THE BOOK OF MIGRATORY BIRDS 83 



Besides the nictitating membrane in the eye of birds, 

 which is not altogether peculiar to them, there is another 

 singular part of the organ whose use has not hitherto been 

 clearly ascertained. It is called by the French Academi- 

 cians the Purse (Marsupium) and the Comb (Pecten 

 plicatum). It arises in the back of the eye, and, proceed- 

 ing apparently through a slit in the retina, it passes 

 obliquely into the vitreous humour, where it terminates, 

 reaching in some species to the capsule of the lens. 

 Numerous blood-vessels run in the folds of the membranes 

 which compose it, and the black pigment by which it is 

 covered suggests the idea that it is chiefly destined to 

 absorb the rays of light when they are too strong or 

 dazzling. If this be the fact, it may serve the eagle in 

 good stead when gazing, if he ever does so, on the sun. 



It is the manifest opinion of others that it serves to 

 assist in producing the internal changes of the eye; but 

 this has been opposed by Crampton, who has shown that 

 the changes in question at least in the ostrich and several 

 large birds, are produced by a peculiar circular muscle in 

 the eyeball. Buffon is of opinion that, on account of this 

 expansion of the optic nerve, birds must have a vastly 

 more perfect sight than other animals, embracing also a 

 much wider range. Hence it is that a sparrow-hawk, 

 while he hovers in the air, espies a lark sitting on a clod, 

 though at twenty times the distance at which it could be 

 perceived by a man or dog. 



The kite, which soars to so amazing a height as totally 

 to vanish from our sight, can yet distinguish small lizards, 

 field-mice, and birds, and from this lofty station he selects 

 his prey. This prodigious extent of vision is, moreover, 

 conjoined with equal accuracy and clearness, inasmuch as 

 the eye can dilate and contract, can be shaded or uncovered, 

 depressed or protruHed, readily assuming the precise con- 

 dition adapted to the distance of an object and the quantity 

 of light. 



Had they, indeed, been formed with eyes like the mole, 



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