The Sense of Sight. 



257 



The Structure of the Retina. The retina is very complicated in its 

 structure. No less than eight layers have been distinguished, as shown 

 in Fig. 100. The rays of light pass through the retina, and produce 

 their effect on the rods and cones which constitute the outer (back) 

 layer ; and the nerve impulses aroused by the light must return through 

 the thickness of the retina to be conveyed along the nerve fibers of the 

 innermost layer of the retina, to the optic nerve. 



Inner or Vitreous Surface 



Internal Limiting Layer 

 Layer of Nerve Fibers 

 Layer of Nerve Cells 



Inner Molecular Layer 



Inner Nuclear Layer 



Outer Molecular Layer 



Outer Nuclear Layer 



External Limiting Layer 



Layer of Rods and Cones 



Layer of Pigment Cells 



Outer or Choroid Surface 



Fig. 100. Section of the Retina. (Waller.) 



Importance of the Retina. The chief structure in the 

 eye is the retina. Without this all else is useless. If light 

 falls on the retina, nerve impulses pass along the fibers of 

 the optic nerve to the brain, and we have a sensation of 

 light. But in order to see anything distinctly, the light 

 must fall on the retina in such a way as to form a distinct 

 image of that object. If the lens be removed, or becomes 

 opaque, as in "cataract," we fail to see distinctly, though 

 we may be able to tell light from darkness. Light from 



