BEWICK'S SWAN. 



321 



the cavity being at this time formed, although the loop of 

 the tube is not yet sufficiently elongated to occupy the whole 

 space. The third and right-hand one of the three represen- 

 tations below, from a still younger bird, possesses only the 

 vertical insertion of the fold of the trachea ; yet in this 

 specimen the cavity in the posterior portion of the sternum 

 already exists to a considerable extent. 



The above are the peculiarities of structure belonging to 

 the tube and the sternum. The bronchige are very short ; 

 but the flexible part intervening between the bone of divari- 



cation and the bronchial rings is considerable, producing an 

 effect to be hereafter noticed ; this elongated, flexible, and 

 delicate portion being defended on each outer side by a dis- 

 tinct membrane, attached to the whole edge of the bone of 

 divarication, and to a slender semicircular bone on each side, 

 by which it is supported. 



The muscles of voice with which this bird is provided 

 pass down — as usual — one on each side of the trachea, till 

 the tube is about to enter the cavity in the keel ; they then 

 qait that part of the tube to be attached to the ascending 



VOL. IV. T T 



