NERVE 



153 



upon parts of the retina which are insensitive {Practical 

 Physiology). 



(4) The Central Spot— Of all parts of the retina the 

 central spot is the most sensitive to differences of illumina- 

 tion in bright light. 



The cones, which entirely replace the 

 rods on the central spot of the eye, are 

 the more specialised elements of the 

 retina, and they react more particularly 

 to bright light, which soon exhausts the 

 rods. The rods, again, react to faint 

 illumination, and are readily fatigued by 

 bright light. This explains wdiy it is 

 that, when we go out into a dark night 

 from a brightly-Hghted room, we can 

 see nothing at first, but after a time, 

 when the rods have recovered, we begin 

 to see objects more distinctly. The eye -pio. 73.— To show that 

 must become adapted to the dark, and the hindmost layer 



, 1 , .• • 1 ,i. • 1 of the retina is stim- 



the adaptation is better in some people ^j^^^^j (p,,,.^,-,^,.-, 

 than in others. 



The rods seem incapable of giving rise 

 to colour sensation, and when the solar 

 spectrum is looked at in a very dim light, 

 it appears as a greyish band of illumina- 

 tion with the red end wanting, because 

 the slow red vibrations fail to stimulate 

 the rods. It is because the blue end of the spectrum is the 

 more active in faint illumination that the illusion of a 

 moonlight scene may be got by looking through a blue glass, 

 while looking through a yellow glass gives the idea of 

 sunlight and brilliant illumination (Practical Physio- 

 logy). 



(5) Modes of Stimulation. — The rods and cones are gener- 

 ally stimulated by the ethereal light vibration, but they 

 may be stimulated by mechanical violence or by sudden 

 changes in an electric current. But, however stimulated, 

 the kind of sensation is always of the same kind — a visual 

 sensation (see p. 138). 



Images). a, source 

 of light ; h, blood- 

 vessel of retina ; c, 

 shadow of vessel on 

 rods and cones ; d, 

 image of shadow 

 mentally projected 

 on to the wall. 



I 



