Pigment 

 layer 



Retina 



r 



y Focusing 

 muscles that make the lens 



-T/^/is brought about by 



v^" // 



y 



more convex 

 for near vision 





Iris and pupil 

 Diaphragm 



more flat 

 for distant vision 



Life 



THE VERTEBRATE EYE 



The eye, like the camera, has a lens at one end and a sensitive surface at the 

 other end. In front of the lens a diaphragm regulates the amount of light admitted. 

 In the eye the sensitive surface (retina) is backed by a layer of pigment and con- 

 nected with the optic nerve 



at some distance — sight, hearing, and smell — give opportunity to discover 

 food or enemies while there is still a little time before action is imperative. 

 Accordingly, these senses act in many situations without bringing about an 

 immediate reaction. Now, as we have already observed, a stimulus may 

 lead to an immediate reflex, but the reflex seldom exists by itself. On the 

 one hand, the stimulus may start impulses that are transmitted to higher 

 levels of the nervous system, as well as to the usual effectors. On the other 



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