422 



THE EYE IN EVOLUTION 



pit-like fovepe of lizards ; the temporal fovea is shallower and some- 

 what reminiscent of the human fovea (Figs. 525-7). In the central pit, 

 single cones containing yellow oil-droplets predominate and rods are 

 excluded. In the deep fovea of the Lacertilians and the shallow fovea 

 of the Primates, the cones are slim and elongated, 

 the nuclear layers are pushed away from the 

 central area and the nerve fibres aggregated to 

 form a layer of Henle ; in Birds, on the other 

 hand, a considerable proportion of the nuclei is 

 retained, a circumstance which would seem to 

 sujjport Walls's (1937) suggestion that the 

 purpose of the fovea is not so much to remove 

 cellular impediments to the incident light as 

 to scatter it over a wider area.^ In the band- 

 shaped areas of greater acuity the retina is thicker than usual so that 

 it projects into the vitreous owing to an enormous increase in the 

 number of nuclei in the bipolar layer, a considerable increase in the 

 outer nuclei and a lengthening of the visual cells (Fig. 528)^ At the edge 

 of the fovea this thickening of the retinal layers is further increased to 

 form a definite ridge owing to the lateral displacement of cells from 

 the foveal pit (O'Day, 1940). 



The 02:)tic nerve is of the usual vertebrate type with a variable 



Fig. 529. — The Decus- 

 sation AT THE ChTASMA 



OF A Bird. 



Fig. 530. — The Milky Eagle Owl, Bubo lacteus. 



i his bird is unusual ; showing the greater development of the upper lid 

 ■ moves preferentially (photograph by Michael Soley). 



1 p. 658. 



