86 The Life Story of the Fish 



one of the striking features of the vertebrates is that many 

 nerves cross from one side of the body to the other. The 

 nerves from the organs of touch in your left hand, for in- 

 stance, go to the right side of your brain. The fish applies this 

 principle faithfully to his eyes: the right eye connects with 

 the left side of the brain, and vice versa. All the fibers of the 

 right optic nerve cross those of the left on the way from the 

 eye to the brain and this is called complete decussation (com- 

 plete crossing). 



In the human, this has been changed. The nerve tract 

 from the left eye starts out toward the right side of the 



brain in orthodox fashion, but when it gets to the point where 

 it should cross the tract from the right eye, some of its fibers 

 turn aside from their course, join the other tract, and end 

 in the left side of the brain. The remaining fibers continue 

 in the original direction and end in the right side of the 

 brain (see Diagram L). This is called incomflete decussation 

 of the optic nerves. 



Complete decussation is found in those animals in which 



