RESPIRATION IN MAMMALIA. 



233 



the action of the intercostal muscles, which produces an ap- 

 proximation of the ribs, tends to raise them, and to bring 

 them more at right angles with the spine; the sternum, also, 

 to which the other extremities of the ribs are articulated, is 

 elevated by this motion, and, consequently, removed to a 

 greater distance from the spine; the general result of all these 

 actions being to increase the capacity of the chest. 



Thus, there are two ways in which the cavity of the 

 thorax can be dilated; namely, by the action of the dia- 

 phragm, and by the action of the intercostal muscles. It is 

 only in peculiar exigencies that the whole power of this ap- 

 paratus is called into action; for in ordinary respiration the 



diaphragm is the chief agent employed, and the principal 

 effect of the action of the intercostal muscles is simply to fix 

 the ribs, and thus give greater purchase to the diaphragm. 

 The muscles of the ribs are employed chiefly to give active 

 VOL. II. 30 



