328 



THE VITAL FUNCTIONS. 



and thu8 give greater purchase to the diaphragm. 

 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 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 cheet, 



