6 
INTRODUCTION. 
The structure of birds and their habits of life, are wonder¬ 
fully adapted to the various functions tney 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 ti 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 frcm being 
stopped or interrupted by the rapidity of their motion through 
^ a resisting medium. Were it possible for man to move with ^ 
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