The Breath of a Bird 



167 



The swollen rim of this opening suffices to close it, and 

 there is no elaborate trap-door arrangement as in mam- 

 mals, only a few backwardly directed fleshy points. 

 Birds have no trace of an "Adam's apple." The vocal 

 chords and other adjuncts to the voice of mammals 

 are entirely absent in birds, not a single note or song 

 being produced in the upper 

 throat. 



Passing down the neck 

 from this orifice is the wind- 

 pipe, which follows the 

 course of the oesophagus, or 

 food canal, passes to one 

 side of the crop and be- 

 tween the two branches of 

 the wish-bone, and finally 

 divides into two equal parts 

 called bronchi, which carry 

 the air directly to the lungs. 



Comparison of the two 

 tubes which traverse the 



throat and neck of birds shows them to be very different 

 in appearance and structure, and consideration of their 

 respective functions gives us the key to this dissimilarity. 

 The only occasion for the oesophagus to. open is to permit 

 the passage of food, and thus a limp, fleshy canal answers 

 all requirements. The windpipe, on the contrary^ must 

 always be wide open, and not only this, but it must be kept 

 open no matter what the pressure upon it. Tn addition, 

 it must be flexible, yielding to every motion of the neck, 



Fig. 127. — Windpipe and oesophagus ot 

 bird compared; the former always dis- 

 tended; the latter soft and collapsed. 



