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HAND-BOOK OF PHYSIOLOGY. 



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 



FIG. 390. 



distal portion of one nerve to the corresponding portion of the other 

 nerves, as if the} 7 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 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 convergence of the optic 

 axes becomes greater. Thus, if any figure of three dimensions, an out- 

 line 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. 391) will be the picture presented to the right eye, and 

 B that seen by the left eye. Wheatstone has shown that on this circum- 

 stance 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 of a cube, A, B, 



