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- 



