408 



THE EYE IN EVOLUTION 



The pengiiin 



Eudyptes 



The house-sparrow 

 Passer domesticus 



Melopsiffacus undulatus ; white in the jackdaw, Corvus, and the crane, Grus ; 

 and so on. In the rock-pigeon, Columba livia, it appears to be scarlet because of 

 the richness of the superficial blood-vessels. In the honey-buzzard, Pernis 

 apivorus, a layer of guanine-containing cells in the yellow iris makes the tissue 

 opaque to transmitted light and a brilliant white to reflected light. Sexual 

 differences occur in a few species ; thus the male breeding blackbird, Euphagus 

 cyanoce2)halus, has a yellow, the female a brown iris ; again, in the rock-hopper 

 pengviin, Eudyptes cristatus, the colour of both the iris and the beak varies from 

 red to yellow with the seasons (Mann, 1931 ; Lienhart, 1936 ; and others). 



The pupil is always circular in Birds and very motile ; it responds 

 relatively poorly, however, to changes in light-intensity, but actively 

 to accommodation and, particularly in captive wild birds, so dramatic- 

 ally to emotional factors such as excitement or fear that it has been 

 claimed to be under voluntary control. In domesticated birds, on the 

 other hand, less alert and more placid on close examination, the 

 ordinary response to light becomes relatively more conspicuous. There 

 is sometimes an apparent consensual light reflex, slow in its onset and 

 irregular in its degree ; Levine (1955) suggested that the reaction was 

 due to light sliining through the head to stimulate the retina of the 

 other eye directly, and in birds such as the owl wherein the visual 

 axes are parallel, no such reaction can be seen. 



The vascular pattern of the iris is typical of the Sauropsida and 

 conforms to the general plan seen in lizards (Mann, 1929-31) (Plate XI, 

 Figs. 1 and 5). Several arteries enter at the periphery, run in a deep 

 plane for some distance circumferentially and supply the rich capillary 

 plexus associated with the sphincter muscle ; thence radial veins run 

 superficially towards the periphery, sometimes raised up from the sur- 

 face of the iris in high relief, sometimes largely obscured by j)igment and 

 sometimes completely so (the falcon, Falco subbuteo, or the shearwater, 

 Puffitius). The sphincteric capillary plexus is usually prominent but 

 is variable in extent ; it may be so broad as to occupy almost the 

 entire surface of the iris (as in the oriental eagle-owl, Biibo orientalis, 

 or the rock-hopjDer penguin, Ei(di/2^tes crisfafvs, or the pigeon, Columba) 

 or may be reduced to a minimum so that the surface is largely occupied 

 by the radial veins (as in the duck, Dendrocygna) . 



At the angle of the anterior chamber the circumferential ciliary 

 venous sinus forms a complex system lying in connective tissue close 

 to the inner surface of the sclera, sometimes separated from it by the 

 anterior end of Crampton's muscle. Two annular vessels encircle the 

 eye associated with at least one large artery and sometimes with two 

 (in the sparrow. Passer domesticus), and draining into the subcon- 

 iiMK'tival veins. Only occasionally, as in the kestrel, Falco tinnunculus, 



^ the bull-finch, Pyrrhula, is the circle incomplete (Lauber, 1931). 



The lens usually has a relatively flat anterior surface in diurnal 



ij , almost plane in some species such as parrots (Psittaciformes), 



