402 THE iMECHx\i\lCAL FUNCTIONS. 



as well as their mode of progression, are so opposite 

 to those of fishes, that we shoiihl expect to find 

 great corresponding differences in their conforma- 

 tion. These two classes of vertebrata, accordingly, 

 are remarkably contrasted with respect to the struc- 

 tnre of their skeletons. In fishes we have seen 

 that the chest and all the viscera are carried as far 

 forw ards as possible ; the respiratory organs and 

 the centre of circulation being close to the head, 

 the neck having disappeared, and the trunk being 

 continued into the lengthened tail, in which the 

 chief bulk of the muscles are situated. In birds, on 

 the contrary, the ribs, and the viscera which they 

 protect, are placed as far back along the spinal 

 column as possible ; and a long and flexible neck 

 extends from the trunk to the head, which is thus 

 carried considerably forwards. These circum- 

 stances are very apparent in the skeleton of the 

 Swan, represented in Fig. 224. In a fish, pro- 

 gressive motion is effected principally by the move- 

 ments of the tail, which impels the body alternately 

 from side to side: in a bird, the only instruments 

 of motion are the wings, which are affixed to the 

 fore part of the trunk, and are moved by muscles 

 situated in that region. In the fish, the spine is 

 flexible nearly 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 with 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 



