BEWICK S SWAN. 



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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 direction from vertical to horizontal, and reaching within 

 half an inch of the posterior edge, is reflected back after 

 making a considerable curve, till it once more reaches the 

 keel, again traversing which, in a line immediately over the 

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

 merrythought ; where, turning upwards, and afterwards back- 

 wards, it enters the body of the bird to be attached to the 

 lungs in the usual manner. This is the state of develope- 

 ment in the oldest bird I have yet met with. The degree 

 next in order, or younger, differs in having the horizontal 

 loop of the trachea confined to one side only of the cavity in 

 the sternum, both sides of which cavity are at this time form- 

 ed, but the loop of the tube is not yet sufficiently elongated 

 to occupy the whole space ; and the third in order, that on 

 the right of the three representations shown on the next page, 

 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 con- 

 siderable extent. 



