EXTERNAL ANATOMY POWERS OP SIGHT. 
4.') 
miniature representations of the quills of the porcu- 
pine. On the second, or under eyelid, these lashes do 
not appear. The third, or 
internal eyelid, is called the 
nictitatory membrane. It is 
thin and semitransparent, 
and is drawn over the ball 
of the eye at pleasure, as we 
should draw the curtain of a 
window on a sunny day, and 
for thesamepurpose; namely, 
to dimiidsh the light when the rays are too powerful : 
it is often seen when a bird has been just wounded, and 
in the agonies of death ; hut at other times, excepting in 
the owls, it is not externally apparent. 
(50). 1'lie power of .siyht, in all birds, is well known 
to very great, and far exceeds what is possessed by 
man, and perhaps by any class of animals. The high 
development of this faculty will appear absolutely ne- 
cessary to birds, when we consider, for a moment, the 
various purposes it is intended to accomplish, as the 
speedy and ready discernment of their food, the deter- 
mination of their route when upon long journeys, or in 
migrating, and to aid the celerity of their movements. 
It is well known, even to a proverb, that eagles and fal- 
cons possess this faculty to an astonishing degree ; but 
the necessity of the gift does not appear to have been 
touched upon. If the powers of vision in these rapa- 
cious birds had merely been equal to those of other trilies, 
it is very questionable whether they could long have 
existed; they would have been compelled to have hunted 
out their prey, so near, as to scare it away ; for it is well 
known that all the small birds, immediately upon dis- 
covering a hawk, set up a note of alarm, heard and un- 
derstood by their companions, both as a signal of danger 
and for immediate concealment. It is essential, thwe- 
fore, to the very existence of rapacious birds, that they 
should have the power of reconnoitring at a distance, 
beyond the ken of their quarry, and even of fixing and 
