194 RESPIRATION. 



when any bronchial tube is closed or obstructed, the 

 supply of air is lost for all the cells opening into it or its 

 branches. 



Mechanism of Respiration. 



For the proper understanding of the mechanism by 

 which air enters and is expelled from the lungs, the follow- 

 ing facts must be borne in mind : 



The lungs form two distinct hollow bags (communicating 

 with the exterior through the trachea and larynx), and are 

 always closely in contact with the inner surface of the 

 chest-walls, while their lower portions are closely in con- 

 tact with the diaphragm, or muscular partition which 

 separates the chest from the abdomen (figs. 3 1 and 65). The 

 lungs follow all movements of the parts in contact with them ; 

 and for the evident reason that the outer surface of the 

 lung-bag not being exposed directly to atmospheric pres- 

 sure, while the inner surface is so exposed, the pressure 

 from within preserves the lungs in close contact with the 

 parts surrounding them, and obliterates, practically, the 

 pleural space, and must continue to do so, until from some 

 cause or other say from an opening for the admission of 

 air through the chest-walls, the pressure on the outside of 

 the lung equals or exceeds that on the interior. Any such 

 artificial condition of things, however, need not here be 

 considered. 



For the inspiration of air into the lungs it will be evi- 

 dent from the foregoing facts, that all that is necessary is 

 such a movement of the side-walls or floor of the chest, or 

 of both, that the capacity of the interior shall be enlarged. 

 By such increase of capacity there will be of course a 

 diminution of the pressure of the air in the lungs, and a 

 fresh quantity will enter through the larynx and trachea 

 to equalise the pressure on the inside and outside of the 

 chest. For the expiration of air, on the other hand, 

 it is also evident, that, by an opposite movement which 



