aia 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, 
