214 The Bird 



shutter, and in fact our bird has not one, but three 

 eyelids we call them. So "between winks" all day our 

 bird is taking snapshots, inconceivably more perfect and 

 continuous than any cinematograph ever produced. We 

 have but two eyelids, and every time we wink these 

 shoot toward each other, moisten the surface of the eye- 

 ball, clear it of dust, and are back in their places so quickly 

 that we are not aware of any interruption of our vision. 

 The upper lid has most to do with covering the eye. In 

 almost all birds this condition is unusual and the lower 

 lid comes far upward over the eyeball. Perhaps the most 

 notable exception to this is among the Great Horned Owls, 

 where the action of the two lids is like that of our own. 



When birds are sleepy these lids close, but usually 

 in winking, the third eyelid, or nictitating membrane, alone 

 is drawn across the eye. This lid is a delicate, semi- 

 transparent sheet of tissue, which, when not in use, lies 

 snugly packed away in folds at the inner corner of the 

 eye, held back out of sight by its own elasticity. It is 

 drawn across the front of the eye by a slender thread of 

 tendon which is suspended, pulley-like, from a muscle 

 which keeps it from pushing against the optic nerve. 



When you see an owl in the daytime with eyes dull 

 and glazed, this third eyelid is drawn partly across them, 

 diluting the strong glare of light and yet enabling the 

 bird to distinguish much that is going on. When an eagle 

 turns his head upward and looks full at the sun, it is not 

 " unwinkingly, " but with the help of this eyelid shield. 



It is interesting to know that this membranous lid 

 is found in many other creatures, from sharks to monkeys, 



