RESPIRATION AND EXCRETION 



333 



diaphragm 



The chest cavity (a) at the time of a full breath ; 

 (6), after an expiration. Explain how the 

 cavity for lungs is made larger. 



Breathing. In every 

 full breath there are two 

 distinct movements, in- 

 spiration (taking air in) 

 and expiration (forcing 

 air out) . In man an in- 

 spiration is produced by 

 the contraction of the 

 muscles between the diaphragm 

 ribs, together with the 

 contraction of the dia- 

 phragm, the muscular 

 wall just below the heart 

 and lungs ; this results 

 in pulling down the dia- 

 phragm and pulling upward and outward of the ribs, thus making 

 the space within the chest cavity larger. The lungs, which lie 



within this cavity, are filled by 

 the air rushing into the larger 

 space thus made. That this 

 cavity is larger than it was at 

 first may be demonstrated by a 

 glance at the accompanying 

 figure. An expiration is simpler 

 than an inspiration, for it re- 

 quires no muscular effort; the 

 muscles relax, the breastbone 

 and ribs sink into place, while 

 the diaphragm returns to its 

 original position. 



A piece of apparatus which illus- 

 trates to a degree the mechanics of 

 breathing may be made as follows : 

 Attach a string to the middle of a 

 piece of sheet rubber. Tie the 

 Apparatus to show the mechanics of rubber over the large end of a bell 

 breathing. jar. Pass a glass Y-tube through a 



