THE CEREBRAL HEMISPHERES 163 



distinguishing features of this cortex are much less 

 well developed than in the opossum, and in the mouse 

 they are scarcely at all evident. Isenschmid (191 1, p. 

 40) calls attention to the fact that mice use their eyes 

 less than rats, and from his account it seems probable 

 that house mice, and especially albino mice, lack a 

 specifically differentiated visual cortex altogether. 

 Sugita (191 7-1 91 8) finds that the visual cortex is bet- 

 ter developed (thicker) in wild Norway rats than in 

 domesticated albinos, a feature correlated with the 

 well-known poor vision of the albinos. 



Waugh (1910) has made a critical experimental 

 study of vision in mice. Their retinas lack cones. The 

 experiments give evidence of poorly developed color 

 discrimination (none in albinos). Discrimination of 

 differences in brightness is fairly good, but discrimina- 

 tion of forms is poor. In the albino rat vision is at a 

 primitive level. Color vision is absent (Watson and 

 Watson, 1913), pattern vision is poor, perhaps absent, 

 but the animal can distinguish differences in bright- 

 ness and size and gross differences in the direction 

 of single lines within the visual field (Lashley, 

 1912). 



The supposed auditory area of the rat (Fig. 38, 

 p) is comparable with the temporal area (Fig. 2^, 

 a. temp.) of the opossum and probably some adjoin- 

 ing parts. The excitable cortex of the opossum is less 

 extensive than that of the rat (cf. Figs. 37 and 40); 

 clearly in both cases no specific "motor area" com- 



