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 the 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 uj) 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 cMed pe^mcB 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 outgrowths 
on each side called harhs, which send off secondary pro- 
cesses called iariiiles. 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 are acute in birds, 
particularly that of sight. Birds have three eyelids, be- 
sides an ujiper and lower lid, a membrane which can be 
drawn over the eye, and is called the ^^nictitating mem- 
brane." This covers the whole front of the eye-ball like a 
curtain. 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 the 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 
