524 



THE SENSE OF SIGHT. 



on which the images of these objects fall, must be identical. 

 All points of the retina in each eye which receive rays of light 

 from lateral objects only, can have no corresponding identi- 

 cal points in the retina of the other eye ; for otherwise two ob- 

 jects, one situated to the right and the other to the left, would 

 appear to lie in the same spot of the field of vision. It is 

 probable, therefore, that there are, in the eyes of animals, 

 parts of the retinse which are identical, and parts which are 

 not identical, i. e., parts in one which have no corresponding 

 parts in the other eye. And the relation of the retinse to each 

 other in the field of vision may be represented as in Fig. 190. 

 The cause of the impressions on the identical points of the 

 two retinse giving rise to but one sensation, and the perception 

 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 corresponding nerves of the two sides of 

 the body to refer their sensations as one to one spot. 



FIG. 190. 



Many attempts have been made to explain this remarkable 

 relation between the eyes, by referring it to anatomical rela- 

 tion 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 retinse 

 from the other root, naturally lead to an attempt to explain 



