THE SENSES. 231 



it, the image of the object must fall, in both eyes, on the outer portion 

 of the retinae. Thus, the image of the object a (Fig. 389) will fall at a' 

 in one, and at a" in the other: and these points a' and a" must be iden- 

 tical. So, also, for distinct and single vision of objects, 

 1) or c, the points fl'and V or c f c", in the two retinae, on 

 which the images of these objects fall, must be identi- 

 cal. All points of the retina in each eye which receive 

 rays of light from lateral objects only, can have no 

 corresponding identical points in the retina of the other 

 eye; for otherwise two objects, one situated to the right 

 and the other to the left, would appear to lie in the same FlG . 



spot of the field of vision. It is probable, therefore, that 

 there are in the eyes of animals, parts of the retinas which are identical, 

 and parts which are not identical, i.e.) parts in one which have no cor- 

 responding parts in the other eye. And the relation of the two retinae 

 to each other in the field of vision may be represented as in Fig. 389. 



Binocular Vision. The cause of the impressions on the identical 

 points of the two retinae giving rise to but one sensation, and the percep- 

 tion of a single image, must either lie in the structural organization of the 

 deeper or cerebral portion of the visual apparatus, or be the result of a 

 mental operation; for in no other case is it the property of the corre- 

 sponding 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 shows the inadequateness of this theory to explain the phenome- 

 non, unless it be supposed that each fibre in each cerebral portion of the 

 optic nerves divides in the optic commissure into two branches for the 

 identical points of the two retinae, as is shown in A, Fig. 390. 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. 390, 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. 390, 0, coming 

 from identical points of the two retinae, are in the optic commissure 



