CH. LVIII.] RETINAL CHANGES DURING ACTIVITY 847 



as indicating a polished surface, because a polished surface reflects 

 rays irregularly, so that the two eyes receive stimuli of unequal 

 intensity. 



Changes in the Retina during Activity. 



The method by which a ray of light is able to stimulate the 

 endings of the optic nerve in the retina in such a manner that a 

 visual sensation is perceived by the cerebrum, is not yet understood. 

 It is supposed that the change effected by the agency of the light 

 which falls upon the retina is in fact a physico-chemical alteration in 

 the protoplasm, and that this change stimulates the optic nerve-end- 

 ings. The discovery of a certain temporary reddish-purple pigmenta- 

 tion of the outer limbs of the retinal rods in certain animals (e.g. 

 frogs) which had been killed in the dark, forming the so-called rhodopsin 

 or visual purple, appeared likely to offer some explanation of the 

 matter, especially as it was also found that the pigmentation dis- 

 appeared when the retina was exposed to light, and reappeared when 

 the light was removed, and also that it underwent distinct changes 

 of colour when other than white light was used. It was also found 

 that if the operation were performed quickly enough, the bleached 

 image of a bright object (optogram) might be fixed on the retina by 

 soaking the retina of an animal which has been killed in the dark, in 

 alum solution. 



The rhodopsin is derived in some way from the black pigment 

 (melanin or fuscin) of the polygonal epithelium of the retina, since 

 the colour is not renewed after bleaching, if the retina is detached 

 from its pigment layer. 



Certain pigments, not sensitive to light, are contained in the inner 

 segments of the cones. These are oil globules of various colours, red, 

 green, and yellow, called chromophanes, and are found in the retinae 

 of marsupials (but not other mammals), birds, reptiles, and fishes. 

 Nothing is known about the yellow pigment of the yellow spot. 



Another change produced by the action of the light upon the 

 retina is the movement of the pigment cells. On being stimulated by 

 light the granules of pigment in the cells which overlie the outer 

 part of the rod and cone layer of the retina pass down into the 

 processes of the cells, which hang down between the rods: these 

 melanin or fuscin granules are generally rod-shaped, and look almost 

 like crystals. In addition to this, a movement of the cones and possibly 

 of the rods occurs, as has been already mentioned ; in the light the 

 cones shorten, and in the dark they lengthen (Engelmann). 



Red light has no action on visual purple ; the maximum bleach- 

 ing effect takes place in greenish-yellow light. Now, when the living 

 eye is brought into a condition of " dark adaptation," that is, when 



