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HANDBOOK OF PHYSIOLOGY. 



point a of the one retina, and a' of the other are identical. So, also, if 

 the object /? be so situated that its image falls in both eyes at the same 

 distance from the central point of the retina, namely, at 1) in the one 

 eye, and at 1}' in the other,' ft will be seen single, for it affects identical 

 parts of the two retinae. The same will apply to the object f. 



In quadrupeds, the relation between the identical and non-identical 

 parts of the retina cannot be the same as in man ; for the axes of their 

 eyes generally diverge, and can never be made to meet in one point of 

 an object. When such an animal regards an object situated directly in 

 front of 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. 438) will fall 

 at a' 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 



Fig. 437. 



Fig. 437. Diagram to show the simultaneous action of the eyes in viewing objects in dif- 

 ferent directions. 



Fig. 438. Diagram to show the corresponding parts of the retina in the horse. 



points V and b" or c' 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 iateral objects only, can have no corre- 

 sponding 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 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 fig. 439. 



The cause of the impressions on the identical points of the two retinae 

 giving rise to but one sensation, and the perception of a single image, 



