BREATHING 



I6 9 



a joint. When the ribs are raised, they push out the breast- 

 bone in front, and thus the cavity of the chest is enlarged. 

 This enlargement by means of the side walls takes place at 

 the same time that the diaphragm descends, so that the 

 chest is enlarged in all its dimensions (Fig. 104). 



An extra quantity of air then rushes into the lungs and 

 we get an inspiration. 



Immediately following the inspiration, the diaphragm 

 relaxes, and, of 

 course, rises ; and, 

 at the same time, 

 another set of in- 

 tercostal muscles 

 begins to pull the 

 ribs and breastbone 

 down. 



These combined 

 movements dimin- 

 ish the cavity of the 

 chest, and conse- 

 quently the air is 

 driven out. This 

 makes an expiration. 



FIG. 1 06. 



Showing the walls of air sacs with blood capillaries 

 injected. The dark lines are the capillaries 

 magnified about 30 times. The white spaces 

 are air sacs cut across. 



Experiment 52. To 

 show how the size of 

 the chest varies during 

 respiration. Stand 



erect with shoulders well thrown back. While breathing naturally, 

 allow some friend to pass a tape around the body just under the arms, 

 bringing the ends of the tape together across the front of the chest. 

 Take the exact measure (Fig. 104). 



Repeat as before, while taking a long, deep inspiration. Hold the 

 breath and measure as before. Note the difference in measurement. 



