THE SENSES 817 



Frogs and rabbits can undoubtedly see at a time when, 

 by continued exposure to bright sunlight, the purple must 

 have been completely bleached. And although the absence 

 of the pigment in the eye of the bat might seem to afford 

 a ready explanation of the proverbial ' blindness ' of that 

 animal, such a hasty deduction would be at once corrected 

 by the fact that birds with as sharp vision as the pigeon are 

 equally devoid of visual purple. The pigmented retinal 

 epithelium is undoubtedly sensitive to light, and has im- 

 portant relations to the formation of the visual purple. 

 When the eye is exposed to light, the pigmented cells push 

 down processes between the rods. In the dark they draw 

 them back again, so that while it is easy to separate the 

 retina without the pigmented layer from the eye of an 

 animal kept in the dark, the hexagonal epithelium always 

 adheres to a retina which has shortly before death been acted 

 upon by light. The precise meaning of these changes of 

 form in the pigmented cells is unknown. Some have sup- 

 posed that they alone contain the essential visual substance, 

 and that, by mechanical pressure on the rods and cones 

 caused by alterations in their form or bulk under the 

 stimulus of light, they set up impulses in the optic nerve. 

 By others it has been plausibly urged that in bright light 

 the processes that stretch in among the rods serve as 

 insulators to confine the excitation, by preventing the lateral 

 passage of scattered light from one element to another. 

 But it may be that the movements are related rather to the 

 formation of photo-chemical substances to act as stimuli to 

 the end-organs of the optic nerve. 



The pigmented epithelium is known to be concerned in the 

 regeneration of the visual purple. When a frog is curarized, oedema 

 occurs between the retina and the choroid, so that the former mem- 

 brane is separated from the hexagonal epithelium. If the frog is 

 now exposed to sunlight till the visual purple is bleached, and the 

 retina then taken out and placed in the dark, no regeneration of the 

 pigment takes place. When the same experiment is repeated on a 

 non-curarized frog, the visual purple is restored in the dark, and 

 may be seen under the microscope in the rods. The only difference 

 in the two experiments is that in the latter the pigmented epithelium 

 adheres to the retina, and it must therefore have a hand in the 

 regeneration of the pigment. Even the visual purple of a retina from 



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