AS TO SOUNDNESS. 151 



is attempted when the cavity of the chest is expanding in 

 all directions, by the receding of the diaphragm, and by 

 the ribs being pulled outwa?'ds and forwards by the in- 

 tercostal muscles. 



NoTV let us stop to enquire why this air rushing into 

 this would-be vacuum does not do so with violence 

 and concussion as when it rushes into the mouth and 

 cavity of a fired cannon ? Because it meets with a 

 pressure or resistance nearly equal to its own, namely, the 

 elastic force of the elastic tissue (yellow elastic tissue) 

 which pervades the whole lungs. It has to stretch this 

 concourse of elastic tissue. If we take a piece of elastic 

 and grasp each end of it, we find that the more we stretch 

 it the more it pulls and trys to resume its unstretched 

 length. So it is with the lungs. The air stretches them 

 out by its even pressure in all directions, thjen a limit is 

 put to this by the walls of the chest, and the elastic force 

 in the lung, which was so able to ofter such steady resist- 

 ance to the ingress of air, gets supplemented by two very 

 trivial forces which, together, amount to very little, and so 

 the air is expelled from the chest. These forces are the 

 pressure of the residual gas in the intestines which has 

 been compressed by the receding diaphragm., and a less 

 force still, namely, the straightening of the costal carti- 

 lages, which have been be)it when the ribs were moved 

 outwards and forwards. These two supplementary forces 

 being of the most trivial character, it follows that the 

 elastic force of the lung tissue is of itself nearly equal to 

 the pressure of the atmosphere. 



