The Senses. 309 



Whether this be the function of these black bodies or 

 not, does not affect the accuracy of the statement that 

 binocular vision in the horse can only occur when a 

 powerful internal squint is produced, and the eyeballs 

 directed well to the front. Monocular vision is otherwise 

 the only one of which animals with their eyes situated on 

 the lateral side of the head are capable, when from the 

 position of the object both eyes cannot be directly concen- 

 trated upon it. Monocular vision would appear to be as 

 perfect as binocular, though on this point it is difficult to 

 speak with certainty. Though ordinary vision is mono- 

 cular, yet the right eye will blink when an attempt is made 

 to strike the left, though the left cannot possibly see what 

 is going on. 



In man binocular vision is perfect, and the explanation 

 afforded is that any part of one retina corresponds to the 

 same part of its fellow ; so that if the retinae be laid over 

 one another, the left portion of one will lie exactly over 

 the left portion of the other, and their right upper and 

 lower parts will equally correspond. But the temporal 

 side of one eye corresponds to the nasal side of the other. 

 For instance, in Fig. 35, the two circles represent the two 

 retina? divided into quadrants, L being the left, and R the 

 right eye ; a and c in the left eye, correspond to a' c' in the 

 right eye ; and b and d in the left, correspond to b' and d' 

 in the right eye. 



When the two images of an object fall on corresponding 

 points of the retina, then vision is binocular and only one 

 object is seen ; thus, if the rays fall on the right side of one 

 retina, they must fall on the right side of its fellow; in 

 Fig. 35 this is shown, v 1 from x to x', and x to x are the 

 two visual axes. If the centre x of an object be looked at 

 the corresponding points which lie on the right side of one 

 retina, lie on the right side of the other also, and con- 

 versely. Owing, then, to the manner in which the human 



eyeballs are so prominent that it is possible that here they are not 

 needed. 



