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 outwards and forwards by the in- 
‘tercostal muscles. 
Now 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 zearly egual to its own, namely, the 
elastic force of the elastic tissue (yellow elastic tissu 
which pervades the whole lungs. It has to stretch this 
concourse of elastic tissue. If we take a piece of elas, 
and grasp each end of it, we find that the more we str¢tch 
it the more it pulls and trys to resume its unstretdhed 
length. So it is with the lungs. The air stretches them 
out by its even pressure in all directions, then a limit is 
put to this by the walls of the chest, and the elastic force 
in the lung, which was so able to offer 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 dent 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. 
