350 PHYSIOLOGY CHAP. 



latter is found to be electrically negative to the former, while it is 

 positive to the postero-lateral part of the eyeball. In the isolated 

 frog's retina, again, the periphery is negative to the point at which 

 the optic nerve enters and to the outer surface, but positive to 

 the inner surface of the retina. According to Kiihne and Steiner, 

 the outer surface is negative to the inner surface of the retina. 



The magnitude of the current of rest differs in different cases ; 

 it falls rapidly in preparations from warm-blooded animals, much 

 more slowly in the cold-blooded ; in the isolated frog's retina it 

 lasts many hours. 



According to Holmgren, the current of rest exhibits a variation 

 when the " dark " retina is suddenly illuminated ; or when the 

 previously illuminated retina is darkened. The strength, direction, 

 and course of this current of action vary considerably with the 

 kind of animal and form of the preparation. The frog lends itself 

 to these experiments better than any other animal, on account of 

 the long persistence of its current of rest. 



The typical reaction in a frog's eye, as little injured as possible, 

 is as follows : Sudden illumination produces after a brief latency 

 a positive oscillation of the current of rest, which attains its 

 maximum after a few seconds, and then slowly diminishes if the 

 illumination is continued. A sudden transition to darkness pro- 

 duces another positive variation, the after-effect, but this is transient, 

 and is followed by a slow return to the position of rest. 



In mammals, birds, and reptiles, even under normal conditions 

 that is, with the retina intact illumination gives rise to a 

 negative, darkness to a positive oscillation (Holmgren) ; to this 

 rule there are, however, exceptions (Nagel and Himstedt.) 



These photo-electrical reactions are probably phenomena con- 

 comitant with the chemical changes produced by the stimulus of 

 light in the sensitive elements of the retina. But little or nothing 

 is at present known as to their functional significance and specific 

 cause. It is highly probable that the electromotive phenomena 

 originate in the sensitive elements of the retina. They were, in 

 fact, observed by Beck in the retina of Cephalopods (Eledone 

 moschala), which consists only of rods and cones, as the elements 

 of the other retinal layers are only found in the optic nerve and 

 ganglion. 



IV. Stimulation of the retina produces neural impulses, which 

 on transmission to the brain excite psycho-physical phenomena, 

 i.e. physiological processes intimately connected with states of 

 consciousness. These are the specific visual sensations, which are 

 sometimes associated with the sensation of " dazzle." 



The fundamental differences in the sensations that reach us 

 through the eyes are differences in brightness or luminosity and in 

 colour : in other words, visual sensations may be colourless or 

 coloured. The former are distinguished from one another only by 



