16 A COURSE ON ZOOLOGY. 



of air are provided for by particular movements of 

 inspiration and expiration. Movements of inspiration 

 are those by which air is drawn into the lungs ; move- 

 ments of expiration are those by which it is expelled. 

 The mechanism by which both these movements are pro- 



m.. 



STBUCTURE OF THE CHEST. The left side of the chest shows only the bony 

 parts ; cv, vertebral column ; a, clavicle ; c, c, ribs ; st, sternum ; m, muscles 

 that raise the upper ribs ; mi, intercostal muscles ; d, the diaphragm. 



duced is exceedingly simple, and its action is much like 

 that of a bellows. 



Inspiration. When the chest expands that is, when 

 the lifting of the ribs and the breast-bone (sternum) and 

 the lowering of the diaphragm increase the capacity 

 of the thorax, and, as a consequence, the rarefaction of 

 the gas which this cavity already contains equilibrium 

 must be re-established between the external and the 



