RKSPIHATION IN BIRDS. 297 



ing 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 

 Oivl, 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 ex- 

 pansion 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 circidates 

 in those organs. It is evident that if the lungs of 

 birds had been constructed on the plan of those of 

 quadrupeds, they must have been twice as large to 

 obtain the same amount of aeration in the blood ; 

 and consequently must have been twice as heavy, 

 which would have been a serious inconvenience in 

 an animal formed for flying. | The diffusion of so 



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

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

 — Hunter on the Animal Economy, p. 81. 



f I must mention, however, that the correctness of this view of 

 the subject is contested by Dr. Macartney, who thinks it probable 

 that the air, on its return from the large air-cells, passes directly by 

 the large air-holes into the bronchia, and is not brought a second 

 time into contact with the blood. 



