umonia. The light yellowish
portions represent healthy lung tissue; the red represents the disease. It
will be noticed that the lines between the lobules are quite faint,
indicating little or no inflammation of the connective tissue between the
lobules. The healthy lung tissue is seen to be raised above the level of
the diseased portion. In contagious pleuropneumonia the exact reverse is
the case, the diseased portions being very much larger than the healthy.

PLATE XXXI. Contagious pleuropneumonia. Appearance of a cow's lung affected
with contagious pleuropneumonia when sections or slices are made of it and
cut surfaces examined.

Fig. 1. Transverse section through the right principal lobe in a case of
acute pleuropneumonia. The area drawn includes the air tubes, veins, and
arteries, and illustrates the great thickening of the interlobular
connective tissue into broad whitish bands and of the walls of the air
tubes, veins, and arteries: _a_, air tube cut obliquely; _a'_, air tube cut
directly across; _b_, arteries cut across; _c_, large vein completely
occluded by a thrombus or plug formed during life. The great thickening of
the walls of the artery and vein in this disease is especially brought out
by stating that in the healthy lung they are so thin as to be easily
overlooked.

Fig. 2. Transverse section of the principal lobe in a case of acute
pleuropneumonia, illustrating the different kinds of hepatization or
consolidation of the lung. These are indicated by the different colors from
dark red to reddish yellow. This variation of color is regarded by some as
the real marbling characteristic of pleuropneumonia, while the whitish
bands penetrating the lung tissue in all directions constitute the true
marbling according to other observers.

PLATE XXXII. Contagious pleuropneumonia. This illustrates what are called
infarctions. The right half of the figure shows nearly normal lung tissue.
The left represents a blackish mass, in which the lung tissue is filled
with blood and solidified. This is caused by the plugging of the vein
carrying away the blood from this portion. The heart forces the blood
through the artery into the tissue at considerable pressure, but owing to
the fact that its return is prevented, the minute blood vessels rupture and
the air vesicles become distended with blood, which coagulates and causes
the firmness of the tissue.

* * * * *

On the other hand, it is known