the axis about which it rotates the eye is necessary. 

 The plane of traction is found by constructing a plane through the 

 middle of the points of origin and insertion of the muscle and through 

 the center .of rotation of the eye. The axis of rotation is always- per- 

 pendicular to the plane of traction, and passes through the center of 

 rotation. 



Measurement has yielded the following data: i. The internal 

 rectus (Fig. 302, I) turns the eye almost exactly inward, and the external 

 rectus (E) outward. The plane of traction lies, therefore, in the plane 



