EXPIRATION. 385 



rounded and distended appearance. In some of these cases 

 the elasticity of the lungs is so far lost, that when the chest 

 is opened after death, they are actually protruded, instead of 

 collapsed. 1 



Little more need be said concerning the passive move- 

 ments of the thoracic walls. When the action of the inspi- 

 ratory muscles 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 

 by the simple elasticity of the parts. 



Action of Muscles in Expiration. The following are the 

 principal muscles concerned in expiration : 



Muscles of Expiration. 



ORDINARY RESPIRATION. 



Muscle. Attachments. 



Osseous portion of Internal Intercostals . .Inner borders of the ribs. 



Inira-costales Inner surfaces of the ribs. 



Triangularis Sterni Ensiform cartilage, lower borders of 



sternum, lower three or four costal 



cartilages cartilages of the second, 



third, fourth, and fifth ribs. 



1 In old cases of emphysema, the chest generally becomes rounded and dis- 

 tended, presenting constantly the appearance which it has in forced inspiration. 

 This is explained in the following way : Emphysema is generally preceded and 

 accompanied by a difficulty in respiration, from some cause which is more or less 

 constant. This gives rise to frequent violent movements of inspiration, when the 

 lungs and chest are distended to their utmost capacity. In this condition, expi- 

 ration is difficult, and the chest collapses but imperfectly. Gradually, as the per- 

 manent dilatation of the air-cells gains ground, the lungs lose their elasticity, and 

 offer considerable resistance to the collapse of the thoracic walls. But difficult 

 breathing, and consequent violent elevation of the ribs, becomes more and more 

 frequent ; the chest is constantly dilated, the lungs following, of course, but refus- 

 ing to collapse in expiration, until the chest becomes permanently distended. In 

 this condition, the lungs press downward, as well as laterally, and the movements 

 of the diaphragm are considerably restricted. 

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