356 



THE EYE IN EVOLUTION 



Ch la m yclosa w ru s 



some (Iguanids) are vegetarian ; they are mostly terrestrial, some 

 arboreal, a few amphibious (the iguanid, A7nblyrhynchus cristatus of the 

 Galapagos Islands). Only exceptionally in sluggish limbless types are 

 the eyes poorly developed — the Anguida^ (slow-worms) and the 

 degenerate Amphisbeenidse of subterranean habits. 



The EYEBALL is almost spherical although the an tero -posterior 

 axis is the shortest, but there is a marked concavity, the corneo-scleral 

 sulcus, in the region of the junction of these two tissues (Fig. 421). The 

 sclera is relatively thin and is supported over most of its extent by a 

 scleral cartilage which, starting from the posterior pole, usually reaches 

 to the equator or beyond (Fig. 422) ; occasionally, as in the chameleon, 



Fig. 421. — Diagram of the Eye of a Lizard. 



A, annular pad ; C, conus ; Ch, choroid ; CM, ciliary muscle ; F, fovea 

 P, pectinate ligament ; S, scleral cartilage ; 8c, sclera ; SM, sphincter muscle 

 SO, sclei'al ossicles ; VS, ciliary venous sinus ; Z, zonule. 



Gecko 



it is confined to a small disc in the foveal region. Anteriorly, and lying 

 superficial to the cartilage when it is prolonged forwards, is a ring of 

 some 14 scleral ossicles distributed around the deep corneo-scleral 

 sulcus sometimes imbricated in 2 or 3 layers ; these, noted by such 

 early writers as Zinn (1754) and Soemmerring (1818), support and 

 maintain the convexity of the globe in this region thus approximating 

 the ciliary body to the lens. The cornea is circular and thin and has the 

 usual layering characteristic of Vertebrates apart from the absence of 

 Descemet's membrane and its endothelium in some geckos ; its inner 

 third merges with the pectinate ligament and gives rise to the ciliary 

 muscle. 



The uvea in general is thin. The choroid forms a tenuous layer 

 without distinctive characteristics. The ciliary body varies in shape — 

 narrow and angular in the geckos, broad and romided in the chameleon 

 — and has no ciliary processes but abuts directly on the annular pad 



