28 EVOLUTION OF THE HUMAN EYE 



considerably greater than it is in other mammals and in 

 fishes, which have rounder lenses. 



Focal distance 



Focal distance of the crystal- Relation: 

 of the cornea. line lens. 1 to 



Man 31.2mm. 49.2mm. 1.60mm. 



Crow 25.8 " 36.7 " 1.42 " 



Horse 78.8 " 64.4 " 0.82 " 



Dog 33.8 " 22.9 " 0.60 " 



Whale (out of water) . . . 314.3 " 40.5 " 0.13 " 



Carp 38.0 6.6 " 0.17 " 



Most of the Ungulata have markedly oval cornea?, their 

 lateral diameters measuring more than the vertical. In 

 sheep there is a difference of 3.5 mm. between the two, and 

 in the ox 5 mm. Associated with oval cornea? they have 

 oval pupils elongated laterally. According to Eversbusch, 5 

 this is due to the presence of a check ligament, extending 

 from the pupillary margin to the periphery, which can be 

 seen as an elevation on the posterior surface of the iris. I 

 have examined the iris of several different species of Ungu- 

 lates, but have been unable to find any such check ligament. 

 The elongation of the pupil can quite adequately be explained : 

 First, by the elongation of the cornea, to which the root of 

 the iris is attached laterally; and secondly, by the length 

 of the iris in the vertical plane being greater than in the 

 horizontal. In a horse's iris I find there is 2 mm. difference 

 in the length of the iris horizontally and vertically (Fig. 4). 

 There is also, in these animals with a horizontally oval pupil, 

 a difference in the length of the ciliary body horizontally 

 and vertically. In the horse the distance from the root of 

 the iris to the ora serrata measures 5 mm. horizontally and 

 10 mm. vertically. To put it another way, the retina extends 

 farther forward laterally than vertically in the Ungulata, 

 which have pupils and cornese elongated laterally. The 

 combined effect of these three anatomical features is to pro- 



