isi. 



THE PHILOSOPHY 



* air to be of great ule in refpiratidn ; for, as we obferved irt 



* the viper, that the air, in its pafTage to and from thefe cells, 

 « muft certainly have a conliderable efFe£l upon the blood in 



< the lungs, by allowing a much greater quantity of air to 



< pafs in a given time, than if there was no fuch conftru(5tioii' 



< of parts. And this opinion will not appear to be ill found- 



< ed, if we confider, that, both in the bird and the viper, the 



< furface of the lungs is fmall in comparifon to what it is in 

 ' many other animals which have not this extenfion of cavi- 



* ty. We muft not, however, give up the idea of fuch ftruc- 



< ture being of ufe in flying ; for I believe we may fet it down 



< as a general rule, that, in the birds of longeft and higheft 



< flight, as eagles, this extenfion, or difFufion of air, is carried 



< farther than in the others ; and this opinion is ftrengthen- 



* ed, by comparing this ftru£lure with the refpiratory or- 



< gans in the flying infects, which are compofed of cells dif- 



< fufed through the whole body -, and thefe are extended 



* even into the head and down the extremities, while there 



< is no fuch ftrufture in thofe that do not fly, as the fpider,'&c. 



Though Mr. Hunter's modefty has not permitted him to 

 draw his conclufion in a pofitive manner, he feems to have 

 proved decidedly, that one ufe of the general diffufion of air 

 through the bodies of birds is to prevent their refpiration 

 from being flopped or interrupted by the rapidity of their mo- 

 tion through a refifting medium. The refiftance of the air 

 increafes in proportion to the celerity of the motion. Were 

 it poflible for man to move with a fwiftnefs equal to that of 

 a fwallow, the refiftance of the air, as he is not provided with 

 internal refervoirs fimilar to thofe of birds, would foon fuffb- 

 cate him. Neither does the difiiculty he mentions, with re- 

 gard to the ftrudure of the oftrich, feem to contradict his 

 theory •, for though, as formerly remarked, the oftrich does 

 not fly, he runs with aftonifhing rapidity. 



