THEORIES OF PHOTOMECHANICAL CHANGES 89 



total internal reflection prevents this dispersion of light. 

 This is supported by the fact that in most rod retinas where 

 the rods are long and slender, the pigment is either very 

 sparse or entirely wanting. Certainly in the mammalian 

 eyes which I have examined (mouse, bat, guinea-pig, dog, 

 rabbit, cat, monkey, man) the rods are slender elements with 



Fig. 61. A, dark-adapted, and B, light-adapted retina of Ameiurus nebu- 

 losus. In A the pigment is withdrawn toward the choroid, the cone myoid 

 elongates, the rod myoid shortens. In B the pigment moves forward toward 

 the external limiting membrane, the rods elongate and the cones shorten. 

 (Redrawn from Arey, 1919. J. Comp. Neur., v. 30.) 



no refractive bodies, and the pigment is not only scarce but 

 is entirely wanting in the regions of those eyes which possess 

 a tapetum. In nocturnal lemurs (Galago and Nycticebus) 

 which have long attenuated rods and possess also a choroidal 

 tapetum, there is complete absence of pigment in the fundus. 

 On the other hand, the owl monkey, Nyctipithecus, which 

 has a pure rod retina similar to the nocturnal lemurs, does 

 have a narrow band of pigment extending throughout the 

 epithelial pigment layer. 



When we examine the gecko where the rods are large, with 

 prominent 'barrel-shaped^ ellipsoids and a large paraboloid, 



