330 THE VITAL FUNCTIONS. 



lungs, and extending all down the sides of the 

 body. Numerous air-cells also exist between 

 the muscles, and underneath the skin ; and the 

 air penetrates even into the interior of the bones 

 themselves, filling the spaces usually occupied 

 by the marrow, and thus contributing materially 

 to the lightness of the fabric*. All these cells 

 are very large and numerous in birds which 

 perform the highest and most rapid flight, such 

 as the eagle. The bill of the Toucan, which is 

 of a cellular structure, and also the cells between 

 the plates of the skull in the Owl, are, in like 

 manner, filled with air, derived from the lungs : 

 the barrels of the large quills of the tails and 

 wings are also supplied with air from the same 

 source. 



In birds, then, the air is not merely received 

 into the lungs, but actually passes through them, 

 being drawn forwards by the muscles of the ribs 

 when they elevate the chest, and produce an 

 expansion of the subjacent air-cells. The chest 

 is depressed, for the purpose of expiration, by 

 another set of muscles, and the air driven back : 

 this air, consequently, passes a second time 

 through the lungs, and acts twice on the blood 

 which circulates in those organs. It is evident 

 that if the lungs of birds had been constructed 



* In birds, not formed for extensive flight, as the gallinaceous 

 tribes, the humerus is the only bone into which air is introduced. 

 — Hunter on the Animal Ecenomy, p. 81. 



I 



