112 THE PHIL0S0PH¥ 



CHAPTER III. 



Of the Refpiration of Animals — Air necejjary to the exiflence of 

 all animated beings — The various modifications of the organs 

 employed by Nature for the tranfmiffion of Air into animal 

 bodies, 



IT is foreign to the defign of this chapter to 

 mention the different kinds of air •, to unfold its compofi- 

 tion ; or to recapitulate the innumerable benefits derived 

 from it in the animal and vegetable kingdoms, in the arts of 

 life, and in the texture and cohefion of inanimate bodies, 

 For our purpofe, it is fufficient to obferve, that by air is 

 meant that common elaftic fluid which pervades this globe, 

 and which by its weight, its prelTure in all directions, and its 

 compreflibility, infinuates itf^lf into every vacuity, and is 

 neceflary to the exiflence of every animal and vegetable 

 being. 



In man, and the larger land animals, air is taken into the 

 body by the lungs. When an animal infpires, the external 

 air pafTes through the apertures of the mouth and nofe into 

 the trachea or wind-pipe, and thence directly into the lungs. 

 This air, by inlinuating itfelf into the numerous cells of the 

 lungs, necefTarily inflates them, and, when retained for a 

 fecond or two, produces an uneafy fenfation. To remove 

 this difagreeable feeling, the animal inftin£tively, by the ex- 

 ertion of particular mufcles deflined by Nature for that 

 purpofe, forces out the air, and thus removes the offending 

 caufe. The lungs, after the air is thrown out, inflead of be- 

 ing inflated, coUapfe ; and, if a frefla fupply is not foon tak- 

 en in, a fimilar uneafy fenfation is felt, which obliges the 

 animal again to infpire. This alternate reception and rejec- 

 tion of air goes on during the life of the animal, and is dif- 



