The Senses and the Nervous System 8i 



So, when the stream is flowing or there is a ripple on the 

 pool, not only does your line cease to be conspicuous as it cuts 

 the water, but you yourself, instead of being the active pro- 

 tagonist in a single, steady picture, become merely another 

 moving object in a kaleidoscope of pictures, all of which are 

 in constant motion. 



So much for what is visible under water. The next ques- 

 tion is, how much of what is visible is the fish capable of 

 seeing? 



The fish's eye is fundamentally very like our own (see 

 Figure 12). It consists in essence of a curved lens inside a 

 transparent covering called a cornea, both of which bend the 

 incoming light rays so as to form an image on a screen com- 

 posed of sensitive rods and cones called a retina. From the 

 retina this image is carried to the brain by the optic nerve. 

 Every word said so far would apply equally well to the 

 human eye, and yet there are between the fish's visual 

 machinery and ours two minor difl?erences, two major dif- 

 ferences, and one surprising resemblance. 



Starting from the outside, the first difference is that the 

 fish has no true eyelids. This seems to us like a very un- 

 comfortable state of affairs, for we are not able to go for 

 more than a few seconds without bringing our lids down 

 across our eyes. If we did not do this, the surface of the eye 

 would become dry, dirt would collect on it, and we should 

 not be able to see. This constant moistening and cleaning of 

 the cornea is a more essential function of our lids than the 

 one we are likely to think of first, the closing of our eyes in 

 sleep. Our lids are more important to see with than to sleep 

 with. 



But the fish lives under water. Water is in constant con- 

 tact with the surface of its eye, and keeps it constantly moist 

 and clean. It therefore has no need of a lid. And the truth 

 of all this is demonstrated by the Central American four- 



