THE AN V RAN EYE 



595 



the cup, to maintain continuity with the substantia propria of the cornea. 

 The cartilage cup is usually thickest in the proximity of the optic nerve, 

 and terminates anteriorly a little ahead of the rectus insertions. It is less 

 extensive in this direction in Bnjo than in Rana, Pelo bates, and Alytes. 

 It is soft and perforate in Discoglossus, discontinuous in some hylids 

 (common tree-frogs), and lacking in at least one of them (Pseudacris, 

 whose sclera is entirely fibrous, at least in the adult) . In one microhylid, 

 Hypopachus incrassatus, a bony ring replaces the cartilage anteriorly. 

 The cornea is very broad, and has a ca. five-layered epithelium and a 



Fig. 173 — The ventral ciliary process and associated structures in a frog, Rana pipiens. x 50. 



c- cornea; ce- ciliary epithelium; cm- ciliary muscle; cp- ciliary process; cs- canal of Schlemm; 

 »'- iris; /- limbus cornea»; plm- protractor lentis muscle; r- sensory retina; s- sclera; c, Z- 

 zonule fibers. 



homogeneous substantia propria — both, thinner apically than peripher- 

 ally. Both Descemet layers can be made out, though they are very thin. 



The chorioid, apart from its choriocapillaris layer, consists largely of 

 two pigmented membranes held apart by numerous broad, flat, pig- 

 mented stmts, and enclosing between them a system of flat veins which 

 tend to have parallel, vertical courses. The outer surface of the chorioid 

 lacks an argentea, but may bear patches of xantholeucophores, guanin- 

 fiUed iridocytes, etc., which show through the translucent sclera. 



The ciliary body occupies a narrow zone and (in section) is essen- 

 tially triangular, with the base of the triangle against the sclera just 



