STRUCTURE OF BIRDS. 201 



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

 creased 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 tlie abdomen; they con- 

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

 the hollows of the bones, so that the bird's body is lightened 

 and buoyed up by air. 



The most striking external feature of birds is the pres- 

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

 on the other, is clothed with feathers, though the scales on 

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

 should be borne in mind that feathers are only modified 

 scales or hairs. The ordinary feathers are coWq A. pennm or 

 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 outgrowtlis 

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

 cesses called barhules. The barbules and booklets are com- 

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

 bules interlock. Down is formed of feathers with soft, 

 free barbs called plumules. 



Over the tail-bone {coccyx) are usually sebaceous glands, 

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

 ing or "preening" its feathers. In some birds, as the cock 

 and turkey, the head and neck are ornamented with naked 

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



The sense of sight, smell, and hearing ai'e acute in birds, 

 particularly that of sight. Birds have three eyelids, be- 

 sides an upper and lower lid, a membrane which can be 

 drawn over the eye, and is called the "nictihating mem- 

 brane." This covers the whole front of the eye-ball like a 

 curtflin. With this it is said the eagle can look directly at 

 the sun. The eyes of hawks and eagles are provided with 

 a ring of bony plates occupying the front of tlie sclerotic. 

 By means of this ring the eye can adjust itself like a tele- 

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



