MUSCLES OF INSPIRATION. 129 



the cavity which they before had completely filled. 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 tonicitv 

 of the muscles which have been put on the stretch will restore the chest to what we may 

 call 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 respiration, 

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

 as speaking, 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, as follows : 



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 sternum, or the vertical di- 

 ameter, by pressing up the abdominal viscera behind 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 ex- 

 piration. 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 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 downward ; in 

 expiration, the elastic lungs exert a suction force upon the diaphragm, drawing it up- 

 ward. This antagonism is one of the causes of the great power of the diaphragm as an 

 inspiratory muscle. 



The elasticity of the lungs operates chiefly upon the diaphragm in reducing the capa- 

 city of the chest; for the walls of the thorax, by virtue of their own elasticity, have a 

 reaction which succeeds the movements produced by the inspiratory muscles. A simple 

 experiment, which we have often performed in public demonstrations, illustrates the 

 expiratory influence of the elasticity of the lungs. If, in an animal just killed, we 

 open the abdomen, seize hold of the vena cava as it passes through the diaphragm, and 

 make traction, we imitate the action of this muscle sufficiently to produce at times an 

 audible inspiration ; on loosing our hold, we have expiration, as it is in a measure accom- 

 plished in natural respiration, by virtue of the resiliency of the lungs, carrying the dia- 

 phragm up into the thorax. Although this is the main action of the lungs themselves 

 in expiration, their relations to the walls of the thorax are important. By virtue of 

 their elasticity, they assist the passive collapse of the chest. When they lose this prop- 

 erty to any considerable extent, as in vesicular emphysema, they offer a notable resistance 

 to the contraction of the thorax ; so much, indeed, that in old cases of this disease the 

 movements are much restricted, and the chest presents a characteristic rounded and dis- 

 tended appearance. 



Little more need be said concerning the passive movements of the thoracic walls. 

 When the action of the inspiratory muscle ceases, the ribs regain their oblique direction, 

 the intercostal spaces are narrowed, and the sternum, if it have been elevated and drawn 

 forward, falls back to its place simply by virtue of the elasticity of the parts. 



Action of Muscles in Expiration. The following are the principal muscles concerned 

 in expiration : 

 9 



