232 



HABITS OF BIRDS. 



it passes into the keel of the breast-bone, which keel 

 is seldom composed of a single bone, but consists of 

 two sides, with a third resting upon them by way of 

 cover, the whole forming a sort of sheath or tunnel. 

 When the windpipe arrives at the extremity of this 

 sheath, it bends downwards in a serpentine form 

 resembling the letter S, and again, beneath the por- 

 tion just described, it emerges from the sheath, and 

 rising upwards over the middle bend of the shoulder- 

 blade, it winds, thus supported, in the manner of a 

 trumpet. Upon its passing under the cavity of the 

 chest, and just before it reaches the lungs, as if to 

 form another vocal organ (larynx), it is transversely 

 cut (being as broad as the small bone is long), and a 



The breast-bone of a wild Swan, with part of the keel removed to show 

 the convolution of the trachea within it. 



The point of the keel-bone, showing the opening through which the trachea 

 enters and returns. 



