BLOOD SYSTEM. 



363 



the thorax expands, air rushes in through the nostrils 

 into the lungs, expanding them and keeping them in con- 

 tact with the ribs. If the thorax contracts, it squeezes 

 out the air through the nostrils. But if there be an 

 opening into the thorax (by wound), then, on expand- 

 ing the thorax, the air rushes in there, gets into the cav- 

 ity instead of into the lungs, and the lungs do not fill. 



Mode of expanding and contracting- the 

 Thorax. — This is done in two ways — viz., by the ribs 

 and by the diaphragm. There are therefore two kinds 

 of respiration — viz., costal or thoracic, and diaphrag- 

 matic or abdominal. 



Costal Respiration. — The ribs do not run straight 

 around the chest horizontally, but slope downward all 

 the way. The simple lifting of the ribs, therefore, 

 increases the whole transverse section of the chest. 

 Now, between the ribs 

 and connecting them there 

 are two sheets of muscu- 

 lar fibers, external and in- 

 ternal. The fibers of the 

 external sheet, ex, run 

 obliquely downward and 

 forward, those of the in- 



■int. 



.blC- 



Fig. 246. — Diagram illustrating action of 

 the intercostal muscles : a a, fiber of 

 exterior sheet when passive ; a a', 

 when contracted, b' b' a fiber of inte- 

 rior sheet when stretched, and b b 

 when contracted. 



Fig. 245. — Showing two ribs 

 and the intercostals be- 

 tween : ex, external, and 

 int. internal sheet. 



terior sheet, int, downward and backward (Fig. 245). 

 The external sheet raises the ribs and expands the chest, 

 the internal sheet pulls down the ribs and contracts the 



