MECHANISM OF EMPHYSEMA. 595 
upon one unsupported assertion by Laennec; whereas, if the 
real cause of emphysema were such as above described, no 
singer or wind-instrument player could, in all probability, remain 
long exempt from this disease. But it would require further to 
be known, whether an increased liability to emphysema in this 
class is not accompanied with a similar proclivity to other pul- 
monary affections, before the question could be decided on such 
grounds. 
But the most serious objection to the expiration-theory of this 
disease is, that the expiratory act is mechanically incapable of 
producing distention of the lung, or of any part of it The act 
of expiration tends entirely towards emptying the air-vesicles by 
the uniform pressure of the external parietes of the thorax upon 
the whole pulmonary surface ; and even when the air- vesicles 
are maintained at their maximum or normal state of fulness by 
a closed glottis, any further distention of them by the expiratory 
force is as much out of the question as would be the further dis- 
tention of a bladder blown up and tied at the neck, by hydrosta- 
tic or equalised pressure applied to its entire external surface. 
The air-vesicles can sustain no distending pressure from the 
column of air within the tubes, as that air only becomes com- 
pressed in virtue of a force acting on the exterior of the lung, 
which opposes exactly as much resistance without as it creates 
pressure within. It is singular that a theory so radically un- 
sound, and so devoid of direct proof, as this of the production of 
emphysema by expiration, should have been allowed to main- 
tain a place in medical literature. 
The only theories of emphysema which remain, are those 
which refer it to the act of inspiration. The most usual form 
assumed by these theories, is the supposition that emphysema 
of the lung is a physiological compensation for the occlusion of 
a diseased portion of lung; — a view not only giving no real ex- 
planation, but totally inconsistent with the fact, that in truly 
morbid emphysema there is always a diminished respiratory 
surface, and consequent dyspnoea. Dr. Williams, however, and 
some others, have placed the inspiration-theory in a more ten- 
able position, — supposing that, when certain portions of the lung 
are occluded, the air is brought by inspiration to penetrate with 
greater force, and in greater volume, into the remaining parts. 
This view is certainly near the truth, and is quite consistent 
with clinical stethoscopic experience. But it is clogged in Dr. 
Williams’s work with a reference to the incompetent expiration- 
theory of Laennec, as if the author did not see his way clearly 
to the explanation of all cases of emphysema by his own. More- 
over, it is not the whole truth; because certain obstructive le- 
sions have, as we have seen, no appreciable influence in causing 
