BIRDS. 



199 



Class VII. — Aves (Birds). 



General Characters of Birds. — The graceful, pleasing 

 lines of the bird's body; its clothing of feathers; the 

 toothless jaws encased with horn to form the bill, and tlie 

 remarkable change of tlie fore-limbs into wings — these are 

 the marks whicli sejiarate tlie birds from other vertebrates. 

 Besides this, tliey are warm-l)looded, and their bones are 

 compact and in many cases hollow, tlius combining light- 

 ness with strength; hence birds are the most active and 

 volatile creatures among all the back-boned animals. The 

 vertebral column is so adapted that birds can lly in any di- 

 rection, particularly in a vertical one; and it is the strengih 



Fig. 241. — Various curves of the wing of a bird at different points in its length. 

 After ]\larey. 



and flexibility of its spinal column that enables tlie lark to 

 rise up and slioot high into the air. Birds can turn the 

 head around and look directly back, as seen in the owls: 

 tliis 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 breastbone, with 

 its keel, which is very high and thin, serving for the at- 

 tachment of the muscles of flight. Thus in all resjiects we 

 see a complete adaptation of the bird's body to its life in 

 the air. The wings are attached exactly at the highest 



