n 



RESPIRATION 



lateral walls of llic (best are thrust out. In this way the chest 

 is increased from side to side. 



An inspiration, then, is caused by the contraction of the 

 diaphragm and of the external intercostal muscles at the same 

 time, whereby the chest is increased in size from above down- 

 wards, from behind forwards, and from side to side. 



Expiration. An expiration takes place when these muscles 

 relax, the diaphragm returns to its more arched form, and the 



Fir,. 57. Four ril)s with intercostal muscles shown. 



ii, til.-- ; /, cartilages ; < , junction of ril>s with cartilages ; d, e, sternum ; A, external 

 int(.-irn>t:il muscles : ill the middle space this muscle has been removed lo show />, 

 the internal intercostal muscle of that space. 



ribs and sternum return to their position of rest. While inspira- 

 tion is taking place, the costal cartilages connecting the ribs to 

 the sternum, as well as other parts of the thoracic walls, being 

 elastic, bend, give or are put on the stretch, and when the 

 muscular effort, which causes the inspiration, ceases, the 

 elasticity leads to a return of the chest walls to their original 

 condition. The position of the ribs, sternum, and diaphragm 



