MECHANISM OF RESPIKATIOX. 161 



ready supply of fresh air to the central than the peripheral 

 portion of the lungs. The cells of adjacent lobules do not 

 communicate ; and those 1 of the same lobule, or proceeding 

 from the same intercellular passage, do so as a general rule 

 only near angles of bifurcation ; so that, 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 following 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 contact with 

 the diaphragm, or muscular partition which separates the 

 chest from the abdomen (Figs. 31 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 pressure, while the inner sur- 

 face is so exposed, the pressure from within preserves the lungs 

 in close contact with the parts surrounding them, and obliter- 

 ates, 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 evident 

 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 move- 

 ment which shall contract the capacity of the chest, the pres- 

 sure 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 circum- 



