XIII 



WHAT DO ANIMALS SEE? 



The eyes of most animals are at the sides of their heads, 

 a position that gives them a very wide field of view, so wide 

 that in many grazing mammals which have to keep a sharp 

 look-out for enemies it extends all round, and the animal 

 can see behind it as well as in front. Such animals need less 

 to concentrate on any particular part of the image than to 

 detect any movement round them that might be made by 

 a predator. Consequently their retinas consist almost entirely 

 of rods, which give great sensitivity but less acuity and no 

 colour vision. Thus most mammals are practically colour- 

 blind, and once they have detected a suspicious movement 

 they bring their other senses, especially hearing and smell, 

 into play to help locate and examine whatever has alerted 

 them. 



In animals whose eyes are directed forwards the field of 

 view is restricted, but in compensation for the loss of all- 

 round vision they gain stereoscopic sight in which the pic- 

 tures sent to the brain from each eye overlap and are com- 

 bined into one, and not just joined to each other at the edges. 

 Stereoscopic sight gives depth to the picture, so that distances 

 can be judged much more accurately, and the appreciation 

 of the three-dimensional picture thus obtained is increased 

 by the acuity given by the preponderance of the cones over 

 the rods. The preponderance of cones also gives colour vision 

 which itself aids acuity by giving an additional means of 

 separating the components of the picture. 



In very dim light the cones give no response and animals 

 with mixed retinas then make use of the rods. At night our 

 foveas, which contain no rods, are practically blind and con- 

 sequently if we wish to see any particular object we must not 

 look at it, but just beside it; if we look at it and bring the 



image on to the fovea it disappears, but if we bring the 



146 



