Chap, xxni.] MECHANISM OF RESPIRATION. 285 



indicated by the left-hand figure the walls of the 

 indiarubber bags are pressed 011 from without by the 

 atmospheric pressure, and from within by the pressure 

 in the cavity of the flask. Those two forces are in 

 equilibrium, as indicated by the level of mercury in 

 the two limbs of the manometer, and the bags are 

 collapsed. Now let the leather bottom be pulled 

 down by the knob 5, the air in the flask is at once 

 rarefied to fill the increased space ; pressure is, there- 

 fore, lowered, as indicated by the rise of mercury in 

 the limb of the manometer next to the flask, and by 

 the forcing inwards of- the indiarubber part of the 

 opposite wall. But this diminution of pressure does 

 not continue, for the atmospheric pressure being 

 constant, and opposed by a diminished resistance, 

 distends the indiarubber bags. As they distend the 

 increased space gets occupied by their increased 

 volume, and the diminution of pressure gets less and 

 less, as indicated by the fall of the mercury towards 

 its former level. When the bags are sufficiently dis- 

 tended equilibrium is re-established, the mercury is 

 again equal in both limbs, and the indiarubber part of 

 the wall is no longer pressed inwards. If now the 

 leather bottom be forced upwards, a rapid rise of 

 mercury in the off-limb of the manometer, and a 

 bulging outwards of the indiarubber wall, indicate 

 increase of pressure in the cavity of the flask. But 

 at once the indiarubber bags, pressed upon, become 

 diminished in size, and expel the air they contain. 

 Thus the increase of pressure is no more constant 

 than was the decrease. As the bags diminish in 

 volume the mercury falls in the ofi-limb, till, when they 

 have been restored to their former size, the level is 

 again what it was at first. Thus alternate distension 

 and collapse of the indiarubber bags can be produced 

 by variations of the pressure in the cavity of the 

 flask, just as the alternate distension and diminution 



