28 A HISTORY OF BIRDS 



as to be invisible to human eyes. The hovering Kestrel again 

 from a considerable height is enabled to detect the presence of 

 creatures so small as mice on the ground beneath him. 



In the structure of the eye birds resemble the reptiles. 

 In both the sclerotic coat is more or less cartilaginous, while 

 the region surrounding the face of the eyeball lodges a number 

 of overlapping, bony plates, which in Accipitres and Owls attain 

 a large size, and gives this region of the eye a tubular shape. 

 In the Woodpeckers and their allies, and in the Passeres, the 

 region surrounding the entrance of the optic nerve also lodges 

 bony plates. In the Lammergeier the outer surface of the 

 sclerotic region immediately surrounding the iris is of a vivid 

 vermilion colour. No other bird has this region similarly 

 coloured, and this may be cited as another of the many char- 

 acters which birds — in common with other animals — display 

 that appear to have no direct relation to the struggle for 

 existence. 



In^addition to \h& fovea centralis, answering to the " yellow 

 spot " of mammals, the spot of most acute visuality, " many 

 birds," says Dr. Gadow, " possess a second foveas more towards 

 the outer or temporal side of the eye. One pair of these fovese 

 seems to be used for monocular, the other for binocular sight, 

 so that the whole field of vision of birds possesses three points 

 where vision is most acute." 



The exposed surface of the eye in birds is protected by a 

 large semi-transparent membrane known as the nictitating 

 membrane. This is formed by a reduplication of the con- 

 junctiva, and worked by two muscles whereby the membrane 

 is drawn over the eye from the outer lower towards the upper 

 inner angle. In some birds, as in Owls, this is kept in constant 

 motion, so that the iris is incessantly being covered and un- 

 covered by this curious curtain. In other birds it is less often 

 used, but in many, as in Corvidae, for example, it appears to be 

 employed extensively during moments of pleasurable excite- 

 ment. 



The eyelids are not conspicuous, and, as a rule, the lower 

 lid only is movable, rising upwards to close the eye. Only in 

 a few species are eyelashes present, such, for example, as the 

 Ostrich and the Hornbills, where they are of exceptional length, 

 and in the Amazon Parrots. 



