SINGLE VISION. 



669 



tural organization of the deeper or cerebral portion of the 

 visual apparatus, or be the result of a mental operation ; 

 for in no other case is it the property of the corresponding 

 nerves of the two sides of the body to refer their sensa- 

 tions as one to one spot. 



Fig. 183. Fig. 184. 



Many attempts have been made to explain this remark- 

 able relation between the eyes, by referring it to anatomi- 

 cal relation between the optic nerves. The circumstance 

 of the inner portion of the fibres of the two optic nerves 

 decussating at the commissure, and passing to the eye of 

 the opposite side, while the outer portion of the fibres con- 

 tinue their course to the eye of the same side, so that the 

 left side of both retinse is formed from one root of the 

 nerves, and the right side of both retinse from the other 

 root, naturally lead to an attempt to explain the pheno- 

 menon by this distribution of the fibres of the nerves. 

 And this explanation is favoured by cases in which the 

 entire of one side of the retina, as far as the central point 

 in both eyes, sometimes becomes insensible. But Miiller 

 shows the inadequateness of this theory to explain the 



