Air-sacs are outgrowths from the lungs of birds. They lie in the chest and abdominal region, are in direct connection with the branches of the bronchial tubes, and are usually nine in number. The air entering those reservoirs is warmed and moistened, while by their dilatation
Tr, trachea; l, lungs; a, peritracheal air-sac with its diverticula (b and c) into the humerus (d) and between the pectoral muscles; e, intermediate air-sacs; f, abdominal or posterior air-sacs.
(From Claus, after Heider.) the specific gravity of the bird can be greatly lessened; the bird in fact thus employing the principle of Montgolfier's original hot-air balloon. In most birds some of the air-sacs communicate with an extensive series of air-spaces in the bones and under the skin. Hints of these air-sacs, so characteristic of birds, are to be found in some reptiles.