360 



THE SPECIAL SENSES. 



lights (night vision). Color blindness, therefore, whether total or 

 partial, may be regarded as an affection or lack of normal develop- 

 ment of the cones. On the other hand, those interesting cases in 



Fig. 152. Schema of the structure of the human retina (Greeff): I, Pigment layer; 

 II, rod and cone layer; ///, outer nuclear layer; IV, external plexiform layer; V, layer 

 of horizontal cells; VI, layer of bipolar cells (inner nuclear) ; VII, layer of amacrinal cells 

 (without axons) ; VIII, inner plexiform layer ; IX, ganglion cell layer ; X , nerve fiber 

 layer; 6, fiber of Muller. 



which the vision, while good in daylight, is faulty or lacking in dim 

 lights (night blindness, hemeralopia) may be referred to a defective 

 functional activity of the rods, probably from lack of formation of 

 visual purple. 



Theories of Color Vision. A number of theories have been 

 proposed to explain the facts of color vision. None of them has 

 been entirely successful in the sense that the explanations it affords 



