EVOLUTION AND SIGNIFICANCE OF FOVEA 117 



LIZARD 



0.5 mm.- 



the retina is less than that of the vitreous, an assumption 

 which Walls questions. He justly calls attention to the 

 fact that in an afoveate area centralis, the retina is thicker 

 than the less speciaUzed retina beyond its Umits; yet, in 

 spite of this, the resolving power of the area centralis retinae is 

 obviously much greater, 

 indicating therefore that 

 a thin spot per se is 

 apparently not so im- 

 portant. In comparing 

 the profile of the highly 

 developed fovea of di- 

 urnal saurians with that 

 of man, he makes a 

 distinction between the 

 two types, that of the 

 saurians being convexi- 

 clivate — that of man, 

 concaviclivate. He re- 

 gards the shallow fovea 

 as seen in such forms as 

 the woodpecker, pigeon, 

 fowl, and owls as a mark 



« /. 1 ^ I'^GJ. 80. Outline drawings of the fovea 



Ot tOVeai degeneracy centralis from various retinae. (Redrawn 



0.5 



SPHENODON 



-0.5mm: 



THEORETICAL 



from Walls, 1937. Arch. Ophth., v. 18.) See 

 text for explanation of theoretical fovea. 



brought about by noctur- 

 nality or domestication. 

 Similarly, the human fovea is regarded as ' crude ' — either 

 never having developed to a high degree or else being 

 degenerate. He argues further, that if removal of tissue 

 were desirable per se, then the concaviclivate form such 

 as in man, Sphenodon, and the poor-sighted birds would 

 be superior to the convexiclivate (sharp-sighted) forms of 

 the diurnal saurians, and these forms, upon such a basis, 

 should have a profile similar to the theoretical one shown 

 in Figure 80. Discussing then the theory that the foveal 

 depression has developed merely to remove tissue, so that 

 light can fall unimpeded upon the visual cells, he produces 



