RESPONSE OF RETINA TO STIMULUS OF LIGHT 417 



the more to the less excitable. The experimental arrange- 

 ment used in this investigation is shown in fig. 248. In such 

 an experiment, equi-alternating shocks are given to the pre- 

 paration by means of a secondary coil included in the circuit. 

 The existing current, from nerve to cornea, may be balanced 

 previously by a potentiometer. Whether the record be taken 

 under balanced or under ordinary conditions, it is found that 

 in a normal eyeball the responsive current is from the nerve to 



the cornea, showing that the nerve 

 is relatively the more excitable of 

 the two. Fig. 249 gives a series 

 of such responses. It will also 

 be noticed that the excitatory 

 current is in the same direction 



FIG. 248. Experimental 

 Arrangement for Deter- 

 mination of Differential 

 Excitability of Optic 

 Nerve and Cornea 



C, injury current, or so-called 

 current of rest ; R, re- 

 sponsive current. 



FIG. 249. Series of Photographic Records of 

 Excitatory Responses in Frog's Eye to 

 Equi-alternating Electric Shocks at Inter- 

 vals of One Minute 



Current of response from nerve to cornea. 



as the existing current, and thus constitutes a positive varia- 

 tion of it. This positive variation of the current in the eyeball 

 thus indicates a true excitatory reaction of the optic nerve. 



From the fact that the nerve is more excitable than the 

 cornea, it is clear that the sectioning of it for isolation of the 

 eyeball, acting as an intense stimulus, will result in its excita- 

 tory negativity, which will persist for a time and slowly dis- 

 appear. Owing to this fact, a current flows from the more 

 excited nerve to the less excited cornea. That this current 



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