8i8 A MANUAL OF PHYSIOLOGY 



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 

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

 from 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 normal saline, 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 difference 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 

 cannot be concerned in colour vision, or, at least, they cannot be 

 essential to it, for in the human retina they do not exist. 



The yellow pigment of the macula lutea does not belong to the 

 layer of rods and cones ; it only exists in the external molecular layer 

 and the layers in front of it ; in the fovea centralis it is absent. 



Time necessary for Excitation of the Retina by Light Fusion 

 of Stimuli. Whatever the exact nature of retinal excitation may be, 

 it is called forth by exceedingly slight stimuli. A lightning flash, 



although it may last only th of a second, lasts long enough 



3 1,000,000 



to be seen. A beam of light thrown from a rotating mirror on the 



eye stimulates when it only acts for th of a second. The 



0,000,000 



minimum stimulus in the form of green light corresponds, as we have 

 already seen (p. 593), to a quantity of work equivalent to no 



more than -L erg, that is, about ~ gramme-millimetre, or -L, 



milligramme-millimetfe, which is the work done by th 



10,000,000 



of a milligramme in falling through a millimetre ; and it cannot be 

 doubted that a portion even of this Lilliputian bombardment is 

 wasted as heat. So quickly, too, is the stimulus followed by the 

 response that no latent period has as yet ever been measured. It is 

 certain, however, that there is a latent period, as surely as there is a 

 latent period in the excitation of a naked nerve-trunk, although this 

 also has never been experimentally detected. The analogies, in fact, 

 between a muscular contraction and a retinal excitation are numerous 



