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HANDBOOK OF PHYSIOLOGY. 



According to a third theory, the fibres a and ', fig. 440, 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 

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

 the case of the identical fibres b and V. 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 retinae of the two eyes. It is supposed, indeed, that these fibres may 

 connect the corresponding parts of the two retinae, and may thus explain 

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

 cerebral hemispheres are believed to be connected together by the com- 

 missural 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 



"TV 



Fig. 441. Diagrams to illustrate how a judgment of a figure of three dimensions is obtained. 



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 projec- 

 tion of it is seen by each eye, these perspectives being more dissimilar as- 

 the convergence 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 eye successively while the 

 head is kept perfectly steady, A (fig. 441) will be the picture presented 

 to the right eye, and B that seen by the left eye. Wheatstone has shown 

 that on this circumstance depends in a great measure our conviction, 

 of the solidity of an object, or of its projection in relief. If different 

 perspective drawings of a solid body, one representing the image seen by 

 the right eye, the other that seen by the left (for example, the drawing. 



