928 FUNCTIONS OF RODS AND CONES. [BOOK m. 



the dim light in which these creatures move calls for increased 

 and not diminished appreciation of small differences of colour. 

 The coloured globules intercalated between the outer and inner 

 limbs of cones in some of the lower animals, such as birds and 

 reptiles, have probably no closer relation to colour vision than 

 has the yellow pigment of our own macula lutea. 



The close resemblance in their general features, apart from 

 form, between the rods and cones, suggests that their functions 

 differ in degree rather than in kind, and this view is supported 

 by the rod-like character assumed by the cones in the macula 

 lutea and especially in the fovea centralis. But we can hardly 

 expect to be able to differentiate the functions of the two, so 

 long as we know so little about either. 



With regard to what goes on in the other layers of the retina 

 our ignorance is complete. We may fairly suppose that the 

 events which take place in the inner limbs of the rods and 

 cones are different from those which take place in the optic 

 fibres. We may conclude that the latter are of the nature of 

 nervous impulses, though we may here repeat what we have 

 already urged, namely, that it is hazardous to infer that the 

 little we know of motor nervous impulses may be applied with 

 little or no modification to sensory nervous impulses ; but as to 

 the nature of the events in the inner limbs of rod and cones, or 

 as to what happens in the intervening layers of the retina, we 

 know nothing. 



579. The little objective knowledge wjiich we possess 

 concerning retinal processes is almost limited to the detection 

 of electric currents. The retina and optic nerve like other 

 nervous structures develope electric currents which may be 

 spoken of as currents of rest and currents of action. They 

 may be shewn by placing one electrode on the retina of a 

 bisected eye, or on the cornea of a whole one, and the other on 

 the optic nerve, or hind part of the eye-ball or on the cortical 

 visual centre or even on some distant part of the body. They 

 are also manifested by the isolated retina itself. The phenom- 

 ena appear somewhat complicated by the appearance now of 

 positive, now of negative variations ; but this fact comes out 

 clearly that the incidence of light on the irritable retina devel- 

 opes an electric change, the magnitude of which is to a certain 

 extent proportionate to the intensity of the light acting as a 

 stimulus. The changes gradually diminish and disappear as 

 the retina gradually loses its irritability. We may add that 

 these electric phenomena appear to be quite independent of the 

 condition of the visual purple. 



