486 



THE EYE IN EVOLUTION 



Primate 

 (squirrel monkey) 



characteristics of cones. There is, however, no displacement of the bipolar cells 

 or nerve fibres and no true fovea. 



A fovea occurs only in the Primates, appearing first in Tarsius ; 

 it and Nyctipithecus have a pure-rod fovea (Polyak, 1957). All the 

 Anthropoidea except Nyctipithecus have a central area and a well- 

 formed pure-cone fovea of the same type as man, which the retinal 

 vessels approach and encircle but do not invade (Fig. 642) (Woollard, 

 1926).! 



The 02ytic disc in the majority of Placentals is circular as in man, 

 but in some Carnivores (Canidse, as the wolf, jackal, fox) it is kidney- 

 shaped and in many Ungulates and all Equidse it is horizontally oval. 





|li!Sr*'i 



Fig. 642. — The Fovea of a Primate. 

 Macaca rhesus ( X 114) (Katharine Tansley). 



In most Sciurida3 this is exaggerated to form a unique type — a long, 

 thin, tape-like structure stretching horizontally across the fundus 

 above the axis of vision — which reaches its greatest development in the 

 marmot (Fig. 635) ; this arrangement gives excellent uj^ward vision 

 for the arboreal family of squirrels. The optic disc lies on the level of 

 the surface of the retina except in Carnivores and the flying squirrel, 

 Pteromys, wherein it is sunk to form a deep pit. It varies considerably 

 in colour ; usually white or jDink, it is red in the Equidse, bright red in 

 the hedgehog and mole, pink surrounded by a green ring in the seal. 



The 02Jtic nerve is of the standard type seen in man, the only 

 excejitional feature being the enormously thick accessory sheath 



1 Bliimenbach (1805), Albers (1808), and Soemmerring (1818) in several of the 

 Simians; Slonaker (1897) in the gorilla; Wolfrum (1908) in Macacus ; Franz (1912) 

 in JJylobates ; Woollard (1925-27) in several of the Anthropoidea ; Detwiler (1943) 

 in il marmoset and the rhesus monkey. 



