EVOLUTION OF THE HUMAN EYE 



DIAMETERS OF THE LENS IN MAMMALS. 



Man . 



Chimpanzee . 



Monkey . 



Civet cat 



Virginian fox 



Horse 



Antelope 



Sheep 



Pig ' 



Rabbit 



Rat . 



Hedgehog 



Sooty phalanger 



Great ant-eater 



Rat kangaroo 



Antero-posterior. 



4.5 mm. 



4 



3.5 " 



5 



6 

 11 

 11 

 10 



R ^ 



7 <^ 



4 



3.5 " 



7 

 6 

 8 





 



Lateral. 



9 mm. 



7 " 



6 



6.5 " 



8 

 20 

 14 

 12.5 " 



9 



9 



4.5 " 



3.5 " 



9.5 " 



8 

 10 



From this it will be seen that in man the antero-posterior 

 diameter of the lens measures nearly half that of the lateral 

 diameter. In monkeys it is a little more than half, while 

 in all the other mammals the proportional difference between 

 the two diameters is considerably less, and in some of them 

 the lens is almost spherical (Fig. 19). 



In man and monkeys the ciliary processes are separated 

 from the sides of the lens by a definite space the so-called 

 circumlental space. In all other mammals the ciliary pro- 

 cesses lie in contact with the sides of the lens. 



It seems natural to infer that the flattening of the lens 

 from before backward in man's eye has resulted from the 

 growth of the ciliary body away from the sides of the lens, 

 and the traction which is thereby produced on its anterior 

 and posterior capsule through the suspensory ligament. 

 An examination of embryonic human eyes .in different 

 stages of development confirms this view. The ciliary 

 processes, when first formed, are in contact with the sides 

 of the lens, and with the growth of the eyeball gradually 

 become separated from it. In association with the gradual 



