The Senses and the Nervous System 87 



the eyes typically face in different directions — in which, to 

 be technical, the optic axes are not parallel. This includes the 

 fish, the amphibians, the reptiles, and the birds. In the mam- 

 mals, the nerve-fibers begin to stray even in those forms 

 where the eyes look in opposite directions, and as the lines of 

 sight become more nearly parallel, more and more of them 

 are affected. In the horse a small fraction fail to decussate, 

 in the cat the number is larger, and by the time we reach 

 the higher apes we find half of them leaving the path on 

 which they started and joining the tract from the other eye. 

 These being animals which left the ground some aeons ago 

 to seek safety in the trees, this arrangement of the nerves 

 and the placing of the eyes so that both face in the same 

 direction are thought to provide the unerring judgment of 

 distance needed in swinging from limb to limb, and are 

 termed an "arboreal adaptation." The phrase is suggestively 

 descriptive, but does not tell us how the machinery works. 

 It is not difficult to cook up a theory which explains every- 

 thing. In animals whose eyes face in diametrically opposite 

 directions, each eye has a separate field of vision. Anything on 

 the animal's left side is seen by the left eye only, and its 

 presence is therefore signaled to the right side of the brain. 

 In man, the two eyes face in the same direction. Anything to 

 the left, unless very far to the left, is seen by both eyes. If 

 decussation were complete, it would be signaled to both sides 

 of the brain, which jnight cause confusion. If, however, fibers 

 from that part of each retina which registers objects on the 

 left go only to the right side of the brain, the original sys- 

 tem is restored. Objects to the left register on the right half 

 of each retina. Nerve-fibers from the right half of the right 

 retina, as well as from the right half of the left retina, must 

 therefore go to the right side of the brain in order to register 

 in that side of the brain objects seen to the left — in other 

 words, objects affecting the side of the body which is under 



