STRUCTURE OP BIRDS. 



95 



the Birds, the instruments of motion are fixed to the fore 

 part of the trunk, the impulse being given by the verti- 

 cal, or up and down action of the wings at the same in- 

 stant. In the Fish, the spine is flexible, especially to- 

 wards the tail, while in the Bird this part is rigid through 

 the body, having motion only in that part which forms 

 the neck. 



Birds change the centre of gravity. It is by means 

 of the neck, that the Bird is enabled to change its centre 

 of gravity according to circumstances. In the act of 

 flying, this centre must be between the articulations of 

 the wings ; while in resting on its legs, it must be be- 

 tween the feet. Had not Birds the power of adjusting 

 the centre of gravity, they could neither fly with precis- 

 ion through the air, nor rest secure on their feet. 

 Fie. 70. 



Skeleton of a Swan. The skeleton of a Swan rep- 

 resented by Fig. 70, will not only serve to show in what 



