CLASS AVES. 



Ill 



bone of the bird of flight," says Owen, " was first solid, next 

 a marrow-bone, and finally became the case of an air-cell." 



The Plumage is a model of adaptation. The quills are 

 hollow, and like the bones, filled with air. They are compos- 

 ed of the same substance, chemically, as the hair of mammals 

 and the scales of fishes. The delicate filaments of a feathei 

 are covered with grasping barbs of a microscopic fineness. 

 The under side of a feather is concave, to resist an upward 



rig. m. 



PLTTMAGE OF A BIRD.!, crown ; 3, forehead ; S, nostrils ; U, upper mandible : 5, fowzr 

 do. ; 6, throat ; 7, neck ; 8, spurious quills ; 9, occiput ; 10, ear ; 11, nape ; is, 

 breast ; 13, middle coverts; 1U, large do. ; 15, belly ; 16, tibia ; 17, tarsus ; 18, inner 

 toe ; 10, ndddle do. ; %0, outer do. ; 21, thumb; 88, under tail-coverts ; 23, tail; 24. 

 primaries ; 25, secondaries ; 26, tertiaries. 



pressure. On the tail is a gland containing oil, with which 

 the bird lubricates its plumage and makes it waterproof. The 

 whole plumage is renewed (molted) once or twice a year. 



The Wing has a motion somewhat like the stroke of an 

 oar ; it strikes the air with the broad side, but, in returning, 

 presents only the sharp edge. The flight of a bird offers 



