PARTS OF ANIMALS, II. xii.-xiii. 



quadrupeds which have horny scales. One vivi- 

 parous animal, tlie Seal, has no ears but only auditory 

 passages ; but this is because, though a quadruped, 

 it is deformed." 



XIII. Man, the Birds, and the Quadrupeds (both Eyes, 

 viviparous and oviparous) have a protective covering 

 for their eyes. The viviparous quadrupeds have 

 two eyelids to each eye (which also enable them 

 to blink) ; some of the birds, especially the heavily 

 built ones, and the oviparous quadrupeds, when 

 they close their eyes, do so with the lower eyelid ; 

 birds, however, can blink, with the aid of a mem- 

 brane that comes out of the corner of the eye. The 

 reason for the existence of these protective cover- 

 ings is that the eye is fluid in order to ensure 

 keenness of vision. If the eye had been con- 

 structed with a hard skin it would of course have 

 been less liable to injury by impact from without, 

 but its vision would have been duller. For this 

 cause the skin round the pupil is left thin and fine, 

 and the safety of the eye is ensured by the addition 

 of the eyelids. The movement of the eyelids known 

 as blinking is a natural and instinctive one, not 

 dependent on the will, and its object is to prevent 

 things from getting into the eyes. All animals 

 that have eyelids do it, but human beings blink 

 most of all, because they have the thinnest and 

 finest skin. 



Now the eyelid is encased ^vith skin ; and that is 

 why, like the tip of the foreskin, it \\'ill not unite 

 again once it has been cut, because both of them 

 are skin and contain no flesh. 



We said just now that some birds and the ovi- 

 parous quadrupeds close the eye with the lower 



183 ' 



