922 PHOTOCHEMISTRY OF THE RETINA. [Book hi. 



it is especially sensitive, are just those which are absorbed by 

 the pigment. 



§ 576. Photochemistry of the Retina. Such considerations 

 as 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 

 fallings 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. And it must be remembered that in 

 such chemical changes electrical events may intervene and that 

 in a special way. 



One of the most striking features in the structure of the 

 retina is the abundance of black pigment, fuscin, 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 impor- 

 tance, 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 suffused 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 on the 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 exami- 

 nation with the microscope it is found that the purple colour is 

 confined exclusively 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 dis- 

 solving 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. 



