-g 4 POSITIONS OF THE EYEBALL. 



sagittal 

 right and 



nrblonzed forward would intersect tue visual uaue m tue mcuiau ^. *-~ .._ i~~-- ~ --5 

 e n be (1) w ised or lowered-the field which it traverses being called the visual field 

 ft BUckfeM'V it is part of a spherical surface with the point of rotation of the eye m its 

 centre ^ lWding from the primary position of both eyes, which is characterised by both 

 vXVnes being parallel with each other and horizontal, then the elevation of the visual plane 

 eTnbe determined by the angle which this forms with the plane of the primary position This 

 Zl7is called the aigU of elevation-it is positive when the visual p ane is raised (to the fore- 

 ffi and negative when it is lowered (chinwards). (2) From the primary position, the visual 

 in? can be turned laterally in the visual plane. The extent of this latera deviation is measured 

 by the angle of lateral rotation, i.e., by the angle which the visual line forms with the 

 median line of the visual plane ; it is said to be positive when the posterior part of the visual 

 line is turned to the right, negative when to the left. The following are the positions of the 

 eyeball : 



1. Primary position [or " position of rest "], in which both the lines of vision 

 are parallel with each other, and the visual planes are horizontal. The three axes of 

 the eyeball coincide with the three fixed axes of the co-ordinate system in the orbit. 



2. Secondary positions are due to movements of the eye from the primary posi- 

 tion. There are two different varieties (a) where the visual lines are parallel, 

 but are directed upwards or downwards. The transverse axis of both eyes remains 

 the same as in the primary position ; the deviations of the other two axes expressed 

 by the amount of the angle of elevation of the ine of vision, (b) The second 

 variety of the secondary position is produced by the convergence or divergence of the 

 lines of vision. In this variety the vertical axis, round which the lateral rotation 

 takes place, remains as in the primary position ; the other axes form angles ; the 

 amount of the deviation is expressed by the " angle of lateral rotation." The eye, 

 when in the primary position, can be rotated from this position 42 outwards, 45 

 inwards, 34 upwards, and 57 downwards (Schnurmann). 



3. Tertiary position is the position brought about by the movements of the eye, 

 in which the lines of vision are convergent, and are at the same time inclined up- 

 wards or downwards. 



[Listing's Law is that which expresses the movements of the eyeball. "When the eyeball 

 moves from the primary position, or position of rest, the angle of rotation of the eye in the 

 second position is the same as if the eye were turned about a fixed axis perpendicular to both 

 the first and the second positions of the visual line (Helmholtz).] 



All the three axes of the eye are no longer coincident with the axes in the primary 

 position. The exact direction of the visual lines is determined by the amount of 

 the angle of lateral rotation and the angle of elevation. There is still another 

 important point. The eyeball is always rotated at the same time round the line 

 of vision and round its axis ( Volkmann, Hering). As the iris rotates round the 

 visual line like a wheel round its axis, this rotation is called " circular rotation " 

 (" Raddrehung ") of the eye, which is always connected with the tertiary positions. 

 Even oblique movements may be regarded as composed of (1) a rotation round 

 the vertical axis, and (2) round the transverse axis ; or it may be referred to rota- 

 tion round a single constant axis placed between the above-named axes, passing 

 through the point of rotation of the eyeball, and at right angles to the secondary 

 and primary direction of the visual axis (line of vision) {Listing). The amount 

 of circular rotation is measured by the angle which the horizontal separation line of 

 the retina forms with the horizontal separation of the retina of the eye in the primary 

 position. This angle is said to be positive, when the eye itself rotates in the same 

 direction as the hand of a watch observed by the same eye, i.e., when the upper end 

 of the vertical line of separation of the retina is turned to the right. 



According to Donders, the angle of rotation increases with the angle of elevation and the 

 angle of lateral rotation it may exceed 10. "With equally great elevation or depression of 

 the visual plane, the rotation is greater, the greater the elevation or depression of the line of 

 vision. 



