22 



GENERAL SCIENCE 



breathing, involving as it does the full use of the diaphragm, 

 is the rhythmic pressure exerted downward upon the stomach 

 and liver. This promotes digestion, and tends to prevent 

 constipation. Over the motions of this muscle which is so 

 largely concerned in breathing a person has but slight and 

 indirect control, making the action in large part involuntary. 

 The muscles of the side walls of the chest are, however, 

 largely under control. By throwing back the shoulders, 



and by causing the muscles be- 

 tween the ribs to contract and 

 pull the ends of the lower ribs 

 apart, the chest cavity can be 

 considerably enlarged at will. 

 An easy erect position, whether 

 sitting or walking, gives larger 

 space for the lungs than where 

 one goes about stooped and 

 with drooping shoulders. Per- 

 sistent efforts to fully inflate 

 the lungs in breathing is one 

 of the ways of maintaining 

 health. It is the unused por- 

 tions of the lungs that are most 

 likely to become diseased. 



The lung capacity in use is about 200 cubic inches (3200 

 cc.). Only about one-eighth of this amount passes in and 

 out at each breath 1 . The reserve supply of air not exhaled 

 at every breath serves to prevent sudden changes of tempera- 



1 The following tabulation exhibits approximately the air volumes con- 

 cerned in breathing: 



Air passing in and out at each breath (Tidal Air) 20-30 cu. in. 



Additional air that can be but seldom is taken in (Comple- 



mental Air) 120 cu. in. 



Air that can be forced out after a full inspiration 150-250 cu. in. 



Air that cannot be driven out (Residual Air) 100 cu. in. 



FIG. 9. Wrong sitting position. 

 The lungs cannot be fully inflated 

 in breathing. 



