356 THE SPECIAL SENSES. 



perceived, and a central zone sensitive to red and green.* The 

 outlines of 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 miite 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 ganghon cells, and in the fovea at least this connection is such 

 i,nat each cone connects with a single nerve cell and eventually per- 

 haps with a single optic nerve fiber. The rods, on the contrary, 

 end in a single knob-like swelling, and a number of them make con- 

 nections with the same nerve cell. Histologically, therefore, the 

 conduction paths for the cones seem to be more direct than in 

 the case of the rods. Each cone makes connection with the brain 

 through a separate fiber and possibly, therefore, when stimulated 

 may give its own distinctive sensation. Hence, where the cones 

 are most numerous, as in the fovea, visual acuity is most devel- 

 oped. On the other hand, several rods send impulses to the brain 

 through a common fiber and there is a single sensation for the 

 whole group. Hence, where rods are numerous and cones scanty 

 visual acuity is imperfect. The rods, however, possess the visual 

 purple, which is lacking in the cones, and in the eye of the totally 

 color-blind, in the dark-adapted eye in dim lights, in the color- 

 blind peripheral area of the normal eye, and in the eyes of most 

 distinctly night-seeing animals, such as the mole and the owl, vision 

 seems to be effected solely by the rods. These latter facts find 

 their simplest explanation perhaps in the view advocated by Pari- 

 naud, Franklin, von Kries,t and others, according to which the 

 perception of color is a function of the cones alone, while the rods 

 are sensitive only to light and darkness, and by virtue of their power 

 of adaptation in the dark through the regeneration of their visual 

 purple, they form also the special apparatus for vision in dim 



* Two recent writers (Ferree and Rand, "American Journal of Physiological 

 Optics," July, 1920) state that the color blindness on the periphery of the 

 retina is relative and not absolute. With a sufficient intensity of stimulation 

 they find that the fields for red, blue, and yellow are coextensive with that of 

 white. Green has a more restricted field, the periphery of the retina being 

 entirely devoid of this sense. 



t Von Kries, " Zeitschrift f . Psychologie u. Physiol, d. Sinnesorgane," 9, 

 81, 1895. 



