xvi 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 vertebre ; 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 aérial 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 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 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 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 
