160 HUMAN PHYSIOLOGY 



The student, when he reads a description of the 

 several layers of the retina, has the strongest feeling 

 that it is all ''wrong side out." The cellular elements 

 on which the light undoubtedly acts are not arrayed 

 upon the inner surface but are on the outside, next to 

 the middle coat. To reach them the light must pass 

 through a tangle of nerve fibers and cells other than the 

 true receptors. It must even pass a network of blood- 

 vessels. The layer which finally translates the radiant 

 energy into nerve-impulses is that of the rods and cones. 

 At the fovea the overlying matter is reduced and the 

 exposure of the sensitive units is correspondingly direct. 



The rods and cones constitute a mosaic pavement in 

 which the individual members are placed with striking 

 regularity. The cones are rather more advanced and 

 elaborate in appearance than the rods and there is 

 little doubt that they have superior properties. In the 

 fovea there is a central group of cones with no rods, 

 farther out the cones are scattered among rods which 

 greatly outnumber them, and still farther from the 

 fovea no cones but only rods are to be found. This 

 distribution is associated with contrasted powers of 

 vision in these three regions. 



The fovea and a zone extending some distance out- 

 side it have the capacity to differentiate all the colors. 

 Beyond the area with complete color vision there is a 

 tract not stimulated characteristically by reds and 

 greens though distinguishing yellows and blues. Still 

 beyond, all color comparison is lost and only light and 

 shade can be recognized. We say that the outlying 

 region of the retina is totally color-blind. The same 

 part is without cones, so it has been natural to infer that 

 the cones are adapted to discriminate color while the 

 rods are affected in the same way by all varieties of 

 light. If this is so their reactions can indicate nothing 

 more than the degree of illumination. 



Color-blindness. What is ordinarily meant by this 

 expression is not a complete inability to compare colors 



