26 



EVOLUTION OF THE HUMAN EYE 



of the eyeball and the cornea of mammals are shown in the 

 following table (Fig. 3.) : 



Diameters of the eyeball. 



Diameters of the cornea. 



In man and monkeys, where a central spot of distinct 

 vision is of the greatest importance, the size of the cornea 

 relatively to the size of the eyeball is comparatively small. 

 Thus in all mammals below man the diameter of the cornea 

 measures more than half the antero-posterior diameter of 

 the globe. In the chimpanzee it is about half and in man 

 considerably less than half. 



Mammals that have a proportionally large cornea have 

 also a proportionally large lens which is nearly spherical in 

 shape, there being only a very slight increase in the size of 

 the lateral diameter over that of the antero-posterior ( vide 

 table, p. 70). In man and monkeys, where the cornea la 

 proportionally small, the lens is also proportionally small 

 and considerably flattened from before backward. In 

 monkeys the antero-posterior diameter of the lens is a little 

 more than half the lateral diameter and in man about half. 



Emmert 2 has estimated the relation of the size of the lens 

 to the size of the eyeball as follows: 



