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A MANUAL OF PHYSIOLOGY 



(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 cephalopod 

 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 



FIG. 291. DIAGRAM SHOWING DIRECTION OF SHOCK IN GYMNOTUS. 



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

 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 (Dewar and McKendrick). (For 

 ' visual purple,' see p. 934. 



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 men- 

 tioned 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 Amer- 

 ica. Another of the electric 

 fishes, Malapterurus electvi- 

 cus, although found in many 



of the AfricanC rivers, the Nile in particular, and known for ages, 

 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 



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

 descending has not been reproduced. 



FIG. 292. DIAGRAM SHOWING DIRECTION 

 OF SHOCK IN MALAPTERURUS. 



