RESPIRATION 397 



The costo- vertebral and costo-chondral and the chondro-sternal articula- 

 tions are diarthrodial in character and endow the thoracic walls with a con- 

 siderable degree of mobility. The costo-vertebral joints are two in number, 

 the first being formed by the beveled head of the rib and the bodies of the 

 two adjoining vertebrae; the second, by the tubercle of the rib and the trans- 

 verse process. The costo-chondral and the chondro-sternal articulations, 

 as their names imply, are formed by the ribs, cartilages, and sternum. 



The muscles which complete the formation of the thoracic walls are 

 as follows: the diaphram, the intercostales externi and interni, the levatores 

 costarum, the triangularis sterni, and the infra-costales. 



The diaphragm is the musculo-membranous sheet which closes the. in- 

 ferior opening of the thorax and completely separates its cavity from that of 

 the abdomen. It consists of two muscles which arise from the bodies of 

 the first three or four lumbar vertebrae and neighboring fascia, from the 

 border of the six lower ribs, and from the ensiform cartilage. From this 

 extensive origin the muscle-fibers pass centrally to be inserted into a com- 

 mon tendon. As the direction of the fibers is from below upward and in- 

 ward, the diaphragm is somewhat dome shaped. Its inferior border is for 

 a short distance in contact with the sides of the thorax. 



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. 183 ancl 184). Their fibers, which are arranged 

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

 forward. The point of attachment, 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 attached to the ribs from the tubercle to the anterior extremity of 

 the cartilages. Their fibers, which are also arranged in parallel bundles, 

 are directed from above downward and from before backward (Figs. 183 and 

 184). The portions of the internal intercostals between the cartilages are 

 frequently termed inter car tilaginei. 



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 

 vertebrae 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 im- 

 plies, 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 

 adherent to the surface of the entire lung as far as its root; the outer portion 

 is reflected over the inner wall of the thorax, the superior surface of the dia- 



