THE MAMMALIAN EYE 253 



condition which most nearly approaches the human for sharp 

 vision. 



Acute sight, however, depends also upon the shape of the 

 lens, cornea and pupil. A large cornea permits more oblique 

 rays to enter the eye, while large spherical lenses correct the 

 distortion which might result from the unequal refraction. The 

 size of the pupil is perhaps of not so much importance as its 

 power of dilation. The round pupil is characteristic of the 

 higher types of eyes. It has been found that horizontally oval 

 or round forms react less readily to mydriatics and less quickly 

 to light than vertically oval or round forms. The pupils of 

 ungulates are very unresponsive to either drugs or light. 



We need not pause long to consider divergence. Laterally 

 placed eyes, of course, preclude stereoscopic vision and parallel 

 axes are found only in man and monkeys. The divergence is 

 greatest, according to Johnson, in rabbits where the fields of 

 the two eyes probably overlap both in front and behind. It 

 is said that a squirrel can see any one approaching directlv 

 behind it. 



This ability to see on all sides gives the animal greater security 

 from enemies and is accompanied in the wild state by hyper- 

 metropia in all animals below simia, according to Johnson. But 

 both he and others say that refraction becomes variable under 

 domestication . 



Johnson also remarks that there is complete absence of con- 

 vergence below simia and that monkeys converge for only a 

 slight period. He believes that animals below simia do not move 

 their eyes for visual purposes or for purposes of convergence. 

 Harris disputes this and contends that there is a degree of con- 

 vergence and of binocular vision especially in animals which 

 pursue their prey and have to follow it with their eyes. 



Harris studied stereoscopic and binocular vision by means 

 of experimental work to demonstrate the incomplete or complete 

 decussation of the optic nerve fibers and also made microscopic 

 studies of degeneration. He believes that where there is incom- 

 plete decussation there is at least the possibility of binocular 

 vision. In these cases the eyes are set rather far forward. He 

 also used the light reflex to confirm nerve section and says 

 that " Sluggishness of pupil reaction seems generally to be 



