VERTEBKATA. 325 



Class IY. — Aves. 



Birds form the most clearly defined class in the whole 

 Animal Kingdom. The Eagle and Hummer, the Ostrich 

 and Duck, widely as they seem to be separated by size, 

 form, and habits, still exhibit one common type of struct- 

 ure. On the whole, Birds are more closely allied to Rep- 

 tiles than to Mammals. • In number, they approach the 

 Fishes, ornithologists having determined eight thousand 

 species, or more. 



A Bird is an air-breathing, egg-laying, warm-blooded, 

 feathered Vertebrate, with two limbs (legs) for perching, 

 walking, or swimming, and two limbs (wings) for flying 

 or swimming. Organized for flight, it is gifted with a 

 light skeleton, very contractile muscular fibre, and a res- 

 piratory function of the highest development. 



The skeleton is more compact than those of Reptiles 

 and Mammals, at the same time that it is lighter, and the 

 bones are harder and whiter. It contains fewer bones 

 than usual, many parts being anchylosed together, as the 

 skull-bones, the dorsal vertebrae, and bones of the tarsus 

 and metatarsus. The lumbar vertebrae are united to the 

 ilia. The neck is remarkably long (containing from nine 

 to twenty-four vertebrae) and flexible, enabling the head 

 to be a most perfect prehensile organ. The ribs are gen- 

 erally jointed in the middle, as well as with the backbone 

 and sternum. The last, where the muscles of flight orig- 

 inate, is highly developed. The skull articulates with 

 the spinal column by a single condyle, and with the lower 

 jaw, not directly, as in Mammals, but through the inter- 

 vention of a separate bone, as in Reptiles. 



All Birds always have four litnbs, while every other 

 vertebrate class shows exceptions. The fore-limbs are fit- 

 ted for flight. They ordinarily consist of nine separate 

 bones, and from the hand, fore-arm, and humerus are de- 



