THE PERCEPTION OF FORM 657 



retina, the higher will be the acuity. For this reason the potential 

 visual acuity of the tiger-snake, Notechis, with its immensely bulky 

 cones, or of some deep-sea Teleosteans (as the pike-perch, Stizostedion) 

 in which the visual cells are so large that the retinal mosaic can be 

 seen ophthalmoscopically (Figs. 345,348), is necessarily much inferior to 

 that of the chameleon which has 756,000 visual cells per sq. mm. at 

 the fovea, or the hawk, Buteo, which is said to have a foveal density 

 of 1,000,000 cones per sq. mm. (Rochon-Duvigneaud, 1933). In this 

 respect the sauropsidan retina, particularly that of lizards and birds, 

 is supreme, and considerably more effective than that of any mammal : 

 the cone population at the human fovea is approximately 200,000 

 per sq. mm. 



In order to promote visual acuity a specialized area centralis 

 is frequently developed wherein the receptor elements are more closely 

 packed than elsewhere in the retina. Such an area, as we have seen, 

 is found in varying states of differentiation in representatives of most 

 of the classes of Vertebrates and is characteristic of diurnal types. It 

 is absent in the primitive Cyclostomes, in Selachians except the dogfish, 

 Mustelus, in the coelacanth, Chondrosteans, Holosteans, in Urodeles, 

 in nocturnal lizards and snakes, and in Mammals except some Rodents 

 particularly the squirrel family (Sciuridae), the Ungulates, Carnivores 

 and Primates. In location such an area may be central or temporal ; 

 in shape, rounded, band-like or (exceptionally) crescentic or ring- 

 shaped (Anurans) ; it is usually single but sometimes is duplicated. In 

 it the visual elements have become slender and closely packed, an 

 increase in receptor elements which involves a corresponding increase 

 in the number of bipolar and ganglion cells in the retina and therefore 

 in the thickness of this tissue. 



The following are provided with an area centralis (macula) without a 

 fovea : dogfish, Mustelus (central and round), most Teleosteans (mainly temjDoral 

 in location, except in Hippocampus where it is central), Anurans (crescentic 

 in shape over the optic papilla), Crocodilians (horizontal band), Chelonians 

 (central, round), rabbits and squirrels (ill-defined, horizontal band). Ungulates 

 (sometimes a broad horizontal band, usually temporal, sometimes a temporal 

 round area, sometimes a combination of both), most Carnivores (well-defined and 

 central), nocturnal Prosimians and Nyctipithecus (central and rovmd). Two 

 teleostean fishes have two areas without a fovea, the killifish, Fundulus, with 

 two ventro-temporal horizontal ridges, and the guppy, Lebistes, with an axial 

 and a ventral area. 



There is evidence that the area centralis in certain sjiecies acts as a device 

 to increase sensitivity rather than acuity, the visual elements, mainly rods, being 

 multiplied for this purjaose.^ This is seen j^articularly in nocturnal, or, at any 

 rate, not strictly diurnal types — the Crocodilians, the echidna, the opossum, 

 and perhaps most Ungulates and some Carnivores. Such a function would 

 certainly seem to apply to the pure-rod fovese of the deep-sea teleost, 

 Bathytroctes, of the gecko, Sphcerodactylus parkeri, and o{ Sphcnodon. 



1 p. 673. 



S.O.— VOL.t. 42 



