338 



THE SPECIAL SENSES. 



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

 shown in the diagram given in Fig. 145, the cones terminate in the 

 external nuclear layer in arborizations which connect with the 

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

 that 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 



06t 081 



Fig. 144. Perimeter chart showing the highly restricted color fields in the left eye 

 of a typical case of so-called red-green color blindness. The ability to distinguish red and 

 green, by whatever characteristics of intensity 9r color they possessed extended for a very 

 short distance outside the fovea. It is interesting that the ability to distinguish blue was 

 in this case limited as compared with a normal eye. 



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

 the case of the rods. These latter elements, moreover, possess the 

 visual purple, which is lacking in the cones. Lastly, 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 facts find 

 their simplest explanation perhaps in the view advocated by Pari- 

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

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

 * Von Kries, " Zeitschrift f. Psychologic u. Physiol. d. Sinnesorgane," 9, 

 81, 1895. 



