BIRDS, 
199 
Class VII.— Aves {Birds). 
General Characters of Birds. — The graceful, pleasing 
lines of the bird's body; its clothing of feathers; tlie 
toothless jaws encased with horn to form the bill, and the 
remarkable change of the fore-limbs into wings — these are 
the marks which separate the birds from other vertebrates. 
Besides this, they are warm-blooded, and their bones are 
compact and in many cases hollow, thus combining light- 
ness with strength; hence birds are the most active and 
volatile creatures among all tlie back-boned animals. Tlie 
vertebral column is so adapted that birds can fly in any di- 
rection, particularly in a vertical one; and it is the strength 
Fig. 241.— Various curves of the wing of a bird at different points in its length. 
After Marey. 
and flexibility of its spinal column that enables the lark to 
rise up and shoot high into the air. Birds can turn the 
head around and look directly back, as seen in the owls: 
this is owing to the unusually free articulation of the first 
neck-vertebra to the skull; thus the bird can reach every 
part of its body with its bill. 
The most striking difference from other animals is in the 
modification of the fore-leg into a limb which supports a 
broad expanse of feathers, and ends in two rudimentary 
toe-bones. Another peculiarity in the skeleton connected 
with the power of flight is the very large breast bone^ with 
its keel, which is very high and thin, serving for the at- 
tachment of the muscles of flight. Thus in all respects we 
see a complete adaptation of the bird's body to its life in 
the air. The wings are attached exactly at the highest 
