Mr. Yarrell on a new Species of Wild Swan. 447 



length. Having arrived at the end of the keel, the tube then 

 gradually inclining upwards and outwards passes into a cavity 

 in the sternum destined to receive it, caused by the separation 

 of the parallel horizontal plates of bone forming the posterior 

 flattened portion of the breast bone, and producing a convex 

 protuberance on the inner surface. The tube also changing its 

 position from vertical to horizontal, and reaching within half an 

 inch of the posterior edge, is reflected back after making a con- 

 siderable curve, till it once more reaches the keel (Tab. XXV. 

 Fig. 3.), again traversing which, in a line immediately over the 

 first portion of the tube, it passes out under the arch of the os 

 furcntorium ; where turning upwards and afterwards backwards, 

 it enters the body of the bird to be attached to the lungs in the 

 usual manner (Tab. XXIV. Fig. 1.). This is the state of de- 

 velopement in the most perfect bird I have yet met with. The 

 degree next in order below, differs in having the horizontal loop 

 of the trachea confined to one side only of the cavity of the ster- 

 num, both sides of which cavity are at this time formed, but the 

 loop of the tube is not yet sufficiently elongated to occupy the 

 whole space (Tab. XXV. Fig. 2.) ; and the third in order, 

 being that of a still younger bird, possesses only the vertical 

 insertion of the fold of the trachea (Tab. XXV. Fig. 1.); yet 

 even in this specimen the cavity in the posterior portion of the 

 sternum already exists to a considerable extent, and will be ob- 

 served to be more capacious on that side to which, judging by 

 the preceding example, the loop of the trachea is first to be de- 

 termined. 



These are the peculiarities of structure which belong to the 

 tube and sternum. The bronchiae are very short ; but the flex- 

 ible part intervening between the bone of divarication and the 

 bronchial rings is considerable, producing an effect to be here- 

 after noticed. This elongated, flexible, and delicate portion, 



VOL. XVI. 3 M being 



