BIRDS. 6 



last named organs are likewise proportionally 

 very large, arranged along on each side of the 

 spine, and occupying the hollows between the 

 bases of the ribs. By this great development of 

 the respiratory system, the blood is more rapidly 

 and effectually oxygenised, and muscular energy 

 greatly increased for the action of flight; while 

 by the animal heat thus evolved the air contained 

 in the complex apparatus described is rarified, 

 and so the body is increased in bulk, but rela- 

 tively diminished in weight. 



The wings of a Bird correspond to the arms and 

 hands of man ; but the hand is composed of but 

 two fingers and a thumb, all of which are rudi- 

 mentary. From the bones of the hand arise the 

 primaries, or quill-feathers properly so called, 

 which are ten in number ; these are the largest 

 and strongest feathers of the wing, and the cha- 

 racter and power of flight is indicated by their 

 form, stiffness, and relative length. From the 

 principal bone of the fore-arm arise the second- 

 aries, the number of which varies in different 

 species; they are usually shorter, broader, and 

 more flexible than the primaries; and are less 

 removed in form from the general clothing feathers 

 of the body. The bone of the upper -arm (hume- 

 rus) gives rise to another series of feathers, called 

 tertiaries : these, in some birds, particularly the 

 Plovers, Curlews, &c., are greatly lengthened; 

 they are, however, still weaker in their structure 



