182 



INTEODUCTOEY CHAPTEE. 



The position renders a double system of breathing necessary. 

 Vital heat in animals is always in proportion to their respiration, 

 for the oxygen of the air^ which penetrates every cell and cavity 

 of their bones, feathers, and body, warming and giving increased 

 activity to their circulation, and specific lightness to their bodies, 



from its rich organisation enables 

 birds to live in the coldest atmo- 

 spheric regions. In Fig. 46 the 

 respiratory organs of a Pigeon are 

 represented. The trachea, or wind- 

 pipe, is composed of many bony rings, 

 varying in different species. In the 

 Falcons it is slightly flattened, and 

 tapers in a small degree ; but in many 

 genera it presents dilatations and 

 contractions, and in others it is vari- 

 ously curved, two slender muscles, 

 which run along its sides towards the 

 sternum, serving to contract it. In 

 many of the song-birds several pairs 

 of small muscles are attached to the lower larynx where the tube 

 bifurcates, by which they are enabled to control this organ, which 

 is the producer of their note. 



The trachea carries the air to the lungs in a Pigeon, and sepa- 



Fig 46. 



rates into two branches in the breast, where it abuts on the 

 aerial sacs, and on the two lungs (Fig. 47). The air carried by 



