558 THE MECHANICAL FUNCTIONS. 



with respect to the structure of their skeletons. 

 In fishes we have seen that the chest and all the 

 viscera are carried as far forwards as possible ; 

 the respiratory organs and the centre of circula- 

 tion 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 con- 

 trary, the ribs, and the viscera which they pro- 

 tect, 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 cir- 

 cumstances are very apparent in the skeleton of 

 the swan, represented in Fig. 224. In the fish, 

 progressive motion is effected principally by the 

 movements of the tail, which impels the body 

 alternately from side to side : in the 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 im- 

 moveable in the trunk, and is capable of exten- 

 sive motion only in the neck. 



In order that the body may be exactly ba- 

 lanced 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 re- 



