40 



Light and the Eye \1 : 3 



Light enters the eye through the cornea, whose microscopic structure 

 is shown in Figure 8. First, the light passes through an outer layer of 

 epithelial cells. These cells are separated by a thin membrane from an 

 inner fibrous layer which in thickness comprises most of the cornea. 

 These fibers are very similar to the fibers in the sclera. Those in the 

 cornea are unique in that they are arranged in an orderly fashion. It 

 appears that it is this orderliness of the fibers of the cornea that is 

 responsible for its transparency as contrasted with the opacity of the 

 sclera. Inside the fibrous layer of the cornea is another very thin 

 limiting membrane and finally a lining of cells called endothelial 

 cells. 



Epithelium 

 ^T Bowman's Membrane 



Substantia 

 Propria 

 {Fibers) 



Membrane of Descemet 

 Corneal Endothelium 



Figure 8. Histology of the cornea. After Schaffer, in A. A. 

 Maximow and W. Bloom, Textbook of Histology (Philadelphia: 

 W. B. Saunders Company, 1957). 



As noted previously, the shape of the cornea is responsible for the 

 major refraction of the eye. Any large irregularities or abrasions would 

 reduce the acuity of vision. The usefulness of the eye depends on keeping 

 the cornea clear and transparent. If a large object approaches the 

 cornea, the eyelids are closed by a reflex action. Smaller particles are 

 removed by blinking and through tear formation. The outer epithelial 

 layer of the cornea is very highly innervated ; the nerves terminate in 

 bare nerve endings. Any slight disturbance stimulates these endings, 

 resulting eventually in the blinking reflex. All persons normally 

 blink quite frequently; this cleans and moistens the outer surface of 

 the cornea which otherwise would become dehydrated and lose its 

 transparency. 



It always appears surprising when one first encounters the idea that 



