266 



ADAPTATIONS TO SPACE AND MOTION 



The entrance of the cornea into the optical picture, together with the 

 elongation of the axis of the eyeball, has permitted the lens to flatten 

 somewhat in land forms. The ratio of its equatorial and axial diameters 

 is aroimd 1.3 : 1 instead of 1 : 1 as in fishes and aquatic salamanders. The 

 lens does not owe this slight flattening to tension in the zonule as in our- 



Fig. 106 — The amphibian eye and its accommodation. 



a, anuran eye in vertical seaion. x 11/4. Based largely on Rand pipiens. 



ac- area centralis (marked by local concentration of visual cells) ; io- inferior oblique; ir- in- 

 ferior rectus; //-lower lid; /rn, /m- lens muscles (protractors); «- optic nerve; «wz- niaitating 

 membrane (transparent, independently movable portion of lower lid — not homologous with 

 sauropsidan and mammalian nictitans); pn, pn- pupillary nodules (urodeles have only the 

 ventral one, and only the ventral lens muscle); sc- scleral cartilage; so- superior oblique; 

 ST- superior rectus; «/- upper lid; z- zonule (fibers are embedded in vitreous) . 



b, anterior segment of Bufo sp., in relaxation, x 3, From Franz, after Beer. 



c, same as b; in accommodation; note forward movement of lens. 



selves, however; for, despite the fact that the amphibian lens is in such a 

 position that it might be changed in shape, it is rather firm and is changed 

 only in position, for accommodatory purposes, just as in all fishes. 



The amphibian mechanism of accommodation is a close imitation of 

 that of the elasmobranchs; but the muscles involved are mesodermal 



