150 ORIGIN, INSERTION, AND USE 



INTERNAL COSTAL REGION. 



DIAPHRAGM. 



The diaphragm is that fleshy and tendinous expansion 

 which divides the cavity of the chest from that of the abdo- 

 men. 



Origin. — It arises by fleshy digitations from the carti- 

 lages of the eighth, and those of all the posterior ribs, with 

 the exception of the two last ; also from the ensiform carti- 

 lage. From these points of attachment, they converge like 

 the radii of a circle, and terminate about midway between 

 the ribs and spine, in a thin expansion of tendon. The 

 crura, or appendages of this muscle, are two in number, and 

 lie by the side of the aorta, which vessel takes its course 

 between them ; the right, much the longer of the two, 

 arises above the artery, from the inferior part of all the 

 bodies of the lumbar vertebrae ; the left, or shorter, has a 

 tendinous origin from the under part of the body of the first 

 lumbar vertebra, and by a separate tendinous slip from that 

 of the second. They unite and decussate each other oppo- 

 site to the seventeenth dorsal vertebra, form a fleshy belly, 

 and this again separates into two portions previously to its 

 insertion, in order to admit of the passage of the oesophagus. 



Insertion. — Into the upper part of the cordiform tendon. 

 About the centre of this tendon, is a perforation for the pas- 

 sage of the vena cava posterior ; so that there are, all together, 

 three openings in the diaphragm. 



Use. — It acts in opposition to the abdominal muscles in 

 the act of respiration. 



STERNO-COSTALIS INTERNUS. 



This muscle lines the sternum inwardly ; it is interposed 

 between it and the cartilages of the ribs and pleura. 



Origin. — By tendinous roots from the upper half of the 

 sternum. 



Insertion. — Into the cartilages of the true ribs. 



Use. — To contract the cavity of the chest. 



