110 



RESPIRATION 



latter down when other muscles, especially the diaphragm, tend to raise it 

 during inspiration. The diaphragm is by far the most important of the 



FIG. 63 



-,0 



I 





E' 



Diagram to show the actions of the intercostal muscles: S C, spinal column; D E, sternum; 

 A D, one rib; B E, the next rib; / M, an external intercostal muscle-fiber in its relaxed state; 

 I' M', the same in its contracted condition. Its shortening helps to raise the ribs and advance 

 the sternum into the position A D' E' B. The internal intercostals act on the same principle, 

 their contraction lowering the ribs. 



FIG. 64 



. The intercostal muscles, etc.: A, lateral view; B, rear view; 1, the levatores costarum, short 

 and long; 2, the external intercostals; 3, the internal intercostals seen on removal of the external 

 set. The internal layer is seen to be deficient toward the spine. (Cloquet.) 



muscles of respiration. When it contracts, its dome-shape central portion 

 flattens out somewhat. This increases the contents of the thorax. The 



