152 VITAL FUNCTIONS. 



derstood by the plan, Fig. 94, which represents the lungs 

 and air cells of an Ostrich. The trachea t, is seen to di- 

 vide into bronchia, which pass to the lungs on each side. 

 These, after entering the lungs, divide into numerous 

 branches, and pass quite through their substance, open- 

 ing on the outside by many apertures, which may be 

 seen at I Z, these parts being the true lungs of the bird. 

 They are small, and thin, forming the dark substance al- 

 ways seen in carving a fowl, along the back, and be- 

 tween the ribs. 



These apertures admit the air into several large air 

 cells, c c c c, which occupy a considerable proportion of 

 the interior bulk. These cells enclose some ol the prin- 

 cipal viscera, as the liver, stomach, and heart, and extend 

 down the sides the whole length of the body. Numer- 

 ous air cells also exist in other parts, with which these 

 are connected by little punctures seen at c c. 



The air vessels thus described, not only communicate 

 with numerous others in different parts of the body, but 

 also with the interior of the bones, which especially in 

 the Eagle, and other birds that are much on the wing, 

 are left hollow, and without marrow, for this purpose. 



In consequence of the large quantity of air consumed 

 by the respiration of Birds, the temperature of their bo- 

 dies is several degrees higher than that of any other an- 

 imal. 



" The peculiarities of structure in the respiratory 

 system of birds," says Roget, " have probably a relation 

 to the capability we see them possess, of bearing with 

 impunity, very quick and violent changes of atmospheric 

 pressure. Thus the Condor of the Andes is often seen 

 to descend rapidly from a height of above 20,000 feet, 

 to the edge of the sea, where the air is more than twice 

 the density of that which the bird had been breathing. 

 We are as yet unable to trace the connection which 

 probably exists between the structure of the lungs, and 

 this extraordinary power of accommodation to such 

 great and sudden variations of atmospheric pressure." 



What is there peculiar in the respiration of birds ? Ho w do the lungs 

 of birds differ from those of quadrupeds 7 What is said of the air cell* 

 surrounding the lungs of birds 1 



