SWINE PLAGUE 55 
surface shows grayish lines usually arranged in curves and circles. 
These, so far as determined, represent the cut outlines of the inter- 
lobular and peribronchial tissue infiltrated with cells. It has already 
been stated that these lines may represent the paths along which the 
swine-plague bacteria invade the lungs from the pleural surface. 
The cut ends of the bronchi of the ventral lobes are frequently 
occluded with thick, whitish pus; in the other lobes a reddish froth 
is usually present. Rarely they also contain thick glairy mucus in 
which particles of dry pus and lung worms are imbedded. The con- 
tents of the air tubes in the ventral lobes may have been derived 
from the overdistended alveoli, or else a broncho-pneumonia may 
have preceded the swine-plague pneumonia. 
In microscopic sections of diseased lung tissue the alveoli and 
smallest air tubes are found distended with cell masses consisting 
chiefly of leucocytes. Usually there is very little fibrin and very few 
red corpuscles in the alveoli, even in cases in which the disease was 
quite recent. It may be that the stage represented in ordinary 
croupous-pneumonia by the presence of fibrin in connection with the 
cellular elements is very brief, and that it is speedily replaced by 
large numbers of leucocytes. The large predominance of these 
elements in some portions of the lungs, as well as beginning fatty 
degeneration, is probably the cause of the regular mottling of the 
lungs, as seen from the surface. The little yellowish hazy dots 
represent alveoli surrounded by the hyperemic walls. 
The necrotic and caseous changes so frequent in swine plague are 
most interesting. The latter are usually quite small and disseminated 
in large numbers over the diseased lobes. The former represent 
larger masses from a marble to a horsechestnut in size. They repre- 
sent tissue which has been destroyed by the rapid multiplication of 
swine-plague bacteria in particular localities. Hence they are found 
in all stages of the pneumonia. The large caseous masses may be 
considered as the result of a slow death of larger areas of lung tissue, 
due primarily to the gradual overdistention of the tissue by leuco- 
eytes, and hence the gradual cutting off of the blood supply. One is 
a rapid death due directly to highly virulent bacteria, the other a slow 
death, or a kind of dry suppuration in the later stages of the pneu- 
monia, characteristic of the pig, and due indirectly to the irritation 
of perhaps more attenuated races of bacteria. In some cases there 
are extensive hemorrhages in the interlobular connective tissue. 
