THE BONES. 35 



It is evident that a creature thus furnished with bones so 

 much lighter than those of other animals, must have a 

 prodigious advantage in raising itself in the air. But, 

 besides this superior lightness of the skeleton, these bones, 

 from their hollow structure, act as pipes supplying air in 

 abundance; and thus not only rendering the bird still 

 lighter, but enabling it to breathe at heights, at which a 

 human being would be gasping for breath. Travellers 

 who have ascended very high mountains find, that when 

 they get near the summits, the air becomes so rarefied and 

 thin, that it is as much as they can do to proceed at 

 a slow pace. Those birds, too, whose habits never lead 

 them into the more elevated regions of the atmosphere, and 

 are, therefore, not so abundantly provided with additional 

 capacities for retaining air, have been observed to suffer 

 severely, as was proved by a Mr. Robertson, who took 

 two birds up with him in a balloon, one of which actually 

 died at the height of 15,600 feet ; whereas others better 

 provided with air-cells appear to feel no such incon- 

 venience. 



Mont Blanc, the most elevated mountain in Europe, is 

 not quite three miles high; and yet on the top breathing 

 is extremely difficult; nevertheless, the Condor of South 

 America, the largest bird gifted with the power of flight, 

 will dart upwards suddenly from the deepest valleys to a 

 considerable height above the summit of even the lofty 

 mountain of Chimboraco, which is one-fourth part higher 

 than Mont Blanc. Humboldt, the celebrated traveller, who 

 has given the besl account of these regions, says, that he has 

 frequently seen this enormous bird soaring without an effort, 

 and enjoying itself at an elevation much higher than that of 

 the clouds in our atmosphere. 



In tracing the bones of the skeleton down the back, 

 another remarkable difference is perceptible. In men and 

 animals, the whole back-bone is more or less moveable, and 

 can be bent ; whereas in birds the parts more immediately 

 connected with the back are either altogether consolidated 

 or stiffened, so as to allow little or no play in the joints. 



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