OF NATURAL HISTORY. 123 



not fo liberally fupplied v/ith thefe cells as the oflricli •, and 

 that the bat had no fach peculiarity of ftrudure. With re- 

 gard to the oftrich, though it is not intended to fly, it runs 

 with amazing rapidity^ and, confequently, requires fimilar 

 refources of air. 



He next conje(Shired, from analogy, that the air-cells in 

 birds ought to be coniidered as an appendage to the lungs ; 

 becaufe in the fnake, viper, and feveral other amphibious 

 animals, the lungs are continued, in the form of two bags, 

 through the whole abdomen, the upper part of which can on- 

 ly perform the cifice of refpiration with any degree of efFe£l ; 

 becaufe the lower part has comparatively few air-veflels. 



* The air,^ fays Mr. Hunter, < mufl pafs through this upper 

 « part before it gets to the lower in infpiration, and mufk alfo 

 ' repafs in expiration ; fo that the refpiratory furface has 

 « more air applied to it than what the lungs of themfelves 



< could contain. There is, in fact, a great fimilarity between 



< birds and that clafs of animals called amphihicus ; and, al- 



< though a bird and a Ihake are not the fame in the conilruc- 



< tion of the refpiratory organs, yet the circumfiance of the 



< air palling in both beyond the lungs, into the cavity of the 



< abdomen, naturally leads us to fuppofe, that a ftrudiure fo 

 <limilar is defigned in each to anfwer a fimilar purpofe. 

 « This analogy is ftill farther fupported by the lungs in both 



< confiiling of large cells. Now, in amphibious animals, the 



< ufe of fuch a conformation of lungs is evident j for it is in 

 « confequence of this fl:ru6lure that they require to breathe 

 « lefs frequently than others. Even confidering the matter 



* in this light, it may ftili, in birds, have fome conne^ftion 



< with flying, as that motion may eafily be imagined to ren- 

 « der frequency of refpiration inconvenient, and a refervoir 



< of air may therefore become Angularly ufeful. Although 

 « we are not to confider this flirudlure in birds to be an exten- 



< iion of lungs, yet I can eaiily conceive this accumulation of 



