472 ELECTRO-PHYSIOLOGY CHAP. 



positive to all other points of the retina. In leading off, on the 

 other hand, from the fibrous layer (internal surface), the entrance 

 of the optic nerve is invariably negative to every peripheral 

 point. Like Holmgren, Kiihne and Steiner further observed, on 

 leading off simultaneously from internal and external surface of 

 the fresh retina, an " ingoing " current, the former being negative 

 to the latter. The retina was so arranged between the electrodes 

 " that the lower of the two supported the membrane on a clay 

 cap curving upwards, while the other made contact with the 

 opposite surface of the retina by a blunt point." The E.M.F. of 

 this current, which is at first considerable, diminishes rapidly, 

 and at times disappears completely, although in most cases it 

 remains for some time longer at a medium height. 



The variations of the retinal current under the influence of 

 light are of far greater interest than its intrinsic behaviour. 

 Here, again, it is to Holmgren that we owe the fundamental 

 observations : he showed that the retinal current invariably 

 gives a positive deflection, if light falls on an eye that had 

 previously been in the dark, or when light is shut off from it. 

 This occurs in the frog without exception, while in reptiles 

 (snakes), birds, and mammals Holmgren finds 011 the contrary 

 a negative effect at the impact of light, a positive variation at 

 darkness. The mere alteration of intensity of illumination makes 

 an effective stimulus. 



Dewar and M'Kendrick discovered, independent of Holmgren, 

 that a positive variation resulted from illumination of the eyes 

 of vertebrates (of all classes), as also of Crustacea, correspond- 

 ing to an increase of E.M.F. in the rest -current of 3-10 / Q . 

 During part of the experiments the lead-off (frog) was not 

 merely from the bulb, but from this, and a portion of the brain 

 still connected with it by the optic nerve. Here, too, the impact 

 of light was followed by a strong positive variation. The same 

 occurs in the pigeon on leading off from optic lobe and cornea 

 of the opposite eye. The variation in this case is nearly doubled 

 if the two retinse are simultaneously illuminated, nor is it 

 altogether absent on leading off from lobe and cornea on the 

 same side. Of coloured lights, yellow was the most effective, 

 then came green, red, and blue. Lastly, Dewar and M'Kendrick 

 eventually believed that they had discovered relations between 

 stimulus and effect, corresponding witli Fechner's law. 



