936 A MANUAL OF PHYSIOLOGY 



pressions as such and of their intensity, without any distinction 

 of quality or colour, with which the rods have to do. They are, 

 then, on the hypothesis under discussion, elements concerned 

 in achromatic sensations under conditions of feeble illumination 

 (twilight vision). The cones are supposed on this theory to be 

 more highly developed elements than the rods, their function 

 being connected especially with the perception of colour, but 

 also with the perception of achromatic sensations under daylight 

 conditions. 



The pigmented retinal epithelium is undoubtedly sensitive to light, 

 and has important relations to the formation of the visual purple. 

 When the eye is exposed to light, black pigment migrates along the 

 processes of the epithelial cells between the rods, even as far as the 

 external limiting membrane. In the dark the pigment moves back 

 again, and gathers around the outer portions of the rods, where the 

 visual purple is being regenerated. The precise meaning of the 

 changes in the pigmented cells is obscure. 



The pigmented epithelium is known to be concerned in the 

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

 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 

 purple 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 

 which the epithelium has been detached will, after being bleached, 

 be restored if the retina is simply laid again on the epithelial surface. 

 And it does not seem to be the black pigment of the hexagonal cells 

 which is the agent in this restoration, for it takes place in the 



Eigment-free retinae of albino rabbits or rats. Even a retina isolated 

 "om the pigmented epithelium, and then bleached, may, to a certain 

 extent, develop new visual purple in the dark. This is even true 

 when it has been kept in the dark in a saturated solution of sodium 

 chloride, and is then, after washing with physiological salt solution, 

 bleached by light. Here the regeneration of the pigment cannot be 

 the result of vital processes, but must be due to chemical changes 

 in products formed from the original pigment by the action of light. 

 No such regeneration takes place in a retina which, after having been 

 bleached in situ, is removed without the pigmented epithelium and 

 placed in the dark ; and the only probable explanation of the differ- 

 ence is that in this case the photo-chemical substances from which 

 visual purple can be formed have been absorbed into the circulation, 

 and have so escaped. 



The inner segments of the cones of certain animals (birds, reptiles, 

 amphibia, and some fishes) contain globules of various colours, 

 ranging over almost the whole spectrum, and including, besides, the 

 non-spectral colour, purple. The globules are composed chiefly of 

 fat with the pigments (chromophanes, as they have been called) 

 dissolved in it. The function of these globules is unknown. They 



