xvii THE SENSE ORGANS 33 1 



as three varieties being distinguished (M. Schultze, van 

 Genderen Stort). Some of these are movable, expanding 



Fig. 92. — Inner surface of the inner half of the eye of Rana esculenta, show- 

 ing the area centralis as a light-colored band above the papilla optica 

 where the optic nerve enters. (After Gaupp.) 



in the dark until they may reach the periphery, and contract- 

 ing under the influence of light. Ordinarily most of the 

 cones do not reach the outer surface of the retina. 



The retina is marked by a thickening or papilla where the 

 optic nerve enters. Above this papilla there extends in an 

 antero-posterior direction a thickened portion of the retina 

 known as the area centralis, which probably marks the 

 region of most acute vision, like the yellow spot in the eye 

 of man. The cones are relatively more numerous in this 

 region than in other parts (1 c. : 2. 4 r. instead of 1 c. : 3 

 or 4 r., as elsewhere), and the rods are of relatively small 

 size. 



The action of light upon the retina results in the produc- 

 tion of several visible changes. In addition to the contraction 



