VITAL CAPACITY. 2O/ 



301 pounds avoirdupois. To this must be added about 

 150 Ibs. for the elastic resistance of the lungs themselves, 

 so that the total force to be overcome by the muscles in 

 the act of inspiring 2OO cubic inches of air is more than 

 450 Ibs. 



In tranquil respiration, supposing the amount of breath- 

 ing air [to be twenty cubic inches, the resistance of the 

 walls of the chest would be equal to lifting more than 

 IOO 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 adaj^jn some measure, 

 the quantity of air admitted to the lungs, and to each part 

 of them, according to the supply of blood. Another pur- 

 pose probably served by the muscular fibres of the bron- ; 

 chial 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 

 (Gairdner). 



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 body, Under such a 

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



