THE VERTEBRATE EYE 13 



Lens 



The human lens is a biconvex structure with a rounded 

 border (equator). The anterior surface is curved as a flat- 

 tened eUipsoid ; the posterior surface is more highly curved as 

 a paraboloid. It is placed behind the pupil and its anterior 

 surface is adjacent to the posterior surface of the iris. It is 

 supported by the zonular fibers and its equator is separated 

 from the ciliary body by a free interval 0.5 mm. wide. 

 Numerous measurements of the equatorial and the sagittal 

 diameters of the lens have been made in individuals of all 

 ages. These are listed in tabular form by Duke-Elder (1939) 

 to whom the reader is referred. 



The lens in all classes of vertebrates has a common origin, 

 viz., from an invaginated lens placode in the surface epi- 

 thelium. In this sense then the lens in all vertebrates con- 

 forms to the same general plan. As one ascends the evolu- 

 tionary scale from fishes to man, one can say in general 

 that the lens becomes progressively less spherical and more 

 biconvex. This does not hold for aquatic forms in which 

 the spherical shape is maintained. Such forms require a 

 greater refracting power than terrestrial and avian forms. 

 This is due to the fact that the refractive index of the cornea 

 and that of water are about equal, a condition which prac- 

 tically eliminates the converging power of the cornea. 



Fig. 12. Diagrammatic section through the 

 eye of the marine fish, Serranus scriba. Fo, 

 fovea. (Photographed from Kahmann, 1936. v. 

 Graefe's Arch. f. Ophth., Bd. 135.) 



In fishes the lens tends to be almost spherical and may 

 protrude into the anterior chamber so as almost to touch the 

 cornea (Figure 12). In amphibians the anterior and pos- 

 terior surfaces become somewhat flattened so that an ovoid 

 shape results. In reptiles and birds, the convexity becomes 



