388 



THE EYE IN EVOLUTION 



accommodation ; the dilatator fibres lie beneath these and rmi radially 

 towards and sometimes into the ciliary body. The pupils are usually 

 very active since they assume the light-protective function in the 

 absence of movable lids ; in some types, however, the contraction is 

 slight {Pytho7i) or even absent (the European grass-snake, Trojndonotus 

 natrix ; the Madagascar sharjj-nosed snake, Heterodon madagas- 

 cariensis). Probably because of the imjDermeability of the corneal 

 spectacle, the instillation of miotic or mydriatic drugs is without effect 

 (Johnson, 1927). In nocturnal and burrowing snakes (with few excep- 



FiG. 472. — The Ciliary Reuion of the Grass Snake, T/toPinoyoTUs 



.\Arji/x yATRix. 



Showing cr, ciliary roll ; o, ora serrata ; va, hyaloid venous arc ; vs, ciliary 



venous siiuis { X 108) (Katharine Tansley). 



tions such as the coral snake, Elups), the constricted aperture is a 

 vertical slit or ellipse ; in diurnal types it is circular except in some 

 Asian and African tree-snakes (Opisthoglyi^hs). 



In these (the East Indian long-nosed tree-snake, Dryophis, and its relative 

 Dryophio])s, the African bird-snake, Thelotornis, and the emerald tree-snake, 

 Passerita) the pupil is a horizontal slit shajaed like a key-hole with the slot of 

 the key-hole extending on the nasal side almost to the limbus, weW beyond the 

 equator of the lens. As occurs in many teleostean Fishes,^ the pupil thus shows 

 a phakic and an aphakic area (Fig. 808). On contraction of the pvipil the central 

 part closes completely leaving two small pvipillary apertures, a larger temporal 

 (phakic) and a smaller nasal (aphakic) aperture. It is significant that at least 

 in some of these snakes a temporal fovea occurs and their vision is said to be 

 veiy acute (Plate IX, Fig. 5). 



'i'be lens is subspherical (1- 1-1-25), is firmer in consistency than 

 in ot: Heptiles. is provided with sutures, and instead of an equatorial 



1 p. 304. 



