Sauropsida: Class Aves 525 



The function of the numerous small air-spaces remote from the 

 lungs is somewhat problematic. The one thing certain is that the 

 entire system of air-sacs, large and small, reduces the specific gravity 

 of the body as a whole — an obvious advantage in flight. Because of the 

 dorsal position of the major air-sacs, the heavy viscera being below 

 them, the bird's center of gravity is low in the body — also an advantage 

 and one which is augmented by the great reduction of muscles along 

 the vertebral column of the trunk, and by the extremely ventral posi- 

 tion of the muscles of flight. The heavy mass of muscle on the lower 

 side of the sternum must be important as ballast. In this connection, 

 the fact that the elevator muscle (subclavius) of the wing is on the 

 sternum, instead of on the back as in land tetrapods, is of interest. 

 In swimming birds the air-sacs are doubtless important as "floats," 

 serving in a manner somewhat analogous to that of the swim-bladders 

 of fishes. It is very probable, too, that the air-sacs, especially the 

 larger, are important in connection with the regulation of the tempera- 

 ture of the body. The mechanism of heat regulation in the bird is not 

 fully known. In mammals the rate of loss of heat from the surface of 

 the body is controlled by increasing or diminishing the flow of blood 

 to the skin and by variation in the output of perspiration. Birds do 

 not perspire, having, like reptiles, a dry skin. The plumage, enclosing 

 innumerable minute particles of air among the dry horny filaments of 

 the small and deeper downy feathers, is an efficient conservator of the 

 heat of the body. On a cold day the bird "fluffs up" the plumage, 

 increasing the volume of air confined within it and thereby increasing 

 the thickness of the insulating layer. A considerable amount of heat 

 must be lost at the inner surfaces of lungs and the larger air-sacs. In 

 control of the bird's temperature, the feathers, skin, and respiratory 

 surfaces are all concerned. 



The vocal organs of vertebrates are always developed in some part 

 of the respiratory passages. Except in birds, it is the enlarged and 

 modified anterior region of the trachea which becomes the "voice box" 

 or larynx. In birds the corresponding region of the trachea appears as 

 a weakly developed larynx, but the vocal function shifts to a "lower 

 larynx" or syrinx situated at the region of junction of the trachea and 

 right and left bronchi. In some birds it is entirely within the posterior 

 end of the trachea; in others it is a duplex structure involving the 

 adjacent anterior ends of the two bronchi; most commonly it is 

 bronchotracheal, all three tubes participating in its structure. It 

 consists essentially of a resonance chamber formed by enlargement of 

 the air-tubes and containing membranous structures so placed that 

 they may be caused to vibrate by the passing of air through the 

 chamber (Fig. 411). The walls of the trachea and bronchi are strength- 



