322 



EESPIEATION 



takes place simultaneously, as does the expansion. The 

 combination of these two movements is responsible for 

 the filling and emptying of the lungs with air. Now, it is 

 easy to see how the return to position of the ribs and 

 diaphragm might squeeze the lungs, and cause them to 

 expel the air; but, as the walls of the chest cavity are not 

 fastened to the walls of the lungs directly, it requires a 

 more detailed study to understand how this outward 

 movement causes the lungs to expand and become filled 



with air. How does the en- 

 largement of the chest cavity 

 cause the limgs to fill with 

 air? 



Causes of inhaling and ex- 

 haling. — ^A simple mechanical 

 device will make clear the 

 relation of the chest enlarge- 

 ment to the inflation of the 

 lungs. (See Ex. LIX.) An 

 ordinary bell jar, open at the 

 top and bottom, will repre- 

 sent ■ the chest cavity. A 

 piece of sheet rubber stretched 

 across the bottom will repre- 

 sent the diaphragm. Finally, 

 a toy rubber balloon tied to a 

 glass tube will be the lung,, 

 and the tube the trachea. 

 To make the comparison com- 

 plete we shall put the balloon into the bell jar with the 

 tube fitted into the stopper. Our apparatus will now 

 look like Figure 151. If we have made our connections 



Handle 



BelfJar 

 ^Balloon 



Rubber 

 Diaphragm 



Fig. 151 — Apparatus to illustrate 

 breathing movements antl their 

 effect upon the lungs. The rubber 

 diaphragm corresponds to the dia- 

 phragm in the body ; the handle to 

 the tendon ; the baWoon to the 

 lungs ; the tube to the trachea ; the 

 bell jar to the walls of the thorax. 

 As the handle is lowered the air 

 flows down the tube and inflates 

 the balloon. 



