560 THE MECHANICAL FUNCTIONS. 



circumstances, and to adjust its position with 

 considerable nicety ; otherwise there would be 

 danger of the equilibrium being destroyed, and 

 the body oversetting. The principal means of 

 effecting these adjustments consist in the mo- 

 tions of the head and neck, which last is, for 

 that purpose, rendered exceedingly long and 

 flexible. The number of cervical vertebra is 

 generally very considerable ; in the mammalia, 

 as we have seen, there are always seven, but in 

 many birds there are more than twice that num- 

 ber. In the swan (Fig. 224), there are twenty- 

 three, and they are joined together by articu- 

 lations, generally allowing free motion in all di- 

 rections ; that is, laterally, as well as forwards 

 and backwards. This unusual degree of mobi- 

 lity is conferred by a peculiar mechanism, 

 which is not met with in the other classes of 

 vertebrated 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 inter- 

 vening cartilage.* 



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

 quence of the positions of the oblique processes, 



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

 phical Transactions for 1823, p. 277. 



