1 86 Mayow 



rushes into the expanded chest and lungs of the 

 breathing animal and inflates them. 



And indeed the force with which air that has not 

 yet expanded seeks to enlarge its volume, is exactly 

 equal to the pressure of the atmosphere, inasmuch 

 as it depends upon it, and increases or diminishes 

 according as this pressure is greater or less. For the 

 elastic force of the air seems to be due to this, that 

 the air, especially that near the earth, is compressed 

 and its volume diminished by the weight of the 

 superincumbent air ; hence it is that it always strives 

 to expand, just as a fleece, when the force which 

 compressed it is withdrawn, instantly unfolds and 

 expands with a certain motion of restitution. And 

 this can be confirmed by a well enough known experi- 

 ment. 



For if a bladder with most of the air pressed out of 

 it, and tied by a tight ligature round the sphincter^ 

 be placed in a glass from which the air is afterwards 

 exhausted, we shall at once see the bladder swell and 

 ^ become greatly distended, a rather pretty sight by the 

 way. For although a very little air was contained in 

 the bladder, yet when the external air (by the pres- 

 sure of which it was reduced to small volume) is 

 removed, it immediately expands and inflates the 

 bladder, indeed sometimes violently bursts it. And^ 

 Y^n fact, the inflation of the lungs is eff"ected in a not 

 very different way. For as soon as the sides of the 

 thorax (which by compressing the lungs make them 

 shrink) are drawn outwards, the air at the entrance to 

 the lungs is immediately driven into them, whether 

 by atmospheric pressure or in consequence of its own 

 elastic force, and distends them. 



But we may illustrate the inflation of the lungs in 

 this way by yet another example. Let us then 



