BIRDS. 5(J1 



the upper vertebrae of the neck bend with more 

 facility forwards than backwards ; while those 

 in the lower half of the neck bend more readily 

 backwards : hence, in a state of repose, the 

 neck naturally assumes a double curvature, like 

 that of the letter S, as is well seen in the graceful 

 form of the swan's neck. By extending the 

 neck in a straight line, the bird can, while 

 flying, carry forwards the centre of gravity, so 

 as to bring it under the wings ; and when resting 

 on its feet, or floating on the water, it can trans- 

 fer that centre backwards, so as to bring it to- 

 wards the middle of the body, by merely 

 bending back the neck into the curved form 

 which has just been described ; and thus the 

 equilibrium is, under all circumstances, pre- 

 served by movements remarkable for their 

 elegance and grace.* 



Another advantage arising from the length 

 and mobility of the neck is, that it facilitates the 

 application of the head to every part of the sur- 

 face of the body. Birds require this power in 

 order that they may be enabled to adjust their 

 plumage, whenever it has by any accident be- 

 come ruffled. In aquatic birds, it is necessary 



* The great mobility of the neck enables the bird to employ 

 its beak as an organ of prehension for taking its food : an object 

 which was the more necessary, in consequence of the conversion 

 of the fore extremities into wings, of which the structure is in- 

 compatible with any prehensile power, such as is often possessed 

 by the anterior extremity of a quadruped. 



VOL. I. O O 



