ACCOMMODATION IN AMPHIBIANS 267 



ones, rather than ectodermal. The amphibian zonule is more distinctly 

 fibrous than that in lower forms, but is not so well extricated from the 

 vitreous. Vitreous substance fills that space* between the radial zonule 

 fibers which, in a mammalian eye, would be an aqueous-filled 'canal of 

 Hannover' with its contents communicating freely with those of the pos- 

 terior chamber (see p. 19). There is scarcely any ciliary body — it is just 

 wide enough to form an attachment for the peripherally converged fibers 

 of the zonule. The more-or-less radial 'ciliary folds' of frogs are really 

 iris folds on the back face of that organ, Mid-ventrally however there is 

 one heavy fold which deserves to be called a ciliary process, and it is to 

 this that the ventral (in salamanders, the only) protractor lentis muscle 

 inserts, running from its origin at the sclerocorneal junction through the 

 root of the iris. The protractors are thus not connected directly to the 

 lens, but their pull is communicated to it by the ciliary 'processes' and 

 the bundles of zonule fibers attached thereto. 



The delicate zonule fibers fan out to the equatorial zone of the lens 

 from the neighborhood of the annular hyaloid vessel which lies on the 

 minute ciliary body. Since the site of origin of the fibers is so narrow, the 

 lens can move forward and backward without much hindrance from them. 



Unlike the elasmobranchs which also pull the lens forward, the 

 anurans have a canal of Schlemm, though a discontinuous one consisting 

 of a dorsal and a ventral crescent. Accommodation is too rapid, however, 

 to afford time for aqueous to escape from in front of the advancing lens 

 by diffusing into the canal, and there is no convenient open slit through 

 the iris root mid-ventrally, for this slit (see p. 265) has been occupied by 

 the ventral protractor lentis muscle. Hence the urodeles (which lack the 

 canal) have produced a nodule of hard connective tissue at the mid-ven- 

 tral point of the pupil margin, which lifts the iris free of the lens capsule 

 locally, and allows the aqueous to flow around to the back of the lens as 

 the latter moves forward. Anurans ordinarily have a dorsal pupillary 

 nodule as well (Fig. 106a, pn), corresponding to a second, dorsal, pro- 

 tractor lentis muscle. The ciliary muscle is no better developed in am- 

 phibians than in teleosts, and is no more obviously useful in any way. 



Newts are emmetropic under water; and in the air, where the cornea 

 comes into optical play, they would become very strongly myopic. Frogs 

 are emmetropic in air. Under water, they of course become strongly 

 hypermetropic, and are quite unable to compensate therefor with their 

 limited range of forward lens-movement. No amphibian has as much as 

 five diopters of accommodation, and many apparently have none at all 



