THE VISUAL APPARATUS 



207 



The peripheral ends of the rods contain a pigment, the visual 

 'purple or rhodopsin, which is chemically changed by light rays 

 and has been supposed to function as the exciting agent for ner- 

 vous impulses of sensibility to light in the rod cells. But recent 

 experiments go to show that the visual purple is concerned with 



Istratum 

 f pigment! 



Istratum 

 [opticum 



Membrana limitans interns 



Fig. 99. Diagrammatic section through the human retina to illustrate 

 the ten layers as commonly enumerated. (After Schultze, from Cunning- 

 ham's Anatomy.) 



the adaptation of the eye to different intensities of light rather 

 than with the specific receptor function itself. The brown pig- 

 ment of the pigment layer is probably also concerned with light 

 adaptation. 



The exact mechanism through the agency of which the rods 

 and cones are excited to nervous activity by light is still obscure; 



