REPTILES 



361 



light the pupil assumes the form of a vertical slit with several paired 

 notches on its margins ; on contraction in bright light the slit com- 

 pletely closes leaving only a row of stenopoeic openings down its 

 length, which, acting together, would produce an image of considerable 

 clarity without any dioptric mechanism or accommodative adjustment 

 (Fig. 425) (Beer, 1898 ; Lasker, 1034). Such an arrangement is un- 

 doubtedly of considerable visual value, and Johnson (1027) after 

 repeated observation concluded that to some extent the movements 

 of this exceedingly sensitive pupil were under voluntary control.^ 



The lens is typically sauropsidan (Beer, 1898 ; Rabl, 1898). In 

 size it is voluminous, particularly in nocturnal types ; in shape it is 

 flattened antero -posteriorly with a low curvature on its anterior surface 

 and a high convexity posteriorly except in nocturnal types, particularly 

 the gecko, wherein it is almost spherical ; in consistency it is soft and 

 readily mouldable with a tliin capsule ; and, as in Cyclostomes, 

 sutures are usually absent for the fibres terminate in one circumscribed 

 area anteriorly and posteriorly. The most characteristic feature, 

 however, is the equatorial annular pad,^ formed by the radial growth 

 of the subcapsular epithelium in this region which elongates to such 

 an extent that it abuts against the ciliary body. In most lizards the 

 pad is marked, in the chameleon enormous, the thickest known among 

 Sauropsida (Figs. 426-8). The zonular fibres arising from a broad 

 area of the ciliary body are attached to this structure. In one diurnal 

 gecko [Lygodaciylus) the lens is coloured with a yellow pigment. 



The retina of lizards shows many interesting peculiarities.^ The 

 pigment epithelium is well formed with numerous long, fine processes 

 dipping dowTi permanently between the outer segments of the visual 

 cells. The extent of the migration of pigment with variations of light is 

 small (3/x in SceJoporus) ; and the contraction of the cones on exposure 

 to light is also minimal (Detwiler, 1016-23). 



As seen ophthalmoscopically, the fundus of lizards varies in its 

 appearance in the different genera, but it sho\\s the same general 

 characteristics (Plate VI, Figs. 1 to 5). The background tends to 

 be uniform — usually slate-grey (as in the alligator lizard. Anolis 

 alligator), sometimes dark or almost black (as in Lacerta galloti). brick- 

 red in the nocturnal geckos (grey in diurnal types), and exceptionally 

 green (as in the iguanid. Conolojyhus cristafus) or variegated (as grey in 

 the upper half and dark red below in the iguanid, Metopoceros cornutus). 

 Sometimes it is heavily besprinkled with white spots {Lacerta galloti), 



^ Compare the pupils of seals and sea-lions, p. 470. 



* An annular pad situated laterally is marked in Chelonians, Crocodilians 

 and lizards (thin in geckos and snake-lizards). It is vestigial in Monotremes and 

 some Marsupials. It is situated anteriorly in Ophidians. 



3 Krause (1863-93). Schultze (1866-67), Ranvier (1889), Hess (1912). Franz (1913), 

 Rochon-Duvigneaud (1917-43), Verrier (1930-32). Kahmann (1933), Walls (1934-42), 

 Underwood (1951). 



Anolis 



