there may be others which approach the
normal lung tissue in color, are soft, and float in water. From these a
milky, purulent fluid may often be expressed. These different shades are
represented in Plate XXXI, fig. 2, within a small compass. Some authorities
are inclined to consider these variations in color on the same cut surface
as the so-called marbling of pleuropneumonia. It matters not whether we
regard the bands between the lobules or the varying shades of the lobules
themselves as the marbling, provided either or both are peculiar to
contagious pleuropneumonia. If we examine the blood vessels appearing on
such cut surface they will usually be found plugged within the firmly
hepatized regions. The artery contains a dark, soft, removable clot, the
vein a grayish-pink, granular, fragile plug (thrombus), which adheres
firmly to the wall of the vein, and if this is slit open, indications of a
diseased condition of the inner coat will be readily detected. When large
regions of the lung tissues are hepatized, the main air tube and its
branches are usually filled with grayish, cylindrical branched masses of
fibrin that are easily removed, as they do not adhere to the mucous
membrane.

The views of pathologists differ as to the nature of the earliest changes
in pleuropneumonia, and it is not within the scope of this work to present
controverted or imperfectly developed theories. In the foregoing
description we have taken as a type the acute pleuropneumonia in its fully
developed phase, which can scarcely be mistaken for any other disease. We
have seen that there is an inflammatory condition of the connective tissue
between the lobules, resulting in the exudation of coagulable lymph. This
inflammation is equally marked around the blood vessels and air tubes. It
leads to inflammatory changes in the inner wall of the veins, and these
cause the deposition of thrombi or plugs in the vessels, which prevent the
return of the blood. The blood pumped into the lung tissue through the
artery, but unable to get out by way of the vein, leaves the mesh-work of
capillaries around the air vesicles, enters the latter, and produces the
firm, hepatized condition so characteristic of this disease. If we bear in
mind that the veins in different parts of the lung tissue are plugged at
different times, and that, therefore, the affected regions are in different
stages of disease, it will be easily understood how the different shades of
color fr