Chapter 15 

 AMPHIBIANS 



See also pages: 257, 265-8, 272-3, 407, 436 accotnmo- 

 53-60, 176-7, 216-7 visual cells 'Nation, refraction 



101 Fig. 33 274 scleral bone 



102 Fig. 36 293 visual fields 

 105 Fig. 37a ^^^ ^^^ movements 

 109 regeneration of retina 3^9-40 median eyes 



118 embryology ^44 movement-perception 



123 Fig. 49a 367 optic nerve 



134-9 origin, relationships ^75 lens and ultraviolet 



145, 164, 208, 344, 368 habits 407 aquatic adaptations 



146-53 photomechanical changes 415 thick sclera 



148 Fig. 64, retina 421 Fig. 143a 



150, 157-8, 161, 218-21, 223-4 pupil 428-9 comparison with elasmobranchs 



184, 187, 305 area centralis 436 amphibious adaptations 



193-6 value of oil-droplets 446 lens epithelium 



200-2 oil-droplets 450, 453, 458 spectacles 



210, 300, 390, 407, 458 cave salamanders 490-4, 518-9 vision, color vision 



230, 240 eyeshine, lack of tapetum 525-8, 535-40 dermal color changes 



251 Fig. 99 543-9 coloration of eye 



In the fishes, the only important property of the cornea is its trans- 

 parency to light (and, perhaps, to water). But when the vertebrates 

 took over the dry land, the cornea at once presented advantages and 

 disadvantages, which had to be dealt with. To remain transparent, suc- 

 culent, and safe from injury, it had to be moistened by new glands and 

 wiped and shielded by lids (see also pp. 418-9). In exchange for these 

 attentions, the cornea offered the eye an opportunity to improve its 

 methods of operation : the outer surface of the cornea, now exposed to 

 air, became an important refractive surface. Some of the burden of focus- 

 ing the image on the retina being thus taken off of the lens, the latter 

 could now recede behind the iris. It then became easier to give the pupil 

 extensive mobility; and, the lens being brought into the plane of the 

 ciliary body, it became possible to discard lens-moving muscles and in- 

 stead use the ciliary muscle for accommodation. 



If the fishes had attempted to obtain these benefits, the withdrawal 

 of the lens deep into the eyeball would have disastrously restricted the 



592 



