xii INTRODUCTION. 



If we draw a comparison between these inhabitants of the 

 air and the earth, we shall perceive that, instead of the large 

 head, formidable 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 shoulders, 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 slender bony legs and feet. So that, in short, we perceive 

 in the whole conformation of this interesting tribe, a structure 

 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 

 clad ; and though the wings (or great organs of aerial motion 

 by which they swim, as it were, in the atmosphere) are formed 

 of such hght materials, yet the force with which they strike the 

 air is so great as to impel their bodies with a rapidity unknoAvn 

 to the swiftest quadruped. The same grand intention of form- 

 ing 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 hght 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 pulmonary 

 organs, but passes through, and is then conveyed into a num- 

 ber of membranous cells on either side the external region of 

 the heart, communicating with 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 lightening and elevating the 

 body, but also appears necessary to prevent the stoppage or 



