309 
tion. Hence, then, we conclude, that the nitrogen 
gas, obtained in these experiments, is derived from 
the residual air existing in the cells of the lungs, and 
that its displacement is occasioned by the mechanical 
substitution of another gas. 
608. Abiding, however, by this conclusion, and 
yielding, as we are disposed to do, assent to the ac- 
curacy of the results which these experiments indi- 
cate, we lay ourselves open to difficulties of another 
kind ; for on comparing the state and capacity of the 
lungs, both in man and in the Guinea pig, with the 
actual volume of air which they respectively afford, 
how, it may be asked, are we able to account for the 
retention of a quantity of air in the lungs, apparently 
superior in volume to the capacity of the containing 
organs ? Even at the ordinary state of temperature 
and density in which the air enters the lungs, this 
difficulty presents itself; and exposed, as this air af- 
terwards is, to so great an increase of temperature, 
its volume, in conformity with the mechanical con- 
stitution of gaseous bodies, will be proportionally 
augmented. To us there occurs no other mode of 
relieving ourselves from this difficulty than by sup- 
posing that the air, instead of expansion, really suffers 
a reduction of its elasticity, in the lungs. 
609. We have seen (114.), that the air, on enter- 
ing the lungs, is diffused through an infinite number 
of small cells, whose diameters do not exceed the T V 
part of an inch ; and if these cells, as some have sup- 
posed, possess a contractile power, either derived 
from their own structure, or from the action of the 
thick vascular plexus that is distributed upon them r 
