RESPIRATION OF BIRDS. 



147 



chest, by a set of muscles for this purpose. When the 

 chest is depressed, this air is again expelled through the 

 same vessels by which it was admitted ; so that in these 

 Animals, the same portion of air passes twice through the 

 lungs. This is a wonderful provision, and one in which 

 we cannot but behold Creative wisdom and design. 



444. The habits of Birds require that levity should be 

 combined with strength in their confirmation. Had the 

 lungs been constructed like those of Quadrupeds and Man, 

 where the air is merely taken in and thrown out, a 

 considerable addition of weight must have been the conse- 

 quence. But by the very peculiar structure of the whole 

 apparatus, which allows the air to be twice breathed, 

 the lungs could be reduced to a very diminutive size, 

 and still the aeration of the blood be as perfect as in 

 Quadrupeds ; and this is the admirable plan adopted in 

 these Animals. 



445. Lungs of the Ostrich. This mechanism will be 



Fig. 94. 



