THE PERCEPTION OF SIGHT. 277 



when we see the hands, which feel, moving in the field of 

 vision. Or, secondly, we may assume with Fick x that, 

 since all impressions upon the retina must be conveyed to 

 the brain in order to be there perceived, the nerves of 

 sight and those of feeling are so arranged in the brain as 

 to produce a correspondence between the notions they 

 suggest of upper and under, right and left. This sup- 

 position has, however, no pretence of any anatomical facts 

 to support it. 



The second difficulty for the Intuitive Theory is that, 

 while we have two retinal pictures, we do not see double. 

 This difficulty was met by the assumption that both retinae 

 when they are excited produce only a single sensation in 

 the brain, and that the several points of each retina corre- 

 spond with each other, so that each pair of corresponding or 

 * identical ' points produces the sensation of a single one. 

 Now there is an actual anatomical arrangement which 

 might perhaps support this hypothesis. The two optic 

 nerves cross before entering the brain, and thus become 

 united. Pathological observations make it probable that 

 the nerve-fibres from the right-hand halves of both retinas 

 pass to the right cerebral hemisphere, those from the left 

 halves to the left hemisphere. 2 But although correspond- 

 ing nerve-fibres would thus be brought close together, it 

 has not yet been shown that they actually unite in the 

 brain. 



1 Ludwig Fick, late Professor of Medicine in the University of Marburg, 

 the brother of Prof. Adolf Fick, of Zurich. 



2 We may compare the arrangement to that of the reins of a pair of 

 horses : the inner fibres only of each optic nerve cross, so that those which 

 run to the right half of the brain are the outer fibres of the right and the 

 inner of the left retina, while those which run to the left cerebral hemi- 

 sphere are the outer fibres of the left and the inner of the right retina : 

 just as the inner reins of both horses cross, so that the outer rein of the off 

 horse and the inner of the near one run together to the driver's right hand, 

 while the inner rein of the off and the outer of the near horse pass to his 

 left hand. TE. 



