AMPHIBIANS 335 



and in one species it is replaced anteriorly by a ring of bone {Hypo- 

 pachus incrassatus). The cornea in the larval stage has the duplex form 

 of many fishes, with the dermal ^^ortion separate from the scleral ; 

 fusion, however, takes place in the adult so that the fully metamor- 

 phosed structure has the tyj)ical vertebrate characteristics of a regu- 

 larly-layered epithelium, a homogeneously stratified substantia propria 

 and Descemet's membrane with its tenuous endothelium. 



The uvecd tract is well developed (Fig. 399). The choroid consists 

 essentially of a choriocapillaris external to which the heavily pigmented 

 tissue is divided into two strata separated by a layer of large veins, 

 traversed by broad pigmented bands running obliquely (the vascular 

 layer of Gaupp, 1904) ; there is no true argentea or tapetum, but a 



Fig. 396.— The Frog, Ba.\a. 



The disc-sliapcd patch behind and below the eye marks the position 

 of the ear. 



certain degree of " eye-shine " is contributed by cells containing a 

 carotenoid yellow pigment and others with crystals of guanine. 



The ciliari/ body is well-formed and triangular in shape. On the 

 internal aspect the double laj-er of epithelium is plicated into numerous 

 meridional ciliary folds rmmmg anteriorly to the back of the iris and 

 from these the fibres of the zonule take origin (Teulieres and Beauvieux, 

 1931). Dorsally and ventrally these folds are hyper tro jellied, two or 

 three neighbouring folds dorsally, a single fold ventrally, and in most 

 species are continued to the pupillary margin where they thicken to 

 form the dorsal and ventral pupillary nodules : their function may be 

 to keejD the iris away from the lens and thus to allow the aqueous 

 humour to flow backwards when the lens is drawTi forwards in accom- 

 modation (v. Hess, 1912). The mass of the triangular ciliary body is 

 occupied by a meshwork of vascularized pigmented tissue ; and from 

 the scleral aspect of the triangle in the dorsal and ventral regions a 

 ciliary muscle (or tensor choroidese) forms two crescentic slips of 



