The Senses and the Nervous System 89 



field of vision of the left eye is limited by the lines E"D and 

 E"A. But these two fields overlap. Anything within the 

 limits AOC is visible to both eyes. In other words, AOC is 

 a common field of vision for the two eyes. It is a very much 

 smaller common field than we have. Ours varies around 90 

 degrees, depending on our features. If our eyes are far apart 

 and our nose is flat, it is larger j if our eyes are close together 

 and our nose high, it is smaller. A dish-faced person can see 



Diagram N 



more than a hawk-nosed one. In the fish, the nose is very 

 large, and the position of the eyes gives him no help in see- 

 ing around it. His common field of vision is small. In the 

 trout it is probably somewhere in the neighborhood of 30 

 degrees. In some species it is much less. But whatever it is, 

 anything within that field of vision is visible to both eyes. 

 And it seems probable, using the same line of reasoning, that 

 this field extends upward as well as forward, and that the fish 

 sees with both eyes whatever is directly over his head. 



While all this is theoretical, observation bears it out. If a 

 piece of food is at the point X in Diagram N, the fish ap- 

 proaches it directly. He does not turn his head to one side to 

 look at it. This means that he must be able to see what is 

 directly in front of him. If he can see what is in front of him 

 with one eye, he can obviously see it with the other eye also, 

 and there must be a common field of vision. 



