2 
INTRODUCTION. 
If we draw a comparison between these inhabitants of the air and 
the earthy 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 slender 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 organs, but passes through, and is then conveyed into a 
number 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 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 
