THE BONES. 
35 
It is evident that a creature thus furnished with hones 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 Chimborazo, which is one-fourth part higher 
than Mont Blanc. Humboldt, the celebrated traveller, who 
has given the best 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. 
d 2 
