714 COMPARATIVE ELECTRO-PHYSIOLOGY 



phase here negative maximum the response overshoots 

 in the negative direction (figs. 245, 247). The direct and 

 after-effects in this Type III., therefore, may be represented 

 by the formula (+ ...). Between these two extremes 

 lie instances of an intermediate Type II., which has cases (a) 

 and (), according as the stimulus is removed at maximum 

 of the first or negative phase, or at maximum of the second 

 or positive phase. The formula of Type II. (a) is thus 

 ( + .. ), while that of Type II. (b) is ( ... ). 



The response of the retina furnishes us with the most 

 striking examples of the action of stimulus of light. The 

 true character of this response has been supposed hitherto 

 to be unlike that of other tissues, for while excited nerve 

 and muscle were said to show response by ' negative 

 variation/ the response of the retina was referred to as by 

 ' positive variation.' This furnishes us with an instance of 

 the confusion which is apt to result from making the 

 so-called resting-current the standard of reference. On 

 testing for the natural current, by making connections with 

 the longitudinal surface of the optic nerve, and with the 

 cornea, in an undetached eyeball of frog, I found that it 

 flowed from the cornea to the nerve. But when the eye is 

 detached, by section of the optic nerve, the after-effect of 

 excitation on the more excitable nerve reverses this current, 

 the nerve becoming relatively galvanometrically negative. 

 The normal effect of transmitted excitation from the retina 

 would now make the nerve still more galvanometrically 

 negative, and this would appear as a positive variation of the 

 reversed natural current. Hence, the responsive positive 

 variation, met with in the eye under light, is in reality the same 

 normal excitatory response, by galvanometric negativity, with 

 which we are already familiar. 



I have also shown, by means of equi-alternating electric 

 shocks, that under normal conditions the optic nerve is more 

 excitable than the cornea, and that the retina is more 

 excitable than the optic nerve. The eyeball and retina 

 have often been found by different observers to exhibit 



