240 The Animal Mind 



Some of the most important conditions of distance per- 

 ception in our own experience are lacking in the lower 

 vertebrates and in invertebrates. Stereoscopic vision, the 

 appearance of solidity given to objects by the fact that the 

 visual fields of the two eyes combine, thus producing blend- 

 ing of two slightly different views of the object looked at, 

 has been held to be dependent on the partial crossing of 

 the optic nerves on their way to the brain, whereby each 

 retina sends nerve fibres to both hemispheres of the brain. 

 This arrangement does not appear in the animal kingdom 

 below the birds; whatever function it plays in space 

 perception is, then, absent from reptiles, amphibians, fish, 

 and invertebrates. Certainly stereoscopic vision cannot 

 exist in animals whose eyes are so placed that the same 

 object cannot be seen by both, as is the case with most 

 fishes. In birds whose eyes are situated too far toward 

 the sides of the head for the same object to cast its images 

 on the foveas or centres of the two retinas, there appears 

 to be a secondary fovea in each eye, so placed as to suggest 

 that it serves binocular vision, while the primary fovea is 

 used for monocular vision. In certain mammals the eyes 

 are placed so far towards the sides of the head that the 

 binocular field is very small. This is probably the reason 

 why rodents do not have a more accurate perception of dis- 

 tance. The writer made some simple tests on the use of 

 binocular and monocular vision by the rabbit (756). 

 When the animal was sitting quietly, two bits of food 

 of equal size and kind were held at equal distances from 

 the rabbit's nose, one straight in front, the other directly 

 to the right or left of the rabbit's head. In forty-eight 

 out of fifty trials, the rabbit turned towards and secured 

 the food at the side rather than that in front, thus showing 

 its dependence on monocular rather than binocular vision. 



