384 



TEXT-BOOK OF PHYSIOLOGY. 



The intercostales externi, eleven in number on each side, occupy the 

 spaces between the ribs to which they are attached from the tubercle to the 

 anterior extremity (Figs. 187 and 188). Their fibers, which are arranged 

 in parallel bundles, are directed from above downward and from behind 



forward. The point of attach- 

 ment, therefore, of any given 

 bundle of fibers to the rib 

 above, lies nearer the vertebral 

 column, nearer the fulcrum, 

 than the point of attachment 

 below. 



The intercostales interni, 

 eleven in number, occupy the 

 spaces between, and are at- 

 tached to the ribs from the 

 tubercle to the anterior extrem- 

 ity of the cartilages. Their 

 fibers, which are also arranged 

 in parallel bundles, are direc- 

 ted from above downward and 

 backward (Figs. 187 and 188). 

 The portions of the internal 

 intercostals between the carti- 

 lages are frequently termed in- 

 tercartilaginei. 



The levatores costarum are 

 twelve in number on either side. 

 They arise from the tips of the 

 transverse processes of the last 

 cervical and the thoracic verte- 

 brae with the exception of the 

 last. From the point of origin 

 the fibers pass ' downward and 

 outward in a diverging manner 

 to be inserted into the ribs 

 between the tubercle and the 

 angle. Their action, as their 

 name implies, is to elevate the 

 posterior portion of the ribs. 

 The triangularis sterni arises from the side of the posterior surface of the 

 lower third of the sternum and is inserted by fleshy slips into the cartilages 

 of the ribs from the second to the sixth. 



From the fact that the inferior opening of the thorax as well as the inter- 

 costal spaces are completely closed by the foregoing muscles, and from the 

 further fact that the superior is closed by fascia except at those points through 

 which pass the trachea, blood-vessels and esophagus, the cavity of the thorax 

 is absolutely air-tight. 



The Pleurae. Each lung is surrounded by a closed invaginated serous 

 sac, the pleura, of which the inner portion is reflected over and is closely 



FIG. 187. SHOWING THE SITUATION, THE POINTS 

 OF ATTACHMENT, AND DIRECTION OF THE INTERCOSTAL 

 MUSCLES, i. The intercostales externi. 2. The inter- 

 costales interni. 3. The intercartilaginei. (Deaver.) 



