VERTEBRATA. 
325 
Class IY.—Aves. 
Birds form tlie 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 limbs, 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- 
