﻿EXTERNAL ANATOMY. POWERS OF SIGHT. 45 



miniature representations of the quills of the porcu- 

 pine. On the second, or under eyelid, these lashes do 

 ^^^^^^ not appear. The third, or 



W~ internal eyelid, is called the 



to diminish 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 ; but at other times, excepting in 

 the owls, it is not externally apparent. 



(50). The power of sight, in all birds, is well known 

 to be 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 tribes, 

 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, there- 

 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 



\ 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 the same purpose ; namely, 



nictitatory membrane. It is 



