PARTS OF ANIMALS, II. xiii. 



eyelid only. This is due to the hardness of the 

 skin which surrounds the head, (a) The heavily 

 built birds are not great fliers, and so the material 

 which would have supplied growth for the wings has 

 been diverted, resulting in thickness of the skin. 

 These creatures, then, use only the bottom eyelid to 

 cover the eye ; whereas pigeons and such use both 

 eyelids. (6) With regard to the oviparous quad- 

 rupeds : As the horny scales with which they are 

 covered are in every case harder than hair, so their 

 skin also is harder than ordinary skin. And as the 

 skin on their heads is hard, they can have no upper 

 eyelid ; but lower dovvn the skin has some flesh with 

 it, and so they have a lower eyelid that is thin and 

 extensible. 



Now the heavily built birds blink not with this 

 lower eyelid, because its motion is slow, but A\ith 

 the membrane above mentioned, whose motion is 

 swift, as is requisite. This blinking or nictitating 

 begins at the corner of the eye nearest the nostrils, 

 because it is better that the membranes should have 

 one place of origin rather than two, and in these birds 

 this is where the eye and nostril are conjoined ; also, 

 the front is more a place of origin than the side. 



The oviparous quadrupeds do not blink in this 

 way, because, unlike birds, which have to use their 

 eyes over great distances, they go upon the ground, 

 and therefore there is no need for them to have 

 fluid eyes or great accuracy of sight. The crook- 

 taloned birds are sharp-sighted, for they view their 

 prey from above, and that also explains why they 

 fly to a greater height than other birds. The 

 birds that remain on the ground, however, and do 

 not fly much (e.g. barn-door fowls and the like) are 



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