















BIRD’S-EYE VIEWS. 509 
which, though contracting in opposite directions, and mutu- 
ally antagonistic as far as the nerve is concerned, eventually 
‘txert their force in the same direction, and work harmo- 
 Miously for a common purpose. When the tendon of the 
Pyramidalis is loosened by relaxation of the two muscles, the 
tictitating membrane is set free, and returns to its hiding- 
place by virtue of its own elasticity, just as the curtain of a 
wach window, after being forcibly drawn down, rolls itself 
P again when the lever that sets a spring in action is 
Moved, 
We understand the mechanism of the nictitating membrane 
than we do its use. Birds can wink with this one 
alone, as might be expected from its name, wherein 
they beat mammals, that cannot wink without moving both 
s. Ifwe menace a bird’s eye with the finger, we see that 
* hictitating is the first of the lids to rush to its defence. 
membrane is believed to be chiefly subservient to 
regulating the amount of light to be admitted to the eye. 
om eagle is, probably, able to soar aloft directly in the sun’s 
Nys, by drawing this covering over its eyes. Owls habit- 
vad sit, in the daytime, with drawn curtains to shut out 
: glare of light. It is also quite possible that many, or 
birds that are rapid flyers, make great use of this mem- 
M guarding against various dangers to which the eye 
Muld be exposed in their dashing career. A screen is 
fore the eye, which, while not preventing sight, as 
>of the outer lids would, opposes the entrance of any 
“icles of matter, 
Three lids of the casket that holds the gem have been 
“ad yet there is still another covering of the jewel 
_ A very delicate filmy membrane, not very apparent 
no. JY inspection, is laid over the front of the ball, 
ound which it is reflected over on to the inside of the 
wet lids. This is the conjunctiva, so-called because it 
‘ 

