Chapter 17 

 BIRDS 



See also pages: 



47 Fig. 21b 



50, 178 amacrine cells 



79, 127-8 photosensory ependyma 



102 zapfensubstanz 



102 Fig. 35 



1 18-9 embryology 



134-5, 139 origin, relationships 



150, 156, 162 photomechanical changes 



156, 158, 162, 220-1, 226 pupil 



169-70, 172-4, 205-9, 307-10, 341-2, 



344-5, 438-42 habits, visual acuity 

 176, 215-6 visual cells 

 179-80 blind spot 



182 Fig. 75 b 



183 Fig. 76 



187-9, 307-10, 324, 442 area centralis, 

 fovea; 



192-7, 200-1, 203 oil-droplets and their 



significance 

 212-3 tubular eyes 

 230, 240-1 eyeshine, tapetum 

 251 Fig. 100 

 257, 269-82, 438-42 accommodation, 



refraction 

 274 scleral ossicles 

 289-91, 295-6, 300, 307-10, 320, 323 



visual fields 

 307-10, 329 eye movements 

 307-10, 320, 323-4, 327, 331 binocularity 

 339-40 median eyes 

 341-2 monocular stereopsis 

 344-5, 354, 365-7 movement perception 

 419-25 adnexa 



438-42 amphibious adaptations 

 466,497-504,519-20 color vision 

 524 dermal color changes 

 545-51 coloration of eye 



The avian eye contains no feature of any importance which does not 

 also occur in some reptiUan group, and practically all of its features 

 occur in the lizards — not because the birds came from lizards (their an- 

 cestors were certain of the dinosaurs), but because nothing material has 

 needed to be changed, in the eye, during the descent of the lizards and 

 birds from their immensely remote common ancestors, the eosuchians. 



Though the birds comprise a whole vertebrate class, containing thou- 

 sands of species divided among many orders, the eye is as uniform 

 throughout the group as it is in any one order or suborder of reptiles 

 or amphibians. 



The Eye as a Whole — The great size of the bird eyeball — the primary 

 basis of the paramount eye-mindedness of the group — ^goes unrealized 

 by the casual observer, for only the relatively small cornea shows in the 

 circular lid-opening. Only the tiniest of birds, such as hummingbirds, 

 warblers, and finches, have eyes as small (6-8mm.) as those of the aver- 



641 



