BIRDS. 557 



which contribute still further to diminish its 

 specific gravity : and by means of canals which 

 open into the air passages of the lungs, this air 

 finds a ready outlet when it becomes rarefied by 

 the ascent of the bird into the higher regions of 

 the atmosphere.* 



The conditions in which a bird is placed with 

 regard to the density of the surrounding me- 

 dium, as well as their mode of progression, are 

 so opposite to those of fishes, that we should 

 expect to find great corresponding differences in 

 their conformation. These two classes of verte- 

 brata, accordingly, are remarkably contrasted 



* This air, being contained in the interior of the body, which 

 preserves a very elevated temperature, must be constantly in a 

 state of greater rarefaction than the cooler external air ; a con- 

 dition which must contribute in some slight degree to render the 

 whole body lighter than it would otherwise have been. It ap- 

 pears to me, however, that considerably greater importance has 

 been attached to this circumstance than it really possesses. 

 Many have gone so far as to represent the condition of a bird as 

 approaching to that of a balloon filled with a lighter gas than 

 atmospheric air: and have been lavish in their expressions of 

 admiration at the beauty of the contrivance which thus con- 

 verted a living structure into an aerostatic machine. A little 

 sober consideration will suffice to show that the amount of the 

 supposed advantages resulting to the bird from the diminution 

 of weight, occasioned by the difference of temperature between 

 the air included in its body and the external atmosphere, is per- 

 fectly insignificant. Any one who will take the trouble to calcu- 

 late the real diminution of weight arising from this cause, under 

 the most favourable circumstances, will find that, even in the case 

 of the largest bird, it can never amount to more than a few 

 grains. 



