406 FOODS CIRCULATED AND USED IN THE BODY 



PROBLEM V. WHAT ARE THE MECHANICS OF BREATHING? 



Demonstration 4. To show the mechanics of breathing. 



Pass a glass Y tube through a rubber stopper. Fasten two small toy- 

 balloons to the branches of the tube. Put the stopper in the small 

 end of a bell jar. Adjust the tube so that the balloons hang free 



in the jar. Attach a 

 string to the middle of a 

 piece of sheet rubber. 

 Tie the rubber over the 

 large end of the jar. 



To what structures in 

 our bodies may the bal- 

 loons and rubber sheet be 

 compared? Pull down 

 the string gently. What 

 effect does the lowering 

 of the sheet rubber have 

 on the balloon? Why? 

 Push the rubber into the 

 bell jar to form an arch. 

 What happens to the 

 balloons? Why? Ex- 

 plain how this experi- 

 ment may be compared 

 to breathing? 



Spinal 

 Coitxmn 



lung 

 Cavity 



^diaphrag 



The pleura. The 

 lungs are inclosed in a 

 thin, elastic, membra- 

 nous sac, the pleura. 

 This membrane is com- 

 posed of moisfc tissue. 

 In breathing, when the 

 lungs become larger, the smooth, moist surface of the pleura 

 prevents the friction that otherwise would occur between the 

 lung and the walls of the chest. 



The mechanics of breathing. In every breath there are two 

 -movements, inspiration (taking air in) and expiration (forcing air 

 out). An inspiration is produced by the contraction of muscles 

 between the ribs, together with the contraction of the diaphragm, 

 the muscular wall forming the floor of the chest cavity; this 

 results in pulling the diaphragm down and pulling the ribs up- 

 ward and outward, thus increasing the space within the chest 



