6.j8 A MANUAL OF PHYSIOLOGY 



on the cornea and the other on the cut optic nerve, it is found that 

 a current due to the injured nerve passes in the eye from optic nerve 

 to cornea. The current has the same direction if the anterior electrode 

 is placed on the retina itself, the front of the eyeball being cut away, 

 or if one electrode is in contact with the anterior and the other with 

 the posterior surface of the isolated retina. There is nothing of 

 special interest in this ; but the important point is that if light be now 

 allowed to fall upon the eye, or better, the isolated retina, an elec- 

 trical change is caused, generally first a positive and then a negative 

 variation, succeeded by another positive movement when the light is 

 cut off. 



The variations depend upon the retina alone, and do not occur 

 when it is removed. Bleaching of the visual purple does not much 

 affect them, so that they are not connected with chemical changes in 



FIG. 217. DIAGRAM SHOWING DIRECTION OF SHOCK IN GYMNOTUS. 



this substance. Of the spectral colours, yellow light causes the largest 

 variation ; blue, the least ; but white light is more powerful than 

 either. (Dewar and McKendrick.) (For ' visual purple ' see 

 Chapter XIII.) 



Electric Fishes. Except lightning, the shocks of these fishes were 



probably the first mani- 

 festations of electricity ob- 

 served by man. The Tor- 

 pedo, or electrical ray, of 

 the coasts of Europe was 

 known to the Greeks and 

 Romans. It is mentioned 

 in the writings of Aris- 

 totle and Pliny, and had the 

 honour of being described 

 FIG. 218. DIAGRAM SHOWING DIRECTION in verse 1,500 years before 

 OF SHOCK IN MALAPTERURUS. Faraday made the first really 



exact investigation of the 



shock of the Gymnotus, or electric eel, of South America. Another 

 of the electric fishes, Malapterurus e.'ectricus, although found in 



