34 
Dr. Carson on the 
surface well secured to the part of the chest at which it 
passes, but affording, in consequence of its cartilaginous and 
incompressible structure, an open passage into the interior sub- 
stance of the lungs, and conceive that no substance should 
exist between the exterior surface of the lungs, and the in- 
ternal surface of the chest. The pressure of the atmosphere, 
jt is evident will, in these circumstances, necessarily bring the 
external surface of the lungs into contact with the whole of 
the interior surface of the chest ; but as the walls of the chest 
are strong and incompressible, and as the lungs are pliant 
and dilatable, the lungs are necessarily expanded to the di- 
mensions of the chest, which remain nearly the same. But 
the lungs, though dilatable, are powerfully elastic ; and, when 
distended to the dimensions which they are thus forced to 
occupy, their substance is stretched far beyond its natural 
condition. As soon, however, as the full weight of the atmo- 
sphere shall be allowed to ponderate upon the external surface 
of the lungs, and an equal pressure shall be sustained by 
their exterior surface and the interior surface of the air vessels, 
of which their substance is composed, a condition which will 
arise as soon as the air shall have obtained a free passage 
through the walls of the chest to the external surface of the 
lungs ; these organs will then shrink into the dimensions 
which are prescribed to them by the nature of their structure. 
To ascertain, then, the extent^of the power required to distend 
the lungs to the dimensions which they occupy in the sound 
system, or, in other words, the extent of that force by which 
the walls of the chest are pressed, or, in popular language, 
sucked inwards, in consequence of the resilience of the lungs, 
is the object of the following experiments. 
