xii. 26 THE EYE 351 



sense of most of the forms. For clear vision it is essential that the 

 surface of the eye be protected, kept moist and free of particles, and 

 for these purposes the eyelids and lachrymal glands are present. The 

 upper lid is fixed, but the lower is very mobile and folded to make a 

 transparent structure, the nicitating membrane, able to move rapidly 

 across the surface of the eye. 



nm 



C % 



Fig. 207. The amphibian eye and its accommodation. 



a, anuran eye in vertical section, ac. area centralis; to. inferior oblique; ir. inferior rectus; 

 //. lower lid; Im. lens muscles (protractors); n. optic nerve; nm. nictitating membrane; 

 pn. pupillary nodules; sc. scleral cartilage; so. superior oblique; sr. superior rectus; ul. upper 

 lid; z. zonula Zinnii. b, anterior segment of Bufo in relaxation. C, in accommodation; note 

 forward movement of lens. (From Walls, after Franz and Ueer.) 



The eyeball is almost spherical, with a rounded cornea. The lens is 

 farther from the cornea than in fishes and is flattened, more so in 

 anurans than in urodeles. These modifications allow focusing of a 

 more distant image. There is an iris, with a rapidly moving aperture, 

 operated by powerful circular (sphincter) and radial (dilatator) 

 muscles. Although these muscles are partly actuated by a nervous 

 mechanism they are also directly sensitive to light, and the pupil of 

 the isolated eye of the frog shows wide excursions with change of 

 illumination. 



Accommodation is effected by protractor lentis muscles, attached 

 to the fibres by which the lens is supported (Fig. 207). These muscles 



