Il6 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



SECTION III. 

 The Retina and its Functions. 



Thus far, as much as possible, we have confined our- 

 selves to the eye as an instrument for making an image, 

 and have compared it with the camera in order to show 

 the beauty of its adaptation for that purpose. But in 

 both the camera and the eye the image is only a means 

 to accomplish an end — to make a photogram in one case 

 and accomplish vision in the other. In both cases there 

 must be a sensitive screen to receive the image — the 



Fig. 8i. — A view of the two eyes, with optic nerves : ch, optic chiasm ; 

 rr , nerve roots; n and «', right and left optic nerves. (After Helm- 

 holtz.) 



iodized plate in the one, and the living retina in the 

 other. In both cases, too, the most wonderful changes 

 take place in these sensitive screens. Before we can un- 



