THE OPHIDIAN EYE 629 



The chorioid is extremely thin (except in the limicolous 'rainbow 

 snakes', Farancia and Abastor), consisting of httle more than a chorio- 

 capillaris (with no large-vessel layer) and a few tightly-packed layers of 

 pigment cells external to it. In contrast to other reptiles, the chorioid 

 and sclera are firmly fused, as if they had never completed their embry- 

 ological differentiation from each other. 



Anterior to the ora, the chorioid continues unchanged (including its 

 choriocapillaris) for a short space, where it is lined with the flat zone 

 of the tall ciliary epithelium. This 'orbicular' zone is always very narrow 

 (except in Python and Epicrates) , and upon it lies an annular 'hyaloid' 

 vein (except in boas; present, however, in Python). The orbicular zone 

 itself is lacking in Charina and Constrictor (= Boa) , and much reduced 

 in Lichanura. Immediately in front of the ora in these snakes, and an- 

 terior to the orbiculus in all others, lies the ciliary body proper, which 

 from its shape (Fig. 181) is perhaps best called the 'ciliary roll'. It forms 

 an annular fold, consisting of the two tall columnar layers of ciliary 

 epithelium with a core of deeply pigmented uveal tissue containing small 

 blood vessels. From this core, strands of connective tissue sweep for- 

 ward onto the inner surface of the cornea, petering out on the posterior 

 slope of the corneal thickening. These strands have much the same rela- 

 tionships as those which compose the corneal meshwork tissue of man 

 (see Fig. 5, mt; p. 10), which is so often mistakenly called a 'pectinate 

 ligament' : in both snake and man, the direction of these fibers is exactly 

 at right angles to that of a true pectinate ligament (see Fig. 109, pi; 

 p. 275). 



The ciliary roll serves for the attachment of the zonule (v./.), and is 

 often taller and thinner (and sometimes sharp-edged) on the nasal side. 

 In Python and the larger boas {Constrictor, Epicrates) it has this shelf- 

 like character throughout its circular course. 



In the Colubridze and all of the higher families derived from them, 

 the standard location of the venous canal of Schlemm is in the cornea, 

 toward the rear of the corneal thickening. It may be separated from the 

 anterior chamber only by the 'pectinate ligament' (c/. man!), but most 

 often it lies completely surrounded by dense fibrous tissue. It sometimes 

 branches (the branches then recombining) along its course and the sec- 

 ondary canals thus formed lie farther posteriorly, often in the core of 

 the ciliary roll. Its connections are chiefly if not solely with the chorio- 

 capillaris of the orbiculus. The Boidae show more variation : in Con- 

 strictor and Eryx no canal can be made out at all, and Lichanura, Trop- 



