86 



THE VITAL PROCESSES 



this account pass into the lungs. As the thorax is 

 enlarged the air in the lungs expands, and there is pro- 

 duced within them a place of slightly less air pressure 

 than that of the atmosphere on the outside of the body. 

 This difference causes the air to flow into the lungs. 



When the thorax is diminished in size, the air within the 

 lungs is slightly compressed. This causes it to become 

 denser and to exert on this account a pressure slightly 

 greater than that of the atmosphere on the outside. The 

 air now flows out until the equality of the 

 pressure is again restored. Thus the thorax, 

 by making the pressure within the lungs first 

 slightly less and then slightly greater than 

 the atmospheric pressure, causes 

 the air to move into and out 

 of the lungs. 



Breathing is well illustrated by 

 means of the common hand bellows, 

 its action being similar to that of the 

 thorax. It will be observed that when 

 the sides are spread apart air flows into 

 B the bellows. When they are pressed 

 FIG. 41. -Diagram illustrat- together the air flows out. If an air- 

 ing the bellows principle in ti S ht sack were hun g ln the bellows 

 breathing. A. The human bel- with its mouth attached to the project- 

 lows. B. The hand bellows, ing tube, the arrangement would 

 Compare part for part. resemble closely the general plan of 



the breathing organs (Fig. 41). One 



respect, however, in which the bellows differs from the thorax should 

 be noted. The thorax is never sufficiently compressed to drive out 

 all the air. Air is always present in the lungs. This keeps them 

 more or less distended and pressed against the thoracic walls. 



How the Thoracic Space is Varied. One means of vary- 

 ing the size of the thoracic cavity is through the move- 

 ments of the ribs and their resultant effect upon the walls 



