STRUCTURE OF BIRJD8. 



The power of remaining a long time in mid-air is in- 

 3ed by the large air-cells, which are pockets filled with 

 air. There are nine of these air-sacs three near the clavi- 

 cle, four in the thorax, and two in the abdomen; they i 

 nect with the ends of some of the air-tubes, and also with 

 the hollows of theboi that the bird's body is light 



ami buoyed up by air. 



The mosl striking external feature of birds is tie p 

 ence of feathers; no reptile on the one hand, or mammal 

 on the other, is clothed with feathers, though t: 

 the legs and feet of birds are like those of reptiles, and it 

 should be borne in mind that feathers are only mod 

 scales or hairs. The ordinary feather- ailed y ■ 



contour feathers, as they determine by their arrangement 

 the outline of the body. They are. like hairs, developed 

 in sacs in the skin; the quill is hollow, partly imbedded in 

 the skin : this merges into the shaft, leaving the outgrowths 

 on each side called barbs, which send off secondary pro- 

 cesses called barbules. The barbules and hookh I com- 

 monly serrated, and end in little hoik- by which the bar- 

 bules interlock. Down is formed of feathers with - 

 free barbs called plumul 



Over the tail-bone (coccyx) are usually - us elands, 



which secrete an oil, used by the bird in oiling and di 

 ing or xt preening" En some birds, as the < 



and turkey, the head and neck are ornamented with naked 

 folds of the skin called "combs" and " wattles." 



The Bense of sight, smell, and hearing are acute in bi 

 particularly that of Bight. Birds have thn 



- an upper and lower lid. a membrane which can he 

 drawn over the eve, and is called the "nici tating mem- 

 brane." This covers the whole front of the eye-hall HI 

 curtain. With this it is said the eagle cai y at 



the sun. The eyes of hawks and are provided with 



a ring of bony plates occupying the front of tie 

 By means of this ring the eye tan If 1 1 k • 



scope so as to take in both near and distant objects. The 



