EXPIRATION 103 



est ramifications of the bronchial tubes, which give them great elas- 

 ticity. The thoracic walls are also very elastic, particularly in young 

 persons. After the muscles which increase the capacity of the thorax 

 cease their action, the elasticity of the costal cartilages and the tonicity 

 of the muscles that have been put on the stretch restore the chest to 

 what may be called its passive dimensions. This elasticity is likewise 

 capable of acting as an inspiratory force when the chest has been com- 

 pressed in any way. There are also certain muscles, the action of 

 which is to draw the ribs downward and which, in tranquil respiration, 

 are antagonistic to those which elevate the ribs. Aside from this, 

 many operations, such as speaking, blowing, singing etc., require power- 

 ful, prolonged or complicated acts of expiration, in which many muscles 

 are brought into play. 



Expiration may be considered as depending on two causes : 



1. The passive influence of the elasticity of the lungs and thoracic 

 walls. 



2. The action of certain muscles, which either diminish the trans- 

 verse and antero-posterior diameters of the chest by depressing the ribs 

 and sternum, or the vertical diameter, by pressing the abdominal viscera 

 upward against the diaphragm. 



Influence of the Elasticity of the Pulmonary Structure and Walls of 

 the Chest. It is easy to understand the influence of the elasticity of 

 the pulmonary structure in expiration. From the collapse of the lungs 

 when openings are made in the chest, it is seen that even after the most 

 complete expiration, these organs have a tendency to expel part of their 

 gaseous contents, which can not be fully satisfied until the chest is 

 opened. They remain partially distended, on account of the impossi- 

 bility of retraction of the thoracic walls beyond a certain degree; and by 

 virtue of their elasticity, they exert a suction force on the diaphragm, 

 causing it to form a vaulted arch, or dome, above the level of the lower 

 circumference of the chest. When the lungs are collapsed, the dia- 

 phragm hangs loosely between the abdominal and thoracic cavities. 

 In inspiration and in expiration, then, the relations between the lungs 

 and diaphragm are reversed. In inspiration, the descending diaphragm 

 exerts a suction force on the lungs, drawing them downward ; in expira- 

 tion, the elastic lungs exert a suction force on the diaphragm, drawing 

 it upward. This antagonism is one of the causes of the great power 

 and importance of the diaphragm as an inspiratory muscle. 



The elasticity of the lungs operates chiefly on the diaphragm in 

 reducing the capacity of the chest ; for the walls of the thorax, by rea- 

 son of their own elasticity, have a reaction which succeeds the move- 

 ments produced by the inspiratory muscles. Although this is the main 



