BIRDS. 495 



form which has just been described ; and thus the 

 equilibrium is, under all circumstances, preserved by 

 movements remarkable for then* elegance and grace. 

 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 ne- 

 cessary, in consequence of the conversion of the 

 fore extremities into wings, of which the structure 

 is incompatible with any prehensile power, such 

 as is often possessed by the anterior extremity of a 

 quadruped. Another advantage arising from the 

 length and mobility of the neck is, that it facili- 

 tates 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 flexibility 

 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 

 further provision made for the accomplishment 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 artictdation consists of a ball 

 and socket joint ; a rounded tubercle of the occi- 

 pital bone being received into a hemispherical de- 

 pression in the first vertebra of the neck. In the 

 mammalia the plan is changed, and there are two 

 articular surfaces, one on each side of the spinal 

 canal, formed on processes corresponding to the 



