34 EVOLUTION OF THE HUMAN EYE 



an increase in divergence of the optic axes, so that animals 

 with a wide divergence are still able to see binocularly. 



A horse with eyes set prominently forward, large cornese 

 and laterally oval pupils, can see behind and observe the out- 

 kick of its hind leg without turning its head. With a visual 

 axis piercing the cornea 50 to 63 inside its center, with 

 the corpus nigrum capable of closing in the middle part of 

 the oval pupil and leaving a circular one looking forward, 

 and with about one-sixth of the fibers of the optic nerve 

 turning into the optic tract of the same side while the rest 

 decussate, a horse evidently possesses binocular stereoscopic 

 vision, which must be of the greatest assistance to it in the 

 estimation of size and distance in jumping. Every horse- 

 man knows that an object at which a horse is most likely 

 to shy is one situated at the side and of a light color. To 

 prevent shying we put blinkers on harness horses, so that 

 they may not see these laterally placed objects, but only 

 those which they view binocularly straight in front of them. 

 In training a shying horse it is customary to walk the animal 

 up to any object it has shied at with its head toward the 

 object, which it can then view binocularly. 



Closely associated with the varying degrees of binocular 

 vision we meet with varying degrees of decussation of the 

 fibers of the optic nerve at the chiasma. This matter has 

 been dealt with very fully by Dr. Wilfred Harris. 8 He con- 

 siders that binocular vision is associated with carnivorous 

 habits; it is met with in carnivorous fishes, amphibia and 

 birds. In all these there is total decussation of the optic 

 nerve fibers at the chiasma, so that they do not have stereo- 

 scopic vision as in mammals, in many of whom, owing to 

 semi-decussation of the fibers at the chiasma, visual im- 

 pressions from the t\vo eyes are received on the same side 

 of the brain. 



