On Respiration 185 



beyond doubt), it follows that the air, passed through 

 the nostrils and the trachea, up to the bronchia or 

 gates of the lungs, presses against the lungs from 

 within and seeks an entrance into them. Hence it is 

 that when the inner sides of the thorax (which by 

 compressing the lungs from without were resisting 

 the pressure of this air) are drawn outwards by 

 muscles whose function it is to dilate the chest, and 

 the space in the thorax is enlarged, the air which i?^ 

 nearest the bronchial inlets, now that every obstacle 

 is removed, rushes under the full pressure of the 



atmosphere into the cavities of the lungs, and by 



inflating them occupies and fills the space of the 

 expanded chest. 



The structure of the lungs is adapted for their 

 inflation as thus described, for their substance is 

 composed, as the eminent Dr Malpighi has noted, of 

 very fine membranes, which form an almost infinite 

 number of spherical vesicles whose mutual connection 

 is such that there is easy access from the trachea to 

 those nearest to it, and from these again to others. 

 Consequently when these vesicles are inflated by an 

 inrush of air, the whole substance of the lungs must 

 necessarily expand. 



Nor is it only the pressure of the atmosphere, but 

 also the elastic force of the air by which it tends to 

 expand indefinitely, that serves to inflate the lungs 

 and cause inspiration, as takes place when the air, on 

 the nostrils being closed, is drawn into the lungs (in 

 the manner already described) from a glass placed in 

 the mouth, and also when a small animal breathes in 

 a glass that has been completely closed. For though 

 the weight of the atmosphere does not, owing to the 

 interposition of the glass, press or impel the air which 

 it contains, still the air, in virtue of its elastic force. 



