THE BONES. 37 



breath. Travellers who have ascended very high moun- 

 tains 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 re- 

 taining 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 breath- 

 ing 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 Chimbon^o, which is one- 

 fourth part higher than Mont Blanc. Humboldt, the 

 celebrated traveller, who has given the best account of 

 those 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 move- 

 able, and can be bent ; whereas in birds the parts more 

 immediately connected with the back, are either alto- 

 gether 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 remarked. 

 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 



