RESPIRATION IN BIRDS. 



295 



the diaphragm, and by the action of the intercostal 

 muscles. It is only in peculiar exigencies that the 

 whole power of this apparatus is called into action ; 

 for in ordinary respiration the diaphragm is the 

 chief agent employed, and the principal effect of 

 the action of the intercostal muscles is simply to fix 

 the ribs, and thus give greater purchase to the dia- 

 phragm. The muscles of the ribs are employed 

 chiefly to assist the diaphragm, when, from any 

 cause, a difficulty arises in dilating the chest. 



In Birds the mechanism of respiration proceeds 

 upon a different plan, of which an idea may be 

 derived from the view given of the lungs of the 

 Ostrich, at l, l, Fijj^. 377. The construction of the 



lungs of birds is such as not to admit of any change 

 in their dimensions ; for they are very compact in 



