BODIES OF BIRDS. 249 



the original animal economy is interwoven with 

 an entirely new machinery of motion. 



The first object will be attained by enlarging 

 the body of the bird without increasing the weight 

 in any thing like the same proportion — and one 

 very obvious means will be found in extending the 

 trachea or windpipe. In examining the tame and 

 wild swan, the most careless observer will detect 

 the provision for flight in the latter, by the confor- 

 mation of the windpipe, which is curiously convo- 

 luted within the sternum or breast-bone. 



This sketch from the male crane will illustrate 

 what we mean. The light and hollow air-tube fills 

 a space in the interior of the bone, by which the 

 surfaces are extended for the attachment of the 

 muscles of the wings, and thus two objects are at- 

 tained through it. 



In attempting to explain the reason of such de- 

 viations from the common forms of parts, we are 

 liable to fall into the same mistakes as we find oc- 

 casionally in commentators on the Sacred Volume. 



