THE PERCEPTION OF FORM 



659 



expense of the visual acuity. Pumphrey therefore suggested that foveas developed 

 along two possible lines — one, the shallow fovea towards greater acuity as in 

 man, and the other, the convex-clivate fovea for the purposes of rapid alignment 

 of the fixation object, as in birds of prey. 



A relatively inefficient fovea of the first type is seen in a number of Teleosts,^ 

 in Sphenodon, in A?nyda among the turtles, in two types of tree-snake, ^ in most 

 ground-feeding and domesticated and many nocturnal birds, in the temporal 

 fovea of bifoveate birds (except the eagle), in Tarsius and the Simians. A deep 

 fovea combining tenuity of the retina with magnification of the image is seen 

 in its highest form in lizards, in the central fovea of predatory birds, in the 

 temporal foveas of the eagle and the swift, Micropus, and in the marmoset, 

 Hapale. In some water-birds (gulls, shearwater, flamingo) the fovea is hori- 

 zontally oval and trough -like. 



Swift 



Figs. 792 to 795. — The Shape of the Fovea. 



Fig. 792. — Sphenodon. 



Fig. 793. — ^A primate. 



Fig. 794. — The chameleon. 



Fig. 795.— a hawk. 



In its position the fovea is usually central, subserving lateral vision when the 

 eyes are so placed, and binocular vision when the visual axes are frontally 

 directed. A temporal fovea, situated far out in the periphery of the retina, 

 subserving forward vision with laterally placed ej^es, is found in Teleosts, the 

 foveate snakes, in the owl and bifoveate birds ; only in wing-feeding passerine 

 and predatory birds, and in the arboreally active lizard, Anolis, are two foveae 

 fovmd, a central for uniocular vision and a temporal for binocular vision.' 



In the structural basis for visual acuity the degree of summatio7i iri 

 the retina, that is, the number of visual elements connected to a single 

 optic nerve fibre, is a factor as important as the density of the retinal 

 mosaic. In general, in the interests of sensitivity ^ many rods are 

 associated with a single ganglion cell ; in the interests of acuity in 

 ideal circumstances each cone would relay through a bipolar cell to an 

 individual ganglion cell, the impulse from which would be relayed to the 

 brain by a separate nerve-fibre. Each visual element would thus have 



1 p. 309. 



3 See further p. 684. 



p. 388. 

 p. 609. 



Flamingo 



Anolis 



