THE ANURAN EYE 597 



are credited in the literature with having been demonstrated, by elec- 

 trical stimulation of the eye, to move the lens forward; but a little read- 

 ing reveals that these experiments were made (by Beer) long years before 

 the Russian anatomist Tretjakoff discovered the actual protractor lentis 

 muscles. Since the accommodation of amphibians is 'positive', not 'neg- 

 ative' as in the teleosts, any use of the ciliary muscles as a 'tensor chor- 

 ioideae' (see p. 584) would seem only to interfere with accommodation. 

 And yet, they are present in the sectors occupied by the accommoda- 

 tory muscles: 



The actual (or important) muscles of accommodation are two, a 

 ventral and a dorsal (Figs. 172, 173, pp. 594, 595). They are of meso- 

 dermal origin embryologically, with no phylogenetic relationship to any 

 muscles outside the Amphibia (though they are supplied by a branch of 

 the oculomotor nerve, like the ectodermal teleostean retractor lentis). 

 Each runs from the periphery of the cornea through the iris root (the 

 ventral one passing through the old embryonic fissure) to insert within 

 the ciliary 'triangle' near its posterior face, in the body of the median 

 ciliary process. Their action is to draw forward these important anchor- 

 ages of the zonule, and thus approximate the lens to the cornea. The 

 large, firm lens is somewhat flattened, more so anteriorly than posteriorly. 

 The ratio of its equatorial to its axial diameter is about 1.3 :1 — a direct 

 optical consequence of the fact that the cornea, unlike that of a fish, is 

 able (in air) to share in the production of the retinal image. This flat- 

 tening is brought about ontogenetically just when it is needed; for the 

 tadpole lens is spherical, and lies closer to the cornea as well, in close 

 imitation of the optical situation in fishes. 



In the iris, the stroma is thinner than the retinal layers — there is just 

 enough of it to hold the blood vessels together; and some of the latter 

 protrude through the anterior mesothelial 'layer' and bulge from the 

 surface of the iris, almost free of it. The stroma contains iridosomes as 

 well as melanophores, but there is no argentea layer. Both retinal layers 

 are pigmented. The cells of the anteriormost are drawn out radially into 

 spindles and constitute the dilatator pupillae. This is lacking only in the 

 region of the small, pigmented sphincter (beneath which the anterior 

 layer is said to be continuous, as unmodified epithelium, to the pupil 

 margin where it joins the squamous posterior retinal layer of the iris). 

 Around the pupillary nodules, whose function is to lift the iris free of 

 the lens to permit the surge of aqueous during accommodation, there 

 are special arrangements of the retinal and uveal tissues. In some forms 



