SINGLE VISIOX. 



669 



phenomenon, unless it be supposed that each fibre in each 

 cerebral portion of the optic nerve divides in the optic 

 commissure into two branches for the identical points of 

 the two retinao, as is shown in fig. 192. But there is no 

 foundation for such supposition. 



By another theory it is assumed that each optic nerve 



contains 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 



Fig. 192. Fig. 193. Fly. 194. 



theory no account is 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 retinae, 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 

 identical fibres I 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 hemisphere. 



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 commissural fibres forming a connection between the 



