198 Report on the Pathological Anatomy of Pleura-pneumonia. 
and set up general inflammation ; and the part of the membrane 
corresponding to the infarction is always cut off from its supply 
of normal nutrition. 
There can be no doubt, however, that in the tissues affected 
by this disease there must exist, or be produced, some material 
with infective properties, and therefore there can be no diffi- 
culty in finding a cause of the inflammation of the susceptible 
serous membrane. 
I am thus forced to believe that the pleurisy is invariably 
secondary to the disease of the lung parenchyma. The next 
question then must be, what is the immediate cause of the 
pulmonary lesion ? 
It has been said that different kinds of morbid changes are 
found side by side in the lung, the immediate exciting causes of 
which must be quite distinct. The most constant of these is the 
opaque consolidation, which is comparatively deep-seated, and 
localised to a conical territory, gradually increasing in extent 
and induration. A second, the most wide-spread change, seems 
to be an interstitial exudation or interlobular pneumonia starting 
from the pleura, or from the last-mentioned lesion. A third, 
perhaps the most distinct and striking, is the black haemorrhagic 
infarction. 
The immediate exciting cause of the last mentioned morbid 
process seems clear enough. We know that, when the lining 
membrane of a blood-vessel is injured, the blood coagulates at 
that point, and this coagulation increases if the irritation con- 
tinues to destroy the smoothness of the inner coat. Thus we have 
thrombosis, leading to vascular plugging. The modus operandi 
of vascular occlusion in causing haemorrhagic infarction of the 
lung is now well understood, and has already been referred to 
(see p. 194). It only remains, then, to search for the origin of 
the irritation of the vessel-wall in order to complete the patho- 
geny of this lesion. 
A ready cause of the disease of the wall of the vessels is 
afforded by the engorged state of the peribrondiial lymph- 
vessels. The entire broncho-vascular system is surrounded by 
one set of lymph-vessels, through which the irritative processes 
can penetrate with great facility (see Fig. 2, p. 190). When the 
lymph-channels around the vessels are thus engorged, one can 
easily understand how the chronic inflammatory processes infect 
their proper walls, and finally reach the intima. 
There appears no reason to doubt that this chronic inflam- 
mation, which takes place in the connective tissue sheath of the 
broncho-vascular system, is the immediate cause of the great 
thickening of the walls of the vessels, which always precedes, 
and is more extensive than, the injury to the inner coat. 
