3S6 THE MECHANICAL FUNCTIONS. 



ly throughout its whole extent; in the bird, it is rigid and 

 immoveable in the trunk, and is capable of extensive motion 

 only in the neck. 



In order that the body may be exactly balanced while the 

 bird is flying, its centre of gravity must be brought precisely 

 under the line connecting the articulations of the wings wdth 

 the trunk; for it is at these points that the resistance of the 

 air causes it to be supported by the wings. When the bird 

 is resting upon its legs, the centre of gravity must, in like 

 manner, be brought immediately over the base of support 

 formed by the toes: it becomes necessary, therefore, to 

 provide means for shifting the centre of gravity from one 

 place to another, according to 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 adjust- 

 ments consist in the motions of the head and neck, which last 

 is, for that purpose, rendered exceedingly long and flexible. 

 The number of cervical vertebrae is generally very consi- 

 derable: in the mammialia,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 together by articulations, generally al- 

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

 as forw^ards and backwards. This unusual degree of mobi- 

 lity is conferred by a peculiar mechanism, w^hich 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 

 intervening cartilage.* 



It is to be observed, how^ever, that in consequence of the 

 positions of the oblique processes, the upper vertebr aeof 



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

 tions for 1833, p. 277. 



