4.20 COMPARATIVE ELECTRO-PHYSIOLOGY 



As, in reference to this latter point, however, it may be 

 urged that the retina is exceptionally sensitive to light, 

 while the reactions of vegetable tissues are generally slug- 

 gish, it may be worth while to point out here that vegetable 

 tissues are not so insensitive as is generally supposed, but 

 are often, on the contrary, highly susceptible to the action of 

 light. It was, for instance, found by Darwin that the coty- 

 ledons of Phalaris canariensis were, in the course of some 

 hours' exposure, curved towards a small -lamp, placed at a 

 distance from them of 12 feet. . The intensity of the stimulus 

 of light was in this case extremely feeble. I have myself 

 observed, again, the remarkable sensitiveness to light of the 

 terminal leaflet of Desmodium^ which, on the mere striking 

 of a match in its vicinity, was thrown into a state of pulsatory 

 movement. 



I shall, in the course of the present chapter, describe 

 certain definite effects on the retina, which will prove, in an 

 unmistakable manner, that, when exposed to light, it under- 

 goes a change of galvanometric negativity. It is now there- 

 fore necessary to show clearly that what has been described 

 as the positive variation is really due to the excitatory nega- 

 tive change of the retina. What has to be demonstrated, 

 then, is the way in which this simple underlying reaction of 

 negativity comes to appear as a positive variation of the 

 opposite-directioned currents of rest in the eyeball and 

 isolated retina respectively. 



As regards experiments on the eyeball, with contacts at 

 nerve and cornea, we have seen that the existing injury- 

 current is from nerve to cornea, and that this undergoes a 

 positive variation when the nerve is stimulated in anyway. 

 This is because the added negativity induced in the nerve 

 gives rise to an increase, or positive variation of, the existing 

 current (fig. 248). Now, when light falls on the eye it acts 

 on both the cornea and the retina. The former, however, 

 is relatively inexcitable, especially to so moderate a stimulus 

 as that of light. But the retina is excited, and its excitatory 

 condition is rapidly transmitted to the optic nerve, which 



