560 ORGANS OF RESPIRATION. 



permeated with air from the pulmonary organs, to a variable 

 extent in different tribes of birds, according to their power of 

 flight, and their necessity for extensive aeration of their fluids. 

 As these cells and cavities are filled with heated air from the 

 lungs, the whole body is thereby rendered more light and 

 buoyant for progression through the colder and denser ex- 

 ternal medium. The highly vascular entosteal lining of the 

 bones, like the vascular mucous lining of the great air-sacs of 

 the trunk, contributes to the aeration of the systemic blood ; 

 and the extensions of the air-sacs thus penetrate the tissues of 

 the body, as they do in insects. When the trachea is tied, birds 

 can still inhale air, and inflate their cavities, by the broken 

 ends of the humerus or femur, and when the bones of the extre- 

 mities are fractured in flight, they are thus quickly precipitated 

 to the earth. The bones which receive air in birds, have a 

 more dry, white, and less oily appearance, than those which 

 permanently preserve their marrow. The air admitted into the 

 interior of the bones during the development and growth of 

 birds, causes, to a variable extent, the gradual absorption of 

 the thin serous marrow, which originally occupied all their 

 cavities. 



The proper cellular lungs of birds, like those of chelonian 

 reptiles, are excluded from the peritoneal cavity, and bound 

 by cellular tissue to the inside of the ribs and the sides of the 

 dorsal vertebne, being covered only on their ventral surface with 

 peritoneum, which is prolonged also from the large bronchial 

 apertures overall the abdominal air-sacs. They were considered 

 by Bartholinus as fixed to the upper and dorsal region of the 

 trunk, in birds, to balance their body in flight. They consist of a 

 single long, flat, nearly triangular, undivided lobe on each side, 

 and have a more light florid red colour than in other vertebrata; 

 their minute cells have a parallel and methodical arrangement, 

 around the bronchi, and freely communicate. The bronchi, 

 thin and membranous, with annulated markings on one side, tra- 

 verse chiefly their ventral and median portions, dividing in each 

 lung, into about seven branches, perforated in their dorsal as- 

 pect with numerous foramina, leading to the minute pulmonary 

 cells, and terminating below in the wide oblique orifices of the 

 great air sacs. Their narrow, thin, inferior, and outer mar- 

 gins are traversed by the radiating peripheral muscular bands, 

 composing the rudimentary diaphragm, as in saurian and 

 chelonian reptiles, and by their thin tendinous expansions ; 



