AVES BIRDS. 



603 



stated concerning the heat and purity of the blood in these crea- 

 tures, we are prepared to find presenting the highest possible con- 

 dition of developement. Birds, in fact, breathe not only with 

 their lungs, but the vital element penetrates every part of the 

 interior of their bodies, bathing the surfaces of their viscera and 

 entering the very cavities of their bones ; so that the blood is 

 most extensively subjected to its influence. The lungs, in fact, 

 are no longer closed bags as those of Reptiles are, but rather 

 resemble spongy masses of extreme vascularity, firmly bound 

 down in contact with the dorsal aspect of the thorax ; their poste- 

 rior surface being fixed to the ribs on each side of the vertebral 

 column, and entering deeply into the intercostal spaces. Such 



lungs are obviously incapable of alternate dilatation and contrac- 

 tion, so that inspiration and expiration must be provided for by a 

 mechanism specially adapted to the emergency. From an exa- 



