THE BONES. 



becomes so rarefied and thin, that it is as much as they can da 

 to proceed at a slow pace. Those birds, too, whose habits 

 never lead them into the more elevated regions of the atmo- 

 sphere, 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 pro- 

 vided with air-cells appear to feel no such inconvenience. 



Mont Blanc, the most elevated mountain in Europe, is not 

 quite three miles high ; and yet on the top breathing is ex- 

 tremely difficult j nevertheless, the Condor of South America, 

 the largest bird gifted with the power of flight, will dart up- 

 wards suddenly from the deepest valleys to a considerable 

 height above the summit of even the lofty mountain of Chim- 

 boraco, 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 backbone is more or less movable, 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. In carving a fowl 

 or any other bird at table, this peculiarity may easily be re- 

 marked. The want of motion in the back, however, is amply 

 compensated by a greater number of bones in the neck, and 

 greater power of moving them, which enables birds to turn 

 their heads in all directions with extraordinary facility. These 

 joints vary in number according to the necessities of the bird ; 

 thus, the Sparrow, which can perch and reach its food close 

 before him, does not require such pliability or length of neck 

 as the Swan, which floats on the water, and must seek its food 

 at a considerable depth beneath; accordingly, we find that, 



