144 'I'HE SENSES OF ANIMALS 



lie at the back, and the hght shines through the layers of 

 cells and vessels before it reaches them. In practice this makes 

 little difference for in life the nerve cells are transparent and 

 the blood vessels are very small ; nevertheless they do inter- 

 fere with the very sharpest sight and part of the retina is 

 arranged to overcome this defect. 



The rods are very sensitive to light but they do not 

 distinguish colours so that the picture they send to the brain 

 is like a black-and-white photograph. Moreover the com- 

 bined messages from many rods travel along single nerve 

 fibres so that the details of the picture sent to the brain are 

 not very sharp. The cones are much less sensitive — it needs 

 about a thousand times more light to stimulate them than 

 the rods — but they are sensitive to colours, and each has its 

 own nerve fibre running to the brain so that the detail of 

 the mental picture is much clearer. The rods are thus more 

 useful when the light is dim and the cones when it is bright. 

 Consequently the retina of nocturnal animals consists mostly 

 of rods, and that of many day animals contains more cones ; 

 in some birds there are practically no rods at all. 



Although the cones need much more light to make them 

 work they give the clearest and most detailed mental picture, 

 and in eyes such as our own which have a mixture of rods 

 and cones there is one place in the retina which contains 

 nothing but cones. This is the. fovea, and when we look at 

 any object we move the eye so that the part of the image 

 we are attending to falls on it. At the fovea the cones are 

 tightly packed, and their nerve fibres run out radially like 

 the spokes of a wheel; the other nerve layers are pushed out 

 to the side so that there is nothing to interfere with the 

 image. Although we are unaware of it, our eyes are con- 

 tinually making slight movements scanning our surround- 

 ings and bringing different parts of the image on to the 

 fovea; when we look steadily at anything we have a less 

 definite picture of the things round it — we see them only out 

 of the "corner of the eye". The cones give acuity, the sharp- 

 ness of sight, whereas the rods are concerned with sensitivity 

 to the intensity of light, a very different thing. 



Owing to the peculiar inversion which makes the nerve 

 fibres run on what we might call the "wrong side" of the 



