O F N A T U R A L II I S T O R Y. 1 1 j 



formation of kings is evident ; for it is in confequence of this 

 ftrudure that they require to breathe lefs frequently than others. 

 Even confidering the matter in this light, it may ftill, in birds, 

 have fome connefl-ion with flying, as tliat motion may eafily be 

 imagined to render frequency of refpiration inconvenient, and a 

 refervoir of air may therefore become fingularly ufeful. Although 

 we are not to confider this ftrudture in birds to be an extenfion of 

 lungs, yet I can eafily conceive this accumulation of air to be of 

 great ufe in refpiration ; for, as we obferved in the viper, that the 

 air, in its paffage to and from thefe cells, muft certainly have a 

 confiderable effed 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 conftrudion of parts. And this opinion will not appear 

 to be ill founded, if we confider, that, both in the bird and the vi- 

 per, the furface of the lungs is fmall in comparifon to what it is 

 in many other animals which have not this extenfion of cavity, — 

 We muft not, however, give up the idea of fuch ftrudure 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 difiufion of air, is carried farther than in the others; 

 and this opinion is ftrengthened, by comparing this ftrudure with 

 the refpiratory organs in the flying infeds, which are compofed of 

 cells diflufed through the whole body ; and thefe are extended 

 even into the head and down the extremities, while there is no 

 fuch ftrudure 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 ftoped or interrupted 

 by the rapidity of their motion through a refitting medium. The 

 refiflence of the air increafes in proportion to the celerity of the 



P 2 motion. 



