248 ESSENTIALS OF PHYSIOLOGY. 



At the outset of the experiment the air in the bottle is at the same 

 pressure as that of the atmosphere, and the bag is collapsed. If a little 

 air is sucked out of the bottle, the pressure falls, and, since the pres- 

 sure within the rubber bag remains unaltered, a difference of pressure 

 is set up on its inner and outer surfaces. The bag expands, air being 

 sucked into it along the glass tube to fill the extra space thus provided, 

 until the pressure within it and outside it becomes nearly equal ; but 

 the pressure outside the bag is finally a little less than atmospheric 

 pressure, because a part of the pressure in the bag is used up in 

 overcoming the tendency of its - stretched elastic wall to collapse. 

 When more air is sucked out of the bottle the rubber bag expands 

 still further, but the pressure in the bottle remains negative, that 

 is, less than atmospheric pressure. When the bottle is opened, 

 the pressure on each side of the bag becomes the same, and it 

 collapses. 



In the body the lungs take the place of the bag, the bottle is 

 represented by the chest, and the changes in the pressure on the outer 

 surface of the lungs are brought about by alteration in the size of the 

 chest cavity. If a small tube, connected with a manometer, is passed 

 through the chest wall of an animal into the pleural cavity, the 

 pressure within the chest is seen to be lower than that of the atmo- 

 sphere ; at the end of expiration the difference is usually about 6 mm. 

 Hg. When the chest enlarges during inspiration, the pressure on 

 the outer surface of the lungs diminishes, and, as the pressure within 

 them remains unchanged, they expand still further. Owing to the greater 

 force required to bring about this additional expansion of the lungs, the 

 pressure in the pleural cavity is further diminished, and amounts on an 

 average to 730 mm. Hg. The negative pressure in the pleural cavity 

 thus varies from -6mm. Hg during expiration to -30 mm. Hg or 

 more during inspiration, and represents the pressure required to over- 

 come the tendency of the expanded lungs to collapse by virtue of their 

 elasticity. When the lungs expand further during inspiration, air rushes 

 in to fill the additional space ; during expiration air is expelled. The 

 expansion of the lungs during inspiration is due almost entirely to 

 the enlargement of the infundibula and atria, and the portions of the 

 lungs which expand most are those in contact with the diaphragm and 

 ribs ; the dorsal and mediastinal surfaces and the apex are much less 

 expansile. 



If the chest is opened, either during life or after death, the pressure 

 on both the outer and the inner surfaces of the lungs is that of the 

 atmosphere, and owing to their elasticity the lungs collapse. In the 

 condition known as emphysema, the elastic tissue of the lungs atrophies 



