88 VERTEBRATE PHOTORECEPTORS 



In bright light the pigment moves distally, surrounding and 

 protecting the rods (which have a low threshold, and which 

 have been rendered sensitive by the accumulation of visual 

 purple in the dark) from too strong light stimulation. This 

 is accomplished by the pigment absorbing the direct and 

 scattered light. The rods elongate, while the less sensitive 



__..„ Fig. 59 (Left). Theoretical _ 



@> ^ light-adapted retina, showing mi- ^^ ^l,^~" 



grated epithelial pigment, elon- *'*' . ^^'^ ■■, 



gated rods, and contracted cones. ■ 



Fig. 60 (Right). Theoretical 



dark-adapted retina, showing con- j | , j 



tracted pigment, elongated cones ^,„' 1^ i_.! 



and contracted rods. ^ f' ^' 



cones are drawn out of the pigment by their myoids and 

 are then made freely accessible to the stronger light stimulus 

 — thus presenting optimum conditions for their stimulation. 

 The theoretical situation is illustrated in Figures 59 and 60 

 and an actual condition as observed by Arey in the eye of 

 Ameiurus is illustrated in Figure 61, A and B. 



It must be assumed in such a theory that the cone cells 

 remain elongated in very dim light, but, according to Garten, 

 working on Abramis, this is not true; consequently the 

 theory doesn't hold. Garten who weakened the theory of 

 Exner and Januschke (1906), maintained that the function 

 of the expanded pigment is to absorb all light which might 

 escape from the visual cells by refraction; that were it not 

 for the optical isolation of the visual cells by the pigment, 

 a great deal of light would be scattered in all directions, on 

 account of the large ellipsoids (fishes) and the strongly re- 

 fractive oil drops (amphibians, reptiles, and birds) and 

 would stimulate the neighboring rods and cones. In the 

 long slender rods such as occur in mammals, however, 



