Chap, hi.] SIGHT. 945 



One position of the eyeball, for reasons which we shall see 

 presently, is called the primary position, and it will be desirable 

 to start from this position. Though its exact determination 

 requires special precautions it may be described as that which is 

 assumed when, with the head erect and vertical, we look straight 

 forwards to the distant horizon ; the visual axes of the two eyes 

 are then parallel to each other and to the median plane. 



Let us now suppose three axes drawn through the centre 

 of rotation, in the three planes of space : — one, the visual axis 

 itself, which we may call the longitudinal axis ; another at 

 right angles to this and horizontal, the horizontal axis ; and a 

 third also at right angles, but vertical, the vertical axis. Cor- 

 responding to these three axes we have three main possible 

 movements of rotation. The eyeball might be rotated round the 

 vertical axis so that the visual axis moved from side to side. It 

 might be rotated round the horizontal axis so that the visual axis 

 moved up and down. And lastly, it might be rotated round the 

 longitudinal axis, the visual axis itself remaining motionless and 

 the pupil turning round like a wheel. 



Now we can easily carry out by an exercise of the will the 

 first and second of these movements. We can easily move the 

 eyes up and down, rotating them on the horizontal axis, as when 

 we look up to the heavens or down to the ground. We can also 

 move the eyes from side to side, rotating them round the vertical 

 axis, as when we look to the right or to the left. We can move 

 the two eyes sideways together in the same direction keeping the 

 visual axes parallel, or we may move them laterally in opposite 

 directions, as when the visual axes being parallel we make them 

 converge, or when convergent bring them back to or towards 

 parallelism. And we can combine rotation round the horizontal 

 axis with rotation round the vertical axis, and so give oblique 

 movements to the eyeball. We can do all this by an exercise of 

 the will, but we cannot by any voluntary effort cany out the 

 third kind of movement, we cannot rotate the eyeball round the 

 visual axis, we cannot twist the eye in a swivel movement round 

 its longitudinal axis. There are certain movements of the eye 

 in which such a swivel rotation, if we may so call it, does to a 

 certain extent take place, and when we induce these movements 

 we do bring about such a swivel rotation ; but we cannot bring 

 about swivel rotation by itself, we can only effect it as part of 

 the particular movements in question. 



And there is a reason why we are thus limited as to our 

 power of moving the eyeball. In both rotation round the hori- 

 zontal axis, and rotation round the vertical axis and in all the 

 various combinations of these two movements which are possible, 

 the two " lines of separation " (§ 589) on both the retinas keep 

 their places ; there is no dislocation of the corresponding regions 

 of the two retinas. Obviously the two retinal circles in the low r er 



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