vii EETINAL EXCITATION 341 



perception, although they differ fundamentally both in function 

 and in structure. 



III. The retina presents marked alterations according as it 

 is exposed to light or darkness. These changes, which represent 

 the reaction of its elements to the luminous stimuli, are sometimes 

 visible macroscopically, sometimes only under the microscope : 

 some are essentially chemical in character, others physical and 

 anatomical. 



The analytical study of these objective changes due to stimula- 

 tion of the retina is of great interest. It forms the starting-point 

 of a new chapter, evolved entirely out of recent researches, and 

 directed to the establishruent of a rational theory of vision more 

 exactly of a physiological theory to explain the physico-chemical 

 process by which the luminous vibrations can produce that excita- 

 tion of the rods and cones which is necessary to evoke sensations 

 of light and colour in the cerebral centres. 



Heinrich Miiller (1851) was the tirst who observed that the 

 rods of the frog's retina are sometimes coloured red. Leydig 

 (1857) stated that a red colour is present in the rods of all 

 amphibia ; M. Schultze found that those of the mouse and owl 

 were also red-. But the discovery of the colouring matter of the 

 rods in the retina of most vertebrates, and of its transformation 

 under the influence of light, was made by Boll, who gave it the 

 name of retinal or visual purple. 



In his memorable paper " On the Anatomy and Physiology 

 of the Eetina," presented to the Accademia dei Lincei, December 

 3, 1876, he clearly demonstrated that the outer segments 

 of the rods secrete a red substance which gradually discolours 

 under the influence of light. When the retina of a frog or rabbit 

 that has been kept for several hours in the dark is examined in 

 the fresh state it looks red, but bleaches fairly rapidly under 

 white or monochromatic light. Decoloration sets in after 5 and 

 is complete at the end of 15 minutes. According to Boll, the 

 visual purple regenerates as fast as it is consumed, but only 

 accumulates in darkness, and reaches its maximum in two hours. 

 Its regeneration appears to be connected with the presence of the 

 pigment epithelium which covers the external surface of the 

 retina ; this may provide the material for the formation of the 

 purple. In the frog's retina, in addition to the rods that contain 

 the purple, Boll detected others, scattered here and there, which 

 are pale green in colour (Fig. 166). He also first described how 

 the pigment from the epithelial cells wanders down between the 

 interstices of the rods and cones under the influence of white and 

 monochromatic light, and he recognised the great importance of 

 this fact in the theory of vision. " The physiological elements," 

 he writes, " which perceive light and colour are highly complex 

 anatomical structures ; they must include the rods and cones on 



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