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396 PHYSIOLOGY CHAP. 



further than to bring them parallel, as in the primary position. 

 When the normal association of the binocular movements is 

 defective, a squint (strabismus) occurs, and the two visual axes no 

 longer lie in the same plane, nor are they capable of converging 

 upon a single point, nor of becoming a parallel to one another. In 

 nystagmus, on the contrary, the normal association of binocular 

 movements is maintained. 



The visual axes can be moved in any direction from the 

 primary position without rotation or wheel movement. This is 

 shown by the method of Euete and Helmholtz, which consists, 

 with the head at rest in the primary position, in fixating two 

 coloured bands in the form of a cross, which are fastened on a wall 

 1-2 m. away, so as to obtain an after-image. If, after sufficiently 

 long fixation, the eye is moved from the primary to a secondary 



position (i.e. upwards or down- 

 wards, to the right or to the left), 

 it will be found that the after- 

 images of the horizontal and 

 vertical parts of the cross remain 

 unchanged, proving that there is 

 no rotation round the visual line. 

 If, on the contrary, the eye is 

 moved from the primary into the 

 tertiary position (as upwards to 

 the right or left, or downwards to 

 the right or left), it is found that 

 the lines of the after-image be- 

 come oblique (Fig. 185). This 



FIG. 185. Projection of the after-image of a effect is not, however, due to the 



rotation of the eye round the 

 visual line, but simply to the fact 

 that the median plane of the eye, which is vertical in the primary 

 position, becomes oblique when the eye is rotated into a tertiary 

 position. 



In all these cases, therefore, the movements of the eye conform 

 to Listing's law. Whenever in the emme tropic and parallel 

 directed eye the visual axis is moved from the primary into any 

 other position, the movements of the eyeball take place round fixed 

 axes, each of which is at right angles to the plane of the move- 

 ments of the visual line ; and both lie in the same plane, at right 

 angles to the primary position of the line of vision. 



Normal binocular movements may be classified in three groups : 



(a) Movements with the two visual axes parallel (long distance 

 vision). 



(&) Movements when the two visual axes converge at a point 

 of the median plane (short distance vision). 



(c) Movements when the two axes are convergent and non- 



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