20 FACULTIES OF BIRDS. 



This singular membrane has been long known, and 

 is well described by the French Academicians, who 

 remark that, in the eyes of the turkey, the optic nerve, 

 which is situated very near the side, spreads into a 

 round space, from the circumference of which a num- 

 ber of black filaments are sent off to form, by their 

 union, a membrane peculiar to the eye of birds. In 

 the eyes of the ostrich they describe the optic nerve 

 as dilated into a sort of funnel of a similar substance. 

 From this funnel a folded membrane takes its origin, 

 forming a sort of purse drawn to a point. This is 

 covered with a black pigment, easily rubbed off *. 



Buffon is of opinion that, on account of this ex- 

 pansion 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 protruded, 

 readily assuming the precise condition adapted to the 

 distance of an object and the quantity of light 'f > . 



In consequence of the rapidity with which birds 

 traverse the air, extent and acuteness of vision 

 appear to be indispensable, in order to direct them 

 in their flight. Had they indeed been formed with 

 eyes like the mole (Talpa lucida, C. BONAPARTE), 

 incapable of seeing more than a few inches' dis- 



* Mem. PHist. des Animaux 

 f Montbeillard, Oiseaux, Prelim. 



