REPTILES 



387 



silver sheen (Plate IX, Figs. 1 and 2), In many sjjecies a clear-cut differentiation 

 in colour occurs — brown and gold in the king -snake, Lampropeltis getulus, silver 

 and gold in the black-and-go Id tree-snake, Boiga dendrophila (Fig. 4). A bright 

 yellow pattern is seen in the four-line snake, Elaphe quatnorUneata (Fig. 3), a 

 silver appearance in the painted tree -snake. Ah oetulla picta, and in the chicken- 

 snake, Elaphe quadrivittata (Fig. 6). 



TJie vascular pattern of the iris is j^ec^^iHar and unique (Mann. 

 1931). The most j^rimitive tj^Des (Boidte) show a fairly well defined 

 arrangement of vessels somewhat resembling that seen m geckos. This 



4: 



->^li*'^ j'i:€^' flf,Tv^i^^^-f:l/*€[:*i»^'''^'^^ 



Fig. 471. — The Posterior ."Segment of the globe of the Copperhead 



Snake. 



r, retina ; p, j^igmentary epithelium'; ch, choroid which became detached 

 from the pigmentary epithelium ; s, fibrous sclera ; v, a vessel of the mem- 

 brana vasculosa retinte ( X 240) (Xorman Ashton). 



is most ajjparent in the pj^thons (Plate IX, Figs. 1 and 2) ; two main 

 arteries enter, one on either side,, and run to the pupillary aperture 

 round which they supj^ly a narrow circumpupillary jjlexus wliile the 

 rest of the iris is occupied by an intermediate network of vessels. In 

 most other .snakes the walls of the vessels are ojsaque so that no blood- 

 flow can be made out ; moreover, they are heavily obscured by j^igment 

 and are arranged in so haphazard a marmer that the interpretation of 

 the vascular arrangements is difficult. 



The musculature of the iris is mesodermal and derived from the 

 ciliary region. Circular fibres predommate, being concentrated into 

 two accumulations, one near the pupil to form a relatively comjjact 

 mass acting as a sphincter, the other at the root acting as a muscle of 



Cop]3crhead 

 (crotalid snake) 



