OF NATURAL HISTORY. 103 



CHAPTER III, 



Of the Refp'iration of Animals — Air neceffary to the exijlence of alt 

 animated beings — The 'various modifications of the organs employed 

 by Nature for the tranfmijjton of Air into animal bodies. 



IT is foreign to the defign of this chapter to mention the different 

 kinds of air ; to unfold its compofuion j 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 in- 

 animate bodies. For our purpofe, it is fufficienc to obferve, that by 

 air is meant that common elaftic fluid which pervades this globe, 

 and which by its weight, its prefTure in all diredions, and its com- 

 prefTibility, infmuates itfelf into every vacuity, and is neceffary to. 

 the exiftence 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 paffes 

 through the apertures of the mouth and nofe into the trachea or 

 wind-pipe, and thence diredtly into the lungs. This air, by infinu- 

 ating itfelf into the numerous cells of the lungs, necelfarily inflates 

 them, and, when retained for a fecond or two, produces an uneafy 

 fenfation. To remove this difagreeable feeling, the animal inftinc- 

 tively, by the exertion of particular mufcles deftined by Nature for 



that: 



