Chap, xxiii.] ATMOSPHERIC PRESSURE. 283 



no communication with the outside air. Suspended in it 

 are the two lungs, which may be considered as two sacs 

 communicating by means of the bronchial tubes and 

 trachea with the external air, there being no connec- 

 tion between the cavities of the sacs and that of the 

 thorax. In what may be called the normal position, 

 the cavity of the chest is completely filled with the 

 lungs, heart, and other thoracic organs ; and there is 

 equilibrium. The walls of the lungs are thus sub- 

 jected to two forces ; one, that of the atmosphere, from 

 without ; the other from the cavity of the thorax, from 

 within ; two equal and opposite forces, that is. By 

 the descent of the muscular floor of the chest (the 

 diaphragm), and by the raising and rotation of the ribs, 

 the extent of the cavity is increased, the thoracic 

 organs are no longer sufficient to fill the enlarged 

 thoracic chamber, and there is thus a tendency to 

 create a void space. The walls of the lungs will no 

 longer be in equilibrium by two equal and opposite 

 forces, for the force acting from the cavity outwards is 

 diminished. Consequently the atmospheric pressure 

 gains the mastery and distends the lungs, till their in- 

 crease in size corresponds to the increase of thoracic 

 space, when equilibrium is again restored. Thus, in- 

 spiration is effected. But the increased size of the 

 chamber has been produced by muscular effort, and as 

 soon as that effort is over the elastic reaction of the 

 thoracic walls, etc., comes into play ; the diaphragm 

 ascends, the ribs proceed to assume their former posi- 

 tion. The play of these forces, all tending to reduce 

 the size of the chest cavity, is too much for the 

 atmospheric pressure. The state of affairs is thus 

 reversed, for the greater force is now acting on the 

 wall of the lungs from within outwards. The 

 diminishing size of the chest cavity, aided by the 

 elasticity of the lung tissue itself, reduces the volume 

 of the lung, air is thus expelled, and the act of 



