EXPIRATION. 123 



marked, and the scaleni, the serratus posticus superior, and sometimes the 

 sterno-mastoid, are brought into action in ordinary respiration. In the 

 different types of respiration, the action of the muscles engaged in ordinary 

 respiration necessarily presents considerable variations. 



Expiration. The air is expelled from the lungs, in ordinary expiration, 

 by a simple and comparatively passive process. The lungs contain a large 

 number of elastic fibres surrounding the air-cells and the smallest ramifica- 

 tions of the bronchial tubes, which give them great elasticity. 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 which 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 compressed in any way. There are also certain muscles, the 

 action of which is to draw the ribs downward and which, in tranquil respira- 

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

 operations, such as speaking, blowing, singing etc., require powerful, pro- 

 longed or complicated acts of expiration, in which many 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 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 ster- 

 num, or the vertical diameter, by pressing up the abdominal viscera 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 pul- 

 monary structure in expiration. From the collapse of the lungs when. open- 

 ings are made in the chest, it is seen that even after the most complete expi- 

 ration, 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 par- 

 tially distended, on account of 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 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 tho- 

 racic cavities. In inspiration and in expiration, then, the relations between 

 the lungs and diaphragm are reversed. In inspiration, the descending dia- 

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

 expiration, the elastic lungs exert a suction force upon the diaphragm, draw- 

 ing 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 upon the diaphragm in reduc- 

 ing the capacity of the chest ; for the walls of the thorax, by reason of their 

 own elasticity, have a reaction which succeeds the movements produced by 

 the inspiratory muscles. Although this is the main action of the lungs 



