330 



THE SPECIAL SENSES. 



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. 143, the cones terminate in the 

 external nuclear layer in arborizations which connect with the 



Fig. 143. 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 Miiller. 



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 



