328 



THE VITAL FUNCTIONS. 



and thus give greater purchase to the diaphragm. 

 The muscles of the ribs are employed chiefly to 

 give active assistance to the diaphragm, when, 

 from any cause, a difficulty arises in dilating the 

 chest. 



In Birds the mechanism of respiration pro- 

 ceeds upon a different plan, of which an idea 

 may be derived from the view given of the lungs 



of the Ostrich, at l. l.. Fig. 377. The construc- 

 tion of the lungs of birds is such as not to admit 

 of any change in their dimensions ; for they are 

 very compact in their texture, and are so closely 

 braced to the ribs, and upper parts of the chest, 



