148 VITAL FUNCTIONS. 



understood by the plan, Fig. 94, which represents the 

 lungs and air cells of an Ostrich. The trachea, t, is seen 

 to divide into bronchia, which pass to the lungs on each 

 side. These, after entering the lungs, divide into numer- 

 ous branches, and pass quite through their substance, 

 opening on the outside by many apertures, which may 

 be seen at /, /, these parts being the true lungs of the bird. 

 They are small, and thin, forming the dark substance 

 always seen in carving a fowl, along the back, and 

 between the ribs. 



446. 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 of the 

 principal viscera, as the liver, stomach, and heart, and 

 extend down the sides the whole length of the body. 

 Numerous" air cells also exist in other parts, with which 

 these are connected by little punctures seen at c c. 



447. The air vessels thus described, not only communi- 

 cate 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 

 bodies is several degrees higher than that of any other 

 Animal. 



448. " The peculiarities of structure in the respiratory 

 systems 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 connexion which pro- 

 bably 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 ! How do the lungs 

 of oirds differ from those of quadrupeds? What is said of the air cells 

 surrounding the lungs of birds? 



