RESPIRATION OP BIRDS. 



151 



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. 



The habits of Birds require that levity should be com- 

 bined with strength, in their conformation. 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 

 consequence. But by the very peculiar structure of the 

 whole apparatus, which allows the air to be twice breath- 

 ed, 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. 



Lungs of the Ostrich. This mechanism will be un- 

 Fig.94. 

 t 



