160 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



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

 light skeleton, very contractile muscular fibers, and a 

 respiratory system 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, ena- 

 bling the head to be a most perfect prehensile organ. 

 The ribs are generally jointed in the middle, as well as 

 with the backbone and sternum. The last, where the 

 muscles of flight originate, 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 intervention of a separate 

 bone, as in reptiles (Fig. 313). 



All birds have four limbs, while every other verte- 

 brate class shows exceptions. The fore limbs are fitted 

 for flight. They ordinarily consist of nine separate 

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

 developed the primary, secondary, and tertiary feathers 

 of the wing. The hind limbs are formed for progres- 

 sion walking, hopping, running, paddling, and also 

 for perching and grasping. The modifications are more 

 numerous and important than those of the bill, wing, or 

 tail. There are twenty bones ordinarily, of which the 

 tibia is the principal ; but the most characteristic is the 

 tarsometatarsus, which is a fusion of the lower part of 

 the tarsus with the metatarsus. The rest of the tarsus 

 is fused with the tibia. The thigh is so short that the 

 knee is never seen outside of the plumage; the first 

 joint visible is the heel. 40 Most birds have four toes 



