250 OF SENSATION, AND THE ORGANS OF THE SENSES. 



mind as double. It is, moreover, easily shown that, in the lower animals whose 

 orbits are not directed forwards as in us, but sideways in a greater or less 

 degree, whenever an object is so situated as to be seen by both eyes, the points 

 of the two retinae on which its images are formed, must be very far from possess- 

 ing this symmetry. 



338. Many attempts have been made to explain the phenomena of single 

 vision, by the peculiar decussation of the optic nerves ; and an interesting 

 correspondence between the varieties in the degree of decussation, and the 

 position of the eyes, in several animals, has been pointed out by Mr. Solly 

 and Mr. Mayo. It is stated by Mr. Mayo that the Optic nerve in Man con- 

 sists of three tracts; of which the internal one is strictly commissural, con- 

 necting together the two retinae anteriorly, and the two optic ganglia posteri- 

 orly; the middle tract decussates, and is believed by Mr. M. to supply that 

 part of the retina which lies on the inner side of each ball, between its ante- 

 rior border and the insertion of the optic nerve ; whilst the external tract does 

 not decussate, but passes on to supply the exterior portion of the retina on the 

 same side. Thus the right optic nerve supplies the right side of each ball ; 

 whilst the left supplies the left side. On the other hand, in most of the Osse- 

 ous Fishes, the decussation is complete ; each nerve passing entirely to the 

 eye of the opposite side. From these and other data, it has been concluded, 

 that each nerve is used in looking towards the opposite side. This is evidently 

 true of the Osseous Fishes, whose two eyes, being directed sideways, have 

 two entirely different spheres of vision. And it is -also true of Man, if Mr. 

 M.'s account of the distribution of the nerve be correct ; since, when we look 

 at an object held directly in front of the face, at the level of the eyes, and at 

 the nearest point for distinct vision, almost the whole of that portion of the 

 right retina, which lies to the outside of the entrance of the optic nerve, is 

 directed to the left ; and the exactly different, complementary, or inner portion 

 of the left retina, which is supplied by the same nerve, is likewise directed to 

 the left. On this supposition, all the rays entering the two eyes from any one 

 point, will be brought to a focus on fibrils belonging to the same nerve ; though 

 these are in Man, as in other animals whose spheres of vision are nearly or 

 partly coincident, distributed to distinct visual organs.* It is obvious, how- 

 ever, that this or any similar explanation must be insufficient to explain the 

 phenomenon of single vision ; since the images formed upon the two retinae 

 are necessarily different, and must be combined or harmonized by an act of 

 the mind, as will be shown in the succeeding paragraphs. 



339. We shall next consider the mode in which our notion of the solid 

 forms and relative projection of objects is acquired ; on which great light has 



recently been thrown by the interesting experiments of Mr. Wheatstone.t It 

 is perfectly evident, both from reason and experience, that the flat picture 

 upon the retina, which is the only object of our sensation, could not itself 

 convey to our minds any notion but that of a corresponding plane surface. 

 In fact, any notion of solidity which might be formed by a person who had 

 never had the use of more than one eye, would entirely depend upon the 

 combination of his visual and tactile sensations. This idea is fully confirmed 

 by the case already referred to, as recorded by Cheselden. The first visual 



* The late Dr. Wollaston was subject to a curious affection of vision, which consisted 

 m his not being able to see more than half of an object, the loss being sometimes on 

 one side, and sometimes on the other. The Author has met with several cases of this 

 disorder, which has been termed hemiopia. Dr. W. thought that they might be explained 

 by the decussation of the optic nerve; but Mr. Mayo states that he has known instances 

 of a parallel affection, involving alternately the centre and the circumference of the retina, 

 and therefore not attributable to any such structural arrangement. 



f Philosophical Transactions, 1838. 



