358 



THE SPECIAL SENSES. 



concentric zones: an extreme peripheral zone devoid of color vis- 

 ion, an intermediate zone in which yellow and blue are perceived, 

 and a central zone sensitive to red and green.* The outlines of 



Fig. 150. Perimeter chart indicating the average fields of vision for blue, red, and 

 green compared with white (gray). Right eye: The outlines of the color fields are repre- 

 sented as smooth since the chart is an average from many determinations. As a matter of 

 fact, in each individual the outline is highly irregular. Normally green (bright green) is the 

 smallest field, green objects outside the limit appearing yellow and farther out colorless 

 (gray). 



the different fields usually show many irregularities, and in some 

 cases it will be found that bright green is perceived over a larger area 

 than the red. The fields are not identical in the two eyes, and in 

 each eye it is, as a rule, more extensive upon the nasal than 

 upon the temporal side of the retina. In the red-green blind the 

 peripheral fields of color vision, judged by the individual's own 

 standards, may be markedly constricted as compared with the nor- 

 mal retina (see Fig. 151). 



Functions of the Rods and Cones. Many facts unite in mak- 

 ing it probable that the rods and cones are different in function. 

 They differ in structure and especially in their connections. As is 

 shown in the diagram given in Fig. 152, the cones terminate in the 

 external nuclear layer in arborizations which connect with the bi- 

 polar ganglion cells, and in the fovea at least this connection is such 



* It is interesting to find (Haycraft) that around the blind spot there is a 

 small zone which, like the periphery of the retina, is completely color-blind, 

 that is, perceives only gray, and external to this the color sense is developed 

 in zones whose order is similar to that on the periphery of the retina. 



