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PHYSIOLOGY. 



it is unable to grow more convex, artificial lenses (eye- 

 glasses) may be used to enable one to see near objects 

 clearly. Most elderly people see fairly well at a distance, 

 but use glasses for reading or any close work. In " near- 

 sighted " eyes, the eyeball is often too long from front to 

 back, so the rays meet in front of the retina. Concave 

 glasses remedy this defect. The eye may also be too 

 short (far-sighted) and need convex glasses. The refract- 

 ing surfaces (cornea and lens) may be unequally curved, 

 causing astigmatism. For most of these defects the 

 oculist can supply suitable glasses. 



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. 90. Diagrammatic Section of the Human Retina. (Waller.) 



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. 90. Of these layers the outermost, the layer of the rods 

 and cones, is the one directly concerned in appreciating the differences 

 in the vibrations of the light. The rays of light pass through the 



