INSPIRATION 141 



can contract the bag will remain distended and will not leave 

 the bellows wall, although it will have a constant tendency 

 to do so. It is also apparent that, since the bag exerts a con- 

 tinual compressing effect on its contents, the pressure inside 

 it will be greater than that outside between it and the bellows 

 wall. Under these conditions there will be a constant ten- 

 dency on the part of the bellows to collapse, and some active 

 force will be necessary to expand it ; when it is made to ex- 

 pand the contained bag will expand with it. Suppose 

 the expansion should be stopped at a certain point and the 

 bellows held (to prevent contraction) ; it is obvious that now 

 the pressure inside the bag is greater, while that outside be- 

 tween its walls and those of the bellows is less, than when 

 the expansion began; that is, the bag has become distended 

 more and is exerting a greater compressing effect upon its 

 contents. If now the bellows be simply released, both the 

 bag and the bellows will contract and the former will empty 

 itself so far as the latter will allow; but when the bellows 

 has reached the limit of its contraction the bag also ceases 

 to contract, although it remains in a constant state of ten- 

 sion. If at any time air be admitted to the bellows proper 

 the bag will at once collapse. 



This illustration can be applied to the mechanical princi- 

 ples obtaining in ordinary respiration. The bellows is the 

 air-tight thorax which cannot contract beyond a certain 

 point ; the rubber bag is the elastic lungs under constant ten- 

 sion, communicating by the trachea with the external air 

 and following, or being followed by, the movements of the 

 thorax; the pressure in the bag and between it and the bel- 

 lows wall represents the intrapulmonary and intrathoracic 

 pressures respectively. 



It will be noticed later that this illustration does not go 

 quite far enough to explain a few of the phenomena of ex- 

 piration, but it could very easily be made to do so. 



Inspiration. Any force which expands the thorax aids 

 in inspiration; and any muscles which increase any of the 



