140 



ELEMENTARY PHYSIOLOGY 



Two sets of muscles, called intercostals, pass between 

 the successive pairs of ribs on each side. The outer set, 

 called external intercostals (Fii,'. 45, A), run from 

 the rib above, obli([uely downwards and forwards, to the 



'N, ^. 



Fio. 45. — View of Four Ribs of the Dog with the Intercostal 



Ml'SCLES. 



a, the bony rib ; 6, the cartilage ; c, the junction of bone and cartilage ; 

 d, uno8sified, (, ossified, portion.s of the stcruum. A, external intercostal 

 muscle ; B, internal intercostal muscle. In the middle interspace, the 

 external intercostal has been removed to show the internal intercostal 

 beneath it. 



rib below. The other set, internal intercostals (Fig. 

 45, B), cross these in direction, passing from the rib 

 above, downwards and backwards, to the rib below. 



The action of these muscles is somewhat puzzling at 

 first, but is readily understood if the fact that when a 



