274 Animal Life cmd 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 human eye. 

 Horizontal section, to show general structure. 



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

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

 fluid, jelly-like material, filling the rest of 

 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. It is, however, indicated 

 in Fig. 32. 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 _ , , A . _ _ _ . , 



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

 section of minute j ^ direction from which the light comes, 



fragment. ° ' 



b., back of retina next but face towards the back of the eyeball, 



the outer coat; l.r.c, , . , n ■, . -, -, , 



layer of rods and cones; where a pigmented substance is developed. 



i.l., intermediate layers; „.. . ,. , , .. „ , .. . 



i.g.c, layer of ganglion- The rays of light are thus locussed through 

 nerve-fibres;"/., front of the retina on to this pigmented substance ; 



retina, the surface turned 



towards the pupii. the ends of the rods and cones are stimu- 

 lated ; and the stimulation is handed on, augmented in 



