INTRODUCTION. 
XIX 
adult bird, many of the larger cylindrical bones have scarcely any marrow, 
but their hollow recesses communicate with the lungs, through the medium 
of the small membranous air cells, before described. Nature, in this 
instance, seems to have operated in a similar manner with respect to birds, 
as with fishes, to facilitate their ascent and descent, in the different 
elements in which they move. We know, that by the dilatation of the air- 
bladder, fishes become specifically lighter than water, and are buoyant 
in it; and it is, perhaps, partly from a similar exertion, that birds float with 
such ease in the light atmosphere, without apparent use of their mechanical 
powers. In the conformation of those birds, that take the longest and 
most extended flights, the air-cells are either proportionally larger, and so 
elastic as to be capable of great expansion, or more generally diflused ; 
and the air, thus retained, is not merely serviceable in rendering them 
buoyant, but at the same time counteracts the resistance of the atmosphere, 
in the rapidity of their flight, and prevents their respiration from being 
impeded by that resistance. Nor here, perhaps, do its uses end. The air, 
in these internal reservoirs, increases, most probably, the strength of the 
voice, propels again into the lungs that which is received by inspiration, assists 
the evacuation of the foeces, and the expulsion of the egg. ‘® The liver, in 
I have frequently inflated these air-cells from the mouth of the bird, and they expand to 
a truly surprizing degree. ^ In the Osprey, one of them, when extended, measured nearly three 
inches in length, and two in depth. 
To descant upon the various uses of the different parts of the economy of birds, on their 
relative proportions and concordance, on the admirable contrivance to adapt each part to its 
intended use, would far exceed the bounds which we have set to our Introduction j and in which 
we propose to give merely a cursory and short description of the feathered race. The few 
particulars which we have now given of their internal anatomy, are only intended as a slight 
introduction to a more 'general essay on their internal and external formation, which we shall 
probably at a future period attempt. The reader will forgive one observation, which demonstrates, 
that so trivial a thing as a feather, discovers the hand of the Deity. The outer web, for instance, 
of a quill-feather, is narrow, the inner one broad. Were but an inversion of this apparently simple 
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