EXPIRATION. 383 



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 speak- 

 ing, blowing, singing, etc., require powerful, prolonged, or 

 complicated acts of expiration, in which numerous muscles 

 are brought into play. 



Expiration may be considered as depending upon two 

 causes : 



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

 the thoracic walls. 



2. The action of certain muscles, which either diminish 

 the transverse and antero-posterior diameters of the chest by 

 depressing the ribs and sternum, or the vertical diameter by 

 pressing up the abdominal viscera behind the diaphragm. 



Influence of ike Elasticity of the Pulmonary Structure 

 and Walls of the Chest. It is easy to understand the in- 

 fluence of the elasticity of the pulmonary structure in expi- 

 ration. 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 cannot be fully satisfied until the 

 chest is opened. They remain partially distended, from the 

 impossibility of collapse of the thoracic walls beyond a certain 

 point ; and by virtue of their elasticity, they exert a suction 

 force upon the floor of the thorax, 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 diaphragm 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 down ; in expiration, the elastic 

 lungs exert a suction force upon the diaphragm drawing it 

 up. This antagonism is one of the causes of the great power 



