BIRDS. 387 



the neck bend with more facility forwards than backwards; 

 while those in the lower half of the neck bend more rcacfily 

 backwards: hence, in a state of repose, the neck naturally as- 

 sumes a double curvature, like that of the letter S, as is well 

 seen in the graceful form of the swan's neck. By extend- 

 ing 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 transfer that centre backwards, so as to bring it 

 towards 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 preserved, 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 surface of the body. Birds require this 

 power in order that they may be enabled to adjust their 

 plumage, whenever it has, by any accident, become ruffled. 

 In aquatic birds, it is necessary that every feather should be 

 constantly anointed with an oily secretion, which preserves 

 it from being wetted, and which is copiously provided for 

 that purpose by glands situated near the tail. The flexibili- 

 ty of the neck alone would have been insufficient for enabling 

 the bird to bring its bill in contact with every feather, in 

 order to distribute this fluid equally over them; and there is, 

 accordingly, a farther provision made for the accomplish- 

 ment of this object in the mode of articulation of the head 

 with the neck. We have seen that, in fishes, and in most 

 reptiles, this articulation consists of a ball and socket joint; 

 a rounded tubercle of the occipital bone being received into 

 a hemispherical depression in the first vertebra of the neck. 

 In the mammalia the plan is changed, and there are two ar- 



* 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 wing^ 

 of which the structure is incompatible with any prehensile power, such as is 

 often possessed by the anterior extremity of a quadruped. 



