402 



THE EYE IN EVOLUTION 



(i rouse 



conformation characteristic of diurnal birds with narrow heads, such 

 as the Columbid?e (doves, pigeons) or the Galhformes (pheasants, 

 grouse, fowls, etc.). Alternatively, in diurnal birds with broader heads, 

 such as the Passeriformes (perching birds such as thrushes, sparrows, 

 swallows and the Corvida? — crow, raven, magpie, jay, etc.) and diurnal 

 birds of prey, such as the Falconiformes (eagle, hawk, falcon), the 



Fig. 494. — The Head of the Owl, Stri.x alvco. 



To show tlie enormous size of the eye in the orbit when the hds and 

 skin are removed (Barany et a1., Brit. J. Ophthal.). 



Thiais 



Raven 



intermediate segment is cone-shaped, sloping backwards at a varying 

 angle to meet the posterior segment, giving the configuration of a 

 globular eye. In nocturnal birds of prey, on the other hand, the 

 intermediate segment runs directly backwards with a marked waist - 

 like concavity before it I'uns outwards to meet the posterior segment 

 at a sharply angulated junction, producing a tubular eye as is seen 

 most tyjDically in the Strigidse (owls) ; in this case, of course, the 

 retina is comparatively much smaller. In each type in the interests 

 of easy binocular vision there is a considerable nasal asymmetry 

 whereby the lens and cornea are centred towards the mid-line, making 

 -lie intermediate segment shorter on the nasal than the temporal side. 

 The maintenance of this non-spherical shape demands skeletal 

 i- )ort (Figs. 495-98). The hemispherical posterior segment is therefore 



