116 PHOTOCHEMISTRY OF THE RETINA. [BOOK in. 



the foregoing may be applied to even the complex organ of vision 

 of the higher animals. If we suppose that the actual terminations 

 of the optic nerve are surrounded by substances sensitive to light, 

 then it becomes easy to imagine how light falling on these 

 sensitive substances should set free chemical bodies possessed 

 of the property of acting as stimuli to the actual nerve-endings 

 and thus give rise to visual impulses in the optic fibres. We say 

 " easy to imagine," but we are, at present, far from being able to 

 give definite proofs that such an explanation of the origin of 

 visual impulses is the true one, probable and enticing as it may 

 appear. 



One of the most striking features in the structure of the retina 

 is the abundance of black pigment, fuscin ( 746), in the retinal 

 epithelium. It is difficult to suppose that the sole function of 

 this pigment is to absorb the superfluous rays of light, and that 

 the rays thus absorbed are put to no use and simply wasted. And 

 indeed it has been shewn that the pigment is sensitive to light ; 

 but the changes in it induced by light are excessively slow. 

 Moreover its presence cannot be of fundamental importance, since 

 vision is not only possible but fairly distinct with albinos in which 

 this pigment is absent. 



Then again, in the vast majority of vertebrate animals, the 

 outer limbs of the rods are suft'used with a purplish-red pigment, 

 the so-called visual purple, which is so eminently sensitive to light 

 that images of external objects may by appropriate means be 

 photographed in it onthe retina. When the eye of a frog or of a 

 rabbit is examined in an ordinary way, with full exposure to light, 

 the retina appears colourless. But if the eye be kept in the 

 dark for some time before it is examined, the retina, if removed 

 rapidly, will be found to be of a beautiful purplish-red or pink 

 colour. Upon exposure to light the colour changes to yellow and 

 then fades away, leaving however the retina, not only white but 

 more opaque than it was before. Upon examination with the 

 microscope it is found that the purple colour is confined exclu- 

 sively to the rods and to the outer limbs of the rods, the inner 

 limbs being wholly devoid of it. 



The colour of the rods is due to' the presence of a distinct 

 pigment, the " visual purple," diffused through the substance of the 

 outer limbs ; and this may be extracted from the rods by dissolving 

 these in an aqueous solution of bile salts. A clear purple solution 

 is thus obtained, which is capable of being bleached by the action 

 of light, and in its general features and behaviour is similar to the 

 pigment as it naturally exists in the retina. 



Visual purple is found as we have said exclusively in the outer 

 limbs of the rods ; it has never yet been found in the cones, and 

 it is accordingly absent from (or exceedingly scanty in) the retinas 

 (such as those of snakes) which are composed of cones only (or 

 contain very few rods), and from the greater part of the macula 



