MECHANISM OF RESPIRATION. 205 



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 equalize 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 

 shall contract the capacity of the chest, the pressure in the 

 interior will be increased, and air will be expelled, until 

 the pressures within and without the chest are again 

 equal. In both cases the air passes through the trachea 

 and larynx, whether in entering or leaving the lungs, 

 there being no other communication with the exterior, and 

 the lung, for the reason before mentioned, remains under 

 all the circumstances described, closely in contact with the 

 walls and floor of the chest. To speak of expansion of the 

 chest, is to speak also of expansion of the lung. 



We have now to consider the means by which the chest- 



