ACCOMMODATION AND CONVERGENCE 77 



of the eye in man and monkeys, which is strikingly distinctive 

 from that in lower mammals, is the elongation of the pars 

 plana of the ciliary body, i. e., the region situated between 

 the hindermost of the ciliary processes and the ora serrata. 

 This is brought out to some extent in the following table, 

 which shows in a series of mammals' eyes the length of a 

 straight line drawn between the root of the iris and the ora 

 serrata in one column and the antero-posterior diameter 

 of the eyeball in the other: 



Antero- Distance 



posterior between root 



diameter of of iris and ora 

 eyeball. serrata. 



mm. mm. 



Man ... .24.8 6 



Chimpanzee . .19 4.L'.~> 



Rhesus monkey . 19.5 5 



Capuchin monkey . .18.5 4.25 



Cat .... -2-2 3.5 



Horse 44 5 to 10 



Ox . . :;:, 6.5 



Antelope . '21 3 to 5 



Sheep 28 3 to 6 



Pig . -2-2 4 



Rabbit ... . . 19 2 



In the Ungulata two measurements are given, because 

 in animals with horizontal oval pupils the distance of the 

 root of the iris from the ora serrata is considerably less 

 where the iris is narrow than where it is broad. 



From this table it will be seen that in man and monkeys 

 the distance between the root of the iris and the ora serrata 

 is about one-quarter of the antero-posterior diameter of 

 the globe, while in all other mammals, except the Ungulata, 

 it is considerably less. In the Ungulata, while it measures 

 nearly one-fourth in the vertical diameter, it is considerably 

 less in the lateral. 



A lens flattened antero-posteriorly has considerably less 

 refractive power than a spherical or nearly spherical one. 

 The lesser refractive power of the lens in man and monkeys, 

 due to flattening of it antero-posteriorly, is to some extent 



