218 RESPIRATION. 



ehial tubes is that of contracting upon and gradually ex- 

 pelling collections of mucus, which may have accumulated 

 within the tubes, and cannot be ejected by forced expiratory 

 efforts, owing to collapse or other morbid conditions of 

 the portion of lung proceeding from the obstructed tubes 

 (Grairdner). 



The muscular action in the lungs, morbidly excited, is 

 probably the chief cause of the phenomena of spasmodic 

 asthma. It may be demonstrated by galvanizing the lungs 

 shortly after taking them from the ftody. Under such a 

 stimulus, they contract so as to lift up water placed in a 

 tube introduced into the trachea (0. J. B. Williams) ; and 

 Volkmann has shown that they may be made to contract by 

 stimulating their nerves. He tied a glass tube, drawn 

 fine at one end, into the trachea of a beheaded animal ; ' 

 and when the small end was turned to the name of a 

 candle, he galvanized the pneumogastric trunk. Each 

 time he did so the flame was blown, and once it was blown 

 out. 



The changes of the air in the lungs effected by these 

 respiratory movements are assisted by the various con- 

 ditions of the air itself. According to the law observed in 

 the diffusion of gases, the carbonic acid evolved in the air- 

 cells will, independently of any respiratory movement, 

 tend to leave the lungs, by diffusing itself into the external 

 air, where it exists in less proportion ; and according to 

 the same law, the oxygen of the atmospheric air will tend 

 of itself towards the air-cells in which its proportion is 

 less than it is in the air in the bronchial tubes or in that 

 external to the body. But for this tendency in the oxygen 

 and carbonic acid to mix uniformly, within and without 

 the lungs, the reserve and residual air would, probably, 

 be very injuriously charged with carbonic acid; for 

 the respiratory movements alone are not enough to empty 

 the air-cells, and perhaps expel only the air which lies 

 in the larger bronchial tubes. Probably also the change 



