STRUCTURE OF BIRDS. 9\ 



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 tow- 

 ard the tail, while in the bird this part is rigid through 

 the body, having motion only in that part w r hich forms 

 the neck. 



260. 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 between 

 the feet. Had not birds the power of adjusting the centre 

 of gravity, they could neither fly with precision through 

 the air, nor rest secure on their feet. 



Fig. 70. 



261. Skeleton of a Swan. The skeleton of a swan 

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



