124 RESPIRATION RESPIRATORY MOVEMENTS. 



themselves in expiration, their relations to the walls of the thorax are impor- 

 tant. By virtue of their elasticity, they assist the passive collapse of the 

 chest. When they lose this property to any considerable extent, as in vesic- 

 ular emphysema, they offer a notable resistance to the contraction of the 

 thorax ; so much indeed, that in old cases of this disease the thoracic move- 

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

 tended appearance. 



Little more need be said concerning the passive movements of the tho- 

 racic 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 mus- 

 cles 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 



inferior 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 downward 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 intercostals in elevating the ribs in 

 inspiration. Between the ribs these muscles are directly antagonistic to the 

 external intercostals. They are more nearly at right angles to the ribs, par- 

 ticularly in that portion of the thorax where the obliquity of the ribs is 



