208 RESPIRATION. 



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

 Yolkmann 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 w r hen the small end was turned to the flame of a 

 candle, he galvanised 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 uniform!} 7 , within and without 

 the lungs, the resen-e 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 

 is assisted by the different temperature of the air 

 within and without the lungs ; and by the action of the 

 cilia on the mucous membrane of the bronchial tubes, 

 the continual vibrations of which may serve to prevent 

 the adhesion of the air to the moist surface of the mem- 

 brane. 



Movement of Blood in the Respiratory Organs. 



To be exposed to the air thus alternately moved into and 

 out of the air-cells and minute bronchial tubes, the blood is 

 propelled from the right ventricle through the pulmonary 



