494 THE MECHANICAL FUNCTIONS. 



rendered exceedingly long and flexible. The nnm- 

 ber of cervical vertebrae is generally very consider- 

 able ; in the mammalia, as we have seen, there are 

 always seven, but in many birds there are more 

 than twice that number. In the swan (Fig. 224), 

 there are twenty-three; and they are joined to- 

 gether by articulations, generally allowing free 

 motion in all directions ; that is, laterally, as well 

 as forwards and backwards. This unusual degree 

 of mobility is conferred by a peculiar mechanism, 

 which is not met with in the other classes of verte- 

 brated animals. A cartilage is interposed between 

 each of the vertebrae, to the surfaces of which these 

 cartilages are curiously adapted, being enclosed 

 between folds of the membrane lining the joint; so 

 that each joint is in reality double, consisting of 

 two cavities, with an intervening cartilage.* 



It is to be observed, however, that in conse- 

 quence of the positions of the oblique processes, 

 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 back- 

 wards: hence, in a state of repose, the neck natu- 

 rally 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 transfer that centre backwards, so as 

 to bring it towards the middle of the body, by 

 merely bending back the neck into the curved 



* See Mr. Henry Earle's paper on this subject in the Philoso- 

 phical Transactions for 1823, p. 277. 



