296 THE NERVOUS SYSTEM 



PHYSIOLOGY OF THE RETINA 



When light falls upon the retina certain changes take 

 p!ace which may be summarised as follows : — 



Structural Change. — The cones shorten and the processes 

 of pigment emerge from the epithehal layer to envelop the 

 ends of the rods. 



Electrical Change. — This occurs on darkening as well as 

 on exposure to light. 



Chemical Change. — In the ends of the rods is a purple 

 pigment, rhodopsin or visual purple. It is bleached by 

 exposure to light. The whole retina, too, takes on an acid 

 reaction. The restitution of the rhodopsin is performed by 

 the pigment cells. 



It is believed that the cones respond to dayUght and 

 the rods to twihght vision, and that only the cones 

 respond to colour. The evidence for such a distinction 

 between the two elements is — (1) twihght vision is most 

 acute at the periphery of the retina where rods are most 

 abundant, and deficient at the fovea where only cones are 

 present ; (2) green rays, which are seen best of all colours at 

 twihght, are those which are most effective in bleaching 

 rhodopsin. Foveal vision further differs from peripheral 

 in being more sharply defined. 



The peripheral hmit of retinal sensitiveness is determined 

 by means of the perimeter. It is found that the extent 

 of the visual field varies for different colours, 



COLOUR VISION 



Of the theories which have been put forward to explain 

 colour vision, the following are the most important. 



Young's Hypothesis. — On this view there are three 

 different substances present in the retina, one responding 

 to red, another to green, a third to blue. When these are 

 stimulated simultaneously fusion in the brain leads to a 

 sensation of white. Different colour sensations are due 



