274 



Animal Life and Intelligence. 



which can be altered in the accommodation of the organ 

 for near or far vision. The space between the lens and 



Fig. 31. The hnman eye. 

 Horizontal section, to show general strnctTire. 



iris and the corneal window of the eye is filled with a 

 watery fluid. Behind the lens there is a transparent, semi- 

 b fluid, jelly-like material, filling the rest of 



|j. the chamber of the eye. At the back of the 

 eye is spread out the sensitive membrane 

 the retina. The structure of this mem- 

 brane is very complicated, and cannot be 

 described here.- 1 It is, however, indicated 

 in Fig. 82. For our present purpose it is 

 sufficient to note that here are the end- 

 organs of the optic nerve ; that these con- 

 , sist of a number of delicate rods and cones ; 



Fig. 32. Retina of , , . , 



the eye. Enlarged and that these rods and cones do not face 

 of minute ^ the Direction from which the light comes, 



'. i.L 



6.,backofretinane:rt but face towards the back of the eyeball, 

 uyeATrodsTnd cones';' where a pigmented substance is developed. 



I.J., intermediate layers ; _., , ,. , , ,, , , ,, , 



I*. c., layer or gan.<iion- The rays of light are thus focussed through 



cells; I.n/.. layer of , , ,. ' . . . . , . , 



nerve-flbre.;/.. front of the retina on to this pigmented substance ; 



retina, the surface turned ,, , .. > -, 



toward, the pupii. the ends of the rods and cones are stimu- 

 lated ; and the stimulation is handed on, augmented in 



