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A TEXTBOOK OF PHYSIOLOGY 



SECTION IV 



THE EFFECTS OF LIGHT FALLING ON THE RETINA 



WHEN those vibrations of the ether which evoke the sensation of 

 light fall upon the retina, certain marked changes occur. In the first 

 place, there is a variation of the resting electrical current. In the 

 resting eye, the current is normally ingoing; when the eye is stimulated 

 by light, the current becomes outgoing (Figs. 349, 350). 



Secondly, there is a movement of the granules in the pigment 

 cells from the centre of the cells into the processes between the rods. 

 In a frog's eye which has been kept in the dark, the pigment layer 



Volt 

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FIG. 349. PLOTTED CURVES CONSTRUCTED FROM ELECTROMETER RECORDS OF EYE- 

 BALL RESPONSES TO THE LIGHT FROM THE RED, GREEN, AND VIOLET REGIONS 

 OF THE SPECTRUM. (Gotch.) 



The upper records are fairly typical, the lower show the most pronounced responses. 



is easily separated from the rods and cones. Afte'r exposure to light, 

 these layers are difficult to separate; the pigment is much more abun- 

 dant between the outer limbs of the rods, and passes as far as the 

 external limiting membrane between the inner limbs (Fig. 351). 



Thirdly, the visual purple present in the outer limbs of the rods is 

 bleached in the parts upon which the light falls. By this means, 

 therefore, a negative image, or optagram, is obtained in the retina 

 of an object, such as a window at which the eye may have been looking 

 (cf. Fig. 352). When light ceases to fall, the visual purple is again 

 regenerated at the expense of the pigment cells. For this purpose, 

 it is necessary for them to be in contact with the rods. If they be in 

 any way detached, regeneration does not take place. 



