THE THORAX. 347 



Insertion. Into the upper border of the eleven lower ribs 

 a little external to the tubercles. Three or four of the 

 lower muscles reach into the cartilages for their insertion. 

 The direction of the fibres is from above downward and 

 forward. 



The Internal. 



Origin. From the inner border of the intercostal groove 

 of the eleven upper ribs and cartilages ; from the sternum 

 around to the angles of the ribs. 



Insertion. The fibres pass downward and backward to 

 the upper surface of the rib and cartilage below. 



Nerve Supply. The intercostal nerves. 



Action. The intercostal muscles act to bring together the 

 ribs between which they are placed. The external inter- 

 costals act upon the ribs alone, the internal upon the ribs 

 and cartilages. Taken together, the action of the intercostal 

 muscles is to elevate the ribs and cartilages. From this 

 action two results follow. 



The chest is enlarged in two directions, transversely and 

 anteroposteriorly. But this is not secured by the move- 

 ment of the ribs upon two axes, as stated by most authori- 

 ties ; such an explanation is not warranted by the construc- 

 tion of the joints which unite the ribs to the spinal column. 

 The reasons for the enlargement are these : First, the ribs 

 move upon an axis passing through their heads and 

 tubercles. This allows the rib to move as if hinged at these 

 points, and the anterior end moves upward and downward. 

 Second, the ribs increase in size from above downward 

 (until the seventh rib is met with). Third, the ribs are not 

 placed horizontally, but slope from behind downward and 

 forward. Fourth, when the intercostal muscles act, they 

 raise the ribs and cartilages into a more nearly horizontal 



