72 



EVOLUTION OF THE HUMAN EYE 



separation of the two structures there is alteration in the 

 shape of the lens from that of a sphere to that which it 

 assumes in the full}' formed eye. 



The following table gives the measurements of the antero- 

 posterior and lateral diameters of the lens in a series of 

 human fetal eyes of different ages, and those recorded by 

 Dub 38 from the ninth month to the twelfth vear: 



Age. Anteroposterior. Lateral. 



Fourth month 2.8 mm. 3.3 mi 



Fifth " 3.5 4 



Sixth " 3.8 " 4.5 " 



Seventh " 4 " 5 



Ninth " 4.3 5.75 " 



To these may be added those given by Dub from the 

 ninth month to the twelfth year: 



From this it will be seen that the lens, when first formed, 

 is spherical in shape, and gradually assumes its flattened 

 condition with the growth of the eyeball. After birth the 

 traction on the sides of the lens causes a diminution in its 

 antero-posterior diameter (Fig. 20). 



If further confirmation is required that flattening of the 

 lens and consequent bowing of its fibers occurs, as the result 

 of traction on its sides, it is to be found in the effects of 

 pathological enlargement of the eyeball, such as occurs in 

 cases of congenital glaucoma— so-called buphthalmos. In 



