"• 13- 47 



the oviparous quadrupeds, and ^ the heavy-bodied birds as well 

 as some others, use only the lower lid to close the eye;^ while 

 birds blink by means of a membrane connected with the canthus. 

 The reason for the eyes being thus protected is that nature has 

 made them of fluid consistency, in order to ensure keenness of 

 vision. For had they been covered with hard skin, they would, it 

 is true, have been less liable to get injured by anything falling 

 into them from without, but they would not have been sharp- 

 sighted. It is then to ensure keenness of vision that the skin 

 over the pupil is fine and delicate ; while the lids are superadded 

 as a protection from injury. It is as a still further safeguard, 

 that all these animals blink, and man most of all ; this action 

 (which is not performed from deliberate intention but from a 

 natural instinct)^ serving to keep objects from falling into the 

 eyes ; and being more frequent in man than in the rest of these 

 animals, because of the greater delicacy of his skin. These lids 

 are made of a roll of skin ; and it is because they are made of 

 skin and contain no flesh that neither they, nor the similarly 

 constructed prepuce, unite again when once cut.* 



As to the oviparous quadrupeds, and such birds as resemble 

 them in closing the eye with the lower lid, it is the hardness of 

 the skin of their heads which makes them do so.^ For such 

 birds as have heavy bodies are not made for flight ; and so the 

 materials which would otherwise have gone to increase the size 

 of the feathers are diverted thence, and used to augment the 

 thickness of the skin.^ Birds therefore of this kind close the 

 eye with the lower lid ; whereas pigeons and the like use both 

 upper and lower lids for the purpose. As birds are covered 

 with feathers, so oviparous quadrupeds are covered with scaly 

 plates ; and these in all their forms are harder than hairs, so that 

 the skin''^ also to which they belong is harder than the skin of 

 hairy animals. In these animals, then, the skin on the head 

 is hard, and so does not allow of the formation of an upper 

 eyelid,^ whereas lower down the integument is of a flesh-like 

 character, so that the lower lid can be thin and extensible. 



The act of blinking is performed by the heavy-bodied birds' 

 by means of the membrane already mentioned, and not by this 

 lower lid. For in blinking rapid motion is required, and such 

 is the motion of this membrane, whereas that of the lower lid is 

 slow. It is from the inner canthus, that is from the one nearest 

 657 b. 



