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A TEXTBOOK OF PHYSIOLOGY 



be used (Fig. 307). When light falls upon the retina, a complex 

 variation ensues, depending upon the strength and duration of the 

 stimulus, upon the condition of the eye, whether adapted to light or 

 dark, fresh or fatigued, and upon the nature of the light, whether 

 white or coloured. In the isolated retina there is first a positive and 

 then a negative variation when light falls on it. When the light is cut 

 off, a positive variation is produced. As the result of a momentary 

 flash there is a short latent period 0-01 second followed by a short 

 negative variation, followed by a large positive variation, quickly 



Inner 

 surface 



FIG. 305. SKIN CURRENT.. 



FIG. 306. GLAND CURRENT. 



followed by another diminution of the positive variation, and then 

 by a long-drawn-out increase of the positive variation. 



Electric Tissues. In some fishes there is developed a tissue capable 

 of producing electricity: These fishes are either elasmobranchs, such 

 as the rays (Raia ocellata, R. laevis, etc.) and the torpedo fish (Tetro- 

 narce), or teleosts, such as the electric eel (Gymnotus), the somewhat 

 similar elongated fish (Mormyrus), the star-gazer (Astroscopus), and 

 the electric catfish (Malapterurus). The electric tissue consists of 



Current of 

 injury from 

 optic nerve. 



Retinal current di- 

 rected in the retina 

 from rod surface to 

 fibre surface. 



FIG. 307. 



a series of plate-like units known as electroplaxes (Fig. 308). The 

 electroplax lies in a compartment of connective tissue embedded in a 

 jelly-like mass, through which the nerve and blood-supply pass to 

 it. In some cases it is to be considered as a single cell with many 

 nuclei, hi others a fusion of cells a syncytium. The discharge is 

 composed of about 200 shocks per second, and the E.M.F. may be 

 sufficient to kill other fish in the neighbourhood. In the discharge 

 of the organ the current flows through the organ from the ventral 

 to the dorsal surface, and through the galvanometer from the dorsal 



