122 FUNCTIONS OF RODS AND CONES. [BOOK in. 



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 w r e 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 what happens in the inter- 

 vening layers of the retina, we know nothing. The double method 

 of connection of the optic fibres with the other retinal layers, on 

 the one hand by the mediation of the ganglionic cells, on the other 

 hand by some more direct way ( 742) with either the inner 

 molecular, or the inner nuclear layer, a like double connection 

 being observed in other parts of the brain, the cerebral cortex and 

 the cerebellar superficial grey matter for example, suggests that 

 the retinal processes are exceedingly complex, and that visual 

 impulses are so to speak largely moulded before they find their 

 way into the optic nerve. 



777. The little objective knowledge which we possess con- 

 cerning 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 phenomena appear somewhat compli- 

 cated by the appearance now of positive, now of negative varia- 

 tions ; but this fact comes out clearly that the incidence of light 

 on the irritable retina developes 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 in- 

 dependent of the condition of the visual purple. 



