REPTILES 



385 



The GLOBE OF THE EYE is tj^icallv sj^lierical or — for the first time 

 among Vertebrates — sliglitly elongated in the direction of the visual 

 axis. The sclera is composed entirely of connective tissue without 

 cartilaginous or osseous supports, varymg considerably in thickness 

 among the different families but usually tliinnest about the ec-[uator 

 where it is most deformed during accommodation. Usually its outer 

 surface is jjigmented with melanojjhores. typically forming a dotted 

 pattern, sometimes a continuous layer, and occasionally {Python) the 



Figs. 468 and 469.— The Ophidian Eye. 



Fig. 468. 



Fig. 469. 



Fig. 468. — Diagram of an ophidian eye. .4, anterior pad ; Ch, choroid ; CR, 

 ciliary roll ; CV, circular vein ; MA, muscle of accommodation ; MV, 

 n:iembrana vasculosa retinae ; ON , optic nerve ; PL, pectinate ligament ; 

 Sc, sclera ; SM, sphincter muscle ; VS, ciliary venous sinus ; Z, zonule. 



Fig. 469. — The eye of the tiger snake, NotecJiis (Norman Ashton). 



whole thickness of the sclera contains pigment cells. The cortiea, with 

 its delicate single-layered epithelium protected by the " spectacle " ^ and 

 without a Bowman's membrane, continues the arc of the sclera and 

 usually shows a peculiar thickening at the corneo -scleral margin 

 (Fig. 470). 



The choroid is unusually thin, the tenuous capillary layer in most 

 species ajDj^earing as if it were fused with the sclera (Fig. 471). The 

 ciliary region starts with a narrow orbicular zone comprised of the two 

 layers of the tall ciliary ei^ithelium (absent in the boas : the common 

 boa, Constrictor, the rubber boa, Charina), anterior to wliich the roll- 

 like ciliary body rises abruptly as an annular fold wherein the ciliary 



1 p. 266, Fig. 279. 



S.O. — VOL. I. 



