168 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 
iy, 
. RNG 
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 aérial progression for which these 
animals are intended,” says McGillivray, “is obvious and _intel- 
