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ELECTRO-PHYSIOLOGY 



followed by a much larger upward movement (positive variation) in 

 the same direction as the resting effect, the fundus becoming relatively 

 more negative to the cornea than before. After the peak B has been 

 reached, the curve sinks first rapidly, then more gradually, but soon 

 mounts again, and reaches a second maximum C, vhichis higher than B 

 (second positive variation). Finally, the curve descends to its original 

 level.* The photo-electric reaction is substantially the same in all 

 vertebrate eyes hitherto investigated. In the cephalopcd retina, too, 

 the only important electrical change on illumination is in the same 

 direction as the resting effect. 



The reaction depends upon the retina alone, and does not occur 

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

 affect it, so that it is not connected with chemical changes in this 

 substance. Its seat must be the layer of rods and cones, since in the 



Fig. 319. Diagram showing Direction of Shock in Gymnotus. 



cephalopods the structure called the retina contains only this layer, the 

 other layers of the vertebrate retina being represented in the optic nerve 

 and ganglion (Beck). Of the spectral colours, yellow light causes the 

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

 either (Dewarand McKendrick). (For 'visual purple, 'see Chap. XVIII.) 

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

 probably the first manifestations of electricity observed by man. The 

 Torpedo, or electrical ray, of the coasts of Europe was known to the 

 Greeks and Romans. It is mentioned in the writings of Aristotle and 



Pliny, and had the honour of 

 being described in verse 1,500 

 years before 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 electricus, al- 

 though found in many of the 

 African rivers, the Nile in par- 

 ticular, and known forages, was 

 scarcely investigated till fifty 

 years ago. 



In all these fishes there is a 



special bilateral organ immediately under the skin, called the electrical 

 organ. It is in this that the shock is developed. It consists of a 

 series of plates arranged parallel to each other. To one side of each 

 plate a branch of the electrical nerve supplying each lateral half of 



* In the figure the last portion of the curve while it is still slowly descending 

 has not been reproduced. 



Fig. 320. Diagram showing Direction of 

 Shock in Malapterurus. 



