354 THE EYE IN EVOLUTION 



general plan broadly comparable with that of the eyes of Fishes. In 

 the eyes of Sauropsida, however, a revolution has occurred. Even 

 among the most primitive Reptiles adaptations of a different character 

 and a much higher order are found, most of them having little apparent 

 evolutionary relationship with the characteristics of the visual organs 

 of surviving Amphibians, and these become perfected in their descen- 

 dants, the Birds. The entire sauropsidan family will be found to have 

 much in common, having evolved a type of eye very different from 

 their ancestors and as different from the mammalian eye which has 

 developed on entirely separate lines. 



Fig. 418. — The Eyes of Reptiles. 



The lizard, 

 Lacerta mo?iitor. 



The tortoise, 

 Testudo tnydas. 



The crocodile, 

 Crocodilus sclerops. 



Reproductions of Soemmerring's engravings (1818). The reproductions 

 are life size and represent the lower half of a horizontal section of the left eye. 



The essential features of the typical reptilian eye are tlie fol- 

 lowing (Fig. 418) : 



An effective, accommodative mechanism de^yending on deformation of 

 the lens — not its to-and-fro movement as m Ichthyojisida. This is 

 effected by a striated ciliary ynuscle arising in the cornea and deriviyig 

 firm leverage from a ring of scleral ossicles — a descendayit of the tensor 

 choroidece of Fishes. To this is added a ventral transversalis muscle 

 emerging from the regioji of the {closed) foetal fissure, the function of ivhich 

 is to swing the lens yiasally and attain the convergence necessary for 

 hinocidar vision — homologous 7vith the j^rotractor lentis of Apiphihians. 

 The lens is necessarily soft and the subcajjsular epithelial cells in the 

 equatorial region have elongated enormously in a radial direction to form 

 an anmdar 2)ad to ivhich are fused the ciliary jifocesses, notv tall ayid 

 well-formed in contrast to the small ciliary folds hitherto found. 



A striated iris musculature giving the iris considerable mobility. 



An avascular retina nourished indirectly by the choroid and, in 

 addition, through the conus pajnllaris {in lizards), or through a inembrana 

 vasculv'-a retince {in snakes). 



A ■■:'tern of iris vascularization co7isisting of deep circumferential 



