6 INTRODUCTION. 



The structure of birds and their habits of life, are wonder- 

 fully adapted to the various functions they are destined to 

 perform. The pointed beak, the long and pliant neck, the 

 gently-swelling shoulder, the expansive wings, the tapering 

 tail, the light and bony feet, 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 dis- 

 posed 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, while the tail serves as a rudder to direct them to the 

 different objects of their pursuit. 



The internal structure of birds is no less wisely adapted to 

 the same purposes. Their lungs have several openings, com- 

 municating with corresponding air bags, or cells, which fill 

 the whole cavity of the body from tl e neck downwards, and 

 into which the air passes and re-pa&es, in the process of 

 breathing. This is not all ; their very bones are hollowed out 

 with the design of receiving air from the lungs, from which 

 air pipes are conveyed to the most solid parts of the body, and 

 even into the quills and plumelets of the feathers which are 

 hollow or spongy for its reception. As all these hollow parts, 

 as well as the cells, are only open on the side communicating 

 with the lungs, the bird requires only to take in a full breath 

 to fill and distend its whole body with air, which, in conse- 

 quence of the considerable heat of its body, is rendered much 

 lighter than the air of the atmosphere. By forcing this air out 

 of the body again, the weight becomes so much increased, that 

 birds of a large size can dart down from great heights in the 

 air with astonishing rapidity. 



This almost universal diffusion of air in the bodies of birds 

 is of infinite use to them, not only in these long and laborious 

 flights, but likewise in preventing their respiration from being 

 stopped or interrupted by the rapidity of their motion through 

 a resisting medium. Were it possible for man to move with 



as* 1 - ' 48® 



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