viii. INTRODUCTION. 



Instead of the large head and formidable jaws, the deep 

 capacious chest, the brawny shoulders, and sinewy legs of 

 the quadrupeds, we observe the pointed beak, the long and 

 pliant neck, the gently swelling shoulder, the expansive 

 wings, the tapering tail, the light and bony feet ; which are 

 all wisely calculated to assist and accelerate their motion 

 through the yielding air. Every part of their frame is 

 formed for lightness and buoyancy ; their bodies are covered 

 with a soft and delicate plumage, so disposed as to protect 

 them from the intense cold of the atmosphere through which 

 they pass : their wings are made of the lightest materials, 

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

 to impel their bodies forward with astonishing rapidity, 

 whilst the tail serves the purposes of a rudder to direct them 

 to the different objects of their pursuit. The internal struc- 

 ture of birds is no less wisely adapted to the same purposes: 

 all the bones are light and thin, and all the muscles, except 

 those which are appropriated to the purpose of moving the 

 wings, are extremely delicate and light ; the lungs are placed 

 close to the back-bone and ribs; the air entering into them 

 by a communication from the windpipe, passes through, and 

 is conveyed into a number of membraneous cells which lie 

 upon the sides of the pericardium, and communicate with 

 those of the sternum. In some birds these cells are con- 

 tinued down the wings, and extended even to the pinions, 

 thigh-bones, and other parts of the body, which can be HI led 

 and distended with air at the pleasure of the animal. 



The extreme singularity of this almost universal diffusion 

 of air through the bodies of birds, has excited a strong 

 desire to discover the intention of Nature in producing a 

 conformation so extraordinary. The ingenious Mr. Hunter 

 imagined that it might be intended to assist the animal in 

 the act of flying, by increasing its bulk and strength, with- 

 out adding to its weight. This opinion was corroborated by 

 considering, that the feathers of birds, and particularly those 

 of the wings, contain a great quantity of air. In opposition 



