1 68 



REPTILES AND BIRDS. 



modified part of the skeleton of Birds. It is a large expanded plate 

 extended over the whole anterior part of the thorax, and even cover- 



Fig. 44. — Lungs of a Pigeon. 



ing more or less what may be considered the abdomen. It varies 

 greatly in different genera ; but in all it is more or less four-sided, and 

 convex externally, forming the basis for the powerful muscles by 



Fig. 45.- — Wing of Eagle. 



which the wings are moved. These wings serve as arms by which 

 the bird guides itself, ascending or descending, according to the 

 impulse given them. " That the anterior form of birds is modified 

 so as to be subservient to the aerial progression for which these 

 animals are intended," says McGillivray, "is obvious and intel- 



