VITAL CAPACITY. 171 



air to be twenty cubic inches, the resistance of the walls of the 

 chest would be equal to lifting more than 100 pounds ; and to 

 this must be added about 70 pounds for the elasticity of the 

 lungs. The elastic force overcome in ordinary inspiration 

 must, therefore, be equal to about 170 pounds. 



It is probable, that in the quiet ordinary respiration, which 

 is performed without consciousness or effort of the will, the 

 only forces engaged are those of the inspiratory muscles, and 

 the elasticity of the walls of the chest and the lungs. It is 

 not known under what circumstances the contractile power 

 which the bronchial tubes possess, by means of their organic 

 muscular fibres, is brought into action. It is possible, as Dr. 

 R. Hall maintained, that it may exist in expiration ; but it is 

 more likely that its chief purpose is to regulate and adapt, in 

 some measure, the quantity of air admitted to the lungs, and 

 to each part of them, according to the supply of blood. 

 Another purpose probably served by the muscular fibres of the 

 bronchial 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 por- 

 tion of lung proceeding from the obstructed tubes (Gairdner). 



The muscular action in the lungs, morbidly excited, is prob- 

 ably the chief cause of the phenomena of spasmodic asthma. 

 It may be demonstrated by galvanizing the lungs shortly after 

 taking them from the body. Under such a stimulus, they 

 contract so as to lift up water placed in a tube introduced into 

 the trachea (C. 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 flame of a candle, he galvanized the pneumo- 

 gastric 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 respi- 

 ratory movements are assisted by the various conditions of the 

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

 gases, the carbonic acid evolved in the air-cells will, inde- 

 pendently 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 bron- 

 chial tubes or iu that external to the body. But for this ten- 

 dency in the oxygen and carbonic acid to mix uniformly, 

 within and without the lungs, the reserve and residual air 



