SINGLE VISION. 



525 



the phenomenon by this distribution of the fibres of the nerves. 

 And this explanation is favored 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 phenomenon, un- 

 less it be supposed that each fibre in each cerebral portion of 

 the optic nerves divides in the optic commissure into two 

 branches for the identical points of the two retinae, as is shown 

 in Fig. 192. But there is no foundation for such supposition. 

 By another theory it is assumed that each optic nerve con- 

 tains exactly the same number of fibres as the other, and that 

 the corresponding fibres of the two nerves are united in the 

 sensorium (as in Fig. 193). But in this theory no account is 



FIG. 192. 



FIG. 193. 



FIG. 194. 



taken of the partial decussation of the fibres of the nerves in 

 the optic commissure. 



According to a third theory, the fibres a and a', Fig. 194, 

 coming from identical points of the two retinse, are in the 

 optic commissure brought into one optic nerve, and in the 

 brain either are united by a loop, or spring from the same 

 point. The same disposition prevails in the case of the iden- 

 tical fibres b and b'. According to this theory, the left half 

 of each retina would be represented in the left hemisphere of 

 the brain, and the right half of each retina in the right hem- 

 isphere. 



Another explanation is founded on the fact, that at the 

 anterior part of the commissure of the optic nerve, certain 

 fibres pass across from the distal portion of one nerve to the 

 corresponding portion of the other nerves, as if they were com- 

 missural fibres forming a connection between the retinae of the 

 two eyes. It is supposed, indeed, that these fibres may con- 

 nect the corresponding parts of the two retinse, and may thus 

 explain their unity of action ; in the same way that corre- 

 sponding parts of the cerebral hemispheres are believed to be 



