The Senses and the Nervous System 85 



do. The reader's surprise is expected to be caused by three 

 facts. First, that each eye needs as many as six muscles to 

 bring about such slight movements as it makes; second, that 

 the fish, with its otherwise elementary muscular system, 

 could develop such complicated musculature; and third, that 

 the same six muscles, controlled by the same nerves, act in 

 just the same way to move the eye of the human as to move 

 the eye of the fish, in spite of the fact that in every other 

 way the human muscular system is so much more advanced 

 than that of the fish. 



So far we have found no basic difference between the eyes 

 of fish and human. The human has made additions and im- 

 provements. He has added the eyelid. He has perfected the 

 iris-diaphragm for governing the amount of light. He has 

 adapted the lens to his environment, and altered the mech- 

 anism whereby it accommodates for distance. But he has not 

 changed the basic principle of a lens focusing light so as to 

 form an image on a sensitive screen. We are, therefore, up to 

 this point, able to form a pretty clear conception of how the 

 fish's apparatus works. It is merely an earlier model of the 

 same machine we are using. 



We now come to a difference so fundamental that it will 

 probably never be possible for science fully to explain its sig- 

 nificance. It has to do with the placing of the eyes, and their 

 connection with the brain. Our eyes face in the same direc- 

 tion, the fish's eyes face in almost diametrically opposite 

 directions. Each of our eyes is connected with both right and 

 left sides of the brain; each of the fish's eyes is connected 

 with only one side of the brain. So little experimental work 

 and so many essays in pure theory have been produced on 

 this question that it is permissible for us to do a little theoriz- 

 ing of our own, provided we keep firmly in mind that much 

 of what follows is hypothesis. 



In the first place, it is necessary for us to understand that 



