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HANDBOOK OF PHYSIOLOGY. 



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

 tion of the retinae. Thus, the image of the object a (Fig. 419) will fall 

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

 identical. So, also, for distinct and single vision of objects b or c, 

 the points V and V or c' and c", in the two retinae, 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 identical points in the retina of the other eye; for other- 

 wise two objects, 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 retinae 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 

 two retinae to each other in the field of vision may be represented as in 

 Pig. 420. 



Binocular Vision. The cause of the impressions on the identical 

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

 ception 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 

 ABC 



FIG. 420. 



FIG. 421. 



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

 responding 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 op- 

 tic 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 phenom- 

 enon by this distribution of the fibres of the nerves. And this explana- 

 tion 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 phenom- 

 enon, unless it be supposed that each fibroin each cerebral portion of the 

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



