Chapter 16 

 REPTILES 



See also pages: 251 Figs, 99, 100 



58-9 visual cells 257,269-83,417 accommodation, refraction 



118-9 embryology 293-6 visual fields 



134-9 origin, relationships 305-7 eye movements 



150 photomechanical changes 339 Fig. 124c, parietal eye 



164-5, 205, 208 habits 365 pecten (conus) 



187, 306-7 area centralis, fovea 419, 450-9 adnexa 



210 fossorial forms 494-7, 518-20 color vision 



240-1 tapeta, eyeshine 538-43 dermal color changes 



The eyes of the various types of reptiles are much alike except for the 

 snakes, which are set sharply off from all the others. The class as a whole 

 exhibits a number of features whose origins cannot be traced by any 

 scrutiny of living amphibians. If a good fairy should offer the compar- 

 ative ophthalmologist a living specimen of any one archaic vertebrate, 

 his choice should certainly be Seymouria, that stegocephalian which was 

 the 'first reptile'. Lacking such a miraculous resurrection, we are no 

 better able to link the exclusive features of the reptilian ocular pattern 

 to the elements of the amphibian plan, than we were to see the origins of 

 the lissamphibian features in any of the so-far-studied lunged fishes. 



The reptiles perfected the terrestrially-adaptive accessory organs which 

 the amphibians had been forced to invent, and also made the most of 

 their opportunity to develop a powerful, lens-squeezing mechanism of 

 accommodation (see pp. 417-23, 592-3). Their most characteristic intra- 

 ocular features are all means to this latter end : the striated ciliary muscle 

 fixed (usually) to the rim of the cornea, the scleral ossicles and the con- 

 cavity which they support, the 'ringwulst' or annular pad of the lens, and 

 the tall ciliary processes which are fused to the lens capsule and are in 

 all probability genetically independent of the uveal folds of modern 

 tailless amphibians. Along with these structures, the reptiles have pro- 

 duced a striated (though ectodermal) iris musculature and a pigmented, 

 richly vascularized, conical protrusion from the optic nerve head (the 

 'conus papillaris'), whose framework is ectodermal (neuroglial) and 

 whose function is to nourish the inner layers of the retina (in lieu of 

 vitreal or intrinsic retinal vessels) by diffusion through the vitreous, after 



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