RESPIRATION IN BIRDS. 235 



terially to the litghtness of the fahric* All these cells are 

 veryjarge 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 Oi^;/, are,in like man- 

 ner, 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, bein|[ drawn for- 

 wards 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 

 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 ani- 

 mal formed for flying.t The diffusion of so large a quantity 

 of air throughout the body of animals of this class presents 

 an analogy with a similar purpose apparent in the confor- 

 mation of insects, where the same object is effected by means 

 of tracheae.:}: 



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

 humerus is the only bone into which air is introduced. — Hunter on tlie 

 Animal Economy, p. 81. 



\ I must mention, however, that the correctness of this view of the sub- 

 ject 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. 



\ The peculiarities of structure in the rcspiratoiy system of birds 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 heiglit of 

 above 20,000 feet, to the edge of the sea, where the air is more than twice 



