INSPIRATION 141 



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

 wall. Under these conditions there will be a constant tendency 

 on the part of the bellows to collapse, and some active force will 

 be necessary to expand it; when it is made to expand the con- 

 tained bag will expand with it. Suppose the expansion to be 

 stopped at a certain point and the bellows held (to prevent con- 

 traction); it is obvious that now the pressure inside the bag is 

 greater, while that outside between its walls and those of the bel- 

 lows 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 tension. 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 principles 

 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 tension, communicating 

 by the trachea with the external air and following, or being fol- 

 lowed by, the movements of the thorax; the pressure in the bag 

 and between it and the bellows wall represents the mtrapulmo- 

 nary 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 expiration, but 

 it could very easily be made to do so. 



Inspiration. Any force which expands the thorax aids in in- 

 spiration; and any muscles which increase any of the thoracic 

 diameters expand the thorax. The diameters increased are 

 chiefly the (i) vertical and (2) antero-posterior. 



The vertical is increased by descent of the diaphragm, which 

 descent is caused by its contraction, since, owing to the intra- 



