a tim 
A į 
TOE 
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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 (Tas. 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 
furcatorium ; where turning upwards and afterwards backwards, 
it enters the body of the bird to be attached to the lungs in the 
usual manner (Tas. 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 (Tas. 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 (Tas. 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 pissiliudiós of structure which belong to the 
tube and sternum. The bronchiz 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. 3M being 
