SENSE ORGANS. 



II 9 



Fig. 84. — Pacillary layer, viewed from 

 the outside surface : A, appearance of 

 usual surfacs ; B, appearance of sur- 

 face of the raised margin of central 

 spot ; C, surface of central spot. 



ing. This is called the central spot on account of its 

 position, and the fovea on account of its depression. 



But the importance of the retina is so great that it 

 must be studied more carefully under a higher magnifi- 

 cation. Fig. 83 is a highly magnified section. Concern- 

 ing the inner or fibrous layer nothing more is revealed. 

 The middle layer is seen to be very complex, consisting 

 of several granular layers and several layers of nucle- 

 ated cells and one layer 

 of very large ganglion- 

 ic cells. The functions 

 of these various layers 

 are not certainly known. 

 The bacillary layer is 

 now seen to contain two 

 kinds of elements — the 

 one slenderer, longer, 

 and more rodlike, the other shorter, stouter, and more 

 conelike. The rods are about one fourteen-thousandth 

 of an inch (one five-hundred-and-sixtieth millimetre), 

 and the cones about one five-thousandth of an inch (one 

 two-hundredth millimetre) in diameter. The rods are 

 usually most numerous. Fig. 84 is a view of the outer 

 surface, showing the larger cones surrounded by the 

 more numerous rods. But the relative number of these 

 is not the same in all parts. 



Distribution of the Rods and Cones.— On the 

 anterior margin of the retina there are no cones, but 

 only rods. As we approach the bottom of the retinal 

 cup the cones become more and more numerous, and at 

 the same time smaller until in the central spot or fovea 

 there are no rods, but only cones, and these have become 

 very small, only about one ten-thousandth of an inch 

 (one four-hundredth millimetre) in diameter (Fig. 84). 

 Further, it must be observed that the fibers of the fibrous 



