^ INTRODUCTION. 



If we draw a comparison between these inhabitants of the air and 

 the earth, we shall perceive that, instead of the large head, formida- 

 ble jaws armed with teeth, the capacious chest, wide shoulders, and 

 muscular legs of the quadrupeds ; they have bills, or pointed jaws 

 destitute of teeth ; a long and pliant neck, gently swelling shoul- 

 ders, immovable vertebrae ; the fore-arm attenuated to a point, and 

 clothed with feathers, forming the expansive wing, and thus fitted 

 for a different species of motion ; likewise the wide-extended tail, 

 to assist the general provision for buoyancy throughout the whole 

 anatomical frame. For the same general purpose of lightness, exists 

 the contrast of sle»der bony legs and feet. So that, in short, we 

 perceive in the whole conformation of this interesting tribe, a struc- 

 ture wisely and curiously adapted for their destined motion through 

 the air. Lightness and buoyancy appear in every part of the 

 structure of birds ; to this end nothing contributes more than the 

 soft and delicate plumage with which they are so warmly clothed ; 

 and though the wings, or great organs of aerial motion by which 

 they swim, as it were, in the atmosphere, are formed of such light 

 materials, yet the force with which they strike the air is so great as 

 to impel their bodies with a rapidity unknown to the swiftest quad- 

 ruped. The same grand intention of forming a class of animals to 

 move in the ambient desert they occupy above the earth, is likewise 

 visible in their internal structure. Their bones are light and thin, 

 and all the muscles diminutive, but those appropriated for moving 

 the wings. The lungs are placed near to the back-bone and ribs ; 

 and the air is not, as in other animals, merely confined to the pul- 

 monary otgans, but passes through, and is then conveyed into a 

 number of membranous cells on either side the external region of 

 the heart, communicating Avith others situated beneath the chest. In 

 some birds these cells are continued down the wings, extending 

 even to the pinions, bones of the thighs, and other parts of the body, 

 which can be distended with air at the pleasure or necessity of the 

 animal. This diffusion of air is not only intended to assist in light- 

 ening and elevating the body, but also appears necessary to prevent 

 the stoppage or interruption of respiration, which would otherwise 

 follow the rapidity of their motion through the resisting atmosphere ; 

 and thus the Ostrich, though deprived of the power of flight, runs 

 almost with the swiftness of the wind, and requires, as he possesses, 

 the usual resources of air conferred on other birds. Were it possi- 

 ble for man to move with the rapidity of a Swallow, the resistance 

 of the air, without some such peculiar provision as in birds, would 



