THE SENSES. 



741 



must either lie in the structural organization of the deeper or cere- 

 bral portion of the visual apparatus, or be the result of a mental opera- 

 tion ; for in no other case is it the property of the corresponding nerves 

 of the two sides of the body to refer their sensations as one to one spot. 



Many attempts have been made to explain this remarkable relation 

 between the eyes, by referring it to anatomical 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 continue their 

 course to the eye of the same side, so that the left side of both retinae is 

 formed from one root of the nerves, and the right side of both retinae 

 from the outer root, naturally led to an attempt to explain 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 has endeavored to show the inadequateness of this theory to ex- 

 plain the phenomenon, unless it be supposed that each fibre in each cere- 

 bral portion of the optic nerves divides in the optic commissure into two 

 A E ^ : c 



Fig. 440. Diagrams to illustrate three theories to explain the action of symmetrical part cf 



the retina. 



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

 fig. 440. 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. 440, B). But 

 in this theory no account is taken of the partial decussation of the fibres 

 of the nerves in the optic commissure. 



