132 HUMAN BIOLOGY 



lungs as completely as possible, then slowly exhale, blowing 

 the breath through the tube. Make sure that only one 

 complete expiration is carried on, and take care that all the 

 expired air is collected in the bottle. 



1. Describe the way the experiment was carried on. 



2. Note the number of quarts occupied by the expired air 



in the bottle, and record this in yovu" notebook. 



3. What do you conclude, therefore, as to the amount of air 



that may be forced out of the two lungs of the individual 

 who performed the experiment? 



4. (Optional.) Ask the pupil who has the highest record of differ^ 



ence in chest measurement before and after inspiration (187) 

 and also the student who has the least diif erence in the two 

 figures, to try the experiment. State whether or not there is a 

 correspondence between chest enlargement and lung capacity. 



190. How air is forced out of the lungs. — As soon as the 

 muscles that cause the upward movement of the ribs and the 

 downward movement of the diaphragm begin to relax, the 

 ribs sink back into their former position, the breastbone 

 is pulled back into place, and the distended wall of the ab- 

 domen presses the organs upward against the diaphragm, 

 which therefore becomes more arched (Fig. 42). In all 

 these ways the walls of the chest cavity close in upon the 

 lungs, and thus help their elastic tissue to force out the air 

 in expiration. Ordinary expiration is thus accomplished 

 without muscular effort. 



IV. Hygiene of the Respiratory Organs 



191. Hygienic habits of breathing. — We have called 

 attention to the admirable provisions in the nose for 

 filtering the air. Air is likewise warmed and moistened by 

 the mucous membrane of the nose. This is necessary, be- 

 cause very cold or very dry air is irritating to the air passages 



