104 RESPIRATION 



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 retraction of the chest. When they lose this property to any 

 considerable extent, as in vesicular emphysema, they offer a notable re- 

 sistance to the contraction of the thorax; so much, indeed, that in old 

 cases of this disease the thoracic movements are restricted, and the chest 

 presents a characteristic rounded and distended 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 has 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 : 



MUSCLES OF EXPIRATION 



Ordinary Respiration 



MUSCLE ATTACHMENTS 



Osseous portion of internal intercostals . . Inner borders of the ribs. 



Infracostales Inner surfaces of the ribs. 



Triangularis sterni Ensiform cartilage, lower borders of sternum, 



lower three or four costal cartilages carti- 

 lages of the second, third, fourth and fifth ribs. 



Auxiliaries 



Obliquus externus External surface and inferior borders of eight in- 

 ferior ribs anterior half of the crest of the 

 ileum, Poupart's ligament, linea alba. 



Obliquus internus Outer half of Poupart's ligament, anterior two- 

 thirds of the crest of the ileum, lumbar fascia 

 cartilages of four inferior ribs, linea alba, 

 crest of the pubis, pectineal line. 



Transversalis ..'.. Outer third of Poupart's ligament, anterior two- 

 thirds of the crest of the ileum, lumbar verte- 

 brae, inner surface of cartilages of six inferior 

 ribs crest of the pubis, pectineal line, linea 

 alba. 



Sacro-lumbalis . . Sacrum angles of six inferior ribs. 



Internal Intercostals. The internal intercostals have different uses 

 in different parts of the thorax. They are attached to the inner borders 

 of the ribs and costal cartilages. Between the ribs they are covered 

 by the external intercostals, but between the costal cartilages they are 

 covered simply by aponeurosis. Their direction is from above down- 

 ward and backward, nearly at right angles to the external intercostals. 

 The action of that portion of the internal intercostals situated between 

 the costal cartilages has already been noted. They assist the external 



