2 : 3/ Light and the Eye 



41 



light can pass through several layers of cells and fibers and still retain 

 its original form. If these layers are arranged in a sufficiently orderly 

 fashion, there is relatively little scattering or absorption of light as it 

 passes through the tissue. 



The so-called "crystalline lens" is also a cellular structure. The 

 cells are long hexagonal columns. Most of the cell nuclei are grouped 

 in a restricted region of the lens which is not active in vision. A typical 

 cross section of a lens is shown schematically in Figure 9. 



(a) 



(b) 



Capsule 



Nuclear Zone 

 of Lens 



Vitreous Humor 



Figure 9. Histology of the lens, (a) Frontal section through 

 the equator of the lens showing the regular arrangement of 

 the cells, (b) Transverse section through the lens. After 

 Schaffer, in A. A. Maximow and W. Bloom, Textbook of 

 Histology (Philadelphia: W. B. Saunders Company, 1957). 



The last cellular structure of the eye, through which incoming light 

 must pass, is the retina. Here, the active photoreceptors are located. 

 There are two types of receptors, called rods and cones. Although not 

 customary, it is technically correct to refer to these rods and cones as 

 transducers. These transducers convert light energy into electrical 

 impulses which travel along the nerve fibers. As noted earlier, the 

 retina may be divided into 10 layers. These are diagrammed in Figure 

 10. Starting from the outermost layer, away from the light, one can 

 list the layers shown in Table I. 



