506 BIRD’S-EYE VIEWS. 
To commence by saying that all birds have eyes, might 
appear at first sight to be superfluous. Yet this statement 
expresses one of the characters of the class Aves; for it is 
not applicable, without some qualification, to any other class 
of Vertebrates. Some representatives of each of the other 
classes either have no eyes at all or else very rudimentary 
ones. ‘There are blind fishes and blind reptiles; and there 
are mammals at least “as blind as a mole.” Among birds, 
the “wingless” species of New Zealand (Apteryx) are said 
to have the smallest eyes of all, and also to want one of the 
most characteristic structures of the avian eye-type,—the 
marsupium, a peculiar organ inside the eye, of which we 
- shall learn something before we have finished our “Views.” 
We will examine first the accessory structures of a bird 
eye,—those that surround and defend it, produce its move 
ments, and keep it in working order; and then we will look 
at the more exquisite mechanism within. ' 
If we hold a dying bird in our hands, we observe that just 
as the last convulsive shiver agitates its frame, the eyes close 
by the uprising of the lower lid. In the primitive theatres 
of classic days, the curtain was lowered from the top to dis 
close the stage, and drawn up when the act was over; ne 
these movements are reversed. Birds follow the classt¢ 
usage, when the curtain rises upon the last scene of their 
life. Here at the outset is one difference between the ey¢ 
of a bird and that of a mammal; and differences will mul 
tiply as we proceed. : 
he movements of the upper lid, in almost all birds, are 
much more restricted than those of the lower. There = 
few exceptions to this rule, and these chiefly furnished i 
the nocturnal raptores (Owls, Strigidæ), and org r 
rostres (Caprimulgide, e. g., Whippoorwill, Nigh 
Both lids are composed of common skin externally, #™ ý 
brane internally (the palpebral part of the conjunctiva, t0 e 
noticed presently), with a layer of fibrous tissue inte iid l 
for greater strength. Besides these tissues, the lowes Si a 
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