598 AMPHIBIANS 



with horizontal pupils (bufonids particularly), the nodules are large 

 enough to meet when the pupil is fully contracted (see Fig. 87c, p. 223). 



The circulation of the eyeball is complex, but because of the wide use 

 of the frog in zoological teaching, we should perhaps consider it in some 

 detail. As in 'ganoids' and in larval teleosts (before their pseudobranchs 

 have differentiated), its main arterial supply is from a branch of the 

 carotid, the 'ophthalmic artery', which on approaching the eye gives off 

 two branches. These puncture the sclera above (or within) the verti- 

 cally-ovoid disc, then diverge nasally and temporally in the chorioid to 

 feed the choriocapillaris. The dorsal halves of both chorioid and ciliary 

 body are drained by two veins which pass out through the sclera and 

 unite as the superior bulbar vein. The ventral halves drain centripetally 

 into a chorioidal venous 'star' from which an 'ophthalmic vein' leaves 

 the sclera posteriorly to join the internal jugular. 



The main trunk of the ophthalmic artery enters the sclera ventro- 

 temporally and runs through the chorioid to the mid-ventral point of 

 the ciliary body, where it gives off two branches and then turns back- 

 ward onto the inner surface of the retina as the 'hyaloid artery'. The 

 two aforementioned branches anastamose around the root of the iris to 

 form a sort of major circle. From this, radial branches set up a plexus in 

 the pupillary zone of the iris, which drains through radial veins (in the 

 iris folds) into a venous rete in the ciliary body. This in turn is con- 

 nected with the veins of the chorioid. 



The hyaloid artery (v.s.) bifurcates nasally and temporally, these 

 branches forming a nearly complete ring which lies just in front of the 

 ora terminalis (Fig. 172, p. 594). Meridional vessels given off from this 

 ring ramify backward over the retinal inner surface to generate a plexus 

 of 'vitreal' vessels. These recombine into veins which assemble into nasal 

 and temporal trunks (paralleling the arterial ring), and these in turn 

 join with a mid-ventral trunk to form the 'hyaloid vein', which turns out 

 through the chorioid alongside the hyaloid artery, and joins the ophthal- 

 mic vein (v.s.) . 



The Retina — The anuran retina is characterized by large, coarse ele- 

 ments reminiscent of those in Protopterus (see Fig. 64b, p. 148). It has 

 the usual layers, and these have average thicknesses relative to each 

 other. The horizontal cells have as fine fibers as the bipolars, and are 

 apparently entirely conductive. The visual cells are thick in ranids, 

 longer and more slender in bufonids and hylids in keeping with the 

 nocturnality of the latter groups. They are of four types: single and 



