109 



the spine. As, among invertebral animals, insects 

 in general, so, among vertebral, birds have the most 

 perfect respiratory apparatus ; although perhaps 

 the real lungs of birds, or those organs in which 

 the blood is purified, are not relatively larger 

 than those of mammiferous animals. It is peculiar 

 to the former, however, to have their lining arid 

 and investing membranes prolonged from various 

 parts of their surface in the form of tubes, which, 

 expanding into bags, envelope almost all the en- 

 trails, so as to keep them constantly surrounded 

 with air ; and similar prolongations, extending 

 also from their back part into the cavity of the 

 bones, serve to inflate these in the same manner. 

 The chief object of this peculiarity appears to be 

 that of giving lightness to the animal, and thus 

 of enabling it to support itself in the air ; and 

 the same object is fulfilled in insects, as we have 

 already seen, in a manner very analogous. But 

 do we ever meet with this general diffusion of 

 air through the bodies of animals not designed 

 for flying in worms, fishes, reptiles or mammi- 

 ferous animals as should at least sometimes 

 have happened, had a blind chance, and not an 

 omniscient Providence presided over their struc- 

 ture ? Another thing remarkable in the respira- 

 tory apparatus of birds, is the provision made by 

 nature, for the necessity which some of the wa- 

 ding and diving kinds are under, of remaining 

 with their heads for a long time under water. 

 She has not given them tubes opening upon the 

 atmosphere, as in some water insects, nor large 



