2 74 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 



/.r.c. 



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- 

 oo T5 |- f sis* ^^ ^ number of delicate rods and cones ; 

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

 frTgmenf '^'''''*^ ^^ *^^ direction from which the light comes, 

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



the outer coat; i.r.c, , . j i i i -tit 



layer of rods and cones; where a pigmented substauce IS developed. 

 zV.CiayT/orglnSn- The ravs of light are thus focussed through 



cells; l.n.f., layer of ,, J ? xi, • • x J l, i 



nerve-fibres; /..front of the rctma ou to this pigmented substance: 



retina, tlie surface turned ,, ti.,i tt i- 



towards the pupil. the cuds of the rods and cones are stimu- 

 lated ; and the stimulation is handed on, augmented in 



