OF NATURAL HISTORY. 71 



through hedges and thickets. To defend their eyes, tlierefore, from 

 external injuries, as well as from too much light when flying in op- 

 pofition to the rays of the fun, they are furnifhed with a membrane 

 called membrana niclitans, which, like a curtain, can at pleafure be 

 drawn over the whole eye. This covering is neither opaque nor 

 pellucid ; but, being fomewhat tranfparent, it allows as many rays to 

 enter as render any object juft vifible, and enable them to diredl their 

 progrefs through the air. It is by the inftrumentality of this mem- 

 brane that the eagle looks at the fun. The feathers of all birds are 

 inferted into the (kin in fuch a manner that they naturally lie back- 

 ward from the head ; and allow the rain to run off their bodies, and, 

 by turning their heads in oppofition to the wind, prevent the wind 

 from rumpling their feathers and retarding their flight. Befide this 

 provifion, the rump of birds terminates in a large gland, which fe- 

 cretes an oily fubftance. When the feathers are too dry, or any way 

 difordered, the animals fqueeze this gland with their bills, extrafl the 

 oil, and with it they befmear and'drefs the feathers. By this means 

 the admiflion of water is totally prevented. Birds have no feparate 

 ribs ; but the breafl;-bone, which is very large, joins the back-bone, 

 and fupplies their place. 



With regard to the external figure of birds, the form of their bo- 

 dies is nicely adapted to their manners and the mode of life they are 

 deftined to purfue. By fl;riking the air with their wings, they move 

 forward in that element, and their tail ferves them as a rudder to di- 

 rtdi their courfe. Their breafl:-bone, infl;ead of being flat, rifes gra- 

 dually from the fplne and terminates in a iharp ridge or keel, which 

 enables them to cut the air with greater facility. For the fame pur- 

 pofe, the heads of birds are proportionally fmaller than thofe of 

 quadrupeds, and moft of them terminate in light fharp-pointed beaks. 

 They are likewife deprived of external ears, and of protuberant no- 

 ftrils. Their tails, inftead af vertebrae, mufcles, and £kin, confifl 

 i eotireljr 



