CRYSTALLINE LENS 



105 



A 



nal pressure of the lens causes the central section of the capsule to 

 assume a different curvature from its peripheral section. 



Histologically the lens is composed of a number of radially arranged 

 fibers each of which is a modified epithelial cell. These fibers are ar- 

 ranged in concentric layers, the more peripheral being soft and nucleated, 

 and of low refractive index, and the fibrous layers between have an 

 intermediate structure and index of refraction. 



Gullst rand's schematic eye simulates the above complex structure by 

 substituting for it a crystalline lens having an outer symmetric double 

 convex cover over a core lens of greater 

 index symmetrically placed with respect 

 to the surrounding outer part. This sim- 

 plified equivalent lens has a refractive 

 index 1.413. It has the same size, shape, 

 and focal length as the complex crystal- 

 line lens of the human eye. 



The anterior pole of the human lens lies 

 3.6 mm behind the vertex of the cornea, 

 and the posterior pole 7.2 mm behind 

 it. Its anterior radius of curvature is 

 10.0 mm, and its posterior radius is —6.0 

 mm. When the lens is accommodated 

 for near vision, its anterior pole A (Fig. 

 Ill— 1) moves forward to within 3.2 mm 

 of the cornea pole; its posterior lens sur- 

 face retains its unaccommodated (focused 

 for parallel light) position. The radii of 

 curvature change so that its anterior radius of curvature at maxi- 

 mum accommodation is reduced to 5.33 mm and its posterior radius to 

 — 5.33 mm. This large posterior change is not universally accepted 

 as representing the complex variations accompanying the rather in- 

 volved change in position of the suspensory ligaments. 



When opacities form in the lens (cataract), this structure is removed 

 by a surgical operation. It is then found that a lens of about 10 diop- 

 ters* must be worn by the patient in order that he may see distinctly, 

 and a 4-diopter spherical lens must be added when a glass for reading is 

 prescribed. 



The optical center 0, of Gullstrand's simplified lens (Fig. Ill— 1), lies 

 5.85 mm from the vertex of the cornea and 5.2 mm from this vertex 

 when the eye is accommodated for near vision. The first and second 



* In dealing with spectacle lenses, it is usual to take the unit of length as 1 meter. 

 The reciprocal of the focal length measured in meters is its power in diopters. 



Fig. III-4. Scale representa- 

 tion of crystalline lens showing 

 changes in thickness of capsule. 

 A, anterior pole. P, posterior 

 pole. (By courtesy of E. F. 

 Fincham [1926].) 



