BREATHING.] 



PHYSIOLOGY. 



99 



rib would move on its fulcrum at the backbone, in 

 such a way that the front end of the rib would rise 

 up, and the sternum would be thrust out a little. 

 When you left off pulling, the sternum, which in 

 being thrust forward had been put on the stretch, 



Fig. 14. — View of Four Ribs of the Dog with the Intercostal Muscles. 



a. The bony rib ; b, the cartilage ; tf, the junction of bone and cartilage ; 

 d, unossified, ^, ossified, portions of the sternum, yl. External intercostal 

 muscle. B. Internal intercostal muscle. In the middle interspace, the 

 external intercostal has been removed to show the internal intercostal 

 beneath it. 



would sink back, and the rib would fall down to its 

 previous position. 



Between the ribs are certain muscles called inter- 

 costal muscles (Fig. i). The exact action of these 

 you will learn at some future time. Meanwhile it 



H z 



