THE SENSES. 



623 



identical points of the two retinae, as is shown in A, Fig. 421. But there 

 is no foundation for such supposition. 



By another theory it is assumed that each optic nerve contains ex- 

 actly 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. 421, B). 

 But in this 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 ', Fig. 421, C, 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 

 ]oop, or spring from the same point. The same disposition prevails in 

 the case of the identical fibres and I'. 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 



FIG. 4,'~\ 



<listal portion of one nerve to the corresponding portion of the other 

 nerves, as if they were commissural fibres forming a connection between 

 the retinae of the two eyes. It is supposed, indeed, that these fibres may 

 connect the corresponding parts of the two retinae, and may thus ex- 

 plain their unity of action; in the same way that corresponding parts of 

 the cerebral hemispheres are believed to be connected together by the 

 commissural fibres of the corpus callosum, and so enabled to exercise 

 unity of function. 



Judgment of Solidity. On the whole, it is probable, that the power 

 of forming a single idea of an object from a double impression conveyed 

 by it to the eyes is the result of a mental act. This view is supported by 

 the same facts as those employed by Wheatstone to show that this power 

 is subservient to the purpose of obtaining a right perception of bodies 

 raised in relief. When an object is placed so near the eyes that to view 

 it the optic axes must converge, a different perspective projection of it 

 is seen by each eye, these perspectives being more dissimilar as the con- 

 vergence of the optic axes becomes greater. Thus, if any figure of three 

 dimensions, an outline cube, for example, be held at a moderate distance 

 before the eyes, and viewed with each eve successively while the head is 



